Optimal water concentration for aqueous Li+ intercalation in vanadyl phosphate†

Development of high-performance aqueous batteries is an important goal for energy sustainability owing to their environmental benignity and low fabrication costs. Although a layered vanadyl phosphate is one of the most-studied host materials for intercalation electrodes with organic electrolytes, little attention has been paid to its use in aqueous Li+ systems because of its excessive dissolution in water. Herein, by controlling the water concentration, we demonstrate the stable operation of a layered vanadyl phosphate electrode in an aqueous Li+ electrolyte. The combination of experimental analyses and density functional theory calculations reveals that reversible (de)lithiation occurs between dehydrated phases, which can only exist in an optimal water concentration.


Introduction
Lithium-ion batteries (LIBs) dominate the battery market for portable electronics and electric vehicles owing to their long lifetime, high efficiency, and high energy densities. However, LIBs are comprised of ammable and costly organic electrolytes, which unexceptionally accompany both safety hazards and high fabrication costs. 1 Batteries that utilize aqueous electrolytes are expected to provide more operational safety, affordability, high power, and environmental benignity, all of which are favorable for large-scale stationary systems and electric vehicle operations. 2,3 Although the intrinsically narrow electrochemical stability window for water as an electrolyte solvent ($1.23 V) imposed severe limitations on any practical applications of aqueous batteries, novel strategies involving highly concentrated aqueous electrolytes have achieved much wider electrochemical potential windows (>3 V), thereby paving a path for the development of more practical aqueous batteries. [4][5][6][7][8] Importantly, by exploiting a specialized solution structure without free water, [9][10][11][12][13][14] highly concentrated aqueous electrolytes can also provide unexpected environments for electrode materials that have been considered 'useless' in conventional dilute aqueous electrolytes. 15 The selection criteria of electrode materials for aqueous batteries are (i) suitable redox potential, (ii) durability against water, (iii) no side reactions, (iv) good reversibility, and (v) low cost. Fig. S1 † lists selected electrode materials that are potentially compatible with aqueous electrolytes. Among them, for instance, hydrated vanadyl phosphate (VOPO 4 $nH 2 O, Fig. 1a) is a versatile layered host for intercalation chemistry, which has been ascertained using various organic electrolyte systems. [16][17][18][19][20][21] However, except for a few very recent reports of its application to aqueous H + /Zn 2+ batteries, 22,23 VOPO 4 $nH 2 O has rarely been considered promising in aqueous systems, due simply to dissolution and decomposition of VOPO 4 $nH 2 O in aqueous electrolytes. VOPO 4 $2H 2 O possesses a bilayer structure where a VOPO 4 layer is composed of corner-sharing VO 6 octahedra and PO 4 tetrahedra, with a layer distance of 7.25Å along the c axis, as illustrated in Fig. 1a. When VOPO 4 $nH 2 O is immersed in liquids that possess high water concentration c(H 2 O) (e.g., pure water or dilute aqueous solutions), surface vanadium atoms are readily coordinated by the water to form soluble vanadium aquo complexes. Furthermore, high-concentration water is prone to intercalate into an interlayer space to exfoliate VOPO 4 layers, accelerating the dissolution process. Alternatively, an extremely low-c(H 2 O) environment (e.g., highly concentrated aqueous solutions) is expected to desorb interlayer water between VOPO 4 layers. Since interlayer water plays an important role in both structural integrity and ion diffusion, the intercalation chemistry may be largely altered via the control of c(H 2 O) of aqueous electrolytes for achieving reversible lithium-ion (de)intercalation in VOPO 4 $nH 2 O. Herein, we demonstrate the stable charge/ discharge operation of VOPO 4 $nH 2 O through accurate control of c(H 2 O) in aqueous Li + electrolytes.  (Fig. 1c) shows a lamellar morphology with a lateral size ranging from 10 to 100 mm.

Results and discussion
The dissolution durability of VOPO 4 $nH 2 O was tested using aqueous Li + electrolytes ( aqueous electrolyte (Fig. 3a) drastically decreases upon cycling, while the potential polarization increases, presumably because of the vanadium dissolution. Upon gradually lowering c(H 2 O) to an optimal range (Li + : H 2 O ¼ 1 : 4), the potential proles exhibit distinct multiple plateaus without substantial polarization, indicating highly reversible (de)lithiation ( Fig. 3b and c). However, when further lowering c(H 2 O) to Li + : H 2 O ¼ 1 : 2.5, the polarization between lithiation/delithiation signicantly increases to reduce the reversible capacity to approximately 85 mA h g À1 (Fig. S3 †). These results are consistent with the CV results (Fig. 2). The reversible capacity of 118 mA h g À1 in the rst cycle (Li + : H 2 O ¼ 1 : 4) corresponds to 0.8 Li + intercalation per formula unit. The reversible reduction/oxidation of vanadium (V 5+ /V 4+ ) upon charge/discharge, which was conrmed by V K-edge X-ray absorption spectroscopy (Fig. S4 †), supports the occurrence of reversible (de)lithiation of VOPO 4 $nH 2 O. Furthermore, owing to the optimal-c(H 2 O) environment, 80% of the initial capacity is retained aer 200 cycles (Fig. 3d), which is much higher than that for a high-c(H 2 O) aqueous electrolyte (7%).
To clarify the phase diagram of Li x VOPO 4 $nH 2 O in the optimal c(H 2 O) environment, in situ XRD experiments were conducted (Fig. 4a-c). Before immersion in the optimal-c(H 2 O) electrolyte, VOPO 4 $nH 2 O in a composite electrode possesses an interlayer distance of 7.2Å. The calculated interlayer distance of VOPO 4 $2H 2 O (7.25Å, Fig. S5 and Table S1 †) agrees with the experimental value, and the water content of VOPO 4 $nH 2 O in a pristine electrode is estimated as n z 2, which is in good agreement with the TG result (the inset of Fig. 1b). However, the in situ XRD pattern for VOPO 4 $nH 2 O immersed in a LiTFSI/ 4H 2 O electrolyte shows the shi of 001 diffraction to a higher diffraction angle (Fig. 4a), indicating the decrease of the interlayer distance to 6.2Å. This value is consistent with the calculated interlayer distance for VOPO 4 $H 2 O (6.16Å, Fig. S5 and Table S1 †). Therefore, the low-c(H 2 O) environment dehydrates VOPO 4 $nH 2 O to a monohydrate phase (n ¼ 1). Upon lithiation/delithiation, a new 001 diffraction, corresponding to an interlayer distance of 5.3Å, emerges/diminishes at the expense of the original 001 diffraction (Fig. 4b), suggesting biphasic (de)lithiation. It is noteworthy that the lithiated/ delithiated monohydrate phases are stable only in the optimal-c(H 2 O) environment; for instance, the lithiated phase immediately becomes hydrated to a dihydrate phase (n ¼ 2) aer being exposed to the ambient atmosphere (Fig. 4c).
A phase diagram of Li x VOPO 4 $nH 2 O in the optimal-c(H 2 O) environment is postulated based on the experimental and simulation results as schematized in Fig. 5. The equilibrium between interlayer water in VOPO 4 $nH 2 O and lowconcentration water in concentrated aqueous electrolytes   drives dehydration of VOPO 4 $nH 2 O to the monohydrate phase (n ¼ 1). Although exhibiting reversible Li + (de)intercalation, the monohydrate phases can only be stabilized in a low-c(H 2 O) environment, and exposure to the ambient atmosphere immediately causes hydration to the dihydrate phase (n ¼ 2).

Conclusions
In summary, the stable operation of VOPO 4 $nH 2 O electrodes in an optimal-c(H 2 O) environment was successfully demonstrated. The reversible (de)lithiation occurs specically in monohydrate phases, which are only stable in the optimal-c(H 2 O) environment. This study provides novel insights into the reaction mechanism and phase diagram of VOPO 4 $nH 2 O in a low-c(H 2 O) environment, and more importantly points to research opportunities to enrich aqueous ion intercalation chemistry by controlling water concentration.
Methods VOPO 4 $nH 2 O was synthesized via a reux method: V 2 O 5 (6 g), H 2 O (144 mL) and H 3 PO 4 (82%, 57.75 g) at 125 C for 16 h. Aer cooling to room temperature, the resulting yellow precipitate was collected by centrifugation and washed three times with water and acetone. Thereaer, the powder was dried under vacuum overnight at 80 C.
X-ray diffraction patterns were recorded on a Bruker AXS D8 Advance X-ray diffractometer using Co Ka radiation. Microstructure analyses were performed using a scanning electron microscope (Hitachi, S-4800) at a beam accelerating voltage of 5 kV. The oxidation states of the samples were measured by V Kedge XANES at the beamline 8B of Photon Factory (PF), High Energy Accelerator Research Organization (KEK) Tsukuba, Japan. The crystal water content of the as-synthesized VOPO 4 -$nH 2 O sample was conrmed by TG (NETZSCH, STA2500). VOPO 4 $nH 2 O electrodes were fabricated by grinding a mixture containing VOPO 4 $nH 2 O, carbon black (super P) and polytetrauoroethylene (PTFE) in a weight ratio of 75 : 10 : 15 for 20 min, and then it was rolled into an electrode lm using a rolling machine with a xed gap of 250 mm. For the preparation of aqueous electrolytes, lithium bis-(tri-uoromethanesulfonyl)imide (LiTFSI) was dissolved in ultrapure water as LiTFSI$nH 2 O (n ¼ 2.5, 4, 6, 8, and 50). The electrochemical performance of VOPO 4 $nH 2 O electrodes was evaluated using a three-electrode system (Ag/AgCl and active carbon as the reference and counter electrodes, respectively). CR2032-type coin cells were assembled to measure the galvanostatic charge/discharge properties of the electrodes. The cell consists of a VOPO 4 $nH 2 O electrode, active carbon anode and glass ber separator (GF/F, Whatman). Electrochemical impedance spectroscopy (VMP3 potentiostat, Biologic) was performed at the open circuit potential with an amplitude of 10 mV in the frequency range of 10 mHz to 200 kHz.
First-principles calculations were performed using the Vienna Ab initio Simulation Package (VASP), 25 based on density functional theory (DFT). 26,27 The exchange-correlation energy is calculated using general gradient approximation (GGA) with the Perdue-Burke-Ernzerhof (PBE) exchange-correlation functional. 28 Furthermore, in our calculations, Hubbard U corrections (GGA + U) were adopted with U À J ¼ 3.1 for vanadium. The effect of van der Waals interactions was estimated and implemented in the optimized exchange van der Waals functional B86b of the Becke (optB86b vdW) functional. 29,30 The plane wave cutoff energy was 580 eV. The convergence condition for the energy is 10 À4 eV, and the structures were relaxed until the force on each atom was less than 0.01 eVÅ À1 . Spin polarization was considered in all calculations. The k-point meshes of 7 Â 7 Â 7 and 14 Â 14 Â 14 in the Monkhorst-Pack sampling scheme were used for geometry optimization and electronic self-consistent computation, respectively. 31

Conflicts of interest
There are no conicts to declare.