CN120419004A

CN120419004A - Nonaqueous electrolyte and lithium secondary battery comprising same

Info

Publication number: CN120419004A
Application number: CN202380079653.0A
Authority: CN
Inventors: 金贤撤; 崔南顺; 朴世原; 文炫奎
Original assignee: Korea Advanced Institute of Science and Technology KAIST; LG Energy Solution Ltd
Current assignee: Korea Advanced Institute of Science and Technology KAIST; LG Energy Solution Ltd
Priority date: 2022-12-01
Filing date: 2023-11-30
Publication date: 2025-08-01
Also published as: KR20240082242A; WO2024117826A1

Abstract

The present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, a separator, and a nonaqueous electrolyte, wherein the negative electrode comprises a silicon-based active material, and the nonaqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, wherein the additive comprises a compound represented by formula 1.

Description

Nonaqueous electrolyte and lithium secondary battery comprising same

Cross Reference to Related Applications

The present application claims priority from korean patent application No. 10-2022-0165981, filed on 1 of 12 months of 2022, the disclosure of which is incorporated herein by reference.

Technical Field

The present invention relates to a nonaqueous electrolyte and a lithium secondary battery including the same.

Background

With the recent development of information society, personal IT devices and computer networks have been developed, and the dependence of the entire society on electric energy has been increased, so that development of a technology for efficiently storing and utilizing electric energy has been required.

Among the developed technologies, a secondary battery is the most suitable technology for various applications, and among them, a lithium secondary battery that can be miniaturized to be suitable for personal IT equipment and has the highest energy density is the focus of attention.

Generally, a lithium secondary battery is prepared by injecting or impregnating a nonaqueous electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a porous separator.

As the positive electrode active material of such a lithium secondary battery, a lithium-containing cobalt oxide, liMnO ₂ having a layered crystal structure, liMn ₂O₄ having a spinel crystal structure, a lithium-containing nickel oxide (LiNiO ₂), and a lithium nickel cobalt manganese transition metal oxide are considered.

Meanwhile, a carbon-based active material such as graphite has been used as a negative electrode active material, but recently, a silicon-based active material is being considered to be used because the silicon-based active material has a higher capacity than the carbon-based active material.

Silicon-based active materials have advantages in that the capacity is high, but have problems in that the volume expansion/contraction during charge and discharge is very large. This large degree of volume expansion/contraction significantly reduces the conductivity of the anode, resulting in a reduction in life performance. Further, in the initial activation process, a solid electrolyte interface layer (hereinafter referred to as "SEI layer") is formed on the anode surface, but the volume expansion degree of the silicon-based active material is large, thereby causing the SEI film to be ruptured and continuously generating a new anode surface. Therefore, as the reaction of forming the SEI film proceeds, the electrolyte is depleted, the thickness of the SEI film increases, resulting in an increase in resistance.

Disclosure of Invention

[ Technical problem ]

An object of the present invention is to provide a nonaqueous electrolyte capable of improving life performance of a lithium secondary battery by forming an SEI film having flexibility and excellent mechanical durability on a negative electrode.

Further, another object of the present invention is to provide a lithium secondary battery comprising the above-described nonaqueous electrolyte.

Technical scheme

The present invention provides a nonaqueous electrolyte comprising a lithium salt, an organic solvent, and an additive. The additive comprises a first additive and a second additive, wherein the first additive comprises a compound represented by the following formula 1, and the second additive comprises fluoroethylene carbonate.

[ 1]

In formula 1 above, R ₁ is selected from aryl groups having 6 to 30 carbon atoms substituted with one or more fluorine groups, and heteroaryl groups having 3 to 30 carbon atoms substituted with one or more fluorine groups. R ₂ is selected from hydrogen, halogen, and alkyl having 1 to 5 carbon atoms, and L ₁ is selected from a direct bond and alkylene having 1 to 5 carbon atoms.

Further, the present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode facing the negative electrode, a separator provided between the negative electrode and the positive electrode, and the above nonaqueous electrolyte.

[ Advantageous effects ]

The nonaqueous electrolyte of the present invention is characterized by containing an acrylic acid ester-based additive (first additive) containing a fluorinated aryl group and a fluorinated ethylene carbonate (second additive) as additives.

When the nonaqueous electrolyte containing the first additive and the second additive is applied to a lithium secondary battery, since an SEI film having flexibility and excellent mechanical durability is formed on the surface of the negative electrode, the SEI film can absorb mechanical stress deposition generated by continuous volume expansion and contraction of the negative electrode, and can prevent acceleration of side reactions of the electrolyte and depletion of the electrolyte caused by rupture of the negative electrode SEI film. Accordingly, overall performance including life characteristics of the lithium secondary battery can be significantly improved. In particular, the nonaqueous electrolyte of the present invention can be preferably used for a negative electrode and a lithium secondary battery containing a silicon-based active material.

Drawings

Fig. 1 is a graph evaluating young's modulus of negative electrode SEI films after activation of lithium secondary batteries in examples 1 to 5 and comparative examples 1 to 4.

Fig. 2 is a graph showing the discharge capacities of examples 1 to 5 as a function of cycles.

Fig. 3 is a graph showing the discharge capacity of comparative examples 1 to 4 as a function of cycle.

Fig. 4 is a graph showing coulombic efficiency versus cycle for examples 1 to 5.

Fig. 5 is a graph showing the coulombic efficiency of comparative examples 1 to 4 as a function of cycle.

Fig. 6 is a graph showing the discharge capacity of example 1 and comparative examples 5to 7 as a function of cycle.

Fig. 7 is a graph showing the coulombic efficiency of example 1 and comparative examples 5to 7 as a function of cycle.

Detailed Description

First, before the description of the present invention, it should be understood that terms or words used in the present specification and claims should not be interpreted as having meanings defined in commonly used dictionaries, but should be interpreted as having meanings and concepts consistent with the technical ideas of the present invention according to the principle that the inventor properly defines term concepts to preferably explain the present invention.

Meanwhile, the terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the invention. Singular forms also are intended to include plural forms unless the context clearly indicates otherwise.

It will be understood that terms, such as "comprises," "comprising," "includes," or "having," when used herein, are intended to specify the presence of stated features, integers, steps, elements, and/or groups, but do not preclude the presence or addition of other features, integers, steps, elements, and/or groups thereof.

In the present specification, unless explicitly stated otherwise, the expression "%" means% by weight.

Before describing the present invention, the expressions "a" and "b" in the specification of "a to b carbon atoms" respectively represent the number of carbon atoms contained in a specific functional group. I.e., the functional group may include "a" to "b" carbon atoms.

In addition, unless otherwise defined in the specification, the expression "substituted" means that at least one of hydrogen bonded to carbon is substituted with an element other than hydrogen, for example, an alkyl group having 1 to5 carbon atoms or a fluorine element.

In the present specification, the average particle diameter (D ₅₀) may be defined as a particle diameter corresponding to 50% of the cumulative volume in the particle diameter distribution curve. The average particle diameter (D ₅₀) can be measured using, for example, a laser diffraction method. The laser diffraction method can measure particle diameters ranging from submicron to several millimeters and can provide results with high reproducibility and high resolution.

Hereinafter, the present invention will be described in more detail.

Nonaqueous electrolyte

The present invention relates to a nonaqueous electrolyte, and more particularly, to a nonaqueous electrolyte for a lithium secondary battery.

The nonaqueous electrolyte of the present invention contains a lithium salt, an organic solvent, and an additive. The additive comprises a first additive and a second additive, wherein the first additive comprises a compound represented by the following formula 1, and the second additive comprises fluoroethylene carbonate.

[ 1]

The nonaqueous electrolyte of the present invention is characterized by containing an acrylic acid ester-based additive (first additive) containing a fluorinated aryl group and a fluorinated ethylene carbonate (second additive) as additives. When the nonaqueous electrolyte containing the first additive and the second additive is applied to a lithium secondary battery, since an SEI film having flexibility and excellent mechanical durability can be formed on the surface of the negative electrode, the SEI film can absorb mechanical stress deposition generated by continuous volume expansion and contraction of the negative electrode, and problems of acceleration of side reactions of the electrolyte and depletion of the electrolyte caused by rupture of the negative electrode SEI film can be prevented. Accordingly, overall performance including life characteristics of the lithium secondary battery can be significantly improved. In particular, the nonaqueous electrolyte of the present invention can be preferably used in a negative electrode and a lithium secondary battery including a silicon-based active material.

(1) Lithium salt

First, a lithium salt will be described as follows.

In the nonaqueous electrolytic solution for a lithium secondary battery of the embodiment of the present invention, any lithium salt commonly used for an electrolytic solution of a lithium secondary battery may be used without limitation, and for example, the lithium salt may include Li ⁺ as a cation and at least one selected from the group consisting of Li 793 and (CF ₃CF₂SO₂)₂N^-. Specifically, the lithium salt may include at least one selected from the group consisting of ：LiCl、LiBr、LiI、LiBF₄、LiClO₄、LiAlO₄、LiAlCl₄、LiPF₆、LiSbF₆、LiAsF₆、LiB₁₀Cl₁₀、LiBOB(LiB(C₂O₄)₂)、LiCF₃SO₃、LiFOB、LiTFSI(LiN(SO₂CF₃)₂)、LiFSI(LiN(SO₂F)₂)、LiCH₃SO₃、LiCF₃CO₂、LiDFOB( lithium difluoro (oxalato) borate), liDFBP (lithium difluoro (bisoxalato) phosphate), liTFOP (lithium tetrafluoro (oxalato) phosphate), liDFP (LiPO ₂F₂)、LiCH₃CO₂ and LiBETI (LiN (SO ₂CF₂CF₃)₂). Specifically, the lithium salt may include one selected from the group consisting of one material or a mixture of two or more thereof ：LiBF₄、LiClO₄、LiPF₆、LiBOB(LiB(C₂O₄)₂)、LiCF₃SO₃、LiTFSI(LiN(SO₂CF₃)₂)、LiFSI(LiN(SO₂F)₂) and LiBETI (LiN (SO ₂CF₂CF₃)₂). More specifically, the lithium salt may include LiPF ₆.

The lithium salt may be appropriately changed within a range of normal use, but may be present in the electrolyte at a concentration of 0.1M to 3.0M, specifically 0.8M to 2.0M, to obtain an optimal film forming effect of the electrode surface corrosion prevention film.

In the case where the concentration of the lithium salt satisfies the above range, the viscosity of the nonaqueous electrolytic solution can be controlled, so that optimal impregnation can be achieved, and the effect of improving the capacity characteristics and cycle characteristics of the lithium secondary battery can be obtained by improving the mobility of lithium ions.

(2) Organic solvents

The organic solvent is a non-aqueous solvent commonly used in lithium secondary batteries, and is not limited as long as it can minimize decomposition due to oxidation reaction or the like during charge and discharge of the battery.

Specifically, the organic solvent may include at least one organic solvent selected from cyclic carbonate-based organic solvents and linear carbonate-based organic solvents.

The cyclic carbonate-based organic solvent is an organic solvent having a high viscosity and a high dielectric constant, and thus is an organic solvent capable of well dissociating a lithium salt in an electrolyte, and specific examples thereof include at least one selected from the group consisting of Ethylene Carbonate (EC), propylene Carbonate (PC), 1, 2-butylene carbonate, 2, 3-butylene carbonate, 1, 2-pentylene carbonate, 2, 3-pentylene carbonate, vinylene carbonate, and more specifically, may include ethylene carbonate.

Further, the linear carbonate-based organic solvent is an organic solvent of low viscosity and low dielectric constant, and may include at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl Methyl Carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, and more particularly, may include Ethyl Methyl Carbonate (EMC).

The organic solvent may be a mixture of a cyclic carbonate-based organic solvent and a linear carbonate-based organic solvent, wherein the cyclic carbonate-based organic solvent and the linear carbonate-based organic solvent may be mixed in a volume ratio of 10:90 to 40:60, specifically 15:85 to 35:65.

The organic solvent may be used without limitation by adding an organic solvent commonly used in nonaqueous electrolytes, if necessary. For example, the organic solvent may further include at least one organic solvent selected from the group consisting of an ester organic solvent, an ether organic solvent, a glycol diether solvent, and a nitrile organic solvent.

The ester organic solvent may include at least one organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, sigma-valerolactone, and epsilon-caprolactone.

Any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether, ethylpropyl ether, 1, 3-Dioxolane (DOL) and 2, 2-bis (trifluoromethyl) -1, 3-dioxolane (TFDOL), and a mixture of two or more thereof may be used as the ether solvent, but is not limited thereto.

The glyme solvent has a high dielectric constant and a low surface tension as compared with the linear carbonate type organic solvent, is a solvent having low reactivity with a metal, and may include at least one selected from dimethoxyethane (glyme, DME), diethoxyethane, diglyme, triglyme, and Tetraglyme (TEGDME), but is not limited thereto.

The nitrile solvent may include one or more selected from acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanonitrile, cyclopentaonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorobenzonitrile and 4-fluorobenzonitrile, but is not limited thereto.

Meanwhile, unless otherwise specified, the rest of the nonaqueous electrolyte except for the lithium salt and the additive may be a nonaqueous organic solvent.

(3) Additive agent

The nonaqueous electrolyte of the present invention contains an additive, and is characterized in that a first additive and a second additive are used together as the additive.

The first additive includes a compound represented by the following formula 1.

[ 1]

The compound represented by formula 1 contained in the first additive is a compound having both a bulky functional group such as an aryl group or a heteroaryl group and an acrylate group in the structure. The compound represented by formula 1 may form a flexible polymer-based SEI film through polymerization of an acrylate group during activation of a lithium secondary battery. In addition, the polymer-based SEI film formed by the first additive has a large volume of aryl or heteroaryl groups, which have a large free volume on the polymer side chains, and thus can minimize the phenomenon of exposure of the anode to the electrolyte while accommodating the change in the anode volume. However, since it is difficult to secure interface stability and durability of the anode using only the first additive, it is necessary to use the second additive while using the first additive, which will be described later.

In the above formula 1, R ₁ is selected from aryl groups having 6 to 30 carbon atoms substituted with one or more fluorine groups, and heteroaryl groups having 3 to 30 carbon atoms substituted with one or more fluorine groups, specifically, aryl groups having 6 to 30 carbon atoms substituted with one or more fluorine groups. In particular, it is phenyl substituted by one or more fluoro groups, more particularly pentafluorophenyl. Meanwhile, the heteroaryl group may refer to a heteroaryl ring substituent containing at least one element selected from N, O and S in the aromatic ring.

In formula 1 above, R ₂ may be selected from hydrogen, halogen (e.g., may be selected from F, cl, br, and I), and alkyl having 1 to 5 carbon atoms. Specifically, R ₂ may be selected from hydrogen and alkyl groups having 1 to 3 carbon atoms, more specifically, it may be selected from hydrogen and methyl groups, even more specifically, it may be hydrogen from the standpoint of being able to easily receive electrons and form an SEI film, and forming a SEI film of high flexibility.

L ₁ may be selected from a direct bond and an alkylene group having 1 to 5 carbon atoms, and in particular, it may be a direct bond.

Specifically, the compound represented by the above formula 1 may include a compound represented by the following formula 2.

[ 2]

In formula 2 above, R ₂ is as defined in formula 1.

More specifically, the compound represented by the above formula 1 may include at least one selected from the compounds represented by the following formulas 3-A and 3-B, and more specifically, may include the compound represented by the following formula 3-B.

[ 3-A ]

[ 3-B ]

In the nonaqueous electrolyte, the content of the first additive may be 0.01 to 10% by weight, specifically 0.5 to 7% by weight, more specifically 1 to 5% by weight, and even more specifically 1.5 to 3.5% by weight. When the content of the first additive is within the above range, it is preferable because an increase in battery resistance due to excessive use of the additive can be prevented while the above flexible SEI film can also be provided to the negative electrode.

The second additive comprises fluoroethylene carbonate (FEC).

During activation of the lithium secondary battery, fluoroethylene carbonate contained in the second additive may be decomposed to form an inorganic SEI film containing LiF or the like. The LiF-containing inorganic SEI film has excellent electrochemical durability and can improve interface stability of a negative electrode. However, since the inorganic-based SEI film does not have sufficient flexibility, the simultaneous use of the above-described first additive and second additive is critical to achieving the effects of the present invention.

In this regard, the nonaqueous electrolyte of the present invention is characterized by using a first additive and a second additive capable of forming a polymer-based/inorganic-based composite SEI film. According to the nonaqueous electrolyte of the present invention, the polymer-based SEI film derived from the first additive completely covers the negative electrode surface to form an SEI film having excellent flexibility and recovery, and the inorganic-based SEI film derived from the second additive can improve durability of the overall SEI film by being distributed on the polymer-based SEI film. Accordingly, the non-aqueous electrolyte of the present invention can form an SEI film having excellent flexibility and recovery and improved durability and strength on the surface of the negative electrode, thereby being capable of preventing consumption of electrolyte caused by side reactions of the electrolyte and increase in resistance caused by increase in thickness of the electrode film during operation of the battery and remarkably improving life performance. In particular, when the nonaqueous electrolyte of the present invention is used together with a negative electrode containing a silicon-based active material, it is capable of forming a soft and durable SEI film on the silicon-based active material that undergoes significant volume expansion during charge and discharge. Therefore, it is preferable because it can prevent the damage of the SEI film due to the volume expansion of the silicon-based active material, and can prevent the problem of the SEI film cracking due to the volume expansion, the acceleration of electrolyte side reaction, and the depletion of electrolyte.

The second additive content in the nonaqueous electrolyte may be 0.01 to 12% by weight, specifically 2.5 to 7.5% by weight. When the content of the second additive is within the above range, it is preferable because it is possible to prevent an increase in battery resistance caused by excessive use of the additive while sufficiently improving durability of the SEI film derived from the first additive and the second additive.

In the nonaqueous electrolyte of the present invention, the ratio of the weight of the first additive to the weight of the second additive may be 0.1 to 1.2, specifically 0.2 to 1.0, more specifically 0.3 to 0.7. When the ratio is within the above range, by optimally controlling the balance of flexibility and durability of the above negative electrode SEI film, the life performance of the lithium secondary battery can be significantly improved.

Lithium secondary battery

Further, the present invention provides a lithium secondary battery comprising the above nonaqueous electrolyte.

Specifically, the lithium secondary battery of the present invention comprises a negative electrode, a positive electrode facing the negative electrode, a separator provided between the negative electrode and the positive electrode, and the above nonaqueous electrolyte.

At this time, the lithium secondary battery of the present invention may be manufactured by a conventional method known in the art. For example, the lithium secondary battery of the present invention may be manufactured by forming an electrode assembly in which a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode are sequentially stacked, then accommodating the electrode assembly in a battery case, and then injecting the non-aqueous electrolyte of the present invention thereinto.

(1) Negative electrode

The negative electrode may include a silicon-based active material.

Although the silicon-based active material exhibits higher capacity than the carbon-based active material, there is a problem in that the degree of volume expansion/contraction caused by charge and discharge is large. However, when the silicon-based active material is used together with the above-described nonaqueous electrolyte, an SEI film having improved flexibility and durability can be formed on the negative electrode, thereby preventing electrolyte side reactions, and realizing a lithium secondary battery having high life performance.

The silicon-based active material may include a compound represented by the following formula 4:

[ 4]

SiOx(0≤x<2)

In formula 4, since SiO ₂ does not react with lithium ions, lithium cannot be stored. Therefore, x is preferably within the above range.

The silicon-based active material may be Si (silicon). Si has an advantage in that its capacity is about 2.5 to 3 times that of silicon oxide (e.g., siO _x (0 < x < 2)), but has a problem in that silicon generates very high volume expansion/contraction upon charge and discharge compared to silicon oxide, and thus is difficult to commercialize. However, by applying the above-described nonaqueous electrolyte, the lithium secondary battery of the present invention can have excellent life performance.

From the standpoint of ensuring structural stability of the active material during charge and discharge, enabling a conductive network to be formed more smoothly to maintain conductivity, or making contact with a binder for binding the active material and the current collector easier, the average particle diameter (D ₅₀) of the silicon-based active material may be 1 μm to 20 μm, preferably 3 μm to 7 μm,

The anode may include an anode current collector and an anode active material layer disposed on at least one surface of the anode current collector, wherein the silicon-based active material may be included in the anode active material layer.

The negative electrode current collector is not particularly restricted so long as it has high conductivity without causing adverse chemical changes in the battery. Specifically, the negative electrode current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like, and an aluminum-cadmium alloy.

The thickness of the negative electrode current collector may be 3 μm to 500 μm.

The negative electrode current collector may form microscopic irregularities on the surface thereof to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector may take various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric body.

The anode active material layer is disposed on at least one surface of the anode current collector. Specifically, the anode active material layer may be disposed on one surface or both surfaces of the anode current collector.

The content of the silicon-based active material in the anode active material layer may be 60 to 90 wt%, preferably 65 to 80 wt%, to sufficiently achieve high capacity of the silicon-based active material in the secondary battery while minimizing the influence of volume expansion/contraction of the silicon-based active material on the battery.

The anode active material layer may contain a conductive agent and/or a binder in addition to the silicon-based active material.

The binder may be used to improve adhesion between the anode active material layer and the anode current collector or binding force between the silicon-based active materials.

Specifically, the binder may include at least one selected from the group consisting of styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluoro rubber, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and Polyacrylamide (PAM), in terms of further improving the electrode adhesion and providing sufficient resistance to volume expansion/contraction of the silicon-based active material.

The binder content in the anode active material layer may be 1 to 30 wt%, specifically 7 to 15 wt%. When the amount is within this range, the binder may be better bonded to the silicon-based active material to minimize the problem of volume expansion of the active material, while the binder may be easily dispersed, and when preparing a slurry for forming the anode active material layer, the coating property and phase stability of the slurry may be improved.

The conductive agent is not particularly limited as long as it can be used to assist and improve conductivity in the secondary battery and has conductivity without causing adverse chemical changes. Specifically, the conductive agent may include at least one selected from the group consisting of graphite, such as natural graphite or artificial graphite, carbon black, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black, conductive fibers, such as carbon fibers or metal fibers, conductive tubes, such as carbon nanotubes, carbon fluoride, metal powders, such as aluminum powder or nickel powder, conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers, conductive metal oxides, such as titanium oxide, and polyphenylene derivatives.

More specifically, the conductive agent may include a first conductive agent containing carbon black and a second conductive agent containing carbon nanotubes, wherein the carbon nanotubes may be single-walled carbon nanotubes.

In the anode active material layer, the content of the conductive agent may be 1 to 20% by weight, and when the content of the conductive agent is within this range, it is preferable in terms of being able to form an excellent conductive network while reducing an increase in resistance caused by the binder.

In terms of achieving high energy density, the thickness of the anode active material layer may be 10 μm to 200 μm, preferably 20 μm to 60 μm.

The negative electrode may be prepared by coating a negative electrode current collector with a negative electrode slurry containing a silicon-based active material and optionally a binder, a conductive agent, and a solvent for forming the negative electrode slurry, followed by drying and rolling.

As for promoting the dispersion of the silicon-based anode active material, the binder and/or the conductive agent, the solvent used to form the anode slurry may include, for example, at least one selected from distilled water, ethanol, methanol and isopropanol, preferably distilled water.

The content of the solvent for forming the anode slurry in the anode slurry may be such that the concentration of solids including the anode active material and optional binder and conductive agent is 15 to 45 wt% in view of the viscosity, coating property and dispersibility of the anode slurry.

(2) Positive electrode

The positive electrode faces the negative electrode.

The positive electrode includes a positive electrode active material.

The positive electrode active material is a compound capable of reversibly intercalating and deintercalating lithium, and specifically may include a lithium transition metal composite oxide containing lithium and at least one selected from transition metals consisting of nickel, cobalt, manganese, and aluminum, and preferably may include a lithium transition metal composite oxide containing lithium and transition metals including nickel, cobalt, and manganese.

For example, the lithium transition metal composite oxide may include lithium manganese-based oxide (e.g., liMnO ₂、LiMn₂O₄, etc.), lithium cobalt-based oxide (e.g., liCoO ₂, etc.), lithium nickel-based oxide (e.g., liNiO ₂, etc.), and the like, Lithium nickel manganese-based oxides (e.g., liNi _1-YMn_YO₂ (where 0< y < 1), liMn _2-ZNi_ZO₄ (where 0< z < 2), etc.), lithium nickel cobalt-based oxides (e.g., liNi _1- _Y1Co_Y1O₂ (wherein 0< y1< 1), etc.), Lithium manganese cobalt-based oxide (e.g., liCo _1-Y2Mn_Y2O₂ (where 0< Y2< 1), liMn _2- _Z1Co_Z1O₄ (where 0< Z1< 2), etc.), a metal oxide (e.g., cobalt oxide), Lithium nickel manganese cobalt-based oxide (e.g., li (Ni _pCo_qMn_r1)O₂ (where 0< p <1,0< q <1,0< r1<1, p+q+r1=1) or Li (Ni _p1Co_q1Mn_r2)O₄ (where 0< p1<2,0< q1<2,0< r2<2, p1+q1+r2=2), etc.) or lithium nickel cobalt transition metal (M) oxide (e.g., li (Ni _p2Co_q2Mn_r3M_s2)O₂ (where M is selected from Al, etc.), Fe. V, cr, ti, ta, mg and Mo, p2, q2, r3 and s2 are the atomic fractions of the respective independent elements, wherein 0< p2<1,0< q2<1,0< r3<1,0< s2<1, p2+q2+r3+s2=1), etc.), any one or two or more of them may be used. Among these materials, the lithium transition metal composite oxide may include LiCoO ₂、LiMnO₂、LiNiO₂, lithium nickel manganese cobalt oxide (e.g., ,Li(Ni_0.6Mn_0.2Co_0.2)O₂、Li(Ni_0.5Mn_0.3Co_0.2)O₂、Li(Ni_0.7Mn_0.15Co_0.15)O₂ or Li (Ni _0.8Mn_0.1Co_0.1)O₂, etc.), or lithium nickel cobalt aluminum oxide (e.g., li (Ni _0.8Co_0.15Al_0.05)O₂, etc.) from the viewpoint of being able to improve the capacity characteristics and stability of the battery. In view of the remarkable improvement effect by controlling the types and content ratios of the respective components constituting the lithium transition metal composite oxide, the lithium transition metal composite oxide may be Li(Ni_0.6Mn_0.2Co_0.2)O₂、Li(Ni_0.5Mn_0.3Co_0.2)O₂、Li(Ni_0.7Mn_0.15Co_0.15)O₂、Li(Ni_0.8Mn_0.1Co_0.1)O₂ or the like, and a mixture of any one or two or more thereof may be used.

More specifically, the positive electrode active material may be a lithium transition metal composite oxide, and the content of nickel may be 60mol% or more based on the total number of moles of transition metal in the lithium transition metal composite oxide. Specifically, the positive electrode active material is a lithium transition metal composite oxide, the transition metal includes nickel and at least one selected from manganese, cobalt, or aluminum, and the content of nickel may be 60mol% or more, specifically 60mol% to 90 mol%, based on the total mole number of the transition metal. When such a lithium transition metal composite oxide having a high nickel content is used together with the above-described nonaqueous electrolytic solution, it is preferable in that the gas phase by-product generated by collapse of the structure can be reduced.

In addition, the positive electrode active material may include a lithium composite transition metal oxide represented by the following formula 5:

[ 5]

Li_1+x(Ni_aCo_bMn_cM_d)O₂

In formula 5, M is at least one selected from W, cu, fe, V, cr, ti, zr, zn, al, in, ta, Y, la, sr, ga, sc, gd, sm, ca, ce, nb, mg, B and Mo, 1+x, a, b, c, and d are atomic fractions of each individual element, where 0≤x≤ 0.2,0.50≤a <1, 0≤b≤0.25, 0≤c≤0.25, 0≤d≤0.1, and a+b+c+d=1.

Preferably, a, b, c and d may be 0.70≤a≤0.95, 0.025≤b≤0.20, 0.025≤c≤0.20 and 0≤d≤0.05, respectively.

Preferably, a, b, c and d may be 0.80≤a≤0.95, 0.025≤b≤0.15, 0.025≤c≤0.15 and 0≤d≤0.05, respectively.

In addition, a, b, c and d may be 0.85≤a≤0.90, 0.05≤b≤0.10, 0.05≤c≤0.10 and 0≤d≤0.03, respectively.

The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer may include the positive electrode active material described above.

The thickness of the positive electrode current collector may be generally 3 μm to 500 μm.

The positive electrode current collector may form microscopic irregularities on the surface thereof to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector may take various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric body.

The positive electrode active material layer may be disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one surface or both surfaces of the positive electrode current collector.

The content of the positive electrode active material in the positive electrode active material layer may be 80 to 99 wt% in view of exhibiting a sufficient positive electrode active material capacity.

The positive electrode active material layer may contain a binder and/or a conductive agent in addition to the positive electrode active material described above.

The binder is a component that facilitates the bonding between the active material and the conductive agent and the bonding with the current collector, may include at least one selected from polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, and may preferably include polyvinylidene fluoride.

The binder content in the positive electrode active material layer may be 1 to 20 wt%, preferably 1.2 to 10 wt%, in terms of ensuring sufficient adhesion between components such as the positive electrode active material.

The conductive agent may be used to assist and improve the conductivity of the secondary battery, without particular limitation, as long as it has conductivity without causing chemical changes. Specifically, the positive electrode conductive agent may include at least one selected from graphite such as natural graphite or artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black, conductive fibers such as carbon fibers and metal fibers, conductive tubes such as carbon nanotubes, fluorocarbons, metal powders such as aluminum powder or nickel powder, conductive whiskers such as zinc oxide or potassium titanate, conductive metal oxides such as titanium oxide, or polyphenylene derivatives, and preferably may include carbon nanotubes in order to improve conductivity.

The content of the conductive agent in the positive electrode active material layer may be 1 to 20 wt%, preferably 1.2 to 10 wt%, in terms of sufficiently securing conductivity.

The thickness of the positive electrode active material layer may be 10 μm to 200 μm, preferably 50 μm to 100 μm.

The positive electrode may be prepared by coating a positive electrode current collector with a positive electrode slurry containing a positive electrode active material and optionally a binder, a conductive agent, and a solvent for forming the positive electrode slurry, and drying and rolling.

The solvent used to form the positive electrode slurry may include an organic solvent, such as NMP (N-methyl-2-pyrrolidone). The solid content of the positive electrode slurry may be 40 to 90 wt%, specifically 50 to 80 wt%.

(3) Diaphragm

The separator is disposed between the positive electrode and the negative electrode.

As the separator, a porous polymer film generally used as the separator may be used, for example, a porous polymer film prepared from polyolefin-based polymers such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer and an ethylene/methacrylate copolymer may be used alone or in a layered manner. In addition, a general porous non-woven fabric, such as a non-woven fabric formed of high melting point glass fiber or polyethylene terephthalate fiber, may be used, but is not limited thereto.

A coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure may be selectively used.

In the lithium secondary battery of the present invention, the young's modulus of the negative electrode SEI film may be 0.36MPa or less, specifically 0.15MPa to 0.36MPa, more specifically 0.18MPa to 0.28MPa, and even more specifically 0.18MPa to 0.21MPa. In the present specification, the young's modulus of the negative electrode SEI film was measured by the following method:

(a) Performing an activation process on the lithium secondary battery, thereby forming an SEI film on the surface of the negative electrode;

(b) Separating the negative electrode from the lithium secondary battery subjected to the activation process, and

(C) The young's modulus of the SEI film formed on the negative electrode surface was measured using a nanoindentation technique of an atomic force microscope.

In step (a), the activation process may refer to a process of assembling a lithium secondary battery with an electrode assembly in which a negative electrode, a separator, and a positive electrode are sequentially stacked, and a non-aqueous electrolyte, enabling it to operate as a lithium secondary battery by performing an electrochemical charging or electrochemical charging-discharging process. Through the activation process, an SEI film may be formed on the surface of the negative electrode.

In step (b), the negative electrode can be separated from the lithium secondary battery by disassembling the lithium secondary battery having undergone the activation process. The cleaning process may be performed after the separation of the negative electrode. The cleaning process may be performed by washing the separated negative electrode with an organic solvent, such as dimethyl carbonate (DMC). A drying process may be performed after washing the separated anode.

In step (c), the young's modulus of the SEI film formed on the negative electrode surface is measured using a nano indentation technique of an atomic force microscope. Force-depth (FD) curves at a plurality of positions (e.g., 10 to 30, specifically 16) may be obtained using a contact mode of an atomic force microscope apparatus, and young's modulus of the SEI film may be calculated using a Smeddon model, wherein a tip half angle (Tip half come angle) may be set to 15 °, and a poisson ratio (v _{Surface of the body}) may be set to 0.5.

As described above, the lithium secondary battery of the present invention is applicable to portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as Hybrid Electric Vehicles (HEVs).

Thus, according to another embodiment of the present invention, there are provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

The battery module or the battery pack may be used as a power source for one or more of large and medium-sized devices such as electric tools, electric vehicles including Electric Vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs), and electric power storage systems.

The shape of the lithium secondary battery of the present invention is not particularly limited, but a cylindrical shape, a prismatic shape, a pouch shape, or a coin shape using a can may be used.

The lithium secondary battery of the present invention can be used not only as a unit cell for a power source of a small-sized device, but also as a unit cell in a large-and-medium-sized battery module including a plurality of unit cells.

Hereinafter, the present invention will be described in more detail according to examples.

This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will fully convey the scope of the invention to those skilled in the art.

Examples

Example 1

(Preparation of nonaqueous electrolyte)

An organic solvent in which Ethylene Carbonate (EC) and Ethylene Methyl Carbonate (EMC) were mixed at a volume ratio of 30:70 was prepared.

LiPF ₆ was dissolved in an organic solvent so that the concentration of LiPF ₆ was 1.3M.

Further, the compound represented by formula 3-B as a first additive and fluoroethylene carbonate (FEC) as a second additive were added to the above-described organic solvent in which lithium salt was dissolved, thereby preparing a nonaqueous electrolyte (see table 1 below).

The nonaqueous electrolyte contained 2% by weight of the compound represented by the formula 3-B, and 5% by weight of fluoroethylene carbonate.

(Preparation of secondary cell)

Positive electrode active material particles (LiNi _0.8Co_0.1Mn_0.1O₂), a conductive agent (carbon black), and a binder (PVdF) were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97.5:1.5:1.5 to prepare a positive electrode mixture slurry (solid content: 78 wt%). The positive electrode mixture slurry was coated on one surface of a 12 μm thick positive electrode current collector (Al thin film) at a loading amount of 24mg/cm ², dried and rolled to prepare a positive electrode (thickness of positive electrode active material: 65 μm).

Silicon as a negative electrode active material, plate-like graphite as a first conductive agent, single-walled carbon nanotubes as a second conductive agent, and Polyacrylamide (PAM) as a binder were added to water as a solvent in a weight ratio of 80.0:9.6:0.4:10.0 to prepare a negative electrode slurry (solid content: 28 wt%). A negative electrode current collector (Cu thin film) of 30 μm thickness was coated with the negative electrode slurry at a loading of 99.65mg/25cm ², dried and rolled to prepare a negative electrode (thickness of negative electrode active material: 37 μm).

An electrode assembly was prepared by stacking a positive electrode, a polyolefin-based porous separator, and a negative electrode in this order.

After the electrode assembly was put into a battery case, the nonaqueous electrolyte prepared as described above was injected to prepare a coin-type lithium secondary battery.

Example 2

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1 except that 1% by weight of the compound represented by formula 3-B was added to the nonaqueous electrolyte.

Example 3

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1 except that 5% by weight of the compound represented by formula 3-B was added to the nonaqueous electrolyte.

Example 4

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1, except that 10% by weight of fluoroethylene carbonate was added to the nonaqueous electrolyte.

Example 5

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1, except that 2% by weight of a compound represented by formula 3-a was added to the nonaqueous electrolyte instead of the compound represented by formula 3-B.

Comparative example 1

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1, except that the compound represented by formula 3-B and fluoroethylene carbonate were not added to the nonaqueous electrolyte.

Comparative example 2

A nonaqueous electrolyte and a lithium secondary battery were produced in the same manner as in example 1, except that fluoroethylene carbonate was not added to the nonaqueous electrolyte.

Comparative example 3

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1, except that the compound represented by formula 3-B was not added to the nonaqueous electrolyte.

Comparative example 4

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1, except that 2 wt% of Vinylene Carbonate (VC) was added in place of fluoroethylene carbonate to the nonaqueous electrolyte.

Comparative example 5

A nonaqueous electrolyte and a lithium secondary battery were produced in the same manner as in example 1, except that 2 wt% of pentafluorophenyl methane sulfonate (the compound represented by formula 6) was added to the nonaqueous electrolyte instead of the compound represented by formula 3-B.

[ 6]

Comparative example 6

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in example 1, except that 2 wt% of pentafluorophenylacetic acid (a compound represented by formula 7) was added to the nonaqueous electrolyte instead of the compound represented by formula 3-B.

[ 7]

Comparative example 7

A nonaqueous electrolyte and a lithium secondary battery were produced in the same manner as in example 1, except that 2 wt% of methyl pentafluorophenyl carbonate (a compound represented by formula 8) was added to the nonaqueous electrolyte instead of the compound represented by formula 3-B.

[ 8]

TABLE 1

Experimental example

Experimental example 1 Young's modulus evaluation of negative electrode SEI film

The lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 4 prepared above were charged to 4.2V/0.05C at a rate of 0.33C under CC/CV conditions of 25 ℃ using electrochemical chargers, and then discharged to 2.5V at a rate of 0.33C under CC conditions, which was set to 1 cycle. Activation is performed by performing charge and discharge for two cycles. Through the activation process, an SEI film is formed on the negative electrode.

The negative electrode is separated from the activated lithium secondary battery.

The Young's modulus of the SEI film formed on the negative electrode surface was measured using a nanoindentation technique of an atomic force microscope (device name: multiMode V, manufacturer: veeco). Specifically, a force-depth (FD) curve of 16 positions was obtained using a contact mode of an atomic force microscope, and young's modulus of the SEI film was calculated using a Smeddon model, wherein a tip half angle was set to 15 °, and a poisson ratio (v _{Surface of the body}) was set to 0.5.

Fig. 1 and 2 show young's moduli (unit: MPa) of the lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 4.

TABLE 2

Referring to table 2, it can be understood that the young's modulus of the negative electrode SEI films formed after activation of the lithium secondary batteries of examples 1 to 5 is lower, 0.36MPa or less, and thus the flexibility of the SEI films is improved. Such flexible SEI films are expected to effectively help improve the lifetime performance of the negative electrode, especially those using silicon-based active materials. Meanwhile, since the first additive and the second additive are used in combination, the SEI films formed on the negative electrodes of examples 1 to 5 have flexibility and excellent durability, and such effects are clearly confirmed in life performance evaluation described below.

In contrast, it was confirmed that in the lithium secondary batteries of comparative examples 1, 3 and 4, the young's modulus of the negative electrode SEI film was very high, which means that the SEI film was not soft but hard. Since the SEI film cannot sufficiently accommodate the volume expansion and contraction of the silicon-based active material, there is a risk of cracking, and thus life performance of the lithium secondary battery is expected to be deteriorated.

Meanwhile, in the case of the lithium secondary battery of comparative example 2, the young's modulus of the negative electrode SEI film is low, but the second additive is not added to the nonaqueous electrolyte, and thus durability is expected to be significantly reduced.

Experimental example 2 cyclic charge/discharge evaluation

Except for experimental example 1, lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 7 were prepared. The lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 4 were charged to 4.2V/1/20C at a rate of 1.0C under CC/CV conditions at 25 ℃ using electrochemical chargers, and then discharged to 2.5V at a rate of 0.5C under CC conditions, which was set as one cycle. 150 charge and discharge cycles were performed, and the capacity retention was measured.

The capacity retention was calculated using the following equation, and the results are shown in table 1 below.

Capacity retention (%) = (discharge capacity after N cycles/discharge capacity after 1 cycle) ×100

In the above formula, N is an integer of 1 or more.

Fig. 2 and 3 show the variation of discharge capacities with cycles of the lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 4. Fig. 4 and 5 show changes in coulombic efficiency with cycles of the lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 4. Fig. 6 shows the variation of discharge capacity with cycles of the lithium secondary batteries of example 1 and comparative examples 5 to 7, and fig. 7 shows the variation of coulombic efficiency with cycles of example 1 and comparative examples 5 to 7. Table 3 shows the discharge capacities and the capacity retention rates of the lithium secondary batteries of examples 1 to 5 and comparative examples 1 to 7 after 150 charge and discharge cycles.

TABLE 3

Referring to fig. 2 to 7 and table 3, it can be confirmed that the lithium secondary batteries of examples 1 to 5 of the present invention exhibited higher discharge capacities and capacity retention rates than the lithium secondary batteries of comparative examples 1 to 7.

From this, it can be understood that the nonaqueous electrolytes of examples 1 to 5 containing both the first additive and the second additive form an SEI film having improved flexibility and durability on a negative electrode containing a silicon-based active material, greatly contributing to improvement of capacity performance and life performance of a lithium secondary battery.

Claims

1. A nonaqueous electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the additive includes a first additive and a second additive, the first additive comprises a compound represented by the following formula 1, and the second additive comprises fluoroethylene carbonate:

[ 1]

In formula 1 above, R ₁ is selected from aryl groups having 6 to 30 carbon atoms substituted with one or more fluorine groups, and heteroaryl groups having 3 to 30 carbon atoms substituted with one or more fluorine groups,

R ₂ is selected from hydrogen, halogen and alkyl having 1 to 5 carbon atoms, and L ₁ is selected from direct bond and alkylene having 1 to 5 carbon atoms.

2. The nonaqueous electrolyte of claim 1, wherein in formula 1, R ₁ is an aryl group having 6 to 30 carbon atoms substituted with one or more fluorine groups.

3. The nonaqueous electrolyte according to claim 1, wherein in formula 1, R ₂ is hydrogen.

4. The nonaqueous electrolyte according to claim 1, wherein the compound represented by formula 1 includes a compound represented by the following formula 2:

[ 2]

Wherein, in formula 2, R ₂ is as defined in formula 1.

5. The nonaqueous electrolyte according to claim 1, wherein the compound represented by formula 1 includes at least one selected from the group consisting of compounds represented by the following formulas 3-a and 3-B:

[ 3-A ]

[ 3-B ]

6. The nonaqueous electrolyte according to claim 1, wherein the content of the first additive in the nonaqueous electrolyte is 0.01 to 10% by weight.

7. The nonaqueous electrolyte according to claim 1, wherein the content of the second additive in the nonaqueous electrolyte is 0.01 to 10% by weight.

8. The nonaqueous electrolyte of claim 1, wherein the weight ratio of the first additive to the second additive is 0.1 to 12.

9. The nonaqueous electrolyte of claim 1, wherein the weight ratio of the first additive to the second additive is 0.3 to 0.7.

10. A lithium secondary battery, comprising:

A negative electrode,

A positive electrode facing the negative electrode,

A separator disposed between the negative electrode and the positive electrode, and

The nonaqueous electrolyte of claim 1.

11. The lithium secondary battery according to claim 10, wherein the negative electrode comprises a silicon-based active material.

12. The lithium secondary battery according to claim 11, wherein the silicon-based active material comprises a compound represented by the following formula 4:

[ 4]

SiO _x, wherein 0≤x <2.

13. The lithium secondary battery according to claim 11, wherein the silicon-based active material is Si.

14. The lithium secondary battery according to claim 10, wherein the young's modulus of the SEI film of the negative electrode is 0.36MPa or less, the young's modulus of the SEI film of the negative electrode being measured by:

(C) The young's modulus of the SEI film formed on the surface of the negative electrode was measured using a nanoindentation technique of an atomic force microscope.