Sodium-ion batteries (SIBs), as a highly promising alternative to its counterpart lithium-ion batteries (LIBs), have attracted increasing attentions due to the low-cost and abundant sodium resources, as well as the compatibility of aluminum current collectors in anode. Due to the heavier atom and larger size of Na atom compared with Li atom, the energy density of SIBs is generally lower than that of LIBs. As a result, the main application area for SIBs would be the large energy storage systems, where cost is the crucial consideration ^[1].

Currently, to obtain cathode materials with good performance is a main challenge up in SIBs field, because of the relatively large size and slow diffusion of Na⁺. Several kinds of materials, such as layered transition metal oxides, tunnel-type oxides, phosphates, fluorides, organic compounds, Prussian blue and its analogues, have been proposed as the cathode material for SIBs ^[2]. Among them, sodium manganese layered oxides Na_xMnO₂ emerge as the most promising ones as they generally have relatively high theoretical capacity, 2D diffusion space for sodium ions and can be easily synthesized. However, Na_xMnO₂ still suffer from the obstacles, i.e. the serious capacity fading and bad rate performance because of ‘Jahn-Teller effect’ of Mn³⁺, and the low energy density as a consequence of the low voltage plateau of Mn³⁺/Mn⁴⁺ redox couple. Substitution Mn with other transition metal elements can be an effective way to overcome these problems ^[3].

In this work, the double metal (Cu and Mg) substitution is applied to enhance the electrochemical performance of P2-type Na_0.67MnO₂. The idea is to combine the advantage of inactive Mg ions (enhancing the stability) and active Cu ions (high voltage plateau of Cu²⁺/Cu³⁺) to improve electrochemical performance of the material. Compare to the un-substituted and single substituted materials, the co-substituted materials exhibits superior electrochemical performance, it can deliver the initial discharge capacity of 85 mAh g^-1 at 180 mA g^-1 with capacity retention of 93% after 500 cycles. Besides, Neutron diffraction, X-ray Absorption Structure (XAS) Spectroscopy and In-situ XRD are combined to obtain its structure information. It is found that the co-substitution of Cu and Mg can effectively suppress the multiple phase transitions during electrochemical process, and the structural stability is largely enhanced.

[1] M. D. Slater, D. Kim, E. Lee, C. S. Johnson, Advanced Functional Materials 2013, 23, 947.

[2] H. Pan, Y.-S. Hu, L. Chen, Energy & Environmental Science 2013, 6, 2338.

[3] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Chemical Society Reviews 2017.