TECHNICAL PAPERS
ELECTROCHEMICAL SCIENCE AND TECHNOLOGY Characterization of LT-LixCOl_yNiyO Electrodes
for Rechargeable Lithium Cells
R. J. Gummow and M. M. Thackeray*
Division of Materials Science and Technology, CSIR, Pretoria 0001, South Africa
ABSTRACT
LT-LiCo0.gNi0102 prepared at 400~ with a structure that is intermediate between an ideal lithiated spinel and a layered structure has been investigated as an electrode in rechargeable lithium cells; it delivers a voltage vs. pure lithium that is significantly lower t h a n the voltage provided by its high-temperature analogue, HT-LiCo09Ni0 ~O~, (synthesized at 900~
The rechargeability of Li/LT-LiCo0 ~Ni010~ cells can be improved by leaching some lithium a n d a small amount of cobalt from LT-LiCo0.~Ni0.~O~ by acid treatment a n d loading the cells in a charged state. The improvement in electrochemical performance is a t t r i b u t e d to the formation of a defect spinel phase Li0.~[Co~.~Ni0.~]O~ in which the lithium ions adopt the tetrahedrat A sites a n d the cobalt a n d nickel ions the B sites of an A[B~]O~ spinel.
Lithium-cobalt-nickel-oxides LiCo~_yNi~O2 (0 -< y -< 1) are of interest as electrodes for rechargeable 4 V lithium cells. 1-6 These electrodes have, in the past, been convention- ally synthesized at 800-900~ and are referred to as HT- LiCo~_yNiyO2 electrodes (HT for high temperature); they have layered rock salt structures. Extensive studies 4'6'7 have shown that electrodes with 0 < y < 1.0 perform better on electrochemical cycling than the end members of the sys- tem HT-LiCoO2 (y = 0) a n d HT-LiNiO2 (y = 1). A disadvan- tage of Li/HT-LiCol_yNiyO2 cells is that it is necessary to charge the cells above 4 V, particularly at low lithium con- tents, which is outside the electrochemical stability w i n - dow of m a n y organic-based electrolytes. It has been re- cently reported, however, that if the synthesis of LiCo~_yNiyO2 materials is carried out at 400~ hereafter referred to as LT-LiCo~ ,NiyO2 (LT for low-temperature), then compounds with a modified structure are produced that have significantly different properties compared to their high-temperature analogues. 8-~6 For 0 -< y -< 0.2, LT- LiCo~_yNiyO2 compounds are essentially single-phase and have spinel-like x-ray- a n d neutron-diffraction patterns.
In l i t h i u m cells, LT-LiCo~_yNiyO2 electrodes discharge most of their capacity at voltages approximately 0.5 V lower t h a n HT-LiCol ,NiyO2 electrodes. This discovery has raised the possibility of developing lithium-cobalt-nickel- oxide electrodes with an enhanced electrolyte stability. Al- though preliminary electrochemical data 9 have indicated that nickel-doped LT-LiCol_yNiyO2 electrodes (0 -< y -< 0.2) perform better t h a n LT-LiCoO2, these data have unfortu- nately also shown that LT-LiCo~_yNiyO2 structures are not particularly stable to repeated lithium extraction and in- sertion reactions. The x - r a y data and electrochemical properties of LT-LiCoQ have been confirmed by Rossen et al. ~,~2
We have attempted to devise methods for improving the cycling properties of LT-LiCOl_yNiyO~ electrodes. In this paper we compare the structural and electrochemical prop- erties of LT-LiCo09Ni0.~O2 (y = 0.1) prepared at 400~ with a product of composition Li0.4Co0.sNi0 ~O2, prepared by chem- ical delithiation of LT-LiCo0.gNi0 ~O~ at room temperature.
Experimental
LT-LiCo0.~Ni0.~O~ a n d a standard L T - L i C o Q sample were synthesized b y reacting stoichiometric quantities of Li2CO~, CoCO~, a n d Ni(NO~)~ - 6 H 2 0 in air at 400~ for five days. T h e precursors were ball-milled in hexane prior to
* Electrochemical Society Active Member.
J. Electrochem. Soc., Vol. 140, No. 12, December 1993 9
firing to ensure t h o r o u g h mixing. T w o partially delithiated s a m p l e s w e r e p r e p a r e d b y reacting I g of LT-LiCo0.gNi0.102 w i t h 20 m l 0 . 6 2 5 M H 2 S O 4 at 22~ for 3 h. T h e s a m p l e s w e r e filtered, w a s h e d w i t h distilled water, a n d dried at 80~ for 24 h. S a m p l e s w e r e heated for a further 24 h at 80~ prior to cell a s s e m b l y to r e m o v e as m u c h surface a n d o c c l u d e d w a t e r as possible. Lithium, cobalt, a n d nickel contents in the partially delithiated s a m p l e s w e r e d e t e r m i n e d b y atomic absorption spectroscopy, a n d the composition w a s f o u n d to be LT-Li0.4Co0.sNi0.102, within experimental error, in b o t h cases.
Two-electrode prismatic lithium cells containing metal- lic lithium anodes, " L T " cathodes, a n d a n electrolyte of either I M LiCIO4 in propylene c a r b o n a t e / d i m e t h o x y e t h a n e ( P C / D M E ) in a i:I volumetric ratio or I M L i C I O 4 in p r o p y - lene carbonate (PC), w e r e a s s e m b l e d as described else- where. 9 C a t h o d e s consisted either of a n intimate m i x t u r e of 20 w e i g h t percent (w/o) of a 1:2 Teflon acetylene black m i x - ture ( T A B ) w i t h 80 w / o active material or alternatively of a m i x t u r e of ethylene propylene diene m o n o m e r ( E P D M ) , acetylene black, a n d active material in a ratio 3:7:90 b y weight. Cells w e r e c h a r g e d at a constant current density of 0.1 m A / c m 2 a n d discharged at 0.2 m A / e m 2 at r o o m t e m p e r - ature (22~ P o w d e r x-ray diffraction patterns w e r e recorded o n a n a u t o m a t e d R i g a k u dilfractometer w i t h C u K ~ radiation, m o n o c h r o m a t e d b y a graphite single crys- tal. D a t a w e r e recorded using a step w i d t h of 0.02 degrees 2(} a n d a scan rate of 0.53 degrees 2{} per minute. Calculated p o w d e r patterns of LT-LiCo0.gNi0 iO2 a n d LT- Li0.4Co0.sNi0.102 w e r e generated b y m e a n s of the p r o g r a m L A Z Y - P U L V E R I X . 13
Results and Discussion
Structural considerations.-- The powder x - r a y diffrac- tion p a t t e r n of LT-LiCo0.gNi0 ~O2 is shown in Fig. la. This p a t t e r n can be indexed either to a trigonal u n i t cell (R3m) with a = 2.833 ,~, a n d c = 13.881 A, 8 or, because the c/a ratio
= 4.90,o to a face-centered cubic u n i t cell (Fd3m) with a = 8.013 A. Although an accurate structure refinement of LT- LiCo0.gNi0.102 has not yet been undertaken, it can be seen from Fig. l a that the observed p a t t e r n of LT-LiCo0.gNi0.~Q strongly resembles the calculated p a t t e r n of a n ideal lay- ered structure (Li)3,{Co0.gNi0.1}~bO2 that has space group symmetry R3m (Fig. lb) and also the calculated p a t t e r n of an ideal lithiated spinel (Li2)~6c [Co~.sNi02104 that has space group symmetry Fd3m (Fig. lc). In fact, the calculated powder patterns of the ideal layered a n d lithiated spinel
The Electrochemical Society, Inc. 3365
3366 J. Electrochem. Soc., Vol. 140, No. 12, December 1993 9 The Electrochemical Society, Inc.
10
' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' '
(a)
lOl
111
311
I '. '. '. ! '. '. ~ !'.
20 30 40
J
(b)
108 11013 2O2 101 116
%
(c)
44O
71 ,,, A =1 6,..
50 60 70 80
2-TH ETA
Fig. 1. The observed and calculated powder x-ray diffraction pot- terns of LT-LiCoo.vNioj02. The observed pattern (a) shows a small amount of a second phase, U, Nil_,O, denoted by an asterisk. Pat- tern (b) was calculated using R3m symmetry, and pattern (c) using Fd3m symmetry.
10
' ' ' ' I ' ' ' ' I ' ' ' ' 1 ' ' ' ' I ' ' ' ' I ' ' ' '
(a}
111
101 I 104 (b)
I I%'
101(:
006 8 105 11(}
311 440 (C)
, ,. ,.,,.: ,!,, l/:^
20 30 40 50 60 70 80
2-THETA
Fig. 2. The observed and calculated powder x-ray diffraction pot- terns of LT-Lio.4Coo.sNio.102. The observed pattern (a) shows a small amount of a second phase, Li, Ni]_~O, denoted by an asterisk. Pat- tern (b) was calculated using R3m symmetry and paltern (c) using Fd3m symmetry.
structures, w h i c h a r e b o t h of t h e r o c k s a l t - t y p e , are i d e n t i - cal; this c r y s t a l l o g r a p h i c a n o m a l y w h i c h h a s a l r e a d y b e e n o b s e r v e d in LT-LiCoO28,10 has b e e n e x p l a i n e d b y Rossen
e t a l . ~1 R e f i n e m e n t of t h e l a t t i c e p a r a m e t e r s of LT-LiCoO2 w i t h x - r a y d a t a h a s s h o w n an i d e a l l y c u b i c - c l o s e - p a c k e d o x y g e n a r r a y ( c / a = 4.90) w h i c h s u p p o r t s a spinel m o d e l for LT-LiCoO2.8,10,1~ By contrast, r e f i n e m e n t of t i m e - o f - f l i g h t n e u t r o n - d a t a g a v e c / a = 4.914 a n d a c a t i o n d i s t r i b u t i o n in LT-LiCoO2 t h a t is i n t e r m e d i a t e b e t w e e n t h e d i s t r i b u t i o n f o u n d in i d e a l l a y e r e d a n d i d e a l spinel structures. 8 T h e s t r u c t u r a l m o d e l of LT-LiCoO2 o b t a i n e d f r o m n e u t r o n d a t a has b e e n t e r m e d " q u a s i - s p i n e l " b e c a u s e of t h e s t r o n g simi- l a r i t y of t h e d i f f r a c t i o n p a t t e r n to those of l i t h i u m - s p i n e l structures. It is o u r b e l i e f t h a t t h e n e u t r o n data, w i t h infor- m a t i o n on t h e p e a k i n t e n s i t i e s d o w n to 0.60 A, has p r o v i d e d t h e m o s t a c c u r a t e analysis of t h e LT-LiCoO2 s t r u c t u r e t h u s far; for this r e a s o n t h e q u a s i - s p i n e l m o d e l for LT-LiCoO2 is preferred, at least f o r those samples p r e p a r e d at 400~ b y the m e t h o d s e m p l o y e d at t h e CSIR.
The a t o m i c a b s o r p t i o n analysis of t h e a c i d - l e a c h e d s a m - ples g a v e a c o m p o s i t i o n LT-Li0.4Co0.sNi0.102 w h i c h , w i t h i n e x p e r i m e n t a l e r r o r for t w o d i f f e r e n t samples, suggests t h a t c o b a l t ions w e r e r e m o v e d f r o m t h e structure, in p r e f e r e n c e to n i c k e l ions, d u r i n g a c i d t r e a t m e n t . The reasons for this are n o t yet u n d e r s t o o d . It is also possible t h a t d u r i n g a c i d t r e a t m e n t s o m e Li + ions w e r e i o n - e x c h a n g e d b y H + ions, t h e e x t e n t of w h i c h w a s n o t d e t e r m i n e d . T h e m e a n o x i d a t i o n s t a t e of t h e c o b a l t a n d n i c k e l cations in t h e d e f e c t spinel p h a s e is 4.0, a s s u m i n g t h a t t h e r e are no p r o t o n s in t h e structure.
The p o w d e r x - r a y d i f f r a c t i o n p a t t e r n of an a c i d - l e a c h e d s a m p l e LT-Li04Co0.~Ni0.102 is s h o w n in Fig. 2a; it c a n be i n d e x e d , l i k e LT-LiCo09Nio ~O2, to e i t h e r a t r i g o n a l u n i t cell (a = 2.826 A, c = 13.845 A, c / a = 4.90) or to a f a c e - c e n t e r e d - cubic u n i t cell (a = 7.993 A). D e l i t h i a t i o n is a c c o m p a n i e d by a 0.5% decrease in u n i t cell volume. T h e p a t t e r n of t h e d e l i t h i a t e d c o m p o u n d is s t r i k i n g l y s i m i l a r to t h a t of t h e p a r e n t c o m p o u n d , b u t c a n be d i s t i n g u i s h e d f r o m it b y a s i g n i f i c a n t difference in t h e r e l a t i v e intensities of t h e {101}
a n d c o m b i n e d {006}/{102} p e a k s of t h e t r i g o n a l u n i t cell, (or alternatively, t h e {311} a n d {222} p e a k s of t h e f a c e - c e n - t e r e d - c u b i c u n i t cell). The c a l c u l a t e d p a t t e r n s of a l a y e r e d s t r u c t u r a l m o d e l (Li0.4)6c(Co0.sNi0.1)3bO2 a n d a spinel m o d e l (Li0.s)8~[Col.sNi0.2]l~dO4, in w h i c h t h e l i t h i u m ions are l o c a t e d in t e t r a h e d r a l sites a n d t h e t r a n s i t i o n m e t a l ions in o c t a h e - d r a l sites, are s h o w n in Fig. 2b a n d c, respectively. A l - t h o u g h t h e profiles of t h e t w o c a l c u l a t e d x - r a y p a t t e r n s seem to be identical, t h e r e are several w e a k p e a k s in t h e p a t t e r n of t h e spinel m o d e l t h a t are n o t p r e s e n t in t h e l a y -
Table I. Calculated intensities of the powder x-ray diffraction patterns of a layered strudural model of LT-(Lio.4)~[Ca0.sNi0.,]3b02
(R3m) and a defect spinel model LT-(Lio.8)8o[Col.~Nio.2116d04 (Fd3m). Intensities were calculated assuming ideal cubic-close-packing of the anion lattice.
LT-Li0.4Co0.sNi0.102 (R3m)
(Layered structure) LT-Li0.sCol.6Ni0.zO4 (Fd3m) (Spinel structure)
hkl 2-theta I hkl 2-theta I
003 19.22 1000.0
101 37.29 624.5
006 39.02 23.5
012 39.02 70.5
104 45.36 781.6
015 49.69 230.7
009 60.12 44.4
107 60.12 133.3
018 66.09 206.6
110 66.09 206.6
113 69.55 70.9
11-3 69.55 70.9
021 78.42 51.8
1010 79.50 14.7
116 79.50 14.7
11-6 79.50 14.7
202 79.50 14.7
111 19.22 1000.0
220 31.64 0.7
311 37.29 624.5
222 39.02 94.1
400 45.36 800.9
331 49.69 230.7
422 56.36 0.2
511 60.12 133.3
333 60.12 44.4
440 66.09 402.8
531 69.55 141.8
442 70.67 0.0
620 75.13 0.1
533 78.42 51.8
622 79.50 58.7
J. Electrochem. Soc.,
Vol. 140, No. 12, December 1993 9 The Electrochemical Society, Inc. 3367
4.2 4.2
4 4
3.8 ,~ 3.8
I ~ 3.6 > 3.6
3.4 e 3.4
3.2 2 3.2
o'}
2.8 > 2.8
2.6 2.6
2.4 2.4
o 20 40 60 8b 16o 12o14o 16o o
Capacity (mAhr/g)
4.2
3.8
>~, 3.6
~5"4 I
2 s.2
~ 3
2.8 2.6 2.4 0
(C) .... ~ ~ 1
10 20
3 040 50 60 70 80 90
Capacity (mAhr/g)
4.4 4.2
4
;-3.8
"-" 3.6 3.4 a 3.2 o
> 3 2.8 2.6 2.4 0
J
(b)
I
2o 4o 60 8o i 0 0 1 2 0 1 4 0 1 6 0
Capacity (mAhr/g)
f (d) % 1
20 40 6'0
8 0160 120 Capacity (mAhr/g)
4
3.9
;,3.7 3.8
"J3.6
;o?,
-- .
> 3.3 o
3.2 3.1 3 0
(e) ~ 4 1
10 20 30 4C) 50 60 70 80 90100 Capacity (mhhr/g)
Fig. 3. Charge and discharge curves for (a) a Li/LT-LiCoO2 cell, (b) a Li/LT-LiCoo.gNio.,O2 cell, (c) a Li/LT-Lio.4Coo.~NiojO2 cell, (d) a Li/LT- Lio.4Coo.~Nio.lO~
cell (2.5 to 4.2 V), and (e) Li/LT-Lio.4Coo.sNi0nO2 cell (3 to 4 V). Cells were
charged at 0.1 mA/cm and discharged at 0.2 mA/cm 2. Cells a-c contained a 1M LiCIO4 in 1:1 propylene carbonate: dimelhoxyethane electrolyte, and cells d and e contained a 1M LiCIO4 in propylene carbonate electrolyte.ered model, as shown, for example, b y the {220} and {422}
reflections in Table I and which can be used to distinguish the two structures from one another. These peaks are not a p p a r e n t in the calculated x - r a y diffraction p a t t e r n s be- cause of their very low intensity relative to the strong peaks; they have, however, been u n a m b i g u o u s l y detected in high quality time-of-flight neutron diffraction d a t a of closely related LT-Li0.4CoO2 samples p r e p a r e d b y acid t r e a t m e n t of LT-LiCoO2. The structure refinement of LT- Li0.4CoO2 has provided unequivocal evidence of a spinel- type structure, not a layered structure. 1~
Therefore, because of the strong analogy between LT- LiCoo.gNi0.~O2 a n d LT-LiCoO2 samples, and between LT-
Li0.4Co0.sNi0.102 and LT-Li0.4CoO2 samples in terms of p r e p a r a t i o n techniques, synthesis temperature, composi- tion, stoichiometry, and x - r a y diffraction patterns, it is highly p r o b a b l e t h a t LT-LiCo0.gNi0.102 has a structure which is quasi-spinel in character and t h a t LT- Li0.4Co0.sNi0.102 is a defect spinel w i t h spinel notation {Li0.s[:]0.2}sa [Co, 6Nio.2D0.2104.
Electrochemical data.--The charge and discharge p r o - files for the first four cycles of Li/LT-LiCoO2, Li/LT- LiCo0.gNi0.102 and Li/LT-Li0ACo0.aNio.]O2 cells are shown in Fig. 3a-e, respectively. Doping of LT-LiCoO2 w i t h nickel improves the initial discharge capacity of LT-LiCoI_~Ni~O2 electrodes from 63 mAh/g for a sample with y = 0 (Fig. 3a)
3368 J. Electrochem. Soc., Vol. 140, No. 12, December 1993 9 The Electrochemical Society, Inc.
to 98 mAh/g for a samp]e with y = 0.1 (Fig. 3b) when cells are charged to an u p p e r voltage limit of 4.2 V and dis- charged to 2.5 V, as reported previously2 However, b o t h cells lose capacity quickly, and after four cycles deliver an electrode capacity of 50 m A h / g or less. In these cells the capacity loss can be a t t r i b u t e d to a m a r k e d increase in polarization during successive charge cycles t h a t p r o b a b l y arises from the i n s t a b i l i t y of the quasi-spinel structures of LT-LiCoO2 and LT-LiCo0.gNi0102 to r e p e a t e d e x t r a c t i o n and insertion of lithium.
By contrast, the d a t a obtained for the charge/discharge profiles of Li/LT-Li0.4Co0.sNi0.102 cells t h a t had been assem- bled in a charged state (Fig. 3c-e) (i.e., the cell was dis- charged on the first cycle), are indicative of a system t h a t is significantly more tolerant to cycling t h a n Li/LT-LiCoO2 and Li/LT-LiCo0.gNi0102 cells. The improved performance of Li/LT-Li0.4Co0.sNi0.102 cells is evident from the following factors: (i) the polarization during both charge and dis- charge is significantly reduced; the cells deliver a p p r o x i - mately 3.4 V for almost the entire length of discharge, and (ii) the rate at which c a p a c i t y is lost on cycling is reduced.
In cells t h a t employed a TAB b i n d e r and an e]ectrolyte of 1M LiC104 in PC/DME, an initial c a p a c i t y of 83 m A h / g was achieved; after four cycles the capacity d r o p p e d to only 67 mAh/g. A significantly higher capacity (>98 mAh/g) was obtained from cells in which an EPDM b i n d e r and a pure propylene carbonate electrolyte were used and in which the u p p e r voltage limit was set at 4.2 V (Fig. 3d). In this cell the capacity d r o p p e d on cycling from 112 m A h / g to 98 mAh/g after four cycles. Cycling of a Li/LT-Li0.4Co0.sNi0.102 cell of this type between 3 and 4 V (Fig. 3e) resulted in a slight reduction of the capacity but failed to improve the cycling stability; the c a p a c i t y d r o p p e d from 95 mAh/g to 81 mAh/g after four cycles.
Conclusions
The difference in the electrochemical performance of Li/
LT-LiCo89Ni0.102 and Li/LT-Lio.~Coo.sNio 102 cells is signifi- cant. It demonstrates t h a t electrochemical delithiation of LT-LiCoo.gNio.~O2 does not result in exactly the same structural f r a m e w o r k t h a t is obtained b y chemical delithi- ation, and therefore supports the hypotheses t h a t LT- LiCoo.aNio.~O2 is quasi-spinel in c h a r a c t e r w i t h a structure which is i n t e r m e d i a t e between an ideal layered and an ideal spinel structure, and t h a t LT-Lio.4Coo.sNio.lO2 is a de- fect spinel in which all the cobalt and nickel ions reside on the B sites of an A[B2104 spinel structure. The spinel phase
is significantly more stable to lithium insertion/extraction reactions t h a n the quasi-spinel phase. It is believed t h a t b y optimizing the processing conditions it may be possible to increase further the rechargeable capacity and s t a b i l i t y of LT-Li[Co2_zNiz]O4 (0 < z < 2) spinel electrodes t h a t operate at 3.4 V vs. pure lithium. A challenge also remains to syn- thesize a l i t h i a t e d spinel phase with a cation distribution, LT-Li2[Co2 zNiz]O4, in which the transition metal cations fully occupy the octahedral B-sites, to allow the fabrication of carbon/LT-Li2[Co2_~Niz]O4 cells in an inherently safe, discharged state.
Acknowledgments
M. Schatz is t h a n k e d for u n d e r t a k i n g the atomic a b s o r p - tion analysis.
Manuscript s u b m i t t e d May 20, 1993; revised m a n u s c r i p t received Sept. 7, 1993.
Division of Materials Science and Technology, CSIR, as- sisted meeting the publication costs of this article.
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