Negative electrode sheet and electrochemical device

Abstract

The application discloses a negative electrode sheet and an electrochemical device. The negative electrode sheet comprises a negative electrode current collector, a negative electrode active material layer and a fast ion conductor layer. The negative electrode active material layer is arranged on at least one side surface of the negative electrode current collector. The fast ion conductor layer is arranged on the surface of the negative electrode active material layer and covers the negative electrode active material layer. In the thickness direction of the negative electrode current collector, the thickness of the negative electrode active material layer is denoted as L', the thickness of the fast ion conductor layer is denoted as L, and L / (L'+L)=b, which satisfies: b<=0.4. The negative electrode sheet can solve the problems of lithium precipitation or sodium precipitation of a lithium ion battery or a sodium ion battery during large-current charging.

Description

TECHNICAL FIELD

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

[0002] Since lithium ion batteries entered the market, they have been widely used due to their long service life, large specific capacity, and no memory effect. When the lithium ion battery is charged, lithium ions migrate to the negative electrode. However, the over-high potential caused by the large current during fast charging will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode to rapidly accept lithium will increase, and the tendency to generate lithium dendrites will increase. Therefore, the negative electrode not only needs to meet the kinetic requirements of lithium ion diffusion during fast charging, but also needs to solve the safety problem caused by the increased tendency of lithium dendrite formation. Similarly, sodium ions will also be deposited during the charging of sodium ion batteries, leading to the formation of sodium dendrites, which will bring the same problems and risks as lithium dendrites.

[0003] Currently, the common solutions for fast charging are: (1) nanosizing the negative electrode material particles to shorten the diffusion path of Li + , but at the same time, it will increase the specific surface area of the negative electrode material, causing more side reactions and gas production problems; (2) thin coating of the negative electrode, but the processing difficulty is large, which will increase the amount of auxiliary materials and reduce the energy density of the battery; (3) pore making or reducing the compaction to improve the kinetics, but it will reduce the compaction density of the electrode sheet and the energy density of the battery; (4) using linear esters in the electrolyte to reduce the viscosity of the electrolyte and improve the kinetics, but the increase of linear esters will cause gas production problems. SUMMARY

[0004] Therefore, the present application provides a negative electrode sheet and an electrochemical device. The negative electrode sheet is configured in the electrochemical device to solve the lithium or sodium deposition problem of lithium ion batteries or sodium ion batteries during fast charging (i.e., "fast charging") to shorten the charging time.

[0005] In a first aspect, the present application provides a negative electrode sheet, which includes a negative current collector, a negative active material layer, and a fast ion conductor layer. The negative active material layer is arranged on at least one side surface of the negative current collector, and the fast ion conductor layer is arranged on the surface of the negative active material layer and covers the negative active material layer. When the negative electrode sheet is configured in the electrochemical device, the fast ion conductor layer of the negative electrode sheet is in contact with the separator, which improves the high-temperature cycle and high-temperature storage performance of the electrochemical device under high rate conditions, and improves the long cycle stability of the electrochemical device.

[0006] In some embodiments, the negative electrode active material layer comprises a negative electrode active material, the fast ion conductor layer comprises a fast ion conductor, the thickness of the negative electrode active material layer is denoted as L', the thickness of the fast ion conductor layer is denoted as L, and L / (L'+L) = b, which satisfies: b≤0.4. When the ratio of the thickness of the fast ion conductor layer to the thickness of the negative electrode active material layer is within the above range, the fast ion conductor in the fast ion conductor layer is in contact with the separator, which is conducive to the rapid insertion of ions into the negative electrode active material layer and avoids the deposition of the negative electrode active material layer near the surface of the separator in the negative electrode sheet, which is a key problem leading to the decline of the fast charging performance of the electrochemical device. By arranging the fast ion conductor layer on the surface of the negative electrode active material layer, the ability of the side of the negative electrode active material layer near the separator to accept ions (such as lithium ions or sodium ions) is improved, thereby improving the fast charging performance of the electrochemical device.

[0007] In some embodiments, the fast ion conductor comprises a niobium composite metal oxide, the mass percentage of the negative electrode active material is denoted as W'%, and the mass percentage of the niobium composite metal oxide is denoted as W% based on the mass of the fast ion conductor layer, and W / (W'+W) = a; which satisfies: a≤0.5 and 80≤W≤95. The niobium composite metal oxide is an oxide of niobium and other metal oxides dissolved together based on Nb2O5 to form a pure phase M-Nb-O compound, where M is other metal ions. The type and content of the fast ion conductor are suitable, which is more conducive to the rapid acceptance of ions by the side surface of the negative electrode active material layer near the separator and avoids the formation of dendrites.

[0008] In some embodiments, 1.3≤a / b≤2.3. When the values of a, b and a / b are within the appropriate ranges, the electrochemical device can achieve both high energy density and improved fast charging performance.

[0009] The above technical solutions can be applied to lithium ion batteries and sodium ion batteries. For lithium ion batteries, a layer of niobium composite metal oxide is coated on, for example, graphite, which utilizes the characteristics of the fast ion conductor of the niobium composite metal oxide to rapidly transport Li + , avoiding the precipitation of Li + on the surface layer of the graphite (the side surface near the separator) during fast charging, thereby achieving the purpose of improving the fast charging performance of the graphite. For sodium ion batteries, a layer of Na + fast ion conductor is coated on, for example, the surface of hard carbon, which utilizes the characteristics of the fast ion conductor to rapidly transport Na + , avoiding the precipitation of Na + on the surface of the hard carbon (the side surface near the separator) during fast charging, thereby achieving the purpose of improving the fast charging performance of the hard carbon.

[0010] Lithium ion battery

[0011] The lithium ion battery comprises a positive electrode sheet, a separator film and a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector, a negative electrode active material layer and a fast ion conductor layer, the negative electrode active material layer comprises a negative electrode active material, and the fast ion conductor layer comprises a niobium composite metal oxide. The positive electrode sheet and the negative electrode sheet are alternately stacked, the separator film is arranged between two adjacent positive electrode sheet and negative electrode sheet, and the fast ion conductor layer of the negative electrode sheet is in contact with the separator film.

[0012] In some embodiments, the negative electrode active material is a first negative electrode active material for a lithium ion battery, and the first negative electrode active material comprises at least one of graphite, hard carbon, silicon-oxygen or silicon-carbon.

[0013] In some embodiments, the niobium composite metal oxide comprises a compound I with a molecular formula of T x Nb y M z O a′ , wherein T is at least one selected from K, Li, Fe, V, W, Cr, Zr, Al, Mg, Zn, Cu, Mo, Na, Ga, P, Tc, Si, Ga, Sn, Ni, Co, Mn, Sr, Y, In, Na or Ti, M is at least one selected from Al, Ti, W, Zr, Nb, In, Ru, Sb, Sr, Y, Ni, Co, Mn, Fe, Gr, Mo, Tc, Sn, Ga, Si, V or Mg, T and M are different, and satisfy: 0

[0014] The structure of the compound I is a Wadsley-Roth section structure and a bronze-like structure, which is conducive to Li +diffusion. During the charge-discharge process, the volume change of the unit cell is ≤10%, thus, the structure stability of the compound I is good, and the cycle performance is good. The solid-phase diffusion coefficient of lithium ions in the lithium electrode is reduced, which is the main speed control step leading to the poor capacity characteristics of the power battery. When the battery is subjected to rapid charging, the smaller diffusion coefficient will hinder the diffusion process of lithium ions in the lithium electrode, thus, the "lithium deposition" is prone to occur on the surface of the negative electrode particles, which causes permanent damage to the battery. The potential of the active material layer near the surface of the separator in the negative electrode sheet is lower, and the risk of lithium precipitation is greater, which further affects the safety performance, cycle performance, gas production, etc. of the lithium ion battery. In the present application, the ni composite metal oxide is introduced into the lithium electrode sheet, and a double-layer coating technology is used, i.e., a layer of lithium electrode material with high capacity and high adhesion is first coated on the current collector, and then a second layer of ni composite metal oxide material is coated thereon, so that the ni composite metal oxide is in contact with the separator. By utilizing the characteristics of the ni composite metal oxide, such as fast lithium intercalation and fast lithium ion conduction, the ability of the active material layer near the surface of the separator to quickly accept ions is improved. Further, the high-temperature cycle and high-temperature storage performance of the lithium ion battery under high rate conditions are improved, the risk of lithium precipitation of the lithium electrode sheet is reduced, and the long cycle stability of the lithium ion battery is improved. That is, the double-layer coating realizes fast ion transmission in the upper layer (fast ion conductor layer) and high energy density in the lower layer (negative electrode active material layer), perfectly balancing the high energy density and fast charging dual cores.

[0015] In some embodiments, based on the mass of the fast ion conductor layer, the mass percentage content of the ni composite metal oxide is 88wt% to 92wt%, at this time, the loss of energy density caused by the use of the fast ion conductor can be reduced, and the following conditions are met: 0.1≤a≤0.3, 0.05≤b≤0.2 and 1.3≤a / b≤1.6. If the value of a is too small (for example, less than 0.1), lithium precipitation will occur during fast charging, resulting in poor high-temperature cycle and high-temperature storage performance under high rate, because the smaller the value of a is, the less the amount of fast ion conductor used, which will cause the negative electrode active material layer to be exposed, which is not conducive to the improvement of the fast charging performance.

[0016] If the value of a is too large (for example, greater than 0.3), the energy density will be reduced. Because the lithium intercalation voltage platform of the fast ion conductor is higher than that of graphite, the energy density of the battery will be reduced to some extent, therefore, while achieving the purpose of ion transmission, the amount of fast ion conductor used should be relatively small. When the values of a, b and a / b are within the above ranges, it is more conducive to balancing the improvement of the fast charging performance of the lithium ion battery and achieving high energy density.

[0017] In some embodiments, the compaction density of the negative electrode sheet is 1.79g / cm 3 to 2.4g / cm 3The compaction density is in a synergic adaptive relationship with the compound I. The greater the proportion of the compound I in the fast ion conductor layer, the higher the compaction density, but the energy density decreases. For the fast ion conductor of the compound I, the compaction density in the above range is more conducive to improving the energy density of the electrochemical device. Preferably, the compaction density of the negative electrode sheet is 1.79 g / cm 3 to 2.28 g / cm 3 .

[0018] In some embodiments, the specific surface area of the compound I is 0.8 m 2 / g to 20 m 2 / g. At this time, it is conducive to further improving the high-temperature cycle and high-temperature storage performance at high rates. Preferably, the specific surface area of the compound I is 0.8 m 2 / g to 1.2 m 2 / g.

[0019] In some embodiments, the platform voltage of the negative electrode sheet to lithium is 0.1 V to 1.0 V. Preferably, the platform voltage of the negative electrode sheet to lithium is 0.4 V to 0.8 V.

[0020] In some embodiments, the specific surface area of the compound I is 0.8 m

[0021] Sodium ion battery

[0022] The sodium ion battery includes a positive electrode sheet, a separator film, and a negative electrode sheet. The negative electrode sheet includes a negative electrode current collector, a negative electrode active material layer, and a fast ion conductor layer. The negative electrode active material layer includes a negative electrode active material, and the fast ion conductor layer includes a niobium composite metal oxide. The positive electrode sheet and the negative electrode sheet are interleaved and stacked, the separator film is arranged between two adjacent positive electrode sheets and negative electrode sheets, and the fast ion conductor layer of the negative electrode sheet is in contact with the separator film.

[0023] In some embodiments, the negative electrode active material is a second negative electrode active material for a sodium ion battery, and the second negative electrode active material includes at least one of hard carbon, antimony, and a mixture thereof.

[0024] In some embodiments, the niobium composite metal oxide includes at least one of a compound II with a molecular formula of Na x′ A y′ Ti z′ O2, wherein A is selected from at least one of Ni, Co, Li, Gr, and satisfies: 0.6 < x' < 0.7, y' + z' = 1. The structure of the compound II is a P2 phase, which can improve the transmission rate of sodium ions and maintain the integrity of the layered structure, and has excellent rate performance and cycle performance.

[0025] In some embodiments, the mixed gravimetric capacity of the negative electrode sheet per unit area in the sodium-ion battery is 240-300 mAh / g.

[0026] The present application introduces Na + fast ion conductor in the sodium battery negative electrode sheet, through a double-layer coating technology, first coating a layer of high-capacity, high-adhesion sodium battery negative electrode material on the current collector, and then coating a second layer of Na + fast ion conductor material on the basis, so that the Na + fast ion conductor material is in contact with the separator, and the characteristics of the Na + fast ion conductor itself, i.e., fast sodium intercalation and fast sodium ion conduction, improve the high-temperature cycle and high-temperature storage performance of the sodium-ion battery under high rate conditions, reduce the risk of sodium precipitation of the negative electrode sheet, and improve the long cycle stability of the sodium-ion battery. The double-layer coating realizes fast ion transmission in the upper layer and high energy density in the lower layer, perfectly balancing the high energy density and fast charging dual cores.

[0027] In some embodiments, the mass percentage of niobium composite metal oxide is 80wt%-88wt% based on the mass of the fast ion conductor layer, which can reduce the loss of energy density caused by the use of fast ion conductor, and satisfy 0.1≤a≤0.3, 0.05≤b≤0.2, and 2≤a / b≤2.3. When the values of a, b, and a / b are within the appropriate ranges, it is beneficial to balance the improvement of high energy density and fast charging performance.

[0028] In some embodiments, the compaction density of the negative electrode sheet is 1.2 g / cm 3 -2.1 g / cm 3 . The compaction density and the compound II are in a synergistic adaptive relationship, i.e., the greater the proportion of the compound II in the fast ion conductor layer, the higher the compaction density, but the energy density will decrease. For the fast ion conductor of the compound II, the compaction density in the above range is more beneficial to improve the energy density of the electrochemical device. Preferably, the compaction density of the negative electrode sheet is 1.6 g / cm 3 -1.7 g / cm 3 .

[0029] In some embodiments, the specific surface area of the compound II is 0.5 m 2 / g-10 m 2 / g. In order to achieve better fast ion transfer effect, the smaller the particle size of the fast ion conductor material, the shorter the ion migration path, but too small particle size will bring a larger specific surface area, thus causing more side reactions, which will deteriorate the high-temperature cycle performance and high-temperature storage performance. Preferably, the specific surface area of the compound II is 0.5 m 2 / g-5 m 2 / g.

[0030] In some embodiments, the plateau voltage of the negative electrode tab against sodium is 0.28V to 0.5V. Preferably, the plateau voltage of the negative electrode tab against sodium is 0.36V to 0.4V.

[0031] In a second aspect, the present application provides an electrochemical device, which comprises a positive electrode tab, a separator film and the above-mentioned negative electrode tab, the positive electrode tab and the negative electrode tab are interleaved and stacked, the separator film is arranged between two adjacent positive electrode tabs and negative electrode tabs, and the fast ion conductor layer of the negative electrode tab is in contact with the separator film.

[0032] By arranging the fast ion conductor layer on the surface of the negative electrode active material layer, when the negative electrode tab is arranged in the electrochemical device, the fast ion conductor layer in the negative electrode tab is in contact with the separator film, so as to improve the ability of the negative electrode active material layer near the separator film to accept ions (such as lithium ions, sodium ions), and further improve the fast charging performance of the electrochemical device. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0034] When the lithium ion battery is charged, lithium migrates to the negative electrode, and the over-high potential caused by the large current of fast charging will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode to rapidly accept lithium will become larger, and the tendency to generate lithium dendrites will become larger. Therefore, when fast charging, the negative electrode not only needs to meet the kinetic requirements of lithium diffusion, but also needs to solve the safety problem caused by the increased tendency of lithium dendrite generation. Therefore, the main technical difficulty of fast charging lithium ion battery is the insertion of lithium ions in the negative electrode, especially on the surface near the separator film.

[0035] In order to solve the above technical problems, the present application provides a negative electrode tab and an electrochemical device. The present application coats a layer of fast ion conductor material on the graphite tab to achieve Li +The application is applicable to lithium ion batteries and plays a great role in the terminal application of lithium batteries. The application is also applicable to solving the sodium precipitation problem of sodium ion batteries. The global reserves of lithium resources are limited, and the content of lithium element in the earth's crust is only 0.0065%. With the development of new energy vehicles, the demand for batteries has increased significantly, and the resource bottleneck has gradually emerged, which limits the large-scale application of lithium ion batteries due to high cost. Sodium resources are very abundant, with an abundance of 2.64% in the earth's crust, which is 440 times that of lithium resources, and the sodium resources are widely distributed and easy to extract. Sodium, as a substitute for lithium, has received increasing attention in the battery field. Sodium ion batteries and lithium ion batteries have similar working principles, and sodium ion batteries also follow the de-intercalation working principle (in the charging process, sodium ions are released from the positive electrode and embedded in the negative electrode, and the process is reversed in the discharging process). Therefore, sodium dendrites may also be formed in sodium ion batteries due to sodium precipitation. The dendrite problem in sodium ion batteries refers to the formation of dendritic sodium deposits during the movement of sodium ions between electrodes during charging and discharging, which may cause internal short circuit or damage to the battery, thereby affecting the performance and life of the battery. The root cause of this problem is that the movement speed of sodium ions between electrodes is slow, which may form irregular deposits on the electrode surface and form dendrites. The formation of sodium dendrites brings the same problems and risks as lithium dendrites.

[0036] In order to shorten the charging time, a large current must be used for charging, at which time the ions need to move quickly to the negative electrode and quickly embed into the negative electrode material (quickly embed into the active material layer), otherwise the sodium precipitation or lithium precipitation problem may occur, which may cause safety problems. The application mainly coats a layer of niobium composite metal oxide on the outer surface of the negative electrode active material layer (the surface on the side away from the current collector), uses the fast ion conductor characteristics of the niobium composite metal oxide to quickly transport Li + or Na + , avoids the accumulation of Li + or Na + on the surface layer of the negative electrode active material layer during fast charging, thereby achieving the purpose of improving the fast charging performance of the electrochemical device.

[0037] Electrochemical device

[0038] The application provides an electrochemical device, which comprises a positive electrode sheet, a negative electrode sheet, a separator film and an electrolyte, wherein the separator film is arranged between the positive electrode sheet and the negative electrode sheet.

[0039] Negative electrode sheet

[0040] The negative electrode sheet includes a negative electrode current collector, a negative electrode active material layer, and a fast ion conductor layer. The negative electrode active material layer is provided on at least one side surface of the negative electrode current collector. The fast ion conductor layer is provided on a surface of the negative electrode active material layer and covers the negative electrode active material layer. In a thickness direction of the negative electrode current collector, a thickness of the negative electrode active material layer is denoted as L', a thickness of the fast ion conductor layer is denoted as L, and L / (L'+L)=b, and b satisfies b≤0.4. For example, b is in a range of 0.001, 0.005, 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or a range between any two of the above values.

[0041] In some embodiments, the negative electrode active material layer includes a negative electrode active material, the fast ion conductor layer includes a fast ion conductor, the fast ion conductor includes a niobium composite metal oxide, a mass percentage of the negative electrode active material based on a mass of the negative electrode active material layer is denoted as W%, a mass percentage of the niobium composite metal oxide based on a mass of the fast ion conductor layer is denoted as W, and W / (W'+W)=a, and a satisfies a≤0.5 and 80≤W≤95. For example, a is in a range of 0.001, 0.005, 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or a range between any two of the above values. For example, W is in a range of 80, 82, 84, 85, 86, 88, 90, 92, 94, 95, or a range between any two of the above values.

[0042] In some embodiments, 1.3≤a / b≤2.3 is satisfied. For example, a / b is in a range of 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.1, 2.3, or a range between any two of the above values.

[0043] Other

[0044] Positive electrode sheet

[0045] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material in the positive electrode active material layer can be selected from one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and a compound obtained by adding other transition metal or non-transition metal to the above compound.

[0046] For example, the positive electrode current collector can use a metal foil or a porous metal plate, such as a foil or a porous plate of aluminum, copper, nickel, titanium, iron, or an alloy thereof, such as an Al (aluminum) foil.

[0047] The positive electrode sheet can be prepared according to conventional methods in the art.

[0048] Separation film

[0049] The type of the separation film is not particularly limited and can be selected as desired. For example, the separation film can be a polyethylene film, a polypropylene film, a polyvinylidene fluoride film, and a multilayer composite film thereof, but is not limited to these materials.

[0050] Electrolyte solution

[0051] The electrolyte solution includes an organic solvent, an electrolyte lithium salt, and an additive. The type of the electrolyte solution is not particularly limited and can be selected as desired.

[0052] Exemplarily, the organic solvent includes one or more, preferably two or more, of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl 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), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), or diethyl sulfone (ESE).

[0053] Exemplarily, the electrolyte lithium salt includes one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bis-trifluoromethanesulfonylimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro bis(oxalato)phosphate), or LiTFOP (lithium tetrafluoro(oxalato)phosphate). Exemplarily, the electrolyte sodium salt includes one or more of NaPF6 (sodium hexafluorophosphate), NaOTF (sodium trifluoromethanesulfonate), NaTFSI (sodium bis-trifluoromethanesulfonylimide), NaBF4 (sodium tetrafluoroborate), NaBOB (sodium bis(oxalato)borate), NaDFOB (sodium difluoro(oxalato)borate), or NaClO4 (sodium perchlorate).

[0054] The electrolyte solution can optionally further include other additives, which can be any additive that can be used for lithium ion secondary batteries, and the present application is not specifically limited, and can be selected according to actual needs. As an example, the additive can be one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), succinonitrile (SN), adiponitrile (ADN), 1,3-propane sultone (PST), tris(trimethylsilyl)phosphate (TMSP), trimethyl borate (TMB), or tris(trimethylsilyl)borate (TMSB).

[0055] The electrochemical device can be prepared according to conventional methods in the art. Illustratively, the above-described positive electrode sheet, the separator, and the negative electrode sheet are stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to function as a separator, to obtain an electrode assembly, which can also be obtained after being wound; the electrode assembly is placed in a packaging case, an electrolyte solution is injected and sealed, to obtain an electrochemical device.

[0056] The electrochemical device of the present application can include any device that undergoes an electrochemical reaction, and specific examples thereof include all kinds of primary batteries or secondary batteries.

[0057] Example 1

[0058] Lithium ion battery

[0059] The lithium ion battery includes a positive electrode sheet, a separator, and a negative electrode sheet, the negative electrode sheet including a negative electrode current collector, a negative electrode active material layer, and a fast ion conductor layer, the negative electrode active material layer including a negative electrode active material, and the fast ion conductor layer including a niobium composite metal oxide. The positive electrode sheet and the negative electrode sheet are alternately stacked, the separator is disposed between adjacent two positive electrode sheets and negative electrode sheets, and the fast ion conductor layer of the negative electrode sheet is in contact with the separator.

[0060] In some embodiments, the negative electrode active material is a first negative electrode active material for a lithium ion battery, and the first negative electrode active material includes at least one of graphite, hard carbon, silicon oxide, or silicon carbon.

[0061] In some embodiments, the niobium composite metal oxide includes a molecular formula of T x Nb y M z O a′at least one of the compounds I, wherein T is selected from at least one of K, Li, Fe, V, W, Cr, Zr, Al, Mg, Zn, Cu, Mo, Na, Ga, P, Tc, Si, Ga, Sn, Ni, Co, Mn, Sr, Y, In, Na or Ti, M is selected from at least one of Al, Ti, W, Zr, Nb, In, Ru, Sb, Sr, Y, Ni, Co, Mn, Fe, Gr, Mo, Tc, Sn, Ga, Si, V or Mg, T and M are different, and satisfy: 0 < x / (x+y+z) < 0.6, 1 < a' / (x+y+z) < 5, 0 < z / (x+y+z) < 0.5. Preferably, the compound I is selected from Nb 16 W5O 55 , Nb 18 W 16 O 93 , TiNb2O7, Nb 16 W5O 93 , Cr 0.5 Nb 24.5 O 62 , Ti2Nb 14 O 39 , TiNb 24 O 62 , TiNb6O 17 , Ni2Nb 34 O 87 , Cu2Nb 34 O 87 , Cr 0.5 Nb 24.5 O 62 , V3Nb 17 O 50 , Zn2Nb 34 O 87 , Al 0.5 Nb 24.5 O 62 , MoNb 12 O 33 , ZrNb 24 O 62 , AlNb 11 O 29 , Mg2Nb 34 O 87 , GaNb 11 O 29 , Mo3Nb 14 O 44 , CrNb 11 O 29 , HfNb 24 O 62 , FeNb 11 O 28 , GaNb49 O 124 , NaNb 13 O 33 , Ni2Nb 34 O 87 , TiNb6O 17 , WNb 12 O 33 , LiNbO3, Li3NbO4, TiCr 0.5 Nb 10.5 O2, VNb9O 25 , KNb5O 13 , K6Nb 10.8 O 30 , PNb9O 25 , Nb 18 W8O 69 , Ti2Nb 10 O 29 , Cr 0.2 Fe 0.8 Nb 11 O 29 , Fe 0.8 Mn 0.2 Nb 11 O 29 , Fe 0.8 V 0.2 Nb 11 O 29 , or Cu 0.02 Ti 0.94 Nb 2.04 O7. More preferably, the compound I is selected from at least one of TiNb2O7, Nb 16 W5O 93 , or Nb 16 W5O 55 .

[0062] In some embodiments, 0.1 < a < 0.3 is satisfied. Exemplarily, the value of a ranges from 0.1, 0.13, 0.15, 0.18, 0.2, 0.23, 0.25, 0.28, 0.3 or a range between any two of the above values.

[0063] In some embodiments, 0.05 < b < 0.2 is satisfied. Exemplarily, the value of b ranges from 0.05, 0.08, 0.1, 0.13, 0.15, 0.18, 0.2 or a range between any two of the above values.

[0064] In some embodiments, 1.3 < a / b < 1.6 is satisfied. Exemplarily, the value of a / b ranges from 1.3, 1.4, 1.5, 1.6 or a range between any two of the above values.

[0065] In some embodiments, the following is satisfied: 88≤W≤92. Illustratively, the value of W ranges from 88, 89, 90, 91, 92, or a range defined by any two of the aforementioned values.

[0066] In some embodiments, the specific surface area of the compound I is 0.8m 2 / g to 20m 2 / g. Illustratively, the specific surface area of the compound I is 0.8m 2 / g, 1m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 2m 2 / g, 5m 2 / g, 8m 2 / g, 10m 2 / g, 15m 2 / g, 20m 2 / g, or a range defined by any two of the aforementioned values.

[0067] In some embodiments, the compaction density of the negative electrode tab is 1.79g / cm 3 to 2.4g / cm 3 . Illustratively, the compaction density of the negative electrode tab is 1.79g / cm 3 , 1.8g / cm 3 , 1.9g / cm 3 , 2g / cm 3 , 2.1g / cm 3 , 2.28g / cm 3 , 2.4g / cm 3 , or a range defined by any two of the aforementioned values.

[0068] In some embodiments, the plateau voltage of the negative electrode tab against lithium is 0.1V to 1.0V. Illustratively, the plateau voltage of the negative electrode tab against lithium is 0.1V, 0.2V, 0.4V, 0.6V, 0.8V, 1.0V, or a range defined by any two of the aforementioned values.

[0069] Example 2

[0070] Sodium ion battery

[0071] The sodium-ion battery comprises a positive electrode tab, a separator film, and a negative electrode tab, the negative electrode tab comprising a negative electrode current collector, a negative electrode active material layer, and a fast-ion conductor layer, the negative electrode active material layer comprising a negative electrode active material, and the fast-ion conductor layer comprising a niobium composite metal oxide. The positive electrode tab and the negative electrode tab are interleaved and stacked, the separator film is arranged between any two adjacent positive electrode tab and negative electrode tab, and the fast-ion conductor layer of the negative electrode tab is in contact with the separator film.

[0072] In some embodiments, the negative active material is a second negative active material for a sodium-ion battery, the second negative active material comprising at least one of hard carbon, Sb, and mixtures therebetween.

[0073] In some embodiments, the niobium complex metal oxide comprises a molecular formula of Na x′ A y′ Ti z′ O2, wherein A is selected from at least one of Ni, Co, Li, Gr, and satisfies: 0.6 < x' < 0.7, y' + z' = 1. Preferably, the compound II is selected from Na 0.66 Ni 0.17 Co 0.17 Ti 0.66 O2, Na 2 / 3 Co 1 / 3 Ti 2 / 3 O2, Na 0.66 Li 0.22 Ti 0.78 O2, P2-Na 0.66 Li 0.22 Ti 0.78 O2, P2-Na 0.62 Cr 0.63 Ti 0.37 O2, or P3-Na 0.63 Cr 0.63 Ti 0.37 O2.

[0074] In some embodiments, 0.1 < a < 0.3 is satisfied. Exemplarily, the value of a ranges from 0.1, 0.13, 0.15, 0.18, 0.2, 0.23, 0.25, 0.28, 0.3, or a range between any two of the above values.

[0075] In some embodiments, 0.05 < b < 0.2 is satisfied. Exemplarily, the value of b ranges from 0.05, 0.08, 0.1, 0.13, 0.15, 0.18, 0.2, or a range between any two of the above values.

[0076] In some embodiments, 2 < a / b < 2.3 is satisfied. Exemplarily, the value of a / b ranges from 2, 2.05, 2.1, 2.15, 2.2, 2.25, 2.3, or a range between any two of the above values.

[0077] In some embodiments, 80 < W < 88 is satisfied. Exemplarily, the value of W ranges from 80, 82, 84, 86, 88, or a range between any two of the above values.

[0078] In some embodiments, the specific surface area of the compound II is 0.5 m 2 / g to 10 m 2 / g. Illustratively, the specific surface area of the compound II is 0.5 m 2 / g, 0.8 m 2 / g, 1 m 2 / g, 3 m 2 / g, 5 m 2 / g, 8 m 2 / g, 10 m 2 / g or a range consisting of any two of the aforementioned values.

[0079] In some embodiments, the compaction density of the negative electrode tab is 1.2 g / cm 3 to 2.1 g / cm 3 . Illustratively, the compaction density of the negative electrode tab is 1.2 g / cm 3 , 1.4 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , 2.0 g / cm 3 , 2.1 g / cm 3 or a range consisting of any two of the aforementioned values.

[0080] In some embodiments, the plateau voltage of the negative electrode tab against sodium is 0.28 V to 0.5 V. Illustratively, the plateau voltage of the negative electrode tab against sodium is 0.28 V, 0.3 V, 0.33 V, 0.36 V, 0.38 V, 0.4 V, 0.45 V, 0.5 V or a range consisting of any two of the aforementioned values.

[0081] Hereinafter, embodiments and comparative examples are presented to more specifically illustrate the embodiments of the present application. Unless otherwise stated, the parts, percentages and ratios listed below are based on weight, and the raw materials used are commercially available or are synthesized according to conventional methods.

[0082] (I) Preparation of lithium ion battery

[0083] Example 1

[0084] (1) Preparation of the positive electrode tab

[0085] The positive electrode active material lithium nickel cobalt manganese oxide (molecular formula LiNi 0.5 Co 0.2 Mn 0.3 O2, abbreviated as NCM523), the positive electrode conductive agent acetylene black, the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight of 2 x 10 5 to 10 x 10 5) mixed according to a mass ratio of 94:3:3, N-methyl pyrrolidone (NMP) was added as a solvent, and stirring was performed in a vacuum stirrer until a positive electrode slurry with a solid content of 75 wt% and a uniform system was obtained. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 6 μm, dried at 110°C, and a positive electrode tab with a single-sided coated positive electrode active material layer was obtained. Then, the above steps were repeated on the other surface of the aluminum foil, and a positive electrode tab with a double-sided coated positive electrode active material layer was obtained. After cold pressing, cutting, and welding of the tabs, a positive electrode tab with a size of 74 mm * 851 mm was obtained for use.

[0086] (2) Preparation of the negative electrode tab

[0087] (2.1) Preparation of the first negative electrode slurry

[0088] The first negative electrode active material (artificial graphite), the negative electrode conductive agent acetylene black, the negative electrode binder styrene-butadiene rubber (SBR, with a weight average molecular weight of 1 x 10 5 to 1.1 x 10 5 , and the thickening agent sodium carboxymethyl cellulose (CMC Na) were mixed according to a mass ratio of 95:2:2:1, deionized water was then added as a solvent, and stirring was performed in a stirrer until a negative electrode slurry with a solid content of 50 wt% and a uniform system was obtained, and the first negative electrode slurry was obtained.

[0089] (2.2) Preparation of the second negative electrode slurry

[0090] The second negative electrode active material (TiNb2O7, with a BET of 1.2 m 2 / g), the negative electrode conductive agent acetylene black, the negative electrode binder styrene-butadiene rubber (SBR, with a weight average molecular weight of 1 x 10 5 to 1.1 x 10 5 , and the thickening agent sodium carboxymethyl cellulose (CMC Na) were mixed according to a mass ratio of 95:2:2:1, deionized water was then added as a solvent, and stirring was performed in a stirrer until a negative electrode slurry with a solid content of 70 wt% and a uniform system was obtained, and the second negative electrode slurry was obtained.

[0091] (2.3) Preparation of the negative electrode tab

[0092] The first negative electrode slurry and the second negative electrode slurry obtained were simultaneously coated on one surface of a copper foil with a thickness of 8 μm by using a double-coating process, dried at 100°C, and a negative electrode tab with a single-sided coated negative electrode active material layer was obtained. Then, the above steps were repeated on the other surface of the copper foil, and a negative electrode tab with a double-sided coated negative electrode active material layer was obtained. After cold pressing, cutting, and welding of the tabs, a negative electrode tab with a size of 76 mm * 867 mm was obtained for use.

[0093] (3) Preparation of the separator

[0094] The porous polyethylene (PE) film with a thickness of 8 μm was used as the separator film, and was slit into appropriate width according to the size of the positive electrode sheet and the negative electrode sheet before use.

[0095] (4) Preparation of electrolyte

[0096] Lithium salt lithium hexafluorophosphate was prepared into electrolyte with a lithium salt concentration of 1.0 mol / L in a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): polypropylene (PP): diethyl carbonate (DEC) = 1:1:1:1, mass ratio) under an environment with a water content of less than 10 ppm.

[0097] (5) Preparation of lithium ion battery

[0098] The positive electrode sheet, the separator film, and the negative electrode sheet were sequentially stacked in order with the separator film in the middle of the positive electrode sheet and the negative electrode sheet to play a role of isolation, and were wound to obtain an electrode assembly. The electrode assembly was placed in an aluminum plastic film packaging bag, and after water was removed at 80°C, the above-mentioned electrolyte was injected and packaged, and a lithium ion battery was obtained after processes such as formation, degassing, and shaping.

[0099] Examples 2 to 15 were the same as Example 1 except that the relevant preparation parameters were adjusted according to Table 1.

[0100] Comparative Examples 1 to 4 had no fast ion conductor layer, and were the same as Example 1 except that the relevant parameters were adjusted according to Table 1.

[0101] (II) Preparation of sodium ion battery

[0102] Example 16

[0103] (1) Preparation of positive electrode sheet

[0104] In a dry room at 25°C and a relative humidity of ≤2%, the positive electrode material NaNi 0.316 Fe 0.332 Mn 0.352 O2, the binder polyvinylidene fluoride, and the conductive agent acetylene black were mixed in a mass ratio of 80:10:10, N-methyl pyrrolidone (NMP) solvent was added, and they were thoroughly stirred and mixed to form a positive electrode slurry with a solid content of 72%; a doctor blade was used to coat a coating layer with a thickness of 200 μm (here, the thickness refers to the sum of the coating thicknesses on the single and double sides of the aluminum foil) on an aluminum foil with a thickness of 10 μm, and then dried at 70°C for 12 h, followed by cold pressing; and then punched into small round sheets with a diameter of 14 mm for use as the positive electrode sheet in the original state.

[0105] (2.1) Preparation of first negative electrode slurry

[0106] The first negative active material hard carbon, the binder polyvinylidene fluoride, and the conductive agent acetylene black were mixed in a mass ratio of 94:3:3, and N-methyl pyrrolidone (NMP) solvent was added. The mixture was stirred thoroughly to obtain a first negative electrode slurry.

[0107] (2.2) Preparation of a second negative electrode slurry

[0108] The second negative active material (P2-Na 0.66 Li 0.22 Ti 0.78 O2, with a BET of 0.6 m 2 / g), the binder polyvinylidene fluoride, and the conductive agent acetylene black were mixed in a mass ratio of 94:3:3, and N-methyl pyrrolidone (NMP) solvent was added. The mixture was stirred thoroughly to obtain a first negative electrode slurry.

[0109] (2.3) Preparation of a negative electrode sheet

[0110] The first negative electrode slurry and the second negative electrode slurry were simultaneously coated on one surface of a copper foil with a thickness of 8 μm using a double-coating process, and the copper foil was dried at 100 °C to obtain a negative electrode sheet coated with a single layer of negative active material. Then, the above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet coated with a double layer of negative active material. The negative electrode sheet was then cold-pressed, cut, and the tabs were welded to obtain a negative electrode sheet with a size of 76 mm * 867 mm.

[0111] (3) Preparation of a separator

[0112] A porous polyethylene (PE) film with a thickness of 8 μm was used as a separator, and the appropriate width was obtained by slitting according to the size of the positive electrode sheet and the negative electrode sheet.

[0113] (4) Preparation of an electrolyte

[0114] In an environment with a water content of less than 10 ppm, sodium hexafluorophosphate and a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): polypropylene (PP): diethyl carbonate (DEC) = 1:1:1:1, mass ratio) were prepared into an electrolyte with a lithium salt concentration of 1.0 mol / L.

[0115] (5) Preparation of a sodium-ion battery

[0116] The positive electrode sheet, the separator, and the negative electrode sheet were sequentially stacked with the separator between the positive electrode sheet and the negative electrode sheet to play a role in isolation, and the electrode assembly was obtained by winding. The electrode assembly was placed in an aluminum plastic film packaging bag, and after removing the water at 80 °C, the above-mentioned electrolyte was injected and packaged. After processes such as formation, degassing, and shaping, a lithium-ion battery was obtained.

[0117] Examples 17 to 26 are the same as Example 1 except that the relevant preparation parameters are adjusted according to Table 2.

[0118] Comparative Example 5 has no fast ion conductor layer and is the same as Example 1 except that the relevant parameters are adjusted according to Table 2.

[0119] (III) Test Section

[0120] 1. Test of mass capacity

[0121] The negative electrode sheets in each example and comparative example were oven-dried, punched into small discs with a diameter of 14 mm, weighed, and the mass M of the active material was calculated according to the proportion of the active material in the electrode sheet. Then, the Li sheet was assembled into a coin cell, and the coin cell was subjected to charge and discharge tests on a blue cell test system (LAND CT2001A). The charge and discharge tests were performed at a current density of 10 mA / g, and the specific discharge process was as follows: discharged at 10 mA / g to 5.0 mV, and then charged at 10 mA / g to 3.0 V after 5 minutes of standing. The capacity C of the coin cell at this time was recorded, and the mass capacity was C / M. Similarly, the mass capacity test in a sodium ion battery only required the replacement of the lithium sheet with a sodium sheet.

[0122] 2. Test of active material vs. Li+ / Li working voltage

[0123] The negative electrode sheets in each example and comparative example were oven-dried, punched into small discs with a diameter of 14 mm, weighed, and the mass M of the active material was calculated according to the proportion of the active material in the electrode sheet. Then, the Li sheet was assembled into a coin cell, and the coin cell was subjected to charge and discharge tests on a blue cell test system (LAND CT2001A). The charge and discharge tests were performed at a current density of 10 mA / g, and the specific discharge process was as follows: discharged at 10 mA / g to 5.0 mV, and then charged at 10 mA / g to 3.0 V after 5 minutes of standing. The capacity C of the coin cell at this time was recorded, and the mass capacity was C / M. Similarly, the mass capacity test in a sodium ion battery only required the replacement of the lithium sheet with a sodium sheet.

[0124] 3. Test of specific surface area BET

[0125] A specific surface area analyzer (Tristar II 3020M) was used to measure the specific surface area of the graphite and niobium composite metal oxide in the negative electrode active material of each example and comparative example by nitrogen adsorption method. The specific test was performed in accordance with the national standard GB / T 19587-2017 “Gas Adsorption BET Method for Determining the Specific Surface Area of Solid Substances”.

[0126] 4. High temperature storage test under high rate

[0127] The lithium ion batteries of each example and comparative example were first charged at a current of 4C until the upper limit voltage was 4.3V at an environment of 25°C, and then stored at 85°C for 12h. The thickness change of the lithium ion battery before and after storage was recorded, and the thickness expansion rate T = (thickness after storage - thickness before storage) / thickness before storage x 100%. T was used to represent the storage performance, and the smaller the T value, the better the storage performance. When charging at a high rate, if lithium is precipitated from the lithium ion battery, the gas production of the lithium ion battery will significantly increase. For sodium ion battery test, only the upper limit voltage of charging was adjusted to 4.0V and the discharge cut-off voltage was adjusted to 2.0V.

[0128] 5. Energy density test under high rate

[0129] The lithium ion batteries prepared in each example and comparative example were first charged at a current of 0.2C to 4.3V, and then charged at a constant voltage of 0.05C at a test temperature of 25°C. After standing for 5 minutes, they were discharged at 0.2C to 2.8V, and the discharge energy was recorded. Then the 0.2C discharge energy density was calculated according to the formula: energy density (Wh / L) = discharge energy (Wh) / volume of electrochemical device. The higher the energy density, the better. For sodium ion battery test, only the upper limit voltage of charging was adjusted to 4.0V and the discharge cut-off voltage was adjusted to 2.0V.

[0130] 6. High temperature cycle test under high rate

[0131] The lithium ion batteries in the examples and comparative examples of the present application were charged and discharged by the following steps, and the cycle capacity retention rate of the lithium ion battery was calculated. First, the first charge and discharge cycle was carried out at an environment of 45°C. The lithium ion battery was charged at a current of 4C until the upper limit voltage was 4.3V, and then discharged at a current of 0.5C until the cut-off voltage was 2.8V. The discharge capacity C0 of the first cycle was recorded. Subsequently, 300 charge and discharge cycles were carried out according to the above charge and discharge process, and the discharge capacity C300 of the 300th cycle was recorded. The cycle capacity retention rate of the lithium ion battery was calculated using the following formula: (C300 / C0) x 100%. When charging at a high rate, if lithium is precipitated, the cycle of the battery will significantly and rapidly decay. For sodium ion battery test, only the upper limit voltage of charging was adjusted to 4.0V and the discharge cut-off voltage was adjusted to 2.0V.

[0132] 7. Pallet compaction density test

[0133] The active material per unit mass per unit volume is obtained. The current collector and the negative electrode plate with an area of A are weighed by an electronic balance, and the weights are recorded as M1 and M2, respectively. The thicknesses of the current collector and the positive electrode plate are measured by a micrometer, and the thicknesses are recorded as C1 and C2, respectively. The compaction density of the positive electrode = [(M2-M1) / (C2-C1)] / A.

[0134] Table 1

[0135]

[0136]

[0137] Note: " / " in Table 1 indicates no relevant preparation parameters.

[0138] In combination with Table 1, Examples 1 to 3 compare the use of three different compounds I, respectively. It can be seen that, when three different niobium composite metal oxides with the same content but different molecular formulas are arranged on the surface of the negative electrode active material layer, under the premise that the parameters a, b and a / b are all appropriate, the fast charging performance of the lithium ion battery can be improved, and the high energy density is also considered. Examples 4 to 6 further change the value of a. It can be seen that, when the value of a is 0.05, the high-temperature cycle at high rate and the high-temperature storage performance are both significantly reduced, i.e., too small a value of a is not conducive to the improvement of the fast charging performance of the lithium ion battery. When the value of a is 0.5, the energy density of the lithium ion battery is significantly reduced, i.e., too large a value of a is not conducive to the realization of high energy density. Examples 7 to 8 change the type of negative electrode active material in the negative electrode active material layer. It can be seen that the technical solution of the present application is not only suitable for graphite negative electrode but also suitable for silicon-containing negative electrode. Under the premise that the type of compound I, the parameters a, b and a / b are all appropriate, the energy density of the lithium ion battery containing silicon at high rate can reach up to 800, and the capacity retention rate after 300 cycles at 4C 45℃ is as high as 87%, and the expansion rate after 12h storage at 4C 85℃ is only 8.6%. Examples 10 to 12 further adjust the specific surface area of the compound I. It can be seen that a suitable specific surface area is conducive to further improving the fast charging performance of the lithium ion battery.

[0139] The surface of the negative electrode active material layer in Comparative Examples 1 to 3 is not arranged with a fast ion conductor layer. It can be obviously seen that the capacity retention rate after 300 cycles at 4C 45℃ of Comparative Examples 1 to 3 is not more than 55%, and the expansion rate after 12h storage at 4C 85℃ is as high as more than 25%. It can be seen that arranging a fast ion conductor layer on the surface of the negative electrode active material layer can significantly improve the fast charging performance of the lithium ion battery.

[0140] In Comparative Example 4, since the negative electrode active material layer contains both the negative electrode active material and the fast ion conductor, at this time, the fast ion conductor can improve the fast charging performance of the lithium ion battery, but cannot well solve the problem of lithium precipitation on the surface of the electrode sheet. The possible reason is that the mixed way causes the kinetic poor negative electrode material (graphite, silicon, etc.) to also be in the surface layer of the electrode sheet. When charging at a high rate, a large amount of lithium cannot be embedded in these kinetic poor negative electrode materials in time, and will be enriched on the surface of these materials, so the problem of lithium precipitation still exists. When the fast ion conductor is placed on the upper layer of the kinetic poor negative electrode material, when charging at a high rate, a large amount of lithium will first be quickly embedded in the fast ion conductor material to avoid the problem of lithium precipitation caused by the inability of lithium to be embedded in time, and is transmitted to the kinetic poor negative electrode material (graphite, silicon, etc.) through the fast ion conductor, providing a buffer time for the lower layer of the negative electrode material to embed lithium.

[0141] Table 2

[0142]

[0143]

[0144] Note: " / " in Table 2 indicates no relevant preparation parameters.

[0145] In combination with Table 2, Examples 16 to 18 compare the use of three different molecular formula compounds II, respectively. It can be seen that when three different molecular formula niobium composite metal oxides with the same content are arranged on the surface of the negative electrode active material layer, under the premise that the parameters a, b and a / b are all appropriate, the fast charging performance of the sodium ion battery can be improved, and the high energy density can also be considered. Examples 19 to 21 further change the value of a. It can be seen that when the value of a is 0.05, the energy density of the sodium ion battery is significantly reduced, and the high-temperature cycle and high-temperature storage performance at a high rate are also significantly reduced, that is, too small a value of a is not conducive to the improvement of the fast charging performance and the energy density of the sodium ion battery. When the value of a is 0.5, the energy density of the sodium ion battery is significantly reduced, that is, too large a value of a is not conducive to achieving high energy density. Examples 22 to 24 further adjust the specific surface area of the compound II. It can be seen that a suitable specific surface area is conducive to further improving the fast charging performance of the sodium ion battery. Comparative Example 5 does not arrange a fast ion conductor layer on the surface. It can be clearly seen that the capacity retention rate of Comparative Example 1 is only 65% after 4C 45°C cycle for 300 cycles, and the expansion rate of 4C 85°C storage for 12h is as high as 30%. Therefore, arranging a fast ion conductor layer on the surface of the negative electrode active material layer can significantly improve the fast charging performance of the sodium ion battery.

[0146] The above merely provides preferred embodiments of the present application, and is not used to limit the present application. Any modification, equivalent replacement, and improvement made in the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector, a negative electrode active material layer, and a fast ion conductor layer; The negative electrode active material layer is disposed on at least one side surface of the negative electrode current collector, and the fast ion conductor layer is disposed on the surface of the negative electrode active material layer and covers the negative electrode active material layer; Along the thickness direction of the negative electrode current collector, the thickness of the negative electrode active material layer is denoted as L′, and the thickness of the fast ion conductor layer is denoted as L, and L / (L′+L)=b, satisfying: b≤0.4; The negative electrode active material layer includes a negative electrode active material, the fast ion conductor layer includes a fast ion conductor, and the fast ion conductor includes a niobium composite metal oxide; Based on the mass of the negative electrode active material layer, the mass percentage of the negative electrode active material is denoted as W′%; Based on the mass of the fast ion conductor layer, the mass percentage of the niobium composite metal oxide is denoted as W%, and W / (W′+W)=a; It satisfies: 1.3≤a / b≤2.

3.

2. The negative electrode sheet according to claim 1, characterized in that, It satisfies: a≤0.5 and 80≤W≤95.

3. The negative electrode sheet according to claim 1, characterized in that, When the negative electrode active material is a first negative electrode active material used in lithium-ion batteries, at least one of the following conditions is met: (1) The niobium composite metal oxide comprises a compound with the molecular formula T x Nb y M z O a′ At least one of compound I, wherein T is selected from at least one of K, Li, Fe, V, W, Cr, Zr, Al, Mg, Zn, Cu, Mo, Na, Ga, P, Tc, Si, Ga, Sn, Ni, Co, Mn, Sr, Y, In, Na or Ti, and M is selected from at least one of Al, Ti, W, Zr, Nb, In, Ru, Sb, Sr, Y, Ni, Co, Mn, Fe, Gr, Mo, Tc, Sn, Ga, Si, V or Mg, and T and M are different and satisfy: 0 < x / (x+y+z) ≤ 0.6, 1 ≤ a′ / (x+y+z) < 5, 0 ≤ z / (x+y+z) ≤ 0.5; （2）0.1≤a≤0.3； （3）0.05≤b≤0.2； (4) 1.3 ≤ a / b ≤ 1.6; （5）88≤W≤92。 4. The negative electrode sheet according to claim 3, characterized in that, The specific surface area of ​​compound I is 0.8 m². 2 / g to 20m 2 / g.

5. The negative electrode sheet according to claim 4, characterized in that, The specific surface area of ​​compound I is 0.8 m². 2 / g to 1.2m 2 / g.

6. The negative electrode sheet according to claim 4, characterized in that, At least one of the following conditions must be met: (1) The compaction density of the negative electrode sheet is 1.79 g / cm³. 3 Up to 2.4 g / cm 3 ; (2) The negative electrode plateau voltage for lithium is 0.1V to 1.0V.

7. The negative electrode sheet according to claim 6, characterized in that, The compaction density of the negative electrode sheet is 1.79 g / cm³. 3 Up to 2.28 g / cm 3 .

8. The negative electrode sheet according to claim 6, characterized in that, The negative electrode plateau voltage for lithium is 0.4V to 0.8V.

9. The negative electrode sheet according to claim 1, characterized in that, When the negative electrode active material is a second negative electrode active material used in sodium-ion batteries, at least one of the following conditions must be met: (1) The niobium composite metal oxide includes the molecular formula Na x′ A y′ Ti z′ At least one of compounds II of O2, wherein A is selected from at least one of Ni, Co, Li, and Gr, and satisfies: 0.6 < x′ < 0.7, y′ + z′ = 1; （2）0.1≤a≤0.3； （3）0.05≤b≤0.2； (4) 2≤a / b≤2.3; （5）80≤W≤88。 10. The negative electrode sheet according to claim 9, characterized in that, The specific surface area of ​​compound II is 0.5 m². 2 / g to 10m 2 / g.

11. The negative electrode sheet according to claim 10, characterized in that, The specific surface area of ​​compound II is 0.5 m². 2 / g to 5m 2 / g.

12. The negative electrode sheet according to claim 10, characterized in that, At least one of the following conditions must be met: (1) The compaction density of the negative electrode sheet is 1.2 g / cm³. 3 Up to 2.1 g / cm 3 ; (2) The plateau voltage of the negative electrode plate relative to sodium is 0.28V to 0.5V.

13. The negative electrode sheet according to claim 12, characterized in that, The compaction density of the negative electrode sheet is 1.6 g / cm³. 3 Up to 1.7 g / cm 3 .

14. The negative electrode sheet according to claim 12, characterized in that, The negative electrode plateau voltage relative to sodium is 0.36V to 0.4V.

15. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode, a separator, and a negative electrode as described in any one of claims 1 to 14; The positive electrode and the negative electrode are stacked alternately, and the separator is disposed between two adjacent positive and negative electrode sheets. The fast ion conductor layer of the negative electrode is in contact with the separator.

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