Ionic Conductivity of LiHf 2 ( PO 4 ) 3 with NASICON-type Structure and its Possible Application as Electrolyte in Lithium Batteries

Este trabalho reporta medidas de condutividade iônica realizadas para o LiHf2(PO4)3 calcinado a 1100 °C. As respostas devidas aos grãos in te rior e junção puderam ser identificadas tanto nas curvas de impedância, como na parte real das curvas de condutividade vs. freqüência. A energia de ativação, associada ao movi men to dos íons Li no in te rior dos grãos, é 0,33 eV, enquanto que aquela associada à condutividade to tal cc, está na faixa de 0,36-0,47 eV. os resultados desta última dependem da contribuição relativa devida ao in te rior e à junção de grão. A possível aplicação do LiHf2(PO4)3 como eletrólito foi testada para a pilha Li/LiHf2(PO4)3/LiMn2O4. Observou-se que o potencial de equilíbrio aumenta de 0,076 V a 2,217 V, quando a temperatura varia de 28 a 148 °C.


In tro duc tion
The world wide in crease in con sumer elec tron ics and the chal lenge of the elec tri cal car ac count for the sig nif i cant ef fort to de velop high en ergy re charge able bat ter ies.Among the bat ter ies those based on lith ium show the best per for mance.The solid elec tro lytes have sev eral ad vantages as com pared with liq uid elec tro lytes.In par tic u lar, they do not pro duce ei ther cor ro sion or passivation of the elec trodes, and the safety of the bat tery is im proved.
The ionic con duc tiv ity re ported for this com pound by sev eral au thors is rather vari able.Some of them 15,29 have as cribed the con duc tiv ity to mo tion of Li + ions in side the grains, while oth ers [12][13][14]16 have re ported the to tal con ductiv ity, i.e. that as so ci ated with grain in te rior and grain bound ary, or else they do not iden tify which of these con tribu tions is dom i nant. Inany case the con duc tiv ity val ues seem to be af fected by the lack of res o lu tion be tween grain in te rior and grain bound ary re sponse, and sig nif i cant dif -Ar ti cle fer ences in the ac ti va tion en ergy (0.32-0.44 eV) have been also re ported [12][13][14][15][16]28,29 .
The aim of this pa per is to study the ionic con duc tiv ity of LiHf 2 (PO 4 ) 3 as well as the pos si ble ap pli ca tion of this ma te rial as an elec tro lyte in solid state bat ter ies.

Ex per i men tal
LiHf 2 (PO 4 ) 3 was pre pared by cal ci na tion of a stoichiometric mix ture of Li 2 CO 3 (Fluka, >99%), HfO 2 (Aldrich, 99.9%), and (NH 4 ) 2 HPO 4 (Fluka, >99%).The reagents were pre vi ously dried at 100 ºC for 12 h, then stoichiometric amounts of these com pounds were thoroughly mixed and cal cined in a plat i num cru ci ble at the follow ing tem per a tures: 300, 600, 700, 800, 900, and 1000 ºC.Ther mal treat ments were ac cu mu la tive up to 1000 ºC.At each de fined tem per a ture the time spent was 10 h ex cept at 300 ºC where the time was 6 h.The mix ture was ground before each ther mal treat ment.
X-ray pow der dif frac tion pat terns were taken at room tem per a ture in a PW-1710 Philips diffractometer with Cu Kα ra di a tion.The scan was car ried out in steps of 0.02º, count ing for 0.5 s at each step.The peaks were fit ted with Kα1-K α2 dou blets, and the po si tion of each peak was taken to be that of the K α 1 com po nent, λ =1.5405981 Å.
Elec tri cal con duc tiv ity mea sure ments were car ried out by the com plex im ped ance method us ing a 1174 Solartron fre quency re sponse an a lyzer.The pel lets ( ca. 6 mm di am eter and 1 mm thick ness) ob tained in the last ther mal treatment of the prep a ra tion pro ce dure were ad di tion ally cal cined at 1100 ºC for 10 h.Gold elec trodes were de posited on the two faces of the pel lets by vac uum evap o ra tion.The fre quency range used was 10 -1 -10 5 Hz.The mea surements were car ried out on the pel lets heated at in creas ing tem per a ture in the range 40-300 ºC.
The use ful ness of LiHf 2(PO 4)3 as a solid elec tro lyte in lith ium bat ter ies has been tested by us ing the Li/LiHf 2(PO 4)3/LiMn 2O4 cell.The com pos ite cath ode is formed by LiMn 2O4 as ac tive ma te rial (50% by mass), carbon black as elec tronic con duc tor (25% by mass), eth yl ene pro pyl ene diene mono mer, EPDM, as binder (10% by mass) and LiHf 2(PO 4)3 (15% by mass).This com pos ite elec trode was made by mix ing the pow ders in cyc lo hex ane, evap o rat ing the sol vent, and press ing the mix ture at 1 Ton cm -2 to ob tain a disk of 0.28 cm 2 .As elec tro lyte, a sintered pel let of the NASICON com pound was used.A lith ium foil was used as an ode.The dif fer ent com po nents were placed in a sealed Swagelock cell un der a dried at mosphere con tain ing <2ppm H 2O. The cell equi lib rium po tential was mea sured in the range 25-150 ºC.To that end the sealed cell was im mersed in a sil i cone bath and the measure ments were car ried out at each tem per a ture af ter half an hour of sta bi li za tion.

Re sults and dis cus sion
The LiHf 2 (PO 4 ) 3 sam ple pre pared in this work showed a pow der X-ray dif frac tion pat tern which was in dexed on the ba sis of a rhombohedral cell with hex ag o nal pa ram e ters a H = 8.8285 (5), c H = 22.019(1) Å.These pa ram e ters agree well with other re ported for the rhombohedral R 3c phase [11][12][13][14][15][16] .Our pa ram e ters were ob tained from a least squares re fine ment pro ce dure in which the 2 Θ val ues of 41 un am big u ously in dexed re flec tions were used as ob servable data.
The im ped ance plots (imag i nary -Z" vs. real Z') recorded at dif fer ent tem per a tures are shown in Fig. 1.At low tem per a tures (40 ºC) two arcs and one spike are ob served.The arcs found at high and low fre quency are as cribed to grain in te rior and grain bound ary re sponse, re spec tively, in agree ment with the assignement re ported for other ceramic ma te ri als 30 .The ca pac i tance as so ci ated with these arcs is 30 pF for the high-frequency arc and 64 nF for the low-frequency one.The spike is as cribed to the block ing ef fect of Li + ions at the elec trode sur face.The ca pac i tance of the spikes is in the range 1-10 µF.When the sam ple is heated at higher tem per a tures the arcs dis ap pear from the plots, first the high-frequency arc and then the The real part of the con duc tiv ity ( σ) vs an gu lar frequency ( ω=2 πf) is plot ted in Fig. 2. A dispersive re gime (I) due to the elec trode re sponse is ob served at low fre quency.At in ter me di ate and high fre quency two pla teaus (II and III) are de tected.The pla teau II is af fected by the grain bound ary dc re sponse, while the other (III) is as cribed to the dc re sponse of the grain in te rior, in agree ment with the in for ma tion de duced from the im ped ance plots.The ex peri men tal data have been fit ted by us ing the power ex pressions: i) σ = E ω e for the elec trode re sponse, ii) σ = σ' dc + Bω P for the grain bound ary re sponse, and iii) σ = σdc + A ω n for the grain in te rior re sponse.σ dc and σ' dc are the dc conduc tiv i ties cor re spond ing to each pla teau.E ω e , B ωP, and Aω n are the ac con duc tiv i ties of the three re sponses.A and E are tem per a ture-dependent pa ram e ters , and e, p, and n are the slopes of the three dispersive re gimes.The solid curve in Fig. 2 shows a good fit of the ex per i men tal data.Then, by this pro ce dure, the dc con duc tiv ity cor re spond ing to mo tion of Li + ions in side the grains, through the bor der of the grains, as well as the to tal dc value of the pel let, have been de ter mined.
Vari a tion of dc con duc tiv ity (log 10 σ dc ) vs in verse temper a ture (1000/T) is shown in Fig. 3.In this fig ure we have in cluded the val ues of the grain in te rior (open cir cles) and to tal (open tri an gles) con duc tiv ity.All the ex per i men tal data are well fit ted by Arrhenius equa tions, σa = σ0 exp(-E/KT), where σ 0 is a pre-exponential fac tor, E the acti va tion en ergy, and K the Boltzmann con stant.The to tal dc con duc tiv ity (solid line) is close to that of the grain in te rior (dot ted line) for tem per a tures above 160 ºC, in di cat ing that the to tal con duc tiv ity is dom i nated by the grain in te rior response.How ever, be low that tem per a ture the to tal con ductiv ity dif fers from the grain in te rior one be cause of the grain bound ary con tri bu tion.The ac ti va tion en ergy as so ci ated with the to tal con duc tiv ity is also a pa ram e ter whose value de pends on the rel a tive grain in te rior and grain bound ary con tri bu tions.Thus, a value (0.36 eV) close to that cor respond ing to the grain in te rior re sponse (0.33 eV) is obtained when the to tal con duc tiv ity is dom i nated by the grain in te rior con tri bu tion (above 160 ºC); the ac ti va tion en ergy is higher (0.47 eV) when the to tal con duc tiv ity is clearly af fected by the grain bound ary re sponse (be low 160 ºC).
Finally, we have tested the pos si ble ap pli ca tion of LiHf 2 (PO 4 ) 3 as an elec tro lyte in solid state lith ium bat teries.Fig. 4 shows the vari a tion of the equi lib rium po ten tial (E eq. ) of the Li/LiHf 2 (PO 4 ) 3 /LiMn 2 O 4 cell vs. tem per a ture.At 28 ºC the E eq. value is quite small (0.076 V).When the cell tem per a ture in creases E eq. in creases monotonically.The slope is larger in the range 60-110 ºC.Above 110 ºC the slope trends to level, and at 150 ºC E eq. reaches 2.217 V.
Al though this value is lower than that nor mally found ( ca. 3.2 V) when a liq uid elec tro lyte, such as 1M LiClO 4 dissolved in pro pyl ene car bon ate, is used 31,32 , the ob served increase in E eq. shows that the re sponse of the LiHf 2 (PO 4 ) 3 elec tro lyte is im proved with the cell tem per a ture, and hence it is po ten tially use able in solid state lith ium bat teries.The in crease with tem per a ture of E eq. is con sis tent with the in crease in Li + con duc tiv ity, as de duced from im pedance data.
Fig ure 3. Tem per a ture de pend ence of dc con duc tiv ity in an Arrhenius plot.The grain in te rior and to tal con duc tiv i ties are rep resented by cir cles and tri an gles, re spec tively.
Fig ure 2. Fre quency de pend ence of the real part of the con duc tiv ity at dif fer ent tem per a tures.The ex per i men tal data re corded at 40 ºC are fit ted by us ing the ex pres sion σ=1/(1/ σe+1/ σgb +1/ σgi ), where σe, σgb and σgi are the con duc tiv i ties for the elec trode, grain bound ary, and grain in te rior, re spec tively.

Con clu sions
Grain in te rior and grain bound ary re sponse are dis tinguished in a LiHf 2 (PO 4 ) 3 pel let pre vi ously cal cined at 1100 ºC.The to tal dc con duc tiv ity and its as so ci ated ac ti vation en ergy are very sen si tive to the rel a tive con tri bu tion of grain in te rior and grain bound ary.For Li + mo tion in side the grains the dc con duc tiv ity at rt and the ac ti va tion en ergy are 1.3 x 10 -5 ohm -1 cm -1 and 0.33 eV, re spec tively.These data to gether with the ob served in crease in the equi lib rium poten tial with the cell tem per a ture show that LiHf 2 (PO 4 ) 3 can be used as an elec tro lyte in solid state Li-batteries.

Ac knowl edge ment
Fi nan cial sup port by CICYT (Pro ject MAT 95-0899) is grate fully ac knowl edged.

Fig ure 1 .
Fig ure1.Im ped ance plots ob tained at two tem per a tures for the LiHf 2(PO 4)3 pel let pre vi ously sintered at 1100 ºC.Se lected fre quencies (in hertz) are marked in the plot ob tained at 40 ºC.

Fig ure 4 .
Fig ure4.Vari a tion of the equi lib rium po ten tial as a func tion of temper a ture for the Li/LiHf 2(PO 4)3/LiMn 2O4 cell.