Electrolyte composition and anode-free lithium secondary battery comprising the same

Abstract

The present invention relates to an electrolyte composition and a non-anode lithium secondary battery comprising the same. Specifically, the invention relates to an electrolyte composition comprising a lithium salt; an organic solvent; and an acidic additive, wherein the acidic additive comprises a fluorinated carboxylic acid, and a non-anode lithium secondary battery comprising the electrolyte composition.

Description

Technology Field

[0001] The present invention relates to an electrolyte composition and a negative electrode lithium secondary battery comprising the same. Specifically, the present invention relates to an electrolyte composition comprising a lithium salt, an organic solvent, and an acidic additive, and a negative electrode lithium secondary battery comprising the same.

[0002] Here, the electrolyte composition can be an electrolyte composition for a negative electrode lithium secondary battery. Background Technology

[0003] Anode-free lithium-ion batteries are attracting attention as next-generation energy storage systems with high energy density. Unlike conventional lithium-ion batteries, anode-free lithium-ion batteries use a negative electrode current collector that does not contain lithium, such as copper, and the negative electrode relies on lithium supplied from the positive electrode during the initial cycle.

[0004] However, anode-free lithium secondary batteries have significant problems such as low Coulomb efficiency, unstable lithium plating and peeling, lithium dendrite growth, and rapid capacity reduction. These problems are mainly attributed to uncontrolled interfacial reactions between precipitated lithium metal and the electrolyte, and continuous lithium consumption during the formation of the solid electrolyte interphase (SEI).

[0005] High-concentration electrolytes containing lithium salts such as lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), and lithium perchlorate (LiClO4), and ether-containing molecules and / or fluorine-containing molecules have been reported to improve the stability of lithium metal by controlling the solvation structure and suppressing side reactions. However, such an electrolyte system alone is still insufficient to fully stabilize lithium precipitation behavior, particularly in the configuration of anode-free lithium secondary batteries under actual high-rate charge / discharge conditions.

[0006] Therefore, an electrolyte composition is required that can further improve lithium utilization efficiency, interfacial stability, and long-term cycle performance in anode-free lithium secondary batteries without causing additional disadvantages such as increased impedance, gas generation, or degradation of characteristics. Prior art literature

[0007] Republic of Korea Published Patent No. 10-2024-0029494 The problem to be solved

[0008] The present invention aims to solve the aforementioned problems by providing an electrolyte composition capable of improving cycle stability and capacity retention rate by stabilizing lithium plating / stripping, and a negative electrode lithium secondary battery comprising the same.

[0009] In addition, the present invention aims to provide an electrolyte composition capable of improving Coulomb efficiency and a negative electrode lithium secondary battery comprising the same.

[0010] In addition, the present invention aims to provide an electrolyte composition capable of suppressing an increase in internal resistance during long-term cycling and a negative electrode lithium secondary battery comprising the same.

[0011] In addition, the present invention aims to provide an electrolyte composition capable of preventing failure caused by localized lithium accumulation (lithium precipitation) and a negative electrode lithium secondary battery comprising the same.

[0012] In addition, the present invention aims to provide an electrolyte composition capable of preventing the deterioration of rate capability and the induction of side reactions, and a negative electrode lithium secondary battery comprising the same. means of solving the problem

[0013] The present invention provides an electrolyte composition for achieving the aforementioned objectives, comprising a lithium salt; an organic solvent; and an acidic additive, wherein the acidic additive comprises a fluorinated carboxylic acid. The fluorinated carboxylic acid, which is the acidic additive, adsorbs and reacts with the electrode surface, thereby homogenizing lithium deposition and suppressing electrolyte decomposition even during long-term charging and discharging, which can improve the battery life.

[0014] The above-mentioned fluorinated carboxylic acid may include a fluorinated aliphatic carboxylic acid having at least one carboxyl group. The carboxyl group promotes interfacial reactions, thereby improving the uniformity of lithium deposition and stabilizing the electrode interface.

[0015] The above-mentioned fluorinated carboxylic acid may include at least one of hexafluoroglutaric acid, tetrafluorosuccinic acid, octafluoroadipic acid, perfluorosuberic acid, difluoroacetic acid, difluoropropanoic acid, perfluoropropionic acid, heptafluorobutyric acid, perfluorovaleric acid, and undecafluorohexanoic acid derivatives. Accordingly, the range of material selection is wide, and the interface of the electrode can be stabilized even under various battery conditions.

[0016] The above-mentioned fluorinated carboxylic acid may include hexafluoroglutaric acid. Accordingly, internal resistance is suppressed, and battery life can be improved.

[0017] The weight ratio of the acidic additive may be greater than 0 wt% and less than or equal to 2.0 wt% with respect to the electrolyte composition. Therefore, even if a small amount of acidic additive is included, the electrode interface can be stabilized.

[0018] Preferably, the weight ratio of the acidic additive may be 0.9 wt% to 1.1 wt% with respect to the electrolyte composition. More preferably, the weight ratio of the acidic additive may be 1 wt% with respect to the electrolyte composition. Through this ratio of acidic additive, battery life improvement and oxidation safety can be achieved simultaneously.

[0019] The above lithium salt comprises at least one selected from the group consisting of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluorosulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium perchlorate (LiClO4), and combinations thereof, and may have a concentration of 2.0 M to 6.0 M, and the organic solvent may comprise at least one of an ether-containing molecule and a fluorine-containing molecule.

[0020] A non-anode lithium secondary battery having a positive active material, a separator, and an electrolyte provided between a positive current collector and a negative current collector, wherein the electrolyte comprises a lithium salt; an organic solvent; and an acidic additive, and the acidic additive may comprise a fluorinated carboxylic acid.

[0021] The above-mentioned fluorinated carboxylic acid may include hexafluoroglutaric acid.

[0022] The weight ratio of the acidic additive may be 0.9 wt% to 1.1 wt% with respect to the electrolyte composition.

[0023] Preferably, the weight ratio of the acidic additive may be 1 wt% with respect to the electrolyte composition.

[0024] An artificial SEI layer of metal is coated on the above-mentioned negative current collector, and the artificial SEI layer can be formed as an Ag-Ti system. Accordingly, interfacial safety, charge / discharge uniformity, and battery life can be improved.

[0025] The artificial SEI layer described above can be coated on the cathode current collector in the form of a laminate of Ag and Ti, rather than being an alloy of Ag and Ti. Therefore, ease of manufacturing is ensured, and a uniform artificial SEI layer can be provided.

[0026] The artificial SEI layer can be formed by alternately and repeatedly coating Ag and Ti on the cathode current collector. Accordingly, a uniform artificial SEI layer can be formed, and damage to the electrode surface can be reduced. Long-term cycle life can be improved.

[0027] In the Ag-Ti system of the artificial SEI layer, the volume of Ag may be larger than the volume of Ti. Preferably, the Ag-Ti system of the artificial SEI layer may contain Ag:Ti in a volume ratio of 10:1 based on the total volume of the Ag-Ti system. Accordingly, the operational efficiency and interfacial stability of the artificial SEI layer can be maximized. Effects of the invention

[0028] According to the present invention, an electrolyte composition capable of improving cycle stability and capacity retention rate by stabilizing lithium plating / stripping, and a non-anode lithium secondary battery comprising the same can be provided.

[0029] In addition, according to the present invention, an electrolyte composition capable of improving Coulomb efficiency and a negative electrode lithium secondary battery containing the same can be provided.

[0030] In addition, according to the present invention, an electrolyte composition capable of suppressing an increase in internal resistance during long-term cycling and a negative electrode lithium secondary battery comprising the same can be provided.

[0031] In addition, according to the present invention, an electrolyte composition capable of preventing failure due to localized lithium accumulation and a negative electrode lithium secondary battery comprising the same can be provided.

[0032] In addition, according to the present invention, an electrolyte composition capable of preventing rate capability degradation and side reactions, and a negative electrode lithium secondary battery containing the same can be provided.

[0033] In particular, the present invention can stabilize the plating and stripping behavior of lithium metal, suppress the increase in interfacial resistance, and improve battery performance during long-term charge and discharge cycles by precisely controlling the reaction at the electrode interface using an acidic additive containing a fluorinated carboxylic acid. Brief explanation of the drawing

[0034] FIG. 1 is a conceptual diagram showing a non-anode lithium secondary battery in an embodiment of the present invention. Figure 2 is a graph comparing the maintenance performance of cell discharge capacity as the cycle increases. Figure 3 is a graph comparing the change in cell resistance with increasing cycles. Figure 4 is a graph comparing the change in Coulomb efficiency with increasing cycles. Figure 5 is a graph comparing the maintenance performance of cell discharge capacity according to additive concentration. Figure 6 is a graph comparing oxidation stability according to additive concentration. Figure 7 is a graph comparing the lifespan performance according to whether or not artificial SEI is coated. Figure 8 is a diagram showing a comparison of capacity retention rates according to the number of charge and discharge cycles. Figure 9 is a diagram showing a comparison of the number of charge and discharge cycles of a battery until the State of Health (SOH) reaches 80%. Figure 10 is a conceptual diagram showing a non-cathode lithium secondary battery having a negative electrode current collector without an artificial SEI layer as a comparative example. Specific details for implementing the invention

[0035] Hereinafter, an electrolyte composition according to an embodiment of the present invention and a negative electrode lithium secondary battery including the same will be described in detail with reference to the attached drawings. The attached drawings illustrate exemplary forms of the present invention and are provided only to explain the present invention in more detail; the technical scope of the present invention is not limited thereby.

[0036] Additionally, identical or corresponding components are assigned the same reference number regardless of drawing symbols, and redundant descriptions thereof are omitted; furthermore, for the convenience of explanation, the size and shape of each illustrated component may be exaggerated or reduced.

[0037] In addition, in describing the present invention, detailed descriptions of related prior art are omitted if it is determined that such descriptions could obscure the essence of the invention.

[0039] FIG. 1 is a conceptual diagram showing a non-anode lithium secondary battery in an embodiment of the present invention, and FIG. 10 is a conceptual diagram showing a non-anode lithium secondary battery having a negative current collector without an artificial SEI layer as a comparative example.

[0040] For example, during charging, lithium ions separated from the positive electrode active material move toward the negative electrode current collector and are reduced on the negative electrode current collector to form a negative electrode layer (i.e., a lithium layer). Additionally, during discharging, the negative electrode layer is oxidized to generate lithium ions, and the generated lithium ions can be reduced in the positive electrode active material.

[0041] In the present invention, the negative electrode active material and the negative electrode plate are not provided separately. That is, the present invention relates to a negative electrode-free lithium secondary battery.

[0042] Referring to FIGS. 1 and 10, a non-negative lithium secondary battery may include a negative electrode current collector (10), a positive electrode current collector (20), a positive electrode active material (30), an electrolyte (40), and a separator (50). For example, a liquid electrolyte (40) may be provided between the positive electrode current collector (20) and the negative electrode current collector (10).

[0043] The material of the above negative electrode current collector (10) can be used without special restrictions as long as it is a material having high conductivity without causing chemical changes in the lithium secondary battery. For example, the material of the negative electrode current collector (10) may be copper, iron, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. Preferably, the negative electrode current collector (10) may be formed of copper or stainless steel.

[0044] The above positive current collector (20) may be formed of aluminum, an aluminum polymer composite, etc. A separator (50) and a positive active material (30) may be disposed between the above negative current collector (10) and the above positive current collector (20).

[0045] The separator (50) is disposed between the negative current collector (10) and the positive plate formed by the positive active material (30), thereby separating the negative current collector (10) and the positive active material (30) (or positive plate) from each other. Additionally, the separator (50) can provide a passage for the movement of lithium ions and can be used without special limitations as long as it is typically used as a separator in a lithium secondary battery.

[0046] For example, it is desirable that the separator (50) has low resistance to the movement of ions through the electrolyte. For example, the separator (50) may be formed from at least one of polyethylene, polypropylene, or a copolymer of polyethylene and polypropylene, and it is also possible to form a multilayer film of two or more layers thereof.

[0047] The above positive active material (30) is a nickel-cobalt-aluminum oxide containing lithium (Li(NiCoAl)O2, NCA), a nickel-cobalt-manganese oxide containing lithium (Li(NiCoMn)O2, NCM or NMC), a lithium manganese oxide (LiMn2O4, LMO), a lithium iron phosphate (LiFePO4, LFP), a lithium cobalt oxide (LiCoO2, LCO), and a manganese-iron-phosphate containing lithium (LiMn x Fe 1-x It may include at least one of Mn4PO4, LMFP.

[0048] When charging the lithium secondary battery, lithium ions are separated from the positive active material (30), and a negative electrode layer (60) can be formed on the negative electrode current collector (10).

[0049] For example, if the positive active material (30) is formed of a nickel-cobalt-manganese oxide containing lithium, lithium ions can be separated from the positive active material (30) through the following chemical formula 1. The separated lithium ions can be reduced on the negative current collector (10) to form a negative electrode layer (60).

[0050] Chemical formula 1:

[0051]

[0052] An anode plate can be formed by compressing the above-mentioned positive active material (30). That is, the anode plate can be formed by compressing the positive active material powders. The compression of the positive active material powders can be performed by a previously known method such as roll compression.

[0053] With the positive active material (30) placed on the positive current collector (20), the positive active material (30) and the positive current collector (20) can be compressed together. In this case, the positive plate can be attached to the positive current collector (20) at the same time as the positive plate is formed. Alternatively, the positive plate may be attached to the positive current collector (20) after the positive active material (30) is formed into a positive plate through compression.

[0054] Additionally, the anode plate may further include at least one of a binder material and a conductive material. For example, the anode plate may be formed by compressing the mixture in the state where at least one of the binder material and the conductive material is mixed with the anode active material (30).

[0055] For example, the binder material may include a polymer, and known materials may be used as the binder material and the conductive material.

[0056] Meanwhile, as the charging and discharging of the non-anode lithium-ion battery are repeated, dendrites may form on the negative electrode layer during charging. The comparative example shown in FIG. 10 (b) illustrates a state in which dendrites have formed on the negative electrode layer (60).

[0057] In order to suppress the occurrence of such dendrites, an artificial SEI layer (70) may be coated on the negative electrode current collector (10) of the negative electrode lithium secondary battery according to an embodiment of the present invention. Here, artificial SEI (Artificial Solid Electrolyte Interphase) may also be referred to as ASEI.

[0058] The artificial SEI layer (70) can also be expressed as an artificial SEI coating layer, an artificial SEI film, an artificial SEI thin film, etc.

[0059] As illustrated in FIG. 1(b), the occurrence of dendrites in the negative electrode layer (60) can be suppressed by coating a metal artificial SEI layer (70) on the negative electrode current collector (10). For convenience of explanation, FIG. 1(b) conceptually depicts a film formed by the artificial SEI layer. In reality, the drawing candidate '70' shown in FIG. 1(b) may be a film formed by the reaction of at least a portion of the artificial SEI layer with the electrolyte (or electrolyte system) during the initial charging of the battery.

[0060] For example, the formation of a membrane through the reaction of at least a portion of the artificial SEI (70) layer and the electrolyte (40) can be represented as shown in the main reaction formula 2 below.

[0061] Chemical formula 2 (no side reactions):

[0062]

[0063] This artificial SEI layer (70) can be coated on the negative current collector (10) in advance before the negative current collector (10) comes into contact with the electrolyte (40). That is, it is preferable that the artificial SEI layer (70) be coated on the negative current collector (10) in advance before manufacturing a non-negative lithium secondary battery. This is to distinguish it from the natural SEI generated on the negative current collector (10) as the electrolyte comes into contact with the negative current collector (10).

[0064] The inventors of the present invention were able to recognize through experiments that when an artificial SEI layer (70), which is a metal coating layer, is coated on a negative current collector (10), the occurrence of dendrites can be significantly suppressed.

[0065] The artificial SEI layer described above can be formed from an Ag-Ti system. Here, the Ag-Ti system does not refer to an alloy of Ag and Ti, but rather to a state in which Ag and Ti are laminated (i.e., coated) on a cathode current collector.

[0066] In the Ag-Ti system, the stacking order of Ag and Ti on the negative current collector can be arbitrary. For example, Ag may be stacked first on the negative current collector, followed by Ti. Conversely, Ti may be stacked first on the negative current collector, followed by Ag.

[0067] In addition, Ag and Ti can be repeatedly and alternately stacked on the negative current collector. That is, Ag and Ti can be stacked on the negative current collector in multiple layers. The above Ag-Ti system can also be expressed as an Ag-Ti layer, an Ag-Ti coating layer, an Ag-Ti film, an Ag-Ti thin film, etc.

[0068] Ag-Ti-based coatings for forming an artificial SEI layer on a cathode current collector can be performed through known methods such as roll coating, spray coating, slot die coating, blade coating, dry coating, and vacuum coating.

[0069] According to the present invention, by coating an artificial SEI layer (70) formed of an Ag-Ti system onto a negative electrode current collector (10), the occurrence of dendrites in the negative electrode layer (60) during battery charging can be suppressed.

[0070] Meanwhile, the inventors of the present application have discovered through experiments that the performance and lifespan of the battery can be improved through the composition of the electrolyte.

[0071] The above electrolyte (40) may be a liquid electrolyte. The above liquid electrolyte may be a non-aqueous electrolyte solution. The non-aqueous electrolyte solution may comprise a lithium salt, a solvent, and an acidic additive.

[0072] The lithium salt may include at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluorosulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium perchlorate (LiClO4), and combinations thereof. The lithium salt may have a concentration of 2.0 M to 6.0 M. Since various materials for lithium salts are disclosed, a more specific description of the lithium salt is omitted.

[0073] The organic solvent may include at least one of a molecule containing an ether bond and a molecule containing one or more fluorine atoms within its molecular structure. For example, the organic solvent may be an organic solvent system containing ether-containing molecules and / or fluorine-containing molecules.

[0074] Here, the ether-containing molecule may refer to an organic molecule containing an ether bond (-O-) within the molecule, and the fluorine-containing molecule may refer to an organic molecule containing one or more fluorine atoms within the molecular structure. The fluorine-containing molecule may include structures ranging from low-level fluorine substitution to high-level fluorine substitution.

[0075] Such organic solvent systems can provide a highly stable solvation environment suitable for high-concentration lithium salt electrolytes and lithium metal electrochemical reactions. As various organic solvents are publicly available, a more detailed description of the organic solvent is omitted.

[0076] The acidic additive may be a compound having an acidity capable of providing protons in an electrolyte environment. The acidic additive may decompose or react within the electrolyte to form an interfacial film (protective film) containing LiF (Lithium Fluoride) at the electrode interface. Additionally, the acidic additive may perform the function of suppressing side reactions on the cathode surface and inducing the homogenization of lithium deposition.

[0077] The above acidic additive may include a fluorinated carboxylic acid. Specifically, the fluorinated carboxylic acid may include a fluorinated aliphatic carboxylic acid having at least one carboxyl group. That is, the fluorinated carboxylic acid may include an acid having a fluorinated aliphatic carbon skeleton and at least one carboxyl functional group.

[0078] For example, the fluorinated carboxylic acid may include at least one of hexafluoroglutaric acid, tetrafluorosuccinic acid, octafluoroadipic acid, perfluorosuberic acid, difluoroacetic acid, difluoropropanoic acid, perfluoropropionic acid, heptafluorobutyric acid, perfluorovaleric acid, and undecafluorohexanoic acid.

[0079] The acids listed above are merely non-limiting examples of fluorinated carboxylic acids having a molecular structure structurally related to hexafluoroglutaric acid. The scope of the invention is not limited thereto, and other fluorinated carboxylic acids satisfying the above structural definition may also be used.

[0080] In a preferred embodiment, the acidic additive may include hexafluoroglutaric acid.

[0081] In an embodiment of the present invention, the acidic additive may be included in a weight ratio of approximately greater than 0 wt% and less than or equal to 2.0 wt%, preferably 0.9 wt% to 1.1 wt%, and more preferably 1 wt% with respect to the electrolyte composition. In the following description, the weight ratio of the additive may refer to a weight ratio based on the total electrolyte composition or a weight ratio relative to the total amount of the electrolyte composition.

[0082] The inventors of the present application conducted comparative experiments on the performance of an electrolyte comprising ether-containing molecules and an electrolyte comprising fluorine-containing molecules, and also conducted comparative experiments on the performance of the fluorine-containing molecule-based electrolyte and the ether-containing molecule-based electrolyte with an additive (e.g., an acidic additive) included.

[0083] In the case of ether-containing molecule-based electrolytes, lithium ion mobility (conductivity) is good, but there are problems such as low oxidation stability, many side reactions, and unstable interfaces. In contrast, fluorine-containing molecule-based electrolytes have the advantages of very high chemical / electrochemical stability, stable interfaces, high oxidation stability at high voltages, and high lifespan performance.

[0084] In the graphs of FIGS. 2 to 4 below, the solid line represents the characteristics of a fluorine-containing molecule-based electrolyte containing an additive according to an embodiment of the present invention, the dashed line represents the characteristics of a fluorine-containing molecule-based electrolyte without an additive (Comparative Example 1), the dotted line represents the characteristics of an ether-containing molecule-based electrolyte without an additive (Comparative Example 2), and the dotted line represents the characteristics of an ether-containing molecule-based electrolyte containing an additive (Comparative Example 3). The additive may be included in a weight ratio of greater than 0 wt% to 2.0 wt% or less with respect to the electrolyte composition. Preferably, the additive may be included in a weight ratio of 0.9 wt% to 1.1 wt% with respect to the electrolyte composition. More preferably, the additive may be included in a weight ratio of 1 wt% with respect to the electrolyte composition.

[0085] In addition, in the graphs of FIGS. 2 to 4, the fluorinated electrolyte containing the additive according to the embodiment of the present invention is denoted as 'Electrolyte B + Add', Comparative Example 1 is denoted as 'Electrolyte B', Comparative Example 2 is denoted as 'Electrolyte A', and Comparative Example 3 is denoted as 'Electrolyte A + Add'.

[0086] Comparative Example 1 represents a fluorine-containing molecule-based electrolyte without additives, Comparative Example 2 represents an ether-containing molecule-based electrolyte without additives, and Comparative Example 3 represents an ether-containing molecule-based electrolyte with additives. Here, the additive may refer to the aforementioned acidic additive.

[0088] Figure 2 is a graph comparing the cell discharge capacity retention performance (also called 'capacity retention rate' or 'lifetime performance') as the cycle increases.

[0089] Referring to FIG. 2, in all of the examples and comparative examples 1 to 3, the capacity retention rate gradually decreases as the charge-discharge cycle of the battery increases.

[0090] In the case of an ether-containing molecule-based electrolyte (i.e., Electrolyte A), the capacity retention rate decreases rapidly with increasing cycles. In contrast, in the case of a fluorine-containing molecule-based electrolyte (i.e., Electrolyte B), the capacity retention rate decreases gradually despite increasing cycles.

[0091] In particular, in the case of a fluorine-containing molecule-based electrolyte (i.e., Electrolyte B + Add) containing an additive according to an embodiment of the present invention, the capacity retention rate with increasing cycles decreased most gradually compared to Comparative Examples 1 to 3. For example, in the case of a fluorine-containing molecule-based electrolyte containing an additive according to an embodiment of the present invention, a capacity retention rate of 93% or more can be achieved at 100 cycles, a capacity retention rate of approximately 90% can be achieved at 150 cycles, and a capacity retention rate of approximately 83% or more can be achieved at 200 cycles.

[0093] Figure 3 is a graph comparing the change in cell resistance (e.g., Direct Current Internal Resistance) with increasing cycles.

[0094] Referring to FIG. 3, in all of the examples and comparative examples 1 to 3, the cell resistance gradually increases as the charge / discharge cycle of the battery increases.

[0095] In the case of ether-containing molecule-based electrolytes, cell resistance increases rapidly with increasing cycles. In contrast, in the case of fluorine-containing molecule-based electrolytes, cell resistance increases gradually despite increasing cycles.

[0096] In particular, in the case of the fluorine-containing molecule-based electrolyte containing the additive according to the embodiment of the present invention, the cell resistance increased most gradually with increasing cycles compared to Comparative Examples 1 to 3. That is, in the case of the fluorine-containing molecule-based electrolyte containing the additive according to the embodiment of the present invention, the increase in resistance with increasing charge-discharge cycles is the smallest, and it exhibits the best resistance characteristics.

[0097] For example, in the case of a fluorine-containing molecule-based electrolyte containing an additive according to an embodiment of the present invention, the cell resistance can be maintained at 250 mΩ or less even after approximately 200 cycles or more.

[0099] Figure 4 is a graph comparing the change in Coulomb efficiency with increasing cycles.

[0100] Referring to FIG. 4, in all of the examples and comparative examples 1 to 3, the Coulomb efficiency gradually decreases as the charge-discharge cycle of the battery increases.

[0101] In the case of ether-containing molecule-based electrolytes, Coulomb efficiency decreases rapidly with increasing cycles. In contrast, in the case of fluorine-containing molecule-based electrolytes, Coulomb efficiency decreases gradually despite increasing cycles.

[0102] In particular, in the case of a fluorine-containing molecule-based electrolyte containing an additive according to an embodiment of the present invention, the Coulomb efficiency decreased most gradually with increasing cycles compared to Comparative Examples 1 to 3. For example, in the case of a fluorine-containing molecule-based electrolyte containing an additive according to an embodiment of the present invention, the Coulomb efficiency can be maintained at 99% or higher even after approximately 200 cycles or more.

[0104] The inventors of the present application conducted comparative experiments on a fluorine-containing molecule-based electrolyte containing an additive according to an embodiment of the present invention by varying the concentration (i.e., the content of the additive) of the additive.

[0105] Figure 5 is a graph comparing cell life performance (discharge capacity retention performance) according to additive concentration.

[0106] In the graph of FIG. 5, the solid line represents Example 1, in which the electrolyte composition contains an additive in a weight ratio of 0.9 wt% to 1.1 wt%, preferably 1 wt%. The dashed line represents Example 2, in which the electrolyte composition contains an additive in a weight ratio of 0.4 wt% to 0.6 wt%, preferably 0.5 wt%. The dotted line represents Example 3, in which the electrolyte composition contains an additive in a weight ratio of 1.9 wt% to 2.1 wt%, preferably 2 wt%.

[0107] In all of Examples 1 to 3, the discharge capacity gradually decreases as the charge-discharge cycle of the battery increases. However, in all of Examples 1 to 3, a discharge capacity of 180 mAh or more was maintained based on 100 cycles.

[0108] In particular, in the case of Example 1, the discharge capacity was maintained at approximately 180 mAh based on 200 cycles, but in the case of Examples 2 and 3, the discharge capacity decreased to 150 mAh or less based on 200 cycles.

[0109] That is, regarding the change in discharge capacity according to the charge / discharge cycle, Example 3 maintained a relatively higher level of discharge capacity compared to Example 2. And, Example 1 maintained the highest level of discharge capacity compared to Examples 2 and 3.

[0111] In addition, the inventors of the present application conducted comparative experiments on oxidation stability by varying the concentration (i.e., the content of the additive) of the additive in a fluorine-containing molecule-based electrolyte containing the additive according to an embodiment of the present invention.

[0112] Figure 6 is a graph comparing oxidation stability according to additive concentration. Specifically, Figure 6 shows the results of a Linear Sweep Voltammetry-Oxidation (LSV_OXI) experiment in which the current is measured while the potential is changed in one direction at a constant rate.

[0113] In the graph of FIG. 6, the solid line represents the oxidation stability of the electrolyte composition according to Example 1, in which the electrolyte composition contains an additive in a weight ratio of 0.9 wt% to 1.1 wt%, preferably 1 wt%. The dashed line represents the oxidation stability of the electrolyte composition according to Example 2, in which the electrolyte composition contains an additive in a weight ratio of 0.4 wt% to 0.6 wt%, preferably 0.5 wt%. The dotted line represents the oxidation stability of the electrolyte composition according to Example 3, in which the electrolyte composition contains an additive in a weight ratio of 1.9 wt% to 2.1 wt%, preferably 2 wt%. The dotted line represents the oxidation stability of the electrolyte composition according to Example 4, in which no additive is included.

[0114] In FIG. 6, an increase in current may mean that the electrolyte decomposes, causing side reactions or generating gas. The electrolyte must not decompose even at high voltage so that the battery can be operated stably at high voltage. Generally, for battery operation, the high voltage can be 5.5V or higher.

[0115] Referring to FIG. 6, in all embodiments, a similar pattern of current increase was observed up to a voltage of 5.5V without a rapid increase in current.

[0116] Meanwhile, at a voltage of 5.5V or higher, the increase in current was greater in the order of Example 4, Example 2, Example 1, and Example 3. That is, at a voltage of 5.5V or higher, oxidation stability was higher in the order of Example 3, Example 1, Example 2, and Example 4. These results may mean that oxidation stability increases as the concentration (or content) of the additive included in the fluorine-containing molecule-based electrolyte increases.

[0117] However, referring to FIG. 5, it can be seen that the lifespan performance of Example 1 is significantly superior to that of Example 3 in terms of cell lifespan performance (discharge capacity retention performance) according to additive concentration. Therefore, in order to improve oxidation stability along with improving the battery lifespan performance, Example 1 is more preferable than Example 3.

[0119] Figure 7 is a graph comparing the lifespan performance (or capacity retention) according to whether artificial SEI is coated.

[0120] FIG. 7 compares a comparative example (Bare Su anode) in which an artificial SEI layer is not coated on the cathode current collector and an embodiment of the present invention (Surface modified anode) in which an artificial SEI layer is coated on the cathode current collector. The comparative example is indicated by a dotted line, and the embodiment of the present invention is indicated by a solid line.

[0121] In the comparative example and example shown in FIG. 7, a fluorine-containing molecule-based electrolyte composition was used in which the aforementioned acidic additive was included as an electrolyte in a weight ratio of more than 0 wt% and less than or equal to 2.0 wt%, preferably 0.9 wt% to 1.1 wt%.

[0122] Referring to FIG. 7, in 150 charge-discharge cycles, the comparative example showed a capacity retention rate of approximately 80%, while the embodiment of the present invention showed a capacity retention rate of approximately 90%. In addition, in the comparative example, the capacity retention rate was 70% in approximately 175 charge-discharge cycles, whereas in the embodiment of the present invention, the capacity retention rate was 70% in approximately 250 charge-discharge cycles.

[0123] In this way, compared to the case where an artificial SEI layer is not coated on the cathode current collector, the capacity retention rate and lifespan performance can be improved when an artificial SEI layer is coated on the cathode current collector.

[0125] FIG. 8 is a diagram showing a comparison of capacity retention rates according to the number of charge-discharge cycles. Specifically, FIG. 8 shows the change in capacity retention rate (vertical axis) according to the number of charge-discharge cycles (horizontal axis) by varying the ratio of Ag to Ti in an Ag-Ti system forming an artificial SEI layer (70).

[0126] In FIG. 8, the dotted line represents a reference in which no artificial SEI layer is provided on the cathode current collector, the single-dotted line represents an example in which Ag:Ti is included in a ratio of 4:1 based on the total volume of the Ag-Ti system in the artificial SEI layer Ag-Ti system, the two-dotted line represents an example in which Ag:Ti is included in a ratio of 16:1 based on the total volume of the Ag-Ti system in the artificial SEI layer Ag-Ti system, and the solid line represents an example in which Ag:Ti is included in a ratio of 10:1 based on the total volume of the Ag-Ti system in the artificial SEI layer Ag-Ti system.

[0127] Referring to Fig. 8, it can be seen that compared to a reference without an artificial SEI layer formed of an Ag-Ti system, the decrease in capacity retention rate according to the number of charge-discharge cycles is relatively small when an artificial SEI layer formed of an Ag-Ti system is included.

[0128] In other words, when the same number of charge and discharge cycles are repeated, the case including an artificial SEI layer formed of Ag-Ti exhibits a higher capacity retention rate compared to the reference that does not include an artificial SEI layer formed of Ag-Ti.

[0129] In other words, when comparing the number of charge / discharge cycles to reach a specific capacity retention rate (e.g., 80% capacity retention rate), more charge / discharge cycles can be secured when an artificial SEI layer formed of Ag-Ti is included compared to a reference that does not include an artificial SEI layer formed of Ag-Ti.

[0130] In this way, when including an artificial SEI layer formed from an Ag-Ti system, the capacity and lifespan of the battery can be relatively increased.

[0131] Meanwhile, the inventors of the present invention conducted an experiment to compare the case including an artificial SEI layer formed of an Ag-Ti system and the case not including it, in relation to the relationship between the battery State of Health (SOH) and the number of charge and discharge cycles.

[0133] FIG. 9 is a diagram showing a comparison of the number of charge-discharge cycles of a battery until its State of Health (SOH) reaches 80%. Specifically, FIG. 9 shows the change in the number of charge-discharge cycles (vertical axis) of a battery until its State of Health reaches 80% according to the ratio of Ag (horizontal axis) in an Ag-Ti system forming an artificial SEI layer (70), by varying the ratio of Ag to Ti.

[0134] Generally, the replacement time for electric vehicle (EV) batteries is considered when the State of Health (SOH) reaches 70–80%. In other words, an SOH of 80% means that the battery is aging normally and is still usable, but it implies a state where driving range and charging speed may decrease compared to the initial state.

[0135] Referring to Fig. 9, it can be seen that compared to a reference without an artificial SEI layer formed of an Ag-Ti system, the number of charge-discharge cycles up to SOH 80% is greater when an artificial SEI layer formed of an Ag-Ti system is included.

[0136] In other words, when comparing the number of charge and discharge cycles to reach a specific battery health state (e.g., SOH 80%), more charge and discharge cycles can be secured when an artificial SEI layer formed of Ag-Ti is included compared to a reference that does not include an artificial SEI layer formed of Ag-Ti.

[0137] In this way, when including an artificial SEI layer formed from an Ag-Ti system, the capacity and lifespan of the battery can be relatively increased.

[0138] Meanwhile, as in the example, since Ag and Ti are stacked alternately in a laminate form on the negative current collector in one layer or multiple layers, the volume of Ag and Ti can be determined based on the thickness of the Ag and Ti stacked on the negative current collector.

[0139] The inventors of the present invention conducted an experiment to compare the change in capacity retention rate according to the number of charge-discharge cycles based on the volume of Ag and the volume of Ti in an Ag-Ti system forming an artificial SEI layer (70).

[0140] Referring to FIGS. 8 and 9, the Ag-Ti system of the artificial SEI layer may contain 5 to 25% of the volume of Ti based on the total volume of the Ag-Ti system. In other words, the artificial SEI layer may contain 5 to 25% of the volume of Ti based on the total volume of the artificial SEI layer.

[0141] Preferably, the Ag-Ti system of the artificial SEI layer may contain approximately 9% of the volume of Ti based on the total volume of the Ag-Ti system. In other words, the artificial SEI layer may contain approximately 9% of the volume of Ti based on the total volume of the artificial SEI layer.

[0142] In addition, the Ag-Ti system of the artificial SEI layer may contain Ag:Ti in a ratio of 4:1 to 16:1 based on the total volume of the Ag-Ti system. In other words, the artificial SEI layer may contain Ag:Ti in a ratio of 4:1 to 16:1 based on the total volume of the artificial SEI layer.

[0143] Preferably, the Ag-Ti system of the artificial SEI layer may contain Ag:Ti in a ratio of 10:1 based on the total volume of the Ag-Ti system. In other words, the artificial SEI layer may contain Ag:Ti in a ratio of 10:1 based on the total volume of the artificial SEI layer.

[0144] In this way, when an artificial SEI layer formed of an Ag-Ti system in a specific ratio is coated on the negative electrode current collector of a lithium secondary battery, the occurrence of dendrites is suppressed, and the capacity and lifespan of the lithium secondary battery can be increased.

[0146] The preferred embodiments of the present invention described above are disclosed for illustrative purposes only, and those skilled in the art with ordinary knowledge of the present invention will be able to make various modifications, changes, and additions within the spirit and scope of the present invention, and such modifications, changes, and additions should be considered to fall within the scope of the following claims. Explanation of the symbols

[0147] 10 Cathode Current Collector 20 positive current collector 30 positive electrode active material 40 electrolytes 50 separators 60 cathode layers 70 artificial SEI layer

Claims

Claim 1 A non-anode lithium secondary battery having a positive active material, a separator, and an electrolyte provided between a positive current collector and a negative current collector, wherein the composition of the electrolyte comprises a lithium salt; an organic solvent; and an acidic additive, wherein the acidic additive comprises a fluorinated carboxylic acid, and wherein an artificial solid electrolyte interphase (SEI) layer of metal is coated on the negative current collector, and wherein the artificial solid electrolyte interphase layer is formed of an Ag-Ti system. Claim 2 A non-anode lithium secondary battery according to claim 1, characterized in that the fluorinated carboxylic acid comprises hexafluoroglutaric acid. Claim 3 A non-anode lithium secondary battery according to claim 1, wherein the weight ratio of the acidic additive is 0.9 wt% to 1.1 wt% with respect to the composition of the electrolyte. Claim 4 A non-cathode lithium secondary battery according to claim 1, characterized in that the artificial solid electrolyte interface layer is not an alloy of Ag and Ti, but rather Ag and Ti are coated on a negative electrode current collector in a laminate form. Claim 5 A non-cathode lithium secondary battery according to claim 4, wherein the artificial solid electrolyte interface layer is formed by alternately and repeatedly coating Ag and Ti on the negative electrode current collector. Claim 6 A non-anode lithium secondary battery according to claim 1, characterized in that, in the Ag-Ti system of the artificial solid electrolyte interface layer, the volume of Ag is greater than the volume of Ti. Claim 7 A non-anode lithium secondary battery according to claim 6, characterized in that the Ag-Ti system of the artificial solid electrolyte interface layer contains Ag:Ti in a volume ratio of 10:1 based on the total volume of the Ag-Ti system. Claim 8 delete Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete Claim 15 delete Claim 16 delete Claim 17 delete

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