ORIGINAL PAPER

Effect of vanadium doping on electrochemical performanceof LiMnPO4 for lithium-ion batteries

Lele Su & Xiaowei Li & Hai Ming & Jason Adkins &

Mangmang Liu & Qun Zhou & Junwei Zheng

Received: 29 July 2013 /Revised: 20 October 2013 /Accepted: 23 October 2013 /Published online: 12 November 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract A series of LiMn1-xVxPO4 samples have been syn-thesized successfully via a conventional solid-state reactionmethod. The active materials are characterized by x-ray dif-fraction, x-ray photoelectron spectroscopy, and scanning elec-tron microscopy. The electrochemical performances of thesamples are tested using cyclic voltammetry, electrochemicalimpedance spectroscopy, and charge/discharge measurementtechniques. It is confirmed that the samples are in single phasewhen the content of vanadium (x ) is lower than 0.05. If thatcontent is higher than 0.1, the samples are shown to contain anadditional conductive phase of Li3V2(PO4)3. The vanadiumdoping significantly enhances the electrochemical propertiesof LiMnPO4. It is underlined that the optimal ratio for a low-vanadium doping with the best electrochemical performanceis 0.1 and this material exhibits a corresponding initial chargeand discharge capacity of 98.9 and 98.1 mAh g−1 at 0.1 Cunder 50 °C. The capacity retention is higher than 99 % after30 cycles. The dramatic electrochemical improvement of theLiMnPO4 samples is ascribed to the strengthened ability oflithium-ion diffusion and enhanced electronic conductivity forthe V-doped samples.

Keywords Lithium-ion batteries . LiMnPO4. Vanadium

doping . Electrochemical properties

Introduction

With increasing reliance on new intermittent forms of energy(e.g., solar and wind), energy-storage devices, such as batte-ries, will play an ever more important role in improving ourutilization of new component materials and boosting theirstorage efficiency [1, 2]. Theoretically, lithium-ion batteries(LIBs) could satisfy increased consumer demands due to theirhigh energy and power density [3, 4]. The cathode material,which is a crucial factor of the performance and safety of theLIBs [5], has the greatest potential for improvement [6, 7].Besides the well-known LiCoO2, LiNiO2, and LiMn2O4

[8, 9], lithium transition metal phosphates (LiFePO4,LiCoPO4, LiMnPO4), as positive electrodes in LIBs, havebeen the focus of many recent studies due to their non-toxicity [10], high theoretical capacity, reversibility, and ther-mal and electrochemical stabilities [11–14]. Among thesecathode materials used today, LiMnPO4 is lower in cost com-pared with LiCoPO4. In addition, LiMnPO4 has a higher redoxpotential (4.1 V vs. Li+/Li) [15] than that of LiFePO4 (3.4 V vs.Li+/Li) [16], indicating that LiMnPO4 has a higher-energydensity [17] than that of LiFePO4 [18]. Also, LiMnPO4 canprovide a moderate working voltage compatible to presentelectrolyte systems [17, 19, 20]. Therefore, LiMnPO4 has beenproposed as an excellent candidate for positive-electrode ma-terials for the next-generation LIBs [21].

However, the electrochemical performance of LiMnPO4 isstill far from satisfaction [22] because of the intrinsically lowionic and electronic conductivity (<10−10 S cm−1) [23] and theinterfacial strain between its lithiated and delithiated phase [8].Recent studies attempt to overcome such restrictions ofLiMnPO4 [24–26]. The primary synthetic approach for im-proving the electrochemical character is to minimize the par-ticle size or obtain uniform particle size distribution with

L. Su :H. Ming :M. Liu :Q. Zhou : J. ZhengCollege of Chemistry, Chemical Engineering, and Materials Science,Soochow University, Suzhou 215123, People’s Republic of China

X. Li (*) :H. Ming : J. Adkins : J. ZhengInstitute of Chemical Power Sources, Soochow University,Suzhou 215006, People’s Republic of Chinae-mail: lixiaowei@suda.edu.cn

J Solid State Electrochem (2014) 18:755–762DOI 10.1007/s10008-013-2315-9

preferential morphology [27]. For instance, Delacourt et al.synthesized nano-structured LiMnPO4 particles as small as100 nm, and obtained a reversible capacity of 70 mAh g−1 at0.1 C [28]. Carbon coating has been verified to be anotherefficient way to improve electronic conductivity [29], whichcan decrease the boundary resistance of crystals and therebyimprove the electrochemical performance of the materials[30]. As reported by Bakenov and Taniguchi, C-LiMnPO4,with a reversible capacity of 112 mAh g−1 at 0.1 C wasobtained via a combination of spray pyrolysis synthesis usingN2/H2 (97/3 vol.%) gas and wet ball-milling with acetyleneblack as a carbon source [12]. Although the above resultsexemplifies improved electrochemical performance by mini-mizing the particle size or using carbon coating, the chemicaldiffusion ability of lithium ions within the crystal structure isdiminished [31]. Moreover, minimizing the particle size andthe use of carbon coating may decrease the tap densitydramatically [32, 33]. The third approach involves dop-ing metal cations in the lattice of LiMnPO4 [34] to improveits conductivity and increase the diffusion ability of lithiumions. For example, solid-state synthesis of LiMnPO4 andLiMn1- yMyPO4 (M = Mg, Ca, Zn, etc.) samples at high tem-perature has been reported in patents byValence Technology Inc[35]. However, the cation doping (e.g., Mg, Ca, Zn) wouldreduce the ratio of the active material, leading to a decrease incapacity. Thus, researchers are focusing on the element dopingusing transition metals in the LiMnPO4 matrix, which can notonly enhance the conductivity, but also maintain or enhance thecapacity of thematerial. Among all of the elements, vanadium isa particularly attractive element to dope into the olivinelattice of LiMnPO4 since the introduction of vanadium canlead to the formation of an additional and more conductivephase Li3V2(PO4)3 [36], thus enhancing the electrochemicalconductivity of the LiMnPO4. And it has reported that theelectrochemical activity of LiMnPO4 can be significantlyenhanced by V doping. Yang et al. reported that the dis-charge capacity at 0.05 C in the voltage range of 2.7–4.5 Vwas 102 mAh g−1 for LiMn0.95V0.05PO4, and 62 mAh g−1

for pure LiMnPO4 [27]. Also, the electrochemical perfor-mance of LiFePO4 and LiCoPO4 can be improved via Vdoping [37, 38].

However, using vanadium as a component in batteries forportable electronics presents additional safety concerns tohumans or the environment, therefore the desirable vanadiumcontent is one which provides the best performance with theleast amount. Particularly, due to the toxic and environmen-tally unfriendly nature of vanadium, it is significant to inves-tigate the optimal ratio for a low-vanadium doping with thebest electrochemical performance.

On the basis of the above discussion, the research hereinfocus on investigating the effect of different contents of Vdoping, trying to find out an optimal ratio of vanadium, andenhancing the electrochemical properties of LiMnPO4.

Experimental

Material synthesis

The LiMnPO4 samples were synthesized via a simple solid-state method using lithium carbonate (Li2CO3, Sinopharm,AR), manganese acetate (Mn(CH3COO)2

.4H2O, Sinopharm,AR), and ammonium dihydrogen phosphate (NH4H2PO4,Sinopharm, AR) as starting materials. The reagents weresufficiently mixed by ball-milling for 3 h, using absoluteethanol as the milling media. Meanwhile, a certain amountof glucose was added, which served as the reducing agent.After completion of the ball-milling, the ethanol was evapo-rated; then, the mixed precursors were heated at 650 °C for 3 hwith a heating rate of 5 °C min−1 under a nitrogen atmospherein a tube furnace. Finally, the product of LiMnPO4 wasobtained. For vanadium-doped LiMnPO4, a stoichiometricamount of ammonium metavanadate (NH4VO3, Sinopharm,AR) was added to the starting reagents before ball-milling,and the preparation process was similar to that of LiMnPO4. Aseries of LiMn1-xVxPO4 products with different molar ratiosof vanadium (x =0.02, 0.05, 0.1, 0.2, 0.3, and 0.4) wereobtained.

Physical characterization

X-ray diffraction (XRD) patterns were acquired on aRigaku (Ultima IV) D/max-γA x-ray diffractometer withCu Kα radiation (λ=0.154178 nm) to identify the crystalstructure of the as-prepared materials. SEM images weretaken on a FEI-quanta 200F scanning electron microscopewith acceleration voltage of 30 kV. Energy dispersive x-rayspectroscopy (EDX) mapping was obtained with a SEM(Hitachi S-4700, Japan). X-ray photoelectron spectroscopy(XPS) was obtained by using a Kratos Axis ultra-DLD x-ray photoelectron spectrometer with a monochromatizedMg Kα x-ray (hν=1283.3 eV).

Electrochemical measurements

The electrochemical performance of the pure LiMnPO4 andLiMn1-xVxPO4 as cathodes were evaluated using CR2016-type half cells. The cathodes were prepared by mixing activematerials, Super P and polyvinylidene fluoride in a weightratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) solvent toform homogeneous slurry. The slurry was then pasted ontoaluminum foil to make the working electrodes, and thereafter,dried at 120 °C for hours to remove the NMP. After drying, thecoated aluminum foil was punched into 14-mm diameterdisks. The active material on every disk is about 3.2 mg.The half cells were assembled in an argon-filled glove box(Vigor, China) using metallic Li foil as the counter and refer-ence electrode, 1M LiPF6 in a mixture of ethylene carbonate

756 J Solid State Electrochem (2014) 18:755–762

and diethyl carbonate in the ratio of 1:1 (w/w, Zhangjiagang,China) as the electrolyte, and Celgard 2300 as the separator.The cells cycled in a constant current-constant voltage modeat 0.1 C to 4.5 V, held at 4.5 V until 0.01 C, and thendischarged to 2.5 V at a certain rate (where 1 C is170 mAh g−1) on a LAND CT2001A battery testing system(Wuhan, China). The cyclic voltammetry (CV) was record-ed with a scan rate of 0.1 mV s−1 between 2.5 and 4.5 Vandelectrochemical impedance spectroscope (EIS) was carriedout in a frequency range from 0.01 to 100 kHz with ACsignal of 5 mV via a CHI660D (Chenhua, Shanghai) elec-trochemistry workstation system.

Results and discussion

Characterization of pristine LiMnPO4 and V-doped LiMnPO4

The XRD patterns of the pure and V-doped samples areapplied to investigate the structural properties. According tothe XRD patterns in Fig. 1, it is evident that the main diffrac-tions peaks of all products are consistent with the standardLiMnPO4 (JCPDS, No. 33–0804), with an ordered olivinestructure indexed by Pnmb Space Group. Comparing theseseven patterns, there is no Li3V2(PO4)3 peaks in patterns ofpristine LiMnPO4 (curve a) and low V-doped LiMnPO4

(curve b, c), indicating that the V atoms are doped into theLiMnPO4 host lattice when the content of V is lower than0.05. The lattice constants of these three samples are listed inTable 1. The V-doped samples shrink the a - and c -axis yetexpand the b -axis, revealing that V atoms are introduced intoLiMnPO4 matrix structure. The cell volume of V-dopedLiMnPO4 shrink compared with that pristine LiMnPO4,which can be attributed to the comparability between the

atomic radii of the V3+ (0.78 Å) [1] and Mn2+ (0.8 Å) [39].Similar phenomena have been reported for V-doped (theoxidation state of V is +3) LiFePO4 [40]. However, whilecontinuing to increase the V content in LiMn1-xVxPO4, (withmore than 0.1), some other peaks emerge, which could beinduced by the Li3V2(PO4)3 [40]. Moreover, the relative peakintensity of the Li3V2(PO4)3 increases with the increasing ofthe vanadium content, indicating more and more content ofLi3V2(PO4)3 is produced. Similar phenomena were reportedby Xia et al. [20]. It is worth noting that no diffraction peaksfrom carbon are observed, which indicates that the residualcarbon is amorphous or the carbon layer on the samples isvery thin.

In order to indentify the oxidation state of V in thesamples, XPS characterization is carried out. As shown inFig. 2a, the XPS survey spectrum of sample possesses themain peaks centered at about 54, 133, 189, 286.4, 516.8/522.9, 534, and 642.1/653.9 eV, corresponding to the Li1s,P2p, P2s, C1s, V2p, O1s, and Mn2p, respectively. No otherpeaks are detected, indicating high purity of the samples.The binding energies of the V2p are 516.8 and 522.9 eV(Fig. 2b), which are assigned to the V2p3/2 and V2p1/2respectively, in accordance with that observed inLi3V2(PO4)3 [5, 41]. Hence, it can be concluded that theoxidation state of V in the sample is +3. Combined with theXRD analysis, it elucidates that V5+ has been reduced toV3+ in the presence of carbon coating under a floweringnitrogen atmosphere.

SEM images of pristine and typical V-doped (x =0.1)LiMnPO4 samples are shown in Fig. 3. It is revealed thatsome of the small particles congregate together to form somelarge secondary particles in both samples, however, V dopingslightly change the particle size and morphology of the sam-ples. For example, the pristine sample presents irregularblocks with a wide size distribution ranging from 400 to1,000 nm (Fig. 3a), while V-doped LiMnPO4 is between 300and 500 nm (Fig. 3b). The small particle size may allowimproved diffusion of lithium ions in samples. It implies thatV doping can effectively inhibit the crystal growth of samplesduring the thermal treatment, which can enhance the elec-trochemical performance of the active material by shorten-ing the lithium ion diffusion path. Figure 3c and d are theelemental mapping of Mn and V elements in the 0.1-V

Fig. 1 XRD patterns of the prepared samples of different vanadiumcontents: x =0, 0.02, 0.05, 0.1, 0.2, 0.3, and 0.4 (a–g), labeling displaypeaks of the Li3V2(PO4)3 phase

Table 1 Lattice constants of pristine and V-doped LiMnPO4 (x=0.02and 0.05)

x a (Å) b (Å) c (Å) V (Å3)

0 10.4387 [49] 6.0790 [50] 4.7621 302.1879 [51]

0.02 10.4482 6.0783 4.7562 302.0539

0.05 10.4523 6.0752 4.7481 301.6672

J Solid State Electrochem (2014) 18:755–762 757

doped sample, which confirms that V and Mn are in ahomogeneous distribution.

Electrochemical performance of different samples

Figure 4 shows the CV curves of the samples with variousvanadium contents to further understand the electrochemicalprocess. A slow scanning rate of 0.1 mV s−1 is employed inthis study. As displayed in Fig. 4a and b, only a couple ofredox peaks at 4.3 and 3.93 Vare observed both in the pristineLiMnPO4 and the low V doping (x =0.02), corresponding tothe phase-transition process between LiMnPO4 and MnPO4

[42]. The results are in accordance with XRD results that V is

doped into the LiMnPO4 host lattice when its content is low.Referring to other samples, three additional redox potentialpairs around 3.65/3.55, 3.70/3.60, and 4.12/4.0 V are ob-served. To our knowledge, the three visible redox peakssimultaneously appear only if vanadium exists in the form ofLi3V2(PO4)3. Nevertheless, combined with the valence +3 ofvanadium in the doped LiMnPO4 (x =0.02), it is reasonable tospeculate that when the vanadium content reaches its limit inthe host lattice, the excessive vanadium would produce theLi3V2(PO4)3 phase, although such a small amount ofLi3V2(PO4)3 could not be detected in the correspondingXRD patterns. Conclusively, the CV curves of the V-dopedsamples (x =0.05, 0.1, 0.2, and 0.4) observed here are simply

Fig. 2 a XPS full spectrum and b high-resolution spectrum of the V2p region of the 0.1-V doped LiMnPO4

Fig. 3 SEM images of pristineand typical V-doped LiMnPO4. ax=0, b x =0.1, along with EDXmapping of c manganeseand d vanadium

758 J Solid State Electrochem (2014) 18:755–762

superimposed profiles of LiMnPO4 and Li3V2(PO4)3. In therange of 2.5–4.5 V, Li3V2(PO4)3 can intercalate/de-intercalatebetween two Li+ reversibly based on the V3+/V4+ redox cou-ple [32]. The two anodic peaks observed at 3.65 and 3.70 Vcorrespond to the de-intercalation of the first Li+ with twosteps, while the second Li+ is removed via a single stepcorresponding to the last anodic peak observed at 4.12 V[31]. It is found the peak current density of the anodic andreductive peak ofMn is enhanced byV doping from Fig. 4a–f,which may result from the increased lithium-ion diffusion,facilitating the kinetic process of the electrochemical reactionsindicative of a better electrochemical performance.

The typical charge/discharge curves of electrodes withvarious contents of V are measured by a constant current-constant voltage charge/discharge test between 2.5 and4.5 V (vs. Li+/Li). The discharge characteristics of the firstcycle at 0.1 C under 50 °C are shown in Fig. 5. As seen in theprofiles of samples, a single flat plateau at about 4.25/3.95 V isobserved on samples with vanadium content up to 0.05, whichcorrelates with the intercalation/de-intercalation of Li+ into/from LiMnPO4. In contrast, with an increase in the vanadiumconcentration to 0.1, the charge plateau voltage decreasesfrom 4.25 to 4.16 V, while the discharge plateau voltageextends from 3.95 to 4.0 V. This feature implies that the

Fig. 4 CV profiles of the various samples (x=0–0.4) at the scanning rate of 0.1 mV s−1 and in a potential window of 2.5–4.5 V (vs. Li+/Li) at roomtemperature

J Solid State Electrochem (2014) 18:755–762 759

polarization of electrodes can be reduced by V doping.Besides these, three additional typical charge/discharge pla-teaus around 3.60/3.57, 3.68/3.65, and 4.09/4.0 V forintercalation/de-intercalation of two Li+ reversibly into/fromLi3V2(PO4)3 based on the V3+/V4+ redox couples [43] aredistinctly observed. These plateaus are in good accordancewith the charge/discharge profiles of Li3V2(PO4)3 and agreewell with the CV profiles in Fig. 4. Also, the vanadiumcontent is found to have a remarkable effect on the electro-chemical performance of the samples in this investigation.Noticeably, the capacities of the V-doped samples are higherthan that of the pure LiMnPO4. The capacities of the V-dopedsamples reach 135 mAh g−1 when the content of V increasesto 0.4. Xia etal [20] has reported the discharge capacities of (1-x)LMP·LVP composites, where the composite with x =0.4 hasthe best performance, which is in good agreement with ourresults. On the basis of the above analysis, both LiMnPO4 andLi3V2(PO4)3 contribute to the improved capacity.

Figure 6 shows the cycling behaviors of pure and V-doped(x =0–0.4) LiMnPO4 at 0.1 C at 50 °C. It is found that thecapacities reliably retained up to 30 cycles in all samples, andalmost no obvious capacity loss can be observed. However, V-doped samples exhibit a better discharge capacity and bettercycle life. More importantly, the materials show excellentcharge–discharge efficiency. For instance, that efficiency isalmost 99 % for the 0.1 V-doped sample. It is suggested thatthe addition of vanadium improves the conductivity of thesamples, which may help to obtain the reversible capacity ofthe samples.

In order to further investigate the influence of V content onthe electrochemical performance of LiMnPO4, the dischargecapacity of LiMnPO4 in the samples is calculated bysubtracting the capacity of Li3V2(PO4)3 from the total capac-ity according to the reported literature [44]. Figure 7 presentsthe discharge capacity of the LiMnPO4 with varying molar

ratios of vanadium. It is evident that the capacity of theLiMnPO4 increases with the increasing of the vanadium con-tent. When the molar ratio of V is in the range of 0 to 0.1, thecapacity of LiMnPO4 increases rapidly compared to the inter-val between 0.1 and 0.4. When V content is higher than 0.1,the capacity increasing of LiMnPO4 is negligible. Therefore, itis reasonable to conclude that the optimal amount of vanadi-um would be 0.1, which may provide good electrochemicalperformance of LiMnPO4 with relatively low V doping sincevanadium is toxic and the lowest concentration of V is desir-able in this application (LIBs).

EIS measurements are performed on V-doped (x =0.02,0.1, and 0.4) and pure LiMnPO4 electrodes (after 3 cycles inthe fully discharged state) to elucidate the possible mechanismof the improved electrochemical performance of the LiMnPO4

sample (Fig. 8). As has been well established, each spectrumin Fig. 8 shows a semicircle and an inclined straight line in thehigh-medium and low-frequency regions, respectively [45].

Fig. 5 The initial charge/discharge profiles of samples (x=0-0.4) in thevoltage range of 2.5–4.5 V at 50 °C

Fig. 6 Cyclic performance of samples (x=0–0.4) at 0.1 C at 50 °C

Fig. 7 Correlation between the discharge capacity of LiMnPO4 and Vcontent

760 J Solid State Electrochem (2014) 18:755–762

An intercept at the Z ′ axis in high frequency corresponds tothe ohmic resistance (R e), which represents the resistance ofthe electrolyte and the electrode. The semicircle in high-medium frequency can be attributed to the charge-transferresistance of the electrochemical reaction (R ct), while theinclined line in the low frequency represents lithium-ion dif-fusion in the active cathode material [31]. The detailed resultsof equivalent circuit are displayed in Table 2. The Rct value ofpure LiMnPO4 is 95.36 Ω, which is larger than that of the V-doped samples. The electronic conductivityσ e of the samplesare also listed in the table. The higher content of vanadium, thesmaller value of Rct, and the larger σ e. This indicates that Vdoping can decrease the charge-transfer resistance and in-crease the electronic conductivity of the samples. The diffu-sion of lithium ions could be calculated from the low-frequency spots according to the literature [31]. The resultclearly shows that the diffusion of lithium ions in sampleshave ameliorated significantly via V doping.

The possible mechanism for the electrochemical perfor-mance of pure and V-doped LiMnPO4 can be explained asfollows. Batteries fabricated from pure LiMnPO4 cannot becharged or discharged effectively due to the lower conductiv-ity [46] which limits the reversibility of lithium ion in theolivine LiMnPO4 structure [27]. V doping can improve theelectronic and ionic conductivity of the samples, because the

uniformly distributed smaller particles and V doping in thelattice of LiMnPO4 both improve the conductivity of thematerials [33, 37]. In our experiment, the pure LiMnPO4

sample has the largest Rct value and the slowest lithium ionicdiffusion. When the content of the vanadium is less than 0.1,vanadium has been doped into the lattice of LiMnPO4, whichimproves the intrinsic properties of electronic and ionic con-ductivity of the LiMnPO4, resulting in the dramatic change ofRct and DLi

+ . This can explain the better electrochemicalperformance of LiMnPO4 (x <0.1). A similar argument hasbeen reported for the V-doped LiFePO4 [47]. While the con-tent of the vanadium is higher than 0.1, the excess of vanadi-um induces the phase of Li3V2(PO4)3, which distributesevenly with LiMnPO4. The open three-dimensionalframework of Li3V2(PO4)3 material has a better conductiv-ity than LiMnPO4, making the lithium ion diffusion in theLiMn1-xVxPO4 samples faster [48]. But this improvement ofthe conductivity only occurs on the surface of LiMnPO4 par-ticles, which only induces a relatively small increase of capac-ity, as shown in Fig. 7. The electrochemical results demonstratea synergistic effect of LiMnPO4 and V doping on the electro-chemical performance of different samples. It is better tounderstand that using an ideal amount of vanadium to dopethe lattice of LiMnPO4 can greatly enhance the capacity ofLiMnPO4, while the use of excessive amounts of the vanadiumonly produces a slight effect on the capacity of LiMnPO4.

Conclusions

The LiMn1-xVxPO4 samples with different vanadium contentsare successfully prepared via traditional solid-state method.Compared with pure LiMnPO4, V-doped samples show anenhanced electrochemical performance. The enhanced prop-erties of LiMnPO4 are attributed to the introduction of vana-dium, which can not only increase the conductivity ofLiMnPO4 but also accelerate the lithium-ion diffusion. Inorder to make the cathode materials economically and envi-ronmentally friendly, 0.1-M ratio of vanadium in LiMnPO4

sample is the optimal ratio, which exhibits the specific dis-charge capacity of 98.9 mAh g−1 at 0.1 C under 50 °C. Ourresults are helpful for the better understanding and the en-hancement mechanism and effect upon the electrochemicalperformance of LiMnPO4, which provides advantageous in-sights on the rational of future design of LiMnPO4-based highpower LIBs with high charge/discharge capacities and bettercycle performance.

Acknowledgments Financial supports from the Nature Science Foun-dation of China (Nos. 20873089, 20975073), Nature Science Foundationof Jiangsu Province (Nos. BK2011272), Industry-Academia CooperationInnovation Fund Projects of Jiangsu Province (Nos. BY2011130), andkey laboratory of lithium-ion battery materials of Jiangsu Province aregratefully acknowledged.

Fig. 8 EIS plots of different samples (x=0, 0.02, 0.1, and 0.4)

Table 2 The value of Rct, σ e and DLi+ for pure and V-doped samples(x =0.02, 0.1, and 0.4)

x Rct (Ω) σ e (S cm−1) (×10−10) DLi+ (cm2 s−1) (×10−16)

0 95.36 4.1 1.80

0.02 74.89 5.2 3.42

0.1 55.34 7.0 5.15

0.4 43.63 8.9 7.43

J Solid State Electrochem (2014) 18:755–762 761

References

1. Hong J, Wang XL, Wang Q, Omenya F, Chernova NA,WhittinghamMS (2012) J Phys Chem C 116:20787–20793

2. Wang Y, Cao G (2008) Adv Mater 20:2251–22693. Ni J, Gao L (2011) J Power Sources 196:6498–65014. Manjunatha H, Suresh GS, Venkatesha TV (2011) J Solid State

Electrochem 15:431–4455. Zhang LL, Liang G, PengG, Zou F, HuangYH, CroftMC, Ignatov A

(2012) J Phys Chem C 116:12401–124086. Wang L, Li ZC, Xu HJ, Zhang KL (2008) J Phys Chem C 112:308–

3127. Yang S, Song Y, Ngala K, Zavalij PY, Whittingham MS (2003) J

Power Sources 119:239–2468. Pivko M, Bele M, Tchernychova E, Logar NZ, Dominko R,

Gaberscek M (2012) Chem Mater 24:1041–10479. Bini M, Ferrari S, Capsoni D, Massarotti V (2011) Electrochim Acta

56:2648–265510. Pei Z, Zhang X, Gao X (2013) J Alloys Compd 546:92–9411. Zheng JC, Li XH, Wang ZX, Li JH, Li LJ, Wu L, Guo HJ (2009)

Ionics 15:753–75912. Bakenov Z, Taniguchi I (2010) Electrochem Commun 12:75–7813. Bakenov Z, Taniguchi I (2010) J Power Sources 195:7445–745114. Fang H, Li L, Yang Y, Yan G, Li G (2008) Chem Commun 1118–

112015. Wang D, Buqa H, Crouzet M, Deghenghi G, Drezen T, Exnar I,

Kwon NH, Miners JH, Poletto L, Grätzel M (2009) J Power Sources189:624–628

16. Molenda J, Ojczyk W, Marzec J (2007) J Power Sources 174:689–694

17. Barpanda P, Djellab K, Recham N, ArmandM, Tarascon JM (2011) JMater Chem 21:10143–10152

18. Oh SM, Oh SW, Myung ST, Lee SM, Sun YK (2010) J AlloysCompd 506:372–376

19. Wang F, Yang J, Gao P, NuLi Y,Wang J (2011) J Power Sources 196:10258–10262

20. Li G, Azuma H, Tohda M (2002) Electrochem Solid-State Lett 5:A135–A137

21. Wang F, Yang J, NuLi Y, Wang J (2013) Electrochim Acta 103:96–102

22. Bramnik NN, Ehrenberg H (2008) J Alloys Compd 464:259–26423. Oh SM, Jung HG, Yoon CS, Myung ST, Chen Z, Amine K, Sun YK

(2011) J Power Sources 196:6924–692824. Kim JK, Chauhan GS, Ahn JH, Ahn HJ (2009) J Power Sources 189:

391–39625. Ma J, Qin QZ (2005) J Power Sources 148:66–7126. Yang G, Ni H, Liu H, Gao P, Ji H, Roy S, Pinto J, Jiang X (2011) J

Power Sources 196:4747–4755

27. Oh SM, Oh SW, Yoon CS, Scrosati B, Amine K, SunYK (2010) AdvFunct Mater 20:3260–3265

28. Dettlaff-Weglikowska U, Sato N, Yoshida J, Roth S (2009) PhyStatus Solidi B 246:2482–2485

29. Bakenov Z, Taniguchi I (2011) Mater Res Bull 46:1311–131430. Wang Y, Yang Y, Yang Y, Shao H (2010) Solid State Commun 150:

81–8531. Zhang LL, Liang G, Ignatov A, Croft MC, Xiong XQ, Hung IM,

Huang YH, Hu XL, Zhang WX, Peng YL (2011) J Phys Chem C115:13520–13527

32. Hong J, Wang CS, Chen X, Upreti S, Whittingham MS (2009)Electrochem Solid-State Lett 12:A33–A38

33. Kandhasamy S, NallathambyK,Minakshi M (2012) Prog Solid StateChem 40:1–5

34. Zhao Y, Cao GS, Xie J, Zhao XB (2012) J Solid State Electrochem16:1271–1277

35. Chen G, Wilcox JD, Richardson TJ (2008) Electrochem Solid-StateLett 11:A190–A194

36. Yang MR, Ke WH, Wu SH (2007) J Power Sources 165:646–65037. Sun CS, Zhou Z, Xu ZG,Wang DG,Wei JP, Bian XK, Yan J (2009) J

Power Sources 193:841–84538. Wang F, Yang J, NuLi Y, Wang J (2010) J Power Sources 195:6884–

688739. Hu C, Yi H, Fang H, Yang B, Yao Y, Ma W, Dai Y (2010)

ElectrochemCommun 12:1784–178740. Omenya F, Chernova NA, Upreti S, Zavalij PY, NamKW, Yang XQ,

Whittingham MS (2011) Chem Mater 23:4733–474041. WangH, Li Y, HuangC, ZhongY, Liu S (2012) J Power Sources 208:

282–28742. Doan TNL, Bakenov Z, Taniguchi I (2010) Adv Powder Techol 21:

187–19643. Liu H, Gao P, Fang J, Yang G (2011) Chem Commun 47:9110–911244. RenMM, Zhou Z, Gao XP, PengWX,Wei JP (2008) J Phys Chem C

112:5689–569345. Wang Y, Chen Y, Cheng S, He L (2011) Korean J Chem Eng 28:964–

96846. Rangappa D, Sone K, Zhou Y, Kudo T, Honma I (2011) J Mater

Chem 21:15813–1581847. Lin H, Wen Y, Zhang C, Zhang L, Huang Y, Shan B, Chen R (2012)

Solid State Commun 152:999–100348. Ren M, Zhou Z, Li Y, Gao XP, Yan J (2006) J Power Sources 162:

1357–136249. Xiao J, Chernova NA, Upreti S, Chen X, Li Z, Deng Z, Choi D, Xu

W, Nie Z, Graff GL, Liu J, WhittinghamMS, Zhang JG (2011) PhysChem Chem Phys 13:18099–18106

50. Du K, Zhang LH, Cao YB, Peng ZD, Hu GR (2012) Mater ChemPhys 136:925–929

51. Koleva V, Stoyanova R, Zhecheva E (2010) Mater Chem Phys 121:370–377

762 J Solid State Electrochem (2014) 18:755–762