WO2018081708A1

WO2018081708A1 - Silicon composite electrodes with dynamic ionic bonding

Info

Publication number: WO2018081708A1
Application number: PCT/US2017/059039
Authority: WO
Inventors: Jeffrey S. Moore; Scott R. White; Nancy R. Sottos; Sen KANG; Ke Yang
Original assignee: The Board Of Trustees Of The University Of Illinois
Priority date: 2016-10-31
Filing date: 2017-10-30
Publication date: 2018-05-03

Abstract

A dynamic bonding approach was developed to increase the cycle lifetimes and reliability of silicon composite anodes through the restoration of the electrical interfaces. Silicon composite anodes with increased capacity retention were achieved through dynamic ionic bonding at the interface between silicon particles and the polymeric binder. Amine groups were covalently attached to a silicon particle surface. During anode fabrication ionic bonds readily form between the amine groups on the silicon particle surface and the carboxyl groups on poly(acrylic acid) binder. The dynamic ionic bonds effectively mitigate large volume changes of silicon anodes during lithium intercalation. Silicon composite anodes can thus achieve improvements in cycle life and capacity retention.

Description

SILICON COMPOSITE ELECTRODES WITH DYNAMIC IONIC BONDING

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 62/415,362, filed October 31, 2016, which is incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with government support under Grant No. ANL 9F-31921 EFRC awarded by the United States Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Lithium-Ion Batteries (LIB) are widely used in portable electronic devices, electric vehicles, and grid-scale energy storage due to the high energy density, high power density, and high operating voltages. Graphite is the active material for most commercial anodes and has a capacity of 372 mAh/g. The growing need for anode materials with higher capacity has led to the investigation of silicon due to its high gravimetric capacity (4200 mAh/g) and volumetric capacity (9786 mAh/cm³). However, silicon (Si) undergoes large volume change (>400%) upon lithium intercalation, resulting in the destruction of the conductive network, an unstable solid electrolyte interface (SEI) layer on the Si particle surfaces, and rapid capacity decrease. Materials strategies to overcome the large volume change and enhance capacity retention of Si anodes include the use of Si particles dispersed in a rigid carbon matrix, Si thin films, nanostructured Si, and nanoparticle Si composite anodes with polymer binders.

Polyvinylidene fluoride (PVDF) is one of the most common binders in graphite composite electrodes, but has weak van der Waals interactions with Si particles. In contrast, polymer binders containing carboxyl groups, such as carboxylmethyl cellulose (CMC), poly(acrylic acid), and alginate, form strong hydrogen bonds with the native silica layer on Si particle surfaces, resulting in much better capacity retention of the Si anodes. Bridel et al. suggest that the hydrogen bonds undergo a self-healing process that is crucial for cycling stability of Si anodes {Chemistry of Materials, 2009, 22, 1229). Furthermore, Cui et al. recently incorporated a hydrogen bonding based self-healing polymer binder in a composite electrode containing Si microparticles {Nat Chem, 2013, 5, 1042). The self-healing composite anodes showed a cycle life of 130 cycles with 70 % capacity retention at a cycle rate of 0.42 A/g, which was attributed to the dynamic hydrogen bonding.

Other types of dynamic bonds such as metal-ligand coordination bonds, π- π stacking, and ionic bonds can impart self-healing properties to polymers. Ionic bonding involves two oppositely charged ions and is weaker than covalent bonding and stronger than hydrogen bonding. Suo et al. {Nature, 2012, 489, 133) developed a stretchable and tough hydrogel by constructing alginate-polyacrylamide matrix with covalent crosslinks and ionic bond networks. The carboxylate/Ca²⁺ ionic bond unzipped upon stretching the film and re-zipped to heal the internal damage. Similarly, Wei et al. prepared self-healing hydrogels consisting of ferric ions and crosslinked poly(acrylic acid) (PAA) {Polymer Chemistry, 2013, 4, 4601). The migration of Fe³⁺ in the hydrogel allowed the reformation of carboxylate/Fe³⁺ ionic bond at the cut interface. A self-healing anticorrosion coating was developed by Andreeva et al. via layer-by-layer deposition of polyelectrolytes (poly(ethyleneimine) as polycation and poly(styrene sulfonate) as polyanion) on an Al substrate {Advanced Materials, 2008, 20, 2789). Lyon et al. described a self-healing hydrogel film containing multilayered polyanion microgel and polycation {Angew. Chemie Int. Ed., 2010, 49, 767). After damage, the solvation of the film in water permitted the reformation of the ionic bonds, resulting in fast recovery for multiple cycles. However, these matrices still fall short on effectively achieving of longer cycle life spans and greater discharge capacities that are desirable for electrochemical cells.

Accordingly, what is needed is a solution to the problems associated with large volume changes in the silicon composite anode due to lithium intercalation.

SUMMARY

The disclosure provides a versatile approach to incorporate dynamic ionic bonds to Si composite anodes for autonomous healing of the electrical interfaces during anode operation. Ionic bonds were incorporated in Si anodes by mixing amine functionalized Si nanoparticles with poly(acrylic acid). The formation of ionic bonds was confirmed by X-ray photoelectron spectroscopy and Raman spectroscopy. The Si anodes with ionic bonding showed excellent cycle life and rate capability.

Accordingly, the invention provides a composite self-healing silicon anode comprising:

a) a functionalized silicon particle comprising: a silica particle having a layer of silanol moieties on the surface of the silica particle; and

a plurality of amino(alkyl)silyl moieties covalently bonded to the silanol moieties by a primary Si-O-Si bond, wherein one or more of the amino(alkyl)silyl moieties are cross-linked to a proximal amino(alkyl)silyl moiety by a secondary Si-O-Si covalent bond;

b) a polymer binder comprising a plurality of acid moieties, wherein a dynamic ionic bond is at an interface between the acid moiety of the polymer binder and the amino portion of the amino(alkyl)silyl moiety of the functionalized silicon particle;

c) a conductive filler particle; and

d) a network of conductive electrical contacts in a composite of one or more functionalized silicon particles, the polymer binder and one or more conductive filler particles, wherein the composite has dynamic interfacial ionic bonds;

wherein the conductive electrical network of dynamic interfacial ionic bonds can self- heal when one or more ionic bonds break.

The invention also provides an electrochemical energy storage cell comprising:

i) a self-healing silicon anode;

ii) a cathode;

iii) an electrolyte disposed between the anode and the cathode; and iv) an ion permeable separator;

wherein the self-healing silicon anode comprises:

a) a functionalized silicon particle comprising:

a silica particle having a layer of silanol moieties on the surface of the silica particle; and

c) a conductive filler particle; and

d) a network of conductive electrical contacts in a composite of one or more functionalized silicon particles, the polymer binder and one or more conductive filler particles, wherein the composite has dynamic interfacial ionic bonds; wherein the conductive electrical network of dynamic interfacial ionic bonds can self- heal when one or more ionic bonds break, thereby prolonging the lifespan of the energy storage cell. BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the specification and are included to further demonstrate certain embodiments or various aspects of the invention. In some instances, embodiments of the invention can be best understood by referring to the accompanying drawings in combination with the detailed description presented herein. The description and accompanying drawings may highlight a certain specific example, or a certain aspect of the invention. However, one skilled in the art will understand that portions of the example or aspect may be used in combination with other examples or aspects of the invention.

Figure 1. Effect of increasing the APS/Si weight ratio on the amino coverage of Si- H2 nanoparticles.

Figure 2A-2B. Chemical characterization of functionalized S1O2 particles and composite films, (a) XPS core-level N Is spectra of (i) S1O2- H2 and (ii) S1O2- H2/PAA composite film, (b) Raman spectra of (i) PAA film, (ii) partially neutralized PAA (80% degree of neutralization) film, (iii) S1O2/PAA composite film, and (iv) S1O2- H2/PAA composite film.

Figure 3A-3B. Cycling performance of Si composite electrodes, (a) Initial voltage- capacity profile of Si composite anodes at a current density of 175 mA/g between 0.01 and 1.0 V vs Li/Li⁺. (b) Discharge capacity (solid symbols) and columbic efficiency (open symbols) of Si composite anodes at a current density of 2.1 A/g between 0.01 and 1.0 V vs Li/Li⁺. The anodes were pre-cycled at a current density of 175 mA/g (results were not shown) for 1 cycle. All cycling experiments used 1 M L1PF6 EC/DMC (1 : 1 by volume) electrolyte with 10 wt% FEC additive. The capacity was normalized by the weight of Si particles.

Figure 4. Electrochemical cycling performance of Si composite anodes with different amine coverages on Si particles. The anodes were cycled at a current density of 2.1 A/g between 0.01 and 1.0 V vs Li/Li⁺. The anodes were pre-cycled at a current density of 175 mA/g (results were not shown) for 1 cycle. The capacity was normalized by the weight of Si particles.

Figure 5. Discharge capacity (large squares) and columbic efficiency (small squares) of S1-NH2/PAA composite anode at various current densities between 0.01 and 1.0 V vs Li/Li⁺. The capacity was normalized by the weight of Si particles. Figure 6. Impedance test of S1- H2/PAA composite anode. The anode was cycled at a current density of 2.1 Ah/g between 0.01 and 1.0 V vs Li/Li⁺. The circled number refers to the cycle number of galvanostatic cycling and impedance was measured after completion of the entire discharge/charge cycle.

Figure 7. Synthetic route for amine functionalized Si particles. APS was first hydrolyzed to produce silanol (Si-OH) groups, and then silanol groups condense with the hydroxyl groups on Si particles to form covalent Si-O-Si bonds by releasing H2O molecules.

Figure 8. Formation of interfacial ionic bonds between Si nanoparticles and PAA binder in Si composite anodes. The yellow spheres, black spheres, and polymer chains represent S1- H2, carbon black, and PAA binder, respectively.

Figure 9. Cross-sectional SEM images of S1-NH2/PAA composite anode before and after cycling. The anode was cycled at a current density of 2.1 Ah/g between 0.01 and 1.0 V vs Li/Li⁺ for 400 cycles.

Figure lOA-lOC. XPS spectra of S1- H2/PAA anode surface before (bottom curves) and after 400 cycles (top curves), (a) Nls; (b) Fls; (c) Cls. The anode was cycled at a current density of 2.1 Ah/g between 0.01 and 1.0 V vs Li/Li⁺.

Figure 11. Impedance test of Si/PAA composite anode. The anode was cycled at a current density of 2.1 Ah/g between 0.01 and 1.0 V vs Li/Li⁺. The circled number refers to the cycle number of galvanostatic cycling and impedance was measured after completion of the entire discharge/charge cycle.

DETAILED DESCRIPTION

Silicon (Si) composite electrodes with ionic bonding are developed to increase cycle lifetimes and reliability through the restoration of interfaces between active Si nanoparticles and the binder. Amine groups are covalently attached to Si nanoparticle surface. A network of Si particles with interfacial ionic bonds is achieved by combining the particles with a poly(acrylic acid) (PAA) binder. The formation of ionic bonds between amine groups on Si particles and carboxylic acid groups on the PAA binder is characterized by XPS and Raman spectroscopy. The Si composite anodes with ionic bonding demonstrate long term cycling stability with a cycle life of 400 cycles and 80 % capacity retention at a current density of 2.1 mA/g and good rate capability. The dynamic ionic bonds effectively mitigate the deterioration of electrical interfaces in the composite anodes, as suggested by stable impedance over 300 cycles. Definitions

The following definitions are included to provide a clear and consistent understanding of the specification and claims. As used herein, the recited terms have the following meanings. All other terms and phrases used in this specification have their ordinary meanings as one of skill in the art would understand. Such ordinary meanings may be obtained by reference to technical dictionaries, such as Hawley 's Condensed Chemical Dictionary 14^th Edition, by R.J. Lewis, John Wiley & Sons, New York, N.Y., 2001.

References in the specification to "one embodiment", "an embodiment", etc., indicate that the embodiment described may include a particular aspect, feature, structure, moiety, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, moiety, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, moiety, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect or connect such aspect, feature, structure, moiety, or characteristic with other embodiments, whether or not explicitly described.

The singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to "a compound" includes a plurality of such compounds, so that a compound X includes a plurality of compounds X. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as "solely," "only," and the like, in connection with any element described herein, and/or the recitation of claim elements or use of "negative" limitations.

The term "and/or" means any one of the items, any combination of the items, or all of the items with which this term is associated. The phrases "one or more" and "at least one" are readily understood by one of skill in the art, particularly when read in context of its usage. For example, the phrase can mean one, two, three, four, five, six, ten, 100, or any upper limit approximately 10, 100, or 1000 times higher than a recited lower limit. For example, one or more substituents on the surface of a functionalized silicon particle can be one, one to one hundred, one hundred to about one thousand, or the number of substituents necessary for surface coverage of about 0.005 mmol/g Si to about 10 mmol/g Si.

As will be understood by the skilled artisan, all numbers, including those expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth, are approximations and are understood as being optionally modified in all instances by the term "about." These values can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the descriptions herein. It is also understood that such values inherently contain variability necessarily resulting from the standard deviations found in their respective testing measurements. When values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value without the modifier "about" also forms a further aspect.

The term "about" can refer to a variation of ± 5%, ± 10%, ± 20%, or ± 25% of the value specified. For example, "about 50" percent can in some embodiments carry a variation from 45 to 55 percent, or as otherwise defined by a particular claim. For integer ranges, the term "about" can include one or two integers greater than and/or less than a recited integer at each end of the range. Unless indicated otherwise herein, the term "about" is intended to include values, e.g., weight percentages, proximate to the recited range that are equivalent in terms of the functionality of the individual ingredient, composition, or embodiment. The term about can also modify the end-points of a recited range as discussed above in this paragraph.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. It is therefore understood that each unit between two particular units are also disclosed. For example, if 10 to 15 is disclosed, then 11, 12, 13, and 14 are also disclosed, individually, and as part of a range. A recited range (e.g., weight percentages or carbon groups) includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, all language such as "up to", "at least", "greater than", "less than", "more than", "or more", and the like, include the number recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio. Accordingly, specific values recited for radicals, substituents, and ranges, are for illustration only; they do not exclude other defined values or other values within defined ranges for radicals and substituents. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

One skilled in the art will also readily recognize that where members are grouped together in a common manner, such as in a Markush group, the invention encompasses not only the entire group listed as a whole, but each member of the group individually and all possible subgroups of the main group. Additionally, for all purposes, the invention encompasses not only the main group, but also the main group absent one or more of the group members. The invention therefore envisages the explicit exclusion of any one or more of members of a recited group. Accordingly, provisos may apply to any of the disclosed categories or embodiments whereby any one or more of the recited elements, species, or embodiments, may be excluded from such categories or embodiments, for example, for use in an explicit negative limitation.

An "effective amount" refers to an amount effective to bring about a recited effect, such as an amount necessary to form products in a reaction mixture. Determination of an effective amount is typically within the capacity of persons skilled in the art, especially in light of the detailed disclosure provided herein. The term "effective amount" is intended to include an amount of a compound or reagent described herein, or an amount of a combination of compounds or reagents described herein, e.g., that is effective to form products in a reaction mixture. Thus, an "effective amount" generally means an amount that provides the desired effect.

Embodiments of the Invention

Various embodiments of the invention include a composite self-healing silicon anode comprising:

a) a functionalized silicon particle comprising:

c) a conductive filler particle; and d) a network of conductive electrical contacts in a composite of one or more functionalized silicon particles, the polymer binder and one or more conductive filler particles, wherein the composite has dynamic interfacial ionic bonds;

In various embodiments the filler particles may be evenly distributed between functionalized silicon particles and the polymer binder, or may be unevenly distributed between the functionalized silicon particles and the polymer binder.

In one embodiment, the dynamic ionic bonds mitigate volume changes in the silicon composite anode upon passage of electrical current through the anode. In various other embodiments, the acid moieties comprise a carboxylic acid or a sulfonic acid, and the dynamic interfacial ionic bonds mitigate volume changes in a silicon anode from lithium intercalation.

In some embodiments, the volume change is a large volume change, such as greater than 200%, greater than 300%, or greater than 400%), of the volume of the silicon composite anode.

In another embodiment, the functionalized silicon particle is about 10 nanometers (nm) to about 10 micrometers (μπι) in diameter. In various embodiments, the particles can be about 20 nm to about 900 nm, or about 900 nm to about 5 μπι. The diameters can also be about 30 nm to about 1 μπι, about 30 nm to about 500 nm, about 30 nm to about 200 nm, or about 30 nm to about 100 nm. The diameters can also be about 900 nm to about 10 μπι, about 1 μπι to about 5 μπι, about 5 μπι to about 10 μπι.

In various embodiments, the alkyl portion of the amino(alkyl)silyl moiety can be an optionally branched alkyl group. The alkyl can be a (Ci-C2o)alkyl or a (C2-C2o)alkyl. Alkyl groups include straight chain and branched alkyl groups and cycloalkyl groups having from 1 to about 20 carbon atoms, and typically from 1 to 12 carbons or, in some embodiments, from 1 to 8 carbon atoms. Examples of straight chain alkyl groups include those with from 1 to 8 carbon atoms such as methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, and n- octyl groups. Examples of branched alkyl groups include, but are not limited to, isopropyl, iso-butyl, sec-butyl, t-butyl, neopentyl, isopentyl, and 2,2-dimethylpropyl groups. Examples of cycloalkyl groups include, for example, (C3-C8)cycloalkyls such as cyclopropyl, cyclopentyl, and cyclohexyl, optionally including alkylene linkers to other moieties such as the amino portion and the silyl portion of the amino(alkyl)silyl moiety. In some embodiments, the alkyl can be substituted with one or more substituents. Substituted alkyl groups can be alkyl groups substituted by one or more amino, hydroxy, cyano, carboxy, nitro, thio, alkoxy, and halogen groups. In other embodiments, the alkyl can be an alkyl optionally interrupted by oxygen or nitrogen atoms, for example, -CH2CH2(XCH2CH2X)nCH2CH2- wherein each X is independently -O- or - H-, and n is 1 to 20. Accordingly, in some embodiments, the alkyl moiety of the amino(alkyl)silyl moiety can be:

-CH₂CH2(OCH2CH20)nCH₂CH2- when X = O, or

-CH2CH2( HCH2CH2 H)nCH₂CH2- when X= H.

In various embodiments, n of the interrupted alkyl moieties can be 1, 2, 3, 4, 5, 5-10, or 10-20.

The amino portion of the amino(alkyl)silyl moiety can be a primary amine, a secondary amine, a tertiary amine, or the amino portion can be a heterocyclic ring wherein the heterocyclic ring contains at least one basic nitrogen atom and the heterocyclic ring is attached to the alkyl moiety of the amino(alkyl)silyl moiety. The heterocyclic ring can be a heteroaryl group or a non-aromatic heterocycle, such as piperidine, morpholine, or pyrrolidine. Specific examples of the amino(alkyl)silyl moiety include, but are not limited to, 2-aminoethylsilyl, 3-aminopropylsilyl, 4-aminobutylsilyl, 2-(heteroaryl)ethylsilyl, 3- (heteroaryl)propylsilyl, and 4-(heteroaryl)butylsilyl. Examples of the heteroaryl include imidazole, pyrazole, triazole, and tetrazole. Various examples of suitable amino(alkyl)silyl moieties wherein the amino moiety is a heteroaryl group include the groups shown in Scheme 1 below.

Scheme 1. Examples of Amino Moieties for Ionic Bond Formation

The amino moiety of the amino(alkyl)silyl moiety can also be other nitrogen- containing heteroaryl groups or non-aromatic heterocyclic groups. Heterocyclic groups include aromatic and non-aromatic ring compounds containing 3 or more ring members, of which, one or more is a nitrogen, optionally also including O or S. In some embodiments, heterocyclic groups include 3 to about 20 ring members, whereas other such groups have 3 to about 15 ring members. A heterocyclic group designated as a C2-heterocyclyl can be a 5-ring with two carbon atoms and three heteroatoms, a 6-ring with two carbon atoms and four heteroatoms and so forth. Likewise a C4-heterocyclyl can be a 5-ring with one heteroatom, a 6-ring with two heteroatoms, and so forth. The number of carbon atoms plus the number of heteroatoms sums up to equal the total number of ring atoms. A heterocyclic ring can also include one or more double bonds. A heteroaryl ring is an embodiment of a heterocyclic group. The phrase "heterocyclic group" includes fused ring species including those comprising fused aromatic and non-aromatic groups. For example, a dioxolanyl ring and a benzdioxolanyl ring system (methylenedioxyphenyl ring system) are both heterocyclic groups within the meaning herein. The phrase also includes polycyclic ring systems containing a heteroatom such as, but not limited to, quinuclidyl. Heterocyclic groups can be unsubstituted, or can be substituted with one or more substituents as discussed above for alkyl moieties. Nitrogen-containing heterocyclic groups include, but are not limited to, pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl, pyrrolyl, pyrazolyl, triazolyl, tetrazolyl, pyridinyl, indolyl, dihydroindolyl, azaindolyl, indazolyl, benzimidazolyl, azabenzimidazolyl, benzoxazolyl, benzothiazolyl, benzothiadiazolyl, imidazopyridinyl, isoxazolopyridinyl, thianaphthalenyl, purinyl, xanthinyl, adeninyl, guaninyl, quinolinyl, isoquinolinyl, tetrahydroquinolinyl, quinoxalinyl, and quinazolinyl groups. Representative substituted heterocyclic groups can be mono- substituted or substituted more than once, such as, but not limited to, piperidinyl or quinolinyl groups, which are 2-, 3-, 4-, 5-, or 6-substituted, or disubstituted with groups such as those listed above.

Heteroaryl groups are aromatic ring compounds containing 5 or more ring members, of which, one or more is a heteroatom such as, but not limited to, N, O, and S; for instance, heteroaryl rings can have 5 to about 8-12 ring members. A heteroaryl group designated as a C2-heteroaryl can be a 5-ring with two carbon atoms and three heteroatoms, a 6-ring with two carbon atoms and four heteroatoms and so forth. Likewise, a C4-heteroaryl can be a 5-ring with one heteroatom, a 6-ring with two heteroatoms, and so forth. The number of carbon atoms plus the number of heteroatoms sums up to equal the total number of ring atoms. Nitrogen-containing heteroaryl groups include, but are not limited to, groups such as pyrrolyl, pyrazolyl, triazolyl, tetrazolyl, oxazolyl, isoxazolyl, thiazolyl, pyridinyl, indolyl, azaindolyl, indazolyl, benzimidazolyl, azabenzimidazolyl, benzoxazolyl, benzothiazolyl, benzothiadiazolyl, imidazopyridinyl, isoxazolopyridinyl, thianaphthalenyl, purinyl, xanthinyl, adeninyl, guaninyl, quinolinyl, isoquinolinyl, tetrahydroquinolinyl, quinoxalinyl, and quinazolinyl groups. Heteroaryl groups can be unsubstituted, or can be substituted with groups as is discussed above. Representative substituted heteroaryl groups can be substituted one or more times with groups such as those listed above.

Additional examples of heteroaryl groups include but are not limited to indenyl, N- hydroxytetrazolyl, N-hydroxytriazolyl, N-hydroxyimidazolyl, indolyl, oxadiazolyl, isoxazolyl, quinazolinyl, xanthenyl, isoindanyl, acridinyl, pyrrolyl (2-pyrrolyl), pyrazolyl (3- pyrazolyl), imidazolyl (1-imidazolyl, 2-imidazolyl, 4-imidazolyl, 5-imidazolyl), triazolyl (1,2,3-triazol-l-yl, l,2,3-triazol-2-yl l,2,3-triazol-4-yl, l,2,4-triazol-3-yl), oxazolyl (2- oxazolyl, 4-oxazolyl, 5-oxazolyl), thiazolyl (2-thiazolyl, 4-thiazolyl, 5-thiazolyl), pyridyl (2- pyridyl, 3-pyridyl, 4-pyridyl), pyrimidinyl (2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl, 6- pyrimidinyl), pyrazinyl, pyridazinyl (3- pyridazinyl, 4-pyridazinyl, 5-pyridazinyl), quinolyl (2-quinolyl, 3-quinolyl, 4-quinolyl, 5-quinolyl, 6-quinolyl, 7-quinolyl, 8-quinolyl), isoquinolyl (1-isoquinolyl, 3-isoquinolyl, 4-isoquinolyl, 5-isoquinolyl, 6-isoquinolyl, 7- isoquinolyl, 8-isoquinolyl), benzo[b]furanyl (2-benzo[b]furanyl, 3-benzo[b]furanyl, 4- benzo[b]furanyl, 5-benzo[b]furanyl, 6-benzo[b]furanyl, 7-benzo[b]furanyl), 2,3-dihydro- benzo[b]furanyl (2-(2,3-dihydro-benzo[b]furanyl), 3-(2,3-dihydro-benzo[b]furanyl), 4-(2,3- dihydro-benzo[b]furanyl), 5-(2,3-dihydro-benzo[b]furanyl), 6-(2,3-dihydro-benzo[b]furanyl), 7-(2,3-dihydro-benzo[b]furanyl), benzo[b]thiophenyl (2-benzo[b]thiophenyl, 3- benzo[b]thiophenyl, 4-benzo[b]thiophenyl, 5-benzo[b]thiophenyl, 6-benzo[b]thiophenyl, 7- benzo[b]thiophenyl), 2,3-dihydro-benzo[b]thiophenyl, (2-(2,3-dihydro-benzo[b]thiophenyl), 3-(2,3-dihydro-benzo[b]thiophenyl), 4-(2,3-dihydro-benzo[b]thiophenyl), 5-(2,3-dihydro- benzo[b]thiophenyl), 6-(2,3-dihydro-benzo[b]thiophenyl), 7-(2,3-dihydro- benzo[b]thiophenyl), indolyl (1-indolyl, 2-indolyl, 3-indolyl, 4-indolyl, 5-indolyl, 6-indolyl, 7-indolyl), indazole (1-indazolyl, 3-indazolyl, 4-indazolyl, 5-indazolyl, 6-indazolyl, 7- indazolyl), benzimidazolyl (1-benzimidazolyl, 2-benzimidazolyl, 4-benzimidazolyl, 5- benzimidazolyl, 6-benzimidazolyl, 7-benzimidazolyl, 8-benzimidazolyl), benzoxazolyl (1- benzoxazolyl, 2-benzoxazolyl), benzothiazolyl (1-benzothiazolyl, 2-benzothiazolyl, 4- benzothiazolyl, 5 -benzothiazolyl, 6-benzothiazolyl, 7-benzothiazolyl), carbazolyl (1- carbazolyl, 2-carbazolyl, 3-carbazolyl, 4-carbazolyl), 5H-dibenz[b,f]azepine (5H- dibenz[b,f]azepin-l-yl, 5H-dibenz[b,f]azepine-2-yl, 5H-dibenz[b,f]azepine-3-yl, 5H- dibenz[b,f]azepine-4-yl, 5H-dibenz[b,f]azepine-5-yl), 10,1 l-dihydro-5H-dibenz[b,f]azepine (10,1 l-dihydro-5H-dibenz[b,f]azepine-l-yl, 10,1 l-dihydro-5H-dibenz[b,f]azepine-2-yl,

10,11 -dihydro-5H-dibenz[b,f]azepine-3 -yl, 10, 11 -dihydro-5H-dibenz[b,f]azepine-4-yl, 10, 11- dihydro-5H-dibenz[b,f]azepine-5-yl), and the like.

In various embodiment, the polymer binder comprises carboxymethyl cellulose, polyacrylic acid, alginic acid, hyaluronic acid, or a combination thereof.

In various embodiments, the filler particles comprise carbon black, graphite, carbon nanotubes, fullerenes, charcoal, fly ash, graphene, activated carbon, or a combination thereof.

In additional embodiments, the composite comprises nanoparticles in an amount of about 30 wt.% to about 90 wt.%, polymer binder in an amount of about 5 wt.% to about 50 wt.%), and filler particles in an amount of about 0.1 wt.%> to about 30 wt.%>. The composite can also comprise nanoparticles in an amount of about 50 wt.%> to about 70 wt.%>, polymer binder in an amount of about 15 wt.%> to about 30 wt.%>, and filler particles in an amount of about 15 wt.%) to about 30 wt.%>. In another embodiment the composite comprises nanoparticles in an amount of about 60 wt.%>, polymer binder in an amount of about 20 wt.%>, and filler particles in an amount of about 20 wt.%>.

In some embodiments, the amino(alkyl)silyl moieties are covalently bonded to the silanol moieties on the silica particle at about 0.005 mmol/g Si to about 10 mmol/g Si, about 0.05 mmol/g Si to about 5 mmol/g Si, about 0.1 mmol/g Si to about 1 mmol/g Si, or about 0.005 mmol/g Si to about 1 mmol/g Si. In other embodiments, the cycle life of the composite anode is greater than 200 cycles, greater that 250 cycles, greater than 300 cycles, greater than 350 cycles, greater than 400 cycles, greater than 450 cycles, greater than 500 cycles, greater than 550 cycles, or greater than 600 cycles. In can also be greater than 50 cycles, greater than 100 cycles, or greater than 150 cycles.

In various embodiments, the discharge capacity is greater than about 60 percent of capacity retention after more than 200 cycles, or greater than about 70 percent of capacity retention after more than about 200 cycles. The discharge capacity can also be greater than 60 percent of capacity retention after more than 300 cycles, greater than about 50 percent of capacity retention after more than 200 cycles, greater than about 50 percent of capacity retention after more than 300 cycles, greater than about 80 percent of capacity retention after more than 400 cycles, greater than about 70 percent of capacity retention after more than 400 cycles, greater than about 60 percent of capacity retention after more than 400 cycles, or greater than about 50 percent of capacity retention after more than 400 cycles.

In other embodiments, the capacity of the anode is more than about 700 mAh/g, more than about 1000 mAh/g, more than about 1100 mAh/g, more than about 1500 mAg/g, more than about 2000 mAg/g, more than about 2500 mAg/g, more than about 3000 mAg/g, more than about 3500 mAg/g, or more than about 4000 mAg/g.

Embodiments of the invention also include an electrochemical energy storage cell comprising:

i) a self-healing silicon anode;

ii) a cathode;

iii) an electrolyte disposed between the anode and the cathode; and

iv) an ion permeable separator;

wherein the self-healing silicon anode comprises:

a) a functionalized silicon particle comprising:

b) a polymer binder comprising a plurality of acid moieties, wherein a dynamic ionic bond is at an interface between the acid moiety of the polymer binder and the amino portion of the amino(alkyl)silyl moiety of the functionalized silicon particle; c) a conductive filler particle; and

wherein the conductive electrical network of dynamic interfacial ionic bonds can self- heal when one or more ionic bonds break, thereby prolonging the lifespan of the energy storage cell.

In one embodiment of the energy storage cell, the cell is a lithium-ion battery. In various embodiments, the cell is a component of a single-cell lithium-ion battery, or a multi- cell lithium-ion battery. In some embodiments, the cells are assembled into a battery having 2 cells, 3 cells, 4 cells, 5 cells, more than 5 cells, more than 10 cells, more than 20 cells, more than 50 cells, more than 100 cells, more than 200 cells, more than 500 cells, or more than 1000 cells.

In other embodiments of the cell, the electrolyte comprises lithium hexafluorophosphate (LiPF₆), ethylene carbonate, and dimethyl carbonate. Embodiments of the cell may also include rechargeable cells.

In various embodiments of the energy storage cell the cycle life of the cell is greater than about 200 cycles, greater that 250 cycles, greater than 300 cycles, greater than 350 cycles, greater than 400 cycles, greater than 450 cycles, greater than 500 cycles, greater than 550 cycles, or greater than 600 cycles. In can also be greater than 50 cycles, greater than 100 cycles, or greater than 150 cycles.

In various embodiments of the cell, the discharge capacity is greater than about 60 percent of capacity retention after more than 200 cycles, or greater than about 70 percent of capacity retention after more than about 200 cycles. The discharge capacity can also be greater than about 60 percent of capacity retention after more than 300 cycles, greater than about 50 percent of capacity retention after more than 200 cycles, greater than about 50 percent of capacity retention after more than 300 cycles, greater than about 80 percent of capacity retention after more than 400 cycles, greater than about 70 percent of capacity retention after more than 400 cycles, greater than about 60 percent of capacity retention after more than 400 cycles, or greater than about 50 percent of capacity retention after more than 400 cycles.

In other embodiments of the cell, the capacity is more than about 700 mAh/g, more than about 1000 mAh/g, more than about 1100 mAh/g, more than about 1500 mAg/g, more than about 2000 mAg/g, more than about 2500 mAg/g, more than about 3000 mAg/g, more than about 3500 mAg/g, or more than about 4000 mAg/g.

Thus, in various embodiments of the apparatus and methods, a self-healing silicon composite anode comprises:

a) functionalized silicon particles comprising:

a silica layer comprising a plurality of silanol moieties on the surface of each particle; and

a plurality of substituents comprising an amino(alkyl)silyl moiety linked to the silica layer by a covalent Si-O-Si bond, wherein one or more of the amino(alkyl)silyl moieties are cross-linked to proximal substituents by one or more covalent Si-O-Si bonds; b) a polymer binder comprising a plurality of acid moieties wherein the acid moieties comprise carboxylic acid or sulfonic acid; and

c) conductive filler particles;

wherein the functionalized silicon particles, the polymer binder and the filler particles form a conductive electrical network of dynamic interfacial ionic bonds, wherein the ionic bonds are between the interface of the amino portion of the amino(alkyl)silyl moieties of the particles and the acid moieties of the polymer binder in the silicon composite anode, wherein when the ionic bonds break they can reform to self-heal the network.

The invention therefore provides functionalized silicon particles having silicon particles that have been modified on the surface by bringing physical, chemical or biological characteristics different from the ones originally found on the surface of a material. Silicon particles possess a thin native silica (SiC ) layer, which contains active silanol (Si-O-H) groups, covering the particle's surface, that can react to form covalent bonds with chemical reagents comprising functional groups, thus forming functionalized silicon particles. The component of the reagent which covalently bonds to the particles surface is referred to as a substituent. The density of the substituents covering the particle's surface can be varied by changing reaction conditions, as understood by persons skilled in the art. Examples of functional groups on the substituent can be but are not limited to amines, hydroxyls, ethers, carboxylic acids, esters, and amides. The functional groups are tethered by a linker of variable length to the silicon particle. The linker may be an alkyl chain comprising carbon atoms, and can optionally comprise heteroatoms, for example but not limited to oxygen, nitrogen, silicon, or combination thereof. In addition, the substituent may be crosslinked to proximal substituents at the atoms distal to the functional group on the linker. Proximal substituents are substituents that are within a spatial volume which are favorable for bond formation between atoms of individual substituents. Crosslinking can then occur between substituents over an area of the particle.

In one example (Figure 7), a 3-aminopropylsilyl substituent is covalently bonded to the silicon particle's surface, wherein the 3-amino moiety is a primary amine functional group tethered by the propylsilyl linker-moiety wherein the silyl moiety is covalently bonded to silanol on the particle's surface. Furthermore, the silyl moiety can be crosslinked, at least in part, by covalent Si-O-Si bonds to a proximal substituent' s silyl moiety along both dimensions of the particle's surface. One example of a silyl moiety is a silanol. A silanol is a functional group that includes the connectivity Si-O-H.

A polymer binder holds or draws other materials together to form a cohesive whole mechanically, chemically, or electrostatically. The backbone of the polymer may comprise repeating units of, for example but not limited to hydrocarbons, carbohydrates, proteins, or combination thereof. Furthermore, the polymer is functionalized with a plurality of functional groups which are complimentary to the functional groups on the functionalized silicon particles, such that electrostatic interactions forming ionic bonds can occur between the binder and functionalized silicon particle. For example, Figure 8 shows a polymer chain comprising carboxylic acids that forms ionic bonds at the interface between the binder and the amine on the functionalized silicon particle. When both the binder and functionalized silicon particles are combined with conductive carbon-based filler particles an electrical network is formed in the whole composite. The electrical network of an anode comprising the composite could partially unravel upon, for example, charging an electrical cell due to intercalation of ions, and upon discharge of the cell the ionic bonds would dynamically reform by electrostatic forces and self-heal to close the void as the ions are released.

A cell is a basic electrochemical unit that contains the basic components, such as electrodes, separator, and electrolyte. A battery or battery pack is a collection of cells or cell assemblies which are ready for use, as it contains an appropriate housing, electrical interconnections, and possibly electronics to control and protect the cells from failure. In this regard, the simplest battery is a single cell with perhaps a small electronic circuit for protection.

A rechargeable battery, storage battery, secondary cell, or accumulator is a type of electrical battery which can be charged, discharged into a load, and recharged many times, while a non-rechargeable or primary battery is supplied fully charged, and discarded once discharged. It is composed of one or more electrochemical cells. Rechargeable batteries are produced in many different shapes and sizes, ranging from button cells to megawatt systems connected to stabilize an electrical distribution network. Devices which use rechargeable batteries include, but are not limited to, automobile starters, portable consumer devices, light vehicles (such as motorized wheelchairs, golf carts, electric bicycles, and electric forklifts), tools, uninterruptible power supplies, and battery storage power stations.

One type of rechargeable battery is the lithium-ion battery which normally has an anode made of graphite. Using an anode made of silicon (Si) can increase the capacity, because the Si-anode can accept much more lithium-ion than a graphite-anode. However, problems occur when charging the battery because the Si-anode expands resulting in a small lifespan for the Si-anode. This situation is mitigated or eliminated by various embodiments described herein.

Surface functionalization of Si nanoparticles

Functionalization of Si materials with amine groups has been widely reported. Si nanoparticles possess a thin native SiC layer, which contains active hydroxyl groups that can react with (3-aminopropyl)trimethoxysilane (APS) to covalently bond the amino groups onto Si particle surface (Figure 7). Different amounts of APS were added to the functionalization solution to systematically vary the amine coverage on Si particles. The resulting surface amine coverage is summarized in Figure 1 for 5 different APS/Si weight ratios. Increasing the amount of APS from 1 : 100 to 1 : 10 of APS/Si ratio (by mass) tripled the amine coverage from ca. 0.016 mmol/g Si to ca. 0.05 mmol/g Si. Further increase in the APS/Si ratio had no effect on the coverage. We hypothesized that at 1 : 10 APS/Si weight ratio, the Si surface was saturated with amino groups due to the steric effect which limited further growth of APS onto Si surfaces.

The amine functionalized Si (denoted as S1- H2), conductive filler particles (carbon black), and PAA were mixed in water and dried to form the electrode. During the mixing process, the amine from the S1- H2 deprotonated the carboxylic acid from the PAA polymer and formed the ionic ammonium carboxylate salt at the Si particle/PAA polymer binder interface (Figure 8). The formation of ionic bonds was confirmed by XPS and Raman spectroscopy using amino functionalized silica (denoted as S1O2- H2). Silica particles were used instead of S1- H2 to provide the transparency needed to detect the Raman signal of PAA. PAA signal could not be detected using a S1- H2/PAA composite film due to the strong light scattering by Si particles.

Figure 2a compares the Nls core-level XPS spectra of S1O2- H2 and the S1O2- H2/PAA composite film. Two characteristic peaks corresponding to free amine (- H2, 399.6 eV) and hydrogen bonded amine (401.8 eV) were observed in S1O2- H2. After blending with PAA, the free amine peak at 399.6 eV was greatly reduced. A new peak at 401.9 eV appeared which is attributed to protonated amine (ammonium, - H3⁺), suggesting the formation of the cationic species of the ionic bonds.

The formation of the anionic species (carboxylate, -COO^") was confirmed by Raman spectroscopy, as shown in Figure 2b. The composition of the particle/PAA (3 : 1 by weight) composite film was the same as the Si anode. Characteristic Raman peaks of PAA (indicated by the black dashed lines) were observed at 1690 cm^"1, 1457 cm^"1, 850 cm^"1, that are attributed to C=0 stretching, CH₂ deformation, and C-COOH stretching, respectively. When PAA was partially neutralized (80 % degree of neutralization, Figure 2b-i), new peaks (indicated by the red dashed lines) appeared at 1710 cm^"1, 1417 cm^"1, 903 cm^"1 (C=0 stretching, CH₂ deformation, and C-COONa stretching, respectively) due to the formation of carboxylate. These three new peaks were present in the spectrum of Si0₂- H₂/PAA but not in the spectrum of Si0₂/PAA, confirming the existence of carboxylate in Si0₂-NH₂/PAA. Cycling performance of Si composite anodes

The Si composite anodes were assembled into coin cells with Li counter electrodes to evaluate their electrochemical properties. The anode composition was 60 wt% Si particles, 20 wt% polymer binder, and 20 wt% conductive filler particles for all the tests. Both PAA and CMC-Na binders were investigated. 10 wt% fluoroethylene carbonate (FEC) was used as additive in battery electrolyte (1 M LiPF₆ in ethylene carbonate/dimethyl carbonate, EC/DMC, 1 : 1 by volume) to promote the formation of stable solid electrolyte interface (SEI) layer on Si particles. The charge/discharge process was performed between 1-0.01V vs. Li/Li⁺. Figure 3a shows the voltage profile of different Si composite anodes during the initial charge/discharge cycle at a current density of 175 mAh/g. For all the anodes, lithiation and delithiation occurred at 0.1 V and 0.25 V, respectively, consistent with the previous reports for Si composite anodes. The specific discharge capacity for all anodes ranged from 2300 to 2700 mAh/g. The initial columbic efficiency (ICE, charge capacity/discharge capacity x l00%) for Si anodes with ionic bonding (Si- H₂/PAA) was 7 %. With CMC-Na and PAA binders, plain Si nanoparticles formed hydrogen bonding with the binders, and the ICE values were 76% for CMC-Na containing anode and 70% for PAA containing anode. The irreversible capacity loss from SEI formation (the decomposition of electrolyte on Si particle surfaces) primarily occurred in the first cycle. The methyl functionalized Si (denoted as Si- CH₃)/PAA anode was tested as a control. Due to the absence of ionic bonding or hydrogen bonding, the weak van der Waals interaction between Si-CFb and PAA resulted in a vulnerable conductive network and significant SEI growth which lowered the ICE to 58 %. The cycling stability and columbic efficiency of the Si composite anodes are summarized in Figure 3b. Anodes of conventional CMC-Na and PAA binders with no ionic bonding exhibited a significant capacity decrease of -65% after 400 cycles. For the Si- CH3/PAA control anode, an even faster capacity fade of -95% was presented due to the weak bonding of S1-CH3 with PAA binder. In contrast, the S1- H2/PAA anode with ionic bonding exhibited excellent cycling stability. The discharge capacity after 400 cycles remained 1177 mAh/g, which corresponds to 80% capacity retention. These results suggest that the ionic bonding is more effective than hydrogen bonding in preserving the integrity of conductive network and repairing the damage in the conductive network caused by the volume change during anode cycling.

An increase of capacity was consistently observed for many anodes during the initial (0-50) cycles (Figure 3b). Though similar results have been reported previously, the origin of this increase is not well understood. The increased capacity is likely due to the slow wetting of the electrolyte into the composite anodes during cycling.

The morphology change of S1- H2/PAA anodes upon cycling was characterized using

SEM, as shown in Figure 9 (cross-sectional images). Prior to cycling, discrete Si particles and carbon black particles were observed in the composite anode with uniform distribution. Upon cycling, the anode surface became smooth due to the formation of SEI layer on both Si and carbon black particle surface. The composition of the SEI layer was further studied by XPS and the results are summarized in Figure 10. The Nls core-level XPS spectra of pristine and cycled anodes are shown in Figure 10a. The formation of ionic bonds was confirmed by the presence of protonated amine (- H3⁺) in pristine anode. After 400 cycles, no Nls signal was detected, suggesting the formation of SEI layer on the Si particle surface. Similarly, in Figure 10c, the Cls signals from carbon black and PAA disappeared after cycling, further demonstrating the formation of SEI layers on carbon black surface and on PAA binder.

To further explore the effect of ionic bonding to the cycling performance of Si anodes, we tested Si anodes with different concentrations of ionic bonding. The XPS characterization revealed that all the amino groups were converted to ammonium ions in the composite anode. Hence, the concentration of ionic bonding in the anodes was controlled by the amine coverage on Si particle surface. Figure 4 shows the relation of cycling stability of Si anodes with different amine coverages on Si particles. Initially, all of the anodes showed similar capacity ranging from 2394 mAh/g to 2609 mAh/g. The capacity retention after 100 cycles, however, started to differ. Significant improvement on anode capacity retention was observed when the amine coverage increased from 0.016 mmol/g Si to 0.050 mmol/g. The increased capacity retention of Si anodes clearly correlated with the increase of amine coverage on Si particles and the increase of ionic bonding concentration in the Si anodes.

The capacity of Si anodes with ionic bonding was also evaluated at different cycling rates. Figure 5 shows the capacity change and columbic efficiency with respect to different cycling rates. The current density increased from 0.21 A/g (C/20) to 4.2 A/g (1C) with 5 different cycling rates, and then moved back to 0.21 A/g. After the initial 10 cycles at 0.21 A/g, the reversible capacity remained above 2000 mAh/g. The initial gradual increase of capacity is consistent with the cycling results in Figure 3b and Figure 4. As expected, a decrease of capacity was observed with increasing current density. At the highest current density of 4.2 A/g, the anode still retained a high capacity of ~ 1150 mAh/g. When the current density was brought back to 0.21 A/g, the capacity was fully recovered to ~ 2100 mAh/g, suggesting no damage of anode structure during the entire cycling test.

The resistance change of the Si composite anodes with ionic bonding was characterized by Electrochemical Impedance Spectroscopy (EIS). The measurements were performed at the fully delithiated state of the anodes by holding the voltage at 1 V vs. Li/Li⁺ for 6 h after the charging process at different cycle numbers. The Nyquist plots of the Si- H2/PAA anodes are shown in Figure 6. The response resembles a semicircle in the high- frequency range followed by a line in the low-frequency range within each plot. The diameter of the semicircle represents the charge transfer resistance of the cell. In the first 10 cycles, the charge transfer resistance decreased possibly due to the slow wetting of electrolyte into the anode structure and increase of Si particle conductivity after Li⁺ doping. After the initial 10 cycles, the charge transfer resistance remained the same throughout the remainder of the test, indicating the anode maintained a robust conductive network with stable SEI layer. Moreover, the stable resistance over 300 cycles also explained the superior capacity retention (Figure 3b) of the Si anodes. In comparison, the Si composite anodes (Si/PAA) without ionic bonding showed steady impedance increase over 300 cycles (Figure 11), suggesting deterioration of the conductive network.

Accordingly, this disclosure provides a novel dynamic bonding approach to increase the cycle lifetimes and reliability of Si composite anodes through the restoration of the electrical interfaces. Si composite anodes with increased capacity retention were achieved through dynamic ionic bonding at the interface between Si nanoparticles and the polymeric binder. Amine groups were covalently attached to Si nanoparticle surface. During anode fabrication, the ionic bonds were readily formed between the amine groups on Si particle surfaces and the carboxyl groups on poly(acrylic acid) binder, as confirmed by X-ray photoelectron and Raman spectroscopy. The dynamic ionic bonds effectively mitigated the large volume change of Si anodes during lithium intercalation. We achieved Si composite anodes with a cycle life of 400 cycles and 80 % capacity retention at a current density of 2.1 mA/g (C/2). The Si composite anodes with ionic bonding also demonstrated good rate capability with a high capacity of 1150 mAh/g at 4.2 mA/g (1C).

The following Examples are intended to illustrate the above invention and should not be construed as to narrow its scope. One skilled in the art will readily recognize that the Examples suggest many other ways in which the invention could be practiced. It should be understood that numerous variations and modifications may be made while remaining within the scope of the invention.

EXAMPLES

Example 1. Characterization of Anodes

Materials. Trimethoxymethylsilane (TMS), (3-aminopropyl)trimethoxysilane (APS), sodium carboxymethyl cellulose (CMC-Na, average Mw -700,000), poly(acrylic acid) (PAA, average Mw -1,250,000), ninhydrin, and sodium hydroxide were used as received from Sigma Aldrich. Silicon nanoparticles (99 %, 100 nm, plasma synthesized) were purchased from MTI Corporation. Battery electrolyte containing 1 M lithium hexafluorophosphate (LiPF₆) in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 by volume) was purchased from BASF. Carbon black (Regal 400R) was obtained from Cabot Corporation. Fluoroethylene carbonate (FEC) and lithium metal (0.75 mm in thickness) were purchased from Alfa Aesar. The battery separator film was a trilayer polypropylene- polyethylene-polypropylene membrane made by Celgard and was purchased from MTI Corporation.

Methods

Electrochemical Characterization of Si Anodes. The Si anodes were dried at 80 °C in argon atmosphere for 8 h and assembled into 2032 type stainless steel coin cells with Li metal as counter electrodes in Ar-filled glove box. The electrolyte contains 1 M LiPF₆ in EC/DMC with 10 wt% FEC.

The cells were electrochemically cycled between 0.01 and 1.0 V vs. Li/Li⁺ at C/24 rate for the first formation cycle and different rates for the subsequent cycles using a battery test station (Arbin BT2000). Impedance was measured on a potentiostat system (Bio-Logic VSP) with a frequency range of 200 kHz to 0.1 Hz at 1 V with 10 mV amplitude. The cells were discharged at 1 V for 6 h prior to impedance measurement. Characterization of Si Anodes Upon Cycling. SEM (Hitachi 4800) and XPS were used to characterize the morphological and structural change of Si anodes upon cycling. The cycled coin cell was disassembled in argon atmosphere. The cycled anode was rinsed with DMC three times to remove residual electrolyte and dried in argon atmosphere prior to SEM and XPS characterization.

Example 2. Surface Functionalization of Si Particles and Amine Coverage

The silicon nanoparticles (1.0 g) were pre-dispersed in ethanol (60 mL). An appropriate amount of APS was then added into the stirring suspension. After 2 days, the amine functionalized silicon particles were centrifuged, rinsed with ethanol for 3 times, and vacuum dried. Silica nanoparticles were also functionalized with amine following the same procedure for Raman and XPS characterization. Trimethoxymethyl silane was used to make methyl functionalized Si particles for control samples.

The coverage of amino groups on the surface of Si nanoparticles was characterized by a ninhydrin titration method. Ninhydrin is known to react with amine to produce a purple color complex. The concentration of the complex (equal to the concentration of amino groups) is determined by measuring the UV-vis absorbance (@ 588 nm) of the reaction solution. The amine functionalized Si (0.02 g), denoted as S1- H2, was dispersed in ethanol (2.8 mL) and sonicated for 20 min. A ninhydrin solution (0.35 w/v % ninhydrin in ethanol, 1.0 mL) was then added to the suspension. After 10 min of sonication, the mixture was heated at 80 °C for 20 min. Upon cooling down, the mixture was centrifuged and the supernatant was collected for UV-vis (UV-2401 PC, Shimadzu, Japan) measurement. A linear calibration curve was created by measuring the UV-vis absorbance (@ 588 nm) of reaction solution of hexylmethylenediamine and ninhydrin with five known concentrations. The concentration of amino groups on S1-NH2 was determined from the calibration curve.

Example 3. Preparation of Si Anodes and Structural Characterization of Ionic Bonds

Si anodes were prepared with a weight ratio of 60:20:20 of Si nanoparticles, polymer binder, and carbon black. Two different binders were investigated: PAA and CMC-Na. The binder (30 mg) was dissolved in deionized water (3 g), and then Si (90 mg) and carbon black (30 mg) particles were added to the solution subsequently. The mixture was homogenized (OMNI GLH-01) for 1 h and magnetic stirred for 2 days. The slurry was cast on Cu foil (thickness ~ 8 μπι) with a doctor blade and air dried. X-ray Photoelectron Spectroscopy (XPS, Kratos Axis ULTRA) and Raman Spectroscopy (Horiba LabRAM HR 3D) were used to characterize ionic bond formation in Si anodes. Films with different compositions were made by casting the particle or particle/polymer suspensions onto plastic substrate followed by air drying.

While specific embodiments have been described above with reference to the disclosed embodiments and examples, such embodiments are only illustrative and do not limit the scope of the invention. Changes and modifications can be made in accordance with ordinary skill in the art without departing from the invention in its broader aspects as defined in the following claims.

All publications, patents, and patent documents are incorporated by reference herein, as though individually incorporated by reference. No limitations inconsistent with this disclosure are to be understood therefrom. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention.

Claims

What is claimed is:

1. A composite self-healing silicon anode comprising:

a) a functionalized silicon particle comprising:

c) a conductive filler particle; and

2. The composite of claim 1 wherein the acid moieties comprise a carboxylic acid or a sulfonic acid, and the dynamic interfacial ionic bonds mitigate volume changes in a silicon anode from lithium intercalation.

3. The composite of claim 1 wherein the functionalized silicon particle is about 10 nanometers to about 10 micrometers in diameter.

4. The composite of claim 1 wherein the amino moiety of the amino(alkyl)silyl moiety is a primary amine, a secondary amine, a tertiary amine, or the amino moiety is contained in a heterocyclic ring wherein the heterocyclic ring contains at least one nitrogen atom.

5. The composite of claim 4 wherein the amino(alkyl)silyl moiety is 2-aminoethylsilyl,

3- aminopropylsilyl, 4-aminobutylsilyl, 2-(heteroaryl)ethylsilyl, 3-(heteroaryl)propylsilyl, or

4- (heteroaryl)butylsilyl, wherein heteroaryl is imidazole, pyrazole, triazole, or tetrazole.

6. The composite of claim 1 wherein the alkyl portion of the amino(alkyl)silyl moiety is optionally branched (C2-C₂o)alkyl, or -CH₂CH2(XCH2CH2X)nCH2CH₂- wherein each X is independently -O- or - H-, and n is 1 to 20.

7. The composite of claim 1 wherein the polymer binder comprises carboxymethyl cellulose, polyacrylic acid, alginic acid, hyaluronic acid, or a combination thereof.

8. The composite of claim 1 wherein the filler particles comprise carbon black, graphite, carbon nanotubes, fullerenes, charcoal, fly ash, graphene, activated carbon, or a combination thereof.

9. The composite of claim 1 wherein the composite comprises functionalized silicon particles in an amount of about 30 wt.% to about 90 wt.%, polymer binder in an amount of about 5 wt.%) to about 50 wt.%, and filler particles in an amount of about 0.1 wt.%> to about 30 wt.%.

10. The composite of claim 9 wherein the composite comprises functionalized silicon particles in an amount of about 50 wt.%> to about 70 wt.%, polymer binder in an amount of about 15 wt.%) to about 30 wt.%, and filler particles in an amount of about 15 wt.% to about 30 wt.%.

11. The composite of claim 1 wherein the amino(alkyl)silyl moieties are covalently bonded to the silanol moieties on the silica particle at about 0.005 mmol/g Si to about 10 mmol/g Si.

12. The composite of claim 1, wherein a self-healing silicon anode has a cycle life of greater than 200 cycles.

13. The composite of claim 12 having a discharge capacity of greater than 60 percent of capacity retention after more than 200 cycles.

14. The composite of claim 13 having a capacity of more than 1000 mAh/g.

15. An electrochemical energy storage cell comprising:

i) a self-healing silicon anode;

ii) a cathode;

iii) an electrolyte disposed between the anode and the cathode; and

iv) an ion permeable separator;

wherein the self-healing silicon anode comprises:

a) a functionalized silicon particle comprising:

c) a conductive filler particle; and

16. The energy storage cell of claim 15 wherein the cell is a component of a single-cell lithium-ion battery, or a multi-cell lithium-ion battery.

17. The energy storage cell of claim 16 wherein the electrolyte comprises lithium hexafluorophosphate (LiPF₆), ethylene carbonate, and dimethyl carbonate.

18. The energy storage cell of claim 16 wherein the cell is rechargeable.

19. The energy storage cell of claim 18 having a cycle life of greater than 200 cycles.

20. The energy storage cell of claim 19 having a discharge capacity of greater than 60 percent of capacity retention after more than 200 cycles.

21. The energy cell of claim 19 having a capacity of more than 1000 mAh/g.