Introduction

Rechargeable aqueous batteries with neutral electrolytes have attracted intensive scientific attention as promising alternatives for large‐scale energy storage technologies. The utilized water‐based electrolytes offer significant advantages of high ionic conductivity (≈1 S cm⁻¹), simplified manufacture, low cost, and intrinsic safety. ^[1] In particular, Zn metal batteries (ZMBs) with mild aqueous electrolytes have recently stood out, due to the direct use of Zn metal anodes with a high specific capacity (≈820 mA h g⁻¹) and a low stripping/plating potential (−0.76 V vs. standard hydrogen electrode). ^[2] Numerous efforts have been devoted to exploring Zn²⁺‐host cathode materials of ZMBs, which has brought Mn‐compounds, V‐compounds, Prussian blue materials into the spotlight. ^[3] However, the Zn²⁺‐intercalation chemistry generally shows sluggish kinetics and unsatisfactory cycling stability. ^[4] In aqueous electrolytes, Zn²⁺ tends to form a large‐size hydrated state (Zn(H₂O)₆ ²⁺ with 5.5 Å) due to the strong Zn²⁺‐water interaction. ^[5] The intercalation of Zn²⁺ into cathode hosts thus requires large de‐solvation and intercalation energy. Besides, bivalent Zn²⁺ imposes a strong repulsive force with the hosts, leading to the large Zn²⁺‐diffusion energy barriers within the hosts and the undesired structure distortion of hosts. ^[6]

Apart from Zn²⁺, non‐metal hydronium (H₃O⁺) has also been recognized as favorable charge carrier ions for aqueous batteries. Assigned to the small ionic size (≈1.0 Å) and light molecular mass, H₃O⁺ intercalation presents attractive high‐kinetics and highly reversible behaviors. ^[7] The partial involvement of H₃O⁺ intercalation was also discovered for the charge‐storage mechanism of ZMB cathodes. ^[8] For example, Sun et al. uncovered the consequent intercalation of H₃O⁺ and Zn²⁺ for ϵ‐MnO₂ cathode in a mixed ZnSO₄/MnSO₄ electrolyte. ^[9] The charge‐transfer resistance of ϵ‐MnO₂ in the H₃O⁺‐intercalation step is three orders of magnitude smaller than that in the Zn²⁺‐intercalation step. A similar phenomenon was also observed for polyaniline‐intercalated MnO₂ nanolayers, in which the diffusion coefficient of H₃O⁺ (5.84×10⁻¹² cm² s⁻¹) was substantially higher than that of Zn²⁺ (7.35×10⁻¹⁴ cm² s⁻¹). ^[10] These findings inspire that selective H₃O⁺ intercalation into cathodes would bring the constructed ZMBs with significant performance advance in terms of capacity, kinetics, as well as cycle life. However, thus far, pure H₃O⁺‐intercalation behavior for layered/tunneled cathodes has only been observed in corrosive acidic electrolytes.[ ^7b , ¹¹ ] It remains a grand challenge to achieve selective H₃O⁺ intercalation in mild neutral electrolytes.

In this study, we, taking orthorhombic MoO₃ (α‐MoO₃) as an example, for the first time demonstrate the feasibility of selective H₃O⁺ intercalation in a neutral ZnCl₂ electrolyte. α‐MoO₃ is selected due to its typical layered structure with distorted [MoO₆] octahedra bilayers weakly bonded by van der Waals force. ^[12] The complete redox of Mo⁴⁺/Mo⁶⁺ allows α‐MoO₃ with an attractive theoretical capacity of 372 mA h g⁻¹. Selective H₃O⁺‐intercalation chemistry is modulated for α‐MoO₃ through a water‐proton co‐intercalation strategy (denoted WP‐MoO₃), which further tackles the low‐capacity, poor‐rate, and short‐life issues faced by pristine α‐MoO₃ with Zn²⁺‐intercalation chemistry. H₂O molecules located between WP‐MoO₃ interlayers impose a high Zn²⁺‐intercalation energy barrier by blocking Zn²⁺ diffusion pathways (Figure 1). Meanwhile, H₃O⁺ intercalation/diffusion can be smoothly achieved within WP‐MoO₃ interlayers through a well‐known Grotthuss mechanism (proton jumping between water molecules). ^[13] In contrast to Zn²⁺‐intercalation α‐MoO₃, selective H₃O⁺‐intercalation WP‐MoO₃ depicts substantially enhanced redox depth (1.92 vs. 0.99 e⁻ per Mo atom; 357 vs. 184 mA h g⁻¹), rate capability (77.5 % vs. 42.2 % from 0.4 to 4.8 A g⁻¹), and cycling stability (83 % vs. 13 % over 1000 cycles).

Figure 1

Schematic illustration of the Zn²⁺‐intercalation chemistry for α‐MoO₃ and the selective H₃O⁺‐intercalation chemistry for WP‐MoO₃.

Results and Discussion

α‐MoO₃ nanoparticles (Figure S1) were first synthesized through a sol‐gel method. WP‐MoO₃ electrode was obtained from α‐MoO₃ electrode through a controllable and time‐efficient (≈6.1 min) electrochemical linear sweep voltammetry (LSV) method (Figure S2) in a three‐electrode cell with an electrolyte of 1 M H₂SO₄. Compared with α‐MoO₃ electrode, WP‐MoO₃ displays an apparent color change from dark grey to purple (Figure S3), which is attributed to the formation of Mo⁴⁺/Mo⁵⁺ (Figure S4). ^[14] Almost no morphological variation was observed between α‐MoO₃ and WP‐MoO₃. The amount of intercalated H⁺ can be estimated by calculating the amount of charge transfer (Figure 2 a). The overall intercalation process presents four stages, referring to potential windows (vs. saturated calomel electrode (SCE)) of 0.3–−0.1 V (Stage I), −0.1–−0.34 V (Stage II), −0.34–−0.53 V (Stage III), and −0.53–−0.72 V (Stage IV). Approximately, the intercalated H⁺ numbers per MoO₃ unit are 0.25, 0.75, 0.25, and 0.75 at Stage I, Stage II, Stage III, and Stage IV, respectively. X‐ray diffraction (XRD) spectra uncover that the peak position corresponding to the interlayer spacing of α‐MoO₃ gradually shifts towards negative at Stage I and II and keeps almost unchanged at Stage III and IV (Figure 2 b & S5). This peak of WP‐MoO₃ is located at 12.0°, indicating the interlayer distance expansion from 6.7 Å for α‐MoO₃ to 7.6 Å (Figure S6). Besides, new peaks at 32.5°, 35.0°, and 37.1° are observed for WP‐MoO₃, which indicates a monoclinic phase of proton‐intercalated MoO₃. ^[15] The widened interlayer distance of WP‐MoO₃ was also evidenced by high‐resolution transmission electron microscopy (HRTEM) images, in which the interlayer spacings are determined to be 6.7 Å and 7.6 Å for MoO₃ and WP‐MoO₃, respectively (Figure 2 c & S7).

Figure 2

a) The intercalated proton amount as a function of potential during the water‐proton co‐intercalation process. b) XRD patterns of α‐MoO₃ electrode at different intercalation stages. c) HRTEM images of α‐MoO₃ and WP‐MoO₃. Scale bars: 5 nm. d) O 1s XPS spectra of α‐MoO₃ and WP‐MoO₃. e) Illustration of different O atoms in WP‐MoO₃. f) Normalized Mo K‐edge XANES spectra of MoO₃ standard, Mo foil standard, α‐MoO₃, and WP‐MoO₃. g) Radial distribution functions of α‐MoO₃ and WP‐MoO₃ obtained from the k ² χ(k) by Fourier transform.

Figure 2 d compares the O 1s X‐ray photoelectron spectroscopy (XPS) spectra of MoO₃ and WP‐MoO₃ electrodes. Two peaks located at 532.6 eV and 531.2 eV are observed for α‐MoO₃, which correspond to lattice O in MoO₃ (denoted O2) and O in adsorbed H₂O (denoted O1). ^[16] Notably, WP‐MoO₃ shows an O1 peak with the substantially enhanced intensity, verifying the intercalation of H₂O into WP‐MoO₃. Thermogravimetric analysis (TGA, Figure S8) results of both electrodes also suggest the intercalated H₂O molecules in WP‐MoO₃. More interestingly, an additional XPS peak at 530.2 eV (denoted O3) is observed for WP‐MoO₃. This peak can be assigned to the terminal O of [MoO₆] bilayers, which splits from O2 peak due to the formation of the hydrogen bond with the intercalated H₂O/H₃O⁺ (as illustrated in Figure 2 e). Furthermore, synchrotron‐based X‐ray absorption near‐edge spectra (XANES) measurements were performed to investigate the localized coordination environments of Mo sites in α‐MoO₃ and WP‐MoO₃. The Mo K‐edge XANES spectra of α‐MoO₃ and WP‐MoO₃, as well as standard Mo foil and MoO₃ as references, are displayed in Figure 2 f. In comparison with α‐MoO₃, the slightly negative‐shifted rising‐edge of WP‐MoO₃ around 20 015 eV suggests the enriched electron densities around Mo sites. ^[17] This result is consistent with the analysis of O K‐edge XANES spectra, which witness the decreased peak intensity of WP‐MoO₃ at the energy region of 530–540 eV (Figure S9). ^[18] In addition, the Mo pre‐edge of WP‐MoO₃ around 20 007 eV, referring to the O 1s‐Mo 4d electron transfer, is obviously decreased compared with that of α‐MoO₃, reflecting the interaction between the terminal O of [MoO₆] bilayers and the intercalated species (i.e., H₂O and H₃O⁺). ^[19] To acquire the detailed bonding and coordination information, corresponding R space curves after k ²[χ(k)]‐weighted Fourier transform of the extended X‐ray absorption fine structure (EXAFS) and quantitatively fitting spectra are presented in Figure 2 g & S10. Two representative peaks at 1.16 and 1.69 Å can be assigned to the scattering of Mo=O and Mo−O bonds in [MoO₆] bilayers, respectively. ^[20] These two peaks are also indicated in Figure 2 e. Both Mo=O and Mo−O bonds show negligible length difference between α‐MoO₃ (1.72 & 1.96 Å) and WP‐MoO₃ (1.73 & 1.97 Å), implying the water‐proton pre‐intercalation causes minor distortion of octahedron [MoO₆] layers. Moreover, the coordination number of Mo−O (from 1.8 to 1.4) and Mo=O (from 2.0 to 1.4) decreases due to the water‐proton co‐intercalation, which again verifies the interaction between the terminal O of [MoO₆] bilayers and the intercalated H₂O/H₃O⁺.

The ion‐intercalation behaviors of α‐MoO₃ and WP‐MoO₃ electrodes were explored in two‐electrode cells with Zn foil as the anode and 2 M ZnCl₂ aqueous solution as the electrolyte. Figure 3 a and Figure 3 b present the galvanostatic charge/discharge (GCD) curves of α‐MoO₃ and WP‐MoO₃ electrodes after three‐cycle activation at 0.4 A g⁻¹. Both electrodes show two mainly ion‐intercalation stages in their discharge curves, which agrees well with the cyclic voltammetry (CV) curves (Figure S11). In detail, α‐MoO₃ electrode presents a discharge curve consisting of one plateau region (Plateau I, ≈0.62 V) and one slope region (Slope I, 0.25–0.35 V), while WP‐MoO₃ electrode presents two plateaus around 0.68 V (Plateau I) and 0.30 V (Plateau II). Impressively, WP‐MoO₃ electrode exhibits a high redox depth of 1.92 e⁻ per Mo atom, close to the full conversion of Mo⁴⁺/Mo⁶⁺. By contrast, a shallow redox depth of 0.99 e⁻ per Mo atom is achieved by α‐MoO₃ electrode.

Figure 3

GCD profiles of a) α‐MoO₃ and b) WP‐MoO₃ electrodes at a current density of 0.4 A g⁻¹. Electrode mass change versus charge during the discharge (ion‐intercalation) processes of c) α‐MoO₃ and d) WP‐MoO₃ electrodes. EDX elemental mapping of e) α‐MoO₃ and f) WP‐MoO₃ at the fully discharged state. Scale bars: 50 nm.

Electrochemical quartz crystal microbalance (EQCM) was employed to in‐operando monitor the mass evolution of MoO₃ and WP‐MoO₃ along with the continuous ion intercalation. In consistent with the discharge curves, the mass change of both electrodes during ion intercalation depicted two stages. At the first stage of α‐MoO₃, the average weight increase calculated by the curve slope (Figure 3 c) is 45 g per mol charge, which suggests the co‐intercalation of Zn²⁺ and H₂O into α‐MoO₃ (Zn²⁺⋅1.5 H₂O in average). Meanwhile, the average weight increase of α‐MoO₃ at the second stage is 18 g per mol charge, close to the weight of H₃O⁺ (19 g per mol charge). Clearly, Zn²⁺/H₂O co‐intercalation plays the dominant role in the charge‐storage mechanism of α‐MoO₃. By contrast, WP‐MoO₃ depicts the weight increase of 19 and 18 g per mol charge at the two stages (Figure 3 d), verifying the selective H₃O⁺‐intercalation chemistry of WP‐MoO₃ in 2 M ZnCl₂ electrolyte. This result means although Zn²⁺ is the mainly charge carrier in the electrolyte, the charge carrier only consists of H₃O⁺ in WP‐ MoO₃ electrode, which comes from the pre‐stored H⁺ in WP‐MoO₃ and the slight hydrolysis of ZnCl₂ in the electrolyte. However, the local Zn²⁺ concentration near the surface of WP‐MoO₃ should be increased during discharging, because Zn²⁺ tends to migrate to the Helmholtz layer of WP‐MoO₃ due to the electrostatic interaction.

The interesting H₃O⁺‐intercalation behavior of WP‐MoO₃ is further supported by the Energy‐dispersive X‐ray spectroscopy (EDX) elemental mapping analysis. The fully discharged α‐MoO₃ presents the even distribution of Mo and Zn over the sample (Figure 3 e & S12), while Zn distribution is barely observed in WP‐MoO₃ (Figure 3 f & S12). The Zn/Mo atomic ratio of the discharged WP‐MoO₃ is calculated to be 0.02, which contrasts with the high Zn/Mo atomic ratio of the discharged α‐MoO₃ (0.48). Moreover, the discharged α‐MoO₃ and WP‐MoO₃ were annealed in air at 500 °C and subjected for XRD measurements (Figure S13). Peaks refer to ZnMoO₃ are only detected for the discharged α‐MoO₃, rather than for the discharged WP‐MoO₃. All these results identify the successful modulation of intercalating charge carriers for α‐MoO₃, which brings the obtained WP‐MoO₃ with exceptional selective H₃O⁺‐intercalation chemistry in a neutral electrolyte.

To understand the origin of the selective H₃O⁺‐intercalation behavior, the first three CV cycles of WP‐MoO₃ electrode were recorded as shown in Figure 4 a. In the initial cycle, only a small cathodic peak is observed during the discharge process, whereas the charge process displays four anodic peaks corresponding to the extraction of the pre‐intercalated H₃O⁺ and H₂O. Afterwards, WP‐MoO₃ electrode presents almost the identical 2^nd and 3^rd cycles (Figure S14), which include three anodic peaks and three cathodic peaks. The cathodic peaks and anodic peaks can be assigned to the H₃O⁺ intercalation and de‐intercalation, respectively. Based on the CV curves, the charge transfer number of the first two pairs of redox peaks is calculated to be 1.495 (close to 1.5), implying the conversion between Mo⁶⁺ and Mo⁵⁺/Mo⁴⁺ (1:1). Meanwhile, the charge transfer of the third pair of redox peaks is 0.498 (close to 0.5), which refers to the conversion between Mo⁵⁺/Mo⁴⁺ (1:1) and Mo⁴⁺. In this regard, the structure of WP‐MoO₃ after the first cycle (denoted WP‐MoO₃‐c) is of significance to induce the selective H₃O⁺ intercalation of WP‐MoO₃ electrode. The Mo 3d XPS spectrum of WP‐MoO₃‐c uncovers the existence of Mo⁵⁺ in WP‐MoO₃‐c (Figure S15), indicating that H⁺ ions were not fully extracted from the lattice. This result is further supported by the observation of O3 peak in the O 1s XPS of WP‐MoO₃‐c (Figure 4 b). The partial extraction of H₂O was verified by the larger O1 peak intensity of WP‐MoO₃‐c than that of α‐MoO₃, as well as the TGA analysis (Figure S16). Moreover, the interlayer distance of WP‐MoO₃‐c identified by the XRD peak position only slightly decreased from 7.6 Å for WP‐MoO₃ to 7.4 Å, which remains to be significantly larger than that of α‐MoO₃ (6.7 Å) (Figure 4 c & S17).

Figure 4

a) Initial three CV cycles of WP‐MoO₃ at 0.7 mV s⁻¹. b) O 1s XPS spectra of α‐MoO₃ and WP‐MoO₃‐c. c) XRD patterns of α‐MoO₃, WP‐MoO₃, and WP‐MoO₃‐c. d) Simulated PDOS of α‐MoO₃ and WP‐MoO₃‐c. Calculated e) Zn²⁺‐intercalation energy and f) H₃O⁺‐intercalation energy for α‐MoO₃ and WP‐MoO₃‐c.

Based on the quantitive analysis of XPS results, the structures of α‐MoO₃ and WP‐MoO₃‐c (Figure S18) were simulated with the density functional theory (DFT) method. In WP‐MoO₃‐c, the bonding interaction between the residual intercalant (i.e., H₂O and H₃O⁺) and the terminal O of [MoO₆] bilayers is proved by the formant arise at −10 eV in projected density of states (PDOS) of terminal O‐p orbital with H‐s orbital (Figure 4 d). This bonding interaction greatly influence the electron density of terminal O atoms, which agrees well with the O 1s XPS results of our samples. Additionally, WP‐MoO₃ has a band gap of 0.04 eV, which is remarkably smaller than that of α‐MoO₃ (2.18 eV). The Fermi level of WP‐MoO₃ also shifts to the conduction band, favoring the excitation of charge carriers to the conduction band and thus the improved electronic conductivity (Figure S19). ^[21]

Subsequently, the Zn²⁺‐intercalation energy (Figure 4 e) and H₃O⁺‐intercalation energy (Figure 4 f) were calculated for α‐MoO₃ and WP‐MoO₃‐c. As expected, a substantially enlarged Zn‐intercalation energy is uncovered for WP‐MoO₃‐c (−0.11 eV) in comparison to α‐MoO₃ (−1.49 eV), while the H₃O‐intercalation energy of WP‐MoO₃‐c (−3.93 eV) is notably lower than that of α‐MoO₃ (−1.61 eV). These results confirm the thermodynamically preferable H₃O⁺ intercalation into WP‐MoO₃‐c. Besides, the residual H₂O and H₃O⁺ located in the interlayer space and bonded with terminal O atoms of [MoO₆] bilayers block the Zn²⁺‐diffusion pathways in WP‐MoO₃‐c (Figure 5 a) and hinder the charge transfer through the interaction with [MoO₆] bilayers (Figure S20 & 21). In the case of H₃O⁺ intercalation into WP‐MoO₃‐c, charge carrier diffusion can be efficiently achieved through a well‐established Grotthuss mechanism (Figure 5 b), in which protons can be fast transported by “jumping” through water molecules. ^[22] Proton conductivities measurement (Figure S22) shows that WP‐MoO₃‐c owns a proton conductivity value of 4.3×10⁻³ S cm⁻¹ at 318 K and 100 % humidity, which is much higher than α‐MoO₃ (6.2×10⁻⁵ S cm⁻¹). Moreover, WP‐MoO₃‐c also shows an activation energy (E _a) of 0.28 eV, which suggests the Grotthuss conduction mechanism (E _a<0.4 eV). ^[23] H₂O molecules between the interlayers serve as the proton transport intermedia, providing a hydrogen‐bonding network for the high‐kinetics charge carrier diffusion. ^[24] Moreover, the charge transfer from H₃O⁺ to WP‐MoO₃‐c can be also through Grotthuss mechanism without breaking the hydrogen bonding interaction between H₂O and the terminal O of [MoO₆] bilayers (Figure 5 c). Thereby, the unique van der Waals structure of WP‐MoO₃‐c favors the selective H₃O⁺‐intercalation behavior both thermodynamically and kinetically.

Figure 5

a) Schematic illustration of Zn²⁺ inserted WP‐MoO₃‐c. b) Schematic illustration of the Grotthuss proton‐conduction mechanism. Protons are transported by rearranging bonds along a water chain. c) Schematic illustration of the charge transfer of H₃O⁺ via Grotthuss mechanism.

The selective H₃O⁺ intercalation of WP‐MoO₃ electrode motivated us to assess the electrochemical performance of ZMB devices assembled by coupling Zn anodes with WP‐MoO₃ cathodes (denoted Zn/WP‐MoO₃). ZMB devices based on Zn²⁺‐intercalation α‐MoO₃ cathodes were also constructed for comparison (denoted Zn/α‐MoO₃). GCD curves at various current densities were collected to evaluate both devices (Figure S23). As shown in Figure 6 a, Zn/α‐MoO₃ only exhibits a specific capacity of 184.0 mA h g⁻¹ at 0.4 A g⁻¹ (based on the cathode) and a capacity retention of 42.2 % at 4.8 A g⁻¹. By contrast, Zn/WP‐MoO₃ delivers a much larger specific capacity (356.8 mA h g⁻¹ at 0.4 A g⁻¹) and a greatly improved rate capability (77.5 % from 0.4 to 4.8 A g⁻¹). When the current density returns to 0.4 A g⁻¹ after the rate tests, Zn/WP‐MoO₃ can still show a specific capacity of 345.6 mA h g⁻¹. The influence of different amounts of intercalated H⁺ (from 0.25 to 2.0 per MoO₃) on the electrochemical performance was also investigated by preparing a series of WP‐MoO₃ electrodes with different cut‐off potentials (Figure S24). In brief, the larger amount of intercalated H⁺ results in the higher specific capacity of the obtained Zn/WP‐MoO₃ devices. Importantly, Zn/WP‐MoO₃ delivers the maximum energy density of 198.0 Wh kg⁻¹ (based on the cathode) at a power density of 0.28 kW kg⁻¹, as well as the peak power density of 6.7 kW kg⁻¹ at a high energy density of 104.5 Wh kg⁻¹. These energy and power densities significantly outperform those of the recently reported ZMBs based on cathodes like MnO₂, ^[3c] ZnMn₂O₄, ^[25] VS₂, ^[26] ZnHCF, ^[27] and pyrene‐4,5,9,10‐tetraone ^[28] (Figure S25). It should be noticed that the energy contribution of plateau II (≈0.30 V) is only about 16.8 % in Zn/WP‐MoO₃ (Figure S26), while most of the energy is contributed by plateau I (≈0.68 V). The outstanding performance of Zn/WP‐MoO₃ originates from H₃O⁺ charge carriers, which allow the high‐kinetics diffusion and the efficient charge transfer. Figure 6 b displays the calculated charge carrier diffusion coefficients of Zn/α‐MoO₃ and Zn/WP‐MoO₃ devices based on a galvanostatic intermittent titration technique (GITT, Figure S27). Both devices exhibit low charge carrier diffusion coefficients at voltages associated with ion‐intercalation stages. As expected, Zn/WP‐MoO₃ with H₃O⁺ charge carriers achieves high diffusion coefficients of 1.6×10⁻⁸ cm² s⁻¹ at 0.57 V and 4.4×10⁻⁹ cm² s⁻¹ at 0.29 V, which are several orders of magnitude higher than those of Zn/α‐MoO₃ with Zn²⁺ as the main charge carrier (1.2×10⁻⁹ cm² s⁻¹ at 0.58 V and 1.3×10⁻¹² cm² s⁻¹ at 0.22 V). In addition, the low charge transfer resistance (R _ct) of Zn/WP‐MoO₃ derived from electrochemical impedance spectroscopy (EIS, Figure S28) reflects the efficient charge transfer enabled by H₃O⁺ charge carriers. As shown in Figure 6 c, R _ct of Zn/WP‐MoO₃ ranges from 48.9 to 124.5 Ω, which contrasts with that of Zn/α‐MoO₃ (48.1 to 667.3 Ω).

Figure 6

a) Rate performance, b) ion diffusion coefficient as a function of the voltage during the discharge, c) R _ct at different voltages, and d) cycling performance at 3.2 A g⁻¹ of Zn/α‐MoO₃ and Zn/WP‐MoO₃ devices. e) Ex situ XRD of WP‐MoO₃ during one discharge/charge cycle of Zn/WP‐MoO₃. f) Radial distribution functions obtained from the k ² χ(k) by Fourier transform of XANES of WP‐MoO₃ cathode at the charged (WP‐MoO₃‐A) and discharged (WP‐MoO₃‐D) states.

The cycling stability of Zn/α‐MoO₃ and Zn/WP‐MoO₃ was evaluated at a current density of 3.2 A g⁻¹. Zn/WP‐MoO₃ presents impressive coulombic efficiencies of nearly 100 %, which indicates its high charge/discharge reversibility. In contrast with the fast capacity decay of Zn/α‐MoO₃ (13 % capacity retention after 1000 cycles), Zn/WP‐MoO₃ can maintain 83 % of the initial capacity after 1000 cycles (Figure 6 d). The outstanding cycling performance of Zn/WP‐MoO₃ is assigned to the negligible structure distortion of WP‐MoO₃ during the repeated H₃O⁺ intercalation/extraction. In the ex‐situ XRD tests of WP‐MoO₃ cathode during one discharge/charge cycle of Zn/WP‐MoO₃, the peak position located at 12.3° of WP‐MoO₃ only experiences slight shift, indicating the little volume expansion/shrinkage of WP‐MoO₃ in direction which is perpendicular to interlayer (Figure 6 e). Moreover, Mo‐K edge XANES (Figure S29) and corresponding Fourier transforms of the Mo K‐edge k ² χ(k) spectra (Figure 6 f) confirm that both Mo=O and Mo−O bonds of WP‐MoO₃ cathode have no change in bonding length at the fully charged and discharged stages. All these results imply that the selective H₃O⁺‐intercalation chemistry is able to empower electrodes with large specific capacity, high charge‐storage kinetics and long cycle life.

Conclusion

In summary, we have uncovered an exceptional selective H₃O⁺‐intercalation chemistry for WP‐MoO₃ electrode in a neutral ZnCl₂ electrolyte. The interesting charge‐carrier‐selection behavior of WP‐MoO₃ originated from the interlayer species (i.e., H₂O, H₃O⁺) of WP‐MoO₃, which allowed the fast‐kinetics transport and charge transfer of H₃O⁺ while blocking Zn²⁺ intercalation. This study provides a novel charge‐carrier‐modulation concept through the strategic van der Waals structure engineering, which opens a promising prospect for developing high‐kinetics and long‐life battery technologies.

Conflict of interest

The authors declare no conflict of interest.