Electronic structure and NMR spectra of sodium clusters in sodium ion batteries

In recent years, due to the resource and cost issues of lithium in lithium-ion batteries (LIB), there has been growing interest in resource-rich sodium-ion batteries (NIB) with equivalent electrode potential. Among them, NIBs that use Na metal oxide as the positive electrode and hard carbon (HC) as the negative electrode has attracted attention in particular. To develop a NIB with high capacity, high efficiency, long life, and acceptable safety, it is essential to elucidate the state of the sodium ion and the mechanism of charge and discharge on the electrode. The states of sodium electrochemically inserted in HC samples have been experimentally reported using solid ²³Na-NMR ^[1].In this study, we use Na clusters in HC pores as a model and discuss the dependence of chemical shift on Na clusters, the dependence of 3s orbitals on electron density, and the effect of relativistic effects on chemical shift. DFT calculations were performed at the B3LYP/6-31G(d) level using Gaussian 16 and at the B3LYP/6-31G(d) level using ADF.As a result, the value of the chemical shielding depends on the interaction between Na size of clusters from NMR analysis, and the effect of relativistic effects must be taken into account.

[1] R. Morita, et al., Carbon, 2019, 145, 71