WO2019234666A1

WO2019234666A1 - A process for producing an anode for lithium-ion batteries

Info

Publication number: WO2019234666A1
Application number: PCT/IB2019/054703
Authority: WO
Inventors: Donato Vincenzi; Silvio FUGATTINI; Alfredo ANDREOLI; Damiano GIUBERTONI; Remo PROIETTI ZACCARIA; Umair GULZAR; Lin Chen
Original assignee: Universita' Degli Studi Di Ferrara; Fondazione Bruno Kessler; Fondazione Istituto Italiano Di Tecnologia; Universita' Degli Studi Di Genova
Priority date: 2018-06-07
Filing date: 2019-06-06
Publication date: 2019-12-12
Also published as: IT201800006103A1; CN112789748A

Abstract

A process for producing an anode for lithium-ion batteries, comprising the following steps: depositing by plasma-enhanced chemical vapor deposition (PECVD) an active material (1) directly onto a metal substrate (2) intended to form a current collector, the active material (1) consisting substantially of a germanium film, and subjecting the film (1) to anodic oxidation to make it porous or increase its porosity.

Description

A process for producing an anode for lithium-ion batteries

The present invention generally concerns the field of lithium-ion batteries.

Lithium batteries are one of the most widely used technologies for storing electrical energy, due to their high gravimetric capacity, i.e. the amount of charge accumulated per unit of mass. In particular, lithium-ion batteries are used in portable devices, in the automotive sector, in the aerospace sector and recently also in domestic accumulators used to store the energy produced by photovoltaic panels. In all these areas, devices with an increasingly high ratio of accumulated charge to mass are required.

The gravimetric capacity, the parameter used to measure this ratio, is expressed in mAh/g and represents one of the key factors to be improved to obtain higher performing batteries. Another fundamental aspect is the maintenance of this gravimetric capacity for hundreds or thousands of charge and discharge cycles.

The use of lithium in batteries is mainly due to the property of being one of the most electropositive elements (about 3.4 V compared to the standard hydrogen electrode) and the least dense among metals (0.53 g/cm3). These features allow one to obtain respectively large differences in potential for the single cell (3.6 V open circuit) and a much higher gravimetric capacity (3860 mAh/g) relative to other metals (260 mAh/g in the case of lead). Another property of lithium, of great importance for use in batteries, lies in the high diffusivity of the atom linked to the small ionic radius that allows high charge and discharge speeds to be obtained.

Primary batteries use lithium metal as an anode directly, but, despite numerous attempts, it has not been possible to create a reliable secondary battery with an electrode made of Li. There are two main causes of this failure: the short circuits caused by the dendrites that form on the anode during the charging process and the high reactivity of lithium which puts at risk the safety of the device.

The solution to this limitation was the development of lithium-ion batteries, wherein lithium metal is replaced by intercalation anodic materials capable of accommodating and releasing lithium ions during charging/discharging. This limits the reactivity of the metal, resulting in inherently safer devices. In the batteries in common use, the lithium source is typically contained in the cathode, formed of oxides such as LiFeP04, LiMn204 and LiCo02, while the anode is made of graphite.

A lithium-ion battery typically consists of a combination of an anode and a cathode (one of which contains chemically bound lithium), an electrolyte and a separator - usually polymeric.

The gravimetric capacity of the complete device depends on the features of all the elements of which it is composed, but a field of research with ample room for improvement concerns the study of new anodic materials as an alternative to graphite. Despite being an inert compound, easy to process and relatively economical, the use of carbon in its various forms constitutes, in effect, a limitation to the performance of the battery due to the relatively low theoretical gravimetric capacity (372 mAh/g).

The gravimetric capacity of a material depends essentially on the type of alloy it forms with lithium: in the case of graphite, the ratio obtained is one lithium atom for every six carbon atoms (LiC6). The search for new materials is based on the identification of substances that may maximize the content of lithium in the alloy within the anode. There are materials, typically semiconductors, with a significantly higher gravimetric capacity than that of graphite. Silicon (4200 mAh/g) and germanium (1600 mAh/g) are the two most common examples. In both, the ratio between lithium atoms and semiconductor atoms is 22 to 5 (Li22Si5, Li22Ge5) but, because of the lower density of Si compared to Ge, the first results in having a greater gravimetric capacity than the second. Despite this, Ge is of particular interest, as it has a diffusivity of lithium 400 times greater and an electrical conductivity typically 10,000 times higher. This allows the germanium to withstand faster charging and discharging regimes than silicon, and to achieve overall higher performing batteries.

However, these materials have a limitation that still prevents their use in commercial devices: the high percentage of lithium ions that accumulate in the anode during the lithiation process causes a large volumetric variation, up to 400%, which quickly leads to the detachment and/or pulverization of the electrode.

In order to create reliable devices that use semiconductors as anodic material, it is necessary to create a structure that allows the volumetric variation to be accommodated reversibly and guarantees the mechanical and electrical adhesion between the anodic material and the substrate material acting as a current collector.

The solution to this twofold technical problem consists in a process defined by the non-trivial combination of: a semiconductor material, a metal substrate that acts as a current collector, a deposition technique, a nano-structuring technique of the same semiconductor material.

Some of the solutions proposed to increase the gravimetric capacity of the anode while maintaining the reliability of the device, are based on the dispersion of these semiconductors within fluid suspensions containing carbon in different forms (nano-particles, nano-tubes, nano-wires, etc.). This allows the overall gravimetric capacity of the anode to be improved by taking advantage of the properties of semiconductors while maintaining a structure that can withstand over time the changes in volume between charging and discharging. Below are some of the patent publications that follow this approach to making innovative anodes for lithium-ion batteries.

In this regard, CN 105070891 describes a mesoporous anodic material composed of germanium or germanium oxide particles dispersed in a carbon matrix. This material is made by means of a chemical process from a germanium salt dissolved in ethanol to which carbon is added. The deposition of the germanium-containing compound on the current collector is done by mechanical methods.

US 20140242460 A1 describes an anode for lithium-ion batteries containing particles composed of a carbon and silicon core covered with a phosphorus-based alloy and a metal chosen from among Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Pd, Ag, In, Sn and W. This expedient allows a resilient matrix containing silicon to be created able to withstand the large variations in volume encountered while creating the alloy between lithium and silicon. US 2010216026 A1 describes an active material composed of mesoporous fullerene-shaped carbon deposited by CVD (Chemical Vapor Deposition) to increase the surface area capable of accumulating lithium ions when it is filled by the electrolyte. The CVD technique is used to deposit the carbonaceous material.

US 9583761 B2 describes an anodic material for lithium-ion batteries and the method for making it. This material contains germanium powder, boron carbide and carbon with the addition of tungsten carbide. The average particle size is 20-100 nm and the mixture may also contain silicon.

CN 105529442 A describes a compound of germanium nanoparticles in a graphite multilayer. It is produced through simpler chemical processes than other techniques and allows the capacity of the battery to be improved with respect to common graphite anodes. However, the gravimetric capacity of this material remains lower than that theoretically obtainable with an anode composed solely of germanium.

US 2003096167 A1 describes an anodic material composed of a carbon core covered with a mixture of elements which may also contain germanium or silicon.

US 2014302385 Al describes an anodic material for lithium batteries consisting of a germanium structure covered with a layer of graphene deposited by CVD (Chemical Vapor Deposition) without making use of catalysts. The germanium structure deposited with the VLS (Vapor-Liquid-Solid) technique may be formed of nano-wires, nano-tubes, germanium film or germanium substrate.

CN 106099066 describes an anodic material composed of graphene and germanium dioxide. Graphene sheets are coated with nano-particles of germanium dioxide to increase their gravimetric capacity.

The addition of silicon or germanium within compounds containing carbon, tin/tin oxide or other metals helps to increase the gravimetric capacity of the anode while still remaining far from the theoretical gravimetric capacities of these two semiconductors (4200 mAh/g for silicon and 1600 mAh/g for germanium). An alternative way is therefore to create nano- structures based solely or almost solely on these two semiconductors, in an attempt to create a stable anode that may withstand the changes in volume that it undergoes in the process of lithiation and de-lithiation. The following patent documents are cited in this regard.

US 2017033356 Al describes an anodic material formed by nano-wires composed mainly of germanium on the inside and silicon on the outside. It is made using the VLS (Vapor- Liquid-Solid) technique starting from a silicon substrate on which nano-particles of gold are evaporated. These particles have the function of being a catalyst for the growth of nanowires that takes place within a CVD (Chemical Vapor Deposition) reactor using as process gas respectively GeC14 and SiC14.

WO 2012028858 Al describes an anodic material for lithium-ion batteries consisting of a variety of particles and elongated elements formed of silicon, tin, aluminum, germanium or a combination thereof. Inside the compound one may also find carbon in various forms (graphite, graphene or conductive carbon), as well as a binder. The current collector is a material chosen from among copper, aluminum or nickel.

CN 105417505 describes an anodic germanium-based material made from a solution containing a germanium salt and not through vapor phase deposition.

US 2015171426 Al describes an anodic material formed of germanium oxide nano-particles (GeOx with x between 0.01 and 1.99) and a lithium ion battery anode consisting of this material, a conductive additive and a binder.

US 9368836 B2 describes an anodic material composed of nanostructures of various kinds (nano-tubes, nano-wires, nano-flakes) of 99.99% silicon.

US 2015010830 Al describes a binder-free anode composed of nanoparticles formed from at least one material chosen from silicon, germanium and antimony. CN 103943836 describes an anode for lithium-ion batteries formed from a three-dimensional structure of hollow germanium nano-tubes made with the magnetron sputtering technique.

US 2014093754 A1 describes a complete battery composed of a carbon monofluoride cathode, electrolyte and an anodic material based on silicon, germanium or tin.

US 2016043385 A1 describes an anodic material composed of a structure of nano- wires of silicon, germanium or tin. This anodic material does not require binders to adhere to the current collector and is obtained by the VLS (vapor liquid solid) technique. The process gases are phenylsilane and diphenylsilane.

CN 106159247 describes an anode for lithium-ion batteries formed from a porous layer of germanium obtained from particles.

JP2004327330 describes an anode for lithium-ion batteries fonned of a porous layer of silicon (Si) or an alloy of silicon and germanium (SiGe). The deposition of the active material may be done by means of techniques known to a person skilled in the art, such as plasma- enhanced chemical vapor deposition (PECVD) using silane (SiH₄) as a precursor gas. The porous structure may be made through techniques known to a person skilled in the art such as anodic oxidation by means of hydrofluoric acid.

US2010221606 describes an energy storage device containing an electrode made by deposition of a semiconductor film on a metal substrate acting as a current collector. This semiconductor film is subsequently anodized in order to obtain a porous structure. The metal substrate is a generic conductor film, with copper as the only specific example. The deposition technique of the semiconductor film is generally chosen from those known to a person skilled in the art, such as, for example, physical vapor deposition, chemical vapor deposition and plasma-enhanced chemical vapor deposition. The semiconductor film may be formed from silicon, germanium, an alloy of silicon and germanium (SiGe) or gallium arsenide (GaAs). The anodizing process for the creation of the porous structure uses hydrofluoric acid and acetic acid as electrolyte. DE 102015120879 describes a method for manufacturing a lithium-ion battery anode made by depositing a layer of silicon, possibly p-doped, on a copper metal substrate. Before silicon deposition, a layer of material is deposited on the metal substrate in order to increase the adhesion between the silicon film and the same copper substrate. The silicon layer is then made porous by an electrochemical attack with hydrofluoric acid. The deposition technique may be one of those known to a person skilled in the art as chemical vapor deposition (CVD) or physical vapor deposition (PVD).

One of the aims of the present invention is to propose a method for creating a semiconductor- based anode that allows an anodic structure to be obtained that may both accommodate reversibly the volumetric variation, both to ensure mechanical and electrical adhesion between the anodic material and the substrate material acting as a current collector.

The subject of the present invention is a process for obtaining a binder-free anode for lithium-ion batteries, defined by the combination of:

germanium as active material,

plasma-enhanced chemical vapor deposition (PECVD) of the active material directly on a metal substrate acting as a current collector,

said metal substrate consisting of molybdenum or stainless steel or a combination or alloy thereof

anodic oxidation of the active material to make it porous or increase its porosity.

The term“directly" means both the case wherein the film of active material is in direct contact with the metal substrate and the case wherein the metal substrate is covered with a thin layer of material that may promote better adhesion of the active material on said metal substrate.

The use of an anode made by this process inside lithium-ion secondary batteries allows the gravimetric capacity (i.e. the accumulated charge per unit of mass) of these devices to be improved mainly for two reasons: the first concerns the use of a semiconductor (germanium) as an anodic intercalation material while the second advantage consists in being binder-free. The term“binder" refers to any substance used in the manufacturing process of the anode for the purpose of incorporating, agglomerating and compacting the active material. This substance consists totally or partially of material that does not actively participate in the chemical reactions that take place inside the battery, thus decreasing the overall gravimetric capacity thereof. Making a binder-free anode is, therefore, an additional element of advantage that is added to the use of the semiconductor (germanium) as an anodic intercalation material.

The temperature conditions and properties of the plasma inside the PECVD reactor allow the creation of alloys or layers with intermediate composition (amorphous and/or crystalline) between the germanium and the metal substrate. Obtaining these alloys or intermediate composition layers (amorphous and/or crystalline) has guided the choice of the non-trivial combination of deposition technique, active material and metal substrate. These alloys or intermediate composition layers (amorphous and/or crystalline) guarantee the mechanical and electrical adhesion between the active material and the metal substrate, keeping it stable for thousands of charge and discharge cycles.

Moreover, the anodic oxidation process allows a desired level of porosity in the active material to be obtained in a simple and industrially scalable way, such as to minimize the effects of the volume variations the material is subjected to in the process of lithiation and de-lithiation and to ensure that the gravimetric capacity is maintained for thousands of charge and discharge cycles. The materials chosen as metal substrate do not need protective films to prevent the damage thereof in case of contact with the electrolytic solution during anodic oxidation. This simplifies the anode manufacturing process and is a further advantage of the present invention.

Further features and advantages of the proposed process will be presented in the following detailed description, which refers to the accompanying drawings, provided only by way of non-limiting example, wherein:

Figure 1 is a schematic representation of a binder-free anode made of semiconductor material (germanium) according to the present invention, before (left) and after (right) an anodic oxidation treatment;

Figure 2 is a schematic representation of an example of a PECVD (plasma-enhanced chemical vapor deposition) reactor that may be used in the process according to the present invention;

Figure 3 is a schematic representation of another example of a PECVD reactor that may be used in the process according to the present invention;

Figure 4 is a schematic representation of an example of anodic oxidation equipment that may be used in the process according to the present invention;

Figure 5 is a schematic representation of another example of anodic oxidation equipment that may be used in the process according to the present invention; and

Figure 6 is a schematic representation of a lithium-ion battery.

With reference to the figures, a manufacturing process of an anode for lithium-ion batteries according to the present invention substantially comprises two parts: the plasma-enhanced chemical vapor deposition (PECVD) of the semiconductor material 1 (germanium) on a metal substrate 2 (molybdenum or stainless steel), and the subsequent creation of a porous structure 3 by anodic oxidation, using in particular a solution containing hydrofluoric acid. Both of these techniques are easily scalable for industrial production on large surfaces.

Prior to deposition, the metal substrate 2 may be treated with chemical agents known to a person skilled in the art in order to remove any trace of impurities and/or to promote the adhesion of the semiconductor film 1 to the metal substrate 2. ln a particular embodiment, the metal substrate 2 may in turn be deposited on an insulating material using techniques known to a person skilled in the art to reduce the overall mass of the anode and increase the flexibility thereof. This insulating material may be made of polyethylene terephthalate, poly(4,4’ - oxydiphenylene-pyromellitimide) or other plastic materials.

The porous structure 3 contains pores having a size that varies between 10 nm and 2 pm depending on the anodizing conditions. The ratio between the mass of the semiconductor film 3 following the formation of the pores and that of the film 1 before anodic oxidation is typically between 0.2 and 0.8, preferably between 0.4 and 0.6. Plasma-enhanced chemical vapor deposition (PECVD) takes place in a vacuum chamber 1 1 maintained dynamically at a pressure of 10-1 to 10-4 mbar using hydrogen or argon, but preferably hydrogen, as the activation gas. Inside the deposition chamber 1 1 , there is a radiofrequency source 12 located in the proximity of a first nozzle 4 that allows the release of a controlled flow of the activation gas. This gas, due to the electromagnetic field generated by the source 12, ionizes, creating a cloud of high-energy plasma 14. Through a second nozzle 5, the process gas GeH4 (germane) is injected into the plasma 14, which provides this process gas with an energy to dissociate the molecule by precipitating the atoms of the semiconductor on the substrate. Near the second nozzle 5 from which the process gas exits, the metal substrate 2 on which the semiconductor film 1 is deposited and a heater 7 which regulates the temperature of the substrate are positioned. The substrate 2 is typically supported by mechanical means 6 that allow this substrate to be held in place and at the same time allow its heating by said heater 7.

The film thickness typically ranges from 0.01 to 40 pm, more preferably between 0.1 and 20 pm and even more preferably between 0.2 and 10 pm. Due to the heater 7 inside the process chamber 1 1 , it is possible to set the temperature of the substrate during deposition to a value between 250°C and 650°C, and more preferably between 300°C and 500°C. The metal substrate 2 may be made alternatively of molybdenum or stainless steel and is typically less than 100 pm thick.

To insert the sample into the process chamber, a pre-chamber 9 is used, called a load-lock. The load-lock 9 is equipped with at least one external communication valve 13 of such a shape and size as'to allow the insertion of the substrate 2 therein.

To proceed with the deposition, the substrate 2 is inserted into the load-lock 9, which is initially at ambient pressure and then is brought to vacuum pressure by a pump system 16. Once the pressure of the load-lock is balanced with the pressure in the deposition chamber, the valve 8 is opeiied to put the load-lock 9 in communication with the process chamber 1 1. Using mechanical means 10, the substrate 2 is taken from the load-lock 9 and correctly positioned on the support 6 inside the deposition chamber 1 1. Said mechanical means 10 are subsequently retracted and the valve 8 is closed before the deposition begins. The advantage of using the PECVD technique to deposit semiconductors consists in the high deposition rate (up to 10 nm/s), at least one order of magnitude higher than that achievable with other techniques such as Molecular Beam Epitaxy (MBE) or chemical vapor deposition (CVD). In particular, chemical vapor deposition (CVD) differs from PECVD because the energy supplied to the atoms of the reagent gas is only of a thermal type, while in PECVD the greatest contribution of energy is provided by the radio frequency source. This allows the process gas to be provided with much greater energies that promote the dissociation of the molecules by increasing the deposition rate. Implementing this technique for deposition on large surfaces makes it suitable for mass production and therefore very competitive in the field of lithium-ion batteries.

Figure 3 shows an example of a PECVD reactor equipped with a“roll-to-roll” system for the deposition of semiconductors over large surfaces. The operation of the PECVD reactor is the same as described with reference to Figure 2, with the difference that the metal substrate 2 is not positioned on a static support system 6, but wound around a first reel 24 and has one of the ends attached to a second reel 25. During deposition, said reels 24 and 25 rotate around parallel axes so that the metal substrate 2 is unwound from the reel 24 and simultaneously wound around the reel 25. In this way, the entire metal substrate 2 flows through the area between the heater 7 and the plasma cloud 14, allowing the semiconductor film 1 to be deposited over the entire surface of the substrate 2. In this way, it is possible to deposit the semiconductor film 1 on large surface metal substrates 2 without the need to use large vacuum chambers 1 1 or to slow down the process to allow the insertion of new small metal substrates 2 from the load-lock 9.

Following deposition, the semiconductor film 1 needs to be nanostructured in order to make it more resistant to the volume variations that occur during the lithiation and de-lithiation process. The creation of pores in the semiconductor film 1 by anodic oxidation in hydrofluoric acid has been shown to be able to preserve the integrity of this anode for thousands of charge/discharge cycles, maintaining a real gravimetric capacity greater than 1000 mAh/g and sustaining charge/discharge rates up to 10C. The C-rate is the unit of measurement used to indicate the speed of charge or discharge of the battery, defined as the ratio between the charge accumulated in the battery divided by the number of hours required to charge/discharge it. 1 C corresponds to charging/discharging the battery in one hour, 2C in half an hour, C/2 in two hours. A C-rate of 10C thus corresponds to charging/discharging it in 6 minutes.

The setup required to create pores is a traditional electrolytic cell of adequate size connected to a current generator (Figures 4 and 5). The simplicity of this technique and the possibility to process large surface samples make this approach to nanostructuring easily scalable at the industrial level. The system is composed of:

• a metal electrode 18 electrically connected to the metal substrate 2 on which the semiconductor film 1 is deposited.

• an electrolytic cell 17 made of a hydrofluoric acid-resistant material (preferably PTFE or PP) that houses the sample by placing a portion of the semiconductor film 1 in contact with the electrolytic solution 21.

• an electrolytic solution 21 containing hydrofluoric acid in a liquid solution.

• an electrode 22, preferably made of graphite or platinum, in contact with the electrolyte 21 to which the semiconductor film 1 is exposed.

• a current generator 23 able to run a substantially direct current in the setup shown in Figure 4 equipped with means that allow the module to be modified or the direction of the current supplied by the same generator 23 to be reversed. The generator 23 may be equipped with a system capable of keeping the intensity of current flowing inside the electrochemical cell constant by varying the difference in applied potential. This solution allows the dissolution rate of the semiconductor film 1 to be kept constant within the electrolytic solution 21. In a particular embodiment, the generator 23 may reverse the current flow inside the electrolytic cell 21 in order to promote the dissolution of the gaseous species that form on the surface of the semiconductor 1 during the process of pore formation. This solution allows the uniformity of dissolution of the semiconductor film 1 inside the electrolytic solution 21 to be increased and thus a more homogeneous porous film 3 to be obtained.

Figure 4 also shows a fastening system 20 that compa'cts the elements 17, 19, 1 , 2 and 18 in order to prevent the leakage of the electrolytic solution 21. The electrolytic cell 17 may be equipped with gaskets 19 made of material resistant to the electrolytic solution 21 that are kept in contact with the semiconductor film 1 due to the pressure exerted by the fastening system 20.

In a particular embodiment, the equipment described in Figure 4 may also be equipped with an agitator or recirculation system that keeps the chemical composition of the electrolytic solution 21 within the cell 17 uniform. This solution helps to make the porous layer 3 more homogeneous. The equipment described in Figure 4 may also be equipped with a pH meter to monitor the electrolytic solution 21 inside the cell 17 during the pore formation process and to ensure that the electrolytic solution 21 keeps its chemical composition substantially unchanged.

Figure 5 shows an alternative device to the one shown in figure 4. The semiconductor film 1 faces the electrolytic solution 21 contained in the half-cell 26. The metal substrate 2, on the other hand, faces a second electrolytic solution 33 contained in the half-cell 27. In some embodiments, the second electrolytic solution 33 may coincide with the first electrolytic solution 21. The material used to make the half-cells 26 and 27 is resistant to the hydrofluoric acid (preferably PTFE or PP) and to any other chemical agents contained in the electrolytic solutions 21 and 33. The two half-cells 26 and 27 are pressed against the semiconductor film

1 and the substrate 2 respectively by a mechanical fastening system, not shown in the figure.

The two half-cells 26 and 27 are possibly equipped with gaskets 19 to prevent the escape of the electrolytic solutions 21 and 33 from these cells ln both half-cells 26 and 27, an electrode 22, preferably made of graphite or platinum, is immersed. The electrode immersed in the half-cell 26 is connected to the negative pole of the generator 23, while the electrode immersed in the half-cell 27 is connected to the positive pole of the generator 23.

In a particular embodiment, the equipment described in Figure 5 may also be equipped with an agitator or a recirculation system that keeps the chemical composition of the electrolytic solution 21 within the half-cell 26 uniform. This solution helps to make the porous layer 3 more homogeneous. In addition to this variant, the half-cell 27 may also be equipped with an agitator or recirculation system that allows a constant current distribution to be maintained on the surface of the metal substrate 2 during the pore formation process. The equipment described in Figure 5 may also be equipped with a pH meter to monitor the electrolytic solution 21 inside the half-cell 26 during the pore formation process and to ensure that the electrolytic solution 21 keeps its chemical composition substantially unchanged.

The electrolytic solution 21 and possibly 33 may contain hydrofluoric acid (HF), hydrochloric acid (HC1), nitric acid (HN03), phosphoric acid (H3P04), sulfuric acid (H2S04), acetic acid (CH3COOH), hydrogen sulfide (H2S), formic acid (HCOOH) or any mixture of these or some of these.

With reference to Figure 6, a lithium-ion battery is schematically represented. The negative pole of the battery 28 is connected to the metal current collector 2 on which the porous semiconductor layer 3 is deposited, while the positive pole 29 is connected to the cathode 32. Between the anode 3 and the cathode 32 there is the electrolyte 31 and a separator 30. In some embodiments, the electrolyte 31 may contain an additive in order to modify the electrochemical characteristics thereof.

The cathode 32 may be a material chosen from LiMn204, LiFeP04. LiNi02, LiNi l -y- zCoyAlz02 (with y between 0 and 1 and z between 0 and 1 ), LiNil -y-z MnyCoz02 (with y between 0 and 1 and z between 0 and 1 ), LiMn3/2Ni 1/204, LiFe04, LiFel/2Mnl/2P04, LiM02 (with M able to be an element chosen from Co, Ni, Mn Fe, V, Cr), LMNC (Li2Mn03 LiM02 with M able to be an element chosen from Co, Ni, Mn Fe, V, Cr), LFP (Lithium- iron-phosphorus), LFMP (Lithium-iron-manganese-phosphorus), LiTiS2, Li2Mn03, LiCo204, LiMnP04, LiCoP04, LiFeS04F, LiVP04F.

The electrolyte 31 may be LiTFSI in DME/DOL, LiC104, LiPF6, LiPF6 in DMC, DEC, EC, EMC (or a combination thereof), LiC104 in DMC, DEC or EC (or a combination thereof), POE.

In a preferred embodiment, one or more additives may be added to the electrolyte 31 chosen from FEC (fluoroethylene carbonate), EC (ethylene carbonate), PC (propylene carbonate), VC (vinylene carbonate), graphene, SWCNHs (single-walled carbon nanohoms) such as those described in W0 20l 7l 03758AL The separator 30 is typically made of polyolefin or glass fiber. In a particular embodiment no separator 30 is used, typically in the case wherein the electrolyte is a polymeric material in solid or gel form. The battery described schematically in Figure 6 may be produced using as a casing a coincell 2032, a cylindrical casing 18650, a cylindrical casing 21700, a pouch casing or a prismatic casing.

Claims

1. A process for producing an anode for lithium-ion batteries, comprising the following steps:

depositing by plasma-enhanced chemical vapor deposition an active material (1 ) directly onto a metal substrate (2) intended to form a current collector, said active material ( 1 ) consisting substantially of a germanium film, and said metal substrate (2) consisting substantially of molybdenum or stainless steel or an alloy thereof, and

subjecting the film (1 ) to anodic oxidation to make it porous or increase its porosity.

2. A process according to claim 1 , further comprising, before depositing the active material (1),

coating the metal substrate (2) with an adhesion layer to improve adhesion of said active material (1 ) to the metal substrate (2).

3. A process according to claim 1 or 2 wherein the active material ( 1 ) is deposited by adding, to the process gas, one or more chemical species having the function of changing the electrical resistivity of the active material.

4. A process according to one of the preceding claims, wherein the deposition of the active material (1 ) is obtained by roll-to-roll processing.

5. A process according to one of the preceding claims, wherein the anodic oxidation is performed by dissolution of the active material (1) within a liquid solution containing hydrofluoric acid, HF.

6. A process according to one of the preceding claims, wherein the metal substrate (2) has a thickness of less than 100 pm.

7. A lithium-ion battery comprising a cathode (32), an electrolyte (31 ) and an anode (2, 3), wherein the anode comprises a metal substrate (2) acting as current collector, as well as an active material (3) substantially consisting of a porous film of germanium and deposited directly onto the metal substrate (2), wherein the anode is produced with a process according to one of the preceding claims.

8. A battery according to claim 7, further comprising a separator (30) interposed between the anode (2, 3) and the cathode (32).

9. A battery according to claim 7, wherein the battery has no separator interposed between the anode (2, 3) and the cathode (32).