Half-cell and full-cell applications of horizontally aligned reduced oxide graphene/V₂O₅ sheets as cathodes for high stability lithium-ion batteries

Abstract

2D hybrid sheets of V₂O₅ and reduced graphene oxide (rGO/V₂O₅) have been synthesized using a hydrothermal method. With the addition of tiny amount of rGO content (6.25%), the proper conductivity and accessibility of hybrid nanosheets for lithium ion migration are obviously promoted. Compared with the bare V₂O₅ sheets, the rGO/V₂O₅ hybrid sheets exhibited enhanced electrochemical performance in terms of improved cycling stability (nearly 81.5% capacity retention after 200 cycles), high reversible capacity of 249 mA h g⁻¹ at a current density of 100 mA g⁻¹ and good rate capability for the half cell. Significantly, a Li-metal-free full battery is assembled successfully, with rGO/V₂O₅ hybrid sheets and lithiated graphite for cathodes and anodes, respectively. This full Li-ion battery exhibits a superior specific capacity of 213 mA h g⁻¹ at a current density of 100 mA g⁻¹ and a high specific energy density of 106.5 W h kg⁻¹ as a result. The good electrochemical performance suggests that this unique 2D hybrid material could be a promising candidate as a cathode material for lithium-ion batteries in the near future.

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1. Introduction

As far as two-dimensional (2D) structural materials are concerned, their unique structural and surface properties endow them with more potential in catalysis and energy devices,^1,2 because compared to one-dimensional materials, they can obviously increase the transmission rate of electrons and easily be used to fabricate complex structures.³ For example, MoS₂, one of the transition-metal dichalcogenides, exhibits unique physical, optical and electrical properties correlated with its 2D ultrathin atomic layer structure.^4–6 Graphene is another attractive material due to its unique 2D atomic thickness structure which leads to various extraordinary properties.^7–10 The experimental discovery of 2D gated graphene in 2004 by Novoselov et al. is a seminal event in electronic materials science, causing a huge outburst of research activity in the study of the electronic properties of graphene. A large number of its material parameters have been found, such as mechanical stiffness, strength and elasticity, very high electrical and thermal conductivity, and many others.^11,12

Vanadium oxides, especially vanadium pentoxide (V₂O₅), have been extensively studied as high capacity cathode materials for lithium-ion batteries (LIBs) in the past decades.^13–15 The theoretical capacity of V₂O₅ with two Li intercalations/de-intercalations is about 294 mA h g⁻¹, and even only one Li⁺ ion insertion/extraction per unit formula, it also can deliver a comparable capacity (147 mA h g⁻¹) compared to the commercialized cathode materials such as LiCoO₂ (140 mA h g⁻¹) and LiMn₂O₄ (146 mA h g⁻¹). However, the practical application of V₂O₅ as a cathode material has been hindered by its low ionic diffusivity (10⁻¹² to 10⁻¹³ cm² s⁻¹)^16,17 and moderate electrical conductivity (10⁻² to 10⁻³ S cm⁻¹).^18,19 Various nanostructured V₂O₅ materials such as 1D nanotubes/wires/rods^20–24 and hollow/porous spheres^25–32 have been synthesized to overcome their kinetic limitation by shortening Li⁺ ion diffusion distance and increased surface area. Currently, an effective approach to enhance the electrochemical performance is to hybridize nanostructured V₂O₅ with carbonaceous materials.^33–36 Since the hybridization of carbon can increase electrical conductivity, prevent the vanadium dissolution, and alleviate the aggregation of the particles.

In terms of LIBs system, the use of lithium-metal anode is well known to have some critical defects including chemical reactivity in commonly used organic electrolytes and the dendritic growth of lithium during long cycles. As a result, poor cycling and safety performance can be easily noted in LIBs. Recently, carbon-type anode materials have been suggested as alternatives to replace the lithium-metal anode.³⁷ Nevertheless, it is a major challenge to effectively combine carbon-type anodes with Li-metal-free cathodes to form a full cell for carbon-type anodes can only couple with lithium metal oxide cathodes.

In this work, we demonstrate a facile and green method to prepare 2D hybrid sheets of V₂O₅ and reduced graphene oxide (rGO), which was applied to the half and full cell applications. The abundant oxygen functional groups in GO act as anchoring spots for the growth of V₂O₅, the reduction of GO to rGO can be simultaneously achieved (Fig. 1).³⁸ When evaluated as a cathode material for LIBs, the V₂O₅ sheets decorated by only a small quantity of rGO manifest significantly improved electrochemical performance in terms of specific capacity, cycling stability, and rate capability. What's more, we also successfully construct full cell system with carbon-type anodes instead of traditional lithium-metal anode, demonstrating the potential application of this cathode materials.

Fig. 1 Schematics of the fabrication process of rGO/V₂O₅ hybrid sheets.

2. Experimental section

Preparation of rGO/V₂O₅ hybrid sheets

Graphite oxide was prepared from natural graphite (SP-1) by a modified Hummers method.³⁹ V₂O₅ was obtained by the dehydration of HVO₃, which was produced by an ion exchange process that used ion exchange resin to replace the Na⁺ in NaVO₃ by H⁺.⁴⁰ The rGO/V₂O₅ hybrid sheets were prepared from the hydrothermal method at 180 °C for 2.5 h, and then annealed at 400 °C under Ar gas flow (Fig. 1).

Preparation of rGO

Dried graphite oxide (90 mg) was suspended in distilled water (90 mL) with strong stirring and ultrasonication at least for 1 h. Reduction was carried out by hydrazine hydrate (the weight ratio of hydrazine hydrate/GO = 1) at 95 °C for 6 hours. Stirring and intermittent ultrasonication should be used throughout the whole preparation process. Upon completion, the rGO powder was immoderately isolated via filtration, and washed with distilled water for four times, then dried at 60 °C for 24 h to remove residual solvent.

Structure characterization

The microstructure and morphology of the hybrid sheets were investigated by field emission scanning electron microscopy (FESEM, SU8010, Japan) and field emission transmission electron microscopy (FETEM, FEI Tecnai G220, America). Surface elemental analysis was performed on an X-ray photoelectron spectrometer (XPS, Kratos Axis Ultra Dld, Japan).

Characterization. The working electrode slurry was prepared by dispersing rGO/V₂O₅, acetylene black and polytetrafluoroethylene (PTFE; 60 wt% dispersion in water) in ethanol with a weight ratio of 7 : 2 : 1. The slurry was spread on titanium foil disks and dried in a vacuum oven at 60 °C overnight prior to coin cells assembly. Lithium foil was used as the counter and reference electrode, and 1.0 M LiPF₆ in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate (4 : 3 : 3 vol%) was used as the electrolyte. The assembly of the cell was conducted in an Ar-filled glove box followed by a six hours aging treatment before the test. Cyclic voltammetry (CV) measurement was conducted at 0.1 mV s⁻¹ within the range of 2–4 V on a CHI 660E (Chenhua Shanghai, China) electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was also performed on this electrochemical workstation over the frequency range from 100 kHz to 0.01 Hz. The cycle life and rate capability of the cells were tested within a voltage window of 2–4 V (vs. Li⁺/Li) by using a battery testing system (LAND CT 2001A, Wuhan, China) at room temperature.

The full cells were assembled by coupling in a LIR2025 coin-type cell and galvanostatic tests were carried out at a current density of 900 mA g⁻¹ within 1.5 and 3.5 V (vs. excess graphite weight). The graphite loading was 2 mg cm⁻² and the rGO/V₂O₅ nanosheets loading was 1 mg cm⁻² that the capacity ratio of negative and positive was calculated to be 2. Prior to being used in full cells, the graphite-based anode was pre-lithiated for 6 h by directly pressing it against lithium metal wetted by the LiPF₆ electrolyte. The energy density (E) of the samples was calculated using the formula, E = 1/4CV, where C is the capacity (mA h g⁻¹) and V is the average working potential (V) of the electrode. The calculation of the corresponding power density (P) of the samples follows like that, P = E/Δt, where E is the energy density and Δt (h) is the discharge time.

3. Results and Discussion

To investigate the morphology of the 2D hybrid sheets of rGO/V₂O₅, SEM and TEM images were performed and results are shown in Fig. 2. Top view SEM image of the hybrid sheets in Fig. 2A reveals a wrinkled surface, which can increase the accessible surface area for ion interaction from the electrolyte. Fig. 2B shows the cross-section SEM image of rGO/V₂O₅, it can be found from the high resolution image that the composite are actually consisted of well-defined layers with the thickness of ∼30 nm for each individual sheet. Scanning electron microscopy mapping analysis of rGO/V₂O₅ unambiguously confirms the homogeneous existence of V (Fig. 2D), O (Fig. 2E) and C (Fig. 2F) elements. The lattice fringes are clearly displayed by the high resolution TEM image (Fig. 2G) with a spacing of 0.26 nm corresponding to the d-spacing of (310) plane of rGO/V₂O₅.

Fig. 2 (A) Top view SEM image of rGO/V₂O₅ hybrid sheets. (B) The cross-section SEM image and high-resolution SEM image of rGO/V₂O₅ (C) SEM image and corresponding elemental mapping of (D) vanadium, (E) oxygen, and (F) carbon, indicating the homogeneous dispersion of V, O, and C in all of the hybrid sheets. (G) TEM image collected at the edge of the hybrid sheets.

XPS spectra of the hybrid sheets were investigated as shown in Fig. 3A. In the survey region from 200 to 600 eV, it was evident that V, O, and C elements all exist in the sample. From the more careful inspection of higher-resolution spectra and elemental analysis, two peaks are found, which assigned to V 2p_3/2 and V 2p_1/2 at the binding energy of 517.52 eV and 524.62 eV, respectively (Fig. 3B), implying the formation of the V₂O₅ phase in the nanocomposite matrix.⁴¹ Furthermore, a detailed analysis of the C 1s region indicated that it can be further divided into three peaks, which were located at 284.7, 286.0 and 288.8 eV and assigned to C–C/C=C, C (epoxy and alkoxy), and C=O groups (Fig. 3C).⁴² The O 1s peak located at 530.2 eV assigned to V–O is attributed to the oxygen ions in V₂O₅.⁴³ The O 1s peak located at 531.8 eV assigned to H–O is attributed to the oxygen ions in rGO (Fig. 3D). Therefore, the XPS results further revealed that the hybrid sheets are indeed composed of rGO and V₂O₅.

Fig. 3 (A) Full XPS spectra of rGO/V₂O₅ hybrid sheets. The high resolution (B) V 2p, (C) C 1s and (D) O 1s XPS spectra.

Fig. 4A presents the CV curves of rGO/V₂O₅ hybrid sheets electrode for the first cycle. To avoid irreversible formation of ω-phase Li_xV₂O₅ (x > 2), a potential window ranging from 2 to 4 V (vs. Li/Li⁺) was applied at a scan rate of 1 mV s⁻¹. Three pairs of redox peaks at 2.7, 2.9, and 3.8 V in anodic sweep, 3.7, 2.8 (R1), and 2.4 V (R3) in cathodic sweep distinguished from CVs (Fig. 4A) can be assigned to the different stages during lithium reversibly intercalating into V₂O₅ according to V₂O₅ + xLi⁺ + xe⁻ ↔ Li_xV₂O₅.⁴⁶ More specifically, the R1, R2, and R3 reduction peaks are recognized as the continuous formation of different lithiated-V₂O₅ phases, i.e., γ-Li_xV₂O₅ (0.35 < x < 0.7), δ-Li_xV₂O₅ (x = 1), and ε-Li_xV₂O₅ (1 < x < 3), respectively.⁴⁹

Fig. 4 (A) Representative CV curves of an electrode based on the rGO/V₂O₅ obtained at a voltage range of 2 to 4 V (vs. Li⁺/Li) and potential scan rate of 1 mV s⁻¹. (B) Voltage profiles plotted for the first, second, fifth, tenth, 30th cycles at a current density of 100 mA g⁻¹. (C) Charge/discharge capacities and the efficiency of the prepared coin cell at a current density of 1500 mA g⁻¹. (D) Charge/discharge capacity at various rates for 35 cycles.

Galvanostatic charge/discharge measurements are applied to discover more electrochemical properties. The individual 1st, 2nd, 5th, 10th and 30th cycle charge/discharge curves of the rGO/V₂O₅ hybrid sheets at a low current density of 100 mA g⁻¹ in the voltage window of 2–4 V are shown in Fig. 4B. Good reversible plateau regions can be observed and the discharge/charge plateaus agree well with the peaks shown in the CV curves (Fig. 4A). The three plateaus observed in the discharge curves at 3.7, 2.8, and 2.5 V indicate the multi-step Li⁺ ion intercalation process. A high initial discharge capacity of ∼249 mA h g⁻¹ is obtained within the voltage range. As is shown in Fig. 4B, the discharge and charge plateaus are generally remain stable over the repeated cycles, which indicate the good structural reversibility of the hybrid sheets of rGO/V₂O₅.

Effect of rGO dopant is investigated by doping with different amounts of rGO into V₂O₅. The cycle performances and capacity of doped V₂O₅ are compared with the pure V₂O₅ at a current density of 300 mA g⁻¹. Fig. 4C, with the rGO content increased, the hybrid sheets electrodes are more stable toward cycle performance but inferior toward capacity behavior. It is obvious that the optimal loading of 6.25% (with the rGO contents of 6.25 wt%) rGO/V₂O₅ delivers a larger capacity of 220 mA h g⁻¹ and 97.8% capacity retention after 30 cycles. These results further prove that V₂O₅ and rGO have the effect at a proper of synergistically enhancing the electrochemical activities.

Fig. 4D shows the rate performance of the rGO/V₂O₅ hybrid sheets. The rGO/V₂O₅ composite performs discharge capacities of 249, 212, 170, 143 and 113 mA h g⁻¹ for five cycles at current densities of 100, 300, 600, 900, 1200 and 1500 mA g⁻¹, respectively. When the current density comes back to 100 mA g⁻¹, the discharge capacity still remained 220 mA h g⁻¹. The above results show that the 2D rGO/V₂O₅ hybrid sheets structure effectively reduces the diffusion length for lithium ions and enables the high rate performance of LIBs.

CV tests of the lithium-ion battery with rGO/V₂O₅ at 0.1, 0.2, 0.5, 0.8 and 1 mV s⁻¹ in a voltage range of 2–4 V vs. Li⁺/Li, are displayed in Fig. 5A, from which the increased cathode–anode peak differences with the increasing of scan rates indicate an enlarged irreversibility at high current densities. As can be seen from Fig. 5B, the peak current density of the intensive cathodic/anodic reaction i_p is proportional to the square root of the sweep rate v^1/2, which shows the linear semi-infinite diffusion in cathodic and anodic processes. Consequently, the Randles–Sevcik equation (eqn (1)) can be applied,^44,48 based on which the Li⁺ ion diffusion coefficient D_Li⁺ can be calculated.

i_p/m = 0.4463n^1.5F^1.5CSR^−0.5T^−0.5D^0.5v^0.5 (1)

where m is the mass of active cathode material (g), n is the number of electrons in reaction, F is the Faraday constant (96 485 C mol⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature (K), S is the surface area per unit weight of active materials, D is the diffusion constant (cm² s⁻¹), v the rate at which the potential is swept (V s⁻¹) and C is the concentration in mol cm⁻³. Herein, the rGO/V₂O₅ could produce an anodic and cathodic D_Li⁺ value of 1.05 × 10⁻¹¹ cm² s⁻¹ and 1.03 × 10⁻¹¹ cm² s⁻¹, respectively.

Fig. 5 (A) CV curves at different scan rates of rGO/V₂O₅ in a voltage range of 2–4 V vs. Li⁺/Li. (B) Linear relationship between the redox peak current and scan rate. (C) Cycling performance of pure V₂O₅ and doped V₂O₅ at a current density of 300 mA g⁻¹ for 30 cycles. (D) Nyquist plots of the two prepared different materials electrodes from 0.01 Hz to 100 kHz and the fitting results for rGO/V₂O₅. Inset: a simplified equivalent circuit. (E) Kinetic parameters obtained from equivalent circuit fitting of experimental data from rGO/V₂O₅ lithium ion batteries.

As is shown in Fig. 5C, the charge–discharge capacity and corresponding coulombic efficiency as a function of cycle number at a constant current density of 1500 mA g⁻¹. The sample exhibits remarkable capacity retention upon long-time cycling with a high coulombic efficiency of nearly 100%. A specific discharge capacity of 106 mA h g⁻¹ can be retained in the 200th cycle, which corresponds to 81.5% of the initial discharge capacity of 130 mA h g⁻¹. The capacity fading rate is about 0.0925% per cycle, which is lower than the results of previous reported materials.^16,45,46

Fig. 5D shows the EIS profiles of the as-prepared coin cells. The Nyquist plots of the electrodes all show a form with a semicircle at higher frequency region and a spike at lower frequency which is a characteristic of the capacitive behavior. In the equivalent circuit (inset of Fig. 5D), R_e represents the equivalent series resistance that includes all ohmic resistance due to the electrolyte and other parts of the cell. R_CT and R_f stand for the charge transfer resistance through the electrode/electrolyte interface and the contacts in between hybrid sheets, respectively, while CPE refers to constant phase elements, revealing the non-ideal capacitance due to the surface roughness.⁴⁷ It is clear that the charge transfer resistance (R_CT) for rGO/V₂O₅ is reduced to 62 Ω, whereas a lower conductivity is found in V₂O₅ with R_CT of 121 Ω. These phenomena illustrate that there is an optimal loading of rGO in the V₂O₅ composite to promote proper conductivity and accessibility for lithium ion migration, which improve the rate performance and cycle ability.

Various kinds of the electrochemical performances of the full cell with rGO/V₂O₅ cathode and lithiated graphite anode are shown in Fig. 6. Cyclic voltammetry test is studied within the voltage range of 1.5–3.5 V and the scan rate is 0.5 mV s⁻¹. As shown in Fig. 6A, the first, second, fifth and tenth CV curves are selected to show two pairs of well-defined anodic and cathodic peaks assigned at 2.44, 2.37 and 2.55, 2.64 V respectively, and the narrow potential gaps between the peaks demonstrate the low polarization between the electrode and electrolyte. Charge/discharge studies of the full cell are conducted at different current densities between 1.5 and 3.5 V (Fig. 6B). It is evident that the discharge trace composed of a sharp decay followed by a distinct plateau then monotonous curves which are attributed to the formation of solid solution, a two-phase region and interfacial storage, respectively. What is worth mentioning is that the full cell delivers a discharge capacity of 213 mA h g⁻¹ at the initial current density of 100 mA g⁻¹, which results a high specific energy density of 106.5 W h kg⁻¹. It can also be obviously seen that even at the high current density of 900 mA g⁻¹, the full battery is able to reach a power density of 450 W kg⁻¹, which indicates Li⁺ can intercalate and de-intercalate both cathode and anode fleetly. The extended galvanostatic cycling and coulombic efficiency (CE) of the full cell is shown in Fig. 6C which exhibits an excellent cycling performance after 500 cycles (a capacity loss of 0.088% per cycle) with a coulombic efficiency of more than 98%.

Fig. 6 (A) CV curves of the full cell consisted of V₂O₅ cathode and lithiated graphite anode at scan rate of 0.5 mV s⁻¹ in a voltage range of 1.5–3.5 V. (B) Typical charge/discharge rate curves at different current densities. (C) Plot of cycling performance and coulombic efficiency at a current density of 900 mA g⁻¹ at room temperature condition.

4. Conclusion

In summary, rGO/V₂O₅ hybrid sheets consisted of well-defined layers are successfully prepared by using a simple hydrothermal method. Compared with V₂O₅ sheets, the rGO/V₂O₅ hybrid sheets exhibit a significant improvement in electrochemical performance as a cathode material for LIBs in terms of excellent capacity (249 mA h g⁻¹ at 100 mA g⁻¹), cycle life (200 cycles with 81.5% capacity retention). The impedance analysis (62 Ω) also shows that the rGO/V₂O₅ hybrid sheets composite were more amenable to Li⁺ ion diffusion. With this hybrid sheets cathode materials and lithiated graphite anode materials, an ideal full battery is assembled which delivers a high capacity of 213 mA h g⁻¹ and an outstanding energy density of 106.5 W h kg⁻¹. As a result, the obtained good electrochemical performance enables a further development in such composite material for promisingly safe, green, and powerful energy storage.

Acknowledgements

We acknowledge the support from the “Thousand Talents Program”, the Natural Science Foundation of Jiangsu Province of China (no. BK20140315), the National Natural Science Foundation of China (no. 51402202), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† The first two authors contributed equally to this work.

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