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Synthesis and Size Control of Single Crystal Ni-Rich Layered Oxide Cathodes Using Statistical Analysis-Guided Molten Salt Method

Rechargeable Li ion batteries have been widely applied in daily lives. The demand for Li ion batteries with higher energy density has been increasing especially with the thriving development of electric vehicles in the past decade. Li(Ni1−x−yMnxCoy)O2 (NMC) is one of the most successful commercial cathodes with various transition metal (Ni, Mn, and Co) ratios. Higher Ni content in the composition of NMC gives rise to higher energy density but at the price of structural and thermal stability. Recently, synthesizing single-crystal cathode particles has been extensively investigated as a strategy to enhance the stability of NMC cathodes. Most commercial NMC cathode particles possess polycrystalline structure, which is prone to induce intergranular cracking and parasitic reactions with the electrolyte upon cycling due to grain boundaries and large surface area. In contrast, single crystal NMC cathodes show higher cycling stability than their polycrystalline counterparts. However, the synthesis of single crystal normally requires higher temperatures and longer calcination duration than that of the polycrystalline counterparts for solid state synthesis. Alternatively, molten salt synthesis of single crystals is less energy-intensive and capable of tuning particle morphology and size. However, there is not yet a systematic guideline for the molten salt synthesis of single crystal Ni-rich NMC cathodes. Herein, we studied the effect of different synthetic parameters on the performance of single crystal Ni-rich NMC cathodes. We applied statistical analysis tools to correlate the synthetic conditions with cathode properties and optimized the synthetic parameters. Furthermore, we investigated the effect of single crystal size on the cathode performance. Our findings provided new insights into the synthesis and understanding of single crystal Ni-rich cathodes.