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Cite this:Phys.Chem.Chem.Phys.,

2017, 19, 5155

Investigations on Zr incorporation intoLi3V2(PO4)3/C cathode materials for lithiumion batteries

Hua-Bin Sun,†a Ying-Xian Zhou,†a Lu-Lu Zhang,*a Xue-Lin Yang,*a

Xing-Zhong Cao,b Hanu Arave,c Hui Fangc and Gan Liangc

Li3V2(PO4)3/C (LVP/C) composites have been modified by different ways of Zr-incorporation via

ultrasonic-assisted solid-state reaction. The difference in the effect on the physicochemical properties

and the electrochemical performance of LVP between Zr-doping and ZrO2-coating has also been

investigated. Compared with pristine LVP/C, Zr-incorporated LVP/C composites exhibit better rate

capability and cycling stability. In particular, the LVP/C-Zr electrode delivers the highest initial capacity of

150.4 mA h g�1 at 10C with a capacity retention ratio of 88.4% after 100 cycles. The enhanced

electrochemical performance of Zr-incorporated LVP/C samples (LVZrP/C and LVP/C-Zr) is attributed to

the increased ionic conductivity and electronic conductivity, the improved stability of the LVP structure,

and the decreased charge-transfer resistance.

Introduction

As friendly storage devices, lithium ion batteries have manyadvantages, such as high safety, long lifetime, excellent electro-chemical properties and so on.1–3 Cathode materials have asignificant impact on the electrochemical performance of fullcells and have received widespread attention. Since the firstreport of LiFePO4 by Goodenough’s group in 1997,4 polyanioncathode materials have attracted much attention due totheir low cost, high safety and environmental friendliness.Compared with LiFePO4 (10�14–10�16 cm2 s�1), monoclinicLi3V2(PO4)3 (LVP) exhibits a higher lithium ion diffusion coefficientof 10�9–10�10 cm2 s�1.5 In addition, as we know, energy density isan important indicator to measure the quality of commercial cells,and the energy density of a cathode depends on its voltage andcapacity. Generally speaking, multi-electron cathodes can providelarger capacity and higher energy density in comparison with a‘‘single-electron’’ cathode.6,7 Compared with a ‘‘single-electron’’cathode such as LiFePO4, Li3V2(PO4)3 has a larger theoretical specificcapacity (197 mA h g�1 for complete extraction of three Li+ ions)

and higher operating voltages (B3.55, 3.65 and 4.0 V),8 leadingto a higher energy density (727 W h kg�1).9 Compared with othermulti-electron cathodes such as VOPO4

6 and Li3Mo4P5O24,7 LVPalso has larger reversible capacity (B130 mA h g�1 at 0.1C within3–4.5 V).10 However, its poor intrinsic electronic conductivity,low ionic conductivity and fast capacity fading at high voltage(44.6 V) delay the commercialization process of LVP.11,12 Nowa-days, tremendous efforts have been devoted to overcoming theabove problems, such as metal ion doping (Fe,13,14 Mg,15,16

Al,17,18 Cr,19 La,20 Ce,21,22 Mo,23 Sn,24 Y,25 Mn,26 Nb,27 Co,28

Bi,29 Zr,30,31 Na,32 Ca,33 etc.), carbon coating,34–36 oxide introduction(SiO2,37,38 MgO,39 RuO2,40 Al2O3,41 ZrO2,42 etc.) and particle sizereduction.43–45 Among them, some research of Zr-modificationhas been carried out. For example, Han et al.30 found thatZr-doping can improve the capacity and cycling performanceof LVP due to the formation of the LiZr2(PO4)3-like ionicconductor, which can facilitate Li+ ion migration and stabilizethe LVP structure. Xu et al.31 used first-principles calculationsto further reveal that Zr-doping does not change the LVPcrystal structure and positively affects the electronic struc-ture by lowering the band gap and increasing the DOS nearthe Fermi level. Besides as a dopant, Zr is also used as acoating element to modify LVP. For instance, Han et al.42

synthesized ZrO2-coated LVP/C and obtained improved cyclingperformance. In addition, ZrO2 has also been successfullyapplied to modify other cathode materials, such as LiCoO2,46,47

LiNiO2,48 LiMn2O4,49,50 LiFePO451 and LiNi1/3Co1/3Mn1/3O2.52–55

However, there are few reports on the difference in the effecton the physicochemical properties and the electrochemical

a College of Materials and Chemical Engineering, Hubei Provincial Collaborative

Innovation Center for New Energy Microgrid, China Three Gorges University,

8 Daxue Road, Yichang, Hubei 443002, China. E-mail: zlljoy@126.com,

xlyang@ctgu.edu.cnb Key Laboratory of Nuclear A Techniques, Institute of High Energy Physics,

Chinese Academy of Sciences, Beijing 100049, Chinac Department of Physics, Sam Houston State University, Huntsville, Texas 77341,

USA

† H. B. Sun and Y. X. Zhou contributed equally to this work.

Received 13th November 2016,Accepted 10th January 2017

DOI: 10.1039/c6cp07760a

rsc.li/pccp

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performance of electrode materials between Zr-doping andZrO2-coating.

In this study, we synthesized Zr-doped LVP/C and ZrO2-coated LVP/C composites, and systematically compared the effectof Zr-doping and ZrO2-coating on the physicochemical propertiesand the electrochemical performance of LVP by X-ray diffraction(XRD), transmission electron microscopy (TEM), X-ray photo-electron spectroscopy (XPS), positron annihilation lifetimespectroscopy (PALS), electrochemical impedance spectroscopy(EIS), cyclic voltammetry (CV) and constant current charge/discharge measurements.

ExperimentalSample synthesis

To prepare the Zr-doped LVP/C composite, stoichiometric lithiumcarbonate (Li2CO3), ammonium metavanadate (NH4VO3), ethanolzirconium ((CH3CH2O)4Zr) and ammonium dihydrogen phosphate(NH4H2PO4) (molar ratio of Li : V : Zr : P = 3.06 : 1.93 : 0.07 : 3) wereused as raw materials and then ball-milled for 10 h in ethanol. Afterdrying, the mixture was pre-calcined at 350 1C for 6 h in a nitrogenatmosphere to obtain gray-white powders. Subsequently, 15 wt%glucose, which acts as not only a carbon source but also a reductiveagent, was added to the resulting grey powders and further ball-milled for 6 h in ethanol. Finally, the above mixture was sintered at700 1C for 10 h with a heating rate of 3 1C min�1 in a nitrogenatmosphere and then cooled down slowly to achieve the Zr-dopedLVP/C composite (denoted as LVZrP/C). For comparison, pristineLVP/C was prepared by the same process without (CH3CH2O)4Zr.

To obtain the ZrO2-coated LVP/C composite, the pristineLVP/C powders were first dispersed in (CH3CH2O)4Zr ethanolsolution (3 wt% of LVP/C powders) by the ultrasonic method for2 h. Then, the mixture was continuously stirred using a magneticforce stirrer at 60 1C until it became dry. Subsequently, theprecursor was calcined at 600 1C for 5 h in a nitrogen atmospherewith a heating rate of 3 1C min�1, and then the ZrO2-coated LVP/Ccomposite (denoted as LVP/C-Zr) was obtained.

Sample analysis

The phase and crystalline structure of the as-prepared sampleswere characterized using an X-ray diffractometer (XRD, RigakuRINT-2000) with Cu-Ka radiation and a graphite monochromator.The morphology was observed with a field emission scanningelectron microscope (FESEM, JSM-7500F, JEOL) and a trans-mission electron microscope (TEM, JEM-2100, JEOL). Thecarbon content was evaluated using an IR carbon/sulfur systemequipped with a high frequency induction combustion furnace(HW2000B, China). The oxidation state and distribution of keyelements (V, Zr and C) in samples were studied using an X-rayphotoelectron spectrometer (XPS, PHI Quantera, U-P) assistedby Ar-ion sputtering. Electrical conductivity was measured by astandard four-probe method using an RTS resistivity measure-ment system (RTS-8, China) on disk-shaped pellets with adiameter of 8 mm and a thickness of about 1.0 mm. Toinvestigate the lattice defects induced by Zr-incorporation,

positron annihilation lifetime spectroscopy (PALS) measurementsof all the three samples were performed by using a high resolutionpositron annihilation lifetime spectrometer. PALS was carried outusing a fast-slow coincident ORTEC system equipped with BaF2 asa detector with a time resolution of 210 ps for the full width athalf-maximum and 22Na as a positron source with an intensity of13 mCi. Each sample was pressed into two disks (diameter: 8 mmand thickness: B1.0 mm), and the 22Na source was sandwichedbetween the two identical disks. The positron lifetime spectrawere de-convoluted and analyzed using the LT-9 software.

Electrochemical measurements

To prepare the electrodes, 75 wt% active material (i.e., LVP/C, LVZrP/C and LVP/C-Zr), 15 wt% conducting agent (acetylene black) and10 wt% binder (polyvinylidene fluoride, PVDF) were mixed inN-methyl pyrrolidinone (NMP) solvent and stirred for 12 h. Theresulting slurry was cast on an aluminum foil and NMP wasevaporated at B60 1C to form a uniform film. After being pressedat a pressure of 6 MPa, the disc electrodes with 14 mm diameterwere dried at 120 1C for 8 h in a vacuum and then transferred intoan argon-filled glove box (Super 1220/750, Mikrouna). Electroche-mical cells were assembled in a CR2025-type coin cell using Celgard2400 as a separator, lithium foil as counter and reference electrodes,and 1.1 M LiPF6/(EC + PC + EMC + DEC) (15 : 20 : 25 : 40 wt%) as anelectrolyte. Constant current charge/discharge measurements wereperformed at various rates on a cell testing system (LAND CT2001A,China) within a voltage range of 3.0–4.8 V. Electrochemical impe-dance spectroscopy (EIS) measurements were performed on anelectrochemical working station (CHI614C, China). Cyclic voltam-metry (CV) was conducted on a multichannel electrochemical work-ing station (Ivium-n-Stat, Netherlands).

Results and discussion

The XRD patterns of LVP/C, LVZrP/C and LVP/C-Zr samples areshown in Fig. 1. As observed in Fig. 1a, it is obvious that all the

Fig. 1 (a) XRD patterns and (b–d) Rietveld refinement results of LVP/C,LVZrP/C, and LVP/C-Zr, respectively.

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three XRD patterns are similar, and all the peaks can beassigned to the monoclinic LVP with space group P21/n (JCPDS,No. 72-7074).6,56 The results indicate that Zr-incorporation(including Zr-doping and ZrO2-coating) does not affect theLVP phase. Moreover, no diffraction peaks for crystalline carboncan be observed, indicating that the residual carbon is amor-phous and/or the amount of carbon is too low to be detected.The carbon content of all samples is similar (2.70, 2.74 and2.61% for LVP/C, LVZrP/C and LVP/C-Zr, respectively). Further-more, Rietveld refinements were also carried out on the threesamples by using the Maud program software,57,58 and theresults are shown in Fig. 1b–d and Table 1. It is obvious thatthe cell volume of LVZrP/C (896.01 Å3) is bigger than that ofpristine LVP/C (891.41 Å3) due to the larger ionic radius of Zr4+

(0.079 nm)31,59 than that of V3+ (0.074 nm),14,60 which demon-strates that Zr has been doped into the LVP crystal lattice to someextent. By contrast, LVP/C-Zr shows similar lattice parameters anda unit cell volume as LVP/C, which indicates that incorporating Zras a coating element has little influence on the LVP structure.However, in both XRD patterns of LVZrP/C and LVP/C-Zr, the ZrO2

diffraction peaks also do not appear because its content is too lowto be detected and/or ZrO2 is amorphous.

To further verify Zr entering into the LVP lattice or existingon the surface of LVP particles, XPS measurements were carriedout assisted by Ar-ion sputtering, and the results are shownin Fig. 2. All the binding energies were calibrated by carbon(EC1s = 284.5 eV). Due to the chemical reduction of Ar-ionsputtering,61 all XPS peaks in the interior shift slightly towardslow binding energy compared to those on the surface. More-over, from the high-resolution XPS spectra of LVZrP/C and

LVP/C-Zr (Fig. 2a1 and b1), it can be clearly seen that the V2p3/2

peaks on the surface are weaker than those in the interior, whichconfirms that there should be some substances (such as carbonand zirconia) coated on the surface of the LVP particle. As shownin Fig. 2a1 and b1, the V2p3/2 peaks at B516.0 eV in both LVZrP/Cand LVP/C-Zr correspond to the +3 state,14 demonstrating thatZr-incorporation does not change the oxidation state of V3+.Interestingly, in Fig. 2a2 and b2, the Zr3d peaks are obviousnot only in the interior but also on the surface. For LVZrP/C(Fig. 2a2), the Zr3d peaks in the interior exhibit almost the sameintensity as those on the surface, which confirms that besidesentering into the LVP lattice, a substantial amount of Zr exists onthe LVP surface due to the atomic diffusion during the high-temperature sintering process. And for LVP/C-Zr (Fig. 2b2), theZr3d peaks on the surface are much stronger than those in theinterior, which confirms that most Zr exists on the LVP surfaceand only a small amount of Zr diffuses into the LVP lattice due tothe atomic diffusion during the sintering process. Fig. 2a2 and b2

present the Zr3d peaks at about 185.0 and 182.5 eV, whichconfirms that the oxidation state of Zr is +4.31,62 When Zrenters into the LVP lattice, Zr4+ ions may substitute for V inthe LVP host crystals, leading to generate the LiZr2(PO4)3 phase(a super-ionic conductor)30 and generate Li+ vacancies to keepthe charge balance; when Zr exists on the LVP surface, it mayform ZrO2 and fabricate a hybrid coating layer with pyrolyticcarbon. Both LiZr2(PO4)3 and Li+ vacancies may accelerate Li+ iontransportation; and the hybrid coating layer composed of C andZrO2 may effectively enhance the electronic and ionic conductivity,respectively. The enhancement of ionic conductivity should beascribed to the intrinsic lattice defects in ZrO2 formed during thehigh-temperature sintering process. Besides, for both LVZrP/C andLVP/C-Zr, the stronger C1s peaks on the surface (Fig. 2a3 and b3)than those in the interior indicate that carbon is also coated on thesurface of LVP particles. That is, ZrO2 and C form a hybrid coatinglayer, which is beneficial for the structural stability because it canmore efficiently alleviate vanadium dissolution in the electrolyte.42

In addition, the distribution of Zr and other major elements

Table 1 Lattice parameters of LVP/C, LVZrP/C and LVP/C-Zr

Sample a (Å) b (Å) c (Å) V (Å3) s Rw

LVP/C 8.6075 8.6028 12.0382 891.41 0.84 10.39LVZrP/C 8.6148 8.6330 12.0477 896.01 0.77 8.31LVP/C-Zr 8.6064 8.6039 12.0381 891.40 0.82 9.06

Fig. 2 XPS spectra of (a) LVZrP/C and (b) LVP/C-Zr.

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(V, P, O and C) in LVZrP/C and LVP/C-Zr particles was furtherinvestigated by EDX mapping analysis. As seen in Fig. 3, the V,P, O, C and Zr elements are uniformly distributed in bothsamples.

As we know, the positron annihilation technique (PAT) is apowerful approach to analyze the defects induced by doping.Table 2 shows the results of positron annihilation lifetimespectroscopy (PALS), and the average lifetime is calculatedby tm = t1 � I1 + t2 � I2. Among the three lifetime components(t1, t2 and t3), the longest lifetime component t3 is expectedfrom the intergranular spacing in powder samples or thecontact surface between the radioactive source and the sample,and the relative intensity I3 is the weakest (o2.5%), so it isignored in the present work. The shorter lifetime (t1) isassigned to the free annihilation of positrons in the defect-free crystal and smaller defects in the bulk of samples, and I1

quantifies the degree of crystal integrity or the abundance ofsmall defects; while the intermediate lifetime component t2 isresponsible for the positrons captured by larger defects nearthe grain boundary and the sample surface, and I2 quantifiesthe abundance of large defects.63–66 Therefore, t1 reflects thechange in the crystal structure, which is also affected by thechange in t2, while t2 reflects the change in larger defects inthe sample. That is to say, both t1 and t2 reveal the intrinsiccharacteristics of samples.67 As seen in Table 2, I1 of all the

three samples indicates that both pristine and Zr-incorporatedLV/C samples are not perfect crystals. LVZrP/C exhibits muchhigher t1 and I1 than LVP/C, which indicates that incorporatingZr as a dopant has a significant influence on the LVP structureand may generate a large amount of smaller defects, such as Li+

vacancies induced by Zr-doping at V sites induced by Zr-dopingat Li sites,17 while LVP/C-Zr only shows a slight increase in t1

and I1, indicating that incorporating Zr as a coating elementhas little influence on the LVP structure, which is consistentwith the XRD results. The slight increase in t1 and I1 for LVP/C-Zris ascribed to the Li+ vacancies induced by the small amount ofZr diffusion into the LVP lattice during the high-temperaturesintering process. Li+ vacancies may contribute to intrinsicionic conductivity, thereby leading to an improved electro-chemical performance. From Table 2, it can also be seen thatboth LVZrP/C and LVP/C-Zr show similar t2 to LVP/C, whichmeans that the size of large defects changes very little. More-over, LVZrP/C shows an obviously reduced I2, which means thatthe decrease of large defects is induced by the increase of smalldefects resulting from Zr-doping, whereas LVP/C-Zr showssimilar I2 values as LVP/C due to the following two aspects:on the one hand, ZrO2 generates some lattice defects during thehigh-temperature sintering process; on the other hand, nano-sized ZrO2 particles may fill in some larger defects on thesurface of samples. So, compared with LVP/C, LVP/C-Zr hasalmost similar I2 as LVP/C. It is generally known that theelectron density is inversely proportional to the positron anni-hilation lifetime.65 That is to say higher electron density meanshigher electronic conductivity. As seen in Table 2, both LVZrP/Cand LVP/C-Zr exhibit decreased average lifetime tm, indicativeof enhanced electronic conductivity, which agrees well with themeasured electronic conductivity (i.e. 0.31 � 10�4 S cm�1 forLVP/C, but 2.27 � 10�4 and 2.86 � 10�4 S cm�1 for LVZrP/C andLVP/C-Zr, respectively).

Fig. 4 shows the morphology and the microstructure of LVP/C,LVZrP/C and LVP/C-Zr powders. As shown in Fig. 4a1–c1, all thesamples exhibit an irregular particle shape, indicating thatZr-incorporation has no apparent effect on the LVP morphology.The distinct lattice fringes of LVP in Fig. 4a2–c2 indicate good

Fig. 3 EDX mapping of (a) LVZrP/C and (b) LVP/C-Zr.

Table 2 Positron annihilation lifetime and relative intensity of samples

Sample LVP/C LVZrP/C LVP/C-Zr

t1 (ps) 157.7 � 6.8 177.1 � 6.0 161.6 � 7.1I1 (%) 37.7 � 2.4 50.9 � 3.1 39.5 � 2.8t2 (ps) 294.1 � 4.7 305.1 � 7.3 289.2 � 5.2I2 (%) 59.8 � 2.4 47.0 � 3.1 58.6 � 2.8t3 (ps) 2234 � 32 2253 � 42 2227 � 38I3 (%) 2.42 � 0.12 2.03 � 0.13 1.93 � 0.11tm (ps) 235.3 233.5 233.3 Fig. 4 TEM images of (a) LVP/C, (b) LVZrP/C, and (c) LVP/C-Zr.

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crystallinity of all the three samples, and all the LVP particles arecoated with an amorphous layer with a thickness of 3–8 nm.Combined with the results of XPS, it is reasonable to assume thatthe amorphous coating layer of LVZrP/C and LVP/C-Zr is justcomposed of carbon and ZrO2.

Galvanostatic charge/discharge measurements were per-formed on the three samples in a voltage range of 3.0–4.8 V.As seen in Fig. 4a, there are four charge plateaus around 3.61,3.69, 4.09 and 4.56 V, corresponding to the phase transitionbetween two adjacent single phases of Li3�xV2(PO4)3 (x = 0, 0.5,1.0, 2.0, 3), respectively.29 The discharge curves consist of anS-shape curve and subsequently two plateaus at B3.63 and3.56 V: the S-shape curve is indicative of a solid solutionmechanism of the intercalation from V2(PO4)3 to Li2V2(PO4)3,and the two plateaus are associated with the two-phase transi-tions from Li2V2(PO4)3 to Li2.5V2(PO4)3 and from Li2.5V2(PO4)3 toLi3V2(PO4)3

40 in turn. As shown in Fig. 5 and Table 3, bothLVZrP/C and LVP/C-Zr show much higher capacity than LVP/C.For example, LVP/C-Zr exhibits the highest initial capacity of170.2 mA h g�1 at 1C, 168.1 mA h g�1 at 5C and 150.4 mA h g�1

at 10C; and LVZrP/C also delivers a higher initial capacity of160.7 mA h g�1 at 1C, 152.0 mA h g�1 at 5C and 133.0 mA h g�1

at 10C than the pristine LVP/C electrode (156.2 mA h g�1 at 1C,135.9 mA h g�1 at 5C and 121.4 mA h g�1 at 10C).

Moreover, after 100 cycles, the LVP/C-Zr electrode ownscapacity retention ratios of 77.8% at 1C, 82.3% at 5C and88.4% at 10C; and the LVZrP/C electrode can retain capacityretention ratios of 81.1% at 1C, 84.9% at 5C and 87.7% at 10C;whereas LVP/C only retains capacity retention ratios of 73.8%at 1C, 75.8% at 5C and 83.8% at 10C. Obviously, whether

introducing Zr as a dopant or a coating element is very usefulto enhance the rate capability and cycling stability of LVP.Comparatively, introducing Zr as a coating element has a morepositive influence on the rate capability.

To investigate the reasons for the effect of different ways ofZr incorporation on the electrochemical performance of LVP,the EIS measurements were performed with a frequency rangeof 0.01 Hz to 100 kHz. Fig. 6 shows the EIS spectra of LVP/C,LVZrP/C and LVP/C-Zr. All the three EIS curves are composed ofa small intercept, a depressed semicircle and a straight line,which demonstrates that the electrochemical process is con-trolled by both charge transfer and diffusion of Li+ ions. Theintercept at the Z0 axis in the high frequency region representsthe ohmic resistance (Rc), corresponding to the resistance ofthe electrolyte. The semicircle in the high frequency region isrelated to the charge transfer resistance (Rct) and the double-layer capacitance between the electrolyte and the cathode (Cdl).The inclined line is assigned to Warburg impedance associatedwith lithium ion diffusion within the electrode (Zw). All the EIScurves can be fitted by an equivalent circuit composed of‘‘R(C(RW))’’ using the ZSimpWin program.68,69 Obviously, bothLVZrP/C and LVP/C-Zr show decreased charge-transfer resistance(36.21 O for LVZrP/C and 30.12O for LVP/C-Zr) than LVP/C (47.08O).In order to further explore the impact of Zr-incorporation on theelectrochemical kinetics of LVP, the EIS spectra of all the threesamples are also tested after various cycles at 5C. As shown in Fig. 7,

Fig. 5 (a) The initial charge/discharge curves and (b) the cycle perfor-mance profiles of the as-prepared samples at 1, 5, and 10C.

Table 3 Discharge capacity and capacity retention ratio at 1, 5 and 10C(unit: mA h g�1)

Samples LVP/C LVZrP/C LVP/C-Zr

1C 1st 156.2 160.7 170.2100th 115.2 130.3 132.5Capacity retention (%) 73.8 81.1 77.8

5C 1st 135.9 152.0 168.1100th 103.0 129.0 138.3Capacity retention (%) 75.8 84.9 82.3

10C 1st 121.4 133.0 150.4100th 101.7 116.6 132.9Capacity retention (%) 83.8 87.7 88.4

Fig. 6 EIS spectra of the as-prepared samples and the locally amplifiedSEI spectra in the inset.

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it is obvious that, after different cycles, the Nyquist plots for LVP/C,LVZrP/C and LVP/C-Zr samples are similar. As listed in Table 4, it isclearly seen that the Rct value of all three samples after cycling ismuch bigger than that of the fresh cells due to the electrolytepenetration into the electrode, the structural change, the solidelectrolyte interface (SEI) film formation and so on.70 In addition,the Rct value of LVZrP/C after 1, 50, 100 and 150 cycles is 70.18, 65.32,61.01 and 38.09 O, respectively, and that of LVP/C-Zr is 101.0, 75.60,75.40 and 92.97 O, respectively; whereas the Rct value of LVP/C is129.7, 89.85, 98.96 and 123.7 O, respectively. As a comparison withLVP/C, LVZrP/C and LVP/C-Zr show a lower Rct value at each cycle,indicative of improved electrode kinetics, leading to improved ratecapability and cycling stability. Two things may account for thereduced Rct value of LVZrP/C and LVP/C-Zr: one is the generatedLiZr2(PO4)3 phase (a super-ionic conductor) and Li+ vacancies, whichmay enhance the electronic and ionic transmission; the other is thehybrid coating layer of ZrO2 and C on the LVP surface, which can

more efficiently alleviate vanadium dissolution in the electrolyte as aresult of improved structural stability.

Compared with LVZrP/C, LVP/C-Zr exhibits a higher Rct

value due to the high content of ZrO2 in the coating layer. Allthe three samples show decreased resistance in the 50th cyclecompared to the 1st cycle, which can be attributed to theelectrode formation/surface modification.71 With the increaseof cycle number (from 50th to 100th), compared with LVP/C,both LVZrP/C and LVP/C-Zr exhibit a slightly reduced Rct value,which is ascribed to the enhanced structural stability inducedby the hybrid layer coating of ZrO2 and C. After 150 cycles,LVZrP/C shows a continuously reduced Rct value, but bothLVP/C-Zr and LVP/C show an increased Rct value, which canbe explained by the positron annihilation lifetime. The highert1 and t2 values of LVZrP/C demonstrate that LVZrP/C has moresmall and large defects for Li+ ion transmission, resulting in areduced Rct value.

Fig. 8 shows the CV curves of the three samples at differentscan rates within the voltage range of 3.0–4.3 V (vs. Li/Li+). It isapparent that all electrodes exhibit a similar shape to threeredox couples.72 The first two anodic and cathodic peaks

Fig. 7 EIS spectra of (a) LVP/C, (b) LVZrP/C and (c) LVP/C-Zr electrodesafter various cycles at 5C.

Table 4 Charge transfer resistance of LVP/C, LVZrP/C and LVP/C-Zr aftervarious cycles at 5C

Samples

Rct (O)

1st 50th 100th 150th

LVP/C 129.7 89.85 98.96 123.7LVZrP/C 70.18 65.32 61.01 38.09LVP/C-Zr 101.0 75.60 75.40 92.97

Fig. 8 CV curves and the corresponding linear fitting results of the Ip vs.v1/2 relationships of (a) LVP/C, (b) LVZrP/C, and (c) LVP/C-Zr samples atdifferent scan rates.

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correspond to the extraction/insertion of the first Li+ ion withtwo steps, corresponding to the phase transition process ofLi3V2(PO4)3 - Li2.5V2(PO4)3 - Li2V2(PO4)3.25 The third coupleis attributed to the extraction/reinsertion of the second Li+ ion,associated with the V4+/V3+ redox reaction.73 CV curves are oftenused in determining the lithium ion diffusion coefficients (D)of electrode materials through the following equation:38,74,75

Ip = 2.69 � 105n3/2AD1/2v1/2C (1)

where Ip is the current intensity (A), n is the number ofelectrons per species reaction, A is the surface area of theelectrode (cm2), D is the lithium ion diffusion coefficient(cm2 s�1), v is the scan rate (V s�1), and CLi+ is the concentrationof lithium ions (mol cm�3). Here, to simplify the calculation, weselected 1 � 10�3 mol cm�3 as the value of CLi+. The lithium iondiffusion coefficients were calculated using eqn (1) and theresults are shown in Table 5. Obviously, the Zr-incorporatedLVP/C electrodes including LVZrP/C and LVP/C-Zr have largerlithium ion diffusion coefficients than pristine LVP/C, espe-cially LVP/C-Zr owns the largest lithium ion diffusion coeffi-cients. The results imply that LVP/C-Zr should present the bestelectrochemical performance, which is consistent with theresults in Fig. 5. The reason for the improved electrochemicalperformance of Zr-incorporated LVP/C samples (LVZrP/C andLVP/C-Zr) is attributed to the following factors: (1) the generatedLi+ vacancies and LiZr2(PO4)3 are beneficial for building fastionic pathways to accelerate Li+ ion transportation and reactionkinetics; (2) ZrO2 existing on the surface of LVP could providesome lattice defects to facilitate both electron and Li+ iontransportation; (3) the hybrid coating layer of (C + ZrO2) couldefficiently alleviate vanadium dissolution in the electrolyte toimprove the stability of the LVP structure.

Conclusions

Zr-incorporated Li3V2(PO4)3 composites were successfully pre-pared via a solid-state reaction. Our results reveal that whetherintroducing Zr as a dopant or a coating element, Zr exists notonly in the LVP lattice but also on the LVP surface due to theatomic diffusion during the high-temperature sintering pro-cess. Zr4+ substitutes for V sites in the LVP lattice, which formthe LiZr2(PO4)3 phase and generate vacancies to accelerate Li+

ion transportation. Some Zr exists in the form of ZrO2, whichmay generate some lattice defects to facilitate both electron andLi+ ion transportation. The resulting ZrO2 and pyrolytic carbonform a hybrid layer coating on the LVP surface, which mayefficiently alleviate vanadium dissolution in the electrolyteto improve the stability of the LVP structure. Therefore, the

Zr-incorporated LVP/C electrodes exhibit better rate capabilityand cycling stability than pristine LVP/C, especially LVP/C-Zrdelivers the highest initial capacity of 150.4 mA h g�1 at 10Cwith a capacity retention ratio of 88.4% after 100 cycles.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (51572151, 51302153 and 51272128) andthe Outstanding Youth Science and Technology InnovationTeam Project of Hubei Educational Committee (T201603).

Notes and references

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Table 5 Lithium diffusion coefficients of the as-prepared samples

Samples Peak c/cm2 s�1 Peak c0/cm2 s�1

LVP/C 6.71 � 10�12 6.56 � 10�12

LVZrP/C 16.24 � 10�12 15.68 � 10�12

LVP/C-Zr 35.50 � 10�12 30.77 � 10�12

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