Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-08-03
2001-02-06
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C429S231100, C429S231800, C429S336000, C429S337000, C429S342000
Reexamination Certificate
active
06183907
ABSTRACT:
The present invention relates to a novel binder for producing an electrode for use in an electrochemical system with a non-aqueous electrolyte, in particular a rechargeable cell with an organic electrolyte.
The present invention also relates to any electrochemical system comprising at least one such electrode.
An electrode in a conventional rechargeable cell with an organic electrolyte contains an electrochemically active material which constitutes a host structure which cations, for example lithium cations, can insert into and leave from during cycling.
Each electrode is obtained by depositing on a current collector a paste containing the electrochemically active material, possibly conductive additives, a polymer binder and a diluent.
The first purpose of using a polymer binder to produce the electrode is to ensure cohesion of the active material, which is in powder form, without masking a large portion of the electrochemically active surface. This effect depends on the wetting properties of the binder. A good bond with the active material is generally guaranteed by the existence in the polymer binder of groups giving rise to chemical bonds or to hydrogen bonds, such as hydroxyl groups, carboxyl groups and amide groups. A compromise must be reached, however, since too strong an interaction of the binder with the active material leads to too much coverage which reduces the active surface and, as a result, the capacity at high rates.
The polymer binder must also allow the paste to adhere to the current collector.
The polymer binder must also endow the electrode with sufficient flexibility to enable it to be manipulated, in particular to withstand winding when producing spiral wound cells.
The polymer binder must accommodate the dimensional variations in the active material during charge-discharge cycles.
These four aims must be satisfied before the cell is assembled and throughout its operation.
The polymer binder must also possess certain electrochemical properties.
In particular, the polymer binder must be compatible with the electrolytes used, preferably without necessitating a cross-linking step which would complicate the electrode formulation and the method used.
Finally, since the reducing agents and oxidizing agents used as the active materials are very powerful, the binder must be as unreactive as possible in order to tolerate extreme operating conditions without degrading.
It thus appears difficult for a single polymer binder to satisfy all of these aims, and some of them appear to be contradictory.
As an example, the electrochemical stability of the electrode on cycling necessitates strong bonds between the polymer binder and the active material. The presence of functional groups accomplishes this function. However, strong inter-chain bonds generally lead to a mechanically rigid structure.
In conventional lithium cells, the binder usually used is a polymer such as polytetrafluoroethylene (PTFE), which has excellent stability in the electrolyte.
However, the antiadhesive properties of PTFE preclude the use of a thin conductive support such as a strip, which is indispensable for producing high energies per unit volume.
Further, the stability of PTFE as regards strong reducing agents is low because of the presence of many fluorine atoms.
Polyvinylidene fluoride (PVDF) and its copolymers, described in document EP-0 492 586, are used as electrode binders in a preparation process employing a metal strip.
This process consists of dissolving the PVDF in a solvent then adding the active material to obtain a paste. The metal strip is thinly coated with the paste comprising the active material and the polymer solution. The solvent is then eliminated by drying the electrode. Dense, thin electrodes are thus obtained.
The mechanical and electrochemical properties of PVDF mean that a good compromise between the multiple criteria described above can be achieved.
However, adhesion to the strip remains poor because of the low surface tension of PVDF, and adhesion promoters have to be added.
Further, the stability of PVDF towards reducing agents is not satisfactory because of the presence of fluorine, leading to safety problems in the event of thermal runaway.
More recently, other polymers have been proposed as a binder for the electrode.
Polyacrylonitrile (PAN) produces excellent electrochemical stability because of its strongly polar groups but the electrode is rigid.
Similarly, polyacrylic acid (PAAc), polyacrylamide (PAA) and their copolymers lead to extremely stable but rigid structures which necessitate the addition of a plasticizer.
Elastomers such as ethylene/propylene/diene terpolymer (EPDM), styrene/butadiene rubber (SER), acrylonitrile/butadiene rubber (NBR) and styrene/butadiene/styrene (SBS) block copolymers or styrene/acrylonitrile/styrene (SIS) block copolymers give the electrode excellent mechanical properties but greatly reduce capacity during cycling.
Document EP-0 606 533 describes polyimides which can produce a good compromise between the mechanical and electrochemical properties but which require a high temperature cross-linking step. Further, polyimides are expensive polymers.
Similarly, mixtures of polymers with complementary properties have been proposed as an electrode binder in which the formulation comprises:
at least one polymer belonging to the non-fluorinated elastomer group which provides the electrode with flexibility, such as ethylene/propylene/diene (EPDM) terpolymer, polyurethane (PU), neoprene, polyisobutylene (PIB) and butyl rubber, and
a polymer which is capable of making strong bonds with the active material, containing groups which can form hydrogen bonds such as OH, COOH, CONH
2
and NH groups, for example polyvinylalcohol, polyacrylic acid, phenolic resins, polyamides and their copolymers.
The main drawback of such mixtures is the difficulty of producing a homogeneous paste because of the incompatibility of the constituents, along with the difficulty of predicting the distribution of binders in the final electrode, the different components having different solubility or coagulation properties during the solvent elimination step.
The present invention concerns a binder which can satisfy all of the criteria described above.
Document U.S. Pat. No. 5,262,255 describes the use of acrylonitrile-butadiene rubber as a binder for a negative electrode.
Document JP-04 342 966 describes the use of a styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) binder with a degree of polymerization in the range 100 to 2 000 and an average molecular weight in the range 25 000 to 400 000.
The binder represents 0.5% to 5% by weight of the electrode.
As mentioned above, electrodes for conventional rechargeable organic electrolyte cells are obtained by depositing a paste containing the active material, the binder and a diluent on a current collector.
After depositing this paste, the electrode is dried to eliminate the diluent. The electrode obtained is thus constituted by active material and binder. With the aim of optimizing the electrochemical performance of the electrode, in particular its capacity per unit mass, attempts have been made to reduce the proportion of binder.
The present invention is based on the discovery that for a comparable, low amount of binder (3% by weight or less), the electrochemical performance of an electrode containing an acrylonitrile-butadiene/carboxymethylcellulose rubber are better than an electrode containing a styrene-butadiene/carboxymethylcellulose binder, in particular in terms of its capacity per unit mass and aging on cycling.
The present invention is also based on the discovery that when the carboxymethylcellulose used in association with the acrylontrile-butadiene rubber has an average molecular weight (MW) of less than 130 000 the electrochemical performance of the electrode containing the binder is insufficient for the envisaged applications.
The present invention provides a binder for an electrode in an electrochemical system with a non-aqueous electrolyte that contains acrylonitrile-butadiene rubber and carboxymeth
Barusseau Sylvie
Martin Florence
Simon Bernard
Alcatel
Sughrue Mion Zinn Macpeak & Seas, PLLC
Weiner Laura
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