Influence of polymeric binders on mechanical properties and microstructure evolution of silicon composite electrodes during electrochemical cycling

Polymeric binders are a critical component to enhance mechanical integrity, maintain electronic conductivity, and achieve long durability of silicon (Si)-based electrodes. A fundamental understanding of the relationship between binder properties and mechanical degradation of Si electrodes is indispensable to developing durable Si-based electrodes. Using an environmental nanoindentation system, we measured the mechanical properties of Si composite electrodes made with different binders, including polyvinylidene fluoride (PVDF), Nafion, sodium-carboxymethyl cellulose (Na-CMC), and sodium-alginate (SA), as a function of the state-of-charge and cycle numbers under both dry and wet conditions. In contrast to electrodes made of Si alone, both elastic modulus (E) and hardness (H) of Si composite electrodes increase with lithium concentration within each cycle. E and H continuously decrease during long-term cycling. The mechanical property evolution of Si composite electrodes can be correlated with the porosity and irreversible thickness changes, which are largely determined by the mechanical properties of binders, instead of the adhesion between binders and Si. Electrodes under wet conditions have smaller E and H values than those under dry conditions because binders soften in the electrolyte. These findings not only provide useful mechanical parameters for battery modeling, but also may help design high performance and durable Si-based electrodes.