Electrolyte and lithium-ion battery

Abstract

Electrolyte and Lithium-ion Battery The invention relates to an electrolyte for a lithium-ion battery comprising, in mass percentages expressed relative to the mass of said electrolyte: - between 8% and 20% of a lithium salt, - between 0.25% and 2% of methylene methane disulfonate, - between 0.25% and 2% of ethylene sulfate, - a complementary additive, the mass percentage of which does not exceed 5%, - q.s. of a non-aqueous organic solvent. The invention also relates to a lithium-ion battery comprising this electrolyte. Figure for the abstract: Fig. 2.

Description

Title of the invention: Lithium-ion electrolyte and battery

[0001] The present invention relates to an electrolyte and a lithium-ion battery comprising this electrolyte.

[0002] In the context of the present invention, a lithium-ion battery is a device used for the electrochemical storage of energy and its release as needed. Following the English term "battery," the term "lithium-ion battery" is also used to designate this type of electrical accumulator. It is an electrical generator consisting of two electrical conductors (namely the electrodes) in contact with an ionic conductor (the electrolyte), which may be in liquid, gel, or solid form.

[0003] The principle of the lithium-ion battery is based on the reversible exchange of lithium ions between a cathode (generally a lithium transition metal oxide such as cobalt or manganese dioxide) and an anode (generally graphite) during charge and discharge cycles, with very good cycle life. The electrolyte is aprotic (generally a dissolved lithium hexafluorophosphate salt, hereinafter abbreviated as "LiPF6") to passivate the anode and prevent degradation of the highly reactive electrodes.

[0004] The lithium-ion battery has the following advantages in particular: - high energy density thanks to the properties of lithium, - low self-discharge, - good cycle life.

[0005] This is why the lithium-ion battery is widely acclaimed for mobile applications (telephony, automotive) and in systems exploiting renewable energies (solar, wind).

[0006] More specifically, with the increasing consumption of portable electronic devices, electric vehicles, and renewable energy storage, the development of lithium-ion batteries with high energy density and power, which are safe and low-cost, has become essential. Research and development have therefore focused primarily on the development of new electrode materials and new electrolyte compositions to obtain increasingly efficient lithium-ion batteries.

[0007] Over time and with the number of charge and discharge cycles, the capacity of a lithium-ion battery tends to degrade and its internal resistance to increase, so that it becomes unusable.

[0008] The physico-chemical phenomenon responsible for this aging of the lithium-ion battery is as follows: when the graphite of the electrode is in contact with During the initial charging of the battery, a layer of lithium is deposited on the electrode of the electrolyte, naturally reducing the amount of lithium ions available in the electrolyte solution. This so-called "passivation layer" electrically insulates the electrode from the electrolyte, preventing and / or restricting any further reaction between the electrode and the electrolyte. This slightly decreases the battery's capacity and increases its internal resistance. This passivation layer thickens over time and with each charge cycle, further increasing the internal resistance and decreasing the lithium-ion battery's capacity.

[0009] Thus, the alteration of capacity, relative to its nominal value, is one of the visible effects of aging in a lithium-ion battery and contributes to reducing the battery's performance. In this regard, within the scope of the present invention, the capacity of a lithium-ion battery is defined as the amount of charge that can be supplied by the battery during discharge. It is the integral of the current that can be delivered in one hour (Ah) and that allows the battery to go from a fully charged state to a 0% charge state. The capacity measurement is performed by galvanostatic cycling at a constant current density.

[0010] The inventors of the present invention sought to overcome this drawback by developing a new lithium-ion battery electrolyte composition for which the retention of its capacity during charge and discharge cycles or during storage at high temperature is superior to that of known lithium-ion batteries in the state of the art.

[0011] In the context of the description of the present invention, the following abbreviations are used: - BS for butane sultone; - CMC for carboxymethylcellulose; - DEC for diethyl carbonate; - DMC stands for dimethyl carbonate; - DTD for ethylene sulfate; - EC for ethylene carbonate; - EMC for ethyl and methyl carbonate; - FEC for fluoroethylene carbonate; - LCO for LiCoO2; - LiBOB for lithium bis(oxalato)borate; - LiDFOB for lithium difluoro(oxalato)borate; - LFP for LiFePO4; - LiFSI for lithium bis(fluorosulfonyl) imide; - LiPF6 for lithium hexafluorophosphate; - LiTFSI for LiN(SO2CF3)2; - LMO for LiMn2O4; - MMDS for methylene methane disulfonate; - NCA for Li(Ni,Co,Al)O2; - NMC for Li(Ni,Mn,Co)O2; - NMP stands for N-methyl-2-pyrrolidone; - PC for propylene carbonate; - PES for prop-l-ene-l,3-sultone; - PS for 1,3-propane sultone; - PVDF stands for polyvinylidene fluoride; - SBR stands for styrene-butadiene; - TMS for trimethylene sulfate; - VEC stands for vinylethylene carbonate; - VC stands for vinylene carbonate.

[0012] The invention has as its primary object an electrolyte which is characterized in that it comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 8% and 20%, preferably between 11% and 16%, of at least one lithium salt, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of MMDS, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of DTD, - at least one additional additive, the mass percentage of which does not exceed 5%, preferably 2.5%, and if the additional additive is FEC, the mass percentage of FEC is less than 0.5%, preferably less than 0.25%, - Qsp of at least one non-aqueous organic solvent.

[0013] “Qsp” is the acronym for “Quantity sufficient for” to mean that the mass percentage of solvent in the electrolyte composition is such that, added to the percentages of all the other constituents of said electrolyte composition, a total of 100% is obtained.

[0014] The inventors have discovered in a completely surprising way that the association of MMDS and DTD in quantities such that their mass percentages are between 0.25% and 2% with at least one complementary additive whose mass percentage does not exceed 5% and if it is FEC, said mass percentage is less than 0.5%, in the composition of an electrolyte made it possible to obtain a lithium-ion battery whose capacity retention remains superior during cycling or during storage to that of known lithium-ion batteries of the state of the art.

[0015] In other words, selecting a mass content of between 0.25% and 2% MMDS and between 0.25% and 2% DTD, in combination with at least one additive at a mass content as described above in the electrolyte composition of a lithium-ion battery, has an effect on its capacity, as well as on its capacity retention. Indeed, this selection, in synergy with the additive(s), makes it possible to obtain excellent capacity retention during cycling or during storage of the lithium-ion battery. This is very beneficial for the performance of the lithium-ion battery, which therefore has an increased lifespan compared to that of known state-of-the-art lithium-ion batteries.

[0016] The additional additive can advantageously be chosen from the group consisting of PS, VC, FEC, VEC, PES, BS and TMS, taken alone or in a mixture of these.

[0017] Preferably, the additional additive is PS and / or VC. Indeed, tests using electrolytes according to the invention comprising PS and / or VC have been very conclusive. VC passively treats the graphite electrode. PS limits the generation of gas (which is to be avoided).

[0018] As described above, the total mass percentage of the additional additive(s) does not exceed 5%, preferably 2.5%. If the additional additive is FEC, the mass percentage of FEC is less than 0.5%, preferably less than 0.25%.

[0019] For additional additives other than FEC, the mass percentage of each of the additional additives may advantageously be between 0.25% and 2%, preferably between 0.5% and 1.5%. In other words: - the mass percentage of PS can be between 0.25% and 2%, preferably between 0.5% and 1.5%, - the mass percentage of VC can be between 0.25% and 2%, preferably between 0.5% and 1.5%, - the mass percentage of VEC can be between 0.25% and 2%, preferably between 0.5% and 1.5%, - the mass percentage of BS can be between 0.25% and 2%, preferably between 0.5% and 1.5%, - the mass percentage of TMS can be between 0.25% and 2%, preferably between 0.5% and 1.5%, - the mass percentage in PES can be between 0.25% and 2%, preferably between 0.5% and 1.5%.

[0020] In one embodiment of the invention, the total mass percentage of the complementary additive(s) may be between 0.25% and 5%, preferably between 1% and 2.5%.

[0021] In a preferred embodiment of the invention, the electrolyte comprises PS and VC as complementary additives in the following mass percentages: - between 0.25% and 2%, preferably between 0.5% and 1.5% of PS, - between 0.25% and 2%, preferably between 0.5% and 1.5% of VC.

[0022] Lithium salt ensures the ionic conductivity of lithium ions within the lithium-ion battery.

[0023] The lithium salt can be selected from the group consisting of LiFSI, LiPF6, LiBF4, LiTFSI, LiClO4, LiAsF6, LiBOB and LiDFOB, taken alone or in a mixture thereof. Preferably, the lithium salt is a mixture of LiFSI and LiPF6.

[0024] Lithium salt LiPF6 offers the best compromise between electrochemical and thermal stability, ionic conductivity and passivation of the aluminum foil used as a current collector for the positive electrode.

[0025] In one embodiment of the invention, with regard to the lithium salt, the electrolyte comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 1.5% and 4%, preferably between 2% and 3.5% of LiFSI, - between 8.5% and 16%, preferably between 10% and 14% of LiPF6.

[0026] The electrolyte according to the invention may comprise one or more non-aqueous organic solvents. It enables the proper functioning of the lithium-ion battery and optimizes the conductivity of the electrolyte.

[0027] This may, for example, consist of one or more non-aqueous organic solvents selected from cyclic or linear-chain carbonate esters. These cyclic or linear-chain carbonate esters allow the conductivity and viscosity of the electrolyte according to the invention to be adjusted in order to improve the cycling and power performance of the lithium-ion battery.

[0028] More specifically, the cyclic carbonate ester can be chosen from the group consisting of EC, PC, 1,2-butylene carbonate and 2,3-butylene carbonate.

[0029] The linear chain carbonate ester can be selected from the group consisting of DMC, DEC, dipropyl carbonate, dibutyl carbonate, EMC, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate and ethyl propyl carbonate.

[0030] Thus, the non-aqueous organic solvent can be chosen from the group consisting of EC, PC, 1,2-butylene carbonate, 2,3-butylene carbonate, DMC, DEC, dipropyl carbonate, dibutyl carbonate, EMC, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate and ethyl propyl carbonate, taken alone or in mixtures thereof.

[0031] Preferably, the non-aqueous organic solvent is chosen from the group consisting of EC, EMC and DMC, taken alone or in a mixture thereof. These solvents allow both good ionic conductivity of the electrolyte and do not induce degradation of the electrode materials (in particular graphite), and this in synergy with the complementary additives described above and in the mass percentages detailed above.

[0032] In one embodiment of the invention, the non-aqueous organic solvent is a mixture of solvents comprising, in mass percentages expressed relative to the total mass of said solvent: - 30% EC, - 70% EMC.

[0033] In one embodiment of the invention, the non-aqueous organic solvent is a mixture of solvents comprising, in mass percentages expressed relative to the total mass of said solvent: - 30% EC, - 40% EMC, - 30% DMC.

[0034] The non-aqueous organic solvent may further comprise at least one carboxylic acid ester to improve the conductivity of the electrolyte and decrease its viscosity.

[0035] The carboxylic acid ester may be selected from the group consisting of methyl formate, ethyl formate, propyl formate, isopropyl formate, methyl propanoate, ethyl propanoate, propyl propanoate, isopropyl propanoate, methyl acetate, ethyl acetate, propyl acetate and isopropyl acetate, taken alone or in a mixture of these.

[0036] Preferably, the mass percentage of carboxylic acid ester expressed in relation to the total mass of the electrolyte does not exceed 30%.

[0037] In a preferred embodiment of the invention, the electrolyte comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 8% and 20%, preferably between 11% and 16%, of at least one lithium salt chosen from LiFSI and LiPF6, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of MMDS, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of DTD, - between 0.25% and 4%, preferably between 0.5% and 2%, of at least one additional additive chosen from PS and VC, - Qsp of at least one non-aqueous organic solvent selected from EC, EMC, and DMC.

[0038] In a preferred embodiment of the invention, the electrolyte comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 1.5% and 4%, preferably between 2% and 3.5%, of LiFSI, - between 8.5% and 16%, preferably between 10% and 14%, of LiPF6, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of MMDS, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of DTD, - between 0.25% and 4%, preferably between 0.5% and 2%, of at least one additive complementary option chosen from PS and VC. - Qsp of at least one non-aqueous organic solvent chosen from EC, EMC and DMC.

[0039] In a preferred embodiment of the invention, the electrolyte comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 1.5% and 4%, preferably between 2% and 3.5%, of LiFSI, - between 8.5% and 16%, preferably between 10% and 14%, of LiPF6, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of MMDS, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of DTD, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of PS, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of VC, - between 10% and 50%, preferably between 20% and 40%, of EC, - between 10% and 80%, preferably between 60% and 80%, of EMC, - between 0% and 60%, preferably between 0% and 40%, of DMC.

[0040] The invention also relates to a lithium-ion accumulator comprising an electrolyte according to the invention as described above.

[0041] More specifically, the lithium-ion battery comprises a positive electrode, a negative electrode, a separator between the two electrodes and an electrolyte according to the invention as described above.

[0042] The positive electrode comprises a positive current collector and a layer of positive active material. The characteristics of the positive electrode are readily apparent to those skilled in the art. For example, the positive active material can be selected from the group consisting of LFP, NMC, NCA, LCO, and LMO. Preferably, the positive active material is NMC.

[0043] The negative electrode comprises a negative current collector and a layer of negative active material. For example, the negative active material may be selected from the group consisting of graphite, silicon, silicon oxide, a silicon alloy, tin, tin oxide, a tin alloy, and lithium titanate. Preferably, the negative active material is graphite.

[0044] Advantageously, the nominal voltage of the lithium-ion battery does not exceed 3.8 V, more preferably 3.6 V.

[0045] The invention will be better understood with the aid of the detailed description set forth below with reference to the accompanying drawing which represents, by way of non-limiting example, experimental data on the evolution of capacity retention of a lithium-ion battery according to the invention and a comparative lithium-ion battery.

[0046] [Fig-1] The [Fig. 1] is a graph showing the evolution of the capacity retention of lithium-ion accumulators according to the invention and comparative as a function of the number of charge and discharge cycles of said accumulators.

[0047] [Fig.2] Fig.2 is a graph showing the evolution of the retention of the lithium-ion battery capacity according to the invention and comparison based on the number of weeks of storage at 60°C of said batteries.

[0048] EXPERIMENTAL SECTION:

[0049] Experiments were carried out with an electrolyte according to the invention and a comparative electrolyte.

[0050] The electrolyte according to the invention comprised, in mass percentages expressed relative to the total mass of the electrolyte: - 12.6% LiPF6, - 3.1% LiFSI, - 0.5% MMDS, - 1% DTD, - 0.5% PS, - 0.5% VC, - 24.5% EC, - 57.3% EMC.

[0051] The comparative electrolyte comprised, in mass percentages expressed relative to the total mass of the electrolyte: - 13.5% of LiPF6, - 3.1% LiFSI, - 1% VC, - 1% PS - 24.4% EC, - 57% EMC.

[0052] For several years, VC and PS have been known and commonly used additives in electrolytes for lithium-ion batteries. Therefore, the comparative electrolyte described above is particularly interesting for comparing the performance obtained with a lithium-ion battery comprising an electrolyte according to the invention (hereinafter referred to as the "lithium-ion battery according to the invention") with that of a lithium-ion battery comprising this comparative electrolyte (hereinafter referred to as the "comparative lithium-ion battery").

[0053] The lithium-ion batteries according to the invention and comparative were prepared according to steps 1) to 4) described just below.

[0054] More specifically, only the electrolyte preparation step differed depending on whether the lithium-ion battery according to the invention (namely step 1a) or the comparative lithium-ion battery (namely step 1b) was being prepared. In other words, steps 2) to 4) were identical for these two lithium-ion batteries.

[0055] a) Preparation of the electrolyte according to the invention:

[0056] Under a controlled atmosphere, the solvents EC and EMC were mixed together. Then, the additives DTD, MMDS, VC, and PS were added to the solvent mixture. The lithium salts LiPF6 and LiFSI were dissolved in the solvent and additive mixture. The quantities of these various constituents of the electrolyte according to the invention were appropriately chosen to obtain the electrolyte according to the invention as described above.

[0057] 1b) Preparation of the comparative electrolyte:

[0058] Under a controlled atmosphere, solvents EC and EMC were mixed together. Then, additives VC and PS were added to the solvent mixture. Lithium salts LiPF6 and LiFSI were dissolved in the solvent and additive mixture. The amounts of these different constituents of the comparative electrolyte were chosen appropriately to obtain the comparative electrolyte as described above.

[0059] 2) Preparation of the positive electrode:

[0060] The active material NMC was mixed with a binding agent (PVDF) and a conducting agent (carbon black) according to the following mass contents expressed in relation to the total mass of the mixture of NMC, PVDF and carbon black: - 90% NMC, - 5% PVDF, - 5% carbon black.

[0061] Next, this mixture was dispersed in NMP until a homogeneous dispersion was obtained. This solution was mixed to obtain an electrode paste which was then deposited uniformly with a thickness of 200 µm onto an aluminum foil, then dried at room temperature, then at 100°C for one hour and finally calendered to obtain the positive electrode.

[0062] 3) Preparation of the negative electrode:

[0063] Graphite was mixed with CMC and a binding agent (SBR) according to the following mass contents expressed in relation to the total mass of the mixture of graphite, CMC and SBR: - 92% graphite, - 4% CMC, - 4% SBR.

[0064] This mixture was then dispersed in water (namely the solvent) to obtain a homogeneous dispersion. This electrode paste was then deposited uniformly with a thickness of 200 pm on a copper sheet, then dried at room temperature, then at 70°C for one hour and finally calendered to obtain the negative electrode.

[0065] 4) Preparation of the lithium-ion battery:

[0066] The positive and negative electrodes and a polypropylene separator were assembled and then placed in a plastic pouch cell. The electrolyte was introduced into the pouch, which was then vacuum-sealed to obtain the lithium-ion battery.

[0067] Aging experiments by galvanostatic cycling.

[0068] Aging experiments by galvanostatic cycling were carried out as follows: the lithium-ion batteries according to the invention and comparator were subjected to charge and discharge cycles at constant current (approximately 1 mA / cm2) at 45°C and the capacity of these batteries was measured regularly at 25°C during the 400 charge and discharge cycles.

[0069] Table 1 below details the capacity retention C relative to its nominal value “Co” as a function of the number of charge and discharge cycles for: - the lithium-ion battery according to the invention (referred to as “Invention”) and - the comparative lithium-ion battery (referred to as “Comparative”). [Tables 1] C / Ce % Number of charge and discharge cycles a 5Û ISO 15Q 200 250 300 35$ 400 Comparison 100.0 95.8 93.3 90.3 89.5 86.5 85.9 82.8 83.1 invention 96.4 94.3 93.® 92.9 92.5 91.4 90.1 88.3

[0070] The graph in [Fig.1] thus represents the evolution of the capacity retention (ordinate axis: C / Co (%)) as a function of the number of charge and discharge cycles of the lithium-ion batteries according to the invention and comparison.

[0071] In view of Table 1 and the graph in [Fig.1], it can be seen that after 400 charge and discharge cycles, the capacity retention of the lithium-ion battery according to the invention is much higher than that of the comparative lithium-ion battery (namely 88.3% versus 83.1%).

[0072] Aging experiments by storage:

[0073] Aging experiments by storage were carried out as follows: the lithium-ion batteries according to the invention and the comparative one were stored at 60°C and the internal resistance of these batteries was measured regularly for 12 weeks at 25°C.

[0074] Tables 2 and 3 below detail the capacity retention C relative to its nominal value “Co” as a function of the number of weeks of storage at 60°C for: - the lithium-ion battery according to the invention (referred to as “Invention”) and - the comparative lithium-ion battery (referred to as “Comparative”).

[0075] [Tables2] C / Q % Number of weeks in storage 0 1 2 4 6 Comparison 100.0 96.5 91.9 89.5 86.3 Invention 100.0 97.5 94.1 91.6 89.3

[0076] [Tables3] % Number of weeks of storage 10 12 14 Comparison 81.26 77.5 73.3 Invention 86.54 86.0 83.6

[0077] The graph in [Fig.2] thus represents the evolution of capacity retention (ordinate axis: C / Co (%)) as a function of the number of weeks of storage of the lithium-ion batteries according to the invention and comparison.

[0078] In view of Tables 2 and 3, as well as the graph in [Fig.2], it can be seen that after 14 weeks of storage, the capacity retention of the lithium-ion battery according to the invention is much higher than that of the comparative lithium-ion battery (namely 83.6% versus 73.3%).

[0079] These experiments demonstrate that the electrolyte according to the invention makes it possible to obtain lithium-ion batteries with higher performance in terms of their capacity.

Claims

Demands

1. Electrolyte characterized in that it comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 8% and 20%, preferably between 11% and 16%, of a mixture of lithium bis(fluorosulfonyl)imide (hereinafter abbreviated as "LiFSI") and lithium hexafluorophosphate (hereinafter abbreviated as "LiPF6"), - between 0.5% and 1.5% of methylene methane disulfonate (hereinafter abbreviated as MMDS), - between 0.25% and 2%, preferably between 0.5% and 1.5%, of ethylene sulfate (hereinafter abbreviated as DTD), - at least one additional additive, the mass percentage of which does not exceed 2.5%, and if the additional additive is fluoroethylene carbonate (hereinafter abbreviated as "FEC"), the mass percentage of FEC is less than 0.5%, preferably less than 0.25%, - Qsp of at least one non-aqueous organic solvent,"Qsp" is the acronym for "Quantity Sufficient to" to signify that the mass percentage of solvent in the electrolyte is such that, added to the percentages of all the other constituents of the electrolyte, a total of 100% is obtained.

2. Electrolyte according to claim 1, characterized in that the additional additive is selected from the group consisting of 1,3-propane sultone (hereinafter abbreviated PS), vinylene carbonate (hereinafter abbreviated VC), FEC, vinylethylene carbonate, prop-l-ene-l,3-sultone, butane sultone and trimethylene sulfate, taken alone or in mixtures thereof.

3. Electrolyte according to claim 2, characterized in that the additional additive is PS and / or VC.

4. Electrolyte according to any one of claims 1 to 3, characterized in that the non-aqueous organic solvent is selected from the group consisting of ethylene carbonate (hereinafter abbreviated EC), propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, dimethyl carbonate (hereinafter abbreviated DMC), diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethyl and methyl carbonate (hereinafter abbreviated EMC), methyl and propyl carbonate, methyl and isopropyl carbonate, methyl and butyl carbonate and ethyl and propyl carbonate, taken alone or in mixtures thereof.

5. Electrolyte according to claim 4, characterized in that the non-aqueous organic solvent is selected from the group consisting of EC, EMC and DMC, taken alone or in mixtures thereof.

6. Electrolyte according to claim 1, characterized in that the electrolyte comprises, in mass percentages expressed relative to the mass of said electrolyte: - between 8% and 20%, preferably between 11% and 16%, of a mixture of LiFSI and LiPF6, - between 0.5% and 1.5% of MMDS, - between 0.25% and 2%, preferably between 0.5% and 1.5%, of DTD, - between 0.5% and 2%, of at least one complementary additive selected from PS and VC, - Qsp of at least one non-aqueous organic solvent selected from EC, EMC and DMC.

7. Lithium-ion battery comprising an electrolyte according to any one of claims 1 to 6.