Unraveling divalent pillar effects for the prolonged cycling of high-energy-density cathodes

Abstract

Solid-state batteries (SSBs) have attracted significant interest owing to their relatively high energy density and nonflammability. LiNi_0.5Mn_1.5O₄ (LNMO) is a promising candidate for cathodes in SSBs because of its high working voltage of 5 V (vs. Li/Li⁺). However, the inadequate durability of LNMO prevents it from being commercially viable. Although LNMO is known to be stabilized by doping with Mg rather than transition metal atoms, the stabilization mechanism of Mg-doped LNMO is not well understood yet. Here, we examine the stabilization mechanism of a Mg-doped LNMO structure using first-principles calculations. We show that the doped Mg atoms act as pillars in the LNMO structure and effectively mitigate lattice misfits of the interfaces during two-phase reactions without affecting the redox reaction. In addition, the doped Mg atoms lower the phase transition barrier in the biphasic region, thereby alleviating sudden structural shifts, which are accompanied by abrupt phase transitions. Consequently, Mg-doped LNMO undergoes less destructive reactions than pristine LNMO and therefore, less cyclability degradation. Additionally, a solid-solution reaction, which has a higher thermodynamic phase stability than the biphasic reaction, occurs in the later part of the delithiation process only when Mg atoms are doped into pristine LNMO. Mg doping also mitigates Ni ion migration that leads to capacity fading through the blocking of Li ion diffusion and acceleration of Mn dissolution. The doped Mg atoms efficiently improve the cycle performance of LNMO. Our findings are useful for the development of long-lasting cathode materials and spinel structures.