ABSTRACT
LiNi1-x-yCoxAlyO2 (NCA), a Nickel rich layered oxide, possessing nano micro hierarchical architecture, can deliver a high specific capacity of 220 mAh/g by increasing the upper cut-off voltage ≥ to 4.3V. However, voltage fade and poor cycling capabilities at high C-rates due to phase irreversibility and surface structure reconstruction pose a serious issue. In LiNi1-x-yCoxAlyO2, partial substitution of Co3+ and Al3+ at Ni sites enhances the structural integrity during electrochemical charge/discharge cycles. Specifically, Al3+ being electrochemically inactive in NCA minimizes the irreversible phase transition and protects the cell from thermal runaway. However, the segregation of Al is observed in NCA forming Al-rich secondary phases when α/β interstratified Ni0.8Co0.15Al0.05(OH)2 is used as Ni, Co, and Al precursor during solid-state heat treatment. The investigation of Aluminium distribution in NCA is extremely challenging. In the present work, the use of a combination of versatile techniques such as X-ray diffraction, energy-dispersive X-ray mapping, and vacuum Fourier transform infrared spectroscopy has been used to identify the distribution of aluminium in NCA.
Considering the segregation and high diffusion of Al in bulk oxide, Ni0.8Co0.135Al0.065O2 with concentration gradient structure having Ni-rich core and increased Al/Ni ratio shell has been synthesized by customized addition of Ni/Co/Al during co-precipitation. Varying the concentration of Al3+ through the nano aggregates retains the phase reversibility and preserves the surface structure. The formation of concentration gradient structure across the hierarchical structure has been analyzed by scanning electron microscopy, elemental analysis, X-ray diffraction and Fourier transform infrared spectroscopy. The graded NCA with high Al/Ni ratio at the surface shows excellent reversibility of the phase leading to low impedance, confirming the reduced surface reconstruction during the initial cycles. Therefore, the specific capacity of graded NCA is 65% higher than that of pristine NCA at 10C. Both in half cell and full cell configurations, the graded NCA exhibits superior first cycle reversibility and specific capacity. Specifically, in the full cell configuration, the capacity retention of graded NCA is 91.5% while that of pristine NCA is 83% after 150 cycles when cycled between 3 to 4.3V. The sustainability of phases in concentration graded and pristine NCA during charge/discharge was investigated under different controlled temperature conditions. The graded LiNi0.8Co0.135Al0.065O2 with higher Al concentration renders excellent reversibility of H2↔H3 phase transition even when cycled at higher temperatures (50°C, 60°C, and 70°C). Similarly, concentration graded LiNi0.8Co0.135Al0.065O2 could still maintain the reversible phase transitions of H1↔M and M↔H2 even at -10°C (sub-zero temperature). The pristine LiNi0.8Co0.135Al0.065O2 exhibits poor electrochemical performance when cycling at both high and low temperatures. Thus, the formation of a concentration gradient without Al segregation enabled the better electrochemical performance of graded NCA compared to pristine NCA.