First-Principles Analysis for Phase Stability of Li-Intercalated Graphite in Li-Ion Battery

Abstract

Introduction The rechargeable Li-ion battery (LIB) is a successful energy storage device because of its high energy density and long cycle life. In order to improve its performance, quantitative understanding of elementary reactions in the LIB such as the reaction of Li intercalation from electrolyte solution into graphite and crystal structure change in graphite during the reaction must be a great help. Structure change of Li-intercalated graphite during the charge/discharge processes was recently observed by operando X-ray diffraction measurements using a synchrotron radiation at SPring-8.1 The operando XRD data at LiC18 charge/discharge state exhibited two peaks corresponding two different C-C grid distances in the same plane of graphite, while no change in interlayer distance was detected during the appearance and disappearance of these peaks; it implies only in-plane structure transition such as Li/6C layer to Li/9C layer changes.1 An AA to AB stack transition in graphite was characterized by (101) peak disappearance in the XRD data at the composition between LiC63–LiC72.2 To interpret and discuss these experimental results, we conduct first-principles calculations for the phase stability of graphite. Methods and Models The formation enthalpy ∆H f of Li-intercalated graphite is defined as∆H f (Li n C m ) = ( E DFT(Li n C m ) − nE DFT(Li) − mE DFT(C) ) / (m+n) (1),where E DFT are the total energies of Li-intercalated graphite Li n C m , simple metal Li, and AB stacked graphite C from density functional theory (DFT) calculations. Formation enthalpy as a function of C-ratio, x C = m / (m + n) (2),was used to investigate the phase stability of Li n C m . Quantum ESPRESSO with a plane-wave basis3 and van der Waals functional4 was used to obtain DFT energies. Our previous study using this functional well reproduced the experimental lattice constants and electromotive forces (EMFs) of Li-intercalated graphite.5 It is known that Li-intercalated graphite exhibits stages from 1 to 8. These stages may take different combination of in-plane configurations and interlayer configurations. In-plane configurations considered are Li/6C, Li/9C, Li/18C, and Li/24C, which means the ratios of Li/C atoms are located on a single layer as shown in Fig. 1 (a). Three interlayer structures are considered here; (1) graphite layers stacked with AA overlap, (2) with AB, and (3) mixed (Li including layers are AA stacked, the other layers are AB stacked). Conventional LiC6 intercalated phase corresponds to the stage 1 of Li/6C AA stack structure (denoted as s1-Li/6C-AA, here s1, s2, s3, ... represents stage 1, stage 2, stage 3, ..., respectively). Results The calculated formation enthalpies ∆H f are plotted as a function of x C in Fig. 1 (b). At the composition of LiC6 (x C = 0.857), LiC12 (x C = 0.923), and C (x C = 1), respective most stable phases are s1-Li/6C-AA, s2-Li/6C-AA, and AB stack graphite ones as we could expect. At the composition of LiC18 (x C = 0.947), s2-Li/9C-AA structure is more stable than the s3-Li/6C-AA structure, which is consistent with the experimental observation.1 AB stack structures are more stable than AA at x C > 0.97 from the theoretical calculation, while the AA-AB transition has been observed at 0.984 < x C < 0.986 by experiments. At higher stages (x C > 0.94), mixed stack structure is predicted to be more stable than simple AA and AB structures. The formation enthalpies of s2-Li/9C-mixed and s3-Li/6C-mixed are almost the same (within 0.01 meV/atom) and these configurations are predicted to be the most stable at LiC18. However, these mixed stackings have not been detected by experiments. In the presentation, we will also present the theoretical EMF values of the Li-intercalated graphite electrode and compare them with experimental results. Acknowledgments This work was supported in part by the Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 Project (RISING2) administrated by the New Energy and Industrial Technology Development Organization (NEDO). References S. Takagi, K. Shimoda, H. Fujimoto, H. Kiuchi, T. Naka, T. Murata, K.-I. Okazaki, T. Fukunaga, E. Matsubara, Z. Ogumi, and T. Abe, “Operando Structure Analysis of Graphite Electrode by Synchrotron Radiation Diffraction” The 60th Battery Symposium, 1B20 (2019).H. Fujimoto , S. Takagi , H. Kiuchi , K. Shimoda , K.-I. Okazaki, T. Murata , T. Abe, Z. Ogumi, and E. Matsubara, “Operando analysis of charge/discharge reaction mechanism of graphite anode of Li ion battery using synchrotron radiation diffraction, Second report”, The 59th Battery Symposium, 2E16 (2018).P. Giannozzi et al., J. Phys.: Condens. Matter 21, 395502 (2009); ibid. 29, 465901 (2017).I. Hamada, Phys. Rev. B 89, 121103(R) (2014).J. Haruyama, T. Ikeshoji, and M. Otani, J. Chem. Phys. C 122, 9804 (2018). Figure 1