Individual Effects of Flux Species as a Reaction Field on Coprecipitation Precursor toward the Design of Fine, Mono-Dispersed LiNi0.5Co0.2Mn0.3O2 Single Crystals

High-output lithium-ion batteries that have excellent high-rate capacity and durability are indispensable for high-energy devices. LiNi0.5Co0.2Mn0.3O2 (NCM523) is a representative active material and is typically used as a secondary particle. However, the practical use of NCM523 in high-output applications remains challenging, owing to insufficient capacity and cycle durability resulting from grain boundary resistance and cracks. Mono-disperse, fine particle characteristics are used for crystal designs for high-output performance. The single-crystal growth of NCM523 has been introduced using various methods; however, these methods primarily focused on NCM523 itself. Therefore, a guideline for crystal control, focusing on particle size and aggregation nature, is still necessary to achieve the optimal NCM523 design for an electrode structure. Precursor choice and reaction field design are important factors to consider to achieve mono-dispersed fine crystals. In this study, we focused on coprecipitation hydroxides as a precursor and molten flux as a reaction field and we studied the effect of fluxes on NCM523 crystal growth. We observed that flux species provide a unique contribution to the growth manner of NCM523. For example, borate-based flux disaggregated secondary particles, whereas chloride-based flux developed the crystal face in primary particles. Some fluxes provided mono-disperse NCM523 crystal particles with a size of less than 1 μm, which exhibited a 1st capacity over 110 mAh·g–1@5C and 100th capacity over 120 mAh·g–1@1C as high-output characteristics. The synergistic effect of the coprecipitate and flux on crystal growth was interpreted based on the growth manner observations. The results show that the size of NCM523 single crystals can be finely controlled in the submicron to several micron range. Coprecipitates can be made at many metal ions, including Mn, Co, and Ni; therefore, our suggested flux guideline for crystal design can be applied to other battery materials, such as NCM and LiCoO2. This insight contributes to the development of high-power battery electrodes based on particle-morphologic design.