Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Flat-type unit cell and specific unit cell components
Reexamination Certificate
1999-07-12
2002-08-13
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Flat-type unit cell and specific unit cell components
C429S237000, C429S127000
Reexamination Certificate
active
06432576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a lithium secondary battery comprising a stack of:
a negative electrode having negative electrode material and a negative current collector;
a positive electrode having positive electrode material and a positive current collector;
a separator sandwiched between the negative and the positive electrode;
a non-aqueous electrolyte solution between the negative and the positive electrode
The invention also relates to a method of manufacturing a lithium secondary battery comprising a stack of a negative electrode, a separator, and a positive electrode, which method comprises applying negative electrode material onto a negative current collector so as to form the negative electrode, applying positive electrode material onto a positive current collector so as to form the positive electrode, and arranging a separator between the negative and the positive electrode so as to be contiguous therewith.
2. Background Art
The need for high-energy density secondary (i.e. rechargeable) batteries is increasing, due to a growing market for lightweight, portable cordless consumer products, e.g. CD-players, mobile telephones, laptop computers and video cameras. For acceptable portability, these batteries should contain the necessary amount of energy at the smallest possible weight and volume. The present rechargeable batteries on the market, e.g. nickel-cadmium (NiCd) and nickel-metalhydride (NiMH), do not meet all these requirements. Moreover, the use of cadmium as the negative electrode material should be avoided for environmental reasons.
A very interesting material for use in batteries is lithium. Lithium is the lightest of all metals, which promises an extremely high theoretical energy density of metallic lithium. Lithium is a leading contender in the field of a battery negative electrode materials, since it has a large negative thermodynamic potential. The use of lithium has no negative environmental consequences. Therefore, rechargeable lithium batteries are very promising, especially when weight is an important factor.
A rechargeable lithium battery consists of a positive and a negative electrode separated by a polymeric film to prevent electronic contact in an organic electrolyte. A lithium transition metal oxide can be used as the positive electrode, and metallic lithium as the negative electrode. The electrolyte is a lithium salt in a non-aqueous organic solvent with good ionic conductivity and negligible electric conductivity. During charging, lithium ions are transported from the positive electrode towards the negative (lithium) electrode. During discharging the lithium ions are transported in the reverse direction and inserted back into the positive electrode.
A battery using lithium metal for the negative electrode encounters the problem of short-circuits in the battery caused by the repetition of the charge/discharge cycles. Repetition of charge/discharge cycles leads to a repetition of the dissolution and precipitation of lithium metal, and dendrites of lithium metal can grow on the surface of the negative electrode. The dendrite grows penetrating through the separator between the negative and the positive electrode, and comes into contact with the positive electrode, resulting in a short-circuit.
Use of a lithium metal alloy, e.g. Li—Al, for the negative electrode instead of lithium metal decreases such growth of dendrites, and improves the charge/discharge cycle characteristic.
A more advanced and safer approach to lithium rechargeable batteries consists of replacing a lithium metal or alloy-type negative electrode by a lithium intercalating compound. When another lithium intercalating compound is used as a positive electrode, this leads to a lithium metal-free rechargeable battery; such a battery is called a Li-ion battery. During charging, lithium ions deintercalate from the positive electrode, and move into the non-aqueous electrolyte. Then the negative electrode intercalates these ions. During discharging the process is reversed. Both electrodes exhibit the so-called intercalation reaction, also known as the host-guest reaction. It does not involve an electrolyte concentration change, nor any dissolution of the active materials into the electrolyte. Therefore, Li-ion batteries sometimes bear the name “rocking-chair batteries”. Carbon materials are good hosts for use as a negative electrode, because they are able to intercalate and deintercalate lithium ions during charging and discharging, respectively, of the battery. In such a negative electrode of carbon the growth of dendrites is prevented, and the problem of a short-circuit in the battery is solved.
In more recent years a new rechargeable lithium battery has been developed which is based on laminates. These very thin and flexible lithium-ion polymer batteries meet the demand for thin batteries of flexible shape for the portable equipment market. Because of the flexibility, it is possible in principle to fold the battery into any desired shape; the battery design is no longer limited to cylindrical or prismatic shape. However, as batteries get thinner, application of external pressure, which is needed to maintain good contact between the electrodes and the electrolyte, becomes difficult. In order to solve this problem, polymer binder is mixed with the electrode materials to produce flexible sheets of negative and positive electrodes. The material for the porous separator is chosen from the same polymer. The sheets servings as the negative electrode, separator, and positive electrode, all of polymeric composition, are laminated together by applying heat and pressure to form a single sheet of battery material. In order to activate the battery, the permeable laminate is immersed in an electrolyte salt solution.
A lithium secondary battery of the type mentioned in the opening paragraph is known from United States patent U.S. Pat. No. 5,478,668. The known battery is a unitary planar laminated structure comprising a polymeric anode layer, a polymeric cathode layer and a polymeric separator layer. In a preferred embodiment, the polymer in the three layers is the same, e.g. a copolymer of vinylidene fluoride and hexafluoropropylene. Lamination of the layers is carried out by applying heat and pressure. The polymer in the layers also comprises a plasticizer, which is extracted by a solvent. The laminate thus treated is then activated by penetration of an electrolyte solution. The use of the same (co)polymer in the three layers ensures a good adhesion, and therefore a good contact, between these layers. The battery obtained is composed of one continuous polymeric phase in which in the electrode regions the active electrode materials are homogeneously dispersed.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a lithium secondary battery which can be produced in a thin and flexible form, and in which the contact between the electrodes and the electrolyte is maintained in an alternative way. Moreover, it is an object of the invention to provide a method of manufacturing such a battery.
These objects are achieved in a battery as specified in the opening paragraph, characterized in that the negative electrode material and the positive electrode material are provided with a pattern of holes, the holes being filled with a polymeric material which sticks and presses the negative electrode, the positive electrode and the separator contiguously together. The holes in the electrode materials are macroscopic holes having a diameter of e.g. 1 mm. In a typical example, the pattern of the holes forms a rectangular two-dimensional array with a mutual hole distance of 5 mm. The holes, at least those facing the separator, are filled with polymeric material, which contacts the separator. The dimensions of the holes and the pattern are chosen in such a way that the active surface of the electrodes amounts preferably to at least 90%, because the holes filled with polymeric material reduces the capacity of the electrodes: in these fill
Bartlett Ernestine C.
Chaney Carol
Koninklijke Philips Electronics , N.V.
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