Local coordination and migration-network topology shape Li-ion transport and delithiation in the low-energy ɛ-LiMnO2 polymorph

Abstract

In rocksalt-derived oxide cathodes, the local Li-migration environment around an O 4 tetrahedral intermediate is commonly classified by the number of face-sharing transition-metal (TM) neighbors. In LiMnO 2 , the TM species is Mn, and 0-TM denotes the absence of face-sharing Mn neighbors. However, migration and delithiation may also depend on higher-shell coordination and tetrahedral connectivity. Using the recently reported low-energy ε -LiMnO 2 polymorph as a model, we examine these factors through bond-valence site-energy and bond-valence pathway analyses combined with first-principles calculations. The resulting migration maps and tetrahedral statistics reveal distinct topologies and dimensionalities across four LiMnO 2 polymorphs. Although the ε phase and the lithiated-spinel phase Li 2 Mn 2 O 4 (hereafter spinel) have identical tetrahedral-type fractions, their 0-TM motifs form quasi-one-dimensional chains and a three-dimensional network, respectively. Climbing-image nudged elastic band calculations further distinguish the two structures: ε -phase barriers are 0.35–0.36 eV, compared with 0.41–0.53 eV in spinel, a difference that may be associated with distinct next-nearest corner-sharing shells. Ab initio molecular dynamics yields an apparent activation energy of 0.32 eV, while direction-resolved mean-squared displacements show preferential Li migration along c , supporting low-barrier quasi-one-dimensional diffusion. Delithiation calculations show that differences in 0-TM connectivity and Li–Li separation between the ε phase and spinel are associated with Li-site evolution and calculated voltage steps, suggesting that motif connectivity may influence voltage response. These results link the local environments and spatial connectivity of 0-TM motifs to Li migration and delithiation, providing insights into the design of metastable cathode structures.