Understanding cation interlayer (non)migration in alkali-ion Cr layered oxides

Abstract

Suppressing transition metal (M) migration is desirable for nonhysteretic and reversible capacities of layered cathodes in alkali(A)-ion batteries; however, it still poses some serious challenges. A unified picture of the cation (non)migration based on an in-depth understanding of two Cr layered oxide models, divided into LiCrO₂ for lithium-ion batteries (LIBs) and NaCrO₂ for sodium-ion batteries (SIBs), is proposed herein to harness the theoretical full potential of the Li-based compounds. We investigated the thermodynamic phase (in)stabilities depending on the cation migration for both Cr oxides; unlike the Li model exhibiting a severe biphasic reaction, NaCrO₂ intriguingly showed a monophasic reaction without the cation migration. These underpinned the electrochemical distinction between LiCrO₂ and NaCrO₂ in experiment, and further leading to providing a structural dissimilarity between the MO₂ and AO₂ layers that restrains the M migration upon charging/discharging for AMO₂ cathodes. The interesting dependency on the guest ion was deeply understood by the metallic feature derived from the Cr−O decoordination; therefore, the Na ion in the AO₂ layer played a critical role in repelling the cation interlayer migration. With the dissimilarity concept, our design rule inspired by the Na oxide is considered to be an intriguing pathway in modulating (non)cation migration for high-energy-density cathodes in LIBs.