Speaker
Description
The recent surge of Li-ion batteries has triggered an increased interest to investigate Na-ion battery materials [1,2], because Na is more abundant than Li, resulting in lower material costs. Although many Na transition metal oxides are available as a cathode material for the Na-ion battery, there is, at present, no suitable anode material [1]. The most common anode materials for the Li-ion battery are not compatible for the Na-ion battery, because graphite is electrochemically inactive in an intercalation and deintercalation reaction of Na$^+$ ions.
Since non-graphitizable carbon (hard carbon) is electrochemically active as a Na insertion host, hard carbon is heavily investigated as an anode material for the Na-ion battery. However, the relationship between the structure of hard carbon and dynamics of Na insertion is still not fully clarified despite huge efforts in the past decade [2]. We have therefore attempted to study the microscopic structural nature of sodium intercalated hard carbon (NaC$_x$) with small-angle neutron scattering (SANS) and the dynamics of Na diffusion in NaC$_x$ with muon spin rotation and relaxation ($\mu^+$SR).
The transverse field $\mu^+$SR measurements on NaC$_x$ clearly showed a motional narrowing behavior above around 150 K, which indicates that Na+ starts to diffuse above 150 K. The zero field and longitudinal field measurements clarified the presence of the two muon sites ($\mu$1 and $\mu$2). Since the Na concentration around the $\mu$1 site is higher than that around the $\mu$2 site, the $\mu$1 site locates in the graphene layer and the $\mu$2 site in the amorphous region [3]. At the presentation, we will also discuss the results of SANS on NaC$_x$.
[1] N. Yabuuchi et al., Chem. Rev. 114, 11636-11682 (2014).
[2] K. Kubota et al., Chem. Mater. 32, 2961-2977 (2020).
[3] K. Ohishi et al., ACS Phys. Chem. Au 2, 98-107 (2022).
