Negative electrode for primary lithium electrochemical element

Abstract

An electrochemical element (1) comprising: - a positive electrode (5) comprising a liquid active material (6) selected from SO2, SOCl2, SO2Cl2 and a mixture of several of these; - a negative electrode (3) comprising a lithium foil or a lithium-based alloy coated on at least one face with a layer of amorphous alumina Al2O3. (Shorthand figure: Figure 4)

Description

Title of the invention: Negative electrode for a primary lithium electrochemical cell Technical field of the invention

[0001] The technical field of the invention is that of primary lithium electrochemical cells, in particular that of the negative electrodes (anodes) used in such cells. The technical field is also that of methods for preparing negative electrodes for primary lithium electrochemical cells. Background to the invention

[0002] The terms “element” or “electrochemical element” will be used interchangeably in the following. The term “primary” designates a non-rechargeable electrochemical element, also called a battery, as opposed to the term “secondary” which designates a rechargeable electrochemical element, also called an accumulator.

[0003] The family of primary lithium electrochemical elements includes elements whose positive active material (cathodic) is solid and elements whose positive active material is liquid. The present invention relates to elements whose positive active material is liquid.

[0004] The best known primary cells comprising a liquid positive active material are those of the lithium / sulfur dioxide (Li / SO2), lithium / thionyl chloride (Li / SOCl2) and lithium / sulfuryl chloride (Li / SO2Cl2) types. The positive electrode may be in the form of a porous carbon block, for example consisting of carbon black grains compressed together, in the presence of a binder, impregnated with liquid positive active material. SO2 being gaseous at room temperature, it is present in the cell in a dissolved form in an organic solvent, such as acetonitrile. SOC12 or SO2C12 being liquid at room temperature, they constitute both the solvent of the electrolyte and the positive active material of the cell. A lithium salt is dissolved in the solvent. The negative electrode is a sheet of lithium metal or a lithium alloy.

[0005] During the discharge of such an element, the oxidation of metallic lithium into lithium ions occurs at the negative electrode according to the following reaction: 2 Li —> 2 Li+ + 2 e

[0006] At the positive electrode, the reduction of the liquid active material oxidizing compound occurs in the pores of the carbon block, the cathodic reaction support. In the case where the liquid active material is SO2, the reduction reaction is as follows: 2 S02 + 2 e —> S2O42

[0007] In the case where the liquid active material is SOC12, the reduction reaction is as follows: 2 SOC12 + 4 e S + SO2 + 4 Cl

[0008] [Fig. 1] schematically represents an example of the structure of an element (1) of cylindrical format with liquid positive active material. In this example, the assembly of the electrodes is said to be of the “coil” type. The element comprises a container (2) of cylindrical format. A sheet (3) of lithium or lithium alloy is pressed against the internal wall of the container. This sheet is the negative electrode of the element. A separator (4) is placed against the lithium sheet. It is permeable to ions and isolates the lithium or lithium alloy sheet from the positive electrode. The positive electrode is arranged in the remaining internal volume of the container. It is in the form of a cylindrical block of carbon (5), the liquid positive active material (6) being located in the pores of the carbon. The side wall of the container and the bottom of the container are at the potential of the lithium or lithium alloy sheet.The bottom of the container serves as the negative terminal of the element. It is electrically insulated from the carbon block by a sheet of electrical insulation. The top opening of the container is closed by a cover (7) generally made of metal. The center of the cover receives a nail (8) having a head and a stem (9). The head of the nail serves as the positive current output terminal. The head of the nail is insulated from the cover by a glass insulator (10) of annular shape (glass-metal type junction). The stem of the nail is driven over most of the height of the carbon block. It serves to conduct current from the carbon block to the positive current output terminal.

[0009] Another assembly of the "spiral" type electrodes consists of superimposing at least one positive electrode, at least one separator and at least one negative electrode. The assembly is wound to form a spiral-shaped electrochemical bundle. In this case, the carbon is no longer in the form of a central block crossed by a nail but in the form of a thin layer deposited on a current collector which is a metal strip or a metal fabric.

[0010] When the element is filled with the liquid positive material, a passivation layer of lithium chloride LiCl is formed on the surface of the lithium sheet. The formation of this layer is detrimental to the proper functioning of the element because it becomes resistant to the passage of lithium ions during the discharge of the element. It causes a sudden transient voltage drop at the start of the discharge. This transient voltage drop is all the more marked when the element is discharged at low temperature or at high current. It has therefore been sought to limit the reaction between the lithium sheet and the electrolyte of a primary element with liquid positive active material.

[0011] Document KR 101556748 describes a lithium-thionyl chloride element Li-SOCl 2 wherein a protective layer is deposited on the surface of the lithium foil. The protective layer comprises at least two types of compounds selected from: - a fluorinated resin of the polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) type, - a ceramic which may be alumina, - a glassy compound such as silica SiO2, and - possibly a binder. To deposit the protective layer, several methods are described. In the absence of a binder, the ceramic or glassy compound is sprayed onto the surface of the lithium foil and the assembly is compressed to adhere the ceramic or glassy compound to the lithium foil. In the presence of a binder, the selected compounds and the binder are mixed and dispersed in an organic solvent. The dispersion is applied to the lithium foil by spraying or immersion. The solvent is then evaporated to obtain the deposit. Regardless of the method used in this document, the thickness of the deposited layer must be between 10 and 300 μm, which is thick.

[0012] A means is sought to obtain a protective layer on the lithium sheet which is as effective as that described in document KR 101556748 but which has a lower thickness. Summary of the invention

[0013] To this end, the invention proposes an electrochemical element comprising: - a positive electrode comprising a liquid active material chosen from SO2, SOC12, SO2C12 and a mixture of several of these; - a negative electrode comprising a lithium or lithium-based alloy sheet covered on at least one of the faces by a layer of amorphous alumina A12O3.

[0014] The alumina layer may have a thickness ranging from 1 to 100 nm or from 5 to 35 nm or from 5 to 20 nm or from 1 to 10 nm.

[0015] According to one embodiment, the coverage rate of the face of the lithium sheet by the alumina ranges from 50 to 100%.

[0016] According to one embodiment, the electrochemical element comprises a cylindrical container.

[0017] The lithium or lithium alloy sheet may be pressed against an internal wall of the container, the alumina layer being deposited only on the face of the lithium sheet which is oriented towards the inside of the container. The positive electrode consists of a porous carbon block whose pores contain SO2 SOC12, SO2C12 or a mixture of several of these.

[0018] The electrochemical element may comprise a cylindrical container, in which the positive electrode, a separator and the negative electrode are superimposed and wound to constitute a spiral electrochemical bundle.

[0019] The invention also relates to a method for manufacturing an electrochemical element as described above, said method comprising a step of depositing a layer of amorphous alumina Al2O3 on at least one face of a lithium sheet or a lithium-based alloy by the radiofrequency cathode sputtering technique.

[0020] The invention is based on the discovery that the radiofrequency cathode sputtering technique makes it possible to produce a very fine deposit of alumina on the surface of a lithium or lithium alloy sheet and that this deposit, despite its fineness, makes it possible to protect the lithium sheet against reaction with the electrolyte. Brief description of the figures

[0021] Embodiments of the invention are described below in more detail with reference to the following figures.

[0022] [Fig. 1] is a schematic representation of the structure of a primary lithium element according to a preferred embodiment of the invention, in which the assembly of the electrodes is of the “coil” type.

[0023] [Fig.2] is an X-ray diffraction spectrum of alpha alumina (dotted line spectrum).

[0024] [Fig.3] is an X-ray diffraction spectrum from amorphous alumina (solid line spectrum). For comparison, the X-ray diffraction spectrum from alpha alumina has been superimposed as a dotted line.

[0025] [Fig.4] is a graph representing the thickness of the alumina layer as a function of the duration of application of the radiofrequency cathode sputtering process.

[0026] [Fig.5] represents the Nyquist diagrams for different primary elements with liquid positive active matter.

[0027] [Fig.6] represents the variation of the voltage of the elements Al, B1 to B5 of the examples during a discharge at 20°C for 5 minutes under a current pulse of C / 60. The elements were previously stored for one month at 20°C.

[0028] [Fig.7] represents the variation of the voltage of the elements Al, B1 to B5 of the examples, during a discharge at 20°C for 5 minutes under a current pulse of C / 60. The elements were previously stored for one month at 20°C, discharged under the conditions of the tests of [Fig.6], again stored for one week at 20°C and finally discharged at 20°C for 5 minutes at the regime of C / 60.

[0029] [Fig.8] represents the variation of the voltage of the elements Al, B1 to B5 of the examples during a discharge at 20°C for 5 minutes at the C / 60 regime. The elements were previously stored for one month at 50°C.

[0030] Description of embodiments of the invention

[0031] The invention is characterized by the deposition of a layer of amorphous alumina on a lithium or lithium alloy foil by the radiofrequency sputtering technique ("RF sputtering"). This technique leads to the deposition of a layer of amorphous alumina. The term amorphous designates a disordered arrangement of the aluminum and oxygen atoms of the alumina, as opposed to the term crystalline which designates an ordered arrangement of these atoms. Alumina in crystalline form is not suitable for the present invention because it is not a conductor of lithium ions. The invention therefore excludes the alumina from being present in one of the alpha, beta, gamma and delta crystalline forms.

[0032] X-ray diffraction technique is the commonly used technique to detect whether a material is crystalline or amorphous. The presence of crystalline phases leads to the appearance of peaks on the X-ray diffraction spectrum at well-defined positions. For the alpha crystalline variety (corundum) of alumina which is the most common crystalline variety of alumina, the presence of peaks on the X-ray diffraction spectrum is noted at angles 20 of 25°, 35°, 38°, 43°, 52° and 57° to within plus or minus 1°, where 0 represents the angle of incidence of the X-ray beam relative to the surface of the alumina sample. The wavelength of the incident X-ray beam is the Kal line of copper.

[0033] [Fig. 2] shows in dotted line an X-ray diffraction spectrum of alpha alumina. The peaks at the 20° positions mentioned above are visible. The solid peak at the 33° position is the peak of silicon, this material having served as a substrate for the deposition of alpha alumina. In [Fig. 3], the X-ray diffraction spectrum of an amorphous alumina sample (solid line) obtained by radiofrequency cathode sputtering is superimposed for comparison with the X-ray diffraction spectrum of alpha alumina (dotted line). It is clearly noted that the X-ray diffraction spectrum of amorphous alumina does not show any peaks at the 20° positions mentioned above.

[0034] A conventional sputtering process does not allow alumina to be deposited because alumina is an electrical insulator. The invention uses a radiofrequency-enhanced sputtering process ("RF sputtering"). In a conventional sputtering process, the electric field resulting from the application of a potential difference between the target and the substrate is continuous. In the process of the invention, it is replaced by an alternating electric field, for example sinusoidal in shape, at high frequency, of the order of 50 to 300 kHz, preferably 50 to 150 kHz. The voltage applied between the target and the substrate is of the order of a thousand Volts peak to peak. The power can range from 50 to 100 w.

[0035] The method according to the invention generally comprises the following steps: In an enclosure, a secondary vacuum of the order of 106 Torr is created. An inert gas such as argon is injected. The target consists of amorphous alumina in the form of an ingot or sintered powder. The substrate is a lithium or lithium alloy sheet. It may be lithium alloyed with one or more of the elements chosen from Mg, Al, Zn, Si, B, Ge, Ga, In and Sn. Preferably, these are the elements Al, Zn and Mg. An alternating potential difference is applied between the target and the substrate. This potential difference creates a plasma composed of electrons, ions, photons, and neutrons. The positively charged species in the plasma are attracted to the alumina target and collide with it. This collision causes the aluminum and oxygen atoms in the alumina to be pulverized and redeposited on the lithium or lithium alloy foil. The advantage of this technique is that the sign of the polarization between the target and the substrate changes very quickly due to the high frequency used, which prevents the accumulation of positive charges on the target surface.The accumulation of positive charges on the target surface is undesirable because it creates electrostatic repulsion forces that repel the positive plasma species, thereby slowing down the collision of the positively charged plasma species with the target alumina.

[0036] The method according to the invention makes it possible to produce very thin layers with a thickness ranging from 1 to 100 nm, unlike the method described in document KR101556748 which leads to a deposit with a thickness ranging from 10 to 300 μm, therefore at least 100 times higher. The methods of the prior art involving the preparation of a dispersion containing alumina, a binder and a solvent, then the evaporation of the binder do not make it possible to obtain a deposit as thin as that of the invention due to the large dimensions of the binder particles.

[0037] Preferably, the layer deposited on the lithium sheet does not contain any compounds other than amorphous alumina. The method described in KR101556748 uses at least two compounds chosen from a fluorinated resin, a ceramic which may be alumina and a vitreous compound whereas only alumina is necessary in the method according to the invention. The method according to the invention does not require the use of a binder to promote the adhesion of the alumina to the lithium sheet.

[0038] The thickness of the alumina layer formed is proportional to the duration of application of the electric field for a power of 65 W. Table 1 gives, as an indication, the thicknesses of alumina layers as a function of the duration of application of the process. [Tables 1] Thickness of the alumina layer (nm) Approximate duration of application of the process (min) 1 1-3 10 9-11 15 14-16 20 17-19 40 32-35 [Fig.4] illustrates the proportionality between the thickness of the deposited alumina layer and the duration for which the process is applied. The thickness of the resulting alumina layer can be estimated using transmission electron microscopy.

[0039] The lithium or lithium alloy sheet thus covered with amorphous alumina serves as a negative electrode for an element. The format of the element can be of any type, for example cylindrical, prismatic, button or pocket type.

[0040] The cylindrical format is preferred and its structure will be described in the following. The element conventionally comprises a cylindrical format container, generally metallic, having a bottom and opposite the bottom, a circular shaped opening.

[0041] According to a preferred embodiment of the invention, the mounting of the electrodes is of the “coil” type. A negative electrode consisting of a lithium sheet or a lithium alloy covered on at least one of its faces with a layer of amorphous alumina obtained by the method according to the invention is inserted into the container and pressed against the internal wall of the container. It can cover part or all of the internal wall of the container.

[0042] Preferably, the only face of the lithium or lithium alloy sheet covered with alumina is that which is oriented towards the inside of the container. The face of the lithium or lithium alloy sheet in contact with the wall of the container may be bare.

[0043] A separator is arranged in contact with the lithium sheet. It is resistant to the electrolyte. It can be made of a layer of polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyester such as polyethylene terephthalate (PET), poly(butylene) terephthalate (PBT), cellulose, polyimide, glass fibers or a mixture of layers of different natures. The separators mentioned can be coated with a ceramic layer and / or polyvinylidene difluoride (PVdF) or poly(vinylidene fluoride-hexafluoropropylene (PVdF-HFP) or acrylates. Preferably, the separator comprises glass fibers.

[0044] A cylindrical porous carbon block is inserted into the remaining volume of the container. It may be carbon black grains compressed together in the presence of a binder, typically PTFE, or a carbon aerogel.

[0045] A generally metallic cover is welded onto the edge of the circular opening of the container. The cover is first provided with a glass-metal type junction comprising a metal base into which a nail-shaped terminal is inserted. The head of the nail serves as the terminal of the element, for example the positive terminal. The stem of the nail is inserted into the center of the carbon block and serves to conduct current from the carbon block to the current output terminal. The terminal is insulated from the base by a glass electrical insulator. Such an arrangement of the electrodes is sometimes called a "coil" type arrangement. The structure of the element obtained differs from that illustrated in [Fig.l] in that a layer of amorphous alumina is deposited on the face of the lithium foil facing the inside of the container.

[0046] Electrolyte is introduced into a hole made through the cover of the element. The electrolyte conventionally comprises a lithium salt whose anion may be chosen from chlorate, perchlorate, trihaloacetate, halide, (boro)hydride, hexafluoroarsenate, hexafluorophosphate, (tetra)chloroaluminate, (tetra)fluoroborate,

[0047] (tetra)bromochloroaluminate, (tetra)bromoborate, tetrachlorogallate, closoborate and mixtures thereof. In the case where the liquid positive active material is thionyl chloride SOC12, lithium tetrachloroaluminate salt LiAlCl4 or lithium tetrachlorogallate LiGaCl4 is preferred. The salt concentration is generally between 0.1 M and 2 M, preferably between 0.5 M and 1.5 M.

[0048] The solvent of the electrolyte consists of the liquid oxidant chosen from SO2, SOCl2, SO2Cl2, alone or in a mixture. The amorphous alumina layer protects the lithium sheet caused by the introduction of these oxidizing compounds into the container. It limits the formation of the passivation layer consisting of LiCl. The opposite face of the lithium sheet is protected by the wall of the container.

[0049] In a second embodiment of the invention, the assembly of the electrodes is said to be of the “spiral” type. In this embodiment, at least one positive electrode, at least one separator and at least one negative electrode are superimposed. Said at least one positive electrode, said at least one separator and said at least one negative electrode are wound to constitute an electrochemical bundle of spiral shape.

[0050] Said at least one negative electrode consists of a lithium sheet or a lithium alloy covered on at least one of its faces with a layer of amorphous alumina obtained by the method according to the invention.

[0051] Said at least one separator may be made of the same materials as those mentioned in connection with the “reel” type assembly described above.

[0052] Said at least one positive electrode comprises a current collector support which may be in the form of a metal strip or a metal fabric. A mixture comprising porous carbon and at least one binder is deposited on the support. The mixture of porous carbon and binder is compressed onto the support during calendering. The electrode obtained has a certain flexibility, which allows it to be wound with said at least one separator and with said at least one negative electrode to form the spiral electrochemical bundle.

[0053] The electrochemical bundle is introduced into the cell container. The metal foil of the positive electrode may be electrically connected to a current output terminal located on a cover of the cell. The lithium or lithium alloy foil may be electrically connected to a wall of the cell container, for example the bottom. The generally metallic cover is welded to the edge of the circular opening of the container. The electrolyte is introduced into the cell through a filling orifice generally located through the cover. The lithium salt(s) and the solvent of the electrolyte may be identical to those described in connection with the “coil” type assembly described above.

[0054] The element according to the invention is preferably of the primary type and is applicable to all conventional fields of application. It is particularly suitable for supplying measuring devices (water meters for example).

[0055] EXAMPLES

[0056] Different primary electrochemical elements of the Li / SOCl2 type in cylindrical format were manufactured. Each of these elements comprises a positive electrode consisting of a porous carbon block impregnated with SOCl2. Lithium tetrachloroaluminate Li AICI4 was previously dissolved in SOCl2. Its concentration is 0.6 M in all the elements. The negative electrode is either a bare lithium sheet, i.e. not coated with amorphous alumina for the reference elements A1-A3, or a lithium sheet coated with amorphous alumina deposited by radiofrequency cathode sputtering for the elements B1-B5 according to the invention.

[0057] Table 2 summarizes the characteristics of the different manufactured elements.

[0058] [Tables2] Element Negative electrode structure Thickness of amorphous alumina layer (nm) Re(Z) (ohm) Al* Uncoated lithium foil - 40 A2* - 50 A3* - 60 B1 Amorphous alumina coated lithium foil 20 «10 B2 15 «10 B3 10 «10 B4 1 «10 B5 40 «10 * example outside of invention

[0059] The elements were stored for one week at 20°C and then their complex impedance spectrum was plotted. The different spectra obtained were superimposed on [Fig.5]. The complex impedance measurements were carried out in potentiostatic mode with a PARSTAT SOLARTRON potentiostat-galvanostat equipped with an impedance spectroscopy module using a potential amplitude of 5 mV around the open-circuit voltage in a frequency range between 0.01 Hz and 150 kHz. The real part Re(Z) of the complex impedance Z was recorded and reported in Table 2.

[0060] We note the following results: - The value of the real part Re(Z) of the complex impedance of elements Al to A3 is higher than that of elements B1 to B5, which indicates a higher impedance caused by the formation of lithium chloride LiCl on the surface of the upper negative electrode for elements Al to A3. In addition, the formation of LiCl seems random for elements Al to A3 because the values ​​of Re(Z) are scattered.

[0061] - A small thickness of amorphous alumina is sufficient to slow down the growth of LiCl. In In fact, element B4, whose negative electrode is coated with only 1 nm of amorphous alumina, nevertheless has a significantly lower impedance than that of elements Al and A3.

[0062] After one month of storage at 20°C, the elements Al, B1 to B5 underwent a discharge test to evaluate the extent of the polarization. The discharge consisted of a discharge pulse at a current of C / 60 for 5 minutes at a temperature of 20°C. [Fig.6] is a superposition of the discharge voltage curves of the different elements tested. A transient voltage drop is noted which occurs in the first moments of the discharge and which comes from the polarization of the negative electrode. The voltage drop in the first moments is all the more marked as the LiCl passivation layer is thick. The voltage drop is approximately 2 V during the first millisecond of discharge for the reference Al element while it is only 0.5 V for elements B1 to B5. It is noted again that a low thickness of alumina of 1 nm is sufficient to limit the polarization of the element.

[0063] Elements A1, B1 to B5 were stored for an additional week at 20°C and underwent the same discharge test. [Fig.7] is a superposition of the discharge curves of the different elements tested. It is noted that the voltage drop of elements B1 to B5 during the transient phase of the first millisecond is more moderate than that of element A1

[0064] New Al elements, B1 to B5 were manufactured and stored for one month at 50°C in order to evaluate whether the benefit of the amorphous alumina coating also manifested itself after storage of the element at a temperature above 20°C. They then underwent the same discharge test as that described previously. [Fig. 8] is a superposition of the discharge curves of the different elements tested. The voltage of the Al element outside the invention drops rapidly to become zero after only 5 ms. By comparison, the voltage drop is much more moderate for elements B1 to B5. It is noted that a large thickness of the alumina layer does not necessarily lead to a better reduction in polarization. Element B4 in which the thickness of the alumina layer is 1 nm gives a better result than element B5 in which the thickness of the alumina layer is 40 nm.

Claims

Claims

1. An electrochemical element (1) comprising: - a positive electrode (5) comprising a liquid active material (6) chosen from SO2, SOCl2, SO2Cl2 and a mixture of several of these; - a negative electrode (3) comprising a lithium or lithium-based alloy sheet covered on at least one of the faces by a layer of amorphous alumina Al2O3 having a thickness ranging from 1 to 100 nm.

2. An electrochemical element according to claim 1, wherein the alumina layer has a thickness ranging from 5 to 35 nm.

3. An electrochemical element according to claim 2, wherein the alumina layer has a thickness ranging from 5 to 20 nm.

4. An electrochemical element according to claim 1, wherein the alumina layer has a thickness ranging from 1 to 10 nm.

5. Electrochemical element according to one of the preceding claims, in which the coverage rate of the face of the lithium sheet by the alumina ranges from 50 to 100%.

6. Electrochemical element according to one of claims 1 to 5, comprising a container (2) of cylindrical format.

7. An electrochemical element according to claim 6, wherein the lithium or lithium alloy sheet is pressed against an internal wall of the container, the alumina layer being deposited only on the face of the lithium sheet which is oriented towards the interior of the container.

8. An electrochemical element according to one of claims 6 or 7, wherein the positive electrode consists of a porous carbon block whose pores contain SO2 SOCl2, SO2Cl2 or a mixture of several of these.

9. An electrochemical element according to claim 6, wherein the positive electrode, a separator and the negative electrode are superimposed and wound to constitute a spiral electrochemical bundle.

10. Method for manufacturing an electrochemical element (1) according to one of the preceding claims, comprising a step of depositing a layer of amorphous alumina Al2O3 on at least one face of a sheet (3) of lithium or a lithium-based alloy by the radiofrequency cathode sputtering technique.

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