CA1263141A - Cathode including a non fluorinated linear chain polymer as the binder, method of making the cathode, and lithium electrochemical cell containing the cathode - Google Patents

Abstract

ABSTRACT

A cathode suitable for use in a lithium electrochemical cell is made from a mixture of active cathode material, carbon, and non fluorinated linear chain polymer by a method including the steps of (a) dissolving the non fluorinated linear chain polymer in a non polar solvent at a temperature near the melting point of the polymer; (b) adding the active cathode material and carbon and evaporating the solvent; and (c) grinding the dried mixture into a fine powder and making it into a cathode by pressing the powdered mixture onto both sides of an expanded metal screen and then cutting to the desired dimensions. The cathode can be combined with lithium as the anode and a solution of 0.8 mol dm-3 LiAlCl4 in a mixed organic solvent of 24 mass percent 4-butyrolactone in 1, 2 dimethozyethane as the electrolyte to provide a mechanically stable, relatively inexpensive lithium electrochemical cell having good cell performance.

Description

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This invention relates to a cathode including a non fluorinated linear chain polymer as the binder, to a method of making the cathode, and to a lithium electrochemical cell containing the cathode.
Existing technology for the fabrication of cathodes for use in lithium primary and secondary cells utilizes Teflon, a trademark for polytetrafluoroethylene as the binding material.
Teflon is expensive although inert, and its use results in cathode structures of poor mechanical stability. These problems do not easily lend themselves to the large scale production of cathodes in manufacturing.
; The general object of the invention is to provide a cathode suitable for use in lithium electrochemical cells.
Another object of the invention is to provide such a cathode that ~1; will be mechanically stable and relatively inexpensive to manufacture. A further object of the invention is to provide such a cathode that will be suitable for use in a primary or secondary .
lithium cell.
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It has now heen Eound that the a~orementioned object~
can be attained by replacing the Teilon binder with polypropylene (PP).
More particularly, according to the invention, a cathode for use in lithium primary or secondary cells can be prepared from a mixture of about 75 to 90 mass percent TiS2, about 10 mass percent carbon, and about 2 to 10 mass percent of polypropylene pressed onto both sides of an expanded metal screen. The polymer is first dissolved in a non polar solvent such as Decali ~ (Decahydronaphthalene) or tetrachloroethylene at a temperature near the melting point of the polymer (100 to 150~C). The active cathode material and carbon are added and the solvent evaporated.
The following is a specific preparation of a cathode structure utilizing TiS2 as the active cathode material. The procedure is performed in an argon filled dry box. Poly-propylene powder is dissolved near its crystalline melting temperature of about 100 to 120C in a small volume of decahydronaphthalene of about 5 mils decahydronaphthalene for abou~ 0.1 to 0.2 gm (PP), the solution being stirred continuously during heating. Once the (PP) i5 dissolved, the solution is removed from the heat and cooled below 100C and the active cathode material and carbon powders are then

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added quickly b~fore the polymer oolution oool~ oompletely. The powd~red material~ and ~olutlons are ~tlrred vigorou~ly by hand untll the aollltlon i~ ~b~orbed and the mix beoome~ ~ranula~ l~nd ha~ coolad to room tamperQture. Tha mlxtura 1~ then driad ln a vaouum ovon at about 120 to 150DC for 12 hour~ in order to remo~e the deoahydronaphthnlene. The dr$ed mixture i9 then ground into B fine powder and made into cathodes by pressing the powdered mi~ture onto both sidas of an eYpanded metal s¢reen and then out to d~ired dimension~.
Although flat plate type eleotrodes have been prep~red in the ~oregoing embodlment to demonstrate the use of the non fluorinated linear ohain polymers as binding materialo, the method re~ults ln moderately fle~ib~e ~truotures whioh ma~e~ the method equally adaptable to the preparation of rolled eleotrodes, eithar oold-rolled or rolled through heated rollers using a powdered mi~ or P slurry mi~ture using a non reaotive organic solvent.
The TA~E shows typioal result~ for a Li-TiS2 cell in which call ; perPormance is studied a~ a funotion of colvent composition, oathode compo~ition, and cathode preparationJ that is, cold-pressing or hot-pres~ing. Per~ormance i~ found to be equal to that obtained from cell~ utlli~ing catho~es o~ poor meohanical properties, that i~, ba~ed on te~lon bindsrs. The TABLE showc that one can operat~ with a wide range of (PP) content in the eleotrode. More ~pacifically~ one can use less binder and ~till obtain a good meohan~oally stable oathode. In fact, one oan go down to as little as 1 to 3 wei~ht percent o~ binder which allows one to put in more actiYe cathode material and improve the performanoe of the cell.

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Tho drawing lllustrat~ a oycling prorile for a reohargeable lithium oell u~ing a ~ cathode ~nd an eleotrolyte oon~l~tin~ of 0.8 mol dm~3 LiAlCl4 in a mixed organlo nolvent of 24 ma~o pero0nt 4-butyrolaotone in 1,2 dimethoxyethane (24~ 4-BL in DME). ~ho oathode (80 maas ~ Ti52~ lO
mas~ ~ carbon and lO ma~ ~ (PP) io prepared a~ de~oribed in the de~oription of the preferred embodiment.
Referring to the drawing, the cell u~ed for tha drawing ia cyoled at 25C at a ourrent den~lty of 2.0 m~ om~2, and exour~ions are shown for di~ohar~es at 5.0mAom~2 and at 2.0 mAom~2 at lowar temperature~
of -20C and -30C. Cycling i~ ~topped after 33 oyoles~ ~he drawing also shows that after excursions to high0r ¢urrent den~itie~ Rnd/or lower temperatursa, cell performAnoe reoovers exceptionally well. In the drawing, the ordinate, percent osthode utilization, i8 an indioator as to how the oell is performing.
Any non fluorinated linear ohaln polymer oan be u~ed in the cathode that i8 stable in tha eleotrolyte of the lithlum oell~ The linear ohain polymer~ ara inert in a wide varlety of non aqueous ~olvents including ether~ and lactones. Suitable polymers inolude (PP) ~nd (PE).
Active cathode materials that c~n be u~ad include metal halidas1 metal oxides, arld metal sulfidea of which ~iS2 is preferred.
Pure carbon can be used for those cells in whioh the solvent ~erves ~s the depol~ri~er.
Any carbon blacX oan be used ~s the carbon for the oathode that enhanos~ the conduotivity of the eleotrodes. The particular oarbon blaok u~ed~in the de3cription of the preferred embodiment is Shawinigan Black* but other high surface area carbons or graphite can also be used.

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'rhe inv~n-tion even con-templatea a c~thode m~de Prom a mixture of polymer and aotlve oathcdd material with no o~rbon pre~cn-t.
'rhe ~leotrolyte u~ed in the llthium oell muat be compatible with the oathode~ made aooording to the invention. Suitable eleotrolyte~
inolude a solution of an inorganio lithium salt in a pure or mixed organio solv2nt.
In the method o~ the invention, meohanioally ~table struoture~
oan be ea~ily prepared by oold-pressing or oold rolling, and by hot-preaaing or hot-rolling. Moreover, sintering temperature~ below 170C
should be used ~ince some oathode materialj are subjeot to deoomposition above 200C.
Thus, it has been demonstrated that non fluorinated linear chain polymers ~uch a~ (PP) or (PE) can be used to prepare meohanioally ~table oathode~ for nonaqueous lithium oells. ~he uae of the non fluorinated linear chain polymera as binders re~ults in low c08t oathodes giving equal electroch mical per~ormance as do Teflon bonded cathodes, but the use of the non fluorinated linear chain polymers resulta in cathodea having great mechanioal stability that can be ~abricated in aeveral forma such as plates or rolls, and that oan be mads as thin as 0.5 mm or less~
We wish it to be understood that we do not de~ire to be ~ limited to ths exaot details a~ de~oribed for obvious modi~1cations will; occur,to a per~on skilled in the art.

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cathode for use in lithium electrochemical cells, said cathode comprising a mixture of about 75 to 90 mass percent TiS2, about 10 mass percent carbon, and about 2 to 10 mass percent of polypropylene pressed onto both sides of an expanded metal screen.

2. A cathode according to claim 1, in the form of a rolled configuration.

3. A lithium electrochemical cell comprising lithium as the anode, a solution of 0.8 mol dm-3 LiAlCl4 in a mixed organic solvent of 24 mass percent 4-butyrolactone in 1, 2-dimeth-oxyethane as the electrolyte, and a mixture of about 75 to 90 mass percent TiS2, about 10 mass percent carbon and about 2 to 10 mass percent of polypropylene pressed onto both sides of an expanded metal screen as the cathode.

4. Method of preparing a cathode for use in a lithium electrochemical cell from a mixture of TiS2, carbon and poly-propylene, said method including the steps of (a) dissolving polypropylene at 100° to 130°C in a small volume of decahydronaphthalene contained in an argon filled dry box while stirring the solution continuously during heating;
(b) removing the solution from the heat, cooling to below 100°C, and quickly adding the carbon and TiS2 before the polymer solution cools completely;

(c) stirring the powdered materials and solution vigorously until the solution is absorbed and the mix becomes granular and has cooled to room temperature;
(d) drying the mixture in a vacuum oven at 120°C to 150°C for 12 hours in order to remove the decahydronaphthalene;
(e) grinding the dried mixture into a fine powder and making into a cathode by pressing the powdered mixture onto both sides of an expanded metal screen.

5. A method according to claim 4, including the additional step of (f) forming the cathode into a rolled configuration.