简介概要

Failure mechanism of bulk silicon anode electrodes for lithium-ion batteries

来源期刊：Rare Metals2013年第3期

论文作者：Tao Li Juan-Yu Yang Shi-Gang Lu Han Wang Hai-Yang Ding

文章页码：299 - 304

摘要：Silicon has been investigated extensively as a promising anode material for rechargeable lithium-ion batteries. Understanding the failure mechanism of silicon-based anode electrodes for lithium-ion batteries is essential to solve the problem of low coulombic efficiency and capacity fading on cycling and also to further commercialize this very new energetic material in cells. To reach this goal, the structure changes of bulk silicon particles and electrode after cycling were studied using ex-situ scanning electron microscopy （SEM） and X-ray diffraction （XRD） techniques. The SEM images indicated that the microstructural changes of the bulk silicon particles during cycling led to a layer rupture of the electrode and then the breakdown of the conductive network and the failure of the electrode. The result contributes to the basic understanding of the failure mechanism of a bulk silicon anode electrode for lithium-ion batteries.

Rare Metals 2013,32(03),299-304+2

Failure mechanism of bulk silicon anode electrodes for lithium-ion batteries

Tao Li Juan-Yu Yang Shi-Gang Lu Han Wang Hai-Yang Ding

National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials, General Research Institute for Nonferrous Metals

R&D Center for Vehicle Battery and Energy Storage, General Research Institute for Nonferrous Metals

作者简介：Tao Li e-mail:taolyking@163.com;Shi-Gang Lu e-mail:slu@grinm.com;

收稿日期：6 September 2012

基金：financially supported by the National Natural Science Foundation of China (Nos. 51004016 and 51004017);the National High Technology Research and Development Program of China (Nos.2012AA110102 and 2011AA11A269);

Failure mechanism of bulk silicon anode electrodes for lithium-ion batteries

Abstract：

Silicon has been investigated extensively as a promising anode material for rechargeable lithium-ion batteries. Understanding the failure mechanism of silicon-based anode electrodes for lithium-ion batteries is essential to solve the problem of low coulombic efficiency and capacity fading on cycling and also to further commercialize this very new energetic material in cells. To reach this goal, the structure changes of bulk silicon particles and electrode after cycling were studied using ex-situ scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The SEM images indicated that the microstructural changes of the bulk silicon particles during cycling led to a layer rupture of the electrode and then the breakdown of the conductive network and the failure of the electrode. The result contributes to the basic understanding of the failure mechanism of a bulk silicon anode electrode for lithium-ion batteries.

Keyword：

Silicon; Anode; Lithium-ion battery; Electrochemical properties;

Received： 6 September 2012

1 Introduction

As an anode material for lithium-ion batteries,silicon possesses a maximum capacity of 4,200 m Ah?g^-1[1–4].However,its commercial use has been hindered because of two major problems[5,6].One is the low electrical conductivity,which is inherited from bulk silicon(6.7 9 10^-4S?cm^-1)and the presence of oxide layer on its surface[7,8].The other is the drastic volume change during lithium insertion and extraction,leading to an extremely bad cyclic performance[9,10].

Understanding the large capacity fading observed during cycling of silicon electrode is a complex issue[11,12].Ryu et al.[13]studied the failure modes of silicon electrode with the galvanostatic intermittent titration technique(GITT)and found that the Si electrode degraded in the dealloying period with an abrupt increase in internal resistance,which was caused by a breakdown of the conductive network.Kalnaus et al.[14]described the fracture of Si particles due to internal stress formed during the intercalation of lithium ions by means of a thermal analogy model and brittle fracture damage parameter and found that the stress was calculated following the diffusion equation and the elasticity equation with an appropriate volumetric expansion term.Obrovac and Krause[15]demonstrated a cycling method and found that cycling stability could be improved by restricting either the upper or the lower cutoff voltage,which reduced the amount of volume change,the tendency for particle aggregation,and the extent of structural change.

Previous studies on the failure mechanism of bulk silicon electrode focused on the indirect means,such as internal impedance change,numerical modeling,and cycling method.However,some pieces are still missing from the full understanding of the failure mechanism of silicon electrode,for example the structure changes of silicon particles and electrode.Therefore,it is important to study the electrochemical reaction of lithium with silicon in great detail to further implement this very new energetic material in commercial cells.The prime concern in this work is to explore the structure changes of silicon particles and the electrode during cycling using combination techniques,such as scanning electron microscopy(SEM)and X-ray diffraction(XRD)observations.This study was carried out using commercial microsized Si particles and an original formulation based on polyvinylidene fluoride(PVDF)binder to prepare the electrode slurry.

2 Experimental

Composite electrodes were made of commercial microsized Si(Beijing Haoyun Industry Co.,LTD),super P carbon black(CB,Timcal)as a conductive additive,and PVDF as a binder.Measured amounts of 65 wt%silicon powder,25 wt%super P,and 10 wt%PVDF binder were added into an eggshaped hardened steel container.An additional amount of N-methyl pyrrolidinone(NMP)solvent was added in order to give the mixture an appropriate viscosity.A shaker was used to shake the container at 5,000 shakes per min for 30 min.Then,the slurry was spread on a copper foil.The wet electrodes were dried at 80°C for 10 h in a vacuum furnace to remove the organic solvent.Electrodes of 1.4 cm in diameter were punched from the coated copper foils.

Coin cells(2032)with a metallic Li counter electrode were used to evaluate the electrochemical performance of the materials.Porous polypropylene(celgard 2300)was used as the separator.The electrolyte was 1.0 mol?L^-1Li PF₆in a mixture of dimethyl carbonate(DMC),DEC,and EC,1:1:1(volume fraction ratio)(Li-battery grade,Zhangjiagang Guotai-Huarong New Chemical Materials Co.,Ltd).Cycling tests of the half cells were performed in the voltage range of 0.005 V–2.000 V with a constant current of 200 m A?g^-1at room temperature with a computer-controlled LAND CT2001A 8-channel battery cycler.After the given cycling,the cells were carefully disassembled,and the electrodes were rinsed in DMC solvent to remove residual electrolyte and then dried in a vacuum atmosphere.Ex-situ SEM was carried out on a Hitachi S4800 microscope to study the electrode morphology before and after cycling.XRD patterns were recorded on a Philip X Pert-MPD using Cu-Ka radiation.

3 Results and discussion

3.1 Material characterization

Figure 1 shows the SEM image and XRD pattern of the silicon powder.The average particle size of the bulk silicon was in the range of 3 lm–40 lm.The XRD pattern of the bulk Si powder exhibited the diffraction peaks of pure Si phase(JCPDS file No.27-1402)and some Si C impurities(introduced by industrial silicon metallurgy).

3.2 Electrochemical behavior

The cycling behavior of silicon powder electrode is shown in Fig.2.A large amount of irreversible capacity was observed in the first 8 cycles.The electrode delivered a specific delithiated capacity of 1,716 m Ah?g^-1even if the lithiated capacity was as high as 2,762 m Ah?g^-1with coulombic efficiency as low as 62%in the 1st cycle.In the3rd cycle,the lithiated and delithiated capacity was 1,011and 755 m Ah?g^-1,respectively.The reversible lithiated capacity of the electrode in the 8th cycle was only282 m Ah?g^-1,which accounted for 10%of the 1st lithiated capacity.Moreover,the coulombic efficiency increased upon cycling from 62%(the 1st cycle)to 90%(the 8th cycle),while the lithiated and delithiated capacity decayed rapidly.Furthermore,the coulombic efficiency after 8 cycles decreased slowly,while the lithiated and delithiated capacity hardly had changes.However,the curve of coulombic efficiency appeared to have a significant jump in the 8th cycle.Such a significant change could presumably result from a severe change of the electrode structure,which will be discussed in Sects.3.3 and 3.4.Finally,the delithiated capacity in each cycle was comparable to the lithiated capacity.For instance,the 2nd lithiated capacity(1,654 m Ah?g^-1)was close to the 1st delithiated capacity(1,716 m Ah?g^-1),while the 3rd lithiated capacity(1,011 m Ah?g^-1)was comparable to the 2nd delithiated capacity(1,075 m Ah?g^-1),which could indicate that almost all of the empty Li?storage sites generated in the previous delithiated reaction were utilized for the next lithiated reaction.This observation strongly suggests that the bulk silicon electrode degrades in the delithiated period rather than in the lithiated process.

Fig.1 SEM image a and XRD pattern b of bulk silicon powder

The selected galvanostatic lithiated–delithiated voltage profiles with silicon electrode are shown in Fig.3.A broad gently sloping plateau was present at about 0.1 V in the 1st lithiated process.During the 1st delithiated process,the potential increased slowly with a sloping curve until about0.4 V.Then,a flat plateau at 0.4 V was followed by an upwardly sloping region.The 0.4 V plateau was the characteristic plateau of the silicon materials.During the subsequent delithiated processes,the characteristic 0.4 V plateaus became weaker gradually and finally disappeared The disappearance of the 0.4 V plateaus may be caused by the increase in ohmic resistance,which resulted in an earlier approach to the discharge cut-off limit.

Fig.2 Cycling behavior of silicon electrode cycled with a constant current of 200 m A?g-1in the voltage range of 0.005 V–2.000 V at room temperature

Fig.3 Selected galvanostatic lithiated–delithiated voltage profiles with Si electrode cycled with a constant current of 200 m A?g-1in the voltage range of 0.005 V–2.000 V at room temperature

3.3 Structure change of the silicon particles

Figure 4 shows the SEM images and XRD patterns of the electrodes’surfaces before cycling,after the 1st full lithiation,after the 1st cycle,after the 3rd cycle,after the 7th cycle,and after the 15th cycle.As shown in Fig.4a,the bulk Si particles ranging from 3 lm to 40 lm were well proportioned and dispersed in super P carbon materials(with a particle size about 40 nm)before cycling.However,the large silicon particles cracked to several small parts after the 1st full lithiation,as shown in Fig.4b,C(the partial enlarged SEM images of Fig.4b).From Fig.4c–f the small silicon parts further cracked and pulverized to smaller pieces and agglomerated upon cycling.The partially enlarged SEM images of Fig.4f with the electrode after 15 cycles and the silicon particle completely pulverized to small pieces is shown in Fig.4D.The pulverization of the bulk silicon particles was studied by Kalnaus et al[14]using a thermal analogy model and brittle fracture damage parameter.The cracking of Si particles in the first lithiation started from the center of the particle where the prediction places the maximum of the damage parameter due to the internal stress formed during the intercalation of lithium ions[14].The subsequent delithiation would introduce the surface cracks in those blocks producing the burst of emissions corresponding to the charging part of the cycle.Then,during the next discharge,the internal cracks would appear again,and so on,until the block reaches a critically small size to develop enough stress for fracture[14].The geometry of the internal crack produced during the first discharge depends on the distribution of initial internal defects in the particle and is impossible to predict within the generalized homogeneous spherical body approach[14].In other words,it is impossible to quantify the amount of blocks produced by the initial fracture of a particle[14].

From the XRD results of the electrodes showed in Fig.4B,the crystal silicon almost disappeared after the 1st lithiation.This result was consistent with the finding of Li and Dahn[16],which confirmed that crystalline silicon became amorphous during the 1st lithiation by in situ XRD The cracking and the following pulverization of silicon particles may lead to the breakdown of the conductive network of the electrode and increase ohmic polarization which could be detected from the voltage profiles in Fig.3

Fig.4 SEM images A and XRD B of the surface of electrodes:a before cycling,b after 1st full lithiation,c after 1st cycle,d after 3rd cycle,e after 7th cycle,f after 15th cycle;C partial enlarged detail of b,and D partial enlarged detail of f

3.4 Structure change of the silicon electrode

The SEM images of the cross section and surface of the electrode from the vertical direction are shown in Fig.5.In Fig.5a,the thickness of the electrode before cycling was about 60 lm with materials tightly adhered to the Cu foil.Upon cycling,the electrode expanded to 150 lm approximately after the 1st cycle in Fig.5b and about 260 lm after the 3rd cycle in Fig.5c.However,in Fig.5d,e,there was almost no change in thickness from the 3rd cycle to the15th cycle.It should be noted that the upper layer(close to the separator)of the electrode gradually delaminated from the substrate with cycling,as shown in the SEM image of the electrode after the 30th cycle in Fig.5f.From the partially enlarged image of Fig.5f in Fig.5g,the upper layer(close to the separator)of the electrode was made up of agglomerates,which consisted of small silicon pieces and carbon materials.From the SEM images of the electrode surface in the vertical direction with the bottom layer of electrode(close to the Cu foil,Fig.5f)as shown in Fig.5h,i,the silicon particles flaked off leaf by leaf as the electrochemical reactions were processing continuously and the previous clear edge of the silicon particles became smooth,looking like a‘‘cobble.’’Moreover,the flaked silicon particles grew along the direction perpendicular to the surface of the electrode,indicating that the electrochemical reaction of the silicon particle has certain orientation.The upper layer(close to the separator),middle layer,and bottom layer(close to the Cu foil)of the electrode cross-sectional SEM images after five cycles are shown in Fig.5B.The porosity of the upper layer is higher than that of both the middle layer and the bottom layer.The gradually decreased porosity of the electrode from the upper layer to the bottom layer indicates that the electrochemical reactions that occurred on upper layer of the electrode are more violent than that which occurred on the bottom layer.This phenomenon is familiar for the porous electrode reaction,which was called‘‘nonuniform polarization in the direction of thickness’’by professor Zha[17]It may be the reason for the layer pision of the electrode and the following breakdown of conductive network and failure of electrode,which eventually leads to an extremely bad cyclic performance.

3.5 Discussion

Based on the above results,the illustration of the structure change of silicon particles and electrode with cycling is shown in Fig.6,aiming to explain the failure mode of the silicon electrode.There are two vital points with the failure of bulk silicon electrode:(1)cracking and pulverization of the silicon particles;(2)structure change of the electrode which resulted from cracking and pulverization of silicon particles,as well as nonuniform polarization of the electrode electrochemical reaction.Before cycling,the bulk silicon dispersed in super P with a large contacting area among components.Due to the large volume change during cycling,the bulk silicon cracked and pulverized to small pieces.The cracking and pulverization of the silicon particles will first damage the electrode architecture and further result in the loss of electrical contact between the silicon phase and the conductive matrix.The disconnection of some silicon particles with the surrounding matrix will lead to irreversible capacity consequently.The nonuniform polarization of the electrode electrochemical reaction in the direction of thickness may result in the layer rupture of the electrode,the following breakdown of the conductive network,and eventually the failure of the electrode.Thus,the reversible capacity of the bulk silicon electrode decreased quickly upon the electrochemical reaction.

Fig.5 Cross-sectional SEM images of electrode before cycling Aa,after 1st cycle Ab,after 3rd cycle Ac,after 7th cycle Ad,after 15th cycle Ae,after 30th cycle Af;Ag partial enlarged detail of Af;Ah surface SEM image from vertical direction with the bottom layer of Af;Ai enlarged SEM image of Ah;B1 upper layer(close to the separator)of cross-sectional SEM images for electrode after 5th cycle;B2 middle layer of cross-sectional SEM images for electrode after 5th cycle;B3 bottom layer(close to the Cu foil)of cross-sectional SEM images for electrode after 5th cycle

Fig.6 Structure change illustration of silicon particle and electrode with cycling

4 Conclusion

The failure mechanism of bulk silicon anode electrodes has been identified from the point of view of structure changes of silicon particles and electrode.The following points are summarized:The severe volume change of the silicon material played an important role in the failure of the sil icon electrode,which resulted in the cracking and pulver ization of silicon particles and structure change of the electrode.The cracking and pulverization of silicon parti cles will damage the electrode architecture,resulting in the loss of electrical contacts and then capacity fading.The structure change of the electrode resulted from the non uniform polarization of the electrode electrochemica reaction in the direction of thickness,which may result in the layer rupture of the electrode and then the breakdown of conductive network and failure of the electrode.Furthe understanding of the failure mechanism of bulk silicon anode electrodes will contribute to the implementation o this very new energetic material in commercial cells.