Positive Electrode Materials in the Li-Mn-Ni-O System Exhibiting Anomalous Capacity Growth during Extended Cycling

Single- and multi-phase compositions in the Li-Mn-Ni-O system with a Ni:Mn ratio of 1:5 and varying Li contents were synthesized and characterized by XRD and galvanostatic charge/discharge measurements. As the Li content relative to Ni and Mn was decreased, the materials were observed to be either single-phase layered, two-phase layered-rocksalt, or three-phase layered-rocksalt-spinel, depending on their location within the Li-Mn-Ni-O phase diagram. SEM analysis revealed significant differences in primary and secondary particle size, shape, and morphology depending on Li content. Differential capacity analysis showed that the samples underwent significant structural rearrangements with extended cycling of half-cells to 4.9 V, accompanied by the evolution of a spinel phase exhibiting a 3 V plateau on discharge. Surprisingly, each composition exhibited an anomalous, gradual increase in capacity with continual cycling, with several compositions producing capacities in excess of 300% of their initial values over 150 cycles. The magnitude of capacity growth was strongly dependent on the lower voltage limit during cycling, with significant growth occurring only when discharging below 3 V. This apparent capacity "growth" phenomenon is not well understood at present, but appears to be strongly associated with the phase transformation of the layered phases in each composition to a new "spinel-like" phase when discharging below 3 V.