Lithium metal secondary battery and battery module including the same

Abstract

A lithium metal secondary battery is disclosed, comprising: an electrode assembly including a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode; a non-aqueous electrolyte in which the electrode assembly is impregnated; and a battery case in which the electrode assembly and the non-aqueous electrolyte are contained, wherein the negative electrode includes a negative electrode collector and a lithium metal layer formed on at least one surface of the negative electrode collector, and the charging / discharging conditions of the lithium metal secondary battery include charging the lithium metal secondary battery under a pressurized state and discharging the lithium metal secondary battery under a non-pressurized or pressurized state, and when the lithium metal secondary battery is discharged under a pressurized state, the pressure applied during the discharge is controlled to be less than the pressure applied during the charging. A battery module including the lithium metal secondary battery is also disclosed.

Description

Technical Field

[0001] The present disclosure relates to a lithium metal secondary battery and a battery module including the same. More specifically, the present disclosure relates to a lithium metal secondary battery having improved cycle characteristics and a battery module including the same.

[0002] This application claims the priority of Korean Patent Application No. 10-2018-0015293 filed in Korea on February 7, 2018, the disclosure of which including the specification and drawings is incorporated herein by reference. Background Art

[0003] With the rapid development of electrical, electronic, communication and computer industries, the demand for high-capacity batteries is increasing. In order to meet such demand, lithium metal secondary batteries using lithium metal or lithium alloy as negative electrodes with high energy density have received much attention.

[0004] Lithium metal secondary batteries refer to secondary batteries that use lithium metal or lithium alloy as the negative electrode. Lithium metal has a mass of 0.54 g / cm 3 Due to its low density and extremely low standard reduction potential (SHE: based on standard hydrogen electrode) of -3.045 V, it has attracted the most attention as an electrode material for high energy density batteries.

[0005] Such lithium metal secondary batteries have not been commercialized due to their poor cycle characteristics. This is because dendritic plating of lithium occurs during charging, resulting in an increase in the surface area of ​​the electrode and an increase in side reactions with the electrolyte.

[0006] Therefore, a method for physically preventing dendritic plating of lithium is needed. Summary of the invention

[0007] Technical issues

[0008] The present disclosure is directed to providing a lithium metal secondary battery capable of providing improved cycle characteristics by applying a predetermined pressure during charging to physically prevent dendritic plating of lithium, and a battery module including the lithium metal secondary battery.

[0009] Technical Solution

[0010] In one aspect of the present disclosure, a lithium metal secondary battery according to any one of the following embodiments is provided.

[0011] According to a first embodiment, there is provided a lithium metal secondary battery comprising:

[0012] An electrode assembly, comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode;

[0013] A nonaqueous electrolyte in which the electrode assembly is impregnated; and

[0014] A battery case containing an electrode assembly and a non-aqueous electrolyte,

[0015] wherein the negative electrode comprises a negative electrode current collector and a lithium metal layer formed on at least one surface of the negative electrode current collector,

[0016] The charge / discharge conditions of the lithium metal secondary battery include charging the lithium metal secondary battery under a pressurized state and discharging the lithium metal secondary battery under a non-pressurized or pressurized state, and when the lithium metal secondary battery is discharged under a pressurized state, the pressure applied during discharge is controlled to be less than the pressure applied during charging.

[0017] According to a second embodiment, there is provided a lithium metal secondary battery according to the first embodiment, wherein the pressure applied during charging is 3-150 psi.

[0018] According to a third embodiment, there is provided a lithium metal secondary battery according to the first or second embodiment, wherein the pressure applied during charging is 40-100 psi.

[0019] According to a fourth embodiment, there is provided the lithium metal secondary battery according to the first to third embodiments, wherein when the lithium metal secondary battery is discharged in a pressurized state, the pressure applied during discharge is 1-10 psi.

[0020] According to a fifth embodiment, there is provided a lithium metal secondary battery according to the first to fourth embodiments, wherein the current density during charging is 0.01-4 mA / cm 2 .

[0021] According to a sixth embodiment, there is provided a lithium metal secondary battery according to the first to fifth embodiments, wherein the current density during charging is 0.05-3.5 mA / cm 2 .

[0022] According to a seventh embodiment, there is provided the lithium metal secondary battery according to the first to sixth embodiments, wherein the temperature during charging is 25-45°C.

[0023] According to an eighth embodiment, there is provided a lithium metal secondary battery according to the first to seventh embodiments, which has a current density of 0.05-3.5 mA / cm2 at a temperature of 25-45° C. and a pressure of 40-100 psi. 2 The battery is charged at a charging current density of 100 psi and discharged in a non-pressurized state or at a pressure of 1-10 psi.

[0024] According to a ninth embodiment, there is provided the lithium metal secondary battery according to the first to eighth embodiments, which is a pouch type lithium metal secondary battery.

[0025] In another aspect of the present disclosure, a battery module according to any one of the following embodiments is provided.

[0026] According to a tenth embodiment, there is provided a battery module including a plurality of unit cells and a module case for accommodating the unit cells, wherein the unit cells are the lithium metal secondary batteries as defined in any one of the first to ninth embodiments.

[0027] According to an eleventh embodiment, there is provided a battery module as defined in the tenth embodiment, wherein the lithium metal secondary battery is a pouch type lithium metal secondary battery.

[0028] According to a twelfth embodiment, there is provided a battery module as defined in the tenth or eleventh embodiment, wherein the module case includes a rubber material.

[0029] According to a thirteenth embodiment, there is provided the battery module as defined in any one of the tenth to twelfth embodiments, wherein a portion of the module case in contact with the large-area surface of the unit battery includes a rubber material.

[0030] In another aspect, a battery pack according to the following embodiments is provided.

[0031] According to a fourteenth embodiment, there is provided a battery pack including the battery module as defined in the tenth embodiment.

[0032] Beneficial Effects

[0033] According to the present disclosure, the lithium metal secondary battery is charged in a pressurized state and discharged in a non-pressurized state or a pressurized state. Therefore, dendritic plating of lithium during charging can be physically prevented, thereby improving the cycle characteristics of the battery.

[0034] In addition, pressure is applied only during charging, and pressure may not be applied during discharging or pressure of a predetermined level may be applied. Therefore, higher efficiency can be provided compared to charging / discharging under pressure application. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, are used to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure should not be construed as being limited to the accompanying drawings.

[0036] Figure 1is a scanning electron microscope (SEM) image showing a cross-sectional view of a negative electrode of the lithium metal secondary battery according to Comparative Example 1 after repeated cycles.

[0037] Figure 2 is a SEM image showing a cross-sectional view of a negative electrode of the lithium metal secondary battery according to Example 5 after repeated cycles. DETAILED DESCRIPTION

[0038] Hereinafter, the preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in the specification and the appended claims should not be interpreted as being limited to the general meaning and dictionary meaning, but should be interpreted according to the meaning and concept corresponding to the technical aspects of the present disclosure on the basis of the principle of allowing the inventor to appropriately define the terms for the best interpretation.

[0039] Therefore, the description provided herein is only for the preferred embodiments for illustrative purposes, and is not intended to limit the scope of the present disclosure, so it should be understood that other equivalent substitutions and modifications may be made without departing from the scope of the present disclosure.

[0040] A lithium metal secondary battery according to an embodiment of the present disclosure includes: an electrode assembly including a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode; a non-aqueous electrolyte in which the electrode assembly is impregnated; and a battery case in which the electrode assembly and the non-aqueous electrolyte are contained, wherein the negative electrode includes a negative electrode collector and a lithium metal layer formed on at least one surface of the negative electrode collector, and the charging / discharging conditions of the lithium metal secondary battery include charging the lithium metal secondary battery under a pressurized state and discharging the lithium metal secondary battery under a non-pressurized or pressurized state, and when the lithium metal secondary battery is discharged under a pressurized state, the pressure applied during the discharge is controlled to be less than the pressure applied during the charging. In conventional lithium metal secondary batteries, dendritic plating of lithium is formed on the surface of the negative electrode during charging. As a result, there are problems such as a decrease in charging capacity due to an increase in the distance between the negative electrode and the positive electrode, or repeated deformation due to charging / discharging, resulting in degradation of internal components and a decrease in cycle life.

[0041] In order to improve the above-mentioned problems, the lithium metal secondary battery according to the present disclosure is placed under controlled charge / discharge conditions, so that it is charged under pressure using a pressurizing device to suppress its appearance expansion, and is discharged without pressurization (in a non-pressurized state) or in a pressurized state where the pressure is less than the pressure applied during charging.

[0042] According to the present disclosure, by charging under pressure, dendritic plating of lithium that may be generated during charging of a lithium metal secondary battery can be physically prevented. Side reactions with the electrolyte caused by an increase in the electrode surface area due to dendritic plating of lithium can also be suppressed, thereby improving the cycle characteristics of the battery.

[0043] In addition, the lithium metal secondary battery according to the present disclosure is not subjected to pressure all the time, but is subjected to pressure application only during battery charging, and is not subjected to pressure application during battery discharge. In addition, the lithium metal secondary battery is subjected to pressure application during battery charging, and is also subjected to pressure application within a predetermined range during battery discharge. Therefore, unnecessary pressure is not used during discharge to provide convenience of use. In addition, the charge / discharge conditions are controlled to apply a lower pressure during discharge than the pressure applied during charging. As a result, when the two electrodes are unexpectedly spaced apart from each other due to an expansion phenomenon after completing a charging cycle, they can be compressed to strengthen the contact between the two electrodes, thereby improving battery performance.

[0044] As used herein, “pressurizing a lithium metal secondary battery” or “applying pressure to a lithium metal secondary battery” means applying pressure in a thickness direction of the lithium metal secondary battery or applying pressure to a large surface area thereof.

[0045] According to an embodiment of the present disclosure, the pressure applied during charging of the lithium metal secondary battery can be 3-150psi, or 40-100psi. When the pressure applied during charging is within the above range, dendritic plating of lithium during charging can be effectively prevented, and leakage of non-aqueous electrolytes, increase in battery resistance, and compression of separators caused by excessive pressurization can be avoided, thereby ensuring battery safety.

[0046] When the lithium metal secondary battery is discharged under pressure, the pressure applied during discharge may be 1-10 psi, or 5-10 psi. When the pressure applied during discharge is within the above range, the space between the two electrodes can be minimized, thereby reducing battery resistance.

[0047] In addition, the lithium metal secondary battery can be pressurized by a charger provided with a pressurizing device (pressure applying device). Here, the charger provided with the pressurizing device may include a pressurizing device formed integrally with the charger, or may be a conventional charger connected to the pressurizing device.

[0048] According to an embodiment of the present disclosure, a charger provided with a pressurizing device may include: a storage member accommodating a secondary battery cell; a pressurizing member, the pressurizing member facing the storage member, the secondary battery cell being inserted therebetween, and the pressurizing member being spaced apart from the storage member at a variable distance; a pressurizing tool, the pressurizing tool being configured to pressurize the secondary battery cell accommodated in the storage member along the thickness direction of the secondary battery cell by pushing the pressurizing member toward the storage member or pulling the pressurizing member from the storage member; a measuring device, the measuring device being configured to measure the pressure applied to the pressurizing member, and / or the distance between the storage member and the pressurizing member at predetermined time intervals; a control unit, the control unit being configured to receive the pressure value applied to the pressurizing member and the distance between the storage member and the pressurizing member at predetermined time intervals, and to keep the distance constant or to change the distance to keep the pressure constant; and a charging unit, the charging unit being configured to charge the secondary battery cell accommodated in the storage member. Here, the storage member, the pressurizing member, the pressurizing tool, the measuring device, and the control unit form a pressurizing device, wherein the charging unit may be integrally formed with the pressurizing device, or a conventional charger may be connected to the pressurizing device.

[0049] According to the pressure control mode in the charger provided with the pressurizing tool, the pressurizing mode during the charge / discharge of the lithium metal secondary battery can be divided into a constant pressure pressurizing mode, a constant distance pressurizing mode, and a combined constant pressure / constant distance pressurizing mode.

[0050] In other words, in the constant pressure pressurization mode, the pressure applied to the pressurization member remains constant. In the constant distance pressurization mode, the distance between the storage member and the pressurization member remains constant. In the combined constant pressure / constant distance pressurization mode, the pressure and distance are controlled simultaneously or sequentially.

[0051] For example, in the constant distance pressurization mode, the distance between the storage member and the pressurization member remains constant. Therefore, as the lithium metal secondary battery swells during charging, a higher pressure can be applied to the lithium metal secondary battery. In the constant pressure pressurization mode, the pressure fixed by the pressurization tool is applied to the lithium metal secondary battery, so the distance between the storage member and the pressurization member can be changed during pressurization. When the constant pressure pressurization mode is used, the optimal voltage range may be slightly higher than the constant distance pressurization mode.

[0052] According to an embodiment of the present disclosure, when a lithium metal secondary battery is discharged in a non-pressurized state, the lithium metal secondary battery can be removed from the charger after completing charging in a pressurized state using a charger provided with a pressurizing tool, or the pressure of the charger can be controlled to 0 psi.

[0053] According to another embodiment of the present disclosure, when a lithium metal secondary battery is discharged under a pressurized state, the pressure applied to the lithium metal secondary battery can be regulated to be lower than the pressure applied during charging after completing charging under a pressurized state using a charger provided with a pressurizing tool.

[0054] The current density during charging can be 0.01-4 mA / cm 2 , or 0.05-3.5mA / cm 2 When the current density during charging is within the above range, delocalization of current is reduced and lithium plating is uniformly formed to prevent battery degradation.

[0055] In addition, the temperature during charging is 25-45° C. When the temperature during charging is within the above range, activation energy for chemical reaction is satisfied to reduce resistance, and thus the capacity retention rate is improved.

[0056] According to an embodiment of the present disclosure, the lithium metal secondary battery can be used at a temperature of 25-45° C. and a pressure of 40-100 psi at a flow rate of 0.05-3.5 mA / cm 2 The battery can be charged at a current density of 1000 psi and discharged in a non-pressurized state or at a pressure of 1-10 psi.

[0057] In addition, the negative electrode includes a negative electrode current collector and a lithium metal layer formed on the negative electrode current collector. The lithium metal layer is a sheet metal and may have a width controllable according to the electrode shape to facilitate the manufacture of the electrode. The lithium metal layer may have a thickness of 0-300 μm. Here, "a lithium metal layer having a thickness of 0 μm" means that after the lithium metal secondary battery is assembled, no lithium metal layer is formed on the negative electrode current collector. Even if a lithium metal layer is not formed on the negative electrode current collector, lithium ions may be transported from the positive electrode during charging to generate a lithium metal layer on the surface of the negative electrode current collector. Therefore, a lithium metal secondary battery can be operated.

[0058] In addition, non-limiting examples of the negative electrode current collector include foils made of copper, gold, nickel, copper alloys, or combinations thereof.

[0059] In addition, the positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on either or both surfaces thereof. Here, non-limiting examples of the positive electrode current collector include foils made of aluminum, nickel, or a combination thereof. The positive electrode active material contained in the positive electrode active material layer may be any one selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2、LiNi 1-x-y-z Co xM1 y M2 z O2 (wherein M1 and M2 each independently represent any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, x, y, and z each independently represent the atomic ratio of the elements forming the oxide, and 0 ≤ x < 0.5, 0 ≤ y < 0.5, 0 ≤ z < 0.5, and 0 < x + y + z ≤ 1), and combinations thereof.

[0060] In addition, the positive electrode active material layer may further include a conductive material to improve conductivity. Herein, the conductive material is not particularly limited as long as it is a conductive material that does not cause a chemical change in the lithium metal secondary battery. Generally, carbon black, graphite, carbon fiber, carbon nanotube, metal powder, conductive metal oxide, or organic conductive material can be used. Commercially available products of such conductive materials include acetylene black (available from Chevron Chemical Company or Gulf Oil Company), Ketjen black EC series (available from Armak Company), Vulcan XC-72 (available from Cabot Company), and Super P (available from MMM Company). For example, acetylene black, carbon black, or graphite can be used.

[0061] In addition, various types of binders can be used, which are used to hold the positive electrode active material on the positive electrode current collector and interconnect the active material particles. Specific examples of such binders include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like.

[0062] In addition, the separator may include a porous polymer substrate. The porous polymer substrate can be any porous polymer substrate conventionally used in lithium secondary batteries, and specific examples thereof include polyolefin-based porous membranes or non-woven fabrics, but are not limited thereto.

[0063] Herein, specific examples of the polyolefin-based porous membrane may include those formed from polymers including polyethylene (such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutene, and polyisopentene alone or in combination.

[0064] In addition to the polyolefin-based nonwoven fabric, specific examples of the nonwoven fabric may include those formed from polymers including polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, or the like, either alone or in combination. The nonwoven fabric structure may be a spunbond nonwoven fabric or a meltblown nonwoven fabric including long fibers.

[0065] The thickness of the porous polymer substrate is not particularly limited, but may be 1 to 100 μm, or 5 to 50 μm.

[0066] In addition, the size and porosity of the pores present in the porous polymer substrate are not particularly limited. However, the pore size and porosity may be 0.001-50 μm and 10-95%, respectively.

[0067] In addition, the electrolyte salt contained in the non-aqueous electrolyte that can be used in the present disclosure is a lithium salt. Any lithium salt conventionally used in electrolytes for lithium secondary batteries can be used without particular limitation. For example, the anion of the lithium salt can be selected from F - , Cl - Br - ,I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N -CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - 、(CF3CF2SO2)2N - Any one of the groups constituted, and their combinations.

[0068] Specific examples of the organic solvent that may be contained in the nonaqueous electrolyte may include those conventionally used for electrolytes of lithium secondary batteries without particular limitation. For example, ethers, esters, amides, linear carbonates, or cyclic carbonates may be used alone or in combination.

[0069] Typical examples of the organic solvent may include carbonate compounds such as cyclic carbonates, linear carbonates, or mixtures thereof.

[0070] Specific examples of cyclic carbonate compounds include any one selected from the group consisting of the following compounds: ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentenyl carbonate, 2,3-pentenyl carbonate, vinylene carbonate, halides thereof, and combinations thereof. Specific examples of such halides include fluoroethylene carbonate (FEC), but are not limited thereto.

[0071] In addition, specific examples of the linear carbonate compound may include any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, and combinations thereof, but are not limited thereto.

[0072] Specifically, ethylene carbonate and propylene carbonate as cyclic carbonates in carbonate organic solvents have high dielectric constants and dissociate lithium salts in electrolytes well. In addition, when such cyclic carbonates are used in combination with low-viscosity, low-dielectric-constant linear carbonates (such as dimethyl carbonate and diethyl carbonate) in an appropriate ratio, an electrolyte with high electrical conductivity can be prepared.

[0073] In addition, among the organic solvents, specific examples of the ether may include any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, and combinations thereof, but are not limited thereto.

[0074] Among the organic solvents, specific examples of the ester include any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, ε-caprolactone, and combinations thereof, but are not limited thereto.

[0075] According to the manufacturing process of the final product and the characteristics required by the final product, the injection of the non-aqueous electrolyte can be carried out in an appropriate step in the process for manufacturing the lithium secondary battery. In other words, the injection of the non-aqueous electrolyte can be carried out before the assembly of the lithium secondary battery or in the last step of the assembly of the lithium secondary battery.

[0076] According to the lithium secondary battery of the present disclosure, in addition to the conventional winding step, it can also undergo the lamination or stacking step of the separator and the electrode and the folding step. In addition, the battery housing may have a cylindrical, prismatic, pouch or coin shape. According to an embodiment of the present disclosure, the lithium metal secondary battery can be a cylindrical lithium metal secondary battery, a prismatic lithium metal secondary battery, a pouch lithium metal secondary battery, or a coin lithium metal secondary battery, specifically a pouch lithium metal secondary battery.

[0077] Furthermore, a battery module according to the present disclosure includes a plurality of unit cells and a module case accommodating the unit cells, wherein the unit cells are the above-mentioned lithium metal secondary batteries.

[0078] According to an embodiment of the present disclosure, the battery module may include two or more pouch type unit batteries and a module case accommodating the pouch type unit batteries, wherein the module case may include a rubber material.

[0079] The pouch-type lithium metal secondary battery according to the present disclosure experiences a volume increase during charging and a volume reduction during discharging. According to the present disclosure, pressurization during charging can prevent a significant increase in volume. However, this volume increase cannot be completely prevented. In addition, the volume reduction during discharge is inevitable.

[0080] According to the present disclosure, the module case may include a rubber material in order to mitigate increase / reduction in volume during charge / discharge of the unit batteries.

[0081] Here, the module case may include the rubber material as a whole, or only a portion contacting a large surface area of ​​the unit battery may include the rubber material.

[0082] Furthermore, in another aspect, a battery pack including the battery module, and a device including the battery pack as a power source are provided.

[0083] Here, specific examples of the device may include, but are not limited to: a power tool driven by an electric motor; an electric vehicle, including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or the like; an electric cart, including an electric bicycle (E-bike) and an electric scooter (E-scooter); an electric golf cart; a power storage system; or the like.

[0084] Each embodiment will be described more fully hereinafter so that the present disclosure can be easily understood. However, the following embodiments can be implemented in many different forms and should not be construed as being limited to the exemplary embodiments set forth therein. On the contrary, these exemplary embodiments are provided to make the present disclosure thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0085] 1. Manufacturing of lithium metal secondary batteries

[0086] (1) Manufacturing of positive electrode

[0087] First, 95 parts by weight of LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, 2.5 parts by weight of Super P as a conductive material, and 2.5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a positive electrode active material slurry. Next, the positive electrode active material slurry was coated on one surface of an aluminum current collector to a thickness of 65 μm, followed by drying, pressing and punching into a predetermined size to provide a positive electrode.

[0088] (2) Manufacturing of negative electrode

[0089] A lithium metal foil having a thickness of 20 μm was attached to one surface of a copper current collector having a thickness of 10 μm to obtain a negative electrode.

[0090] (3) Manufacturing of lithium metal secondary batteries

[0091] A separator (a polyethylene-based porous polymer substrate having a porous coating containing a mixture of aluminum oxide and a PVDF binder on both surfaces) is inserted between the positive electrode and the negative electrode to form an electrode assembly. Next, the electrode assembly is inserted into a pouch-type battery case. Then, an electrolyte including 1% by weight of vinylene carbonate (VC) as an additive and 1M LiPF6 dissolved in a solvent containing fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) mixed in a volume ratio of 30:70 is injected into the battery case. Afterwards, the battery case is completely sealed to obtain a lithium metal secondary battery.

[0092] 2. Determination of capacity retention rate of lithium metal secondary batteries

[0093] While changing the pressure conditions during charge / discharge as shown in Table 1 below, the lithium metal secondary battery was charged to 4.25 V under constant current / constant voltage (CC / CV) conditions and discharged at 2 mA / cm 2 The battery was discharged at a current density of 1.50 V (discharge temperature 25° C.) to 3 V. The above charge / discharge cycle was repeated 50 times. Table 1 shows the capacity retention after 50 cycles.

[0094] Here, the lithium metal secondary battery is charged / discharged by using a charger connected to a constant voltage pressurizing device.

[0095] Specifically, a pressurizing device is prepared. The pressurizing device includes: a storage member for accommodating a secondary battery cell; a pressurizing member, the pressurizing member facing the storage member, a secondary battery cell inserted therebetween, and the pressurizing member and the storage member are spaced apart at a variable distance; a pressurizing tool, the pressurizing tool being configured to pressurize the secondary battery cell accommodated in the storage member along the thickness direction of the secondary battery cell by pushing the pressurizing member toward the storage member or pulling the pressurizing member from the storage member; a measuring device, the measuring device being configured to measure the pressure applied to the pressurizing member and / or the distance between the storage member and the pressurizing member at predetermined time intervals; and a control unit, the control unit being configured to receive the pressure value applied to the pressurizing member and the distance between the storage member and the pressurizing member at predetermined time intervals, and to keep the distance constant or to change the distance to keep the pressure constant. A lithium metal secondary battery is mounted on the pressurizing device. Then, the lithium metal secondary battery is connected to a charger (PNE Solution Co., PESC05) and charged / discharged under the conditions shown in Table 1. Here, the pressure applied to the lithium metal secondary battery during charge / discharge is controlled by a pressurizing device. In Table 1, the pressurizing condition during charging refers to the pressure (pressurizing force) applied to the lithium metal secondary battery during charging, and the pressurizing condition during discharging refers to the pressure (pressurizing force) applied to the lithium metal secondary battery during discharging.

[0096] [Table 1]

[0097]

[0098] As shown in Table 1, Comparative Examples 1-4 in which no pressure was applied during charging of the lithium metal secondary battery provided a capacity retention rate of 0 after 50 cycles, while Examples in which pressure was applied during charging showed a capacity retention rate of at least 15%.

[0099] Specifically, Examples 9-23 in which pressurization was applied at pressures of 40 psi and 100 psi during charging showed a high capacity retention rate of at least 64%.

[0100] In addition, by comparing Example 10, Example 18, and Example 19, in which charging / discharging was performed at the same pressurization conditions, temperature, and current density during charging and at different pressurization conditions of 0psi, 5psi, and 10psi during discharging, it can be seen that they show an equivalent capacity retention rate of 75-79%. This indicates that when the lithium metal secondary battery according to the present disclosure satisfies the pressurization conditions during charging, even when discharging is performed without pressurization (in a non-pressurized state) or at a pressure lower than that applied during charging, improved cycle characteristics can be provided by physically preventing dendritic plating of lithium.

[0101] In addition, under the same pressurization conditions during charging, a higher capacity retention rate is provided at a higher temperature. Referring to Examples 13-17, it can be seen that under the same pressurization conditions and temperature during charging, when the current density during charging is 0.05-2 mA / cm 2 In other words, when the temperature is high, the activation energy for the chemical reaction is satisfied and the chemical reaction is accelerated to reduce resistance and provide a slightly increased capacity retention rate. As the current density decreases, the delocalization of the current decreases, and a lithium plating layer is uniformly formed to prevent battery degradation.

[0102] Figure 1 is a scanning electron microscope (SEM) image showing a cross-sectional view of the negative electrode after 10 cycles were repeated in the lithium metal secondary battery according to Comparative Example 1. Figure 2 is a SEM image showing a cross-sectional view of the negative electrode after 10 cycles were repeated in the lithium metal secondary battery according to Example 5.

[0103] Reference Figure 1 In the case of Comparative Example 1, the lithium metal grew in a dendrite-like manner to form pores. In addition, when observed from the bottom, a copper foil with a thickness of 13 μm, a lithium metal with a thickness of 13.8 μm, and a dendrite with a thickness of 236 μm were observed. In contrast, in the case of the negative electrode according to Example 5, Figure 2 In the figure, it can be seen that the dendrite layer has a smaller thickness, no pores are formed, and the lithium metal layer has a larger thickness. Therefore, it can be seen that the lithium metal secondary battery according to the present disclosure can physically prevent dendritic plating of lithium by charging under pressurized conditions and discharging under non-pressurized or pressurized conditions.

[0104] It should be understood that the detailed description is given by way of illustration only, and that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims. Therefore, the embodiments disclosed herein are not intended to limit the scope of the present disclosure, but are for illustrative purposes only, and the scope of the present disclosure is not limited thereto. It should also be understood that the scope of the present disclosure is defined by the following claims, and all equivalent forms thereof are also included within the scope of the present disclosure.

Claims

1. A method for placing a lithium metal secondary battery under controlled charge / discharge conditions, The lithium metal secondary battery is a pouch-type lithium metal secondary battery including lithium metal or lithium alloy as a negative electrode, comprising: An electrode assembly, the electrode assembly comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode; A nonaqueous electrolyte in which the electrode assembly is impregnated; and a battery case containing the electrode assembly and the nonaqueous electrolyte, wherein the negative electrode comprises a negative electrode current collector and a lithium metal layer formed on at least one surface of the negative electrode current collector, The method comprises: The lithium metal secondary battery is charged / discharged after the battery housing is completely sealed, wherein the charging / discharging conditions include: charging the lithium metal secondary battery under pressure, wherein the pressure applied during charging is 40-100 psi, and the current density during charging is 0.01-4 mA / cm 2 ; and discharging the lithium metal secondary battery in a non-pressurized state, and wherein the pressurized state is performed by a pressurizing device in a constant pressure pressurizing mode, or a combined constant pressure / constant distance pressurizing mode; The pressurizing device includes: a storage member that accommodates the lithium metal secondary battery; a pressurizing member, which faces the storage member, the lithium metal secondary battery is inserted between the pressurizing member, and the pressurizing member is spaced apart from the storage member by a variable distance; a pressurizing tool, which is configured to pressurize the lithium metal secondary battery accommodated in the storage member along the thickness direction of the lithium metal secondary battery by pushing the pressurizing member toward the storage member or pulling the pressurizing member from the storage member; a measuring device, which is configured to measure the pressure applied to the pressurizing member and / or the distance between the storage member and the pressurizing member at predetermined time intervals; and a control unit, which is configured to receive the pressure value applied to the pressurizing member and the distance between the storage member and the pressurizing member at predetermined time intervals, and keep the distance constant or change the distance to keep the pressure constant; and a charging unit, which is configured to charge the lithium metal secondary battery accommodated in the storage member.

2. The method according to claim 1, wherein the current density during charging is 0.05-3.5 mA / cm 2 .

3. The method of claim 1, wherein the temperature during charging is 25-45°C.

4. The method according to claim 1, wherein the temperature is 25-45°C, the pressure is 40-100 psi, and the flow rate is 0.05-3.5 mA / cm 2 The battery is charged at a charging current density of 1000 Ω and discharged in a non-pressurized state.

5. A battery module, comprising: A plurality of unit cells and a module case for accommodating the unit cells, wherein the unit cells are lithium metal secondary batteries having improved cycle characteristics obtained by the method as defined in any one of claims 1 to 4. The battery module according to claim 5 , wherein the module case comprises a rubber material. 7 . The battery module according to claim 5 , wherein a portion of the module case in contact with a large-area surface of the unit battery comprises a rubber material.

8. A battery pack comprising the battery module as defined in claim 5.

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