GB2296368A - Electrochemical cell - Google Patents

Description

1 wELECTROCMMICAL CELL' 2296368 THIS INVENTION relates to an

electrochemical cell. It relates also to a method of manufacturing a positive electrode active mass precursor, to a positive electrode active mass precursor, to a method of making a shaped artefact, to a shaped artefact, and to a positive electrode precursor for an electrochemical cell.

According to a first aspect of the invention, there is provided a method of manufacturing a positive electrode active mass precursor, which method comprises reacting a transition metal halide aqueous solution with an alkali metal hydroxide and, optionally, a metal-free base, to produce an alkali metal halide-containing precipitate; separating the alkali metal halide-containing precipitate from residual liquid by removing residual liquid to a degree which adjusts, or allows adjusting of, the proportions of soluble and insoluble reaction products; k OPSIMM SPEC MSK 44GSK13 D 1M 2 optionally, washing said precipitate to adjust the content of soluble reaction products therein; if necessary, adjusting the water content of the precipitate to render it suitable for shaping; and drying the precipitate and reducing it at temperatures up to the melting point of the alkali metal halide, to form an intimate mixture of transition metal and alkali metal halide having a mean particle size of less than 2 Am.

According to a second aspect of the invention, there is provided a positive electrode active mass precursor, when made by the method according to the first aspect of the invention.

According to a third aspect of the invention, there is provided a method of making a shaped artefact which comprises reacting a transition metal halide aqueous solution with an alkali metal hydroxide and, optionally, a metal-free base, to produce an alkali metal halide-containing precipitate; separating the alkali metal halide-containing precipitate from residual liquid by removing residual liquid to a degree which adjusts, or allows adjusting of, the proportions of soluble and insoluble reaction products; optionally, washing said precipitate to adjust the content of soluble reaction products therein; if necessary, adjusting the water content of the precipitate to render it suitable for shaping; A DPSIMEIL SPEC DISK 44%GSK^ EM 1995 lk 3 shaping the precipitate into a form suitable for location in a cathode compartment of an electrochemical cell housing; and drying the precipitate and reducing it at temperatures up to the melting point of the alkali metal halide, to form an intimate mixture of transition metal and alkali metal halide having a mean particle size of less than 2 gm.

According to a fourth aspect of the invention, there is provided a shaped artefact, when made by the method according to the third aspect of the invention.

The shaped artefact is thus suitable for use as a positive electrode or cathode precursor of an electrochemical cell as hereinafter described.

According to a fifth aspect of the invention, there is provided a positive electrode precursor, which comprises a shaped artefact is according to the fourth aspect of the invention, or a positive electrode active mass precursor according to the second aspect of the invention.

The cathodic or positive electrode active mass precursor does thus not have a definite shape, which distinguishes it from the positive electrode precursor which has a definite shape and occupies the space provided for a cathode in an electrochemical cell housing. The positive electrode or cathode precursor can thus be charged to form the cathode of the resultant cell.

A:WPSIME11. SPEC DISK 44GSK\.Mil D 1995 4 According to a sixth aspect of the invention, there is provided an electrochemical cell which includes a cell housing having an anode and a cathode, with the cathode comprising, at least initially, a positive electrode precursor according to the fifth aspect of the invention.

is The cell can thus be an alkali metal/metal halogenide rechargeable electrochemical cell containing also a molten salt electrolyte, with said molten salt electrolyte comprising at least one salt of said alkali metal. The electrolyte may be molten at the cell operating temperature, and the alkali metal in the cathode may be substituted by an alkali metal alloy. In particular, if the alkali metal is molten at the cell operating temperature, the cell further comprises a solid electrolyte separator separating liquid alkali metal from molten salt electrolyte, with said solid electrolyte being an ionic conductor for ions of said alkali metal, or in the case of an alkali metal alloy, for ions of at least one of said alkali metals.

Such cells are typified by the so-called ZEBRA cell, which incorporates a sodium metal anode and a cathode containing a transition metal chloride, eg iron chloride or nickel chloride.

Generally, an active mass precursor is thus a chemical compound or a mixture of compounds which upon further treatment or processing, eg by chemical or electrochemical redox reaction which may be effected by charging, yields the active mass which A WPSISMEIL SPEC DISK 44XGSK11 D. 1993 is electrochemically active for discharge of the electrode of the cell or battery.

Such a precursor may also comprise constituents which are not transformed into the active mass proper but which are functional in other ways such as furnishing metallic conduction pathways, furnishing mechanical support for the active mass, porifiers, expanders, charge reserve, getters and other additives known in the art to enhance electrode performance.

Thus, a positive active mass precursor for the above type of cells which combines at least one transition metal with an alkali metal halide capable of entering the chemical constitution of the molten salt electrolyte, eg combining nickel or iron with sodium chloride, and a reducing agent such as aluminium, is known. Upon charging, the precursor mix is transformed into the metal halide or halides, ie the positive active mass proper, and the reducing agent is halogenated in situ to form eg aluminium chloride which combines with excess sodium chloride to form the sodium aluminium chloride molten electrolyte, and an equivalent amount of alkali metal such as sodium at the negative electrode.

The present invention thus provides a novel method of manufacturing a positive active mass precursor for such cells. In the method, the alkali metal hydroxide used must be selected so that the alkali metal cations are capable of entering the A MPSIMEIL SPEC DISK 44%GSK^ 1 D. 1995 6 constitution of the molten salt electrolyte when used in an electrochemical cell as hereinbefore described.

The precursor may contain a compound which, when heated, induces reducing conditions for the reduction step and/or under reducing atmosphere becomes or remains electronically conducting.

The invention accordingly provides an electrode precursor and shaped artefact of controllable porosity and pore size distribution, consisting of finely divided metal particles in intimate admixture with at least one alkali metal halide and optional additives known in the art to enhance electrode performance.

According to a preferred version of the invention, the metal component of the precursor consists of finely divided particles of iron and/or nickel and up to 50 mol % of at least one element selected from the group comprising As, Sb and Bi, and, optionally, a minor proportion of aluminium. The finely divided alkali metal halide consists of sodium chloride with minor additions of sodium fluoride, sodium bromide, and any other dopants known in the art. All the additives, from As, Sb and Bi to aluminium and dopant halogenides, may be present already during the precipitation step, or may be added during any step preceding shaping, singly or jointly, provided that an intimate uniform mixture is created.

AAMMEIL SPEC DISK 44XGSKk.AI 1 r 1 ows 7 The preferred application of the electrode precursor of the invention is thus for manufacturing positive electrodes in discharged or overdischarged state in sodium/nickel chloride or sodium/iron chloride batteries. The intimate dispersion of the metal and the metal halide components is designed for, and is capable of providing a large reaction surface as well as a continuous electronically conducting matrix to allow high reaction rates, ie high power density of the electrochemical cell.

Heretofore the positive electrode precursors of such cells were usually prepared by physically mixing the metallic components in particulate form with the alkali metal halides in particulate form. Thus the smallest primary particle size of the metallic component was limited by the availability and the pyrophoric is nature of fine metal powders.

In the case of iron, the metal is deactivated or passivated by surface oxidation processes when handled in air, and more rapidly so when in contact with sodium halides during the processing. By deactivation is meant a degradation of electrochemical performance for charge acceptance which results in reduced capacity yield upon discharge. It will be appreciated that the metal, eg iron, is converted into metal halogenide during charge in this type of cell. Because of this passivation of fine metal powders in air a heat treatment in reducing environment has in the past been effected to remove oxide layers from the metallic A,kDPSISMEIL SPEC DISK 44GSK11 D. 1995 8 surfaces and to decrease the specific resistivity of the electrode. Thus, a reducing step similar to that of the method of manufacturing the active mass precursor according to the invention may also be necessary in the prior state of the art manufacturing process in spite of forming the precursor from a metal powder.

For manufacturing the active mass precursor according to the invention, transition metal chloride solutions obtained from recycling processes of sodium/iron chloride and/or sodium/nickel chloride cells may be used.

In a preferred version of the invention an acidic, aqueous solution of the transition metal salt, particularly of the transition metal chlorides FeC13 and NiC12, may be neutralised during the reaction stage in up to two steps:

In a f irst step, up to the stoichiometric quantity of aqueous sodium hydroxide solution may be added to the metal chloride solution under agitation. Reaction of the sodium hydroxide produces a mixture of sodium chloride in suspension with a precipitate of metal oxyhydroxides and/or hydroxides and/or oxychlorides and/or hydroxychlorides, depending on the course of the reaction.

If the ratio of sodium hydroxide to metal chloride is chosen to be less than stoichiometric, residual metal chloride anions may A:0PSIME1L SPEC MSK 4IbGSK11 D.O 19% 9 be neutralised in a second step to a pH of approximately 7 by the addition of aqueous ammonia solution or an organic base, for example alkyl ammonium compounds. In addition to the metal hydroxides and/or oxychlorides and/or hydroxychlorides that are formed in the first stage of the hydrolysis, the hydrolysis product is enriched by dissolved and/or suspended ammonium chloride or organic base chloride which provides a basis for obtaining a modified pore structure as set out in more detail hereinafter. Therefore, this second precipitation step may be performed to modify the pore structure of the product.

After the precipitation step or steps, the liquid is essentially a more or less saturated alkali salt solution, and the invention contemplates removing the solution by filtration, centrifuging or other mechanical separation means known in the art, to such a degree that the salt content of the remaining solid/liquid residue or "cake" approximates the intended proportions as far as possible. An optional washing step with water may be used to reduce further the salt content of the residue after said separation.

After these operations the wet crude product contains the intended proportions of transition metal compounds and alkali metal halogenides in intimate admixture, but it may not yet be suitable for shaping. Therefore, the water content of the mix may have to be adjusted by suitable means, e.g. lowered by evaporation or A:kDPSIWEIL SPEC DISK 44kGSK.Mi I D. M5 increased by water addition e.g. in a kneader or pug mill, to achieve the proper consistency.

According to the invention the paste or powder mixture thus formed is then shaped into a body which is suitable for use as an electrode. Depending on the viscosity of the material, shaping may involve extrusion, uniaxial pressing, or densification by sintering in a mould.

Suitable shapes include particulate agglomerates, granules and pellets as well as consolidated bodies which may be free-standing in the form of a sheet or extrudate, or may contain an electronically conducting matrix or current collector.

Subsequent to the shaping process the powder or paste artefacts thus formed are subjected to a heating and reduction process at temperatures below the melting point of the alkali metal salt is mixture or sodium chloride. It is known from thermal analysis that the resultant temperature rise results first in loss of physically bound water (drying), followed by loss of chemically bound water (dehydroxylation) prior to reduction, and thus the heating and reduction process may be split into the above steps or stages if this should appear useful, and different apparatus and atmospheres may be employed for said stages. Thus, drying and dehydroxylation may be carried out in flowing air while reduction is performed in a reducing atmosphere. The process conditions for the individual steps depend on the particular A:WPSIMISIL SPEC DISK 44%G51(11 D IM 11 chemistry of the compounds present in the mixture, and may be obtained from literature.

This process generates water vapour and small amounts of hydrochloric acid (HCl) corresponding to the oxychloride content, and ammonia derived from the optional second precipitation step, and the liberation of these gases render the resultant product porous. The nature of the porosity of the artefact after reduction can be controlled by the composition of the hydrolysis mixture formed in the neutralisation stage, as indicated above, namely by adding a second precipitation step e.g. with ammonia or organic base.

The reduction can be induced by heating to about 3000C, when the metal is nickel, or to about 5000C, when the metal is iron, in a hydrogen atmosphere. After the reduction step the precursor artefact consists of a solid porous body made up by an intimate mixture of:

sodium chloride and metal and, optionally, dopants known in the art, and, optionally, a component providing enhanced electronic conductivity.

It is within the scope of the invention to add a component which when heated to temperatures below the melting point of sodium chloride under reducing atmosphere remains, or becomes, an electronically conducting material. Preferably, this material AAMMEIL SPEC DISK 44GSKXRAll D% 1995 12 may be the same metal as the metal formed in the reduction step of the precursor manufacture, or may be a refractory hard metal compound of a transition metal with at least one non-metal of the group consisting of B, C, N, Si and P. The electronically conducting component may have a relatively low surface area when designed to function as an at least partially contiguous current collector eg assuming the shape of a fibre network, expanded metal sheet and similar shapes known in the art for current collectors, or may have a relatively high surface area such as carbon powders and fibres also known in the art as conductivity enhancing additives, either pre-fabricated or generated in situ by pyrolysis of suitable compounds such as furfuryl alcohol.

Depending on its nature, said conductivity enhancing component may be added before or during precipitation or any other step before the mixture loses mouldability by drying.

The invention is now described by way of non-limiting illustration with reference to the following non-limiting examples.

EXAMPLE 1

1 mol of a 40 weight percent aqueous solution of FeC13 (Fluka, Buchs /Switzerland) was boiled and 0.924 mol of a 50 weight percent sodium hydroxide solution (Hoechst AG, Frankfurt (Main) /Germany) was added dropwise while stirring. The solution was allowed to cool down to 250C and neutralised with 2.076 mol of ammonia aqueous solution. After lowering the temperature to A OPSISMEIL SPEC DISK 44XGSKAI D IM 13 5OC the resultant brownish slurry was dripped into liquid nitrogen. The solid material which formed was separated from the liquid nitrogen and dried at a temperature below -300C for two days under reduced pressure. The agglomerated particles thus obtained were densified by uniaxial pressing at 2,55 kilo bar.

By sintering under hydrogen at 6100C for 1 hour (1.5 hours at temperatures higher than 500OC) at an oxygen partial pressure of 2.10-22 bar both NH4C1 volatilisation and reduction of the iron oxide was achieved. The material was not pyrophoric, which is believed to be a consequence of the dispersion of fine iron powder in NaCl.

Electron microscopic photographs of the chemically produced Fe/NaCl mixture revealed that the size of the spherical Fe (<l jum) and cubic NaCl particles (<2 gm) was considerably reduced by this chemical preparation route as compared to mixtures of commercially available powders. In such powder mixtures the particle size of iron is typically in the range of 5 gm to 30 gm.

The charge acceptance of the finely divided powder mixture was tested in an electrochemical cell with the iron/sodium chloride mixture used as cathode and an aluminium plate as anode, with the cathode and anode separated by a porous glass disk and immersed in molten NaAM4 at 2600C. The cathode material was contacted with Ni-felt and Ni powder (Inco W 201) and cycled between 0,4 and 0,9 V, measured against an aluminium reference electrode, with a constant current of 20 mA applied to a pellet of 1 cm A DPSIMEIL SPEC DISK 44GSK1 1 D. 1993 14 cross section and 2mm thickness. The theoretical capacity of the pellet was restricted to 21,01 mAh by the content of NaCl. The measured charge was 20,07 mAh. Thus, more than 95% of the NaCl incorporated was consumed corresponding to 45% of the iron transformed to FeC12.

EXAMPLE 2:

1 mol of a 40 weight percent aqueous solution of NiC12 (OMG Europe GmbH, Dasseldorf, Germany) was neutralised with 2 mol of a 50 weight 6 NaOH aqueous solution (Hoechst AG, Frankfurt/ Germany). The resultant green slurry contained Ni(OH)2 precipitate and NaCl in solution. Subsequently, 1 mol of NaCl, ie half of the sodium chloride, was separated from the remaining cake by filtration. Drying of the moist cake and dehydroxylation, ie conversion of Ni (OH) 2 to NiO was performed by slowly increasing the temperature to 3000C in flowing air. The mean size of the primary NiO particles was 0,2Am as measured by electron microscopy. Reduction with hydrogen at temperatures up to 5000C produced nickel and sodium chloride as a finely divided mixture.

The invention also extends to a composition of matter comprising an intimate mixture of transition metal in fine particulateform. having a primary particle size as measured by electron microscopy of less than 1 micron and fine particles of sodium chloride having a particle size as measured by electron microscopy of less than 2 microns.

AAMIMEIL SPEC DISK 4AGSK D. 1995 is

Claims (15)

CLAIMS:

1. A method of manufacturing a positive electrode active mass precursor, which method comprises reacting a transition metal halide aqueous solution with an alkali metal hydroxide and, optionally, a metal-free base, to produce an alkali metal halide-containing precipitate; separating the alkali metal halide-containing precipitate from residual liquid by removing residual liquid to a degree which adjusts, or allows adjusting of, the proportions of soluble and insoluble reaction products; optionally, washing said precipitate to adjust the content of soluble reaction products therein; if necessary, adjusting the water content of the precipitate to render it suitable for shaping; and drying the precipitate and reducing it at temperatures up to the melting point of the alkali metal halide, to form an intimate mixture of transition metal and alkali metal halide having a mean particle size of less than 2 Am.

2. A method according to Claim 1, wherein the transition metal is selected from the group consisting Of iron (Fe), nickel (Ni) and mixtures thereof, the alkali metal is sodium (Na), and the halogen of the halide is chlorine, with the reaction of the solution with the hydroxide comprising adding up to the stoichiometric quantity of aqueous sodium hydroxide solution to the transition metal chloride solution under agitation, and A.ADMIWEIL SPEC DISK 44XGSKI.RAI 1 D IM 16 neutralizing any residual metal chloride anions to a pH of approximately 7 by the addition of aqueous ammonia solution or an organic base.

3. A method according to Claim 1 or Claim 2, wherein the separation is effected by mechanical separation means and wherein the water adjustment, when employed, comprises evaporation or water addition in a kneader or pugmill.

A positive electrode active mass precursor, when made by the method as claimed in any one of Claims 1 to 3 inclusive.

5. A composition of matter comprising an intimate mixture of transition metal in fine particulate form having a primary particle size as measured by electron microscopy of less than 1 micron and fine particles of sodium chloride having a particle size as measured by electron microscopy of less than 2 microns.

6. A precursor according to Claim 4, wherein the metal component of the precursor consists of finely divided particles of iron and/or nickel and up to 50 mol % of at least one element selected from the group comprising As, Sb and Bi, and, optionally, a minor proportion of aluminium, with the finely divided alkali metal halide consisting of sodium chloride with minor additions of sodium fluoride and/or sodium bromide.

17

7. A method of making a shaped artefact which comprises reacting a transition metal halide aqueous solution with an alkali metal hydroxide and, optionally, a metal-free base, to produce an alkali metal halide-containing precipitate; separating the alkali metal halide-containing precipitate from residual liquid by removing residual liquid to a degree which adjusts, or allows adjusting of, the proportions of soluble and insoluble reaction products; optionally, washing said precipitate to adjust the content of soluble reaction products therein; if necessary, adjusting the water content of the precipitate to render it suitable for shaping; shaping the precipitate into a form suitable for location in a cathode compartment of an electrochemical cell housing; and drying the precipitate and reducing it at temperatures up to the melting point of the alkali metal halide, to form an intimate mixture of transition metal and alkali metal halide having a mean particle size of less than 2 Am.

8. A shaped artefact, when made by the method as claimed in Claim 7.

9. A positive electrode precursor, which comprises a shaped artefact as claimed in Claim 8, or a positive electrode active mass precursor as claimed in Claim 4 or Claim 6 or a composition of matter as claimed in Claim 5.

10. An electrochemical cell which includes a cell housing having an anode and a cathode, with the cathode comprising, at least initially, a positive electrode precursor as claimed in Claim 9.

18 A novel method of manufacturing a positive electrode active mass precursor, substantially as described and exemplified herein.

12. A novel positive active mass precursor, substantially as described and exemplified herein.

13. A novel method of making a shaped artefact, substantially as described and exemplified herein.

14. A novel shaped artefact, substantially as described and exemplified herein.

15. A novel electrochemical cell, substantially as described and exemplified herein.

AAWS M.FAL SPEC DISK 441GSK91 0. IM