IN SITU XAFS STUDY ON CATHODE MATERIALS FOR LITHIUM-ION BATTERIES FROM THE STANDPOINT OF INDUSTRIAL USE

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Fig. 1. Schematic drawing of the coin cell. IN SITU XAFS STUDY ON CATHODE MATERIALS FOR LITHIUM-ION BATTERIES FROM THE STANDPOINT OF INDUSTRIAL USE LiNi 0.8 Co 0.2 O 2 is one of the current candidates f o r a c a t h o d e m a t e r i a l o f a d v a n c e d r e c h a r g e a b l e b a t t e r i e s w i t h h i g h c a p a c i t y . T h e s t a b i l i t y o f L i N i 0 . 8 C o 0 . 2 O 2 i s s u p e r i o r t o L i N i O 2 , e x h i b i t i n g a s i n g l e - p h a s e r e g i o n u p o n o x i d a t i o n f r o m 3 . 0 V t o 4 . 1 V . T h e c a p a c i t y f a d i n g , h o w e v e r , o c c u r s n o t only during charge/discharge cycling but also when ba tt er ie s ar e ke pt at hi gh te mp er at ur es , cr ea ti ng d i f f i c u l t p r o b l e m s f o r p r a c t i c a l u s e . T o o v e r c o m e t h e p r o b l e m s o f c a p a c i t y f a d i n g , i t i s e s s e n t i a l t o u n d e r s t a n d t h e e l e c t r o n i c a n d s t r u c t u r a l c h a n g e s a c c o m p a n i e d b y t h e c a p a c i t y f a d i n g . F o r t h i s p u r p o s e , i n s i t u X A F S a n a l y s i s i s v e r y u s e f u l , because it gives information on both the local and 3 mm 20 mm Be window Be window Separator Cathode Anode Cell no. Condition Capacity (relative value) 1 Initial state (no treatment) NA 2 After one charge/discharge cycle* 100 3 After 515 charge/discharge cycles* 9.5 4 After keeping at 80 ° C for 3 days** 64.3 5 After keeping at 60 ° C for 25 days** 14.7 * The charge/discharge cycling have been done at rate of 1 mA/cm 2 in the range of voltage from 3.0 V to 4.1 V. ** Kept at charged state which corresponds to a voltage of 4.1 V. Table 1 . The conditions of cells. e l e c t r o n i c s t r u c t u r e s u r r o u n d i n g a n a b s o r b i n g a t o m w i t h o u t dis ass emb ly of the bat ter y for a m e a s u r e m e n t . S o m e s t u d i e s applying in situ XAFS analysis to L i N i O 2 o r L i N i 0 . 5 C o 0 . 5 O 2 h a v e b e e n a l r e a d y r e p o r t e d [ 1 - 3 ] . Nakai et al. revealed that the oxidation of the Ni ion occurred and the Jahn-Teller distortion of the NiO 6 octahedron decreased with Li deintercalation. The in situ XAFS analysis of the capacity faded battery, however, has not been reported. In this study we p r e p a r e d t h e b a t t e r i e s w i t h v a r i o u s c a p a c i t i e s t o mea sur e the in sit u Ni and Co K -ed ge abs orp tio n spectra of LiNi 0.8 Co 0.2 O 2 [4] . Figure 1 shows a drawing of the coin cell newly d e v e l o p e d f o r i n s i t u X A F S m e a s u r e m e n t s i n a transmission mode. The use of 0.4 mm Beryllium windows allow X-rays to penetrate the cell. XAFS data can therefore be obtained at various voltages without removing the cathode material from the cell. Fig. 3. Graphical comparison of the edge peak energies (E P ) as a function of the voltages. Fig. 2. Ni K-edge XANES spectra of LiNi 0.8 Co 0.2 O 2 8330 8335 8340 8345 8350 8355 0 0.2 0.4 0.6 1 0.8 Photon Energy (eV) Normalized Absorbance (a.u.) 1. Initial state 2. After one cycle (3.0 V) 3. After one cycle (4.5 V) 4. After 515 cycles (3.0 V) 5. After 515 cycles (4.5 V) 8346 8347 8348 8349 8350 E p (eV) Voltage (V) 3.5 3 4 4.5 1. Initial state 2. After one cycle 3. After 515 cycles 4. 80 ° C × 3 days 5. 60 ° C × 25 days T o i n v e s t i g a t e t h e c h a n g e s r e s u l t i n g f r o m c y c l i n g a n d k e e p i n g a t h i g h t e m p e r a t u r e s , w e p r e p a r e d c e l l s a t d i f f e r e n t c y c l i n g s t a t e s , a n d k e e p i n g temperature and time conditions. The cells used in this study are summarized in Table 1 . Ni an d Co K -e dg e XA FS da ta we re co ll ec te d u s i n g b e a m l i n e B L 1 6 B 2 . I n c i d e n t X - r a y s w e r e m o n o c h r o m a t i z e d u s i n g a S i ( 1 1 1 ) d o u b l e - c r y s t a l monochromator. The harmonic content of the beam w a s m i n i m i z e d u t i l i z i n g a R h - c o a t e d S i m i r r o r , i n c l i n e d t o 5 m r a d . T h e X - r a y i n t e n s i t i e s w e r e m o n i t o r e d u s i n g i o n i z a t i o n c h a m b e r s f i l l e d w i t h nitrogen gas for the incident beam and a mixture of argon (25%) and nitrogen (75%) for the transmitted beam. Figure 2 shows the Ni K -edge XANES spectra of LiNi 0.8 Co 0.2 O 2 for several samples. In both edges, c h e m i c a l s h i f t s o f t h e e d g e p e a k e n e r g y w e r e found. A graphical comparisons of the edge peak e n e r g i e s a s a f u n c t i o n o f t h e c e l l v o l t a g e s a r e s h o w n i n F i g . 3 . T h e e d g e p e a k e n e r g y ( E p ) i s defined here as the energy at maximum height of t h e e d g e j u m p . C o n t i n u o u s s h i f t s t o w a r d h i g h e r e n e r g i e s i n d i c a t e t h e i n c r e a s e s i n t h e a v e r a g e oxidation states of Ni upon lithium removal [2 ] . For c a p a c i t y f a d e d s a m p l e s , t h e r a n g e s o f c h e m i c a l shifts upon charging are less than that in “after one c y c l e ” . A g o o d c o r r e l a t i o n b e t w e e n t h e r a n g e o f c h e m i c a l s h i f t a n d t h e c a p a c i t y o f t h e c e l l w a s found. Fourier-transformation of the Ni K -edge EXAFS s p e c t r a f o r s e v e r a l s a m p l e s a r e d e m o n s t r a t e d i n F i g . 4 . T h e f i r s t p e a k a t a p p r o x i m a t e l y 1 . 5 Å co rr es po nd s to Ni -O in te ra ct io ns , an d th e se co nd p e a k a t a p p r o x i m a t e l y 2 . 5 Å c o r r e s p o n d s t o N i - N i i n t e r a c t i o n s . The Ni-O peak height of the sample in its initial state is lower than that o f t h e c a p a c i t y f a d e d s a m p l e s . T h i s p h e n o m e n o n i s e x p l a i n e d b y t h e l o c a l J a h n - T e l l e r d i s t o r t i o n o f t h e N i O 6 o c t a h e d r o n , d u e t o t h e lo w sp in Ni 3+ . Th e di st or te d Ni O 6 o c t a h e d r a l c o o r d i n a t i o n , s u c h a s 4(s hor ter ) + 2(l ong er) Ni- O bon ds, c a u s e s t h e a p p a r e n t d e c r e a s e i n the Ni-O peak height resulting from i n t e r f e r e n c e o f t h e i m a g i n a r y a n d real part of the FT [2] . The graph o f t h e h e i g h t s o f t h e N i - O p e a k p l o t t e d a s a f u n c t i o n o f t h e c e l l voltage ( Fig. 5 ), reveals that, upon c h a r g i n g , t h e e x t e n t o f t h e l o c a l d i s t o r t i o n i s r e d u c e d , a n d t h e d i s t o r t e d N i O 6 o c t a h e d r o n b e c o m e s a r e g u l a r o c t a h e d r o n . Fig. 6. A comparison of the averages of Ni-O distances as a function of the voltages. F i g . 5 . A c o m p a r i s o n o f t h e h e i g h t s o f N i - O peaks in FT spectra as a function of the voltages. 3.5 3 4 4.5 4 6 8 10 12 14 16 Voltage (V) FT Magnitude 1. Initial state 2. After one cycle 3. After 515 cycles Voltage (V) 3.5 3 4 4.5 1.85 1.86 1.87 1.88 1.89 1.90 1.91 1.92 Ni-O distance (Å) 1. Initial state 2. After one cycle 3. After 515 cycles 0 1 2 3 4 5 0 5 10 15 20 30 25 r (Å) FT Magnitude Ni - O Ni - Ni 1. Initial state 2. After one cycle 3. After 515 cycles T h i s p h e n o m e n o n r e s u l t s f r o m t h e g r a d u a l changes of the average valence of Ni from 3+ to 4+, making the shape of Fig. 5 similar to that of Fig. 3 . The averages of the Ni-O distances deduced f r o m q u a n t i t a t i v e a n a l y s i s o f E X A F S d a t a a r e s h o w n i n F i g . 6 . T h e c u r v e - f i t t i n g w e r e p e r f o r m e d w i t h t h e c o o r d i n a t i o n n u m b e r s o f oxygen fixed to 6. “After one cycle”, the Ni-O d i s t a n c e o b v i o u s l y d e c r e a s e s u p o n c h a r g i n g , while in “after 515 cycles”, the distance changes only slightly. The change of the Ni-O distance o r i g i n a t e s m a i n l y f r o m t h e c h a n g e i n t h e i o n - r a d i i a c c o m p a n i e d b y t h e c h a n g e i n o x i d a t i o n from Ni 3+ to Ni 4+ . Takamasa Nonaka TOYOTA Central R & D Labs., Inc. E-mail: nonaka@mosk.tytlabs.co.jp References [1] A . N . M a n s o u r e t a l . , J . E l e c t r o c h e m i c a l S o c . 1 4 6 ( 8 ) (1999) 2799. [2] I. Nakai et al. , J. Power Sources 68 (1997) 536. [3] I. Nak ai and T. Nak ago me, Ele ctr och emi cal and Solid-State Lett. 1 (1998) 259. [4] T. Nonaka, C. Okuda, Y. Ukyo and T. Okamoto, J. Synchrotron Rad. 8 (2001) 869. Fi g. 4. Fo ur ie r tr an sf or ma ti on of Ni K-EXAFS spectra for LiNi 0.8 Co 0.2 O 2 .