Relaxation Rietveld Stage Analysis of Li Inserted Graphite

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.[6]  synthesized Li-GICs and analyzed the layered structures crystallographically by the one-dimensional Rietveld method.

In this study, we changed current when inserting lithium into graphite electrochemically and analyzed stage structures at the relaxation time by using the one-dimensional Rietveld method. We investigated the effect of the insertion current on the relaxation process.

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 using two electrode cell. EC/DMC (2:1 volume ratio) with 1 mol∙dm^-3 LiPF₆ was used as the electrolyte. We prepared three samples with different current rate and discharging hour(G1:0.01C-100h, G2:0.02C-50h, G3:0.04C-25h). 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 mole fraction changes of stage1 for samples( G1, G2, G3). The mole fraction of stage1 decreased and that of stage2 increased with the relaxation time for all samples. 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. The mole fraction change with relaxation time became the smaller for the lower current rate.

 Fig.2-4 shows lattice parameter of c-axis(Fig.2) in stage2 and two kinds of interlayer distance of stage2(Fig3-4) in three samples. The wider one (D_w) increased, and the narrower one (D_n) and lattice parameter 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 stage 2, 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). The relaxation of the lattice constant, D_w, and D_n were different with the insertion current. It is considered that the change of insertion current effect on the manner of Li insertion.

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)