P-type NaxNi0.22Co0.11Mn0.66O2 materials: linking synthesis with structure and electrochemical performance

P-type layered oxides are promising cathode materials for sodium-ion batteries and a wide variety of compounds have been investigated so far. Nevertheless, detailed studies on how to link synthesis temperature, structure and electrochemistry are still rare. Herein, we present a study on P-type NaxNi0.22Co0.11Mn0.66O2 materials, investigating the influence of synthesis temperature on their structure and electrochemical performance. The change of annealing temperature leads to various materials of different morphologies and either P3-type (700 C), P3/P2-type (750 C) or P2-type (800–900 C) structure. Galvanostatic cycling of P3-type materials revealed high initial capacities but also a high capacity fade per cycle leading to a poor long-term cycling performance. In contrast, pure P2-type NaxNi0.22Co0.11Mn0.66O2, synthesized at 800 C, exhibits lower initial capacities but a stable cycling performance, underlined by a good rate capability, high coulombic efficiencies and high average discharge capacity (117 mA h g

1) and discharge voltage (3.30 V vs. Na/Na+) for 200 cycles. Introduction The interest in layered sodium-transition metal oxides, NaxMO2 (M ¼ transition metal, Ni, Co, Mn, Fe, Cr and others) is increasing signicantly in recent years.1–8 In fact, Na-based batteries appear to be a promising alternative when energy density is not a critical issue, e.g. for large scale energy storage and low-cost applications.9 Additionally, sodium is highly abundant and homogeneously distributed in the Earth's crust and sea water and, consequently, cheap.8,10,11 However, one great advantage of Na batteries is certainly the feasible replacement of copper with aluminium current collectors since Na does not alloy with this metal.12 This could represent substantial cost and weight savings for batteries.11,13 In layered NaxMO2, Na cations can be accommodated in trigonal prismatic (P), tetrahedral (T) or octahedral (O) sites between the MO6 octahedra. P-type materials nowadays are attracting interest due to their promising performance in sodium-ion batteries. Additionally, they were intensively investigated in the past due to the facile preparation of layered lithium oxides via ion-exchange.4,14–17 P2-type materials.