Solid-state lithium (Li) batteries using spinel-oxide electrode materials such as LiNi0.5Mn1.5O4 are promising power supplies for mobile devices and electric vehicles. Here, we demonstrate stable battery cycling between the Li0Ni0.5Mn1.5O4 and Li2Ni0.5Mn1.5O4 phases with working voltages of approximately 2.9 and 4.7 V versus Li/Li+ in solid-state Li batteries with contamination-free clean Li3PO4/LiNi0.5Mn1.5O4 interfaces. This clean interface has the effect of doubling the capacity of conventional battery cycling between the Li0Ni0.5Mn1.5O4 and Li1Ni0.5Mn1.5O4 phases. We also investigated the structural changes between the Li0Ni0.5Mn1.5O4 and Li2Ni0.5Mn1.5O4 phases during battery cycling. Furthermore, we found an inhomogeneous distribution of the Li2Ni0.5Mn1.5O4 phase in the LiNi0.5Mn1.5O4 electrode, induced by spontaneous Li migration after the formation of the Li3PO4/LiNi0.5Mn1.5O4 interface. These results indicate that the formation of a contamination-free clean Li3PO4/LiNi0.5Mn1.5O4 interface is key to increase the battery capacity.
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