Controlling side reactions and self-discharge in high-voltage spinel cathodes: the critical role of surface crystallographic facets

Abstract

Instabilities resulting from side reactions between the high-voltage cathode and the electrolyte are major barriers to meeting the calendar and cycle life requirements in lithium-ion batteries for vehicular applications. The present study reports a new approach for minimizing the effect of these reactions. LiMn_1.5Ni_0.5O₄ (LMNO) with two distinct morphologies, octahedron with (111) and plate with (112) surface facets, were synthesized in a similar size and investigated for structural changes and electrochemical stability during long-term cycling and storage in the presence of a liquid carbonate electrolyte. Bulk and surface analyses using ICP, XRD, FTIR, soft and hard XAS revealed that in the charged state, the high-valent transition metals in Mn_1.5Ni_0.5O₄ (MNO) oxidatively decompose the electrolyte which results in electron transfer from the electrolyte to the cathode. As a compensation mechanism, Li⁺ ions are re-inserted into MNO and the cathode self-discharges. Surface facets where the local redox reactions occur heavily influence the reaction kinetics and selectivity which ultimately determine the nature of the products and rate of self-discharge. Significantly lower self-discharge was observed on octahedra with the (111) facets, benefiting from their ability for promoting sufficient passivation after the initial interaction with the electrolyte. The importance of particle engineering reported in this work has a broad implication in the development of next generation cathode materials with improved performance.