Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2000-12-20
2003-07-22
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S308000, C252S062200
Reexamination Certificate
active
06596440
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention concerns a polymer electrolyte, a battery cell comprising said polymer electrolyte and a method of producing said polymer electrolyte. The polymer electrolyte is suitable for electrochemical devices such as, batteries, particularly for secondary batteries of high energy density.
BACKGROUND OF THE INVENTION
A battery is usually composed of a number of elementary units called electrochemical cells. Each of these cells consists of a positive electrode, a negative electrode and an electrolyte solution, in which the two electrodes are immersed with or without the interposition of a separator. The most important function of the separator is to prevent electronic contact between different plates and to absorb the electrolyte. Moreover, it is also important to keep the resistance as low as possible.
By the term “battery” is meant herein, a collection of two or more cells connected together with electrically conductive material, placed in a case.
There are two main types of batteries, primary batteries and secondary batteries; however in the following, only secondary batteries will be considered. Secondary batteries can be charged by a source of electrical energy, from which batteries the energy can be recovered. Secondary batteries are also called accumulators, or rechargeable batteries. The latter term will be used in the following.
Rechargeable batteries are often used as power supply in portable communication equipment, such as cellular phones, personal pagers, portable computers and other electrical devices, such as smart cards, calculators etc.
In a rechargeable battery, ions of a source electrode material move between electrodes through an intermediate electrolyte during the charge and discharge cycles of the cells. During discharge, the electricity-producing reactions cause reversible changes in the composition of die electrodes and the electrolyte. During charging, these changes can be reversed to the original conditions. The electrochemical reactions take place both at the negative electrode (which is the anode in the discharging mode and the cathode in the charging mode) and at the positive electrode of the electrochemical cell.
Research and development are now being made on lightweight and high-voltage secondary batteries having improved design flexibility. The main battery characteristics to be improved by new research are size, weight, energy density, lower discharge rates, cost, environmental safety and working life. Lithium metal secondary batteries are promising power sources because of their high energy density. In general, such a lithium battery employs lithium metal as its negative electrode and an organic solution containing a lithium salt as its electrolyte. Dendrites (crystals with tree-like branches) are commonly generated on the lithium metal surface during repeated charge and discharge cycles when lithium metal is used as negative electrode in a lithium secondary battery, which results in a short circuit within the battery.
Most attention has now been focused on lithium ion secondary batteries using a negative electrode comprising a carbon material being a host for inserted lithium ions. These systems utilize an intercalation and de-intercalation reaction of the lithium ions in the host. The lithium ion secondary batteries generally have a lower theoretical negative electrode capacity than the lithium metal secondary battery, but is superior in cycle characteristic and system reliability. Frequently, lithium ion secondary battery cells employ organic electrolytic solutions as their electrolytes. However, the use of an organic liquid electrolyte imposes problems associated with the reliability of the battery system, e.g. leakage of the electrolyte out of the battery, vaporization of the solvent of the electrolyte, and dissolution of electrode material in the electrolytic solution. Since the electrolyte contains a flammable organic solvent the leakage of the solvent may result in ignition. While better manufacturing techniques have decreased the occurrences of leakage, lithium ion secondary battery cells still can leak potentially dangerous electrolytes. Battery cells using liquid electrolytes are also not available for all designs and do not have sufficient flexibility.
Conversely, solid polymer electrolytes are free from problems of leakage. They have, however, inferior properties as compared to liquid electrolytes. For example, conventional solid polymer electrolytes have ionic conductivities in the range of 10
−6
to 10
−4
S/cm at ambient temperature whereas acceptable ionic conductivities are >10
−3
S/cm. High ionic conductivity is necessary to ensure a battery system capable of delivering usable amounts of power for a given application. It is also necessary for the high rate operation demanded by, for example, mobile phones. Accordingly, present solid polymer electrolytes are not adequate for many high performance battery systems. By “solid” polymer electrolyte is meant a polymer electrolyte without any solvent or plasticizer, while a polymer “gel” electrolyte comprises a solvent or a plasticizer, in this application. When using the term “polymer electrolyte” both solid and gel electrolyte are included.
While solid polymer electrolytes are intended to replace the combination of liquid electrolytes and separators used in conventional batteries, the limitations described here have prevented them from being fully implemented. One class of polymer electrolytes, namely polymer gel electrolytes, have shown some promises. There is however a disadvantage with a poor compatibility with the anode. The reason for poor compatibility is the building up of a passivating layer on the surface of the anode.
Polymer gel electrolytes are formed by tapping an electrolyte, i.e., an organic solvent mixture containing dissolved lithium salt, in a polymer matrix. Such a polymer matrix consists of, for example, poly(acrylontrile) (PAN), poly(methyl methacrylate) (PMMA), a copolymer of poly(vinylidene fluoride) (PVdF) and hexafluoropropene (HFP) (Kynarflex®). The immobilization procedure varies from case to case and includes UV crosslinking, casting and gelation. There is no molecular interaction between these polymers and the electrolyte and the electrolyte solution, and the polymer gel electrolyte can be considered as a two-phase system.
Polymer gel electrolytes have promising properties in terms of conductivity and electrochemical stability (wide operational voltage window). In principle, these properties make the polymer gel electrolytes suitable for use in different types of high-energy lithium batteries. Many of the polymer gel electrolytes can be considered as basically two-phase materials where the polymer is a passive component acting as a rigid matrix incorporating regions of electrolyte. These is two-phase materials do not offer sufficient long-term stability due to phase separation. Another disadvantage of conventional polymer gel electrolyte is that the reliability of the battery cells is low due to their poor chemical compatibility with the electrodes. The reason for the poor compatibility is the build-up of passivation films, mainly at the interface between the negative electrode and the polymer gel electrolyte, because of the high content of organic solvent. The passivation film consists of a primary inorganic layer and a second layer of organic nature. The second layer is probably not evenly distributed over the electrode surface, and areas with varying thickness are present This second layer increases in thickness with cycling of the battery cell, and this increase is regarded as the main problem when using polymer gel electrolytes in secondary lithium polymer batteries due to the concurrent loss of capacity. Approaches to reduce the problem, by addition of inorganic and organic additives, or replacing the reactive organic solvents with less reactive ones, have not been successful.
U.S. Pat. No. 5,587,253 discloses a lithium ion battery with an electrolyte/separator composition comprisi
Gavelin Patrik
Wesslén Bengt
Jenkens & Gilchrist P.C.
Ryan Patrick
Telefonaktiebolaget LM Ericsson (publ)
Yuan Dah-Wei D.
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