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(Digital Presentation) Particle Size and Transition-Metal Chemistry Determine the Impact of Li-Excess on Disordered Rock-Salt Li-Ion Cathode Materials

The development of Co-free Li-excess disordered rock-salt (DRX) cathodes (e.g., Li1.2Mn0.4Ti0.4O2) for Li-ion batteries and interpretation through the percolation theory of Li diffusion have directed researchers to consider “Li-excess” (x > 1.1 in Li
 x
 TM
 2-x
 O
 2
 ; TM = transition metal) as being essential to achieving high performance [1]. While the percolation theory provided an important insight into Li-transport in DRX materials, it leaves a critical impression that those DRXs without Li-excess will not deliver high capacity and thus are unsuitable as Li-ion cathode materials. However, the majority of DRX materials are made into pulverized nanoparticles whose small particle sizes (diameter < 150 nm) should render Li transport easy to achieve even with slow intrinsic Li diffusivity, making us question if Li-excess is necessary for designing DRX cathodes [2]. Moreover, the introduction of Li-excess requires a high degree of oxygen redox (along with TM-redox) in the materials to store electrons during cycling, leading to various O-redox-related side reactions (e.g., O-loss) that trigger fast capacity/voltage loss.
 In this presentation, we show that Li-excess is not necessary for some DRX-cathodes demonstrated by Li1.05Mn0.90Nb0.05O2 (M90) and Li1.20Mn0.60Nb0.20O2 (M60), which both deliver high capacity (>250 mAh/g) regardless of their Li-excess level [3]. By contextualizing this finding within the broader space of DRX materials and confirming with DFT calculations, we reveal that the percolation effect is not crucial at the nanoparticle scale, which most DRX materials developed so far have assumed by having the pulverized nanoparticle morphology [2]. Instead, Li-excess is required to decrease the charging voltage of certain DRX cathodes, which otherwise would experience difficulties in charging due to their very high TM-redox potential. Our findings show the double roles of Li-excess – modifying the cathode voltage and improving Li diffusion – that must be simultaneously considered to understand the necessity of Li-excess for high-capacity DRX cathodes.
 [1] J. Lee & G. Ceder et al., Science 
 343, 519-522 (2014)
 [2] H. Li & J. Lee et al., Joule 
 6, 53-91 (2022)
 [3] J. Lee & J. Li et al., Adv. Energy Mater., 2100204 (2021)