Electrolyte for lithium secondary battery

Abstract

The present invention relates to a sulfolane-based electrolyte composition suitable for lithium secondary batteries and the use of the electrolyte composition in lithium secondary battery cells, said electrolyte composition comprising: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount (x) of 39.0 vol% < x < 47.5 vol%, respectively weight, fluorinated ethylene carbonate (FEC) in an amount (y) of 0 < y < 15 vol% (corresponding to an amount of 0 < y < 14.0 wt%), and sulfolane, wherein the molar ratio (z) of SL / LiTFSI is 2 < z < 3.5, relative to the total volume, respectively weight, of the electrolyte composition.

Description

Technical Field

[0001] This invention relates to electrolyte compositions for lithium metal-based or lithium-ion batteries. Specifically, the invention relates to electrolyte compositions suitable for lithium secondary batteries and their application in lithium secondary battery cells, said electrolyte composition comprising: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount (x) of 39.0 vol% ≤ x ≤ 47.5 vol% relative to the total volume of said electrolyte composition, and fluoroethylene carbonate (FEC) in an amount (y) of 1.0 ≤ y ≤ 15.0 vol% (equivalent to an amount (y) of 1.0 ≤ y ≤ 14.0 wt%), and the remaining volume of said electrolyte being composed of a suitable solvent such as sulfolane (SL), wherein the molar ratio (z) of SL / LiTFSI is 2.0 ≤ z ≤ 3.5. Background Technology

[0002] The three main functional components of a lithium-ion battery are the anode, cathode, and electrolyte. In a conventional lithium-ion battery, the anode is made of carbon, the cathode is a transition metal oxide such as cobalt, nickel, or manganese, and the electrolyte is a non-aqueous solvent containing lithium salts. Other lithium-ion batteries, such as those based on lithium iron phosphate cathodes, are also available on the market.

[0003] The electrolyte should conduct lithium ions, which act as carriers between the cathode and anode when the battery carries current through external circuitry. Currently used electrolyte solvents decompose during initial charging to form a solid mesophase layer that is electrically insulating but provides sufficient ionic conductivity. This mesophase prevents further decomposition of the electrolyte in subsequent charge / discharge cycles.

[0004] Such electrolyte solvents typically consist of a mixture of organic carbonates (such as ethylene carbonate (EC), dimethyl carbonate (DMC), and propylene carbonate (PC)), and the lithium salt is typically composed of hexafluorophosphate LiPF6. WO 2016 / 204278 A1 discloses a non-aqueous electrolyte composition comprising various lithium salts and sulfolane.

[0005] The rapidly expanding market for lithium-ion rechargeable batteries, coupled with increasing demand for smaller and lighter batteries suitable for portable electronic devices and exhibiting high energy density, has led to a surge in the development of batteries with higher capacity and the ability to operate safely and stably at high voltages. The capacity of batteries for portable electronic devices has now plateaued, primarily due to the stability of the electrolyte, which limits the operating voltage. The operating voltage of commercial batteries for portable electronic devices currently varies between 4.2V and a maximum of 4.4V. For very high-end portable electronic devices such as cutting-edge mobile phones, batteries are required to operate at a voltage of at least 4.4V (and preferably not exceeding 4.5V). Furthermore, some electrolyte compositions used in rechargeable lithium-ion battery cells present safety concerns due to their flammability.

[0006] Therefore, the object of the present invention is to provide a stable, safe and high energy density battery that exhibits good cycle life (which may be, for example, sufficient to achieve high or excellent cycle life) and is capable of achieving high coulombic efficiency (i.e., at least 93%, preferably at least 98%), preferably in a higher voltage range (i.e. at voltages above 4.4V) relative to the conventional cutoff voltage or operating voltage (limited to 4.4V).

[0007] This objective has been achieved by using a sulfolane (SL)-based electrolyte composition suitable for lithium secondary batteries, the electrolyte composition comprising: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount (x) of 39.0 vol% ≤ x ≤ 47.5 vol% relative to the total volume of the electrolyte composition, and ethylene fluoride carbonate (FEC) in an amount (y) of 1.0 ≤ y ≤ 15.0 vol% (equivalent to an amount (y) of 1.0 ≤ y ≤ 14.0 wt%), and the remaining volume of the electrolyte being composed of a suitable solvent such as sulfolane (SL), wherein the molar ratio (z) of SL / LiTFSI is 2.0 ≤ z ≤ 3.5, wherein the volume % is defined as the volume of a particular component divided by the volume of LiTFSI (M: 287.08 g / mol, ρ: 1.33 g / cm³). 3 ), FEC (M: 106.05g / mol, ρ: 1.45g / cm 3 ) and SL (M: 120.17 g / mol, ρ: 1.26 g / cm) 3 The total volume of ). Attached Figure Description

[0008] Figure 1 Experimental results on the relationship between cycle efficiency and the changing molar ratio of sulfolane (SL) to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with a fixed content of 10.0 vol% fluoroethylene carbonate (FEC).

[0009] Figure 2 Experimental results on the relationship between cycle efficiency and volume change % of fluoroethylene carbonate (FEC) with a fixed molar ratio of sulfolane (SL) to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) of 3.0 to 1.0.

[0010] Figure 3 Voltage distribution described in Section 3 of the embodiments. Detailed Implementation

[0011] This invention relates to a sulfolane (SL)-based composition suitable for lithium secondary batteries, the composition comprising: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount (x) of 39.0 vol% ≤ x ≤ 47.5 vol% relative to the total volume and corresponding weight of the electrolyte composition; ethylene fluoride carbonate (FEC) in an amount (y) of 1.0 ≤ y ≤ 15.0 vol% (equivalent to an amount (y) of 1.0 ≤ y ≤ 14.0 wt%); and sulfolane (SL), wherein the molar ratio (z) of SL / LiTFSI is 2.0 ≤ z ≤ 3.5.

[0012] For clarity, those skilled in the art can calculate the volume % or volume percentage and weight % or weight percentage of each component described herein, as well as the molar ratio between each component described herein, from the physical data available for each component in the composition described herein.

[0013] For clarity, unless otherwise stated, volume % or volume percentage and weight % or weight percentage are based on the total volume of the electrolyte composition herein.

[0014] According to the present invention, the electrolyte composition comprises lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount (x) of 39.0 vol% ≤ x ≤ 47.5 vol% (equivalent to an amount of 37.9 wt% ≤ x' ≤ 48.9 wt%) relative to the total volume of the electrolyte composition. LiTFSI is a well-known chemical compound (CAS: 90076-65-6). More preferably, the electrolyte composition comprises lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount (x) of 39.2 vol% ≤ x ≤ 47.5 vol% (equivalent to an amount of 38.1 wt% ≤ x' ≤ 48.9 wt%) relative to the total volume of the electrolyte composition.

[0015] According to the present invention, the electrolyte composition further comprises: fluoroethylene carbonate (FEC) in an amount (y) of 1.0 ≤ y ≤ 15.0% by volume (corresponding to an amount of 1.0 ≤ y' ≤ 14.0% by weight) relative to the total volume, respectively weight, of the electrolyte composition. FEC is a well-known chemical compound (CAS: 114435-02-8).

[0016] In one embodiment, there may be present: FEC in an amount (y) of 1.0% by volume ≤ y, 1.0% by volume < y, 2.0% by volume ≤ y, 2.0% by volume < y, 2.5% by volume ≤ y, 2.5% by volume, 5.0% by volume ≤ y, 5.0% by volume < y or 10.0% by volume ≤ y relative to the total volume of the composition. Depending on the respective amounts of SL and LiTFSI in the composition, the electrolyte composition corresponds to an electrolyte composition comprising: fluoroethylene carbonate (FEC) in an amount (y') of 1.0% by weight ≤ y', 1.0% by weight < y', 2.1% by weight ≤ y', 2.1% by weight < y', 2.6% by weight < y', 2.6% by weight < y', 5.2% by weight ≤ y', 5.2% by weight < y' or 9.8% by weight ≤ y' relative to the total weight of the composition.

[0017] In one embodiment, there may be present: FEC in an amount (y) of y ≤ 15.0% by volume or y < 15.0% by volume. Depending on the respective amounts of SL and LiTFSI in the composition, the electrolyte composition corresponds to an electrolyte composition comprising: fluoroethylene carbonate (FEC) in an amount (y') of approximately 14.0% by weight ≤ y' or 14.0% by weight < y' relative to the total weight of the composition.

[0018] In a preferred embodiment, the amount (y) of FEC may be 10.0 ≤ y ≤ 15.0% by volume (corresponding to an amount of 9.8 ≤ y' ≤ 14.0% by weight) relative to the total volume, respectively weight, of the electrolyte composition.

[0019] In a more preferred embodiment, the amount (y) of FEC may be 10.0% by volume (corresponding to an amount (y') of 9.8% by weight) relative to the total volume, respectively weight, of the electrolyte composition.

[0020] According to the present invention, the electrolyte composition further comprises sulfolane (SL). SL is a well-known chemical compound (CAS: 126-33-0).

[0021] According to the present invention, the electrolyte composition comprises an SL / LiTFSI molar ratio (z) of 2.0 ≤ z ≤ 3.5.

[0022] In one embodiment, the electrolyte composition comprises SL / LiTFSI with a minimum molar ratio (z) of 2.0 < z or 2.5 ≤ z.

[0023] In one embodiment, the electrolyte composition comprises SL / LiTFSI with a maximum molar ratio (z) of ≤ 3.5.

[0024] In a preferred embodiment, the maximum molar ratio (z) of SL / LiTFSI can be 2.0 < z ≤ 3.5.

[0025] In a more preferred embodiment, the molar ratio (z) of SL / LiTFSI can be 2.5 ≤ z ≤ 3.5.

[0026] In an even more preferred embodiment, the molar ratio (z) of SL / LiTFSI can be 2.5 < z < 3.5.

[0027] In an even more preferred embodiment, the molar ratio (z) of SL / LiTFSI can be 2.5 < z ≤ 3.0.

[0028] In an even more preferred embodiment, the molar ratio (z) of SL / LiTFSI can be 3.

[0029] In a particularly preferred embodiment, the electrolyte composition can comprise: fluoroethylene carbonate (FEC) with an amount (y) of 10.0 ≤ y ≤ 15.0 vol% (equivalent to an amount of 0 ≤ y’ ≤ 14.0 wt%) relative to the total volume and correspondingly the weight of the electrolyte composition, and SL / LiTFSI with a molar ratio (z) of 2.5 ≤ z ≤ 3.5.

[0030] In a particularly preferred embodiment, the electrolyte composition can comprise: fluoroethylene carbonate (FEC) with an amount of 10.0 vol% (equivalent to an amount (y’) of 9.8 wt%) relative to the total volume and correspondingly the weight of the electrolyte composition, and SL / LiTFSI with a molar ratio (z) of 2.5 ≤ z ≤ 3.5.

[0031] In a particularly preferred embodiment, the electrolyte composition can comprise: fluoroethylene carbonate (FEC) with an amount of 10.0 vol% (equivalent to an amount (y’) of 9.8 wt%) relative to the total volume and correspondingly the weight of the electrolyte composition, and SL / LiTFSI with a molar ratio (z) of 2.5 < z ≤ 3.0.

[0032] In a particularly preferred embodiment, the electrolyte composition may comprise: 10.0 vol% (equivalent to 9.8 wt%) of fluoroethylene carbonate (FEC) relative to the total volume of the electrolyte composition and SL / LiTFSI in a molar ratio (z) of 3.0.

[0033] There are no particular limitations on the method of preparing electrolyte compositions; they can be prepared, for example, by mixing the components.

[0034] The present invention also relates to a lithium secondary battery cell comprising an electrolyte composition according to the present invention.

[0035] For clarity, a lithium secondary battery cell includes at least an anode, a cathode, and an electrolyte, as well as optionally a separator.

[0036] The electrolyte relates to the electrolyte of the invention as described herein.

[0037] There are no particular limitations on the cathode material, and examples include transition metal compounds or specialized metal compounds thereof with structures capable of diffusing lithium ions, and lithium oxides. Specifically, LiCoO2, LiNiO2, LiMn2O4, LiFePO4, etc., may be mentioned.

[0038] A cathode can be formed by molding the cathode materials listed above together with known conductive aids or binders, or by molding the positive electrode active material together with known conductive aids or binders into an organic solvent such as pyrrolidone. It can also be obtained by applying a mixture and attaching it to a current collector such as aluminum foil, followed by drying.

[0039] In a preferred embodiment, the cathode is a copper foil (cathode) relative to the lithium foil (anode).

[0040] There are no particular restrictions on the anode material, as long as it is capable of inserting and extracting lithium. Examples include lithium metal, Sn-Cu, Sn-Co, Sn-Fe, or Sn-An alloys such as -Ni, and metal oxides such as Li4Ti5O. 12 Or Li5Fe2O3, natural graphite, artificial graphite, boronized graphite, mesophase carbon microspheres, carbon materials such as pitch-based carbon fiber graphitized materials, carbon-Si composites, or carbon nanotubes.

[0041] A diaphragm is typically inserted between the cathode and anode to prevent short circuits between them. There are no particular limitations on the material and shape of the diaphragm, but it is preferred that the electrolyte composition can easily pass through it and that the diaphragm is an insulating and chemically stable material. Examples include microporous membranes and sheets made of various polymeric materials. Specific examples of polymeric materials include polyolefin polymers, nitrocellulose, polyacrylonitrile, polyvinylidene fluoride, polyethylene, and polypropylene. From the perspective of electrochemical and chemical stability, polyolefin polymers are preferred.

[0042] In a preferred embodiment, the membrane is a polypropylene membrane with a thickness of 40.0 μm and a porosity of 48% (e.g., Cellguard 2075-1500M). This membrane is described in the following article: International Journal of Electrochemistry, 2018, Article ID 1925708, page 7, https: / / doi.org / 10.1155 / 2018 / 1925708.

[0043] The optimal operating voltage of the lithium secondary battery of the present invention is not particularly limited by the combination of positive and negative electrodes, but can be used at an average discharge voltage of 2.4 to 4.5V. Preferably, the lithium secondary battery cell has a high operating voltage, i.e., an operating voltage higher than or equal to 4.4V and preferably lower than or equal to 4.5V.

[0044] Example

[0045] 1. Description of button cell manufacturing process

[0046] The test included a CR2025 button cell battery. The battery was fabricated by placing the positive casing, positive electrode (pre-soaked in electrolyte), cellguard membrane, 50 μL electrolyte droplet, negative electrode, spacer, wave spring, and negative casing on top of each other in this order. A manual roll press from MTI Corp. was used at 80 kg / cm². 2 Rolling is performed under pressure.

[0047] The electrolyte composition is obtained by adding fluoroethylene carbonate (FEC) according to the invention at a total volume metric (y) relative to the electrolyte to sulfolane (SL) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a molar ratio (z) of 3.0 to 1.0.

[0048] 2. passivation scheme

[0049] The passivation of the lithium samples was performed in two steps. First, the cell described in Section 1 above was constructed such that it was symmetrical (Li metal was chosen for both the anode and cathode). Second, at 0.60 mA / cm²...2 The current density is maintained for 2 hours / half cycle, which cycles the battery 5 times, resulting in 1.20 mAh / cm³. 2 The capacity was then determined. Afterward, the battery was left to stand for 12 hours before being disassembled, and the passivated Li electrode containing SEI was extracted from the lithium battery.

[0050] 3. Description of methods for measuring coulombic efficiency

[0051] To determine the charge-discharge cycle performance of a button cell containing a passivated lithium electrode, the following conditions were applied: the cell configuration consisting of copper foil as the cathode and lithium foil as the anode was used, and the coulombic efficiency was measured using a Biologic VMP-3 potentiostat. Initially, 0.38 mA / cm² was used. 2 A constant current is used to plate a certain amount of lithium metal (approximately 1 mg / 50 μL electrolyte, corresponding to a capacity of 3.80 mAh) onto a copper foil, and then this certain amount of lithium metal is completely removed by applying a reverse current up to a potential of 0.50 V, resulting in Q_clean, which is used to calculate CE_1st = Q_clean / Q_initial. Figure 1 and Figure 2 The efficiency of the first cycle in the process.

[0052] Subsequently, using the same current density, another lithium metal of approximately 1 mg / 50 μL electrolyte (corresponding to a capacity of 3.80 mAh) (initial 2Q) was plated onto the copper foil.

[0053] After that, at 0.380 mA / cm 2 The current density was used for 50 cycles (n), and each cycle cycled 12.5% ​​of the total (3.80 mAh, Q initial) capacity (0.475 mAh in our setting).

[0054] After completing the 50th cycle, apply 0.380 mA / cm 2 The current density strips the remaining lithium from the copper electrode to achieve a cutoff voltage of 0.5V (thus obtaining Q final).

[0055] Figure 3 The typical voltage distribution of the above procedure is shown.

[0056] CE is calculated using the following general formula:

[0057]

[0058] Since the Q-loop, the initial Q, and n are known (see the description of the experiment above), the formula can be simplified to:

[0059]

[0060] 4. Experimental tests and results

[0061] To test the relationship between cycle efficiency and the molar ratio of sulfolane (SL) to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the molar ratio was gradually varied from 2:1 to 4:1 in increments of 0.5, while maintaining a constant 10 vol% FEC content. Coulombic efficiency was measured during the first charge-discharge cycle and subsequent charge-discharge cycles. Experimental results are shown in… Figure 1 middle.

[0062] Figure 1 The cycling efficiency of the electrolyte composition is shown to depend on the molar ratio of SL / LiTFSI.

[0063] The electrolytes according to the invention with a molar ratio of SL / LiTFS of 2:1 to 4:1 exhibit a significantly high cycle efficiency of over 90%.

[0064] The electrolyte composition according to the invention with a molar ratio of SL / LiTFSI of 2.5 to 3.5 exhibits the best cycling efficiency, with the maximum value at a molar ratio of SL / LiTFSI of 3.

[0065] The cycling efficiency of electrolyte compositions with a molar ratio exceeding 4:1 does indeed decrease significantly to the point where they cannot be cycled.

[0066] To test the dependence of cycle efficiency on the amount of fluoroethylene carbonate (FEC), the amount of FEC (based on volume percentage relative to the total volume of the electrolyte composition) was gradually varied from 0 vol% to 15 vol% in increments of 2.5 vol% while maintaining the SL:TFSI molar ratio constant at 3:1. Coulombic efficiency was measured during the first charge-discharge cycle and in subsequent charge-discharge cycles. The experimental results are shown in… Figure 2 middle.

[0067] Figure 2 The cycling efficiency of the electrolyte composition is shown to depend on the amount of FEC added.

[0068] The electrolyte according to the invention at a certain molar ratio exhibits a significantly high cycle efficiency of over 90%.

[0069] The electrolyte compositions according to the invention with FEC values ​​of 10 vol%, 12.5 vol%, and 15 vol% exhibited the best cycling efficiency (the experimental results for FEC values ​​of 12.5 vol% and 15 vol% are the same as those for FEC value of 10 vol%, and therefore have been omitted for readability).

[0070] FEC-containing electrolyte compositions exceeding 15% by volume exhibit significantly reduced cycle efficiency and lead to unstable lithium plating behavior and battery failure.

[0071] Figure 1 and 2 The results described are summarized in Tables 1 and 2 below:

[0072] Table 1 :

[0073] SL / LiTFSI CE (%) in the first cycle CE (%) 2.0 93.00 93.40 2.5 95.46 98.30 3.0 96.68 98.37 3.5 94.50 98.20

[0074] Table 2 :

[0075] FEC volume % CE (%) in the first cycle CE (%) 0.0 19.28 68.44 2.5 94.78 90.46 5.0 95.87 91.97 7.5 96.68 97.46 10.0 96.88 98.37

Claims

1. A sulfolane (SL)-based electrolyte composition suitable for lithium secondary batteries, the electrolyte composition comprising: - lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in an amount x of 39.0 vol.% < x < 47.5 vol.%, and - fluoroethylene carbonate (FEC) in an amount y of 1.0 < y < 15.0 vol.%, the remaining volume of the electrolyte composition consisting of sulfolane (SL), and wherein the molar ratio z of SL / LiTFSI is 2.0 < z < 3.

5.

2. The electrolyte composition according to claim 1, wherein the amount y of FEC is 2.0 < y < 15.0 vol.%.

3. The electrolyte composition according to claim 1 or 2, wherein the amount y of FEC is 2.5 < y < 15.0 vol.%.

4. The electrolyte composition according to claim 1 or 2, wherein the amount y of FEC is 10.0 < y < 15.0 vol.%.

5. The electrolyte composition according to claim 1 or 2, wherein the amount of FEC is 10.0 vol.%.

6. The electrolyte composition according to claim 1 or 2, wherein the molar ratio z of SL / LiTFSI is 2.0 < z < 3.

5.

7. The electrolyte composition according to claim 1 or 2, wherein the molar ratio z of SL / LiTFSI is 2.5 < z < 3.

5.

8. The electrolyte composition according to claim 1 or 2, wherein the molar ratio z of SL / LiTFSI is 2.5 < z < 3.

5.

9. The electrolyte composition according to claim 1 or 2, wherein the molar ratio z of SL / LiTFSI is 2.5 < z < 3.

0.

10. A lithium secondary battery cell comprising the electrolyte composition according to any one of claims 1 to 9. ​

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