Electrochimic Act 146 (2014) 585 590 Contents lists ville t ScienceDirect Electrochimic Act journl homepge: www.elsevier.com/locte/electct Octgonl prism shped lithium iron phosphte composite prticles s positive electrode mterils for rechrgele lithium-ion ttery Keqing Ding,, Hongo Gu,, Chuno Zheng,LuLiu, Likun Liu, Xingru Yn, Zhnhu Guo, College of Chemistry nd Mterils Science, Heei Norml University, Shijizhung 050024, P.R. Chin Integrted Composites Lortory (ICL), Dn F. Smith Deprtment of Chemicl Engineering, Lmr University, Beumont, TX 77710, USA rticle info strct Article history: Received 1 July 2014 Received in revised form 21 August 2014 Accepted 26 August 2014 Aville online 21 Septemer 2014 Keywords: Octgonl prism shpe lithium iron phosphte composites multi-wlled cron nnotues Lithium ion ttery For the first time, octgonl prism shped lithium iron phosphte (LiPO 4 ) composite prticles supported on the multi-wlled cron nnotues (MWNTs) (denoted s OP-LiPO 4 /MWNTs) re prepred y using oiling reflux ssisted clcintion method. Interestingly, sphericl LiPO 4 composite prticles (indexed s S-LiPO 4 /C) re produced y the sme procedure in the presence of ctivted cron. It is oserved tht the edge length of OP-LiPO 4 prticles is out 400 nm s compred to the prticle size of S-LiPO 4 ( 150 nm) smple. Cyclic voltmmetry (CV) tests demonstrte tht t the scn rte of 0.5 mvs 1 the potentil seprtion of the OP-LiPO 4 smple (out 250 mv) is much smller thn tht of S-LiPO 4 (out 443 mv). More importntly, the initil dischrge cpcities t 0.1 nd 1.0 C for the OP-LiPO 4 /MWNTs re 160.58 nd 116.71 mahg 1, higher thn tht of S-LiPO 4 /C (151.29 nd 95.69 mahg 1 t 0.1 nd 1.0 C). 2014 Elsevier Ltd. All rights reserved. 1. Introduction Lithium ion tteries (LIBs) re one of the most dvnced secondry rechrgele energy storge devices due to its higher power nd energy density [1 3]. The development of higher energy LIBs is essentil for the further commerciliztion of portle electronic devices [4], such s lptops, personl digitl ssistnts, cellulr phones, nd utomotives [5]. Recently, olivine structured lithium iron phosphte (LiPO 4 ) hs een thought s promising cthode cndidte for the higher power LIBs especilly since its discovery y Goodenough group [6]. LiPO 4 hs theoreticl specific cpcity of 170 mah g 1 nd reltively high redox potentil of 3.4 3.5 V vs. Li/Li + rising from the chemicl vlence trnsition of the 2+ / 3+ couple [7,8]. However, the disdvntges of LiPO 4 including the lower electronic conductivity (10 9 Scm 1 ) nd lower lithium ion diffusion coefficient (10 18 m 2 s 1 ) effectively limit its wide pplictions [9,10]. Thus, mny methods hve een developed to resolve ove prolems. Summrily, the following three solutions were proposed so fr, (i) nchoring conductive lyer on the surfce of the prticles; (ii) doping second (or third) metl ion into the Corresponding uthor. Tel.: +86-311-80787400; fx: +86-311-80787401. Corresponding uthor. Tel.: +(409) 880-7654/7195; fx: +(409) 880-2197. E-mil ddresses: dkeqing@263.net (K. Ding), zhnhu.guo@lmr.edu (Z. Guo). lttice of crystllite LiPO 4 ; (iii) controlling the morphologies of the resultnt LiPO 4 prticles. Up to now, mny works concerning the morphologies of the LiPO 4 prticles hve een pulished. For exmple, Xing et l. [11] reported the synthesis of LiPO 4 prticles tht hve 3D conductive network nd porous structure, nd he ddressed tht the s-prepred smple could deliver dischrge cpcity of 95 mah g 1 t rte of 20 C. Sun et l. [12] synthesized LiPO 4 microspheres tht consist of nnopltes or nnoprticles y using novel solvotherml method, nd fter mixing with cron mterils the products exhiited lrger dischrge cpcity vlues (140 mah g 1 t 0.1 C) nd etter cycling performnce (110 mah g 1 t 5 C nd 86 mah g 1 t 10 C fter 1000 cycles). Chen et l. [13] fricted the monodisperse porous LiPO 4 /C microspheres with dimeter rnge of 1.0 1.5 m vi microwve-ssisted hydrotherml pproch comined with crotherml reduction. Wng et l. [14] fricted nno-sized core-shell structured LiPO 4 /C nnocomposites using polyniline (PANI) s the cron precursor. Lim et l. [15] synthesized the nnowire nd hollow prticles of LiPO 4, in which the nnowire smple showed dischrge cpcity of 157 mah g 1 t 0.2 C nd the hollow smple exhiited dischrge cpcity of 165 mah g 1 t 0.2 C. Murlignth et l. [16] reported tht the LiPO 4 nnorods could e prepred y rpid microwve-solvotherml method within 5 minutes t the temperture of 300 C. Zhou et l. [17] http://dx.doi.org/10.1016/j.electct.2014.08.141 0013-4686/ 2014 Elsevier Ltd. All rights reserved.
586 K. Ding et l. / Electrochimic Act 146 (2014) 585 590 descried the synthesis of highly-conductive 3D cron nnotue network interlced with porous LiPO 4 through n in-situ sol-gel process, which exhiited dischrge cpcity of 159 mah g 1 t the current density of 10 ma g 1. Toprkci et l. [18] prepred the LiPO 4 /CNT/C composite nnofiers through comined method (electrospinning nd sol-gel techniques), nd the s-prepred smples displyed lrge cpcity vlue (165 mah g 1 t 0.1 C), extended cycle life, nd excellent rte cpility. It cn e deduced from ove pulictions tht the cron sources plyed key role in controlling the morphology of the resultnt LiPO 4 prticles. To the est of our knowledge, there is no pper reporting the preprtion of octgonl prism shped LiPO 4 huge prticles nd the method of oiling reflux ssisted clcintion process s well. In this communiction, MWNTs nd ctivted cron hve een utilized s cron sources in the oiling reflux ssisted clcintion process for prepring LiPO 4, with n intention to evlute the influence of cron sources on the morphologies of the resultnt smples. As result, octgonl prism shped nd sphericl prticles of LiPO 4 were produced, respectively, in the presence of MWNTs nd ctivted cron. The otined LiPO 4 composites hve een chrcterized y Fourier trnsform infrred (FT-IR), X- ry diffrction (XRD), Energy Dispersive X-Ry Spectroscopy (EDX), nd scnning electron microscope (SEM). The electrochemicl performnces of the LiPO 4 composites s cthode mterils for the LIBs re evluted y cyclic voltmmetry (CV), electrochemicl impednce spectroscopy (EIS), nd glvnosttic chrge-dischrge mesurement. 2. Experimentl LiOH H 2 O (99.0 wt%), phosphoric cid (H 3 PO 4, 85 wt%), - 2 O 3, ctivted cron, oxlic cid ( 99.5 wt%), nd glucose were ll otined from Tinjin Yongd Chemicl Regent Co., Ltd (Chin). Acetylene lck, polyvinylidene fluoride (PVDF, inder), N-methyl- 2-pyrrolidone (NMP), electrolyte solution (ethylene cronte (EC)/diethyl cronte (DEC)/dimethyl cronte (DMC) = 2: 5: 11 (volume) with 1 mol L 1 LiClO 4 ), nd polypropylene (PP, M w = 2400) were purchsed from Tinjin Lingnuo S&T Development Co., Ltd (Chin). Multi-wlled cron nnotues (MWNTs) with n verge dimeter of 30 50 nm were purchsed from Shenzhen Nnotech Port Co., Ltd. (Chin). All of the chemicls were nlyticl grde nd used s-received without ny further tretment. Douly distilled wter ws used to prepre the queous solutions. The procedure for the preprtion of LiPO 4 composites filled with MWNTs or ctivted cron is s follows. Strting mterils of LiOH H 2 O, H 3 PO 4, - 2 O 3 nd MWNTs in molr rtio of 1.1: 1: 0.5: 0.6 were mixed with 90 ml distilled wter in three-neck round-ottom flsk. 5 g of oxlic cid nd 0.4 g glucose were dded into the ove suspension solution, nd then the suspension solution ws heted to oiling reflux, nd this process ws kept for out one hour until the solution volume ecme 30 40 ml. It should e noted tht fter the oiling reflux process, green solution ws otined, indictive of the 2+ formtion. After cooling down to the room temperture, the solution ws plced in n oven nd dried t 150 C for 3 h. Susequently, the produced powders were ground in mortr nd pressed into slice with dimeter of round 1 cm. And then the smples were trnsferred into crucile tht ws covered y cron powders. Finlly, the crucile ws plced in n electronic furnce t 700 C for 5 h. The finl product ws denoted s OP-LiPO 4 /MWNTs. The products mixed with ctivted cron were lso fricted ccording to the sme procedure, nd correspondingly, the resultnt smples were nmed s S-LiPO 4 /C. Trnsmittnce (%) 469 501 549 577 636 1053 1094 1138 400 600 800 1000 1200 Wvenumer (cm -1 ) Fig. 1. FT-IR spectr of () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples. Fourier trnsform infrred (FT-IR) spectr of the smples were otined on JASCO 8900 (Hitchi Ltd., Jpn) in the rnge of 400 to 1200 cm 1 t resolution of 4 cm 1. X-ry diffrction (XRD) nlysis ws crried out with Bruker AXS D8 ADVANCES with Cu- K rdition source ( = 1.5406 Å). Dt were collected in rnge of 15 to 75. The morphologies of the synthesized products were oserved on field emission scnning electron microscope (SEM, S-4800, Hitchi Ltd., Jpn). Energy Dispersive X-ry Spectroscopy (EDX) spectrum nlysis ws crried out on n instrument of EDAX (PV-9900, USA). Electrochemicl mesurements were crried out on CHI 660B electrochemicl worksttion (Shnghi Chenhu Apprtus, Chin) connected to personl computer. In the EIS mesurement, the excittion voltge pplied to the cells ws 5 mv nd the frequency rnge ws from 0.1 to 1 10 5 Hz. All the experiments were crried out t room temperture. The cthode ws prepred s follows. The fricted OP- LiPO 4 /MWNTs or S-LiPO 4 /C powders, polyvinylidene fluoride (PVDF), cetylene lck were mixed in n-methyl-2 pyrrolidone (NMP) solution with weight rtio of 8: 1: 1. After the ove slurries were uniformly coted on the luminum foil with dimeter of 10 mm, the prepred electrodes were put into vcuum oven nd dried t 120 C for 12 h. After tht, the dried smples were pressed y hydrulic presser t 5 MP nd cut into disks efore trnsferring into nitrogen-filled glove ox. Two-electrode electrochemicl cells, consisting of lithium metl foil s the negtive electrode, Celgrd 2400 seprtor, nd n electrolyte of 1 M LiClO 4 in ethylene cronte (EC):diethyl cronte(dec):dimethyl cronte (DMC) (2:5:11, vol.), were ssemled in nitrogen-filled glove ox. The electrochemicl cycle tests were performed using CT-3008W- 5V20mA-S4 testing system (Shen Zhen Newwre Technology Ltd., Chin) t vrious rtes (1C=170mAhg 1 ) etween 2.75 nd 4.2 V t room temperture. 3. Results nd discussion Fig. 1 shows the FT-IR spectr of OP-LiPO 4 /MWNTs nd S- LiPO 4 /C smples. The min sorption nds re in the region of 400 1200 cm 1. The strong nds t 636, 968, 1053, 1094, nd 1138 cm 1 correspond to the symmetricl nd symmetricl virtion of PO 4 3 [19]. The peks locted t 469, 501, 549, nd 577 cm 1 re ssigned to the ending virtion of P-O nd. These chrcteristic peks (469, 501, 549, 577 636, 968, 1053, 1094, nd 1138 cm 1 ) re consistent with the results of LiPO 4 in the literture [19] very well. Fig. 2 shows the EDX spectr of LiPO 4 /MWNTs nd LiPO 4 /C smples. Only, P, O elements re oserved in the OP- LiPO 4 /MWNTs nd S-LiPO 4 /C smples, indicting tht there 968
K. Ding et l. / Electrochimic Act 146 (2014) 585 590 587 C C O O P P 0 2 4 6 8 10 E (kev) Fig. 2. EDX spectr of () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples. re no other impurities such s SO 2-4 nd NO- 3 introduced during the formtion of LiPO 4 [20]. Li element cnnot e detected y this technique due to the low tomic weight of Li [21]. The weight percentges of, P, O elements in the OP-LiPO 4 /MWNTs smple re oserved to e 22.10, 16.87 nd 56.21%, respectively. The weight percentges of, P, O elements in the S-LiPO 4 /C smple re 19.30, 14.00, nd 59.24%, respectively. This result indicted tht the components of the OP-LiPO 4 /MWNTs nd S-LiPO 4 /C re close to ech other. The X-ry diffrction (XRD) ptterns of OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re shown in Fig. 3. The XRD ptterns of OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re consistent with the stndrd pttern of LiPO 4 (JCPDS#01-083-2092) very well, indicting single phse of olivine LiPO 4 structure with the orthorhomic crystl structure[22]. Menwhile, no other diffrction peks re oserved in the XRD ptterns of ove two smples. These indicted tht oth the OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples hve only the olivine crystl structure. In ddition, no diffrction pek of cron structure (normlly locted t 2 =25 corresponding to the (0 0 2) fcet of the cron mterils [23]) is Intensity(.u.) (2 0 0) (1 0 1) (2 1 0) (0 1 1) (1 1 1)(2 0 1) (0 2 0) (3 0 1) (3 1 1) (1 2 1) (4 1 0) (1 0 2) (2 2 1) (4 0 1) (1 1 2) (2 0 2) (0 2 2) (1 3 1) (2 2 2) (6 1 0) (3 3 1) (4 3 0) 20 30 40 50 60 70 2θ ( o ) (0 4 0) (5 1 2) (3 1 3) 01-083-2092 Fig. 3. XRD ptterns of () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples. oserved proly due to its disordered stte nd/or low content in the composites. This indicted tht the ddition of MWNTs nd ctivted cron hs no effect on the crystl structure of olivine LiPO 4. Fig. 4 shows the SEM imges of OP-LiPO 4 /MWNTs nd S- LiPO 4 /C smples. Interestingly, the LiO 4 composites exhiited different morphologies when using different cron sources. It is cler tht for the OP-LiPO 4 /MWNTs smple, octgonl prism shped huge prticles (circled prt) were found in the MWNTs, Fig. 4( & c). Close inspection reveled tht the length of the huge prticle ws out 400 nm nd the dimeter of its section ws round 130 nm. The shpe of OP-LiPO 4 prticles presented here is novel, s hs never een reported previously. Interestingly, the S-LiO 4 prticles prepred in the presence of ctivted cron showed ll-like structure with n verge dimeter round 150 nm, Fig. 4( & d). The electrochemicl performnce of the s-prepred ttery cell ws chrcterized y the cyclic voltmmetry (CV), glvnosttic chrge/dischrge, nd EIS mesurements. Fig. 5 shows the CVs of Fig. 4. SEM imges of () OP-LiPO 4/MWNTs, nd () S-LiPO 4/C smples; Enlrged SEM imges of (c) OP-LiPO 4/MWNTs, nd (d) S-LiPO 4/C smples.
588 K. Ding et l. / Electrochimic Act 146 (2014) 585 590 0.8 I (ma) 0.4 0.0-0.4 Cpcity (mah g -1 ) 150 120 90 60 2.8 3.2 3.6 4.0 E (V) (vs. Li/Li + ) Fig. 5. CV curves of () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples. the ttery cell ssemled y the s-prepred OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples t potentil scnning rte of 0.5 mv s 1. Both smples exhiited one pir of well-defined redox pek, which corresponds to the extrction nd insertion of Li ion during the electrochemicl rection [24]. The oxidtion nd reduction pek potentils for OP-LiPO 4 /MWNTs smple re 3.573 nd 3.323 V, respectively. For the S-LiPO 4 /C smple, the oxidtion nd reduction pek potentils re 3.655 nd 3.212 V, respectively. The pek potentil intervls etween oxidtion nd reduction peks ( E p, E p = E p E pc, where E p nd E pc re the pek potentils for nodic nd cthodic rection, respectively) for OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re pproximtely 250 nd 443 mv, respectively. Generlly, the more rodened the pek intervl, the higher the electrode polriztion. And smller pek potentil seprtion indictes etter reversiility of ttery cell [25]. Therefore, the reversiility for the intercltion/deintercltion process of the Li ions in the OP-LiPO 4 /MWNTs is much etter thn tht in the S- LiPO 4 /C, even though the pek current of the OP-LiPO 4 /MWNTs is slightly lower thn tht of S-LiPO 4 /C smple. Fig. 6 shows the initil chrge-dischrge curves of the prepred cells t the rtes of 0.1 nd 1 C with potentil window rnging from 2.75 to 4.2 V. The cpcity vlues of the OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples in the initil dischrge cycle t the rte of 0.1 C re 160.58 nd 151.29 mah g 1, respectively, which is much etter thn the results otined y Ziolkowsk et l. (122 mah g 1 for the Sn-modified LiPO 4 mteril) [26]. In the chrge/dischrge profile, only one voltge plteu is oserved, which corresponds Voltge (V) 4.0 3.6 3.2 2.8 1C 1C 1C 1C 0 40 80 120 160 Cpcity (mah g -1 ) Fig. 6. Chrge nd dischrge curves of the ttery cell () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples t different rtes. 0 5 10 15 20 25 30 Cycling numer Fig. 7. Cycling performnce of the dischrge cpcity for the cell () OP- LiPO 4/MWNTs nd () S-LiPO 4/C smples t rte of 1C. to the extrction/insertion process of Li ions nd two phse trnsformtion rections etween LiPO 4 nd PO 4 [27]. This result is consistent with tht otined from the CV curve, Fig. 5. The voltge intervl ettween the chrge nd dischrge voltge plteus is normlly relted to the polriztion loss [28] nd the reversiility of ttery cell [29]. In Fig. 6, the voltge intervls etween the chrge nd dischrge voltge plteus for OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re 50.9 nd 52.9 mv, respectively. Therefore, the reversilility of the cell ssemled y OP- LiPO 4 /MWNTs smple ws etter thn tht in S-LiPO 4 /C smple [30]. After incresing the rte to 1 C, the dischrge cpcity vlues of OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples for the initil dischrge cycle dropped to 116.7 nd 95.69 mah g 1, respectively. Thus, it cn e concluded tht the dischrge cpcity of OP- LiPO 4 /MWNTs smple is much higher thn tht of S-LiPO 4 /C smple. The dischrge potentil plteu of OP-LiPO 4 /MWNTs smple is locted t out 3.36 V, nd no ovious flt dischrge potentil plteu is oserved in the cse of S-LiPO 4 /C smple. Thus, the electrochemicl properties of OP-LiPO 4 /MWNTs smple re superior to those of S-LiPO 4 /C smple in terms of the dischrge cpcity vlue, dischrge voltge plteu nd reversiility even t higher rte of 1C. The cycling performnce of the prepred cells mde from OP- LiPO 4 /MWNTs nd S-LiPO 4 /C smples is lso investigted. Fig. 7 shows the dischrge cpcities of OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples during the 30 chrge-dischrge cycles t current rte of 1 C. The cpcity vlues for oth ttery cells re well mintined even fter 30 cycles. For exmple, in the whole testing period, the dischrge cpcity vlue of OP-LiPO 4 /MWNTs smple ws t round 120 mah g 1, higher thn tht of S-LiPO 4 /C smple (round 100 mah g 1 ). This result sustntilly indicted tht oth smples hve excellent cycling stility. Fig. 8 shows the rte cpilities of these two smples mesured t, 0.5C, 1C nd 1.5C rtes during every five cycles. It is evident tht the dischrge cpcity vlue decresed with incresing C rte. This is minly due to the incresed IR voltge loss nd higher concentrtion polriztion t the electrode/electrolyte interfce to meet the fst rection kinetics t higher C rtes [31]. Evidently, the dischrge cpcity vlues of OP-LiPO 4 /MWNTs nd S-LiPO 4 /C t 0.1 C nd 1.5 C re, 160 nd 90 mah g 1, 150 nd 76 mah g 1, respectively. Tht is to sy, the dischrge cpcity vlue of OP-LiPO 4 /MWNTs smple is lwys higher thn tht of S-LiPO 4 /C when pplied y the sme current rtes. Interestingly, when reset to dischrging rte fter 20 cycles, the dischrge cpcities for OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re still 160 nd 152 mah g 1, respectively.
K. Ding et l. / Electrochimic Act 146 (2014) 585 590 589 Cpcity (mah g -1 ) 200 160 120 80 40 0.5C 1C 0 5 10 15 20 25 Cycling numer 1.5C Fig. 8. Cycling performnce of the dischrge cpcity for the ttery cell () OP- LiPO 4/MWNTs nd () S-LiPO 4/C smples t different rtes. These vlues re lmost the sme s the dischrge cpcities fter 5 cycles () efore the high-rte dischrge (0.5C, 1C nd 1.5C), indicting good cpcity retention. The EIS mesurements were crried out in the frequency rnge from 0.1 to 10 5 Hz. The inset of Fig. 9 shows the typicl Nyquist plots. Generlly, in the Nyquist plots, the semi-circle t the high frequency region stnds for the chrge-trnsfer-limited process t the interfce of electrodes nd the inclined line t the low frequency region represents the Wrurg fctor eing ssocited with the diffusion-limited process of the Li ions in LiPO 4 [32]. And the numericl vlue of the dimeter of the semicircle on the Z xis is pproximtely equl to chrge trnsfer resistnce (R ct ). Thus, vlues of R ct for the OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re estimted to e 50 nd 81, respectively. This illustrtes tht the intercltion/deintercltion process of Li ion in the OP- LiPO 4 /MWNTs smple ttery cell is much esier thn tht in the S-LiPO 4 /C smple ssemled cell. Normlly, the diffusion coefficient (D) of Li ions in the solid phse of ttery cell cn e clculted from Eqution (1) [33]: D = (R 2 T 2 )/(2A 2 n 4 F 4 c 2 2 ) (1) where R is the gs constnt (8.314 J mol K 1 ), T is the sulute temperture (K); A is the effective surfce re per unit of the thode; n is the trnsfer numer of electrons per molecule during oxidiztion; F is the Frdy constnt; c is the concentrtion of Li ions in Z'(Ω) 250 200 150 100 50 -Z''(Ω) 150 100 50 0 0 40 80 120 160 Z'(Ω) 0.2 0.4 0.6 0.8 1.0 1.2 ω -1/2 Fig. 9. Z s function of ω 1/2 of () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples t low frequency rnge. Inset shows the Nyquist plots of () OP-LiPO 4/MWNTs nd () S-LiPO 4/C smples. the solid phse, which normlly is 7.69 10 3 mol cm 3 [34]; is the Wrurg prefctor, which cn e determined from the slope of the Z ω 1/2 curve sed on the following reltionship [34], in which R s is the resistnce of SPE(solid polymer electrolyte), nd R ct is the chrge trnsfer resistnce. Z = R s + R ct + ω 1/2 (2) Fig. 9 shows the reltionship etween Z nd the reciprocl squre root of frequency (ω 1/2 ) in the low frequency region. The corresponding vlues D of Li ions in OP-LiPO 4 /MWNTs nd S-LiPO 4 /C smples re clculted to e 1.45 10 14 nd 5.53 10 15 cm 2 s 1, respectively. This result indictes tht the diffusion process of Li ions (which is the slowest process) into OP-LiPO 4 /MWNTs smple is esier thn tht in S-LiPO 4 /C smple. This result is consistent with the results otined in Nyquist plots, in which the higher conductivity is oserved in OP- LiPO 4 /MWNTs smple. The decresed chrge trnsfer resistnce nd incresed Li ion diffusion coefficient should e responsile for the higher dischrge cpcity (160.58 mah g 1 t ) in the OP-LiPO 4 /MWNTs composite. Generlly, it is thought tht smller LiPO 4 nnoprticles re eneficil for the diffusion of Li ions due to their ility of shortening the diffusion pth of Li-ions [35]. However, in this work, the lrger prticles of OP-LiPO 4 composites showed significntly enhnced electrochemicl performnce s compred to the smller prticles of S-LiPO 4, which is rther opposite to the previous proposition tht in LiPO 4 -sed cthode mterils the electrode resistnce depends solely on the men prticle size. How does one under this interesting result? Up to now, severl fctors, such s, electricl conductivity, prticle size nd crystllinity of crystl prticles, hve een thought s the min fctors tht cn gretly ffect the electrochemicl performnce of cthode mteril. In this work, new concept, i.e., order degree of rrngement of prticles, ws tenttively proposed. Tht is to sy, due to the well defined crystl structure nd the huge prticle size, the OP-LiPO 4 prticles, when eing pressed to form n electrode, cn e rrnged much more orderly s compred to the cse of S-LiPO 4 prticles. Additionlly, it hs een reported tht Li-ions diffusion minly proceeded long the shortest xis (c-xis) in lttice [36]. Thus, the direction selectivity of Li-ions diffusion in the OP-LiPO 4 huge prticles structured cthode ws superior to tht in the cthode constituted y smller prticles of S-LiPO 4. These novel concepts proposed here, i.e., order degree of rrngement nd direction selectivity of Li-ions diffusion, proly cn e well utilized to interpret some fcts tht lrger prticles of LiPO 4 s cthode mterils could lso exhiit excellent electrochemicl performnce s reported in Ref.[17] nd [29]. How does one under the formtion process of such huge prticles OP-LiPO 4 in this novel work? According to our previous work [37], during the oiling reflux process, ll metl ions my spontneously ssemled on the surfce of MWNTs due to the vn der Wls forces or the so clled chemicl interction etween metl ions nd the orgnic groups tht existed on the surfce of MWNTs, leding to the formtion of so-clled self-ssemled monolyer [38]. And during the nneling process, due to the well rrnged ssemly of metl ions nd nions, orderly rryed molecules re produced in the vicinity of MWNTs. As result, crystl prticles of LiPO 4 would grow up long their preferentil growth direction, leding to the genertion of huge crystl prticles of OP- LiPO 4. In other words, in ove process, MWNTs cted s n inititor for the ordered rrngement of metl ions, which my led to the ordered ssemly of prepred molecules. Or in other words, ordered rrngement of ions produced ordered ssemly of molecules, nd the well rrnged molecules could produce well ccumulted nnoprticles. Consequently, huge prticles with
590 K. Ding et l. / Electrochimic Act 146 (2014) 585 590 specil morphology were fricted. Evidently, the morphology of the ctivted cron ws rther different from tht of MWNTs, exhiiting vrious properties when eing employed in this developed process. It is expected tht the prepred OP-LiPO 4 huge prticles hve potentil pplictions in the field of micro- electronic devices. 4. Conclusions The octgonl prism shped LiPO 4 /MWNTs nd sphericl LiPO 4 /C composites hve een prepred vi oiling reflux ssisted nneling method. The FT-IR spectr, EDX spectr, XRD mesurement, nd SEM technique hve confirmed the successful friction of LiPO 4 composites. The unique crystl structure ws thought s the min reson for the higher vlue of dischrge cpcity nd etter cycling performnce exhiited y the synthesized octgonl prism shped LiPO 4 composites. 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