Unraveling the overlithiation mechanism of LiMn2O4 and LiFePO4 using lithium-metal batteries

With the solid-state battery (vs. Li) application, the overlithiation mechanism of the different cathode materials is worthy to investigate. In this study, both LiMn2O4 and LiFePO4 cathode materials at different over-discharge conditions were tested using half cell (vs. Li) and anode-free systems. The cells were dismantled to study the electrode structure, surface morphology, and compositional changes. The study shows that LiMn2O4 and LiFePO4 still maintain good crystal morphology during the deep over-discharge process, showing better over-discharge resistance capability with different overlithiation mechanisms. As shown by X-ray diffraction and X-ray photoelectron spectroscopy with Ar-ion etching, the new phase, Li2Mn2O4, appears starting from 2.5 V. Until the voltage is less than 0.2 V, the framework structures of LiMn2O4 are deteriorated, and further overlithiation caused decomposition into Li2MnO2 and Li2O. LiFePO4 essentially maintains its olivine-type structure, but below 0.2 V, direct overlithiation causes decomposition into Li2O and Fe metal. Furthermore, overlithiated decomposition of LiMn2O4 and LiFePO4 occurs at very low voltages approximately 0.43 and 0.56 V, respectively. Additionally, the deep over-discharge also leads to the decay of the electrolyte structure, associated with LiF, Li2CO3 and LixPOyFz by-products. The detailed overlithiation mechanism will provide important theoretical guidance for practical applications.