Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-09-12
2003-07-08
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C429S218100, C429S231300
Reexamination Certificate
active
06589499
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lithium metal oxides for use as positive electrode materials for lithium and lithium-ion secondary batteries, and to methods of making lithium metal oxides.
BACKGROUND OF THE INVENTION
Lithium metal oxides of the formula LiMO
2
, wherein M is a transition metal, are important cathode (positive electrode) materials for rechargeable lithium and lithium-ion batteries. Examples of LiMO
2
compounds include LiCoO
2
, LiNiO
2
, and LiMnO
2
. Presently, LiCoO
2
is used in most commercial lithium and lithium-ion batteries as a cathode material.
LiMO
2
compounds can have different crystal structures and phases, even within the same compound. For example, LiCoO
2
synthesized at greater than 700° C. has a hexagonal layered structure analogous to &agr;-NaFeO
2
. LiCoO
2
synthesized at around 400° C., however, has a cubic spinel-like structure analogous to Li
2
Ti
2
O
4
. Both structures have essentially the same FCC (face centered cubic) closed packed arrangement for oxygen except the layered structure has a small distortion in the direction perpendicular to the layers. Additionally, the two structures differ in cation arrangement.
It has been determined that the cubic spinel-like LiCoO
2
turns into hexagonal layered LiCoO
2
when heated to temperatures above 700° C. Therefore, phase transformation between the two structures is possible and the layered structure is energetically favored only at high temperatures. Layered LiCoO
2
also has an energetically favored tendency of changing into spinel LiCo
2
O
4
when 50% of the lithium ions are removed from the LiCoO
2
during electrochemical charging. See A. van der Ven et al., Phys, Rev. B 58, 2975 (1998); and H. Wang et al., J. Electrochem. Soc., 146, 473 (1999). The spinel-like LiCoO
2
and spinel LiCo
2
O
4
also have essentially the same atom arrangement except that lithium is at the octahedral
16
c
site in spinel-like LiCoO
2
and at tetrahedral
8
a
site in spinel LiCo
2
O
4
.
The tendency of the phase transformation from hexagonal layered LiMO
2
to cubic spinel-like LiMO
2
is not unique to LiCoO
2
. Layered LiMnO
2
also turns into spinel-like LiMnO
2
only after a few cycles in an electrochemical cell. Although a cubic spinel-like LiNiO
2
has not been experimentally observed, Li
0.5
NiO
2
(50% delithiated LiNiO
2
) will indeed turn into LiNi
2
O
4
spinel.
The electrochemical performance of LiMO
2
compounds having a cubic spinel-like structure has been found to be particularly poor, especially compared to layered structures. Moreover, the mere presence of the cubic spinel-like structural phase within the layered phase or on the surface of the layered phase has also been found to be detrimental to battery performance. In particular, the presence of cubic spinel-like phases within the layered crystal structure impedes the diffusion of lithium ions during the charge and discharge cycles of the rechargeable lithium or lithium-ion battery. Furthermore, because the cubic spinel-like phase is energetically favored and only kinetic limitations prevent large scale phase transformation, the presence of localized cubic spinel-like structures can act as a seed for phase transformation to readily occur in the LiMO
2
compound. Therefore, even the minor presence of cubic spinel-like phases, even at levels that cannot be detected by bulk techniques, such as powder x-ray diffraction (XRD), can cause problems in battery cycling.
SUMMARY OF THE INVENTION
The present invention provides lithium metal oxides that are substantially single-phase compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases. Therefore, the lithium metal oxides of the invention have more consistent electrochemical performance than prior art compounds. In addition, the lithium metal oxide compounds of the invention have good structural stability and maintain their structure through cycling. Therefore, the lithium metal oxides of the invention are useful for rechargeable lithium and lithium ion secondary batteries.
The lithium metal oxides of the invention have the formula Li
&agr;
M
&bgr;
A
&ggr;
O
2
, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.51 ≦N ≦+3.5, 0.90 ≦&agr;≦1.10 and &bgr;+&ggr;=1. As measured using powder x-ray diffraction, the Li
&agr;
M
&bgr;
A
&ggr;
O
2
compounds according to the invention preferably have no diffraction peaks at a smaller scattering angle than the diffraction peak corresponding to Miller indices (003). In addition, the ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (110) to the integrated intensity of the diffraction peak corresponding to Miller indices (108) using powder x-ray diffraction is preferably greater than or equal to 0.7, more preferably greater than or equal to 0.8. The ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (102) to the integrated intensity of the diffraction peak corresponding to Miller indices (006) using powder x-ray diffraction is preferably greater than or equal to 1.0, more preferably greater than or equal to 1.2. The average oxidation state of the dopants N is preferably about +3.
In one preferred embodiment of the invention, the Li
&agr;
M
&bgr;
A
&ggr;
O
2
compound is LiCoO
2
. As measured using electron paramagnetic resonance, the LiCoO
2
compounds of the invention typically have a change in intensity from the peak at about g=12 to the valley at about g=3 of greater than 1 standard weak pitch unit, and more typically of greater than 2 standard weak pitch units.
In addition to the Li
&agr;
M
&bgr;
A
&ggr;
O
2
compounds above, the present invention is also directed to the dilithiated forms of these compounds resulting from the electrochemical cycling of these compounds. Specifically, the present invention includes Li
&agr;−x
M
&bgr;
A
&ggr;
O
2
compounds wherein 0≦x≦&agr; that are derived by electrochemically removing x Li per formula unit from a compound having the formula Li
&agr;
M
&bgr;
A
&ggr;
O
2
, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5 ≦N ≦+3.5, 0.90 ≦&agr;≦1.10 and &bgr;+&ggr;=1. The Li
&agr;−x
M
&bgr;
A
&ggr;
O
2
compounds are substantially single-phase lithium metal oxide compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases.
The present invention further includes lithium and lithium ion secondary batteries including a positive electrode comprising a compound having the formula Li
&agr;
M
&bgr;
A
&ggr;
O
2
, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5 ≦N ≦+3.5, 0.90 ≦&agr;≦1.10 and &bgr;+&ggr;=1. The Li
&agr;
M
&bgr;
A
&ggr;
O
2
compound used in the positive electrode has a substantially single phase, hexagonal layered crystal structure and is substantially free of localized cubic spinel-like structural phases.
The present invention further includes a method of preparing compounds having a substantially single phase, hexagonal layered crystal structure that are substantially free of localized cubic spinel-like structural phases. A lithium metal oxide having the formula Li
&agr;
M
&bgr;
A
&ggr;
O
2
, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5 ≦N ≦+3.5, 0.90 ≦&agr;≦1.10 and &bgr;+&ggr;=1, is provided at a temperature of at least about 600° C., and preferably of greater than 800° C. The lithium metal oxide is then cooled at a rate of greater than 8° C./min, preferably between 8° C./min and 140° C./min, more preferably between 10° C./min and 100° C./min. The lithium metal oxide can be synthesized at a temperature of at least about 600
Gao Yuan
Wang Hugh H.
Yakovleva Marina
Bos Steven
FMC Corporation
Myers Bigel & Sibley Sajovec, PA
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