Patent Publication Number: US-2023146812-A1

Title: Negative electrode plate, secondary battery, battery module, battery pack, and electric apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/CN2021/129506, filed on Nov. 9, 2021, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This application relates to the technical field of lithium batteries, and in particular, to a negative electrode plate, a secondary battery, a battery module, a battery pack, and an electric apparatus. 
     BACKGROUND 
     In recent years, lithium-ion batteries are being applied to increasingly extensive fields. For example, lithium-ion batteries are extensively applied in storage power systems such as hydropower, thermal power, wind power, and solar power stations, and many fields such as electric tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace. Currently, the extensive application of lithium-ion batteries poses higher requirements on comprehensive performance of batteries. More application scenarios require that traction batteries should have both high energy density and good charge-discharge characteristics. A negative electrode is one of most critical components of a lithium-ion battery whose design directly affects performance of the battery, especially charge-related characteristics of the battery. How a battery having both desirable energy density and kinetic performance can be obtained by optimizing the design of the negative electrode plate is currently a common challenge in the industry. 
     SUMMARY 
     An objective of this application is to provide a negative electrode plate to meet an existing requirement. This application further provides a secondary battery, and a battery module, battery pack, and electric apparatus using such negative electrode plate. The inventors of this application have found that by using two or more active material layers in a negative electrode and adjusting a relationship between layer spacings and that between coating weights of the active material layers, it is possible to obtain a battery negative electrode having both higher energy density and better charge-discharge kinetic performance. 
     To achieve the foregoing objective, a first aspect of this application provides a negative electrode plate, including a negative electrode current collector and active material layers disposed on at least one surface of the negative electrode current collector, where the active material layers include a first active material layer and a second active material layer disposed on a surface of the first active material layer, the first active material layer includes a first active material, the second active material layer includes a second active material, and the active material layers satisfy α×CW 2 ≤CW 1 , where 
     α is a relative factor of layer spacings, where 
     
       
         
           
             α 
             = 
             
               
                 d 
                 ⁢ 
                 2 
               
               
                 d 
                 ⁢ 
                 1 
               
             
           
         
       
     
     and 1≤α≤1.12; 
     d 1  is a layer spacing corresponding to a d002 peak of the first active material, in units of nm; 
     d 2  is a layer spacing corresponding to a d002 peak of the second active material, in units of nm; 
     CW 1  is mass per unit area of the first active material layer disposed on the negative electrode current collector, in units of g/m 2 ; and 
     CW 2  is mass per unit area of the second active material layer disposed on the negative electrode current collector, in units of g/m 2 . 
     Therefore, by using two or more active material layers in a negative electrode and adjusting a relationship between layer spacings and that between coating weights of the active material layers, it is possible to obtain a battery negative electrode having both higher energy density and better charge-discharge kinetic performance. More specifically, the inventors of this application have found that when two active material layers are applied on one side of the current collector of the negative electrode plate with the active material layer spacing of the second active material layer larger than the active material layer spacing of the first active material layer, during migration of lithium ions from a positive electrode to the negative electrode, because of the large spacing of the surface material layer, impedance to intercalation of lithium ions is low so that lithium ions can be quickly intercalated into the surface active material. Therefore, precipitation of lithium ions on the surface of the negative electrode plate due to an excessive charging rate is avoided. Because a larger spacing of the negative active material layer means a lower capacity of the material, a heavier coating weight of the second active material layer has greater impact on the overall extractable capacity of the negative electrode plate. Taking the extractable capacity of the negative electrode plate and the charging window both into account, this application defines the coating weight of the second active material layer. A larger active material layer spacing of the second active material layer represents a stronger charging capability and a lower capacity, and correspondingly, just a small coating weight is enough to achieve a good charging level. When the active material layer spacing of the second active material layer is smaller, its charging capability becomes weaker, and the capacity is higher. In this case, in order that the electrode plate has relatively good charge performance on the whole, a proportion of the coating weight of the second active material layer to a total coating weight of the electrode plate should be increased. The inventors of this application have unexpectedly found that, with a structure with two active material layers used, the first active material layer having a small active material layer spacing and a high capacity makes the electrode plate and the battery have relatively high energy density; and the second active material layer having a large active material layer spacing allows a lower impedance to lithium intercalation on the surface of the negative electrode, avoiding deposition of lithium ions on the surface of the negative electrode plate during high-rate charging, and increasing the battery charging window. Therefore, the negative electrode of the battery whose active material layers satisfy the foregoing relationships has both desirable energy density and charge-discharge kinetic performance. 
     In any one of the embodiments of this application, the active material layers satisfy 
     
       
         
           
             
               CW 
               2 
             
             ≥ 
             
               
                 3 
                 17 
               
               ⁢ 
               
                 
                   α 
                   ⨯ 
                   
                     CW 
                     1 
                   
                 
                 . 
               
             
           
         
       
     
     To take both the energy density and charge performance of the battery into account, avoiding overdesign of one performance causing inadequate design of the other, the inventors further define the relationship between coating weights and that between layer spacings of the first active material layer and the second active material layer based on an actual test results. When these relationships are satisfied, both rate performance and energy density of the negative electrode plate and the battery can be improved properly. 
     In any one of the embodiments of this application, 
     
       
         
           
             
               CW 
               ⁢ 
               1 
             
             
               CW 
               ⁢ 
               2 
             
           
         
       
     
     is inversely proportional to 
     
       
         
           
             
               
                 d 
                 ⁢ 
                 
                   1 
                   ⨯ 
                   Da 
                 
                 ⁢ 
                 50 
               
               
                 d 
                 ⁢ 
                 
                   2 
                   ⨯ 
                   Db 
                 
                 ⁢ 
                 50 
               
             
             , 
                
             
               
                 and 
                     
                 0.2 
               
               ≤ 
               
                 
                   CW 
                   ⁢ 
                   2 
                 
                 
                   
                     CW 
                     ⁢ 
                     2 
                   
                   + 
                   
                     CW 
                     ⁢ 
                     1 
                   
                 
               
               ≤ 
               0.45 
             
             , 
           
         
       
     
     where 
     Da50 is a volume median particle size of the first active material, in units of μm; and 
     Db50 is a volume median particle size of the second active material, in units of μm. 
     Based on the layer spacing and particle size of each active material layer, the coating weights of the active material layers are adjusted, so that the electrode plate and the battery has both fast charge performance and high energy density. 
     In any one of the embodiments of this application, the volume median particle size Da50 of the first active material and the volume median particle size Db50 of the second active material satisfy 
     
       
         
           
             
               0 
               .2 
             
             ≤ 
             
               
                 D 
                 ⁢ 
                 b 
                 ⁢ 
                 50 
               
               
                 D 
                 ⁢ 
                 a 
                 ⁢ 
                 50 
               
             
             ≤ 
             
               0.8 
               . 
             
           
         
       
     
     By specifying a relative relationship between the particle sizes of the active materials in the two active material layers, the kinetic performance is further improved, and processability is facilitated when a coating amount of the second layer is small. 
     In any one of the embodiments of this application, the layer spacing d 1  corresponding to the d002 peak of the first active material is within a range of 0.335-0.3362 nm, and the layer spacing d2 corresponding to the d002 peak of the second active material is within a range of 0.3356-0.38 nm. 
     In any one of the embodiments of this application, the volume median particle size Da50 of the first active material is within a range of 8-20 μm, and the volume median particle size Db50 of the second active material is within a range of 4-12 μm. 
     In any one of the embodiments of this application, the mass per unit area CW 1  of the first active material layer disposed on the negative electrode current collector is within a range of 80-200 g/m 2 , and the mass per unit area CW 2  of the second active material layer disposed on the negative electrode current collector is within a range of 10-110 g/m 2 . 
     In any one of the embodiments of this application, the first active material is a natural graphite or artificial graphite material, and/or the second active material is an artificial graphite material. 
     In any one of the embodiments, soft carbon or hard carbon is contained in the first active layer and/or the second active layer. 
     A second aspect of this application provides a secondary battery, where the secondary battery includes the negative electrode plate according to the first aspect of this application. 
     A third aspect of this application provides a battery module, where the battery module includes the secondary battery according to the second aspect of this application. 
     A fourth aspect of this application provides a battery pack, where the battery pack includes one of the secondary battery according to the second aspect of this application and the battery module according to the third aspect of this application. 
     A fifth aspect of this application provides an electric apparatus, where the electric apparatus includes at least one of the secondary battery according to the second aspect of this application, the battery module according to the third aspect of this application, or the battery pack according to the fourth aspect of this application. 
     The battery module, the battery pack, and the electric apparatus in this application include the secondary battery provided in this application, and therefore have at least the same advantages as the secondary battery. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       To describe the technical solutions in the embodiments of this application more clearly, the following briefly describes the accompanying drawings required in the embodiments of this application. Apparently, the accompanying drawings in the following description show merely some embodiments of this application, and a person of ordinary skill in the art may still derive other drawings from these accompanying drawings without creative efforts. 
         FIG.  1    is a schematic diagram of an embodiment of a negative electrode plate according to this application; 
         FIG.  2    is a schematic diagram of an embodiment of a secondary battery according to this application; 
         FIG.  3    is a schematic exploded view of an embodiment of a secondary battery according to this application; 
         FIG.  4    is a schematic diagram of an embodiment of a battery module according to this application; 
         FIG.  5    is a schematic diagram of an embodiment of a battery pack according to this application; 
         FIG.  6    is an exploded view of  FIG.  5   ; and 
         FIG.  7    is a schematic diagram of an embodiment of an electric apparatus using a secondary battery as a power source according to this application. 
     
    
    
     The accompanying drawings are not necessarily drawn to scale. Reference signs in the accompanying drawings are described as follows: 
       1 . battery pack; 
       2 . upper box body; 
       3 . lower box body; 
       4 . battery module; 
       5 . secondary battery; 
       51 . housing; 
       52 . electrode assembly; and 
       53 . cover plate. 
     DESCRIPTION OF EMBODIMENTS 
     A negative electrode plate, a secondary battery, a battery module, a battery pack, and an electric apparatus in this application are described below in detail with reference to the accompanying drawings as appropriate. However, unnecessary detailed descriptions are omitted in some cases. For example, well-known matters are not described in detail and substantially identical structures are not repeatedly described. This is to prevent the following description from becoming unnecessarily long and to facilitate ease of understanding by a person skilled in the art. In addition, the accompanying drawings and the following description are provided to help a person skilled in the art fully understand this application, and are not intended to limit the subject matter described in the claims. 
     “Ranges” disclosed in this application are defined in a form of lower and upper limits, where given ranges are defined by selecting lower and upper limits and the selected lower and upper limits define boundaries of special ranges. Ranges defined in this method may or may not be inclusive of end values, and any combinations may be used, that is, any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are provided for a specific parameter, it is understood that ranges of 60-110 and 80-120 can also be envisioned. In addition, if low limit values of a range are given as 1 and 2, and upper limit values of the range are given as 3,4, and 5, the following ranges can all be envisioned: 1-3,1-4,1-5,2-3,2-4, and 2-5. In this application, unless otherwise specified, a value range of “a-b” is a short representation of any combination of real numbers between a and b, where both a and b are real numbers. For example, a value range of “0-5” means that all real numbers within the range of “0-5” are listed herein, and “0-5” is just a short representation of a combination of these values. In addition, when a parameter is expressed as an integer greater than or equal to 2, this is equivalent to disclosure that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12. 
     Unless otherwise specified, all the embodiments and optional embodiments of this application can be mutually combined to form new technical solutions. 
     Unless otherwise specified, all the technical features and optional technical features of this application can be mutually combined to form a new technical solution. 
     Unless otherwise specified, all the steps in this application can be performed sequentially or randomly, and in some embodiments, performed sequentially. For example, a method includes steps (a) and (b), which indicates that the method may include steps (a) and (b) performed in sequence, or may include steps (b) and (a) performed in sequence. For example, that the method may further include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or steps (a), (c), and (b), or steps (c), (a), and (b), or the like. 
     Unless otherwise specified, “include” and “contain” mentioned in this application are inclusive. For example, terms “include” and “contain” can mean that other unlisted components may also be included or contained. 
     Unless otherwise specified, 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 one of the following conditions satisfies the condition “A or B”: A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present). 
     Secondary Battery 
     A secondary battery, also referred to as a rechargeable battery or a storage battery, is a battery whose active material can be activated for continuous use through charging after the battery is discharged. 
     Usually, the secondary battery includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. During charging and discharging of the secondary battery, active ions (such as lithium ions) are intercalated and deintercalated back and forth between the positive electrode plate and the negative electrode plate. The separator is disposed between the positive electrode plate and the negative electrode plate to mainly prevent short circuit between the positive electrode and the negative electrode and to allow active ions to pass through. The electrolyte is between the positive electrode plate and the negative electrode plate to conduct the active ions. 
     [Negative Electrode Plate] 
     A negative electrode is one of most critical components of a lithium-ion battery. Design of the negative electrode directly affects performance of the battery, and especially charge-related characteristics of the battery. How a battery having both desirable energy density and kinetic performance can be obtained by optimizing the design of the negative electrode plate is currently a common challenge in the industry. 
     To resolve the foregoing problem, the inventor has performed plenty of research and provided a negative electrode plate. The negative electrode plate in this application uses two or more active material layers, and by adjusting a relationship between layer spacings and that between coating weights of the active material layers, the negative electrode plate can have both relatively high energy density and good charge-discharge kinetic performance. 
     The negative electrode plate in this application includes a negative electrode current collector and active material layers disposed on at least one surface of the negative electrode current collector, where the active material layers include a first active material layer and a second active material layer disposed on a surface of the first active material layer, the first active material layer includes a first active material, the second active material layer includes a second active material, and the active material layers satisfy α×CW 2 ≤CW 1 , where 
     α is a relative factor of layer spacings, where 
     
       
         
           
             α 
             = 
             
               
                 d 
                 ⁢ 
                 2 
               
               
                 d 
                 ⁢ 
                 1 
               
             
           
         
       
     
     and 1≤α≤1.12; 
     d 1  is a layer spacing corresponding to a d002 peak of the first active material, in units of nm; 
     d 2  is a layer spacing corresponding to a d002 peak of the second active material, in units of nm; 
     CW 1  is mass per unit area of the first active material layer disposed on the negative electrode current collector, in units of g/m 2 , and if the first active material layer and the second active material layer are disposed on both sides of the negative electrode current collector, the mass should be mass of the first active material layers on the two sides; and 
     CW 2  is mass per unit area of the second active material layer disposed on the negative electrode current collector, in units of g/m 2 , and if the first active material layer and the second active material layer are disposed on both sides of the negative electrode current collector, the mass should be mass of the second active material layers on the two sides. 
     Therefore, in this application, by using two or more active material layers in a negative electrode and adjusting a relationship between layer spacings and that between coating weights of the active material layers, it is possible to obtain a negative electrode of a battery with both higher energy density and better charge-discharge kinetic performance. More specifically, the inventors of this application have found that when two active material layers are applied on one side of the current collector of the negative electrode plate with the active material layer spacing of the second active material layer larger than the active material layer spacing of the first active material layer, during migration of lithium ions from a positive electrode to the negative electrode, because of the large spacing of the surface material layer, impedance to intercalation of lithium ions is low so that lithium ions can be quickly intercalated into the surface active material. Therefore, precipitation of lithium ions on the surface of the negative electrode plate due to an excessive charging rate is avoided. Because a larger spacing of the negative active material layer means a lower capacity of the material, a heavier coating weight of the second active material layer has greater impact on the overall extractable capacity of the negative electrode plate. Taking both the extractable capacity of the negative electrode plate and the charging window into account, this application defines the coating weight of the second active material layer. A larger active material layer spacing of the second active material layer represents a stronger charging capability and a lower capacity, and correspondingly, just a small coating weight is enough to achieve a good charging level. When the active material layer spacing of the second active material layer is smaller, its charging capability becomes weaker, and the capacity is higher. In this case, in order that the electrode plate has relatively good charge performance on the whole, a proportion of the coating weight of the second active material layer to a total coating weight of the electrode plate should be increased. The inventors of this application have unexpectedly found that, with a structure with two active material layers used, the first active material layer having a small active material layer spacing and a high capacity makes the electrode plate and the battery have relatively high energy density; and the second active material layer having a large active material layer spacing allows a lower impedance to lithium intercalation on the surface of the negative electrode, avoiding deposition of lithium ions on the surface of the negative electrode plate during high-rate charging, and increasing the battery charging window. Therefore, the negative electrode of the battery whose active material layers satisfy the foregoing relationships has both desirable energy density and charge-discharge kinetic performance. 
     In some embodiments, the active material layers satisfy 
     
       
         
           
             
               CW 
               
                 2 
                   
               
             
             ≥ 
             
               
                 3 
                 17 
               
               ⁢ 
               α 
               × 
               
                 
                   CW 
                   1 
                 
                 . 
               
             
           
         
       
     
     The weight of the second active material layer is adjusted based on the layer spacing and the weight of the first active material layer. On the whole, to enable the negative electrode plate to have both high energy density and good kinetic performance, the coating weight of the second active material layer should satisfy 
     
       
         
           
             
               CW 
               
                 2 
                   
               
             
             ≥ 
             
               
                 3 
                 17 
               
               ⁢ 
               α 
               × 
               
                 
                   CW 
                   1 
                 
                 . 
               
             
           
         
       
     
     The inventors of this application have found that, when the coating weight of the active material layer on the negative electrode plate is smaller and the active material layer spacing is larger, internal resistance of the electrode plate and the battery is lower, and charge-discharge performance is better, but a smaller weight and larger spacing of the active material layer causes energy density of the battery to decrease under the same battery capacity due to the use of more auxiliary materials such as foil and more materials. To take both the energy density and charge performance of the battery into account, avoiding overdesign of one performance causing inadequate design of the other, the inventors further define the relationship between coating weights and that between layer spacings of the first active material layer and the second active material layer based on an actual test results. When these relationships are satisfied, both rate performance and energy density of the negative electrode plate and the battery can be improved properly. 
     In some embodiments, 
     
       
         
           
             
               CW 
               ⁢ 
               1 
             
             
               CW 
               ⁢ 
               2 
             
           
         
       
     
     is inversely proportional to 
     
       
         
           
             
               
                 d 
                 ⁢ 
                 1 
                 × 
                 Da 
                 ⁢ 
                 50 
               
               
                 d 
                 ⁢ 
                 2 
                 × 
                 Db 
                 ⁢ 
                 50 
               
             
             , 
             
               
                 and 
                     
                 0.2 
               
               ≤ 
               
                 
                   CW 
                   ⁢ 
                   2 
                 
                 
                   
                     CW 
                     ⁢ 
                     2 
                   
                   + 
                   
                     CW 
                     ⁢ 
                     1 
                   
                 
               
               ≤ 
               0.45 
             
             , 
           
         
       
     
     where 
     Da50 is a volume median particle size of the first active material, in units of μm; and 
     Db50 is a volume median particle size of the second active material, in units of μm. 
     Based on the layer spacing and particle size of each of the active material layers, when the coating weights of the active material layers and the active material particle sizes of the active material layers are adjusted to satisfy that 
     
       
         
           
             
               CW 
               ⁢ 
               1 
             
             
               CW 
               ⁢ 
               2 
             
           
         
       
     
     is inversely proportional to 
     
       
         
           
             
               
                 d 
                 ⁢ 
                 1 
                 × 
                 Da 
                 ⁢ 
                 50 
               
               
                 d 
                 ⁢ 
                 2 
                 × 
                 Db 
                 ⁢ 
                 50 
               
             
             , 
             
               
                 and 
                 ⁢ 
                      
                 that 
                 ⁢ 
                 
                       
                     
                 
                 0.2 
               
               ≤ 
               
                 
                   CW 
                   ⁢ 
                   2 
                 
                 
                   
                     CW 
                     ⁢ 
                     2 
                   
                   + 
                   
                     CW 
                     ⁢ 
                     1 
                   
                 
               
               ≤ 
               0.45 
             
             , 
           
         
       
     
     the electrode plate and the battery has both good fast charge performance and high energy density. 
     In some embodiments, the volume median particle size Da50 of the first active material and the volume median particle size Db50 of the second active material satisfy 
     
       
         
           
             0.2 
             ≤ 
             
               
                 Db 
                 ⁢ 
                 50 
               
               
                 Da 
                 ⁢ 
                 50 
               
             
             ≤ 
             
               0.8 
               . 
             
           
         
       
     
     By specifying a relative relationship between the particle sizes of the active materials in the two active material layers, the kinetic performance is further improved, and processability is facilitated when a coating amount of the second layer is small. 
     In some embodiments, the layer spacing d 1  corresponding to the d002 peak of the first active material is within a range of 0.335-0.3362 nm, and the layer spacing d 2  corresponding to the d002 peak of the second active material is within a range of 0.3356-0.38 nm. 
     In some embodiments, the volume median particle size Da50 of the first active material is within a range of 8-20 μm, and the volume median particle size Db50 of the second active material is within a range of 4-12 μm. 
     In some embodiments, the mass per unit area CW 1  of the first active material layer disposed on the negative electrode current collector is within a range of 80-200 g/m 2 , and the mass per unit area CW 2  of the second active material layer disposed on the negative electrode current collector is within a range of 10-110 g/m 2 . 
     In this application, the volume median particle size D50 of materials has a meaning known in the art, and can be measured by using a method and an instrument known in the art. For example, it may be measured by using a laser particle size analyzer (for example, Mastersizer 2000E made by Malvern Panalytical in the UK) in accordance with GB/T 19077-2016 Particle Size Analysis-Laser Diffraction Methods. 
     In some embodiments, the negative electrode plate further includes a conductive agent and a binder. Types and contents of the agents are not specifically limited and may be selected based on an actual requirement. For example, the conductive agent is one or more of superconducting carbon, carbon black (for example, acetylene black or Ketjen black), carbon dots, carbon nanotube, graphene, and carbon nanofiber. The binder may include one or more of styrene-butadiene rubber (SBR), water soluble unsaturated resin SR-1B, water-borne acrylic resin (for example, polyacrylic acid PAA, polymethylacrylic acid PMAA, and sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). Types of other optional additives may be the same or different. For example, the other optional additives may include a thickener (for example, sodium carboxymethyl cellulose CMC-Na) and a PTC thermistor material. 
     In the negative electrode plate in this application, the negative electrode current collector may use a metal foil or a composite current collector. As an example of the metal foil, a copper foil may be used. The composite current collector may include a polymer material matrix and a metal material layer formed on at least one surface of the polymer material matrix. For example, a metal material may be selected from one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. For example, the polymer material matrix may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE). 
     In some embodiments, the active material layer is disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two opposite surfaces in its thickness direction, and the active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector. 
       FIG.  1    shows a negative electrode current collector with active material layers on its surface, where two surfaces of the negative electrode current collector shown in  FIG.  1    both have a first active material layer and a second active material layer. 
     Certainly, a negative electrode plate  10  in this application may also have other embodiments. For example, the negative electrode plate  10  includes a negative electrode current collector  11 , a first active material layer  121  disposed on one side of the negative electrode current collector, and a second active material layer  122  disposed on the first active material layer  121 . 
     In addition, the negative electrode plate in this application does not exclude additional functional layers other than the negative electrode film layer. For example, in some embodiments, the negative electrode plate in this application may further include a conductive primer layer (which is, for example, formed by a conductive agent and a binder) disposed between the negative electrode current collector and the second negative electrode film layer. In some other embodiments, the negative electrode plate in this application further includes a protection layer covering a surface of the first negative electrode film layer. 
     In some embodiments, a preparation method of the negative electrode plate of this application may include the following steps: 
     1. Prepare a slurry A containing a first active material and a slurry B containing a second active material separately. 
     2. Calculate CW 1  and CW 2  based on a layer spacing of the first active material and a layer spacing of the second active material. 
     3. Apply the slurry A on a current collector and dry it to obtain an electrode plate A coated with a first active material layer. 
     4. Apply the slurry B on the surface of the electrode plate A and dry it, and then perform cold-pressed and slitting to obtain the negative electrode plate described in this application. 
     Coating amounts of the first active material layer and a second active material layer respectively satisfy CW 1  and CW 2 . 
     In some embodiments, the slurry A includes the first active material and one or more of a conductive agent, a binder, and a thickener. 
     In some embodiments, the slurry B includes the second active material and one or more of a conductive agent, a binder, and a thickener. 
     For a specific method for preparing a negative electrode, reference may be made to a specific embodiment provided in this application. Details are not described herein. 
     [Positive Electrode Plate] 
     The secondary battery includes a positive electrode plate. The positive electrode plate usually includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector and including a positive electrode active material. For example, the positive electrode current collector has two opposite surfaces in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector. 
     In the positive electrode plate in this application, the positive electrode current collector may use a metal foil or a composite current collector. As an example of the metal foil, an aluminum foil may be used as the positive electrode current collector. The composite current collector may include a polymer material matrix and a metal material layer formed on at least one surface of the polymer material matrix. For example, a metal material may be selected from one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. For example, the polymer material matrix may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE). 
     In the positive electrode plate in this application, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material may be a known positive electrode active material for secondary batteries in the art. For example, the positive electrode active material may include one or more of lithium transition metal oxide, olivine-structured lithium-containing phosphate, and respective modified compounds thereof. Examples of the lithium transition metal oxide may include but are not limited to one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds. Examples of the olivine-structured lithium-containing phosphate may include but are not limited to one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds. This application is not limited to these materials, and other conventionally well-known materials that can be used as a positive electrode active material for secondary batteries may also be used. One of these positive electrode active materials may be used alone, or two or more of them may be used in combination. 
     In the positive electrode plate in this application, the modified compounds of the positive electrode active materials may be obtained by making a doping modification, a surface coating modification, or both a doping modification and a surface coating modification to the positive electrode active materials. 
     In the positive electrode plate in this application, the positive electrode film layer usually includes the positive electrode active material and optionally, a binder, and optionally, a conductive agent. The positive electrode film layer is usually formed by applying a positive electrode slurry onto the positive electrode current collector, followed by drying and cold-pressing. The positive electrode slurry is usually formed by dispersing the positive electrode active material and optionally, the conductive agent, and optionally, the binder and any other component in a solvent and stirring them evenly. The solvent may be but is not limited to N-methylpyrrolidone (NMP). For example, the binder for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylic resin. For example, the conductive agent for the positive electrode film layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofiber. It should be noted that the composition or parameters of the positive electrode film layer described in this application are all the composition or parameter ranges of a single-side film layer of the positive electrode current collector. When the positive electrode film layer is disposed on two opposite surfaces of the positive electrode current collector, the composition or parameters of the positive electrode film layer on any one of the surfaces satisfies this application, that is, being considered to fall within the protection scope of this application. 
     [Electrolyte] 
     An electrolyte conducts active ions between the positive electrode plate and the negative electrode plate. For the secondary battery in this application, there is no specific limitation on a type of the electrolyte, and the electrolyte may be selected based on a requirement. For example, the electrolyte may be selected from at least one of a solid electrolyte or a liquid electrolyte (that is, a liquid electrolyte). 
     In some embodiments, the electrolyte is a liquid electrolyte. The electrolyte includes an electrolyte salt and a solvent. 
     In some embodiments, a type of the electrolyte salt is not specifically limited, and may be selected based on an actual requirement. For example, the electrolyte salt may be selected from one or more of LiPF 6  (lithium hexafluorophosphate), LiBF 4  (lithium tetrafluoroborate), LiClO 4  (lithium perchlorate), LiAsF 6  (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bistrifluoromethanesulfonimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium dioxalate borate), LiPO 2 F 2  (lithium difluorophosphate), LiDFOP (lithium difluoro(dioxalato)phosphate), and LiTFOP (lithium tetrafluoro oxalato phosphate). In some embodiments, a type of the solvent is not specifically limited, and may be selected based on an actual requirement. For example, the solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), methyl sulfonyl methane (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE). 
     In some embodiments, optionally, the solvent is a non-aqueous solvent. 
     In some embodiments, optionally, the electrolyte further includes an additive. For example, the additive may include a negative electrode film-forming additive, or may include a positive electrode film-forming additive, or may include an additive capable of improving some performance of the battery, for example, an additive for improving overcharge performance of the battery, an additive for improving high-temperature performance of the battery, or an additive for improving low-temperature performance of the battery. 
     [Separator] 
     A secondary battery using a liquid electrolyte and some secondary batteries using solid electrolytes further include a separator. The separator is disposed between the positive electrode plate and the negative electrode plate to provide separation. The separator is not limited to any specific type in this application, and may be any commonly known porous separator with good chemical stability and mechanical stability. In some embodiments, a material of the separator may be selected from one or more of a glass fiber, non-woven, polyethylene, polypropylene polyethylene, and poly (vinylidene difluoride). The separator may be a single-layer thin film or a multilayer composite thin film. When the separator is a multilayer composite thin film, each layer may be made of the same or different materials. 
     In some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be made into an electrode assembly through a winding process or a lamination process. 
     In some embodiments, the secondary battery may include an outer package. The outer package may be used for packaging the electrode assembly and the electrolyte. 
     In some embodiments, the outer package of the secondary battery may be a hard shell, for example, a hard plastic shell, an aluminum shell, or a steel shell. The outer package of the secondary battery may alternatively be a soft package, for example, a soft bag. A material of the soft package may be plastic, for example, one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), and the like. 
     This application does not impose special limitations on a shape of the secondary battery, and the secondary battery may be cylindrical, rectangular, or of any other shape.  FIG.  2    shows a secondary battery  5  with a rectangular structure as an example. 
     In some embodiments, referring to  FIG.  3   , the outer package may include a housing  51  and a cover plate  53 . The housing  51  may include a base plate and a side plate connected onto the base plate, and the base plate and the side plate enclose an accommodating cavity. The housing  51  has an opening connected to the accommodating cavity, and the cover plate  53  is configured to cover the opening to seal the accommodating cavity. A positive electrode plate, a negative electrode plate, and a separator may be made into an electrode assembly  52  through a winding process or a lamination process. The electrode assembly  52  is packaged in the accommodating cavity. The electrolyte infiltrates into the electrode assembly  52 . There may be one or more electrode assemblies  52  in the secondary battery  5 , and the quantity may be adjusted based on a requirement. 
     In some embodiments, the secondary battery may be assembled into a battery module. The battery module may include a plurality of secondary batteries and a specific quantity may be adjusted based on application and capacity of the battery module. 
       FIG.  4    shows a battery module  4  as an example. Referring to  FIG.  4   , in the battery module  4 , a plurality of secondary batteries  5  may be sequentially arranged in a length direction of the battery module  4 . Certainly, the secondary batteries may alternatively be arranged in any other manner. Further, the plurality of secondary batteries  5  may be fastened using fasteners. 
     Optionally, the battery module  4  may further include a housing with an accommodating space, and the plurality of secondary batteries  5  are accommodated in the accommodating space. 
     In some embodiments, the battery module may be further assembled into a battery pack, and a quantity of battery modules included in the battery pack may be adjusted based on application and capacity of the battery pack. 
       FIG.  5    and  FIG.  6    show a battery pack  1  as an example. Referring to  FIG.  5    and  FIG.  6   , 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  is configured to cover the lower box body  3  to form an enclosed space for accommodating the battery modules  4 . The plurality of battery modules  4  may be arranged in the battery box in any manner. 
     Electric Apparatus 
     An embodiment of this application further provides an electric apparatus. The electric apparatus includes at least one of the secondary battery, the battery module, or the battery pack provided in this application. The secondary battery, the battery module, or the battery pack may be used as a power source for the electric apparatus, or an energy storage unit of the electric apparatus. The electric apparatus may be, but is not limited to, a mobile device (for example, a mobile phone or a notebook computer), an electric vehicle (for example, a battery electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf vehicle, or an electric truck), an electric train, a ship, a satellite, an energy storage system, and the like. 
     The secondary battery, the battery module, or the battery pack may be selected for the electric apparatus based on requirements for using the apparatus. 
       FIG.  7    shows an electric apparatus as an example. The electric apparatus is a full electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or the like. To satisfy a requirement of the electric apparatus for high power and high energy density, a battery pack or a battery module may be used. 
     In another example, the electric apparatus may be a mobile phone, a tablet computer, a notebook computer, or the like. The electric apparatus is usually required to be light and thin, and the secondary battery may be used as a power source. 
     EXAMPLES 
     Examples below more specifically describe the content disclosed in this application, and these examples are merely used for explanatory description. It is apparent for a person skilled in the art to make various modifications and variations within the scope of the content disclosed in this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight, all reagents used in the examples, the first active material, and the second active material are commercially available or synthesized in a conventional manner. All instruments used in the examples are commercially available. 
     Examples 1 to 9 
     Lithium-ion batteries including the negative electrode plate in this application in Examples 1 to 9 were prepared according to the following method. 
     Preparation of Negative Electrode Plate 
     1. According to Table 1, a first active material of a first active material layer, a conductive agent Super-P, a binder SBR, and a thickener CMC were mixed at a mass ratio of 96:1:2:1, and then fully stirred and evenly mixed in a deionized aqueous solvent system, to obtain a slurry A. 
     2. According to Table 1, a second active material of a second active material layer, a conductive agent Super-P, a binder SBR, and a thickener CMC were mixed at a mass ratio of 96:1:2:1, and then fully stirred and evenly mixed in a deionized aqueous solvent system, to obtain a slurry B. 
     3. According to Table 1, a Cu foil was coated with the slurry A and dried, to obtain an electrode plate A coated with the first active material layer. 
     4. According to Table 1, a surface of the electrode plate A was coated with the slurry B and dried, and then the electrode plate A was cold-pressed and slit, to obtain a negative electrode plate having two active material layers. 
     Coating amounts of the first active material layer and the second active material layer respectively satisfied CW 1  and CW 2 . 
     Preparation of Positive Electrode Plate 
     A positive electrode active material LiFePO 4 , a conductive agent acetylene black, and a binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 96:2:2. A solvent N-methylpyrrolidone (NMP) was added. Then the resulting mixture was stirred evenly by a vacuum mixer to obtain a positive electrode slurry. The positive electrode slurry was evenly applied on an aluminum foil of the positive electrode current collector. The aluminum foil was dried at room temperature and then continued to be dried in an oven. Then cold-pressing and slitting were performed to obtain a positive electrode plate. 
     Preparation of Electrolyte 
     Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1:1 to obtain an organic solvent, and then a fully dried lithium salt LiPF 6  was evenly dissolved in the organic solvent to obtain an electrolyte whose concentration was 1 mol/L. 
     Preparation of Separator 
     A polyethylene film was used as a separator. 
     Preparation of Secondary Battery 
     The positive electrode plate, the separator, and the negative electrode plate were sequentially stacked so that the separator was located between the positive electrode plate and the negative electrode plate to provide separation. Then the resulting stack was wound to obtain an electrode assembly. The electrode assembly was placed into an outer package and dried. Then an electrolyte was injected, and processes such as vacuum packaging, standing, formation, and shaping were performed to obtain a secondary battery. 
     Comparative Examples 1 to 3 
     A method for preparing secondary batteries of Comparative Examples 1 to 3 is the same as that of Examples 1 to 13, except for a process of preparing the negative electrode plate. Details are given in Table 1. 
     Tests 
     Energy density: At 25° C., the lithium-ion battery was charged to 4.2V with a constant current of 1C, then charged with a constant voltage of 4.2V until the current was less than 0.05C, and then discharged to 2.8V at 0.1C to obtain discharge energy Q. Mass of the battery was measured as M. Energy density=Q/M. 
     Charge time: The charge time described in this application is a time for constant charging at 25° C. to 85% SOC using a lithium precipitation window rate. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Data table of examples and comparative examples 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 Negative electrode active material layer 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 First 
                   
                   
                   
                 Second 
                   
                   
                   
                   
               
               
                   
                 active 
                 CW 1   
                 Wh/kg 
                 Da50 
                 active 
                 CW 2   
                 D2 
                 Db50 
               
               
                 No. 
                 material 
                 g/m 2   
                 nm 
                 μm 
                 material 
                 g/m 2   
                 nm 
                 μm 
                 α 
               
               
                   
               
               
                 Example 1 
                 Artificial 
                 176 
                 0.3358 
                 18 
                 Soft 
                 44 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Comparative 
                 Artificial 
                 176 
                 0.3358 
                 18 
                 Hard 
                 44 
                 0.3805 
                 8 
                 1.133 
               
               
                 Example 1 
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Comparative 
                 Artificial 
                 88 
                 0.3362 
                 18 
                 Artificial 
                 132  
                 0.3358 
                 8 
                 0.999 
               
               
                 Example 2 
                 graphite 
                   
                   
                   
                 graphite 
               
               
                 Comparative 
                 Artificial 
                 220 
                 0.3358 
                 18 
                 Artificial 
                 / 
                 / 
                 / 
                 One 
               
               
                 Example 3 
                 graphite 
                   
                   
                   
                 graphite 
                   
                   
                   
                 layer 
               
               
                 Example 1 
                 Artificial 
                 176 
                 0.3358 
                 18 
                 Soft 
                 44 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 2 
                 Artificial 
                 154 
                 0.3358 
                 18 
                 Soft 
                 66 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 3 
                 Artificial 
                 110 
                 0.3358 
                 18 
                 Soft 
                 90 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 4 
                 Artificial 
                 176 
                 0.3358 
                 18 
                 Soft 
                 35 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 5 
                 Artificial 
                 80 
                 0.3358 
                 18 
                 Soft 
                 70 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 1 
                 Artificial 
                 176 
                 0.3358 
                 18 
                 Soft 
                 44 
                 0.3700 
                 8 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 6 
                 Artificial 
                 176 
                 0.3358 
                 20 
                 Soft 
                 44 
                 0.3700 
                 4 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 7 
                 Artificial 
                 176 
                 0.3358 
                 15 
                 Soft 
                 44 
                 0.3700 
                 12 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 8 
                 Artificial 
                 176 
                 0.3358 
                 22 
                 Soft 
                 44 
                 0.3700 
                 4 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                 Example 9 
                 Artificial 
                 176 
                 0.3358 
                 12 
                 Soft 
                 44 
                 0.3700 
                 12 
                 1.102 
               
               
                   
                 graphite 
                   
                   
                   
                 carbon 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Negative electrode active material layer 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 α × 
                   
                 CW 2  ≥ 
                 CW 2 / 
                   
                 Energy 
                 Charge 
               
               
                   
                 α × 
                 CW 2  ≤ 
                 3/17α × 
                 3/17α × 
                 (CW 2  + 
                 Db50/ 
                 density 
                 time 
               
               
                 No. 
                 CW 2   
                 CW 1 ? 
                 CW 1   
                 CW 1 ? 
                 CW 1 ) 
                 Da50 
                 Wh/kg 
                 min 
               
               
                   
               
               
                 Example 1 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 0.44 
                 185 
                 45 
               
               
                 Comparative 
                 49.857 
                 Yes 
                 35 
                 Yes 
                 0.2 
                 0.44 
                 165 
                 43 
               
               
                 Example 1 
               
               
                 Comparative 
                 131.843  
                 No 
                 16 
                 Yes 
                 0.6 
                 0.44 
                 187 
                 82 
               
               
                 Example 2 
               
               
                 Comparative 
                 / 
                 / 
                 / 
                 / 
                 / 
                 / 
                 185 
                 90 
               
               
                 Example 3 
               
               
                 Example 1 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 0.44 
                 185 
                 45 
               
               
                 Example 2 
                 72.722 
                 Yes 
                 30 
                 Yes 
                 0.3 
                 0.44 
                 182 
                 45 
               
               
                 Example 3 
                 99.166 
                 Yes 
                 21 
                 Yes 
                 0.45 
                 0.44 
                 180 
                 43 
               
               
                 Example 4 
                 38.565 
                 Yes 
                 34 
                 Yes 
                 0.17 
                 0.44 
                 180 
                 47 
               
               
                 Example 5 
                 77.129 
                 Yes 
                 16 
                 Yes 
                 0.47 
                 0.44 
                 176 
                 40 
               
               
                 Example 1 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 0.44 
                 185 
                 45 
               
               
                 Example 6 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 0.20 
                 184 
                 42 
               
               
                 Example 7 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 0.80 
                 185 
                 51 
               
               
                 Example 8 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 0.18 
                 181 
                 46 
               
               
                 Example 9 
                 48.481 
                 Yes 
                 34 
                 Yes 
                 0.2 
                 1.00 
                 185 
                 50 
               
               
                   
               
            
           
         
       
     
     In this application, by using two or more active material layers in a negative electrode and adjusting a relationship between layer spacings and that between coating weights of the active material layers, it is possible to obtain a negative electrode of a battery with both higher energy density and better charge-discharge kinetic performance. 
     According to Table 1, from the comparison between Examples 1 to 9 and Comparative Examples 1 to 3, it can be seen that the secondary batteries obtained by using two active material layers in the negative electrode and adjusting the relationship between layer spacings and that between coating weights of the active material layers have both higher energy density and better charge-discharge kinetic performance. In short, the negative electrode materials prepared by using the method of the present disclosure all satisfy that the charge time is less than or equal to 75 minutes and that 0.3C energy density is greater than or equal to 170 Wh/kg. 
     Comparative Examples 1 to 3 do not conform to the relationship between layer spacings and that between coating weights of active material layers as defined in this application. Specifically, in comparison between Comparative Example 1 and Example 1 of this application, the second active material in Comparative Example 1 uses hard carbon with a larger layer spacing, and its capacity density is much lower than that in Example 1 of this application. In other words, when the active material layer spacing of the second active material layer is larger, a charging capability is stronger, but the capacity is lower, and obviously, energy density and charge-discharge kinetic performance cannot be both guaranteed. In comparison with Example 1, in Comparative Example 2, the relative factor a of layer spacings is not within the range defined in this application (1≤α≤1.12), and therefore energy density and charge-discharge kinetic performance cannot be both guaranteed. In comparison with Example 1, Comparative Example 3 has a single layer of coating, showing that given a same coating amount, a single layer of coating is unable to guarantee both energy density and charge-discharge kinetic performance. 
     Further, in comparison with Examples 1 to 3, CW 2/ (CW 2 +CW 1 ) in Example 5 is beyond the range defined in this application, and as a result, energy density cannot be guaranteed although charge-discharge kinetic performance is good; whereas, CW 2/ (CW 2 +CW 1 ) in Example 4 is beyond the range defined in this application, and as a result, charge-discharge kinetic performance cannot although energy density is good. 
     Further, in comparison with Example 1, Example 6, and Example 7, Db50/Da50 in Example 8 and that in Example 9 are beyond the ranged defined in this application, and as a result, energy density and charge-discharge kinetic performance cannot be both guaranteed. 
     The foregoing descriptions are merely specific embodiments of this application, but are not intended to limit the protection scope of this application. Any equivalent modifications or replacements readily figured out by a person skilled in the art within the technical scope disclosed in this application shall fall within the protection scope of this application. Therefore, the protection scope of this application shall be subject to the protection scope of the claims. 
     It should be noted that this application is not limited to the foregoing embodiments. The foregoing embodiments are merely examples, and embodiments having substantially the same composition as the technical idea and exerting the same functions and effects within the scope of the technical solutions of this application are all included in the technical scope of this application. In addition, without departing the scope of the essence of this application, various modifications that can be conceived by a person skilled in the art and applied to the embodiments, and other forms constructed by combining some of constituent elements of the embodiments are also included in the scope of this application.