Self-healing alginate-carboxymethyl chitosan porous scaffold as an effective binder for silicon anodes in lithium-ion batteries
来源期刊:Rare Metals2019年第9期
论文作者:Zhao-Hui Wu Juan-Yu Yang Bing Yu Bi-Meng Shi Chun-Rong Zhao Zhang-Long Yu
文章页码:832 - 839
摘 要:Polymer binder plays a pivotal role in electrochemical performance of high-capacity silicon(Si)anode that usually suffers from severe capacity fading due to enormous substantial volume change of Si during cycling.In an effort to find efficient polymer binder that could mitigate such capacity fading,alginate-carboxymethyl chitosan(Alg-C-chitosan)composite polymer was investigated as a low-cost watersoluble binder for silicon anodes in lithium-ion batteries.The electrostatic interaction between carboxylate(-COO-)of Alg and protonated amines(-NH3+)of C-chitosan forms a selfhealing porous scaffold structure.Synergistic effect on the enhanced porous scaffold structure and self-healing electrostatic interaction of Alg-C-chitosan binder effectively can tolerate the tremendous volume change of Si and maintain an integrated electrode structure during cycling process.The Si nanopowder electrodes with Alg-C-chitosan composite binder exhibit an excellent cycling stability,with a capacity of750 mAh·g-1 remaining after 100 th cycling.In addition,an extraordinary areal capacity of 3.76 mAh·cm-2 is achieved for Si-based anodes with Alg-C-chitosan binder.
稀有金属(英文版) 2019,38(09),832-839
Zhao-Hui Wu Juan-Yu Yang Bing Yu Bi-Meng Shi Chun-Rong Zhao Zhang-Long Yu
Research and Development Center for Vehicle Battery and Energy Storage,General Research Institute for Nonferrous Metals
作者简介:Juan-Yu Yang e-mail:juanyuyang@163.com;
收稿日期:16 April 2015
基金:financially supported by the National Natural Science Foundation of China (No. 51404032);the National High Technology Research and Development Program of China(No. 2013AA050904);
Zhao-Hui Wu Juan-Yu Yang Bing Yu Bi-Meng Shi Chun-Rong Zhao Zhang-Long Yu
Research and Development Center for Vehicle Battery and Energy Storage,General Research Institute for Nonferrous Metals
Abstract:
Polymer binder plays a pivotal role in electrochemical performance of high-capacity silicon(Si)anode that usually suffers from severe capacity fading due to enormous substantial volume change of Si during cycling.In an effort to find efficient polymer binder that could mitigate such capacity fading,alginate-carboxymethyl chitosan(Alg-C-chitosan)composite polymer was investigated as a low-cost watersoluble binder for silicon anodes in lithium-ion batteries.The electrostatic interaction between carboxylate(-COO-)of Alg and protonated amines(-NH3+)of C-chitosan forms a selfhealing porous scaffold structure.Synergistic effect on the enhanced porous scaffold structure and self-healing electrostatic interaction of Alg-C-chitosan binder effectively can tolerate the tremendous volume change of Si and maintain an integrated electrode structure during cycling process.The Si nanopowder electrodes with Alg-C-chitosan composite binder exhibit an excellent cycling stability,with a capacity of750 mAh·g-1 remaining after 100 th cycling.In addition,an extraordinary areal capacity of 3.76 mAh·cm-2 is achieved for Si-based anodes with Alg-C-chitosan binder.
Keyword:
Binder; Alginate; Carboxymethyl chitosan; Self-healing; Scaffold; Silicon anode;
Received: 16 April 2015
1 Introduction
Silicon is a promising active material for anode in lithiumion batteries,because it has the highest theoretical capacity of~4200 mAh·g-1 for Li4Si22
As one of the major components of electrodes,polymeric binder is used to bind active materials and conduct additives together onto the current collector.The properties of polymeric binder play a vital role in maintaining the electrode structure during cycling,especially for siliconbased anodes.The most conventional binder poly(vinylidene fluoride)(PVDF) is widely used in comumercialized lithium-ion batteries.The linear-type PVDF binder with inferior tensile strength attached to Si particles via weak van der Waals forces only cannot ensure good cycling performance to Si-based anodes
Moreover,it has been observed that polymeric binders with enhanced mechanical properties can help suppress volume expan sion.Pol yamide imide copol ymer is a kind of polymer with the best comprehensive mechanical properties,for instance polyamide imide (PAI)
Here the development of an effective self-healing Alg-C-chitosan porous scaffold structure binder was reported.Alginate and chitosan are important bio-derived materials due to extensive sources and low price
2 Experimental
2.1 Materials
Si nanopowders (SiNPs) with average particles size of100 nm were purchased from Shanghai Shuitian Materials Technology Co.,China.Alg (mol mass of 8×105-12×105 g·mol-1,medium viscosity) was purchased from Sigma-Aldrich,USA,dissolved in distilled water (2 wt%)standby.Alg is a linear polysaccharide copolymer that consists of two sterically different repeating units,(1,4)-β-D-mannuronic acid (M) andα-L-guluronic (G) in varying proportions.The ratio of M-to-G monoblocks in the Na alginate sample was 1.56.Carboxymethyl chitosan (carboxymethylation degree≥60%) was purchased from Nanjing Doulai Biological Technology Co.,China,and dissolved in distilled water (5wt%) standby.PVDF (molar mass of~3×106 g·mol-1) in N-methyl-2-pyrrolidone(NMP) and CMC/SBR (1:1 by weight) in distilled water was used as alterative binders for comparison.
For consistency,all electrodes contained SiNP,conducting additives (including 28 wt%super-P and 2 wt%carbon nanotubes (CNTs)),and polymeric binder with the weight ratio of 50:30:20.The electrode slurries were thoroughly mixed using a laboratory stirrer (FLUKO high shear dispersion emulsifying machine) for at least 1 h,coated on a piece of copper foil with~8μm in size,dried in air first at room temperature and then in vacuum at 105℃for at least8 h prior to use.The density of SiNP electrode was managed to be~0.56 g·cm-3.The alginate-C-chitosan composite binder was formed in the process of slurry making.Active materials and conducting additives were firstly added to Alg aqueous solution and roughly mixed,and then C-chitosan was added with the same amount of Alg and fully mixed.
2.2 Measurements
Electrochemical tests were performed by using 2032 cointype half cells assembled with lithium metal as the counter electrode in an Ar-filled glove box.The electrolyte consisted of 1 mol·L-1 LiPF6 in a mixture of ethylene carbonate,diethyl carbonate,and dimethyl carbonate (volume ratio of EC:DEC:DMC of 1:1:1).For long-term cycling tests,10 vol%fluoroethylene carbonate (FEC) were added into the electrolyte solution.To clarify the effect of enhanced binder on the electrochemical performance,the cycling performance of Alg-C-chitosan binder was evaluated compared to the results for PVDF,CMC+SBR (1:1by weight),Alg,and C-chitosan.Galvanostatic cycling tests of the SiNP electrodes were conducted at a constant current density of 100 mA·g-1 in the voltage range of0.02-1.00 V at room temperature (25℃) on a CT2001A battery testing system.The specific capacity was calculated on the basis of the weight of the active materials.
Scanning electron microscopy (SEM) was performed on Hitachi S-4800,Japan.The sample was achieved by mixing Alg (2 wt%) and carboxymethyl chitosan (5 wt%) solution,and homogeneous blend solution of two polymers was formed under stirring at room temperature for 30 min and then dried in air at room temperature till water was evaporated thoroughly.X-ray diffraction (XRD) patterns were performed on X-PerPRO MPO diffractometer (PANalytical)with Cu Kαradiation source.The relative intensities were recorded within 2θrange of 5°-80°at a scanning rate of 8(°)·min-1.Fourier transform infrared spectroscopy (FTIR)spectra were recorded on a Bruker Vector 22 FTIR spectrometer with 32 scans in the wavenumber range of500-4000 cm-1.The samples and KBr were fully dried prior to submission of samples by FTIR analyses to exclude the influence of water.The mechanical properties of the binders(Alg-C-chitosan,Alg,C-chitosan,and PVDF) were measured by a universal testing machine (CMT5000,MST).The specimens were cut to pieces with the same dimensions of1 cm×10 cm×100μm,and the stretching velocity was fixed at 5 mm·min-1.
3 Results and discussion
3.1 Alg-C-chitosan polymer binder
FTIR analysis was used to evaluate the interactions of Alg with C-chitosan.The FTIR spectra of C-chitosan,Alg,and Alg-C-chitosan are shown in Fig.1a.The characteristic peak of alginate is seen at 1620 cm-1,corresponding to carbonyl (C=O) bond.The C-chitosan spectrum shows characteristic bands of amide-I (1640 cm-1),amide-II(1560 cm-1),and amino group (1173 cm-1).Chitosan used in these experiments is deficiently deacetylated,and the double amide peaks correspond to the partial N-deacetylation of chitin
SEM image of the cross sections of Alg-C-chitosan film in Fig.1c shows that this complex polymer has high condensed orientation and forms a porous scaffold structure.Figure 1d presents the wide-angle XRD patterns of Alg,C-chitosan,and Alg-C-chitosan complex.The diffractogram of Alg consists of two crystalline peaks at2θ=13.7°and 23.0°
Fig.1 FTIR spectra of C-chitosan,alginate,and alginate-C-chitosan complex a,schematic illustration showing cross-linking process between Alg and C-chitosan b,SEM image of cross section of Alg-C-chitosan film c and XRD patterns of Alg,C-chitosan,and Alg-C-chitosan complex d
Table 1Tensile strength and elongation of polymer binders
It is well recognized that the mechanical property of polymeric binder is a major parameter that determines cycling performance of a Si anode
3.2 Characterization of SiNP anode
As shown in Fig.2a,the first cycle delithiation takes place around 0.4 V,which is consistent with previously reported values for other Si anodes;the first cycle lithiation potential shows a plateau profile at 0.02-0.10 V,consistent with the behavior of crystalline Si
It is clear that the electrochemical performance of SiAlg-C-chitosan electrode is distinctly superior to the corresponding conventional electrodes (Fig.2).Silicon composite anodes with the self-healing Alg-C-chitosan porous scaffold binder shows higher ICE of 74.5%and superior capacities of 2565 and 1910 mAh·g-1 for the first insertion and extraction of lithium,respectively.Furthermore,this electrode also gives an excellent cycling stability,with a capacity of 750 mAh·g-1 remaining after 100th cycling,and the capacity retention compared with those of Si-Alg and Si-C-chitosan increases by 17.5%and 15.5%,respectively.
To confirm that the self-healing Alg-C-chitosan porous scaffold binder is robust enough for mechanical and electrical support,the thickness variation and surface topography change of the silicon electrodes during cycling were investigated by SEM.Figure 3 shows cross-sectional SEM images of Si-PVDF,Si-CMC+SBR,Si-Alg,and SiAlg-C-chitosan electrodes before cycling and after 5th cycling.The thickness of Si-Alg-C-chitosan electrode increases to 16.9μm (Fig.3h) from its origin state of15.4μm (Fig.3g) after 5th cycling,which is a~9.7%volume change.In contrast,the Si-PVDF,SiCMC+SBR,and Si-Alg electrodes undergo thickness increase of 66.5%,27.2%,and 20.1%,respectively.The smallest thickness change of the Si electrode with Alg-C-chitosan binder after 5th cycling is owing to the excellent mechanical properties of rigidly porous structure and selfhealing electrostatic interaction of Alg-C-chitosan binder,which effectively alleviates the volume change of silicon and allows the reversible deformation of electrode during cycling.Moreover,the Si-PVDF electrode is delaminated from a Cu current collector and a large number of cracks could be observed form cross-sectional SEM images of SiPVDF (Fig.3b).It suggests that the poor mechanical property,weak binding force with active material,and Cu current collector of PVDF could be not enough to keep the Si-based electrode structure well,leading to the degradation of electrical conducting network,isolation of SiNP,and inferior electrochemical performance.
Fig.2 Comparison of the first voltage profiles of Si anode with different binders a and cycling performance of Si anode with different binders b
Fig.3 SEM images of cross sections showing thickness change of a Si-PVDF,c Si-CMC+SBR,e Si-Alg and g Si-Alg-C-chitosan before cycling and b Si-PVDF,d Si-CMC+SBR,f Si-Alg and h Si-Alg-C-chitosan at end of 5th cycling
This can be further proved by the surface morphology change of the silicon electrodes.Figure 4 shows the surface microstructures of Si electrodes before cycling and after30th cycling.It is found that all of the samples exhibit uniform particle distributions in their initial states before cycling.The clear changes are noted that (1) the electrodes lost the pores and change to the dense morphology and (2)some of the cracks are observed in the surface morphology of the Si-PVDF,Si-CMC+SBR,and Si-Alg electrodes after cycling.The dense morphology is considered to be due to pulverization of silicon particles from intrinsic volume change and constantly forming SEI on newly exposed surfaces subsequently
This excellent electrochemistry performance of the Si anode with self-healing Alg-C-chitosan porous scaffold binder can be explained by several reasons.First,carboxylic and amino function groups of the Alg-C-chitosan binder could interact with the hydroxyl groups on the silicon oxide surfaces by strong hydrogen bonding interactions.Second,synergistic effect on the enhanced mechanical properties of the porous scaffold structure and self-healing electrostatic interaction of Alg-C-chitosan binder retard the breakdown of polymeric binder due to the structural stress produced by the volume expansion of silicon nanoparticles during the lithiation process (Fig.5).Third,the cross-linking 3D network structure of the Alg-C-chitos an binder restricts active materials and conductive agent from sliding,which are caused by continuous volume expansion/shrinkage of SiNP during cycling process.Fourth,the porous structure of the binder scaffold allows the Si particles to expand without the deformation of the electrode structure and increases ion transport in electrodes.
Fig.4 SEM images of surface micro structures of a Si-PVDF,c Si-CMC+SBR,e Si-Alg and g Si-Alg-C-chitosan electrodes before cycling and b Si-PVDF,d Si-CMC+SBR,f Si-Alg and h Si-Alg-C-chitosan electrodes after 30th cycling
Fig.5 Schematic illustrations of working mechanism of a PVDF and b Alg-C-chitosan porous scaffold binder on keeping mechanical integrity of Si electrode in cycling processes,and c details for self-healing working mechanism
Furthermore,the self-healing Alg-C-chitosan porous scaffold binder was used to design a practical anode setting.The working electrodes are made by a typical slurry method with Si/Gr@C composite,super-P,and Alg-C-chitosan binder with a mass ratio of 8:1:1,and the mass loading of active material (Si/Gr@C composite) is>5 mg·cm-2.The cells were measured at current density of 40 mA·g-1 in the potential range of 0.005-2.000 V (vs.Li/Li+).As displayed in Fig.6,the electrode exhibits charge/discharge capacities of 752/943 mAh·g-1 in the initial cycle with a relatively high ICE of 80.0%.The ICE value increases to~98.0%at the third cycle and finally stabilizes at~99.5%in subsequent cycles.Moreover,after 160th cycling,the electrode preserves 60.6%of the original capacity (~455 mAh·g-1).It is worth mentioning that the areal capacity of~3.76 mAh·cm-2 is superior compared with that in most reports
Fig.6 Cycling performance of Si-Gr@C composite electrode
4 Conclusion
A self-healing alginate-carboxymethyl chitosan porous scaffold polymer based on the low-cost water-soluble alginate and carboxymethyl chitosan bio-derived precursors,as an excellent binder for silicon anodes in lithium batteries,was introduced.The electrostatic interaction between c arboxy late (-COO-) of Alg and protonated amines
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51404032) and the National High Technology Research and Development Program of China (No.2013AA050904).
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