Abstract
In this work we demonstrate a new in operando neutron scattering method and identify several new processes occurring in Li1+δMn2O4 (0 ≤ δ ≤ 1) spinel. New reaction mechanisms are proposed involving solid solution regions within both cubic and tetragonal spinel phases in addition to a continuous phase transition between them. In operando neutron powder diffraction (NPD) is an ideal tool to follow the light elements such as lithium and oxygen in cathode materials which are often the key to understanding mechanism of cycling a battery material. Here we report a novel methodology to prepare an extremely thick electrode with ~ 378 mg/cm2 loading density suitable for the in operando NPD studies. Enabled by such thick electrode, the reaction mechanisms driven by the lithiation of Li1+δMn2O4 (0 ≤ δ ≤ 1) spinels were re-examined. The metal oxygen M-O (M = Li and Mn) bond lengths in both end members LiMn2O4 and Li2Mn2O4 experiences pronounced changes larger than dictated by change in lattice parameters, due to the locally-formed Jahn-Teller distorted Mn3+. First-principles density functional theory calculations confirmed these metastable intermediates and further proposed atomistic reaction pathways for the phase transition by coupling a global structure search algorithm. These findings redefine the conventional understandings on two-phase reactions of this cathode material.
