WO2005029552A2

WO2005029552A2 - Sn-c structures prepared by plasma-enhanced chemical vapor deposition

Info

Publication number: WO2005029552A2
Application number: PCT/US2004/030371
Authority: WO
Inventors: Ping Liu; John A. Turner
Original assignee: Midwest Research Institute
Priority date: 2003-09-17
Filing date: 2004-09-16
Publication date: 2005-03-31
Also published as: WO2005029552A3

Abstract

An exemplary depositing method includes use of a plasma-enhanced chemical vapor depostion (PECVD) technique to deposit various exemplary tin and carbon compositions onto a surface. Such compositions are optionally suitable for use as anodes for lithium-ion batteries. Method parameters such as electromagnetic energy or power and temperature (e.g., substrate temperature, etc.) can affect characteristics of deposited compositions. Various exemplary compositions allow for interaction between tin and lithium and carbon and lithium to thereby increase lithium storage capacity. Various exemplary compositions exhibit superior long term cycling stability and are suitable for use as an anode or an anode material.

Description

Sn-C Structures Prepared By Plasma-Enhanced Chemical Vapor Deposition

Contractual Origin of the Invention: The United States Government has rights in this invention pursuant to Contract

No. DE-AC36-99GO10337 between the United States Department of Energy and the Midwest Research Institute. Technical Field: The subject matter disclosed herein generally relates to compositions suitable for use in batteries and methods of making the same. Background Art:

Lithium-ion batteries play an important role in many electricity-powered devices. For example, electric vehicles, hybrid electric vehicles, and fuel cell vehicles all depend on rechargeable batteries to deliver acceptable performance. On a smaller scale, lithium-ion batteries are useful as a power source for MOS memory chips (e.g., standby power), micro-electromechanical systems (MEMS), microsensors, smart cards, miniature transmitters, etc. Much research on lithium-ion batteries is aimed at understanding better mechanisms associated with lifespan, tolerance and cost. In particular, traditional lithium-ion batteries are known to suffer power fade and capacity loss, which may be a result of degrading anode performance. Some approaches to achieving improved anode performance involve the use of amorphous tin composite oxides which exhibit higher capacity than carbon-based materials as negative electrodes.

Regarding amorphous tin composite oxides, while such a composite structure can help minimize volume expansion issues, other significant issues still exist. For example, tin composite oxide decomposition by lithium may occur through an initial irreversible process that forms intimately mixed lithium oxide (Li₂O) and metallic tin (Sn). This initial irreversible process results in a significant capacity loss during a battery's first cycle as it necessitates a loss of two lithium ions for each molecule of lithium oxide (Li₂O) formed: a reaction that allows for reduction of tin oxide (SnO₂) to metallic tin (Sn). Thus, for such a battery to generate power, additional lithium is required, which participates in an alloying reaction that forms lithium-tin alloy (e.g., Li₄.₄Sn). The resulting lithium-tin alloy is typically embedded in a composite structure matrix composed predominantly of lithium oxide (Li₂O). While such a composite structure and corresponding alloying reaction have some promise to improve cyclability of a lithium-ion battery's anode, the required loss of lithium during lithium oxidation and tin oxide reduction hinders commercial viability of this approach.

Another approach employs a Sn-X alloy, with X being a metal that (i) does not form an alloy with lithium and (ii) acts as a "spectator" atom. According to this approach, an alloying reaction creates lithium-tin alloy domains dispersed in a matrix composed of the metal X without any irreversible oxide decomposition or formation. A particular study used the Sn-X approach with iron (Fe) as the metal X wherein a Sn-Fe alloy acted as a reversible anode with tin domains dispersed inside a non-active Fe matrix. However, issues still existed, for example, stability issues related to domain size, especially for higher capacities obtained with a larger cycling voltage range. Regarding carbon-based materials, some research has focused on tin and carbon composite materials for anode applications, which can potentially exploit lithium storage capacities of both tin and carbon while maintaining a dispersed tin structure. However, progress has only been made on materials that have predominantly separate tin and carbon phases and hence separate tin and carbon domains, offering marginal improvement in cycling stability.

A need exists for compositions or materials that take advantage of aforementioned benefits while minimizing or eliminating aforementioned detriments. Various exemplary compositions and methods for making such compositions are disclosed herein that address this need and/or other needs. Disclosure of the Invention:

An exemplary depositing method includes use of a plasma-enhanced chemical vapor deposition (PECVD) technique to deposit various exemplary tin and carbon compositions onto a surface. Such compositions are optionally suitable for use as anodes for lithium-ion batteries. Method parameters such as electromagnetic energy or power and temperature (e.g., substrate temperature, etc.) can affect characteristics of deposited compositions. An exemplary composition made at approximately 50°C and approximately 150 W RF power density exhibited a reversible lithium storage capacity of approximately 599 mAh/g. Structural characterization by infrared (IR) spectroscopy, ellipsometry, and X-ray electron spectroscopy (XPS) indicated that various exemplary composition have a polymer-like structure. Various exemplary compositions allow for interaction between tin and lithium and carbon and lithium to thereby increase lithium storage capacity. Various exemplary compositions exhibit superior long term cycling stability and are suitable for use as an anode or an anode material. Brief Description of the Drawings:

Features and advantages of the described implementations can be more readily understood by reference to the following description taken in conjunction with the accompanying drawings.

Fig. 1 is an illustration of an exemplary battery that includes an anode.

Fig. 2 is a schematic diagram of an exemplary apparatus for making various exemplary compositions.

Fig. 3 is a plot of exemplary performance versus time for an exemplary tin and carbon composition.

Fig. 4 is a cycle plot of exemplary cycles of Fig. 3.

Fig. 5 is a plot of exemplary performance versus cycle number for various exemplary tin and carbon compositions. Description of the Preferred Embodiments: The following description includes the best mode presently contemplated for practicing various described implementations. This description is not to be taken in a limiting sense, but rather is made merely for the purpose of describing the general principles of the various implementations. The scope of the described implementations should be ascertained with reference to the issued claims. Exemplary Tin and Carbon Compositions

An exemplary tin and carbon composition has an ability to interact with an alkali metal such as lithium. In particular, the exemplary tin and carbon composition has an ability to react with lithium in a reversible reaction. For example, in the following reversible reaction, presented herein as Equation 1 :

Sn_xC_y + (4.4x+yz)l_i «→ Li₄.₄Sn + yI_i₂C (1 ) x represents a stoichiometric amount of tin in a tin and carbon composition, y represents a stoichiometric amount of carbon in the tin and carbon composition, and the quantity (4.4x + yz) represents a stoichiometric amount of lithium that reacts with the tin and carbon composition. This reversible reaction proceeds forward to form a lithium and tin composition and a lithium and carbon composition. Note that the forward or reverse reactions do not involve oxygen; hence, Equation 1 does not include decomposition or formation of lithium oxide or tin oxide.

An exemplary tin and carbon composition has a tin to carbon ratio, optionally defined by the stoichiometric parameters x and y of Equation 1. An exemplary tin and carbon composition may have a relatively low content of tin and a relatively high content of carbon, resulting in a tin to carbon ratio of less than 1. Of course, other exemplary tin and carbon compositions may have a tin to carbon ratio greater than 1 and, in some instances, greater than approximately 50. Arrangement of the tin atoms and the carbon atoms in such a composition typically yields a density of approximately 2 grams per cubic centimeter. In general, a composition having a density of approximately 2 grams per cubic centimeter may accommodate volume changes associated with formation of tin and alkali metal alloys more effectively than a higher density composition. Further, the arrangement of the atoms typically corresponds to a single phase. For example, an exemplary tin and carbon composition has a predominantly amorphous phase. Further, such an amorphous phase typically has a polymer-like structure.

An exemplary tin and carbon composition includes one or more terminal side chains in a polymer-like structure. A terminal side chain may include a carbon atom bound to one or more other atoms. The other atoms may include tin, lithium, hydrogen, etc. For example, a terminal side chain may be a methyl group (i.e., a carbon bound to three hydrogen atoms and at least one other atom). A methyl group includes sp³ bonded carbons. Thus, an exemplary tin and carbon composition having methyl groups as terminal side chains should exhibit characteristics of sp³ bonds. However, for a tin and carbon composition formed into or as a film, the percentage of sp³ bonds should be relatively low and for a tin and carbon composition formed into or as a bulk solid, the percentage of sp³ bonds should be even lower than for a film. The relative percentage of bonds associated with one or more terminal side chains typically corresponds to bonds at an outer surface of a film or a bulk solid. Of course, for a porous film or a porous solid, terminal side chains may exist within the film or solid where measurement An exemplary tin and carbon composition includes one or more terminal side chains in a polymer-like structure. A terminal side chain may include a carbon atom bound to one or more other atoms. The other atoms may include tin, lithium, hydrogen, etc. For example, a terminal side chain may be a methyl group (i.e., a carbon bound to three hydrogen atoms and at least one other atom). A methyl group includes sp³ bonded carbons. Thus, an exemplary tin and carbon composition having methyl groups as terminal side chains should exhibit characteristics of sp³ bonds. However, for a tin and carbon composition formed into or as a film, the percentage of sp³ bonds should be relatively low and for a tin and carbon composition formed into or as a bulk solid, the percentage of sp³ bonds should be even lower than for a film. The relative percentage of bonds associated with one or more terminal side chains typically corresponds to bonds at an outer surface of a film or a bulk solid. Of course, for a porous film or a porous solid, terminal side chains may exist within the film or solid where measurement of such internal bonds may be problematic. Yet further, an exemplary tin and carbon composition has terminal side chains that are optionally replaceable and/or modifiable by any of a variety of physical and/or chemical processes (e.g., radiation, heat, chemical reaction, etc.).

An exemplary tin and carbon composition has a refractive index of approximately 1.6. The refractive index is a ratio of electromagnetic wave velocity in a vacuum to that in an isotropic medium. For an exemplary tin and carbon composition, refractive index is measured optically by ellipsometry. An exemplary tin and carbon composition has a relatively large optical band gap of approximately 4 eV or more; however, other exemplary tin and carbon compositions may have substantially smaller optical band gaps (e.g., less than 3 eV).

While various exemplary metal and carbon compositions discussed herein refer specifically to tin as the metal, other metals (e.g., Group XIV metals, such as, silicon and germanium and Group XIII metals, such as, aluminum, gallium and indium), may be suitable in addition to tin or as replacements for tin. For example, various exemplary compositions optionally include Si-C and another X-C, where X is a metal selected from Group XIII and/or Group XIV metals. Exemplary Electrodes that include a Tin and Carbon Composition

An exemplary electrode includes a tin and carbon composition wherein the tin and carbon composition may react with one or more ions that participate in an overall electrochemical reaction. For example, Equation 1 includes reaction of lithium with a tin and carbon composition. In this example, such an electrode may serve as an anode of a lithium-ion battery.

Fig. 1 shows an exemplary ion battery 100 (e.g., a lithium-ion battery, etc.) that includes a substrate 104 supporting a cathode current collector 108 and an anode current collector 112. A cathode material 116 contacts the cathode current collector 108 to allow for electrical conduction between the cathode material 116 and the cathode current collector 108 while an anode material 120 contacts the anode current collector 112 to allow for electrical conduction between the anode material 120 and the anode current collector 112. An electrolyte material 124 is disposed at least partially between the cathode material 116 and the anode material 120. The electrolyte material 124 may also contact either or both of the current collectors 108, 112. The battery 100 further includes a protective coating 128, which may protect various materials from exposure to fluids (e.g., gas, liquid, etc.) or other environmental constituents. In particular, a protective coating may protect various battery materials from oxygen exposure. The exemplary battery 100 may have a thickness "d" on the order of 10 μm. Of course other thickness and/or arrangements are possible (e.g., other layered arrangements, cylindrical arrangements, spiral arrangements, etc.).

In general, at an anode of a lithium-ion battery (e.g., the anode material 120), lithium loses an electron to form a lithium ion which can then migrate across an electrolyte (e.g., the electrolyte material 124) toward a cathode (e.g., the cathode material 116) of the battery, which functions as a lithium ion acceptor. During operation, particularly during discharge, electrons liberated from lithium at an anode may help to create a potential between an anode and a cathode and may become available at the cathode. With respect to the exemplary battery 100, such a potential may be measured between the cathode current collector 108 and the anode current collector 112. Of course, during "recharge" or charge the discharge process occurs substantially in reverse due to, for example, a potential applied across a cathode and an anode (e.g., applied via the current collectors 108, 112). While metals other than lithium may be suitable (e.g., for electron release and ion migration), lithium has a relatively low ionization energy (i.e., loses an outer shell electron relatively easily).

Anodes used in lithium-ion batteries are often characterized based on lithium storage capacity given as milliampere hours per gram (mAh/g). Metallic tin has a maximum lithium storage capacity of approximately 990 mAh/g, which corresponds to a composition of Li .₄Sn. Thus, any composition that includes tin and one or more other atoms will have a lithium storage capacity associated with any tin that is less than that of metallic tin. Various exemplary electrodes that include a tin and carbon composition typically have an initial lithium storage capacity of at least approximately 40% that of metallic tin (e.g., at least approximately 400 mAh/g). Of course, lithium storage capacity may decrease with respect to repeated charge-discharge cycles or other factors (e.g., time, temperature, environment, etc.). Further, various exemplary electrodes that include a tin and carbon composition may have lithium storage capacity associated with tin and additional lithium storage capacity associated with carbon. Of course, if such a composition includes one or more other atoms or constituents (e.g., other than tin and carbon, etc.) then yet additional lithium storage capacity may be associated with such one or more other atoms or constituents. In electrodes having irreversible lithium ion storage capacity, such irreversible storage capacity may also be given in units of mAh/g. Various exemplary tin and carbon compositions react with lithium in a manner somewhat analogous to that proposed for SnFeC₂, shown in Equation 2:

SnC_x + yLi <→ Li_ySn + xC (2), where x and y are stoichiometric factors.

Various exemplary tin and carbon compositions may have a tin to carbon atomic ratio of less than 0.5 yet have a initial lithium storage capacity greater than 33% (e.g., 0.5/(0.5 + 1)) that of metallic tin (e.g., greater than 330 mAh/g); thus, carbon appears to be involved in electrochemical reactions associated with lithium storage.

Various exemplary tin and carbon compositions exhibit no or little evidence of metallic tin domains or formation of metallic tin domains. In some instances, such exemplary compositions exhibit no or little evidence of metallic tin domains or formation of metallic tin domains due to control of or limitation of potentials. For example, when an electrode includes such an exemplary composition, a generated potential and/or an applied potential may be controlled or otherwise limited. Yet other exemplary compositions may exhibit evidence of some degree of metallic tin domains or formation of metallic tin domains. Such evidence may appear as an artifact (e.g., plateaus, etc.) in potential (e.g., voltage) with respect to time (e.g., charge-discharge, etc.). Lack of such artifacts may indicate a lack of metallic tin domains or formation of metallic tin domains.

Various exemplary electrodes that include a tin and carbon composition exhibit minimal lithium storage capacity loss with respect to charge-discharge cycling. For example, an exemplary electrode that includes a tin and carbon composition exhibits an initial lithium storage capacity of approximately 600 mAh/g and a lithium storage capacity of approximately 430 mAh/g after 800 charge-discharge cycles (see, e.g., further below for details). Such an exemplary electrode exhibits a lithium storage capacity of approximately 480 mAh/g after approximately 200 charge-discharge cycles and a lithium storage capacity of approximately 440 mAh/g after approximately 600 charge-discharge cycles. Thus, in this example, the exemplary electrode loses less than approximately 10% of its lithium storage capacity over approximately 400 charge- discharge cycles.

Exemplary Methods of Making Exemplary Tin and Carbon Compositions

An exemplary method of making an exemplary tin and carbon composition includes depositing using a plasma deposition technique. Such an exemplary method may deposit atomically dispersed tin in conjunction with carbon. According to such depositing techniques, one or more source materials in a plasma or associated with a plasma decomposes or otherwise reacts and deposits as a composition onto a surface. For example, for making of an exemplary tin and carbon composition, an organotin may suffice as a source material wherein organotins include alkyltins (e.g., methyltin, ethyltin, propyltin, butyltin, etc.) and other molecules containing carbon and tin (e.g., dibutyltin, tetrabutylin, tetrapropyltin, butyltripropyltin, dibutyldipropyltin, tributylpropyltin, tributylpentyltin, etc.). Of course, tin is optionally supplied in one source material and carbon in another source material. In the example of an organotin, the organotin may decompose and deposit onto a surface as a tin and carbon composition.

Often, a carrier fluid (e.g., a gas) carries the one or more source materials to a plasma location. Argon gas or another relatively inert fluid may suffice as a carrier fluid. The carrier fluid and the one or more source materials enter a plasma location. In general, a plasma is an ionized gas supported by an electromagnetic energy supply.

Various techniques may serve to form and maintain a plasma suitable for use in a depositing method. Plasma techniques suitable for depositing methods include plasma enhanced chemical vapor deposition (PECVD), two-step deposition and remote deposition schemes, electron cyclotron resonance (ECR) deposition, and other microwave and/or radio frequency techniques.

An exemplary apparatus suitable for making a tin and carbon composition includes a plasma chamber having one or more radio frequency electrodes positioned within to supply electromagnetic energy to form and maintain the plasma. The exemplary apparatus typically includes a holder for a substrate. A holder may also serve as an electrode. Where a holder holds an electrically conductive substrate, the substrate may serve as an electrode. The exemplary apparatus further includes an inlet to supply one or more source materials to the plasma and typically an outlet. In general, the exemplary apparatus can maintain a pressure within the plasma chamber.

During operation of such an apparatus, the one or more source materials decompose or otherwise react and deposit as a composition onto a surface of the substrate. Doping of a composition may also occur as part of a depositing method. For example, a carrier gas may carry a doping material to a plasma chamber which then deposits onto a surface of a substrate in the chamber.

A variety of factors can affect depositing of the one or more source materials and the nature of the deposited composition. For example, chamber temperature, substrate temperature, chamber pressure (e.g., pressure differential between inlet and outlet, etc.), energy supplied to the plasma, supply rate of the one or more source materials, ratio of the one or more source materials, nature of the substrate, etc. may all affect depositing and the nature of the deposited composition.

Fig. 2 shows an exemplary apparatus 200 suitable for making an exemplary tin and carbon composition. The apparatus 200 includes a deposition chamber 204 (e.g., a PECVD chamber, etc.) having a first electrode 208 and a second electrode 212 for forming and maintaining a plasma 216 substantially disposed between the first electrode 208 and the second electrode 212. The deposition chamber 204 may be referred to at times as a plasma chamber. The deposition chamber 204 further includes a substrate 220 having a surface for supporting deposition of an exemplary composition 224 thereon. The substrate may have a composition and a thickness suitable for use in any of a variety of electrode or battery arrangements such as, but not limited to, planar, cylindrical arrangements and spiral arrangements. Of course, a substrate may be flexible and/or pliable to aid in formation a particular arrangement. During operation (i.e., a depositing method), a low deposition chamber temperature can allow for a wider variety of substrates, including flexible or pliable substrates.

A controller 230 operatively controls one or more parameters relevant to operation of the deposition chamber 204. For example, the controller 230 may control a pump 234 for maintaining a pressure (e.g., positive pressure or vacuum) within the deposition chamber 204. The controller 230 may also control supply of materials to the deposition chamber 204.

Regarding materials that may be supplied or otherwise provided to the deposition chamber 204, such materials include a source material 238, a carrier gas 240, and optionally one or more other gases 244, 248. Of course, other materials in solid, liquid, gas, plasma form may be supplied or otherwise provided to the deposition chamber 204 and optionally controlled via the controller 230. According to the exemplary apparatus 200, the source material 238 resides in a temperature controllable unit 236 (e.g., a bath, an oven, etc.). The controller 230 may control temperature of the temperature controllable unit 236. Of course, such a unit may allow for pressure, flow or other control as well. The temperature controllable unit 236 has an inlet for a carrier gas 240 and an outlet for a mixture of carrier gas 240 and source material 238. The mixture can flow to the deposition chamber 204 wherein the mass or volumetric flow rate of the mixture is controlled via a valve 242, which may be controllable via the controller 230. Mass or volumetric flow rate of the other gases 244, 248 are controlled via valves 246, 250, respectively, which may be controllable via the controller 230. Exemplary Method using Tetrabutyltin

An exemplary depositing method used tetrabutyltin (e.g., Sn(Bu)₄) as a source material. Argon was used as a carrier gas to carry the source material to a plasma chamber. The argon gas was supplied to a vessel containing source material at a pressure of approximately 15 psi, which could maintain a flow rate to the plasma chamber of the source material and carrier gas of approximately 20 standard cubic centimeters per minute (seem) with a plasma chamber pressure of approximately 800 mTorr. The temperature of the source material was maintained at approximately 50°C. Where necessary, hydrogen and oxygen gases were introduced to the plasma chamberto modulate carbon and hydrogen content in a deposited film. Such gasses may help in removing carbon more efficiently, in addition, in some instances, it is possible to use a small amount of oxygen not form any significant tin-oxide. A copper foil substrate was positioned in the plasma chamber. In general, copper is relatively inert and stable at reductive voltages and hence does not readily form alloys with lithium in a reducing environment. Copper foil may serve as a current collector for carbon-based anodes in commercial lithium-ion batteries. Prior to positioning the substrate in the plasma chamber, the substrate was rinsed with acetone and blow-dried with nitrogen gas. An electromagnetic energy supply supplied radio frequency energy to electrodes of the plasma chamber at a power of approximately 150 W. Deposition of an exemplary tin and carbon composition onto a surface of the substrate was achieved. For the aforementioned parameters, a deposition rate, given in thickness of the composition per minute, of approximately 114 A per minute was achieved over a period of approximately 20 minutes (e.g., an average rate of approximately 2 A per second).

The aforementioned exemplary depositing method, identified as EDM 1 , and various other exemplary depositing methods are presented in Table 1 , below. Table 1. Exemplary Depositing Methods (EDMs)

Data presented in Table 1 indicate that deposition rate changes with respect to temperature and power. For example, at approximately 50°C, deposition rate generally increases with respect to an increase in power (EDM 1 and EDM 3). For a constant power of approximately 150 W, deposition rate first increases with respect to an increase in temperature (e.g., EDM 1 to EDM 7) and then decreases after approximately 100°C with no deposition observed at a temperature of approximately 250°C (e.g., EDM 4 to EDM 6 to EMD 5). With respect to power, a power density may be determined based on a chamber volume. In these examples, a chamber diameter of approximately 24 cm was used with an inter-electrode gap of approximately 3.2 cm, which gives a volume of approximately 120 cm³. Hence, the power density for 50 W RF would be approximately 0.41 Wcm^"3, for 150 W RF would be approximately 1.24 Wcm^" ³, and for 250 W RF would be approximately 2.07 Wcm^"3. In these examples, the chamber pressure was approximately 800 mTorr.

Data presented in Table 1 indicate that density of the deposited compositions (e.g., as films) changes with respect to power. For example, density is generally higher for compositions prepared at high power values (e.g., EDM 4 and EDM 6). Data presented in Table 1 indicate that carbon content of resulting compositions changes with respect to temperature. For example, an increase in temperature generally results in a decrease in carbon content as indicated by Sn:C ratio (e.g., EDM 1 versus EDM 4). The relationship between carbon content in resulting compositions and temperature may be attributed to carbon entities (e.g., carbon containing decomposition products) having a higher volatility than tin entities (e.g., tin containing decomposition products). In general, such differences in volatility become more evident with respect to increasing substrate temperature, typically as a higher ratio of tin to carbon (Sn:C ratio). Studies that examined PECVD of carbon films using hydrocarbons as source material indicate that properties of the films depend on power density. For example, at very high power densities, hard coatings were obtained having a relatively high percentage of sp³ bonded carbons. Such films are typically highly insulating and very inert towards chemical reactions. In contrast, at low power densities, polymer-like soft amorphous carbon films were formed. The exemplary depositing methods (EDM 1-7) presented in Table 1 may be categorized as associated with low power densities; hence, exemplary tin and carbon compositions generally belong to the latter polymeric- like category with respect to carbon structure (i.e., carbon within the exemplary compositions are not characteristic of hard coatings having a high percentage of sp³ bonded carbons).

With respect to data presented in Table 1 and elsewhere herein, infrared (IR) spectra were obtained on Nicolet, Model 510 FT-IR spectrometer; X-ray photoelectron spectroscopy (XPS) were performed on a Philips 3600 ESCA system; optical properties of exemplary compositions were examined with a n&k analyzer (n&k Technologies, Inc.); and electrochemical tests were performed in a one-compartment three-electrode cell with lithium as both the counter and the reference electrodes wherein a solution of 1 M of LiCIO₄ in propylene carbonate was used as the electrolyte and wherein electrodes were cycled between 0.01 and 1.2 V at a current density of 10 μA/cm². Infrared (IR) spectral analyses were performed in the transmittance mode on exemplary compositions deposited on silicon wafers. IR data indicate that at approximately 3425 cm^"1, broad absorption bands can be attributed to absorbed water, which is indicative of a polymer-like structure. An absorption band for C-H stretching modes for the exemplary composition associated with EDM 1 can be de-convoluted to three peaks with wave-numbers of 2963, 2926, and 2873 cm^"1. All three bands can be attributed to sp³ CH₃ carbons according to previous assignments. The dominance of CH₃ carbon again is evidence of a polymer-like structure rather than a hard carbon structure. In hard carbon structures, side chain termination (CH₃) is rare due to formation of a three-dimensional cross-linking of a structural carbon skeleton. The exemplary compositions associated with EDM 1-7 exhibit evidence of a polymer-like structure, despite different temperatures and/or powers. In general, various prior studies indicate that the power densities employed in EDM 1-7 (e.g., at the given pressure) are well below values necessary for formation of hard carbon structures. IR data for exemplary compositions associated with EDM 4 and EDM 6 indicates weaker C-H peaks and smaller water absorptions, which is consistent with elemental data provided by XPS analyses. The exemplary compositions associated with EDM 4 and EDM 6 have a relatively high concentration of tin (e.g., Sn:C greater than approximately 30). Consequently, carbon species observed in the IR spectra are mostly likely on the surface of the exemplary compositions.

Optical measurements performed on the exemplary compositions associated with EDM 1 through EDM 4, by ellipsometry, yield refractive index values of approximately 1.6, which is typical of polymer-like films. Exemplary compositions prepared at low temperatures have extremely large optical band gaps of over 4 eV (e.g., EDM 1 and EDM 2). The band gap is substantially smaller for samples prepared at higher temperatures (e.g., EDM 4 and EDM 6).

Further information was obtained by XPS analysis. XPS analysis plots of C 1s and Sn 3d were acquired for the exemplary composition associated with EDM 1. The exemplary composition associated with EDM 1 was exposed to air during transfer for XPS analysis and the XPS analysis exhibits three peaks associated with different types of surface carbons with binding energies at approximately 286.34 eV, approximately 288.47 eV, and approximately 290.19 eV, respectively. The peak corresponding to approximately 286.34 eV corresponds to surface contamination carbons. A large deviation (e.g., at approximately 1.84 eV) from a normal value of 284.5 eV indicates significant charging of the exemplary composition during XPS analysis. The two peaks exhibited at higher energies indicate oxidized forms of carbon such as those in C-O bonds. In another XPS analysis plot, a single peak from Sn 3d_5/2 was observed at approximately 487.80 eV. After taking into account the charging effect, a value of approximately 485.96 eV was obtained, which indicates some form of an oxidized tin. Again, noting that the exemplary composition was exposed to air (i.e., an oxygen containing gas) prior to XPS analysis.

A further analysis occurred after sputtering the exemplary composition with approximately 1 keV Ar⁺ for about one minute. XPS analysis indicates the presence of two types of tin with binding energies of approximately 484.44 eV and approximately 486.05 eV. The first peak value at 484.44 eV is very close to that of metallic tin, indicating that the sample is partially reduced during sputtering, while the second peak at 486.05 eV can be assigned as tin bonding to carbon. XPS analysis also indicates the presence of two different types of carbon after sputtering, with binding energies at approximately 284.30 eV and approximately 285.32 eV. The low value of the first peak indicates a reduced form of carbon and can be assigned to carbon-tin bonds. Notice that the value is higher than that of carbon in carbides. The second peak at 285.32 eV can then be assigned to carbon bonding to carbon or hydrogen. The XPS results thus corroborate data acquired with IR spectral analysis. Overall, data indicate that a tin and carbon composition has been deposited via an exemplary depositing method (e.g., one that uses PECVD).

Performance of Exemplary Compositions With regard to lithium storage capacity, deposition power has an influence and, in particular, power density. For example, exemplary compositions associated with EDM 1 and EDM 2 were prepared at the same temperature (approximately 50 °C) and pressure (approximately 800 mTorr) but at different power densities (approximately 1.24 Wcm^"3 versus approximately 0.41 Wcm^"3). The composition associated with EDM 2 exhibits little or no capacity while the exemplary composition associated with EDM 1 exhibits a capacity of approximately 599 mAh/g (see, e.g., Table 1). Of course at some point, power density becomes an important variable, which may depend on other variables as well. Trends exhibited by various examples, at the specified conditions, indicate that a power density greater than approximately 0.4 Wcm^'3 should be used. However, a power density of approximately 1.2 Wcm^"3 is likely not a lower limit given the specified conditions; hence, in one example, a power density of greater than approximately 0.8 Wcm^'3 may be used.

Fig. 3 shows a plot of voltage versus time for the first four charge-discharge cycles for the exemplary composition associated with EDM 1. During the first discharge, little reaction is observed above 0.5 V (i.e., discharge from approximately 3.25 V to approximately 1 V occurs quickly). This observation is in remarkable contrast to either tin oxide or carbon anodes. During the first discharge of tin oxide, a plateau at around 0.8 V is attributed to the irreversible formation of lithium oxide (see, e.g., Background section). Because the exemplary depositing method, EDM 1 , was used to prepare the exemplary tin and carbon composition in a substantially oxygen free environment, relatively little or no tin oxide forms during the depositing method; thus, tin oxide formation the associated irreversible reaction are avoided.

The plot 300 of Fig. 3 bears some resemblance to charge-discharge cycles exhibited by Sn₂FeC_x compositions prepared by ball milling. Further, the plot 300 does not exhibit any well-defined plateaus in cycles 2, 3 or 4.

Fig. 4 shows a differential plot 400 of the discharge curve of Fig. 3 that exhibits peaks at 0.25 and 0.40 V, which are relatively broad compared to those of SnFeC alloy compositions. The consequence of this exhibited behavior indicates that no significant phase transition takes place which would compromise cycling stability. Compositions made via depositing methods that used a higher power density of approximately 2.07 Wcm^"3 resulted in similar discharge profiles (not shown) when compared to the example EDM1 , although the specific capacity was much higher. Analyses of the discharge profiles of an exemplary composition made using the exemplary depositing method EDM 7 (e.g., temperature of approximately 100°C) indicated a charge-discharge behavior that was similar to an exemplary composition made using the exemplary depositing method EDM 1. More specifically, the charge- discharge behavior was similarly without distinct plateaus even though EDM 7 used a higher deposition temperature and resulted in a composition having a much higher

Sn/C ratio (see, e.g., Table 1). However, analyses on other compositions indicated that further increases in temperature can result in fundamental changes in composition properties (see, e.g., Table 1). In particular, an exemplary composition made using the exemplary depositing method EDM 4 (temperature approximately 150°C) exhibited well- defined charge-discharge plateaus and indications that structural changes take place during the first few cycles. For example, a peak at 0.3 V diminished with cycling while a peak at 0.48 V grew. Elemental analyses of the exemplary composition made using EDM 4 revealed a relatively small content of carbon (e.g., Sn:C of approximately 33.5). Profiles of charge-discharge curves for this particular composition have similarities to profiles of elemental tin. Thus, while EDM 4 made a tin and carbon composition, the composition was not substantially free of metallic or elemental tin. Analyses of an exemplary composition made using the exemplary depositing method EDM 6 (temperature of approximately 200°C) exhibited an even higher density and an even higher Sn:C ratio than compositions made using lower temperature methods (see, e.g., Table 1 ). This composition also exhibited more clearly resolved peaks on differential curves.

As already mentioned, metallic tin domains typically grow in certain compositions in response to electrochemical cycling until they reach a "saturation" size. Such domain size growth contributes to the transformation of the charge-discharge curves and is closely related to the capacity degradation during cycling. Hence, various exemplary compositions discussed herein have reduced or limited tin content (see, e.g., Sn:C ratios of Table 1 ). Of course, control of cycling potential range may also reduce or minimize growth of metallic tin domains. More specifically, exemplary tin and carbon compositions made using the exemplary depositing methods EDM 1 , 3 and 7 exhibited relatively low tin content and relatively high carbon content, resulting in Sn:C ratios of less than approximately 10 and more specifically less than approximately 8. These three exemplary compositions exhibited relatively smooth and featureless charge-discharge curves (i.e., without any substantial plateaus).

In contrast, exemplary tin and carbon compositions made using the exemplary depositing methods EDM 4 and EDM 6 exhibited relatively high tin content (e.g., Sn:C ratios greater than approximately 30 yet less than approximately 71). These two compositions also exhibited charge-discharge curves with plateaus as detailed by the sharp peaks on corresponding differential curves. Thus, while these two compositions are beneficial in comparison to conventional compositions, they include more tin domains than the previously mentioned three exemplary compositions (i.e., those made using EDM 1 , 3 and 7). For example, various exemplary compositions optionally include a Sn-C structure with tin domains. Fig. 5 shows a plot 500 of performance for various exemplary tin and carbon compositions described in Table 1 and prepared using aforementioned exemplary depositing methods (i.e., EDM 1-7) versus cycle number. In the plot 500, performance is shown as lithium storage capacity in units of mAh/g while a cycle refers to a charge- discharge cycle (or discharge-charge cycle). Performance is shown for an exemplary composition 1 made using the exemplary depositing method EDM 1 ; an exemplary composition 3 made using the exemplary depositing method EDM 3; an exemplary composition 4 made using the exemplary depositing method EDM 4; an exemplary composition 6 made using the exemplary depositing method EDM 6; and an exemplary composition 7 made using the exemplary depositing method EDM 7. Performance data shown in plot 500 indicates that an exemplary composition made using the exemplary depositing method EDM 1 has better long term stability when compared to exemplary compositions made using exemplary depositing methods EDM 2-7. More specifically, EDM 1 can produce a composition that after 800 cycles has a capacity of approximately 400 mAh/g.

As described herein, various exemplary methods use a PECVD technique to prepare high-capacity, high-stability tin and carbon thin-film electrodes for use as anodes in lithium batteries. Various exemplary depositing methods can generate a tin and carbon composition having substantially no separate metallic tin phase. When used as anodes in a lithium-ion battery, the lithium may potentially interact with both tin and carbon to thereby increase lithium storage capacity. Further, according to various exemplary compositions, a lithium-tin compound may coexist with a lithium-carbon matrix to thereby enhance cycling stability of an anode. Yet further, various exemplary depositing methods can create tin and carbon compositions that exhibit little or no irreversible formation of lithium oxide. Of the exemplary compositions made using the exemplary depositing methods, a composition having superior long term performance was made at a relatively low temperature of approximately 50°C. This exemplary composition, as well as others, is suitable for use as an anode for thin-film lithium-ion batteries.

Claims

1. A composition comprising predominantly a single phase of Sn-C.

2. The composition of claim 1 further comprising an amorphous single phase.

3. The composition of claim 1 further comprising lithium ion conductivity.

4. The composition of claim 1 further comprising a polymer-like structure.

5. The composition of claim 1 further comprising terminal side chains.

6. The composition of claim 5 wherein the terminal side chains include C-H bonds.

7. The composition of claim 5 wherein the terminal side chains include CH₃.

8. The composition of claim 1 further comprising refractive index of approximately 1.6 or more.

9. The composition of claim 1 further comprising a density of approximately 2 grams per cubic centimeter.

10. The composition of claim 1 further comprising a specific capacity of greater than approximately 0.4 ampere hours per gram.

11. The composition of claim 1 further comprising a metal to carbon ratio of less than approximately 10.

12. The composition of claim 1 further comprising one or more other Group XIII or Group XIV metals.

13. The composition of claim 1 further comprising a bandgap of greater than approximately 3 electron volts.

14. The composition of claim 1 having substantially no metallic tin domains.

15. The composition of claim 14 wherein charge-discharge cycle profiles for the composition lack substantial plateaus and thereby indicate substantially no metallic tin domains.

16. The composition of claim 1 having a low percentage of sp³ bonded carbons.

17. The composition of claim 16 wherein the sp³ bonded carbons are predominantly associated with terminal side chains.

18. The composition of claim 1 having infrared spectra wave numbers of approximately 2963 cm^"1, 2926 cm^"1, and 2873 cm^"1.

19. The composition of claim 1 wherein the terminal side chains occur primarily on an outer surface of the composition.

20. The composition of claim 1 comprising a binding energy of approximately 486 eV.

21. The composition of claim 20 wherein the binding energy corresponds to tin- carbon bond.

22. The composition of claim 1 comprising a binding energy that corresponds to a carbon-metal bond.

23. The composition of claim 1 further comprising lithium.

24. The composition of claim 1 further comprising a doping material.

25. The composition of claim 1 further comprising a lithium-metal compound in a matrix of Sn-C.

26. The composition of claim 1 wherein the composition does not include any substantial amount of oxygen.

27. The composition of claim 1 wherein the composition does not include any substantial amount of tin-oxides.

28. An anode comprising predominantly a single phase of Sn-C.

29. A lithium ion battery comprising: an electrolyte; a cathode; an anode, wherein the anode comprises predominantly a single phase of Sn-C.

30. The lithium ion battery of claim 29 wherein the battery maintains a capacity of greater than approximately 0.4 mAh/g for approximately 800 cycles.

31. The lithium-ion battery of claim 29 wherein discharge behavior resembles that of a SnFeC alloy.

32. A method comprising: providing a substrate; providing metal and carbon to a deposition chamber; maintaining the substrate at a temperature of less than approximately 150°C; providing electromagnetic energy at a power density of more than approximately 0.8 Wcm^"3 to a plasma in the deposition chamber; and depositing a single phase of Sn-C from the plasma onto the substrate.

33. The method of claim 32 wherein the depositing step deposits the single phase of Sn-C at a rate of greater than approximately 2000 Angstroms in approximately 20 minutes.

34. The method of claim 32 wherein the temperature comprises a temperature in a range from approximately 50°C to approximately 100°C.

35. The method of claim 32 wherein the temperature is approximately 50°C and the power density is approximately 1.2 Wcm^"3.

36. The method of claim 32 wherein the temperature is approximately 50°C and the power density is approximately 2 Wcm^"3.

37. The method of claim 32 wherein the temperature is approximately 100°C and the power density is approximately 1.2 Wcm^"3.

38. The method of claim 32 wherein the power density is approximately 1.2 Wcm^"3.

39. The method of claim 32 wherein the power density is approximately 2 Wcm^"3.

40. The method of claim 32 wherein the deposition chamber has a pressure of approximately 800 mtorr.

41. The method of claim 32 wherein the depositing occurs without forming any substantial metallic domains.

42. The method of claim 32 wherein the providing step provides tin and carbon as an organotin.

43. The method of claim 42 wherein the organotin is tetrabutyltin.

44. The method of claim 32 wherein the depositing step includes decomposition of a carbon and metal compound in a deposition chamber environment substantially free of hydrogen and oxygen gases.

45. The method of claim 32 wherein the providing step include providing the tin and the carbon via a carrier gas.

46. The method of claim 32 wherein the carrier gas comprises argon.

47. The method of claim 32 wherein at a power density of approximately 150 W deposition rate increases with respect to increasing temperature to approximately

100°C and then decreases with increasing temperature.

48. The method of claim 32 wherein the depositing step deposits a low percentage of sp³ bonded carbons.

49. An anode comprising predominantly a single phase of X-C, wherein X is a metal selected from a group consisting of Sn, Al, Ga, In, and Ge.

50. The anode of claim 49 further comprising Si-C.