Negative electrode sheet, electrochemical device, and electronic device

Abstract

The application discloses a negative electrode sheet, an electrochemical device and an electronic device. The negative electrode sheet comprises a negative electrode current collector, a first active material layer and a second active material layer. The second active material layer is located between the negative electrode current collector and the first active material layer. The first active material layer comprises a first active material and a target compound. The target compound comprises A x B y wherein 0 The target compound improves the ion conductivity of the first active material layer, thereby improving the kinetic performance of the first active material layer, improving polarization, preventing lithium precipitation, improving the initial efficiency without losing the energy density, and without reducing the adhesion between the second active material layer and the negative electrode current collector.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and in particular to a negative electrode sheet, an electrochemical device, and an electronic device. BACKGROUND

[0002] In recent years, with the rapid development of electronic products and electric vehicles, the requirements for electrochemical devices (such as lithium ion batteries) are also increasingly high.

[0003] During the charging and discharging process of the electrochemical device, lithium ions are easily deposited on the surface of the negative electrode sheet, causing safety hazards. In order to solve the above problem, the mass content of the active material in the negative active material layer is usually reduced, thereby reducing the surface deposition of lithium. This method will cause the loss of the overall capacity of the electrode sheet and the loss of energy density. Although the current technology for improving the electrochemical device can inhibit the deposition of lithium to some extent, it is not satisfactory, and further improvement is expected. SUMMARY

[0004] The present application provides a negative electrode sheet, comprising: a negative current collector, a first active material layer, and a second active material layer.

[0005] The second active material layer is located between the negative current collector and the first active material layer.

[0006] The first active material layer comprises a first active material and a target compound, and the target compound comprises A x B y ;

[0007] Wherein, 0 < x < 4, 0 < y < 8, A comprises at least one of the group consisting of Li, Na, Mg, Ca, Zn and Cs, and B comprises at least one of the group consisting of N, S and Si.

[0008] In some embodiments, A x B y is at least one selected from the group consisting of Li3N, Li2S, Na3N, Na2S, Ca3N2, CaS, Mg3N2 and MgS.

[0009] In some embodiments, the mass content of the target compound is 0.1% to 20% based on the total mass of the first active material layer and the second active material layer.

[0010] In some embodiments, the thickness of the first active material layer is h1, the thickness of the second active material layer is h2, and 10% < h1 / (h1+h2) < 90%.

[0011] In some embodiments, 10% < h1 / (h1+h2) < 50%.

[0012] In some embodiments, the first active material has a gravimetric capacity of c1 and an average particle size of d1; the second active material layer includes a second active material, the second active material has a gravimetric capacity of c2 and an average particle size of d2; and c1 x d1 is greater than or equal to c2 x d2.

[0013] In some embodiments, the second active material layer includes a second active material, and the gravimetric capacity of the first active material is greater than or equal to the gravimetric capacity of the second active material.

[0014] In some embodiments, the second active material layer includes a second active material;

[0015] The average particle size of the first active material is greater than or equal to the average particle size of the second active material.

[0016] In some embodiments, the second active material layer includes a second active material;

[0017] In some embodiments, the second active material layer includes a second active material;

[0018] The negative electrode sheet provided in the embodiments of the present application includes a first active material layer and a second active material layer, the first active material layer includes a target compound, the ionic conductivity of the target compound is in the order of 10 -4 S / cm to 10 -2 S / cm, the presence of the target compound improves the ionic conductivity of the first active material layer, thereby improving the kinetic performance of the first active material layer, improving polarization, preventing lithium precipitation, and improving the initial efficiency. At the same time, compared with the prior art, this way does not reduce the mass content of the active material in the negative active material layer, so the energy density is not lost. BRIEF DESCRIPTION OF DRAWINGS

[0019] The above and other features, advantages, and aspects of the present disclosure will become more apparent with reference to the following detailed description in conjunction with the accompanying drawings, in which like reference numerals denote like elements. Throughout the drawings, the same or similar elements are denoted with the same or similar reference numerals. It is to be understood that the drawings are schematic, and elements and elements are not necessarily drawn to scale.

[0020] Figure 1 FIG. 1 is a schematic diagram of a negative electrode sheet according to an embodiment of the present disclosure.

[0021] FIG. 2(a) is a schematic diagram of another negative electrode sheet according to an embodiment of the present disclosure.

[0022] FIG. 2(b) is a schematic diagram of another negative electrode sheet according to an embodiment of the present disclosure.

[0023] FIG. 2(c) is a schematic view of another negative electrode tab according to an embodiment of the present disclosure.

[0024] FIG. 2(d) is a schematic view of another negative electrode tab according to an embodiment of the present disclosure.

[0025] FIG. 3(a) is a schematic view of the relationship between voltage and capacity of Example 1, Comparative Example 1 and Comparative Example 2 of the present disclosure at a discharge rate of 0.2C.

[0026] FIG. 3(b) is a schematic view of the proportion of constant current charging in the charging process of Example 1, Comparative Example 1 and Comparative Example 2 of the present disclosure at different rates. DETAILED DESCRIPTION

[0027] Embodiments of the present application will be described in more detail below. Although certain embodiments of the present application are shown, it is to be understood that the present application can be embodied by various forms and should not be interpreted as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present application will be thoroughly and completely understood. It should be understood that the embodiments of the present application are only for exemplary purposes and are not intended to limit the scope of protection of the present application.

[0028] An electrochemical device, such as a lithium ion battery, can experience lithium plating during cycling, which can result in reduced safety performance of the electrochemical device and create a safety hazard. By using a double-layer coating, lithium plating can be inhibited. In the upper layer of the double-layer coating, a graphite with fast lithium ion acceptance speed and good kinetic performance is used, and in the lower layer of the double-layer coating, a graphite with slow lithium ion acceptance speed is used. However, the graphite with fast lithium ion acceptance speed and good kinetic performance has a low capacity, and using the above two types of graphite for double-layer coating can result in a decrease in energy density. Therefore, further improvements are desired.

[0029] In an embodiment of the present application, a negative electrode tab is provided, as shown in Figure 1 The negative electrode tab includes a negative current collector 10, a first active material layer 11 and a second active material layer 12. The second active material layer 12 is located between the negative current collector 10 and the first active material layer 11.

[0030] The negative current collector 10 can be made of copper foil or other materials. In some embodiments, the negative current collector can be made of aluminum foil or steel foil.

[0031] The first active material layer 11 includes a first active material and a target compound. The first active material can be, for example, artificial graphite, natural graphite, coke, silicon oxide compounds and silicon-containing metals, etc.

[0032] In some embodiments, the average particle size of the first active material is 6 μm to 20 μm, and in other embodiments, the average particle size of the second active material is 6 μm to 20 μm. An excessively large average particle size can result in insufficient rate performance, and an excessively small average particle size can result in decreased cycle performance.

[0033] In some embodiments of the present disclosure, the target compound is included in the first active material layer 11. The target compound includes: A x B y ; wherein A includes a metal element, and B includes a non-metal element. In particular, 0 < x ≤ 4, 0 < y ≤ 8, and the valence of x × A = the valence of y × B. In some embodiments, x and y are positive integers.

[0034] In some embodiments, A includes at least one of the group consisting of Li, Na, Mg, Ca, Zn, and Cs; and B includes at least one of the group consisting of N, S, and Si. The above A x B y has good ionic conductivity, can better improve the kinetic performance of the first active material layer 11, and the ionic radius of the above A element is not less than that of Li ion, thereby ensuring the conduction performance of Li ion. In some embodiments, A x B y is obtained by decomposition of A x1 B y1 O z1 . 0 < x1 ≤ 4, 0 < y1 ≤ 4, 0 < z1 ≤ 8, and the valence of x1 × A = (the valence of y1 × B + the valence of z1 × O). In some embodiments, x and y are positive integers. In some embodiments, A x1 B y1 O z1 may include at least one of LiNO3, NaNO3, Ca(NO3)2, Mg(NO3)2, Li2SO4, or Li2SiO3. By adding the compound as shown above, the kinetic performance of the first active material layer 11 can be significantly improved, thereby preventing lithium precipitation and improving charge performance. In some embodiments, A x B y may be, for example, at least one of the group consisting of Li3N, Li2S, Na3N, Na2S, Ca3N2, CaS, Mg3N2, and MgS.

[0035] The ionic conductivity of the target compound is in the range of 10 -4 S / cm to 10 -2The target compound is present in an amount of 1% to 20% by mass in the first active material layer 11 and the second active material layer 12. If the amount of the target compound is too low, the improvement in the kinetic performance of the first active material layer 11 is not significant. If the amount of the target compound is too high, the performance is not significantly improved, but the cost is increased. The amount of the target compound can be measured by scanning electron microscopy or transmission electron microscopy to measure the distribution of each element, and the amount of the target compound can be determined based on the amount of the element.

[0036] In some embodiments, the decomposition potential of the target compound is greater than or equal to 1.95 V. In some embodiments, the target compound needs to remain relatively stable during the cycle of the negative electrode sheet and cannot be decomposed during the cycle, and thus the decomposition potential of the target compound needs to be greater than or equal to 1.95 V.

[0037] In some embodiments, the amount of the target compound is 1% to 20% by mass in the total amount of the first active material layer 11 and the second active material layer 12. If the amount of the target compound is too low, the kinetic performance of the first active material layer 11 can not be significantly improved. If the amount of the target compound is too high, the performance is not significantly improved, but the cost is increased. The amount of the target compound can be measured by scanning electron microscopy or transmission electron microscopy to measure the distribution of each element, and the amount of the target compound can be determined based on the amount of the element.

[0038] In some embodiments, the amount of the target compound is 0.1% to 20% by mass in the total amount of the first active material layer 11 and the second active material layer 12. If the amount of the target compound is too low, the improvement in the kinetic performance of the first active material layer 11 is not significant. If the amount of the target compound is too high, the cost is increased. In some embodiments, the amount of the target compound can be measured by X-ray photoelectron spectroscopy to measure the amount of the target compound in the first active material layer and the second active material layer.

[0039] The second active material layer includes a second active material. The second active material can include at least one of artificial graphite, natural graphite, a silicon oxide compound, or a silicon-containing alloy.

[0040] The second active material and the first active material can be the same kind of active material or different kinds of active material. The second active material and the first active material can be the same kind of material. For example, the second active material and the first active material are both graphite. The second active material and the first active material can be different kinds of material, for example, the second active material is graphite or silicon dioxide.

[0041] The average particle size of the second active material and the average particle size of the first active material can be the same or different. In the present application, the average particle size of the second active material and the average particle size of the first active material are considered to be the same if the difference between the average particle size of the second active material and the average particle size of the first active material is no more than 10%. In some embodiments, the average particle size of the second active material can be tested by measuring the Dv50 of the second active material using a Malvern particle size analyzer, which is the average particle size of the second active material. Similarly, the average particle size of the first active material can be tested by measuring the Dv50 of the first active material using a Malvern particle size analyzer, which is the average particle size of the first active material. In some embodiments, the average particle size of the first active material and the average particle size of the second active material can be tested as follows:

[0042] (1) A cross-section of the electrode along the thickness direction is scanned using a scanning electron microscope to obtain a SEM photo. Specifically, sampling: disassemble the electrochemical device to be tested, take out the electrode, soak in dimethyl carbonate (DMC) solution for 6 hours, and remove the residual electrolyte. Finally, dry the electrode in a drying oven; sample preparation: cut the dried electrode with a blade to obtain the cross-section of the active material layer along the thickness direction; use a hot plate to stick the test sample to paraffin, and use an ion polisher IB-195020 CCP to polish the test section until the surface is smooth to obtain the SEM test sample; test: use a scanning electron microscope (SEM) JEOL6390 to obtain the SEM photo of the active material layer of the electrochemical device to be tested. (2) In the obtained SEM photo, the average particle size of the first active material and the second active material is determined. Specifically, in the SEM photo, randomly select n (for example, 100) first active material particles, respectively determine the particle size of each first active material particle, and obtain n particle sizes. Calculate the arithmetic mean of the obtained n particle sizes, which is the average particle size of the first active material. Similarly, the average particle size of the second active material can be calculated from the SEM photo.

[0043] In the SEM photo, the particle size of a single particle (for example, a first active material particle, a second active material particle) can be determined as follows: determine the cross-sectional area of the particle; determine the diameter of a circle equal to the cross-sectional area as the particle size of the particle. In some embodiments, the particle size of the particle can be determined as follows: determine the length of the longest diagonal of the particle, determine the length of the shortest diagonal of the particle, and the particle size of the particle is the arithmetic mean of the length of the longest diagonal and the length of the shortest diagonal; in some embodiments, the length of the longest diagonal of the particle is the particle size of the particle. In some embodiments, the length of the longest side of the particle is the particle size of the particle. In some embodiments, the cross-sectional area, the longest diagonal, the shortest diagonal, and / or the longest side of the particle can be determined by scanning the cross-section of the particle using a scanning electron microscope.

[0044] In some embodiments, the average particle size of the first active material is greater than or equal to the average particle size of the second active material. In some embodiments of the present application, the average particle size of the first active material is greater than the average particle size of the second active material, in which case the specific surface area of the first active material is smaller than the specific surface area of the second active material, and the gap of the first active material layer 11 is larger, so that lithium ions are more easily introduced into the second active material layer 12 to be embedded in the second active material during charging, thereby delaying the speed of lithium embedding on the surface of the negative electrode sheet, increasing the speed of lithium embedding inside the negative electrode sheet, preventing lithium precipitation on the surface, and improving the kinetic performance.

[0045] The gram capacity of the second active material and the gram capacity of the first active material can be the same or different. In the present application, the difference between the gram capacity of the second active material and the gram capacity of the first active material is not greater than 10%, i.e., the gram capacity of the second active material and the gram capacity of the first active material are considered to be the same. In some embodiments, the gram capacity of the second active material and the gram capacity of the first active material are the same. Further, in some embodiments, the second active material and the first active material are the same kind of material with the same gram capacity. For example, the second active material and the first active material are the same kind of graphite with the same gram capacity. In some embodiments, the gram capacity of the second active material and the gram capacity of the first active material are different. For example, in some embodiments, the gram capacity of the first active material is greater than or equal to the gram capacity of the second active material. In some embodiments, because the gram capacity of the first active material is greater than or equal to the gram capacity of the second active material, the first active material can accommodate more lithium ions, thereby increasing the capacity of the negative electrode sheet, and because the target compound is contained in the first active material layer, polarization can be inhibited and the kinetic performance can be improved. In some embodiments, the gram capacity of the first active material and the gram capacity of the second active material are 330 mAh / g to 371 mAh / g.

[0046] In some embodiments, the gram capacity of the first active material is c1, the average particle size of the first active material is d1; the second active material layer includes a second active material, the gram capacity of the second active material is c2, and the average particle size of the second active material is d2; and c1xd1≥c2xd2. In some embodiments, when the first active material and the second active material satisfy the above formula, the improvement effect on the kinetic performance is more optimal.

[0047] In some embodiments, the gram capacity of the first active material is greater than or equal to the gram capacity of the second active material. In some embodiments, because the gram capacity of the first active material is greater than or equal to the gram capacity of the second active material, the first active material can accommodate more lithium ions, thereby increasing the capacity of the negative electrode sheet, and because the target compound is contained in the first active material layer, polarization can be inhibited and kinetic performance can be improved at the same time. In some embodiments, the gram capacity of the first active material and the second active material is 330 mAh / g to 371 mAh / g.

[0048] The second active material layer 12 is located between the negative electrode current collector 10 and the first active material layer 11.

[0049] The first active material layer 11 and the second active material layer 12 on the negative electrode current collector 10 can be coated in a double-layer coating manner,

[0050] In some embodiments, the double-layer coating manner can be used, the second active material layer slurry is coated on the negative electrode current collector 10 to form the second active material layer 12, and then the first active material layer slurry is coated on the second active material layer 12 to form the first active material layer 11. The additive A is added to the slurry of the first active material layer x1 B y1 O z1 The additive A is not added to the second active material layer 12 x1 B y1 O z1 The sheet, the additive A x1 B y1 O z1 For example, at least one of LiNO3, NaNO3, Ca(NO3)2, Mg(NO3)2, Li2SO4, or Li2SiO3, which has a decomposition potential of 0.3-1.0 V, can be included. A x1 B y1 O z1 During the formation of the electrochemical device, the additive A can participate in the formation of a high ionic conductivity SEI (solid electrolyte interphase) film. A x B y high ionic conductivity SEI film containing the additive A is formed in the first active material layer. Because the additive A x1 B y1 O z1 may not be completely decomposed, in some embodiments, the additive A x1 B y1 O z1 may also be contained in the first active material layer 11 of the negative electrode sheet. By forming the additive A x B yhigh ionic conductive SEI film, improves the kinetic performance of the first active material layer 11, improves the polarization of the negative electrode sheet, prevents lithium precipitation, improves the charging capacity, is conducive to improving the initial efficiency of the electrochemical device, and does not cause capacity loss. In addition, because A x1 B y1 O z1 is not added in the second active material layer 12 x1 B y1 O z1 , on the one hand, A x1 B y1 O z1 can be more fully decomposed to form an SEI film, and on the other hand, A x1 B y1 O z1 decomposition will not cause the adhesion between the second active material layer 12 and the negative electrode current collector 10 to decrease.

[0051] In some embodiments, the thickness of the first active material layer 11 is h1, the thickness of the second active material layer 12 is h2, and 10%≤h1 / (h1+h2)≤90%. In some embodiments, when the thickness of the first active material layer 11 is too low in the total thickness of the first active material layer 11 and the second active material layer 12, lithium ions can accumulate near the first active material layer 11 of the second active material layer 12, causing lithium ions to be unable to be embedded in the deeper interior of the second active material layer 12, resulting in the charging capacity of the second active material layer 12 being unable to be fully utilized. When the thickness of the first active material layer 11 is too high in the total thickness of the first active material layer 11 and the second active material layer 12, A x1 B y1 O z1The gas generated during decomposition can affect the adhesion between the second active material layer 12 and the negative electrode current collector. In some embodiments, 10%≤h1 / (h1+h2)≤50% when in this range, the performance of the electrochemical device is better. In some embodiments, a scanning electron microscope (SEM) can be used to measure the thickness of the active material layer (for example, the thickness h1 of the first active material layer, or the thickness h2 of the second active material layer). It should be understood that in some embodiments, there is no obvious boundary between the first active material layer and the second active material layer, and the thickness of the first active material layer is measured as follows: a sample of a predetermined size (for example, 5 cm x 1 cm x thickness, where the thickness is the thickness of the electrode sheet) is cut from the electrode sheet to be tested, three test points are selected in the SEM photo in a direction perpendicular to the thickness of the electrode sheet, and the thickness of the first active material layer corresponding to each of the three points is tested, denoted as h11, h12, h13, and the average of h11, h12, h13 is calculated, which is the thickness h1 of the first active material layer. Similarly, the thickness h2 of the second active material layer can be measured.

[0052] In some embodiments, the first active material layer and the second active material layer can further include a conductive agent. The conductive agent can include at least one of conductive carbon black, Ketjen black, flake graphite, graphene, carbon nanotubes, or carbon fibers.

[0053] In some embodiments, an electrochemical device is also provided, further comprising: a positive electrode sheet, a separator, and the negative electrode sheet of any of the embodiments. The separator is disposed between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer is disposed on one side or both sides of the positive electrode current collector. In some embodiments, the positive electrode current collector can be an Al foil, and of course, other positive electrode current collectors commonly used in the art can also be used. In some embodiments, the thickness of the positive electrode current collector can be 1 μm to 200 μm. In some embodiments, the positive electrode active material layer can be coated only on a partial area of the positive electrode current collector. In some embodiments, the thickness of the positive electrode active material layer can be 10 μm to 500 μm. In some embodiments, the positive electrode active material layer can further include a conductive agent. In some embodiments, the conductive agent in the positive electrode active material layer can include at least one of conductive carbon black, Ketjen black, flake graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the positive electrode active material layer has a positive electrode active material, and the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer can be 70 to 98: 1 to 15: 1 to 15. It should be understood that the above is only an example, and the active material layer of the positive electrode can use any other suitable material, thickness, and mass ratio.

[0054] In some embodiments, the separator film includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene have good effects on preventing short circuit and can improve the stability of the battery through the shutdown effect. In some embodiments, the thickness of the separator film is in a range of about 5 μm to 500 μm.

[0055] In some embodiments, the surface of the separator film can further include a porous layer disposed on at least one surface of the substrate of the separator film. The porous layer can be a polymer layer or an inorganic layer, or a layer formed by mixing polymer and inorganic matter. For example, the inorganic layer includes inorganic particles selected from at least one of aluminum oxide (Al2O3), silicon oxide (SiO2), magnesium oxide (MgO), titanium oxide (TiO2), hafnium dioxide (HfO2), tin oxide (SnO2), cerium dioxide (CeO2), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate, and a binder.

[0056] In some embodiments, the pores of the separator film have a diameter in a range of about 0.01 μm to 1 μm. The binder of the porous layer is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer of the surface of the separator film can improve the heat resistance, oxidation resistance, and electrolyte infiltration performance of the separator film, and enhance the adhesion between the separator film and the electrode sheet.

[0057] Figure 2(a) 、 2(b) Figures 2(c) and 2(d) are schematic diagrams of optional structures of the negative electrode sheet according to some embodiments of the present application. As shown in Figure 2(a), a second active material layer having a second active material is coated on the negative current collector by double-layer coating, and a first active material layer having a first active material is disposed on the second active material layer. In the first active material layer, A x1 B y1 O z1 After formation, as shown in Figure 2(b), an SEI film having high lithium ion conductivity is formed on the surface of the first active material, and the composition of the SEI film includes A x B yan inorganic layer, thereby reducing the polarization of the electrochemical reaction and improving the kinetic performance. In some embodiments, as shown in FIG. 2(c), a second active material layer having a second active material is coated on the negative current collector, and a first active material layer having a first active material is coated on the second active material layer, wherein the gravimetric capacity of the first active material is greater than the gravimetric capacity of the second active material, and A x1 B y1 O z1 The high-capacity first active material in the first active material layer is conducive to improving the capacity of the electrochemical device, and because A x1 B y1 O z1 is added to the first active material layer, after formation of the electrochemical device, the first active material layer forms a high-ionic-conductivity SEI film, thereby improving the polarization of the first active material layer and further improving the kinetic performance. In other embodiments of the present application, as shown in FIG. 2(d), a second active material layer having a second active material is coated on the negative current collector, and a first active material layer having a first active material is coated on the second active material layer, wherein the particle size of the first active material is greater than the particle size of the second active material, and A x1 B y1 O z1 In the case of a lithium ion battery, for example, during charging, the particles on the surface of the negative electrode will first intercalate ions, causing the state of charge of the particles on the surface of the negative electrode to be higher than the state of charge of the particles inside the negative electrode, resulting in the surface of the negative electrode being prone to ion precipitation. Therefore, in some embodiments of the present application, the particle size of the first active material is greater than the particle size of the second active material, and the increase in particle size reduces the reaction area of the first active material, thereby slowing the rate of increase in the state of charge of the first active material, inhibiting the precipitation of surface ions, and further improving the kinetic performance of the electrochemical device, and A x1 B y1 O z1 can be conducive to improving the polarization of the electrochemical reaction on the surface and further improving the kinetic performance.

[0058] It should be understood that Figure 2(a) , 2(b) FIGS. 2(c) and 2(d) are schematic diagrams of some embodiments of the present application, which are intended to enable a better understanding of the technical solutions provided by the present application by the skilled person, and are not used to limit the present application. In some embodiments, the actual position of the SEI film is different from that shown in the figures. For example, the SEI film can not be uniformly formed on the surface of the active material particles. In some embodiments, the SEI film further contains other substances. In some embodiments, the first active material particles and / or the second active material particles have shapes and distribution conditions different from those shown in the figures.

[0059] In some embodiments of the present application, the electrochemical device is a wound lithium ion battery or a stacked lithium ion battery.

[0060] In some embodiments, the electrochemical device can further include an electrolyte. The electrolyte can be one or more of a gel electrolyte, a solid-state electrolyte, and an electrolytic solution including a lithium salt and a non-aqueous solvent. The lithium salt is selected from one or more of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, or lithium difluoroborate. For example, the lithium salt is selected as LiPF6because it can give a high ionic conductivity and improve the cycle characteristics.

[0061] The non-aqueous solvent can be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof.

[0062] The carbonate compound can be a chain carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound, or a combination thereof.

[0063] Examples of the chain carbonate compound are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and a combination thereof. Examples of the cyclic carbonate compound are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), or a combination thereof. Examples of the fluorinated carbonate compound are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, trifluoromethyl ethylene carbonate, or a combination thereof.

[0064] Examples of the carboxylic acid ester compound are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone, methylvaleronolactone, caprolactone, methyl formate, or a combination thereof.

[0065] Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof.

[0066] Examples of other organic solvents are dimethylsulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, formamide, dimethylformamide, acetonitrile, and phosphates or combinations thereof. Exemplary phosphates can include one or more of trimethyl phosphate, triethyl phosphate, and trioctyl phosphate in combination.

[0067] Embodiments of the present application also provide an electronic device comprising the above-mentioned electrochemical device. The electronic device of embodiments of the present application is not particularly limited and can be any electronic device known in the art. In some embodiments, the electronic device can include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, and a home-use large storage battery, etc.

[0068] Some specific examples and comparative examples are listed below to better illustrate the present application, in which lithium ion batteries are used as examples. In order to facilitate the illustration of the technical effects of the present application, the difference between each example and comparative example is only in the negative electrode sheet, and the following examples are only used as illustrative examples, which should not limit the scope of protection of the present application.

[0069] Example 1

[0070] (1) Preparation of positive electrode sheet

[0071] Lithium cobalt oxide (LiCoO2), Super P (conductive carbon), and polyvinylidene fluoride were mixed in a mass ratio of 97:1.4:1.6, N-methyl pyrrolidone (NMP) was added, and the system was stirred under the action of a vacuum stirrer until it was uniform to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry was 72wt%; the positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil; the aluminum foil was dried at 85°C, then cold-pressed, cut into pieces, and then dried at 85°C under vacuum for 4h to obtain a positive electrode sheet.

[0072] (2) Preparation of negative electrode sheet

[0073] Artificial graphite, Super P, sodium carboxymethyl cellulose, and styrene butadiene rubber were mixed, deionized water was added, and a slurry of the second active material layer was obtained under the action of a vacuum stirrer; the slurry of the second active material layer was uniformly coated on the negative electrode current collector copper foil to form a second active material layer;

[0074] The artificial graphite, LiNO3, Super P, sodium carboxymethyl cellulose, butadiene rubber are mixed, wherein the LiNO3 accounts for 1% of the total mass of the mixture after mixing. Deionized water is added, and a slurry of the first active material layer is obtained under the action of a vacuum stirrer; the slurry of the first active material layer is coated on the side of the second active material layer away from the negative electrode current collector to form the first active material layer, which is dried at 85°C, and then subjected to cold pressing, cutting, and drying at 120°C for 12h under vacuum conditions to obtain a negative electrode sheet, wherein the artificial graphite used in the first active material layer and the second active material layer is the same kind of graphite, and the thickness of the first active material layer and the second active material layer accounts for 50% and 50% of the total thickness of the first active material layer and the second active material layer, respectively.

[0075] (3) Preparation of electrolyte

[0076] In a dry argon atmosphere glove box, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0077] (4) Preparation of separator film

[0078] A polyethylene (PE) separator film is selected and coated with a ceramic layer and an adhesive layer.

[0079] (5) Preparation of lithium ion battery

[0080] The positive electrode sheet, the separator film, and the negative electrode sheet are stacked in order, with the separator film between the positive electrode sheet and the negative electrode sheet to play a separating role, and then wound to obtain an electrode assembly; Al tabs are used as positive electrode tabs and Ni tabs are used as negative electrode tabs, with the tab position located in the middle of the electrode sheet to reduce ohmic polarization, and after welding the tabs, the electrode assembly is placed in an outer packaging foil aluminum plastic film, the prepared electrolyte is injected into the dried electrode assembly, and the electrode assembly is subjected to vacuum packaging, standing, formation, shaping, and capacity testing processes to obtain a lithium ion battery.

[0081] Each of the examples and comparative examples is based on the parameter changes in the preparation of the negative electrode sheet in step of Example 1, and the specific changed parameters are shown below. Compared with Example 1, in Examples 2 to 25, the content of the additive changes, and the corresponding change is the content of the active material (the content of artificial graphite). The mass percentage content of Super P, sodium carboxymethyl cellulose, and butadiene rubber in the active material layer as a whole remains unchanged.

[0082] In Example 2, LiNO3 accounts for 2% of the total mass of the mixture after mixing

[0083] Example 2 differs from Example 1 in that LiNO3 comprises 2% of the total mass of the mixture after mixing.

[0084] Example 3 differs from Example 1 in that LiNO3 comprises 4% of the total mass of the mixture after mixing.

[0085] Example 4 differs from Example 1 in that LiNO3 comprises 7% of the total mass of the mixture after mixing.

[0086] Example 5 differs from Example 1 in that LiNO3 comprises 10% of the total mass of the mixture after mixing.

[0087] Example 6 differs from Example 1 in that LiNO3 comprises 15% of the total mass of the mixture after mixing.

[0088] Example 7 differs from Example 1 in that LiNO3 comprises 20% of the total mass of the mixture after mixing.

[0089] Example 8 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 1% of the total mass of the mixture after mixing.

[0090] Example 9 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 2% of the total mass of the mixture after mixing.

[0091] Example 10 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 4% of the total mass of the mixture after mixing.

[0092] Example 11 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 7% of the total mass of the mixture after mixing.

[0093] Example 12 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 10% of the total mass of the mixture after mixing.

[0094] Example 13 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 15% of the total mass of the mixture after mixing.

[0095] Example 14 differs from Example 1 in that NaNO3 is used instead of LiNO3, and NaNO3 comprises 20% of the total mass of the mixture after mixing.

[0096] Example 15 differs from Example 1 in that Mg(NO3)2 is used instead of LiNO3, and Mg(NO3)2 comprises 1% of the total mass of the mixture after mixing.

[0097] Example 16 differs from Example 1 in that Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 2% of the total mass of the mixture after mixing.

[0098] Example 17 differs from Example 1 in that Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 4% of the total mass of the mixture after mixing.

[0099] Example 18 differs from Example 1 in that Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 7% of the total mass of the mixture after mixing.

[0100] Example 19 differs from Example 1 in that Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 10% of the total mass of the mixture after mixing.

[0101] Example 20 differs from Example 1 in that Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 15% of the total mass of the mixture after mixing.

[0102] Example 21 differs from Example 1 in that Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 20% of the total mass of the mixture after mixing.

[0103] Example 22 differs from Example 1 in that the first active material layer and the second active material layer use different types of artificial graphite, the gram capacity of the artificial graphite of the first active material layer x the average particle size of the artificial graphite of the first active material layer > the gram capacity of the artificial graphite of the second active material layer x the average particle size of the artificial graphite of the second active material layer, and Mg(N03)2is used instead of LiN03, and the Mg(N03)2is 20% of the total mass of the mixture after mixing.

[0104] Example 23 differs from Example 1 in that the first active material layer and the second active material layer use different types of artificial graphite, the gram capacity of the artificial graphite of the first active material layer x the average particle size of the artificial graphite of the first active material layer > the gram capacity of the artificial graphite of the second active material layer x the average particle size of the artificial graphite of the second active material layer, LiN03is 5% of the total mass of the mixture after mixing, and the thickness of the first active material layer and the second active material layer is 40% and 60% of the total thickness of the first active material layer and the second active material layer, respectively.

[0105] Example 24 differs from Example 1 in that the first active material layer and the second active material layer use different artificial graphites, the gram capacity of the artificial graphite of the first active material layer x the average particle size of the artificial graphite of the first active material layer ≥ the gram capacity of the artificial graphite of the second active material layer x the average particle size of the artificial graphite of the second active material layer, LiNO3 accounts for 5% of the total mass of the mixture after mixing, and the thickness of the first active material layer and the second active material layer accounts for 30% and 70% of the total thickness of the first active material layer and the second active material layer, respectively.

[0106] Example 25 differs from Example 1 in that the first active material layer and the second active material layer use different artificial graphites, the gram capacity of the artificial graphite of the first active material layer x the average particle size of the artificial graphite of the first active material layer ≥ the gram capacity of the artificial graphite of the second active material layer x the average particle size of the artificial graphite of the second active material layer, LiNO3 accounts for 5% of the total mass of the mixture after mixing, and the thickness of the first active material layer and the second active material layer accounts for 20% and 80% of the total thickness of the first active material layer and the second active material layer, respectively.

[0107] Comparative Example 1 differs from Example 1 in that only the second active material layer is coated on the copper foil in Comparative Example 1, and the first active material layer is not coated.

[0108] Comparative Example 2 differs from Example 1 in that only the first active material layer is coated on the copper foil in Comparative Example 2, and the second active material layer is not coated.

[0109] Comparative Example 3 differs from Example 1 in that the artificial graphite in the first active material layer and the artificial graphite in the second active material layer in Comparative Example 3 are different types of artificial graphites, LiNO3 is not added to the first active material layer, and the gram capacity of the artificial graphite of the first active material layer x the average particle size of the artificial graphite of the first active material layer ≥ the gram capacity of the artificial graphite of the second active material layer x the average particle size of the artificial graphite of the second active material layer.

[0110] The test methods for the various parameters of the present application are described below.

[0111] Initial efficiency test:

[0112] The capacity discharged in the first cycle / the amount of electricity charged in the first cycle of the lithium ion battery after the first cycle of charging and formation is the initial efficiency. The specific parameters of the formation are as follows: the battery is warmed up to 60℃, then charged at a rate of 0.5C (2h full charge) for 100min, and then cooled to 25℃.

[0113] Cycle test:

[0114] The lithium ion battery after formation was charged at 3C constant current to 4.2V at 25℃, then charged at constant voltage to the current of 0.05C, and after standing for 5min, discharged at 1C to 2.8V, and the above charging and discharging was repeated for 1000 cycles; the discharge capacity of the first cycle was recorded as D0, and the discharge capacity of the 1000th cycle was recorded as D1;

[0115] 1000 cycle capacity retention rate (%) = D1 / D0 x 100%.

[0116] 25℃ 3C constant current charging ratio:

[0117] The proportion of the constant current charging capacity in the total charging capacity during the charging process is the 3C constant current charging ratio at 25℃, which is used to represent polarization. The smaller the polarization of the lithium ion battery, the higher the 3C constant current charging ratio at 25℃ will be.

[0118] The performance test results of the lithium ion batteries in Examples 1 to 25 and Comparative Examples 1 to 3 are shown in Table 1.

[0119] Table 1

[0120]

[0121]

[0122] As can be seen from Comparative Examples 1 to 25 and Comparative Examples 1 to 3, the initial efficiency, 1000 cycle capacity retention rate and 3C constant current charging ratio at 25℃ of Examples 1 to 25 are all higher than those of Comparative Examples 1 to 3. This is because no double coating is adopted in Comparative Example 1, and no LiNO3 additive is added. The additive is a substance that can decompose and generate a target compound after the electrochemical device is charged and discharged, so lithium ions are easy to accumulate on the surface of the negative electrode plate, causing lithium precipitation, resulting in capacity loss, reducing the initial efficiency, and deteriorating the kinetic performance, causing the cycle performance and kinetic performance to decrease. Although LiNO3 is added in Comparative Example 2, no double coating is adopted, which can improve the kinetic performance to some extent, but the gas generated during the decomposition of LiNO3 causes the adhesion between the active material layer and the negative electrode plate to decrease, and because LiNO3 is distributed on the surface and inside of the active material layer, the internal LiNO3 is not fully decomposed, and the overall decomposition of LiNO3 is not uniform, which causes the initial efficiency loss and cycle performance decrease. No LiNO3 compound is added in Comparative Example 3, so lithium precipitation is easy to occur, resulting in insufficient initial efficiency, cycle performance and kinetic performance. Double coating is adopted in Examples 1 to 25, and A x1 B y1 O z1, the first active material layer is located on the upper layer of the second active material layer, so A x1 B y1 O z1 can be fully decomposed, and A x B y , the ion conductivity of A x B y is higher than 10 -3 mS cm -1 , thereby improving the ion conductivity of the first active material layer, so that the lithium ions can be transmitted through the first active material layer to the second active material layer located inside the negative electrode tab more quickly compared to the case where A x B y is not contained in the first active material layer, thereby preventing the accumulation of lithium ions in the first active material layer, inhibiting lithium precipitation, and improving the kinetic performance of the first active material layer and the cycle performance. In addition, since A x1 B y1 O z1 is added to the raw material of the first active material layer, A x1 B y1 O z1 generates gas during decomposition, thereby improving the porosity of the first active material layer and further improving the kinetic performance of the first active material layer. The first active material layer is on the second active material layer and away from the current collector, and the second active material layer does not contain A x1 B y1 O z1 , so the decomposition of A x1 B y1 O z1 will not affect the adhesion between the second active material layer and the current collector. Therefore, in some embodiments of the present application, the negative electrode tab is defined to include the first active material layer and the second active material layer, and A x B y is contained in the first active material layer.

[0123] As can be seen from Comparative Examples 1 to 7, as the amount of LiNO3 added to the raw material of the first active material layer increases, the initial efficiency of the lithium ion battery gradually increases, the capacity retention rate at 1000 cycles fluctuates within a certain range, and the 3C constant current charge at 25°C increases first and then decreases slightly. This may be because as the amount of LiNO3 increases, the ion conductivity of the first active material layer gradually increases, thereby facilitating the improvement of the initial efficiency and reducing the polarization. The addition of LiNO3 can improve the cycle performance, but when the amount reaches a certain degree, the change in the amount has no obvious effect on the cycle performance. Examples 8 to 14 and Examples 15 to 21 show similar characteristics, and it can be seen that the addition of different types of A x B yAll can achieve improvement effect.

[0124] As can be seen from Comparative Example 21 and Example 22, the initial efficiency, 1000 cycle performance and 3C constant current charging ratio of Example 22 are all better than those of Example 21, so when the gram capacity c1 of the first active material, the average particle size d1 of the first active material, the gram capacity c2 of the second active material and the average particle size d2 of the second active material satisfy c1xd1≥c2xd2, the initial efficiency, cycle performance of the lithium ion battery can be further improved and polarization can be inhibited. When the gram capacity of the first active material is equal to the gram capacity of the second active material, the particle size of the first active material is larger than that of the second active material, and the large particles in the upper layer can reduce the tortuosity of the upper layer channel, thereby improving the migration speed of lithium ions and reducing the unevenness of lithium intercalation in the direction of the pole piece. If the particle sizes of the first active material and the second active material are similar (for example, equal), the gram capacity of the first active material is larger, and more lithium can be intercalated, so the real potential drops slowly during the constant current charging process, thereby making the upper layer less likely to precipitate lithium, improving the cycle performance and inhibiting polarization.

[0125] As can be seen from Comparative Example 23 to Example 25, as the ratio of the thickness of the first active material layer to the total thickness of the first active material layer and the second active material layer gradually decreases, the initial efficiency of the lithium ion battery gradually decreases, the cycle performance and 3C constant current charging ratio at 25°C fluctuate within a certain range, and the overall performance of the lithium ion battery remains in a good range. Therefore, when the ratio of the thickness of the first active material layer to the total thickness of the first active material layer and the second active material layer is within the range of 20% to 50%, good performance can be maintained.

[0126] FIG. 3(a) is a schematic diagram of the voltage and capacity relationship of the lithium ion battery of Example 1 of the present disclosure, Comparative Example 1 and Comparative Example 2 under 0.2C rate discharge. FIG. 3(b) is a schematic diagram of the constant current charging proportion (CC proportion) of the lithium ion battery of Example 1 of the present disclosure, Comparative Example 1 and Comparative Example 2 under different rate conditions. As shown in FIG. 3(a), the capacity of the lithium ion battery of Example 1 of the present application is greater than that of Comparative Example 1 and Comparative Example 2. As can be seen from FIG. 3(b), the constant current charging proportion of Example 1 is higher than that of Comparative Example 1 and Comparative Example 2 under different rate conditions, because the first active material layer and the second active material layer are prepared by double-layer coating in Example 1 of the present application, and A x1 B y1 O z1 additive, A x B yThe kinetic performance is improved, and the adhesion between the second active material layer and the negative electrode current collector is not reduced, the improvement of the kinetic performance improves the charge and discharge performance, lithium ions can be better embedded in the negative electrode, thus increasing the proportion of constant current charging and reducing the charging time. Due to the improvement of the kinetic performance, the polarization and lithium precipitation are reduced, and the capacity loss caused by lithium precipitation is inhibited.

[0127] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

1. A negative electrode sheet, characterized by, The negative electrode sheet includes: a negative current collector, a first active material layer, and a second active material layer; the second active material layer is located between the negative current collector and the first active material layer; The first active material layer includes a first active material, A x1 B y1 O z1 and a target compound in a solid electrolyte interface film on the surface of the first active material, the target compound including: A x B y ; wherein, 0 < x ≤ 4, 0 < y ≤ 8, A includes a metal element, the metal element includes at least one of a combination consisting of Na, Mg, Ca, Zn, and Cs, B includes a non-metal element, the non-metal element includes at least one of a combination consisting of N and S, 0 < x1 ≤ 4, 0 < y1 ≤ 4, 0 < z1 ≤ 8; a gram capacity of the first active material is c1, an average particle size of the first active material is d1; the second active material layer includes a second active material, a gram capacity of the second active material is c2, an average particle size of the second active material is d2; and the first active material and the second active material are the same kind of active material with different gram capacities, c1×d1 > c2×d2.

2. The negative electrode sheet according to claim 1, characterized by A x B y at least one selected from the group consisting of Na3N, Na2S, Ca3N2, CaS, Mg3N2, and MgS.

3. The negative electrode sheet according to claim 1, wherein a mass content of the target compound based on a total mass of the first active material layer and the second active material layer is 0.1% to 20%.

4. The negative electrode sheet according to claim 1, wherein a thickness of the first active material layer is h1, a thickness of the second active material layer is h2, and 10% ≤ h1 / (h1+h2) ≤ 90%.

5. The negative electrode sheet according to claim 4, characterized by 10% ≤ h1 / (h1+h2) ≤ 50%.

6. The negative electrode sheet according to claim 1, wherein a gram capacity of the first active material is greater than or equal to a gram capacity of the second active material.

7. The negative electrode sheet according to claim 1, wherein an average particle size of the first active material is greater than or equal to an average particle size of the second active material.

8. An electrochemical device, characterized by, The electrochemical device includes a positive electrode sheet, a separator, and the negative electrode sheet according to any one of claims 1 to 7, the separator is located between the positive electrode sheet and the negative electrode sheet.

9. An electronic device, comprising: The electrochemical device includes the electrochemical device according to claim 8.

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