A Case Study of\nan Energy Barrier in Li-Ion Battery\nCathode Material Using DFT and Post-HF Approaches

With the ever-increasing interest\ntoward energy storage materials,\nan accurate understanding of the underlying physicochemical processes\nbecomes mandatory for enabling accurate and predictive simulations.\nIn this study, we apply multilevel quantum chemical calculations on\na benchmark material commonly adopted as a cathode in Lithium batteries,\nLi<sub>0.5</sub>Co<sub>0.5</sub><sup>+3</sup>Ni<sub>0.5</sub><sup>+4</sup>O<sub>2</sub>. We estimate the Lithium hopping barrier, a key quantity\nfor the estimate of Li diffusion coefficient, at different levels:\nHartree–Fock (HF), density functional theory (DFT), periodic\nlocal Møller–Plesset perturbation theory of second order,\ncomplemented with a coupled cluster correction evaluated using an\nembedded-fragment approach. The post-HF methods were used here not\nonly for benchmarking the key quantities themselves but also for assessing\nthe accuracy of different functionals and probing the influence of\nthe long-range and static correlation. For the given system and quantity\nin question, we observe that obtained results do not significantly\nvary across different DFT functionals or post-HF methods, which is\nrather uncommon. Such an agreement between the employed methods suggests\nthat static correlation, even if prominent in this system, cancels\nout in the studied energy differences. In fact, the values of the\nT1 diagnostics, which test the reliability of the single-reference\ndescription, do vary from one fragment to another. But for certain\nfragments they are fairly small and of similar magnitude, indicating\nthe applicability of such fragments for the correction. Our best estimate\nof the reaction barrier is about 0.85 eV.