English

• 1. INTRODUCTION

  Rechargeable aqueous Zn-ion batteries (ZIBs) are recognized as a promising candidate for large-scale electrochemical energy storage owing to their low cost, low toxicity, the capability of utilizing high-capacity Zn metal anodes, and intrinsic safety from the aqueous nature^[1]. Despite these merits, the development of ZIBs has been largely hindered by the divalent nature of Zn²⁺. The divalent Zn²⁺ typically interacts strongly with cathode lattice through electrostatic interactions, which not only initiates structural degradation and thereby limits the cycle life to cathode materials but also results in sluggish Zn²⁺ migration within cathode hosts^{[2, 3]}. In this context, it is crucial to explore high-performance cathode materials that could conquer the divalency-induced issues and achieve long-term cycling and fast-charging capabilities.

  In the past decade, a tremendous amount of inorganic cathode materials have been intensively investigated. Among them, Mn-based^[3-6] and V-based^[7-9] materials represent two main types of cathode hosts. The former typically delivers high battery voltages and energy density but suffers from Mn²⁺-dissolution issues and low cyclability, while the latter often exhibits good cyclability but provides relatively low battery voltage (typically 0.6~0.8 V)^{[10, 11]}. Very recently, it has been demonstrated that introducing PO₄^3- functional groups into V-based cathode materials can raise their operating voltage via inductive effect^{[12, 13]}. It has also been demonstrated that lattice water molecules can effectively shield the electrostatic interactions between Zn²⁺ and V-based cathode hosts and thus accelerate the solid-state Zn²⁺ migration for achieving improved fast-charging capability^[14]. And when incorporating both abundant lattice water and PO₄^3- groups in a cathode skeleton, fast-charging capability, and relatively high battery voltage plateau can be achieved simultaneously^[15]. Apart from the inorganics, organic materials including sustainable quinone analogs^{[16, 17]} and polyaniline^[18] represent another type of cathode host for ZIBs. These organics possess a higher degree of structural flexibility and often exhibit better cycling stability compared to inorganics. Given the above, V-based inorganic-organic hybrid (IOH) cathode materials that combine abundant lattice water, PO₄^3- groups and organic components may achieve fast charging, high battery voltage, and stable cycling performance concurrently^[19].

  Herein, we for the first time report an IOH cathode material K₂[(VO)₂(HPO₄)₂(C₂O₄)]·4H₂O (KVPCO). The IOH exhibits a lattice-water-rich layered structure and is capable of delivering a high discharging voltage of 0.8 V on average, long cycle lives (90% and 81.5% capacity retention over 600 and 1500 cycles at current densities of 200 and 1000 mA/g, respectively), and ultrafast charging (~90% state of charge in 1 minute). We have also revealed the underlying reaction mechanism through ex-situ X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses.

  2. EXPERIMENTAL

  2.1 Synthesis of the title complex

  K₂[(VO)₂(HPO₄)₂(C₂O₄)]·4H₂O is prepared by room temperature solution method. The mixture brown aqueous solution of vanadium pentoxide, tartaric acid, and potassium hydroxide in the mole ratio 1:1.4:4 is formed by magnetic stirring. While the solution turns clear, 20 molars of phosphoric acid are slowly added and stirred continuously for another 12 hours, which resulted in a bluish-green precipitate. The precipitate is repeatedly washed with water and dried at 80 ℃ for 8 h in a vacuum.

  2.2 Materials characterization

  X-ray powder diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer with Cu/Kα radiation source (λ = 1.54184 Å) at a 2θ range of 5~65° and a scanning rate of 2 °/min. The operating voltage and current are 40 kV and 40 mA, respectively. The microscopic size and morphology were characterized by a field-emission scanning electron microscope (FESEM, HITACHI SU-8010).

  2.3 Electrochemical experiment

  The electrochemical performance is tested using CR2032 coin-type cells. The cathode electrode is fabricated by mixing K₂[(VO)₂(HPO₄)₂(C₂O₄)]·4H₂O, and the solution of CNT and r-GO was dissolved in NMP according to a weight ratio of 7:2:1. The mixed solution is filtrated onto filter paper and dried at 80 ℃ for 12 h in a vacuum. Zinc foil and glass fiber are used as the anode and separator, respectively. 80 μL 3 M Zn(CF₃SO₃)₂ aqueous solution is employed as the electrolyte. The cycling tests are performed on a battery testing system (LANHE CT-2001A) at different current densities from 0.2 to 1.8 V. The GITT is performed after 10 cycles of activation. The battery is charged (or discharged) at a current density of 30 mA/g for 60 min, and is left to rest for 10 h. Then the steps were repeated until the charging (or discharging) voltage reached 1.8 V (0.2 V). Rate scan cyclic voltammetry is performed on an electrochemical workstation (Bio-Logic SP-300) at a scanning rate from 0.02 to 0.15 mV/s. All electrochemical tests are conducted at a constant temperature of 28 ℃.

  3. RESULTS AND DISCUSSION

  KVPCO was synthesized via an aqueous-phase roomtemperature coprecipitation method, which is simple yet scalable and suitable for massive production (Fig. 1a). XRD pattern and Rietveld refinement confirm the phase purity of the as-synthesized KVPCO (Fig. 1b). Transmission Electron Microscope (TEM) imaging reveals that the obtained KVPCO particles are in nanosize with an average diameter of ~22 nm (Fig. 1c). As confirmed by thermogravimetric analysis (TGA), 2.2% of weight loss from 25 to 70 ℃ is observed, which is mainly attributed to the physically absorbed water. And at 120 ℃, near 13.4% weight loss emerges due to the loss of 4 mol lattice water. The amount of lattice water molecules reaches two per V-redox center (Fig. 1d), which is substantially higher than those of previous reports (typically < 1.2). KVPCO crystallizes in the triclinic system (P$ \overline 1 $) and exhibits a two-dimensional structure with K-ions and lattice waters filling between two [(VO)₂(HPO₄)₂(C₂O₄)]^2- IOH layers (Fig. 1e). The oxalate units within the IOH layers provide certain structural flexibility that is absent with inorganic materials. This lattice-water-rich feature favors the shielding of electrostatic interactions between Zn²⁺ and KVPCO cathode host, and thereby the achievement of fast-charging performance.

  Figure 1

  

  Figure 1.  (a) Schematics of the synthesis process; (b) XRD Rietveld refinement (R_wp = 9.54%, and R_p = 7.02%); (c) TEM image and HR-TEM of the KVPCO (inset); (d) Thermo gravimetric data; (e) Projection view of the KVPCO structure

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  Galvanostatic cycling analysis shows that, at a current density of 10 mA/g, KVPCO delivers an initial discharging capacity of 12 mAh/g with voltage plateau absent, suggesting negligible Zn-ion intercalation (Fig. 2a). Under the reverse charging process, a capacity of 38.6 mAh/g and a voltage plateau at an average potential of 1.67 V are observed, which may correspond to the electrochemical extraction of K⁺-ions from KVPCO. Upon eight-cycle activation, the reversible capacity reaches 78 mAh/g. Figs. 2b~2d depict the long-term cycling performance. KVPCO achieves high capacity retentions of 90% over 600 cycles at a current density of 200 mA/g and 83% over 1500 cycles at 1 A/g. Equally important, the flat voltage plateaus remain stable during 600 cycles (Fig. 2b), demonstrating the high reversibility of the Zn²⁺-ions intercalation chemistry.

  Figure 2

  

  Figure 2.  (a) Initial 10 charge-discharge curves at a current density of 10 mA/g; (b) Voltage profiles of the 11^th, 50^th, 100^th, 200^th, 400^th, and 600^th cycles at 200 mA/g; (c) Long-term cyclability at the current density of 200 mA/g; (d) Long-term cyclability at the current density of 1 A/g

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  Apart from long-term cyclability, the lattice-water-rich layered structure renders KVPCO excellent rate capability. The reversible capacity reaches 80, 81.3, 79.8, 77.6, 75, 73.3, and 72.2 mAh/g at current densities of 50, 200, 500, 1000, 2000, 3000, and 5000 mA/g, respectively (Fig. 3a). That is, the Zn-ion batteries constructed with the KVPCO cathode can realize a 90% state of charge at 53.2 C (1 C = 94 mAh/g), surpassing all the previously reported cathod materials in ZIBs (Fig. 3b). Remarkably, the voltage plateaus exhibit negligible change under both deep and fast charging conditions (Fig. 3a), demonstrating fast Zn-ion intercalation kinetics. Galvanostatic intermittent titration technique (GITT) reveals that the average Zn-ion diffusion coefficients (D_Zn-ion) reach as high as 3.202 × 10^-10 and 5.205 × 10^-10 cm²/s (Fig. 3c), respectively, for the charging and discharging process. Rate-scan cyclic voltammetry (CV) analysis (Fig. 3d) is conducted to further understand the fast reaction kinetics based on equation (1)^[20]:

  $ i = av^{b} $

  (1)

  Figure 3

  

  Figure 3.  (a) Rate performance of KVPCO/CNT-GO electrodes; (b) Comparison of the capacity retention-rate among reported cathode; (c) GITT; (d) Rate-scan CV; (e) Log(peak current) versus log(scan rate) and corresponding b-values

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  Where i is the peak current (A), a and b are adjustable parameters, and v refers to scan rate (V/s). The b values corresponding to peaks A1, A2, C1, and C2 are determined to be 0.878, 0.901, 0.913, and 0.786, respectively (Fig. 3e). These b values indicate that the electrochemical reaction kinetics is capacitance dominant for the KVPCO cathode. This capacitance-dominant Zn²⁺ intercalation reactions and the fast Zn²⁺ diffusion within the cathode host could be responsible for the achieved ultrafast charging capability.

  Ex-situ XRD and X-ray photoelectron spectroscopy (XPS) analyses are performed to provide a mechanistic understandding of this new Zn²⁺ intercalation chemistry. Fig. 4a depicts the crystal structural evolution of the KVPCO cathode in response to (de)zincation. Upon initial discharge to 0.2 V, Bragg diffraction (010) and (002) shift to lower 2θ angles by 0.33° and 0.34°, respectively, and peak (022) disappears (Fig. 4a). Considering the tiny initial capacity of 12 mAh/g and the absence of V⁴⁺ reduction depicted in XPS spectra (Fig. 4b), the observed Zn²⁺ could be attributed to partial K–Zn ions exchange rather than electrochemical Zn²⁺ intercalation. Upon reverse charge to 1.8 V, the above-mentioned Bragg peaks shift back or re-appear to their original positions, accompanying the oxidation of V⁴⁺ to V⁵⁺ (Fig. 4b). These observations suggest the extraction of Zn²⁺ and K⁺ from the KVPCO structure. Note that there are still K-2p bands existing in XPS spectra, indicating incompleted K⁺ extraction. During the subsequent two cycles, (010), (002), and (022) peaks follow a similar evolution trend, while the diffraction (-210) disappears upon full charge at the third cycle and never reappears after then (Fig. 4a). XPS analysis reveals that K⁺ cations are completely removed from the KVPCO lattice after three-cycle activation (Fig. 4b), yielding Zn_0.54[(VO)₂(HPO₄)₂(C₂O₄)]·4H₂O. After activation for 8 cycles, compared to the original structure, (010) and (002) diffraction peaks slightly shift to lower angles by 0.8° and 0.39°, and three new Bragg peaks appear at 19°~23°. It is observed that, during the subsequent cycles, the above-mentioned diffraction peaks remain unchanged except for the (022) peak which disappears upon discharge and re-appears on charge. Besides, the electrochemically oxidized V⁵⁺ cations completely convert into V⁴⁺ cations upon full discharge, yielding Zn_0.98[(VO)₂(HPO₄)₂(C₂O₄)]·4H₂O. The demonstrated high structural stability and redox reversibility are directly responsible for the achieved long-term cycling stability.

  Figure 4

  

  Figure 4.  (a) Ex-situ XRD patterns of the KVPCO/CNT-GO electrodes of the first three cycles, 8^th cycle, and 10^th cycle; (b) XPS spectra of KVPCO

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  4. CONCLUSION

  In conclusion, we have demonstrated the capability of adopting inorganic-organic hybrids (IOH) as a new type of cathode material for aqueous ZIBs. The unique lattice-water-rich layered structure enables the mitigations of the two key issues facing ZIBs, that is, sluggish solid-state Zn²⁺ diffusion kinetics and cycling instability. The IOH cathode is capable of achieving high cycling stability with 81.5% capacity retention over 1500 cycles and ultrafast charging capability (~90% state of charge about 1 minute). IOH materials may pave the way for the development of low-cost, ultrafast-charging, and ultralong-lifespan aqueous ZIBs for grid-scale electrochemical energy storage.

  
  
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• 

  Figure 1  (a) Schematics of the synthesis process; (b) XRD Rietveld refinement (R_wp = 9.54%, and R_p = 7.02%); (c) TEM image and HR-TEM of the KVPCO (inset); (d) Thermo gravimetric data; (e) Projection view of the KVPCO structure

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  Figure 2  (a) Initial 10 charge-discharge curves at a current density of 10 mA/g; (b) Voltage profiles of the 11^th, 50^th, 100^th, 200^th, 400^th, and 600^th cycles at 200 mA/g; (c) Long-term cyclability at the current density of 200 mA/g; (d) Long-term cyclability at the current density of 1 A/g

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  Figure 3  (a) Rate performance of KVPCO/CNT-GO electrodes; (b) Comparison of the capacity retention-rate among reported cathode; (c) GITT; (d) Rate-scan CV; (e) Log(peak current) versus log(scan rate) and corresponding b-values

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  Figure 4  (a) Ex-situ XRD patterns of the KVPCO/CNT-GO electrodes of the first three cycles, 8^th cycle, and 10^th cycle; (b) XPS spectra of KVPCO

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