2.1. Material Preparation and Characterization

All the chemicals in the analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd., Wuhan, China. All of them were used without further purification. The LiCoO₂–LiFeO₂ heterostructure composite (abbreviated as LCF) was synthesized via a facile hydrothermal method. Typically, 0.02 M lithium acetate was dissolved in deionized water. Then, 0.01 M cobalt nitrate and 0.01 M iron nitrate in the same volume were added, following magnetic stirring. Subsequently ammonium hydroxide with a concentration of 2 M, as precipitator, was dropwise added to pH up to 9–10. After stirring for 2 h, the solution was transferred into the Teflon reactor at 120 °C for 4 h. The precipitate was filtered and washed several times by distilled water, followed by drying at 120 °C to obtain the precursor and then calcined at 720 °C for 2 h to obtain the final LiCoO₂–LiFeO₂ composite. To clarify the effect of composition, the LiCoO₂–LiFeO₂ composites with various weight ratios of 8:2, 5:5, and 2:8, denoted as LCF2, LCF, and LCF8, respectively, are designed and synthesized in the same procedure.

The preparation process of samarium doped ceria (SDC) has been reported in a previous publication [22]. The commercial nickel cobalt aluminum–lithium oxides (Ni₀.₈Co₀.₁₅Al₀.₀₅Li-oxide, NCAL) used in this study, were purchased from Tianjin Bamo Sci.&Tech. Joint Stock Ltd., Tianjin, China.

The structure of the as-synthesized LCF sample was detected by X-ray diffraction (XRD, D-max-2500 X-ray diffractometer, Japanese Rigaku Corp., Tokyo, Japan) with filtered Cu–Kα radiation. The data were recorded in the 2 theta range of 20–90° and analyzed using Jade 6.5 software. The morphology of the LCF sample was characterized on a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS), transmission electron microscope (Hitachi HF-7700, Tokyo, Japan) and the high-resolution transmission electron microscope (Titan ETEM G2, Hillsboro, OR, USA).