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 LiMn 2 O 4 and LiFePO 4 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 LiMn 2 O 4 and LiFePO 4 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, Li 2 Mn 2 O 4 , appears starting from 2.5 V. Until the voltage is less than 0.2 V, the framework structures of LiMn 2 O 4 are deteriorated, and further overlithiation caused decomposition into Li 2 MnO 2 and Li 2 O. LiFePO 4 essentially maintains its olivine-type structure, but below 0.2 V, direct overlithiation causes decomposition into Li 2 O and Fe metal. Furthermore, overlithiated decomposition of LiMn 2 O 4 and LiFePO 4 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, Li 2 CO 3 and Li x PO y F z by-products. The detailed overlithiation mechanism will provide important theoretical guidance for practical applications.