WO2022145030A1

WO2022145030A1 - Salt containing electrolytes that promote the formation of proton-conducting rechargeable batteries

Info

Publication number: WO2022145030A1
Application number: PCT/JP2020/049281
Authority: WO
Inventors: Kwohsiung YOUNG; Ryohei Yoshida
Original assignee: Kawasaki Motors, Ltd.
Priority date: 2020-12-29
Filing date: 2020-12-29
Publication date: 2022-07-07

Abstract

Provided are ionic liquids with one or more salt additives for use as electrolyte in a proton-conducting rechargeable battery. The ionic liquids include a proton conductive ionic liquid that in some aspects is formed of an aprotic ionic liquid and one or more organic acids, where ionic liquid further includes one or more salt additives suitable to produce a maximum discharge capacity of a proton conducting electrochemical cell housing the electrolyte of greater than 1000 mAh/g of the anode electrochemically active material, wherein electrode electrochemically active material is associated with a stainless steel current collector.

Description

SALT CONTAINING ELECTROLYTES THAT PROMOTE THE FORMATION OF PROTON-CONDUCTING RECHARGEABLE BATTERIES

This disclosure relates to batteries, more specifically rechargeable batteries that cycle protons between the anode and the cathode in the generation of an electrical current that may be used to power one or more devices.

The low equivalent weight alkali metals, such as lithium, render them particularly attractive as a battery component. Lithium provides greater energy per weight than previously used nickel and cadmium. An important challenge in the development of rechargeable lithium metal batteries, however, is effective cell cycling. On repeated charge and discharge, lithium “dendrites” are gradually produced on the surface of the lithium metal electrode. These may eventually grow to such an extent that they contact the cathode causing an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. Moreover, silicon that is commonly used as an anode material in lithium ion batteries due to very high theoretical specific capacity (4000 mAh/g), undergoes a dramatic volumetric lattice expansion when cycling with lithium. This expansion of as much as 400% further reduces cycle life and prevents the materials from being effectively used in many systems.

An alternative and attractive technology for rechargeable batteries relies on the cycling of the very low molecular weight hydrogen atom. It is known that some materials such as metal hydride alloys are capable of absorbing and desorbing hydrogen. When paired with an appropriate cathode material, these hydrogen storage materials can be employed in fuel cells and metal hydride batteries.

Silicon is also an attractive anode material in proton conducting batteries that it theoretically provides high gravimetric energy of hydrogen storage. The alkaline aqueous electrolyte typically used in the conventional metal hydride battery, however, is corrosive to silicon based materials making efforts at producing proton conducing rechargeable batteries employing Si as an anode active material difficult. Hydroxide ion is also much heavier than hydrogen ion (proton). Collectively, the use of Si based electrochemically active materials has been hampered by these negative consequences of electrolyte reactions.

It was also found that when Si was used for proton-conducting batteries, that the cells showed relatively poor capacity and formation characteristics. As such, there is a need for improved materials for use in proton conducting electrochemical cells that employ hydrogen storage materials and processes of their manufacture or activation. As will be explained herein below, the present disclosure addresses these needs by providing ionic salts that may be used in proton-conducting electrochemical cells, and include one or more salt additives that were found to dramatically improve discharge capacity of the batteries and allow the use of previously difficult to employ but stable current collector materials. These and other advantages of the disclosure will be apparent from the drawings, discussion, and description which follow.

The following summary is provided to facilitate an understanding of some of the innovative features unique to the present disclosure and is not intended to be a full description. A full appreciation of the various aspects of the disclosure can be gained by taking the entire specification, claims, drawings, and abstract as a whole. The inventions as described herein are presented in the claims that follow.

Proton conducting batteries have numerous advantages including fast ion conduction, high energy density, relatively low cost and improved safety profiles relative to lithium ion batteries. Among the challenges of bringing proton conducing batteries to the marketplace has been improving capacity. Provided herein are proton conducing batteries that employ ionic liquid electrolytes supplemented with one or more salt additives that allow the use of stainless steel current collectors that are not susceptible to corrosion by electrolytes used in proton conducting batteries. The presence of these salt additives dramatically improves discharge capacity of the electrochemical cells relative to otherwise identical cells not using the salt additives.

As such, provided are ionic liquids for use in a proton-conducting rechargeable batteries that employ these ionic liquids as electrolytes. An ionic liquid for use in proton-conducting rechargeable batteries includes: a proton conductive ionic liquid, the proton conductive ionic liquid optionally comprising an aprotic ionic liquid and one or more organic or inorganic acids; and two or more salt additives suitable to produce a maximum discharge capacity of a proton conducting electrochemical cell housing the electrolyte greater than 1000 mAh/g of the chemically active material, optionally 1100 mAh/g of the anode electrochemically active material, wherein the anode electrochemically active material is associated with a stainless steel current collector.

The additives are optionally an pH buffer salt, metallic salt, or combinations thereof, and may collectively be termed a salt additive herein. The electrolyte may include 2, 3, 4, or more such salts. In some aspects, the metallic salt additive is an organic or inorganic salt of Fe, Ni, or combinations thereof. Optionally, the metallic salt additive is a phosphate, carbonate, or sulfate of Fe, Ni, or combinations thereof. Optionally, alone or in addition to a metallic salt additive that is a phosphate, carbonate, or sulfate of Fe, Ni, or combinations thereof, the electrolyte includes a pH buffer salt additive may have a pKa value in water between 1 and 14. In some aspects, the pH buffer salt additive is a phosphate, carbonate, or sulfate of potassium or sodium.

In some aspects, in a proton conductive ionic liquid that also includes one or more aprotic compounds. The one or more aprotic compounds may include a cation selected from the group consisting of ammonium ions and phosphonium ions. Optionally, the ammonium ion is an imidazolium ion, the phosphonium ion is a pyrrolidinium ion, or both are present. When an ammonium ion is present, the ammonium ion is optionally an alkylimidazolium wherein the alkyl has 1-6 carbons. In any of the foregoing, the aprotic compound may include an anion selected from the group consisting of methides, nitrate, carboxylates, imides, halides, borates, phosphates, phosphinates, phosphonates, sulfonates, sulfates, carbonates and aluminates. Optionally, a said carboxylate is an acetate. In specific aspects, an aprotic compound includes dimethylimdiazolium, 1-ethyl-3-methylimidazolium, 1,2,3-trimethylimidazolium, tris-(hydroxyethyl)methylammonium, 1,2,4-trimethylpyrazolium, or combinations thereof. In any of the foregoing or as otherwise provided herein the electrolyte optionally includes less than 5 weight percent water.

Also provided are proton-conducting rechargeable batteries that include the electrolyte as provided in any aspect above or otherwise herein. A proton conducting battery may also include an anode including anode electrochemically active material comprising Si_xM_1-x wherein M comprises one or more non-Si group 14 elements, and wherein 0 ＜ x ≦ 1, wherein a microstructure of the anode electrochemically active material is amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, or a combination of polycrystalline, nanocrystalline and amorphous. Optionally, the electrochemical cell has a maximum discharge capacity in excess of 1500 mAh/g per weight of anode electrochemically active material.

In the above batteries, or in Si containing alloys for use in proton conducting batteries, the anode active material may contain, in some aspects, 1-3 different group 14 elements, optionally two group 14 elements, optionally one group 14 element. A group 14 element in an anode electrochemically active material is optionally Si. In some aspects, the anode active material contains no metals or metalloids other than one or more group 14 elements. Optionally, the anode electrochemically active material includes Si and one or more non-Si group 14 elements, optionally C, Ge, or combinations thereof. The non-Si group 14 elements are optionally present at 50 atomic percent or less relative to the total group 14 elements in the anode electrochemically active material. In addition, in some aspects and anode electrochemically active material further includes one or more non-group 14 element containing hydrogen storage materials, optionally at 50 weight percent or less.

The batteries of either or both of the preceding two paragraphs may have a discharge capacity above 800 mAh/g of anode electrochemically active material above 1 Volt vs. Ni(OH)₂ based cathode, optionally above 1000 mAh/g above 1 Volt, optionally above 1500 mAh/g above 1 Volt. In some aspects, a battery has a maximum discharge capacity of the rechargeable battery is above 3500 mAh/g of anode electrochemically active material.

In any of the aspects of any of the preceding paragraphs of this section, the battery optionally includes a cathode with a cathode electrochemically active material that can absorb and desorb a proton. Optionally, the cathode electrochemically active material may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, a hydride thereof, an oxide thereof, a hydroxide thereof, an oxyhydroxide thereof, or any combination of the foregoing. In some aspects, the cathode electrochemically active material includes Ni at greater than or equal to 10 atomic percent relative to all metals in the cathode electrochemically active material, optionally at equal to or greater than 80 atomic percent, optionally 90 atomic percent. Optionally, the cathode electrochemically active material includes a hydroxide of Ni, Co, Mn, Zn, Al, or combinations thereof.

The cathode, anode, and electrolyte of any of the foregoing paragraphs are optionally presented in a housing. The anode and cathode are optionally separated by a separator. The anode includes an anode current collector, and the cathode includes a cathode current collector, whereby the anode current collector and the cathode current collector are electrically associated by one or more electron conducting conduits.

The proton conducting batteries are capable of achieving excellent capacity and dramatically pushing the technologies closer to theoretical maximums.

FIG. 1 illustrates an x-ray diffraction (XRD) pattern of a silicon sample used for an anode electrochemically active material according to some aspects as provided herein with a polycrystalline microstructure; FIG. 2 illustrates an XRD pattern of a silicon sample used for an anode electrochemically active material according to some aspects as provided herein with a mixture of polycrystalline, nanocrystalline and amorphous Si; FIG. 3 illustrates an XRD pattern of a silicon sample used for an anode electrochemically active material according to some aspects as provided herein with a nanocrystalline and amorphous microstructure; FIG. 4 illustrates a test cell as used to characterize anode electrochemically active materials and electrolytes as provided herein; FIG. 5 illustrates the discharge voltage profile at cycle 28 for sample 1 (polycrystalline Si) and sample 2 (mixture of polycrystalline, nanocrystalline and amorphous Si); FIG. 6 illustrates the discharge voltage profile at cycle 31 for sample 3 (nanocrystalline and amorphous Si); FIG. 7 illustrates an XRD pattern of a SiGe alloy provided according to some aspects as provided herein illustrating a combination of nanocrystalline and amorphous microstructure; FIG. 8 illustrates cycling of an exemplary battery including a SiGe anode illustrating high discharge capacity rapidly reached during formation; FIG. 9 illustrates a discharge voltage profile at cycle 27 of an exemplary battery including a SiGe anode; and FIG. 10 illustrates discharge capacity over the first 14 cycles using cells constructed with heat treated or control (no anneal) polycrystalline Si as an anode electrochemically active material.

Provided are ionic liquids that can be used as an electrolyte in proton conducting rechargeable batteries employing stainless steel current collectors. By employing particular additives to the ionic liquids, it was found that the electrolytes produce electrochemical cells with dramatically improved discharge capacity. It was found that the addition of one or more suitable additives, such as a suitable organic or inorganic salt or combination of salts, dramatically improves the formation of a proton conducting electrochemical cell employing the ionic liquid as an electrolyte, and thereby improves the discharge capacity achievable by the cell. In the otherwise identical cell, it was found that the addition of one or more such additives could boost the maximum capacity above 1000 mAh/g of anode electrochemically active material relative to the discharge capacity on stainless steel current collectors in the absence of such additives.

As such, provided herein are electrolytes and proton conducting rechargeable batteries that employ these electrolytes that use stainless steel current collectors and are capable of achieving discharge capacities of 1000 mAh/g of an anode electrochemically active material or greater.

The proton conducting batteries as provided herein differ from traditional metal hydride batteries for many reasons. This new generation of proton conducting batteries operate traditionally by cycling hydrogen between the anode and the cathode. The anodes thereby form a hydride of one or more elements in the anode during charge. This hydride is formed reversibly such that during discharge the hydride becomes the elemental portion of the anode electrochemically active material generating both a proton and an electron. The half reaction that takes place at the anode can be described as per the following half reaction:

where M as provided herein is or includes one or more group 14 elements.

The corresponding cathode reaction half reaction is typically:

wherein M_c is any suitable metal(s) for use in a cathode electrochemically active material, optionally Ni.

As used herein, the term “battery” or “cell” may be used interchangeably. Optionally, a batter is a collection of two or more cells, wherein each cell may function as a proton conducting battery.

As used herein, an “anode” includes an electrochemically active material that acts as an electron acceptor during charge.

As used herein, a “cathode” includes an electrochemically active material that acts as an electron donor during charge.

As used herein, an “electrochemically active” material is one that includes one or more elements that are able to reversibly absorb a hydrogen ion.

When atomic percentages (at%) are presented and not otherwise defined, the atomic percentages are presented on the basis of the amount of all elements in the described material other than hydrogen and oxygen.

An electrolyte as provided herein includes a proton conductive ionic liquid, the proton conductive ionic liquid optionally includes an aprotic ionic liquid and one or more acids, and one or more salt additives suitable to produce a maximum discharge capacity of a proton conducting electrochemical cell housing the electrolyte of greater than 1000 mAh/g of anode electrochemically active material. The one or more additives suitable to produce a maximum capacity of a proton conducting electrochemical cell housing the electrolyte to a discharge capacity of greater than 1000 mAh/g per weight of anode electrochemically active material are optionally a pH buffer salt, a metallic salt, or combinations thereof. A pH buffer salt additive is optionally an organic or inorganic salt of an alkali metal or alkali earth metal, optionally potassium or sodium. A metallic salt is optionally an organic or inorganic salt of a transition metal, optionally Ni or Fe. In some aspects, both pH buffer salt and a metallic salt are present in the electrolyte. Optionally, an electrolyte includes a pH buffer salt and both salts of nickel and iron. In some aspects, an electrolyte includes one or more pH buffer salts in addition to a Ni²⁺ metallic salt and a Fe²⁺ metallic salt.

An electrolyte as provided herein includes one or more metallic salt additives. A metallic salt additive is an organic or inorganic salt of a transition metal. Illustrative examples of a metallic salt additive suitable to produce a proton conducting electrochemical cell housing the electrolyte to a discharge capacity of greater than 1000 mAh/g per weight of anode electrochemically active material optionally includes phosphates, carbonates, or sulfates of a transition metal. Optionally, a metallic salt includes phosphates, carbonates, or sulfates of nickel or iron. Optionally, a metallic salt is a phosphate, carbonate, or sulfate of Ni²⁺, Fe²⁺, or combinations thereof. Optionally, a metallic salt is a sulfate of Ni²⁺, Fe²⁺, or combinations thereof. Optionally, sulfates of both Ni²⁺ and Fe²⁺ are present in an electrolyte. Illustrative examples of nickel salts include divalent nickel salts, optionally nickel phosphates and nickel sulfate. Illustrative examples of iron salts include divalent iron salts, optionally iron phosphates and iron sulfate.

In addition to a metallic salt, an electrolyte may further include a pH buffer salt additive. A pH buffer salt is an organic or inorganic salt that optionally includes those with a pKa in water of 1-14, optionally 3-13, optionally 7-13, optionally 3-8. Illustrative examples of a pH buffer salt additive suitable to produce a proton conducting electrochemical cell housing the electrolyte to a discharge capacity of greater than 1000 mAh/g per weight of anode electrochemically active material optionally includes phosphates, carbonates, or sulfates of potassium or sodium. Specific illustrative examples of potassium salts include, but are not limited to potassium phosphate such as mono- or dipotassium phosphate, potassium carbonate, potassium sulfate, among others. Illustrative examples of sodium salts include, but are not limited to sodium mono-, di-, tetra-phosphate, sodium bicarbonate, and sodium hydrogen sulfate.

The salt additive suitable to produce a proton conducting electrochemical cell housing the electrolyte to a discharge capacity of greater than 1000 mAh/g per weight of anode electrochemically active material may be present in an electrolyte at 0.01 to 1 moles/kg (m), optionally 0.01 to 0.2 m, optionally 0.1 to 1 m. Optionally, a metallic salt is present at a level relative to a pH buffer salt of 1-4x by molar concentration. Optionally, a nickel or iron salt is present at a level relative to a pH buffer salt of 2-3x by molar concentration.

The electrolytes as provided herein are a proton conducting electrolyte, optionally a non-aqueous proton conducting electrolyte. The electrolyte is disposed between the anode electrochemically active material and the cathode electrochemically active material in a proton conducting battery employing such an electrolyte and allows the flow or other transfer of protons between the anode and the cathode. The ionic liquid is optionally nonaqueous, which in the context of the present disclosure means that the ionic liquid includes less than 10 weight percent (wt%) water, optionally less than 5 wt% water, optionally less than 1 wt% water. In some aspects, a ionic liquid includes less than 100 ppm water, less than 50 ppm water, optionally less than 10 ppm water.

The ionic liquid optionally includes one or more aprotic compounds alone or in combination with one or more proton sources such as an organic acid. An aprotic compound is any compound suitable for use in an electrolyte and that is not otherwise detrimentally reactive with any other component of an electrochemical cell. Illustrative examples of an aprotic compound include ammonium or phosphonium compounds, optionally where the ammonium or phosphonium includes one or more linear, branched or cyclic substituted or non-substituted alkyl groups connected to a nitrogen or phosphorous.

An aprotic compound optionally includes an ammonium or phosphonium compound with 1, 2, or more linear, branched or cyclic substituted or non-substituted alkyl groups bound to a positively charged nitrogen or phosphorus atom. Optionally, the aprotic compounds include one such alkyl, optionally two such alkyls that may be the same or different. Optionally, the ammonium or phosphonium compound alkyl is or includes 1-6 carbon atoms, optionally 1-4 carbon atoms and may be branched, linear or cyclic. In some aspects, the nitrogen or phosphorous is a member of a 5 or 6 membered ring structure that may have one or more pendant groups extending from the central ring. Optionally, ammonium ion is an imidazolium ion. Optionally, a phosphonium ion is a pyrrolidinium ion.

In some aspects, an ammonium or phosphonium includes 1 or 2 linear or cyclic, substituted or unsubstituted alkyls of 1-6 carbon atoms. Optionally, the alkyl includes 2, 3, 4, 5, or 6 carbons. In some aspects, the aprotic compound includes 1 or 2 alkyls of 1-6 carbons. A substitution in an alkyl is optionally a nitrogen, oxygen, sulfur, or other such element. Optionally, an ammonium or phosphonium includes a ring structure with 5-6 members where the ring is substituted with an N, an O, or P.

Illustrative examples of an aprotic compound for use as an electrolyte include, but are not limited to 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1,3-dimethylimdiazolium, 1-ethyl-3-methylimidazolium, 1,2,3-trimethylimidazolium, tris-(hydroxyethyl)methylammonium, 1,2,4-trimethylpyrazolium, or combinations thereof.

The aprotic compound optionally includes one or more anions in conjunction with the aprotic compound. Illustrative examples of an anion include but are not limited to methides, nitrate, carboxylates, imides, halides, borates, phosphates, phosphinates, phosphonates, sulfonates, sulfates, carbonates and aluminates. Further illustrative examples may be found in U.S. Patent Nos: 6,254,797 and 9,006,457. In specific exemplary aspects, an anion includes carboxylates such as an acetate, phosphates such as a hydrogen, alkyl, or fluorophospate, phophinates such as alkyl phosphinates, among others. Illustrative examples of such aprotic compounds include but are not limited to acetates, sulfonates, or borates of 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1,3-dimethylimdiazolium, 1-ethyl-3-methylimidazolium, 1,2,3-trimethylimidazolium, tris-(hydroxyethyl)methylammonium, 1,2,4-trimethylpyrazolium, or combinations thereof. Specific examples include diethylmethylammonium trifluoromethanesulfonate (DEMA TfO), 1-ethyl-3-methylimidazolium acetate (EMIM Ac) or 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM TFSI).

In addition to the aprotic compound, an electrolyte is optionally further supplemented with an organic acid that serves as a proton donor. The presence of the organic acid improves the overall proton conductivity of the electrolyte and thereby improves the function of the proton conducting battery employing the electrolyte. Optionally, an organic acid is a carboxylate. Illustrative examples of a carboxylate include those with 0-10 or more carbons attached to a terminal carboxylic acid. Specific illustrative examples include an acetic acid such as acetic acid or haloacetic acid (e.g. with 1-3 fluorine or chlorine atoms). Optionally an organic acid is acetic acid.

The organic acid is optionally present in the electrolyte at 1-5 moles/kg (m). Optionally, the organic acid is present at 3-4 m. Optionally, the organic acid is present at a concentration that is 3-3.5 m.

Also provided are proton-conducting electrochemical cells that utilize the ionic liquids as provided herein as an electrolyte and are capable of employing stainless steel current collectors that, when in combination with an electrolyte as provided herein, produce cells that demonstrate excellent capacity, optionally at or above 1000 mAh/g of anode electrochemically active material. A proton conducting electrochemical cell as provided herein includes a cathode, an anode, and an electrolyte as also described herein. An anode includes an anode electrochemically active material that serves to reversibly absorb a proton. A cathode includes a cathode electrochemically active material that serves to reversibly absorb a proton. The anodes or cathodes include a current collector upon which the anode electrochemically active material and cathode electrochemically active material are coated or otherwise contacted to.

A current collector may be formed of steel such as stainless steel, nickel-plated steel, aluminum optionally an aluminum alloy, nickel or nickel alloy, copper or copper alloys, or other such material. For an anti-corrosion property in acid electrolyte, a current collector is may be formed of stainless steel. Optionally, both the current collector of the anode and the cathode are formed of stainless steel. In contrast to Ni-plated current collectors, stainless steel current collectors do not possess the necessary ions to activate a Si containing anode electrochemically active material. It was found that addition of the metallic salt additives of the present disclosure into the electrolyte produced improved cell formation when stainless steel current collectors are used and achieving a discharge capacity in excess of 1000 mAh/g of anode electrochemically active material. Without the addition of the additives to the electrolyte, stainless steel current collectors could not produce sufficiently high discharge capacities, typically not exceeding 950 mAh/g, even with the addition of pH buffer salts to the electrolyte. Without being limited to one particular theory, it is believed that nickel plated current collectors also suffer from corrosion by electrolyte. This corrosion releases Ni²⁺and Fe²⁺ into the electrolyte that improves cell formation and resulting high discharge capacities. When stainless steel is substituted for the nickel plated current collector, these ions are not available in the system. The additives of this disclosure serve to supplement Ni²⁺and Fe²⁺ ions thereby dramatically improving discharge capacities achieved by the cells.

A current collector is optionally in the form of a sheet, and may be in the form of a foil, solid substrate, porous substrate, grid, foam or foam coated with one or more metals, or other form known in the art. In some aspects a current collector is in the form of a foil. Optionally, a grid may include expanded metal grids and perforated foil grids.

The anode electrochemically active material as used in an anode according to some aspects of a battery as provided herein includes one or more group 14 elements. The anode electrochemically active materials as provided herein according to some aspects, has a microstructure that is amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, or a combination of polycrystalline, nanocrystalline and amorphous.

An anode electrochemically active material optionally includes one or more group 14 elements. Group 14 elements include carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). In some aspects, a group 14 element excludes Pb. Optionally, a group 14 element is C, Si, Ge, or any combination thereof. In some aspects, an anode electrochemically active material includes Si. Optionally, an anode electrochemically active material includes C. Optionally, an anode electrochemically active material includes Ge.

In some aspects, an anode electrochemically active material includes two or more group 14 elements. Si is an expensive element to source for use in large quantities. It was found that some Si may be substituted by other non-Si group 14 elements and provide robust capacity to a proton conducting battery employing the materials as anode electrochemically active materials. In addition, some such alloys may be sourced as waste materials used by other industries, such as the solar industry that readily produces large amounts of SiGe alloys as a waste product. As such, in some aspects, an anode electrochemically active material includes two group 14 elements. Optionally, an anode electrochemically active material includes three group 14 elements. In some aspects, an anode electrochemically active material includes Si and C. Optionally, an anode electrochemically active material includes Si and Ge. Optionally, an anode electrochemically active material includes C and Ge. Optionally, an anode electrochemically active material includes Si, C, and Ge.

An anode electrochemically active material optionally includes Si_xM_1-x wherein M comprises one or more non-Si group 14 elements, and wherein 0 < x< 1. As described above, M is optionally C, Ge, or any combination thereof. Optionally, M is C. Optionally, M is Ge. Optionally, x is 0.5 or greater, optionally x is 0.55 or greater, optionally x is 0.6 or greater, optionally x is 0.65 or greater, optionally x is 0.7 or greater, optionally x is 0.71 or greater, optionally x is 0.72 or greater, optionally x is 0.73 or greater, optionally x is 0.74 or greater, optionally x is 0.75 or greater, optionally x is 0.76 or greater, optionally x is 0.77 or greater, optionally x is 0.78 or greater, optionally x is 0.79 or greater, optionally x is 0.8 or greater, optionally x is 0.85 or greater, optionally x is 9 or greater, optionally x is 0.95 or greater, optionally x is 0.96 or greater, optionally x is 0.97 or greater, optionally x is 0.98 or greater, or optionally x is 0.99 or greater.

An anode electrochemically active material according to some aspects includes Si and one or more non-Si group 14 elements, optionally C and/or Ge. The non-Si group 14 elements are optionally present at 50 atomic percent or less relative to all group 14 elements in the anode electrochemically active material. Optionally, the non-Si group 14 elements are optionally present at 45 atomic percent or less, optionally 40 atomic percent or less, optionally 35 atomic percent or less, optionally 30 atomic percent or less, optionally 29 atomic percent or less, optionally 28 atomic percent or less, optionally 27 atomic percent or less, optionally 26 atomic percent or less, optionally 25 atomic percent or less, optionally 24 atomic percent or less, optionally 23 atomic percent or less, optionally 22 atomic percent or less, optionally 21 atomic percent or less, optionally 20 atomic percent or less, optionally 15 atomic percent or less, optionally 10 atomic percent or less, optionally 5 atomic percent or less, optionally 4 atomic percent or less, optionally 3 atomic percent or less, optionally 2 atomic percent or less, or optionally 1 atomic percent or less.

In some aspects, an anode electrochemically active material includes Si and Ge, wherein the Ge is present at 50 atomic percent or less relative to all group 14 elements in the anode electrochemically active material. Optionally, the Ge is present at 45 atomic percent or less, optionally 40 atomic percent or less, optionally 35 atomic percent or less, optionally 30 atomic percent or less, optionally 29 atomic percent or less, optionally 28 atomic percent or less, optionally 27 atomic percent or less, optionally 26 atomic percent or less, optionally 25 atomic percent or less, optionally 24 atomic percent or less, optionally 23 atomic percent or less, optionally 22 atomic percent or less, optionally 21 atomic percent or less, optionally 20 atomic percent or less, optionally 15 atomic percent or less, optionally 10 atomic percent or less, optionally 5 atomic percent or less, optionally 4 atomic percent or less, optionally 3 atomic percent or less, optionally 2 atomic percent or less, or optionally 1 atomic percent or less.

In other aspects, an anode electrochemically active material includes Si and C, wherein the C is present at 50 atomic percent or less relative to all group 14 elements in the anode electrochemically active material. Optionally, the C is present at 45 atomic percent or less, optionally 40 atomic percent or less, optionally 35 atomic percent or less, optionally 30 atomic percent or less, optionally 29 atomic percent or less, optionally 28 atomic percent or less, optionally 27 atomic percent or less, optionally 26 atomic percent or less, optionally 25 atomic percent or less, optionally 24 atomic percent or less, optionally 23 atomic percent or less, optionally 22 atomic percent or less, optionally 21 atomic percent or less, optionally 20 atomic percent or less, optionally 15 atomic percent or less, optionally 10 atomic percent or less, optionally 5 atomic percent or less, optionally 4 atomic percent or less, optionally 3 atomic percent or less, optionally 2 atomic percent or less, or optionally 1 atomic percent or less.

It is appreciated that an anode electrochemically active material may include one or more other non-group 14 elements. Illustrative examples of non-group 14 elements include, but are not limited to lithium, boron, sodium, magnesium, and aluminum. Optionally, when a non-group 14 element(s) is present, the element is at 50 atomic percent or less, optionally 20 at% or less, optionally 10 at% or less, optionally 5 at% or less, optionally 4 at% or less, optionally 3 at% or less, optionally 2 at% or less, optionally 1 at% or less.

The Si component of the anode electrochemically active material is characterized by a microstructure. The microstructure of Si in an anode electrochemically active material is optionally amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, amorphous, or a combination of polycrystalline, nanocrystalline and amorphous. Optionally, a microstructure is not solely amorphous.

Optionally, a microstructure of the Si material in the anode electrochemically active material is or includes polycrystalline. Polycrystalline silicon is formed of multiple small silicon crystals or crystallites. The multiple crystallites are typically randomly arranged. Illustratively, polycrystalline Si may be obtained from any recognized commercial supplier, illustratively Wacker Chemi or Hemlock Semiconductor, among others.

Optionally, a microstructure of the Si in the anode electrochemically active material is a combination of nanocrystalline and amorphous. Nanocrystalline silicon is a form of silicon with a paracrystalline structure that typically includes an amorphous phase, but the nanocrystalline silicon differs from amorphous Si in that the nanocrystalline silicon also includes grains of crystalline silicon within the amorphous phase. Typical sources of nanocrystalline silicon include Strem (USA) and Cenate (Norway).

In some aspects, a microstructure of the Si in the anode electrochemically active material is a combination of polycrystalline, nanocrystalline and amorphous. When polycrystalline Si is present in a mixture of other microstructure silicon(s), the percentage (mass) of polycrystalline phase is 20 percent or less. Optionally, the percentage of polycrystalline Si is 15 percent or less, optionally 10 percent or less, optionally 5 percent or less.

It was found that exposure to air alters Si containing anode electrochemically active materials to show reduced maximum discharge capacity, often nearly 10-fold lower than when fresh Si containing anode electrochemically active materials are employed. As such, in some aspects, Si containing anode electrochemically active materials are subjected to a treatment step prior to assembly into an anode where the treatment step substantially prevents or reverses electrochemical degradation of the Si containing anode electrochemically active materials resulting in much greater maximum discharge capacities. Treatment processes include providing an anode electrochemically active material that includes Si in at least a portion of the material, and treating the anode electrochemically active material in an atmosphere including hydrogen for a treatment time and a treatment temperature sufficient to reduce or reverse electrochemical performance degradation of the anode electrochemically active material when exposed to air. Reduction in performance degradation is measured relative to an otherwise identical anode electrochemically active material that is not treated according to the methods as provided herein where full performance is that found by an idealized equivalent Si containing material that was not previously exposed to oxygen and measured at cycle 1. In some aspects, the reduction in performance degradation is optionally greater than about 1%, optionally greater than about 10%, optionally greater than about 20%, optionally greater than about 30%, optionally greater than about 40%, optionally greater than about 50%, optionally greater than about 60%, optionally greater than about 70%, optionally greater than about 80%, optionally greater than about 90%, optionally greater than about 95%, optionally greater than about 99%. In some aspects, the reduction in performance degradation is eliminated such that full activity is achieved by treating the electrochemically active materials as provided herein.

The steps of treating include subjecting the anode electrochemically active materials to an atmosphere that includes hydrogen. The amount of hydrogen in the atmosphere is optionally similar to or identical to that of air at sea level. In some aspects, the atmosphere is air. Optionally, the atmosphere includes hydrogen that is substantially purified and may include one or more other inert gasses in addition to the hydrogen. The amount of hydrogen is optionally at a weight percent of the total atmosphere contents of or in excess of 1%, optionally greater than about 5%, optionally greater than about 10%, optionally greater than about 20%, optionally greater than about 30%, optionally greater than about 40%, optionally greater than about 50%, optionally greater than about 60%, optionally greater than about 70%, optionally greater than about 80%, optionally greater than about 90%, optionally greater than about 95%, optionally greater than about 99%. In some aspect, the atmosphere is 100 weight percent hydrogen.

The atmosphere may include one or more additional gasses that are substantially non-reactive with Si. Illustrative examples include nitrogen and argon, but other non-reactive gasses may be present as well. It is not necessary that an atmosphere exclude oxygen in the processes making the treatment process somewhat surprising in its ability to reduce the degradation normally occurring due to oxygen exposure. In some aspects, however, an atmosphere excludes oxygen or includes oxygen at less than or equal to 1 weight percent.

A treatment process for anode electrochemically active materials includes treating the anode electrochemically active material in an atmosphere including hydrogen for a treatment time. A treatment time is optionally equal to or greater than 1 minute, optionally 20 minutes, optionally 40 minutes, optionally 1 hour, optionally 2 hours, optionally 3 hours, optionally 5 hours. In some aspects, a treatment time does not exceed 2 hours, optionally 3 hours, optionally 5 hours.

A treatment process for anode electrochemically active materials is in an atmosphere including hydrogen where the treating is at a treatment temperature. It was found that treatment temperatures are critical for maximum activity of the treated electrochemically active material. Treatment temperatures of between 100 °C and 300 °C yielded optimal activity of an electrochemical cell employing the treated anode electrochemically active material in the anode. Temperatures may exceed 300 °C, but the improvement begins to decline. In some aspects, a treatment temperature should not exceed 600 °C. As such, a treatment temperature is optionally from 100 °C to 600 °C, or any value or range therebetween. Optionally, a treatment temperature is from 100 °C to 300 °C, optionally 100 °C to 200 °C. In some aspects a treatment temperature is less than 600 °C but at or greater than 100 °C, optionally 150 °C, optionally 200 °C, optionally 250 °C, optionally 300 °C, optionally 350 °C, optionally 400 °C, optionally 450 °C, optionally 500 °C, optionally 550 °C.

An anode electrochemically active material optionally includes one or more non-group 14 element containing hydrogen storage materials. If a non-group 14 element containing hydrogen storage material is present in an anode electrochemically active material, the non-group 14 element containing hydrogen storage material is optionally present at 50 weight percent or less. Optionally the non-group 14 element containing hydrogen storage material is present at 40 weight percent or less, optionally 30 weight percent or less, optionally 20 weight percent or less, optionally 10 weight percent or less, optionally 5 weight percent or less, optionally 3 weight percent or less, optionally 20 weight percent or less, optionally 1 weight percent or less, optionally 0.1 weight percent or less, optionally 0.01 weight percent or less.

Illustrative examples of a non-group 14 element containing hydrogen storage material that may be included in an anode electrochemically active material include any material known in the art as capable of electrochemically and reversibly storing hydrogen. Illustrative examples of such materials are the AB_x class of hydrogen storage materials where A is a hydride forming element, B is a non-hydride forming element and x is from 1-5. Illustrative examples include the AB, AB₂, AB₃, A₂B₇, A₅B₁₉, and AB₅ type materials as they are known in the art. A hydride forming metal component (A) optionally includes but is not limited to titanium, zirconium, vanadium, hafnium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, yttrium, or combinations thereof or other metal(s) such as a mischmetal. A B (non-hydride forming) component optionally includes a metal selected from the group of aluminum, chromium, manganese, iron, nickel, cobalt, copper, and tin, or combinations thereof. In some aspects, AB_x type materials that may be further included in an anode electrochemically active material are disclosed, for example, in U.S. Patent 5,536,591 and U.S. Patent 6,210,498. Optionally, non-group 14 element containing hydrogen storage materials are as described in Young, et al., International Journal of Hydrogen Energy, 2014; 39(36):21489-21499 or Young, et al., Int. J. Hydrogen Energy, 2012; 37:9882. Optionally, non-group 14 element containing hydrogen storage materials are as described in U.S. Patent Application Publication No: 2016/0118654. In some aspects, a non-group 14 containing hydrogen storage material includes hydroxides, oxides, or oxyhydroxides of Ni, Co, Al, Mn, or combinations thereof, optionally as described in U.S. Patent No. 9,502,715. Optionally, a non-group 14 containing hydrogen storage material includes a transition metal such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Au, Cd, or combinations thereof, optionally as disclosed in U.S. Patent No: 9,859,531.

The anode electrochemically active material is presented in a powder form, meaning that the anode electrochemically active material is a solid at 25 degrees Celsius (°C) and free of any substrate. Despite prior belief to the contrary, it was found that solid group 14 elements may be used to form hydrides in the solid state and be useful for hydrogen storage or battery applications. The powder is held together by a binder that associates the powder particles in a layer that is coated on a current collector in the formation of an anode.

An electrochemical cell as provided herein also includes a cathode that includes a cathode electrochemically active material. A cathode electrochemically active material has the capability to absorb and desorb a hydrogen ion in the cycling of a proton conducting battery so that the cathode active material functions in pair with the anode electrochemically active material to cycle hydrogen and produce an electrical current. Illustrative materials suitable for use in a cathode electrochemically active material include metal hydroxides. Illustrative examples of metal hydroxides that may be used in a cathode electrochemically active material include those described in U.S. Patent Nos: 5,348,822; 5,637,423; 5,366,831; 5,451,475; 5,455,125; 5,466,543; 5,498,403; 5,489,314; 5,506,070; 5,571,636; 6,177,213; and 6,228,535.

In some aspects, a cathode electrochemically active material includes a hydroxide of Ni alone or in combination with one or more additional metals. Optionally, an electrochemically active material includes Ni and 1, 2, 3, 4, 5, 6, 7, 8, 9, or more additional metals. Optionally, a cathode electrochemically active material include Ni as the sole metal.

Optionally, a cathode electrochemically active material includes one or more metals selected from the group of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, a hydride thereof, an oxide thereof, a hydroxide thereof, an oxyhydroxide thereof, or any combination of the foregoing. Optionally, a cathode electrochemically active material includes one or more of Ni, Co, Mn, Zn, Al, Zr, Mo, Mn, a rare earth, or combinations thereof. In some aspects, a cathode electrochemically active material includes Ni, Co, Al, or combinations thereof.

A cathode electrochemically active material may include Ni. Ni is optionally present at an atomic percentage relative to the total metals in the cathode electrochemically active material of 10 atomic percent (at%) or greater. Optionally, Ni is present at 15 at% or greater, optionally 20 at% or greater, optionally 25 at% or greater, optionally 30 at% or greater, optionally 35 at% or greater, optionally 40 at% or greater, optionally 45 at% or greater, optionally 50 at% or greater, optionally 55 at% or greater, optionally 60 at% or greater, optionally 65 at% or greater, optionally 70 at% or greater, optionally 75 at% or greater, optionally 80 at% or greater, optionally 85 at% or greater, optionally 90 at% or greater, optionally 91 at% or greater, optionally 92 at% or greater, optionally 93 at% or greater, optionally 94 at% or greater, optionally 95 at% or greater, optionally 96 at% or greater, optionally 97 at% or greater, optionally 98 at% or greater, optionally 99 at% or greater. Optionally the sole metal in the cathode electrochemically active material is Ni.

An anode electrochemically active material, a cathode electrochemically active material, or both are optionally in a powder or particulate form. The particles may be held together by a binder to form a layer on a current collector in the formation of the anode or cathode. A binder suitable for use in forming an anode, a cathode or both is optionally any binder known in the art suitable for such purposes and for the conduction of a proton.

Illustratively, a binder for use in the formation of an anode, a cathode, or both includes but is not limited to polymeric binder materials. Optionally a binder material is an elastomeric material, optionally styrene-butadiene (SB), styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS) and styrene-ethylene-butadiene-styrene block copolymer (SEBS). Illustrative specific examples of a binder include, but are not limited to polytetrfluoroethylene (PTFE), polyvinyl alcohol (PVA), teflonized acetylene black (TAB-2), styrene-butadiene binder materials, or/and carboxymethyl cellulose (CMC). Illustrative examples may be found in U.S. Patent No: 10,522,827. The ratio of electrochemically active material to binder is optionally from 4:1 to 1:4. Optionally, the ratio of electrochemically active material to binder is 1:3 to 1:2.

A cathode, anode or both may further include one or more additives intermixed with the electrochemically active materials. An additive is optionally a conductive material. A conductive material is optimally a conductive carbon. Illustrative examples of a conductive carbon include graphite. Other examples are materials that contain graphitic carbons, such as graphitized cokes. Still other examples of possible carbon materials include non-graphitic carbons that may be amorphous, non-crystalline, and disordered, such as petroleum cokes and carbon black. A conductive material is optionally present in an anode or a cathode at a weight percent (wt%) of 0.1 wt% to 20 wt%, or any value or range therebetween.

An anode or a cathode may be formed by any method known in the art. For example, an anode electrochemically active material or a cathode electrochemically active material may be combined with a binder, and optionally conductive material, in an appropriate solvent to form a slurry. The slurry may be coated onto a current collector and dried to evaporate some or all of the solvent to thereby form an electrochemically active layer on the surface of the current collector.

A proton conducing electrochemical cell may include a separator interposed between an anode and a cathode. A separator may be permeable to a hydrogen ion so as to not appreciably or unacceptably restrict ion transfer between the anode and the cathode. Illustrative examples of a separator include but are not limited to materials such as nylons, polyesters, polyvinyl chloride, glass fibers, cotton, among others. Illustratively, a separator may be polyethylene or polypropylene.

The anode, cathode, separator, and electrolyte may be housed in a cell case (e.g. housing). The housing may be in the form of a metal or polymeric can, or can be a laminate film, such as a heat-sealable aluminum foil, such as an aluminum coated polypropylene film. As such, an electrochemical cell as provided herein may be in any known cell form, illustratively, a button cell, pouch cell, cylindrical cell, or other suitable configuration. In some aspects, a housing in is in the form of a flexible film, optionally a polypropylene film. Such housings are commonly used to form a pouch cell. The proton conducting battery may have any suitable configuration or shape, and may be cylindrical or prismatic.

The current collector or substrates may include one or more tabs to allow the transfer of electrons from the current collector to a region exterior of the cell and to connect the current collector(s) to a circuit so that the electrons produced during discharge of the cell may be used to power one or more devices. A tab may be formed of any suitable conductive material (e.g. Ni, Al, or other metal) and may be welded onto the current collector. Optionally, each electrode has a single tab.

The resulting proton conducting batteries as provided herein in any aspect described optionally has a discharge capacity of the rechargeable battery above 1000 mAh/g of the anode electrochemically active material above 1 Volt vs. Ni(OH)₂ cathode. Optionally, the discharge capacity is measured following cell formation, optionally at

cycle

20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30. The battery optionally has a discharge capacity at or in excess of 1000 mAh/g, optionally 1100 mAh/g, optionally 1200 mAh/g, optionally 1300 mAh/g, optionally 1400 mAh/g, optionally 1500 mAh/g, optionally 1600 mAh/g, optionally 1700 mAh/g, optionally 1800 mAh/g, optionally 1900 mAh/g, optionally 2000 mAh/g.

In some aspects, a proton conducting battery as provided herein has a maximum capacity of or in excess of 1000 mAh/g where grams is the weight of the anode electrochemically active material and as measured against a Ni(OH)₂ cathode. Optionally, the maximum capacity, is or is in excess of 1100 mAh/g, optionally 1200 mAh/g, optionally 1300 mAh/g, optionally 1400 mAh/g, optionally 1500 mAh/g, optionally 1600 mAh/g, optionally 1700 mAh/g, optionally 1800 mAh/g, optionally 1900 mAh/g, optionally 2000 mAh/g, optionally 2500 mAh/g, optionally 3000 mAh/g, optionally 3500 mAh/g, optionally 4000 mAh/g, optionally 4500 mAh/g, optionally 5000 mAh/g, optionally 5500 mAh/g, optionally 6000 mAh/g, optionally 6500 mAh/g.

In particular aspects, an electrochemical cell as provided herein includes a cathode comprising a cathode electrochemically active material capable of storing and releasing hydrogen, an anode, the anode comprising an anode electrochemically active material of one or more group 14 elements, the anode electrochemically active material in the powder form and associated by a binder, wherein a microstructure of the anode electrochemically active material is amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, or a combination of polycrystalline, nanocrystalline and amorphous, and a non-aqueous electrolyte that includes an ammonium aprotic compound and a carboxylic acid additive, optionally where the anode electrochemically active material includes Si, and optionally includes a buffer salt additive and one or more salt additives of nickel and iron.

In other aspects, an electrochemical cell as provided herein includes an electrolyte comprising a pH buffer salt and a salt of nickel and iron, as well as a cathode comprising a cathode electrochemically active material comprising Ni, an anode, the anode comprising an anode electrochemically active material of Si and one or more non-Si group 14 elements, the anode electrochemically active material in the powder form and associated by a binder, wherein a microstructure of the anode electrochemically active material is amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, or a combination of polycrystalline, nanocrystalline and amorphous, and a non-aqueous electrolyte that includes an ammonium aprotic compound and a carboxylic acid additive, optionally where the anode electrochemically active material includes Si, and optionally includes a pH buffer salt and one or more salt additives of nickel and iron.

In some aspects, an electrochemical cell as provided herein includes a cathode comprising a cathode electrochemically active material including Ni as a predominant, an anode, the anode comprising an anode electrochemically active material of one or more group 14 elements, the anode electrochemically active material in the powder form and associated by a binder, wherein a microstructure of the anode electrochemically active material is amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, or a combination of polycrystalline, nanocrystalline and amorphous, and a non-aqueous electrolyte that includes an ammonium aprotic compound and an acetic acid additive, optionally where the anode electrochemically active material includes Si as a predominant by atomic percentage, and optionally includes one pH buffer salt and one or more salt additives of nickel and iron as provided herein.

EXPERIMENTAL

Example 1
A series of silicon containing compositions were obtained from commercial sources. Polycrystalline silicon was obtained from Alfa Aesar (US), Fijifilm (Japan), Hongwu (China), Silican (Taiwan) and Paraclete (US). Amorphous/nanocrystalline silicon was obtained from Cenate (Norway) and Strem (US). To confirm the microstructure of each of the silicons used, each sample was subjected to analysis by x-ray diffraction (XRD) utilizing a Philips X’Pert Pro x-ray diffractometer with Cu-K_α as radiation source. Sample 1 included the polycrystalline Si. A diffraction pattern is illustrated in FIG. 1. The diffraction illustrates sharp peaks at about 48° and 56° which is classically characteristic of polycrystalline silicon. The material does not appear to include any amorphous or nanocrystalline silicon. Sample 2 analyses are illustrated at FIG. 2 and includes a mixture of polycrystalline with nanocrystalline and amorphous silicon. The nanocrystalline is revealed by the broad peak at about 29° and the amorphous present as the broad peak at about 52°. It is clear from the small sharp peaks at about 48° and 56° that some amount of polycrystalline Si is also present in the sample. Sample 3 as depicted in FIG. 3 illustrates a mixture of nanocrystalline Si and amorphous Si in the absence of any polycrystalline Si.

Anodes were constructed from each of the sample silicon containing materials. The Silicon materials were in powder form and combined with a TAB-2 binder in the dry form at a weight ratio of 1:3. The materials were pressed into a Ni mesh substrate as a current collector. Ni(OH)₂ cathodes were made by standard methods using commercially sourced and sintered Ni(OH)₂. To test the electrochemical properties, the anodes were tested by forming an electrochemical cell within an all-teflon Swagelock tee. The cell used for electrochemical analyses is illustrated in FIG. 4 and includes a central gland 1 capped with ferrules 2 at both ends secured by collars 3. The sample 4 is sandwiched between two current collector rods 5 made from Ni-plated steel (NS) or stainless steel (SS). The top channel is covered with a parafilm 6 as a pressure vent device. The sample is a sandwich of an anode, cathode, and separated by a standard separator. The cell is flooded with an electrolyte including EMIM/AC with 3.33 m acetic acid that included one or more salt additives as provided herein.

The cell was cycled with a charge rate of 700 mA/g, charge time: 20 hours and a discharge rate of 70 mA/g to a discharge cut-off of 1 V or 0 V. The discharge provides following cell formation at cycle 28 for samples 1 and 2 (FIG. 5), and cycle 31 for sample 3 (FIG. 6) demonstrate high capacity in excess of 3800 mA/g (of Si) for all samples tested. Both the polycrystalline Si anode and the mix of nanocrystalline Si and amorphous Si showed maximum discharge capacity in excess of 5500 mAh/g. Results for all three samples and conditions are illustrated in Table 1.

Example 2
Polycrystalline silicon materials that also include one or more non-Si group 14 elements were obtained from commercial sources. An alloy of Si and Ge was obtained from Ge Solartech (US). The SiGe alloy was subjected to analyses using a Varian Liberty 100 inductively coupled plasma optical emission spectrometer (ICP-OES) in accord with principles known in the art. Microstructure of the alloys was studied utilizing a Philips X’Pert Pro x-ray diffractometer and a JEOL-JSM6320F scanning electron microscope with energy dispersive spectroscopy (EDS) capability. The alloy tested consisted of Si_0.78Ge_0.22. The structure of the alloy is illustrated in FIG. 7 illustrating the combination of nanocrystalline (27.7°) and amorphous (51.7°).

The SiGe alloy was formed into an anode and tested against a sintered Ni(OH)₂ as in Example 1 using the same apparatus design and using an EMIM/Ac electrolyte supplemented with 3.33 m acetic acid and 0.1 m K₂HSO₄. The proton conducting battery was charged at 700 mAh/g for 20 hours followed by discharge at 70 mAh/g to a cut-off of 0 V. The discharge capacity of this cell during the first 27 cycles is illustrated in FIG. 8. The cell was capable of reaching a discharge capacity of 1185 mAh/g of the SiGe anode electrochemically active material. This discharge capacity is further illustrated in FIG. 9 illustrating the discharge profile at cycle 27. These studies demonstrate that an alloy of Si with a non-Si group 14 element can readily function as a hydride forming material for use in a proton conducting battery and do so with excellent discharge capacity.

Example 3
The silicon samples of Example 1 were either left untreated as a control, or were treated in an air atmosphere for various treatment times and at various treatment temperatures. Some samples were treated for 2 hours. Treatment temperatures were 100 °C, 300 °C and 600 °C. Following treatment surface (about 1 micron) oxygen content for each of the treated samples was measured by x-ray energy dispersive spectroscopy (EDS) with electron energy at 15 keV. Results are illustrated in Table 2.

Results demonstrate that treatment at 100 °C showed a modest reduction in resulting oxygen content suggesting that temperatures less than 100 °C would produce little to no function effect of the treatment. Treatment at 300 °C and 600 °C both showed significant decreases in final oxygen content in the material.

The cell was cycled with a charge rate of 700 mA/g, charge time: 20 hours and a discharge rate of 70 mA/g to a discharge cut-off of 1 V or 0 V. The initial capacity at cycle 1 for samples coated onto stainless steel current collectors and treated at various temperatures for 2 hours are illustrated in Table 3. Resulting capacity versus cycle number are plotted in FIG. 10.

Samples treated at 100 °C, which in Table 2 showed a very small change in oxygen content, revealed an appreciable jump in electrochemical activity producing a doubling of activity relative to cells constructed with untreated anode electrochemically active material. This activity was further increased by treatment to 300 °C. Treatment at 600 °C still showed an appreciable improvement, but the resulting electrochemical activity was lower than treatment at 100 °C or 300 °C, likely due to some initial changes in the microstructure of the Si in the samples. As illustrated in FIG. 5, the sample treated at 600 °C showed a deep drop in discharge capacity over the first 7 cycles that began to recover as cell formation progressed. These data suggest that treatment above 600 °C may show lower benefits than treatment at lower temperatures.

Example 4
It was found that the addition of particular salts to an electrolyte of a proton conducting battery employing a Si containing anode could stabilize and improve the formation of the cell and thereby improve the electrochemical characteristics of the cell. The cells of Examples 1 and 2 are tested with or without the addition of one or more salt additives supplemented into the electrolyte. These studies were done using potassium salts simply due to the ready solubility of potassium salts, but are expected to show similar function for sodium salt additives. Also, tested were salts of nickel and iron. Tests were performed using electrodes with nickel plated or stainless steel current collectors.

An EMIM/Ac electrolyte including acetic acid at 3.33 m was further tested as is, or through the addition of a pH buffer salt such as K₂HPO₄, KH₂PO₄, KHCO₃, KHSO₄, or K₂C₂O₄ at concentrations of 0.1 or 0.05 m. The electrolytes were studied in cells including polycrystalline Si anodes of Sample 1 of Example 1. The cells were charged at 700 mAh/g for 20 hours followed by discharge at 70 mAh/g to a cut-off of 0 V and studied for capacity out to 31 cycles. Results using nickel-plated steel current collectors are presented in Table 4.

While excellent maximum capacity is reached in the absent of a salt additive, the cells demonstrated greater than three-fold increases in maximum capacity with the addition of pH buffer salt additives. The capacity of the cell using the K₂HPO₄ additive showed a remarkable maximum capacity of over 6800 mAh/g of the Si anode material.

Similar studies were performed directly comparing the results achievable on nickel plated steel and stainless steel. Anode electrochemically active material was polycrystalline Si and the electrolyte base was EMIM/Ac with 3.3 m acetic acid. Data was collected at maximum discharge capacity achieved between cycle 20 and cycle 50. As is illustrated in Table 5, substitution stainless steel dramatically reduced the maximum discharge capacity of the cell.

The substitution of the stainless steel dramatically reduced the free Fe and Ni in the electrolyte. This suggests that the electrolyte serves to liberate Ni and Fe from a nickel plated steel current collector leading to increased concentrations of Ni and Fe in the electrolyte. Without being limited to one particular theory, it is believed that the presence of these additional Ni and Fe ions in the electrolyte assists in cell formation and helps produce significantly increased discharge capacities.

To further test this theory, cells were constructed as above employing the same electrolyte with 0.1 m potassium phosphate or potassium carbonate, no additive but including either Ni sulfate or iron sulfate to determine of one of these salts could affect cell formation, as well as the combination of potassium phosphate or carbonate with both nickel and iron sulfates. All cells were constructed using stainless steel current collectors for both electrodes and tested in the cells using polycrystalline Si heat treated for 2 hours at 300 °C as the anode electrochemically active material and the Ni(OH)₂ cathode materials as described above. Results achieved in the first 40 cycles are illustrated in Table 6.

The results indicate that the capacity of the system on a stainless steel current collector was 810 mAh/g. The addition of the potassium phosphate alone did appreciably increase the discharge capacity. Testing with both potassium phosphate and potassium carbonate with either iron sulfate or nickel sulfate alone did not improve the capacity. However, when either potassium phosphate or potassium carbonate was combined with both nickel and iron sulfates, the capacity significantly increased to greater than 1000 mAh/g. These results indicate that capacity of cells with electrodes formed on stainless steel current collectors can be improved by the addition of salts of Ni and Fe to the electrolyte.

The foregoing description of particular aspect(s) is merely exemplary in nature and is in no way intended to limit the scope of the invention as claimed below, its application, or uses, which may, of course, vary. The disclosure is provided with relation to the non-limiting definitions and terminology included herein. These definitions and terminology are not designed to function as a limitation on the scope or practice of the invention but are presented for illustrative and descriptive purposes only. While the processes or compositions are described as an order of individual steps or using specific materials, it is appreciated that steps or materials may be interchangeable such that the description of the invention may include multiple parts or steps arranged in many ways as is readily appreciated by one of skill in the art.

It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

It will be understood that, although the terms “first,” “second,” “third” etc. may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. Thus, “a first element,” “component,” “region,” “layer,” or “section” discussed below could be termed a second (or other) element, component, region, layer, or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms, including “at least one,” unless the content clearly indicates otherwise. “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. The term “or a combination thereof” means a combination including at least one of the foregoing elements.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Patents, publications, and applications mentioned in the specification are indicative of the levels of those skilled in the art to which the invention pertains. These patents, publications, and applications are incorporated herein by reference to the same extent as if each individual patent, publication, or application was specifically and individually incorporated herein by reference.

In view of the foregoing, it is to be understood that other modifications and variations of the present invention may be implemented. The foregoing drawings, discussion, and description are illustrative of some specific embodiments of the invention but are not meant to be limitations upon the practice thereof. It is the following claims, including all equivalents, which define the scope of the invention.

Claims

An electrolyte for use in a proton-conducting rechargeable battery comprising:
a proton conductive ionic liquid, the proton conductive ionic liquid optionally comprising an aprotic ionic liquid and one or more acids; and
two or more metallic salt additives suitable to produce a maximum discharge capacity of a proton conducting electrochemical cell housing the electrolyte of greater than 1000 mAh/g of anode electrochemically active material, wherein electrode electrochemically active material is associated with a stainless steel current collector.
The electrolyte of claim 1 wherein the electrochemical cell has a maximum discharge capacity in excess of 1100 mAh/g per weight of anode electrochemically active material.
The electrolyte of claim 1 wherein two metallic salts are present in said ionic liquid.
The electrolyte of claim 1 wherein the metallic salt is an organic or inorganic salt of Fe, Ni, or combinations thereof.
The electrolyte of claim 1 wherein the metallic salt is a phosphate, carbonate, or sulfate of Fe, Ni, or combinations thereof.
The electrolyte of claim 1 wherein the metallic salt comprises a sulfate of Ni and a sulfate of Fe.
The electrolyte of any one of claims 1-6, further comprising a pH buffer salt additive, optionally wherein the pH buffer salt additive is a phosphate, carbonate, or sulfate of potassium or sodium.
The electrolyte of any one of claims 1-6 wherein the proton conductive ionic liquid comprises one or more aprotic compounds and one or more acids.
The electrolyte of claim 8 wherein the aprotic compound contains a cation selected from the group consisting of ammonium ions and phosphonium ions.
The electrolyte of claim 9 wherein said ammonium ion is an imidazolium ion, or wherein the said phosphonium ion is a pyrrolidinium ion.
The electrolyte of claim 9 wherein said ammonium ion is an alkylimidazolium wherein said alkyl has 1-6 carbons.
The electrolyte of claim 11 wherein alkylimidazolium is a dialkylimidazolium with each alkyl a linear or branched alkyl having between 1-4 carbon atoms.
The electrolyte of claim 9 wherein said aprotic compound comprises an anion selected from the group consisting of methides, nitrate, carboxylates, imides, halides, borates, phosphates, phosphinates, phosphonates, sulfonates, sulfates, carbonates and aluminates.
The electrolyte of claim 8 wherein the aprotic compound comprises 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1,3-dimethylimdiazolium, 1-ethyl-3-methylimidazolium, 1,2,3-trimethylimidazolium, tris-(hydroxyethyl)methylammonium, 1,2,4-trimethylpyrazolium, or combinations thereof.
The electrolyte of any one of claims 1-6 wherein the electrolyte comprises less than 5 weight percent water.
A proton-conducting rechargeable battery comprising:
a cathode comprising a cathode electrochemically active material capable of storing and releasing hydrogen;
an anode comprising an anode electrochemically active material comprising Si_xM_1-x wherein x comprises one or more non-Si group 14 elements, and wherein 0 ＜x ≦ 1, wherein a microstructure of the anode electrochemically active material is amorphous, polycrystalline, a mixture of nanocrystalline and amorphous, or a combination of polycrystalline, nanocrystalline and amorphous; and
the electrolyte of any one of claims 1-6 between the anode and the cathode.
The battery of claim 16 wherein the electrochemical cell has a maximum discharge capacity in excess of 1500 mAh/g per weight of anode electrochemically active material.
The battery of claim 16 wherein two additives are present in said ionic liquid.
The battery of claim 16 wherein the metallic salt is a salt of Fe, Ni, or combinations thereof.
The battery of claim 16 wherein the metallic salt is a phosphate, carbonate, or sulfate of Fe, Ni, or combinations thereof.
The battery of claim 16 wherein the metallic salt comprises a sulfate of Ni and a sulfate of Fe.
The battery of any one of claims 16-21, further comprising a pH buffer salt, wherein the pH buffer salt additive is a phosphate, carbonate, or sulfate of potassium or sodium.
The battery of any one of claims 16-21 wherein the proton conductive ionic liquid comprises one or more aprotic compounds and one or more acids.
The battery of claim 23 wherein the aprotic compound contains a cation selected from the group consisting of ammonium ions and phosphonium ions.
The battery of claim 24 wherein said ammonium ion is an alkylimidazolium wherein said alkyl has 1-6 carbons.
The battery of claim 25 wherein alkylimidazolium is a dialkylimidazolium with each alky having between 1-3 carbon atoms.
The battery of claim 23 wherein said aprotic compound comprises an anion selected from the group consisting of carboxylates, imides, methides, nitrate, bifluoride, halides, borates, phosphates, phosphinates, phosphonates, sulfonates, sulfates, carbonates and aluminates.
The battery of claim 27 wherein said carboxylate is an acetate.
The battery of claim 23 wherein the aprotic compound comprises and ion of 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1,3-dimethylimdiazolium, 1-ethyl-3-methylimidazolium, 1,2,3-trimethylimidazolium, tris-(hydroxyethyl)methylammonium, or 1,2,4-trimethylpyrazolium.
The battery of any one of claims 16-21 wherein the electrolyte comprises less than 5 weight percent water.
The battery of any one of claims 16-21 wherein the anode electrochemically active material comprises two or more group 14 elements.
The battery of any one of claims 16-21 wherein the anode electrochemically active material comprises Si and one or more non-Si group 14 elements.
The battery of claim 32 wherein the one or more non-Si group 14 elements is C, Ge, or combinations thereof.
The battery of claim 32 wherein the anode electrochemically active material comprises one or more non-Si group 14 elements, wherein an amount of non-Si group 14 elements is 50 atomic percent or less relative to the total group 14 elements in the anode electrochemically active material, optionally wherein the amount of non-Si group 14 elements is less than 40 atomic percent.
The battery of any one of claims 16-21 wherein the discharge capacity of the rechargeable battery is greater than 800 mAh/g per weight of anode electrochemically active material above 1 Volt vs. Ni(OH)₂, optionally greater than 1000 mAh/g above 1 Volt, optionally greater than 1500 mAh/g above 1 Volt.
The battery of any one of claims 16-21 wherein the cathode electrochemically active material comprises Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, a hydride thereof, an oxide thereof, a hydroxide thereof, or an combination of the foregoing.
The battery of claim 37 wherein the cathode electrochemically active material comprises Ni.
The battery of claim 36 wherein the cathode electrochemically active material comprises Ni at greater than or equal to 10 atomic percent relative to all metals in the cathode electrochemically active material.
The battery of claim 37 wherein Ni is present at equal to or greater than 80 atomic percent, optionally 90 atomic percent.
The battery of claim 37 wherein the cathode electrochemically active material comprises a hydroxide of Ni, Co, Mn, Zn, Al, or combinations thereof.