Negative electrode sheet and manufacturing method thereof, electrode assembly, and secondary battery

Abstract

The present application provides a negative electrode plate, comprising: a current collector; a first active material layer, disposed on at least one surface of the current collector; and a second active material layer, disposed on a surface of the first active material layer away from the current collector; the first active material layer comprises a first active material, the second active material layer comprises a second active material, and the second active material has a higher lithium insertion potential and a higher lithium extraction potential than the first active material. The negative electrode plate according to the present application can improve the safety and cycle characteristics of a secondary battery.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular to a negative electrode plate and a method for manufacturing the negative electrode plate, an electrode assembly having the negative electrode plate, a secondary battery, a battery module, a battery pack and an electrical device. Background Art

[0002] Secondary batteries are widely used due to their reliable performance, no pollution, and no memory effect. For example, as environmental protection issues are increasingly valued and new energy vehicles become more popular, the demand for power secondary batteries will show explosive growth. However, as the application scope of secondary batteries becomes wider and wider, the performance of secondary batteries is also severely challenged.

[0003] However, for secondary batteries, as the energy density of the designed battery increases, some parts of the negative electrode plate are prone to lithium deposition due to insufficient lithium insertion capacity. As lithium dendrites continue to form, the separator is punctured, causing a short circuit in the battery and reducing the safety performance of the battery. Therefore, it is urgent to design and develop a secondary battery that does not reduce the energy density of the battery and can effectively reduce the risk of lithium deposition. Summary of the invention

[0004] The present application is made in view of the above-mentioned problems, and its purpose is to provide a negative electrode plate and its manufacturing method, a secondary battery equipped with the same, as well as a battery module, a battery pack and an electrical device. The negative electrode plate is partially double-coated with different active materials, which can reduce the risk of lithium plating of the secondary battery, improve the safety performance of the secondary battery, and extend the service life of the battery without reducing the energy density of the battery.

[0005] In order to achieve the above-mentioned objectives, the first aspect of the present application is to provide a negative electrode plate, including: a current collector; a first active material layer, arranged on at least one surface of the current collector; and a second active material layer, arranged on the surface of the first active material layer away from the current collector; the first active material layer includes a first active material, the second active material layer includes a second active material, and the lithium insertion potential and the lithium deintercalation potential of the second active material are both higher than those of the first active material.

[0006] By locally coating a second active material layer with high lithium insertion and delithiation potentials on the first active material layer, the negative electrode lithium plating of the secondary battery can be improved, the safety performance of the secondary battery can be improved, the battery life can be extended, and the energy density of the battery will not be reduced.

[0007] In some embodiments, there are multiple second active material layers, and the multiple second active material layers are arranged at intervals. Thus, the second active material layer can be selectively selected to coat the position where lithium is easily deposited, and the safety risk of lithium deposition caused by insufficient lithium insertion capacity of the position where lithium is easily deposited in the secondary battery can be addressed, thereby improving the safety performance, cycle characteristics and energy density of the secondary battery.

[0008] In some embodiments, when the lithium insertion capacity of the negative electrode sheet is in the range of 0 to 300 mAh / g, the lithium insertion potential of the second active material is higher than that of the first active material. By making the lithium insertion potentials of the second active material and the first active material satisfy the above relationship within a specific lithium insertion capacity range, it is ensured that the lithium from the positive electrode material can be firstly inserted into the second active material, thereby reducing the risk of lithium over-insertion of the second active material, reducing the risk of lithium plating at the negative electrode of the secondary battery, and improving the safety performance of the secondary battery.

[0009] In some embodiments, when the delithiation capacity of the negative electrode is in the range of 10 to 300 mAh / g, the delithiation potential of the second active material is higher than that of the first active material. By making the delithiation potentials of the second active material and the first active material satisfy the above relationship within a specific lithium insertion capacity range, the lithium ions embedded in the second active material can be only partially or not removed during the subsequent charge and discharge process of the battery, and the unremoved lithium ions will not be able to be reduced and generate lithium dendrites, that is, the second active material can enrich and lock the lithium ions that may have been reduced to generate lithium dendrites on the surface of the negative electrode, thereby improving the lithium precipitation of the secondary battery due to insufficient local lithium insertion capacity of the negative electrode, and improving the safety performance of the secondary battery.

[0010] In some embodiments, the gram capacity of the second active material is higher than that of the first active material, thereby further increasing the lithium insertion capacity of the negative electrode, improving the lithium precipitation improvement effect, and improving the safety performance of the secondary battery.

[0011] In some embodiments, the ratio of the mass of the second active material to the mass of the first active material is 0.1 to 2, and can be 0.1 to 1. Thus, the second active material layer is coated with less second active material, and the change in the mass of the secondary battery after adding the second active material layer is negligible, which can improve the safety performance of the secondary battery without reducing the energy density of the secondary battery.

[0012] The second aspect of the present application is to provide an electrode assembly, comprising a negative electrode sheet, a positive electrode sheet and a separator in any of the above embodiments; the positive electrode sheet comprises a positive electrode active material layer; wherein the capacity a of the second active material layer, the capacity b of the first active material layer and the capacity c of the positive electrode active material layer satisfy the formula (1):

[0013] a+b=nc, n=1.07~5 Formula (1)

[0014] The units of a, b and c in formula (1) are mAh / cm 2 .

[0015] The capacity a of the second active material layer, the capacity b of the first active material layer and the capacity c of the positive electrode active material layer can further increase the lithium insertion capacity of the negative electrode, improve the negative electrode lithium plating of the secondary battery, and improve the safety performance and cycle characteristics of the secondary battery.

[0016] In some embodiments, the negative electrode sheet, the separator, and the positive electrode sheet are wound into a winding structure along a winding direction; the winding structure includes a plane portion and a corner portion connecting the plane portion; and the second active material layer is at least partially located at the corner portion. Thus, lithium deposition at the corner portion of the secondary battery can be improved, thereby improving the safety of the secondary battery.

[0017] In some embodiments, the corner portion includes a first corner portion and a second corner portion close to the winding start end of the electrode assembly; the second active material layer is located at the first corner portion and the second corner portion. Thus, lithium deposition at the inner corner of the secondary battery can be further improved, thereby improving the safety of the secondary battery.

[0018] In some embodiments, the current collector of the negative electrode plate further includes a lug portion; and the second active layer is also located on a side of the planar portion connected to the lug portion. Thus, lithium deposition near the lug portion of the negative electrode plate can be improved, thereby improving the safety of the secondary battery.

[0019] In some embodiments, the negative electrode sheet, the separator, and the positive electrode sheet are stacked in sequence to form a laminate structure; the laminate structure includes a center portion and an edge portion disposed around the center portion; and the second active material layer is at least partially located at the edge portion. Thus, lithium deposition at the edge of the negative electrode sheet can be improved, thereby improving the safety of the secondary battery.

[0020] In some embodiments, the current collector of the negative electrode plate further includes a lug portion; and the second active material layer is also located on a side of the edge portion connected to the lug portion. Thus, lithium deposition near the lug portion of the negative electrode plate can be improved, thereby improving the safety of the secondary battery.

[0021] The third aspect of the present application is to provide a method for manufacturing a negative electrode sheet, which is used to manufacture the negative electrode sheet in any of the above embodiments. According to the method for manufacturing a negative electrode sheet of the present application, a double-layer coating method is adopted, which has high process controllability, flexibility and simplicity. For negative electrode materials of different systems, the double-layer coating method is used to form a composite negative electrode to avoid the process difficulties caused by differences in material systems.

[0022] A fourth aspect of the present application is to provide a secondary battery, which includes the electrode assembly according to the second aspect of the present application.

[0023] A fifth aspect of the present application is to provide a battery module, which includes the secondary battery according to the fourth aspect of the present application.

[0024] A sixth aspect of the present application is to provide a battery pack, the battery module comprising the battery module according to the fifth aspect of the present application.

[0025] The seventh aspect of the present application is to provide an electrical device, which includes at least one of the secondary battery according to the fourth aspect of the present application, the battery module according to the fifth aspect of the present application, and the battery pack according to the sixth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a cross-sectional schematic diagram of a negative electrode sheet according to one embodiment of the present application.

[0027] Figure 2 It is a schematic top view of a negative electrode sheet according to one embodiment of the present application.

[0028] Figure 3 It is a schematic top view of an electrode assembly according to one embodiment of the present application.

[0029] Figure 4 yes Figure 3 A detailed schematic diagram of a corner portion of an electrode assembly according to an embodiment of the present application is shown.

[0030] Figure 5 It is a schematic top view of the negative electrode sheet of the electrode assembly according to one embodiment of the present application.

[0031] Figure 6 It is a schematic top view of an electrode assembly according to one embodiment of the present application.

[0032] Figure 7 yes Figure 6 A schematic top view of a negative electrode sheet of an electrode assembly according to an embodiment of the present application is shown.

[0033] Figure 8 is a schematic diagram of a secondary battery according to one embodiment of the present application.

[0034] Fig. 9 yes Figure 8 An exploded view of a secondary battery according to an embodiment of the present application is shown.

[0035] Fig.10 is a schematic diagram of a battery module according to an embodiment of the present application.

[0036] Fig.11 It is a schematic diagram of a battery pack according to one embodiment of the present application.

[0037] Fig.12 yes Fig.11 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0038] Fig.13 FIG. 1 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present application as a power source.

[0039] Fig.14 1 is a lithium insertion and extraction curve diagram of the first active material and the second active material according to one embodiment of the present application.

[0040] Description of reference numerals:

[0041] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly; 54 connecting member; 55 electrode terminal; 6 positive electrode sheet; 7 negative electrode sheet; 9 separator; 100 current collector; 101 first active material layer; 102 second active material layer; 111 plane portion; 112 corner portion; 112a first corner portion; 112b second corner portion; 100a pole ear portion; DETAILED DESCRIPTION

[0042] Below, the embodiments of the positive electrode active material and its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack and electrical device of the present application are specifically disclosed with appropriate reference to the drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0043] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a particular parameter, it is understood that a range of 60 to 110 and 80 to 120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In the present application, unless otherwise specified, the numerical range "a to b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0044] If not otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0045] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0046] If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0047] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0048] At present, from the perspective of market development, the application of power batteries is becoming more and more extensive. Power batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, as well as military equipment and aerospace and other fields. With the continuous expansion of the application field of power batteries, the market demand is also constantly expanding.

[0049] The inventors have noticed that in the actual use of power batteries, lithium deposition is prone to occur in some parts of the negative electrode due to insufficient lithium insertion capacity. As lithium dendrites continue to form, the isolation membrane is punctured, causing a short circuit in the battery and reducing the safety performance of the battery.

[0050] In order to alleviate the lithium deposition of the negative electrode sheet and take into account the problem of improving the energy density of the battery, the inventor has designed a negative electrode sheet after in-depth research, which includes: a current collector, a first active material layer disposed on at least one surface of the current collector, and a second active material layer disposed on the surface of the first active material layer away from the current collector. The first active material layer includes a first active material, and the second active material layer includes a second active material, and the lithium insertion potential and lithium de-lithiation potential of the second active material are both higher than those of the first active material.

[0051] The above-mentioned negative electrode plate is coated with a second active material layer with high lithium insertion potential and lithium desorption potential at a position where lithium is easily deposited, which can improve the negative electrode lithium deposition of the secondary battery, improve the safety performance of the secondary battery, extend the battery life, and does not reduce the energy density of the battery.

[0052] The secondary battery disclosed in the embodiment of the present application can be used in, but not limited to, electrical devices such as vehicles, ships or aircraft. The embodiment of the present application provides an electrical device using a secondary battery as a power source, and the electrical device can be, but not limited to, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, a battery car, an electric car, a ship, a spacecraft, and the like. Among them, the electric toy can include a fixed or mobile electric toy, for example, a game console, an electric car toy, an electric ship toy, an electric airplane toy, and the like, and the spacecraft can include an airplane, a rocket, a space shuttle, a spacecraft, and the like.

[0053] [Pole structure]

[0054] like Figure 1 , 2 As shown, in one embodiment of the present application, a negative electrode plate 7 is proposed, including: a current collector 100; a first active material layer 101, arranged on at least one surface of the current collector 100; and a second active material layer 102, arranged on the surface of the first active material layer 101 away from the current collector 100; the first active material layer 101 includes a first active material, and the second active material layer 102 includes a second active material, and the lithium insertion potential and the lithium desorption potential of the second active material are both higher than those of the first active material.

[0055] The first active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, and the like.

[0056] The inventors of the present application have found that by locally coating the second active material layer 102 with high lithium insertion and removal potentials on the first active material layer 101, lithium can be first embedded in the second active material coated on the upper layer, and the lithium embedded in the second active material can be prevented from being removed and re-embedded in the first active material to cause lithium precipitation, thereby improving negative electrode lithium precipitation and improving the safety and cycle stability of the secondary battery. In addition, the overall increase in the lithium insertion capacity of the negative electrode also increases the gram capacity of the positive electrode of the battery and improves the energy density of the battery.

[0057] In some embodiments, there are multiple second active material layers 102, and the multiple second active material layers 102 are arranged at intervals. Thus, the second active material layer can be selectively coated on a position where lithium is easily deposited, which can improve the lithium deposition of the negative electrode of the secondary battery and improve the cycle characteristics and energy density of the secondary battery.

[0058] “Interval arrangement” means that there is a gap between at least two of the plurality of second active material layers. According to actual production needs, the spacing between any two adjacent second active material layers may be equal or unequal, and this application does not limit this.

[0059] In some embodiments, within the range of 0 to 300 mAh / g of the lithium insertion capacity of the negative electrode plate 7, the lithium insertion potential of the second active material is higher than that of the first active material. By making the lithium insertion potentials of the second active material and the first active material satisfy the above relationship within a specific lithium insertion capacity range, it is ensured that the lithium from the positive electrode material can be firstly inserted into the second active material, thereby improving the lithium insertion capacity of the negative electrode, reducing the risk of lithium plating at the negative electrode of the secondary battery, and improving the safety performance of the secondary battery.

[0060] In some embodiments, within the range of 10 to 300 mAh / g of the lithium desorption capacity of the negative electrode plate 7, the lithium desorption potential of the second active material is higher than that of the first active material. By making the lithium desorption potentials of the second active material and the first active material satisfy the above relationship within a specific lithium insertion capacity interval, the lithium ions embedded in the second active material can be only partially or not extracted during the subsequent charge and discharge process of the battery, and the unextracted lithium ions will not be able to be reduced and generate lithium dendrites, that is, the second active material can enrich and lock the lithium ions that may have been reduced to generate lithium dendrites on the surface of the negative electrode plate, thereby improving the lithium precipitation of the secondary battery due to insufficient local lithium insertion capacity of the negative electrode, and improving the safety performance of the secondary battery.

[0061] In some embodiments, the second active material has a higher specific capacity than the first active material. Specifically, the second active material has a higher specific capacity than the first active material at a potential > 0.5 V (vs Li / Li +) potential interval is higher than that of the first active material. Thus, the lithium insertion capacity of the negative electrode can be further increased, the lithium deposition of the secondary battery caused by insufficient local lithium insertion capacity of the negative electrode can be improved, and the safety performance of the secondary battery can be improved. The second active material includes but is not limited to one or more of transition metal oxides, lithium titanate, etc.

[0062] In some embodiments, the ratio of the mass of the second active material to the mass of the first active material is 0.1 to 2, and can be 0.1 to 1. Thus, the second active material layer is coated with less second active material, and the change in the mass of the secondary battery after adding the second active material layer is negligible, which can improve the safety performance of the secondary battery without reducing the energy density of the secondary battery.

[0063] In addition, the negative electrode active material of the present application adopts a double-layer coating method, which has high process controllability, flexibility and simplicity. For negative electrode materials of different systems, the double-layer coating method is used to form a composite negative electrode to avoid process difficulties caused by differences in material systems.

[0064] [Electrode assembly]

[0065] like Figures 3 to 6 As shown, in one embodiment of the present application, an electrode assembly 52 is provided, comprising a negative electrode plate 7, a positive electrode plate 6 and a separator 9; the positive electrode plate 7 comprises a positive electrode active material layer; wherein the capacity a of the second active material layer, the capacity b of the first active material layer and the capacity c of the positive electrode active material layer satisfy the formula (1):

[0066] a+b=nc, n=1.07~5 Formula (1)

[0067] The unit of a, b and c in the formula (1) is mAh / cm2.

[0068] Here, the capacity is calculated as follows: Capacity = active material gram capacity (mAh / g) * mass of active material in the active material layer (g / cm2)

[0069] When n is less than 1.07, the lithium insertion capacity provided by the second active material is insufficient and cannot effectively reduce the risk of lithium plating. When n is greater than 5, although the risk of lithium plating of the secondary battery can be reduced, the second active material is enriched with excessive lithium, which can easily cause lithium loss. At the same time, the battery mass increases, resulting in a decrease in the battery energy density.

[0070] By keeping the positive and negative electrode capacity ratio within the above range, the lithium insertion capacity of the negative electrode can be further increased, the negative electrode lithium plating of the secondary battery can be improved, and the safety performance, cycle characteristics and energy density of the secondary battery can be improved.

[0071] like Figure 3As shown, the electrode assembly 52 may be a wound structure. Specifically, the positive electrode sheet 6 and the negative electrode sheet 7 are each one, and the positive electrode sheet 6 and the negative electrode sheet 7 are strip-shaped structures. The positive electrode sheet 6, the separator 9 and the negative electrode sheet 7 are stacked in sequence and wound more than two times to form the electrode assembly 52. ​​The electrode assembly 52 may be flat.

[0072] In some embodiments, Figure 3 As shown, the negative electrode sheet 7, the separator 9 and the positive electrode sheet 6 are wound into a winding structure along the winding direction; the winding structure includes a plane portion 111 and a corner portion 112 connected to the plane portion 111; the second active material layer 102 is at least partially located at the corner portion 112. Thus, lithium deposition at the corner of the secondary battery can be improved, thereby improving the safety of the secondary battery.

[0073] In some embodiments, Figure 4 As shown, the corner portion 112 includes a first corner portion 112a and a second corner portion 112b close to the winding start end of the electrode assembly; the second active material layer 102 is located at the first corner portion 112a and the second corner portion 112b. Thus, lithium deposition at the inner corner of the secondary battery can be further improved.

[0074] In some embodiments, Figure 5 As shown, the current collector 100 of the negative electrode sheet 7 also includes a pole ear portion 100a; the second active material layer 102 is also located on the side of the plane portion 111 connected to the pole ear portion 100a. Thus, lithium deposition near the negative pole ear portion of the secondary battery can be improved, thereby improving the safety of the secondary battery.

[0075] Alternatively, if Figure 6 As shown, the electrode assembly 52 may also be a laminated structure. Specifically, a plurality of positive electrode sheets 6 and a plurality of negative electrode sheets 7 are provided, the plurality of positive electrode sheets 6 and negative electrode sheets 7 are alternately stacked, and the separator 9 separates the positive electrode sheets 6 and the negative electrode sheets 7.

[0076] In some embodiments, Figure 7 As shown, the negative electrode sheet 7, the separator 9 and the positive electrode sheet 9 are stacked in sequence to form a laminate structure; the laminate structure includes a central portion and an edge portion arranged around the central portion; the second active material layer is at least partially located at the edge portion. Thus, lithium deposition at the edge of the negative electrode sheet without an active material layer at the position opposite to the positive electrode sheet of the secondary battery can be improved.

[0077] In some embodiments, Figure 7 As shown, the current collector of the negative electrode sheet 7 also includes a pole ear portion 100a; the second active material layer 102 is also located on the side of the edge portion connected to the pole ear portion. Thus, lithium deposition near the negative pole ear portion of the secondary battery can be improved, thereby improving the safety of the secondary battery.

[0078] [Secondary battery]

[0079] Figure 8 A secondary battery 5 of a square structure is shown. In the present application, the secondary battery includes a lithium-ion battery, a lithium-sulfur battery, a sodium-lithium-ion battery, a sodium-ion battery or a magnesium-ion battery, etc. The concept of the present application will be described below mainly by taking a lithium-ion battery as an example. It should be understood that any other appropriate type of rechargeable battery is applicable.

[0080] Reference Fig. 9 , the secondary battery 5 of one embodiment of the present application includes a shell (including a shell 51 and a cover plate 53), an electrode assembly 52. ​​Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The present application has no particular restriction on the shape of the shell 51, which may be cylindrical, square or any other shape. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 may be one or more, and those skilled in the art can select according to specific actual needs.

[0081] There are two electrode terminals 55 disposed on the cover plate 53 . There are two connecting members 54 , one connecting member 54 connects one electrode terminal 55 and the positive electrode sheet 6 of the electrode assembly 52 , and the other connecting member 54 connects the other electrode terminal 55 and the negative electrode sheet 7 of the electrode assembly 52 .

[0082] In addition, the secondary battery, battery module, battery pack, and electric device of the present application will be described below with reference to the drawings as appropriate.

[0083] In one embodiment of the present application, a secondary battery is provided.

[0084] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is set between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0085] [Positive electrode]

[0086] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

[0087] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0088] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0089] In some embodiments, the positive electrode active material may adopt a positive electrode active material for a battery that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al0.05 O2) and its modified compounds, etc. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0090] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0091] In some embodiments, the positive electrode film layer may further include a conductive agent, which may include, for example, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0092] In some embodiments, the positive electrode sheet can be prepared in the following manner: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0093] [Negative electrode]

[0094] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0095] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0096] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0097] In some embodiments, the negative electrode first active material may adopt a negative electrode active material for a battery known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0098] In some embodiments, the second negative electrode active material may include at least one of the following materials: transition metal oxides (eg. Fe2O3, Fe3O4, Mn2O3, Co3O4, NiO, ZnO), lithium-containing transition metal oxides (eg. Li4Ti5O12), etc. One or more of them, it can be understood that any one of the above substances alone or any multiple substances are mixed as the second negative electrode active material layer material, it can be achieved that the lithium insertion potential is higher than the lithium insertion potential of the first active material in the range of 0 to 300 mAh / g of the negative electrode lithium insertion capacity, and the lithium desorption potential is higher than the lithium desorption potential of the first active material in the range of 10 to 300 mAh / g of the negative electrode lithium desorption capacity. Preferably, the gram capacity of the second negative electrode active material layer material may be greater than that of the first negative electrode active material layer material. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0099] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl chitosan (CMCS), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0100] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0101] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0102] In some embodiments, a negative electrode sheet containing a first active material can be prepared in the following manner: the components for preparing a negative electrode slurry containing a first active material, such as a first negative electrode active material, a conductive agent, a binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry of the first active material; the negative electrode slurry is coated on a negative electrode collector, and after drying, cold pressing and other processes, a negative electrode sheet containing a first active coating can be obtained.

[0103] In some embodiments, a negative electrode sheet comprising the first and second active materials can be prepared in the following manner: the components for preparing the second active material negative electrode slurry, such as the second active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a second active material negative electrode slurry; the second active material negative electrode slurry is coated on the negative electrode sheet comprising the first active coating, and after drying and other processes, a positive electrode sheet can be obtained.

[0104] [Electrolytes]

[0105] The electrolyte plays a role in conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs.

[0106] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0107] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0108] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0109] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0110] [Isolation film]

[0111] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0112] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0113] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0114] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0115] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0116] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape.

[0117] In some embodiments, secondary batteries may be assembled into a battery module. The number of secondary batteries contained in the battery module may be one or more, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery module.

[0118] Fig.10 4 is an example of a battery module. Fig.10 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0119] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0120] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

[0121] Fig.11and Fig.12 1 is a battery pack 1 as an example. Fig.11 and Fig.12 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0122] In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0123] As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0124] Fig.13 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the high power and high energy density requirements of the power consumption device for the secondary battery, a battery pack or a battery module can be used.

[0125] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be light and thin, and a secondary battery may be used as a power source.

[0126] Example

[0127] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0128] 1. Preparation of secondary batteries

[0129] <Example 1>

[0130] (1) Preparation of negative electrode materials

[0131] The graphite as the first negative electrode active material is mixed with the conductive agent Super P, the thickener CMC, and the adhesive styrene butadiene rubber (SBR) in deionized water to prepare the first negative electrode slurry. The solid content in the negative electrode slurry is 53wt%, and the mass ratio of graphite, Super P, CMC and adhesive styrene butadiene rubber (SBR) in the solid component is 96:2:1:1.

[0132] The second negative electrode slurry is prepared by mixing NiO as the second negative electrode active material, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP). The solid content in the negative electrode slurry is 43wt%, and the mass ratio of NiO, Super P, CMC, and the binder styrene butadiene rubber (SBR) in the solid component is 92:6:2.

[0133] (2) Preparation of negative electrode

[0134] The first negative electrode slurry is coated on the current collector copper foil and dried at 85°C to form the first negative electrode active material layer. Then the second negative electrode slurry is coated on the first negative electrode active material layer at a specified position and dried at 85°C. Then, after cold pressing, trimming, cutting, and slitting, it is dried at 120°C under vacuum conditions for 12 hours to form a negative electrode sheet.

[0135] (3) Preparation of positive electrode

[0136] The positive electrode material lithium iron phosphate, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are made into positive electrode slurry in N-methylpyrrolidone (NMP). The solid content in the positive electrode slurry is 64wt%, and the mass ratio of lithium iron phosphate, SuperP, and PVDF in the solid component is 97:1:2. The positive electrode slurry is coated on the current collector aluminum foil and dried at 85°C and then cold pressed. Then, after trimming, cutting, and striping, it is dried under vacuum conditions at 85°C for 4 hours to make a positive electrode sheet.

[0137] (4) Preparation of lithium-ion batteries :

[0138] Polyethylene film (PE) is used as a separator. The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to isolate the positive and negative electrodes, and the bare battery cell is wound, and the tabs are welded. The bare battery cell is placed in an outer package, and the prepared electrolyte is injected into the dried battery cell, and the lithium-ion battery is prepared by packaging, standing, forming, shaping, and capacity testing.

[0139] <Example 2> to <Example 11>, <Comparative Examples 1 to 3>

[0140] The secondary batteries of Examples 2 to 11 and Comparative Examples 1 to 3 were prepared in a similar manner to the secondary battery of Example 1, but the composition of the second active material and the mass ratio of the second active material to the first active material were adjusted. The different product parameters are detailed in Table 1.

[0141] Next, the testing process of the relevant parameters of the electrode assembly is described.

[0142] [1] Method for measuring the lithium insertion potential of the first and second active materials

[0143] After the negative electrode material powder is made into a pole piece according to a certain formula, it is assembled into a pole piece-lithium plate semi-button battery and the total charge and discharge capacity of the negative electrode material at different potentials is obtained by charging and discharging at a small rate, and then the total charge and discharge capacity of the second cycle is divided by the mass of the active material of the pole piece to obtain the potential-charge and discharge capacity curve of the material. Here, the lithium insertion potential is the potential value corresponding to the corresponding capacity of the potential-capacity curve during the second cycle of discharge.

[0144] [2] Method for measuring the delithiation potential of the first and second active materials

[0145] After the negative electrode material powder is made into a pole piece according to a certain formula, it is assembled into a pole piece-lithium plate semi-button battery and the total charge and discharge capacity of the negative electrode material at different potentials is obtained by charging and discharging at a small rate, and then the total charge and discharge capacity of the second cycle is divided by the mass of the active material of the pole piece to obtain the potential-charge and discharge capacity curve of the material. Here, the de-lithiation potential is the potential value corresponding to the corresponding capacity of the potential-capacity curve during the second cycle of charging.

[0146] [3] Method for measuring the capacity of the first and second negative electrode active material layers

[0147] After the material powder is made into an electrode according to a certain formula, it is assembled into a semi-button battery of an electrode-lithium sheet and the button capacity is obtained through small-rate charge and discharge. The capacity is then divided by the mass of the active material in the electrode to obtain the gram capacity parameter. Here, the capacity is calculated as follows: Capacity (mAh / cm2) = gram capacity of active material (mAh / g) * mass of active material in the active material layer (g / cm2).

[0148] [4] Method for measuring the capacity of the positive electrode active material layer

[0149] After the material powder is made into an electrode according to a certain formula, it is assembled into a semi-button battery of an electrode-lithium sheet and the button capacity is obtained through small-rate charge and discharge. The capacity is then divided by the mass of the active material in the electrode to obtain the gram capacity parameter. Here, the capacity is calculated as follows: Capacity (mAh / cm2) = gram capacity of active material (mAh / g) * mass of active material in the active material layer (g / cm2).

[0150] 2. Battery performance test

[0151] The secondary batteries prepared in the above-mentioned 1. [1] to [4] were subjected to performance tests by the following method. The results are shown in Table 2.

[0152] [1] Lithium deposition at negative electrode

[0153] The formed battery is charged at 25°C at a constant current of 2C to 3.65V, then charged at a constant voltage of 3.65V to a current less than 0.05C, and then discharged at 1C to 2.5V. After 10 cycles, it is charged at a constant current of 0.33C to 3.65V, and then charged at a constant voltage of 3.65V to a current less than 0.05C to obtain a fully charged battery.

[0154] Disassemble the battery after 10 cycles, observe the lithium deposition on the convex surface of the negative electrode, and record it in the column "Lithium deposition on the convex surface of the negative electrode" in the table below.

[0155] The evaluation criteria after observation are as follows.

[0156] No lithium precipitation: The surface of the fully charged negative electrode in the corner area is golden yellow. When wiped with dust-free paper, there is no gray metallic lithium powder on the paper.

[0157] Mild lithium deposition: The surface of the fully charged negative electrode in the corner area is dark yellow. When wiped with dust-free paper, there is gray metallic lithium powder on the paper.

[0158] Gray spots: The surface of the fully charged negative electrode in the corner area is partially gray, with no golden color showing through.

[0159] Severe lithium plating: The surface of the fully charged negative electrode in the corner area is all gray, with no golden color showing through.

[0160] In the present application, the preferred order of lithium deposition on the negative electrode convex surface is no lithium deposition > gray spots > slight lithium deposition > severe lithium deposition.

[0161] [Table 1]

[0162]

[0163] In Table 1, the lithium insertion or lithium desorption potential refers to the potential corresponding to a lithium insertion or lithium desorption capacity of 100 mAh / g. Fig.14 As shown, the lithium insertion potential of the first active material refers to the potential corresponding to the specific capacity of 100 mAh / g on the lithium insertion curve of the first active material. The lithium desorption potential of the first active material refers to the potential corresponding to the specific capacity of 100 mAh / g on the lithium desorption curve of the first active material. Similarly, the lithium insertion or desorption potential of the second active material can be obtained. The above potentials are all relative to Li / Li + The unit is V. In addition, Fig.14It can be seen that in the range of lithium insertion capacity of 0 to 300 mAh / g, the lithium insertion potential of the second active material is higher than that of the first active material, and in the range of lithium desorption capacity of 10 to 300 mAh / g, the lithium desorption potential of the second active material is higher than that of the first active material.

[0164] [Table 2]

[0165]

[0166] In Table 2, the unit of gram capacity is mAh / g, and the unit of capacity is mAh / cm 2 .

[0167] III. Analysis of test results of various embodiments and comparative examples

[0168] From Table 1 above, we can see that:

[0169] According to the comparison between Examples 1 to 11 and Comparative Example 1, it can be seen that by providing the negative electrode material with a first active material layer and a second active material layer, and the second active material has a higher lithium insertion potential and a lithium extraction potential than the first active material, the negative electrode lithium deposition can be greatly improved. In Comparative Example 1, no second active material layer is provided, and slight lithium deposition occurs in the negative electrode.

[0170] According to the comparison between Examples 1 to 6 and Comparative Example 2, by making the mass ratio of the second active material in the negative electrode material reach a certain content or more, the negative electrode lithium deposition can be greatly improved. However, too much second active material in Comparative Example 2 will increase the battery mass. From the perspective of ensuring the battery energy density, the mass ratio of the second active material to the first active material is preferably less than 2, and more preferably less than 1.

[0171] According to the comparison between Examples 1 to 6 and Comparative Example 3, the lithium deposition of the negative electrode can be further improved by increasing the mass ratio of the second active material in the negative electrode material to a certain content or more. In Comparative Example 3, the mass of the second active material is relatively low, and gray spots appear on the negative electrode.

[0172] From Table 2 above, we can see that:

[0173] According to the comparison between Examples 1 to 11 and Comparative Examples 1 to 3, it can be seen that the negative electrode lithium plating can be greatly improved by satisfying a+b=nc, n=1.07 to 5 among the capacity a of the second active material layer, the capacity b of the first active material layer and the capacity c of the positive electrode active material layer.

[0174] According to the results of Examples 1 to 11, it can be seen that the lithium deposition improvement effect can be enhanced by making the gram capacity of the second negative electrode active material greater than the gram capacity of the first negative electrode active material.

[0175] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode sheet, wherein: The negative electrode plate comprises: current collector; a first active material layer disposed on at least one surface of the current collector; and a second active material layer, disposed on a surface of the first active material layer away from the current collector; The first active material layer includes a first active material, the second active material layer includes a second active material, and the lithium insertion potential and the lithium desorption potential of the second active material are both higher than those of the first active material; There are a plurality of second active material layers, and the plurality of second active material layers are arranged at intervals.

2. The negative electrode sheet according to claim 1, wherein: When the lithium insertion capacity of the negative electrode plate is in the range of 0 to 300 mAh / g, the lithium insertion potential of the second active material is higher than that of the first active material.

3. The negative electrode sheet according to claim 1 or 2, wherein: When the delithiation capacity of the negative electrode sheet is in the range of 10 to 300 mAh / g, the delithiation potential of the second active material is higher than the delithiation potential of the first active material.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: The second active material has a higher gram capacity than the first active material.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The ratio of the mass of the second active material to the mass of the first active material is 0.1-2.

6. The negative electrode sheet according to claim 5, wherein: The ratio of the mass of the second active material to the mass of the first active material is 0.1-1.

7. An electrode assembly, wherein: The electrode assembly comprises a negative electrode sheet, a positive electrode sheet and a separator according to any one of claims 1 to 6; the positive electrode sheet comprises a positive active material layer; The capacity a of the second active material layer, the capacity b of the first active material layer and the capacity c of the positive electrode active material layer satisfy the formula (1): a+b=nc, n=1.07~5 Formula (1) The units of a, b and c in formula (1) are mAh / cm 2 .

8. The electrode assembly according to claim 7, wherein: The negative electrode sheet, the separator and the positive electrode sheet are wound into a winding structure along a winding direction; the winding structure includes a planar portion and a corner portion connecting the planar portion; and the second active material layer is at least partially located at the corner portion.

9. The electrode assembly according to claim 8, wherein: The corner portion includes a first corner portion and a second corner portion close to the winding start end of the electrode assembly; the second active material layer is located at the first corner portion and the second corner portion.

10. The electrode assembly according to claim 8 or 9, wherein: The current collector of the negative electrode plate further includes a pole ear portion; the second active material layer is also located on a side of the planar portion connected to the pole ear portion.

11. The electrode assembly according to claim 7, wherein: The negative electrode sheet, the isolation membrane and the positive electrode sheet are stacked in sequence to form a laminate structure; the laminate structure includes a central portion and an edge portion arranged around the central portion; the second active material layer is at least partially located at the edge portion.

12. The electrode assembly according to claim 11, wherein: The current collector of the negative electrode plate further includes a pole ear portion; the second active material layer is also located on a side of the edge portion connected to the pole ear portion.

13. A method for manufacturing a negative electrode sheet, used for manufacturing the negative electrode sheet according to any one of claims 1 to 6.

14. A secondary battery, wherein: The secondary battery includes the electrode assembly according to any one of claims 7 to 12.

15. A battery module, wherein: The battery module includes the secondary battery according to claim 14 .

16. A battery pack, wherein: The battery pack includes the battery module according to claim 15.

17. An electrical device, wherein: The electric device includes at least one selected from the group consisting of the secondary battery of claim 14 , the battery module of claim 15 , and the battery pack of claim 16 .

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