In Situ Stress Measurements during Electrochemical Cycling of Lithium-Rich Cathodes

Abstract

Lithium-rich layered oxides (Li1+xM1-xO2, M= Ni, Mn, Co) are attractive cathode materials for lithium ion batteries due to their high reversible capacity originating from their high lithium and manganese content. During cycling, structural changes cause compositional stresses that may lead to mechanical degradation. In this study, we seek to characterize this degradation using Li1.2Mn0.55Ni0.125Co0.125O2 (LR-NMC) thin-films prepared by sputtering on sapphire substrates and characterized using x-ray diffraction (XRD), raman spectroscopy, and scanning electron microscopy (SEM). A multibeam optical stress sensor (MOSS) is used to quantitatively measure the average stress in the cathode film during electrochemical cycling. A unique and unpredicted stress signature is observed during the first charge. Initially, a tensile stress is observed, consistent with volume contraction from lithium removal, however, the stress reverses and becomes compressive with continued charging beyond ~4V vs Li/Li+, indicating volume expansion; this phenomenon is present in the first cycle only. The origin of this irreversible stress during the first charge is not clear; cracking, oxygen loss, and the formation of a surface phase are explored as possible causes. Subsequent cycling is reversible, with volume expansion during lithiation and volume contraction during delithiation. Raman spectroscopy provides evidence of the layered-to-spinel phase transition after cycling. After reannealing in an oxygen environment, the original raman spectra is recovered. Annealed samples show the same first cycle behavior after re-cycling. Atomistic modelling in conjunction with experimental results aids in understanding the effects of stress on kinetics and phase transformations, guiding the development of improved lithium rich cathodes.