WO2024065276A1

WO2024065276A1 - Secondary battery, manufacturing method therefor, and electric apparatus

Info

Publication number: WO2024065276A1
Application number: PCT/CN2022/122131
Authority: WO
Inventors: 黄瑛; 刘江; 姚斌; 王红
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2022-09-28
Filing date: 2022-09-28
Publication date: 2024-04-04

Abstract

A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and an separator, the separator being arranged between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising modified graphite, and the modified graphite comprising graphite and anions located between lamellae of the graphite.

Description

Secondary battery and preparation method thereof, and power-using device

Technical Field

The present application relates to the field of secondary batteries, and more specifically to secondary batteries and methods for preparing the same, and electrical devices.

Background technique

The statements herein merely provide background information related to the present application and do not necessarily constitute prior art.

During the first charge and discharge process of a lithium-ion battery, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. This passivation layer is an interface layer that has the characteristics of a solid electrolyte. It is an electronic insulator but an excellent conductor of lithium ions. Lithium ions can freely embed and detach through this passivation layer. Therefore, this passivation film is called a "solid electrolyte interface" (solid electrolyte interface), or SEI film for short.

The formation of the SEI film will consume some active lithium ions, increase the irreversible capacity of the first charge and discharge, and reduce the cycle performance of the battery. Therefore, in order to compensate for the performance loss caused by the formation of the SEI film, the battery often needs to be replenished with lithium. However, traditional lithium replenishment technology often requires the introduction of lithium replenishers that have a negative impact on the battery's energy density or stability, or requires complex processes to achieve lithium replenishment. Therefore, how to further simplify the lithium replenishment of secondary batteries and avoid excessive negative impacts on other battery properties has always been a problem that needs to be solved in the field of secondary batteries.

Summary of the invention

According to various embodiments of the present application, a secondary battery and a method for preparing the same, and an electrical device are provided.

In a first aspect of the present application, a secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet;

The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes modified graphite, and the modified graphite includes graphite and anions located between layers of the graphite.

By embedding anions between the layers of graphite as a positive electrode additive, these anions embedded in the graphite layers are not easy to escape during the cycle, and the lithium ions in the corresponding electrolyte are easily embedded in the negative electrode to maintain the charge balance, thereby achieving lithium replenishment in a simple and efficient manner without significantly changing the existing battery preparation process, which can effectively improve the battery's cycle performance. In addition, the modified graphite with embedded anions is also conducive to enhancing conductivity, which can further improve the battery's electrical performance.

In some embodiments, the anion is a monovalent anion; optionally, the monovalent anion includes one or more of PF ₆ ^- , ClO ₄ ^- , TFSI ^- , FSI ^- and NO ₃ ^- . Suitable types of anions can be better embedded in the graphite layer without affecting the battery performance, and the release rate during the cycle is lower, which is more conducive to improving the lithium replenishment efficiency.

In some embodiments, the X-ray diffraction spectrum of the modified graphite has a characteristic peak in the range of 10° to 20°. After the anions that are difficult to be removed are embedded in the interlayer of the graphite during the cycle, an irreversible phase change occurs, so it can be detected that the obtained modified graphite has a characteristic X-ray diffraction peak in the range of 10° to 20°.

In some embodiments, the positive electrode active material used in the positive electrode active material layer includes one or more of lithium iron phosphate and _ternary positive _electrode materials; optionally, the ternary positive electrode material includes _{LiaNixCoyMzO2} , wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, Mo, and B _, _{0.95≤a≤1.2} , x>0, y>0, z>0, and x+y+z＝1. The working voltage of the appropriate type of positive electrode active material is more matched with the voltage required for inserting anions into the graphite interlayer, avoiding the failure of anion insertion into the graphite interlayer or the decomposition of the electrolyte due to voltage mismatch during preparation.

In some embodiments, the mass percentage of the graphite in the positive electrode active material layer is 1% to 8%; alternatively, the mass percentage of the graphite in the positive electrode active material layer is 2% to 4%. The mass percentage of graphite in the positive electrode active material layer is controlled within a suitable range, and while meeting the lithium replenishment demand, it does not occupy too much space of the positive electrode active material, causing the battery capacity to decrease, thereby achieving a balance between lithium replenishment and battery capacity.

In some embodiments, the Dv50 particle size of the graphite is 10 μm to 20 μm; optionally, the Dv50 particle size of the graphite is 16 μm to 18 μm. Controlling the particle size of the graphite within a suitable range can make the positive electrode slurry more uniform during slurry preparation, the compaction density of the pole piece more appropriate, and the ion diffusion path moderate, thereby further improving the effects of lithium supplementation and conductivity.

In a second aspect of the present application, a method for preparing a secondary battery according to one or more of the above embodiments is provided, comprising the following steps:

Mixing the graphite, other raw materials for preparing the positive electrode active material layer, and a solvent to prepare a positive electrode slurry;

The positive electrode slurry is coated on at least one surface of a positive electrode current collector, and dried and pressed to obtain a positive electrode sheet;

The positive electrode sheet, the negative electrode sheet and the isolation membrane are assembled, and an electrolyte is injected to perform a formation treatment at a voltage of 4.5V to 4.8V.

The voltage of the formation treatment is controlled to be 4.5V~4.8V, which can make the anions smoothly embedded in the interlayer of graphite and not easy to escape, and will not cause the decomposition of the electrolyte and affect the battery performance.

In some embodiments, the electrolyte solute includes one or more of LiPF ₆ , LiClO ₄ , LiTFSI, LiFSI and LiNO _3. Suitable electrolyte solute species provide suitable anion species, which can be better embedded in the interlayer of graphite during formation and are not easily removed during circulation, thereby better achieving lithium replenishment.

In some embodiments, the concentration of lithium ions in the electrolyte is 1 mol/L to 1.5 mol/L; alternatively, the concentration of lithium ions in the electrolyte is 1.2 mol/L to 1.3 mol/L. In the present application, the electrolyte not only provides basic electron and ion conduction functions, but also provides a source of lithium ions after formation, playing a role in lithium supplementation. Therefore, the electrolyte having a suitable lithium ion concentration is an important prerequisite for maintaining the basic performance of the battery while achieving lithium supplementation.

In some embodiments, the current of the formation treatment is less than 0.1 C. Controlling the formation current within a suitable range can make the anions more uniformly embedded in the interlayers of the graphite, reduce the escape of anions during circulation, and help improve the lithium replenishment efficiency.

In some embodiments, when the chemical formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity range of the anions embedded in the graphite is 100mAh/g to 200m Ah/g.

In some embodiments, when the chemical formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity of the anions released from the graphite is in the range of 20mAh/g to 100mAh/g.

By using appropriate chemical formation conditions, the specific capacity of anions embedded in graphite can be made greater than the specific capacity of anions released, thereby achieving lithium replenishment.

In a third aspect of the present application, an electrical device is provided, comprising a secondary battery as described in one or more of the aforementioned embodiments.

The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better describe and illustrate the embodiments or examples of the inventions disclosed herein, reference may be made to one or more drawings. The additional details or examples used to describe the drawings should not be considered as limiting the scope of the disclosed inventions, the embodiments or examples currently described, and any of the best modes of these inventions currently understood.

FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 1 .

FIG. 3 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

Description of reference numerals:

1: secondary battery; 11: housing; 12: electrode assembly; 13: cover plate; 2: electrical device.

FIG. 4 is an X-ray diffraction spectrum of the graphite raw material used in the positive electrode in each embodiment and comparative example of the present application.

FIG5 is an X-ray diffraction pattern of graphite in the positive electrode of the battery prepared in Example 1 of the present application (Test 1).

FIG6 is an X-ray diffraction pattern of graphite in the positive electrode of the battery prepared in Comparative Example 2 of the present application (Test 2).

FIG. 7 is an X-ray diffraction pattern of graphite in the positive electrode of the battery prepared in Comparative Example 3 of the present application (Test 3).

8 is an X-ray diffraction pattern of graphite at the positive electrode of the battery prepared in Example 1 of the present application after being subjected to a 2.5V voltage formation treatment (Test 4).

Detailed ways

In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

The "range" disclosed in this application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a specific parameter, it is understood that the range of 60 to 110 and 80 to 120 is also expected. In addition, if the

minimum range values

1 and 2 are listed, and if the maximum range values 3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In this application, unless otherwise specified, the numerical range "a to b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 to 5" means that all real numbers between "0 to 5" have been fully listed in this article, and "0 to 5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

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

Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

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

If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

As the market demand for long battery life gradually increases, how to effectively improve the battery cycle performance has become a research hotspot in the field of secondary batteries. The formation of SEI film is an important reason for the damage to the battery cycle performance. Therefore, additional lithium supplementation has become an effective measure to improve the battery cycle performance. The current lithium supplementation methods can be roughly divided into two categories: positive electrode lithium supplementation and negative electrode lithium supplementation. Positive electrode lithium supplementation often introduces a certain amount of lithium salt lithium supplementation agent into the positive electrode active material layer, but these lithium salt lithium supplementation agents often cause the collapse of the corresponding position of the active material layer after being used for lithium supplementation. Therefore, it is necessary to strengthen the positive electrode active material layer or the entire pole piece structure accordingly, and it will occupy the position of the positive electrode active material. In addition to lithium supplementation, it has no other functions, which greatly reduces the capacity of the battery; negative electrode lithium supplementation often uses lithium as a lithium supplementation agent. The nature of lithium is very active and easy to react with water and oxygen. Therefore, it is also necessary to specially design the pole piece structure or improve the preparation process to achieve it, and there are certain hidden dangers in terms of safety.

Based on the above background, the first aspect of the present application provides a secondary battery, including a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet;

The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes modified graphite. The modified graphite includes graphite and anions located between graphite layers.

In some embodiments, the anion is a monovalent anion; optionally, the monovalent anion includes one or more of PF ₆ ^- (hexafluorophosphate ion), ClO ₄ ^- (perchlorate ion), TFSI ^- (bistrifluoromethanesulfonyl imide ion), FSI ^- (bisfluorosulfonyl imide ion) and NO ₃ ^- (nitrate ion). Suitable types of anions can be better embedded in the graphite interlayer without affecting the battery performance, and the release rate during the cycle is lower, which is more conducive to improving the lithium replenishment efficiency.

In some embodiments, the X-ray diffraction spectrum of the modified graphite has a characteristic peak in the range of 10° to 20°. Optionally, the X-ray diffraction spectrum of the modified graphite has a characteristic peak at any of the following values or in the range of any two values: 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18° or 19°. After the anions that are difficult to be removed during the interlayer embedding cycle of graphite, an irreversible phase change occurs, so it can be detected that the obtained modified graphite has an X-ray diffraction characteristic peak in the range of 10° to 20°.

In some embodiments, the positive electrode active material used in the positive electrode active material layer includes one or more of lithium iron phosphate and ternary positive electrode materials; optionally, the ternary positive electrode material includes Li _a Ni _x Co _y M _z O ₂ , wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, Mo, and B, 0.95≤a≤1.2, x>0, y>0, z>0, and x+y+z＝1. Further optionally, M is Mn, and the LiNi _x Co _y Mn _z O ₂ material includes one or more of NCM ₅₂₃ , NCM ₆₂₂ , and NCM _811. The working voltage of the suitable type of positive electrode active material is more matched with the voltage required for inserting anions into the graphite interlayer, so as to avoid the failure of anion insertion into the graphite interlayer or the decomposition of the electrolyte due to voltage mismatch during preparation.

In some embodiments, in the positive electrode active material layer, the mass percentage of graphite is 1% to 8%; alternatively, in the positive electrode active material layer, the mass percentage of graphite can be, for example, 3%, 5%, 6% or 7%, and can also be 2% to 4%. The mass percentage of graphite in the positive electrode active material layer is controlled within a suitable range, while meeting the lithium replenishment demand, it will not occupy too much space of the positive electrode active material, causing the battery capacity to decrease, and achieve a balance between lithium replenishment and battery capacity.

In some embodiments, the Dv50 particle size of the graphite is 10 μm to 20 μm; alternatively, the Dv50 particle size of the graphite can be, for example, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm or 19 μm, and can also be 16 μm to 18 μm. Controlling the particle size of the graphite within a suitable range can make the positive electrode slurry more uniform during slurry preparation, the compaction density of the pole piece more suitable, and the ion diffusion path moderate, thereby further improving the effects of lithium supplementation and conductivity.

In this application, Dv50 refers to the particle size corresponding to the cumulative volume distribution number of particles reaching 50% in the volume cumulative distribution curve of particle size. Its physical meaning is that the volume proportion of particles with a particle size smaller than (or larger than) this particle size value is 50%. As an example, Dv50 can be conveniently measured by a laser particle size analyzer with reference to GB/T 19077-2016 particle size distribution laser diffraction method, such as the Mastersizer 2000E laser particle size analyzer of Malvern Instruments Ltd., UK.

Mixing graphite, other raw materials of the positive electrode active material layer and a solvent to prepare a positive electrode slurry;

Applying the positive electrode slurry on at least one surface of the positive electrode current collector, drying and pressing to obtain a positive electrode sheet;

The positive electrode sheet, the negative electrode sheet and the separator are assembled, and the electrolyte is injected, and a chemical treatment is performed at a voltage of 4.5 V to 4.8 V. The voltage of the chemical treatment can also be, for example, 4.55 V, 4.6 V, 4.65 V, 4.7 V or 4.75 V.

The voltage of the formation treatment is controlled to be 4.5V to 4.8V, which can make the anions smoothly embedded in the interlayer of graphite and not easy to escape, and will not cause the electrolyte to decompose and affect the battery performance.

In some embodiments, the concentration of lithium ions in the electrolyte is 1 mol/L to 1.5 mol/L; alternatively, the concentration of lithium ions in the electrolyte can be, for example, 1.1 mol/L or 1.4 mol/L, or 1.2 mol/L to 1.3 mol/L. In the present application, the electrolyte not only provides basic electron and ion conductivity, but also provides a source of lithium ions after formation, thereby playing a role in lithium supplementation. Therefore, the electrolyte having a suitable lithium ion concentration is an important prerequisite for maintaining the basic performance of the battery while achieving lithium supplementation.

In some embodiments, the current of the formation treatment is less than 0.1 C. Optionally, the current of the formation treatment may be, for example, 0.09 C, 0.08 C, 0.07 C, 0.06 C, 0.05 C, 0.04 C, 0.03 C, 0.02 C or 0.01 C. Controlling the formation current within a suitable range can make the anions more uniformly embedded in the interlayer of the graphite, reduce the escape of anions during circulation, and help improve the lithium replenishment efficiency.

In some embodiments, when the formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity of the anions embedded in the graphite ranges from 100mAh/g to 200m Ah/g. When the formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity of the anions embedded in the graphite can also be, for example, 110mAh/g, 120mAh/g, 130mAh/g, 140mAh/g, 150mAh/g, 160mAh/g, 170mAh/g, 180mAh/g or 190mAh/g.

In some embodiments, when the formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity of the anions released from the graphite is in the range of 20mAh/g to 100mAh/g. When the formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity of the anions released from the graphite can also be, for example, 30mAh/g, 40mAh/g, 50mAh/g, 60mAh/g, 70mAh/g, 80mAh/g or 90mAh/g.

In this application, "specific capacity" is mass specific capacity, which refers to the capacity value of anions that can be extracted or embedded in a unit mass of graphite. For example, the specific capacity of anions embedded in graphite is 110 mAh/g, which means that the capacity value of anions that can be embedded in 1g of graphite is 110 mAh.

By using appropriate chemical formation conditions for treatment, the specific capacity of anions embedded in graphite can be made greater than the specific capacity of anions released, thereby achieving lithium replenishment. The specific capacity of anions embedded in or released from graphite is related to the specific type of anions. For example, in some embodiments, when the anion is PF ₆ ^- , the specific capacity of PF ₆ ^- embedded in the positive electrode graphite is 140mAh/g, the specific capacity of PF ₆ ^- released is 20mAh/g, and the lithium replenishment capacity is 120mAh/g; in some embodiments, when the anion is FSI ^- , the specific capacity of FSI ^- embedded in the positive electrode graphite is 130mAh/g, the specific capacity of FSI ^- released is 20mAh/g, and the lithium replenishment capacity is 110mAh/g.

In a third aspect of the present application, an electrical device is provided, comprising a secondary battery according to one or more of the aforementioned embodiments.

In addition, the secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

In one embodiment of the present application, a secondary battery is provided.

Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is set between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

Positive electrode

The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

In some embodiments, the positive electrode active material may be a positive electrode active material for a battery known in the art. As an example, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to _, lithium cobalt oxide (such as _LiCoO2 ), lithium nickel oxide (such as _LiNiO2 ), lithium manganese oxide (such as _LiMnO2 , _LiMn2O4 ), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel _cobalt manganese oxide (such as LiNi1 _/ 3Co1 _/ _3Mn1 _/ _3O2 (also referred to as _NCM333 ), _{LiNi0.5Co0.2Mn0.3O2} (also referred _to _as _NCM523 ₎ _, _{LiNi0.5Co0.25Mn0.25O2} (also referred _to as _NCM211 ₎ , _{LiNi0.6Co0.2Mn0.2O2} (also referred to as _NCM622 ), _{LiNi0.8Co0.1Mn0.1O2} (also referred to _as _NCM811 ), lithium _nickel _cobalt aluminum oxide ( _such as LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) and at least one of modified compounds thereof. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO ₄ (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

In some embodiments, the positive electrode film layer may also optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

In some embodiments, the positive electrode film layer may further include a conductive agent, which may include, for example, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

In some embodiments, the positive electrode sheet can be prepared in the following manner: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

Negative electrode

The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

In some embodiments, the negative electrode active material may also include negative electrode active materials for batteries known in the art, such as artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

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

In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

Electrolytes

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

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

In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl imide), lithium bis(trifluoromethanesulfonyl imide), lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

Isolation film

In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

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

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

In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. For example, FIG1 is a secondary battery 1 of a square structure as an example.

In some embodiments, referring to FIG. 2 , the outer package may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can form an electrode assembly 12 through a winding process or a lamination process. The electrode assembly 12 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 12. The number of electrode assemblies 12 contained in the secondary battery 1 can be one or more, and those skilled in the art can select according to specific actual needs.

In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto. Among them, the mobile device may be, for example, a mobile phone, a laptop computer, etc.; the electric vehicle may be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but are not limited thereto.

As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

Fig. 3 shows an example of an electric device 2. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.

As another example, the device may be a mobile phone, a tablet computer, a laptop computer, etc.

Example 1

(1) Preparation of positive electrode sheet

Lithium iron phosphate, graphite (Dv50 particle size of 16.9 μm), conductive agent acetylene black, and binder PVDF (polyvinylidene fluoride) are mixed in a weight ratio of 92:2:4:2, and a solvent N-methylpyrrolidone is added, and the mixture is stirred and mixed to obtain a positive electrode slurry, which is then coated on both surfaces of the positive electrode current collector aluminum foil, and dried and cold pressed to obtain a positive electrode sheet;

(2) Preparation of negative electrode sheet

Graphite, conductive agent acetylene black, binder SBR (styrene-butadiene rubber latex), binder CMC (sodium carboxymethyl cellulose) are mixed in a weight ratio of 95:1.5:3.1:0.4, deionized water is added as a solvent, and the mixture is stirred and mixed to obtain a negative electrode slurry, which is then coated on both surfaces of the negative electrode current collector copper foil, and dried and cold pressed to obtain a negative electrode sheet;

(3) Preparation of electrolyte

In an argon atmosphere glove box with a water content of <10ppm, EC (ethylene carbonate), PC (polycarbonate), and DMC (dimethyl carbonate) were mixed at a weight ratio of EC:PC:DMC=3:3:3, and then LiPF ₆ , VC, DTD, and PS were added and stirred to obtain an electrolyte, wherein the concentration of lithium ions was 1.2 mol/L, and the mass percentages of VC, DTD, and PS were 3%, 1%, and 1%, respectively;

(4) Preparation of isolation membrane

PP/PE/PP composite film is used as the isolation film.

(5) Preparation of lithium-ion secondary batteries

The positive electrode sheet prepared in step (1), the isolation film in step (4), and the negative electrode sheet prepared in step (2) are stacked in order, so that the isolation film is located between the positive and negative electrodes to play an isolating role, thereby obtaining a bare battery cell; the bare battery cell is placed in an outer package, the electrolyte prepared in step (3) is injected and packaged, and chemical formation is carried out at a voltage of 4.8V and a current of 0.05C. At this time, the specific capacity of PF ₆ ^- embedded in the positive electrode graphite is 140mAh/g, and the specific capacity of PF ₆ ^- released is 20mAh/g, thereby obtaining a lithium-ion secondary battery with a lithium supplement of 120mAh/g.

Example 2

The method is basically the same as Example 1, except that in step (1), the weight ratio of lithium iron phosphate, graphite, conductive agent acetylene black, and binder PVDF is 93:1:4:2.

Example 3

The method is basically the same as Example 1, except that in step (1), the weight ratio of lithium iron phosphate, graphite, conductive agent acetylene black, and binder PVDF is 86:8:4:2.

Example 4

The method is basically the same as Example 1, except that the Dv50 particle size of the graphite in step (1) is 20 μm.

Example 5

The method is basically the same as Example 1, except that the Dv50 particle size of the graphite in step (1) is 10 μm.

Example 6

The method is basically the same as Example 1, except that the lithium ion concentration of the electrolyte obtained in step (3) is 1 mol/L.

Example 7

The method is basically the same as Example 1, except that the lithium ion concentration of the electrolyte obtained in step (3) is 1.5 mol/L.

Example 8

The method is basically the same as Example 1, except that the current formed in step (5) is 0.15C.

Example 9

The method is basically the same as Example 1, except that the lithium iron phosphate in step (1) is replaced by NCM523, the solute of the electrolyte in step (3) is replaced by LiFSI, and the formation voltage in step (5) is adjusted to 4.5 V;

During formation, the specific capacity of FSI ^- embedded in the positive electrode graphite is 130 mAh/g, and the specific capacity of FSI ^- released is 20 mAh/g, thus obtaining a lithium-ion secondary battery with a lithium supplement of 110 mAh/g.

Comparative Example 1

The method is basically the same as Example 1, except that step (1) does not contain graphite, and the weight ratio of lithium iron phosphate, conductive agent acetylene black, and binder PVDF is 94:4:2.

Comparative Example 2

Basically the same as Example 1, except that the formation voltage in step (5) is 4.0V.

Comparative Example 3

Basically the same as Example 1, except that the formation voltage in step (5) is 4.9V.

Characterization Tests:

(1) X-ray diffraction (XRD) test:

XRD tests were performed on the graphite raw material, the graphite of the positive electrode in the battery prepared in Example 1, Comparative Example 2 and Comparative Example 3, respectively recorded as graphite raw material, test 1, test 2 and test 3, and the obtained spectra are shown in Figures 4 to 7 respectively;

Repeat steps (1) to (5) in Example 1, then lower the formation voltage to 2.5 V, and perform an XRD test on the graphite of the positive electrode in the prepared battery, recorded as Test 4, and the obtained spectrum is shown in FIG8 ;

(2) Cyclic performance test:

At a temperature of 25°C, charge to 3.65V with a constant current of 0.5C, then charge to 0.05C with a constant voltage, let stand for 10 minutes, discharge to 2.0V with a constant current of 1C, let stand for 10 minutes, and cycle the test under this test condition until the cell capacity decays to 80% SOH, and stop the test. Record the number of cycles when stopping. The results are listed in Table 1.

Table 1

Analyzing the data in Table 1, compared with Example 1, the amount of graphite added in Example 2 is reduced, and the capacity that can be used to supplement lithium in SEI is reduced, and the battery cycle performance is reduced; the amount of graphite added in Example 3 is increased, although the capacity that can be used to supplement lithium is increased, but it occupies the space of the positive electrode active material, so it will also cause the overall battery cycle performance to decrease; in Example 4, the Dv50 particle size of the graphite is larger, which reduces the compaction density of the pole piece, so that the amount of active material that can be accommodated in the same volume is reduced, thereby reducing the cycle performance; in Example 5, the Dv50 particle size of the graphite is smaller, although the compaction density is high, but the ion diffusion path is increased, the polarization is increased, and a lower The rate and time required to replenish lithium are longer, so less lithium is replenished under the same conditions and the cycle performance is improved less; in Example 6, the concentration of lithium ions in the electrolyte is low. During the charging and lithium replenishment process, after part of the lithium salt is consumed, the concentration of lithium ions in the electrolyte is lower than 1 mol/L, and the ion diffusion rate between the electrode layers is reduced, which ultimately reduces the cycle performance of the battery; in Example 7, the concentration of lithium ions in the electrolyte is high, the electrolyte viscosity is high, the infiltration is difficult, and the ion diffusion rate between the electrode layers is low, which also leads to a decrease in the cycle performance of the battery; in Example 8, the formation current is too large, resulting in excessive polarization, resulting in incomplete formation reaction and incomplete lithium replenishment reaction, and the expected lithium replenishment efficiency cannot be achieved.

In Comparative Example 1, no graphite was added, and lithium replenishment could not be achieved; in Comparative Example 2, the formation voltage was too low, and the anions could not be normally embedded in the graphite, so lithium replenishment could not be achieved; in Comparative Example 3, the formation voltage was too high, resulting in irreversible decomposition of part of the electrolyte, which would seriously deteriorate the battery performance.

Comparing Figures 4 to 7, it can be seen that relative to the XRD spectrum of the graphite raw material (Figure 4), the positive electrode graphite prepared in Example 1 is formed by embedding anions to form modified graphite, and its spectrum (Test 1, Figure 5) has characteristic peaks in the range of 10° to 20°; in Comparative Example 2, due to the low formation voltage, the anions cannot be normally embedded in the graphite, and no modified graphite is formed, and its XRD spectrum (Test 2, Figure 6) does not have characteristic peaks in the range of 10° to 20°; in Comparative Example 3, although its XRD spectrum (Test 3, Figure 7) is in the range of 10° to 20°, the negative ions are embedded in the graphite, and the modified graphite is not formed. In the range of 10° to 20°, characteristic peaks similar to those in Example 1 appeared. However, due to the excessively high formation voltage, the electrolyte partially decomposed irreversibly, which seriously deteriorated the battery performance. In Test 4 (FIG. 8), steps (1) to (5) in Example 1 were repeated, and then the formation voltage was lowered to 2.5 V. The XRD spectrum showed characteristic peaks similar to those in Example 1 in the range of 10° to 20°, indicating that in the scheme of the present application, the embedding of anions into graphite is irreversible. Even if the voltage is subsequently reduced, the anions can still be retained in the graphite, thereby continuously replenishing lithium.

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

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

Claims

A secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet;

The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes modified graphite, and the modified graphite includes graphite and anions located between layers of the graphite.
The secondary battery according to claim 1, characterized in that the anions are monovalent anions; optionally, the monovalent anions include one or more of PF 6 - , ClO 4 - , TFSI - , FSI - and NO 3 - .
The secondary battery according to any one of claims 1 to 2, characterized in that the X-ray diffraction spectrum of the modified graphite has a characteristic peak in the range of 10° to 20°.
The secondary battery according to any one of claims 1 to 3, characterized in that the positive electrode active material used in the positive electrode active material layer includes one or more of lithium iron phosphate and a ternary positive electrode material; optionally, LiaNixCoyMzO2 , wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, Mo, and B, 0.95≤a≤1.2, x > 0, y>0, z>0, and x+y+z=1.
The secondary battery according to any one of claims 1 to 4, characterized in that, in the positive electrode active material layer, the mass percentage of the graphite is 1% to 8%; optionally, in the positive electrode active material layer, the mass percentage of the graphite is 2% to 4%.
The secondary battery according to any one of claims 1 to 5, characterized in that the Dv50 particle size of the graphite is 10 μm to 20 μm; optionally, the Dv50 particle size of the graphite is 16 μm to 18 μm.
The method for preparing a secondary battery according to any one of claims 1 to 6, comprising the following steps:

Mixing the graphite, other raw materials for preparing the positive electrode active material layer, and a solvent to prepare a positive electrode slurry;

The positive electrode slurry is coated on at least one surface of a positive electrode current collector, and dried and pressed to obtain a positive electrode sheet;

The positive electrode sheet, the negative electrode sheet and the isolation membrane are assembled, and an electrolyte is injected to perform a formation treatment at a voltage of 4.5V to 4.8V.
The preparation method according to claim 7, characterized in that the solute of the electrolyte includes one or more of LiPF 6 , LiClO 4 , LiTFSI, LiFSI and LiNO 3 .
The preparation method according to any one of claims 7 to 8 is characterized in that, in the electrolyte, the concentration of lithium ions is 1 mol/L to 1.5 mol/L; optionally, in the electrolyte, the concentration of lithium ions is 1.2 mol/L to 1.3 mol/L.
The preparation method according to any one of claims 7 to 9 is characterized in that the current of the chemical formation treatment is less than 0.1C.
The preparation method according to any one of claims 7 to 10, characterized in that when the chemical formation treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity range of the anions embedded in the graphite is 100mAh/g to 200mAh/g.
The preparation method according to any one of claims 7 to 11 is characterized in that when the chemical treatment is carried out at a voltage of 4.5V to 4.8V, the specific capacity range of the anions released from the graphite is 20mAh/g to 100mAh/g.
An electrical device comprising the secondary battery according to any one of claims 1 to 6.