Development of lithium ion kinetic, bottleneck for microscale particle of disordered rock-salt(DRX) cathode material by Cr doping

Abstract

With the increasing demand for electric vehicles (EVs), interest in affordable and high-energy-density batteries has been growing. To reduce battery costs while enhancing energy density, research on cathode materials is crucial. Among these, disordered rock-salt (DRX) has been proposed as a next-generation cathode material. It utilizes Mn as a redox center in a Li-rich environment and offers the application of various transition metals, providing advantages in both cost and energy density. However, the intrinsic Li+ diffusivity in DRX materials is typically low (DLi: 10-16–10-15 cm2/s) unless 0-TM percolation is organically established, as the Li-ion pathway is randomly formed. Therefore, conventional DRX materials have been utilized in nanometer sizes to minimize the Li-ion pathway. Attaining high capacity and rate capability with micrometer-sized Mn-DRX particles, which necessitate long-range Li diffusion, has not yet been accomplished. Instead, the synthesized particles typically undergo a pulverization process using a planetary ball mill or shaker mill, often combined with conductive carbon black, to create an Mn-DRX-carbon black composite. Following this process, the Mn-DRX particles are reduced to nano size (e.g., d < 200 nm), typically resulting in the broadening of XRD peaks. The DRX-carbon black composite powder is then mixed with binders to synthesize the Mn-DRX cathode film. So far, the most frequently used weight ratio between the Mn-DRX, carbon black, and binder in the cathode film has been 70:20:10, with a minimum carbon loading of 10 wt %, which is unacceptably high for practical cathodes. So, even though DRX has a high energy density per weight, it inevitably has a low energy density per volume. Such energy density per volume can be improved through sizing up; however, DRX requires higher Li-ion conductivity compared to the existing nanoscale materials to achieve this upscaling. Therefore, in this study, I introduce DRX material in a more practical environment by growing particle size (micro-scale) following the previous study of improving Li+ kinetic characteristics through Cr doping. Li1.2Mn0.4Ti0.4O2 (LMTO) and the Cr-doped material Li1.2Mn0.2Ti0.4Cr0.2O2 (LMTCO) are compared to evaluate the Li+ kinetic differences between the existing DRX material (LMTO) and the Cr-doped DRX material (LMTCO) in terms of size. As a result, the increase in capacity ratio due to the effect of Cr tetrahedral migration is minimal in the original nano-sized DRX materials but increased more significantly in the larger sub-micro-sized DRX materials. It demonstrated superior energy density per volume compared to the existing nanoscale materials, and this improved effect was more pronounced in the Cr-doped materials. This finding is supported by various analyses, suggesting that this strategy and design could allow for an approach to slightly larger sizes in DRX.