Hybrid superlattice cathodes unlocking diffusion-barrier-free proton storage for high-rate Zn–MnO₂ batteries

Abstract

Diffusion-controlled Zn²⁺ intercalation often suffers from strong lattice repulsion when using a MnO₂ cathode in Zn-ion batteries, leading to slow reaction kinetics and irreversible phase transitions. Boosting intercalation-barrier-free Grotthuss proton storage in competition with Zn²⁺ in the MnO₂ host provides a highly promising path to develop high-kinetics and stable Zn-ion batteries, but this remains challenging. Here we incorporate tetraamino-benzoquinone (TABQ) into a nickel-doped δ-MnO₂ host to design a two-dimensional hybrid superlattice cathode (Ni–TABQ@δ-MnO₂), which triggers ultrafast proton transfer via Grotthuss topochemistry. Conductive Ni–TABQ effectively modulates the electronic properties of δ-MnO₂ through π–d electron coupling, enabling a transition from semiconducting to metallic behavior and markedly increasing the current response from 205 to 305 pA. Furthermore, the intermolecular H-bonding network between the coordination water of Ni–TABQ and lattice oxygen of δ-MnO₂ allows H₃O⁺ to transfer protons through the continuous breaking and reformation of O–H bonds. Accordingly, dynamic proton hopping within the Ni–TABQ@δ-MnO₂ cathode shows an ultralow energy barrier (0.124 eV) compared to Zn²⁺ intercalation (0.741 eV), leading to superior rate capacities (453 mA h g⁻¹ at 0.2 A g⁻¹; 151 mA h g⁻¹ at 10 A g⁻¹) and a long lifespan (8000 cycles). This study gives new insights into the design of diffusion-barrier-free proton-conductive hybrid superlattice cathodes for advanced energy storage.

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Broader context

Aqueous rechargeable Zn–MnO₂ batteries have gained significant attention due to their inherent safety, low cost and favorable operating voltages, making them promising candidates for large-scale energy storage. However, the large size and high desolvation energy of [Zn(H₂O)₆]²⁺ ions result in sluggish intercalation kinetics and diminished (dis)charge rates in Zn–MnO₂ batteries. Additionally, the strong lattice repulsion between Zn²⁺ ions and the MnO₂ host often leads to irreversible phase transitions and structural collapse, severely compromising the rate capacity and cycling stability. In this work, we design a hybrid superlattice cathode (Ni–TABQ@δ-MnO₂), which triggers ultrafast proton transfer via Grotthuss topochemistry. Conductive Ni–TABQ effectively modulates the electronic properties of δ-MnO₂ through π–d electron coupling, and the intermolecular H-bonds between coordination water (Ni–TABQ) and lattice oxygen (δ-MnO₂) facilitate dynamic proton transfer through the continuous breaking and reformation of the H-bonding network. The unique proton hopping processes empower Zn–MnO₂ to deliver a high-rate and ultrastable charge storage process. This study may shed new light on the design of diffusion-barrier-free proton-conductive hybrid superlattice cathodes for advanced energy storage.

Introduction

Aqueous rechargeable zinc-ion batteries (ZIBs) have gained significant attention due to their inherent safety, low cost, and the high theoretical capacity of zinc anodes (820 mA h g⁻¹) coupled with a favorable redox potential (−0.76 V vs. standard hydrogen electrode), making ZIBs promising candidates to act as large-scale energy storage solutions.^1–5 Among the various Zn²⁺-hosting inorganic cathode materials, manganese-based oxides show significant properties, such as a high theoretical capacity, favorable operating voltage and natural abundance.^6–10 In particular, layered δ-MnO₂ has a two-dimensional open crystal structure and large interlayer spacing enabling efficient charge-carrier (de)intercalation, distinguishing it among manganese-based oxides and making it an ideal candidate for high-performance energy storage.^11–15 However, the large hydrated structure and high desolvation energy of [Zn(H₂O)₆]²⁺ ions pose significant challenges, triggering sluggish intercalation kinetics and (dis)charge rates in ZIBs.^16–18 Additionally, strong lattice repulsion between Zn²⁺ ions and the δ-MnO₂ host lattice often triggers irreversible phase transitions and structural collapse, further limiting the capacity performance and cycling stability.^19–22 Addressing these issues requires innovative approaches to optimize the charge storage behavior of manganese-based cathodes and stabilize their structures during cycling.

Compared with sluggish Zn²⁺ storage, protons (H⁺), having the smallest size and lightest weight, can enable fast (de)intercalation kinetics and reduce structural degradation in Mn-based hosts during cycling, significantly improving the rate performance and high-power energy storage of ZIBs.^23–26 Therefore, activating superior H⁺ storage offers a compelling solution to overcome the electrochemical limitations of Zn²⁺, with significant potential to improve the rate performance and lifespan of Zn–MnO₂ batteries.^27–30 For instance, Pan et al. introduced the Grotthuss proton transport mechanism in α-MnO₂, highlighting the rapid proton migration within the [2 × 2]-tunnel structure.³¹ He et al. showed that the lattice water in NiMn₃O₇·3H₂O facilitates hydrated proton migration to interlayer spaces, where protons bind with O²⁻ ions in the Mn–O layer, thereby enabling efficient charge transport and structural stability during cycling.³² These findings highlight the critical role of the crystal structure in facilitating proton migration and enhancing electrochemical performance. However, during prolonged cycling, competition with Zn²⁺ insertion and the structural instability of reported Mn-based host materials frequently undermine the efficiency of Zn–MnO₂ energy storage systems, leading to decreased proton storage, especially at high current densities. Therefore, it is of great significance to precisely design Mn-based cathodes with more favorable crystal lattice structures for better selective proton intercalation for the purpose of propelling high-performance Zn–MnO₂ batteries.

In this work, a hybrid superlattice Ni–TABQ@δ-MnO₂ cathode is designed to enhance proton transfer by taking advantage of the Grotthuss topochemistry mechanism. Incorporated conductive Ni–TABQ can modulate the electronic properties of δ-MnO₂ via π–d electron coupling, establishing efficient conductive pathways. Moreover, rich intermolecular H-bonds between coordination water (Ni–TABQ) and lattice oxygen (δ-MnO₂) facilitate proton conduction for rapid redox reactions, and the formed intermolecular H-bonding network allows H₃O⁺ to transfer protons through the continuous breaking and reformation of O–H bonds. This dynamic hopping of protons significantly reduces the charge-transfer energy barrier compared to the (de)intercalation of Zn²⁺ (0.082 eV vs. 0.741 eV), enabling fast and stable energy storage. As a result, the Ni–TABQ@δ-MnO₂ cathode demonstrates excellent performance, including high capacity, large-current tolerance, and an ultralong lifespan. This study paves the way for the development of Grotthuss proton-conductive hybrid superlattice cathodes towards advanced energy storage.

Results and discussion

During the cyclic charging and discharging of a layered δ-MnO₂ cathode, zinc-ion insertion induces strong lattice repulsion and irreversible phase transitions, resulting in rapid structural degradation and performance decline. A widely adopted approach to mitigate these effects is cation pillar engineering.^24,33 While cation pre-intercalation modestly enhances the (de)intercalation kinetics of Ni–δ-MnO₂, it leads to limited improvement of its intrinsic electronic properties. Additionally, cation pillar dissolution during ion exchange over periods of extended cycling compromises structural integrity, leading to a sharp decline in capacity retention (Fig. S1†). To address these challenges, TABQ was selected due to its conjugated planar structure and easy combination with Ni²⁺ ions to form a π–d electronic coupling configuration; these are beneficial for boosting electronic conductivity and triggering ultrafast proton transfer via Grotthuss topochemistry, thereby optimizing the electrochemical properties. We innovatively incorporate TABQ into Ni–δ-MnO₂ to form a two-dimensional conductive metal–organic framework (Ni–TABQ). This framework further integrates with the δ-MnO₂ host to construct an organic–inorganic hybrid superlattice structure (Ni–TABQ@δ-MnO₂). The hybrid superlattice cathode combines the superior electronic conductivity of the organic components with the high-energy storage capabilities of the δ-MnO₂ lattice, offering a promising solution for zinc-ion energy storage. Furthermore, the superlattice architecture enables precise control over the lattice arrangement and interfaces of the organic and inorganic components, significantly enhancing the proton transfer efficiency. This synergistic integration optimizes the electronic properties and rate performance of the Ni–TABQ@δ-MnO₂ cathode, making it a robust candidate for use in high-rate and stable energy storage systems (Fig. 1a).

Fig. 1 (a) A schematic diagram of the three energy storage mechanisms at the cathode interface. (b) Rietveld-refined crystal diffraction profiles of Ni–TABQ@δ-MnO₂. (c) SEM and TEM (inset) images of Ni–TABQ@δ-MnO₂. (d) FT-IR spectra. HRTEM images of (e) δ-MnO₂, (f) Ni–δ-MnO₂, and (g) Ni–TABQ@δ-MnO₂ (inset: inverse fast Fourier transform). Structural and electronic property analysis of Ni–TABQ@δ-MnO₂: (h) IGM scatter plots, (i) differential electron density isosurfaces, and (j) an ELF map of the optimized crystal structure.

Rietveld refinement of the crystal diffraction profiles (Fig. 1b) shows that the hybrid superlattice Ni–TABQ@δ-MnO₂ cathode retains the same monoclinic phase (C2/m space group) as δ-MnO₂, with refined lattice parameters of α = γ = 90°, β = 116.99°, a = 10.38, b = 5.84, and c = 8.09 Å, achieving a satisfactory agreement factor (R_wp = 6.47%). Morphological characterization shows that δ-MnO₂, Ni–δ-MnO₂ and Ni–TABQ all feature nanoflower structures composed of nanosheets (Fig. S2a–f and S3†). Notably, Ni–TABQ@δ-MnO₂ retains its morphology after incorporating the TABQ component into the Ni–δ-MnO₂ framework (Fig. 1c and Fig. S2g–i†), which is helpful for the exposure of active sites and fast ion (de)intercalation. Fourier transform infrared (FT-IR) spectra (Fig. 1d) and thermogravimetric analysis results (Fig. S4†) further confirm that TABQ has been successfully incorporated into the Ni–δ-MnO₂ host.³⁴ High-resolution transmission electron microscopy (HRTEM) images provide detailed insights into the crystal structure and diffraction planes of manganese-based materials. For δ-MnO₂, a monoclinic crystal structure with an interlayer spacing of 0.672 nm along the (100) plane is observed, which expands to 0.695 nm upon pre-intercalation with Ni²⁺ cation pillars (Fig. 1e and f). Further characterization of Ni–TABQ@δ-MnO₂ based on HRTEM and fast Fourier transformation (FFT) patterns (Fig. 1g) confirms the successful incorporation of TABQ into the (001) plane of Ni–δ-MnO₂. The integration of Ni–TABQ induces the formation of a hybrid superlattice structure, further expanding the interlayer spacing to 0.705 nm. XRD patterns substantiate these changes, showing broadened (001) peaks and their shift to lower 2θ angles (Fig. S5†), indicative of improved crystallinity and interlayer expansion.³⁵ HRTEM maps reveal the distinct elemental signals of Ni and N, originating from Ni–TABQ, further confirming the uniform integration of TABQ into the Ni–δ-MnO₂ crystal framework to form the hybrid superlattice Ni–TABQ@δ-MnO₂ cathode (Fig. S6†).

Density functional theory (DFT) simulations were conducted to investigate the intermolecular forces and electronic properties of Ni–TABQ@δ-MnO₂. The independent gradient model based on Hirshfeld partitioning (IGM), analyzed as a function of electron density, reveals the robust hydrogen bonding interactions (H⋯O) between the coordination water molecules in Ni–TABQ and the lattice oxygen in δ-MnO₂ (Fig. 1h).^36–38 These strong interactions establish high-speed channels for proton transfer across the interlayers, significantly enhancing the ion transport efficiency. Differential electron density isosurfaces demonstrate charge transfer from Ni–TABQ to the lattice oxygen of δ-MnO₂, further confirming that interlayer H-bonds facilitate electron transfer during electrochemical redox processes (Fig. 1i and Fig. S7†). Electron localization function (ELF) maps provide additional insights into the bonding characteristics of the hybrid superlattice, showing pronounced electron localization around the coordination water molecules, highlighting strong H-bonding interactions (Fig. 1j).^39,40 Overall, the hybrid superlattice structure strengthens H-bonding and the electronic properties of Ni–TABQ@δ-MnO₂, underlining the synergistic effect of Ni–TABQ on the structural and electrochemical properties of δ-MnO₂.

Band structure and density of state calculations were performed to elucidate the electronic properties of δ-MnO₂ and Ni–TABQ@δ-MnO₂ (Fig. S8†). The layered δ-MnO₂ crystal structure restricts electron conduction, resulting in a wide bandgap (0.955 eV) and semiconductor-like behavior (Fig. 2a and b).^41,42 After introducing Ni–TABQ MOFs as interlayer electron carriers, metallic properties emerge, markedly enhancing the conductivity (Fig. 2c and d). Through the hybridization of Ni-3d and N-2p orbitals, Ni–TABQ MOFs enable π–d electron coupling to establish an electronic conductive network that facilitates both ion and electron transfer (Fig. 2e).^43,44 Conductive atomic force microscopy (CAFM) was employed to evaluate the local current distribution on the surface of manganese-based materials, providing precise insights into their conductivity behavior (Fig. S9†). As depicted in Fig. 2f, δ-MnO₂ demonstrates a peak current of 205 pA, accompanied by an uneven current distribution, indicating its limited intrinsic conductivity. Upon doping with Ni²⁺ cation pillars, the peak current slightly increases to 229 pA, reflecting a moderate improvement in conductivity (Fig. 2g). In contrast, the most significant enhancement of peak current (307 pA) is observed in Ni–TABQ@δ-MnO₂ (Fig. 2h). This improvement is accompanied by a notably uniform current distribution across the surface, highlighting the superior electronic transport pathways established by the conductive Ni–TABQ. The enhanced conductivity can be attributed to the π–d electron coupling induced electron transfer in the hybrid superlattice structure, which collectively facilitated faster ion and electron mobility.

Fig. 2 Electronic structure and orbital analysis. The density of states and band structure of (a and b) δ-MnO₂ and (c and d) Ni–TABQ@δ-MnO₂. (e) Orbital hybridization of conductive Ni–TABQ MOFs. Conductive atomic force microscopy mapping images of (f) δ-MnO₂, (g) Ni–TABQ@δ-MnO₂ and (h) Ni–TABQ@δ-MnO₂. Electrochemical property analysis. (i) EIS spectra. (j) The relationship between Z′ and ω^−0.5. (k) E_a values.

To evaluate the electrochemical conductivity properties of δ-MnO₂-based cathodes, electrochemical impedance spectroscopy (EIS) analysis was carried out (Fig. 2i and Fig. S10a†). It reveals that the hybrid superlattice Ni–TABQ@δ-MnO₂ cathode achieves a significantly lower charge transfer resistance (R_ct) of 48.8 Ω, outperforming both Ni–δ-MnO₂ (88.2 Ω) and δ-MnO₂ (118.5 Ω). The real impedance component Z′ and angular frequency ω further verify the superior charge transfer kinetics of the hybrid superlattice structure (Fig. 2j).^45,46 The activation energy (E_a) for charge transfer was obtained using the Arrhenius equation (eqn (S5) and (S6)†). The Ni–TABQ@δ-MnO₂ cathode demonstrates an exceptionally small E_a value of 11.88 kJ mol⁻¹ (Fig. 2k and Fig. S10b†), which is significantly lower than those of Ni–δ-MnO₂ (19.10 kJ mol⁻¹) and δ-MnO₂ (21.87 kJ mol⁻¹). These results underscore the high ionic conductivity, efficient ion diffusion pathways, and reduced reaction barrier of the hybrid superlattice structure of Ni–TABQ@δ-MnO₂. The incorporation of Ni–TABQ enhances the charge transfer efficiency and ion transport, highlighting its role in enabling high-kinetics charge storage, and positioning the hybrid superlattice Ni–TABQ@δ-MnO₂ as a promising cathode material for advanced energy storage.

Zn–MnO₂ batteries were constructed, which involve a Zn metal anode, δ-MnO₂-based cathodic materials, and 2 M ZnSO₄ + 0.2 M MnSO₄ aqueous electrolyte (Fig. S11†). Cyclic voltammetry (CV) curves of Ni–TABQ@δ-MnO₂ (Fig. 3a) exhibit pronounced redox peaks and reduced polarization, indicating its superior redox activity compared to δ-MnO₂ and Ni–δ-MnO₂. The Ni–TABQ@δ-MnO₂ cathode achieves an impressive specific capacity of 453 mA h g⁻¹ at 0.2 A g⁻¹, outperforming δ-MnO₂ (236 mA h g⁻¹), Ni–δ-MnO₂ (346 mA h g⁻¹) and Ni–TABQ (12 mA h g⁻¹) (Fig. 3b and Fig. S12†). The Ni–TABQ@δ-MnO₂ cathode also demonstrates excellent rate capacities of 370, 331, 274, 218, 183, and 152 mA h g⁻¹ at current densities of 0.5, 0.8, 1, 3, 5, and 10 A g⁻¹, respectively (Fig. 3b and c). Significantly, the Zn∥Ni–TABQ@δ-MnO₂ battery achieves a record energy density of 586 W h kg⁻¹ (calculated based on the mass of active cathode material; Fig. 3d and Table S1†), which is superior to reported Mn/V-based and organic cathode materials.^47–54 At mass loadings of 2.9 and 5.0 mg cm⁻², the TABQ@δ-MnO₂ cathode delivers capacities of 423 and 362 mA h g⁻¹, respectively. Even at a high mass loading of 10.2 mg cm⁻², it retains a remarkable capacity of 289 mA h g⁻¹, highlighting its exceptional large-scale energy storage potential (Fig. S13†). Furthermore, the TABQ@δ-MnO₂ cathode maintains 88.76% of its initial capacity (328 mA h g⁻¹) after 200 cycles at 0.5 A g⁻¹, with high coulombic efficiency (Fig. 3e). Conversely, Ni–δ-MnO₂ and δ-MnO₂ exhibit inferior cycling stability, with low-capacity retention of 45.16% and 34.08% in the absence of the TABQ component, due to the dissolution of pre-intercalated Ni²⁺ cations and structural degradation. Remarkably, even at a high current density of 5 A g⁻¹ for 8000 cycles, the capacity retention remains at 93.82%, underscoring the robust structural and electrochemical stability of the hybrid superlattice cathode (Fig. 3e, Fig. S14 and S15†). Three integrated Zn∥Ni–TABQ@δ-MnO₂ batteries can power an LED toy (Fig. S16†), demonstrating the application promise of Zn–MnO₂ batteries.

Fig. 3 Electrochemical performances of Zn–MnO₂ batteries. (a) CV curves. (b) GCD profiles. (c) Rate capabilities. (d) Voltage-capacity contour plots of reported Zn batteries. Cycling performance at (e) 0.5 and (f) 3 A g⁻¹. Charge storage kinetics of the Ni–TABQ@δ-MnO₂ hybrid superlattice cathode. (g) Calculated b values. (h) Capacitive contribution. (i) Ratios of capacitive and diffusion-controlled contributions at various scan rates.

In addition, the redox kinetics of the Zn∥Ni–TABQ@δ-MnO₂ cell were analyzed by using CV curves and Dunn's method.^55,56 Across scan rates in the range of 0.2–1.0 mV s⁻¹, CV profiles maintain consistent shapes with minor shifts in redox peaks (Fig. S17†), indicating the excellent electrochemical reversibility of the Zn∥Ni–TABQ@δ-MnO₂ battery. Three distinct redox signals (denoted as P_R1, P_R2, and P_O1) are observed, corresponding to the Zn²⁺/H⁺ co-intercalation redox reaction. The power-law relationship between the peak current (i) and scan rate (v) yields b-values of 0.56, 0.77, and 0.53 for the three peaks (Fig. 3g), demonstrating a rapid surface-dominated proton charge storage process coupled with Zn²⁺ intercalation during discharging. Further analysis reveals that the rapid surface capacitive contribution dominates charge storage, accounting for 75.66–94.47% of the total capacity across all scan rates (Fig. 3h and i). However, the b-values and rapid surface capacitive contributions of δ-MnO₂ and Ni–δ-MnO₂ cathodes indicate their slow reaction kinetics processes (Fig. S18 and S19†). These findings underscore the critical role of the conductive Ni–TABQ component in modulating the electronic properties of the hybrid superlattice cathode, facilitating high-power energy storage.

To further identify the charge-storage mechanism of the Ni–TABQ@δ-MnO₂ cathode, spectroscopic characterization was performed to delve into the proton storage behavior in specific potential states (Fig. 4a). Based on ex situ XRD patterns (Fig. 4b and Fig. S20†), the peak intensities of HMnO₂ at 20.8° and 32.7° increase during discharging (state A → D) and disappear during subsequent charging (state D → G),^32,57 verifying the highly reversible phase transition process. HRTEM images in state D further confirm the evolution of HMnO₂, accompanied by the appearance of the (1, 1, −1) crystal plane with a width of 0.342 nm (Fig. 4c). Operando pH measurements were conducted to accurately monitor the H⁺ redox behavior in the Ni–TABQ@δ-MnO₂ cathode during the discharge/charge process (Fig. S21†). The pH value is significantly boosted from 2.47 to 3.01 during discharging from 1.80–1.25 V. Subsequently, it slightly increases in the voltage range of 1.25–0.80 V, with pH values of 3.01 to 3.09 (Fig. 4d). Such results imply a strong proton-coupled charge storage process in the Ni–TABQ@δ-MnO₂ cathode over the voltage range of 1.80–1.25 V (state A → C). In the subsequent recharging procedure, the pH value returns back to its initial level. In addition, the reversible formation of Zn₄(OH)₆(SO₄)·5H₂O (ZHS) functions (Fig. S22†) as a pH buffer and holds the solution pH value constant within a certain range.

Fig. 4 The Grotthuss proton transfer behavior of the Zn∥Ni–TABQ@δ-MnO₂ battery. (a) GCD profiles with selected specific (dis)charge states. (b) Ex situ XRD patterns. (c) A HRTEM image of the Ni–TABQ@δ-MnO₂ cathode in the fully discharged state. (d) In situ pH variations. (e) The electrode mass change versus charge during the discharge (ion-intercalation) process. (f) The ion diffusion coefficient during the discharge process. Ex situ (g) O 1s and (h) Zn 2p XPS spectra. (i) Element maps of Ni–TABQ@δ-MnO₂ and δ-MnO₂ cathodes in the fully discharged state.

An electrochemical quartz crystal microbalance (EQCM) was employed for the operando monitoring of the mass change of Ni–TABQ@δ-MnO₂ with continuous ion intercalation. Consistent with the GITT results, the mass change of the Ni–TABQ@δ-MnO₂ electrode during ion intercalation involves two stages. In the first stage (state A → C), the average weight increase calculated based on the curve slope (Fig. 4e) is 19.72 g per mol charge, close to the weight of H₃O⁺ (19 g per mol charge), suggesting hydrated proton (H₃O⁺) intercalation in Ni–TABQ@δ-MnO₂. Meanwhile, the average weight increase in the second stage (state C → D) is 32.94 g per mol charge, which suggests the intercalation of Zn²⁺ in Ni–TABQ@δ-MnO₂. The galvanostatic intermittent titration technique (GITT) is applied to measure the ion diffusion coefficient (D; Fig. 4f and Fig. S23†). The higher D values of ∼10⁻⁹ cm² s⁻¹ during discharging (state A → C) are ascribed to the fast-kinetics incorporation of H⁺, while the lower D values of ∼10⁻¹¹ cm² s⁻¹ (state C → D) belong to the slow intercalation of Zn²⁺ in the Ni–TABQ@δ-MnO₂ cathode. These results verify the two-step H⁺/Zn²⁺ co-intercalation redox process. Thanks to the mixed superlattice structure of Ni–TABQ@δ-MnO₂, protons can be transferred via H₂O media through Grotthuss topochemistry, which is beneficial for rapid charge storage and high-power energy storage.

High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the O 1s signal was carried out to elucidate the redox behavior of the Ni–TABQ@δ-MnO₂ cathode during cell operation. In the pristine state, two peaks at 530.1 and 531.7 eV are observed, corresponding to lattice oxygen (O1) and oxygen from adsorbed H₂O (O2), respectively.⁵⁸ Notably, an additional peak at 533.8 eV (O3) emerges during the charge/discharge process. This peak is attributed to the terminal oxygens of [MnO₆] bilayers, and it splits from the O2 peak due to H-bond formation with intercalated H₂O/H₃O⁺ (as illustrated in Fig. 4g). Compared with Ni–δ-MnO₂ and δ-MnO₂ cathodes, the Mn 2p XPS spectra of Ni–TABQ@δ-MnO₂ reveal the largest fluctuations of Mn valence due to enhanced redox activity from proton involvement (Fig. S24a and b†). This confirms that the extra capacity contribution originates from the multielectron redox activity of Mn sites and is enabled by a strong proton–electron coupling process. The Ni–TABQ@δ-MnO₂ cathode in HOTF–H₂O electrolyte exhibits the highest proton storage capacity of 233.2 mA h g⁻¹ (Fig. S24c†), which outperforms Ni–δ-MnO₂ (95.6 mA h g⁻¹) and δ-MnO₂ (52.3 mA h g⁻¹), highlighting the key role of Ni–TABQ incorporation in improving the electrochemical capacity. Operando FT-IR spectra exhibit the generation/disappearance of O–H signals, which derive from reactions between the lattice oxygen of Ni–TABQ@δ-MnO₂ and protons (Fig. S24d†). Such a result confirms the participation of protons through distinct shifts in the O–H stretching bands during the (dis)charge process. Simultaneously, the Zn 2p signal intensity increases during the discharge process (state A → D) due to Zn²⁺ intercalation into the Ni–TABQ@δ-MnO₂ host and weakens during the recharge process (state D → G) as Zn²⁺ ions are extracted (Fig. 4h). Compared to pristine δ-MnO₂, the hybrid superlattice Ni–TABQ@δ-MnO₂ cathode exhibits a reduced Zn²⁺ intercalation capacity due to steric hindrance imposed by Ni–TABQ. Elemental distribution maps of Mn, O, and Zn, along with their relative proportions, further validate the selective ionic storage behavior (Fig. 4i and Fig. S25a and b†). Moreover, coupled plasma optical emission spectroscopy (ICP-OES) analyses of the Ni–TABQ@δ-MnO₂ cathode at various rates confirmed that proton storage becomes more dominant with increasing current density, reaching 93.91% at 10 A g⁻¹ (Fig. S25c and Table S2†). Accordingly, the structural design of hybrid superlattice Ni–TABQ@δ-MnO₂ can not only suppress excessive Zn²⁺ intercalation but also promote efficient proton storage through Grotthuss topochemistry, enabling fast and stable charge storage performance.

To investigate the mechanisms of proton transport and charge storage, DFT calculations were performed to simulate the structural evolution of the Ni–TABQ@δ-MnO₂ cathode during electrochemical reactions. As shown in Fig. 5a, the hybrid superlattice Ni–TABQ@δ-MnO₂ exhibits highly protophilic reactivity (H₃O⁺, −1.65 eV) and weaker zincophilic behavior (Zn²⁺, −0.26 eV), while δ-MnO₂ demonstrates stronger Zn²⁺ adsorption (−1.17 eV) but relatively limited interaction with H₃O⁺ (−0.85 eV). The pronounced protophilic activity of Ni–TABQ@δ-MnO₂ facilitates efficient Grotthuss proton conduction. The interaction bonding types were further explored using IGMH and ELF maps. Hydrated protons exhibit strong hydrogen bonding within the hybrid superlattice framework of Ni–TABQ@δ-MnO₂ (Fig. 5b), whereas Zn²⁺ predominantly forms Zn–O ionic bonds (Fig. 5c). In contrast with the rigid Zn–O ionic bonds, the dynamic and flexible nature of H-bonds enhances the proton transport kinetics and charge storage. Differential electron density isosurfaces and Bader charge analysis reveal the charge transfer behavior between charge carriers and the Ni–TABQ@δ-MnO₂ host (Fig. 5d). Notably, H₃O⁺ enables better charge transfer to the Ni–TABQ@δ-MnO₂ host (0.824 e) in comparison to Zn²⁺ (0.413 e), further confirming its superior protonophilic reactivity. To evaluate ion diffusion pathways and energy barriers, the climbing-image nudged elastic band (CI-NEB) method was employed. Fig. 5e and Fig. S26† illustrate the migration trajectories of Zn²⁺ and H⁺ ions. The calculated activation energy barriers for Zn²⁺ and H⁺ migration are 0.741 and 0.124 eV, respectively. The main barrier to Zn²⁺ ion transport in the aqueous electrolyte originates from the spatial steric hindrance imposed by Ni–TABQ layers within the hybrid Ni–TABQ@δ-MnO₂ superlattice structure. In contrast, the hydrogen-bonding network primarily facilitates proton transport through the Grotthuss mechanism, which affords fast redox kinetics at Mn sites but does not directly influence Zn²⁺ migration. This ultra-low energy barrier for H⁺ diffusion underscores the superior proton storage performance of the Ni–TABQ@δ-MnO₂ cathode.

Fig. 5 Theoretical simulations of proton storage behaviors in the Ni–TABQ@δ-MnO₂ hybrid superlattice cathode. (a) The uptake energy of Zn²⁺ and H₃O⁺ in the optimized crystal structure. (b) Plots of IGMH versus sign(λ₂)ρ and the corresponding gradient isosurfaces of H₃O⁺ ions. (c) ELF maps of Zn²⁺ ions. (d) Differential charge isosurfaces and Bader charge transfer. (e) Migration energy barriers of Zn²⁺ and H⁺ ions. (f) The Grotthuss topochemistry model of proton storage. (g) The diffusion-free H⁺ transport mechanism in the hybrid superlattice Ni–TABQ@δ-MnO₂ cathode.

Here, the Grotthuss proton transfer mechanism, also known as proton hopping, is demonstrated, representing an efficient and dynamic pathway for proton conduction in H-bonding systems (Fig. 5f). Unlike conventional Zn²⁺ ionic diffusion, where ions physically migrate across the host material, the Grotthuss mechanism relies on the rapid relay of protons through hydrogen-bonding networks. This process involves the transient breaking and reformation of H-bonds, allowing protons to “hop” between adjacent water molecules, contributing to diffusion-free H⁺ transport (Fig. 5g). As a result, the net transport of protons occurs without the significant mass movement of ionic carriers, leading to faster proton mobility and reduced energy barriers. The strong protophilic nature of the hybrid superlattice TABQ@δ-MnO₂ framework plays a critical role in enabling Grotthuss topochemistry. The hydrated protons (H₃O⁺) form dynamic and flexible H-bonds within Ni–TABQ@δ-MnO₂, creating an ideal environment for Grotthuss proton transfer. This well-connected H-bonding network facilitates the rapid and continuous hopping of protons, significantly enhancing the proton transport kinetics. Compared to rigid and slow Zn²⁺ transport through Zn–O ionic bonds, proton transfer via Grotthuss topochemistry ensures higher charge storage efficiency and faster response times during electrochemical reactions. Consequently, the Grotthuss mechanism contributes directly to the superior proton storage capabilities, high ionic conductivity, and enhanced energy storage performance of the Ni–TABQ@δ-MnO₂ cathode, making this a promising strategy for advanced energy storage systems.

Conclusions

We designed a hybrid superlattice Ni–TABQ@δ-MnO₂ cathode to enhance proton transfer through Grotthuss topochemistry, offering a promising solution for ultrahigh-rate Zn-ion batteries. By incorporating two-dimensional Ni–TABQ, the electronic properties of δ-MnO₂ are effectively modulated via π–d electron coupling, forming a highly efficient conductive network. The abundant intermolecular H-bonds between coordination water (Ni–TABQ) and lattice oxygen (δ-MnO₂) facilitate proton conduction with rapid redox reactions. Through the dynamic hopping of H₃O⁺ ions, driven by the continuous breaking and reformation of O–H bonds within the intermolecular H-bonding network, Ni–TABQ@δ-MnO₂ achieves a significantly reduced charge transfer energy barrier (0.082 eV) compared to the sluggish Zn²⁺ diffusion process (0.741 eV). This fast-kinetics proton transport mechanism mitigates the lattice repulsion and phase instability associated with Zn²⁺ intercalation, ensuring a Ni–TABQ@δ-MnO₂ cathode with exceptional electrochemical metrics, including high capacity, excellent rate performance, and an ultralong lifespan. These findings underscore the importance of hybrid superlattice cathode structures with efficient proton transfer for advanced energy storage systems.

Data availability

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

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 22272118, 22172111 and 22309134), the Science and Technology Commission of Shanghai Municipality, China (No. 22ZR1464100, 20ZR1460300, and 19DZ2271500), China Postdoctoral Science Foundation (2022M712402), Shanghai Rising-Star Program (23YF1449200), Zhejiang Provincial Science and Technology Project (2022C01182), and the Fundamental Research Funds for the Central Universities (2023-3-YB-07).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5eb00022j

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