Abstract
Spherical composite oxide Li1.5Ni0.25Mn0.75O2.5 (or 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2) powder as a Li-ion battery cathode has been synthesized by high-temperature calcination of a mixture containing Li2CO3 and (Ni0.25Mn0.75)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 Li1.5Ni0.25Mn0.75O2.5 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 Li1.5Ni0.25Mn0.75O2.5 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.
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