Suppressing Jahn–Teller Distortion in Manganese Oxides for High-Performance Aqueous Zinc-Ion Batteries

Abstract

Manganese oxides (MnO_x) have been confirmed as the most promising candidates for aqueous zinc-ion batteries (AZIBs) due to their cost-effectiveness, high theoretical capacity, high voltage platforms, and environmental friendliness. However, in practical applications, AZIBs are hindered by the Jahn–Teller distortion (JTD) effect, primarily induced by Mn³⁺ (t_2g³e_g¹) in octahedral coordination, which leads to severe structural deformation, rapid capacity fading, and poor cycling stability. This review systematically outlines the fundamental mechanisms of JTD in MnO_x cathodes, including electronic structure changes, lattice distortions, and their side effects on Zn²⁺ storage performance. Furthermore, we critically discuss advanced strategies to suppress JTD, such as cation/anion doping, interlayer engineering, surface/interface modification, and electrolyte optimization, aimed at enhancing both structural stability and electrochemical performance. Finally, we propose future research directions, such as in situ characterization, machine learning-guided material design, and multifunctional interfacial engineering, to guide the design of high-performance MnO_x hosts for next-generation AZIBs. This review may provide a promising guideline for overcoming JTD challenges and advancing MnO_x-based energy storage systems.

1. Introduction

The growing demand for large-scale and sustainable energy storage systems has driven extensive research into safe, cost-effective, and environmentally friendly battery systems. Currently, lithium-ion batteries (LIBs) dominate the market due to their long cycle life, high energy density, and superior specific capacity, making them the preferred choice for large-scale energy storage and electric vehicles [1,2,3,4]. However, their widespread adoption faces significant challenges, including the scarcity of lithium resources and safety concerns associated with flammable organic electrolytes. Therefore, new energy storage systems are urgently needed to meet the demand for sustainable and eco-friendly energy systems.

Among these, aqueous zinc-ion batteries (AZIBs) have emerged as a promising candidate for LIBs due to their high theoretical capacity (820 mAh g⁻¹ and 5855 mAh cm⁻³), high energy density, low cost, eco-friendliness, and ease of recycling [5,6,7,8]. Among various cathode materials, manganese oxides (MnO_x) have emerged as one of the most promising candidates owing to their cost-effectiveness, high theoretical capacity, multiple oxidation states, environmental benignity, and abundant natural reserves [9,10,11,12]. However, in practical application, MnO_x cathodes in AZIBs are significantly hampered by the Jahn–Teller distortion (JTD) effect, which originates from the electronic configuration of Mn³⁺ (t_2g³e_g¹) in octahedral coordination, resulting in elongation or compression of Mn-O bonds, and thus, inducing asymmetric lattice distortions and destabilizing the host structure [13,14,15,16]. Although Mn (IV) can undergo multi-electron transfer processes to deliver higher capacity, during the discharge process, partial reduction of Mn (IV) to Mn (III) occurs alongside disproportionation reactions (2Mn³⁺ → Mn²⁺ + Mn⁴⁺) and exacerbate the capacity fading. When MnO₂ serves as the cathode material, the dissolution of Mn²⁺ induces structural collapse in the crystal framework, resulting in diminished ion storage sites. Concurrently, Mn²⁺ dissolution reduces active material content, thereby compromising the cathode’s reversible capacity. Therefore, the JTD effect will trigger a severe structural collapse, compromised cyclability, and deteriorate Zn²⁺ diffusion kinetics [17,18,19]. Nowadays, to address the challenges, extensive efforts have been devoted to suppressing JTD through strategies such as heteroatom doping (e.g., cation, anion, or oxygen vacancies), interlayer engineering, surface modification, and electrolyte optimization [20,21,22,23,24,25]. These approaches aim to stabilize the MnO_x framework, enhance intrinsic conductivity and charge/mass transport efficiency, and improve Zn²⁺ storage reversibility.

In this review, we systematically elucidated the mechanism of Jahn–Teller distortion in MnO₂, influencing factors, and demonstrated adverse effects on the structure while analyzing how these structural changes affect electrochemical performance. Then, we introduced current common strategies for suppressing Jahn–Teller distortion and finally outlined future directions for mitigating this phenomenon.

2. Fundamentals of Jahn–Teller Effect in MnO_x

Jahn–Teller distortion is an important phenomenon in transition metal oxides, particularly in MnO₂-based materials, where structural deformation arises from the inherent electronic instability of specific transition metal ions and their tendency to minimize energy in octahedral coordination environments [26,27,28,29,30]. As a cornerstone of crystal field theory, this effect explains how Mn³⁺-containing octahedral complexes (t_2g³e_g¹ configuration) undergo spontaneous symmetry breaking through tetragonal distortion to achieve system stabilization. The distortion originates from the degeneracy of e_g orbitals (dz² and dx² − y²) in high-spin Mn³⁺ (d⁴) ions, where asymmetric electron occupation triggers lattice deformation to lift orbital degeneracy and lowers the overall energy state [1,31,32,33,34].

In MnO₂ cathodes for aqueous zinc-ion batteries, this effect manifests through three critical aspects. Firstly, the electronic structure, the labile e_g electron in Mn³⁺, induces unequal Mn-O bond lengths [35], such as elongation and/or compression. Secondly, structural consequences trigger irreversible phase transitions (layered → spinel) and MnO₆ octahedra tilting [36]. Ultimately, electrochemical impact accelerates Mn dissolution via disproportionation and deteriorates Zn²⁺ diffusion [37]. The aforementioned aspects will be discussed in detail below.

2.1. Electronic and Structural Basis

In manganese dioxide (MnO₂), the Jahn–Teller (JT) effect plays a pivotal role in determining the electronic and structural properties, which arises from the electronic instability of Mn³⁺ (3d⁴) ions in octahedral coordination, and thus significantly influencing their electrochemical behaviors [38]. Universally, the valence state of Mn is +4 in MnO₂. However, during battery operation, the cathode gains electrons, and the valence of Mn is reduced from Mn⁴⁺ to Mn³⁺. For Mn³⁺ (3d⁴), under the influence of an octahedral crystal field with sixfold coordination, the 3d orbitals split into two energy levels [39,40]: t_2g (triply degenerate, lower energy) and e_g (doubly degenerate, higher energy). According to the Aufbau principle, the four 3d electrons preferentially occupy the lower-energy t_2g orbitals before filling the higher-energy e_g orbitals [41,42]. As illustrated in Figure 1 [43], three electrons occupy the t_2g orbitals, while the remaining electron resides in the e_g orbitals. The e_g orbitals (comprising dz² and dx² − y²) host only one electron, creating an asymmetric electron distribution [44,45]. This imbalance induces directional shielding effects: the dz² orbital provides stronger shielding along the axial (z-axis) direction. The dx² − y² orbital exhibits reduced electron shielding in the equatorial plane compared to the dz² orbital, which would create the asymmetry of the electron and establishes unequal electrostatic repulsion between Mn³⁺ ions and the surrounding oxygen atoms [46,47]. Therefore, this imbalance drives the characteristic Jahn–Teller distortion, where the MnO₆ octahedron undergoes either axial elongation or compression to achieve a lower-energy configuration. During the electrochemical reduction of MnO₂, the conversion from Mn⁴⁺ to Mn³⁺ promotes this distortion process, then elongates the Mn-O bonds along the y-axis direction [48,49]. The resulting structural deformation generates substantial localized lattice strain that accumulates with cycling, eventually leading to the degradation as follows: a microstructural collapse that exposes fresh active surfaces to the electrolyte, initiation of the Mn³⁺ disproportionation reaction (2Mn³⁺ → Mn²⁺ + Mn⁴⁺ [50]) at the electrode–electrolyte interface, and irreversible phase transformation to the thermodynamically stable spinel-type MnO₂. Finally, the capacity will fade due to both active material loss and increased charge transfer resistance, originating from the dissolved Mn²⁺ species migrating into the electrolyte solution and structural reorganization of the remaining Mn⁴⁺ [51].

This comprehensive degradation mechanism highlights the critical interplay between electronic structure (orbital asymmetry), atomic-scale structural changes (Jahn–Teller distortion), and macroscopic electrochemical performance in MnO_x cathodes. The understanding of these coupled phenomena provides essential insights for developing stabilization strategies targeting each degradation pathway.

2.2. Impact of Jahn–Teller Effect on Electrochemical Implications in AZIBs

Based on this fundamental understanding, we will investigate the correlation between Jahn–Teller distortion effects and the electrochemical performance of MnO₂ cathodes in AZIBs [13,14,52]. As one of the most promising host materials for AZIBs, MnO₂’s intrinsic properties govern the performance by the following key aspects such as the specific capacity, determined by accessible redox-active sites and their utilization efficiency, cycling stability, dictated by structural integrity, and the rate capability, influenced by ion transport kinetics [17,53,54,55].

The charge-storage mechanism in MnO₂ mainly relies on the reversible insertion/extraction of Zn²⁺/H⁺ ions [56]. However, during the reversible process, strong electrostatic interactions with the host lattice will be created during migration due to the substantial ionic radius and high charge density of Zn²⁺. These interactions aggravated the inherent Jahn–Teller distortion of Mn³⁺-containing octahedra, with each electrochemical cycle progressively accumulating lattice strain. This cyclic distortion ultimately leads to manganese dissolution via disproportionation and structural collapse through bond-length fatigue, both contributing to capacity fading [57,58,59,60]. The specific capacity, cycling performance, and dissolved Mn²⁺ comparison of the modified MnO₂ cathode materials of aqueous Zn-ions batteries are summarized in

Table 1. Recent capacity retention and Mn dissolution of the cathode.

Cathode	Specific Capacity	Cycling Performance	Dissolved Mn2+	Refs.
δ-MnO2	125 mAh g−1 at 0.2 A g−1	14.3% after 200 cycles at 1 A g−1	2.5 mg L−1 after 50 cycles	[61]
α-MnO2/MGS	382.2 mAh g−1 at 0.3 A g−1	94% after 3000 cycles at 3 A g−1	0.42 mg L−1 after 1 cycle	[62]
Se-MnO2	386 mAh g−1 at 0.1 A g−1	78% after 5000 cycles at 3 A g−1	0.71 mg L−1 after 300 cycles	[63]
Al-MnO2	379 mAh g−1 at 0.2 A g−1	87% after 1000 cycles at 1 A g−1	0.12 mg L−1 after 50 cycles	[61]
δa-MnO2	175 mAh g−1 at 0.5 A g−1	91% after 500 cycles at 1 A g−1	0.54 mg L−1 after 100 cycles	[60]
Mn2O3@PPy	353.9 mAh g−1 at 0.5 A g−1	82% after 500 cycles at 1 A g−1	0.27 mg L−1 after 200 cycles	[64]
BMO	348 mAh g−1 at 0.1 A g−1	60% after 2000 cycles at 1 A g−1	0.015 mg L−1 after 100 cycles	[65]
δ-MnO2 (ZS-DOP electrolyte)	160 mAh g−1 at 1 A g−1	80% after 70 cycles at 7.5 mA g−1	1.2 mg L−1 after 100 cycles	[66]
AMO	400 mAh g−1 at 0.1 A g−1	94.5% after 2000 cycles at 2 A g−1	0.25 mg L−1 after 300 cycles	[67]

.

Jahn–Teller effect impacts capacity through the following interrelated mechanisms [68]. First, manganese dissolution reduces the quantity of electroactive material [69]. Then, lattice distortion decreases the accessibility of remaining Zn²⁺ storage sites by modifying coordination environments [70,71]. Finally, the induced strain field disrupts ion migration pathways, increasing the activation energy for Zn²⁺ diffusion in severely distorted regions, which can be confirmed by recent density functional theory calculations [72].

In conclusion, the structural evolution studies reveal that the Jahn–Teller effect drives irreversible phase transitions in MnO₂ polymorphs, which reduce both Zn²⁺ storage sites and diffusion coefficients by an order of magnitude, consistent with the phenomenon observed experimentally. The cumulative Jahn–Teller distortion during cycling initiates a cascade of degradation processes in MnO₂ cathodes, including local lattice strain accumulation, manganese dissolution, structural collapse, and detrimental phase transitions. These interrelated phenomena collectively limit the cycle life and restrict the practical capacities of MnO₂ in most reported systems. Currently, the mitigation strategies mainly focus on orbital engineering through transition metal doping and strain–relief structural designs, which will be discussed in subsequent sections.

3. Mitigation Strategies to Suppress the Jahn–Teller Effect

To overcome these limitations, strategic modification of MnO₂ has become imperative, with the central objective of stabilizing the crystal lattice against Jahn–Teller-induced deformations.

Figure 2 shows the currently reported modification strategies to suppress the Jahn–Teller effect mentioned. It focuses on the following fundamental aspects of the distortion mechanisms: strengthening the main structure through cationic/anionic doping [73], electronic structure modulation to reduce e_g orbital degeneracy, and interface stabilization via surface engineering [74]. These strategies simultaneously address the root causes of Jahn–Teller effects and improve zinc-ion diffusion kinetics and charge transfer efficiency. Notably, recent advances also combine the above approaches with electrolyte optimization to promote synergistic stabilization effects [75]. This multi-dimensional modification paradigm will demonstrate promising improvement strategies toward advanced MnO₂-based materials.

3.1. Strategic Cationic Doping for Structural Stabilization

Nowadays, to suppress the Jahn–Teller distortions and enhance the structural stability of the MnO₂ based materials, cation doping has been confirmed as one of the most effective strategies, which fundamentally operates through the strategic substitution of Mn⁴⁺ in MnO₆ octahedra with foreign cations, thus simultaneously modulates bond strength through altered orbital hybridization, optimizes the local crystal field environment, and regulates valence electron distribution to effectively suppress Jahn–Teller distortions. For example, Zhao et al. [61] employed electrochemical oxidation to synthesize Al-doped δ-MnO₂ (Al_xMnO₂, x ≈ 0.1), where controlled Al³⁺ substitution created beneficial cation vacancies (Figure 3b) that enabled three-dimensional Zn²⁺ diffusion with reduced energy barriers while providing additional redox-active sites. In addition, the stabilization mechanism is confirmed through the comprehensive characterizations. First, inductively coupled plasma (ICP) analysis demonstrated a significant reduction in Mn dissolution. Second, density functional theory (DFT) calculations revealed a dramatic increase in Mn dissolution energy, indicating enhanced lattice cohesion. Third, the variation in Mn-O bond length during Zn²⁺ insertion was markedly suppressed, with the decrease in bond length, ultimately delivering exceptional electrochemical performance (327.9 mAh g⁻¹ at 1 A g⁻¹ with 92.4% capacity retention over 1000 cycles) (Figure 3c). In addition to Al³⁺ doping discussed in the aforementioned work, other cation dopants such as Ni also exhibit inhibitory effects on Jahn–Teller distortion. Liang et al. [79] prepared Ni-doped α-MnO₂ (Ni-MnO₂) as shown in Figure 3d. Compared to Mn⁴⁺, Ni²⁺ has a smaller atomic radius; its occupation in the lattice causes the surrounding O atoms to be arranged more closely, thereby Ni²⁺ doping resulting in shorter and more uniform chemical bond lengths compared to Mn-O bonds at equivalent positions. The electron localization function (ELF) diagram shown in Figure 3e also shows that the electron localization around the Ni²⁺ is enhanced compared to the Mn ion at the same position, indicating that the Ni-O has a stronger interaction and the material’s stability is improved. The analysis of the results is shown in Figure 3f. Ni-MnO₂ has extremely excellent cycle stability. This optimized bond length effectively suppressed Jahn–Teller distortion, significantly enhancing the material’s cycling performance. Collectively, these findings validate the effectiveness of cationic doping in mitigating Jahn–Teller distortions while simultaneously enhancing both the capacity and cycling stability in AZIBs.

3.2. Anionic Doping for Enhanced Structural Resilience

The incorporation of anionic species at the O sites of MnO₆, attributed to the varying covalency of Mn-X interactions, where X represents an anion such as F⁻, S²⁻, and PO₄³⁻, has also been confirmed as one of the most promising approaches to suppress the JTD and improve the electrochemical properties. By introducing these species, the distortion of MnO₆ octahedra can be effectively suppressed. Thus, Ye et al. [63] explored the anion species of Se to mitigate Jahn–Teller distortions of MnO₂ (Figure 4a). The mechanism of doping effect and inhibition of Jahn–Teller distortions are evidenced by comprehensive experimental and computational analyses. As shown in Figure 4e, the charge density differentials reveal a substantial weakening of Zn²⁺ host electrostatic coupling in Se-MnO₂, directly correlating with reduced lattice strain and suppressed octahedral distortion. The stabilization effects are quantitatively demonstrated through comparative bond length analysis (Figure 4b), confirming effective suppression of both bond elongation and manganese dissolution, which are the hallmark manifestations of Jahn–Teller distortion. The stabilization mechanism of the doping effect to suppress the Jahn–Teller is primarily reflected through the following aspects: optimized ion adsorption energetics (Figure 4d) and inhibited H⁺ co-intercalation that typically exacerbates structural degradation. These synergistic improvements enhanced the electrochemical performance of Se-MnO₂, which maintains 98.5% capacity retention over 1000 cycles and delivers 325 mAh g⁻¹ reversible capacity, confirming that anionic doping is a potent strategy for developing stable MnO₂ cathodes. Similar to the Se element doping, other anion dopants also substitute oxygen positions in the original [MnO₆] octahedra. Zhao et al. [80] synthesized S-MnO₂ (Figure 4f). The roles of these anion dopants in cathode materials are analogous: they regulate the electronic structure of MnO₂ and weaken the electrostatic interactions between Zn²⁺ and the host material. As shown in Figure 4g, S-MnO₂ has a lower diffusion energy barrier of Zn²⁺, and the charge density difference distribution in Figure 4h indicates the same result. In sharp contrast, the insertion of Zn in S-MnO₂ possessed a smaller charge depletion area, indicating less electronic interaction between the inserted Zn²⁺ and S-MnO₂. The capacity fading caused by Jahn–Teller distortion is mitigated (Figure 4i). The introduction of anion dopants primarily modulates the electronic structure while structurally exerting dual effects: weakening Zn²⁺-host electrostatic interactions and strengthening Mn-O bonds. These synergistic actions collectively suppress Jahn–Teller distortion and enhance cycling stability.

3.3. Interlayer Engineering for Structural Stabilization

The strong electrostatic interactions between Zn²⁺ ions and the host lattice interlayers serve as a primary driver of Jahn–Teller distortion. Therefore, it is crucial to make efforts to mitigate these interactions to regulate the structural stability of the MnO₂ host. Interlayer engineering has emerged as a multifaceted strategy to counteract Jahn–Teller distortions in MnO₂ cathodes through simultaneous geometric confinement and electronic structure modulation. Among the various modulation strategies, the pre-intercalation of pillaring structures between the layers is an effective approach to simultaneously stabilize the interlayer framework and expand the interlayer spacing. Zhang et al. [76] demonstrated this concept by introducing Cu²⁺ pillars (CuMO) into the MnO₂ structure, as shown in Figure 5a, achieving an expanded interlayer spacing of 0.7 nm. This structural modification yielded remarkable electrochemical performance, with CuMO delivering enhanced specific capacity and maintaining stable cycling over 700 cycles at 5 A g⁻¹ in Figure 5b. The reduced Zn²⁺ insertion energy in CuMO (Figure 5c) directly confirms the weakened electrostatic interactions between the host structure and Zn²⁺ ions, facilitating easier ion intercalation. Additionally, the charge-discharge mechanism of CuMO (Figure 5d) reveals the mechanism of the Cu²⁺ pillars maintaining structural integrity during the electrochemical processes according to the stable lattice spacing observed before and after cycling. This dual functionality of Cu²⁺ pillars—both expanding the interlayer space and providing robust structural support—ensures the preservation of crystal structure integrity while simultaneously mitigating the detrimental effects of Jahn–Teller distortion. In addition to ion pillars, other species can also be confirmed as the ideal interlayer structures, such as Na⁺ [81], K⁺ [82], and PVA [83], et al. have also been demonstrated as effective interlayer engineering strategies for MnO₂.

Beyond inorganic atoms, organic molecules also serve as effective interlayer pillars. PVP-pillared δ-MnO₂ synthesized by Zhang et al. [84] in Figure 5e,g indicates PVP expanded the interlayer spacing and weakened the electrostatic interactions between Zn²⁺ and the host structure. Figure 5g shows a conspicuous terrace-shaped superlattice structure. In MnO₂/MXene, the superlattice significantly reduced ion diffusion barriers, and the diagram of the PVP-MnO₂ hybrid superlattice in Figure 5h shows the increased electron entropy triggers more t_2g–e_g transitions for better reactivity, while the selective proton Grotthuss intercalation behavior is thus stemmed from the hybrid superlattice structure and optimized charge distribution. This inhibition of Zn²⁺ insertion provided high specific capacity (Figure 5i) and improved the rate ability of the material (Figure 5j).

3.4. Hierarchical Surface–Interface Stabilization Strategies

In practical applications, the MnO₂ cathodes demand an integrated stabilization approach coordinating atomic-scale electronic modulation with macroscopic structural engineering to address the multiscale challenge of Jahn–Teller distortion in AZIBs. At the atomic level, surface doping induces orbital hybridization, which will reduce the e_g orbital splitting energy and prevent localized strain accumulation during cycling. Therefore, surface coating is one of the effective measures to simultaneously enhance the intrinsic conductivity and suppress the volume expansion/Mn dissolution to mitigate structural collapse caused by Jahn–Teller effects. Carbon-based materials have emerged as the most promising candidates for this application due to their exceptional electrical conductivity, remarkable structural versatility, and excellent cost-performance ratio.

Wu et al. [62] developed a graphene-coated MnO₂ nanowire composite (MGS) through a controlled hydrothermal synthesis process (Figure 6a). The conformal graphene coating serves as both an efficient electron conductor and structural stabilizer. This core-shell architecture demonstrates excellent rate capability (89% capacity retention at 3 A g⁻¹, Figure 6c), as well as outstanding cycling stability (92% capacity retention over 3000 cycles at 3 A g⁻¹, Figure 6d).

The kinetic process was analyzed with galvanostatic intermittent titration technique (GITT), revealing a two-stage Zn²⁺ insertion mechanism and Figure 6b plays the two-step intercalation mechanism of MGS cathode: an initial rapid intercalation phase (DP I) with high diffusion coefficient at the MnO₂/graphene interface, and a slower diffusion-limited phase (DP II) with Zn²⁺ penetrating into the 2 × 2 tunnels of α-MnO₂. The kinetic transition at 1.30 V causes reactant accumulation, with DP II producing substantially greater lattice strain that typically triggers structural damage and Mn dissolution in bare MnO₂. However, the graphene coating in MGS effectively suppresses these degradation pathways, as evidenced by a 78% reduction in Mn²⁺ dissolution compared to uncoated MnO₂ nanowires.

Jiang et al. [85] prepared a β-MnO₂ material coated with thin graphite films via a P-milling strategy, as shown in Figure 6e. The pores within β-MnO₂ facilitated electrolyte infiltration, while the graphite-integrated structure significantly enhanced the material’s conductivity and suppressed Mn dissolution. Consequently, the β-MnO₂@C cathode exhibited exceptional rate capability and cycling stability (Figure 6f). As illustrated in the discharge mechanism schematic (Figure 6g), during discharge, zinc ions precipitate within the pores between the graphite layers and β-MnO₂. The graphite coating preserves the structural integrity of β-MnO₂, maintains its ion storage sites, and ensures high capacity retention in β-MnO₂@C (Figure 6h).

It suggested that the comprehensive protective mechanism, combining strain redistribution, electronic connectivity maintenance, and dissolution suppression, shows a generalizable modification approach, which can be successfully extended to other coating materials, including conductive polymers (e.g., PEDOT) and metal oxides (e.g., Al₂O₃). Therefore, the surface engineering strategies demonstrate the validity of stabilizing Jahn–Teller-active electrode materials in rechargeable battery systems.

3.5. Electrolyte Optimization in Suppressing Jahn–Teller Effect

In addition to the electron and structural factors, electrolyte engineering has also emerged as one of the effective strategies for mitigating Jahn–Teller distortions in MnO₂ cathodes through interfacial chemistry control and bulk electrolyte modulation. Recent advances demonstrate that it can effectively stabilize the Mn³⁺ state through tailoring electrolyte formulations by modifying the solvation structure of Zn²⁺ ions and creating protective interfacial layers.

Acetate-based electrolytes, for instance, exhibit unique advantages through the formation of adsorption-induced passivation layer on MnO₂ surfaces. For example, Zeng et al. [86] study the MnO₂ charge/discharge mechanism (Figure 7a). The reaction pathway shifts from single-electron to two-electron transfer by replacing conventional sulfate anions with acetate groups, enabling direct Mn⁴⁺ → Mn²⁺ conversion and, thus, suppressing Jahn–Teller-active Mn³⁺ intermediates (Figure 7c). This change is mainly attributed to the modification effect of the acetates, as shown below. First, the H₂O* adsorption energy is weakened (Figure 7b) while the Mn²⁺ discharge product is stabilized. Then, the transformed reaction kinetics yield exceptional electrochemical performance, including remarkable rate capability (Figure 7d) (70 mA cm⁻² with minimal capacity loss) and ultra-stable cycling over 4000 cycles. The acetate electrolyte simultaneously addresses the challenges of eliminating Jahn–Teller distortion, suppressing manganese dissolution, and enhancing proton transfer kinetics, providing a new paradigm in aqueous zinc battery design.

Beyond suppressing Mn dissolution by adding Mn²⁺-containing salts, high-concentration electrolytes also effectively mitigate Mn dissolution. Huang et al. [87] developed a 46.5 M NH − Ac − NH₃ − Zn(Ac)₂ − Mn(Ac)₂ buffered solution (HCDCE electrolyte) for aqueous zinc-ion batteries. Figure 7e illustrates the mechanism of HCDCE: the complexation between NH₄Ac/NH₃ and Zn²⁺ weakens Zn²⁺ solvation, reducing structural damage from electrostatic interactions during Zn²⁺ insertion/extraction. Additionally, the NH₄⁺−NH₃ buffer pair maintains stable electrolyte pH (Figure 7f,g), minimizing by-product formation and preserving electrochemical activity. This endows the material with exceptional cycling stability (4500 cycles with near-100% capacity retention, Figure 7h) and rate capability (Figure 7i).

Modification of electrolytes is achieved through three effective approaches: applying Le Chatelier’s principle by adding homologous salt ions to suppress Mn dissolution, utilizing high-concentration salt solutions to reduce Zn²⁺ solvation, and employing ionic buffer pairs to maintain pH stability. This strategy simultaneously addresses the challenges of eliminating Jahn–Teller distortion, suppressing manganese dissolution, and enhancing proton transfer kinetics, providing a new paradigm in aqueous zinc battery design.

4. Summary and Perspectives

In this review, we systematically studied the fundamental mechanisms and external triggers of Jahn–Teller distortions, analyzed the unfavorable effects on MnO₂ cathodes, and provided targeted mitigation strategies. In AZIBs, while the MnO₂-based host has achieved remarkable progress, the critical challenges for commercial implementation still need to be resolved. Then, we would suggest future prioritized research as the following directions:

First, it should deeply investigate the correlation between Jahn–Teller effects and charge storage mechanisms. Currently, the understanding remains incomplete due to the complex, condition-dependent reaction pathways observed in MnO₂ cathodes. Therefore, the advanced in situ characterization platforms combining synchrotron X-ray diffraction, electrochemical mass spectrometry, and atomic force microscopy could provide unprecedented insights into real-time structural evolution and ion transport dynamics.

Second, we should focus on the design of comprehensive regulation strategies. Although Jahn–Teller distortion can arise from multiple factors, its fundamental origin lies in the intrinsic electronic structure of MnO₂. Thus, cationic/anionic doping strategies that modulate electronic configurations play pivotal roles in suppressing Jahn–Teller distortion. Notably, the electrostatic interactions between Zn²⁺ and the host framework, coupled with structural degradation induced by water molecule intrusion, exacerbate Jahn–Teller effects. Consequently, integrating ion doping with interfacial engineering (e.g., pillar interlayer design) and electrolyte optimization to achieve multi-pronged suppression of Jahn–Teller distortion is critical for advancing MnO₂-based electrodes. Firstly, synergistic doping-pillaring simultaneously tunes electronic structures and mitigates Zn²⁺-framework electrostatic forces. Secondly, doping-electrolyte coupling modifies charge/discharge mechanisms while optimizing electron delocalization.

Thirdly, research on by-products, intermediate products, and electrolyte additives should be strengthened. During the charge/discharge process, MnO₂ cathode surfaces tend to generate by-products or intermediate products, which can affect battery performance and even alter the charge/discharge mechanisms. Currently, there is a lack of systematic studies on the types, structures, and influencing factors of these intermediate products. Electrolyte additives can effectively suppress surface-side reactions and stabilize material structures during charge/discharge cycles. While the effect of a single additive is limited, effective synergistic combinations of multiple additives may yield better results.

Fourth, Jahn–Teller distortion plays a critical role in battery systems utilizing MnO₂-type octahedral frameworks as electrode materials, owing to structural deformation in transition metal complexes with octahedral coordination under specific electronic states to achieve energy minimization. This phenomenon manifests broadly across various battery systems, necessitating strategies to enhance the structural stability of manganese-based materials. Representative examples include suppression of Jahn–Teller distortion in LiMn₂O₄ spinel cathodes for Li-ion batteries, structural stabilization of layered NaMnO₂ oxides for Na-ion batteries, and optimization of MnO₂-based cathodes for Ca-ion batteries. Thus, mitigating Jahn–Teller distortion not only enhances the performance of zinc-ion battery cathodes but also serves as a universal strategy for advancing multivalent ion battery technologies.