FR3009439A1 - USE OF BORATES AS INHIBITOR FOR DEGRADING ORGANIC ELECTROLYTES IN ELECTROCHEMICAL BATTERIES - Google Patents

Abstract

The invention relates to the use of one or more compounds comprising a borate functional group, in complexed or non-complexed form, for inhibiting the action of alcoholates on the degradation of the electrolyte of electrochemical cells. The invention also relates to an electrolyte comprising one or more particular compounds having a borate function. Finally, the invention relates to an electrochemical cell comprising the aforementioned electrolyte.

Description

The invention relates to the field of electrochemical cells, in particular in lithium-ion batteries, and more particularly to the electrolyte domain of these cells. More specifically, the invention relates to the use of compounds belonging to the family of borates as an additive in the electrolyte of electrochemical cells for inhibiting the action of alcoholates on the degradation of this electrolyte. The invention also relates to an electrolyte comprising one or more compounds having a borate function. Finally, the invention relates to an electrochemical cell comprising the aforementioned electrolyte. Due to their high energy density, lithium-ion batteries have become widely accepted in the field of batteries and allow in particular to consider the development of electric vehicles with a range greater than 200 km.

However, these batteries have the disadvantage of having a still low cyclability, of the order of 500 to 1000. Cyclability is expressed in number of cycles (a cycle corresponds to a charge and a discharge). It characterizes the life of the battery, that is to say the number of times it can restore the same level of energy after each new recharge. The drop in the level of available energy as the charging and discharging cycles can have several origins, such as the formation of a passivation layer on the electrodes and the degradation of the electrolyte in particular.

The degradation of the electrolyte is notably due to a cascade of irreversible reactions in the electrolyte and in particular to the chemical action of the alcoholates (formed by decomposition of the solvent) on the solvent itself, leading to degradation products. The electrolyte thus becomes, during the cycles, "polluted" and performs less well its function of ionic conductor. In addition, the generated degradation products are deposited on the surface of the electrodes. This leads to batteries that lose, at each cycle, energy capacity and power until the battery becomes unusable. In order to improve the cycling of lithium-ion batteries, Aurbach et al (Electrochim Acta, 47, (2002), 1423-1439) have proposed the use of vinylene carbonate (or 1,3-dioxol-2-one). ) as an additive of the electrolyte. The additive reacts with the surface of the electrodes and forms a passivation layer. However, this film can increase the resistivity of the electrode, which decreases the capacity in terms of battery power. Thus, there is a need to use new compounds that can inhibit the action of alcoholates on the electrolyte solvent to stabilize it, in order to improve the cyclability of these batteries, without compromising their power. It has now been discovered that the use of compounds comprising a borate function as an additive for electrolytes of lithium-ion batteries makes it possible to respond to the technical problems mentioned above. In particular, these compounds, once dissolved in the electrolyte, act as a complexing agent with respect to the alkoxides. The alkoxides are more preferably captured relative to other anions that may be present in the electrolyte (fluoride ion in particular). These compounds thus make it possible to trap alcoholates and thus prevent the formation of irreversible cascade reactions. The other elements of the electrolyte therefore remain intact. The result is that batteries comprising such an electrolyte have better cyclability, that is, they can be used for a greater number of cycles at a maintained energy capacity. In addition, the technical effect is obtained with a low concentration of compounds having a borate function.

Finally, the targeted compounds can be used in combination with other additives and are soluble in the electrolyte. In addition, their synthesis is already known as explained below. Other characteristics and advantages of the invention will emerge more clearly on reading the description, the examples and the single figure which follow. The single figure describes the standardized energy capacity of two batteries, one according to the invention and a comparative one, as a function of the number of charging and discharging cycles.

The subject of the invention is therefore the use of one or more compounds comprising a borate functional group, in a complexed or non-complexed form, for inhibiting the action of alcoholates on the degradation of the electrolyte of electrochemical cells. The borate-containing compound or compounds may be in complexed form or not. When the said compound (s) are in non-complexed form, they are preferably chosen from the compounds as defined in the following formula (I): ## STR2 ## where Y is an alkyl group or a nitro group, Z1 and Z2 independently of one another, being a CH2 group or a C = O group.

Preferably, Y is selected from a C 1 -C 3 alkyl group or a nitro group. In a particularly preferred manner, Y is chosen from a methyl group, an ethyl group or a nitro group. Most preferably, the one or more compounds according to formula (I) are preferably selected from the compound with Y = CH 3 and Z 1 = Z 2 = a group C = O.

When the one or more compounds are in complexed form, they are preferably chosen from the compounds as defined in the following formula (II): A (II) - Y 'being an alkyl group or a nitro group, - Z'i and Z'2 independently of one another, being a group CH2 or a group C = O, - A is a metal cation, preferably selected from Na, K and Li.

Preferably, Y 'is chosen from a C 1 -C 3 alkyl group or a nitro group. In a particularly preferred manner, Y 'is chosen from a methyl group, an ethyl group or a nitro group. Most preferably, the compound or compounds according to formula (II) are preferably chosen from compounds with Y '= CH3, Z'l = Z'2 = a group C = O and A = Li, Na or K Whether or not the compounds having a borate function are in complexed form or not, the mode of action for inhibiting alcoholates is the same. It consists of complexing the alcoholate ions formed. In the case of complexed compounds, the fluorine atom is replaced by the alcoholate molecule. The synthesis of these compounds comprising a borate function, whether they are in complexed or non-complexed form, can be carried out in particular from document US 2011 / 0171112. The compounds obtained are generally in solid form. They are generally soluble in polar aprotic solvents such as carbonate esters or ethers.

In a particular embodiment, the compound or compounds comprising a borate function may be grafted or encapsulated by means of a mineral filler. The inorganic filler is preferably selected from Al 2 O 3, TiO 2, BaTiO 3, SiO 2, ZrO 2, LiAlO 2, MgO. The effectiveness of the alcoholate inhibition remains equivalent to the unencapsulated version of the compounds comprising a borate function. According to the invention, the one or more compounds comprising a borate functional group represent preferably from 0.1% to 10% by weight, more preferably from 0.5% to 5% by weight, and in particular from 1.5% to 3% by weight. % by weight, based on the total weight of the electrolyte. The compound or compounds comprising a borate function used according to the invention react with the alcoholates to inhibit their harmful action on the solvent of the electrolyte.

The alcoholate (s) are, in particular, the compounds of formula (III) below: R-O M + (III) - R being an alkyl group, and preferably a methyl group. M being a metal cation, preferably lithium. In a preferred manner, the alkoxide that is to be inhibited in the electrolyte is lithium methanolate or ethanolate. In general, the alcoholates are derived from the electrochemical reduction of the electrolyte solvent, or chemical reactions between the electrolyte solvent degradative products. This concerns in particular solvents such as carbonate esters or ethers. Once formed, the alcoholate (s) react with the compounds contained in the electrolyte, and more particularly react with the solvent molecules of the electrolyte. The presence of one or more compounds used according to the invention makes it possible to avoid this reaction.

Preferably, the electrolyte comprises one or more polar aprotic solvents preferably chosen from carbonate esters and ethers, γ-butyrolactone and mixtures thereof. As carbonate ester, mention may especially be made of dimethyl carbonate, diethyl carbonate, ethyl and methyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and a mixture of these compounds. It is known to those skilled in the art that solvents capable of being co-interposed, such as propylene carbonate with lithium in the negative graphite electrode, can not be used pure and must be diluted to prevent this phenomenon. . As ether, mention may especially be made of tetrahydrofuran, methyl ethers of oligoethylene glycols of general formula CH30 (CH2CH2O) .CH3 with 1 <n <4 and a mixture of these compounds.

Preferably, the compound or compounds comprising a borate function are used according to the invention in the lithium-ion battery electrolyte. However, the borate functional compound (s) may also be used in sodium ion or potassium ion battery electrolyte or any other system in which alcoholates may adversely affect battery capacity. The subject of the invention is also an electrochemical cell electrolyte comprising one or more compounds comprising a borate functional group chosen from the compounds of formula (I) or (II) described above. Preferably, in the electrolyte described above, the compound or compounds comprising a borate function represent from 0.1% to 10% by weight, preferably from 0.5% to 5% by weight, and in particular from 1.5% to 3% by weight, based on the total weight of the electrolyte.

The invention also relates to an electrochemical cell comprising an anode, a cathode and an electrolyte, said electrolyte being as defined above. The anode and the cathode can be chosen from all the electrodes known to those skilled in the art. Finally, the invention relates to a battery comprising the electrolyte as defined above. The examples which follow have for the invention without however limiting it. Examples The following examples are intended to show the effectiveness of compounds having a borate function as an alcoholate inhibitor. For this purpose, the presence of methanolate is examined, for example, by the presence or absence of compounds 11 or 1, depending on the solvent mixture used: [Intermediate A Me0 + 0 0 DMC [Intermediate A] + These compounds li and ri are obtained by reacting an alkoxide with a first solvent which is an ethylene carbonate molecule (EC). Intermediate A thus formed, then reacts with the second solvent (dimethyl carbonate (DMC) or diethyl (DEC)) to form compound 11 or the compound. Thus the presence of these compounds is an indicator of the presence of methanolate or ethanolate (and more generally alcoholates) in the electrolyte.

The two solvent systems are studied: ethylene carbonate / dimethyl carbonate on the one hand, and ethylene carbonate / diethyl carbonate on the other hand.

Example 1: Electrolyte of ethylene carbonate / dimethyl carbonate type (EC / DMC) Two electrolytic solutions are prepared according to Table 1 below: Table 1: AB electrolytic solution (1) EC / DMC solvent EC / DMC (1) / 1 m / m (2)) (1/1 m / m (2)) LiPF6 LiPF6 electrolyte salt (1 mol / L) (1 mol / L) Borate compound Compound 1 (3) - 3% by weight (1) Commercial electrolyte known as LP30 (2) Mass ratio (3) The compound of formula (I) with Y = CH3 and Z1 = Z2 = the group C = O. Two electrochemical cells are prepared from the electrolytic solutions A and B above. The cells are of Swagelok® type. The negative electrode is based on graphite. The commercial positive electrode used in this example is based on a LiMn 2 O 4 / LiN 3 Cl 3 Mh 3 ClO / 302 mixture (1: 1 by mass). Two Whatman GF / D borosilicate separators, impregnated with 1600, of electrolyte A or B, are placed between the two electrodes. The cells are then subjected to a charge for 6 hours at C / 50 (a complete charge in 50h) then the composition of the electrolyte is analyzed by GC / MS (gas chromatography associated with mass spectrometry) after opening. the cell. The results are collated in Table 2 below: Table 2: Compound studied Amount (relative percentage determined by the integration of the product peaks in GC / MS) Solution A Solution B Ethylene carbonate 100% 100% Compound li 1% 21% This table shows the disappearance of the compound 1 i in the electrolyte solution A containing the additive unlike the electrolyte solution B does not contain it. Therefore, this proves that the compound 11 could not be formed in the medium and therefore that the alcoholates are inhibited by the additive. EXAMPLE 2 Electrolyte of the ethylene carbonate / diethyl carbonate type (EC / DEC) Two electrolytic solutions are prepared according to Table 3 below: TABLE 3 Electrolytic solution CD Solvent EC / DEC EC / DEC (3/7 v / v (1)) (3/7 v / v (1)) Electrolytic salt LiPF6 LiPF6 (1 mol / L) (1 mol / L) Compound with borate function Compound 2 (2) - 1.5% by weight (1) Ratio by volume (2) the compound (I) with Y = C2H5 and Z1 = Z2 = the group C = O, 1.5% by weight In the same way as for example 1 above, two cells are prepared and loaded until reaching a full charge (50 hours at C / 50). The composition of their electrolyte is analyzed.

The results are summarized in Table 4 below: Table 4: Compound studied Amount (relative percentage determined by the integration of product peaks in GC / MS) Solution C Solution D Ethylene carbonate 100% 100% Compound 1 ' 1 <0.1% 30% This table shows the disappearance of the compound in the electrolyte solution C containing the additive unlike the electrolytic solution D does not contain it.

Therefore, this proves that the compound 1 'i could not be formed in the medium and therefore that the alcoholates are inhibited by the additive. In these two cases, the compounds comprising a borate function trap alcoholates and prevent their degradation action vis-à-vis solvents by chemical reactions. Thus, from the first charge, the beneficial action of the compound having a borate function is observed by the absence of degradation products.

Example 3 Cyclability Two batteries from the electrolytes of Example 1 above are prepared. They are assembled in the form of button batteries. The negative electrode is based on graphite. The commercial positive electrode used, in this example, is based on a mixture LiMn 2 O 4 / LiNi 3 MoI 3 ClO / 302, (1: 1 by mass). The separator is made of Whatman GF / D borosilicate. Each cell contains 150 liters of electrolyte.

Charge / discharge cycles are carried out on these batteries and the standardized energy capacity of these batteries is studied. The cyclability results are collated in Figure 1. This single figure shows the normalized energy capacity levels of the two batteries as a function of charge and discharge cycles. Curve 2 of this figure represents the evolution of the energy capacity of the battery according to the invention comprising the electrolyte A. Curve 1 of this figure represents the evolution of the energy capacity of the comparative battery comprising the electrolyte B. The beneficial action of the compound having a borate function results in a better long-term cyclability of the batteries containing this compound in their electrolyte.

Claims (15)

REVENDICATIONS1. Use of one or more compounds comprising a borate function, in a complexed or non-complexed form, for inhibiting the action of alcoholates on the degradation of the electrolyte of electrochemical cells. 2. Use according to the preceding claim, characterized in that the one or more compounds comprising a borate functional group are chosen from the following compounds of formulas (I) or (II): Z 0 i Y OB (1) Z 2 O -Y and Y independently of one another, being an alkyl group or a nitro group, - Z1, Z2, Z'1, Z'2 independently of one another, being a CH2 group or a C = O group . - A being a metal cation, preferably selected from Na, K and Li. 3. Use according to the preceding claim, characterized in that Y and Y 'independently of one another, are selected from a C 1 -C 3 alkyl group or a nitro group. 4. Use according to the preceding claim, characterized in that Y and Y 'independently of one another, are selected from a methyl group, an ethyl group or a nitro group. 5. Use according to any one of the preceding claims, characterized in that the compound or compounds comprising a borate function are chosen from compounds of formulas (I) and (II) with Y = Y '= CH3, Zi = Z2 = Z'i = = a group C = 0 and A = Li, Na or K. 6. Use according to any one of the preceding claims, characterized in that the one or more borate compounds are grafted or encapsulated by means of a mineral filler. 7. Use according to the preceding claim, characterized in that the inorganic filler is selected from A1203, TiO2, BaTiO3, SiO2, ZrO2, LiA1O2, MgO. 8. Use according to any one of the preceding claims, characterized in that the one or more compounds comprising a borate function represent from 0.1% to 10% by weight, preferably from 0.5% to 5% by weight, and in particular from 1.5% to 3% by weight, relative to the total weight of the electrolyte. 9. Use according to any one of the preceding claims, characterized in that said alcoholates are the compounds of formula (III) below: R-O M + (III) - R being an alkyl group, and preferably a methyl group. M being a metal cation, preferably lithium. 10. Use according to any one of the preceding claims, characterized in that the electrolyte comprises one or more polar aprotic solvents preferably chosen from carbonate esters and ethers. 11. Use according to any one of the preceding claims, characterized in that the one or more compounds comprising a borate function are in the lithium-ion type battery electrolyte. An electrochemical cell electrolyte comprising one or more compounds having a borate function as defined in any one of claims 2 to 7. 13. Electrolyte according to the preceding claim, characterized in that the one or more compounds comprising a borate function represent from 0.1% to 10% by weight, preferably from 0.5% to 5% by weight, and in particular from 1 to , 5% to 3% by weight, based on the total weight of the electrolyte. An electrochemical cell comprising an anode, a cathode and an electrolyte, said electrolyte being as defined in any one of claims 12 or 13. 15. Lithium-ion battery comprising an anode, a cathode and an electrolyte, said electrolyte being as defined in any one of claims 12 or 13.