Rhombohedral NASICON-structured Li₂NaV₂(PO₄)₃ with single voltage plateau for superior lithium storage

Abstract

A single rhombohedral NASICON-structured cathode material Li₂NaV₂(PO₄)₃ has been synthesized through a facile sol–gel route for the first time. The results suggest that the Na⁺ sources, i.e. anionic surfactants including sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfonate (SDS) and sodium oleate (NaOL) can induce the Na⁺ ions to occupy all of the A1 sites in formula unit [V₂(PO₄)₃]³⁻. Almost all the monoclinic phase of Li₃V₂(PO₄)₃ can be transformed into the rhombohedral phase of Li₂NaV₂(PO4)₃ with Li⁺ partially substituted by Na⁺ ions offered by SDBS, SDS or NaOL. In the absence of monoclinic Li₃V₂(PO₄)₃, the rhombohedral Li₂NaV₂(PO₄)₃ exhibits only one voltage plateau around 3.76 V versus lithium metal during the charge–discharge process, which is consistent with the single couple V⁴⁺/V³⁺ redox peaks in the cyclic voltammetry (CV) curves. Furthermore, the Li₂NaV₂(PO₄)₃ obtained using SDBS as a Na⁺ source displays remarkably high-rate capability (80 mA h g⁻¹ at 5 C and 68 mA h g⁻¹ at 10 C) and excellent cyclability, 500 cycles at 2 C (about 93% of the initial capacity is retained) due to no fundamental change occurring in the host three-dimensional framework and the porous nanosheets structure with carbon coating.

---

Introduction

Energy storage systems have become extremely important to integrate intermittent energies into the grid due to the rapid development of renewable energy sources. During the past few decades, lithium-ion batteries (LIBs) had been developed as promising energy storage technologies owing to the long cycle life, high capacity retention and low cost. Following the introduction of LiFePO₄ by J. B. Goodenough,¹ lithium transition metal phosphates LiMPO₄ (M = Co, Mn, Ni, Fe)^2–15 and Li₃V₂(PO₄)₃ (ref. 16–19) have attracted both research and technological interest as potential cathode materials for lithium-ion batteries. All of these materials contain mobile Li⁺ ions and redox-active metal sites surrounded by a tetrahedral (PO₄)³⁻ anion network. Compared with traditional lithium layered oxides, phosphate based polyanion materials display better electrochemical performance, thermal and chemical stability.²⁰ As an excellent candidate among the mentioned cathode materials, Li₃V₂(PO₄)₃ with a Na superionic conductor framework is of especial interest because of its structural stability and fast ion migration induced by the three-dimensional framework structure.

The NASICON-structured Li₃V₂(PO₄)₃ has two different crystallographic forms depending on the site distribution of the [V₂(PO₄)₃]³⁻ units:^21,22 the monoclinic form with space group P2₁/n and the rhombohedral form with space group R3̄c. As a cathode material, monoclinic LVP usually has a high operating voltage and a larger theoretical specific capacity, 197 mA h g⁻¹, than rhombohedral LVP,²³ but it exhibits four voltage plateaux with three lithium ions extraction/insertion in charge–discharge curves when tested in the range from 3.0 to 4.8 V. On the contrary, rhombohedral LVP has a theoretical specific capacity of 133 mA h g⁻¹ and shows only one voltage plateau around 3.76 V with two lithium ions extraction/insertion in the charge–discharge process between 3.0 and 4.3 V.²⁴ Usually, an electronic device can run normally only with a single supply voltage since a single plateau can give a stable output power. Therefore, rhombohedral LVP material with a single V⁴⁺/V³⁺ voltage plateau is more suitable for the application of portable electronic devices and electric vehicles. However, the rhombohedral LVP could not be obtained directly since monoclinic LVP is more stable than rhombohedral LVP.^25,26 Lithium ion-exchange of the corresponding rhombohedral Na₃V₂(PO₄)₃ (NVP) is a feasible method to obtain rhombohedral LVP at present and this method is also employed for the preparation of rhombohedral Li₂NaV₂(PO₄)₃ (LNVP).²⁷ The rhombohedral LNVP is homeotypic to rhombohedral LVP with a single voltage plateau around 3.76 V when two lithium ions extraction/insertion in the charge–discharge process between 3.0 and 4.3 V. Though rhombohedral LVP or LNVP with a single plateau is more attractive than monoclinic LVP as a cathode material, the synthetic method (ion-exchange) is extremely complex and NVP would be dissolved partially in LiNO₃ even below 40 °C during the preparation procedures. In addition, the Na⁺ guests in NVP can stabilize the rhombohedral structure of the [V₂(PO₄)₃]³⁻ framework while the smaller Li⁺ ions could not support the larger interstitial free space, so, compared with the monoclinic LVP obtained by the sintering process (usually no obvious decay after 100 cycles at 5 C in the range from 3.0 to 4.3 V),^16,17 the cycling stability of the rhombohedral LVP or LNVP obtained by lithium ion-exchange is usually poor (obvious decay at high rate).²⁸

Recently, a hybrid phase composite Li₂NaV₂(PO₄)₃ (rhombohedral LVP, rhombohedral NVP and monoclinic LVP coexistent with a ratio of 59 : 31 : 10) was synthesized by a solid state reaction method.²⁹ This study proved that rhombohedral LVP could be formed directly when Li⁺ ions are partially substituted by Na⁺ ions. However, multiple voltage plateaux could still be observed in the charge–discharge curves because the monoclinic LVP was generated simultaneously during the solid state reaction.

In this paper, single rhombohedral phase Li₂NaV₂(PO₄)₃ has been synthesized through a simple sol–gel route for the first time and the hybrid phase Li₂NaV₂(PO₄)₃ is also synthesized using the same method for comparison. Anionic surfactant SDBS and sodium acetate (NaAc) are employed as two different types of Na⁺ source to prepare LNVP precursors via a facile sol–gel route. After calcining the precursors under the same condition, rhombohedral phase Li₂NaV₂(PO₄)₃ (denoted as LNVP-1) and hybrid phase Li₂NaV₂(PO₄)₃ (denoted as LNVP-2) are obtained, respectively. We speculate that an anionic surfactant can induce Na⁺ ions to occupy all of the A1 sites in the formula unit of [V₂(PO₄)₃]³⁻ and hence the monoclinic phase of Li₃V₂(PO₄)₃ can be transformed into the rhombohedral phase of Li₂NaV₂(PO₄)₃ while NaAc or NaF²⁹ can not introduce this change. To verify this result, we also use other anionic surfactant SDS and NaOL as Na⁺ source to synthesize Li₂NaV₂(PO₄)₃ (denoted as LNVP-3 and LNVP-4) under the same preparation route and both of the final products are proved to be single rhombohedral phase. The as prepared rhombohedral Li₂NaV₂(PO₄)₃ materials exhibit superior high rate performance as well as excellent cyclability, indicating their promising application as a superior cathode material for rechargeable lithium-ion batteries.

Experimental

Preparation of the materials

All the chemicals were of analytical grade and used without further purification. The Li₂NaV₂(PO₄)₃ composites were prepared via a facile sol–gel route. In a typical procedure, 0.5 mmol SDBS was dissolved in a mixture of 20 mL deionized water and 10 mL absolute ethanol. At the same time, a stoichiometric ratio of NH₄VO₃, oxalic acid, LiAc and NH₄H₂PO₄ (2 : 3: 2: 3) were dissolved in 20 mL deionized water under magnetic stirring to form a clear blue solution. Afterwards, this blue solution was added dropwise to the SDBS solution. The final mixture solution was heated gently with continuous stirring to remove the excess water and ethanol at 75 °C and blue LNVP precursor was obtained. The precursor was first sintered at 350 °C for 4 h under N₂, followed by milling and then heating at 750 °C for 8 h under N₂. Finally, a grey powder of rhombohedral LNVP was obtained. Hybrid phase LNVP was prepared by using 0.5 mmol NaAc and 0.5 mmol CTAB instead of SDBS as the Na⁺ and carbon sources via the same preparation route. In addition, another type of ionic surfactant SDS and NaOL were employed to replace SDBS to prepare rhombohedral phase LNVP under the same conditions. The final products were denoted as LNVP-1, LNVP-2, LNVP-3 and LNVP-4, respectively.

Characterization

The morphology and microstructure of the products were investigated using field-emission scanning electron microscopy (FESEM, LEO 1430VP, Germany) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010). The crystal structure of the samples was collected by X-ray diffraction (XRD) (Bruker D8 advance) with Cu K-alpha radiation. The angular resolution in 2θ scans was 0.4° over a 2θ range of 10–60°.

Electrochemical measurements

The electrochemical characterization was carried out in two-electrode electrochemical cells. The working electrodes were prepared by milling a mixture of 80 wt% active materials, 10 wt% acetylene black and 10 wt% poly(vinyl difluoride) (PVDF) in N-methylpyrrolidinone (NMP) to from a homogeneous slurry. The slurry of the mixture was pasted uniformly on an aluminum foil current collector and the electrode was then dried under vacuum at 110 °C for 12 h. Test cells were assembled in an argon-filled glovebox using Li foil as both the counter and reference electrode, polypropylene (PP) film as the separator. The electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 v/v). The electrochemical performance was tested at various current densities in the voltage range of 3.0–4.3 V. Cyclic voltammetry (CV) studies were carried out on an electrochemical workstation (CHI660C) between 3.0 and 4.3 V at scans rate of 0.025 mV s⁻¹, 0.05 mV s⁻¹ and 0.1 mV s⁻¹, respectively.

Result and discussion

Structural characterization

X-ray diffraction (XRD) patterns of the as-synthesized LNVP-1 and LNVP-2 are presented in Fig. 1. The sharp diffraction peaks indicate good crystallinity of these two samples. The patterns of LNVP-1 can be well-indexed to the rhombohedral structure with R-3c space group, which is consistent with previous reports.²⁷ Compared with LNVP-1, the pattern of LNVP-2 shows two extra peaks at 26.5° and 27.4° because of the existence of the monoclinic phase.²⁹ Furthermore, the patterns of LNVP-3 and LNVP-4 (Fig. S1†) show no extra peaks at 26.5° and 27.4° similar to LNVP-1, which means there is no monoclinic phase in these three samples. No diffraction peaks from carbon could be detected from the XRD patterns, which demonstrates that the residual carbon has an amorphous structure. In addition, the lattice parameters of LNVP-1, LNVP-3 and LNVP-4 are in good agreement with the results of Li₂NaV₂(PO₄)₃ prepared by lithium ion-exchange (Table S1†).

Fig. 1 XRD patterns of the LNVP-1 and LNVP-2 nanoparticles.

The rhombohedral LNVP has a three-dimensional framework structure and alkali metal ions can easily diffuse through the well-defined ion channels. As illustrates in Fig. 2, each VO₆ octahedron is linked with three PO₄ tetrahedrons through common corners, forming the framework anion [V₂(PO₄)₃]³⁻ unit.

Fig. 2 Schematic illustration of the rhombohedral LNVP structure.

With the [V₂(PO₄)₃]³⁻ units aligned along the c axis and interconnected by a PO₄ tetrahedral along the a axis, an open 3D framework with facile ions migration is formed. Each formula unit of LNVP contains one A1 site and two A2 sites. The one A1 site is occupied by Na⁺ ion so the rhombohedral structure of [V₂(PO₄)₃]³⁻ framework could be maintained because the radius of the Na⁺ ion is suitable at this site. The two A2 sites, which are occupied by Li⁺ ions, allow easily accomodate and transport the Li⁺ ions.²⁷ However, if Li⁺ ions take the A1 position, the rhombohedral structure would transform into a monoclinic structure at high temperature since the radius of Li⁺ ion is too small to support the A1 site, so rhombohedral LVP can not be formed directly. Furthermore, during the charge–discharge process, only the two Li⁺ ions in the A2 site can be extracted/inserted from the NASICON framework to achieve the V⁴⁺/V³⁺ redox couple while the one Na⁺ ion in A1 site is immobilized.

Morphological analysis

The morphology and microstructure of the as-synthesized LNVP-1 and LNVP-2 samples are characterized by field-emission scanning electron microscopy. The low magnification image of LNVP-1 in Fig. 3a shows a porous nanosheets structure. These porous structures can serve as channels for fast lithium supply and facile electron transport, which facilitate the kinetics of the electrochemical reaction. From higher magnification image (Fig. 3b), it can be seen that the size of nanoparticles ranges from 200 to 400 nm. In comparison, the particles of LNVP-2 are severely agglomerated bulk structures (Fig. 3c) with a size ranging from 1 to 2 μm (Fig. 3d). In addition, the FESEM images of LNVP-3 and LNVP-4 (Fig. S2†) display an agglomerated structure consisting of nanoparticles.

Fig. 3 FESEM images of LNVP-1 (a and b) and LNVP-2 (c and d).

An amorphous carbon layer on the surface of a single particle can be observed clearly in the HRTEM image of sample LNVP-1 (Fig. 4a). The selected area electron diffraction (SAED) pattern (Fig. 4b) demonstrates the single crystalline nature of the nanoparticles, which is a typical diffraction pattern of the rhombohedral phase.²¹ This SAED pattern provides evidence that the substitution of Li⁺ for Na⁺ offered by SDBS can transform almost all the monoclinic phase into rhombohedral phase.

Fig. 4 HRTEM image of the LNVP-1 nanoparticles (a) and the corresponding SAED pattern (b).

Electrochemical behavior

In order to identify that the anionic surfactants can inhibit the formation of a monoclinic structure, a coin-type cell was employed to evaluate the electrochemical behavior of LNVP-1 and LNVP-2 (Fig. 5). Only the V⁴⁺/V³⁺ couple is apparent in the voltage range from 3.0 to 4.3 V. Fig. 5a and b display the CV curves for the LNVP-1 and LNVP-2 at scan rates of 0.025 mV s⁻¹, 0.05 mV s⁻¹ and 0.1 mV s⁻¹ respectively. We can clearly distinguish that only one oxidation peak is located around 3.8 V in Fig. 5a, which is consistent with the one charge plateau in the charge curves shown in Fig. 5c. During the following reduction process, the corresponding one reduction peak near 3.7 V is in good agreement with the one discharge plateau in the discharge curves (Fig. 5c). This one couple of redox peaks proves the absence of a monoclinic phase in LNVP-1. Compared with Fig. 5a, the curves in Fig. 5b present four couples of V⁴⁺/V³⁺ redox peaks. The three auxiliary oxidation peaks around 3.6 V, 3.7 V and 4.1 V are caused by the extraction of Li⁺ ion in the monoclinic phase while the main oxidation peak around 3.8 V is caused by the extraction of Li⁺ ion in the rhombohedral phase. Four corresponding reduction peaks appear around 3.56 V, 3.66 V, 3.71 V and 4.03 V, respectively. This observation demonstrates the coexistence of monoclinic and rhombohedral phase in LNVP-2. The CV curves of the LNVP-3 and LNVP-4 (Fig. S3†) also present only one couple of redox peaks, which indicate that there is no monoclinic phase in LNVP-3 and LNVP-4.

Fig. 5 Electrochemical characterization of LNVP-1 and LNVP-2. CV and galvanostatic charge–discharge curves of LNVP-1 (a and c) and LNVP-2 (b and d). Rate capability of LNVP-1 and LNVP-2 at different current densities (e). Cycling performance of LNVP-1 and LNVP-2 at 2 C rate (f).

Unlike the LNVP-1 with a one voltage plateau (Fig. 5c), four voltage plateaux could be observed in the charge–discharge curves of LNVP-2 (Fig. 5d). The short plateaux around 3.6 V, 3.68 V and 4.10 V correspond to the reversible reaction of the first and second Li⁺ ion in the monoclinic phase, and the long plateau near 3.76 V is caused by Li⁺ insertion/extraction in the rhombohedral phase. These four voltage plateaux indicate that the monoclinic and rhombohedral phases are coexistent in LNVP-2 samples. In addition, from the charge–discharge curves of LNVP-3 and LNVP-4 (Fig. S3†), we notice that there is only one observed voltage plateau around 3.76 V. This feature confirms the absence of a monoclinic phase in LNVP-3 and LNVP-4.

To evaluate the rate capability, the LNVP-1 and LNVP-2 electrodes are cycled at various current densities from 0.1C to 10C over a voltage window of 3.0–4.3 V (Fig. 5e). At the lower rate, LNVP-1 and LNVP-2 electrodes show no obvious difference in specific capacity. However, the LNVP-1 electrode shows better rate capability at high rates and deliver discharge capacities of 80 mA h g⁻¹ and 68 mA h g⁻¹ at 5 C and 10 C rates, respectively, much higher than that of the LNVP-2 electrode. To test the cycling performance, LNVP-1 and LNVP-2 electrodes are charged–discharged at 2 C for 500 cycles. As shown in Fig. 5f, the capacity of LNVP-1 has no obvious fading after 500 cycles (nearly 93% of the initial discharge capacity is retained) while the LNVP-2 has a rapid capacity decay. The excellent rate capacity and cycling ability of the LNVP-1 can be attributed to the small crystal size and porous nanosheets structure, which possess short distances for Li⁺ ions diffusion and a large electrode-electrolyte contact area for high Li⁺ ions flux across the interface. Furthermore, the carbon shell is closely attached onto the small LNVP particles, which would also effectively enhance the conductivity of the resultant materials.

Conclusions

In summary, single rhombohedral Li₂NaV₂(PO₄)₃ has been synthesized via a simple sol–gel method by using anionic surfactant SDBS, SDS and NaOL as the starting materials. These anionic surfactants not only work as a morphology regulator, but also offer both Na⁺ and carbon sources. According to the XRD analysis, SAED pattern and the electrochemical characterization, we speculate that anionic surfactant can induce Na⁺ ions to occupy all of the A1 sites in formula unit of [V₂(PO₄)₃]³⁻ and hence monoclinic structure can be totally transformed into a rhombohedral structure. Moreover, LNVP-1 showed high rate capability and excellent cycling ability owing to no fundamental change occurring in the host three-dimensional framework and the porous nanosheets structure with carbon coating.

Acknowledgements

This work was supported financially by the National Program on Key Basic Research Project of China (973 Program, no. 2014CB239701), National Natural Science Foundation of China (no. 21173120, 51372116) and Natural Science Foundations of Jiangsu Province (no. BK2011030).

Notes and references

1. A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc., 1997, 144, 1188–1194.
2. I. C. Jang, C. G. Son, S. M. G. Yang, J. W. Lee, A. R. Cho, V. Aravindan, G. J. Park, K. S. Kang, W. S. Kim, W. I. Cho and Y. S. Lee, J. Mater. Chem., 2011, 21, 6510–6514.
3. J. J. Schneider, J. Khanderi, A. Popp, J. Engstler, H. Tempel, A. Sarapulova, N. N. Bramnik, D. Mikhailova, H. Ehrenberg, L. A. Schmitt, L. Dimesso, C. Förster and W. Jaegermann, J. Inorg. Chem., 2011, 4349–4359.
4. P. Nie, L. Shen, F. Zhang, L. Chen, H. Deng and X. Zhang, CrystEngComm, 2012, 14, 4284–4288.
5. C. V. Ramana, A. Ait-Salah, S. Utsunomiya, U. Becker, A. Mauger, F. Gendron and C. M. Julien, Chem. Mater., 2006, 18, 3788–3794.
6. B. Lung-Hao Hu, F. Y. Wu, C. T. Lin, A. N. Khlobystov and L. J. Li, Nat. Commun., 2013, 4, 1687.
7. M. Maccario, L. Croguennec, F. Le Cras and C. Delmas, J. Power Sources, 2008, 183, 411–417.
8. F. Omenya, N. A. Chernova, R. Zhang, J. Fang, Y. Huang, F. Cohen, N. Dobrzynski, S. Senanayake, W. Xu and M. S. Whittingham, Chem. Mater., 2012, 25, 85–89.
9. Y. Wang, B. Sun, J. Park, W.-S. Kim, H.-S. Kim and G. Wang, J. Alloys Compd., 2011, 509, 1040–1044.
10. J. Yang, J. Wang, Y. Tang, D. Wang, X. Li, Y. Hu, R. Li, G. Liang, T.-K. Sham and X. Sun, Energy Environ. Sci., 2013, 6, 1521.
11. A. Kraytsberg and Y. Ein-Eli, Adv. Energy Mater., 2012, 2, 922–939.
12. A. V. Murugan, T. Muraliganth, P. J. Ferreira and A. Manthiram, Inorg. Chem., 2009, 48, 946–952.
13. X. Rui, X. Zhao, Z. Lu, H. Tan, D. Sim, H. H. Hng, R. Yazami, T. M. Lim and Q. Yan, ACS Nano, 2013, 7, 5637–5646.
14. M. S. Whittingham, Chem. Rev., 2004, 104, 4271–4302.
15. S. Kandhasamy, A. Pandey and M. Minakshi, Electrochim. Acta, 2012, 60, 170–176.
16. M. M. Ren, Z. Zhou, X. P. Gao, W. X. Peng and J. P. Wei, J. Phys. Chem. C, 2008, 112, 5689–5693.
17. H. Liu, P. Gao, J. Fang and G. Yang, Chem. Commun., 2011, 47, 9110–9112.
18. J. Kim, J.-K. Yoo, Y. S. Jung and K. Kang, Adv. Energy Mater., 2013, 3, 1004–1007.
19. J. Su, X.-L. Wu, J.-S. Lee, J. Kim and Y.-G. Guo, J. Mater. Chem. A, 2013, 1, 2508–2514.
20. A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., 2001, 148, A224–A229.
21. D. Morgan, G. Ceder, M. Y. Saïdi, J. Barker, J. Swoyer, H. Huang and G. Adamson, Chem. Mater., 2002, 14, 4684–4693.
22. C. Masquelier and L. Croguennec, Chem. Rev., 2013, 113, 6552–6591.
23. H. Huang, S. C. Yin, T. Kerr, N. Taylor and L. F. Nazar, Adv. Mater., 2002, 14, 1525–1528.
24. J. Gaubicher, C. Wurm, G. Goward, C. Masquelier and L. Nazar, Chem. Mater., 2000, 12, 3240–3242.
25. C. Masquelier, C. Wurm, J. Rodríguez-Carvajal, J. Gaubicher and L. Nazar, Chem. Mater., 2000, 12, 525–532.
26. S. C. Yin, H. Grondey, P. Strobel, M. Anne and L. F. Nazar, J. Am. Chem. Soc., 2003, 125, 10402–10411.
27. B. L. Cushing and J. B. Goodenough, J. Solid State Chem., 2001, 162, 176–181.
28. Y. Lu, L. Wang, J. Song, D. Zhang, M. Xu and J. B. Goodenough, J. Mater. Chem. A, 2013, 1, 68–72.
29. Y. Tang, C. Wang, J. Zhou, Y. Bi, Y. Liu, D. Wang, S. Shi and G. Li, J. Power Sources, 2013, 227, 199–203.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra44951c

---