Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1997-12-24
2003-02-04
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C429S224000, C429S221000
Reexamination Certificate
active
06514640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to secondary (rechargeable) alkali-ion batteries. More specifically, the invention relates to materials for use as electrodes for an alkali-ion battery. The invention provides transition-metal compounds having the ordered olivine, the modified olivine or the rhombohedral NASICON structure and containing the polyanion (PO
4
)
3−
as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.
2. Description of the Related Art
Present-day lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li
+
ions to the Li
+
-ion electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li
+
ions are inserted reversibly from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible. On charge, removal of electrons from the cathode by an external field releases Li
+
ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li
+
ions back into the anode to restore it to its original composition.
Present-day rechargeable lithium-ion batteries use a coke material into which lithium is inserted reversibly as the anode and a layered or framework transition-metal oxide is used as the cathode host material (Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/or Ni are expensive and may degrade due to the incorporation of unwanted species from the electrolyte. Oxides such as Li
1±x
[Mn
2
]O
4
, which has the [M
2
]O
4
spinel framework, provide strong bonding in three dimensions and an interconnected interstitial space for lithium insertion. However, the small size of the O
2−
ion restricts the free volume available to the Li
+
ions, which limits the power capability of the electrodes. Although substitution of a larger S
2−
ion for the O
2−
ion increases the free volume available to the Li
+
ions, it also reduces the output voltage of an elementary cell.
A host material that will provide a larger free volume for Li
+
-ion motion in the interstitial space would allow realization of a higher lithium-ion conductivity &sgr;
Li
, and hence higher power densities. An oxide is needed for output voltage, and hence higher energy density. An inexpensive, non-polluting transition-metal atom would make the battery environmentally benign.
SUMMARY OF THE INVENTION
The present invention meets these goals more adequately than previously known secondary battery cathode materials by providing oxides containing larger tetrahedral oxide polyanions forming 3D framework host structures with octahedral-site transition-metal oxidant cations, such as iron, that are environmentally benign.
The present invention provides electrode material for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte. The cell may additionally include an electrode separator. As used herein, “electrochemical cell” refers not only to the building block, or internal portion, of a battery but is also meant to refer to a battery in general. Although either the cathode or the anode may comprise the material of the invention, the material will preferably be useful in the cathode.
Generally, in one aspect, the invention provides an ordered olivine compound having the general formula LiMPO
4
, where M is at least one first row transition-metal cation. The alkali ion Li
+
may be inserted/extracted reversibly from/to the electrolyte of the battery to/from the interstitial space of the host MPO
4
framework of the ordered-olivine structure as the transition-metal M cation (or combination of cations) is reduced/oxidized by charge-compensating electrons supplied/removed by the external circuit of the battery in, for a cathode material, a discharge/charge cycle. In particular, M will preferably be Mn, Fe, Co, Ti, Ni or a combination thereof. Examples of combinations of the transition-metals for use as the substituent M include, but are not limited to, Fe
1−x
Mn
x
, and Fe
1−x
Ti
x
, where 0<x<1.
Preferred formulas for the ordered olivine electrode compounds of the invention include, but are not limited to LiFePO
4
, LiMnPO
4
, LiCoPO
4
, LiNiPO
4
, and mixed transition-metal compounds such as Li
1−2x
Fe
1−x
Ti
x
PO
4
or LiFe
1−x
Mn
x
PO
4
, where 0<x<1. However, it will be understood by one of skill in the art that other compounds having the general formula LiMPO
4
and an ordered olivine structure are included within the scope of the invention.
The electrode materials of the general formula LiMPO
4
described herein typically have an ordered olivine structure having a plurality of planes defined by zigzag chains and linear chains, where the M atoms occupy the zigzag chains of octahedra and the Li atoms occupy the linear chains of alternate planes of octahedral sites.
The present invention additionally provides electrode material for a rechargeable electrochemical cell including an anode, a catrode and an electrolyte where the material has a modified olivine structure. The pristine olivine structure of LiMPO
4
may be modified either on the anionic site or on the cationic site to provide an alternative lithium insertion-type. It is also envisioned that the pristine olivine structure may be modified on both the anionic and the cationic sites. Preferably, the structure is modified by aliovalent or isocharge substitutions to provide better lithium ion diffusitivity and electronic conductivity.
In general, “isocharge substitutions” refers to substitution of one element on a given crystallographic site with an element having a similar charge. For example, Mg
2+
is considered similarly isocharge with Fe
2+
and V
5+
is similarly isocharge with P
5+
. Likewise, PO
4
3
tetrahedra can be substituted with VO
4
3
tetrahedra. “Aliovalent substitution” refers to substitution of one element on a given crystallographic site with an element of a different valence or charge. One example of an aliovalent substitution would be Cr
3+
or Ti
4+
on an Fe
2+
site. Another example would be Li
+
on a Fe
2+
site. These cathode materials will generally have an olivine structure based on iron or manganese derivatives whose general formula is:
Li
x+y
M
1−(y+d+t+q+r)
D
d
T
t
Q
q
R
r
[PO
4
]
1−(p+s+v)
[SO
4
]
p
[SiO
4
]
s
[VO
4
]
v
where
M may be Fe
2+
or Mn
2+
or mixtures thereof;
D may be a metal in the +2 oxidation state, preferably Mg
2+
, Ni
2+
, Co
2+
, Zn
2+
, Cu
2+
, or Ti
2+
;
T may be a metal in the +3 oxidation state, preferably Al
3+
, Ti
3+
, Cr
3+
, Fe
3+
, Mn
3+
, Ga
3+
, Zn
3+
, or V
3+
;
Q may be a metal in the +4 oxidation state, preferably Ti
4+
, Ge
4+
, Sn
4+
, or V
4+
;
R may be a metal in the +5 oxidation state, preferably V
5+
, Nb
5+
, or Ta
5+
;
In this preferred embodiment, M, D, T, Q and R reside in octahedral sites. The additional coefficients may be defined as follows: x represents the degree of intercalation during operation of the electrode material; y represents the fraction of lithium ions on the initial Fe
2+
sites; d represents the fraction of divalent ions (noted as D) on the initial Fe
2+
sites; t represents the fraction of trivalent ions (noted as T) on the initial Fe
2+
sites; q represents the fraction of tetravalent ions (noted as Q) on the initial Fe
2+
sites; r represents the fraction of pentavalent ions (noted as R) on the initial Fe
2+
sites; p represents the frac
Armand Michel
Goodenough John B.
Masquelier Christian
Nanjundaswamy Kirakodu S.
Padhi Akshaya K.
Board of Regents , The University of Texas System
Chaney Carol
Katten Muchin Zavis & Rosenman
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