Chemistry Letters 1997
`
`725
`
`Synthesis of Solid Solutions in a System of LiCo0z-Li2Mn03 for Cathode Materials of
`Secondary Lithium Batteries
`
`Koichi Numata, * Chie Sakaki, t and Shoji Yamanaka t
`Battery Materials Research Laboratory, Mitsui Mining and Smelting Co., Ltd., Takehara, Hiroshima 725
`t Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima, Hiroshima 739
`
`(Received March 26, 1997; CL-970220)
`
`Lithium-manganese-cobalt oxide, Li(Li,13Mn,,,Co,.,)O, (0 :S
`x 5 I) was prepared as a solid solution between the two kinds of
`layer structured end members, LiCoO, and Li,MnO,. Excess
`lithium carbonate or lithium hydroxide should be added to the
`mixture on calcination at 900 - 1000 °C to obtain the solid
`solutions. The resulting layer structured solid solutions can be
`used as a cathode material for secondary lithium batteries.
`
`Since the success of the development of LiCoO, as a cathode
`material for lithium ion battery, much attention has been paid to
`the improvement of this cathode.' ·' LiCoO, has a layered
`structure with the a-NaFeO, type with lithium atomic layers
`sandwiched between the CoO, octahedral layers. The interlayer
`lithium ions are deintercalated and intercalated in the charge and
`discharge process, respectively. However, only about 70% ofLi
`ions are used as an active part for the cathode.' More lit!Jium
`ions could be deintercalated, but a higher charge voltage should
`be applied, which would result in the decomposition of
`electrolyte solutions. In addition, the higher level of Li
`deintercalation causes the degradation of the cathode due to the
`higher concentration of reactive Co''.'
`In an effort to lower the charge voltage for deintercalation
`and to stabilize the LiCoO, electrode against the higher
`deintercalation, substitution of cobalt with other transition
`metals has been attempted. Stoyanova et a!.' reported the effect
`of Mn-substitution for Co. They tried to synthesize solid
`solutions in a system of LiCoO,-LiMnO,, and showed that the
`crystal structure varied with the increase of x in LiMn,Co,_,O,;
`trigonal layered structure (a-NaFeO, type) for x :S 0.2, a rock
`salt structure for 0.2 < x < 0.7, and a tetragonal structure for x <::
`0. 7. In this study, attention is paid to prepare a new solid
`solution system of LiCoO,-Li,MnO, as a lithium battery
`cathode. As shown in Figure I, Li,MnO, is isostructural with o;(cid:173)
`NaFeo;' where Li and (LiMn'',) layers alternately occupy the
`octahedral sites between the oxygen packed layers of Na and Fe,
`respectively. LiCoO, also has an a-NaFeO, structure, and the
`formation of the solid solutions in the system LiCoO,-Li,MnO,
`is expected. In addition, manganese is much more abundant and
`less expensive than cobalt, the substitution of Mn for Co may be
`favorable in the cost of the fabrication of cathode.
`
`c
`
`1
`
`LiCo02
`
`• Li
`
`@ Co
`
`@) Mn
`
`Qo
`
`Li2Mn0,
`
`The solid solutions Li(Liu,MnMCo,.,)O, (0 :S x :S I) were
`prepared by calcination of the stoichiometric mixtures of lithium
`carbonate, basic cobalt carbonate, and manganese carbonate
`with excess amount of lithium carbonate or lithium hydroxide at
`900 and 1000 °C for 20 h in an alumina crucible. The
`compositions of the product will be designated by the formula
`Li(Li.,,Mn,,13Co,_,)O,-yLi,O, where y denotes the excess amount
`of lithium added.
`The X-ray powder diffraction (XRD) patterns of the samples
`were measured by a diffractometer with graphite
`monochromated Cu-Ka radiation. Chemical analysis was
`performed by an ICP (inductively coupled plasma) method after
`dissolving the sample in a mixed solution of cone. HCI and
`cone. HNO, (3 : 1). The obtained solid solution was compressed
`into a cathode disk (10.4 mm in diameter) with 4 wt% of
`polytetrafluoroethylene powder and 6 wto/o of acetylene black. A
`lithium battery cell was constructed with lithium metal anode,
`and porous · fiber separator soaked with I M (I M = I mol dm-')
`LiBF, solution in a mixture of propylene carbonate and
`dimethoxy ethane. The cells were charged and discharged at a
`constant current density of I mNcm'.
`When a simple mixture of the constituents which correspond
`to solid solution Li(Li, 13Mn,,,Co,.,)O, was calcined at
`temperatures ranging from 900 to I 000 °C, only a mixture of
`two crystalline phases of LiCoO, and Li2Mn0, was obtained in
`the whole composition range of 0 < x < I. However, when
`excess amount of lithium was added before the calcination, solid
`solutions were obtained over the whole composition range.
`Figure 2 shows typical XRD patterns with a nominal
`composition of x = 0.4. The samples with x > 0.2 had some
`
`i!
`
`-~
`~
`c ·o;
`c:
`!l
`.=
`
`(b)
`
`cLiCoO,
`•Li,MnO,
`
`D
`
`••
`20
`
`30
`
`40
`50
`60
`29 I deg. Cu Ka
`Figure 2. X -ray diffraction patterns of the products obtained by
`calcination of a mixture with composition of Li(Li,,MnMCo,_
`,)0,-yLi,O (x = 0.4) at 1000 °C, followed by washing with water
`:(a) y = 0 (LiCoO, + Li,MnO,) and (b) y = 1/12 (solid solution).
`
`Figure 1. Crystal structure models of LiCoO, and Li,MnO,.
`
`Copyright© 1997 The Chemical Society of Japan
`
`SONY EXHIBIT 1021
`
`Page 1 of 2
`
`

`
`726
`
`0.505 ...----- - -- - -- -- - - . 0.860
`c
`
`Chemistry Letters 1997
`
`§ 0.500
`.._
`u
`d'
`~ 0.495
`"' 0 u
`" u
`~ 0.490
`
`0.855 s
`-E:
`~
`;g
`0 u
`8
`·~
`....:l
`
`0.850
`
`0.845
`
`0.485 ' - - - - ' - - - - - '- ---1..- -----' 0.840
`0
`0.25
`0.50
`0.75
`1.00
`x in Li(Liv3Mn,.,Co,.,)O,
`
`the lattice constants against x in
`Figure 3. Dependence of
`Li(Li,.,Mn,.,Co,.JO, for the solid solutions obtained by calcina(cid:173)
`tion at I 000 °C.
`
`XRD peaks characteristic of the Li,MnO, structure such as 020
`and 110 reflections in a range of 20- 30° in 28 (Figure 2 (b)).
`These peaks disappeared for the samples x ~ 0.2 with the
`LiCoO, structure by systematic extinctions. The XRD
`reflections of LiCoO, can also be indexed by a monoclinic
`symmetry like Li,MnO,. The solid solutions have a monoclinic
`symmetry over the whole composition range, and they can have
`a higher symmetry, trigonal one, when x ~ 0.2. The amount of
`excess lithium, y in Li(Li,,Mn,.,Co,.,)O, ·yLi,O required to
`obtain solid solution varied with the composition x and the
`calcination temperature. In the preparation of the solid solution
`with x = 0.4, for example, at 900 °C, the amount y was about
`1/6, but at higher temperature of 1000 °C, addition of only 1/12
`of lithium in y was sufficient. Excess lithium could be added in
`a form of lithium carbonate or lithium hydroxide LiOH·H,O. On
`calcination, these compounds form molten phases. It is likely
`that the liquidous phase enhances the mutual diffusion to form
`the solid solutions. The excess lithium was removed by washing
`with water after calcination.
`Figure 3 shows the lattice parameters determined on the
`basis of a monoclinic symmetry for the solid solutions obtained
`by calcination at 1000 °C. The parameter p was 109.2 ± 0.1 o for
`every sample. As can be seen from the figure, the lattice
`constants change continuously with the composition, indicating
`the formation of uniform solid solutions. It is interesting to note
`that in the solid solution system of LiCoO,-LiMnO, or LiCo,.
`,Mn,O,, the solid solutions with a layered structure were formed
`only in a region of x < 0.2; the solid solutions with 0.2 < x < 0.7
`
`Table 1. Chemical analysis data of Li(Li, ,Mn""Co,.JO,
`
`X
`
`0. 11
`0.31
`0.52
`0.71
`0.90
`
`Li,O
`16.60/(16.17)
`18.31/(17.94)
`20.1 0/(20.02)
`22. 72/(22.1 0)
`24.91/(24.33)
`
`Found/(Calculated) (wt%)
`Co,O,
`MnO,
`76.20/(77 .26)
`6.48/ (6 .57)
`62 .84/(62 .68)
`19.42/(19.:17)
`45.48/(45.65)
`34.21/(34.:12)
`28 .79/(28 .57)
`49.72/(49.33)
`I 0.37/(1 0.25)
`66.20/(65.42)
`
`Total
`
`99.28
`100.6
`99.79
`101.2
`101.5
`
`0
`
`20
`
`x=O.O
`100 120 140 160
`80
`60
`40
`Capacity I Ah kg·'
`
`Figure 4. Charge-discharge curves for the sample with compo(cid:173)
`sition of x = 0 (LiCoO,) and 0.1 for Li(Li.,Mn,,,Co,.,)O,.
`
`adopted the rock salt structure. In the system of this study, all of
`the solid solutions have a layered structure.
`Table 1 shows the chemical analysis data of the solid
`solutions . It is assumed that cobalt cations were trivalent and
`manganese cations were tetravalent . The compositions
`determined were in good agreement with those calculated on the
`basis of the solid solutions Li(Li,.,Mn""Co 1.,)0,.
`The performance of the solid solution as the cathode of the
`lithium battery was examined. Figure 4 shows the charge and
`discharge curves for the sample with a composition of x = 0.1
`for Li(Li,nMn,,,;Co,.,)O, in comparison with those of LiCoO, .
`Two samples were charged up to 4.3 V and discharged down to
`3.0 V. LiCoO, showed a small jump at about 4.1 V in the
`discharge curve due to a transformation from trigonal to
`monoclinic.' This change was not observed by the formation of
`the solid solution. It should be noted that a remarkable high
`discharge and charge rates of LiCoO, were kept after the
`formation of solid solutions, though the discharge capacity
`decreased by 10.%.,The maintenance ratio of discharge capacity
`at the tenth cycle was 98 .% for two samples shown in Figure 4.
`More detailed electrochemical and physical characteristics of
`the solid solutions are under investigation and will be reported
`near future.
`
`References and Notes
`I
`J. R. Dahn, E. W . Fuller, M. Obrovac, and U. von Sacken,
`Solid State Ionics , 69, 265 (1994).
`2 R. Kanno, H. Kubo, Y. Kawamoto, T. Kamiyama, F. Izumi,
`Y. Takeda, and M. Takano, J. Solid State Chern., 110, 216
`(1994).
`3 F . Coowar, J. M . Tarascon, W . R . McKinnon, and D.
`Guyomard, Mat. Sci. Forum, 152-153,213 (1994).
`4 K. Mizushima, P. C . Jones, P. J. Wiseman, and J. B.
`Goodenough, Mat. Res. Bull., 15, 783 (1980)
`5 G. Amatucci, J. Tarascon, and L. Clein, Solid State Ionics,
`83, 167 (1996).
`6 R. Stoyanova, E . Zhecheva, and L. Zarkova, Solid State
`Ionics, 73, 233 (1994).
`7 P. Strobel and B. Lambert-Andron, J. Solid State Chern., 75,
`90 (1988).
`J. N. Reimers and J. R. Dahn, J. Electrochem. Soc., 139,
`2091 (1992).
`
`8
`
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