Lithium secondary battery comprising a GEL polymer electrolyte and a method for preparing the same

Abstract

The present invention relates to a lithium secondary battery comprising an anode, a cathode and a gel polymer electrolyte; wherein the cathode comprises a cathode active material; the cathode active material comprises lithium manganese rich oxide; the anode comprises an anode active material; the anode active material comprises silicon; the gel polymer electrolyte comprises a cross-linkable-monomer-based polymer and a lithium salt; cross-linkable-monomer comprises at least three radically polymerizable groups; and the gel polymer electrolyte comprises at least one additive selected from the group consisting of tris(trimethylsilyl) borate (TMSB) and fluoroethylene carbonate (FEC); and the invention relates further to a method for preparing the lithium secondary battery.

Description

LITHIUM SECONDARY BATTERY COMPRISING A GEL POLYMER ELECTROLYTE AND A METHOD FOR PREPARING THE SAME

[0001] The present invention relates to a lithium secondary battery comprising a gel polymer electrolyte and a method for preparing the same.

[0002] The use of silicon (Si)-rich anodes and lithium-manganese-rich oxides (e.g. Li1.34Ni0.35Mn0.65O2, LMR) as cathode material places special demands on the electrolyte.

[0003] The silicon-rich anode expands during lithiation, i.e. the formation of a LixSiyalloy during the charging process, causing the protective layer formed around the particles to become brittle. This leads to a continuous reformation of the protective layer at the brittle spots and thus to the consumption of electrolyte and active Li+. The stress evoked on the particles during volume expansion can lead to cracks and breakage. Overall, the volume changes can lead to contact losses between the current collector and the particles, which reduces the amount of available active material and, therefore, the cycle life of the cells. One challenge for the gel polymer electrolyte is to form a stable protective layer to enable permanent contact between the active material and the current collector and to limit particle cracking.

[0004] Due to the cathode material used and the associated high cell voltage of up to 4.65 V, a stable protective layer must also be built up on the cathode side to protect the electrolyte from continuous decomposition. The high cell voltage also leads to instabilities within the cathode material and thus to so-called transition metal dissolution. Transition metals that are soluble in the electrolyte can migrate to the anode and favor the deposition of lithium metal there. In addition to loss of battery capacity, the high reactivity of the lithium metal poses an operational safety risk. Here too, the gel polymer electrolyte has the task of forming a stable protective layer to reduce such processes.

[0005] In addition to improving contact with the active materials and forming protective layers, the gel polymer is intended to increase the general safety of rechargeable batteries. Polymerization traps the liquid electrolyte in the cell within the polymer matrix and prevents the liquid from leaking in the event of damage to the rechargeable battery. At the same time, the gel polymer electrolyte must also have good ionic conductivity to ensure good performance at elevated C rates. The design of the gel polymer electrolyte is of crucial importance here. Furthermore, the gel polymer electrolyte must be designed that it completely wets the active materials on both electrodes, which can be challenging, especially with high mass loadings.

[0006] By using LMR as the cathode material and Si-rich anodes, the energy density of the cells can be increased compared to systems with graphite and NMC or LFP. The use of microparticulate Si in the anode also leads to a cost reduction compared to nanoparticulate Si, as the energy-intensive step of particle milling is removed. In addition, the smaller surface area of the microparticulate Si leads to fewer side reactions in the first cycle, which improves the Coulomb efficiency. A disadvantage, however, is the increased risk of particle breaking during volume expansion, which reduces the cycle life. To counteract this, the degree of lithiation of the silicon electrodes can be controlled by the choice of the N / P ratio (capacitance of the anode divided by the capacitance of the cathode), avoiding complete lithiation of the silicon electrodes and reducing volume expansion. (D. Jantke et al. J. Electrochem. Soc. 2019, 166, A3881-3885)

[0007] Gel polymers are attractive solutions for increasing both the performance and safety of batteries. The in-situ polymerization process, which ensures improved contact with the active materials and simplifies cell production, is particularly noteworthy here. Better contact with the active material can reduce the interface resistance and improve the performance at higher applied currents. This is particularly advantageous for electrodes with high mass loadings, as the liquid electrolyte can first penetrate the electrode and wet all particles before polymerization. A proven method is radical polymerization, in which acrylates, methacrylates or molecules with vinyl groups, for example, are polymerized in a single temperature step using radical initiators such as azo polymerization initiators.

[0008] Acrylates with 3 or more functional groups are popular monomers for in-situ polymerization, as they form a highly branched network due to their numerous active groups. Examples are pentaerythritol tetraacrylate (PETA), trimethylolpropane triacrylate (TMTA) and dipentaerythritol penta / hexaacrylate. Li et al. used a gel polymer electrolyte consisting of 1M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC): diethyl carbonate (DEC) : ethyl methyl carbonate (EMC) (1:1:1 volume %), 1.5 weight % PETA and 0.1 weight % AIBN in cells with LiNi0.8Co0.15Al0.05O2and a composite of graphite-Si. A three-layer (polypropylene| polyethylene|polypropylene) film was used as a separator. The constructed cells showed improved properties in terms of lifetime, gas formation and safety in so-called nail penetration tests. (X. Li et at. J. Mater. Chem. A, 2017, 5, 18888). Shen et al. combined a mixture of 1M LiPF6in EC:DEC (1:1 volume %), PETA and AIBN (mass ratio 95:5:0.3) with an electrospun polyacrylonitrile separator. This setup was tested in different systems such as NMC811|Li, Li|Li, NMC811|SiC-Gr and LFP|Li. (Z. Shen et al. ACS Appl. Mater. Interfaces 2022, 14, 41022-41036). PETA was also studied together with other monomers that contain a polymerizable double bond and are thus incorporated into the network during polymerization. Hu et al. added hexafluoro butyl acrylate as a monomer and were able to prepare a non-flammable electrolyte from a combination of PETA, hexafluoro butyl acrylate, 1,3-dioxolane, dimethyl ether, lithium nitrate and lithium bis(trifluoromethyl sulfonyl)amide, which also prevents thermal runaway of anode-free lithium-sulfur cells under mechanical stress (A. Hu et al. Adv. Mater. 2023, 35, 2304762). Wu et al. combined PETA and diethyl allyl phosphate with the fluorinated liquids fluoroethylene carbonate (FEC), methyl 2,2,2-trifluoroethyl carbonate and 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether to produce a non-flammable gel polymer electrolyte that is stable at high cell voltages as demonstrated for Li-rich oxides (Li1+xM1-xO2, M = Mn, Ni, and Co). (J. Wu et al. Nat. Commun. 2021, 12, 5746) The combination of PETA with inorganic materials such as silicon dioxide or aluminum fluoride results in a gel polymer electrolyte which positively influences Li ion transport and SEI formation and thus increases the performance in Li|Cu, Li|Li and LFP|Li cells. (C. Guo et al. Adv Funct. Mater. 2023, 33, 2301111 and X. Zhang et al. Nano Energy, 2023, 115, 108700) Bok et al. demonstrated that the combination of patterned silicon with a TMTA-based gel polymer of 1M LiPF6 in EC:DEC with 10 weight % FEC increased the cycle life under continuous charging / discharging compared to a liquid electrolyte. (T. Bok et al. RSC Adv. 2016, 6, 6960) The improved performance was partly due to a lower volume expansion of the electrode.

[0009] Therefore, there is the object of the present invention to provide an electrolyte for use in a lithium secondary battery overcoming drawbacks of the prior art. Especially, it is an object of the present invention to provide a lithium secondary battery with Si-rich anodes and lithium-manganese-rich oxide cathodes with improved performance.

[0010] The above problem is solved in accordance with the independent claims. Further embodiments result from the sub claims and / or the following detailed description.

[0011] Especially, in order to achieve the above objects, the present disclosure provides, in a first embodiment, a lithium secondary battery comprising an anode, a cathode and a gel polymer electrolyte;

[0012] wherein

[0013] - the cathode comprises a cathode active material;

[0014] - the cathode active material comprises lithium manganese rich oxide;

[0015] - the anode comprises an anode active material;

[0016] - the anode active material comprises silicon;

[0017] - the gel polymer electrolyte comprises a cross-linkable-monomer-based polymer and a lithium salt;

[0018] - cross-linkable-monomer comprises at least three radically polymerizable groups; and

[0019] - the gel polymer electrolyte comprises at least one additive selected from the group consisting of tris(trimethylsilyl) borate (TMSB) and fluoroethylene carbonate (FEC).

[0020] According to a second embodiment of the present disclosure, in the first embodiment, cross-linkable-monomer comprises three to six radically polymerizable groups.

[0021] According to a third embodiment of the present disclosure, in the first embodiment or in the second embodiment, the radically polymerizable groups are independently selected from the group consisting of acrylate and (meth)acrylate.

[0022] According to a fourth embodiment of the present disclosure, in any of the first to the third embodiments, the cross-linkable-monomer is pentaerythritol tetraacrylate (PETA).

[0023] According to a fifth embodiment of the present disclosure, in any of the first to the fourth embodiments, the lithium manganese rich oxide is Li1.34Ni0.35Mn0.65O2.

[0024] According to a sixth embodiment of the present disclosure, in any of the first to the fifth embodiments, the lithium secondary battery further comprises a separator between the anode and the cathode.

[0025] According to a seventh embodiment of the present disclosure, in any of the first to the sixth embodiments, the separator comprises poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP).

[0026] According to an eighth embodiment of the present disclosure, in any of the first to the seventh embodiments, the additive is tris(trimethylsilyl) borate (TMSB).

[0027] According to a ninth embodiment of the present disclosure, in any of the first to the eighth embodiments, the gel polymer electrolyte further comprises a non-aqueous solvent.

[0028] According to a tenth embodiment of the present disclosure, in any of the first to the ninth embodiments, the non-aqueous solvent comprises ethylene carbonate (EC), ethyl methylcarbonate (EMC) or a mixture thereof.

[0029] According to an eleventh embodiment of the present disclosure, in any of the first to the tenth embodiments, the lithium salt is LiPF6.

[0030] The present disclosure further provides, in a twelfth embodiment, a method for preparing a lithium secondary battery according to any of the preceding claims, comprising the steps of:

[0031] - polymerizing a reaction mixture comprising a cross-linkable-monomer comprising at least three radically polymerizable groups in the presence of the lithium salt with an initiator to obtain the gel polymer electrolyte; and

[0032] - arranging the gel polymer electrolyte between the anode and the cathode.

[0033] According to a thirteenth embodiment of the present disclosure, in the twelfth embodiment, the amount of cross-linkable-monomer is 1.5 to 2.5 wt.-%, based on the total weight of the reaction mixture.

[0034] According to a fourteenth embodiment of the present disclosure, in the thirteenth embodiment, the initiator is azobisisobutyronitrile (AIBN).

[0035] It was surprisingly found that a gel polymer electrolyte based on a polymer comprising at least three radically polymerizable groups in combination with specific additives, Si-rich anodes and lithium-manganese-rich oxide cathodes results in improved battery performance compared to other combinations such as that using liquid electrolytes, especially results in improvements at elevated temperatures. A highly porous poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) separator supports the use at high cell voltages.

[0036] Figure 1. shows the results of polymerization tests with the gelling agent PETA in different concentrations in the liquid electrolyte 1M LiPF6EC:EMC (3:7 volume / volume).

[0037] Figure 2. shows long-term cycling of gel polymers in LMR||Si cells.

[0038] Figure 3. shows the specific discharge capacity of a gel polymer electrolyte and a non-polymerized liquid in LMR||Si button cells.

[0039] Figure 4. shows the cell voltage from the first cycle of the cells in Figure 3.

[0040] Figure 5. shows cyclic voltammetry measurement of a total of 3 cycles on a Si-rich anode as working electrode with 1M LiPF6EC:EMC (3:7 volume / volume), 2 weight-% PETA and 0.1　weight-% AIBN as gel polymer electrolytes.

[0041] Figure 6. shows results of determination of oxidative stability of various electrolytes against carbon-coated Al electrodes.

[0042] Figure 7. shows long-term cycling of a gel polymer and a liquid electrolyte in LMR||Si cells at 45 °C.

[0043] Figure 8. shows C-rate tests between 0.05C and 5C of LMR||Si coin cells with different electrolytes and separators.

[0044] Figure 9. shows long-term cycling of LMR||Si cells with a liquid electrolyte and a gel polymer electrolyte in different electrolyte formulations.

[0045] Figure 10. shows long-term cycling of LMR||Si cells with the most promising electrolyte formulations.

[0046] Figure 11. shows contact angle measurements of (A) a commercial PP separator compared to (B) a fabricated PVdF-HFP membrane using 1M LiPF6EC:EMC (3:7 v / v) as the liquid electrolyte.

[0047] Figure 12. shows SEM surface image of the PVdF-HFP membrane.

[0048] The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and should be construed in a sense and concept consistent with the technical idea of the present disclosure, based on the principle that the inventor can properly define the concept of a term to describe this invention in the best way possible.

[0049] Unless otherwise restricted, a detailed description defining or specifying the elements may be applied to all inventions and is not limited to descriptions of particular inventions. That is, the present disclosure also refers to combinations of the embodiments even if they are disclosed separately.

[0050] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms "a," "an," and "the" comprise plural referents unless the context clearly dictates otherwise. It is to be understood that the terms such as "comprise" or "have" as used in the present specification, are intended to designate the presence of stated features, numbers, steps, operations, components, parts or combinations thereof, but not to preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof. The term "comprises" explicitly, even if not necessarily limited accordingly, includes the meaning "essentially comprising" and "consists of".

[0051] The term "essentially comprises" as used herein has the meaning of "comprising at least 70 %", preferably of "comprising at least 80 %", most preferred of "comprising at least 90 %". If reference is made to the amount of a constituent in a mixture of material, % is wt %, relative to the total weight of the respective mixture. For example, a material essentially comprising silicon comprises the silicon in an amount of at least 70 wt% with respect to the total weight of the material.

[0052] Additionally, the terms "about" and "substantially" as used herein are used in the sense of at, or nearly at, when given the manufacturing and material tolerances inherent in the stated circumstances and are used to prevent the unscrupulous infringer from unfairly taking advantage of the present disclosure where exact or absolute figures are stated as an aid to understanding the present disclosure.

[0053] As used herein, "A and / or B" means "A and B, or A or B".

[0054] The anode as referred to herein may also be referred to as negative electrode. The cathode as referred to herein may also be referred to as positive electrode.

[0055] Hereinafter, the present disclosure will be described in more detail.

[0056] The present invention relates to a lithium secondary battery. The lithium secondary battery comprises an anode, a cathode and a gel polymer electrolyte.

[0057] Gel polymer electrolyte

[0058] The gel polymer electrolyte comprises a cross-linkable-monomer-based polymer and a lithium salt.

[0059] The expression cross-linkable-monomer-based polymer refers to a polymer obtainable by crosslinking a cross-linkable-monomer.

[0060] The cross-linkable-monomer comprises at least three radically polymerizable groups. The cross-linkable-monomer may comprise three to six radically polymerizable groups. The cross-linkable-monomer may comprise three to five radically polymerizable groups. The cross-linkable-monomer may comprise three or four radically polymerizable groups. The cross-linkable-monomer may comprise four or five radically polymerizable groups. The cross-linkable-monomer may comprise four radically polymerizable groups.

[0061] The radically polymerizable groups may be independently selected from the group consisting of acrylate and (meth)acrylate.

[0062] The cross-linkable monomer comprising at least three radically polymerizable groups may be selected from the group consisting of tris(2-(meth)acryloethyl)isocyanurate, trimethylolpropane tri(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol di(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth) acrylate and a mixture of two or more thereof.

[0063] The cross-linkable-monomer comprising at least three radically polymerizable groups may be pentaerythritol tetraacrylate (PETA).

[0064] The amount of cross-linkable-monomer-based polymer in the gel polymer electrolyte may be from 0.1 to 10 wt.-%, from 0.5 to 5 wt.-%, from 1 to 4 wt.-%, from 1.5 to 3 wt.-%, or from 1.5 to 2.5 wt.-%, such as about 2 wt.-%, based on the total weight of the gel polymer electrolyte.

[0065] The lithium salt can be used without limitation as long as it is commonly used in an electrolyte solution for a lithium secondary battery. For example, the lithium salt may be LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, tetra-phenyl lithium borate, lithium imide, etc. or a mixture of two or more thereof. Preferably, the lithium salt is LiPF6, (CF3SO2)2NLi, LiN(SO2F)2or a mixture thereof. Most preferred, the lithium salt is LiPF6.

[0066] The concentration of the lithium salt in the gel polymer electrolyte may be 0.2 to 2 M, preferably 0.4 to 2 M, more preferably 0.4 to 1.7 M, most preferred from 0.8 to 1.2 M, such as about 1 M, depending on various factors such as the exact composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and the like. When the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte may be lowered and thus the performance of the electrolyte may be deteriorated. When the concentration of the lithium salt is more than 2 M, the viscosity of the electrolyte may increase and thus the mobility of the lithium ion may be reduced.

[0067] The gel polymer electrolyte comprises at least one additive selected from the group consisting of tris(trimethylsilyl) borate (TMSB) and fluoroethylene carbonate (FEC), that is, either tris(trimethylsilyl) borate (TMSB) or fluoroethylene carbonate (FEC) or both. The additive may be tris(trimethylsilyl) borate (TMSB).

[0068] In case that the gel polymer electrolyte comprises tris(trimethylsilyl) borate (TMSB), the TSMB may be comprised in the gel polymer electrolyte in amount from 0.1 to 5 wt.-%, from 0.8 to 1.5 wt.-%, from 0.2 to 4 wt.-%, from 0.5 to 3 wt.-%, from 0.7 to 2 wt.-%, or from 0.8 to 1.2 wt.-%, such as about 1 wt.-%, based on the total weight of the gel polymer electrolyte.

[0069] In a first embodiment in which the gel polymer electrolyte comprises fluoroethylene carbonate (FEC), the fluoroethylene carbonate (FEC) is contained in the gel polymer electrolyte in amount from 0.1 to 5 wt.-%, from 0.8 to 1.5 wt.-%, from 0.2 to 4 wt.-%, from 0.5 to 3 wt.-%, from 0.7 to 2 wt.-%, or from 0.8 to 1.2 wt.-%, such as about 1 wt.-%, based on the total weight of the gel polymer electrolyte. In this first embodiment, the fluoroethylene carbonate (FEC) which is contained in the gel polymer electrolyte in said amounts can be considered as an electrolyte additive.

[0070] In a second embodiment in which the gel polymer electrolyte comprises fluoroethylene carbonate (FEC), the fluoroethylene carbonate (FEC) is contained in the gel polymer electrolyte in an amount from 15 to 35 wt.-%, or from 25 to 23 wt.-%, such as about 30 wt.-%, based on the total weight of the gel polymer electrolyte. In this second embodiment, the fluoroethylene carbonate (FEC) which is contained in the gel polymer electrolyte in said amounts can be considered as a co-solvent.

[0071] The gel polymer electrolyte may further comprise a non-aqueous solvent.

[0072] The non-aqueous solvent may be used without limitation, and for example, may be ether, ester, amide, linear carbonate, cyclic carbonate, etc. may be used alone or in combination of two or more. Among them, linear carbonates and cyclic carbonates are preferred.

[0073] For example, the acyclic ether may be, but is not limited to, at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether, ethylpropyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethylmethylether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methylethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methylethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methylethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methylethyl ether.

[0074] As an example, the cyclic ether may be, but is not limited to, at least one selected from the group consisting of 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,4-dimethoxy benzene, and isosorbide dimethyl ether.

[0075] Examples of the ester of the organic solvent may be, but is not limited to, any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, and a mixture of two or more thereof.

[0076] Specific examples of the linear carbonate compound may representatively be, but is not limited to, any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, or a mixture of two or more thereof.

[0077] In addition, specific examples of the cyclic carbonate compound may be any 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, vinylethylene carbonate, and halides thereof, or a mixture of two or more thereof. Examples of such halides comprise, but are not limited to, fluoroethylene carbonate (FEC) and the like. If fluoroethylene carbonate (FEC) is comprised in the non-aqueous electrolyte as a (co-)solvent, the non-aqueous electrolyte comprises at least one further non-aqueous solvent which is different from fluoroethylene carbonate (FEC).

[0078] The non-aqueous solvent may comprise, essentially comprise or consist of ethylene carbonate (EC), ethyl methylcarbonate (EMC) or a mixture thereof.

[0079] The non-aqueous solvent may be ethyl methylcarbonate (EMC) or may be a mixture of a mixture of ethylene carbonate (EC) and ethyl methylcarbonate (EMC) in a ratio (vol.) from 10:90 to 50:50, 15:85 to 45:55, 20:80 to 40:60, 25:75 to 35:65, or 28:72 to 32:68, such as about 30:70.

[0080] If the non-aqueous solvent is ethyl methylcarbonate (EMC), the non-aqueous electrolyte may comprise fluoroethylene carbonate (FEC) as a co-solvent, wherein a in a ratio (vol.) of FEC:EMC may be from 10:90 to 50:50, 15:85 to 45:55, 20:80 to 40:60, 25:75 to 35:65, or 28:72 to 32:68, such as about 30:70.

[0081] The amount of the non-aqueous solvent may balance the amounts of the further constituents of the gel polymer electrolyte and may be 70wt.-% or mor, 80 wt.-% or more, or more 90 wt.-% or more, based on the total weight of the gel polymer electrolyte.

[0082] Cathode

[0083] The lithium secondary battery according to the present invention comprises a cathode. The cathode comprises a cathode active material. The cathode may comprise a cathode current collector and a cathode active material. The cathode may be manufactured by coating the cathode active material on the cathode current collector.

[0084] The cathode comprises lithium manganese rich oxide (LMR). In detail, the cathode active material may comprise the lithium manganese rich oxide (LMR). The positive electrode active material may comprise, for example, lithium manganese oxide such as formula Li1+xMn2-xO4(x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, LiNi1-xMxO2(M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3); lithium manganese composite oxide represented by formula LiMn2-xMxO2(M = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01 to 0.1) or Li2Mn3MO8(M = Fe, Co, Ni, Cu or Zn); LiMn2O4with partial substitution of alkali earth metal ion for Li. The lithium manganese rich oxide may be represented by the formula Li1.34Ni0.35Mn0.65O2.

[0085] The cathode active material may further comprise a conductive material which may be added in an amount of 0.1 weight% to 30 weight% based on the total weight of the cathode active material. The conductive material is not limited to any particular type when it has conductive properties while not causing a chemical change to the corresponding battery, and may include, for example, conductive materials, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers or metal fibers; fluorocarbon, metal powder such as aluminum powder and nickel powder; conductive whiskers such as oxide zinc and potassium titanate; conductive metal oxide such as titanium oxide; and polyphenylene derivatives.

[0086] The cathode active material may further comprise a binder and may be added in an amount of 0.1 weight% to 30 weight% based on the total weight of the mixture comprising the cathode active material. Examples of the binder may comprise polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regeneratedcellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, fluorine rubber and various types of copolymers.

[0087] In general, the cathode current collector may be manufactured with the thickness of 3 ㎛ to 500 ㎛, and is not limited to a particular type and may include any material having high conductivity without causing any chemical change to the corresponding battery, for example, one selected from stainless steel, aluminum, nickel, titanium, and aluminum or stainless steel surface treated with carbon, nickel, titanium or silver, and specifically aluminum. The current collector may have macrotexture on the surface to improve the adhesion strength of the positive electrode active material, and may come in various types, for example, films, sheets, foils, nets, porous bodies, foams and non-woven fabrics.

[0088] Anode

[0089] The lithium secondary battery according to the present invention further comprises an anode. The anode comprises an anode active material. The anode active material may be a porous anode active material (= anode active materials having pores). The anode may essentially comprise or consist of the anode active material. Alternatively, only a part of the anode may essentially comprise or consist of the anode active material, for example one or more layers of the anode.

[0090] In one embodiment the anode (= negative electrode) comprises essentially comprises or consists of a current collector and an anode active material layer, wherein the anode active material layer is provided on at least one surface of the current collector. In such a case, the current collector is a negative electrode current collector. The anode active material layer comprises, essentially comprises or consists of the anode active material.

[0091] The anode may be manufactured by coating the anode active material on a negative electrode current collector and drying.

[0092] The negative electrode current collector may be manufactured with the thickness of 3 ㎛ to 500 ㎛. The negative electrode current collector is not limited to a particular type and may comprise any material having conductive properties without causing any chemical change to the corresponding battery, for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless-steel surface treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloy. The negative electrode current collector may have microtexture on the surface to improve the adhesion strength of the anode active material, and may come in various types, for example, films, sheets, foils, nets, porous bodies, foams and non-woven fabrics.

[0093] For example, the porous anode active material may comprise silicon, silicon-containing alloys; carbons such as non-graphitizing carbon and graphite-based carbon; metal composite oxides such as LixFe2O3(0≤x≤1), LixWO2(0≤x≤1), SnxMe1-xMe'yOz(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Groups 1, 2 and 3 elements of the periodic table, halogen; 0<x≤1; 1≤y≤3; 1≤z≤8); tin-containing alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; and conductive polymers such as polyacetylene; preferably comprises, essentially comprises or consists of silicon, wherein the silicon is preferably microcrystalline silicon (μ-Si). It is preferred that the porous anode active material microcrystalline silicon (μ-Si) in an amount of at least 80 wt.-%.

[0094] The porous anode active material may include a binder. The binder included in the porous anode active material is usually added in an amount of 0.1 weight% to 30 weight% based on the total weight of the mixture comprising the porous anode active material. In an exemplary embodiment of the present application, the negative electrode binder may comprise at least one selected from the group consisting of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, polyacrylic acid and a material in which the hydrogen thereof is substituted with Li, Na, Ca, or the like, and may also comprise various polymers thereof.

[0095] The porous anode active material may have a porosity of at least about 10%, at least 20%, or at least 25%. The porous anode active material may have a porosity of 90% or less, 80 % or less, 70% or less, 60% or less, 50% or less 40% or less 35% or less, or 30 % or less. Preferably, porous anode active material has a porosity from 20% to 35%, more preferably from 25% to 30%, such as about 30%. Such a porosity of the porous anode active material is advantageous in view of the amount of Li which can be deposited in the porous anode active material and in view of the achieved capacity of a lithium secondary battery using such a porous anode active material.

[0096] The term "porosity" used in the present specification refers to a fraction of voids in a structure over the total volume and is indicated in %, and may be used interchangeably with void fraction, degree of porosity or the like. In the present disclosure, the porosity may be measured by mercury permeation method (Hg porosimeter) according to ASTM D-2873 in the version at the priority date of the present application.

[0097] Separator

[0098] The lithium secondary battery according to the present invention may further comprise a separator. The separator may be arranged between the anode and the cathode. The separator may be made of a porous non-conductive or insulating material and enables transport of lithium ions between the anode and the cathode. The separator may be used without special limitation, may be one conventionally used as a separator in a conventional lithium secondary battery. The separator may be an independent member such as a film.

[0099] The separator may be made of a porous substrate, and the porous substrate may be used as long as it is a porous substrate commonly used for a lithium-sulfur battery, and porous polymer films may be used alone or by laminating them, and for example, a nonwoven fabric or a polyolefin-based porous membrane made of glass fibers, polyethylene terephthalate fibers, etc. having a high melting point may be used, but is not limited thereto.

[0100] The material of the porous substrate is not particularly limited in the present disclosure, and any material can be used as long as it is a porous substrate commonly used in an electrochemical device. For example, the porous substrate may comprise at least one material selected from the group consisting of polyolefin such as polyethylene and polypropylene, polyester such as polyethyleneterephthalate and polybutyleneterephthalate, polyamide, polyacetal, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, polyethylenenaphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, poly(p-phenylene benzobisoxazole), and polyarylate.

[0101] The separator may be a porous substrate comprising, essentially comprising or consisting of an at least partially fluorinated polymer, wherein the at least partially fluorinated polymer is preferably selected from the group consisting of polytetrafluoroethylene, polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and a mixture of two or more thereof.

[0102] The separator may be a porous substrate comprising, essentially comprising or consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP).

[0103] The thickness of the separator is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness range of the separator is not particularly limited to the above-mentioned range, when the thickness is excessively thinner than the lower limit described above, mechanical properties are deteriorated and thus the separator may be easily damaged during pre-lithiation.

[0104] The separator may have pores.

[0105] It may be provided that 90 % or more of the pores of the separator have an average diameter in the range selected from the group consisting of from 0.001 μm to 50 μm, from 0.01 μm to 20 μm, from 0.05 μm to 10 μm, from 0.1 μm to 5 μm, and from 0.2 μm to 2 μm. The diameter of a pore in this regard is the largest diameter of the pore determined by SEM.

[0106] SEM measurements as referred to herein are scanning electron microscopy with a Zeiss Crossbeam 550 workstation (Carl Zeiss Microscopy GmbH), equipped with a field emission gun, at an acceleration voltage of 3 kV and a working distance of 5.1 mm. Images were recorded with an in-lens detector using the SmartSEM software (Version 6.07).

[0107] The separator having pores may have a porosity selected from the group consisting from 10 to 95% by volume, from 20 to 90% by volume, from 30 to 85% by volume, from 40 to 80% by volume, from 50 to 75 % by volume, and from 55 to 70 % by volume. The porosity is calculated by taking the weight and dimensions (volume) of the prepared membranes as well as the density (such as 1.78kg / m3for PVdF-HFP) into account. With the weight of the membrane and density the volume can be calculated. From the measured thickness and area of a membrane the total volume of the membrane is calculated. Subtracting the volume from e.g. PVdF-HFP from the total volume gives the pore volume. The porosity in % is the pore volume divided by the total volume (multiplied by 100 for %).　A balance as well as a thickness gauge　are used for determining the necessary parameters.

[0108] A contact angle of a liquid electrolyte solution (preferably 1M LiPF6EC:EMC (3:7 v / v)) on the separator may be in a range selected from the group consisting of from 1° to 40°, from 5° to 35°, from 10° to 30°, from 15° to 30°, from 15° to 25°, and from 16° to 22°. Contact angle measurements were carried out on a DSA 100 by Kruss.

[0109] Lithium secondary battery

[0110] The present invention relates to a lithium secondary battery, especially a lithium-ion secondary battery.

[0111] The lithium secondary battery according to the present disclosure can be manufactured by lamination, stacking, and folding processes of the separator and the electrodes, in addition to the usual winding process.

[0112] The shape of the lithium secondary battery is not particularly limited and may be various shapes such as a cylindrical shape, a laminate shape, and a coin shape.

[0113] Also, the present disclosure provides a battery module comprising the lithium-sulfur battery described above as a unit battery.

[0114] The battery module may be used as a power source for medium to large-sized devices requiring high temperature stability, long cycle characteristics, high capacity characteristics, and the like.

[0115] Examples of such medium to large-sized devices may comprise, but are not limited to, a power tool powered and moved by an electric motor; an electric car including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; an electric two-wheeled vehicle including an electric bike (E-bike) and an electric scooter (E-scooter); an electric golf cart; a power storage system, etc.

[0116] Method for preparing the lithium secondary battery

[0117] According to a further aspect, the present invention relates to a method for preparing a lithium secondary battery according to the present invention, that is, the lithium secondary battery as described herein above.

[0118] The method for preparing a lithium secondary battery comprises a step of polymerizing a reaction mixture comprising a cross-linkable-monomer comprising at least three radically polymerizable groups in the presence of the lithium salt with an initiator to obtain the gel polymer electrolyte.

[0119] The cross-linkable-monomer may comprise three to six radically polymerizable groups. The cross-linkable-monomer may comprise three to five radically polymerizable groups. The cross-linkable-monomer may comprise three or four radically polymerizable groups. The cross-linkable-monomer may comprise four or five radically polymerizable groups. The cross-linkable-monomer may comprise four radically polymerizable groups.

[0120] The radically polymerizable groups may be independently selected from the group consisting of acrylate and (meth)acrylate.

[0121] The cross-linkable monomer comprising at least three radically polymerizable groups may be selected from the group consisting of tris(2-(meth)acryloethyl)isocyanurate, trimethylolpropane tri(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol di(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth) acrylate and a mixture of two or more thereof.

[0122] The cross-linkable-monomer comprising at least three radically polymerizable groups may be pentaerythritol tetraacrylate (PETA).

[0123] The amount of cross-linkable-monomer-based polymer in the reaction mixture may be from 0.1 to 10 wt.-%, from 0.5 to 5 wt.-%, from 1 to 4 wt.-%, from 1.5 to 3 wt.-%, or from 1.5 to 2.5 wt.-%, such as about 2 wt.-%, based on the total weight of the reaction mixture.

[0124] The lithium salt can be used without limitation as long as it is commonly used in an electrolyte solution for a lithium secondary battery. For example, the lithium salt may be LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, tetra-phenyl lithium borate, lithium imide, etc. or a mixture of two or more thereof. Preferably, the lithium salt is LiPF6, (CF3SO2)2NLi, LiN(SO2F)2or a mixture thereof. Most preferred, the lithium salt is LiPF6.

[0125] The concentration of the lithium salt in the reaction mixture may be 0.2 to 2 M, preferably 0.4 to 2 M, more preferably 0.4 to 1.7 M, most preferred from 0.8 to 1.2 M, such as about 1 M, depending on various factors such as the exact composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and the like. When the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte may be lowered and thus the performance of the electrolyte may be deteriorated. When the concentration of the lithium salt is more than 2 M, the viscosity of the electrolyte may increase and thus the mobility of the lithium ion may be reduced.

[0126] The reaction mixture further comprises at least one additive selected from the group consisting of tris(trimethylsilyl) borate (TMSB) and fluoroethylene carbonate (FEC), that is, either tris(trimethylsilyl) borate (TMSB) or fluoroethylene carbonate (FEC) or both. The additive may be tris(trimethylsilyl) borate (TMSB).

[0127] In case that the reaction mixture comprises tris(trimethylsilyl) borate (TMSB), the TSMB may be comprised in the reaction mixture in amount from 0.1 to 5 wt.-%, from 0.8 to 1.5 wt.-%, from 0.2 to 4 wt.-%, from 0.5 to 3 wt.-%, from 0.7 to 2 wt.-%, or from 0.8 to 1.2 wt.-%, such as about 1 wt.-%, based on the total weight of the reaction mixture.

[0128] In a first embodiment in which the reaction mixture comprises fluoroethylene carbonate (FEC), the fluoroethylene carbonate (FEC) is contained in the reaction mixture in amount from 0.1 to 5 wt.-%, from 0.8 to 1.5 wt.-%, from 0.2 to 4 wt.-%, from 0.5 to 3 wt.-%, from 0.7 to 2 wt.-%, or from 0.8 to 1.2 wt.-%, such as about 1 wt.-%, based on the total weight of the reaction mixture. In this first embodiment, the fluoroethylene carbonate (FEC) which is contained in the reaction mixture in said amounts can be considered as an electrolyte additive.

[0129] In a second embodiment in which the reaction mixture comprises fluoroethylene carbonate (FEC), the fluoroethylene carbonate (FEC) is contained in the reaction mixture in an amount from 15 to 35 wt.-%, or from 25 to 23 wt.-%, such as about 30 wt.-%, based on the total weight of the reaction mixture. In this second embodiment, the fluoroethylene carbonate (FEC) which is contained in the reaction mixture in said amounts can be considered as a co-solvent.

[0130] The reaction mixture may further comprise a non-aqueous solvent.

[0131] The non-aqueous solvent may be used without limitation, and for example, may be ether, ester, amide, linear carbonate, cyclic carbonate, etc. may be used alone or in combination of two or more. Among them, linear carbonates and cyclic carbonates are preferred.

[0132] For example, the acyclic ether may be, but is not limited to, at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether, ethylpropyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethylmethylether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methylethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methylethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methylethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methylethyl ether.

[0133] As an example, the cyclic ether may be, but is not limited to, at least one selected from the group consisting of 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,4-dimethoxy benzene, and isosorbide dimethyl ether.

[0134] Examples of the ester of the organic solvent may be, but is not limited to, any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, and a mixture of two or more thereof.

[0135] Specific examples of the linear carbonate compound may representatively be, but is not limited to, any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, or a mixture of two or more thereof.

[0136] In addition, specific examples of the cyclic carbonate compound may be any 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, vinylethylene carbonate, and halides thereof, or a mixture of two or more thereof. Examples of such halides comprise, but are not limited to, fluoroethylene carbonate (FEC) and the like. If fluoroethylene carbonate (FEC) is comprised in the non-aqueous electrolyte as a (co-)solvent, the non-aqueous electrolyte comprises at least one further non-aqueous solvent which is different from fluoroethylene carbonate (FEC).

[0137] The non-aqueous solvent may comprise, essentially comprise or consist of ethylene carbonate (EC), ethyl methylcarbonate (EMC) or a mixture thereof.

[0138] The non-aqueous solvent may be ethyl methylcarbonate (EMC) or may be a mixture of a mixture of ethylene carbonate (EC) and ethyl methylcarbonate (EMC) in a ratio (vol.) from 10:90 to 50:50, 15:85 to 45:55, 20:80 to 40:60, 25:75 to 35:65, or 28:72 to 32:68, such as about 30:70.

[0139] If the non-aqueous solvent is ethyl methylcarbonate (EMC), the non-aqueous electrolyte may comprise fluoroethylene carbonate (FEC) as a co-solvent, wherein a in a ratio (vol.) of FEC:EMC may be from 10:90 to 50:50, 15:85 to 45:55, 20:80 to 40:60, 25:75 to 35:65, or 28:72 to 32:68, such as about 30:70.

[0140] The amount of the non-aqueous solvent may balance the amounts of the further constituents of the reaction mixture and may be 70 wt.-% or mor, 80 wt.-% or more, or more 90 wt.-% or more, based on the total weight of the reaction mixture.

[0141] The initiator may be a radical a radical initiator. The initiator may be an azo-type initiator, an acetophenone-based compound, a biimidazole-based compound, a triazine-based compound, or an oxime-based compound. The azo-type initiator may be selected from the group consisting of 2,2'-azobis-isobutyronitrile (AIBN), 2,2'-azobis(2-methylbutyronitrile) (AMBN), 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN), 1,1'-azobis(1-cyclohexanecarbonitrile) (ACHN), dimethyl-2,2'-azobisisobutyrate (MAIB), 4,4'-azobis(4-cyanovaleric acid) (ACVA), 1,1'-azobis(1-acetoxy-1-phenylethane), 2,2'-azobis(2-methylbutylamide), 1,1'-azobis(methyl 1-cyclohexanecarbonate), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4,4-trimethylpentane) and 2-cyano-2-propylazoformamide. The acetophenone-based compound usable as the radical initiator is selected from the group consisting of 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)-phenyl-(2-hydroxy-2-propyl) ketone, 1-hydroxycyclohexyl phenyl ketone, benzoin methyl ether, benzoin ethyl ether, benzoin isobutyl ether, benzoin butyl ether, 2,2-dimethoxy-2-phenyl acetophenone, 2-methyl-(4-methylthio)phenyl-2-morpholino-1-propan-1-one, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 2-(4-bromo-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one, the biimidazole-based compound is selected from the group consisting of 2,2-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl biimidazole, 2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetrakis(3,4,5-trimethoxyphenyl)-1,2'-biimidazole, 2,2'-bis(2,3-dichlorophenyl)-4,4',5,5'-tetraphenyl bimidazole and 2,2'-bis(o-chlorophenyl)-4,4,5,5'-tetraphenyl-1,2'-biimidazole, the triazine-based compound is selected from the group consisting of 3-{4-[2,4-bis(trichloromethyl)-s-triazin-6-yl]phenylthio}propionic acid, 1,1,1,3,3,3-hexafluoroisopropyl-3-{4-[2,4-bis(trichloromethyl)-s-triazin-6-yl]phenylthio}propionate, ethyl-2-{4-[2,4-bis(trichloromethyl)-s-triazin-6-yl]phenylthio}acetate, 2-epoxyethyl-2-{4-[2,4-bis(trichloromethyl)-s-triazine-6-yl]phenylthio}acetate, cyclohexyl-2-{4-[2,4-bis(trichloromethyl)-s-triazine-6-yl]phenylthio}acetate, benzyl-2-{4-[2,4-bis(trichloromethyl)-s-triazine-6-yl]phenylthio}acetate, 3-{chloro-4-[2,4-bis(trichloromethyl)-s-triazine-6-yl]phenylthio}propionic acid, 3-{4-[2,4-bis(trichloromethyl)-s-triazine-6-yl]phenylthio}propionamide, 2,4-bis(trichloromethyl)-6-p-methoxystyryl-s-triazin, 2,4-bis(trichloromethyl)-6-(1-p-dimethylaminophenyl)-1,3-butadienyl-s-triazine and 2-trichloromethyl-4-amino-6-p-methoxystyryl-s-triazine, and the oxime-based compound includes 1,2-octadione-1-(4-phenylthio)phenyl-2-(o-benzoyloxime) (Ciba Geigy, CGI 124), and ethanone-1-(9-ethyl)-6-(2-methylbenzoyl-3-yl)-1-(o-acetyloxime) (CGI242), oxime OX-03 (Ciba Geigy), NCI-831 (ADEKA), PI-102 (LG Chem), PBG 304, PBG 305, PBG 3057 (Tronly), and the like.

[0142] The initiator may be selected from the group consisting of 2,2'-azobis-isobutyronitrile (AIBN), 2,2'-azobis(2-methylbutyronitrile) (AMBN), 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN), 1,1'-azobis(1-cyclohexanecarbonitrile) (ACHN), dimethyl-2,2'-azobisisobutyrate (MAIB), 4,4'-azobis(4-cyanovaleric acid) (ACVA), 1,1'-azobis(1-acetoxy-1-phenylethane), 2,2'-azobis(2-methylbutylamide), 1,1'-azobis(methyl 1-cyclohexanecarbonate), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4,4-trimethylpentane) and 2-cyano-2-propylazoformamide.

[0143] The initiator may be selected from the group consisting of 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN), ,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis-isobutyronitrile (AIBN).

[0144] The initiator may be 2,2'-azobis-isobutyronitrile (AIBN).

[0145] The content of the initiator may be from 0.01 to 5 wt.-%, from 0.05 to 1 wt.-%, from 0.05 to 0.15 wt.-%, such as about 0.1 wt.-%, based on the total weight of the reaction mixture.

[0146] The gel polymer electrolyte obtained by the polymerizing of the reaction mixture is arranged between the anode and the cathode.

[0147] The gel polymer electrolyte may be obtained by the polymerizing of the reaction mixture first and may be arranged thereafter between the anode and the cathode.

[0148] Preferably, the reaction mixture is placed between the anode an the cathode first and thereafter polymerized to obtain the gel polymer electrolyte, that is, the gel polymer electrolyte may be obtained by in situ polymerization of the reaction mixture.

[0149] In addition to the gel polymer electrolyte, a separator may be arranged between the anode and the cathode. According to one embodiment, a separator and the reaction mixture may be arranged between the anode and the cathode first and thereafter the reaction mixture may be polymerized to obtain the gel polymer electrolyte, that is, the gel polymer electrolyte may be obtained by in situ polymerization of the reaction mixture between the anode and the cathode in the presence of the separator, wherein preferably the separator is a porous separator and the reaction mixture is preferably at least partially impregnated into the porous separator before polymerizing.

[0150] The at least partially impregnating (wetting) of the reaction mixture into the porous separator before polymerizing is favorable to support hat the liquid phase containing the gelation agent fully permeates all active materials within the electrodes before the liquid phase solidifies during polymerization. The wetting time (= time of partially impregnating the reaction mixture into the porous separator before polymerizing) may be at least one hour or at least three hours, preferably at least 6 hours. The rection mixture may be least partially impregnated into the porous separator before polymerizing at a temperature from room temperature to 100°C, preferably from 40 to 80°C, more preferably from 50 to 70°C, such as about 60 °C.

[0151] The polymerizing of the reaction mixture may be performed at a temperature from 30 to 80°C, or from 50°C to 80°C. The polymerizing of the reaction mixture may be performed for 2 to 20 hours.

[0152] Further aspects of the invention

[0153] The following further aspects of the invention may be, separately or in combination with each other or parts of the forgoing generalized specification of the invention, material for realizing the invention.

[0154] The cross-linkable-monomer-based polymer may be produced by adding 1.5 to 3 weight % of a cross-linkable-monomer comprising at least three radically polymerizable groups, such as PETA, and an initiator, such as AIBN, to a liquid electrolyte mixture of a lithium salt, a carbonate-based solvent and at least one additive selected from TMSB and FEC. As shown in Figure 1, a minimum amount of 1.5 weight % PETA is favorable for sufficient gelation.

[0155] The polymerization of the cross-linkable-monomer comprising at least three radically polymerizable groups may take place after cell assembly by means of a temperature step that may be carried out between 50 and 80 °C for a period of 2 to 20 hours. The selected temperature depends on the initiator and must not be too high in order to avoid decomposition of lithium salt, such as LiPF6. AIBN may be used as an initiator. Alternative initiators can be, for example, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) and 2,2'-azobis(2,4-dimethylvaleronitrile), which also allow lower polymerization steps due to their lower half-lives t1 / 2of approx. 30 °C and 51 °C, respectively.

[0156] The combination of a separator with the gel polymer electrolyte is favorable to prevent a short circuit before polymerization. Commercially available separators, e.g. polyolefin-based separators, or specially manufactured separators with a (macro)porous structure can be used as separators. The latter can particularly improve performance at increased discharge currents, as shown in Figure 8.

[0157] The gel polymer electrolyte as described herein results in improved performance of cells with a silicon-rich anode and a lithium-manganese-rich cathode. In addition to safety can be increased by using the gel polymer electrolyte, which can prevent the electrolyte from leaking. The cross-linkable-monomer comprising at least three radically polymerizable groups, such as PETA, may also involved in the formation of a protective layer, especially on the anode (Figure 5), and thus improves the performance of the lithium secondary battery as shown in Figures 2 to 5.

[0158] The use of a gel polymer electrolyte using cross-linkable-monomer-based polymer with cross-linkable-monomers comprising at least three radically polymerizable groups improves the performance at elevated temperatures as shown in Figure 7.

[0159] By combining such a gel polymer electrolyte, a highly porous separator and suitable additives such as TMSB, the electrochemical performance of LMR cathodes with microcrystalline Si electrodes in lithium secondary batteries can be increased (as shown in Figures 9 and 10). By increasing the N / P ratio (increasing the capacity of the anode), the roll-over failure during long-term cycling can be further delayed.

[0160] The appropriate choice of electrolyte and the required amount of PETA, initiator (e.g. AIBN) to gel the liquid phase in the lithium secondary battery is favorable to achieve the technical effects described herein.

[0161] Production of the optimized electrolyte formulation by mixing adapted quantities of Li salt, carbonate-based solvents and additives is favorable to achieve the technical effects described herein.

[0162] A highly porous separator is favorable to improve the fast-charging capability especially together with the selection of a suitable material.

[0163] The assembly of a lithium secondary battery, selection of the appropriate time for wetting of the active material by the liquid phase and selection of the appropriate temperature and time for in-situ polymerization are favorable to achieve the technical effects described herein.

[0164] Different liquid or gel polymer electrolytes were prepared and tested as follows

[0165] Preparation Exmaple 1: 1M LiPF6EC:EMC(3:7 voumne%)

[0166] 1.519 g LiPF6were placed in a 10 mL volumetric flask, a mixture of EC and EMC of 30:70 volume % (approx. 8 g) was added and stirred until the salt was completely dissolved. The remaining amount of the mixture was then added and the volumetric flask was filled up to the 10 mL mark. To prepare the gel polymer electrolyte, the liquid / PETA / AIBN components were mixed in a ratio of 97.9 / 2 / 0.1 weight %.

[0167] Preparation Exmaple 2: 1M LiPF6EC:EMC with FEC(10:1)

[0168] A solvent mixture EC:EMC (30:70 volume %) was mixed with FEC (10:1 weight %). Then 1.519 g LiPF6was placed in a 10 mL volumetric flask, the mixture (approx. 8 g) was added and stirred until the salt was completely dissolved. The remaining amount of the mixture was then added and the volumetric flask was filled up to the 10 mL mark. To prepare the gel polymer electrolyte, the liquid / PETA / AIBN components are mixed in a ratio of 97.9 / 2 / 0.1 weight %.

[0169] Preparation Exmaple 3: 1M LiPF6EC:EMC+1weight% TMSB

[0170] 1.980 g 1M LiPF6 EC:EMC (3:7 volume %) was mixed with 0.020 g TMSB. To prepare the gel polymer electrolyte, the components 1M LiPF6 EC:EMC / TMSB / PETA / AIBN were mixed in a ratio of 96.9 / 1 / 2 / 0.1 weight %.

[0171] Preparation Exmaple 4: 1M LiPF6FEC:EMC+1weight% TMSB

[0172] 1.519 g LiPF6was placed in a 10 mL volumetric flask, a mixture of FEC and EMC of 30:70 volume % (approx. 8 g) was added and stirred until the salt was completely dissolved. The remaining amount of the mixture was then added and the volumetric flask was filled up to the 10 mL mark. For the liquid electrolyte, 1.980 g 1M LiPF6FEC:EMC (3:7 volume %) was mixed with 0.020 g TMSB. To prepare the gel polymer electrolyte, the components 1M LiPF6FEC:EMC / TMSB / PETA / AIBN are mixed in a ratio of 96.9 / 1 / 2 / 0.1 weight %.

[0173] The formulated electrolytes according to Preparation Examples 1 to 4 with PETA and AIBN were stored in a cool place to prevent polymerization.

[0174] Preparation Example 5: Highly porous PVdF-HFP separator

[0175] A highly porous PVdF-HFP separator having a porosity of 60% was produced by preparing a solution of 10 weight % PVdF-HFP in a mixture of water:acetone (weight ratio 1:8). To dissolve PVdF-HFP completely, the solution was heated to 70 °C in a closed vessel, cooled briefly and then placed on a glass plate and coated with a doctor-blade. After evaporation of the solvent acetone, the membrane was removed from the glass plate and dried at 60 °C in a vacuum oven. The presence of the non-solvent water results in formation of macropores.

[0176] Battery preparation

[0177] Battery preparation is described here exemplarily for coin-type cell formats. The negative electrode (anode) is placed onto a spacer and positioned in the bottom part of a coin cell, which includes a spring. The electrode surface is first wetted with 15 μL of liquid electrolyte, followed by placing either the PVdF-HFP membrane or a commercial separator onto the electrode. An additional 20 μL of electrolyte is then applied on top of the separator / membrane. Subsequently, the cathode, another spacer, and the coin cell casing are added. The assembled cells are rested for at least 6 hours to ensure complete wetting of the electrode particles. Cells containing a polymerizable component are then heat-treated at 60 °C for 6 hours to solidify the liquid phase.

[0178] Test results

[0179] Figure 1 shows the results of polymerisation tests with the gelling agent PETA in different concentrations in the liquid electrolyte 1M LiPF6 EC:EMC (3:7 volume / volume). Shown are different mixtures of the gelling agent with the liquid electrolyte, from left to right the following amounts of PETA were added: 1) 0 weight %, 2) 0.5 weight %, 3) 1 weight %, 4) 1.5 weight %, 5) 2.0 weight %, 6) 2.5 weight %, 7) 3.0 weight %. The amount of PETA was added to the electrolyte in weight percent (weight %) and polymerization was carried out at 60 °C for 6 hours. As an initiator for the radical polymerization, 0.1 weight % of AIBN was added. From 1.5 weight %, a gel polymer electrolyte is formed by the formation of a branched polymer network by PETA monomers. A solid phase is formed from a PETA content of 1.5 weight %.

[0180] Figure 2 shows long-term cycling of gel polymers in LMR||Si cells. X is the number of cycles and Q is the specific discharge capacity in mAh g-1. The gel polymer consists of 1M LiPF6in EC:EMC (3:7 volume / volume) with X weight % PETA and 0.1 weight % AIBN:

[0181] 1) liquid electrolyte without PETA and AIBN,

[0182] 2) 1.5 weight % PETA,

[0183] 3) 2.0 weight % PETA,

[0184] 4) 2.5 weight % PETA,

[0185] 5) 3 weight % PETA.

[0186] Lithium manganese rich (LMR) cathodes and silicon rich anodes were used as electrodes. As the polymerization was carried out in situ, the electrolyte mixture with the PETA monomer was placed on a polyolefin separator and the finished button cell was polymerized at 60 °C for 6 hours. The best performance is achieved with cells containing 2 weight % PETA. Higher amounts of PETA result in a decrease in performance.

[0187] Figure 3 shows comparison of

[0188] 1) gel polymer electrolyte and

[0189] 2) non-polymerized liquid in LMR||Si button cells.

[0190] X is the number of cycles and Q is the specific discharge capacity in mAh g-1. In both cases the electrolyte used was 1M LiPF6EC:EMC (3:7 volume / volume), 2 weight % PETA and 0.1 weight % AIBN, but one cell was not polymerized at 60 °C for 6 hours. A polyolefin-based separator was again used. Only the cell with the gel polymer electrolyte has some longevity, while the cell with the non-polymerized electrolyte leads to rapid cell death.

[0191] If PETA is not polymerized in the electrolyte, there is a rapid decrease in specific discharge capacity combined with a rapid cell death, which underlines the importance of the polymerization step.

[0192] Figure 4 shows a closer look at the cell voltage Z from the first cycle of the cells in Figure 3 with

[0193] 1) gel polymer electrolyte and

[0194] 2) non-polymerized liquid.

[0195] The drop right at the beginning of the charging step indicates the decomposition of non-polymerized PETA monomers on the electrode surfaces. Since almost all of the monomers in the non-gelated electrolyte formulation are unpolymerized, the voltage drop here is correspondingly greater. The strong reaction presumably leads to the formation of thick protective layers that increase the interfacial resistance and thus reduces the capacity.

[0196] The voltage profile of the first cycle of the cells in Figure 3 shows a clear decomposition / reaction at the beginning of the charging step for the non-gelated electrolyte. This effect can only be observed in small quantities for the gel polymer, as most PETA monomers are already polymerized.

[0197] Figure 5 shows cyclic voltammetry measurement of a total of 3 cycles with

[0198] 1) indicating the first cycle,

[0199] 2) the second cycle,

[0200] 3) the third cycle

[0201] on a Si-rich anode as working electrode with 1M LiPF6EC:EMC (3:7 volume / volume), 2 weight % PETA and 0.1 weight % AIBN as gel polymer electrolytes. P is the potential of the working electrode vs. Li|Li+in V and J is the measured current density in mA cm-2. The electrolyte was polymerized at 60 °C for 6 hours prior to measurement. Several peaks can be seen in the first cycle, indicating decomposition of the electrolyte on the electrode surface. A stable SEI appears to be formed in the first cycle, as no further decomposition is seen in the two subsequent cycles. Lithium metal was used as the reference and counter electrode.

[0202] Figure 6 shows the determination of the oxidative stability of various electrolytes against carbon-coated Al electrodes. P is the potential of the working electrode against Li|Li+in V and J is the measured current density in mA　cm-2. In total, the following 4 electrolytes were tested:

[0203] 1) reference electrolyte 1M LiPF6EC:EMC (3:7 volume / volume),

[0204] 2) 1M LiPF6EC:EMC (3:7 volume / volume) with 2　weight-% PETA and 0.1 weight-% AIBN, gelled for 6 hours at 60　°C,

[0205] 3) 1M LiPF6EC:EMC (3:7 volume / volume) with 2　weight-% PETA and 0.1　weight-% AIBN, gelled for 12 hours at 60　°C,

[0206] 4) 1M LiPF6EC:EMC (3:7 volume / volume) with 2　weight-% PETA and 0.1　weight-% AIBN, non-gelled.

[0207] All electrolytes show very similar behaviour in terms of oxidative stability, indicating that the presence of unreacted PETA monomers does not lead to a significantly stronger reaction at the electrode surfaces. Lithium metal was used as the reference and counter electrode. The comparison of different electrolytes in the determination of oxidative stability shows that below a potential of 5　V　vs.Li|Li+no strong decomposition of PETA occurs at the cathode.

[0208] Figure 7 shows long-term cycling of

[0209] 1) gel polymer and

[0210] 2) liquid electrolyte in LMR||Si cells at 45 °C.

[0211] X is the number of cycles and Q is the specific discharge capacity in mAh g-1. The gel polymer consists of 1M LiPF6EC:EMC (3:7 volume / volume), 2 weight % PETA and 0.1 weight % AIBN, the liquid electrolyte of 1M LiPF6EC:EMC (3:7 volume / volume), lithium manganese rich cathodes and silicon rich anodes were used as electrodes. Polymerization was carried out in situ at 60 °C for 6 hours. Long-term cycling at 45 °C shows that the gel polymer electrolyte significantly improves the performance in LMR||Si cells in contrast to the liquid electrolyte.

[0212] Figure 8 shows C-rate test between 0.05C and 5C of LMR||Si coin cells with different electrolytes and separators. The C-rate refers to the capacity of the cathode material, J is the current density in mA cm-2and Q is the specific discharge capacity in mAh g-1. In total, the following separator / electrolyte combinations were tested:

[0213] 1) single layer polyolefin separator with 1M LiPF6EC:EMC (3:7 volume / volume) as electrolyte,

[0214] 2) porous PVdF-HFP separator with 1M LiPF6EC:EMC (3:7 volume / volume) as electrolyte,

[0215] 3) porous PVdF-HFP separator with 1M LiPF6EC:EMC (3:7 volume / volume) with 2 weight % PETA and 0.1 weight % AIBN as gel polymer electrolytes.

[0216] The C-rate performance can be improved by using a highly porous separator. Up to a C-rate of 1C, the performance of the gel polymer electrolyte combined with a high porosity separator is comparable to a commercial olefin separator combined with a liquid electrolyte. The use of a highly porous PVdF-HFP separator improves capacity retention at higher applied current densities.

[0217] Figure 9 shows long-term cycling of LMR||Si cells with

[0218] a) liquid electrolyte and

[0219] b) gel polymer electrolyte in different electrolyte formulations.

[0220] X is the number of cycles and Q is the specific discharge capacity in mAh g-1. The combination of PETA with different additives was tested. The electrolyte formulations were

[0221] 1) 1M LiPF6EC:EMC (3:7 volume / volume) with FEC (10:1 weight / weight),

[0222] 2) 1M LiPF6EC:EMC (3:7 volume / volume) + 1 weight % TMSB,

[0223] 3) 1M LiPF6FEC:EMC (3:7 volume / volume) + 1 weight % TMSB.

[0224] For the gel polymer electrolytes, an additional 2 weight % PETA and 0.1 weight % AIBN were added to the mixture and the cells were polymerized at 60 °C for 6 hours. A polyolefin was used as separator. The combination of PETA for polymerization of the liquid electrolyte with functional additives shows a synergistic effect which can improve the longevity of the cell.

[0225] The addition of selected additives such as FEC and TMSB to a PETA-based gel polymer electrolyte increases the performance during long-term cycling. The gel polymer electrolyte of 1M LiPF6in FEC:EMC (3:7 volume %) with 1 weight % TMSB, 2 weight % PETA and 0.1 weight % AIBN improves the performance to over 300 cycles.

[0226] Figure 10 shows long-term cycling of LMR||Si cells with the most promising electrolyte formulation

[0227] 1) 1M LiPF6FEC:EMC (3:7 volume / volume) + 1 weight % TMSB + 2 weight % PETA + 0.1 weight % AIBN and

[0228] 2) 1M LiPF6FEC:EMC (3:7 volume / volume) + 1 weight % TMSB.

[0229] X represents the number of cycles and Q the specific discharge capacity in mAh g-1. In contrast to Figure 9, a silicon electrode with a higher mass loading was tested here (N / P ratio: 2.3). The calculated capacity of 11.9 mAh cm-2increases the N / P ratio and thus reduces the degree of lithiation of the silicon electrode, which has a positive effect on the performance. In a direct comparison between gel polymer electrolyte and liquid electrolyte, a shift in the so-called roll-over effect can be observed in favor of the gel polymer electrolyte.

[0230] By selecting a silicon anode with a higher capacity (11.9　mAh　cm-2) and thus increasing the N / P ratio, the performance with the most promising electrolyte from Figure 9 can be further increased to over 400 cycles. An improvement is also recognizable in comparison to the liquid electrolyte 1M LiPF6in FEC:EMC (3:7 volume-%) with 1　weight-% TMSB.

[0231] Contact angle measurements of (A) a commercial PP separator compared to (B) a fabricated PVdF-HFP membrane using 1M LiPF6EC:EMC (3:7 v / v) as the liquid electrolyte are shown in Fig. 11. The image was captured immediately after placing the drop, formed at the tip of the syringe, onto the substrate surface. For the PVdF-HFP membrane, the droplet spreads widely across the membrane surface, resulting in a lower contact angle and demonstrating superior wettability. Contact angle measurements were carried out on a DSA 100 by Kruss.

[0232] A SEM surface image of the PVdF-HFP membrane to visualize the pore size and distribution is shown in Fig. 12. Most pores (>90%) are inside the range of 0.2　μm to 2　μm.

[0233] The features disclosed in the foregoing description and in the dependent claims may, both separately and in any combination thereof, be material for realizing the aspects of the disclosure made in the independent claims, in diverse forms thereof.

Claims

1.A lithium secondary battery comprising an anode, a cathode and a gel polymer electrolyte;wherein- the cathode comprises a cathode active material;- the cathode active material comprises lithium manganese rich oxide;- the anode comprises an anode active material;- the anode active material comprises silicon;- the gel polymer electrolyte comprises a cross-linkable-monomer-based polymer and a lithium salt;- cross-linkable-monomer comprises at least three radically polymerizable groups; and- the gel polymer electrolyte comprises at least one additive selected from the group consisting of tris(trimethylsilyl) borate (TMSB) and fluoroethylene carbonate (FEC).2.The lithium secondary battery according to claim 1, wherein cross-linkable-monomer comprises three to six radically polymerizable groups.3.The lithium secondary battery according to claim 1 or 2, wherein the radically polymerizable groups are independently selected from the group consisting of acrylate and (meth)acrylate.4.The lithium secondary battery according to any of the preceding claims, wherein the cross-linkable-monomer is pentaerythritol tetraacrylate (PETA).5.The lithium secondary battery according to any of the preceding claims, wherein the lithium manganese rich oxide is Li1.34Ni0.35Mn0.65O2.6.The lithium secondary battery according to any of the preceding claims, wherein the lithium secondary battery further comprises a separator between the anode and the cathode.7.The lithium secondary battery according to claim 6, wherein the separator comprises poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP).8.The lithium secondary battery according to any of the preceding claims, wherein the additive is tris(trimethylsilyl) borate (TMSB).9.The lithium secondary battery according to any of the preceding claims, wherein the gel polymer electrolyte further comprises a non-aqueous solvent.10.The lithium secondary battery according to claim 9, wherein the non-aqueous solvent comprises fluoroethylene carbonate (FEC), ethylene carbonate (EC), ethyl methylcarbonate (EMC) or a mixture thereof.11.The lithium secondary battery according to any of the preceding claims, wherein the lithium salt is LiPF6.12.A method for preparing a lithium secondary battery according to any of the preceding claims, comprising the steps of:- polymerizing a reaction mixture comprising a cross-linkable-monomer comprising at least three radically polymerizable groups and at least one additive selected from the group consisting of tris(trimethylsilyl) borate (TMSB) and fluoroethylene carbonate (FEC) in the presence of the lithium salt with an initiator to obtain the gel polymer electrolyte.13.The method for preparing a lithium secondary battery according to claim 12, wherein the amount of cross-linkable-monomer is 1.5 to 2.5 wt.-%, based on the total weight of the reaction mixture.14.The method for preparing a lithium secondary battery according to claim 12 or 13, wherein the initiator is azobisisobutyronitrile (AIBN).

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