Novel Copper-Containing Layered Oxide Cathode for Room-Temperature Stationary Sodium-Ion Batteries
Authors: Hu Yong-Sheng , Mu Linqin , Xu Shuyin , Li Yunming , Chen Liquan 
Published: February 2020

With the tremendous development of renewable energies such as solar and wind powers, the smooth integration of their energies into the grid, thus improving the grid reliability and utilization, critically needs large-scale energy storage systems with long-life, high efficiency, high safety and low cost. Among the various energy storage technologies, electrochemical approach represents one of the most promising means to store the electricity in large-scale because of the flexibility, high energy conversion efficiency and simple maintenance. Due to the highest energy density among practical rechargeable batteries, lithium-ion batteries have been widely used in the portable electronic devices and would undoubtedly be the best choice for the electric vehicles. However, the rarity and non-uniform distribution of lithium in the Earth’s crust (0.0065%) may limit their large scale application in renewable energy. In this regard, room-temperature sodium-ion batteries with lower energy density compared with lithium-ion batteries have been reconsidered particularly for such large-scale applications, where cycle life and cost are more essential factors than energy density owing to the abundant sodium resources (2.75%) and potentially low cost as well as similar “rocking-chair” sodium storage mechanism as lithium1-11.

Searching for suitable electrode materials to satisfy the long-term stability requirement is an important step to realize the large-scale energy storage. Recently, many layered Na
x
MO2 (M: 3d transition metals) oxides have been proposed as positive electrode materials for sodium-ion batteries. Amongst them, in general, only layered oxides containing Ni or Co transition metal show promising Na storage performance in terms of high storage capacity, high rate capability and long cycling stability. However, Ni and Co are toxic and their oxides are relatively expensive, which would certainly increase the cost of the battery and is unfavorable for large-scale energy storage applications. Herein, we found that Cu2+/Cu3+ redox couple in such layered oxides is electrochemically active and highly reversible in sodium-ion batteries12. Take P2-Na0.68Cu0.34Mn0.66O2 as the first example, this material shows a reversible capacity of ca. 70 mAh/g with an average storage voltage of 3.8 V vs. Na+/Na. To the best of our knowledge, this is the first time to realize the reversible change of Cu2+/Cu3+ redox couple with high Na storage voltage and small polarization. Copper is harmless, and is already very common in our daily life. In addition, the cost of copper oxide is only half of that of nickel oxide. Based on this important finding, it is possible to use copper to design new layered oxides with similar Na storage performance as that of Ni or Co containing layered oxides. Therefore, we further optimize a series of air-stable NaaDxMnyFezCu1-x-y-zO2 (D(Dopant): Mg, Al, etc.) layered oxides13,14. The preliminary results are very promising and will be presented in this talk (Figure 1).

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Figure 1. The typical charge/discharge curves of the air-stable NaaDxMnyFezCu1-x-y-zO2 (D: Mg, Al, etc.) positive electrodes (a) and full cell using hard carbon as the negative electrode (b).

Figure 1