Design and synthesis of carbon-based nanomaterials for electrochemical energy storage

ZHU Cheng-yu, YE You-wen, GUO Xia, CHENG Fei

（National-local Joint Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization, Tianjin Key Laboratory of Chemical Process Safety, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China）

Abstract： Because of damage to the environment and the energy crisis, the storage and use of sustainable energy, such as solar and wind, has become urgent.Much attention has been given to the use of electrochemical energy storage (EES) devices in storing this energy.Electrode materials are critical to the performance of these devices, and carbon-based nanomaterials have become extremely promising components because of their unique and outstanding advantages.The structure design and controllable synthesis of electrode materials determine the electrochemical performance of EES to a large extent.In this review, strategies for carbon-based materials of different dimensionalities are summarized and their uses in different EES devices are given, providing an in-depth understanding of the relationship between material structure and electrochemical performance.Prospects for the design and synthesis of carbon-based nanomaterials with exceptional performance for EES devices are given.

Key words: Electrochemical energy storage；Carbon-based materials；Different dimensions；Lithium-ion batteries

1 Introduction

With the rapid economic development, traditional fossil fuels are further depleting, which leads to the urgent development and utilization of new sustainable energy sources such as wind, water and solar energy[1-2].In view of the intermittence of these emerging energy sources, it is essential one of the top priorities to meet the rapid development requirements of energy storage and conversion devices[3-7].Furthermore, in the wake of the gradual popularity of smart devices and 5G systems, the miniaturization, flexibility, and durability design of energy storage devices have become utterly desirable.The electrochemical energy storage (EES) devices, among the outstanding ones, including lithium-ion batteries (LIBs), sodiumion batteries (SIBs), potassium-ion batteries (PIBs)and super-capacitors (SCs), have aroused great interest of researchers[8-11].Nevertheless, it is generally expected for various classes of EES devices to possess comprehensive performance composed of high power/energy density, steady cycle-ability, excellent safety, which largely rests on electrode materials.The related researches on advanced nanostructured materials have been extensively conducted about the acquirement of electrodes with high specific surface area, fast electron and ion transport, and long-lasting structural stability, so as to realize efficient energy storage[12-13].

Among the numerous energy storage nanomaterials, carbon nanomaterials esstentially occupy a place for their high abundance, excellent conductivity and stability, and low environmental pollution[14-16].So far,there have been many reports on the synthesis and application of different types of carbon materials (e.g.graphene[17-18], activated carbon (AC), carbon nanotubes (CNTs)[19-20], carbon nanofibers (CNFs)[21-22],etc.) in the field of EES devices.Notably, benefitting from their outstanding properties, these devices built by carbon nanomaterials electrodes always own superior power density and stable capacity under a longcycle process.However, compared to other types of nanomaterials such as metal oxide/sulfide, siliconbased materials, phosphorus based materials, etc., the theoretical capacity of carbon materials is generally lower.On the contrary, these non-carbon electrode materials are excluded from practical applications due to their poor cycle stability resulting from the irreversible reactions[23-24].In this context, advanced carbonbased nanomaterials (CBNMs) for EES that combinecarbon materials with other high-capacity materials are designed and fabricated to conquer the above shortcomings deriving from optimization in conductive, electrochemical, and nano-structural attributes through the cooperative effect of the different components[25-29].

CBNMs, as highly representative electrode materials for the EES domain, have received increasing attention in recent years because of their fascinating performance.These CBNMs own abundant pore structure, large specific area, prominent conductivity and good stability, which are particularly beneficial for the improvement of electrochemical performance.On the other hand, it is ready to adjust the structure of carbon-based materials to form nanomaterials with different dimensions such as zero-dimensional (0D)nanoparticles or nanospheres, one-dimensional (1D)nanofibers or nanorods, two-dimensional (2D)nanosheets or nanofilms, three-dimensional (3D) complex forms.The small size effect of 0D CBNMs increases the specific surface area and reactivity of the materials, provides more active sites, as well as weakens the absolute volume change[30-31].1D CBNMs own linear transport path of ions and electrons, which can lead to the promotion of ions transport and the acceleration of charge transfer between electrode and electrolyte[32].For 2D CBNMs, the adjustable layer spacing, large specific surface area and nanosheets flexibility endow them the potential for electrode materials with excellent performance[33-34].3D CBNMs possess high interconnection conductivity and outstanding structural mechanical stability, which are essential to excellent electrochemical performance[35-36].The design of these structural dimensions[37-42]can not only improve the electrochemical performance of the electrode materials, but also provide suitable choices for different application conditions and environments.

Rested on the many advantages of CBNMs, numerous efforts have been devoted to realizing their advanced development in the field of EES applications[43-46].However, from the perspective of different dimensional structures in CBNMs, there are few relevant contents to comprehensively summary the relationship between dimensional structures and electrochemical properties.Therefore, it is significantly helpful and necessary to sort out, discuss and summarize CBNMs for the EES applications in recent years, in the view of the design and synthesis of different dimensional structures (from 0D to 3D).In this article, we aim to capture the recent advances in CBNMs consisting of dimensional structure for EES devices.As shown in Fig.1, CBNMs in different dimensions are classified, mainly including: 0D, 1D, 2D and 3D, meanwhile the advanced achievements about these different-dimensional materials are discussed.The practical application and optimization effects of these CBNMs in diverse EES devices such as LIBs,SIBs, PIBs and SCs, are presented.In addition, the key challenges and outlook for the development of CBNMs are proposed.It is hoped that this review can facilitate in-depth understanding of the relationship between material structures of different dimensions and provide some meaningful guidance for high-performance CBNMs in energy storage.

2 Design and synthesis of advanced CBNMs with different dimensions for EES devices

Carbon-based materials play a vital part for actual production and life, and because of the good mold-ability, it can form different microstructures.Notably,the carbon-based materials themselves include pure carbon materials (such as graphene, carbon nanotubes,etc.) and carbon-based composites, and various carbon-based nanocomposites are also designed and prepared using pure carbon materials of different dimensions.These advanced CBNMs can be itemized in different dimensions (0D, 1D, 2D, 3D), which all exhibit unique characteristics and physicochemical properties.The material definitions of different dimensional structures are as follows: 0D nanomaterials refer to materials in the range of 1-100 nm in three dimensions, such as: nanospheres, quantum dots; 1D nanomaterials usually possess a high ratio of length to diameter, including nanotubes, nanorods, nanowires; 2D materials refer to materials in which electrons can only move freely (planar motion) on the nanometer scale (1-100 nm) in two dimensions, such as:graphene, MoS2; 3D materials refer to a composite material composed of one or more basic structural units in 0D, 1D and 2D.Beyond doubt, it has been proved that CBNMs have exhibited excellent potential for the EES devices.So far, there has been a lot of researches on the application of CBNMs in the field of energy storage.Therefore, in this part, we will introduce the practical applications and optimization effects of various CBNMs in different EES devices including LIBs, SIBs, PIBs and SCs.

2.1 Advanced CBNMs for LIBs

As a representative of the rapidly developing rechargeable battery technology, LIBs realize charges storage using lithium ions as charge carriers.During the charging process, lithium ions are transferred from the cathode electrode to the anode electrode through the electrolyte, and the opposite is true when discharging.Some lithium-ion battery anode electrode materials, such as graphite, have been commercially applied in electronic devices.Meanwhile, the increasing demand for energy storage further stimulates the accelerated development of fresh electrode materials with larger capacity and better cycle performance.Although significant attempts have been paid to modify the energy density, the current commercial electrode materials of LIBs cannot match the urgent demands for excellent properties energy storage especially in large capability, high rate performance and absolute security.Structure engineering of electrode materials is essential to the enhancements of conductivity and cycling life.Currently, due to appropriate structural engineering, CBNMs own excellent lithium storage performances, and they are one of the most promising materials for LIBs according to their abundant resource, high conductivity, and easy engineering.Dimensional design of CBNMs provides great opportunity to improve their performance and expand the application in LIBs.

2.1.1 Design and synthesis of 0D CBNMs for LIBs

0D CBNMs, such as nanocages, nanoparticles,nanospheres, quantum dots (QDs) and so on, have also been researched and applied in LIBs in addition to luminescent materials and solar cells in recent years.0D CBNMs generally possess large specific surface area, excellent electrical conductivity and good mobility[47], which are essential properties governing the electrochemical performance of electrodes.A class of 0D carbon-based materials, namely nanoparticles coated with carbon layers, is widely used in the field of LIBs.For instance, Zhang et al[48]designed and prepared SnOx(x= 0, 1, 2) quantum dots@carbon hybrid (SQD@C), by a binary oxide-induced surface-targeted coating of ZIF-8 followed by pyrolysis approach (Fig.2a).In this products, SnOx

quantum dots (under 5 nm) were dispersed homogeneously throughout the nitrogen-containing carbon nanocage.Used as the anode of LIBs, the SnOx@C revealed an ultrahigh reversible capacity of 1 824 mAh g?1at a current density of 0.2 A g?1, and superior capacities of 1 408 and 850 mAh g?1even at high current densities of 2 and 5 A g?1, respectively (Fig.2b and c).For another example, Cheng et al.[49]reported an effective approach of coating nano metal carbonate with polydopamine, followed by pyrolysis and carbonization to prepare metal-oxide@carbon.In this process, nanosized particles were obtained by decomposition of the confined carbonate, meanwhile the generated CO2gases mainly plays two roles: on theone hand, creating conditions for the generation of interconnected mesopores through the particles and micropores in the carbon layer; on the other hand, offering gentle oxidizing atmosphere to facilitate the formation of pure oxidized phase.Recently, Cao et al.[50]designed and fabricated the novel dual carbon shells coated SnO2hollow nanospheres (C@SnO2@C) for LIBs anode in which the double carbon shells played a role in conducting electricity, and provide a protective shell to retard the aggregation of nano-sized SnO2and structural collapse.Due to the advantages of dually coated structure based space confined protectionstrategy, the C@SnO2@C hollow nanospheres exhibited a large specific capacity during first cycle(1 483.9 mA h g?1at 0.2 A g?1) and a reversible capacity of 713.8 mAh g?1after 300 cycles, corresponding to a capacity declining of 0.071% per cycle from 2ndto 300thcycle.

Apart from metal oxides/sulfide coated with carbon layers, some non-metal element nanomaterials possessing higher theoretical capacity, are also conducted to coat with carbon layers for improved energy storage properties.Silicon nanomaterials, as emerging high-capacity materials, are very likely to become one of the outstanding under the post-graphite anode era in the near future.However, its low conductivity and large volume change critically limit its application in energy storage materials[51-52].Mi et al.[53]prepared yolk-shell structured silicon@void@carbon (Si@void@C) composites using a self-sacrifice template strategy (Fig.2d).The images of SEM and TEM (Fig.2e, f and g) showed that the carbon layer is coated on the outside of the silicon nanospheres and there is a void between the silicon and the carbon layers.The yolk-shell structured silicon@void@carbon (Si@void@C) composites exhibited an enhance cycle stability for lithium-ion batteries, which delivered a capacity of 854.1 mA h g?1at a current density of 0.2 A g?1after 200 cycles.Wang et al.[54]synthesized a porous silicon microsphere@C (pSiMS@C) core-shell composite, which was composed of an amorphous carbon shell and the porous silicon microsphere core.And when the pSiMS@C electrode was used as anode, a high reversible specific capacities of 1 485.3 mA h g?1was obtained at 0.5 A g?1, and a good capacity retention (1 027.8 mA h g?1after 500 cycles at 1 A g?1).This 0D yolk-shell structure can provide a satisfactory void for the expansion of the silicon as well as more active sites.Moreover, the formation of carbon layers framework can decrease the diffusion distances of lithium ions and electrons.Notably, it’s ready to find out through these similar reports, in the carbon-coated 0D nanomaterials, the buffering effect and enhancement of electron conduction of the overall structure of the active material surrounded by the conductive carbon layer play a key role to the improvement of electrochemical performance.

0D pure carbon nanomaterials have also received attention in the field of LIBs.Zhao et al.[55]prepared graphene hollow nanospheres (GDY-HNS) by simple solvothermal synthesis technology, in which CuO nanospheres are both the template and the source of catalyst.GDY-HNSs show enhanced electrochemical properties, including higher reversible specific capacity, improved rate capacity and cycling capability compared with most of other carbon materials, even in a relatively high mass loading of 2.0 mg cm?2.Yang et al.[56]directly carbonized zeolitic imidazolate framework-8 nanospheres which were prepared through an emulsion-based interfacial reaction, to obtain N-doped hollow carbon nanospheres (NHCNSs).The NHCNs showed a high surface area of 1 083 m2g?1and high N content of 16.6 at%, which displayed a high capacity of 2 053 mA h g?1at 100 mA g?1and 879 mA h g?1at 5 A g?1after 1 000 cycles, indicating an excellent cyclin-life.

2.1.2 Design and synthesis of 1D CBNMs for LIBs

1D nanomaterials, such as nanotubes, nanorods,nanofibers generally own large length-to-diameter aspect ratios.After CNTs were the earliest 1D nanomaterials discovered by Iijima[57], 1D CBNMs with homologous nanostructures have been extensively researched.For 1D nanomaterials, the structural characteristics mainly embody nanoscale and microscale joint effects, high aspect ratio and oriented growth direction[58].Therefore, these characteristics make 1D materials able to form a more effective contact with the electrolyte, and shorten Li-ion diffusion distances.When used as an electrode materials for lithium-ion batteries, its structural advantages can effectively buffer volume changes and significantly alleviate pulverization during cycling.In addition, the existence of carbon materials in 1D CBNMs delivers confinement effects and improves the electronic conductivity of the materials.The successful construction and application of 1D CBNMs provide more possibilities and options for high-performance LIBs.

Similar to 0D carbon-based materials, it is a common form in 1D CBNMs that the carbon layers are coated, embedded or encapsulated with the main body materials for capacity contribution as the core.Polymerization-assisted synthesis using organic-inorganic hybrid materials as precursors and electrospinning has been widely conducted in the construction of 1D CBNMs.For instance, Gao et al.[59]fabricated MoC/C nanowires (MoC/C NWs) via pyrolyzing MoOx/p-methylaniline precursor under argon flow at 700 °C (Fig.3a).In these nanowires, the η-MoC nanocrystallites (5-6 nm) were uniformly distributed into carbon NWs framework.The MoC/C nanowires exhibited excellent cyclic stability even at high current densities (455.4 mA h g?1after 2 000 cycles, with average capacity loss rate of only 0.004% per cycle).Joo et al.[60]introduced a universal method to compound metal sulfides into carbon nanofibers (CNFs) through a mature electrospinning process.In this hybrid, a series of metal sulfide nanoparticles, such as Co9S8,MnS, NiS, and Cu1.96S, were evenly distributed in CNF networks.The composites as anodes for LIBs,exhibited outstanding cycling ability and excellent rate performance.Furthermore, 1D nanostructures can also be constructed with the help of metal organic frameworks (MOFs).Dai et al.[61]developed metal?organic framework (MOF)-structured porous ZnCo2O4/C nanofibers by electrospinning, followed by in situ growth and annealing (Fig.3b).As shown in Fig.3c and d after in situ growth, the ZIF-67 particles were anchored on the PAN fibers.In this work, the ZnCo2O4/C nanofibers showed fascinating properties including robust pores, high specific surface area (148.7 m2g?1), the abundant channels offered by the MOF structure, the intrinsic stability of the 1D fiber and excellent conductivity.After 100thcycling, a large specific capacity of 1 145 mA h g?1was displayed for ZnCo2O4/C, meanwhile it exhibited discharge capacities of 1 501, 1 206, 701, and 450 mA h g?1at different current densities.

Carbon layer encapsulating or coating on 1D nanostructures which are effectively synthesized by hydrothermal method can also greatly improve the conductivity and stability of the overall structure.For instance, Gao et al.[62]constructed graphene foam N, S co-doping with 1D ultra-long α-MnS nanowires coated graphene scrolls (α-MnS@GSC/GF) (Fig.3e).This interesting combination of structures mainly included: first crimping graphene sheets to coat MnO2nanowires through a hydrothermal reaction, and then performing subsequent vulcanization process.The images of SEM and TEM (Fig.3f, g, h) clearly displays the 1D ultra-long linear morphology of the α-MnS@GSC nanocables.Based on the unique structural merits, the α-MnS@GSC/GF electrode revealed remarkably enhanced rate (406 mA h g?1at 2 000 mAg?1) and cycling capability (519 mAhg?1after 400 cycles at 1 000 mA g?1).Yang et al.[63]fabricated yolk-shell MnOxnanostructures within carbon nanofibers in a botryoid morphology by electrospinning a manganese-based metal organic framework (Mn-MOF).As an anode of LIBs, the optimized ysMnOx@NC-2, exhibited a high reversible capacity of 880 mA h g?1at the current density of 0.1 A g?1,which was based on the overall weight of the positive and negative electrode.

In addition to the 1D nanostructure coated with the carbon layers, the active materials growing on the surface of 1D carbon material as a supported framework is also a widely used electrode material.Various 1D carbon materials such as carbon nanotubes and carbon nanofibers could become candidates as the supported frameworks.Lou et al.[64]designed a hierarchical hollow growth of siphonate molybdenum disulfide flake on CNTs, which delivered a very high specific capacity of ～ 1 320 mA h g?1at a current density of 0.1 A g?1, exceptional rate capability, and an ultra-long cycle life of up to 1 000 cycles.

2.1.3 Design and synthesis of 2D CBNMs for LIBs

Since graphene was discovered and synthesized in 2004, the popularity of 2D nanomaterials has continued, and materials with similar structures such as MoS2, Mexne, carbon nanosheets, etc., have been studied successively.For large specific surface area,2D CBNMs can significantly reduce the local current density and thus inhibit the growth of Li dendrite.Thelength of transportation could be effective shortened by the interaction between 2D structures and Li+or electrons, significantly improving electrochemical performance of LIBs.

Chen et al.[65]designed a unique heterostructure with 2D SnSe2nanosheets supported on graphene,where the chemical coupling effect is formed between graphene and SnSe2by a one-step hydrothermal method (Fig.4a).The chemically coupled bond between graphene and SnSe2nanosheets formed in the 2D/2D“SnSe2-on-graphene” heterostructure hybrid can effectively prevent the agglomeration of tin atoms generated during electrochemical reactions.The 2D/2D SnSe2/graphene heterostructure revealed a specific capacity of 490.9 mA h g-1at a current rate of 0.1 C,showing an ultra-long cycling stability with capacity retention of 59.3% after 1 500 cycles at 0.5 C, and it was able to work normally at special temperatures range (?30 - 70 °C).Wang et al.[66]reported ultrathin amorphous carbon nanosheet material which were fabricated by a molten-salt medium (Fig.4b).The amorphous petroleum-asphalt-derived molten-salt carbons (MSCs) nanosheets materials delivered a high reversible capacity, outstanding rate performance, and superior cycling ability (579 and 396 mA h g?1at 2 and 5 A g?1after 900 cycles).Chen et al.[67]prepared successive porous micro-spheres constructed from defect-rich, interlayer-expanded, and few-layered MoS2/C nanosheets (MoS2/C-CPM) via a facile CS2-templating hydrothermal method.For the MoS2/CCPM anode, the reversible capacities of 832, 752,540, 436 and 344 mA h g?1were recieved at 0.1, 0.2,0.5, 1 and 2 A g?1, respectively.Jia et al.[68]sythesized an oxygen-deficient SnO2?δ/C composite with rational nanoarchitecure design containing porous SnO2?δnanofllake arrays coated with carbon as anode of LIBs(Fig.4c).The coating of the carbon layer could effectively increase the conductivity of SnO2?δ, while greatly reducing the agglomeration effect and volume change during the charging and discharging process.As a result, the as-prepared porous SnO2?δ/C nanosheet arrays could show a large reversible specific capacity of 1 378.6 mA h g?1with a high initial coulombic efficiency of about 74.3% at a current density of 0.1 A g?1.And in our work[69], a MnO@C hybrid with crosslinked nanoflake structure was sythesized.In Fig.4d and e, the thin carbon nanosheets form an interconnected open structure.When as anode applied for LIBs, the prepared MnO@C hybrid retains an excellent capacity (868 mA h g?1at 0.2 A g?1over 300 cycles and 668 mA h g?1at 1 A g?1over 500 cycles),indicating a high Li-storage capacity and superior cycling stability.In addition, we[70]further used biomass kelp as a precursor by a similar method to obtain a porous network-structure MnO/C hybrid which owned abundant porous structure and good conductivity, resulting in better rate performance and cycle stability.

In addition to metal compound-based 2D carbonbased materials, some non-metallic elements and their oxides have also shown great potential to become high-performance materials in their composites with carbon materials in recent years.Furthermore, Mu et al.[71]prepared a graphene-like carbon/silicon composite (NRC/Si) with a high-nitrogen content through self-assembly of citric acid, melamine, and the amino carboxyl group of Si-NH2.The 2D carbon nanosheets in the NRC/Si composite could effectively buffer the volume change of silicon material and their high-nitrogen doping promted the electronic conductivity and facilitates the charge transfer during cycling process.The NRC/Si as anode material for LIBs showed good cycle stability and rate capability, delivering stable cycling performance (1 000 mA h g?1after 300 cycles at 2 000 mA g?1and 572 mA h g?1even at 5 000 mA g?1, respectively).Recently, Liu and coworkers[72]constructed by wrapping SiOxinto PVA derived carbon layer and N-doped carbon (NC)nanosheets derived from exfoliated chitin nanosheets.The SiOxnanoparticles tightly encapsulated by the chitin nanosheets and PVA are transformed to robust SiOx@NC composites after pyrolysis.When applied for the anode of LIBs, the prepared SiOx@NC-2 anode delivers a stable cycling performance and competitive rate capability.After 300 cycles at 500 mA g?1,the SiOx@NC-2 still exhibited a relatively high capacity of 602 mA h g?1.

2.1.4 Design and synthesis of 3D CBNMs for LIBs

Since a large part of three-dimensional CBNMs are assembled or combined by other dimensions of nanomaterials in different ways, they have the same properties as other types of nanomaterials, such as large specific surface area and porous structure.Apartfrom these characteristics, three-dimensional interconnected structures are frequently contained in 3D CBNMs, which effectively increases the transfer rate of ions and electrons between nanoparticles.Moreover,the shortage of 1D and 2D nanomaterials derived from evident aggregation could be meaningfully conquered by 3D nanostructure possessing large advantage, and specially, some 3D CBNMs be utilized as free-standing electrodes without deviation, greatly improving the efficiency of preparations process.

Ke et al.[73]used a large-scale and controllable method for preparing ultrathin MoS2nanosheets adhered on 3D hierarchical nanohybrids with SnS QDs,which were decorated on a carbon nanosheet network(MoS2@SnS-QDs/CNN).MoS2@SnS-QDs/CNN-2 provided a reversible capacity of 713 mA h g?1after 1 000 cycles at a current density of 2 A g?1.Cheng et al.[74]developed an effective method to synthesize high LiFePO4loading cathode material with steady cycling and good rate capability for LIBs by encapsulating LiFePO4nano-crystallites inside the coralloid,conductive and interconnected carbon structure(Fig.5a).As shown in Fig.5a, the carbon framework consists of many branches, which interpenetrates to form a 3D open porous structure with a coral-like morphology and the LiFePO4/C composites inherit the morphology of the carbon framework, which further confirms that LiFePO4crystallites nucleated and grew within the mesopores without affecting the external morphology of the carbon support.Based on the advantages, the LiFePO4/C cathode material exhibited a stable and high reversible capacity of 144.6 mA h g?1at 0.1 C and 60.4 mA h g?1at 20 C (1 C=170 mA g?1).Moreoveer, it could maintain 96.7% of its initial capacity at 10 C over 1 000 cycles with an ultrahigh specific power of 5 114 W kg?1.

In recent years, as a good carrier for the preparation of three-dimensional network structure, biomassderived carbon has attracted the attention of researchers due to the characteristics of clean, non-polluting and abundant reserves.Various researchers have synthesized carbon-based materials using biomass precursors that shows 3D nanostructure.For instance,Han et al.[75]reported 3D interconnected Fe3O4@C core@shell nanocomposites utilizing chitosan as both structural directing agent and carbon precursor to realize small particle size and an in-situ formed carbon coating layer.In this work, an conformal and continuous chitosan coating layer can be spontaneously formed on Fe3O4particles due to the strong coordination between hydroxyl and amino groups of chitosan and Fe3+ions, which then converted into N-doped carbon after calcination.These composites delivered a reversible capacity of 1 116.1 mA h g?1at 100 mA g?1and 587.2 mA h g?1at 2 000 mA g?1, as well as stable cycling life.

Lee et al.[76]designed and synthesized a gyroid 3D network Si@SiOx/C nanoarchitecture (3D-Si@Si-Ox/C), which had structural stability and superior ion/electron transport.The 3D-Si@SiOx/C anode could still maintain 85% of the discharge capacity(1 342 mA h g?1) at a high current density of 4 A g?1(compared to that at 0.2 A g?1).Furthermore, it owned a higher cycle stability of 83.3% even after 100 cycles than the pure Si particles.Yang et al.[77]designed a facile strategy to fabricate three kinds of 3D hierarchical architectures employing manganese 1, 3, 5-benzenetricarboxylate (Mn-BTC) for the templates (Fig.5b).Among these three samples, the optimized MnSe/C delivered a high reversible capacity of 885 mA h g?1at 0.2 Ag?1and a capacity of 880 mA h g?1at a rate of 1 A g?1after 1 000 cycles as the anode material for LIBs.Wang et al.[78]introduced a Ge QDs uniformly distributed into 3D N-doped carbon framework(3DOP Ge@NC) material as a binder-free anode for LIBs using polystyrene colloidal nanospheres as a template (Fig.5c).A maximum discharge capacity of～1 160 mA h g?1was obtained, and the obtained 3DOP Ge@NC still maintained a high capacity of～500 mA h g?1at 40 A g?1implying its superior rate capability.In addition, this composite also showed long cycling-life (only ＜10% capacity fading after 1 200 cycles at 5 000 mA g?1).

On the whole, in the field of LIBs, CBNMs (regardless of carbon composites or pure carbon materials) are still a fascinating choice for the pursuit ofgood performance, such as high rate performance, excellent cycle stability, etc.CBNMs with different dimensional structures all exhibit their own advantages.The use of CBNMs in LIBs can effectively fulfill the increasing conductivity and large specific surface area of electrode materials, thereby satisfying a higher reaction rate.However, a high specific surface area for CBNMs will also lead to a decrease in tap density, an increase in side-reactions and unsafety.Therefore, a careful design of surface and structure to balance the high reactivity and side-reactions of the CBNM electrodes is an effective measure to improve its defects.It is worth noting that the design and synthesis of CBNMs in combination with different dimensional structures may be one of the best ways to achieve further breakthroughs in performance.

2.2 Advanced CBNMs for SIBs

Compared with metal lithium, the adjacent metal sodium in the periodic table has the characteristics of high resource reserves and low cost, which promotes sodium-ion batteries to become a strong competitor for the next generation of new rechargeable battery electrode materials.Similar to LIBs, SIBs are also a rocking chair battery that stores and transforms energy through the movement of sodium ions between the cathode and anode electrodes.Sodium also has a low oxidation-reduction potential (only 0.3 V higher than lithium).In recent years, these advantages have made rechargeable SIBs a promising alternative to LIBs and have received widespread attention.However, SIBs have the following disadvantages:(1) The relative molecular mass of sodium is greater than that of lithium, leading to inferior theoretical energy density than that of LIBs; (2) The radius of Na+is greater than that of lithium, which makes it more difficult for the intercalation/deintercalation of sodium ions during charging and discharging[79-80].Thus,the rational design and structural engineering optimization of electrode materials play critical roles to improve the performance of SIBs.Lots of efforts have been carried out to develop Na+storage materials with high capacity and fast diffusion kinetics for SIBs.Among various electrode materials, CBNMs possess unique microstructure and morphology, which enable them to have higher sodium storage capacity and better rate-capacity.

2.2.1 Design and synthesis of 0D CBNMs for SIBs

0D CBNMs including carbon dots, nanospheres,etc.exhibit a large specific surface area and increase the connection of the materials and the electrolyte.Moreover, 0D nanostructure can well accomodate to the strain of sodium ion extraction/intercalation during cycling procedure, and reduce the swelling effect.Wang et al.[81]reported the design of S-doped porous hollow carbon nanospheres confined SnS particles(SnS@SPC) which possessed a large surface area of 135.8 m2g?1.For the obtained yolk-shelled SnS@SPC anode materials of SIBs, a high reversible capacity of 512 mA h g?1at 0.1 A g?1and good cycling-life(100 cycles with a capacity retention of 75%) were obtained.Xu et al.[82]synthesized rambutan-like hybrid hollow spheres of carbon confined Co3O4nanoparticles via a simple one-pot hydrothermal method followed by post-annealing (Fig.6a).When tested in sodium-ion batteries, the hollow structured composite electrode possessed a high reversible capacity of 712 mA h g?1at a current density of 0.1 A g?1, and also maintained a considerable capacity of 223 mA h g?1even at A high current density of 5 A g?1.Liu et al.[83]constructed a new type of Sb@C nanosphere anode, which is composed of Sb hollow yolk entirely confined by a well-conductive carbon layer.As for Na-ion storage, the Sb@C composite maintained a reversible capacity of ～ 280 mA h g?1at 1 000 mA g?1after 200 cycles.Yang et al.[84]designed Bi@Void@C nanospheres (Bi@Void@C) with carbon coated hollow structure (Fig.6b) showing an excellent rate capacity of 173 mA h g?1at a ultrahigh current density of 100 A g?1and long-cycle life(198 mA h g?1at 20 A g?1over 10 000 cycles).

Some 0D pure carbon nanomaterials such as amorphous carbon (soft carbon and hard carbon) can also be used in SIBs.Wang et al.[85]presented an amorphous carbon (MAC) with rich structural defects and heteroatomic doping of multiple active site decoration as anode material for SIBs.The designed MAC-600 sample could deliver an outstanding high rate performance and after 10 000 cycles, the capacity remained over 200 mAh g?1at 10 A g?1.Zhang et al.[86]synthesized a range of hard carbon spheres (HCS)with controlled architectures by carbonizing synthetic phenolic resin under a wide temperature range (900-2 800 °C).HCS treated at 1 900 °C (HCS-1900)showed a reversible capacity of 295 mA h g?1and anultra-large capacity of 248.2 mA h g?1(84% of the reversible capacity) at low-voltage platform, in which an intercalation mechanism is proposed for Na ion storage.Lu et al.[87]synthesized ultrathin Al2O3-coated hard carbon materials through direct atomic layer deposition.When used as an anode for sodium-ion batteries, the optimized Al2O3-coated hard carbon electrode had a high reversible capacity of 355 mA h g?1,a high ICE of 75% and considerable cycle life (a capacity decay of 9.3% over 150 cycles).

2.2.2 Design and synthesis of 1D CBNMs for SIBs

1D CBNMs display fast Na+diffusion kinetics,and their straight channels can impactfully boost electrical transport.Moreover, 1D CBNMs provide a large specific surface area, conductive to the improvement of specific capacity, hamper active materials from arising the agglomeration and pulverization, and endow long cycling performance.Liu et al.[88]developed a CNT-backboned mesoporous carbon confining red P composite (P@TBMC), in which P was encapsulated completely within the pores, and homogenous distributed.In these hydrides, red P as active materials was able to furnish high capacity and the multilaminar CNTs braced the structure as skeleton frame, and quickened the electrons transport.The P@TBMC composite showed a high reversible desodiation capacity (～1 000 mA h g?1at 0.05 A g?1), superior rate performance (～430 mA h g?1retained at 8 A g?1), and excellent cycling steadiness (no capacity decay for 800 cycles at 2.5 A g?1).For another ex-ample, Lou et al.[89]introduced a N-doped carboncoated bullet-like Cu9S5hollow particles, which was synthesized by a template-engaged strategy through two-step (anion/cation) ion exchange and polydopamine coating (Fig.7a).In this work, the authors used bullet-like ZnO particles as a template to obtain carbon-coated ZnS through anion exchange and polydopamine coating and carbonization, followed by cation exchange to finally obtain a bullet-like hollow nanomaterial of Cu9S5@C.The structure feature of coating the hollow nanoparticles with carbon shell can efficiently increase the electronic conductivity and ion transport efficiency, thereby enhancing the electrochemical kinetics mechanism.Benefitting from their unique structure and composition, the bullet-like Cu9S5@C hollow particles exhibit enhanced Na+storage properties with high specific capacity(＞300 mA h g?1), excellent rate capability (237 mA h g?1at 5 A g?1), and ultra-long cyclic life (79%capacity retention over 4 000 cycles).

Wang et al[90]designed and prepared a hybrid of Fe7S8nanoparticles/N-doped carbon nanofibers(Fe7S8/N-CNFs) via electrospinning (Fig.7b).Fe7S8/N-CNFs showed a good reversible capacity of 649.9 mA h g?1at 0.2 A g?1after 100 cycles, and superior cycling stability for 2 000 cycles at 1 A g?1with only 6.04% capacity decay.Sun and co-workers[91]combined electrospinning and ball milling process to prepare tin sulfide and multilayer CNTs with outer CNFs encapsulation (SnS?Sn/MCNTs@CFs), as a SIBs anode.After 1 000 cycles, a reversible capacity of 359.3 mA h g?1remained.Chen et al.[92]developed MnFe2O4@C nanofibers (MFO@C) applied for the SIBs anode.In this composite, the distribution of Mn-Fe2O4nanodots (～3.3 nm) were very uniform and porous N-doped CNFs wrapped these nanodots well.The different redox potentials of manganese and iron make them able to become self-buffering matrix and lessen the pulverization and agglomeration effect during the discharge/charge.Derived from these advantages, the obtained MFO@C anode delivered a capacitance of 305 mA h g?1even at 1 A g?1implying its good rate performance and only a capacity decay of 10% after 4 200 cycles, indicating superior cycle stability.

2.2.3 Design and synthesis of 2D CBNMs for SIBs

2D CBNMs were mentioned in SIBs electrodes materials, which have large specific surface area conducive to ionic adsorption, as well as own high electrical conductivity and adjustable interlayer spacing to facilitate the intercalation of electrons and the transfer of Na ions.Liu et al.[93]reported a simple phosphorusamine-based method for the scalable synthesis of nanoscale red phosphorus (NRP), with the utlization of commercial red phosphorus as a source.The NRP was deposited in-situ on reduced graphene oxide(rGO) having a homogeneous disposition quantitatively by this phosphorus-amine method.The binding between NRP/Na3P and rGO effectively stabilized the NRP on rGO during the cycling processes, therefore enabling the NRP?rGO composite to possess a large capacity of 2 057 mA h g?1at a current density of 0.1 A g?1and remarkable long-cycling performance(capacity decline ＜14.5% after 1 400 cycles at 2 A g?1and 34.4% after 5 000 cycles at 5 A g?1).Wang et al.[94]reported a novel ternary B2O3/BC2O/Cnanosheets with mesoporous structure, where the ultrafine PbTe nanodots are uniformly embedded(PbTe/BC)(Fig.8a).In this work, The PbTe/BC composite presented 2D nanosheets with the thickness of approximately 10-56 nm and had a smooth surface.These existing boracic phases remarkably improved the electrical conductivity of carbon matrix, even provided ample structural defects and more reaction active sites.PbTe/BC anode displayed a sodium storage capacitance of more than 1 000 mA h g?1at 0.1 A g?1, good rate performance of 230 mA h g?1and lowcapacitance decay of ～10% after 500 cycles at 10 A g?1.Xu et al.[95]fabricated N/S co-doped graphene sheets (PDMcT/RGO) via carbonization of poly(2,5-Dimercapto-1,3,4-thiadiazole) (PDMcT) on graphene oxides nanosheets.When the composite was used as anode of SIBSs, a capacity of 240 mA h g?1at 500 mA g?1, improved rate performance of 144 mA h g?1at 10 A g?1were obtained.Moreover，it still maintained 153 mA h g?1over 5 000 cycles at 5 000 mA g?1, indicating a good cycling stability.Liu et al.[96]proposed an uncomplicated and scalable method to synthesize layered BP/graphene composite(BP/rGO) by pressurization at room temperature(Fig.8b).BP/rGO anode prepared with this method achieved specific charge capacities of 1 460.1,1 401.2, 1 377.6, 1 339.7, 1 277.8, 1 123.78 and 720.8 mA h g?1in different current density.In a longcycling performance, the capacity of BP/rGO anodes stabilized at 1 250 and 640 mA h g?1at 1 and 40 A g?1(as shown in Fig.8c) over 500 cycles, respectively, implying significant performance improvement for sodium-ion battery anodes.

2.2.4 Design and synthesis of 3D CBNMs for SIBs

The electrochemical performance of SIBs were significantly boosted by 3D interlinked structure design, which combines conducting networks with promoted redox activity of active materials.The threedimensional carbon nanostructure and the relevant nanocomposite materials have an abundant open space and fine hierarchical structure, which make SIBs tend to possess a high capacity, low cost and stable performance.Furthermore, the successive porous structure can also quicken the diffusion of Na+, thereby promoting charge transport.The unique structure accompanied by the exist of micro/nano-pores can make the electrode materials able to well settle in large volume expansion during charge/discharge processes.

Xia et al.[97]successfully synthesized a flexible SnSe/C nanofiber membrane by electrospinning technology and subsequent calcination.The SnSe/C nanofiber membrane could withstand a 180° bending angle with no damage, implying its superior mechanical flexibility.SnSe nanoparticles were uniformly disposed along the carbon nanofiber framework and the composite revealed 290 mA h g?1at 200 mA g?1after 200 cycles in sodium ion battery.Chen et al.[98]designed and prepared a flexible free-standing carbonnetworks/Fe7S8/graphene (CFG) electrode with hierarchical structure (Fig.9a), in which Fe7S8microparticles were dispersed on 3D-crosslinked flexible conductive substrate, and tightly attached to high conductive graphene film, via a simple and scalable synthesis method.The as-obtaind freestanding electrode CFG represented high areal capacity (2.12 mA h cm?2at 0.25 mA cm?2) and good cycle steadiness after 5 000 cycles (47.5% capacity decay).The assembled all-flexible sodium-ion battery possessed remarkable performance (high areal capacity of 1.42 mA h cm?2at 0.3 mA cm?2and superior energy density of 144 W h kg?1), quite close to the requirement of practical application.Wang et al.[99]prepared 3D carbon framework (3DCF) anode materials for SIBs, by electrolysis followed by subsequent calcination process.The prepared 3DCFs owned the unique structure with interconnected, porous structure and abundant defects,significantly improving the Na-storage performance.As the anode material for SIBs, 3DCFs displayed high specific capacity and rate capability, and excellent long-term cyclability.Lou et al.[100]demonstrated the delicate design and synthesis of hierarchical microboxes composed of SnS nanoplates coated with nitrogen-doped carbon (NC) (SnS@NC) (Fig.9b),which showed a specific capacities of 607 mA h g?1at the current densities of 0.1 A g?1,even at high current density of 1 A g?1still retained 456 mA h g?1.And thecycling performance of the SnS@NC electrode at a current density of 0.5 A g?1retained a high discharge capacity of 443 mA h g?1over 100 cycles without apparent capacity fading.

Despite some similarities between SIBs and LIBs, the greater ion mass and radius of sodium ions make them far away from practical applications.Thanks to their unique physicochemical natures, CBNMs have shown great potential for application under field of sodium ion energy storage in recent years.At present, regarded as the first choice for the practical application of sodium ion battery electrode materials,cheap and stable pure carbon nanomaterials have been widely studied.For pure carbon nanomaterials, it might still be not ready to realize the precise control for the pore structure, morphology, interlayer spacing,and heteroatom doping, which are expected in the research of SIBs.The composite nanomaterials of carbon and other high-capacity materials (metal oxides,sulfides, and non-metallic elements etc.), owning higher specific capacity are promising electrode materials.The structural design of carbon-based composites in different dimensions and the accurate control of the size and crystal structure of active substances will reduce the interfacial resistance between materials and improve the stability of composites, which will effectively promote their applications in electrode materials of SIBs.

2.3 Advanced CBNMs for PIBs

Embodying similar characteristics to SIBs, PIBs also belong to the types of rocking chair batteries.During charging, K+acts as an ionic charge carrier,which can transfer from the positive electrode materials to the negative electrode through the electrolyte.When discharging, the opposite process is true.The standard electrode potential of K is ?2.93 V, closer to the standard electrode potential of Li (?3.04 V) than that of Na (?2.71 V), which makes PIBs theoretically have a higher energy density.Although K+has a larger ionic radius than Na+, the solvation radius of K+is smaller than Na+, which indicates that K+has better kinetic properties[101].Based on the above advantages,PIBs have become another research hotspot in the field of energy storage after SIBs.However, there are some problems in the field of PIBs currently, such as the volume effect caused by a larger ion radius, poor reaction kinetics, excessive first-turn side reactions,and low energy density, which significantly restrict the development and commercialization of PIBs[102].Similar to LIBs and SIBs, the selection of electrode materials is an important factor in promoting potassium-ion batteries.CBNMs have been widely applied in PIBs owing to their above-mentioned advantages.

2.3.1 Design and synthesis of 0D CBNMs for PIBs

0D nanomaterials generally signify a class of materials with sizes of 1 to 100 nm in three dimensions.0D CBNMs are studied in PIBs field due to structural features and properties such as large surface area offering ample sites for ionic reaction, physical and chemical stability able to buffer large volume expansion and so on.

On the basis of the density functional theory(DFT) calculations, Gan et al.[103]introduced a nitrogen/phosphorus co-doped carbon nanoparticles with increased interlayer (NP-CNPs).In this work, as shown in Fig.10a, while Pluronic F127 was the template for the internal pores of the polymer nanoparticles, sodium phosphite, hexamethylenetetramine and resorcinol were used as the phosphorus source, nitrogen source and carbon source respectively.The specific surface area of NP-CNPs reached 558 m2g?1,and the authors adopt DFT calculations to prove that N/P co-doping had a critical influence on the interlayer space, volume change, and the energy barrier of carbon, resulting in the improved electrochemical performance for the K+storage.So the NP-CNPs anode displayed a specific capacity of 270 mA h g?1at 0.2 A g?1after 300 cycles, and a great rate capability(157 mA h g?1at high rate of 5 A g?1), as well as an ultralong cycle life (low decay of 13.6% after 4 000 cycles at 1 A g?1).In another work, Zheng et al.[104]reported a Sb@CSN composite anode, where Sb nanoparticles evenly embeded carbon sphere network (CSN), for PIBs (Fig.10b).Combining theoret-ical calculations with electrochemical features confirmed a reversible continuous phase transition in the process of K+storage.In this work, the Sb@CSN anode revealed a large specific capacity of 551 mAh g?1at 0.1 A g?1, and the capacity decay was slow after 100 cycles.Moreover, it still retained a 504 mA h g?1capacity at a higher current density of 0.2 A g?1over 220 cycle.

Wang and his co-works[105]prepared a MoSe2/C hybride with special pistachio-shuck-like nanostructure (PMC) as an advanced PIBs anode.The PMC exhibited a specific capacity of 322 mA h g?1at 0.2 A g?1over 100 cycles, and could still maintain 226 mAh g?1at 1.0 A g?1over 1 000 cycles.Li et al.[106]proposed hollow graphitized carbon nanocages(HGCNs) via structure engineering, as efficient anode materials for PIBs (Fig.10c).The HGCNs were synthesized using a one-pot, self-template and self-graphitization approach where direct pyrolysis of nickel citrate converts citrate chains into carbon-shell and simultaneously reduces Ni2+to form Ni metal core.The HGCNs obtained at 1 000 °C (HGCN-1000), owned its highly graphitized carbon-cage and rich porosity,with a potassium ion capacity of 402.2 mA h g?1at a current density of 30 mA g?1.The synergistic structure of hollow structure and graphitized carbon layer makes HGCN-1000 possess an amazing stable structure, with a bit capacity decease rate of 4.1% at 2 000thcycle under 1 A g?1.Ding et al.[107]reported sulfurgrafted hollow carbon spheres (SHCS) prepared through an in situ sulfuration process.In this work, a high content of S (38 wt%) is chemically integrated into a carbon host in SHCS for KIB anodes.The SHCS electrode showed good rate performance, obtaining 202, 160 and 110 mAh g?1at 1.5, 3 and 5 A g?1, respectively and only decayed 7% of the initial capacity after 1 000 cycles at 3 A g?1.

2.3.2 Design and synthesis of 1D CBNMs for PIBs

1D CBNMs possess an effective interfacial contact with electrolyte, and short ions diffusion distances.Used as electrodes for PIBs, 1D CBNMs can well accommodate the large volume change, and significantly hamper the structure collapse of electrode materials during K+storage.Advanced studies have lately been conducted for anode materials with superior properties by constructing and preparing 1D CBNMs.Adams et al.[108]prepared CNFs through electrospinning perform as excellent anode materials for PIBs.Here, two kinds of materials were obtained by nitrogen doped raw CNFs and plasma oxidized carbon nanofibers (CNF-O) respectively.And N-rich carbon nanofibers exhibited a stable capacitance of 170 mA h g?1over 1 900 cycles at 1 C rate (1 C =279 mA g?1).Yu et al.[109]proposed a metal octahedral CoSe2, which was penetrated by nitrogen-doped CNTs as a soft backbone for the anode of PIBs(Fig.11a).The metallic nature of CoSe2compositing with the highly conductive N-doped CNTs bring about the great facilitation of electron transport, which improved the rate capability.The CNTs acted as a skeleton to suppress the accumulation, fix the active materials, and make the integral structure tend to be steady.Beneficial from these nice structure advantages, a capacity of 253 mA h g?1at 0.2 A g?1could be obtained and the CoSe2/CNTs still retained 173 mA h g?1at 2.0 A g?1over 600 cycles, implying good rate capability.

Heteroatom-doped 1D carbon materials are also widely used in potassium ion batteries.Xu et al.[110]conducted polypyrrole (PPy) nanofibers as a precursor for direct carbonization and obtained N-doped CNFs (NCNFs) owning a large N-doping content of 13.8%.In this work, NCNFs exhibited a homogeneous morphology of the interlinked nanofibers, whose diameter is 70-80 nm.The NCNFs anode delivered reversible capacities of 248 mA h g-1at 25 mA g-1and 101 mA h g-1at 20 A g-1, and maintained 146 mA h g-1at 2 A g-1over 4 000 cycles.Hao et al.[111]fabricated the N-doped carbon nanofibers(NCFs) by direct pyrolysis of bio-waste chitin.Significant properties for electrochemical performances like specific surface areas, porosity and N-doped existential forms could be adjusted by different thermal treatments temperature (500, 700 and 900 °C).NCFs thermal treated at 700 °C yielded reversible capacities of 240.1, 211.3, 167.5, 153.5, 136, 123.8, 109.3 and84.7 mA h g?1at different current density (0.1-5 C,1 C = 279 mA h g?1) (Fig.11b).The specific capacities reached 208.1 and 231.2 mA h g?1when current rate returned to 0.2 and 0.1 C, indicating little capacity loss compared with initial corresponding current rates.Moreover, the electrode showed a considerable capacity of 215.2 mA h g?1at 0.2 C after 100 cycles,excellent rate capability and a remarkable cycling stability (105.6 mA h g?1at 2 C after 500 cycles).

2.3.3 Design and synthesis of 2D CBNMs for PIBs

As mentioned earlier, 2D CBNMs has a high specific surface area offering ample active sites, good conductivity improving the electron transfer efficiency of the material, and the adjustable interlayer spacing conducive to the rapid extraction/intercalation of potassium ions.Meanwhile, 2D nanostructure could extremely refrain the agglomeration effect of active material, and contribute to the utilization and reaction of K+.

As a representative of 2D CBNMs, graphene is widely used as an electrode material for energy storage, deriving from its high surface area, extraordinary mechanical strength, as well as high electrical conductivity[112-115].Functional P/O co-doped graphene(PODG) was prepared by a thermal annealing method using triphenylphosphine and graphite oxide as raw matereials[116].For the PODG electrode materials, the entry of oxygen and phosphorus into carbon skeleton leaded to various vacancies and defects in the graphene sheets, which could shorten the pathways of K+diffusion.The ultrathin 2D sheets with wrinkle characteristics undertook successive, businesslikeelectron transfer and the large surface area of graphene provided abundant touch area between electrolyte and materials.The PODG delivered a high reversible capacity of 474 mA h g?1at 0.05 A g?1and the specific capacity maintained 385, 235 and 160 mA h g?1at current densities of 0.5, 1 and 2 A g?1for 600thcycle, respectively, implying outstanding cycle-life.Moreover, using graphene nanosheets as a substrate to grow other materials such as metal sulfides and oxides on the surface can yet be regarded as an effective strategy for the preparation of 2D CBNMs.Zhang et al.[117]realized a stress-dispersed structure with Co3Se4nanocrystallites orderly anchored on graphene sheets by a two-step hydrothermal treatment (Fig.12a), which could effectively lessen the structural deterioration.The features to reduce the contact stress by the well-dispersed Co3Se4nanocrystallites during K+intercalation, combined with the highly conductive graphene matrix, gave a more reliable and efficient anode architecture than its two seperated counterparts.The optimized electrode delivers excellent cycling stability (301.8 mA h g?1at 500thcycle under 1 A g?1), as well as an outstanding rate capacity (203.8 mA h g?1at 5 A g?1).Xu et al.[118]introduced the formation of multi-scale hierarchical engineering carbon nanosheets with multi-level structure of carbon nanosheets through guest-doped inclusions and high-temperature covalent organic framework (COF)-guest interactions (Fig.12b).The resulting carbon material showed homogeneous co-doping of nitrogen and phosphorus, a large carbon layer interspacing of ≈0.4 nm, and abundant micro/meso/macro-pores.And applied for the PIBs anode, the carbon materials displayed a high reversible potassium capacity of 404 mA h g?1at 100 mA g?1as well as considerable longterm stability (179 mA h g?1at 1 A g?1after 2 000 cycles).Xiong et al.[119]synthesized an integrated hybrid architecture composed of ultrathin Cu3P nanoparticles (～20 nm) confined in porous carbon nanosheets (Cu3P/NPCSs) as a new anode material for PIBs through a rational self-designed self-templating strategy.The Cu3P/NPCSs architectures could deliverreversible capacities of 149 mA h g?1over 2 000 cycles with 0.003% decay per cycle at 1.0 A g?1and good rate capabilities up to 10 A g?1with a specific capacity of 125 mA h g?1.Bai et al.[120]introduced a new self-template and recrystallization-self-assembly strategy for the one-step preparation of CoPNPPCS hybrids, which showed reversible capacities of ≈174,134, 123, 94, 74 and 54 mA h g?1at different current densities and reversible capacityances of 127 and 114 mA h g?1after 1 000 cycles at 0.1 and 0.5 A g?1,respectively, without detectable capacitance fading.

2.3.4 Design and synthesis of 3D CBNMs for PIBs

In alkali metal ion batteries, most of CBNMs usually use carbon materials as a skeleton or frame, in which other active materials are compounded or doped.3DCBNMs contain materials of other dimensions, which provides more options for material structure and performance.In addition, 3D interconnected nanostructures can generally promote the transmission of electrons and ions and improve the structural stability of materials.Based on these advantages, 3D CBNMs have gradually been widely applied in potassium ion batteries.

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Li et al.[121]introduced a hierarchical and condensible porous CNF foam (CNFF) as free-standing electrode (Fig.13a).The resulting CNFF showed a three-dimensional (3D) network with hierarchical micro- and meso-pores structures and a high specific surface area (778.75 m2g?1).The CNFF electrode steadily retained a capacitance of 158 mA h g?1after 2 000 cycles at a high rate of 1 A g?1, which only had a decrease of 0.006% per cycle.Furthermore, it also could keep a capacity of 141 mA h g?1up to 2.0 A g?1for 1 500 cycles, and a capacity of 122 mA h g?1at 5.0 A g?1for an additional 1 000 cycles.Zhou et al.[122]carbonized 3D order macroporous ZIF-8 to synthesize 3D interconnected nitrogen doped hierarchical porous carbon (N-HPC) (Fig.13b).Attributed to the unique structure, the N-HPC exhibited excellent rate performance (94 mA h g?1at 10.0 A g?1), remarkablecycle-life (157 mA h g?1at 12 000thcycle under 2.0 A g?1) and good specific discharge capacitance(292 mA h g?1at 0.1 A g?1).Recently, Huang et al.[123]reported a Cu9S5embedded in 3D ordered macroporous carbon framework (3DOM Cu9S5@C) prepared through a sulfidation and subsequent ion exchange strategy with Zn-based metal?organic frameworks as a precursor for an advanced PIBs anode (Fig.13c).In the interlinked 3D ordered macroporous structure,there were rapid transport channels for the large potassium ions and an ample touch area between solid materials and the liquid electrolyte, conducive to promote the ionic diffusion kinetics of batteries.The 3DOM Cu9S5@C showed a remarkable K+storage rate capacitance of 170 mA h g?1at 2.0 A g?1and an excellent cycling steadiness of 316 mA h g?1under 100 mA g?1at 200thcycle.Du et al.[124]developed a simple method for designing an amorphous 3D carbon material of PIBs anode with superabsorbent polymers (SAPs) stripped from the baby diaper with higher efficiency and environmentally friend.In this work,this amorphous 3D carbon material exhibited a reversible potassium storage capacity of 270.4 mA h g?1at the current density of 50 mA g?1after 100 cycles.At the same time, at a current density of 1.0 A g?1, the electrode material also showed extremely long-terms cycling-life with specific capacity of 161.7 mA h g?1over 2 000 cycles.Zhang et al.[125]developed a new,general strategy of direct pyrolysis of supermolecules to synthesize carbonaceous anodes (3D-NTC) containing superlarge marginal-nitrogen doping (16.8%)for PIBs.The prepared 3D-NTC possessed a 3D skeleton consisted of carbon nanosheets, turbine layered crystal structure.Meanwhile, the 3D-NTC anode displayed a specific capacitance of 473 mA h g?1, boosting rate capability, and a long cycle life after 500 cycles with a low capacity decay of 6.9%.

As an emerging battery system, PIBs have drawn widespread attention derived from their low cost, long life, and high energy density advantages.Although the potassium ion battery system has certain advantages over the lithium ion system, it is still in its infancy,and the problems caused by the excessively large potassium ion size are still facing researchers severely.CBNMs with different dimensional structures possessing unique advantages, can provide an extremely effective choice for the modification of PIBs electrode materials.Similar to SIBs, pure carbon materials have an important position in the research of potassium ion batteries.The regulation of the interlayer spacing and heteroatom doping of carbon nanomaterials are key means to improve its potassium storage performance.For carbon matrix composites, combining the advantages of different dimensional structures,designing and synthesizing new hybrid carbon composites to improve the storage performance of potassium ions is an important method to obtain potassium ion electrode materials with high magnification, high capacity and long cycle life.

2.4 Advanced CBNMs for SCs

As one of the most promising EES devices, SCs also called electrochemical capacitors, own the properties of high power density, rapid charge storage efficiency, and excellent cycle stability, etc.Compared with batteries, SCs possesses higher power density but lower specific capacitance and energy density.There-fore, increasing the energy density of SCs has become the focus of current research.According to the calculation formula of energy density:E= 1/2CV2, it can be seen that for high energy density, there are two main methods, namely increasing the capacity of the electrode material and widening the voltage window between the positive and negative electrodes[126-131].According to different energy storage principles, the electrode materials of SCs are mainly composed of three types: electric double layer capacitance (EDLC),pseudocapacitance (PC) and intercalation pseudocapacitance (IPC)[132].In EDLC, the charge storage is mainly achieved by the physical adsorption of ions on the intersurface between electrode materials and electrolytes.In PC, energy storage originates from the rapid faradic reaction which take place on the surface of the electrode materials[133].This kind of energy storage mechanism dominated by redox reaction can usually realize higher specific capacity, compared to EDLC.Belonging to a fresh type of SCs mechanism,IPC relies on the intercalation/de-intercalation of cations (e.g.Li+, Na+, K+, and H+) in the body of active materials, nevertheless the diffusion of ions within the crystalline skeleton can not restrict dynamics mechanism[134].On the one hand, from the point of view of the charge storage mechanism, excellent properties such as high specific surface area, good conductivity, and stable structure are necessary conditions for constructing SCs with good performance.Nevertheless, CBNMs are promisingly one of the ideal candidates as electrode materials of SCs that can meet the above conditions.On the other hand, the electrode materials with different voltage windows are selected as the positive and negative electrodes to assemble asymmetric/hybrid SCs for widening the overall operating voltage of the device and thus achieve the improvement of energy density.In this respect, the different material combinations contained in CBNMs also have certain advantages for the construction of asymmetric/hybrid capacitors.

2.4.1 Design and synthesis of 0D CBNMs for SCs

0D CBNMs display a high specific surface area(hundreds to thousands m2g?1) with tunable pore size and distribution, which are critical factors that govern the performance of SCs.Zhu et al.[135]prepared carbon QDs and NiCo2O4sphere composites(CQDs/NiCo2O4) through the reflux route, and then carrying out post-annealing process.Owing to the merits of structure including a large specific surface area, high mesoporosity and superior electronic conductivity, the prepared CQDs/NiCo2O4hybrid exhibited a maximum specific capacitance of 856 F g?1at 1 A g?1, and it obtained a 60.8% capacity retention rate at 100 A g?1.In addition, the composite only decreased 1.25% of the initial capacitance at 10 000thcycle under 5 A g?1indicating considerable cycling stability.Pan et al.[136]synthesized a hollow spherical carbon (HSC) through carbonization and NaOH activation of corncob lignin.In this work, the specific surface area of HSC-1000 reached 1 261.7 m2g?1and the HSC, which was featured by hierarchically porous structure, revealed high rate capability and good cycling stability when it is directly used as electrode material for SCs.Wu and co-workers[137]fabricated nickel nanoparticles partially distributed into graphitic porous carbon (NiOF) via directly carbonizing nickelorganic framework.The carbon structure and surface area were both affected by the calcination temperature, and thereby changing the capacitive performance of NiOF electrodes.The specific surface area of hybrid went up with the calcination temperature, and it peaked at 800 °C.Due to the excellent porous structure, NiOF obtained at 800 °C showed a long cyclelife, delivered a specific capacitance of 886 F g?1at 1 A g?1and still held 746 F g?1at 30 A g?1.

0D pure carbon nanomaterials such as graphene quantum dots (GQDs) and heteroatom doped carbon nanoparticles etc.have also received attention in the field of SCs.Zhou et al.[138]discussed a series of preparation conditions and methods about GQDs including: chemical oxidation, electrochemical exfoliation,hydrothermal/solvothermal treatment, microwave/ultrasound assisted methods, carbonization of organic precursors, chemical vapour deposition, stepwise organic synthesis in their review study.Liu and coworkers[139]prepared GQDs by a solvothermal meth-od using graphene oxide (GO) as a raw material and it shows fascinating performance with high power density, superior rate performance as well as steady cycling nature for micro-supercapacitor.The small size effect and surface effect of carbon nanoparticles make them obtain a large specific surface area and supply sufficient sites for ion desorption, meanwhile the combination of these properties with high conductivity of carbon material is extremely beneficial to the performance improvement of electrode materials.

2.4.2 Design and synthesis of 1D CBNMs for SCs

1D CBNMs have desirable features including straight electric pathway, reduced ion transfer length,which are suitable for use in SCs.For example, Li et al.[140]prepared the MnO2/CNTs composites via a reformative one-pot reaction method, in which crossbonded MnO2sheets evenly grew on CNTs.The composite showed a maximum specific capacitance of 201 F g?1and when the current density was even enlarged to 20 A g?1,its rate retention achieved 70%.Furthermore, the capacity of MnO2/CNTs had barely fallen after 10 000 cycles at 1 A g?1, demonstrating the excellent cycle stability.Huo et al.[141]proposed mesoporous VN nanowires coated by nitrogen-doped carbon (MVN@NC NWs) as a free-standing electrode for supercapacitors (Fig.14a).The N-doped carbon layer hamper the pulverization of the MVN NWs core in the alkaline condition and improved the conductivity, conducive to the electochemical performance.Based on these advantages, the composite delivered a remarkable area capacitance of 282 mF cm?2at 1.44 mA cm?2and outstanding cycle steadiness with～8.2% decrease of initial capacitance after 12 000 cycles.Furthermore, the all-solid-state flexible supercapacitor assembled from MVN@CN NWs films and alkaline gel electrolyte could deliver a volumetric capacitance of 10.9 F cm?3, high energy density of 0.97 mW h cm?3and power density of 4.13 W cm?3.Mai et al.[142]proposed a graphene scrolls coating MnO2nanowire (MGSs) by a simple self-scroll tactics (Fig.14b).There existed high internal hollow between the MnO2nanowires core and outer graphene shell in the obtained composites.The SC composed of the MGSs electrodes delivered a high reversible capacitance of 317 F g?1at 1 A g?1.The considerable electrochemical properties should be ascribed to 1D nanostructures with the graphene-encapsulated MnO2nanowires, supplying successive charge transfer channels and enough space for volume change during cycling.

2.4.3 Design and synthesis of 2D CBNMs for SCs

2.4.4 Design and synthesis of 3D CBNMs for SCs

3D CBNMs not only have diverse structures to meet different needs, but also show unique morphology with large porosity and high specific surface area, which enables the charge to shuttle more freely between the electrode and the electrolyte.Based on the mechanical and physical natures, some 3D CBNMs are used as free-binder electrodes for SCs at first hand, possessing high specific capacitance, reliable rate capability and strong cycle-life.Yu et al.[147]reported polyaniline (PANI) nanowire grown on rGO foam (RGO-F/PANI) via an in-situ polymerization process (Fig.16a).The ASC device with high energy and power densities was assembled from RGOF/PANI electrodes.Owing to the flourishing structure engineering and developed electrochemical properties of the hybrid, this device showed a maximum energy and power density of 17.6 W h kg?1and 98 kW kg?1and a prosperous cycle steadiness with only 20% capacitance decay after 5 000 cycles.

In another work, Xiong et al.[148]designed and prepared a hierarchical structure consisting of Ni-Co hydroxide nanopetals (NCHPs) attached on a frivolous free-supporting graphene petal foam (GPF) via a facile procedure for pseudocapacitive electrode applictions.For the GPF/NCHP electrodes a high volumetric capacitances of 765 F cm?3was obtained, same as an areal capacitance of 15.3 F cm?2.The two-terminal asymmetric solid-state SCs with 3D GPF/NCHPs as positive electrodes were assembled and the device exhibited boosting energy and powerdensities (≈10 mW h cm?3, ≈3 W cm?3), good longterm cycle life (no decay over 15 000 cycles) and high rate capability (a capacitance retention of ≈60% at 100 mA cm?2).Saeed et al.[149]synthesized a fresh 3D graphene-CNTs/MoO3hybrid as a binder free electrode material with improved structure and excellent properties (Fig.16b).The hybrid electrode delivered large specific capacitance of 1 503 F g?1at 1 A g?1and 798.93 F g?1at 10 A g?1, as well as low capacitance decrease of 3.5% over 10 000 cycles.The ASC equipment assembled from GF-CNTs/MoO3and GF-CNTs also exhibited a maximum energy density of 75.27 W h kg?1at a power density of 816.67 Wk g?1and exceptional cycling capability with 94.2% of the initial capacitance after 10 000 cycles.

Kumar et al.[150]reported hierarchical nanohoneycomb-like CoMoO4-MnO2core-shell and Fe2O3nanosheet arranged on 3D graphene foam (GF) and utilized them as binder-free electrodes for SCs.In the typical preparation process, the 3D GF was formed by a solution pouring method at first, and then the Co-MoO4-MnO2or Fe2O3nanosheets grew on the 3D GF by a hydrothermal method.The asymmetric supercapacitor (ASC) assembled from CoMoO4-MnO2@GF as cathode and Fe2O3@GF as anode delivered uncommon reversible capacity of 237 F g?1and a high rate performance of 61% even up to 20 A g?1.Moreover,the ASC device also showed a flourishing energy density of 84.4 W h kg?1and a large power density of 16 122 W kg?1as well as the capacitance was only 7.9% decay over 10 000 cycles.Zhu et al.[151]also introduced a simple, fast and efficient strategy to compound sodium anthraquinone-2-sulfonate (AQS) with reduction graphene oxide (rGO) to obtain the 3D AQS/rGO hydrogels.The preparation process only needs two steps: firstly mixing the AQS powder and GO solution uniformly, and then adding the reducing agent to obtain the AQS/rGO hydrogel under low temperature and normal pressure in a short time.The AQS/rGO hydrogel exhibited a high specific capacit-ance of 387.43 F g?1at 1 A g?1and an excellent cycling stability of 91.11% after 10 000 cycles at 10 A g?1.Besides, the maximum energy density of the ASC assembled by AQS/rGO hydrogel was 15.98 W h kg?1at the power density of 325.02 W kg?1.

Among the electrode materials of SCs, CBNMs with different dimensions occupy an important position.Especially after the concept of hybrid capacitor was put forward, the asymmetric devices assembled by carbon-based composite material with pseudo-capacitance characteristics and pure carbon material with EDLCs have made up for the defect of insufficient energy density of SCs to a certain extent.There is still room for improvement in the cycle stability of pseudocapacitance carbon-based composite materials.In CBNMs, dimensional structure design, precise control of pore structure and optimization of overall structure engineering are the key means to further improve the performance of supercapacitors and prepare highperformance devices.

3 Summarize and perspective

EES systems including LIBs, SIBs, PIBs, SCs and so on, are reliable, available, and efficient renewable energy storage devices.It is very clear that the nature of electrode materials is the key element affecting the performance of devices.Aimed at catering to the urgent needs of advanced EES devices, the evolvement of new electrode materials is imperative.The design and synthesis of electrode materials with excellent characteristics, such as high specific surface area, stable structure, excellent moldability, etc., have become the basis for research and development of high-performance energy storage devices.The unique morphology, excellent conductivity, structural stability and good modelability of CBNMs make them ideal electrode materials for energy storage.Notably, the structural design of carbon-based materials with different dimensions can not only obtain boosting porosity, improved ion channels, and increased specific surface area, which are essential for the rate performance and cycling stability, but also can well meet the needs of different situations and conditions.In this light,CBNMs are still promising candidates for the construction of advanced EES devices, and the controllable design and synthesis of their dimensional structures is an effective and necessary way to realize their considerable applications for energy storage.

Numerous previous studies have fully indicated the great potential of CBNMs around the background of EES.Inevitably, the design of carbon-based materials with different dimensional structures greatly improves the electrochemical performance of electrode materials, but also brings some problems.Firstly, for 0D CBNMs, the small size effect and increase of surface potential energy brought by nanostructure make materials prone to agglomeration and thus reduce the cyclic stability of materials.Meanwhile, due to the large specific surface area and low vibration density of 0D carbon-based materials, the increases of ions consumption in electrolyte and the occurrence of irreversible reaction lead to the decrease of Coulombic efficiency in initial cycles.Secondly, although 1D CBNMs theoretically possess short charge transmission pathway and good strain characteristics, when they are actually used as electrode materials, ion transmission are often hindered due to the 1D structure of anisotropic interleaving, especially at high current density.Moreover, the too large length of one-dimensional structure is also ready to result in a fracture in the cycle process.For 1D CBNMs, the effect of appropriate diameter length ratio on their electrochemical properties has not been pointed out and verified.After that, for 2D CBNMs, effectively solving the agglomeration and stacking effect between layers to maintain the stability of two-dimensional structure is still a knotty problem, despite some studies have given some methods, such as heteroatom doping and so on.And the adjustment of layer spacing of 2D carbon-based materials has not been systematically researched and described to obtain its general method.Finally, 3D CBNMs containing interconnection network structure can overcome the problems caused by other low dimensions to a certain extent, such as agglomeration effect and poor structural stability.However, their porous structure is uncontrollable and complex pre-paration process is usually required.

To solve these existing problems, it is still better to start with structural design.For example, for 0D CBNMs, attaching them to the surface of 1D or 2D structure is very effective in solving the agglomeration effect.And combining a large-size structure can also avoid the defects deriving from the small size effect.Another example is the design of the same-directional structure for one-dimensional materials, which can greatly reduce the low charge transfer efficiency caused by the different-directional arrangement.In general, combining the advantages of different dimensional structures to design and synthesize CBNMs is an effective way to further improve the electrochemical performance of CBNMs for EES devices.In addition, theoretical calculations, such as density functional theory (DFT) calculations, are organically integrated into the structure design and synthesis of materials, so as to better explore the structure-activity relationship between suitable structures and electrochemical properties.

Moreover, it is necessary to point out that although the performance reported in many studies far exceeds the commercial ones, the economic problems caused by large-scale and standardization have left a huge gap between experimental study and industrial production.The effects of multidimensional structures on the electrodes performances require a deeper understanding, conductive to hearten more strategies for design of developed carbon-based nanostructures with fascinating properties.In-depth investigation of the structure-activity relevance between the dimensional structure, porosity and the properties of the electrode materials is very likely to find a solution to these problems.It should be paid more attention that the precise control of dimensional configuration and compounding patterns in order to optimize the reaction and transfer of ions and electrons.On the other hand, the integration of active substances should be also an essential aspect.Specially, the interaction mode between carbon materials and active substances is very important to the electrochemical performance,which also needs more attention to study and understand.Identifying the electrode active materials that construct the basic performance of energy storage devices requires more effort in order to obtain the ideal electrode material properties including: wide electrochemical window, good rate performance and excellent cycle stability.

Imbalance between low output and high cost,cumbersome preparation process are increasingly becoming serious problems.The construction steps of CBNMs should be further simplified and minifying expense by introducing facile, low-cost, feasible approaches as well as selecting sustainable raw materials.Besides, whether for batteries or capacitors, their functionalization is aimed at being wearable and flexible and CBNMs will draw increasing attention as it can be readily expanded to the construction of flexible EES devices.Under reasonable majorization and development, the diverse structure of CBNMs will always meet the needs of different applications, so it will certainly play an important position in the practical application of EES devices in the near future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21805067), the State Key Laboratory of Fine Chemicals, Dalian University of Technology (KF 2008), and Science and Natural Science Foundation of Hebei Province(B2021202043).