Note Electrochemistry , (in press) The 64th special issue "Frontiers of Carbon Materials" Relaxation Analysis of Graphite Anode Materials after Charge-Discharge Cycles Shigeomi TAKAI, a, * Takashi KITAMURA, a Takeshi YABUTSUKA, a and Takeshi YAO b a Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan b Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan * Corresponding author: stakai@energy.kyoto-u.ac.jp ABSTRACT Relaxation analysis has been carried out on fresh and charge-discharge cycled graphites to evaluate the effect of cycle process on the relaxation behavior after lithium insertion. While the formed stage I at charging partly transforms into stage II during relaxation, the cycled samples exhibit smaller fraction of transformation. The charge-discharge cycles also restrict the relaxation variation in c-axis of stage II, even though lithium occupation ordering similarly occurs between the fresh and cycled samples. Variation in c-length of stage II would be the results of stage transformation from I into II, and charge-discharge process enables to follow the equilibrium stages rapidly at charging. © The Author(s) 2020. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium provided the original work is properly cited. [DOI: 10.5796/electrochemistry.20-64049]. Uploading "PDF file created by publishers" to institutional repositories or public websites is not permitted by the copyright license agreement. Keywords : Graphite, Stage Structure, Relaxation Analysis, Li-GIC 1. Introduction Graphite has been employed as the practical anode material for lithium ion batteries due to its sufficiently low voltage, high specific capacity and low cost. 1–3 The electrode reaction of graphite is based upon the lithium ion intercalation into graphene interlayers showing stage structures, where lithium ions typically occupy every n graphene interlayers at stage n. 4–6 On charging, the stage number of Li-GIC (lithium graphite intercalation compound) decreases toward stage I with the ideal composition of LiC 6 . 7,8 We have recently reported the structural relaxation behavior after the electrochemical lithium insertion into as well as extraction from graphite. 9 Just after the lithium insertion, the formed stage I with some lithium occupation defects partly changes into stage II within 50 hours of relaxation time, i.e. defective stage I changes into the ideal stages I and II as shown in Fig. 1(a). Moreover, stage II with the stacking of higher and lower lithium concentration interlayers orders into the ideal stage II as Fig. 1(b) during the relaxation. Essentially the similar relaxation phenomena have been observed even after the lithium extraction. Namely, lithium ions migrate in Li-GIC in rather random manner during the charging or discharging process, while they would occupy the graphene interlayers in ordered manner thorough the structural relaxation. The widely accepted model for lithium migration in Li-GICs to form the stage structure has been proposed by Daumas and Hérold (D-H model), 10 in which in-plane lithium diffusion in the graphene interlayer enables the formation of stage structured domains. In terms of this model, the rate of stage change is associated with the in-plane lithium migration, which might be attributed by the charge- discharge cycles prior to the experiments due to the formation of preferred path. On the other hand, concerning that the stage change is accompanied by the shifts of graphene layers, the charge- discharge process before the last charging might facilitate the stage change by providing flexibility of graphene layer or some defects in Li-GIC. However, most of studies on the effect of preceding charge- discharge cycles to the electrode properties addressed to the surface condition including SEI formation, and no structural relaxation of bulk Li-GIC seems not to be investigated, although few studies focusing on the defect formation of graphene have been reported. 11 In the present study, relaxation analysis has been carried out on the fresh and cycled graphite anode materials to clarify the contribution of charge-discharge process before the experiment to the relaxation phenomena. 2. Experimental Active material of natural graphite powder (LB-GC, Nippon Graphite) was mixed with PVdF in the weight ratio of 93:7 to form the working electrode by spreading on the copper foil current collector, followed by drying in vacuum. The lithium metal counter- electrode and 1 M LiPF 6 in EC/DMC solution (2:1 v/v, Kishida Chem.) were employed to fabricate the test cell (Hosen). The electrode has been charged with the current density of 0.1 C for 10 hours (nominal capacity) using a galvanostat (HJ-SM8, Hokuto Denko), which is referred as “1st charge” or “fresh sample” in this paper. Additional Li-GICs after charge-discharge cycles have been also prepared. Lithium ions were inserted and extracted up to the nominal capacity with the rate of 0.1 C for 1 time and 5 times with 1 minute of rest after every step, and finally lithium ions were inserted into the cycled electrode with 0.1 C rate for 10 hours like “1st charge”. These samples are presently referred as “2nd charge” and “6th charge” according to the number of charge process. Relaxation analysis study has been carried out as the previous reports. 12–17 After the termination of lithium insertion, the charged electrode with a copper current collector was immediately separated from the cells to prevent unfavorable local cell reaction, 18 and washed with EC and EC/DMC solvents followed by drying in an Ar gas filled glove box. The electrode was then mounted on a sealed holder to set on an X-ray diffractometer (Ultima IV, Rigaku) using CuKA radiation. The diffracted X-ray was counted in the 2ª range between 11° to 53° at a rate of 10° min ¹1 with 0.01° step. The measurements were carried out after various relaxation times. The Rietveld refinements were carried out using RIEVEC code 19,20 with one-dimensional calculation along c-axis assuming Electrochemistry Received: April 9, 2020 Accepted: May 28, 2020 Published online: July 3, 2020 The Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.20-64049 Advance Publication by J-STAGE i