Construction of a flexible, integrated rechargeable Li battery based on a coaxial anode with a carbon fiber core encapsulated in FeNiMnO4 and a nitrogen-doped carbon sheath

ZOU Yi-ming, SUN Chang-chun, LI Shao-wen, BAI Miao, DU Yu-xuan,ZHANG Min, XU Fei,*, MA Yue,*

（1. International Research Center for Composite and Intelligent Manufacturing Technology, Institute of Chemical Power Sources,School of Science, Xi'an University of Technology, Xi'an 710048, China;2. State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China）

Abstract： A coaxial anode with a carbon fiber core encapsulated in nanocrystalline FeNiMnO4 with a nitrogen-doped carbon sheath was prepared using carbon fiber cloth as the core, FeNiMnO4 nanocrystallite arrays as the first coating layer and nitrogendoped carbon derived from F127 (a kind of triblock copolymer)-resorcinol-melamine gel as the outer layer. After annealing at 600 °C it was used as the anode material of an all solid flexible lithium ion battery using LiFePO4 as the cathode material and boron nitride modified polyethylene oxide as the electrolyte. The battery had a large areal capacity of ～1.40 mAh cm-2 and satisfactory cycling stability under different bending and strain states. Annealing below 600 °C leads to incomplete carbonization of the nitrogen-doped carbon and thus a low electrical conductivity while above 600 °C aggregation of FeNiMnO4 nanocrystallites and their detachment during cycling are observed under bending and strain.

Key words: Flexible solid-state lithium battery；Quaternary oxide；Integrated configuration；Mechanical flexibility；Coaxial structure

1 Introduction

The multibillion-dollar industry of lithium-ion batteries (LIBs) have documented their commercial success in consumer electronics. To meet the soaring requirements from the end users like electric vehicles and grid-scale energy storage, intensive research has been devoted to the technological breakthrough of the battery formats toward higher energy/power densities at the lower production cost and higher operational safety[1,2]. Meanwhile, the emerging markets of wearable electronics, integrated package, and smart cards,etc., have motivated the fundamental research and technological innovation of the flexible energy storage systems[3,4].

Unlike stationary power devices, the development of the flexible battery configuration is somewhat challenging in terms of the indispensable balance among the mechanical flexibility of the electrodes, current collectors, and the shape-conformability of the separator[5,6]. On the device level, all the components need to coherently maintain their characteristic electrochemical behaviors even upon the continual bending or twisting while preserving the lightweight arrangement and structural robustness of the battery configuration[7-9]. Therefore, some researchers attempted to directly decorate the electroactive species, for instance thin layers in the form of nanoparticles, nanoneedles, or nanosheets, loaded on the metallic foil or 3D framework serving as binder-free electrodes[10-12,6]. However, the insecure adhesion force between the electroactive materials and the current collectors deteriorates the structural integrity when they are subjected to the long-term cycles. Besides, the sparse distribution of electroactive materials on the metallic foil sacrifices the areal mass loading of electroactive materials[13,14]. Through pyrolysis of the high-content emulsion of the sulphated polymers, our previous studies have functionalized the carbon framework for the enhanced loading level of the electroactive Sn or Fe species. Despite the areal capacity up to 1.5-1.7 mAh cm-2, the integrated electrodes failed to afford sufficient mechanical flexibility due to the rigidity of the macroporous carbonaceous current collector[15,16]. An alternative pathway is to integrate the spinel-type oxides like Fe3O4, CoFe2O4and Co3O4particles within the flexible substrates,i.e.,graphene oxide aerogels or 3D graphene foam[17-19]. However,the electroactive species direct exposure to the reactive electrolytes degrades and depletes the Li+reservoir irreversibly. Besides these electrode innovations,the component compatibility of the powered devices is more challenging upon the flexing states,i.e.,regulation the interfacial property of the flexible electrode in a more rational manner towards the improved cycling efficiency, more importantly, harmony system integration.

Structural engineering and chemical composition of the electrode manipulate the battery chemistry.Among many candidate anode materials, transitional metal oxides could render the high reversible Li+storage capacities (～500-1 000vs372 mAh g-1of commercial graphite) based on spatially varied conversion reactions. Despite other merits of the safe lithiation potential, corrosion resistance, and low cost, the large polarization and unstable interfacial reactivity undermine reversibility of the conversion reaction and compromise the cycling life[20,21]. Investigation thus has focused on the synthesis of ternary transitional metal oxides (AxByO4, A, B = Co, Fe, Ni, Mn, Zn),which have an evidently boosted electrical conductivity and more multifarious redox reactions than binary oxides because of the existence of multiple valent cations. So far, Yanget al.synthesized ZnFeO4nanoparticles confined in hierarchical porous active carbon fibers derived from cotton[22]. Huanget al.prepared 3D sandwich-structured NiMn2O4@reduced graphene oxide nanocomposites[23]. FeMnO3microcube delivering an average diameter of ～1.0 μm was accomplished by Houet al.[24]. However, the difficulties exist in the tedious synthetic procedures to allow multiple metal cations (ternary (quaternary or multinary) oxides) to be incorporated according to the preset compositional ratio and microstructure without mass relocation, particle agglomerations, or parasitic phase separation during the high-temperature phase transition.

Herein, we constructed a flexible and integrated electrode based on the quaternary oxide nanocrystallines (FeNiMnO4) with the precise control of the cation distribution on preferential atomic sites and regulation of the interfacial properties. In the coaxial architecture design, the quaternary oxides were supported on the carbon cloth nanoarrays by a facial chemical coprecipitation method. Meanwhile, the N-doped carbon layer derived from the quasi-gelstate tricopolymer gelation, i.e., F127-resorcinolmelamine encapsulated oxide nanocrystallines(CC@FeNiMnO4-600). The 3D coaxial configuration offers several key merits for the flexible device: (1) a low-strain structure derived from the mechanical flexible carbon cloth and N-doped carbon coating, (2) the nanocrystallines with the pre-determined spatial cation occupancy, (3) the carbon encapsulation which avoids the direct contact of oxide nanocrystallines to the electrolyte, (4) the high electrical conductivity of the coaxial carbon network. With the assist of M?ssbauer characterization, we further identify the preferential occupancy of Fe at the tetrahedral sites to stabilize the cubic spinel structure. When evaluated as a LIB anode, the CC@FeNiMnO4-600 electrode demonstrated a large capacity of ～1.40 mAh cm-2with an ultra-robust cyclability at 1 mA cm-2. The CC@FeNiMnO4-600 anode was further integrated with the few-layer boron nitride (FL-BN) modified polyethylene oxide (FL-BN/PEO) eletrolyte in the solid-state battery, which demonstrates the intimate interfacial contact even under various bending states. Under the guidance of the crystal field theory, the methodology presented here expands the landscape of nanocrystalline multinary oxides, with the tailored chemical composition and electrochemical interfacial behavior, in the potential fields of energy storage,catalysts, and nanoscale additive manufacturing.

2 Experimental

2.1 Preparation of the CC@FeNiMnO4 composites

Carbon cloth (CC, Sigma-Aldrich) was pretreated by concentrated hydrochloric acid (HCl,36.0%～38.0%) for 6 h, then washed with deionized water and ethyl alcohol 3 times and dried at 60 °C for 2 h. After that, CC was processed with the UV light treatment for 0.5 h. The pretreated CC was shaped into round pieces with a diameter of 12 mm. Afterwards, 0.125 mol FeCl3、(≥99.8%), 0.125 mol Ni(NO3)2、(≥99.8%), 0.125 mol Mn(CH3COO)2、(≥99.8%) and a certain amount of polyvinyl acetate(PVA, Aladdin) were added into 250 mL deionized water to form a clear solution upon intensive magnetic stirring. The CC piece was added into the mixed solution and moved to a Teflon-lined stainless autoclave at 180 °C for 6h. The CC was recovered and washed with water and ethyl alcohol alternatively, followed by drying at 60 °C for 2 h. Quasi-gel-state F127-resorcinol-melamine was mixed with previously obtained samples and then treated at 600 °C for 2 h under argon atmosphere at a 5 °C min-1heating rate. The quasi-gel-state F127-resorcinol-melamine was prepared through the polycondensation of organic monomers according to the previous study[25]. Specifically, 0.22 g of resorcinol (99%, Aladdin), 0.75 g of Pluronic F127 (Mw=12 600, Aladdin) and 0.26 g of melamine (Aladdin, 99%) were dissolved into 200 mL deionized water under stirring. Then 0.06 g of NaS2O8was added to initiate the polymerization reaction,which was further treated for 12 h at 80 °C, yielding the quasi-gel-state F127-resorcinol-melamine. The asprepared sample was designated as CC@FeNiMnO4-600. For comparison purposes, the samples were conducted without N-doped carbon layers and different annealed temperatures (450, 500, 550, 600, 650 and 700 °C) while maintaining other experimental parameters. The as-obtained were designated as CC@FeNiMnO4-600 w/o NC, CC@FeNiMnO4-450, CC@FeNiMnO4-500, CC@FeNiMnO4-550, CC@FeNiMnO4-600, CC@FeNiMnO4-650 and CC@FeNiMnO4-700.

2.2 Preparation of FL-BN/PEO solid polymer electrolyte (SPE)

First, 0.5 g of PEO (Mw=1 000 000, Aladdin) and 0.12 g of LiClO4(99.99%, Aladdin) were added into 15 mL of anhydrous acetonitrile (Aladdin, 99.5%)with magnetic stirring for 1 h, followed by the incorporation of FL-BN with further stirring overnight. The FL-BN was obtained from normal boron nitride(Aladdin) treated by a high-pressure homogenizer,and the dispersion solvent is deionized water while the processing time is 0.5 h. After that, the solution was subjected to ultrasonic treatment for 0.5 h and then magnetic stirring to yield a homogeneous solution.The as-obtained viscous solution was cast in a special Teflon mold and dried at Fume hood for 24 h to yield the FL-BN/PEO SPE. Then the membrane was vacuum dried at 50 °C for 12 h. The electrolyte was obtained with diameter and thickness of 19 mm and～120 μm, respectively. Finally, the product was stored in a glove box prior to use.

2.3 Characterization

Powder X-ray diffraction (XRD) curves were performed by a transmission-mode X-ray diffractometer (STADIP STOE) with a position-sensitive detector and CuKα1radiation (0.154 05 nm) with the 2-Theta range of 10°-70°, and 40 kV and 40 mA. The morphology of samples was studied by field emission scanning electron microscopy (SEM, Quanta 600 FEG)at 15 kV coupled with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscope(TEM, FEI Talos F200X) measurements were performed at 200 kV. The X-ray photoelectron spectra(XPS) were tested using a spectrometer (Thermo escalab 250XI) and AlKα radiation (hv= 1 486.6 eV).The peak areas were integrated using a weighed leastsquare fitting of model curves. Thermogravimetric analysis (TGA, Q50, TA Instruments) was carried out at a heating rate of 10 °C min-1from ambient temperature to 800 °C. Raman spectra were obtained on a Raman spectrometer (HORIBA, France) with a 532 nm laser source.57Fe Mo?ssbauer spectroscopy was implemented with the aid of a constant acceleration spectrometer. The source, 57CoRh, was held at room temperature. 8 mg active material was mixed with a given amount of boron nitride as the absorber,and spread evenly on the absorber disc (13 mm in diameter). The folded spectra showing a velocity span of±10 mm/s or less were least-square fitted with Lorentzian lines with the software Recoil. The center shift,being the sum of the true isomer shift and the secondorder Doppler shift, is given relative to metallic iron(α-Fe) at room temperature. The magnitude of the magnetic splitting is provided as the peak separation in the symmetrical sextet.

2.4 Electrochemical measurements

Coin cells with a CR2016 type were assembled in an argon-filled glove box. The lithium metal and polypropylene film (Celgard 2400) act as the counter electrode and the separator, respectively. The surface area of the CC@FeNiMnO4-600 was 1.1 cm2. The flexible pouch cell (120 mAh) was assembled with LiFePO4(8.4 mg cm-2) as the cathode, CC@FeNiMnO4(1.5 mAh cm-2) as the anode and liquid electrolyte in a glove box. The electrolyte was 1 mol L-1LiPF6in a mixed solution of diethyl carbonate (DEC),ethylene carbonate (EC) with additive of 2% fluoroethylene carbonate (FEC). The galvanostatic cycling was carried out by the Neware instrument at the voltage window of 0.01-3 V (VversusLi+/Li). Allsolid-state flexible pouch cell was assembled with LiFePO4as the cathode, CC@FeNiMnO4as the anode and FL-BN/PEO as the electrolyte in a glove box.Before the flexible test, the pouch cell was charged to 3.4 V at 55 °C. The electrochemical impedance spectroscopy (EIS) was obtained by a Biologic VMP3 potentiostat with the frequency range from 7 MHz to 1 Hz. The ionic conductivity (σ) was calculated according to the equation 1, whereRis the resistance from the EIS,dthe thickness of the electrolyte membrane, andSthe surface area of the stainless-steel electrode.

3 Results and discussion

The conventional preparation of anodes needs the uniform mixing of carbon additive, electroactive material and binder by the slurry-casting strategy, so as to electrically connect the reactant particles to the metallic (nickel/copper) current collectors. In sharp contrast, we directly develop the coaxial architecture to afford the electron transport network and engineer the interfacial chemistry of oxide particles in the preparation step. As schematically shown in Scheme 1,our synthesis started with the acid functionalization of the carbon cloth with negatively charged -OH,-COOH, and epoxy groups. Subsequently, the surface activated CC was immersed in the precursor solution (Process I). In brief, mixed metal cations of Fe,Mn and Ni at the pre-determined ratios were anchored with the hydroxyl functional groups of CC, forming an “M-impregnated CC framework (M=Fe, Ni, Mn)”upon the vacuum drying. During the calcination process, the mixed M+cations at the equal-atomic ratio partially transformed to the amorphous phase accompanied by the oxidative decomposition of the hydroxyl groups. Subsequently, we employed F127-resorcinol-melamine, to conformally encapsulate the oxide surface (Process II). Pyrolysis treatment was done in N2atmosphere to complete the carbonization, during which ultrafine FeNiMnO4nanocrystals were uniformly encapsulated within the N-doped coating layer.The quasi-gel-state nature of the F127-resorcinolmelamine permited its facile infiltration into the oxide interior structure. During the pyrolysis process, the sufficient lone pair electrons that are tremendously present in the tri-copolymer could closely coordinate with the empty d orbitals of the transitional metal atoms (Scheme 1), the N-doped carbon layer with nano-scale thickness was thus created while retaining the macroporous channels for ready electrolyte access towards electrode.

To figure out the effect of calcination temperature on the crystallinity and microstructure of the quintenary oxides, a variety of CC@FeNiMnO4coaxial electrodes were fabricated under different calcination temperatures at 450, 500, 550, 600, 650 and 700 °C. The scanning electron microscope (SEM) images of the CC@FeNiMnO4-600 composite, shown in Figs. 1a and 1b at different magnifications as an example, demonstrate the fibrous nanoarray with the oxide nanocrystals on the surface. It is noted that the uniformly distributed porous channels situated between the oxide aggregates, suggesting the excellent percolation of this coaxial structure upon the wetting of liquid electrolyte. The detailed microstructure of the oxides was further estimated by high-resolution TEM (HRTEM). As shown in Fig. 1c, the amorphous carbon layer has conformally coated on the surface of the oxide nanocrystallines. Some light-contrast regions (noted by the yellow arrows of Fig. 1c) exhibit well-distributed mesopores. Incorporated within the“solid” oxides, the mesopores were derived from the lattice shrinkage upon the thermal dehydration of metal hydroxide precursors. From the selected area in Fig. 1c (highlighted by a white square), we can observe the lattice fringe spaced by 0.25 nm apart, which is assignable to (311) planes of the isostructural cubic spinel phase similar to Fe3O4. However, the N-doped carbon layer cannot be observed in Fig. S1 (CC@FeNiMnO4-600 w/o NC composite). EDX (Figs. 1d-1j) reveals the elemental maps of Ni, Fe and Mn, confirming the uniform distribution of these cations across the oxide nanocrystallines. In contrast, the elemental maps of C and N wrapped on the oxide nanocrystallines imply the N-doped carbon interfacial layer. For the comparison purpose, Figs. S2a and S2b show SEM images of the CC@FeNiMnO4-500, the blurred surface and irregular oxide coating reveal the amorphous oxides and the incomplete carbonization of the tri-copolymer precursor. The CC@FeNiMnO4-550 composite (Figs. S2c-S2d) demonstrates the uneven coverage of the oxide nanocrystallines on the surface. SEM images of CC@FeNiMnO4-650 and CC@FeNiMnO4-700, as shown in Figs. S2e-S2f and Figs. S2g-S2h, clearly reveal the particle agglomeration and the enhanced crystallinity as compared to the CC@FeNiMnO4-600 composite. We chose 600 °C as the optimal calcination temperature for the coaxial electrode construction based on these morphological analysis.

Fig. 2 exhibits the chemical composition of the as-developed CC@FeNiMnO4-600 composite. As shown in Fig. 2a, the well-defined peaks situated at 641.3 and 653.6 eV are ascribed to the spin-orbital splitting of the Mn2p3/2and 2p1/2emissions, respectively. Direct peak fitting of the Mn2p peak is not straightforward to identify the Mn valency due to the notable broadening effect in these strongly electron correlated systems. Nevertheless, the satellite peak at 647.3 eV is the characteristic of trivalent Mn, as highlighted in Fig. 2a. In the Ni2p spectra (Fig. 2b), the splitting of the main peak and the ‘shake-up’ satellite(ΔEm,s) positively associates with the intensity of coulombic attraction between the 2p and 3d electrons and the hybridization degree between ligand p and cation 3d levels[26]. The ΔEm,sof 6.2 eV demonstrates the divalent Ni in the spinel structure. Furthermore, the Fe 2p3/2peak of Fig. 2c located at ～711.0 eV and the presence of a broad satellite peak demonstrate the potential existence of trivalent iron in the CC@FeNiMnO4-600 composite. The C1s XPS spectrum(Fig. 2d) of the CC@FeNiMnO4-600 could be deconvoluted into two peaks at the 284.5 and 288.4 eV, attributed to C-C and C＝O, respectively. Fig. 2e shows “pyridinic”, “pyrrolic” and “graphitic” nitrogen with BE values of ～398.4, ～400.5 and ～401-403 eV, respectively[27]. The O1s spectrum shows 2 dominant peaks at 531.1 and 529.0 eV, designated to the C-O b and M-O bonding (M=Fe, Ni, Mn), respectively (Fig. 2f).

As displayed in Fig. 3a, the diffraction peaks of FeNiMnO4are correlated to the standard isostructural cubic spinel phase as Fe3O4(JCPDS No. 65-3107).The peaks at 18.3°, 30.1°, 35.46°, 37.08°, 43.1°,53.46°, 56.98° and 62.58° correspond to the (111),(220), (311), (222), (400), (422), (511) and (440)planes of the isostructural compound of cubic spinel Fe3O4(JCPDS No. 65-3107), respectively. 2 broad peaks shown at 26.3° and 43.5° in the XRD patterns are assigned to (002) and (101) crystal planes of graphitic carbon, respectively. As the calcination temperature increases, the peaks indexed to the cubic spinel phase gradually become more pronounced in accompany with the suppression of the broad graphitic carbon peak. The Raman spectra of CC@FeNiMnO4exhibit 2 peaks at 1 341 and 1 568 cm-1,ascribed to theD-band andG-band of carbon fibers,respectively (Fig. 3b). The largeID/IGratio (0.9-1.3)indicates a large number of defects generated in the carbon fibers upon the oxide loadings. Additionally,the peaks at 510, 530 and 540 cm-1correspond to the spinel oxide of the FeNiMnO4. M?ssbauer spectroscopy was also employed for quantitative analysis of the Fe chemical environment of the CC@FeNiMnO4-600 composite (Fig. 3c). Fitting results indicate the obvious presence of two sets of sharp and resolved sextets with hyperfine parameters, ascribing to the Fe atoms positioned at the tetrahedral sites (Sextet 2,86.6%) and the octahedral sites (Sextet 1, 12.5%). The specific structure detail can be observed in Fig. S3.Based on the isomer shift analysis of the smaller than 0.5 mm s-1, divalent Fe is not detected within the detecting limit of the M?ssbauer spectrum, implying the preferential occupancy of Fe at the tetrahedral sites to stabilize the cubic spinel structure.

Fig. 4a displays the discharge-charge curves of the CC@FeNiMnO4-600 galvanostatically cycled at 1 mA cm-2. The discharge and charge capacities for the 1stcycle are 1.96 mAh cm-2and 1.52 mAh cm-2,respectively, demonstrating the relative higher Columbic efficiency (CE) up to 75% compared to that of the CC@FeNiMnO4-600 w/o NC with a CE of ～60%. This CE value is 98.3% in the 2ndcycle and maintained higher than 99.5 % from the 2ndcycle onwards. The superimposability for the 2nd, 50th, 100thand 200thprofiles suggest the structural stability with the repetitive discharge-charge cycling. Long-term cyclability of the CC@FeNiMnO4-600 electrode and CC@FeNiMnO4-600 w/o NC electrode were also measured (Fig. 4b). With the continuous cycling to the 200thcycle, a capacity of 1.28 mAh cm-2is still achieved with only ～10% deterioration. The CC@FeNiMnO4-600 electrode shows an excellent cyclability, especially at high current densities. Fig.S4 shows that the reversible capacity of pure CC electrode is only 0.18 mAh cm-2at 1 mAh cm-2, in which the metal oxide (FeNiMnO4) coated by N-doped carbon layer contributes to the majority of the capacity(87%) of the CC@FeNiMnO4-600 electrode. Fig. 4c validates the crucial role of the N-doped carbon layer for the rate performance. When the CC@FeNiMnO4-600 electrode is cycled at 0.2 mA cm-2, the capacity is maitained at 1.81 mAh cm-2after 10 cycles. The stepwise increase in the current density to 0.4, 1, 2 and 4 mA cm-2results in the stable capacities of 1.52,1.40, 1.28 and 0.77 mAh cm-2after 10 cycles at each step. When the current density is switched back to its initial value of 0.2 mA cm-2, a capacity of 1.66 mAh cm-2is still recoverable. However, for the CC@FeNiMnO4-600 w/o NC electrode, the capacity is 1.75,1.24, 0.97, 0.52 and 0.34 mAh cm-2at 0.2, 0.4, 1, 2and 4 mA cm-2, respectively, lower than the CC@FeNiMnO4-600 electrode. Based on the capacity comparison shown in Fig. 4d, the CC@FeNiMnO4-600 electrode approaches the optimal coaxial configuration in terms of the higher retrievable capacities compared to other composite electrodes, especially at the higher current densities.

In order to reveal the effect of the composite structure on the cycle stability, the morphology of the post-mortem electrode was characterized. After 100 cycles, the main building blocks of the oxide nanocrystals are still observable as shown in (Fig. 5a).However, the polymeric species derived from the electrolyte decomposition cover the oxide surface and the interparticle void space is still preserved. The observable Mn and Fe signals from the EDX element mapping suggest that the formed SEI layer is relatively uniform and homogenous. In sharp contrast, a non-uniform inorganic species is shown on the CC@FeNiMnO4-600 w/o NC after 100 cycles(Fig. 5b). In addition, the relatively weak signal of Mn and Fe from the oxide and the stronger P and F signals derived from the SEI layer species (e.g., LiF,PEO type polymer and LiPF6salt) corroborate the formation of a thicker, heterogenous SEI layer on the electrode, which irreversibly depletes the Li+and compromised the cycling efficiency. Fig. 5c compares the sum of the irreversible capacity loss (ICL) of CC@FeNiMnO4-600 and CC@FeNiMnO4-600 w/c NC electrodes. It can be seen that the difference between two electrodes becomes more evident upon the continuous cycling (0.31 mAh cm-2of the ICL difference for the initial cycle, 0.35 mAh cm-2for the first 10 cycles, 0.56 mAh cm-2for 50 cycles,0.77 mAh cm-2for 100 cycles and 1.23 mAh cm-2for 200 cycles). This result signifies the effectiveness of the N-doped carbon encapsulation in enhancing the reversibility of the CC@FeNiMnO4-600 electrode.

Fig. 6a shows the current-potential response of the CC@FeNiMnO4-500, 550, 600 and 650 electrodes (5×1 cm2) with different loadings. When they are swept between -0.3 and 0.3 V at 10 mV s-1, no difference is detected for all the integrated composite films in flat, bent and twisted states, demonstrating the electrical continuity for the integrated configuration.Specifically, the resistance or conductivity of the CC@FeNiMnO4-500, 550, 600, 650 and CC electrodes films are almost the same by the deformation,indicative of the promising mechanical flexibility(Fig. 6b). Such good flexibility ias supposed to be due to the close coupling of mechanically robust FeNiMnO4layer and CC nanoarrays. In contrast, the integrated electrodes with relative thick loading (FeNiMnO4) demonstrate a significant conductivity change upon flexing. Fig. 5c displays the cycling performance of the integrated CC@FeNiMnO4-600//liquid electrolyte‖LiFePO4pouch cell at 1 mA cm-2in various bending states. Under the initial flat state, the pouch cell could deliver a stable capacity of 1.39 mAh cm-2with ～95% retention over 20 cycles. Upon the mechanical loading from the 20thto the 40thcycle,it is found that the pouch cell could keep 92% of the reversible capacity. Upon the mechanical loadings at the bent states from the 60thcycle to the 80thcycle, the pouch cell could still deliver a stable capacity of 1.26 mAh cm-2with a 87% capacity retention. As the pouch cell returns to the flat state from the 80thcycle to the 100thcycle, the initial capacity could still be recoverable. The mechanical stability of the pouch cell attributes to the excellent flexibility of the coaxial CC@FeNiMnO4-600 configuration.

The solid-state pouch cell configuration was also constructed by pairing the CC@FeNiMnO4-600 anode with the composite polymer electrolyte, namely CC@FeNiMnO4-600// FL-BN/PEO SPE//LiFePO4pouch cell. The detailed properties of FL-BN/PEO SPE are elaborated in Figs. S5-S7. This electrolyte presents unique properties combining high thermal and electrochemical stabilities. SEM images of FLBN/PEO and PEO SPE are shown in Fig. S5. At room temperature (～25 °C), the ionic conductivity of the FL-BN/PEO sample is calculated as 7.84×10-5S cm-1,an order of magnitude higher than PEO SPE (Fig. S6).Additionally, the FL-BN additive is found to have significant effects on the thermal responses and the TGA curve in Fig. S7a suggests good thermal stability of the FL-BN/PEO SPE. In Fig. S7b, the load-extension curves demonstrate excellent toughness property. The SEM and TEM images of FL-BN are shown in Fig. 7a, indicating that the BN possesses a unique lamellar structure. Fig. 7b demonstrates the excellent flexibility and toughness of FL-BN/PEO SPE.Figs. 7c-7f demonstrate the open circle voltage (OCV)response of the CC@FeNiMnO4-600//FL-BN/PEO SPE//LiFePO4pouch cell at various geometrical states. There was nearly no difference in the OCV with different states. As shown in Figs. 7d-7e, the allsolid-state pouch cell shows relative stabilized voltage even at the bend or cut states. Abuse cutting test of the 1 mAh pouch cell was also carried out. It is noticed that the cell remains operational (shortly afterward)and still lights up the LED light.

4 Conclusions

In summary, we successfully developed the integrated electrode through the coaxial integration of quaternary oxide FeNiMnO4nanoarrays with the tailored phase and composition onto the carbon cloth.Meanwhile, the quasi-gel like tri-copolymer derived N-doped carbon was coated on the surface to regulate the interfacial electrochemistry. Therefore, this electrode configuration demonstrates the simultaneous electrochemical performance and mechanical robustness for flexible lithium-ion storage. The preferential occupancy of the Fe in tetrahedral sites helps to stabilize the cubic spinel structure. When employed as a free-standing electrode, the CC@FeNiMnO4-600 composite anode demonstrates a high areal capacity of 1.40 mAh cm-2, an average CE of 99%, a robust cyclability at 1 mA cm-2and a high rate capability up to 4mA cm-2. Additionally, the CC@FeNiMnO4-600 electrode demonstrates the interfacial compatibility with FL-BN/PEO SPE in the integrated, stacked configuration, showcasing the potential use in the flexible allsolid-state energy storage system.

Acknowledgments

This work is financially supported by National Natural Science Foundation of China (52173229 and 51972270), Natural and Scientific Program funded by Education Department of Shaanxi Provincial Government (18JK0579), Research Fund of the State Key Laboratory of Solidification Processing (NWPU),China (2021-TS-03), Fundamental Research Funds for the Central Universities (3102019JC005) and Key Research and Development Projects of Shaanxi Province(2019ZDLGY04-05).

Supplementary data

Additional characterization of materials (TEM and SEM images, EIS, TGA and Tensile test results),additional electrochemical performance (charge-discharge curves) can be found online.