An electrochemical device and an electronic device

Abstract

The application provides an electrochemical device and an electronic device, the electrochemical device comprising a positive electrode, a negative electrode and an electrolyte, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a surface layer and an inner layer, the mass percentage content a of sodium in the surface layer satisfying 0.05%<=a<=0.15% based on the mass of the negative electrode active material layer, the mass percentage content b of sodium in the inner layer satisfying 0<b<=0.05%, and a>b; the electrolyte comprising an additive, the additive comprising at least one of lithium difluorophosphate, vinylene carbonate or 1,3-propane sultone; and the mass percentage content of the additive being 0.001% to 8% based on the mass of the electrolyte. By regulating a, b and the relationship between a and b within the above range, selecting the above additive and regulating the mass percentage content of the additive within the above range, the cycle stability of the lithium ion battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and particularly relates to an electrochemical device and an electronic device. BACKGROUND

[0002] Lithium ion batteries are widely used in portable electronic products, electric transportation, energy storage and other fields due to their high energy density, good cycle performance, environmental protection, safety and no memory effect. With the continuous research of lithium ion batteries with higher energy density and power density, sodium sources may be introduced into the electrolyte, electrode material and electrode material formula in the battery system. Sodium is easy to block the lithium ion transmission channel due to its high standard potential, which affects the stability of the negative electrode interface film and thus the cycle stability of the lithium ion battery. Therefore, how to improve the cycle stability of the lithium ion battery has become a problem to be solved. SUMMARY

[0003] The present application aims to provide an electrochemical device and an electronic device to improve the cycle stability of the lithium ion battery.

[0004] The first aspect of the present application provides an electrochemical device, which comprises a positive electrode, a negative electrode and an electrolyte, the negative electrode comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, the negative electrode active material layer comprises a surface layer and an inner layer, the surface layer is a region with a depth of 20 μm from the surface of the negative electrode active material layer to the inside of the negative electrode active material layer, and the inner layer is a region except the surface layer in the negative electrode active material layer; the mass percentage content a of sodium in the surface layer satisfies 0.05%≤a≤0.15% based on the mass of the negative electrode active material layer, the mass percentage content b of sodium in the inner layer satisfies 0

[0005] In some embodiments of the present application, 0.05%≤a≤0.1%, 0

[0006] In some embodiments of the present application, the electrolyte satisfies one of the following conditions based on the mass of the electrolyte:

[0007] (I) the mass percentage content A of the lithium difluorophosphate satisfies 0.01%≤A≤2%;

[0008] (II) the mass percentage content B of the vinylene carbonate satisfies 0.01%≤B≤5%;

[0009] (III) the mass percentage content C of the 1,3-propane sultone satisfies 0.01%≤C≤8%.

[0010] By selecting the electrolyte with the above characteristics, it is beneficial to obtain a stable SEI film to promote uniform deposition of lithium and thus improve the cycle stability of the electrochemical device.

[0011] In some embodiments of the present application, 0≤A / C≤1. By regulating A / C in the above range, it is beneficial for lithium difluorophosphate and 1,3-propane sultone to form a synergistic effect, obtain a stable SEI film, stabilize the deposition of sodium in the negative active material layer, promote uniform deposition of lithium, and thus improve the cycle stability of the electrochemical device.

[0012] In some embodiments of the present application, the additive includes lithium difluorophosphate, and the electrolyte includes F - , the mass percentage content D of the F - satisfies 0.05%≤D≤2%, and 0.2≤D / A≤2 based on the mass of the electrolyte. By regulating D and D / A in the above range, it is beneficial to obtain a fluorine-containing SEI film to promote uniform deposition of lithium and thus improve the cycle stability of the electrochemical device.

[0013] In some embodiments of the present application, the positive electrode includes a positive active material layer, the positive active material layer includes a positive active material, and the positive active material has a P63mc crystal structure; the positive active material includes Li x Na z Co 1-y M yO2, 0.6 < x < 1.01, 0≤y<0.15, 0≤z<0.03, M includes at least one of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr or Zr. By selecting the positive active material Li x Na z Co 1-y M y O2, and regulating x, y, z within the above range, the cycle stability of the electrochemical device is improved.

[0014] In some embodiments of the present application, 0.001≤z / (1-y)≤0.02. By regulating z / (1-y) within the above range, the cycle stability of the electrochemical device is improved. x Na z Co 1-y M y Na, Co and M in Li

[0015] In some embodiments of the present application, the additive includes 1,3-propane sultone, the mass percentage of the 1,3-propane sultone in the electrolyte is C, 0.001≤z / (1-y)C≤0.012. By regulating z / (1-y)C within the above range, the cycle stability of the electrochemical device is improved. x Na z Co 1-y M y Li

[0016] In some embodiments of the present application, the electrolyte satisfies one of the following conditions:

[0017] (Ⅳ) the mass percentage of the lithium difluorophosphate A satisfies 0.1%≤A≤1%;

[0018] (Ⅴ) the mass percentage of the vinylene carbonate B satisfies 0.2%≤B≤2%;

[0019] (Ⅵ) the mass percentage of the 1,3-propane sultone C satisfies 0.5%≤C≤5%.

[0020] By selecting the electrolyte with the above characteristics, a more stable SEI film is obtained, further promoting uniform deposition of lithium, thereby further improving the cycle stability of the electrochemical device.

[0021] The second aspect of the present application provides an electronic device comprising the electrochemical device provided by the first aspect of the present application. The electrochemical device provided by the present application has good cycle stability. Therefore, the electronic device of the present application has good cycle stability.

[0022] The present application provides an electrochemical device and an electronic device. The electrochemical device comprises a positive electrode, a negative electrode and an electrolyte. The negative electrode comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector. The negative electrode active material layer comprises a surface layer and an inner layer. The surface layer is a region with a depth of 20 μm from the surface of the negative electrode active material layer to the inside of the negative electrode active material layer. The inner layer is a region of the negative electrode active material layer excluding the surface layer. The mass percentage of sodium in the surface layer a satisfies 0.05%≤a≤0.15%, and the mass percentage of sodium in the inner layer b satisfies 0

[0023] Of course, implementing any product or method of the present application does not necessarily require all the advantages described above to be achieved at the same time. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0025] It should be noted that in the specific embodiments of the present application, lithium ion batteries are used as examples of electrochemical devices to explain the present application, but the electrochemical devices of the present application are not limited to lithium ion batteries.

[0026] The first aspect of the present application provides an electrochemical device, the electrochemical device comprising a positive electrode, a negative electrode and an electrolyte, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a surface layer and an inner layer, the surface layer being a region of the negative electrode active material layer from the surface of the negative electrode active material layer to a depth of 20 μm into the negative electrode active material layer, the inner layer being a region of the negative electrode active material layer excluding the surface layer; the mass percentage of sodium in the surface layer a satisfies 0.05%≤a≤0.15%, preferably 0.05%≤a≤0.1%, based on the mass of the negative electrode active material layer, the mass percentage of sodium in the inner layer b satisfies 0

[0027] The inventors have found that when a and b are too small (for example, a is less than 0.05% and b is 0), sodium is not easy to cause the static shielding effect, which affects the uniform deposition of lithium, thereby affecting the cycle performance of the electrochemical device; when a and b are too large (for example, a is greater than 0.15% and b is greater than 0.05%), sodium is easy to block the transmission channel of lithium ions, affecting the transmission of lithium ions during the cycle process, and affecting the formation of SEI film, thereby affecting the cycle performance of the electrochemical device. By adjusting a, b and the relationship between a and b within the above range, the transmission performance of lithium ions is improved, and the formation of SEI film is promoted, thereby improving the cycle stability of the electrochemical device. The inventors have also found that when the mass percentage of the additive is too small (for example, less than 0.001%), the effect of improving the film quality of the SEI film is not obvious, thereby being not conducive to improving the cycle stability of the electrochemical device; when the mass percentage of the additive is too large (for example, greater than 8%), the formed SEI film is unstable, which affects the uniform deposition of lithium, and further affects the cycle performance of the electrochemical device. By selecting the above additive and adjusting the mass percentage of the additive within the above range, a stable SEI film is obtained, so as to promote the uniform deposition of lithium, thereby improving the cycle stability of the electrochemical device.

[0028] The application has no particular restriction on the way of regulating the content of sodium in the negative active material layer, as long as the purpose of the application can be achieved. Illustratively, during the preparation of the negative electrode sheet, after the negative current collector surface is coated with the slurry, the distribution of the sodium-containing material (such as sodium carboxymethyl cellulose) in the negative active material layer can be regulated by regulating the heating rate, heating time, drying temperature, and drying time during the drying of the slurry, and a multi-stage heating method or a single-stage heating method can be used, thereby regulating the mass percentage content a of sodium in the surface layer and the mass percentage content b of sodium in the inner layer. For example, the sodium content of the surface layer of the negative active material layer is higher and the sodium content of the inner layer is lower when the heating rate during drying is increased, the heating time is extended, the drying temperature is increased, and the drying time is extended; the sodium content of the surface layer of the negative active material layer is lower and the sodium content of the inner layer is higher when the heating rate during drying is reduced, the heating time is reduced, the drying temperature is reduced, and the drying time is reduced. Those skilled in the art can understand that during the drying process of the slurry, the solvent in the slurry gradually evaporates with heating, and after the solvent evaporates, the sodium-containing material such as sodium carboxymethyl cellulose remains in the negative electrode sheet. Due to the fact that the solvent diffuses from the bottom to the surface during the drying process and then evaporates, the distribution of sodium carboxymethyl cellulose in the negative active material layer is prone to have a content gradient. For example, at a higher drying temperature, such as 85°C to 150°C, the solvent evaporates quickly and sodium carboxymethyl cellulose migrates quickly, resulting in a content gradient in its distribution, so that the sodium content in the surface layer of the negative active material layer is higher and the sodium content in the inner layer is lower. Further, the drying temperature can be 100°C to 130°C.

[0029] The application has no restriction on the drying form, and those skilled in the art can select according to actual needs, as long as the purpose of the application can be achieved. For example, vacuum drying can be used.

[0030] In some embodiments of the application, the mass percentage content A of lithium difluorophosphate satisfies 0.01%≤A≤2%, preferably 0.1%≤A≤1%, for example A can be 0.1%, 0.5%, 1%, 1.5%, 2%, or a range composed of any two of the above values. By regulating A within the above range, it is beneficial to obtain a stable SEI film to promote uniform deposition of lithium, thereby improving the cycle stability of the electrochemical device.

[0031] In some embodiments of the application, the mass percentage content B of vinylene carbonate satisfies 0.01%≤B≤5%, preferably 0.2%≤B≤2%, for example B can be 0.01%, 1%, 2%, 3%, 4%, 5%, or a range composed of any two of the above values. By regulating B within the above range, it is beneficial to obtain a stable SEI film to promote uniform deposition of lithium, thereby improving the cycle stability of the electrochemical device.

[0032] In some embodiments of the present application, the mass percentage content C of 1,3-propane sultone satisfies 0.01%≤C≤8%, preferably 0.5%≤C≤5%, for example, C can be 0.01%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or a range between any two of them. By regulating C in the above range, it is beneficial to obtain a stable SEI film, promote uniform deposition of lithium, and thus improve the cycle stability of the electrochemical device.

[0033] In some embodiments of the present application, 0≤A / C≤1. By regulating A / C in the above range, it is beneficial to form a synergistic effect between lithium difluorophosphate and 1,3-propane sultone, obtain a stable SEI film, promote uniform deposition of lithium, and thus improve the cycle stability of the electrochemical device.

[0034] In some embodiments of the present application, the additive includes lithium difluorophosphate, and the electrolyte includes F - , the mass percentage content D of F - satisfies 0.05%≤D≤2% based on the mass of the electrolyte, and 0.2≤D / A≤2. The inventors have found that lithium difluorophosphate can lose fluorine to form F - , and by regulating D and D / A in the above range, it is beneficial to obtain a fluorine-containing SEI film, and thus improve the cycle stability of the electrochemical device.

[0035] In some embodiments of the present application, the positive electrode includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material has a P63mc crystal structure; the positive electrode active material includes Li x Na z Co 1-y M y O2, 0.6 x Na z Co 1-y M y O2, and regulating x, y, and z in the above range is beneficial to improve the cycle stability of the electrochemical device.

[0036] In some embodiments of the present application, 0.001≤z / (1-y)≤0.02. By regulating z / (1-y) in the above range, it is beneficial to Lix Na z Co 1-y M y Na, Co and M in O2 synergistically act to improve the cycle stability of the electrochemical device.

[0037] In some embodiments of the present application, the additive includes 1,3-propane sultone, and the mass percentage of 1,3-propane sultone based on the mass of the electrolyte is C, 0.001≤z / (1-y)C≤0.012. By regulating z / (1-y)C within the above range, Li x Na z Co 1-y M y O2 and 1,3-propane sultone synergistically act to improve the cycle stability of the electrochemical device.

[0038] In some embodiments of the present application, the electrolyte further includes other additives, and the other additives include at least one of glutaronitrile, succinonitrile, adiponitrile, 1,3,6-hexanetricarbonitrile, lithium tetrafluoroborate, and the mass percentage of the other additives based on the mass of the electrolyte is 0.01% to 20%.

[0039] In some embodiments of the present application, the electrolyte includes an organic solvent. The organic solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone.

[0040] In some embodiments of the present application, the electrolyte includes a lithium salt, and the mass percentage of the lithium salt based on the mass of the electrolyte is 10% to 15%, and the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, or lithium difluoro(oxalato)borate. The content of the organic solvent can be selected according to specific circumstances, such as the mass percentage of the organic solvent based on the mass of the electrolyte is 60% to 85%.

[0041] The electrochemical device of the present application further includes an electrode assembly including a positive electrode, a negative electrode, and a separator.

[0042] The negative current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative current collector can include, but is not limited to, a copper foil, an aluminum foil, a nickel foil, or a carbon-based current collector, etc. The negative active material layer of the present application includes a negative active material. The kind of the negative active material is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative active material can include, but is not limited to, at least one of lithium metal, natural graphite, artificial graphite, or a silicon-based material including at least one of silicon, a silicon oxide compound, a silicon carbon compound, or a silicon alloy. In the present application, the thickness of the negative current collector and the negative material layer is not particularly limited, as long as the object of the present application can be achieved. For example, the thickness of the negative current collector is 1 μm to 200 μm, and the thickness of the single-sided negative material layer is 40 μm to 200 μm. In the present application, the negative active material layer can be disposed on one surface in the thickness direction of the negative current collector, or on both surfaces in the thickness direction of the negative current collector. It should be noted that the "surface" herein can be the entire area of the negative current collector, or a partial area of the negative current collector, which is not particularly limited in the present application, as long as the object of the present application can be achieved. The negative active material layer of the present application can further include a conductive agent and a binder.

[0043] In the present application, the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the positive current collector can include an aluminum foil or an aluminum alloy foil, etc. The thickness of the positive current collector and the positive material layer is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the thickness of the positive current collector is 1 μm to 200 μm. The thickness of the single-sided positive material layer is 40 μm to 200 μm. In the present application, the positive active material layer can be disposed on one surface in the thickness direction of the positive current collector, or on both surfaces in the thickness direction of the positive current collector. It should be noted that the "surface" herein can be the entire area of the positive current collector, or a partial area of the positive current collector, which is not particularly limited in the present application, as long as the object of the present application can be achieved. The positive active material layer of the present application can further include a conductive agent and a binder.

[0044] The conductive agent and the binder are not particularly limited as long as the object of the present application can be achieved. For example, the conductive agent can include at least one of conductive carbon black (Super P), acetylene black, ketjen black, flake graphite, graphene, carbon nanotube, carbon nanowire, or carbon fiber. The binder can include at least one of polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride-hexafluoropropylene, a copolymer of styrene-acrylate, a copolymer of styrene-butadiene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, sodium carboxymethyl cellulose (CMC-Na), polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyaniline, polyimide, polyamide-imide, polysiloxane, epoxy resin, polyester resin, polyurethane resin, or polyfluorene, etc.

[0045] The separator film of the present application is not particularly limited as long as the object of the present application can be achieved. For example, the separator film can include at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, aramid, etc., and polyethylene and polypropylene can prevent short circuiting and can improve the stability of the electrochemical device through a shutdown effect. For example, the polyethylene can include at least one of high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. The surface of the separator film can include a porous layer disposed on at least one surface of the separator film, and the porous layer can include at least one of inorganic particles or a binder, and the porous layer can improve the heat resistance, oxidation resistance, and electrolyte impregnation properties of the separator film and enhance the adhesion between the separator film and the electrode sheet. For example, the inorganic particles can include at least one of aluminum oxide (Al2O3), silicon oxide (SiO2), magnesium oxide (MgO), titanium oxide (TiO2), hafnium dioxide (HfO2), tin oxide (SnO2), cerium dioxide (CeO2), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate, etc. For example, the binder in the porous layer can include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene, etc.

[0046] The electrochemical device of the present application is not particularly limited and can include any device in which an electrochemical reaction occurs. In an embodiment of the present application, the electrochemical device can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium metal secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0047] The preparation process of the electrochemical device is well known to those skilled in the art, and the present application is not particularly limited, for example, can include but is not limited to the following steps: stacking the positive electrode, the separator and the negative electrode in order, and winding, folding, etc. as needed to obtain a wound structure of the electrode assembly, placing the electrode assembly into a packaging bag, injecting an electrolyte into the packaging bag and sealing to obtain an electrochemical device; or stacking the positive electrode, the separator and the negative electrode in order, then fixing the four corners of the entire stack structure with tape to obtain a stack structure of the electrode assembly, placing the electrode assembly into a packaging bag, injecting an electrolyte into the packaging bag and sealing to obtain an electrochemical device. In addition, a current protection element, a guide plate, etc. can also be placed in the packaging bag as needed to prevent the pressure inside the electrochemical device from rising, overcharging and discharging. The present application does not limit the packaging bag, and those skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved. For example, an aluminum plastic film packaging bag can be used.

[0048] The second aspect of the present application provides an electronic device comprising the electrochemical device provided by the first aspect of the present application. The electrochemical device provided by the present application has good cycle stability. Therefore, the electronic device of the present application has good cycle stability.

[0049] The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. In some embodiments, the electronic device can include but is not limited to a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery or a lithium ion capacitor, etc.

[0050] Embodiment

[0051] Hereinafter, embodiments and comparative examples are given to more specifically describe the embodiments of the present application. Various tests and evaluations were carried out according to the following methods.

[0052] Test method and apparatus :

[0053] Cycle performance test:

[0054] The lithium ion battery was charged at 25°C with a constant current of 1C to a voltage of 4.55V, then charged at a constant voltage of 4.55V to a current of 0.05C, then rested for 5min, then discharged at a constant current of 0.5C to a voltage of 3.0V, and rested for 5min, which was the first cycle of charge and discharge process, and the discharge capacity of this cycle was recorded as the initial capacity. The lithium ion battery was subjected to 500 cycles of charge and discharge test according to the above method, and the discharge capacity of the 500th cycle was detected and recorded. Capacity retention rate (%) = discharge capacity of the 500th cycle / initial discharge capacity x 100%.

[0055] Sodium content test:

[0056] The discharged lithium ion battery was disassembled to obtain a negative electrode, and the negative electrode was punched into small round sheets with an area of 1545.25mm 2 . A part of the small round sheets was digested (the small round sheets were weighed in a digestion tank, 68% concentrated nitric acid (10ml of 68% concentrated nitric acid was added for every 0.5000g of small round sheets) was added, the digestion device was assembled, and the digestion was carried out in a microwave digestion instrument. The "68% concentrated nitric acid" refers to the volume fraction of concentrated nitric acid is 68%), and then inductively coupled plasma optical emission spectrometry (ICP-OES) test was carried out to obtain the content of sodium element, and the average value of 5 test results was taken as C1. Another part of the small round sheets obtained by punching was obtained, and the small round sheet surface layer of 20μm thick negative active material layer was removed by double-sided tape or scraper or precision laser cutting machine, and the remaining obtained electrode sheet was digested and then subjected to ICP-OES test to obtain the content of sodium element, and the average value of 5 test results was taken as C2. The mass percentage content of sodium in the surface layer a=C1-C2; the mass percentage content of sodium in the inner layer b=C2.

[0057] Additive and F - Content test:

[0058] The discharged lithium ion battery was disassembled to obtain an electrolyte, the electrolyte was centrifuged to obtain supernatant, and then filtered to obtain a filtrate, which was subjected to gas chromatography (GC-MS) test and ion chromatography (IC) test respectively, according to which the mass percentage content B of the additive such as vinylene carbonate and / or the mass percentage content C of 1,3-propane sultone was obtained by GC-MS test; and the mass percentage content A of lithium difluorophosphate and / or the mass percentage content D of F - in the electrolyte was obtained by IC test.

[0059] Positive active material Li x Na z Co 1-y M y Element content test in O2:

[0060] The lithium ion battery after discharging was disassembled to obtain a positive electrode, the positive electrode was punched into small circular sheets with an area of 1545.25 mm 2 After digestion, ICP-OES test was performed, and the contents of elements such as Li, Na, Co and M were recorded. The average value of 10 test results of each element was calculated as the mass percentage content of each element.

[0061] Example 1-1

[0062] Preparation of the negative electrode

[0063] The negative electrode active material artificial graphite, conductive carbon black (Super P), carboxymethyl cellulose sodium (CMC-Na), binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 96.4:1.5:0.5:1.6, deionized water was added, and the mixture was stirred uniformly under the action of a vacuum stirrer to obtain a negative electrode slurry, wherein the solid content of the negative electrode slurry was 54 wt%. The negative electrode slurry was uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 10 μm, and the copper foil was dried at 85°C for 1.5 h to obtain a negative electrode sheet with a negative electrode material layer on one side with a thickness of 50 μm. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode with a double-sided coated negative electrode material layer. Then, after cold pressing, sheet cutting and slitting, the negative electrode was dried under vacuum at 100°C for 0.5 h to obtain a negative electrode with a size of 76 mm x 867 mm.

[0064] Preparation of the positive electrode

[0065] Sodium carbonate and cobalt trioxide tetraoxide were uniformly mixed in a molar ratio of Na to Co of z:(1-y)=0.02:1, sintered at 800°C for 46 h in an oxygen atmosphere, to obtain Na 0.02 CoO2with a P63mc structure; Na 0.02 CoO2and lithium nitrate were uniformly mixed in a molar ratio of Na to Li of 0.02:0.73, reacted at 300°C for 6 h in an air atmosphere, washed with deionized water several times after the reaction, and dried at 100°C for 6 h to obtain Li 0.73 Na 0.02 CoO2with a P63mc structure.

[0066] The positive electrode active material Li 0.73 Na 0.02CoO2, conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6, N-methyl pyrrolidone (NMP) was added, and the system was stirred to be homogeneous and transparent under the action of a vacuum stirrer to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry was 72 wt%. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 13 μm, and the aluminum foil was dried at 120 °C for 1 h to obtain a positive electrode sheet with a single-sided positive electrode material layer with a thickness of 60 μm. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided positive electrode material layer. Then, after cold pressing, sheet cutting, and slitting, the positive electrode sheet was dried at 120 °C under vacuum for 1 h to obtain a positive electrode sheet with a size of 74 mm x 851 mm.

[0067] <Preparation of electrolyte>

[0068] Under a dry argon environment, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were first mixed in a mass ratio of 1:1:3 to obtain an organic solvent, then lithium salt LiPF6 was added to the organic solvent, and finally an additive was added to obtain an electrolyte. The mass percentage of the lithium salt was 14.4%, the mass percentage of the lithium difluorophosphate additive was 0.05%, and the rest was the organic solvent.

[0069] <Preparation of separator film>

[0070] Aluminum oxide (Al2O3) was added to NMP to prepare a ceramic layer slurry with a solid content of 30 wt% and was stirred uniformly. The ceramic layer slurry was uniformly coated on one surface of a polyethylene substrate with a thickness of 9 μm, and after drying, a single-sided ceramic layer-coated separator film was obtained. The above steps were repeated on the other surface of the separator film to obtain a double-sided ceramic layer-coated separator film. Then, PVDF was added to NMP and stirred uniformly to prepare a binder slurry with a solid content of 30 wt%, 2.5 mg / 15 40.25 mm 2 of PVDF was coated on the surface of the ceramic layer, and the adhesive layer was obtained after drying at 85 °C for 4 h. Similarly, 2.5 mg / 15 40.25 mm 2 of PVDF was coated on the other surface of the polyethylene substrate to obtain an adhesive layer, and the separator film was obtained after drying at 85 °C for 4 h.

[0071] <Preparation of lithium ion battery>

[0072] The positive electrode, the separator, and the negative electrode prepared above are stacked in order, with the separator between the positive electrode and the negative electrode to play a role of separation, and are wound to obtain an electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film outer package, is placed in a vacuum oven at 80°C for 12h to remove water, is injected with the electrolyte prepared above and is sealed, is subjected to processes of formation, degassing, edge cutting, and the like to obtain a lithium ion battery.

[0073] Examples 1-2 to 1-29

[0074] Except that the dry temperature of the negative electrode slurry is adjusted according to Table 1 to adjust the values of a and b in the <preparation of the negative electrode>, and except that the types and mass percentages of the additives are adjusted according to Table 2 in the <preparation of the electrolyte>, the mass percentage of the organic solvent is changed accordingly, and the mass percentage of the lithium salt is unchanged, the rest is the same as in Example 1-1.

[0075] Table 1

[0076] Drying temperature a(％) b(％) Example 1-1 85 0.051 0.04 Example 1-2 100 0.06 0.037 Example 1-3 120 0.08 0.02 Example 1-4 125 0.08 0.016 Example 1-5 130 0.1 0.013 Example 1-6 128 0.1 0.014 Example 1-7 140 0.12 0.008 Example 1-8 150 0.15 0.004 Example 1-9 148 0.15 0.005 Example 1-10 98 0.06 0.035 Example 1-11 120 0.08 0.02 Example 1-12 98 0.06 0.035 Example 1-13 120 0.08 0.02 Example 1-14 122 0.08 0.016 Example 1-15 138 0.12 0.009 Example 1-16 135 0.11 0.010 Example 1-17 102 0.06 0.033 Example 1-18 120 0.08 0.02 Example 1-19 125 0.08 0.016 Example 1-20 138 0.12 0.009 Example 1-21 139 0.12 0.008 Example 1-22 148 0.15 0.005 Example 1-23 120 0.08 0.02 Example 1-24 120 0.08 0.02 Example 1-25 129 0.1 0.012 Example 1-26 120 0.08 0.02 Example 1-27 129 0.1 0.011 Example 1-28 120 0.08 0.02 Example 1-29 129 0.1 0.011 Comparative Example 1-1 160 0.2 0.002 Comparative Example 1-2 60 0.012 0.1 Comparative Example 1-3 120 0.08 0.02

[0077] Example 2-1

[0078] <Preparation of the positive active material>

[0079] Except that the mass percentage of the additive 1,3-propane sultone and the mass percentage of the organic solvent are adjusted according to Table 3 in the <preparation of the electrolyte>, the mass percentage of the lithium salt is unchanged, and the rest is the same as in Example 1-1.

[0080] Example 2-2

[0081] <Preparation of the positive electrode>

[0082] Cobalt chloride and aluminum sulfate (a source of M elements) are added to deionized water in a molar ratio of Co to Al of (1-y):y=0.985:0.015, a precipitating agent sodium carbonate and a complexing agent ammonia are added, and the pH value is adjusted to 7 to precipitate the cobalt chloride and the aluminum sulfate; then the precipitate is sintered at 600°C for 7h, and is ground to obtain (Co 0.985 Al 0.015 )3O4 powder; the precipitating agent sodium carbonate is mixed with the (Co 0.985 Al 0.015 )3O4 powder in a molar ratio of Na to Co of z:(1-y)=0.002:0.985; the uniformly mixed powder is sintered at 800°C for 46h in an oxygen atmosphere to obtain Na 0.002 Co 0.985 Al 0.015 O2. The Na 0.002 Co 0.985 Al 0.015O2 and lithium nitrate were mixed uniformly in a molar ratio of Na to Li of z:x = 0.002:0.76, and reacted at 300°C for 6h in an air atmosphere. After the reaction, the product was washed with deionized water several times and dried at 100°C for 6h to obtain Li 0.76 Na 0.002 Co 0.985 Al 0.015 O2.

[0083] The positive electrode active material Li 0.76 Na 0.002 Co 0.985 Al 0.015 O2, conductive carbon black (Super P), and a binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6, N-methyl pyrrolidone (NMP) was added, and the system was stirred to be uniform and transparent under the action of a vacuum stirrer to obtain a positive electrode slurry. The solid content of the positive electrode slurry was 72wt%. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 13μm, and the aluminum foil was dried at 120°C for 1h to obtain a positive electrode tab with a positive electrode material layer thickness of 80μm. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode tab with a positive electrode material layer coated on both sides. Then, after cold pressing, tabbing, and slitting, the positive electrode tab was dried at 120°C under vacuum for 1h to obtain a positive electrode tab with a size of 74mm×851mm.

[0084] In addition to preparing the positive electrode according to the above method, the mass percentage content of the additive 1,3-propane sultone and the mass percentage content of the organic solvent were changed according to Table 3 in the preparation of the electrolyte, and the mass percentage content of the lithium salt remained unchanged, and the rest was the same as Example 1-1.

[0085] Examples 2-3 to 2-15

[0086] In addition to adjusting the type and content of the M source (MgSO4 when M is Mg; TiO2 when M is Ti), the contents of x, y, and z in the preparation of the positive electrode according to Table 3, and adjusting the mass percentage content of the additive 1,3-propane sultone and the mass percentage content of the organic solvent according to Table 3 in the preparation of the electrolyte, and the mass percentage content of the lithium salt remained unchanged, and the rest was the same as Example 1-1.

[0087] Comparative Examples 1-1 to 1-2

[0088] Example 1-1 was repeated except that in the preparation of the negative electrode, the drying temperature of the negative electrode slurry was adjusted according to Table 1 to adjust the values of a and b, and in the preparation of the electrolyte, no additive was added according to Table 2, the mass percentage content of the organic solvent was changed accordingly, and the mass percentage content of the lithium salt was unchanged.

[0089] Comparative Example 1-3

[0090] Example 1-1 was repeated except that in the preparation of the negative electrode, the drying temperature of the negative electrode slurry was adjusted according to Table 1 to adjust the values of a and b, and in the preparation of the electrolyte, the type and mass percentage content of the additive were adjusted according to Table 2, the mass percentage content of the organic solvent was changed accordingly, and the mass percentage content of the lithium salt was unchanged.

[0091] Comparative Example 2-1

[0092] Preparation of the positive electrode

[0093] Cobalt chloride and aluminum sulfate were added to deionized water in a molar ratio of Co to Al of 0.985:0.015, a precipitant sodium carbonate and a complexing agent ammonia water were added, and the pH value was adjusted to 7 to precipitate the cobalt chloride and aluminum sulfate; then the precipitate was sintered at 600°C for 6h, and ground to obtain (Co 0.985 Al 0.015 )3O4 powder. The precipitant sodium carbonate was mixed with the (Co 0.985 Al 0.015 )3O4 powder in a molar ratio of Na to Co of 0.01:0.985; the uniformly mixed powder was sintered at 700°C for 40h in an oxygen atmosphere to obtain Na 0.01 Co 0.985 Al 0.015 O2. After the lithium carbonate and Na 0.01 Co 0.985 Al 0.015 O2 were uniformly mixed in a molar ratio of Li to Co of 0.58:0.985, they were sintered at 1000°C for 12h in an air atmosphere, cooled, ground, and sieved to obtain Li 0.58 Na 0.01 Co 0.985 Al 0.015 O2 with R3m structure.

[0094] The positive electrode active material Li 0.58 Na 0.01 Co 0.985 Al 0.015O2, conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6, N-methyl pyrrolidone (NMP) was added, and the system was stirred to be homogeneous and transparent under the action of a vacuum stirrer, to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry was 72 wt%. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 13 μm, and the aluminum foil was dried at 120 °C for 1 h, to obtain a positive electrode sheet with a positive electrode material layer thickness of 80 μm. The above steps were repeated on the other surface of the aluminum foil, to obtain a positive electrode sheet with a positive electrode material layer on both sides. Then, after cold pressing, sheet cutting, and slitting, the positive electrode sheet was dried at 120 °C under vacuum for 1 h, to obtain a positive electrode sheet with a size of 74 mm x 851 mm.

[0095] In addition to preparing the positive electrode according to the above method, and adjusting the mass percentage content of the additive 1,3-propane sultone and the mass percentage content of the organic solvent in the electrolyte according to Table 3 in <Preparation of electrolyte>, the mass percentage content of the lithium salt remains unchanged, and the rest is the same as Example 1-1.

[0096] The preparation parameters and performance tests of each example and comparative example are shown in Tables 2 to 3.

[0097] Table 2

[0098]

[0099]

[0100] Note: " / " in Table 2 indicates that the corresponding substance or parameter does not exist.

[0101] As can be seen from Examples 1-1 to 1-29 and Comparative Examples 1-1 to 1-3, when the relationship of a, b, a and b, the type of additive, and the mass percentage content of the additive are within the scope of the present application, the capacity retention rate of the lithium ion battery is good, indicating that the cycle stability of the lithium ion battery is good.

[0102] As can be seen from Examples 1-24 to 1-25 and Examples 1-28 to 1-29, when the value of A / C in the electrolyte is within the scope of the present application, the capacity retention rate of the lithium ion battery is high, indicating that the cycle stability of the lithium ion battery is good.

[0103] As can be seen from Examples 1-3 to 1-4, Example 1-9, Example 1-11, Example 1-24, and Example 1-28, when the value of D / A in the electrolyte is within the scope of the present application, the capacity retention rate of the lithium ion battery is high, indicating that the cycle stability of the lithium ion battery is good.

[0104] As can be seen from Example 1-1 to Example 1-11, when the mass percentage content of lithium difluorophosphate is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0105] As can be seen from Example 1-12 to Example 1-16, when the mass percentage content of vinylene carbonate is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0106] As can be seen from Example 1-17 to Example 1-22, when the mass percentage content of 1,3-propane sultone is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0107] As can be seen from Example 1-23, when the electrolyte contains both lithium difluorophosphate and vinylene carbonate, and the mass percentage content of the above two additives is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0108] As can be seen from Example 1-24 to Example 1-25, when the electrolyte contains both lithium difluorophosphate and 1,3-propane sultone, and the mass percentage content of the above two additives is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0109] As can be seen from Example 1-26 to Example 1-27, when the electrolyte contains both vinylene carbonate and 1,3-propane sultone, and the mass percentage content of the above two additives is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0110] As can be seen from Example 1-28 to Example 1-29, when the electrolyte contains lithium difluorophosphate, vinylene carbonate and 1,3-propane sultone, and the mass percentage content of the above three additives is within the range of the present application, the capacity retention of the lithium ion battery is high, indicating that the lithium ion battery has good cycle stability.

[0111] Table 3

[0112]

[0113]

[0114] As can be seen from Example 2-1 to Example 2-15, and Comparative Example 2-1, when the lithium ion battery contains Li x Na z Co 1-y M y O2, and Lix Na z Co 1-y M y When x, y, z in O2 are within the scope of the present application, the capacity retention rate of the lithium ion battery is good, indicating that the cycle stability of the lithium ion battery is good.

[0115] As can be seen from Examples 2-1 to 2-15, when Li x Na z Co 1-y M y When x, y, z, z / (1-y), C, z / (1-y)C are within the scope of the present application, the capacity retention rate of the lithium ion battery is high, indicating that the cycle stability of the lithium ion battery is good.

[0116] As can be seen from Examples 2-1 to 2-15, when Li x Na z Co 1-y M y When M is contained in O2, and M is within the scope of the present application, the capacity retention rate of the lithium ion battery is good, indicating that the cycle stability of the lithium ion battery is good.

[0117] It should be noted that in this document, the terms "comprising", "containing", or any other variant thereof are intended to cover non-exclusive inclusions, such that a process, method, article, or apparatus that comprises a list of elements does not only include those elements, but can also include other elements not expressly listed, or inherent to such process, method, article, or apparatus.

[0118] Each embodiment in the specification is described in a relevant manner, and the same or similar parts between each embodiment can be referred to each other, and each embodiment mainly explains the difference from other embodiments.

[0119] The above only describes the preferred embodiments of the present application, and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application is included in the protection scope of the present application.

Claims

1. An electrochemical device, comprising a positive electrode, a negative electrode, and an electrolyte, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a surface layer and an inner layer, the surface layer being a region of the negative electrode active material layer from the surface of the negative electrode active material layer to a depth of 20 μm into the negative electrode active material layer, the inner layer being a region of the negative electrode active material layer other than the surface layer; a mass percentage content a of sodium in the surface layer satisfies 0.05%≤a≤0.15%, a mass percentage content b of sodium in the inner layer satisfies 0 The negative active material layer includes sodium carboxymethyl cellulose, the positive electrode includes a positive active material layer, the positive active material layer includes a positive active material, the positive active material includes Li x Na z Co 1-y M y O2, 0.6 < x < 1.01, 0 ≤ y < 0.15, 0 ≤ z < 0.03, M includes at least one of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr, or Zr; The electrolyte comprises an additive, the additive comprising at least one of lithium difluorophosphate, vinylene carbonate, or 1,3-propane sultone; a mass percentage content of the additive is 0.001% to 8% based on a mass of the electrolyte.

2. The electrochemical device of claim 1, wherein, 0.05%≤a≤0.1%, 0 3. The electrochemical device of claim 1, wherein, The electrolyte satisfies one of the following conditions based on a mass of the electrolyte: (I) a mass percentage content A of the lithium difluorophosphate satisfies 0.01%≤A≤2%; (II) a mass percentage content B of the vinylene carbonate satisfies 0.01%≤B≤5%; (III) a mass percentage content C of the 1,3-propane sultone satisfies 0.01%≤C≤8%.

4. The electrochemical device of claim 3, wherein, 0≤A / C≤1.

5. The electrochemical device of claim 3, wherein, The additive includes lithium difluorophosphate, the electrolyte includes F - , the mass percentage content D of the F - of 0.05%≤D≤2%, 0.2≤D / A≤2 based on the mass of the electrolyte.

6. The electrochemical device of claim 1, wherein, The positive electrode active material has a P63mc crystal structure.

7. The electrochemical device of claim 1, wherein, 0.001≤z / (1-y)≤0.

02.

8. The electrochemical device of claim 1, wherein, The additive comprises 1,3-propane sultone, a mass percentage content of the 1,3-propane sultone is C based on a mass of the electrolyte, and 0.001≤z / (1-y)C≤0.

012.

9. The electrochemical device according to any one of claims 1 to 8, wherein, The electrolyte satisfies one of the following conditions based on a mass of the electrolyte: (IV) a mass percentage content A of the lithium difluorophosphate satisfies 0.1%≤A≤1%; (V) a mass percentage content B of the vinylene carbonate satisfies 0.2%≤B≤2%; (VI) a mass percentage content C of the 1,3-propane sultone satisfies 0.5%≤C≤5%. 10.An electronic device comprising the electrochemical device according to any one of claims 1 to 9.

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