Patent Description:
Various anode compositions have been introduced for use in lithium-ion batteries. Such compositions are described, for example, in <CIT> and <CIT>.

<CIT> discloses a method of making nanostructured alloy particles by milling a millbase in a pebble mill containing milling media. The millbase comprises: (i) silicon, and (ii) at least one of carbon or a transition metal, and wherein the nanostructured alloy particles are substantially free of crystalline domains greater than <NUM> nanometers in size. A method of making a negative electrode composition for a lithium ion battery including the nanostructured alloy particles is also disclosed.

<CIT> discloses a method of making an alloy which can be used in electrode compositions for lithium-ion batteries. The alloy comprises alloying components including ferrosilicon; and at least one of a metallic element or a metallic compound.

<CIT> discloses a negative electrode material which is composed of Fe-Si group compound particles. Application of this material is useful to restrict a volume change of the Fe-Si group compound particle when occluding or discharging lithium ions.

In a first aspect of the invention, an electrochemically active material is provided. The electrochemically active material includes an active phase comprising silicon; and an inactive phase having a Scherrer Grain Size of greater than <NUM> nanometers, said inactive phase being β-FeSi<NUM>. The electrochemically active material includes an additional inactive phase having a Scherrer Grain Size of less than <NUM> nanometers. The inactive β-FeSi2 phase, and the additional inactive phase of the material having a Scherrer Grain Size of less than <NUM> nanometers, together, present in the electrochemically active material in an amount of greater than <NUM> vol. %, based on the total volume of the electrochemically active material, as determined by XRD. The term "inactive phase" as used above refers to a phase that does not electrochemically react or alloy with lithium at voltages between <NUM> V and <NUM> V versus lithium metal during charging and discharging in a lithium ion battery. The term "active phase" refers to a phase that can electrochemically react or alloy with lithium at voltages between <NUM> V and <NUM> V versus lithium metal during charging and discharging in a lithium ion battery.

In a second aspect of the invention, an electrode composition is provided. The electrode composition includes the electrochemically active material as hereinbefore defined and a binder.

In a third aspect of the invention, a negative electrode is provided. The negative electrode includes a current collector and the above-described electrode composition.

In a fourth aspect of the invention, an electrochemical cell is provided. The electrochemical cell includes the above-described negative electrode, a positive electrode comprising a positive electrode composition comprising lithium, and an electrolyte comprising lithium.

In a fifth aspect of the invention, an electronic device comprising the electrochemical cell as hereinbefore defined is provided.

In a sixth aspect of the invention, a method of making an electrochemical cell is provided. The method includes providing a positive electrode comprising a positive electrode composition comprising lithium, providing a negative electrode as described above, providing an electrolyte comprising lithium, and incorporating the positive electrode, negative electrode, and the electrolyte into an electrochemical cell.

The above summary of the present disclosure is not intended to describe each embodiment of the present disclosure. The details of one or more embodiments of the disclosure are also set forth in the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and from the claims.

Silicon (Si) based alloys are a promising alternative to graphite as anode materials for next generation high energy density lithium ion batteries due, at least in part, to their higher energy density. However, adequate cycle life remains a significant challenge to commercialization of silicon based alloys.

Over the last several years, several design parameters have emerged for Si-based materials for Li-ion batteries. When micron sized Si is fully lithiated it is known to form the crystalline Li<NUM>Si<NUM> phase. The existence of this crystalline phase has been correlated with poor cycle life and its presence can be established by dQ/dV analysis of the voltage curve in a half cell. Active/inactive alloys are a well-established approach for suppressing the formation of Li<NUM>Si<NUM>. When the domain sizes of the active phase (e.g., Si) and the inactive phase (e.g. a metal silicide) are sufficiently small, the Si domains remain amorphous throughout lithiation and delithiation and the formation of Li<NUM>Si<NUM> is suppressed. Recent discoveries have shown that the suppression of the formation of Li<NUM>Si<NUM> is due strain/voltage coupling, where the strain stemming from the inactive phase lowers the lithiation potential and the formation of Li<NUM>Si<NUM> is avoided.

Surprisingly, it has been found that a key parameter for the design of a Si-based active/inactive material is the lattice mismatch between the inactive phase and Li<NUM>Si<NUM>. The greater the lattice mismatch between the inactive phase and crystalline Li<NUM>Si<NUM>, the greater the suppression of the formation of Li<NUM>Si<NUM> and, in turn, the better the cycling. Generally, the present disclosure is directed to active/inactive materials with large lattice mismatches between the inactive phase and Li<NUM>Si<NUM>, resulting in enhanced suppression of the Li<NUM>Si<NUM> phases and, consequently, improved cycling.

As a Si-based material is cycled, the formation of the crystalline Li<NUM>Si<NUM> phase can increase with cycle number. A material in which the presence of the Li<NUM>Si<NUM> phase, as determined by dQ/dV analysis, increases with cycle number is said to have an unstable microstructure. Cycling at elevated temperatures generally promotes microstructure changes. Therefore, an efficient way of quantifying the stability of the microstructure of Si-based material is by cycling the material at <NUM> and monitoring the presence of crystalline Li<NUM>Si<NUM>. The materials in accordance with some embodiments of the present disclosure are found to have surprisingly stable microstructures even when cycled at <NUM>.

As used herein, the singular forms "a", "an", and "the" include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended embodiments, the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

As used herein, the recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. <NUM> to <NUM> includes <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>).

Unless otherwise indicated, all numbers expressing quantities or ingredients, measurement of properties and so forth used in the specification and embodiments are to be understood as being modified in all instances by the term "about. Each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

In some embodiments, the present disclosure relates to an electrochemically active material for use in an electrochemical cell (e.g., a lithium ion battery). For example, the electrochemically active material may be incorporated into a negative electrode for a lithium ion battery.

The electrochemically active material of the present invention comprises one or more active phases and one or more inactive phases. The active phases may be in the form of or include an active chemical element, an active alloy, or combinations thereof. The active phases may include silicon and one or more additional active chemical elements such as, for example, magnesium (Mg), calcium (Ca), strontium (Sr), silver (Ag), zinc (Zn), boron (B), aluminum (Al), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), or combinations thereof. In accordance with the present invention, the active phase comprises silicon. In some embodiments, the active phase may include silicon and Sn. In some embodiments, the active phase may consist essentially of silicon.

In some embodiments, active phase may account for at least <NUM> vol. % or at least <NUM> vol. % of the active material based on the total volume of the active material; or between <NUM> vol. % and <NUM> vol. %, between <NUM> vol. % and <NUM> vol. %, between <NUM> vol. % and <NUM> vol. %,, between <NUM> vol. % and <NUM> vol. %, or between <NUM> vol. % and <NUM> vol. %, based on the total volume of the active material.

The electrochemically active material according to the present invention further includes an electrochemically inactive phase. The electrochemically active phase and the electrochemically inactive phase may share at least one common phase boundary. In various embodiments, the electrochemically inactive phase may be in the form of or include one or more electrochemically inactive chemical elements, including transition metals (e.g., titanium, vanadium, chromium, manganese, iron, cobalt, nickel), alkaline earth metals, rare earth metals, or combinations thereof. In various embodiments, the electrochemically inactive phase may be in the form of an alloy. In various embodiments, the electrochemically inactive phase may include a transition metal or combination of transition metals. In some embodiments, the electrochemically inactive phase may include one or more active chemical elements, including tin, carbon, gallium, indium, silicon, germanium, lead, antimony, bismuth, or combinations thereof. In some embodiments, the electrochemically inactive phase may include compounds such as silicides, aluminides, borides, carbides, nitrides, phosphates or stannides. The electrochemically inactive phase may include oxides, such as titanium oxide, zinc oxide, silicon oxide, aluminum oxide or sodium-aluminum oxide. In some embodiments, the electrochemically inactive phase may include TiSi<NUM>, B<NUM>Si, Mg<NUM>Si, VSi<NUM>, β-FeSi<NUM>, Mn<NUM>Si<NUM>, SiC, or combinations thereof.

In some embodiments, inactive phase may account for between <NUM> vol. % and <NUM> vol. %, between <NUM> vol. % and <NUM> vol. %, or between <NUM> vol. % and <NUM> vol. % of the active material, based on the total volume of the active material.

In some embodiments, the electrochemically active material may be represented by the following formula (I):.

where x, y, and z are atomic percentages, x + y +z = <NUM>, x is <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>; M is one or more transition metal elements selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, B, and C; y is <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>; and z is <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>.

In some embodiments, M is or includes iron, and the electrochemically active material includes at least an active phase that includes silicon, an iron di-silicide (FeSi<NUM>) inactive phase, and a silicon carbide (SiC) inactive phase. In such embodiments, the silicon phase may be present in the active material in an amount of between <NUM> to <NUM> vol% or between <NUM> to <NUM> vol. %; the FeSi<NUM> phase may be present in the active material in an amount of between <NUM> to <NUM> vol% or between <NUM> to <NUM> vol. %; and the SiC phase may be present in the active material in an amount of between <NUM> to <NUM> vol% or between <NUM> to <NUM> vol. %, based on the total volume of the active material.

In some embodiments, each of the phases of the electrochemically active material (i.e., the active phase, inactive phase, or any other phase of the active material) may include or be in the form of one or more grains. In some embodiments, the Scherrer grain size of each of the phases of the active material is no greater than <NUM> nanometers, no greater than <NUM> nanometers, no greater than <NUM> nanometers, no greater than <NUM> nanometers, or no greater than <NUM> nanometers. As used herein, the Scherrer grain size of a phase of an active material is determined, as is readily understood by those skilled in the art, by X-ray diffraction and the Scherrer equation.

As discussed above, it was discovered that lattice mismatch to crystalline Li<NUM>Si<NUM> is relevant to cycling performance. In this regard, the electrochemically active material in accordance with the present invention comprises one or more inactive phases having a Scherrer grain size of greater than <NUM> nanometers, greater than <NUM> nanometers, or greater than <NUM> nanometers. Further, in some embodiments, each inactive phase of the electrochemically active material having a Scherrer grain size of greater than <NUM>, <NUM>, or <NUM> nanometers may have a lattice mismatch to crystalline Li<NUM>Si<NUM> of greater than <NUM>%, greater than <NUM> %, or greater than <NUM> %; or between <NUM> and <NUM>%, between <NUM> and <NUM>%, or between <NUM> and <NUM>%.

Silicides are the most common inactive phases in silicon based alloys. Table <NUM> below lists the lattice mismatch of some common silicides to Li<NUM>Si<NUM>. Among them, TiSi<NUM>, B<NUM>Si, Mg<NUM>Si, VSi<NUM>, β-FeSi2 have a very large lattice mismatch with Li<NUM>Si<NUM>, and Mn<NUM>Si<NUM> also shows a significant mismatch. It has been discovered that phase mismatches are beneficial to prohibit Li<NUM>Si<NUM> crystallization when such mismatched phases are in a nano-crystalline form (e.g., Scherrer grain sizes between <NUM> and <NUM> nanometers, or <NUM> and <NUM> nanometers), as opposed to amorphous or nearly amorphous (e.g., Scherrer grain sizes less than <NUM> nanometers). For example, in a Si-Fe-C material prepared by mechanical milling, the silicon carbide (SiC) inactive phase may be substantially amorphous with a Scherrer grain sizes of less than <NUM> nanometers, while the iron di-silicide (FeSi<NUM>) inactive phase may be nano-crystalline with Scherrer grain sizes in the range of <NUM>-<NUM> nanometers. Though the lattice mismatch between SiC and Li<NUM>Si<NUM> is smaller than <NUM>%, it will not facilitate Li<NUM>Si<NUM> crystallization as the SiC is substantially amorphous. For the nanocrystalline inactive phase FeSi<NUM>, however, it has been discovered that lattice mismatch is important. If α-FeSi<NUM> is present, its neighboring Li<NUM>Si<NUM> has a tendency to crystallize. Thus, it has been discovered that the total volume of β-FeSi2 and SiC should be sufficiently high enough such that most of the Si domains are neighbored by β-FeSi2 and SiC rather than α-FeSi<NUM>.

In accordance with the present invention, the one or more inactive phases having a Scherrer grain size of greater than <NUM> nanometers (e.g. <NUM> or <NUM> nanometers) include one or more silicides. For example, such inactive phases include TiSi<NUM>, B<NUM>Si, Mg<NUM>Si, VSi<NUM>, β-FeSi<NUM>, Mn<NUM>Si<NUM>, or combinations thereof. In accordance with the present invention, the one or more inactive phases having a Scherrer grain size of greater than <NUM> nanometers (e.g. <NUM> or <NUM> nanometers) consists of β-FeSi<NUM>.

The electrochemically active material of the present invention includes one or more inactive silicide phases having a Scherrer grain size of greater than <NUM> nanometers (e.g. <NUM> or <NUM> nanometers), the inactive silicide phase(s), and an additional inactive phase having a Scherrer Grain Size of less than <NUM> nanometers (e.g. <NUM> or <NUM> nanometers), collectively, these phases are present in the electrochemically active material in an amount of greater than <NUM> vol. %, for example <NUM> vol. %, or <NUM> vol. %, based on the total volume of the electrochemically active material; for example between <NUM> and <NUM> vol. %, or between <NUM> and <NUM> vol. %, based on the total volume of the electrochemically active material.

In some embodiments, the phases may be distributed substantially homogeneously throughout the active material, including the surface and bulk of the material.

In some embodiments, the electrochemically active material may take the form of particles. The particles may have a diameter (or length of longest dimension) that is no greater than <NUM>, no greater than <NUM>, no greater than <NUM>, no greater than <NUM>, no greater than <NUM>, or even smaller; at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, or at least <NUM> or even larger; or <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>.

In some embodiments the electrochemically active material may take the form of particles having low surface area. The particles may have a surface area that is less than <NUM><NUM>/g, less than <NUM><NUM>/g, less than <NUM><NUM>/g, less than <NUM><NUM>/g, less than <NUM><NUM>/g, or even less than <NUM><NUM>/g.

In some embodiments, the active material (e.g., in the form of particles) may bear on an exterior surface thereof a coating that at least partially surrounds the active material. By "at least partially surrounds" it is meant that there is a common boundary between the coating and the exterior of the active material. The coating can function as a chemically protective layer and can stabilize, physically and/or chemically, the components of the active material. Exemplary materials useful for coatings include amorphous carbon, graphitic carbon, LiPON glass, phosphates such as lithium phosphate (Li<NUM>PO<NUM>), lithium metaphosphate (LiPO<NUM>), lithium dithionate (LiS<NUM>O<NUM>), lithium fluoride (LiF), lithium metasilicate (LiSiO<NUM>), and lithium orthosilicate (Li<NUM>SiO<NUM>). The coating can be applied by milling, solution deposition, vapor phase processes, or other processes known to those of ordinary skill in the art.

The present disclosure is further directed to negative electrode compositions for use in lithium ion batteries. The negative electrode compositions comprises the above-described electrochemically active materials. The electrochemically active materials may be present in the negative electrode compositions in an amount of between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, or between <NUM> wt. % and <NUM> wt. %, based upon the total weight of the electrode composition. Additionally, the negative electrode compositions in accordance with the present invention comprises one or more additives such as binders, conductive diluents, fillers, adhesion promoters, thickening agents for coating viscosity modification such as carboxymethylcellulose, polyacrylic acid, polyvinylidene fluoride, lithium polyacrylate, carbon black, or other additives known by those skilled in the art.

In illustrative embodiments, the negative electrode compositions may include an electrically conductive diluent to facilitate electron transfer from the composition to a current collector. Electrically conductive diluents include, for example, carbons, powdered metal, metal nitrides, metal carbides, metal silicides, and metal borides, or combinations thereof. Representative electrically conductive carbon diluents include carbon blacks such as Super P and Super S carbon blacks (both from Timcal , Switzerland), Shawinigan Black (Chevron Chemical Co. , Houston, Tex. ), acetylene black, furnace black, lamp black, graphite, carbon fibers, carbon nanotubes, and combinations thereof. In some embodiments, the amount of conductive diluent in the electrode composition may be at least <NUM> wt. %, at least <NUM> wt. %, or at least <NUM> wt. %, or at least <NUM> wt. % based upon the total weight of the electrode coating; less than <NUM> wt. less than <NUM> wt. %, or less than <NUM> wt. % based upon the total weight of the electrode composition, or between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, or between <NUM> wt. % and <NUM> wt. %, based upon the total weight of the electrode composition.

In some embodiments, the negative electrode compositions may include graphite to improve the density and cycling performance, especially in calendered coatings, as described in <CIT>, which is herein incorporated by reference in its entirety. The graphite may be present in the negative electrode composition in an amount of greater than <NUM> wt. %, greater than <NUM> wt. %, greater than <NUM> wt. %, greater than <NUM> wt. % or even greater, based upon the total weight of the negative electrode composition; or between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, or between <NUM> wt. % and <NUM> wt. %, based upon the total weight of the electrode composition.

In accordance with the present invention, the negative electrode compositions comprise a binder. Suitable binders include oxo-acids and their salts, such as sodium carboxymethylcellulose, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, methyl acrylate/acrylic acid copolymers, lithium methyl acrylate/acrylate copolymers, and other optionally lithium or sodium neutralized polyacrylic acid copolymers. Other suitable binders include polyolefins such as those prepared from ethylene, propylene, or butylene monomers; fluorinated polyolefins such as those prepared from vinylidene fluoride monomers; perfluorinated polyolefins such as those prepared from hexafluoropropylene monomer; perfluorinated poly(alkyl vinyl ethers); perfluorinated poly(alkoxy vinyl ethers); or combinations thereof. Other suitable binders include polyimides such as the aromatic, aliphatic or cycloaliphatic polyimides and polyacrylates. The binder may be crosslinked. In some embodiments, the amount of binder in the electrode composition may be at least <NUM> wt. %, at least <NUM> wt. %, at least <NUM> wt. %, or at least <NUM> wt. % based upon the total weight of the electrode coating; less than <NUM> wt. %, less than <NUM> wt. %, or less than <NUM> wt. %, based upon the total weight of the electrode composition; or between <NUM> wt. % and <NUM> wt. %, between <NUM> wt. % and <NUM> wt. %, or between <NUM> wt. % and <NUM> wt. %, based upon the total weight of the electrode composition.

The present disclosure is further directed to negative electrodes for use in lithium ion electrochemical cells. The negative electrodes of the present invention include a current collector having disposed thereon the above-described negative electrode composition. The current collector may be formed of a conductive material such as a metal (e.g., copper, aluminum, nickel), or a carbon composite.

The present disclosure further relates to lithium ion electrochemical cells. In addition to the above-described negative electrodes, the electrochemical cells include a positive electrode, an electrolyte, and a separator. In the cell, the electrolyte may be in contact with both the positive electrode and the negative electrode, and the positive electrode and the negative electrode are not in physical contact with each other; typically, they are separated by a polymeric separator film sandwiched between the electrodes.

In some embodiments, the positive electrode may include a current collector having disposed thereon a positive electrode composition that includes a lithium transition metal oxide intercalation compound such as LiCoO<NUM>, LiCO<NUM>Ni<NUM>O<NUM>, LiMn<NUM>O<NUM>, LiFePO<NUM>, LiNiO<NUM>, or lithium mixed metal oxides of manganese, nickel, and cobalt in any proportion. Blends of these materials can also be used in positive electrode compositions. Other exemplary cathode materials are disclosed in <CIT>) and include transition metal grains in combination with lithium-containing grains. Suitable transition metal grains include, for example, iron, cobalt, chromium, nickel, vanadium, manganese, copper, zinc, zirconium, molybdenum, niobium, or combinations thereof with a grain size no greater than about <NUM> nanometers.

In various embodiments, useful electrolyte compositions may be in the form of a liquid, solid, or gel. The electrolyte compositions may include a salt and a solvent (or charge-carrying medium). Examples of solid electrolyte solvents include polymers such as polyethylene oxide, polytetrafluoroethylene, fluorine-containing copolymers, and combinations thereof. Examples of liquid electrolyte solvents include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate, fluoroethylene carbonate (FEC), tetrahydrofuran (THF), acetonitrile, and combinations thereof. In some embodiments the electrolyte solvent may comprise glymes, including monoglyme, diglyme and higher glymes, such as tetraglyme Examples of suitable lithium electrolyte salts include LiPF<NUM>, LiBF<NUM>, LiClO<NUM>, lithium bis(oxalato)borate, LiN(CF<NUM>SO<NUM>)<NUM>, LiN(C<NUM>F<NUM>SO<NUM>)<NUM>, LiAsF<NUM>, LiC(CF<NUM>SO<NUM>)<NUM>, and combinations thereof.

In some embodiments, the lithium ion electrochemical cells may further include a microporous separator, such as a microporous material available from Celgard LLC, Charlotte, N. The separator may be incorporated into the cell and used to prevent the contact of the negative electrode directly with the positive electrode.

The disclosed lithium ion electrochemical cells can be used in a variety of devices including, without limitation, portable computers, tablet displays, personal digital assistants, mobile telephones, motorized devices (e.g., personal or household appliances and vehicles), instruments, illumination devices (e.g., flashlights) and heating devices. One or more lithium ion electrochemical cells of this disclosure can be combined to provide a battery pack.

The present disclosure further relates to methods of making the above-described electrochemically active materials. In some embodiments, the materials can be made by methods known to produce films, ribbons or particles of metals or alloys including cold rolling, arc melting, resistance heating, ball milling, sputtering, chemical vapor deposition, thermal evaporation, atomization, induction heating or melt spinning. The above described active materials may also be made via the reduction of metal oxides or sulfides. In some embodiments, the electrochemically active materials of the present disclosure may be made in accordance with the methods discussed in <CIT>.

The present disclosure further relates to methods of making negative electrodes that include the above-described negative electrode compositions. In some embodiments, the method may include mixing the above-described electrochemically active materials, along with any additives such as binders, conductive diluents, fillers, adhesion promoters, thickening agents for coating viscosity modification and other additives known by those skilled in the art, in a suitable coating solvent such as water or N-methylpyrrolidinone to form a coating dispersion or coating mixture. The dispersion may be mixed thoroughly and then applied to a foil current collector by any appropriate coating technique such as knife coating, notched bar coating, dip coating, spray coating, electrospray coating, or gravure coating. The current collectors may be thin foils of conductive metals such as, for example, copper, aluminum, stainless steel, or nickel foil. The slurry may be coated onto the current collector foil and then allowed to dry in air or vacuum, and optionally by drying in a heated oven, typically at about <NUM>° to about <NUM> for about an hour to remove the solvent.

The present disclosure further relates to methods of making lithium ion electrochemical cells. In accordance with the present invention, the method includes providing a negative electrode as described above, providing a positive electrode that includes lithium, and incorporating the negative electrode and the positive electrode into an electrochemical cell comprising a lithium-containing electrolyte.

In accordance with the compositions and methods of the present disclosure, electrochemically active materials having improved cycle performance may be obtained. In some embodiments, lithium-ion electrochemical cells that incorporate the negative electrodes of the present disclosure may prohibit the Li<NUM>Si<NUM> formation significantly during lithiation to <NUM> mV vs. Li/Li+ at elevated temperatures, such as <NUM> or greater which, in turn, may improve the capacity retention by <NUM>%, <NUM>%, or <NUM>% or greater.

The following examples are offered to aid in the understanding of the present disclosure and are not to be construed as limiting the scope of the appended claims. Unless otherwise indicated, all parts and percentages are by weight.

The following test methods and protocols were employed in the evaluation of the illustrative examples that follow.

The lattice mismatch between Li<NUM>Si<NUM> and its neighboring inactive phases was calculated as follows. The lattice of Li<NUM>Si<NUM> is cubic, and all inactive phases of interests are among cubic, tetragonal, hexagonal or orthorhombic lattices. As the minimal lattice mismatch is usually between lattice planes with low Miller indices, only (<NUM>) and (<NUM>) planes of cubic Li<NUM>Si<NUM> were considered. For cubic, tetragonal or orthorhombic inactive phases, (<NUM>), (<NUM>), (<NUM>), (<NUM>), (<NUM>) and (<NUM>) planes were used to calculate the lattice mismatch. For hexagonal or rhombohedral inactive phases, (<NUM>), (<NUM>), (<NUM>), (<NUM>), (<NUM>), (<NUM>) and (<NUM><NUM><NUM>) planes were considered in lattice mismatch calculation. All the lattice planes considered are either square or rectangle. Therefore, the lattice mismatch between two lattice planes can be calculated by: <MAT> where <MAT> is the lattice constant of Li<NUM>Si<NUM> and <MAT> the lattice constant of the inactive phase. Integer multiples of the unit cells were used to calculate the lattice mismatch, and the minimum <MAT> was found using a Python script. For example, NiSi<NUM> (<NUM>) plane is a square lattice with a lattice constant <MAT> while for Li<NUM>Si<NUM> (<NUM>), <MAT>. All the integer multiples of <MAT> and <MAT> were used to calculate the lattice mismatch. The minimum lattice mismatch between NiSi<NUM> (<NUM>) and Li<NUM>Si<NUM> (<NUM>) planes was found to be e = <NUM>% when: <MAT> Lattice mismatch of each inactive phase plane to Li<NUM>Si<NUM>(<NUM>) or (<NUM>) plane was calculated as is described above. The minimum mismatch among all those combinations is defined as the lattice mismatch between the inactive phase and Li<NUM>Si<NUM> as listed in Table <NUM>.

A Siemens D500 diffractometer equipped with a copper target X-ray tube and a diffracted beam monochromator was used for the X-Ray Diffraction (XRD) measurements. The emitted X-rays utilized were the Cu Kα1 (λ=<NUM>Å) and Cu Kα2 (λ=<NUM>Å). The divergence and anti-scatter slits used were set both at 1o, while the receiving slit was set at <NUM>°. The X-ray tube was powered to <NUM> kV at <NUM> mA. The scan ranges from <NUM>° to <NUM>° with a step of <NUM>°. The dwelling time of each step was <NUM> seconds. The X-ray diffraction pattern was quantitatively analyzed by FullProf Rietveld refinement program (a free software developed by Laboratoire Léon Brillouin, France).

Si alloy composite particles of the present invention were prepared by mechanical milling. Using the weights of each precursor provided in Table <NUM>, silicon powder (available from Elkem Silicon Materials, Norway), iron powder (available from North American Hoganas Inc. , Hollsopple, PA) and graphite powder (available from Asbury Graphite Mills Inc. , NJ) were milled together in an <NUM>-feet diameter pebble mill with <NUM> <NUM>/<NUM>" steel media under an argon atmosphere. The mill was cooled by <NUM> chilling water at a flow rate of <NUM>-<NUM> gallon per minute. After milling for <NUM> hours, the powder was discharged and sieved for further characterization.

A binder solution was prepared as follows: <NUM> wt% aqueous solution of polyacrylic acid (PAA) (<NUM> MW, available from Sigma Aldrich), de-ionized water, and lithium hydroxide monohydrate (available from Sigma Aldrich) were mixed in a <NUM>:<NUM>:<NUM> weight ratio, and placed in a shaker for <NUM> hours. The resulting solution is a <NUM> wt% lithium polyacrylate (LiPAA) aqueous binder solution.

Electrodes comprising the Si alloy composite particles and lithium polyacrylate (LiPAA) with a <NUM>/<NUM> weight ratio were made by placing <NUM> of each of Examples <NUM>-<NUM>, <NUM> of a <NUM>% LiPAA aqueous solution prepared above in a <NUM>-milliliter tungsten carbide vessel with four tungsten carbide balls (<NUM> diameter) and mixing in a planetary micro mill (PULVERISETTE <NUM>, available from Fritsch GmbH, Idon-Oberstein, Germany) at a speed setting of two for one hour. The resulting slurry was then coated onto a copper foil using a coating bar with a <NUM>" gap and dried under vacuum at <NUM>° C. for two hours. Coin cell electrodes were then punched from this foil.

Electrochemical <NUM> coin cells were made with the composite particle electrodes versus a lithium foil counter/reference electrode. The electrolyte contains <NUM> wt % FEC and <NUM> wt % Selectilyte LP <NUM> (<NUM> LiPF<NUM> in EC:EMC <NUM>:<NUM> w/w solution, available from BASF, Independence, OH). Two pieces of Celgard <NUM> microporous membranes (available from Celgard LLC, Charlotte, N. ) served as the separator. After crimping the cells closed, they were additionally sealed around the edges with Torr Seal (Varian, Inc. , Palo Alto, CA) to prevent any leakage at <NUM>.

The coin cells were then cycled at <NUM> using a Maccor <NUM> Series charger (available from Maccor Inc, Tulsa, OK). The first cycle was performed at C/<NUM> with a C/<NUM> trickle at 5mV and a delithiation up to <NUM>. 5V, subsequent cycles were performed at C/<NUM> with a C/<NUM> trickle at <NUM> mV and a delithiation up to <NUM> V.

X-ray diffraction was used to identify α-FeSi<NUM>, β-FeSi<NUM> phases as well as Si and SiC in the synthesized composites. <FIG> shows the diffraction pattern of Example <NUM>. The diffraction peak of α-FeSi<NUM> (<NUM>) at around <NUM>° and β-FeSi<NUM> (<NUM>) diffraction peak around <NUM>° were fit to calculate the volume ratio of β-FeSi<NUM> to α-FeSi<NUM>. Results are listed in Table <NUM>.

The voltage profiles during cycling were used to characterize the stability of the alloy composites. The derivative of capacity by voltage (dQ/dV) of Example <NUM> during delithiation versus voltage is provided in <FIG>. It shows two peaks in range of <NUM>-<NUM> V (P1) and <NUM>-<NUM> V (P2) respectively. The voltage curve is considered stable when the intensity of P2 is not changing significantly over cycling. Therefore, the ratio of P2 intensity at cycle <NUM> to P2 intensity at cycle <NUM>, i.e. P2(<NUM>th)/P2(<NUM>nd), was used to measure the voltage stability.

Table <NUM> lists the volume content of β-FeSi<NUM>+SiC, the ratio of P2(<NUM>th) to P2(<NUM>nd), and the first lithiation capacity of each composite. The results in Table <NUM> show that when the content of β-FeSi<NUM>+SiC is above <NUM>% in volume, P2(<NUM>th)/P2(<NUM>nd) becomes close to <NUM> indicating that the voltage curve is stable. On the other hand, there is no correlation found between the <NUM>st lithiation capacity and the voltage stability. These results lead to the conclusion that a higher volume of β-FeSi<NUM>+SiC can stabilize the voltage curve of Si alloys.

Claim 1:
An electrochemically active material comprising:
an active phase comprising silicon;
an inactive phase having a Scherrer Grain Size of greater than <NUM> nanometers, said inactive phase being β-FeSi<NUM> and.
an additional inactive phase having a Scherrer Grain Size of less than <NUM> nanometers,
wherein
(i) the inactive β-FeSi<NUM> phase, and (ii) the additional inactive phase of the material having a Scherrer Grain Size of less than <NUM> nanometers, are, together, present in the electrochemically active material in an amount of greater than <NUM> vol. %, based on the total volume of the electrochemically active material, as determined by the method defined in the description;
wherein the term "inactive phase" refers to a phase that does not electrochemically react or alloy with lithium at voltages between <NUM> V and <NUM> V versus lithium metal during charging and discharging in a lithium ion battery; and
the term "active phase" refers to a phase that can electrochemically react or alloy with lithium at voltages between <NUM> V and <NUM> V versus lithium metal during charging and discharging in a lithium ion battery.