Mechanistic Understanding of LMR-NMC Synthesis via In Situ Characterization

Lithium- and manganese-rich nickel-manganese-cobalt oxides (LMR-NMCs) are a promising cathode material for lithium-ion batteries. Despite their advantages – low cost due to low Ni and Co content, and high specific energy – they face significant barriers to commercialization, including gas evolution, voltage fade, and poor rate capability¹.<br/><br/>However, every cathode material requires unique processing in order to optimize its properties and performance. For example, lithium iron phosphate (LFP) is commercially viable only when synthesized via procedures (e.g. sol-gel and hydrothermal synthesis) that mitigate its major flaws (i.e. low conductivity)². Analogous work to understand and modify the synthesis of LMR-NMCs may be necessary for their commercialization. In this study, we establish a deeper understanding of the LMR-NMC synthesis process, and ultimately propose mechanistically informed and scalable process modifications to mitigate degradation and improve performance.<br/><br/>Prior work on the synthesis of LMR-NMC materials has established that co-precipitation leads to higher performing materials than solid state or sol-gel synthesis routes³, and investigated the role of the Li:TM ratio and synthesis temperature in determining LMR-NMC performance^4,5. Yet work to understand and tune other aspects of LMR-NMC synthesis is lacking.<br/><br/>Here, using a commercially-optimized co-precipitated TM (transition metal) precursor, we present a detailed study of calcination of LMR-NMCs. Our suite of <i>in situ</i> characterization techniques, including thermogravimetric analysis (TGA), <i>in situ</i> x-ray diffraction (XRD), and <i>in situ</i> x-ray absorption spectroscopy (XAS), paints the most detailed picture to date of LMR-NMC synthesis. We identify key moments in this process that warrant more detailed characterization – e.g. before, during, and after a fast lithiation event. We freeze the synthesis at these points through rapid quenching, enabling characterization via ex situ techniques that include scanning electron microscopy, XRD, XAS, and scanning transmission x-ray microscopy.<br/><br/>Our <i>in situ</i> work suggests potential cost-saving and performance-improving modifications to LMR-NMC synthesis. For example, we identify thermally-driven changes to the TM precursor that involve no reaction with the Li source, which our work shows could be accomplished prior to mixing with the Li source. We further establish the relationship between Mn oxidation and lithiation, indicating modifications to oxygen activity may modify lithiation. Finally, we investigate the often-overlooked role of lithium source morphology in calcination, revealing that this parameter determines whether the reaction between TM precursor and Li source follows a solid-solid or solid-liquid pathway. We demonstrate the implications for LMR-NMC morphology and rate performance of each synthetic pathway.<br/><br/>Ultimately, our study suggests that modifications to LMR-NMC synthesis could enable the improved performance necessary for commercialization.<br/><br/>1. Manthiram, Arumugam. “A reflection on lithium-ion battery cathode chemistry.” <i>Nature communications </i>11.1 (2020): 1-9.<br/>2. Satyavani, T. V. S. L., A. Srinivas Kumar, and PSV Subba Rao. “Methods of synthesis and performance improvement of lithium iron phosphate for high rate Li-ion batteries: A review.” <i>Engineering Science and Technology, an International Journal</i> 19.1 (2016): 178-188.<br/>3. Menon, Ashok S., et al. “Synthetic Pathway Determines the Nonequilibrium Crystallography of Li-and Mn-Rich Layered Oxide Cathode Materials.” <i>ACS Applied Energy materials</i> 4.2 (2021): 1924-1935.<br/>4. Hua, Weibo, et al. “Structural insights into the formation and voltage degradation of lithium- and manganese-rich layered oxides.” <i>Nature communications</i> 10.1 (2019): 1-11.<br/>5. Pimenta, Vanessa, et al. “Synthesis of Li-rich NMC: a comprehensive study.” <i>Chemistry of Materials</i> 29.23 (2017): 9923-9936.