Ceramics International 42 (2016) 16557–16562

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Effect of Fe-doping followed by CþSiO2 hybrid layer coating onLi3V2(PO4)3 cathode material for lithium-ion batteries

Hua-Bin Sun a, Lu-Lu Zhang a,n, Xue-Lin Yang a,n, Yun-Hui Huang b, Zhen Li a,Ying-Xian Zhou a, Xiao-Kai Ding a, Gan Liang c

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 Chinab School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei, 430074 Chinac Department of Physics, Sam Houston State University, Huntsville, TX, 77341 USA

a r t i c l e i n f o

Article history:Received 17 May 2016Received in revised form12 July 2016Accepted 12 July 2016Available online 13 July 2016

Keywords:Lithium ion batteryLithium vanadium phosphateFe-dopingHybrid layer coating

x.doi.org/10.1016/j.ceramint.2016.07.07542/& 2016 Elsevier Ltd. All rights reserved.

esponding authors.ail addresses: zlljoy@126.com (L.-L. Zhang), xly

a b s t r a c t

A novel Li3V2(PO4)3 composite modified with Fe-doping followed by CþSiO2 hybrid layer coating (LVFP/C-Si) is successfully synthesized via an ultrasonic-assisted solid-state method, and characterized by XRD,XPS, TEM, galvanostatic charge/discharge measurements, CV and EIS. This LVFP/C-Si electrode shows asignificantly improved electrochemical performance. It presents an initial discharge capacity as high as170.8 mA h g�1 at 1 C, and even delivers an excellent initial capacity of 153.6 mA h g�1 with capacityretention of 82.3% after 100 cycles at 5 C. The results demonstrate that this novel modification withdoping followed by hybrid layer coating is an ideal design to obtain both high capacity and long cycleperformance for Li3V2(PO4)3 and other polyanion cathode materials in lithium ion batteries.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Polyanion monoclinic lithium vanadium phosphate(Li3V2(PO4)3, LVP) cathode material has received extensive atten-tion because it behaves the highest theoretical capacity(197 mA h g�1) in all the reported transition metal phosphate [1–4]. However, the low electronic conductivity (�2.4�10�7 S cm�1

at room temperature) and the rapid capacity fading at high voltage(44.6 V) of LVP [5,6] limit its large-scale applications. Nowadays,much effort has been made to overcome these shortages, such assurface coating [7–14], metal ionic doping (Fe [14,,15], Cr [16], W[17], Ti [18], Co [19], Ni [20]), particle size reducing [21–23] and soon. Among them, metal ionic doping has been regarded as an ef-fective way to enhance the intrinsic electronic conductivity [16].Especially, Fe-doping has been successfully applied to modify LVP[14,15]. For instance, Ren et al. [14] first reported Fe-doping canimprove the electrochemical performance and cyclic stability ofLVP. Our group [15] also systematically investigated the influenceof Fe-doping on the electrochemical performances of LVP and re-vealed the modification mechanism. Besides metal ionic doping,carbon coating has also been proved to be another effective way tomodify LVP [21,24]. However, excess carbon will hamper lithium

ang@ctgu.edu.cn (X.-L. Yang).

ion diffusion as well as decrease the tap density. Recently, it hasbeen reported [9,10,25,26] that carbon and oxide co-coating canremarkably enhance the electrochemical performance of LVP,especially the rate capability and cycle stability. There are twomain reasons: on the one hand the coexistence of carbon andoxide in the surface layer enables high conductivity for both Li ionsand electrons and thus facilitates the charge transfer reactions atthe electrode interface [4,25], on the other hand carbon and oxideco-coating can give more effective protection for active materialagainst direct contact with the electrolytes and thus improves thestructural stability of LVP [4,10]. However, it is difficult to improvethe intrinsic electronic conductivity of LVP only by carbon and/oroxide coating. As for doping, only single doping can't effectivelyprevent vanadium ion from dissolution into electrolyte so that it isnot easy to obtain desirable cycling performance of LVP whencharged to high voltage (44.6 V).

Considering the integration of high capacity and long cycleperformance, in this work, we design a novel modified method ofLVP with Fe-doping followed by CþSiO2 hybrid layer coating forthe first time (labeled as LVFP/C-Si). In LVFP/C-Si, Fe-doping cansignificantly enhance both electric conductivity and lithium iondiffusion coefficient, while the amorphous CþSiO2 hybrid coatinglayer can improve the structural stability of LVP. Under the com-mon influence of doping and coating, this novel LVFP/C-Si cathodematerial exhibits an excellent rate capability with good capacity

H.-B. Sun et al. / Ceramics International 42 (2016) 16557–1656216558

retention, e.g., delivering an initial capacity as high as153.6 mA h g�1 with capacity retention of 82.3% after 100 cycles at5 C.

2. Experiment

Li3V2�xFex(PO4)3/C (x¼0, 0.05) composites were synthesizedby traditional solid state method as follows: firstly, lithium car-bonate (Li2CO3), ammonium metavanadate (NH4VO3), ferric oxide(Fe2O3) and ammonium dihydrogen phosphate (NH4H2PO4) wereball-milled for 10 h in alcohol. Considering the volatilize of Li2CO3

during sintering process, the amount of Li2CO3 is a little more thanstoichiometric ratio, i.e., the molar ratio of Li:V:Fe:P¼3.06:2�x:x:3 (x¼0 and 0.05). After dried, the mixture was pre-sintered at350 °C for 6 h with a rate of 3 °C min�1 in N2 atmosphere to getthe precursor. Then, glucose (15 wt% of active materials), as carbonsource and reducing agent, was added into the above precursorand followed by ball-milling for another 6 h in alcohol. Finally, theresulting powder was sintered at a higher temperature of 700 °Cfor 10 h in N2 atmosphere to obtain the carbon coated Li3V2(PO4)3and Li3V1.95Fe0.05(PO4)3 composites (denoted as LVP/C and LVFP/C,respectively).

To incorporating Si, LVP powders (i.e., LVP/C and LVFP/C, re-spectively) were dispersed in a TEOS/alcohol solution (�3 wt%TEOS of powders) by ultrasonic method for 2 h. Then, the mixturewas stirred by a magnetic stirrer at �60 °C to evaporate alcohol.Subsequently, the precursor was heat-treated at 600 °C for 5 h inN2 atmosphere to obtain the Si-incorporated LVP and LVFP com-posites, respectively (denoted as LVP/C-Si and LVFP/C-Si). Theoverall synthesis process for LVFP/C-Si is schematically illustratedin Fig. 1.

The phase and crystalline structure of the LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si were studied by X-ray diffraction (XRD, RigakuRINT-2000) with Cu-kα radiation in a 2θ range of 10–70°. Themorphology of composites was analyzed by transmission electronmicroscopy (TEM, JEM-2100, JEOL). Combined with Ar-ion sput-tering, X-ray photoelectron spectroscopy (XPS, PHI Quantera, U-P)was performed to analyze the valence state and distribution of keyelements (V, Fe, C and Si) in samples. The residual carbon contentin the four samples was measured with an IR carbon/sulfur ana-lyzer (HW2000B, China). The Si content of LVP/C-Si and LVFP/C-Siwas tested by gravimetric method. Electronic conductivity wasmeasured by RTS resistivity measurement system (RTS-8, China)on disk-shaped pellets with diameter of 8 mm and thickness of�1.0 mm.

Electrochemical performance of the as-prepared samples (i.e.,LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si, respectively) was in-vestigated by assembling CR2025 coin cells. The active materialswere mixed with acetylene black conductor and PVDF binder in

Fig. 1. Schematic illustration of the

N-methyl pyrrolidinone solvent at a weight ratio of 75:15:10. Theobtained slurry was uniformly cast on an Al foil (with a thicknessof 20 mm). After evaporating solvent, the electrode was punchedinto disk (φ14 mm) and further dried at 120 °C for 8 h in vacuum.The particle loading of the positive electrode (including the re-sidual carbon) is about 0.748 mg cm�2. The prepared electrodewas put into an argon-filled glove box (Super 1220/750, Mikrouna)to assemble CR2025-type coin cells, in which Celgard 2400 wasused as separator, lithium foil as counter and reference electrode,and 1 M LiPF6/(ECþDMC, 1:1 vol%) as electrolyte.

To compare the specific capacity and cyclic performance of allthe samples, galvanostatic charge/discharge measurements wereperformed on a cell testing system (LAND CT2001A, China) in thevoltage range of 3.0–4.8 V at 25 °C. Cyclic voltammetry (CV) andelectrochemical impedance spectra (EIS) tests were monitored onan electrochemical working station (CHI614C, China) to reveal theelectrochemical behavior.

3. Results and discussion

XRD patterns of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si wereshown in Fig. 2. It is obvious that all the diffraction peaks of thefour samples can be well indexed as a monoclinic structure with aspace group of P21/n (JCPDS, No. 72-7074) and agree well with theprevious reports [14,15, 27, 28], which indicating Fe and/or Si in-corporating did not change crystal structure of Li3V2(PO4)3. Nodiffraction peaks for crystalline silicon are observed in the XRDpatterns of LVP/C-Si and LVFP/C-Si, demonstrating that silicon inboth samples is amorphous or their content is too low to be de-tected. The Si content of LVP/C-Si and LVFP/C-Si is about 0.36 and0.25 wt%. Similarly, no diffraction peaks for crystalline carbon in allthe four XRD patterns are also indicative of amorphous residualcarbon. The carbon content of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si is about 3.01, 2.69, 2.87 and 1.99 wt%, respectively. The de-creased carbon in both LVFP/C and LVFP/C-Si is caused by the re-duction of Fe3þ in Fe2O3. Moreover, the results of electronic con-ductivity measurements show that the three modified compositesexhibit higher electronic conductivity (0.37�10�3 S cm�1 for LVP/C-Si, 5.12�10�3 S cm�1 for LVFP/C, and 6.14�10�3 S cm�1 forLVFP/C-Si) than LVP/C (0.24�10�3 S cm�1), especially Fe-dopingis more effective for improving the electronic conductivity. Fur-thermore, the cell volume of all samples was calculated by Maudsoftware. As seen in Table 1, the cell volume of LVFP/C and LVFP/C-Si is 890.44 Å3 and 889.61 Å3, respectively, which is smaller thanthat of LVP/C (892.16 Å3) and LVP/C-Si (891.46 Å3). It is possible toinfer that Fe enters into the crystal lattice because of the largerionic radius of V3þ (0.078 nm) than that of Fe2þ (0.076 nm) [15].To further support this point, XPS measurements assisted by Ar-ion sputtering were taken in the next part.

synthesis process for LVFP/C-Si.

Fig. 2. XRD patterns of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si.

Table 1The lattice parameters of samples.

Sample a (Å) b (Å) c (Å) V (Å3)

LVP/C 8.6122 8.6017 12.0433 892.16LVP/C-Si 8.6083 8.5998 12.0419 891.46LVFP/C 8.6024 8.6009 12.0348 890.44LVFP/C-Si 8.6038 8.5928 12.0330 889.61

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Fig. 3 shows the high-resolution XPS spectra of V2p3/2, Fe2p3/2,C1s and Si2p1/2 of LVFP/C and LVFP/C-Si. The binding energy ofboth samples were checked by carbon (C1s¼284.5 eV). As shownin Fig. 3, the V2p3/2 (�516.7 eV) and Fe2p3/2 (�710 eV) mainpeaks for LVFP/C-Si are located as similar as that for LVFP/C, whichconfirm that Si-incorporation doesn't change the oxidation state ofV3þ and Fe2þ . As seen in Fig. 3b4, the Si2p1/2 peak at �102.9 eVconfirms Si with tetravalence, corresponding to SiO2 [10,29].Noting that, due to the chemical reduction of Ar-ion sputtering[30], all the XPS peaks in the interior shift slightly towards lowbinding energy compared to those at the surface. Here, we shouldpoint out that the weaker V2p3/2 peak and the stronger C1s andSi2p1/2 peaks at the surface are enough to prove that C and SiO2

mainly exist on the surface; moreover, the stronger Fe2p3/2 peaksin the interior confirm Fe-doping and the weaker Fe2p3/2 peaks atthe surface is indicative of the existence of trace FeO. Fe-doping is

Fig. 3. High-resolution XPS spectra of sam

effective for improving the intrinsic conductivity, and trace FeO atthe surface is beneficial to Liþ absorption and diffusion because ofthe defects formed during sintering process.

Fig. 4 shows the TEM images of LVFP/C and LVFP/C-Si powders.It is obvious that both samples exhibit irregular particles with anaverage size of �200 nm, demonstrating that Si-incorporating hasnearly no influence on morphology. Fig. 4c and d exhibit theHRTEM image and the corresponding EDX spectrum of LVFP/C-Si,respectively. The HRTEM image shows the lattice spacing ofd¼0.695 nm and 0.323 nm, which corresponds to the (011) and(122) planes of P21/n-LVP crystals. And an amorphous coatinglayer with a thickness of �12 nm is also clearly observed. Com-bined the EDX spectrum (Fig. 4d) with the above XPS spectrum ofSi2p1/2 (Fig. 3b4), we can reasonably infer that the coating layer is ahybrid one, which is composed of C and SiO2.

Fig. 5 displays the initial charge/discharge profiles and cycleperformance of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si at 1 C and5 C (1 C¼197 mA g�1). As shown in Fig. 5a, all samples exhibitfour charge plateaus around 3.69, 3.70, 4.10, and 4.55 V, which isrelated with four Liþ extraction steps from Li3V2(PO4)3 (i.e.,Li3V2(PO4)3-Li2.5V2(PO4)3-Li2V2(PO4)3-LiV2(PO4)3- V2(PO4)3)[9,31]. During the discharge process, an S-shaped curve and twosubsequent plateaus (about 3.65 V and 3.58 V) are observed. TheS-shaped curve corresponds to a solid solution behavior of phasetransformation from V2(PO4)3 to Li2V2(PO4)3, and the two sub-sequent plateaus are attributed to two Liþ insertion steps intoLi2V2(PO4)3 (i.e., Li2V2(PO4)3-Li2.5V2(PO4)3-Li3V2(PO4)3) [9,31].Moreover, for LVP/C-Si and LVFP/C-Si electrodes, the charge pla-teaus shift downward, while the S-shaped curves and dischargeplateaus shift upward, indicating better reversibility of Liþ ex-traction and insertion reactions. As seen in Fig. 5, the modified LVPsamples (LVP/C-Si, LVFP/C and LVFP/C-Si) show better electro-chemical performance. For example, at 1 C, the initial dischargecapacity of LVP/C-Si, LVFP/C and LVFP/C-Si is 152.6, 163.9 and170.8 mA h g�1, respectively, which is obviously higher than LVP/C(142.9 mA h g�1). Furthermore, these modified samples also ex-hibit better cycle stability, i.e., after 100 cycles at 5 C, LVFP/C, LVP/C-Si and LVFP/C-Si electrodes still deliver desirable capacity of114.2, 119.3 and 121.9 mA h g�1 with capacity retention ratio of79.7%, 85.5% and 82.3%, respectively, which are also higher thanLVP/C (102.9 mA h g�1, 72.4%). Here, it must be said that, con-sidering the penetration of electrolyte into the electrode, solidelectrolyte interphase (SEI) formation of electrode and structuralstability can be completed in the first cycle, we choose the secondcycle to calculate the capacity retention ratio. It's not hard to see

ples: (a) LVFP/C, and (b) LVFP/C-Si.

Fig. 4. (a) TEM image of LVFP/C, (b, c) TEM images of LVFP/C-Si, and (d) EDX spectrum of LVFP/C-Si.

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that, compared with the 100th capacity (5 C) for LVP/C, the in-crease rate of capacity for LVFP/C, LVP/C-Si and LVFP/C-Si is 11.0%,15.9% and 18.5%, respectively. Therefore, modification of LVP withFe-doping or CþSiO2 hybrid layer coating, especially co-mod-ification with Fe-doping followed by CþSiO2 hybrid layer coatingcan effectively enhance the rate capability and cycle performance.

In order to further investigate the electrochemical reaction offour samples during cycling process, CV measurements wereperformed in the cut-off voltage of 3.0–4.8 V vs. Li/Liþ at a scan-ning rate of 0.05 mV s�1. As shown in Fig. 6, it is apparent that allthe CV curves are very similar. There are four oxidation peaks andthree reduction peaks, corresponding to the phase transformationsin LixV2(PO4)3 (x¼3, 2.5, 2.0, 1.0, 0) [32,33]. The C3 peaks are at-tributed to the solid solution behavior of phase transformationfrom V2(PO4)3 to Li2V2(PO4)3 during discharge process [9]. More-over, as shown in Fig. 6 and Table 2, LVP/C-Si, LVFP/C and LVFP/C-Sishow stronger current peaks and smaller potential differencesbetween anodic and cathodic peaks than LVP/C, indicating thatmodification with Fe-doping and/or CþSiO2 hybrid layer coatingcan obtain lower electrode polarization and better electrochemicalreversibility of Liþ ion extraction/insertion. More interestingly,from the enlarged patterns shown on the right side of Fig. 6b,another oxidation/reduction peaks around 3.4 V can be detected inboth LVFP/C and LVFP/C-Si electrodes. These oxidation/reductionpeaks are just characteristic of Fe2þ/Fe3þ redox couples inLiFePO4, which is also consistent with the result of XPS.

To study the reason for the improved electrochemical proper-ties of the modified samples, the EIS measurements (0.01–100 Hz)were performed and the results were shown in Fig. 7. All EIScurves of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si are composed of an

intercept, a semicircle and a sloping line [10], and can be fitted byan equivalent circuit composed of “R(C(RW))” using the ZSimpWinprogram. As shown in Fig. 7a and Table 3, LVFP/C, LVP/C-Si andLVFP/C-Si exhibit an obviously decreased charge-transfer re-sistance (40.19, 35.60 and 29.95Ω, respectively) than LVP/C(47.08Ω), indicative of faster kinetics of cell reaction. Moreover,the sloping line at low frequency is related to lithium ion diffusionin LVP, thus the lithium ion diffusion coefficient can be calculatedfrom the sloping line according to the following equation [10,34]:

δ=+D A n CR T /2 FLi2 2 2 4 4 2 2

Here, R is the gas constant, T is the absolute temperature, A is thesurface area of LVP cathode, n is the number of electrons permolecule during oxidation, F is the Faraday constant, C is theconcentration of lithium ion in LVP, and δ is the Warburg coeffi-cient which is related with Z′:

δω′ = + + −Z R Re ct1/2

here, ω is the frequency at low frequency. To obtain δ, the re-lationship between Z′ and ω�1/2 of LVP/C, LVP/C-Si, LVFP/C andLVFP/C-Si are shown in Fig. 7b. The linear fitting results show thatthe calculated lithium ion diffusion coefficient of LVP/C is1.67�10�12 cm2 s�1, while that of LVFP/C, LVP/C-Si and LVFP/C-Siis 1.92�10�12, 2.01�10�12 and 16.4�10�12 cm2 s�1, respec-tively. Obviously, lithium ion diffusion in LVP becomes faster aftermodification, especially after co-modification with doping andcoating, thus this novel LVFP/C-Si cathode modified with Fe-dop-ing followed by CþSiO2 hybrid layer coating exhibits the highestcapacity and the best cycle ability.

Fig. 5. (a) The initial charge/discharge profiles, and (b) the corresponding cycle performance profiles at 1 C and 5 C.

Table 2Peak potentials and difference between reduction and oxidation potentials for theas-prepared samples.

Samples EA1 EA2 EC1 EC2 ΔEA1-C1 ΔEA2-C2

LVP/C 3.621 3.698 3.579 3.640 0.042 0.058LVP/C-Si 3.614 3.692 3.579 3.644 0.035 0.048LVFP/C 3.620 3.695 3.580 3.642 0.040 0.053LVFP/C-Si 0.612 0.690 0.581 0.640 0.031 0.050

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4. Conclusions

In summary, Li3V2(PO4)3 cathode material is successfullymodified with Fe-doping followed by CþSiO2 hybrid layer coating(LVFP/C-Si). This novel co-modification does not change themonoclinic structure of P21/n-Li3V2(PO4)3. LVFP/C-Si particles arecoated with an amorphous CþSiO2 hybrid layer; and most Feenters the lattice, meanwhile trace Fe exists in the form of FeO atthe surface of Li3V2(PO4)3. Compared with LVP/C, LVP/C-Si andLVFP/C, this co-modified LVFP/C-Si cathode exhibits the highestspecific capacity and the best cycle stability at high C-rate, i.e.,LVFP/C-Si presents an initial discharge capacity as high as170.8 mA h g�1 at 1 C, and even delivers a desirable initial capacityof 153.6 mA h g�1 with capacity retention of 82.3% after 100 cyclesat 5 C. The high capacity and long cycle performance are attributed

Fig. 6. CV profiles of LVP/C, LVP

to the common effect of doping and hybrid layer coating: dopingincreases the intrinsic conductivity of LVP, and trace FeO at thesurface is beneficial to Liþ absorption and diffusion; CþSiO2 hy-brid layer coating effectively prevents vanadium ion from dis-solution into electrolyte when charged to high voltage (44.6 V).

/C-Si, LVFP/C and LVFP/C-Si.

Fig. 7. (a) EIS spectra, and (b) the relationship between the Z′ and ω�1/2 at low frequency region of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si electrodes.

Table 3EIS parameters of LVP/C, LVP/C-Si, LVFP/C and LVFP/C-Si cathodes.

Sample Rct (Ω) δ (Ω cm2 s�1/2) DLiþ (cm2 s�1)

LVP/C 47.08 94.73 1.67�10�12

LVP/C-Si 35.60 86.39 2.01�10�12

LVFP/C 40.19 88.30 1.92�10�12

LVFP/C-Si 29.95 30.20 16.4�10�12

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Acknowledgments

This work was supported by the NSFC (No. 51302153, 51572151and 51272128), the Outstanding Youth Science and TechnologyInnovation Team Project of Hubei Educational Committee (No.T201603), and the Opening Project of CAS Key Laboratory of Ma-terials for Energy Conversion (No. CKEM131404).

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