Lithium secondary battery

Abstract

A lithium secondary battery includes: a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The positive electrode includes a lithium transition metal complex oxide. The lithium transition metal complex oxide includes at least Ni, and the ratio of Ni among all metal elements other than Li is 90 mol% or more. In the negative electrode, lithium metal is precipitated at the time of charging, and lithium metal is dissolved at the time of discharging. The nonaqueous electrolyte includes: a nonaqueous solvent, lithium ions, a fluorine-containing oxalate complex anion, and a nitrate anion.

Description

Technical Field

[0001] This disclosure relates to lithium secondary batteries. Background Technology

[0002] Lithium-ion batteries are known as high-capacity rechargeable batteries. In lithium-ion batteries, carbon materials are used, for example, as the negative electrode active material. Carbon materials are charged and discharged by reversibly inserting and deintercalating lithium ions.

[0003] On the other hand, lithium-ion batteries (also known as lithium metal batteries) that use lithium metal as the negative electrode active material have an even higher theoretical capacity density. In lithium-ion batteries, lithium metal is deposited on the negative electrode current collector during charging, and the deposited lithium metal dissolves in the non-aqueous electrolyte during discharging.

[0004] However, in lithium-ion batteries, it is difficult to control the precipitation morphology of lithium metal. When lithium metal precipitates in a dendritic form, the specific surface area of ​​the negative electrode increases, leading to increased side reactions with the non-aqueous electrolyte. Furthermore, the formation of inactive lithium that is not conducive to charge and discharge results in a decrease in discharge capacity.

[0005] Patent Document 1 discloses a non-aqueous electrolyte secondary battery comprising: a positive electrode having a positive current collector and a positive electrode composite material layer formed on the current collector; a negative electrode having a negative current collector; and a non-aqueous electrolyte, wherein lithium metal is deposited on the aforementioned negative electrode current collector during charging and dissolves in the aforementioned non-aqueous electrolyte during discharging, wherein the aforementioned non-aqueous electrolyte contains a lithium salt with oxalate complex as an anion. By adding a lithium salt with oxalate complex as an anion to the non-aqueous electrolyte, lithium metal is uniformly deposited on the negative electrode, and the swelling of the negative electrode is specifically suppressed.

[0006] Patent Document 2 discloses an ion-electrochemical battery, which is a lithium-ion electrochemical battery, comprising: a metal oxide cathode; an anode that is lithium metal or a lithium metal alloy; a separator between the anode and the cathode; a non-aqueous electrolyte containing one or more non-aqueous solvents and one or more lithium salts; and a nitrogen-containing substance, which includes an inorganic nitrate and is soluble in the electrolyte. Patent Document 2 states: "During operation or cycling of the electrochemical battery, the nitrogen-containing compound can form a uniform ion-conducting surface layer on the lithium anode. The formation of the ion-conducting surface layer facilitates uniform lithium deposition on the anode during battery charging by suppressing dendrite formation and the growth of high surface area lithium on the anode."

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. 2018 / 179782

[0010] Patent Document 2: Japanese Patent Application Publication No. 2019-24009 Summary of the Invention

[0011] To achieve higher capacity lithium-ion secondary batteries, it is desirable to use lithium transition metal composite oxides containing a large amount of Ni as the positive electrode active material. Therefore, the use of lithium transition metal composite oxides containing at least Ni, with Ni accounting for more than 90 mol% of all metal elements except Li, has been investigated. However, in this case, the methods proposed in Patent Documents 1 and 2 are insufficient to adequately improve the reduction in discharge capacity during repeated charge-discharge cycles of lithium-ion secondary batteries.

[0012] One aspect of this disclosure relates to a lithium secondary battery comprising: a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises a lithium transition metal complex oxide, the lithium transition metal complex oxide comprising at least Ni, and Ni accounts for more than 90 mol% of all metal elements except Li, wherein lithium metal is deposited during charging and dissolved during discharging, and the non-aqueous electrolyte comprises: a non-aqueous solvent, lithium ions, fluorinated oxalate complex anions, and nitrate anions.

[0013] According to this disclosure, the decrease in discharge capacity during charge-discharge cycles of a reusable lithium secondary battery can be suppressed. Attached Figure Description

[0014] Figure 1 A partial cross-sectional view is shown for illustrative purposes as an example of a lithium secondary battery of this disclosure. Detailed Implementation

[0015] The lithium secondary battery disclosed herein includes: a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and a non-aqueous electrolyte. In the negative electrode, lithium metal is deposited during charging and dissolved during discharging. Specifically, the negative electrode has at least a negative electrode current collector, on which lithium metal is deposited. The lithium secondary battery of this disclosure is also referred to as a lithium metal secondary battery.

[0016] In lithium (metal) secondary batteries, for example, over 70% of the rated capacity is due to the deposition and dissolution of lithium metal. The movement of electrons in the negative electrode during charging and discharging is primarily based on the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70-100% (e.g., 80-100%, 90-100%) of the movement of electrons (or, from another perspective, current) in the negative electrode during charging and discharging is based on the deposition and dissolution of lithium metal. That is, the negative electrode of this embodiment differs from negative electrodes where the movement of electrons during charging and discharging is primarily based on the absorption and release of lithium ions generated by the negative electrode active material (graphite, etc.).

[0017] In this study, the positive electrode incorporates a lithium transition metal composite oxide as the active material. The lithium transition metal composite oxide contains at least Ni, and the proportion of Ni in all metal elements except Li is 90 mol% or more. Hereinafter, the lithium transition metal composite oxide containing at least Ni, and the proportion of Ni in all metal elements except Li being 90 mol% or more, will also be referred to as composite oxide A. Composite oxide A exhibits particularly high capacity among lithium transition metal composite oxides.

[0018] The composite oxide A preferably also contains Al. The crystal structure of lithium transition metal composite oxides with high Ni content tends to become unstable. In contrast, Al improves the stability of the crystal structure of composite oxide A, thus contributing to improved thermal stability and durability. In composite oxide A, the proportion of Al among all metal elements except Li should be 10 mol% or less. From the viewpoint of further increasing capacity, the proportion of Al can be 7 mol% or less, or even 5 mol% or less. On the other hand, from the viewpoint of the stability of the crystal structure and high durability of composite oxide A, the proportion of Al can be 0 mol% or more, preferably 1 mol% or more, and more preferably 3 mol% or more.

[0019] The lithium transition metal composite oxide also preferably contains Co. Co contributes to the stability of the crystal structure of the composite oxide A, thereby improving power characteristics, durability, and capacity. In the composite oxide A, the proportion of Co among all metal elements except Li should be 10 mol% or less. From the viewpoint of further increasing capacity, the proportion of Co can be 7 mol% or less, or 5 mol% or less. On the other hand, from the viewpoint of improving the stability of the crystal structure and power characteristics of the composite oxide A, the proportion of Co is preferably 0 mol% or more, more preferably 1 mol% or more, and more preferably 3 mol% or more.

[0020] Next, the non-aqueous electrolyte comprises: a non-aqueous solvent, lithium ions, oxalate complex anions with fluorine (or fluorine groups), and nitrate anions.

[0021] Fluorine-containing oxalate complex anions (hereinafter also referred to as F-containing oxalate complex anions) have the effect of suppressing the dendritic deposition of lithium metal. It should be noted that in this embodiment, even when using non-fluorine-containing oxalate complex anions instead of F-containing oxalate complex anions, it is difficult to suppress the dendritic deposition of lithium metal. Therefore, a sudden short circuit occurs midway through charge-discharge cycles, sometimes resulting in a significantly shorter cycle life than intended.

[0022] During charging, protruding precipitates may form at the negative electrode. These protruding precipitates can lead to the elongation of dendritic precipitates in the lithium metal. If protruding precipitates (hereinafter also referred to as dendrite precursors) are placed, it is difficult to suppress the formation of dendritic precipitates.

[0023] In contrast, the oxalate complex anions containing F decompose at high potentials compared to other components in non-aqueous electrolytes, forming a thin and uniform coating on the surface of lithium metal. It is believed that lithium metal is primarily deposited between the coating and the negative electrode current collector.

[0024] However, if the oxalate complex anion containing F is included in the non-aqueous electrolyte, the degradation of the composite oxide A contained in the positive electrode becomes easier to occur. In particular, the degradation caused by the oxalate complex anion containing F in composite oxide A with low Co content and high Ni content is significant. Furthermore, it is believed that the stability of the negative electrode improves, thus making the impact of positive electrode degradation on battery performance more readily apparent.

[0025] On the other hand, when nitrate anions are present in the non-aqueous electrolyte, the degradation of the Ni-rich composite oxide A is suppressed. Nitrate anions significantly inhibit the decrease in discharge capacity during repeated charge-discharge cycles. This is attributed to the adsorption of nitrate anions on composite oxide A, which suppresses the side reactions between oxalate complex anions and composite oxide A.

[0026] Furthermore, the uniformity of the coating derived from F-containing oxalate complex anions is particularly significantly improved when nitrate anions are present in a non-aqueous electrolyte. The highly uniform coating derived from nitrate anions and F-containing oxalate complex anions covers a larger area of ​​lithium metal and exhibits high flexibility. It is believed that by forming a flexible coating over a large portion of the lithium metal surface, the lithium metal is firmly pressed onto the coating. This pressing effect suppresses the elongation of dendritic precipitates. Additionally, the flexible coating can easily follow changes in the surface shape of the lithium metal during dissolution. That is, the coating is frequently in contact with the lithium metal, facilitating the pressing effect. As a result, the formation of dendrite precursors is significantly suppressed, and the reduction of dendrite precursors significantly improves the suppression of lithium metal precipitation in a dendritic form.

[0027] The oxalate complex anion containing F can be derived from any oxalate complex salt containing F. For example, lithium oxalate complex salts containing F can be used as oxalate complex salts, but are not limited thereto.

[0028] The oxalate complex anion containing fluorine (F) preferably contains boron (B) or phosphorus (P). Examples of boron-containing F-containing oxalate complex anions include difluorooxalate borate anion (BF2(C2O4)).- (Hereinafter also referred to as FOB anion). Examples of phosphorus-containing F-containing oxalate complex anions include LiPF4(C2O4) and LiPF2(C2O2)2. Among these, boron-containing F-containing oxalate complex anions are more suitable than phosphorus-containing F-containing oxalate complex anions in forming a stable coating on the surface of lithium metal at high temperatures, and FOB anions are the most preferred.

[0029] The concentration C1 of the F-containing oxalate complex anion in the non-aqueous electrolyte can be, for example, above 0.1 mmol / L and below 500 mmol / L, above 10 mmol / L and below 300 mmol / L, or above 80 mmol / L and below 150 mmol / L.

[0030] The concentration C2 of nitrate anion in non-aqueous electrolytes can be, for example, above 0.1 mmol / L and below 50 mmol / L, above 0.5 mmol / L and below 20 mmol / L, or above 1 mmol / L and below 10 mmol / L.

[0031] Even a small amount of nitrate anion can reduce the influence of F-containing oxalate complex anions on the positive electrode. Therefore, the ratio of concentration C2 to concentration C1, C2 / C1, can be, for example, 0.01 or more and less than 1, 0.01 or more and less than 0.6, or 0.01 or more and less than 0.5.

[0032] The non-aqueous solvent of the non-aqueous electrolyte is expected to include ether compounds. Ether compounds exhibit high resistance to reduction at the negative electrode and are less prone to side reactions with lithium metal. The ether compounds are expected to constitute, for example, more than 50% by volume of the total non-aqueous solvent, and can be more than 70% or more, or even more than 90% by volume; all non-aqueous solvents can be ether compounds. It should be noted that here, all substances in the non-aqueous electrolyte other than dissociated ionic substances are considered non-aqueous solvents when calculating the ether compound content.

[0033] The content of each component of non-aqueous electrolytes can be determined using methods such as high performance liquid chromatography, gas chromatography-mass spectrometry (GC-MS), NMR, inductively coupled plasma mass spectrometry (ICP-MS), and elemental analysis.

[0034] Examples of ether compounds include dialkyl ethers having 1 to 5 carbon atoms. Dialkyl ethers having 1 to 5 carbon atoms can be, for example, of the general formula:

[0035] R1-(OCH2CH2)n-OR2

[0036] The chain-like ether compound shown is also referred to as ether compound A. R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms, preferably an alkyl group having 1 to 2 carbon atoms. In addition, n is 1 to 4, preferably 1 to 2. By using ether compound A as the main component of the non-aqueous solvent, the solubility of lithium salt in the non-aqueous electrolyte is increased, ensuring high fluidity and high lithium-ion conductivity of the non-aqueous electrolyte.

[0037] Here, the main component of the non-aqueous solvent is, for example, a component that accounts for more than 20% by volume of the non-aqueous solvent. Ether compound A is, for example, expected to account for more than 20% by volume and less than 80% by volume of the non-aqueous solvent.

[0038] Examples of ether compounds include tetrahydrofuran, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. Ether compounds can be used alone or in combination of two or more.

[0039] Fluorinated ether compounds can be used as ether compounds. Examples of fluorinated ether compounds include difluoroalkyl ethers having 1 to 4 carbon atoms. Fluorinated ether compounds can, for example, account for 20% to 80% by volume of the non-aqueous solvent.

[0040] The fluorination rate of fluorinated ether compounds can be above 60% or even 100%. Here, the fluorination rate of fluorinated ether compounds refers to the ratio of fluorine atoms to hydrogen atoms in the total number of fluorine atoms contained in the fluorinated ether compound, expressed as a percentage (%).

[0041] By using fluorinated ether compounds, the interaction between oxygen and lithium ions in the ether skeleton can be reduced. The fluorine atoms in the fluorinated ether compounds, due to their strong electronegativity, attract electrons from the ether molecule towards the core. The introduction of fluorine into the ether compound lowers the orbital energy levels of the non-shared electron pairs of oxygen in the ether skeleton, which would normally interact with lithium ions. This reduces orbital overlap, thus weakening the interaction between lithium ions and the ether. Lithium ions are less easily captured by the fluorinated ether compound, and therefore are readily reduced to lithium metal at the negative electrode surface. This allows for the formation of a uniform SEI coating, suppressing the formation of dendritic lithium metal. Consequently, side reactions between lithium metal and the non-aqueous electrolyte are suppressed, and the charge-discharge reaction proceeds more uniformly.

[0042] Specific examples of fluorinated ether compounds include 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. Fluorinated ether compounds can be used alone or in combination of two or more.

[0043] When the non-aqueous solvent contains ether compounds, the charge-discharge reaction at the negative electrode becomes more uniform, but the effects of positive electrode degradation become more apparent. Therefore, the effect of suppressing the degradation of Ni-rich composite oxide A generated by nitrate anions is relatively greater. In other words, when the positive electrode contains composite oxide A, the non-aqueous electrolyte contains F-containing oxalate complex anions, and ether compounds, nitrate anions have a significant impact on improving the charge-discharge cycle characteristics of lithium-ion secondary batteries.

[0044] The lithium secondary battery of this disclosure will now be described in detail for each of its constituent elements.

[0045] (positive electrode)

[0046] The positive electrode contains a positive electrode active material. The positive electrode typically comprises: a positive electrode current collector and a positive electrode binder held by the positive electrode current collector. The positive electrode binder may contain the positive electrode active material as a necessary component, and may also contain binders, thickeners, conductive agents, etc., as optional components. The positive electrode typically comprises a layered positive electrode binder (hereinafter referred to as the positive electrode binder layer) held by the positive electrode current collector. The positive electrode binder layer can be formed by coating a positive electrode slurry containing the components of the positive electrode binder dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying it. The dried coating film can be calendered as needed.

[0047] As the positive electrode active material, at least the aforementioned composite oxide A is used. Composite oxide A is a lithium transition metal composite oxide with a layered rock salt-type structure. Composite oxide A only needs to be the main component of the positive electrode active material.

[0048] Here, the main component of the positive electrode active material refers to a component that accounts for more than 50% by mass of the positive electrode active material. The composite oxide A is expected to account for more than 50% by mass of the positive electrode active material, more preferably more than 70% by mass, and even more preferably more than 90% by mass.

[0049] The composition of composite oxide A can be, for example, made from Li. α Ni (1-x1-x2-x3-y) Co x1 Mn x2 Al x3 M y O 2+β(0.95 ≤ α ≤ 1.05, 0.9 ≤ 1 - x1 - x2 - x3 - y ≤ 0.99, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.1, 0 ≤ x3 ≤ 0.1, 0 ≤ y ≤ 0.1, -0.05 ≤ β ≤ 0.05), where M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y.

[0050] From the viewpoints of thermal stability and durability, the ratio (atomic ratio) of Al, x3, is preferably such that 0 < x3 ≤ 0.1, and more preferably 0.01 ≤ x3 ≤ 0.1.

[0051] From the viewpoints of power characteristics and durability, the ratio (atomic ratio) of Co, x1, is preferably such that 0 < x1 ≤ 0.1, and more preferably 0.01 ≤ x1 ≤ 0.1.

[0052] From the viewpoints of high capacity and stability, the ratio (atomic ratio) of Ni, (1 - x1 - x2 - x3 - y), is preferably such that 0.9 ≤ 1 - x1 - x2 - x3 - y ≤ 0.95.

[0053] As the binder, for example, fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aromatic polyamide resins), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic - methacrylic acid copolymer, ethylene - acrylic acid copolymer, or their salts), vinyl resins (e.g., polyvinyl acetate), and rubber - like materials (e.g., styrene - butadiene copolymer rubber (SBR)) can be mentioned.

[0054] As the thickener, for example, cellulose derivatives such as cellulose ethers can be mentioned. As cellulose derivatives, carboxymethyl cellulose (CMC) and its modified products, methyl cellulose, etc. can be mentioned. The modified products of CMC also include salts of CMC.

[0055] As the conductive agent, for example, conductive fibers and conductive particles can be mentioned. As conductive fibers, carbon fibers, carbon nanotubes, metal fibers, etc. can be mentioned. As conductive particles, conductive carbon (carbon black, graphite, etc.), metal powders, etc. can be mentioned.

[0056] The positive electrode current collector is selected according to the type of non - aqueous electrolyte secondary battery. As the positive electrode current collector, a metal foil can be used. As the material of the metal foil, for example, stainless steel, aluminum, aluminum alloy, titanium, etc. can be exemplified. The thickness of the positive electrode current collector is not particularly limited and can be, for example, 1 to 50 μm.

[0057] (Negative electrode)

[0058] The negative electrode has a negative current collector. During charging, lithium metal is deposited on the negative current collector, and during discharging, the lithium metal dissolves. The lithium ions that form the lithium metal are supplied by a non-aqueous electrolyte, and lithium ions are supplied to the non-aqueous electrolyte from the positive electrode. The negative electrode may contain a lithium-ion storage layer (a layer that embodies capacity through the storage and release of lithium ions generated by the negative electrode active material (graphite, etc.)) loaded on the negative current collector. In this case, the open-circuit potential of the negative electrode when fully charged relative to the lithium metal (the dissolution potential of lithium) can be below 70mV. When the open-circuit potential of the negative electrode when fully charged relative to the lithium metal is below 70mV, lithium metal exists on the surface of the lithium-ion storage layer when fully charged. That is, the negative electrode embodies the capacity generated by the deposition and dissolution of lithium metal.

[0059] Here, "fully charged" refers to the state of charge when the battery's rated capacity is set to C, for example, charging the battery until it reaches a state of charge of 0.98 × C or higher. The open-circuit potential of the negative electrode when fully charged can be determined as follows: A fully charged battery is decomposed under an argon atmosphere, the negative electrode is removed, and a battery cell is assembled using lithium metal as the counter electrode. The non-aqueous electrolyte of the battery cell can have the same composition as the non-aqueous electrolyte in the decomposed battery.

[0060] The lithium-ion absorption reservoir is a layered structure containing a negative electrode additive that includes the negative electrode active material. In addition to the negative electrode active material, the negative electrode additive may further include binders, thickeners, conductive agents, etc.

[0061] Examples of anode active materials include carbonaceous materials, Si-containing materials, and Sn-containing materials. An anode may contain one type of anode active material or a combination of two or more. Examples of carbonaceous materials include graphite, easily graphitized carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon).

[0062] As an adhesive, conductive agent, etc., it can be used, for example, as exemplified in the positive electrode. The shape and thickness of the negative electrode current collector can be selected separately from the shape and extent of the positive electrode current collector. Examples of materials for the negative electrode current collector (metal foil) include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0063] (Non-aqueous electrolyte)

[0064] Non-aqueous electrolytes encompass liquid non-aqueous electrolytes (i.e., non-aqueous electrolyte solutions), gel electrolytes, and solid electrolytes; they are electrolytes excluding aqueous solutions. Gel electrolytes and solid electrolytes can be non-flowing electrolytes formed by combining non-aqueous electrolytes with gelling agents or matrix materials.

[0065] Non-aqueous electrolytes contain: a non-aqueous solvent, lithium ions, fluorine-containing oxalate complex anions, and nitrate anions. For example, a non-aqueous electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The lithium salt may include an fluorine-containing oxalate complex lithium and lithium nitrate. The fluorine-containing oxalate complex anions and nitrate anions do not necessarily originate from the lithium salt; for example, the nitrate anion may also originate from nitrates other than lithium salts.

[0066] Lithium salts may also include, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB. 10 Cl 10 Lithium, including lower aliphatic carboxylic acids, lithium borate, and lithium imide, can be used alone or in combination of two or more.

[0067] It should be noted that, as lithium imide, examples include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2) (hereinafter also known as LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate fluorosulfonylimide (LiN(CF3SO2)(FSO2)), lithium trifluoromethanesulfonate nonafluorobutanesulfonylimide (LiN(CF3SO2)(C4F9SO2)), and lithium bis(pentafluoroethylsulfonyl)imino (LiN(C2F5SO2)2).

[0068] The total concentration of lithium salts in non-aqueous electrolytes can be, for example, above 0.5 mol / L and below 2 mol / L.

[0069] The ether compound described above can be used as a non-aqueous solvent, but other compounds may also be used. For example, cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, etc., can be used as at least a portion of the non-aqueous solvent. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0070] (Separator)

[0071] The desired arrangement is to sandwich the separator between the positive and negative electrodes. The separator should have high ion permeability, moderate mechanical strength, and good insulation. Suitable materials for the separator include, for example, microporous membranes, woven fabrics, and nonwoven fabrics. Polyolefins (e.g., polypropylene, polyethylene) are preferred as the material for the separator.

[0072] (other)

[0073] As an example of the structure of a lithium secondary battery, the following structures can be cited: an electrode assembly formed by winding the positive and negative electrodes together with a separator, and a structure in which a non-aqueous electrolyte is housed in a casing. Alternatively, a stacked electrode assembly formed by layering the positive and negative electrodes with a separator, or other forms of electrode assembly, can be used. Lithium secondary batteries can be of any shape, such as cylindrical, square, coin-shaped, button-shaped, or laminated.

[0074] Using partial sectional views Figure 1 The diagram schematically illustrates the configuration of an example of the lithium secondary battery of this disclosure. Figure 1 The lithium secondary battery 100 shown is a cylindrical secondary battery.

[0075] The lithium secondary battery 100 is a wound battery, comprising a wound electrode assembly 40 and a non-aqueous electrolyte. The electrode assembly 40 includes a strip-shaped positive electrode 10, a strip-shaped negative electrode 20, and a separator 30. The separator 30 is disposed between the positive electrode 10 and the negative electrode 20. A positive electrode lead 13 is connected to the positive electrode 10. A negative electrode lead 23 is connected to the negative electrode 20.

[0076] One end of the positive lead 13 is connected to the positive electrode 10, and the other end is connected to the sealing body 50. The sealing body 50 includes the positive terminal 50a. Typically, the sealing body 50 includes a mechanism that acts as a safety valve when the internal pressure of the battery rises.

[0077] One end of the negative lead 23 is connected to the negative electrode 20, and the other end is connected to the bottom of the housing (shell body) 60. The housing 60 functions as the negative terminal. The housing 60 is a bottomed cylindrical container. The housing 60 is made of metal, for example, iron. The inner surface of the iron housing 60 is usually nickel-plated. Resin upper insulating ring 81 and lower insulating ring 82 are respectively arranged at the upper and lower parts of the electrode assembly 40. The electrode assembly 40 and non-aqueous electrolyte are arranged inside the housing 60. The housing 60 is sealed by a sealing body 50 and a gasket 70.

[0078] [Example]

[0079] The present disclosure will now be described in detail based on embodiments and comparative examples, but the present invention is not limited to the following embodiments.

[0080] Example 1

[0081] (1) Production of the positive electrode

[0082] The positive electrode active material particles (composition: LiNi) 0.9 Co 0.05 Al 0.05 100 parts by weight of O2, 1 part by weight of acetylene black, 1 part by weight of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) are mixed to obtain a positive electrode slurry. Next, the positive electrode slurry is coated on both sides of an aluminum foil (positive electrode current collector). After the coating is dried, it is calendered to form a positive electrode additive layer (95 μm thick, 3.6 g / cm³) on both sides of the aluminum foil. 3 ), thus obtaining the positive electrode.

[0083] (2) Fabrication of the negative electrode

[0084] The negative electrode (negative current collector) is made by cutting electrolytic copper foil (10μm thick) into specified dimensions.

[0085] (3) Preparation of non-aqueous electrolyte

[0086] LiN(FSO2)2 (i.e., LiFSI) was dissolved at a concentration of 1 mol / L in a non-aqueous solvent mixture containing DME (1,2-dimethoxyethane) and PC (propylene carbonate) in a volume ratio of 2:1. Lithium difluorooxalate borate (LiFOB), which is an F-containing oxalate complex lithium salt, was further dissolved at a concentration of 100 mmol / L, and lithium nitrate was dissolved at a concentration of 5 mmol / L to prepare a non-aqueous electrolyte.

[0087] (4) Fabrication of lithium secondary batteries

[0088] An aluminum tab is attached to the positive electrode, and a nickel tab is attached to the negative electrode. Next, the positive and negative electrodes and the separator are arranged with a separator between them, and then wound together in a spiral shape. The wound electrode assembly is housed in a bag-shaped outer casing formed of a laminate containing an aluminum layer. After injecting a non-aqueous electrolyte into the casing, the casing is sealed to obtain a battery cell A1 for evaluating a lithium secondary battery.

[0089] Example 2

[0090] The concentration of lithium nitrate in the non-aqueous electrolyte was changed to 50 mmol / L, and battery cell A2 was prepared in the same manner as in Example 1.

[0091] Example 3

[0092] In a non-aqueous solvent, dimethyl carbonate (DMC) was used instead of PC to fabricate battery cell A3 in the same manner as in Example 1.

[0093] Example 4

[0094] The concentration of LiFOB in the non-aqueous electrolyte was changed to 500 mmol / L, and battery cell A4 was prepared in the same manner as in Example 1.

[0095] Example 5

[0096] In a non-aqueous solvent, DMC was used instead of DME, and battery cell A5 was fabricated in the same manner as in Example 1.

[0097] Comparative Example 1

[0098] The non-aqueous electrolytes do not contain LiFOB or lithium nitrate, and battery cells B1 are prepared in the same manner as in Example 1.

[0099] Comparative Example 2

[0100] The non-aqueous electrolyte does not contain lithium nitrate, and battery cell B2 is prepared in the same manner as in Example 1.

[0101] Comparative Example 3

[0102] The non-aqueous electrolyte does not contain LiFOB, and battery cell B3 is prepared in the same manner as in Example 1.

[0103] Comparative Example 4

[0104] The composition of the positive electrode active material was changed to LiNi 0.5 Co 0.2 Al 0.3 O2 was used to fabricate battery cell B4 in the same manner as in Example 1.

[0105] Comparative Example 5

[0106] The non-aqueous electrolyte does not contain lithium nitrate, and battery cell B5 is prepared in the same manner as in Example 5.

[0107] Comparative Example 6

[0108] The non-aqueous electrolyte does not contain LiFOB, and battery cell B6 is prepared in the same manner as in Example 5.

[0109] Comparative Example 7

[0110] The non-aqueous electrolyte does not contain lithium nitrate, and battery cell B7 was fabricated in the same manner as in Comparative Example 4.

[0111] Comparative Example 8

[0112] The battery cell B8 was fabricated in the same manner as in Example 1, using lithium dioxaborate (LiBOB) instead of LiFOB in a non-aqueous electrolyte.

[0113] Table 1 shows the composition of the non-aqueous electrolyte and the positive electrode active material.

[0114] [Table 1]

[0115]

[0116] <Battery Review>

[0117] (charge-discharge cycle characteristics)

[0118] For the battery cells used in the evaluation, at a temperature of 25°C, they were charged with a constant current of 0.3 It until the voltage reached 4.1V. Then, they were charged with a constant voltage of 4.1V until the current reached 0.05 It. Next, the battery cells were discharged with a constant current of 0.3 It until the voltage reached 2.5V. This charge-discharge cycle was repeated 100 times, and the ratio of the discharge capacity of the 100th cycle to the discharge capacity of the 1st cycle (C1) was calculated as the capacity retention rate (R). 100 The results are shown in Table 2. The discharge capacity (C1) is expressed as a relative value (index) with the value obtained in battery cell A1 of Example 1 as 100.

[0119] (Short-circuit lifetime (dendritic formation))

[0120] After 100 cycles and subsequent repetition of the above charge-discharge cycles, a short circuit caused by lithium dendrite formation was observed. The number of cycles (N) until the voltage suddenly dropped was determined. The results are shown in Table 2.

[0121] [Table 2]

[0122]

[0123] Comparing Example 1 and Comparative Example 2, it can be understood that the presence of fluorinated oxalate complex anions in the non-aqueous electrolyte significantly suppresses the formation of lithium dendrites. Furthermore, the charge-discharge cycle characteristics (R...) are improved. 100 The charge-discharge cycle characteristics (Ri) deteriorated. In contrast, in examples where the non-aqueous electrolyte contained oxalate complex anions and nitrate anions with fluoride, the charge-discharge cycle characteristics (Ri) were significantly improved. 100 Significant improvement.

[0124] Furthermore, comparing Comparative Examples 4 and 7, which both use lithium transition metal composite oxides with low Ni content, it can be understood that the presence or absence of nitrate anions has a negligible impact on charge-discharge cycle characteristics. Moreover, comparing the results of Comparative Examples 2 and 7, which do not use nitrate anions, with those of Comparative Example 7 (R), which uses lithium transition metal composite oxides with low Ni content... 100 =61.2) Compared to Comparative Example 2 (R), which uses a composite oxide A with a high Ni content, 100The charge-discharge cycle characteristics (=58.6) become quite low. This effect of reducing nitrate anions in improving charge-discharge cycle characteristics is particularly significant when using composite oxide A with high Ni content.

[0125] Furthermore, comparing the improvement effect of Example 5 (which contains no ether compounds in its non-aqueous electrolyte) to that of Example 2 (which contains ether compounds in its non-aqueous electrolyte), the improvement effect of Example 1 compared to Example 2 is significant. This result demonstrates that, in the case of a non-aqueous electrolyte containing ether compounds, the effect of nitrate anions is particularly evident.

[0126] It should be noted that the results of Comparative Example 8 show that even if the non-aqueous electrolyte contains LiBOB, it is impossible to expect an effect on suppressing dendrites or improving charge-discharge cycle characteristics.

[0127] Industrial availability

[0128] This disclosure can be used in lithium secondary batteries where lithium metal is deposited during charging and dissolved during discharging.

[0129] Explanation of reference numerals in the attached figures

[0130] 10 Positive electrode

[0131] 13 Positive lead

[0132] 20 Negative electrode

[0133] 23 Negative lead

[0134] 30 separators

[0135] 40 electrode groups

[0136] 50 sealing bodies

[0137] 50a Positive End Particle

[0138] 60 Shell (shell body)

[0139] 70 gasket

[0140] 81 Upper insulating ring

[0141] 82 Lower insulating ring

[0142] 100 Lithium Secondary Battery

Claims

1. A lithium secondary battery, comprising: a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode comprises a lithium transition metal composite oxide. The lithium transition metal composite oxide contains at least Ni, and the proportion of Ni in all metal elements except Li is more than 90 mol%. In the negative electrode, lithium metal is deposited during charging and dissolved during discharging. The non-aqueous electrolyte comprises: a non-aqueous solvent, lithium ions, fluorine-containing oxalate complex anions, and nitrate anions. The oxalate complex anion containing fluorine includes boron; The concentration C1 of the fluorinated oxalate complex anion in the non-aqueous electrolyte is greater than 0.1 mmol / L and less than 500 mmol / L. The concentration C2 of the nitrate anion in the non-aqueous electrolyte is greater than 0.1 mmol / L and less than 50 mmol / L. The ratio of concentration C2 to concentration C1 is: C2 / C1 is greater than 0.01 and less than 1. The non-aqueous solvent comprises more than 50% by volume of an ether compound.

2. The lithium secondary battery according to claim 1, wherein, The lithium transition metal composite oxide also contains Al.

3. The lithium secondary battery according to claim 1, wherein, The lithium transition metal composite oxide also contains Co.

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