Cycling effects on surface morphology, nanomechanical and interfacial reliability of LiMn2O4 cathode in thin film lithium ion batteries

In this study, the effects of Li+ intercalation/deintercalation processes on surface morphology, nanomechanical and interfacial properties of sputtered LiMn2O4 cathode film are studied using various techniques, including ex situ field emission scanning electron microscope (FESEM), atomic force microscope (AFM), nanoindentation tests combining with focused ion beam (FIB) sectioning. The results reveal that the spinel LiMn2O4 cathode film shows good cycling performance and morphology stability, accompanying with a slight increase of surface roughness mainly due to the agglomeration of nano-grains caused by phase transformations. The cathode film mechanically fails due to the stress induced by the lattice parameter change upon Li+ intercalation/deintercalation and lattice mismatch related to inhomogeneous phase transformation as well as the Jahn–Teller distortion in the 4 V region. The induced stress in the film causes the fragmentation of grains and the initiation/propagation of micro-cracks, leading to the mechanical degradation. The non-uniformity, micro-cracks and nanomechanical degradation have harmful effects on the electrical contact of LiMn2O4 cathode film, resulting in the capacity fading of the lithium ion battery.