CN115663268A - Lithium-sulfur battery without negative electrode and preparation method thereof - Google Patents

Abstract

The invention discloses a lithium-sulfur battery without a negative electrode and a preparation method thereof, wherein the lithium-sulfur battery without the negative electrode is SeS ₂ -PAN// Cu battery; the positive electrode is a positive plate obtained by lithium supplement treatment of a grade plate coated with a selenium sulfide-polyacrylonitrile material; the electrolyte being locally highThe electrolyte with high concentration is composed of LiFSI, DMC and HFE; the preparation method comprises the following steps: preparing a grade piece coated with a selenium sulfide-polyacrylonitrile material; preparing local high-concentration electrolyte; li// SeS is assembled by taking the graded sheet as a positive electrode, lithium foil as a counter electrode and local high-concentration electrolyte as battery electrolyte ₂ -a PAN battery; the battery is used for supplementing lithium to the grading piece; disassembling the grade sheet after the lithium supplement treatment to be used as a positive plate; and assembling the positive plate, the copper current collector and the local high-concentration electrolyte under the argon atmosphere to obtain the lithium-sulfur battery without the negative electrode. The battery prepared by the method can improve the sulfur utilization rate, the coulombic efficiency and the battery cycle life.

Description

Cathode-free lithium-sulfur battery and preparation method thereof

Technical Field

The invention relates to the technical field of lithium batteries. In particular to a lithium-sulfur battery without a negative electrode and a preparation method thereof.

Background

Among lithium metal batteries, the lithium-sulfur battery system (Li-S) is one of the most potential batteries due to the reversible electrochemical reaction of elemental sulfur with elemental lithium: li + S → Li ₂ S _n (n = 2-8). The theoretical capacity of the sulfur element is up to 1675mAh/g, the energy density (2567 Wh/kg) is more than 6 times higher than that of the traditional lithium ion battery (387 Wh/kg), and the sulfur element has high natural content, low cost and low toxicity, so that a plurality of scholars have great interest in the sulfur element. Although the chemical reaction of elemental lithium and elemental sulfur has been shown to achieve high theoretical energy densities, poor energy retention during cycling is the most challenging problem for lithium sulfur battery systems. If sulfur is simply slurry cast with conductive additives and polymer binders in the manner of conventional lithium ion battery electrode fabrication, rapid capacity fade is observed and complete failure is observed in as few as a few tens of cycles. The lithium-sulfur battery also has a big challenge of self-discharge phenomenon, and before the assembled lithium-sulfur battery is applied with no voltage, the sulfur positive electrode can react with lithium ions in the electrolyte to reduce a part of S8 into polysulfide which is dissolved in the electrolyte and shuttled to the lithium metal negative electrode to generate side reaction.

In pursuit of higher energy density and power density, a non-negative electrode lithium metal battery without an excessive lithium source is increasingly gaining attention. In 2000, neudecker et al demonstrated for the first time the feasibility of operating a non-negative cell. Qian et al have further produced lithium metal batteries without negative electrodes and elucidated their application prospects, from which they have attracted a great deal of interest. The lithium metal battery without the negative electrode is characterized in that lithium metal is electroplated on a copper current collector during charging, and the lithium metal is stripped from the copper current collector and returns to a positive electrode material during discharging. However, the challenge of the lithium metal battery without the negative electrode is very difficult, the coulombic efficiency is one of the most important indexes of the lithium metal battery without the negative electrode, and the coulombic efficiency directly determines the actual cycle life and the reversibility of the battery because no redundant lithium metal source exists. Coulombic inefficiencies are mainly the generation of dead lithium and the irregular growth of lithium dendrites.

With the advancement of technology, the practical development and application of high energy density batteries are in need. The lithium metal battery without the negative electrode has ultrahigh energy density, but has the problem of low coulombic efficiency and short cycle life. Lithium sulfur batteries have high theoretical specific capacity and low cost, however, slow reaction kinetics produce shuttle effects and electrode instability at high loadings limit their applications. If the lithium sulfur battery is combined with a lithium metal battery without a negative electrode, the prepared lithium sulfur battery without the negative electrode generally has the problems of limited active lithium source, low utilization rate of positive active substances under the condition of high load and the like, so that the problems of low coulombic efficiency, rapid attenuation of high-load capacity positive electrode capacity, short cycle life and the like of the lithium sulfur battery without the negative electrode are caused.

Disclosure of Invention

Therefore, the technical problem to be solved by the invention is to provide a non-negative lithium sulfur battery and a preparation method thereof, so as to solve the problems of low coulombic efficiency, rapid attenuation of high-load capacity positive electrode capacity, short cycle life, sulfur utilization rate and the like of the non-negative lithium sulfur battery prepared by the existing method.

In order to solve the technical problems, the invention provides the following technical scheme:

a lithium-sulfur battery without negative electrode is SeS ₂ -PAN// Cu battery; the positive electrode is a positive plate obtained by lithium supplement treatment of a grade plate coated with a selenium sulfide-polyacrylonitrile material; the electrolyte is locally high in concentrationThe electrolyte with high concentration locally consists of lithium bis (fluorosulfonyl) imide LiFSI, dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE.

The preparation method of the grade piece coated with the selenium disulfide-polyacrylonitrile material of the lithium-sulfur battery without the negative electrode comprises the following steps:

step (A-1): uniformly mixing selenium disulfide powder and polyacrylonitrile powder, grinding, and sieving to obtain mixed raw material powder;

step (A-2): placing the mixed raw material powder in a quartz tube for vacuum tube sealing; then placing the sealed quartz tube in a tube furnace for first heat treatment, and cooling to room temperature after the heat treatment is finished; crushing the quartz tube and taking out the solid product in the quartz tube;

step (A-3): placing the solid product in a tube furnace for secondary heat treatment, and obtaining a selenium sulfide-polyacrylonitrile material after the heat treatment is finished;

step (A-4): mixing a carboxymethyl cellulose aqueous solution and styrene-butadiene rubber emulsion to obtain a mixed binding solution; mixing and grinding the selenium disulfide-polyacrylonitrile material and the conductive carbon black, and then sieving to obtain mixed powder; adding the mixed bonding liquid into the mixed powder, and uniformly stirring and mixing to obtain mixed slurry;

step (A-5): and coating the mixed slurry on a positive current collector, and drying to obtain the graded piece coated with the selenium sulfide-polyacrylonitrile material.

In the lithium sulfur battery without the negative electrode, in the step (A-1), the mass ratio of the selenium disulfide powder to the polyacrylonitrile powder is 1 (1-5); grinding for 20-50 min, and sieving with a 300-mesh sieve after grinding;

in the step (A-2), the first heat treatment method comprises: heating to 370-400 ℃ at the heating rate of 5 ℃/min, and then preserving heat for 6-9 h;

in the step (A-3), the second heat treatment method comprises: heating to 300-350 ℃ at the heating rate of 5 ℃/min, and then preserving the heat for 2-5 h.

In the lithium-sulfur battery without the negative electrode, in the step (A-4), the volume ratio of the carboxymethyl cellulose aqueous solution to the styrene-butadiene rubber emulsion in the mixed binding liquid is (20-30) to 1; the mass fraction of the carboxymethyl cellulose in the carboxymethyl cellulose aqueous solution is 1 to 6 weight percent; the mass fraction of the styrene-butadiene rubber in the styrene-butadiene rubber emulsion is 40-50 wt%; mixing and grinding the selenium disulfide-polyacrylonitrile material and the conductive carbon black in the mixed powder according to the mass ratio of (4-9) to (1) for 20-50 min, and sieving the mixture through a 300-mesh sieve after grinding; the mass ratio of the selenium disulfide-polyacrylonitrile material to the carboxymethyl cellulose in the mixed slurry is (15-20) to 1; the grade piece coated with the selenium disulfide-polyacrylonitrile material prepared according to the proportion can give consideration to both conductivity, capacitance and energy density.

In the step (A-5), the positive current collector is an aluminum foil; the coating amount of the mixed slurry on the positive current collector is 1-3mg/cm ² Or the coating amount of the mixed slurry on the positive current collector is 5-7mg/cm ² (ii) a The drying temperature is 55-65 ℃, and the drying time is 10-12 h.

In the lithium sulfur battery without the negative electrode, in the step (a-1), the mass ratio of the selenium disulfide powder to the polyacrylonitrile powder is 1; grinding for 30min, and sieving with 300 mesh sieve;

in the step (A-2), the first heat treatment method comprises: heating to 380 ℃ at the heating rate of 5 ℃/min, and then preserving heat for 8h; under the heat treatment condition, the selenium sulfide and the polyacrylonitrile can be fully bonded;

in the step (A-3), the second heat treatment method comprises: heating to 350 ℃ at the heating rate of 5 ℃/min, and then preserving heat for 4h; under the heat treatment condition, the excess SeS not bonded with polyacrylonitrile can be removed ₂ If the holding time is too short, the removal of sulfur will be insufficient, and SeS will be affected ₂ -properties of PAN material;

in the step (A-4), the volume ratio of the carboxymethyl cellulose aqueous solution to the styrene-butadiene rubber emulsion in the mixed binding liquid is 24; the mass fraction of the carboxymethyl cellulose in the carboxymethyl cellulose aqueous solution is 2wt%; the mass fraction of the styrene-butadiene rubber in the styrene-butadiene rubber emulsion is 48wt%; mixing and grinding the selenium sulfide-polyacrylonitrile material and the conductive carbon black in the mixed powder for 30min according to the mass ratio of 8; the mass ratio of the selenium disulfide-polyacrylonitrile material to the carboxymethyl cellulose in the mixed slurry is 16; if the particle size of the solid components in the mixed slurry is too large, the uniformity of the slurry components is poor, and the coating effect on the positive current collector is influenced;

in the step (A-5), the drying temperature is 60 ℃ and the drying time is 12h. If the drying temperature is too high, the chips are prone to cracking, and if the drying time is too short, trace amounts of water in the chips cannot be removed.

The specific operation method of the grade piece lithium supplement treatment of the lithium sulfur battery without the negative electrode coated with the selenium disulfide-polyacrylonitrile material comprises the following steps:

step (C-1): under the argon atmosphere with oxygen and moisture content less than or equal to 1ppm, the Li// SeS is obtained by assembling by taking the grade piece coated with the selenium disulfide-polyacrylonitrile material as a positive electrode, a lithium foil as a counter electrode and local high-concentration electrolyte as battery electrolyte ₂ -a PAN battery;

step (C-2) assembling the Li// SeS ₂ First discharge of PAN battery to 0.28V for Li// SeS ₂ SeS of PAN batteries ₂ -PAN positive electrode deep lithiation; 0.28V is the Li-Al alloying reaction point position, and no lithium-aluminum alloying reaction occurs when the voltage is higher than 0.28V; discharging to 0.28V for the first time, and enabling the lithium aluminum to perform alloying reaction to generate Li-Al alloy;

step (C-3): the discharge was continued with a small current of 50 μ A [ too much current would lead to insufficient alloying reaction, too little current would be detrimental to the uniform deposition of lithium-aluminum alloy ], so that lithium was present in SeS ₂ Depositing on PAN positive electrode to form lithium-aluminum alloy, the deposition amount is 0.5-3 mAh/cm ² ；

Step (C-4): after the discharge is completed, li// SeS ₂ And (4) disassembling the positive electrode of the PAN battery to obtain the positive plate after lithium supplement treatment.

The preparation method of the local high-concentration electrolyte of the lithium-sulfur battery without the negative electrode comprises the following steps:

step (B-1), pretreatment of the solvent: adding a molecular sieve into two solvents of dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE respectively, standing for 1-3 days to remove trace water in the two solvents;

step (B-2), pretreatment of lithium salt: putting lithium bis (fluorosulfonyl) imide LiFSI into an oven for drying;

and (B-3) sequentially pouring dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE into the pretreated lithium bis (fluorosulfonyl) imide LiFSI, and magnetically stirring and uniformly mixing to obtain the local high-concentration electrolyte.

In the lithium sulfur battery without the negative electrode, the volume ratio of dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE in the local high-concentration electrolyte is 1: (1-5); the mass concentration of the lithium bis (fluorosulfonyl) imide LiFSI in the local high-concentration electrolyte is 1.0-1.5 mol/L.

In the lithium sulfur battery without the negative electrode, the volume ratio of dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE in the local high-concentration electrolyte is 1:3; the mass concentration of the lithium bis (fluorosulfonyl) imide LiFSI in the local high-concentration electrolyte is 1.2mol/L.

A preparation method of a lithium-sulfur battery without a negative electrode comprises the following steps:

step A: preparing a grade sheet coated with a selenium disulfide-polyacrylonitrile material;

and B, step B: preparing local high-concentration electrolyte, wherein the local high-concentration electrolyte consists of lithium bis (fluorosulfonyl) imide LiFSI, dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE;

and C: under the argon atmosphere with oxygen and moisture content less than or equal to 1ppm, the Li// SeS is obtained by assembling by taking the grade piece coated with the selenium disulfide-polyacrylonitrile material as a positive electrode, a lithium foil as a counter electrode and local high-concentration electrolyte as battery electrolyte ₂ -a PAN battery; using Li// SeS ₂ Discharging the PAN battery to perform lithium supplement treatment on the grade piece coated with the selenium disulfide-polyacrylonitrile material; after the discharge is completed, li// SeS is added ₂ Disassembling the grade sheet coated with the selenium sulfide-polyacrylonitrile material after lithium supplement treatment in the PAN battery, and taking the grade sheet as a positive plate of the lithium-sulfur battery without the negative electrode;

step D: under the atmosphere of argon with oxygen and moisture content less than or equal to 1ppm, the positive plate and the copper current collector are utilizedAnd local high-concentration electrolyte to obtain SeS ₂ PAN// Cu cell, namely, a lithium sulfur cell without negative electrode.

The technical scheme of the invention achieves the following beneficial technical effects:

the local high-concentration electrolyte used in assembling the cathode-free lithium-sulfur battery has better adaptability with the cathode material of the cathode-free lithium-sulfur battery prepared by the preparation method. The SeS prepared by the lithium supplementing treatment method of the invention ₂ The PAN positive electrode grade sheet is subjected to lithium supplement treatment, and the lithium supplement amount of the lithium supplement treatment is controlled, so that the SeS can be used ₂ The PAN positive electrode level sheet has better adaptability with local high-concentration electrolyte, and meanwhile, the problems of short cycle life, low sulfur utilization rate, low coulombic efficiency and the like of a lithium-sulfur battery without a negative electrode can be effectively solved. SeS assembled according to the invention ₂ The PAN// Cu cathode-free lithium-sulfur battery still has ultrahigh coulombic efficiency and capacity retention rate after being cycled for multiple times, and polarization is small in the cycling process and stable in cycling.

Drawings

FIG. 1 SeS in an embodiment of the invention ₂ -XRD patterns of PAN powder and PAN powder;

FIG. 2a SeS in an embodiment of the present invention ₂ A real shot image after mixing with PAN powder;

FIG. 2b SeS in an embodiment of the present invention ₂ -PAN powder mixed live shot;

FIG. 2c SeS in an embodiment of the present invention ₂ SEM picture of PAN powder (5 μm);

FIG. 2d SeS in an embodiment of the present invention ₂ SEM picture of PAN powder (500 nm);

FIG. 3a SeS in an embodiment of the present invention ₂ EDS diagram of PAN powder (1 μm);

FIG. 3b SeS in an embodiment of the present invention ₂ -EDS elemental map of PAN powder (S element);

FIG. 3c SeS in an embodiment of the present invention ₂ -EDS elemental map of PAN powder (C element);

FIG. 3d SeS in an embodiment of the present invention ₂ -EDS elemental map (Se element) of PAN powder;

FIG. 4 shows an embodiment of the present inventionSeS ₂ -thermogravimetric plots of PAN powder and PAN powder;

FIG. 5a is a photograph of a wetting of a lithium foil with a conventional ether electrolyte in accordance with an embodiment of the present invention;

FIG. 5b is a photograph of a wet out of a common ether electrolyte in an embodiment of the present invention on a separator;

FIG. 5c is a photograph of a wet out of a conventional ether electrolyte on a copper foil according to an embodiment of the present invention;

FIG. 5d is a photograph of a wet-out of a lithium foil with a locally high concentration of electrolyte in an embodiment of the present invention;

FIG. 5e is a photograph of a wet out of a localized high concentration electrolyte on a separator in an embodiment of the invention;

FIG. 5f is a photograph of a wet out of a localized high concentration electrolyte on a copper foil in an embodiment of the present invention;

FIG. 6a is an EIS impedance plot of a conventional ether electrolyte and a localized high concentration electrolyte in an embodiment of the invention;

FIG. 6b is a bar graph of the conductivity of a conventional ether electrolyte and a localized high concentration electrolyte in an embodiment of the present invention;

FIG. 7 is a graph of a linear scan of a conventional ether electrolyte and a localized high concentration electrolyte in an embodiment of the invention;

FIG. 8a is a graph of the circulating coulombic efficiencies of the conventional ether electrolyte and the local high concentration electrolyte in an example of the present invention;

FIG. 8b is a graph of the overpotential curve for a conventional ether electrolyte in an embodiment of the present invention;

FIG. 8c is a graph of the overpotential for a localized high concentration of electrolyte in an embodiment of the present invention;

FIG. 8d EIS plots of a normal ether electrolyte and a locally high concentration electrolyte in an example of the invention;

FIG. 9 is a plot of Coulomb efficiency of Li// Cu half-cells at different current densities for localized high concentrations of electrolyte in an embodiment of the invention;

FIGS. 10 a-10 f are overpotential graphs of Li// Cu half-cells at different current densities in an embodiment of the present invention;

FIG. 11a is an SEM image of a conventional ether electrolyte Li// Cu half cell cycled 50 times in accordance with an embodiment of the present invention;

FIG. 11b is an SEM image of a partial high concentration electrolyte Li// Cu half cell of an embodiment of the invention cycled 50 times;

FIG. 12a shows the low Li// SeS loading for two electrolytes in an example of the present invention ₂ -a graph of the cyclic performance of the PAN;

FIG. 12b high load Li// SeS for two electrolytes in an example of the present invention ₂ -a graph of the cyclic performance of the PAN;

FIG. 12c is an EIS diagram of two electrolytes in an example of the present invention;

FIG. 12d is a graph of rate performance of a localized high concentration electrolyte in an embodiment of the present invention;

FIG. 13 illustrates a local high concentration electrolyte system Li// SeS in an embodiment of the present invention ₂ -cyclic voltammogram of a PAN battery;

FIG. 14a shows an example of an ether electrolyte Li// SeS ₂ SEM images (100 μm) after PAN positive cycling;

FIG. 14b local high concentration electrolyte Li// SeS in an embodiment of the invention ₂ SEM images (100 μm) after PAN positive cycling;

FIG. 14c Li// SeS Ether electrolyte in an example of the present invention ₂ SEM images (500 μm) after PAN positive cycling;

FIG. 14d local high concentration electrolyte Li// SeS in an example of the invention ₂ SEM images (500 μm) after PAN positive cycling;

FIG. 15 shows Li// SeS for different lithium supplements in the examples of the present invention ₂ -a graph of the cyclic performance of the PAN battery;

FIGS. 16a to 16d are graphs showing Li// SeS with different lithium supplementation amounts in the examples of the present invention ₂ -a charge-discharge profile of a PAN battery;

FIG. 17a is a SEM image of a planar aluminum foil in an embodiment of the present invention;

FIG. 17b example of the present invention at 1mAh/cm ² SEM images of SEM aluminum current collectors of lithium replenishment;

FIG. 17c shows 2mAh/cm in an example of the present invention ² SEM images of SEM aluminum current collectors of lithium replenishment;

FIG. 17d 3mAh/cm in an example of the present invention ² SEM images of SEM aluminum current collectors of lithium replenishment;

FIG. 18a is a photomicrograph of a lithium-plated surface of a copper foil at different resting times in accordance with an embodiment of the present invention;

FIG. 18b photo of a lithium aluminum alloy surface on copper foil for different resting times according to an embodiment of the present invention;

FIG. 19 Coulomb efficiency plots for lithium and lithium aluminum alloys plated on copper for different air exposure times in accordance with an embodiment of the present invention;

FIG. 20a is a charge and discharge graph of a first cycle rest 15 day cycle for a Li-Cu battery in an example of the present invention;

FIG. 20b is a graph of the charge and discharge cycles for the first cycle rest 15 day cycle for a Li-Al battery in an example of the present invention;

FIG. 21a shows Li// SeS with low Li 1mAh loading in the example of the present invention ₂ -a cyclic performance map of the PAN battery;

FIG. 21b Low-load Li// SeS without lithium supplement in the example of the present invention ₂ -a cyclic performance map of PAN batteries;

FIG. 21c shows Li// SeS with high Li 1mAh loading in the example of the present invention ₂ -a cyclic performance map of the PAN battery;

FIG. 21d high-capacity Li// SeS without lithium supplement in the example of the present invention ₂ -a cyclic performance map of PAN batteries;

FIG. 22a is a Z- ω signal obtained by EIS low frequency range analysis without lithium supplementation in an embodiment of the present invention ^-1/2 A drawing;

FIG. 22b is a graph showing Z- ω obtained by low frequency range analysis of EIS for 1mAh lithium supplementation in the example of the present invention ^-1/2 Drawing (A)

FIG. 22c lithium 1mAh and lithium Li// SeS without supplementation in the examples of the present invention ₂ -a graph of the variation of the diffusion coefficient of lithium ions for different discharge states of the PAN battery;

FIG. 22d lithium 1mAh supplement and Li// SeS supplement not in the example of the invention ₂ -PAN battery charge and discharge profile;

FIG. 23a lithium 1mAh and non-lithium SeS supplementation in an example of the present invention ₂ -a graph of cycling performance of PAN// Cu cells;

FIG. 23b lithium 1mAh supplement and lithium SeS supplement in the example of the invention ₂ -coulombic efficiency profile for PAN// Cu cells;

FIG. 23c non-lithiated SeS in the example of the present invention ₂ Discharge curve for PAN// Cu cellsLine drawing;

FIG. 23d lithium 1mAh and SeS without lithium supplement in the example of the invention ₂ Discharge profile of PAN// Cu cells.

Detailed Description

First part of experimental materials, instruments and experimental methods

1.1 chemical reagents and laboratory instruments

1.1.1 chemical reagents

The chemical reagents mainly used in the experimental process and related information are shown in table 1.

TABLE 1 Experimental chemicals and reagent parameters

The raw materials of the reagent used in the experiment are all directly used and are not subjected to any purification treatment.

1.1.2 Experimental instruments

The instrumentation used primarily during the study is shown in table 2.

TABLE 2 Experimental instruments

1.2 Structure and morphology characterization

1.2.1 scanning Electron microscopy analysis (SEM)

Scanning Electron Microscope (SEM) is a powerful tool for studying the microstructure and morphology of materials, and its principle is to generate signals by the interaction of accelerated high-energy electron beams on a sample in a high vacuum state, and to collect information such as secondary electrons, and to image them by data processing. In this example, the morphology of lithium deposition and the morphology of the SeS2-PAN positive electrode were characterized by using a scanning electron microscope of the S-3400n type from Hitachi, japan.

1.2.2 X-ray diffraction (XRD) analysis

An X-Ray diffractometer (X-Ray Diffraction, abbreviated as XRD) diffracts a material by using X-rays, analyzes a Diffraction pattern, and obtains the components of the material, and the structural morphology of atoms or molecules in the material. XRD is mostly used for physical phase analysis and crystal structure analysis. An X-ray diffractometer Smart Lab 9KW produced in daily life used for the experiment has a test range of 10-80 degrees and a scanning speed of 3 degrees/min, and a Cu target and a high-sensitivity D/teX UItra250 detection system are adopted in a continuous scanning mode.

1.2.3 Thermogravimetric (TG) analysis

Thermogravimetric analysis (TG) is a thermal analysis technology for measuring the relation between the mass of a sample to be measured and the temperature change under the control of a program, and is used for researching the thermal stability and components of a material. And analyzing the information of the thermogravimetric curve to obtain the chemical and physical changes of the material along with the temperature change, thereby analyzing the decomposition temperature, the composition, the thermal stability and the like of the material. The experiment uses a thermo-gravimetric analyzer STA449F3 Jupiter of German Chinesota, the testing temperature range is 25-1000 ℃, the heating rate is 10 ℃/min, and the sample is tested in the air and nitrogen atmosphere.

1.3 electrochemical testing

1.3.1 preparation of electrodes

The manufactured SeS ₂ PAN powder, ground through 300 mesh sieve. CMC and 5wt% CMC aqueous solutions were prepared by adding carboxymethyl cellulose (CMC) powder to deionized water to yield 2wt% CMC and 5wt% CMC aqueous solutions. The prepared CMC aqueous solution and SBR (styrene butadiene rubber emulsion) with the solid content of 48wt percent are mixed according to the weight ratio of 1:1, was mixed. In this example, 625. Mu.L of the 2wt% CMC aqueous solution and 26. Mu.L of the SBR emulsion were sucked up with a pipette and mixed, magnetically stirred for 6 hours, and thereafter 200mg of SeS was weighed ₂ Mixing and grinding PAN powder and 25mg of conductive carbon black for 30 minutes, sieving by a 300-mesh sieve after grinding, pouring into the prepared CMC and SBR mixed solution, and magnetically stirring for 12 hours to obtain uniformly mixed slurry A; active material powder (SeS) sieved in slurry A ₂ -PAN), conductive carbon black (Super P), styrene Butadiene Rubber (SBR) and carboxymethylcellulose (CMC) in a mass ratio of 80:10:5:5; another slurry B was prepared in the same way, using a 5% by weight aqueous CMC solution and the sieved active substance powder (SeS) in slurry B ₂ -PAN), conductive carbon black (Super P), styrene Butadiene Rubber (SBR) and carboxymethylcellulose (CMC) in a mass ratio of 75:15:5:5; will subsequentlyCoating the slurry A on an aluminum foil according to the coating thickness of 100um to prepare a low-load grade sheet (1-3 mg/cm) ² ) (ii) a Coating the slurry B on an aluminum foil according to the coating thickness of 400 mu m to prepare a high-load-level sheet (5-7 mg/cm) ² ). The two kinds of loading-capacity tablets are respectively placed in a vacuum drying oven to be dried for 12 hours at the temperature of 60 ℃, and are punched into tablets with the diameter of 12mm by a puncher and then are weighed for later use.

1.3.2 Assembly of the Battery

The cell assembly process was carried out in a glove box with an atmosphere of high purity argon. The oxygen and water contents in the glove box are respectively controlled at O ₂ ＜1ppm，H ₂ O is less than 1ppm. During assembly, the button (CR 2032) battery is assembled in sequence according to the order of the negative electrode shell, the elastic sheet, the gasket, the negative electrode, 25 muL of electrolyte, the diaphragm, 25 muL of electrolyte, the positive electrode sheet, the gasket and the positive electrode shell. The assembled battery needs to be kept still for more than 12 hours to ensure that the electrolyte fully soaks the electrode material. A porous polypropylene film (PP) with the diameter of 19mm is used as a diaphragm. The negative electrode shell, the positive electrode shell, the elastic sheet and the gasket are all made of stainless steel.

1.3.3 cycle Rate test

The cycle test is a charge-discharge test under constant current and in a given working voltage range, and the cycle performance can show the cycle life, the cycle stability, the capacity retention rate in the cycle process and the first discharge capacity of the battery. The multiplying power performance refers to constant-current charging and discharging of the battery under different current densities, the multiplying power performance shows the performance of the battery under large current, high multiplying power corresponds to low capacity, and the performance of the battery with higher capacity under the same multiplying power is better. The battery rate performance is directly related to the migration capacity of lithium ions at the anode and cathode, the electrolyte and the interface between the anode and the cathode and the electrolyte, and factors influencing the migration speed of the lithium ions. The experiment was performed using a blue charging and discharging battery tester (Land CT 2001A) from blue electronic corporation, wuhan city. And when the test is carried out at room temperature, the constant-current charge-discharge voltage range is 1-3V.

1.3.4 Cyclic voltammetry test (CV)

The cyclic voltammetry test is to control the electrode potential to scan in a given range in a periodic change along with time, so that different redox reactions occur in the given voltage range to form a current-voltage curve, and required information such as peak current, peak area, reduction oxidation potential and the like can be obtained according to the curve, so that the chemical reaction of the electrode in a given voltage interval can be deduced. The experiment used a Princeton model PMC1000A electrochemical workstation with a voltage range of 1-3V and a scan rate of 0.4mV/S.

1.3.5 AC impedance test (EIS)

The ac impedance method is to apply sine wave voltage (current) disturbance to the electrode, and the electrode correspondingly generates a series of signals to obtain an ac impedance spectrogram. The alternating current impedance spectrogram can read information such as electrode impedance and the like, and can calculate relevant reaction kinetics and the like. The experimental apparatus uses a Princeton PMC1000A type electrochemical workstation, the testing frequency is 0.01-100kHz, and the amplitude is 5mV.

Second part of local high-concentration electrolyte is used for lithium-sulfur battery

2.1 introduction to

The low coulombic efficiency and the growth of lithium dendrites of the lithium metal negative electrode have been key problems hindering the development of the lithium metal battery, and among various factors affecting the lithium metal negative electrode, the electrolyte plays a leading role. Electrolyte of conventional commercial lithium ion battery lithium hexafluorophosphate (LiPF) of 1M ₆ ) The solvent is carbonate solvent]The coulombic efficiency is low due to the strong side reaction with lithium metal, and is not suitable for the lithium metal battery. In recent years, it has been reported that a high-concentration ether electrolyte is more compatible with a lithium metal negative electrode than most electrolytes. The reason is the high concentration of Li ⁺ Even at high current densities, the ions in the high concentration electrolyte may promote rapid deposition/exfoliation of lithium, and the reduced number of solvent molecules around the lithium metal negative electrode that are not coordinated may mitigate side reactions between the electrolyte and the lithium metal negative electrode. However, the high concentration electrolyte has a certain disadvantage, for example, most of the high concentration electrolytes cannot be operated in a high voltage system because of poor compatibility with the positive electrode. And the high-concentration electrolyte has high cost, poor ionic conductivity and high viscosity, and is not suitable for wide production and application. This example uses lithium bis (fluorosulfonyl) imide (LiFSI) as the lithium salt, dimethyl carbonate (DMC) as the solvent, 1, 2-tetrafluoroethyl 2, 3-Tetrafluoropropyl ether (HFE) is used as a diluent to formulate a localized high concentration electrolyte suitable for use in a non-negative lithium sulfur battery.

2.2 preparation of cathode Material and electrolyte

2.2.1 preparation of selenium disulfide (SeS 2-PAN) cathode Material

Selenium disulfide (SeS) ₂ PAN) positive electrode material has simpler preparation principle and SeS ₂ Reacting with PAN particles in a small closed space after high-temperature sublimation and high-temperature heat preservation to ensure that the SeS is ₂ And PAN to create a chemical bond to tightly bind. Raw materials of selenium disulfide powder and Polyacrylonitrile (PAN) powder are mixed in a ratio of 1:4, grinding for 30min, and sieving with a 300-mesh sieve. And putting the ground powder into a quartz tube, and performing vacuum tube sealing operation. Then the sealed quartz tube is placed in a tube furnace, and the temperature is raised to 380 ℃ at the heating rate of 5 ℃/min for heat treatment and heat preservation for 8h. And after the quartz tube is cooled, crushing the quartz tube to obtain black powder, and then placing the quartz tube in a tube furnace to heat treat for 4 hours at the temperature of 350 ℃ at the heating rate of 5 ℃/min under the argon atmosphere so as to remove the excessive selenium sulfide. Finally, obtaining a black powder finished product, and adding a conductive agent and a binder into the finished product powder to manufacture the electrode.

2.2.2 electrolyte configuration

The preparation of the electrolyte needs to ensure anhydrous conditions, firstly, a part of the purchased solvent is taken out and added into a molecular sieve to be kept stand for two days to remove trace water in the solvent, a part of lithium salt is taken out and dried for 6 hours at 60 ℃ in a heating table to be completely dried, and then the preparation of the electrolyte can be carried out. All the above operations are carried out in a glove box. For comparing local high-concentration electrolyte, ether electrolyte 1M LiTFSI +0.75M LiNO commonly used for lithium-sulfur battery is selected ₃ DOL/DME as a control.

(1)1M LiTFSI+0.75M LiNO ₃ DME/DOL (volume ratio 1: 0.574g of LiTFSI and 0.102g of LiNO were weighed ₃ Put into a 10mL glass bottle and magnetically stirred for 12h with 1mL DME and 1mL DOL to give a clear liquid.

(2) 1.2M LiFSI-DMC/HFE (volume ratio 1: 0.448g LiFSI was weighed into a 10mL glass vial, and was magnetically stirred for 6h with the addition of 0.5mL DMC and 1.5mL HFE to give a clear liquid.

2.3 selenium disulfide (SeS) ₂ PAN) structural morphology characterization of the cathode material

2.3.1 selenium disulfide (SeS) ₂ -PAN) Positive electrode Material XRD

As shown in FIG. 1 as SeS ₂ XRD of PAN powder and PAN powder, PAN powder without any treatment being the (110) crystal plane of PAN at 2 θ =17 ° and the amorphous region of PAN at 2 θ =25 °. SeS ₂ Disappearance of PAN crystalline characteristic peaks on PAN. SeS ₂ Without any treatment, it is of a crystalline structure. SeS ₂ SeS on PAN ₂ The characteristic peaks all disappeared, indicating SeS from the original ring structure ₂ The molecules forming a chain Se _x S _y Structure, which is similar to the structure of an S chain covalently bonded to a polymer backbone. SeS formed by heat-treating the two materials in a sealed tube at 380 ℃ for 8h ₂ PAN composite without PAN and SeS ₂ The crystal structure signal of (a) becomes amorphous. SeS ₂ The PAN powder as a whole presents an amorphous phase. SeS ₂ PAN to amorphous mainly due to PAN and SeS ₂ C-S and C-Se bonds are formed by high-temperature reaction, and the XRD result shows that SeS ₂ The amorphous structure is formed by the uniform bonding of S-Se, C-S and C-Se bonds to PAN.

2.3.2 selenium disulfide (SeS) ₂ -PAN) Positive electrode Material SEM

As shown in FIG. 2a as SeS ₂ And mixed and milled photo of PAN powder, seS ₂ As an orange-red powder, PAN is a white powder. FIG. 2b is SeS ₂ SeS after high temperature reaction with PAN ₂ Optical photographs of PAN sample powders. As can be seen, the color of the powder before the high temperature treatment was SeS ₂ The product is orange red, and after high-temperature heat treatment, the product is changed into black powder, and the SeS is illustrated by very obvious color change ₂ And PAN to form a composite material. FIG. 2c is SeS ₂ SEM image of PAN, from which the SeS can be seen ₂ PAN powder is an agglomerate of nanoscale particles forming a diameter of 2-5 μm, the primary particles being round. FIG. 2d higher magnification observation of SeS ₂ SEM images of PAN materials, seS can be found ₂ PAN material is spherical particles with a diameter of about 200nm and a uniform shape and size indicating SeS ₂ And PAN compounding was successful. FIGS. 3a to 3d are SeS ₂ X-ray Spectroscopy (EDS) of PAN Material, good coincidence of the signals of the three elements carbon, sulphur and selenium, indicating a homogeneous distribution of Se and S elements on the carbon substrate material, further indicating SeS ₂ And PAN are very uniformly combined.

2.3.3 selenium disulfide (SeS 2-PAN) cathode Material thermal weight loss

We are right to SeS ₂ Thermal stability of PAN Material was analyzed and the results are shown in FIG. 4 as SeS ₂ -thermogravimetric curve of PAN powder, rising from 25 ℃ to 800 ℃ at a rate of rise of 10 ℃/min. The results show that untreated PAN powder decomposes at around 300 ℃ with a weight loss of 64% at 800 ℃. SeS ₂ PAN starts to decompose at around 400 ℃ SeS ₂ And C-S bonds and C-Se bonds in PAN are broken, resulting in SeS ₂ PAN decomposition. See SeS ₂ Higher thermal decomposition temperature of PAN and better thermal stability. It can also be seen that SeS is observed at 800 ℃ ₂ The thermal weight loss of PAN material is 56%, which indicates SeS ₂ PAN and SeS in PAN Material ₂ The specific gravity of (a) is basically 1:1.

2.4 local high concentration electrolyte physical Property characterization

2.4.1 local high concentration electrolyte wettability characterization

As shown in FIG. 5a and FIG. 5d, the local high concentration electrolyte 1.2M LiFSI-DMC/HFE and the common ether electrolyte 1M LiTFSI +0.75M LiNO are respectively ₃ Optical photographs of the wettability test of the lithium foil surface with both electrolytes DOL/DME. Under the same condition, 15uL electrolyte is respectively dripped on the surface of the lithium foil, and obviously, the wettability of the ester local high-concentration electrolyte on lithium metal is better than that of the ether electrolyte. Similarly, as shown in fig. 5b and fig. 5e, for wettability tests of the two electrolytes on the surface of the separator, the electrolyte with ester local high concentration is dropped on the surface of the separator to wet the separator in a short time, while the wettability of the electrolyte with ether is slightly inferior to that of the electrolyte with local high concentration. Fig. 5c and 5f show wettability tests of two electrolytes on the surface of a copper foil, and similar to wettability on the surface of a diaphragm, ester local high-concentration electrolytes have good wettability on the surface of the copper foil. The excellent wettability of the local high-concentration electrolyte is mainly attributed to the DMC solutionCompared with DME solvent, the viscosity of the electrolyte is much lower, the viscosity directly influences the conductivity of the electrolyte, and the electrolyte has high viscosity and low conductivity. The viscosity is determined by the intrinsic property of the solvent and the concentration of the lithium salt, and the viscosity of the selected ester solvent DMC is lower than that of ether DME. The concentration of the lithium salt is diluted by the diluent, and the diluent can greatly reduce the overall concentration and further reduce the overall viscosity under the condition of not damaging the original solvation structure of the high-concentration electrolyte.

2.4.2 local high concentration electrolyte conductivity test

The conductivity is a very key index for judging the performance of the electrolyte, and the high conductivity is favorable for reducing the internal resistance of the battery and improving the cycling stability and the cycling life. Both stages used stainless steel to assemble stainless steel// stainless steel pair cells to test the impedance of both electrolyte systems as shown in figure 6a. The conductivity can be calculated by the formula δ = d/RS, where d is the membrane thickness 25 μm and S is the contact area 2cm ² And R is impedance. The impedance of the two electrolytes is 2.2 omega and 5.3 omega respectively, and the conductivity is calculated to be 5.633 multiplied by 10 ^-4 S/cm and 2.365X 10 ^-4 S/cm. As shown in fig. 6b, the conductivity of the local high-concentration carbonate electrolyte is doubled compared with that of the common ether electrolyte, and the viscosity of the ether solvent is high, which is not favorable for ion transport. And the thinner with lower donor number and dielectric constant in the local high-concentration electrolyte system can not be coordinated with the cation in the high-concentration electrolyte. The local high-concentration electrolyte system maintains the original solvate characteristics of the high-concentration electrolyte, improves the ionic conductivity and the wettability, and reduces the viscosity result to be consistent with the performance of the wettability test in the upper section.

2.4.3 local high concentration electrolyte chemical stability test

When the voltage reaches a certain value, the electrolyte can generate decomposition reaction, thereby losing the original performance and causing the collapse of the battery. The higher the cut-off voltage of the battery and the higher the corresponding capacity, the higher the voltage at which the decomposition reaction of the electrolyte occurs, the better the battery system. The electrochemical window is a common way to assess the electrochemical stability of the electrolyte. To test the electrochemical stability of the localized high concentration electrolyte, the cells were assembled from stainless steel gaskets and lithium foil and subjected to linear operation over a voltage range of 1-6VScanning (LSV). The decomposition of the electrolyte occurs with an increase in current. Supply current from 10 ^-6 A changes to 10 ^-5 A is defined as the occurrence of severe decomposition reactions and the potentials of the two electrolytes are recorded, yielding figure 7. It can be seen from the figure that the current of the ether electrolyte reaches 10 at 3.88V ^{- 5} A shows that the electrolyte has obvious decomposition reaction, and the carbonate locally high-concentration electrolyte reaches 10 at 4.13V current ^{- 5} A. It is evident that the local high concentration electrolyte decomposition voltage is higher than that of ethers. Therefore, the high-voltage stability of the local high-concentration electrolyte is better than that of the common ether electrolyte, and the working voltage of the lithium-sulfur battery can be met.

2.5 local high concentration electrolyte Li// Cu Battery testing

2.5.1 local high concentration electrolyte Li// Cu cell electrochemical test

Firstly, the common ether electrolyte 1M LiTFSI +0.75M LiNO is selected ₃ DOL/DME (volume ratio 1) with a local high concentration electrolyte of 1.2M LiFSI-DMC/HFE (volume ratio 1. The coulombic efficiency of the Li// Cu half-cell can be directly used for judging the lithium metal plating/stripping efficiency and the active lithium loss of each cycle, and the coulombic efficiency is also an important index for judging the cycle life and the cycle stability of the cell.

The circulation performance data of the Li// Cu half-cell of the two electrolytes are shown in figure 8a, and the current density of the Li// Cu half-cell is set to be 0.5mA/cm ² The deposition amount is 1mAh/cm ² . The Li// Cu half-cell cycles are performed 5 times at 0-1V with a small current of 50 muA, so as to remove impurities on the surface of the copper foil with the small current. 1M LiTFSI +0.75M LiNO ₃ The first CE of Li// Cu half-cell of-DOL/DME electrolyte after 5 times of impurity removal was 94.6%, while the first CE of 1.2M LiFSI-DMC/HFE electrolyte of carbonate type local high concentration electrolyte was 95.9%, it can be seen that the first CE of local high concentration electrolyte is slightly higher than that of common ether electrolyte, and the stable average CE is 98.98%, also higher than that of common ether electrolyte (98.6%). As shown in FIG. 8b and FIG. 8c, the charging and discharging curves of the 10 th cycle of the two electrolytes show that the overpotential of the conventional ether electrolyte system is high, up to 71mV, and higher than that of the carbonate local high concentration electrolyte (46)mV), which is consistent with the coulombic efficiency laws for both electrolytes. When lithium is deposited on the surface of copper, the polarization is large, the side reactions are more, the impedance is large, and the CE of the corresponding carbonate local high-concentration electrolyte is higher than that of the common ether electrolyte. The high CE and low overpotential exhibited by the locally high concentration electrolyte is attributed to the locally high concentration electrolyte having the characteristics of a high concentration electrolyte and low viscosity per se and good wettability with lithium metal. On the one hand, the solvates in the electrolyte exist mainly in the form of contact ion pairs and aggregated ion pairs in the electrolyte. The unique solvation structure has few free solvent molecules, and improves the oxidation stability of the electrolyte. The change in solvating structure results in a change in the Lowest Unoccupied Molecular Orbital (LUMO) energy level composed of anions, enhancing the reduction stability of the high concentration electrolyte system. At the same time, the interfacial reaction will be dominated by anions rather than solvents. Therefore, the anion will be preferentially reduced and decomposed to form SEI with more inorganic components to inhibit dendritic growth; on the other hand, low viscosity, good wettability, low dielectric constant and excellent coordination ability are the effects of diluent addition.

FIG. 8d is an AC impedance spectrum of different Li// Cu cells. The ac impedance spectrum typically has two regions, the half-circle corresponding to the high and medium frequency regions, primarily reflecting the diffusion of lithium ions through the SEI and charge transfer impedance, and the diagonal line corresponding to the low frequency region ion diffusion impedance. It can be seen from the figure that the semi-circular arc of the local high-concentration electrolyte is smaller than that of the common ether electrolyte, and the impedance of the local high-concentration electrolyte is much smaller than that of the common ether electrolyte by 20 Ω. The local high-concentration electrolyte has high conductivity and small overpotential in the circulation process, and the local high-concentration electrolyte has small corresponding polarization and is beneficial to long circulation of the battery.

Fig. 9 and 10a to 10f show the cycle curve and the charge and discharge curve at different current densities. At a low current density of 0.25mA/cm ² The deposition amount is 0.5mAh/cm ² The lower localized high concentration electrolyte exhibited a higher stable average CE of about 98.9%. Increasing the current density to 0.5mA/cm ² And the deposition amount is 1mAh/cm ² The average CE was stabilized at about 98.9%, and the curves were charged and discharged from FIGS. 10a and 10bThere was some slight difference in overpotential at the two current densities obtained by the wire of 38mV and 53mV, respectively. It can be seen that the current density was 0.5mA/cm ² And the average CE value difference is almost the same when the circulation is stable under the current density below, which shows that the local high-concentration electrolyte has better compatibility with the lithium metal cathode under the low current density. The current density of the lithium-sulfur battery without negative electrode due to subsequent high loading is about 0.5mA/cm ² And the deposition amount is 3mAh/cm ² Therefore, a cyclic stability test of a small current density and a large deposition amount was particularly performed, and the stable cyclic average CE was 99.1%. FIG. 10c shows that the overpotential is 48mV, which is slightly higher than the CE at low deposition amount, and the overpotential is 1mAh/cm ² The deposition amount is slightly smaller. This is because the size of lithium decreases with increasing current density and overpotential, and at the same current density, the size of lithium increases with increasing amount of deposited lithium, especially more pronounced at low current densities, and an increase in size of lithium indicates a decrease in the amount of dead lithium and lithium dendrites formed and a more uniform lithium deposition. Further continuously increasing the current density to 1mA/cm ² And the deposition amount is 1mAh/cm ² The stable average CE was about 98.34% compared to 0.5mA/cm ² The following small current density stable average CE slightly dropped and slightly increased the overpotential by 100mV, as shown in FIG. 10d. With further increase in current density to 2mA/cm ² Deposition amount of 1mAh/cm ² And 3mA/cm ² Deposition amount of 1mAh/cm ² At this time, the Li// Cu half cell cycle life was only 60 and 40 times at both current densities, but still maintained a considerable coulombic efficiency of 97.7% and 97.6% over the cycle life, but increasing the current density resulted in a severe overpotential increase of polarization to 204mV and 170mV as shown in fig. 10e and fig. 10 f. The battery is damaged under a large current density due to a large amount of dead lithium generated by polarization increase during a current density increasing cycle, and electrolyte consumption is accelerated, thereby causing battery failure.

2.5.2 local high concentration electrolyte Li// Cu battery lithium deposition morphology characterization

After the cycle performance of the two electrolytes is researched, in order to further clarify the difference of the two electrolytes, the appearance of the deposited lithium metal is photographed by using 1mA/cm ² Current density of 1mAh/cm ² SEM of lithium negative electrodes after 50 cycles of both electrolytes, the results are shown in fig. 11a to 11 b. In fig. 11a, which is an SEM of a lithium metal negative electrode after a normal ether electrolyte is circulated, it can be seen that the surface of lithium metal is cracked to be uneven, and the crystal grains are fine and the appearance looks uneven. Compared with the carbonate-based local high-concentration electrolyte in fig. 11b, after 50 cycles, the surface of the lithium negative electrode is blocky, and has fewer gaps and a flat and dense appearance. The smooth and compact shape can reduce the side reaction of lithium metal and electrolyte, almost all the coordination of the solvent and ions in a local high-concentration electrolyte system is realized, and almost no free solvent exists in the electrolyte. The decrease in the activity of the coordinating solvent can suppress the continuous decomposition of the solvent on the surface of the negative electrode. Thereby improving CE, improving cycle life and preventing excessive irregular lithium dendrite growth to improve safety of the lithium metal battery.

2.6 testing of the Positive electrode Performance of a locally high-concentration electrolyte

2.6.1 local high concentration electrolyte Anode electrochemical Performance

A good electrolyte ensures high CE and also considers the compatibility of the positive electrode, so that two electrolytes are used for assembling Li/SeS ₂ PAN battery test positive electrode suitability. FIG. 12a shows a low loading (1-3 mg/cm) ² ) Li// SeS of ₂ The cycle performance of the PAN battery shows that the first discharge capacity of the common ether electrolyte is 720mAh/g, the capacity is basically stable in the first 40 cycles, the capacity tends to decline from the 40 th cycle to slightly increase from the 50 th cycle, but the overall capacity shows a slow decrease trend. The cause of the capacity decrease is mainly the dissolution of polysulfide in ether electrolyte of lithium sulfur batteries, although SeS ₂ PAN has a faster reaction rate than SPAN (elemental sulfur complexed with polyacrylonitrile), but still has a small amount of polysulfide dissolved. The local high-concentration electrolyte shows more stable capacity at low-load first discharge capacity of 684mAh/g in long circulation, but the slow reduction is also caused because of the carbonate solvent and SeS ₂ Very little free SeS in the PAN cathode material ₂ Nucleophilic addition to nucleophilic sulfide anionAnd (4) reacting. FIG. 12b is a high loading (6-8 mg/cm) ² ) Li// SeS of ₂ Cycling performance of PAN batteries, see SeS at high load ₂ The compatibility of the PAN anode and the common ether electrolyte is poor, the capacity is greatly attenuated, and on one hand, polysulfide is dissolved in the electrolyte and shuttled to the lithium metal cathode to generate side reaction with the lithium metal; another important reason is the insulating elemental sulfur and Li ₂ S and Li ₂ Deposition of Se causes deterioration of the electrode conductive contact, and thus the active material utilization rate is greatly reduced. And the first discharge capacity of the local high-concentration electrolyte is 632mAh/g under the condition of high load, the second circulation capacity is 500mAh/g, and the stable capacity of about 481mAh/g is kept in the circulation process, although the situation of slow reduction exists, compared with the circulation stability of the ether electrolyte, the local high-concentration electrolyte has more stable performance, which is attributed to the fact that the reaction kinetics at the interface of the local high-concentration electrolyte is improved.

Fig. 12c is an ac impedance spectrum of two electrolyte systems under high loading, and it can be seen that the impedance after the circulation of the common ether electrolyte is 169.4 Ω, while the impedance after the circulation of the local high concentration electrolyte of carbonates is only 47.9 Ω and the curve has two arcs, which indicates that a Solid Electrolyte Interface (SEI) is formed. The small polarization and rapid SEI formation of the localized high concentration electrolyte further demonstrate the faster reaction rate of the localized high concentration electrolyte and the improved interfacial reaction kinetics due to the low viscosity, high conductivity and good wettability of the localized high concentration electrolyte. Fig. 12d shows the rate capability of two electrolytes, the current density is 0.1A/g, 0.2A/g, 0.5A/g, 1A/g, 2A/g, 5A/g, the specific capacity is 553mAh/g, 543mAh/g, 532mAh/g, 521mAh/g, 504mAh/g, 463mAh/g, when the current density recovers to 0.1A/g specific capacity and then returns to 550mAh/g, it shows that the capacity can be kept at an appreciable level under different rates, and after different rate cycles, the initial current density is recovered, the capacity can be kept at an initial capacity level, and excellent rate capability is shown.

As shown in FIG. 13 is SeS ₂ Cyclic voltammograms of PAN anodes at a scan rate of 0.4mV/s under a localized high concentration electrolyte system. In the negative direction of the sweeping processIn the case of SeS, the reduction peak was 1.66V, which is the reduction stage ₂ To discharge product Li ₂ S and Li ₂ The process of Se conversion. The oxidation peak appears at 2.28V and is Li ₂ S and Li ₂ Conversion of Se back to SeS ₂ The process of (1). In the continuous scanning process, the peak symmetry and the contact ratio are good, and the method embodies that the reversibility of the anode is high in a local high-concentration electrolyte system, and the reaction kinetics is fast.

2.6.2 Li//SeS ₂ -PAN Positive electrode morphology characterization

In order to observe the failure reason of the two systems of the electrolyte anodes after circulation, after circulation is carried out under the current density of 0.1A/g, the circulating batteries are disassembled in a glove box, and then SeS is carried out ₂ PAN positive electrodes were soaked in the respective electrolyte-system solvents in order to dissolve the lithium salts on the surface of the positive electrodes, followed by air-drying SEM. SeS after cycling at high loading using a common ether electrolyte, as shown in FIG. 14a ₂ The surface of the PAN positive electrode is cracked, and the size of the crack reaches about 10 mu m. Fig. 14c is a SEM of a lower magnification ether electrolyte system and an optical photograph of the electrode with a cracked surface. The electrode structure is damaged to a great extent, so that the internal conductive contact of the electrode is poor, the utilization rate of active substances is reduced, and the rapid attenuation life of the battery capacity using the ether electrolyte is short when high load is used. And FIG. 14b is the SeS after cycling at high loading using localized high concentration electrolyte ₂ The surface of the PAN positive electrode is provided with rugged particles, but the surface is not obviously cracked, so that the structural collapse of the positive electrode material is not caused. Fig. 14d is an SEM and optical photograph of a low magnification local high concentration electrolyte system, where only a few fine cracks of the positive electrode were observed after cycling, illustrating that the integrity of the positive electrode structure was maintained stable at the interface during local high concentration electrolyte cycling. And the analysis of the scanning result of the anode after circulation is consistent with the circulation performance.

2.7 this part summary

This section explores the application of SeS to lithium-sulfur batteries ₂ -carbonate based localized high concentration electrolyte system of PAN positive electrode material. The added diluent HFE does not change the solvation structure of the lithium salt in the initial solvent, but can be charged with a high concentration of lithiumThe solvent and the lithium ion solvate in the electrolyte system are mixed and dissolved, and the addition of the diluent does not influence the coordination of the initial lithium salt and the solvent in the high-concentration electrolyte system, and can greatly reduce the overall concentration of the electrolyte, so that the viscosity of the electrolyte is reduced while the characteristics of the high-concentration electrolyte are maintained, and the cost is reduced. Furthermore, the DMC solvent is strongly coordinated to the lithium salt in the electrolyte, and there is not much free solvent in the electrolyte. The reduction in the solvent coordination activity can suppress the continuous occurrence of the side reaction of the solvent in the lithium metal negative electrode. The experiment explores the electrochemical performance and the intrinsic property of the carbonate local high-concentration electrolyte, and summarizes the following conclusions:

(1) The conductivity of the self-developed carbonate local high-concentration electrolyte is 5.633 multiplied by 10 ^-4 S/cm, and the wettability and high-voltage stability of the positive electrode, the negative electrode and the separator are also very excellent compared with ether electrolyte.

(2) Li// Cu half cells tested for compatibility with lithium metal, cells using localized high concentration electrolyte at current density of 0.5mA/cm ² Deposition amount of 1mAh/cm ² The lower cycle 100 times had a high coulombic efficiency of 98.9%. Meanwhile, the overpotential is 46mV, which is much smaller than that of ether electrolyte (71 mV). At a current density of 0.5mA/cm ² And the deposition amount is 3mAh/cm ² The lower-cycle stable average coulombic efficiency reaches 99.1%, the cycle performance is more excellent under a large deposition amount, and the possibility is provided for further development of the lithium metal battery without the negative electrode.

(3) Novel carbonate local high-concentration electrolyte in lithium-sulfur battery and SeS ₂ The compatibility of the PAN anode is that the PAN anode is cycled for 50 times under the current density of 0.2A/g, the first discharge capacity is 632mAh/g, the second discharge capacity is 500mAh/g, the capacity retention rate is 71.3%, and the capacity retention rate is calculated from the second circle to reach 90%. The battery rate performance using the local high-concentration electrolyte is also excellent, and the impedance is small.

The third part of positive electrode lithium supplement research is applied to the lithium-sulfur battery without the negative electrode

3.1 introduction to

The energy density of the traditional lithium ion battery is lower than 300Whkg ^-1 Far from meeting the requirements of emerging markets such as electric automobiles and the like on battery energyStandards and requirements for density. Lithium metal batteries, which are newly discovered and considered as the negative electrode of the next-generation secondary lithium battery, are theoretically higher in energy density than lithium metal batteries and are also theoretically possible, and are currently the highest energy density lithium metal battery system. The removal of the negative electrode material will result in an energy density of the lithium metal battery exceeding 400Wh/kg at the battery level, compared to conventional lithium metal batteries. But with the attendant lack of a negative electrode to provide a source of lithium, the cycle life of a lithium metal battery without a negative electrode presents significant challenges. The development of positive electrode lithium-rich materials indicates a feasible way for the lithium source scarcity problem of lithium metal batteries without negative electrodes. Wu et al propose a sodium-rich cathode with a super-ionic conductor structure that supplements an excess sodium source to achieve superior performance in a sodium metal battery without a negative electrode. Kim et al, by deeply sodium-treating the SPAN positive electrode material, through a simple electrochemical activation operation, make the SPAN positive electrode material exhibit a high reversible capacity. Also, the harsh conditions and safety issues of lithium metal battery assembly due to poor air/moisture stability of lithium metal limit the transition of lithium metal batteries from the laboratory to the commercialization thereof. Lithium-rich materials are lithium compounds that have better air/water stability than lithium metal. The lithium alloying reaction can also improve the air stability of the electrode, and meanwhile, the lithium alloy also has the advantages of rapid lithium ion diffusion dynamics, low nucleation barrier and the like. Xu et al use Li _x Sn alloy anode for improving cycle stability of battery and Li _x Sn alloys possess good air/water stability.

The high energy density of lithium metal batteries without negative electrodes has been attracting researchers' attention, but the problems facing them are also various, with short cycle life and low coulombic efficiency being the two most formidable challenges. Due to the inevitable presence of Li on the surface of the lithium foil ₂ O、Li ₂ CO ₃ And a passivation layer consisting of LiOH, hydrocarbons and carbonates, leads to uneven distribution of the affinity of the surface for lithium ions, which is not favorable for uniform nucleation and growth of the Li coating. The irreversible loss of active lithium cycling on the copper foil is lower than that on the lithium foil, thereby realizing higher capacity retention rate of the lithium metal battery without the negative electrode.And Lin indicates that the capacity of the lithium metal battery without the negative electrode decays rapidly in the first ten cycles, so that the lithium is supplemented in the positive electrode and the active lithium lost in the first ten cycles is compensated by discharging to the negative electrode for the first time, and the method has great significance for improving the lithium metal battery without the negative electrode. The high specific capacity of lithium-sulfur batteries has also attracted the attention of researchers, and non-negative lithium-sulfur batteries combining the high specific capacity and the non-negative lithium-sulfur batteries have great potential, but few results are reported at present.

The part uses self-developed carbonate-based local high-concentration electrolyte, seS ₂ -a PAN positive electrode material, performing a deep lithiation operation on the positive electrode. And selecting the optimal lithium supplement amount and exploring the influence rule of the lithium supplement amount on the cycle stability by adopting the electrochemical performance so as to manufacture the stably-cycled long-life lithium-sulfur battery without the negative electrode.

3.2 Positive electrode lithium supplement Experimental section

The lithium supplement of the positive current collector is to firstly assemble the well-assembled Li// SeS ₂ First discharge of PAN battery to 0.28V, with the aim of SeS ₂ Deep lithiation of PAN positive electrodes. Then, the discharge is continued by using a small current of 50 mu A to carry out 0.5mAh/cm ² ，1mAh/cm ² ，2mAh/cm ² ，3mAh/cm ² The four deposition amounts compensate lithium on the aluminum current collector with a small current aimed at forming a lithium aluminum alloy on the current collector. And then carrying out normal charge-discharge circulation of 1-3V.

3.3 optimization of the amount of lithium added to the positive electrode

Lithium supplement at positive electrode current collector, substantially at SeS ₂ -forming a PAN positive electrode to form a lithium aluminium alloy on a current collector after deep lithiation. The lithium metal and aluminum undergo an alloying reaction at around 0.28V, but the lithium metal undergoes a volume expansion when alloyed with the aluminum current collector. The current collector structure will collapse due to the violent volume expansion, so the amount of the lithium supplement is measured in advance to select a proper amount of the lithium supplement. Four lithium supplement amount tests are respectively carried out for 0.5mAh/cm ² ，1mAh/cm ² ，2mAh/cm ² ，3mAh/cm ² And high loading of the chips (6-8 mg) were used in unison. FIG. 15 shows the cycle performance of four lithium supplement amounts, and it can be seen that the lithium supplement amount is 0.5mAh/cm ² First discharge capacity1088mAh/g, then 40 cycles average capacity at 575mAh/g capacity remains steady with no significant drop. When the lithium supplement amount is 1mAh/cm ² The first discharge capacity was 1160mAh/g, and then the 40-cycle average capacity was stable at 570mAh/g with no significant drop. Continuously increasing the lithium supplement amount to 2mAh/cm ² The first discharge capacity of 1391mAh/g is found, which proves that the lithium supplement is successful, but the capacity begins to fluctuate around 25 cycles, and the electrode structure is unstable due to excessive lithium supplement. The lithium supplement amount is increased to 3mAh/cm ² It is found that the first discharge capacity is 1363mAh/g, the average circulating capacity is only 494mAh/g, and the capacity begins to fluctuate around 26 cycles.

In order to more intuitively see the relationship between the amount of lithium supplement and the battery failure and observe the voltage change in the charging and discharging curve after lithium supplement, FIG. 16a shows that lithium supplement is 0.5mAh/cm ² The charging and discharging curves of (1) can show that the charging capacity of the second circle is 927mAh/g, the discharging capacity of the second circle is 605mAh/g, the discharging capacity of the 40 th circle is 561mAh/g, and the capacity retention rate is 92.7%. Lithium supplement of 0.5mAh/cm ² Proves that the lithium supplement of the anode can improve the SeS ₂ Feasibility of this measure of reversible capacity of the PAN positive electrode. FIG. 16b shows that lithium is supplemented at 1mAh/cm ² The charge-discharge curve shows that the charge capacity of the second circle is 1160mAh/g, the discharge capacity of the second circle is 603mAh/g, the discharge capacity of the 40 th cycle is 566mAh/g, and the capacity retention rate is 93.8 percent. FIG. 16c shows that lithium is supplemented by 2mAh/cm ² The charge-discharge curves of (1) show a charge capacity of 1170mAh/g and a discharge capacity of 588mAh/g in the second cycle, although 20 cycles before the cycle show 1mAh/cm of lithium supplement ² The cycling behavior was similarly stabilized, but the capacity began to be unstable after only 25 cycles. The 40 th cycle charging and discharging curve is branched, the subsequent charging capacity is multiplied, the charging and discharging capacity is greatly fluctuated, and the coulomb efficiency is unstable, so that the battery can not normally run. This is presumed to occur because a large amount of lithium metal enters the SeS during the lithiation in the positive electrode discharge ₂ PAN positive electrode and aluminum current collector cause the positive electrode to be fragile in structure, and the electrode is damaged due to large volume change of the current collector, and then the electrode is not damagedThe broken charge-discharge cycle and the continuous change of the volume lead to the collapse of the anode structure, so that the service life of the battery is greatly reduced. FIG. 16d shows that lithium is supplemented by 3mAh/cm ² The charging and discharging curve of (2) shows that the charging capacity of the second circle is 1363mAh/g and the discharging capacity of the second circle is 548mAh/g. Simultaneously supplementing lithium by 2mAh/cm ² Similarly, at cycle 40, it can be seen that the charging curve has been jagged, resulting in a cell with a charging capacity far outside the normal capacity range, a sudden drop in coulombic efficiency, and a cell with a collapsed electrode structure that cannot be cycled normally.

In order to further select a proper lithium supplement amount, the Li-Al half cell is assembled, lithium-aluminum alloy with different amounts is generated by discharging at a small current of 50 muA, then the cell is disassembled and soaked in a DMC solvent for about 3min, and the SEM of the surface of the aluminum foil is shot in an air drying mode. Fig. 17a is an SEM of a planar aluminum foil without any treatment of the aluminum foil for comparison of the volume change and surface flatness of the lithium aluminum alloy. As shown in FIG. 17b, 1mAh/cm of lithium was added ² The SEM of the surface of the aluminum current collector of (a) shows that the surface has fine cracks, but the whole has no tendency of significant volume expansion or structural collapse. And FIG. 17c shows the lithium supplement of 2mAh/cm ² The SEM of the surface of the aluminum current collector of (a) shows that the volume expansion cracking is severe, and aluminum foil can also be seen. FIG. 17d shows that lithium is supplemented by 3mAh/cm ² The SEM of the surface of the aluminum current collector of (a) shows that the lithium aluminum alloy expands in volume and cracks very severely and the aluminum current collector is not visible, which is consistent with the previous electrochemical performance, and excessive volume expansion results in a reduced cycle life. The four lithium supplement amounts are combined, and the final optimal value is 1mAh/cm in consideration of two conditions of battery cycling stability and the maximum lithium supplement amount ² The amount of lithium supplement of (a) is a main research target.

3.4 Positive electrode lithium-supplement cell Performance

3.4.1 lithium air-filling and chemical stability of the positive electrode

The lithium metal battery has poor stability in air and water due to its high activity, so that the assembling conditions and safety of the lithium metal battery become very harsh, which greatly limits the way the lithium metal battery goes from the laboratory to the commercialization. When the anode adopts an aluminum current collector, the aluminum current collector has alloying reaction with lithium metal at 0.28V, and the lithium aluminum alloy is compared with simple substance lithiumThe chemical property is stable, so that the lithium source is supplemented to the positive electrode in the form of lithium-aluminum alloy safely and reliably. To test the air stability of the positive electrode for lithium supplementation, 1mAh/cm was deposited on the aluminum and copper current collectors ² The lithium metal is disassembled, the deposited copper foil and aluminum foil are placed in the air for preservation for 3, 6, 12 and 24 hours and then are reassembled, and the lithium metal is charged to 1V to calculate the coulombic efficiency.

As shown in fig. 18a, which is an optical photograph after the disassembly of the Li// Cu cell, and fig. 18b, which is an optical photograph after the disassembly of the Li// Al cell, 0h can be seen that the lithium metal of the copper foil surface exhibits a metallic luster and the coulombic efficiency is 95.7%. The lithium aluminum alloy presents 94% of bright silver coulombic efficiency of the aluminum foil, and it can be seen that under the condition of not contacting air, the coulombic efficiency of lithium metal and the lithium aluminum alloy is almost the same, even the lithium metal is higher, so that the lithium metal and the lithium aluminum alloy are not greatly different under a closed oxygen-free and water-free environment. As the time of exposure to air increased, the lithium metal color of the copper foil surface gradually faded out at 3h and a thin gray film was formed on the intermediate surface, the coulombic efficiency also decreased to 68.9%. The aluminum foil shows the same bright silver metallic color as the initial aluminum foil, and the coulombic efficiency is 93 percent, and basically does not decrease, so that the stability of the lithium aluminum alloy in the air is relatively better. When the copper foil is kept still for 6 hours in the air, the lithium metal on the copper foil is gray, the coulombic efficiency is reduced to 41.2 percent, obviously, the lithium metal and gases such as oxygen in the air and the like are subjected to chemical reaction, the lithium metal does not have a simple substance form in the nature and is subjected to chemical reaction with almost all gases in the air, and the reaction product of the lithium metal and the oxygen in the air has Li ₂ O and Li ₂ O ₂ LiOH, a reaction product with water, and CO ₂ Reaction to form Li ₂ CO ₃ Even with N ₂ Reaction to form Li ₃ And N is added. The aluminum current collector still presents bright silvery metallic luster, and the coulombic efficiency is slightly reduced when the coulombic efficiency is 87.1% compared with that of the aluminum current collector for 3 hours. After 12h, the surface of the copper foil is gray black, and the coulombic efficiency is reduced to 0.5%, which means that the surface of the copper foil is not provided with an active lithium source capable of performing electrochemical reaction. The aluminum foil keeps bright silver metallic luster and does not change much, the coulombic efficiency is 79.8 percent, and compared with lithium metal and lithium-aluminum alloyThe air stability is much better. And when 24h, active lithium is completely removed, the copper foil is basically exposed and is orange red, and a lithium metal extraction experiment cannot be carried out. The aluminum foil has little shade after 24 hours, the color is kept as it is, and the coulombic efficiency is kept at a high level of 78.5%. Fig. 19 is a graph comparing the coulombic efficiencies of the two batteries with different air retention times, and it is found that the coulombic efficiency of the lithium aluminum alloy is much higher than that of the lithium metal coulombic efficiency except that the lithium aluminum alloy is not in contact with air, so that the air stability of the lithium aluminum alloy formed by lithium supplement of the positive electrode is better than that of the lithium metal, and the feasibility and the advantage of lithium supplement of the positive electrode are further proved.

Chemical stability is also an important indicator for evaluating the compatibility of electrolytes with electrodes. Chemical stability, i.e., the spontaneous chemical reaction at the interface when the electrolyte and electrode materials are in direct contact, measures whether the interface undergoes further decomposition when the cell is operated within a certain voltage window. By depositing 1mAh/cm on equal area copper and aluminum current collectors ² Lithium metal, left standing for 15 days, was subjected to a lithium metal extraction experiment and tested for electrochemical stability as shown in fig. 20a and 20b. FIG. 20a is a graph showing the charge and discharge curves of Li-Cu cell before and after standing, tested with 1mAh/cm of copper surface deposition ² The lithium metal can be extracted by 0.7925mAh, and the coulombic efficiency is 79%. The initial coulombic efficiency of the Li-Cu battery is 96%, and the coulombic efficiency of the lithium source lost due to actual reaction after being applied and kept stand for 15 days is divided by the initial coulombic efficiency, and the calculated value is 82%. FIG. 20b is a charge and discharge curve of Li-Al battery before and after standing with aluminum current collector deposition of 1mAh/cm ² The lithium-aluminum alloy is generated, the extraction rate is 0.8254mAh, and the coulombic efficiency is 82%. The initial coulombic efficiency of the Li-Al battery is 92 percent, and the actual self-discharge coulombic efficiency is calculated to be 89 percent and is 7 percent higher than that of lithium metal. Meanwhile, the charging platform of the Li-Cu battery rises sharply after standing for 15 days, and the curve can be seen to have fluctuation in the initial stage. It is known that the reaction of lithium metal with the electrolyte during the period of standing causes an increase in the internal resistance of the battery. And the change of the charging platform of the Li-Al battery after standing for 15 days is small, which indicates that the side reaction between the lithium-aluminum alloy and the electrolyte is slight. By comprehensive comparison, the chemical stability of the lithium-aluminum alloy is better than that of lithium metal.

3.4.2 electrochemical Properties of the Positive electrode for lithium supplement

Low loading Li// SeS assembled first using a local high concentration electrolyte of carbonates ₂ -a PAN battery. Lithium is not supplemented and supplemented by 1mAh/cm ² Cycling performance of the cell as shown in fig. 21a, the cell without lithium supplement showed a first discharge capacity of 594mAh/g, a second discharge capacity of 505mAh/g, and a discharge capacity of 513mAh/g after 90 cycles, with stable capacity. The first discharge capacity of the battery for replenishing lithium by 1mAh is 1393mAh/g, the second discharge capacity is 596mAh/g, and the high discharge capacity of 560mAh/g is still obtained after the battery is circulated for 100 times. From the general rule, the low load amount of Li// SeS ₂ The PAN battery has good cycling stability in lithium supplement and lithium non-supplement due to the excellent performance of the carbonate local high-concentration electrolyte. At the same time, it can be seen that SeS for the lithium supplement operation ₂ Ultra-high reversible capacity during PAN positive cycling, indicating that deep lithiation of the positive electrode can serve to activate SeS ₂ The effect of the PAN positive electrode. High loading of Li// SeS assembled using localized high concentration electrolytes of carbonates ₂ PAN battery, no lithium supplement and 1mAh/cm lithium supplement ² The cycle performance of (c) is shown in fig. 21 b. The first discharge capacity of the battery without lithium supplement is 632mAh/g, the second discharge capacity is 500mAh/g, the capacity is maintained at about 500mAh/g in a circulating mode, the capacity is slowly reduced, the capacity begins to be rapidly reduced after the 45 th circulating discharge capacity reaches 465mAh/g, the capacity reaches 406mAh/g after the 70 th circulating discharge capacity, the capacity is slowly reduced to 90 th circulating capacity and only 386mAh/g is obtained, and the capacity retention rate is 77.3% calculated from the second discharge capacity. Batteries without lithium supplementation exhibit an absolute low capacity and poor cycling stability. The reason for the capacity reduction is that the sulfur anode with high loading capacity has a common problem, the internal conductive contact of the electrode is poor due to the slow reaction kinetics of the thickness of the electrode film and the continuous change of the volume of the electrode in the circulation process, the utilization rate of the sulfur is low, the polarization of the lithium metal cathode is serious in the continuous circulation process, and the capacity reduction is caused by the continuous accumulated impedance. The first discharge capacity of the battery for replenishing lithium by 1mAh is 1129mAh/g, the second discharge capacity is 557mAh/g, the tenth cycle discharge capacity is reduced to 529mAh/g, and the reduction of the ten cycle capacity is presumed to be carbonic acidThe ester solvent and lithium polysulfide react with each other at an early stage and a nucleophilic reaction surface forms a positive electrode electrolyte interface (CEI) to prevent the lithium polysulfide from continuously diffusing. However, since lithium supplementation improves interfacial reaction kinetics, a flat and robust CEI cannot be generated at once during cell cycling, and several cycles are passed to generate a consolidated CEI. Then, the capacity tends to be stabilized to a discharge capacity of 508mAh/g at the 90 th cycle, the capacity retention rate is 91.1% calculated from the second cycle, and is 13.8% higher than the Coulomb efficiency without lithium supplement, and excellent cycle stability is also shown under the condition that the absolute value of the discharge capacity is high. After lithium is supplemented, the reaction kinetics of the anode is improved, and a large amount of lithium ions can rapidly react and diffuse.

FIG. 21c shows that lithium is added by 1mAh/cm ² The current density is 0.1A/g, 0.2A/g, 0.5A/g, 1A/g, 2A/g and 5A/g. The average capacity without lithium supplement is 572mAh/g, 524.4mAh/g, 495mAh/g, 474mAh/g and 509mAh/g when the current density is restored to 0.1A/g. 1mAh/cm of lithium supplement ² The average capacities of the capacitors are 670mAh/g, 548mAh/g, 533mAh/g, 512mAh/g and 477mAh/g respectively, and the capacities are 561mAh/g when the current density is recovered to 0.1A/g. It can be seen that the capacity is still high at high current density after lithium supplement, but the battery without lithium supplement cannot be directly restored to the normal level when the low current is restored after the high current density circulation, and the total capacity without lithium supplement is low. In conclusion, the rate performance of 1mAh for lithium supplement is excellent. The excellent performance of rate capability may be due to improved SeS by lithium supplementation ₂ Kinetics of electrochemical reaction of PAN positive electrode with locally high concentration electrolyte of carbonate type, which will be discussed in detail in the next section of the relevant study. FIG. 21d shows that lithium is added by 1mAh/cm ² The alternating current impedance spectrogram of the lithium battery is not supplemented, the battery impedance of the lithium battery is 40.06 omega, and the lithium is supplemented by 1mAh/cm ² The battery impedance of (2) is only 21.59 omega, and the SeS can be reduced by lithium supplement ₂ The internal resistance of the PAN anode and the impedance value of the lithium supplement battery are just consistent with the cycle life and the stability of the PAN anode.

3.4.3 mechanism of improving stability by lithium supplement of positive electrode

In the upper section, after lithium supplement is found, the capacity value of the positive electrode is improved, and more importantly, the lithium supplement activation capacity is improved, so that the cycling stability is improvedAnd (5) effect. This effect occurs because lithium supplementation improves the kinetics of the electrochemical reaction taking place. To investigate the reason further, the lithium ion diffusion coefficient (D) was calculated _Li+ ) The reaction kinetics is proved to be improved, the lithium ion diffusion coefficients under different discharge states are further evaluated based on EIS results, an EIS low-frequency area is used for reacting lithium ion diffusion, and the lithium ion diffusion coefficients can be calculated through low-frequency area impedance, and the formula is as follows:

D _Li ＝1/2{[Vm/(FAσ _w )]dE/dx} ² ；

vm-molar volume (196) in formula (lb); F-Faraday constant (9.65X 10) ⁴ ) (ii) a Area of A-grade tablet (1.1304 cm) ² )；σ _w Is the Warburg coefficient; from the low frequency region Z' -omega ^-1/2 The graph shows that omega is the angular frequency of the low frequency region, and the Warburg coefficient of the low frequency region can be obtained by the following formula: z' = R + σ _w ω ^-1/2 (ii) a The dE/dx can be obtained from the slope of the discharge curve at 25%, 50%, 75%, and 100%,4 depths of discharge, respectively.

FIGS. 22a and 22b are Z' -omega of low frequency region of 1mAh without lithium supplement and with lithium supplement at different depth of discharge ^-1/2 The graph shows that the slope reflects the Warburg coefficient, the Warburg coefficients of 1mAh lithium supplement at four discharge depths are all higher than those of batteries without lithium supplement, and the corresponding lithium ion diffusion coefficients are also higher than those of batteries without lithium supplement measures. FIG. 22c shows the lithium ion diffusion coefficients of the four states of 25%, 50%, 75% and 100% calculated from EIS, regardless of the average lithium ion diffusion coefficient without lithium supplementation or the lithium ion diffusion coefficient (D) at each depth of discharge _Li ＝6.871×10 ^-9 ) All are lower than the lithium ion diffusion coefficient (D) of 1mAh for lithium supplement _Li ＝5.768×10 ^-8 ) This indicates that the lithium ion is in the SeS for lithium supplement ₂ The reaction rate on the PAN electrode is fast, and the lithium supplement is proved to improve the reaction kinetics. The rapid reaction kinetics proves the excellent cycle performance of the upper-section lithium supplement battery.

In order to further understand the reason of the lithium supplement measures for improving the circulation stability, the lithium supplement is not carried out and the lithium supplement is 1mAh/cm ² High load amount of SeS ₂ Overpotential of the PAN electrode in a local high concentration electrolyte of carbonates. As figure 22d for capacity takingOver-potential of quantitative median analysis without lithium supplement SeS ₂ The overpotential of the PAN anode is 290mV, and the discharge platform reduces the internal resistance and increases the kinetic difference. SeS for supplementing lithium by 1mAh ₂ PAN Positive electrode overpotential 260mV less than non-lithium supplement SeS ₂ PAN positive electrode, and the larger the capacity is, the larger the overpotential difference is, further indicating that lithium is supplemented by 1mAh/cm ² SeS of ₂ The PAN anode has small polarization in the battery under the condition of high load capacity, and the lithium ion reaction rate is high, so that the cycling stability under the condition of high load capacity is improved.

3.5 non-negative pole lithium sulfur battery

3.5.1 non-negative electrode lithium Sulfur Battery Assembly

The self-developed high coulombic efficiency and lithium supplement of the carbonate-based local high-concentration electrolyte are utilized to improve the capacity and the reaction kinetics, and the non-negative electrode lithium-sulfur battery with maximized specific energy is assembled. Loading first with high load of Li// SeS ₂ PAN battery, discharged to 0.28V at a current density of 0.1A/g, with SeS first ₂ -PAN positive electrode complete lithiation. Then, lithium is supplemented by a small current of 50 mu A to obtain 1mAh/cm ² Lithium, forming a lithium aluminum alloy. Disassembling the battery, and adding lithium to SeS ₂ Assembly of PAN Positive electrode with Cu into SeS ₂ PAN// Cu cathode-free lithium-sulfur batteries.

3.5.2 negative-electrode-free lithium-sulfur Battery Performance testing

FIG. 23a shows a high load of non-negative electrode lithium sulfur battery SeS ₂ The cycling performance of the PAN// Cu battery shows that the first discharge capacity of the battery without lithium supplement is only 454mAh/g, the capacity after 50 cycles is only 207mAh/g, and the capacity retention rate is 37.9 percent. Figure 23b sees its average coulombic efficiency of 98.5%. The non-negative electrode battery without lithium supplement has loss of active lithium at both positive and negative electrodes, so capacity decreases rapidly although coulombic efficiency is not so low. And 1mAh/cm of lithium is supplemented ² The first charging capacity of the battery is 990mAh/g, the first discharging capacity is 555mAh/g, the capacity is not discharged completely in the first discharging, and the positive electrode stores a lithium source and provides an active lithium source for the subsequent lithium source lost because the coulomb efficiency of the negative electrode is less than 100%. After 50 cycles, the capacity is also 509mAh/g, and the capacity retention rate is up to 91.6 percent. The first coulombic efficiency was 33% because the first charge, the supplemental lithium source was plated onto the Cu current collector asSubsequently unavailable lithium due to dead lithium or the like acts as an active lithium source. The average coulombic efficiency from the second turn to the 50 th turn is up to 99.8% except for the first coulombic efficiency. During the period, the coulombic efficiency exceeds 100 percent for many times, because the lithium supplement operation continuously provides redundant lithium sources, and the reaction kinetics of the positive electrode after the lithium supplement are improved, the positive electrode cannot have large capacity reduction. Furthermore, it has been reported that lithium metal is deposited on both the copper foil and the lithium foil, and the copper foil has a higher peeling and delithiation efficiency than the lithium foil, i.e., the irreversible loss of active lithium on the copper foil is less than that of the lithium foil. The formation of reduced sulfur species on the lithium surface is in dynamic equilibrium with dissolved polysulfides, believed to have a stabilizing effect on lithium deposition, which is also why coulombic efficiency is higher than for Li// Cu half cells.

FIGS. 23c and 23d are the SeS without and with 1mAh of lithium supplementation, respectively ₂ -charge and discharge curves for PAN// Cu cells. It can be seen from fig. 23c that the initial charge capacity of the normal versus lithiation to 1V and then assembled non-negative cell was low, leaving only 280mAh/g of discharge capacity by the 30 th cycle, almost half of the initial charge capacity. More importantly, the charging platform is obviously raised in the process of each charging and discharging cycle, which indicates that the internal polarization of the battery is serious. This phenomenon corresponds to the fast capacity fading and low coulombic efficiency of the lithium battery in fig. 23a and 23b without being supplemented. After the lithium is supplemented in a reverse observation mode, the battery not only has ultrahigh first discharge capacity, but also has the 30 th cycle 541mAh/g which is almost the same as the first discharge capacity. And the charge and discharge platform has no obvious migration in the circulating process, the polarization is very small, and the lithium supplementing operation is further proved to improve the lithium ion migration reaction kinetics.

3.6 summary of this section

SeS is performed by the key point that the lithium and the aluminum can generate alloying reaction at 0.28V ₂ Advanced physicochemical of PAN positive electrode and replenishment of lithium source on aluminum current collector. The following conclusions can be summarized for the lithium supplement amount, the electrochemical performance and the assembled lithium-sulfur battery without the negative electrode:

(1) When the lithium supplement amount is 1mAh/cm ² When the lithium ion battery is used, the comprehensive performance is best determined as the best lithium supplement amount, li// SeS ₂ -the PAN cell second cycle discharge capacity 603mAh/g, the 40 th cycle discharge capacity566mAh/g, capacity retention rate 93.8%. The coulombic efficiency of the aluminum current collector after lithium supplement is 78.5% after the aluminum current collector is placed in the air for 24 hours, and the lithium metal completely fails after the lithium metal is placed in the air for 24 hours; the Li// Al battery still has 89 percent of coulombic efficiency after being discharged once and standing for 15 days, which shows that the electrode has excellent air and chemical stability after lithium supplement.

(2) The best lithium supplement is 1mAh/cm ² Under the condition, the cycle performance is also excellent. The first discharge capacity is 1129mAh/g, the second discharge capacity is 557mAh/g, 508mAh/g still exist in the 90 th cycle, and the capacity retention rate is 91.1% calculated from the second cycle. By supplementing lithium not only activates SeS ₂ The PAN positive electrode enables the PAN positive electrode to have ultrahigh reversible capacity, improves reaction kinetics, improves cycle stability and simultaneously shows excellent rate performance. Lithium supplement is calculated to be 1mAh/cm ² Diffusion coefficient of lithium ion, D _Li ＝5.768×10 ^-8 And the polarization is small in the circulating process, so that the lithium supplement is proved to be 1mAh/cm ² SeS of ₂ PAN positive electrode increases the interfacial reaction rate improving the reaction kinetics during cycling.

(3) Assembly SeS ₂ The PAN// Cu cathode-free lithium-sulfur battery has the advantages that the lithium supplement measures enable the cathode-free lithium-sulfur battery to have a sufficient lithium source, the capacity retention rate is ultrahigh in coulombic efficiency of 99.87% and 91.6% during 50 cycles, and polarization is small and stable in cycling during cycling.

4. Conclusion

The embodiment takes the manufacture of a high specific energy non-cathode lithium sulfur battery as a center, finds a carbonate local high-concentration electrolyte system suitable for the lithium sulfur battery and optimizes the system, inhibits polysulfide dissolution and ensures the sulfur utilization rate under the condition of high load capacity; and a proper anode material is searched, and deep lithiation lithium supplement is carried out on the anode, so that the service life and the cycling stability of the lithium-sulfur battery without the cathode are ensured. The main contents and results of the research are as follows:

(1) 1.2M LiFSI-DMC/HFE (volume ratio 1 ⁺ This will slow down the side reactions between the electrolyte and the lithium metal. And a diluentThe solvation structure of lithium salt under the original high-concentration condition is not damaged, so that the local high-concentration electrolyte has the advantages of high-concentration electrolyte and low viscosity and cost. The assembly test shows that the Li// Cu half cell has a current density of 0.5mAh/cm ² The deposition amount is 1mAh/cm ² The average coulombic efficiency of the next 100 cycles is 98.9 percent, and the current density is 0.5mAh/cm ² The deposition amount is 3mAh/cm ² The average coulombic efficiency for the next 100 cycles was 99.1%.

(2) Li// SeS assembly was tested using local high concentration electrolyte ₂ The capacity retention rate of the PAN battery is 76.6% after 100 cycles at a current density of low load of 0.1A/g, and the capacity retention rate is 97.1% from the second cycle. High load of Li// SeS ₂ The PAN battery has a 50-cycle capacity retention of 71.3% at a current density of 0.2A/g, the capacity retention being calculated from the second cycle as 90%. Compared with the common ether electrolyte system, the carbonate local high-concentration electrolyte has the advantages that the capacity, the rate capability and the cycle life of the battery are improved, and the overall performance of the battery is improved.

(3) For high load SeS ₂ The PAN positive electrode is subjected to a lithium replenishment operation so that the prefabricated lithium source has excellent air and chemical stability. The lithium-aluminum alloy formed by lithium supplement still has 78.5 percent of coulombic efficiency (lithium metal 0%) after standing in air for 24 hours, and only 11 percent of active lithium is lost (lithium metal 18%) after standing in the electrolyte for 15 days. Simultaneously, the lithium supplement operation improves the capacity, and the SeS after the lithium supplement ₂ The PAN positive electrode internal resistance reduction reaction kinetics are improved. The first discharge capacity is up to 1129mAh/g, the second discharge capacity is 557mAh/g, and the capacity retention rate after 90 times of circulation is calculated from the second circulation is 91.1 percent and is far higher than the capacity retention rate (77.3 percent) of a lithium battery which is not compensated. SeS ₂ The reaction kinetics are improved after the lithium supplement operation of the PAN positive electrode, and the lithium ion diffusion coefficient (D) of 1mAh for lithium supplement is calculated through EIS _Li ＝5.768×10 ^-8 )。

(4) SeS to be supplemented with lithium ₂ The negative-electrode-free lithium metal battery is assembled by the PAN positive electrode and the copper foil, the first discharge capacity is 555mAh/g, the capacity retention rate is 91.6 percent after 50 times of circulation, the average coulombic efficiency is as high as 99.87 percent, and the charge-discharge level is in the whole circulation processThe stage stationary polarization is very small and cyclic.

The non-negative electrode lithium sulfur battery has ultrahigh energy density, but the practical application of the non-negative electrode lithium sulfur battery is limited by low coulombic efficiency and short cycle life. In the embodiment, the high-capacity non-negative-electrode lithium-sulfur battery is manufactured by improving an electrolyte system and a positive electrode, the reaction kinetics is improved, the sulfur utilization rate is increased, and the coulombic efficiency is increased.

Claims (10)

1. A lithium-sulfur battery without negative electrode is characterized in that SeS ₂ -PAN// Cu battery; the positive electrode is a positive plate obtained by lithium supplement treatment of a grade plate coated with a selenium disulfide-polyacrylonitrile material; the electrolyte is a local high-concentration electrolyte, and the local high-concentration electrolyte consists of lithium bis (fluorosulfonyl) imide LiFSI, dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE.

2. The lithium sulfur battery of claim 1, wherein the step of preparing the grade coated with the selenium disulfide-polyacrylonitrile material comprises the steps of:

step (A-1): uniformly mixing selenium disulfide powder and polyacrylonitrile powder, grinding, and sieving to obtain mixed raw material powder;

step (A-2): placing the mixed raw material powder in a quartz tube for vacuum tube sealing; then placing the sealed quartz tube in a tube furnace for first heat treatment, and cooling to room temperature after the heat treatment is finished; crushing the quartz tube and taking out a solid product in the quartz tube;

step (A-3): placing the solid product in a tube furnace for secondary heat treatment, and obtaining the selenium sulfide-polyacrylonitrile material after the heat treatment is finished;

step (A-4): mixing a carboxymethyl cellulose aqueous solution and styrene-butadiene rubber emulsion to obtain a mixed binding solution; mixing and grinding the selenium disulfide-polyacrylonitrile material and the conductive carbon black, and then sieving to obtain mixed powder; adding the mixed bonding liquid into the mixed powder, and stirring and mixing uniformly to obtain mixed slurry;

step (A-5): and coating the mixed slurry on a positive current collector, and drying to obtain the graded piece coated with the selenium sulfide-polyacrylonitrile material.

3. The lithium sulfur battery without the negative electrode as claimed in claim 2, wherein in the step (A-1), the mass ratio of the selenium disulfide powder to the polyacrylonitrile powder is 1 (1-5); grinding for 20-50 min, and sieving with a 300-mesh sieve after grinding;

in the step (A-2), the first heat treatment method comprises: heating to 370-400 ℃ at the heating rate of 5 ℃/min, and then preserving heat for 6-9 h;

in the step (a-3), the second heat treatment method comprises: heating to 300-350 ℃ at the heating rate of 5 ℃/min, and then preserving the heat for 2-5 h.

4. The lithium-sulfur battery without the negative electrode as claimed in claim 2, wherein in the step (a-4), the volume ratio of the carboxymethyl cellulose aqueous solution to the styrene-butadiene rubber emulsion in the mixed binder solution is (20-30): 1; the mass fraction of the carboxymethyl cellulose in the carboxymethyl cellulose aqueous solution is 1 to 6 weight percent; the mass fraction of the styrene-butadiene rubber in the styrene-butadiene rubber emulsion is 40 to 50 weight percent; mixing and grinding the selenium disulfide-polyacrylonitrile material and the conductive carbon black in the mixed powder according to the mass ratio of (4-9) to 1 for 20-50 min, and sieving the mixture through a 300-mesh sieve after grinding; the mass ratio of the selenium disulfide-polyacrylonitrile material to the carboxymethyl cellulose in the mixed slurry is (15-20) to 1;

in the step (A-5), the positive current collector is an aluminum foil; the coating amount of the mixed slurry on the positive current collector is 1-3mg/cm ² Or the coating amount of the mixed slurry on the positive current collector is 5-7mg/cm ² (ii) a The drying temperature is 55-65 ℃, and the drying time is 10-12 h.

5. The lithium sulfur battery without a negative electrode according to claim 2, wherein in the step (a-1), the mass ratio of the selenium disulfide powder to the polyacrylonitrile powder is 1; grinding for 30min, and sieving with 300 mesh sieve;

in the step (A-2), the first heat treatment method comprises: heating to 380 ℃ at the heating rate of 5 ℃/min, and then preserving heat for 8h;

in the step (a-3), the second heat treatment method comprises: heating to 350 ℃ at the heating rate of 5 ℃/min, and then preserving heat for 4h;

in the step (A-4), the volume ratio of the carboxymethyl cellulose aqueous solution to the styrene-butadiene rubber emulsion in the mixed binding solution is 24; the mass fraction of the carboxymethyl cellulose in the carboxymethyl cellulose aqueous solution is 2wt%; the mass fraction of styrene-butadiene rubber in the styrene-butadiene rubber emulsion is 48wt%; mixing and grinding the selenium disulfide-polyacrylonitrile material and the conductive carbon black in the mixed powder for 30min according to the mass ratio of 8; the mass ratio of the selenium disulfide-polyacrylonitrile material to the carboxymethyl cellulose in the mixed slurry is 16;

in the step (A-5), the drying temperature is 60 ℃, and the drying time is 12h.

6. The lithium-sulfur battery without the negative electrode as claimed in claim 1, wherein the specific operation method of the graded lithium supplement treatment coated with the selenium disulfide-polyacrylonitrile material is as follows:

step (C-1): under the argon atmosphere with oxygen and moisture content less than or equal to 1ppm, the Li// SeS is obtained by assembling by taking the grade piece coated with the selenium disulfide-polyacrylonitrile material as a positive electrode, a lithium foil as a counter electrode and local high-concentration electrolyte as battery electrolyte ₂ -a PAN battery;

step (C-2) assembling the Li// SeS ₂ First discharge of PAN battery to 0.28V for Li// SeS ₂ SeS for PAN batteries ₂ -PAN positive electrode deep lithiation;

step (C-3): the discharge was continued with a small current of 50. Mu.A to make the lithium in SeS ₂ Depositing on PAN positive electrode to form lithium-aluminum alloy, the deposition amount is 0.5-3 mAh/cm ² ；

Step (C-4): after the discharge is completed, li// SeS is added ₂ And (4) disassembling the positive electrode of the PAN battery to obtain the positive electrode plate after lithium supplement treatment.

7. The lithium sulfur battery without a negative electrode of claim 1, wherein the local high concentration electrolyte is prepared by a method comprising:

step (B-1), pretreatment of the solvent: adding molecular sieve into two solvents of dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE respectively, and standing for 1-3 days to remove trace water in the two solvents;

step (B-2), pretreatment of lithium salt: putting lithium bis (fluorosulfonyl) imide LiFSI into an oven for drying;

and (B-3) sequentially pouring dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE into the pretreated lithium bis (fluorosulfonyl) imide LiFSI, and magnetically stirring and uniformly mixing to obtain the local high-concentration electrolyte.

8. The lithium sulfur battery without negative electrode as claimed in claim 1, wherein the volume ratio of dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE in the local high concentration electrolyte is 1: (1-5); the mass concentration of the lithium bis (fluorosulfonyl) imide LiFSI in the local high-concentration electrolyte is 1.0-1.5 mol/L.

9. The lithium sulfur battery of claim 8, wherein the volume ratio of dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE in the locally high concentration electrolyte is 1:3; the mass concentration of the lithium bis (fluorosulfonyl) imide LiFSI in the local high-concentration electrolyte is 1.2mol/L.

10. A preparation method of a lithium-sulfur battery without a negative electrode is characterized by comprising the following steps:

step A: preparing a grade piece coated with a selenium sulfide-polyacrylonitrile material;

and B: preparing local high-concentration electrolyte, wherein the local high-concentration electrolyte consists of lithium bis (fluorosulfonyl) imide LiFSI, dimethyl carbonate DMC and 1, 2-tetrafluoroethyl 2, 3-tetrafluoropropyl ether HFE;

and C: under the argon atmosphere with oxygen and moisture content less than or equal to 1ppm, the grade piece coated with the selenium sulfide-polyacrylonitrile material is taken as a positive electrode, the lithium foil is taken as a counter electrode, and local high-concentration electricity is generatedThe electrolyte is a battery electrolyte and assembled to obtain Li// SeS ₂ -a PAN battery; using Li// SeS ₂ Discharging the PAN battery to perform lithium supplement treatment on the grade piece coated with the selenium disulfide-polyacrylonitrile material; after the discharge is completed, li// SeS ₂ Disassembling the grade sheet coated with the selenium sulfide-polyacrylonitrile material after lithium supplement treatment in the PAN battery, and taking the grade sheet as a positive plate of the lithium-sulfur battery without the negative electrode;

step D: assembling the SeS by using a positive plate, a copper current collector and a local high-concentration electrolyte under an argon atmosphere with oxygen and moisture contents less than or equal to 1ppm ₂ PAN// Cu cell, namely, a lithium sulfur cell without negative electrode.

Images