Non-aqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery according to the present invention is provided with a positive electrode, a negative electrode and a lithium ion-conductive nonaqueous electrolyte; the negative electrode contains graphite; the open circuit potential of the negative electrode in a fully charged state is 70 mV or less with respect to lithium metal; the nonaqueous electrolyte contains a solvent, cations and anions; the solvent contains a fluorine-containing cyclic carbonic acid ester; the cations contain lithium ions; and anions contain oxalate complex anions.

Description

Technical Field

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

Background Art

[0002] Non-aqueous electrolyte secondary batteries are used, for example, in applications such as ICT including personal computers and smartphones, in-vehicle use, and power storage. In such applications, further higher capacity is required for non-aqueous electrolyte secondary batteries. As a high-capacity non-aqueous electrolyte secondary battery, a lithium-ion battery is known. However, the increase in the capacity of lithium-ion batteries is reaching its limit.

[0003] As a non-aqueous electrolyte secondary battery with a higher capacity than a lithium-ion battery, a lithium secondary battery is promising. In a lithium secondary battery, lithium metal is deposited on the negative electrode during charging and dissolved in the non-aqueous electrolyte during discharging. Note that a lithium secondary battery may be called a lithium metal secondary battery. However, in a lithium secondary battery, lithium metal is likely to be deposited in a dendrite shape on the negative electrode during charging, and the discharge capacity is likely to deteriorate with the charge-discharge cycle.

[0004] Therefore, Patent Document 1 proposes a secondary battery in which the capacity of the negative electrode includes a capacity component due to the insertion and extraction of lithium and a capacity component due to the deposition and dissolution of lithium metal, and is represented by the sum thereof. This uses a carbon material capable of inserting and releasing lithium ions into the negative electrode, and deposits lithium metal on the surface of the carbon material during charging.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] It is difficult to deposit lithium metal in a good state on the surface of a carbon material capable of occluding and desorbing lithium, and there are limitations in improving the cycle characteristics of secondary batteries.

[0007] One aspect of the present disclosure provides a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a lithium-ion conductive non-aqueous electrolyte. The negative electrode contains a carbon material that occludes and releases lithium ions. The open circuit potential of the negative electrode at full charge is 70 mV or less with respect to lithium metal. The non-aqueous electrolyte contains a solvent, a cation, and an anion. The solvent contains a fluorine-containing cyclic carbonate. The cation contains lithium ions. The anion contains an oxalate complex anion.

[0008] In the non-aqueous electrolyte secondary battery, the cycle characteristics are improved.

Brief Description of the Drawings

[0009]

Figure 1

Embodiments for Carrying Out the Invention

[0010] A lithium secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a lithium-ion conductive non-aqueous electrolyte. The negative electrode contains a carbon material that occludes and releases lithium ions. The negative electrode exhibits capacitance by occluding and releasing lithium ions with respect to the carbon material. Further, the open circuit potential (OCV) of the negative electrode at full charge is 70 mV or less with respect to lithium metal. Since lithium metal deposits on the negative electrode with an OCV of 70 mV or less with respect to lithium metal, lithium metal exists on the surface of the carbon material at full charge. That is, the negative electrode also exhibits capacitance by deposition and dissolution of lithium metal.

[0011] The fully charged state means that when the rated capacity of the battery is C, the battery is charged until, for example, the state of charge (SOC) reaches 0.98×C. The OCV of the negative electrode at full charge can be measured by disassembling the fully charged battery in an argon atmosphere, taking out the negative electrode, and assembling a cell with lithium metal as the counter electrode. The non-aqueous electrolyte of the cell may have the same composition as the non-aqueous electrolyte in the disassembled battery. For example, the non-aqueous electrolyte used in Example 1 described later may be used as a model non-aqueous electrolyte. Reference The non-aqueous electrolyte used in Example 1 described later may be used as a model non-aqueous electrolyte.

[0012] The non-aqueous electrolyte contains a solvent, a cation, and an anion. The solvent contains a fluorine-containing cyclic carbonate. The cation contains lithium ions. The anion contains at least an oxalate complex anion. Here, the fluorine-containing cyclic carbonate and the oxalate complex anion cooperate to improve the precipitation state of lithium metal on the carbon material and improve the cycle characteristics.

[0013] Specifically, when the fluorine-containing cyclic carbonate and the oxalate complex anion coexist, the fluorine-containing cyclic carbonate preferentially forms a high-quality film on the surface of the carbon material. Such a film is considered to be thin, flexible, and homogeneous. Once the film derived from the fluorine-containing cyclic carbonate is formed, the oxalate complex anion is appropriately decomposed on the surface of the carbon material, and the film components derived from the oxalate complex anion are appropriately deposited on the surface of the carbon material. The resulting hybrid film suppresses the precipitation of dendrites and leads to a more uniform precipitation state of lithium metal.

[0014] In addition, the oxalate complex anion has the effect of stabilizing lithium ions in the non-aqueous electrolyte and plays a role in inhibiting the local precipitation of lithium metal. As a result, the precipitation of dendrites is further suppressed. It is considered that the easy coordination of the oxalate complex anion to encapsulate lithium ions affects the stabilization of lithium ions.

[0015] When using a fluorine-containing cyclic carbonate alone, a sufficient effect of improving cycle characteristics cannot be obtained, and neither can a sufficient effect be obtained with an oxalate complex anion alone. In the case of using an oxalate complex anion alone, the oxalate complex anion reacts more on the surface of the carbon material and is likely to form a thick and non-uniform film. It is difficult to sufficiently suppress the precipitation of dendrites with such a film.

[0016] Examples of the fluorine-containing cyclic carbonate include fluoroethylene carbonate (FEC), 4,5-difluoroethylene carbonate (DFEC), trifluoropropylene carbonate, and the like. These may be used alone or in combination of two or more.

[0017] Examples of the oxalate complex anion include B(C2O4)2 - , BF2(C2O4) - , P(C2O4)3 - , PF2(C2O4)2 - and at least one selected from the group consisting of PF4(C2O4) - can be used. These are considered to have a great effect of stabilizing lithium ions and suppressing the local precipitation of lithium metal. Among them, an oxalate complex anion containing fluorine is preferable. The oxalate complex anion may be included in the non-aqueous electrolyte as a lithium oxalate salt (i.e., a lithium salt).

[0018] The OCV of the negative electrode at full charge may be less than 20 mV with respect to lithium metal. In this case, a non-aqueous electrolyte secondary battery with a higher capacity can be obtained. It can be said that a considerable amount of lithium metal is deposited on the negative electrode with an OCV of less than 20 mV with respect to lithium metal. For example, in the negative electrode, when the capacity C1 expressed by the insertion and extraction of lithium ions into the carbon material and the capacity C2 expressed by the deposition and dissolution of lithium metal satisfy C1:C2 = 100:10, the OCV of the negative electrode with respect to lithium metal is generally less than 20 mV. Also, when C1:C2 = 100:20 is satisfied, the OCV of the negative electrode with respect to lithium metal is generally less than 5 mV. When the negative electrode contains a carbon material, the OCV of the negative electrode at full charge exceeds 0 mV. The OCV of the negative electrode at full charge may be 5 mV or more and 20 mV or less with respect to lithium metal. Further, from the viewpoint of obtaining a non-aqueous electrolyte secondary battery with an even higher capacity, the OCV of the negative electrode at full charge may be 5 mV or less with respect to lithium metal, or may be approximately 0 mV.

[0019] The OCV of the negative electrode at full charge may be 20 mV or more and 70 mV or less, or 20 mV or more and 50 mV or less with respect to lithium metal. In this case, a non-aqueous electrolyte secondary battery with more excellent cycle characteristics can be obtained. Not much lithium metal is deposited on the negative electrode with an OCV of 20 mV or more with respect to lithium metal, and the carbon material bears most of the capacity. However, even a small amount of lithium metal has a large contribution to the capacity considering the capacity density of lithium metal. Also, from the viewpoint of achieving both cycle characteristics and high capacity, the OCV of the negative electrode at full charge may be more than 0 mV and 70 mV or less, or 5 mV or more and 70 mV or less with respect to lithium metal.

[0020] The configuration of the non-aqueous electrolyte secondary battery will be described more specifically below.

[0021] [Negative electrode] The negative electrode includes, for example, a negative electrode mixture layer containing a negative electrode active material and a negative electrode current collector that supports the negative electrode mixture layer. The negative electrode active material includes at least a carbon material that can occlude and release lithium ions. However, the design capacity Cn borne by the carbon material of the negative electrode with respect to the design capacity Cp of the positive electrode satisfies Cn / Cp < 1, and may satisfy Cn / Cp < 0.8.

[0022] For the negative electrode current collector, for example, a metal foil can be used. As the metal constituting the negative electrode current collector, a metal that does not react with lithium metal is preferable, and examples include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements.

[0023] The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry in which a negative electrode mixture is dispersed in a dispersion medium onto the surface of the negative electrode current collector and drying it. The dried coating film may be rolled if necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0024] The negative electrode mixture contains the negative electrode active material as an essential component and may contain, as optional components, a binder, a conductive agent, a thickening agent, and the like. As the binder, conductive agent, and thickening agent, for example, known materials can be used. Further, the negative electrode active material contains, as an essential component, a carbon material that can occlude and release lithium ions.

[0025] Examples of the carbon material that can occlude and release lithium ions include graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among them, graphite, which is excellent in charge-discharge stability and has a small irreversible capacity, is preferable. Further, the negative electrode active material may contain materials other than the carbon material, but it is preferable that 80 mass% or more (more preferably 90 mass% or more) of the negative electrode active material is graphite.

[0026] Graphite is a carbon material with a developed graphite-type crystal structure, and may be, for example, graphite with an interplanar spacing d002 of the (002) plane measured by X-ray diffraction method of 3.4 Å or less. The crystallite size of graphite may be 100 Å or more. The crystallite size is measured, for example, by the Scherrer method.

[0027] The average particle size of the carbon material that occludes and releases lithium ions is, for example, 10 to 30 μm, and may also be 15 to 25 μm. When using a carbon material within the above particle size range, it is usually difficult to deposit lithium metal in a good state on the surface of the negative electrode mixture layer. On the other hand, when a fluorine-containing cyclic carbonate and an oxalate complex anion coexist in a non-aqueous electrolyte, it is possible to improve the deposition state of lithium metal. The average particle size of the carbon material means the particle size (volume average particle size) at which the volume integration value becomes 50% in the particle size distribution measured by the laser diffraction scattering method.

[0028] Examples of materials other than the carbon material that can be used as the negative electrode active material include alloy-based materials. An alloy-based material is a material containing at least one kind of metal capable of forming an alloy with lithium, and examples thereof include silicon, tin, silicon alloys, tin alloys, and silicon compounds.

[0029] As the alloy-based material, a composite material having a lithium ion conductive phase and silicon particles dispersed in the phase may be used. As the lithium ion conductive phase, a silicate phase, a silicon oxide phase in which 95 mass% or more is silicon dioxide, a carbon phase, or the like may be used.

[0030] [Positive Electrode] The positive electrode includes a positive electrode mixture layer containing a positive electrode active material and a positive electrode current collector. The positive electrode mixture layer can be formed by dispersing a positive electrode mixture containing a positive electrode active material, a binder, a conductive agent, etc. in a dispersion medium, applying the positive electrode slurry to the surface of the positive electrode current collector, and drying it. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0031] For the positive electrode current collector, for example, a metal foil can be used. Examples of the metal constituting the positive electrode current collector include aluminum (Al), titanium (Ti), alloys containing these metal elements, and stainless steel.

[0032] The positive electrode active material contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, etc. as optional components. As the binder, conductive agent, etc., known materials can be used, for example.

[0033] Examples of the positive electrode active material include layered rock salt type composite oxides. For example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b M 1-b O c (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Ni, Cu, Zn, Al, Cr, Pb, Sb and B), LiMPO 4、 (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B), etc. Here, 0 < a ≤ 1.1, 0 ≤ b ≤ 0.9 and 2 ≤ c ≤ 2.3. The a value indicating the molar ratio of lithium increases or decreases by charge and discharge.

[0034] Among them, a layered rock salt type composite oxide containing nickel element is preferable. Nickel is advantageous for increasing the capacity and reducing the cost. Such a composite oxide has, for example, the general formula: Li a Ni x Co y M 1-x-y O2. Cobalt is advantageous for extending the battery life, etc.

[0035] The above general formula satisfies, for example, 0.97 < a ≤ 1.2, 0.5 ≤ x ≤ 1.0 and 0 ≤ y ≤ 0.1. In order to obtain a higher capacity, 0.8 ≤ x ≤ 1.0 may be satisfied. Here, M is preferably at least one selected from the group consisting of Mn, Al, W, Mg, Mo, Nb, Ti, Si and Zr. Among them, it is preferable that M contains at least Al. Aluminum is advantageous for improving the thermal stability, etc. The atomic ratio Co / Al may be set to 0 to 1.0. It is considered that manganese, tungsten, niobium, magnesium, zirconium, etc. contribute to the stabilization of the crystal structure.

[0036] Examples of binders to be included in the positive electrode active material and / or the negative electrode active material include, for example, fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, polyvinylpyrrolidone, polyethersulfone, rubber particles, and the like. These may be used alone or in combination of two or more.

[0037] Examples of conductive agents to be included in the positive electrode active material and / or the negative electrode active material include, for example, carbon black such as acetylene black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride, and the like. These may be used alone or in combination of two or more.

[0038] [Non-aqueous electrolyte] The cations and anions contained in the non-aqueous electrolyte are derived from, for example, oxalate complex salts (hereinafter also referred to as the first salts) and salts other than the first salts (hereinafter also referred to as the second salts). The second salt contains at least a lithium salt. That is, the non-aqueous electrolyte is obtained by dissolving an oxalate complex salt (the first salt) and a lithium salt (the second salt) in a solvent. The oxalate complex salt may be used alone or in combination of two or more. The lithium salt may also be used alone or in combination of two or more.

[0039] As the first salt (oxalate complex salt), lithium oxalate salt may be used. As the lithium oxalate salt, for example, at least one selected from the group consisting of LiB(C2O4)2, LiBF2(C2O4), LiP(C2O4)3, LiPF2(C2O4)2, and LiPF4(C2O4) can be used.

[0040] Examples of the second salt (lithium salt) include, for example, lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10etc.), lithium salts of fluorinated acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.), lithium salts of fluorinated acid imides, etc. can be used. As the fluorinated imide anion, bis(fluorosulfonyl)imide anion (N(SO2F)2 - )(FSI), bis(trifluoromethylsulfonyl)imide anion (N(SO2CF3)2 - )(TFSI), bis(perfluoroethylsulfonyl)imide anion (N(SO2C2F5)2 - ) etc. can be mentioned.

[0041] The solvent may contain a fluorinated cyclic carbonate as an essential component. The concentration of the fluorinated cyclic carbonate in the non-aqueous electrolyte may be, for example, 0.1% by mass or more and 20% by mass or less, or may be 5% by mass or more and 20% by mass or less.

[0042] As optional components other than the fluorinated cyclic carbonate, for example, cyclic carbonates not containing fluorine, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, etc. are used. Examples of the cyclic carbonate not containing fluorine include propylene carbonate (PC), ethylene carbonate (EC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc. Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), etc. The solvent may be used alone or in combination of two or more.

[0043] The concentration of lithium ions in the non-aqueous electrolyte may be, for example, 0.5 mol / liter or more and 3.5 mol / liter or less, 1 mol / liter or more and 2 mol / liter or less, or 1 mol / liter or more and 1.5 mol / liter or less. By setting the lithium ion concentration within the above range, a non-aqueous electrolyte excellent in ionic conductivity and having appropriate viscosity can be obtained.

[0044] The concentration of the oxalate complex anion in the non-aqueous electrolyte may be, for example, 0.8 mass% or more and 4.0 mass% or less, 1.0 mass% or more and 3.0 mass% or less, or 1.3 mass% or more and 2 mass% or less. In this case, in addition to improving the cycle characteristics, gas generation due to the decomposition of the oxalate complex anion is significantly suppressed, and it becomes easier to obtain a non-aqueous electrolyte excellent in ionic conductivity and having appropriate viscosity.

[0045] From the viewpoint of obtaining a non-aqueous electrolyte with even better lithium ion conductivity, for example, when the non-aqueous electrolyte contains LiPF6 as a lithium salt, PF6 - The ratio of the molar-based content of the oxalate complex anion to the molar-based content of the PF6 ion in the non-aqueous electrolyte may be 0.1 or more and 0.5 or less.

[0046] Also, from the viewpoint of making the deposition state of lithium metal via the carbon material better and significantly improving the cycle characteristics, the mass ratio of the fluorine-containing cyclic carbonate ester to the oxalate complex salt in the non-aqueous electrolyte may be 0.025 or more, 0.25 or more, or 1.25 or more.

[0047] The mass ratio of the fluorine-containing cyclic carbonate ester to the oxalate complex salt in the non-aqueous electrolyte may be 25 or less.

[0048] That is, the ratio of the molar-based content of the fluorine-containing cyclic carbonate ester to the molar-based content of the oxalate complex anion contained in the non-aqueous electrolyte may be 0.02 or more (more preferably 0.2 or more), and from another viewpoint, the ratio may be 13 or less.

[0049] The contents of cations, anions, and various solvents in the non-aqueous electrolyte can be measured, for example, by using NMR, ion chromatography, or the like.

[0050] The non-aqueous electrolyte may be a liquid electrolyte or a gel. The gel-like non-aqueous electrolyte contains a liquid electrolyte and a matrix polymer. Examples of the matrix polymer include fluororesin, acrylic resin, polyether resin, and the like.

[0051] [Separator] A separator may be interposed between the positive electrode and the negative electrode. The separator has high ion permeability and appropriate mechanical strength and insulation. As the separator, a microporous thin film, a woven fabric, a non-woven fabric, or the like can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0052] [Non-aqueous electrolyte secondary battery] The type and shape of the non-aqueous electrolyte secondary battery are not particularly limited. For example, it can be appropriately selected from various shapes such as cylindrical, coin-type, rectangular, sheet-type, and flat-type. The form of the electrode group is also not particularly limited and may be, for example, a wound type or a laminated type.

[0053] As an example, FIG. 1 shows a longitudinal sectional view of a cylindrical non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery 100 includes a wound electrode group 40 and a non-aqueous electrolyte (not shown). The electrode group 40 includes a strip-shaped positive electrode 10, a negative electrode 20, and a separator 30, respectively. A positive electrode lead 13 is connected to the positive electrode 10, and a negative electrode lead 23 is connected to the negative electrode 20. One end of the positive electrode lead 13 in the length direction is connected to the positive electrode 10, and the other end is connected to the sealing plate 90. The sealing plate 90 is provided with a positive electrode terminal 15. One end of the negative electrode lead 23 is connected to the negative electrode 20, and the other end is connected to the bottom of the battery case 70 that serves as a negative electrode terminal. The battery case (battery can) 70 is made of metal and is formed of, for example, iron. An upper insulating ring 80 and a lower insulating ring 60 made of resin are disposed above and below the electrode group 40, respectively. In the illustrated example, a cylindrical battery including a wound electrode group has been described, but the present embodiment is not limited to this case and can be applied.

[0054] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0055] [Comparative Example 1, Reference Example 1, Example 2] (1) Fabrication of negative electrode Artificial graphite (average particle diameter: 25 μm), acetylene black, and polyvinylidene fluoride (PVdF) were mixed at a predetermined mass ratio, and N-methyl-2-pyrrolidone (NMP) was added to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of an electrolytic copper foil serving as a negative electrode current collector, and after drying the coating film, it was rolled to form a negative electrode mixture layer on both sides of the copper foil.

[0056] (2) Preparation of non-aqueous electrolyte Lithium difluorooxalate borate (LiBF2(C2O4)) (hereinafter referred to as LiFOB) as a first salt and LiPF6 as a second salt were dissolved in a solvent containing fluoroethylene carbonate (FEC) to prepare a non-aqueous electrolyte.

[0057] The concentration of LiPF6 in the non-aqueous electrolyte was set to 1.0 mol / liter.

[0058] The concentration of LiFOB in the non-aqueous electrolyte was set to 0.5 mol / liter (4 mass%).

[0059] The concentration of FEC in the non-aqueous electrolyte was set to 5 mass%.

[0060] The solvent components other than FEC were ethylene carbonate (EC) and dimethyl carbonate (DMC), and the volume ratio of EC to DMC was 30:70.

[0061] (3) Preparation of half cells Using the above negative electrode, a counter electrode of a sufficient amount of lithium metal foil, and the above non-aqueous electrolyte, a half cell A1 was obtained.

[0062] [Comparative Examples 2 to 4] In the preparation of the non-aqueous electrolyte, FEC and LiFOB were not used. A half cell B1 was prepared in the same manner as half cell A1 except for the above.

[0063] [Evaluation 1] In a thermostat at 25°C, each half cell was charged, rested for 20 minutes after charging, and discharged under the following conditions. This charge-discharge was defined as one cycle, and the ratio of the charge capacity to the discharge capacity (charge-discharge efficiency) was determined. Charge-discharge was performed for 10 cycles, and the charge-discharge efficiency at the 10th cycle was designated as Rm1 (%). The evaluation results are shown in Table 1.

[0064] (Charge) Constant current charging was performed until the state of charge (SOC) reached 100% (Comparative Example 1), 110% ( Reference Example 1), and 120% (Example 2) at a current of 0.1 It. When the charge capacity Cc when the open circuit voltage after charging was 90 mV was set to 100, the charge capacities Cc at the cut-off voltages of 20 mV and 5 mV were 110 and 120, respectively.

[0065] (Discharge) Constant current discharge was performed until the cell voltage reached 1.0 V at a current of 0.1 It.

[0066]

Table 1

[0067] In Table 1, from the comparison between Comparative Example 1 and Comparative Example 2 (both with Cc = 100 and fully charged OVC 90 mV), it can be understood that when only the carbon material bears the negative electrode capacity, there is no difference in charge-discharge efficiency (i.e., cycle characteristics) regardless of the presence or absence of LiFOB and FEC. That is, the degradation of cycle characteristics is a phenomenon peculiar to the case where lithium metal bears a part of the negative electrode capacity. On the other hand, Reference When comparing Example 1 and Comparative Example 3 (both with Cc = 110 and fully charged OVC 20 mV), an improvement effect of charge-discharge efficiency of about 0.2% due to the coexistence of LiFOB and FEC can be seen at the 10th cycle. And when comparing Example 2 and Comparative Example 4 (both with Cc = 120 and fully charged OVC 5 mV), a significant (2.1%) improvement effect of charge-discharge efficiency due to the coexistence of LiFOB and FEC can be seen.

[0068] [Example 3] (1) Preparation of the positive electrode Lithium nickel composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O2), acetylene black, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) were mixed at a predetermined mass ratio to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, which is the positive electrode current collector. After drying the coating film, it was rolled to form a positive electrode mixture layer on both sides of the aluminum foil.

[0069] (2) Preparation of the negative electrode Comparative Example 1 and Reference Example 1, Example The same negative electrode slurry as that used in 2 was prepared. Next, the negative electrode slurry was applied to the surface of an electrolytic copper foil, which is the negative electrode current collector. After drying the coating film, it was rolled to form a negative electrode mixture layer on both sides of the copper foil.

[0070] (3) Preparation of the non-aqueous electrolyte LiFOB was dissolved as the first salt and LiPF6 was dissolved as the second salt in a solvent containing FEC to prepare a non-aqueous electrolyte.

[0071] The concentration of LiPF6 in the non-aqueous electrolyte was set to 1.0 mol / liter.

[0072] The concentration of LiFOB in the non-aqueous electrolyte was set to 0.5 mol / liter (4 mass%).

[0073] The concentration of FEC in the non-aqueous electrolyte was set to 2 mass%.

[0074] The solvent components other than FEC were EC and DMC, and the volume ratio of EC to DMC was 30:70.

[0075] (4) Design capacity Assuming that the OCV of the negative electrode at full charge is about 5 mV with respect to lithium metal, the ratio of the design capacity Cn borne by artificial graphite (carbon material) of the negative electrode to the design capacity Cp of the positive electrode: Cn / Cp was set to 100 / 120 = 0.83.

[0076] (5) Fabrication of the battery Lead tabs were attached to each electrode, and an electrode group was fabricated by winding the positive electrode and the negative electrode spirally through a separator so that the leads were located at the outermost peripheral part. The electrode group was inserted into a laminate film exterior body with an aluminum foil as a barrier layer, together with a reference electrode surrounded by a separator, vacuum dried at 105 °C for 2 hours, then an electrolytic solution was injected, and the opening of the exterior body was sealed to obtain Battery A10.

[0077] [Example 4] In the preparation of the non-aqueous electrolyte, Battery A11 was fabricated in the same manner as Battery A10, except that the concentration of LiFOB in the non-aqueous electrolyte was set to 1.6 mass% and the concentration of FEC was set to 5 mass%.

[0078] [Example 5] In the preparation of the non-aqueous electrolyte, Battery A12 was fabricated in the same manner as Battery A10, except that the concentration of FEC in the non-aqueous electrolyte was set to 5 mass%.

[0079] [Comparative Example 5] A battery B10 was fabricated in the same manner as battery A10, except that FEC and LiFOB were not used in the preparation of the non-aqueous electrolyte.

[0080] [Comparative Example 6] A battery B11 was fabricated in the same manner as battery A10, except that FEC was not used in the preparation of the non-aqueous electrolyte.

[0081] [Comparative Example 7] A battery B12 was fabricated in the same manner as battery A10, except that LiFOB was not used and the concentration of FEC in the non-aqueous electrolyte was set to 5% by mass.

[0082] [Evaluation 2] In a thermostat at 25 °C, each battery was charged, rested for 20 minutes after charging, and then discharged under the following conditions. This charge-discharge cycle was defined as one cycle, and 50 charge-discharge cycles were performed. The value obtained by dividing the discharge capacity at the 50th cycle by the discharge capacity at the first cycle was determined as the capacity retention rate Rm2 (%). The evaluation results are shown in Table 2. (Charge) Constant current charging was performed at a current of 0.3It until the battery voltage reached 4.2V, and then constant voltage charging was performed at a voltage of 4.2V until the current value reached 0.02It. (Discharge) Constant current discharge was performed at a current of 0.3It until the battery voltage reached 2.85V.

[0083]

Table 2

[0084] In Table 2, when comparing Example 3 and Comparative Example 5, the improvement effect of cycle characteristics due to the coexistence of LiFOB and FEC is more remarkable (2.5%). Also, when comparing Comparative Example 5 and Comparative Example 6, it can be seen that with only LiFOB, the improvement effect of cycle characteristics (0.8%) is small. Furthermore, when comparing Comparative Example 5 and Comparative Example 7, it can be understood that even with only FEC, an improvement effect of cycle characteristics (1.1%) is observed, but it is not so remarkable. The sum of the respective effects of LiFOB and FEC is 1.9%, which is about 30% lower than the effect (2.5%) when LiFOB and FEC are used in combination.

[0085] Next, when comparing Examples 3 to 5, it can be understood that the greater the mass ratio of FEC to LiFOB in the non-aqueous electrolyte, especially when it is 1.25 or more, the more remarkable the effect of improving cycle characteristics becomes.

Industrial Applicability

[0086] The non-aqueous electrolyte secondary battery according to the present disclosure has excellent cycle characteristics, and thus can be used in electronic devices such as mobile phones, smartphones, tablet terminals, electric vehicles including hybrids and plug-in hybrids, and household storage batteries combined with solar cells.

Explanation of Signs

[0087] 10 Positive electrode 13 Positive electrode lead 15 Positive electrode terminal 20 Negative electrode 23 Negative electrode lead 30 Separator 40 Electrode group 60 Lower insulating ring 70 Battery case 80 Upper insulating ring 90 Sealing plate 100 Lithium secondary battery

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a lithium ion-conductive non-aqueous electrolyte, wherein the negative electrode contains a carbon material that occludes and releases lithium ions, the open circuit potential of the negative electrode at full charge is more than 0 mV and less than 20 mV with respect to lithium metal, the non-aqueous electrolyte contains a solvent, a cation, and an anion, the solvent contains a fluorine-containing cyclic carbonate, the cation contains lithium ions, and the anion contains an oxalate complex anion.

2. The oxalate complex anion is B(C ２ O ４ ) ２ － , B.F. ２ (C ２ O ４ ) － , P(C ２ O ４ ) ３ － , P.F. ２ (C ２ O ４ ) ２ － and P.F. ４ (C ２ O ４ ) － 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolyte is at least one selected from the group consisting of:

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the concentration of the oxalate complex anion in the non-aqueous electrolyte is 0.8% by mass or more and 4.0% by mass or less.

4. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the concentration of the fluorine-containing cyclic carbonate in the non-aqueous electrolyte is 0.1% by mass or more and 20% by mass or less.

5. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein when the oxalate complex anion is derived from an oxalate complex salt, the mass ratio of the fluorine-containing cyclic carbonate to the oxalate complex salt in the non-aqueous electrolyte is 0.025 or more.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the open circuit potential of the negative electrode at full charge is more than 0 mV and 5 mV or less with respect to lithium metal.

7. The positive electrode contains a composite oxide represented by the general formula: Li ａ Ni x Co y M １－ｘ－ｙ O ２ and includes The general formula is 0.97 < a ≤ 1.2, 0.5 ≤ x ≤ 1.0, and 0 ≤ y ≤ 0.1 are satisfied, and M is at least one selected from the group consisting of Mn, Al, W, Mg, Mo, Nb, Ti, Si, and Zr. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 6.

8. The non-aqueous electrolyte secondary battery according to claim 7, wherein 0.8 ≤ x ≤ 1.0 is satisfied.

9. The non-aqueous electrolyte secondary battery according to claim 7 or 8, wherein M contains Al, and the atomic ratio Co / Al is 0 to 1.0.

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