Relaxation Stage Analysis of Li Inserted Graphite By Means of One-Dimensional Rietveld Method

Introduction

 Recently, we have conducted the relaxation analysis for various electrode materials such as γ-Fe₂O₃[1-2], LiMn₂O₄[3], LiFePO₄[4], LiCoO₂[5] after termination of electrochemical Li insertion/extracttion. Relaxation analysis makes transition of electrode material from kinetic state to equilibrium state clear.

 Graphite is widely used as a negative electrode material for lithium ion rechargeable batteries. When lithium ion is inserted into graphite, various lithium-graphite intecalation compounds (Li-GICs) are formed. Previously Yao et al. synthesized Li-GICs and analyzed the layered structures crystallographically by the one-dimensional Rietveld method[6].

 Recently, we inserted lithium into graphite electrochemically and investigated the relaxation process by means of the XRD [7].

 In this study, by using the one-dimensional X-ray Rietveld analysis, we investigated the relaxation process of lithium inserted graphite.

Experiment

 We prepared the working electrode by mixing natural graphite powder (LB-CG, Nippon Kokuen) and PVdF with a ratio of 93 : 7 (weight ratio). Lithium foil was used as the counter electrode. We inserted lithium electrochemically at a rate of 0.1C for 10 h using two electrode cell. EC/DMC (2:1 volume ratio) with 1 mol∙dm-3 LiPF₆was used as the electrolyte. After the termination of Li insertion, we immediately removed the working electrode from the cell in a glove box to avoid the local cell action between the electrode material and the current collector.

 XRD patterns from 11 ° to 53° in 2θ were measured by using CuKα radiaton (UltimaIV, Rigaku corp., Japan) for various relaxation time. We analyzed XRD patterns by the one-dimensional Rietveld method [6] using RIEVEC program[1-6]. Interlayer distances and scale factors were obtained.

Results and Discussion

 XRD profiles of the samples for each relaxation time after Li insertion were well fitted by the Rietveld calculation. Fig.1 shows relative mole fraction changes calculated from scale factor and unit cell volume[8] obtained. The mole fraction of stage1 decreased and that of stage2 increased with the relaxation time. It is considered that defective stage1 was formed at the lithium insertion process, and at the relaxation time, it separated to stage1 without defect and stage2.

 Fig.2 shows two kinds of interlayer distance of stage2. The wider one (D_w) increased and the narrower one (D_n) decreased with the relaxation time. Generally, the structure of stage2 is presented as the stack of Li-inserted graphene layer and Li-not-inserted graphene layer. However, from the point of symmetry, stack of graphene layers with two different Li concentration at the interlayer makes stage2 structure. In this study, it is considered that, at Li insertion process, Li-rich interlayer(D_w) and Li-lean interlayer(D_n) stacked to construct stage2, and that, at the relaxation time, structure of stage2 changed to stack of Li–fully-inserted graphene layer(D_w) and Li-not-inserted graphene layer(D_n).

References

[1] S.Park, M.Oda, and T.Yao, Solid State Ionics, 203, 29-32 (2011).

[2] S.Park, S.Ito, K. Takasu and T.Yao, Electrochemistry, 80 (10) 804-807 (2012).

[3] I.S.Seo, S.Park, and T.Yao, ECS Electrochem. Lett., 2 (1) A6-A9 (2013).

[4] S.Park, K.Kameyama, and T.Yao, Electrochemical and Solid-State Letters, 15 (4) A49-A52 (2012).

[5] I.Seo, S.Nagashima, S.Takai and T.Yao, ECS Electrochem. Lett., 2 (7) A72-A74 (2013).

[6] T.Yao, N.Ozawa, T.Aikawa, and S.Yoshinaga, Solid State Ionics, 175,199-202 (2004)

[7] T.Kitamura, S.Park, S.Takai, T.Yao, 224th Meet. Electrochem. Soc., Abstract 147 (2013)

[8] R.J.Hill, The Rietveld Method, R.A.Young, Editor, p. 95, Oxford Univ. press, Oxford (1995).