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Appl. Phys. A 59, 173-178 (1994) Applied SolidsPh~ic~ A andSurfaces Springer-Verlag 1994

Lithium intercalation in MoO3: A comparison between crystalline anddisordered phasesC. Julien, A. Khelfa, J. P. Guesdon, A. Gorenstein*Laboratoire de Physiqu e des Solides, associ~ au CN RS, Universit~ Pierre et Marie Curie, 4, place Jussieu, F-75252 Paris Cedex 05,France(Fax: + 33-1/44274541, E-m ail: MI@frunip.62)Received 18 December 1993/Accepted 21 April 1994

Abstract. We report properties of l i thium-intercalatedMoO3 crystall ine and thin-f i lm which are potential cath-ode materials for high energy density batter ies. Dis-charge and charge reactions of MoO3 electrodes in anon-aqueous Li+-electrolyte have been studied. Thekinetically accessible discharge range amounts to0_

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174 C. Ju l i en e t a l .

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F i g . 1 , S c h e m a t i c r e p r e s e n t a t i o n o f t h e c r y s t a l s tr u c t u r e o f M o O 3s h o w i n g i n f i n it e o f M o O 6 o c t a h e d r a p e r p e n d i c u l a r t o t h e b - a x i s

ing a t t = 0 obeyes the fo l low ing equa t ionE* = ( I W R T / F 2 c* A ) [ (4 t fi zD ) I /2 - t / O], (1 )where E * i s the vo l tage o f the ce l l wi th re fe rence to theopen c i rcu i t vo l tage , I i s the cu rren t in tens i ty , t i s thet ime, c* i s the L i + ion con cen t r a t ion in the ac t ive mater i -a l , W i s the therm ody nam ic fac to r , A and 6 a re thesu rface and the th ickness o f the e lec t rode , T is the tem-pera tu re , F = 96 500 As m ol - 1 and R = 8 .314 J K - 1 mol - 1.The thermodynamic fac to r i s def ined as W = ~( ln a * ) /Q(ln c* ) in w h ich a* i s the ac t iv i ty o f the in te rca lan tspec ies in the so l id so lu t ion e lec t rode . W is a l so expressedb y t h e r a t i ow = D / D 0 , ( 2)between the chemica l d i f fus ion coeff ic ien t D, and thec o m p o n e n t d i f fu s i o n c o e f fi c ie n t Do .

2 Res ul t s and d i scus s ion

fo rm ed in the second zone a t a bo u t 650 C . Smal l c rys ta lso f M oOa h a v e t h e s h a p e o f e l o n g a t e d p l a t e le t s w i t h (0 1 0 )perpend icu lar to the basa l ac-p lane and (001) a long thelonges t edge .

1 . 2 F i l m p r e p a r a t io nF i l m s o f M o O3 we re p re p a re d b y f l as h e v a p o ra t i o n o fp r e - b a k e d m o l y b d e n u m o x id e p o w d e r ( A l p h a P r o d u c ts ,Pura t ron ic 99 .9995%) on s i l i ca g lass subs t ra tes main -t a i n e d i n t h e t e m p e ra t u re r a n g e 3 0 -3 0 0 C i n a v a c u u mb e t t e r t h a n 5 x 1 0 - 6 T o r t . D u r i n g e v a p o r a t i o n a b o a t o fm o l y b d e n u m w a s m a i n t a i n e d a t 1 00 0 C a n d t h e r a t e o fdepo s i t ion was 4 nm/s . The th ickness o f the f i lms was inthe range 0 .5 -0 .6 gm.F l a s h -e v a p o ra t e d M o Oa f i l m s a r e u n i fo rm l y t h i c kwi t h a b l u e c o l o u r . T h e c o l o u r o f t h e s a m p l e s s u g g e s tstha t the f i lms con ta in a nu mb er o f oxygen vacanc ies [34 ].T h e c o l o u r o f t he s e s a m p l e s v a r i e d f ro m l ig h t b l u e t odeep b lue wi th increas ing subs t ra te t em pera tu re (Ts ).T h i s i s a t t r i b u t e d t o t h e i n c re a s e o f t h e l a c k o f o x y g e natom s w i th increas ing T~ dur ing the depos i t ion .

1 . 3 E l e c t r o c h e m i c a l m e a s u r e m e n t sE l e c t ro c h e m i c a l m e a s u re m e n t s o f l it h i u m i n t e r c a l a t io ni n M o O3 s a m p l e s h a v e b e e n c a r r i e d o u t b y c o u l o m e t r i ct i t ra t ion in L i /M oO 3 ga lvan ic ce l l s wi th 1 M L iC104 inP ro p y l e n e C a rb o n a t e (P C ) s o l u t i o n a s a n o n -a q u e o u se lec t ro ly te .The k ine t ic parameters , i . e . , the chemica l d i f fus ionc o e f fi c ie n t a n d t h e t h e rm o d y n a m i c f a c t o r h a v e b e e n d e -t e rm i n e d b y t h e M o d i f i e d Ga l v a n o s t a t i c In t e rm i t t e n tT i t r a t i o n T e c h n i q u e (M GI T T ) f ro m t h e v a r ia t i o n o f t h ece l l vo l tage versus t ime dur ing the re laxa t ion per iodfo l lowing a long d i scharge o r charge [35 ]. Th is m etho ds ta tes tha t the evo lu t ion o f the vo l tage a f te r c i rcu i t open-

M o O3 c a t h o d e m a t e r i a l s h a v e b e e n c h a ra c t e r i z e d b yX- ra y d i f f r a ct i o n m e a s u re m e n t s u s i n g a C u K~ r a d i a t io n .F i g u re 2 s h o ws t h e X- ra y d i f f ra c t i o n p a t t e rn s o f M o O 3crys ta l and M oO a th in f ilms. X-ra y d i f f rac t ion da ta o f theM o O 3 c ry s t a l i n d i c a te t h a t m o l y b d e n u m t r i o x id e c ry s ta l -l iz es i n t h e o r t h o rh o m b i c s t ru c t u re w i t h a = 0 .3 96 2 n m ,b = 1 .3 85 8 n m a n d c = 0 .3 6 9 6 nm . T h e d i a g ra m o f t h eMoO3 crys ta l (F ig . 2 , cu rve a ) shows the (0k0) p redomi-nan t re f lec t ion l ines wh ich sugges t tha t th i s compound i so f a l a y e re d s t ru c tu re p a c k e d i n t h e d i r e c t io n o f t h eb-axis .X- ra y d i f f r a c t i o n p a t t e rn s o b t a i n e d o n f r e s h l y d e -pos i ted MoO3 f i lms a re shown in F ig . 2 (cu rves b and c ) .

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2 b 3 b 4 0 5 o 6 b z o 80Angle 28 (degree)F i g . 2 . X - r a y d i f f ra c t i o n d i a g r a m s o f M o O a . ( a ) cr y s t a ll i n e o r t h o r -h o m b i c M o O 3 m a t e r i a l , ( b) th i n f i l m g r o w n a t T ~= 120 C a nd (c) thinf i lm g r ow n a t T~= 250 C

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1 . 5 ' l l I i I a , I , i i i ' .0.0 0 .5 1 .0 1 .5C o m p o s i f i o n i n L i x M O 03F i g . 3. D i s c h a r g e c u r v e s o f L i / M o O 3 c el ls . T h e M o O 3 a c t i v e c a t h o d em a t e r i a l i s ( a ) c r y s t a l l i n e e - M o O 3 p h a s e , ( b ) t h i n f i l m g r o w nat T~ = 120 C an d (c) th in f i lm gro wn a t T~ = 250 C

T h e s u b s t r a t e t e m p e r a t u r e a f fe c t s w i d e l y th e s t r u c t u r e o fM o O 3 f i lm s , a s r e p o r t e d e l s e w h e r e [ 3 6 ]. A t T s = 3 0 C ,M o O 3 f i lm s a r e c o m p e t e l y a m o r p h o u s a n d t h e p o l y c ry s -t a l l in e p h a s e s t a r t s t o g r o w f o r T ~> 1 00 C . U p o n i n -c r e a s i n g t h e s u b s t r a t e t e m p e r a t u r e t h e c r y s t a l l i t e s i z ei n c r e a s e s a n d t h e c h a r a c t e r i s t i c c r y s t a l l i n e p e a k s o fM o O 3 a p p e a r . T h e X - r a y d i f f r a c t i o n s p e c t r u m o f f il m sd e p o s i t e d a t T ~= 1 2 0 C e x h i b i t s b o t h ( 0 k 0 ) a n d (Okl)o r i e n t a t i o n s r e p r e s e n t i n g a n e - # m i x e d p h a s e f i lm ( F ig . 2 ,cu rve b ). W e obse rve t ha t a f i l m depo s i t ed a t T~ = 250 Ci s r a t h e r a p o l y c r y s t a ll i n e m a t e r i a l w i t h a n o r t h o r h o m b i cs t r u c t u r e w i t h a p r e d o m i n a n t ( 0 k 0 ) o r i e n t a t i o n ( F i g . 2 ,c u r v e c ). T h e l a y e r e d s t r u c t u r e h a s b e e n c l e a r l y o b s e r v e db y s c a n n i n g e l e c t r o n m i c r o s c o p y .

F i g u r e 3 s h o w s t h e d i s c h a r g e v o l t a g e c u r ve s o f L i /M o O 3 c e ll s a s a f u n c t i o n o f th e l i t h i u m c o n c e n t r a t i o n i nt h e i n t e r c a l a t e d a c t i v e c a t h o d e m a t e r i a l s . A n i n i t i a l v o l t -a g e o f 3. 2 V w a s m e a s u r e d f o r M o O a t h i n -f i lm c a t h o d ec e l l s , w h i c h i s h i g h e r t h a n t h a t r e c o r d e d o n t h e g a l v a n i cc e ll u s in g a c r y s ta l l in e c a t h o d e . T h e m e a s u r e d p o t e n t i a l sa r e i n t h e r a n g e 3.2-2.5 V f o r t h i n - f il m b a t t e r i e s ( F i g . 3 ,c u r v e s b a n d c ) , w h e r e a s t h e r a n g e 2 . 7 - 1 . 5 V i s o b t a i n e dw i t h a c r y s t a l l i n e c a t h o d e ( F i g . 3 , c u r v e a ) .C o m p a r i n g t h e d i s c h a r g e c u r v e s f o r c r y st a ls a n d f il m s ,w e m a y r e m a r k t h a t :( i ) t h e c e l l v o l t a g e d e c r e a s e s s t e a d i l y w i t h t h e d e g r e e o fL i i n s e rt i o n in t h e h o s t m a t e r i a l s a n d r e a c h e s a v a l u e o f2 .45 V fo r f i l ms d epo s i t ed a t T~= 120 C an d 2 .80 V fo rf i lm s d e p o s i t e d a t T~= 250 C (at x = 1 .0) ,( ii) t h e d i s c h a r g e o c c u r s s t e a d i l y i n th e c o m p o s i t i o n a lr a n g e 0 _ x < 1 .5 , a n d( ii i) 1 .5 e l e c t r o n s a r e t r a n s f e r r e d i n t o t h e h o s t m a t e r i a l .T h e e l e c t r o c h e m i c a l p r o c e s s i s a c la s s i c a l i n t e r c a l a t i o nm e c h a n i s m f o r t h e l i t h i u m i n s e r t i o n [ 3 7 ] . T h e s i m u l -t a n e o u s i n j e c t io n o f io n s a n d e l e c tr o n s i n to M o O 3 f o r m sa l i th i u m b r o n z e a c c o r d i n g t o t h e r e a c t i o n1.5 L i + + 1 .5 e - +M OO 3 ~=~ Li t s + ( M O O 3 ) 1 ' 5 - . (3 )T h e r e d u c t i o n p r o c e s s , a c c o m p a n i e d b y c o l o u r a t i o n o fM o O a , i n v o lv e s t h e i n j e c t io n o f l i t h i u m c a t i o n a n d e le c -t r o n i n t o t h e h o s t a c c o r d i n g t o ( 3 ) .

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F i g . 4 . Chem ical diffusion coefficient of lithium ions in LixMoO3cathodes: (a) crystall ine e-MoO3 p hase , (b) thin fi lm grow n atT~= 120C and (c) thin film gro wn at T~=250 C

I t is o b s e r v e d i n F i g . 3 t h a t t h e v o l t a g e i s l o w e r i n c e l lsus i ng M oO 3 f i l ms dep os i t e d a t T~ = 120 C t ha n i n ce l l sw i t h f i lm s d e p o s i t e d a t T ~= 2 5 0 C . T h i s r e s u l t c a n b ea t t r i b u t e d t o t h e f a c t t h a t L i d if f u s i o n c o u l d b e l i m i t e db y t h e g r a i n b o u n d a r y e f fe c ts w h i c h a f f e c t t h e d i s c h a r g ec u r v e . A s e c o n d p o s s i b i l i ty is t h e p r e s e n c e o f t h e m i x e da -l? p h a s e w h i c h m a y r e d u c e t h e s t a n d a r d p o t e n t i a l. T h ed i s c h a r g e c u r v e o f t h e c e l l f a b r i c a t e d b y e m p l o y i n gM oO 3 f i l ms depo s i t ed a t T~= 250 C (F i g . 3 , cu rve c ) i sq u i t e s t a b l e b e c a u s e o f t h e u n i q u e l a y e r e d s t r u c t u r e o fc ~- M oO 3 w i t h l a r g e g r a i n s i z e. A t h i r d e x p l a n a t i o n m a yb e o x y g e n - d e f e c t s i n t h e h o s t s t r u c t u r e i n v o l v i n g a l o w e rFermi l eve l i n such a f i l m.F i g u r e 4 s h o w s t h e c h e m i c a l d i f f u s i o n c o e f f i c i e n t o fL i + io n s a s a f u n c t i o n o f t h e d e g r e e o f i n t e r c a l a t i o n i nL i x M o O 3 m a t e r i a l s . I n t h e s e e x p e r i m e n t s ( 1 ) i s v a l i d : ( i )i f t h e d u r a t i o n o f t h e p r e c e d i n g d i s c h a r g e i s lo n g e r t h a na c r i t i ca l t i me t o = 6 2 /4 D , a nd ( ii ) f o r a l i m i t ed pe r i odo f r e l a x a t i o n t < to o f t h e e l e c t r o c h e m i c a l c e ll . I n t h ec o m p o s i t i o n a l r a n g e 0 . 2_ < x _< 1 .2 t h e v a l u e s o f t h e c h e m i -c a l d i f f u s i o n c o e f f ic i e n t o f l i t h i u m v a r i e s f r o m 9 x 1 0 -1 1t o 1 x 10 9 c m 2 s - 1 i n t h e M o O a c r y s t a l . T h e c o m -p o s i t i o n a l d e p e n d e n c e o f D i s r a t h e r a q u a d r a t i c f u n c t i o nw h i c h i s d u e t o t h e n a t u r e o f t h e e m p t y s i te s in t h e h o s ts t r u c t u r e , a n d t h e v a r i a t i o n s o f t h e c h e m i c a l d i f fu s i v i t yo f t h e i n t e r c a la t e d a l k a l i i o n s c a n b e m o d e l e d . I n t h em o d e l o f B a s u a n d W o r r e l l [ 3 8 ] , t h e c h e m i c a l d i f fu s i v i t yi s r e l a t e d t o t h e c o m p o s i t i o nD = f ix 2 ( 1 - x ) + x ( 1 - x 2 ) , (4)w h e r e f i i s a n i n t e r a c t i o n p a r a m e t e r r e l a t e d t o t h e r e p u l -s iv e i n t e r a c t i o n e n e r g y b e t w e e n a l k a l i io n s . A c c o r d i n g t o

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17 6 C . J u l i e n e t a l .( 4) , D s h o w s a m a x i m u m a t t h e h a l f - f il l in g s i te n u m b e r .A l t h o u g h t h e u n c e r t a in t i e s i n t h e c h e m i c a l d i f f u s io n d a t as h o w n i n F i g . 4 p r e l u d e a n a d e q u a t e t e s t o f (4 ), t h em a x i m u m i n t h e L i ~ M o O 3 d a t a s u g g e s t s a n i n t e r a c t i o ne n e r g y l o w e r t h a n 0 . 2 e V w h i c h i s c o n s i s t e n t w i t h t h ev a lu e o b t a i n e d f r o m t h e r m o d y n a m i c m e a s u r e m e n t s.T h e v a l u e o f t h e c h e m i c a l d i f f u s i o n c o e f f i c i e n t i nM o O 3 t h i n f il m s i s l o w e r t h a n i n c r y s t a l s (F i g . 4 , c u r v e sb , c ). F o r M o O 3 f i lm s p r e p a r e d a t Z = 1 2 0 C a m a x i -m u m v a lu e o f 5 x 1 0 - 1 2 c m 2 s - 1 i s o b t a i n e d a t x = 0 . 8( F i g . 4, c u r v e b ). D , w h i c h i s a f u n c t i o n o f th e c o m -p o s i t i o n x , v a r i e s a c c o r d i n g t o a q u a d r a t i c l a wD = ~ x ( 2 - x ) , ( 5)w h e r e ~ = 2 x 10 - 1 2 c m 2 s - 1 . T h e c h e m i c a l d i f f u s i o nc o e f f i c i e n t b e h a v i o u r i n M o O 3 t h i n f i l m d e p o s i t e d a t2 5 0 C d i f f e r s f r o m t h e p r e v i o u s o n e . W e o b s e r v e t h a t Dh a s a n a l m o s t c o n s t a n t v a l u e o f 1 .5 x 1 0 - l a c m 2 s - * i nt h e r a n g e 0 . 1 < x < 1 .2 a n d i n c r e a s e s a t h i g h e r c o n c e n t r a -t i o n o f i n t e r c a l a n t i o n s . W e t e n t a t i v e l y a t t r i b u t e t h i sc o m p l e x b e h a v i o u r t o t h e p o l y c r y s ta l l in e s ta t e o f M o O 3f il m s g r o w n a t h i g h s u b s t r a t e t e m p e r a t u r e f o r w h i c h t h ee n h a n c e m e n t f a c t o r i s h i g h . I n t h i s c a s e , th e i n t e r c a l a t i o np r o c e ss i s p a r t l y c o n t r o l l e d b y t h e n u m b e r o f i o n o c -c u p a n c i e s i n t h e h o s t l a t t i c e o f t h e c r y s t a l l i t e i n t h e f i lm .F i g u r e 5 s ho w s t h e t h e r m o d y n a m i c fa c t o r , W (i n al o g a r i t h m i c s c a le ) , a s a f u n c t i o n o f t h e d e g r e e o f i n t e r-c a l a t i o n i n L i ~ M o O s m a t e r i a l s . I n t h e c o m p o s i t i o n a lr a n g e 0 . 2 < x < 1 .4 , W v a ri e s f r o m 1 .8 to 3 0 in M o O 3c r y s t a l s ( F i g . 5 , c u r v e a ) . C o n s i d e r i n g t h a t M o O 3 i s al a y e r e d h o s t f o r t h e i n t e r c a l a n t , t h e m o d e l o f i o n - i o ni n t e r a c ti o n c a n b e a p p l i e d. A r m a n d [ 3 9] h a s p r o p o s e ds u c h a m o d e l t o d e s c r ib e t h e v a r i a t i o n o f t h e c h e m i c a lp o t e n t i a l i n i n t e r c a l a t i o n c o m p o u n d s . T h e t h e r -m o d y n a m i c f a c t o r is re l a t e d t o t h e i n t e r a c t i o n f a c t o r, g ,a s

E x p e r i m e n t a l d a t a a r e w e l l f i tt u s i n g ( 6 ) w h i c h p r o v i d e sa n i n t e r a c t i o n f a c t o r 9 = 7 .5 . T h i s v a l u e i s o f t h e s a m eo r d e r a s i n TiS2 [40] .T h e t h e r m o d y n a m i c f a c t o r o f M o O 3 t h i n f i lm s va r ie sf r o m 5 0 to 8 0 0 i n th e c o m p o s i t i o n a l r a n g e 0 . 2 < x < 1 .5 .W e o b s e r ve a q u a s i - li n e a r v a r i a t io n o f th e c o m p o s i t i o nd e p e n d e n c e o f W w h i c h m a y b e a s s o c i a t e d w i t h t h eo x y g e n - d e f e c t s i n t h e h o s t l a tt i c e . I t i s a f a c t t h a t n u m e r -o u s o f s u c h d e f e c t s e x is t in t h e f i l m s t ru c t u r e , e v e n w h e nt h e c r y s t a l l i n i t y i s i m p r o v e d b y d i f f e r e n t c o n d i t i o n s o fp r e p a r a t i o n . H e r e , w e r e m a r k t h a t W is t w o o r d e r s o fm a g n i t u d e h i g h e r t h a n t h e t h e r m o d y n a m i c f a c t o r i n t h eM o O 3 c r y s t a l . U s i n g ( 6 ) a f it o f e x p e r i m e n t a l d a t a i so b t a i n e d w i t h a h i g h v a l u e o f t h e i n t e r a c t i o n f a c t o r( g = 4 0 0 ). C o n s i d e r i n g t h a t a n M o O 3 f il m i s a n o x y g e n -d e f i c i e n t m a t e r i a l , t h e m o d e l o f c h a r g e t r a n s p o r t i n i n te r -n a l d e f e c t - m a t e r i a l s c a n b e a p p l i e d [ 41 , 4 2] . D e f e c t s a r eL i i n t e r s t i t i a l s , L i * , a n d c o n d u c t i o n e l e c t r o n s , e ' , f o re x a m p l e . I t h a s b e e n s h o w n [ 4 1] t h a t , i n a s o l i d s o l u t i o nw h e r e n o i n t e r n a l d e f e c t r e a c t i o n s o c c u r , t h e t h e r -m o d y n a m i c f a c t o r is re l a t e d t o t h e d e f e c t c o n c e n t r a t i o n( i f d i l u t e defec t s e x i s t ) asW = ( i 0 L i I -~ - ( i ~ )e ~ 1 , (7 )w h e re ~b~-~ = CLi*/eL~+ an d ~bg I = c o , / c u + . E q u a t i o n ( 7 )p r o v i d e s a l in e a r v a r i a t io n o f th e t h e r m o d y n a m i c f a c t o rw i t h t h e d e g r e e o f i n t e r c a l a t i o n , a s s h o w n i n F i g . 5 (c u r v eb ) . T h e l a r g e i n c r e a s e o f W m a y b e a l s o a s s o c i a t e d w i t ht h e d e c r e a s e o f t h e e l e c t r o n i c m o b i l i t y i n L i ~ M o O 3 f il m .F o r t h e c a s e i n w h i c h t h e c h e m i c a l d i f f u s i o n i s p r e d o -m i n a n t l y d e t e r m i n e d b y i o n i c s p e c ie s , (2 ) y ie l d s , w i t h t h es u b s t i t u t i o n o f D o b y t h e c o n d u c t i v i t y a~ a n d t h e t h e r -m o d y n a m i c f a c t o r , f o r t h e p a r t ia l c o n d u c t i v i t y [4 3]a~ to = ( q c * D / W ) ( F / R T ) , (8 )

w= [ ( 1 - x ) - l + g x ] . (6 )1 0 - 3 i I l l O n i l i i t

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F i g . 5. T h e r m o d y n a m i c f a c t o r o f L i x M o O 3 c a t h o d e s : ( a) c r y st a ll i nee - M o O 3 p h a s e a n d ( b ) t h i n f i lm g r o w n a t T~ = 2 5 0 C . D a s h e d l i n e sr e p r e s e n t t h e f i t

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1 0 - 8 F , , , ~ r - - ~ - ~ ' -, i i , r ~ ,0.0 0.5 1.0 1.5[omposifion in LixNO09Fig. 6. Partial ionic condu ctivityof LixMoO3 cathodes: (a) crystallinee-MoO 3 phase and (b) thin film grown at T~= 250 C

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5/6

Lithium intercalation in Mo O3 177w h e r e t o i s t h e t r a n s f e r e n c e n u m b e r te = o -e /( ae + a i ) . B yc o m b i n i n g t h e c h e m i c a l d i f f u s i o n c o e f f i ci e n t a n d t h e t h e r -m o d y n a m i c f a c t o r v a l u e s , t h e l i t h i u m i o n c o n d u c t i v i t i e sa i i n L i x M o O 3 m a t e r i a l s a r e s h o w n i n F i g . 6 . D i s c h a r g i n gt h e c el l r e s u lt s in t h e f o r m a t i o n o f a m i x e d c o n d u c t o r ; t h ec o m p o u n d is a n e l e c tr o n i c a n d i o n ic c o n d u c t o r . I f th es a m p l e i s p r e d o m i n a n t l y a n e l e c t r o n i c c o n d u c t o r , i .e .a o >> a i , t h e i o n i c c o n d u c t i v i t y c a n b e d e d u c e d f r o m ( 8 ).T h e c o n d u c t i v i t y is f o u n d t o i n c r e a s e w i t h l it h i u m c o n -c e n t r a t i o n i n L i x M o O 3 c r y s t a l t o r e a c h a b o u t1 .5 x 1 0 - ~ S c m - I a t x = 0 . 6 ( F i g . 6 , c u r v e a ). O n e c a nr e m a r k t h a t t h e i on i c t r a n s p o r t p a r a m e t e r s i n L i x M o O 3c r y s t a l a r e c o m p a r a b l e w i t h t h o s e o f L i x T i S 2 [3 8] . F i g u r e6 ( c u r v e b ) d i s p l a y s t h e l i t h i u m i o n i c c o n d u c t i v i t y o fM o O 3 t h i n - f il m c a t h o d e . T h e i o n i c c o n d u c t i v i t y is es ti -m a t e d w i t h a n a v e r a g e v a l u e o f 9 x 1 0 - 9 g c m - 1 a tx = 0 . 8. W e r e m a r k t h a t t h e L i + i o n ic c o n d u c t i v i t y i sm u c h s m a l le r t h a n f o r M o O 3 c r ys ta l s. T h i s b e h a v i o u rm a y b e a t t r i b u t e d t o t h e d i s o r d e r e d s t r u c t u r e o f t h e fi lm .I n s u c h a m a t e r i a l t h e c o n d u c t i o n p a t h s a r e n o t d e f i n e da s w e l l a s t h e y a r e i n t h e c r y s t a l l i n e m a t e r i a l b e c a u s e i nt h i n f i l m : ( i ) t h e v a n d e r W a a l s p l a n e i s n o t q u a s i - i n f i n i t e ,( ii ) t h e p o t e n t i a l b a r r i e r s a r e m o r e i m p o r t a n t , a n d ( ii i) t h eL i + io n s c a n b e t r a p p e d b y t h e s t r u c t u r a l d e f e c t s .E l e c t r o c h e m i c a l p r o p e r t i e s o f L i ~ M o O 3 m a t e r i a l s c a nb e i l l u st r a t e d b y a n e l e c t r o n i c b a n d s c h e m e , a s s h o w n i nF i g . 7 . I n t h e p h y s i c i s t s d e s c r i p t i o n , t h e L i / M o O 3 c e l lv o l t a g e c a n b e v i e w e d a s t h e d i f f e re n c e o f t h e F e r m ie n e rg i e s b e t w e e n a n o d e a n d c a t h o d e . I n M o O 3 t h e r e ar ef i v e O ( p ~ ) a n d t h r e e M o ( t 2 g ) o r b i t a l s w h i c h i n t e r a c t t of o r m rc a n d re* b a n d s [ 44 ] w h i c h f o r m t h e v a l e n c e a n dc o n d u c t i o n b a n d s , r e sp e c ti v e ly . T h e a n t i b o n d i n g~ * s t a te s h o l d t h e e x t r a e l e c t r o n s s u p p l i e d b y t h e i n s e r t e dl i th i u m . T h e n a r r o w i n g o f t h e c o n d u c t i o n b a n d i s e x p e c t -e d [ 4 4 ] t o l e a d t o a n i n c r e a s e i n t h e e f f e c ti v e e l e c t r o nm a s s w h i c h w i ll a f f ec t t h e p o s i t i o n o f t h e F e r m i e n e r g yi n L i ~ M o O 3 p h a s e s . I n L i - i n t e r c a l a t e d M o O 3 c r y s t a l s t h eF e r m i e n e r g y l ie s e i t h e r w i t h t h e d o n o r l ev e l n e a r t h eb o t t o m o f th e c o n d u c t i o n b a n d o r i n th e c o n d u c t i o nb a n d i t s e l f ( F i g . 7 a ) , w h e r e a s i n M o O 3 t h i n f i lm s t h ed e n s i t y -o f - s t at e s m i g h t s h o w a b a n d i n t h e m i d d l e o f t h eg a p a n d t h e F e r m i l e v e l l ie s i n t h e b a n d . c ~- M oO 3 is a

( a ) ( b )

. . . . . EFg

D e n s i t q o f s f a l e sFig. 7a, b. Schem atic representation o f the electronic band structureof (a) ~-MoO3 crystal and (b) MoO 3 thin film. The energy of theFermi levelE v lies with the localizedelectronic band upon Li insertionin the host material

s e m i c o n d u c t o r w i t h a l ar g e b a n d g a p o f 3.1 e V . T h u s , t h eh i g h e s t v o l t a g e d i s p l a y e d i n t h i n - f i l m c e l l s c a n b e a t t r i b -u t e d b y t h e d i f f e r e n c e b e t w e e n F e r m i l e v el s ( a b o u t 1 e V )i n t h e c r y s t a l a n d t h i n - f i l m .

3 Conc lus ion

I n t h i s w o r k , w e h a v e s t u d i e d t h e l i t h i u m i n t e r c a l a t i o np r o c e s s in M o O 3 h o s t m a t e r i a l f o r t w o f o r m s o f i t, t h ec r y s t al l i n e o r t h o r h o m b i c c ~-M o O a a n d t h e p o l y e r y s t a l l i n eM o O 3 f i l m s . T h e k i n e t i c a l l y a c c e s s i b l e d i s c h a r g e r a n g ea m o u n t s t o 0 _ < x _< 1. 5 f o r t h e L i i n s e r t i o n i n M o O 3 a c t i v ec a t h o d e m a t e r i a l s. I n M o O 3 c r y s t a ll i n e c a t h o d e s t h ec h e m i c a l d i f f u s i o n c o e f f i c i e n t i s 1 x 10 . 9 c m 2 s - 1 a tx = 0 . 7 a n d t h e t h e r m o d y n a m i c f a c to r r e a c h es 30 atx = 1 .4 . V a l u e s o f t h e c h e m i c a l d i f f u s i o n c o e f f i c i e n t a n dt h e r m o d y n a m i c f a c t o r a r e s im i l a r to t h o s e o f t h e L i/ T i S 2s y s t e m .

I n M o O 3 f i l m s , t h e c h e m i c a l d i f f u s i o n c o e f f i c i e n tr a n g es f r o m 1 0 - i t t o 1 0 - 1 2 c m 2 s - 1 a n d t h e th e r -m o d y n a m i c f a c t o r is tw o o r d e r s o f m a g n i t u d e h i g h e rt h a n i n M o O 3 c r y s t a l s . T h i s v a l u e s c l e a r l y d e p i c t t h ew e a k e r i o n i c t r a n s p o r t i n t h e d i s o r d e r e d p h a s e s . H o w -e v e r , t h e c e l l v o l t a g e r e m a i n s h i g h i n f i lm s b e c a u s e d o n a t -i n g e l e c tr o n s a r e t r a p p e d i n t h e l o c a li z e d b a n d l o c a t e d i nt h e b a n d g a p o f d i s o r d e re d s e m i c o n d u c t o r s . M o O 3 f il m sa r e i n t e r e s t in g a s p o t e n t i a l e l e c t r o a c t i v e m a t e r i a l s f o rs o l id - s t at e l i t h iu m m i c r o b a t t e r i e s .

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39 .40 .41 .

42.43 .44.