FR2615658A1 - INSOLUBLE MIXED HEAVY METAL POLYSULFIDE CATHODES - Google Patents

Abstract

AS CATHODES WITH VERY HIGH ENERGY DENSITY, IN NON-AQUEOUS ELECTROCHEMICAL CELLS, POLYSULFIDES OF MIXED HEAVY METALS WHICH ARE INSOLUBLE IN NON-AQUEOUS ELECTROLYTES AND COMPLY WITH THE FORMULA: M(1)NM(2)M(S) )-2, IN WHICH M(1) AND M(2) ARE DIFFERENT HEAVY METAL ATOMS, N AND M ARE INTEGER NUMBERS RESPECTIVELY REPRESENTING THE VALENCE STATES OF M(1) AND M(2), W AND X ARE INTEGER NUMBERS RESPECTIVELY REPRESENTING THE STOECHIOMETRY OF M(1) AND M(2) IN POLYSULFIDE, S IS SULFUR, NW MX 2Z AND Y IS GREATER THAN 4.5.

Description

The present invention relates to metal swellings and to

  peculiar to metal polysulfides used as cathode materials in non-aqueous electrochemical cells. Elemental sulfur has an extremely high theoretical electrochemical capacity (1.6 A-hr / g) and has in fact been used as a cathode in electrochemical cells. However, such

  sulfur use has been limited by various drawbacks which

  strongly limit the possibility of effectively reaching such a high capacity and which, moreover, have prevented the use of sulfur in numerous battery applications. Sulfur is almost an insulator having a very low ionic and electronic conductivity, while at least an electronic conductivity of the cathode material is necessary to achieve a reasonable use of this material. As a result, sulfur cathodes required

  massive inclusions, reducing capacity and formed by conduc-

  electronic donors without cathodic activity. In applications

  in solid electrolyte batteries it was still necessary to provide other conduc-

  ionic contents of low capacity or without cathodic activity, reducing agents

of capacity.

  In addition to its low conductivity, sulfur has a relatively high vapor pressure and dissolution rate with the resulting tendency to reduce battery life by internal short-circuiting, especially when stored at temperatures high. As a result, sulfur cathodes have generally been used only in high temperature cells, in which the sulfur is found in the molten state during operation with increased conductivity, and in which

  the molten sulfur is necessarily completely enclosed.

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  - 2 - To use at least partially the inherent capacity of sulfur, sulphides of

  metals, such as PbS, AgS, etc., especially in electric cells

  solid trolyte. Although these materials do not exhibit the detrimental high vapor pressure or dissolution rate of elemental sulfur, they also do not exhibit higher capacities.

  or less close to the theoretical capacity of sulfur.

  Disulfides of metals, such as FeS2, CoS2 and NiS2, due to their relatively higher sulfur content, give higher capacities than monosulfide materials and have been used effectively in batteries, especially in batteries operating at high temperatures. The theoretical capacity of FeS2 for example is 0.730 A-hr / g, a capacity of about 0.700 A-hr / g being actually obtained. The capabilities of

  these subjects could not yet compare favorably

  that of elemental sulfur.

  Another class of metal sulfides includes transition metal intercalating compounds. This class is best exemplified by titanium disulfide (TiS2). Cathodes formed from these materials are most suitable for rechargeable cells because of the total reversibility of intercalation reactions with alkali metal ions. However, these materials give less primary capacity than other metal sulfides since the sulfur itself does not participate in the reaction

electrochemical cells.

  Another class of sulfides are alkali metal polysulfides, such as Li2Sx and Na2Sx, or x41.

  of metals which are used as cathodes in electro-

  chemicals. These materials have in fact shown relatively good capabilities but several significant disadvantages. support their use. Non-aqueous electrolytes from cells with alkali metal polysulfide cathodes tend to become

  increasingly viscous with a concomitant loss of conductivity

  speed and a significantly reduced capacity of discharge rate. In addition, the alkali metal polysulfides are at least partially

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- 3 -

  soluble in common electrolyte solvents and are therefore

  quent, prone to open battery discharge

  over extended periods of time.

  U.S. Patent No. 4,481,267 to the Applicant describes a new class of materials formed by metal polysulfides, which are used as

  cathodes in nonaqueous electrochemical cells. These poly-

  Metal sulfides are characterized by a sulfur / transition metal atomic ratio, which is greater than 3.5 / 1. Examples of such polysulfides are CoS45, NiS45, CuS37 and FeS 5 These materials come closest to the theoretical capacity of sulfur, presenting

  theoretical capacities slightly higher than 1.0 A-hr / g.

  These heavy metal polysulfides can pre-

  trim by precipitation from a mixture of an aqueous solution of metal chloride with an aqueous solution of polysulfide. Of

  suitable polysulfide solutions are prepared from poly-

  ammonium sulfide or sodium polysulfide for example. In

  a pending patent application in the United States of America M-

  3912, a process for the preparation of heavy metal polysulphides using ammonium polysulphide is described. This process produces products which, except heavy metal polysulfide, are volatile and can be removed by heating, thereby simplifying separation. When polysulfides prepared by this process or by the process described in U.S. Patent No. 4,481,267 are not heated, they have a sulfur / metal ratio

  ranging from 3.5 / 1 to 5/1. Although these materials have catholic activity

  As described in US Patent No. 4,481,267, it has been found that there is less degradation on storage if the heavy metal polysulfides are first heated to

  a constant weight value before being transformed into cathodes.

  Heating under vacuum to a constant weight removes weakly bound sulfur and produces polysulfides - with a proportion of

  sulfur between 3.5 / 1 and 4.5 / 1.

  In general, metal polysulfides

  heavy mixtures of the present invention correspond to the formula stoechio-

2 6 1 5 65 8

  - 4 - metric M (l) M (2) S o M (1) and M (2) are atoms of different heavy metals, S denotes sulfur, and y is greater than or equal to 4.5. When mixed heavy metals are used in the preparation of the polysulfide, it has been found that it has a higher sulfur content after heating under vacuum than when a single heavy metal is used. This in turn gives the heavy metal polysulfides

  mixed higher electrochemical capacity.

  The present invention was made when a solution containing salts of two different heavy metals was mixed with an aqueous solution of sodium polysulfide. More particularly, when an aqueous equimolar solution of FeSO4 and CoSO4 was mixed with an excess of an aqueous solution of sodium polysulfide, there was a precipitation of a material which, after washing and drying, was found to be Co2.3Fe 0.7S15. This material has a sulfur / metal atomic ratio of 5/1 after drying under vacuum at 110 C, which is higher than the ratio normally found in simple heavy metal polysulfides after drying under vacuum at 110 C. of concentration

  salts in the initial solution brings about a change in the composition

  sition of mixed heavy metal polysulfide. In addition, a change of metals in the salts leads to mixed heavy metal polysulfides of a different composition. It is obvious that any metal salt which is soluble in water could be used in the preparation of a mixed heavy metal polysulfide. Such salts are in particular the salts of copper, titanium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium,

cobalt, rhodium and nickel.

  In addition to the preparation from aqueous solutions, it is also possible to prepare mixed heavy metal polysulfides from organic solvents, such as dimethylformamide or diethyl ether. Instead of ammonium polysulfide, sulfur could be supplied from H2S and

  from elemental sulfur itself.

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  An advantage of the present invention is that mixed heavy metal polysulfides can be prepared and used

  as cathodically active materials in electrochemical cells.

  The electrolyte used in batteries could be aqueous, non-aqueous or solid. When the electrolyte is nonaqueous, the anode can be

  made of an alkali or alkaline earth metal, such as lithium.

  An additional advantage is that the polysulfides

  mixed heavy metals can be prepared more economically

  mique as simple heavy metal polysulfides. For example, leo es Co27 is a polysulfide which is suitable as a cathode material but is expensive due to the cost of cobalt. By replacing part of the cobalt with a different and cheaper heavy metal, like

  iron, the cost of cathode material decreases.

  Another advantage is that mixed heavy metal polysulfides do not always show properties similar to those of the corresponding simple metal polysulfides. As

  example, Fe3S8 is very sensitive to air and breaks down easily

  lement. On the other hand, a cobalt-iron polysulphide does not have

this issue.

  An object of the present invention is to provide materials formed by mixed heavy metal polysulfides, which are useful as cathodically active substances in

electrochemical cells.

  Another object of the present invention is to provide materials formed by polysulfides, which are less expensive to produce than the previously known polysulfide materials.

  while ensuring at least the same capacity.

  These goals and advantages will become apparent from the following Examples. It should be understood that these Examples are

  simply illustrative and that we could prepare other poly-

  mixed heavy metal sulfides. Accordingly, the details described in these examples are not intended to constitute any limitations of the present invention. Unless otherwise indicated,

  all parts are parts by weight.

6 -

Example I

  An aqueous solution is prepared which is equimolar

  in FeSO 4 and in CoSO4. An aqueous solution of poly-

  sodium sulfide. The added amount of sodium polysulfide is less than the amount required to precipitate all of the Co2 + and Fe2 + ions. A black precipitate is formed which is separated by filtration. The black precipitate is washed and then dried under vacuum at 110 ° C. until a constant weight is obtained. The mixed polysulfide resulting from heavy metals was analyzed and it was found that it corresponds to the formula Co2.3Fe0 7S15. This polysulfide has a sulfur / metal atomic ratio of 5/1, a value which is higher than that found

  generally in simple heavy metal polysulfides.

Example 2

  A solution of (NH4) 2MoS4 in dimethylformamide (DMF) is prepared. To this solution, we add a solution

  of elemental sulfur in DMF. Add a sufficient amount

2 -2

  health of sulfur to bring the anion MoS42 to the anion MoS 2. The anion of polysulfide.precipitates in the form of (NH4) 2MoS9 and one separates

42 9

  this by filtration. The (NH4) MoS9 is then added to a solution of CuCl2 in DMF. CuCI2 is in molar excess with respect to (NH4) 2MoS9. Diethyl ether is added until the solution becomes cloudy. This solution is then cooled and crystals of CuMoS9 are formed. This mixed metal polysulfide

  heavy has a sulfur / metal atomic ratio of 4.5 / 1.

  Example 3 An aqueous ammoniacal solution is prepared

  NH4VO3. This solution is then saturated with H2S for.

  form (NH4) V3S4 which precipitates as dark crystals.

  These dark crystals are separated from the solution by filtration. The separated crystals are then dissolved in a dilute aqueous solution of sodium hydroxide. Then sulfur is added with stirring to form (NH4) 3VS8. A solution of cobalt (II) chloride complexed with ammonia is then added to form Co3 (VS)

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  Comparative Example A Batteries of the pellet type were prepared, having an external diameter of 24.5 mm and a height of 3 mm, these batteries each containing an anode formed from a lithium sheet (440 mA- hr) pressed on a nickel grid welded to the internal bottom of the battery compartment. This cell contained a 0.75 M LiCIO4 electrolyte in propylene carbonate and demethoxyethane (1/1 by volume ratio). The cathode consisted of 100 mg of Co257

  shaped like a disc with a surface area of 3 cm2.

  One battery was discharged at a high discharge rate (500.f) and gave 800 mA-hr / g up to a blocking voltage of I V. Another battery was discharged at a low discharge rate (2 kSI) and gave 1000 mA-hr / g up to a blocking voltage of I V. These two discharges have a main voltage plateau at 1.8 V and a shorter plateau at 1.4 V. Comparative example B Pellet-type batteries were made in the same manner as for the batteries of Comparative Example A, except that the cathode consisted of 100 mg of Fe3S8. A battery has been discharged

  at a high rate (500A) and another battery at a low rate (2 k L).

  Each discharge gave approximately the same capacity of 900 mA-hr / g up to a blocking voltage of 1 V. The discharges had two voltage plates of approximately equal duration, the first at 1.6 V and the second at 1.4 V. Example 4 Batteries of the pellet type were manufactured in the same manner as for the batteries of Comparative Examples A and B, except

  that the cathode material consisted of 100 mg of Co Fe S15.

2.3 0.7 15 '

  One battery was discharged at a high discharge rate (500f0) and gave 1100 mA-hr / g up to a blocking voltage of I V. A second battery was discharged at a low discharge rate (2 k) and gave 1300 mA-hr / g up to a blocking voltage of I V. The two discharges had two voltage plates of roughly equivalent duration, the first at 1.8 V and the second at 1, 4 V. These results provide two proofs that Co 2, 3Fe0,7S15 is a

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  -8- distinct new material and not a mixture of cobalt and iron polysulfides. The first proof is that the capacity is greater than the sum of the capacities of the necessary quantities of cobalt and iron polysulfides. The second is the absence of a 1.6 V voltage plateau which would exist if there were an iron polysulfide

any present.

  Materials formed by metal polysulfides

  heavy mixtures of the present invention are suitable for use

  both in aqueous electrochemical cells and in nonaqueous electrochemical cells, since these materials are insoluble in common aqueous and non-aqueous solvents. Such solvents are propylene carbonate, acetonitrile, dimethoxyethane, dioxolane, range-butyrolactone, tetrahydrofuran, methyl formate, dimethyl sulfoxide, sulfur dioxide, aqueous alkaline solutions, etc. In addition, such mixed heavy metal polysulfides are useful as high cathodes.

  capacity in solid electrolyte batteries, in which the electro-

  lyte consists of metallic salts with ionic conduction in the state

solid, such as Lil.

  To take full advantage of the high energy densities of the materials formed by polysulphides according to the present invention, it is preferable that these materials are used in batteries.

  non-aqueous with alkali or alkaline metal anodes

  earthy, for example lithium, these materials giving batteries having typical voltages between 1.5 and 2 V. Although the mixed heavy metal polysulfides cited in the Examples are limited to only two different heavy metals, it is possible to prepare metal polysulfides

  mixed heavy consisting of three or more different heavy metals.

  This could be done by preparing a solution of three or more heavy metal salts, from which the mixed polysulfide of

heavy metal is precipitated.

  The preceding examples are given by way of illustration.

  lustration of the present invention. Changes could be made to specific heavy metals,

  component reports, battery structure, component parts

  supervision of these, etc., without thereby departing from the scope of the

present invention.

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Claims (11)

CLAIMS I. Electrochemical cell comprising an anode, an electrolyte and a solid active cathode, characterized in that this cathode consists of one or more mixed heavy metal polysulfides corresponding to the formula: M (l) w + nM (2) X + m (Sy) z-2 in which M (l) and M (2) denote different heavy metal atoms, n and m are whole numbers representing the valence states of M (I) and M ( 2), w and x are whole numbers respectively representing the stoichiometry of M (l) and M (2) in the polysulfide, S denotes sulfur, nw + mx = 2z and y is greater than 4.5.   2. Electrochemical cell according to claim 1, characterized in that the aforementioned heavy metals are chosen from the group comprising copper, vanadium, molybdenum, iron and cobalt.   3. Electrochemical cell according to one or the other   of claims I and 2, characterized in that the anode consists   of a metal chosen from alkali and alkaline earth metals.   4. Electrochemical cell according to claim   3, characterized in that the anode is made of lithium.   5. Electrochemical cell according to any one   claims 1 to 4, characterized in that one of the metals heavy is cobalt.   6. Electrochemical cell according to any one   claims 1 to 4, characterized in that one of the metals heavy is molybdenum.   - 7. Electrochemical cell according to any one   claims 1 to 4, wherein one of the heavy metals is vanadium.   8. Electrochemical cell according to any one   claims 1 to 4, characterized in that one of the metals heavy is iron.   9. Electrochemical cell according to any one   claims 1 to 4, characterized in that one of the metals 2 6 1 5 6 5 8 - 1I1 - heavy is copper.   O. Electrochemical cell according to any one   of claims I to 4, characterized in that M (l) is iron and M (2) is cobalt.   11. Electrochemical cell according to any one   of the preceding claims, characterized in that the electrolyte   is a fluid and the mixed heavy metal polysulfide is substantially insoluble in this electrolyte.   12. Electrochemical cell according to claim   11, characterized in that the fluid electrolyte is non-aqueous.   13. Electrochemical cell according to any one   of claims I to 10, characterized in that the electrolyte is solid matter.   14. Non-aqueous electrochemical cell, characterized in that it comprises a lithium anode, a non-aqueous liquid electrolyte, and a cathode made up of one or more mixed heavy metal polysulphides having an empirical stoichiometric formula   M (l) M (2) Sy, in which M (1) and M (2) are different heavy metals   rents selected from the group comprising copper, vanadium, molybdenum, iron and cobalt, S is sulfur and is greater there or equal to 4.5.