Co3V2O8 Nanoparticles Supported on Reduced Graphene Oxide for Efficient Lithium Storage.

Co3V2O8 (CVO) with high theoretical specific capacity derived from the multiple oxidation states of V and Co is regarded as a potential electrode material for lithium-ion batteries (LIBs). Herein, reduced graphene oxide (rGO)-supported ultrafine CVO (rGO@CVO) nanoparticles are successfully prepared via the hydrothermal and subsequent annealing processes. The CVO supported on 2D rGO nanosheets possess excellent structural compatibility for the accommodation of volume variation to maintain the structural integrity of an electrode during the repeated lithiation/delithiation process. On the other hand, the rGO, as a highly-conductive network in the rGO@CVO composite, facilitates rapid charge transfer to ensure fast reaction kinetics. Moreover, the CV kinetic analysis indicates that the capacity of rGO@CVO is mainly dominated by a pseudocapacitive process with favorable rate capability. As a result, the rGO@CVO composite exhibits improved specific capacity (1132 mAh g−1, 0.1 A g−1) and promising rate capability (482 mAh g−1, 10 A g−1).


Introduction
With the increasing environmental concerns and high demand for sustainable energy, there is great urgency to develop novel materials for high-performance energy storage and conversion [1][2][3]. Lithium-ion batteries (LIBs), as outstanding energy storage devices, have been widely used, owing to their high energy densities, long lifespans and environmental friendliness [4,5]. However, the traditional carbon-based anode shows limited specific capacity, which hinders the energy density of LIBs for further improvement [6][7][8]. Therefore, the investigation of high-efficiency anode materials for LIBs is highly essential for future applications [9][10][11].
Various types of anode materials, including carbonaceous, alloy reaction materials and transition metal oxides, have been widely investigated in field of LIBs [12][13][14]. Metal vanadate materials originated from multivalence of vanadium (V 5+ , V 4+ , V 3+ , V 2+ ) have displayed high electrochemical kinetics [15,16]. Due to the bimetallic synergistic effect between the active constituents Co and V, cobalt vanadates have been regarded as considerable electrode materials in lithium storage [17]. Moreover, the advantages of the high theoretical specific capacity and low cost of metal vanadate have attracted more attention to it for perspective anode materials for LIBs [18,19]. Among them, Co 3 V 2 O 8 (CVO) can absorb 15. 4 Li + during the first discharge process with higher lithium ion storage [19]. However, the unavoidable pulverization and the agglomeration of bulk CVO material during cycling for lithium storage result in poor electrochemical performance and cycle life [20]. To address these weak points, compositing CVO with carbon materials is a common tactic with which to enhance the conductivity and serve as volume buffer matrix for large volume change [21][22][23]. As a typical was homogenously dispersed into NMP solution and then coated on Cu foil, which was dried at 60 °C for 24 h as the working electrode. The charge/discharge tests were performed by the Neware battery testing system in the voltage between 0.01 and 3.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured on the CHI 660E electrochemical workstation (Chenhua Inc., Shanghai, China).

Results and Discussion
X-ray diffraction was performed to characterize the structure and crystallization of the CVO nanoparticles and rGO@CVO composite. As depicted in Figure 1a, the diffraction peaks of the CVO nanoparticles appear at 18.8°, 35.9°, 43.3°, 57.7° and 62.0°, which match well with the (120), (221), (122), (042), (162) and (442) facets of CVO (JCPDS number . The XRD peaks of rGO@CVO composite are similar to those of CVO nanoparticles but with weaker intensity and broader shape, indicating that the addition of rGO could decrease the crystal size of CVO nanoparticles [28,29]. The chemical oxidation of the samples was investigated by XPS measurement. In high-resolution XPS of V 2p spectra (Figure 1b) [30], the major signal of V 2p3/2 two peaks centered at 517.5 and 516.7 eV are ascribed to the V 5+ and V 4+ . The larger peak area of V 5+ indicates the majority of V 5+ in the rGO@CVO composite. In Figure 1c, the detailed Co 2p spectrum possesses two spin-orbit doublets that are characteristic of Co 2+ and Co 3+ . In addition, two characteristic satellites at 803.1 and 786.7 eV indicate the high-spin Co 2+ of the rGO@CVO composite [31]. These results confirm that the mixture of V 5+ , V 4+ , Co 2+ and Co 3+ in the rGO@CVO composite. The C 1s spectrum is divided into three peaks at 289.   the uniform size of about 60-80 nm. The SEM morphologies of rGO@CVO composites are displayed in Figure 2c,d. The rGO nanosheets act as a support skeleton with obvious wrinkles. Furthermore, the high-resolution SEM image shows that ultrafine CVO nanoparticles uniformly cover the surfaces of rGO nanosheets with an average size of about 10-20 nm. The average size of CVO nanoparticles in the rGO@CVO composite is much smaller than that of the pure CVO, which is in accordance with the XRD results. The original SEM image and corresponding EDX mappings reveal the uniform distribution of Co, V and O elements on the surface of rGO in rGO@CVO composite (Figure 2e). Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 10 with the uniform size of about 60-80 nm. The SEM morphologies of rGO@CVO composites are displayed in Figure 2c,d. The rGO nanosheets act as a support skeleton with obvious wrinkles. Furthermore, the high-resolution SEM image shows that ultrafine CVO nanoparticles uniformly cover the surfaces of rGO nanosheets with an average size of about 10-20 nm. The average size of CVO nanoparticles in the rGO@CVO composite is much smaller than that of the pure CVO, which is in accordance with the XRD results. The original SEM image and corresponding EDX mappings reveal the uniform distribution of Co, V and O elements on the surface of rGO in rGO@CVO composite ( Figure 2e).  The morphological and structural characterizations suggest that the hierarchical rGO@CVO composite is prepared successfully via a facile two-step method. Inspired by the combination of the ultrafine structure of CVO nanoparticles and the high conductivity of rGO, the electrochemical property of the rGO@CVO for lithium storage was investigated. Figure 4a exhibits the cycling performance of rGO@CVO and CVO electrodes at the current density of 0.5 A g −1 . The rGO@CVO electrode delivers the specific capacity of 738 mAh g −1 with no obvious decay after 100 cycles. In contrast, the specific capacity of CVO electrode sharply decreases at the first 10 cycles and gradually reduces to only 340 mAh g −1 at the end of 100 cycles. Additionally, after 300 cycles, the rGO@CVO electrode keeps a high reversible capacity of 633 mAh g −1 at high rate of 2 A g −1 (Figure 4b). The favorable cycling performance suggests that the incorporation of CVO nanoparticles with rGO can accommodate volume variations to ensure high reversible capacity and cycling stability. Figure 4c displays the rate capability of rGO@CVO and CVO electrodes. As the current density increases from 0.1 to 10 A g −1 , the specific capacity of rGO@CVO electrode decreases from 965 to 482 mAh g −1 . Nearly 50% capacity retention is achieved at a high rate, which reveals the superior rate performance of the rGO@CVO electrode. Meanwhile, after 100 cycles, the specific capacity gradually increases up to 1132 mAh g −1 at 0.2 A g −1 , which could be ascribed to the full activation process at a high rate. In contrast, the CVO electrode shows a negligible capacity at a high rate (14 mAh g −1 , 10 A g −1 ). As shown in Table  1, the rGO@CVO electrode in lithium storage exhibits considerable electrochemical performance. The promising cycling and rate performance of the rGO@CVO electrode can be attributed to 2D rGO nanosheets possessing excellent structure compatibility to protect the structural integrity of rGO@CVO electrode, thereby maintaining long-term cycling stability during charging and discharging processes. Moreover, the rGO, as a highly-conductive network in the rGO@CVO composite facilitates fast electron and ion transport to ensure a promising rate capability. The morphological and structural characterizations suggest that the hierarchical rGO@CVO composite is prepared successfully via a facile two-step method. Inspired by the combination of the ultrafine structure of CVO nanoparticles and the high conductivity of rGO, the electrochemical property of the rGO@CVO for lithium storage was investigated. Figure 4a exhibits the cycling performance of rGO@CVO and CVO electrodes at the current density of 0.5 A g −1 . The rGO@CVO electrode delivers the specific capacity of 738 mAh g −1 with no obvious decay after 100 cycles. In contrast, the specific capacity of CVO electrode sharply decreases at the first 10 cycles and gradually reduces to only 340 mAh g −1 at the end of 100 cycles. Additionally, after 300 cycles, the rGO@CVO electrode keeps a high reversible capacity of 633 mAh g −1 at high rate of 2 A g −1 (Figure 4b). The favorable cycling performance suggests that the incorporation of CVO nanoparticles with rGO can accommodate volume variations to ensure high reversible capacity and cycling stability. Figure 4c displays the rate capability of rGO@CVO and CVO electrodes. As the current density increases from 0.1 to 10 A g −1 , the specific capacity of rGO@CVO electrode decreases from 965 to 482 mAh g −1 . Nearly 50% capacity retention is achieved at a high rate, which reveals the superior rate performance of the rGO@CVO electrode. Meanwhile, after 100 cycles, the specific capacity gradually increases up to 1132 mAh g −1 at 0.2 A g −1 , which could be ascribed to the full activation process at a high rate. In contrast, the CVO electrode shows a negligible capacity at a high rate (14 mAh g −1 , 10 A g −1 ). As shown in Table 1, the rGO@CVO electrode in lithium storage exhibits considerable electrochemical performance. The promising cycling and rate performance of the rGO@CVO electrode can be attributed to 2D rGO nanosheets possessing excellent structure compatibility to protect the structural integrity of rGO@CVO electrode, thereby maintaining long-term cycling stability during charging and discharging processes. Moreover, the rGO, Nanomaterials 2020, 10, 740 6 of 11 as a highly-conductive network in the rGO@CVO composite facilitates fast electron and ion transport to ensure a promising rate capability.  To further illustrate the favorable rate performance of rGO@CVO electrode, the reaction kinetics of the rGO@CVO electrode were investigated by CV measurement. As illustrated in Figure 5a, the CV curves of rGO@CVO electrode show almost the same trend shape with an increased sweep rate. Generally, the linear relation between i (current density: mA) and v (scan rate: mV s −1 ) can be assigned as follows, by the equation of i = av b , where a and b are constants [25,33]. The slope of the fitted line (log (i) = log(a) + blog(v)) determines the b value, which is applied to analyze the capacitive process of the electrode. If the b value is close to 1, the surface pseudocapacitance-controlled process (interfacial Li + storage) is dominant. The b value approaching to 0.5 represents a dominant diffusioncontrolled electrochemical process [24,34]. Based on anodic and cathodic peaks of rGO@CVO electrode, in Figure 5b, the b values are calculated to be 0.76, 0.93, 0.94, 0.88 and 0.82, respectively. The high b values demonstrate a behavior controlled by pseudocapacitance, further implying fast Na + transport. The surface pseudocapacitive portion of rGO@CVO electrode can be performed  To further illustrate the favorable rate performance of rGO@CVO electrode, the reaction kinetics of the rGO@CVO electrode were investigated by CV measurement. As illustrated in Figure 5a, the CV curves of rGO@CVO electrode show almost the same trend shape with an increased sweep rate. Generally, the linear relation between i (current density: mA) and v (scan rate: mV s −1 ) can be assigned as follows, by the equation of i = av b , where a and b are constants [25,33]. The slope of the fitted line (log (i) = log(a) + blog(v)) determines the b value, which is applied to analyze the capacitive process of the electrode. If the b value is close to 1, the surface pseudocapacitance-controlled process (interfacial Li + storage) is dominant. The b value approaching to 0.5 represents a dominant diffusion-controlled electrochemical process [24,34]. Based on anodic and cathodic peaks of rGO@CVO electrode, in Figure 5b, the b values are calculated to be 0.76, 0.93, 0.94, 0.88 and 0.82, respectively.
The high b values demonstrate a behavior controlled by pseudocapacitance, further implying fast Na + transport. The surface pseudocapacitive portion of rGO@CVO electrode can be performed quantitative calculation according to the equation of i = k 1 υ 1/2 + k 2 υ × (i/υ 1/2 = k 1 + k 2 υ 1/2 ) [24,35]. Typically, at 0.6 mV s −1 (Figure 5c), ≈78.4% of the total capacity in the rGO@CVO electrode comes from the surface's capacitive contribution. Moreover, Figure 5d shows that the surface's pseudocapacitive contribution in the rGO@CVO electrode is obviously improved from 68.9% (0.2 mV s −1 ) to 84.9% (1.0 mV s −1 ). The high ratio of pseudocapacitance contribution can effectively enhance the reversible capacity and rate capability at a high current density for lithium storage.
Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 10 quantitative calculation according to the equation of i = k1υ 1/2 + k2υ × (i/υ 1/2 = k1 + k2υ 1/2 ) [24,35]. Typically, at 0.6 mV s −1 (Figure 5c), ≈78.4% of the total capacity in the rGO@CVO electrode comes from the surface's capacitive contribution. Moreover, Figure 5d shows that the surface's pseudocapacitive contribution in the rGO@CVO electrode is obviously improved from 68.9% (0.2 mV s −1 ) to 84.9% (1.0 mV s −1 ). The high ratio of pseudocapacitance contribution can effectively enhance the reversible capacity and rate capability at a high current density for lithium storage. The EIS measurement was applied to evaluate the reaction kinetics of rGO@CVO composite at the electrode/electrolyte interface for LIBs. In the EIS profile, the semicircle in the high frequency region represents the charge-transfer resistance (Rct), implying the resistance on the electrodeelectrolyte interface [36,37]. As shown in Figure 6a, the Rct values of rGO@CVO and CVO electrodes in pristine state are about 55 and 144 Ω. After 100 charge/discharge cycles, the Rct of rGO@CVO and CVO electrodes increase to 102 Ω and 334 Ω, which could be attributed to the formation of stable SEI film and structural stability (Figure 6b) [38]. The smaller Rct values of pristine and cycled rGO@CVO electrode demonstrate that the rGO@CVO electrode possesses stable structure and good electrical conductivity, revealing the fast electron transfer and reaction kinetics of rGO@CVO electrode. The Na + diffusion coefficient (DNa+) is calculated by the equation of D = R 2 T 2 /(2n 4 A 2 F 4 C 2 b 2 ) [39,40]. The b is the Warburg factor, which is the slope of the line Z' = Re + Rct + bω −1/2 [41]. In the analysis of the equations, the b value dominates DNa+; the smaller the b value, the larger the DNa+. Obviously, the b value of the rGO@CVO electrode is much lower than for the CVO electrode, which demonstrates that the rGO@CVO electrode possesses the larger DNa+ and fast diffusion kinetics to guarantee favorable electrochemical performance at high rate for efficient lithium storage (Figure 6c,d). The EIS measurement was applied to evaluate the reaction kinetics of rGO@CVO composite at the electrode/electrolyte interface for LIBs. In the EIS profile, the semicircle in the high frequency region represents the charge-transfer resistance (R ct ), implying the resistance on the electrode-electrolyte interface [36,37]. As shown in Figure 6a, the R ct values of rGO@CVO and CVO electrodes in pristine state are about 55 and 144 Ω. After 100 charge/discharge cycles, the R ct of rGO@CVO and CVO electrodes increase to 102 Ω and 334 Ω, which could be attributed to the formation of stable SEI film and structural stability (Figure 6b) [38]. The smaller R ct values of pristine and cycled rGO@CVO electrode demonstrate that the rGO@CVO electrode possesses stable structure and good electrical conductivity, revealing the fast electron transfer and reaction kinetics of rGO@CVO electrode. The Na + diffusion coefficient (D Na+ ) is calculated by the equation of D = R 2 T 2 /(2n 4 A 2 F 4 C 2 b 2 ) [39,40]. The b is the Warburg factor, which is the slope of the line Z' = R e + R ct + bω −1/2 [41]. In the analysis of the equations, the b value dominates D Na+ ; the smaller the b value, the larger the D Na+ . Obviously, the b value of the rGO@CVO electrode is much lower than for the CVO electrode, which demonstrates that the rGO@CVO electrode possesses the larger D Na+ and fast diffusion kinetics to guarantee favorable electrochemical performance at high rate for efficient lithium storage (Figure 6c,d).

Conclusion
In summary, we successfully synthesized the hierarchical rGO@CVO composite for lithium storage. The CVO supported on 2D rGO nanosheets possess excellent structural compatibility for accommodation of volumetric variation to maintain structural integrity of electrode during the repeated lithiation/delithiation process. On the other hand, the rGO, as the highly-conductive network in the rGO@CVO composite, facilitates rapid charge transfer to ensure promising rate capability. As expected, the rGO@CVO composite for lithium storage exhibits a highly-reversible capacity (1132 mAh g −1 , 0.1 A g −1 ) and considerable rate performance (482 mAh g −1 , 10 A g −1 ).

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In summary, we successfully synthesized the hierarchical rGO@CVO composite for lithium storage. The CVO supported on 2D rGO nanosheets possess excellent structural compatibility for accommodation of volumetric variation to maintain structural integrity of electrode during the repeated lithiation/delithiation process. On the other hand, the rGO, as the highly-conductive network in the rGO@CVO composite, facilitates rapid charge transfer to ensure promising rate capability. As expected, the rGO@CVO composite for lithium storage exhibits a highly-reversible capacity (1132 mAh g −1 , 0.1 A g −1 ) and considerable rate performance (482 mAh g −1 , 10 A g −1 ).