CN115513513A - Secondary battery and electric equipment - Google Patents

Abstract

The embodiment of the application discloses secondary battery and consumer, secondary battery is through adding first lithium salt and the cooperation of manganese iron lithium oxide positive pole active material in electrolyte, and in battery charge-discharge cycle in-process, first lithium salt in the electrolyte takes place reduction reaction in preference to organic solvent and cosolvent in the electrolyte on negative pole active material layer, constructs the SEI layer that one deck ionic conductivity is higher for the negative pole piece, restraines Mn ²⁺ And deposition on the SEI film layer is carried out, so that the dissolution of Mn in the positive active material is avoided, and the high-temperature cycle and high-temperature storage capacity of the secondary battery are improved.

Description

Secondary battery and electrical equipment

technical field

The present application relates to the field of battery technology, in particular to a secondary battery and electrical equipment.

Background technique

LMFP (lithium manganese iron phosphate) is a new generation of cathode material developed on the basis of LFP (lithium iron phosphate). Although LMFP and LFP have the same specific capacity density, LMFP has a higher operating voltage, so its energy density will be get a certain boost.

LMFP has lower electronic conductivity and ionic conductivity, which will cause greater polarization, and because Mn ³⁺ /Mn ^{2 +} is a two-phase reaction, it has a larger lattice mismatch and is less affected by polarization. Big. The John-Teller effect of manganese ions leads to the dissolution of manganese, which reduces the cycle life of the battery. Due to the existence of the John-Teller effect, Mn ³⁺ is enriched on the surface of the cathode particles, distorting the MnO ₆ octahedron, resulting in lattice distortion and reduced structural stability, affecting stability and cycleability. At the same time, the dissolved manganese ions will undergo a reduction reaction and precipitate at the negative electrode, causing damage to the SEI film, causing more active lithium to be consumed during the repair of the SEI film, thereby affecting the cycle life of the battery. At the same time, the lithium salt LiPF ₆ (lithium hexafluorophosphate) commonly used in the electrolyte is prone to the generation of HF (hydrofluoric acid) at high temperatures, and the fluorine-containing solvent FEC (fluoroethylene carbonate) will also generate HF, and the presence of HF will promote Mn dissolution process. At the same time, the positive electrode material produces a manganese-deficient phase due to the dissolution of manganese, and the diffusion kinetics of lithium ions slows down during the subsequent charge-discharge process, which increases the polarization voltage of the secondary battery, resulting in irreversible capacity loss, and eventually leads to a rapid charge-discharge cycle. Capacity fading, poor rate performance.

Contents of the invention

The embodiments of the present application provide a secondary battery and electrical equipment, which can solve the problems of capacity fading and rate performance degradation caused by the dissolution of manganese ions in the existing lithium manganese iron phosphate secondary battery.

The first aspect of the present application provides a secondary battery, including a positive electrode sheet and an electrolyte, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes manganese iron lithium oxide; The total molar weight of manganese element and iron element in the product is 100%, the mole percentage of the manganese element is n1%, and the mole percentage of the iron element is n2%, satisfying n1>n2; the electrolytic The liquid includes a first lithium salt, and the first lithium salt includes at least one of lithium nitrate, lithium nitrite, lithium borate, lithium phosphate, lithium fluoroborate, lithium sulfate, and lithium chloride.

Optionally, based on the total mass of the electrolyte, the content of the first lithium salt is less than or equal to 1.5 wt%.

Optionally, the content of the first lithium salt is 0.1wt%-1wt%.

Optionally, n1≥60.

Optionally, n1-n2≤40.

Optionally, the electrolyte solution further includes a second lithium salt, and the second lithium salt includes lithium hexafluorophosphate; the mass ratio of the first lithium salt to the second lithium salt is (0.01˜0.13):1.

Optionally, the mass ratio of the first lithium salt to the second lithium salt is (0.02˜0.09):1.

Optionally, the mass ratio of the first lithium salt to the second lithium salt is (0.02˜0.04):1.

Optionally, the electrolyte solution further includes an organic solvent, and the organic solvent includes cyclic carbonates and chain carbonates.

Optionally, the electrolyte solution further includes a film-forming additive, and the film-forming additive includes at least one of vinylene carbonate, 1,3-propane sultone, and fluoroethylene carbonate.

Optionally, the electrolyte solution further includes a co-solvent, and the co-solvent includes at least one of ether solvents, dimethyl sulfoxide, sulfolane, tetramethylurea, and amide compounds.

The second aspect of the present application provides an electric device, the electric device includes the secondary battery provided by the first aspect of the present application, and the electric device includes but not limited to an electric vehicle and an energy storage device.

The beneficial effect of the present application is to provide a secondary battery and electrical equipment. The secondary battery is combined with the lithium manganese iron phosphate positive electrode active material by adding the first lithium salt in the electrolyte, during the battery charge and discharge cycle process , the first lithium salt in the electrolyte is preferential to the organic solvent and co-solvent in the electrolyte to undergo a reduction reaction on the negative electrode active material layer, building a SEI layer with higher ion conductivity for the negative electrode sheet, and inhibiting Mn ²⁺ The deposition on the SEI film layer can avoid the dissolution of Mn in the positive electrode active material, and improve the high-temperature cycle and high-temperature storage capacity of the secondary battery.

detailed description

The following will clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without making creative efforts belong to the scope of protection of this application. In addition, it should be understood that the specific implementations described here are only used to illustrate and explain the present application, and are not intended to limit the present application.

In the detailed description and claims, a list of items linked by the term "at least one of" may mean any combination of the listed items. For example, if the items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if the items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements. The term "at least one of" has the same meaning as the term "at least one of".

In this specification, the numerical range shown using "-" means the range which includes the numerical value described before and after "-" as a minimum value and a maximum value, respectively.

The embodiment of the present application provides a secondary battery and electrical equipment. The secondary battery is coordinated with the positive electrode active material of lithium manganese iron phosphate by adding the first lithium salt in the electrolyte. During the charging and discharging cycle of the battery, the electrolyte The first lithium salt in the electrolyte is preferential to the organic solvent and co-solvent in the electrolyte to undergo a reduction reaction on the negative electrode active material layer, constructing a layer of SEI layer with higher ion conductivity for the negative electrode sheet, and suppressing Mn ²⁺ in the SEI film The deposition on the layer, thereby avoiding the dissolution of Mn in the positive electrode active material, improves the high-temperature cycle and high-temperature storage capacity of the secondary battery.

In an embodiment of the present application, a secondary battery is provided, and the secondary battery includes a positive pole piece, a negative pole piece, a separator, an electrolyte, and a casing.

I. Positive pole piece

The positive pole piece includes a positive current collector and a positive active material layer disposed on the positive current collector.

Positive electrode active material layer

The positive active material layer may be one or more layers. Each layer of the multilayer positive active material may contain the same or different positive active material. The positive electrode active material is any substance capable of reversibly intercalating and deintercalating metal ions such as lithium ions.

In some embodiments, the positive electrode active material includes iron manganese lithium oxide, based on the total molar mass of manganese and iron elements in the iron manganese lithium oxide as 100%, the molar percentage of the manganese element is n1 %, the mole percentage of the iron element is n2%, satisfying n1>n2.

In some embodiments, n1≥60, then n2≤40.

In some embodiments, n1-n2≤40.

In some embodiments, the lithium iron manganese oxide includes doped and/or coated lithium iron manganese phosphate.

In some embodiments, the general structural formula of lithium iron manganese oxide includes Li _a M _y Mn _x Fe _1-xy PO ₄ , wherein 0.95≤a≤1.1, 0≤y≤0.05, 0.55≤x≤0.8, and M includes One or more of Mg, Al, Ti, Co.

In some embodiments, a is 1, y is 0, and x is 0.6, then the structural formula of iron manganese lithium oxide in the positive electrode active material is LiMn _0.6 Fe _0.4 PO ₄ ; a is 1, y is 0, x is 0.7, Then the structural formula of lithium iron manganese oxide in the positive electrode active material is LiMn _0.7 Fe _0.3 PO ₄ .

In some embodiments, the structural formula of lithium iron manganese oxide is LiMn _0.6 Fe _0.4 PO ₄ . If the manganese content is too high, the electronic conductivity and ionic conductivity of the positive electrode active material are low, and the John-Teller effect of manganese is serious, that is, the structure of the positive electrode material will be destroyed due to high manganese content, and the manganese dissolution of the positive electrode material will be more serious. And destroy the SEI layer of the negative electrode, affecting the capacity retention rate and rate performance of the secondary battery at 45 °C, and the low manganese content will limit the improvement of the energy density of the battery.

In addition, the positive electrode active material layer also includes a positive electrode conductive agent and a positive electrode binder.

Positive electrode conductive agent

The kind of positive electrode conductive agent is not limited, and any known conductive agent can be used. Examples of positive electrode conductive agents may include, but are not limited to, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; carbon materials such as amorphous carbon such as needle coke; carbon nanotubes; graphene and the like. The above-mentioned positive electrode conductive agents can be used alone or in any combination.

positive electrode binder

The type of positive electrode binder used in the production of the positive electrode active material layer is not particularly limited, and in the case of the coating method, any material can be dissolved or dispersed in the liquid medium used in electrode production. Examples of positive electrode binders may include, but are not limited to, one or more of the following: polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic Resin-based polymers such as polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, and ethylene-propylene rubber; styrene - Butadiene-styrene block copolymer or its hydrogenated products, ethylene-propylene-diene terpolymer (EPDM), styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene - Thermoplastic elastomeric polymers such as styrene block copolymers or their hydrogenated compounds; syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer Polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer and other fluorine-based polymers; alkali metal ions (especially Lithium ion) ion-conductive polymer composition, etc. The above positive electrode binders may be used alone or in any combination.

Positive current collector

The type of the positive electrode collector is not particularly limited, and it can be any known material suitable for use as the positive electrode collector. Examples of positive current collectors include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; carbon materials such as carbon cloth and carbon paper; composite materials formed of polymers and metal layers. In some embodiments, the positive current collector is a metal material. In some embodiments, the positive current collector is aluminum.

The form of the positive electrode collector is not particularly limited. When the positive electrode current collector is a metal material, the form of the positive electrode current collector may include, but not limited to, metal foil, metal cylinder, metal strip, metal plate, metal foil, expanded metal, stamped metal, foamed metal, and the like. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector may include, but not limited to, a carbon plate, a carbon film, a carbon cylinder, and the like. In some embodiments, the positive current collector is a metal foil. In some embodiments, the metal foil is mesh. The thickness of the metal foil is not particularly limited. In some embodiments, the thickness of the metal foil is greater than 1 μm, greater than 3 μm, or greater than 5 μm. In some embodiments, the thickness of the metal foil is less than 1 mm, less than 50 μm, or less than 20 μm. In some embodiments, the thickness of the metal foil is within the range formed by any two values above.

II. Electrolyte

The electrolyte includes lithium salts, organic solvents, film-forming additives and co-solvents.

lithium salt

In some embodiments, the lithium salt includes a first lithium salt including lithium nitrate, lithium nitrite, lithium borate, lithium phosphate, lithium fluoroborate, lithium sulfate, lithium chloride, and lithium fluoride. at least one.

In some embodiments, based on the total mass of the electrolyte, the content of the first lithium salt is less than or equal to 1.5 wt%. Specifically, the content of the first lithium salt can be 0.1wt%, 0.2wt%, 0.3wt%, 0.5wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.2wt%, 1.3wt%, 1.5wt% or a range of any two of them.

In some embodiments, the content of the first lithium salt is 0.1wt% to 1.0wt%, specifically, the content of the first lithium salt can be 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt% , 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, or any two of them.

In some embodiments, the lithium salt further includes a second lithium salt, and the second lithium salt includes lithium hexafluorophosphate; the mass ratio of the first lithium salt to the second lithium salt is (0.01˜0.13):1.

In some embodiments, the mass ratio of the first lithium salt to the second lithium salt is (0.02˜0.09):1.

In some embodiments, the mass ratio of the first lithium salt to the second lithium salt is (0.02˜0.04):1.

Organic solvents

In some embodiments, the organic solvent includes cyclic carbonates and chain carbonates.

Specifically, the organic solvent is one or more of EC (ethylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate), PC (polycarbonate), and EMC (ethyl methyl carbonate). a mixed solvent. Among them, EC (ethylene carbonate) and PC (polycarbonate) are cyclic carbonates. DEC (diethyl carbonate), DMC (dimethyl carbonate) and EMC (ethyl methyl carbonate) are chain carbonates.

Film-forming additives

In some embodiments, the film-forming additive includes at least one of vinylene carbonate, 1,3-propane sultone, and fluoroethylene carbonate (FEC). In some embodiments, fluoroethylene carbonate (FEC) is preferred.

Co-solvent

In some embodiments, the co-solvent includes at least one of ether solvents, dimethyl sulfoxide, sulfolane, tetramethylurea, and amide compounds. The ether solvent includes but not limited to at least one of tetraethylene glycol dimethyl ether, ethylene glycol dimethyl ether, propylene glycol methyl ether, and ethylene glycol monomethyl ether.

It can be understood that the choice of co-solvent includes but is not limited to the above range, as long as it can dissolve the first lithium salt. Compared with other options, the above-mentioned co-solvent has a higher solubility for the first lithium salt, and can better introduce the first lithium salt into the electrolyte.

During the charge-discharge cycle of the secondary battery, the first lithium salt in the electrolyte is preferentially reduced to the organic solvent on the negative electrode sheet, and a layer of SEI layer with higher ion conductivity is constructed for the negative electrode sheet. The SEI layer can Avoid the structural damage caused by the dissolution of Mn in the cathode material LMFP caused by the deposition of Mn ²⁺ in the electrolyte, and avoid the lithium loss and gas generation caused by the rupture and repair of the SEI layer.

In a secondary battery in which the negative electrode active material includes graphite and the positive electrode active material includes lithium manganese iron phosphate (LMFP), the first lithium salt is the electronic insulation of the SEI layer formed by the negative electrode sheet, thereby avoiding the deposition of manganese on the surface of the negative electrode active material layer , destroying the SEI layer on the surface of the negative electrode active material layer. The presence or absence of manganese metal can be checked by performing EDS scanning on the surface of the negative electrode sheet, so as to observe the inhibitory effect of the secondary battery provided by the embodiment of the present application on the deposition of Mn ²⁺ .

By adding the first lithium salt (such as lithium nitrate), film-forming additives and co-solvents, a nitrogen-rich SEI layer is formed at the negative electrode, which can reduce the deposition of Mn ²⁺ in the electrolyte, and at the same time isolate the negative electrode from the electrode. The contact of the electrolyte reduces a large number of side reactions and inhibits the occurrence of gas production. Where LiNO ₃ is introduced into the organic solvent of the electrolyte through a co-solvent, the more co-solvent, the higher the concentration of lithium salt (such as lithium nitrate). The higher the concentration of lithium nitrate, the denser the SEI layer will be, and the better the protection effect on the negative electrode will be. However, too dense SEI layer is not conducive to the diffusion of lithium ions, and the rate performance of the battery will be reduced. After adding lithium nitrate, a better SEI layer is formed, which reduces the deposition of Mn ²⁺ , thus avoiding the dissolution of Mn. SEI protects the negative electrode and avoids the deposition of Mn ions. At this time, the concentration of Mn ions in the electrolyte reaches saturation, and electrolysis It is difficult for the liquid to continue to dissolve Mn ions, thereby avoiding the dissolution of Mn in the positive electrode, stabilizing the structural stability of the LMFP positive electrode material, which can be beneficial to the performance of the LMFP and improve the high-temperature cycle and high-temperature storage performance of the battery.

III. Negative pole piece

The negative electrode sheet includes 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 includes a negative electrode active material, and the negative electrode active material includes graphite.

The negative pole piece is a single-sided pole piece or a double-sided pole piece. When the negative pole piece is a single-sided pole piece, the negative active material layer is arranged on one surface of the negative electrode current collector. When the negative pole piece is a double-sided pole piece, the negative electrode The active material layer is arranged on both surfaces of the negative electrode current collector. A single-sided negative pole piece area and a double-sided negative pole piece area may also exist on the negative pole piece.

Negative electrode collector

In some embodiments, the negative electrode current collector includes, but is not limited to, metal foil, metal cylinder, metal strip, metal plate, metal film, expanded metal, stamped metal, foamed metal, and the like. In some embodiments, the negative electrode current collector is a metal foil. In some embodiments, the negative electrode current collector is copper foil. As used herein, the term "copper foil" includes copper alloy foil.

In some embodiments, the negative electrode current collector is a conductive resin. In some embodiments, the conductive resin includes a film obtained by evaporating copper on a polypropylene film.

Negative electrode active material layer

The negative active material layer may be one or more layers, and each of the multiple negative active material layers may contain the same or different negative active materials. The negative electrode active material is any substance capable of reversibly intercalating and deintercalating metal ions such as lithium ions. In some embodiments, the chargeable capacity of the negative active material is greater than the discharge capacity of the positive active material, so as to prevent lithium metal from being deposited on the negative sheet during charging.

In some embodiments, the thickness of the negative active material layer refers to the thickness of the negative active material layer coated on a single side of the negative current collector. In some embodiments, the thickness of the single-sided negative electrode active material layer is more than 15 μm. In some embodiments, the thickness of the single-sided negative electrode active material layer is more than 20 μm. In some embodiments, the thickness of the single-sided negative electrode active material layer is more than 30 μm. In some embodiments, the thickness of the single-sided negative electrode active material layer is less than 150 μm. In some embodiments, the thickness of the single-sided negative electrode active material layer is less than 120 μm. In some embodiments, the thickness of the single-sided negative electrode active material layer is less than 100 μm. In some embodiments, the thickness of the negative electrode active material layer is within a range consisting of any two values above. When the thickness of the negative electrode active material layer is within the above range, the electrolyte can soak into the vicinity of the negative electrode current collector interface, improving the charge and discharge characteristics of the secondary battery at high current density; at the same time, the volume ratio of the negative electrode current collector to the negative electrode active material Within an appropriate range, the capacity of the secondary battery can be ensured.

In some embodiments, the negative active material layer includes a negative active material, a conductive agent, a binder, and a dispersant.

negative active material

In some embodiments, the negative electrode active material can be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, silicon-based material, and tin-based material, more preferably graphite.

conductive agent

In some embodiments, the conductive agent includes one or more of carbon black, graphite, carbon fiber, carbon nanotube or graphene, preferably carbon black.

binder

The binder can improve bonding between negative active materials. The type of the binder is not particularly limited, as long as it is a stable material to the electrolyte solution or the solvent used in electrode production. In some embodiments, the binder includes sodium carboxymethylcellulose and styrene-butadiene rubber.

Dispersant

In some embodiments, the dispersant includes diethylhexanol, which is an environmentally friendly organic compound with low price and wide sources. Its surface tension is low and it is easy to adsorb and spread on the surface of the liquid. The material is subject to Shearing and friction of mechanical force, at the same time, there will be internal friction between the particles. Under the action of various forces, the raw material particles tend to be highly dispersed, which makes the slurry more uniform and the dispersion effect is good. The prepared dry electrode sheet Uniform thickness, avoiding problems such as wrinkling that affect the performance of electrical performance, enhance the stability of the pole piece, improve the transmission efficiency of lithium ions between the positive and negative electrodes, reduce electrochemical polarization, accelerate the non-Faraday reaction process, and meet the power battery rate performance and cycle performance. life requirements.

IV. Isolation film

In order to prevent short circuit, a separator is usually arranged between the positive pole piece and the negative pole piece. In this case, the electrolytic solution of the present application is usually used by permeating the separator.

V. Application

An embodiment of the present application also provides an electric device, including the above-mentioned secondary battery. As a typical application, the secondary battery can be used in electric toys, electric tools, battery cars, electric vehicles, energy storage equipment, ships, spacecraft and the like.

The preparation method of the secondary battery provided by the present application is described below in conjunction with specific examples:

Example 1

Electrolyte preparation:

Dissolve the first lithium salt LiNO ₃ in co-solvent DMSO (dimethyl sulfoxide) to obtain a uniform mixed solution. Dissolve 1 mol of LiPF ₆ in 1 L of mixed solvent of EC, DEC and FEC, the volume ratio of EC, DEC and FEC in the mixed solvent is 4.5:4.5:1 to obtain an electrolyte with a LiPF ₆ concentration of 1 mol/L. The electrolyte solution containing LiNO ₃ is prepared by adding the solvent containing the first lithium salt into the electrolyte solution containing LiPF ₆ . Wherein, the content of _LiNO3 in the obtained electrolytic solution is 0.5wt%, and the mass ratio of _LiNO3 to _LiPF6 is 0.04.

Preparation of positive electrode sheet:

Mix the positive electrode active material LiMn _0.6 Fe _0.4 PO ₄ , the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) according to the mass ratio of 96.5:2.3:1.2, add the solvent N-methylpyrrolidone (NMP), and stir in vacuum until The system is uniform, and the positive electrode slurry is obtained, and then evenly coated on the upper and lower surfaces of the positive electrode current collector aluminum foil, dried at room temperature, transferred to an oven, and then cold pressed under the condition of a compacted density of 2.3g/cm ³ , After slitting, the positive electrode sheet is obtained.

Preparation of negative electrode sheet:

Mix negative electrode active material graphite, conductive agent CNT (carbon nanotube), thickener CMC (carboxymethyl cellulose), and binder SBR (styrene-butadiene latex) at a mass ratio of 96.5:0.8:0.9:1.8 and add solvent Deionized water was vacuum stirred until the system was uniform to obtain the negative electrode slurry, which was then evenly coated on the upper and lower surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven to continue drying to a compacted density of 1.7g/ The negative electrode sheet is obtained after cold pressing and cutting under the condition of cm ³ .

Separator preparation:

Choose polyethylene film as the isolation film.

Preparation of lithium-ion batteries:

The above-mentioned positive and negative electrode sheets, polyethylene film separator and negative electrode sheet are stacked in the order of one layer of positive electrode sheet, one layer of separator, and one layer of negative electrode sheet, and the battery core is obtained by winding, and the electrolyte is added after drying. Lithium-ion batteries are obtained through processes such as vacuum packaging, standing still, formation, and volume separation.

Example 2

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is LiNO ₃ with a content of 0.4wt%, and the mass ratio of LiNO ₃ to LiPF ₆ is 0.03.

Example 3

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is LiNO ₃ with a content of 0.2 wt%, and the mass ratio of LiNO ₃ to LiPF ₆ is 0.02.

Example 4

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The positive electrode active material includes lithium manganese iron phosphate material with a structural formula of LiMn _0.7 Fe _0.3 PO ₄ .

Example 5

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is LiNO ₃ with a content of 1.0 wt%, and the mass ratio of LiNO ₃ to LiPF ₆ is 0.09.

Example 6

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The positive electrode active material includes lithium manganese iron phosphate material with a structural formula of LiMn _0.8 Fe _0.2 PO ₄ .

Example 7

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is LiNO ₃ with a content of 0.1 wt%, and the mass ratio of LiNO ₃ to LiPF ₆ is 0.01.

Example 8

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is LiNO ₃ with a content of 1.5 wt%, and the mass ratio of LiNO ₃ to LiPF ₆ is 0.13.

Example 9

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The positive electrode active material includes lithium manganese iron phosphate material with a structural formula of LiMn _0.9 Fe _0.1 PO ₄ .

Example 10

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is lithium nitrite.

Example 11

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is lithium borate.

Example 12

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The first lithium salt in the electrolyte is lithium chloride.

Comparative example 1

A secondary battery was prepared according to the method of Example 1, except for the following differences, the rest were the same as Example 1:

The electrolyte does not contain the aforementioned first lithium salt.

Next, a method for testing the secondary battery of the present invention will be described.

High temperature cycle performance test of battery

Stand still for 10 minutes, 1C constant current charge to 4.25V, then constant voltage charge to 0.05C, and then 1C constant current discharge to 2.8V, which is a charge and discharge cycle. The first discharge capacity is 100%, and the number of cycles when the discharge capacity drops to 80% is the cycle life.

End condition: 3000cycles or the discharge capacity is lower than 80%. At 45°C, the lithium-ion batteries prepared in Examples and Comparative Examples were fully charged at 1C rate and fully discharged at 1C rate, and the actual capacity at this time was recorded, and then tested at 45°C with the above cycle test method, and the test was completed. When the high temperature cycle curve is drawn. Due to the long cycle time, the simulation curve after 500 cycles is used as the high-temperature cycle curve to judge the high-temperature cycle performance of the battery.

Wherein, Table 1 is a list of test results after the high-temperature cycle performance test of the secondary batteries prepared in the examples and the secondary batteries prepared in the comparative examples. Table 2 is a list of room temperature discharge rate performance test results of the secondary batteries prepared in the various examples and the secondary batteries prepared in the comparative examples. Table 3 is a list of room temperature charge rate performance test results of the secondary batteries prepared in each example and the secondary batteries prepared in comparative examples.

The room temperature discharge rate test method of the secondary battery is as follows:

At 25°C, take 5 secondary batteries of Comparative Example 1 and Examples 1-12 respectively, charge them at a constant current rate of 1C to 4.25V, and then charge them at a constant voltage of 4.25V until the current reaches 0.05C. And the rate of 0.33C, 0.5C, 1C, 1.5C, 2C and 3C is discharged to 2.8V respectively, and the discharge capacity of the secondary battery is measured.

Discharge capacity ratio (%) at different rates=discharge capacity at different rates/0.33C discharge capacity×100%.

Charge capacity ratio (%) at different magnifications = charge capacity at different magnifications/0.33C charge capacity×100%.

Table 1

Table 2

table 3

As can be seen from Table 1, with reference to Examples 1 and 4, when the first lithium salt is added in the electrolytic solution, and the first lithium salt adopts LiNO ₃ (lithium nitrate), and the content of LiNO ₃ in the electrolytic solution is 0.5wt% , The capacity retention rate of the secondary battery is the highest after 500 cycles at 45°C, as high as 99%. In addition, the first lithium salt adopts LiNO ₃ (lithium nitrate), the content of LiNO ₃ in the electrolyte is 0.5wt%, and the structural formula of lithium manganese iron phosphate in the positive electrode active material is LiMn _0.6 Fe _0.4 PO ₄ or LiMn _0.7 Fe _0.3 At PO ₄ , the capacity retention rate of the secondary battery at 45°C after 500 cycles is the highest, reaching 99%.

Referring to Examples 2, 5 and ₇ , when the LiNO content in the electrolyte is lower than 0.5wt% or higher than 0.5wt% in the secondary battery, the capacity retention of the secondary battery after 500 cycles at 45°C The rate cannot reach 99%. For example, when the content of LiNO ₃ in the electrolyte exceeds 0.5wt% to 1.0wt% or even 1.5wt%, it is necessary to add additional solvent to dissolve too much LiNO ₃ . Adding too much solvent will affect the The cycle capacity retention rate of the secondary battery, too much _LiNO3 (lithium nitrate) will easily cause lithium nitrate to re-precipitate in the electrolyte, resulting in a decrease in the cycle capacity retention rate to 97%, while the _LiNO3 content in the electrolyte is too low, For example, when it is only 0.1wt% as shown in Example 7, a dense SEI film layer cannot be formed on the negative electrode sheet, resulting in Mn deposition on the surface of the negative electrode sheet, causing the cycle capacity retention rate of the secondary battery to drop to 93%.

In addition, referring to Examples 6 and 9, when the structural formula of lithium manganese iron phosphate in the positive electrode active material is LiMn _0.8 Fe _0.2 PO ₄ or LiMn _0.9 Fe _0.1 PO ₄ , even if the content of LiNO ₃ in the electrolyte is 0.5 wt%, It will also affect the capacity retention rate of the secondary battery after 500 cycles at 45°C, causing the capacity retention rate of the secondary battery to drop to 95% and 96% after 500 cycles at 45°C.

In addition, referring to Comparative Example 1, the only difference between Comparative Example 1 and Example 1 is that the first lithium salt (LiNO ₃ ) was not added in Comparative Example 1, resulting in the The capacity retention rate dropped to 86%, and the high-temperature cycle performance was poor.

In addition, referring to Examples 10-12, the first lithium salt in the corresponding secondary battery does not use nitrate, and the capacity retention rate of the prepared secondary battery after 500 cycles at 45°C is 90%. , 89%, and 89%.

Since the higher the concentration of lithium nitrate, the denser the SEI layer is, the better the protection effect on the negative electrode is. However, too dense SEI layer is not conducive to the diffusion of lithium ions, and the rate performance of the battery will be reduced.

As can be seen from Table 2, with reference to Examples 1 and 4, when the first lithium salt is added in the electrolytic solution, and the first lithium salt adopts LiNO ₃ (lithium nitrate), and LiNO _The content in the electrolytic solution is 0.5wt% , and the structural formula of lithium manganese iron phosphate in the positive electrode active material is LiMn _0.6 Fe _0.4 PO ₄ or LiMn _0.7 Fe _0.3 PO ₄ , the room temperature discharge capacity ratio of the secondary battery is always maintained at 100% under different rate discharge capacities, indicating that the two The rate performance of the secondary battery has not declined. With reference to Example 5, when the _LiNO3 content in the electrolyte rose to 1wt%, the rate performance of the secondary battery at room temperature dropped to 99% at 1.5C discharge capacity ratio, The discharge capacity ratio at 3C drops to 98%, indicating that the discharge rate performance of the secondary battery decreases, because too much LiNO ₃ causes the SEI layer to be too dense and is not conducive to lithium ion diffusion, and the discharge rate performance of the secondary battery decreases. Refer to Examples 7. When the content of LiNO ₃ in the electrolyte is only 0.1wt%, the rate performance of the secondary battery at room temperature drops to 99% at 0.5C and 98% at 1C %, because too little LiNO ₃ leads to insufficient compactness of the SEI layer, the discharge rate performance of the secondary battery decreases, but because of the addition of LiNO ₃ , the decline is not large.

In addition, referring to Examples 6 and 9, when the structural formula of lithium manganese iron phosphate in the positive electrode active material is LiMn _0.8 Fe _0.2 PO ₄ or LiMn _0.9 Fe _0.1 PO ₄ , even if the content of LiNO ₃ in the electrolyte is 0.5 wt%, The discharge capacity ratio of the corresponding secondary battery drops to 99% at 0.5C. Referring to Examples 10-12, the first lithium salt in the corresponding secondary battery does not use nitrate, and the discharge capacity ratio of the prepared secondary battery drops significantly.

Referring to Comparative Example 1, since Comparative Example 1 did not add LiNO ₃ to the electrolyte, the discharge rate performance of the secondary battery at room temperature gradually decreased with the increase of the discharge rate, and the decline was severe, and the discharge capacity ratio of 3C decreased to 92%, the discharge rate performance of the secondary battery drops significantly.

As can be seen from Table 3, with reference to Example 1, when the first lithium salt is added in the electrolyte, and the first lithium salt adopts LiNO ₃ (lithium nitrate), and the content of LiNO ₃ in the electrolyte is 0.5wt%, and When the structural formula of lithium manganese iron phosphate in the positive electrode active material is LiMn _0.6 Fe _0.4 PO ₄ or LiMn _0.7 Fe _0.3 PO ₄ , the charge rate performance of the secondary battery will not drop much, and the charge capacity ratio of 3C will drop to 84%. Refer to the implementation Example 5, when the _LiNO3 content in the electrolyte solution increased by 1.0wt%, the charge capacity ratio of 3C dropped to 80%. With reference to Example 7, when the _LiNO3 content in the electrolyte solution decreased by 0.1wt%, the 3C The charge capacity ratio drops to 75%.

Referring to Comparative Example 1, since Comparative Example 1 did not add LiNO ₃ to the electrolyte, the charge rate performance of the secondary battery at room temperature gradually decreased with the increase of the charge rate, and the decline was serious, and the charge capacity ratio of 3C decreased to 68%, the charge rate performance of the secondary battery drops significantly.

In addition, referring to Examples 6 and 8, when the structural formula of lithium manganese iron phosphate in the positive electrode active material is LiMn _0.8 Fe _0.2 PO ₄ or LiMn _0.9 Fe _0.1 PO ₄ , even if the content of LiNO ₃ in the electrolyte is 0.5 wt%, The corresponding charge capacity ratio of the secondary battery drops significantly. Referring to Examples 10-12, the first lithium salt in the corresponding secondary electromagnetic does not use nitrate, and the discharge capacity ratio of the prepared secondary battery drops significantly.

The above is a detailed introduction of a secondary battery and electrical equipment provided by the embodiment of the present application. In this paper, specific examples are used to illustrate the principle and implementation of the present application. The description of the above embodiment is only for helping understanding The method of this application and its core idea; at the same time, for those skilled in the art, according to the idea of this application, there will be changes in the specific implementation and scope of application. In summary, the content of this specification should not be understood For the limitation of this application.

Claims (10)

1. A secondary battery comprises a positive pole piece and electrolyte, and is characterized in that the positive pole piece comprises a positive active material, and the positive active material comprises manganese iron lithium oxide;

the total molar weight of manganese and iron in the lithium manganese iron oxide is 100%, the molar percentage of the manganese is n1%, the molar percentage of the iron is n2%, and n1 is more than n2;

the electrolyte comprises a first lithium salt, and the first lithium salt comprises at least one of lithium nitrate, lithium nitrite, lithium borate, lithium phosphate, lithium fluoroborate, lithium sulfate and lithium chloride.

2. The secondary battery according to claim 1, wherein the content of the first lithium salt is less than or equal to 1.5wt% based on the total mass of the electrolyte.

3. The secondary battery according to claim 1, wherein n 1. Gtoreq.60.

4. The secondary battery according to claim 1, wherein n1-n2 ≦ 40.

5. The secondary battery of claim 1, wherein the general structural formula of the lithium iron manganese oxide comprises Li _a M _y Mn _x Fe _1-x-y PO ₄ Wherein a is more than or equal to 0.95 and less than or equal to 1.1,0 and less than or equal to 0.05,0.55 and less than or equal to 0.8, and M comprises one or more of Mg, al, ti and Co.

6. The secondary battery of claim 1, wherein the electrolyte further comprises a second lithium salt comprising lithium hexafluorophosphate;

the mass ratio of the first lithium salt to the second lithium salt is (0.01-0.13): 1.

7. the secondary battery according to claim 1, wherein the electrolyte further comprises an organic solvent, and the organic solvent comprises a cyclic carbonate and a chain carbonate.

8. The secondary battery of claim 1, wherein the electrolyte solution further comprises a film forming additive comprising at least one of vinylene carbonate, 1,3-propane sultone, and fluoroethylene carbonate.

9. The secondary battery of claim 1, wherein the electrolyte further comprises a co-solvent comprising at least one of an ether solvent, dimethyl sulfoxide, sulfolane, tetramethylurea, and an amide compound.

10. An electric device comprising the secondary battery according to any one of claims 1 to 9.