WO2022198845A1 - Pre-lithiated negative electrode plate and preparation method therefor, and lithium ion battery - Google Patents

Abstract

The present invention relates to the technical field of lithium ion batteries, and in particular to a pre-lithiated negative electrode plate and a lithium ion battery. The pre-lithiated negative electrode plate is characterized by comprising a negative electrode plate body and an ultra-thin lithium net which is combined with the negative electrode plate body by means of rolling, wherein the thickness of the ultra-thin lithium net is 5-500 μm; the surface density of the ultra-thin lithium net is 0.5-100 g/m²; and the ultra-thin lithium net is prepared from a lithium base material by means of a rolling-cut stretching process. According to the present invention, an ultra-thin lithium net is prepared by using the characteristic of the elongation of a lithium base material being high and by creatively using a rolling-cut stretching method, and the ultra-thin lithium net is combined with a negative electrode plate body to prepare the pre-lithiated negative electrode plate. According to the rolling-cut stretching process, the temperatures of a cutter and a rolling-cut table of a device are controlled by using a cooling liquid, thereby alleviating the problem of metal lithium sticking to the cutter. The ultra-thin lithium net is used for pre-lithiation such that grid-shaped pre-lithiation can be formed on the negative electrode plate, and the stress of pre-lithiation expansion can be gradually released, thereby improving the structural stability of the negative electrode plate, and improving the cycle performance of the lithium ion battery.

Description

A pre-lithium negative electrode sheet and preparation method thereof, and lithium ion battery technical field

The invention relates to the technical field of lithium ion batteries, in particular to a pre-lithium negative electrode sheet, a preparation method thereof, and a lithium ion battery.

Background technique

During the first charging process of the battery, the electrolyte will form SEI on the surface of the negative electrode, consuming part of the active lithium, resulting in a low first efficiency of the battery and affecting the energy density of the battery, especially when the negative electrode uses Si, SiO, Li-SN, SnO, The effect is particularly obvious when the alloy anode such as SnO ₂ is used. In order to improve the energy density of batteries, SiC and SiO anode materials are more and more used in lithium-ion battery systems. The industry directly supplements active lithium to the anode by pre-lithiation of the anode electrode, thereby supplementing the activity caused by the formation of SEI. Lithium loss increases the first efficiency of the battery. The supplemented metal lithium can be appropriately excessive, and the excess metal lithium can be used as backup lithium to supplement the loss of active lithium caused by the damage repair of the SEI in the later cycle of the battery, thereby improving the service life of the battery.

Due to the structural design of the pole piece, the surface density of the added metal lithium is very small, and the thickness of the metal lithium is 1 to 10 μm, and the preparation of ultra-thin metal lithium strips is still the current technical bottleneck. Due to the low strength of metal lithium , and has self-adhesion, it is quite difficult from calendering to film coating to rolling. How to realize the pre-lithiation of the negative pole piece is one of the technical problems in the current industry, which directly affects the mass production application of high energy density solutions.

The Chinese patent document discloses the "Lithium-ion battery pole piece lithium powder processing system", and its application publication number is CN204668390U. This utility model disperses lithium powder on the surface of the pole piece, and combines it with the negative electrode by rolling. This method requires the use of Stabilized surface-coated small particle lithium powder. However, this method has the problems of uneven pre-lithium, high cost, and safety risks caused by lithium powder dust, and is now rarely used.

The Chinese patent document discloses "calendering mechanism and pole piece lithium replenishing device", and its application publication number is CN207038626U. This utility model is rolled to the required thickness by differential rolling, and the scheme has high production efficiency; However, this solution has the problems of high cost, complicated process and low efficiency.

The Chinese patent document discloses "metal lithium mesh and lithium ion battery using the same", and its application publication number is CN208368618U. This utility model does not specify the method for making lithium mesh, and the method for preparing lithium mesh is the key technology for realizing the scheme. ; In the specification, the preparation method of the lithium mesh is simply explained, including the method of mold casting and mechanical punching. However, the patent applies the metal lithium mesh to the negative electrode to supplement lithium or use the metal lithium as the negative electrode. The lithium ion battery solves the problem. When it is difficult to process ultra-thin metal lithium strips below 50 μm, there is no alternative solution to meet the needs of lithium replenishment; metal lithium is soft and low in strength, and it is difficult to prepare ultra-thin metal lithium meshes by precision casting, and ultra-thin lithium meshes have delamination. Difficulty in moulding and forming. At the same time, the use of casting method to prepare ultra-thin lithium mesh requires high processing precision and high cost; Time-efficient operation, and adjusting the mesh area requires changing the mold, which is extremely costly.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned problems in the prior art, the present invention provides a pre-lithium negative electrode sheet with high structural stability.

The invention also provides a preparation method of a pre-lithium negative electrode sheet, which is simple to operate, easy to control the process conditions, and easy to realize industrialization.

The present invention also provides a lithium ion battery including the above-mentioned pre-lithium negative electrode sheet, which utilizes the lithium mesh pre-lithium to form a grid-like pre-lithium on the negative electrode electrode sheet, so that the profit of the pre-lithium expansion is gradually released, and the structural stability of the electrode sheet is improved. The lithium-ion battery exhibits better cycle performance.

In order to achieve the above object, the present invention adopts the following technical solutions:

A pre-lithium negative electrode sheet, comprising a negative electrode sheet body and an ultra-thin lithium mesh rolled and compounded with the negative electrode sheet body, the ultra-thin lithium mesh having a thickness of 5-100 μm and a width of 10-1000 mm; the ultra-thin lithium mesh The surface density of the mesh is 0.5-20 g/m ² ; the ultra-thin lithium mesh is prepared from a lithium base material through a rolling-cutting and stretching process.

The invention utilizes the characteristics of soft texture and high elongation of the lithium base material (metal lithium has low hardness and the elongation is more than 50%), and creatively adopts the method of rolling cutting and stretching to prepare an ultra-thin lithium mesh, which is compounded with the negative electrode body. Pre-lithium anode. The advantage of using ultra-thin lithium mesh pre-lithium over lithium foil pre-lithium is that the use of ultra-thin lithium mesh can form grid-like pre-lithium on the negative pole piece, the stress of pre-lithium expansion can be gradually released, and the structure of the negative pole piece can be improved. stability and improve the cycle performance of lithium-ion batteries.

The parameters of the ultra-thin lithium mesh are very critical. Too thin thickness will increase the difficulty of processing, and the lithium mesh will easily break and cannot be combined with the negative electrode sheet. If the thickness is too thick, the areal density will be too large, exceeding the required pre-lithium amount of the negative electrode; It is related to the negative electrode width of the required pre-lithium.

Preferably, the lithium base material is pure metal lithium or a lithium alloy, the lithium alloy includes lithium and an alloy element, and the alloy element is selected from one of Mg, Al, B, Si, Ca, Na, Zr or several.

Preferably, the ultra-thin lithium mesh has uniformly distributed meshes, and the meshes are in the shape of a rhombus, a hexagon or a special shape.

Preferably, the ultra-thin lithium mesh is prepared by rolling cutting equipment, the rolling cutting equipment includes a cutting knife and a rolling cutting table, and a temperature adjusting mechanism is arranged in the cutting knife and the rolling cutting table; the cutting knife has several The cutter head is detachably connected with the cutter, and the shape of the cutter head can be adjusted. The temperature during hobbing has a great influence on the mechanical properties of metal lithium, and if the temperature of the cutter is too high, the problem of sticking will occur, and if the temperature is too low, the elongation will decrease. The temperature adjustment mechanism can control the temperature of the equipment cutting knife and the hobbing table, and improve the problem of metal lithium sticking to the knife.

Preferably, holes are provided in the cutter and the hobbing table, and cooling liquid flows through the holes.

Preferably, the negative electrode sheet body includes a negative electrode current collector and a negative electrode active slurry layer located on the negative electrode current collector, and the negative electrode active slurry layer includes a negative electrode active material; the negative electrode active material is selected from natural graphite, artificial graphite, One or more of mesocarbon microspheres, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO ₂ and Li-Al alloy.

A preparation method of a pre-lithium negative electrode sheet, comprising the following steps:

(1) Preparation of ultra-thin lithium mesh by rolling the lithium substrate through a rolling process;

(2) Rolling and compounding the negative electrode sheet body with the ultra-thin lithium mesh obtained in step (1) to obtain a pre-lithium negative electrode sheet.

Preferably, in step (1), the temperature during rolling and stretching is -20 to 60° C.; in step (2), the pressure during rolling and compounding is 0 to 50 t. The conditions of the hobbing and stretching process in step (1) are very critical, and the temperature has a great influence on the mechanical properties of metallic lithium, and if the temperature of the cutter is too high, the problem of sticking will occur, and if the temperature is too low, the elongation will be reduced; therefore; The temperature of hobbing and stretching should be controlled within the range of -20 to 60 °C to ensure the mechanical properties and elongation of the lithium substrate while avoiding the occurrence of sticking to the knife. The temperature can be optimized according to the material of the lithium substrate and the process conditions. . In step (2), if the pressure of rolling compounding is too high, the pole piece will be deformed, and if the pressure is too low, the contact between the lithium mesh and the negative electrode piece will be less, which will affect the diffusion of metallic lithium in the negative electrode piece. The rolling compounding pressure of the present invention is preferably 0～ 50t.

A lithium ion battery comprising the above-mentioned pre-lithium negative electrode sheet, the lithium ion battery further comprises a positive electrode sheet, a separator and an electrolyte, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active slurry on the positive electrode current collector layer; the positive electrode active slurry layer includes a positive electrode active material; the active material is selected from LiCoO ₂ , LiNi _x A _y B _(1-xy) O ₂ , LiMPO ₄ , Li _1-x Q _y' L _z' One or more of C _(1-y'-z') O ₂ ; wherein A, B are each independently selected from one of Co, Al, Mn, and A and B are different; LiMPO ₄ has olivine type structure, M is selected from one or more of Co, Ni, Fe, Mn, V, Q, L, C are independently selected from one of Co, Ni, Fe, Mn, and Q, L, C varies.

Preferably, 0<x<1, 0<y<1 and x+y<1; 0<x'<1, 0<y'<1, 0<z'<1 and y'+z'<1.

Therefore, the present invention has the following beneficial effects:

(1) Taking advantage of the high elongation rate of the lithium substrate, an ultra-thin lithium mesh is creatively prepared by rolling and stretching, and the pre-lithium negative electrode sheet is prepared by compounding it with the negative electrode body;

(2) The hobbing and drawing process uses coolant to control the temperature of the equipment cutter and hobbing table to improve the problem of metal lithium sticking to the knife;

(3) The use of ultra-thin lithium mesh pre-lithium can form grid-like pre-lithium on the negative pole piece, and the stress of pre-lithium expansion can be gradually released, improving the structural stability of the negative pole piece and improving the cycle performance of lithium ion batteries.

Description of drawings

FIG. 1 is a schematic diagram of the structure of the rolling cutting equipment of Example 1. FIG.

FIG. 2 is a schematic structural diagram of the ultra-thin lithium mesh prepared in Example 1. FIG.

FIG. 3 is an enlarged view of A in FIG. 2 .

In the figure: cutter 1, hobbing table 2, cutter head 3, first coolant inlet 4, first coolant outlet 5, second coolant inlet 6, second coolant outlet 7.

Detailed ways

The technical solutions of the present invention will be further specifically described below through specific embodiments and in conjunction with the accompanying drawings.

In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following examples are conventional methods in the art unless otherwise specified.

Example 1

(1) Preparation of ultra-thin lithium mesh:

Using pure metal lithium as the lithium base material, using the rolling cutting equipment as shown in Figure 1, a rolling cutting and drawing process is used to prepare an ultra-thin lithium mesh. The rolling cutting equipment includes a cutter 1 having several cutter heads 3 with diamond-shaped cutting surfaces And the rolling cutting table 2, the cutter 1 is provided with a first hole, the bottom of one end of the cutter 1 is provided with a first cooling liquid inlet 4 that communicates with the first hole, and the top of the other end is provided with a first hole. The first cooling liquid outlet 5; the rolling cutting table is provided with a second hole, one end of the cutter 1 is provided with a second cooling liquid inlet 4 communicated with the second hole, and the other end is provided with a second hole communicated with the second hole. Two cooling liquid outlets 5; cooling liquid flows through the first hole and the second hole. In the process of hobbing and stretching, the cutter will generate heat, and the temperature has a great influence on the mechanical properties of metal lithium. If the temperature of the cutter is too high, it will cause the problem of sticking. If the temperature is too low, the elongation will be reduced, which will affect the processing performance. , so the temperature of rolling and stretching should be controlled within the range of -20 to 60 °C, and adjusted according to the equipment;

Fig. 2 is a schematic diagram of the structure of the ultra-thin lithium mesh prepared in the present embodiment, the mesh is in the shape of a diamond, and Fig. 3 introduces the structural parameters of the ultra-thin lithium mesh, wherein a is the short pitch, b is the long pitch, c is the stem width, and the thickness is d. The specific parameters are shown in Table 1.

(2) Preparation of negative electrode body:

Mix the negative electrode active material silicon carbon composite material, binder styrene-butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC), and conductive agent acetylene black Super-P in a mass ratio of 94:3:2:1 , add deionized water, stir in a vacuum mixer until stable and uniform, and obtain a negative electrode slurry. The positive electrode slurry is uniformly coated on a copper foil with a thickness of 8 μm, and the coated copper foil is dried in a blast oven at 120° C., and then cold-pressed and cut to obtain a negative electrode body.

(3) Preparation of pre-lithium negative electrode sheet:

The ultra-thin lithium mesh obtained in step (1) is rolled and compounded with the negative electrode sheet body prepared in step (2). The process conditions of roll compounding should be adjusted according to the rolling equipment. Too high pressure will lead to deformation of the pole piece. If the pressure is too low, there is little contact between the lithium mesh and the negative electrode sheet, which affects the diffusion of metallic lithium in the negative electrode sheet. The rolling compound pressure range used in this example is 0-50t, and the pressure should be adjusted according to the equipment conditions, and the adjustment of the pressure range should not affect the The protection of this method; after rolling and compounding, the pre-lithium negative electrode sheet is obtained by placing it at room temperature for more than 48 hours under the condition of dew point <-40 °C.

(4) Preparation of positive electrode sheet:

The positive active material lithium nickel cobalt manganate (LiNi _0.8 Mn _0.1 Co _0.1 ), the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black Super-P were mixed in a mass ratio of 96:2:2, and N-methyl methacrylate was added. pyrrolidone, and stirred in a vacuum mixer until stable and uniform to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil with a thickness of 12 μm, and the coated aluminum foil was dried in a blast oven at 120° C., and then subjected to cold pressing and slitting to obtain a positive electrode sheet.

(5) Assembly of lithium-ion battery:

In the form of laminations, the positive electrode sheet, the diaphragm, and the pre-lithium negative electrode electrode are laminated to form a battery cell, and the tabs are welded in one direction; then the aluminum plastic film is heat-sealed, the electrolyte is injected, and the sealing is performed by heat sealing; Hot pressing-precharging-evacuating-forming-distributing capacity to make lithium-ion batteries. The battery charge and discharge cut-off voltage is 4.25-2.8V.

Example 2-4

The difference between Examples 2-4 and Example 1 is that the thickness of the ultra-thin lithium mesh is different, see Table 1 for details, and the rest of the processes are exactly the same.

Example 5

The difference between Example 5 and Example 1 is that the mesh shape of the ultra-thin lithium mesh is hexagonal, as shown in Table 1, and the rest of the processes are exactly the same.

Examples 6-8

The difference between Examples 6-8 and Example 5 is that the thickness of the ultra-thin lithium mesh is different, see Table 1 for details, and the rest of the processes are completely the same.

Comparative Example 1

The difference between Comparative Example 1 and Example 1 is that the negative electrode body is directly used without pre-lithium, and the rest of the processes are completely the same.

Comparative Example 2

The difference between Comparative Example 2 and Example 1 is that the pre-lithium negative electrode sheet is prepared by rolling and compounding the lithium foil with a thickness of 5 μm and the negative electrode sheet body, and the rest of the process is exactly the same.

Table 1. Pre-Lithium Parameters for Examples 1-8 and Comparative Examples 1 and 2

Numbering	Pre-lithium method	mesh shape	Areal density	Converted thickness of lithium foil	Pre-Lithium Capacity
Example 1	5μm lithium mesh	diamond hole	0.9612	1.8	3710.23
Example 2	10μm lithium mesh	diamond hole	1.7088	3.2	6595.97
Example 3	15μm lithium mesh	diamond hole	2.5632	4.8	9893.95
Example 4	20μm lithium mesh	diamond hole	3.4176	6.4	13191.94
Example 5	5μm lithium mesh	Hexagonal hole	0.5874	1.1	2267.36
Example 6	10μm lithium mesh	Hexagonal hole	1.1214	2.1	4328.60
Example 7	15μm lithium mesh	Hexagonal hole	1.7088	3.2	6595.97
Example 8	20μm lithium mesh	Hexagonal hole	2.2428	4.2	8657.21
Comparative Example 1	Not pre-lithium	-	-	-	-
Comparative Example 2	5μm lithium foil	-	2.67	5	10306.20

The lithium ion batteries prepared in Examples 1-8 and Comparative Examples 1 and 2 were charged and discharged, and the test method was as follows:

(a) First Coulomb efficiency

The first charge-discharge test was carried out on the battery prepared by the above method. The battery was charged with a current of 0.05C, charged to 4.25V, and the charged power was recorded as FCC, and then the battery was discharged with a current of 0.1C to 2.8V and discharged. Denoted as FDC, where: first effect = FDC/FDC, the higher the value, the higher the efficiency of battery energy storage

(b) Lithium evolution from negative electrode

Lithium precipitation in the negative electrode means that during the charging process, lithium ions are extracted from the positive electrode and deposited in the form of metallic lithium on the surface of the negative electrode. Lithium precipitation in the negative electrode will seriously affect the safety of the battery, so it should be strictly avoided. The reason for the lithium evolution of the negative electrode may be that the negative electrode has no space to accommodate more lithium, or that the lithium intercalation rate of the negative electrode is lower than the migration rate of lithium ions. The negative electrode does not have enough space to hold more lithium.

(c) Discharge gram capacity

As mentioned above, FDC is the amount of electricity discharged by the battery for the first time, and the active material mass of the positive electrode material in the battery is m, then the discharge gram capacity of the positive electrode material in the battery is:

Discharge gram capacity = FDC/m, for the same design, the higher the value, the higher the energy density of the battery and the higher the utilization of active material.

(d) Cycle performance test

Carry out a cycle test on the above battery, charge it to 4.25V at a constant current of 0.3C, turn it to a constant voltage charge to stop the current <= 0.05C, and then discharge it at 0.5C to stop 2.8V, such a cycle, the cycle capacity retention rate is the nth cycle. , the ratio of the discharge capacity to the first discharge capacity. Under the same number of cycles, the higher the value, the smaller the capacity decay of the battery.

Table 2. Performance of Li-ion Batteries of Examples 1-8 and Comparative Examples 1 and 2

It can be seen from Table 2 that when the lithium mesh is selected in Examples 3 and 7, when the thickness of the pre-lithium converted lithium foil exceeds 3.2 μm, the first effect reaches 84%, and the discharge gram capacity of the positive electrode material reaches about 188, reaching the highest value, more pre-lithium content can slightly improve the first effect and gram capacity, but there will be lithium precipitation in the negative electrode, which may lead to safety problems.

The cycle capacity retention rate results show that Example 3 and Example 7 can show better cycle capacity retention rate, and when lithium mesh is selected, it shows a higher capacity retention rate.

The use of ultra-thin lithium mesh can achieve more precise pre-lithiation. At the same time, pre-lithium with lithium mesh can form grid-like pre-lithium on the negative pole piece, and the stress caused by the expansion of pre-lithium can be gradually released, which further improves the structural stability of the negative pole piece and improves the cycle performance of the lithium ion battery.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention in any form. There are other variations and modifications under the premise of not exceeding the technical solutions described in the claims.

Claims (10)

1. A pre-lithium negative electrode sheet, characterized in that it comprises a negative electrode sheet body and an ultra-thin lithium mesh rolled and compounded with the negative electrode sheet body, wherein the thickness of the ultra-thin lithium mesh is 5-100 μm; the surface of the ultra-thin lithium mesh is The density is 0.5-20 g/m ² ; the ultra-thin lithium mesh is prepared from a lithium base material through a rolling-cutting and stretching process.
2. A pre-lithium negative electrode sheet according to claim 1, wherein the lithium base material is pure metal lithium or a lithium alloy, the lithium alloy includes lithium and an alloy element, and the alloy element is selected from Mg, Al , one or more of B, Si, Ca, Na, Zr.
3. The pre-lithium negative electrode sheet according to claim 1, wherein the ultra-thin lithium mesh has uniformly distributed meshes, and the meshes are in the shape of a rhombus, a hexagon or a special shape.
4. A pre-lithium negative electrode sheet according to claim 1, characterized in that, the ultra-thin lithium mesh is prepared by rolling cutting equipment, and the rolling cutting equipment comprises a cutter and a rolling cutting table, and the cutting knife and rolling cutting A temperature adjustment mechanism is arranged in the table; the cutter has several cutter heads, the cutter heads are detachably connected with the cutter, and the shape of the cutter head can be adjusted.
5. The pre-lithium negative electrode sheet according to claim 4, wherein the cutter and the hobbing table are provided with a channel, and a cooling liquid is passed through the channel.
6. The pre-lithium negative electrode sheet according to claim 4, wherein the negative electrode sheet body comprises a negative electrode current collector and a negative electrode active slurry layer located on the negative electrode current collector, and the negative electrode active slurry layer comprises a negative electrode active slurry layer. material; the negative electrode active material is selected from natural graphite, artificial graphite, mesocarbon microspheres, hard carbon, soft carbon, silicon, silicon carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, One or more of SnO ₂ and Li-Al alloys.
7. A method for preparing a pre-lithium negative electrode sheet as described in any one of claims 1-6, characterized in that, comprising the following steps:

   (1) Preparation of ultra-thin lithium mesh by rolling the lithium substrate through a rolling process;

   (2) Rolling and compounding the negative electrode sheet body with the ultra-thin lithium mesh obtained in step (1) to obtain a pre-lithium negative electrode sheet.
8. The method for preparing a pre-lithium negative electrode sheet according to claim 7, wherein the temperature during roll-cutting and stretching in step (1) is -20 to 60°C; The pressure is 0 ~ 50t.
9. A lithium ion battery comprising the pre-lithium negative electrode sheet according to any one of claims 1-6, further comprising a positive electrode sheet, a separator and an electrolyte, and the positive electrode sheet comprises a positive electrode current collector and is located in the positive electrode A positive electrode active slurry layer on the current collector; the positive electrode active slurry layer includes a positive electrode active material; the active material is selected from LiCoO ₂ , LiNi _x A _y B _(1-xy) O ₂ , LiMPO ₄ , Li ₁ One or more of _-x Q _y' L _z' C _(1-y'-z') O ₂ ; wherein A and B are each independently selected from one of Co, Al and Mn, and A and B is different; LiMPO ₄ has an olivine structure, M is selected from one or more of Co, Ni, Fe, Mn, V, and Q, L, C are independently selected from Co, Ni, Fe, Mn One, and Q, L, C are different.
10. The lithium-ion battery according to claim 9, wherein 0<x<1, 0<y<1 and x+y<1; 0<x'<1, 0<y'<1, 0<z '<1 and y'+z'<1.

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