CN111987278B - Composite diaphragm for lithium metal secondary battery and preparation method and application thereof - Google Patents

Abstract

The invention provides a composite diaphragm for a lithium metal secondary battery, and a preparation method and application thereof. The coating is transferred to the surface of a lithium metal cathode in the assembly process of the battery, and in-situ conversion reaction occurs in the subsequent battery circulating electrochemical process to generate a mixed electron ion conductor layer, and meanwhile, the distribution of ions and electrons on an interface is adjusted, so that the ion concentration gradient is relieved, uniform lithium deposition is guided, and the generation of lithium dendrites is inhibited. The lithium metal secondary battery using the composite diaphragm shows excellent cycle and rate performance. The composite diaphragm of the invention avoids the complex procedures and harsh operating environment of the prior art for directly modifying lithium metal, has simple preparation technology, easily obtained raw materials and extremely high practical and large-scale prospects.

Description

Composite separator for lithium metal secondary battery, preparation method and application thereof

technical field

The invention belongs to the field of electrochemical power sources, and in particular relates to a functional composite diaphragm of a lithium metal secondary battery, a preparation method thereof, a lithium metal secondary battery assembled by using the diaphragm, and its application in an energy storage device.

Background technique

Lithium-ion secondary batteries have been widely used in consumer electronics and communications. However, due to its limited theoretical capacity, its development and utilization are approaching the limit. With the rapid development of hybrid electric vehicles and smart grids in the future, people put forward higher and higher performance requirements for lithium-ion batteries in the energy storage link, such as high energy density and high safety.

Lithium metal has attracted much attention due to its high theoretical specific capacity (3860mAh/g) and the lowest negative electrode electrochemical potential (-3.04V, hydrogen standard potential), and has broad development prospects. Lithium metal secondary batteries can provide much higher energy density than conventional batteries and are regarded as one of the most important energy storage technologies for the next generation.

However, during the battery cycle process, the uneven deposition of metallic lithium on the surface of the lithium metal negative electrode results in the formation of lithium dendrites, which are easy to pierce the separator and cause great safety hazards. On the other hand, the reaction activity between the lithium metal anode and the electrolyte is extremely high. After the interfacial solid electrolyte layer (SEI) continues to collapse and disintegrate, a new SEI is formed again, which continuously consumes the electrolyte and generates harmful by-products. cycle life. Therefore, protecting the lithium metal anode has become a key challenge for the commercialization of lithium metal batteries.

At present, the most common means to protect the metal lithium negative electrode is to add electrolyte additives, or to modify a protective layer on the lithium negative electrode to prevent the continuous reaction between the electrolyte and lithium metal, and to uniformly deposit lithium; or to make the lithium negative electrode into a three-dimensional collection fluid, increasing the specific surface area, decreasing the current density, and guiding the uniform deposition of lithium. However, these methods all involve complex preparation processes and harsh manufacturing environments, which are not conducive to large-scale actual production.

Therefore, there is an urgent need to develop a simple and low-cost method for protecting the lithium metal negative electrode, which can achieve the effect of suppressing metal lithium dendrites during the cycle of the negative electrode, improve the safety and cycle stability of the battery, and promote the development of lithium metal batteries. Practical.

SUMMARY OF THE INVENTION

Therefore, the purpose of the present invention is to solve the problem that the lithium metal negative electrode in the existing secondary battery is easy to generate lithium dendrites, pierce the diaphragm, and cause serious safety hazards, and provide a simple and easy-to-operate functional composite diaphragm, which can uniformly The deposition of lithium ions can inhibit the generation of lithium dendrites, improve the cycle stability and safety of the battery, and avoid the tedious procedures and harsh operating environment of the direct modification of lithium metal in the prior art.

In one aspect, the present invention provides a composite separator for a lithium metal secondary battery, the composite separator comprising a separator base layer and a functional coating coated on at least one surface of the separator base layer, wherein the functional coating layer Contains particles of substances capable of reacting with lithium metal.

According to the composite separator provided by the present invention, the substance particles capable of reacting with lithium metal include at least one metal nitride and/or metal fluoride. Wherein, the metal nitride can be selected from magnesium nitride, copper nitride, strontium nitride, dysprosium nitride, lanthanum nitride, cerium nitride, ytterbium nitride, iron nitride, manganese nitride, titanium nitride, One or more of vanadium nitride, cobalt nitride, nickel nitride, molybdenum nitride, germanium nitride and aluminum nitride. The metal fluoride may be selected from magnesium fluoride, copper fluoride, strontium fluoride, dysprosium fluoride, lanthanum fluoride, cerium fluoride, ytterbium fluoride, iron fluoride, manganese fluoride, titanium fluoride, fluoride One or more of vanadium, cobalt fluoride, nickel fluoride, molybdenum fluoride, germanium fluoride, and aluminum fluoride.

According to the composite separator provided by the present invention, the thickness of the functional coating may be 0.1-30 μm, preferably 0.5-5 μm.

In a preferred embodiment, the particle size of the substance particles is 5 nm˜5 μm, preferably 200˜500 nm.

According to the composite separator provided by the present invention, the separator base layer is not particularly limited, and a separator commonly used in secondary batteries in the art can be used. For example, the membrane base layer may be selected from polyethylene membranes, polypropylene membranes, polyethylene/polypropylene membranes, alumina polyethylene membranes, and ceramic fiber paper membranes.

The composite separator provided by the present invention is a functional separator, which can transfer the coating compound pre-coated on the separator to the surface of lithium metal during the battery assembly process to protect the lithium metal. In this way, the traditional and cumbersome lithium metal protection scheme is greatly avoided, and the steps are simple, which is beneficial to production and saves costs. More importantly, this coating can in-situ convert to form a mixed electron-ion conductor layer during the electrochemical reaction, and simultaneously adjust the electron and ion distribution at the lithium anode interface, ease the ion concentration gradient, uniform lithium deposition, and avoid lithium dendrites. production, improve the safety and cycle stability of the battery.

On the other hand, the present invention also provides a preparation method of the composite diaphragm, and the preparation method comprises the following steps:

(1) disperse the active material particles and the binder in an organic solvent to obtain a coating slurry;

(2) Coating the coating slurry prepared in step (1) on at least one surface of the diaphragm base layer, and drying to obtain the composite diaphragm.

According to the preparation method provided by the present invention, the active material particles include at least one metal nitride and/or metal fluoride, preferably, the metal nitride is selected from magnesium nitride, copper nitride, and strontium nitride , dysprosium nitride, lanthanum nitride, cerium nitride, ytterbium nitride, iron nitride, manganese nitride, titanium nitride, vanadium nitride, cobalt nitride, nickel nitride, molybdenum nitride, germanium nitride and nitrogen One or more of aluminum fluoride; the metal fluoride can be selected from magnesium fluoride, copper fluoride, strontium fluoride, dysprosium fluoride, lanthanum fluoride, cerium fluoride, ytterbium fluoride, iron fluoride, One or more of manganese fluoride, titanium fluoride, vanadium fluoride, cobalt fluoride, nickel fluoride, molybdenum fluoride, germanium fluoride and aluminum fluoride.

According to the preparation method provided by the present invention, wherein the binder is selected from polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyethylene oxide, polyvinyl alcohol , one or more of polytetrafluoroethylene and polyamide; preferably, the solvent is selected from dimethyl sulfoxide, N-methylpyrrolidone, N,N-dimethylformamide, N,N- One or more of dimethylacetamide, diethyl ether, acetonitrile, cyclohexane, dichloromethane, acetone, ethanol and methanol.

In the preparation method of the present invention, the content of the active material particles and the binder in the coating slurry is not particularly limited. In some embodiments, the mass fraction of active material particles in the coating slurry is 1-50 wt.%, preferably 10-20 wt.%. In some embodiments, the mass fraction of the binder in the coating slurry is 0.1-20 wt.%, preferably 1-10 wt.%.

In a preferred embodiment of the present invention, the coating slurry is uniformly mixed by high-energy ball milling, and the ball milling speed is 300-800 rpm, preferably 400-600 rpm; the ball milling time is 3-10 hours, preferably 4- 8 hours.

According to the preparation method provided by the present invention, the drying temperature may be 10-80°C, preferably 30-60°C.

In yet another aspect, the present invention also provides a lithium metal secondary battery, the lithium metal secondary battery includes a positive electrode, a lithium negative electrode, an electrolyte, and a separator, wherein the separator is the above-mentioned composite separator, and the composite separator faces toward The surface of the lithium negative electrode is coated with a coating.

According to the lithium metal secondary battery provided by the present invention, the positive electrode may be a conventional positive electrode used for lithium metal secondary batteries, for example, may include a positive electrode active material, a conductive additive, a binder, and the like.

The present invention has no particular limitation on the positive electrode active material. Generally, any positive electrode active material used in lithium metal secondary batteries can be used, such as nickel cobalt lithium manganate, lithium cobalt oxide, lithium manganate, lithium nickel manganate, nickel cobalt aluminum Lithium oxide, sulfur-Ketjen black composite material, sulfur-Super P composite material, sulfur-carbon nanotube composite material, sulfur-acidified carbon nanotube composite material, selenium-Ketjen black composite material, selenium-Super P composite material, One or more of selenium-carbon nanotube composite materials and selenium-acidified carbon nanotube composite materials.

The conductive additive used for the positive electrode can be one or more of Super P, Ketjen black, graphene, and conductive carbon nanotubes; the binder can be polyvinylidene fluoride, carboxymethyl cellulose One or more of sodium, styrene-butadiene rubber/sodium carboxymethyl cellulose, and sodium alginate.

In some embodiments of the present invention, the mass of the positive active material in the positive electrode accounts for 70% to 90% of the total mass of the positive electrode, the conductive additive accounts for 5% to 20% of the total mass of the positive electrode, and the binder accounts for the total mass of the positive electrode. 5% to 20%.

According to the lithium metal secondary battery provided by the present invention, the negative electrode is a metal lithium negative electrode.

According to the lithium metal secondary battery provided by the present invention, the electrolyte includes a solvent and a lithium salt. Wherein, the solvent can be selected from 1,3 dioxolane, 1,4 dioxane, tetrahydrofuran, trimethylolpropane triglycidyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether At least one of ether, methyl carbonate, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the lithium salt can be lithium hexafluorophosphate, lithium dioxalate borate, trifluoromethane At least one of lithium sulfonate, lithium bis(trifluoromethanesulfonate)imide, and lithium perchlorate. Preferably, the concentration of the lithium salt is 0.5-3M.

In yet another aspect, the present invention also provides an energy storage device comprising the above-mentioned lithium metal secondary battery.

Compared with the prior art, the lithium metal secondary battery functional composite separator provided by the present invention has the following advantages:

1. The conventional complex procedures for modification of the lithium metal anode or the lithium metal interface and harsh environmental conditions are avoided.

2. The coating is applied to the separator, which is stored and used as a conventional battery component. After the obtained functional composite separator is assembled as a lithium metal secondary battery separator, the coating can be transferred to the lithium metal surface, which simplifies the process, saves the cost, and has a high commercialization prospect.

3. The in-situ conversion reaction occurs between the coating and lithium metal during the electrochemical cycle to generate electronic conductors and fast ion conductors. The electronic conductor can distribute electrons evenly, and at the same time, the fast ion conductor part can quickly conduct lithium ions, alleviate the concentration gradient, promote the uniform deposition of lithium, effectively improve the dendrite problem of the metal negative electrode, and improve the safety and stability of the battery.

Description of drawings

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein:

Fig. 1 (a) is the SEM image of the coating surface of the composite diaphragm prepared in Example 1 of the present invention;

Figure 1 (b) is a cross-sectional SEM image of the composite diaphragm prepared in Example 1 of the present invention;

Figure 2(a) is a diagram of the Coulombic efficiency and cycle performance of the battery prepared in Example 1 of the present invention at a rate of 0.5C;

Figure 2(b) is the first cycle charge-discharge curve of the battery prepared in Example 1 of the present invention at a rate of 0.5C;

Figure 3(a) shows the Coulomb efficiency and cycle performance of the battery prepared in Comparative Example 1 at a rate of 0.5C;

Figure 3(b) is the first cycle charge-discharge curve of the battery prepared in Comparative Example 1 at a rate of 0.5C.

FIG. 4 is a schematic diagram of the composite separator for a lithium metal secondary battery of the present invention.

Detailed ways

The present invention will be further described in detail below with reference to the specific embodiments, and the given examples are only for illustrating the present invention, rather than for limiting the scope of the present invention.

The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources.

Example 1

This example is used to illustrate the preparation of the composite separator of the present invention and its application in nickel cobalt lithium manganate (NCM622) batteries

(1) Mix polyvinylidene fluoride (mass fraction 2 wt.%), magnesium nitride particles (particle size is about 200 nm, mass fraction 20 wt.%) and N-methylpyrrolidone, and perform high-energy ball milling, rotating speed 450 rpm, ball milling time 4 Hour. After the mixing was completed, the solution was scraped on the surface of a commercial polyethylene membrane with a spatula with a gap of 1 μm. The separator was placed at 50° C. for drying treatment to obtain a composite separator coated with magnesium nitride on its surface.

Figure 1(a) is a scanning image of the topography of one side coated with magnesium nitride, and Figure 1(b) is a scanning image of the cross-section of the composite diaphragm. It can be seen from Figure a that the magnesium nitride particles are uniformly coated on the surface of the separator, and the cross-sectional view b shows that the magnesium nitride particles are thin and uniform, and are closely decorated on the base layer of the separator.

(2) Under high-purity argon, NCM622 was used as positive active material (200 mg), Super P was used as conductive additive (25 mg), and PVDF was used as binder (25 mg), and the positive electrode sheet was prepared at a mass ratio of 8:1:1. After the above-mentioned pole pieces are obtained, a composite separator, an electrolyte (1M LiPF6 dissolved in EC:DEC:DMC=1:1:1) and a lithium negative electrode are stacked in sequence.

(3) Completely seal the battery case and let it stand for 6 hours to test the battery performance.

Battery performance test

The electrochemical performance of the battery is tested in a battery test system. The test temperature is 25°C, and the test voltage range is 0.5C and 2.8-4.3V voltage conditions for the cycle test, where 1C=200mA hg ^-1 .

Figure 2 shows the coulombic efficiency and cycle performance of the NCM622 battery prepared in Example 1 at a rate of 0.5C (Figure (2a)) and the first cycle charge-discharge curve at a rate of 0.5C (Figure (2b)). It can be seen that the NCM622 battery using the composite separator is stable in cycling, with an initial discharge capacity of 170.1mAh/g. After 100 cycles, the capacity is still 159.1mAh/g, and the retention rate is 93.5%, indicating that the use of the composite separator improves the battery’s performance. Cycling stability and capacity retention.

Example 2

The composite separator and battery were prepared in the same manner as in Example 1, except that the particle size of the coating active material used in step (1) was about 50 nm.

Example 3

The composite separator and battery were prepared in the same manner as in Example 1, except that the coating active material used in step (1) had a particle size of about 1 μm.

Example 4

The composite separator and battery were prepared in the same manner as in Example 1, except that the particle size of the coating active material used in step (1) was about 2 μm.

Example 5

The composite separator and battery were prepared in the same manner as in Example 1, except that the particle size of the coating active material used in step (1) was about 500 nm.

It can be seen from the comparison of Examples 1-5 that the particle size of the coating active material has an influence on the battery performance, and 200-500 nm is preferred.

Example 6

The composite separator and battery were prepared in the same manner as in Example 1, except that the coating blade gap used in step (1) was about 5 μm.

Example 7

The composite separator and battery were prepared in the same manner as in Example 1, except that the coating blade gap used in step (1) was about 0.5 μm.

Example 8

The composite separator and battery were prepared in the same manner as in Example 1, except that the coating blade gap used in step (1) was about 8 μm.

The comparison of Examples 6-8 shows that the thickness of the coating also has a certain influence on the battery performance, and the thickness is preferably 0.5-5 μm, and 0.5-5 μm is more preferred.

Example 9

The composite separator and battery were prepared in the same manner as in Example 1, except that the active material in step (1) was copper nitride.

Example 10

The composite separator and battery were prepared in the same manner as in Example 1, except that the active material in step (1) was magnesium fluoride.

It can be seen from the comparison of Examples 1, 9 and 10 that various nitrides and fluorides can achieve the effect of the comparison number.

Example 11

The composite separator and battery were prepared in the same manner as in Example 1, except that the mass fraction of the active material in the slurry in step (1) was 5%.

Example 12

The composite separator and battery were prepared in the same manner as in Example 1, except that the mass fraction of the active material in the slurry in step (1) was 15%.

Example 13

The composite separator and battery were prepared in the same manner as in Example 1, except that the mass fraction of the active material in the slurry in step (1) was 25%.

The comparison of Examples 11, 12, 13 and Example 1 shows that the mass fraction of the active material in the slurry has a certain influence on the battery performance, but the influence is small.

Example 14

The composite separator and battery were prepared in the same manner as in Example 7, except that the slurry solvent in step (1) was acetone.

Example 15

The composite separator and battery were prepared in the same manner as in Example 7, except that the slurry solvent in step (1) was dimethyl sulfoxide.

Example 16

The composite separator and battery were prepared in the same manner as in Example 7, except that the base film used in step (1) was a polypropylene separator.

Example 17

The composite separator and battery were prepared in the same manner as in Example 1, except that the positive electrode material used in step (2) was NCM811.

Comparative Example 1

A battery was prepared in the same manner as in step (2) of Example 1, except that a commercial polyethylene separator was used.

Figure 3(a) and Figure 3(b) are the cycle performance and the corresponding first cycle charge-discharge curves, respectively. It can be seen that the initial discharge capacity is normal, which is 168.1mAh/g. After 100 cycles, the discharge capacity is only 147.1mAh/g, and the capacity retention rate is 87.5%, which is much lower than that of the composite diaphragm battery. It is contradicted that the composite separator has a remarkable effect on regulating the uniform deposition of lithium ions and stabilizing the lithium anode interface.

The lithium battery obtained by the embodiment and the comparative example is carried out to the electrochemical performance test, and the results are shown in Table 1 below:

The present invention creatively transfers the modification and protection of lithium metal to the modification of the separator. During the assembly process, the protective layer is directly transferred to the surface of the lithium metal negative electrode, and during the subsequent electrochemical cycle, the in situ occurs. The conversion reaction forms a mixed conductor layer, and at the same time, the electron ions are regulated to make them evenly distributed and guide the uniform deposition of lithium. In turn, the cycling stability and kinetic performance of the battery are improved. The cycle specific capacity of the full battery matched with the NCM622 cathode at 0.5C is significantly higher than that of the unmodified ordinary separator, and the capacity decay is significantly suppressed. The charge-discharge plateau curve shows that the polarization is significantly reduced after modification. This shows that the formed mixed conductor layer effectively promotes the interfacial ion transport and guides uniform lithium deposition, thereby suppressing the side reactions, reducing polarization, stabilizing the interface structure, and improving the coulombic efficiency, cycle stability and stability of the battery. rate performance. The method has the advantages of simple operation, good uniformity and remarkable effect, and is suitable for large-scale production.

The above specific embodiments are only schematic illustrations of the content of the present invention, and do not represent the limitation of the content of the present invention. Those skilled in the art can modify the specific structure in the application according to the main idea of the present invention and the actual research system. The protection scope of the present invention shall be subject to the protection scope required by the claims.

Claims (4)

1. A method of preparing a composite separator for a lithium metal secondary battery, the composite separator comprising a separator base layer and a functional coating layer coated on at least one surface of the separator base layer, wherein the functional coating layer comprises active material particles capable of reacting with lithium metal, and the functional coating layer has a thickness of 0.5 to 1 μm, the method comprising the steps of:

(1) dispersing active substance particles and a binder in an organic solvent to obtain coating slurry, wherein the active substance particles comprise magnesium nitride or copper nitride, the particle size is 200-500 nm, and the organic solvent is N-methylpyrrolidone;

(2) and (2) coating the coating slurry prepared in the step (1) on at least one surface of the diaphragm base layer, and drying to obtain the composite diaphragm.

2. The production method according to claim 1, wherein the binder is one or more selected from polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyethylene oxide, polyvinyl alcohol, polytetrafluoroethylene, and polyamide.

3. A lithium metal secondary battery comprising a positive electrode, a lithium negative electrode, an electrolyte and a separator, wherein the separator is the composite separator obtained by the preparation method according to claim 1, and the surface of the composite separator facing the lithium negative electrode is coated with a functional coating.

4. An energy storage device comprising the lithium metal secondary battery of claim 3.

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