FR2722119A1 - Graphite carbon anode for rechargeable lithium cell - Google Patents

Abstract

A carbon anode, for a rechargeable lithium power cell, consists of a graphitic material which, before electrical cycling, contains more than 10 (pref. more than 14)% of a first graphite phase of rhombohedral crystalline structure. Also claimed are: (i) prodn. of the above graphitic material; and (ii) a rechargeable lithium power cell using the above anode.

Description

Process for producing a usable carbonaceous material
as the generator electrode active material
Rechargeable lithium electrochemical
The present invention relates to a process for producing a carbonaceous material that can be used as a rechargeable lithium electrochemical generator electrode active material. It also extends to the product obtained by this process.

 In the manufacture of rechargeable electrochemical generator anodes, the use of carbonaceous materials capable of reversibly inserting lithium into their structure has grown considerably recently.

During the first charge, the intercalation of lithium in the carbon material is performed correctly after passivation of its surface, due to the inevitable reduction of the low potential electrolyte. This passivation irreversibly consumes part of the initial capacitance of the cathode which is all the greater since the electrochemical surface of the material to be passivated is large.

 At present work is turning towards reducing this irreversible loss of capacity in the first cycle. A notable progress has been made by the rhombohedral phase enrichment of the carbonaceous material. A known method of enrichment is the grinding of the material; the formation of the rhombohedral phase has been attributed to the simultaneous effect of shear and pressure stresses. But this method has the main disadvantage of producing small particles, so to increase the surface of the material.

The irreversible loss of capacity is thus increased.

 The present invention relates in particular to a method of manufacturing a carbonaceous material which makes it possible to increase the proportion of rhombohedral phase that it contains without substantially increasing the electrochemical surface.

To this end, the present invention proposes a process for producing, from graphite, a carbonaceous material that can be used as an electrochemical rechargeable lithium electrode electrode active material, characterized in that it comprises the following steps:
said powdery graphite is introduced into a liquid maintained at a given temperature,
ultrasonic waves are generated in said liquid by the vibration of a room for a fixed duration,
said carbonaceous material is separated from the liquid and dried.

 Ultrasonic waves are inaudible by the human ear, that is to say that they are of frequency higher than about 16kHz. Their propagation in the liquid medium causes the formation of bubbles or cavitation effect. The characteristics of the cavitation effect obtained depend on the parameters of implementation of the method.

 The wave frequency should not be too high otherwise the cavitation effect does not have time to occur.

Most commercial devices used deliver a frequency between 20 and 50kHz.

 It is best to use a high viscosity liquid: higher power will be needed to produce cavitation, but the resulting effect will be increased. For example, water or aqueous mixtures containing an alcohol, a ketone, a salt or even acetonitrile will be used.

 A decrease in temperature also increases the viscosity of the liquid. Cavitation will be more difficult to obtain at low temperatures but its effects will be greater. Preferably, said temperature is less than 20 ° C and greater than the solidification point of the liquid used (0 C for aqueous mixtures).

 The part immersed in the liquid that generates the ultrasonic waves is usually called "sonotrode". It can be full or hollow, and of variable profile; it is usually metallic. Increasing the amplitude of the vibration of the part, and therefore of the generated wave, produces an increase in the cavitation effect. Preferably, the amplitude of said vibration is greater than 8Oun. In practice we are limited by the power of the device used.

 The duration of application of the ultrasonic waves is of course a parameter which depends on the kinetics of the reaction envisaged. Preferably in the manufacturing method according to the invention, said duration is greater than 10 minutes.

 Available commercial devices can generate frequencies in a continuous mode or in a pulsed mode.

These two modes of ultrasonic wave generation can be used in the method according to the invention and lead to very similar results under the same conditions of application.

 According to a first embodiment of the present invention, said liquid is acetonitrile.

 According to a second embodiment of the present invention, said liquid is water or an aqueous mixture containing a compound chosen from an alcohol, such as, for example, ethanol, a ketone, for example acetone, and a salt, as for example potassium iodide.

 The method of the present invention has the advantage of being easy to implement, especially since the choice of the liquid does not offer any difficulty for the separation of the powder. This treatment is fast and causes negligible loss of material.

 The subject of the present invention is also a carbon material produced by this process and comprising, prior to electrical cycling, at least a first phase consisting of graphite with a rhombohedral crystal structure in a proportion greater than 10%.

Other features and advantages of the present invention will appear on reading the following examples of embodiments, given of course by way of illustration but in no way limiting, with reference to the appended drawing:
FIG. 1 shows a characteristic part of the commercial graphite diffraction diagram, the abscissa indicates the degree of diffraction angle in degrees with a copper anticathode of wavelength k = 15.4 nm,
FIG. 2, similar to FIG. 1, shows this part of the diagram for the carbonaceous material produced by the process according to the invention,
FIG. 3 shows in section a rechargeable lithium electrochemical generator containing the material produced by the method according to the invention,
FIG. 4 represents the first charge and discharge curve of an electrode comprising the carbonaceous material produced by the method of the present invention in comparison with analogous curves of an electrode comprising a material of the prior art. The mass capacity C of the material in mAh / g is plotted on the abscissa and the voltage V in volts is on the ordinate.

EXAMPLE 1 Prior art
From commercial graphite A 'in powder form and containing less than 5% of rhombohedral phase, a rhombohedral phase enrichment treatment is carried out by the method of the prior art. The powdered graphite is milled with a knife mill for a minimum of 15 minutes.

 A powder of a carbon material A containing 14% of rhombohedral phase is then obtained.

EXAMPLE 2
From commercial graphite B 'in powder form and containing less than 5% of rhombohedral phase, a carbonaceous material is produced by the process according to the present invention. The X-ray diffraction pattern of graphite B '(FIG. 1) has, in the spectral zone between 40 and 48 degrees, the lines [100] 1 and [101] 2 relating to the hexagonal phase.
The graphite powder is introduced into a container containing acetonitrile MeCN, at a rate of 40cana of liquid per gram of graphite. This container is placed in a tank thermostated at 20 ° C. A sonotrode, connected to a 600W power device continuously delivering ultrasonic waves with a frequency of 20kHz, is immersed in the container. The vibration of the sonotrode with an amplitude of 120Am causes the propagation of ultrasonic waves in the liquid, which produces the suspension of the graphite powder and creates bubbles by the effect of cavitation.

 After about 10 minutes of treatment, the sonotrode is removed. The powder is then separated from the liquid by filtration and dried under vacuum at 110 ° C. for 2 hours.

 The X-ray diffraction pattern of the carbon material B obtained (FIG. 2) has, in the spectral zone between 40 and 48 degrees, the lines [100] 1 and [101] 2 relating to the hexagonal phase similar to graphite and the lines [ 101] 3 and [012] 4 of the rhombohedral phase.

 After treatment, the carbonaceous material B contains 13% of rhombohedral phase.

EXAMPLE 3
From the same commercial graphite B 'as that used in Example 2, a carbonaceous material is produced by the process according to the present invention under conditions similar to those described in Example 2, with the exception of the duration of the treatment. which is 2mn, 5mn and 60mn respectively for materials D, E and F. The results obtained are as follows:
TABLE I
Powder duration of rhombohedral phase treatment
minutes
D 2 5
E 5 9
B 10 13
F 60 12
It is clear that under these conditions, a treatment time of 10 minutes is sufficient to obtain the maximum enrichment.

EXAMPLE 4
From the same commercial graphite B 'as in Example 2, a carbonaceous material is produced by the process according to the present invention under conditions similar to those described in Example 2 except for the treatment temperature. The tank is thermostated at 10C and 48 C respectively for the preparation of materials G and H.

The results obtained are as follows:
TABLE II
Powder temperature of the rhombohedral phase treatment C
G l 13
B 20 13
H 48 7
It can be seen that the treatment should preferably be carried out at low temperatures, less than or equal to room temperature.

EXAMPLE 5
From the same commercial graphite B 'as in Example 2, a carbonaceous material is produced by the process according to the present invention under conditions similar to those described in Example 2 with the exception of the liquid medium used which is water, an aqueous solution containing 20% by volume of ethanol C2H5OH, an aqueous solution containing 20% by volume of acetone (CH3) 2CO, an aqueous solution of potassium iodide KI at 0.5 mol / liter.

The rhombohedral phase levels in the powders obtained are of the order of 12 to 13% for all the samples.

EXAMPLE 6
With the materials A ', A, B' and B of Examples 1 and 2, mixtures are made each comprising 90% of one of the carbonaceous materials, 5% of acetylene black of reference flysl and 5% of polytetrafluoroethylene ( PTFE). An electrode is made by embedding this mixture on a grid or metal fabric, here in copper but one could just as well use nickel. After drying, a circular anode 16 mm in diameter and 0.3 mm in thickness is cut out, which contains a quantity of carbonaceous material of the order of 50 mg.

 As shown in FIG. 3, a button-type electrochemical generator containing the previously made anode 11 and a counter-electrode 12 which is a lithium disk of 22 mm in diameter and weighing about 10 mm in diameter are assembled as a test cell. 110 mg. The two electrodes are separated by a polypropylene microporous separator 13 and by an electrolyte reservoir 14 made of polypropylene fibers (PP). The electrolyte used is composed of an organic solvent, which is an equivolumic mixture of ethylene carbonate and dimethyl carbonate (EC / DNC), in which the lithium trifluoromethanesulfonimide (LiTFSI) is dissolved at a concentration of 1 mole /liter.

The assembly is disposed in a cup 15 closed in a sealed manner by a lid 16, via a seal 17.

 The electrochemically active carbon material surface wetted by the electrolyte is evaluated by measuring the double layer capacity expressed in millifarad per gram of carbon (mF / g). This measurement implements the known method of complex impedances that is applied to the test cells. The results are obtained with an accuracy of + 5% on the calculated value; they are collected in Table III.

TABLE III
Rhombohedral Phase Powder Double Layer Capacity
% mF / g
A '<5 400
A 14 1500
B '<5,245
B 13 256
The development of the carbonaceous material B using the method of the present invention makes it possible to preserve an electrochemical surface of the same order of magnitude as the initial material B ', which is not the case for the material A, obtained from the A 'graphite with the method of the prior art whose surface has been multiplied by about four.

EXAMPLE 7
With the materials B 'and B of Example 2, two electrodes are produced as described in Example 6 which are each placed in a generator similar to that shown in FIG.

An electrochemical cycling is carried out of the generators whose first cycle is shown in FIG. 4 for materials B '(curves 21-22) and B (curves 31-32). A first lithium intercalation (curves 21 and 31) is first carried out in the anode at a rate of 40 mA / g of carbon to a potential of 0 volts relative to Li / Li + lithium. Then this lithium is disintercalated (curves 22 and 32) at the same speed up to 2 volts. The difference in capacitance ac between these two stages of cycling corresponds to the loss of capacity due to passivation of the anode (passivation capacity). The results obtained are shown in Table IV below:
TABLE IV
Rhombohedral phase powder passivation ability
% mAh / g
B '<5 505
B 13,420
The material produced by the process according to the invention has a loss of irreversible capacity decreased compared to a commercial carbon.

 Naturally, the present invention is not limited to the embodiments described, but it is capable of many variants accessible to those skilled in the art without departing from the spirit of the invention. In particular, it will be possible without departing from the scope of the invention to vary the parameters within the ranges indicated.

Claims (7)

1. / A process for producing, from graphite, a carbonaceous material that can be used as a rechargeable lithium electrochemical generator electrode active material, characterized in that it comprises the following steps:  said powdery graphite is introduced into a liquid maintained at a given temperature,  ultrasonic waves are generated in said liquid by the vibration of a room for a fixed duration,  said carbonaceous material is separated from the liquid and dried. 2. The method of claim 1, wherein said temperature is less than 20 ° C. 3. The method of claim 1, wherein the amplitude of said vibration is greater than 80gm. 4. The method of claim 1, wherein said duration is greater than 10 minutes. 5. The process according to one of the preceding claims, wherein said liquid is acetonitrile. 6. / A method according to one of claims 1 to 4, wherein said liquid is water or an aqueous mixture containing a compound selected from an alcohol, a ketone and a salt. 7. / carbon material manufactured by the process according to one of the preceding claims, comprising before electric cycling at least a first phase made of graphite rhombohedral crystal structure in a proportion greater than 10%.