Despite widespread usage of Li-ion batteries in portable electronics, large scale utilization of Li-ion batteries in electric vehicles and grid-level energy storage is only possible if cost, safety, energy density, specific power and cycle life of Li-ion batteries are further improved. Enhancing the cycle life is especially critical for wider market penetration of Li-ion batteries which requires detailed understanding of the degradation mechanisms. The interplay between various battery subcomponents such as cathode, anode, separator, electrolyte and current collector determines the degradation mechanisms of Li-ion batteries that can have both chemical and mechanical origins that are closely interrelated. Chemical degradation mechanisms such as solid electrolyte interphase formation and electrolyte decomposition have been studied extensively; however, mechanical degradation mechanisms attracted less attention. Mechanical degradation is associated with the volume changes and stress generated during Li de-/intercalation in an active material which can result in formation of cracks, delamination and isolation of the active particles causing capacity and power losses.

Our previous work on composite LiFePO₄ cathode by nanoindentation revealed that elastic modulus deteriorates by aging as well as shows spatial variation on the electrode surface [1]. Binder is the compliant component in a composite electrode; therefore, degradation in elastic modulus is associated with the binder not the active material. To determine the mechanical degradation in the active material, thin film LiFePO₄ cathode (i.e., no binder or carbon coating/additive as in a composite electrode) was synthesized by physical vapor deposition, and employed in determining the changes in the mechanical properties of the active material due to aging. The preliminary AFM experiments on thin film LiFePO₄ cathode showed significant changes in particle morphology and roughness upon cycling as shown in Fig. 1. On the other hand, mechanical properties of the thin film LiFePO₄ did not show as significant deterioration as in a composite LiFePO₄ cathode indicating mechanical degradation in binder is more problematic than the mechanical degradation in the active material.  

References

[1] Demirocak, D.E., Bharat B., Probing the aging effects on nanomechanical properties of a LiFePO₄ cathode in a large format prismatic cell, Journal of Power Sources, 2015, 280, 256-262.