Thermodynamic Analysis of Li-Intercalated Graphite by First-Principles Calculations with Vibrational and Configurational Contributions

• Published in: Journal of physical chemistry. C 2021-12, Vol.125 (51), p.27891-27900
• Main Authors: Haruyama, Jun, Takagi, Shigeharu, Shimoda, Keiji, Watanabe, Iwao, Sodeyama, Keitaro, Ikeshoji, Tamio, Otani, Minoru
• Format: Article
• Language: English
• Subjects: C: Energy Conversion and Storage
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.1c08992

Li-ion batteries require quantitative analysis of the crystal structure change in the graphite electrode during charge/discharge reactions to improve their performance. Herein, we investigated the thermodynamically stable phases of Li x C6 and their structure transitions by first-principles free-energy calculations including the vibrational free energy and configurational entropy. We considered the in-plane configurations of six structures with different Li concentrations in the Li layer and the interlayer configurations of AA, AB, and mixed stacks for eight stages. The contributions of the vibrational free energy and configurational entropy were estimated to be less than −20 meV/Li x C6 for 0 ≤ x ≤ 1/3. The formation free energy predicts that the AA-Li/9C-s2 structure with x = 1/3 (LiC18) and AB-stack phase with 0 ≤ x ≤ 0.05 (C–LiC120) are stable. The formation free energy also suggests the stable phase of the mixed-stack structures for intermediate Li compositions of 0.05 < x < 1/3 (LiC120–LiC18). The AB-stack–mixed-stack transition at x ≈ 0.05 obtained herein is consistent with recent X-ray diffraction observations. We also discuss the relation between the computational entropy change in Li x C6 and the results of electrochemical measurements, which are in quantitative agreement, especially with the entropy jump at x ≈ 0.05.