WO2011024149A1

WO2011024149A1 - Secondary electrochemical cell including inter-electrode scavenger

Info

Publication number: WO2011024149A1
Application number: PCT/IB2010/053870
Authority: WO
Inventors: Shalom Luski; Arieh Meitav; Doron Aurbach; Eli Lancry
Original assignee: Etv Motors Ltd.
Priority date: 2009-08-31
Filing date: 2010-08-29
Publication date: 2011-03-03

Abstract

Disclosed are secondary electrochemical cells (26) and methods of making secondary electrochemical cells including an inter-electrode scavenger component of alkali metal (18) positioned between the positive and negative electrodes (12,14) of the electrochemical cells (26). Also disclosed are separators (16) and methods of making separators (16) useful for such secondary electrochemical cells. Also disclosed are methods for balancing the electrodes of secondary electrochemical cells (26).

Description

SECONDARY ELECTROCHEMICAL CELL INCLUDING INTER-ELECTRODE

SCAVENGER

RELATED APPLICATION

The present application gains priority from U.S. Provisional Patent Application No.

61/238,402 filed 31 August 2009 and from U.S. Provisional Patent Application No. 61/241,177 filed 10 September 2009, both which are included by reference as if fully set- forth herein. FIELD AND BACKGROUND OF THE INVENTION

The invention, in some embodiments, relates to the field of electrochemical cells and, more particularly but not exclusively to secondary electrochemical cells, especially secondary alkali-metal ion electrochemical cells such as lithium and sodium ions electrochemical cells.

A secondary electrochemical cell generally includes a negative electrode comprising a negative active material with a reduction potential, a positive electrode comprising a positive active material with an oxidation potential and an electrolyte that allows movement of ions between the electrodes, electrically- insulating the positive electrode from the negative electrode is a separator that is permeable to the passage of ions in the electrolyte. The sum of the reduction potential and the oxidation potential is the standard electrochemical cell potential of the electrochemical cell.

A well-known type of secondary electrochemical cell is the lithium-ion secondary electrochemical cell. A typical lithium-ion secondary electrochemical cell includes a lithium- ion intercalating material (typically a carbonaceous material such as graphite or hard carbon) as the negative active material and a lithium-ion containing material (e.g., LiCoθ2) as the positive active material. During electrochemical cell charging, the positive active material is oxidized, releasing lithium ions into the electrolyte (e.g., LiCoθ2 > Lii_-xCθ2 + xLi⁺ + xe^~) and electrons from the circuit charge the negative active material while lithium ions from the electrolyte are intercalated in the negative active material (xLi⁺ + xe" +6C > Li_xCo). During electrochemical cell discharge, the positive active material is reduced, lithium ions are released from the negative active material and reintegrated in the positive active material.

A lithium-ion secondary electrochemical cell is assembled in an uncharged state and must be charged (a process called formation) for use. During the first few charging events (formation charge) of a lithium-ion secondary electrochemical cell, components of the electrolyte are reduced on the surface of the negative active material and oxidized on the surface of the positive active material, forming a solution-electrolyte interphase (SEI) on the active materials. When the SEI is permeable to lithium ions, non-soluble and non-electrically conductive, the SEI constitutes a protective layer on the positive and negative active materials, preventing deposition of reactive species formed in the electrochemical cell that leads to irreversible capacitance loss.

SUMMARY OF THE INVENTION

Aspects of the invention relate to secondary electrochemical cells that, in some aspects, have advantages over known secondary electrochemical cells.

Some embodiments of the invention relate to secondary electrochemical cells comprising a scavenger component of alkali metal (e.g., lithium metal, sodium metal) positioned between the positive electrode and the negative electrode. According to an aspect of some embodiments of the invention there is provided a secondary electrochemical cell, comprising:

a. a positive electrode including a positive active material;

b. a negative electrode including a negative active material facing the positive electrode;

c. a separator positioned between the positive electrode and the negative electrodes and electrically- insulating the positive electrode from the negative electrode;

d. an electrolyte contacting the positive electrode, the negative electrode and the separator; and

e. an inter-electrode scavenger component of alkali metal positioned between the positive electrode and the negative electrode, electrically- insulated from the positive electrode and the negative electrode, and in contact with the electrolyte. In some embodiments, the scavenger component is a scavenger electrode.

Some embodiments of the invention relate to methods of making secondary electrochemical cells comprising a scavenger component of alkali metal (e.g., lithium metal, sodium metal) positioned between the positive electrode and the negative electrode. According to an aspect of some embodiments of the invention there is also provided a method of making an electrochemical cell, comprising:

a. providing a positive electrode including a positive active material,

b. positioning a negative electrode including a negative active material facing the positive electrode; c. positioning a separator between the positive electrode and the negative electrode to electrically insulate the positive electrode and the negative electrode; and

d. positioning an inter-electrode scavenger component of alkali metal between the positive electrode and the negative electrode so that the scavenger component is electrically- insulated from the positive electrode and the negative electrode.

Some embodiments of the invention relate to separators useful for secondary electrochemical cells comprising a scavenger component of alkali metal (e.g., lithium metal, sodium metal) as well as secondary electrochemical cells comprising such separators. According to an aspect of some embodiments of the invention there is also provided a separator suitable for use with a secondary electrochemical cell having a laminated structure, comprising:

a. a first electrically- insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte;

b. facing the first layer, a second electrically- insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte; and

c. an inter-electrode scavenger component of alkali metal positioned between the first layer and the second layer.

Some embodiments of the invention relate to methods of making separators useful for secondary electrochemical cells comprising a scavenger component of alkali metal (e.g., lithium metal, sodium metal). According to an aspect of some embodiments of the invention there is also provided a method of making a separator having a laminated structure suitable for use with a secondary electrochemical cell, comprising:

a. providing a first electrically- insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte;

b. positioning a second electrically- insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte facing the first layer; and

c. positioning an inter-electrode scavenger component of alkali metal between the first layer and the second layer.

Some embodiments of the invention relate to methods for balancing the capacity of electrodes in an alkali-metal ion secondary electrochemical cell. According to an aspect of some embodiments of the invention there is also provided a method for balancing the capacity of electrodes in an alkali- metal ion secondary electrochemical cell, comprising: a. positioning a balancing electrode including an alkali metal between a positive electrode and a negative electrode of the electrochemical cell so that the balancing electrode is electrically- insulated from the positive electrode and from the negative electrode;

b. bringing the positive electrode to a given capacity by passing electrical current and alkali metal ions between the balancing electrode and the positive electrode; and

c. bringing the negative electrode to a given capacity, by passing electrical current and alkali metal ions between the balancing electrode and the negative electrode

wherein the given capacity of the negative electrode is within about 5% of the given capacity of the positive electrode, thereby balancing the positive electrode and the negative electrode.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. In case of conflict, the patent specification, including definitions, will control.

As used herein, the terms "comprising", "including", "having" and grammatical variants thereof are to be taken as specifying the stated features, integers, steps or components but do not preclude the addition of one or more additional features, integers, steps, components or groups thereof. These terms encompass the terms "consisting of and

"consisting essentially of.

As used herein, the indefinite articles "a" and "an" mean "at least one" or "one or more" unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE FIGURES

Some embodiments of the invention are described herein described with reference to the accompanying figures. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the invention may be practiced. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the invention. For the sake of clarity, some objects depicted in the figures are not to scale.

In the Figures:

FIGS. IA, IB and 1C are schematic depictions of an embodiment of a secondary electrochemical cell as described herein including a scavenger electrode; FIGS. 2A and 2B are schematic depictions of an embodiment of a secondary electrochemical cell as described herein including a scavenger component attached to a separator;

FIG. 3 is a schematic depiction of an embodiment of a secondary electrochemical cell as described herein including a scavenger electrode;

FIG. 4 is a schematic depictions of an embodiment of a secondary electrochemical cell as described herein including a particulate scavenger component;

FIG. 5 is a schematic depictions of an embodiment of a secondary electrochemical cell as described herein including a jelly- roll electrode assembly;

FIG. 6 is a schematic depictions of a stage in an embodiment of a method of making a jelly-roll electrode assembly as described herein; and

FIGS. 7A and 7B compare the performance of a secondary electrochemical cell including a scavenger component as described herein (Figure 7A) to that of a secondary electrochemical cell without such a scavenger component (Figure 7B).

DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

Aspects of the invention relate to secondary electrochemical cells, especially alkali- metal (lithium, sodium) secondary electrochemical cells.

The principles, uses and implementations of the teachings of the invention may be better understood with balancing to the accompanying description and figures. Upon perusal of the description and figures present herein, one skilled in the art is able to implement the teachings of the invention without undue effort or experimentation.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details set forth herein. The invention can be implemented with other embodiments and can be carried out in various ways. It is also understood that the phraseology and terminology employed herein is for descriptive purpose and should not be regarded as limiting.

As noted above, lithium-ion secondary electrochemical cells are assembled uncharged. One or more positive electrodes bearing a positive active material, one or more negative electrodes bearing a negative active material and one or more separators are assembled to constitute a laminated electrode assembly where a positive active material layer on a positive electrode faces a negative active material layer on a negative electrode, with a separator positioned between the two electrodes to electrically insulate the two electrodes one from the other. The laminated electrode assembly having a desired laminated structure (e.g., flat, stacked, jelly-roll) is placed inside a electrochemical cell- container. An electrolyte is added to saturate the electrodes and the separators before or after assembly of the electrode assembly and before or after placement of the electrode assembly in the electrochemical cell- container. The electrochemical cell- container is sealed so that a positive contact functionally associated with the positive electrode or electrodes and a negative contact functionally associated with the negative electrode or electrodes are accessible from outside of the sealed electrochemical cell- container.

For electrochemical cell- charging, the contacts are functionally associated with an electrical power source that charges the electrochemical cell by oxidizing a component of the positive active material releasing lithium ions from the positive active material into the electrolyte, loading the negative active material with electrons and intercalating lithium ions from the electrolyte into the negative active material.

During electrochemical cell discharge, the contacts are functionally associated with an electrical load. Electrons move from the negative active material to the positive active material, leading to reduction of a component of the positive active material, release of lithium ions intercalated in the negative active material to the electrolyte and reintegration of lithium ions from the electrolyte into the positive active material.

During the first few charging events (formation), components of the electrolyte are reduced on the surface of the negative active material and oxidized on the surface of the positive active material, forming an ion-permeable electrically- insulating insoluble solution- electrolyte interphase (SEI) at both electrodes. The exact nature of an SEI is determined by the nature of the electrolyte components that are reduced or oxidized at the electrodes. It is known to add additives to the electrolyte to generate an SEI having advantageous properties, see for example Abe K et al Journal of Power Sources 2008, 184, 449-455. Formation of the SEI uses charge and therefore reduces the capacity of the electrochemical cell. However, as the SEI is electrically- insulating, once an active material is completely coated with a dense SEI layer, no further substantial reduction or oxidation occurs so electrochemical cell capacity remains substantially constant. Additionally, once a dense SEI layer is formed, further reduction/oxidation reactions with components of the electrolyte or impurities and gas- formation inside the electrochemical cell are substantially prevented.

Generally, it is preferred that an SEI be thin: the thicker an SEI the greater the electrochemical cell internal impedance and the slower the possible maximal charge and discharge rates. Further, if an SEI is imperfect or not homogenous, the SEI-formation reactions may continue, using charge and increasing SEI thickness with the concomitant disadvantages, including electrochemical cell-capacity loss and increased internal impedance.

Two known families of lithium-ion positive active materials are composite lithium metal oxides with a spinel structure known as LMS (such as LiMn2θ4 and LiMnCH) and

LiNMS (such as LiNiMnCoθ2). The use of LMS and LiNMS as positive active materials in secondary electrochemical cells is promising due to the exceptionally high oxidation potential such materials exhibit, in some cases no less than about 4.0 V versus Li/Li⁺.

However, it has been found that lithium-ion secondary electrochemical cells comprising a lithium- ion containing positive active material having a high oxidation potential, e.g., an oxidation potential greater than 4.0 V vs. lithium (e.g., LMS or LiNMS) have inferior performance. Specifically, it is seen that there is a continuous irreversible capacity loss with each charge / discharge cycle that renders the electrochemical cell unusable.

Although not wishing to be held to any one theory, the Inventors hypothesize that in such electrochemical cells electrolyte oxidation processes that occur on the surface of the positive active materials at high oxidation potentials, e.g., greater than 4.0 V vs Li/Li+ in some instances produce soluble products, some positively charged, that migrate through the electrolyte to deposit on the surface of the negative active material. Additionally, reduction and oxidation of components of some positive active materials having high oxidation potentials, e.g., greater than 4.0 V vs Li/Li+, leads to the production of soluble metal cations that migrate through the electrolyte to deposit on the surface of the negative active material. Formation of such soluble metal cations is exceptionally significant when the positive active material includes manganese, for example with composite lithium metal oxides with a spinel structure such as LMS and LiNMS. For example, in some such electrochemical cells, Mn³⁺ cations in the positive active material undergo disproportionation reactions, producing insoluble Mn⁴⁺ and soluble Mn²⁺ cations. The soluble Mn²⁺ cations subsequently migrate towards the negative electrode during a following charging cycle.

During the formation cycles, the soluble positively-charged entities (e.g., produced by oxidation of electrolyte components or metal cations from the positive active material) on the surface of the negative active material are reduced on the surface of the negative electrode and interfere with the formation of the desired thin, dense and homogenous negative electrode SEI. Additionally, the metal cations reduced on the negative electrode potentially form conductive paths through the SEI, from the negative active material to the electrolyte. As a result, the negative electrode SEI is ineffective in stopping further reduction reactions of electrolyte components at the negative electrode. During subsequent charge/discharge cycles, further reduction reactions may use charge, leading to an increasing electrode imbalance and concomitant permanent capacitance loss, formation of gas inside the electrochemical cell and increasing electrochemical cell internal impedance.

Aspects of the invention relate to secondary electrochemical cells comprising a scavenger component of alkali metal (e.g., lithium metal, sodium metal) positioned between the positive electrode and the negative electrode and electrically- insulated therefrom.

It has been found that the presence of such a scavenger component positioned between the positive electrode and the negative electrode of a secondary electrochemical cell, especially an alkali-metal secondary electrochemical cell (e.g., analogous to well-known lithium ion electrochemical cells or sodium ion electrochemical cells such as disclosed in US Patent 6,872,492) increases electrochemical cell performance, depending on the embodiment, for example, in terms of a reduced extent of capacity loss during charge / discharge cycles, electrochemical cell cyclability, electrochemical cell life-time, maximal charge rate, maximal discharge rate and internal impedance.

Although not wishing to be held to any one theory, it is currently believed that the undesirable soluble positively-charged entities (e.g., produced by oxidation of electrolyte components or metal cations from the positive active material) do not reach the negative electrode, but rather are neutralized by interaction with the scavenger component, for example trapped on the surface of the scavenger component and in some instances even reduced by interaction with the scavenger component while oxidizing the alkali metal of the scavenger component forming e.g., Li⁺ (when the scavenger component comprises lithium metal) or Na⁺ (when the scavenger component comprises sodium metal). Apparently, the undesirable species are subsequently no longer able to interfere with the formation of or to damage the negative electrode SEI. As a result, the formed SEI is presumably more homogeneous, includes fewer imperfections, is denser and/or is thinner.

Thus, according to an aspect of some embodiments of the teachings herein there is provided a secondary electrochemical cell, comprising:

a. a positive electrode including a positive active material;

b. a negative electrode including a negative active material facing the positive electrode; c. a separator positioned between the positive electrode and the negative electrodes and electrically- insulating the positive electrode from the negative electrode;

d. an electrolyte contacting the positive electrode, the negative electrode and the separator, configured to allow migration of ions between the electrodes and through the separator; and

e. an inter-electrode scavenger component of alkali metal (e.g., lithium or sodium metal) positioned between the positive electrode and the negative electrode, electrically-insulated from the positive electrode and the negative electrode, and in contact with the electrolyte.

Generally, such an electrochemical cell further comprises a positive contact functionally associated with the positive electrode and a negative contact functionally associated with the negative electrode.

In some embodiments, the scavenger component constitutes a portion of a scavenger electrode of the electrochemical cell, that functions as an inter-electrode scavenger as described above, that can be used as a reference electrode known in the art, and, optionally may be used as a balancing electrode to balance the capacity of the positive and negative electrodes as described herein. In some such embodiments, the electrochemical cell further comprises a scavenger contact functionally associated with and thereby in electrical contact with the scavenger component.

The positive electrode, the negative electrode, the separator and the scavenger component together constitute components of the electrode assembly of the electrochemical cell. An electrode assembly of any suitable arrangement may be used in implementing a electrochemical cell herein, for example, a "flat" electrode assembly having a single positive electrode layer and a single negative electrode layer separated by a separator, a "stacked" electrode assembly including a plurality of alternating positive electrode and negative electrode layers separated by separators, or a "flat" or "stacked" electrode assembly that is wound in a spiral to make a "jelly-roll" electrode assembly.

The electrode assembly is typically contained inside a electrochemical cell- container. The positive contact, the negative contact and the scavenger contact (if present) are accessible from outside the electrochemical cell- container. Any suitable electrochemical cell- container may be used, for example, depending on the embodiment, the electrochemical cell is a pouch cell, a button cell, a coin cell or a rigid-can cell.

In some embodiments, an electrochemical cell as described herein is a component of a battery, that is to say there is a battery comprising at least one electrochemical cell as described herein. Generally, such a battery comprises a battery-shell defining a volume in which one or more electrochemical cells are contained. A battery of any suitable structure maybe used. Positive electrode and positive active material

Any suitable positive electrode including a positive electrode support having a height, a breadth and a thickness and bearing any suitable positive active material on at least one face thereof may be used in implementing embodiments of the teachings herein. In some embodiments, a positive electrode as described herein is between 30 and 350 micrometer thick, typically between 50 and 200 micrometers thick.

Any suitable positive active material may be used in implementing the teachings herein. In some embodiments, the electrochemical cell is a lithium-ion electrochemical cell and the positive active material is a lithium-ion containing positive active material. In some embodiments, the electrochemical cell is a sodium-ion electrochemical cell and the positive active material is a sodium- ion containing positive active material.

The oxidation potential of the positive active material is any suitable oxidation potential. That said, the teachings herein are exceptionally useful when the positive active material is high, that is to say at least about 4.0 V vs. Li/Li+, at least about 4.2 V vs. Li/Li+, at least about 4.4 V vs. Li/Li+ and even at least about 4.6 V vs. Li/Li+.

Known suitable positive active materials include: LiNio.5Mm.5O4 (oxidation potential

4.75 V vs Li/Li+), LiCoPO4 (oxidation potential 4.8V vs Li/Li+), LiNiVO4 (oxidation potential 4.8 V vs Li/Li+), and LiNiPCH (oxidation potential 5.1V vs Li/Li+).

In some embodiments, the positive active material is selected from the group consisting of spinels and olivines.

A positive active material of any suitable composition may be used in implementing the teachings herein.

That said, the teachings herein are exceptionally useful when the positive active material includes manganese: as discussed above such active materials may produce soluble Mn²⁺ that negatively- affect electrochemical cell performance, which in some embodiments are neutralized by the presence of a scavenger component as described herein. Thus, in some embodiments, the positive active material comprises manganese ions. Typical suitable positive active materials comprise:

lithium manganese phosphates for example LiMnPCH;

positive active materials known as LiNMS (such as LiNiMnCoθ2) having the formula: Li(i+r)Ni(o.5-r)Mn(i.5-x)MxO(4-δ)Tδ or Li(i+r)Ni(o.5)Mn(i.5-x-r)MxO(4-δ)Tδ; where M represents a cation such as Al, Ti, Cr, Fe, Zn, Mg and the like;

where T represents an anion such as F;

r is between 0 and 0.2;

x is between 0 and 0.2; and

δ is between 0 and 0.2

and

positive active materials known as LMS (such as LiMn2U4 and LiMnCH) having the formula:

LiMn(2-_x)M_xO(4-δ)Tδ,

where M represents a cation such as Al, Ti, Cr, Fe, Zn, Mg and the like;

where T represents an anion such as F;

x is between 0.01 and 0.2; and

δ is between 0 and 0.2

In some embodiments, suitable positive active materials include materials such as lithium metal oxides, lithium nickel oxides, lithium cobalt oxides, lithium iron oxides, LiMnθ4, LiNiMnCoθ2, LiMCoAlC>2, LiCoC>2, LiMC>2, LiCoi-_xNi_xC>2 (O.Ol≥x≥l), mixtures of LiCoθ2 with LiNiθ2, LiFePCH, LiFeSCH and Li2FePCHF. In some embodiments, the positive active materials include an amount of other cations, such as cations of Al, Ti, Cr, Fe, Zn, Mg and the like.

In some embodiments, suitable positive active materials include materials such as lithium metal phosphates, (e.g., Li(Mn₅Ni, Co)PCH with any suitable ratio of the different metal cations) including lithium manganese phosphates (e.g., LiMnPCH), lithium nickel phosphates (e.g., LiNiPCH), lithium cobalt phosphates (e.g., LiCoPCH) and lithium nickel manganese phosphates (e.g., LiNio.5Mno.5PO4).

Any suitable positive electrode support, such as known in the art, may be used in implementing the teachings herein. Typically, a positive electrode support also acts as a current collector to transport electrons between the positive contact of the electrochemical cell and the positive active material. Suitable positive electrode-support include meshes, foils and plates of materials such as aluminum, aluminum alloys, gold, gold alloys, platinum, platinum alloys, titanium, titanium, alloys and combinations thereof. In some embodiments, a positive electrode support is permeable to the passage of alkali metal ions, e.g., a porous micromesh such as copper micromesh. In some embodiments, a positive electrode support is impermeable to the passage of alkali metal ions, e.g., a solid copper foil.

A positive electrode is generally functionally associated with a positive contact, for example a wire or a strip of conductive material, integrally formed or attached, for example by welding, to the positive electrode support, to transport electrons to and from the positive electrode. A positive contact is generally accessible (electrically contactable) from outside the electrochemical cell- container of the electrochemical cell.

Any suitable method may be used for producing a positive electrode, for example as described in US patent publication 2008/0254367 or WO 2006/073277. Generally, a positive electrode is made by applying a layer of a slurry comprising the positive active material, a conductive material, a binder and a solvent to at least one face of an electrode-support. The slurry is dried, leaving a layer of positive active material attached to the electrode-support.

For example, powdered positive active material is kneaded together with a conductive material such as acetylene black or carbon black, a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR) or carboxymethyl cellulose (CMC) to give a positive active material composition. The positive active material composition is mixed with a solvent such as 1 -methyl- 2-pyrrolidone to form a slurry. At least one face of a positive electrode-support is coated with a layer of the slurry, and the coated electrode-support heated at between about 50⁰C and about 250⁰C under vacuum for a sufficient time for drying, for example between 1 and 24 hours, providing a positive electrode.

Negative electrode and negative active material

Any suitable negative electrode including a negative electrode support having a height, a breadth and a thickness and bearing any suitable negative active material on at least one face thereof may be used in implementing embodiments of the teachings herein. In some embodiments, a negative electrode as described herein is between 30 and 350 micrometer thick, typically between 50 and 200 micrometers thick.

Any suitable negative active material may be used in implementing the teachings herein. In some embodiments, the electrochemical cell is a lithium-ion electrochemical cell and the negative active material is a lithium intercalating negative active material. In some embodiments, the electrochemical cell is a sodium-ion electrochemical cell and the negative active material is a sodium intercalating negative active material, for example as described in US Patent 6,872,492.

Some embodiments include at least one negative active material selected from the group consisting of metals (e.g., tin, aluminum), silicon, silicates, SnO₂, TiO₂ and intermetallic alloys. In some embodiments include at least one negative active material that is a carbonaceous materials (e.g., a lithium- intercalating material that is primarily carbon) such as cokes, graphites, hard carbons, soft carbons, fired organic polymers, carbonaceous fibers or mixtures thereof.

Any suitable negative electrode support, such as known in the art, may be used in implementing the teachings herein. Typically, a negative electrode support also acts as a current collector to transport electrons between the negative contact of the electrochemical cell and the negative active material. Suitable negative electrode-support include meshes, foils and plates of materials such as copper, copper alloys, nickel, nickel alloys, gold, gold alloys, platinum, platinum alloys, titanium, titanium, alloys and combinations thereof. In some embodiments, a negative electrode support is permeable to the passage of alkali metal ions, e.g., a porous micromesh such as copper micromesh. In some embodiments, a negative electrode support is impermeable to the passage of alkali metal ions, e.g., a solid copper foil.

A negative electrode is generally functionally associated with a negative contact, for example a wire or a strip of conductive material, integrally formed or attached, for example by welding, to the negative electrode support, to transport electrons to and from the negative electrode. A negative contact is generally accessible (electrically contactable) from outside the electrochemical cell- container of the electrochemical cell.

Any suitable method may be used for producing a negative electrode, for example as described in US patent publication 2008/0254367 or WO 2006/073277. Generally, a negative electrode is made by applying a layer of a slurry comprising the negative active material, a conductive material, a binder and a solvent to at least one face of an electrode-support. The slurry is dried, leaving a layer of negative active material attached to the electrode-support.

For example, powdered carbonaceous negative active material is mixed with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), styrene-butadiene copolymer (SBR), acrylonitrile- butadiene copolymer (NBR) or carboxymethyl cellulose (CMC) to give a negative active material composition. The negative active material composition is mixed with a solvent such as 1 -methyl- 2-pyrrolidone to form the slurry. At least one face of a negative electrode- support is coated with a layer of the slurry, and the coated electrode-support heated at between about 50⁰C and about 250⁰C under vacuum for a sufficient time for drying, for example between 1 and 24 hours, providing a negative electrode.

Electrolyte

An electrolyte is a medium that allows movement of ions, e.g., lithium or sodium ions, into and out of the positive and negative active materials and through the separator.

Any suitable electrolyte may be used for implementing the teachings herein such as known in the art, for example a liquid or gel electrolyte.

In some embodiments, an electrolyte comprises one or more alkali-metal salts in a non-aqueous solvent including one or more solvent components. In some embodiments, an electrolyte comprises two, three or more different alkali-metal salts. In some embodiments, the concentration of the alkali- metal salts in the electrolyte are between about 0.1 M and about 3 M, in some embodiments between about 0.5 M and about 1.5 M.

In some embodiments, an electrolyte includes at least one lithium salt, for example in embodiments related to lithium- ion electrochemical cells. Typical lithium salts include lithium salts selected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂Fs)₂, LiC(SO₂CFs)₃, LiPF₄(CF₃)₂, LiPF₃(C₂Fs)₃, LiPF₃(CF₃),, LiPF₃(iso-C₃F₇)₃, LiPF₅(iso-C₃F₇), lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiDFOB) and combinations thereof.

In some embodiments, an electrolyte includes at least one sodium salt, for example in embodiments related to sodium- ion electrochemical cells. Typical sodium salts include sodium salts discussed in U.S. Patent 6,872,492, which is included by reference as if fully set- forth herein.

In some embodiments, an electrolyte comprises at least one non-aqueous solvent including one or more components. In some embodiments, one or more solvent components are selected from the group consisting of cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); linear carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), dipropyl carbonate (DPC); lactones such as gamma-butylo lactone (GBL); ethers such as tetrahydrofuran (THF), 2-methyl-tetrahydrofuran, 1,4-dioxane, 1,2- dimethoxyethane, 1 ,2-diethoxyethane, and 1,2-dibutoxyethane; nitriles such as acetonitrile; esters such as methyl propionate, methyl pivalate and octyl pivalate; N-methyl-2-pyrrolidone (NMP), sulfolane and adiponitrile and combinations thereof. In some embodiments an electrolyte comprises a mixture of two, three or more different non-aqueous solvents. In some embodiments, an electrolyte further comprises one or more additives for modifying the characteristics of the electrolyte such as increased safety or formation of an advantageous SEI. Any suitable electrolyte additive may be used in implementing the teachings herein. In some embodiments, an electrolyte includes at least one SEI forming- additive. Some embodiments include at least one additive (e.g., negative-electrode SEI- forming additives) such as listed in US patent publication 2008/0254367, Gnanaraj JS (Electrochem. Comm. 2003, 5, 940-945) or Abe K et al in J Power Sources 2008, 185, 449- 455 and references cited therein, which are included by reference as if fully set forth herein.

Typical additives include propargyl methyl sulfate (PMS), propargyl methyl carbonate (PMC), allyl methanesulfonate (AMS), vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene carbonate (EC), ethylene sulfite, propylene sulfite, vinylene ethylene carbonate (VEC), vinyl acetate (VA) and fluorinated ethylene carbonate (FEC). In some embodiments lithium salts are added as SEI-forming additives, for example LiDFOB and LiBOB.

Typically, an electrolyte is made by mixing the components together.

Scavenger Component

As noted above, a feature of some embodiments of the invention is an inter-electrode scavenger component of alkali metal positioned between the positive electrode and the negative electrode of an electrochemical cell. The scavenger component is electrically- insulated from the positive electrode and the negative electrode, and in contact with the electrolyte. Apparently, as noted above, when the electrochemical cell is operated undesirable soluble positively- charged entities (e.g., produced by oxidation of electrolyte components or metal cations from the positive active material) do not reach the negative electrode, but are reduced by interaction with the scavenger component, while oxidizing the alkali metal of the scavenger component forming an alkali metal ion (e.g., Li⁺ when the scavenger component comprises lithium metal or Na⁺ when the scavenger component comprises sodium metal).

In some embodiments, the alkali metal of the scavenger component comprises lithium metal, for example, in some embodiments where the electrochemical cell is a lithium ion electrochemical cell but also in some embodiments where the electrochemical cell is a sodium ion electrochemical cell. In some embodiments the scavenger component comprises substantially pure lithium metal while in some embodiments the scavenger component comprises alloyed lithium metal. In some embodiments, the scavenger component includes not less than about 50%, not less than about 90%, not less than about 95%, not less than about 98%, not less than about 99%, not less than about 99.9%, not less than about 99.99% and even not less than about 99.999% lithium metal by weight.

In some embodiments, the alkali metal of the scavenger component comprises sodium metal, for example, in some embodiments where the electrochemical cell is a sodium ion electrochemical cell. In some embodiments the scavenger component comprises substantially pure sodium metal while in some embodiments the scavenger component comprises alloyed sodium metal. In some embodiments, the scavenger component includes not less than about 50%, not less than about 90%, not less than about 95%, not less than about 98%, not less than about 99%, not less than about 99.9%, not less than about 99.99% and even not less than about 99.999% sodium metal by weight.

The scavenger component is of any suitable form, e.g., one or more particles (generally having a dimension of not more than about 1 mm in size, e.g., filaments, threads, strips, chips, flakes, buttons, knobs or powders), sheets (including strips, ribbons, plates and foils) and wires. In some embodiments, the scavenger component is of a combination of two or more forms. In some embodiments, the scavenger component is permeable to the passage of ions (e.g., a mesh, a net, a foam, a plurality of individual particles). In some embodiments, the scavenger component is impermeable to the passage of ions.

For example, in some embodiments, the scavenger component is a plurality of small (e.g., not more than about 1 mm in size, not more than about 100 micrometers in size, not more than about 50 micrometers in size, not more than about 20 micrometers in size, not more than about 10 micrometers in size and even not more than about 5 micrometers in size) individual particles dispersed in the separator. In some such embodiments, the particles are separate. In some such embodiments, such particles are physically connected, for example are sintered so that the scavenger component comprises a sheet of sintered lithium powder.

For example, in some embodiments, the scavenger component is an impermeable sheet (e.g., a foil and the like) contained inside the separator. In some such embodiments, the surface area of the sheet is not more than about 40%, not more than about 30%, not more than about 20%, and even not more than about 10% of the surface area of a corresponding positive electrode.

For example, in some embodiments, the scavenger component is a permeable (to the passage of lithium ions) sheet, e.g., a mesh, net or perforated foil contained inside the separator.

For example, in some embodiments, the scavenger component is one or more wires (or the like) contained inside the separator. In some such embodiments, the scavenger component comprises physically connected wires constituting, for example, a mesh, net or web. In some such embodiments, the scavenger component comprises physically separate wires, for example a single wire, two or more wires, for example two or more parallel wires.

In some embodiments, the scavenger component is a layer (e.g., a coating) on a scavenger substrate. A scavenger substrate is a component of some material that is substantially inert to the conditions inside the electrochemical cell. In some embodiments, the scavenger component is a layer of alkali metal deposited on a substrate, for example, by vapor deposition, plasma vapor deposition or chemical vapor deposition or lamination.

In some embodiments, the scavenger substrate is electrically not-conductive, e.g., a mesh of non-conductive filaments (e.g., PTFE) or a porous sheet of non-conductive material (e.g., PTFE). For example, a 20 micrometer thick sheet of porous material (such as used in making a separator) is partially coated, on one or more sides, for example using vapor deposition, with a 2 to 5 micrometer layer of lithium or sodium so that the pores of the material are not blocked. In some embodiments, the separator of the electrochemical cell is the scavenger substrate and the scavenger component is thereby attached to the separator.

In some embodiments, e.g., when the scavenger component is a scavenger electrode, the scavenger substrate is electrically conductive, e.g., a mesh of copper or silver wires.

The thickness (the dimension perpendicular to the positive and negative electrodes) of the scavenger component is any suitable dimension and is generally not more than about 300 micrometers. Since the scavenger component is positioned between the positive and negative electrodes, it is preferred that the scavenger component (and, if present, together with scavenger substrate) be as thin as possible, to reduce the bulk of the electrochemical cell and to keep the energy density of the electrochemical cell as high as possible. Typically, a scavenger component is not more than about 100 micrometers thick, not more than 20 micrometers thick, not more than about 10 micrometers thick and even not more than about 5 micrometers thick.

The amount of alkali metal constituting the scavenger is any suitable amount and may be adjusted by trial and error depending on the nature of the active materials and electrolyte. That said, in some embodiments, the amount of alkali metal constituting the scavenger is at least 10%, at least 20% and even at least 50% of intercalating sites of the negative active material. In some embodiments, there is no upper limit to the amount of alkali metal making up a scavenger in a given electrochemical cell. That said, a greater amount increases the bulk of electrochemical cell, reducing the energy density of the electrochemical cell. Thus, in some embodiments, the amount of alkali metal constituting the scavenger is not more than about 1000% of intercalating sites of the negative active material.

Separator

Like known electrochemical cells, embodiments of an electrochemical cell described herein comprise a separator positioned between the positive electrode and the negative electrodes and electrically-insulating the positive electrode from the negative electrode. Any suitable separator, such as known in the art, may be used for implementing the teachings herein, especially separators suitable for use for alkali-metal ion electrochemical cells such as lithium ion electrochemical cells.

Generally, a separator is a sheet having a height, a breadth, a thickness and is permeable to the passage of Li⁺ and/or Na⁺ ions.

Typically, there is at least one layer of separator positioned between every positive electrode and negative electrode to prevent physical contact (with concomitant short circuit) of the positive electrode and negative electrode.

Typical separators comprise one or more sheets of suitable materials such as microporous polyolefϊns (e.g., polyethylene or polypropylene film, fluorinated polyolefϊn films), other microporous films, woven fabrics and non-woven fabrics. Suitable sheets are commercially available, for example from Such separators are commercially available, e.g, from Ube Industries, Tokyo, Japan or Celgard LLC, Charlotte, North Carolina, USA.

As is known in the art, it is preferred that a separator be as thin and porous as possible in order to allow maximal power density and minimal internal resistance, but must also be physically strong enough to maintain physical integrity, to increase electrochemical cell reliability without short-circuits. In some embodiments, separators are made of one or more sheets of separator material so that the separator is typically between about 5 and about 200 micrometers thick, more typically between about 10 and about 60 micrometers thick, preferably between about 20 and about 50 micrometers thick.

Since the scavenger component of an electrochemical cell is also positioned between the electrodes of the electrochemical cell, a scavenger component and a corresponding separator are generally physically close together.

In some embodiments, the scavenger component is contained inside the separator, so that the separator electrically insulates the scavenger components from the electrodes. For example, the separator is a laminated structure comprising two layers (e.g., individual sheets or a folded-over sheet) of separator material as described above with the scavenger component positioned therebetween.

In some embodiments the scavenger component is physically separate from the separator. For example, in some such embodiments where the scavenger component is contained inside the separator, the scavenger component is positioned between two individual layers of separator material constituting the separator.

In some embodiments the scavenger component is attached to a part of the separator. For example, in some such embodiments where the scavenger component is contained inside the separator, the separator comprises two individual layers of separator material facing one another, and the scavenger component is a coating (e.g., by vapor deposition) on a facing face of one or both individual layers. In some such embodiments, the outer faces of the layers that contact the electrodes are contacted by electrically- insulating separator material so that the electrodes and the scavenger component are all insulated one from the other.

In some embodiments, the scavenger component is a component of the separator. For example, in some embodiments as described above, subsequent to placing a scavenger component between two layers of separator, the two layers are connected (e.g., by welding, adhesive) making a laminated structure having two connected layers of separator material with a scavenger component contained therebetween, the laminated structure constituting a scavenger.

Some embodiments herein relate to separators useful for secondary electrochemical cells comprising a scavenger component of alkali metal (e.g., lithium metal, sodium metal) as well as secondary electrochemical cells comprising such separators. Thus, according to an aspect of some embodiments of the invention there is also provided a separator suitable for use with a secondary electrochemical cell (especially an alkali metal secondary electrochemical cell) having a laminated structure, comprising:

a. a first electrically- insulating separator layer, permeable to the passage of alkali metal ions (e.g., Li⁺ and/or Na⁺) in an electrolyte;

b. facing the first layer, a second electrical insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte; and

As discussed above, in some embodiments, the scavenger component is physically separate from the first and second separator layer. In some embodiments, the scavenger component is attached to at least one of the first separator layer and the second separator layer. In some embodiments, the first separator layer and the second separator layer are mutually attached so as to contain the scavenger component therebetween.

In some embodiments, the scavenger component is configured to constitute a portion of a scavenger electrode of an electrochemical cell, and further comprising a scavenger contact functionally associated with and thereby in electrical contact with the scavenger component.

A separator as described herein may be made by any suitable method. In some embodiments, a separator is made according to a method as described herein. According to an aspect of some embodiments, there is provided a method of making a separator having a laminated structure suitable for use with a secondary electrochemical cell, comprising:

Positioning of the scavenger component is as described above, and includes placing or distributing a scavenger component, such as described above, randomly or in an orderly fashion between the two separator layers.

In some embodiments, the method further comprises: attaching the scavenger component to at least one of the first layer and the second layer. For example as described above, in some embodiments, a layer of alkali metal is deposited (e.g., by vapor deposition, lamination, adhesion) on one face of a sheet of a separator material and the sheet of separator material used as the first, the second or both the first and second separator layer.

In some embodiments, the method is performed during the making of an electrochemical cell. In some embodiments, the method is performed separately from making of an electrochemical cell, and subsequently making the electrochemical cell includes positioning the already-made separator between the electrodes of the electrochemical cell.

In some embodiments, the method further comprises: attaching the first layer to the second layer, for example, by welding (plasma welding, ultrasonic welding) or the use of adhesive. An advantage of such attaching is that the separator can then be made separately from an electrochemical cell and, in some embodiments, ensures a desirable distribution of scavenger component in the separator. Some embodiments of the invention relate to methods of making secondary electrochemical cells comprising a scavenger component as described herein. Generally, such an electrochemical cell is made in the usual way as known in the art, with the addition of properly positioning the scavenger electrode between the positive and negative electrodes. Thus according to an aspect of some embodiments there is also provided a method of making an electrochemical cell, comprising:

a. providing a positive electrode including a positive active material;

b. positioning a negative electrode including a negative active material facing the positive electrode;

c. positioning a separator between the positive electrode and the negative electrode to electrically insulate the positive electrode and the negative electrode; and

Generally, the positive electrode is functionally associated with a positive contact and the negative electrode is functionally associated with a negative contact, and the method further comprises sealing the positive electrode, negative electrode, separator and the inter- electrode scavenger inside a electrochemical cell- container such that the positive contact and the negative contact are accessible and provide electrical contact from outside the electrochemical cell- container with a respective component inside the electrochemical cell- container. In embodiments where the scavenger component constitutes a portion of a scavenger electrode and is functionally associated with a scavenger contact, the sealing is such that the scavenger contact is accessible and provides electrical contact from outside the electrochemical cell- container with the scavenger component inside the electrochemical cell- container.

For example, in a typical embodiment where the electrochemical cell is a flat pouch electrochemical cell, a plurality of sheets electrolyte are stacked facing each other to make a laminated structure in order: a flexible aluminized sheet of foil as one side of the electrochemical cell- container, a sheet constituting the positive electrode including a positive active material functionally associated with a conductive tab constituting a positive contact, a first separator sheet, an inter-electrode scavenger component (e.g., a mesh of lithium wire), a second separator sheet, a sheet constituting the negative electrode including a negative active material functionally associated with a conductive tab constituting a negative contact, and a second flexible aluminized sheet of foil as a second side of the electrochemical cell- container. Generally, the separator sheets and the electrochemical cell- container sheets are slightly higher and broader than the electrode sheets and the scavenger component and the stacking is such that there is a margin of the separator sheets and the electrochemical cell- container sheets around the electrode and scavenger component sheets, where a portion of the tabs constituting the electrode contacts pass the margins. The margins of the electrochemical cell- container and separator sheets are secured under vacuum in the usual way so that the electrodes, scavenger components and electrolyte are sealed within a pouch made of the electrochemical cell- component sheets with the electrode contacts protruding from between the seam between the electrochemical cell- container sheets. In some embodiments, the electrode and separator sheets are impregnated with electrolyte prior or during the stacking. In some embodiments, electrolyte is added just-prior to sealing the electrochemical cell- container.

In some such embodiments that include a scavenger electrode, the scavenger electrode is stacked relative to the other components so that a portion of the scavenger contact passes the margins of the electrochemical cell- container sheets and protrudes from the seam between the electrochemical cell- container sheets. In such a way, the scavenger contact provides electrical contact to the scavenger electrode from outside the electrochemical cell- container. For example, in a related embodiment, the scavenger component comprises a copper mesh or perforated copper foil as a scavenger electrode support having a height and breadth approximately that of the electrodes on which lithium has been deposited (e.g., by lamination, vapor deposition or adhesion, see US patent 5,470,357) and includes a copper tab as a scavenger contact. During sealing of the electrochemical cell- container, the copper tab passes the margin so as to protrude from the seams between the electrochemical cell- container sheets

In some such embodiments, the separator including the scavenger component is provided as a part of a separator sheet (e.g., a laminated separator as described herein). In some such embodiments, the single separator component is stacked between the two electrode sheets.

In some embodiments where the electrochemical cell is a stacked electrochemical cell, that is includes multiple positive and negative electrodes, the method is implemented substantially as described above, but with alternately placing a positive electrode sheet and a negative electrode sheet, separated by a first separator sheet, a scavenger component, and a second separator sheet or, alternately a separator sheet that includes the scavenger component as a part of the separator. As is known in the art, when an electrochemical cell includes a "jelly-roll" electrode assembly, typically at least one sheet of positive electrode, at least one sheet of positive electrode, and at least two sheets of separator are all secured to a mandrel (typically a round or planar mandrel), where one separator sheet is located between and separates any two electrode sheets, and an electrode sheets is located between and separates any two separator sheets. The mandrel is rotated and the sheets are wound around each other in a spiral fashion where one separator sheets is positioned between any positive electrode and negative electrode layer. When the electrode assembly is of the desired size, the electrode assembly is placed inside a electrochemical cell- container (e.g., a flexible pouch) with removal of some, none or all of the mandrel, and the electrochemical cell- container sealed.

In some embodiments where the separator already includes a scavenger component, for example the laminated separator as described herein, a separator sheet including the scavenger component is wound about the mandrel as described above.

In some embodiments, a given separator sheet is replaced with two separator layers (e.g., two separator sheets or a folded separator sheet) and during rotation of the mandrel a scavenger component is intermittently or continuously placed between the sheets.

For example, particulate scavenger component is distributed (e.g., sprinkled or sprayed) between any two separator layers, in some embodiments continuously distributed.

For example, a scavenger component comprising multiple discrete parts such as pieces of foil, knobs, buttons and the like are intermittently, e.g., every half rotation of the mandrel, placed between each the two layers of both separators.

For example, a scavenger component comprising one or more continuous components such as wires, strips, meshes and the like is place between each the two layers of both separators and wound around the mandrel together with the other components of the electrode assembly.

An embodiment of a secondary electrochemical cell as described herein, electrochemical cell 10, is depicted in Figures IA (side cross section), IB (perspective) and 1C (front view of some of the electrode assembly).

Electrochemical cell 10 comprises a positive electrode 12, a negative electrode 14, a separator 16 and a scavenger component 18 constituting an electrode assembly contained inside a electrochemical cell container 20. Positive electrode 12 includes a positive active material on a sheet of copper that functions as a positive electrode support a copper tab integrally formed therewith that functions as a positive contact 22.

Negative electrode 14 includes a negative active material on a sheet of copper that functions as a negative electrode support a copper tab integrally formed therewith that functions as a negative contact 24.

Separator 16 includes two separate sheets 16a and 16b of electrically-insulating separator material permeable to the passage of lithium ions in an electrolyte, positioned between positive electrode 12 and negative electrode 14, electrically- insulating the two electrodes one from the other.

Positioned between separator sheets 16a and 16b and therefore between electrodes 12 and 14 is a laminated scavenger electrode 26. Scavenger electrode 26 includes a scavenger support 28 of 25 micrometer thick copper foil integrally formed with a copper tab that functions as a scavenger contact 30. The surface area of a face of scavenger support 28 is 20% of the surface area of a face of either electrode 12 or 14. To the face of scavenger support 28 facing positive electrode 12 is attached by pressing a 75 micrometer thick layer of 99.999% lithium metal as a scavenger component 18.

Electrochemical cell- container 20 is a standard flexible pouch-shell container fashioned from two layers 20a and 20b of aluminized polymer foil.

Positive electrode 12, negative electrode 14 and separator 16 are all saturated with a liquid or gel electrolyte including a lithium salt (e.g., IM LiF₆ electrolyte salt in a nonaqueous solvent comprising ethylene carbonate and dimethyl carbonate, 1 :2).

For use, electrochemical cell 10 may be first charged as known in the art, or in accordance with the method of balancing the capacity of electrodes as discussed below.

During charge and discharge of electrochemical cell 10, soluble positively-charged entities produced at positive electrode 12 (e.g., produced by oxidation of electrolyte components or metal cations from the positive active material, e.g., Mn²⁺) enter the electrolyte and migrate towards negative electrode 14. Prior to reaching negative electrode

14, the entities encounter and are reduced on scavenger component 18 producing harmless products and lithium ions, and are thus neutralized. Additionally, impurities that are in the electrochemical cell due to the manufacture process (e.g., impurities in the electrolyte such as water or HF) react with scavenger component 18 and are neutralized.

If desired, scavenger electrode 26 may be used as a reference electrode in the usual way, or to balance the capacity of electrodes 12 and 14 as discussed below. In Figures 2, an embodiment of a separator 16 including a scavenger component as described herein is depicted alone (Figure 2A) and in an electrochemical cell 36 (Figure 2B, in side cross section).

In Figure 2A, it is seen that separator 16 comprises a single sheet 38 of electrically- insulating ion-permeable separator material (e.g., 15 micrometer thick sheet of polyethylene) to which first face a 2 micrometer thick partial layer 40 of lithium metal has been attached by vapor deposition so that sheet 38 functions as a scavenger support. Sheet 38 is folded over so that separator 16 is between 30 - 35 micrometers thick and comprises two lithium metal layers 40a and 40b that constitute a scavenger component 18 contained inside two layers of separator material 16a and 16b.

In Figure 2B, separator 16 is depicted as a component of electrochemical cell 36, held inside electrochemical cell container 20 together with a positive electrode 12 and a negative electrode 14, together constituting an electrode assembly.

In Figure 3, an embodiment of an electrochemical cell 42 comprising a separator 16 including a scavenger electrode 26 as described herein is depicted in side cross-section.

Scavenger electrode 26 of electrochemical cell 42 comprises a square mesh of 15 micrometer diameter copper wires (as a scavenger support 28) coated by vapor deposition with a 2 micrometer thick layer of lithium metal that constitutes a scavenger component 18. The distance between any two parallel copper wires is 5 mm.

Scavenger electrode 26 is contained inside separator 16, between two layers 16a and

16b of a folded sheet of separator material. Separator 16 including scavenger electrode 26 is held inside electrochemical cell container 20 together with a positive electrode 12 and a negative electrode 14, together constituting an electrode assembly. A 1 cm broad 25 micrometer thick strip of copper foil 44 passes through the seam of electrochemical cell container 20 to contact scavenger electrode 26, to function as a scavenger contact 30.

In a non-depicted embodiment similar to electrochemical cell 42, a separator includes a scavenger component that is not a component of a scavenger electrode. In the embodiment, instead of a copper mesh, a mesh of polyethylene fibers (a scavenger support) is coated, for example by vapor deposition with a layer of lithium metal. In such an embodiment, there is no separator contact 30.

In Figure 4, an embodiment of an electrochemical cell 46 comprising a separator 16 including a scavenger component 18 as described herein is depicted in side cross-section.

Separator 16 is made up of two separate sheets 16a and 16b of separator material containing a scavenger component 18 of electrochemical cell 42 comprising a lithium powder having an average particle size of 5 micrometers. During manufacture, one of the sheets of separator material (e.g., 16a) is placed on a surface and the lithium powder distributed (e.g., by scattering or spraying) on one face of the sheet. Much of the powder settles in pores on the face of the sheet of separator material. A face of the second separator sheet (e.g., 16b) is contacted with the face of the first separator sheet, so that the lithium powder is held therebetween. The two sheets are mutually connected, for example with an adhesive or welding to make a unified separator component having a scavenger component held therebetween.

Separator 16 including scavenger component 16 is held inside electrochemical cell container 20 together with a positive electrode 12 and a negative electrode 14, together constituting an electrode assembly.

In some of the embodiments depicted above, a scavenger component 18 is contained inside a separator 16 between two unconnected layers 16a and 16b of separator material. In some such embodiments, the two layers 16a and 16b are connected, for example with the use of an adhesive or welding to make a unified separator component having a scavenger component held therebetween, making assembly of the respective electrochemical cell more simple.

As is known to one skilled in the art, it is advantageous to make an electrode assembly of a secondary electrochemical cell having a "jelly- roll" electrode assembly where the positive electrode, the negative electrode and two separator layers are spirally wound and then placed in a electrochemical cell container. In some embodiments, the teachings herein are applied to an electrochemical cell having a jelly- roll electrode assembly.

In Figure 5, an embodiment of an electrochemical cell 48 comprising a jelly-roll electrode assembly is depicted in side cross section, electrochemical cell 48 is analogous to known electrochemical cells comprising jelly-roll electrode assemblies, and includes a spiral wound positive electrode 12 and a spiral wound negative electrode 14, electrically separated one from the other by spiral wound separators 16 and 16', all contained inside electrochemical cell container 20.

In contrast to the art, separators 16 and 16' both comprise two layers of separator material with a scavenger component held therebetween (16a, 16b and 18 for separator 16 and 16'a, 16'b and 18' for separator 16'). The scavenger component can be any suitable scavenger component as described herein, e.g., lithium on a copper foil support (like in electrochemical cell 10), lithium metal attached to a face of a separator component (like in electrochemical cell 36), lithium metal attached to a face of separator support such as a mesh (like in electrochemical cell 42) or a lithium powder (like in electrochemical cell 46).

The method of making a jelly-roll electrode assembly in accordance with the teachings herein is substantially analogous to the method for making prior-art jelly-roll electrode assemblies and was discussed in detail hereinabove.

In some embodiments, a separator comprising a scavenger component as described herein is pre-made. In some such embodiments, winding of the separator layers with the positive and negative electrodes is performed substantially the same as known in the art.

In some embodiments, the scavenger component is properly positioned during the winding process. For example, in some such embodiments, a positive electrode, a negative electrode, and two separatorss are secured to the mandrel, so that the electrodes are each separated by a separator and the separators are each separated by an electrode. Each of the separators is provided as two separator sheets. Two scavenger component dispensers are provided, each configured to dispense a scavenger component between two separator sheets corresponding to a single separator. As the mandrel is rotated, the scavenger component is dispensed from the scavenger component dispensers to be held between the two sheets making up each scavenger component.

In Figure 6, a a stage in an embodiment of a method of making a jelly- roll electrode assembly as described herein is schematically depicted, specifically the beginning of winding the various components of the electrode assembly around a mandrel 50.

An end of a first separator 16 made of two separator sheets 16a and 16b each fed from a separate spindle (not depicted) and guided by rollers 52 is secured to mandrel 50.

An end of a positive electrode 12 is fed from a spindle (not depicted) and guided by a roller 52 is secured to mandrel 50 contacting first separator 16.

An end of a second separator 16' made of two separator sheets 16'a and 16'b each fed from a separate spindle (not depicted) and guided by rollers 52 are secured to mandrel 50 contacting positive electrode 12.

An end of a negative electrode 14 is fed from a spindle (not depicted) and guided by a roller 52 is secured to mandrel 50 contacting second separator 16'.

In such a way, negative electrode 14 is always separated from positive electrode 12 by either separator 16 or 16' while separator 16 is always separated from separator 16' by one of the two electrodes 12 or 14.

Positioned between separator sheets 16a and 16b is scavenger component 18, a lithium wire or ribbon (and in some embodiments, a plurality of lithium wires or ribbons, in some embodiments parallel), dispensed from a scavenger component dispenser 54. Similarly, positioned between separator sheets 16'a and 16'b is scavenger component 18', a lithium wire or ribbon (and in some embodiments, a plurality of lithium wires or ribbons, in some embodiments parallel), dispensed from a scavenger component dispenser 54.

As mandrel 50 is rotated in the usual way, the various components of the electrode assembly, including scavenger components 18 and 18' are spirally wound to constitute the electrode assembly. When an electrode assembly of a desired size is achieved, rotation is stopped and the electrode assembly processed further, as known in the art, including placing inside a electrochemical cell container.

The above method can be modified for positioning most if not all types of scavenger components by appropriate modification of the scavenger component dispensers 54. For example, in some embodiments, a scavenger component dispenser is configured to dispense an alkali metal powder as a scavenger component.

In the specific embodiments depicted above with reference to the figures, the scavenger components 18 comprise lithium metal. As is clear from the description herein, in some embodiments a scavenger component comprises sodium metal.

Electrode Capacity Balancing

One of the challenges of operating alkali-metal ion secondary electrochemical cells such as lithium ion and sodium ion secondary electrochemical cells relates to balancing the capacity of the electrodes.

Alkali-metal ion secondary electrochemical cells are assembled uncharged. Generally the positive electrode, the negative electrode and the separator or separators are assembled in the desired laminated structure of an electrode assembly and placed inside a electrochemical cell- container. The electrochemical cell- container is then filled with electrolyte to saturate the electrodes and separators and the electrochemical cell- container sealed so that the positive and negative contacts are apparent outside of the sealed electrochemical cell- container.

For charging, the contacts are functionally associated with an electrical power source that charges the electrochemical cell by transferring alkali- metal ions from the positive active material at the positive electrode into the electrolyte and from the electrolyte to be intercalated in alkali- metal- ion accepting sites of the negative active material at the negative electrode while oxidizing a component of the positive active material (e.g., Co³⁺ to Co⁴⁺).

During discharge, the contacts are functionally associated with an electrical load. Alkali- metal ions are transferred from the negative active material at the negative electrode to the electrolyte while alkali-metal ions from the electrolyte enter alkali-metal-accepting sites on the positive active material at the positive electrode while reducing a component of the positive active material (e.g., Co³⁺ to Co⁴⁺).

In order to get a balanced electrochemical cell, where the positive electrode and the negative electrode have the same number of alkali- metal- ion accepting sites and therefore the same capacity, the amount of negative active material and positive active material used in making a given electrochemical cell is carefully calculated.

In practical implementations, during any charge- discharge cycle, but especially during the first few charge-discharge (formation) cycles some charge is lost by oxidation and reduction reactions (of electrolyte components) forming the surface-electrolyte interphase

(SEI) on the surface of the positive and negative active materials. Since the amount of charge lost at the two electrodes is different there is a capacity imbalance.

As a result, in practical terms, the electrochemical cell can subsequently be only partially charged or discharged and the effective energy density of the electrochemical cell is lower than theoretically possible, typically in the order of up to about 10%.

No less importantly, capacity imbalance is a critical problem when assembling a battery from a number of individual electrochemical cells where the capacity of the entire battery will be determined by the electrode of the electrochemical cell having the lowest capacity.

Embodiments of electrochemical cells including a scavenger electrode as described herein may be operated to overcome this challenge when the scavenger electrode is operated as a balancing electrode against each one of the positive and negative electrodes.

Thus according to an aspect of some embodiments of the invention, there is also provided a method of balancing the capacity of electrodes in an alkali- metal ion secondary electrochemical cell, comprising:

a) positioning a balancing electrode including an alkali metal between a positive electrode and a negative electrode of the electrochemical cell in an electrolyte so that the balancing electrode is electrically- insulated from the positive electrode and from the negative electrode;

b) bringing the positive electrode to a given capacity by passing electrical current and alkali metal ions between the balancing electrode and the positive electrode; and

c) bringing the negative electrode to a given capacity by passing electrical current and alkali metal ions between the balancing electrode and the negative electrode wherein the given capacity of the negative electrode and the given capacity of the positive electrode is substantially the same, thereby balancing the positive electrode and the negative electrode. In some embodiments, the given capacity of the negative electrode is within about

5%, within about 3%, within about 2% and even within about 1% of the given capacity of the positive electrode.

Specifically, implementation of the method of balancing the capacity of the electrodes may be seen as charging each of the positive electrode and the negative electrode separately with reference to the balancing electrode. As a result, both the positive electrode and the negative electrode are brought to a desired and substantially equal given capacity. In some embodiments, the given capacity is the maximal possible capacity so that the energy density of the electrochemical cell is maximal. In some embodiments, the given capacity is some other capacity, for example the maximal capacity of a second electrochemical cell with which the electrochemical cell is associated to make a battery.

In some embodiments, the method is implemented so that upon completion, the electrochemical cell is charged, that is to say, the negative active material of the negative electrode is filled to the desired capacity with intercalated lithium ions and the positive active material of the positive electrode is appropriately oxidized.

In some embodiments, the method is implemented so that upon completion, the electrochemical cell is discharged, that is to say, the negative active material of the negative electrode is substantially empty of intercalated lithium ions and the positive active material of the positive electrode is appropriately reduced and is filled to the desired capacity with intercalated lithium ions.

A person having ordinary skill in the art is familiar with methods of charging/discharging an electrochemical cell, that is to say passing electrical current and alkali metal ions between any two electrodes (e.g., the balancing electrode and either the positive electrode or the negative electrode) to given a desired capacity.

In some embodiments, 'a' is prior to 'b', that is to say, the positive electrode is first brought to the given capacity and subsequently, the negative electrode is brought to the given capacity.

In some embodiments, 'b' is prior to 'a', that is to say, the negative electrode is first brought to the given capacity and subsequently, the positive electrode is brought to the given capacity. In some embodiments, 'a' and 'b' are substantially contemporaneous, that is to say, alternately passing electrical current and alkali metal ions between the balancing electrode and the positive electrode or between the balancing electrode and the negative electrode.

In some embodiments, the method is implemented as a formation cycle, that is to say the initial charge and discharge of a newly assembled electrochemical cell.

In some embodiments, the method if implemented in a "mature electrochemical cell", that is to say an electrochemical cell that has already undergone a number of charge and discharge cycles. In some such embodiments, the electrochemical cell has developed a capacity imbalance between the positive and negative electrodes. In some such embodiments, the electrochemical cell has developed a capacity imbalance relative to a second electrochemical cell of a battery of which the electrochemical cell is a component.

The term "balancing electrode" is non-limiting and is used for clarity to differentiate that electrode from the positive and negative electrodes of an electrochemical cells. In some embodiments, the balancing electrode is substantially the same as a scavenger electrode described above. In some embodiments an electrochemical cell is provided with an electrode that is functional as both a balancing electrode and as a scavenger electrode may perform the same functions: as an inter-electrode scavenger component to neutralize harmful entities present in the electrolyte as described above and, when desired, balancing the the capacity of the electrodes.

In some embodiments, e.g., when the electrochemical cell is a lithium ion electrochemical cell, the alkali metal is lithium and the alkali metal ion is a lithium ion as described above.

In some embodiments, e.g., when the electrochemical cell is a sodium ion electrochemical cell, the alkali metal is sodium and the alkali metal ion is a sodium ion as described above.

EXAMPLES

Reference is now made to the following example, which together with the above description illustrate some embodiments of the invention in a non limiting fashion.

Electrochemical cells and the various components thereof are made, tested and examined using methods analogous to the known in the art, for example as described in Gnanaraj JS (Electrochem. Comm. 2003, 5, 940-945), in Aurbach D et al (J Power Sources 2006, 162(2), 780-789), Abe K et al (J. Power Sources 2008, 184, 449-455) and US 2008/0254367 which are included by balancing as if fully set-forth herein. Flat pouch electrochemical cells analogous to electrochemical cell 10 depicted in Figures 1 are made using methods known in the art. Various positive electrodes 12 are fashioned comprising various positive active materials including positive active materials having an oxidation potential no less than about 4.0V versus Li/Li+, having either a spinel or olivine crystal structure, for example LiMosMni sC^ and LiMn₂OzI. Various suitable negative electrodes 14 are fashioned comprising various negative active materials such as carbon black.

Various suitable scavenger components 32, including scavenger components that are components of a scavenger electrode 26 are made. In one embodiment, a scavenger electrode is fashioned by pressing together 25 micrometer thick copper foil as a scavenger support 28 with 75 micrometer thick lithium foil to produce a laminated scavenger electrode as depicted in Figures 1.

Various suitable separators 16 are used, each separator sheet 16a or 16b typically having a thickness of between 10 and 30 micrometers.

Various suitable fluid electrolytes are used, for example EQDMC (1:2) IM LiPF₆.

The dimensions of the electrodes 12 and 14 are typically 100 micrometer thick, 2 cm wide by 4 cm long. The dimensions of the two separator sheets 16a and 16b are typically 10- 30 micrometer thick, 2.5 cm wide by 4.5 cm long.

The electrochemical cells are assembled from the separate components in the usual way and as described herein, where the scavenger electrode 26 is positioned between the two separator layers 16a and 16b that are positioned between the two electrodes 12 and 14.

The electrochemical cells are tested in the usual way, including repeated charge / discharge cycles.

Typical results are depicted in Figure 2A and compared to the results depicted in Figure 2B of a similar electrochemical cell that instead of scavenger electrode 26 has an inert sheet of polyethylene having the same dimensions as the scavenger electrode.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Citation or identification of any balancing in this application shall not be construed as an admission that such balancing is available as prior art to the invention.

Section headings are used herein to ease understanding of the specification and should not be construed as necessarily limiting.

Claims

WHAT IS CLAIMED IS:

1. A secondary electrochemical cell, comprising:

a. a positive electrode including a positive active material;

b. a negative electrode including a negative active material facing said positive electrode;

c. a separator positioned between said positive electrode and said negative electrodes and electrically- insulating said positive electrode from said negative electrode;

d. an electrolyte contacting said positive electrode, said negative electrode and said separator; and

e. an inter-electrode scavenger component of alkali metal positioned between said positive electrode and said negative electrode, electrically- insulated from said positive electrode and said negative electrode, and in contact with said electrolyte.

2. The electrochemical cell of claim 1, wherein said scavenger component constitutes a portion of a scavenger electrode of the electrochemical cell, and further comprising a scavenger contact functionally associated with and thereby in electrical contact said scavenger component.

3. The electrochemical cell of any of claims 1 to 2, wherein said scavenger component is physically separate from said separator.

4. The electrochemical cell of any of claims 1 to 2, wherein said scavenger component is attached to a part of said separator.

5. The electrochemical cell of any of claims 1 to 4, wherein said scavenger component is contained inside said separator.

6. A method of making an electrochemical cell, comprising:

a. providing a positive electrode including a positive active material,

b. positioning a negative electrode including a negative active material facing said positive electrode;

c. positioning a separator between said positive electrode and said negative electrode to electrically insulate said positive electrode and said negative electrode; and d. positioning an inter-electrode scavenger component of alkali metal between said positive electrode and said negative electrode so that said scavenger component is electrically- insulated from said positive electrode and said negative electrode.

7. The metho d o f claim 3 ,

wherein said positive electrode is functionally associated with a positive contact, wherein said negative electrode is functionally associated with a negative contact, and wherein said scavenger component constitutes a portion of a scavenger electrode and is functionally associated with a scavenger contact, and further comprising:

sealing said positive electrode, said negative electrode, said separator and said inter- electrode scavenger inside a electrochemical cell- container such that said positive contact, said negative contact and said scavenger contact provide electrical contact from outside said electrochemical cell- container with a respective component inside said electrochemical cell- container.

8. A separator suitable for use with a secondary electrochemical cell having a laminated structure, comprising:

b. facing said first layer, a second electrically-insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte; and

c. an inter-electrode scavenger component of alkali metal positioned between said first layer and said second layer.

9. The separator of claim 8, wherein said scavenger component is physically separate from said first layer and from said second layer.

10. The separator of claim 8, wherein said scavenger component is attached to at least one of said first layer and said second layer.

11. The separator of any claim 9 to 10, wherein said first layer and said second layer are mutually attached so as to contain said scavenger component therebetween.

12. The separator of any of claims 8 to 11, wherein said scavenger component is configured to constitute a portion of a scavenger electrode of an electrochemical cell, and further comprising a scavenger contact functionally associated with and thereby in electrical contact with said scavenger component.

13. A secondary electrochemical cell, comprising a separator of any of claims 8 to 12.

14. A method of making a separator having a laminated structure suitable for use with a secondary electrochemical cell, comprising:

b. positioning a second electrically- insulating separator layer, permeable to the passage of alkali metal ions in an electrolyte facing said first layer; and

c. positioning an inter-electrode scavenger component of alkali metal between said first layer and said second layer.

15. The method of claim 14, further comprising: attaching said scavenger component to at least one of said first layer and said second layer.

16. The method of any of claims 14 to 15, further comprising: attaching said first layer to said second layer.

17. The electrochemical cell, separator or method of any of claims 1 to 16, wherein said alkali metal of said scavenger component comprises lithium metal.

18. The electrochemical cell or method of claim 17, wherein the electrochemical cell is a lithium ion electrochemical cell.

19. The electrochemical cell or method of claim 17, wherein the electrochemical cell is a sodium ion electrochemical cell.

20. The electrochemical cell, separator or method of any of claims 1 to 16, wherein said scavenger component comprises sodium metal.

21. The electrochemical cell or method of claim 20, wherein the electrochemical cell is a sodium ion electrochemical cell.

22. The electrochemical cell, separator or method of any of claims 1 to 21, wherein said scavenger component comprises at least one form selected from the group consisting of particles, sheets and wires.

23. The electrochemical cell, separator or method of any of claims 1 to 22, wherein said scavenger component is permeable to the passage of ions.

24. The electrochemical cell, separator or method of any of claims 1 to 23, wherein said scavenger component is a layer on a scavenger substrate.

25. The electrochemical cell, separator or method of claim 24, wherein said scavenger substrate is substantially electrically not-conductive.

26. The electrochemical cell, separator or method of any of claims 24 to 25, wherein said scavenger substrate is said separator.

27. The electrochemical cell, separator or method of claim 24, wherein said scavenger substrate is electrically conductive.

28. A method for balancing the capacity of electrodes in an alkali- metal ion secondary electrochemical cell, comprising:

a. positioning a balancing electrode including an alkali metal between a positive electrode and a negative electrode of the electrochemical cell so that said balancing electrode is electrically- insulated from said positive electrode and from said negative electrode;

b. bringing said positive electrode to a given capacity by passing electrical current and alkali metal ions between said balancing electrode and said positive electrode; and c. bringing said negative electrode to a given capacity, by passing electrical current and alkali metal ions between said balancing electrode and said negative electrode wherein said given capacity of said negative electrode is within about 5% of said given capacity of said positive electrode, thereby balancing said positive electrode and said negative electrode.

29. The method of claim 28, wherein said alkali metal is lithium and said alkali metal ion is lithium ion.

30. The method of claim 28, wherein said alkali metal is sodium and said sodium metal ion is sodium ion.

31. The method of any of claims 28 to 30, wherein 'a' is prior to 'b'.

32. The method of any of claims 28 to 30, wherein 'b' is prior to 'a'.

33. The method of any of claims 28 to 30, wherein 'a' and 'b' are substantially contemporaneous.