KR20240005312A - Lithium sulfur battery - Google Patents

Abstract

The present application relates to a lithium sulfur battery with high performance by optimizing electrolyte donation degree and cathode adsorption capacity.

Description

Lithium sulfur battery {LITHIUM SULFUR BATTERY}

The present application relates to a lithium sulfur battery with high performance by optimizing electrolyte donation degree and cathode adsorption capacity.

Lithium-sulfur (Li-S) batteries, which theoretically have a high capacity of 1675 mAh/g, are attracting attention as next-generation energy storage devices. However, Li-S batteries are unable to fully utilize their theoretical capacity due to inherent problems occurring in the sulfur conversion reaction, delaying their commercial use. Specifically, when a solid product of sulfur or lithium sulfide (Li ₂ S) is generated through a liquid lithium polysulfide (LiPS) intermediate during the charge/discharge process, the conversion reaction occurs due to the high solubility of LiPS. There is a problem with this limitation. Additionally, the electrically/ionically non-conductive properties of sulfur and Li ₂ S result in high energy barriers to conversion.

Previous research has paid much attention to introducing advanced materials (in particular, materials effective for adsorption and/or catalytic conversion of LiPS) into the cathode. Various oxides such as Co ₃ O ₄ , NiO, WO ₃ , TiO ₂ , MoO ₃ , and V ₂ O ₅ have been investigated due to their ease of synthesis and morphology control. The polar surface of the oxide strongly adsorbs LiPS, mitigating the so-called ‘shuttle’ effect. Additionally, on the oxide surface, LiPS is capable of chemical bonding through the formation of thiosulfate groups and thiosulfate-polythionate conversion. Transition metal compounds such as CoS ₂ , MoS ₂ , ZnS ₂ , TiN, and ZrB ₂ have also been demonstrated to be polar conductive compared to oxides. MOFs such as MIL-100(Cr), Co ₃ S ₄ , and Zn-MOF, which can bind strongly to LiPS by Lewis acid-base interactions, have also been applied.

Additionally, increasing electrolyte donation (i.e., strong interaction of Li cations) has been investigated as an alternative approach to improve sulfur utilization. During the discharge process, LiPS dissolved in the electrolyte is randomly deposited as Li ₂ S on the cathode surface, often electrically passivating the cathode. It has been demonstrated that the growth of Li ₂ S under a high donor electrolyte environment is controlled to form particulate morphologies and is also accompanied by high sulfur availability. The growth was described as solution diffusion-dominated growth based on solvation of sulfide or solution-phase transition based on sulfide radicals. In particular, Liu's group demonstrated almost complete sulfur utilization by obtaining deposition of particulate Li ₂ S using high donor number (DN) electrolytes containing solvents such as dimethyl sulfoxide (DMSO). . The growth of particulate Li ₂ S may also include lithium salts such as lithium bromide or lithium bis(trifluoromethanesulfonyl)imide; Alternatively, it was observed in high donor electrolytes using 1,3-dimethyl-2-imidazolidinone (DMI) solvent.

Although these efforts each approach overcome limitations associated with the intermediate (LiPS) or the final product (Li ₂ S), no method has been developed to apply these approaches simultaneously.

Huilin Pan, Kee Sung Han, Mark H. Engelhard, Ruiguo Cao, Junzheng Chen, Ji-Guang Zhang, Karl T. Mueller, Yuyan Shao, Jun Liu, "Addressing Passivation in Lithium-Sulfur Battery Under Lean Electrolyte Condition", Adv. Funct. Mater. 2018, 28, 1707234.

The present application relates to a lithium sulfur battery with high performance by optimizing electrolyte donation degree and cathode adsorption capacity.

However, the problem to be solved by the present application is not limited to the problems mentioned above, and other problems not mentioned can be clearly understood by those skilled in the art from the description below.

One aspect of the present application is a cathode including a carbonaceous substrate, sulfur, and metal oxide; anode; A lithium sulfur battery comprising an electrolyte containing a solvent, a first lithium salt and a second lithium salt, wherein the metal oxide has an adsorption energy for lithium polysulfide of less than 3.5 eV, and the anion of the first lithium salt is donated. A lithium sulfur battery is provided, wherein the number is 15 kcal/mol or more.

The lithium sulfur battery according to the embodiments of the present application has the feature of improving the discharge capacity of the cell by optimizing the degree of electrolyte donation and the adsorption of the oxide substrate. Specifically, by constructing a cathode cell containing weak adsorbents NiO and MgO in an electrolyte with a high donor number, the adsorbent adsorbs lithium polysulfide (LiPS), improving the conversion efficiency of LiPS to Li ₂ S, Li ₂ S is adsorbed to the cell in the form of three-dimensional (3D) particles, improving current transfer efficiency.

Lithium sulfur batteries according to embodiments herein may have a battery capacity of at least about 1200 mAh/g, at least about 1250 mAh/g, at least about 1300 mAh/g, at least about 1350 mAh/g, or about 1400 mAh at 0.2 C-rate. It can provide high discharge capacity of /g or more.

1 is a scanning electron microscope of a carbon nanotube (CNT) composite coated with MgO, NiO, Fe ₂ O ₃ , Co ₃ O ₄ , and V ₂ O ₅ , respectively, in an example of the present application. (scanning electron microscopy; SEM) image (a) and transmission electron microscopy (TEM) image (b), and X- of MgO, NiO, Fe ₂ O ₃ , Co ₃ O ₄ , and V ₂ O ₅ X-ray diffraction (XRD) analysis spectrum (c).
Figure 2 is a digital camera image of a low donor number electrolyte (a) and a high donor number electrolyte (b) containing various types of oxides after adsorption of Li ₂ S ₆ for 3 hours, a low donor number electrolyte in an embodiment of the present application. Li ₂ S ₆ adsorption amount and Li 2 S ₆ binding energy (c), and _{Li 2 S 6} angle according to density functional theory (DFT) calculations for different types of oxides in electrolyte and high donor electrolyte. This is a schematic diagram (d) showing the optimized atomic configuration on the oxide surface.
Figure 3 shows the low donor electrolyte (a) of MgO/CNT, NiO/CNT, Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cathode cell in one embodiment of the present disclosure. ) and galvanostatic discharge/charge profiles in high donor number electrolytes (b), comparison of discharge capacity of each oxide cell in low and high donor number electrolytes (c), and MgO/CNT, NiO/CNT, Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cathode cells at various C-rate conditions from 0.2 C to 3 C in low-donor electrolyte (d) and high-donor electrolyte (e). This is the rate capability.
Figure 4 shows the constant current discharge/charge profile (a) and discharge capacity (b) of the MgO/CNT cell depending on the concentration of LiNO ₃ in one embodiment of the present application.
Figure 5 shows the Li of MgO/CNT, NiO/CNT, Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cathode cells in low donor number electrolytes, in one embodiment of the present disclosure. After complete discharge of ₂ S nucleation-growth, chronoamperogram (a), dimensionless it (current-time) curve (b), and SEM image (c) are shown (dotted lines in (b), respectively). It curves obtained from 2D and 3D models).
Figure 6 shows the Li of MgO/CNT, NiO/CNT, Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cathode cells in a high donor number electrolyte in one embodiment of the present application. After complete discharge of ₂ S nucleation-growth, chronoamperogram (a), dimensionless it (current-time) curve (b), and SEM image (c) are shown (dotted lines in (b), respectively). It curves obtained from 2D and 3D models).
Figure 7 is an SEM image of a weak adsorbent catalyst (MgO and NiO) and a strong adsorbent catalyst (Co ₃ O ₄ and V ₂ O ₅ ) cathode cell after the charging process, according to an example of the present application.
Figure 8 is a schematic diagram showing the mechanism by which LiPS is converted to Li ₂ S in an oxide with weak adsorption capacity (a) and strong adsorption capacity (b) in a high donor electrolyte environment, according to an embodiment of the present application.

Hereinafter, with reference to the attached drawings, implementation examples and embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present application may be implemented in various different forms and is not limited to the implementation examples and examples described herein. In order to clearly explain the present invention in the drawings, parts that are not related to the description are omitted, and similar parts are given similar reference numerals throughout the specification.

Throughout this specification, when a part is said to be “connected” to another part, this includes not only the case where it is “directly connected,” but also the case where it is “electrically connected” with another element in between. do.

Throughout this specification, when a member is said to be located “on” another member, this includes not only the case where the member is in contact with the other member, but also the case where another member exists between the two members.

Throughout the specification of the present application, when a part “includes” a certain element, this means that it may further include other elements rather than excluding other elements, unless specifically stated to the contrary.

As used herein, the terms “about,” “substantially,” and the like are used to mean at or close to a numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and to aid understanding of the present application. It is used to prevent unscrupulous infringers from unfairly exploiting disclosures in which precise or absolute figures are mentioned.

The terms “step of” or “step of” as used throughout the specification herein do not mean “step for.”

Throughout this specification, the term "combination(s) thereof" included in the Markushi format expression means a mixture or combination of one or more selected from the group consisting of the components described in the Markushi format expression, It means containing one or more selected from the group consisting of the above components.

Throughout this specification, description of “A and/or B” means “A or B, or A and B.”

Hereinafter, embodiments of the present application have been described in detail, but the present application may not be limited thereto.

One aspect of the present application is a cathode including a carbonaceous substrate, sulfur, and metal oxide; anode; A lithium sulfur battery comprising an electrolyte containing a solvent, a first lithium salt and a second lithium salt, wherein the metal oxide has an adsorption energy for lithium polysulfide of less than 3.5 eV, and the anion of the first lithium salt is donated. A lithium sulfur battery is provided, wherein the number is 15 kcal/mol or more.

In one embodiment of the present application, the carbonaceous substrate may be one commonly used as a battery substrate, and non-limiting examples include carbon nanotube (CNT), graphite, and graphene. ), carbon fiber, graphite, and activated carbon, but may not be limited thereto.

In one embodiment of the present application, the sulfur may include one or more selected from S ₈ and sulfur compounds, but may not be limited thereto.

In one embodiment of the present application, the loading amount of sulfur in the cathode may be about 1 mg/cm ² to about 8 mg/cm ² , but may not be limited thereto.

In one embodiment of the present application, the metal oxide may have an adsorption energy for lithium polysulfide (LiPS) of less than 3.5 eV. In one embodiment of the present application, the lithium polysulfide is Li ₂ S _n (n≥2), and may be one or more selected from Li ₂ S ₂ , Li ₂ S ₄ , Li ₂ S ₆ , and Li ₂ S ₈ . However, it may not be limited thereto. Throughout this specification, metal oxides with an adsorption energy of less than 3.5 eV for lithium polysulfide (LiPS) are referred to as weak adsorbents. In one embodiment of the present application, the metal oxide may adsorb LiPS more strongly than the carbonaceous substrate and promote the conversion of LiPS to Li ₂ S.

In one embodiment of the present application, the metal oxide may include one or more selected from NiO and MgO, but may not be limited thereto.

In one embodiment of the present application, the weight of the metal oxide may be 20 wt% to 50 wt% of the weight of the cathode, but may not be limited thereto.

In one embodiment of the present application, the diameter of the metal oxide may be about 30 nm to about 50 nm, but may not be limited thereto.

In one embodiment of the present application, the specific surface area of the metal oxide calculated using the Barrett-Emmett-Teller (BET) method is about 85 m ² /g to about 110 m ² /g. It may be, but may not be limited to this.

In one embodiment of the present application, the anode may contain Li.

In one embodiment of the present application, the anion of the first lithium salt may have a donation number of about 15 kcal/mol or more. In one embodiment of the present application, the anion of the first lithium salt may have a donation number of about 15 kcal/mol or more, about 16 kcal/mol or more, about 17 kcal/mol or more, or about 18 kcal/mol or more.

In one embodiment of the present application, the concentration of the first lithium salt in the electrolyte may be about 0.5 M to about 1.8 M, but may not be limited thereto. In one embodiment of the present application, the concentration of the first lithium salt in the electrolyte may be about 0.5 M to about 1.8 M, or about 0.5 M to about 0.9 M, but may not be limited thereto.

In one embodiment of the present application, the donation number of the electrolyte may be adjusted by adjusting the concentration of the first lithium salt.

In one embodiment of the present application, the first lithium salt may include one or more selected from LiNO ₃ , LiBr, LiCl, LiI, LiCN, LiOAc (lithium acetate), and LiOTf (lithium triflate). It may not be limited.

In one embodiment of the present application, the second lithium salt is lithium bis(trifluoromethane)sulfonimide (LiTFSI), LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN ( SO ₂ C ₂ F ₅ ) ₂ , Li(CF ₃ SO ₂ ) ₂ N, LiN(SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiCl, LiI, and LiB(C ₂ O ₄ ) ₂ (lithium bis(oxalato) borate (LiBOB)), but may not be limited thereto. In one embodiment of the present application, In this case, the first lithium salt controls the donor water of the electrolyte, and the second lithium salt may be a source of lithium ions.

In one embodiment of the present application, the concentration of the second lithium salt may be about 0.1 M to about 1 M, but may not be limited thereto. In one embodiment of the present application, the concentration of the second lithium salt may be about 0.1 M to about 1 M, or about 0.1 M to about 0.5 M, but may not be limited thereto.

In one embodiment of the present application, the sum of the concentrations of the first lithium salt and the second lithium salt may be about 0.5 M to about 2 M. In one embodiment of the present application, the sum of the concentrations of the first lithium salt and the second lithium salt may be preferably about 1 M. In one embodiment of the present application, the concentration ratio of the first lithium salt and the second lithium salt may be about 5:5 to about 9:1.

In one embodiment of the present application, the solvent may be an organic solvent, but may not be limited thereto. In one embodiment of the present application, the organic solvent is commonly used in lithium sulfur batteries, and is selected from carbonate-based solvents, ester-based solvents, ketone-based solvents, alcohol-based solvents, and aprotic solvents as non-limiting examples. It may include one or more things. In one embodiment of the present application, the organic solvent is dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methyl ethyl carbonate ( MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide ( decanolide), mevalonolactone, caprolactone, dimethyl ether, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, cyclohexanone, It may include one or more selected from ethyl alcohol, isopropyl alcohol, nitriles, dimethylformamide, and 1,3-dioxolane, but may not be limited thereto.

In one embodiment of the present application, the lithium sulfur battery may have a discharge capacity of about 1200 mAh/g or more at a rate of 0.2 C. In one embodiment of the present application, the lithium sulfur battery has a discharge capacity of about 1200 mAh/g or more, about 1250 mAh/g or more, about 1300 mAh/g or more, about 1350 mAh/g or more at a rate of 0.2 C, or about It may be more than 1400 mAh/g.

Hereinafter, the present application will be described in more detail using examples. However, the following examples are merely illustrative to aid understanding of the present application, and the content of the present application is not limited to the following examples.

[Example]

Preparation of metal oxide nanoparticles/CNTs

CNT films were prepared by filtering the CNT aqueous solution (5 wt% JENO Inc.). The CNT film was immersed in 0.01% HCl (Merck) aqueous solution to remove residual metal. This process is necessary to exclude parasitic capacity (about 10%) due to impurities. V ₂ O ₅ , Co ₃ O ₄ , Fe ₂ O ₃ , MgO, and NiO nanoparticles were prepared by thermal reduction of the respective metal chloride precursors. Specifically, vanadium (Ⅲ) chloride (97%, Merck), cobalt (Ⅱ) chloride hexahydrate (98%, Merck), iron (Ⅱ) chloride ( iron (Ⅱ) chloride) (97%, Merck), magnesium chloride hexahydrate (98%, Merck), and nickel (Ⅱ) chloride hexahydrate (98%, Merck) ) were used as precursors for V ₂ O ₅ , Co ₃ O ₄ , Fe ₂ O ₃ , MgO, and NiO, respectively. The ethanol solution containing the precursor was dispersed in the CNT film. The precursor-coated CNT film was dried at 80°C for 6 hours and annealed at 400°C for 2 hours in a mixed gas (O ₂ :N ₂ = 18%:82%) environment.

Preparation of low and high donor number electrolytes

0.9 M lithium bis(trifluoromethane) sulfonimide (LiTFSI) (Sigma-Aldrich), and 0.1 M lithium nitrate (LiNO ₃ , Alfa Aesar) at 1:1 v/v% Low donor electrolyte by dissolving in 1,3-dioxolane (DOL) (Sigma-Aldrich)/1,2-dimethoxyethane (DME) (Sigma-Aldrich) was manufactured. The high donor electrolyte was prepared by dissolving higher concentrations of LiTFSI and LiNO ₃ by setting the concentrations of LiTFSI and LiNO ₃ to 0.3 M and 0.7 M, respectively.

Adsorption analysis

Li ₂ S ₆ was prepared by reacting lithium sulfide (Li ₂ S, Alfa Aesar) and elemental sulfur (S ₈ , Alfa Aesar) at a molar ratio of 8:5 in DOL/DME (5:5 v/v) solvent. The Li ₂ S ₆ solution was diluted to a concentration of 0.2 mM using low and high donor water. To measure the adsorption capacity, a 40 mg oxide sample was immersed in a 0.2 mM Li ₂ S ₆ solution and the UV-Vis spectrum of the solution was measured over time.

Electrochemical measurements

Loading of the active material was prepared by drying the oxide/CNT host electrode in a sulfur-dissolved carbon disulfide solution (0.0226 g/mL). Li-S battery cells were assembled with a sulfur-loaded oxide/CNT cathode, lithium foil anode (1 mm, Honzo), and separator (Celgard 2400). Assembly of the cell was performed in a glove box in an Ar environment. The sulfur area loading at the cathode is 2 mg/cm ² . The ratio of electrolyte to sulfur is 15 μL/mg. All electrochemical measurements were performed using a Maccor 4300 system. The galvanostatic experiment was controlled in a voltage range of 1.9 V (vs. Li/Li ⁺ ) to 2.8 V (vs. Li/Li ⁺ ), and the current density was 0.2 C to 3 C. Cyclic voltammetry is performed by cycling over a voltage range of 1.9 V (vs. Li/Li ⁺ ) to 2.8 V (vs. Li/Li ⁺ ) at various scan rates from 0.2 mV/s to 0.5 mV/s. Obtained. Chronoamperometry was measured by applying 2.05 V to a cell containing an electrolyte containing Li ₂ S ₆ . To observe the morphology of the deposited Li ₂ S, the cell was disassembled and soluble residues were removed using acetonitrile.

Material Characterization

Morphology and microstructure were characterized by field-emission scanning electron microscopy (FE-SEM) (JSM-7100F) and transmission electron microscopy (TEM, JEM-3010, JEOL). The crystal structure of the metal oxide was studied using X-ray diffraction (XRD) (Davinci D8 Advance diffractometer) at a scan speed of 0.05°/s in a scan range of 10° to 80°. X-ray photoelectron spectroscopy (XPS) was collected using a Leybold spectrometer with an Al Kα monochromatic beam (1486.6 eV) (150 W input power, ESCALAB250 XPS system, Theta Probe XPS system). The specific surface area of the metal oxide was obtained by the Barrett-Emmett-Teller (BET) method (ASAP 2020, Micrometrics Inc.). Thermogravimetric analysis (TGA-50H, SHIMADZU) was performed to confirm the metal oxide content of the metal oxide/CNT film.

1. Fabrication of oxide/CNT host cathode

Sulfur host cathodes of CNT films decorated with nanoparticles of MgO, NiO, Fe ₂ O ₃ , Co ₃ O ₄ , and V ₂ O ₅ were prepared, respectively. The oxides exhibit various binding energies for LiPS and the discharge capacity of the cell. Oxide nanoparticle coatings of CNTs are obtained by sol-gel reaction of metal chloride precursors in the presence of CNTs. The oxide content of the composite film was controlled to be similar to each other at approximately 30 wt% for each sample. SEM and TEM images of each oxide nanoparticle-CNT composite are shown in Figure 1 a and b, respectively. The size of each oxide nanoparticle was strictly controlled to ensure that each oxide sample had a similar BET surface area. The size of each oxide nanoparticle ranges from 30 nm to 50 nm, and the BET surface area of each oxide complex ranges from 85 m ² /g to 110 m ² /g.

The XRD peak for each oxide nanoparticle is shown in Figure 1c, where cubic phase Co ₃ O ₄ , trigonal phase Fe ₂ O ₃ , cubic phase MgO, cubic phase NiO, and orthorhombic phase V ₂ O ₅ were identified. HR-TEM images confirm the lattice spacing of each oxide (inset of Figure 1b), with d-spacing being 0.2290 nm for Co ₃ O ₄ ; for Fe ₂ O ₃ , 0.2524 nm; for MgO, 0.2040 nm; for NiO, 0.2078 nm; and for V ₂ O ₅ , 0.4021 nm, cubic (311) of Co ₃ O ₄ , cubic (110) of Fe ₂ O ₃ , cubic (200) of MgO, cubic (200) of NiO, and V ₂ It corresponds to each of the orthorhombic (110) crystal phases of O ₅ .

2. Exploration of the adsorption properties of oxides

The donacy of the electrolyte was controlled by adding a highly electron-donating LiNO ₃ salt with a high donor number (DN) (22.2 kcal/mol) to the DOL:DME electrolyte. Donation values for DOL and DME are 18 kcal/mol and 20 kcal/mol, respectively. It has been demonstrated that sufficiently high concentrations of NO ₃ ⁻ stabilize the sulfide species anion and increase its solubility by the common ion effect on Li ⁺ . Specifically, electrolytes containing 0.1 M LiNO ₃ and 0.7 M LiNO ₃ were prepared in the DOL:DME-based electrolyte, respectively, and were designated as low-donor electrolyte and high-donor electrolyte, respectively. Radical formation due to stabilization of LiPS was observed in a high donor electrolyte environment and was also confirmed in experiments.

The adsorption capacity of LiPS for each nanoparticle of MgO, NiO, Fe ₂ O ₃ , Co ₃ O ₄ , and V ₂ O ₅ was compared. Specifically, each oxide nanoparticle was dispersed in a low-donor electrolyte and a high-donor electrolyte in which 2 mM Li ₂ S ₆ was dissolved, and the adsorption of Li ₂ S ₆ over time was analyzed using UV-Vis spectra. . Digital camera images of low-donor and high-donor electrolyte samples after 3 hours of adsorption are shown in Figure 2a and b, respectively. In the low donor electrolyte, the orange color faded in the following order: V ₂ O ₅ , Co ₃ O ₄ , Fe ₂ O ₃ , MgO, and NiO samples. Quantitative adsorption amount (μmol/m ² ) is analyzed as the peak intensity in the 420 nm region corresponding to the absorption of Li ₂ S ₆ in the UV-Vis spectrum. The relative Li ₂ S ₆ adsorption of each oxide is shown in Figure 2c, consistent with the adsorbent properties identified by color. Specifically, the adsorption capacity of V ₂ O ₅ , which showed the strongest adsorption, was 4.6 times higher than that of NiO, which showed the weakest adsorption. V ₂ O ₅ , Co ₃ O ₄ , and Fe ₂ O ₃ show relatively strong adsorption capacity, while MgO and NiO show weak adsorption capacity.

In high donor number electrolytes, the order of adsorption capacity of each oxide does not differ from that in low donor number electrolytes, but the adsorption capacity decreases. In particular, oxides with strong adsorption capacity show relatively less reduced adsorption capacity, and the adsorption amount of V ₂ O ₅ in the high-donor electrolyte is similar to that in the low-donor electrolyte. However, the adsorption capacity of NiO decreased by approximately 35% in high donor electrolyte. This represents a trade-off between stabilization of LiPS by high donor salts and adsorption of LiPS by oxides.

The atomic interactions between the oxide and Li ₂ S ₆ were analyzed using first principles calculations based on density functional theory (DFT). Optimal binding configuration of Li ₂ S ₆ on the surfaces of Co ₃ O ₄ (400), Fe ₂ O ₃ (110), MgO (200), NiO (200), and V ₂ O ₅ (001); And the calculated binding energy (BE) is shown in d in Figure 2; and c, respectively. Due to the composition of many SS linear chains, the bonds of each oxide show significant differences from each other (Figure 2d). Nevertheless, the favorable binding is determined by the interaction of Li and O, and only a few of the S atoms bind with the oxygen on the surface. The strongest BE was calculated for V ₂ O ₅ , where Li forms bridges with two oxygens at the surface, showing stable VS and O-Li bonds. The Li-S bond length of Li ₂ S ₆ adsorbed on V ₂ O ₅ is 2.4 Å, which indicates a greater strain rate than other oxides. On the other hand, on the NiO surface with low BE, Li is combined with one oxygen. The Li-S bond length of Li ₂ S ₆ adsorbed on NiO shows a small value of 2.2 Å. BE for Li ₂ S ₆ decreases in the following order: V ₂ O ₅ (001), Co ₃ O ₄ (400), Fe ₂ O ₃ (110), MgO (200), and NiO (200), and the values were compared in Figure 2c. In the present invention, when the binding energy (adsorption energy) of the metal oxide to Li ₂ S ₆ is relatively compared, the metal oxide with an adsorption energy of less than 3.5 eV is a pharmaceutical adsorbent, and has an adsorption energy of 3.5 eV to 4.5 eV. Metal oxides were defined as intermediate adsorbents, and metal oxides with adsorption energies exceeding 4.5 eV were defined as strong adsorbents. The relative binding energy trend of the metal oxide to LiPS (Li ₂ S ₆ ) calculated using density functional theory is the quantitative adsorption amount of the metal oxide to LiPS (Li ₂ S ₆ ) obtained through UV-Vis spectrum ( μmol/m ² ) is consistent with the trend.

3. Electrochemical performance of oxide/CNT cathode cells in low or high donor electrolytes.

A Li-S battery cell was fabricated using a CNT composite cathode coated with nanoparticles of MgO, NiO, Fe ₂ O ₃ , Co ₃ O ₄ , or V ₂ O ₅ , respectively. Here, each cell was fabricated with either a low-donor electrolyte or a high-donor electrolyte. The sulfur mass loading is approximately 2 mg/cm ² .

The charge/discharge curves of various oxide/CNT cells using low donor electrolytes are shown in Figure 3a. The discharge capacities of MgO/CNT, NiO/CNT, Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cells were 895 mAh/g, 815 mAh/g, and 920 mAh at 0.5 C, respectively. /g, 983 mAh/g, and 994 mAh/g. The discharge capacity of the bare CNT cell is 750 mAh/g, which is significantly lower than that of the cell containing oxide nanoparticles. The discharge capacity of the oxide/CNT cell decreases in the order of V ₂ O ₅ , Co ₃ O ₄ , Fe ₂ O ₃ , MgO, and NiO, which is consistent with the order of adsorption capacity of oxides. The above results are explained by a general mechanism in which the strong adsorption of LiPS on the oxide surface suppresses the shuttle effect, resulting in high sulfur utilization. For the above cells, the rate capability is evaluated at an increase in C-rate from 0.2 C to 3 C. The rate capabilities are 85.85% for MgO/CNT cells, respectively; for NiO/CNT cells, 84.69%; for Fe ₂ O ₃ /CNT cells, 75.98%; for Co ₃ O ₄ /CNT cells, 77.93%; and for V ₂ O ₅ /CNT cells, 87.53%.

The charge/discharge profile of each oxide/CNT cell containing high donor electrolyte is shown in Figure 3c. The discharge capacities at 0.5 C for MgO/CNT, NiO/CNT, Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cells were 1238 mAh/g, 1112 mAh/g, respectively. 975 mAh/g, 882 mAh/g, and 686 mAh/g. The adsorption capacity of the oxide no longer determines the discharge capacity of the cell. Cells containing strong adsorbents, Co ₃ O ₄ and V ₂ O _{5 ,} show somewhat lower capacity compared to cells containing weak adsorbents, MgO and NiO. Compared to low donor water electrolytes, the discharge capacity of oxide/CNT cells shows wider variation in high donor water electrolyte environments. In the low donor water electrolyte the capacity ranges from 815 mAh/g to 994 mAh/g, while in the high donor water electrolyte the capacity ranges from 686 mAh/g to 1238 mAh/g. This is due to differences in the growth mode of Li ₂ S on the oxide/CNT substrate in a high-donor electrolyte environment.

An increase in electrolyte donation decreases the capacity of cells containing strong adsorbents, Co ₃ O ₄ and V ₂ O ₅ , but increases the capacity of cells containing weak adsorbents, MgO and NiO. The V ₂ O ₅ /CNT cell exhibits significantly lower capacity in high donor electrolyte, which is only 69% of the capacity achieved with low donor electrolyte. From previous studies, it has been confirmed that the presence of LiNO ₃ salt in the electrolyte improves capacity by stabilizing the anode. The poor capacity of V ₂ O ₅ /CNT cells in high concentration LiNO ₃ salt-electrolyte indicates that this stabilizing effect is not dominant.

The MgO/CNT cell shows the highest capacity, which is 25% higher than the highest capacity V ₂ O ₅ cell at low donor electrolyte. This means a synergy effect between the adsorption capacity of the oxide and the solvating capacity of the electrolyte.

Rate capability at increasing C-rate is shown in Figure 3d and e. The rate capacity in high donor water electrolytes also depends on the discharge capacity and thus differs from the results obtained in low donor water electrolytes.

In addition, the concentration of LiO ₃ was set to 0.1 M to 1 M to control the number of electrolyte donors, and the discharge capacity of the MgO/CNT cell was measured (FIG. 4 a and b). At this time, compared to the 0.1 M LiNO ₃ electrolyte, it was confirmed that the discharge capacity was improved by 20% for 0.5 M, 54% for 0.7 M, and 30% for 0.9 M.

4. Li in oxide/CNT host electrode ₂ S growth analysis

We analyzed the formation of Li ₂ S in an oxide/CNT cell using chronoamperometry. The conversion of liquid LiPS to solid-state Li ₂ S, so-called precipitation, is known to be the rate-limiting step, and the process has a high energy barrier due to the significant reduction in entropy. The IT transient profile under potentiostatic discharge was obtained by applying a constant voltage (2.05 V), sufficient to convert higher-order LiPS to insoluble Li ₂ S. To determine the growth mode of Li ₂ S on each oxide substrate, the transient profiles were compared to those of a classical electrochemical deposition model.

In particular, the Bewick, Fleischman, and Thirsk (BFT) model, which describes 2D nucleation-growth, or the Scharifker-Hills (SH) model, which describes 3D diffusion-dominated growth. was used. In each model, immediate or gradual nucleation was considered. The it curves of all oxide/CNT cells in low donor electrolytes correspond to 2D growth with immediate nucleation. SEM images of each oxide/CNT after full discharge show the typical morphology of 2D-grown Li ₂ S overlying the CNT for all samples (Figure 5 a to c).

The it curves of MgO/CNT and NiO/CNT cells upon application of high donor electrolyte are consistent with the 3D growth mode observed in Figure 6 a to c. Characteristically, the time to reach the local maximum in the it curve (t _m ) may be significantly delayed in the sample. This indicates delayed coalescence between Li ₂ S nuclei due to solvation of Li ₂ S in the high donor salt. In previous studies, 3D growth of Li ₂ S resulted in partially solvated Li ₂ S precipitates; Alternatively, it has been demonstrated that it is obtained by bulk diffusion of Li ₂ S produced by solution-phase reduction by sulfur anion radicals. SEM image showing Li ₂ S deposited after complete discharge showing particulate Li ₂ S in the sample. Each curve for Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT corresponds to 2D growth as in low donor electrolyte. The SEM image in Figure 6c clearly shows 2D-grown Li ₂ S on the surfaces of Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT. In Figure 7, the shapes of cells containing MgO, NiO, Co ₃ O ₄ , and V ₂ O ₅ were compared after being charged. MgO and NiO cells with particulate Li ₂ S growth show smaller sulfur residues compared to Co ₃ O ₄ and V ₂ O ₅ cells with 2D Li ₂ S growth.

The precipitation capacity for the Li ₂ S nucleation-growth process was calculated from the it curve. The precipitation capacity is 1.166 mAh/cm ² for MgO; for NiO, 1.054 mAh/cm ² ; For Fe ₂ O ₃ , 0.782 mAh/cm ² ; for Co ₃ O ₄ , 0.742 mAh/cm ² ; and for V ₂ O ₅ , it is 0.708 mAh/cm ² . The results show that MgO/CNT and NiO/CNT cells with 3D Li ₂ S growth provide high capacity, whereas Fe ₂ O ₃ /CNT, Co ₃ O ₄ /CNT, and V ₂ O ₅ /CNT cells with 2D growth. The cells indicate that they provide relatively low capacity (Table 1). Additionally, the order of deposition capacity for each oxide/CNT cell is consistent with the order of full discharge capacity for each oxide/CNT cell. The capacity of the oxide/CNT cell in high donor electrolytes is determined by the contribution of the precipitation capacity, which depends on the growth type of Li ₂ S.

oxide electrolyte growth mode Li ₂ S precipitation capacity _tm (s) i _m MgO high number of donations 3D 583 640 2.48 low number of donations 2D 242 260 3.03 NiO high number of donations 3D 527 690 1.66 low number of donations 2D 319 370 2.76 _{Fe2O3 _} high number of donations 2D 391 530 1.81 low number of donations 2D 446 450 2.47 Co ₃ O ₄ high number of donations 2D 371 570 2.26 low number of donations 2D 452 320 3.64 V ₂ O ₅ high number of donations 2D 354 650 1.35 low number of donations 2D 511 320 3.19

The present inventors propose a discharge mechanism and capacity according to the combination of electrolyte donation degree and adsorption capacity. First, weak adsorbent substrate (i.e. MgO and NiO) cathode cells with high donor electrolytes provide the highest discharge capacity. We obtained a high discharge capacity of 1394 mAh/g at 0.2 C in the MgO/CNT cell. This capacity is higher than all oxide/CNT cells with low donor electrolyte. The results immediately indicate a synergistic effect between the LiPS adsorption capacity of the oxide and the solvation power of the electrolyte. This discharge capacity is superior to the results obtained with conventional composite oxide cathodes. The adsorption capacity of MgO or NiO adsorbs LiPS better than the CNT substrate, promoting the conversion of LiPS to Li ₂ S. On the other hand, the adsorption capacity of MgO or NiO is weak compared to the solvation power by high donor salts, allowing partial solvation of Li ₂ S and subsequent 3D growth, as shown in Figure 6. These two actions result in high capacitance in the substrate. MgO/CNT cells provide higher capacity than NiO/CNT cells. This is explained by the superior adsorption capacity of MgO for LiPS in the competitive adsorption of LiNO ₃ and LiPS. The adsorption of LiNO ₃ weakens the solvation of Li ₂ S, resulting in incomplete sulfur utilization during the 3D growth process.

Second, applying strongly adsorptive oxides such as Fe ₂ O ₃ , Co ₃ O ₄ , and V ₂ O ₅ together with high donor electrolyte results in reduced capacity. The 3D growth of Fe ₂ O ₃ /CNT, V ₂ O ₅ /CNT, and Co ₃ O ₄ /CNT is suppressed even in a high donor electrolyte environment. The strong adsorption of the substrate to sulfide species inhibits the solvation of Li ₂ S. In particular, it is noteworthy that the respective capacities of the V ₂ O ₅ /CNT and Co ₃ O ₄ /CNT cells were lower than those achieved in the respective cells using low donor electrolytes. We hypothesize that high donor salts are strongly adsorbed to these strong adsorbent substrates, and that the adsorbed NO ₃ ^- anions can act as a sheath to prevent conversion of LiPS, as shown in Figure 8. Compared to weak adsorbent substrates, strong adsorbent substrates also strongly adsorb NO ₃ ^- and are therefore greatly affected by coating. It has been reported that the binding of LiNO ₃ to the cathode substrate is stronger than that of LiPS. Additionally, in a previous study, Lu and co-workers suggested that the shell, which strongly solvates the polysulfide anion, provides a barrier to the reduction reaction. This effect is also supported by the lower capacity obtained in the V ₂ O ₅ cell, which has a stronger adsorption capacity than Co ₃ O ₄ .

Third, oxide composite cathodes with low donor electrolytes provide only moderate capacity. This is typical of most previous composite cathodes. Stronger adsorbent substrates adsorb more polysulfide species, in which case lower polysulfide molecule loss and higher conversion result in higher discharge capacity. The discharge capacity of the cell depends on the adsorption capacity of the oxide.

In conclusion, we reveal the influence of the interaction between the donicity of the electrolyte and the adsorptivity of the oxide substrate on Li-S battery performance. Electrolyte donation and oxide adsorption cancel each other out for partial solvation of the discharge product Li ₂ S. The oxide with strong adsorption capacity not only weakens solvation by the electrolyte, causing surface-passivating of Li ₂ S, but also adsorbs the electrolyte salt, forming a barrier to the conversion of Li ₂ S. This actually causes a decrease in cell capacity. A weak adsorbent oxide substrate that can sustain particulate Li ₂ S formation is a selection criterion for high cell performance. The MgO/CNT cathode together with the electrolyte containing high concentration of LiNO ₃ salt represents the optimal combination. The cell provides a high discharge capacity of 1394 mAh/g, which is approximately 25% higher than the capacity achieved with strong adsorbent oxide cathodes alone. The present inventors believe that understanding the interaction of advanced materials and electrolytes will become the cornerstone for the practical use of Li-S batteries.

The description of the present application described above is for illustrative purposes, and those skilled in the art will understand that the present application can be easily modified into other specific forms without changing its technical idea or essential features. Therefore, the embodiments described above should be understood in all respects as illustrative and not restrictive. For example, each component described as single may be implemented in a distributed manner, and similarly, components described as distributed may also be implemented in a combined form.

The scope of the present application is indicated by the claims described below rather than the detailed description above, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present application. .

Claims (12)

a cathode comprising a carbonaceous substrate, sulfur, and metal oxide;
anode; and
An electrolyte comprising a solvent, a first lithium salt, and a second lithium salt.
A lithium sulfur battery comprising,
The metal oxide has an adsorption energy for lithium polysulfide of less than 3.5 eV,
The anion of the first lithium salt has a donation number of 15 kcal/mol or more,
Lithium sulfur battery.
According to claim 1,
A lithium sulfur battery, wherein the carbonaceous substrate includes one or more selected from carbon nanotubes, graphite, graphene, carbon fiber, graphite, and activated carbon.
According to claim 1,
A lithium sulfur battery, wherein the metal oxide includes one or more selected from NiO and MgO.
According to claim 1,
A lithium sulfur battery, wherein the weight of the metal oxide is 20 wt% to 50 wt% of the weight of the cathode.
According to claim 1,
A lithium sulfur battery, wherein the metal oxide has a diameter of 30 nm to 50 nm.
According to claim 1,
A lithium sulfur battery, wherein the concentration of the first lithium salt is 0.5 M to 1.8 M.
According to claim 1,
The first lithium salt is LiNO ₃ , LiBr, LiCl, LiI, LiCN, LiOAc, and LiOTf, and includes one or more selected from LiOTf.
According to claim 1,
The second lithium salt is LiTFSI, LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN(SO ₂ C ₂ F ₅ ) ₂ , Li(CF ₃ SO ₂ ) ₂ N, LiN(SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiCl, LiI, and LiB(C ₂ O ₄ ) ₂ , a lithium sulfur battery comprising one or more selected from the group consisting of.
According to claim 1,
A lithium sulfur battery, wherein the concentration of the second lithium salt is 0.1 M to 1 M.
According to claim 1,
In one embodiment of the present application, the concentration ratio of the first lithium salt and the second lithium salt is 5:5 to 9:1, a lithium sulfur battery.
According to claim 1,
The solvent is dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, Methyl propionate, ethyl propionate, decanolide, mevalonolactone, caprolactone, dimethyl ether, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran A lithium sulfur battery comprising one or more selected from cyclohexanone, ethyl alcohol, isopropyl alcohol, nitriles, dimethylformamide, and 1,3-dioxolane.
According to claim 1,
A lithium sulfur battery having a discharge capacity of 1200 mAh/g or more at a rate of 0.2 C.