Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-04-23
2002-05-21
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100
Reexamination Certificate
active
06391493
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 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 is 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.
In another aspect, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials comprise a rhombohedral NASICON material having the formula Y
x
M
2
(PO
4
)
3
, where 0≦x≦5. Preferably, the compounds of the invention will be useful as the cathode of a rechargeable electrochemical cell. The alkali ion Y may be inserted from the electrolyte of the battery to the interstitial space of the rhombohedral M
2
(XO
4
)
3
NASICON host framework as the transition-metal M cation (or combination of cations) is reduced by charge-compensating electrons supplied by the external circuit of the battery during discharge with the reverse process occurring during charge of the battery. While it is contemplated that the materials of the invention may consist of either a single rhombohedral phase or two phases, e.g. orthorhombic and monoclinic, the materials are preferably single-phase rhombohedral NASICON compounds. Generally, M will be at least one first-row transition-metal cation and Y will be Li or Na. In preferred compounds, M will be Fe, V, Mn, or Ti and Y will be Li.
Redox energies of the host M cations can be varied by a suitable choice of the XO
4
polyanion, where X is taken from Si, P, As, or S and the structure may contain a combination of such polvanions. Tuning of the redox energies allows optimization of the battery voltage with respect to the electrolyte used in the battery. The invention replaces the oxide ion O
2−
of conventional cathode materials by a polyanion (XO
4
)
m−
to take advantage of (1) the larger size of the polyanion, which can enlarge the free volume of the host interstitial space available to the alkali ions, and (2) the covalent X—O bonding, which stabilizes the redox energies of the M cations with M-O-X bonding so as to create acceptable open-circuit voltages V
oc
with environmentally benign Fe
3+
/Fe
2+
and/or Ti
4+
/Ti
3+
or V
4+
/V
3+
redox couples.
Preferred formulas for the rhombohedral NASICON electrode compounds of the invention include, but are not limited to those having the formula Li
3+x
Fe
2
(PO
4
)
3
, Li
2+x
FeTi(PO
4
)
3
, Li
x
TiNb(PO
4
)
3
, and Li
1+x
FeNb(PO
4
)
3
, where 0<x<2. It will be understood by one of skill in the art that Na may be substituted for Li in any of the above compounds to provide cathode materials for a Na ion rechargeable battery. For example, one may employ Na
3+x
Fe
2
(PO
4
)
3
, Na
2+x
FeTi(PO
4
)
3
, Na
x
TiNb(PO
4
)
3
or Na
1+x
FeNb(PO
4
)
3
, where 0<x<2, in a Na ion rechargeable battery. In this aspect, Na
+
is the working ion and the anode and electrolyte comprise a Na compound.
Compounds of the invention having the rhombohedral NASI
Goodenough John B.
Masquelier Christian
Nanjundaswamy K. S.
Pahdi Akshaya K.
Chaney Carol
Katten Muchin & Zavis
The University of Texas Systems
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