Lithium-ion rechargeable battery

Abstract

To provide a lithium secondary battery with excellent durability.SOLUTION: The lithium secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode has an intermediate layer and a positive electrode mixture layer laminated in sequence on a positive electrode current collector. The intermediate layer includes a pre-doping agent, a conductivity aid, and a binder.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a lithium secondary battery. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to energy efficiency in order to ensure that many people have access to affordable, reliable, sustainable, and advanced energy. For example, lithium metal anodes are attracting attention because, compared to graphite anodes in lithium-ion secondary batteries, they have a higher capacity density, and therefore, when used as the anode in lithium secondary batteries, they result in a higher voltage.

[0003] However, during the initial charge and discharge of a lithium secondary battery, the electrolyte decomposes at the interface between the lithium metal negative electrode and the electrolyte, forming an SEI film. At this time, lithium ions are consumed, irreversible capacity is generated, and the capacity of the lithium secondary battery decreases.

[0004] Therefore, it is known that a pre-doping agent is added to the positive electrode composite layer to compensate for the irreversible capacity (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-147863 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, since the pre-doping agent does not contribute to the capacity during the charge-discharge cycle of the lithium secondary battery, the intercalation and release of lithium ions in the positive electrode composite layer becomes non-uniform. As a result, the growth of lithium metal in the negative electrode becomes non-uniform, which reduces the durability of the lithium secondary battery.

[0007] An object of the present invention is to provide a lithium secondary battery having excellent durability.

Means for Solving the Problems

[0008] One aspect of the present invention relates to a lithium secondary battery including a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode has an intermediate layer and a positive electrode composite layer sequentially laminated on a positive electrode current collector. The intermediate layer contains a pre-doping agent, a conductive auxiliary agent, and a binder.

[0009] On the negative electrode current collector, a lithium metal layer, a lithium alloy layer, or a negative electrode composite layer may be formed.

[0010] The intermediate layer may have a thickness of 1 μm or more and 20 μm or less.

[0011] The intermediate layer may have a ratio of the mass of the conductive auxiliary agent to the total mass of the pre-doping agent and the conductive auxiliary agent of 0.01 or more and 0.10 or less.

[0012] The pre-doping agent may have a median diameter (D ) of 10 μm or less.

[0013] The pre-doping agent may have an initial irreversible capacity of 50% or more and an initial charging capacity of 300 mAh / g or more.

[0014] The pre-doping agent has the formula Li a Me b X (1-b) O4 (where a is 5 or more and 6 or less, b is 0.8 or more and 1 or less, Me is one or more elements selected from the group consisting of Co, Mn, and Fe, and X is a metal element other than Co, Mn, Fe, and Ni). It is a metal oxide represented by and having an inverse fluorite structure, and the surface may be coated with the conductive auxiliary agent.

Advantages of the Invention

[0015] According to the present invention, a lithium secondary battery with excellent durability can be provided. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described below.

[0017] The lithium secondary battery of this embodiment comprises a positive electrode, a negative electrode, and an electrolyte. Here, the positive electrode has an intermediate layer and a positive electrode composite layer sequentially laminated on a positive electrode current collector. The intermediate layer also contains a pre-doping agent, a conductive additive, and a binder.

[0018] In this embodiment, the lithium secondary battery contains a pre-doping agent in the intermediate layer, which uniformly absorbs and releases lithium ions in the positive electrode composite layer during charge-discharge cycles. As a result, the growth of lithium metal in the negative electrode becomes uniform, and therefore, the lithium secondary battery of this embodiment has excellent durability.

[0019] The lithium secondary battery in this embodiment is not particularly limited, but examples include lithium-ion secondary batteries and lithium metal secondary batteries.

[0020] The lithium secondary battery of this embodiment can be manufactured, for example, by sandwiching a separator between the positive electrode and the negative electrode, and then pouring an electrolyte solution into the separator.

[0021] [Positive electrode] As mentioned above, the positive electrode consists of an intermediate layer and a positive electrode composite layer sequentially laminated on the positive electrode current collector.

[0022] (Middle class) The pre-doping agent is not particularly limited as long as it can release lithium ions during the initial charge and discharge of the lithium secondary battery, but for example, formula Li a Me b X (1-b) O4 (where a is 5 or more and 6 or less, b is 0.8 or more and 1 or less, Me is one or more elements selected from the group consisting of Co, Mn, and Fe, and X is a metal element other than Co, Mn, Fe, and Ni.) It is a metal oxide represented by and having an inverse fluorite structure.

[0023] Specific examples of the pre-doping agent include, for example, Li6MnO4, Li5FeO4, Li6CoO4, etc.

[0024] The median diameter (D 50 ) of the pre-doping agent is preferably 10 μm or less, and more preferably 4 μm or less. When the median diameter (D 50 ) of the pre-doping agent is 10 μm or less, the charge capacity density of the lithium secondary battery of the present embodiment is improved. Note that the median diameter (D 50 ) of the pre-doping agent is not particularly limited, but is preferably, for example, 0.5 μm or more.

[0025] The initial irreversible capacity of the pre-doping agent is preferably 50% or more, and more preferably 90% or more. When the initial irreversible capacity of the pre-doping agent is 50% or more, an excessive increase in the film thickness of the positive electrode is suppressed.

[0026] The initial charge capacity of the pre-doping agent is preferably 300 mAh / g or more, and more preferably 500 mAh / g or more. When the initial charge capacity of the pre-doping agent is 300 mAh / g or more, an excessive increase in the film thickness of the positive electrode is suppressed.

[0027] The conductive assistant is not particularly limited, and examples thereof include acetylene black, furnace black, etc.

[0028] The ratio of the mass of the conductive additive to the total mass of the pre-doping agent and conductive additive in the intermediate layer is preferably 0.01 or more and 0.10 or less, and more preferably 0.025 or more and 0.05 or less. When the ratio of the mass of the conductive additive to the total mass of the pre-doping agent and conductive additive in the intermediate layer is 0.01 or more, the initial resistance of the lithium secondary battery of this embodiment is reduced, and when it is 0.10 or less, the durability of the lithium secondary battery of this embodiment is improved.

[0029] The pre-doping agent is preferably coated on the surface with a conductive additive. This reduces the initial resistance of the lithium secondary battery in this embodiment.

[0030] The binder is not particularly limited, but examples include polyvinylidene fluoride and polytetrafluoroethylene.

[0031] The thickness of the intermediate layer is preferably 1 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. If the thickness of the intermediate layer is 1 μm or more, the durability of the lithium secondary battery of this embodiment is improved, and if it is 20 μm or less, the increase in resistance of the lithium secondary battery of this embodiment is suppressed.

[0032] (Positive electrode composite layer) The positive electrode composite layer contains a positive electrode active material and may further contain conductive additives, binders, etc., as needed.

[0033] The positive electrode active material is not particularly limited as long as it is capable of intercalating and releasing lithium ions, but examples include LiCoO2, Li(Ni 5 / 10 Co 2 / 10 Mn 3 / 10 )O 2、 Li(Ni) 6 / 10 Co 2 / 10 Mn 2 / 10 )O 2、 Li(Ni) 8 / 10 Co 1 / 10 Mn 1 / 10 )O 2、 Li(Ni) 0.8 Co 0.15 Al 0.05)O 2、 Li(Ni) 1 / 6 Co 4 / 6 Mn 1 / 6 )O 2、 Li(Ni) 1 / 3 Co 1 / 3 Mn 1 / 3 )O 2、 Examples of lithium composite oxides include LiCoO4, LiMn2O4, LiNiO2, and LiFePO4.

[0034] The conductive additive is not particularly limited, but examples include acetylene black and furnace black.

[0035] The binder is not particularly limited, but examples include polyvinylidene fluoride and polytetrafluoroethylene.

[0036] (Positive electrode current collector) The positive electrode current collector is not particularly limited, but examples include metal foil. Examples of metals that make up the metal foil include aluminum.

[0037] The positive electrode current collector may have through holes. In this case, by press molding, a portion of the intermediate layer fills the through holes of the positive electrode current collector, thus enabling the positive electrode to be made thinner. Examples of positive electrode current collectors having through holes include foamed metal, metal mesh, expanded metal, perforated metal, and metal nonwoven fabric.

[0038] [Negative electrode] The negative electrode may have a lithium metal layer, a lithium alloy layer, or a negative electrode composite layer formed on the negative electrode current collector.

[0039] (Lithium metal layer or lithium alloy layer) The materials that make up the lithium alloy are not particularly limited, but examples include tin, bismuth, antimony, zinc, copper, etc., and two or more of these materials may be used in combination.

[0040] In this embodiment, if the lithium secondary battery is an anode-free lithium-ion secondary battery, the initial negative electrode consists only of a negative electrode current collector.

[0041] (Negative electrode composite layer) The negative electrode composite layer contains a negative electrode active material and may further contain conductive additives, binders, etc., as needed.

[0042] The negative electrode active material is not particularly limited as long as it is capable of intercalating and releasing lithium ions, but examples include metallic lithium, lithium alloys, metal oxides, metal sulfides, metal nitrides, Si, SiO, and carbon materials. Examples of carbon materials include artificial graphite, natural graphite, hard carbon, and soft carbon.

[0043] The conductive additive is not particularly limited, but examples include acetylene black and furnace black.

[0044] The binder is not particularly limited, but examples include sodium carboxymethylcellulose, styrene-butadiene rubber, and sodium polyacrylate.

[0045] (Negative electrode current collector) The negative electrode current collector is not particularly limited, but examples include copper foil.

[0046] [Electrolyte] An electrolyte solution is a solution in which an electrolyte is dissolved in a solvent.

[0047] The electrolyte is not particularly limited, but examples include lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate, lithium hexafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, and two or more may be used in combination.

[0048] The solvent is not particularly limited, but examples include ethylene carbonate, propylene carbonate, dimethyl ether (DME), fluoroethylene carbonate, dimethyl carbonate, hydrofluoroether, ethyl methyl carbonate, diethyl carbonate, etc., and two or more may be used in combination.

[0049] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Examples]

[0050] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0051] [Synthesis of pre-doping agents 1] Using an agate mortar, 5.583 g of Li2O and 4.418 g of MnO were mixed, and then fired in an Ar atmosphere at 900°C for 12 hours to obtain Li6MnO4.

[0052] [Synthesis of Pre-doping Agents 2] Using an agate mortar, 65 g of Li2O4 and 4.97 g of Fe2O3 were mixed, and then fired in an Ar atmosphere at 800°C for 12 hours to obtain Li5FeO4.

[0053] [Post-treatment of pre-doping agents] The pre-doped material was ground at 800 rpm for 1 hour using a 1 mm diameter ball and a ball mill. Next, a predetermined amount of acetylene black (AB) as a conductive additive (see Table 1) was added, and the mixture was dispersed at 400 rpm for 30 minutes to obtain a pre-doped material whose surface was coated with AB.

[0054] After dissolving polystyrene in butyl acetate, a pre-doped material coated with AB was dispersed in the solution. Next, the solvent was removed using a rotary vaporizer to obtain the pre-doped material coated with polystyrene. Subsequently, it was calcined at 700°C under an Ar atmosphere to obtain the pre-doped material supported with AB.

[0055] [Median diameter of pre-doping agent (D 50 )] Using a particle size distribution analyzer (Beckman Coulter) that measures particle size distribution by laser diffraction and scattering, the pre-doping agent was dispersed in isopropyl alcohol, and then the median diameter (D) of the pre-doping agent was measured. 50 ) was measured.

[0056] [Initial irreversible volume and initial charge volume of pre-doped agent] When evaluating the initial performance of lithium metal secondary battery cells, as described later, positive electrodes were fabricated without adding pre-doping agents, and the discharge capacity and charge capacity per gram of positive electrode were measured. Subsequently, the initial irreversible capacity and initial charge capacity of the pre-doping agent were calculated from the increase in discharge capacity and charge capacity when the pre-doping agent was added.

[0057] [Examples 1-9] A pre-doping agent coated with AB was pre-mixed with polyvinylidene fluoride (PVDF) as a binder and N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The pre-mixed mixture was then wet-mixed using a rotary-orbit mixer to obtain an intermediate layer slurry (see Table 1). Next, the intermediate layer slurry was applied to a 15 μm thick aluminum foil and dried to form an intermediate layer. The thickness of the intermediate layer was then adjusted by roll pressing.

[0058] Acetylene black (AB) (3.0 parts by mass), polyvinylidene fluoride (PVDF) (1.5 parts by mass), and polyvinylpyrrolidone (PVP) (appropriate amount) and N-methyl-2-pyrrolidone (NMP) (appropriate amount) as dispersants were premixed, and then the premixed mixture was wet-mixed using a rotary-orbit mixer to obtain a slurry. Next, Li1Ni 0.8 Co 0.1 Mn 0.1O2(NCM811) (95.5 parts by mass) was mixed with a slurry, and the mixture was dispersed using a planetary mixer to obtain a paste for the positive electrode composite layer. Here, NCM811 has a median diameter of 12 μm. Next, the paste for the positive electrode composite layer was applied to the intermediate layer and dried to form the positive electrode composite layer. Next, after roll pressing, it was dried in a vacuum at 120°C to obtain a positive electrode plate. Next, the positive electrode plate was punched out to a size of 30 mm × 40 mm to form the positive electrode.

[0059] [Comparative Example 1] A positive electrode was obtained in the same manner as in Example 1, except that no intermediate layer was formed, and a pre-doping agent coated with AB was added so that the solid content in the positive electrode composite layer was 5% by mass.

[0060] [Comparative Example 2] The positive electrode was obtained in the same manner as in Example 1, except that a pre-doping agent not coated with AB (see Table 1) was used instead of a pre-doping agent whose surface was coated with AB.

[0061] [Fabrication of the negative electrode] A rolled copper foil with a thickness of 8 μm was punched out to a size of 34 mm x 44 mm and used as the negative electrode.

[0062] [Separator] A polyethylene microporous membrane coated with alumina was used as the separator.

[0063] [Electrolyte] After dissolving LiFSI in DME to a concentration of 4 mol / L, LiNO3 was dissolved to a concentration of 1% by mass to obtain the electrolyte.

[0064] [Cell creation] With the separator positioned so that the alumina-coated surface of the separator is in contact with the positive electrode, and the separator is sandwiched between the positive and negative electrodes, the electrolyte was poured in, and then the cell was vacuum-sealed using aluminum laminate to obtain an anode-free lithium metal secondary battery cell.

[0065] Next, we evaluated the initial performance of lithium metal secondary battery cells.

[0066] [Initial capacity] After leaving the lithium metal secondary battery cells at the measurement temperature (25°C) for 1 hour, constant current charging was performed at 12.4 mA to 4.3 V, followed by constant voltage charging at 4.3 V for 1 hour. Next, after leaving the lithium metal secondary battery cells for 30 minutes, constant current discharge was performed at 12.4 mA to 2.65 V. This procedure was repeated four times, and the discharge capacity during the fourth discharge was measured and defined as the initial capacity. The current value required to complete discharge in 1 hour was defined as 1C for the obtained discharge capacity.

[0067] [Initial resistance] After measuring the initial capacity of a lithium metal secondary battery cell, it was left at the measurement temperature (25°C) for 1 hour. Then, it was charged with a constant current of 0.2C, and the state of charge (SOC) was adjusted to 50% and left for 10 minutes. Next, the voltage was measured when pulse discharge was performed at 0.5C for 10 seconds. The current was plotted on the x-axis, and the voltage when pulse discharged at 0.5C for 10 seconds was plotted on the y-axis. Next, after leaving the lithium metal secondary battery cell for 10 minutes, supplemental charging was performed to restore the SOC to 50%, and then it was left for another 10 minutes. The above operations were performed for each C rate of 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C, and plotted in the same manner as above. The slope of the approximate straight line obtained from the plot using the least squares method was calculated and taken as the initial resistance of the lithium metal secondary battery cell.

[0068] Next, we evaluated the durability performance of lithium metal secondary battery cells.

[0069] [Capacity after durability] As a durability test for lithium metal secondary battery cells, 50 charge-discharge cycles were performed in a constant temperature bath at 45°C, where constant current charging was performed at 1C to 4.2V, followed by constant current discharging at 2C to 2.65V. Next, the lithium metal secondary battery cells were left in a constant temperature bath at 25°C for 24 hours, then constant current charging was performed at 0.33C to 4.3V, followed by constant voltage charging at 4.3V for 1 hour. After leaving the lithium metal secondary battery cells for 5 minutes, constant current discharging was performed at 0.33C to 2.5V, and the discharge capacity was measured and defined as the capacity after durability.

[0070] [Resistance after durability] Except for using lithium metal secondary battery cells whose capacity after durability was measured, the resistance after durability was determined in the same manner as the initial resistance.

[0071] [Capacity maintenance rate] The ratio of the capacity after durability to the initial capacity was calculated and defined as the capacity retention rate.

[0072] [Resistance increase rate after endurance] The ratio of the resistance after endurance to the initial resistance was calculated and defined as the resistance increase rate.

[0073] [shortcut] The above durability test was performed on 10 lithium metal secondary battery cells, and the number of cells that experienced a short circuit was determined.

[0074] Table 1 shows the evaluation results for lithium metal secondary battery cells.

[0075] [Table 1]

[0076] Table 1 shows that the lithium metal secondary battery cells of Examples 1 to 9 have high durability (capacity retention rate, resistance increase rate, and short circuit resistance). In contrast, the lithium metal secondary battery cell of Comparative Example 1 has low durability because it does not have an intermediate layer. Furthermore, the lithium metal secondary battery cell of Comparative Example 2 has low durability, low initial capacity, and high initial resistance because the intermediate layer does not contain a conductive additive.

Claims

1. Equipped with a positive electrode, a negative electrode, and an electrolyte, The positive electrode has an intermediate layer and a positive electrode composite layer sequentially laminated on a positive electrode current collector. The aforementioned intermediate layer consists only of a pre-doping agent, a conductive additive, and a binder. The pre-doping agent is Li ６ MnO ４ or Li ６ CoO ４ A lithium secondary battery, wherein the surface is coated with the aforementioned conductive additive.

2. The lithium secondary battery according to claim 1, wherein the negative electrode has a lithium metal layer, a lithium alloy layer, or a negative electrode composite layer formed on a negative electrode current collector.

3. The lithium secondary battery according to claim 1 or 2, wherein the intermediate layer has a thickness of 1 μm or more and 20 μm or less.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein the ratio of the mass of the conductive additive to the total mass of the pre-doping agent and the conductive additive is 0.01 or more and 0.10 or less.

5. The aforementioned pre-doping agent has a median diameter (D ５０ A lithium secondary battery according to any one of claims 1 to 4, wherein the diameter of the ) is 10 μm or less.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein the pre-doping agent has an initial irreversible capacity of 50% or more and an initial charge capacity of 300 mAh / g or more.

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