Negative electrode sheet and preparation method thereof, secondary battery, battery pack, and electrical device

Abstract

The present application relates to a negative electrode plate and a preparation method thereof, and a secondary battery, a battery pack, and an electrical device. The negative electrode plate comprises: a negative electrode collector and a negative electrode film layer. The negative electrode collector comprises a main region and an edge region located on at least one side of the main region. The negative electrode film layer is arranged on the main region and the edge region of the negative electrode collector. The porosity of the negative electrode film layer in the edge region is greater than the porosity in the main region.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode plate and a preparation method thereof, a secondary battery, a battery pack, and an electrical device. Background Art

[0002] With the improvement of the performance of portable electronic products and the advent of the 5G era, the loss of battery capacity in a short period of time has intensified, and with it comes a strong demand for battery fast charging capabilities. Super fast charging products are widely favored by the market, but high-rate charging and discharging causes a significant increase in battery temperature, which seriously affects product safety and reliability, while reducing customer experience.

[0003] Therefore, how to make the battery have both high-rate charge and discharge performance and safety performance is a key challenge in the field of battery design. Summary of the invention

[0004] Based on this, it is necessary to provide a negative electrode sheet and a preparation method thereof, as well as a secondary battery, a battery pack, and an electrical device that can enable the battery to have both high rate charge and discharge performance and safety performance.

[0005] The present application is implemented through the following technical solutions.

[0006] In a first aspect of the present application, a negative electrode sheet is provided, comprising:

[0007] A negative electrode current collector, comprising a main body region and an edge region located on at least one side of the main body region; and

[0008] The negative electrode film layer is arranged on the main area and the edge area of ​​the negative electrode current collector, and the porosity of the negative electrode film layer in the edge area is greater than the porosity in the main area.

[0009] The negative electrode plate of the present application sets the porosity of the negative electrode film layer in the edge area to be greater than the porosity of the negative electrode film layer in the main area, so that the edge area has a stronger electrolyte reabsorption rate, and the electrolyte infiltrated in the main area can quickly diffuse to the edge area with lower porosity, thereby improving the wettability of the electrolyte in the edge area, allowing the electrolyte to be reabsorbed to the top edge area, alleviating and improving the degree of lithium precipitation in the edge area, reducing the temperature rise of the battery under high-rate charge and discharge, ensuring the safety, reliability and service life of the secondary battery, and making it have both high-rate charge and discharge performance and safety performance.

[0010] In some embodiments, the negative electrode film layer includes a negative electrode active material layer and an optional undercoat layer, and the undercoat layer and / or the negative electrode active material layer has a porosity in the edge region that is greater than a porosity in the main region.

[0011] In some embodiments, the difference between the porosity of the undercoat layer and / or the negative electrode active material layer in the edge region and the porosity of the main region is 2% to 9%.

[0012] In some embodiments, the porosity of the undercoat layer and / or the negative electrode active material layer in the edge region is 28% to 32%, and the porosity of the main region is 23% to 26%.

[0013] In some embodiments, the negative electrode film layer includes a primer layer and a negative electrode active material layer, and the primer layer is located between the negative electrode current collector and the negative electrode active material layer.

[0014] In some embodiments, the thickness of the negative electrode film layer in the edge region and the main region is 80 μm to 150 μm; and / or

[0015] The compaction density of the negative electrode film layer in the edge area and the main area is 1.50-1.70 g / cm 3 .

[0016] In some embodiments, there are at least two edge regions, and at least two edge regions are located on two opposite sides of the main region.

[0017] In some embodiments, based on the total area of ​​the negative electrode film layer, the area of ​​a single edge region accounts for ≤10%; optionally 5%-10%.

[0018] In a second aspect of the present application, a method for preparing the above-mentioned negative electrode sheet is provided, the preparation method comprising the following steps:

[0019] forming the negative electrode film layer on the main area and the edge area of ​​the current collector;

[0020] Among them, the coatings applied in the edge area and the main area for forming the negative electrode film layer are respectively a first coating and a second coating, the first coating contains a loosening agent, the second coating contains or does not contain a loosening agent, and the mass content of the loosening agent in the first coating is greater than the mass content of the loosening agent in the second coating.

[0021] In some embodiments, the difference between the mass content of the leavening agent in the first coating and the mass content of the leavening agent in the second coating is 5% to 25%.

[0022] In some embodiments, the mass content of the loosening agent in the first coating is 10% to 25%;

[0023] And / or, the mass content of the leavening agent in the second coating is 0-10%.

[0024] In some embodiments, the loosening agent includes at least one of sodium bicarbonate, calcium bicarbonate, oxalic acid, sodium oxalate, calcium oxalate and ammonium salt;

[0025] And / or, the type of the leavening agent in the first coating is the same as or different from the type of the leavening agent in the second coating.

[0026] A third aspect of the present application provides a secondary battery comprising the negative electrode sheet of the first aspect of the present application.

[0027] A fourth aspect of the present application provides a battery module, comprising the secondary battery of the third aspect of the present application.

[0028] A fifth aspect of the present application provides a battery pack comprising the battery module of the fourth aspect of the present application.

[0029] In a sixth aspect of the present application, there is provided an electrical device comprising at least one selected from the secondary battery of the third aspect of the present application, the battery module of the fourth aspect of the present application, and the battery pack of the fifth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of a secondary battery according to one embodiment of the present application.

[0031] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of the present application is shown.

[0032] Figure 3 It is a schematic structural diagram of an electrode assembly without a tab according to one embodiment.

[0033] Figure 4 FIG. 1 is a schematic diagram of a surface of a negative electrode current collector of a negative electrode sheet according to an embodiment of the present invention.

[0034] Figure 5 It is a schematic diagram of a battery module according to one embodiment of the present application.

[0035] Figure 6 It is a schematic diagram of a battery pack according to one embodiment of the present application.

[0036] Figure 7 yes Figure 6 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0037] Figure 8 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.

[0038] Description of reference numerals:

[0039] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 521 electrolyte insufficient area; 501 first temperature measuring point; 502 second temperature measuring point; 503 third temperature measuring point; 53 cover plate; 6, electrical device; 7 negative electrode current collector; 72 main area; 74: edge area. DETAILED DESCRIPTION

[0040] Below, the embodiments of the pole piece and its manufacturing method, secondary battery, battery module, battery pack and electric device of the present application are specifically disclosed in detail 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.

[0041] 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.

[0042] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0043] 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.

[0044] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0045] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean 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.

[0046] 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).

[0047] The term "and / or" as used herein includes any and all combinations of one or more related listed items. In the description of the present application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the feature. In the description of the present application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0048] Secondary battery

[0049] A secondary battery is a battery that can be recharged to activate the active materials after being discharged and continue to be used.

[0050] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the battery charging and discharging process, active ions are embedded and extracted 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. Furthermore, the secondary battery may also include a separator.

[0051] In some examples, the secondary battery is a lithium-ion battery.

[0052] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. Figure 1 The secondary battery 5 is a square structure as an example.

[0053] 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.

[0054] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. 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 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 pole piece, the negative pole piece and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. 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 can be one or more, and those skilled in the art can select according to specific actual needs.

[0055] When secondary batteries are charged and discharged at high rates, the temperature rise of the battery increases significantly, which seriously affects the safety and reliability of the product and the service life of the battery. Taking lithium-ion batteries as an example, the technical personnel of this application found that this is mainly because the degree of lithium deposition in the edge area of ​​the negative electrode plate of the lithium-ion battery is more serious than that in the main area of ​​the negative electrode plate, and as the charging rate increases, the degree of lithium deposition in the edge area becomes more serious. The technical personnel of this application further analyzed the reasons and found that this is mainly because during the charging process of the secondary battery, the negative electrode plate expands, causing the electrolyte on the negative electrode plate side to be squeezed and squeezed out. Due to the reduced reabsorption speed of the electrolyte, the electrolyte has an uneven distribution problem. Specifically, the electrolyte at the bottom of the secondary battery is difficult to be reabsorbed to the top of the secondary battery, especially the edge areas on both sides of the top, which are difficult to be infiltrated by the electrolyte. Figure 3 As shown, the electrode assembly 52 is prone to having an electrolyte insufficient area 521 .

[0056] In order to solve the safety performance of secondary batteries under high-rate charge and discharge, the present application provides a negative electrode plate.

[0057] The negative electrode plate includes a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector.

[0058] The negative electrode current collector includes a main region and an edge region located on at least one side of the main region, the negative electrode film layer is arranged on the main region and the edge region of the negative electrode current collector, and the porosity of the negative electrode film layer in the edge region is greater than the porosity in the main region.

[0059] Without wishing to be limited to any theory, the negative electrode plate of the present application sets the porosity of the negative electrode film layer in the edge area to be greater than the porosity of the negative electrode film layer in the main area, so that the edge area has a stronger electrolyte reabsorption rate, and the electrolyte infiltrated in the main area can quickly diffuse to the edge area with lower porosity, thereby improving the wettability of the electrolyte in the edge area, allowing the electrolyte to be reabsorbed to the top edge area, alleviating and improving the degree of lithium precipitation in the edge area, reducing the temperature rise of the battery under high-rate charge and discharge, ensuring the safety, reliability and service life of the secondary battery, and making it have both high-rate charge and discharge performance and safety performance.

[0060] 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.

[0061] like Figure 4 As shown, the negative electrode current collector 7 includes a main region 72 and an edge region 74 located at at least one side of the main region 72. In the negative electrode film formed on the negative electrode current collector 7, the porosity in the edge region 74 is greater than the porosity in the main region 72.

[0062] It is understood that the shape of the negative electrode current collector is generally rectangular or square, among which rectangular negative electrode current collectors are more commonly used to form rectangular negative electrode sheets, which are convenient for winding to form electrode assemblies and further form wound batteries.

[0063] Furthermore, if Figure 4 As shown, the negative electrode current collector 7 has a longitudinal direction extending along the length direction and a transverse direction perpendicular to the longitudinal direction. The edge region 74 and the main region 72 are sequentially arranged in the transverse direction of the negative electrode current collector 7 .

[0064] It is worth noting that the longitudinal direction here refers to the length direction of the negative electrode current collector 7, that is, Figure 4 The lateral direction refers to the direction perpendicular to the length direction of the negative electrode current collector 7, that is, Figure 4 Horizontal direction in .

[0065] Specifically, the secondary battery is an M6T structure battery, which refers to a battery structure in which the tabs are led out from both sides. Figure 3 As shown, the tabs are led out from the left and right sides of the wound electrode assembly 52 .

[0066] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a 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.).

[0067] 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.

[0068] Furthermore, both the main region and the edge region contain a conductive agent.

[0069] In some embodiments, the negative electrode film layer may further include a binder, which 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) and carboxymethyl chitosan (CMCS).

[0070] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0071] In some embodiments, the negative electrode film layer includes a negative electrode active material layer and an optional primer layer. The primer layer may not contain negative electrode active materials. In this case, the porosity of the primer layer and / or the negative electrode active material layer in the edge region is greater than its porosity in the main region. For example, the porosity of the primer layer in the edge region is greater than the porosity of the primer layer in the main region. For example, the porosity of the negative electrode active material layer in the edge region is greater than the porosity of the negative electrode active material layer in the main region.

[0072] Further, the thickness of the undercoat layer is 1 μm to 10 μm. Further, the thickness of the negative electrode active material layer is 75 μm to 140 μm.

[0073] When the negative electrode film layer includes a primer layer and a negative electrode active material layer, the primer layer is located between the negative electrode current collector and the negative electrode active material layer. Further, the thickness of the negative electrode film layer in the edge area and the main area is 80 μm to 150 μm; further, the compaction density of the negative electrode film layer in the edge area and the main area is 1.50 to 1.70 g / cm 3Further controlling the thickness and compaction density of the edge area and the main area can, on the one hand, meet the porosity requirements and reduce the temperature rise of the battery, and on the other hand, better take into account the charge and discharge rate performance of the secondary battery.

[0074] In some embodiments, the difference between the porosity of the undercoat layer and / or the negative electrode active material layer in the edge region and the porosity of the main region is 2% to 9%, such as 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, such as 3% to 7%. That is, the difference between the porosity of the undercoat layer in the edge region and the porosity of the main region is 2% to 9%; and / or, the difference between the porosity of the negative electrode active material layer in the edge region and the porosity of the main region is 2% to 9%.

[0075] In some embodiments, the porosity of the undercoat layer and / or the negative electrode active material layer in the edge region is 28% to 32%, such as 28%, 29%, 30%, 31%, 32%, and the porosity of the undercoat layer and / or the negative electrode active material layer in the main region is 23% to 26%, such as 23%, 24%, 25%, 26%. Further controlling the porosity of the undercoat layer and / or the negative electrode active material layer in the edge region and the main region within the above given range can further improve the rate charge and discharge performance and safety performance of the secondary battery.

[0076] In some embodiments, there are at least two edge regions, and at least two edge regions are located on opposite sides of the main region. Setting edge regions on both sides of the main region can solve the problem of insufficient electrolyte areas on both sides of the top of the secondary electrode, and further improve the rate charge and discharge performance and safety performance of the secondary battery.

[0077] In some embodiments, based on the total area of ​​the negative electrode film layer, the total area of ​​the edge zone accounts for ≤20%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 12%, 15%, 18%, 20%; optionally, the total area of ​​the edge zone accounts for 5% to 20%, further 10% to 20%.

[0078] Further, the area ratio of a single edge region is ≤10%, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%; optionally, the area ratio of a single edge region is 5% to 10%, and further 5% to 7%. Further controlling the area ratio of the edge region within the above given range so that the area of ​​the edge region covers the electrolyte-deficient areas on both sides of the top of the secondary electrode as much as possible can further improve the rate charge and discharge performance and safety performance of the secondary battery.

[0079] Furthermore, the edge region and the main region are preferably rectangular in shape, but it is understood that the shape is not limited thereto. The size of a single edge region in the above lateral direction accounts for ≤10% of the size of the negative electrode collector in the above lateral direction, and can be 5% to 10%.

[0080] It can be understood that, in addition to the primer layer and the negative electrode active material layer, in other examples, the negative electrode plate may further include other film layers.

[0081] In some embodiments, the negative electrode active material may adopt the negative electrode active material for the 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, tin-based materials, lithium titanate, etc. Among them, the silicon-based material can be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material can be selected from at least one of elemental tin, tin oxide compounds, and tin 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 can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0082] In some embodiments, the method for preparing the negative electrode film layer comprises the following steps:

[0083] S10: forming a negative electrode film layer on the current collector.

[0084] The coatings applied to the edge area and the main area for forming the negative electrode film layer are respectively the first coating and the second coating. The first coating contains a loosening agent, and the second coating contains or does not contain a loosening agent. The mass content of the loosening agent in the first coating is greater than the mass content of the loosening agent in the second coating.

[0085] The above-mentioned preparation method uses a first coating and a second coating to form a negative electrode film layer in the edge area and the main area respectively, and controls the different contents of the loosening agent in the first coating and the second coating to control the porosity of the formed negative electrode film layer in the edge area to be greater than the porosity in the main area.

[0086] It can be understood that the first coating material and the second coating material can form a negative electrode film layer in the corresponding area of ​​the negative electrode current collector by a coating method.

[0087] In some embodiments, coating can be performed using a coating machine. It is understood that there is no particular limitation on the type of coating machine, and a commercially available coating machine can be used.

[0088] Furthermore, the coating may be performed by transfer coating, spin spray coating, dip coating, etc. It is understood that after the coating step, after drying, cold pressing, etc., a negative electrode film layer can be formed on the negative electrode current collector to obtain a negative electrode sheet.

[0089] In some embodiments, the difference between the mass content of the loosening agent in the first coating and the mass content of the loosening agent in the second coating is 5% to 25%, for example 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%.

[0090] Furthermore, the mass content of the leavening agent in the first coating is 10% to 25%.

[0091] Furthermore, the mass content of the loosening agent in the second coating is 0-5%, such as 0, 1%, 2%, 3%, 4% or 5%. Preferably, the mass content of the loosening agent in the second coating is 0.

[0092] In some embodiments, the loosening agents in the first coating and the second coating each independently include at least one of sodium bicarbonate, calcium bicarbonate, oxalic acid, sodium oxalate, calcium oxalate and ammonium salt. These loosening agents can be decomposed into gas during the steps of coating and baking the first coating and the second coating to form the negative electrode film layer, thereby making the negative electrode film layer have a loose and porous structure. The loosening agent content in the first coating in the edge area is higher, and the porosity of the film layer formed by it is larger.

[0093] Furthermore, the type of the loosening agent in the first coating is the same as or different from that of the loosening agent in the second coating.

[0094] In some embodiments, the first coating and the second coating each independently further optionally contain at least one of a conductive agent and a binder.

[0095] In some embodiments, the negative electrode film layer includes a negative electrode active material layer and an optional undercoat layer. When the negative electrode film layer includes an undercoat layer and a negative electrode active material layer, the undercoat layer is located between the negative electrode current collector and the negative electrode active material layer.

[0096] In some embodiments, when the porosity of the undercoat layer and the negative electrode active material layer in the edge region is greater than that in the main region, the undercoat layer and the negative electrode active material layer are formed by coating in two layers, and different coating materials are used for each layer for zone coating.

[0097] Positive electrode

[0098] The positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector.

[0099] 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.

[0100] 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.).

[0101] The above-mentioned positive electrode film layer contains positive electrode active materials. In some embodiments, the positive electrode active material may adopt positive electrode active materials for batteries that are well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates containing olivine structure, lithium transition metal oxides 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 can also be used. These positive electrode active materials can 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 / 3Mn 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 Al 0.05O2) 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] Electrolytes

[0106] The electrolyte plays the role of conducting ions between the electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0107] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] Isolation film

[0112] 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.

[0113] 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.

[0114] In some embodiments, the electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0115] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0116] 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.

[0117] Figure 5 This is a battery module 4 as an example. Figure 5 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.

[0118] 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.

[0119] 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.

[0120] Figure 6 and Figure 7 1 is a battery pack 1 as an example. Figure 6 and Figure 7 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.

[0121] 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.

[0122] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0123] Figure 8 The power consumption device 6 is used as an example. The power consumption device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the power consumption device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module can be used.

[0124] 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 thin and light, and a secondary battery may be used as a power source.

[0125] In order to make the purpose, technical solutions and advantages of the present application more concise and clear, the present application is described with the following specific embodiments, but the present application is by no means limited to these embodiments. The embodiments described below are only preferred embodiments of the present application and can be used to describe the present application, and cannot be understood as limiting the scope of the present application. It should be pointed out that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

[0126] In order to better illustrate the present application, the present application is further described below in conjunction with the embodiments. The following are specific embodiments.

[0127] Comparative Example 1

[0128] Negative electrode:

[0129] Copper foil is used as the negative electrode current collector;

[0130] The preparation of the negative electrode film layer on the negative electrode sheet is as follows:

[0131] SP: SBR: CMC were mixed in a mass ratio of 0.645: 0.323: 0.032 to prepare a primer coating, which was applied to the entire current collector using a gravure plate and dried at 40° C. for 6 h to obtain a negative electrode current collector with a primer coating having a thickness of 3 μm.

[0132] Graphite: SP: SBR: CMC were mixed in a mass ratio of 96.8: 0.7: 1.3: 1.2 to prepare a negative electrode active slurry, and then the negative electrode active slurry graphite: SP: SBR: CMC was coated on the bottom coating of the negative electrode current collector to form a negative electrode active material layer with a thickness of 124 μm; dried and cold pressed to obtain a negative electrode sheet.

[0133] Positive electrode sheet: Provide the following components by mass percentage: 96.2% positive electrode active material, 1.1% PVDF, 2.7% conductive carbon. Among them, the positive electrode active material is NCM523 and carbon-coated LFP (abbreviation of lithium iron phosphate material) in a mass ratio of 2:8. Dissolve the above components in NMP (N-methylpyrrolidone) solution, stir evenly, apply on aluminum foil, dry, and cold press to obtain a positive electrode sheet.

[0134] A porous polymer film made of polyethylene (PE) is used as an isolation membrane. The electrolyte contains ethylene carbonate and ethyl methyl carbonate in a mass ratio of 3:7, and also contains LiPF6 at a concentration of 1 mol / L.

[0135] The positive electrode sheet, the separator and the negative electrode sheet are stacked in order, the separator is placed between the positive and negative electrodes to play a role of isolation, and then wound and put into a shell to obtain a battery cell with an M6T structure. The battery cell is placed in an outer package, the electrolyte used in the preparation is injected and packaged to obtain a secondary battery.

[0136] Example 1

[0137] It is basically the same as Comparative Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0138] like Figure 4As shown, the lateral size of the negative electrode current collector is 10mm+163m+10mm, which is respectively divided into 10mm±0.5mm from the left edge, 163mm±0.5mm in the middle and 10mm±0.5mm from the right edge.

[0139] SP: SBR: CMC were mixed in a mass ratio of 0.645: 0.323: 0.032 to prepare coating A1, which was coated by gravure coating at 163 mm ± 0.5 mm in the middle and 10 mm ± 0.5 mm from the right edge (i.e., coated at Figure 4 The main body area 72 and the edge area 74 on the right are shown;

[0140] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.445: 0.323: 0.032: 0.2 to obtain coating B1, which was coated with a gravure coating at a distance of 10 mm ± 0.5 mm from the left edge (i.e. Figure 4 The left edge area 74 is shown, and then dried at 40° C. for 6 h to obtain a negative electrode current collector with a primer layer.

[0141] Embodiment 2:

[0142] It is basically the same as Example 1, except that the coating process of the primer coating on the negative electrode plate is to adjust the areas where the two different primer coatings are applied, as follows:

[0143] SP:SBR:CMC were mixed in a mass ratio of 0.645:0.323:0.032 to prepare coating A2, which was coated by gravure coating at 163 mm ± 0.5 mm in the middle and 10 mm ± 0.5 mm from the left edge;

[0144] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.445: 0.323: 0.032: 0.2 to prepare coating B2, which was coated on a surface 10 mm ± 0.5 mm from the right edge by gravure coating and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0145] Embodiment 3:

[0146] It is basically the same as Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0147] The coating A3 was prepared by mixing SP:SBR:CMC in a mass ratio of 0.645:0.323:0.032 and applied by gravure coating at the middle of 163±0.5 mm;

[0148] SP:SBR:CMC:Ca(HCO3)2 was mixed in a mass ratio of 0.445:0.323:0.032:0.2 to prepare coating B3, which was coated by gravure coating at a distance of 10 mm±0.5 mm from the right and left edges, and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0149] Example 4

[0150] It is basically the same as Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0151] SP:SBR:CMC:NaHCO3 was mixed in a mass ratio of 0.545:0.323:0.032:0.1 to prepare coating A4, which was applied by gravure coating at the middle of 163±0.5 mm;

[0152] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.445: 0.323: 0.032: 0.2 to prepare coating B4, which was coated on a surface 10 mm ± 0.5 mm from the left edge by gravure coating and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0153] Example 5

[0154] It is basically the same as Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0155] The coating A5 was prepared by mixing SP:SBR:CMC in a mass ratio of 0.645:0.323:0.032 and applied by gravure coating at the middle of 163±0.5 mm;

[0156] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.545: 0.323: 0.032: 0.1 to prepare coating B5, which was coated by gravure coating at a distance of 10 mm ± 0.5 mm from the right and left edges and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0157] Example 6

[0158] It is basically the same as Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0159] The coating A6 was prepared by mixing SP:SBR:CMC in a mass ratio of 0.645:0.323:0.032 and applied by gravure coating at the middle of 163±0.5 mm;

[0160] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.445: 0.323: 0.032: 0.15 to prepare coating B6, which was coated by gravure coating at a distance of 10 mm ± 0.5 mm from the right and left edges and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0161] Example 7

[0162] It is basically the same as Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0163] The coating A7 was prepared by mixing SP:SBR:CMC in a mass ratio of 0.645:0.323:0.032 and applied by gravure coating at the middle of 163±0.5 mm;

[0164] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.445: 0.323: 0.032: 0.2 to prepare coating B7, which was coated by gravure coating at a distance of 10 mm ± 0.5 mm from the right and left edges and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0165] Example 8

[0166] It is basically the same as Example 1, except that the composition and coating process of the primer coating on the negative electrode plate are as follows:

[0167] The coating A8 was prepared by mixing SP:SBR:CMC in a mass ratio of 0.645:0.323:0.032 and applied by gravure coating at the middle of 163±0.5 mm;

[0168] SP: SBR: CMC: NaHCO3 was mixed in a mass ratio of 0.445: 0.323: 0.032: 0.25 to prepare coating B8, which was coated by gravure coating at a distance of 10 mm ± 0.5 mm from the right and left edges and dried at 40°C for 6 h to obtain a negative electrode current collector with a primer layer.

[0169] Some parameters of each embodiment and comparative example are shown in Table 1 below.

[0170] The coating area formed by coating materials A1 to A8 in Table 1 is Area A, and the coating area formed by coating materials B1 to B8 is Area B. The compaction density of Area A and Area B in Table 1 refers to the compaction density of the entire negative electrode film layer.

[0171] The secondary batteries prepared in the above comparative examples and embodiments were tested for fast charge temperature rise performance (i.e., 2.2C charge, 1C discharge temperature test), room temperature cycle performance, and lithium precipitation. The results are shown in Table 1. The specific test steps are as follows:

[0172] (1) Fast charging temperature rise performance:

[0173] The temperature rise was tested synchronously during the 2.2C and 1C discharge and charging processes. The specific test steps are as follows.

[0174] The temperature of the bare cell inside the cell prepared in each embodiment or comparative example is tested by a temperature sensing line, as follows Figure 3 The three points arranged as shown are: the first temperature measuring point 501 (denoted as 1#, the upper edge of the battery cell), the second temperature measuring point 502 (denoted as 2#, the middle of the battery cell), and the third temperature measuring point 503 (denoted as 3#, the lower edge of the battery cell). The specific operation steps are as follows:

[0175] 1) Use an electric drill to drill holes in the non-engraved area of ​​the top cover, and the inner diameter of the holes should be controlled at about 4 to 6 mm.

[0176] 2) Grind away the burrs at the punching points; clean the top cover to remove particulate impurities.

[0177] 3) When the temperature sensing wire is 5cm beyond the top cover, use a blade to peel off the outer insulation layer of the temperature sensing wire to expose the inner wire.

[0178] 4) Perform laser soft connection welding on the perforated top cover and the battery cell.

[0179] 5) Insert the prepared temperature-sensing wire into the surface of the battery cell through the small hole, and lay a wire to mark a number on the temperature-sensing wire.

[0180] 6) The punching position should be appropriate and should not be blocked by the adapter and the tab. The burrs at the punching position should be cleaned and the particulate impurities should be removed to avoid short circuit in the battery cell.

[0181] 7) Wrap the temperature sensing wire probe with yellow tape, and then stick it on the JR area where it needs to be arranged.

[0182] 8) After the temperature sensing wire is attached, use blue glue to tie and fix the battery cells.

[0183] 9) Apply glue about 1 cm wide on the temperature sensing line on the top cover of the battery cell. After applying the glue, let it stand for 5 to 10 minutes until it solidifies.

[0184] (II) Room temperature cycle performance (capacity retention rate):

[0185] In an environment of 25°C, the first charge and discharge are carried out, and constant current and constant voltage charging is carried out at a charging current equivalent to 2.2C until the upper limit voltage is 4.4V. Then, constant current discharge is carried out at a discharge current of 1C until the final voltage is 2.5V, and the first discharge capacity value C1 is recorded. Subsequently, the 2nd to 2000th OK charge and discharge cycles are carried out under the same test conditions as the first charge and discharge, the discharge capacity value during the cycle is recorded, and the cycle capacity retention rate is calculated. The capacity retention rate of the 2000th cycle = (discharge capacity of the 2000th cycle / discharge capacity of the first cycle) × 100%.

[0186] (III) Lithium precipitation test:

[0187] The secondary battery was subjected to a cyclic charge and discharge procedure, with the charge and discharge rates being 2.2C and 1C, and the number of cycles being 2000. After the cyclic charge and discharge procedure was completed, the secondary battery was disassembled to evaluate the degree of lithium deposition of the negative electrode sheet.

[0188] The evaluation method of lithium precipitation degree is as follows:

[0189] No lithium deposition: There is no lithium deposition area in a single electrode assembly;

[0190] Primary lithium deposition: Maximum area of ​​a single lithium deposition area ≤ 5*5mm 2 , the number of lithium deposition areas of a single electrode assembly is ≤1;

[0191] Secondary lithium deposition: 5*5mm 2 <The maximum area of ​​a single lithium deposition area ≤ 10*10mm 2 , the number of lithium deposition areas of a single electrode assembly is ≤1;

[0192] Level 3 lithium plating: does not meet the judgment conditions of the first two levels.

[0193] (IV) Porosity and compaction density test

[0194] Porosity is defined by the following equation: Porosity = (1-(mass of porous material [g] / (volume of porous material [cm 3 ]× material density)))×100[%]. The porosity is determined by the gas adsorption method according to GB / T 21650.2-2008.

[0195] Compacted density refers to the ratio of the electrode coating surface density to the coating thickness, and is one of the reference indicators of the material energy density. Test method: The electrode coating surface density can be determined according to the energy density design requirements. After the electrode is dried and cold pressed, use a vernier caliper to measure the total thickness of the electrode, and the coating thickness can be calculated by deducting the thickness of the current collector. Based on the two parameters of coating surface density and coating thickness, the compacted density of the electrode can be calculated.

[0196] Table 1

[0197]

[0198] As can be seen from Table 1 above, each embodiment adds a loosening agent to the edge area of ​​the bottom coating, and adds a lower content of loosening agent or no loosening agent in the middle. Compared with the secondary battery prepared by uniformly coating the bottom coating without adding a loosening agent in Comparative Example 1, it can significantly reduce the temperature rise of the battery during rapid charging, and can also reduce the lithium precipitation and thus reduce the degree of lithium precipitation, so the capacity retention rate of the battery can be improved to a certain extent. The capacity retention rate and lithium precipitation degree of 2000 cycles at a 2.2C charge rate and a 1C discharge rate indicate that the secondary battery prepared in the embodiment of the present application can take into account both high rate charge and discharge performance and safety performance.

[0199] From the comparison between Examples 1 to 2 and Example 7, it can be seen that, when other conditions remain basically unchanged, compared with Examples 1 to 2 in which the loosening agent is added only to the edge area on one side, Example 7 has a loosening agent added to the upper left edge area of ​​the base coating, and its lithium plating degree is lower and the temperature rise amplitude of fast charging is smaller, and at the same time, a relatively higher battery capacity retention rate can be achieved.

[0200] From the comparison between Example 3 and Example 7, it can be seen that when other conditions remain basically unchanged, compared with the calcium bicarbonate used in Example 3, the loosening agent sodium bicarbonate used in Example 7 has a lower degree of lithium precipitation in the battery and a smaller temperature rise during fast charging, while achieving a relatively higher battery capacity retention rate.

[0201] From the comparison between Example 4 and Example 7, it can be seen that when other conditions remain basically unchanged, compared with Example 4, in which a loosening agent is also added to the middle bottom coating of the negative electrode, Example 7 does not add a loosening agent to the middle bottom coating of the negative electrode. The degree of lithium plating of the battery is lower and the temperature rise during fast charging is smaller, and a relatively higher battery capacity retention rate can be achieved.

[0202] The other conditions of Examples 5 to 8 remain basically unchanged, and only the content of sodium bicarbonate is changed. It can be seen from Examples 5 to 8 that the mass content of sodium bicarbonate is preferably 10% to 20%.

[0203] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0204] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims, and the description and drawings may be used to interpret the contents of the claims.

Claims

1. A negative electrode plate, characterized in that: include: A negative electrode current collector, comprising a main body region and an edge region located on at least one side of the main body region; and The negative electrode film layer is arranged on the main area and the edge area of ​​the negative electrode current collector, and the porosity of the negative electrode film layer in the edge area is greater than the porosity in the main area.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode film layer comprises a negative electrode active material layer and an optional undercoat layer, and the undercoat layer and / or the negative electrode active material layer has a porosity in the edge region that is greater than a porosity in the main region.

3. The negative electrode sheet according to claim 2, characterized in that: The difference between the porosity of the undercoat layer and / or the negative electrode active material layer in the edge region and the porosity of the main region is 2% to 9%.

4. The negative electrode sheet according to claim 2, characterized in that: The undercoat layer and / or the negative electrode active material layer has a porosity of 28% to 32% in the edge region and a porosity of 23% to 26% in the main region.

5. The negative electrode sheet according to claim 2, characterized in that: The negative electrode film layer includes an undercoat layer and a negative electrode active material layer, wherein the undercoat layer is located between the negative electrode current collector and the negative electrode active material layer.

6. The negative electrode sheet according to claim 5, characterized in that: The thickness of the negative electrode film layer in the edge region and the main region is 80 μm to 150 μm; and / or The compaction density of the negative electrode film layer in the edge area and the main area is 1.50-1.70 g / cm 3 .

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: There are at least two edge regions, and at least two edge regions are located at two opposite sides of the main region.

8. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: Based on the total area of ​​the negative electrode film layer, the total area of ​​the edge region accounts for ≤20%.

9. The negative electrode sheet according to claim 8, characterized in that: Based on the total area of ​​the negative electrode film layer, the total area of ​​the edge region accounts for 5%-20%.

10. A method for preparing a negative electrode sheet according to any one of claims 1 to 9, characterized in that: The preparation method comprises the following steps: forming the negative electrode film layer on the main area and the edge area of ​​the current collector; Among them, the coatings applied in the edge area and the main area for forming the negative electrode film layer are respectively a first coating and a second coating, the first coating contains a loosening agent, the second coating contains or does not contain a loosening agent, and the mass content of the loosening agent in the first coating is greater than the mass content of the loosening agent in the second coating.

11. The preparation method according to claim 10, characterized in that: The difference between the mass content of the leavening agent in the first coating and the mass content of the leavening agent in the second coating is 5% to 25%.

12. The preparation method according to claim 10, characterized in that: The mass content of the loosening agent in the first coating is 10% to 25%; And / or, the mass content of the leavening agent in the second coating is 0-5%.

13. The preparation method according to any one of claims 10 to 12, characterized in that: The loosening agent includes at least one of sodium bicarbonate, calcium bicarbonate, oxalic acid, sodium oxalate, calcium oxalate and ammonium salt; And / or, the type of the leavening agent in the first coating is the same as or different from the type of the leavening agent in the second coating.

14. A secondary battery, characterized in that: It comprises the negative electrode sheet as described in any one of claims 1 to 9.

15. A battery module, characterized in that: Comprising the secondary battery as claimed in claim 14.

16. A battery pack, characterized in that: Comprising the battery module as claimed in claim 15.

17. An electrical device, characterized in that: The battery comprises at least one selected from the group consisting of the secondary battery as claimed in claim 14, the battery module as claimed in claim 15, and the battery pack as claimed in claim 16.

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