Electrode active material grown by plasma-enhanced chemical vapour deposition

Abstract

A method of growing an electrode active material uses plasma-enhanced chemical vapour deposition (PECVD). The electrode active material is grown by providing a seeding layer (31, Figure 3) comprising a first precursor for providing charge carriers to a reactor 41 and exposing the seeding layer to a plasma phase of a second precursor at a temperature above a cracking temperature of the seeding layer. The first precursor may comprise an alkali metal, such as lithium, sodium or potassium, which may be bound in a coordination complex with an organic ligand. Suitable materials include lithium acetate and sodium acetate. The second precursor may comprise silicon or germanium and can be silane, silylene, trichlorosilane, germane or germylene, for example. The seeding layer may comprise a further precursor comprising Tin, such as Tin (II) Acetylacetonate. The grown electrode active material comprises an alloy of the charge carriers and elements from the second precursor, for example a lithium-silicon alloy or lithium-silicon-tin alloy. When the electrode active material is in use, the charge carriers can be extracted from the electrode active material in the presence of an electric field, leaving vacancies for later insertion of the charge carriers.

Description

TECHNOLOGICAL FIELD 5 Embodiments of the present disclosure relate to growing an electrode active material by plasma-enhanced chemical vapour deposition. In particular, but not exclusively, the electrode active material is an anode active material. 10 BACKGROUND 15 CXI CXI 20 Some types of electrical energy storage means, such as secondary cells (rechargeable batteries), comprise active materials at the anode electrode (oxidation site during discharge cycle) and at the cathode electrode (reduction site during discharge cycle). The active materials are configured to enable reversible insertion and extraction of charge carriers such as metal ions (e.g. Lithium). The electrode active material typically comprises crystalline structures and / or a mixture of amorphous and crystalline structures configured to accept charge carriers. A conventional Lithium-ion cell has a carbonaceous anode, and a metal oxide cathode. Research of improved active materials is ongoing. Some active materials have been found to offer higher charge storage capacity, but have also been found to lack cyclability due to significant volume expansion upon charge carrier insertion, which 25 induces material stresses. The active material fabrication method also significantly affects the technical viability of the new material. High-temperature processes are energy-inefficient, and low-productivity processes are difficult to scale. 30 SUMMARY Embodiments of the disclosure provide a method, an electrode active material, an electrode, and a secondary cell. According to various, but not necessarily all, embodiments there is provided a method of growing an electrode active material (e.g. anode active material) using plasma-enhanced chemical vapour deposition (PECVD), the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a 5 first precursor for providing charge carriers, the first precursor comprising a coordination complex of a charge carrier bound to an organic ligand, wherein the organic ligand is suitable for cracking, and wherein the charge carrier comprises an alkali metal; exposing the seeding layer to a plasma phase of a second precursor, at a 10 temperature above a cracking temperature of the seeding layer, to grow the electrode active material, the second precursor comprising at least one of: Silicon; Germanium; or a Silicon or Germanium containing compound, wherein the Silicon or Germanium containing compound is suitable for dissociation in the plasma phase, wherein the grown electrode active material comprises an alloy of the charge 15 carriers and elements from the second precursor, so that when the electrode active material is in use, the charge carriers can be extracted (dealloyed) from the electrode active material in the presence of an electric field, leaving vacancies for later insertion (alloying) of the charge carriers. 20 The resulting active material has improved cyclability. It is believed that this is because the electrode active material is pre-alloyed with the charge carriers. Therefore, when the charge carriers are extracted during the first discharge cycle, vacancies formerly occupied by the charge carriers remain. The vacancies (porosity) can accommodate subsequent insertion of the charge carriers without causing significant volumetric 25 expansion. By contrast, if the active material had not been pre-alloyed with the charge carriers, then significant swelling damage may occur upon first insertion of the charge carriers. A significant advantage is that this improved active material is produced using a low-30 energy and flexible fabrication method. PECVD is a chemical vapour deposition process for growing thin film amorphous, poly-crystalline or mono-crystalline nano- or micro-structures. PECVD offers significantly lower temperatures than annealing or Low Pressure CVD. PECVD offers a high deposition rate for quickly growing thick coatings. PECVD offers more uniform deposition than sputtering processes, which can help to 35 avoid coagulation and internal battery short circuits. PECVD was previously most commonly used for fabricating thin film transistors or solar cells (e.g. amorphous Silicon films). However, as shown herein, PECVD may now facilitate the production of improved electrode active materials, and more specifically, pre-alloyed swell-resistant electrode active materials for secondary cells. Rather than using a generic growth-promoting catalyst on the seeding layer, we have found that some elements can act as both a catalyst and as functional charge carriers. For example, the charge carriers may be Lithium, Sodium or Potassium. 10 The Silicon or Germanium can bind to the charge carriers to grow into crystals. Silicon and Germanium are promising active materials, offering a greater charge carrier storage capacity than carbonaceous active materials and not requiring a binder. Silicon or Germanium nanostructures expand significantly upon charge carrier insertion unless there are vacancies for accommodating the charge carriers, as provided in the 15 present method. CXI The Silicon or Germanium containing compound may be suitable for dissociation in the plasma phase under radio frequency energy. The Silicon or Germanium can be supplied to the PECVD reactor as a gas. Example gases include Silane; Silylene; 20 Trichlorosilane; Germane; or Germylene. The PECVD reactor creates a plasma of these gases, for example via radio frequency discharge. The plasma comprises Silicon-containing radicals or Germanium-containing radicals, which readily react with the seeding layer to grow a crystalline thin film on the substrate at low required temperatures. 25 In an example implementation, the reaction of Lithium with Silicon-containing radicals would grow a Li-Si (Lithium-Silicon) alloy active material, which offers high cyclability, high charge-storage capacity, and low manufacturing energy requirements. 30 The charge carrier may be bound to the organic ligand via a metalorganic or organometallic bond, so that the precursor can be stored and handled safely. The ligand should crack (break) from the charge carriers at a low temperature in the PECVD reactor, exposing the elemental charge carriers that act as nucleation sites for growing the electrode active material. The ligand should be chosen to minimise secondary 35 reactions that could create impurities. The ligand may be acetic acid. Examples include Lithium Acetate, which is a salt of lithium and acetic acid, or Sodium Acetate, which is a salt of Sodium and acetic acid. In the above examples, the grown alloy is a binary alloy (not counting 5 impurities / dopants) consisting of the charge carriers and the elements of the second precursor. In some, but not necessarily all examples, the alloy may comprise one or more additional different types of elements, to improve desired properties. Examples of useful additional elements include Tin, Aluminium, Antimony, or Magnesium. These elements have a higher charge storage capacity than Carbon. The dispersed additional 10 element in the alloy further improves the battery capacity without compromising cyclability or flexibility / malleability. The further precursor may be provided as part of the seeding layer. The further precursor may comprise a metalorganic or organometallic coordination complex with an organic ligand. For example, the ligand may comprise Acetylatone, which can form Tin (II) Acetylacetonate. 15 The term ‘alloy’ herein refers to a combination of elements comprising more than 1000 parts per million of each element. In other words, each precursor is provided in sufficient concentrations to be considered an alloy rather than a dopant. Dopants may also be added but they are outside the scope of this disclosure. CM 20 1— The method may be configured to grow nanostructures, at a smaller scale than microstructures. Nanostructures can accommodate large strains and volume changes without pulverization, and a short insertion distance for the charge carriers. The nanostructures grown by the method may comprise nanocrystals, nanowires, and / or 25 nanoparticles. The term ‘nanowire’ herein refers generally to nanostructures that have a constrained nano-scale thickness or diameter, and an unconstrained length. This covers nanostructures that would be described as nanorods (low length:width ratio) and nanowires (high length:width ratio). Nanowires provide a one-dimensional electrical path for efficient charge transport. Chemical vapour deposition is good for 30 growing nanowires that are electrically interconnected. Optimal ranges of PECVD control parameters have been identified. PECVD supports a high growth rate of the alloy using a low growth temperature of between approximately 200 Celsius to approximately 400 Celsius. This is significantly lower 35 than the temperature required by Low-pressure CVD (around 600 Celsius). The 5 optimum temperature depends on the thermal decomposition (cracking) temperature of the precursor(s) on the seeding layer, and on the desired nanostructure shape. 10 15 CXI CXI 20 The reactor pressure during the growth may be controlled to be a value from the range approximately 100 mTorr to approximately 900mTorr. Lower pressures can produce planar nanocrystals. Higher pressures within the range have been found to produce nanowires, which become straighter as pressure increases. Straighter nanowires are considered to provide improved battery cyclability and capacity. A pressure range for growing relatively straight nanowires is from approximately 500mTorr to approximately 900mTorr. The plasma phase of the second precursor may be a radio frequency-induced plasma phase, and wherein radio frequency power during the growth may be controlled to be a value from the range approximately five Watts to approximately 25 Watts. Higher powers within the range have been found to create smaller diameter nanowires, at a slower growth rate. Although small diameter nanowires are useful, they should not be too small so that more Lithium can be stored. Accordingly, an RF power range for growing suitably small nanowires is no greater than approximately 10 watts. According to various, but not necessarily all, embodiments there is provided examples as claimed in the appended claims. BRIEF DESCRIPTION 25 Some examples will now be described with reference to the accompanying drawings in which: FIG.1 schematically illustrates an example of a method; FIG. 2 schematically illustrates an example of a substrate; FIG. 3 schematically illustrates an example of a seeded substrate; 30 FIG. 4 schematically illustrates an example of a PECVD apparatus; FIG. 5A schematically illustrates an example of nanostructures of the alloy, with the charge carriers pre-inserted; FIG. 5B schematically illustrates the nanostructures of FIG. 5A after extraction of the charge carriers during a cell discharge cycle; and 35 FIG. 6 schematically illustrates an example of a secondary cell. DETAILED DESCRIPTION FIG.1 illustrates a method 10 of growing the electrode active material using PECVD. 5 At block 11, the seeding layer is prepared, for example by deposition onto a substrate. At block 12, the seeding layer is provided to a PECVD reactor. At block 13, the electrode active material is grown. First, at block 11 of the method 10 of FIG. 1, the seeding layer is prepared. The seeding 10 layer comprises the first precursor for providing the charge carriers. In an example implementation, the first precursor comprises Lithium acetate hydrate or Sodium acetate, anhydrous. 20 15 Preparation of the seeding layer may comprise depositing the first precursor directly or indirectly onto a substrate of material with a high melting point, which is configured to act as a current collector foil of the battery. In some examples, the substrate comprises Copper foil or Carbon paper. Carbon paper is useful if the battery is flexible. In an example implementation, the Carbon paper is treated to have a hydrophobic surface, e.g. using Polytetrafluoroethylene, to better function as an electrode backing layer. FIG. 2 shows an example of the substrate. FIG. 3 shows a coated substrate 30 comprising the substrate 20 and the seeding layer 31. The substrate 30 may be a continuous layer as shown. In other implementations, the substrate 30 may be a grid or mesh. The thickness of the substrate 30 may be at least 25   100 micrometres. The particular deposition method depends on the desired results. For a patterned electrode and / or moderate precision, printing (e.g. inkjet, screen-print, k-bar coating) or paint-brushing may be used. For uniform coverage over large areas, dip-coating, 30 wire-bar coating, or spin-coating methods may be used. If high precision or a thin layer is not a requirement, spray coating or drop-casting may be used. K-bar coating, inkjet printing, screen printing and paint-brushing are readily scalable. The seeding layer precursor (e.g. Lithium Acetate) may be in aqueous solution during deposition of the seeding layer. The predetermined concentration of the precursor will affect the quantity of charge carriers in the finished product. 10 15 CXI CXI 20 25 30 Dip-coating and spin-coating enable precise control of the seeding layer thickness by varying immersion time (spinning speed in the case of spin-coating), the concentration of first precursor in aqueous solution, temperature, dip speed, and choice of solute. A precise seeding layer thickness is desirable for many secondary cell applications, particularly those requiring a high specific capacity (Wh / kg) and consistent thermal properties such as automotive, aerospace or portable electronics applications. The required seeding layer thickness depends on the desired ratio of elements in the grown alloy. The seeding layer thickness may be at least approximately 15 nanometres, comprising a minimum functional seeding layer thickness of 5 nanometres, and 10 nanometres that can be sacrificially etched away by the plasma. Although PECVD for growing Silicon films normally favours a thin seeding layer, to prevent the creation of traps, this is less of a problem in the present disclosure. Therefore, the seeding layer could be much thicker for some implementations, for example two hundred micrometres or more. This enables the use of PECVD to grow thick electrodes for large batteries. In some, but not necessarily all examples, the atomic ratio of the charge carriers (e.g. Lithium) to the crystal-forming elements (e.g. Silicon) in the grown alloy may vary from 1 (e.g. LiSi) to 4.4 (e.g. Li22Si5). Increasing the proportion of charge carriers may result in a thicker film. The atomic ratio within the grown alloy can be determined by spectroscopic techniques such as Secondary Ion Mass Spectroscopy. In some examples, the alloy is to further comprise an additional element having a greater charge capacity than Carbon. Examples include Tin, Aluminium, Antimony or Magnesium. A precursor such as Tin (II) Acetylacetonate, Aluminum nitrate hydrate, etc., may be deposited onto the substrate, forming part of the seeding layer. The deposition may be part of step 11 or a separate step. The additional element may be added in sufficient concentrations to form a ternary alloy, or may be added in smaller concentrations to act as a dopant. Then, at block 12 of the method 10 of FIG. 1, the coated substrate 30 is added to the PECVD reactor. FIG. 4 schematically illustrates an example of a PECVD apparatus 40 comprising the reactor 41. Only the main components are shown. FIG. 4 represents just one example of a variety of usable PECVD designs. 5 The illustrated PECVD apparatus 40 comprises a sealable reactor chamber 41, a gas inlet 43 for receiving the gaseous second precursor, a radio frequency source 42, and a pair of electrodes 44, 45 between which is generated the radio frequency field which excites the second precursor to a plasma state. The coated substrate 30 is placed 10 between the electrodes 44, 45, for exposure of the seeding layer 31 to the plasma. In an example implementation, the gaseous second precursor comprises elements of Silicon and / or Germanium, configured to form the crystalline nanostructures. In 20 15 gaseous precursor form, the elements may be bound to elements such as Hydrogen, for example Silane, Silylene, Trichlorosilane, Germane, Germylene, or other compounds that can dissociate under radio frequency energy. Other gases are possible. The radio frequency source 42 is configured to generate sufficient radio frequency energy to dissociate the gas (e.g. Silane gas) into radicals (e.g. elemental Silicon) that can react with the charge carrier nucleation sites on the seeding layer, to grow alloyed micro- or nano-structures. The plasma-generating excitation frequency may be from tens of kilohertz to tens of megahertz. A non-limiting example is approximately 13.6MHz which is reserved for industrial, scientific and medical (ISM) purposes. 25 Then, at block 13 of the method 10 of FIG. 1, the method comprises exposing the seeding layer to a plasma phase of the second precursor, at a temperature above the cracking temperature of the seeding layer, to grow the electrode active material. The temperature may be controlled using a heater configured to heat the coated substrate 30. 30 Experiments have found the suitable growth-temperature range to be between 200 Celsius and 400 Celsius, wherein the precise temperature depends on the cracking temperature of the compound used in the seeding layer. Experiments have found that a radio frequency power of between 5 Watts and 25 Watts will form a plasma containing Silicon or Germanium radicals, that react with the Lithium or Sodium (and optionally Tin / Aluminium / Antimony / Magnesium) at the nucleation sites, to grow into a substantially poly-crystalline thin film containing micro-or nano-structures. The RF power range may be no greater than approximately 10 watts, to ensure nanowires are small but not too small. 10 Experiments have found that the reactor pressure may be between 100mTorr and 900mTorr, depending on the desired nanostructure shapes. Higher reactor pressures within the above range can produce nanowires 50. Nanowires are considered to be less sensitive to volumetric expansion than other nanostructure shapes such as nanocrystals or nanoparticles. Straight nanowires are favoured, which are achievable using a pressure of at least approximately 500mTorr or above. However, the other nanostructure shapes are possible. 15 CXI CXI 20 FIG. 5A schematically illustrates an arbitrary number of crystalline nanostructures 50 grown using the above method, forming the electrode active material. The shape shown is for illustrative purposes only. The nanostructures 50 are shaded in solid, to indicate that there are initially no / few vacancies in the alloyed structure. This is because the charge carriers have been pre-alloyed within the crystalline structure. 25 30 FIG. 5B schematically illustrates de-alloyed nanostructures 51 after a discharge cycle in a battery. The presence of vacancies is indicated by the dot-hatched shading of the nanostructures 51. The charge carriers have been extracted in the presence of an electric field (discharge cycle). The vacancies in the nanostructures are for accommodating the charge carriers during the next charge cycle without causing a damaging level of volumetric expansion of the anode. FIG. 6 schematically illustrates an example of a secondary cell 60 containing the electrode active material. The secondary cell 60 comprises a cathode 61 (e.g. Lithium foil), an anode 65, and an electrolyte 62 and separator 63 therebetween. The anode 65 comprises the electrode active material 50 produced by the above method 10. The secondary cell 60 may be a cell battery or a thin film battery, for example. The electrode active material produced by PECVD can be made sufficiently thin to be resiliently flexible, for flexible battery applications or for spooling into cylindrical shapes. Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described. 10 15 CXI CXI 20 25 30 The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one..” or by using “consisting”. Although references are made herein to targeted properties such as nanocrystalline structures, nanowires, and particular alloys, these have an inclusive not an exclusive meaning. For example, the material is substantially, but not exclusively nanocrystalline, and substantially, but not exclusively comprises nanowires. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. For example, the method may be applied to the production of a cathode active material. Further, elements other than Silicon or Germanium may suffer from swelling problems that can be alleviated by the method described herein. Further, charge carrier elements other than Lithium or Sodium are known, such as Potassium, that can be usable in the described method. 5 Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those 10 functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. 20 15 The term ‘a’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially 25 the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. 30 In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as 35 described. CXI CXI Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance it should be understood that the Applicant may seek protection via the claims in respect of any patentable feature or combination of features 5 hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon. l / we claim:

Claims

1. A method of growing an electrode active material using plasma-enhanced chemical vapour deposition, the method comprising:5          providing a seeding layer to a reactor, wherein the seeding layer comprises afirst precursor for providing charge carriers, the first precursor comprising a coordination complex of a charge carrier bound to an organic ligand, wherein the organic ligand is suitable for cracking, and wherein the charge carrier comprises an alkali metal;10          exposing the seeding layer to a plasma phase of a second precursor, at atemperature above a cracking temperature of the seeding layer, to grow the electrode active material, the second precursor comprising at least one of: Silicon; Germanium; or a Silicon or Germanium containing compound, wherein the Silicon or Germanium containing compound is suitable for dissociation in the plasma phase,15          wherein the grown electrode active material comprises an alloy of the chargecarriers and elements from the second precursor, so that when the electrode active material is in use, the charge carriers can be extracted from the electrode active material in the presence of an electric field, leaving vacancies for later insertion of the charge carriers.CM 201—2. The method of claim 1, wherein the electrode active material is an anode active material.

3. The method of claim 1 or 2, wherein the Silicon or Germanium containing 25 compound is suitable for dissociation in the plasma phase under radio frequency energy.

4. The method of any preceding claim, wherein the second precursor comprises gaseous: Silane; Silylene; Trichlorosilane; Germane; orGermylene.

305. The method of any preceding claim, wherein the charge carriers of the first precursor comprise Lithium and / or Sodium and / or Potassium.

6. The method of any preceding claim, wherein the coordination complex 35 comprises:Lithium Acetate, orSodium Acetate.

7. The method of any preceding claim, wherein the seeding layer further 5 comprises a further precursor, and wherein the further precursor comprises Tin.

8. The method of claim 7, wherein the further precursor comprises Tin (II) Acetylacetonate.10   9. The method of any preceding claim, wherein the alloy comprises more than1000 parts per million of the charge carriers.

10. The method of any preceding claim, wherein the alloy is a Lithium-Silicon alloy or Lithium-Silicon-Tin alloy.1511. The method of any preceding claim, wherein the temperature during the growth is controlled to be a value from the range approximately 200 Celsius to approximately 400 Celsius.20   12. The method of any preceding claim, wherein reactor pressure during the growthis controlled to be a value from the range approximately 100 mTorr to approximately 900mTorr.

13. The method of any preceding claim, wherein the plasma phase of the second 25 precursor is a radio frequency-induced plasma phase, and wherein radio frequency power during the growth is controlled to be a value from the range approximately five Watts to approximately 25 Watts.

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