CN114127986A

CN114127986A - Negative pole piece, electrochemical device and electronic device

Info

Publication number: CN114127986A
Application number: CN202180004451.0A
Authority: CN
Inventors: 林小萍; 陈茂华; 谢远森
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2021-03-17
Filing date: 2021-03-17
Publication date: 2022-03-01
Anticipated expiration: 2041-03-17
Also published as: WO2022193188A1; CN114127986B

Abstract

The application provides a negative pole piece, an electrochemical device and a related electronic device thereof, wherein the negative pole piece comprises a functional layer; the functional layer comprises M conducting layers and N dielectric layers which are arranged in a stacked mode, and the conducting layers comprise lithium wetting materials; wherein M is more than or equal to 1, and N is more than or equal to 1. The functional layer of the negative pole piece can relieve the volume expansion of the negative pole piece in charge-discharge circulation and inhibit the formation of lithium dendrites, and the electrochemical device comprising the negative pole piece has good cycle life and safety performance, so that the electrochemical device can be widely applied to electronic devices.

Description

Negative pole piece, electrochemical device and electronic device

Technical Field

The application relates to the technical field of lithium batteries, in particular to a negative pole piece, an electrochemical device and an electronic device.

Background

Lithium batteries have the characteristics of high energy density, high power density and the like, and are increasingly widely used in electric vehicles and mobile electronic devices. With the continuous development of science and technology, in addition to energy density and power density, people pay more attention to the cycle life and the safety performance of lithium batteries.

In the lithium battery charging process, lithium ions are continuously generated at the negative electrode to be subjected to electronic reduction, lithium dendrites are easily generated in the process, and the lithium dendrites penetrate through the diaphragm to cause short circuit of the lithium battery, so that safety risk is brought. Moreover, the continuously generated lithium dendrites are easy to fall off, so that 'dead lithium' which cannot continuously participate in the reaction is generated, and the capacity of the lithium battery is reduced in the circulation process. In addition, along with the lithium cell is constantly discharging and charging, volume expansion and shrink take place for lithium constantly, and the negative pole structure of lithium cell is easily destroyed, leads to the lithium cell capacity to sharply descend, produces the risk of potential safety hazard occasionally seriously.

Therefore, how to improve the cycle life and the safety performance of the lithium battery is an urgent problem to be solved in the technical field of lithium batteries.

Disclosure of Invention

In order to solve the technical problem, the application provides a negative pole piece, through designing functional layer in the negative pole piece, alleviates the volume expansion of negative pole piece, suppresses the formation of lithium dendrite to make the electrochemical device including this negative pole piece have good cycle life and security performance.

The application also provides an electrochemical device, which comprises the negative pole piece, so that the cycle life and the safety performance of the electrochemical device are excellent.

The application also provides an electronic device, which comprises the electrochemical device, so that the electronic device is long in service life, strong in cruising ability, high in safety performance and good in user experience.

The application provides a negative pole piece, wherein the negative pole piece comprises a functional layer; the functional layer comprises M conducting layers and N dielectric layers which are arranged in a stacked mode, and the conducting layers comprise lithium wetting materials;

wherein M is more than or equal to 1, and N is more than or equal to 1.

The functional layer of the negative electrode plate comprises a dielectric layer and a conductive layer attached with a lithium wetting material. On one hand, the dielectric layer can reduce the electron concentration of the functional layer and prevent lithium ions from forming dendrites under a larger current density, and the dielectric layer can also reduce the electron concentration of the surface of the functional layer, force the lithium ions to enter the interior of the functional layer for deposition and reduce the reduction amount of the lithium ions on the surface of the functional layer, so that the formation of the lithium dendrites on the surface of the functional layer is inhibited; on the other hand, the lithium wetting material can induce lithium ions to enter the functional layer for deposition, so that the formation of lithium dendrites on the interface of the negative electrode is reduced; therefore, the cycle life and safety performance of the electrochemical device can be improved. In addition, the three-dimensional framework formed by the lamination of the conducting layer and the dielectric layer can reduce the extrusion and expansion damage of lithium dendrites formed in the functional layer to the negative pole piece, and relieve the volume expansion of the negative pole piece in the circulating process, so that the circulating life and the safety performance of the electrochemical device can be further prolonged. And the conducting layer has equipotential property, when the lithium dendrite contacts the conducting layer, the conducting layer can disperse the electric field at the top of the lithium dendrite, further restrain the continuous growth of the lithium dendrite, and realize the self-correction of the dendrite, thereby improving the cycle life and the safety performance of the electrochemical device.

In the negative electrode sheet described above, the content of the lithium-wetting material is distributed in a gradient in the stacking direction.

According to the method, the deposition position of lithium ions is controlled through distribution of the content of the lithium wetting material, the lithium ions are further induced to be deposited at the position far away from a negative electrode interface, and generation of interface lithium dendrites is inhibited, so that the safety performance and the cycle life of the electrochemical device are improved.

In the above negative electrode tab, the negative electrode tab further includes a lithium metal layer or a current collector, the functional layer is disposed on a functional surface of the lithium metal layer or a functional surface of the current collector, and an extending direction of the lithium metal layer or an extending direction of the current collector is perpendicular to the stacking direction;

the content of the lithium wetting material is graded in a direction away from the lithium metal layer or current collector.

When the negative pole piece is implemented according to the mode, the dielectric layer can effectively reduce the electron concentration on the surface of the functional layer, lithium ions are forced to enter the functional layer for deposition, and the negative pole interface lithium deposition is reduced, so that the formation of negative pole interface lithium dendrites is inhibited, and the cycle life and the safety performance of the electrochemical device are improved. In addition, the lithium wetting material has high content in the area close to the lithium metal layer or the current collector and low content close to the negative electrode interface, and further reduces the deposition of lithium on the negative electrode interface, so that the formation of lithium dendrites on the negative electrode interface is inhibited, and the safety performance of the electrochemical device is further improved.

In the above negative electrode tab, the M conductive layers and the N dielectric layers are alternately stacked.

Compared with other stacking modes of M conducting layers and N dielectric layers, the mode that the conducting layers and the dielectric layers are stacked in a staggered mode enables lithium dendrites generated in the functional layer to be more easily contacted with the conducting layers, so that the conducting layers carry out self-error correction on the lithium dendrites, further growth of the lithium dendrites in the functional layer is restrained, and the cycle life and the safety performance of an electrochemical device comprising the negative pole piece can be improved.

In the above negative electrode tab, the first edge layer and/or the second edge layer of the functional layer is a dielectric layer.

The lithium wetting material is distributed in the conductive layer, so that when the first edge layer and/or the second edge layer of the functional layer are/is the dielectric layer, lithium ions are more prone to be deposited in the functional layer, the deposition of the lithium ions on the negative electrode interface is reduced, and the formation of lithium dendrites on the negative electrode interface is inhibited, so that the cycle life and the safety performance of an electrochemical device comprising the negative electrode plate can be effectively improved.

In the above negative electrode sheet, the conductivity of the conductive layer is 10^-7S/cm to 10⁶S/cm, and/or,

the dielectric layer has a conductivity of less than 10^-8S/cm。

When the conductivity of the conducting layer and/or the dielectric layer is within the range, the current density in the functional layer is more favorable for the deposition of lithium ions in the functional layer, and the formation of lithium dendrites is inhibited, so that the electrochemical device comprising the negative pole piece has excellent cycle life and safety performance.

In the negative electrode sheet as described above, the functional layer has a porosity of 5% to 90%, optionally 50% to 90%.

When the porosity of the functional layer is 50-90%, the functional layer can induce lithium ions to enter the interior of the electrochemical device and provide deposition sites for the lithium ions on the premise of not influencing the volumetric energy density of the electrochemical device, so that the formation of interface lithium dendrites is inhibited, and the cycle life and the safety performance of the electrochemical device are improved.

In the negative electrode plate, the mass of the lithium wetting material is 5 to 95 percent, optionally 30 to 60 percent of the mass of the functional layer.

The content of the lithium wetting material is too low, the capability of inducing lithium ions to enter the internal deposition is low, the capability of improving the cycle life and the safety performance of the electrochemical device by the function is limited, and the content of the lithium wetting material is too high, so that the structural stability of a functional layer and the energy density of the electrochemical device are negatively influenced; when the content of the lithium wetting material is 30-60%, lithium ions can be induced to enter the functional layer for deposition on the premise of not influencing the energy density of the electrochemical device and the structural stability of the functional layer, so that the retention probability of the lithium ions on a negative electrode interface is reduced, the formation of interface lithium dendrites is reduced, and the cycle life and the safety performance of the electrochemical device are improved.

In the above negative electrode plate, the functional layer further includes lithium metal, and the content of the lithium metal in the functional layer is 0.25mg/cm²To 25mg/cm²。

Lithium metal in the functional layer can continuously supplement lithium for the negative pole piece in the circulating process, and irreversible lithium loss in the charging and discharging process is counteracted, so that the circulating life of the electrochemical device comprising the negative pole piece is further prolonged.

The application also provides an electrochemical device, which comprises the negative pole piece.

The electrochemical device comprises the negative pole piece, so that the electrochemical device has more excellent cycle life and safety performance.

The present application also provides an electronic device, wherein the driving source or the energy storage unit of the electronic device is the electrochemical device.

The driving source or the energy storage unit of the electronic device adopts the electrochemical device, so that the cruising ability, the cycle life and the safety performance are excellent, and the user experience sense is excellent.

Drawings

FIG. 1 is a schematic view of functional layers in an embodiment of the present application;

FIG. 2 is a schematic view of functional layers in another embodiment of the present application;

fig. 3 is a schematic structural diagram of a negative electrode tab in an embodiment of the present application;

FIG. 4 is a schematic structural diagram of a negative electrode tab according to another embodiment of the present disclosure;

FIG. 5 is a schematic structural diagram of a negative electrode tab according to yet another embodiment of the present application;

FIG. 6 is a scanning electron microscope image of the negative electrode plate in example 8 of the present application;

FIG. 7 is a scanning electron micrograph of a negative electrode tab lithium deposit in example 11 of the present application;

FIG. 8 is a scanning electron micrograph of lithium deposition on the negative electrode plate of comparative example 1 of the present application.

Detailed Description

In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application, and it is obvious that the described embodiments are some but not all of the embodiments of the present application. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

A first aspect of the present application provides a negative electrode tab, which includes a functional layer; the functional layer comprises M conducting layers and N dielectric layers which are arranged in a laminated mode, and the conducting layers comprise lithium wetting materials;

wherein M is more than or equal to 1, and N is more than or equal to 1.

Fig. 1 is a schematic view of a functional layer in an embodiment of the present application, as shown in fig. 1, in the embodiment, the functional layer includes a conductive layer a and a dielectric layer b, which are stacked, and the conductive layer a includes a lithium wetting material c; in fig. 1, M is 1 and N is 1.

Fig. 2 is a schematic view of a functional layer in another embodiment of the present application, as shown in fig. 2, in this embodiment, the functional layer includes a conductive layer a1, a conductive layer a2, and a dielectric layer b, which are stacked, and the conductive layer a1 and the conductive layer a2 include a lithium wetting material c; in fig. 2, M is 2 and N is 1.

Generally, in the charging process, electrons flow to the negative electrode plate, meet with Li + in the negative electrode plate, and the Li + obtains electrons, so that lithium metal is deposited in the negative electrode plate, and the opposite is true in the discharging process. Therefore, in the continuous charging and discharging process, lithium is continuously deposited and dissolved in the negative pole piece, and the volume of the lithium is increased and reduced, so that the volume of the negative pole piece is expanded and contracted. In addition, lithium metal tends to form lithium dendrites on the negative electrode sheet during lithium deposition, creating a risk of puncturing the separator and causing short circuits.

The functional layer of the negative pole piece comprises a dielectric layer and a conductive layer attached with a lithium wetting material, wherein the dielectric layer can reduce the electron concentration on the surface of the functional layer and prevent lithium ions from forming dendrites under higher electron concentration; the dielectric layer can prevent electrons from being transmitted to the surface of the functional layer, and lithium ions are forced to enter the functional layer for deposition, so that the deposition amount of the lithium ions on the negative electrode interface is reduced, and the formation of lithium dendrites on the negative electrode interface is inhibited; and under the induction of the lithium wetting material, lithium ions can rapidly enter the functional layer when migrating to the negative electrode, so that the probability of the lithium ions staying at the negative electrode interface is further reduced, the integrity of the negative electrode interface is maintained by further inhibiting the growth of interface lithium dendrites, potential safety hazards caused by the fact that the interface lithium dendrites grow to puncture a diaphragm are avoided, and the deterioration of the cycle performance of the electrochemical device caused by the growth of the lithium dendrites and even the formation of 'dead lithium' is effectively avoided.

In addition, the structure of the negative pole piece functional layer is equivalent to a framework formed by laminating a conductive layer and a dielectric layer, so that a certain space is formed inside the functional layer. After lithium ions enter the inside of the functional layer, even if the lithium ions are reduced and grown into lithium dendrites, the skeleton structure of the functional layer can also provide a certain growth space for the lithium dendrites, so that the volume of the lithium dendrites is reduced to a certain extent, the negative pole piece is extruded and expanded to be damaged, and the cycle performance and the safety performance of the electrochemical device are maintained. It is worth mentioning that the conductive layer also has a certain "self-error correction function". Specifically, even if lithium dendrites grow rapidly inside the functional layer, when the lithium dendrites grow to contact the conducting layer, the electric field distribution concentrated at the top of the lithium dendrites can be dispersed and tend to be uniform due to the equipotentiality of the conducting layer, so that the further growth of the dendrites is restrained, the self-error correction of the dendrites is realized, and the cycle performance and the safety performance of the electrochemical device are effectively improved by further restraining the volume expansion of the negative pole piece.

Therefore, the negative pole piece of the application not only reduces the formation of lithium dendrite in the functional layer, but also inhibits the growth of interface lithium dendrite through the functional layer formed by the conductive layer and the dielectric layer which comprise the lithium wetting material, and effectively improves the volume expansion of the negative pole piece in the long-term application process, thereby being beneficial to the optimization of the cycle performance and the safety performance of an electrochemical device.

In addition, because the negative pole piece of this application can effectively avoid the formation of lithium dendrite, reduces the loss of lithium in cycle process, therefore electrochemical device can also have good coulombic efficiency.

The material of the conductive layer is not strictly limited as long as effective conduction of electrons can be achieved. In a specific implementation, the conductive layer may be a conductive material, such as a metal material, e.g., copper, nickel, chromium, titanium, tungsten, zirconium, aluminum, and alloys thereof; the conductive layer may also be made of a semiconductor material, such as carbon-based materials, e.g., solid carbon spheres, hollow carbon spheres, porous carbon, single-layer carbon nanotubes, multi-layer carbon nanotubes, pure carbon fibers, doped carbon fibers (including doped carbon fibers of oxides, sulfides, carbides, nitrides, simple metals, functional groups, etc.), graphene cages, doped graphene, and doped graphene derivatives. The present application can select, but is not limited to, the above materials.

The material of the dielectric layer is not strictly limited as long as the dielectric layer can block electron conduction. In a specific implementation process, the dielectric layer may be made of a high molecular material, for example, at least one of polyvinylidene fluoride, polyimide, polyamide, polyacrylonitrile, polyethylene glycol, polyphenylene oxide, polypropylene carbonate, polymethyl methacrylate, polyethylene terephthalate, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-chlorotrifluoroethylene, polyvinylidene fluoride-hexafluoropropylene, and polyvinylidene fluoride; the dielectric layer may also be an inorganic material, such as at least one of sodium silicate and derivatives thereof, aluminum oxide and derivatives thereof, and silicon oxide and derivatives thereof. The present application can select, but is not limited to, the above materials.

The lithium-wetting material is not strictly limited as long as it has a good affinity for lithium. The lithium wetting material can be a simple metal, such as silver, aluminum, gold, barium, calcium, indium, platinum, silicon, tin, zinc, cobalt, gallium; the lithium wetting material may also be a nitride such as molybdenum nitride, a sulfide such as molybdenum sulfide, tin sulfide. In addition, the conductive layer can be modified to include hydroxyl (-OH), ester (-COOR), carboxyl (-COOH), and amino (-NH)₂) Sulfo (-SO)₃H) The lithium-philic functional group is iso-philic to include a lithium wetting material in the conductive layer.

The method for obtaining the conductive layer is not limited strictly, and as an alternative, the conductive layer can be obtained by combining electrostatic spinning and heat treatment. Specifically, a spinning solution is prepared from a precursor capable of generating a conductive substance and a solvent, electrostatic spinning is performed using an electrostatic spinning device to obtain a fiber film, and then the fiber film is heat-treated in an inert atmosphere to obtain a conductive layer.

As another possible embodiment, a conductive paste including a conductive substance and a binder may be prepared, and then a suitable substrate is selected, and the conductive paste is coated on the substrate to obtain a conductive layer.

The method for obtaining the dielectric layer is not strictly limited in the present application, and the dielectric layer can also be obtained by electrostatic spinning or coating, which is not described herein again.

The method for preparing the conductive layer by combining the electrostatic spinning and the thermal treatment is not strictly limited, and as an implementable method, the method can adopt the process of preparing the conductive layer by combining the electrostatic spinning and the thermal treatment, and the precursor of the lithium wetting material is added into the spinning solution for electrostatic spinning to obtain the fiber film containing the precursor of the lithium wetting material, and then when the fiber film is thermally treated, the precursor of the lithium wetting material is converted into the lithium wetting material, so that the conductive layer contains the lithium wetting material. As another practicable manner, an amount of lithium wetting material may be added to the conductive paste during coating to prepare the conductive layer, such that the resulting conductive layer includes the lithium wetting material.

The method for laminating the conductive layers and the dielectric layers to obtain the functional layer is not strictly limited, and as an optional implementation mode, the functional layer can be obtained by laminating M conductive layers and N dielectric layers in a hot pressing mode. As another optional mode, the functional layer formed by stacking the conductive layer and the dielectric layer may be obtained by coating, specifically, selecting a suitable substrate, and sequentially stacking and coating slurry for forming M conductive layers and N dielectric layers on the surface of the substrate.

On the basis of the structure of the negative pole piece, the content distribution of the lithium wetting material in the conducting layer is further limited, so that the safety performance and the cycle life of the electrochemical device are further improved. In some embodiments of the present application, the content of the lithium wetting material in the negative electrode sheet is distributed in a gradient in the stacking direction.

In the application, the content of the lithium wetting material is distributed in a gradient mode in the stacking direction and comprises a gradient increasing mode and a gradient decreasing mode, wherein the gradient increasing mode means that the content of the lithium wetting material in the j-th conducting layer is smaller than that in the (j +1) -th conducting layer, the gradient decreasing mode means that the content of the lithium wetting material in the j-th conducting layer is larger than that in the (j +1) -th conducting layer, and j is larger than or equal to 1 and smaller than M. According to the method, the content distribution of the lithium wetting material is controlled, and the position of lithium ions deposited in the functional layer can be controlled. It can be understood that lithium ions tend to deposit at a position where the content of the lithium wetting material is large, and therefore, in the specific implementation process, the content of the lithium wetting material in the conductive layer far away from the negative electrode interface is controlled to be larger than the content of the lithium wetting material in the conductive layer close to the negative electrode interface, so that the deposition of the lithium ions at the position close to the negative electrode interface is reduced, the formation of lithium dendrites at the negative electrode interface is inhibited, the integrity of the negative electrode interface is maintained, the formation of the lithium dendrites at the interface is prevented from puncturing a diaphragm to cause short circuit, and therefore the safety performance of the electrochemical device can be further improved.

In the functional layer shown in fig. 1, for example, M is 1, the content of the lithium wetting material is distributed in a gradient manner in the stacking direction inside the one conductive layer a. As an applicable manner, in the process of obtaining the conductive layer a by combining the electrostatic spinning and the heat treatment, during the spinning, a spinning solution with a gradient change in the concentration of a lithium wetting material precursor is added into an electrostatic spinning device at different time periods, so that the content of the lithium wetting material in the thickness direction of the conductive layer a is distributed in a gradient manner, and then the conductive layer a and the dielectric layer b are laminated in the thickness direction of the conductive layer a to obtain the functional layer, so that the content of the lithium wetting material in the conductive layer a is distributed in a gradient manner in the laminating direction.

When the functional layer comprises two or more conductive layers (M is more than or equal to 2), the content of the lithium wetting material of each conductive layer is distributed in a gradient mode in the laminating direction, and the content of the lithium wetting material in each conductive layer is distributed uniformly.

For example, in the functional layer shown in fig. 2, where M is 2, the content of the lithium wetting material is uniformly distributed in the conductive layer a1, the content of the lithium wetting material is uniformly distributed in the conductive layer a2, and the content of the lithium wetting material in the conductive layer a1 is greater than the content of the lithium wetting material in the conductive layer a2, so that the content of the lithium wetting material in the negative electrode tab is distributed in a gradient manner in the stacking direction.

When the functional layer comprises two or more conductive layers, the content of lithium wetting material precursors in M spinning solutions is changed in a gradient manner in the process of preparing M conductive layers by combining electrostatic spinning and heat treatment, and then the conductive layers are laminated with the dielectric layer according to the increasing or decreasing order of the content of the lithium wetting material precursors in the spinning solutions during lamination. As another practicable manner, in the process of coating to obtain the conductive layer, the content of the lithium wetting material in the M conductive pastes in the stacking direction may be made to vary in a gradient manner, so that the content of the lithium wetting material is distributed in a gradient manner in the stacking direction.

Of course, when the functional layer includes two or more conductive layers, the content of the lithium wetting material in one conductive layer may also be distributed in a gradient in the stacking direction, and, in consideration of the induced deposition of lithium ions, the content of the lithium wetting material in one conductive layer may be distributed in a gradient in the stacking direction in the same tendency as the content of the lithium wetting material in a different conductive layer is distributed in a gradient in the stacking direction, for example, with an increasing gradient or with a decreasing gradient.

In some embodiments herein, the mass of the lithium wetting material may be 5% to 95%, alternatively 30% to 60%, of the mass of the functional layer.

Generally, the more mass of the lithium wetting material in the functional layer, the better the lithium affinity of the functional layer, and thus, the more favorable the lithium ions can be induced to rapidly enter the functional layer for deposition. However, the improvement of the quality of the lithium wetting material is not beneficial to the improvement of the quality energy density of the electrochemical device and the structural stability of the functional layer, so that the quality of the lithium wetting material can be controlled to be in the range of 30-60% of the quality of the functional layer, the probability of lithium ions on a negative electrode interface is reduced, the formation of lithium dendrites on the negative electrode interface is inhibited, and the cycle life and the safety performance of the electrochemical device are improved on the premise of not influencing the quality energy density of the electrochemical device. In addition, in the aspect of selection of lithium wetting materials, the uniformity of the cycle life, the safety performance and the mass energy density of the electrochemical device can be further realized by selecting some materials with good lithium-philic effect and low density.

The negative pole piece of the application also comprises a lithium metal layer or a current collector. Generally, the lithium metal layer or the current collector is in a sheet shape, and two large and opposite surfaces of the lithium metal layer or the current collector are functional surfaces. When the negative electrode sheet of the present application includes a lithium metal layer or a current collector, the functional layer is disposed on the functional surface.

The arrangement mode of the functional layer on the functional surface is not strictly limited, for example, the functional layer can be arranged on the functional surface by hot pressing; as described above, when the functional layer is obtained by coating, a lithium metal layer or a current collector may be used as the substrate. In some embodiments, the functional layer may also be placed directly on the functional surface of the lithium metal layer or current collector without the functional layer forming a substantial connection with the functional surface. It can be understood that substantial connection can be formed between the functional layer and the functional surface by means of hot pressing or coating, which is beneficial to further keeping the structural stability of the negative electrode plate and the stability of the conductive network in the charging and discharging processes.

In a specific implementation process, whether the negative electrode sheet includes a lithium metal layer or a current collector may be selected according to the use of the negative electrode sheet. For example, when the negative electrode plate of the present application is used in a button cell, the functional layer can be directly used as the negative electrode of the cell without using a lithium metal layer or a current collector, so that the weight of the button cell can be lighter, and the mass energy density of the button cell can be further improved. When the negative electrode sheet of the present application is used for a winding type battery, the negative electrode sheet includes a lithium metal layer or a current collector so as to provide a tab. In the present application, the current collector may be, but is not limited to, a copper foil.

In order to further improve the inducing effect of the functional layer on lithium ion deposition and the blocking effect of electron transmission, as a preferred embodiment, when the negative electrode plate comprises a lithium metal layer or a current collector, the functional layer is disposed on the functional surface of the lithium metal layer or the functional surface of the current collector, and the extending direction of the lithium metal layer or the extending direction of the current collector is perpendicular to the stacking direction;

the content of lithium wetting material decreases in a gradient in a direction away from the lithium metal layer or current collector.

Fig. 3 is a schematic structural diagram of a negative electrode tab in an embodiment of the present application, and as shown in fig. 3, the negative electrode tab includes a lithium metal layer or a current collector d and a functional layer e, and the functional layer e is disposed on a functional surface of the lithium metal layer or the current collector d. The functional layer e comprises a conductive layer a, a dielectric layer b and lithium wetting materials c, wherein M is 1, and N is 1, the lamination direction is perpendicular to the extension direction of the lithium metal layer or the extension direction of the current collector, the arrow direction in the figure is the lamination direction, and the content of the lithium wetting materials is gradually reduced in the direction far away from the lithium metal layer or the current collector d.

When the negative pole piece is implemented according to the mode, the extending direction of the dielectric layer is parallel to the extending direction of the lithium metal layer or the current collector, and the area for blocking electron transfer by the dielectric layer is large, so that the dielectric layer can effectively reduce the electron concentration in the functional layer, thereby effectively preventing the combination of lithium ions and electrons and inhibiting the formation of lithium dendrites in the functional layer; the dielectric layer can effectively reduce the electron concentration on the surface of the functional layer, so that lithium ions are forced to enter the functional layer for deposition, and the formation of lithium dendrites on the interface of the negative electrode is reduced; the cycle life and safety performance of the electrochemical device can be improved. In addition, the content of the lithium wetting material is low in the region far away from the lithium metal layer or the current collector and is high in the region close to the lithium metal layer or the current collector, so that lithium ions are more prone to be deposited in the region close to the lithium metal layer or the current collector, the formation of lithium dendrites in the region close to the negative electrode interface is reduced, the lithium dendrites are prevented from puncturing the diaphragm to cause safety risks, and the safety performance of the electrochemical device is improved.

The inventors found in the course of research that the thickness of the functional layer in the stacking direction has a certain influence on the electrical performance of the electrochemical device, that the energy density of the electrochemical device decreases when the thickness of the functional layer in the stacking direction is too large, and that the functional layer has a limited improvement in the cycle life and safety performance of the electrochemical device when the thickness is too small. When the thickness of the functional layer is between 50 μm and 100 μm, the electrochemical device can have not only a good energy density but also excellent cycle life and safety.

In addition, under the condition that the thickness of the functional layer is constant, the number of layers of the conducting layer and the dielectric layer is reasonably controlled, and the cycle life and the safety performance of the electrochemical device are further improved. Specifically, as the number of layers of the conductive layer and the dielectric layer increases within a certain range, the cycle life and safety performance of the electrochemical device tend to increase first and then remain substantially unchanged, and thus, for economic reasons, the number of layers M of the conductive layer is generally 4 to 6, and/or the number of layers N of the dielectric layer is generally 4 to 6.

In addition to the above-described manner in which the stacking direction is perpendicular to the extending direction of the lithium metal layer or the current collector, the stacking direction may be parallel to the extending direction of the lithium metal layer or the extending direction of the current collector or may form an arbitrary acute angle.

Fig. 4 is a schematic structural diagram of a negative electrode tab in another embodiment of the present application, and as shown in fig. 4, the negative electrode tab includes a lithium metal layer or a current collector d and a functional layer e, and the functional layer e is disposed on a functional surface of the lithium metal layer or the current collector d. Wherein functional layer e comprises conductive layer a1, conductive layer a2, dielectric layer b1, dielectric layer b2, and lithium wetting material c; in fig. 4, M is 2, N is 2, the stacking direction is parallel to the extending direction of the lithium metal layer or the extending direction of the current collector, and the arrow direction in the figure is the stacking direction.

Fig. 5 is a schematic structural diagram of a negative electrode tab according to another embodiment of the present disclosure, as shown in fig. 5, the negative electrode tab includes a lithium metal layer or a current collector d, and a functional layer e, where the functional layer e is disposed on a functional surface of the lithium metal layer or the current collector d. Wherein functional layer e comprises conductive layer a1, conductive layer a2, dielectric layer b1, dielectric layer b2, and lithium wetting material c; m is 2, N is 2, the stacking direction forms an acute angle with the extending direction of the lithium metal layer or the extending direction of the current collector, and the arrow direction in the figure is the stacking direction.

In the research process, the inventor finds that compared with the embodiment shown in fig. 4 and 5, the embodiment shown in fig. 3 is more beneficial to the functional layer to exert the capability of inhibiting the formation of interfacial lithium dendrites, so that the cycle life and the safety performance of the electrochemical device can be better.

It should be added that, no matter whether the stacking direction is perpendicular to, parallel to, or at any acute angle with the extending direction of the lithium metal layer or the current collector, the functional layer disposed on the functional surface of the lithium metal layer or the current collector includes both cases where the functional layer is disposed on one functional surface and two functional surfaces of the lithium metal layer or the current collector. In the specific implementation process, one functional layer is arranged on one functional surface of the lithium metal layer or the current collector, or two functional layers are arranged on the two functional surfaces, and the relation between the stacking direction of the functional layers arranged on the different functional surfaces and the extending direction of the lithium metal layer or the current collector can be selected according to the requirement of the negative pole piece.

In the present application, the stacking order of the M conductive layers and the N dielectric layers has a certain influence on the performance of the functional layer. As a preferred embodiment, M conductive layers and N dielectric layers of the present application are alternately stacked.

Compared with the mode that i (i is more than or equal to 2 and less than or equal to M) conductive layers are laminated and then k (k is more than or equal to 2 and less than or equal to N) dielectric layers are laminated, the conductive layers and the dielectric layers are arranged in a staggered and laminated mode, so that lithium dendrites generated in the functional layers can be more easily contacted with the conductive layers, the conductive layers can be more favorable for self-error correction of the lithium dendrites, further growth of the lithium dendrites is inhibited, the risk that the lithium dendrites puncture a diaphragm to cause short circuit is reduced, energy density loss caused by the lithium dendrites is reduced, and the safety performance and the cycle life of an electrochemical device are improved.

In a preferred embodiment, the first and/or the second edge layer of the functional layer is a dielectric layer.

In the present application, the first and/or second edge layer of the functional layer refers to the first and/or last layer of the functional layer in the stacking direction, respectively.

It can be understood that when M is 1, the first edge layer and/or the second edge layer of the functional layer must be the dielectric layer regardless of how the conductive layer and the dielectric layer are stacked. When M is larger than or equal to 2, the first edge layer and/or the second edge layer of the functional layer are dielectric layers.

Since the lithium wetting material is distributed in the conductive layer, when the first edge layer and/or the second edge layer is the dielectric layer, lithium ions are more likely to be deposited inside the functional layer, and thus, the amount of lithium deposited at the negative electrode interface is reduced, the formation of lithium dendrites at the negative electrode interface is suppressed, and the cycle life and safety of the electrochemical device can be further improved.

The inventor finds that the conductivity of the conductive layer is 10 in the research process^-7S/cm to 10⁶S/cm, and/or the dielectric layer has a conductivity of less than 10^-8At S/cm, the electrochemical device has more excellent energy density, cycle life and safety performance.

The current density in the functional layer can be optimized through controlling the conductivity of the conductive layer and/or the conductivity of the dielectric layer, lithium ions can be deposited in the functional layer more conveniently, and formation of lithium dendrites is inhibited. When the conductivity of the conductive layer and/or the conductivity of the dielectric layer are/is within the above range, the current density in the functional layer is not too high, so that the risk of generation of lithium dendrites in the functional layer is increased, and the current density is not too low, so that a large amount of lithium cannot be subjected to electron deposition in the functional layer, and the capacity of the electrochemical device is reduced; therefore, the safety performance, the cycle performance and the capacity of the electrochemical device can be organically unified.

When the material of the conductive layer is selected from conductive materials, the conductivity of the conductive layer is 10^-3S/cm to 10⁶S/cm; when the material of the conductive layer is selected from semiconductor material, the conductivity of the conductive layer is 10^-7To 10³. In a specific implementation, the conductivity of the conductive layer and/or the dielectric layer can be adjusted by selecting the material of the conductive layer or the material of the dielectric layer, selecting suitable parameters during the preparation of the conductive layer and/or the dielectric layer, adding a certain amount of conductive agent or insulating material during the preparation of the conductive layer and/or the dielectric layer, and the like, so that the conductivity of the conductive layer and/or the dielectric layer is within the above range.

The porosity of the functional layer has an effect on the deposition of lithium metal in the functional layer, and the inventors found in the course of their research that the porosity of the functional layer is between 5% and 90%, and further preferably between 50% and 90%.

It can be understood that the higher the porosity of the functional layer is, the more favorable the shuttle of lithium ions in the functional layer is, the lithium ions are induced to enter the interior of the functional layer more rapidly for deposition, and the functional layer can have a larger specific surface area to provide more sites for the deposition of the lithium ions. However, the large porosity of the functional layer often means that the functional layer has a large volume, which is not favorable for the increase of the volumetric energy density of the electrochemical device. Therefore, the porosity of the functional layer is controlled within the range of 50% to 90%, so that lithium ions can be induced to enter the functional layer and deposit on the premise of not influencing the volume energy density of the electrochemical device, the formation of interface lithium dendrites is inhibited, and the cycle life and the safety performance of the electrochemical device are improved.

In specific implementation, the material of the conductive layer and the dielectric layer with appropriate porosity, the thickness of the conductive layer and the dielectric layer, the preparation process (e.g., electrostatic spinning temperature, spinning solution concentration) for matching the appropriate conductive layer and the dielectric layer, and the lamination process (e.g., hot pressing pressure and temperature) of the conductive layer and the dielectric layer can be selected so that the porosity of the functional layer is 50% to 90%.

The good mechanical strength of functional layer can realize the stability of structure in the functional layer charge-discharge cycle, avoids the functional layer to take place the breakage in the cycle process for the functional layer can continuously exert the effect, guarantees the stable structure of negative pole piece. In the specific implementation process, the mechanical strength of the functional layer can be adjusted by selecting materials of the conductive layer and the dielectric layer with proper strength, combining the preparation process of the conductive layer and the dielectric layer, the lamination process of the conductive layer and the dielectric layer and the like.

On the basis of the negative pole piece, the functional layer of the negative pole piece further comprises lithium metal, and the content of the lithium metal in the functional layer is 0.25mg/cm²To 25mg/cm²I.e. from 0.25 to 25mg of lithium metal per square centimeter of area of the functional layer. The area of the functional layer refers to the area of the surface of the functional layer closest to the functional surface of the current collector.

By including a certain content of lithium metal in the functional layer, lithium lost in discharge cycle can be replenished, and the cycle life of an electrochemical device comprising the negative electrode plate is further prolonged.

In a specific implementation, the lithium metal can be included in the functional layer in the above-mentioned content by a melting method, a physical vapor deposition method, an electrochemical method, or the like.

The second aspect of the present application also provides an electrochemical device, which includes the above negative electrode sheet. It can be understood that the electrochemical device generally includes a positive electrode sheet, an electrolyte (electrolyte), a separator, and the like, in addition to a negative electrode sheet.

The electrochemical device is not strictly limited, and for example, the electrochemical device may be a wound battery, a laminated battery, a solid-state battery, or the like. In a specific implementation process, the cathode pole piece, the anode pole piece, the electrolyte (electrolyte), the diaphragm and the like can be matched to form a proper electrochemical device according to requirements.

In some embodiments of the present application, the negative electrode sheet may have only the functional layer, and at this time, the extending direction of the functional layer may be parallel to the extending direction of the positive electrode sheet, so that the negative electrode sheet of the electrochemical device is opposite to the positive electrode sheet, and is matched with the diaphragm and the electrolyte (electrolyte), thereby obtaining the electrochemical device with excellent cycle life and safety performance.

In other embodiments of the present application, the negative electrode plate includes a functional layer and a current collector, the functional layer is disposed on a functional surface of the current collector, and at this time, by making an extending direction of the current collector parallel to an extending direction of the positive electrode plate and matching with the separator and the electrolyte (substance), an electrochemical device with excellent cycle life and safety performance is obtained.

The electrochemical device provided by the application has the advantages that the functional layer included by the negative pole piece can effectively reduce the volume expansion of the negative pole piece in charge-discharge circulation and the formation of lithium dendrites, so that the electrochemical device has excellent cycle life and safety performance.

The third aspect of the present application also provides an electronic device comprising the electrochemical device described above. The electrochemical device may serve as a driving source or an energy storage unit of the electronic device. The electronic device in the present application may be, but is not limited to, a mobile device (e.g., a smart watch, a mobile phone, a notebook computer, etc.), an electric vehicle (e.g., an electric bicycle, a hybrid electric vehicle, a power-assisted bicycle, etc.), and the like.

The electronic device comprises the electrochemical device, so that the electronic device has excellent cruising ability, service life and safety performance, and good user experience.

The present application is further illustrated below with reference to examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present application. In the following examples, the starting materials used are commercially available unless otherwise indicated.

The button cells of examples 1-17 were prepared as follows.

(1) Preparation of negative pole piece

A: preparing a conductive layer: dissolving 0.8 part by mass of polyacrylonitrile in 10 parts by mass of N, N-dimethylformamide, and then adding tin tetrachloride to obtain a spinning solution containing tin tetrachloride at a certain concentration; injecting the spinning solution into an electrostatic spinning device for electrostatic spinning to obtain a polyacrylonitrile fiber membrane, and setting the working state of the electrostatic spinning device to control the thickness of the polyacrylonitrile fiber membrane in the spinning process; then, placing the polyacrylonitrile fiber membrane in a muffle furnace, heating the muffle furnace to 230 ℃ at a heating rate of 1 ℃ per minute in an air atmosphere, keeping for 2 hours, pre-oxidizing the polyacrylonitrile fiber membrane, heating the pre-oxidized polyacrylonitrile fiber membrane to 800 ℃ at a heating rate of 5 ℃ per minute in an argon atmosphere in a tubular furnace, keeping for 4 hours, and distributing tin dioxide (lithium wetting material) in the obtained carbon fiber membrane to obtain a conducting layer;

in the process of injecting the spinning solution into the electrostatic spinning device, when the concentration of the tin tetrachloride in the spinning solution is not changed (namely the concentration of the tin tetrachloride is not adjusted), the polyacrylonitrile fiber membrane with the content of the tin tetrachloride uniformly distributed along the thickness direction of the polyacrylonitrile fiber membrane can be obtained;

in the process of injecting the spinning solution into the electrostatic spinning device, the concentration of tin tetrachloride in the spinning solution is adjusted at different time periods to enable the concentration of the tin tetrachloride to present gradient change, so that the polyacrylonitrile fiber membrane with the gradient distribution of the content of the tin tetrachloride along the thickness direction of the polyacrylonitrile fiber membrane can be obtained;

b: preparing a dielectric layer: dissolving 0.8 part by mass of polyvinylidene fluoride in 10 parts by mass of N, N-dimethyl and amide to obtain a spinning solution, injecting the spinning solution into an electrostatic spinning device for electrostatic spinning to obtain a polyvinylidene fluoride fiber membrane, setting the working state of the electrostatic spinning device, and controlling the thickness of the polyvinylidene fluoride fiber membrane; placing the polyvinylidene fluoride fiber membrane in a vacuum drying oven, and keeping the polyvinylidene fluoride fiber membrane at 80 ℃ for 12 hours to obtain a dielectric layer;

c: preparation of the functional layer: punching M conductive layers and N dielectric layers into wafers with the diameter of 18mm, laminating the M conductive layers and the N dielectric layers in a staggered manner by adopting a hot pressing mode to obtain a functional layer, and pre-supplementing lithium in the functional layer by utilizing an electrochemical method to ensure that the content of lithium metal in the functional layer is 4.1mg/cm²；

D: arranging the functional layer obtained in the step C on one functional surface of the copper foil of the negative current collector in a hot pressing mode to obtain a negative pole piece;

the laminating direction of the conducting layer and the dielectric layer is vertical to the extending direction of the copper foil current collector.

(2) Preparation of positive pole piece

Mixing the positive active material lithium iron phosphate, conductive carbon black and polyvinylidene fluoride according to the mass ratio of 97.5: 1.0: 1.5 mixing, adding into N-methyl pyrrolidone, blending to obtain positive electrode slurry with solid content of 0.75, uniformly stirring, uniformly coating the positive electrode slurry on a functional surface of a current collector aluminum foil, drying the aluminum foil at 90 ℃ to obtain a positive electrode piece, and punching and cutting the positive electrode piece into sheets with the diameter of 14 mm;

wherein the loading amount of the positive active material on the positive pole piece is 1mg/cm²。

(3) Preparation of the electrolyte

In a dry argon atmosphere, firstly, mixing dioxolane and dimethyl ether according to the volume ratio of 1:1 to obtain a mixed solution, then adding lithium bistrifluoromethanesulfonimide into the mixed solution and uniformly mixing to obtain the electrolyte with the concentration of the lithium bistrifluoromethanesulfonimide being 1M.

(4) Button cell assembly

Selecting a CR2032 battery case, wherein the diameter of the battery case is 20mm, the height of the battery case is 3.2mm, and the thickness of the sheet is 0.25 mm; assembling the negative pole piece, the positive pole piece and the diaphragm according to a button cell assembly method commonly used in the field, and performing side sealing, top sealing, liquid injection and packaging after assembling to finally obtain the button cell;

wherein the separator is a polyethylene film having a thickness of 15 μm.

Example 18

The button cell of this example was prepared by the same procedure as in example 1, except that no pre-lithium supplement was performed after the functional layer was obtained in step C of (1) preparing the negative electrode tab.

Example 19

The preparation of the button cell of this embodiment is basically the same as the preparation steps of embodiment 1, and the only difference is that in this embodiment, when the functional layer obtained in step C is disposed on one functional surface of the copper foil of the negative current collector in a hot-pressing manner in step D of (1) preparing the negative electrode tab to obtain the negative electrode tab, the stacking direction of the dielectric layer and the conductive layer is parallel to the extending direction of the current collector.

Example 20

The preparation of the button cell of this embodiment is basically the same as the preparation steps of embodiment 1, the only difference is that in (1) the preparation step D of the negative electrode plate, before the functional layer is disposed on the copper foil of the negative current collector, the functional layer is rolled by using a 1.5T roller, and the rolling speed is 1 m/min.

Comparative example 1

The button cell of this comparative example was prepared as follows:

(1) preparation of negative pole piece

Dissolving 0.8 parts by mass of polyacrylonitrile in 10 parts by mass of a solvent to obtain a spinning solution, injecting the spinning solution into an electrostatic spinning device for electrostatic spinning to obtain a polyacrylonitrile fiber membrane, and setting the working state of the electrostatic spinning device so that the thickness of the polyacrylonitrile fiber membrane is 60 mu m; then, placing the polyacrylonitrile fiber membrane in a muffle furnace, heating the muffle furnace to 230 ℃ at a heating rate of 1 ℃ per minute in the air atmosphere, keeping the temperature for 2 hours, pre-oxidizing the polyacrylonitrile fiber membrane, heating the pre-oxidized polyacrylonitrile fiber membrane to 800 ℃ at a heating rate of 5 ℃ per minute in a tube furnace under the argon atmosphere, keeping the temperature for 4 hours to obtain a carbon fiber membrane, and punching the carbon fiber membrane into sheets with the diameter of 18mm to obtain a guide tubeA conductive layer is arranged on one functional surface of the copper foil of the negative current collector through hot pressing, and lithium is pre-supplemented in the functional layer by an electrochemical method, so that the content of lithium metal in the functional layer is 4.1mg/cm²Obtaining a negative pole piece;

the processes of the preparation of the positive electrode plate, the preparation of the electrolyte and the button assembly in the comparative example are the same as those in example 1.

Comparative example 2

The button cell of this comparative example was prepared as follows:

(1) preparation of negative pole piece

Dissolving 0.8 parts by mass of polyvinylidene fluoride in 10 parts by mass of a solvent to obtain a spinning solution, injecting the spinning solution into an electrostatic spinning device for electrostatic spinning to obtain a polyvinylidene fluoride fiber membrane, and setting the working state of the electrostatic spinning device so that the thickness of the polyvinylidene fluoride fiber membrane is 60 mu m; placing the polyvinylidene fluoride fiber membrane in a vacuum drying oven, keeping the polyvinylidene fluoride fiber membrane at 80 ℃ for 12 hours, then punching the polyvinylidene fluoride fiber membrane into sheets with the diameter of 18mm to obtain a dielectric layer, and arranging the dielectric layer on one functional surface of a negative current collector copper foil through hot pressing to obtain a negative pole piece;

the processes of the preparation of the positive electrode plate, the preparation of the electrolyte and the assembly of the button cell in the comparative example are the same as those in example 1.

Comparative example 3

The button cell of this comparative example was prepared as follows:

(1) preparation of negative pole piece

Dissolving 0.8 mass part of polyacrylonitrile in 10 mass parts of N, N-dimethyl amide solvent, and then adding tin tetrachloride to ensure that the concentration of the tin tetrachloride in the spinning solution is 0.4M; injecting the spinning solution into an electrostatic spinning device for electrostatic spinning to obtain a polyacrylonitrile fiber membrane, and setting the working state of the electrostatic spinning device to ensure that the thickness of the polyacrylonitrile fiber membrane is 60 mu m; then, placing the polyacrylonitrile fiber membrane in a muffle furnace, heating the muffle furnace to 230 ℃ at a heating rate of 1 ℃ per minute in an air atmosphere, keeping for 2 hours, pre-oxidizing the polyacrylonitrile fiber membrane, heating the pre-oxidized polyacrylonitrile fiber membrane to 800 ℃ at a heating rate of 5 ℃ per minute in a tube furnace under an argon atmosphere, keeping for 4 hours, uniformly distributing tin dioxide in the obtained carbon fiber membrane along the thickness direction of the carbon fiber membrane, punching the carbon fiber membrane into sheets with the diameter of 18mm to obtain a conductive layer, and arranging the conductive layer on one functional surface of a negative current collector copper foil through hot pressing to obtain a negative pole piece;

Comparative example 4

The comparative example adopts copper foil with the diameter of 18mm as a negative pole piece;

In each of the specific examples and comparative examples, parameters such as the number of conductive layers (M), the number of dielectric layers (N), the thickness of the conductive layers before hot pressing, the thickness of the dielectric layers before hot pressing, the thickness of the functional layers in the stacking direction, the composition of the negative electrode sheet, the concentration of tin tetrachloride in the spinning solution added to the electrospinning device at different spinning time periods during the preparation of each conductive layer, and the content distribution of the lithium wetting material in the conductive layers in the direction away from the current collector are shown in table 1.

TABLE 1

The thickness of the functional layer in the lamination direction in table 1 was obtained by observation with a scanning electron microscope after preparing a sample of the negative electrode sheet using the argon ion polishing CP method.

The electrical conductivity of the conductive layer and the dielectric layer, the porosity of the functional layer, and the quality of the lithium wetting material in each of examples and comparative examples are shown in table 2 based on the quality of the functional layer.

TABLE 2

In table 2, the detection method of the relevant parameters is as follows:

1. conductivity:

A. turning on the resistivity of the four probes, calibrating the instrument by using a standard resistance method, and determining that the instrument can be normally used;

B. setting the thickness of the film according to the thickness of the sample, selecting the shape of the probe to be square, and selecting a voltage gear and a probe spacing according to the thickness and the material of the sample;

C. connecting the test probe, and making the probe contact with the sample by finely adjusting the probe platform up and down,

D. after the resistance data displayed by the four-probe resistance meter is stable, reading the conductivity data to obtain the conductivity of the sample;

2. porosity of the functional layer:

testing the porosity of the functional layer by using a mercury intrusion method according to the national standard GB/T21650.1-2008;

3. content of lithium wetting material:

according to the national standard GB/T27761-2011, the content of the tin dioxide in the material is tested by using a thermogravimetric analyzer.

Test examples

The following parameter tests were performed on the negative electrode sheet and the laminated button cell of each of the above examples and comparative examples.

1. The negative pole piece comprises:

the microscopic morphology of the negative electrode plate obtained in example 8 was observed using a scanning electron microscope, and the results are shown in fig. 6;

as can be seen from fig. 6:

fig. 6 includes a copper foil 101, a lithium metal layer 201, a dielectric layer 301, a conductive layer 401, a dielectric layer 501, and a conductive layer 601, and thus, the functional layer of the present invention is formed by laminating the dielectric layer and the conductive layer and has a three-dimensional structure;

2. lithium deposition state in the negative electrode sheet:

charging the negative electrode plate obtained in example 11 and comparative example 1 to 3.7V at a constant current of 0.2C, then charging to 0.025C at a constant voltage, standing for 5min, and discharging to 2.55V at a constant current of 0.5C; circulating for 10 times, standing for 5min, and constant-current charging to 3.7V full-charge state with 0.1C rate; preparing a sample of the negative pole piece by adopting an argon ion polishing CP method, and observing the sample by using a scanning electron microscope, wherein the results are respectively shown in fig. 7 and fig. 8;

as can be seen from a comparison of fig. 7 and 8:

in the negative electrode plate of fig. 7, the deposition of lithium in the functional layer is very dense, and the deposition amount is large; in the negative pole piece of fig. 8, the deposition of lithium ions is very loose, and the deposition amount is less; the comparison between fig. 7 and fig. 8 shows that the negative electrode plate of the present application can effectively induce lithium deposition inside the functional layer, and the lithium deposition is very dense; moreover, as can be seen from fig. 7, lithium is less likely to be deposited in a region near the negative electrode interface, and therefore, the formation of negative electrode interface lithium dendrites is suppressed; therefore, when the stacking direction is perpendicular to the extending direction of the current collector and the gradient of the lithium wetting material is gradually reduced in the stacking direction, the negative pole piece can induce lithium ions to deposit in an area close to the current collector, the probability of lithium ion deposition on a negative pole interface is reduced, and the formation of lithium dendrites on the negative pole interface is inhibited; therefore, the electrochemical device comprising the negative pole piece can show better cycle life and safety performance; this can also be confirmed by comparing the cycle count and the volumetric expansion of the button cell of example 11 with those of comparative example 1 in table 3;

3. cycle performance:

charging to 3.7V at a constant current of 0.2C rate at 20 deg.C by a blue electrochemical workstation, then charging to 0.025C at constant voltage, standing for 5min, and then discharging to 2.55V at a constant current of 0.5C; standing for 5min, which is a cycle, wherein the cycle performance is the cycle number when the discharge capacity is attenuated to 80% of the first-cycle discharge capacity, and the result is shown in table 3;

4. volume expansion ratio:

charging to 3.7V at constant current of 0.2C rate at 20 deg.C by blue electrochemical workstation, charging to 0.025C at constant voltage, standing for 5min, and discharging to 2.55V at constant current of 0.5C; standing for 5min, which is a cycle, stopping testing when the discharge capacity of the battery is attenuated to 80% of the first-cycle discharge capacity, disassembling the battery, taking out a negative pole piece, soaking the negative pole piece in a dimethyl carbonate solution, preparing a sample of the negative pole piece by adopting an argon ion polishing CP method, observing the section of the negative pole piece (vertical to the functional surface of a current collector) by using a scanning electron microscope to obtain the thickness T of the section of the negative pole piece after the cycle, wherein the section thickness of the negative pole piece before the cycle is T, and the capacity retention rate (T-T)/T, and the result is shown in Table 3;

5. coulombic efficiency:

at 20 ℃, a blue electrochemical workstation is utilized to charge the mixture to 3.7V at a constant current of 0.2C, then the mixture is charged to 0.025C at a constant voltage and stands for 5min, then the mixture is discharged to 2.55V at a constant current of 0.5C and stands for 5min, which is a cycle, the coulombic efficiency is an average value of the ratio of the discharge capacity to the charge capacity of each cycle when the capacity is attenuated to 80% of the discharge capacity of the first cycle, and the result is shown in a table 3.

TABLE 3

From table 3, it can be seen that:

1. examples 1 to 17 and 19 show that the batteries of examples have a larger volume expansion ratio with a smaller number of cycles than the comparative examples, and that the cycle performance and safety performance of the electrochemical device can be improved when the negative electrode sheet of the electrochemical device satisfies the technical scheme of the present application, because: on one hand, the dielectric layer can reduce the electron concentration of the functional layer and prevent lithium ions from forming dendrites under high current density, so that the formation of lithium dendrites is inhibited; the dielectric layer can also reduce the electron concentration on the surface of the functional layer, and lithium ions are forced to enter the functional layer for deposition, so that the formation of lithium dendrites on the interface of the negative electrode can be reduced; on the other hand, the lithium wetting material can also induce lithium ions to deposit in the functional layer, so that the formation of lithium dendrites on the interface of the negative electrode is reduced; in addition, the three-dimensional framework formed by laminating the conducting layer and the dielectric layer can reduce extrusion and expansion damage of lithium dendrites formed in the functional layer to the negative pole piece and relieve volume expansion of the negative pole piece in the circulation process; although the cycle life of example 18 is inferior to that of each comparative example in which lithium is pre-compensated, the coulombic efficiency of example 18 is not much different from that of comparative examples 1 and 2 without pre-compensation of lithium, thus demonstrating that the functional layer of the present application can suppress the formation of lithium dendrites, thereby reducing the consumption of lithium ions and contributing to the improvement of the coulombic efficiency of the electrochemical device; in contrast, comparative example 3 includes a lithium wetting material, and the presence of the lithium wetting material can further inhibit the formation of lithium dendrites and further reduce the consumption of lithium ions, compared to comparative example 1 and comparative example 2, so the coulombic efficiency of the electrochemical device of comparative example 3 is improved;

2. example 1 compared to example 3, the battery of example 3 had a greater number of cycles and a smaller volume expansion, and the improvement effect on the cycle life and safety performance of the electrochemical device was better when the content of the lithium wetting material was graded in the stacking direction, because: the lithium wetting material has higher content in the area close to the current collector, can more effectively induce lithium ions to enter the functional layer for deposition, and inhibit the formation of lithium dendrites on the negative electrode interface, so that the electrochemical device has better cycle life and safety performance; a comparison of example 4 and example 6 can also lead to the same conclusion;

3. example 9 the battery of example 11 exhibited a smaller volume expansion ratio and the safety performance of the electrochemical device was more excellent than that of example 11 because: the lithium wetting material is distributed on the conducting layer, so when the first edge layer and/or the second edge layer of the functional layer are/is the dielectric layer, lithium ions are more prone to deposit on the conducting layer inside, the formation of lithium branches on a negative electrode interface is inhibited, and the safety performance of the electrochemical device is improved;

4. example 9 the battery of example 9 had superior cycle life and safety performance compared to example 10, because the dielectric layer was effective in suppressing the formation of interfacial lithium dendrites when the layer of the functional layer closest to the negative electrode interface was the dielectric layer;

5. example 18 compared to example 15, the number of cycles of the battery obtained in example 18 was significantly greater than that of example 15, because lithium metal can replenish lithium lost during cycling when included in the functional layer of the negative electrode tab, and thus, the cycle life of the electrochemical device can be significantly improved;

6. example 19 compared with the remaining examples, the battery obtained in example 19 has a small number of cycles, a large volume expansion rate, and shows poorer cycle performance and safety performance, because the lamination direction of the functional layer is perpendicular to the extension direction of the current collector in the remaining examples, and the dielectric layer can more effectively reduce the electron concentration of the functional layer and inhibit the formation of lithium dendrites at the negative electrode interface, thereby more effectively improving the cycle life and safety performance of the electrochemical device;

7. examples 16 and 17 compared with example 13, examples 16 and 17 were inferior to example 13 in both cycle performance and safety performance because: too high a content of lithium wetting material (example 17), the structural stability of the functional layer is reduced, which is not favorable for the cycle life and safety performance improvement of the electrochemical device, and the energy density of the electrochemical device is also affected; the lithium wetting content is too low (example 16), the inductivity to lithium metal is limited, and the cycle life and the safety performance of the electrochemical device are improved to a limited extent, so that when the mass of the lithium wetting material is controlled to be 30-60% of the mass of the functional layer, the cycle life and the safety performance of the electrochemical device are improved;

8. example 20 compared with example 13, the electrochemical device obtained in example 20 was inferior in both cycle performance and safety performance because the porosity of the functional layer was low and lithium did not smoothly enter the interior of the functional layer for deposition, so lithium tended to deposit at the negative electrode interface, interfacial lithium dendrites were easily formed, and the cycle performance and safety performance of the electrochemical device were not good.

Finally, it should be noted that: the above embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A negative electrode plate, wherein the negative electrode plate comprises a functional layer; the functional layer comprises M conducting layers and N dielectric layers which are arranged in a stacked mode, and the conducting layers comprise lithium wetting materials;

wherein M is more than or equal to 1, and N is more than or equal to 1.

2. The negative electrode tab of claim 1, wherein the content of the lithium wetting material is distributed in a gradient in the stacking direction.

3. The negative electrode tab according to claim 1 or 2, wherein the negative electrode tab further comprises a lithium metal layer or a current collector, the functional layer is disposed on a functional surface of the lithium metal layer or a functional surface of the current collector, and an extending direction of the lithium metal layer or an extending direction of the current collector is perpendicular to the stacking direction;

4. The negative electrode tab of any one of claims 1-3, wherein the M conductive layers and the N dielectric layers are alternately stacked.

5. The negative electrode tab of any one of claims 1 to 4, wherein the first and/or second edge layer of the functional layer is a dielectric layer.

6. The negative electrode tab of claim 1, wherein the conductive layer has a conductivity of 10^-7To 10⁶S/cm, and/or,

the dielectric layer has a conductivity of less than 10^-8S/cm。

7. The negative electrode tab of any one of claims 1 to 6, wherein the functional layer has a porosity of 5% to 90%, optionally 50% to 90%.

8. The negative electrode tab of any one of claims 1 to 7, wherein the mass of the lithium wetting material is 5 to 95%, optionally 30 to 60%, of the mass of the functional layer.

9. The negative electrode tab of any one of claims 1 to 8, wherein the functional layer further comprises lithium metal, and the content of the lithium metal in the functional layer is 0.25mg/cm²To 25mg/cm²。

10. An electrochemical device comprising the negative electrode sheet of any one of claims 1 to 9.

11. An electronic device, wherein a drive source or an energy storage unit of the electronic device is the electrochemical device according to claim 10.