CN116365011A

CN116365011A - Lithium secondary battery, method of manufacturing the same, and electronic apparatus

Info

Publication number: CN116365011A
Application number: CN202111638167.7A
Authority: CN
Inventors: 张磊; 杜木挺; 孙华军; 屈丽辉
Original assignee: BYD Co Ltd
Current assignee: BYD Co Ltd
Priority date: 2021-12-28
Filing date: 2021-12-28
Publication date: 2023-06-30

Abstract

The application provides a lithium secondary battery, a preparation method thereof and electronic equipment, wherein the lithium secondary battery comprises a positive electrode, a negative electrode, electrolyte and a diaphragm positioned between the positive electrode and the negative electrode, the positive electrode comprises a current collector and a positive electrode active material layer arranged on the surface of the current collector, the positive electrode active material layer comprises a positive electrode active material and elemental sulfur, and the elemental sulfur covers the active site of the surface of the positive electrode active material. The positive electrode active material in the lithium secondary battery has good stability under the working condition of high voltage, thereby ensuring the battery to have longer service life.

Description

Lithium secondary battery, method of manufacturing the same, and electronic apparatus

Technical Field

The application relates to the technical field of lithium ion secondary batteries, in particular to a lithium secondary battery, a preparation method thereof and electronic equipment.

Background

Compared with other chargeable battery systems, the lithium ion secondary battery has the advantages of high working voltage, light weight, small volume, no memory effect, low self-discharge rate, long cycle life, high energy density and the like, and is widely applied to mobile terminal products such as mobile phones, notebook computers, tablet computers and the like. The energy density of the lithium ion secondary battery can be improved by adopting a mode of improving the battery voltage, however, along with the increase of the working voltage, the activity of oxygen atoms in the crystal lattice of the surface area of the particles of the positive electrode active material is improved, and the reaction of the positive electrode active material and the electrolyte is accelerated, so that the cycle capacity retention rate of the battery is rapidly reduced under high voltage, and the life cycle of the battery is obviously shortened. Therefore, it is necessary to provide a new lithium secondary battery to suppress side reactions of the positive electrode active material and the electrolyte under high voltage conditions and to improve the cycle performance of the battery.

Disclosure of Invention

In view of this, the present application provides a lithium secondary battery in which a positive electrode active material has good stability under high-voltage operating conditions, thereby ensuring a long service life of the battery.

The first aspect of the application provides a lithium secondary battery, which comprises a positive electrode, a negative electrode, electrolyte and a diaphragm arranged between the positive electrode and the negative electrode, wherein the positive electrode comprises a current collector and a positive electrode active material layer arranged on the surface of the current collector, the positive electrode active material layer comprises a positive electrode active material and elemental sulfur, and the elemental sulfur covers an active site on the surface of the active material.

In the lithium secondary battery, the active site of the positive electrode active material in the positive electrode active material layer is covered with elemental sulfur, and the low-valence sulfur coating layer can stabilize O in the surface lattice of the active material ^2- Inhibit O ^2- The reaction of converting into oxygen ensures that the lattice structure of the active material has good stability; on the other hand, for the separated oxygen, the low-valence sulfur can react with the oxygen to form a sulfur-containing compound, so that the separated oxygen is prevented from reacting with the electrolyte, and the sulfur-containing compound can be used as an SEI film to separate the active site of the active material from the electrolyte, so that the direct contact of the active site and the electrolyte is avoided, and the oxidative decomposition of the electrolyte is relieved; the battery can effectively exert capacity and has good cycle performance.

Optionally, the upper limit of the charging voltage of the lithium secondary battery is 4.3V to 4.4V.

In a second aspect, the present application provides a method for preparing a lithium secondary battery, comprising:

providing a positive plate, wherein the positive plate comprises a current collector and an active material layer arranged on the surface of the current collector, and the active material layer comprises an active material and elemental sulfur; assembling the positive plate, the negative plate, the diaphragm and the electrolyte into a battery and forming to obtain a preactivated battery;

performing at least one sulfur resetting treatment on the preactivated battery to obtain a lithium secondary battery; the sulfur reset process includes a first process and a second process that are performed sequentially, the first process including: discharging to 1.0V-1.5V with the current of 0.1-1C; the second process includes: charging to 3.0V-3.5V with a current of 0.1-1C.

The positive electrode plate containing elemental sulfur is assembled into a battery and is formed, so that electrolyte infiltrates an active material layer and the active material to be electrically activated to form electrochemical active sites, and a pre-activated battery is obtained; the pre-activated cell is then subjected to a sulfur reset treatment to cause sulfur to adhere to the active sites of the active material, specifically, elemental sulfur is converted to polysulfide during the discharge of the first treatment and dissolved in the electrolyte of the active material layer, and polysulfide is reduced to elemental sulfur at the electrochemically active sites of the active material during the charge of the second treatment. The elemental sulfur can be coated on the surface of the active site of the active material through sulfur resetting treatment, so that the lattice structure stability of the active material is improved, and the battery can effectively exert capacity and has good cycle performance.

Optionally, the mass ratio of the elemental sulfur to the active material is 0.1% -0.5%.

Optionally, the active material comprises NCM positive electrode material, NCA positive electrode material, NCMA positive electrode material, lithium nickel manganese oxide, liCoO ₂ One or more of a positive electrode material and a lithium-rich manganese-based layered oxide.

Optionally, each of the sulfur reset treatments further comprises: and before the second treatment, standing the preactivated battery subjected to the first treatment for 1-100 s.

Optionally, the first process includes: the discharge was carried out at a current of 0.1 to 1C to 1.5V.

Optionally, the second processing includes: charging to 3.5V with a current of 0.1-1C.

Optionally, the number of sulfur reset treatments is 1 to 10.

Optionally, the number of sulfur reset treatments is 2 to 8.

Optionally, the forming includes: charging to 4.25-4.5V with current of 0.05-0.5C, discharging to 3.0-3.5V with current of 0.05-0.5C.

Optionally, the preparation method of the positive plate comprises the following steps: providing a pole piece precursor, wherein the pole piece precursor comprises a current collector and an active material precursor layer arranged on the surface of the current collector, and the active material precursor layer comprises an active material; and fumigating the electrode plate precursor by sulfur vapor to obtain the positive electrode plate.

Optionally, the fumigation temperature is 110-180 ℃.

The preparation method of the lithium secondary battery is simple in steps, and the low-valence sulfur coating layer is formed at the active site of the positive electrode active material through charging and discharging of the battery, so that the stability of the positive electrode active material under a high-pressure condition is improved, side reactions with electrolyte are not easy to occur, and the lithium secondary battery prepared by the method has good cycle performance and good application prospect.

In a third aspect, the present application provides an electronic device comprising a lithium secondary battery according to the first aspect of the present application.

Drawings

Fig. 1 is a schematic view of a method for manufacturing a lithium secondary battery according to an embodiment of the present application;

fig. 2 is a schematic structural diagram of a positive plate according to an embodiment of the present disclosure;

fig. 3 is a schematic diagram of a method for preparing a positive electrode sheet according to an embodiment of the present application;

fig. 4 is a process diagram of the preparation of the lithium secondary battery provided in example 1;

fig. 5 is a scanning electron microscope image of the positive electrode sheet of example 1;

FIG. 6 is a scanning electron microscope image of the positive electrode sheet of comparative example 2;

fig. 7 is a graph showing the initial capacity comparison of the preactivated batteries of example 1 and comparative example 2;

fig. 8 is a comparative view of the valence state of sulfur in the positive electrodes of the lithium secondary batteries of example 1 and comparative example 2;

fig. 9 is a graph showing the capacity retention ratio of the lithium secondary batteries provided in example 1 and comparative example 2.

Detailed Description

The following description of the embodiments of the present application will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all, of the embodiments of the present application. All other embodiments, which can be made by one of ordinary skill in the art based on the embodiments herein without making any inventive effort, are intended to be within the scope of the present application.

The energy density of the battery can be increased by increasing the voltage of the lithium secondary battery, but under the high-voltage working condition of the lithium secondary battery, oxygen atoms in the crystal lattice of the surface area of the positive electrode active material particles are easy to separate from the crystal lattice of the material, and the dissolution of the oxygen atoms can damage the stability of the crystal structure of the active material on one hand, so that active components in the active material are lost, the electrochemical activity of the active material is reduced, and the cycle performance of the battery is not facilitated; on the other hand, the separated oxygen reacts with the electrolyte, so that the electrolyte is decomposed to generate gas, and the battery performance is reduced. In order to improve the electrochemical performance of the battery, the application provides a preparation method of the lithium secondary battery, and the lithium secondary battery obtained by the preparation method has good stability under a high-voltage working condition and can effectively improve the cycle performance of the battery.

Referring to fig. 1, fig. 1 is a schematic diagram of a method for preparing a lithium secondary battery according to an embodiment of the present application, where the method for preparing a lithium secondary battery includes:

step 100: providing a positive plate, wherein the positive plate comprises a current collector and an active material layer arranged on the surface of the current collector, and the active material layer comprises an active material and elemental sulfur;

step 200: assembling the positive plate, the negative plate, the diaphragm and the electrolyte into a battery, and performing formation to obtain a pre-activated battery;

step 300: and (3) carrying out sulfur reset treatment on the pre-activated battery to obtain the lithium secondary battery.

In step 100 of the present application, the positive electrode sheet includes a current collector and an active material layer disposed on a surface of the current collector, the active material layer including an active material and elemental sulfur. Referring to fig. 2, fig. 2 is a schematic structural diagram of a positive plate according to an embodiment of the present application. In fig. 2, the positive electrode sheet 100 includes a current collector 10 and an active material layer 20 disposed on a surface of the current collector 10, the active material layer including an active material and elemental sulfur. In the embodiment of the application, in the active material layer, elemental sulfur is dispersed among active material particles, and in the subsequent treatment process, the elemental sulfur can be transferred to the active site surface of the active material, so that effective protection is formed.

In some embodiments of the present application, the preparation method of the positive electrode sheet includes: and providing a pole piece precursor, wherein the pole piece precursor comprises a current collector and an active material precursor layer arranged on the surface of the current collector, fumigating the pole piece precursor by sulfur vapor to enable sulfur to enter the active material precursor layer, and obtaining an active material layer, thus obtaining the positive pole piece. The fumigation method is favorable for depositing sulfur among pores of the active material particles, and improves the uniformity of sulfur distribution in the active material layer. In some embodiments of the present application, a method for preparing a pole piece precursor includes: and mixing the active material, the conductive agent, the binder and the solvent to form positive electrode slurry, coating the positive electrode slurry on the surface of a current collector, and drying to obtain the pole piece precursor. Compared with the method for directly mixing the elemental sulfur with the active material to prepare the active material layer, the method for preparing the active material layer by sulfur fumigation is beneficial to improving the structural stability of the positive plate. Specifically, if elemental sulfur is mixed with an active material to prepare a pole piece slurry, the elemental sulfur reduces the bonding strength of the binder to the active material, so that the stability of the active material layer is poor, and the active material is easy to fall off.

Referring to fig. 3, fig. 3 is a schematic diagram of a preparation method of a positive electrode sheet according to an embodiment of the present application, in fig. 3, a pole piece precursor 12 is fixed on an upper surface inside a container 200, sulfur powder 13 is laid on a bottom of the container, and sulfur sublimates into gas by heating the sulfur powder during the preparation process, and because an active material layer is in a porous structure, the gaseous sulfur enters the active material layer and condenses into solid sulfur at pores of the active material layer. In some embodiments of the present application, the sulfur powder includes sublimed sulfur, wherein the content of sulfur in the sublimed sulfur is greater than or equal to 99.9%, and the sublimed sulfur has high purity, which is beneficial to reducing the introduction of impurities into the positive electrode sheet. In some embodiments of the present application, the preparation of the positive electrode sheet is performed in an inert gas environment. In some embodiments of the present application, the temperature at which the pole piece precursor is fumigated is 110 ℃ to 180 ℃, wherein the fumigated temperature refers to the temperature of sulfur vapor. The fumigation temperature may be, but is not limited to, specifically 110 ℃, 120 ℃, 125 ℃, 130 ℃, 135 ℃, 140 ℃, 150 ℃, or 180 ℃. The control of the fumigation temperature is favorable for the uniform deposition of sulfur in the pore structure inside the active material layer, so that the sulfur can be fully coated on the active site of the active material in the sulfur resetting treatment process, and the sulfur is not easy to oxidize at the temperature, so that the introduction of impurities is reduced, and the sulfur can fully protect the active material. In some embodiments of the present application, the pole piece precursors are fumigated under normal pressure conditions at a temperature of 130 ℃ to 150 ℃. In some embodiments of the present application, the pole piece precursors are fumigated under reduced pressure at a temperature of 120 ℃ to 140 ℃. The fumigation is carried out under the condition of reduced pressure, which is beneficial to shortening the fumigation time and improving the production efficiency. The preparation method of the positive plate is simple and easy to implement, the positive plate has good stability, the requirements on storage environment are relatively relaxed, chemical reagents are not used in the preparation process, and the development direction of a clean factory is met.

In some embodiments of the present application, the elemental sulfur is present in an amount of 0.10% to 0.50% by mass relative to the active material. The mass ratio of elemental sulfur to active material may be, but is not limited to, specifically 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, or 0.5%. In the application, the function of sulfur is to improve the cyclic reversible capacity retention rate of the positive electrode active material under the high-voltage working condition, and the positive electrode active material cannot be effectively protected when the sulfur content is too low. When the mass ratio of elemental sulfur to the active material is more than 0.50%, the effect of increasing the sulfur content on improving the cycle performance of the battery is small, and because of poor conductivity of sulfur, excessive sulfur can increase the battery impedance and reduce the battery energy density. When the mass ratio of elemental sulfur to the active material is controlled to be 0.10% -0.50%, sulfur can effectively stabilize oxygen atoms in surface lattices of the positive electrode active material, stability of the surface structure of the active material under a high-voltage working condition is improved, and the capacity of the active material is normally exerted, so that the energy density of a battery is not influenced.

In some embodiments of the present application, the active material comprises an NCM positive electrode material (nickel cobalt manganese ternary positive electrode material), an NCA positive electrode material (nickel cobalt aluminum ternary positive electrode material), an NCMA positive electrode material (nickel cobalt manganese aluminum quaternary positive electrode material), lithium nickel manganese oxide, liCoO ₂ One or more of a positive electrode material and a lithium-rich manganese-based layered oxide. The material has good cycle performance and multiplying power performance under the high-voltage working condition, and is beneficial to preparing the battery with high energy density.

In step 200 of the present application, the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte are assembled into a battery and formed, thereby obtaining a pre-activated battery. The positive plate is assembled into a battery and is formed into an active material capable of activating the positive plate, so that the active material is activated to form an activation site (a site for inserting and extracting lithium), and the sulfur can smoothly migrate to the activation site of the active material during sulfur resetting treatment, thereby effectively protecting the active material. In this application, specific parameters of battery formation may be set according to properties of a battery, and in some embodiments of the present application, formation includes constant current charging and constant current discharging, where constant current charging includes: charging to 4.25V-4.5V with a current of 0.05-0.5C, and constant current discharging comprises: discharging to 3.0V-3.5V with the current of 0.05-0.5C. Under this formation condition, the active material can be sufficiently activated to exert capacity. In some embodiments of the present application, the battery is allowed to stand for 1min to 20min after constant current charging, and is allowed to stand for 1min to 20min after constant current discharging.

In step 300 of the present application, a sulfur reset process is performed on the pre-activated battery to obtain a lithium secondary battery, wherein the sulfur reset process includes a first process and a second process that are performed sequentially. In this embodiment, the first process includes: discharging to 1.0V-1.5V with a current of 0.1-1C, and in the discharging process of the first treatment, obtaining electrons from sulfur on the surface of the active material layer and reacting with lithium ions to generate lithium polysulfide, wherein the reaction equation is as follows:

2Li ⁺ +xS+2e ^- →Li ₂ S _x

wherein Li is ₂ S _x The value of x is 3-8, and through the reaction, sulfur which is difficult to dissolve in the electrolyte can be converted into lithium polysulfide which is easy to dissolve in the electrolyte, and the lithium polysulfide is dissolved in the electrolyte in the positive plate.

In an embodiment of the present application, the second process includes: charging to 3.0V-3.5V with a current of 0.1-1C, wherein in the charging process of the second treatment, lithium polysulfide loses electrons at the active site of the active material and forms elemental sulfur, and the reaction equation is as follows:

Li ₂ S _x -2e ^- →2Li ⁺ +xS

through the above reaction, the lithium polysulfide in the electrolyte is converted into a relatively uniform low-valence sulfur coating layer at the active site (lithium intercalation/deintercalation site) of the active material, which can stabilize the O in the active material lattice ^2- The stability of the active material crystal lattice is improved, and the sulfur coating layer can separate active oxygen from electrolyte, so that the oxidative decomposition of the electrolyte is inhibited, the side reaction between the surface of the active material and the electrolyte under the high-voltage working condition is reduced, and the circulating capacity stability of the active material is improved.

In some embodiments of the present application, the first process comprises: the discharge was carried out at a current of 0.1 to 1C to 1.5V. In the discharge process, li which is difficult to dissolve in the electrolyte is easily generated when the cut-off voltage is lower than 1.5V ₂ S _x (x=1 to 2), which is unfavorable for the sufficient diffusion of sulfur element to the active site of the active material, sulfur is effective for Li when the cut-off voltage of discharge is 1.5V ₂ S _x The conversion rate of (x=3 to 8) is high,is beneficial to shortening the migration time of sulfur and improving the speed of sulfur resetting treatment. In some embodiments of the present application, the pre-activated battery after the first treatment is allowed to stand for 1s to 100s before the second treatment, and the time for which the pre-activated battery after the first treatment is allowed to stand may be, but is not limited to, 1s, 10s, 20s, 30s, 45s, 60s, 80s, or 100s. The pre-activated battery subjected to the first treatment is kept stand, so that lithium polysulfide can be effectively diffused to the surface of an active material, and the lithium polysulfide can be oxidized into elemental sulfur at the active site of the active material as soon as possible in the second treatment. In some embodiments of the present application, the time for the pre-activated battery subjected to the first treatment to stand is 1s to 60s, and when the time for the pre-activated battery subjected to the first treatment to stand is 1s to 60s, lithium polysulfide can migrate to the surface of the active material more sufficiently, and the lithium polysulfide is not easy to diffuse to the negative electrode side of the battery, so that the loss of active materials in the battery can be reduced. In some embodiments of the present application, the discharge current in the first treatment process is 0.3C to 1C, and the reaction between polysulfide and solvent can be reduced when discharging at a higher current, thereby ensuring that polysulfide can be sufficiently converted into elemental sulfur in the second treatment.

In some embodiments of the present application, the second process comprises: charging to 3.5V with a current of 0.1-1C. And when the charging cut-off voltage is 3.5V, the lithium polysulfide is favorable for fully forming a low-valence sulfur coating layer at the active site of the active material, so that the active material is effectively protected. In some embodiments of the present application, the charging current in the second treatment process is 0.3C to 1C, and when charging with a higher current, the reaction of polysulfide into sulfur can be accelerated, the existing time of polysulfide can be shortened, and the diffusion of polysulfide into the negative electrode and the reaction of polysulfide with the electrolyte can be suppressed. In some embodiments of the present application, the second treatment is to charge to 3.5V at a current of 1C and then leave the pre-activated cell after the second treatment for 30s. In some embodiments of the present application, the second treatment is charging to 3V at a current of 0.3C, and the pre-activated cell after the second treatment is allowed to stand for 60s.

In some embodiments of the present application, the number of sulfur reset treatments is greater than 1, and multiple sulfur reset treatments on the preactivated cell facilitate the formation of a uniform low-cost sulfur coating on the active material surface. In some embodiments of the present application, the number of sulfur reset treatments is less than or equal to 10. In some embodiments of the present application, the number of sulfur reset treatments is 2 to 8. When the number of sulfur reset treatments is 2-8, the elemental sulfur can be fully coated at the active sites on the surface of the active material.

The positive plate, the negative plate, the diaphragm and the electrolyte are assembled into a battery and formed to obtain the preactivated battery, sulfur in the positive plate is redistributed through sulfur resetting treatment, and the sulfur forms a low-price sulfur coating layer on the surface of the active material, so that the active material is stabilized. The formation and sulfur resetting treatment in the preparation method are realized in the sealed environment of the battery, which is beneficial to reducing side reactions (such as water absorption failure of lithium polysulfide, decomposition of lithium polysulfide to generate hydrogen sulfide and the like) generated during the optimization of the positive plate. In the use process of the battery, the low-valence sulfur can also react with active oxygen to form a sulfur-containing compound, the sulfur-containing compound can be used as an SEI film to separate the active site of the active material from the electrolyte, so that the direct contact of the active site and the electrolyte is avoided, the oxidative decomposition of the electrolyte is relieved, and the battery can effectively exert capacity and has good cycle performance.

The application also provides a lithium secondary battery prepared by the preparation method of the lithium secondary battery. In this embodiment, the lithium secondary battery includes positive pole, negative pole, electrolyte and lie in the diaphragm between positive pole and the negative pole, and the positive pole includes the current collector and sets up the positive pole active material layer at the current collector surface, and positive pole active material layer includes positive pole active material and elemental sulfur, and elemental sulfur covers the active site department on active material surface. The lithium secondary battery may operate under high voltage conditions, and in some embodiments, the upper limit of the charging voltage of the lithium secondary battery is 4.3V to 4.4V and the lower limit of the discharging voltage of the lithium secondary battery is 3.2V to 3.0V. The lithium secondary battery provided by the application has good cycle performance under the high-voltage working condition, and has wide practical application prospect.

In some embodiments of the present application, the negative electrode of the lithium secondary battery includes one or more of a carbon-based negative electrode and a lithium negative electrode. Wherein the carbon-based negative electrode may include one or more of graphite, hard carbon, soft carbon, and graphene; the lithium negative electrode may include metallic lithium or a lithium alloy, and the lithium alloy may be specifically at least one of a lithium silicon alloy, a lithium sodium alloy, a lithium potassium alloy, a lithium aluminum alloy, a lithium tin alloy, and a lithium indium alloy. In some embodiments of the present application, the current collector of the negative electrode comprises copper foil, and the negative electrode active material comprises one or more of natural graphite, artificial graphite, hard carbon, and soft carbon; the binder comprises one or more of polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), and styrene-butadiene latex (SBR); the conductive agent comprises one or more of acetylene black, ketjen black, super-P, carbon nanotubes, carbon nanofibers, activated carbon and graphene. In the present application, the preparation method of the negative electrode may employ any method known in the art.

In this application, the separator of the lithium secondary battery may be any separator known to those skilled in the art, for example, the separator may be one or more of a polyolefin microporous film, polyethylene terephthalate, polyethylene felt, glass fiber felt, or ultra fine glass fiber paper.

In the present application, the electrolyte of the lithium secondary battery includes a solution of an electrolyte lithium salt in a nonaqueous solvent. In an embodiment of the present application, the electrolyte lithium salt includes lithium hexafluorophosphate (LiPF ₆ ) Lithium perchlorate (LiClO) ₄ ) Lithium tetrafluoroborate (LiBF) ₄ ) Lithium hexafluoroarsenate (LiAsF) ₆ ) Lithium hexafluorosilicate (Li) ₂ SiF ₆ ) Lithium tetraphenyl borate (LiB (C) ₆ H5) ₄ ) Lithium chloride (LiCl), lithium bromide (LiBr), lithium chloroaluminate (LiAlCl) ₄ ) Lithium fluorocarbon sulfonate (LiC (SO) ₂ CF ₃ ) ₃ )、LiCH ₃ SO ₃ 、LiN(SO ₂ CF ₃ ) ₂ And LiN (SO) ₂ C ₂ F ₅ ) ₂ One or more of the following. In some embodiments of the present application, the nonaqueous solvent includes one or more of a chain acid ester and a cyclic acid ester. In some embodiments of the present application, the chain acid ester includes one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), and dipropyl carbonate (DPC). Some embodiments of the applicationWherein the chain acid ester includes a fluorine-, sulfur-or unsaturated bond-containing chain organic ester. In some embodiments of the present application, the cyclic acid ester includes one or more of Ethylene Carbonate (EC), propylene Carbonate (PC), vinylene Carbonate (VC), gamma-butyrolactone (gamma-BL), and sultone. In some embodiments of the present application, the cyclic acid esters include fluorine-, sulfur-or unsaturated bond-containing cyclic organic esters. In some embodiments of the present application, the nonaqueous solvent includes one or more of a chain ether and a cyclic ether solution. In some embodiments of the present application, the cyclic ether includes one or more of Tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1, 3-Dioxolane (DOL), and 4-methyl-1, 3-dioxolane (4-MeDOL). In some embodiments of the present application, the cyclic ether includes a fluorine-, sulfur-or unsaturated bond-containing cyclic organic ether. In some embodiments of the present application, the chain ether includes one or more of Dimethoxymethane (DMM), 1, 2-Dimethoxyethane (DME), 1, 2-Dimethoxypropane (DMP), and Diglyme (DG). In some embodiments of the present application, the chain ether includes a fluorine-, sulfur-or unsaturated bond-containing chain organic ether. In the embodiment of the application, the concentration of the electrolyte lithium salt in the electrolyte is 0.1mol/L to 15mol/L. In some embodiments of the present application, the concentration of the lithium salt of the electrolyte is 1mol/L to 10mol/L.

The application also provides electronic equipment, which comprises the lithium secondary battery provided by the application, wherein the lithium secondary battery supplies power for the electronic equipment.

The technical scheme of the application is further described in the following several embodiments.

Example 1

A method of manufacturing a lithium secondary battery, comprising:

1) Preparation of a positive plate:

weighing 100g of active material (nickel cobalt manganese ternary positive electrode material), 2.3g of conductive agent (carbon black) and 2.5g of binder (vinylidene fluoride, PVDF), adding into a proper amount of N-methyl pyrrolidone, stirring for 60min at a rotating speed of 1500rpm by adopting an IKA stirrer, adding a proper amount of N-methyl pyrrolidone to adjust viscosity, stirring for 120min at a rotating speed of 1800rpm, and sieving with a 150-mesh screen to form stable and uniform slurry;the sizing agent is evenly coated on the surface of an aluminum foil with the thickness of 15 mu m, and is dried at the temperature of 120 ℃ to obtain a pole piece precursor, wherein the surface density of an active material layer in the pole piece precursor is 150g/m ² The compacted density of the active material layer was 3.3g/cm ³ 。

Referring to fig. 4, fig. 4 is a diagram illustrating a process for preparing a lithium secondary battery according to example 1, in fig. 4, a container 200 includes a beaker at the bottom and a surface dish at the top, the surface dish is covered on the beaker to form a closed space, a pole piece precursor 12 is fixed on the surface dish by a polyimide tape 14, and sulfur powder 13 is laid at the bottom of the beaker. The preparation method of the positive plate comprises the following steps: weighing 2g of sublimed sulfur powder, transferring to the bottom of a beaker, and paving; covering a surface dish fixed with a pole piece precursor on a beaker opening to form a closed container, putting the container into an oven, and cooling the container at the oven temperature of 130 ℃ for 2 hours at constant temperature to obtain the positive pole piece.

Taking a lithium sheet as a negative plate and taking polyethylene with the thickness of 18 mu m and coated with ceramic on both sides as a diaphragm, winding the prepared positive plate, the negative plate and the diaphragm to prepare a battery cell, packaging, and then injecting 1mol/L lithium hexafluorophosphate electrolyte (DMC: EMC: DEC: EC with the volume ratio of 15:40:15:30) into the battery cell, wherein the additive is Vinylene Carbonate (VC) with the mass percentage of 2 percent).

2) Formation of the battery:

constant current charging is carried out on the battery, and the conditions of the constant current charging are as follows: charging the battery to 4.3V at 0.1C, and performing constant current discharge on the battery after standing the battery for 10min, wherein the constant current discharge conditions are as follows: the 0.1C was discharged to 3.0V and the cell was allowed to stand for 10min. And (3) carrying out repeated constant-current charging and constant-current discharging on the battery, wherein the repeated times are 5 times, obtaining a preactivated battery, and taking the capacity data of the battery as initial capacity when the 5 th discharging is completed.

3) Sulfur reset treatment:

sulfur resetting treatment is carried out on the pre-activated battery, wherein the sulfur resetting treatment comprises a first treatment and a second treatment which are sequentially carried out, and the conditions of the first treatment are as follows: discharging to 1.5V at 0.3C, standing the battery for 1min, and performing a second treatment on the battery, wherein the second treatment condition is as follows: the battery was left to stand for 1min at 0.3C to 3.5V. And carrying out sulfur reset treatment on the battery for 5 times to obtain the lithium secondary battery.

Example 2

The difference from example 1 is that in example 2, when the positive electrode sheet was prepared, the temperature of the oven was 120℃and the heating time was 2 hours.

Example 3

The difference from example 1 is that in example 3, when the positive electrode sheet was prepared, the temperature of the oven was 150℃and the heating time was 15 minutes.

Example 4

The difference from example 1 is that the conditions of the sulfur reset treatment in example 4 are:

the conditions for the first treatment were: discharging the battery to 1.0V at 1C, standing the battery for 30s, and then performing a second treatment on the battery, wherein the condition of the second treatment is as follows: the battery was left to stand for 60s at 0.3C to 3.0V. And carrying out sulfur reset treatment on the battery for 5 times to obtain the lithium secondary battery.

Example 5

The difference from embodiment 1 is that the conditions of the sulfur reset process in embodiment 5 include:

the conditions for the first treatment were: discharging to 1.5V at 0.1C, standing the battery for 60s, and performing a second treatment on the battery, wherein the condition of the second treatment is as follows: the 0.3C was charged to 3.0V and the cell was left to stand for 60s. And carrying out sulfur reset treatment on the battery for 5 times to obtain the lithium secondary battery.

Comparative example 1

The difference from example 1 is that comparative example 1 did not perform sulfur reset treatment on the preactivated battery, and a lithium secondary battery was obtained after the battery was formed.

Comparative example 2

The difference from example 1 is that comparative example 2 was not fumigated with the electrode sheet precursor, that is, comparative example 2 was a lithium secondary battery obtained by winding a positive electrode sheet, a negative electrode sheet and a separator into a battery cell, injecting an electrolyte, and forming.

Effect examples

To verify the performance of the preparation method made by the present application, the present application also provides effect examples.

1) According to the method of GB/T20123, the content of elemental sulfur in the positive plates of examples 1-5 and comparative examples 1-2 is measured by using a high-frequency infrared carbon sulfur analyzer and a combustion furnace, and the mass ratio of elemental sulfur to active material is calculated according to the mass of the active material, and the test result is shown in Table 1.

TABLE 1 Positive plate parameter tables of examples 1-5 and comparative examples 1-2

Experimental group	Mass ratio of elemental sulfur to active material (%)
		Example 1	0.380
Example 2	0.103
		Example 3	0.627
Example 4	0.380
		Example 5	0.380
Comparative example 1	0.380
		Comparative example 2	0.086

2) According to the method of GB/T17359, the morphology of the positive electrode sheet of example 1 and comparative example 2 and the mass percentage and atomic percentage of sulfur element in the positive electrode sheet are characterized by using a Scanning Electron Microscope (SEM) and an X-ray spectrometer (SEM-EDS, oxford EDX spectrometer), and referring to fig. 5 and 6, fig. 5 is a scanning electron microscope image of the positive electrode sheet of example 1, fig. 6 is a scanning electron microscope image of the positive electrode sheet of comparative example 2, and it can be seen from fig. 5 and 6 that the morphology of the active material particles is substantially unchanged after fumigation of the electrode sheet precursor in example 1. The mass percentage and atomic percentage of sulfur element in the active material layer of the positive electrode sheet were characterized under the test conditions of 15kV and 2000x, so as to obtain the relative content of sulfur at the depth of 3 μm on the surface of the active material layer, and the test results are shown in tables 2 and 3.

Table 2 table of element content of positive electrode sheet active material layer of example 1

Element(s)	Mass percent (%)	Atomic percent (%)
			C	18.84	34.31
O	32.54	44.49
			F	3.46	3.99
Al	0	0
			S	0.31	0.21
Mn	11.94	4.75
			Co	3.47	1.29
Ni	29.44	10.97
			Total amount of	100	100

Table 3 comparative example 2 positive electrode sheet active material layer element content table

Element(s)	Mass percent (%)	Atomic percent (%)
			C	19.35	35.08
O	32.21	43.84
			F	3.21	3.68
Al	0.77	0.62
			S	0	0
Mn	11.6	4.6
			Co	3.13	1.16
Ni	29.71	11.02
			Total amount of	100	100

As can be seen from tables 2 and 3, in example 1, sulfur was successfully introduced into the active material layer by fumigating the precursor of the electrode sheet, the sulfur content in the active material layer was greatly increased, and the relative content of sulfur at a depth of 3 μm on the surface of the active material layer in the positive electrode sheet was not much different from the content ratio of sulfur in the entire active material layer, which indicates that sulfur was more uniformly distributed in the active material layer, and that sulfur was not significantly condensed.

3) The initial capacities of the preactivated batteries of example 1 and comparative example 2, which are capacity data of the preactivated battery at the completion of formation, were compared. Referring to fig. 7, fig. 7 is a graph showing the initial capacities of the pre-activated batteries of example 1 and comparative example 2, and as can be seen from fig. 7, the capacities of comparative example 2 and example 1 are not greatly different, which illustrates that the method of the present invention has no effect on the capacity of the battery.

4) The lithium secondary batteries of example 1 and comparative example 2 were disassembled, the positive electrode of the lithium secondary battery was taken out, the valence state of sulfur in the positive electrode of the lithium secondary battery was characterized, referring to fig. 8, fig. 8 is a graph comparing the valence states of sulfur in the positive electrodes of the lithium secondary batteries of example 1 and comparative example 2, and as can be seen from fig. 8, the peak of sulfur appears in the low valence state region in the positive electrode of the lithium secondary battery of example 1, while the positive electrode of the lithium secondary battery of comparative example 2 does not have the peak position of low valence sulfur, which means that the method of the present application can reset sulfur to the surface of the active material, thereby effectively protecting the active material.

5) The lithium secondary batteries of examples 1-5 and comparative examples 1-2 were subjected to capacity retention test under the following conditions,

step 1: constant current charging: charging to 4.3V at 0.45 mA;

step 2: constant-current constant-voltage charging: charging to 4.4V with 0.45mA, cutting off 0.05mA, and standing for 2min;

step 3: floating charge cycle: 4.4V constant voltage charging, 0.05mA cut-off, and standing for 2min, repeating for 100 times;

step 4: constant current discharge: discharge to 3.0V at 0.45 mA.

Repeating the steps 2-4 for 10 times, and comparing the floating charge attenuation degree by taking the discharge capacity data of the step 4.

Referring to fig. 9, fig. 9 is a graph showing the capacity retention ratio of the lithium secondary batteries provided in example 1 and comparative example 2, and it can be seen from fig. 9 that the lithium secondary battery of example 1 has a reversible capacity retention ratio improved by 3.5% after 1000 times of float cycles compared to the lithium secondary battery of comparative example 2. This shows that the preparation method of the lithium secondary battery provided by the invention can effectively improve the cycle performance of the battery. Referring to table 4, table 4 is a summary table of capacity retention rates of the lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 2 of the present application.

Table 4 summary of capacity retention rate tables of lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 2

Experimental group	Capacity retention (%)
		Example 1	95.8
Example 2	93.7
		Example 3	93.5
Example 4	94.6
		Example 5	94.1
Comparative example 1	92.2
		Comparative example 2	91.7

As can be seen from table 4, the battery of example 1 has improved capacity retention after the sulfur reset treatment compared with the battery of comparative example 1, which suggests that the sulfur reset treatment can effectively improve the stability of the active material. In the battery sulfur resetting process of example 4, the first process employs a higher discharge current and lowers the lower limit voltage, accelerating the reaction of sulfur into lithium polysulfide, but under this condition, sulfur may not be completely discharged, resulting in lower capacity retention than in example 1. In the sulfur resetting treatment of the battery of example 5, the charge-discharge current was low, the charge-discharge time was long, and the sulfur resetting effect was reduced due to migration of part of the lithium polysulfide to the negative electrode. The battery of comparative example 1 adopts the positive plate subjected to sulfur fumigation treatment, and the capacity retention rate of the positive plate is also superior to that of the battery of comparative example 2, which shows that the capacity retention rate of the battery can be improved by carrying out sulfur fumigation on the pole piece and adding a proper amount of sulfur into the pole piece.

The foregoing is a preferred embodiment of the present application and is not to be construed as limiting the scope of the present application. It should be noted that modifications and adaptations to the principles of the present application may occur to one skilled in the art and are intended to be comprehended within the scope of the present application.

Claims

1. The lithium secondary battery comprises a positive electrode, a negative electrode, electrolyte and a diaphragm positioned between the positive electrode and the negative electrode, and is characterized in that the positive electrode comprises a current collector and a positive electrode active material layer arranged on the surface of the current collector, the positive electrode active material layer comprises a positive electrode active material and elemental sulfur, and the elemental sulfur covers active sites on the surface of the positive electrode active material.

2. The lithium secondary battery according to claim 1, wherein an upper limit of a charging voltage of the lithium secondary battery is 4.3V to 4.4V.

3. A method for manufacturing a lithium secondary battery, comprising:

providing a positive plate, wherein the positive plate comprises a current collector and an active material layer arranged on the surface of the current collector, and the active material layer comprises an active material and elemental sulfur;

assembling the positive plate, the negative plate, the diaphragm and the electrolyte into a battery and forming to obtain a preactivated battery;

4. The method according to claim 3, wherein the mass ratio of the elemental sulfur to the active material is 0.1% to 0.5%.

5. The method of preparing as claimed in claim 3 or 4, wherein each of the sulfur reset treatments further comprises: and before the second treatment, standing the preactivated battery subjected to the first treatment for 1-100 s.

6. The method according to any one of claims 3 to 5, wherein the number of sulfur resetting treatments is 1 to 10.

7. The method of any one of claims 3 to 6, wherein the method of producing the positive electrode sheet comprises: providing a pole piece precursor, wherein the pole piece precursor comprises a current collector and an active material precursor layer arranged on the surface of the current collector, and the active material precursor layer comprises an active material; and fumigating the electrode plate precursor by sulfur vapor to obtain the positive electrode plate.

8. The method of claim 7, wherein the fumigation is at a temperature of 110 ℃ to 180 ℃.

9. The method of any one of claims 3-8, wherein the forming comprises: charging to 4.25V-4.5V with current of 0.05-0.5C, discharging to 3.0V-3.5V with current of 0.05-0.5C.

10. The method of any one of claims 3 to 9, wherein the active material comprises NCM positive electrode material, NCA positive electrode material, NCMA positive electrode material, lithium nickel manganese oxide, liCoO ₂ One or more of a positive electrode material and a lithium-rich manganese-based layered oxide.

11. An electronic device, characterized in that the electronic device comprises the lithium secondary battery according to claim 1 or 2.