THERMAL STUDY ON Li BIS(OXALATEBORATE) LiBOB

Transcription

1 THERMAL STUDY ON Li BIS(OXALATEBORATE) LiBOB L. Lrush-Asrf, E. Zinigrd, G. Slitr, M. Sprecher, nd D. Aurch Br-Iln University, Rmt-Gn, 52900, Isrel ABSTRACT. The therml stility of the lithium is(oxltoorte) (LiBOB) in the temperture rnge of 40 to 350 ºC ws studied y ccelerting rte clorimetry (ARC), differentil scnning clorimetry (DSC) nd therml grvimetric nlysis. An endothermic rection ws detected with n onset t ~ 293 ºC involving complete irreversile decomposition of the LiBOB in which gseous products re formed. In the first stge Li 2 C 2 O 4 (crystlline), B 2 O 3 (glss), CO nd CO 2 gses re formed. The next stge is n irreversile formtion of lithium triorte LiB 3 O 5 (glss). Introduction The new lithium slt is(oxltoorte) ws recently proposed y Lishk et l.[1] nd Xu nd Angell [2] s very promising nd dvntgeous electrolyte for rechrgele Li ion tteries, insted of the prolemtic LiPF 6 which is the commonly used Li slt in the electrolyte solutions for commercil Li ion tteries. During the lst ten yers, numer of ppers ppered in the literture, demonstrting good performnce of grphite nodes nd trnsition metl cthodes in solution contining LiBOB (compred to the ehvior in solutions sed on the commonly used slt LiPF 6 ). It ws lso demonstrted tht the therml stility of electrolyte solutions sed on LiBOB for Li ion tteries, is much higher compred to systems sed on other Li slts (e.g. LiPF 6, LiBF 4 ).[3] Hence, it is cler tht the intrinsic therml ehvior of LiBOB is interesting nd importnt in connection to R & D of novel nd sfer Li ion tteries. Indeed, in recent yers, some reports on therml studies of LiBOB ppered, s summrized elow. According to thermogrvimetric experiments, lithium-is(oxltoorte) is fully stle up to pproximtely 300 ºC [1]. Differentil therml nlysis comined with thermogrvimetric nlysis (DTA-TGA) using scnning rte of 10 ºC min -1, showed tht LiBOB is stle up to 302 ºC. Beyond this temperture, it decomposes rther thn melts [2]. This slt is chemiclly stle in orgnic solutions ut it slowly decomposes y hydrolysis to LiBO 2 nd LiOOCCOOH [2]. Amine et l.[4] presented thermogrvimetric curve for LiBOB in the temperture rnge 20 to 600 ºC. Bi-To Yu [5] reported out the stility of LiBOB in the temperture rnge ºC, nd out LiBOB decomposition to Li 2 CO 3, B 2 O 3, CO 2 nd CO when the temperture is higher thn 300 ºC. They oserved weight loss relted to two processes t tempertures out 320 ºC nd 450 ºC. It ws reported tht the decomposition of LiBOB t 302 ºC produces B 2 O 3 nd CO 2 [6]. Despite of the intensive studies of LiBOB nd its solutions on oth electrochemicl performnce nd therml ehvior, there is not yet comprehensive picture of the intrinsic therml ehvior of this slt nd cler nlysis of ll the products of its therml rections. Therefore, the present work ims t providing etter nd more complete picture of the therml ehvior of LiBOB including rigorous nlysis of its decomposition products, nd comprehensive understnding of its clorimetric response. 3-46

2 Experimentl Highly pure LiBOB slt ws otined from Chemetll Inc. nd could e used s received. The pristine slt nd its therml dissocition products were chrcterized y Fourier trnsform infrred (FTIR) spectroscopy, (Mgn 860 Spectrometer from Nicolet Inc., plced in glove ox under H 2 O nd CO 2 -free tmosphere), element nlysis y n inductively coupled plsm (ICP) spectrometer (Ultim 2, Joon Yvon Hori), opticl microscope (AX70/AX70A, Olympus Inc.), scnning electron microscopy (SEM) (JEOL JSM 840), nd X-ry diffrction using D8 Advnce System from Brucker Inc. XRD ptterns were otined using Cu K α rdition (λ=1.54å) t 40 ma nd 40 kv. We used the cell which enled the isoltion of the sensitive smples from tmospheric gses y polyester Mylr type film 90 μ thick trnsprent for X-ry rdition. An ccelerting rte clorimeter (ARC) from Arthur D Little Inc., Model 2000, differentil scnning clorimeter (DSC) from Mettler Toledo Inc., Model DSC 822, nd thermogrvimetry (TG, DTA) (SDT 2960, TA instrument) were used for the therml nlysis of LiBOB. In the ARC mesurements, out 1 grm of slt ws loded in titnium flsk (8.2 ml volume) in n rgon filled glove ox (VAC Inc.) nd ws trnsferred to the ARC under purified Ar tmosphere The slt ws heted from 40 to 300 ºC in 5 ºC increments t the rte of 2 ºC min -1 in the serch for self-heting t sensitivity threshold of 0.02 ºC min -1. The controller ws progrmmed to wit 15 min for the smple nd the clorimeter tempertures to equilirte, nd then to serch during 20 min for temperture increse of 0.02 ºC min -1. After the ARC experiments, the rection vessel ( titnium flsk) ws cooled with liquid nitrogen until the pressure ws close to the tmospheric pressure. The gseous products were relesed through specilly designed high-pressure vlve. For NMR nlysis the gs ws collected into CD 3 CN solution. TGA-DTA mesurements were conducted under pure nitrogen tmosphere in lumin cruciles, 70 μl in volume. DSC mesurements in confined volume were conducted in hermeticlly seled, gold-plted stinless steel cruciles 30 μl in volume (Mettler Toledo Inc.), over temperture rnge of ºC. The cruciles were filled with out 10 mg of the slt, nd then seled in glove ox under n rgon tmosphere. The heting/cooling rtes were 0.5, 1 nd 20 ºC min -1. Results The significnt pressure increse during ARC experiment t n onset of 265ºC nd the negtive slop in the temperture steps ppering t out 268 ºC, my e explined y n endothermic decomposition of LiBOB. The gseous products of this rection were collected nd nlyzed (see lter). Figure 1 presents TGA nd DTA curves, relted to heting LiBOB nd Li oxlte smples. This figure, relted to LiBOB, shows two endothermic peks t out 100 ºC nd 310 ºC, respectively, in the DTA curve, ccompnied y weight loss. The first endotherm is ttriuted to the loss of dsored wter. The wter content in the smples is ner 0.06 % ccording to the TGA results (Fig. 1). The second endotherm with n onset t out 293 ºC (Fig. 1) is relted to LiBOB decomposition. The weight loss clculted fter the process completion, sed on the TGA nd the ARC mesurements, is 61.2±0.5 %. Fig. 2 shows typicl DSC curves otined from LiBOB (,, c) nd mixtures of B 2 O 3 -Li oxlte (d) in closed cruciles. One endothermic irreversile process is distinguished in the temperture rnge 210 to 390 C (Fig. 2). It ssocites with 3-47

3 LiBOB decomposition. The onset of the endothermic dissocition of LiBOB is fixed t out 293 C. The decomposition het is out 38 kjmol -1 (196 Jg -1 ). Weight ( %) c d Temperture difference (mv) Temperture (ºC ) Fig. 1. TGA (, c) nd DTA (, d) curves otined y heting LiBOB smples t rte of 1 C min -1 ( mg) (, ), nd Li oxlte (5.255 mg) (c, d). d 0.2 W g W g -1 c Fig. 2. DSC curves otined y heting LiBOB smples of different weights (indicted) t different heting /cooling rtes (indicted): () 0.5 C min -1, mg, () 1 C min -1, mg, (c) 20 C min -1, mg, nd (d) Li 2 C 2 O 4 -B 2 O 3 mixture smple (1.717 mg ) heted t rte of 1 C min -1. Fig. 2 (,, c) shows typicl DSC curves t different heting/cooling rtes. These results provide further s evidence for the endothermic rection of LiBOB rther thn phse trnsition. The onset nd pek tempertures of the endotherm shift to higher vlues s the heting rte is higher (Fig. 2). This ehvior is typicl for chemicl processes, rther thn to phse trnsition. The shpe of the pek does not chnge t higher rtes, nd no indiction for the existence of n ccompnying process is oserved. Figure 2d showing the therml ehvior of B 2 O 3 -Li oxlte mixture reflects n endothermic rection etween these two species t the sme temperture rnge s tht 3-48

4 LiBOB decomposition with rection het round 67.9 kj per mole of recting B 2 O 3. This dt is very importnt for the discussion see lter. Fig. 3. SEM microgrphs of: () pristine LiBOB smples; () LiBOB fter heting to 350 C. A scle ppers in ech microgrph. Figs. 3, 4 show SEM nd opticl microgrphs of pristine nd heted LiBOB powder. The pristine white powder consists of elongted round prticles with mm length nd mm dimeter. Surprisingly, the prticles' shpe nd size did not chnge noticely fter heting to 350 C (Figs. 3 nd 3) in spite of the loss of weight of out 60 %. They re non trnsprent under trnsmitted light (Fig. 4) nd don't demonstrte non homogeneity under polrized light (Fig. 4). Fig. 4. Photomicrogrphs from opticl microscope of the powder otined fter heting of LiBOB smples to 350 C: () trnsmitted light (x 10); () polrized light (x 5). 3-49

5 Lin (Counts) Thet - Scle Fig. 5. XRD ptterns of: () pristine LiBOB powder, () powder otined y heting LiBOB to 350 C, (c) sme s () fter storge t 300 C during 10 dys. Line mrks of Li oxlte re shown for comprison. Fig. 5 shows XRD ptterns of the pristine slt nd the solid decomposition product (collected fter heting). The XRD ptterns of the pristine LiBOB correltes well with literture dt [7]. No oservtion of pristine LiBOB peks fter heting to 350 C indictes complete LiBOB decomposition. Three powders, fter heting in closed volume in ARC om, in DSC cruciles, nd in opened lumin cruciles in TGA tests, show identicl XRD ptterns. All peks correspond to monoclinic lithium oxlte 8. A further storing of this product during 10 dys t 300 C did not cused ny structurl chnges (confirmed y XRD nd SEM). There is good greement etween the XRD mesurements nd the FTIR spectr (Fig. 6). As seen in Figure 6, the IR nds of LiBOB pristine re not detected fter heting to 350 C ut ll the expected IR peks of Li oxlte cn e seen in the spectr of the therml product. 1 c Asornce Wvenumers (cm (cm-1) ) 500 Fig. 6. FTIR spectr of: () pristine LiBOB powder, () the powder otined y heting LiBOB heting to 350 C, nd (c) Li oxlte powder. 3-50

6 Fig. 7. FTIR spectrum of gs phse tht is formed in the ARC clorimetric om due to therml decomposition of LiBOB (). Spectr of CO 2 () nd CO (c) re presented for comprison. Figure 7 compres FTIR spectr of the gseous product of LiBOB therml decomposition to spectr of CO 2 nd CO. From these studies, it is cler tht the gseous product of LiBOB decomposition comprises oth CO 2 nd CO. The finl solid product of the LiBOB therml decomposition ws dissolved in cidic solution nd ws nlyzed y ICP. From this nlysis it ws found tht the solid end product contins 9.3% Li nd 13.9% B. Discussion Bsed on the ove mentioned results the following mechnism of LiBOB decomposition t ~ 300 C my e suggested: 2LiBC 4 O 8 (cryst) Li 2 C 2 O 4 (cryst)+ B 2 O 3 (glss)+3co(gs)+3co 2 (gs); (1) Li 2 C 2 O 4 (cryst)+3b 2 O 3 (glss) 2LiB 3 O 5 (glss)+co(gs)+co 2 (gs); (2) The overll therml l decomposition process of LiBOB is: 3LiBC 4 O 8 (cryst) Li 2 C 2 O 4 (cryst)+lib 3 O 5 (glss)+5co(gs)+5co 2 (gs) (3) The clculted weight loss due to complete decomposition of LiBOB ccording to eqution 1 is 55.7 %. However the TGA nd the ARC experiments showed higher weight loosed due to the therml decomposition processes. The weight loss demonstrted in the TGA dt (Fig. 1) is ner 62 %. The further exposure of the smples to ir t room temperture, fter heting to 350 C, did not cuse ny further chnge in mss during 24 hours. Nevertheless, it is known tht oron oxide rects slowly with wter to form oric cid. FTIR spectr of the LiBOB smples fter heting to 350 C indicted the sence of B 2 O 3 s finl decomposition product (Fig. 6). Hence, n dditionl process with gs evolution is needed to explin weight loss round 62%. This second (to eqution 1) process cnnot e prtil 3-51

7 decomposition of lithium oxlte, ecuse it decomposes to cronte only ner 500 C ccording to our TGA-DTA mesurements (Fig. 1) nd literture dt [9]. Hence, we suggest rection 2 ove s follow up process tht very well explins weight loss mesured herein. As suggested y eqution 2, ll the B 2 O 3 produced y the first process (eq. 1) is consumed y the second step. In this follow up rection, only 1/3 of the Li oxlte formed y the first process is consumed. This correltes well with the fct tht the finl products of LiBOB decomposition contin Li oxlte, s evidenced y oth XRD nd FTIR mesurements (Figs. 5, 6). The overll processes, descried y eqution 3, suggest loss of mss of out 61.6% (compred to the initil mss of LiBOB) due to the formtion of CO nd CO 2. This correltes very well with the experimentl results (DTA-TGA, Fig. 1) tht show mss loss of out 62% when LiBOB thermlly decomposes. Li triorte, LiB 3 O 5, is formed s non crystlline glss phse nd cnnot e identified y X-ry diffrction. Fig. 6 presents FTIR spectr of pristine LiBOB nd of smple fter heting to 350 C, nd of Li oxlte for comprison. The spectrum of the decomposed smple clerly shows Li oxlte nds t 449, 516, 775, 1330, nd 1665 cm -1, nd three new nds, t 1087, 1382 nd 866 cm -1 (not relted to Li oxlte). These peks cn e ssigned indeed to the strong nds of LiB 3 O 5 t 1372, 1087 nd 860 cm The composition of the end solid product fter heting LiBOB to 623 K ccording to the ICP nlysis (9.3 % Li nd 13.9 % B) correltes well with the solid product suggested y eqution 3 ove, nmely, mixture of Li 2 C 2 O 4 +LiB 3 O 5 (9.4 % Li nd 14.5 % B). We checked the possiility of the rection suggested y eqution 2 y DSC mesurements. Mixtures contining Li 2 C 2 O 4 :B 2 O 3 1:3 mol/mol were thoroughly grinded in gte mortr nd pestle nd then were mesured y DSC in closed cruciles up to 500 C. A pronounced rod endothermic pek ppers in the sme temperture rnge in which LiBOB decomposes (Fig. 2). The onset of this pek is ner 210 C tht is y 80 C lower thn the onset of the LiBOB decomposition pek (eq. 1). The het of the endothermic rection etween Li oxlte nd B 2 O 3 is out 67.9 kj per mol of the product LiB 3 O 5 s mesured herein. According to eqution (3) only 1/3 mol of LiB 3 O 5 is formed y decomposition of 1 mole of LiBOB. Therefore, out 67.9/3~23 kj per mole of LiBOB is sored for formtion of 1/3 mol of LiB 3 O 5. This vlue is prt of the het of LiBOB decomposition, out 38 kj mol -1. Consequently, the het of the first decomposition step of LiBOB, eqution (1), is ner 38-23=15 kj mol -1. XRD mesurements of the product of heting the Li 2 C 2 O 4 -B 2 O 3 mixture (up to 350 C) showed no evidence of crystlline phse. The temperture of the endotherm relted to the rection of Li oxlte nd B 2 O 3 is too low for the oxlte decomposition. Therefore this process cn e relted only to the rection 2. Hence, rection 2 egins immeditely when Li 2 C 2 O 4 nd B 2 O 3 pper s result of LiBOB decomposition (eq. 1). In order to complete the discussion, it is importnt to mention tht the element nlysis of the finl solid product of LIBOB decomposition which showed 9.3% Li nd 13.9% B cn not e unmiguously relted only to mixture of Li 2 C 2 O 4 -B 2 O 3 1:1. One my suggest tht such n tomic rtio my relte lso to the following mixture: Li 2 O 2B 2 O 3 (Li 2 B 4 O 7 )+Li 2 O 4B 2 O 3 (Li 2 B 8 O 13) insted of LiB 3 O 5. However, such mixture contining 75% B 2 O 3 should undergo slow crystlliztion upon storge t elevted tempertures 11. The end product of the LiBOB decomposition ws stored t 300 C during 10 dys followed y XRD mesurements. (This is the highest temperture in which mixture contining Li oxlte cn e heted nd remin 3-52

8 stle). No evidence for the formtion of ny new crystlline phse except the existing Li oxlte ws oserved y X-ry diffrction fter storge t elevted tempertures (Fig. 5c). Conclusion The results of ARC, DSC nd TGA experiments nd their comprehensive nlysis demonstrte the complete irreversiility of the endothermic decomposition of LiBOB under conditions of constnt nd open volume. The onset of decomposition is fixed t 293±4 C; het of decomposition is 37.8±3.1kJ mol -1. Decomposition commences with formtion of Li oxlte (crystlline) nd oron oxide in the first step. The endothermic rection etween Li 2 C 2 O 4 nd B 2 O 3 results in the second step the formtion of LiB 3 O 5 s glss mteril. These two steps re ccompnied y the development of CO nd CO 2. REFERENCES 1. Lishk U., Wietelmnn U., nd Wegner M., Germn Pt. DE C1, Xu W.nd A. Angel C., Electrochem Solid-Stte Lett., , E1-E4. 3. Cmpion C. L., Li W., nd. Lucht B. L, Journl Electrochem. Soc (12) A2327-A Amine K., Liu J., Kng S., Belhrouk I., Hyung Y., Vissers D.,. Heriksen G, Journl of Power Sourses , Yu B.-T., Qui W.-H.,. Li F.-S, nd Xu G.-X., Electrochem Solid-Stte Lett., (1) A1-A4. 6. Xu K., Zhng S., Jow T. R., Xu W., nd Angel C. A., Electrochem Solid-Stte Lett., (1) A26-A Zvlij P. Y., Yng S., nd Whittinghm M. S., Act Cryst.. B59, JCPDS-ICDD, Girgis M. M., nd Elwd A. M., Thermochimic Act, (2) Moryc U., nd Ptk W. S., Journl of Moleculr Structure, , Sstry B. S. R., nd Hummel F. A., Journl of the Americn Cermic Society, (1)