Engineering Particle Size for Multivalent Ion Intercalation: Implications for Ion Battery Systems

Here we present our recent understandings and engineering opportunities on the two-faceted nature of the size effect of cathode particles on electrochemically driven phase transformation pathways and reaction mechanisms. We have been using spinel λ-MnO₂particles as a model cathode material and Mg- and Zn-ion insertion as our focus of multivalent ion battery systems. We find that small, nanoscale cathode particles consistently favor a solid-solution-type phase transition and uniform ion distribution upon discharge. This phase transformation pathway facilitates fast charge insertion kinetics and mechanical stability compared to the multiphase transition pathway in large, micron-sized particles. Meanwhile, when it comes to the electrochemical reaction mechanism, the cathode particle size effect diverges for different systems. Whereas nanoscale cathode particles exhibit superior discharge capacity and cycling performance for Mg-ion-insertion systems, they suffer from a severe side reaction of Mn dissolution in aqueous Zn-ion batteries. Micron-sized λ-MnO₂particles instead show enhanced cycling performance for Zn-ion insertion because of decreased side reaction sites per mass and accommodation of an interpenetrating network of amorphous MnO_xnanosheets. Regarding the mechanistic understanding of the size effect, we discuss insights provided by high-resolution imaging methods such as scanning transmission electron microscopy and scanning electron diffraction, which are capable of monitoring structural changes in cathode particles upon multivalent ion insertion. Together we highlight the opportunities in both fundamentally understanding the electrochemically driven phase transformation in insertion materials and engineering high-performance electrode materials, not by composition variation but by tailoring of the "size"-and potentially the shape, exposed facets, surface chemistry, and mesoscale assemblies-of the cathode particles. The particle size effects are transferrable and have potential applications in both multivalent and monovalent ion batteries.