Particle Size and Crystal Structure Engineering of λ-MnO₂ Particles as Cathodes for Zinc-Ion Batteries

We use spinel λ-MnO₂ particles as a model system to study the effect of particle size and crystal structure engineering on Zn-ion diffusion and electrochemical performance in Zn-ion batteries. Through X-ray diffraction and energy-dispersive X-ray spectroscopy analysis, we demonstrate that Zn-ion insertion is enhanced in small nanoparticles (NPs) compared to large micron-sized particles due to larger surface area and solid-solution type phase transition pathway. Meanwhile, poor Zn-ion insertion/extraction reversibility leads to the poor cycling stability of NPs. To improve the cycling performance of λ-MnO₂NPs, crystal structure engineering is employed to create defects in the spinel lattice. The crystal structure change of λ-MnO₂ is studied by a collocated four-dimensional scanning transmission electron microscopy. Results show that single-crystalline λ-MnO₂ will turn into more disordered polycrystals, which can enhance Zn-ion diffusivity. As a result, the cycling performance of λ-MnO₂NPs is significantly improved, with a high capacity retention of over 94% after 100 cycles. Our work pinpoints the distinctive impacts of particle size and defects on the ion-diffusion process and cathode performance in Zn-ion batteries, providing guidance for the design of high-performance cathode materials for multi-valent ion batteries.