Unraveling the voltage-fade mechanism in high-energy-density lithium-ion batteries: Origin of the tetrahedral cations for spinel conversion

High-voltage layered lithium- and manganese-rich (LMR) oxides have the potential to dramatically enhance the energy density of current Li-ion energy storage systems. However, these materials are currently not used commonly; one reason is their inability to maintain a consistent voltage profile (voltage fade) during electrochemical cycling. This report rationalizes the cause of this voltage fade by providing evidence of layered to spinel (LS) structural evolution pathways in the host Li_1.2Mn_0.55Ni_0.15Co_0.1O₂ oxide. By employing neutron powder diffraction, we show that LS structural rearrangement in the LMR oxide occurs through a tetrahedral cation intermediate via the following: (i) diffusion of lithium atoms from octahedral to tetrahedral sites of the lithium layer [(Li_Lioct → Li_Litet] which is followed by the dispersal of the lithium ions from the adjacent octahedral site of the metal layer to the tetrahedral sites of lithium layer [Li_TMoct → Li_Litet]; (ii) migration of Mn from the octahedral sites of the transition-metal layer to the "permanent" octahedral site of lithium layer via tetrahedral site of lithium layer [Mn_TMoct → Mn_Litet → Mn_Lioct)]. These findings open the door to potential routes to mitigate this "atomic restructuring" in the high-voltage LMR composite oxide by manipulating their composition/structure for practical use in high-energy-density lithium-ion batteries.