NH₄⁺-mediated interfacial chemistry for collaborative dual-pathway high-mass-loading energy storage

Abstract

The deposition/dissolution mechanism of MnO₂ with two-electron transfer is promising for high-energy aqueous energy storage. However, this reaction is severely limited by the kinetically unfavorable dissolution step, a challenge that is greatly exacerbated as the deposit thickens. Herein, by refining NH₄⁺-mediated interface chemistry, we achieve the precise configuration of MnO₂ with controlled hydroxylation, which guides the reversible MnO₂/Mn²⁺ conversion under high-mass-loading conditions. The partially hydroxylated surface further creates a kinetically favorable microenvironment for NH₄⁺ storage, ultimately leading to energetic dual-pathway storage behaviors. This enables a remarkable areal capacitance of 13.8 F cm⁻² and sound cycling stability over 6000 cycles under high-mass-loading conditions (27.1 mg cm⁻²). Theoretical calculations reveal that the controlled partial hydroxylation of MnO₂ promotes electronic conduction and lowers the adsorption energy of NH₄⁺, outperforming both highly hydroxylated and pure MnO₂. The adsorbed NH₄⁺ delivers intimate interfacial electronic interaction with partially hydroxylated MnO₂ to trigger local charge redistribution, substantially lowering the MnO₂/Mn²⁺ conversion energy barrier of the nonspontaneous rate-determining step at the NH₄⁺-proximal site. Our findings highlight the significance of the interfacial microenvironment governing the collaborative dual-pathway storage chemistry, which provides guidance for boosting high-mass-loading energy storage.

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Introduction

The escalating global energy crisis and climate change highlight the great demand for developing clean and sustainable energy techniques,^1–3 e.g., aqueous electrochemical energy storage with inherent safety, environmental friendliness, and cost-effectiveness.^4,5 MnO₂ has been widely considered as an economic and highly promising cathode alternative in a near-neutral environment, featuring a wide operating voltage and high theoretical capacitance.^6,7 However, the sluggish kinetics and the poor stability occur under commercial-level mass loading (≥10 mg cm⁻²), posing significant challenges for sustaining efficient practical applications of MnO₂ electrodes so far.^8,9

Tremendous efforts have been made to tailor the reactivity and kinetics behaviors of the electrode materials by controlling the microstructure and local reaction environment toward optimal carrier-intercalation/diffusion and interface behaviors.^7,9–13 For instance, it was reported that the structural stability issue can be partially relieved by harnessing the physical confinement effect and abundant interfacial interactions.^13,14 In another case, robust electronic coupling between the heteroatom and the Mn atom was developed to suppress the structure deformation and Jahn–Teller effect during cycling.^15,16 Despite the progress, under practical scenarios with substantially increased electrode thickness and non-active substances, significant kinetics limitations still exist to compromise the storage capability, stemming from the super-long diffusion distance and complicated interfacial redox process.^17–19 Beyond that, the dynamic reconstruction of the surface/interface happens with deactivation and dissolution, creating a varied local reaction microenvironment over time that requires better control.

In contrast to intercalation chemistry, the MnO₂/Mn²⁺ deposition/dissolution chemistry provides a new opportunity for highly efficient energy storage. The in situ formation and interface reset during the charge/discharge process can bypass the undesired compositional/structural evolution and considerable local strain accumulation, responsible for breaking the limitations for energetic and sustainable high-mass-loading applications. Moreover, the topological structure and the spatial arrangement of MnO₂ can be tailored by refining the cation environment and the reactivity. For better practice, the high concentration of Mn²⁺ is preferred to boost the deposition behaviors and thereby the charges stored. However, a high-concentration pure Mn²⁺ electrolyte leads to the suppressed dissolution of MnO₂ according to Le Chatelier's principle. At the same time, the deposited thick MnO₂ features highly compromised mass/charge-transfer rates with an inferior utilization rate of active species. An open question appears regarding how to enable better control of the deposition/dissolution chemistry with enhanced conversion kinetics under high Mn²⁺ concentration conditions. Besides, it is predicted that synergistically coupling the cation storage with the MnO₂/Mn²⁺ conversion chemistry may potentially boost high-mass-loading energy storage with ultra-efficient utilization of active species. The mechanistic insights can help the step-forward optimization of high-mass-loading energy storage techniques for better applications.

Herein, by introducing NH₄⁺ into a dynamic environment, we overcome the kinetics limitation of MnO₂/Mn²⁺ conversion and trigger its synergistic coupling with NH₄⁺ (de)intercalation chemistry toward an optimal integrated reaction system. This dual-pathway storage mechanism enables a high areal capacitance of 13.8 F cm⁻² for electrodes with a superhigh mass loading of 27.1 mg cm⁻², along with a good capacitance retention of 80% after 6000 cycles. The theoretical and experimental results indicate that NH₄⁺ acts as the modulator at the reaction interface to precisely control the hydroxylation degree of MnO₂, creating a kinetically favorable reaction microenvironment with fast electronic conduction and enhanced interfacial NH₄⁺ adsorption. Furthermore, the adsorption of NH₄⁺ on the reconstructed interface triggers a local charge redistribution, which substantially reduces the energy barrier of the nonspontaneous rate-determining step during dissolution of MnO₂, thereby promoting an energetic dual-pathway storage mechanism. This work offers novel insights into cationic environment engineering for mutual manipulation of deposition/dissolution and intercalation chemistry, and provides a promising avenue for tailoring better high-mass-loading energy storage.

Results

Refining the interfacial microenvironment for favoring MnO₂/Mn²⁺ conversion

Our previous study has demonstrated that dissolved Mn^x+ assists in the progressive formation of the electrode–electrolyte interphase, providing the energetic dual-cation storage mechanisms.²⁰ We herein demonstrate the key role of NH₄⁺ in regulating the hydroxylation degree of the derived high-mass-loading MnO₂, which is responsible for enhanced intrinsic capability of NH₄⁺ storage (Scheme 1). More importantly, optimized MnO₂ dissolution chemistry is revealed with NH₄⁺ in the electrode–electrolyte interface, leading to the collaborative dual-pathway energy storage. The reaction system and the new concept in the collaborative interface chemistry can help break the bottleneck toward energetic high-mass-loading energy storage (Scheme 1).

Scheme 1 Schematics of collaborative dual-pathway high-mass-loading energy storage.

We start with the formation of an active MnO₂ substrate through phosphate-assisted in situ reconstruction.²⁰ By studying its charge/discharge properties in 0.5 M Mn²⁺ electrolyte, we found an increase of the CV curve area and thereby the areal capacitance value for the first 10 cycles (Fig. 1a and b), along with the increased active mass with cycling (SI Fig. 1). However, further cycling leads to a significant performance decline, indicating an unstable storage interface derived from poorly controlled deposition/dissolution chemistry. Increasing the Mn²⁺ concentration to 1 M leads to an increment of the areal capacitance, but the poor sustainability of the reaction system remains unsolved. In sharp contrast, we discovered that the areal capacitance exhibits a continuous increment during electrochemical cycling in the NH₄⁺–Mn²⁺ mixed electrolyte, which achieves the highest value after 60 CV cycles (Fig. 1a and b). The derived 60-cycled electrode in NH₄⁺–Mn²⁺ mixed electrolyte was denoted as Rec-NM-MnO₂ for further study.

Fig. 1 Regulation of electrolytes and substrates. (a) CV curves of the activated MnO₂ substrate in different electrolytes. (b) Performance evolution of the active MnO₂ substrate with cycling in different electrolyte environments. (c) Comparison of areal capacitance for Rec-NM-MnO₂ in different electrolytes and the ratio of the capacitance measured in different electrolytes to that measured in NH₄⁺–Mn²⁺ mixed electrolyte. (d) Comparison of CV curves at a scan rate of 2 mV s⁻¹ for the Rec-NM-MnO₂ electrode in different electrolytes. (e) Comparison of the areal capacitance for the Rec-NM-MnO₂ electrode with previously reported high-mass-loading MnO₂-based electrodes. (f) Capacitive contributions for Rec-NM-MnO₂. (g and h) SEM images of Rec-NM-MnO₂ (g) before and (h) after electrochemical cycling. (i) HRTEM image of Rec-NM-MnO₂.

To further test the superiority of the mixed electrolyte system, we applied the bare carbon substrate (electrochemically exfoliated graphene, EG) to eliminate the influence of active MnO₂. A fast increase of areal capacitance with cycling was observed by using the pure EG substrate, ultimately achieving a decent areal capacitance of 9.3 F cm⁻² at 2 mA cm⁻² (SI Fig. 5). This result reflects the superiority of NH₄⁺–Mn²⁺ mixed electrolyte to sustain the deposition/dissolution behaviors of MnO₂. Of note, a highly improved capacitance was demonstrated by applying the active MnO₂ electrode as the substrate (SI Fig. 5), attributed to more abundant nucleation sites and a larger accessible area for better deposition/dissolution chemistry. It delivers an appealing areal capacitance with a high value of 13.8 F cm⁻² at 2 mA cm⁻² and maintains an areal capacitance of 2.9 F cm⁻² at 50 mA cm⁻² (Fig. 1c and SI 3d), superior to many state-of-the-art high-mass-loading electrodes (Fig. 1e and SI Table S1).^6,21–29 Moreover, it displays good cycling stability with a retention of 80% after 6000 cycles (SI Fig. 3i).

The analysis of capacitive contribution and the b value by Dunn's method indicates a diffusion-controlled behavior during the charge-storage process (Fig. 1f and SI 2b). An asymmetric supercapacitor (Rec-NM-MnO₂//active carbon) was configured and investigated in the NH₄⁺–Mn²⁺ mixed electrolyte. It delivers a high areal capacitance of 3.1 F cm⁻² at 4 mA cm⁻² (SI Fig. 6d), alongside a maximum energy density of 1070 µWh cm⁻², which is superior to many previously reported results (SI Fig. 6e).^30–36 Besides, it can operate stably for 500 cycles at a current density of 120 mA cm⁻² (SI Fig. 6f). Please refer to SI Note 2 for details.

Revealing the reconstructed high-mass-loading electrode structure

To reveal the reconstructed structure, we first collected scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images. Before cycling, it demonstrates a core–shell heterostructure with ultra-thin nanosheets uniformly distributed (Fig. 1g and SI 7). It is noted that the unique structure with a large active area creates rich nucleation sites for the energetic MnO₂ deposition process, in good agreement with our electrochemical results (SI Fig. 5).²⁰ After 60 cycles in the NH₄⁺–Mn²⁺ mixed electrolyte, the Rec-NM-MnO₂ microstructure features the dense stacking of nanosheets with an enlarged size and thickness compared to before, causing an increase of the overall diameter of the heterostructure from 114 nm to 225 nm and thereby an increment of mass loading from 7.4 mg cm⁻² to 27.1 mg cm⁻² (Fig. 1g, h, SI 7a and b and 8a–d). The accumulation of mass loading is further indicated by an increase of the charge transfer resistance (R_ct) from 1.10 Ω to 6.12 Ω (SI Fig. 9a–c and Table S2). Notably, the system ohmic impedance (R_s) of the Rec-NM-MnO₂ electrode is as low as 0.61 Ω (SI Fig. 9a–c and Table S2), compared to the pristine electrode (0.49 Ω). Meanwhile, the Warburg factor (σ) of the samples after different cycles exhibits slight variation in the range of 3.6 to 5.55 (SI Fig. 9d), demonstrating a favorable ion diffusion capability in the thick electrode. The lattice spacing of 0.21 nm in HRTEM images can be indexed to the (101) plane of ε-MnO₂ (Fig. 1i and SI 8e).³⁷ One detail is the uniform distribution of N elements on the pristine active substrate, as shown by elemental mapping, which might come from intercalated NH₄⁺ and the dopamine-derived N-doped carbon layer (SI Fig. 8f).

Ex situ X-ray diffraction (XRD) measurements were carried out to further reveal the structural evolution with cycling. As shown in Fig. 2a, the pristine active substrate shows diffraction peaks located at 36.2, 43.0, and 65.6°, indexed to the (110), (11-2), and (020) planes of δ-MnO₂ (JCPDS card no. 43-1456).^20,22,38 However, after cycling, the as-made Rec-NM-MnO₂ electrode exhibits diffraction peaks at 37.1, 42.4, and 66.2°, which are matched well with the (100), (101), and (110) planes of ε-MnO₂ (JCPDS card no. 30-0820), in good accordance with the HRTEM results.^39,40 The transformation of the main structure of the material is further reflected by the gradual shift of diffraction peaks with cycling (SI Fig. 10). Of note, there is an increase in the relative peak intensity of MnO₂ compared with that of the EG substrate, attributed to the gradual accumulation of ε-MnO₂ upon electrochemical cycling. These results indicate that, despite the enhanced kinetics, the active substrate features a higher deposition rate than that of the dissolution one during the early stage of cycling. The reconstructed reaction interface modulated by the accumulated ε-MnO₂ might play a key role in the highly improved electrochemical energy storage behaviors.

Fig. 2 Structure and composite analysis during electrochemical cycling. (a–e) Ex situ investigations of the active MnO₂ substrate: (a) XRD patterns; (b) ΔE value of Mn 3 s peaks and the AOS of Mn; characteristic content analysis based on (c) the O 1s spectra and (d) the N 1s spectra; (e) FTIR spectra. (f) Operando Raman contour maps of the active MnO₂ substrate during electrochemical cycling. (g) Schematic illustration of the development of the high-mass-loading partially hydroxylated ε-MnO₂ electrode.

Ex situ X-ray photoelectron spectroscopy (XPS) measurement was conducted to study the evolution of the interfacial electronic structure. The peak separation energy (ΔE) of Mn 3 s increases from 4.91 eV for pristine MnO₂ to 5.13 eV after the first two cycles (Fig. 2b and SI 11a), suggesting the decrease of the average oxidation state (AOS) of Mn from 3.42 to 3.18 with the accumulation of ε-MnO₂. Subsequently, the AOS of Mn increases to 3.49, demonstrating that surface oxidation happens with further cycling. This is also the case in the pure 0.5 M Mn(NO₃)₂ electrolyte (SI Fig. 12). The O 1s fine spectra can be decoupled and fitted into three components at 530.1, 531.6, and 533.2 eV, assigned to M–O–M, M–OH, and H–O–H, respectively (Fig. 2c and SI 11b).⁴¹ The content ratio between M–O–M and M–OH bonds exhibits a reduction after the first two cycles and a gradual increase in the subsequent cycles. This suggests the initial formation of MnO₂ with a high hydroxylation degree (H-MnO₂(h)), but the hydroxylation degree shows a slight decrease with further cycling, yielding the partially hydroxylated MnO₂ (H-MnO₂(p)) for the Rec-NM-MnO₂ electrode. In the early stage, it is believed that a considerable amount of H⁺ was produced with the hydrolysis of Mn²⁺ and the deprotonation effect of NH₄⁺, which promotes the transformation from M–O–M to M–O–H bonds with discharging.⁴² The similar evolution of M–O–M and M–O–H bonds is found during deposition in the pure 0.5 M Mn²⁺ electrolyte (SI Fig. 13). However, a relatively low hydroxylation degree was demonstrated, attributed to the inferior H⁺ supply during hydroxylation.

The N 1s spectra of the pristine active substrate show two main peaks at 400.2 and 407.3 eV, attributed to the –NH– bond derived from dopamine (PDA)-derived carbon and NO₃⁻ from the NaNO₃ electrolyte, respectively (SI Fig. 11c).^20,43,44 Note that a new peak located at 401.4 eV, ascribed to the –N⁺H– bond, appears at the second cycle, which originates from the NH₄⁺ intercalated in MnO₂. Besides, the content ratio between –NH– and –N⁺H– bonds gradually decreases with cycling (Fig. 2d and SI 11c), indicative of the varied main N source and the gradual accumulation of NH₄⁺-intercalated MnO₂, which is also evidenced by XPS full-spectra (SI Fig. 11d and e). Moreover, it is also demonstrated by the ex situ Fourier transform infrared spectra (FTIR), where the characteristic absorptions indexed to the N–H bending vibration (1400 cm⁻¹), N–H⋯O bending vibration (3100–3180 cm⁻¹) and N–H stretching vibration (3180–3300 cm⁻¹) show an increase in intensity, while the peak indexed to the O–H stretching vibration (3300–3600 cm⁻¹) decays with cycling (Fig. 2e and SI 11f), which is consistent with the O 1s results (Fig. 2c).^23,45

We focused on the first two cycles to study the structural evolution of the active substrate by ex situ characterization (SI Fig. 14–16). XRD results show periodic changes in the characteristic diffraction peaks (SI Fig. 14). There is a dynamic increase/decrease for the ratio of M–O–M/M–OH with reversible oxidation/reduction (SI Fig. 16), indicating H⁺ participation in the early stage (please refer to SI Fig. 14–16 for details).

To gain more insights into the microstructure transformation, operando Raman characterization was performed. The pristine active substrate demonstrates three main peaks at 645 cm⁻¹ (ν₁), 571 cm⁻¹ (ν₂), and 497 cm⁻¹ (ν₃), associated with the symmetric stretching vibration of the Mn–O bond along the d_z² orbital within [MnO₆] octahedra, the stretching vibration in the basal plane along the d_x²–y² orbital, as well as the weak Mn–O vibration mode, respectively (SI Fig. 17a).^46,47 The intensity of these major peaks (ν₁, ν₂, and ν₃) weakened during the first several cycles of deposition (Fig. 2f and SI 17b). This phenomenon might originate from the generation and accumulation of new substances, i.e., hydroxylated MnO₂ on the surface of the original MnO₂ as discussed (Fig. 2c and SI 11b). As the hydroxylation degree slightly decreases with cycling, the characteristic peaks of MnO₂ demonstrate increasing intensity (Fig. 2f and SI 17b). We further focus on the operando Raman spectra during the 10th cycle with the well-defined characteristic peaks. As shown in Fig. 2f and SI 17d, with charging, ν₁ and ν₂ shift negatively and positively, respectively, and both of them shift back to the original locations upon discharging, indicating a highly reversible lattice deformation with NH₄⁺ insertion/extraction.^8,23,47–52 A similar evolution trend was observed during the 1st cycle, except for the spectra in the potential range from 0 to 0.19 V. This unique oscillation phenomenon indicates a significant change in the surface chemical environment with the voltage applied (Fig. 2f and SI 17c).

XPS depth profiles were collected to gain more insights into the surface-accumulated hydroxylated ε-MnO₂ of the derived Rec-NM-MnO₂ electrode. A high content of Mn–OH (>20%) was found after different etching times, indicative of the considerable hydroxylation degree in the bulk structure upon cycling (Fig. 3f and SI 18b). It is further found that the outermost surface of the electrode demonstrates a higher hydroxylation degree.

Fig. 3 Study of the energy storage mechanism of Rec-NM-MnO₂. (a–e) Ex situ characterization of the Rec-NM-MnO₂ electrode at different voltages: (a) XRD patterns; (b) ΔE value of Mn 3 s peaks and the AOS of Mn; (c) N 1s spectra and position variation of the peak relative to the –N⁺H- bond; (d) O 1s spectra and corresponding content analysis; (e) FTIR spectra. (f) The content analysis of the O 1s spectra for Rec-NM-MnO₂ with different etching times. (g and h) Operando characterization of Rec-NM-MnO₂: (g) Raman contour map and (h) EIS spectra.

The above results have evidenced the interface reconstruction with the accumulation of H-MnO₂(p) with high mass loading when cycling in the mixed NH₄⁺–Mn²⁺ electrolyte (Fig. 2g), responsible for a significant increase in electrochemical performance (Fig. 1e and SI Table S1).

Understanding the role of NH₄⁺ in the reconstructed interface

After understanding the promising nature of H-MnO₂(p), we further performed a series of electrochemical analyses and in situ/ex situ characterization experiments to gain insights into two questions: (1) what is the key role of NH₄⁺ in boosting high-mass-loading energy storage? (2) How does NH₄⁺ help overcome the inherent dissolution kinetics limitations?

We first studied the electrochemical behaviors of the Rec-NM-MnO₂ electrode in the pure 0.5 M (NH₄)₂SO₄ or 0.5 M Mn(NO₃)₂ electrolyte. The electrode exhibits a higher areal capacitance value in the mixed electrolyte than its counterparts, reinforcing an effective collaborative storage mechanism (Fig. 1c, SI 3a–d and 4). It displays a pronounced oxidation current by using the NH₄⁺–Mn²⁺ or 0.5 M Mn(NO₃)₂ electrolyte (Fig. 1d and SI 3e), attributed to Mn²⁺-to-MnO₂ deposition in the Mn²⁺ rich reaction environment.^53,54 In contrast, the reduction current from 1.2 V to 0.9 V increases a lot with NH₄⁺ introduction (Fig. 1d and SI 3e), suggesting a unique role of NH₄⁺ in promoting MnO₂ dissolution, which contributes to an increased discharge capacitance.^53,54

This is also demonstrated by the charge evolution analysis of the Rec-NM-MnO₂ electrode (SI Fig. 3f and g), wherein an obvious charge accumulation was observed in the pure Mn²⁺ electrolyte, indicative of inadequate dissolution dynamics. In contrast, gradual charge consumption was found when cycling in the pure NH₄⁺ electrolyte, implying promoted MnO₂ dissolution with NH₄⁺ introduction. This is further evidenced by significantly weakened charge accumulation in NH₄⁺–Mn²⁺ mixed electrolyte in comparison to that in the pure Mn²⁺ electrolyte.⁵⁵ In addition, with further discharging, an obvious reduction peak was observed in NH₄⁺- Mn²⁺ or 0.5 M (NH₄)₂SO₄ electrolyte (Fig. 1d and SI 3e), which might originate from the extensive intercalation of NH₄⁺ in the NH₄⁺-rich environments.^23,54 We have further studied the electrochemical behaviors in a series of mixed electrolyte environments, evidencing the unique role of NH₄⁺ in boosting charge-storage capability of the reaction system (SI Fig. 3h).

Then, we studied the energy storage mechanism of the Rec-NM-MnO₂ electrode in the NH₄⁺–Mn²⁺ mixed electrolyte. According to ex situ XRD patterns, the diffraction peaks, relative to the (110) and (020) planes of ε-MnO₂, demonstrate a positive shift of 0.6° and 0.5° with charging, respectively, and return to the original locations upon discharging, which might originate from the interaction between NH₄⁺ and ε-MnO₂ (Fig. 3a).⁵⁶ This is accompanied by the reversible change of the average oxidation state (AOS) of Mn (Fig. 3b and SI 18a). The characteristic peak indexed to the –NH⁺- bond displays a negative shift of ∼0.3 eV with charging (Fig. 3c), and shifts back upon discharging, implying the reversible deintercalation/intercalation of NH₄⁺ and its potential interaction with MnO₂. Another detail is the negligible change of Mn–O–H content upon discharging, indicative of the negligible contribution of H⁺ storage after interface reconstruction, which is responsible for the enhanced cycling stability (Fig. 3d).⁵⁷ This phenomenon is attributed to the accumulation of ε-MnO₂ with partial hydroxylation during cycling, which leads to the fast consumption of free H⁺ in the electrolyte environment. Moreover, the FTIR spectra demonstrate the reversible decrease/increase of peak intensity for the N–H/N–H⋯O bending vibration and N–H stretching vibration during charging/discharging, while the peak indexed to the O–H stretching vibration shows a negligible change, reinforcing that NH₄⁺ serves as the charge carrier during the energy storage process (Fig. 3e and SI 18c).

We further collected the operando Raman spectra of the Rec-NM-MnO₂ electrode for tracking the NH₄⁺ storage behaviors. With charging, ν₁ shifts positively from 639 cm⁻¹ to 650 cm⁻¹ (Fig. 3g and SI 19), ascribed to the oxidation of the Mn element and enhancement of the Mn–O stretching vibration along the d_z² orbital of [MnO₆] octahedra due to the extraction of NH₄⁺.^23,47–50 Moreover, the ν₂ peak shows an enhanced intensity and a negative shift of 8 cm⁻¹ from 580 cm⁻¹ to 572 cm⁻¹ during charging, originating from the increase of Mn(IV) in the [MnO₆] framework and reduced Jahn–Teller disorder, accompanied by NH₄⁺ deintercalation.^8,47,52 Both of them shift back to the pristine locations upon discharging, indicative of the reversible structural change with charge carrier deintercalation/intercalation. Operando electrochemical impedance spectra (EIS) of the Rec-NM-MnO₂ electrode demonstrate that the semicircle diameter indexed to the charge transfer resistance (R_ct) decreases with charging, while the slope in the low-frequency region relative to the ionic diffusion process increases (Fig. 3h and SI 2a). The reversed changes were observed with discharging, further confirming the highly reversible evolution of reaction dynamics.

The above results indicated that NH₄⁺ serves as the charge carrier during the charge/discharge process of the Rec-NM-MnO₂ electrode. To further study the effect of NH₄⁺ in the mixed reaction system, the electrochemical behaviors of Rec-NM-MnO₂ were further studied in the pure Mn²⁺ electrolyte. In this case, the capacitance gradually falls behind that in the NH₄⁺–Mn²⁺ mixed electrolyte with cycling (Fig. 4a). Ex situ XPS results demonstrate the irreversible change of the AOS of Mn after one charge/discharge cycle in the pure Mn²⁺ electrolyte, resulting in a higher oxidation state of Mn (SI Fig. 20), indicative of an unsatisfactory dissolution dynamics. The fast MnO₂ accumulation but unfavorable dissolution kinetics, i.e., an irreversible deposition/dissolution process, serve as the main cause of a fast discharge capacitance decay. The inconspicuous variation in the content of the Mn–O–H bond also manifests the negligible contribution of H⁺ storage in the pure Mn²⁺ electrolyte (SI Fig. 21).

Fig. 4 Understanding the MnO₂ dissolution behavior assisted by NH₄⁺. (a) The capacitance ratio of the Rec-NM-MnO₂ electrode in the 0.5 M Mn(NO₃)₂ or 0.5 M (NH₄)₂SO₄ electrolyte to that in the NH₄⁺–Mn²⁺ mixed electrolyte at 2 mA cm⁻². (b) The digital images of (NH₄)₂SO₄ electrolyte before and after cycling. (c) Operando Raman contour map of Rec-NM-MnO₂ in 0.5 M (NH₄)₂SO₄ electrolyte. (d–f) Ex situ XPS characterization of the Rec-NM-MnO₂ electrode during the first two GCD cycles in the pure 0.5 M (NH₄)₂SO₄ electrolyte: (d) O 1s spectra; (e) content analysis of the O 1s spectra; (f) Mn 3 s spectra. (g) Schematic diagram of the electrochemical behavior for Rec-NM-MnO₂ in the NH₄⁺–Mn²⁺ mixed electrolyte.

Of particular note, the capacitance of Rec-NM-MnO₂ in the pure NH₄⁺ electrolyte displays a dramatic decline, with only 52% of the capacitance retained compared with that in the mixed electrolyte after 8 cycles (Fig. 4a). At the same time, we noticed the significantly darkened brown color of the electrolyte (Fig. 4b). This detail indicates the fast dissolution of active Mn species into Mn ions, accompanied by the irreversible disproportionation of Mn³⁺ in the pure NH₄⁺ electrolyte, which triggers the sharp decrease of capacitance. Operando Raman characterization was employed to further study the microstructure evolution after adopting the pure NH₄⁺ electrolyte. ν₁ demonstrates a positive shift of 15 cm⁻¹ during charging (Fig. 4c and SI 22), analogous to the scenario in the mixed electrolyte, suggesting the extraction of NH₄⁺ and the oxidation of Mn centers.^23,47–50 Nevertheless, no reverse shift is observed for the ν₁ peak in the discharge stage (Fig. 4c and SI 22), indicating the emergence of irreversible oxidation/dissolution processes, in good accordance with our discussion, as shown in Fig. 4a. This is in good agreement with our electrochemical results in the pure NH₄⁺ electrolyte, e.g., the considerable increase of the reduction current (Fig. 1d), the variation of charge accumulation (SI Fig. 3f and g), and the decreased discharge capacitance (Fig. 4a), which concurrently evidence an NH₄⁺-promoted dissolution dynamics.^58,59

Through the ex situ XPS measurement, we noticed a considerable change in the content of the Mn–O–H bond during charging/discharging(Fig. 4d and e), indicating a significant proton-storage process along with local structural distortion, which destabilizes the structure and facilitates the dissolution of MnO₂ during electrochemical processes. Besides, the most significant variation range of the AOS of Mn (0.47) was found in the pure NH₄⁺ electrolyte (Fig. 4f and SI 23a). The periodic shifts of the –N⁺H– peak are associated with the intercalation and deintercalation of NH₄⁺ during the cycles (SI Fig. 23b and c). The intercalated NH₄⁺ could also increase local structural disorder, facilitating the interaction of MnO₂ with H⁺ for enhanced dissolution kinetics.⁴⁷

In addition, in the NH₄⁺–Mn²⁺ mixed electrolyte system, NH₄⁺ competes with Mn²⁺ for weakly bound water molecules in the outer solvation sheath through hydrogen bonds and ion–dipole interactions (with solvated H₂O), which breaks the compact and ordered solvation configuration of [Mn(H₂O)₆]²⁺ in the pure Mn²⁺ electrolyte.^60,61 This reconstruction increases the diversity and weakens the aggregation of solvation structures, thereby elevating the solvation-configurational entropy.^62,63 It has been reported that high solvation-configurational entropy can effectively reduce the desolvation energy barrier and accelerate interfacial charge-transfer kinetics.^64,65 Meanwhile, the optimized solvation environment is expected to facilitate fast and reversible transport of both NH₄⁺ and Mn²⁺ by reducing the desolvation energy barrier and weakening the electrostatic aggregation between cationic solvation clusters.^61,66 Additionally, the weakly aggregated solvation clusters can also suppress local ion concentration polarization and the subsequent severe interfacial side reactions, thus stabilizing the electrode/electrolyte interface and maintaining the structural integrity of partially hydroxylated MnO₂, contributing to good cycling stability.^67,68

Therefore, with NH₄⁺ introduced into the reconstructed reaction interface, a synergistic energy storage mechanism integrating the MnO₂ deposition/dissolution process coupled with NH₄⁺ storage was enabled (Fig. 4g). This NH₄⁺-assisted cationic environment not only overcomes the dissolution kinetics limitation but also brings additional capacitance contribution through NH₄⁺ storage, leading to sustainable and collaborative dual-pathway high-mass-loading energy storage.

Deciphering the dual-pathway storage mechanism

The X-ray absorption near-edge structure (XANES) characterization was conducted to gain more insights into the evolution of active Mn species. In the Mn K-edge spectra, the absorption edge energy of Rec-NM-MnO₂ shows a positive shift to higher energy upon charging (Fig. 5a), indicating the increased oxidation state. Of particular note, the Mn species fabricated by 60 CV cycling in the pure Mn²⁺ system exhibits a higher valence at 0 V, which is even closer to that in the NH₄⁺–Mn²⁺ mixed system at 1.2 V (Fig. 5a, SI 24a and b). This is mainly attributed to the sluggish dissolution dynamics for MnO₂ in the pure Mn²⁺ system, resulting in a higher oxidation state at the same discharged state, in good accordance with the ex situ XPS analysis. As shown in the Fourier-transformed (FT) k³-weighted extended XAFS (FT-EXAFS) spectra in Fig. 5b, the peaks located at 1.42 and 2.43 Å indexed to Mn–O and Mn–Mn bonds, respectively, exhibit almost no change in shifts after charging to 1.2 V in NH₄⁺–Mn²⁺ mixed electrolyte, implying the structural stability of Rec-NM-MnO₂ during cycling, in good accordance with the results of the ex situ O 1s spectra (Fig. 3d).^69,70 The wavelet transform (WT) analysis further corroborates that the intensity of the Mn–Mn bond experiences a slight increase as the operating voltage ascends (Fig. 5c). In the NH₄⁺–Mn²⁺ mixed system, the slightly lower intensity of peaks at 0 V than that at 1.2 V is ascribed to insertion of NH₄⁺ and the structural distortion derived from the decline of Mn valence (SI Fig. 24c). Furthermore, the R-space curve in the pure Mn²⁺ electrolyte shows a high similarity to that in NH₄⁺–Mn²⁺ mixed electrolyte (SI Fig. 24d), demonstrating the same coordination environment of MnO₂ generated, consistent with the XRD analysis (SI Fig. 14c).

Fig. 5 Fine structure characterization and theoretical calculations for understanding the integrated storage mechanism. (a) Mn K-edge XANES spectra of the Rec-NM-MnO₂ electrode in NH₄⁺–Mn²⁺ mixed electrolyte and the counterpart electrode derived in pure 0.5 M Mn(NO₃)₂ electrolyte at different states. (b) The k³-weighted Fourier transformation of the Mn K-edge EXAFS spectra. (c) Wavelet transformation of the k³-weighted Mn K-edge EXAFS signals. (d) Density of states for pure MnO₂, H-MnO₂(h), and H-MnO₂(p). (e) Differential charge density of NH₄⁺ adsorbed on H-MnO₂(p) (the cyan and yellow colors correspond to charge depletion and accumulation, respectively). (f) Adsorption energies of NH₄⁺ on the surface of pure MnO₂, H-MnO₂(h), and H-MnO₂(p). (g) Relative energy profile of the dissolution processes for the sites near (below) and far (above) from the adsorbed NH₄⁺.

Our study has demonstrated the formation of partially hydroxylated MnO₂ (H-MnO₂(p)) as the true active species in the NH₄⁺–Mn²⁺ mixed system to guide the deposition/dissolution process. For further clarification of the working mechanisms, density functional theory (DFT) simulations were conducted to study the influence of the hydroxylation degree on the interfacial reaction dynamics. As shown in SI Fig. 25, we chose pure ε-MnO₂ (named pure MnO₂), H-MnO₂(p), and H-MnO₂(h) (highly hydroxylated MnO₂) as models for the calculations. The H-MnO₂(p) and H-MnO₂(h) structures are modelled as ε-MnO₂ with small and large amounts of hydroxyl groups adsorbed on the (101) plane, respectively, according to our aforementioned experimental results. The density of states (DOS) calculation results demonstrate that the band gap decreases in the following sequence: pure MnO₂ > H-MnO₂(h) > H-MnO₂(p) (Fig. 5d), indicating the enhanced electron transfer capability of H-MnO₂(p).

To gain more information on the superiority of the storage capacitance of H-MnO₂(p) in the NH₄⁺–Mn²⁺ mixed system, the charge density difference (CDD) analysis and adsorption energy for NH₄⁺ on pure MnO₂, H-MnO₂(h), and H-MnO₂(p) were studied. As depicted in Fig. 5e, at an isosurface level of 0.002 e Å⁻³, a pronounced charge-transfer behavior appears from the NH₄⁺ side to the MnO₂ side, implying an intimate interfacial electronic interaction between NH₄⁺ and H-MnO₂(p). In addition, the calculated NH₄⁺ adsorption energy of H-MnO₂(p) was considerably lower than that of pure MnO₂ and H-MnO₂(h) (Fig. 5f), suggesting its kinetically favorable NH₄⁺ adsorption stemming from the tailored interface microenvironment. We further calculated the Gibbs free energy to identify the synergistic modulation mechanism of NH₄⁺ ions during the MnO₂ discharge process. As demonstrated in Fig. 5g, this process begins with a proton binding to a lattice oxygen atom on the MnO₂ surface, followed by an attack from a second proton. Remarkably, the second proton attack is more favorable at the nearby site, with an energy of −3.03 eV compared to −2.22 eV at the distal site (Fig. 5g). This is followed by the in situ formation of a structural H₂O molecule, which is a nonspontaneous process serving as the rate-determining step (RDS). Of note, the energy barrier for this RDS is markedly lower at the NH₄⁺-proximal site (0.51 eV) compared to the distal site (1.13 eV), underscoring the critical role of NH₄⁺ in promoting dissolution kinetics. Also, the proximal site demonstrates a low energy barrier for H₂O molecule release from the MnO₂ surface, signifying the facilitated dissolution behavior along with the interfacial proton/electron transfer.

Conclusions

In summary, we discovered NH₄⁺-mediated interface chemistry for dynamically reshaping the hydroxylation degree of MnO₂ toward an optimal reconstructed interface, which ensures fast Mn²⁺/MnO₂ conversion chemistry, and at the same time, creates a kinetically favorable microenvironment for efficient NH₄⁺ storage. The as-enabled dual-pathway storage mechanism triggers a superior areal capacitance of 13.8 F cm⁻² and good cycling stability at a superhigh mass loading of 27.1 mg cm⁻². Theoretical calculations indicate that local partial hydroxyl groups help strengthen the electronic conduction and facilitate the interfacial adsorption for NH₄⁺ storage. The adsorbed NH₄⁺ triggers the redistribution of local electrons and thereby results in a considerably reduced uphill energy barrier of the nonspontaneous rate-determining step of the dissolution process, enabling a collaborative dual-pathway storage behavior. This research establishes a study paradigm focusing on the synergistic dual-pathway mechanism by refining the interfacial microenvironment for better high-mass-loading energy storage.

Author contributions

W. G. and Q. Z. supervised the research. J. W. and W. G. designed the experimental routes. J. W. carried out the material fabrication and most of the characterization experiments. J. W. and W. G. conducted the main data analysis and the paper writing. J. W., M. S., N. H., and Y. Y. carried out electrochemical measurements. J. W., W. G., G. Z., M. C., and W. X. performed operando Raman characterization. All co-authors contributed to discussions on the relative results and improving the paper.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: the detailed experimental procedures for electrode fabrication, materials characterization, electrochemical measurements, and density functional theory (DFT) computational methods, mass variation data, charge storage dynamics analysis, electrochemical performance comparisons across different electrolytes and substrates, characterization of asymmetric supercapacitors, scanning and transmission electron microscopy images, operando electrochemical impedance spectroscopy, ex situ X-ray diffraction patterns, X-ray photoelectron spectra, Fourier-transform infrared spectra, operando Raman spectra, X-ray absorption near-edge structure data, additional interpretive analysis, performance comparisons with literature reports, and fitted equivalent circuit parameters from impedance measurements. See DOI: https://doi.org/10.1039/d6sc02038k.

Acknowledgements

The authors thank the National Natural Science Foundation of China (Grant No. 51673156 for Q. Z., 52202301 for W. G., and 22375161 for W. G.), China Postdoctoral Science Foundation (2022TQ0256 for W. G. and 2022M722587 for W. G.), and Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX2024093 and CX2025094) for their great support. We also thank eceshi (https://www.eceshi.com) for the theoretical calculations and the Anhui Absorption Spectroscopy Analysis Instrument Co, Ltd for XAFS measurements and analysis.

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Footnote

† These authors contributed equally to this work.

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