The increasing reliance of human society on fossil fuels has led to severe global environmental challenges [
1,
2]. There is a pressing need to explore new energy storage solutions. Aqueous zinc-ion batteries (AZIBs) are promising candidates for future energy storage systems and are anticipated to become strong alternatives to lithium-ion batteries, owing to their enhanced safety characteristics and high energy density [
3–
7]. Among them, AZIBs have attracted intensity attentions due to the inherently high theoretical capacity of Zn (820 mAh/g) and non-toxicity [
8–
12]. The manganese oxides as the cathode materials received significant interest due to their low cost, high discharge potentials, and satisfactory rate and cycle performance [
13–
16]. MnO
2 stands out as a cost-effective option, given the abundance and affordability of manganese, which helps reduce production expenses [
17,
18]. Its high discharge potential significantly boosts the energy density of AZIBs, allowing for greater energy density. As a non-toxic material, it meets the growing demand for sustainable energy storage solutions [
19,
20].
However, the reaction mechanism of the Mn-based AZIBs remains controversial. Three kinds of charge storage mechanisms have been reported so far: (1) The reversible Zn
2+ intercalation/deintercalation in/from Mn-based cathode [
21,
22], (2) the reversible proton reaction accompanied by deposition of zinc hydroxide sulfate species [
23,
24] and (3) the successive intercalation/deintercalation of both H
+ and Zn
2+ at different charge/discharge stages [
25–
27], whatever the case, these mechanisms are usually related to one-electron transfer of Mn
4+/Mn
3+ with the capacity at about 300 mAh/g, which limits both capacities and output voltages [
28]. The multivalent nature of Mn (+2, +3 and +4) offers potential for a two-electron Mn
4+/Mn
2+ reaction with a theoretical capacity of 616 mAh/g and a high theoretical voltage [
29–
33]. Despite efforts to enhance the MnO
2/Mn
2+ reaction, a high proton concentration is required to maintain the high redox voltage governed by the Nernst equation where protons are reactants. This two-electron transfer typically occurs in strongly acidic electrolytes, which facilitates the dissolution of MnO
2 from the electrode [
34,
35]. However, such strong acidity poses several challenges. For one, it can lead to the corrosion of the zinc metal anode, compromising the overall integrity and lifespan of the battery [
36–
39]. For another, this acidic environment can promote hydrogen evolution, which not only decreases the efficiency of the battery but also contributes to gas bubble formation that can disrupt operation. Moreover, the issue of poor reversibility persists due to incomplete dissolution of MnO
2 from the electrode surface during cycling [
40,
41]. This incomplete dissolution can hinder the effectiveness of the charge-discharge process, resulting in diminished capacity and performance over time. Addressing these challenges is essential for optimizing the use of MnO
2 in aqueous zinc-ion batteries and ensuring their long-term viability as an energy storage solution. Therefore, developing a mild electrolyte that promotes MnO
2 dissolution from the electrode while protecting the Zn metal anode from parasitic reactions is crucial for enhancing Mn-based AZIBs.
Herein, Al2(SO4)3 was introduced into the electrolyte as the additive to realize the two-electron reaction of Mn2+/MnO2 deposition/dissolution. Al3+ undergoes hydrolysis to supply necessary protons to promote MnO2 dissolution and increasing electrode potential according to Nernst’s equation. Besides, Al3+ can form an electrostatic shielding layer on the Zn surface, thus preventing the formation of Zn dendrite and ensuring long stability by inhibiting parasitic reactions. As a result, the Zn//MnO2 battery can achieve a high reversible redox reaction (1.6 V vs. Zn/Zn2+). The mild anode-friendly electrolyte enables outstanding stability and high plating/stripping Columbic efficiency of Zn anode. The Zn//MnO2 full battery demonstrates long cycling stability of 1900 cycles without capacity decay. This work holds promise for significantly enhancing the electrochemical performances of Zn//MnO2 batteries.
In three-electrode configuration, a pretreated carbon cloth served as the working electrode, a platinum plate as the counter electrode, saturated calomel electrode (SCE) as the reference electrode and 0.1 mol/L MnSO
4 + 0.4 mol/L ZnSO
4 solution with different concentration of Al
2(SO
4)
3 additives as the electrolyte to explore its effects on the redox process. The mechanism of deposition/dissolution is that Mn
2+ are oxidized during the charging process and then deposited on the carbon cloth electrode to form a solid MnO
2 layer. In the following discharge phase, these MnO
2 redissolve back into the electrolyte, promoting the reversibility of the reaction [
42]. This behavior is illustrated by the cyclic voltammetry (CV) curves shown in
Fig. 1a, which shows a distinct single-reduction peak at 0.6 V when Al
3+ are present in the system. In contrast, in the absence of Al
3+, a lower reduction peak was observed, which is consistent with previously reported data on a one electron transfer mechanism associated with the MnO
2 cathode [
43]. The difference suggests that the presence of Al
3+ significantly affects electrochemical behavior. These results are further confirmed by the constant voltage charge and current discharge behavior shown in
Fig. 1b. Notably, only one elevated discharge plateau is observed for the Al
3+ electrolyte, aligning well with the previously reported two-electron transfer mechanism [
44–
48]. Furthermore, the discharge voltage plateau increases with the concentration of Al
3+, remaining stable at 0.4 mol/L, which suggests that 0.4 mol/L Al
2(SO
4)
3 is the optimal concentration. Subsequently, a long cycling test was conducted using 0.4 mol/L Al
2(SO
4)
3 to evaluate performance stability. As depicted in
Fig. 1c, the cycling stability and Coulombic efficiency (CE) of MnO
2 cathode are substantially enhanced by introducing Al
2(SO
4)
3 into the electrolyte. Thus, it could be deduced that the Al
3+ plays a beneficial role of the MnO
2 deposition/dissolution during charge/discharge process. Overall, the addition of Al
3+ not only optimizes the discharge characteristics but also promotes a sustainable and effective electrochemical reaction, making it a valuable additive for improving the performance of manganese-based energy storage systems.
In addition, as shown in
Fig. 1d, the charge and discharge curve of the three-electrode system in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte for the first 100 cycles shows that both the capacity and discharge voltage increased with the number of cycles. This behavior underlines the positive impact of Al
3+ on the electrochemical performance and efficiency of the system, confirming its effectiveness as an additive in improving battery performance. As shown in
Fig. 1e, the Al
3+ ensures a more uniform and stable deposition of MnO
2, which is crucial for maintaining electrode integrity and performance. In a word, during discharging, the presence of Al
3+ enhances the electrochemical process by providing protons that facilitate the hydrolysis of MnO
2. This reaction not only promotes the dissolution of MnO
2 but also supports a high capacity through efficient two-electron transfer linked to the Mn
4+/Mn
2+ redox couple, achieving an impressive capacity of approximately 616 mAh/g. This synergy between the electrolyte components optimizes the overall electrochemical performance, making it a promising approach for advanced energy storage applications.
Ex-situ scanning electron microscope (SEM) characterizations of the cathode at different charge/discharge states are obtained to confirm the morphological and charge storage mechanism of electrodes with/without Al
3+. Typical SEM images of the cathode in the electrolyte with Al
3+ after the first charge in
Fig. 2a exhibit the uniformly electrodeposited MnO
2 layer coating on the surface of the carbon cloth. When discharged to 0.6 V, there is a sign of MnO
2 coating layer dissolving and peeling off from the carbon cloth. After full discharged to 0 V, the MnO
2 on the cathode disappears, restoring the pristine morphology of the clean carbon substrate. Fig. S1 (Supporting information) shows the enlarged SEM image of the carbon cloth after discharging to 0 V. It can be seen that almost no MnO
2 remains on the carbon cloth after discharge. The element mapping images of the cathode in the Al
2(SO
4)
3 + MnSO
4 + ZnSO
4 electrolyte is presented in Fig. S2 (Supporting information). At the full charge state, the Mn and O elements is uniform distribution on the carbon cloth. With the cutoff voltage of discharge decreasing, the contents of Mn and O elements gradually decrease with discharge, corresponding to the gradual dissolution of MnO
2 from the carbon cloth. By contrast, the cathode without Al
3+ after the first charge are shown in
Fig. 2b, from which the nonuniformly electrodeposited MnO
2 is coated on the surface of the carbon cloth. After discharging, there still remained many MnO
2 residual on the surface of carbon cloth even after discharging to 0 V in without Al
3+ electrolyte. As shown in
Fig. 2c and Fig. S3 (Supporting information), after charging at 1.0 V for 0.5 mAh/cm
2, new diffraction peaks appear at 37° and 67°, corresponding to the (110) and (2
0) crystal facets of
γ-MnO
2 (PDF #30–0820, space group:
P63/
mmc), respectively. And it is gradually diminished in intensity until they vanish entirely upon reaching a full discharge to 0 V. These observations further support that Al
3+ promotes more uniform MnO
2 deposition during charging and facilitates more thorough MnO
2 dissolution during discharge. The whole chemical process during cycling without or with Al
3+ can be summarized in
Fig. 2d. During charging, Mn
2+ is oxidized to MnO
2. During discharging, Al
3+ promotes the dissolution of MnO
2 into Mn
2+ by providing H
+, effectively consuming accumulated MnO
2, thereby enhancing Coulombic efficiency, power density, and capacity of the battery.
In addition to the cathode, ensuring the reaction stability of the Zn metal anode in the electrolyte is also important for the overall cell performance [
49–
51]. The long-term stability and reversibility of Zn anode in the different electrolytes were further investigated by using Zn//Zn symmetrical batteries, as displayed in
Figs. 3a and b. Exceptional Zn plating/stripping behavior with excellent cycling durability (1000 h at 1 mA/cm
2 with a capacity of 1 mAh/cm
2) in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte is illustrated. In contrast, the symmetrical cells exhibit unstable voltage fluctuation and shorter lifespans (< 100 h) in the MnSO
4 + ZnSO
4 electrolyte. This indicates that the additive of Al
3+ promotes the uniform deposition of Zn
2+ and improves the anti-corrosion performance of the Zn anode. It is probably due to that the Al
3+ accumulates on the surface of Zn anode in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte forming an electrostatic shielding layer to induce a compact and uniform Zn plating. The conclusion is supported by the SEM results in
Fig. 3c, the Zn anode maintains a smooth and dense morphology without Zn dendrites or "dead" Zn in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte after 1000 h of cycling. On the contrary, the Zn anode exhibits a rough and uneven morphology after cycling for nearly 100 h in the MnSO
4 + ZnSO
4 electrolyte (Fig. S4 in Supporting information). The digital graphs of the Zn anode after cycling 20 h clearly show smoother deposition in MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte compared to MnSO
4 + ZnSO
4 electrolyte (Fig. S5 in Supporting information). As shown in Fig. S6 (Supporting information), the XRD patterns of zinc foil cycled in with Al
2(SO
4)
3 electrolyte, were consistent with the initial pure zinc, and no additional by-products were observed. However, in the without Al
2(SO
4)
3 electrolyte, peaks corresponding to a by-product appeared in the XRD pattern of the zinc foil after cycling. These results manifest that the Al
2(SO
4)
3 introducing into the electrolyte will not only promote the MnO
2 deposition/dissolution but also effectively inhibit the parasitic reaction of zinc anode, greatly improving the overall stability of the Zn//MnO
2 batteries. Besides, the Zn anode demonstrates a higher hydrogen evolution reactions (HER) overpotential in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte compared to the MnSO
4 + ZnSO
4 electrolyte (
Fig. 3d), indicating effective mitigation of HER due to the existence of Al
3+. This is mainly due to the fact that the electrostatic shielding effect provided by Al
3+ inhibits the formation of Zn dendrites, thereby protecting the Zn metal from corrosion and HER during the cycle [
52–
54].
The electrochemical stability of the Zn anode with Al
3+ is further evaluated through CE performance measurements with Zn//Cu cells. Zn//Cu cells with Al
3+ additive exhibits stable cycling for > 1200 cycles with an impressive average CE of 99.77% (
Fig. 3e). In contrast, the Zn//Cu cell exhibits significantly lower CE and cycling stability when cycled in the MnSO
4 + ZnSO
4 electrolyte. The corresponding charge and discharge curves of the Zn//Cu cells in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte is recorded in
Fig. 3f. The presence of Al
3+ significantly improves CE and extends the cycling stability of the Zn//Cu cells. As shown in Fig. S7 (Supporting information), the diffusion mechanisms of Zn deposition on the anode surface are examined using chronoamperometry (CA) in Zn//Zn cells. When the Zn electrode is placed in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte, the current density remains steady after a short period of 2D diffusion. This behavior facilitates the deposition of fine-grained, uniformly distributed Zn. In contrast, when the Zn electrode is immersed in the MnSO
4 + ZnSO
4 electrolyte, the current density continues to increase gradually even after 50 s, indicating that a 2D diffusion process is still ongoing. This continuous rise in current density leads to the formation and growth of Zn dendrites. In summary, the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte ensures a uniform Zn deposition morphology, outstanding stability and compatibility of the Zn anode, enabling a dendrite-free and highly stable Zn anode with high CE. The excellent cycling stability and reversibility of Zn in the MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte are crucial for high-performance Zn//MnO
2 batteries.
The full batteries of Zn//MnO
2 in MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte were investigated to check the practicality (
Fig. 4a). The CV curves of Zn//MnO
2 full batteries at different scan rates (from 0.5 mV/s to 2 mV/s) is depicted in
Fig. 4b. At different scanning speeds, the shapes of CV curves are basically the same, indicating the fast reaction kinetics of this configuration in MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte. As shown in
Fig. 4c, CV curve of the Zn//MnO
2 batteries in MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte shows a completely different reduction peak compared to that in the MnSO
4 + ZnSO
4 electrolyte system, indicating a different charge storage mechanism. And the closed area of the batteries in MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte is much higher than in MnSO
4 + ZnSO
4 electrolyte, further implying that the Al
3+ great enhances the two-electron reactions of the Mn
2+/MnO
2. As shown in
Fig. 4d, the discharge voltage and CE of the Zn//MnO
2 batteries in MnSO
4 + ZnSO
4 + Al
2(SO
4)
3 electrolyte is increased with each cycle. The Zn//MnO
2 system also exhibits superior rate performance (
Fig. 4e), maintaining a discharge capacity close to 0.5 mAh/cm
2 even at a high current density of 10 mA/cm
2. In addition, as shown in the Fig. S8 (Supporting information), the discharge voltage in with Al
2(SO
4)
3 electrolyte is about 1.6 V, which is higher than many previous reports. It is indicated that the Al
2(SO
4)
3 significantly facilitate the deposition/dissolution mechanism of Mn
2+/MnO
2, thus improving the electrochemical performance [
55–
58]. The corresponding discharge curves at different current densities in
Fig. 4f demonstrate that nearly 99% of the charge capacity is preserved, even at the high current density of 10 mA/cm
2. The long-term cycling performance and CE of the Zn//MnO
2 battery at 2 mA/cm
2 are presented in
Fig. 4g. The battery with Al
3+ exhibits a stable discharge capacity without noticeable decay for 1900 cycles at a charge capacity of 0.5 mAh/cm
2, reaching a high CE of nearly 99%.
Fig. 4h shows the charge and discharge curves of the Zn//MnO
2 batteries for the first 200
th cycles in the Al
2(SO
4)
3 + MnSO
4 + ZnSO
4 electrolyte. The presence of Al
3+ significantly improves discharge voltage and prolongs the cycling stability of the Zn//MnO
2 battery. The superior cycling stability and rate capability are attributable to the highly reversible MnO
2/Mn
2+ deposition/dissolution reaction mechanism induced by Al
3+.
In conclusion, owing to the effect of Al3+, the two-electron Mn4+/Mn2+ redox reaction is remarkably inspired, leading to a higher capacity with a higher voltage and superior cycling reversibility compared to the conventional Mn4+/Mn3+ redox reaction. In addition, the presence of Al3+ ensures outstanding stability and compatibility of the Zn anode and consequently enables a dendrite-free Zn anode with a high plating/stripping CE. As results, the assembled aqueous Zn//MnO2 battery demonstrates a high discharge voltage (1.6 V) and a long cycling stability (1900 cycles without decay). The anode-friendly electrolyte facilitates stable Zn plating/stripping and high Coulombic efficiency. This new strategy of enhancing Zn//MnO2 battery via Al3+ in the electrolyte provides a promising solution for large-scale energy storage applications.
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