Electrochem ical Performance Stud ies of Tin Ox ide Prepared by Sol-Gel 3

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1 CH EM. R ES. CH IN ESE U. 2004, 20 (5), Electrochem ical Performance Stud ies of Tin Ox ide Prepared by Sol-Gel 3 M A Yue, ZHOU D e2feng, ZHAN G J ing2p ing, HAO J ie, X IE H ai2m ing ZHAN G X i2yan, SU Zhong2m in and W AN G Rong2shun 3 3 Institu te of F unctiona l M a teria l Chem istry, F acu lty of Chem istry, N ortheast N orm a l U n iversity, Chang chun , P. R. Ch ina R eceived A ug. 18, 2003 SnO 2 p repared w ith the so l2gel technique at different temperatures w as used as an anode m aterial of the lith ium 2ion battery. W e obtained the qualititave relationsh ip betw een the electrochem ical perfo rm ance of tin oxide anode m aterial and each of its carbon component, particle size, structure and surface mo rpho logy in de2 tail. It w as found that the best electrochem ical perfo rm ance can be reached by contro lling the physical charac2 teristics of SnO 2, w h ich w as affected greatly by the treatm ent temperature. T he SnO 2 obtained at 600 w as pure. W hen it w as used as the anode m aterial of the lith ium 2ion battery, it demonstrated a h igh initial specific capacity of 770 ma h g. Ke yw o rds L ith ium 2ion battery, A node, SnO 2 A rtic le ID (2004) In troduction Since the in troduction of comm ercial lith ium 2 ion batteries fo r the po rtab le devices in the 1990 s, the search fo r new anode m aterials w ith imp roved energy den sity and cycle2ab ility ha s never been stopped [1 5 ]. Graph ite u sed as the early comm ercial anode m aterial fo r lith ium 2ion batteries over o ther k inds of m aterials in term s of its flat in tercalation po ten tial and h igh cycle2ab ility [6, 7 ] can t keep pace w ith the dem and today becau se of its low capacity. In resen t years, the great effo rts have been m ade to find novel m aterials w ith h igh specific capacity [8, 9 ]. Since Fu ji [10 ] announced the comm ercialization of tin ox ide as negative electrodes of lith ium 2ion batteries, the tin ox ide anode has attracted m uch atten tion due to its h igh specific capacity, w h ich is abou t tw ice that of graph ite, and con sidered as the best candidate fo r lith ium 2ion battery anode m ateri2 al [11 13 ]. W hen SnO 2w as u sed as the anode m aterial of the lith ium 2ion batteries, tin w o rk s as the virtu2 al part, and its reversib le capacity is based on the fo rm ation and decompo sition of lith ium tin alloys, L isn, L i7sn3, L i5sn 2, L i3sn5, L i7sn2 o r L i22sn 5 [14 17 ]. T he oxygen in SnO 2 can com b ine w ith lith ium to fo rm L i2o in the first cycle, w h ich can be the buffer fo r tin aggregation. L ith ium in2 tercalated in tin diox ide in the first cycle can be de2 scribed by the fo llow ing reaction s [11, 18, 19 ] : 4L i + + SnO 2 + 4e L i2o + Sn (1) xl i + + Sn + x e Charge D ischarge L ix Sn, 0 < x 4. 4 (2) T he differen t syn thetic techn iques and treat2 m en t temperatu res can m ake SnO 2 have differen t carbon con ten ts, partical sizes, crystallin ities and the mo rpho logies, w h ich can fu rther affect its elec2 trochem ical perfo rm ance. W e p repared SnO 2 by the so l2gel techn ique at differen t temperatu res, aim ing at investigating the detail relation sh ip betw een the physical p roperty and electrochem ical perfo rm ance. Exper im en ta l 1 M a ter ia l Prepara tion A 250 ml ethano l so lu tion of 20 g SnC l2 2H 2O w as vigo rou sly stirred fo r 30 m in, refluxed fo r 1 h, then aged fo r 2 h. H aving left under 70 fo r 2 d in a w ater bath, the gelatin i m ix tu re w as dried at 120 to get the so lid p recu rso ṙ T he tin ox ide w as even tually ob tained by calcin ing the p re2 cu rso r in a ir fo r 2 h under 300, 400, 500 and 600, respectively. A ll the chem icals u sed w ere of reagen t g rade and comm ercia lly ava ilab le and u sed w ithou t fu rther purification. 2 M a ter ia l Character iza tion s T he T G2D TA characterization of the p recu rso r 3 Suppo rted by the Innovation Fund fo r Sm all T echno logy2based F irm s (N o. 00C ). 3 3 To w hom co rrespondence should be addressed. E2m ail: w angrs@ nenu. edu. cn

2 6 48 CH EM. R ES. CH IN ESE U. V o l. 20 w as perfo rm ed on an SD T 2960 in strum en ṫ T he X2ray diffraction (XRD ) characterization w as per2 fo rm ed on a D gm ax2vb diffractom eter w ith Cu K Α radiation. T he mo rpho logies of the samp les w ere exam ined by scann ing elect ron m icro scop y ( SEM ) on a H itach i S2570 in strum en ṫ T he su rface area of SnO 2 pow der w as m easu red by the B runauer, Em 2 m ett, and T eller (BET ) techn iques on a M i2 crom eritics A SA P2010M in strum en ṫ T he carbon con ten t of the samp les w as determ ined on an E le2 m en tary A nalysen system e Gm bh V arioel elem en2 tal analyzer. 3 Electrode Prepara tion and Electrochem ica l Character iza tion T he tin ox ide w as m ade in to slu rry w ith 82% SnO 2 pow der, 10% ca rbon b lack, and 8% po lyvinylidene di2fluo ride (PVD F) in N 2m ethyl py2 ro lidinone (NM P ), and w as coated on copper fo ils ( th ickness: 30 Λm; area: abou t 1 cm 2 ), then the coa ted fo ils w ere d ried in vacuum overn igh t a t 120 and p ressed betw een stain less steel p lates at 6 M Pa. T he electrochem ical cells w ere assem b led w ith tin ox ide as the po sitive electrode, a po lyp ropylene m icro po rou s separato r, ethylene carbonate (EC ) and diethyl carbonate (D EC) [V (EC) V (D EC ) = 1 2 ] as the electro lyte, and a lith ium fo il as the negative electrode. T he cell assem b ly w as carried ou t in an argon2filled glove box. T he cells w ere cy2 cled w ith a PCBT D 2A battery tester betw een 1 mv and 2 V at a con stan t cu rren t den sity of 015 ma gcm 2 and a con stan t temperatu re of ( ). Results and D iscuss ion 1 Therm ograv im etry-d ifferen tia l Therma l Ana lyses(tg-d TA) T he D TA and T G analyses w ere perfo rm ed in air w ith A l2o 3 as the reference m aterial. T he D TA in F ig11 suggests that the endo therm ic peak around F ig. 1 TG-D TA curves of the SnO 2 precursor. 150 show s the dehydrate p rocess of the p recu r2 so r, w h ile the exo therm ic peak from 180 to 400 co rresponds w ell to the decompo sition of the p re2 cu rso ṙ T he T G show s that the p recu rso r began to lo ss its w eigh t from 50 to 414. F rom the componen t of the p recu rso r and the D TA resu lts, it can be concluded that the w eigh t lo ss is due to the evo lving of H 2, H 2O, CH 4, CO and CO 2, w h ich are the decompo sition p roducts of the calcination p rocess. Since the w eigh t lo ss still con tinued around 300, there m u st have been som e con ten t of carbon undecompo sed in the sys2 tem. W ith the increasing of temperatu re, the car2 bon con ten t w as low er, w h ich w as also testified by the elem en ta l ana lyzer. T here is 212% ca rbon in the tin ox ide ob tained at 300, w h ile there is ju st 0199% carbon at XRD Ana lysis T he X2ray diffraction (XRD ) pattern s of the samp les calcined at differen t temperatu res are p re2 sen ted in F ig12. In the scan range from 20 to 90, all the samp les show the th ree m ajo r peak s (100), (101) and (211), w h ich co rrespond w ell w ith that of Cassiterite phase (JCPD file ). T he peak in ten sity of the samp les increases w ith the rising of temperatu re, w h ich demon strates that the crys2 ta llin ity of the sam p les is im p roved by the h igh temperatu re. F ig. 2 XRD pattern s of SnO 2 powder treated at differen t temperatureṡ T he peak half w idth of tin ox ide also tu rn s to be narrow from 300 to 600. A cco rding to Scher2 rer fo rm u la, d = kκgβco sη, w e can calcu late crystal p a rt icle sizes of SnO 2 to be 412, 814, 1314, 1814 nm, respectively at 300, 400, 500, 600. T h is show s the crystal particle size increases w ith the rising of temperatu re. 3 SEM Ana lysis F igs. 3 (A ) (C ) show the real mo rpho logies of the samp les ob tained at 300, 400 and 500, re2

3 N o. 5 M A Yue et al. 649 spectively. F rom F ig13 w e can t see the ex istence of sm all crystal particles, w h ich have the sm all size co rresponding to tho se show n by the XRD, bu t a b ig and compact b lock, w h ich show s the severe ag2 gregation of the crystal particles. H ere w e can see that becau se of the particle aggregating, the real particle sizes of the samp les are m ain ly decided by the aggregating degree rather than the sm all crystal size show n in the XRD. F ig. 3 SEM m icrographs( 6000) of SnO 2 powder treated at differen t temperatureṡ (A ) 300 ; (B) 400 ; (C) 500 ; (D ) 600. F igs13 (A ) (C ) show that from 300 to 500, the real particle size of the b lock also becom e larger w ith the increasing of temperatu re, w h ich obeys the sam e chang ing ru le of crysta l p a rt icle size becau se of the severe aggregation. F ig13 (D ) show s the SEM m icrograph of the tin ox ide calcined at 600, although there still ex ist aggregates, a lo t of single sm all round SnO 2 particles can be found and begin to separate from the b ig aggregation b lock. Compared w ith the par2 ticles ob tained at 500, the aggregating degree and the real particle size of SnO 2 seem s low er and sm aller. H ere w e p ropo se that w ith the rising of tem 2 peratu re, the SnO 2 crystals con tinue to grow to be2 com e mo re and mo re in tegrates, w h ich can also be seen from the XRD pattern s in F ig12. A t 600 th is crystal grow s to a very fu ll ex ten t, then fo r the purpo se of the crystal itself grow ing, the crys2 ta l p a rt icles agg rega t ing severely in a b ig flake b lock at 500 begin to grow tow ard itself, then the sm all round particles w ere separated from the b ig flake b lock w h ich m ade the aggregating degree low er, and fu rther m ade the real particle size of 600 sm aller than that of BET Ana lysis F rom T ab le 1 w e can see tha t from 300 to 500 the specific su rface area of the samp les de2 creases w ith the increasing of temperatu re, and the specific su rface area of SnO 2 ob tained at 600 is b igger than that of the one ob tained at 500. A ll the above resu lts agree w ell w ith the changing ru le of the real particle sizes go t from SEM, th is is be2 cau se the b igger the real particle is, the sm aller the specific su rface area w ill be. Table 1 The BET of SnO 2 powder treated at differen t tem peratures tg Specific surface areag(m 2 g - 1 ) Electrochem ica l Performance 5. 1 T he Cha racteristic of the F irst Cy cle Cu rve T he sim ila r first2cycle cha rge and d ischa rge

4 6 50 CH EM. R ES. CH IN ESE U. V o l. 20 cu res of SnO 2 heated at differen t tempeatu res are show n in F ig14. T he p lateau around 018 V in the charging cu rves disapears in the latter cycles, co r2 respond ing w ell to the irreversib le fo rm a t ion of L i2o and so lid2elect ro lyte in terface ( SE I) in the first charging p rocess. T hey can give a to tal theo2 retical irreversib le capacity of abou t 712 ma hg g [20 ]. T he p lateau at 015 V in the discharge cu rves is due to the reversib le alloying p rocess of lith ium and tin, and it is the sou rce of reversib le capacity, w h ich is abou t 783 ma h g [20 ]. H ere SiO 2 treated at 600 has the largest capacity. F ig14 F irst-cycle charge and discharge curves of SnO 2 prepared a t d ifferen t tem pera tureṡ 5. 2 T he Cy cle2ability of T in O x id e A nod e F ig. 5 show s the rela t ion cu rve of reversib le capacity and cycle num ber. Fo r the tin ox ide ob2 tained at 300, after 10 cycles the reversib le ca2 pacity ju st rem ain s 55% that of the first cycle, the average capacity fade rate is 4. 5% per cycle. Fo r the SnO 2 go t at 400, 500 and 600, the fade rate is 219%, 312%, and 317% per cycle, respective2 ly. T he tin ox ide anodes have a poo r cycleab ility becau se the alloying p rocess of lith ium and tin can cau se a large vo lum e change in the tin region s, and the vo lum e can expands as m uch as 259% that of pure Sn [11, 21 ]. T hough the fo rm ing of L i2o m atrix can p reven t the aggregation to som e ex ten t, som e of the L i2o w ill decompo se w hen the po ten tial is F ig. 5 Compar ison of cycleability of SnO 2 samples obta ined at differen t temperatureṡ over 113 V [11 ], and the cycle life of SnO 2 w ill re2 duce as w ell. H ere SiO 2 trated at 400 show s the best cycleab ility. 6 The Effect of Trea tm en t Tem pera ture on the Electrochem ica l Performance T ab le 2 g ives the first2cycle elect rochem ica l perfo rm ances of SnO 2 fired at differen t tempera2 tu res. T he SnO 2 treated at 300 gives very low charge and discharge capacities. T he discharge ca2 pacity is ju st 605 ma h g, w h ich is the low est among all tho se of the samp les, it is due to its h igh carbon con ten ṫ T he carbon in the system can t in2 tercalate lith ium, becau se it can t fo rm the layer structu re ju st like graph ite o r o ther flak iness car2 bon, then cau ses the low er capacity. Table 2 The F irst-cycle electrochem ical performance of SnO 2 samples obta ined at differen t temperatures tg 1st charge capacity 1st discharge capacityg Irreversible capacityg A s fo r SnO 2 treated at 400 and 500, their charge and discharge capacities reduce w ith the ris2 ing of temperatu re, w h ich obeys the changing ru le of the specific su rface area fo llow ed by the temper2 [22 ] atu re. Sato et a l. have ever repo rted tha t the specific capacity of carbon anode m aterial is direct2 ly p ropo rtional to its specific su rface area. W e have found th is ru le is also fit fo r the tin ox ide anode. F rom T ab le 2 w e can see that from 400 to 500, the irreversib le cap acity is reduced by 91 ma h g, w h ich is tw ice the reduction of the reversib le capacity, 42 ma h g. T hen the specif2 ic su rface a rea ha s m o re influence on the irre2 versib le capacity in conclu sion. H ere w e p ropo se tha t it is becau se the reversib le cap acity ju st in2 cludes the fo rm ing of L ix Sn, w h ile there are tw o parts of irreversib le capacity, the fo rm ing of L io 2 and the so lid electro lyte in terface (SE I), and they a re affected by the sp ecific su rface a rea severely, then m ake the influence of specific su rface area on the irreversib le capacity mo re rem arkab le than on the reversib le cap acity. T he th ird type of SnO 2 treated at 600 gives the h ighest discharge capacity, 770 ma h g, w h ich is clo se to the theo ret ica l va lue 783 ma h g. W e p ropo se that it is due to the ex is2 tence of sm all particles w h ich are separated from

5 N o. 5 M A Yue et al. 651 the aggregate b lock. It en larges the specific su rface a rea of the system, and their sep a ra t ing from the b lock fo rm s som e sm all in terspaces in the b lock. So in th is m aterial no t on ly lith ium can alloy w ith tin bu t also it is in tercalated in to the in terspaces in the b lock, then m ak ing the large capacity po ssib le. Conclus ion s T he tin ox ides w ith differen t carbon con ten ts, su rface mo rpho logies and specific su rface areas w ere ob tained by firing the p recu rso r at differen t treatm en t temperatu res w ith the so l2gel techn ique, and their d ifferen t elect rochem ica l p erfo rm ances co rrespond w ell to their characteristics. T he SnO 2 w ith a h igh carbon con ten t ob tained at the low er calcin ing temperatu re gives the low est capacity, the imp roved crystallin ity of the tin ox ide ob tained at a h igher temperatu re can abate the particle ag2 gregation and m ake the ex istence of sm all separate particles and in terspaces in the b ig b lock, then im 2 p roving the capacity. T he specific su rface area of the t in ox ide can affect bo th reversib le and irre2 versib le cap acit ies, and the influence on the irre2 versib le capacity is mo re rem arkab le. Re fe re nce s [ 1 ] N oelm., Suryanarayanan V., J. P ow er S ou rces, 2002, 111 (2), 193 [ 2 ] T irado J. L., M a ter. S ci. E ng. R., 2003, 40 (3), 103 [ 3 ] W inter M., Besenhard J. O., Spah r M. E., et a l., A d v. M ater, 1998, 10 (10), 725 [ 4 ] X ie Z. W., Zhang X. Y., T ian Y. M., et a l., Chem. J. Chinese U niversities, 2001, 22 (7), 1110 [ 5 ] X ie Z. W., Q u X. G., Zhu D. X., et al., Chem. J. Ch i2 nese U niversities, 2001, 22 (12), 2048 [ 6 ] W u S. H., You J. K., L in Z. G., Ch inese J ou rnal of P ow 2 er S ources, 1998, 22 (1), 35 [ 7 ] T ang Z. Y., Zhuang X. G., Zhai Y. M., Ch inese J ou rnal of P ow er S ou rces, 2000, 24 (2), 108 [ 8 ] M a S. H., J ing X. B., W ang F. S., E lectrochem istry, 1997, 3 (3), 293 [ 9 ] L iu M., Yin G. P., Sh i P. F., B attery B im onth ly, 2001, 31 (4), 195 [ 10 ] Ido ta Y., M ish im a M., M iyak i M., et al., U S P atent, , 1997 [ 11 ] Courtney İ, D ahn J. R., J. E lectrochem. S oc., 1997, 144 (6), 2045 [ 12 ] L iu W., H uang X., W ang Z., et al., J. E lectrochem. S oc., 1998, 145 (1), 5 [ 13 ] Courtney İ A., D ahn J. R., J. E lectrochem. S oc., 1997, 144 (9), 2943 [ 14 ] W en C. J., H uggins R. A., J. S olid S tate Chem., 1980, 35, 376 [ 15 ] Boukamp B. A., L esh G. C., H uggins R. A., J. E lec2 trochem. S oc., 1981, 128, 725 [ 16 ] W en C. J., H uggins R. A., J. S olid S tate Chem., 1981, 128, 1181 [ 17 ] A nania., C rouch2baker S., H uggins R. A., J. E lec2 trochem. S oc., 1987, 134, 3098 [ 18 ] Retoux R., B rousse T., Sch leich D. M., J. E lectrochem. S oc., 1999, 146 (7), 2472 [ 19 ] B rousse T., Retoux R., H erterich U., et al., J. E lec2 trochem. S oc., 1998, 145 (1), 1 [ 20 ] N am S. C., Yoon Y. S., Cho W. İ, et al., E lectro2 chem istry Comm unications, 2001, 3 (1), 6 [ 21 ] V eeraraghavan V., D urairajan A., H aran B., J. E lec2 trochem. S oc., 2002, 149 (6), A 675 [ 22 ] Sato Y., N akano T., Kobayakaw a K., J. P ow er S ou rces, 1998, 75 (2), 27