Promoting Proton Migration Kinetics by Ni²⁺ Regulating Enables Improved Aqueous Zn-MnO₂ Batteries

1 Introduction

With the development trend of renewable energy technology to large-scale penetration, aqueous rechargeable zinc ion batteries (ZIBs) have broad prospects in energy storage applications due to the high theoretical capacity of Zn metal anode (820 mAh g⁻¹), low oxidation–reduction potential (−0.76 V vs SHE), and intrinsic incombustibility of water electrolytes.^[1-7] At present, most of research works are focused on the exploration of cathode materials, such as Mn-based oxides,^[8-11] V-based oxides,^[11-13] Prussian blue analogs,^{[14, 15]} organic compounds,^{[16, 17]} etc. Among them, polymorphic MnO₂ (such as α-, β-, γ-, and σ-) has high theoretical capacity (Mn⁴⁺/Mn³⁺, ~308 mAh g⁻¹)^[18-22] and high-voltage window (1.2–1.4 V),^[23-25] which make the aqueous Zn-MnO₂ batteries have outstanding advantages such as high energy density, high safety, low cost, and ecofriendliness.^{[7, 15, 20, 26-29]}

Although Zn²⁺ has a relatively small ionic radius (0.74 Å) and relatively high ionic conductivity (≈1⁻¹⁰ mS cm⁻¹),^[30] it will cause a strong electrostatic interaction with the host lattice of MnO₂, thus showing slow diffusion rate and limited reversibility. Meanwhile, the Jahn–Teller distortion in the [MnO₆] octahedron and the disproportionate dissolution of Mn(III) further make the host lattice framework unstable, resulting in the irreversible dissolution of manganese oxides.^[31-33] Therefore, achieving the reversible intercalation of Zn²⁺ or H⁺ with high capacity and long-cycle life is still a challenge for aqueous ZIBs.

The electronic state regulation of manganese oxide by doping transition metal elements (Zn, K, Ni, Cu, and Co) is considered to be an important and effective strategy to significantly improve electrochemical performance.^{[22, 34-40]} On the one hand, the strategy can optimize the charge/ion state and electronic band gap, and promote the rapid migration of H⁺, thereby improving rate performance and nanofluidic iontronics.^[41-44] On the other hand, it stabilizes the crystal structure of the MnO₂ cathode materials by the coordination of guest ions with neighboring host atoms. For example, our team recently reported that the multivalent cobalt-doped manganese oxide (Co-Mn₃O₄) can precisely regulate the electronic state of Mn-O, effectively suppressing the Jahn–Teller effect. This work improved the cycle stability of Mn₃O₄ (after 250 cycles at 0.2 A g⁻¹, the capacity retention rate is as high as 90%) and increased the discharge capacity (362 mAh g⁻¹ at 0.1 A g⁻¹).^[33] Zhou et al. adjusted the electronic structure by pre-embedding K⁺ in the 2 × 2 tunnel structure of α-MnO₂ to improve storage capacity.^[45] In addition, Qin's group reported that Ni²⁺-doped Mn₂O₃ could stabilize the Mn-O bond and promote electron rearrangement. The Zn//Mn₂O₃ battery exhibited excellent cycle stability (the capacity retention rate reached 85%, more than 2500 cycles at 1.0 A g⁻¹).^[22] Therefore, it is a significant measure to construct manganese oxide cathode materials with stable ion (Zn²⁺/H⁺) migration channels by the electronic state regulation.^[46]

Herein, we facilely synthesize a Ni²⁺-regulated γ-MnO₂ (designated as Ni-MnO₂) with excellent proton migration kinetics as cathode material for high-performance ZIBs. The introduction of Ni²⁺ can ameliorate H⁺ storage activity in the tunnel and accelerate electrochemical reaction kinetics. Meanwhile, Ni²⁺ doping lowers the diffusion barrier of H⁺ and selectively induces the ion intercalation, thus endowing the aqueous battery rapid kinetics. First principle calculations further verify that the Ni-MnO₂ allows the one-dimensional H⁺ migrate rapidly along the tunnel due to reduction of the activation energy caused by Ni²⁺ regulating. Compared to pristine γ-MnO₂, the Ni-MnO₂ electrode material delivers ultra-high rate performance (56 mAh g⁻¹ at 10 A g⁻¹) and ultra-long-cycle life (the capacity retention rate exceeds 100% after 11 000 cycles at 3.0 A g⁻¹). The Ni²⁺ doping significantly improves the electrochemical performance of γ-MnO₂, which provides a new route for the development of superb aqueous Zn-MnO₂ batteries.

2 Results and Discussion

To understand the feasibility that Ni²⁺ doping can selectively induce ion intercalation, the H⁺ intercalation barrier energies in the MnO₂ and Ni-MnO₂ model were calculated by density functional theory (DFT) (Figure 1a). Compared with the pristine MnO₂, the introduction of Ni²⁺ can effectively reduce H⁺ diffusion barrier in the {2 × 1} tunnel, thereby accelerating the proton reaction kinetics. Although it has a lower barrier energy in the {1 × 1} tunnel, it is not the most stable diffusion position. Ni-doped γ-MnO₂ spike-like nanoarrays (Ni-MnO₂) are prepared by galvanostatic electrodeposition and finally uniformly coated on carbon cloth.^[47] Figure 1b shows a simple synthesis process of Ni-MnO₂ electrode. Figure 1c and Figure S1, Supporting Information show the doping type Ni-MnO₂ formed by Ni²⁺ replacing part of the Mn⁴⁺ lattice position in γ-MnO₂. The smaller layer spacing structure (2.3 Å × 2.3 Å + 4.6 Å × 2.3 Å) of γ-MnO₂ is considered to be a random intergrowth of {1 × 1} tunnels of pyrolusite and {2 × 1} tunnels of ramsdellite composed of [MnO₆] octahedral units with edge or corner sharing, so the volume expansion rate of the crystal is low after doping.^{[48, 49]} As a result, the overall crystal form of Ni-MnO₂ is not easy to transform. The scanning electron microscopy (SEM) of Ni-MnO₂ shows that the nanoarrays are densely and uniformly coated on the surface of the carbon fiber, and the deposition thickness is about 1.5 μm. Furthermore, it can see that the spike-like nanotopography by magnifying a certain part under high magnification (Figure 1d). For intuitive comparison, undoped γ-MnO₂ is prepared by electrodeposition, as shown in Figure S2, Supporting Information. Transmission electron microscopy (TEM) is used to analyze the microstructure of Ni-MnO₂ (Figure 1e). High-resolution transmission electron microscope (HR-TEM) shows lattice fringes with a pitch of 0.242 nm, which can be assigned to the (131) crystal plane of γ-MnO₂ (Figure 1f). In addition, the interpolated selected area electron diffraction (SAED) pattern shows three fuzzy and bright diffraction rings, corresponding to the (131), (300), and (421) crystal planes of γ-MnO₂. According to energy dispersive X-ray spectroscopy (EDX) element detection, Mn, Ni, and O elements are uniformly distributed on a single nanosheet, and the atomic ratio of Ni:Mn is close to 1:6 (Figure 1g and Figure S3, Supporting Information). Furthermore, the amount of Ni doping in Ni-MnO₂ is quantitatively analyzed by the inductively coupled plasma optical emission spectroscopy (ICP-OES), and the results are shown in Table S1. The proportion of Ni content is in line with the characteristics of metal ion doping. To prove that the crystal structure of γ-MnO₂ does not be changed after Ni doping, X-ray diffraction (XRD) analysis is carried out on the crystal and phase information (Figure 1h). In comparison with pristine γ-MnO₂, the diffraction peaks corresponding to the three main crystal planes have neither obvious shift nor other new impurity peaks after Ni doping, but the peak intensity is slightly weakened (JCPDS No. 14-0644).^[25] This confirms that Ni doping will reduce the crystallinity of the material.

Ni-MnO₂ was further analyzed by X-ray photoelectron spectroscopy (XPS) for the valence state (Figure 1i,j). The target elements Ni, Mn, and O were detected, respectively, which confirmed the successful doping of Ni. The high-resolution Mn 2p spectrum is shown in Figure 1i, and its high spin–orbit resolution peak is resolved as Mn⁴⁺ (2p_3/2, 642.2 eV; 2p_1/2, 653.8 eV).^{[21, 50-52]} Compared with the Mn 2p spectrum of pristine γ-MnO₂ (Figure S4, Supporting Information), the spin–orbit peak of Ni-MnO₂ shifts to the left. It shows that the hybridization of foreign ions will change the bonding state of the crystal structure. In Ni 2p spectrum (Figure 1j), the spin–orbit resolution peak is resolved as Ni²⁺ (2p_3/2, 853.4 eV; 2p_1/2, 870.2 eV), which indicates that Ni is doped into γ-MnO₂ with +2 valence.^{[22, 53-56]} The XPS results are consistent with the above analysis, and Ni-MnO₂ meets the experimental requirements in the overall structure design.

The electrochemical performance of Ni-MnO₂ cathode material is carried out in the coin cell, which utilized zinc foil as the anode and 2 M ZnSO₄ + 0.2 M MnSO₄ as the electrolyte.^{[1, 38]} Cyclic voltammetry (CV) curves of Ni-MnO₂ are obtained at the scan rate of 0.3 mV/s under 0.6–1.85 V (Figure 2a). During the initial cathode cycle, the contact surface area between electrolyte and active site of electrode material is small, and only a small amount of H⁺ is intercalated into the host for energy storage. As the number of scanning cycles increases, the winding area of CV curve gradually increases, and the shape of the curve tends to overlap after the fifth cycle. In particular, there is only a pair of obvious reduction/oxidation peaks in the entire cathodic scanning process, corresponding to 1.32 V/1.58 V, respectively, which is related to the intercalation of H⁺. With the scan rate increasing from 0.3 to 5 mV/s, the reduction peak/oxidation peak of CV curve shifts to lower or higher potentials, resulting in a deeper polarization (Figure 2b). According to the typical method (log i = log a + blog v),^[44] the b values of Ni-MnO₂ are calculated to be 0.671 (peak 1) and 0.654 (peak 2), respectively, indicating that the electrochemical kinetics of electrode is related to the diffusion control process and the capacitance effect, but the diffusion control behavior is the main process (Figure 2d). Further fitting calculation (i(V) = k₁ν + k₂ν^1/2)^[57] reveals that the capacitance contribution of the electrode material is only 44.58% (Figure S5, Supporting Information). Figure 2c shows galvanostatic charge–discharge (GCD) curves of Ni-MnO₂ at different current densities. The specific capacity reaches 224 mAh g⁻¹ at 0.1 A g⁻¹. In addition, with the increase of current density, even increasing to 10 A g⁻¹, the capacity remains as high as 56 mAh g⁻¹, thus exhibiting excellent rate performance. When returning to 0.1 and 0.2 A g⁻¹ again, the capacity retention rate of the electrode still exceeds 95% (Figure 2e). Furthermore, we perform long-cycle performance tests. It can cycle more than 180 cycles at 0.1 A g⁻¹, and the capacity has not decayed significantly (Figure S6a, Supporting Information). In addition, after 7000 cycles at 2.0 A g⁻¹, the capacity retention rate of Ni-MnO₂ was close to or exceeded 100% (Figure 2g). Compared with reported cathode materials for zinc-based batteries (Figure 2f), such as ZnMn₂O₄,^[28] CNT@MnO₂,^[52] Na_0.95MnO₂,^[58] γ-MnO₂,^[49] H₂V₃O₈,^[59] FeHCF,^[15] β-MnO₂,^[26] α-MnO₂@TiO₂,^[38] etc., Ni-MnO₂ delivers high energy density of 346.4 Wh kg⁻¹ and outstanding power density of 14.11 kW kg⁻¹ (based on the active mass of the cathode load, about 2.0 mg).

Based on γ-MnO₂ and Ni-MnO₂, various electrochemical measures were used to explore the performance differences caused by Ni²⁺ doping. Figure 3a is the rate performance comparison of γ-MnO₂ and Ni-MnO₂. Specifically, with the current density increases stepwise from 0.1 to 3.0 A g⁻¹, the Ni-MnO₂ shows excellent rate performance, indicating its improved electrical conductivity. When it returns to 0.1, 0.2 A g^-1 cycles, the capacity retention rate exceeds 100%, which further reflects the outstanding stability of the electrode structure. Conversely, the γ-MnO₂ exhibits extremely poor reversibility, which eventually leads to a rapid decline in electrochemical performance. To better explain the reaction kinetic mechanism, the galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were used for rigorous experimental research. Figure 3b is the GITT of γ-MnO₂ and Ni-MnO₂ during the first cycling, respectively. Correspondingly, the diffusion coefficient D is calculated according to Fick's second law ($D=\frac{4}{\pi \tau }{\left(\frac{m_B{v}_M}{M_{B}S}\right)}^2{\left(\frac{\mathrm{\Delta }{E}_S}{\mathrm{\Delta }{E}_{\tau }}\right)}^2$)^[33] and shown in Figure 3c. The diffusion coefficient of Ni-MnO₂ is one order of magnitude higher than that of γ-MnO₂ at the discharge stage, which demonstrates that Ni²⁺ doping can promote H⁺ intercalation. Besides obtaining electrochemical impedance spectroscopy (EIS) to learn more about resistance and conductivity (Figure 3d), by fitting the EIS data of Ni-MnO₂, R_S (0.35 Ω), R₁ (71.27 Ω), and R₂ (84.86 Ω) are calculated. In contrast, γ-MnO₂ has higher R_S (0.56 Ω), R₁ (266.1 Ω), and R₂ (172.9 Ω). Furthermore, Ni-MnO₂ electrode exhibits the greater linear slope in the low-frequency region, which reflects the faster transfer/diffusion kinetics at the electrode–electrolyte interface.

Figure 3e shows the curves of γ-MnO₂ and Ni-MnO₂, which could indicate the difference in ion (de)intercalation mechanism between them. The CV curve of γ-MnO₂ has two obvious reduction peaks (1.24 and 1.37 V), corresponding to the intercalation process of Zn²⁺ and H⁺, respectively (Mn⁴⁺ → Mn³⁺). For Ni-MnO₂, there is only a strong reduction peak at 1.35 V, that is, the intercalation reaction of H⁺. The charge–discharge curves of first 6 cycles are shown in Figure 3f. As the current density is constant in the discharge process, H⁺ intercalation in Ni-MnO₂ may make the voltage change approximately linearly, resulting in discharge platform tilt or not obvious inflection point. However, γ-MnO₂ will undergo the conversion reaction (phase transformation). When Zn²⁺/H⁺ enters the host, a new fixed phase will be produced (such as spinel ZnMn₂O₄, layered Zn_xMnO₂, and MnOOH), and the potential corresponding to this new phase is fixed. As the discharge process goes on, the content of this new phase is continuously accumulated, thus presenting an obvious discharge platform in the discharge process.^[43] Combined with Figure S7, Supporting Information, the γ-MnO₂ electrode exhibits two obvious discharge platforms, which correspond to the intercalation process of Zn²⁺ and H⁺, respectively. Besides, γ-MnO₂ has an obvious side reaction process in the low potential range (0.6–0.8 V), which is unfavorable for the reversibility of rechargeable batteries. But for Ni-MnO₂, the discharge platform corresponding to Zn²⁺ intercalation is shorter and not clear, and the whole process is mainly the long discharge platform corresponding to the H⁺ intercalation. Besides, 2 M ZnSO₄/dimethyl sulphoxide (DMSO), 0.2 M MnSO₄/H₂O, and (2 M ZnSO₄ + 0.2 M MnSO₄)/H₂O are used to explore the capacity contribution of Zn²⁺ and H⁺ intercalation. It can be seen from Figures S8a,b (Supporting Information) that the discharge platform corresponding to H⁺ intercalation is between 1.35–1.37 V, and the discharge platform corresponding to Zn²⁺ intercalation is around 1.24 V, which corresponds well to the information feedback in Figure 3e. Meanwhile, the capacity contribution brought by H⁺ intercalation accounted for the main part (the capacity exceeds 130 mAh g⁻¹) in Ni-MnO₂. Figure S8c,d, Supporting Information again confirm that the introduction of Ni selectively promotes H⁺ intercalation and improves structural stability. During the charging and discharging process, the pH value of the electrolyte where Ni-MnO₂ is located changes more than that of γ-MnO₂, implying that the H⁺ intercalation process is more prominent and highly reversible (Figure S9, Supporting Information). This reveals that H⁺ storage behavior would be favorably regulated by doped Ni²⁺. Importantly, we analyzed the dissolved Mn and Ni concentrations in the 2 M ZnSO₄ electrolyte by ICP-OES based on the three-electrode tank test system. After 100 cycles, the dissolution of Mn in Ni-MnO₂ can be effectively alleviated, while Ni is basically insoluble (Figure 3g). To intuitively understand the improvement of structural stability by Ni doping, we performed ex situ SEM characterization of MnO₂ and Ni-MnO₂ (Figure S10, Supporting Information). Ni-MnO₂ nanomorphology coated on the carbon cloth did not dissolve after 100 charge–discharge cycling, while the dissolution of MnO₂ nanoarrays was extremely serious (Figures S6a and S11, Supporting Information). As a result, the capacity retention rate of γ-MnO₂ electrode was only 43.5% after 100 cycles at 0.1 A g⁻¹, suggesting its extremely poor stability. The γ-MnO₂ and Ni-MnO₂ were then tested for long-life performance. Excitingly, compared with γ-MnO₂ (Figure 3h), Ni-MnO₂ exhibits excellent cycle stability (the specific capacity maintains 63.90 mAh g⁻¹ after 11 000 cycles at 3.0 A g⁻¹). Therefore, Ni²⁺ doping can promote rapid proton migration and improve structural stability.

Ex situ SEM, XRD, and XPS were used to analyze the energy storage mechanism of Ni-MnO₂ electrodes. Figure 4a shows the schematic diagram of Ni-MnO₂ as the cathode and assembled into coin cell. Figure 4b shows the energy storage mechanism of Ni-MnO₂. The energy storage contribution of Ni-MnO₂ is mainly due to the H⁺ (de)intercalation during charging and discharging. Meanwhile, this process will also be accompanied by the reversible formation and dissolution of the byproduct zinc hydroxy sulphate (ZnSO₄·3[Zn(OH)₂]) on the surface of electrode.^{[44, 46]} Next, we use ex situ XPS to characterize and analyze the valence states of Ni-MnO₂ at different states. According to Figure 4c, with the discharge from the initial state to 0.6 V, H⁺ gradually intercalated into the host, the spin–orbit peak of Mn shifted to the left, and the Mn⁴⁺ in Ni-MnO₂ was reduced to Mn³⁺ (641.9 eV).^[51] When charged to 1.85 V, H⁺ is deintercalated from the host, and Mn³⁺ is oxidized to Mn⁴⁺ (642.3 eV), returning to the original Ni-MnO₂ crystal structure. More importantly, the spin–orbit peak corresponding to Ni²⁺ in Ni-MnO₂ did not shift, and the +2 valence is always maintained during the cycle (Figure 4d). Combined with the analysis about the reversible valence transition of Mn⁴⁺ ↔ Mn³⁺ and the ICP-OES of dissolved Ni (Figure 3g), which show that Ni plays a role of "structural pillar" to a certain extent. The reason for weakening of the Ni spin–orbit peak is related to the interference caused by ZnSO₄·3[Zn(OH)₂] coated on the electrode surface (Figure 4e–h). For comparison, we also performed ex situ XPS tests on γ-MnO₂ (Figure S12b, Supporting Information). During the entire charge and discharge, the spin–orbit peak of Mn 2p in γ-MnO₂ has an irreversible shift, implying its poor cyclic reversibility.

Furthermore, the structure of Ni-MnO₂ and γ-MnO₂ was characterized by ex situ XRD (Figure 4g and Figure S12a, Supporting Information). From the initial state to the first discharge to 0.6 V, the strong diffraction peak of ZnSO₄·3[Zn(OH)₂] (JCPDS No. 44-0675) was detected. In addition, combined with the corresponding EDX mapping of Ni-MnO₂ and ZnSO₄·3[Zn(OH)₂] in Figure S13, Supporting Information, it was proved that there was a large amount of H⁺ insertion.^[27] After charging to 1.85 V, the obvious intrinsic diffraction peaks were detected. This is due to the deintercalation of H⁺ from the Ni-MnO₂ host, the ZnSO₄·3[Zn(OH)₂] coated on the surface begins to be dissolved, and the pH value in the electrolyte is reduced to the original level, and the inner layer Ni-MnO₂ nanoarray is exposed again. The reversible dissolution/generation of large-size flaky ZnSO₄·3[Zn(OH)₂] or the appearance/disappearance of the intrinsic diffraction peaks of Ni-MnO₂ during the cycle strongly indicates its high structural stability and cycle reversibility. To reduce the test interference of the discharge product ZnSO₄·3[Zn(OH)₂], Ni-MnO₂ is characterized by the XRD based on the three-electrolyte tank at the discharge state. Figure S14, Supporting Information shows that multiple diffraction peak signals were detected during the discharge phase, including ZnSO₄·3[Zn(OH)₂] (The diffraction peak signal is weakened, illustrating that the pH value fluctuates less in large-volume electrolyte environment), MnOOH,^{[7, 25]} and a very small amount of ZnMn₂O₄.^{[28, 60, 61]} This indicates that the doped Ni²⁺ has an excellent promotion effect on the intercalation of H⁺ and strongly confirms the existence of H⁺ storage regulation mechanism.

First principle calculation was used to further verify the optimization of Ni²⁺ doping on the energy storage mechanism. H⁺/electrons can quickly migrate and conduct in the Ni-MnO₂ (Figure 5a). The band gap of Ni-MnO₂ electrode is basically unchanged after the introduction of Ni²⁺, but the impurity level produced will increase the carrier concentration (Figure 5b,c). The electron distribution of the corresponding impurity level (Figure 5d) intuitively shows that the octahedrons around Ni²⁺ have higher density of electron distribution. Therefore, the introduction of Ni²⁺ will improve the surrounding electron concentration. Figure 5e shows the proton migration pathways derived by applying the soft BV bond valence parameter approach to the Ni-MnO₂ crystal structure.^[62] The model predicts the one-dimensional H⁺ paths that are strictly separated along the tunnel. Figure 5f is the comparison of H⁺ self-diffusion paths of γ-MnO₂ and Ni-MnO₂ calculated by soft BV. Lower activation energy and favorable doping sites can promote proton migration. In addition, in this process, for protons, the adsorption of path 1 is more stable, and the barrier is slightly higher; the diffusion barrier of path 2 is lower, but it is not the most stable position. Combined with the information of Figures S15 and S16, Supporting Information, Ni²⁺ doping can effectively lower the activation energy (0.23 eV → 0.19 eV), thereby facilitating the one-dimensional H⁺ diffusion along the tunnel. Moreover, Ab initio molecular dynamics (AIMD) calculation further illustrates that Ni²⁺ doping will reduce the absorption energy (144 meV → 88 meV) of H⁺ diffusion to overcome the barrier (Figure 5g), which is consistent with the diffusion barrier results by soft BV. Figure 5h is the contour map of soft BV. Two paths of H⁺ diffusion in the Ni-MnO₂ lattice: path1 {1 × 2} tunnel and path 2 {1 × 1} tunnel, but path1 {1 × 2} tunnel adsorption will be more stable.

Zn//Ni-MnO₂ battery is assembled into the flexible package to illustrate the feasibility of its practical application (Figure 6a). Combined with Figure 6b and Figure S17, Supporting Information, Zn//Ni-MnO₂ soft-pack battery exhibits excellent rate performance. The battery delivers a discharge capacity of up to 238.7 mAh g⁻¹ at 0.1 A g⁻¹, and as the current density increases to 7.0 A g⁻¹, the capacity still exceeds 50 mAh g⁻¹. The soft-pack battery tested in Figure 6b is then cycled to 4000 cycles at 1.0 A g⁻¹ and still has ideal capacity retention (Figure S18, Supporting Information). Furthermore, it can be seen from Figure S19, Supporting Information and Figure 6c that the open circuit potential of single soft-packed battery is 1.85 V, and two batteries in series can obtain an output potential close to 3.7 V. In addition, the capacity contribution obtained by two soft-pack batteries in parallel is nearly twice that provided by single battery. To avoid the difference in performance test caused by the activation of electrode materials, we conducted a long-life test on the initialized Zn//Ni-MnO₂ soft-pack battery. After 1100 cycles at 2.0 A g⁻¹, the capacity retention rate still reaches 81.8% (Figure 6d). This indicates that the soft-pack battery has a good electrochemical stability in the energy storage of large-scale devices. After bending at various angles (0°, 90°, and 180°), the electrochemical performance of the battery has not attenuated and it works normally (Figure 6e,f). Especially, the soft-packaged battery is bent 180°, the indicator light (1.85 V) can still be normally lit. Two batteries in series can also light up the indicator light of the model car (more than 3 V). After more than 30 min, the car lights are still bright (Figure 6g).

3 Conclusion

In summary, Ni²⁺-doped MnO₂ was successfully synthesized for aqueous ZIBs. The introduction of Ni²⁺ can effectively regulate the H⁺ storage behavior and endow the Ni-MnO₂ electrode material with excellent proton migration kinetics. Meanwhile, Ni²⁺ could lower the diffusion barrier of H⁺ and selectively induce the ion intercalation. Density functional theory clearly reveals that Ni²⁺ doping significantly ameliorates structural stability and enhances conductivity of electrons. The resultant Zn//Ni-MnO₂ cell delivers excellent rate performance and ultra-long-cycle stability (After 11 000 cycles at 3.0 A g⁻¹, the capacity retention rate exceeds 100%). In addition, the flexible soft-packaged battery also displays superior electrochemical performance. The revealed feasibility of proton transmission along a one-dimensional path has greatly stimulated the further development of Zn-MnO₂ batteries and nanofluidic ionic devices.

4 Experimental Section

Conflict of interest

The authors declare no competing financial interest.