Cathode materials with high energy density for lithium-ion batteries are highly desired in emerging applications in automobiles and stationary energy storage for the grid. Lithium transition metal oxide with concentration gradient of metal elements inside single particles was investigated as a promising high-energy-density cathode material. Electrochemical characterization demonstrated that a full cell with this cathode can be continuously operated for 2500 cycles with a capacity retention of 83.3%. Electron microscopy and high-resolution X-ray diffraction were employed to investigate the structural change of the cathode material after this extensive electrochemical testing. It was found that microstrain developed during the continuous charge/discharge cycling, resulting in cracking of nanoplates. This finding suggests that the performance of the cathode material can be further improved by optimizing the concentration gradient to minimize the microstrain and to reduce the lattice mismatch during cycling.

A novel concentration-gradient Li[Ni 0.83 Co 0.07 Mn 0.10 ]O 2 cathode material was successfully synthesized via co-precipitation, in which the core Li[Ni 0.9 Co 0.05 Mn 0.05 ]O 2 was encapsulated completely with a stable concentration-gradient layer having reduced Ni content. The electrochemical and thermal properties of the concentration-gradient Li[Ni 0.83 Co 0.07 Mn 0.10 ]O 2 were studied and compared to those of the core Li[Ni 0.9 Co 0.05 Mn 0.05 ]O 2 material alone. The concentration-gradient material had a superior lithium intercalation stability and thermal stability compared to the core material. The high capacity was delivered from the Ni-rich core Li[Ni 0.9 Co 0.05 Mn 0.05 ]O 2 , and the improved thermal stability was achieved by the Ni-depleted concentration-gradient layer with outer surface composition of Li[Ni 0.68 Co 0.12 Mn 0.20 ]O 2 . The concentration-gradient materials open a new era for the development of advanced Li-ion batteries with high energy density, long cycle life, and improved safety.

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