Solid State Ionics 269 (2015) 30–36

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Solid State Ionics

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High rate performance of LiF modified LiFePO4/C cathode material

Yuan Gu ⁎, Xiangjun Zhang, Shigang Lu, Danping Jiang, Aide WuR&D Center for Vehicle Battery and Energy Storage, General Research Institute for nonferrous Metals, Beijing 100088, China

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.ssi.2014.11.0070167-2738/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 March 2014Received in revised form 4 November 2014Accepted 6 November 2014Available online xxxx

Keywords:Lithium ion batteriesCathode materialsLiFePO4

LiF modificationLi-ion diffusion coefficient

Nonstoichiometric LiF modified F-LiFePO4/C cathode material is successfully synthesized by solid-state-reactionusing LiF as the additive and FePO4 and Li2CO3 as precursors. The synthesized F-LiFePO4/C powders are character-ized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM),transmission electron microscope (TEM), Raman spectrum, X-ray photoelectron emission microscopy (XPS)and potentiostatic intermittent titration technique (PITT). The XRD results show that the LiF additive modifica-tion can improve the crystallinity of LiFePO4 without introducing any impurities. LiF modification has little influ-ence on LiFePO4 crystal structure based on lattice parameters, calculated byMDI Jade 6. TheDSC curve shows thata LiF additive can lower the LiFePO4 formation temperature by about 100 °C. F1s signal and a C\F bondhave beendiscovered on the surface of the modified sample via XPS analysis, indicating that LiF has an influence onthe chemical reaction. The final particle size varied with LiF additive content based on SEM and HRTEM images.F-LiFePO4/C samples have better and clearer crystal fringes than the untreated LiFePO4/C sample from HRTEM,which suggests that LiF modification can improve the crystallinity of LiFePO4. Charge–discharge curves indicatethat the F-LiFePO4/C sample has excellent capacities at high discharge rates: 130.0 mAh g−1 at 5 C. PITT indicatesthat the F-LiFePO4/C sample has a higher Li+ ion diffusion coefficient than the LiFePO4/C sample.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Recently, more attention has been given to olivine-structuredLiFePO4 as one of the most promising cathode materials for lithium-ion batteries. Compared with commercial LiCoO2 and LiNiO2, LiFePO4

materials have several advantages such as lower cost, excellent heatstability, lower toxicity, and higher thermal stability [1,2]. Moreover,the discharge potential is extremely stable, which makes it valuable.However, twomain obstacles prevent the commercialization of LiFePO4:(a) the lowelectronic conductivity (10−10 Sm−1); and (b) thepoor lith-ium ion conductivity (10−17–10−14 cm2 S−1) limits the rate capabilityof the material [3–6]. To overcome these problems, various methodshave been developed. For instance, Carbon coating has been consideredas the most effective and popular approach to improve the electronicconduction of LFP [7–16]. In addition, reducing particles to nanometerscale is also an effective method due to shorter lithium-ion transportdistances [17–21].

As another effective method, recently, doping has drawn more at-tention in the electrochemical performance enhancement of LiFePO4

[22]. Cation doping is thought to be an effective way and is widelystudied because cation doping can increase lithium ion conductivity[23–26]. However, anion doping in LiFePO4 is rarely attempted, al-though anionic doping has been reported in other cathode materialssuch as LiMn2O4 and LiNiO2. Yang [27] reported that their Cl doped

LiFePO4/C exhibited a reversible discharge capacity of 92.3 mAh g−1 at15 C [27]. Fluorine substitution is demonstrated as an effective way toenhance the cycling life for layered structure cathode materials andLiMn2O4 spinel [28–31]. Also, Sun reported that F doping at the oxygensite of LiNiO2 and Li [Ni1/3Co1/3Mn1/3]O2 could improve high rate capac-ity and cycle stability compared to an undoped sample [31–33]. Someresearches on fluorine doped LiFePO4/C, especially synthesized byusing solid state reaction, have been reported [34–40]. In their worksthey believe F can enter into the LiFePO4 lattice, which can improve itselectrochemical performance.

In this work, LiF modified F-LiFePO4/C cathode materials are synthe-sized by solid-state reaction. In order to study the origin of the improve-ment F-LiFePO4/C, the structural, electrochemical properties and Li+

diffusion coefficient of this material are investigated in detail.

2. Experimental

2.1. Materials preparation

The LiFePO4/C sample is synthesized by conventional solid-state re-action. The precursors are prepared by mixing the reactants of FePO4,Li2CO3 in stoichiometric proportions. A suitable amount of glucose isused as reductive agent and carbon source for coating. A certain amountof LiF as the fluorine source is added to yield themolar ratio of LiFePO4:F = 1:0.05. The mixture is ball-milled in ethanol for 4 h. The precursormixtures are dried and formed into powders by spray-drying. Theprecursor powders are calcined subsequently at 650 °C for 10 h

Fig. 1. X-ray diffraction patterns of LiFePO4/C and F-LiFePO4/C.

Table 1Lattice parameters of LiFePO4/C and F-LiFePO4/C deduced from the analysis of X-ray dif-fraction pattern for our samples.

Samples a (nm) b (nm) c (nm) V (nm3)

LiFePO4/C 1.0315 0.6005 0.4689 0.2904F-LiFePO4/C 1.0318 0.6001 0.4690 0.2904Δ +0.03% −0.07% +0.02%

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in argon atmosphere and cooled down directly to room temperature(25 °C) in the furnace. The obtained powders are named as LiFePO4/Cand F-LiFePO4/C.

2.2. Materials characterization

Powder X-ray diffraction (XRD)measurements are carried out usingPANalytical, X'PertPRO MPD the (2θ range from 10°to 90° with CuK_radiation (λ = 15.4 nm).

Differential scanning calorimetry (DSC) curves were obtained ina DSC-50 cell (METTLER) with Al crucibles under dynamic argonatmosphere (100 mL min−1). The heating rate is 10 K min−1 in thetemperature ranges from 303 K to 973 K.

Ramanmeasurements were carried out with an EZRaman LE RamanAnalyzer system from Optronics using 785 nm laser excitation coupled

Fig. 2. X-ray diffraction patterns of (a) LiFePO4/C precursor

to a Leica optical microscope. The spectrometer was calibrated usingsilicon wafer and diamond powder standards to a frequency accuracyof 1 cm−1.

The binding energy of fluorine ion is investigated by X-ray photo-electron spectroscopy (XPS). XPS spectra are obtained using a PHI-5000C ESCA system (Perkin-Elmer) with AlK_ radiation (1486.6 eV);the base pressure of the analyzer chamber is about 5 × 10−7 Pa. Thebinding energies are calibrated by using containment carbon (C1s =284.6 eV).

2.3. Electrochemical measurements

The electrochemical characteristics of the LiFePO4 and F-LiFePO4/Csamples are measured by using R2032 button cells assembled in anargon-filled glove box using lithium foil as the anode, and Celgard2400 as the separator. Cathode material is formulated by mixing84 wt.% active materials, 6 wt.% super-P and 10 wt.% PVDF binder toform a homogeneous slurry. The obtained slurry is then cast on the Alcurrent collector and dried 12 h in a vacuum oven at 100 °C. Lithiummetal foil is used as an anode. LiPF6 (1 M) in a 1:1 (v/v) mixture ofdimethyl carbonate (DMC) and ethylene carbonate (EC) is used as theelectrolyte. The charge–discharge test is conducted on a battery testsystem (LAND CT2001A model, Wuhan Jinnuo Electronics Co., Ltd.) inthe potential difference range of 2.0–4.2 V vs. Li/Li+ under a constantcurrent condition. The Cyclic voltammetry measurements (CV) andPotentiostatic intermittent titration technique (PITT) are performed tomeasure the Li+ diffusion coefficient using a CHI 600A ElectrochemicalAnalyzer (Chenhua, China), by applying a potential step of 10 mV, andrecording the current as a function of time between 2.0 V and 4.2 V.All electrochemical measurements are carried out at room temperature.

3. Results and discussion

3.1. Crystal structure analysis

Fig. 1 shows theXRDpatterns of LiFePO4/C and LiFmodified LiFePO4/Csamples. The XRD pattern of LiFePO4/C indicates that the synthesizedmaterials bear an olivine structure with space group Pnma. In Fig. 1 wealso report the indices (h k l) of LiFePO4 standard data. By comparison,the XRD pattern of F-LiFePO4/C does not show any discrepancy fromthat of LiFePO4/C, which indicates that LiF modification does not alterthe crystal structure. Also, no obvious secondary peaks (for instance:Fe2O3, LiF and Li3PO4) have been discovered. Since XRD is not sufficientlysensitive to small amounts of contaminant, we cannot make sure that F

; (b) F-LiFePO4/C at 400 °C, 450 °C, 500 °C and 550 °C.

Fig. 3. DSC curves of LiFePO4/C and F-LiFePO4/C samples.

32 Y. Gu et al. / Solid State Ionics 269 (2015) 30–36

can be doped into the crystal of LiFePO4, or just remain as an additive orother phase in the sample. Therefore, detecting change after LiFmodifica-tionwill require other experiments. The crystal lattice parameters are cal-culated by MDI Jade 6 and provided in Table 1. The change of latticeparameters of a, b and c is very slight which can ascribe to the allowederror. LiFmodification can barely affect the space structure and lattice pa-rameters of LiFePO4 based onXRD results. The crystallinity of two samplesprovided in Fig. 1 indicates that LiF modification can enhance the crystal-linity of LiFePO4. LiF may act as flux during the chemical reaction processas indicated by previous research [41–43]. Fig. 2(a) and (b) shows

Fig. 4. XPS spectra of (a) F1 s and (b) C1 s for un

Fig. 5. SEM of: (a) LiFePO4/C; (b) F-LiFePO4/C (5% LiF molar r

the X-ray diffraction patterns of LiFePO4/C and F-LiFePO4/C sinteredat 400 °C, 450 °C, 500 °C and 550 °C, respectively. LiF modificationdoes influence the crystallinity and LiFePO4 formation temperature:sample with LiF addition shows better crystallinity, sharper peaksand less impurity at 450 °C compared with sample without LiFmodification; it even shows a complete LiFePO4 phase at 500 °C. Incontrast, the sample without LiF modification starts showing arough LiFePO4 phase at 550 °C.

3.2. DSC analysis

LiF addition can lower the chemical reaction temperature which isalso confirmed in DSC curves (Fig. 3). The endothermic peaks 1 and 1′around 150 °C are associated to release of physically adsorbed waterand glucose melting process. The LiF additive has no significant influ-ence on the glucose melting process. Exothermic peaks 2 and 2′ arethe formation of LiFePO4 around 450 °C and 570 °C for the LiF treatedand untreated precursors, respectively. The crystallization peak shiftedtoward lower temperature about 100 °C with an LiF additive. Lowersintering temperature can enhance the crystallinity of LiFePO4 powderand be beneficial to form smaller particle size (no particle fusion).

3.3. XPS analysis

The XPS spectrum of LiFePO4/C and F-LiFePO4/C samples is shown inFig. 4. The F1s peak is located at the binding energy of 684.5 eV is differ-ent from LiF particles (686.52). The binding energymay be attributed tothe F\Fe bond [34]. Fig. 4(b) shows the C–C peak at 284.5 eV. An obvi-ous C–F peak at 290 eVwas discovered. It is hard to detect the remainingLiF in the final sample from XRD and XPS, suggesting that there is a

treated LiFePO4/C and F-LiFePO4/C samples.

atio); and (c) F-LiFePO4/C (10% LiF molar ratio) samples.

Fig. 6. HRTEM images and SAED of: (a)–(b) LiFePO4/C; (c)–(d) F-LiFePO4/C samples.

33Y. Gu et al. / Solid State Ionics 269 (2015) 30–36

reaction between the fluorine and carbon [44]. Also, a small amount offluorine makes a bond with Fe in LiFePO4 crystal. Combining DSC, XRDand XPS results, LiF affects the chemical formation of LiFePO4 signifi-cantly and barely has influence on the space structure of LiFePO4.

3.4. Micro-morphology and HRTEM analysis

The SEM of all samples is shown in Fig. 5(a)–(c). 10% LiF molar ratioF-LiFePO4/C is also synthesized to study the LiF modification influenceon particle size (Fig. 5(c)). The particle size varies with LiF content.The average primary particle size ranges from 200 to 300 nm forLiFePO4/C sample (Fig. 5(a)). The primary particles are linked togetherinto secondary particles by the carbon network which is from glucosedecomposition reaction. The secondary particles have sphere-likemorphology with an average particle size around 2–3 μm. The average

Fig. 7. Charge–discharge curves of LiFePO4/C and F-LiFePO4/C samples at 0.1 C rate in therange of 2.0–4.2 V at 25 °C.

primary particle size of F-LiFePO4/C (5% LiF molar ratio) ranges from50 to 200 nm, which is the smallest one among all samples (Fig. 5(b)).F-LiFePO4/C (10% LiF molar ratio) has medium particle size rangesfrom 100 to 300 nm (Fig. 5(c)). The small particles shorten the diffu-sion path of Li+ ions inside the LiFePO4 crystal. The change in particlesize may be caused by a lower chemical reaction temperature due toLiF addition from DSC curve as mentioned above. Also, carbon blackfrom glucose prevents the further growth of small LiFePO4 particlesgenerated at low temperature. The influence of C\F bond on themorphology of final particles is unclear. Porous features are observedon the surface of all samples, which would benefit electrolyte diffu-sion through the secondary particles to improve the electrochemicalperformance.

Themicrostructure of LiFePO4/C and F-LiFePO4/C (5% LiFmolar ratio)is further characterized by HRTEM, as shown in Fig. 6(a)–(d). It is clearthat a uniform carbon (b3 nm) coated on the surface of LiFePO4

particles, which is due to generation of carbon on the particle surfacesthrough decomposition of glucose. We do observe much smaller parti-cles (darker part) after LiF modification than untreated sample.HRTEM has the same results as SEM. It is obvious that the F-LiFePO4/Csample shows much better and clearer lattice fringes than theLiFePO4/C sample (Fig. 6(b) and (d)), due to higher crystallinity. Valuesof 0.350 nm and 0.353 for d111 are obtained by Gatan Digital-Micrograph for LiF treated and untreated, respectively. The similard111 value confirms that LiF modification has little influence on LiFePO4

lattice structure and parameters. Therefore, the addition of LiF at thepreparation of LiFePO4/C influences not only the particle size but alsothe crystallinity [43].

3.5. Raman analysis

Raman is used to collect the Raman spectra at room temperature(not shown). The two peaks on the Raman spectrum are the D and Gbands. The D band characterizes the disorder in carbon and the Gband is assigned to graphitic structure. The ratio between the area ofD and G peaks can quantify the order of carbon on the LiFePO4 surface.AD/AG values of LiFePO4/C and F-LiFePO4/C samples are: 2.79 and 2.81respectively. The AD/AG ratios of two samples are almost the same

Fig. 8. Discharge curves of (a): LiFePO4/C and (b): F-LiFePO4/C samples at different rates in the range of 2.0–4.2 V at 25 °C.

34 Y. Gu et al. / Solid State Ionics 269 (2015) 30–36

whichmeans that they have similar graphitization. LiF modification hasno effect on graphitization of glucose decomposition.

3.6. Electrochemical properties

Fig. 7 shows the charge–discharge potential profiles of the two dif-ferent samples between 2.0 and 4.2 V using a constant current densityof 0.1 C. All the samples exhibit a flat potential plateau. For theLiFePO4/C sample, the initial specific charge capacity is 160.7 mAh g−1

and the discharge capacity is 151.7 mAh g−1. The coulombic efficiencyis ca. 94.4%, while the F-LiFePO4/C sample shows a charge capacity of162.2 mAh g−1 and a discharge capacity of 156.2 mAh g−1. Thecoulombic efficiency of F-LiFePO4/C is ca. 96.3%. Since the first charge–discharge capacity of the untreated sample is close to the theoreticalvalue (170 mAh g−1) and the coulombic efficiency is around 95%, it is

Table 2The discharge capacities of two samples at different rates.

0.2 0.5 1 C 2 C 5 C 10 C

LiFePO4/C/mAh·g−1 148.1 140.9 134.0 124.5 107.9 86.3F-LiFePO4/C/mA h−1 155.3 153.1 149.6 143.7 130.0 112.2

Fig. 9. Typical cycling behavior for F-LiFePO4/C and LiFePO4/C samples at various rates ofdischarge in the range of 2.0–4.2 V at 25 °C.

not clear that LiF modification can affect electrochemical performanceof LiFePO4/C especially first charge–discharge capacity. In order tostudy the influence of LiFmodification on electrochemical performance,high rate discharge is characterized.

Fig. 8(a) and (b) shows the curves of LiFePO4/C and F-LiFePO4/Csamples at different discharge rates. The average discharge potentialof LiFePO4/C decreases from 3.42 V (0.2 C) to 2.86 V (10 C)with increas-ingdischarge rate. By comparison, the average discharge potential of theF-LiFePO4/C sample decreases from 3.39 V (0.2 C) to 2.96 V (10 C) withincreasing discharge rate. The discharge capacities of two samples atdifferent rates are shown in Table 2. From Fig. 8(a) and (b) andTable 2, at low discharge rate, two samples show similar performance.LiFePO4/C even has a higher discharge potential plateau at 0.2 C.Howev-er, with increasing discharge rate, the F-LiFePO4/C sample exhibits bet-ter electrochemical performance than the LiFePO4/C sample. Higherdischarge capacities suggest that LiF modification could improve theLi+ conductivity of LiFePO4/C due to high crystallinity. This assumptioncan be verified by the PITT and CV test below. On the other hand, smallerparticles of F-LiFePO4/C can also BENEFIT the high rate discharge [14,45]. The cycle stability of the F-LiFePO4/C and LiFePO4/C samples athigh current densities is shown in Fig. 9. The measurements are carriedout at different current densities from 0.1 to 10 C during the prolongedcycling in the potential range of 2.5–4.2 V at 25 °C. There is only a smallamount of fading observed in high rate tests. No obvious capacity loss is

Fig. 10. CV plots of F-LiFePO4/C and LiFePO4/C samples in the range of 2.0–4.2 V at 25 °C.

Fig. 11. Curves of I/t (a) and d ln(I)/dt (b) of F-Li1 − xFePO4/C samples.

35Y. Gu et al. / Solid State Ionics 269 (2015) 30–36

observed after 40 cycles, which proves the good cycling stability of thesamples, whether modified or not.

3.7. Cyclic voltammetry

CV measurements are performed between 2.0 and 4.2 V vs. Li/Li+ ata scan rate of 0.05 mV s−1. All samples show a pair of oxidation and re-duction peaks (Fig. 10), which are in accordance with the typical CVcharacteristics of the LiFePO4 sample. The oxidation and reductionpeaks of F-LiFePO4/C and LiFePO4/C samples are centered at 3.34/3.53and 3.29/3.61 V, respectively. The peak separation of the F-LiFePO4/Csample is narrower and the peak shape is shaper than LiFePO4/C. Thisresult indicates that the F-LiFePO4/C sample has a better reversibilityin the electrochemical process of oxidation–reduction.

3.8. Li+ diffusion coefficient

PITT [46] is valid in measuring the Li+ diffusion coefficient in Li1− xFePO4 on the basis of the following assumptions: no consideration ofohmic potential drop, double-layer charging, charge-transfer kinetics,and phase transformation in one-dimensional diffusion of a solid solu-tion electrode [47,48]. According to the Fick's second law, the following

Fig. 12. Li-ion diffusion coefficient of F-Li1 − xFePO4/C samples.

equation can be used to estimate the Li+ diffusion coefficient in a solidsolution electrode:

DLiþ ¼ −d ln Ið Þdt

4L2

π2DLiþ t

.4L2

N0:075 ð1Þ

where L (cm) is the characteristic length of the electrode material, I(t)(A) is the transient current during the voltage step, t (s) is the time ateach voltage step.

For a givenDLiþ , data must be taken for a long enough time to satisfyDLiþ t

�4L2N0:075 [49]. Fig. 11(a) shows the I/t plots of F-Li1 − xFePO4/C

(x = 1, 0.9875, 0.9750, 0.75, 0.50, 0.1875, 0.125, 0.0625) which agreewith previous reports [49]. From Fig. 11(a) two ranges can be observed:(a) a sharp decrease in current in range 1 which means charge thedouble layer; and (b) current slowly decreases with time in range 2which accords with the Cottrell law, indicating Li+ diffusion in theF-Li1 − xFePO4/C. As shown in the Fig. 11(b) a straight line is obtainedfor a long time region. From the linear relationship between ln(I) andt (Fig. 11(b) range 2), Li+ diffusion coefficients of F-Li1− xFePO4/C at dif-ferent discharge states are estimated. The Li+ diffusion coefficients inthe discharge states obtained by PITT are plotted in Fig. 12. The Li+ dif-fusion coefficient of F-Li1 − xFePO4/C is about 10−12 cm2 S−1. The Li+

diffusion coefficient of LiFePO4/C sample is also measured followingthe same process which is around 10−15 cm2 S−1 (not shown) that isclose to previously reported (10−17–10−14 cm2 S−1) [5,6]. LiFmodifica-tion can improve the Li+ diffusion coefficient of LiFePO4/C.

4. Conclusions

Nonstoichiometric LiF modified F-LiFePO4/C materials are success-fully synthesized by solid state reaction. XRD analysis indicates thatLiF additive does not alter the crystal structure of LiFePO4 nor induceobvious impurity phases. The F-LiFePO4/C is more favorable for Li+ todiffuse into lattice due to the higher crystallinity and smaller particlesize. LiF may react with carbon precursor and affect the final morpholo-gy of LiFePO4/C particles, but LiF treatment does not affect graphitiza-tion of glucose decomposition. LiF modification can lower LiFePO4

formation temperature, where LiF acts as a flux. Formation of LiFePO4

at low temperature can decrease particle size and prevent particlefusion. Further work is required to study the chemical mechanism ofLiF or F during LiFePO4 formation reaction and glucose carbonization.The F-LiFePO4/C sample has excellent rate capacities and stable cycleability. PITT exhibits that the F-LiFePO4/C sample has a higher Li+ diffu-sion coefficient than theuntreated sample. Therefore, LiFmodification is

36 Y. Gu et al. / Solid State Ionics 269 (2015) 30–36

an effective, easy and cheap way to attain high capacity and high ratecapacity for LiFePO4.

Acknowledgments

This project was funded by the National 863 project of China (NO.2008AA11A103).

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