Degradation Analysis of 18650-Type Lithium-Ion Cells By in-Situ and Operando Neutron Diffraction
Authors: Shiotani Shinya , Naka Takahiro , Morishima Makoto , Yonemura Masao , Kamiyama Takashi , Ishikawa Yoshihisa , Ukyo Yoshio , Uchimoto Yoshiharu , Ogumi Zempachi 
Published: February 2020

Introduction

　Lithium-ion battery (LIB) is one of the most promising energy-storage systems for electric vehicles and renewable energy storage. For the acceleration of development for higher performance LIB, the non-disassembling evaluation technique for practical batteries is required. Neutron diffraction is considered to be the most effective non-disassembling method for the analysis of practical batteries because of the remarkable features of neutrons, e.g. sensitivity to light atoms (lithium and oxygen) and high penetration depth. In this study, the neutron diffraction technique was used to analyze the degradation of 18650-type Li-ion cells. Neutron measurements were conducted for cells before and after degradation tests, either in the charged and discharged state (in-situ) or during discharge (operando). The degradation factors are discussed based on the obtained data.

Experimental

　18650-type Li-ion cells with a nominal capacity of 1800 mAh were fabricated using LiNi1/3Co1/3Mn1/3O2 and graphite as the cathode and anode materials, respectively. Ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 v/v) containing 1M LiPF6 was used as the electrolyte.

 The cells were degraded by carrying out cycle and storage tests. In the cycle tests, cells were cycled 200 times at 50 °C, 400 times at 25 °C, 400 times at 0 °C (Charge: 4.2 V, CCCV, 0.5C, Discharge: 2.7 V, CC, 1C). In the storage tests, the cells were stored at 50 °C for 1000 h at 4.2 V.

　The neutron diffraction experiments either at the constant voltage (in-situ) or during the discharge process (operando) were performed using the special environment neutron powder diffractometer, SPICA [1], in MLF/J-PARC. In-situ data for fresh and degraded cells were acquired at state of charge (SOC) 100% (i.e., charged to 4.2 V) and 0% (i.e., discharged to 2.7 V). Operando data for fresh and degraded cells were taken during discharge at the rate of 0.1C (180 mA).

　By using the Z-Rietveld program [2], the data acquired from in-situ measurements were analyzed by the Rietveld method to refine the crystal structures of both electrodes. The data collected from operando measurements were analyzed by the profile fitting technique to evaluate a dspace of the reflection of electrode materials.

 

Results and discussion

 The capacity losses due to degradation were estimated by electrochemical investigation. Regardless of the degradation conditions, the capacity losses were approximately 15%.

 Fig. 1 shows examples of Rietveld refinement of neutron diffraction patterns from the fresh cell at SOC 100%. The other state and degraded cells were analyzed in a same way, and the Li contents in cathode and anode active materials were determined. Fig. 2 shows the Li content in cathode for the fresh and degraded cells, and Fig. 3 shows that in anode. Regarding anode at SOC 0%, the structure and composition in both the fresh and degraded cells could be characterized by the stage structure of graphite models. Therefore, the Li content in anode at SOC 0% for both the fresh and degraded cells is assumed to be 0 (Li0C6).

 The difference in Li content between SOC 0% and 100% is considered to be equal to the amount of active Li-ions which contribute to charge/discharge. By comparing the amount of active Li-ions in the fresh and degraded cells, the loss of active Li-ions due to degradation was estimated. We found that the amount of active Li-ions in the degraded cells was reduced by 14.4% and 13.7% in the cathode and anode, respectively. This reduction was in good agreement with the capacity loss obtained by electrochemical measurements. These results suggest that capacity loss is mainly caused by the loss of active Li-ions due to side reactions such as SEI growth or Li dendrite formation.

 In the presentation, the results of the operando measurements will be shown and the difference of reaction behavior due to degradation will be reported.  

Acknowledgement

 This work was supported by the Research and Development Initiative for Science Innovation of New Generation Batteries (RISING) project of the New Energy and Industrial Technology Development Organization (NEDO). Neutron diffraction experiments were carried out under the S-type project with Proposal No. 2014S10.

Reference

[1] M. Yonemura, K. Mori, T. Kamiyama, T. Fukunaga, S. Torii, M. Nagao, Y. Ishikawa, Y. Onodera, D. S. Adipranoto, H. Arai, Y. Uchimoto and Z. Ogumi, J. Phys.: Conf. Ser.502 (2014) 012053.

[2] R. Oishi, M. Yonemura, Y. Nishimaki, S. Torii, A. Hoshikawa, T. Ishigaki, T. Morishima, K. Mori and T. Kamiyama, Nucl. Instrum. Methods Phys. Res., Sect. A 600, 94 (2009).

Figure 1