(Invited) Binder Investigation for Silicon Electrode of Lithium-Ion Batteries

Abstract

Though the binder is not an active material and does not contribute capacity to lithium ion batteries (LIBs), it is an important ingredient for maintaining the integrity of the electrode, which helps to improve electrical contact between the active materials and conductive carbon black, improves the Li+ transport efficiency, reduces the internal resistance of Li-ion batteries and effectively depresses the crack formations and phase separation.Polyvinylidene fluoride (PVDF) is a polymer that is commonly used as a binder for both anode and cathode materials in LIBs. However, it is not suitable for Si electrode due to its poor binding capability and stability. Viewed as a next generation anode material, Si has been extensively studied due to its extremely high theoretical specific capacity of 3578 mAh/g (assuming Li15Si4 as intermediate product). However, silicon undergoes huge volume variations during repeated discharge and charge, which requires a strong binder to maintain the electrode mechanical integrity. Developing a high strength binder for silicon anodes has been considered as an efficient pathway to alleviate various capacity decay pathways.Polyacrylic acid (PAA) and its lithiated version (LiPAA), rich with carboxy function group, can provide stronger binding and are widely used as binder for Si electrode. In this work, we optimized Si electrode formulation by varying the binder composition. The optimized Si electrodes were electrochemically tested in both half cells and full cells. With improvement in electrochemical performance, cracks at the electrode level can still be observed for the cycled electrodes, which suggests that a stronger binder is required to mitigate volume expansion of Si electrodes and hold the particles together.We further studied the polyimide (PI) materials as binders for Si anodes in an attempt to achieve more stable cycling performance. In this work, we explore various PI crosslinking conditions and their effects on electrochemical performance. The optimized silicon electrode exhibits a higher tensile strength than that of conventional PAA binder. The strong adhesion of the PI binder suppresses the structural collapse of the Si negative electrode during lithiation/delithiation, enabling good capacity retention and stable cycle life.