Patent Application
20190023574
Some embodiments relate to a method of manufacturing a material for a lithium battery, to the materials that can be obtained by the process of some embodiments, and to a battery, in particular a lithium battery, as well as to the devices incorporating a battery, in particular a lithium battery.
In the description below, the reference numerals between square brackets ([ ]) refer to the list of references given at the end of the text.
It is of scientific and industrial interest to further improve or enhance the electrochemical properties of electrical batteries, in particular lithium batteries, in order to obtain ever greater capacities, energies, powers and lives, above all in the current economic and ecological context.
With an objective of capacity, the strategy of some market players has been to increase the fluorination rate x in CFx in order to reach or even slightly exceed CFI, the precursors being inexpensive graphite or petroleum cokes. The method of fluorination is the reaction with molecular fluorine F2 at high temperatures, usually 500 to 600° C. in the case of the graphite precursor. By this synthesis, the defect rate generated is high for cokes and the faradic yield of the battery is below 100%. Furthermore, the implementation procedures are costly.
The use of controlled fluorination methods, which are described in the following documents:
The use of nanometric forms of carbon also encourages the diffusion of ions during electrochemical processes.
By controlling fluorination, decomposition under a fluorine atmosphere, which always competes against fluorination at high temperatures, can be limited. This decomposition produces perfluorinated volatile species CF4, C2F6 and hence results in a loss of both fluorine and carbon.
Therefore, it may be advantageous to identify new materials that will address or overcome some or all of the flaws, drawbacks and obstacles of the related art; in particular, a process enabling cost reduction and improved or enhanced electrochemical properties of electrical batteries must be developed.
Some embodiments address or solve the above-mentioned problems by providing a process of manufacturing a material for an electrochemical cell enabling a material to be obtained that has enhanced or improved electrochemical capacities.
Some embodiments include: grinding fluorinated carbon nanofibres (CNFs) having the formula CFx with 0.2<x<1, the grinding being achieved by frictional impacts for a period of 2 to 100 hours, for example of 2 to 10 hours, with a grinding pressure on the particles ranging from 0.29×106 Pa to 4.8×106 Pa, for example from 0.31×106 Pa to 3.2×106 Pa, for example from 0.8×106 Pa to 2.4×106 Pa, for example from 0.8×106 Pa to 1.6×106 Pa, for example from 0.8×106 Pa to 1.2×106 Pa.
Fluorinated carbons, having a chemical composition CFx with x<1, used in the related art particularly as a lithium battery electrode, have a maximum theoretical capacity of less than 865 mAh/g linked to the rupture of C—F bonds and the reduction of carbon atoms, the limit corresponding to the highest fluorination rate, namely a composition C/F=1. This limit has recently been largely exceeded, as described in the document Ahmad et al, “Pushing the theoretical limit of Li—CFx batteries using fluorinated nanostructured carbon nanodiscs,” Carbon, 94 (2015) 1061-1070 [6], but by implementing a second electrochemical phenomenon consuming lithium for the formation of a species intercalated in neoformed carbon by electrochemical defluorination. This second electrochemical phenomenon is only possible if the carbonaceous matrix has reconstructed itself around a reinforcement such as, for example, a central tube for nanotubes or a central disc for carbon nanodiscs, which requires a more complex process and additional costs. Furthermore, more lithium is consumed.
The inventors of the presently disclosed subject matter unexpectedly observed that the electrochemical performance of some fluorinated materials is enhanced or improved simultaneously in terms of potential discharge (increased), discharge profile (reduced or even no initial ohmic drop), energy density (increased) and capacities with values that have been measured at nearly 300% times higher than that of the original fluorinated material when the fluorinated material is subjected to grinding.
The inventors of the presently disclosed subject matter therefore experimentally studied the grinding of fluorinated carbon nanofibres and have developed the process of some embodiments. They have observed from numerous experiments carried out that the process of some embodiments in fact has the effect of opening up the carbon nanofibres in the material. Some embodiments of the presently disclosed subject matter are therefore the very first to have in some way created an intrinsic reinforcement of the material, in the sense previously described as regards extra capacity, thanks to the process of some embodiments, by mechanically grinding the fluorinated carbon nanofibres.
For example, in an electrochemical battery or cell, the enhancement of improvement of the electrochemical properties of the fluorinated material by the process of some embodiments enables an enhancement or improvement in battery life. Generally speaking, when the fluorination rate x in CFx increases, the material becomes less and less electronically conductive. At x>0.7, the fluorinated carbon is insulating leading to an ohmic drop at the start of discharge of the battery. Electrochemical defluorination during the operation of the battery then forms conductive carbon and the potential increases.
The experimental results achieved by the inventors of the presently disclosed subject matter show that the material obtained by the process of some embodiments enables this conduction from the very start of the discharge, and the ohmic drop is thus reduced or even eliminated. Overvoltage is also reduced throughout the discharge, increasing the energy density in synergy with the increase in capacity.
Thus, some embodiments also relate to a material of an electrochemical cell that can be obtained by the process of some embodiments.
According to some embodiments, the grinding by frictional impact can be performed using beads or balls, for example as described in the examples below. According to some embodiments, “frictional impact” or “impact and friction” refer to an interaction that opposes the relative movement between two systems in contact. The grinding according to some embodiments can be performed using any appropriate methods to carry out some embodiments, known by one of ordinary skill in the art, for example using a planetary ball mill or using an impact crusher with beads.
According to some embodiments, grinding can be performed under a vacuum or under an atmosphere chosen from a neutral atmosphere, possibly fluorinated, advantageously or preferably under an argon atmosphere, possibly fluorinated.
According to some embodiments, grinding can for example be performed at a temperature ranging from 0 to 15° C. and 55 to 60° C., i.e., the lower boundary of this temperature range being between 0 and 15° C., and the upper boundary of this temperature range being between 55 and 60° C. The temperature may for example be from 15 to 35° C.
According to some embodiments, the fluorinated carbon nanofibres having the formula CFx with 0.2<x<1 used for implementing some embodiments can be obtained by any process known to a person of ordinary skill in the art. The fluorination methods that can be used may, for example, involve fluorinating agents known to a person of ordinary skill in the art, chosen for example from F2, a solid fluorinating agent, etc. The fluorination methods that can be used may, for example, include one of those described in the following documents: Carbon nanofibres fluorinated using TbF4 as fluorinating agent [2]; Zhang et al, Carbon nanofibres fluorinated using TbF4 as fluorinating agent [3] or Zhang et al, New synthesis methods for fluorinated carbon nanofibres and applications [1].
These non-fluorinated nanofibres are also commercially available. For example, they may be materials marketed by the MER Corporation (US), under commercial reference Catalog#MRCSD or any other equivalent material.
According to some embodiments, the fluorinated carbon nanofibres that can be used to implement the process of some embodiments can for example have a diameter of 50 to 200 nm, advantageously or preferably from 110 to 170 nm, and a length from 5 to 20 μm, advantageously or preferably from 5 to 9 μm.
According to some embodiments, advantageously or preferably, the central non-fluorinated carbon part represents from 3 to 65% of the volume of nanofibres, advantageously or preferably from 10 to 60%, for example to 20 and 53%, for example for materials CF0.68 to CF0.31.
According to some embodiments, advantageously or preferably, the 13C MAS NMR spectrum has a chemical shift band of 120 to 135 ppm/tetramethylsilane (TMS).
According to some embodiments, the grinding step can advantageously include an alternation of periods of grinding (B) and pausing (P) without grinding. This alternation according to some embodiments can also be regarded as being a succession of grinding cycles each including a period of grinding (B) and a pause (P) without grinding, P and B being as defined above.
According to some embodiments, the experiments performed by the inventors of the presently disclosed subject matter involved, for example, an alternation or one or more cycles including:
These experiments in fact show that this alternation of grinding and pausing, or repetition of grinding cycles according to some embodiments, unexpectedly allows further enhancement or improvement of the electrochemical performance of the materials obtained thanks to the process of some embodiments.
The process of some embodiments may further include a controlled fluorination or controlled “post-fluorination” step of the product derived from the process according to some embodiments. This step may for example be carried out like the aforementioned fluorination step. It makes it possible to boost the fluorination level of the material obtained after applying the inventive process, which leads to an increased capacity.
According to some embodiments, an analysis carried out on an example of material obtained by the process of some embodiments shows that the material of some embodiments includes three main classes of fibers that can be identified as follows:
i. those of lengths less than 1 μm,
ii. those of lengths between 1 and 3 μm, and
iii. those of lengths greater than 3 μm
The most numerous are fibers of lengths less than 1 μm which represent 50 to 70%, namely about 60%, out of all of the SEM images analyzed, those ranging from 1 to 3 μm represent from 20 to 40%, namely about 30% of the populations represented; and those greater than 3 μm represent from 5 to 15%, namely about 10% of the populations represented. This example of distribution of the main classes of fibers is given purely to illustrate an instance in the light of the experiments outlined above. If the duration of grinding varies, this distribution also varies.
Some embodiments also relate to an electrode including a material according to some embodiments.
Some embodiments also relate to a battery including a material or an electrode according to some embodiments.
According to some embodiments, the battery can be for example a lithium battery.
It may, for example, be a battery chosen from a button battery, a laboratory battery, a cylindrical battery or a spiral-wound battery.
Some embodiments also relate to a device including a battery according to some embodiments, for example a device chosen from a portable telephone, a meter, oil drilling communication equipment, a pressurized device, a low or high temperature device, a watch, a pacemaker, a drug or medication injector or a neuro-stimulator.
The exceptional properties of the materials of some embodiments make them the materials of choice for the manufacture of new generations of batteries, with enhanced or improved electrochemical properties, including for uses in difficult, or even extreme, conditions, for example under pressure or at low and high temperatures.
Other advantages will emerge to a person of ordinary skill in the art from the examples described below, illustrated by the accompanying figures, given for the purpose of illustration.
In these figures, “Qexp” represents the experimental capacity expressed in mAh/g; “Qth” represents the theoretical capacity in mAh/g calculated by considering the fluorination rate achieved by NMR, in the manner described in document F. Chamssedine, Marc Dubois, Katia Guérin, J. Giraudet, F. Masin, D. A. Ivanov, L. Vidal, R. Yazami, and A. Hamwi, Reactivity of Carbon Nanofibers with Fluorine Gas, Chem. Mater., 2007, 19 (2), pp 161-172 [7]; the theoretical capacity “Qth” is expressed in accordance with the following equation: Qth=(96500*x)/(3.6*(12+19*x), where x corresponds to the fluorination rate F/C for a compound CFx and 12 and 19 are the molar masses of carbon and fluorine respectively.
The indications of time in hours (h) express the total grinding time when implementing the process of some embodiments; “η” represents the ratio expressed as a percentage between the theoretical capacity and the capacity measured during the experiments; “rpm” expresses the number of rotations per minute on implementing some embodiments in a grinder as described in the examples below.
In this first example, 200 mg of nanofibers (from the MER Corporation, under commercial reference MRCSD) were placed in a nickel basket positioned in the center of a one-liter passivated nickel reactor to be fluorinated by dynamic fluorination (flow of F2 gas in an open reactor).
Dynamic fluorination was performed under a flow of pure gaseous molecular fluorine F2 at a temperature Tf of 420° C., with a flow rate of F2 of 25-30 ml/min for 180 minutes.
The material obtained had an atomic ratio rate F/C of 0.31 determined by NMR.
This material had been ground by a Retsch PM100 (trademark) planetary ball mill in a 50 ml stainless steel bowl with four 10 mm-diameter stainless steel balls under an argon atmosphere for 6 hours at a grinding speed of 350 rpm with 1 min pauses in grinding every 9 min.
These experiments were repeated with different starter CFx materials, with x being between 0.2 and 1. For example, in order to obtain a CFx material where x=0.68, dynamic fluorination was performed under a flow of pure gaseous molecular fluorine F2 at a temperature Tf of 420° C., with a flow rate of F2 of 25-30 ml/min for 180 minutes.
The SEM photographs in the accompanying
The photographs in
The material obtained according to Example 1 was placed in suspension in a mixer with glass vessel by mixing with 10% by weight of polyvinylidene fluoride (PVDF) in a liquid medium of propylene carbonate.
The suspension was then deposited on the stainless steel electrode pads heated to 80° C. over a surface of around 1 cm2, then the pads were degassed under primary vacuum at 120° C. for 1 hour. 50 mg of cathode material, CFx/PVDF 90/10 was obtained.
The mass of the pads was then estimated to be between 2 and 5 mg. The pads were returned to the glove box and incorporated into a laboratory lithium battery consisting of or including metallic lithium, Celgard (registered trademark) microporous separators, wetted by LiPF6 PC/EC/3 DMC 1M electrolyte, as shown in the accompanying
In
This electrochemical laboratory cell has two stainless steel electrodes consisting of or including a piston at each end.
The battery thus constituted is taken out of the glove box and connected to a VMP2 (commercial reference) potentiated/galvanostatic made by Bio-Logic Science Instruments SAS (France). After a relaxation period of 5 h, a 10 to 20 mA/g reduction current is applied with a cut-off voltage of 1.5 V vs. Li+/Li.
After the period of relaxation, the battery was discharged until the stopping potential of 1.5 V and a density of 10 mA/g (reduction) or 20 mA/g.
This example presents different experiments carried out on different materials obtained according to the process of some embodiments. In particular, the process of some embodiments used in order to obtain the materials tested in this example is that described in Example 1, and the measurements of electrochemical performance are made using a device as described in Example 2.
The variation in potential as a function of time is then measured. This duration multiplied by the current density allows the capacity to be determined. In 3 electrochemical tests performed on this material, the capacities 970, 1684 and 2339 mAh/g were obtained. The “theoretical” capacity of this material, corresponding to the capacity measured on the fluorinated carbon nanofibre material used to implement the process of some embodiments, i.e. before grinding, is 465 mAh/g. A marked phenomenon of extra capacity is therefore observed in these different experiments.
Tables I and II below summarize the experiments carried out in the context of some embodiments.
13C NMR
13C NMR
These different experiments show that the ohmic drop is limited or non-existent with the materials of some embodiments. Thus, batteries made using a material according to some embodiments are immediately operational. The potential has been increased to 2.75 V with the materials of some embodiments.
The NMR analyses show a defluorination during grinding. However, the electrochemical properties of the materials are markedly enhanced or improved thanks to the process of some embodiments.
A limitation of heating of the battery is also observed thanks to the materials of some embodiments, as well as enhanced or improved battery life and power, which allows them to be the material of choice for the manufacture of a new generation of batteries, including batteries that must operate at high temperatures, for example at temperatures ranging from 150 to 180° C., or at a low temperature, for example at −20° C.
During grinding, the grains of powder are highly stressed: they deform and some break, while others stick together. The combined action of this plastic deformation, breakage and sticking together leads to the formation of very homogenous powders consisting of or including agglomerates.
In the case of the materials obtained by the process of some embodiments, for example a material CF0.51, the fluorinated carbon nanofibers are still visible, despite the presence of some agglomerates, as shown by the scanning electron microscope (SEM) image in the accompanying
These agglomerates have an average size of 1.65 μm. In
According to the protocol described in Example 1, other materials according to some embodiments are made by varying the degree of fluorination and/or the grinding time and/or the grinding speed and/or the grinding cycles and/or the grinding temperature in order to obtain different fluorinated materials according to some embodiments, having a degree of fluorination CFx where x ranges from 0.2 to less than 1.
For example, the experiments with the following parameters are implemented, without being vertically related to each other:
Measurements on the materials are made using a device such as that described in Example 2:
The following observations are made regarding the materials currently obtained:
The material obtained according to Example 1 was placed in suspension as described in Example 2, except that the battery used differed because button batteries were used.
A high discharge potential of around 2.8 V was observed in the button battery, with a very limited ohmic drop, which confirms the enhanced or improved electrochemical properties of the materials of some embodiments.
The 13C NMR analyses show a defluorination of the fluorinated nanofibers during grinding, which implies a reduction in the fluorination rate F/C and theoretical capacity of the material.
An increased extra-capacity and enhanced or improved electrochemical performance are achieved for laboratory batteries and an extra-capacity is for this reason observed in the button battery.
The grinder used in this example is a planetary ball mill, which produces mechanical stresses of the frictional impact type. The starting materials are comparable to those of Example 3 above, namely CF0.43 and CF0.76 versus CF0.31 and CF0.68 in Example 3.
The balls exert a centrifugal force that is determined on the one hand from characteristics specific to the latter, namely their mass, density, diameter, etc., and on the other hand from grinding conditions and accessories, in particular rotation speed and diameter of the bowl, and lastly taking account of the physicochemical characteristics of the material to be ground, in particular the diameter of the particles or fluorinated nanofibers and their stiffness.
From this force, and using the mechanical formula described below, the pressure of a ball on a particle to be ground can be determined in order to model what happens during the implementation of some embodiments.
Three ball diameters were studied: 10 mm-15 mm-20 mm, which corresponds to respective pressures of about 1×106 Pa, 2×106 Pa and 4×106 Pa.
Four different grinding durations were studied: 3 h, 6 h, 12 h and 18 h.
A speed of 350 rpm was set for all of the experiments.
In light of the electrochemical performance obtained in this example, optimal grinding conditions for fluorinated nanofibers with a fluorination rate F/C≈0.4 were determined in the context of this example:
For fluorinated nanofibers with a fluorination level F/C≈0.8, the optimal grinding conditions were:
During grinding, the grains of powder are highly stressed: they deform, and some break, while others stick to each other. The combined action of this plastic deformation, breakage and sticking together leads to the formation of very non-homogenous powders consisting of or including agglomerates.
In the case of the materials obtained by the process of some embodiments, for example a material CF0.51, the fluorinated carbon nanofibers are still visible, despite the presence of some agglomerates, as shown by the scanning electron microscope (SEM) image in the accompanying
These agglomerates have an average size of 1.65 μm. In
The most numerous are the fibers smaller than 1 μm accounting for 50 to 70%, i.e. about 60% in all of the SEM images analyzed, and those between 1 and 3 μm constitute 20 to 40%, i.e. around 30% of the populations represented, and those larger than 3 μm represent from 5 to 15% in general, i.e. around 10% of the populations represented.
Post-fluorination experiments were conducted on materials comparable to those of Example 3, and the electrochemical performance of the materials was studied.
The appended
The best performance observed in these experiments was under argon atmospheres.
The inventors of the presently disclosed subject matter have observed that post-fluorination enhances or improves the performance in terms of the experimental capacity obtained, the fluorination level having slightly increased. Indeed, fluorination by XeF2 done after grinding induces a surface fluorination of the nanofibers that have been broken or been reduced to small pieces. The intrinsic carbon (not fluorinated) initially located in the core of the fiber is brought back to the surface during grinding and is more accessible to the atomic fluorine derived from the XeF2 fluorinating agent. This surface “re-fluorination,” slight but nevertheless present, makes it possible to increase the fluorination level of the ground material and thus explains how a higher capacity is obtained. The ohmic drop remains limited relative to the non-ground fibers (black curve) of the related art but is more pronounced than for the grinding done under argon, vacuum or nitrogen according to the process of some embodiments.
For the ground CF0.71 (comparable to the CF0.8) CNFs (6 h at 1×106 Pa, speed 350 rpm), the same tendencies were observed, as shown in the appended
The observed extra capacity was visible and repeated on batteries of the Swagelok type, i.e. laboratory batteries and button batteries (industrial batteries).
The inventors of the presently disclosed subject matter reproduce the experimental results in a button battery, since the performance in general is more reproducible, but above all because these are industrial batteries.
A study seeking to study the performance of the material on a larger volume was carried out. Thus, a first batch of ground fluorinated CNFs F/C≈0.8 was synthesized in a 50 ml (1.5 g) bowl, then a second batch in a 250 ml (10 g) bowl.
The electrochemical performance is comparable to that observed in all of the experiments done relative to some embodiments, including those described above, for the two aforementioned batches, as shown in the appended
On average, the discharge potential is 2.6 V for both batches. The average capacity for the small batch (50 ml) is 730 mAh/g and 732 mAh/g for the large batch (250 ml). The results are therefore comparable when the volumes are increased.
The extra capacity phenomenon was not observed on the selected materials, but they nevertheless have a good experimental capacity, equal or close to the theoretical capacity of the non-ground material, a smaller or non-existent ohmic drop, and a high discharge potential compared to the non-ground fluorinated nanofibers of the related art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1559378 | Oct 2015 | FR | national |
This application is a national phase filing under 35 C.F.R. § 371 of and claims priority to PCT Patent Application No. PCT/FR2016/052497, filed on Sep. 29, 2016, which claims the priority benefit under 35 U.S.C. § 119 of French Patent Application No. 1559378, filed on Oct. 2, 2015, the contents of each of which are hereby incorporated in their entireties by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2016/052497 | 9/29/2016 | WO | 00 |
