Origin of Enhanced Performance when Mn-Rich Rocksalt Cathodes transform to $\delta$-DRX

TL;DR

This study investigates how the phase transformation of Mn-rich rock-salt cathodes into a $ ext{delta}$-phase with spinel-like domains enhances electrochemical performance, revealing the role of domain boundaries, defects, and composition in Li-percolation.

Contribution

It provides a detailed analysis of the multi-domain structure, boundary energetics, and defect behavior using machine learning potentials, offering insights for optimizing cathode design.

Findings

Eight variants of Spinel domains explain nanoscale domain formation.

Li-percolation is facilitated by boundary segregation of vacancies.

Ti defects influence domain morphology and electrochemical performance.

Abstract

Most Mn-rich cathodes are known to undergo phase transformation into structures resembling spinel-like ordering upon electrochemical cycling. Recently, the irreversible transformation of Ti-containing Mn-rich disordered rock-salt cathodes into a phase -- named $\delta$ -- with nanoscale spinel-like domains has been shown to increase energy density, capacity retention, and rate capability. However, the nature of the boundaries between domains and their relationship with composition and electrochemistry are not well understood. In this work, we discuss how the transformation into the multi-domain structure results in eight variants of Spinel domains, which is crucial for explaining the nanoscale domain formation in the $\delta$-phase. We study the energetics of crystallographically unique boundaries and the possibility of Li-percolation across them with a fine-tuned CHGNet machine learning interatomic potential. Energetics of $16d$ vacancies reveal a strong affinity to segregate to the boundaries, thereby opening Li-pathways at the boundary to enhance long-range Li-percolation in the $\delta$ structure. Defect calculations of the relatively low-mobility Ti show how it can influence the extent of Spinel ordering, domain morphology and size significantly; leading to guidelines for engineering electrochemical performance through changes in composition.