Patent Application
20110297889
The invention relates to a process for manufacturing a composite comprising a fibrillar carbon-based material and tin oxide. The expression “fibrillar carbon-based material” is understood to mean carbon nanotubes CNTs or carbon nanofibers CNFs or a mixture of the two. The invention also relates to electrodes consisting of said composite and to lithium batteries comprising such electrodes.
The invention applies to the field of the storage of electrical energy in batteries and more particularly in secondary lithium batteries of Li-ion type.
Tin, like silicon, is capable of forming alloys with lithium and of making it possible to obtain capacities that are substantially greater than those that are achieved with graphite.
It is known that the main difficulty in developing these materials lies in the high volume expansion during lithiation, which gives rise to losses of cohesion of the electrodes and losses of electrical contact resulting in a significant reduction in performance.
The increase in portable electronic products has contributed to the growing expansion of the market for batteries, and more particularly for lithium-ion batteries.
Indeed, from several hundreds of thousands in 1995, the worldwide production of lithium-ion batteries reached 500 million in 2000 (compared with 1300 million Ni-MH batteries) then 1700 million in 2005. In 2006, Japan, the leading worldwide producer, itself produced more than 1200 million lithium-ion batteries per annum (ITE Express News, (2005)).
Since the emergence of lithium and lithium-ion batteries, several generations of positive and negative electrodes have successively appeared. In the case of negative electrodes, the most widely used material is carbon graphite due to a long service life owing to the formation of a protective layer during the first cycles of cycling. The reversible capacity of such an anode is 372 mAh/g.
To improve this value, several studies are currently being carried out on various materials such as carbon having a high aspect ratio (e.g. carbon nanotubes) or else metals that can form alloys with lithium (silicon, antimony, tin, etc.).
Some metal oxides may also be used as negative electrode for Li-ion batteries (SiO2, SnO, SnO2, etc.). These materials have a capacity considerably greater than that of carbon graphite, but their service life is very limited due to the change in volume in the course of cycling during the alloying reaction. To overcome this problem, several ideas have been put forward, such as the use of particles of nanoscale size or else the development of carbon/tin or carbon/tin oxide composites.
In this regard, J. Xie et al., published an article entitled: “Synthesis and Characterization of High surface area tin oxide/functionalized carbon nanotubes composites as anode materials”, in Materials Chemistry and Physics, 91, (2005), 274-280. This article presents a synthesis pathway in which tin tetrachloride in the presence of carbon nanotubes that are previously surface-oxidized with permanganate, is reduced in the aqueous phase using urea. The suspension containing the nanotubes is stirred ultrasonically. The urea is then added. Several heat treatments follow one another, the last of which consists in placing the material in a furnace at 600° C. The resulting composites show the presence of small particles deposited on the nanotubes, having a size between 10 and 20 nm. A large number of these particles is not on the surface of the nanotubes, but is in the form of aggregates and is consequently not very effective. Furthermore, the use as a negative electrode material is only suggested with no experimental proof making it possible to attest to the results produced in terms of cycling performance and capacity.
Another article by L. Yuan et al., entitled “Nano-structured SnO2-carbon composite obtained by in-situ spray pyrolysis method as anodes in lithium batteries”, J. of Power Sources, 146, (2005), 180-184, describes the preparation of SnO2 nanoparticles of 5-15 nm, distributed in a carbon-based matrix synthesized by pyrolysis of a spray-dried solution of sucrose of SnCl2.
Contrary to the preceding example, electrochemistry experiments were carried out. The initial discharge capacity is 600 mAh/g, which shows that there is a high irreversibility at the start.
However, the plot of discharge capacity as a function of the number of cycles shows a decrease that is even less pronounced when the carbon/carbon+SnO2 ratio is high. Correlatively, this also means that the electrode has an overall lower capacity.
This solution is not suitable since it does not make it possible to combine both a good cycling performance and a high capacity.
Other publications in which SnO2 and CNT are combined have been described:
The publication by Zhenhai Wen, et al., entitled “In Situ Growth of Mesoporous SnO2 on Multiwalled Carbon Nanotubes: A Novel Composite with Porous-Tube Structure as Anode for Lithium Batteries” Adv. Funct. Mater. 2007, 17, 2772-2778, describes a method for the in situ preparation of CNT/SnO2 composites via a hydrothermal route. The capacity obtained is 350 mAh/g, for 50 cycles.
The publication by Guimin An et al., entitled “SnO2/carbon nanotube nanocomposites synthesized in supercritical fluids: highly efficient materials for use as a chemical sensor and as the anode of a Lithium-ion Battery”, Nanotechnology 18 (2007) 435707, describes composites prepared via a hydrothermal route. These materials comprise, by weight, 40% of SnO2 and 60% of CNTs. The capacity obtained after 30 cycles does not exceed 400 mAh/g.
It is understood that in all these publications, the authors always express the capacity relative to the tin contained in the composite, however, it is the capacity of the electrode which matters in the battery application.
Thus, the current prior art makes it possible to observe that most of the studies do not correspond to the technical problem of developing a process for manufacturing a composite based on SnO2 and on CNTs and more generally on a composite based on SnO2 and on a simple fibrillar carbon-based material, resulting in a composite that has good electrochemical properties.
Indeed, besides the complexity of the processes described, it emerges that for the SnO2/CNT composites proposed, the cycling improves when the proportion of active compound, in this case tin oxide, decreases in the composite, this being accompanied by a reduction in the absolute capacity of the composite.
Reference can also be made to the prior art consisting of documents D1, D2 and D3 below:
Thus, document D1 relates to a process for depositing particles of tin oxide on carbon fibers. It does not describe a process for depositing tin oxide on carbon nanofibers, or on carbon nanotubes. The carbon fibers have a diameter around 10 micrometers. The layer of SnO2 deposited on the surface of a fiber, according to this document, preferably has a thickness of 250 nm.
According to embodiments of the present invention, the carbon-based material consists of CNTs or of CNFs or of a mixture of CNTs and CNFs. The diameters of the CNTs and of the CNFs are not comparable to those of the fibers described in D1 since it is a question of nanometers and not of micrometers. Indeed, for example, at most a diameter of 2.2-2.3 nm is achieved for the singular walled CNTs. The multi-walled CNTs have, for example, an external diameter ranging from 3 to 50 nm and the CNFs have, for example, diameters of 50 to 200 nm.
Furthermore, according to embodiments of the present invention, use is preferably made of multi-walled CNTs having an external diameter ranging from 3 to 50 nm, preferably from 5 to 30 nm and better still from 8 to 20 nm. Indeed, the applicant has observed that the use of multi-walled CNTs makes it possible to obtain a conductivity that is higher, more homogeneous and more stable.
On the contrary, in embodiments of the present invention, the process comprises a nucleation/crystallization phase, which is a physical step since it corresponds to a step of drying then of heat treatment. The drying step leads to an evaporation of the reaction medium (namely water) and therefore physical precipitation. One of the advantages of this physical nucleation step is its ease of industrial implementation (use of a simple evaporator or furnace) and its lack of production of liquid effluent (except the water of the reaction medium); which industrially is advantageous since this leads to less retreatment of the effluents.
Furthermore, the problem that it has been sought to solve in document D1 (WEI et al.), is not the same as that of embodiments of the present invention. Indeed, the problem in D1 is that of obtaining a material having good optical and thermal performances.
In embodiments of the present invention, an exemplary problem solved is the production of a composite comprising a fibrillar carbon-based material (CNT and/or CNF) and tin oxide having a good electronic conductivity, a moderate volume expansion during electrochemical cycling and also a good reversible capacity. In particular, an embodiment of the composite has, in the galvanostatic cycling, a capacity of greater than 600 mAh/g after 60 cycles enabling the production of electrodes.
Document D2 is a publication from 2 Jun. 2008 by Yu-Jin CHEN et al., entitled “High capacity and excellent cycling stability of single-walled carbon nanotubes/SnO2 core-shell structures as Li-insertion material”. The composite described in this document consists of single-walled nanotubes (SWNTs) and of SnO2. Document D2 specifies that the initial discharge capacity of the “core-shell” structures is greater than 1399 mAh/g and that the reversible capacities of these structures are stabilized at around 900 mAh/g after 100 cycles. The document also specifies that the diameter of the tin particles deposited at the surface of the nanotubes is around 2 nm and that the length of the single-walled carbon nanotubes (SWNTs) is around 20 micrometers. Thus, the SWNTs/SnO2 structures have a very large surface area and a very large length/diameter ratio resulting in their high capacity. Indeed, in this case, the reversible capacity of the core/shell structures of nanotubes covered with tin oxide is high.
In D2 (Yu-Jin Chen), the SnO2 content is not taught; it is not therefore possible to compare the charge/discharge results given with those of embodiments of the present disclosure.
According to embodiments of the present invention, the results are given relative to the SnO2/CNT composite which contains around 71% by weight of SnO2.
The nanotubes are then rinsed with distilled water. 1 g of tin chloride is put into a container containing 40 ml of distilled water, then 0.7 ml of 38% hydrochloric acid is added. 10 mg of previously cleaned single-walled carbon nanotubes are put into the prepared solution. Ultrasonic waves are applied to the solution for 3 to 5 minutes, then it is mixed for 30 to 60 minutes at ambient temperature.
The nanotubes thus treated, are rinsed with distilled water. Then these carbon nanotubes covered with tin oxide are filtered.
The applicant has reproduced the experimental conditions described in this document. The plot of discharge capacity as a function of the number of cycles, obtained under these conditions, is illustrated in
The prior art which has just been described moreover consists of theoretical articles with no industrial vision and in particular with no vision of an industrial implementation process.
An exemplary problem that the applicant has sought to solve by embodiments of the present invention is to propose a process for manufacturing a composite comprising a carbon-based fibrillar material and tin oxide without the drawbacks of the deposition processes that have just been described.
The fibrillar carbon/tin oxide composites thus produced according to embodiments of the present invention exhibit good electronic conductivity, moderate volume expansion during electrochemical cycling and also good reversible capacity.
The applicant proposes a process that makes it possible to control the effects of volume expansion during cycling in order not to give rise to excessively high losses in performance.
Furthermore, the process proposed is simple to implement since it requires temperature conditions that are not very high and atmospheric pressure conditions in order to anchor the particles of tin oxide on the surfaces of the carbon-based fibrillar material. This process is more effective than the solutions known to date since the composite obtained has a charge capacity and discharge capacity after several cycles greater than that of composites made of carbon nanotubes and tin oxide from the prior art.
Moreover, the process does not require any technique liable to impair the performances of the fibrillar carbon-based material used, as is the case in the techniques that use ultrasonic waves. The process makes it possible to use a fibrillar carbon-based material such as carbon nanotubes but also carbon fibers or a mixture of carbon nanotubes and carbon nanofibers.
One subject of the present invention is more particularly a process for manufacturing a composite comprising particles of tin oxide and a fibrillar carbon-based material, mainly characterized in that it comprises a synthesis by precipitation/nucleation in a water-alcohol medium of particles of tin hydroxide resulting from a tin salt in the presence of the fibrillar carbon-based material and an acid, in that the fibrillar carbon-based material consists of carbon nanotubes or carbon nanofibers or a mixture of carbon nanotubes and carbon nanofibers and in that the synthesis comprises a dissolving/contacting phase carried out at ambient temperature and at atmospheric pressure, then a nucleation/crystallization phase carried out at a temperature above ambient temperature and finally a heat treatment phase.
In the dissolving/contacting phase, a) the tin salt is dissolved in a water, alcohol and acid mixture and stirred, water is added while maintaining the stirring, b) the fibrillar carbon-based material is added and the mixture is stirred; it being possible for steps a) and b) to be carried out in this order or in the reverse order.
The nucleation/crystallization phase comprises an evaporation to dryness. Specifically, the drying consists in bringing the reaction mixture to a temperature above ambient temperature (typically 25° C. under 1 atm) but below the boiling point of the mixture (typically below 100° C.)
This evaporation to dryness is, for example, carried out at a temperature between 25 and 80° C. or better still from 40° C. to 70° C.
The heat treatment phase consists of heating the product obtained at a temperature much higher than the boiling point of the reaction mixture. This heat treatment phase is carried out in a furnace, under nitrogen or in air, for about ten minutes, at a temperature between 300° C. and 500° C.
The drying ensures the nucleation, whereas the heat treatment instead ensures the crystallization.
The nucleation is carried out according to an embodiment of the invention via a physical step.
The fibrillar carbon-based material can be added during the dissolving/contacting phase in the form of powder or as a prior predispersion.
The prior predispersion may be carried out by milling in water of planetary ball milling type or equivalent.
In the case where the fibrillar carbon-based material is added in the form of powder, the stirring is a vigorous stirring, which may be identical to that which is carried out in the case of a predispersion. This vigorous stirring makes it possible to break up the aggregates and to increase the density of the material.
In the other cases of stirring, the stirring may be carried out by means of a blade (non-vigorous stirring).
According to another feature of an embodiment of the invention, the fibrillar carbon-based material consists of carbon nanotubes or carbon nanofibers or a mixture of carbon nanotubes and carbon nanofibers.
The expression “carbon nanotubes” is understood to mean hollow tubes having one or more concentric graphite plane walls with an external diameter of 2 to 50 nm. The expression “carbon nanofibers” is understood to mean solid fibers of graphitic carbon having a diameter of 50 to 200 nm, but which may often have a thin hollow central channel. For both nanotubes and nanofibers, the length/diameter ratio is much greater than 1, typically greater than 100.
The applicant has observed that it is preferable, in order to obtain the best results, to treat the fibrillar carbon-based material after manufacture (synthesis). This material is treated so as to remove the catalytic residues present. Thus, the tin oxide particles adhere better to the surfaces. This purification treatment consists in carrying out an oxidation that enables the fibrillar carbon-based material to exhibit polar surface functional groups of OH and/or COOH type.
The purification is obtained, for example, by means of a strong mineral acid such as HNO3 or H2SO4.
The acid treatment is followed by a surface oxidation operation using sodium hypochlorite (NaOCl) or aqueous hydrogen peroxide solution (H2O2) or ozone (O3) when the acid chosen for purifying is not sufficiently oxidizing (for example H2SO4).
Embodiments of the invention also relate to the composite obtained by the process as described, the composite mainly being characterized in that it consists of a homogeneous distribution of tin particles over the surfaces of the fibrillar carbon-based material with a virtual absence of tin particles that are not supported by said material.
The composite consists of 20 to 35% by weight of fibrillar carbon-based material and from 65 to 80% by weight of tin oxide particles.
In the case where the fibrillar carbon-based material is a mixture of carbon nanotubes and carbon nanofibers, this mixture consists, preferably at a concentration of 50% by weight, of each of the two constituents.
In the case where the composite described consists of carbon nanotubes and of particles of tin oxide, it has, in galvanostatic cycling, a capacity of greater than 600 mAh/g after 60 cycles.
In the case where the composite consists of carbon nanotubes, carbon nanofibers and of tin oxide particles, it has, in galvanostatic cycling, a capacity of greater than 750 mAh/g after 60 cycles.
The carbon nanotubes are preferably multi-walled CNTs.
Use is preferably made of multi-walled CNTs having an external diameter ranging from 3 to 50 nm, preferably from 5 to 30 nm and better still from 8 to 20 nm since multi-walled CNTs make it possible to obtain a conductivity that is higher, more homogeneous and more stable.
Embodiments of the invention apply to the production of electrodes, comprising a composite as described previously and very particularly to the production of lithium-ion battery negative electrodes.
In particular, an electrode comprises a composite consisting of a mixture of at least 80% by weight of active material (CNT-SnO2) and at most 20% by weight of binder.
The binder may consist of any liquid, or molecular or polymeric paste, which is chemically inert, generally used to make particles of powder adhere to one another, such as for example polyvinylidene difluoride (PVDF), polyvinylpyrrolidone (PVP) or CMC (carboxymethyl cellulose).
Embodiments of the invention apply to the production of lithium-ion batteries having a negative electrode comprising a composite as described previously.
Other features and advantages of the invention will become clearly apparent on reading the following description given by way of non-limiting illustrative example and in conjunction with the figures in which:
The following examples will make the scope of embodiments of the invention better understood.
Specific example of implementation of the process for manufacturing the composite. In this example, use is made, as fibrillar carbon-based material, of CNTs that are purified in order to obtain a better attachment of the tin particles as described previously.
The applicant has observed that the carbon nanotubes after synthesis are not suitable for the process. In order for the particles of tin oxide to adhere, it is necessary that the surface of the nanotubes exhibits polar surface functional groups of OH and/or COOH type. These functional groups are obtained by treatment of the nanotubes in a strong acid such as HNO3 (oxidizing acid) or H2SO4 (not very oxidizing acid), which treatment is followed by a surface oxidation operation using sodium hypochlorite if the acid used for the purification is not sufficiently oxidizing.
Other oxidizers, such as H2O2 or O3 may also be used without compromising the scope of the invention.
This observation is also true when it is desired to incorporate carbon nanofibers into the composition.
Moreover, the applicant has observed that tin oxide particles of the order of a few nanometers gave better results. The particles used are advantageously tin oxide nanoparticles.
In this first example, the following steps are carried out:
It is clear that the reverse order may be followed: a predispersion of nanotubes is firstly prepared, by vigorous stirring, to which nanofibers are optionally added, then the solution of tin salt is added.
The electrochemical performances were characterized in lithium batteries, that is to say that the positive electrode K consists of metallic lithium and the electrolyte E is a lithium salt in an organic solvent having an EC/DMC (ethylene carbonate/dimethyl carbonate) composition of 1/1 by volume, with an LiPF6 concentration equal to 1M.
The negative electrode A consists of a mixture of 80% by weight of active material (CNT/SnO2) and of 20% by weight of PVDF (polyvinylidene difluoride), which is a binder that makes it possible to ensure a good mechanical strength of the electrode. These various constituents are introduced into N-methylpyrrolidone in order to obtain a very homogeneous mixture. This mixture is then coated onto a glass plate by a “doctor BLADE” coating plate. The coating is carried out to a thickness of 150 μm.
Electrodes having a diameter of 11 mm are then cut in this film and dried for several hours (6 to 8 h) at 80° C. under vacuum.
Once in a cell (button cell), the negative electrode A is successively covered by a separator S (polypropylene saturated with electrolyte) and by the positive electrode K which is a pellet of metallic lithium. The electrolyte used is a lithium salt (LiPF6, 1M) dissolved in the mixture of EC/DMC (ethylene carbonate/dimethyl carbonate) organic solvents in volume proportions of 1/1.
Various individual cells thus formed are assembled in a glovebox under a controlled atmosphere in order to form a battery.
The various electrochemical tests are carried out on VMP3 (Biologic SAS). The electrochemical behavior of the CNT/SnO2 composites was studied in galvanostatic mode under a C/10 constant regime in the potential window [0.02-1.2] V (vs. Li+/Li).
This negative electrode consists of the composite synthesized by the process that has been described. The reversible capacity drops at the end of the first cycle but is maintained at around 700 mAh/g for more than 30 cycles. After 60 cycles, the capacity of the composite remains above 600 mAh/g.
This example repeats the test conditions from Example 2 but replacing, in the synthesis, half of the carbon nanotubes, i.e. 0.5 g, with 0.5 g of carbon nanofibers (by way of example, these are carbon nanofibers sold by Showa Denko, the diameter of which is 150 nm).
Before preparation of the composite, these nanofibers were treated in the presence of sodium hypochlorite.
A negative electrode A is then manufactured with this new composite. The reversible capacity drops at the end of the first cycle but is maintained at around 870 mAh/g for more than 30 cycles. After 60 cycles, the capacity of the composite remains above 750 mAh/g.
The nanofibers are capable of ensuring electrical connections over long distances and the carbon nanotubes act more at the local level.
Indeed, the nanotubes appear to play the role of “elastomeric” material for accommodating the volume variations, and also of short-distance electrical connectors between particles whilst the nanofibers appear to play the role of long-distance connectors.
In any case, the nanotubes used are purified so that the ash content is less than 2.5% by weight loss at 900° C. in air, since after synthesis, the nanotubes contain catalytic residues which may reach up to 10% by weight.
Embodiments of the invention presented here makes it possible for a tin oxide SnO2 to obtain a reversible capacity of the order of 850 mAh/g after 50 cycles without detrimental volume expansion. The composite obtained by the process (SnO2 with a fibrillar carbon-based material) also gives the following results:
an increase in the specific surface area owing to the nanoscale size of the SnO2 particles, enabling a reduction in the diffusion length of the lithium during the deintercalation/intercalation of the lithium; and
an increase in the electronic conductivity owing to the addition of the fibrillar carbon-based material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0858459 | Dec 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR09/52408 | 12/4/2009 | WO | 00 | 8/24/2011 |
