Non-aqueous electrolytes and electrochemical devices for lithium-ion batteries

Abstract

This invention provides a non-aqueous electrolyte for lithium-ion batteries and an electrochemical device. The non-aqueous electrolyte comprises an electrolyte, a non-aqueous solvent, and additives, wherein the additives include morpholine derivatives with cyclic structures and cyclic ester additives. During high-voltage charge and discharge, the non-aqueous electrolyte provided by this invention can promote the formation of a dense and stable protective film on the surface of the positive electrode material, reduce the contact between the electrolyte and the positive electrode active material, inhibit the oxidative decomposition reaction of the electrolyte solvent on the surface of the active positive electrode, and reduce the acidity of the battery system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrolyte materials, and particularly relates to a non-aqueous electrolyte for a lithium ion battery and an electrochemical device. BACKGROUND

[0002] With the continuous further expansion of the application field of lithium ion batteries, from electronic products to new application scenarios such as large-scale energy storage power stations and base station power supply, the demand for high-energy lithium ion secondary batteries has become more urgent.

[0003] In order to improve the energy density of lithium ion batteries, the working voltage of lithium ion batteries is generally improved or high-energy-density positive electrode materials are developed. At present, LiNi 0.5 Mn 1.5 O4 as a positive electrode material that is expected to be applied in the field of high working voltage, its charge voltage platform is close to or higher than 5V, but the non-aqueous organic electrolyte is difficult to match the high-voltage positive electrode material, thereby limiting the further application of lithium ion secondary batteries.

[0004] The conventional commercial electrolyte will undergo oxidative decomposition reaction on the surface of the positive electrode of the battery at a high potential of 4.5V or above. The oxidative decomposition reaction of the electrolyte will further cause the performance of the positive electrode material to deteriorate, for example, the surface morphology changes or the structure collapses, etc., ultimately leading to the decline of the cycle performance and storage performance of the lithium ion battery, and the volume expansion phenomenon occurs at the same time. Therefore, the currently commercial electrolyte is difficult to be applied in the high-voltage battery system. For example, 1mol / L of LiPF6 is dissolved in a carbonate solvent, and when charged to 4.5V or above, the activation of the positive electrode material will accelerate the decomposition reaction of the electrolyte, generating CO2 or H2O, etc. The CO2 gas will cause the volume expansion of the battery cell, thereby potentially threatening the safety performance of the battery cell; the presence of H2O will cause the hydrolysis reaction of LiPF6 salt, and the hydrogen fluoride product generated thereby will increase the acidity of the electrolyte system, further intensifying the side reactions inside the battery cell.

[0005] Therefore, in the field, there is an urgent need to develop a non-aqueous electrolyte for a lithium ion battery, which not only has good stability and can reduce the occurrence of side reactions, but also has good cycle stability and rate performance of the lithium ion battery prepared. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a non-aqueous electrolyte for a lithium ion battery and an electrochemical device. The cyclic structure of the morpholine derivative is used as an electrolyte additive, and the amino functional group in the cyclic structure of the morpholine derivative contained in the electrolyte tends to absorb the free F - , thereby reducing the F -It has a high concentration and can be mixed with PF5, thus avoiding the negative impact of acidic systems on battery performance.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a non-aqueous electrolyte for lithium-ion batteries, the non-aqueous electrolyte comprising an electrolyte, a non-aqueous solvent, and additives, wherein the additives comprise morpholine derivatives having a cyclic structure as shown in Formula I and cyclic ester additives:

[0009]

[0010] R1 is selected from C1 to C6 straight-chain alkyl, C1 to C6 branched alkyl or hydrogen atom;

[0011] R2 and R3 are each independently selected from hydrogen atoms, saturated alkyl groups from C1 to C16, unsaturated alkyl groups from C1 to C16, halogen atoms, aryl groups, or ester groups.

[0012] In this invention, R1 is selected from straight-chain alkyl, branched alkyl or hydrogen atom of C1 to C6, for example, it can be methyl, ethyl, propyl, isobutyl or hydrogen atom. For the sake of brevity, the types of substituents in the above range will not be listed one by one.

[0013] In this invention, R2 and R3 are each independently selected from hydrogen atoms, saturated alkyl groups from C1 to C16, unsaturated alkyl groups from C1 to C16, halogen atoms, aryl or ester groups. For example, they can be hydrogen atoms, methyl, ethyl, propyl, vinyl, propenyl, fluorine atoms, chlorine atoms, bromine atoms, iodine atoms, phenyl or ester groups. For the sake of brevity, the types of substituents in the above range will not be listed one by one.

[0014] As a preferred embodiment of the present invention, R2 and R3 are each independently selected from hydrogen atoms, saturated alkyl groups of C1 to C16, unsaturated alkyl groups of C1 to C16 with an unsaturation degree of 1 to 6, halogen substituents, phenyl groups, or ester groups.

[0015] This invention employs morpholine derivatives with cyclic structures as shown in Formula I and cyclic ester additives. The amino functional groups in the cyclic morpholine derivatives contained in the electrolyte tend to absorb free F in the solution. - This reduces the F content in the electrolyte. -The concentration is such that it can be mixed with PF5, avoiding the negative impact of acidic systems on battery performance. Simultaneously, the non-aqueous electrolyte for lithium-ion batteries provided by this invention can promote the formation of a dense and stable protective film on the surface of the positive electrode material during high-voltage charging and discharging, reducing the contact between the electrolyte and the positive electrode active material, inhibiting the oxidative decomposition reaction of the electrolyte solvent on the surface of the active positive electrode, and reducing the acidity of the battery system.

[0016] As a preferred embodiment of the present invention, the morpholine derivative having the cyclic structure shown in Formula I is any one of the following compounds:

[0017]

[0018]

[0019] In this invention, the above-mentioned morpholine derivative additive has the advantages of reducing the acidity of the electrolyte and forming a stable solid electrolyte membrane (CEI) at the positive electrode interface.

[0020] As a further preferred embodiment of the present invention, the morpholine derivative having the cyclic structure shown in Formula I is any one of the following compounds:

[0021]

[0022] This invention provides a method for preparing a morpholine derivative having a cyclic structure as shown in Formula I. The method includes the following steps: dissolving morpholine and a brominated compound with corresponding substituent groups in dimethyl sulfoxide solvent, adding a small amount of CuI as a catalyst and an equivalent amount of K2CO3, heating at 90°C for 24 h, and then distilling under reduced pressure to obtain the morpholine derivative having a cyclic structure as shown in Formula I.

[0023] In this invention, the above-mentioned morpholine derivative additives can not only reduce the acidity of the electrolyte, but also form a stable solid electrolyte membrane (CEI) at the positive electrode interface, and have the advantages of high reactivity and low cost.

[0024] The mass percentage of the morpholine derivative having the cyclic structure shown in Formula I in the non-aqueous electrolyte of the lithium-ion battery is such that it satisfies any one of the following conditions (a) to (b):

[0025] (a) The mass percentage of the morpholine derivative having the cyclic structure shown in Formula I in the non-aqueous electrolyte of the lithium-ion battery is 0.01% to 20%;

[0026] (b) The mass percentage of the morpholine derivative having the cyclic structure shown in Formula I in the non-aqueous electrolyte of the lithium-ion battery is 0.05% to 5%.

[0027] In this invention, the mass percentage of the morpholine derivative having the cyclic structure shown in Formula I in the non-aqueous electrolyte of the lithium-ion battery is 0.01% to 20%, preferably 0.05% to 5%, for example, it can be 0.01%, 0.05%, 0.08%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.5%, 0.8%, 1%, 5%, 8%, 10%, 12%, 15%, 18%, 20%. For the sake of brevity, the values ​​in the above range will not be listed one by one.

[0028] In this invention, by adjusting the mass percentage of the morpholine derivative with the cyclic structure shown in Formula I in the non-aqueous electrolyte of the lithium-ion battery, it is possible to fully exert its role in reducing acidity during electrolyte cycling and to form a CEI protective film of appropriate thickness. If the content is too low, the acidity will not be fully neutralized, while if it is too high, the generated CEI protective film will be too thick, which will deteriorate the battery performance.

[0029] Preferably, the cyclic ester additives include any one or a combination of at least two of the following: cyclic carbonate additives, cyclic sulfonyl lactone additives, or cyclic sulfate additives.

[0030] Preferably, the cyclic carbonate additive includes any one or a combination of at least two of vinylene carbonate, fluoroethylene carbonate, or ethylene ethylene carbonate.

[0031] Preferably, the cyclic sulfonyl lactone additives include 1,3-propanesulfonyl lactone and / or 1,3-propenesulfonyl lactone.

[0032] Preferably, the cyclic sulfate additives include vinyl sulfate and / or propylene sulfate.

[0033] Preferably, the mass percentage of cyclic ester additives in the non-aqueous electrolyte of the lithium-ion battery satisfies any one of the following conditions (c) to (d):

[0034] (c) The mass percentage of cyclic ester additives in the non-aqueous electrolyte of the lithium-ion battery is 0.05% to 20%;

[0035] (d) The mass percentage of cyclic ester additives in the non-aqueous electrolyte of the lithium-ion battery is 0.1% to 5%.

[0036] In this invention, the mass percentage of cyclic ester additives in the non-aqueous electrolyte of the lithium-ion battery is 0.05% to 20%, preferably 0.1% to 5%, for example, it can be 0.05%, 0.08%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.5%, 0.8%, 1%, 5%, 8%, 10%, 12%, 15%, 18%, 20%. For the sake of brevity, the values ​​in the above range will not be listed one by one.

[0037] In this invention, by adjusting the mass percentage of cyclic ester additives in the non-aqueous electrolyte of the lithium-ion battery, an SEI film of suitable thickness can be formed, which is beneficial to improving the electrochemical performance of the battery. If the content is too low, the SEI film will be thin, and it will be easy to puncture the SEI film when lithium is deposited on the negative electrode. Conversely, if the content is too high, the thickness will be too high, which will deteriorate the battery performance.

[0038] Preferably, the electrolyte is a lithium salt.

[0039] Preferably, the lithium salt includes any one or a combination of at least two of LiPF6, LiBF4, LiFSI, LiTFSI, LiBOB, LiODFP, LiODFB, LiPO2F2 or LiCF3SO3.

[0040] Preferably, the concentration of lithium salt in the non-aqueous electrolyte of the lithium-ion battery is from 0.1 mol / L to 2 mol / L, for example, it can be 0.1 mol / L, 0.12 mol / L, 0.15 mol / L, 0.18 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, or 2 mol / L.

[0041] Preferably, the non-aqueous solvent includes any one or a combination of at least two of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, or diethyl carbonate.

[0042] Preferably, the mass percentage of non-aqueous solvent in the non-aqueous electrolyte of the lithium-ion battery is 60% to 85%, for example, it can be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, or 85%.

[0043] In a second aspect, the present invention provides an electrochemical device comprising a positive current collector and a positive active material coated on the positive current collector, a negative current collector and a negative active material coated on the negative current collector, an electrolyte, and a separator, wherein the electrolyte is a non-aqueous electrolyte for lithium-ion batteries according to the first aspect.

[0044] In this invention, the positive electrode active material includes any one or a combination of at least two of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide.

[0045] In this invention, the negative electrode active material includes any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbide, or lithium titanate.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] This invention provides a non-aqueous electrolyte for lithium-ion batteries, which employs a morpholine derivative with a cyclic structure and cyclic ester additives. The amino functional groups in the morpholine derivative contained in the electrolyte tend to absorb free F in the solution. - This reduces the F content in the electrolyte. - The concentration is such that it can be mixed with PF5, avoiding the negative impact of acidic systems on battery performance. Simultaneously, the non-aqueous electrolyte for lithium-ion batteries provided by this invention can promote the formation of a dense and stable protective film on the surface of the positive electrode material during high-voltage charging and discharging, reducing the contact between the electrolyte and the positive electrode active material, inhibiting the oxidative decomposition reaction of the electrolyte solvent on the surface of the active positive electrode, and reducing the acidity of the battery system. Detailed Implementation

[0048] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0049] In the examples and comparative examples, the preparation method of the morpholine derivative having the cyclic structure shown in Formula I includes the following steps: dissolving morpholine and the brominated compound with the corresponding substituent group in dimethyl sulfoxide solvent, adding a small amount of CuI as a catalyst and an equivalent amount of K2CO3, heating at 90°C for 24 h, and then distilling under reduced pressure to obtain the morpholine derivative having the cyclic structure shown in Formula I.

[0050] Example 1

[0051] This embodiment provides a non-aqueous electrolyte for lithium-ion batteries. Based on the total mass of the non-aqueous electrolyte as 100%, the lithium-ion non-aqueous electrolyte includes additives with a mass percentage of 1% of compound A, 1% of vinylene carbonate, 1.5% of 1,3-propanesulfonate lactone, and 1% of vinyl sulfate, respectively. The lithium salt is lithium hexafluorophosphate with a concentration of 1 mol / L, and the balance is a non-aqueous solvent. The non-aqueous solvent is composed of vinyl carbonate, methyl ethyl carbonate, and diethyl carbonate in a mass ratio of 3:5:2.

[0052] The method for preparing the non-aqueous electrolyte for lithium-ion batteries is as follows:

[0053] The electrolyte was prepared in a glove box with a nitrogen content of 99.999%, an actual oxygen content of 0.1 ppm, and a moisture content of 0.1 ppm. Based on a total non-aqueous electrolyte mass of 100%, battery-grade organic solvents of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 3:5:2 were mixed thoroughly. Fully dried lithium hexafluorophosphate was then added to the aforementioned non-aqueous solvent, along with 1% (by mass) of compound A, 1% of vinylene carbonate, 1.5% of 1,3-propanesulfonate lactone, and 1% of vinyl sulfate, respectively, to achieve a lithium hexafluorophosphate concentration of 1 mol / L, thus preparing the non-aqueous electrolyte for lithium-ion batteries.

[0054] This embodiment also provides a lithium-ion battery, and the preparation method of the lithium-ion battery is as follows:

[0055] LiNi, the positive electrode active material 0.5 Mn 1.5 O4, conductive agent acetylene black, and binder polyvinylidene fluoride are mixed thoroughly in an N-methylpyrrolidone solvent system at a mass ratio of 95:3:2. The mixture is then coated onto aluminum foil, dried, and cold-pressed to obtain a positive electrode sheet with a compacted density of 3.5 g / cm³. 3 .

[0056] The negative electrode active material graphite, conductive agent acetylene black, binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose were thoroughly mixed in a deionized water solvent system at a mass ratio of 96:2:1:1. The mixture was then coated onto copper foil, dried, and cold-pressed to obtain the negative electrode sheet with a compacted density of 1.65 g / cm³. 3 .

[0057] A diaphragm was obtained by using 9 μm thick polyethylene as the base membrane and coating the base membrane with a 3 μm thick nano-alumina coating.

[0058] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The stacked electrodes then form a bare cell.

[0059] The bare battery cell is placed into an aluminum-plastic film, then baked at 80°C to remove moisture, and then injected with the appropriate electrolyte and sealed. After that, it goes through processes such as settling, hot and cold pressing, formation, clamping and capacity testing to obtain the finished soft-pack lithium-ion secondary battery.

[0060] Other embodiments and comparative examples are based on the steps of Embodiment 1 with parameter changes. The specific parameters changed are shown in Table 1:

[0061] Table 1

[0062]

[0063]

[0064]

[0065] Test conditions

[0066] The lithium-ion batteries provided in Examples 1 to 10 and Comparative Examples 1 to 3 were subjected to performance tests, and the test methods are as follows:

[0067] (1) Capacity retention during high-temperature cycling at 45℃:

[0068] At 45℃, the lithium-ion battery was charged at a constant current rate of 1C to 5V, then charged at a constant voltage of 5V until the current was less than 0.05C. After resting for 10 minutes, it was discharged at a constant current rate of 1C to 2.8V. The discharge capacity of the lithium-ion battery at this point was measured, which is the discharge capacity of the first cycle. The battery was then subjected to multiple cycles under the above conditions, and the capacity retention rate after 1000 cycles was calculated. The capacity retention rate after cycling was calculated using the following formula:

[0069] Capacity retention rate (%) = (Discharge capacity after 1000 cycles / Discharge capacity after the first cycle) × 100%.

[0070] (2) Volume expansion rate after 30 days of storage at 60℃:

[0071] At 25°C, the lithium-ion battery was charged to 5V at a constant current of 1C, and then charged to 0.05C at a constant voltage. The volume of the lithium-ion battery was measured and recorded as V0. The fully charged battery was then stored in a 60°C oven for 30 days, and the volume after storage was measured and recorded as V1. The volume expansion rate of the lithium-ion battery relative to its initial volume before storage was calculated using the following formula:

[0072] Volume expansion rate (%) = (V1-V0) / V0×100%.

[0073] (3) Acidity of the system under high-temperature cycling at 45℃:

[0074] At 45℃, the lithium-ion battery was charged at a constant current rate of 1C to 5V, then charged at a constant voltage of 5V until the current was less than 0.05C. After resting for 10 minutes, it was discharged at a constant current rate of 1C to 2.8V. The battery was cycled 500 times under the above conditions. After cycling, the battery was discharged to empty using a small light bulb, then disassembled. The battery was immersed in dimethyl carbonate and titrated with a 1mol / L sodium hydroxide standard solution until the bromothymol blue indicator turned blue, which was the titration endpoint. The acidity of the system was then calculated.

[0075] The test results are shown in Table 2:

[0076] Table 2

[0077]

[0078] As can be seen from the data in Table 2, from Examples 1 to 6, morpholine derivatives with the cyclic structure shown in Formula I, as additives, can effectively improve the cycle performance of lithium-ion batteries and reduce battery volume expansion and system acidity. This is because morpholine derivatives with the cyclic structure shown in Formula I can effectively absorb F in the system. - , reduce F - The concentration was reduced, the acidity of the system was improved, and the high-temperature cycling and storage gas generation performance of the high-voltage cell system was effectively improved.

[0079] Comparing Examples 1 and 4 to 6, it can be seen that as the content of morpholine derivatives gradually increases, the high-temperature cycling performance of the battery cell first increases and then decreases, while the high-temperature storage gas generation first decreases and then increases. This indicates that when the content of morpholine derivatives is low, the generated CEI film cannot completely cover the interface between the cathode material and the electrolyte; while when the content is too high, the generated CEI film is thicker, and its improvement on high-temperature cycling and storage gas generation performance is not significant enough.

[0080] Comparing Example 1 with Comparative Examples 1 to 3, it can be seen that without the addition of morpholine derivatives or cyclic ester compounds, complex side reactions occur in the electrolyte due to the high reactivity of the cathode material interface, resulting in poor high-temperature cycle performance and storage performance of the battery cell. Conversely, excessive addition of these additives fails to provide protection, leading to performance degradation. Furthermore, unmodified morpholine derivatives have a poor effect on improving the battery's electrochemical performance and high-temperature gas generation performance.

[0081] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A non-aqueous electrolyte for lithium-ion batteries, characterized in that, The lithium ion battery non-aqueous electrolyte comprises an electrolyte, a non-aqueous solvent and an additive, the additive comprises a morpholine derivative having a cyclic structure shown in Formula I and a cyclic ester additive: Formula I wherein R1 is selected from a hydrogen atom; R2 and R3 are each independently selected from a hydrogen atom, a C1 to C16 saturated alkyl group, a C1 to C16 unsaturated alkyl group, a halogen atom, an aryl group or an ester group; The mass percentage content of the morpholine derivative having a cyclic structure shown in Formula I in the lithium ion battery non-aqueous electrolyte is 0.01% to 20%.

2. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, characterized by The morpholine derivative having a cyclic structure shown in Formula I is any one of the following compounds: A B C D E F G H I J K L M N.

3. The nonaqueous electrolyte for a lithium-ion battery according to claim 2, wherein The morpholine derivative having a cyclic structure shown in Formula I is any one of the following compounds: A G M.

4. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, wherein The mass percentage content of the morpholine derivative having a cyclic structure shown in Formula I in the lithium ion battery non-aqueous electrolyte is 0.05% to 5%.

5. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, wherein The cyclic ester additive comprises any one of a cyclic carbonate additive, a cyclic sulfonate lactone additive or a cyclic sulfate additive or a combination of at least two thereof; The cyclic carbonate additive comprises any one of vinylene carbonate, fluoroethylene carbonate or vinyl ethylene carbonate or a combination of at least two thereof; The cyclic sulfonate lactone additive comprises 1,3-propane sulfonate lactone and / or 1,3-propylene sulfonate lactone; The cyclic sulfate additive comprises vinyl sulfate and / or propylene sulfate.

6. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, wherein The mass percentage content of the cyclic ester additive in the lithium ion battery non-aqueous electrolyte is 0.05% to 20%.

7. The nonaqueous electrolyte for a lithium-ion battery according to claim 6, wherein The mass percentage content of the cyclic ester additive in the lithium ion battery non-aqueous electrolyte is 0.1% to 5%.

8. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, wherein The electrolyte is a lithium salt; The lithium salt comprises any one of LiPF6, LiBF4, LiFSI, LiTFSI, LiBOB, LiODFP, LiODFB, LiPO2F2 or LiCF3SO3 or a combination of at least two thereof; The concentration of the lithium salt in the lithium ion battery non-aqueous electrolyte is 0.1 mol / L to 2 mol / L.

9. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, wherein The non-aqueous solvent comprises any one of vinyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate or diethyl carbonate or a combination of at least two thereof.

10. The nonaqueous electrolyte for a lithium-ion battery according to claim 1, wherein The mass percentage content of the non-aqueous solvent in the lithium ion battery non-aqueous electrolyte is 60% to 85%.

11. An electrochemical device, characterized by, The electrochemical device comprises a positive electrode current collector and a positive electrode active material coated on the positive electrode current collector, a negative electrode current collector and a negative electrode active material coated on the negative electrode current collector, an electrolyte and a separator, the electrolyte is the lithium ion battery non-aqueous electrolyte according to any one of claims 1 to 10.

Images