CN118380650A

CN118380650A - Electrolyte suitable for micron silicon negative electrode and high-nickel positive electrode and application thereof

Info

Publication number: CN118380650A
Application number: CN202410240167.9A
Authority: CN
Inventors: 柯福生; 艾新平; 郭怡业; 缪谋仁
Original assignee: Wuhan University WHU
Current assignee: Wuhan University WHU
Priority date: 2024-03-04
Filing date: 2024-03-04
Publication date: 2024-07-23

Abstract

The invention discloses an electrolyte suitable for a micron silicon negative electrode and a high-nickel positive electrode and application thereof, wherein the electrolyte comprises the following components in a molar ratio of 1-2: 3.3 to 6:3.3 mixing fluorine-containing lithium salt, carbonic ester solvent and ether auxiliary agent. The electrolyte constructed by the invention can form a cathode solid electrolyte phase interface film which is formed by lithium fluoride and sulfide in parallel and vertically and is similar to a reinforced bar mixed soil structure on a micron silicon cathode; and forming a thin and uniform anode solid electrolyte phase interfacial film on the high-nickel anode. Thereby prolonging the service life of the high-capacity rechargeable lithium ion battery. In addition, the electrolyte disclosed by the invention is simple in formula, wide in raw material sources and favorable for realizing commercial production.

Description

Electrolyte suitable for micron silicon negative electrode and high-nickel positive electrode and application thereof

Technical Field

The invention relates to the technical field of lithium ion battery electrolyte, in particular to electrolyte suitable for a micron silicon negative electrode and a high-nickel positive electrode and application thereof.

Background

With the rapid development of electronic devices, electric vehicles, and large-scale energy storage systems, the search for high energy density Lithium Ion Batteries (LIBs) has been driven. Among the negative electrode materials, silicon materials have been widely studied as a substitute negative electrode for next-generation lithium graphite because of their theoretical specific capacities (3579 mAh g ^-1) much higher than graphite (372 mAh g ^-1). However, despite this high specific capacity, the major challenges in achieving a silicon negative electrode in practical lithium ion batteries are its large volume change (300%) during lithiation/delithiation, resulting in pulverization, electrical contact loss, severe capacity fade. In order to overcome the problem, various nanometer silicon-based materials with different morphologies are explored to improve the cycle stability. Compared with nano-sized materials, the micro-sized silicon-based materials have the advantages of low cost, small specific surface area, high tap density and the like, and are more suitable for practical industrial application based on manufacturing technology. For high nickel cathodes, the energy density increases due to the increase in nickel content; but this also results in dissolution of the transition metal at high cut-off voltages. The dissolved transition metal is reduced at the anode, thereby affecting the cycle performance of the anode.

Aiming at the problems of material pulverization and cathode-anode nickel-cathode transition metal dissolution caused by the volume expansion process of the silicon cathode, the construction of a stable electrode/electrolyte interface becomes particularly important. Research shows that the solid electrolyte phase interface film (SEI) rich in lithium fluoride can prevent further side reaction of electrolyte and electrode material, maintain the stability of the morphology of silicon particles and improve the cycle performance of the battery. The stable and uniform positive electrode solid electrolyte phase interface film (CEI) is constructed on the high-nickel positive electrode, so that the dissolution of transition metal can be effectively prevented, and the stable circulation of the transition metal under the high cut-off voltage can be maintained.

Therefore, for lithium batteries constructed with micron silicon cathodes and high nickel anodes, it is necessary to select a suitable electrolyte.

Disclosure of Invention

The invention aims to provide an electrolyte for constructing a stable electrode/electrolyte interface of a micron silicon negative electrode and a high-nickel positive electrode of a high-capacity lithium ion battery, so that the electrochemical performance of a micron silicon negative electrode material and a high-nickel positive electrode material is improved, and the service life of a high-capacity rechargeable lithium ion battery is prolonged.

In order to achieve the above object, the present invention provides an electrolyte suitable for a micron silicon negative electrode and a high nickel positive electrode, comprising the following components in a molar ratio of 1-2: 3.3 to 6:3.3 mixing fluorine-containing lithium salt, carbonic ester solvent and ether auxiliary agent.

Further, the fluorine-containing lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium dioxalate borate, lithium bis (trifluoromethylsulfonyl) imide, lithium difluorooxalate borate and lithium difluorophosphate.

Further, the carbonate-based solvent includes ethylene carbonate, diethyl carbonate, dimethyl carbonate, methylethyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, fluoroethylene carbonate, dimethyl 1-fluorocarbonate, ethyl 2, 2-difluoromethyl carbonate, ethyl 2, 2-trifluoroethyl 2, 2-trifluoroethyl trifluorocarbonate, methyl-2, 2', at least one of 2',2', -isopropyl hexafluorocarbonate, ethyl-2, 2',2',2', -isopropyl hexafluorocarbonate, bis (2, 2-trifluoroethyl) carbonate.

Further, the method comprises the steps of, the ether auxiliary agent comprises ethylene glycol dimethyl ether, dioxolane, 1, 2-tetrafluoroethyl-2, 3-tetrafluoropropyl ether 1, 2-tetrafluoromethyl ether, bis (2, 2-trifluoroethyl) ether at least one of 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether.

Further, the fluorine-containing lithium salt is a lithium bis-fluorosulfonyl imide salt;

the carbonic ester solvent is diethyl carbonate or ethylmethyl carbonate;

the ether auxiliary agent is 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether.

The invention also provides a lithium ion battery, which comprises the electrolyte, a positive electrode plate, a diaphragm and a negative electrode plate.

Further, the mass ratio of the active materials to the conductive carbon and the binder in the positive electrode plate and the negative electrode plate is 3-8: 1:1. the kinds of the conductive carbon and the binder are not necessarily strictly limited, and the conductive carbon may be at least one of conductive carbon black BP2000, carbon nanotubes, and conductive carbon black super P, for example; the binder is at least one of sodium hydroxymethyl cellulose, polyvinyl alcohol, polyacrylic acid, polyvinylidene fluoride and sodium alginate

The invention also provides application of the electrolyte in a micron silicon negative electrode lithium battery or a high nickel positive electrode lithium battery.

Compared with the prior art, the invention has the following beneficial effects:

The electrolyte constructed by the invention can form a cathode solid electrolyte phase interface film which is formed by lithium fluoride and sulfide in parallel and vertically and is similar to a reinforced bar mixed soil structure on a micron silicon cathode; and forming a thin and uniform anode solid electrolyte phase interfacial film on the high-nickel anode. Thereby prolonging the service life of the high-capacity rechargeable lithium ion battery. In addition, the electrolyte disclosed by the invention is simple in formula, wide in raw material sources and favorable for realizing commercial production.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

Fig. 1 is electrochemical performance test results of example 1 and comparative example 1, wherein fig. 1a is a cycle performance test result, fig. 1b is a rate performance test result, and fig. 1c is a specific capacity-voltage curve of example 1;

Fig. 2 is a scanning electron microscope image of a micrometer silicon negative electrode of example 1 and comparative example 1 after 30 weeks of cycling at a current density of 1Ag ^-1, wherein fig. 2a and 2c are the surface and cross section of example 2, respectively, and fig. 2b and 2d are the surface and cross section of comparative example 2, respectively;

Fig. 3 is a transmission electron microscope image of a micrometer silicon negative electrode of example 1 and comparative example 1 after 30 weeks of cycling at a current density of 1Ag ^-1, wherein fig. 3a and 3c are example 2 and fig. 3b and 3d are comparative example 2, respectively;

FIG. 4 is a time-of-flight secondary ion mass spectrometry standard results for the micrometer silicon negative electrodes of example 1 and comparative example 1 after cycling;

FIG. 5 is the results of the cycle 140 test for example 2 and comparative example 2 at a cut-off voltage of 4.9V;

FIG. 6 is a scanning electron microscope image, a transmission electron microscope image and ion concentration in electrolyte after circulation of the high nickel cathode material of the example 2 and the comparative example 2 at a current density of 0.1A g ^-1 for 30 weeks, wherein FIG. 6a is a scanning electron microscope image before circulation, FIG. 6b is a scanning electron microscope image after circulation of the comparative example 2, FIG. 6c is a scanning electron microscope image after circulation of the example 2, FIG. 6d is a transmission electron microscope image after circulation of the comparative example 2, FIG. 6e is a transmission electron microscope image after circulation of the example 2, and FIG. 6f is ion concentration results in electrolyte after circulation;

fig. 7a is the cycling performance results of the lithium ion full cell of example 3 and comparative example 3, and fig. 7b is the specific capacity results of example 3 at different current densities;

FIG. 8 is the electrochemical performance test results of example 4, FIG. 8a is the cycle performance test results, and FIG. 8b is the coulombic efficiency of the cycle;

Fig. 9 is a mechanism diagram of the battery performance enhancement of the present invention.

Detailed Description

The endpoints of the ranges and any values disclosed in the present invention are not limited to the precise range or value, and the range or value should be understood to include values close to the range or value. For numerical ranges, one or more new numerical ranges may be obtained in combination with each other between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point values, and are to be considered as specifically disclosed in the present invention.

The following description of specific embodiments of the present invention and the accompanying drawings will provide a clear and complete description of the technical solutions of embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Example 1

A lithium ion half-cell adopts electrolyte suitable for a micron silicon negative electrode and a high nickel positive electrode, and the preparation method is as follows,

Step 1, in a glove box, according to a molar ratio of 2:3.3:3.3 weighing lithium bis (fluorosulfonyl) imide, diethyl carbonate and 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether, and uniformly mixing to completely dissolve the lithium bis (fluorosulfonyl) imide to obtain an electrolyte suitable for a micron silicon negative electrode and a high-nickel positive electrode;

Step 2, mixing the micron silicon material with conductive carbon black super P and polyacrylic acid according to a mass ratio of 3:1:1, uniformly mixing to obtain slurry, coating the slurry on a current collector copper foil, and drying the copper foil in a vacuum drying oven at 80 ℃ for 12 hours to obtain a negative electrode plate;

and 3, taking the negative electrode plate obtained in the step 2 as a working electrode, taking a metal lithium plate as a counter electrode, taking a polypropylene/polyethylene/polypropylene (PP/PE/PP) three-layer composite membrane as a diaphragm, and assembling the electrolyte obtained in the step 1 into the lithium ion battery in an inert gas atmosphere.

Comparative example 1

Substantially the same as in example 1, except that: the electrolyte of the lithium ion half battery is a commercial electrolyte of 1mol/L lithium hexafluorophosphate, and the specific preparation method comprises the following steps of dissolving lithium hexafluorophosphate in a volume ratio of 1:1, and diethyl carbonate (5 wt% of fluoroethylene carbonate).

To determine the performance impact of the electrolyte on lithium ion half-cells constructed with micron silicon cathodes, the lithium ion half-cells of example 1 and comparative example 1 were tested. FIG. 1a shows the current density cycling performance test results at 1A g ^-1, it can be seen that example 1 still has a reversible specific capacity of 1667 mAh.g ^-1 after 200 weeks of cycling, whereas comparative example 1 has a reversible specific capacity of only 642 mAh.g ^-1; FIG. 1b shows the rate capability test results, and it can be seen that example 1 has better rate capability; fig. 1c shows the specific capacity-voltage curve of example 1.

The morphology of the microsilica cathodes of example 1 and comparative example 1 after 30 weeks of cycling at a current density of 1A g ^-1 was observed using a scanning electron microscope. As can be seen from a comparison of fig. 2a and fig. 2b, the surface of the micro silicon anode material is still dense and stable after the circulation of example 1, whereas the crack appears in comparative example 1, which indicates that the micro silicon anode material is unstable in the electrolyte of comparative example 1, the expansibility is not significantly improved, and fig. 2c and fig. 2d also illustrate the case. The electrolyte suitable for the micron silicon negative electrode and the high-nickel positive electrode in the embodiment 1 of the invention can effectively inhibit the expansion of the silicon negative electrode and improve the stability.

To further verify the effect of the electrolyte on the microsilica cathodes in the cycling test, the morphology of the microsilica cathodes after 30 weeks of cycling at a current density of 1A g ^-1 for example 1 and comparative example 1 was observed using a transmission electron microscope. It can be seen from the combination of fig. 3a and 3c, and fig. 3b and 3d, respectively, that example 1 forms a stable anode solid electrolyte phase interfacial film, thus improving the cycle performance of the battery. In addition, the structure of the circulating micron silicon negative electrode is characterized by using a time-of-flight secondary ion mass spectrum, the generated secondary ions are analyzed to obtain the distribution of the chemical components of the sample in three dimensions, and the distribution is visualized by using Dragonfly software. As can be seen from fig. 4, the electrolyte constructed in example 1 is capable of forming a negative solid electrolyte phase interfacial film of a reinforced concrete-like structure composed of lithium fluoride and sulfide side by side vertically on a micro silicon negative electrode.

Example 2

Step 2, mixing the high-nickel anode material with conductive carbon black super P and polyvinylidene fluoride according to a mass ratio of 8:1:1, uniformly mixing to obtain slurry, coating the slurry on a current collector aluminum foil, and drying the aluminum foil in a vacuum drying oven at 80 ℃ for 12 hours to obtain an anode electrode plate;

And 3, taking the positive electrode plate obtained in the step 2 as a working electrode, taking a metal lithium plate as a counter electrode, taking a polypropylene/polyethylene/polypropylene (PP/PE/PP) three-layer composite film as a diaphragm, and assembling the electrolyte obtained in the step 1 into the lithium ion battery in an inert gas atmosphere.

The high nickel cathode material used in this example was lithium nickel cobalt manganese oxide, specifically Li (Ni _0.83Co_0.12Mn_0.05)O₂), and the high nickel cathode material of the following examples or comparative examples was the same.

Comparative example 2

Substantially the same as in example 2, except that: the electrolyte of the lithium ion half cell is a commercial electrolyte of 1mol/L lithium hexafluorophosphate, and the preparation method is the same as that of comparative example 1, and will not be described again.

To determine the performance impact of the electrolyte on the lithium ion half-cells constructed with the high nickel positive electrode material, the lithium ion half-cells of example 2 and comparative example 2 were tested. Fig. 5 shows the results of the example 2 and comparative example 2 cycle 140 test at a cut-off voltage of 4.9V, and it can be seen that the capacity retention of example 2 after 140 weeks of cycle was 83% and the capacity retention of the control group was only 63%.

The morphology of the high nickel positive electrode materials of example 2 and comparative example 2 was observed using a scanning electron microscope before and after 30 weeks of cycling at a current density of 0.1A g ^-1. From the morphology of fig. 6a before cycling, fig. 6b after cycling of comparative example 2 and fig. 6c after cycling of example 2, it can be seen that the structure of example 2 is more compact and not significantly altered before cycling, whereas the structure of comparative example 2 is significantly loose, which demonstrates that the electrolyte of example 2 can form a stable electrode/electrolyte interface at the high nickel positive electrode surface, as well as the results demonstrated in fig. 6d and 6 e. The concentrations of nickel, cobalt and manganese ions in the electrolyte after circulation were also detected as shown in fig. 6f, and it can be seen that the electrolyte of example 2 has less nickel, cobalt and manganese ions, further illustrating that a thin and uniform interface film of the solid electrolyte phase of the positive electrode is formed and dissolution of the active material in the electrode is effectively prevented.

Example 3

A lithium ion full battery adopts electrolyte suitable for a micron silicon negative electrode and a high nickel positive electrode, and the preparation method thereof is as follows,

step 3, mixing the high-nickel anode material with conductive carbon black super P and polyvinylidene fluoride according to a mass ratio of 8:1:1, uniformly mixing to obtain slurry, coating the slurry on a current collector aluminum foil, and drying the aluminum foil in a vacuum drying oven at 80 ℃ for 12 hours to obtain an anode electrode plate;

And 4, assembling the negative electrode plate (without pre-lithiation or pre-circulation) obtained in the step 2, the positive electrode plate obtained in the step 3 and glass fiber as a diaphragm, and operating the electrolyte obtained in the step 1 in an inert gas atmosphere to form the lithium ion full battery.

Comparative example 3

Substantially the same as in example 3, except that: the electrolyte of the lithium ion full battery is a commercial electrolyte of 1mol/L lithium hexafluorophosphate, and the preparation method is the same as that of comparative example 1, and is not repeated.

To determine the performance impact of the electrolyte on lithium ion full cells constructed of the micron silicon material negative electrode and the high nickel positive electrode material, the lithium ion full cells of example 3 and comparative example 3 were tested. As can be seen from fig. 7a, example 3 has more stable cycle performance than comparative example 3, and as can also be seen from fig. 7b, example 3 can still maintain 80% of reversible specific capacity (161 mAh g ^-1) after the current density is increased from 0.1C to 2C (1c=100 mA g ^-1), which is significant for the rapid charging of lithium ion batteries.

Example 4

Step 1, in a glove box, according to a molar ratio of 2:3.3:3.3 weighing lithium bis (fluorosulfonyl) imide, methyl ethyl carbonate and 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether, and uniformly mixing to completely dissolve the lithium bis (fluorosulfonyl) imide to obtain an electrolyte suitable for a micron silicon negative electrode and a high-nickel positive electrode;

The lithium ion half-cell of example 4 was tested. Fig. 8a shows that example 4 has a reversible specific capacity of 1232mAh g ^-1 after 200 weeks of cycling, which is less than that of 1667mAh g ^-1 of example 1. The average coulombic efficiency, example 4 was 99.4% and was also lower than 99.5% of example 1. This result demonstrates that diethyl carbonate is better as a solvent.

Fig. 9 shows a mechanism of battery performance enhancement of the present invention, which results from the ability of the electrolyte to form a reinforcing bar-like solid electrolyte phase interface film (SEI) consisting of lithium fluoride side by side and perpendicular to sulfide at the negative electrode, while forming a thin and uniform positive electrode solid electrolyte phase interface film (CEI) on the high nickel positive electrode. Thereby improving the cycle life of the high-capacity rechargeable lithium ion battery

Example 5

A lithium ion full cell substantially as in example 3 except that in step 1, the molar ratio 1:3.3:3.3 weighing lithium bis (fluorosulfonyl) imide, diethyl carbonate and 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether.

Example 6

A lithium ion full cell substantially as in example 3 except that in step 1, the molar ratio 1:5:3.3 weighing lithium bis (fluorosulfonyl) imide, diethyl carbonate and 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether.

Finally, it should be noted that: the foregoing description is only illustrative of the preferred embodiments of the present invention, and although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that modifications may be made to the embodiments described, or equivalents may be substituted for elements thereof, and any modifications, equivalents, improvements or changes may be made without departing from the spirit and principles of the present invention.

Claims

1. The electrolyte suitable for the micron silicon negative electrode and the high-nickel positive electrode is characterized by comprising the following components in a molar ratio of 1-2: 3.3 to 6:3.3 mixing fluorine-containing lithium salt, carbonic ester solvent and ether auxiliary agent.

2. The electrolyte of claim 1, wherein the fluorine-containing lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium dioxalate borate, lithium bis (fluorosulfonyl) imide, lithium bis (trifluoromethylsulfonyl) imide, lithium difluorooxalate borate, and lithium difluorophosphate.

3. The electrolyte of claim 1 wherein the carbonate-based solvent comprises at least one of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methylethyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, fluoroethylene carbonate, dimethyl 1-fluorocarbonate, ethyl 2, 2-difluoromethyl carbonate, ethyl 2, 2-trifluoroethyl carbonate, diethyl 2, 2-trifluoro-carbonate, methyl-2, 2',2',2', -isopropyl hexafluorocarbonate, ethyl-2, 2',2',2', -isopropyl hexafluorocarbonate, bis (2, 2-trifluoroethyl) carbonate.

4. The electrolyte according to claim 1, wherein, the ether auxiliary agent comprises ethylene glycol dimethyl ether, dioxolane, 1, 2-tetrafluoroethyl-2, 3-tetrafluoropropyl ether 1, 2-tetrafluoromethyl ether, bis (2, 2-trifluoroethyl) ether at least one of 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether.

5. The electrolyte of claim 1, wherein the fluorine-containing lithium salt is a lithium bis-fluorosulfonyl imide salt;

the carbonic ester solvent is diethyl carbonate or ethylmethyl carbonate;

the ether auxiliary agent is 1, 2-tetrafluoroethyl-2, 2-trifluoroethyl ether.

6. A lithium ion battery, characterized by comprising the electrolyte according to any one of claims 1 to 5, and further comprising a positive electrode plate, a diaphragm and a negative electrode plate.

7. The lithium ion battery according to claim 6, wherein the mass ratio of the active materials in the positive electrode sheet and the negative electrode sheet to conductive carbon and binder is 3-8: 1:1.

8. Use of the electrolyte according to any one of claims 1 to 5 in a micrometer silicon negative electrode lithium battery or a high nickel positive electrode lithium battery.