Method for pretreating an anode for an electrochemical cell of an electric battery

Abstract

A method of pretreating an anode for an electrochemical cell of an electric battery, comprising the following steps: a) providing an anode, b) placing the anode in the presence of a sulfuryl fluoride gas (SO2F2) until a layer comprising the -F and -SO2F groups is formed on the surface of the anode, c) removing the SO2F2 gas to stop the reaction between the SO2F2 and the anode material, and d) recovering the pretreated anode obtained.

Description

Title of the invention: Method for pretreating an anode for an electrochemical cell of an electric battery Technical field

[0001] The present invention relates, in general, to electric batteries, such as an electric battery for a motor vehicle and, more specifically, to the lifespan of an electric battery.

[0002] More specifically, the invention relates to a method for pretreating an anode for an electrochemical cell of an electric battery, as well as to an anode assembly comprising a pretreated anode, an electrochemical cell incorporating such an anode assembly and an electric battery comprising such an electrochemical cell. Previous techniques

[0003] An electrochemical cell of an electric battery comprises a positive electrode called the "cathode", a negative electrode called the "anode", an electrolyte allowing the circulation of ions between the anode and the cathode, and anode and cathode current collectors carrying, respectively, the anode and the cathode and connecting them to the external circuit.

[0004] The performance of a battery depends on the ionic and electronic transport properties.

[0005] Thermodynamic reactions are initiated during the first charging cycle of the electrochemical cell and the first ion exchanges between the electrodes take place. Products resulting from these reactions accumulate on the surface of the electrodes to form a layer called the solid-electrolyte interface or SEI layer from the English "Solid Electrolyte Interphase".

[0006] In a lithium-ion battery, this layer is an essential element for the proper functioning of the electric battery because it conducts lithium ions very well and has the advantage of stopping the catalytic decomposition of the solvent of the liquid electrolyte.

[0007] The quality of the SEI layer determines the lifespan of the battery and its formation is therefore an important step.

[0008] Alkali metal anodes such as lithium are the most attractive candidates due to their high energy density.

[0009] However, their volume is considerably changed during the cycle, which can lead to various mechanical problems in the electrochemical cell. The drastic change in volume results in continuous exposure of the anode to the reactive content of the electrolyte. In addition, the SEI layer is strongly bonded to the anode surface so that the SEI layer can be strongly deformed during cycling.

[0010] It is well known that a high lithium fluoride (LiF) SEI layer formed on an anode exhibits exceptional stability during cycling, suppressing deformation of the SEIs by anode volume change and dendrite formation.

[0011] Conventionally, the LiF-rich SEI layer is formed in situ by the addition of additives and / or solvents, such as fluoroethylene carbonate (FEC).

[0012] FEC or its derivatives provide remarkable cycle stability but its transport properties such as ionic conductivity and transfer number are limited. In other words, the use of such fluorine-containing materials for the formation of the LiF-rich SEI layer therefore limits the design of the electrolyte.

[0013] In addition, the use of fluorine-containing materials to form the LiF-rich SEI layer on an anode causes certain adverse effects.

[0014] In particular, LiPF6 was selected several decades ago for Li-ion batteries because it can serve as a source of LiF for the SEI layer on the anode and as a passivation layer on the current collector. The decomposition of LiPF6 leads, in the presence of water, to the formation of HF. However, HF is a very corrosive compound and can destroy the electrodes, the SEI layer and the passivation layer of the current collector. Statement of the invention

[0015] The present invention therefore aims to overcome the aforementioned drawbacks and to improve the stability of an electrochemical cell during its charge and discharge cycles with a view to increasing its lifetime and the lifetime of the electric battery incorporating such an electrochemical cell.

[0016] In the description of the invention which will be made, the expression "at least one" used must be considered as equivalent to the expression "one or more".

[0017] Furthermore, it is specified that the expression “between ... and ...” used in the present description of the invention must be understood as including each of the limits mentioned.

[0018] The present invention relates to a method for pretreating an anode for an electrochemical cell of an electric battery, comprising the following steps:

[0019] a) providing an anode,

[0020] b) placing the anode in the presence of a sulfuryl fluoride gas (SO2F2) until a layer comprising the -F and -SO2F groups is formed on the surface of the anode,

[0021] c) removing the SO2F2 gas, and

[0022] d) recovering the pretreated anode obtained.

[0023] In step b), the SO2F2 gas reacts with the anode material. The reaction leads to the formation of a layer comprising a compound comprising the anode material and an -F group and a compound comprising the anode material and an -SO2F group.

[0024] The obtained layer with a high content of -F and -SO2F groups is formed on the surface of the anode. Such pretreatment of the anode is particularly advantageous for use of the obtained layer as an SEI layer in an electrochemical cell of an electric battery.

[0025] The pretreatment method according to the invention makes it possible to manufacture, by a simple, rapid, and inexpensive means, an artificial SEI layer on the anode by an ex situ method.

[0026] The resulting SEI layer is rich in fluorine, the SO2F2 gas being a source of both -F group and -SO2F group. Due to the presence of such an SEI layer, the stability of the anode and the electrochemical cell is improved.

[0027] Furthermore, the pretreatment method according to the invention does not require any solvent or additive so that it can be easily adapted for large-scale use.

[0028] In one embodiment, the anode provided in step a) may be an anode made of an alkali metal, such as lithium, potassium or sodium, or of a mixture of alkali metals.

[0029] For example, the alkali metal of the anode may be lithium. The SEI layer thus formed in step b) has a high content of LiF and LiSO2F compounds. The SO 2F2 gas alone acts as a source of LiF and LiSO2F.

[0030] In another embodiment, the anode provided in step a) may be a graphite anode for the formation in the presence of SO2F2 in step b), of a layer comprising the -F and -SO2F groups on the surface of the graphite anode.

[0031] When the anode is made of graphite, step b) is preferably carried out in the presence of at least one base and at least one solvent, in addition to the SO2F2 gas.

[0032] Advantageously, step b) can be carried out in the presence of a solvent such as toluene and a base such as 4-dimethylaminopyridine (DMAP).

[0033] For example, the base, which may be DMAP, may be present in a content of between 1 and 2% by weight.

[0034] The surface of graphite is generally not uniform and contains many defects, such as -OH, -OOC, -OOH groups, etc. The reaction between the graphite anode and the SO2F2 gas allows these defects to be replaced by the -F and -SO 2F groups, in order to improve the stability of the graphite anode.

[0035] The presence of a layer comprising the -F and -SO2F groups replacing the defects on the surface of the graphite makes it possible to improve the reactivity of the entire graphite surface to the electrolyte when the anode is incorporated into an electrochemical cell. In this way, the SEI layer can be formed more homogeneously over the entire surface of the anode.

[0036] Furthermore, the replacement of defects, in particular -OH, by the -F and -SO2F groups makes it possible to avoid the formation of the HF compound which is corrosive, in the gas phase at room temperature, and which has the disadvantage of destructuring the graphite in the electrochemical cell.

[0037] Also, the -SO2F group on the graphite defects is likely to facilitate the formation of the SEI layer on the graphite anode.

[0038] Depending on the desired thickness of the SEI layer, the duration of the contact of the gas with the anode in step b) can be adapted.

[0039] Preferably, step b) is carried out for a duration of between 5 and 60 s.

[0040] According to examples, the duration of step b) may be 5 s, 15 s, 30 s or 60 s.

[0041] Preferably, step c) is carried out by replacing the SO2F2 gas with an atmosphere inert atmosphere, such as argon, to stop the reaction and avoid any side reactions. Replacing the SO2F2 gas with an inert atmosphere also allows for better control of the thickness of the SEI layer formed.

[0042] Advantageously, the anode comprises a first face intended to come into contact with an electrolyte of the electrochemical cell, and a second face opposite the first face.

[0043] The pretreatment method may comprise, prior to step b), a step of masking the second face of the anode in order to form the SEI layer only on the first face intended to come into contact with the electrolyte.

[0044] The invention also relates to an anode assembly for an electrochemical cell of an electric battery comprising an anode and an SEI layer arranged on the surface of the anode and comprising the groups -F and -SO2F.

[0045] The anode of the anode assembly may be an alkali metal, such as lithium, potassium, or sodium.

[0046] Alternatively, the anode of the anode assembly may be made of graphite.

[0047] Advantageously, the SEI layer is arranged on the first face of the anode intended to come into contact with the electrolyte.

[0048] Preferably, the thickness of the SEI layer formed is of the order of a few nm to 100 nm.

[0049] The invention also relates to an electrochemical cell for an electric battery comprising an assembly as previously described.

[0050] The electrochemical cell may comprise a cathode, and an anode assembly as previously described, an electrolyte allowing the circulation of ions between the anode and the cathode and anode and cathode current collectors carrying, res respectively, the anode and the cathode. The SEI layer of the anode assembly is positioned opposite the electrolyte.

[0051] The invention also relates to an electric battery comprising at least one electrochemical cell as previously described.

[0052] The electric battery may incorporate a lithium metal anode, an anode made of an alkali metal other than lithium, for example sodium or potassium, an anode made of a mixture of alkali metals, or a graphite anode.

[0053] The electric battery may be a lithium-ion battery, a lithium-air battery, a lithium-sulfur battery or even a battery incorporating an anode made of an alkali metal other than lithium.

[0054] In one embodiment, the anode assembly may comprise a lithium metal anode and an SEI layer comprising LiF and LiSO2F compounds.

[0055] The presence of the SEI layer, and in particular of the LiF and LiSO2F compounds, leads to stabilization of the anode during the charge and discharge cycles by slowing down the increase in volume of the anode and the formation of dendrites, which results in a prolonged battery life.

[0056] LiF, which confers high mechanical properties to the SEI layer, contributes greatly to improving the stability of the anode.

[0057] LiSO2F also contributes to the stability of the anode and compensates for the low ionic conductivity of LiF by contributing to a uniform transfer of Li+ in the plane of the SEI layer.

[0058] This results in significant stability of the Li metal during cycling obtained by a synergistic effect of LiF and LiSO2F.

[0059] The invention also relates to a motor vehicle comprising at least one electric battery as previously described. Example

[0060] Ex situ formation of an SEI layer on a lithium metal anode

[0061] A lithium metal plate is placed on a glass or polytetrafluoroethylene (PTFE) plate. PTFE adhesive tape is applied to the edges of a first face of the lithium metal plate so that the second face, opposite the first face and arranged in contact with the glass or PTFE plate, is masked. The plate is then placed in a double-necked flask, the flask is closed with a rubber stopper and then placed under an argon atmosphere using a medical needle passing through the stopper.

[0062] Under a hood, a flow of dry SO2F2 gas is inserted into the balloon using a medical needle passing through the stopper. The argon atmosphere is replaced by SO 2F2. After a period of a few seconds to 1 minute, variable depending on the expected thickness, an SEI layer is formed on the first face of the lithium metal plate. The balloon is then purged with argon to stop the reaction.

[0063] The obtained pretreated lithium metal plate is stored in a glove box filled with argon until its use in an electrochemical cell.

Claims

Claims

1. A method of pretreating an anode for an electrochemical cell of an electric battery, comprising the following steps: a) providing an anode, b) placing the anode in the presence of a sulfuryl fluoride gas (SO2F2) until a layer comprising the -F and -SO2 F groups is formed on the surface of the anode, c) removing the SO2F2 gas to stop the reaction between the SO2F2 and the anode material, and d) recovering the pretreated anode obtained.

2. The method of claim 1, wherein the anode provided in step a) is an alkali metal anode.

3. A method according to claim 2, wherein the alkali metal of the anode is lithium metal, the layer formed in step b) comprising LiF and LiSO2F compounds.

4. A method according to claim 1, wherein the anode provided in step a) is a graphite anode, step b) is carried out in the presence of at least one base and at least one solvent.

5. A method according to any preceding claim, wherein step c) is carried out by replacing the SO2F2 gas with an inert atmosphere, such as argon.

6. Method according to any one of the preceding claims, in which step b) is carried out for a duration of between 5 and 60 s.

7. Anode assembly for an electrochemical cell of an electric battery comprising an anode and a layer arranged on the surface of the anode and comprising the groups -F and -SO2F.

8. An anode assembly according to claim 7, wherein the anode is made of an alkali metal, such as lithium, sodium or potassium, a mixture of alkali metals, or graphite.

9. An electrochemical cell for an electric battery comprising an anode assembly as defined in claim 7 or 8.

10. Electric battery comprising at least one electrochemical cell as defined in claim 9.

11. Motor vehicle comprising at least one electric battery as defined in claim 10.