Suppression of Voltage Decay through Ni<sup>3+</sup> Barrier in Anionic-Redox Active Cathode for Na-Ion Batteries

Abstract: The storage of intermittent wind and solar electricity requires grid-level energy storage devices, and due to the abundance and wide distribution of Na resources, Na-ion batteries (NIBs) are much more cost-effective and have shown greater potential for large-scale energy storage than Li-ion batteries (LIBs). However, the lack of suitable cathode hinders the practical use of NIBs, so exploring suitable cathode materials that can maintain a balance between high energy density and cost-effectiveness is essential for NIBs. Ni-Mn based layered oxides are important cathode materials for NIBs, offering relatively high potential through the multi-electron redox reaction of Ni4+/Ni3+/Ni2+ as well as the low-cost and non-toxic nature of Mn4+. P2-Na0.67[Ni0.33Mn0.67]O2 was the first reported Ni-Mn based Na-ion battery cathode with a high capacity of ~160 mAh·g-1 in the voltage range of 2.0-4.5 V, while irreversible P2-O2 phase transition above 4.1 V makes poor capacity retention and limits their applications. Moreover, a dilemma has emerged in that a costly element (Ni) is used for sodium-ion batteries, which is supposed to be low-cost. With the intensive research in recent years, introducing an appropriate amount of anionic redox can effectively improve energy density while simultaneously reducing the amount of high-cost transition metals, such as V, Co, and Ni. However, because of irreversible oxygen loss and Mn4+/Mn3+ redox activation, voltage decay is difficult to avoid for most of these anion-redox materials. In this research, we report a Li-substituted Nax[Ni, Mn]O2 cathode, the designed formula being Na0.85[Li0.2Ni0.15Mn0.65]O2. This material shows a unique combination of both cationic redox (Ni4+/Ni3+/Ni2+) and anionic redox (O2-/O2n-) during charge and discharge, showing a high capacity of ~150 mAh·g-1 (10 mA·g-1, 1.5-4.5 V) with only 0.15 Ni. With an optimized voltage range, the material shows a capacity of ~100 mAh·g-1 and stable cycling performance (80% of initial capacity after 100 cycles at 10 mA·g-1 within 2.5-4.25 V) and high-rate capability (the capacity of 500 mA·g-1 is 80% of 10 mA·g-1, 2.5-4.25 V). Moreover, we demonstrate an effective way to suppress the voltage decay and Mn reduction through Ni3+ as a redox barrier. Specifically, during the discharge process, the Mn4+/Mn3+ reduction process was replaced by the Ni3+/Ni2+ reduction process with higher redox potential in the layered oxides. In addition, the full Ni2+/Ni4+ redox can compensate for the partial oxygen redox loss in the subsequent cycles. We believe that introducing the anion redox through Li substitution and the use of Ni3+ as a redox barrier to suppress the voltage decay will provide a new way in the design of NIBs' cathode materials, with potential benefits such as higher energy density, lower cost, and longer cycle life.