Constructing a surface spinel layer to stabilize the oxygen frame of Li-rich layered oxides

Abstract

With the explosive growth of the electric energy storage industry, it is urgent to develop cathode materials with wide working voltage, high capacity, and low cost. Li-rich layered oxides have been in focus because they work stably in a voltage range of 2.0–4.8 V, deliver a reversible capacity of >300 mA h g⁻¹ and can be Co-free. Nevertheless, the low initial Coulombic efficiency, inferior rate capacity, and rapid fading of the voltage and capacity stand in the way of their large-scale applications. Herein, we use citric acid to treat the surface of Li-rich layered materials. Citric acid not only dissolves the residual lithium compounds but also constructs a surface spinel layer with high crystallinity. In turn, the spinel layer protects the layered cathode from direct exposure to the electrolyte, improves the Li⁺ diffusion coefficient and suppresses the irreversible oxygen release. As a result, the citric acid-lithium rich sample possesses ultrahigh specific capacity (260 mA h g⁻¹, while the pristine sample only possesses 207 mA h g⁻¹) and initial Coulombic efficiency (>90%, 16% higher than that of the pristine sample). Even after 200 cycles, the citric acid-lithium rich sample possesses a high capacity of 202 mA h g⁻¹ with a capacity retention of 81.7%. Furthermore, the voltage decay rate of the citric acid-lithium rich sample from the 10th to 200th cycle is nearly 1.42 mV per cycle, while that of the pristine sample is up to 2.06 mV per cycle. Briefly, this work provides a way to construct a surface stable oxygen frame, which is of great significance to the practical application of Li-rich layered oxide cathodes.