Unveiling the Charge Storage Mechanism of High‐Performance LiV₃O₈ Cathode for Mn²⁺/H⁺ Hybrid Batteries

Abstract

Mn‐based energy storage systems are gaining attention as promising candidates for next‐generation aqueous batteries, owing to their higher theoretical energy density and capacity compared to conventional Zn‐based systems. This advantage is primarily attributed to the lower standard redox potential of the Mn anode (−1.19 V vs SHE) relative to that of Zn (−0.76 V vs SHE). In this study, an Mn²⁺/H⁺ hybrid aqueous battery system utilizing LiV₃O₈ is presented as the cathode material, which delivers a high specific capacity of 204.58 mAh g⁻¹ and excellent capacity retention of 76.2% after 7,000 cycles. The charge storage mechanism of LiV₃O₈ is thoroughly investigated through structural characterization, as well as diffusion pathway and energy barrier analyses. Proton insertion is identified as the dominant charge carrier and is found to induce the formation of Mn(OH)₂ on the electrode surface, as confirmed by spectroscopic techniques. Notably, the Mn//LiV₃O₈ cell achieved an operating voltage of 1.1–0.2 V higher than that of the conventional Zn//LiV₃O₈ cell. This study underscores the potential of Mn²⁺/H⁺ hybrid systems as next‐generation aqueous batteries and offers a comprehensive understanding of the associated reaction mechanisms, providing valuable guidance for the future design of Mn‐based aqueous energy storage technologies.

In this study, LiV₃O₈ is explored as a cathode for an aqueous Mn²⁺/H⁺ hybrid battery. It delivers a high capacity of 204.58 mAh g⁻¹ and retains 76.2% after 7000 cycles. Detailed analyses reveal the charge storage mechanism involving proton intercalation and limited Mn²⁺ diffusion due to high migration barriers.

1. Introduction

Currently, lithium‐ion batteries (LIBs) are the most widely used energy storage systems worldwide, owing to their high energy density and excellent capacity. However, the rapid consumption of lithium resources has raised concerns regarding resource scarcity and increasing costs, thereby underscoring issues related to sustainability and long‐term economic feasibility.^{[ 1 ]} Furthermore, as the global demand for LIBs continues to surge, safety concerns have become more critical, particularly due to the flammable nature of organic electrolytes, which also pose environmental hazards.^{[ 2 ]} To address these limitations, significant efforts have been devoted to the development of next‐generation energy storage systems. Among them, aqueous batteries have attracted considerable attention due to their inherent safety, environmental friendliness, and cost‐effectiveness.^{[ 3 ]} The use of water‐based electrolytes eliminates the need for organic solvents, greatly reducing the risks of fire and explosion, while also minimizing environmental impact. Capitalizing on these advantages, a wide range of aqueous battery systems have been investigated using various metal ions—including monovalent ions such as Li⁺,^{[ 4 ]} Na⁺,^{[ 5 ]} and K⁺,^{[ 6 ]} as well as multivalent ions like Mg²⁺,^{[ 7 ]} Ca²⁺,^{[ 8 ]} and Zn²⁺.^{[ 9 ]} Recently, systems based on multivalent ions have gained increasing interest for their potential to achieve higher energy densities and theoretical capacities.

Among these, manganese (Mn)‐based energy storage systems have emerged as particularly attractive candidates due to Mn's high theoretical capacity, favorable redox potential, and natural abundance.^{[ 10 ]} With a standard reduction potential of −1.19 V (vs SHE), Mn offers a more negative potential compared to Zn (−0.76 V vs SHE), enabling higher operating voltages in aqueous battery designs. Additionally, as a divalent ion, Mn²⁺ provides a high theoretical volumetric and gravimetric capacity (7250 mAh cm⁻³ and 976 mAh g⁻¹, respectively), making it highly competitive among multivalent charge carriers. Mn is also abundant, cost‐effective, and environmentally benign, further enhancing its appeal from both sustainability and economic standpoints. Despite these promising characteristics, Mn‐based aqueous batteries are still in the early stages of development, and relevant studies remain limited. In particular, the identification and optimization of suitable cathode materials for Mn²⁺ storage have proven challenging. Reported cathode candidates to date include Chevrel phase (Mo₆S₈),^{[ 11 ]} bilayered vanadium oxides (e.g., Al₀.₁V₂O₅·1.5H₂O,^{[ 12 ]} Mn₀.₁₈V₂O₅·nH₂O),^{[ 13 ]} V₂O₅,^{[ 11} , ^{14 ]} VO₂,^{[ 15 ]} Ag₀.₃₃V₂O₅,^{[ 16 ]} Ag₀.₁₁V₂O₅,^{[ 17 ]} nickel and manganese hexacyanoferrates (NiHCF,^{[ 11 ]} MnHCF), LiFePO₄,^{[ 18 ]} and organic materials such as graphite,^{[ 19 ]} PTCDA,^{[ 10} , ^{20 ]} and coronene.^{[ 10a ]} To advance Mn‐based aqueous batteries, the development of high‐performance, stable cathode materials is therefore essential.

LiV₃O₈ (lithium vanadium bronze) has recently emerged as a promising cathode material due to its favorable structural and electrochemical properties.^{[ 21 ]} Its crystal structure consists of VO₆ octahedra and VO₅ tetrahedra forming layered frameworks, with lithium ions positioned between these layers. This layered architecture enables a 2D diffusion pathway (Figure 1a), enhancing ion transport and contributing to superior electrochemical performance (Figure 1b). The open‐framework structure of LiV₃O₈ allows for fast ion diffusion and reversible redox reactions, which are advantageous for both high capacity and long‐term cycling stability. Based on these characteristics, we hypothesized that LiV₃O₈ is a strong candidate for Mn²⁺/H⁺ hybrid aqueous battery systems, capable of maintaining capacity and structural integrity under extended cycling conditions.

Figure 1.

a) Crystal structure of LiV₃O₈ along the ac‐plane and b) bc‐plane with diffusion pathway. c) Rietveld refinement of XRD patterns for synthesized LiV₃O₈ powder (red: data points, green: calculated pattern, pink: difference). d) SEM image of LiV₃O₈. e) TEM images of LiV₃O₈ including high‐resolution crystalline planes and f) EDX mapping of each element.

In this study, we demonstrate the electrochemical performance of LiV₃O₈ as a cathode material in Mn²⁺/H⁺ hybrid aqueous batteries. The charge–discharge reaction mechanism was elucidated through a combination of structural, elemental, and spectroscopic analyses, along with diffusion pathway and energy barrier calculations. The LiV₃O₈ cathode delivered a high specific capacity of ≈204.58 mAh g⁻¹ at a current density of 0.1 A g⁻¹, and exhibited excellent cycling stability, retaining 76.2% of its capacity after 7000 cycles at 0.8 A g⁻¹. Furthermore, comparative analysis with Zn//LiV₃O₈ cells revealed that the Mn//LiV₃O₈ system achieved a higher operating voltage of 1.1 V—an increase of 0.2 V over the Zn‐based counterpart. These findings provide valuable insights into the charge storage mechanisms in Mn‐based aqueous systems and highlight the potential of Mn²⁺/H⁺ hybrid batteries as next‐generation energy storage technologies.

2. Results and Discussion

2.1. Materials Synthesis and Characterization of LiV3O8

LiV₃O₈ was synthesized via a sol–gel method using Li₂CO₃, V₂O₅, and a diluted H₂O₂ solution as precursors. The detailed synthesis procedure is provided in the Supporting Information. The crystal structure and phase purity of the synthesized material were examined using X‐ray diffraction (XRD) combined with Rietveld refinement using the GSAS program^{[ 22 ]} (Figure 1c). The analysis confirmed that LiV₃O₈ crystallizes in a monoclinic phase with a P2₁/m space group. The refined lattice parameters were determined to be a = 6.6756(3) Å, b = 3.5963(1) Å, c = 11.813(1) Å, and β = 104.72(1)°, with a unit cell volume of 273.57(1) ų. These results indicate the formation of a well‐defined monoclinic structure without any detectable impurities. Further refinement details are summarized in Table S1 (Supporting Information). Morphological analysis was conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figures 1d and S1 (Supporting Information), SEM images reveal irregularly shaped particles ranging from ≈100 to 400 nm in size. TEM observations (Figure 1e) corroborate these findings and provide further insight into the material's microstructure. High‐resolution TEM images reveal distinct lattice fringes with an interplanar spacing of 0.63 nm, corresponding to the (100) plane—consistent with the XRD data—confirming the high crystallinity of the synthesized LiV₃O₈ (Figure S2, Supporting Information). Elemental composition was analyzed using energy‐dispersive X‐ray spectroscopy (EDX) coupled with TEM (Figure 1f). The EDX mapping confirmed the presence of vanadium (V, purple) and oxygen (O, green), in accordance with the expected stoichiometry. No manganese (Mn, orange) signal was detected for the pristine material.

2.2. Electrochemical Performances of LiV₃O₈

The electrochemical performance of LiV₃O₈ was evaluated using a custom‐designed three‐electrode cell using activated carbon (counter electrode) and Ag/AgCl reference. To examine its charge storage behavior, cyclic voltammetry (CV) was conducted at a scan rate of 0.2 mV s⁻¹ within a potential window of −0.8 to 0.7 V versus Ag/AgCl (Figure 2a). The CV curve displayed a distinct pair of redox peaks, indicative of reversible ion insertion (−0.12 V vs Ag/AgCl) and extraction (0.05 V vs Ag/AgCl) processes. Galvanostatic charge/discharge (GCD) measurements were further performed to assess capacity and cycling behavior (Figure 2b). The first and second discharge capacities were 204.58 and 207.25 mAh g⁻¹, respectively, demonstrating excellent electrochemical activity. The slight increase in capacity during the second cycle suggests an activation process that enhances charge storage performance. Rate capability was evaluated at current densities of 0.1, 0.2, 0.4, and 0.8 A g⁻¹(Figure 2c), yielding corresponding discharge capacities of 207.2, 137.8, 102.8, and 74.3 mAh g⁻¹. Notably, a stable and reversible capacity of 74.3 mAh g⁻¹ was retained even at the 0.8 A g⁻¹ current density, indicating good rate performance. GCD profiles for the first cycle at each rate are shown in Figure S3 (Supporting Information). In a previous study, Mn₀.₁₈V₂O₅·nH₂O^{[ 13 ]} was evaluated as a cathode material for Mn‐ion batteries, delivering a lower specific capacity of 133.7 mAh g⁻¹ at a current density of 0.2 A g⁻¹ compared to the material employed in this work. Furthermore, earlier research on LiV₃O₈^{[ 23 ]} as a Mn‐ion battery cathode was limited to half‐cell configurations using an activated carbon counter electrode, without investigating the use of a Mn‐metal anode. In this study, we not only performed comprehensive material characterization but also carried out full‐cell experiments employing an Mn‐metal anode(Figure 6). Long‐term cycling stability was assessed at 0.8 A g⁻¹ (Figure 2d). The initial discharge capacity was 74.7 mAh g⁻¹, which increased slightly during the initial cycles due to activation, and then gradually decreased to 54.9 mAh g⁻¹ after 7000 cycles, corresponding to a capacity retention of 76.2%. Despite prolonged cycling, the electrode maintained high electrochemical stability. Previous studies conducted cycling tests across a range of current densities and reported excellent capacity retention, even under low current (0.5A g⁻¹) density conditions.^{[ 23 ]} According to the XRD analysis (Figure S4, Supporting Information), the structure of the LiV₃O₈ electrode gradually transformed into an amorphous phase after cycling. This amorphization may partially contribute to capacity fading, as it disrupts the intercalation‐based reaction mechanism. However, as the LVO structure becomes amorphous, the diffusion length decreases, enabling easier ion access and interaction with the active material. The shortened diffusion pathways facilitate surface‐controlled processes and promote capacitive‐dominated reactions during cycling, which collectively help maintain relatively stable capacity over prolonged cycling. Consequently, this structural change does not appear to have significantly compromised the overall performance. Representative GCD profiles at the 1st, 3000th, 5000th, and 7000th cycles are provided in Figure S5 (Supporting Information). Electrochemical impedance spectroscopy (EIS) was conducted to evaluate interfacial resistance (Figure 2e). The internal resistance (R_IR) increased from 2.07 Ω (pristine) to 3.31 Ω (after 7000th), while the charge transfer resistance (R_CEI + R_ct) rose from 4.15 to 11.7 Ω. The low initial resistance is likely due to the reduced de‐solvation energy enabled by the high‐concentration electrolyte, which enhances both the ionic conductivity of the electrolyte and the interfacial contact between the cathode and the electrolyte solution, compared to a low‐concentration system. This increase is attributed to electrolyte decomposition and the formation of insulating species such as Mn(OH)₂. The decreased slope in the Warburg region of the Nyquist plot indicates reduced ion diffusion, likely due to structural amorphization after extended cycling. To further elucidate the reaction kinetics, CV was performed at various scan rates (Figure 2f). As the scan rate increased, both oxidation and reduction peak currents increased, confirming the reversible nature of the redox reactions. The b‐values, derived from the logarithmic relationship between peak current and scan rate (Figure 2g; Figure S6, Supporting Information), were 0.7856 for oxidation and 0.5316 for reduction, indicating mixed diffusion‐ and surface‐controlled kinetics for oxidation and predominantly diffusion‐controlled kinetics for reduction. Figure 2h presents the relative contributions of diffusion‐controlled and capacitive (surface‐limited) processes at different scan rates. As the scan rate increased, the capacitive contribution became more significant, likely due to surface layer formation that limited ion diffusion. A more detailed quantitative breakdown of these contributions is available in Figure S6 (Supporting Information).

Figure 2.

Electrochemical performance analysis of LiV₃O₈: a) CV at a scan rate of 0.2 mV s⁻¹. b) GCD profile at a current density of 0.1 A g⁻¹. c) Rate performance under various current densities. d) Long‐cycle performance and coulombic efficiency with an average of over 7000 cycles. e) Impedance spectroscopy of electrodes: pristine and after 7000 cycles. f) CV curves at various scan rates from 0.2 to 1.0 mV s⁻¹. g) Comparison of b‐values derived from log(scan rate) versus log(current) plots h) Relative contributions of diffusion‐controlled and surface‐limited processes at various scan rates.

Figure 6.

a–c) Comparison of Zn and Mn metal anodes in aqueous electrolyte systems using a LiV₃O₈ cathode: a) Schematic illustration of voltage comparison, b) CV curves recorded at a scan rate of 0.1 mV s⁻¹, and c) GCD profiles under 0.1 A g⁻¹ current density. d) rate performance at various current densities, e) long‐term cycling performance at a current density of 0.4 A g⁻¹, and f) EIS of the pristine cell after 100 cycles.

2.3. Investigation of the Charge Storage Mechanism of LiV₃O₈

To elucidate the charge storage mechanism of the LiV₃O₈ electrode, TEM‐EDX was conducted on samples discharged to various voltages (Figure 3a). In the pristine sample (Figure 3b), vanadium (V, green) and oxygen (O, yellow) were prominently detected, while manganese (Mn, orange) appeared only in trace amounts. This observation is also supported by the corresponding EDX spectrum (Figure S7, Supporting Information), where the V peak is dominant without the Mn signal. In contrast, in the fully discharged sample at −0.8 V (Figure 3c), V and O remained the primary elements, but the Mn signal increased noticeably, as corroborated by the enhanced Mn peak in Figure S8 (Supporting Information). Furthermore, Table S2 (Supporting Information) shows the ICP results of the electrode before and after the 7000 cycled charge sample, clearly indicating an enhancement in the Mn contents which means that Mn is contained in the electrode even after the cycle. These findings suggest that Mn is involved in redox reactions with the LiV₃O₈ cathode during discharge. However, due to the overall low Mn content, the involvement of additional processes—such as side reactions or H⁺ insertion—cannot be ruled out.

Figure 3.

a) GCD profiles from ex situ analysis of samples at different voltage states. b) TEM‐EDX elemental mapping of the pristine electrode and c) fully discharged electrode. d–g) XPS during discharge of (d) V 2p, e) Mn 2p, f) O 1s, and (g) Li 1s.

To further investigate this behavior, X‐ray photoelectron spectroscopy (XPS) was performed on electrodes at various discharge states (Figure 3d–g). Figure 3d displays the V 2p₃/₂ and V 2p₁/₂ peaks corresponding to the V⁵⁺ oxidation state. At −0.7 V, the V⁵⁺ peak is distinctly observed at 517.0 eV; however, as discharge progresses, a slight shift to 516.4 eV is detected, indicating a partial reduction of V⁵⁺ to V⁴⁺. Additionally, the overall intensity of the vanadium peaks gradually decreases with continued discharge, suggesting that vanadium serves as the primary redox‐active species in the LiV₃O₈ structure during the charge storage process. Figure 3e presents the Mn 2p₃/₂ and Mn 2p₁/₂ spectra, which exhibit increasing peak intensities with progressive discharge. This trend confirms that Mn participates in the charge storage process. Nevertheless, the formation of Mn(OH)₂ cannot be ruled out, as it may occur via chemical reactions between Mn²⁺ and OH⁻ in the aqueous electrolyte. To investigate changes in the oxygen environment, O 1s spectra were analyzed (Figure 3f). The intensity of the Metal─O peak—primarily attributed to V─O bonds—decreased with discharge, indicating alterations in the local oxygen coordination.^{[ 18 ]} Simultaneously, the O─H peak intensity increased, suggesting the formation of hydroxyl bonds, likely due to proton (H⁺) insertion into the electrode structure. These observations imply that H⁺ ions, rather than Mn²⁺, are inserted into the host lattice, while Mn²⁺ predominantly forms a Mn(OH)₂ layer on the electrode surface. This surface layer may suppress or obscure signals from the underlying host structure, contributing to the observed attenuation in spectral features. Finally, Li 1s spectra were collected to assess lithium content during discharge (Figure 3g). As anticipated, the Li signal was nearly absent in the discharged state, confirming that most lithium ions are extracted during the charging process.

To investigate structural transformation, ex situ XRD analysis was conducted at various stages of the discharge process (Figure 4a). The XRD pattern of the pristine electrode exhibits distinct reflections corresponding to the (100), (002), and (003) planes, in agreement with the pattern shown in Figure 1c. Notably, no diffraction peaks associated with Mn(OH)₂ or H_xV₃O₈ phases are present in the initial state, confirming the phase purity of the LiV₃O₈ structure. As discharge proceeds, the primary diffraction peaks remain mostly unchanged, indicating that the bulk crystal framework is largely retained. However, new peaks gradually emerge, suggesting the formation of Mn(OH)₂ on the electrode surface and the incorporation of protons into the lattice, resulting in the formation of a hydrated vanadium oxide phase (H_xV₃O₈).^{[ 24 ]} The XRD pattern after charging shows incomplete recovery to the pristine LiV₃O₈ phase, along with the appearance of a new diffraction peak (Figure S4, Supporting Information), indicating that the structure undergoes irreversible structural changes. Additionally, XPS analysis (Figure S9, Supporting Information) conducted after charging reveals that the oxidation states of vanadium and the oxygen peaks return to their pristine states. However, manganese signals remain, and the lithium signal disappears. These observations suggest that an ion exchange reaction may have occurred within the LiV₃O₈ structure during the redox process, in which lithium ions are replaced by manganese ions. Figure 4b presents a schematic illustration of the proposed electrochemical of LiV₃O₈ and side reactions occurring between the saturated MnCl₂ electrolyte, highlighting both the intercalation of protons and water molecules (Figure 4c), as well as the chemical precipitation of Mn(OH)₂ side products. Further confirmation of proton and water intercalation is provided by Fourier‐transform infrared (FTIR) spectroscopy (Figure 4c). Upon full discharge, a broad O─H stretching band appears in the 3200–3600 cm⁻¹ region due to Mn(OH)₂ formation with proton intercalation, along with a new peak ≈1600 cm⁻¹ corresponding to the bending vibration of crystalline water. These features confirm the incorporation of both protons and water into the structure, likely facilitated by hydrogen bonding and hydration effects. In addition, the V═O stretching vibration shifts from 959.6 to 942.2 cm⁻¹, indicating a weakening of the bond strength due to the reduction in the oxidation state of vanadium. Complementary evidence is provided by Raman spectroscopy (Figure 4d). After full discharge, the Raman peaks associated with V═O (478.0 cm⁻¹), V─O─V (540.4 cm⁻¹), and VO₅ (770.0 cm⁻¹) units shift to lower wavenumbers (421.4, 512.9, 743.7 cm⁻¹, respectively), suggesting a weakening of V─O bonds due to vanadium reduction (5+ to 4+). The presence of a small O─H stretching band above 3000 cm⁻¹ further confirms the existence of intercalated protons and/or water molecules. Overall, these findings demonstrate that during the discharge process, protons and water molecules from the MnCl₂‐based aqueous electrolyte are intercalated into the LiV₃O₈ lattice, forming a hydrated vanadium oxide phase denoted as H_x(H₂O)γV₃O₈.^{[ 24 ]} Simultaneously, Mn²⁺ ions in the electrolyte react with OH⁻ to form Mn(OH)₂ precipitates on the electrode surface.

Figure 4.

a) Ex situ XRD patterns and electrochemical profiles captured during discharge at each voltage. b) Schematic illustration of the reaction pathway between LiV₃O₈ and the MnCl₂ electrolyte. c) FTIR spectra of pristine and fully discharged LiV₃O₈ cathode. d) Raman spectra of pristine and fully discharged LiV₃O₈ cathode.

These charge storage reactions can be represented by the following equations:

$$\mathrm{Cathode}:\mathrm{Li}{\mathrm{V}}_3{\mathrm{O}}_8+\mathrm{x}{\mathrm{H}}^{+}+\mathrm{y}{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_{\mathrm{x}}{\left({\mathrm{H}}_2\mathrm{O}\right)}_{\mathrm{y}}{\mathrm{V}}_3{\mathrm{O}}_8+\mathrm{xL}{\mathrm{i}}^{+}$$ (1)

$$\mathrm{Side}\mathrm{reaction}:\mathrm{M}{\mathrm{n}}^{2+}+2\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{Mn}{\left(\mathrm{OH}\right)}_2$$ (2)

The diffusion pathways of Li⁺, Mn²⁺, and H⁺ ions within the LiV₃O₈ structure were calculated using the SoftBV program,^{[ 25 ]} as shown in Figure 5a–c. Each figure presents the ion migration channels along the b‐axis, confirming that ionic transport primarily occurs in this crystallographic direction. In Figure 5a, Li⁺ ions exhibit highly localized and narrow diffusion pathways, indicating limited mobility within the lattice. Likewise, Figure 5b shows that Mn²⁺ ions follow similarly constrained diffusion paths, suggesting poor diffusivity. As shown in Figure S10 (Supporting Information), the Mn diffusion pathways within the structure from different viewing directions are visualized. Consistent with previous observations, Mn migration remains highly restricted.

Figure 5.

Diffusion pathways along the b‐axis of LiV₃O₈ for a) Li⁺, b) Mn²⁺, and c) H⁺. Calculated migration energy barriers for d) Li⁺, e) Mn²⁺, and f) H⁺ diffusion. g) Proton migration energy barriers for Path 1 and Path 2.

In contrast, the diffusion pathways for H⁺ ions in Figure 5c are markedly broader and more interconnected, strongly supporting the hypothesis of active proton intercalation into the LiV₃O₈ structure. These findings are consistent with the charge storage mechanisms previously discussed in Figures 3 and 4. To further quantify ionic mobility, migration energy barriers were calculated for each ion species (Figure 5d–g). The migration energy barrier for Li⁺ along the b‐axis was found to be 1.05 eV (Figure 5d), indicating that Li⁺ transport is energetically unfavorable. For Mn²⁺, the barrier was even higher at 2.74 eV (Figure 5e), reflecting extremely limited mobility within the host structure. In stark contrast, H⁺ ions exhibited a significantly lower energy barrier of 0.135 eV (Figure 5f), suggesting favorable diffusion kinetics and high transport efficiency. Additionally, two distinct diffusion pathways for H⁺ were identified—Path 1 and Path 2—with energy barriers of 0.135 and 0.305 eV, respectively (Figure 5g). Both values are substantially lower than those calculated for Li⁺ and Mn²⁺, further confirming that protons are the most mobile and energetically favorable charge carriers in the LiV₃O₈ lattice. Additionally, Figure S11 (Supporting Information) presents the migration energy barriers calculated for each element. Similarly, Li⁺ and Mn²⁺ exhibit high migration energy barriers, while H⁺ displays two relatively low energy barriers. In results, these diffusion path and barrier analyses demonstrate that LiV₃O₈ is poorly suited for Li⁺ and Mn²⁺ diffusion, while proton transport is highly preferred. Comprehensive structural and electrochemical analyses confirmed that protons, rather than Mn²⁺ ions, serve as the primary charge carriers in this system. Various proton bonding configurations within the host structure were identified, and diffusion pathway mapping along with migration energy barrier calculations further corroborated the preferential proton insertion into the structure. In contrast, Mn²⁺ ions exhibited restricted mobility due to their high migration barriers and were found to engage predominantly in surface redox reactions, suggesting a secondary contribution to the overall charge storage mechanism. To investigate the cation's behavior in greater detail, it would be desirable to present diffusion coefficients and ionic currents through molecular dynamics simulations. This aspect will be addressed in future research. Moreover, quantifying proton insertion remains a significant challenge—not only in our system but also across the broader field of aqueous battery research, including zinc‐ion batteries, where this issue remains unresolved. We believe that electrochemical quartz crystal microbalance with dissipation monitoring (EQCM‐D) represents a promising technique for quantitatively assessing the involvement of protons and manganese. In future studies, we plan to employ EQCM‐D analysis to gain deeper insights into these charge storage mechanisms.

2.4. Comparison of Zn and Mn Metal Anodes in Aqueous Battery Systems

Figure 6 compares the voltage characteristics and electrochemical performance of Zn and Mn metal anodes in aqueous battery systems, demonstrating that Mn‐based batteries can be considered promising candidates for next‐generation aqueous energy storage systems. Figure 6a schematically illustrates their respective operating voltages when paired with a LiV₃O₈ cathode. Zn metal, with a standard electrode potential of −0.76 V (vs SHE), operates at ≈0.71 V in this configuration. In contrast, Mn metal, with a more negative standard electrode potential of −1.19 V (vs SHE), delivers a higher operating voltage of 1.14 V, highlighting its superior voltage output compared to Zn.

The electrochemical behavior of each system was further evaluated using CV at a scan rate of 0.1 mV s⁻¹ (Figure 6b). The Zn‐based cell displays oxidation and reduction peaks at 1.00 and 0.83 V, respectively, whereas the Mn‐based cell shows corresponding peaks at 1.18 and 1.01 V, further confirming its enhanced electrochemical potential. GCD measurements at a current density of 0.1 A g⁻¹ (Figure 6c) reveal a more pronounced charge plateau and a higher discharge capacity for the Mn cell, demonstrating its superior electrochemical performance. The rate capability of the Mn‐based system was evaluated at various current densities (Figure 6d). The capacity remains stable at 0.1 and 0.2 A g⁻¹ but significantly decreases at 0.4 A g⁻¹. Accordingly, long‐term cycling was conducted at 0.4 A g⁻¹ (Figure 6e), where the initial capacity of 72.6 mAh g⁻¹ gradually declined and stabilized at ≈44.3 mAh g⁻¹ after 100 cycles, corresponding to a capacity retention of 61%. The overall electrochemical performance declined when Mn metal was employed as the anode. This degradation is primarily attributed to the hydrogen evolution reaction (HER), which commonly occurs on the Mn surface and promotes the formation of a MnO₂ passivation layer. Figure S12 (Supporting Information) presents SEM–EDS analysis of Mn metal electrodes before and after cycling. The SEM image of the pristine Mn metal reveals a relatively smooth and clean surface. In contrast, the cycled Mn electrode exhibits a surface densely covered with deposited species, indicating substantial morphological changes. EDS analysis of the pristine Mn shows distinct Mn peaks along with a minor oxygen signal, which is likely attributed to surface oxidation upon air exposure. After cycling, a pronounced increase in the oxygen signal is observed, corroborating the formation of a MnO₂ passivation layer on the electrode surface. This insulating film obstructs ion transport and contributes to increased resistance and capacity fading over repeated cycles. EIS provides further evidence of this degradation, as shown in Figure 6f. The internal resistance increased from 2.2 to 3.7 Ω, while the charge transfer resistance rose significantly from 11.7 to 38.7 Ω after 100 cycles. These increases indicate greater polarization and hindered charge transfer, likely due to the buildup of surface byproducts and the formation of passivation layers. Complementary GCD profiles (Figure S13, Supporting Information) display a marked voltage drop during the initial charge phase of the 100th cycle, further corroborating the electrochemical deterioration of the Mn metal anode during extended operation. To advance Mn²⁺/H⁺ hybrid aqueous battery technology, future work should focus on the development of optimized anode materials and electrolyte formulations. Especially, systematic studies targeting HER and parasitic side reactions are essential for improving long‐term performance and stability.

This study presents, for the first time, a functional Mn²⁺/H⁺ hybrid aqueous battery utilizing LiV₃O₈ as the cathode. A high capacity of 204.58 mAh g⁻¹ and an outstanding capacity retention of 76.2% over 7000 cycles were achieved, validating the feasibility of this system for long‐term operation. These results serve as an important benchmark and highlight the potential of Mn‐based aqueous batteries, while also emphasizing the need for strategies to suppress side reactions and mitigate resistance buildup during extended cycling.

To investigate the degradation mechanisms at both the cathode and anode, we conducted XPS analyses. On the cathode side (Figure S14, Supporting Information), the spectra show no notable differences compared to the pristine state (Figure S9, Supporting Information), indicating that the surface chemical composition remains relatively unchanged. However, as evidenced by structural analyses (Figure S4, Supporting Information), the degradation likely stems from structural collapse rather than surface chemical alterations.

For the cycled Mn metal anode (Figure S15, Supporting Information), we examined the possible migration of vanadium species from the cathode via the electrolyte. No vanadium signals were detected, suggesting negligible crossover. In contrast, strong manganese signals were observed. The spectra revealed the coexistence of MnO_x and Mn(OH)_x species, suggesting the formation of a composite passivation layer consisting of manganese oxide and hydroxide. Additionally, MnCl_x signals were identified, likely originating from electrolyte salt residues or decomposition.

In summary, the cathode undergoes degradation primarily due to structural collapse, while the anode experiences surface passivation by manganese‐based oxides and hydroxide species, which may increase interfacial resistance. Together, these degradation pathways influence the cycling stability of the manganese metal‐based cell.

To further improve the performance and long‐term stability of Mn metal‐based batteries, future research should focus on suppressing the HER through two primary strategies. First, electrolyte engineering—such as the incorporation of pH buffers, optimized salts, and functional additives—can help stabilize the electrochemical environment and reduce parasitic reactions. Second, Mn metal engineering through surface modification approaches—including alloying with elements such as Sn or Bi, applying protective organic coatings (e.g., PDDA), or introducing interfacial buffer layers (e.g., TiO₂, ZrO₂)—can effectively mitigate Mn corrosion and passivation. In parallel, the design of advanced cathode materials with improved multivalent ion transport characteristics is also essential for achieving high energy efficiency and prolonged cycle life in Mn‐based aqueous battery systems.

3. Conclusion

In this study, a Mn²⁺/H⁺ hybrid aqueous battery system employing a LiV₃O₈ cathode was systematically investigated. The LiV₃O₈ electrode demonstrated outstanding electrochemical performance, delivering a high specific capacity of 204.58 mAh g⁻¹ and retaining ≈76.2% of its capacity after 7000 cycles in a saturated MnCl₂ electrolyte. An operating voltage of 1.14 V was achieved, highlighting the potential of Mn metal as a promising alternative to conventional Zn‐based anodes in aqueous battery systems. Through a combination of structural, spectroscopic, and computational analyses, the proton dual ion charge storage mechanism involving both Mn²⁺ and H⁺ ions was clearly elucidated. While H⁺ ions exhibited low migration energy barriers and favorable, reversible intercalation behavior, Mn²⁺ ions were limited by sluggish diffusion kinetics due to their high migration energy barrier. Additionally, side reactions associated with HER on the anode side—including the formation of MnO₂ and Mn(OH)₂ passivation layers on the cathode side—contributed to increased impedance and progressive performance degradation. Despite these challenges, this study provides critical insights into the strengths and limitations of LiV₃O₈ as a cathode material for Mn²⁺/H⁺ hybrid aqueous batteries. To further enhance the performance and long‐term stability of Mn metal‐based batteries, future research should focus on suppressing HER through optimized electrolyte salts and additives, Mn metal surface engineering strategies—such as Mn metal anode modification via alloying, coating, or protective interlayers, and the development of advanced cathode materials with improved multivalent ion transport characteristics. With continued progress in materials design and electrochemical optimization, Mn²⁺/H⁺ hybrid aqueous batteries offer strong potential as next‐generation, cost‐effective, and inherently safe solutions for large‐scale energy storage applications.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Pyun J., Lee H., Lee H., Kwon H., Lee H., Hong S.‐T., Lee W.‐J., Chae M. S., Unveiling the Charge Storage Mechanism of High‐Performance LiV₃O₈ Cathode for Mn²⁺/H⁺ Hybrid Batteries. Small 2025, 21, 2504200. 10.1002/smll.202504200

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.