Avoiding Fracture in a Conversion Battery Material through Reaction with Larger Ions

Conversion and alloying electrode materials offer high specific capacity for emerging sodium- and potassium-ion batteries, but the larger volume changes compared to reaction with lithium are thought to limit cyclability. The reaction mechanisms of many materials with Na+ and K+ are unknown, however, and this knowledge is key for engineering mechanically resilient materials. Here, in situ transmission electron microscopy is used to uncover the nanoscale transformations during the reaction of FeS2 electrode materials with Li+, Na+, and K+. Surprisingly, despite larger volume changes during the conversion reaction with Na+ and K+, the FeS2 crystals only fracture during lithiation. Modeling of reaction-induced deformation shows that the shape of the two-phase reaction front influences stress evolution, and unique behavior during lithiation causes stress concentrations and fracture. The larger volume changes in Na- and K-ion battery materials may therefore be managed through understanding and control of reaction mechanisms, ultimately leading to better alkali-ion batteries. [Display omitted] •In situ TEM reveals reaction mechanisms in FeS2 with various alkali ions•Only reaction with lithium causes fracture•Fracture process is driven by evolution of crystal shape and stress concentrations•Finite element modeling and nanoindentation provide insight into chemomechanics High-capacity electrode materials hold promise for next-generation batteries with high energy density. However, such materials often undergo large volume changes during charge and discharge, which can cause mechanical degradation and reduced cycle life. It is therefore critical to understand and control coupled reaction and degradation processes in high-capacity electrode materials. Here we find that FeS2, a battery electrode material that undergoes a conversion-type reaction, fractures during reaction with lithium, but not with larger alkali ions (sodium and potassium). This result is counterintuitive, since larger ions induce larger volume changes, which are generally associated with greater stresses and more significant mechanical degradation. These findings are important since they indicate that large-volume-change electrode materials can be mechanically resilient in emerging sodium- and potassium-ion battery systems, which is a key aspect of attaining long cycle life. Next-generation batteries with high energy density rely on high-capacity electrode materials, but large volume changes and mecha