Biwedge Octahedron-Shaped Li-Excess Nickel-Manganese Oxide Cathode Showing Remarkably Suppressed Potential Fade

Li-ion batteries (LIBs) are foreseen to play a very important role in energy storage for the next decade. Research for advanced Li-ion secondary batteries has been following the direction toward higher energy and power densities. Various cathode materials, such as LiCoO₂, LiMn_0.33Ni_0.33Co_0.33O₂, spinel LiMn₂O₄,olivine LiFePO₄, and Li_1+x(M,Mn)_1-xO_y, having different or similar lattice structures have been developed. Among these materials, the class of layered Li-rich Mn-transition metal oxides (Li_1+x(M,Mn)_1-xO_y) has lately drawn escalated attention due to their potential high capacities beyond 200 mAh/g. In these oxides, the excess Li ion gives rise to the presence of inter-grown Li₂MnO₃ slabs within Li(M,Mn)O₂ matrix to form a layered “composite” structure. These Li₂MnO₃ slabs may be transformed to electrochemical active MnO₂ layers by removal of Li⁺ upon charging above 4.5 V (versus Li/Li⁺) and stabilizing the layered structure under high depth of lithiation/de-lithiation during the charge/discharge cycles. During charge and discharge process,  the layered composite suffers from the well known phenomena of potential fade. The potential fade has been attributable to the migration of the transition metal ions, notably manganese and nickel, into the lithium layers, causing slow transformation to a spinel-like domain within the crystal lattice. The fade results in gradual loss in energy capacity of the electrode.

In this study, a biwedge octahedron-shaped Li_1+x(M,Mn)_1-xO_y powder has been synthesized via a oxalate-based precipitation process followed by high-temperature calcination. The resulting powder shows a large aspect ratio with a width near 10 μm and a thickness of a couple of μm. The large width allows for high powder tap density, while the small thickness enables fast charging-discharging kinetics. More importantly, in either full-cell or half-cell testing, the resulting oxide electrode showed essentially no potential fade after several hundred cycles. The synthesis process and the material and electrochemical characterization results will be presented.