Reduced graphene oxide encapsulated MnO microspheres as an anode for high-rate lithium ion capacitors

JIA Yao,YANG Zhe-wei,LI Hui-jun,WANG Yong-zhen,WANG Xiao-min,,

（1.College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China;2.Shanxi Key Laboratory of New Energy Materials and Devices, Taiyuan 030024, China）

Abstract：Developing an anode material with high-rate Li+ intercalation and stable charge/discharge platform is important for achieving high performance lithium ion capacitors (LICs).Reduced graphene oxide (rGO)-encapsulated MnO microspheres (～2 μm)are obtained by a simple process including solvothermal and calcination techniques.The material contains a large number of mesopores (～2.8 nm diameter).The MnO/rGO has a favorable cycling stability (846 mAh g?1 at 0.1 A g?1 after 110 cycles) and an outstanding rate performance (207 mAh g?1 at 6.4 A g?1).Kinetic analysis reveals that a pseudocapacitive contribution plays a dominant role for the energy storage.The improvement in the pseudocapacitive behavior is ascribed to the fact that the uniform rGO coating on the MnO provides continuous pathways for electron transport,and the mesoporous structure provides numerous migration paths for Li-ions.Furthermore,MnO/rGO//activated carbon (AC) LICs have a high energy density of 98 Wh kg?1 at a relatively high power density of 10 350 W kg?1,and have a capacity retention of 71% after 5 000 cycles at 1.6 A g?1.These outstanding results indicate that the enhanced Li+ intercalation of the anode offsets the kinetic imbalance between the two electrodes.

Key words:MnO microspheres；Reduced graphene oxide；Rate capability；Lithium ion capacitors

1 Introduction

Lithium ion capacitors (LICs) bridge the high energy density of lithium-ion batteries (LIBs) and the high power density of electric double-layer capacitors(EDLCs)[1–4].They have aroused extensive attention in consumer electronics,electric vehicles,smart grids and other areas.Typically,LICs are built on a capacitor-type cathode,a battery-type anode and a lithium salt-contained electrolyte[5].In recent years,some outstanding works about LICs have been reported.Zhang et al.reported a LICs with pre-lithiated hard carbon(HC) anode and activated carbon (AC) cathode,which exhibited an energy density of 85.7 Wh kg?1[6].Auxilia et al.assembled Au@TiO2/RGO//AC LICs showed a capacity retention of 83% over 1 000 times[7].By reason of the slower Li-ions insertion/extraction reaction process in the battery-type anode than the anions adsorption/desorption process on the capacitor-type cathode surface during charge and discharge process,both high energy and power density of LICs are limited[8,9].In most reported LICs,high energy density(＞90 Wh kg?1) was achieved only under conditions of low power output (＜5 kW kg?1)[10–12].In addition,the potential of anode usually shifts to high values during charge/discharge process,leading to the inferior utilization ratio of cathode materials and poor cycling stability of LICs[13].The long and stable charge/discharge platform is favorable for decelerating anode potential moving towards high values,enhancing the cycling performance[8].

With the analysis above,developing an anode material with superior rate performance and stable charge/discharge platform is an effective strategy for achieving high performance LICs.Recently,a variety of battery-type anodes have been explored for LICs application,including graphitic carbon[14],hard carbon[15]and metal oxides (Fe3O4[16],TiO2[17],Nb2O5[10,18],MnO[19,20],etc.).Among them,MnO has received adequate attention owing to its high theoretical capacity(756 mAh g?1),low electrochemical potential at 0.5 V(vs Li/Li+) as well as abundant manganese resources[21,22].Yang et al.reported MnO/C//hierarchical porous carbon (HPC) LICs delivered an energy density of 97.3 Wh kg?1[23].Yang et al.assembled MnO/graphene aerogel//LiFePO4LICs delivering a capacity retention of 52.9% over 1 000 times[24].What’s more,the low and stable discharge platform(0.5 V) during the discharge process could not only avoid sacrificing the voltage window for the devices,but also decelerate anode potential moving towards high values,enhancing the cycling performance.Therefore,MnO is regarded as an ideal anode material for developing high performance LICs.

Unfortunately,MnO suffers from poor rate capability along with unsatisfied cycle life,which is caused by naturally poor electronic conductivity and unstable structure during Li+insertion/extraction[25].In order to ameliorate inferior rate performance and unsatisfied durable life,constructing flexible electrodes[26],designing special structures[27]and hybridizing with carbon materials[28]have been adopted.Among them,the hybridizing MnO with carbon materials can improve its conductivity and structural stability.In addition,MnO with porous structure is favorable for Li+insertion/extraction process.For instance,a CNT@MnO@N-doped carbon with sandwich-like structure displayed 665 mAh g?1at 0.1 A g?1[29].A N-doped MnO/reduced graphene oxide (rGO) composite with rGO coated MnO retained a capacity of 805 and 335 mAh g?1at 0.2 and 1 A g?1,respectively[30].A Sn-MnO@N-doped carbon with yolk-shelled three-dimensional structure presented 757.2 mAh g?1at 0.1 A g?1[31].It is a promising and valid approach to optimize the structure of MnO and compose it with carbon materials,but the unsatisfied rate performance and the complicated preparation processes normally limit its further application in LICs.Additionally,uniformly wrapping rGO along MnO is still a great challenge.Therefore,reasonable construction of MnO structure can improve the cyclic stability and rate capability for further application to LICs.

Herein,we report a facile chemical synthesis of MnO/rGO via combining solvothermal and calcination treatment.The relationship between the mesoporous and coating structure and the lithium-ion storage performance of MnO/rGO is explored,and the kinetic behavior of the MnO/rGO is also emphasized.Furthermore,MnO/rGO//AC LICs were constructed via the pre-lithiated MnO/rGO anode and the AC cathode,and their electrochemical properties were investigated.The remarkable performances of LICs indicate that the MnO/rGO anode with excellent rate capability is beneficial to the kinetics balance between the two electrodes.

2 Experiment

2.1 Synthesis of MnO/rGO

At first,the graphene oxide (GO) solution was obtained via the modified Hummers method[32].Then,GO suspension (3 mg mL?1,15 mL),Mn(CH3COO)2·4H2O (3 mmol) and urea (6 mmol) were dispersed in 30 mL glycerol under magnetic stirring to form a uniform solution.Next,the above mixture solution was placed in the Teflon-lined autoclave at 140 °C for 12 h.The MnCO3/rGO could be gained via alternate centrifugation for three times with deionized water and ethanol and drying in vacuum at 80 °C for 6 h.At last,MnO/rGO was prepared by sintering in N2atmosphere at 800 °C for 4 h.For comparison,pure MnO was also prepared via an identical method without GO solution.

2.2 Material characterization

The morphology and structure were investigated via scanning electron microscopy (SEM,MiRA3-LMH) and high-resolution transmission electron microscopy (HRTEM,JEM-2010).The crystalline structures were determined using X-ray diffraction with Cu-Kα radiation (XRD,Ultima IV,λ=0.154 18 nm).The chemical compositions were analyzed via X-ray photoelectron spectrometer with Al-Kα radiation(XPS,PHI-5 000 versaprobe,hν=1 486.6 eV).The textural properties were investigated using nitrogen adsorption-desorption measurements (Beishide 3H-2000PS2) and all materials were degassed at 200 °C for 10 h before testing.Raman spectrum was performed by a Renishaw invia system using 532 nm laser under ambient condition.Thermogravimetric analysis (TGA,TG 209F3) was performed from 30 to 800 °C (10 °C min?1) in air atmosphere.The electronic conductivity of materials was measured by RTS-9 four-point probe techniques.

2.3 Electrochemical measurement

The slurry of the working electrodes was prepared via grinding MnO/rGO anode or AC cathode materials (80 wt%),Super P conductive carbon black(10 wt%) and polyvinylidene fluoride (10 wt%) binder dissolved in N-methyl-2-pyrrolidone.Then,the above slurry was coated on Cu foil (anode) and Al foil(cathode),and the as-obtained electrodes were dried in a vacuum oven at 120 °C for 6 h.The loading mass of active material is about 0.9-1.3 mg cm?2.The CR2025 coin cells were fabricated with the MnO/rGO working electrode,polypropylene film separator (Celgard 2400),1 mol L?1LiPF6electrolyte in a mixture of EC,EMC and DMC (1∶1∶1 in volume) and metal lithium counter electrode.

The button MnO/rGO//AC LICs were assembled via AC cathode and pre-lithiated MnO/rGO anode in the same half-cells system. The pre-lithiated MnO/rGO electrode was obtained by the electrochemical method that the half-cells were charged and discharged 10 times within an operating voltage of 0.01-3 V at 0.05 A g?1and discharged to the potential of 0.5 V.

The cyclic voltammetry (CV),and the electrochemical impedance spectroscopies (EIS) were tested via the electrochemical workstation (PMC 500).The galvanostatic charge/discharge (GCD) was measured using a LAND CT2001A battery testing system.The power density (P,W kg?1),energy density (E,Wh kg?1) and specific capacitance (C,F g?1) of LICs were calculated via equations (1),(2) and (3)[5]:

whereIis the current (A),mis the total mass of the cathode and anode (g),Vmaxis the maximum working voltage without the IR drop andVminis the minimum working voltage (V),tis the discharge time (s).

3 Results and discussion

3.1 Morphology and structure

Fig.1 presents the fabrication process for MnO/rGO.Firstly,MnCO3/rGO was prepared by the solvothermal method.During the solvothermal condition,the decomposition urea (CO32?) reacts with the Mn2+to form MnCO3precursor.In this stage,the MnCO3microspheres (JCPDS No.44-1 472,Fig.S1)with smooth surface is formed by the aggregation of flaky particles and rGO is uniformly dispersed on the surface of MnCO3.It could be seen that the average size (～3 μm) of MnCO3in MnCO3/rGO is smaller than that of pure MnCO3(2-5 μm) (Fig.S2),because rGO could restrain the growth of MnCO3.Then,MnO/rGO was prepared by calcination of MnCO3/rGO at 800 °C.The pore structure is formed because of the MnCO3decomposition to create CO2and the shrinkage of the flaky particles during the annealing process,thus increasing the specific surface area.

Fig.1 Schematic illustration of the preparation process for MnO/rGO.

The morphology and structure of MnO and MnO/rGO were investigated by SEM,TEM and HRTEM.MnO displays a microspheres structure and the size is about 2 μm (Fig.2a).As for MnO/rGO,it is clearly observed that the MnO microspheres are wrapped and connected by the rGO (Fig.2b).Due to the restriction of the coated rGO,additionally,the size of MnO is smaller than that of pure MnO.Such structure helps to ensure the structural stability of MnO,and the conductive network constructed by rGO can increase the conductivity.The homogeneous distributions of Mn,O and C elements can be revealed by energy dispersion spectrum (EDS) (Fig.2c) which indicates the MnO microspheres are coated uniformly by rGO.It can be further observed that rGO is coated on MnO microspheres from the TEM image (Fig.2d).Besides,the pore structure inside of MnO/rGO was formed due to the MnCO3decomposition to create CO2and the shrinkage of the flaky particles during the annealing process.Porous structure can provide a convenient pathway to transfer Li-ions[33].The SAED pattern (inset Fig.2e) of MnO/rGO shows the diffraction rings,revealing the presence of polycrystalline structure of MnO.From the HRTEM image (Fig.2f),it can be analyzed that the lattice fringes with average distance spacing of 0.26 and 0.34 nm are ascribed to(111) plane of MnO and (002) plane of rGO,respectively.

Fig.2 (a) SEM image of MnO;(b-f) SEM image,EDS mapping,TEM image and inset (e) SAED pattern and HRTEM image of MnO/rGO.

The crystal structures of MnO and MnO/rGO were characterized by XRD (Fig.3a).The diffraction peaks at 34.9°,40.5°,58.8°,70.2° and 73.9° are observed,attributing to the (111),(200),(220),(311) and(222) planes for the cubic phase MnO (JCPDS No.07-0230).Meanwhile,a broad hump at 20°?30° is clearly observed in MnO/rGO,which corresponds to(002) plane of rGO[4,34].Other peaks are not found,demonstrating the high purity of substances.The Raman spectrum (Fig.3b) of MnO/rGO displays the two characteristic peaks at 1 341 and 1 590 cm?1,associating to theA1gvibration mode of disordered carbon (Dbond) and theE2gvibration of sp2carbon (G-bond) of rGO,respectively[35].TheID/IGvalue is 1.06,suggesting a more defective and disordered structure for MnO/rGO.Additionally,the peak of 643 cm?1ascribes to the Mn-O vibration band of MnO.

The element information of MnO/rGO was analyzed via XPS spectrum.In survey spectra (Fig.S3a),the main components of MnO/rGO are carbon (C 1s at～285 eV),oxygen (O 1s at～530 eV) and manganese (Mn 3p,Mn 2p and Mn 2s at～49,～642 and～772 eV,respectively).The Mn 2p spectrum (Fig.3c)depicts two asymmetric peaks located at 641.6 and 653.3 eV,attributing to Mn 2p3/2and Mn 2p1/2of Mn2+[36].The C 1s spectrum (Fig.3d) shows the three energy bands at 284.7,285.6 and 289.3 eV,associating to C＝C,C―O and O＝C―O bonds,respectively[37].Additionally,the binding energies of 530.1,531.3 and 532.8 eV in the O 1s spectrum (Fig.S3b)represent Mn―O,C＝O along with C―O bonds,respectively.

Fig.3 (a) XRD patterns of MnO/rGO and MnO;(b) Raman spectrum of MnO/rGO;(c) Mn 2p and (d) C 1s high-resolution spectra of MnO/rGO.

On the basis of nitrogen adsorption/desorption isotherms as presented in Fig.4a,the specific surface area (SSA) can be obtained via the BET equation and the pore size distribution (PSD) was calculated via the BJH theory.The isotherm (Fig.4a) of MnO/rGO shows a type-IV isotherm with an H3 hysteresis loop,revealing the existence of large numbers of mesopores in MnO/rGO.From the PSD curves in Fig.4b,the mesopores size peak of MnO/rGO is appeared at～2.8 nm and the average pore size is 14.31 nm.The mesoporous structure could provide a convenient transport channel for more Li-ions and buffer volume change during cycling,improving rate capability[38].In addition,MnO/rGO possesses a SSA of 63.1 m2g?1,while pure MnO is only 18.1 m2g?1.The carbon content of MnO/rGO was calculated by the TGA curves(Fig.S4),which is measured to be 13.7 wt% based on the reaction formula[28]:2MnO+1.5C+2O2→Mn2O3+1.5CO2.

3.2 Electrochemical performance

Fig.5a displays the CV curves of MnO/rGO for the first three cycles.At the first cathodic scan,the irreversible peak at～0.68 V is correlated with the formation of the solid electrolyte interface (SEI) film due to the decomposition of electrolyte.The reduction peak at～0.13 V corresponds to the reduction from Mn2+to Mn,and the peak moves to～0.38 V after the second cycle due to the enhanced kinetics after the first Li-ions insertion process[39].The oxidation peak at～1.31 V is related to the oxidation of Mn and Li2O decomposition.It is demonstrated that MnO stores chemical energy via the following reversible reaction: MnO+2Li++2e??Mn+Li2O[36].Additionally,a weak oxidation peak at～2.1 V is regarded to the oxidation from Mn2+to Mn3+or Mn4+,which contributes to rising of capacities during cycling[40].

Fig.5b displays the GCD curves of MnO/rGO for different cycles.It is observed that an obvious discharge platform is located at 0.18 or 0.5 V,which is in agreement with the CV analysis.MnO/rGO shows a charge capacity of 814 mAh g?1for the first cycle and an initial coulombic efficiency of 62.9%.What's more,MnO/rGO shows a high capacity of 327 mAh g?1below 0.5 V occupying 45% of total discharge capacity.The relative long and low discharge plateau could not only avoid the formation of lithium dendrites at high rate,but also decelerate anode potential moving towards high values,enhancing the cycling performance of LICs[8].Fig.5c exhibits the electrochemical cycling capability of as-prepared MnO/rGO and MnO at 0.1 A g?1for 110 cycles.The capacity of both MnO/rGO and MnO during cycling shows a continuous rise due to the oxidation from Mn2+to Mn3+or Mn4+[39].After 110 cycles,MnO/rGO displays a capacity of 846 mAh g?1,but the capacity of pure MnO is 486 mAh g?1.Meantime,the discharge capacity of MnO/rGO electrode is 624 mAh g?1after 550 cycles at 1 A g?1(Fig.S6a).Moreover,the morphology of MnO/rGO electrode after cycling was tested by TEM and SEM (Fig.S5).The size of MnO is almost unchanged and the morphology remained well after cycling,indicating the rGO effectively ensures the structural stability of MnO microspheres during chargedischarge process.

Fig.5 (a) CV curves at 0.1 mV s?1 and (b) GCD curves of MnO/rGO;(c) Cycling performance and (d) Rate capability of MnO/rGO and MnO;(e) Comparison of rate capability of metal oxides or Mn-based anode materials for LIBs anodes;(f) EIS spectra of MnO/rGO and MnO.

Fig.5d displays the rate capability of as-prepared MnO/rGO and MnO at 0.05,0.1,0.2,0.4,0.8,1.6,3.2 and 6.4 A g?1.At 1.6 A g?1,MnO/rGO exhibits a reversible capacity of 483 mAh g?1,which is much higher than MnO (97 mAh g?1).Even at 6.4 A g?1,MnO/rGO still delivers a good reversible capacity of 207 mAh g?1.Compared with the manganese based or other metal oxide anodes reported in literature[20,24,31,41–47](Fig.5e),the as-prepared MnO/rGO exhibits much higher rate capability.

Fig.5f presents the EIS spectra and the simulated equivalent circuit model as shown in the inset.MnO/rGO displays a charge-transfer resistance (Rct)value of 40.4 Ω,much smaller than MnO (327 Ω).And the electronic conductivity of MnO/rGO is 1.49 S cm?1using an RTS-9 type four-point probe tests,while that of MnO is only 1.42×10?4S cm?1,which indicates that uniform rGO coating facilitates the transfer of electron.Additionally,the Li-ions diffusion coefficient value (D) was calculated by the following equation[21]:

whereσis the slope value (σMnO/rGO=243.77,σMnO=859.26) of the fittedZ′-ω?1/2(Fig.S6b).The calculatedDLi+of MnO/rGO electrode is 3.29×10?14cm2S?1,higher than that of pure MnO (2.65×10?15cm2S?1).The enhance of MnO/rGO ion diffusion ability of MnO/rGO is owing to the continuous and convenient pathways supplied by rGO and mesoporous structure.

To further explore the electrochemical kinetics for the enhanced capacity and rate performance,CV tests are performed from 0.1 to 1 mV s?1(Fig.6a).In general,the current response (i) is a function of the sweep rate (v),which could be shown by equations (5)and (6)[48]:

Typically,it can be drawn that the electrode reactions are controlled via diffusion process asb-value is close to 0.5,and the pseudocapacitive process dominated whenb-value tends to 1[49].

Fig.6b displays the logv-logiplots for the cathodic (peak 1) and anodic (peak 2) scans.The calculatedb-values of MnO/rGO are 0.79 and 0.76 for peak 1 and 2,respectively,indicating that the reaction process is mostly dominated by the pseudocapacitive behavior.Furthermore,the equations (7) and (8) are used to further quantitatively differentiate the pseudocapacitive contribution from the capacity[50]:

wherek1v1/2andk2vcorrespond to diffusion control contribution and pseudocapacitive contribution,respectively.

Fig.6c shows the percentage of pseudocapacitive contribution of 71% (0.6 mV s?1),also indicating that the capacity contribution of the MnO/rGO electrode is mainly derived from the pseudocapacitive contribution.Fig.6d displays the pseudocapacitive contributions of MnO/rGO are 54%,60%,67%,74%and 76% at 0.1,0.2,0.4,0.8 and 1 mV s?1,respectively.Obviously,the strengthen in pseudocapacitive contribution of MnO/rGO as the scan rate increasing,which further explains that its excellent rate capability is related to the higher pseudocapacitive contribution.The enhancement of the pseudocapacitive behavior can be ascribed to that the rGO uniformly coating on the MnO microspheres supplies a three-dimensional channels for electron/ions transportation,and the mesoporous structure provides plenty of active sites and convenient transport shifting pathways for Li-ions.

Fig.6 MnO/rGO:(a) CV curves at different scan rates;(b) b-values fitting results;(c) the capacitive contribution at 0.6 mV s?1;(d) the proportion of pseudocapacitive and diffusion contributions.

3.3 Electrochemical performance of LICs

MnO/rGO exhibits a favorable rate performance(207 mAh g?1at 6.4 A g?1),enhancing the kinetics matching ability between the two electrodes for LICs.The MnO/rGO//AC LICs were assembled using AC cathode and MnO/rGO anode.Pre-lithiation of anode in LICs is necessary and the potential of MnO/rGO anode after pre-lithiation by electrochemical method decreases to 0.5 V to keep at the stable plateau and the leaving sufficient utilizable capacity for cathode.During charge and discharge,PF6?1anions are adsorbed/desorbed on AC surface,while Li-ions insertion/extraction in MnO/rGO[51](Fig.7a).Additionally,the electrochemical performance is optimized via four sets for LICs (LIC-1,LIC-2,LIC-3 and LIC-4)based on the mass ratio between cathode and anode(1∶1,2∶1,3∶1 and 4∶1).

To explore the electrochemical behavior of AC as cathode for LICs,the CV and rate performance were recorded in the potential range of 2-4 V.The CV curve (Fig.S7a) of AC shows no clear redox peaks implying that it really behaves as the capacitor-liked features.Fig.S7b exhibits the rate capability of AC at 0.05,0.1,0.2,0.4,0.8,1.6,3.2 and 6.4 A g?1.The specific capacity of AC is 53.3 mAh g?1at 0.05 A g?1.The specific capacity decreases with the increase of current density.

The CV curves (Fig.7b) of LIC-1,LIC-2,LIC-3 and LIC-4 at 2 mV s?1present near-rectangular shape indicative of well capacitance behavior.Among as-assembled LICs,LIC-3 possesses the largest area implying the largest capacity.Furthermore,the CV curves(Fig.S8a) of LIC-3 remain approximately rectangular shape as the sweep rate increases showing excellent rate performance.The GCD curves (Fig.7c) at 0.8 A g?1present the triangular shapes,delivering the well capacitance behavior.The GCD curves (Fig.S8b) of LIC-3 display a linear relationship for different current densities,delivering the excellent performance,which in agreement with the CV analysis.LIC-1,LIC-2,LIC-3 and LIC-4 exhibit the specific capacity of 52,56,63 and 45 F g?1at 0.8 A g?1based on the total mass of two electrodes,respectively.

Fig.7d displays the rate capability of LIC-1,LIC-2,LIC-3 and LIC-4 for different current densities,the capacity is 57,67,69 and 61 F g?1at 0.1 A g?1,and 49,48,60 and 35 F g?1at 1.6 A g?1,respectively.At 12.8 A g?1,LIC-3 shows the highest capacity of 47 F g?1.Fig.7e exhibits the cycling capability and the Coulombic efficiency of LIC-3,showing superior cycling performance with a capacity retention of 71%and the Coulombic efficiency maintains above 99%during cycling.

Fig.7 Electrochemical performance of MnO/rGO//AC LICs:(a) Schematic illustration of working principle;(b) CV curves at 2 mV s?1;(c) GCD curves at 0.8 A g?1 and (d) Rate capability of LICs;(e) Cyclic performance of LIC-3 at 1.6 A g?1;(f) Ragone plots of LIC-3 and similar LICs reported in literature,inset (f) is the digital photo of the LED lighting application.

Fig.7f displays the Ragone plots of LIC-3 which is similar to LICs devices reported in literature.LIC-3 delivers a maximum energy density of 135 Wh kg?1at 47 W kg?1.Even at the ultrahigh power density of 10 350 W kg?1,LIC-3 can still maintain a high energy density of 98 Wh kg?1.Compared with some configurations reported in literatures,such as TiO2@PCNFGAs//AC[17],NixFeyOz@rGO//AC[52],3DC@LTSO//LDAC[12],Fe3O4@C//Ppy@CNT[16],LiMnO2//AC[11],Mn3O4-G//APDC[53],MoO2-CNT//AC[54],Nb2O5film//AC[18],SnO2-rGO//AC[55],and MnO/C//CNS[19],our LIC-3 presents much better energy-power characteristics.Additionally,the“TYUT”logo constituted by 31 LED bulbs could be easily lighted by LIC-3 indicating its practical application.The outstanding performances of MnO/rGO//AC LICs reveal that the superior rate capability of MnO/rGO anode with the rGO coating and the mesoporous structure effectively alleviates the kinetics gap with cathode improving the energy storage performance of LICs device.

4 Conclusion

In summary,MnO/rGO was successfully designed and fabricated as anode for enhanced performance in LICs.In the hybrid structure,the rGO is uniformly coated on MnO microspheres,and the mesoporous structure is existed in the substrate.The uniform coating of rGO on the MnO microspheres can not only ensure the structural stability of MnO microspheres during the conversion reaction,but also increase the conductivity (1.49 S cm?1) and supply more paths for electron transportation,and the optimized structure further promotes the improvement of cycle stability. Meanwhile,the mesoporous structure provides plenty of active sites and convenient transport pathways for the Li-ions,which greatly improves the diffusion rate of Li-ions (D=3.29×10?14cm2S?1).In addition,the excellent electrochemical performances arise from the high pseudocapacitive contribution.Moreover,MnO/rGO//AC LICs exhibit a high energy density of 98 Wh kg?1at 10 350 W kg?1.The high-rate Li+intercalation of the anode and the reasonable design of the device ensure a high performance LICs.Our work presents a promising method to prepare high performance MnO anode for LICs.

Acknowledgements

National Natural Science Foundation of China(U1710256,U1810204,U1810115).