Electrolyte system for lithium batteries, the use thereof,...

Details Electrolyte system for lithium batteries, the use thereof,... Electrolyte system for lithium batteries, the use thereof,...

2001-06-19

2004-01-13

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S199000, C429S200000, C429S330000, C429S331000, C429S326000, C029S623100

Reexamination Certificate

active

06677085

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to electrolyte systems for lithium batteries with enhanced safety, their use, and a method for enhancing the safety of lithium batteries.
Portable high-value electronic devices, such as mobile telephones, laptop computers, camcorders, etc. are enjoying a very fast growing market. An adequate electrical supply for these devices requires light, high-capacity and high-quality power sources. For environmental and economic reasons, secondary rechargeable batteries are overwhelmingly used. There are essentially three competing systems: nickel cadmium, nickel metal hydride, and lithium ion batteries. An additional interesting field of use for these battery systems could be their use in electrically operated vehicles.
Due to its outstanding performance characteristics, the lithium battery has already acquired large market shares, although it was introduced in the market in its current state of the art only in 1994. Despite the triumphant success of the secondary lithium battery, one cannot overlook the fact that it has safety-related problems:
Rechargeable lithium batteries typically contain a compound of lithium oxide and metal oxide as the cathode (e.g., Li
x
MnO
2
or Li
x
CoO
2
) and lithium metal as the anode. The lithium is preferably used in the form of an intercalation compound with graphite or with carbon or graphite fibers. An overview of the use of such batteries is given by K. Brandt (Solid State Ionics 69 (1994), 173-183, Elsevier Science B. V.).
According to the current state of the art, the electrolyte liquids, which are used to achieve high conductivity, are preferably solvent mixtures of at least two or more components. The mixture must contain at least one strongly polar component, which due to its polarity has a highly dissociative effect on salts. The polar components that are typically used are ethylene carbonate or propylene carbonate. These highly polar solvents are relatively viscous and have usually a relatively high melting point, e.g., 35° C. for ethylene carbonate.
To ensure adequate conductivity even at lower temperatures of use, one or more low-viscosity components are generally added as “thinners.” Typical thinners include, for instance, 1,2-dimethoxyethane, dimethyl carbonate ordiethyl carbonate. Usually the thinners are added in a proportion of 40-60% of the total volume. A serious drawback of these thinner components is their high volatility and their low flash point. 1,2-dimethoxyethane has a boiling point (BP) of 85° C., a flash point (FP) of −6° C., and an explosion limit between 1.6 and 10.4% by volume; dimethyl carbonate has a BP of 90° C., and an FP of 18° C. For these “thinners” there are currently no equivalent substitutes.
Since the electrochemical use of electrolyte solutions and, to a far greater extent, the occurrence of faults (short circuits, overcharging, etc.) always generates heat, this implies—particularly if a cell bursts open and solvent spills—a risk of ignition with the corresponding serious consequences. The currently used systems basically avoid this by using costly electronic controls. Nevertheless, some accidents caused by fire are known to have occurred, particularly during manufacture where large amounts of solvents are handled, but also during the use of rechargeable lithium batteries.
A greater source of risk during use is created in electrical vehicle applications. Here, substantially greater amounts of electrolyte liquid per energy storage device are required, and electronic control of many interconnected cells is far more difficult and involves correspondingly greater risks.
To enhance safety, the cathode and anode space can be separated by a microporous separator membrane, which is made in such a way that the current flow is interrupted by the melting of the pores when a certain temperature limit is exceeded. Suitable membranes of this type are found, for instance, in the Celgard® line of Hoechst Celanese Corporation.
The safety of lithium batteries can be further enhanced by pressure relief devices that respond to gas development if the battery is overcharged and, as mentioned above, by electronic monitoring and control devices.
Also recommended are flame-retardant additives containing phosphorus and halogen, but these often have a negative effect on the performance characteristics of the batteries.
All of these measures, however, cannot exclude the possibility that the highly volatile and flammable “thinner” ultimately may be ignited in case of malfunctions and after rupture of the cell cause a fire that is difficult to control with common extinguishing agents. Burning lithium reacts violently not only with water but also with carbon dioxide, which is used in many fire extinguishers.
The following documents are cited as representative of the prior art:
JP-A-7 249432=D1
EP-A-0 631339=D2
EP-A-0 599534=D3
EP-A-0 575591=D4
U.S. Pat. No. 5,169,736=D5
B. Scrosati, ed., 2nd International Symposium on Polymer Electrolytes, Elsevier, London and New York (1990)=D6
U.S. Pat. No. 5,393,621=D7
JP-A-06020719=D8
U.S. Pat. No. 4,804,596=D9
U.S. Pat. No. 5,219,683=D10
JP-A-5 028822=D11, and
EP-A-0-821368=D12.
D1 and D2, for instance, propose highly fluorinated ethers as electrolyte solvents or as additives to other electrolytes. In general, these substances are thermally and chemically very stable and have high flash points. However their solvent power is far too low for the required lithium electrolyte salts, so that they cannot be used alone, and they are poorly miscible with conventional battery solvents.
Partially fluorinated carbonates are also described as electrolytes having an increased flash point (D3). The problem here is that the compounds, which apparently are suitable based on their low viscosity, have only a moderately increased flash point (37° C.) and their electrical conductivities are clearly below those of the prior art (assuming that the reported measurements were taken at room temperature which seems likely since no temperature was specified).
Carbamates are also described as thinners for anhydrous electrolytes (D4). They have higher boiling points compared to the currently used thinners, but only marginally improved flash points.
D8 discloses ester compounds of the formula R
1
COOR
2
as electrolytes for secondary lithium batteries, in which at least one of the groups R
1
and R
2
has a fluorine substitution. A preferred compound is trifluoroacetic acid methyl ester. However, this compound has a boiling point of only 43° C. and a flash point of −7° C., which presents a high safety risk in case of damage.
According to the present state of the art, reduced flammability of the electrolyte solution is primarily achieved by increasing the viscosity of the electrolyte solution with the aid of binders or fillers or the use of polymer electrolytes, which are practically solid at room temperature.
D5, for instance, describes organic or inorganic thickeners (polyethylene oxide, SiO
2
, Al
2
O
3
and others) for solidifying liquid electrolyte solutions.
Polymer electrolytes comprising macromolecules with numerous polar groups, such as polyethylene oxides, as they are known from D6, are also far less flammable due to their low volatility. One also frequently finds diacylated diols or monoacylated diol monoalkyl ethers as the monomer components for producing such a gel-like polymer electrolyte. In these substances the acyl component carries a double bond (i.e., it is, for example, an acrylic acid or methacrylic acid). Examples of such systems include references D11 and D12.
D7 describes polymer electrolytes comprising polar macromolecules formed by polymerization of organophosphorus compounds, which are characterized by their particularly low flammability.
All of these gel-like to solid electrolytes have in common that due to their high viscosity, the mobility of the ions of the salts dissolved in them is far lower than in liquid electrolyte solutions. As a result, particularly

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