Molten Salt Synthesis and Growth Mechanism of Single Crystal Ni-Rich Layered Oxides As Cathode Materials

Responding to the growing demand for high-energy lithium ion batteries (LIBs), it is crucial to develop advanced materials which offer higher energy density than currently available commercial materials. The class of Ni-rich layered oxide has attracted great interest owing to their prime advantage of high reversible capacity.[1] However, conventional polycrystalline cathodes suffer from intergranular cracking that occurs within secondary particles during repeated cycling, leading to the loss of contact between active materials and an increase in the exposed fresh surfaces to further parasitic reactions with electrolytes.[2-3] Consequently, this phenomenon results in a significant impedance rise and rapid deterioration in performance. In response to the challenge of intergranular cracking, the utilization of a single crystal morphology has been proposed as a promising solution. Studies have demonstrated that eliminating grain boundaries allows a single crystal cathode to maintain structural integrity, suppress the formation of intergranular cracks, and ultimately enhance cycle stability.[4-5] Nevertheless, ongoing debates persist regarding gas evolution and rate performances. One reason for this lies in the diverse characteristics of single crystal particles, such as particle size and shape, the degree of agglomeration, and cation disorder, all of which exert a complex influence on their properties. To effectively manage these features, a comprehensive understanding of the synthesis process and the identification of critical factors that affect the properties of single crystal Ni-rich cathodes are necessary. In this study, we elucidated the synthesis mechanism of single crystal LiNi0.95Co0.05O2 (SC95) using the molten salt synthesis (MSS) method, employing a combination of lithium hydroxide (LiOH) and lithium sulfate (Li2SO4) as a salt mixture. Through a comparative analysis of the MSS process against the solid-state synthesis, we closely examined the mechanisms governing crystal growth and morphology evolution during MSS. By conducting a comprehensive analysis, including in situ heating XRD, SEM, and EPMA, we demonstrated that the unique synthesis behavior of MSS stemmed from both the formation process of a layered structure and the grain growth of SC95 proceeded through a liquid-solid phase reaction. The maintenance of the salt mixture in a molten state during the reaction facilitated the uniform coverage of the precursor, resulting in a homogeneous reaction and effective dispersion of SC95. Based on these findings, we successfully prepared discrete micron-sized SC95 particles with monodisperse particle size distribution. The SC95 cathode exhibited superior cyclability over 150 cycles with a capacity retention of 89 % at a 1 C rate. This study provides valuable insights necessary for designing highly dispersed single crystal Ni-rich cathodes. References [1] N. Mohamed and N. K. Allam, RSC Advances 2020, 10, 21662. [2] S. Yin, W. Deng, J. Chen, X. Gao, G. Zou, H. Hou and X. Ji, Nano Energy 2021, 83, 105854. [3] T. Cheng, Z. Ma, R. Gu, R. Chen, Y. Lyu, A. Nie, and B. Guo, Energies 2018, 11, 2712. [4] J. Langdon and A. Manthiram, Energy Storage Mater. 2021, 37, 143-160. [5] J. Sun, C. Sheng, X. Cao, P. Wang, P. He, H. Yang, Z. Chang, X. Yue and H. Zhou, Adv. Funct. Mater. 2022, 32, 2110295. Figure 1