Patent Description:
A separator is one of the key internal components for a lithium battery. The separator is located between a positive electrode and a negative electrode. A major role of the separator is to separate active materials of the positive and negative electrodes to avoid a short circuit caused by contact between the two electrodes, and to allow the rapid transportation of charged ions. In order to improve the heat resistance, the separator is generally coated with a slurry, which not only improves the heat resistance of the separator, but also helps protect positive and negative electrodes of a lithium battery. However, the slurry in the related prior art exhibits poor compatibility with the separator, and thus the slurry is easy to fall off from the separator to weaken the heat resistance of the separator, thereby affecting a service life of the lithium battery.

At present, commercial lithium battery separators are mainly polyolefin (PO) separators based on polyethylene (PE) and polypropylene (PP), including single-layer PP separators, single-layer PE separators, and multi-layer PP/PE/PP composite separators. PO materials have advantages such as high strength, excellent acid and alkali resistance, and excellent solvent resistance, but have a low melting point of <NUM> or lower. When the temperature inside or outside a lithium battery arises and is close to a melting point of the separator, the separator will undergo deformation retraction or melt fracture to cause a short circuit in positive and negative electrodes of the lithium battery, which will accelerate the thermal runaway of the lithium battery. In addition, when a lithium battery is discharged at a high power, the rise of a temperature inside the lithium battery is correlated with impedance, which requires a separator in the lithium battery to have excellent impedance performance.

Currently, in order to improve the thermal performance of a PO separator, the PO separator is often modified through ceramic coating. However, ceramic particles are inorganic particles, and thus a binder is required to improve the adhesion among ceramic particles and the adhesion between ceramic particles and a PO film, thereby avoiding the phenomenon of powder dropping. For example, Chinese patent application No. <CIT> discloses a ceramic slurry, a ceramic separator, and a lithium-ion battery (LIB). The ceramic slurry includes: a ceramic powder; a binder including a polar binder and a non-polar binder; a dispersing agent; and a surfactant. The binder in the ceramic slurry includes too many polar functional groups. Although the polar functional groups can improve the adhesion of the ceramic slurry, the polar functional groups are very easy to cause thermochemical reactions among components in LIB under heat, which leads to the thermal runaway of the LIB and even causes an explosion, indicating great potential safety hazards. A separator coated with a ceramic alone exhibits excellent thermal shrinkage performance, but still has a low breakage temperature.

Currently, aramid-coated LIB separators are mostly prepared by a compounding method. The compounding method is as follows: an aramid solution is coated on a surface of a PO separator, and then a solvent in the aramid solution is removed to obtain an aramid/PO separator. However, a composite film prepared by the compounding method has shortcomings such as large thickness and uneven pore size distribution. Moreover, the compounding method has harsh requirements for a molecular weight of aramid, and aramid needs to be dissolved by a suitable solvent due to its poor solubility, which increases a cost to some extent. For example, Chinese patent applications No. <CIT> and No. <CIT> disclose a process of coating an LIB separator with modified para-aramid, where another monomer is introduced into the traditional para-aramid to improve the comprehensive physical properties of a composite coated film. However, this process is complicated, and a separator prepared by this process has a large thickness, a low porosity, and poor heat resistance, which leads to performance degradation of a battery. Chinese patent application No. <CIT> and Chinese patent No. <CIT> disclose a furyl-containing bio-based aramid polymer, and a use of the furyl-containing bio-based aramid polymer in an LIB separator. The use includes: the furyl-containing bio-based aramid polymer is first dissolved, and then a variety of additives are added to obtain a coating solution; and the coating solution is coated on a film to improve physical properties of the film. However, this process is cumbersome and involves many raw materials, and a coated film prepared by this process does not have a three-dimensional (3D) network structure and exhibits a low electrolyte migration rate.

Chinese patent application No. <CIT> discloses a dissolution system for para-aramid and a use thereof, an aramid-coated separator and a preparation method thereof, and an aramid/ceramic mixture-coated separator and a preparation method thereof. The preparation method of the aramid/ceramic mixture-coated separator is that: a ceramic slurry is mixed with a para-aramid slurry prepared by the dissolution system to obtain an aramid/ceramic mixed slurry; and then the aramid/ceramic mixed slurry is coated on at least one surface of a film. However, when aramid and inorganic ceramic particles are directly coated in combination, there will be poor adhesion between a coating and a substrate; and if a conventional binder is added, the gas permeability and thermal performance of a separator will be affected, which will cause safety problems.

Based on the above-mentioned problems in the prior art, how to improve the adhesion between aramid and ceramic particles without affecting the performance of a separator has become an urgent technical problem to be solved.

In order to solve the above-mentioned technical problems, the present disclosure provides a composite slurry. The composite slurry is prepared as follows: in-situ compounding a nanoceramic particle with an isocyanate through a modification reaction during which hydroxyl on a surface of the nanoceramic particle reacts with an isocyanate group in the isocyanate to obtain a modified nanoceramic particle; and in-situ compounding the modified nanoceramic particle with aramid through a grafting reaction during which an isocyanate group in the modified nanoceramic particle is grafted with an amido bond in the aramid to obtain the composite slurry. The composite slurry has a viscosity of <NUM> mPa·s (cp) to <NUM>,<NUM> mPa·s (cp), the isocyanate includes at least <NUM> isocyanate groups, and the viscosity is tested by a rotational viscometer.

In one embodiment, the nanoceramic particle is one selected from the group consisting of alumina, silicon oxide, and titanium oxide.

In one embodiment, the aramid is one selected from the group consisting of meta-aramid, para-aramid, heterocyclic aramid, and polyamide-imide (PAI).

In one embodiment, the isocyanate has a structural formula of O=C=N-R<NUM>-N=C=O, where R<NUM> has a structural formula of
<CHM>
<CHM>
when R<NUM> is
<CHM>
the isocyanate is <NUM>,<NUM>'-diphenylmethane diisocyanate (MDI); when R<NUM> is
<CHM>
the isocyanate is methyl-<NUM>,<NUM>-diisocyanate (<NUM>,<NUM>-TDI); and when R<NUM> is
<CHM>
the isocyanate is methyl-<NUM>,<NUM>-diisocyanate (<NUM>,<NUM>-TDI).

The present disclosure also provides a preparation method of the composite slurry, including the following steps:.

In one embodiment, a concentration of the isocyanate in the isocyanate-containing toluene solution in the step (<NUM>) is <NUM> mol/L to <NUM> mol/L, and a mass of the nanoceramic particle is <NUM>% to <NUM>% of a mass of the isocyanate-containing toluene solution.

In one embodiment, the stirring in the step (<NUM>) is conducted at <NUM> to <NUM> for <NUM> to <NUM>.

In one embodiment, the inert gas in the step (<NUM>) is nitrogen, the heating is conducted to <NUM> to <NUM>, and the reaction is conducted for <NUM> to <NUM>.

In one embodiment, the organic solvent in the step (<NUM>) is acetone, and the crude modified nanoceramic particle product is washed <NUM> to <NUM> times with the acetone.

In one embodiment, the washed product in the step (<NUM>) is dried at <NUM> to <NUM> for <NUM> to <NUM>.

In one embodiment, the in-situ compounding of the nanoceramic particle with the isocyanate in the step (<NUM>) involves the following reaction principle: An oxygen atom on the surface of the nanoceramic particle adsorbs water in air to produce hydroxyl (-OH) on the surface. Then the hydroxyl reacts with the isocyanate group (-N=C=O) in the isocyanate at the fixed temperature in the step (<NUM>) to produce a polymer A, namely, the modified nanoceramic particle. The surface of the nanoceramic particle has one or more hydroxyl groups (-OH) and the isocyanate includes at least <NUM> isocyanate groups (-N=C=O).

In one embodiment, the aramid-containing polymerization solution in the step (<NUM>) includes the aramid, and a mass ratio of the aramid to the modified nanoceramic particle is (<NUM>-<NUM>):(<NUM>-<NUM>).

In one embodiment, a mass percentage of the aramid in the aramid-containing polymerization solution is <NUM>% to <NUM>%.

In one embodiment, the aramid in the aramid-containing polymerization solution is in a liquid state.

In one embodiment, the aramid-containing polymerization solution is adopted due to the following reason: Solid aramid has low solubility, and different solid aramid products need to be dissolved by different solvents under different dissolution conditions to obtain liquid aramid. In the step (<NUM>), the aramid-containing polymerization solution produced in preparation of aramid is directly used, that is, aramid in a liquid state is directly used before becoming a solid, which can effectively avoid the above problem.

In one embodiment, in the step (<NUM>), the isocyanate group (-N=C=O) in the polymer A and the amido bond (-CO-NH-) in the aramid undergo a grafting reaction when the polymer A and the aramid are mixed, such that the aramid is in-situ compounded on a surface of the polymer A to produce a polymer B and thus the composite slurry is obtained. In this way, the nanoceramic particle and the aramid can be compounded without a binder, and the nanoceramic particle will not fall off.

In one embodiment, the grafting reaction in the step (<NUM>) is conducted at <NUM> to <NUM> for <NUM> to <NUM>.

In one embodiment, a preparation method of a lithium battery separator includes the following steps:.

In one embodiment, the composite slurry, the pore-forming agent, and the polar solvent in the step (<NUM>) are in a mass ratio of (<NUM>-<NUM>):(<NUM>-<NUM>):(<NUM>-<NUM>).

In one embodiment, the pore-forming agent is one or more selected from the group consisting of polyethylene glycol (PEG), tripropylene glycol (TPG), polyvinylpyrrolidone (PVP), anhydrous calcium chloride, anhydrous lithium chloride, methanol, ethanol, and propanol; and the pore-forming agent plays a role of regulating a porosity of the lithium battery separator, and can increase the porosity of the lithium battery separator from <NUM>% to <NUM>% in the prior art to <NUM>% to <NUM>%.

In one embodiment, the polar solvent is one selected from a group consisting of N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N-methylformamide (NMF), and N-ethylpyrrolidone (NEP).

In one embodiment, the coating layer in the step (<NUM>) has a thickness of <NUM> to <NUM>.

In one embodiment, the substrate layer in the step (<NUM>) has a thickness of <NUM> to <NUM>.

In one embodiment, a material of the substrate layer is one selected from a group consisting of dry PE, dry PP, a three-layer PP/PE/PP composite separator, wet PE, and a non-woven fabric.

In one embodiment, a manner for the coating in the step (<NUM>) is one selected from a group consisting of slit-extrusion, roller-coating, scraper-coating, and wire rod-coating, and the coating is conducted at room temperature.

In one embodiment, the degassing in the step (<NUM>) is conducted for <NUM> to <NUM>, and the degassing refers to static degassing or vacuum degassing, where the vacuum degassing is conducted with a vacuum degree of <NUM> mmHg to <NUM> mmHg; a curing agent in the curing tank includes an organic solvent and water, where a volume fraction of the organic solvent in the curing agent is <NUM>% to <NUM>%; the curing is conducted for <NUM> to <NUM> at <NUM> to <NUM>; the water-washing is conducted for <NUM> to <NUM> at <NUM> to <NUM>; the drying is conducted for <NUM> to <NUM> at <NUM> to <NUM>; and the heat-setting is conducted for <NUM> to <NUM> at <NUM> to <NUM>.

In one embodiment, the organic solvent is one selected from the group consisting of DMAc, NMP, DMF, NMF, and NEP.

The present disclosure also provides a lithium battery separator obtained by the method described above.

In one embodiment, a room-temperature capacity of the battery is <NUM> Ah to <NUM> Ah.

<FIG> is a scanning electron microscopy (SEM) image of a surface of the lithium battery separator in Example <NUM> of the present disclosure.

The following examples <NUM>-<NUM> and comparative examples <NUM> and <NUM> are not according to the invention and are present for illustration purposes only.

Among the following examples, Examples <NUM> to <NUM> are examples of preparation of a composite slurry. Examples <NUM> to <NUM> are examples of preparation of lithium battery separators with the isocyanate-modified nanoceramic particle/aramid in-situ composite slurries prepared in Examples <NUM> to <NUM>. Examples <NUM> to <NUM> are examples of preparation of batteries with the lithium battery separators prepared in Examples <NUM> to <NUM>. Comparative Examples <NUM> and <NUM> are comparative examples of batteries prepared with the lithium battery separators in the prior art.

In this example, a preparation method of a composite slurry was provided, including the following steps:.

This step involves the following reaction principle: An oxygen atom on a surface of the nanoceramic particle adsorbs water in air to produce hydroxyl (-OH) on the surface. Then the hydroxyl reacts with an isocyanate group (-N=C=O) in the isocyanate at the fixed temperature in the step (<NUM>) to produce a polymer A, namely, the modified nanoceramic particle. The surface of the nanoceramic particle has one or more hydroxyl groups (-OH) and the isocyanate includes <NUM> isocyanate groups (-N=C=O).

(<NUM>) The modified nanoceramic particle obtained in the step (<NUM>) was mixed with an aramid-containing polymerization solution, and a grafting reaction was conducted at <NUM> for <NUM> to obtain the composite slurry. The slurry has a viscosity of <NUM> mPa·s (cp), and a mass ratio of the aramid to the modified nanoceramic particle was <NUM>:<NUM>.

This step involves the following reaction principle: An isocyanate group (-N=C=O) in the polymer A and an amido bond (-CO-NH-) in aramid undergo a grafting reaction when the polymer A and the aramid are mixed in the step (<NUM>), such that the aramid is in-situ compounded on a surface of the polymer A to produce a polymer B and thus the composite slurry (namely, sample <NUM>) is obtained. In this way, the nanoceramic particle and the aramid can be compounded without a binder, and the nanoceramic particle will not fall off.

In this example, the aramid was meta-aramid, and a preparation process of the aramid-containing polymerization solution was as follows:
<NUM> of m-phenylenediamine (MPD) was fully dissolved in <NUM> of DMAc to obtain a mixed solution, and with the mixed solution kept at room temperature, <NUM> of isophthaloyl chloride (IPC) was added to allow a reaction for <NUM>. A pH of a resulting reaction system was adjusted with calcium hydroxide to <NUM> to obtain a meta-aramid slurry, which was the aramid-containing polymerization solution used in this example, where a mass percentage of meta-aramid in the aramid-containing polymerization solution was <NUM>%.

The aramid-containing polymerization solution is adopted due to the following reason: Solid aramid has low solubility, and different solid aramid products need to be dissolved by different solvents under different dissolution conditions to obtain liquid aramid. In the step (<NUM>), the aramid-containing polymerization solution produced in preparation of aramid is directly used, that is, aramid in a liquid state is directly used before becoming a solid, which can effectively avoid the above problem.

The nanoceramic particle was silicon oxide.

(<NUM>) The modified nanoceramic particle obtained in the step (<NUM>) was mixed with an aramid-containing polymerization solution, and a grafting reaction was conducted at <NUM> for <NUM> to obtain the composite slurry. The slurry has a viscosity of <NUM>,<NUM> mPa·s (cp), and a mass ratio of the aramid to the modified nanoceramic particle was <NUM>:<NUM>.

In this example, the aramid was para-aramid, and a preparation process of the aramid-containing polymerization solution was as follows:
<NUM> of p-phenylenediamine (PPD) was fully dissolved in <NUM> of NMP/CaCl<NUM> (a mass fraction of CaCl<NUM> was <NUM>%) to obtain a mixed solution, the mixed solution was cooled to <NUM>, and then <NUM> of terephthaloyl chloride (TPC) was added to allow a reaction for <NUM>. A pH of a resulting reaction system was adjusted with calcium hydroxide to <NUM> to obtain a para-aramid slurry, which was the aramid-containing polymerization solution used in this example, where a mass percentage of para-aramid in the aramid-containing polymerization solution was <NUM>%.

The nanoceramic particle was titanium oxide.

In this example, the aramid was heterocyclic aramid, and a preparation process of the aramid-containing polymerization solution was as follows:
PPD and <NUM>-(<NUM>-aminophenyl)-<NUM>-aminobenzimidazole were fully dissolved in <NUM> of NMP/CaCl<NUM> (a mass fraction of CaCl<NUM> was <NUM>%) to obtain a mixed solution, the mixed solution was cooled to <NUM>, and then <NUM> of TPC was added to allow a reaction for <NUM>. A pH of a resulting reaction system was adjusted with calcium hydroxide to <NUM> to obtain a heterocyclic aramid slurry, which was the aramid-containing polymerization solution used in this example, where a molar ratio of the PPD to the <NUM>-(<NUM>-aminophenyl)-<NUM>-aminobenzimidazole was <NUM>:<NUM>, and a total molar amount of the PPD and the <NUM>-(<NUM>-aminophenyl)-<NUM>-aminobenzimidazole was <NUM> mol; and a mass percentage of heterocyclic aramid in the aramid-containing polymerization solution was <NUM>%.

In this example, the aramid was PAI, and a preparation process of the aramid-containing polymerization solution was as follows:
In a nitrogen environment, trimellitic anhydride (TMA) and toluene diisocyanate (TDI) were added in a molar ratio of <NUM>:<NUM> to a reactor, then DMF was added, and a resulting mixture was stirred to allow complete dissolution; a resulting solution was heated and controlled at <NUM> to <NUM> to allow a reaction for <NUM>, and then a resulting precipitate was fully washed with water and then oven-dried to obtain a PAI solid; and <NUM> of the PAI solid was taken and added to <NUM> of DMF, and a resulting mixture was stirred for dissolution to obtain a PAI solution, which was equivalent to the aramid-containing polymerization solutions in Examples <NUM> to <NUM>, where a molar ratio of the DMF to the TMA was <NUM>:<NUM>.

In this example, after the reaction is completed, the precipitate (namely, the PAI solid) can be directly obtained, and the PAI adopted in this example is soluble aramid; and to allow a grafting reaction with the polymer A, the PAI needs to be dissolved.

Viscosities of the composite slurries in Examples <NUM> to <NUM> were shown in Table <NUM>:.

A viscosity test was conducted by a rotational viscometer.

In this example, the composite slurry prepared in Example <NUM> was used to prepare a lithium battery separator through the following steps:.

The polar solvent and the organic solvent both were DMAc.

A material of the substrate layer was wet PE, and a thickness of the substrate layer was <NUM>.

The pore-forming agent was PEG, and the pore-forming agent played a role of adjusting a porosity of the lithium battery separator. The porosity of the lithium battery separator was <NUM>%.

The lithium battery separator (sample <NUM>) finally obtained in this example exhibited a peeling strength of <NUM>-<NUM>, a gas permeability of <NUM>/<NUM> cc, a horizontal thermal shrinkage of <NUM>% at <NUM> within <NUM>, a longitudinal thermal shrinkage of <NUM>% at <NUM> within <NUM>, a breakage temperature of <NUM>, and an electrolyte contact angle of <NUM>°.

The polar solvent and the organic solvent both were NMP.

A material of the substrate layer was dry PP, and a thickness of the substrate layer was <NUM>.

The pore-forming agent was TPG, and the pore-forming agent played a role of adjusting a porosity of the lithium battery separator. The porosity of the lithium battery separator was <NUM>%.

In this example, the composite slurry prepared in Example <NUM> was used to prepare a lithium battery separator, and an SEM image of the lithium battery separator was shown in <FIG>; and the preparation of the lithium battery separator included the following steps:.

The polar solvent and the organic solvent both were DMF.

A material of the substrate layer was a three-layer PP/PE/PP composite separator, and a thickness of the substrate layer was <NUM>.

The pore-forming agent was PVP, and the pore-forming agent played a role of adjusting a porosity of the lithium battery separator. The porosity of the lithium battery separator was <NUM>%.

The polar solvent and the organic solvent both were NMF.

The pore-forming agent was anhydrous calcium chloride, and the pore-forming agent played a role of adjusting a porosity of the lithium battery separator. The porosity of the lithium battery separator was improved to <NUM>%.

Properties of the lithium battery separators in Examples <NUM> to <NUM> were shown in Table <NUM>:.

Performance tests were conducted according to test specifications in GB/T <NUM>-<NUM>.

In this example, the lithium battery separator prepared in Example <NUM> was used to prepare a battery, that is, a <NUM> Ah <NUM> ternary lithium battery (sample <NUM>) was assembled. Test results of the battery were shown in Table <NUM>.

In this comparative example, a commercially-available <NUM> PE film was adopted as a lithium battery separator. The lithium battery separator exhibited a gas permeability of <NUM>/<NUM> cc, a melting temperature of <NUM>, a breakage temperature of <NUM>, and an electrolyte contact angle of <NUM>°.

The lithium battery separator was used to assemble a <NUM> Ah <NUM> ternary lithium battery (sample <NUM>). Test results of the battery were shown in Table <NUM>.

In this comparative example, a commercially-available <NUM>+<NUM>+<NUM> PE/ceramic-coated film was adopted as a lithium battery separator. The lithium battery separator exhibited a peeling strength of <NUM>-<NUM>, a gas permeability of <NUM>/<NUM> cc, a horizontal thermal shrinkage of <NUM>% at <NUM> within <NUM>, a longitudinal thermal shrinkage of <NUM>% at <NUM> within <NUM>, a breakage temperature of <NUM>, and an electrolyte contact angle of <NUM>°.

In summary, compared with the commercially-available lithium battery separators in Comparative Examples <NUM> and <NUM>, the lithium battery separators prepared with the composite slurries prepared in Examples <NUM> to <NUM> exhibit significantly-improved properties. The lithium battery separator prepared with the composite slurry in the present disclosure has a peeling strength of higher than or equal to <NUM>-<NUM>, a gas permeability of less than or equal to <NUM>/<NUM> cc, a horizontal thermal shrinkage of less than or equal to <NUM>% at <NUM> within <NUM>, a longitudinal thermal shrinkage of less than or equal to <NUM>% at <NUM> within <NUM>, a breakage temperature of higher than or equal to <NUM>, and an electrolyte contact angle of smaller than or equal to <NUM>°.

Compared with the batteries prepared with the lithium battery separators in the comparative examples of the present disclosure, the batteries prepared with the lithium battery separators in the examples of the present disclosure exhibit significantly-improved properties, that is, the battery prepared with the lithium battery separator including the composite slurry exhibits a capacity retention rate after <NUM> cycles increased from (<NUM>-<NUM>)% to (<NUM>-<NUM>)%, a room-temperature capacity increased from (<NUM>-<NUM>) Ah to (<NUM>-<NUM>) Ah, a high-temperature capacity (<NUM>) increased from (<NUM>-<NUM>) Ah to (<NUM>-<NUM>) Ah, and a low-temperature capacity (-<NUM>) increased from (<NUM>-<NUM>) Ah to (<NUM>-<NUM>) Ah.

Claim 1:
A composite slurry, wherein the composite slurry is prepared as follows:
in-situ compounding a nanoceramic particle with an isocyanate through a modification reaction during which hydroxyl on a surface of the nanoceramic particle reacts with an isocyanate group in the isocyanate to obtain a modified nanoceramic particle; and
in-situ compounding the modified nanoceramic particle with aramid through a grafting reaction during which an isocyanate group in the modified nanoceramic particle is grafted with an amido bond in the aramid to obtain the isocyanate-modified nanoceramic particle/aramid in-situ composite slurry, wherein the isocyanate-modified nanoceramic particle/aramid in-situ composite slurry has a viscosity of <NUM> mPa·s (cp) to <NUM>,<NUM> mPa·s (cp), the isocyanate comprises at least <NUM> isocyanate groups, and the viscosity is determined as described in the description.