Charge Compensation Mechanism of Li2MnO3 Cathode in All-Solid-State Thin Film Battery Investigated By Using Operando HAXPES
Authors: Shimizu Keisuke , Hikima Kazuhiro , Kiuchi Hisao , Suzuki Kota , Hirayama Masaaki , Matsubara Eiichiro , Kanno Ryoji 
Published: December 2020

Lithium battery has been used as a power source for not only small devices but also large equipment like an electric vehicle because of its potentially high energy density and high rate performance. All-solid-state battery is a promising battery form to achieve high energy density. Since solid-electrolyte has a wider potential window than a liquid one, high voltage cathode material can be implemented. Among novel cathode materials, we focus on a Li-rich manganese oxide Li2MnO3, which has a layered rock-salt structure with a honeycomb-ordered LiMn2 layer. Although having a high theoretical capacity of ~459 mAh g-1, the cathode delivers a smaller reversible capacity than the theoretical value. Recently, we have found that an all-solid-state battery with a Li2MnO3 thin film can deliver a reversibly capacity of 270 mAh g-1, and the rate and cyclic performance are extremely high [1]. However, the redox species of the Li2MnO3 during the electrochemical charge/discharge reactions are still unknown for this battery system.

In this study, the charge compensation mechanism of the Li2MnO3 cathode in an all-solid-state thin film battery was investigated by using an operando hard X-ray photoelectron spectroscopy (HAXPES) method. Although X-ray photoelectron spectroscopy is a powerful method to investigate the electronic structure of the cathode, there is an issue that the probing depth is about a few nm when using Al or Mg Ka source. On the other hand, HAXPES has a high probing depth of several 10 nm, which makes it possible to detect a signal from the buried region of the battery.

Solid-electrolyte supported thin film batteries were developed for HAXPES measurement as illustrated in Fig (a). Using solid-electrolyte as substrate, cathode, and anode layers can be separated on the front and the back of the substrate, respectively, which enables us to detect the HAXPES signal of the cathode layer. The battery composed of a polycrystalline Li2MnO3 cathode, an amorphous Li3PO4 buffer layer, a lithium anode, an Al current collector, and a LICGC glass-ceramics solid-electrolyte substrate (manufactured by Ohara Inc.) was prepared by using pulsed laser deposition, magnetron sputtering, and thermal or electron-beam vapor deposition methods. Charge-discharge experiments were performed galvanostatically at a 0.2 C rate between 2.0 and 5.0 V (vs. Li/Li+) at room temperature. After the cell voltage reached at 5.0 V, the voltage was kept for 30 minutes.An operando HAXPES measurement was conducted at BL28XU in SPring-8. The incident photon energy of 7940 eV was used. The operando O1s, Mn3s, and Li1s spectra were measured during the 1st and the 5th charge-discharge processes.

The charge-discharge curves showed a plateau at 4.6 V in the 1st charge and slope-like curves from the 1st discharge. Since these characteristics are also identified in the case of the battery using the Li2MnO3 epitaxial thin film [1], the prepared battery is suitable to conduct a HAXPES measurement for the purpose. The figures (b, c) show O1s operando spectra during the 1st charge-discharge process. During the 1st charge, a new peak at 531.1 eV is generated from 3.7 V in addition to a lattice oxygen O2- at 529.8 eV. According to previous reports, the new peak can be assigned as higher oxidized lattice oxygen such as O- [2]. After 3.7 V, the area of the new peak increases while that of O2- decreases as the voltage increases. During the 1st discharge, the new peak gradually decreases and then disappears at 3.3 V. Therefore, the reversible oxygen redox could contribute the charge compensation for the battery reactions. The average valence state of Mn was analyzed from Mn3s operando spectra. Manganese is reduced from 4+ to 3.5+ during the 1st charge process then Mn is further reduced to 3.1+ from 3.3 V during 1st discharging process. Upon the 5th charge-discharge process, the Mn redox occurred reversibly below 3.5 V.

These results indicate that the oxygen as well as Mn redox reactions occur reversibly in Li2MnO3 cathode. Furthermore, the oxygen redox occurs on the higher voltage region between ~3.3 and 5.0 V and the Mn redox occurs on the lower region between 2.0 and ~3.3 V. Therefore, it is evident that the large reversible capacity of Li2MnO3 thin film results from the oxygen redox activity. Realizing the stable oxygen redox reaction on batteries with a larger scale will be a key for practical application.

[1] K. Hikima et al., Chem. Lett. 48, 192 (2019).

[2] M. Sathiya et al., Nature Materials 12, 827 (2013).

Figure 1