WO2015024862A1

WO2015024862A1 - Process for obtaining composite materials for secondary energy storage devices

Info

Publication number: WO2015024862A1
Application number: PCT/EP2014/067439
Authority: WO
Inventors: Anne-Lise GOFFIN; Shilei CHEN; Maurizio Biso; Riccardo Pieri; Frederic Fouda-Onana
Original assignee: Solvay Sa
Priority date: 2013-08-21
Filing date: 2014-08-14
Publication date: 2015-02-26

Abstract

A process for the manufacture of composite materials comprising nanoparticulate materials capable of intercalating lithium ions in a matrix of porous carbon materials which comprises a) a step of mixing an aqueous dispersion of a carbonizable polymer with an aqueous dispersion of a nanoparticulate material capable of intercalating lithium ions, b) a subsequent step wherein the combined dispersions are co-coagulated, and the coagulate is filtrated and dried and thereafter c) the product from step b) is subjected to a thermal treatment at a temperature in the range of from 300 to 1000°C for a time period of from 10 minutes to 5 hours thereby obtaining a porous composite material comprising nanoparticles capable of intercalating lithium ions in a matrix of conductive porous carbon.

Description

Process for obtaining composite materials for secondary energy storage devices

[0001] This application claims priority to European application No. 13181 190.3, filed on August 21 , 2013, the whole content of this application being incorporated herein by reference for all purposes.

[0002] The present invention relates to a novel process for the manufacture of composite materials useful as electrode materials in secondary energy storage devices and to the composite materials obtained by the process.

[0003] Secondary energy storage devices for storing electrical energy are a need for many applications.

[0004] Currently so called lithium ion batteries are widely used for storage of electrical energy due to their cyclability and capacity which has allowed the development of small batteries useful in many kinds of electrical devices.

[0005] The positive electrode of this kind of batteries usually consists of a metal oxide (e.g. UC0O2) with a layered structure coated on a current collector. During the charge process, the positive material at the cathode is oxidized whereas the material of the negative electrode is reduced. During this process lithium ions are de-intercalated from the interstitial space between the atomic layers of the material of the positive electrode and are intercalated into the interstitial space between the atomic layers of the material of the negative electrode (the anode). During discharge the flow of lithium ions is reversed. It is apparent that for a good cyclability the intercalation and de-intercalation process has to be highly reversible to achieve a sufficient number of charge/discharge cycles.

[0006] In todays battery systems of this type, the anode used is usually based on carbon conductive materials. Graphite may be mentioned in particular. This provides good cycling stability. However, the theoretical capacity of graphite is limited to appr. 370 mAh/g, which is relatively low for high- energy application fields.

[0007] Crystalline silicon provide a theoretical lithium storage capacity which is higher by an order of magnitude (4200 mAh/g) which makes Si an interesting material. There are also other materials based on metal oxides or metal alloys which show good lithium storage capacity but Si provides the highest potential. The replacement of graphitic carbon as the energy storage component in the anode by silicon could result in an increase in anode capacity by a factor of 10 and considerably increase the energy capacity of the total battery.

[0008] However, high-capacity Si anodes show a drastic volume change (up to 300 %) during the alloying/de-alloying reaction with Li⁺, leading to electrode failure. The volume change results in the pulverization of the electrode structure. Additionally, the cracking exposes the silicon nanoparticles to electrolyte leading to the formation of an insulating solid electrolyte interface (SEI) at low potential. Both effects detrimentally affect the electrode performance in term of conductivity, (reversible) capacity and power capability upon cycling.

[0009] Previous studies have shown that the strain induced by the expansion or contraction can be at least partly accommodated by using Si nanoparticles with diameters of 150 nm or less as well as with other Si nanostructures such as nanowires, nanotubes, hollow spheres, nanoparticles and nanoporous silicon. However, the formation of SEI limits the current efficiency in an undesirable manner.

[0010] One approach to limit or reduce the formation of the SEI is to prevent the direct contact of the nanoparticulate Si with the electrolyte. The

modification of the Si surface by coating with an electronically conducting material which at the same time is permeable for lithium ions has been studied and carbon materials have shown to provide some interesting properties in this regard. Carbon can act as a buffering matrix and has been studied mostly in the form of dense materials for this purpose.

[001 1] Conventional dense Si/C electrodes still have some disadvantages when it comes to cyclability, capacity and adsorption of volume expansion during cycling.

[0012] Porous carbon with pre-existing pores providing the volume for the

expansion of the Si and allowing for fast transport of lithium ions would be desirable.

[0013] The synthesis of structures of this type has been described through chemical vapour deposition (CVD) synthesis of Si on carbon black dendritic structures by Yushin et al. in Nature Mat. 9, 353 (2010).

According to the process described annealed carbon black dendritic particles are coated through CVD and the resulting composite particles are mixed with a binder and compacted into an electrode with open

interconnected internal channels. The electrode is transformed into a solid bulk product by annealing in a non-oxidising atmosphere and

simultaneous transformation of a sacrificial binder into carbon.

[0014] Huang et al, J. Phys. Chem. Lett. 2012, 3, 1824-1829 describe crumpled graphene-encapsulated Si nanoparticles for lithium ion battery anodes which are synthesized via a one step capillary driven assembly route in aerosol droplets. Aqueous dispersions of graphene sheets and Si nanoparticles were nebulized to form aerosol droplets by an ultrasonic atomizer, which droplets were subsequently passed through a preheated tube furnace (tube temperature 600°C). During this process, graphene sheets are wrapped around the Si nanoparticles in the form of a crumpled shell. The folds and wrinkles in the wrapped shell are helpful in

accommodating the expansion of the Si nanoparticles upon lithiation. Compared to native Si nanoparticles, the composite capsules showed better cyclability and Coulombic efficiency. Even after 200 cycles, at least 80 % of the initial capacity was still available.

[0015] US 2009/186267 claims anode structures for lithium batteries comprising nanoparticulate Si particulates with crystallite sizes of from 1 to 10 nm, pore sizes from about 1 to 100 nm and a BET specific surface area of from 140 to 400 m²/g, which nanofeatured particles are embedded in a substantially conductive network. Carbon is a preferred material for the network and can be obtained from carbon precursors. Such carbon precursors can be dissolved in a liquid (which might consist of or comprise water), mixed with the Si nanoparticulate powder and thereafter dried. The resulting product is then subjected to heat treatment (usually at a temperature in the range of from 300 to 1000 °C) to form the carbon from the carbon precursor. A number of polymers, including poly vinyl chloride (PVC) is mentioned as carbonizable precursor material. [0016] Zhou et al., Colloids and Surfaces A, Physicochem. Eng. Aspects 316

(2008), 85-88 describe a method for preparing porous carbon by pyrolysis of polyvinylidene chloride (PVDC). According to the method described, PVDC powder was put into a tubular furnace, heated to the carbonization temperature at a rate of 10 °C/min under the protection of nitrogen and kept at the carbonization temperature for one hour. The carbonization temperature varied in the range of from 400 to 900 °C. The results show that the PVDC is carbonized completely under these conditions. It is said that the carbon obtained has a highly porous structure and non- graphitized. The material is believed to offer potential as electrode material for electrical double layer capacitors.

[0017] While the prior art references described above show an interesting

potential for the improvement of SiC composite materials useful for anodes in secondary energy storage devices, the processes described are expensive and very difficult to use in a commercial set-up.

[0018] Accordingly there exists a need for an improved process for the synthesis of composite materials comprising nanoparticles capable of reversibly intercalating lithium ions in a conductive carbon matrix which process is easy to handle and can be used on a commercial scale.

[0019] It was an object of the present invention to provide an improved process of such composite materials useful for anodes in energy storage devices.

[0020] This object is achieved by a process in accordance with claim 1.

[0021] Preferred embodiments of the process in accordance with the present invention are set forth in the dependent claims and in the detailed description hereinafter.

[0022] Another aspect of the present invention relates to composite materials obtainable in accordance with the process of the present invention.

[0023] A third aspect relates to the use of the materials obtained by or obtainable by the process of the present invention as anode materials for secondary lithium ion batteries.

[0024] A fourth aspect relates to anode structures comprising the materials

obtained or obtainable by the process of the present invention.

[0025] In accordance with claim 1 the present invention relates to a process for the manufacture of composite materials comprising nanoparticulate materials capable of intercalating lithium ions in a matrix of porous carbon materials which comprises

a) a step of mixing an aqueous dispersion of a carbonizable polymer with an aqueous dispersion of a nanoparticulate material capable of

intercalating lithium ions,

b) a subsequent step wherein the combined dispersions are co- coagulated, and the coagulate is filtrated and dried and thereafter c) the product from step b) is subjected to a thermal treatment at a temperature in the range of from 300 to 1000 °C for a time period of from 10 minutes to 5 hours

thereby obtaining a porous composite material comprising nanoparticles capable of intercalating lithium ions in a matrix of porous carbon.

[0026] The present invention can also be defined as a process for the

manufacture of composite materials comprising nanoparticulate materials in a matrix of porous carbon materials,

[0027] said nanoparticulate materials being capable of intercalating lithium ions, [0028] said process comprising

intercalating lithium ions,

thereby obtaining a porous composite material comprising nanoparticles in a matrix of porous carbon, wherein the nanoparticles materials are capable of intercalating lithium ions.

[0029] In the step a) of the process in accordance with the present invention two aqueous dispersions are mixed. The term aqueous dispersion as used in the context of the present invention is intended to denote dispersions the solvent of which consists of water and dispersions on the basis of water as main solvent mixed with other solvents depending on the specific dispersant. The other solvents might be organic or inorganic and there is no specific restriction in this regard. The skilled person will select an appropriate solvent combination based on his professional knowledge and best suited for the specific application.

[0030] The first dispersion comprises a carbonizable polymer. The term

carbonizable polymer, when used herein, denotes a polymer which forms carbon upon pyrolysis at a temperature in the range of from 300 to

1000 °C.

[0031] The skilled person knows multiple organic polymers which form graphitic carbon or other types of carbon upon pyrolysis and in principle all these polymers may be used in the process of the present invention. Polymers which form porous carbon materials, especially preferably porous hard carbon materials upon pyrolysis are preferred as they lead to products with better performance properties when used as electrode materials in secondary energy storage devices.

[0032] Usually the eliminated molecules during pyrolysis of carbonizable

polymers include carbon monoxide, carbon dioxide, water and small hydrocarbons, in case of halogen-containing polymers also hydrogen halides. In a number of cases tar is also formed as a by-product. Small hydrocarbons and tar may fill some of the pores in the carbon material which is less desirable as this part of pore volume is no longer available for embedding the nanoparticles of the material capable of intercalating lithium ions. Accordingly, carbonizable polymers for which the eliminated molecules can be completely eliminated from the final product are preferred in accordance with the present invention.

[0033] Preferred carbonizable polymers in the process of the present invention are polymers where hydrogen halides are eliminated during pyrolysis. Vinyl chloride polymers, vinylidene chloride polymers and copolymers and mixtures thereof are especially preferred polymers for the process of the present invention.

[0034] Vinyl chloride polymers comprise, preferably essentially consist of and especially preferably consist of repeating units obtained by polymerisation of vinyl chloride (CH2=CHCI) whereas vinylidene chloride polymers comprise, preferably essentially consist of and especially preferably consist of repeating units obtained by polymerization of vinylidene chloride (CH₂=CCI₂).

[0035] Copolymers comprising both units as described above are also suitable and the repeating units may be present in a ratio of from 1 :99 to 99: 1 , preferably of from 10:90 to 90: 10 in respective copolymers. The

copolymers may have a block structure or the repeating units may be randomly distributed.

[0036] Furthermore, mixtures of respective vinyl chloride polymers and vinylidene chloride polymers may be advantageously used in the process of the present invention.

[0037] In the course of the present invention, it has been found that vinyl chloride polymers, vinylidene chloride polymers or copolymers thereof as well as mixtures of these polymers upon pyrolysis yield hard carbon (as defined below) with a substantially microporous or mesoporous structure.

According to the definition used by lUPAC "microporous" indicates products with pore sizes of up to 2 nm whereas "mesoporous" denotes a pore size range of from 2 to 50 nm.

[0038] Generally, products yielding a high porosity upon pyrolysis are preferred in the process of the present invention as the respective products are particularly well suited for use as electrode materials in secondary energy storage devices.

[0039] Vinyl chloride polymers and vinylidene chloride polymers are commercially available in great variety from a number of commercial suppliers, including Solvay SA. Respective products are available in powder form but also, especially for vinylidene chloride polymers in the form of latices and/or dispersions, eventually stabilized with appropriate additives.

[0040] In accordance with a preferred embodiment, carbonizable polymers with an average particle diameter in the range of from 50 to 300 nm, preferably of from 70 to 200 nm and particularly preferred in the range of from 100 to 180 nm are used. Especially preferred examples are vinylidene chloride polymers in the form of dispersions or latices. The average particle diameter is determined as described below for the nanoparticulate material capable of intercalating lithium ions.

[0041] Especially preferred are carbonizable polymers which upon pyrolysis yield so called hard carbon.

[0042] Hard carbon is generally available from precursors which char before they pyrolyze, i.e. which undergo decomposition before they melt. More generally hard carbon is defined as non-graphitizable carbon which does not yield graphite-like structures even at temperatures exceeding 2500 °C.

[0043] Different to so-called soft-carbons, the layers of carbon atoms in hard

carbons are not neatly stacked and hard carbons show a macroscopically isotropic behaviour. The spacing between different planes of carbon atoms is usually exceeding 0.38 nm, i.e. there is little bonding between different planes. According to the so-called Franklins theory, the interlayer spacing in graphitic structures is appr. 0.344 nm and thus the interlayer distance for hard carbons exceeds this value, indicating the substantial absence of interlayer bonding.

[0044] The material capable of intercalating lithium ions is not subject to particular restrictions and in principle any such material may be used in the process of the present invention. In case of the intended use for secondary energy storage devices, materials capable of intercalating high amounts of lithium ions (thus yielding good mass capacities) and a good cyclability are preferred.

[0045] The term intercalation, when used herein, is intended to designate the

reversible inclusion of a molecule in the interstitial space between the atomic layers of another material. Many layered solids intercalate guest molecules. Intercalation requires energy which is usually supplied by charge transfer processes between the guest and the host solid material. For use in secondary energy storage devices, in particular such devices based on lithium ions, lithium ions are intercalated, i.e. reversibly

incorporated into the anode and cathode materials during the charge and discharge process. The degree of reversibility determines the cyclability of the device, which is an important performance parameter of such devices.

[0046] The term nanoparticulate, as used herein, is intended to refer to small objects which have a weight average particle diameter (measured by dynamic light scattering) of usually less than 300 nm, in particular less than 270 nm in particularly preferred less than 250 nm. In certain cases particle diameters in the range of from 1 to 220 nm, preferably of from 5 to 200 nm and particularly from 10 to 190 nm have proved to provide excellent properties for use in the process of the present invention. The characterization by weight average particle diameter is best applicable to particles having spherical or similar shapes but for rod like materials or fibers or nanotubes, for which the size in one axis is significantly different from the size along another axis, the average diameter is not very suitable. In this case, non-spherical particles are deemed to be included in the term nanoparticulate if the largest size along one axis of the product does not exceed 300nm, preferably does not exceed 250 nm and in particular does not exceed 200 nm.

[0047] In the course of the present invention, it has been recognized that

sometimes bigger particles can hide smaller particles but generally the mean diameter of the particles is in the range of from 20 to 300 nm which could be seen in an SEM analysis performed on a Zeiss Supra 55 field emission gun scanning electron microscope at an accelerating voltage of 4 kV in the in-lens detection mode (contrast due mainly to the topography). The powder was fixed on a double-sided adhesive carbon tab.

[0048] For particle size determination the method as described in ISO Norm

Particles size analysis - Dynamic Light Scattering (DLS), ISO

22412:2008(E) is recommended to be followed. This norm provides i.a. for instructions relating to instrument location (section 8.1.), system

qualification (section 10), sample requirements,, measurement procedure (section 9 points 1 to 5 and 7) and repeatability (section 1 1 ). Measurement temperature is usually at 25 °C and the refractive indices and the viscosity coefficient of the respective dispersion medium used should be known with an accuracy of at least 0.1 %. After appropriate temperature equilibration the cell position should be adjusted for optimal scattered light signal according to the system software. Before starting the collection of the time autocorrelation function the time averaged intensity scattered by the sample is recorded 5 times. In order to eliminate possible signals of dust particles moving fortuitously through the measuring volume an intensity threshold of 1.10 times the average of the five measurements of the average scattered intensity may be set. The primary laser source attenuator is normally adjusted by the system software and preferably adjusted in the range of about 10,000 cps. Subsequent measurements of the time autocorrelation functions during which the average intensity threshold set as above is exceeded should be disregarded.

[0049] Usually a measurement consists of a suitable number of collections of the autocorrelation function (e.g. a set of 200 collections) of a typical duration of a few seconds each and accepted by the system in accordance with the threshold criterion explained above. Data analysis is then carried out on the whole set of recordings of the time autocorrelation function by use of the Contin algorithm available as a software package, which is normally included in the equipment manufacturer's software package.

[0050] Spherical particulate materials or materials resembling a shape similar to spherical are generally preferred as they can be best encapsulated in accordance with the process of the present invention.

[0051] In certain cases it has proved advantageous to use materials capable of intercalating lithium ions with a BET specific surface area of at most 120 m²/g, preferably at most 100 m²/g and most preferably in the range of from 10 to 80 m²/g.

[0052] BET specific surface area and porous volume were determined in

accordance with ISO 9277:2010 by gas adsorption on an ASAP2020 instrument from Micromeritics. Prior to the analysis the samples were pre- treated under vacuum at 1 10°C for 16h. The measurements were perfomed using nitrogen as adsorptive gas at 77K by volumetric method according to ISO 9277:2010 (Determination of the specific surface area of solids by gas adsorption - BET method) and ISO 15901 -3:2007 (pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption-Part 3: analysis of micropores by gas adsorption) standards. Annex C of ISO 9277:2010 standard describes a method which allows to define a range of relative pressures in which the BET theory is applicable for the calculation of the specific surface area of microporous samples This method requires drawing the graph of na(1 -P/Po) vs P/Po (with na the specific adsorbed quantity and P/Po the relative pressure). The application of the BET equation should be limited to the pressure range where the quantity na(1 -P/Po) continuously increases with P/Po. The maximum P/Po value is thus the relative pressure for which the quantity na(1 -P/Po) is maximum. Five points equal and lower than the maximum P/Po value have been chosen to apply the BET theory. The pore volume was calculated from the amount of nitrogen adsorbed at P/Po close to unity or before the liquid condensation (at the plateau).

[0053] A variety of materials capable of intercalating lithium ions has been

described in the literature, Si, metals, metal alloys, metal oxides or metal salts may be mentioned here as preferred materials.

[0054] Just by way of example lithium cobalt oxides, lithium iron phosphates, lithium manganese oxides, cobalt oxide, titanium dioxide, lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium titanate, alloys of antimony and tin, molybdenum disulfide, lithium vanadium oxides, silicon/titanium dioxide composites, cobalt oxide, metal hydrides, copper antimonides, boron-doped silicon nanoparticles, iron oxides and nanoparticulate metal phosphates may be mentioned here.

[0055] In terms of theoretical capacity, Si offers one of the highest values with a theoretical capacity of 4200 mAh/g, which is higher than most other materials. Nanoparticulate Si can intercalate up to 4.4 lithium ions per Si, leading to a product with the stoichiometric formula Li_{4 4}Si.

[0056] Accordingly, silicon is a preferred material capable of intercalating Li ions in the process of the present invention.

[0057] In a number of cases it has found advantageous to use Si nanoparticles having a low content of impurities as it has been found that sometimes impurities can have a disadvantageous influence of the morphology and the performances of the products obtained after pyrolysis.

[0058] Suitable materials capable of intercalating lithium ions suitable for use in the process of the present invention are known to the skilled person and have been described in the literature. A number of suitable materials are also commercially available from different suppliers.

[0059] Si as preferred nanoparticulate material can also be obtained from the saw dust of the manufacture of silicon wafers, which may be advantageous in term of economy.

[0060] In the following the process of the present invention is described in more detail referring to Si as material capable of intercalating lithium ions but it is apparent to the skilled man that Si can be replaced by other materials capable of intercalating lithium ions and the skilled person will adjust the conditions of the process depending on the properties of the material capable of intercalating lithium ions

[0061] In step a) of the process of the present invention, two dispersions, one of them comprising the carbonizable polymer and the other comprising the nanoparticulate silicon are mixed.

[0062] The content of the carbonizable polymer or material capable of

intercalating lithium ions is not subject to particular restrictions and can vary over a wide range. The ratio of carbonizable polymer to material capable of intercalating lithium ions is preferably chosen so that in the final composite the maximum amount of material capable of intercalating lithium is obtained which can be embedded in the carbon matrix without a substantial amount of said material being on the outside of the material when used as electrode in a battery. As described hereinabove, it is advantageous if the carbon matrix substantially entirely encapsulates and/or coats the material capable of intercalating lithium ions as this is beneficial to limit or reduce the detrimental effect of solid electrolyte interfaces (SEI).

[0063] Taking this into account, the skilled person will use the two dispersions in step a) of the process of the present invention in a ratio ensuring that the material capable of intercalating lithium ions is substantially entirely anchored to, encapsulated by or coated with the carbon matrix material. Whereas it is not mandatory that the entire nanoparticulate material capable of intercalating lithium ions in the final product is embedded in or coated with carbon, it is advantageous if at least a certain percentage thereof is. It is possible that a certain, preferably limited amount is anchored to carbon but may be present at the surface. Albeit this part of the nanoparticulate material may be exposed to SEI formation, the performance of the products can still be satisfactory.

[0064] In case of vinyl chloride or vinylidene chloride polymers, copolymers or mixtures latex dispersions comprising the polymers in various

concentrations are commercially available from a number of suppliers. These latex dispersions may or may not comprise additional ingredients to stabilize the dispersion. Any of these products may be used as starting material in the process of the present invention.

[0065] Dispersions of nanoparticulate Si are also available or can be easily

prepared by dispersing suitable amounts of solid Si material in water or an appropriate solvent mixture comprising water as major solvent component. Sometimes, agglomeration appears upon dispersing the nanoparticulate material and in this case it is advantageous to subject the dispersion to a disagglomeration treatment before mixing with the dispersion comprising the carbonizable polymer. Ultrasonication or turbulent mixing in so called ultra-mixers may be mentioned here for this purpose. In some cases ultra- mixing with milling balls has shown to be particularly effective to

disagglomerate the product and to reduce particle size which is generally preferable.

[0066] After mixing the two dispersions, the mixture is subjected to co- coagulation. In this step a solid nano-composite is formed.

[0067] Two different mechanisms have shown certain advantages in the

coagulation step. Depending on its nature, the coagulation can be configured by either coagulating agent which preferably interacts with surfactants which might be present to stabilize the dispersion thereby inducing the neutralization and precipitation of same or it can be triggered by an electrolyte which modifies the ionic strength of the suspension. In both cases, the colloidal stability is reduced and the coagulation is triggered.

[0068] Metal salts, such as potassium chloride, sodium chloride, calcium

dichloride and calcium sulfate, have notably been described in the literature as suitable electrolytes for coagulation of dispersions. There is no specific restriction, however, as far as the nature and chemical composition of the coagulating agent is concerned and the skilled person may select any suitable compound he is aware of for this purpose.

[0069] In certain cases, especially for PVDC and related polymers, alkaline earth metal chlorides and in particular calcium dichloride have proven to be advantageous. In particular calcium dichloride has proved to be suitable to coagulate aqueous PVDC latices as well as aqueous dispersions of silicon nanoparticles. The concentration of the coagulation agent in case of calcium dichloride usually is the in the range of from 0.001 to 5 M, preferably of from 0.005 to 1 M and particularly preferred from 0.01 to 0.5 M.

[0070] After the addition of the coagulating agent, the coagulum is filtrated. In some cases it has proved to be advantageous to wash the coagulum one or several times with water to remove remaining amounts of the

coagulating agent which might have a detrimental influence in the subsequent step.

[0071] Thereafter the coagulum is dried, which may be carried out at room

temperature or at elevated temperature if an acceleration of the drying is desirable. In this case the temperature used in the drying step can be below the onset temperature of carbonization of the carbonizable polymer. Drying at room temperature is preferable in some cases as it is the mildest form of drying enabling evaporation of the water smoothly with a low risk of affecting the composite structure during the drying.

[0072] It is principally also possible to carry out the drying of step b) and the step c continuously without interruption, i.e. the drying may be effected in the course of heating the coagulum to the carbonization temperature so that the final drying part of step b and step c of the process of the present invention basically merge into one continuous step.

[0073] Due to the increased risk of detrimentally influencing the composite

structure, however, it is generally preferred to dry the coagulum at a temperature of 60°C or less, preferably 40 °C or less for a period of time of 1 h to 7 days, preferably of from 12 h min to 96 hours to remove the water.

[0074] After step b, the product is subjected to a thermal treatment at a temperature in the range of form 300 to 1000 °C, preferably of from 400 to 900°C, more preferably in the range of from 450 to 700°C for 10 min to 5 hours, more preferably from 30 min to 3 hours and even more preferably from 45 min to 2 hours to carbonize the polymer and to obtain a porous composite material comprising nanoparticles capable of intercalating lithium ions embedded in a matrix of conductive porous carbon.

[0075] The carbonization is preferably carried out in an inert atmosphere, in

particular under nitrogen or a noble gas like Ar in order to prevent oxidation reactions as far as possible.

[0076] Suitable devices for such carbonization reactions are known to the skilled person; just by way of example, muffle furnaces (provided same can be inertized) or tubular furnaces may be mentioned here, tubular furnaces being generally preferred because they can be better inertized. It has also been found that the type of device used for carbonization may have an influence on the morphology of the final product.

[0077] As a result of the process of the present invention, the nanoparticulate material capable of intercalating lithium ions is substantially anchored to and/or embedded in and surrounded by porous carbon, preferably conductive porous carbon. The term embedded in and surrounded by for the purpose of the present invention is intended to denote that the nanoparticulate material capable of intercalating lithium ions is embedded in carbon capsules and/or coated with carbon. The carbon capsules in which the material is embedded provide free volume allowing for the size expansion of the Si in the course of the intercalation thereby eliminating or at least substantially reducing the risk of fracture or otherwise deterioration of the surface of the material during operation of the storage device if the material obtained in accordance with the process of the present invention is used as electrode material in secondary energy storage devices. This significantly improves cyclability, i.e. the number of charge/discharge cycles available without significant deterioration of the capacity of the device.

[0078] The coating by the surrounding carbon molecules also reduces the risk of the formation of a detrimental solid electrolyte interface as there is substantially no direct contact between the material capable of

intercalating the lithium ions and the electrolyte anymore.

[0079] As mentioned before it is possible that a certain part of the

nanoparticulate material is anchored to porous carbon particles, i.e. may be exposed at the surface of the final product.

[0080] The carbon obtained by the carbonization process, in particular from the carbonization of the preferred vinyl chloride or vinylidene chloride polymers has a porous structure with the pores being available for embedding the nanoparticulate materials. Since no tar or small organic compounds are liberated in the course of carbonization for these polymers, there is no risk of the pores being occupied, which is one of the reasons why vinyl chloride or vinylidene chloride polymers are preferred in the process of the present invention.

[0081] The BET specific surface area of the composite material obtained in

accordance with the process of the present invention is usually in the range of from 100 to 1500 m²/g, preferably in the range of form 250 to 1000 m²/g. The specific surface area is determined in accordance with ISO norm 9277:2010 as described above.

[0082] Vinylidene chloride has proved to be a particularly suitable polymer as it starts carbonization at low temperature which carbonization can be put to completion at relatively low temperatures of e.g. from 550 to 650 °C.

[0083] In accordance with a preferred embodiment of the present invention using vinylidene chloride polymers, the carbonization is carried out by heating the product to be carbonized to a temperature of 600 °C and keeping it at such temperature for a period of 30 min to 2 hours. The degree of completion of the carbonization can be monitored via the carbon residue. PVDC has a carbon content of 24.7 % and thus the carbon content of the pyrolyzed product provides information on the degree of carbonization.

[0084] The nanoparticulate material capable of intercalating lithium ions is not normally affected by the temperatures of the pyrolysis; however, in case of Si it has been observed that a reaction might take place between the Si and the hydrochloric acid liberated in the course of the carbonisation. Depending on the time and temperature of the pyrolysis a more or less significant part of the Si may be converted to Si halides which are not useful as intercalating materials. This can be taken into account by increasing the amount of nanoparticles dispersion mixed with the dispersion of the polymerizable polymer. Increasing this ratio enables to obtain final products with the desired Si content (which should be as high as possible to maximize the capacity of the material).

[0085] Experiments carried out at different temperatures and different

carbonization temperatures have shown that 40 to 75 % of the Si might react with the hydrogen chloride eliminated so that the initial amount of Si introduced has to be increased respectively.

[0086] An initial weight ratio of PVDC (carbon content 24.7 wt%) to Si of 95:5 should yield a final product comprising approximately 20 % of Si (if no Si would be used by reaction with hydrogen halide). Measured contents of Si in the final product for such starting compositions were appr.9 wt% so that appr. 55wt% of the Si reacted with the hydrogen chloride during the carbonization.

[0087] The skilled person can easily adjust the carbonization conditions and the weight ratio of the two dispersions used as starting materials to adjust a desired content of Si in the final product.

[0088] In certain cases it has been found advantageous if the product obtained after step c) is subjected to a grinding or milling operation to modify morphology, if desirable.

[0089] The composite materials obtainable by the process of the present

invention comprising nanoparticulate materials capable of intercalating lithium ions in a matrix of porous carbon are useful for the manufacture of anode structures in secondary energy storage devices, in particular secondary lithium batteries.

[0090] As well known to the skilled person, the terms "secondary batteries" are commonly used to denote rechargeable batteries.

[0091] Accordingly, other aspects of the present invention relate to anode

structures for secondary energy storage devices comprising a composite material obtained or obtainable by the process as hereinbefore described and to secondary energy storage devices, in particular lithium ion batteries comprising such anode structures.

[0092] Secondary energy storage devices and in particular secondary lithiunn ion batteries and the components thereof have been described in great detail in the literature so that no further details need to be given here.

[0093] The process of the present invention yields composite materials which are particularly suitable for anode structures in secondary energy storage devices. Compared to the existing products these materials show better cyclability and stability of the electrode structures comprising the materials.

[0094] Another embodiment of the present invention relates to composite

materials comprising a nanoparticulate material capable of intercalating lithium ions having an average particle diameter in the range of from 1 to 300 nm and a BET specific surface area of less than 120 m²/g, which nanoparticulate material is embedded in and/or anchored to a matrix of hard carbon as described hereinbefore.

[0095] Especially preferably the average diameter of the nanoparticulate material is in the range of from 5 to 200 nm, even more preferred in the range of from 20 to 170 nm.

[0096] The BET specific surface area of the nanoparticulate material capable of intercalating lithium ions preferably is less than 100 m²/g and especially preferred in the range of from 20 to 80 m²/g.

[0097] Suitable nanoparticulate materials capable of intercalating lithium ions as well as suitable carbonizable precursors have been described

hereinbefore in connection with the process of the present invention and reference is made thereto for further details here.

[0098] Preferred materials are the same described above as preferred materials for the process of the present invention.

[0099] In some cases it has been found to be advantageous if at least a part of the composite material has a non-spherical, preferably a flaky shape.

Flaky in this regard means that the particles have an extension in one dimension which is significantly different from the extension in the two other dimensions, i.e. have an aspect ratio significantly differing from 1 (which is the aspect ratio of an ideal spherical particle. Aspect ratios in the range of from 10 to 10 000, preferably of from 20 to 5 000 have been found useful in this regard.

[00100] If the product obtained after pyrolysis has a substantially spherical shape, a grinding or milling step may be performed to modify the particle shape in the desired manner.

[00101] The flaky form of the final product does not exclude that the

nanoparticulate material capable of intercalating lithium ions still is substantially spherical in shape. In fact, the composite material can have a similar structure as the seeds in pulp of a kiwi fruit where the pulp is represented by the hard carbon and the seeds are nanoparticles capable of intercalating lithium ions.

[00102] The pore volume of the composite material in accordance with the present invention preferably in the range of from 0,10 to 1 cm³/g, preferably of from

0.25 to 0.50 cm³/g.

[00103] The specific surface area (BET) as described above is preferably in the range of from 20 to 1000 m²/g, preferably of from 50 to 950 m²/g.

[00104] The composite materials in accordance with the present invention can preferably be obtained in accordance with the process of the present invention described hereinbefore.

[00105] The following example shows a preferred embodiment of the present invention.

[00106] EXAMPLE

[00107] Manufacture of a composite material in accordance with the invention

[00108] 10 g of a poly vinylidene chloride latex available under the tradename Diofan® A736 and having a solids content of 60 wt% was mixed with 237.1 mL of deionised water (the average particle diameter of the PVDC particles was 150 nm, determined as described before).

[00109] Nanosized silicon with an average particle diameter of 190 nm and a Si content of 0,67% was used as second dispersion.

[001 10] The dispersion of the poly vinylidene chloride was mixed with 78.125 mL of the silicon suspension.

[001 1 1] The mixture thus obtained was added dropwise into a stirred solution of 5.55 grams of calcium dichloride in 500 ml of deionised water using a dropping funnel at room temperature. [001 12] The coagulum was filtrated under vacuum and washed two times with deionised water. Thereafter the composite was dried for three days (72 h) at room temperature to avoid PVDC degradation.

[001 13] The dried product was transferred to a tubular furnace heated to a

temperature of 600 °C at 10°C/min and pyrolyzed under Ar for 1 h at

600°C.

[001 14] The pyrolysis yield was 27.7 wt%, indicating a quantitative carbonization and the Si content of the product was determined to 8.6 wt%. Chlorine content was 0.1 wt% and oxygen content 2.3 wt%.

[001 15] The TGA curve showed that the carbonization took place in several steps which could be assigned to water evaporation (at appr. 200°C), and HCI and CO₂ elimination (at 260 to 450 °C, driven to completion at 600°C).

[001 16] The SEM analysis showed that the Si nanoparticles are anchored and/or embedded within the carbon.

[001 17] The specific surface area and the pore volume in accordance with the method described by Brunauer, Emmett and Teller (BET) and determined in accordance with ISO norm 9277:2010 were calculated to 821 m²/g and 0.33 cm³/g.

[001 18] Manufacture of hard carbon (for comparison)

[001 19] 10 g of the PVDC latex Diofan® A736 having a solids content of 60 wt% which was used for the manufacture of the above composite (the average particle diameter of the PVDC particles was 150 nm, determined as described before) were added dropwise into a stirred solution of 5.55 grams of calcium dichloride in 500 ml of deionised water using a dropping funnel at room temperature.

[00120] The PVDC coagulum was filtrated under vacuum and washed two times with deionised water. Thereafter the PVDC was dried for three days (72 h) at room temperature to avoid PVDC degradation.

[00121] The dried product was transferred to a tubular furnace heated to a

temperature of 600 °C at 10°C/min and pyrolyzed under Ar for 1 h at 600°C.

[00122] The pyrolysis yield was 25 wt%, indicating a quantitative carbonization.

Chlorine content was 0.6 wt% and oxygen content 3.3 wt%. [00123] The TGA curve showed likewise that the carbonization took place in several steps which could be assigned to water evaporation (at appr.

200°C), and HCI and CO₂ elimination (at 260 to 450 °C, driven to completion at 600°C).

[00124] The specific surface area and the pore volume in accordance with the

method described by Brunauer, Emmett and Teller (BET) and determined in accordance with ISO norm 9277:2010 were calculated to 907 m²/g and 0.35 cm³/g.

[00125] Manufacture of electrodes

[00126] Electrodes were manufactured, having the following composition:

[00127] 80 wt% of an active material : either the composite material as above examplified (according to the invention) or the hard carbon as above examplified (for comparison);

[00128] 12 wt% of Super-P carbon black commercially available from Imerys S.A.; and

[00129] 8 wt% of a binder composed of sodium carboxymethyl cellulose (CMC) having Mw=250000 (commercially available from Sigma-Aldrich Corporation) and of polyacrylic acid (PAA) having Mw=100000 (also commercially available from Sigma-Aldrich Corporation) in a weight ratio CMCPAA of 81.25: 18.75.

[00130] The electrodes were prepared using the following equipment:

[00131] mechanical mixer: mechanical mixer of the Dispermat series with Stainless-steel lightweight dispersion impeller (for good mixing dispersion state),

[00132] film coater/Doctor Blade: Elcometer 4340 Motorised / Automatic Film Applicator,

[00133] vacuum oven: vacuum drying oven - BINDER APT line VD 53 with vacuum, and

[00134] roll press: Precision 4" Hot Rolling Press/Calender up to 125°C.

[00135] 1.3 g of active material, 2.64 g of CMC (4 wt. % in water), 0.070 g of PAA (Mw 250k, 35 wt% in water) and 0.2 g of Super-P were mixed in a glass Becker. The solid content in the slurry was adjusted to 17% with 5.4 g of a 1/4 ethanol/water solution and then mixed with a mechanical mixer at 1000 rpm for 30 minutes.

[00136] The slurry was then laminated at 200 μηη with a film coater on a copper current collector (10 μηη thick) and the resulting electrode was dried at 80°C under vacuum (about 1 hour) in a vacuum drying oven. Hot rolling calender were used at 90°C to reduce the electrode porosity to 40-50%.

[00137] Electrochemical tests

[00138] The tests made it possible to demonstrate silicon contribution to total active material specific capacity in silicon-hard carbon matrix for LIB anodes.

[00139] Two types of tests were made:

[00140] cycling test in Half Cell (galvanostatic cycling with potential limitation, hereinafter GCPL), and

[00141] cycling test in half cell (cyclic voltammetries, hereiafter CV).

[00142] As equipment for the GCPL and CV tests, a battery test station Bio-Logic

VMP3 was used.

[00143] Cell configuration was as detailed below:

[00144] half cell with Li,

[00145] geometry: coin cell (button cell) CR2032,

[00146] separator: glass microfiber (about 230 μηη) (commercially available from

Whatman), and

[00147] electrolyte: LiPF6 1 M EC:DMC 1 : 1.

[00148] The tests were made at room temperature (23°C). Other testing conditions are detailed in tables 1 and 2 here below.

[00149] Table 1 - GCPL Cycling schedule:

[00150] Table 2 - CV Cycling schedule

Negative cut positive cut off

# cycles Scan rate (mV/s) off voltage voltage

50 0.5 0.01 1 [00151] Si/HC vs. HC GCPL

[00152] Cycling schedule is reported in table 1.

[00153] We observed that Si-HC had a greater discharge capacity (figure 1 ) with respect to HC only and we concluded that this was due to silicon contribution.

[00154] Besides, when we looked at the charge and discharge profile (figure 2), we did not observe big differences between Si-HC and HC. However, when first derivative was applied to the data of figure 2 and we plotted the charge variation (dq) vs. voltage (figure 3), some differences (peaks) appeared at voltages related to the silicon-lithium alloy formation. This is a direct proof of silicon contribution to total delithiation capacity.

[00155] Si/HC vs. HC Cyclic voltammetries

[00156] Cycling schedule is reported in table 2.

[00157] To have a deeper insight in the contribution of the silicon to total capacity, we recorded cyclic voltammetries of Hard Carbon and Silicon-Hard carbon electrodes. As can be clearly seen in figure 4, the electrodes that contained silicon-hard carbon composites exhibited a higher charge transfer capability and peaks related to silicon-lithium alloy formation could be easily identified.

[00158] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Claims

1. A process for the manufacture of composite materials comprising nanoparticulate materials capable of intercalating lithium ions in a matrix of porous carbon materials which comprises

a) a step of mixing an aqueous dispersion of a carbonizable polymer with an aqueous dispersion of a nanoparticulate material capable of intercalating lithium ions,

b) a subsequent step wherein the combined dispersions are co-coagulated, and the coagulate is filtrated and dried and thereafter

c) the product from step b) is subjected to a thermal treatment at a temperature in the range of from 300 to 1000°C for a time period of from 10 minutes to 5 hours thereby obtaining a porous composite material comprising nanoparticles capable of intercalating lithium ions in a matrix of porous carbon.

2. The process of claim 1 wherein Si, metals, metal alloys, metal oxides or metal salts are used as materials capable of intercalating lithium ions.

3. The process of claim 2 wherein Si is used as material capable of intercalating lithium ions.

4. The process of any of claims 1 to 3 wherein the carbonizable polymer is selected from polyvinyl chloride, poly vinylidene chloride and mixtures and copolymers thereof.

5. The process of any of claims 1 to 3 wherein the temperature in step c) is in the range of from 400 to 900°C,

6. The process of any of claims 1 to 4 wherein the temperature in step c) is in the range of from 450 to 700 °C,

7. The process of any of claims 1 to 6 wherein the thermal treatment is applied for a time period of from 30 minutes to 3 hours.

8. The process of any of claims 1 to 7 wherein the carbonizable polymer in the aqueous dispersion has an average particle diameter in the range of from 50 to 300 nm, preferably of from 70 to 200 nm and particularly preferred in the range of from 100 to 180 nm.

9. Composite material comprising nanoparticulate materials capable of intercalating lithium ions in a matrix of porous carbon materials obtainable by the process in accordance with any of claims 1 to 7.

10. Composite material comprising a nanoparticulate material capable of intercalating lithium ions having an average particle diameter in the range of from 1 to 300 nm and a BET specific surface area of less than 120 m²/g, which nanoparticulate material is anchored to and/or embedded in a matrix of hard carbon.

1 1. Composite material in accordance with any of claims 9 or 10 wherein the BET specific surface area of the nanoparticulate material capable of intercalating lithium ions is in the range of from 20 to 80 m²/g.

12. Composite material in accordance with any of claims 9 to 1 1 wherein the particles have a substantially non-spherical shape.

13. Use of the composite material in accordance with any of claims 9 to12 for the manufacture of anode structures in secondary energy storage devices, in particular secondary lithium batteries.

14. Anode structure for secondary energy storage devices comprising a composite material as claimed in any of claims 9 to 12.

15. Secondary energy storage device comprising an anode comprising a composite material as claimed in any of claims 9 to 12.