Lithium secondary battery electrolyte and lithium secondary battery comprising same

Abstract

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery comprising same, and, more specifically, to a lithium secondary battery comprising a lithium secondary battery electrolyte, which comprises, as an additive, a monomer of a conductive polymer selected from pyrrole derivatives, wherein, during charging, the additive donates electrons to a negative electrode and is oxidized so as to form a conductive polymer coating layer on the surface of a positive electrode, thereby reducing initial irreversible capacity and improving cycle performance and rate capability.

Description

Electrolyte for lithium secondary batteries and lithium secondary batteries containing the same

[0001] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same. More specifically, the invention relates to a lithium secondary battery comprising an electrolyte for a lithium secondary battery that includes a monomer of a conductive polymer selected from pyrrole derivatives, aniline derivatives, thiophene derivatives, etc., as an additive, wherein during charging, the additive is oxidized at the positive electrode to form a conductive polymer coating layer, and electrons generated during the formation of the conductive polymer are supplied to the negative electrode, thereby reducing the initial irreversible capacity and improving cycle performance and rate capability.

[0002] This research was conducted with funding from the government (Ministry of Science and ICT) in 2022 and supported by the Korea Institute of Energy Research's "Development of Carbon-based Energy Clean Utilization Technology for Greenhouse Gas Reduction and Clean Air" (22-KIER-4-1-1).

[0003] This research was supported by the Korea Institute of Energy Research (KIER) grant by the Korea government (MSIT) (No. 22-KIER-4-1-1).

[0004]

[0005] Lithium secondary batteries have been adopted as power sources for many portable devices due to their high energy density and ease of design. Recently, as lithium secondary batteries are also being adopted for electric vehicles and power storage, research on lithium secondary battery materials that enable high energy density and long lifespan is expanding. Among these, electrolyte additives are a method that can improve performance without significantly affecting the physical properties of the existing electrolyte, as they are used in small quantities.

[0006] In response to the demand for high energy density, the use of low-potential negative active materials and high-potential positive active materials has exposed the electrolyte to an environment prone to decomposition on the surfaces of the positive and negative electrodes. Therefore, for graphite negatives, a method is already being used to prevent the reduction of the electrolyte by employing an appropriate electrolyte or additive to decompose on the surface of the active material during initial charging, thereby forming a film (Solid-Electrolyte Interphase, SEI).

[0007] The reaction for storing energy by charging a lithium secondary battery proceeds through the simultaneous occurrence of an oxidation reaction in which lithium ions and electrons are released from the cathode material, and a reduction reaction in which lithium ions and electrons enter the anode material. The capacity of a lithium secondary battery that can be reversibly used by charging and discharging is attributed to the total amount of lithium ions stored in the cathode and anode, and is called reversible capacity. Meanwhile, the reaction in which the electrolyte or additives decompose at the anode to form a film is also a reduction reaction, and thus consumes electrons. Since the electrons consumed in the film-forming reaction cannot be used to store lithium in the anode, the total amount of lithium ions stored in the cathode and anode decreases; this is called the initial cycle irreversible capacity, which is attributed to the formation of SEI in the anode of the lithium secondary battery. The initial cycle irreversible capacity of such lithium secondary batteries is a battery in which the irreversible capacity of the anode is greater than the irreversible capacity of the cathode (e.g., for LiFePO4, LiCoO2 cathode materials, graphite anode material or Si (or SiO2)). x Battery systems using graphite anode materials containing ), or with respect to Li(NiCoMn)O2, LiNiO2, Li(NiMn)O2, LiMnO2, Li(NiCoAl)O2, and cathode materials, Si (or SiO x Reduces the reversible capacity of the battery in a battery system using a graphite cathode material containing ).

[0008] Since the initial cycle irreversible capacity caused by the formation of a negative electrode SEI reduces the capacity of lithium secondary batteries, technologies capable of reducing this have been proposed. In the patents 'Method for manufacturing a negative electrode and a secondary battery using the same (Application No. 10-2010-0135412)' and 'Method for manufacturing a negative electrode for a lithium secondary battery including pre-lithiation (Application No. 10-2022-0025665)', a patent was filed to reduce the electrons consumed in forming a negative electrode SEI film by forming an SEI or an artificial SEI on the negative electrode in advance before manufacturing a lithium secondary battery. However, there is a disadvantage that it is difficult to apply this to the current manufacturing process of lithium secondary batteries due to the characteristics of the negative electrode SEI film, which is unstable in air.

[0009] Meanwhile, as cathode active materials operating at even higher voltages are introduced, the risk of electrolyte oxidation on the cathode surface has increased, leading to a growing need for cathode films. However, conventional additives used to form cathode surface films were primarily overcharge inhibitors. In other words, when the voltage rises above a certain level, a thick film is formed on the cathode surface to prevent the passage of lithium ions, thereby stopping the current flow. Recently, research has reported that significantly reducing the concentration of such overcharge inhibitor additives forms a thin film on the cathode surface, which improves lifespan. In the patent application 'Electrolyte containing conductive polymer monomer and lithium secondary battery containing the same (Application No. 10-1999-0015926)', a conductive polymer monomer is included in the electrolyte to form a cathode film; however, while a slight improvement in lifespan is achieved by adding a minute amount of conductive polymer monomer, there is a disadvantage in that cell capacity and lifespan characteristics decrease significantly as the weight fraction of the conductive polymer in the electrolyte increases. In addition, although the reduction and oxidation reactions of electrolyte additives that form the anode film through oxidation and those that form the cathode film through reduction occur simultaneously during the first charging reaction of the battery, the combination of anode and cathode additives must be considered because if these additives influence each other's film formation reactions, it can reduce the battery's lifespan.

[0010] Lithium iron phosphate (LiFePO4) cathode material is gaining attention as a new cathode material for lithium secondary batteries because it is cheaper and more environmentally friendly than currently commercialized ternary cathode materials (NCM, NCA) and lithium cobalt oxide (LCO), and it offers safe battery operation capabilities due to its excellent chemical stability and reversibility. It is a material with excellent lifespan characteristics, having a discharge voltage of 3.4V when Li metal is used as an electrode and being structurally stable.

[0011] However, LiFePO4 has very low electrical conductivity (~10 -9 It has (S / cm), and furthermore, the lithium diffusion coefficients of LiFePO4 and FePO4 are 1.8×10⁻¹⁰, respectively. -14 and 2.2X10 -16 cm 2 Because the low oscillation rate ( / s) presents a problem of poor electrochemical characteristics, the particle size of LiFePO4 must generally be made small, around tens of nanometers. To ensure electrical contact between fine particles, the amount of low-density conductive material in the LiFePO4 cathode is increased, which causes a decrease in battery energy density. Additionally, LiFePO4 cathode material has a very small initial irreversible capacity of less than 2%, and when combined with a graphite anode, which has a large initial irreversible capacity, it reduces the reversible capacity of the battery.

[0012] Therefore, there is a need to develop a secondary battery comprising an electrolyte additive that can be used in conjunction with additives for forming a cathode film, which, in a battery composed of a LiFePO4 cathode and a graphite anode, 1. improves the reversible capacity of the battery by inducing an oxidation reaction on the surface of the cathode material during the first charging reaction to offset the initial irreversible capacity of the anode, 2. enables a reduction in the conductive material content by improving cathode conductivity through the formation of a conductive film on the surface of the cathode active material, 3. improves battery life by suppressing side reactions on the surface of the cathode material, and 4. improves battery life. A secondary battery containing an electrolyte additive with these characteristics is a battery in which the irreversible capacity of the anode is greater than the irreversible capacity of the cathode, for example, a LiFePO4 or LiCoO2 cathode material and a graphite anode material or Si (or SiO2). x A secondary battery using a graphite anode material containing ), Li(NiCoMn)O2, LiNiO2, Li(NiMn)O2, LiMnO2, Li(NiCoAl)O2, anode material and Si (or SiO x It can be applied to secondary batteries using graphite anode materials containing ).

[0013]

[0014] The technical problem to be solved by the present invention is to provide an electrolyte for a lithium secondary battery capable of improving the reversible capacity of the lithium secondary battery by forming a conductive polymer on the surface of the positive electrode through an oxidation reaction during the first charging reaction of the lithium secondary battery, and simultaneously using the electrons generated at this time for the reduction reaction required for the formation of the negative electrode film.

[0015] Another technical objective of the present invention is to provide a lithium secondary battery with improved electrochemical performance and stability by including the above-mentioned electrolyte for the lithium secondary battery.

[0016] The technical problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned technical problems will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0017]

[0018] In order to achieve the above technical problem, one aspect of the present invention is,

[0019] A lithium secondary battery comprising a positive electrode; a negative electrode; and an electrolyte interposed between the positive electrode and the negative electrode, wherein the electrolyte comprises a lithium salt; a solvent; an additive 1; and an additive 2, and the additive 1 comprises a polymer monomer composed of a hydrocarbon derivative comprising a nitrogen or sulfur element and having at least one double or triple bond.

[0020] The above polymer monomer may be characterized by comprising one or more selected from the group consisting of pyrrole, thiophene, aniline, 3,4-ethylenedioxythiophene, and their dimers, trimers, or derivatives.

[0021] The above polymer monomer may be characterized by containing pyrrole.

[0022] The above polymer monomer may be characterized by being included in an amount of 1 to 300 mM relative to the entire electrolyte.

[0023] The above polymer monomer may be characterized by being included in an amount of 0.005 to 3.4 parts by weight per 100 parts by weight of the total electrolyte.

[0024] The above solvent may further comprise one or more organic solvents selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methyl ethyl carbonate (MEC), fluoroethylene carbonate (FEC), methylpropyl carbonate (MPC), ethyl propyl carbonate (EPC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-butylene carbonate, and combinations thereof.

[0025] The above additive 2 may be characterized by further comprising one or more organic substances selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinylethylene carbonate (VEC), propanesulfone (PS), 1,3-propanesulfone (PRS), ethylene sulfate (ESa), succinonitrile (SN), adiponitrile (AN), hexane tricarbonitrile (HTCN), gamma-butyrolactone (GBL), biphenyl (BP), cyclohexylbenzene (CHB), tert-amylbenzene (TAB), and derivatives thereof.

[0026] The above additive 2 may be characterized by being included in an amount of 0.1 to 10 parts by weight per 100 parts by weight of the total electrolyte.

[0027] The above polymer monomer may be characterized by being oxidized on the surface of the positive electrode during charging of the lithium secondary battery to form a conductive polymer film.

[0028] The above anode is LiFePO4(LFP), Li(Mn a Fe b )PO4(0 <a,b<1), LiCoO2및 이들의 조합으로 이루어진 군으로부터 선택되는 하나 이상의 양극 활물질을 포함하는 것을 특징으로 할 수 있다.

[0029] The above cathode may be characterized by comprising one or more cathode active materials selected from carbon-based materials.

[0030] The above anode is Li[Ni a Co b Mn c Al d ]O2(0 <a,b,c,d<1), Li 1+x (Ni,Co,Mn,Al) 1-y O z (0 <x≤1, 0≤y<1, 2≤z≤4), Li 1+x (Ni,Co,Mn,Al) 1-x The cathode comprises one or more positive active materials selected from the group consisting of O2 (0≤x≤0.2) and mixtures thereof, and the negative electrode comprises Si, SiO x It may be characterized by including one or more negative active materials selected from the group consisting of carbon-based materials containing (0 < x < 2) and mixtures thereof.

[0031]

[0032] According to an embodiment of the present invention, a lithium secondary battery can be provided that includes an electrolyte for a lithium secondary battery comprising a monomer of a conductive polymer selected from pyrrole derivatives as an additive, wherein the additive is oxidized at the positive electrode during charging to form a conductive polymer film, thereby providing electrons to the negative electrode, reducing the initial irreversible capacity of the lithium secondary battery, and improving cycle performance and rate capability characteristics.

[0033] The effects of the present invention are not limited to the effects described above, and should be understood to include all effects that can be inferred from the composition of the invention described in the description or claims of the present invention.

[0034]

[0035] FIG. 1 is a schematic diagram showing the charging and discharging process of a lithium secondary battery according to an embodiment of the present invention.

[0036] FIG. 2 is a schematic diagram showing the charging and discharging process of a lithium secondary battery according to a comparative example of the present invention.

[0037] Figure 3 is the result of measuring the positive electrode surface before and after one cycle of charging and discharging for a lithium secondary battery according to one embodiment and a comparative example of the present invention using SEM.

[0038] Figure 4 is the result of Raman analysis of the electrode after 1 cycle of charge and discharge for a lithium secondary battery according to one embodiment and a comparative example of the present invention.

[0039] Figure 5 shows the results of measuring the initial charge / discharge capacity of a lithium secondary battery according to one embodiment and a comparative example of the present invention.

[0040] Figure 6 is the result of measuring the cycle characteristics of a lithium secondary battery according to one embodiment and a comparative example of the present invention.

[0041] Figure 7 shows the results of measuring the rate capability characteristics of a lithium secondary battery according to an embodiment and a comparative example of the present invention.

[0042] Figure 8 shows the durability measurement results of a lithium secondary battery according to one embodiment and a comparative example of the present invention.

[0043] Figure 9 is a graph showing the discharge capacity results according to pyrrole concentration in one embodiment and a comparative example of the present invention.

[0044] FIG. 10 is a graph of the voltage profile results showing charging and discharging in Example 5 and Comparative Example 5 of the present invention.

[0045]

[0046] The present invention will be described below with reference to the attached drawings. However, the present invention may be implemented in various different forms and is therefore not limited to the embodiments described herein. Furthermore, in order to clearly explain the present invention in the drawings, parts unrelated to the explanation have been omitted, and similar parts throughout the specification have been given similar reference numerals.

[0047] Throughout the specification, when it is stated that a part is "connected (connected, in contact, combined)" with another part, this includes not only cases where they are "directly connected," but also cases where they are "indirectly connected" with other members interposed between them. Furthermore, when it is stated that a part "includes" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but rather allows for the inclusion of additional components.

[0048] The terms used herein are merely for describing specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as “comprising” or “having” are intended to indicate the presence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0049]

[0050] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0051]

[0052] Examples 1 to 4. Lithium secondary battery

[0053] An electrolyte for a lithium secondary battery was prepared by adding a polymer monomer as an additive to a mixed organic solvent composed of 30 volume % of ethylene carbonate (EC), 40 volume % of ethylmethyl carbonate (EMC), and 30 volume % of diethyl carbonate (DEC) as shown in Table 1 below, further including 2 weight % of vinylene carbonate (VC), and including 1 M of LiPF6 as a lithium salt.

[0054] Differentiating Additive (High-molecular Monomer) Content (mM) Example 1 Pyrrole 25 Example 2 Pyrrole 50 Example 3 Pyrrole 100 Example 4 Pyrrole 200

[0055] An anode slurry was prepared by mixing LFP (LiFePO4 (Johnson Matthey product), D50: 14 μm) powder, a cathode active material; a binder solution in which 5 wt% of polyvinylidene fluoride (PVdF) was dissolved in N-methylpyrrolidone (NMP); and a conductive material (Denka black) in a weight ratio of 89:8:3. The anode slurry was coated onto a 15 μm thick aluminum foil using a bar coating method. This was placed in a 60°C oven for primary drying for approximately 2 hours, then placed in a 100°C vacuum oven for secondary drying for approximately 2 hours to ensure complete evaporation of the NMP. Subsequently, the material was rolled and punched to obtain a diameter of 1.4 cm and a composite weight of 8.9 mg / cm². 2 , composite density 1.7 g / cm³ 3 A positive electrode was obtained. The capacity of this positive electrode is approximately 1.12 mAh / cm² 2 was.

[0056] A 2032 standard coin cell battery was manufactured using the above anode, graphite cathode (product of LIBEST), polyethylene separator (SC1622, product of W-Scope), and the above electrolyte.

[0057] After charging the above lithium secondary battery to 3.2V at a current density of 0.01C in a constant current charging mode, immediately charging from 3.2V to 4.2V at a current density of 0.1C in a constant current charging mode, switching to a constant voltage charging mode to charge until the current density reached 0.02C, and discharging at 0.1C in a formation cycle, charge and discharge experiments were conducted at various current densities of 0.1C, 0.5C, 1.0C, and 2.0C (1C = 1.12 mA / cm²). 2 (In coin cells, 1C = 1.72 mA).

[0058]

[0059] Comparative Example 1. Lithium secondary battery

[0060] A lithium secondary battery was manufactured as in the above example, but a lithium secondary battery according to Comparative Example 1 was manufactured without including a polymer monomer as an additive in the electrolyte.

[0061]

[0062] Comparative Example 2. Lithium secondary battery

[0063] A lithium secondary battery was manufactured as in Example 2 above, but a lithium secondary battery according to Comparative Example 2 was manufactured without including vinylene carbonate (VC) as an additive in the electrolyte.

[0064]

[0065] Comparative Example 3. Lithium secondary battery

[0066] A lithium secondary battery was manufactured as in Comparative Example 1 above, but a lithium secondary battery according to Comparative Example 2 was manufactured in which vinylene carbonate (VC) was not included as an additive in the electrolyte.

[0067]

[0068] Comparative Example 4. Lithium secondary battery

[0069] A lithium secondary battery was manufactured as in Example 2 above, but the first cycle charging was performed at 0.1C.

[0070]

[0071] Example 5. Lithium secondary battery

[0072] A lithium secondary battery was manufactured as in Example 2 above, wherein the positive electrode active material was LiCoO2 (LIBEST, active material:binder:conductive material = 94:3:3 parts by weight, composite weight 7.35 mg / cm² 2 , composite density 2.8 g / cm³ 3 ) and SiO as the cathode x A lithium secondary battery was manufactured using a graphite electrode containing 10 parts by weight, charged to 4.2V at a current density of 0.1C, charged to 0.02C in a constant voltage charging mode, and discharged to 2V at a current density of 0.1C.

[0073]

[0074] Comparative Example 5. Lithium secondary battery

[0075] A lithium secondary battery was manufactured as in Example 5 above, but without adding a conductive polymer monomer to the electrolyte.

[0076]

[0077] Experimental Example 1. SEM Measurement

[0078] Figure 3 shows the results of measuring the anode surface with SEM when one cycle of charging and discharging was performed on the lithium secondary battery of Example 2 and Comparative Example 1 of the present invention, compared with the anode surface before charging and discharging.

[0079] Referring to Figure 3, in the case of the LiFePO4 anode (Pristine) that was not charged or discharged and the LiFePO4 anode of Comparative Example 1 (without Py) that was charged or discharged without a conductive polymer monomer additive, no product was formed on the surface of the anode material. On the other hand, in the case of the LiFePO4 anode (50 mM Py) that was charged or discharged using an electrolyte containing 50 mM of a conductive polymer monomer additive as in Example 2, it can be seen that fine conductive polymers were formed on the surface of the anode after charging and discharging.

[0080]

[0081] Experimental Example 2. Raman Analysis

[0082] Figure 4 shows the results of Raman analysis of the positive electrode when one cycle of charging and discharging was performed on the lithium secondary battery of Example 4 and Comparative Example 1 of the present invention. Raman analysis was performed using a RENISHAW device with a 532 nm semiconductor laser as a light source. After the first charge and discharge of the lithium secondary battery, the lithium secondary battery was disassembled inside a glove box filled with argon, and the positive electrode was placed in a sealed cell for Raman measurement with a glass window so that the positive electrode did not come into contact with air for measurement.

[0083] Referring to FIG. 4, in the case of the LiFePO4 anode (wo Py) of Comparative Example 1, which underwent charge and discharge without a conductive polymer monomer additive, the typical D-band of the carbon material (1300 cm⁻¹) -2 (near) and G-band (1600 cm) -2 While only a peak originating from the vicinity is observed, in the case of the LiFePO4 anode (200 mM Py) subjected to charge-discharge using an electrolyte containing 50 mM of a conductive polymer monomer additive as in Example 4, 1000 cm⁻¹ after charge-discharge -2 It can be confirmed that characteristic peaks of polypyrrole, a conductive polymer, are observed before and after, and it can be confirmed that a conductive polymer can be coated as proposed in the present invention.

[0084]

[0085] Experimental Example 3. Electrochemical Performance

[0086] For a lithium secondary battery according to an embodiment of the present invention, it was charged to 3.2V at 0.01C and to 4.2V at 0.1C in a constant temperature chamber at 25°C, and then discharged until it reached 2.5V at 0.1C. For a lithium secondary battery according to Comparative Example 1, it was charged to 3.95V at 0.1C and then discharged until it reached 2.5V at 0.1C to measure the discharge capacity (discharge capacity of the first cycle). Then, the discharge capacity at each cycle was measured while repeating the 2.0C charging and 2.0C discharging 400 times in a chamber at 25°C. The initial charge / discharge capacities of the lithium secondary batteries according to Example 2 and Comparative Example 1 were measured and are shown in Fig. 5, and the cycle characteristics were measured and are shown in Fig. 6.

[0087] For the lithium secondary batteries of Examples 1 to 4 and Comparative Example 1, the high-rate charge / discharge characteristics (rate capability) of each battery were measured by charging under constant current (charging to 3.2V at 0.01C and charging to 4.2V at 0.1C) and constant voltage (4.2V, 0.02C cut-off) conditions, resting for 10 minutes, and then discharging at 0.1C until 2.5V was reached. Then, the constant current conditions were changed to 0.1C, 0.5C, 1C, and 2C, respectively, and the charging and discharging were repeated in the range of 2.5V to 3.95V, and the results are shown in FIG. 7.

[0088] Referring to FIGS. 5 and 6, it can be seen that the lithium secondary battery according to Example 2 of the present invention has a higher charge / discharge capacity (147.3 mAh / g) compared to the lithium secondary battery of Comparative Example 1 (136.8 mAh / g) that does not contain a polymer monomer, and it can be confirmed that the cycle performance (capacity retention rate) is also improved.

[0089] Referring to FIG. 7, it can be seen that the lithium secondary batteries of Examples 1 to 4 of the present invention exhibit significantly improved rate capability characteristics compared to the lithium secondary battery of Comparative Example 1, which does not contain a polymer monomer, and in particular, it was confirmed that the lithium secondary batteries according to Examples 1, 2, 3, and 4 exhibit improved high-rate power generation characteristics.

[0090]

[0091] Experimental Example 4. Measurement of durability depending on whether organic solvent is included as an additive

[0092] The durability of the lithium secondary battery according to Example 2 and Comparative Examples 1, 2, and 3 of the present invention was measured (charge-discharge experiment in which the battery was charged to 3.2V with a current density of 0.01C in a constant temperature chamber set to 25 °C, then charged to 4.2V with a current density of 0.1C, then charged in a constant voltage charging mode until the current dropped to 0.02C, followed by a formation cycle of discharging to 2.5V with 0.1C, then charged to 3.95V with a current density of 2.0C, and then discharged to 2.5V with 2C) and is shown in FIG. 8.

[0093] Referring to FIG. 8, it can be seen that the lithium secondary batteries of Comparative Example 1, which includes only VC as an additive; Comparative Example 2, which includes a conductive polymer monomer as an additive but does not include VC; and Comparative Example 3, which does not include any additives, maintain a low durability of 66.3% (Comparative Example 1), 62.2% (Comparative Example 2), and 60.4% (Comparative Example 3) at less than 1000 cycles, respectively, whereas the lithium secondary battery according to Example 2 maintains the highest durability of 83.3%.

[0094]

[0095] Experimental Example 5. Measurement of discharge capacity according to first cycle charging current density

[0096] The discharge capacity of a lithium secondary battery according to Examples 1, 2, 3, and 4 of the present invention and Comparative Examples 1 and 4 was measured (Examples 1, 2, 3, and 4, which perform a charge-discharge experiment in which the battery is charged to 3.2V with a current density of 0.01C in a constant temperature chamber set to 25 °C, then charged to 4.2V with a current density of 0.1C, then charged in a constant voltage charging mode until the current drops to 0.02C, and then discharged to 2.5V with 0.1C, and Comparative Example 4, which does not include a charging process for an initial formation of 0.01C) and is shown in FIG. 9.

[0097] Referring to Fig. 9, it was confirmed that in Examples 1, 2, 3, 4 and Comparative Example 4, which include a conductive polymer monomer and VC, the increase in reversible capacity was higher when the initial charging current density was low.

[0098]

[0099] Experimental Example 6. Measurement of discharge capacity according to first cycle charging current density

[0100] The discharge capacity of the lithium secondary battery according to Example 5 and Comparative Example 5 of the present invention was measured (charge-discharge experiment in which the battery was charged to 4.2V with a current density of 0.1C in a constant temperature chamber set to 25 °C, charged in a constant voltage charging mode until the current dropped to 0.02C, and discharged to 2.0V with a current density of 0.1C) and is shown in FIG. 10.

[0101] Referring to Fig. 10, the anode made of LiCoO2 and SiO x It can be confirmed that the reversible capacity is improved by adding a conductive polymer monomer to the electrolyte in a lithium secondary battery fabricated using a cathode made of a graphite cathode containing [the component]. From Experimental Example 6, it was confirmed that, in addition to lithium secondary batteries using a cathode made of LiFePO4 and a cathode made of graphite, the effect of increasing reversible capacity is generally obtained in lithium secondary batteries where the initial efficiency of the cathode is higher than the initial efficiency of the cathode.

[0102]

[0103] In order to achieve the above technical problem, one aspect of the present invention is,

[0104] A lithium secondary battery comprising a positive electrode; a negative electrode; and an electrolyte interposed between the positive electrode and the negative electrode, wherein the electrolyte comprises a lithium salt; a solvent; an additive 1; and an additive 2, and the additive 1 comprises a polymer monomer composed of a hydrocarbon derivative comprising a nitrogen or sulfur element and having at least one double or triple bond.

[0105]

[0106] According to an embodiment of the present invention, the polymer monomer may be characterized by comprising one or more selected from the group consisting of pyrrole, thiophene, aniline, 3,4-ethylenedioxythiophene, and dimers, trimers, or derivatives thereof. Such a polymer monomer is preferred in that it is electrochemically polymerized at a lithium potential of 1.0 V or higher to form a conductive polymer, and in particular, it is preferable for the polymer monomer to include pyrrole in that it has an oxidation potential lower than that of a non-aqueous organic solvent contained in the electrolyte, thereby oxidizing and / or decomposing at a faster rate than the non-aqueous organic solvent during operation of the lithium secondary battery to form a stable film on the electrode of the lithium secondary battery, for example, as shown in FIG. 1. On the other hand, in the case of conventional lithium secondary batteries that do not contain polymer monomers, as shown in Figures 2 and 5, there is a problem in that the initial irreversible capacity decreases as electrons are consumed due to an oxidation reaction in which lithium from the positive electrode is released when the lithium salt or organic solvent contained in the electrolyte is reduced to form a film.

[0107] According to an embodiment of the present invention, the polymer monomer may be characterized by being included in an amount of 1 to 300 mM with respect to the entire electrolyte. In other examples, the polymer monomer may be included in an amount of 5 mM or more, 20 mM or more, 30 mM or more, or 50 mM or more, or 100 mM or less, 150 mM or less, 200 mM or less, or 300 mM or less with respect to the entire electrolyte. When the content of the polymer monomer is within the above range, a lithium ion conductive film that facilitates lithium ion conduction between the anode and the electrolyte may be formed on the surface of the anode. Since the electrolyte of a lithium secondary battery is a pathway for lithium ions, if the electrolyte reacts to become oxidized or reduced during charging and discharging, the charge and discharge performance of the battery may be degraded.

[0108] According to an embodiment of the present invention, the polymer monomer may be characterized by being included in an amount of 0.005 to 3.4 parts by weight per 100 parts by weight of the total electrolyte.

[0109] According to an embodiment of the present invention, the solvent may further comprise one or more organic solvents selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methyl ethyl carbonate (MEC), fluoroethylene carbonate (FEC), methylpropyl carbonate (MPC), ethyl propyl carbonate (EPC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-butylene carbonate, and combinations thereof, and more preferably, as additive 2, vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinylethylene carbonate (VEC), propanesulfone (PS), 1,3-propanesulfone (PRS), It may be characterized by further including one or more organic materials selected from the group consisting of ethylene sulfate (ESa), succinonitrile (SN), adiponitrile (AN), hexane tricarbonitrile (HTCN), gamma-butyrolactone (GBL), biphenyl (BP), cyclohexylbenzene (CHB), tert-amylbenzene (TAB), and derivatives thereof. In a state where additive 1, which is a conductive polymer monomer, and additive 2, which forms a negative electrode film such as VC or FEC, coexist in the electrolyte, if additive 1 is oxidized at the anode and the electrons are used to reduce additive 2 at the cathode, the lifespan of the lithium secondary battery can be improved by providing a robust oxidizing film and a reducing film to the anode and cathode, respectively.

[0110] According to an embodiment of the present invention, the additive 2 may be characterized by being included in an amount of 0.1 to 10 parts by weight per 100 parts by weight of the total electrolyte. In other examples, the additive 2 may be included in an amount of 0.5 parts by weight or more, 1.0 parts by weight or more, or 1.5 parts by weight or more, or 4.5 parts by weight or less, 4.0 parts by weight or less, 3.5 parts by weight or less, 3.0 parts by weight or less, or 2.5 parts by weight or less, per 100 parts by weight of the total electrolyte. If only additive 1, which is a conductive polymer monomer, is present in the electrolyte and additive 2, which forms a negative electrode film such as VC or FEC, is not present in the electrolyte, it is impossible to form a robust negative electrode film, which can lead to a continuous decomposition reaction of the solvent in the electrolyte at the negative electrode, thereby reducing the lifespan of the lithium secondary battery.

[0111] According to an embodiment of the present invention, the polymer monomer may be characterized by being oxidized on the surface of the positive electrode during charging of the lithium secondary battery to form a conductive polymer film. For example, when pyrrole is included as the polymer monomer according to one embodiment of the present invention, the oxidation potential of the pyrrole is about 1 to 3 V or lower than that of the non-aqueous organic solvent included in the electrolyte. In this case, for example, when operating a lithium secondary battery containing an electrolyte to which the pyrrole has been added, it can be expected that the pyrrole will be polymerized on the surface of the positive electrode by ring opening or polymerization reaction caused by the oxidation of the pyrrole compared to the non-aqueous organic solvent in the electrolyte, thereby forming a conductive polymer film (preferably polypyrrole). The film formed on the surface of the positive electrode can prevent the electrolyte from being oxidized on the surface of the positive electrode by blocking direct contact between the electrolyte and the positive electrode active material, and thus can prevent a decrease in the charge / discharge performance of the battery. By providing a pathway through which lithium ions can move more smoothly through the film formed on the surface of the positive electrode, a lithium secondary battery having lifespan characteristics and high rate characteristics can be obtained. At this time, only lithium ions can pass through the film formed on the surface of the anode, and electrons cannot move. Additionally, by forming a conductive polymer on the surface of the anode material to create a channel for electricity to flow, the electrical conductivity of the anode can be improved and the rate capability of the lithium secondary battery can be improved. At this time, the reaction in which the conductive polymer monomer generated at the anode forms the conductive polymer is an oxidation reaction, and the electrons generated as the conductive polymer monomer is oxidized are used in the negative electrode film formation reaction (a reduction reaction in which additive 2 or the solvent receives electrons), thereby reducing the initial irreversible capacity of the lithium secondary battery and improving the reversible capacity.

[0112] According to an embodiment of the present invention, the thickness of the conductive polymer film formed on the surface of the positive or negative electrode during operation of a lithium secondary battery containing an electrolyte to which the polymer monomer has been added may be 1 to 50 nm.

[0113] According to an embodiment of the present invention, the anode is LiFePO4(LFP), Li(Mn a Fe b )PO4(0 <a,b<1), LiCoO2및 이들의 조합으로 이루어진 군으로부터 선택되는 하나 이상의 양극 활물질을 포함하는 것을 특징으로 할 수 있다. 상기와 같은 리튬인산철(LFP)계 양극 활물질의 경우 올리빈 구조를 가짐으로써, 층상 구조 혹은 스피넬 구조를 갖는 양극 활물질에 비해 리튬 이온의 흡탈착에 의한 구조, 특히 부피의 변화가 작아 사이클 특성이 우수하다는 장점이 있으나 P, O, Fe 간의 강한 결합력(binding force)에 의해 리튬 이온의 전도도가 낮고, 양극의 반전지 초기효율은 높으나 반전지 초기효율이 낮은 음극과 함께 사용할 수밖에 없었기 때문에 충방전 용량 및 초기 비가역 용량에 한계가 있었으나, 본 발명에 따른 리튬 이차전지의 경우 전해액에 첨가된 고분자 단량체가 산화됨에 따른 양극 표면에의 전도성 고분자 피막 형성함을 통해 상기 리튬인산철계 양극 활물질을 사용하는 리튬 이차전지의 초기 비가역 용량을 감소시켜 및 전지의 충방전 용량 특성을 현저히 향상시킬 수 있으며, 높은 전기저항으로 인해 낮았던 율속 특성 역시 향상시킬 수 있다.

[0114] According to an embodiment of the present invention, the cathode may be characterized by comprising one or more cathode active materials selected from carbon-based materials. The carbon-based materials may include, for example, one or more selected from graphite, soft carbon, hard carbon, mesophase pitch carbide, or calcined coke.

[0115] According to an embodiment of the present invention, the anode is Li[Ni a Co b Mn c Al d ]O2(0 <a,b,c,d<1), Li 1+x (Ni,Co,Mn,Al) 1-y O z(0 <x≤1, 0≤y<1, 2≤z≤4), Li 1+x (Ni,Co,Mn,Al) 1-x The cathode comprises one or more positive active materials selected from the group consisting of O2 (0≤x≤0.2) and mixtures thereof, and the negative electrode comprises Si, SiO x It may be characterized by including one or more negative electrode active materials selected from the group consisting of carbon-based materials containing (0 < x < 2) and mixtures thereof. By combining a positive electrode active material having a layered and spinel structure and a silicon-based negative electrode active material as described above, when the initial irreversible capacity of the negative electrode is greater than the initial irreversible capacity of the positive electrode, an additional oxidation reaction is induced at the positive electrode, thereby improving the reversible capacity and electrochemical performance of the lithium secondary battery containing the same.

[0116]

[0117] The foregoing description of the present invention is for illustrative purposes only, and those skilled in the art will understand that other specific forms can be easily modified without altering the technical spirit or essential features of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. For example, each component described as a single unit may be implemented in a distributed manner, and components described as distributed may likewise be implemented in a combined form.

[0118] The scope of the present invention is defined by the claims set forth below, and all modifications or variations derived from the meaning and scope of the claims and equivalent concepts thereof should be interpreted as being included within the scope of the present invention.

[0119]

[0120] According to an embodiment of the present invention, an electrolyte for a lithium secondary battery capable of improving the reversible capacity of the lithium secondary battery can be provided by forming a conductive polymer on the surface of the positive electrode by an oxidation reaction during the first charging reaction of the lithium secondary battery, and simultaneously using the electrons generated at this time for a reduction reaction required for the formation of a negative electrode film.

[0121] In addition, according to one embodiment of the present invention, since a lithium secondary battery with improved electrochemical performance and stability including the electrolyte for the lithium secondary battery can be provided, it can be considered to have industrial applicability.

Claims

1. A lithium secondary battery comprising a positive electrode; a negative electrode; and an electrolyte interposed between the positive electrode and the negative electrode, The above electrolyte is, Lithium salt; menstruum; Comprising additive 1; and additive 2, A lithium secondary battery characterized in that the above additive 1 comprises a polymer monomer composed of a hydrocarbon derivative containing a nitrogen or sulfur element and containing at least one double or triple bond.

2. In Paragraph 1, A lithium secondary battery characterized in that the above polymer monomer comprises one or more selected from the group consisting of pyrrole, thiophene, aniline, 3,4-ethylenedioxythiophene, and dimers, trimers, or derivatives thereof.

3. In Paragraph 1, A lithium secondary battery characterized in that the above polymer monomer includes pyrrole.

4. In Paragraph 1, A lithium secondary battery characterized in that the above polymer monomer is included in an amount of 1 to 300 mM relative to the entire electrolyte.

5. In Paragraph 1, A lithium secondary battery characterized in that the above polymer monomer is included in an amount of 0.005 to 3.4 parts by weight per 100 parts by weight of the total electrolyte.

6. In Paragraph 1, A lithium secondary battery characterized in that the solvent further comprises one or more organic solvents selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methyl ethyl carbonate (MEC), fluoroethylene carbonate (FEC), methylpropyl carbonate (MPC), ethyl propyl carbonate (EPC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-butylene carbonate, and combinations thereof.

7. In Paragraph 1, A lithium secondary battery characterized in that the above additive 2 further comprises one or more organic materials selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinylethylene carbonate (VEC), propanesulfone (PS), 1,3-propanesulfone (PRS), ethylene sulfate (ESa), succinonitrile (SN), adiponitrile (AN), hexane tricarbonitrile (HTCN), gamma-butyrolactone (GBL), biphenyl (BP), cyclohexylbenzene (CHB), tert-amylbenzene (TAB), and derivatives thereof.

8. In Paragraph 7, A lithium secondary battery characterized in that the above additive 2 is included in an amount of 0.1 to 10 parts by weight per 100 parts by weight of the total electrolyte.

9. In Paragraph 1, A lithium secondary battery characterized in that the above polymer monomer is oxidized on the surface of the positive electrode during charging of the lithium secondary battery to form a conductive polymer film.

10. In Paragraph 1, The above anode is LiFePO4(LFP), Li(Mn a Fe b )PO4(0 <a,b<1), LiCoO2및 이들의 조합으로 이루어진 군으로부터 선택되는 하나 이상의 양극 활물질을 포함하는 것을 특징으로 하는, 리튬 이차전지.

11. In Paragraph 1, A lithium secondary battery characterized in that the above-mentioned cathode comprises one or more cathode active materials selected from carbon-based materials.

12. In Paragraph 1, The above anode is Li[Ni a Co b Mn c Al d ]O2(0 <a,b,c,d<1), Li 1+x (Ni,Co,Mn,Al) 1-y O z (0 <x≤1, 0≤y<1, 2≤z≤4), Li 1+x (Ni,Co,Mn,Al) 1-x It comprises one or more positive active materials selected from the group consisting of O2 (0≤x≤0.2) and mixtures thereof, and The above cathode is Si, SiO x A lithium secondary battery characterized by comprising one or more negative active materials selected from the group consisting of carbon-based materials containing (0 < x < 2) and mixtures thereof.

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