Modified silicon coatings for use in lithium-ion battery anodes

Abstract

Provided herein are nanostructures and fabrication methods for lithium-ion battery electrodes. In some embodiments, nanostructure templates coated with a silicon-based coating are provided. The silicon coating is a non-conformal, more porous silicon-rich SiE. x layer and a conformal, denser SiE layer on top of this non-conformal, more porous layer. x In some embodiments, two different deposition processes are used: a non-conformal silicon-rich SiE layer; x A PECVD process is used to deposit the silicon-rich SiE layer, and a thermal CVD process is used to deposit the conformal layer. x The material prevents the growth of silicon crystalline domains, restricts macroscopic expansion, and increases the lithium diffusion rate, significantly improving battery life during charge-discharge cycling of lithium-ion batteries.

Description

[Technical Field]

[0001] A PCT Request Form has been filed contemporaneously with this application as part of this application. Each application to which this application claims benefit or priority as identified in the contemporaneously filed PCT Request Form is incorporated herein by reference in its entirety for all purposes. [Background technology]

[0002] The present invention relates generally to nanostructures, and more particularly to multi-layer silicon nanowire structures useful in battery anodes.

[0003] Much research has been done to find ways to use silicon in lithium battery anodes. Silicon has great promise because it has a lithium capacity ten times higher than current graphite. Unfortunately, however, silicon expands by 400% when it absorbs too much lithium, often resulting in its collapse and a shortened battery life. Summary of the Invention

[0004] One aspect of the present disclosure is an anode for a lithium battery, the anode comprising a substrate, an array of nanowires rooted to the substrate, each nanowire having a surface, and a first layer coating most or all of the surfaces of the nanowires, the layer being made of silicon carbide (SiE). x a first layer comprising a material; and a second layer over the first layer, any exposed surfaces of the nanowires, and the substrate, the second layer being silicon or SiF yand a second layer comprising any of the materials, wherein x is greater than zero and less than 1, y is greater than zero and less than 1, and E and F are each independently selected from the group consisting of nitrogen, carbon, boron, phosphorus, oxygen, magnesium, aluminum, germanium, tin, nickel, copper, and combinations thereof. In some embodiments, x is between 0.01 and 0.5, between 0.01 and 0.3, or between 0.01 and 0.1. In some embodiments, y is between 0.01 and 0.5, between 0.01 and 0.3, or between 0.01 and 0.1. In some embodiments, lower values ​​of x and / or y may be used.

[0005] In some embodiments, the concentration profile of E varies through the thickness of the first layer and / or the concentration profile of F varies through the thickness of the second layer. In some embodiments, the density of the second layer is higher than the density of the first layer. In some embodiments, the average density of the first layer is 2.1 g / cm 3 In some embodiments, the average density of the second layer is less than 2.0 g / cm 3 In some embodiments, the density of the first layer varies throughout the first layer. In some embodiments, the density of the second layer varies throughout the second layer. In some embodiments, the first layer is non-conformal to the nanowire template. In some embodiments, the second layer is conformal to the first layer. In some embodiments, the anode further comprises a third layer on the second layer, the third layer being silicon-free. In some embodiments, the nanowire template comprises silicide nanowires. In some embodiments, the first layer has a thickness at its largest diameter of about 5 to 20 microns. In some embodiments, the second layer has a thickness of 5 to 500 nanometers. In some embodiments, the second layer has a thickness of 5 to 100 nanometers.

[0006] Another aspect of the present disclosure relates to a lithium battery comprising an anode as described herein, a cathode containing lithium, and an electrolyte in ionic communication with both the anode and the cathode.

[0007] Another aspect of the present disclosure is a method of making an anode for a lithium battery, comprising the steps of providing a substrate; growing nanowires from the substrate, each nanowire having a surface; and depositing a first layer using a PECVD process to coat most or all of the surfaces of the nanowires, the first layer being a first silicon-rich SiE. x and depositing a second layer over the first layer, any exposed surfaces of the nanowires, and the substrate using a thermal CVD method, the second layer comprising a second silicon-rich SiE. x The present invention relates to a method comprising the steps of:

[0008] In some embodiments, the PECVD process is an expanding thermal plasma process. In some embodiments, the nanowires are silicide nanowires. In some embodiments, the chamber pressure during the thermal CVD process is less than about 2 Torr.

[0009] These and other aspects of the present disclosure are further described below with reference to the drawings. [Brief explanation of the drawings]

[0010] The foregoing and other aspects will be readily apparent to those skilled in the art from the following description of exemplary embodiments when read in conjunction with the accompanying drawings.

[0011] [Figure 1] FIG. 1 is a schematic diagram of a nanowire with a layer of silicon-based material deposited thereon using PECVD (plasma-enhanced chemical vapor deposition).

[0012] [Figure 2]1 is a schematic diagram of a nanowire having a first silicon-based material layer deposited using PECVD, and then a second silicon-based material layer deposited thereon using thermal CVD, according to one embodiment of the present invention.

[0013] [Figure 3] 1 shows a schematic representation of a non-conformal silicon coating on a template nanowire.

[0014] [Figure 4A] 1 is a schematic representation of a plan view of a partially assembled electrochemical cell using electrodes described herein, according to certain embodiments.

[0015] [Figure 4B] 1 is a schematic representation of a cross-sectional view of an electrode stack of a partially assembled electrochemical cell using electrodes described herein, according to certain embodiments.

[0016] [Figure 5A] 1A-1C are schematic representations of various views of an electrode wound together with two separator sheets to form a cell, according to certain embodiments. [Figure 5B] 1A-1C are schematic representations of various views of an electrode wound together with two separator sheets to form a cell, according to certain embodiments. [Figure 5C] 1A-1C are schematic representations of various views of an electrode wound together with two separator sheets to form a cell, according to certain embodiments.

[0017] [Figure 6A] 1 is a schematic representation of a cross-sectional view of a stack of cells including a plurality of cells according to certain embodiments. [Figure 6B] 1 is a schematic representation of a perspective view of a stack of cells including a plurality of cells according to certain embodiments.

[0018] [Figure 7] 1 is a schematic representation of a cross-sectional view of a rolled cylindrical cell, according to certain embodiments.

[0019] [Figure 8] 1 is a graph showing capacity retention as a function of cycle index for silicon anodes compared to SiEx anodes. DETAILED DESCRIPTION OF THE INVENTION

[0020] Certain embodiments are presented in the context of depositing silicon on silicide nanowires to form anode structures for lithium battery cells. However, those skilled in the art will readily appreciate that the materials and methods disclosed herein have application in numerous other contexts in which tailoring deposition to produce layers or particles with unique characteristics is useful. For example, various embodiments are described herein with reference to nanowires. However, unless otherwise specified, it should be understood that references herein to nanowires include other types of nanostructures, such as nanotubes, nanoparticles, nanospheres, nanorods, nanowhiskers, and the like, as described in U.S. Pat. No. 8,257,866, which is incorporated herein by reference.

[0021] In general, the term "nanostructure" refers to a structure having at least one dimension less than about 1 micron. In some embodiments, the structure has at least one dimension less than 500 nanometers or 100 nanometers. This dimension can be the diameter of the nanostructure (e.g., a silicide template nanowire) or the final coated structure. However, any of the overall dimensions (length and diameter) of the final coated structure need not be nanoscale. For example, the final structure may include a layer that is about 10 microns thick at its largest diameter, coated on a template that is about 100 nanometers in diameter and 20 microns in length. This overall structure is about 10.1 microns in its largest diameter and 20 microns in length, but can be generally referred to as a "nanostructure" because of the dimensions of the template. In specific embodiments, the term "nanowire" refers to a structure with a nanoscale shell disposed on an elongated template structure.

[0022] In various embodiments, nanowires (as an example of nanostructures) have an aspect ratio greater than 1, at least about 2, or at least about 4. In various embodiments, nanowires have an aspect ratio of at least 10, at least 100, or at least 500. Nanowires may be connected to other electrode components (e.g., a conductive substrate, other active material structures, or conductive additives). For example, nanowires may be rooted to a substrate such that one end of the nanowire contacts the substrate.

[0023] The term "silicon-based material" refers to a material that is exclusively silicon or silicon-rich SiE x where E is any element or elements capable of forming an intermetallic or alloy compound with silicon, such as nitrogen, carbon, boron, phosphorus, oxygen, magnesium, aluminum, germanium, tin, nickel, copper, and combinations thereof. Silicon is at least 50 atomic % of the silicon-based material.

[0024] More than one element E may be mixed with silicon. In such cases, x is the sum of the values ​​of the elements (e.g., SiE1 x1 E2 x2 ; x=x1+x2, etc.) In various embodiments, the value of x is less than 1, 0.001 to 0.5, 0.005 to 0.3, 0.01 to 0.3, 0.03 to 3, 0.01 to 0.1, or 0.01 to 0.05.

[0025] As will be understood below, in some embodiments, the nanostructure includes two distinct layers of silicon-rich SiEx, where E and / or x are the same and / or different for each layer. The second SiEx material is alternatively referred to as SiFy in such instances. It is therefore understood that any description of E and x herein can also apply to F and y, respectively. For example, F can be any element or elements capable of forming an intermetallic or alloy compound with silicon, such as nitrogen, carbon, boron, phosphorus, oxygen, magnesium, aluminum, germanium, tin, nickel, copper, and combinations thereof, and y can independently be any value described for x, regardless of the particular value of x.

[0026] In some embodiments, the nanostructures described herein can be fabricated by first growing a nanowire template structure on a substrate. In many embodiments, the nanowire template structure is fabricated from a conductive material. Examples of conductive materials that can be used to form the nanowire template structure include metals and metal silicides. In some embodiments, the conductive template may comprise an oxide. The nanowire template structure is then coated with one or more layers of silicon-based electrode active material. Thermal CVD (chemical vapor deposition), HWCVD (hot wire CVD), PECVD (plasma enhanced chemical vapor deposition), and / or evaporation (with or without thermal or laser assistance) may be used to deposit the silicon-based electrode active material layer.

[0027] Various deposition processes produce different profiles when depositing silicon-based electrode active material layers on nanowire templates. For example, thermal CVD produces a conformal, amorphous silicon-based electrode active material coating. HWCVD (also known as catalytic CVD) creates a dense, non-conformal, amorphous silicon-based electrode active material coating that is thicker at or near the tips of the nanowires and thinner at the base of the nanowires near the substrate. PECVD also produces a non-conformal, amorphous silicon-based electrode active material coating that is thicker at the ends of the nanowires and thinner at the base of the nanowires near the substrate. PECVD coatings have many small voids and are low-density.

[0028] Silicon-rich SiE x includes silicon compounds where x is less than 1, or any range subsumed therein.

[0029] In some embodiments of the present invention, silicon-rich SiE x is deposited on the nanowire template using PECVD in a reaction chamber. Examples of process gases that can be used for such deposition include, but are not limited to, silane (SiH4) diluted with hydrogen or argon and mixed with an element E precursor. Such gases become reactive species and form silicon-rich SiE on the surface of the nanowire template under AC / DC plasma. x The amount of element E, and therefore the value of x, can be controlled by adjusting the ratio of the process gases. The reaction chamber temperature can range from 200°C to 600°C or from 300°C to 500°C. The plasma power can range from 500W to 1000W depending on the chamber size. The pressure in the process chamber can range from 1 to 200mTorr.

[0030] Initially, PECVD can deposit a very thin layer of silicon-based electrode active material, having a thickness of less than 1 micron or 0.1-0.4 microns, on the nanowire template along the entire surface, including the substrate and the root points of the nanowires adjacent to the substrate. However, as deposition continues, more silicon-based electrode active material accumulates at or near the tips of the nanowires in the nanowire template, shadowing the substrate area. As a result, the very thin layer of silicon-based electrode active material on the substrate may be continuous or discontinuous, depending on the density and non-uniformity of the nanowires along the substrate surface.

[0031] Figure 1 shows the silicon-rich SiE grown by PECVD. x 1 is a schematic diagram of a nanowire template 110 with a layer deposited thereon. The nanowire template 110 is rooted in a substrate 120. Silicon-based electrode active material (SiE x ) layer 140 is deposited on nanowire template 110. x Layer 140 is thickest at or near the tips of the nanowires of nanowire template 110, tapers off, and finally becomes SiE at the base of the nanowires. x Note that the SiE x Layer 140 is a non-conformal coating, i.e., SiE x Layer 140 is SiE x The layer 140 does not conform to the shape on which it is deposited. x In some configurations, a thin continuous SiE x A thin, non-continuous SiE layer is present on the substrate 120. In some configurations, x The layer resides on a substrate 120. In some configurations, some or all of the nanowire region (the base of the nanowire) adjacent the substrate is covered with a thin (0.1-0.4 micron) SiE x Has a coating.

[0032] Figure 2 is a schematic diagram of a nanowire template with two silicon-based electrode active material layers deposited thereon, according to one embodiment. Two different deposition methods are used to provide an optimal silicon-based electrode active material coating. The nanowire template 210 is rooted on a substrate 220. PECVD is used to deposit silicon-rich SiE x A first silicon-based electrode active material layer 240 containing SiE is deposited on the nanowire template 210. x Layer 240 is thickest at or near the tips of the nanowires of nanowire template 110, tapers off, and finally becomes SiE at the base of the nanowires. x In some embodiments, the first silicon layer 240 has a thickness of 0.5 to 50 microns, 0.5 to 20 microns, or 10 to 20 microns at or near the tips of the nanowires. x A second silicon-based electrode active material layer 230 containing element "E" is deposited on the first layer 240 of nanowires. (As indicated above, the identity and / or amount of element "E" may be the same or different from that in the first silicon-based electrode active material layer 240, and the material may be silicon-rich SiE in layer 240.) x To distinguish it from the silicon-rich SiF y In some embodiments, the second SiE x The layer 230 has a thickness of 5 to 500 nm, 10 to 200 nm, or 10 to 90 nm. x Layer 230 is a conformal coating, i.e., a second SiE x Layer 230 is a second SiE x The second SiE layer 230 conforms to the shape on which it is deposited. x Layer 230 is conformal to the surface of first silicon-based electrode active material layer 240, substrate 220, and any exposed portions of nanowire template 210. xLayer 230 has a generally uniform thickness, and the resulting structure has much more silicon-based electrode active material at or near the tips of the nanowires than at the root ends (due to the non-conformal nature of first silicon-based electrode active material layer 240).

[0033] First SiE x The layer 240 has a surface roughness and porosity. x A second SiE layer having a smoother surface than that of layer 240. x The thickness of the layer 230 is x The surface roughness of the layer 240 may be reduced. x Layer 230 reduces the total surface area of ​​the coated nanowires. Reduced surface area means there is less surface on which an SEI (solid electrolyte interface) layer can form as the battery cycles. Less SEI means less lithium is consumed, leaving more lithium available for cycling. In some configurations, the first SiE x As the surface roughness of the layer increases, the second SiE x Increasing the thickness of layer 230 is useful. In some configurations, the second SiE x The thickness of the layer 230 is 5 to 500 nm, 10 to 200 nm, or 10 to 90 nm.

[0034] The structures described herein have many advantages. In some embodiments, SiE x There is more material near the tip of the nanowire than at the base, but at the base there is less SiE x A thin layer of material is still present. Having such a thin silicon layer at the base strengthens the mechanical connection between the nanowires of the nanowire template and the substrate, which helps ensure that the nanowires do not separate from the substrate during cycling.

[0035] Another advantage is that PECVD SiE x The layer is thermal CVD SiE xThe problem is that the SiE layer is not as dense as the PECVD layer. x The layer may contain a large amount of voids and pores, and such defects can be very useful in that they provide space into which the silicon-based electrode active material can expand as it absorbs lithium during charging of the battery cell.

[0036] In some configurations, even small amounts of additional element E support and buffer the expansion of silicon-based electrode active materials as they absorb lithium ions, thereby reducing cracking of the silicon-based electrode active materials and improving the recoverability and cycle life of battery cells. Element E also dramatically improves lithium ion transport through silicon-based electrode active materials. In some embodiments, element E combines with silicon to create a structure that disperses nanosized silicon domains within the material matrix. The silicon grains or domains maintain their nanosized dimensions for a greater number of cycles due to the physical separation of the silicon grains by the structure. Furthermore, unlike SiO2, which reacts with lithium upon lithiation to irreversibly form lithium silicate compounds, resulting in very high first-cycle losses and cell capacity loss, silicon-rich SiE x The material does not exhibit high first cycle losses and cell capacity fade. In some embodiments, the level of E in the silicon-rich layer is at least 0.005, 0.01, 0.05, 0.07, 0.1, or 0.15.

[0037] In some embodiments, the level of E in the silicon-rich layer is maintained at a level such that a sufficient amount of silicon active material is available. x The level x of E in is less than or equal to 0.3, 0.2, 0.15, 0.1, 0.07, 0.05, 0.03, or 0.01. In some embodiments, only one of the two layers is SiE x The first layer may be pure silicon or may contain another element.

[0038] In some embodiments, the first SiE is deposited using PECVD. x The silicon layer is amorphous and has a density of 2.25 g / cm 3 Less than 2.10 g / cm 3 Less than or 1.70 g / cm 3 The second SiE deposited using thermal CVD has an average density of less than 1000 nm and may contain many small voids. x The layer is amorphous and has a density of 2.0 g / cm 3 Higher or 2.25g / cm 3 In some embodiments, the densities of the two layers may be described in terms of the density difference between the layers, rather than their absolute densities. x The layer has an average density at least 0.05 g / cm 3 higher than that of the first layer. 3 , at least 0.1 g / cm 3 , at least 0.2 g / cm 3 , at least 0.3 g / cm 3 It has a high average density. As will be appreciated by those skilled in the art, the density of amorphous silicon-based electrode active materials is lower than the density of the same material in crystalline or polycrystalline form.

[0039] According to various embodiments, the nanostructures described herein are formed by first forming a SiE x Second SiE on top of layer x In some embodiments, the first SiE layer may be characterized as a silicon dioxide film, and the second layer may be characterized as having a density higher than that of the first layer. Such a structure may be used to form an anode in a lithium battery cell. In some embodiments, the first SiE layer may be characterized as a silicon dioxide film. x The layer is low density amorphous and may contain some voids, all of which are removed by the SiE x As the SiE absorbs lithium ions, xThe amorphous silicon layer serves to provide space into which the nanowires can expand. This is an advantage over crystalline or polycrystalline silicon-based electrode active materials, which have higher densities and can experience stress cracking when absorbing lithium ions. In addition, lithium ions diffuse more easily through amorphous materials than through crystalline or polycrystalline silicon-based electrode active materials. Therefore, the density of each layer can be tailored depending on the silicon cycling capacity, power or cycling rate requirements, and nanowire template density.

[0040] In certain embodiments, an anode for a lithium battery cell is formed from a nanowire template structure rooted on a conductive substrate that can serve as a current collector for the anode. Examples of conductive substrate materials include copper, metal oxide-coated copper, stainless steel, titanium, aluminum, nickel, chromium, tungsten, other metals, metal suicides and other conductive metal compounds, carbon, carbon fiber, graphite, graphene, carbon mesh, conductive polymers, doped silicon, or combinations of the above, including multilayer structures. The substrate may be formed as a foil, film, mesh, foam, laminate, wire, tube, particle, multilayer structure, or any other suitable form. In certain embodiments, the substrate is a metal foil having a thickness of about 1 micron to 50 microns, or more specifically, about 5 microns to 30 microns.

[0041] The nanowires may be physically and conductively attached to the substrate. The physical attachment may be more than mere mechanical contact; for example, it may result from coating a binder with the discrete nanostructures on the substrate. In some embodiments, the physical attachment results from fusing the nanostructures to the substrate or from directly depositing the nanostructures or portions of the nanostructures on the substrate, for example, using CVD techniques or vapor-liquid-solid CVD growth. In some embodiments, the physical attachment results from ballistic impalement of the nanowires onto the substrate. In various embodiments, the physical attachment includes metallic bonding, such as forming an alloy (e.g., a silicide) between two bonding materials. In other embodiments, the nanowires are grown from the substrate using other nanowire growth techniques that produce structures with similar shapes and dimensions.

[0042] In many embodiments, the nanowires of the nanowire template comprise a metal or metal silicide and are electronically conductive. In some embodiments, the nanowires comprise one or more oxides. Conductive templates can be useful for providing an electron transport path from a silicon-based electrode active material to a substrate or current collector. In various embodiments, the nanowires in the nanowire template have diameters between 10 nanometers and 100 nanometers and lengths between 10 microns and 100 microns. Lithium battery cell anodes comprising nanowire templates are further described in U.S. Pat. No. 7,816,031, incorporated herein by reference.

[0043] In some embodiments, the silicon-based material nanostructures are generally circularly symmetric. Note that arrays of generally circularly symmetric nanowires include arrays in which asymmetry may be introduced due to two nanowires being close enough together that their coatings abut one another.

[0044] FIG. 3 shows two SiE films deposited using different deposition methods as described herein. x FIG. 1 shows a schematic cross-sectional view of a nanowire coated with a layer. Dimensions dl, d2, and h are shown. dl is the maximum diameter of the coating, d2 is the bottom diameter of the coating, and h is the height of the coated nanowire. The non-conformal coating (either a porous non-conformal coating alone or a porous non-conformal coating conformally coated with a dense coating) may be characterized in some embodiments by the following ratios: dl / h is 1 / 2 to 1 / 9, d2 / h is 1 / 400 to 1 / 70, and dl / d2 is 50:1 to 1.5:1. In various embodiments, dl is 4 to 15 microns or 4 to 12 microns, d2 is 0.2 to 2 microns, and h is 20 to 50 microns or 30 to 40 microns.

[0045] In one example, nanowires having a diameter of about 10-50 nm and a length of about 10-25 microns are coated with silicon-rich silicon nitride such that, after coating, the diameter at the base of the nanostructure is 100-400 nm, the maximum diameter is 2-20 microns, and the total height of the anode is 20-50 microns.

[0046] In some embodiments, a non-conformal layer of silicon-based electrode active material deposited by PECVD may include a hydrogen content of at least 10%. In some embodiments, a conformal layer of silicon-based electrode active material deposited by thermal CVD may include a hydrogen content of 7% or less, or 5% or less.

[0047] In some embodiments, the non-conformal SiE x The layers may be deposited by evaporation or physical vapor deposition (PVD), or hot wire chemical vapor deposition (HWCVD) instead of or in addition to PECVD.

[0048] In a PECVD deposition process, a plasma may be generated either within the chamber in which the substrate is located or upstream of the chamber and supplied to the chamber. Any type of plasma may be used, including capacitively coupled plasma, inductively coupled plasma, and conductively coupled plasma. Any plasma source may be used, including DC, AC, RF, and microwave sources.

[0049] PECVD process conditions can vary depending on the particular process and tool used. A fairly wide range of temperatures may be used, e.g., 180° C. to 600° C. Pressures are typically low for plasma processes, e.g., in the range of 1 mTorr to 400 Torr, or 10 mTorr to 100 mTorr, depending on the process.

[0050] In some implementations, the PECVD process used to form the novel structures described herein is an expanding thermal plasma chemical vapor deposition (ETP-CVD) process. In such a process, plasma-generating gases are passed through a direct current arc plasma generator to form a plasma with a web or other substrate containing the nanowire template in an adjacent vacuum chamber. Silicon-based source gases, along with the generated radicals, are injected into the plasma. The plasma is expanded through a diverging nozzle and injected into the vacuum chamber toward the substrate, forming amorphous SiE. x A non-conformal layer of SiE is formed on the nanowire template. Examples of plasma generating gases include, but are not limited to, argon (Ar) and ammonium (NH), and nitrogen (N). In some embodiments, ionized argon and NH / N species in the plasma collide with silane molecules to form radical species of silicon precursor, resulting in a SiE layer on the nanowire template. x An example range of voltage and current for a DC plasma source is 60-80 volts or 50-70 amps.

[0051] In some embodiments, the conformal dense silicon layer is deposited using atomic layer deposition (ALD) instead of or in addition to thermal CVD. Any suitable thermal CVD process, such as low-pressure CVD (LPCVD), may be used. The temperature may be increased as high as the thermal budget allows, as long as care is taken to ensure that metal silicide does not form around the interface between the nanowire and the substrate. In some embodiments, the chamber pressure during the thermal CVD process is kept low, e.g., between 100 mTorr and 2 Torr, to prevent gas-phase reactions and non-conformal deposition. Higher pressures, e.g., greater than 2 Torr or 500 Torr, may result in non-conformal deposition.

[0052] Any suitable silicon source to be combined with the elemental E source may be used to produce non-conformal and conformal SiE x Examples of silicon sources include, but are not limited to, silane (SiH), dichlorosilane (HSiCl), monochlorosilane (HSiCl), trichlorosilane (HSiCl), and silicon tetrachloride (SiCl). Examples of nitrogen sources include, but are not limited to, ammonium (NH) and nitrogen (N) to form silicon-rich silicon nitride layers. Other elements can be introduced into the plasma from gas precursors (CH, GeH, BH, etc.) or vaporized liquid precursors, as in the case of organometallic precursors.

[0053] Further discussion of depositing active material layers with controlled density can be found in US patent application Ser. No. 13 / 277,821, which is incorporated herein by reference.

[0054] Furthermore, in some embodiments, a non-Si-dominant layer may be the outermost shell of the nanostructure. Examples of layers include metal oxides such as aluminum oxide, titanium oxide, cobalt oxide, and zirconium oxide, metal nitrides, and silicon nitride or carbon-based layers. In some embodiments, a thin layer of any of these may be deposited in addition to or instead of the dense Si layer described above.

[0055] In some embodiments, the surface of the outer SiEx layer may be chemically modified by gas or solution phase treatment / exposure to add or remove elements to create 1-10 nm thick layers of different chemical compositions, e.g., oxides or halides.

[0056] According to various embodiments, the first SiE x layer and second SiE x The layers each have a uniform density. In some embodiments, deposition conditions may be adjusted during deposition to create a density gradient in one or both layers. For example, one or both layers may be denser toward the outer portions of the layer. In such embodiments, the average density of the layer may be used to characterize the density of the layer.

[0057] According to various embodiments, the first SiE x layer and second SiE x Each layer has a uniform concentration of the E element. xrefers to the total amount of Si and E in a layer. However, in some embodiments, deposition conditions may be adjusted during deposition to create a concentration gradient in one or both layers. In one example, one or both layers may have an increasing E concentration toward the outer portions of the layer. In another example, one or both layers may have a decreasing E concentration toward the outer portions of the layer. In yet another example, one or both layers may have a varying E concentration, with both increases and decreases throughout the layer. In such embodiments, the average E concentration of the layer may be used to characterize the E concentration of the layer. The concentration profile of E is said to vary through the thickness of the layer. This includes concentration profiles that include one or more regions of a flat (uniform) profile and one or more regions of increase and / or decrease. assembly

[0058] 4A is a plan view of a partially assembled electrochemical cell using electrodes described herein, according to certain embodiments. The cell includes a positive electrode active layer 402, which is shown covering most of a positive current collector 403. The cell also includes a negative electrode active layer 404, which is shown covering most of a negative current collector 405. A separator 406 is present between the positive electrode active layer 402 and the negative electrode active layer 404.

[0059] In one embodiment, the negative electrode active layer 404 is slightly larger than the positive electrode active layer 402 to ensure that lithium ions released from the positive electrode active layer 402 are captured by the active material of the negative electrode active layer 404. In one embodiment, the negative electrode active layer 404 extends in one or more directions beyond the positive electrode active layer 402 by at least about 0.25 millimeters to 7 millimeters. In a more specific embodiment, the negative electrode active layer 404 extends in one or more directions beyond the positive electrode active layer 402 by about 1 millimeter to 2 millimeters. In certain embodiments, the edges of the separator 406 extend beyond at least the outer edges of the negative electrode active layer 404 so that the negative electrode is completely electronically insulated from other battery components.

[0060] 4B is a cross-sectional view of an electrode stack 400 of a partially assembled electrochemical cell using the electrodes described herein, according to certain embodiments. There is a positive current collector 403 with a positive electrode active layer 402a on one side and a positive electrode active layer 402b on the other side. There is a negative current collector 405 with a negative electrode active layer 404a on one side and a negative electrode active layer 404b on the other side. Between the positive and negative electrode active layers 402a and 404a is a separator 406a. The separator sheets 406a and 406b serve to maintain mechanical separation between the positive and negative electrode active layers 402a and 404a and act as a sponge to absorb liquid electrolyte (not shown), which will be added later. The ends of the current collectors 403 and 405 are free of active material and can be used to connect to the appropriate terminals of the cell (not shown).

[0061] Together, the electrode layers 402a, 404a, the current collectors 403, 405, and the separator 406a are said to form an electrochemical cell unit. The complete stack 400 shown in FIG. 4B includes electrode layers 402b, 404b, and an additional separator 406b. The current collectors 403, 405 can be shared between adjacent cells. Repeating such stacks can result in a cell or battery with a capacity greater than that of a single cell unit.

[0062] Another way to create a large-capacity battery or cell is to create one very large cell unit and then wind it to create multiple stacks. The cross-sectional schematic in FIG. 5A shows how long and thin the electrodes can be wound together with two separator sheets to form a battery or cell, sometimes referred to as a jellyroll 500. The jellyroll is shaped and sized to fit the interior dimensions of a curved, often cylindrical, case 502. The jellyroll 500 includes a positive electrode 506 and a negative electrode 504. The white space between the electrodes is the separator sheet. The jellyroll can be inserted into the case 502. In some embodiments, the jellyroll 500 can include a mandrel 508 in its center. The mandrel 508 establishes the initial winding diameter and prevents the inner turns from occupying the central axis area. The mandrel 508 can be made of a conductive material and, in some embodiments, can be part of the cell terminal. 5B shows a perspective view of a jelly roll 500 with positive and negative tabs 512, 514 extending from a positive and negative current collector (not shown), respectively. The tabs may be welded to the current collectors.

[0063] The length and width of the electrodes depend on the overall dimensions of the cell and the thickness of the active layer and current collector. For example, a conventional 18650-type cell with a diameter of 18 mm and a length of 85 mm may have electrodes approximately 300-1000 mm long. Shorter electrodes for lower rate and / or higher capacity applications will be thicker and have fewer turns.

[0064] Cylindrical designs can be used for some lithium-ion cells, especially when the electrodes may expand during cycling, potentially exerting pressure on the casing. It is useful to use a cylindrical casing that is as thin as possible while still being able to adequately support the pressure on the cell (with a sufficient safety margin). Prismatic (flat) cells can be rolled similarly, but their cases may be flexible and therefore bend along their long sides to accommodate internal pressure. Also, pressure may not be uniform within different parts of the cell, leaving corners of prismatic cells empty. Empty pockets within lithium-ion cells should be avoided because the electrodes tend to squeeze unevenly into these pockets during expansion. Furthermore, electrolyte may concentrate in empty pockets, leaving dry areas between the electrodes and adversely affecting lithium ion transport between the electrodes. However, for certain applications, such as those dictated by a rectangular form factor, prismatic cells are appropriate. In some embodiments, prismatic cells use rectangular stacks of electrode and separator sheets to avoid some of the difficulties encountered with wound prismatic cells.

[0065] 5C shows a top view of a wound prismatic jellyroll 520. Jellyroll 520 includes a positive electrode 524 and a negative electrode 526. The white space between the electrodes is a separator sheet. Jellyroll 520 is enclosed in a rectangular prismatic casing 522. Unlike a cylindrical jellyroll, the winding of a prismatic jellyroll begins with a flat, extended section in the center of the jellyroll. In one embodiment, the jellyroll may include a mandrel (not shown) in the center of the jellyroll, and the electrodes and separator are wound onto this mandrel.

[0066] FIG. 6A shows a cross-sectional view of a stack of cells comprising multiple cells (601a, 601b, 601c, 601d, and 601e), each comprising a positive electrode (e.g., 603a, 603b), a positive current collector (e.g., 602), a negative electrode (e.g., 605a, 605b), a negative current collector (e.g., 604), and an inter-electrode separator (e.g., 606a, 606b). Each current collector is shared by adjacent cells. Stacked cells can be fabricated in nearly any shape, making them particularly well-suited for prismatic batteries. Current collector tabs typically extend from the stack and connect to the battery terminals. FIG. 6B shows a perspective view of a stack of cells comprising multiple cells.

[0067] Once the electrodes are configured as described above, the cell is filled with an electrolyte. The electrolyte in a lithium-ion cell can be a liquid, solid, or gel. Lithium-ion cells with a solid electrolyte are called lithium polymer cells.

[0068] A typical liquid electrolyte contains one or more solvents and one or more salts, at least one of which contains lithium. During the first charging cycle (sometimes called the formation cycle), the organic solvent in the electrolyte may partially decompose on the negative electrode surface, forming an SEI layer. This interface is typically electrically insulating but ionically conductive, allowing lithium ions to pass through. This interface also prevents electrolyte decomposition during subsequent charging subcycles.

[0069] Some examples of suitable non-aqueous solvents for some lithium-ion cells include cyclic carbonates (e.g., ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinyl ethylene carbonate (VEC)), vinylene carbonate (VC), lactones (e.g., γ-butyrolactone (GBL), γ-valerolactone (GVL), and α-angelicalactone (AGL)), linear carbonates (e.g., dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), dipropyl carbonate (DPC), methyl butyl carbonate (NB), and methyl methyl butyl carbonate (MLC)). C) and dibutyl carbonate (DBC)), ethers (e.g., tetrahydrofuran (THF), 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, and 1,2-dibutoxyethane), nitrites (e.g., acetonitrile and adiponitrile), linear esters (e.g., methyl propionate, methyl pivalate, butyl pivalate, and octyl pivalate), amides (e.g., dimethylformamide), organic phosphates (e.g., trimethyl phosphate and trioctyl phosphate), and organic compounds containing an S═O group (e.g., dimethyl sulfone and divinyl sulfone), and combinations thereof.

[0070] The non-aqueous liquid solvents can be used in combination. Examples of such combinations include cyclic carbonate-linear carbonate, cyclic carbonate-lactone, cyclic carbonate-lactone-linear carbonate, cyclic carbonate-linear carbonate-lactone, cyclic carbonate-linear carbonate-ether, and cyclic carbonate-linear carbonate-linear ester. In one embodiment, the cyclic carbonate can be combined with a linear ester. Alternatively, the cyclic carbonate can be combined with a lactone and a linear ester. In a specific embodiment, the volume ratio of the cyclic carbonate to the linear ester is about 1:9 to 10:0, preferably 2:8 to 7:3.

[0071] Salts used in liquid electrolytes include LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiCF3SO3, LiC(CF3SO2)3, LiPF4(CF3)2, LiPF3(C2F5)3, LiPF3(CF3)3, LiPF3(iso-C3F7)3, LiPF5(iso-C3F7), and lithium salts containing cyclic alkyl groups (e.g., (CF2)2(SO2) 2x Li and (CF2)3(SO2) 2x Li), and combinations thereof. Common combinations include LiPF and LiBF, LiPF and LiN(CFSO), and LiBF and LiN(CFSO).

[0072] In one embodiment, the total concentration of salt in the liquid non-aqueous solvent (or combination of solvents) is at least about 0.3 M, and in a more specific embodiment, the salt concentration is at least about 0.7 M. The upper limit of the concentration may be determined by the upper limit of solubility or may be no more than about 2.5 M. In a more specific embodiment, it may be no more than about 1.5 M.

[0073] Solid electrolytes are typically used without a separator, as they function as a separator themselves. Solid electrolytes are electrically insulating, ionically conductive, and electrochemically stable. In solid electrolyte configurations, a lithium-containing salt is used, which can be the same as in the liquid electrolyte cell described above, but it is held inside a solid polymer composite rather than dissolved in an organic solvent. Examples of solid polymer electrolytes include polyvinylidene fluoride (PVDF) or polyvinylidene chloride or copolymers of their derivatives, poly(chlorotrifluoroethylene), poly(ethylene-chlorotrifluoroethylene), or poly(fluorinated ethylene-propylene), polyethylene oxide (PEO) and oxymethylene-linked PEO, trifunctional urethane-crosslinked PEO-PPO-PEO, poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP), triol-type PEO crosslinked with difunctional urethane, poly((oligo)oxyethylene)methacrylate-co-alkali, and the like. Examples of suitable ion-conducting polymers include those prepared from monomers containing atoms with lone pairs of electrons available for bonding with lithium ions of the electrolyte salt and transferring between them during conduction, such as lithium metal methacrylates, polyacrylonitrile (PAN), polymethyl methacrylate (PNMA), polymethylacrylonitrile (PMAN), polysiloxanes and their copolymers and derivatives, acrylate-based polymers, other similar solvent-free polymers, different polymers formed by combining the aforementioned polymers either by condensation or crosslinking, and physical mixtures of any of the aforementioned polymers. Other less conductive polymers that can be used in combination with the above polymers to improve the strength of thin laminates include polyester (PET), polypropylene (PP), polyethylene naphthalate (PEN), polyvinylidene fluoride (PVDF), polycarbonate (PC), polyphenylene sulfide (PPS), and polytetrafluoroethylene (PTFE).

[0074] FIG. 7 shows a cross-sectional view of a wound cylindrical cell, according to one embodiment. The jelly roll includes a spirally wound cathode 702, anode 704, and two separator sheets 706. The jelly roll is inserted into a cell casing 716, and a cap 718 and gasket 720 are used to seal the cell. Note that in certain embodiments, the cell is not sealed until after subsequent operations. In some cases, the cap 718 or cell casing 716 includes a safety device. For example, a safety vent or burst valve may be used to release excess pressure if it builds up within the battery. In certain embodiments, a one-way gas release valve is included to release oxygen released upon activation of the cathode material. A positive temperature coefficient (PTC) device may also be incorporated into the conductive path of the cap 718 to reduce potential damage if the cell is shorted. The outer surface of the cap 718 may be used as the positive terminal, while the outer surface of the cell casing 716 may function as the negative terminal. In an alternative embodiment, the polarity of the battery is reversed, with the outer surface of cap 718 used as the negative terminal, and the outer surface of cell casing 716 then serving as the positive terminal. Tabs 708 and 710 may be used to establish connections between the positive and negative electrodes and the corresponding terminals. Suitable insulating gaskets 714 and 712 may be inserted to prevent the possibility of internal short circuits. For example, Kapton™ film may be used for internal insulation. During manufacture, cap 718 may be crimped onto cell casing 716 to seal the cell. However, prior to this operation, electrolyte (not shown) is added to fill the porous spaces of the jelly roll.

[0075] While lithium-ion cells typically use rigid cases, lithium polymer cells may be packaged in flexible foil-type (polymer laminate) cases. A variety of materials can be selected for these cases. For lithium-ion batteries, Ti-6-4, other Ti alloys, Al, Al alloys, and 300 series stainless steels are suitable materials for the positive conductive case section and end cap, while commercially pure Ti, Ti alloys, Cu, Al, Al alloys, Ni, Pb, and stainless steel are suitable materials for the negative conductive case section and end cap.

[0076] In addition to the battery applications mentioned above, the nanostructures may also be used in fuel cells (e.g., anodes, cathodes, and electrolytes), active materials in heterojunction solar cells, various forms of current collectors, and / or absorbing coatings. Experiments and Results

[0077] The nanowire template on the metal foil is exposed to silane, argon, and ammonium or nitrogen gas in a PECVD plasma chamber, resulting in the deposition of silicon-rich SiN on the nanowire template. x The first layer, a SiN layer, was deposited using thermal CVD. x On the layer, silicon-rich SiN x A second coating of 1000 .ANG. was deposited to form the anode for the lithium battery cell. In addition to the anode prepared above, the cell included a lithium cobalt oxide cathode, a separator, and a carbonate-based electrolyte with LiPF.sub.6 salt.

[0078] Figure 8 shows the silicon-rich SiN x 1 is a graph showing capacity retention as a function of cell cycle number for both a SiN cell and a cell with an anode containing only silicon. Both cells were charged and discharged at a rate of C / 2. x SiN for cells x The capacity retention profile deteriorated much slower than that for the Si-only cells. xThe cell life cycle is approximately 75% longer than that of a Si-only cell. x Cycling data from cells containing such anodes indicates that the cycle life of such anodes is increased compared to anodes made from pure silicon.

[0079] The present invention has been described herein in considerable detail to provide those skilled in the art with information relating to the application of the novel principles and the construction and use of such specific components as may be required. It should be understood, however, that the invention can be practiced with different equipment, materials, and devices, and that various modifications to both the equipment and operating procedures can be implemented without departing from the scope of the invention itself.

Claims

1. 1. An anode for a lithium battery, comprising: A substrate; an array of nanowires anchored to the substrate, each nanowire having a surface; a first layer coating most or all of the surface of the nanowire, the first layer comprising SiE ｘ a first layer comprising a material; and a second layer over the first layer and any exposed surfaces of the nanowires and the substrate, the second layer being silicon or SiF ｙ a second layer comprising any of the materials; Equipped with x is greater than zero and less than one, y is greater than zero and less than one, E and F are nitrogen, the anode.

2. 2. The anode of claim 1, wherein x is from 0.01 to 0.

5.

3. 2. The anode of claim 1, wherein x is from 0.01 to 0.

3.

4. 2. The anode of claim 1, wherein x is from 0.01 to 0.

1.

5. 5. The anode of claim 1, wherein a concentration profile of E varies through the thickness of the first layer and / or a concentration profile of F varies through the thickness of the second layer.

6. 6. The anode according to claim 1, wherein the density of the second layer is higher than the density of the first layer.

7. The average density of the first layer is 2.1 g / cm ３ 7. The anode according to claim 1, wherein the anode temperature is lower than 1000°C.

8. The average density of the second layer is 2.0 g / cm ３ 7. The anode according to claim 1, wherein the anode has a temperature of 1000° C. or higher.

9. 9. The anode of claim 1, wherein the density of the first layer varies throughout the first layer.

10. 10. The anode of claim 1, wherein the density of the second layer varies throughout the second layer.

11. The anode of claim 1 , wherein the first layer is non-conformal to the nanowires.

12. 12. The anode of claim 1, wherein the second layer is conformal to the first layer.

13. 13. The anode of claim 1, further comprising a third layer on the second layer, the third layer being silicon-free.

14. The anode of claim 1 , wherein the nanowires comprise silicide nanowires.

15. 15. The anode of any one of claims 1 to 14, wherein the first layer has a thickness at its largest diameter of about 5 to 20 microns.

16. 16. The anode of claim 1, wherein the second layer has a thickness of 5 to 500 nanometers.

17. 16. The anode of claim 1, wherein the second layer has a thickness of 5 to 100 nanometers.

18. An anode according to any one of claims 1 to 17; a cathode containing lithium; and an electrolyte in ionic communication with both the anode and the cathode; A lithium battery comprising:

19. 1. A method of making an anode for a lithium battery, comprising: providing a substrate; growing nanowires from the substrate, each nanowire having a surface; depositing a first layer using PECVD to coat most or all of the surface of the nanowire, the first layer being a first silicon-rich SiE ｘ and depositing a second layer over the first layer, any exposed surfaces of the nanowires, and the substrate using a thermal CVD method, the second layer comprising a second silicon-rich SiE ｘ and wherein E is nitrogen.

20. 20. The method of claim 19, wherein the PECVD process is an expanding thermal plasma process.

21. 21. The method of claim 19 or 20, wherein the nanowires are silicide nanowires.

22. 22. The method of any one of claims 19 to 21, wherein the chamber pressure during the thermal CVD process is less than about 2 Torr.

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