Estimation of the discharge capacities and lithium-ion storage site of pitch-derived hard carbon obtained by different heat-treatment temperature

Hard carbon (HC) is one of the most promising materials for the negative electrodes of lithium (Li)-ion batteries, as it shows unique electrochemical performance. HC has a structure with small crystalline size and a higher discharge (delithiation) capacity than graphite. However, because of its complex structure, the quantitative relationship between the discharge capacities and Li-ion storage sites was not clear though it is important to realize HC with higher capacity. In this study, heat-treatment temperature was adopted to control HC structure. The structural and compositional parameters of pitch-derived HC, such as the lattice constant, crystalline size, pore size, and hydrogen (H) content were closely analyzed. Then, the amounts of Li stored in pores and interlayer spaces, and at the edge H of HC were estimated as the discharge capacities for each site. The estimated discharge capacities corresponded to the experimental ones at 0–0.2 V, 0.2–0.9 V, and 0.9–2.5 V, respectively. Based on this estimation, it was suggested that the suitable pores for Li storage are relatively large, and especially are pores larger than 1.2 nm. The quantitative relationship between the discharge capacities and Li-ion storage sites revealed in this study will play a guiding role in improving the electrochemical properties of HC. • Structural and compositional parameters of hard carbon (HC) were analyzed. • The experimental Li discharge capacities were divided into three regions. • They corresponded to capacities of pore, interlayer spaces, and edge H sites of HC. • Suitable pores to store Li was suggested to be larger than 1.2 nm.