Cathode Properties Crystal and Electronic Structures of αLi2MnO3-(1-α)Li(Mn10/24Ni7/24Co7/24)O2 in Li Ion Batteries with TiNb2O7 anode
Authors: Aida Takuma , Ishibashi Chiaki , Kitamura Naoto , Idemoto Yasushi 
Published: December 2024

Introduction
Lithium ion batteries (LIBs) are currently used in many small devices due to their high energy density. The solid solution material αLi2MnO3-(1-α)Li(Mn10/24Ni7/24Co7/24)O2 (α = 0.5, 0.4) has attracted attention as a cathode material because it exhibits high energy density and high capacity of 200 mAh/g or more when charged to 4.6 V vs. Li/Li+ or higher. However, there are some problems such as irreversible capacity due to oxygen desorption during initial charging and degradation of cycle characteristics. In our previous research, Li metal has been used for the anode1). However, it is desirable to develop a new oxide anode material with high safety and high capacity for practical use. In this study, we focused on αLi2MnO3-(1-α)Li(Mn10/24Ni7/24Co7/24)O2 cathode and used TiNb2O7 (TNO) anode2), which is one of the candidates for an anode material, in addition to Li metal to evaluate battery characteristics of LIB. In addition, the purpose of this study was to investigate the relationship between battery properties and changes in average and electronic structure due to different anodes by examining the changes in average and electronic structure during the charge-discharge process.
Experiments
αLi2MnO3-(1-α)Li(Mn10/24Ni7/24Co7/24)O2 (α = 0.5 and 0.4) were synthesized by a coprecipitation method. The precursor was obtained by adding 1 mol/L of each metal nitrate to lithium hydroxide monohydrate dissolved in distilled water and then drying the precipitate. Lithium hydroxide monohydrate corresponding to the amount of lithium deficient in the precursor was added, and the samples were obtained by calcining (600°C, Air, 15 h) and firing (950°C, Air, 15 h). The obtained samples were subjected to powder X-ray diffraction to identify the phases and ICP to determine the metal composition. Charge-discharge tests using Li metal as the anode (2.0~4.8 V vs. Li/Li+, 0.1C) and TNO as the anode (0~3.3 V, 0.1C) were conducted using an HS cell to investigate battery characteristics. The powder before charging and discharging and after 5 cycles of charging and discharging of these batteries were analyzed by synchrotron X-ray diffraction measurements (BL19B2, SPring-8) and neutron diffraction measurements (iMATERIA, J-PARC) using Rietveld method (RIETAN-FP, Z-Code) for average structure analysis. In addition, electronic and local structures were analyzed by XAFS (BL14B2, SPring-8). The battery characteristics and changes in average and electronic structure after charge-discharge were discussed for the batteries using TNO and Li as anode.
Results and Discussion
The sample was synthesized by the coprecipitation method, and the phase was identified by powder X-ray diffraction measurement, and it was found to have a monoclinic C2/m structure. The composition was confirmed to be controlled by ICP measurement. From the results of charge-discharge tests, it was indicated that when the anode was TNO, the capacities of the samples with α = 0.5 and 0.4 tended to increase with the number of cycles in both cases. In order to clarify the cause of this, synchrotron X-ray diffraction measurements were performed to analyze the crystal structures. The samples with α=0.5 and 0.4 were also analyzed using Rietveld analysis to determine the average structures after discharge (Fig.1). The results showed mixing of Ni in the samples. The crystal structures were examined in detail by comparing the distortion parameters calculated by the metal-oxygen bond lengths and bond angles, and by calculating the BVS. As a result, it was found that an increase in distortion due to discharge was observed. Furthermore, XAFS was used to examine the relationship between the battery properties and the electronic and local structures before and after charging and discharging.
References
1) Y. Kiribayashi, N. Ishida, N. Kitamura, Y. Idemoto, Abstracts of the 59th Battery Symposium, 1C13 (2018).
2)N. Takami, et al., J. Power Sources, 396, 429-436 (2018)

Figure 1