Establishing the AZIB testing environment.
Figure 1a demonstrates two methods of operation for an AZIB MnO2 cathode. In the first instance, MnO2 (δ-MnO2 is depicted) could be used solely as an intercalation host for Zn2+ cations. The cathode active material exists only in the solid phase, and the instability from MnO2 dissolution is an obstacle to achieving the desired performance. In the second instance, the instability of MnO2 is embraced by incorporating Mn2+ in the electrolyte to facilitate cycles of MnO2 dissolution and redeposition, in addition to Zn2+ intercalation. The cathode material used in this study was nanostructured δ-MnO2 (see experimental details in Methods and characterization in Supplementary Fig. 1–2, Supplementary Table 1). The δ-MnO2 particles were encased within layers of carbon nanotubes (δ-MnO2/CNT), which provide a high surface area, conductive, and robust support for the active material (Supplementary Fig. 3). Either 0.5 M ZnSO4 (Zn2+(aq)-only) or 0.5 M ZnSO4/0.5 M MnSO4 (Zn2+(aq)-Mn2+(aq)) electrolytes were used to explore the two mechanisms of operation. In Zn2+(aq)-only electrolyte, the capacity of the δ-MnO2/CNT cathode decreased by 54% over 100 cycles, while in Zn2+(aq)-Mn2+(aq) electrolyte, the capacity increased from ~ 185 mAh/g to nearly 500 mAh/g (Fig. 1b,c). Such high capacity has only been observed a few times in Zn/MnO2 AZIBs31–34, so the mechanism behind this spike warranted further investigation.
Upon disassembling a battery with Zn2+(aq)-Mn2+(aq) electrolyte, a thick black film was observed on the stainless steel (SS) current collector. This was an immediate indication about the consequences of using electrochemically active material in the liquid phase of an AZIB. Considering that the research on AZIBs has often adopted coin cell fabrication techniques from Li-ion battery research, including the use of SS components26,27,33−38, the testing methodology needs thorough evaluation to prevent interference with the true phenomena during cycling. Therefore, potential side reactions with the supporting current collector were studied to eliminate their influence on cathode performance. This approach guarantees that the impact of MnO2 dissolution and redeposition on phase changes, capacity, and stability of the active material can be delineated and studied.
Batteries were assembled without a MnO2 cathode to isolate the interface between either a SS or Ti current collector and the Zn2+(aq)Mn2+(aq) electrolyte. Using 50 µL of electrolyte, the capacity of a battery with a SS electrode dramatically increased over the course of 100 cycles (Fig. 2a). In contrast, the capacity on a Ti cathode dropped steeply over the first 30 cycles to a negligible value. If only 10 µL electrolyte was used, the initial capacity was negligible on both Ti and SS. However, the capacity still increased over time on SS while it decreased on Ti. The increase in capacity on SS is attributed to the catalytic effect of nickel, a component in SS, on the electrodeposition of manganese oxide32,39−41. The capacity from SS cells was less reproducible than Ti due to the variability of these catalytic conditions. Since it is impossible to control the electrolyte distribution within a coin cell after assembling, it is clear that Ti is the more stable and reproducible choice for AZIB studies due to its minimal effect on the cathode performance.
In concurrence with the results shown in Fig. 2a, the impact of the current collector material on the observed AZIB performance is highly dependent on the volume of electrolyte used. Recent studies have included a SS current collector at the positive electrode without reporting the precise amount of electrolyte used26,27,33−36. However, it is critical to control the volume of electrolyte because adding a Mn2+ salt makes the liquid phase an active material that contributes capacity. The contrast between current collectors is drastic when 50 µL electrolyte is used (Fig. 2b). The δ-MnO2/CNT cathode with SS exhibits a steep rise in capacity over 100 cycles, while the capacity increases much less dramatically with a Ti current collector. In addition to MnO2 deposition on the metallic current collector, X-ray diffraction (XRD) confirmed that ε-MnO2 electrodeposits on bare CNTs in this voltage region (Fig. 2c). To reduce the impact of the current collector, only 10 µL electrolyte was used, although cathodes with SS still had more improvement in capacity than Ti current collectors. It is clear that Ti should be used to support the active material in AZIB research and the volume of electrolyte should be selected after consideration of the electrode area, active mass, and geometry of the battery (see Methods for additional detail). By eliminating the increased capacity from side reactions with SS, the mechanisms for capacity evolution in AZIBs can be fairly evaluated.
Stage I of transformation of δ-MnO 2 cathode.
Since MnO2 dissolution is prevalent under the electrochemical conditions of interest in AZIBs, the potential for MnO2 redeposition must be leveraged to attain stable battery performance. It is important to note that in Zn2+(aq)Mn2+(aq) electrolyte, dissolution and redeposition can occur each cycle according to the following reaction:
The Coulombic efficiency (CE) provides information on whether the equilibrium lies further to the left or right of equation (1) in a given cycle. Initially, the δ-MnO2/CNT cathode is charged to the upper potential limit (1.9 V vs Zn/Zn2+), which does not appreciably change the crystal structure (Supplementary Fig. 4). The first cycle begins with discharge, initiating Stage I of the transformation of δ-MnO2. Therefore, CE is calculated here as the ratio of the discharge capacity to the following charge capacity (see Supplementary Note 1 and Supplementary Fig. 5 for additional detail). Within the first five cycles, the capacity peaks and the CE rises above 100% (Fig. 3a,b), indicating considerable dissolution of δ-MnO2 in the slightly acidic Zn2+(aq)-Mn2+(aq) electrolyte (pH ≈ 5.5). Enhanced dissolution is supported by cyclic voltammetry (CV, Fig. 3c). In Zn2+(aq)-only electrolyte, the Zn2+‑intercalation peak occurs at ~1.37 V and MnO2 dissolution occurs below 1.2 V. The narrower reduction wave in the CV is attributed to Zn2+ intercalation due to the transport-related issues of solid-state Zn2+ diffusion, while the broader wave is attributed to MnO2 dissolution due to the availability of excess solid MnO2 at the electrochemical interface. In Zn2+(aq)-Mn2+(aq) electrolyte, MnO2 dissolution is also indicated by substantial reduction current below 1.2 V in the first five cycles. From cycle 5 to 10, the magnitude of dissolution current decreases and the CV begins to stabilize, indicating that dissolution is most notable in the first 5 cycles (Supplementary Fig. 6).
Given the extensive MnO2 dissolution taking place, it is important to establish its relationship to possible changes in the crystal structure. XRD shows that δ-MnO2 begins converting to tetragonal spinel ZnxMn2O4 after just the first discharge (Fig. 3d). XRD also exposes the presence of Zn4(OH)6SO4·4H2O (zinc hydroxide sulfate, ZHS) after discharge at cycles 1 and 5. This is further proof of the substantial MnO2 dissolution occurring in the first five cycles. According to equation (1), MnO2 dissolution consumes protons, raising the pH of the electrolyte42,43. Various hydrates of ZHS have been observed to precipitate out of ZnSO4 solutions at a pH above ~5.543-45. Therefore, the presence of ZHS indicates more alkaline pH caused by MnO2 dissolution in the first five cycles. Because the precipitation of ZHS acts as a buffer, it is estimated the pH increases to a maximum of ~6.0 based on ex situ precipitation experiments (Supplementary Fig. 7). In addition, transmission electron microscopy (TEM) shows that the architecture of interconnected sheets has completely transformed by cycle 5 due to dissolution (Fig. 3e,f). Between cycles 5 to 10, the formation of the spinel ZnxMn2O4 phase likely reduces dissolution, shifting the equilibrium of equation (1) towards deposition. The increase in ε-MnO2 deposition acidifies the electrolyte, dissolving ZHS. By cycle 10, ZHS is no longer present in XRD, δ-MnO2 barely remains, and the ZnxMn2O4 crystal domains are growing.
Stage II of transformation of δ-MnO 2 cathode.
The local minimum in capacity at cycle 10 concludes Stage I, in which the instability of the active material was exemplified. Stage II provides further insight into the potential for leveraging the dynamic cathode during battery operation. The impact of the Zn2+(aq)-Mn2+(aq) electrolyte in Stage II is clear from CV, which is essentially steady by 30 cycles, while the current is constantly decreasing even after 30 cycles in Zn2+(aq)-only electrolyte (Supplementary Fig. 8). During Stage II, the capacity initially increases and the CE approaches a limit of 99.9% by cycle 100 (Fig. 4a). The lower CE at cycle 10 indicates that ε-MnO2 electrodeposition during charging is dominant. The result is a buildup of active material on the cathode that explains the steady increase in the capacity normalized to initial active mass, which peaks after approximately 55 cycles. The deviation of CE from 100% indicates that MnO2 dissolution/redeposition is not a fully reversible process under these conditions. This is attributed to the formation of ZnxMn2O4 and decreased dissolution of this phase.
As the Zn2+(aq)-Mn2+(aq) electrolyte is depleted of Mn2+, the capacity begins to fade. The XRD shows that in the discharged state at cycle 30, δ-MnO2 has completely disappeared leaving only tetragonal spinel ZnxMn2O4 (Fig. 4b). However, as the capacity fades, XRD indicates that some cubic zinc manganese oxide is forming after discharge46,47. This suggests intercalation of Zn2+ above a 1:2 Zn:Mn ratio, which was confirmed by inductively coupled plasma-mass spectrometry (ICP-MS) of the cathode after discharge at 100 cycles (Supplementary Table 2). The charged state at 100 cycles exhibits only tetragonal ZnMn2O4 structure and composition, suggesting that the Zn2+-intercalation capacity is due to intercalation of Zn2+ into tetragonal spinel ZnMn2O4 (Supplementary Fig. 9, Supplementary Table 2). This is a low capacity process (~ 49 mAh/g theoretical based on ICP-MS), counteracting the potential for high capacity due to fresh ε-MnO2 deposition and dissolution (616 mAh/g theoretical).
Comparison of the performance and phase changes of δ-MnO2 in Zn2+(aq)-only electrolyte provides additional insight into the underlying mechanisms occurring over the course of 100 cycles. Dissolution of the MnO2 host causes the capacity to drop continuously after the first 10 cycles in Zn2+(aq)-only electrolyte (Fig. 4c). Despite the dissolution, the XRD after 100 cycles indicates highly crystalline tetragonal spinel ZnxMn2O4, as well as reflections associated with ZHS (Fig. 4b, Supplementary Fig. 10). The presence of ZHS proves that the equilibrium is shifted strongly towards dissolution of MnO2 in Zn2+(aq)-only electrolyte. Dissolution is also evident from analysis of the CE, which rises above 100% for the majority of the first 30 cycles, and TEM that shows complete loss of the interconnected sheets of the synthesized architecture (Supplementary Fig. 11–12). It is clear that the mechanism by which δ-MnO2 converts to tetragonal ZnxMn2O4 does not require redeposition of ε-MnO2. However, high capacity is facilitated by the constant redeposition/dissolution of ε-MnO2 in Zn2+(aq)Mn2+(aq) electrolyte because of the difficulty in reversibly intercalating Zn2+ in the spinel structure48–50. This provides novel understanding of the fundamental processes in a Zn/MnO2 battery and the potential benefits of limiting Zn2+ intercalation.