WO2015078791A1 - Electrolyte compositions for lithium batteries - Google Patents

Abstract

A method for manufacturing an improved electrolyte composition for lithium batteries is herein provided. The method comprises using a hydrofluoroether with a well-defined (F+H)/H molar ratio. The method allows reducing the flammability of electrolyte composition without negatively affecting the solubility of the lithium salt contained in the composition.

Description

Electrolyte compositions for lithium batteries Cross-reference to related applications

This application claims priority from European patent application No. 13194867.1, filed on November 28, 2013. The whole content of this application is incorporated herein by reference for all purposes.

Technical Field

The present invention relates to lithium ion batteries, in particular to electrolyte compositions for such batteries.

Background Art

In view of the growing need to ensure environment protection and to reduce costs, rechargeable (secondary) batteries have raised increasing interest and have been developed in recent years. Among them, lithium ion batteries (or cells) proved particularly suitable for use in the sector of mobile electronic devices (e.g. high quality camcorders, portable computers, mobile telephones and the like), due to the fact that their energy density is about twice as high as that of metal hydride batteries and about three times higher than that of nickel cadmium cells. Lithium ion batteries have also been proposed for use in the automotive field, for example in electrically driven town automobiles.

Rechargeable lithium batteries generally contain a compound composed of lithium and a metal oxide as cathode (for example Li_xMnO₂ or Li_xCoO₂) and lithium metal as anode, the lithium preferably being used as intercalation compound with graphite or in combination with carbon or graphite fibers (graphite-based anodes). They also contain an electrolyte solution containing a lithium salt: the most widespreadly used salt is LiPF₆, which is endowed with high conductivity.

Unfortunately, lithium ion batteries involve safety risks: indeed, critical operating states, like overcharging, overdischarging or short circuit would result in the opening of the cell and outbreak of fire.

In order to reduce this risk, organic solvents containing heteroatoms with strongly polarizing action, such as, for example, nitrogen, oxygen or sulphur, for producing aprotic electrolyte solutions are used. Among such solvents, ethers (like 1,2-dimethoxyethane), esters (typically ethylene carbonate and propylene carbonate), nitriles (for example acetonitrile), lactones and sulfones can be mentioned. Ethylene carbonate and propylene carbonate have been widely used in view of the fact that they are endowed with low volatility and are less inclined to form volatile mixtures. However, they are also highly viscous and the electrolyte solution prepared therefrom has low conductivity. Furthermore, propylene carbonate proved not suitable for graphite anodes, while ethylene carbonate showed good performances. In order to decrease viscosity, a further component usually referred to as “thinner”, is usually added to such solvents; a commonly used thinner is dimethyl carbonate. Nevertheless, such thinners, in particular dimethyl carbonate, are volatile and imply the risk of ignition and explosion if air enters in the battery.

The electrolyte solution contained in rechargeable lithium batteries can also comprise safety measures, including a microporous separation diaphragm between the catode and the anode, pressure release switches and flame-retarding additives. However, such measures do not remove the risk that the volatile components, in particular dimethyl carbonate, ignite themselves and also the negative electrode; in such case, the resulting fire would not be extinguishable, because burning lithium violently reacts not only with water, but also with the substances normally contained in standard fire extinguishers.

Furthermore, LiPF₆, often used in the electrolyte solution, decomposes at a temperature of about 80°C and upon contact with moist generates HF, which may in turn lead to the formation of hydrogen, which is highly flammable. The aforementioned ethylene carbonate is typically used to dissolve LiPF₆; however, ethylene carbonate is solid at room temperature (m.p. of about 30°C), so it has to be mixed with dimethyl carbonate in order to ensure that the electrolyte solution remains liquid at room temperature and in a wide range of working conditions. As noted above, dimethyl carbonate is highly flammable. In order to avoid the use of dimethyl carbonate, attempts were made in the past to replace LiPF₆ with (per)fluoropolyether lithium salts, but the results were not satisfactory.

There is therefore the outstanding need to provide electrolyte solutions for lithium ion batteries endowed with high safety.

With the aim of meeting such need, US 5,916,708 HOECHST AKTIENGESELLSHAFT discloses electrolyte compositions for lithium ion batteries comprising, as essential component of the electrolyte mixture, a hydrofluoroether of the formulae:
(I) RO-[(CH₂)_mO]_n-CF₂-CFH-X
and/or
(II) X-CFH-CF₂O-[(CH₂)_mO]_n-CF₂-CFH-X
wherein:
- R is a straight-chain alkyl group containing 1 to 10 carbon atoms or a branched alkyl group containing 3 to 10 carbon atoms,
- X is fluorine, chlorine or a perfluoroalkyl group containing 1 to 6 carbon atoms, which may also contain ethereal oxygen,
- m is an integer from 2 to 6 and n is an integer from 1 to 8.

In US 5,916,708 it is stated that the hydrofluoroethers of the formulae (I) and (II) have a surprisingly low viscosity, so they “can be used as thinning agents for low-flammability, highly viscous components, for example ethylene carbonate and propylene carbonate”. It is further stated that by using the hydrofluoroethers of the formulae (I) and (II) in an amount from 5 to 70% by volume, preferably from 20 to 50% by volume relative to the total volume of the electrolyte system, “aprotic electrolyte systems can be produced which are virtually no longer flammable”. Since the compounds have a tendency to thermally decompose above 200°C, tertiary aliphatic amines can be added in amount ranging from 0.1 to 1.0% wt relative to the partially fluorinated ether. Examples 1 – 3 specifically disclose safety battery electrolyte solutions containing a hydrofluoroether of the formula:
HC₂F₄OCH₂CH₂OC₂F₄H
a lithium salt [either LiN(CF₃SO₂)₂ or LiPF₆] and ethylene carbonate and/or propylene carbonate.
In all examples, it is pointed out that the electrolyte solution has:
- favourable low temperature behaviour down to -50°C ;
- good conductivity;
- stability in a wide electrochemical window;
- favourable behaviour in the intercalation of lithium in carbons.
It is further stated that the electrolyte solutions are extremely difficult to ignite.

US 2008160419 3M INNOVATIVE PROPERTIES COMPANY 20080703 discloses electrolyte solutions for electrochemical devices, including lithium secondary batteries, and methods for their manufacture. The solutions contain a supporting electrolyte salt, such as LiPF₆, and a solvent composition including a cyclic carbonic acid ester, such as ethylene carbonate, and at least one fluorine-containing solvent having a boiling point of at least 80°C. Hydrofluoroethers are cited as fluorine-containing solvents, including one having formula:
CF₃-CFHCF₂-O-CH₂CH(OCF₂CFHCF₃)-CH₂-O-CF₂CFHCF₃.
US 2009130567 3M INNOVATIVE PROPERTIES COMPANY 20090521 discloses a non-aqueous mixed solvent for use in a non-aqueous electrolytic solution for electrochemical energy devices (including lithium-ion batteries), said solvent comprising an aprotic solvent and at least one fluorinated ether having boiling point of at least 80°C. This document further discloses electrolytic solutions containing such solvents and electrochemical energy devices containing such solutions. Inter alia, the two following fluorinated ethers:
CF₃CFHCF₂OC₂H₄OCF₂CFHCF₃ (HFE3)
and
CF₃CFHCF₂OC₃H₆OCF₂CFHCF₃ (HFE4)
are therein specifically disclosed.

WO WO 2012/084745 A SOLVAY SPECIALTY POLYMERS ITALY S.P.A. 20120628 discloses, inter alia, certain hydrofluoroether mixtures obtained by partial fluorination of hydrofluoroethers, said hydrofluoroethers being obtained by addition of a hydrogen-containing alcohol, preferably a fluorine-free alcohols, to a perfluorinated or a fluorinated olefin.

Summary of invention

The Applicant has now found out that the performances of the electrolyte solutions disclosed in US 5,916,708 can be further improved by replacing a hydrofluoroether (HFE) of formula (I) and/or (II) as defined therein with certain hydrofluoroether mixtures having a higher fluorination degree, said mixtures being obtainable with the process disclosed in WO2012/084745 A. Indeed, the Applicant observed that such hydrofluoroether mixtures and the electrolyte solutions obtainable therefrom are not flammable and maintain favourable properties in terms of solubility of the lithium salt, working temperature range, conductivity and oxidative stability at high voltage.

Accordingly, the present invention relates to a method for manufacturing an electrolyte composition for lithium ion batteries, said method comprising using a hydrofluoroether mixture [mixture (M1)] complying with at least one of average formulae (M1-a) - (M1-d) as defined here below:
(M1-a) X-CFYCF₂O[(CY₂)_mO]_n-CF₂CFY-X
(M1-b) RO[(CY₂)_mO]_n-CF₂CFY-X
(M1-c) X-CFYCF₂O(CY₂)_a(CYR¹)(CY₂)_b-OCF₂CFY-X
(M-1d) RO(CY₂)_a(CR¹Y)(CY₂)_b-OCF₂CFY-X
wherein:
- R is an optionally fluorinated straight-chain alkyl group containing from 1 to 10 carbon atoms or an optionally fluorinated branched alkyl group containing from 3 to 10 carbon atoms;
- R¹ is -OCF₂CFY-X or an optionally fluorinated straight-chain alkyl group containing from 1 to 10 carbon atoms or an optionally fluorinated branched alkyl group containing from 3 to 10 carbon atoms;
- X is a perfluoroalkyl group containing from 1 to 6 carbon atoms, which may also contain one or more ethereal oxygens;
- m is a positive number ranging from 2 to 6;
- n is a positive number ranging from 1 to 8;
- a and b are 0 or a positive number ranging from 1 to 6, with the proviso that at least one of a and b is different from 0 and that a and b are both different from 0 when R¹ is -OCF₂CFY-X;
- each Y is independently selected from hydrogen and fluorine, with the proviso that not all Y are fluorine and not all Y are hydrogen and that the molar ratio (F+H)/H in any one of formulae (M1-a) - (M1-d) ranges from 2.5 to 6.5.

For the sake of clarity, when ranges are indicated in the present description, extremes are included.

Preferably, in mixtures (M1-a) - (M1-d) above:
- R is an optionally fluorinated straight-chain alkyl group containing from 1 to 4 carbon atoms or a branched optionally fluorinated alkyl group containing 3 or 4 carbon atoms;
- X is a perfluoroalkyl group containing from 1 to 3 carbon atoms;
- m ranges from 2 to 4;
- n is 1;
- a and b are 1 or 2, preferably 1.

Preferably, the mixtures used in the compositions of the invention comply with average formulae (M1-a) and (M1-c) above.
More preferably, the mixtures used in the compositions of the invention comply with average formula (M1-a) above, wherein X is F of CF₃, m ranges from 2 to 4, n is 1 and Y is as defined above. Even more preferably, in formula (M1-a), X is F, m is 2, n is 1 and Y is as defined above. A most preferred mixture according to the present invention is a mixture (M1-a) having average raw formula C₆H₄F₁₀O₂, wherein the molar ratio (F+H)/H is 3.5.

For the avoidance of doubt, throughout the present description, the expression “average formula” is intended to denote a mixture comprising hydrofluoroethers with different amount and position of hydrogen and fluorine atoms (groups Y). For example, with particular reference to mixture (M1-a) having average raw formula C₆H₄F₁₀O₂, different amount of fluorine and hydrogen atoms means that the mixture comprises also hydrofluoroethers of formula C₆H₃F₁₁O₂ and C₆H₅F₁₁O₂, while different position of hydrogen/fluorine atoms means that a hydrofluoroether of raw formula C₆H₄F₁₀O₂ may include, for example, a hydrofluoroethers of formulae: HCF₂CF₂OCHFCHFOCF₂CF₂H, HCF₂CF₂OCHFCH₂OCF₂CF₃ and CF₃CF₂OCH₂CH₂OCF₂CF_3.

Mixtures (M1-a) - (M1-d) are obtainable by partial fluorination of hydrofluoroether precursors (HFE-1).In greater detail, mixtures (M1-a) can be obtained by partial fluorination of HFEs complying with formula (HFE1-a) here below:
(HFE1-a) X-CFHCF₂O[(CH₂)_mO]_n-CF₂CFH-X
wherein X, m and n are as defined above.

Mixtures (M1-b) can be obtained by partial fluorination of HFEs complying with formula (HFE1-b) here below:
(HFE1-b) R-CFHCF₂O[(CH₂)_mO]_n-CF₂CFH-X
wherein R, X, m and n are as defined above.

Mixtures (M1-c) can be obtained by partial fluorination of HFEs complying with formula (HFE1-c) here below:
(HFE-1c) X-CFHCF₂O(CH₂)_a(CHR¹)(CH₂)_b-OCF₂CFH-X
wherein X, R¹, a and b are as defined above.

Mixtures (M1-d) can be obtained by partial fluorination of HFEs complying with formula (HFE1-d) here below:
(HFE1-d) RO(CH₂)_a(CR¹H)(CH₂)_b-OCF₂CFH-X
wherein R, R¹, a and b are as defined above.

The partial fluorination is conveniently carried out following the process disclosed in WO 2012/084745, i.e. by reacting a hydrofluoroether precursor (HFE-1), with fluorine in the presence of at least one (per)haloolefin comprising at least one carbon-carbon double bond and having at least one fluorine or chlorine atom on either one of the carbon atoms of said double bond, the amount of (per)haloolefin being from 0.1 to 30 mol % with respect to the hydrogen atoms contained in the hydrocarbon compound. Indeed, the method disclosed in this prior art allows conveniently controlling the amount of fluorine introduced, thereby obtaining mixtures (M1-a) - (M1-d) wherein the molar ratio (F+H)/H ranges from 2.5 to 6.5, extremes included.

Hydrofluoroether precursors (HFE-1) can be prepared by addition reaction of a hydrogen-containing alcohol with a fluorinated or perfluorinated olefin. Typically, the reaction is carried out by addition of the olefin to an alcohol, optionally in the presence of a polar aprotic solvent, in the presence of a base. Alternatively, the hydrofluoroether precursor (HFE-1) may be prepared by radical addition of a hydrogen-containing alcohol, preferably a fluorine-free alcohol, to a perfluorinated or fluorinated olefin, followed by etherification of the partially fluorinated alcohol thus obtained according to methods well known in the art.

Notable examples of hydrofluoroether precursors (HFE-1) deriving from the reaction of a fluorine-free alcohol with a fluorinated olefin are: HCF₂CF₂OCH₂CH₂OCF₂CF₂H,
HCF₂CF₂OCH₂CH₂CH₂OCF₂CF₂H,
HCF₂CF₂OCH₂CH₂CH₂CH₂OCF₂CF₂H,
CF₃CFHCF₂OCH₂CH₂OCF₂CFHCF₃, CF₃CFHCF₂OCH₂CH₂CH₂OCF₂CFHCF₃,
CF₃CFHCF₂OCH₂CH₂CH₂CH₂OCF₂CFHCF₃,
HCF₂CF₂OCH₂CH₂OCH₂CH₂OCF₂CF₂H, CF₃CFHCF₂OCH₂CH₂OCH₂CH₂OCF₂CFHCF_3,
HCF₂CF₂OCH₂CH(OCF₂CF₂H)CH₂OCF₂CF₂H.

The method according to the invention is particularly suitable for reducing flammability without negatively affecting the solubility of the lithium salt contained in the composition. Indeed, hydrofluoroether mixtures (M-1a) - (M1-d) as defined above, in particular a hydrofluoroether mixture (M-1a) having average raw formula C₆H₄F₁₀O₂, are less flammable than the hydrofluoroethers of US 5,916,708, but they are able to better dissolve the lithium salt.

In a further aspect, the present invention relates to an electrolyte composition for lithium ion batteries comprising a hydrofluoroether mixture complying with at least one of formulae (M1-a) - (M1-d) as defined above, more preferably a hydrofluoroether mixture (M-1a), even more preferably a hydrofluoroether mixture (M1-a) having average raw formula C₆H₄F₁₀O₂ in admixture with one or more polar organic solvents and one or more lithium salts.

In the electrolyte composition, the weight amount of hydrofluoroether mixture with respect to the overall weight of the composition typically ranges from 1.5% to 50%, preferably from 10% to 40%, more preferably from 15% to 30%.

The one or more polar organic solvents typically comprise heteroatoms like nitrogen, oxygen or sulphur. Examples of solvents are ethers (like 1,2-dimethoxyethane), esters (typically cyclic carbonates including ethylene carbonate and propylene carbonate), nitriles (for example acetonitrile), lactones and sulfones; preferably, the solvent is ethylene carbonate or propylene carbonate or a mixture thereof. Ethylene carbonate is preferred in compositions for use in Li-batteries with graphite-based electrodes. According to a preferred embodiment, one or more linear carbonates, typically dimethyl carbonate, are used in admixture with one or more other organic solvents belonging to the aforementioned groups in order to reduce viscosity. Preferably, dimethyl carbonate is used in admixture with ethylene carbonate, propylene carbonate or a mixture thereof.

The lithium salt is preferably selected from LiN(SO₂CF₃)₂, LiPF₆ and mixtures thereof, typically at concentrations ranging from 0.5 to 2 M.

A preferred electrolyte composition according to the present invention contains:
- a hydrofluoroether mixture (M-1a) having average raw formula C₆H₄F₁₀O₂, preferably in an amount of 20% wt;
- ethylene carbonate, preferably in an amount ranging from 40% wt to 50% wt;
- dimethyl carbonate, preferably in an amount from 30% wt to 40% wt;
- LiN(SO₂CF₃)₂ or LiPF₆, preferably at a 1M concentration,
weight percentages being referred to the overall weight of the composition.

According to a preferred aspect, the composition further comprises at most 5% wt with respect to the overall weight of the composition of a hydrofluoroether mixture [mixture (M2)] complying with at least one mixture of average formulae (M2-a) – (M2-d) below:
(M2-a) X-CFYCF₂O[(CY₂)_mO]_n-CF₂CFY-X
(M2-b) RO[(CY₂)_mO]_n-CF₂CFY-X
(M2-c) X-CFYCF₂O(CY₂)_a(CYR¹)(CY₂)_b-OCF₂CFY-X
(M2-d) RO(CY₂)_a(CR¹Y)(CY₂)_b-OCF₂CFY-X
wherein X, Y, R, R¹, a and b are as defined in formulae (M1-a) – (M1-d) above with the proviso that not all Y are fluorine and not all Y are hydrogen and that the molar ratio (F+H)/H in any one of formulae (M2-a)- (M2-d) above is higher than 6.5. Indeed, mixtures (M2), although less soluble in polar organic solvents used to prepare the compositions, are also not flammable and can be added to the compositions of the invention to reduce flammability and, therefore, to increase safety.

In a further aspect, the invention relates to a lithium ion battery, typically a lithium ion secondary battery, containing an electrolyte composition as defined above.

The lithium ion battery according to the invention typically comprises a cathode which can be prepared in a form where a cathode active material is bound to a positive current collector according to a conventional method. Non-limiting examples of the cathode active material include conventional cathode active materials known in the art, which can be used in the cathode of the conventional electrochemical devices, as well as lithium-adsorbing materials, such as LiCoO₂, LiNiO₂, LiMnO₂, LiNi_xCo_1-xO₂ (0 < x < 1), Li_xCo_1-yAl_yO₂ (0 < x < 1, 0<y<1), LiMn₂O_{4 ,} LiFePO₄ and Li(Fe_xMn_1-x)PO₄ (0 < x < 1). Non-limiting examples of the positive current collector include foils made of aluminium, nickel or a combination thereof.

The lithium-ion battery according to the invention typically comprises an anode which can be prepared in a form where an anode active material is bound to a negative current collector in the same manner as in the preparation of the cathode. Non-limiting examples of the anode active material include conventional anode active material known in the art, which can be used in the anode of the conventional electrochemical devices, as well as lithium-adsorbing materials, such as lithium alloys, carbon, petroleum coke, graphite or other carbons. Non-limiting examples of the negative current collector include foils made of copper, gold, nickel, copper alloy or a combination thereof.

Representative anodes used in the present invention for preparing a secondary battery include the following:
- alkaline or alkaline-earth metal, including lithium, sodium, magnesium or calcium;
- graphitic carbons able to intercalate alkaline or alkaline-earth metal, typically existing in forms such as powders, flakes, fibers or spheres (for example, mesocarbon microbeads) hosting at least one alkaline or alkaline-earth metal;
- alkaline or alkaline-earth metal alloy compositions, including silicon-based alloys, germanium-based alloys;
- alkaline or alkaline-earth metal titanates, advantageously suitable for intercalating alkaline or alkaline-earth metal with no induced strain.

The invention and its advantages are illustrated in greater detail in the following experimental section by means of non-limiting examples.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Experimental section

Materials

Hydrofluoroether mixture (M-1a) having average raw formula C₆H₄F₁₀O₂ [herein after “(M1-a^*)”] was prepared by fluorination of a (HFE1-a) of formula CF₂HCF₂OCH₂CH₂OCF₂CF₂H (C₆H₆F₈O₂) (herein after “precursor of (M1-a*)”, as disclosed in example 1 of WO 2012/084745. A comparative HFE mixture with a higher fluorine content than (M1-a*), having raw formula C₆H₂F₁₂O₂ (herein after “(M2-a*) was obtained by fluorination of C₆H₄F₁₀O_2, following the same procedure as disclosed in WO 2012/084745.

EC (ethylene carbonate, ≥99%) was purchased from Fluka^® and DMC (dimethylene carbonate, 99+%) was purchased from Aldrich^®. LP30 LiPF₆ 1M in EC DMC 1:1 w/w from BASF was used as standard electrolyte.

LiPF₆ (lithium esafluorophosphate, Aldrich^®, battery grade, ≥99.99%) and LiN(SO₂CF₃)₂ (lithium bis(trifluoromethane)sulfonimide, LiTFSI, 99.99%, Rhodia) were used as lithium salts.

Lithium Manganese Nickel Oxide (LMNO, NANOMYTE® SP-10 from NEI Corporation, USA) was used as active material for working electrodes in galvanostatic charge/discharge tests. The separators used in coin cells were glass-fiber Whatman^® separator (for galvanostatic cyclation tests and polyolefin Tonen separator for LSV with steel electrode.

Cell construction, liquids, salts and dried electrodes were located in a glove-box (LabMaster MBRAUN) under argon atmosphere (H₂O and O₂ < 1 ppm).

Methods

1. Miscibility

Miscibility with organic carbonates and salts was evaluated by visual inspection.

2. Electrochemical methods

Electrochemical experiments were performed using a multichannel potentiostat/galvanostat/EIS analyzer VMP3 (BioLogic). Charge–discharge cycling tests (galvanostatic cyclations) were conducted using a two-electrode configuration. The electrochemical cells were CR2032 coin cells assembled in an argon-filled glove box using a glass-fiber separator (Whatman^® ) filled with the electrolyte composition and lithium foil as counter and reference electrode. Lithium Manganese Nickel Oxide (LMNO, NANOMYTE^® SP-10 from NEI Corporation, USA) was used as active material. Charge-discharge cycling tests were performed progressively increasing C-rates, from C/10-D/10 (charge-discharge) to C/5-2D, concluding with stability cycles at C/5-D/3. LiMn_1.5Ni_0.5O₄ cut offs were 4.95-3.5 V vs. Li. Charge-discharge cycling tests were carried out at room temperature.

Galvanostatic cyclations were conducted in order to investigate the electrolyte in terms of battery performance.

The charge/discharge protocol applied to the LMNO cells is set out below.
C/10-D/10 (3 cycles)
C/5-D/5 (3 cycles)
C/5-D/3 (3 cycles)
C/5-1D (5 cycles)
C/5-2D (5 cycles)
C/5-D/3 (several cycles)

3. Ionic conductivity from room temperature to low temperatures and freezing test.

The ionic conductivity of electrolyte compositions was measured in a sealed steel conductivity cell through electrochemical impedance spectroscopy covering a frequency range from 200 MHz to 200 kHz with a perturbation amplitude of ±5mV. To evaluate conductivities at different temperatures the measuring cell was immersed in a thermostated bath at the chosen temperature, and conductivity was measured after an equilibration time of 20 min in order to stabilize the value.

The freezing test was carried out by equilibrating the electrolyte compositions at the given temperature for 60 minutes, and then visually evaluating the aspect (i.e. if still liquid flowing or frozen).

4. Oxidation stability

Oxidation stability of the electrolyte compositions was checked by linear sweep voltammetry (LSV) in coin cell 2032 with steel working electrode and lithium counter and reference electrode, with PP separator (Tonen^®). LSV is based on the application of an increasing voltage to the cell, from the starting open-circuit value to an extreme value like 7 V vs. Li at ramp 1 mV/s, registering the current provided by the cell: high values of current are sign of redox reactions related to electrolyte or salt degradation. Oxidation stability measurements were made at room temperature.

5. Flammability

Flammability was evaluated according to the UNI EN ISO 3680:2005 method.

Results and discussion

1. Miscibility

Different amounts of (M1-a*) and (M2-a*) were added to standard electrolyte LP30 in order to obtain electrolyte compositions containing 10%, 20% and 50% wt (M1-a*) and (M2-a*) respectively. (M2-a*) was not miscible with LP30 components, while (M1-a*) was completely miscible. The compositions were allowed to stand at room temperature for at least one month and no changes were observed. It was observed that (M2-a*) was instead miscible with LP30 in amounts up to 5% wt; thus, small amounts of this HFE can be added to the compositions of the invention.

Based on this result, electrolyte compositions were prepared using (M1-a*) (compositions EF1 – EF3, EF7 and EF8, according to the invention) or its precursor (composition EF0); standard electrolyte compositions (S1 and S2, without hydrofluoroether) were also prepared. Table 1 below reports the ingredients of the electrolyte compositions obtained as single homogeneous phase at room temperature (RT), and their conductivity measured at RT. Table 1

Electrolyte composition	(M1-a*) (%weight)	Ethylene carbonate (%weight)	Dimethyl carbonate (%weight)	Kind and amount of lithium salt	Conductivity (mS cm -1 )
EF1	20	40	40	LiN(SO2CF3)2 1M	6.7
EF2	20	50	30	LiN(SO2CF3)2 1M	6.2
EF3	20	40	40	LiPF6 1M	8.1
EF7	50	25	25	LiPF6 1M	4.1
EF8	50	25	25	LiN(SO2CF3)2 1M	4.0
EF0	20 of precursor C6H6F8O2	40	40	LiPF6 1M	8.3
S1	0	50	50	LiN(SO2CF3)2 1M	6.3
S2 (LP30)	0	50	50	LiPF6 1M	11.1

Table 1 reports also the results of measurements of the ionic conductivity of the electrolyte compositions by Impedance Spectroscopy. Such results show that the conductivity of electrolyte compositions EF1 – EF3 and EF7, although slightly lower than that of standard electrolyte compositions S1 and S2, is suitable for use in Li-batteries, as it is still well above 1 mS cm^-1.

2. Capacity and efficiency in cells with LMNO electrode

The performance of electrolyte compositions according to the invention and of reference electrolyte S2 was evaluated in terms of capacity and efficiency vs. cycle in cells with LMNO electrodes. The results are illustrated in Tables 2a – 2c below. Capacities are normalized to the initial value measured at C/10 and all cells started with the expected capacities.

Table 2a

Electrolyte composition	C/10-D/10 cycle 1
	Capacity retention	Coulombic efficiency
EF3	100%	85%
EF7	100%	89%
S2 (LP30)	100%	89%

Table 2b

Electrolyte composition	C/5-2D cycle 19
	Capacity retention	Coulombic efficiency
EF3	99%	96%
EF7	95%	97%
S2 (LP30)	99%	98%

Table 2c

Electrolyte composition	C/5-D/3 cycle 27
	Capacity retention	Coulombic efficiency
EF3	100%	99%
EF7	100%	99%
S2 (LP30)	100%	98%

If a battery contains a suitable electrolyte composition what is expected from this cyclation test is that the cell maintains capacity values close to 100% of the initial value over cyclation, with little decrease when the C-rates reach high values and with complete recovery of capacity when the C-rates returns to the mild values of C/5-1D.

The results show that the electrolyte compositions according to the invention allow achieving performances that are comparable to those achieved with a standard electrolyte.

3. Conductivity from room to low temperatures and freezing test

Tables 3a and 3b below report the conductivity of the electrolyte compositions at temperatures from RT to -43°C. Table 3a

	Conductivity (mS cm-1)
Electrolyte composition	RT	12°C	2°C
EF0	8.3	6.1	4.4
EF1	6.7	4.9	3.8
EF3	8.1	5.6	4.4
EF7	4.1	2.9	2.0
S1	6.3	4.8	3.8
S2 (LP30)	11.1	8.5	6.5

Table 3b

Electrolyte formulation	-13°C	-28°C	-43°C
EF0	2.5	1.11	0.06
EF1	2.1	0.94	0.07
EF3	2.4	1.10	0.07
EF7	1.0	0.34	0.07
S1	2.3	0.23	0.01
S2 (LP30)	3.8	0.22	0

Table 4 below reports the physical state of the electrolyte compositions at the given temperatures. Table 4

	Physical state
Electrolyte composition	0°C	-15°C	-30°C	-45°C
EF0	Liquid	Liquid	Liquid	Liquid
EF1	Liquid	Liquid	Liquid	Frozen
EF3	Liquid	Liquid	Liquid	Frozen
EF8	Liquid	Liquid	Liquid	Liquid
S2 (LP30)	Liquid	Liquid	Frozen	Frozen

All electrolyte compositions based on the hydrofluoroether mixtures according to the invention show advantages in performances at very low temperatures compared to standard electrolyte compositions S2.

4. Oxidation stability

The results reported in Table 5 below show that the oxidation stability of the electrolyte compositions according to the invention is higher than that of standard electrolyte composition S2. Table 5

	Generated current (μA/cm2)
Electrolyte composition	4V	5V	6V	7V
EF3	0.4	8.5	11.2	15.2
EF7	0.5	7.0	7.1	9.6
S2 (LP30)	0.7	19.5	14.6	21.3

5. Flammability tests

The flammability of (M1-a*) and of its precursor was evaluated according to method UNI EN ISO 3680:2005). The results are reported in Table 6 below.

Table 6

Hydrofluoroether	Flash point
C6H6F8O2 (precursor of M1-a*)	64°C
C6H4F10O2 (M1-a*)	Not flammable
C6H2F12O2 (M2-a*)	Not flammable

Claims (15)

1. A method for the manufacture of electrolyte compositions for lithium ion batteries comprising using a hydrofluoroether mixture [mixture (M1)] complying with at least one of average formulae (M1-a) - (M1-d) as defined here below:

   (M1-a) X-CFYCF₂O[(CY₂)_mO]_n-CF₂CFY-X

   (M1-b) RO[(CY₂)_mO]_n-CF₂CFY-X

   (M1-c) X-CFYCF₂O(CY₂)_a(CYR¹)(CY₂)_b-OCF₂CFY-X

   (M-1d) RO(CY₂)_a(CR¹Y)(CY₂)_b-OCF₂CFY-X

   wherein:

   - R is an optionally fluorinated straight-chain alkyl group containing from 1 to 10 carbon atoms or an optionally fluorinated branched alkyl group containing from 3 to 10 carbon atoms;

   - R¹ is -OCF₂CFY-X or an optionally fluorinated straight-chain alkyl group containing from 1 to 10 carbon atoms or an optionally fluorinated branched alkyl group containing from 3 to 10 carbon atoms;

   - X is a perfluoroalkyl group containing from 1 to 6 carbon atoms, which may also contain one or more ethereal oxygens;

   - m is a positive number ranging from 2 to 6;

   - n is a positive number ranging from 1 to 8;

   - a and b are 0 or a positive number ranging from 1 to 6, with the proviso that at least one of a and b is different from 0 and that a and b are both different from 0 when R¹ is -OCF₂CFY-X;

   - each Y is independently selected from hydrogen and fluorine, with the proviso that not all Y are fluorine and not all Y are hydrogen and that the molar ratio (F+H)/H in any one of formulae (M1-a) - (M1-d) ranges from 2.5 to 6.5.
2. A method according to claim 1 comprising using hydrofluoroether mixtures complying with at least one of average formulae (M1-a) - (M1-d) wherein:

   - R is an optionally fluorinated straight-chain alkyl group containing from 1 to 4 carbon atoms or a branched optionally fluorinated alkyl group containing 3 or 4 carbon atoms;

   - X is a perfluoroalkyl group containing from 1 to 3 carbon atoms;

   - m ranges from 2 to 4;

   - n is 1;

   - a and b are 1 or 2.
3. A method according to claim 2 comprising using a hydrofluoroether mixture (M-1a) wherein X is F of CF₃, m ranges from 2 to 4 and n is 1.
4. A method according to claim 3 wherein hydrofluoroether mixture (M-1a) is a mixture having average raw formula C₆H₄F₁₀O₂.
5. A method according to any one of claims 1 to 4 comprising mixing a hydrofluoroether mixture complying with at least one of average formulae (M1-a) - (M1-d), one or more polar organic solvents and one or more lithium salts.
6. The method according to claim 5, wherein the organic solvent is ethylene carbonate or propylene carbonate or a mixture thereof, optionally in admixture with dimethyl carbonate.
7. The method according to claim 5 or 6 wherein the lithium salt is LiN(SO₂CF₃)_2, LiPF₆ or a mixture thereof.
8. An electrolyte composition for lithium batteries comprising a hydrofluoroether mixture complying with at least one of average formulae (M-1a) - (M1-d) as defined in claim 1 or 2, one or more polar organic solvents and one or more lithium salts.
9. An electrolyte composition according to claim 8 wherein the hydrofluoroether mixture (M-1) is a mixture (M-1a) as defined in claim 3.
10. An electrolyte composition according to claim 8 wherein hydrofluoroether mixture (M-1a) is a mixture having average raw formula C₆H₄F₁₀O₂.
11. An electrolyte composition according to any one of claims 8 – 10 wherein the organic solvent is ethylene carbonate or propylene carbonate or a mixture thereof, optionally in admixture with dimethyl carbonate.
12. An electrolyte composition according to any one of claims 8 – 11 wherein the lithium salt is LiN(SO₂CF₃)_2, LiPF₆ or a mixture thereof.
13. An electrolyte composition according to any one of claims 10 – 12 which comprises:

    - a hydrofluoroether mixture (M-1a) having raw average formula C₆H₄F₁₀O₂;

    - ethylene carbonate;

    - dimethyl carbonate and

    - LiN(SO₂CF₃)₂ or LiPF₆.
14. An electrolyte composition according to any one of claims 8 – 13 further comprising a hydrofluoroether mixture [mixture (M2)] complying with at least one mixture of average formulae (M2-a) – (M2-d) below:

    (M2-a) X-CFYCF₂O[(CY₂)_mO]_n-CF₂CFY-X

    (M2-b) RO[(CY₂)_mO]_n-CF₂CFY-X

    (M2-c) X-CFYCF₂O(CY₂)_a(CYR¹)(CY₂)_b-OCF₂CFY-X

    (M2-d) RO(CY₂)_a(CR¹Y)(CY₂)_b-OCF₂CFY-X

    wherein:

    - R is an optionally fluorinated straight-chain alkyl group containing from 1 to 10 carbon atoms or an optionally fluorinated branched alkyl group containing from 3 to 10 carbon atoms;

    - R¹ is -OCF₂CFY-X or an optionally fluorinated straight-chain alkyl group containing from 1 to 10 carbon atoms or an optionally fluorinated branched alkyl group containing from 3 to 10 carbon atoms;

    - X is a perfluoroalkyl group containing from 1 to 6 carbon atoms, which may also contain one or more ethereal oxygens;

    - m is a positive number ranging from 2 to 6;

    - n is a positive number ranging from 1 to 8;

    - a and b are 0 or a positive number ranging from 1 to 6, with the proviso that at least one of a and b is different from 0 and that a and b are both different from 0 when R¹ is -OCF₂CFY-X;

    - each Y is independently selected from hydrogen and fluorine,

    with the proviso that not all Y are fluorine and not all Y are hydrogen and that the molar ratio (F+H)/H in any one of formulae (M2-a)- (M2-d) above is higher than 6.5.
15. A lithium ion battery comprising an electrolyte composition as defined in any one of claims 8 to 14.