(Invited) Novel Approaches for the Study of Local Processes in Rocksalt Oxyfluoride Cathodes (2020)

Cation-disordered rocksalt (DRX) lithium transition metal oxides have recently emerged as a new class of high energy density lithium-ion cathodes, but in most cases suffer from rapid performance degradation upon electrochemical cycling. Unlike layered lithium transition metal oxides, these DRXs are amenable to bulk fluorination (as demonstrated by 19 F NMR spectroscopy) and significant improvements in their electrochemical properties, especially cyclability, have been observed upon partial oxygen substitution by fluorine. 1 We present here our findings on the impact of fluorine doping on the local cation order and structural and electrochemical properties of several DRX cathodes, using a combination of solid-state NMR experiments and theoretical calculations. Specifically, the impact of F insertion on the distribution of Li and F species in the as-synthesized cathodes is evaluated, and changes in the local structure upon electrochemical cycling are monitored. Ex situ 19 F NMR spectra collected on cathode samples stopped at different stages of (dis)charge are particularly difficult to interpret, due to the presence of paramagnetic transition metals leading to very large shifts and broad resonances. In addition, the intrinsic disorder on the cation sublattice leads to a large number of F environments with similar shifts, resulting in broad and overlapping signals. To assist the interpretation of the NMR data, we use Monte Carlo simulations and ab initio calculations of the paramagnetic NMR parameters to determine the distribution of F environments in the materials and predict the chemical shift for these various F sites. Monte Carlo simulations clearly indicate short-range order in the as-synthesized materials, with the incorporation of F in octahedral anion sites with at least five Li nearest-neighbors, in good agreement with 19 F NMR results (see Fig. 1). Yet, first principles calculations of NMR parameters reveal that 19 F NMR signals from F nuclei directly bonded to the redox-active (and paramagnetic) transition metal species are too broad to be observed. 2 Hence, all experimentally-observed 19 F NMR signals arise from F sites that are not directly bonded to the paramagnetic metal. These initial insights clearly reveal the strength of our combined experimental and theoretical approach but also suggest that further insight into short-range order in these complex materials requires new experimental (NMR) approaches and complementary techniques. Figure 1. a) Distribution of F environments in the model Li 1.166 Ni 0.333 Ti 0.5 O 1.833 F 0.166 DRX structure obtained from Monte Carlo simulations at an equilibration temperature of 973 K. This distribution is used as a proxy for the distribution of F environments in the Li 1.15 Ni 0.45 Ti 0.3 Mo 0.1 O 1.85 F 0.15 material synthesized at 700°C. b) 19 F spin echo NMR spectrum obtained at 11.7 T and at a magic angle spinning (MAS) speed of 60 kHz. The observed 19 F signal arises from F nuclei with no Ni nearest-neighbor. Figures adapted from Clément et al. 2 References 1. Lee, J. K. Papp, R. J. Clément, S. Sallis, D.-H. Kwon, T. Shi, W. Yang, B. D. McCloskey & G. Ceder, Nat. Commun. , 8 , 981 (2017). 2. J. Clément, D. Kitchaev, J. Lee, and Gerbrand Ceder, Chem. Mater. , 30 , 6945–6956 (2018). Figure 1