Negative electrode sheet and preparation method thereof, battery and electrical equipment

Abstract

A negative electrode sheet and a preparation method thereof, as well as a battery and an electrical device. The negative electrode sheet includes a current collector and n active layers. The current collector has a first surface. The n active layers are stacked sequentially along a first direction perpendicular to the first surface. Each active layer includes a first hard carbon material and / or a second hard carbon material. The content of the first hard carbon material in the m+1th active layer is less than that in the mth active layer, the content of the second hard carbon material in the m+1th active layer is greater than that in the mth active layer, the mth active layer is closer to the first surface than the m+1th active layer, and the conditions 1≤m≤n-1 are satisfied, where m and n are both positive integers. Furthermore, the condition b1<b2 is satisfied. This negative electrode sheet enables a battery to achieve both energy density and chargeability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet and a preparation method thereof, a battery and an electrical device. Background Art

[0002] Sodium-ion batteries cost less than lithium-ion batteries, but the energy density of existing sodium-ion battery systems is far lower than that of lithium-ion batteries, which limits the application scenarios of sodium-ion batteries.

[0003] Among them, sodium-ion battery negative electrodes have significant room for improvement in gram capacity and compaction. However, increasing the gram capacity of the negative electrode requires increasing its plateau region around 0V, which will increase the corresponding plateau capacity ratio. However, this will result in a decrease in the slope capacity ratio, which means a decrease in power performance. It is difficult to achieve both. Summary of the Invention

[0004] The purpose of the present invention is to provide a negative electrode sheet and a preparation method thereof, a battery and an electrical device to solve the problem that it is difficult to balance the power performance and battery capacity of sodium ion batteries.

[0005] To achieve the purpose of the present invention, the present invention provides the following technical solutions:

[0006] In a first aspect, the present invention provides a negative electrode sheet, comprising:

[0007] a current collector having a first surface;

[0008] n active layers are stacked sequentially on the first surface along a first direction, wherein the first direction is perpendicular to the first surface;

[0009] Each of the active layers includes a first hard carbon material and / or a second hard carbon material; a content of the first hard carbon material in the (m+1)th active layer is less than a content of the first hard carbon material in the (m)th active layer, a content of the second hard carbon material in the (m+1)th active layer is greater than a content of the second hard carbon material in the (m)th active layer, the (m)th active layer is closer to the first surface than the (m+1)th active layer, and 1≤m≤n-1 is satisfied, where m and n are both positive integers;

[0010] The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1<b2;

[0011] Furthermore, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery during sodium insertion at a voltage above 0.1 V vs. Na+ / Na.

[0012] In one implementation, the following condition is also satisfied: 2≤n≤4.

[0013] In one embodiment, the reversible gram capacity of the first hard carbon material is a1, which satisfies: a1≥290 mAh / g.

[0014] In one embodiment, the following is also satisfied: 20%≤b1≤40%.

[0015] In one embodiment, the following is also satisfied: 40%≤b2≤70%.

[0016] In one embodiment, the thickness of the active layer is h, satisfying: h≥20 μm.

[0017] In one embodiment, the following is also satisfied: 30 μm ≤ h ≤ 60 μm.

[0018] In one embodiment, the platform area capacity proportion of the first hard carbon material is e1, satisfying: 60%≤e1≤80%; the platform area capacity proportion of the second hard carbon material is e2, satisfying: 30%≤e2≤60%, and the capacity of the platform area of ​​the first hard carbon material and the second hard carbon material is the capacity of the battery during the sodium insertion process from 0V vs.Na+ / Na to 0.1V vs.Na+ / Na.

[0019] In a second aspect, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0020] providing a current collector having a first surface;

[0021] stacking n active layers in sequence on the first surface along a first direction, wherein the first direction is perpendicular to the first surface;

[0022] Each of the active layers includes a first hard carbon material and / or a second hard carbon material; a content of the first hard carbon material in the (m+1)th active layer is less than a content of the first hard carbon material in the (m)th active layer, a content of the second hard carbon material in the (m+1)th active layer is greater than a content of the second hard carbon material in the (m)th active layer, the (m)th active layer is closer to the first surface than the (m+1)th active layer, and 1≤m≤n-1 is satisfied, where m and n are both positive integers;

[0023] The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1<b2;

[0024] The capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery during sodium insertion at a voltage above 0.1 V vs. Na+ / Na.

[0025] In a third aspect, the present invention provides a battery comprising a positive electrode sheet, a separator, and a negative electrode sheet as described in any one of the first aspects, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet.

[0026] In a fourth aspect, the present invention provides an electrical device, comprising an electrical device and the battery as described in the third aspect, wherein the battery supplies power to the electrical device.

[0027] The negative electrode sheet of the present invention is provided with multiple active layers, and the active layers are provided with a first hard carbon material for improving the battery capacity and a second hard carbon material for improving the battery rate. The first hard carbon material decreases gradiently in the active layer in the direction away from the current collector, and the second hard carbon material increases gradiently in the active layer in the direction away from the current collector, that is, in the direction close to the electrode. The first hard carbon material and the second hard carbon material with gradient change are provided in the multi-layer active layers, so that the ion exchange rate in the part close to the electrode is improved, and the ion accommodation capacity in the part close to the current collector is improved. The corresponding sodium ion battery can take into account the improvement of power performance and battery capacity, so that the battery capacity is improved while having better rate performance and shorter fast charging time. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 is a side view of a negative electrode sheet and a separator according to an embodiment;

[0030] Figure 2 The present invention is a flow chart of a method for preparing a negative electrode sheet according to an embodiment.

[0031] Description of reference numerals:

[0032] 100-negative electrode sheet, 10-current collector, 11-first surface, 20-active layer, 200-separator, X-first direction. DETAILED DESCRIPTION

[0033] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0034] It should be noted that when a component is referred to as being "fixed to" another component, it may be directly on the other component or there may be an intermediate component. When a component is referred to as being "connected to" another component, it may be directly connected to the other component or there may be an intermediate component.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which this invention pertains. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0036] The following embodiments of the present invention are described in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0037] refer to Figure 1 The present invention provides a negative electrode sheet 100, comprising a current collector 10 and n active layers 20. The current collector 10 has a first surface 11. The n active layers 20 are stacked in sequence on the first surface 11 along a first direction X, and the first direction X is perpendicular to the first surface 11. Each active layer 20 comprises a first hard carbon material and / or a second hard carbon material. The content of the first hard carbon material in the m+1th active layer 20 is less than the content of the first hard carbon material in the mth active layer 20, the content of the second hard carbon material in the m+1th active layer 20 is greater than the content of the second hard carbon material in the mth active layer 20, the mth active layer 20 is closer to the first surface 11 than the m+1th active layer 20, and satisfies 1≤m≤n-1, and m and n are both positive integers.

[0038] The first hard carbon material and the second hard carbon material have specific special properties, specifically: the slope region capacity ratio of the first hard carbon material is b1, the slope region capacity ratio of the second hard carbon material is b2, and b1<b2;

[0039] The capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery during the sodium insertion process at a voltage above 0.1V vs. Na+ / Na.

[0040] When the electrode sheet is a negative electrode sheet 100 , the corresponding current collector 10 may be any one of copper foil, composite copper foil or carbon-coated copper foil, or any one of aluminum foil, composite aluminum foil or carbon-coated aluminum foil.

[0041] The first and second hard carbon materials can be resin carbon (such as phenolic resin, epoxy resin, polyfurfuryl alcohol resin, etc.), organic polymer carbon (such as polyvinyl alcohol, polyvinyl chloride, polyvinylidene fluoride, polyacrylonitrile, etc.), carbon black (such as acetylene black prepared by chemical vapor deposition), or biomass carbon (such as plant residues and shells), without limitation. Hard carbon materials, as negative electrode materials for sodium-ion batteries, are suitable for sodium ion intercalation and deintercalation due to their large interlayer spacing and irregular structure.

[0042] In one embodiment, the reversible specific capacity of the first hard carbon material is a1, which satisfies the following: a1 ≥ 290 mAh / g. a1 can be 290 mAh / g, 300 mAh / g, 350 mAh / g, 400 mAh / g, etc., without limitation.

[0043] In one embodiment, the following is also satisfied: 20%≤b1≤40%. b1 can be 20%, 25%, 30%, 35%, or 40%, without limitation. Optionally, b1 also satisfies 30%≤b1≤40%.

[0044] The first hard carbon material has a1≥290mAh / g and the slope area capacity ratio b1 satisfies 20%≤b1≤40%, making the first hard carbon material a capacity-type hard carbon material, so that the energy density of the battery is greater and more electricity can be stored.

[0045] In one embodiment, the following conditions are satisfied: 40% ≤ b2 ≤ 70%. b2 can be 40%, 50%, 60%, or 70%, without limitation. Alternatively, 40% ≤ b2 ≤ 60%. Furthermore, if b1 is 40%, b2 cannot be 40%. The second hard carbon material is a rate-sensitive hard carbon material, enabling the battery to release or absorb energy more quickly, resulting in faster charging and discharging, and thus improving the operating efficiency of the device.

[0046] The total mass of the first hard carbon material and the second hard carbon material in each active layer 20 may be the same or different.

[0047] The number of layers n of the active layer 20 satisfies 1≤m≤n-1. Optionally, it also satisfies 2≤n≤4. Specifically, n can be 2, 3, or 4. Multiple active layers 20 are stacked on the current collector 10, and as they move away from the current collector 10 along the first direction X, the content of the first hard carbon material in the active layer 20 decreases gradually, while the content of the second hard carbon material in the active layer 20 increases gradually. Figure 1 The active layer 20 directly stacked on the first surface 11 is the first active layer 20, the active layer 20 farthest from the first surface 11 is the nth active layer 20, and the nth active layer 20 is in close contact with the diaphragm 200. Optionally, the hard carbon material in the first active layer 20 is all the first hard carbon material, and the hard carbon material in the nth active layer 20 is all the second hard carbon material.

[0048] The negative electrode sheet 100 of the present invention is provided with multiple active layers 20, and the active layers 20 are provided with a first hard carbon material for increasing the battery capacity and a second hard carbon material for increasing the battery rate. The first hard carbon material decreases gradiently in the active layer 20 in the direction away from the current collector 10, and the second hard carbon material increases gradiently in the active layer 20 in the direction away from the current collector 10, that is, in the direction close to the electrode. The first hard carbon material and the second hard carbon material with gradient changes are provided in the multi-layer active layer 20, so that the ion exchange rate in the part close to the electrode is improved, and the ion accommodation capacity in the part close to the current collector is improved. The corresponding sodium ion battery can have better rate performance and shorter fast charging time while improving the battery capacity.

[0049] In one embodiment, the thickness of the active layer 20 is h, which satisfies: h ≥ 20 μm. Specifically, 30 μm ≤ h ≤ 60 μm. h can be 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, without limitation.

[0050] When h≥20 μm, the active layer 20 can provide more electrochemical reaction area, thereby improving the capacity and energy density of the battery.

[0051] When h > 60 μm, an excessively thick active layer 20 may increase the transmission distance of electrons and ions, thereby increasing the battery's internal resistance and charge / discharge time. An excessively thick active layer 20 is also susceptible to lithium dendrites, leading to reduced battery performance. When h is 30 μm ≤ 60 μm, the active layer 20 can balance the battery's capacity, energy density, and charge / discharge speed while also reducing the battery's weight, thereby improving its energy density and portability.

[0052] In one embodiment, the platform area capacity proportion of the first hard carbon material is e1, satisfying: 60%≤e1≤80%; the platform area capacity proportion of the second hard carbon material is e2, satisfying: 30%≤e2≤60%, and the capacity of the platform area of ​​the first hard carbon material and the second hard carbon material is the capacity of the battery during the sodium insertion process from 0V vs.Na+ / Na to 0.1V vs.Na+ / Na.

[0053] e1 can be 60%, 65%, 70%, 75%, 80%, etc., without limitation. e2 can be 30%, 40%, 50%, 60%, etc., without limitation. Specifically, the sum of e1 and b1 is 100%, and the sum of e2 and b2 is 100%.

[0054] The first hard carbon material has a large platform area capacity, so that the first hard carbon material has a higher specific capacity; the second hard carbon material has a large platform area capacity, so that the second hard carbon material has better rate performance.

[0055] refer to Figure 1 and Figure 2 The present invention also provides a method for preparing a negative electrode sheet 100, comprising the following steps:

[0056] Step S10, providing a current collector 10, wherein the current collector 10 has a first surface 11;

[0057] In step S20 , n active layers 20 are sequentially stacked on the first surface 11 along a first direction X, where the first direction X is perpendicular to the first surface 11 .

[0058] Each active layer 20 includes a first hard carbon material and / or a second hard carbon material; a content of the first hard carbon material in the (m+1)th active layer 20 is less than a content of the first hard carbon material in the (m)th active layer 20, a content of the second hard carbon material in the (m+1)th active layer 20 is greater than a content of the second hard carbon material in the (m)th active layer 20, the (m)th active layer 20 is closer to the first surface 11 than the (m+1)th active layer, and 1≤m≤n-1 is satisfied, where m and n are both positive integers;

[0059] The capacity of the slope area of ​​the first hard carbon material accounts for b1, and the capacity of the slope area of ​​the second hard carbon material accounts for b2, satisfying: b1<b2; wherein, the capacity of the slope area of ​​the first hard carbon material and the second hard carbon material is the capacity of the battery sodium insertion process above 0.1Vvs.Na+ / Na.

[0060] In step S20 , n active layers 20 are sequentially stacked on the first surface 11 along the first direction X by a multi-layer coating method.

[0061] The present invention provides a battery, comprising a positive electrode sheet, a separator 200 and the negative electrode sheet 100 as described in any of the aforementioned embodiments, wherein the separator 200 is disposed between the positive electrode sheet and the negative electrode sheet 100 .

[0062] The battery can be a square shell battery, a cylindrical battery, or other types such as a prismatic battery, without limitation.

[0063] The battery also includes a shell, which is provided with a receiving cavity, and the positive electrode sheet, the diaphragm 200 and the negative electrode sheet 100 are accommodated in the receiving cavity. The shell is made of a material with high structural strength, and can be specifically made of metal materials, high-strength plastics, ceramics, etc., and metal materials such as aluminum, aluminum alloys, magnesium alloys, iron and iron alloys. The shell includes a bottom plate and side plates. The shell can be an integrated structure, that is, the bottom plate and the side plates are an integrated structure made by an integrated molding process. The integrated molding process can be specifically stamping, casting, etc., without limitation. The shell can also be a split structure, and the side plates and the bottom plate can be connected and fixed by welding, bonding, clamping, screwing, etc. The wall thickness of the shell can be roughly uniform everywhere, that is, the thickness of the side plates can be roughly uniform, and the thickness of the bottom plate and the side plates can also be roughly the same.

[0064] The positive electrode material of the battery may be a sodium positive electrode active material, and in specific embodiments, includes one or more of sodium transition metal oxides, Prussian blue / white compounds, polyanion compounds, mixed metal oxides, and layered oxides.

[0065] The separator 200 can be any one of woven membrane, non-woven membrane (non-woven fabric), microporous membrane, composite membrane, rolled membrane, etc., without limitation. Several layers of the positive and negative electrode sheets 100 are separated by several layers of separator 200.

[0066] The positive electrode material, conductive agent and binder are uniformly mixed and dispersed in NMP (N-methylpyrrolidone), coated on a foil, and baked to obtain a positive electrode sheet. The conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 (carbon 60) and carbon nanotubes; the type of binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives, without limitation. This application does not specifically limit these materials, and appropriate materials can be selected according to actual application requirements.

[0067] The present invention provides an electrical device, comprising an electrical device and the battery as described in the above embodiment, wherein the battery supplies power to the electrical device.

[0068] The preparation process of a battery according to an embodiment of the present invention is as follows:

[0069] The first hard carbon material and the second hard carbon material are uniformly mixed with a conductive agent, a binder, etc. in deionized water according to a certain proportion, and are uniformly coated on the surface of the current collector 10 , and then baked and rolled to form the negative electrode sheet 100 .

[0070] The positive electrode sheet, the separator 200 and the negative electrode sheet 100 are wound in sequence and in an orderly manner to obtain a pole core.

[0071] After the core is shelled, the electrolyte is injected, and after aging, aging, formation and capacity separation, a complete secondary sodium ion battery is obtained.

[0072] The technical solution of this application is described in detail below through specific embodiments.

[0073] Example 1

[0074] This embodiment provides a negative electrode sheet 100 , which includes a current collector 10 and two active layers 20 .

[0075] The first active layer 20 of the two active layers 20 is disposed on the first surface 11 of the current collector 10, and the second active layer 20 is disposed on the surface of the first active layer 20 facing away from the current collector 10. The mass ratio of the first hard carbon material to the second hard carbon material in the negative electrode sheet 100 is 1:1. The first hard carbon material is resin carbon, and the second hard carbon material is biomass carbon. The thickness h of both the first active layer 20 and the second active layer 20 is 40 μm ± 2 μm.

[0076] The first hard carbon material has a1 of 315.6 mAh / g, the second hard carbon material has a2 of 280.4 mAh / g, the first hard carbon material has a1 of 34.3%, and the second hard carbon material has a2 of 43.4%.

[0077] The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 6:4.

[0078] The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 4:6.

[0079] Example 2

[0080] This embodiment provides a negative electrode sheet 100, which is substantially the same as that of the first embodiment, except that:

[0081] The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 8:2.

[0082] The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 2:8.

[0083] Example 3

[0084] This embodiment provides a negative electrode sheet 100, which is substantially the same as that of the first embodiment, except that:

[0085] The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 10:0.

[0086] The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 0:10.

[0087] Example 4

[0088] This embodiment provides a negative electrode sheet 100, which is basically the same as that of embodiment 3, except that:

[0089] The second hard carbon material is resin carbon;

[0090] a2 of the second hard carbon material is 262.6 mAh / g, and b2 of the second hard carbon material is 60%.

[0091] Example 5

[0092] This embodiment provides a negative electrode sheet 100, which is substantially the same as that of the fourth embodiment, except that:

[0093] a2 of the second hard carbon material is 248.8 mAh / g, and b2 of the second hard carbon material is 70%.

[0094] Example 6

[0095] This embodiment provides a negative electrode sheet 100, which is basically the same as that of embodiment 3, except that:

[0096] a1 of the first hard carbon material is 300 mAh / g, and b1 of the first hard carbon material is 36%.

[0097] Example 7

[0098] This embodiment provides a negative electrode sheet 100, which is basically the same as that of embodiment 3, except that:

[0099] a1 of the first hard carbon material is 290 mAh / g, and b1 of the first hard carbon material is 39%.

[0100] Comparative Example 1

[0101] This comparative example provides a negative electrode sheet 100, which is substantially the same as that of Example 1, except that:

[0102] There is only one active layer 20 , and the thickness of the active layer 20 of Comparative Example 1 is the same as the total thickness of the two active layers 20 of Example 1. The mass ratio of the first hard carbon material to the second hard carbon material in the active layer 20 is 5:5.

[0103] Comparative Example 2

[0104] This comparative example provides a negative electrode sheet 100, which is substantially the same as that of Example 1, except that:

[0105] The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 4:6.

[0106] The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 6:4.

[0107] Comparative Example 3

[0108] This comparative example provides a negative electrode sheet 100, which is substantially the same as that of Example 2, except that:

[0109] The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 2:8.

[0110] The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 8:2.

[0111] Comparative Example 4

[0112] This comparative example provides a negative electrode sheet 100, which is substantially the same as that of Example 3, except that:

[0113] The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 0:10.

[0114] The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 10:0.

[0115] Comparative Example 5

[0116] This comparative example provides a negative electrode sheet 100, which is substantially the same as that of Example 4, except that:

[0117] There is only one active layer 20 , and the thickness of the active layer 20 in Comparative Example 5 is the same as the total thickness of the two active layers 20 in Example 4.

[0118] Comparative Example 6

[0119] This comparative example provides a negative electrode sheet that is substantially the same as Example 5, except that:

[0120] There is only one active layer 20 , and the thickness of the active layer 20 in Comparative Example 6 is the same as the total thickness of the two active layers 20 in Example 5.

[0121] Comparative Example 7

[0122] This comparative example provides a negative electrode sheet that is substantially the same as Example 6, except that:

[0123] There is only one active layer 20 , and the thickness of the active layer 20 of Comparative Example 7 is the same as the total thickness of the two active layers 20 of Example 6.

[0124] Comparative Example 8

[0125] This comparative example provides a negative electrode sheet that is substantially the same as Example 7, except that:

[0126] There is only one active layer 20 , and the thickness of the active layer 20 in Comparative Example 8 is the same as the total thickness of the two active layers 20 in Example 7.

[0127] Table 1 shows the distribution of the mass ratio of the first hard carbon material and the second hard carbon material in each layer on the current collector 10 in the negative electrode sheets of Examples 1-7 and Comparative Examples 1-8, Table 2 shows the data of the reversible gram capacity and slope area ratio of the first hard carbon material and the second hard carbon material in Examples 1-7 and Comparative Examples 1-8, and Table 3 shows the comparison of the energy density and charging capacity of the batteries corresponding to Examples 1-7 and Comparative Examples 1-8.

[0128] Table 1

[0129]

[0130]

[0131] Table 2

[0132]

[0133] Table 3

[0134]

[0135]

[0136] With reference to Tables 1-3, it can be seen from the comparison of Examples 1-3 that, for the battery with the double active layer 20, as the amplitude of the gradient change of the first hard carbon material and the second hard carbon material increases, the gram capacity of the battery increases and the fast charging time of the battery decreases.

[0137] By comparing Example 1 and Comparative Example 2, Example 2 and Comparative Example 3, and Example 3 and Comparative Example 4, it can be seen that when the mass ratios of the first hard carbon material and the second hard carbon material in the corresponding active layer 20 are swapped, the fast charging time of the battery is increased.

[0138] By comparing Examples 1-3 and Comparative Example 1, it can be seen that when the gram capacity of the batteries is similar, the fast charging time of the battery having only the first active layer 20 is longer than the fast charging time of the battery having two active layers 20 .

[0139] Comparing Examples 4 and 5, it can be seen that the larger the slope region ratio b2 of the second hard carbon material, the smaller the reversible specific capacitance a2 of the second hard carbon material. Comparing Examples 6 and 7, it can be seen that the larger the slope region ratio b1 of the first hard carbon material, the smaller the reversible specific capacitance a1 of the first hard carbon material.

[0140] Comparing Examples 1-3 with Comparative Examples 2-4, it can be seen that the gram capacity of the batteries of Examples 1-3 is similar to that of the batteries of Comparative Examples 2-4, but the fast-charging time of the batteries of Examples 1-3 is shorter than that of the batteries of Comparative Examples 2-4. The maximum difference in fast-charging time is 9.2 minutes between Example 3 and Comparative Example 4. Therefore, the battery of the present invention improves the battery's charging capacity while maintaining the battery's gram capacity. In other words, the battery of the present invention can balance battery capacity and power performance.

[0141] By comparing Example 4 and Comparative Example 5, Example 5 and Comparative Example 6, Example 6 and Comparative Example 7, and Example 7 and Comparative Example 8, it can be seen that when the reversible gram capacity and the proportion of the slope area of ​​the first hard carbon material and the second hard carbon material remain unchanged, changing the distribution of the first hard carbon material and the second hard carbon material has little effect on the gram capacity of the battery, but the fast charging time of the battery using the negative electrode sheet of the present invention (double-layer coating and the negative electrode sheet with the first hard carbon material closer to the current collector) is significantly reduced. Therefore, it can be considered that the batteries of Examples 4-7 reduce the fast charging time while ensuring the battery capacity, that is, the battery of the present invention can take into account both battery capacity and power performance.

[0142] Comparing Examples 3-5, it can be seen that changing the slope region proportion b2 of the second hard carbon material reduces the battery's gram capacity, while correspondingly improving the battery's fast-charging time. Furthermore, when the battery's gram capacity decreases by 5.25%, the battery's fast-charging time is shortened by 26.2%, demonstrating that the battery of the present invention balances both battery capacity and power performance. Comparing Examples 1-5, it can be seen that batteries with a higher proportion of the first hard carbon material have a higher gram capacity but a longer fast-charging time; batteries with a higher proportion of the second hard carbon material have a shorter fast-charging time but a higher gram capacity. Simultaneously adding the first and second hard carbon materials can balance the battery's gram capacity and fast-charging performance. Further enhancing the battery's fast-charging performance by gradient designing the first and second hard carbon materials separately can further improve the battery's fast-charging performance.

[0143] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship of terms such as "center", "up", "down", "left", "right", "vertical", "horizontal", "inside" and "outside" is based on the orientation or positional relationship described in the accompanying drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0144] The above disclosure is only a preferred embodiment of the present invention, and certainly cannot be used to limit the scope of the rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that: include: a current collector having a first surface; n active layers are stacked sequentially on the first surface along a first direction, wherein the first direction is perpendicular to the first surface; Each of the active layers includes a first hard carbon material and / or a second hard carbon material; a content of the first hard carbon material in the (m+1)th active layer is less than a content of the first hard carbon material in the (m)th active layer, a content of the second hard carbon material in the (m+1)th active layer is greater than a content of the second hard carbon material in the (m)th active layer, the (m)th active layer is closer to the first surface than the (m+1)th active layer, and 1≤m≤n-1 is satisfied, where m and n are both positive integers; The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1<b2; The capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery during sodium insertion at a voltage above 0.1 V vs. Na+ / Na.

2. The negative electrode sheet according to claim 1, characterized in that: Also satisfies: 2≤n≤4.

3. The negative electrode sheet according to claim 1, characterized in that: The reversible gram capacity of the first hard carbon material is a1, which satisfies the following conditions: a1≥290 mAh / g.

4. The negative electrode sheet according to claim 1, characterized in that: Also meets: 20%≤b1≤40%.

5. The negative electrode sheet according to claim 1, characterized in that: Also meets: 40% ≤ b2 ≤ 70%.

6. The negative electrode sheet according to claim 1, characterized in that: The thickness of the active layer is h, which satisfies: h≥20 μm.

7. The negative electrode sheet according to claim 6, characterized in that: Also meets: 30μm≤h≤60μm.

8. The negative electrode sheet according to claim 1, characterized in that: The platform area capacity proportion of the first hard carbon material is e1, satisfying: 60%≤e1≤80%; the platform area capacity proportion of the second hard carbon material is e2, satisfying: 30%≤e2≤60%, and the capacity of the platform area of ​​the first hard carbon material and the second hard carbon material is the capacity of the battery sodium insertion process from 0V vs.Na+ / Na to 0.1V vs.Na+ / Na.

9. A method for preparing a negative electrode sheet, characterized in that: include: providing a current collector having a first surface; stacking n active layers in sequence on the first surface along a first direction, wherein the first direction is perpendicular to the first surface; Each of the active layers includes a first hard carbon material and / or a second hard carbon material; a content of the first hard carbon material in the (m+1)th active layer is less than a content of the first hard carbon material in the (m)th active layer, a content of the second hard carbon material in the (m+1)th active layer is greater than a content of the second hard carbon material in the (m)th active layer, the (m)th active layer is closer to the first surface than the (m+1)th active layer, and 1≤m≤n-1 is satisfied, where m and n are both positive integers; The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1<b2; The capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery during sodium insertion at a voltage above 0.1 V vs. Na+ / Na.

10. A battery, characterized in that: The invention comprises a positive electrode sheet, a separator and the negative electrode sheet according to any one of claims 1 to 8, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet.

11. An electrical device, characterized in that: The device comprises an electric device and the battery as claimed in claim 10, wherein the battery supplies power to the electric device.

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