Correlation Between Reversible Anionic Redox and Open-Circuit-Voltage Hysteresis in Co-Free Li-Rich Layered Oxides

Co-free and Li-rich nickel manganese layered oxides (LR-NMs) are considered next-generation cathode active materials for Li-ion batteries, owing to their high energy density at low cost. However, this materials class suffers from an open-circuit-voltage (OCV) hysteresis, resulting in low energy efficiency. In this study, LR-NMs with varying Li and Mn contents (Li 1+ δ [Ni x Mn y ] 1- δ O 2 with x / y = 0.65/0.35, 0.50/0.50, 0.35/0.65 ( x + y = 1), and δ = 0.10–0.20) are prepared and characterized by X-ray diffraction. Their composition-dependent OCV hysteresis is determined by charge/discharge tests with intermittent OCV periods. We demonstrate that increasing Li and/or Mn content increases the contribution of the reversible anionic redox at similar discharge capacities, concomitant with an increased OCV hysteresis between charge and discharge in the cycles following the first-cycle activation. This is supported by a linear increase of the OCV hysteresis with the anionic redox contribution for the LR-NM materials synthesized and examined in this study (seven different compositions, each prepared at three different calcination temperatures). Considering that the increase of the OCV hysteresis with increasing overlithiation δ and/or Mn content y results in an intrinsically inferior energy efficiency, possible strategies for future LR-NM materials development are discussed.