简介概要

Improving anode performances of lithium-ion capacitors employing carbon-Si composites

来源期刊：Rare Metals2019年第12期

论文作者：Ya-Bin An Si Chen Min-Min Zou Lin-Bin Geng Xian-Zhong Sun Xiong Zhang Kai Wang Yan-Wei Ma

文章页码：1113 - 1123

摘要：The lithium-ion capacitor is a promising energy storage system with a higher energy density than traditional supercapacitors.However,its cycling and rate performances,which depend on the electrochemical properties of the anode,are still required to be improved.In this work,soft carbon anodes reinforced using carbon-Si composites of various compositions were fabricated to investigate their beneficial influences on the performance of lithium-ion capacitors.The results showed that the specific capacities of the anodes increased significantly by 16.6 mAh g^-1 with 1.0 wt% carbon-Si composite,while the initial discharge efficiency barely changed.The specific capacity of the anode with a 10.0 wt% carbon-Si composite reached 513.1 mAh g^-1,and the initial discharge efficiency was 83.79%.Furthermore,the anodes with 7.5 wt% or lower amounts of carbon-Si composite demonstrated reduced charge transfer resistances,which caused an improvement in the rate performance of the lithium-ion capacitors.Moreover,the use of the optimized amount（7.5 wt%） of carbon-Si composite in the anode could significantly improve the cycling performance of the lithium-ion capacitor by compensating the consumption of active lithium.The capacity retention of the lithium-ion capacitor reached 95.14% at 20 C after 10,000 cycles,while the anode potential remained below 0.412 V,which is much lower than that of a soft carbon anode.

详情信息展示

稀有金属(英文版) 2019,38(12),1113-1123

Improving anode performances of lithium-ion capacitors employing carbon-Si composites

Ya-Bin An Si Chen Min-Min Zou Lin-Bin Geng Xian-Zhong Sun Xiong Zhang Kai Wang Yan-Wei Ma

Institute of Electrical Engineering,Chinese Academy of Sciences

College of Materials Science and Engineering,Beijing Institute of Petrochemical Technology

University of Chinese Academy of Sciences

作者简介：*Yan-Wei Ma e-mail:ywma@mail.iee.ac.cn;

收稿日期：21 March 2019

基金：financially supported by the National Natural Science Foundation of China (No.51721005);the Beijing Municipal Science and Technology Commission (No.Z171100000917007);

Improving anode performances of lithium-ion capacitors employing carbon-Si composites

Ya-Bin An Si Chen Min-Min Zou Lin-Bin Geng Xian-Zhong Sun Xiong Zhang Kai Wang Yan-Wei Ma

Institute of Electrical Engineering,Chinese Academy of Sciences

College of Materials Science and Engineering,Beijing Institute of Petrochemical Technology

University of Chinese Academy of Sciences

Abstract：

The lithium-ion capacitor is a promising energy storage system with a higher energy density than traditional supercapacitors.However,its cycling and rate performances,which depend on the electrochemical properties of the anode,are still required to be improved.In this work,soft carbon anodes reinforced using carbon-Si composites of various compositions were fabricated to investigate their beneficial influences on the performance of lithium-ion capacitors.The results showed that the specific capacities of the anodes increased significantly by 16.6 mAh g^-1 with 1.0 wt% carbon-Si composite,while the initial discharge efficiency barely changed.The specific capacity of the anode with a 10.0 wt% carbon-Si composite reached 513.1 mAh g^-1,and the initial discharge efficiency was 83.79%.Furthermore,the anodes with 7.5 wt% or lower amounts of carbon-Si composite demonstrated reduced charge transfer resistances,which caused an improvement in the rate performance of the lithium-ion capacitors.Moreover,the use of the optimized amount(7.5 wt%) of carbon-Si composite in the anode could significantly improve the cycling performance of the lithium-ion capacitor by compensating the consumption of active lithium.The capacity retention of the lithium-ion capacitor reached 95.14% at 20 C after 10,000 cycles,while the anode potential remained below 0.412 V,which is much lower than that of a soft carbon anode.

Keyword：

Lithium-ion capacitor; Soft carbon; Carbon-Si composite; Cycling performance; Rate performance;

Received： 21 March 2019

1 Introduction

Supercapacitors are one of the most promising energy storaige systems and are endowed with an excellent power density,cycle performance and high availability at various temperatures due to their unique energy storage mechanism of ion adsorption-desorption,which is quite different from that of lithium batteries [ 1, 2, 3] .However,theirenergy density is limited by the inferior capacity of the activated carbon cathode,and theoperating voltage of the cellrequires further improvement to meet the demanding requirements of practical applications.Recently,hybrid supercapacitors,especially lithium-ion capacitors (LICs),have been developed to address this problem.In contrast to conventional supercapacitors,lithium-ion supercapacitors are constructed using an activated carbon cathode based on the ion adsorptiondesorption mechanism and a pre-lithiated anode fabricated using lithium-ion intercalation reactions.Owing to the low charge and discharge potential and high capacity of prelithiated carbon,the operating voltage of a LIC can be significantly improved to realize a hi gher energy density [ 4, 5, 6] .In contrast,owing to the sluggish kinetics of lithiumion intercalation reactions,the performances of LICs,especially their cycling and rate capability,are highly dependent on the electrochemical behavior of the anode.In recent years,various anode materials,such as hard carbon [ 7, 8, 9, 10, 11] ,soft carbon [ 12, 13] ,graphite [ 14, 15, 16, 17, 18] ,graphene [ 19, 20, 21, 22] ,Li₄Ti₅O₁₂ [ 23, 24] ,Fe₃O₄ [ 25, 26] ,MXene [ 27, 28] ,MoO₂ [ 29, 30] ,carbon nanotubes [ 31] ,carbon fiber [ 32] and silicon-based materials [ 33, 34, 35] ,have been widely studied [ 36, 37] .However,it is still a challenge to realize LICs with long cycling life and high rate capability by simply employing a single material as described above.

Silicon is a promising material for application in nextgeneration anodes due to its excellent charge-discharge platforms and extremely high specific capacity;however,its cycling and rate pertormances are relatively poor because of its severe volume expansion and low electronic and ionic conductivity [ 38] .In contrast,soft carbon (SC) is a frequently used commercialized anode material with high conductivity,fast lithium-ion transportation and long cycling performances,but its specific capacity and operating platform still require to be improved.Therefore,the use of a small amount of carbon-Si composite in a soft carbon anode would ameliorate the charge-discharge kinetics of the anode and also provide surplus lithium for slowing the rate of consumption of active lithium in longterm cycling after the pre-lithiaition of the anode.In this work,soft carbon anodes comprising various amounts of carbon-Si composite were fabricated,and their electrochemical behaviors in a half-cell and soft-packed LIC were carefully investigated to understand their influences on the cycling and rate performances of LICs.

2 Experimental

2.1 Preparation of anode and cathode electrode

Firstly,active substances with different contents of carbonSi composite (Si/C,Hitachi Chemical) were prepared by mixing Si/C with soft carbon (Hitachi chemical,Japan)with different proportions (0 wt%,2.5 wt%,5.0 wt%,7.5wt%,10.0 wt%and 100.0 wt%Si/C in total active substances),which were well dispersed with Super C45(IMERYS),carboxymethyl cellulose (CMC,JSR) and polymerized styrene butadiene rubber (SBR) to prepare a homogeneous slurry in a high-speed mixing device according to the mass ratio of 90:5:4:1.A similar method was also used to prepare the cathode slurry by mixing the YP80 (Kuraray),polyvinyl idene fluoride (PVDF,Arkema)and Ketjen black (Lion Corporation) according to the ratio of 85:10:5.Finally,these anode and cathode slurries were coated on Cu foils and A1 foils with a mass loading of4-5 mg·cm^-2 and then dried in an oven at 100℃.

2.2 Assembling of coin-type cell

After being punched into round pieces with a diameter of12 mm,SC-Si/C electrodes were dried in the vacuum oven over 12 h at 120℃and transferred into the glove box.Then,these electrodes were assembled into 2025 coin-type cells with lithium electrode (15.6 mm),Celgard-2400 separator(19 mm) and 50μl electrolyte (1 mol.L^-1 LiPF6 in ternary solvent (weight ratio of propylene carbonate/pthylene carbonate/diethyl carbonate (PC/EC/DEC)=1:1:1).

2.3 Assembling of soft-packed lithium-ion capacitors

Firstly,the anode and cathode electrodes were punched into a rectangular sheet with a size of 35 mm×40 mm.Then,a Z-shaped lamination Was assembled by stacking an anode sheet and a cathode sheet on opposite sides of a cellulose membrane (NKK) alternately.After welding corresponding external tabs to anode and cathode electrodes,they were partially encapsulated in the aluminum plastic film and dried in the vacuum oven over 12 h at120℃.In the next step,3 ml electrolyte was injected,and a piece of Li foil used as a reference electrode was inserted before the cell was sealed.Finally,after lithiation of anode and formation,the soft-packed lithium-ion capacitors were obtained.

2.4 Characterization

Morphologies of soft carbon and Si/C were characterized by a Zeiss Sigma scanning electron microscope (SEM),and the constituents of the two materials were investigated through Bruker D8 Advance X-ray diffractometer (XRD)at a scanning speed of 5 (°) min^-1.Electrochemical performances were tested through the Biological EC-Lab electrochemical workstation;more specifically,cyclic voltammetry (CV) curves of coin-type cells were tested between 0.01 and 3.00 V at a scanning speed of0.05 mV·s^-1,and electrochemical impedance spectroscopy(EIS) was tested between 0.01 and 100.00 kHz with a disturbance of 5 mV.Besides,the charge and discharge performances of the coin cells and soft-packed full cells were also investigated at different current densities using a NEW ARE battery testing system.

Fig.1 Composition and morphology of anodes:a SEM image of soft carbon;b SEM image of Si/C;c,d SEM images of 5.0 wt%Si/C electrode;e XRD patterns of soft carbon and Si/C

3 Results and discussion

3.1 Morphologies and structures

The morphologies of soft carbon and Si/C are shown in Fig.1a,b.From the results,both morphologies are amorphous particles,and their sizes are 2-15μm and 3-10μm,respectively,while the Si/C particles have a narrower dimension distribution and a much rougher coating surface.XRD patterns of SC and Si/C were also investigated to reveal their structure characteristics,and the results are shown in Fig.1d.In the pattern of SC,there is an apparent peak at 26.4,corresponding to the (002) diffraction peak of graphite,while it shifts toward the small angle in Si/C,which indicates a larger interlayer spacing of carbon.There are three other apparent peaks at 28.6°,47.6°and 56.6°,corresponding to (111),(220) and (311) diffraction peak of silicon.As shown in Fig.1c,after being mixed into the electrode,SC and Si/C particles distribute uniformly in the conductive network formed by nano-sized Super C45,leaving numbers of tunnels and space for rapid ion transportation and volume expansion of Si/C.Benefiting from the excellent conductive network and electrode structure,the cycling and rate performances of the composite electrode could be greatly enhanced.

3.2 Performances of anodes and cathodes

Electrodes of various compositions of Si/C as well as the corresponding coin-type cells were then prepared according to the above experimental descriptions.Figure 2 shows the first three discharge-charge curves of these cells.It can be observed from the results that the initial discharge specific capacity of 0 wt%Si/C,2.5 wt%Si/C,5.0 wt%Si/C,7.5 wt%Si/C,10.0 wt%Si/C and 100.0 wt%Si/C at a current density of 50 mA·g^-1 is 347.6,395.1,436.3,461.9,513.1 and 1986.5 mAh·g^-1,and the initial efficiencies of these electrodes are 84.85%,83.03%,83.81%,84.31%,83.79%and 49.42%,respectively.It can be observed that the specific capacities of the 2.5 wt%Si/C,5.0 wt%Si/C and 7.5 wt%Si/C electrodes increase significantly as the Si/C composition increases beyond that of the SC electrode,and these enhanced specific capacities are advantageous in realizing high-specific-capacity LICs.Furthermore,the specific capacities of 2.5 wt%Si/C,5.0 wt%Si/C and 7.5 wt%Si/C exceed the theoretical value calculated based on the proportion of SC and Si/C.This may be because the Si/C particles exhibit a higher specific capacity in the Si/C electrodes as the conductivities of these electrodes are higher than that of 100.00 wt%Si/C electrode.Moreover,almost the full capacity of the Si/C electrode is charged and discharged below 0.5 V,which is much lower than that of SC.In addition,similar to the superposition of SC and Si/C,the curves of 2.5 wt%Si/C,5.0 wt%Si/C and 7.5 wt%Si/C exhibit properties of both SC and Si/C simultaneously,and thus,the capacity in the lower platforms also increases as the Si/C contents increase.These anodes with lower charge and discharge platforms would facilitate a safer potential for the cathode at a specific full-cell voltage,which would exert a pivotal role in the long-term cycling performance of the LIC.

Fig.2 Charge-discharge curves of anodes at 20 mA·g^-1:a 0 wt%Si/C electrode;b 2.5%Si/C electrode;c 5.0%Si/C electrode;d 7.5%Si/C electrode;e 10.0 wt%Si/C electrode;f 100.0 wt%Si/C electrode

CV curves of these electrodes were also investigated.Figure 3 shows irreversible reduction peaks at 0.80 and0.63 V in the initial discharge process of SC and Si/C,respectively,corresponding to the formation of a solid electrolyte interface (SEI) on the surface of the electrodes.The irreversible reduction peak of SC is also found in the case of 2.5 wt%Si/C,5.0 wt%Si/C,7.5 wt%Si/C,10.0 wt%Si/C and 100.0 wt%Si/C electrodes,while the peak in the case of Si/C is not visible.This may be ascribed to the earlier SEI formation of SC,such that the decomposed products of the electrolytes could also deposit on the surface of the Si/C particles at～0.8 V and consequently form a protection layer;then,when the potential reaches0.63 V,the decomposition reaction of the electrolyte on the surface of the Si/C is suppressed.Moreover,the other peaks of C@S,such as the reduction peak at～0.2 V and oxidation peak at～0.25-0.3 V,are also found in the case of 2.5 wt%Si/C,5.0 wt%Si/C,7.5 wt%Si/C,10.0 wt%Si/C and 100.0 wt%Si/C electrodes,and their intensity increases as the Si/C contents increase.Moreover,a reversible oxidation peak at～0.54 V could also be found in 2.5 wt%Si/C,5.0 wt%Si/C,7.5 wt%Si/C,10.0 wt%Si/C and 10.0 wt%Si/C electrodes.However,such a peak is not observed for either SC or Si/C electrode,which may indicate that some interaction between the SC and Si/C changes the charging process of the electrodes.

The cycling performances of coin-type cells were investigated between 0.01 and 3.00 V at 100 mA·g^-1.As shown in Fig.4a,the electrodes with the added Si/C exhibit higher specific capacities and attenuate rapidly in the first 30 cycles than the SC electrode.Their cycling performances then become more stable,and no further attenuation is observed.After 100 cycles,the specific capacities of the SC,2.5 wt%Si/C,5.0 wt%Si/C,7.5 wt%Si/C,10.0 wt%Si/C and 100.0 wt%Si/C electrodes are262.0,274.3,265.3,265.9 and 256.6 mAh·g^-1,respectively,which indicates that these electrodes exhibit a small difference in specific capacities after long-term deep charge and discharge.The rate performances of the cointype cells were tested between 0.01 and 3.00 V at current densities of 20,50,100,200,500 mA·g^-1,1 and 2 A·g^-1.As shown in Fig.4b,the specific capacities of these electrodes are closely related to the Si/C contents at current densities below 200 mA·g^-1;the greater the amount of added Si/C is,the higher specific capacities we are obtained.However,when the current density is greater than500 mA·g^-1,the 7.5 wt%Si/C electrodes exhibit a higher specific capacity,which indicates that excessive Si/C could result in an adverse effect on the rate performances.From the charge-discharge curves of 7.5 wt%Si/C in Fig.4c,the platform of Si/C at 0.25 V could be recognized until the current density reaches 500 mA·g^-1.The electrochemical impedance spectroscopy (EIS) and the equivalent circuit model of these coin-type cells are presented in Fig.4d.In this model,R represents the contact resistances related to the electrical conductivity of the current collector,electrolyte and active materials;Q_s and R_s represent the interface capacitance and resistance of the SEI film,respectively,while Q_ct and R_ct represent the interface capacitance and resistance of the charge transfer process,respectively.According to the fitting result,the charge transfer resistances of the 0 wt%Si/C,2.5 wt%Si/C,5.0 wt%Si/C,7.5 wt%Si/C,10.0 wt%Si/C and100.0 wt%Si/C electrodes are 225.9,220.0,205.5,190.5,229.7 and 244.1Ω,respectively.This indicates that the charge transfer resistance of the electrodes would decrease to a minimum value in the case of 7.5 wt%Si/C.

Fig.3 CV curves of anodes at 0.05 mV·s^-1:a 0 wt%Si/C electrode;b 2.5 wt%Si/C electrode;c 5.0 wt%Si/C electrode;d 7.5 wt%Si/C electrode;e 10.0 wt%Si/C electrode;f 100.0 wt%Si/C electrode

Fig.4 Charge-discharge performances and EIS curves of anodes:a cycling performances;b rate performances;c charge-discharge curves of7.5 wt%Si/C electrode;d EIS curves

The cathode performances were also investigated.Figure 5 a,b shows CV curves and rate performances of the cathode,respectively.The obtained result shows that the cathode exhibits excellent electrochemical capacitive behavior between 2.0 and 4.1 V even at a scan speed of20 mV·s^-1,and its capacitances at 1C (50 mA·g^-1,based on the active material of the cathode) and 100C are 108.6and 51.3 F·g^-1,respectively.

3.3 Performances of soft-packed lithium-ion capacitors

To further understand the electrochemical performances of these anodes,the soft-packed LICs were assembled according to the experimentation.Figure 6a shows their time-voltage curves at 1C (35 mA·g^-1,based on the total active material of the anode and cathode),all of which show a symmetrical triangle,and no platform is observed,which indicates an excellent capacitive behavior.Figure 6b presents the discharge specific capacities of the LICs at various rates (1C-100C),and all the values were calculated based on the total active substances of the cathode.In contrast to the coin-type cells,these LICs exhibit almost the same results at1C (60.3 F·g^-1,102.2 Wh·g^-1 at 107 W·kg^-1),which indicates that the specific capacities of the LICs are more closely related to the cathode material.While at a high discharge rate,the cell comprising 7.5 wt%-Si/C electrode exhibits the highest capacity (33.2 F·g^-1,49.0 Wh·kg^-1 at9.3 kW·kg^-1),which is consistent with the charge transfer resistance results.The cycling performances of activated carbon (AC)//7.5 wt%Si/C and AC//SC were tested at 20C,as presented in Fig.6c.As per the results,the initial discharge specific capacities of AC//0 wt%Si/C and AC//7.5 wt%Si/C are 77.0 and 78.2 F·g^-1,respectively.Then,after 10,000 cycles,the specific capacities attenuate to 66.8and 74.4 F·g^-1,and accordingly,the capacity retention ratios of AC//7.5 wt%Si/C and AC//0 wt%Si/C are 95.14%and 86.8 1%,respectively.It is encouraging to observe that the cycling performance of AC//7.5 wt%Si/C is superior to that of AC//SC.This can be attributed to the high specific capacity and low charge and discharge platform of Si/C.For LICs,the capacity of the anode attenuates faster than that of the cathode because of the respective different energy storage mechanisms.This inevitable capacity fade of the anode would eventually result in an increased potential of the anode in long terms of cycling,which indicates a higher cathode operation potential in the charge and discharge process.Furthermore,the elevated cathode potential could aggravate the decomposition of electrolyte and result in the deterioration of the cycling performance [ 39] .Therefore,the cycling performances of the LIC could be significantly improved by suppressing the potential increase of the anode.Generally,silicon swells severely (up to 320%) and tends to crack [ 38] during the intercalation of lithium,which in turn results in a poor cycling performance.While the lithium in the Si/C particles is not really“dead,”it just loses the effective contact with the conductivity network.Given sufficient time,the lithium is still able to diffuse or transfer to other materials,such as SC particles,under the effect of potential differences in the anode.Therefore,for the Si/C-added electrodes,the concentration of the active lithium in the SC particles decreases continuously during cycling and results in an increase in the anode potential.Meanwhile,benefiting from high capacity and low charge and discharge platform,the fragmented low-active Si/C particles maintain a lower potential and higher active lithium concentration.Bec ause of the potential differences,the active lithium in the Si/C particles could be slowly transferred into SC particles continuously to compensate for the consumption of lithium.In other words,the addition of Si/C provides the anodes with more lithium ions after pre-lithiation,and these redundant lithium ions could effectively slow down the increase in the anode potential caused by the lithium consumption.Consequently,the cycling performances of the LICs are improved.

Fig.5 Electrochemical performances of cathode:a CV curves of cathode at various scan rates;b rate performance of cathode

Fig.6 Charge-discharge performances of lithium-ion capacitors:a charge-discharge curves at 1C;b rate performances;c cycling performances of AC//0 wt%Si/C and AC//7.5 wt%Si/C;d anode potential curves of AC//0 wt%Si/C and AC//7.5 wt%Si/C before and after cycling

To further understand the influences of the added Si/C on cycling performances,the reference electrodes were reinserted into the LICs to investigate potential changes in the anode after 10,000 cycles.As shown in Fig.6d,the potential values of the fresh SC and 7.5 wt%Si/C electrode are 0.002-0.255 V and 0.002-0.243 V,respectively,while after long-term cycling,their potential values increase to0.138-0.484 V and 0.100-0.412 V.Apparently,the potential of the 7.5 wt%Si/C anode is much lower than that of the SC after cycling,which indicates the benefit of the added Si/C to the cycling performance.Moreover,to further evaluate the application prospects of AC//7.5 wt%Si/C,its preparation method,mass loading,energy density,power density and cycling performances were compared with those of the literature referenced in this paper,as presented in Table 1.Considering the high mass loading of the anode and cathode (4-5 mg) as well as the

Table 1 Comparison of preparation methods and performances in various studies referenced in this paper 下载原图

下载原表

CNT:carbon nanotube,SWCNT:single-walled carbon nanotube,MWCNTs:multi-walled carbon nanotubes,PTFE:poly-tetra-fluoro-ethylene,PVDF:polyvinylidene fluoride,CMC:carboxymethyl cellulose,SBR:polymerized styrene butadiene rubber,PAA:polyacrylic acid

Table 1 Comparison of preparation methods and performances in various studies referenced in this paper

4 Conclusion

In this work,it was found that adding a suitable amount of Si/C was advantageous in improving the cycling and rate performances of LICs.Moreover,an appropriate amount of Si/C was found to aid in decreasing the charge transfer resistance of the anode,thus resulting in a higher rate performance of LICs.Moreover,Si/C could significantly enhance the capacities of the anode and lower the charge and discharge platform,especially the capacities at the potential below 0.5 V,which would promote the platform of the anode and provide redundant active lithium.Therefore,in long-term cycling,the Si/C particles may serve as a low-active carrier of lithium and offset the consumption of active lithium,thus resulting in an excellent cycling performance.In particular,the LIC with AC//7.5%Si/C showed the best rate performance with a high capacity retention of 95.14%after 10,000 cycles.