C cathode material with high rate cycling performance for lithium-ion batteries

Journal of Power Sources 357 (2017) 117e125

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

High energy density of Li3
xNaxV2(PO4)3/C cathode material with high rate cycling performance for lithium-ion batteries Zong-Lin Zuo a, Jian-Qiu Deng a, b, c, **, Jin Pan a, Wen-Bin Luo b, *, Qing-Rong Yao c, ***, Zhong-Min Wang a, c, Huai-Ying Zhou a, c, Hua-Kun Liu b a b c

School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China Institute for Superconducting and Electronic Materials, University of Wollongong, Squires Way, Fairy Meadow, NSW 2500, Australia Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China

h i g h l i g h t s  Micro-sized Li2.6Na0.4V2(PO4)3/C increase tap density.  High energy density of 478.8 Wh kg
1.  Excellent high-rate capacity and cycling performance.  Consolidate crystal structure by Na doping.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 February 2017 Received in revised form 11 April 2017 Accepted 29 April 2017

A serials of micro-sized Li3
xNaxV2(PO4)3/C composite has been synthesized by sol-gel method, comprised of numerous primary nanocrystals. This structure can efficiently facilitate lithium-ion transport in secondary aggregated individual particles due to the short diffusion distance among primary nanocrystals, along with a high tap density. With the increasing of Na doping content, the structure evolution occurs in Li3-xNaxV2(PO4)3 from a single-phase structure to a two-phase structure. The appearance of rhombohedral phase can provide a larger free volume of the interstitial space, fastening ionic movement to offer an excellent high rate capability. Furthermore, Na doping can stabilize the rhombohedral structure of the V2(PO4)3 framework, leading to the remarkable cycling stability. Among all the composites, Li2.6Na0.4V2(PO4)3/C presents the best electrochemical performance with a high energy density of 478.8 Wh kg
1, delivering high initial discharge capacities of 121.6, 113.8 and 109.7 mAh g
1 at the rate of 5 C, 10 C and 20 C in a voltage range of 3.0 e 4.3 V, respectively. It also exhibit an excellent high rate cycling performance, with capacity retention of 85.9 %, 81.7 % and 76.5 % after 1000 cycles at the rate of 5 C, 10 C and 20 C in a voltage range of 3.0 e 4.3 V. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lithium-ion batteries Cathode materials Lithium vanadium phosphate Na doping

1. Introduction During the past decades, lithium-ion batteries (LIBs) have become the dominant power sources for portable electronics and the electrification of transportation owing to their high capacity retention, high energy density and long cycling life [1e3]. However,

* Corresponding author. ** Corresponding author. School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China. *** Corresponding author. E-mail addresses: [email protected] (J.-Q. Deng), [email protected] (W.-B. Luo), [email protected] (Q.-R. Yao). http://dx.doi.org/10.1016/j.jpowsour.2017.04.106 0378-7753/© 2017 Elsevier B.V. All rights reserved.

the power and energy densities of lithium-ion batteries cannot satisfy the increasing requirement of EVs and HEVs, especially the high energy density and excellent cycling performance at high rate [4,5]. NASICON-structured monoclinic Li3V2(PO4)3 (LVP) as one of polyanion materials has been considered as a promising cathode material for lithium-ion batteries because of its high theoretical specific capacity (197 mA h g
1), thermal stability, and excellent cycling stability [6,7]. However, the low intrinsic electronic conductivity and slow lithium-ion diffusivity restrict the practical application of Li3V2(PO4)3. To overcome these shortcomings, various strategies have been developed to improve the electronic and ion conductivity of Li3V2(PO4)3, such as carbon coating [8e10], metal ion doping [11e13], and reducing the particle size [14e16].

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For example, carbon coating is an efficient approach to improve the electron conductivity and electrochemical performance [17,18], while particle size decrease can provide the short pathway for both lithium ions and electrons, by increasing the contact area with the electrolyte to enhance electrode kinetics. The low tap density and serious aggregation of nanoparticle, however, degrade the lifecycle performance and energy density of the cells [18,19]. Metal ion doping is also an efficient strategy to improve intrinsic properties of Li3V2(PO4)3, without degrading its electrochemical properties. Besides substitution of V3þ sites with metal ions such as Al3þ, Mn2þ, Ni2þ or Fe3þ [11e13], Naþ ion have also been used to partially replace the Li sites in the LVP structure to enhance the cycling performance and rate capability due to the improved structural stability and increased Li-ion mobility in Li3
xNaxV2(PO4)3 [20e28]. However, the enhanced performance is far away from the requirements for high-power Li-ion batteries, particularly at high rate. Therefore, in this paper, a series of Li3
xNaxV2(PO4)3/C (0.2  x  0.5) composite was successfully synthesized through a simple sol-gel method and the pristine LVP was also synthesized for comparison. The Na doping content has great influence on the phase composition and electrochemical performance by enlarging the interstitial space and stabilizing the crystal frame. Based on the electrochemical performance, Li2.6Na0.4V2(PO4)3/C exhibits the best electrochemical properties. It exhibits an excellent cycling stability at high rate with high energy density, promising as a candidate for future lithium-ion batteries. 2. Experimental details The Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4, 0.5) composite were synthesized by a sol-gel method [29]. Firstly, NH4VO3 (0.01 mol) and C2H2O4 $ 2H2O (0.015 mol) were dissolved in 50 mL of deionized water under vigorous stirring at 70  C to obtain a clear blue solution. Secondly, stoichiometric LiOH $ H2O, NH4H2PO4, Na2CO3 and glucose were added to the solution, and the mixed solution was stirred continually till the green precursor gel was obtained. The amount of glucose was based on 10 wt.% residual carbon content of Li3
xNaxV2(PO4)3/C. The obtained precursor gel was heated at 120  C for 12 h in a vacuum oven to achieve a dry gel. Finally, the gel precursor was pre-heated at 350  C for 4 h and calcined at 750  C for 10 h under Ar/H2 (90:10) flow in a tube furnace to yield black products. Powder X-ray diffraction (XRD) measurements were performed on PLXcel 3D X-ray diffractometer using Cu Ka radiation to identify the crystalline phase of the materials. The crystallographic structure analysis by the Rietveld method was carried out using the program of Fullprof Suite. The carbon content of the samples was determined by thermogravimetric (TG) analysis. X-ray photoelectron spectroscopy was carried out using an ESCALAB-250Xi spectrometer. To measure the electronic conductivities of materials, the Li3
xNaxV2(PO4)3/C powders are pressed into the disks with a diameter of 13 mm and a thickness of about 1 mm under 2 MPa pressure. A thin layer of conductive silver paste was spread over the both upper and lower surfaces. The obtained disks were dried at 60  C for 6 h in a vacuum oven, and then tested by a Keiythley 2400 digital sourcemeter to get the I - V curves of the samples. The electronic conductivities could be calculated by using I/U. The morphology of the samples was observed using field emission scanning electron microscope (FESEM, Hitachi S-4800) and highresolution transmission electron microscope (HRTEM, Tecnai G2 F20, 200 kV). The electrochemical tests of Li3
xNaxV2(PO4)3/C composites were conducted in a 2032-type coin cell. The working electrodes were fabricated by mixing the active material, acetylene black, and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 in

Nmethyl-2-pyrrolidone (NMP). The slurry mixture was pasted onto Al foil and dried at 110  C for 12 h in vacuum condition. The active material loading of the working electrode is about 1.5e2.0 mg cm
2. The electrolyte solution was 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) with a volume ratio of 1:1:1. The coin cells were assembled in an argon-filled glove box using pure lithium plate as the anode and Celgard 2325 as the separator. The galvanostatic chargeedischarge measurements were carried out by an Arbin battery testing system. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were conducted on a Solartron electrochemical workstation. 3. Results and discussion The X-ray diffraction patterns of the Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) samples are shown in Fig. 1. It is obvious that the structure of Li3
xNaxV2(PO4)3/C evolves gradually with the increase of sodium doping content. The diffraction peaks of LVP and Li2.8Na0.2V2(PO4)3 can be indexed to a monoclinic structure (PDF No. 01-078-1106) with a space group of P21/n, which is consistent with the previous results [30]. Li3
xNaxV2(PO4)3/C (x ¼ 0.3, 0.4 and 0.5) samples have a mix phase structure, consisting of monoclinic Li3V2(PO4)3 and rhombohedral Li3V2(PO4)3. The results reveal that a low amount of sodium doping does not change the structure of LVP, while gradual phase transformation occurs with increasing the sodium content up to x ¼ 0.3 and 0.4, from monoclinic LVP into rhombohedral LVP. With further increasing the value of x ¼ 0.5, the rhombohedral LVP exists as the main phase in the sample of Li2.5Na0.5V2(PO4)3/C, in accordance with the reported results in literature [24,31]. No diffraction peaks assigned to carbon phase are observed in XRD patterns, indicating that the residual carbon is amorphous. In order to further investigate the fine structure of the Li3
xNaxV2(PO4)3/C samples, the XRD patterns were refined using the software Fullprof Suite, as shown in Fig. 1(c). The obtained parameters are reliable: Rwp ¼ 8.94%; Rp ¼ 6.02%; and Rexp ¼ 7.32%. The structural information for the Li3
xNaxV2(PO4)3/C samples from the refinement calculation is listed in Table 1. For Li2.6Na0.4V2(PO4)3/C sample, the mole ratio of monoclinic structure and rhombohedral structure is 81:19. The lattice parameters of monoclinic LVP in the Li2.6Na0.4V2(PO4)3/C sample are a ¼ 8.607 Å, b ¼ 8.593 Å and c ¼ 12.034 Å. XPS measurement was performed to investigate the oxidation states of V in the samples. The V2p3/2 XPS spectra of Li2.6Na0.4V2(PO4)3 is displayed in Fig. 1(d). The V2p3/2 peak at a binding energy of 517.02 eV corresponds to the oxidation state of V3þ and is in good agreement with the results reported by Wang [24] and Oh [27]. Fig. 2 presents the SEM images of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5). All the irregular individual micro-sized particles are aggregated by numerous primary nanocrystals. Besides this, a part of particles are agglomerated together and exhibit an accumulation phenomenon. A typical HRTEM image of Li2.6Na0.4V2(PO4)3/C sample is shown in Fig. 2(f). A uniform amorphous carbon layer covers on the surface of the particles, and the thickness of carbon coating layer is about 6.5 nm. This carbon coating layer can efficiently enhance the electronic conductivity of the active materials [32,33]. The carbon content determined from TG analysis is 6.3%, 6.6%, 5.2%, 7.5% and 7.3% in the Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) composites, respectively. The CV curves of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) samples were recorded at a scanning rate of 0.05 mV s
1 in the potential range of 3.0 e 4.3 V, as shown in Fig. 3(a). Three anodic peaks of LVP/C at around 3.60, 3.68 and 4.1 V and three cathodic peaks at 3.55, 3.63 and 4.03 V are observed, corresponding to a sequence of phase transition processes of LixV2(PO4)3 at x ¼ 2.5, 2.0,

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119

Fig. 1. XRD patterns of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) and the V2p3/2 XPS spectra of Li2.6Na0.4V2(PO4)3.

Table 1 The phase composition of Li3
xNaxV2(PO4)3/C samples. Sample

x¼0

x ¼ 0.2

x ¼ 0.3

x ¼ 0.4

x ¼ 0.5

M-LVP (%) R-LVP (%)

100 0

100 0

83 17

81 19

61 39

1.0 [27,30]. The CV curves for Li2.8Na0.2V2(PO4)3 are similar to those of LVP. However, the Li3
xNaxV2(PO4)3/C (x ¼ 0.3, 0.4 and 0.5) samples exhibit four pairs of oxidation and reduction peaks. Three redox couples at around 3.6/3.5 V, 3.7/3.6 V and 4.1/4.0 V are ascribed to the extraction/interaction of lithium ions in the monoclinic phase and another a couple of redox peaks at 3.8/3.7 V is assigned to the reversible reaction lithium ions in the rhombohedral phase [24,34]. From Fig. 3(a), the anodic peak associated to extraction of lithium ions from rhombohedral becomes gradually more intense with the increase of sodium doping content, suggesting the main phase change from monoclinic structure to rhombohedral structure in the composites. CV curves of the Li2.6Na0.4V2(PO4)3/C sample tested at the scan rates of 0.05, 0.1, 0.2, 0.5 and 1 mV s
1 are displayed in Fig. 3(b). Even at relatively high scan rate (1 mV s
1), the well-defined and symmetrical redox peaks can be observed, implying the good cycling and rate performance of the sample. A linear relationship between the highest redox peak current and the square root of scan rate is depicted in Fig. 3(c), indicative of a diffusion controlled lithium ions extraction/interaction process. The chemical diffusion coefficient of lithium ions in the composites can be calculated by the following Randles-Sevcik Eq. (1):

  1=2 * 1=2 ip ¼ 2:69  105 n3=2 ADLiþ CLi y

(1)

Where ip is the peak current (A), n is the charge-transfer number * is the (n ¼ 2), A is the surface area of electrode (1.54 cm2), CLi concentration of lithium ions in the composites, and y is the scan rate (V s
1). Based on the Eq. (1) and the slope of ip vs. y1/2 shown in Fig. 3(c), the chemical diffusion coefficient of lithium ions in

Li3
xNaxV2(PO4)3/C composites (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) is calculated to be 3.24  10
10, 1.12  10
9, 6.29  10
10, 1.39  10
9 and 8.46  10
10 cm2 s
1, respectively. The highest lithium-ion diffusion coefficient of Li2.6Na0.4V2(PO4)3/C among all the composites implies the fastest lithium-ion transport and the best rate performance in this active material. The rate capability of Li3
xNaxV2(PO4)3/C composites (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) was evaluated by galvanostatic chargeedischarge testing at various rates of 0.1 C, 0.5 C, 1 C, 5 C, 10 C and 20 C (1 C ¼ 133 mAh g
1) in the voltage range of 3.0 e 4.3 V. The typical chargeedischarge profiles of Li2.6Na0.4V2(PO4)3/C sample at various C-rates is presented in Fig. 4(a). The chargeedischarge profiles at 0.1 C e 1 C rate clearly exhibit four well defined charge/discharge plateaus at 3.58, 3.67, 3.76 and 4.05 V, in consistent with the redox peaks in the CV curve (Fig. 3(a)). The charge/discharge plateaus at 3.76 V should be attributed to the rhombohedral phase in the asprepared composite. However, the voltage plateaus gradually degrade along with the increased C-rates, which may be attributed to the electrode polarization at the high current density. The rate capability of Li3
xNaxV2(PO4)3/C is displayed in Fig. 4(b). Compared with pristine LVP, all the Na-doped Li3
xNaxV2(PO4)3/C (x ¼ 0.2 e 0.5) samples deliver a high capacity beyond 120 mAh g
1 at 0.1 C. It indicates that Na doping is helpful for improving the discharge capacity of LVP. Among all the Li3
xNaxV2(PO4)3/C samples, the Li2.6Na0.4V2(PO4)3/C sample delivers the best rate capability, implying the optimal Na doping content of x ¼ 0.4. The discharge capacities are 126.7, 127.5, 126.3, 121.3, 118.1 and 112.3 mAh g
1 at 0.1 C, 0.5 C, 1 C, 5 C, 10 C and 20 C, respectively. When the current rate is turned back to 0.5 C, the discharge capacity can recover 124.6 mAh g
1, as high as 97.7 % the first capacity tested at same Crate for the Li2.6Na0.4V2(PO4)3/C sample after 55 cycles, indicating an outstanding capacity retention. This excellent rate capability of Li2.6Na0.4V2(PO4)3/C is superior to the previous nanostructured or doped LVP/C composites [18,35]. Whereas, pristine LVP/C sample delivers the lowest capacities at all applied C-rates and also exhibits the worst rate capability (Fig. 5(b)). The gravimetric energy density of Li3
xNaxV2(PO4)3/C composites (x ¼ 0.2, 0.3, 0.4 and 0.5) is 473.1,

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Fig. 2. SEM images of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5): (a) x ¼ 0, (b) x ¼ 0.2, (c) x ¼ 0.3, (d) x ¼ 0.4 and (e) x ¼ 0.5; (f) HRTEM and SAED images of Li2.6Na0.4V2(PO4)3/C.

470.8, 478.8 and 453.2 Wh kg
1, respectively, which is higher than that of the pristine LVP (406.6 Wh kg
1) and LiMn2O4 (z 430 Wh kg
1), but lower than that of LiFePO4 (z 530 Wh kg
1) [36]. Fig. 5 (aee) show the charge/discharge profiles of the Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) samples at a rate of 1 C in the voltage range of 3.0 e 4.3 V. Three pairs of flat charge/discharge plateaus are observed for the pristine LVP/C and Li2.8Na0.2V2(PO4)3/ C samples. The samples of Li3
xNaxV2(PO4)3/C (x ¼ 0.3, 0.4 and 0.5) display the similar well-defined four charge/discharge plateaus. The discharge plateau at 3.72 V is becoming increasingly long and flat, and other three plateaus are becoming gradually short. Especially, the Li2.5Na0.5V2(PO4)3/C sample exhibits clearly a pair of long charge/discharge platforms at 3.77/3.72 V (Fig. 5(e)), originating from V3þ/V4þ redox couple in rhombohedral LVP. The evolution of charge/discharge profiles is due to the gradual increase of rhombohedral phase in the Li3
xNaxV2(PO4)3/C composites. The corresponding cycle performance of the Li3
xNaxV2(PO4)3/C composites is present in Fig. 5(f). All the samples display superior cycling stability at 1 C, which may be attributed to the uniform carbon coating layer. The carbon coating can enhance the electronic conductivity and improve efficiently cycling life of the samples. All the Na-doped samples have higher capacity than the pristine LVP, which is

ascribed to the fact that the substitution of Li sites by Na with larger radius enlarges the cell volume of LVP and offers larger channel for rapid lithium ion diffusion in active materials, resulting in the high capacity [20e22,27]. In addition, a single-phase reaction mechanism and the high ionic conductivity in rhombohedral phase facilitate the diffusion of lithium ions in the active materials [24,34]. Amongst all the Li3
xNaxV2(PO4)3/C samples, the Li2.6Na0.4V2(PO4)3/ C delivers the highest capacity and the best cycling stability. The initial capacity is 127.9 mA h g
1, extremely close to the theoretical capacity (129.3 mAh g
1). After 200 cycles, the capacity retention of the Li2.6Na0.4V2(PO4)3/C is 95.3 %. The specific capacity and cycling stability are higher than the previous results [34,37]. The cycling performance of the Li3
xNaxV2(PO4)3/C composites at high rates of 5 C, 10 C and 20 C is shown in Fig. 6 (aec). Compared to the pristine LVP, all the Na-doped samples deliver an enhanced initial capacity at high rates. The Li3
xNaxV2(PO4)3/C (x ¼ 0.3, 0.4 and 0.5) samples show superior cycling performance at high rate. This can be attributed to the structural evolution from the monoclinic phase into rhombohedral phase with the Na doping content x  0.3. The rhombohedral structure can provide a larger free volume of the interstitial space, facilitating a faster ionic movement to offer an excellent rate capability. Furthermore, Na doping can stabilize the rhombohedral structure of the V2(PO4)3 framework,

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121

Fig. 3. (a) Cyclic voltammograms of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) at a scan rate of 0.05 mV s
1 in the potential range of 3.0e4.3 V. (b) Cyclic voltammograms of Li2.6Na0.4V2(PO4)3/C at various scan rates. (c) A linear relationship between the highest redox peak current and the square root of scan rate of Li2.6Na0.4V2(PO4)3/C.

Fig. 4. (a) The typical charge
discharge profiles of Li2.6Na0.4V2(PO4)3/C sample at various C-rates. (b) Rate capability of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) at various current rates.

leading to the remarkable cycling stability [34,38]. It is worth noting that compared with previous reported ion-doped LVP/C cathodes as shown in Table 2 [21,23,24,26,39
44], this Li2.6Na0.4V2(PO4)3/C exhibits the best electrochemical performance amongst all the samples. The high initial discharge capacities are 121.6, 113.8 and 109.7 mAh g
1, and the capacity retentions are 85.9 %, 81.7 % and 76.6 % after 1000 cycles at a rate of 5 C, 10 C and 20 C, respectively (Fig. 6(d)). To further calculate the electronic conductivity and ion diffusion model of the Li2.6Na0.4V2(PO4)3/C sample, the electronic conductivity and electrochemical impedance measurements of samples were performed in this work. IeV curves of the samples were obtained using a digital sourcemeter (Keiythley 2400) and the electronic conductivities could be calculated by using I/U. As clearly observed in Table 3, the electronic conductivity of Li2.7Na0.3V2(PO4)3/C and Li2.6Na0.4V2(PO4)3/C is larger than that of LVP/C. Compared with Liþ ion (0.76 Å), the doping of Na with larger

ion radius (1.02 Å) could enlarge a-axis of LVP and result in distortion of V positions and narrowing the band gap, thus increase the electronic conductivity of Na-dope LVP [20,27]. The Li2.6Na0.4V2(PO4)3/C sample presents the highest electronic conductivity (1.53  10
3 S cm
1) among all the samples, which suggests the best electron transport properties and electrochemical performance. The electrochemical impedance spectra of the electrodes were also tested in the fully discharge state of the cells after 1000 cycles at 20 C. The AC voltage amplitude of 5 mV was employed in the frequency range of 0.1 Hze100 kHz. All the Nyquist plots (Fig. 7(a)) demonstrate a semicircle in high-middle frequency and a sloping line in the low frequency. The impedance spectra were simulated by an equivalent circuit model [42], as displayed in Fig. 7(b). The resistor Rs represents to the electrolyte resistance. The semicircle corresponds to the charge-transfer resistance (Rct) and the double layer capacitance (CPE). The sloping line is assigned to Warburg impedance (Zw) associated with lithium-ion diffusion in

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Fig. 5. The charge
discharge profiles of the Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) samples at a rate of 1C in the voltage of 3.0e4.3 V, (a) x ¼ 0, (b) x ¼ 0.2, (c) x ¼ 0.3, (d) x ¼ 0.4, (e) x ¼ 0.5; (f) Cycling performance of Li3
xNaxV2(PO4)3/C at 1C rate.

Fig. 6. The cycling performance at 5 C (a), 10 C (b) and 20 C (c) of Li3
xNaxV2(PO4)3/C samples. (d) A comparison of cycling performance of Li2.6Na0.4V2(PO4)3/C at various rates.

the electrodes. The Rs and Rct are listed in Table 3. Li2.6Na0.4V2(PO4)3/C sample exhibits the lowest Rct (42.1 U), indicating the fastest transport of the electrons in the electrodes and resulting in the best electrochemical performance. Whereas, the Rct of Li2.8Na0.2V2(PO4)3/C is higher than that of the pristine LVP/C,

demonstrating the inferior high-rate performance, which is in good agreement with the results at high rate. In order to further investigate the structural stability of Nadoped samples during the charge-discharge process, ex-situ XRD experiments were performed in the angle range of 10 e 60 , and

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123

Table 2 A comparison of Li2.6Na0.4V2(PO4)3/C in this work to previous ion-doped LVP/C cathodes in the voltage range of 3.0e4.3 V. Cathode

Size

Carbon content

1C (mAh g
1)

Rates

Initial capacity (mAh g
1)

Retention rate

Potential (V)

Li2.9Na0.1V2(PO4)3/C Li2NaV2(PO4)3/C Li2.5Na0.5V1.98Ni0.03(PO4)3/C Li2NaV2(PO4)3/C Li3V1.95Ce0.5(PO4)3/C Li3V1.92Bi0.03(PO4)3/C Li3V1.92Ti0.06(PO4)3/C Li3V2(PO4)3@CMK-3 Li3V2(PO4)3/graphene Li3V2(PO4)3/C Li2.6Na0.4V2(PO4)3/C

0.5e2 mm ~200 nm micrometer 200-400 nm 0.2 e 6 mm micrometer < 1 mm < 50 nm 150 - 400 nm sub-micrometer micrometer

6.5 % 7.87 % e e 3.34 % 3-4% 6.65 % 10.1 % 3.9 % 6.2 % 7.5 %

e 130 e 133 133 133 132 133 140 133 133

5C 1C 1C 2C 10 C 10 C 5C 5C 20 C 10 C 1C 5C 10 C 20 C

113.6 ~105.0 108.0 ~88.0 88.6 ~104.0 ~116.0 ~120.0 89.6 106.0 127.9 121.6 113.8 109.7

99.4 % (80 cycles) 92.8 % (100 cycles) 99.0 % (50 cycles) 93 % (500 cycles) 94.3 % (100 cycles) 90.2 % (500 cycles) ~100.0 % (50 cycles) 80.0 % (300 cycles) 89.5 % (500 cycles) 83.0 % (800 cycles) 95.3 % (200 cycles) 85.9 % (1000 cycles) 81.7 % (1000 cycles) 76.6 % (1000 cycles)

3.0 3.0 3.0 3.0 3.0 3.0 3.2 3.0 3.0 3.0 3.0

e e e e e e e e e e e

4.3 4.4 4.3 4.3 4.3 4.3 4.3 4.3 4.5 4.3 4.3

Ref. [21] [23] [24] [26] [39] [40] [41] [42] [43] [44] This work

Table 3 The electronic conductivities and fitted results of impedance spectra for Li3
xNaxV2(PO4)3/C samples. Sample

x¼0

x ¼ 0.2

x ¼ 0.3

x ¼ 0.4

x ¼ 0.5

s (S cm
1) R s ( U) Rct (U)

3.30  10
4 3.3 75.5

5.80  10
4 6.4 100.0

7.61  10
4 9.2 54.3

1.53  10
3 4.2 42.1

3.15  10
4 4.3 91.0

Fig. 7. (a) The Nyquist plots of Li3
xNaxV2(PO4)3/C samples. (b) The fitted impedance spectra for Li2.6Na0.4V2(PO4)3/C.

the XRD patterns of Li2.6Na0.4V2(PO4)3/C sample were presented in Fig. 8. The diffraction peaks shifted to a higher 2q angle during the first charge process, and come back the original position at 100 % state of discharge (SOD), demonstrating the structure of Na-doped materials can reversibly recover to the original phase after full discharge. It is noted that a small diffraction peak at 33 exists the sample at 50 % and 100 % state of charge (SOC), which is assign to rhombohedral LiV2(PO4)3 [34,45]. The reversible structural evolution of the materials in the charge - discharge process demonstrates the excellent cycling stability of the Na doped-LVP/C sample.

4. Conclusions

Fig. 8. Ex-situ XRD patterns of Li2.6Na0.4V2(PO4)3/C during the initial charge-discharge cycle.

In summary, serials of Li3
xNaxV2(PO4)3/C (x ¼ 0, 0.2, 0.3, 0.4 and 0.5) cathode material have been successfully synthesized by a solegel method. The micro-sized particle is aggregated by numerous primary nanocrystals, which can enhance the energy density and shorten the ion transfer distance during the whole reaction process. With increasing of Na doping content, the single monoclinic structure transforms into a mixed two-phase structure. Meanwhile, the electrochemical properties, especially high-rate cycling

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stability, are dramatically improved with the increasing Na doping content. Among all the cathode materials, Li2.6Na0.4V2(PO4)3/C composite exhibits the best performance, delivering an initial discharge capacity of 127.9 mAh g
1 and a capacity retention of 95.3% after 200 cycles at 1 C rate in the voltage range of 3.0 e 4.3 V. Even at a high rate of 20 C, it also offers a high capacity of 109.7 mAh g
1 and good capacity retention of 76.6 % after 1000 cycles. This enhancement can be primarily attributed to the appearance of rhombohedral phase, providing a larger free volume of the interstitial spaces to fasten ionic movement for offering an excellent rate capability. Meanwhile, Na doping can stabilize the rhombohedral structure of the V2(PO4)3 framework, also leading to the remarkable cycling stability. This work demonstrates that Li2.6Na0.4V2(PO4)3/C is a promising cathode material for future high-power lithium ion batteries, also indicating that a proper amount of Na doping is an effective way to significantly enhance the electrochemical performance of electrode materials. Acknowledgements

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This work was financially supported by the National Natural Science Foundation of China (No. 21363005, 51371061 and 51661009) and the Guangxi Natural Science Foundation (2016GXNSFGA380001).

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