Electrolyte polymers for lithium rechargeable batteries

Details Electrolyte polymers for lithium rechargeable batteries Electrolyte polymers for lithium rechargeable batteries

2000-02-02

2003-10-07

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S323000, C429S212000, C429S249000, C526S242000, C526S255000

Reexamination Certificate

active

06630271

ABSTRACT:

The present invention relates to the use of electrolyte polymers having improved conductivity in electrochemical cells incorporating said electrolyte polymer.
In particular, the invention relates to electrolyte polymers combining high mechanical properties with improved conductivity at room temperature and higher; said improved conductivity remains at high values even after thermal ageing cycles up to 80° C.
More specifically, the electrolyte polymers of the invention are used for the preparation of membranes and electrodes in lithium rechargeable batteries.
The use, in the form of film compositions, of electrolyte polymers to form the separatory membrane of a rechargeable battery cell is known in the art. A separatory membrane constituted by said electrolyte polymers, comprises a polymeric matrix which is ionically conductive due to the incorporation therein of an organic solution of a disassociable lithium salt which supplies the ionic mobility.
A polymeric matrix suitable for the use in a cell of a rechargeable battery should have the properties to be commercially available at low cost, to be electrochemically usable within a large potential range, to have an high melting temperature and low creep, to have good mechanical properties and to be able to absorb an high amount of liquid electrolyte in order to guarantee an high ionic conductivity and at the same time to remain easily processable.
In this field polyethylenoxide-based (PEO) electrolyte polymers wherein the crystalline polymer is swollen with the electrolytic solution, have been developed. The films obtained by said electrolyte polymer have good conductivity but unsuitable mechanical properties and poor electrochemical stability.
An alternative type of electrolyte polymer has been proposed in U.S. Pat. No. 5,296,318, wherein for the preparation of polymeric membranes of lithium rechargeable batteries vinylidenfluoride (VDF)-based copolymers are used. In particular in the above patent, copolymers containing VDF in amounts in the range 75%-92% by weight and hexafluoropropene (HFP) in amounts in the range 8%-25% by weight are described. These copolymers allow the preparation of polymeric electrolytes constituted by the copolymer itself and by a solvent, for example an ethylencarbonate/propylencarbonate (EC/PC) mixture, containing a lithium salt. The polymeric membrane of this composition can be obtained by casting, by dissolving the swollen polymer with an ethylencarbonate and propylencarbonate mixture wherein the lithium salt is dissolved in a solvent, for example tetrahydrofurane (THF). Subsequently the solution coating takes place on a substratum and the solvent (THF) is evaporated. In this way a polymeric film of about 100 micron is obtained. The disadvantage of these copolymers resides in that they show a low conductivity.
In the U.S. Pat. No. 5,418,091 and U.S. Pat. No. 5,456,000 a method for preparing polymeric membranes always containing a VDF/HFP copolymer and a method for preparing the battery itself, is described. In particular the polymeric membranes are prepared by an extraction/activation process whereby a suitable plasticizer is added to the polymer in an amount 20%-70% by weight. In this step the electrolyte polymer is in an unactivated form and the storage of the battery components using the electrolyte polymer itself is possible. In the unactivated form it contains said plasticizer, for example dibutylphthalate, and the activation occurs with the plasticizer extraction from the polymer and its substitution with the electrolyte solution formed by the EC/PC mixture containing the lithium salt.
The optimal amount of HFP used in the above mentioned patents for the polymerization with VDF ranges from 8% to 25% by weight in order to obtain electrolyte polymer compositions having suitable mechanical properties. When the HFP comonomer amount is lower than 8% by weight, the obtained film causes a limited retention of the high boiling solvent containing the lithium salt and therefore a reduced ionic conductivity. On the other hand, when the HFP comonomer amount is higher than 25% by weight, the obtained electrolyte polymer does not produce the formation of a film having a suitable mechanical strenght if it is not subjected to a subsequent crosslinking process by radiation.
In the mentioned patents the whole battery assembling by applying the extraction/activation process also in the anode and cathode preparation, is also described. The two electrodes are obtained by depositing, for example by casting, the polymer containing the plasticizer on the metal substrata. The separator is then placed between the two electrodes and subsequently the polymeric films present on all the battery components are subjected to a melting process, such as for example coextrusion, which favours the adhesion between the separator and the electrodes. The battery is then activated by replacing the plasticizer with the electrolyte solution containing the lithium salt. An example of preparation of these batteries according to the mentioned patents is described by J. M. Tarascon et al. in Solid State Ionic 86-88 (1996) pages 49-54.
The electrolyte polymer is therefore a system comprising the polymer itself, solvents and/or plasticizers, such as ethylencarbonate, propylencarbonate, etc. and by lithium salts, such as LiSO
3
CF
3
, LiN(SO
2
CF
3
)
2
, LiPF
6
, etc.
The drawback of the VDF/HFP electrolyte polymers of the prior art is that, even though they show good mechanical properties, they have not an high ionic conductivity. Another drawback is that the conductivity quickly decreases when the electrolyte polymer is subjected to ageing cycles at high temperatures, for example 80° C.
The need was felt to have available electrolyte polymers having an improved ionic conductivity combined with good mechanical properties; preferably characterized by improved conductivity values even after thermal ageing cycles at high temperatures, up to 80° C.
The Applicant has surprisingly found electrolyte polymers based on VDF copolymers, showing an improved conductivity, preferably maintaining said improved conductivity values even after thermal ageing cycles at high temperatures.
An object of the present invention is therefore the use of vinylidenfluoride (VDF) copolymers for preparing electrolyte polymers, wherein the VDF copolymers have the following polymeric structure (I):
(CH
2
—CF
2
)
m
—(CXY—C(OR
f
)Z)
n
—(CF
2
—CF(CF
3
))
p
  (I)
wherein:
R
f
is a perfluoroalkyl group having a number of carbon atoms from 1 to 3; X, Y, and Z equal to or different from each other are selected from F, Cl or H, preferably F;
m,n and p are integers defining the number of corresponding monomeric units present in the polymer, n or p can be 0 but not simultaneously; the sum of the repeating units m,n,p is such as to give a copolymer having a melt flow index (MFI), measured at 230° C. according to the ASTM D-1238 method under a 10 kg load, in the range 0.1-100, preferably 0.5-50;
said VDF copolymers being obtainable by emulsion polymerization at 75-120° C., preferably 95°-120° C., in the presence of organic initiators.
The VDF amount in the polymeric structure (I) expressed by moles and identifiable by NMR
19P
is in the range 92%-99% by moles and the comonomers amount is in the range 1-8% by moles, preferably the comonomers amount is in the range 2-5% by moles.
In the present invention the VDF comonomers can be selected from hexafluoropropene (HFP) and/or perfluoroalkylvinylethers of formula (II):
CXY═CZOR
f
  (II)
wherein: X, Y, Z, R
f
have the above defined meaning.
The preferred comonomers are perfluoroalkylvinylethers of formula (II) wherein X, Y and Z are fluorine atoms and specifically: perfluoromethylvinylether (MVE) wherein R
f
═CF
3
, perfluoroethylvinylether (EVE) wherein R
f
═CF
2
CF
3
, perfluoropropylvinylether (PVE) wherein R
f
═CF
2
CF
2
CF
3
. Also a mixture of the mentioned comonomers can be used.
The Applicant has surprisingly and unexpectedly found that the use of the above preferred comonomers and of the

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