Structure And Properties Of Polymer Electrolytes And Electrode Binders In Lithium Ion Battery

Lithium ion battery has been widely used in various kinds of electronic equipment because of its high capacity,long cycle life,and memoryless effect.Traditional lithium ion battery employed liquid electrolytes,which contain flammable esters or ethers,thus have severe concerns of leakage.Polymer electrolytes attract significant attention because they are incombustible,easy to process,and take no risk of leakage.Also,they can substitute both membrane and electrolyte,reducing interfacial resistance between electrodes and membranes.The room-temperature ionic conductivity of all-solid-state polymer electrolytes is too low for commercial production.Gel polymer electrolytes possess high ionic conductivity of liquid electrolytes and take no risk of leakage.The addition of liquid electrolytes improves ionic conductivity but sacrifices mechanical properties.Microphase separations of block copolymers decouple conductivity and mechanical properties,therefore provide a possible solution for this contradiction.Electrodes optimization is also an important method to improve battery performance.Although binders make up only a small part of electrode composition,they play an important role in affecting the electrode structures.Improving adhesion stress,elasticity and ionic conductivity of binders could achieve better battery performances.The requirements of binders have something in common with that of gel polymer electrolytes.They both require affinity with liquid electrolytes to promote fast ion transport,while maintaining mechanical properties.In this paper,we synthesize random,triblock,and V-shape gradient copolymers based on styrene and methyl acrylate via reversible addition fragmentation chain transfer radical polymerization.We applied these copolymers as gel polymer electrolytes and binders for LiFePO4 electrodes.The ionic conductivity,battery performances and mechanical properties were evaluated to investigate the relationship between molecular structures and properties.In the research of gel polymer electrolytes,gradient copolymers present bicontinuous morphology and form three dimensional ionic pathways.The weak microphase separation of gradient copolymers could help to minimize the adverse effect resulted from the discontinuity of domain boundary.The gradient copolymer gel electrolyte filled with a regular electrolyte gave the highest ion conductivity of 1.2×10-S cm-1 at room temperature.As for the mechanical tests,poly(styrene)phase were resistant to liquid electrolyte,thus providing mechanical support.Gel polymers based on triblock copolymers with 150 wt%liquid electrolytes still had a modulus of 41.2 MPa.In the research of binders for LiFePO4 electrodes,sodium carboxymethyl cellulose was used as thicker and water is used for dispersion.Compared with traditional polyvinylidene fluoride,binders based on copolymers performed better rate capacity and more stable cycling capacity in this research.Better battery performances for triblock binders than that of random copolymers demonstrated the importance of ion transportation.Meanwhile,for triblock copolymers,when the design molecular weight was higher than entanglement molecular weight,increasing molecular weight do not have significant improvement for battery performance.