An investigation of the lattice parameter and micro-strain behaviour of LiMn2O4 coated with LiMn1.5Ni0.5O4 to attain high-rate capability and cycling stability

This research provides new insights on the lattice parameters, development of micro strain, and electrochemical properties of uniquely-designed core-shell structure of the LiMn2O4@LiMn1.5Ni0.5O4 composite. The investigation of lattice expansion and contraction during cycling, and evolution of micro strain, provided a better mechanistic understanding of Li+ diffusion. The LiMn2O4 (LMO) was successfully interfaced with LiMn1.5Ni0.5O4 (LMNO) to modify the surface of spinel LiMn2O4 particles through solution combustion and Pluronic polymer assisted composite synthesis. The unique synthesis approach significantly enhanced the rate capability. The LMO@LMNO composite was characterized using XRD, XPS, TEM, CV and charge/discharge characterization. The material characterization results confirmed that the LMO@LMNO composite had slightly larger lattice parameter (8.24027 Å) compared to the pristine LMO (8.2402 Å) and LMNO (8.18511 Å). The micro-strain of the composite is 2.1 × 10−3 μm/m (LMO@LMNO); this is higher than the individual components 1.28 × 10−3 μm/m (LMO), and 1.22 × 10−3 μm/m (LMNO). As expected, the micro-strain of the composite increased during composite formation which induces defects and microscopic stresses. The LMO@LMNO sample exhibited superior electrochemical properties at all current rates as compared LMO. The LMNO delivered a low capacity for high current rates compared to LMO and LMO@LMNO samples; this was attributed to the unique synthesis approach. The first cycle capacities of LMO, LMNO, and LMO@LMNO, were 101 mAh g−1, 125 mAh g−1, 123 mAh g−1 (0.2C). As expected, the discharge capacities for LMO, LMNO, and LMO@LMNO reduced after 120 cycles to 95 mAh g−1 (94 %), 122 mAh g−1 (97.3 %) and 119 mAh g−1 (96.2 %), respectively. In brief, composite LMO@LMNO core-shell structure exhibited superior electrochemical properties due to the higher Ni ratios coupled with the formation a surface layer.