Anhydrous Grotthuss mechanism for fast proton transport in a dense oxide-ion array of α-MoO 3

Developing high-power battery chemistry is an urgent task to buffer uctuating renewable energies and achieve a sustainable and exible power supply. Owing to the small size of proton and its ultrahigh mobility in water via the Grotthuss mechanism, aqueous proton batteries are an attractive candidate for high-power energy storage devices. Although Grotthuss proton transfer usually occurs in hydrogen-bonded networks of water molecules, in this work, we discover anhydrous Grotthuss-type proton transport in a dense oxide-ion array of solid α-MoO 3 even without structural water. The fast proton transfer and accumulation that occurs during (de)intercalation in α-MoO 3 is unveiled using both experiments and rst-principles calculations. Coupled with a zinc anode and a superconcentrated dual-ion Zn 2+ /H + electrolyte, the solid-state anhydrous Grotthuss proton transport mechanism realizes an aqueous MoO 3 -Zn battery with both high energy and power densities.


Main
The increasing concern given to the global environment and energy sustainability is driving the research and development of electrochemical energy storage devices that provide power supply with more resilience and exibility. Currently, lithium-ion batteries dominate the power-source market for portable devices and electric vehicles due to their high energy density, high energy e ciency, and long lifetime. 1,2 However, the global maldistribution of lithium resources has impeded their further widespread use. 3 In particular, ammable organic electrolytes in lithium-ion batteries result in a low safety level and high fabrication/maintenance costs, both of which are unacceptable for grid-scale use. 4 Therefore, aqueous rechargeable batteries that contain safe and less expensive aqueous electrolytes are an important future alternative for sustainable development. 5 For the development of aqueous batteries with high energy density, exploiting Zn metal as a negative electrode is a straightforward approach because the Zn metal electrode possesses high theoretical capacities of 820 mAh/g and 5854 mAh/L. While reversible Zn metal plating/stripping is an important issue to be addressed for the development of aqueous Zn-ion batteries 6 , another large challenge also remains in a positive electrode. The intercalation of large hydrated Zn 2+ generally causes damaging structural changes upon charge/discharge, leading to capacity degradation after cycling. 7 Consequently, the concept of aqueous dual-ion batteries has recently been increasingly studied. For example, an aqueous Zn 2+ /Li + dual-ion battery, which consists of a Zn metal plating/stripping negative electrode and Li + intercalation positive electrode (e.g., LiFePO 4 ) with an aqueous dual-ion Zn 2+ /Li + electrolyte, was reported to provide an energy density of approximately 95 Wh/kg with 90% capacity retention after 80 cycles 8 .
In this work, we focus on protons as charge carriers in aqueous dual-ion batteries ( Figure 1). Proton is the smallest and lightest cation; thus, it can be easily (de)intercalate in various structures at a fast rate. 9 Moreover, barrierless H + hopping enables fast H + transport in an electrolyte owing to the Grotthuss mechanism, where protons are transferred through the hydrogen bond network. 10 In recent years, the Grotthuss topochemistry was extended to hydrate solid-state materials; for example, Prussian blue analogs exhibit fast H + (de)intercalation with the assistance of structural water networks. 11 Similarly, the intercalated water layers in transition metal carbide nanosheets (MXenes) facilitate H + storage and fast H + transfer. 12,13 Without structural/con ned water, quinone-based organic compounds, which store H + on carbonyl groups, exhibit both long cycle lives and large capacities. 14 Among various transition metal (TM) oxides (TM= Mn, V, W, Ti, Mo) 15-24 that deliver large capacities upon protonation, orthorhombic MoO 3 (α-MoO 3 ) possesses a unique bilayered structure ( Figure 1) that accommodates various cations, such as Li +25,26 , Na +27 , Ca 2+28 and Mg 2+29 in organic electrolytes. In aqueous electrolytes, the intercalation of bare Zn 2+30, 31 and Al 3+32 has also been studied. However, the H + intercalation behavior in α-MoO 3 with various aqueous electrolytes remains controversial: some reports claim a bare H + intercalation mechanism 33-37 that is consistent with H x MoO 3 bronze obtained through the spillover method 38 , while others report water and H + cointercalation 39,40 . Meanwhile, α-MoO 3 suffers severe dissolution in aqueous electrolytes; therefore, the capacity decays gradually upon cycling, making it di cult to study the detailed H + intercalation mechanism. Although many attempts have been made to solve this issue, for example, the use of highly concentrated electrolytes 40,41 , gel-type or quasisolid-state electrolytes with polymer additives 31 , and electrode surface coating with polymers or ceramics 30,42 , the detailed H + intercalation mechanism in α-MoO 3 has yet to be fully understood. Herein, we provide the full comprehension of H + intercalation in α-MoO 3 as a cathode material for aqueous Zn 2+ /H + batteries. Fast H + transfer in α-MoO 3 through a solid-state anhydrous Grotthuss mechanism realizes aqueous batteries with both high power and high energy densities.
Electrochemical properties of MoO 3 in aqueous Zn 2+ /H + electrolytes α-MoO 3 was synthesized by a previously reported hydrothermal method, 40 and the synchrotron X-ray diffraction pattern con rmed the successful synthesis of a pure α-MoO 3 phase ( Figure S1). To study the H + intercalation mechanism in α-MoO 3 , ZnCl 2 was selected as an electrolyte salt. In addition to its compatibility with a Zn anode, the high solubility of ZnCl 2 enabled the formation of a superconcentrated liquid structure with limited amount of free water molecules, while its Brønsted acidity generated a low pH environment with a high H + concentration. Therefore, the electrochemical properties of α-MoO 3 were evaluated using three aqueous electrolytes: conventional Zn 2+ electrolyte (3 mol kg -1 ZnCl 2 /H 2 O), superconcentrated Zn 2+ electrolyte (32 mol kg -1 ZnCl 2 /H 2 O), and superconcentrated dual-ion (Zn 2+ /H + ) electrolytes (32 mol kg -1 ZnCl 2 + 1 mol kg -1 P 2 O 5 /H 2 O). Note that P 2 O 5 in a dual-ion electrolyte generates H + through hydrolysis (P 2 O 5 + 3H 2 O → 2H 3 PO 4 ). The Raman spectra for the superconcentrated electrolytes indicate that most water molecules are coordinated to Zn 2+ ( Figure S2).
Before testing the electrochemical properties of α-MoO 3 , we evaluated the negative electrode, namely, Zn stripping and plating on a Ti current collector using the three electrolytes ( Figures S3 and   2a). 6 The average coulombic e ciency in the aqueous Zn 2+ /H + dual-ion electrolyte is 99.0% over 200 cycles ( Figure S3), largely outperforming the superconcentrated Zn 2+ electrolyte (95.0% over 200 cycles) and conventional Zn 2+ electrolyte (82.2% over 100 cycles). The improved zinc reversibility after the addition of P 2 O 5 may result from the formation of a Zn 3 (PO 4 ) 2 -based solid electrolyte interphase (SEI) layer. 43 This reversible Zn stripping and plating was used as the counter electrode in this work. aqueous electrolyte 40 , suggesting dominant H + (de)intercalation even when using the Zn 2+ aqueous electrolytes. Indeed, the X-ray uorescence (XRF) elemental analysis of the electrodes after a cathodic scan shows no evident increase in the peak intensity of Zn compared to that of the pristine electrode ( Figure. S4). Considering that the aqua Zn 2+ complex is a Brønsted acid to generate H + , it is most likely that H + (de)intercalation occurs even when using the aqueous Zn 2+ electrolytes. Importantly, while 3 and 32 mol kg -1 ZnCl 2 aqueous electrolytes exhibit steep redox peak degradation upon repeated CV cycling, the 32 mol kg -1 ZnCl 2 + 1 mol kg -1 P 2 O 5 aqueous dual-ion electrolyte exhibits stable CV curves ( Figure   2b). The improved cycle stability should be ascribed to the suppression of α-MoO 3 dissolution and the formation of effective SEI. 43 Indeed, the X-ray photoelectron spectroscopy (XPS) analysis of the Zn metal anode after cycling in the 3 m ZnCl 2 electrolyte evidences the Mo deposition from the Mo ions dissolved in the electrolyte ( Figure S6). Figure 2c shows the charge/discharge curves of the α-MoO 3 electrode with galvanostatic charging followed by 3 h of potentiostatic charging in the aqueous Zn 2+ /H + dual-ion electrolyte. The α-MoO 3 electrode delivers a large capacity of 465 mAh g -1 at a rate of 0.5 A g -1 during the rst discharge, corresponding to 2.5 H + intercalation per formula unit of MoO 3 with an average voltage of approximately 0.9 V. Note that 'discharge' and 'charge' of the α-MoO 3 electrode are de ned as H + intercalation (cathodic process) and deintercalation (anodic process), respectively. Although the galvanostatic charge at 0.5 A g -1 can extract only 1.5 H + , the remaining 1.0 H + can be extracted when applying a constant voltage of 1.3 V for 3 h (Figures S7 and S8). The diffusion coe cient determined by the potentiostatic intermittent titration technique (PITT) shows a signi cant 4.9-fold deceleration in H + diffusion during the deprotonation from H 1.1 MoO 3 to MoO 3 , con rming the trapped nature of ~1.0 H + in MoO 3 ( Figure S9).
Under galvanostatic charging (without a potentiostatic step), the α-MoO 3 electrode in an aqueous Zn 2+ /H + dual-ion electrolyte retains 98% of its initial capacity after 1000 cycles at a rate of 2 A g -1 ( Figure   3a). Furthermore, 62% of the speci c capacity at 1 A g -1 is available at the fast discharge rate of 16 A g -1 (Figure 3b and Figure S10). These performance results indicate the stability of the MoO 3 framework against (de)protonation as well as the fast proton diffusion therein H x MoO 3 (1.0 ≤ x ≤ 2.5). In contrast, the α-MoO 3 electrodes in the aqueous Zn 2+ electrolytes have capacity retentions of only 24.5% and 63.8%, respectively (Figures S11 and S12). Moreover, both the capacity and cycling stability of the α-MoO 3 electrode in the aqueous Zn 2+ /H + dual-ion electrolyte outperform those reported previously for α-MoO 3 electrodes using aqueous electrolytes, including quasi-solid-state Zn 2+ electrolytes 30,31 and concentrated acid electrolytes 33,40 .

MoO 3 host-lattice response to proton intercalation
To clarify the structural evolution of the α-MoO 3 electrode in the aqueous Zn 2+ /H + dual-ion electrolyte, in situ X-ray diffraction (XRD) was performed during the 1 st cycle (Figure 4a). The interlayer distance of α-MoO 3 remains nearly constant (approximately 7.0 Å) during the entire protonation process, while it increases from 7.0 to 7.5 Å and then decreases to 7.0 Å during deprotonation. This asymmetric lattice response is consistent with the asymmetric CV and charge/discharge curves, which also highly resemble those reported in a 6 M H 2 SO 4 electrolyte 33 and in a 4.4 M H 2 SO 4 electrolyte 35 . However, despite the asymmetric structural evolution, the α-MoO 3 structure recovers to the pristine state after a constant voltage is applied. The structure of the fully protonated phase was clari ed using ex situ synchrotron XRD and Rietveld re nement (Figure 4b and 4c). Although it is di cult to determine proton positions using Xrays, the MoO 3 framework only exhibits a slight monoclinic distortion after protonation, which is consistent with a previous report on H 1.68 MoO 3 44 and the in situ XRD results.
Importantly, no water cointercalation occurs in the protonated structure. When a small amount of water (one water or hydronium intercalation in 16 formula units of MoO 3 ) is intercalated, the density functional theory (DFT) calculation predicts that the interlayer distance expands by 13.8% (H 2 O intercalation) and 17.2% (H 3 O + intercalation) ( Figure S13), which are considerably larger than those observed experimentally. Additionally, the interlayer distance after water cointercalation in a 1 M H 2 SO 4 electrolyte has been reported to show an expansion of 11% upon protonation 39  H + transfer in the dense oxide-ion array is described as the solid-state anhydrous Grotthuss mechanism, whose kinetics highly rely on the distance between two adjacent lattice oxide ions. Figure Figure   S19). Therefore, further deprotonation from site C is unfavorable, instead, protons in site A are deintercalated, resulting in asymmetric (de)protonation (Figure 5b). Indeed, after deprotonation to HMoO 3 , all the long-range 1D channels are disrupted so that fast Grotthuss H + transfer is no longer applicable ( Figure S20). The remaining H + can be removed only under long relaxation times, such as potentiostatic charging and PITT ( Figure S21 and S22). However, except for the trapped H + at site C, H x MoO 3 (1.0 ≤ x ≤ 2.5) exhibits fast H + transport through the diffusion channels upon charge/discharge, providing a remarkably high capacity and high-rate capability, as demonstrated in Figure 3.
To summarize, coupled with a zinc metal anode and an aqueous dual-ion Zn 2+ /H + (32 m ZnCl 2 + 1 m P 2 O 5 ) electrolyte, the MoO 3 -Zn battery delivers a large energy density of 413 Wh kg -1 upon discharge at a power density of 0.90 kW kg -1 as well as a peak power density of 10.52 kW kg -1 at an energy density of 217 Wh kg -1 per weight of MoO 3 ; these results are more than double that of a similar MoO 3 -Zn battery with a ZnCl 2 -based electrolyte 39 (energy density of 198 Wh kg -1 at a power density of 0.28 kW kg -1 and power density of 6.7 kW kg -1 at an energy density of 104.5 Wh kg -1 ). Moreover, with the aid of the solidstate anhydrous Grotthuss mechanism, this prototype cell successfully outperforms most aqueous zincion batteries and proton batteries ( Figure S23). Contrary to conventional intercalation chemistry, which requires a porous host that accommodates ion diffusion and storage, the solid-state anhydrous Grotthuss mechanism demonstrated in this work enables fast H + transfer and accumulated H + storage in dense oxide-ion arrays. Therefore, further exploration for other host materials capable of H + intercalation based on the solid-state anhydrous Grotthuss mechanism will be an important challenge for not only fabrications of high-power aqueous H + batteries but also other solid-state ionics applications using protons. Methods α-MoO 3 was synthesized via a simple hydrothermal synthesis approach 40 , where 1 g of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (Wako) was dissolved in 25 mL of water, followed by the addition of 10 mL of HNO 3 solution (3 M). After stirring for 10 min, the transparent colorless solution was poured into a 50 mL Te on TM -lined Parr autoclave and heated at 180 °C for 12 h. After ltration, the white-colored powder was washed with water and ethanol and then dried in an oven at 80 °C in air overnight.
The electrolyte was prepared by weighing ZnCl 2 and P 2 O 5 in an argon-lled glovebox to prevent the absorption of water. After weighing, ultrapure water (Wako) was mixed with salts to form a solution in open air.
The electrochemical performance was tested in PTFE three-electrode beaker cells with Ag/AgCl (in a saturated KCl aqueous solution) as the reference electrode and activated carbon as the counter electrode. In the two-electrode cell tests, zinc foil was attached to a titanium wire, working as both the reference and counter electrode. The zinc stripping and plating tests were conducted in a 2032-type coin cell where a titanium foil and a zinc foil were separated with a glass ber separator (Fisher) presoaked with an electrolyte. The in situ XRD studies were conducted in a custom cell with a Kapton ® membrane window on the cathode side.
The electrochemical performance was studied via cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD), the galvanostatic intermittent titration technique (GITT) and the potentiostatic intermittent titration technique (PITT) using a VMP3 potentiostat (BioLogic) at room temperature.
Powder XRD and in situ XRD studies were conducted with a Bruker-AXS D8 ADVANCE diffractometer using a Co sealed tube (operating at 35 kV, 40 mA) and a linear position-sensitive detector (LYNX-EYE).
VESTA 45 software was used to illustrate the crystal structure. For crystal structure re nement, we used a powder diffractometer at a synchrotron radiation beamline 5S2 of Aichi Synchrotron Radiation Center, Japan. The wavelength used was calibrated by re ning a powder diffraction pattern of the ceria power, and the value was 0.700072(9) Å.
Elemental analysis of the MoO 3 electrodes by energy dispersive X-ray uorescence (XRF) spectroscopy was performed with a JSX-3400RII instrument (JEOL).
The surface chemistry of the Zn electrodes was analyzed via X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe II, ULVAC-PHI) with a monochromatic Al K_X-ray source. The Zn electrode samples were rinsed with pure water and acetone after being extracted from cycled cells.
The coordination states of the water molecules were investigated by Raman spectroscopy (NRS-5100, JASCO). A 532 nm excitation laser was used. The electrolyte solution was sealed in a quartz cell, and the laser was directed through the quartz crystal window. where F is the Faraday constant, μ H is the chemical potential of hydrogen gas at atmospheric pressure, and T = 298.15 K.
In the above equation, E(H 2 ) is the calculated total energy of a hydrogen molecule, k B is the Boltzmann constant and ΔS is obtained from the JANAF thermochemistry tables. 53 We used the climbing image nudged elastic band (CI-NEB) method to calculate proton diffusion pathways in supercell. 54 For the stable structures in the discharge process, an extra proton was added to migrate between two neighboring adsorption sites. In the charge process, a proton was removed from the stable structures to construct a vacancy site for the migration of other protons.

Con icts of Interest
There are no con icts of interest.   Discharge capacity and coulombic e ciency at 2 A g -1 , (b) rate performance at discharge rates from 1 A g -1 to 16 A g -1 . Inset: the corresponding discharge curves.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. SIv2MaMoO3NM.docx