Boosting the ultrastable Li storage performance in electron-sponge-like polyoxovanadates by constructing inorganic 3D structures

Abstract

Polyoxometalates act as an electron sponge, processing multielectron redox reactions and acting as a fast ionic conductor. They show great potential as promising electrode materials for next-generation lithium ion batteries (LIBs). However, there are still some fundamental issues which should be solved before their application can be realized, such as determining the stable structural feature with reversible Li⁺ ion insertion/desertion. In this work, polyoxovanadates (POVs) based materials of K₄Na₂V₁₀O₂₈·nH₂O (KNaV₁₀) and Mg₂(NH₄)₂V₁₀O₂₈·nH₂O (MgV₁₀) have been prepared and used as the electrode material for a Li⁺ ion reservoir. The 10-core polyoxovanadate is demonstrated as a anionic building block and the 3D extended structure has been smartly tuned by counter cations. For MgV₁₀, a 1D tunnel with an approximate size of 3 Å × 10 Å was formed along the a axis by Mg²⁺ ions and [V₁₀O₂₈]⁶⁻ polyanions. The MgV₁₀ shows a higher capacity, cycling stability, and rate performance than that of KNaV₁₀ without tunnels. The capacity of MgV₁₀ is about 160 mA h g⁻¹ at a high discharge rate of 250 mA g⁻¹, while it is only 118 mA h g⁻¹ for KNaV₁₀. Even after 60 discharge/charge cycles at 50 mA g⁻¹, it displayed a capacity of 180 mA h g⁻¹. The 1D tunnel in MgV₁₀ facilitates the Li⁺ ion transport and provides spatial Li storage sites, which promotes the electrochemical performance in LIBs. Moreover, the Mg²⁺ ions remained stable during battery cycling and promoted the 3D structure stability. This work demonstrates promising guidelines for the structural design of POVs based materials for Li storage.

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Introduction

The increasing demand for more advanced consumer electronics, electric vehicles, and stationary facilities calls for next generation rechargeable batteries with higher specific and volumetric capacities than the current state-of-the-art lithium ion batteries (LIBs).^1–3 To date, the distorted rock-salt α-NaFeO₂, spinel, and olivine structured bulk transition metal oxides are still the mainstream of the current cathode materials.^4–6 However, these compounds are still challenged by lower capacity and limited Li⁺ ion transport kinetics, resulting in a relatively low power density output which limits their further applications.^7–9 In this regard, novel electrode materials that process multi-electron redox reactions and enable more rapid Li⁺ ion transportation are needed to satisfy the increasing demand for high power and energy density. Among the potential candidates, the highly redox-active molecular metal oxides, widely known as polyoxometalates (POMs),¹⁰ have been investigated as promising electrode materials for their distinguished physical/chemical properties such as electronic versatility, versatile structures, and unique molecular structures etc.^11–13 Great progress has been achieved. For instance, Song et al. reported the periodic patterning redox-active POMs on individual carbon nanotubes (CNTs), where the chemical activity and battery performance can be tuned at the molecular level with a diverse range of element doping.¹⁴ Ma et al. developed a bottom-up strategy to fabricate hierarchically sandwiched frameworks composed of manganese oxide nanosheets and various guest metal cations, which showed good electrochemical performance for both Li and Na storage.¹⁵ Recently, Lan and Sha et al. fabricated a series of Keggin type POMs/metal organic framework hybrids and explored these as anodes for LIBs. The corresponding electrochemical performance was better than that with simple counter cations.¹⁶ Most of the works have concentrated on composite design and fabrication to improve their conductivity and stability.^17–20 There is still lack of fundamental understanding related to the stability of polyanions, as well as the 3D structures upon discharge/charge cycles. To further expand their application and obtain a more stable high performance from POMs based electrode materials, investigation of the structural effects and structural design guidelines are urgently needed.

Herein, we demonstrate 3D polyoxovanadates (POVs) with multielectron redox properties as high performance electrode materials. The extended 3D structures have been designed and smartly tuned by optimizing the counter cations. The [V₁₀O₂₈]⁶⁻ functions as an “electron sponge”, which can theoretically reversibly adopt 10 electrons between [V₁₀O₂₈]⁶⁻ and [V₁₀O₂₈]¹⁶⁻ during charging/discharging. Our results revealed that the electrochemical properties such as capacities and cyclability of the electrode materials are influenced by the counter-cations (K⁺ or Mg²⁺) directed structural effects, suggesting the crucial role of these guest cations. The fundamental findings presented in this work provide new guidelines for the optimization of the host materials with fast Li⁺ ion transportation and a robust structure for next generation LIBs.

Experimental

Synthesis of KNaV₁₀

KNaV₁₀ was synthesized according to the literature method.²¹ V₂O₅ 1 g (5 mmol) was dissolved in 50 mL of 0.5 M CH₃COOH/CH₃COONa buffer solution (pH = 4.8) with stirring. The turbid solution was heated at 80 °C for 1 h. Then the solution was filtered off and KCl (3.5 g) was added to the solution and the mixture was stirred for another 15 min. The orange single crystals suitable for crystallography were obtained from the solution and the crystal samples air-dried at room temperature for further characterization.

Synthesis of MgV₁₀

MgV₁₀ was synthesized by the following procedure: NH₄VO₃ 2.1 g (17.95 mmol) was dissolved in deionized water (50 mL) and heated at 100 °C for several minutes until the solution became yellow and transparent. After cooling down to room temperature, a Mg(NO₃)₂ solution (1.4 g, 5.5 mmol Mg(NO₃)₂, 10 mL H₂O) was added to the above solution under stirring. The pH of the resulting yellow clear solution was adjusted to 4.4 and evaporated slowly at room temperature. The orange crystals were obtained and air-dried at room temperature. Both of the structures were determined by single crystal X-ray diffraction analysis.

Materials characterization

The crystal structure of POVs was determined by a Bruker SMART CCD APEX II diffractometer with a graphite monochromated Mo K source of (λ = 0.71073 Å) at 293 K. All data were corrected for Lorentz polarization and absorption effects. X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance X-ray diffractometer employing Cu Kα radiation (λ = 0.15406 nm). The thermal stability was tested by thermogravimetric (TG) analysis (Netzsch DSC 214) under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹. The SHELXL2014 programs were used for structural determination and refinement.²² The structure was solved by the least-squares methods. Except for some disordered O atoms, all other non-hydrogen atoms were refined anisotropically by least-squares on F² using Olex2.²³ FTIR characterization was carried out on a BRUKER Vertex 70 spectrometer with a KBr pellet in the 4000–400 cm⁻¹ region. Particle size and morphology were observed by a scanning electron microscopy (SEM, Quanta FEG 250, FEI) operating at 10 kV and a transmission electron microscope (TEM, Tecnai G2 Spirit TWIN, FEI) operating at 120 kV. The surface elemental information of the POMs were analysed by means of an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, ThermoFisher Scientific), using monochromatized Al Kα X-rays as the excitation source and C 1s (284.60 eV) as the reference line.

Electrochemical measurements

The electrochemical properties of POVs were tested with two-electrode 2032 coin-type cells using pure Li foil (Aldrich) as the counter electrodes. The cathode composition was POVs, acetylene black (AB) and polytetrafluroethylene preparation (60% dispersion in H₂O, Sigma Aldrich Co.) at a weight ratio of 60 : 30 : 10. The mixture was then rolled onto the aluminium mesh (100 meshes) and dried at 60 °C under vacuum for 12 h. The electrolyte was LiPF₆ (1 M) in a mixture of dimethyl carbonate and ethylene carbonate (1 : 1, by volume). A microporous Clegard 2400 was used as a separator. The cells were assembled in an Ar-filled glove-box with less than 0.1 ppm of both oxygen and moisture. Galvanostatic discharge/charge measurements were performed in the potential window from 1.0 to 3.8 V versus Li⁺/Li on an Arbin battery test system (BT2043). Cyclic voltammetry (CV) data were collected by using an electrochemical workstation (CHI 760E) in the potential window of 3.8 to 1.0 V at a scan rate of 0.1 mV s⁻¹.

Results and discussion

Single crystal X-ray diffraction analysis reveals that KNaV₁₀ is composed of [V₁₀O₂₈]⁶⁻ polyoxoanion, K⁺, Na⁺, and water molecules. The [V₁₀O₂₈]⁶⁻ polyoxoanion is a 10-core polyoxovanadate anion, which is constructed by ten VO₆ polyhedra that arrange each other by corner- or edge-sharing modes. The K⁺ and Na⁺ ions not only compensate the charge balance, but also coexist with structural water molecules to form a fully filled 3D framework, in which Na⁺ ions coordinate to the [V₁₀O₂₈]⁶⁻ anion by Na–O bonds while K⁺ ions form the dimer units (Fig. S1†). The 3D packing diagram of KNaV₁₀ indicates that there is not any porous channel formed in the solid state.

For complex MgV₁₀, when Mg²⁺ ions and NH₄⁺ ions are used as counter cations in the synthetic design, it is different to the [V₁₀O₂₈]⁶⁻ polyoxoanion in KNaV₁₀ as the Mg²⁺ ions and solvent water molecules in MgV₁₀ link the neighboring [V₁₀O₂₈]⁶⁻ only by diverse hydrogen bonds to form the supermolecular structure.²⁴ We performed TG analysis to analyze the water content in the prepared samples. As shown in the Fig. S2,† there are two weight loss processes corresponding to the water molecules. An early weight loss from 40 °C to 110 °C is associated with the loss of free water molecules. A consecutive steady weight loss from 110 °C to 250 °C corresponds to the loss of crystal water. This is due to the hydrogen bonds formed by the crystal water molecule and is similar to the previous reports.²⁵ Furthermore, omitting the water molecules, an interesting 1D channel can be observed along the a axis with a size of ca. 3 Å × 10 Å (Fig. 1a). These 1D channels may facilitate the Li⁺ ions migration (Fig. 1b). As such, the syntheses of KNaV₁₀ and MgV₁₀ pave the way to the rational design of porous polyoxovanadate materials by selection of suitable cations. Both the experimental and simulated XRD patterns for the two materials are shown in the Fig. S3 and S4.† The main XRD peaks of the prepared bulk materials are consistent with the simulated data, which demonstrates the high purity of prepared compounds.

Fig. 1 (a) The crystal structure of MgV₁₀. Green: vanadium; red: oxygen; pink: magnesium. (b) Expected lithiation process of the MgV₁₀ material.

The morphology and structure of the POVs were studied by SEM, TEM, FTIR and XPS. As shown in Fig. 2a and b, the dehydrated KNaV₁₀ clusters form irregular particles with a size around 1 μm, while the MgV₁₀ demonstrates an interconnected, open and porous structure constructed by numerous nanosheets with a width of about 60 nm (Fig. S5†). The different morphology was associated with the counter cation directed crystallization process.^26,27 The solubility product constant of K⁺ and Na⁺ surpasses that of Mg²⁺, and the crystallization speed of KNaV₁₀ is slower than MgV₁₀. Therefore, MgV₁₀ exhibited a sheet like morphology while KNaV₁₀ displays as particles with a scale of several hundred nm. As we know, the morphology can profoundly affect the electrochemical performance.²⁸ FTIR analyses provided further structural information of the [V₁₀O₂₈]⁶⁻. As shown in Fig. 2c, the strong band located at ∼956 cm⁻¹ could be ascribed to the stretching of the terminal V–O bonds. While the other three bands at around 745, 806, and 845 cm⁻¹ possibly correspond to the bridging antisymmetric vibrations of V–O–V.²¹ The FTIR spectra of KNaV₁₀ and MgV₁₀ are not completely overlapped as observed in Fig. 2c, which suggests that the stretching of the polyanions bands have been deeply influenced by the counter-cations. The XPS of KNaV₁₀ and MgV₁₀ samples feature a couple of peaks located at 517.20 and 524.90 eV, which could be assigned to the V⁵⁺ species from the fitting of the V 2p standard spectra (Fig. 2d).²⁹ The XPS peaks of Na 1s (1071.50 eV), K 2p (292.60 and 295.00 eV),¹⁵ and Mg 1s (1304.70 eV)³⁰ confirm the successful incorporation of these guest cations into the [V₁₀O₂₈]^6– frameworks. Based on these characterizations, the POVs samples of KNaV₁₀ and MgV₁₀ have been successfully synthesized as deigned.

Fig. 2 SEM images of (a) KNaV₁₀ and (b) MgV₁₀. (c) FTIR spectra of the KNaV₁₀ and MgV₁₀ compounds. (d) High-resolution XPS spectra showing the V 2p, K 2p, Na 1s peaks for KNaV₁₀ and V 2p, Mg 1s peaks for MgV₁₀.

The reversible electrochemical performance of POVs has been carefully examined. The CV data for MgV₁₀ reveals the redox properties of the POV during reversible Li⁺ ions insertion/desertion. As shown in Fig. 3a, two redox couples were observed which demonstrated a small voltage separation. The first two cycles of CV curves overlapped well, indicating functional stability as an electrode in LIBs and a higher stability of MgV₁₀ than that of KNaV₁₀ (Fig. S6†). The morphology of electrodes after discharging/charging shown in Fig. S7† also demonstrated that the MgV₁₀ remained stable. Fig. 3b shows the ex situ XPS characterizations of the electrode at different discharge and charge stages. It can be seen that the POV after discharging showed complicated V 2p peaks (highlighted with blue and pink lines), which could be mainly assigned to the V⁴⁺ and V⁵⁺ species from the fitting of the V 2p standard spectra. The presence of the V⁵⁺ species in the discharging state might arise from the incomplete reduction in the POV framework.³¹ Whereas, after charging, the V⁵⁺ species³² increasing clearly and the appearance of the V⁴⁺ species can be attributed to the incomplete oxidation of the V centers, which corresponds to the battery performance. The reversible redox of the V center enables the Li storage in POVs based materials. The intensity of Mg²⁺ ions exhibits no obvious changes in different charging/discharging states. We performed FTIR to further characterize the structure and stability of POVs anions before and after cycles. As shown in Fig. S8 and S9,† the fingerprint spectrum between 800 and 1100 cm⁻¹, which originated from the vibration of the polyanions, experienced no obvious changes. These results demonstrate that the POVs anions remained almost stable during cycles.

Fig. 3 (a) Cyclic voltammograms of MgV₁₀ with a scan rate of 0.1 mV s⁻¹ at the potential range of 3.8–1.0 V. (b) V 2p, Mg 1s, and Li 1s XPS spectra of MgV₁₀ collected at the first discharging (left) and charging (right) states. (c) Galvanostatic discharge/charge profiles and (d, e) the corresponding statistical analysis of the electron number evolution of MgV₁₀ at different cycling stages.

To further study their electrochemical performance, a galvanostatic discharge/charge was carried out to evaluate the Li storage performance. The MgV₁₀ (KNaV₁₀) shows an initial discharging capacity of 198.8 (152.6) mA h g⁻¹ at a current density of 50 mA g⁻¹ (Fig. 3c, Fig. S10†). Accordingly, the theoretical specific capacities³³ of MgV₁₀ and KNaV₁₀ can be calculated as 257.25 and 231.08 mA h g⁻¹, respectively. For the 10 electrons redox reactions,³⁴ the migration of each Li⁺ ion contributes a capacity of around 25.7 (MgV₁₀) and 23.1 (KNaV₁₀) mA h g⁻¹. Therefore, about eight (198.8/25.7)/seven (152.6/23.1) V⁵⁺ centers can be reduced if all of the V species are reduced to V⁴⁺ (Fig. 3d). Whereas, there are about ten (seven) V⁴⁺ centers that undergo electrochemical oxidization during the subsequent charging process for MgV₁₀ (KNaV₁₀). It can be expected, theoretically, that 10-electrons reactions can occur from the 10^th charge process (the theoretical redox electrons are shown in Fig. 3e as dashed lines). With prolonged cycles, the MgV₁₀ can reversible uptake more electrons than that of KNaV₁₀, which reveals that the improved structural stability is brought by the stable Mg²⁺ ions providing support to POV frameworks.

The above discussion disclosed that the POVs underwent a dynamic activation process in the initial 10 cycles. To evaluate the electrochemical properties of the POVs as a cathode material, rate performance and cycling stability were performed. MgV₁₀ shows a higher rate performance than KNaV₁₀ when subjected to various current densities over the voltage range 1.0–3.8 V (Fig. 4a). There is no severe polarization when the MgV₁₀ was subjected to a high charging–discharging current density. The capacity is around 160 mA h g⁻¹ at a high discharge current density of 250 mA g⁻¹, while it is only 118 mA h g⁻¹ for KNaV₁₀. This excellent rate performance could be attributed to the 3D interconnected, porous, and open structured feature (Fig. 2b, Fig. S11†).³⁵ Cycle performance was further conducted at a current density of 50 mA g⁻¹. As shown in Fig. 4b, the MgV₁₀ exhibits a high discharge capacity of around 180 mA h g⁻¹ up to the 60^th cycles. In contrast, the specific capacities of KNaV₁₀ significantly decreased after 10 cycles, and after 60 cycles, the capacity retention is just 63.9% of the 10^th cycle (Fig. S10 and S12†). Such a prominent difference between MgV₁₀ and KNaV₁₀ highlights the effects of the structural stability, large accessible surface area, and more facile active sites in MgV₁₀ that are favorable for fast Li⁺ ions diffusion and storage.

Fig. 4 (a) Schematic illustration of the discharge capacity evolutions of MgV₁₀ and KNaV₁₀ at different current densities. (b) Cycling performance and coulombic efficiency of MgV₁₀ at 50 mA g⁻¹.

Conclusions

In conclusion, we designed and fabricated POVs based electrode materials and systematically investigated the extended stable 3D structure feature for fast Li⁺ ion transportion and storage. The present results indicate that the morphological and structural stability of POVs can be varied through control over the counter-cations. The charging/discharging measurements for the MgV₁₀ indicate a better cyclability and rate performance compared with those of the KNaV₁₀. The structure sustainability and electrochemical properties of POVs can be tuned by the counter-cations. The Mg²⁺ ions remain stable during repeated Li⁺ ion insertion/desertion upon discharge/charge cycles, and appeared as a strong supporting-point to keep the structure stable. This is of vital importance to the design of POVs based electrode materials. Furthermore, this work provides useful guidelines for the POVs based electrode optimization in the future, and enriches the in-depth understanding of the correlations among the chemical make-up, the structure sustainability, and the electrochemical properties of the POMs based electrode for rechargeable batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21603162 and 51671145), Tianjin Sci. & Tech. Program (17JCYBJC21500), major projects of new materials of Tianjin city (16ZXCLGX00120), and the Fundamental Research Funds of Tianjin University of Technology.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00581d

‡ These authors contributed equally.

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