磷酸鐵鋰制備經(jīng)典

1、Journal of Power Sources 195 (2010 28772882Contents lists available at ScienceDirect Journal of PowerSourcesj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j p o w s o urHydrothermal preparation of LiFePO 4nanocrystals mediated by organic acidJiangfeng Ni a ,Masanori Morishi

2、ta b ,Yoshiteru Kawabe b ,Masaharu Watada b ,Nobuhiko Takeichi a ,Tetsuo Sakai a ,a National Institute of Advanced Industrial Science and Technology,Kansai Center,1-8-31Midorigaoka,Ikeda,Osaka 563-8577,Japan bGS Yuasa Corporation,Nishinosho,Kisshoin,Minami-ku,Kyoto 601-8520,Japana r t i c l e i n f

3、o Article history:Received 21July 2009Received in revised form 2November 2009Accepted 2November 2009Available online 10 November 2009Keywords:Lithium ion battery Cathode LiFePO 4Hydrothermal NanocrystalSynchrotron radiationa b s t r a c t© 2009 Elsevier B.V. All rights reserved.1.IntroductionOl

4、ivine LiFePO 4has been regarded as one of the most promis-ing cathode materials for advanced Li ion battery for use in high energy and power system as required for plug-in hybrid electric vehicles (PHEV13.Although it displays plenty of attractive characters such as good electrochemical property,high

5、 thermal stability,and environmental benignity,the low electronic/ionic conductivity makes it not a competent candidate for use in high power devices.Over the last decade,enormous attempts have been made to improve its conductivity to applicable level.A successful strategy is the preparation of LiFe

6、PO 4carbon composite 47,where LiFePO 4particles are incorporated into carbon network,bringing the total conductivity to a level similar to LiCoO 2.This LiFePO 4/C composite is generally synthesized by an in situ carbon-coating route during which the carbon decomposed from organic precursor deposits

7、on the LiFePO 4surface to form a high way for electron movement.An additional benet arisen from this process is the inhibiting of particle fusion and growth by exotic carbon layer,which helps achieve high rate ability.Doping of divalent (Mg 2+,Mn 2+,Ni 2+,Cu 2+or supervalent ions (Al 3+,Cr 3+,Zr 4+,

8、Nb 5+either in Li site(M1or in Fe site (M2has also been demonstrated to improve the electronic conductivity successfully 79,but so far the mecha-nism is still in great controversial 10,11.Recently,the application of cutting-edge nanotechnology in the synthesis of LiFePO 4has been demonstrated,too.Wi

9、th template,LiFePO 4of particular mor-phology such as nanobre or nanowire has been prepared,and it demonstrates potential for high power application 12,13.Although the conductivity problem are solved to a large extent by methods mentioned above,application of LiFePO 4still suffers from tough synthes

10、is procedure resulting from instable nature of Fe(II.The popular strategy to synthesize LiFePO 4is the high-temperature ceramic route under the inert atmosphere,which results in a nal product frequently composed of impurities like Li 3Fe 2(PO 43,Fe 2O 3,and Li 3PO 414,15.And it is also very difcult

11、by such process to obtain ne and homogeneous particle that is regarded as a key factor for the power ability 16.To obtain LiFePO 4with ne particles for high power purpose,it is essential to take advantage of solution synthetic route such pre-cipitation 16,solgel 17,polyol 18,hydrothermal 1925.Among

12、them,hydrothermal reaction receives particular interest due to mild operating temperature,simple process,high crystal-lization of product,and the potential for large-scale production.It is also considerably convenient to control the properties of product by tuning experiment parameters such as tempe

13、rature,pressure,con-centration,and additive.In a pioneering attempt,Whittingham and co-workers displayed the synthesis of LiFePO 4via hydrothermal reaction at 120C,but the product showed a low electrochemical2878J.Ni et al./Journal of Power Sources195 (2010 28772882activity due to a severe displacem

14、ent of Fe in Li site19.A fol-lowing report by them demonstrated that the displacement of Fe and Li could be remarkably suppressed when arising temperature above180C20.However,many displacement-free materials do not show satisfactory electrochemical behavior,probably due to grown particle size20,21or

15、 the presence of impurities22,23. Recently,Nazar and co-workers reported that LiFePO4nanocrystal-lites could be obtained under hydrothermal condition by increasing reagents concentration or adding organic additives polyacrylic acid and ascorbic acid,but not citric acid24.However,their material does

16、not show favorable electrochemical performance as expected,possibly due to grain agglomeration.Special strat-egy involving the application of surfactants been demonstrated effectively control the crystalline property and particles size of LiFePO4,leading to a pronounced improved performance.A good c

17、ase in point is the synthesis of C/LiFePO4core/shell nanomaterial via hydrothermal route using acetylene black as nucleus diox-ane as in the presence of polyethylene glycol25.The resultant material shows an excellent rate ability,but the high car-bon content(52wt.%makes it not suitable for practical

18、 application.2.ExperimentalThe XRD measurements were conducted using a large DebyeScherrer camera with an imaging plate at the BL19B2beam line,the synchrotron radiation facility SPring-8,Japan.A glass cap-illary of0.3mm was used as the holder forlling the sample powder.The diffraction patterns were

19、recorded in the2Ârange of 0.0178with duration of5min.The wavelength =0.7006Åwas calibrated by using CeO2as standard material.The structural anal-yses were performed with the Rietveld method on the program Rietan-200026.SEM and TEM were performed to observe the particle mor-phology and size

20、 distribution.SEM images were taken on a JEOL JSM-6390microscope at an accelerating voltage of30kV.TEM was performed on a JEOL JEM-3000F transmission electron microscope at an accelerating voltage of300kV.Raman spectroscopy was car-ried out to study the surface microstructure,using a RMP-320 (Jascos

21、pectrometer with Ar laser of50mW at532cm1.For the electrochemical test,80wt.%as-prepared LiFePO4mate-rial,10wt.%ketjen black,and10wt.%polyvinylidene were blended in N-methylpyrrolidinon.The blended slurry was then casted on an aluminum foil followed by roll processing.Afterward,the electrode sheet w

22、as tailored to disks shape of12mm with a typical material loading of5mg cm2and dried at120C for6h in vacuum.A2032 cell was assembled using the disk electrode as cathode in a dry room with a dewpoint below60C.The anode is Li foil and the electrolyte is1mol L1LiPF6in ethylene carbonate/diethyl carbon-

23、ate(EC/DEC(1:1,v/v.The cells were tested on a Keisoku Genki program-controlled test system in the voltage range of2.34.2V at 28C.EIS was carried out with a Solartron SI1280B electrochemi-cal unit.The applied frequency range is65kHz to0.01Hz and the oscillation voltage is10mV.3.Results and discussion

24、Fig.1shows the XRD patterns of as-prepared three LiFePO4 samples(F-blank,F-citric,and F-ascorbic.All the patterns show well-resolved diffraction peaks indexed to olivine LiFePO4(JCPDS No.81-1173.This indicates that well-crystallized LiFePO4can be readily obtained via hydrothermal reaction at230C in

25、half hour.By contrast,Nazar et al.found that well-crystallized LiFePO4could be prepared only when hydrothermal reaction extending5h at190C 24.It is also found that the pattern of LiFePO4prepared without additive shows diffraction peaks at10.1,10.6,and11.2(Fig.1a, which can be attributed to orthorhom

26、bic Li3PO4(JCPDS No.25-1030,while these peaks were not visible in those of the samples mediated by organic acid Fig.1b and c.The structural renement has been done for the synchrotron XRD data of as-prepared samples.The R factors and the struc-tural parameters based on the Rietveld renement of all th

27、ree samples are summarized in Table1,and the observed pattern and calculated one of the typical sample F-blank are presentedJ.Ni et al./Journal of Power Sources 195 (2010 287728822879Table 1The R factors and structural parameters of as-prepared LiFePO 4materials.SampleLiFePO 410.3378(16.0027(14.6989

28、(1291.5884.453.381.096100 Table 2lists the atomic parameters of the typical F-ascorbic sam-ple.In LiFePO 4lattice,Li ions occupy 4a site in one set of octahedra,Fe ions occupy 4c site in another set of octahedra,P ions and half O ions occupy 4c site,and the other half O ions occupy 8d site.It is wor

29、thy of noting that our attempt to rene the pattern assuming part Fe ion in Li site failed,indicative of no Fe ion occupying Li site.When LiOH was added to mixture solution of (NH 42Fe(SO 42and H 3PO 4,the solution gradually changed from acidic to neutral and Fe 3(PO 42·8H 2O precipitated,as men

30、tioned in previous report 23.But in our experiment,the feed order was changed;i.e.,theTable 2The atomic parameters for F-ascorbic sample.Here g is the site occupancy and B is the overall isotropic atomic displacement parameter.Atom Site x y z g B (Å2Li 4a 0008d0.16640.04230.281210.525FeSO 4solu

31、tion was added to system of LiOH and H 3PO 4.The process can be expressed as the formation of Li 3PO 4(Eq.(1followed by one unit Fe 2+ion replacing two units Li +ions and crystallizing (Eq.(2.3LiOH +H 3PO 4Li 3PO 4+3H 2O (1Li 3PO 4+FeSO 4LiFePO 4+Li 2SO 4(2From the above discussion,it can be conclud

32、ed that acidic effect inuences the reaction rate,and the chelating effect adjusts the balance.It is both of these two effects,not any single one of them,that lead to the pure material.To conrm this,we carried out two additional experiments without organic acid to simulate the two effects,respectivel

33、y.One case concerns merely the chelating effect which was inspected by using polyvinyl alcohol (PVA.Unfor-tunately,the corresponding XRD pattern (Fig.3adisplays even more intense peaks due to Li 3PO 4than F-blank,probably the con-version being blocked as crystallites are covered by PVA The other cas

34、e concerns merely the acidic environment which was deliberately created by controlling the ratio of LiOH against H 3PO 4to 2.5:1.The pH value is about 4.5,very close to that of the organic2880J.Ni et al./Journal of Power Sources195 (2010 28772882 acid-containing system.This product does not comprise

35、 Li 3PO 4but monoclinic Fe 3(PO 42(JCPDS No.39-0341(Fig.3b,which is con-sistent with the previous report by Kanamura and co-workers 21.So it is reasonable to conclude that the impurity-free material is the result of the convergence acidic effect and chelating effect from organic additive.It is belie

36、ved that the difference in particle among these products is related to the adsorption of organic molecules or their decom-posing derivatives on the crystal surface.adsorbed molecules cover the surface of nanocrystals to prevent them from fusing together,leading to ne and dispersive particles 24,28.S

37、ince the electrochemical behaviors of LiFePO 4,particularly the rate behav-ior,are largely determined by Li ion diffusion length within particle,the decrease of particle size denitely helps utilize the full capacity.Fig.6compares the rate discharge capability of as-prepared three samples.As expected

38、,the mediated LiFePO 4samples dis-play signicantly enhanced performance over the blank LiFePO 4.In detail,F-blank discharges 128mAh g 1at 0.1C,97mAh g 1at 1C,and 62mAh g 1at 5C,whereas F-citric delivers 153mAh g 1at 0.1C,139mAh g 1at 1C,and 95mAh g 1at 5C.Moreover,F-ascorbic is able to deliver 162mAh g 1,154mAh g 1,and 122mAh g 1at 0.1C,1C,and 5C,respectively.It is worth noting that the mediated LiFePO 4also shows a stable discharge plateau upon increasin