Amorphous NaVOPO 4 as a High-Rate and Ultrastable Cathode Material for Sodium-Ion Batteries

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Oct 2021Amorphous NaVOPO4 as a High-Rate and Ultrastable Cathode Material for Sodium-Ion Batteries Yongjin Fang, Jiexin Zhang, Faping Zhong, Xiangming Feng, Weihua Chen, Xinping Ai, Hanxi Yang and Yuliang Cao Yongjin Fang College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072 Google Scholar More articles by this author , Jiexin Zhang College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072 Google Scholar More articles by this author , Faping Zhong National Engineering Research Center of Advanced Energy Storage Materials, Changsha 410205 Google Scholar More articles by this author , Xiangming Feng College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Weihua Chen College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Xinping Ai College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072 Google Scholar More articles by this author , Hanxi Yang College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072 Google Scholar More articles by this author and Yuliang Cao *Corresponding author. E-mail Address: [email protected] College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000520 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail The low cost and profusion of sodium resources make sodium-ion batteries (SIBs) a potential alternative to lithium-ion batteries for grid-scale energy storage applications. However, the use of conventional cathode materials for Na-ion intercalation/deintercalation cannot satisfy the requirements of high-powered and long lifespan performance due to multiphase transition and lattice confinement. Herein, we demonstrate that amorphous NaVOPO4 incorporated reduced graphene oxide hybrid in a SIB is a superior cathode with high-rate performance and stable cycle life. Remarkably, the electrode exhibits high voltage (∼3.5 V vs Na/Na+), high reversible capacity (110 mAh g−1 at 0.05 C), and remarkable cyclability with 96% capacity retention over 2000 cycles. This outstanding performance originates from its amorphous characteristics and the unique hierarchical microflower structure of the hybrid, which undergo a distinct single-phase-like redox reaction without lattice restriction during charge/discharge processes, thus accelerating the intercalation/deintercalation of large-sized Na+ ions and maintaining the integrity and stability of the microflower cathode. This amorphous cathode material with fast ion migration and high structural stability may open a new avenue for exploring advanced electrode materials for efficient sodium storage. Download figure Download PowerPoint Introduction Fast-growing renewable energy technology has stimulated the continuous development of low-cost, affordable, and efficient technology for large-scale electric energy storage. Sodium-ion batteries (SIBs) have drawn growing attention due to the abundance of inexpensive sodium (Na) resources and their similar chemical/electrochemical characteristics to established lithium-ion battery technology.1–6 Over the last decade, tremendous efforts have been devoted to develop high-performance electrode materials for SIBs.7–9 As cathode materials, metal oxides,10–15 polyanions,16–24 and ferricyanides25–27 have been extensively investigated, and they have shown decent Na-storage performance. However, the huge volume/phase variation during (de)sodiation, slow diffusion of Na+ ions in the zigzag lattice frameworks, and insufficient electronic conductivity of these cathode materials have only led to moderate electrochemical properties and sluggish kinetics for Na storage. Exploring cathode materials with rational structure and facile Na-storage processes should be one of the most effective methods to optimize the electrochemical performance of SIBs. Amorphous materials are regarded as promising electrodes considering their abundant active sites and lack of lattice restriction during repeated sodiation/desodiation reactions of large-sized Na+ ions in the amorphous framework.28 Fang et al.29 synthesized carbon-decorated mesoporous amorphous FePO4, which maintained amorphous characteristics at varied states of charge (SOC), leading to long-term cycling stability. This work confirms the efficient Na-storage performance of amorphous structures and their potential application in cathode materials. However, the low working voltage of the FePO4 material results in a low energy density. Also, the Na-vacant characteristic is not convenient and economical, as the Na-deficient material will have to be presodiated to form a Na-containing cathode for practical application. As far as we know, few Na-containing amorphous cathodes have been reported for Na storage because of the metastable characteristic of amorphous materials. Therefore, finding Na-containing amorphous structures with satisfactory Na-storage remains a huge challenge. Herein, for the first time, we propose fabrication of a novel Na-containing amorphous cathode, amorphous NaVOPO4 dispersed on reduced graphene oxide (designated as a[email protected]), using a facile reflux reduction method. The a[email protected] cathode exhibits high voltage (∼3.5 V vs Na/Na+), high reversible capacity (110 mAh g−1 at 0.05 C), and remarkable cyclability with 96 % capacity retention after 2000 cycles. Significantly, ex situ X-ray diffraction (XRD) analysis reveals that the a[email protected] electrode maintains amorphous properties at different SOC, which results in superior rate capability and long-term stability. Furthermore, the pseudocapacitance accounts for a notable fraction of the total capacity in the a[email protected] cathode, demonstrating accelerated sodiation/desodiation processes. Materials and Methods Sample preparation VOPO4·2H2O microplates were first prepared through a reflux method. Typically, V2O5 (4.8 g), H3PO4 (85% 26.6 mL), and H2O (115.4 mL) were refluxed at 115 °C for 20 h. The yellow precipitate was harvested by filtration and washed several times with water and acetone. The resulting sample was dried in an oven at 40 °C for 8 h. To obtain the a-[email protected] microflowers, the as-prepared VOPO4·2H2O microplates (0.5 g) were dissolved into 70 mL of 2 mg mL−1 graphene oxide solution by sonication for 30 min. Next, the mixture was stirred for 0.5 h at 80 °C. Then, 80 mL of 0.35 M NaI ethanol solution was rapidly added to the above solution. The solution was kept under stirring for 5 h. The final product was harvested by centrifugation and washed sequentially with water and ethanol several times. Then the resulting sample was calcinated in Ar at 200 °C for 8 h to obtain the a-[email protected] For comparison, the graphene-free NaVOPO4 microflowers (designed as a-NVOP) were prepared via a similar method by replacing the graphene oxide solution with deionized water. Materials characterization XRD patterns were examined using a Shimadzu XRD-6000 (Shimadzu Co., Kyoto, Japan) diffractometer with Cu Kα. The diffraction data were recorded in the 2θ range of 10–80° with a scan rate of 2° min−1. The morphology and structure of the samples were examined by field-emission scanning electron microscopy (FESEM; ZEISS Merlin Compact; ZEISS, Oberkochen, Germany) and transmission electron microscopy (TEM; JEM-2100; JEOL Ltd., Tokyo, Japan). The composition of the samples and elemental mapping images were analyzed by energy-dispersive X-ray spectroscopy (EDX) attached to the FESEM instrument. Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw inVIA; Renishaw, Wotton-under-Edge, United Kingdom; 532 nm excitation wavelength). The X-ray photoelectron spectroscopy (XPS) measurement of the samples was carried out on an ESCALAB250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA). Electrochemical measurements Electrochemical characterization was carried out using 2016-type coin cells. The working electrodes were prepared by applying the paste containing 80 wt % of active material, 10 wt % of acetylene black, and 10 wt % of poly(vinyl difluoride) (PVdF) binder onto an aluminum foil. The electrolyte was 1.0 mol L–1 NaClO4 dissolved in an ethylene carbonate/diethyl carbonate (EC/DEC, 1∶1 by vol.) solution. Na thin disks were used as anodes. All the cells were assembled in a glovebox with water/oxygen content <0.5 ppm and were tested at room temperature. The galvanostatic charge–discharge tests were conducted on a Neware Cycler CT-4008 (Shenzhen Neware Electronics Co., Shenzhen, China). Cyclic voltammetric measurements were performed on a CHI 660a electrochemical workstation (CH Instruments Co., Shanghai, China). The electrochemical impedance measurements were carried out over the frequency range of 100 kHz to 10 mHz. Results and Discussion The a[email protected] microflowers were prepared via a facile reflux reduction reaction between VOPO4·2H2O and NaI. Briefly, the VOPO4·2H2O precursor prepared by a refluxing method was dispersed in a graphene oxide solution, followed by an in situ reduction process with NaI to obtain the a[email protected] (Figure 1a). During the reaction, the NaI acted as an efficient reductant to reduce the V5+ into V4+ and graphene oxide to reduced graphene oxide simultaneously, and the Na+ ions were inserted into active sites via charge compensation to obtain the Na-containing phase. The formation of the microflower structure may be due to Ostwald’s ripening process.30,31 For comparison, the graphene-free amorphous NaVOPO4 (designated as a-NVOP) microflowers were prepared via a similar method by replacing the graphene oxide solution with deionized water. Figure 1 | (a) Schematic illustration of the synthesis of a[email protected] (b) XRD patterns and (c) Raman spectra of the a-NVOP and a[email protected] microflowers. Download figure Download PowerPoint The XRD peaks of VOPO4·2H2O precursor were assigned to the crystalline tetragonal phase ( Supporting Information Figure S1). And typical SEM images ( Supporting Information Figure S2) showed that the VOPO4·2H2O exhibited coin-like flakes with a typical tightly stacked layered structure. The a-NVOP and a[email protected] microflowers were characterized by powder XRD analysis, as shown in Figure 1b. Interestingly, both XRD patterns had no signals of crystalline diffraction peaks, indicating their amorphous properties. The EDX analysis ( Supporting Information Figure S3) reveals that the a[email protected] sample was composed of Na, V, O, P, and C elements with a Na/V atomic ratio of ca. 0.87 and carbon content of 22.3 wt %. It should be noted that the a[email protected] sample was very stable and maintained an amorphous state even after being calcinated at 400 °C in the Ar atmosphere ( Supporting Information Figure S4). At a higher calcination temperature (>500 °C), the sample became crystallized and gradually transformed into NASICON Na3V2(PO4)3. Raman spectroscopy was conducted to study the surface properties of the a-NVOP and a[email protected] microflowers (Figure 1c). Both samples exhibited a series of peaks near 540 and 950 cm−1, which could be assigned to the symmetric bending modes and stretching vibrations of PO4, respectively.32,33 The bands around 1100 and 750 cm−1 might belong to external modes from V=O bonds.32,34 For the a[email protected] composite, wide and robust bands located at 1356 and 1605 cm−1 were observed, corresponding to the D and G bands of reduced graphene oxide. Besides, the inconspicuous peaks at 2692, 2928, and 3170 cm−1 were generally ascribed as the 2D, D + G, and 2D′ bands of reduced graphene oxide, respectively. The D and G signals indicated the successful reduction of graphene oxide by NaI. Besides, based on the Brunauer–Emmett–Teller (BET) analysis ( Supporting Information Figure S5), the a[email protected] microflowers exhibited a higher surface area (66.5 m2 g−1) than the a-NVOP microflowers (41.9 m2 g−1). Panoramic SEM images demonstrated that the a[email protected] had a grape cluster-like structure (Figure 2a and Supporting Information Figure S6a). During the synthesis, graphene oxide (designated as GO) with functional groups (such as –OH and –COOH) adsorbed reactant ions. As a result, the NaxVOPO4 microflowers that formed were uniformly dispersed on the rGO, resulting in interconnected “grape cluster”-like structures. Enlarged SEM images indicated that each “grape cluster” possessed a microflower-like structure with an average diameter of 1 μm (Figure 2b and Supporting Information Figure S6b). The microflowers were made of randomly assembled ultrathin nanosheets (Figure 2c and Supporting Information Figures S6c and S6d). The microflower structure could be further clearly confirmed by the TEM observation (Figure 2d). The enlarged TEM images verified the nanosheet structure and the incorporation of NaVOPO4 with reduced graphene oxide (Figure 2e and Supporting Information Figures S7a–S7d). The high-resolution TEM (HRTEM) image (Figure 2f) revealed the amorphous characteristic without a discernible lattice fringe. Elemental mappings of an individual a[email protected] “grape cluster” demonstrated that Na, V, O, P, and C elements were uniformly distributed throughout the microstructure (Figure 2g). The a-NVOP material also exhibited a similar nanosheet-assembled microflower structure ( Supporting Information Figure S8). Figure 2 | (a–c) SEM images, (d and e) TEM images, and (f) HRTEM image of the a[email protected] microflowers. (g) Typical SEM image and the elemental mapping images of the a[email protected] microflowers. Download figure Download PowerPoint The Na-storage properties of the a[email protected] microflowers were investigated by assembling coin cells. Figure 3a demonstrates the typical cyclic voltammetry (CV) curves of the a[email protected] microflower electrode at a scan rate of 0.1 mV s−1. The a[email protected] microflower electrode exhibited a couple of broad current peaks located at 3.55 and 3.75 V, respectively, which corresponds to the redox of V4+/V5+. The broad current peaks indicated a single-phase-like redox reaction for a[email protected] microflower electrode, which was very different from the crystalline NaVOPO4 materials with multiple phase transitions during the electrochemical reaction.34 The CV curves from the second to fifth cycles were well overlapped, suggesting superb electrochemical reversibility. Figure 3b displays the typical galvanostatic charge–discharge curves of the a[email protected] microflower electrode at a current rate of 0.05 C (1 C = 145 mA g−1). The a[email protected] microflower electrode exhibited an average working potential of 3.5 V, which was higher than that of NaFePO4 (2.8 V),35 Na3V2(PO4)3 (3.3 V),33 ferrocyanides (3.3 V),36 and most layered metal oxide cathodes (3.0 V).37 The electrode demonstrated a discharge capacity of 110 mAh g−1, corresponding to a high energy density of 385 Wh kg−1 based on the mass of the active material for Na/a[email protected] cell. The a[email protected] exhibited an initial Coulombic efficiency of 89%, which may have resulted from the irreversible decomposition of electrolyte during the charge processes and the formation of the cathode–electrolyte interface layer.34 Impressively, the charge–discharge voltage profiles exhibited a monotonous voltage slope in the 3–4 V window, which could be helpful for easily manipulating the SOC of batteries. The monotonous voltage slope demonstrated a single-phase-like insertion process of Na+ ions in an amorphous structure. The a[email protected] microflower electrode maintained the stable capacities with almost overlapped charge–discharge curves from the second to fifth cycles (Figure 3b), demonstrating high electrochemical reversibility of Na+ ion intercalation/deintercalation in an amorphous structure. Figure 3 | (a) The cyclic voltammograms of the a[email protected] electrode at a scan rate of 0.1 mV s−1. (b) Typical charge–discharge curves of the a[email protected] electrode at 0.05 C. (c) Rate performance of the a[email protected] electrode and (d) the corresponding charge–discharge curves at various current rates. (e) Cycling performance of the a[email protected] electrode at 5 C. Download figure Download PowerPoint The rate capability of the a[email protected] electrode was investigated by galvanostatic charge–discharge at various current rates. As shown in Figure 3c, the electrode can deliver an average reversible capacity of 108, 90, 81, 70, 65, 58, and 50 mAh g−1 at 0.05, 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively. Even at a notable current density of 10 C, a discharge capacity of 38 mAh g−1 can still be achieved. The average discharge potential of the a[email protected] microflower electrode dropped slightly as the current rate increased to 1 C (Figure 3d), and could still maintain an average voltage of 3.2 V even at a high current rate of 5 C. In vast contrast, the capacity of the a-NVOP microflowers dropped rapidly with the increase of current rate ( Supporting Information Figure S9 and Table S1), which was likely due to the higher charge-transfer resistance and lower Na+ diffusion coefficient compared with that of the a[email protected] electrode ( Supporting Information Figures S10 and S11 and Table S2). Moreover, the a[email protected] also exhibited good cycling stability. Specifically, the a[email protected] electrode demonstrated high-capacity retention of 96% over 2000 cycles at 5 C (Figure 3e). The electrode exhibited small polarization during prolonged cycling, indicating the excellent reversibility and stability of the a[email protected] electrode ( Supporting Information Figure S12). At other current rates ( Supporting Information Figure S13), the electrode also exhibited excellent cycling stability (with >70% capacity retention over 2000 cycles). The cycling performance of the a[email protected] microflowers was superior to that of other reported cathode materials, such as crystalline NaVOPO4,34,38–40 metal oxides,41 ferricyanides,42 and some of the polyanions.28 Moreover, the hierarchical microflower structure composed of ultrathin nanosheets of a-NVOP and a[email protected] can be well preserved after a prolonged cycling test ( Supporting Information Figure S14). These results suggest that the amorphous structure is responsible for the enhanced Na-storage properties of the a[email protected] microflower electrode, which avoids the lattice confinement of crystal frameworks, thus effectively accommodating large Na+ ions. Ex situ XPS experiments were performed to reveal the sodiation/desodiation mechanism. We tested the V-element states of the as-prepared a[email protected] microflowers (Figure 4a) as well as the charged and discharged states of the electrode. The electrochemical reactions were associated with the V4+/V5+ reduction. As shown in Figure 4b, the as-prepared a[email protected] microflowers displayed a peak located at a binding energy of 516.6 eV, which corresponds to V4+.43,44 When charged to 4.2 V, the peak showed a distinct shift to 517.4 eV, representing the formation of the oxidation state of V5+.44,45 Upon discharge to 2 V, the XPS peak recovered its initial position. These results further showed that the Na+ ion insertion/extraction processes are highly reversible. Ex situ XRD measurements were also carried out on the a[email protected] microflower electrodes at different SOC (Figures 4c and 4d). Interestingly, at different SOC, the a[email protected] microflower electrodes demonstrated similar XRD patterns without new peaks emerging, indicating that the a[email protected] microflower electrode maintained its amorphous state during the whole charge–discharge reaction, thereby resulting in ultrastable cycling life. Figure 4 | XPS analysis of the (a) a-[email protected] sample and (b) a-[email protected] electrodes at the different charge and discharge states. (c) Typical charge–discharge voltage profiles of the a-[email protected] electrodes. The marks of (i)–(v) indicate the depths of charge and discharge, at which the electrodes are taken for ex situ XRD testing. (d) XRD patterns of the a-[email protected] electrodes at different SOC. Download figure Download PowerPoint To further understand the Na-storage reaction of the a[email protected] microflowers, a kinetic study based on CV testing at different scan rates was performed (Figure 5a). The electrode exhibited similar CV curves with wide redox peaks during both anodic and cathodic processes at various scan rates from 0.1 to 1 mV s−1. The measured peak current (i) and the scan rate (v) obeys the power-law relationship: i = avb, where a and b are the fitting parameters. By determining both the a and b contents, the portion of the current from capacitance and diffusion contributions was differentiated. Specifically, a b value of 0.5 suggests a diffusion-controlled characteristic, whereas a b value of 1.0 implies that the capacitance dominates the charge–discharge process.46 From Figure 5b, the b values are determined to be 0.78 and 0.71 for anodic and cathodic peaks, respectively, suggesting the kinetics of capacitance-dominated characteristics. The total capacitance contribution could be quantified by separating the detailed contribution from the capacitive and diffusion-controlled charge at a fixed potential. For example, at a scan rate of 0.8 mV s−1, the capacitance-controlled charge exists in all ranges, and 60.8% of the total charge is capacitance-controlled (Figure 5c). The contribution ratios between the two different behaviors at other scan rates were also calculated (Figure 5d). The results indicated that the capacitive contribution increases gradually with the elevating of scan rates and finally attains a high value of 63.4% at 1 mV s−1. The origin of the high-rate capability and pseudocapacitive behavior of the a[email protected] might be ascribed to the structural features with a hierarchical microflower structure composed of ultrathin amorphous nanosheets.47 Figure 5 | (a) The CV of the a[email protected] electrode at various scan rates. (b) Determination of the b values. (c) Separation of the capacitive and diffusion currents in the a[email protected] electrode at a scan rate of 0.8 mV s−1 with the capacitive portion shown by the shaded region. (d) Relative contribution ratio of the capacitive and diffusion-controlled charge storage at different scan rates. Download figure Download PowerPoint Conclusion A novel a[email protected] microflower with an outstanding electrochemical performance for SIBs is reported. The cathode exhibits high voltage (∼3.5 V vs Na/Na+), high reversible capacity (110 mAh g−1 at 0.05 C), long lifespan (96% capacity retention ratio after 2000 cycles), and high-rate performance (38 mAh g−1 at 10 C). The electrode demonstrates a capacitance-controlled characteristic and maintains amorphous states throughout the whole charge–discharge process, that is, single-phase-like sodiation/desodiation. We believe that the study on this novel amorphous NaVOPO4 cathode with its fascinating electrochemical properties and unique Na-storage mechanism can open a new avenue for cathode performance optimization, especially for the polyanion-type cathode, promoting the future commercialization of SIBs. Supporting Information Supporting Information is available. Acknowledgments The authors thank the National Key Research Program of China (no. 2016YFB0901500), the National Science Foundation of China (nos. 21673165 and 2197210821333007), and the supercomputing system in the Supercomputing Center of Wuhan University for their financial support.