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Electrochimica Acta 56 (2011) 7385– 7391

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

ffects of tin doping on physicochemical and electrochemical performances ofiFe1−xSnxPO4/C (0 ≤ x ≤ 0.07) composite cathode materials

un Maa,b, Baohua Lia, Hongda Dua, Chengjun Xua, Feiyu Kanga,b,∗

Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen, Guangdong Province 518055, ChinaLaboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

r t i c l e i n f o

rticle history:eceived 11 January 2011eceived in revised form 22 May 2011ccepted 23 May 2011vailable online 24 June 2011

eywords:ithium iron phosphate

a b s t r a c t

Nanocrystalline LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples are synthesized using SnCl4·5H2O as dopant via aninorganic-based sol–gel method. The dependency of the physicochemical and electrochemical propertieson the doping amount of tin are systemically worked out and regular changes are revealed. In the wholeconcentration range, the chemical valence of Fe2+ is not basically changed whereas tin is found in twodifferent oxidation states, namely +2 and +4. The replacement of Fe2+ by supervalent Sn4+ would lead toelectron compensation. Under the synergetic effects between the charge compensation and the crystaldistortion, the electrical conductivities for the bulk samples first increase and then decrease with the

inixed-valence dopinganostructure

increasing amount of Sn doping. Upon the doping amount, the apparent lithium-ion diffusion coefficientand the electrochemical performance also display the similar trends. The doping is beneficial to refine theparticle size and narrow down the size distribution, however optimizing the doping amount is necessary.Compared with other samples, the sample with a doping amount of about 3 mol% delivers the highestcapacities at all C-rates and exhibits the excellent rate capability due to the high electrical conductivity

iffus

and the fast lithium-ion d

. Introduction

Since lithium iron phosphate (LiFePO4) was firstly proposed as aathode material for lithium-ion battery by Padhi et al. [1], in-depthtudies have been carried out to improve or alleviate its severeisadvantages, namely low electric conductivity and slow lithium

on diffusion velocity [2]. The solutions or strategies proposed areainly focused on the synthesis of nanostructured materials [3–7],

he surface modifications of the conductive substances [3,8,9] andost importantly, the doping of isovalent or aliovalent cation at

i-site or/and Fe-site [10–19].The doping effects at Fe-site on the physicochemical and electro-

hemical properties of LiFePO4 have been extensively investigated.he doping elements involved have covered transition and non-ransition metal elements. And the variety have exceeded 20

pecies, e.g. Mn [10], Zr [18], Ti [17], V [13,15], Al [13], Na [19],g [13,14,16] and Sn [20], etc. For the doping effects of the non-

ransition metal elements, the current research interests are mainly

∗ Corresponding author at: Advanced Materials Institute, Graduate Schoolt Shenzhen, Tsinghua University, Shenzhen, Guangdong Province 518055,hina/Laboratory of Advanced Materials, Department of Materials Science and Engi-eering, Tsinghua University, Beijing, 10084, China. Tel.: +86 755 2603 6188;

ax: +86 755 2603 6188.E-mail address: fykang@tsinghua.edu.cn (F. Kang).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.05.091

ion velocity.© 2011 Elsevier Ltd. All rights reserved.

concentrated on the alien cations Al3+, Mg2+ and Na+. Yang and Ke[13] investigated the doping effects of Al3+ and Mg2+ on the electro-chemical performances of LiFe0.95Al0.05PO4 and LiFe0.95Mg0.05PO4,respectively. It was revealed that the discharge specific capacities at0.1 C and 1 C were apparently increased and the lithium-ion diffu-sion velocities were greatly accelerated after doping. Roberts et al.[14] studied the electrochemical performance of nanocrystalliteLiFe1−xMgxPO4 and found that the sample with a doping amountof 10 mol% exhibited the higher electrical conductivity and the bet-ter rate capability. Arumugan et al. [16] reported that the specificcapacity and the cycling stability of the LiFePO4 electrode materialwere enhanced by doping 5 mol% Mg2+. Chu et al. [19] systemicallyinvestigated the doping effects of Na+ on the electrochemical per-formances of LiFe1−xNaxPO4 (0 < x ≤ 0.1). The results indicated thatwhen the doping amount was about 5 mol%, the sample exhibitedthe highest discharge capacity and the discharge specific capacityat 4 C is about 52% of that at 0.1 C. However, for the doping effects ofthe non-transition metal element Sn, the related investigations andreports are very limited. Only Jayaprakash et al. [20] reported thatthe discharge capacity and the cycling stability of LiFe0.98Sn0.02PO4(submicron level) could be effectively enhanced by the low leveldoping of Sn2+ (about 2 mol%).

Up to now, there is no literature involving the mixed-dopingeffects of Sn4+ and Sn2+ on the electrochemical performance ofLiFePO4. And there is no systematic study on the relationshipsbetween the physicochemical and electrochemical performances

7 ica Acta 56 (2011) 7385– 7391

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386 J. Ma et al. / Electrochim

f nanocrystalline LiFePO4 and the doping amount of Sn. However,t is paramount and meaningful to understand comprehensivelyhese fundamental aspects. In this work, the doping effects ofhe non-transition metal element Sn on the physicochemical andlectrochemical performances of nanocrystalline LiFe1−xSnxPO40 < x ≤ 0.07) were systemically investigated. The dependency ofhe physicochemical and electrochemical properties on the dopingmount of Sn was well established. And for the first time, the mixed-alence state of tin ion was noticed and the charge compensationechanism of supervalent cation Sn4+ was studied.

. Experimental

.1. Synthesis of precursor

The pristine and Sn-doped LiFePO4 precursors were synthesizedy an inorganic-based sol–gel route using FeCl2·4H2O (Analyticaleagent, AR, 98.5%), Li2CO3 (AR, 99%) and H3PO4 (AR, >98%) as rawaterials, citric acid (AR, >99.5%) as a carbon source. Here, tin(IV)

hloride pentahydrate (SnCl4·5H2O, AR, >99%) was used as a tinource. Firstly, all reactants were added separately into ethanol toorm pale blue sol, maintaining magnetic stirring. Three hours later,he stirring was stopped and gel was immediately formed. Lastly,fter drying at 358 K in air, the resulting xerogel was pre-sinteredt 623 K for 5 h followed by final sintering at 923 K for 15 h in a tubeurnace under Ar gas atmosphere.

.2. Material characterization

A Rigaku D/max 2500/PC diffractometer (Cu-K� radiation, = 0.15406 nm) was used to characterize crystal structure andhase composition of the as-resulting sample. The diffraction dataas collected at step mode over the angular range of 15–120◦

ith a step size 0.02◦ and a 5 s exposure time at 50 kV, 250 mA.he lattice constants for the samples were refined by Rietveldnalysis using the Maud program. A Pnma space group was cho-en as the model. The chemical valence states of Fe and Sn werenalyzed by an X-ray photoelectron spectroscopy (XPS, Perkinlmer, PHI 5600) equipped with monochromatic Al X-ray source1486.6 eV). The data was analyzed using XPSPEAK41 software.ll data were calibrated by using adventitious C 1s peak with

fixed value of 284.4 eV. A Shirley-type background was usedo subtract the background of each spectrum. The residual car-on content was determined by a chemical analysis. The specificurface area for sample was measured using a multi-point (8)runauer–Emmett–Teller (BET) technique (Micromeritics ASAP020 M + C). The concentrations of Li and Fe for the Sn-dopedamples were determined by an inductively coupled plasma-ptical emission spectroscopy (ICP-OES, Perkin Elmer, Optima100DV). Both the low- and high-resolution images for sam-les were obtained by a transmission electron microscopy (TEM,

OEL JSM-2100F) operated at 200 kV. The energy-dispersive X-raypectrometry (EDS) line scan spectrum was used to identify thexistences and distributions of alien element tin and matrix ele-ents by a scanning transmission electron microscopy (STEM,

OEL JSM-2100F). The morphology of sample was observed andhe particle size distribution was determined by a field emissioncanning electron microscopy (FE-SEM, HITACH S4800) operated at

0 kV. The electrical conductivity was measured by a four-point dcethod. The measured sample was uniaxially pressed into pelletsith 13 mm in diameter at 10 MPa and then Ag paste was coated

n the both sides of the disks.

Fig. 1. XRD diffraction patterns for the LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples.

2.3. Evaluation of electrochemical performance

The cathode electrode was fabricated by mixing the as-synthesized active material with super P and polyvinylidenefluoride (PVDF) at a weight ratio of 90:5:5 homodispersed in N-methyl-2 pyrrolidone (NMP) solution. The coin cell (CR2032) witha metallic lithium anode was assembled in glove-box (Mbraum) toevaluate the electrochemical performance of the as-obtained sam-ple. The microporous polypropylene sheet (Celgard 2500, CelgardInc., USA) was used as the separator. 1 M LiPF6 solution in ethy-lene carbonate and diethyl carbonate (EC/DEC) (volume ratio 1:1)was used as the electrolyte. Cyclic voltammetry (CV) experimentwas carried out over the potential range of 2.5–4.3 V (versus Limetal) at a scanning rate of 0.1 mV s−1 using a VMP3 electrochem-ical workstation (Bio Logic Science Instruments). The galvanostaticcharge/discharge performance test and the rate capability at dif-ferent C-rates were performed using a Land 2001A cell test system(Wuhan, China) at 298 K.

3. Results and discussion

The XRD patterns for the samples with different tin ion content(0–7 mol%) are shown in Fig. 1. It is found that all XRD diffrac-tion peaks match very well with the standard fingerprint of anolivine LiFePO4 and no odd phases could be detected. The crystal-lographic evidence reveals that the introduction of Sn does neithersignificantly change the host lattice structure, nor bring about theformation of the second phase. Table 1 lists the cell parameters ofthe pristine and Sn-doped samples refined by Rietveld analysis. Itis easy to see that the introduction of the alien element Sn leads tothe lattice volume changing and the Li–O bond lengthening. Andwith the increase of the adding amount of Sn, the Li–O bond lengthis first prolonged and then shortened. When the adding amountof Sn is about 3 mol%, the Li–O bond distance reaches a maximum.Obviously, the substitution of Fe by Sn could account for the regularvariation. The result implies that the alien element Sn has been suc-cessfully incorporated into the LiFePO4 host lattice. The presenceand distribution of the doping element Sn within the doped sam-ples could be further verified by means of EDS line scan spectrum.The composition with distance along a line within LiFe0.93Sn0.07PO4sample was shown in Fig. 2. It was found that the Fe, P and Sn

lines showed similar Gaussian distributions of X-rays across everyparticle as expected from the random arrangement of atoms. Theintensities of Fe and P are almost equal while the intensity of Snis apparently lower than that of Fe and P, in good agreement with

J. Ma et al. / Electrochimica Acta 56 (2011) 7385– 7391 7387

Table 1Parameters of the original and Sn-doped samples obtained from XRD Rietveld refinement.

Sample LiFePO4 1 mol% Sn 3 mol% Sn 5 mol% Sn 7 mol% Sn

Lattice constant (nm)a 1.032834(5) 1.032924(3) 1.032708(6) 1.033051(8) 1.032891(1)b 0.600592(5) 0.600691(3) 0.600648(8) 0.600751(3) 0.600693(4)c 0.469134(8) 0.469269(3) 0.469237(9) 0.469169(7) 0.469163(6)

Lattice volume (nm3) 0.29101(0) 0.29116(7) 0.29106(6) 0.29117(0) 0.29109(3)Interatomic distance (nm)

Li–O1 0.219411 0.219606 0.219952 0.219762 0.219473Li–O2 0.191544 0.208730 0.208904 0.208678 0.208187Li–O3 0.208739 0.217912 0.218074 0.217402 0.217704Li–Omean 0.2065(6) 0.2154(2) 0.2156(4) 0.2152(8) 0.2151(2)

Reliability factors

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Rwp (%) 7.82 8.27

Rp (%) 5.47 5.71

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heir stoichiometry in the sample. Other samples with differentoping amount of Sn also display similar results. Combined withhe results of XRD and the features of the sol–gel synthesis, it coulde concluded that the element Sn had been doped uniformly intohe parent phase.

The chemical valences of Fe and Sn for the LiFe1−xSnxPO40 ≤ x ≤ 0.07) samples were characterized by XPS spectra. The Fep core-level XPS spectra for the pristine and Sn-doped samplesre shown in Fig. 3a. It shows that for the undoped sample, the Fep spectra are split into two parts due to spin-orbit coupling andach part consists of a main peak and its satellite or shoulder peak,hich are characteristic for Fe2+ [21,22]. A single low intensity peak

t a bonding energy of about 709.6 eV is due to the formation of Feon with a lower valence than +2 [22]. Compared with the undopedample, the shape of the Fe 2p peaks for the doped samples changesittle, indicating that the doping of Sn does not change significantlyhe chemical valence state of Fe2+. Besides the characteristic peaksf Fe2+, an additional peak at a binding energy of approximately16.0 eV can be found in the XPS spectra, which is assigned to then 3p . With increasing amount of the Sn doping, the correspond-

3/2ng XPS peak intensity gradually increases. The Sn 3d core-level XPSpectra for the doped samples are shown in Fig. 3b. In all spectra,

typical two-peak curves are presented, which are assigned to Sn

ig. 2. Representative STEM–EDS line scan spectrum for the LiFe0.93Sn0.07PO4 sample. Rellotted against the line scan probe position (horizontal axis) and given to STEM image. Ao color in this figure legend, the reader is referred to the web version of the article.)

7.81 7.06 6.145.59 4.80 4.222.51 2.26 2.01

3d5/2 of 487.0 eV and Sn 3d3/2 of 495.5 eV, respectively. The bind-ing energy splitting is about 8.5 eV and the FWHM is more than1.39 eV. The Sn 3d5/2 line could be further fitted by two subpeakswith binding energies of 486.5 eV and 487.1 eV, which correspondsto that of Sn2+ and Sn4+[23], respectively. The result suggests thattin ion with two different chemical valences (Sn2+ and Sn4+) co-exists in the doped samples. Therefore, it could be concluded thatthe doping mode of Sn in this case is a mixed-valence doping. It isvery different from the common cases, where the cation only hav-ing one valence [13,15,20] is doped in the LiFePO4 host lattice. Therelative contents of Sn2+ and Sn4+ in the doped samples could becalculated by their respective intensities. It is found that with thedoping amount of Sn, the concentration ratio of Sn2+ to Sn4+ firstdecreases and then increases. The minimum is found at the dopingamount of approximately 3 mol%, where the concentration of Sn2+

is almost equal to that of Sn4+.From the point of view of ionic radius, the doping of Sn2+ with

a larger ion radius (about 0.093 nm) than Fe2+ actually would tendto form a limited solid solution instead of a continuous one. It hasbeen proved by the work of Jayaprakash et al. [20]. They studied

the doping effects of Sn2+ (0.02 mol) on the LiFe0.98Sn0.02PO4 cath-ode material and found that only some Sn2+ could be doped in theLiFePO4 lattice. However in our case, even when the doping amount

ative atomic% composition (vertical axis) of Fe (green), P (yellow) and Sn (blue) are 1 nm probe was used to trace 144.4 nm scans. (For interpretation of the references

7388 J. Ma et al. / Electrochimica Acta 56 (2011) 7385– 7391

705710715720725730735

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mine the electrical conductivity besides the electron compensationfrom the supervalent doping. The drop in the electrical conductiv-ity might be ascribed to the crystal distortion. Due to the largerionic radius difference between Sn2+ (about 0.093 nm) and Fe2+

ig. 3. XPS core levels of Fe 2p, Sn 3p and Sn 3d for the LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07)amples.

s up to 7 mol%, the as-synthesized samples still preserve the olivinetructure of LiFePO4. It might be related to the co-doping of Sn4+

0.071 nm) with a smaller ion radius than Fe2+, which might beeneficial to increase the solid solubility of Sn2+ in the host lat-ice.

When the isovalent Sn2+ is doped into the lattice to replace Fe2+,n×

Fe is formed without giving rise to any charge compensations.owever, when Fe2+ is substituted by supervalent Sn4+, chargeompensation would occur. In order to determine the type of theharge compensation, the chemical analysis of the LiFe1−xSnxPO40 < x ≤ 0.07) samples was performed by ICP-OES. Table 2 lists the

olar ratios of Fe and Li for the doped samples with different dop-ng amounts of Sn. It is found that the molar ratios are very closeo the theoretical values, which implies that the charge differenceetween Sn4+ and Fe2+ could be balanced by electron. The corre-ponding aliovalent doping mechanism could be represented by

he formula Eq. (1)

nO2 + Fe×Fe → Sn••

Fe + FeO + 2e′ + 12

O2 ↑ (1)

able 2he molar ratios of Fe and Li in the LiFe1−xSnxPO4 (0 < x ≤ 0.07) samples.

Sample 1 mol% Sn 3 mol% Sn 5 mol% Sn 7 mol% Sn

Fe:Li (theoretical) 0.99 0.97 0.95 0.93Fe:Li (experimental) 0.99(2) 0.96(1) 0.94(4) 0.92(6)

Doping Content (mol%)

Fig. 4. The electrical conductivity as a function of the doping amount of tin.

This type of the charge compensation might play an active role inimproving the bulk electrical conductivities for the doped samples.It could be expected that the electrical conductivities of the dopedsamples might be monotonically increased upon the concentrationof Sn4+.

The variation in the electrical conductivity of the sample as afunction of the doping amount of Sn is shown in Fig. 4. It is foundthat the measured results are clearly distinct from our expectationmentioned above. The bulk electrical conductivity first increasesand then decreases as the doping amount of Sn increases. Whenthe doping amount of Sn is approximately 3 mol%, the electricalconductivity for sample is the highest and is one order of mag-nitude higher than others. Because the residual carbon contentfor all samples is almost equal (about 4.5 wt.%) and no conductivephases (e.g. metallic iron phosphides or iron phosphocarbides) aredetected (shown in Fig. 1), the changes of electrical conductivity isattributed to the doping of Sn. On the other hand, the changingtrend also implies that there might be another factor to deter-

Fig. 5. The representative TEM image for the LiFe1−xSnxPO4 (0 < x ≤ 0.07) samples.(Inset: the corresponding high-resolution TEM images.)

J. Ma et al. / Electrochimica Acta 56 (2011) 7385– 7391 7389

F ne sams

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ig. 6. SEM photographs for the LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples. (a) the pristiize distribution).

about 0.074 nm), the doping of Sn2+ would induce larger crystalistortion, which would greatly undermine the electrical conduc-ivity. With the increase of the doping amount of Sn, especially thencreasing concentration of Sn2+, the negative impact induced byhe crystal distortion on the electrical conductivity would become

ore pronounced. Therefore, it could be concluded that by balanc-ng the positive effect of the charge compensation and the negativeffect of the crystal distortion, the maximum electrical conductivityould be achieved, as illustrated by the LiFe0.97Sn0.03PO4 sample.

The representative microstructure of the LiFe1−xSnxPO40 < x ≤ 0.07) samples was shown in Fig. 5. The sample shows a

icrostructure, in which amorphous carbon is uniformly coatedn the surface of the near-spherical nanoparticles and no otherrystalline phase is found. The morphologies, the particle size andize distributions of the samples were observed by SEM images as

ple; (b) 1 mol% Sn; (c) 3 mol% Sn; (d) 5 mol% Sn; (e) 7 mol% Sn (Inset: the relevant

shown in Fig. 6. All samples exhibit that the secondary particle iscomposed of an agglomeration of the quasi-spherical nanosizedparticles, which is consistent with the TEM images. The particlesize is varied with the concentration of dopant. When the dopingamount is about 1 mol%, the particle size of sample is about 58 nm.And when the doping amount is increased to 3 mol%, the particlesize is not significantly changed. However, with a continuedincrease of the doping amount, the particle size is obviouslyincreased. The results are consistent with the variations of thespecific surface area from 34.5, 49.1, 48.4, 35.1 to 33.0 m2 g−1,which correspond to the doping amount of 0, 1, 3, 5 and 7 mol%,respectively. In order to examine the effect of doping amount on

the size distribution of the samples further, the relative span ofthe particle size distribution (PSD) is defined as (D90–D10)/D50. It isfound that the PSD first decreases with Sn doping level up to 3 mol%

7390 J. Ma et al. / Electrochimica Acta 56 (2011) 7385– 7391

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the increase of the C-rate, the doping effect on the electrochem-ical performance becomes more pronounced. What’s more, it isrevealed that under various C-rates, the specific capacities for the

765432100

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ig. 7. CV profiles of the LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples with the scan rate of.1 mV s−1.

nd then increases at the higher level. And the LiFe0.97Sn0.03PO4ample shows the narrowest particle distribution. The variationsn the particle size and the size distribution as a function of theoping amount could be explained by a classical crystallizationheory. According to the classical crystallization theory, the finalarticle size mainly depends on the ratio of nucleation to growthates. A slow or difficult nucleation would lead to a low numberf nuclei, which can grow and reach large polydisperse sizes.therwise, a fast or easy nucleation would form such a largeumber of nuclei that the final particle size will be small andonodisperse. The introduction of dopant might be favorable to

ucleation process, and thus the particle size is refined and theize distribution is narrowed down. With increasing concentrationf dopant, the number of seed nuclei would remarkably increase.hese seed nuclei would begin to compete with each other forhe available reactant to grow. Abnormal growth of crystal wouldccur in such case. As a result, the particle size is increased and theize distribution is broadened. Therefore, in order to effectivelyontrol the particle size and the size distribution, it is necessary toptimize the doping amount of Sn.

All possible electrochemical reactions occurred for theiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples could be identified by CVurves as shown in Fig. 7. It is found that over the potential range of.5–4.3 V, the samples only exhibit the characteristic redox peaks ofiFePO4. Because no additional redox peaks in CV profiles have beenound, the dopant does not participate in the redox reaction. How-ver, upon doping, the current intensities of the redox peaks firstncrease and then decrease. The LiFe0.97Sn0.03PO4 sample showshe largest current intensity, which indicates the fastest lithium-on diffusion velocity. The effect of the doping amount of Sn on theinetics of the LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples could be furtherlarified by calculating the apparent lithium-ion diffusion coeffi-ient. As shown in Fig. 7, the potential intervals of the redox peaksor the samples are within the limit of 58 mV at 298 K, which is ingreement with characteristics for a reversible reaction [24]. Thus,he apparent lithium-ion diffusion coefficients could be calculatedy formula Eq. (2)

p = 0.4463nF(

nF

RT

)1/2CLi�

1/2AD1/2appK (2)

here ip is the current intensity of the oxidation peak, n is the elec-ron number participating in the oxidation process, F is Faraday’sonstant, CLi is the concentration of lithium ion, � is the scan rate, As electrode area per unit mass, Dapp is the apparent diffusion coef-

Fig. 8. The apparent lithium-ion diffusion coefficient calculated as a function of thedoping amount of tin.

ficient of lithum-ion, K is an irreversible constant (0.78) [24–26].The variations in the apparent lithium-ion diffusion coefficients asa function of the doping amount of Sn are shown in Fig. 8. It isfound that the Li+ diffusion coefficient at first increases and thendecreases upon the doping amount of Sn. The sample with a dopingamount of 3 mol% exhibits the highest lithium-ion diffusion coeffi-cient. The enhancement of the apparent Li+ diffusion velocity canpartly be attributed to the optimization of cooperation of the Li–Obond length and the particle size. The elongation of Li–O bond willfacilitate Li+ intercalation/deintercalation within the hose lattice.The small particle size will be beneficial to shorten the diffusiondistance within the particles. The result indicates that the dopingamount is a paramount factor to the enhancement of the kineticsof the lithium-ion diffusion.

The dependency of the rate capability on the doping amount ofSn is shown in Fig. 9. At the low C-rate, the doping effect on theelectrochemical performance is not quite obvious. The pristine andthe doped samples deliver similar capacities. The slight differencesmainly result from the differences of the electrical conductivitiesand the apparent lithium-ion diffusion velocities. However, with

Doping Content (mol%)

Fig. 9. The rate capability of the LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07) samples as a functionof the doping amount of tin.

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amples first increase and then decrease upon the doping amount ofn. When the doping amount is about 3 mol%, the sample exhibitshe highest specific capacities and most excellent rate capability.he result is consistent with the fact that the sample doped with

mol% Sn shows the highest electrical conductivity and the fastestithium ion diffusion velocity. It could be concluded that the rateapability can be greatly enhanced by optimizing the amount of Snoping.

. Conclusion

The pure olivine nanostructured LiFe1−xSnxPO4 (0 ≤ x ≤ 0.07)amples were synthesized using SnCl4·5H2O as dopant by annorganic-based sol–gel route. The tendency of the physicochem-cal and electrochemical performances upon the doping amountf Sn was systemically investigated and regular behavior wasevealed. In the whole concentration range, the chemical valencef Fe2+ is not significantly changed and the doping of the non-ransition element Sn might be a mixed-valence doping. Theresence of the dopant is favorable to reducing the particle sizend homogenizing the size distribution. However it is necessaryo optimize the doping amount. Additionally, with the increasef the doping amount of Sn, the bulk electrical conductivities ofhe samples first increase and then decrease. Correspondingly,he Li+ diffusion coefficient and the electrochemical performancelso show similar trends. In contrast, due to the high electricalonductivity, the fast lithium ion transfer and small particle size,he LiFe0.97Sn0.03PO4 sample exhibits the excellent electrochemicalerformance, especially the rate capability.

cknowledgements

We thank the financial support from National Nature Scienceoundation of China under Grants (No. 50632040 & No. 50802049)nd Shenzhen Technical Plan Projects (NO. JP200806230010A & No.

[

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a 56 (2011) 7385– 7391 7391

SG200810150054A). We also appreciate the financial support fromGuangdong Province Innovation R&D Team Plan.

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