MoS2 decorated lignin-derived hierarchical mesoporous carbon hybrid nanospheres with exceptional Li-ion battery cycle stability

Feng Chen,Long Wu,Zeping Zhou,Jijun Ju,Zhengping Zho,Mingqing Zhong,*,Tirong Kung,*

a College of Material Science and Engineering,Zhejiang University of Technology,Hangzhou 310014,China

b The Key Laboratory of Polymer Processing Engineering of Ministry of Education,South China University of Technology,Guangzhou 510640,China

c Zhijiang College,Zhejiang University of Technology,Hangzhou 310014,China

Key words:Lignin MoS2 Porous carbon nanosphere Electrochemical performance Excellent cycle stability

ABSTRACT Lignin is the most abundant and important macromolecule in organic matter and its yield is second only to cellulose.Lignin is abundant in source,low in price,and has a large number of active groups such as methoxy group and carboxyl group,so it has great utilization value.We used lignin as a carbon source to prepare porous carbon nanosphere(PCN)materials,and in-situ synthesized the Mo S2 on its surface.The high specific surface area[38_TD DIFF](?462.8 m 2/g),large pore volume and good electron conductivity of the porous carbon scaffold facilitated the reversible electro-chemical reaction of Stowards metallic Li,and thus the nano-hybrid showed a high specific energy and excellent cycle stability which still remained 520[49_TD DIFF]m Ah/g after 50 cycles.

As the energy crisis and environmental pollution become more serious,environmentally friendly energy has attracted w ide attention from researchers.Lithium-ion batteries,one of the most important eco-friendly energy sources,have gained focus of current research,especially considering the increasing demand for energy storage equipment caused by the rapid grow th of electronic equipment(mobile phones,computers,etc.)[1–5].Moreover,today[39_TD DIFF]’s lithium-ion batteries are difficult to meet the high performance requirements of products such as electric vehicles.

Most anode materials of lithium ion batteries used in practical applications are carbon materials,such as carbon fiber,natural graphite,and Artificially modified graphite.Lignin,a heterogeneous and amorphous polymer that constitutes a large portion of the cell w alls of vascular plants,is one of the amplest biomass just after cellulose on earth[6–9].It can be obtained from a variety of low-cost w oody plants and even from papermaking wastew ater[10].Lignin is of great application value for it containing a large number of reactive groups,such as methoxy,hydroxyl[40_TD DIFF]6].Its specific structure is not clear yet,we only know it composed of macromolecular structure with benzene and methoxy[11].The estimated natural production of lignin on earth is in the range of[41_TD DIFF]5?108–36?108tons annually and annual production of commercial lignin is more than 70 million tons[12,13].So lignin has attracted great interests in developing value-added products such as sorbents of exhaust gas CO2and electrode materials of supercapacitors and lithium-ion batteries[14–22].Sodium lignosulfonate with relative molecular weight 1000–20000 can be obtained by attaching a sulfonate-containing group such as–SO3Na to lignin[23].Since lignosulfonate contains hydrophilic groups such as sulfonate,carboxylate,and phenolic hydroxyl,it is soluble in various aqueous solutions,as a result,the application of lignosulfonate become w idely.

Molybdenum disul fi de(MoS2)is a typical representative of transition metal sul fi de materials[24,25].The crystal of MoS2belongs to the hexagonal system,and the lamellar MoS2with single sheet composed of three layers of atomic layers.The middle layer molybdenum atoms are bonded to sulfur atoms of upper and lower layers by Mo-S covalent bonds[26].When the MoS2monolayer accumulates in ABAB,we get the most stable lamellar MoS2with a 0.62 nm distance of tw o MoS2layers[26].This distance facilitates the insertion and extraction of lithium ions,so MoS2has a high specific capacity and charge and discharge ef fi ciency[27–31].However,the main draw back with MoS2as the electrode material of lithium ion battery is the poor cycle stability due to the insuperable expansion of MoS2nanoparticles.

On the other hand,carbon materials,such as activated carbon,porous carbon,carbon nanotubes,nano fibers,carbon aerogels,graphene and other carbon materials,especially mesoporous carbon,can improve the dispersion and conductivity of metal nanoparticles[32–36].Meanwhile,the carbon scaffold with high graphitization could endure the volume expansion during the charging-discharging process[37].When lignin is carbonized at high temperature,porous carbon spheres are formed,which exhibits the electrochemical properties of typical carbon materials.Since lignin is derived from biomass and usually discarded as waste,it is promising to use lignin as a cheap alternative material in w oody materials.On the other hand,few studies have investigated the fabrication of lignin-derived LPN and their potential usage in high-performance electrode materials[32,38].

To overcome the unstable specific capacity of common MoS2anode material,we prepared lignin and MoS2as core-shell composites MoS2@porous carbon nanospheres(MoS2@PCN),in which PCN working as both the carbon source and the matrix with layered MoS2binding to the surface of PCN.Ow ing to the unique hierarchical structure,this composite not only reserved all advantages of the spherical structure,but also increased the reversible capacity and the cycling performance.In addition,the graphitized carbon skeleton in PCN acts as a buffer to alleviate the volume expansion of MoS2during charge-discharge.At the same time,the graphitized carbon skeleton in PCN also increases conductivity and acts as a conduit for electron transfer.The experimental details are as follow s.

Lignosulphonate,isopropanol,sodium molybdate hexahydrate,thiourea were purchased from Aladdin Reagents.Porous carbon Nanosphere(PCN)was prepared using a combined carbonization/activation method and a typical process was show n as below.First,0.5 g lignosulphonate was dissolved into 10 m L deionized w ater and then the solution was slow ly dripped into 100 m L isopropyl alcohol.The product was separated by centrifugation and was dried at 60?C for 24 h to obtain LPN.Porous carbon nanosphere(PCN)was obtained by carbonization of LPN at 800?C for 2 h.Secondly,PCN,thiourea,Sodium molybdate hexahydrate were added into 60 m L isopropanol,then the mixture was transferred into a high pressure reactor and the reaction took place at high temperatures and pressures.The resulting product was heated to 800?Cin a nitrogen atmosphere and carbonized for 2 h to obtain MoS2@PCN.The major process steps employed in this work are illustrated in Scheme 1.

Schem e 1.Schematic procedure for the preparation of MoS2@PCN derived from lignosulfonate.

In order to understand the material properties,we chose the follow ing test methods for characterization.

Fourier transform infrared spectroscopy(FTIR)data were collected on a Nicolet 6700(Thermo Nicolet Corporation)with a measuring range 4000 cm?1to 400 cm?1and a resolution of 0.4 cm?1.DLS data were collected on a BI-200SM(American Brookhaven)with w ater as the solvent.Thermogravimetry(TG)test was performed using a TA Instrument SDT Q600 under a N2atmosphere with 10?C/min ramp rate from room temperature to 1000?C.X-ray diffraction(XRD)data were collected on an X’Pert PRO(Malvern Panalytical).Raman spectra were recorded on a LabRam HRUV(HORIBA Jobin Yvon).Scanning electron microscopy(SEM)was carried out with S-4700(HITACHI),and the accelerate voltage was 10 k V and the working distance was 8 mm.Transmission electron microscopy(TEM)was carried out with JEM-100CXII(JEOL)fi eld emission electron microscope at an accelerate voltage of 300 k V.Brunauer-Emmet-Teller(BET)isotherms and specific surface area(i.e.,BET surface area)were performed on an ASAP 2020(Micromeritics).X-ray photoelectron spectroscopy(XPS)analysis was performed on a Kratos AXIS(Shimadzu)employing a monochromatic Al KαX-Ray.Raman spectrum was taken with a Lab RAM HR UV800 Raman spectrometer(JOBIN YVON)using an excitation w avelength of 632.81 nm at room temperature.

The electrodes for LIBs were prepared according to the follow ing steps.Poly[42_TD DIFF](vinylidene fl uoride)andN-methyl pyrrolidone(mass ratio,1:10)were mixed to obtain a conductive gel.A small amount ofN-methyl pyrrolidone was added to evenly mixed MoS2@PCN,acetylene black and conductive gel(mass ratio 8:1:1)and the mixture was stirred for 3 h to form a uniform slurry.The slurry was applied on copper and then dried in a vacuum oven at 60?Cfor 3 h.The dried electrodes were then pressed(15 MPa)and placed in a vacuum oven for an over-night dry at 60?C.Finally,the half cells were assembled in an Ar fi lled glove-box,using 1 mol/L LiPF6solution in a 1:1(v/v)mixture of ethylene carbonate(EC)and dimethyl carbonate(DMC)as electrolyte and Li foil as counter electrode to form a CR2025 button battery.

The adhesive was prepared withPTFE and deionized w ater(1:20 mass ratio).Then MoS2@PCN composite,acetylene black and PTFEbinder were mixed at a weight ratio of 8:1:1,and the resulting slurry waspasted onto an graphite sheet(1 cm?2 cm)and dried at 80?C for 12 h.

Athree-electrode battery system wasbuilt for cyclic voltammogram(CV)testing.The graphite sheet with mixture is the working electrode with Ag/AgCl electrode as a reference electrode,Pt w afer as the counter electrode and KOH(6 mol/L)as the electrolyte[37,39].

Galvanostatic cycling tests were conducted on a New are battery system in the voltage range of 1.0–3.0 V(vs.Li+/Li)at 30?C in a thermostatic desiccator.Cyclic voltammetry(CV)tests were performed on a CHI660D electrochemical work-station at 0.1 m V/s in the voltage range of 1.0–3.0 V.The specific capacitance of the electrode was calculated by the follow ing formula.

w here Cwas the specific capacitance(F/g),I was the current(A),V was the potential w indow(V),v was the scan rate(m V/s),and m was the mass of the sample used for the electrochemical test(g).

The characterization results and discussion are as follow s.

Fig.1.(a)FTIR spectrum of LPN;(b)TG curves in N2 atmosphere of LPN and MoS2@PCN;(c)DLS result of LPN;(d)N2 absorption-desorption isotherms and pore size distribution curves(inner plot)of MoS2@PCN;(e)N2 absorption-desorption isotherms and pore size distribution curves(inner plot)of PCN;(f)The Raman spectra for representative PCN;(g,[30_TD DIFF]h)The XPSspectra of PCN.

Lignosulfonates are rich in reactive groups such as sulfonic acid groups and phenolic hydroxyl groups,and various chemical reactions can be carried out to obtain lignin derivatives[6].In order to understand the functional groups of LPN,LPN wastested by FTIR.The FTIRspectrum of LPNisshow n in Fig.1a.The absorption peak at 3432 cm?1is broad and strong,indicating that the lignin contains hydroxyl groups.The phenolic hydroxyl groups in lignin directly affect the physical and chemical properties.The peak at 1650 cm?1wasattributed to thevibration of thebenzenering C?Cdoublebond.The band at 1250 cm?1assigned to sulfonic acid group wasstrong.In general,lignosulfate containslarge amountsof reactive groupssuch as sulfonic acid groups,hydroxyl groups and the like.Fig.1b show s the weight loss curves of the LPN and MoS2@PCN during the high temperature calcination process in N2atmosphere.In the 0–150?C region,mainly small molecules,e.g.,H2Oand dispersant vaporized.

The residue content is 67.9 w t%.And the weight of the nanocomposite decreased by about 4%w hen the temperature increased to 800?C.This result indicates that the porous structure of the material did not collapse along with the adsorption effect above 800?C.Fig.1c shows DLS results of LPN.We can see that the diameter is between 500–600 nm,slightly larger than the results obtained in SEM.That[46_TD DIFF]is because DLSmeasures the diameter of the hydrated particles.Figs.1d and e show the N2absorptiondesorption isotherms and pore size distribution curves(inner plot)of MoS2@PCN and PCN sample.The detailed porous structure data are summarized in Table 1.The rise in the fi rst stage of Fig.1d indicates that monolayer physisorption formed.With increasing pressure,a straight line appearson the isotherm,which re fl ectsthe establishment of multilayer adsorption.Further pressure increase leads to capillary condensation.The observation of hysteresisloops indicated that the material has a certain pore structure.The BET surfaceareaof PCNis867.6 m2/g(Table1),higher than 462.8 m2/gof MoS2@PCN.This is because MoS2blocked some holes of PCN.With the addition of MoS2,the volume content of mesopores and macropores increase,while the volume content of micropores decreases.A porous structure is essential with incorporation of macropores and mesopores to minimize ion diffusion distance for fast charge transport and increase surface area for promoting electrolyte/electrode contact[39,40].Fig.1f is the Raman spectrum of PCN.The D peak at 1317 cm?1represents the degree of disorder and defects in the carbon material,and the G peak at 1608 cm-1represents the graphite carbon material.The peak area ratio,i.e.,ID/IG=1.07 suggested the degree of graphitization of the material was high.From the above data,it can be seen that after the hightemperature calcination of the LPN,the formed porous carbon spheres have a high degree of graphitization,favoring the electron transport and thus improves the electrochemical performance.Figs.1g and h present the XPSspectra of PCN.The three peaks in Fig.1g represent C?C??C(284.7 eV),C–O(286.1 eV),O??C?C(288.7 eV)respectively.The contents of C–Oand O??C?Care 25.4%and 3.58%,respectively.This indicates that the surface of the PCN contains more oxygen-containing functional groups and LPN is not completely carbonized after high temperature calcination.Oxygencontaining functional groups caused by this incomplete carbonization enhance electrochemical capacitive properties.The four peaks in Fig.1g represent thiophene(164 eV),R-O-S-S-R(165 eV),sulfone(169.4 eV)and inorganic sulfur(170.6 eV).According to semiquantitative analysis by XPS,the relative contents of S and Mo element in the material are 10.22%(atom%)and 14.81%(atom%),respectively.The mass ratio of Mo and Sis about 3:2,in accordance with the composition of MoS2.

Fig.2.(a)SEM of LPN without ultrasonic;(b)SEM of LPN with ultrasonic;(c)SEM of carbonized LPN;(d)SEM of MoS2@PCN;(e, f)TEM of MoS2@PCN.

SEM observation(Fig.2)show s that the surface morphology and structure of LPN,porous carbon spheres and their composites.Figs.2a and b are the LPNs without and with ultrasonic treatment,respectively.Large differences in sizes of LPN can be observed in Fig.2a while uniform size distribution was seen in Fig.2b.That means ultrasonic can reduce the size difference between microspheres.From the Fig.2c,we can be seen that after heating at 800?C,the structure of carbonized LPNremains unchanged.Fig.2d is SEM of MoS2;we can see that the sheet-like MoS2is attached to the surface of carbonized LPN.The diameters of the microspheres are calculated in a range of 200 nm–300 nm.Figs.2e and f show the TEM of the MoS2@PCN.Large amount of pores can be observed in the carbonized LPN,which greatly increases the specific surface area of the carbon spheres and facilitates the transport of ions.From Fig.2e,we can see that the MoS2is grow ing on the surface of the LPN.And the LPN has a large number of pore structures that con fi rms the result of BET results.The sheet-like MoS2on the surface of the material grow s outw ard,and the length is about 70 nm.The distance between the sheetsisabout 100 nm.As show n in Fig.2f,it is calculated that the lamellar distance between the MoS2layers is 0.62 nm,which is almost the same as the general MoS2layer spacing.This distance facilitates the insertion and extraction of lithium ions during charging and discharging.

Fig.S1(Supporting information)showed XRD patterns of MoS2@PCN.All observed diffraction peaks can be systematically indexed to those of the hexagonal phase of MoS2,which are in good agreement with the values of standard card(JCPDSNo.37-1492).The diffraction peak at 14?corresponding to the crystal surface(002)representsa distinct single-layer structure.The peak at 29?is the crystal surface(112)of MoS2.The strength of these tw o peaksis very high,which means that the crystallinity of MoS2with these tw o structures is relatively perfect.Clearly,diffraction peaks of elemental Mo were not detected in the XRDpattern,indicating that no carbon thermal reduction occurs during the second calcinationof MoS2@PCN.The higher peak intensities indicate that the MoS2nanostructures are of high crystallinity.

Table 1 The specific surface area,pore volume,pore size of four samples.

Fig.3.(a)Comparative cycling performance at a current density of 0.1 A/g of PCN,MoS2 and MoS2@PCN;(b)Charge-discharge voltage pro files of MoS2@PCN;(c, d)CV curves of PCN and MoS2@PCN with various scan rates.

Fig.3a show s the cycling performance of three materials at low current charge and discharge conditions.The theoretical specific capacity of graphite is 355[49_TD DIFF]m Ah/g and the specific capacity of PCN used in this work is about 300[49_TD DIFF]m Ah/g.It show s the typical characteristics of carbon materials(good cycle stability).In the fi rst few cycles the specific capacity of Mo S2is high,but after several cycling it falls rapidly.In contrast,MoS2@PCN has a stable cycling performance while the specific capacity is 519[49_TD DIFF]m Ah/g after 50 cycles.Fig.3b is the voltage capacitance curve of MoS2@PCN.During the charging process there is a platform at a voltage of 2100 m V,w here lithium ions embedding in the anode material;and during the discharge process,there are 2 platforms at voltages of 2000 m V and 1100 m V,w here lithium ions escape from the anode material.The electrochemical properties of PCN and MoS2@PCN are measured by using cyclic voltammetry(CV)over the potential w indow of 0-0.7 V in a 6 mol/LKOH aqueous solution with a three-electrode cell system.Figs.3c and d show the CV curve of PCN and MoS2@PCN at different sweep rate of 0.01-0.1 V/s.InFigs.3c and d,the obtained CV curve is symmetrical with the curve of the reducing w ave,indicating that the electron transfer is reversible while this material is as an electrode reaction.The area around the CV curve increases with increasing scan rate,however,the shape of the CVcurves does not change distinctly,re fl ecting the good capacitive performance and good power performance of MoS2@PCN electrode material.And the calculated specific capacitances at different scanning rate are presented in Fig.S2(Supporting information).As scanning rate increases,specific capacitance decreases.Compare the specific capacitances of PCN,the specific capacitances MoS2@PCN is higher.This is also con fi rmed by the above conclusions.

In summary,we have developed a feasible self-assembly and carbonization method to prepare lignin-derived PCN and its efficient embedment of MoS2without any additives.The system is eco-friendly,as it using natural lignosulfonate as the carbon precursor in w ater solution.The obtained Mo-based PCN intermediate can be simply converted to highly crystalline MoS2@PCN via thermal annealing without signi fi cant changes in morphology.The annealed product is phase-pure MoS2and highly graphitized carbon with a high surface area.The hybrid structure hasa high specific surface area(462.8 m2/g)and hierarchical pores,which can enhance Li-ion transportation in the electrode,increase the conductivity,and suppress the deformation of MoS2.More importantly,the hybrid nanosphere anode exhibited and retained a high discharging capacity of 519[49_TD DIFF]m Ah/g at 0.1 A/g after 50 cycles for a Li-ion battery.We think this hybrid nanocomposite may provide a new route to develop high performance Li-ion battery from natural biomass or organizational structures.

Acknow ledgm ents

T.Kuang w ould like to acknow ledge the fi nancial support of National Natural Science Foundation of China(No.51803062),National Natural Science Foundation of Guangdong Province(No.2018A030310379),National Postdoctoral Program for Innovation Talents(No.BX201700079),China Postdoctoral Science Foundation Funded Project(No.2017M620371),and Foundation for Distinguished Young Talents in Higher Education of Guangdong Province(No.2017KQNCX001).F.Chen thanks the fi nancial support of Natural Science Foundation of China(No.51673175),and Natural Science Foundation of Zhejiang Province(Nos.LY16E030012,LY17E030006 and LY18E030009).

Appendix A.Supp lementary data

Supplementary material related to this article can be found,in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.10.007.