Tailoring surface structures in Mn-based Prussian blue analogues for enhanced NH₄⁺ transport and high-performance aqueous batteries

Abstract

Aqueous ammonium-ion batteries (AAIBs) have attracted significant attention, with Prussian blue analogues (PBAs) emerging as promising cathode materials. Although Mn–PBA possesses multiple redox-active centers and high specific capacity in AAIBs, its limited structural stability and inadequate utilization of active sites continue to hinder its broader application. In this work, a novel, direct, and efficient strategy utilizing tannic acid (TA) is employed to achieve omnidirectional modulation of Mn–PBA, leading to the full exposure of active sites within the Mn–PBA–TA framework. As a result, the Mn–PBA–TA cathode exhibits a reversible specific capacity of 120.3 mAh g⁻¹ after 200 cycles at 1 A g⁻¹, demonstrating high active site availability. Furthermore, it retains exceptional cycling stability over 10 000 cycles at a current density of 15 A g⁻¹, with an ultra-low capacity fade of just 0.0036% per cycle. A comprehensive investigation into the NH₄⁺ electrochemical diffusion behavior, redox capability, and structural stability of Mn–PBA–TA is conducted, complemented by theoretical calculations that elucidate a rational NH₄⁺ migration pathway and its associated energy barriers. Based on these insights, a full cell assembled with a quinone–imine organic anode delivers a high-power density output. This study provides valuable insights into the chemical modification of PBAs, paving the way for the development of advanced cathodes in aqueous batteries.

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New concepts

This study introduces a breakthrough in aqueous ammonium-ion batteries (AAIBs) by addressing the limitations of Mn-based Prussian blue analogues (Mn–PBA) through tannic acid (TA)-driven structural modulation. A novel omnidirectional TA-coordination strategy reconstructs the Mn–PBA framework, enabling full exposure of redox-active sites. A hollow structure with a large specific surface area accelerates NH₄⁺ diffusion, stabilizing the lattice during cycling. The optimized Mn–PBA–TA cathode achieves dual breakthroughs: (1) high-capacity retention (120.3 mAh g⁻¹ after 200 cycles at 1 A g⁻¹) through maximized active site utilization, and (2) ultra-long cyclability (10 000 cycles at 15 A g⁻¹ with 0.0036% capacity decay/cycle) via surface-mediated structural resilience. Theoretical modelling further reveals low-energy-barrier NH₄⁺ migration pathways, guiding rational cathode design. Coupled with a quinone–imine anode, the full cell delivers high power density, demonstrating practical feasibility. This work pioneers surface structure-engineered dynamic stabilization in PBAs, offering a universal framework for next-generation aqueous battery cathodes.

1. Introduction

Aqueous rechargeable batteries hold great potential for large-scale, low-cost, and high-safety energy storage applications.^1–4 Since most ammonium salt-containing aqueous electrolytes are nearly neutral, material and device corrosion can be largely neglected.⁵ As a result, aqueous ammonium-ion batteries (AAIBs), which utilize NH₄⁺ as the charge carrier, have recently emerged as a key focus in battery research. Compared to metal ions, ammonium ions have a lower molar mass and smaller hydrated ionic size, enabling rapid diffusion in aqueous electrolytes. Additionally, NH₄⁺ does not form dendrites during charge–discharge cycles, significantly improving battery stability and lifespan.^6,7 Various transition metal oxides, including MnO₂,⁸ MoO₃,⁹ and WO₃,¹⁰ are being developed as anode materials for AAIBs, marking progress in this area. Furthermore, organic electrode materials are increasingly recognized as promising anodes for NH₄⁺ storage in aqueous systems.^11–13 Despite these advances, the development of high-performance cathode materials remains a critical challenge that must be addressed to advance the practical application of AAIBs. Prussian blue analogues (PBAs) are widely used materials with established industrial applications in sodium-ion batteries, biomedicine, and sensing.^14–16 Their distinctive three-dimensional cubic framework and tunable chemical composition make them highly attractive as cathode materials for AAIBs.^17–19 The NH₄⁺ ion has an ideal size that closely matches the cavity dimensions of PBAs. With expansive (100) crystal planes and abundant charge carrier sites (∼4.6 Å in diameter), PBAs facilitate rapid ion insertion and extraction. Additionally, many PBAs exhibit relatively high NH₄⁺ insertion potentials (0.6–1.2 V). Theoretical studies suggest that NH₄⁺ insertion induces a volume change of less than 1%, classifying PBAs as “zero-strain” materials.^20,21 In contrast, sodium-ion insertion leads to significant volume expansion, while potassium-ion insertion reduces structural symmetry, causing lattice distortions.^22,23 Ji et al. was the first to conduct an in-depth study on a “rocking-chair” type ammonium-ion battery using a PBA-based cathode ((NH₄)_1.47Ni[Fe(CN)₆]_0.88).²⁴ With an (NH₄)₂SO₄ electrolyte and a current density of 0.15 A g⁻¹, the PBA electrode achieved a reversible capacity of 57 mAh g⁻¹ after 60 cycles.

This performance, however, remains far below the theoretical capacity of ∼170 mAh g⁻¹, highlighting the limited capacity of PBAs in aqueous ammonium-ion electrochemical storage. Currently, most reported PBA cathodes exhibit specific capacities below 100 mAh g⁻¹, with active site utilization rates under 60%, posing a major challenge for large-scale application. This limitation arises from the single redox-active site ([Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻) and the uneven distribution of active sites within the open framework, leading to significant spatial constraints for ion insertion and extraction. To overcome these challenges, the development of PBAs with multi-electron redox capabilities is crucial for enhancing active site utilization. PBAs incorporating elements such as Mn, V, and Ag²⁵ have demonstrated multiple redox-active centers, significantly improving their specific capacity in aqueous electrochemical energy storage. However, the incorporation of additional ions into the framework raises concerns about structural stability. Moreover, the lack of a fundamental understanding of the relationship between the structure and performance has hindered the optimization of NH₄⁺ storage electrochemical performance in these systems. Addressing these challenges is essential for the advancement of AAIB technology.

Recently, researchers such as Hu et al. and Lu et al. have enhanced the electrochemical stability of Mn-based PBAs by employing partial doping and substitution strategies to suppress and delay phase transitions in aqueous batteries.^26,27 However, this approach requires precise regulation. Currently, the interface between the outer coating and Mn-based PBA active materials often exhibits weak or unstable connections, and the persistent issue of Mn dissolution continues to hinder storage performance, particularly in terms of cycling stability.²⁸ Biocompatible tannic acid (TA) has been widely used for the pretreatment and surface/interface modification of metal–organic framework (MOF) materials.^29–31 As a mild etchant, TA provides small amounts of protons to coordinate with MOFs, while its phenolic hydroxyl groups chelate metal ions, protecting the MOF structure and improving electrochemical stability. Simultaneously, the release of protons accelerates etching, increasing the exposure of active sites. The functionalization of MOFs using TA is both universal and targeted, making it an effective strategy for stabilizing Mn-based PBAs. When applied to Mn–PBA, TA effectively modifies the surface properties and reinforces structural stability during ion deintercalation.

Building on this concept, the present study introduces a novel one-step solvated TA treatment of Mn–PBA, leading to improvements in both surface integrity and overall framework stability, thereby enhancing NH₄⁺ deintercalation. Material characterization reveals that Mn–PBA–TA undergoes controlled etching at both its surface and internal structure, resulting in enhanced thermal stability, an increased specific surface area, and a more uniform Mn element distribution. As a cathode for AAIBs, Mn–PBA–TA demonstrates exceptional cycling stability, sustaining 10 000 charge–discharge cycles at 15 A g⁻¹, as verified by in situ ultraviolet-visible spectroscopy. Detailed kinetic analysis confirms excellent NH₄⁺ ion diffusion in Mn–PBA–TA, while charge–discharge state analyses indicate superior redox reversibility. Moreover, the well-defined NH₄⁺ migration pathway within the Mn–PBA framework further underscores its advantages as an AAIB cathode. Ultimately, a full battery incorporating Mn–PBA–TA as the cathode and an organic anode was successfully constructed, achieving 1000 stable cycles at 1 A g⁻¹.

2. Results and discussion

The Mn–PBA products were synthesized via co-precipitation and subsequently subjected to chemical treatment with tannic acid (TA) etching to yield Mn–PBA–TA materials, as illustrated in Fig. 1a. The etching process induced a structural transformation from a solid cubic framework to a hollow cubic architecture, exposing a greater specific surface area and more active sites.³² Notably, Mn–PBA–TA retained the crystal structure of Mn–PBA, which features both broad NH₄⁺ ion diffusion channels and higher NH₄⁺ ion diffusion energy barriers. Additionally, TA etching effectively mitigated the Jahn–Teller effect associated with Mn³⁺. Scanning electron microscopy (SEM) was employed to analyze the structural characteristics of Mn–PBA and Mn–PBA–TA, as shown in Fig. 1b and c. The Mn–PBA synthesized via one-step co-precipitation exhibited a well-defined cubic morphology with an average particle size of approximately 1.5 μm (Fig. 1b). As a weak organic acid, tannic acid dissociates during the etching process, releasing free protons. These protons diffuse through the three-dimensionally interconnected porous network of the Mn–PBA surface, progressively dissolving the core as they migrate inward. Simultaneously, the planar surface of micrometer-sized Mn–PBA particles allows for uniform adsorption of tannic acid molecules. However, steric hindrance effects due to the molecular size of tannic acid limit its deep penetration into the Mn–PBA framework.^33,34 Consequently, the adsorbed tannic acid primarily protects the outer crystal regions from excessive etching. As depicted in Fig. 1c, the Mn–PBA–TA samples retained a similar morphology and particle size (∼1.65 μm) to Mn–PBA. However, their surfaces exhibited significant nanoparticle aggregation, resulting in a visibly rougher texture. Transmission electron microscopy (TEM) analysis, as shown in Fig. 1d and f, further confirmed the formation of a hollow cubic shell structure with a side length of approximately 1.7 μm and a shell thickness of around 0.2 μm (Fig. 1d). Elemental mapping (Fig. 1e) revealed the uniform distribution of Fe, Mn, K, C, N, and O across the hollow cubic shells. This unique hollow architecture not only increases the specific surface area and enriches active sites but also alleviates volumetric expansion during charge/discharge cycles, thereby enhancing cycling stability and the overall lifespan.

Fig. 1 (a) Schematic of this surface modification strategy and the advantages. SEM images of (b) Mn–PBA and (c) Mn–PBA–TA (insets show the particle size distribution). (d) TEM images of Mn–PBA–TA (inset shows the shell thickness distribution) and the (e) corresponding elemental mapping images.

A comprehensive structural characterization of Mn–PBA and Mn–PBA–TA was performed, as illustrated in Fig. 2. The X-ray diffraction (XRD) patterns (Fig. 2a) reveal diffraction peaks at 17.5°, 24.9°, 29.6°, 35.4°, 39.8°, 43.4°, 44.5°, 51.1°, and 57.6°, corresponding to the (020), (220), (22−2), (040), (240), (24−2), (242), (440), and (062) planes of K_xMnFe(CN)₆·yH₂O, respectively. These peaks align with the standard JCPDS no. 51-1896, confirming that both Mn–PBA and Mn–PBA–TA exhibit a monoclinic structure (space group P21/c(14)). The refined lattice parameters for Mn–PBA and Mn–PBA–TA are a = 10.108 Å, b = 10.104 Å, c = 10.114 Å, with α = γ = 90.0° and β = 92.93°. The atomic structures (inset, Fig. 2a) indicate that both materials retain a 3D open framework (Fe–C≡N–Mn), conducive to ion transport. Notably, Mn–PBA–TA exhibits broader ion transport channels, enhancing ion mobility. Raman spectroscopy (Fig. 2b) further elucidates the structural and compositional features. Peaks in the 440–640 cm⁻¹ and 2000–2200 cm⁻¹ regions correspond to Mn–O and Fe–CN bond vibrations, respectively.²⁶ Due to irregular Raman activity, the Mn–O signal is significantly weaker than the Fe–CN peak. Compared to Mn–PBA, the Mn–PBA–TA spectrum displays a reduced peak intensity and a slight redshift in Mn–O and Fe–CN bands, attributed to the thermal stress during TA etching. Fourier transform infrared (FT-IR) spectroscopy (Fig. 2c) confirms characteristic PBA features, with peaks at 2066, 1392, and 592 cm⁻¹ corresponding to the –C≡N, C=O, and Fe–CN stretching vibrations, respectively.³⁵ Peaks at 1641 and 3436 cm⁻¹ indicate H–O–H and O–H vibrations, suggesting water molecule interactions in both samples. The additional peak at 1514 cm⁻¹ in Mn–PBA–TA corresponds to the C=O stretching, confirming tannic acid adsorption.

Fig. 2 Structural characterisation of Mn–PBA and Mn–PBA–TA. (a) XRD patterns of Mn–PBA and Mn–PBA–TA (inset shows the atomic configuration). (b) Raman spectra, (c) FT-IR spectra, (d) TG curve, (e) N₂ adsorption–desorption isotherms and (f) pore size distribution curves and XPS spectra of (g) C 1s, (h) Fe 2p and (i) Mn 2p, of Mn–PBA and Mn–PBA–TA.

Thermogravimetric analysis (TGA, Fig. 2d) quantifies the water content, revealing that Mn–PBA contains 12.1% adsorbed water and 5.2% crystalline water, whereas Mn–PBA–TA has higher adsorbed water (15.9%) and lower crystalline water (3.4%). The reduction in crystalline water minimizes defects, enhancing structural integrity. Additionally, Mn–PBA–TA demonstrates improved thermal stability during decomposition. Porosity analysis via N₂ adsorption/desorption (Fig. 2e) indicates that Mn–PBA–TA possesses nearly twice the Brunauer–Emmett–Teller (BET) specific surface area of Mn–PBA (17.707 vs. 8.9256 m² g⁻¹). The Barrett–Joyner–Halenda (BJH) pore size distribution (Fig. 2f) reveals a combination of mesopores and micropores in both samples. However, Mn–PBA–TA exhibits a higher proportion of these pores, facilitating enhanced NH₄⁺ extraction and insertion pathways. Elemental composition analysis using X-ray photoelectron spectroscopy (XPS, Fig. S1, ESI†) confirms the presence of K, C, N, O, Mn, and Fe in both materials. The C 1s spectrum (Fig. 2g) exhibits peaks at 284.80 eV (C–C/C=C), 285.10 eV (C–N), 286.76 eV (C≡N), and 288.54 eV (C=O). The Fe 2p spectrum (Fig. 2h) indicates the coexistence of Fe²⁺ (2p_3/2 at 708.48 eV and 2p_1/2 at 721.28 eV) and Fe³⁺ (2p_3/2 at 709.87 eV and 2p_1/2 at 723.80 eV), with satellite peaks at 712.63 eV and 734.75 eV. Similarly, the Mn 2p spectrum (Fig. 2i) confirms the presence of Mn²⁺ (2p_3/2 at 641.34 eV and 2p_1/2 at 653.40 eV) and Mn³⁺ (2p_3/2 at 643.00 eV and 2p_1/2 at 654.73 eV). The Mn–PBA–TA sample exhibits a relative decrease in the Fe³⁺ and Mn³⁺ content after TA etching (Fig. S2, ESI†). Additional XPS spectra (Fig. S3, ESI†) provide further insights into Mn–PBA–TA. The N 1s spectrum (Fig. S3a, ESI†) features peaks at 397.70 eV and 399.50 eV, corresponding to the C–N and C≡N bonds in [Fe(CN)₆]⁴⁻, with an additional peak at ∼398.06 eV confirming Mn–N bonding. The O 1s spectrum (Fig. S3b, ESI†) exhibits peaks at 531.44 eV (Mn–O), 532.28 eV (Mn–O–H), and 533.46 eV (O–H), while the K 2p spectrum (Fig. S3c, ESI†) further supports elemental composition findings. These structural characterizations collectively confirm the successful synthesis of Mn–PBA–TA and elucidate the structural modifications induced by TA etching, which enhance ion transport properties and stability.

For the selection of acidic etching solvents, milder acids including tannic acid, oxalic acid, glycine, and tartaric acid were tested for etching Mn–PBA. The SEM images of the etching products from oxalic acid, glycine, and tartaric acid are shown in Fig. S4–S6 (ESI†). The electrochemical performance of the etched products was evaluated, yielding specific capacities of 121.9, 63.2, 37.3, and 39.4 mAh g⁻¹ at a current density of 1 A g⁻¹, respectively (Fig. 3a). Based on these results, tannic acid was selected as the optimal etching solvent. The electrochemical behavior of Mn–PBA and Mn–PBA–TA was systematically investigated using galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) experiments in a three-electrode setup. As shown in the CV profiles (Fig. 3b), both electrodes exhibited two distinct pairs of redox peaks: 0.62/0.67 V and 0.75/0.77 V for Mn–PBA, and 0.67/0.69 V and 0.77/0.81 V for Mn–PBA–TA, under a scan rate of 1 mV s⁻¹. These peaks correspond to the electrochemical transitions of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺.³⁶ After 20 cycles at a sweep rate of 20 mV s⁻¹, a significant decrease in the peak current density was observed for Mn–PBA, whereas Mn–PBA–TA retained relatively stable peak intensities. This suggests that Mn–PBA–TA exhibits enhanced electrochemical reversibility (Fig. S7, ESI†). The Mn–PBA–TA electrode also demonstrated superior rate performance compared to Mn–PBA (Fig. 3d). Across a current density range of 1 to 15 A g⁻¹, the Mn–PBA–TA electrode achieved specific capacities of 122.1, 98.8, 77.6, 65.1, 58.3, 53, and 47.2 mAh g⁻¹, respectively (Fig. 3c and Fig. S8, ESI†). When the current density was reverted to 1 A g⁻¹, the Mn–PBA–TA electrode recovered a capacity of 121.9 mAh g⁻¹, highlighting its excellent reversibility. The enhanced capacity can be attributed to TA etching, which increases the specific surface area, exposes more active sites, and facilitates faster reaction kinetics. Compared to other reported aqueous aluminum-ion batteries (AAIBs) using Prussian blue analogs (PBAs) and metal oxide cathodes,^37–45 the Mn–PBA–TA cathode exhibited superior rate capability and higher capacity across various current densities (Fig. 3e). The GCD curves for the first three cycles of the Mn–PBA–TA electrode at 1 A g⁻¹ (Fig. S9, ESI†) further confirm its structural reversibility. To evaluate the loss of Mn and Fe active centers during charge–discharge cycling, the dQ/dV curves were analyzed (Fig. 3f and g). During discharge at 1 A g⁻¹ within −0.1 V to 0.9 V, the Mn–PBA–TA electrode displayed reduction peaks at 0.78 V (Mn³⁺/Mn²⁺) and 0.69 V (Fe³⁺/Fe²⁺), with lower dQ/dV values compared to the untreated Mn–PBA electrode. Over cycling, both electrodes exhibited a weakening of the main reduction peaks; however, the Mn–PBA peak at 0.69 V faded completely after 40 cycles, whereas Mn–PBA–TA retained this peak even after 100 cycles.

Fig. 3 Electrochemical performance (the electrochemical test of the half-cell is performed with a three-electrode system, with carbon rods used as the counter electrodes and Ag/AgCl as the reference electrodes.). (a) Specific capacity of products etched by different solvents at 1 A g⁻¹. (b) CV curves of Mn–PBA and Mn–PBA–TA at 1 mV s⁻¹. (c) Rate performance of Mn–PBA–TA. (d) Comparison of the rate performance of Mn–PBA and Mn–PBA–TA. (e) Comparison of the rate performance of Mn–PBA–TA and other published electrodes for AAIBs. Corresponding discharged dQ/dV plots of (f) Mn–PBA and (g) Mn–PBA–TA. (h) Cycling performance of Mn–PBA–TA compared to that of Mn–PBA, MnO₂ and FeHCF samples. (i) In situ UV-vis spectroscopy of the electrolyte for the Mn–PBA–TA electrode during the charge/discharge process (inset shows the electrolyte conditions before and after the cycles). (j) Long term cycling performance at 15 A g⁻¹ of Mn–PBA–TA.

To further examine peak evolution and cycling stability, dQ/dV peak variations at 0.78 V were analyzed over multiple cycles. Linear fitting of the peak values (Fig. S10, ESI†) revealed that Mn–PBA–TA exhibited a smaller slope, indicating slower voltage decay and reduced active center loss. These findings suggest that TA etching effectively mitigates electrochemical polarization, thereby enhancing the cycling stability of Mn–PBA–TA. The cycling stability of Mn–PBA–TA was further assessed. As shown in Fig. 3h, the cycle life of Mn–PBA, Mn–PBA–TA, FeHCF (Fig. S11, ESI†), and commercial MnO₂ (Fig. S12, ESI†) was evaluated at 1 A g⁻¹. After 100 cycles, Mn–PBA, MnO₂, and FeHCF retained capacities of 62.3, 45.84, and 30.85 mAh g⁻¹, corresponding to capacity retentions of 60.3%, 71.3%, and 99%, respectively. In contrast, the Mn–PBA–TA cathode retained a discharge capacity of 120.3 mAh g⁻¹ after 200 cycles, with an impressive capacity retention of 80%, demonstrating its superior long-term cycling stability. Furthermore, a specific capacity of 120 mAh g⁻¹ is achieved at 1 A g⁻¹ in 1 M (NH₄)₂SO₄ electrolyte. This indicates that the capacity contribution primarily originates from NH₄⁺ ions, while dissolved Mn²⁺ ions serve to stabilize the lattice structure of Mn–PBA–TA (Fig. S13, ESI†). Although the Mn–PBA–TA cathode exhibited a higher specific capacity than FeHCF cathodes, its cycling stability was slightly compromised. This trade-off arises from its dual redox-active centers, which enhance charge storage capacity, while Mn³⁺ Jahn–Teller distortion accelerates Mn dissolution, contributing to progressive capacity fading. Nonetheless, Mn–PBA–TA exhibited superior cycling stability compared to Mn–PBA and commercial MnO₂, suggesting that the Mn vacancies introduced by TA etching effectively mitigated the Jahn–Teller effect.

To further investigate dissolution dynamics, in situ UV-visible spectroscopy was employed to monitor Mn and Fe dissolution from Mn–PBA and Mn–PBA–TA during cycling. As shown in Fig. 3i, after prolonged cycling, the Mn–PBA–TA electrode exhibited only a weak absorption peak and slight electrolyte discoloration. In contrast, the Mn–PBA electrode displayed intense absorption peaks and significant electrolyte discoloration (Fig. S14, ESI†), indicating greater metal dissolution. The long-term stability of Mn–PBA–TA was further evaluated at a high current density of 15 A g⁻¹. As illustrated in Fig. 3g, after 10 000 cycles, the Mn–PBA–TA cathode retained a capacity of 34 mAh g⁻¹, corresponding to 64.3% retention. This outstanding durability underscores the exceptional stability of Mn–PBA–TA under high-rate cycling conditions. At this current density, assuming two charge–discharge cycles per day, the system could operate for approximately 13.7 years, highlighting the promising practical application potential of Mn–PBA–TA.

Cyclic voltammetry (CV) measurements at various scan rates (1 to 50 mV s⁻¹) were conducted to comprehensively investigate the ion insertion and extraction kinetics of the Mn–PBA–TA cathode in AAIBs, as illustrated in Fig. 4a. CV analysis revealed two well-defined redox couples across multiple scan rates, demonstrating the coexistence of dual electroactive moieties in the Mn–PBA–TA cathode. These coupled redox systems were quantitatively characterized through sweep-rate-dependent kinetic analysis, where the peak current (i) exhibited a power-law relationship with a scan rate (v) described by the equation i = av^b, where b represents the slope of the linear fit between log(v) and log(i).^46,47 A b value approaching 1.0 suggests a capacitance-controlled process, while a b value near 0.5 indicates a diffusion-dominated mechanism. For peaks 1, 2, 3, and 4, the logarithmic fit yielded b values of 0.97, 0.95, 0.58, and 0.97, respectively, suggesting that the Mn–PBA–TA cathode operates through a combination of capacitive and diffusion-controlled charge storage mechanisms, as depicted in Fig. 4b.⁴⁸ As shown in Fig. S15 (ESI†), capacitive contributions accounted for 78.5% of the NH₄⁺ ion storage at a scan rate of 20 mV s⁻¹. With increasing scan rates from 1 mV s⁻¹ to 50 mV s⁻¹, the pseudo-capacitive contribution rose from 43.6% to 86.5%, demonstrating a growing dominance of capacitance control at higher scan rates (Fig. 4c).

Fig. 4 (a) CV curves at different scan rates. (b) Plots of log(i) and log(v) at specific peak currents. (c) Contribution of capacitance capacity and diffusion-limited capacity to the scan rate. (d) Nyquist plots and fitted lines of Mn–PBA and Mn–PBA–TA electrodes (inset shows the equivalent circuit). (e) Distribution of the relaxation time (DRT) analysis of Mn–PBA and Mn–PBA–TA electrodes calculated using the EIS curve. (f) Ex situ EIS plots of the Mn–PBA–TA electrode at different charging and discharging states and the corresponding charge transfer resistance (R_ct). (g) DRT analysis of the Mn–PBA–TA electrode during the charge and discharge process. (h) Bode plots upon charging and discharging.

Electrochemical impedance spectroscopy (EIS) was employed to elucidate the factors underpinning the superior electrochemical performance of the Mn–PBA–TA cathode (Fig. 4d and Table S1, ESI†). Equivalent circuit modeling (ECM) revealed a low equivalent series resistance (R_s) of 3.07 Ω and a charge transfer resistance (R_ct) of 0.49 Ω, indicating enhanced electrical conductivity, a critical factor contributing to its excellent electrochemical behavior. In the low-frequency region, Mn–PBA–TA exhibited a higher EIS slope, suggesting improved NH₄⁺ ion diffusion capability. Furthermore, a comparison of the Nyquist plots of FeHCF, MnO₂, Mn–PBA, and Mn–PBA–TA electrodes (Fig. S16, ESI†) confirmed that Mn–PBA–TA demonstrated lower charge transfer and diffusion resistance. To mitigate potential biases in impedance fitting due to subjective factors, the distribution of the relaxation time (DRT) method was applied to analyze the impedance data for Mn–PBA and Mn–PBA–TA. The DRT results closely aligned with ECM fitting, identifying two distinct peaks corresponding to charge transfer and ion diffusion processes, respectively. The smaller peak areas and higher frequency appearance of Mn–PBA–TA suggest a lower charge transfer resistance and faster ion diffusion (Fig. 4e).⁴⁹ Both ECM fitting and DRT analysis confirmed that the Mn–PBA–TA electrode exhibits rapid NH₄⁺ ion insertion/extraction kinetics. Further electrochemical kinetic analysis via ex situ EIS measurements demonstrated that the Nyquist plots retained similar features throughout cycling, reflecting the insertion and detachment of NH₄⁺ ions. The charge transfer resistance (R_ct) obtained from ECM fitting gradually decreased during charging and increased during discharging, returning to its initial state at −0.1 V, indicating favorable cycling reversibility (Fig. 4f). Changes in the DRT curves further revealed variations in the reaction intensity during the charge/discharge process, as shown in Fig. 4g. During charging, the residence time of the electrode in the low-frequency region decreased, suggesting enhanced NH₄⁺ transport due to ion detachment. Conversely, during discharging, the residence time increased, implying that continuous NH₄⁺ insertion slowed the reaction kinetics, leading to a delayed overall process.⁵⁰ This behaviour underscores the highly reversible charge storage kinetics of the Mn–PBA–TA electrode. Bode phase analysis of the electrochemical impedance spectroscopy (EIS) data (Fig. 4h) revealed characteristic relaxation frequencies (f₀) spanning 11.25–14.06 Hz, corresponding to remarkably low relaxation time constants (τ₀) of only 0.071–0.089 s. These exceptionally short time constants quantitatively demonstrate ultrafast charge-transfer kinetics at the Mn–PBA–TA electrode–electrolyte interface, indicative of a highly reversible redox mechanism.

To investigate the NH₄⁺ storage mechanism of Mn–PBA–TA, ex situ XRD analysis was conducted to examine the lattice changes under different charge and discharge states (Fig. 5a–c). The XRD spectra exhibited minimal variations throughout the charge/discharge process, indicating the structural stability of Mn–PBA–TA (Fig. 5b). A magnified view of the (020) peak revealed a reversible shift of the Mn–PBA–TA electrode during cycling, which was attributed to the lattice deformation caused by NH₄⁺ intercalation and deintercalation (Fig. 5c).⁵¹ This finding suggests that the surface modification of Mn–PBA with TA effectively mitigates the stress associated with NH₄⁺ insertion/extraction, thereby enhancing structural reversibility during cycling. In addition, ex situ XPS characterization was performed to analyze valence state changes under different charge and discharge conditions (Fig. 5d–f). In the Mn 2p XPS spectra, peaks corresponding to Mn²⁺ were observed at 641.68 and 653.49 eV, while those for Mn³⁺ appeared at 643.35 and 654.89 eV (Fig. 5d). During charging, the intensity of Mn²⁺ peaks decreased, while Mn³⁺ peaks intensified due to NH₄⁺ extraction, indicating partial oxidation of Mn²⁺ to Mn³⁺. Conversely, during discharge, NH₄⁺ insertion facilitated the reduction of Mn³⁺ back to Mn²⁺, as evidenced by the weakening of Mn³⁺ peaks. This reversible Mn²⁺/Mn³⁺ redox process was further validated in Fig. S17 (ESI†).⁵² Similarly, the Fe 2p XPS spectra confirmed the Fe²⁺/Fe³⁺ redox process (Fig. 5e). During charging, Fe²⁺ (708.75 and 721.57 eV) was partially oxidized to Fe³⁺ (710.07 and 724.29 eV), while during discharge, Fe³⁺ was reduced back to Fe²⁺, driven by NH₄⁺ insertion and extraction. This reversible redox behavior, closely linked to NH₄⁺ ion dynamics, was further visualized in Fig. S17 (ESI†).⁵³ The N 1s spectra also captured the NH₄⁺ insertion/extraction process (Fig. 5f). During charging, NH₄⁺ extraction led to a gradual weakening of the N–H peak at 402.34 eV, whereas upon full discharge, its intensity significantly increased, confirming NH₄⁺ re-insertion into the Mn–PBA–TA cathode.²⁸

Fig. 5 Energy storage mechanism and DFT calculations of the Mn–PBA–TA electrode. (a) Typical GCD curves of the electrode. (b) and (c) Ex situ XRD analysis under different charging and discharging states. Ex situ XPS spectra of (d) Mn 2p, (e) Fe 2p and (f) N 1s in redox reactions. (g) Diffusion energy barriers of NH₄⁺ diffusion within the lattice of Mn–PBA–TA and the corresponding structure diagrams at different stages.

The structural evolution of the Mn–PBA–TA electrode at different charge states was further examined using ex situ Raman spectroscopy. Two main regions were analyzed: Mn–O and Fe–CN functional groups. The Raman spectra revealed two prominent bands at ∼480 and 601 cm⁻¹, corresponding to the Mn–O bond vibrations, while a strong peak at 815 cm⁻¹ was attributed to the SO₄²⁻ ions from the electrolyte (Fig. S18a, ESI†). During charging, NH₄⁺ extraction resulted in the gradual weakening of the 480 cm⁻¹ peak and an increase in the 601 cm⁻¹ peak, indicating the oxidation of Mn²⁺ to Mn³⁺ (Fig. S18b, ESI†). During discharge, the process was reversed as NH₄⁺ was reinserted, reducing Mn³⁺ back to Mn²⁺.⁵⁴ Furthermore, the ex situ Raman spectra and contour plots of Mn–PBA–TA across 1900–2400 cm⁻¹ (Fig. S19, ESI†) revealed changes in the Fe oxidation states. During charging, NH₄⁺ extraction reduced the intensities of peaks at 2080 cm⁻¹ and 2118 cm⁻¹, while the peak at 2148 cm⁻¹ intensified, indicating the oxidation of Fe²⁺ to Fe³⁺. During discharge, NH₄⁺ insertion facilitated the reduction of Fe³⁺ back to Fe²⁺.⁵⁵ The well-reversible Mn²⁺/Mn³⁺ and Fe²⁺/Fe³⁺ redox processes were further corroborated by these observations. The dual redox-active centers (Mn–O and Fe–CN) drive the redox mechanism of Mn–PBA–TA in AAIBs, promoting efficient ion insertion/extraction and ensuring the cathode's high electrochemical performance. In addition, ex situ FT-IR spectroscopy was performed on Mn–PBA–TA (Fig. S20, ESI†). The FT-IR spectra exhibited two distinct peaks at 2061 cm⁻¹ and 2130 cm⁻¹, corresponding to the Fe²⁺–CN and Fe³⁺–CN vibrations, respectively.^17,56 As the charge/discharge process progressed, the vibrational peaks of Fe³⁺–CN exhibited a reversible enhancement followed by weakening, confirming the active participation of the [Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻ redox couple in the electrochemical reaction.

Density functional theory (DFT) calculations were conducted to elucidate NH₄⁺ migration pathways and diffusion energy barriers within the Mn–PBA lattice. A cubic Mn–PBA structural model was constructed, followed by simulations of NH₄⁺ migration and energy barrier calculations at various sites. The results indicated that NH₄⁺ diffusion occurs along the [001] axis through the central core of the crystal lattice, with –CN sites in the PBA framework serving as preferential adsorption sites. Fig. 5g illustrates the transition of NH₄⁺ from the initial state (IS) through the transition state (TS) to the final state (FS), providing a detailed representation of the ion's diffusion trajectory. Additional three-view images of NH₄⁺ migration are shown in Fig. S21 (ESI†). Unlike spherical metal cations (e.g., K⁺ and Na⁺), NH₄⁺ possesses a tetrahedral structure, enabling it to undergo twisting and rotation via a “monkey-swinging” diffusion mechanism within the Mn–PBA–TA framework.⁵⁷ During insertion, NH₄⁺ twisting facilitates the formation of N–H⋯N hydrogen bonds between the H atoms and N atoms in the Mn–PBA–TA lattice. The calculated diffusion energy barrier is shown in Fig. 5g. Initially, the system's total energy increases as diffusion begins, reaching a peak (∼1.43 eV) at the transition state (TS), where hydrogen bonds are disrupted. As diffusion proceeds, new hydrogen bonds form, releasing energy and reducing the system's total energy. Despite NH₄⁺ having a relatively large ionic radius (1.43 Å), which could contribute to a high diffusion energy barrier, the Mn–PBA–TA framework effectively accommodates NH₄⁺ diffusion with low energy barriers, ultimately enabling excellent electrochemical performance.

An aqueous NH₄⁺ full cell, based on Mn–PBA–TA//PQANS, was fabricated by integrating the Mn–PBA–TA cathode with the PQANS anode, as shown in Fig. 6a. In this configuration, ammonium ions reversibly shuttle between the Mn–PBA–TA and PQANS. The morphology of the prepared PQANS anode exhibited a uniform spherical shape and demonstrated excellent electrochemical performance in AAIBs (Fig. S22–S24, ESI†).⁵⁸ The CV curves of the Mn–PBA–TA//PQANS full cell at various scan rates, ranging from 1 to 50 mV s⁻¹, within a voltage window of 0–1.6 V, are shown in Fig. 6b. At higher scan rates, the full cell exhibited a pair of reversible redox peaks, confirming the superior redox kinetics of the Mn–PBA–TA//PQANS system. The initial discharge capacity of the full cell at a current density of 1 A g⁻¹ was 42.6 mAh g⁻¹, and the five nearly overlapping charge/discharge curves indicated excellent cycling stability (Fig. 6c). The specific capacities were 57.4, 40.6, 31.7, and 21.8 mAh g⁻¹ at current densities of 0.5, 1, 2, and 5 A g⁻¹, respectively. Upon restoring the current density, the specific capacity gradually returned to 32.2, 42.4, and 55.8 mAh g⁻¹ at 5, 1, and 0.5 A g⁻¹, demonstrating the remarkable rate performance of the full cell (Fig. 6d). Moreover, the Mn–PBA–TA//PQANS full cell exhibited exceptional cycling stability. After 1000 cycles at a current density of 1 A g⁻¹, the cell retained a capacity of 38.4 mAh g⁻¹, corresponding to a capacity retention of 88.5% (Fig. 6e). Considering the total effective mass of both the anode and cathode, the cell achieved an energy density of approximately 36.2 Wh kg⁻¹, with a power density of 849.8 W kg⁻¹. To highlight the performance of the Mn–PBA–TA//PQANS full cell, its capacity, energy density, and cycling stability were compared to those of representative aqueous NH₄-ion, Na-ion, and H-ion batteries, as shown in Fig. 6f, Fig. S25 and Table S2 (ESI†),^59–68 demonstrating superior overall performance. Finally, Fig. 6g shows a digital photograph of the Mn–PBA–TA//PQANS soft pack battery powering a small fan, indicating its great potential for practical applications.

Fig. 6 Electrochemical performance of the Mn–PBA–TA//PQANS full cell. (a) Schematic illustration of the Mn–PBA–TA//PQANS full cell. (b) CV curves of the Mn–PBA–TA//PQANS battery at different scan rates. (c) Initial five GCD curves at a current density of 1 A g⁻¹. (d) Rate performance. (e) Cycling performance at 1 A g⁻¹ based on the total mass of the anode and cathode. (f) A comparison of this NH₄-ion battery with representative aqueous NH₄-ion, Na-ion and H-ion batteries, in terms of capacity and energy density. (g) Digital photograph showing the actual application of the Mn–PBA–TA//PQANS soft package battery.

3. Conclusions

In conclusion, Mn–PBA–TA was successfully synthesized as a cathode material for AAIBs through a TA-assisted etching process. This material features extensive NH₄⁺ diffusion channels and dual redox-active centers. The TA etching method effectively forms a hollow cubic structure, enhancing the exposure of specific surface areas and active sites while mitigating the Jahn–Teller distortion typically observed in Mn³⁺. After 200 cycles at a current density of 1 A g⁻¹, the Mn–PBA–TA cathode exhibited an impressive specific capacity of 120.3 mAh g⁻¹. When subjected to a high current density of 15 A g⁻¹, the Mn–PBA–TA electrode retained its capacity over 10 000 cycles, with only a 0.0036% attenuation per cycle, highlighting its exceptional rate capability and long-term cycling stability. Additionally, a series of ex situ characterization methods confirmed that the energy storage mechanism of the Mn–PBA–TA cathode is primarily driven by a redox reaction between the Mn–O and Fe–CN groups, further supporting its efficient electrochemical behavior. By integrating the Mn–PBA–TA cathode with a quinone–amine polymer anode, a highly stable aqueous NH₄⁺ battery was successfully developed, demonstrating significant potential for practical applications. This study serves as a valuable reference for the rational design of PBAs and their application in AAIBs.

Author contributions

J. Yang, H. Fu, and E. H. Ang developed the conceptual framework and designed the experiments. J. Yang and H. Fu were responsible for material fabrication. J. Yang also performed data measurement and analysis, while L. Ye created the figures. M. Shi handled data analysis. The manuscript was prepared by J. Yang and E. H. Ang. Funding support was provided by J. Yang and E. H. Ang. All authors contributed to the discussions and provided feedback on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All relevant data are within the manuscript and the ESI.†

Acknowledgements

We greatly acknowledge the funding for this project through the China Postdoctoral Science Foundation (2023M741471 and 2022M711686), and the Marine Equipment and Technology Institute, Jiangsu University of Science and Technology (XTCX202405), the National Institute of Education, Singapore, under its Academic Research Fund (RI 1/21 EAH and RI 3/23 EAH), and the Ministry of Education, under its Academic Research Fund, Tier 1 (RG88/23).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00582e

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