Elucidation of the Li Storage State and Dynamics in Various Carbon Electrodes by Using 7Li NMR
Authors: Okabe Jugo , Fang Yuan , Moriguchi Isamu , Furo Istvan 
Published: December 2024

In recent years, both soft carbons and hard carbons have been considered as potential anode materials for lithium-ion batteries (LIBs) and lithium-ion capacitors (LICs) because of their high capacity and superior rate performance compared to graphite electrodes. To design high power carbon anode, revealing the Li storage state and dynamics in carbon electrodes is required. However, the effect of carbon microstructure on the Li storage state remains unclear because of the complex microstructures of these carbon materials, which consist of a mixture of graphitic and amorphous domains. 7Li NMR is a useful measurement to know the Li storage state because it provides direct observation of Li atoms in carbon microstructures and has the advantage of seeing the dynamics of Li without the process of de-solvation on the electrode-electrolyte interface, compared to electrochemical measurements. In this study, we investigated the Li storage state in soft and hard carbons with different graphitization degrees using 7Li NMR. We obtained information about the Li species, domain size of Li sites, and motion properties by comparing temperature-dependent NMR spectra and spin relaxation time.
Graphite, soft carbon (SC), and hard carbon (HC), as well as their graphitized samples by heat treatment at 1500, 2000, and 2500°C, denoted as SCX or HCX (X: Heat treatment temperatures/°C, HTTs), were prepared. The crystallinity of the carbon materials was evaluated using X-ray diffraction (XRD). These carbon samples were electrochemically lithiated by applying a constant current of 50 mA/g to 0.002 V vs. Li/Li+, followed by keeping the potential until a cut-off current of 5 mA/g. The lithiated carbon samples were characterized using 7Li NMR spectroscopy and spin relaxation measurements at different temperatures (103-353 K). The Li self-diffusion coefficient in SC at 343 K was measured by pulsed gradient spin-echo (PGSE) NMR.
The 7Li NMR spectrum of lithiated SC was assigned to Li in the turbostratic structure. As the SC heat-treatment temperature increased, the NMR spectra corresponding to the shrinkage of the carbon layer spacing were obtained. The Li in SC2500 was predominantly stored in a graphitic structure and partially in amorphous structures. On the other hand, the Li state in HC showed intercalated Li in the turbostratic layers and quasi-metallic Li clusters. As the HC heat-treatment temperature increased, the peak of quasi-metallic Li clusters disappeared, and the peak of Li in the turbostratic structures remained. Only HC2500 showed a small peak assigned to Li in the graphitic structure, indicating that HC2500 has a distinct graphitic layer structure, as measured by XRD. The average diffusional displacement in the measurement timescale was estimated to be approximately 1 nm below 200 K 1. The chemical exchange occurring at the temperature suggests that the domain size of the Li sites are a few nanometers.
The spin-lattice relaxation time (T1) was measured at different temperatures, and the activation energy of each Li species for motion in carbon electrodes were estimated. Li species intercalated in the turbostratic layers indicated similar activation energy for all amorphous carbons, providing an evidence that these Li species in various carbon electrodes have similar restriction and motion properties. Li in graphitic structures on HC2500 showed higher activation energy than amorphous structures and is considered unsuitable for high-rate charge-discharge performance.

Y. Fang, K. Peuvot, A. Gratrex, E. V. Morozov, J. Hagberg, G. Lindbergh, and I. Furó, J. Mater. Chem. A, 10, 10069–10082 (2022).

Figure 1