Oriented LiMn2O4 Particle Fracture from Delithiation-Driven Surface Stress

The insertion and removal of Li+ ions into Li-ion battery electrodes can lead to severe mechanical fatigue due to the repeated expansion and compression of the host lattice during electrochemical cycling. In particular, the lithium manganese oxide spinel (LiMn2O4, LMO) experiences a significant surface stress contribution to electrode chemo-mechanics upon delithiation that is asynchronous with the potentials where bulk phase transitions occur. In this work, we probe the stress evolution and resulting mechanical fracture from LMO delithation using an integrated approach consisting of cyclic voltammetry, electron microscopy, and density functional theory (DFT) calculations. High-rate electrochemical cycling is used to exacerbate the mechanical deficiencies of the LMO electrode and demonstrates that mechanical degradation leads to slowing of delithiation and lithiation kinetics. These observations are further supported through identification of significant fracturing in LMO using scanning electron microscopy. DFT calculations are used to model the mechanical response of LMO surfaces to electrochemical delithiation and suggest that particle fracture is unlikely in the [001] direction, due to tensile stresses from delithiation near the (001) surface. Transmission electron microscopy and electron backscatter diffraction of the as-cycled LMO particles further support the computational analyses, indicating that particle fracture instead tends to preferentially occur along {111} planes. This joint computational and experimental analysis provides molecular-level details of the chemo-mechanical response of the LMO electrode to electrochemical delithiation, as well how surface stresses may lead to particle fracture in Li-ion battery electrodes.