Role of carbon content in qualifying Li₃V₂(PO₄)₃/C as a high capacity anode for high rate lithium battery applications

Abstract

Nanocrystalline Li₃V₂(PO₄)₃ has been prepared by the oxalic dihydrazide assisted combustion (ODHAC) method, and the corresponding Li₃V₂(PO₄)₃/C composites containing different concentrations of super P carbon, viz. 10, 20 and 30 wt% have been explored individually as anodes for lithium batteries. Among the chosen composites, Li₃V₂(PO₄)₃/C-20 exhibits superior electrochemical properties, thus recommending a carbon content of 20 wt% as an optimum amount required to improve the electrochemical properties significantly. An initial capacity of 500 mA h g⁻¹ and a progressive capacity of ∼400 mA h g⁻¹ up to 100 cycles have been delivered by a Li₃V₂(PO₄)₃/C-20 anode with an admissible capacity fade of 20%, especially when cycled at a current density of 100 mA g⁻¹. The optimized Li₃V₂(PO₄)₃/C-20 composite thus demonstrates itself as a high capacity anode that is suitable for high rate applications by way of exhibiting appreciable capacity values of 450, 350, 302 and 220 mA h g⁻¹, under the influence of 100, 200, 300 and 400 mA g⁻¹ current density.

---

1. Introduction

Lithium-ion batteries are widely studied energy storage devices for application in portable electronic devices, electric vehicles (EV) and hybrid electric vehicles (HEV).^1–3 The selection of suitable electrodes gains importance when considering how to improve the power and energy density of lithium-ion batteries. Graphite is the most widely used anode material, especially after the commercialization of Sony's lithium-ion battery in 2001. Driven by the low theoretical capacity and sloping voltage plateau of graphite,⁴ certain lithium alloying metal and tin oxides, viz., Mn₃O₄,⁵ Co₃O₄,⁶ SnO₂,⁷ CuO,⁸ Fe₂O₃ (ref. 9) and Fe₃O₄ (ref. 10) have been extensively studied along with the zero strain Li₄Ti₅O₁₂ anode.¹¹ However, issues such as larger irreversible capacity loss, volume expansion and higher intercalation potential of such anodes pose the necessity to identify and explore newer electrode materials as alternative anodes for lithium-ion batteries.

Transition metal phosphates as cathodes are popularly known for their high stability, rate capability and safety. On the other hand, Kalaiselvi et al.¹² in 2004 reported on the anodic behaviour of LiFePO₄ for lithium battery applications, and LiVOPO₄ is another cathode material reported recently for its application as an anode material.¹³

Towards this direction, monoclinic Li₃V₂(PO₄)₃/C, an upcoming cathode with high energy density, rate capability and safety, has intrigued recent researchers to explore the possibility of using it as an anode. This idea gets validated by the possible existence of vanadium in at least three different oxidation states and its ability to undergo insertion and/or alloying reaction with lithium in a wide potential range of 0.05–4.8 V. In this regard, X. H. Rui et al.¹⁴ reported on the performance of a Li₃V₂(PO₄)₃/C anode in the potential range of 3.0–0.0 V to obtain a capacity of 203 mA h g⁻¹ versus Li⁺/Li, and W. F. Mao et al. discussed the possible existence of a Li₃V₂(PO₄)₃ (cathode)‖Li₃V₂(PO₄)₃ (anode) assembly.¹⁵ Herein, the open structure of Li₃V₂(PO₄)₃ allows the easy entry/departure of lithium ions into/from the electrode without causing structural change.

Quite different from such scarcely available reports on the Li₃V₂(PO₄)₃ anode, our present work that deals with the investigation of combustion synthesized Li₃V₂(PO₄)₃/C as a high capacity and high rate anode material is the first of its kind to report on the possibility of extracting a high specific capacity of ∼400 mA h g⁻¹ under 100 mA g⁻¹, wherein the suitability of the Li₃V₂(PO₄)₃/C anode for high rate (400 mA g⁻¹) applications has also been demonstrated. The synergistic effect of the oxalic dihydrazide assisted combustion method (ODHAC) and the optimized addition of super P carbon (20 wt%) is believed to be responsible for the excellent electrochemical properties of the Li₃V₂(PO₄)₃/C anode.

2. Experimental procedure

2.1 Material synthesis of Li₃V₂(PO₄)₃/C

Li₃V₂(PO₄)₃/C was synthesized by the solution assisted combustion (SAC) method using oxalic dihydrazide (ODH), and the details of the ODHAC method are reported elsewhere.¹⁶ Li₃V₂(PO₄)₃ thus obtained was treated with different compositions (10, 20 and 30 wt%) of super P carbon and ball milled for 5 h at 300 rpm. Subsequently, the mixture was heated in a furnace at 700 °C for 2 h to ensure better adherence of carbon to the surface of the Li₃V₂(PO₄)₃ compound. Carbon coated Li₃V₂(PO₄)₃ samples thus prepared were investigated further for their performance as anodes in lithium-ion cell assembly.

2.2 Physical and electrochemical characterization

The structure and phase purity of the synthesized compound were examined with Bruker D8 Advance X-ray diffraction (XRD) using Ni-filtered Cu Kα radiation (λ = 1.5406 Å). Particle size, carbon coating and surface morphology of the synthesized active material were investigated using Tecnai 20 G2 (FEI make) Transmission Electron Microscopy (TEM) and Gemini Field Emission Scanning Electron Microscopy (FESEM). TG/DTA studies were performed using a TA Instruments SDT Q600 thermogravimetric analyzer. Cyclic voltammetry (CV) was carried out using a VMP3 multichannel potentiostat–galvanostat system (Biologic Science Instrument). Charge–discharge studies were carried out using an ARBIN charge–discharge cycler.

2.3 Electrode preparation and coin cell fabrication

Electrochemical characterization was carried out using CR2032 coin cells. Preparation of the electrode and fabrication of coin cells are similar to our earlier reports,¹² and the fabricated cells were subjected to charge–discharge studies galvanostatically in the voltage range of 3.0–0.05 V versus Li⁺/Li at room temperature. Cyclic voltammetry (CV) measurements were performed at a scan rate of 0.2 mV s⁻¹ between 0.05 and 3.5 V for the cells containing Li₃V₂(PO₄)₃/C anode vs. Li metal separated by Celgard separator soaked in the electrolyte consisting of 1 M LiPF₆ dissolved in EC : DMC (1 : 1 v/v).

3. Results and discussion

3.1 Structural characterization – X-ray diffraction, TEM, FE-SEM and elemental mapping

Fig. 1 shows the XRD pattern of Li₃V₂(PO₄)₃ synthesized at 850 °C. The position and intensity of all peaks indicate the formation of polycrystalline Li₃V₂(PO₄)₃ with a monoclinic structure and P21/n space group. Indexing of miller indices (hkl) of respective diffraction peaks of Li₃V₂(PO₄)₃ evidences the absence of impurity peaks. The calculated lattice parameter values, viz., a = 8.59 Å, b = 8.57 Å, c = 12.02 Å and β = 90.5° are in agreement with the literature report.¹⁷ Hence, it is understood that the ODHAC method produces a Li₃V₂(PO₄)₃ compound with the desired purity and crystallinity. Particle size and the presence of carbon on Li₃V₂(PO₄)₃/C have been investigated by TEM studies (Fig. 2), and the corresponding SAED pattern is appended as the inset of Fig. 2a. The TEM image shows the presence of particles of ∼100 nm size (Fig. 2a). Further, TEM evidences the presence of a carbon coating on Li₃V₂(PO₄)₃ particles (Fig. 2b), and the thickness of such a continuous carbon coating is 15 nm (Fig. 2c). SAED pattern (inset of Fig. 2a) confirms the polycrystalline nature of the pristine Li₃V₂(PO₄)₃ product obtained from the ODHAC method. Further, the SAED pattern recorded for the carbon present in the Li₃V₂(PO₄)₃/C composite evidences the amorphous nature of added super P carbon (Fig. 2d).

Fig. 1 XRD pattern of combustion synthesized Li₃V₂(PO₄)₃.

Fig. 2 (a) Typical TEM image recorded for the Li₃V₂(PO₄)₃/C composite; inset: SAED pattern of pristine Li₃V₂(PO₄)₃. (b) Presence of carbon coating on the surface of Li₃V₂(PO₄)₃/C particles. (c) Figure showing the thickness of carbon coating. (d) SAED pattern evidencing the amorphous nature of super P carbon.

The presence of the carbon layer effectively suppresses the growth of Li₃V₂(PO₄)₃ particles during high calcination processes, and the carbon layer is expected to improve the electronic conductivity of pristine Li₃V₂(PO₄)₃, which is of great importance in improving its electrochemical properties. Further, the flaky appearance of the combustion synthesised Li₃V₂(PO₄)₃ compound and the stoichiometry of the Li₃V₂(PO₄)₃/C composite are confirmed from FE-SEM and EDX analysis, respectively (Fig. 3a and b).

Fig. 3 (a) FE-SEM image and (b) EDX spectra recorded for the Li₃V₂(PO₄)₃/C-20 composite; inset: closer view of the flaky morphology; (c–f) elemental mapping of the Li₃V₂(PO₄)₃/C-20 composite as a function of V, P, O and C contents individually; inset of (c): cumulative elemental mapping of the Li₃V₂(PO₄)₃/C-20 composite.

Elemental mapping (Fig. 3c–f) of the Li₃V₂(PO₄)₃/C composite, with a special reference to cumulative elemental mapping (inset of Fig. 3c), confirms the presence of V, P, O and C in the individual particles. The total carbon content of the title composite has been calculated using TG/DTA and is found to be 20 wt% (Fig. 4).

Fig. 4 TG/DTA results of Li₃V₂(PO₄)₃/C-20 composite.

3.2 Cyclic voltammetry studies

Typical cyclic voltammetry (CV) behaviour of the Li₃V₂(PO₄)₃/C composite anode is shown in Fig. 5. The first cycle CV is quite different from the following cycles due to the formation of the SEI layer and is not unusual. Insertion/de-insertion of lithium occurs at different voltages, viz., 1.95/1.72, 1.86/1.77, 1.74/1.90, 1.65/2.02 V, corresponding to the formation of two phase (>1.6 V) and single phase (≤1.6 V) lithium insertion related intermediate compounds with varying lithium content,¹⁴ which is evident from CV cycles. However, a slight shift in peak position of the first and successive CV curves is observed, which is due to the SEI formation-driven unavoidable irreversible capacity loss with respect to the initial cycle. Interestingly, the following 2nd and 3rd cycles also show the presence of four redox pairs in the respective anodic region, thus confirming the reversibility and structural stability of the currently synthesized Li₃V₂(PO₄)₃/C anode.

Fig. 5 Cyclic voltammogram of Li₃V₂(PO₄)₃/C-20 anode.

3.3 Charge–discharge studies

Charge–discharge behaviour of Li₃V₂(PO₄)₃/C anodes consisting of 10, 20 and 30 wt% of super P carbon under a current density of 100 mA g⁻¹ is shown in Fig. 6. The initial charge capacities corresponding to Li₃V₂(PO₄)₃/C composites containing 10, 20 and 30 wt% of carbon are found to be 414, 500 and 418 mA h g⁻¹. After 50 cycles, a nominal specific capacity of 156, 400 and 201 mA h g⁻¹ with a respective capacity retention of 38, 80 and 48% has been exhibited by Li₃V₂(PO₄)₃/C anodes containing 10, 20 and 30 wt% of carbon. From this observation, it is understood that 20 wt% of super P carbon exhibits better cycling performance compared with those of 10 and 30 wt% of super P carbon.

Fig. 6 Cycling behavior of Li₃V₂(PO₄)₃/C anodes with different carbon content (10, 20 and 30 wt%).

Even though it appears to be straightforward that an increase in carbon content would improve the electrochemical behaviour in a linear manner, the observed improvement in the electrochemical performance is not linear with the increasing carbon content.

The probable reason may be understood as follows: it is evident from Fig. 7 that the R_ct value of Li₃V₂(PO₄)₃/C-20 is lower than those of Li₃V₂(PO₄)₃/C-10 and Li₃V₂(PO₄)₃/C-30 anodes, especially upon cycling. Similarly, a higher peak current (i_p) has been exhibited upon cycling by the Li₃V₂(PO₄)₃/C-20 anode in comparison with the Li₃V₂(PO₄)₃/C-10 and Li₃V₂(PO₄)₃/C-30 anodes, thus substantiating the advantageous role of 20 wt% carbon in improving the electrochemical behaviour of the Li₃V₂(PO₄)₃/C anode by offering a desired conducting network to facilitate faster lithium diffusion kinetics.

Fig. 7 Impedance spectra (a and b) and cyclic voltammetry (c and d) of Li₃V₂(PO₄)₃/C composite anodes containing 10, 20 and 30 wt% carbon.

In addition, the gradually reducing diffraction spots (Fig. 8) observed in the SAED pattern with the increasing carbon content clearly evidences the fact that the intensity of the carbon cloud is adversely high in the Li₃V₂(PO₄)₃/C-30 composite, thus impeding the faster diffusion of lithium ions. As a result, Li₃V₂(PO₄)₃/C-20 exhibits better electrochemical performance compared with those of Li₃V₂(PO₄)₃/C-10 and Li₃V₂(PO₄)₃/C-30 anodes.

Fig. 8 TEM images and SAED pattern of Li₃V₂(PO₄)₃/C composites containing (a and b) 10, (c and d) 20 and (e and f) 30 wt% super P carbon.

Based on the above mentioned reasons, further studies such as extended charge–discharge studies and a rate capability test were restricted to Li₃V₂(PO₄)₃/C containing 20 wt% carbon. The voltage profiles of the Li₃V₂(PO₄)₃/C-20 anode corresponding to 1, 10, 20, 30, 40, 50 and 100 cycles are shown in Fig. 9, wherein consistent appearance of voltage plateaus at the respective positions is observed up to 100 cycles, with the exception of the initial discharge curve. The appreciable capacity retention and the admissible fade (∼20%) in capacity of the Li₃V₂(PO₄)₃/C-20 anode, as evident from Fig. 10, are in favour of considering it as a potential anode for lithium battery applications. In other words, a reasonable capacity of 400 mA h g⁻¹ that has been observed up to 100 cycles under the influence of 100 mA g⁻¹ current density substantiates the superiority of the currently synthesized Li₃V₂(PO₄)₃/C-20 anode over conventional graphite.

Fig. 9 Capacity vs. voltage profile of Li₃V₂(PO₄)₃/C-20 anode (100 mA g⁻¹ current density).

Fig. 10 Charge–discharge behavior of the Li₃V₂(PO₄)₃/C-20 anode upon extended cycling (100 mA g⁻¹ current density).

In addition, the currently observed capacity of 400 mA h g⁻¹ under the influence of 100 mA g⁻¹ current density is found to be superior than the capacity value reported for the Li₃V₂(PO₄)₃/C anode, and the irreversible capacity loss of 20 mA h g⁻¹ is also nominal compared with the literature report.¹⁴

Hence, it is understood that the amount of carbon plays a crucial role in deciding the electrochemical performance, and 20 wt% carbon has been identified as the optimum concentration to prepare Li₃V₂(PO₄)₃/C anodes with appreciable specific capacity behaviour.

3.4 Rate capability test

The rate capability behavior of the Li₃V₂(PO₄)₃/C-20 anode was investigated upon progressive cycling as a function of different current densities such as 100, 200, 300 and 400 mA g⁻¹ (Fig. 11).

Fig. 11 Rate capability behaviour of ODHAC synthesized Li₃V₂(PO₄)₃/C-20 anodes.

Li₃V₂(PO₄)₃/C-20 anodes exhibit acceptable capacity values of 450, 350, 302 and 220 mA h g⁻¹ corresponding to a current density of 100, 200, 300 and 400 mA g⁻¹, respectively. Interestingly, the Li₃V₂(PO₄)₃/C-20 anode is found to resume the initial capacity of ∼ 400 mA h g⁻¹ even after subjecting it to higher current densities such as 200, 300 and 400 mA g⁻¹, which is in favour of its suitability and structural stability for high rate applications.

4. Conclusions

The ODHAC method, by virtue of offering a phase pure Li₃V₂(PO₄)₃ compound and its synergy with the addition of the optimized amount of 20 wt% of super P carbon, results in the formation of a Li₃V₂(PO₄)₃/C-20 anode, which in turn is responsible for the improved electrochemical behaviour. Accordingly, a Li₃V₂(PO₄)₃/C-20 anode delivers an appreciable specific capacity of ∼400 mA h g⁻¹ up to 100 cycles under the influence of 100 mA g⁻¹ current density. Inferior electrochemical properties observed with 10 and 30 wt% super P carbon emphasizes the crucial role of the addition of 20 wt% carbon in offering a desirable conducting network to facilitate facile lithium diffusion kinetics and to improve the electrochemical behaviour. The study recommends the suitability of high capacity Li₃V₂(PO₄)₃/C-20 anodes for high rate applications, as evident from the extraction of a nominal capacity of ∼220 mA h g⁻¹ under a 400 mA g⁻¹ condition.

Acknowledgements

Among the authors, V. Mani is thankful to DST, New Delhi and K. Nathiya to CSIR, New Delhi for financial support through CSIR-EMPOWER Project. N. Kalaiselvi is thankful to CSIR, New Delhi and DST, New Delhi for support through CSIR-EMPOWER and Grant-in-aid Project (GAP) respectively.

Notes and references

1. J. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
2. J. M. Tarascon and M. Armand, Nature, 2001, 414, 359–367.
3. C. M. Park, J. H. Kim, H. Kim and H. J. Sohn, Chem. Soc. Rev., 2010, 39, 3115–3141.
4. H. Nozakia, K. Nagaokab, K. Hoshib, N. Ohtaa and M. Inagaki, J. Power Sources, 2009, 194, 486–493.
5. H. Wang, L. F. Cui, Y. Yang, H. S. Casalongue, J. T. Robinson, Y. Liang, Y. Cui and H. Dai, J. Am. Chem. Soc., 2010, 132, 13978–13980.
6. Y. M. Kang, M. S. Song, J. H Kim, H. S. Kim, M. S. Park, J. Y. Lee, H. K. Liu and S. X. Dou, Electrochim. Acta, 2005, 50, 3667–3673.
7. N. Li, C. R. Martin and B. Scrosati, Electrochem. Solid-State Lett., 2000, 3, 316–318.
8. J. Y. Xiang, J. P. Tu, L. Zhang, Y. Zhou, X. L. Wang and S. J. Shi, J. Power Sources, 2010, 195, 313–319.
9. H. Liu, G. Wang, J. Park, J. Wang, H. Liu and C. Zhang, Electrochim. Acta, 2009, 54, 1733–1736.
10. S. Ito, K. Nakaoka, M. Kawamura, K. Ui, K. Fujimoto and N. Koura, J. Power Sources, 2005, 146, 319–322.
11. L. Aldon, P. Kubiak, M. Womes, J. C. Jumas, J. O. Fourcade, J. L. Tirado, J. I. Corredor and C. P. Vicente, Chem. Mater., 2004, 16, 5721–5725.
12. N. Kalaiselvi, C. H. Doh, C. W. Park, S. I. Moon and M. S. Yun, Electrochem. Commun., 2004, 6, 1110–1113.
13. M. M. Ren, Z. Zhou and X. P. Gao, J. App. Electrochem., 2010, 40, 209–213.
14. X. H. Rui, N. Yesibolati and C. H. Chen, J. Power Sources, 2011, 196, 2279–2282.
15. W.-F. Mao, H.-Q. Tang, Z.-Y. Tang, J. Yan and Q. Xu, ECS Electrochem. Lett., 2013, 2(7), A69–A71.
16. K. Nathiya, D. Bhuvaneswari, Ganguli babu, D. Nirmala and N. Kalaiselvi, RSC Adv., 2012, 2, 6885–6889.
17. Y. Li, L. Honga, J. Suna, F. Wua and S. Chen, Electrochim. Acta, 2012, 85, 110–115.

---