CN112825354B - Lithium negative electrode, preparation method thereof and lithium secondary battery - Google Patents
Lithium negative electrode, preparation method thereof and lithium secondary battery Download PDFInfo
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Abstract
The invention relates to a lithium cathode, a preparation method thereof and a lithium secondary battery, and belongs to the technical field of metal lithium cathodes. The invention provides a lithium cathode which sequentially comprises a current collector, a mixed electric conduction layer I, a metal layer and a mixed electric conduction layer II from bottom to top; the metal layer is a metal lithium layer or a metal lithium alloy layer; the mixed conducting layer I and the mixed conducting layer II are mixed conducting material layers containing metal lithium; the weight ratio of the metal lithium in the mixed conductive layer I to the mixed conductive material is 1: 0.05-1, wherein the weight ratio of the metal lithium in the mixed electric conduction layer II to the mixed electric conduction material is 1:0.1 to 10. The mixed conducting layer II contains a mixed conducting material and metallic lithium, and the mixed conducting material provides uniform deposition sites for the metallic lithium, so that the formation and the growth of lithium dendrites are inhibited; the mixed electric conduction layer I and the current collector have good cohesiveness, and meanwhile, the mixed electric conduction layer I and the mixed electric conduction layer II are favorable for relieving the volume effect of the metal lithium in the electrochemical reaction process.
Description
Technical Field
The invention relates to a lithium cathode, a preparation method thereof and a lithium secondary battery, and belongs to the technical field of metal lithium cathodes.
Background
The battery as an energy storage device not only brings the development of the portable electronic industry, but also brings the rapid rise of new energy automobiles, and has become an indispensable core component for the development of the modern society. Lithium ion batteries account for most of the portable electronic product batteries and power batteries due to their advantages of high energy density, high power density, long life, no memory effect, etc. With the progress of electronic equipment and the popularization of new energy automobiles, higher requirements are put forward on the energy density of lithium ion batteries. Metallic lithium is an excellent choice for high energy density lithium batteries due to its super-negative electrode potential (-3.04V relative to standard hydrogen electrode) and extremely high specific capacity (3860 mAh/g), but the volume expansion and lithium dendrite problems of metallic lithium negative electrodes during electrochemical reaction limit the commercial application of metallic lithium negative electrodes.
Lithium dendrites are dendritic lithium deposits that occur in the negative electrode during multiple depositions/precipitations of the lithium ion negative electrode. The lithium dendrite may pierce through the diaphragm when it grows to a certain degree, causing problems of battery short circuit and safety, and the lithium dendrite will increase the side reaction of the electrolyte and the lithium metal, consuming the lithium active substance, and reducing the battery utilization rate. The lithium dendrites that separate from the current collector are dead lithium, which reduces the available active material, decreases the efficiency and cycle life of the battery, and causes a decrease in the capacity of the negative electrode.
The volume expansion and shrinkage of the metallic lithium negative electrode in the charging and discharging process can cause the SEI film to break and grow repeatedly, which can lead to the irreversible consumption of the lithium negative electrode, the behavior of the lithium negative electrode is represented by low coulombic efficiency, and in addition, after the broken and failed non-electronic conductive SEI film is embedded into the metallic lithium bulk phase, the pulverization of lithium can be caused due to the physical isolation effect of the broken and failed non-electronic conductive SEI film, and the formation of 'dead lithium' is accelerated.
Disclosure of Invention
The first purpose of the present invention is to provide a lithium negative electrode which is advantageous in alleviating the volume expansion of the lithium negative electrode and suppressing the formation of lithium dendrites.
A second object of the present invention is to provide a method of preparing a lithium negative electrode.
A third object of the present invention is to provide a lithium secondary battery.
The technical scheme of the invention is as follows:
a lithium negative electrode comprises a current collector, a mixed conductive layer I, a metal layer and a mixed conductive layer II from bottom to top in sequence; the metal layer is a metal lithium layer or a metal lithium alloy layer; the mixed conducting layer I and the mixed conducting layer II are mixed conducting material layers containing metal lithium; the weight ratio of the metal lithium in the mixed conductive layer I to the mixed conductive material is 1: 0.05-1, wherein the weight ratio of the metal lithium in the mixed conductive layer II to the mixed conductive material is 1:0.1 to 10.
The lithium negative electrode of the present invention has a uniform content of metallic lithium in the horizontal direction. In the vertical direction of the lithium cathode, the content of the metal lithium in the metal layer is highest, the content of the metal lithium in the mixed conducting layer I and the mixed conducting layer II can be uniform or gradient distributed, and when the mixed conducting layer I and the mixed conducting layer II are gradient distributed, the content of the metal lithium in one side close to the metal layer is higher.
Conductive carbon black is also known as Super P. The carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes.
The mixed conducting materials in the mixed conducting layer I and the mixed conducting layer II can be the same or different.
The lithium cathode consists of a current collector, a mixed conductive layer I, a metal layer and a mixed conductive layer II, wherein the mixed conductive layer II contains a mixed conductive material and metal lithium, and the mixed conductive material provides uniform deposition sites for the metal lithium, so that the formation and growth of lithium dendrites are inhibited; the mixed electric conduction layer I and the current collector have good cohesiveness, and meanwhile, the mixed electric conduction layer I and the mixed electric conduction layer II are favorable for relieving the volume effect of the metal lithium in the electrochemical reaction process.
Preferably, the weight ratio of the metallic lithium to the mixed conducting material in the mixed conducting layer I is 1:0.2 to 1; the weight ratio of the metal lithium in the mixed conductive layer II to the mixed conductive material is 1:1 to 10.
In order to enable the thickness of the lithium negative electrode to be small and enable the mixed conductive layer II to effectively inhibit the formation and growth of lithium dendrites, the mixed conductive layer I has good cohesiveness with a current collector, and meanwhile, the volume expansion of the lithium negative electrode can be effectively relieved, preferably, the thickness ratio of the metal layer, the mixed conductive layer I and the mixed conductive layer II is 0-1: 0.1 to 1:0.1 to 1.
When the thickness ratio of the metal layer is 0, it means that metallic lithium has completely penetrated into the mixed-conductivity layer i and the mixed-conductivity layer ii.
Preferably, the lithium metal in the mixed electric conduction layer I and the mixed electric conduction layer II is infiltrated from a metal layer.
Preferably, the mixed conducting materials used in the mixed conducting layer i and the mixed conducting layer ii are each independently selected from one or more mixed materials of natural graphite, artificial graphite, soft carbon, hard carbon, silicon oxide, silicon carbon, lithium titanate, carbon black, ketjen carbon, acetylene black, conductive carbon black, graphene and carbon nanotubes.
A method of making a lithium anode, comprising the steps of:
(1) Preparation of three-layer composite and two-layer composite
(a) Coating the slurry A containing the mixed conductive material on the surface of a current collector, drying to form a mixed conductive layer I, then placing a metal belt and the mixed conductive layer I adjacent to each other, and laminating to obtain a three-layer composite material which sequentially comprises the current collector, the mixed conductive layer I and a metal layer from bottom to top; the metal belt is a metal lithium belt or a metal lithium alloy belt;
(b) Coating the slurry B containing the mixed conductive material on the surface of a substrate material, and drying to form a mixed conductive layer II to obtain a double-layer composite material which is composed of the substrate material and the mixed conductive layer II from bottom to top in sequence;
(2) And placing the mixed conductive layer II of the double-layer composite material adjacent to the metal layer of the three-layer composite material, and removing the substrate material through pressing to obtain the lithium cathode.
The components of slurry a containing a mixed electrically conductive material and slurry B containing a mixed electrically conductive material may be the same or different.
The paste a containing the mixed electroconductive material and the paste B containing the mixed electroconductive material may be prepared by mixing the mixed electroconductive material, a binder and a solvent.
The current collector can be a copper foil or a copper mesh with a three-dimensional structure. The surface of the copper foil current collector can be a rough surface or a smooth surface. A copper foil having a rough surface, such as a porous copper foil, is preferable as the current collector.
The thickness of the current collector is 4.5-8 μm.
The substrate material is a release film with smaller release force. The release film can be a polymer film, such as a PP film, a PE film or a PET film, and can also be a metal foil, such as an aluminum foil, a copper foil or a steel foil.
The pressing temperature is lower than the decomposition temperature of each component in the mixed electric conduction layer I and the mixed electric conduction layer II.
The metal strip may be a separate lithium metal strip or a separate lithium metal alloy strip.
According to the preparation method of the lithium cathode, the slurry A is coated on the surface of the current collector to form the mixed conductive layer I, the metal belt is pressed and pressed, the mixed conductive layer II arranged on the surface of the substrate material is pressed and pressed, and the lithium cathode can be prepared.
In order to effectively inhibit the formation and growth of lithium dendrites while the thickness of the lithium negative electrode is small, and to make the lithium negative electrode have good cohesiveness while the thickness is small, and to effectively relieve the volume expansion of the lithium metal negative electrode, in step (1), the thickness ratio of the metal layer, the mixed conducting layer i and the mixed conducting layer ii is preferably 0 to 1:0.1 to 1:0.1 to 1.
Preferably, in the step (1), the metal strip has a thickness of 10 to 100 μm. By reasonably adjusting and optimizing the thickness of the metal belt, the lithium cathode can be ensured to have higher lithium content, the cost can be considered, and meanwhile, the thickness of the lithium cathode is in a proper range.
The content of the solvent in the mixed conductive layer containing the slurry A and the slurry B can be controlled to obtain a mixed conductive layer I and a mixed conductive layer II with specific porosity, and the mixed conductive layer further promotes the lithium element to permeate into the mixed conductive layer I and the mixed conductive layer II, preferably, in the step (1), the porosity of the mixed conductive layer I is 30-90%; the porosity of the mixed electric conduction layer II is 10-50%.
Preferably, in the step (1), the slurry a and the slurry B are respectively and independently composed of the following components in percentage by mass: 90-98 wt% of mixed conductive material and 2-10 wt% of binder.
The binder may be an oil-based binder selected from PVDF (polyvinylidene fluoride), PI (polyimide), and the like, or an aqueous binder selected from styrene-butadiene rubber, polyacrylic acids, and the like.
Preferably, in the step (1) and the step (2), the pressing temperature is 25-180 ℃, and the pressing pressure is 1-30 t. Through the temperature and the pressure of reasonable adjustment and optimization pressfitting, the mixed electric conduction layer is more favorable to lithium to be doped to mixed electric conduction layer I and mixed electric conduction layer II.
The pressing time is short, namely 3-10 s.
A lithium secondary battery includes a positive electrode, an electrolyte, and the lithium negative electrode.
The lithium secondary battery comprising the lithium composite metal negative electrode may be a liquid battery or a solid battery.
The lithium secondary battery with the lithium-containing negative electrode has higher energy density and specific capacity, and the lithium negative electrode in the lithium secondary battery can effectively inhibit the formation and growth of lithium dendrites, relieve the volume effect and prolong the service life of the lithium secondary battery.
The positive electrode material of the lithium secondary battery is a lithium-rich phase material, and preferably, the positive electrode is lithium cobaltate, a ternary material, a lithium-rich positive electrode material, lithium manganate or lithium iron phosphate.
Drawings
Fig. 1 is a schematic flow chart of the preparation of a lithium negative electrode of example 1.
In the figure, 11 is artificial graphite, 12 is artificial graphite, 2 is copper foil, 3 is a PET film, 4 is a metallic lithium tape, and 5 is a lithium negative electrode.
Detailed Description
The present invention will be further described with reference to the following embodiments.
1. Specific examples of the method for producing a lithium negative electrode of the present invention are as follows:
example 1
Fig. 1 is a schematic flow chart of a preparation method of the lithium negative electrode in this example, in fig. 1, 11 is artificial graphite, 12 is artificial graphite, 2 is copper foil, 3 is a PET film, 4 is a metal lithium tape, and 5 is a lithium negative electrode. The method specifically comprises the following steps:
(1) Slurry material
97 parts by weight of artificial graphite, 2 parts by weight of styrene butadiene rubber binder and 1 part by weight of carboxymethyl cellulose are uniformly mixed in water, and the solid content is 30 percent, so that slurry containing the artificial graphite is obtained for later use.
(2) Three-layer composite material and two-layer composite material
(a) Coating the slurry containing the artificial graphite prepared in the step (1) on the surface of a copper foil current collector with the thickness of 8 microns, drying for 1h at 100 ℃ to form a mixed conductive layer I with the porosity of 40% and the thickness of 20 microns, then placing a metal lithium belt with the thickness of 50 microns adjacent to the mixed conductive layer I, and pressing at the temperature of 120 ℃ and under the pressure of 15t to obtain the three-layer composite material sequentially comprising the copper foil, the mixed conductive layer I and the metal lithium layer from bottom to top.
(b) Coating the slurry containing the artificial graphite prepared in the step (1) on the surface of a 36-micron PET film, and drying at 100 ℃ for 1h to form a mixed electric conduction layer II with the porosity of 20% and the thickness of 30 microns to obtain the double-layer composite material sequentially comprising the PET film and the mixed electric conduction layer II from bottom to top.
(3) And (3) placing the mixed conductive layer II of the double-layer composite material prepared in the step (b) in the step (2) and the metal lithium layer of the three-layer composite material prepared in the step (a) in the step (2) adjacently, pressing at the temperature of 120 ℃ and under the pressure of 12t, and removing the PET film to obtain the lithium cathode.
Example 2
The preparation method of the lithium negative electrode of the embodiment specifically includes the following steps:
(1) Slurry material
96 parts by weight of carbon black, 2.5 parts by weight of styrene-butadiene rubber binder and 1.5 parts by weight of carboxymethyl cellulose are uniformly mixed in water, and the solid content is 20 percent, so that slurry containing the carbon black is obtained for later use.
Uniformly mixing 96 parts by weight of multi-walled carbon nanotubes, 2.5 parts by weight of styrene butadiene rubber binder and 1.5 parts by weight of carboxymethyl cellulose in water, and obtaining slurry containing the multi-walled carbon nanotubes with the solid content of 30% for later use.
(2) Three-layer composite material and two-layer composite material
(a) Coating the slurry containing the carbon black prepared in the step (1) on the surface of a porous copper foil current collector with the thickness of 8 microns, drying for 1h at 100 ℃ to form a mixed conductive layer I with the porosity of 70% and the thickness of 30 microns, then placing a metal lithium aluminum alloy in a PET (polyethylene terephthalate) belt loaded with 40 microns of metal lithium aluminum alloy (the mass percentage of aluminum in the metal lithium aluminum alloy belt is 5%) adjacent to the mixed conductive layer I, pressing at 100 ℃ and under the pressure of 10t, removing the PET substrate, and obtaining the three-layer composite material sequentially comprising the porous copper foil, the mixed conductive layer I and the metal lithium aluminum alloy layer from bottom to top.
(b) And (2) coating the slurry containing the multi-walled carbon nano-tubes prepared in the step (1) on the surface of a 36-micron PET film, and drying at 100 ℃ for 1h to form a mixed conductive layer II with the porosity of 40% and the thickness of 30 microns, so as to obtain the double-layer composite material sequentially comprising the PET mixed conductive layer film and the mixed conductive layer II from bottom to top.
(3) And (3) placing the mixed conducting layer II of the double-layer composite material prepared by the mixed conducting layer in the step (b) in the step (2) and the metal lithium-aluminum alloy layer of the three-layer composite material prepared by the step (a) in the step (2) adjacently, pressing at the temperature of 100 ℃ and under the pressure of 8t, and removing the PET film to obtain the lithium cathode.
Example 3
The preparation method of the lithium negative electrode of the embodiment specifically includes the following steps:
(1) Slurry material
Uniformly mixing 3.57 parts by weight of graphene, 1.43 parts by weight of polyvinylidene fluoride and 95 parts by weight of N-methyl pyrrolidone, and obtaining slurry containing graphene with the solid content of 20% for later use.
(2) Three-layer composite material and two-layer composite material
(a) Coating the slurry containing graphene prepared in the step (1) on the surface of a copper mesh (400 mesh) current collector, drying at 90 ℃ for 2 hours to form a mixed conductive layer I with the porosity of 80% and the thickness of 20 microns, then placing a 50-micron metal lithium belt adjacent to the mixed conductive layer I, and pressing at 100 ℃ and under the pressure of 10t to obtain the three-layer composite material sequentially comprising the copper mesh, the mixed conductive layer I and the metal lithium layer from bottom to top.
(b) And (2) coating the slurry containing graphene prepared in the step (1) on the surface of a copper foil, and drying at 90 ℃ for 2 hours to form a mixed conductive layer II with the porosity of 50% and the thickness of 30 microns, wherein the mixed conductive layer is a double-layer composite material consisting of the copper foil and the mixed conductive layer II from bottom to top in sequence.
(3) And (3) placing the mixed conductive layer II of the double-layer composite material prepared by the mixed conductive layer in the step (b) in the step (2) and the metal lithium layer of the three-layer composite material prepared by the step (a) in the step (2) adjacently, pressing at the temperature of 100 ℃ and under the pressure of 8t, and removing the copper foil to obtain the lithium cathode.
2. Examples of the lithium negative electrode of the present invention correspond to the final products of the lithium negative electrode preparation methods examples 1 to 3, respectively.
3. Specific examples of the lithium secondary battery of the present invention are as follows:
example 4
The lithium secondary battery of the present example includes a positive electrode, an electrolyte, and the lithium negative electrode prepared in example 1.
The anode is NCM622, and the electrolyte in the electrolyte is 1M LiPF 6 And the solvent is EC: EMC =3 (mass ratio).
Examples 5 to 6
The lithium secondary batteries of examples 5 to 6 each had a negative electrode made of the lithium secondary battery obtained in example 2 and a negative electrode made of the lithium secondary battery obtained in example 3, and a positive electrode and an electrolyte were the same as those of example 4.
4. Description of the comparative examples
Comparative example 1
The method for preparing the lithium negative electrode of the comparative example specifically includes the following steps:
(1) Slurry material
Uniformly mixing graphene and polyvinylidene fluoride in a mass ratio of 1.
(2) The copper mesh (400 mesh), lithium strip (50 μm), carbon-coated copper foil were aligned and stacked (with the carbon layer of the carbon-coated copper foil placed against the ultra-thin lithium strip), and rolled by a rolling mill.
And (3) after rolling, peeling the copper foil on the upper layer of the sandwich structure (the graphene layer is adhered to the surface of the ultrathin lithium strip after rolling), and finally obtaining the composite metal lithium cathode of the copper mesh/ultrathin lithium strip/graphene sandwich structure. And the ultra-thin lithium strip is embedded into the gaps of the copper mesh after being rolled. The thickness of the copper mesh (including lithium) was about 65 μm and the thickness of the graphene layer was about 10 μm. Lithium metal is deposited in bulk form on the surface of graphene. The thickness of the overall composite negative electrode was 90 μm.
5. Relevant test examples
Test example 1
Batteries were assembled using the lithium anodes obtained in examples 1 to 3 and comparative example 1 as the negative electrode, the positive electrode was NCM622, and the electrolyte in the electrolyte was 1M LiPF 6 The electrolyte solvent is EC: EMC =3 (mass ratio), and the diaphragm is a diaphragm coated with 3 mu mAl 2 O 3 The cycle life of the battery assembled with the lithium negative electrodes obtained in examples 1 to 3 and comparative example 1 as negative electrodes was measured at a test rate of 0.5C, and the results are shown in table 1.
Table 1 performances of batteries assembled with lithium negative electrodes prepared in examples 1 to 3 and comparative example 1 as negative electrodes
| Example 1 | Example 2 | Example 3 | Comparative example 1 | |
| Cycle life (cycle corresponding to 80% capacity retention) | 530 | 510 | 480 | 360 |
Experimental results show that the cycle life of the lithium metal cathode is obviously prolonged by compounding the mixed conductive material. The cycle life of the battery obtained by assembling the lithium negative electrodes prepared in examples 1 to 3 as the negative electrode was 480 weeks or more corresponding to the 80% capacity retention rate, and the cycle life of example 3 was increased by 33% or more corresponding to the 80% capacity retention rate as compared with comparative example 1.
Claims (5)
1. A method for producing a lithium negative electrode, comprising the steps of:
(1) Preparation of three-layer composite and two-layer composite
(a) Coating the slurry A containing the conductive material on the surface of a current collector, drying to form a conductive material layer I, then placing a metal belt and the conductive material layer I adjacently, and pressing to obtain a three-layer composite material which is composed of the current collector, the mixed conductive layer I and a metal layer from bottom to top in sequence; the metal belt is a metal lithium belt or a metal lithium alloy belt;
(b) Coating the slurry B containing the conductive material on the surface of a substrate material, and drying to form a conductive material layer II to obtain a double-layer composite material which is the substrate material and the conductive material layer II from bottom to top in sequence;
(2) Placing the conductive material layer II of the double-layer composite material and the metal layer of the three-layer composite material adjacently, and removing the substrate material through pressing to obtain the lithium cathode which sequentially comprises a current collector, a mixed conductive layer I, the metal layer and the mixed conductive layer II from bottom to top;
the pressing temperature in the step (1) is 100-120 ℃, and the pressing pressure is 10-15 t; the pressing temperature in the step (2) is 100-120 ℃, and the pressing pressure is 8-12t;
the porosity of the mixed conducting layer I is 40%, and the porosity of the mixed conducting layer II is 20%; or the porosity of the mixed electric conduction layer I is 70 percent, and the porosity of the mixed electric conduction layer II is 40 percent; or the porosity of the mixed electric conduction layer I is 80%, and the porosity of the mixed electric conduction layer II is 50%;
the conductive materials used in the mixed conductive layer I and the mixed conductive layer II are respectively and independently selected from one or more mixed materials of natural graphite, artificial graphite, soft carbon, hard carbon, carbon black, ketjen carbon, acetylene black, conductive carbon black, graphene and carbon nano tubes;
the weight ratio of the metal lithium to the conductive material in the mixed conductive layer I is 1:0.2 to 1;
the weight ratio of the metal lithium to the conductive material in the mixed conductive layer II is 1:1 to 10;
the lithium metal in the mixed conducting layer I and the mixed conducting layer II is formed by permeating from a metal layer, the content of the lithium metal in the mixed conducting layer I and the mixed conducting layer II is distributed in a gradient manner in the vertical direction of the lithium cathode, and the content of the lithium metal in the mixed conducting layer I and the mixed conducting layer II is gradually reduced in the vertical direction far away from the metal layer;
the thickness ratio of the metal layer to the mixed electric conduction layer I to the mixed electric conduction layer II is 0 to 1:0.1 to 1:0.1 to 1.
2. The method for producing a lithium negative electrode according to claim 1, wherein the metal tape in step (1) has a thickness of 10 to 100 μm.
3. The method for preparing the lithium negative electrode according to any one of claims 1 to 2, wherein in the step (1), the slurry A and the slurry B are respectively and independently composed of the following components in percentage by mass: 90-98 wt% of mixed conductive material and 2-10 wt% of binder.
4. A lithium secondary battery comprising a positive electrode, an electrolyte and the lithium negative electrode according to claim 1.
5. The lithium secondary battery according to claim 4, wherein the positive electrode is lithium cobaltate, a ternary material, a lithium-rich positive electrode material, lithium manganate, or lithium iron phosphate.
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| CN113437253A (en) * | 2021-06-26 | 2021-09-24 | 宁德时代新能源科技股份有限公司 | Lithium metal negative pole piece, electrochemical device and electronic equipment |
| CN113611868B (en) * | 2021-08-03 | 2023-03-28 | 湖南立方新能源科技有限责任公司 | Metal lithium composite electrode material, preparation method thereof and lithium ion battery |
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