Charge-Discharge Behavior of NMC111 Cathode in Aqueous Zinc Battery
Authors: Kosaki Takahiro , Hayashi Hiroki , Nara Hiroki , Gota Asano , Momma Toshiyuki 
Published: December 2024

Currently, aqueous zinc batteries (AZBs) with zinc anode and aqueous electrolyte are focused on as one of the post-lithium-ion batteries because they can be a safe and inexpensive energy storage device. To date, a number of compounds, such as manganese oxide, vanadium oxide, Prussian blue analogues, and so on, have been investigated as cathode materials for AZBs. Although these materials are promising, suitable materials for cathode are still being studied for practical use. A variety of charge-discharge mechanisms are found in cathode of AZBs, which is influenced by many factors such as the composition, crystal structure, and morphology of the compounds used. Therefore, it is essential to understand the reaction mechanism to study new AZB systems.
Previously, we investigated the applicability of Li(Ni0.33Mn0.33Co0.33)O2 (NMC111) as a cathode for AZBs1. In the report, it is found for the first time that NMC111 can be used as an AZB cathode by delithiation in 1 M KOH aq. (delithiation treatment). In order to implement the NMC cathode in AZB in the future, it is important to understand its delithiation treatment and charge-discharge behavior. During the delithiation process, it was found that water molecules inserted into the interlayer spaces and expanded the interlayer spacing, thereby facilitating the ion diffusion and redox activity2. The purpose of this study was to reveal the charge-discharge behavior of NMC111 cathode by characterization and electrochemical measurement, leading to the improvement of the performance of NMC111 cathode in AZB.
The cathode was prepared by mixing NMC111, acetylene black, Ketjen black, polyvinylidene difluoride (PVdF) (45 : 4 : 1 : 4 in weight ratio) diluted with an amount of N-methyl-2-pyrrolidinone (NMP) and coated it on a Ni porous substrate. A beaker cell was used for delithiation treatment with an NMC111 cathode as the working electrode, a Cu foil as the counter electrode, a Hg/HgO electrode as the reference electrode, and 1 M KOH aq. as the electrolyte. Then a constant current of 170 mA g-1 was applied to an NMC111 cathode for 1 hour to deintercalate Li+ from NMC111. Charge-discharge tests were performed using a beaker cell with a delithiated NMC111 cathode, a Zn plate anode, a Hg/HgO reference electrode, and 1 M KOH (sat. ZnO) electrolyte, with a C rate of 0.1 C (= 17 mA g-1) and cut-off potentials of -0.4－0.7 V vs. Hg/HgO. Then characterizations, X-ray Diffraction analysis (XRD) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), were performed on the NMC111 cathode after charge-discharge tests. In addition, the WE used for Electrochemical Quartz Crystal Microbalance (EQCM) measurements was prepared by using a 6 MHz Au-coated quartz crystal (1.33 cm2) as a substrate, mixing NMC111 and PVdF (99 : 1), diluting it with an appropriate amount of NMP, and loading about 10－50 μg onto the gold. Then, delithiation treatment and electrochemical measurement were performed in EQCM cell.
First, the crystal structures of NMC111 cathodes after charge-discharge test were confirmed by ex-situ XRD. The results showed that there was a single peak attributed to the interlayer spacing of the cathode after charge, while the peak after discharges was split into two peaks (Figure 1). The interlayer spacing was calculated to be 0.702 nm after charging, it changed to 0.694 and 0.714 nm after discharging. These results suggests that the charge-discharge reaction of NMC111 occurs by intercalation of ions. In addition, both an increase and a decrease in the interlayer spacing were observed during the discharge process, which indicated that ions could be inserted into the interlayer in stages. Next, the cathode composition was measured by ICP-OES after charge and discharge to identify the intercalants. The concentration changes of potassium, lithium, and zinc in the NMC111 cathode after charge and discharge were small, less than 5 mAh g-1 converted to specific capacity. Considering that the discharge capacity is about 120 mAh g-1, the dominant intercalants are not Zn2+, K+, or Li+. Thus, insertion of OH-, H+, and H3O+ may have occurred since this battery system doesn't contain other ions. Finally, EQCM measurements could detect the weight of intercalants inserting into the cathode interlayer. The result of intercalant identification by EQCM measurements will be also discussed.
[1] H. Hayashi, et al., 63rd Battery Symposium in Japan, Abstr., 1H22 (2022). [in Japanese]
[2] H. Hayashi, et al., 91st ECSJ Annual Meeting, Abstr., S8-3_3_12 (2024). [in Japanese]
Figure 1 (a) Charge-Disharge curves of 1st cycle (b) XRD profiles at each point

Figure 1