Study on the synthesis–microstructure-performance relationship of layered Li-excess nickel–manganese oxide as a Li-ion battery cathode prepared by high-temperature calcination

Abstract

Spherical composite oxide Li_1.5Ni_0.25Mn_0.75O_2.5 (or 0.5Li₂MnO₃·0.5LiNi_0.5Mn_0.5O₂) powder as a Li-ion battery cathode has been synthesized by high-temperature calcination of a mixture containing Li₂CO₃ and (Ni_0.25Mn_0.75)CO₃, which is synthesized by a continuous co-precipitation method. The evolution in the microstructure of the oxide during calcination is studied mainly by transmission X-ray microscopy (TXM), complemented by X-ray diffraction and thermogravimetry, and it can be divided into three major stages. The first stage takes place below 400 °C, and is characterized by the decomposition of the transition metal carbonates to form amorphous oxides, leading to the formation of internal cracks due to extensive densification. The second stage occurs between 400 and 800 °C, and involves complete Li_1.5Ni_0.25Mn_0.75O_2.5 formation and the development of a unique radially-distributed pore structure. The third stage takes place above 800 °C and during prolonged heating at 900 °C, and is characterized by grain growth and change into a randomly distributed tortuous pore structure. TXM 3D-elemental analysis gives statistical evidence showing heterogeneity in the distributions of Ni and Mn, which causes capacity loss and might be a common problem encountered in the two-step precipitation–calcination process. The electrochemical performance of the resulting Li_1.5Ni_0.25Mn_0.75O_2.5 powder exhibits complex dependence on the microstructure. The radially distributed pore pattern and small grain size produced by moderate heating favor the rate performance of the composite oxide cathode by reducing charge-transfer resistance and enhancing apparent Li ion solid-state diffusivity. A large grain size resulting from prolonged heating, on the other hand, reduces the formation of the spinel MnO₂ domain upon de-lithiation of the Li₂MnO₃ component of the composite oxide.