Robot-Assisted Sol-Gel Exploration of Cation-Disordered Rocksalt (DRX) Materials for Li-Ion Batteries

Rechargeable Li-ion batteries are omnipresent in our daily lives and demand is increasing. To satisfy the tremendous growth, there is a great interest in developing novel high-performance electrode materials at reduced cost, and nickel and/or cobalt free.[1] In this regard, Li-excess cation-disordered rocksalt (DRX) metal oxides have been identified as promising cathode materials with energy densities that can exceed traditional layered cathodes such as LiTMO₂ with TM being a combination of transition metals.[1,2] Some drawbacks of these DRX materials include significant first-cycle capacity loss, limited cycle life, and voltage fade.[1] Several factors have been identified to mitigate these drawbacks, including greater disorder of the TMs as well as controlled particle size and shape.[3] Each of these characteristics can be manipulated by modifying the choice of precursors and synthesis techniques.<br/>Conventionally, solid-state synthesis is employed for the fabrication of electrochemically active materials including Li-ion batteries.[4] The typical sequence of synthesis steps includes mixing, grinding, pelletizing, and annealing. Such reactions are driven by solid-state diffusion, which is predominantly controlled by time and temperature.[4] In comparison, the sol-gel approach to synthesis mixes elements at the molecular level by dissolving precursors in a solvent (e.g. water) with the addition of chelating agents (e.g. citric acid) to form a viscous gel. This is followed by solvent evaporation, gel decomposition, and annealing to form powders or thin films.[4] In this study, we compare the nucleation and crystallization pathway of Li-Mn-Ti oxide (LMTO) deposited both by solid-state as well as sol-gel synthesis. Using automated, robot-assisted synthesis approaches, we systematically investigate the parameter space of both syntheses and the influence of precursors and precursor ratios on phase purity, final composition, oxidation states, reaction temperatures, and sample uniformity.<br/>In the cathodes fabricated by solid-state synthesis, we find strong competition between two intermediate spinel-like phases, LiTM₂O₄ and Li₂TMO₃, depending on the oxidation state of the TMs as well as the Li-TM ratio in the selected precursors. While the intermediates forming in cathodes deposited via the sol-gel method appear to be less sensitive to these parameters, we find a strong dependency on the solvents and chelating agents used for fabrication. Additionally, we observe a strong interdependency between the choice of solvents and the ratio of Li:TMs influencing the reaction temperature of target phases and intermediates. Based on these findings, we will propose reaction pathways as well as a nucleation model for LMTOs prepared by sol-gel synthesis.<br/><br/>References:<br/>[1] J. Zheng <i>et al.</i>, <i>Adv. Energy Mater.</i>, vol. 7, no. 6, p. 1601284, 2017.<br/>[2] Z. Lun et al., Nature Materials, vol. 20, pp. 214-221, 2021.<br/>[3] J. Zheng <i>et al.</i>, <i>Nano Lett.</i>, vol. 14, no. 5, pp. 2628–2635, May 2014.<br/>[4] T. N. L. Doan et al., <i>Front. Energy Res.</i>, vol. 2, 2014.