Study On Modification And Properties Of PVDF Binder For Lithium Ion Battery

Binder belong to inactive material in lithium-ion batteries（LIBs）,but binder was one of the key materials,which has a great influence on the cycle stability and rate performance of LIBs.Polyvinylidene fluoride（PVDF）was one of the most widely used binders for LIBs.PVDF was a semi-crystalline polymer,high crystallinity will imply the high adhesion,but it will also reduce the wettability of electrolyte,resulting in lower Li⁺transfer rate.Furthermore,at high temperature,PVDF was easy to react with lithium and residual water to produce LIF and HF,resulting in poor battery performance.In order to improve the cycling performance of Li Fe PO₄electrode with modified PVDF binder at room temperature and high temperature,inorganic-organic composite materials were prepared and used as filler to modify PVDF binder for LIBs.The main conclusions were as follows:1.The oil soluble silica sol and titanium sol were synthesized by phase transfer method and sol-gel method.Then the composite materials with the different sort of phosphate were prepared by reacting with different phosphate extractant respectively,such as 2-ethylhexyl phosphate mono 2-ethylhexyl ester（P507）,Bis（2-ethylhexyl）phosphate（P204）.And the final products were recorded as Si O₂@P507（SP5）,Ti O₂@P507（TP5）,Si O₂@P204（SP2）and Ti O₂@P204（TP2）,respectively.The results of FT-IR,TGA and XPS showed that phosphate was successfully grafted on the surface of inorganic particles.Through the BET analysis of the synthesized samples,it was found that the synthesized samples were mesoporous materials with uniform pore size,which can promote the absorption of electrolyte.2.Then,the effect of SP5 and TP5 modified PVDF binders on the performance of LIBs were studied.The content of SP5 modified PVDF binder was explored and the results was as follows:The electrochemical performance of cells with SP5P3 binder（SP5accounted for 3 wt%of PVDF）was the best.At 25℃and 1C current density,the initial and 40^thdischarge specific capacities were 129 m Ah/g and 128 m Ah/g,respectively,and the capacity retention rate was 99.2%.At 80℃and 1C current density,the discharge specific capacities of cells with SP5P3 binder in the initial and 35^thcycle were 154m Ah/g and 103 m Ah/g respectively,and the capacity retention rate was 66.9%.The lithium ion diffusion coefficient of cells with SP5P3 binder was increased to 1.1×10^-13cm~2/s.Simultaneously,the content of TP5 modified PVDF binder was explored and the results was as follows:At 25℃and 1C current density,the electrochemical performance of cells with TP5P5 binder（TP5 accounted for 5wt%of PVDF）was the best.At 25℃and 1C current density,and its initial and 40^thdischarge capacity were 131 m Ah/g and132 m Ah/g,respectively.At 80℃and 1C current density,the discharge specific capacities of cells with TP5P5 binder in the initial and 35^thcycle were 157 m Ah/g and108 m Ah/g respectively,and the capacity retention rate was 68.9%.The Li⁺diffusion coefficient of cells with TP5P5 binder increased to 1.34×10^-13cm~2/s.Therefore,the modified PVDF（SP5P3 and TP5P5）can significantly improve the cycling performance at room temperature and high temperature of LIBs.3.Finally,the effect of SP5 and TP5 modified PVDF binders on the performance of LIBs were studied.The content of SP2 modified PVDF binder were explored and the results were as follows:The electrochemical performance of cells with SP2P5 binder（SP2 accounted for 5 wt%of PVDF）was the best.At 25℃and 1C current density,the initial and 40^thdischarge specific capacities were 133 m Ah/g and 130 m Ah/g,respectively,and the capacity retention rate was 97.7%.At 80℃and 1C current density,the discharge specific capacities of cells with SP2P5 binder in the initial and 35^thcycle were 151 m Ah/g and 99 m Ah/g respectively,and the capacity retention rate was 65.6%.The Li⁺diffusion coefficient of cells with SP2P5 binder was increased to 1.67×10^-13cm~2/s.Simultaneously,the content of TP2 modified PVDF binder was explored and the results was as follows:At 25℃and 1C current density,the electrochemical performance of cells with TP2P5 binder（TP2 accounted for 5wt%of PVDF）was the best.At 25℃and 1C current density,and its initial and 40^thdischarge capacity were 133 m Ah/g and133 m Ah/g,respectively.At 80℃and 1C current density,the discharge specific capacities of cells with TP2P5 binder in the initial and 35^thcycle were 153 m Ah/g and105 m Ah/g respectively,and the capacity retention rate was 68.6%.The Li⁺diffusion coefficient of cells with TP5P5 binder increased to 1.75×10^-13cm~2/s.Therefore,the modified PVDF（SP2P5 and TP2P5）can significantly improve the cycling performance at room temperature and high temperature of LIBs.