In Situ Acoustic Diagnostics of Particle-Binder Interactions in Battery Electrodes

The high phase-transformation strain developed upon intercalation in the host particles of a composite battery electrode affects the polymeric binder network mechanically, deteriorating the electrode cycling performance. Here, electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is used to demonstrate a new strain-accommodation mechanism, in high-strain NaFePO₄/PVdF electrodes, via relaxation of the binder network surrounding the intercalation particles. Complete mechanical degradation of the polymer network occurs during long-term cycling of NaFePO₄ electrodes in aqueous solutions (hard and tough behavior). In contrast, in aprotic solutions, a softened binder easily accommodates the high transformation strain, ensuring excellent electrode cycling performance (soft and tough behavior). Quantification of the high-frequency viscoelastic properties of an operating composite electrode linked to the binder's fracture toughness ensures fast and facile screening of the optimal polymeric binder/electrolyte solution combinations. This methodology should be extremely important for optimization of cycling performance of Li-Si anodes undergoing huge volume changes during cycling. Almost all practical composite battery electrodes experience volume changes during cycling, deteriorating their cycling performance. A new key element that the EQCM-D methodology introduces into the energy storage field is a general platform for quantification of high-frequency viscoelastic characteristics of high-strain composite electrodes correlated with their low-frequency resilience and toughness moduli. Fast relaxation ensures effective high-strain accommodation by softened binder in aprotic solution. In contrast, with excessively stiff binder in aqueous solution, the strong electrode/binder interactions tracked dynamically reflect the initial and advanced stages of the mechanical degradation of the polymeric binder up to its complete destruction. The discovered correlation between the fracture toughness of the binder and its high-frequency viscoelastic behavior will serve for a rational design of extremely strained composite alloy-type electrodes such as the Li-Si anode. Tough and brittle characters of the binder in composite battery electrodes stabilize/deteriorate their long-term cycling performance, respectively. Instead of performing conventional long-time cycling tests of battery electrodes followed by their postmortem analysis, the proposed accelerated EQCM-D diagnostic easily distinguishes between the stable and failed cycling modes of strained electrodes by characteristic coupled shifts in the recorded resonance frequency and resonance bandwidth changes on multiple harmonics (EQCM-D signatures).