Solid composite electrolytes for lithium batteries

Details Solid composite electrolytes for lithium batteries Solid composite electrolytes for lithium batteries

2000-06-28

2001-02-20

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S323000, C252S062200

Reexamination Certificate

active

06190806

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to solid composite electrolytes for use in rechargeable lithium batteries, and more particularly, to a polymer-ceramic composite and ceramic-ceramic composite which exhibit enhanced low-temperature ionic conductivity and low temperature dependence of conductivity.
Widespread interest has existed in the use of solid electrolytes for use in lithium batteries and other high-energy-density power sources. Various classes of materials have been proposed for use including polymers, ceramics, and polymer-ceramic composites, particularly polymer electrolytes. Solid polymer electrolytes generally consist of a high molecular weight polymer such as polyethylene oxide complexed with a lithium salt. However, the conductivity of such electrolytes has been marginal for low (ambient) temperature applications. In addition, such electrolytes possess a low cationic transport number, they exhibit poor interfacial stability with lithium electrodes, and they have a very high activation energy (high temperature dependence) for lithium ion conduction at low temperatures.
Polymer-ceramic composite electrolytes are a known sub-class of solid polymer electrolytes which are formed by incorporating a ceramic material in the polymer matrix to enhance conductivity. For example, ceramic additives such as Al
2
O
3
, LiAlO
2
, SiO
2
, and zeolite have been used in small amounts to increase the room temperature conductivity of composite electrolytes. See Capuano et al. “Composite Polymer Electrolytes”,
J. Electrochem. Soc
. 138, 1918 (1991) which teaches the incorporation of &ggr;-Al
2
O
3
and LiAlO
2
in a poly(ethylene oxide) polymer.
The use of lithium nitride (Li
3
N) has also been proposed for use in composites as it has an high ionic conductivity at ambient temperatures of the order of about 10
−3
S cm
−1
. See Skaarup et al. “Mixed Phase Solid Electrolytes”,
Solid State Ionics
, 28-30, 975 (1988), which teaches a polymer composite containing Li
3
N. See also commonly assigned U.S. Pat. No. 5,695,873, which teaches a polymer-ceramic composite electrolyte containing lithium nitride. However, while the incorporation of ceramic materials in composite electrolytes results in increased conductivity as compared with solid polymer electrolytes, up until now such increases have been relatively marginal, even when such electrolytes have been subjected to low temperature annealing. In addition, the processing of such polymer-ceramic composite electrolytes in thin film applications has been limited due to the relatively large volume fraction of lithium nitride used as well as the brittleness of lithium nitride.
Accordingly, there is still a need in the art for a solid composite electrolyte for use in lithium batteries and other electrochemical applications which may be easily manufactured and which exhibits high conductivity at ambient temperatures and low temperature dependence of conductivity.
SUMMARY OF THE INVENTION
The present invention meets those needs by providing solid composite electrolytes which exhibit moderate to high conductivity at ambient temperature and low activation energies for lithium transport, i.e., low temperature dependence of conductivity. The solid composite electrolytes may be used in a variety of electrochemical applications, particularly lithium rechargeable batteries.
According to one aspect of the present invention, a solid composite electrolyte is provided comprising at least one lithium-containing phase and having an activation energy from about 0.10 eV to about 0.50 eV. By activation energy, it is meant the energy needed for transport of lithium ions through the composite electrolyte. In one embodiment of the invention, the solid composite electrolyte is a polymer-ceramic composite electrolyte having an activation energy of about 0.5 eV and comprising from about 30 to 70% by weight poly(ethylene oxide) (PEO), from about 10 to 20% by weight lithium tetrafluoroborate (LiBF
4
), and from about 5 to 40% by weight titanium dioxide (TiO
2
). Preferably, the titanium dioxide is in the form of a powder having a particle size of about 5 to 30 nm, and more preferably, about 17 nm.
In a preferred embodiment of the invention, the polymer-ceramic composite electrolyte preferably comprises about 60% by weight poly(ethylene oxide), about 10% by weight lithium tetrafluoroborate, and about 30% by weight titanium dioxide.
In an alternative embodiment of the invention, the polymer-ceramic composite electrolyte comprises from about 30 to 70% by weight poly(ethylene oxide), from about 10 to 20% by weight lithium tetrafluoroborate, and from about 5 to 40% by weight of a ceramic material selected from the group consisting of MgO, ZnO, SrO, BaO, CaO, ZrO
2
, Al
2
O
3
, SiO
2
, SiC, Si
3
N
4
, and BN.
Preferably, the polymer-ceramic composite electrolyte of the present invention is in the form of a thin film which has been annealed such that the film has a room temperature conductivity of the order of about 10
−5
S cm
−1
to 10
−3
S cm
−1
. By room temperature conductivity, it is meant that the film exhibits high conductivity at temperatures ranging from about −40° to 40° C. The film is preferably about 100 &mgr;m thick.
In another embodiment of the invention, the solid composite electrolyte comprises at least one lithium-containing phase and a phase containing a ceramic material selected from the group consisting of MgO, ZnO, SrO, BaO, CaO, ZrO
2
, Al
2
O
3
, SiO
2
, SiC, Si
3
N
4
, and BN.
In an alternative embodiment of the invention, the solid composite electrolyte is a ceramic-ceramic composite electrolyte which comprises from about 40 to 90% by weight lithium nitride and from about 10 to 60% by weight lithium phosphate. More preferably, the ceramic-ceramic composite electrolyte comprises about 60% lithium nitride and about 40% lithium phosphate.
The ceramic-ceramic composite electrolyte is preferably in the form of a compressed disc which is from about 0.05 to 0.1 cm thick. The compressed disc has preferably been annealed such that it exhibits a room temperature conductivity of the order of about 10
−6
S cm
−1
to about 10
−7
S cm
−1
. The ceramic-ceramic composite electrolyte exhibits a very low activation energy of 0.1 eV.
The solid composite electrolytes of the present invention have been found to exhibit excellent electrode-electrolyte interfacial stability, and because they exhibit low activation energies, they may be effectively used in batteries and other electrochemical devices operating at low temperatures.
Accordingly, it is a feature of the present invention to provide solid composite electrolytes for use in lithium batteries having enhanced low-temperature ionic conductivity and low activation energies. This, and other features and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.


REFERENCES:
patent: 4049881 (1977-09-01), Rao et al.
patent: 5314765 (1994-05-01), Bates
patent: 5576115 (1996-11-01), Capuano et al.
patent: 5695873 (1997-12-01), Kumar et al.
patent: 5728489 (1998-03-01), Gao et al.
Kumar et al., Journal of Power Sources 52 (1994) 261-268, 1994, No Month.
Krawiec et al., Journal of Power Sources 54 (1995) 310-315, 1995, No Month
Quartarone et al., Solid State Ionics 110 (1998) 1-14, 1998, No Month.
Kumar et al., Solid State Ionics 124 (1999) 239-254, 1999, No Month.
SKAARUP et al., “Mixed Phase Solid Electrolytes ”, Solid State Ionics, 28-30 (1998) pp. 975-978. No Month Available.
CAPUANO et al., “Composite Polymer Electrolytes”, J. Electrochem. Soc., vol. 138, No. 7, Jul. 1991. No Month Available.
PHIPPS et al., (Solid States Ionics 5 (1981) 393-396). No Month Available.
KRAWIEC et al., (Journal of Power Sources 54 (1995) 310-315). No Month Available.

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