JP2019075320A - Lithium ion secondary battery - Google Patents

Abstract

To suppress a decrease in the capacity due to charge and discharge cycles and an increase in internal resistance and realize the life extension of a battery in a lithium ion secondary battery using graphitizable carbon as a negative electrode active material and a non-aqueous electrolyte solution as an electrolyte solution.SOLUTION: A lithium ion secondary battery includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, and the negative electrode active material includes graphitizable carbon, and the electrolyte is a non-aqueous electrolytic solution and includes 1,3,2-dioxathiolane 2,2-dioxide (DTD) or the like as an additive.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a lithium ion secondary battery.

  A lithium ion secondary battery is one of secondary batteries expected to be smaller and lighter as a power source for electronic devices. As a negative electrode active material of a lithium ion secondary battery, carbon-based materials typified by graphite (artificial graphite and natural graphite) and amorphous carbon, alloy materials having silicon, tin and the like as main components are studied. Practical application is in progress.

  However, in recent years, as the demand for high input / output characteristics of batteries increases in order to be applied to large-sized products such as electric vehicles, a technique for suppressing a decrease in discharge capacity and an increase in internal resistance of batteries accompanying charge / discharge cycles. Is required.

  Patent Document 1 includes cyclic carbonate and chain carbonate as an electrolytic solution in order to produce a lithium ion secondary battery that achieves excellent first-time discharge capacity and sufficient discharge capacity at low temperature, The use of those containing a glycol sulfate derivative such as 3,2,3-dioxathiolane 2,2-dioxide (DTD) and fluoroethylene carbonate, the use of artificial graphite as the negative electrode active material of the example, etc. ing.

Patent Document 2 describes a nonaqueous solvent, Li ₂ B ₁₂ F _12-x Z _x (Z Is any of hydrogen, chlorine and bromine, and x is any natural number of 10 to 12.) containing 1 to 15% by mass of 1, 2, 3-dioxathiolane 2, 2-dioxide (DTD), etc. It is disclosed that a non-aqueous electrolytic solution containing 0.5 to 6% by mass of the ethylene glycol sulfate derivative of the following is used, artificial graphite is used as a negative electrode active material of the example, and the like.

  In Patent Document 3, a heat-resistant porous material containing inorganic particles such as alumina on the surface of a substrate as a separator in order to improve the capacity retention rate when storing a lithium ion secondary battery at high temperature (60 ° C.) Layers, and using non-aqueous electrolytic solution containing 1,3,2-dioxathiolane-2,2-dioxide (DTD), using artificial graphite as a negative electrode active material of the example, etc. There is.

JP, 2013-229307, A Unexamined-Japanese-Patent No. 2014-103066 Unexamined-Japanese-Patent No. 2016-71998

  The artificial graphite used as the negative electrode active material in Patent Documents 1 to 3 seems to be a material containing graphite as a main component, but its physical properties and the like are not sufficiently and clearly described.

  As a result of experimental investigation by the present inventor, when ordinary artificial graphite is used as a negative electrode active material and 1,3,2-dioxathiolane 2,2-dioxide (DTD) is added to the electrolytic solution, the discharge capacity decreases. , It turned out that DC resistance becomes large.

  The object of the present invention is to suppress the decrease in capacity and the increase in internal resistance associated with charge and discharge cycles in a lithium ion secondary battery using graphitizable carbon as a negative electrode active material and a non-aqueous electrolyte solution as an electrolyte. To extend the life of the

  The lithium ion secondary battery of the present invention has a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolytic solution, the negative electrode active material contains graphitizable carbon, and the electrolytic solution contains It is a non-aqueous electrolytic solution and contains 1,3,2-dioxathiolane 2,2-dioxide (DTD) as an additive.

  According to the present invention, in a lithium ion secondary battery using graphitizable carbon as a negative electrode active material and a non-aqueous electrolytic solution as an electrolytic solution, the decrease in capacity and the increase in internal resistance associated with charge and discharge cycles are suppressed. Excellent output characteristics and long life can be achieved.

It is a schematic cross section which shows 18650 type | mold battery to which this invention is applied. It is a graph which shows the discharge capacity after the charging / discharging cycle of Examples 1-7 and Comparative Examples 1-5. It is a graph which shows the direct current resistance after the charging / discharging cycle of Examples 1-7 and Comparative Examples 1-5.

  The present invention relates to a lithium ion secondary battery having excellent life characteristics.

  In the lithium ion secondary battery of the present invention, graphitizable carbon is used as the negative electrode active material, and a non-aqueous electrolytic solution mixed with an additive represented by the following chemical formula (1) is used as the electrolytic solution. .

(Wherein, R ¹ and R ² each independently represent a functional group selected from the group consisting of a hydrogen atom and a hydrocarbon group having 1 to 5 carbon atoms.)
Hereinafter, embodiments of the present invention will be described. The following description shows specific examples related to the present invention, and the contents of the present invention are not limited to these descriptions.

  The lithium ion secondary battery of the present embodiment includes one having a configuration shown in FIG. 1 described as an example described later.

  A lithium ion secondary battery includes a positive electrode, a negative electrode, and a separator interposed therebetween. The positive electrode, the negative electrode and the separator are enclosed in a battery can (battery container) together with an electrolytic solution.

1) Positive Electrode The positive electrode of the present embodiment is applicable to a lithium ion secondary battery (hereinafter, also simply referred to as "battery") having high capacity and high input / output. The positive electrode is composed of a current collector and a positive electrode mixture provided on both sides thereof. The positive electrode mixture contains a positive electrode active material.

  The positive electrode active material is a layered crystal structure of lithium-nickel-cobalt-manganese composite oxide (hereinafter abbreviated as "NMC material"). The NMC material is a high capacity material as compared to lithium manganate having a spinel structure. From the viewpoint of increasing the capacity of the battery, the content of the NMC material in the positive electrode mixture is preferably 80 to 98 mass% with respect to the total amount of the positive electrode mixture.

  As the NMC material, it is desirable to use one represented by the following composition formula.

Li _{(1 + a)} Mn _x Ni _y Co _(1-x-y-z) M _z O ₂ ... Composition formula In this composition formula, (1 + a) is the composition ratio of Li (lithium) and x is the composition of Mn (manganese) The ratio y represents the composition ratio of Ni (nickel), and (1-x-y-z) represents the composition ratio of Co (cobalt). z represents the compositional ratio of the element M. The composition ratio of O (oxygen) is 2. -0.15 <a <0.15, 0.1 <x <0.5, 0.6 <x + y + z <1.0, 0 <z <0.1. The element M, if including it, is Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge One or more elements selected from the group consisting of (germanium) and Sn (tin) can be selected.

In the following examples, LiMn _1/3 Ni _1/3 Co _1/3 O ₂ with z = 0 was used.

  The positive electrode mixture and the current collector will be described in detail below.

  The positive electrode mixture includes a positive electrode active material, a binder and the like, and is attached to both sides of the current collector. There is no limitation on the method of attachment, but a method of preparing a slurry by dispersing a positive electrode active material, a binder, a conductive material and the like in a solvent, and coating it on a current collector and drying it can be applied.

  The median particle diameter (d50) of the NMC material particles (when the primary particles are aggregated to form the secondary particles, the median diameter of the secondary particles) can be adjusted within the following range. From the viewpoint of the filling property and the mixing property with other materials, 3 to 25 μm is desirable, and 5 to 15 μm is more desirable. The median diameter (d50) described here means the particle size at an integrated value of 50% in the particle size distribution determined by the laser diffraction / scattering method. For example, it is a value measured as d50 using a particle size distribution measuring apparatus (for example, SALD-3000 manufactured by Shimadzu Corporation). Hereinafter, when the particle size is not particularly described, it is represented by the median diameter defined as described above.

The range of the BET specific surface area of the particles of the NMC material is desirably 0.2 to 4.0 m ² / g. When the BET specific surface area is in this range, the mixing property with the binder and the conductive material is good, and the packing density as the electrode can be increased. The BET specific surface area is a surface area per unit mass determined by the BET method.

  As the conductive material for the positive electrode, materials such as graphite and amorphous carbon can be used. One of these materials may be used alone, or two or more of these materials may be used in combination. The content of the conductive material relative to the mass of the positive electrode mixture is desirably 0.01 to 15% by mass. When the content is 0.01% by mass or more, sufficiently high conductivity can be obtained, and when the content is 15% by mass or less, a decrease in battery capacity can be suppressed.

  The binder of the positive electrode active material is not particularly limited, and when the positive electrode mixture is attached by a coating method, a material having good solubility or dispersibility in a dispersion solvent is selected. Specifically, resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polyimide; rubber-like polymers such as SBR (styrene-butadiene rubber), polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyfluoride And fluorine-containing polymers such as vinylidene.

  The range (mass standard) of the content of the binder with respect to the positive electrode mixture is as follows.

  The content of the binder with respect to the positive electrode mixture is preferably 0.1 to 15% by mass. When the content of the binder is 0.1% by mass or more, the positive electrode active material can be sufficiently bound, and when the content is 15% by mass or less, a sufficiently high battery capacity can be obtained.

  The material of the current collector for the positive electrode is not particularly limited, but is preferably a metal material, particularly aluminum.

2) Negative Electrode The negative electrode is composed of a current collector and a negative electrode composite attached to both sides thereof. The method for producing the negative electrode mixture is not particularly limited, but the same method as that for the positive electrode mixture can be applied. The above-mentioned negative electrode mixture contains a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions.

  As the above-mentioned negative electrode active material, graphitizable carbon is suitable. The graphitizable carbon has an interplanar spacing d 002 value of 0.34 nm or more and less than 0.36 nm in the C-axis direction obtained by X-ray diffraction method. In addition, graphitizable carbon is a material having graphitization that is graphitized by heat treatment at 800 ° C. or higher, and is a material that is not completely graphitized. On the other hand, non-graphitizable carbon is characterized in that graphitization hardly progresses even by heat treatment at 2800 ° C. or higher. This is because graphitizable carbon is an atomic arrangement structure that easily forms a layered structure, and has the property of being easily changed to a graphite structure by heat treatment at a relatively low temperature as compared to non-graphitizable carbon. Non-graphitizable carbon is also a material that is not completely graphitized.

  In addition, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon) and graphite are generally distinguished as follows.

  In graphitizable carbon, relatively many functional groups derived from the organic matter of the raw material remain. Therefore, the lattice spacing (d002) of carbon is large, so that SEI (Solid Electrolyte Interface) is easily generated, and lithium difluoro (oxalato) borate easily enters and exits between lattice planes of carbon.

  In non-graphitizable carbon, there is a high probability that the lattice planes of adjacent carbons are not parallel to one another and the lattice spacing (d 002) of carbon is small. In addition, the functional group is less than graphitizable carbon, and the ratio of carbon in the entire material is high. Therefore, SEI is difficult to occur, and lithium difluoro (oxalato) borate is difficult to enter and exit between lattice planes of carbon.

  In graphite, lattice planes of adjacent carbons are parallel, and the lattice plane spacing (d 002) of carbon is small. In addition, in the case of graphite, there is also a problem that the volume change due to the entrance and exit of lithium ions is large. Graphite includes natural graphite obtained by mining, artificial graphite manufactured industrially, and the like.

  Thus, graphitizable carbon is more likely to produce SEI in combination with lithium difluoro (oxalato) borate as compared to non-graphitizable carbon or graphite, and lithium difluoro (oxalato) borate is between the lattice planes of carbon. It can be said to be a desirable material in that it is easy to move in and out, and the volume change due to the movement of lithium ions is small.

  Lithium difluoro (oxalato) borate is one of the additives to be mixed with the electrolytic solution, and is represented by the following chemical formula (2).

  Hereinafter, graphitizable carbon will be described more specifically.

  In the graphitizable carbon used here, the mass ratio in an air stream determined by thermogravimetric analysis (TG) is 75% or more at 550 ° C. with respect to the mass at 25 ° C. (initial mass), and 650 It has a characteristic that it is 20% or less at ° C.

  From the viewpoint of battery performance, the above mass ratio is desirably 85% or more at 550 ° C. and 10% or less at 650 ° C. And it is more desirable that it is 95% or more at 550 ° C and 5% or less at 650 ° C.

  If the above mass ratio is less than 75% at 550 ° C., the input / output performance is degraded. In addition, when the above-mentioned mass ratio exceeds 20% at 650 ° C., the life characteristics deteriorate.

  As an apparatus for TG, for example, TG / DTA 6200 manufactured by SII Nano Technology Co., Ltd. can be used. The measurement conditions can be measured by collecting 10 mg of a sample, flowing dry air at a flow rate of 300 ml / min, and using aluminum oxide (alumina) as a comparison target at a temperature increase rate of 1 ° C./min.

  Specifically, for example, a material exhibiting graphitizability is fired in an inert atmosphere furnace at 800 ° C. or higher, and this is pulverized by a known method (such as a vibration mill or jet mill) to obtain a median diameter of 5 to 5 The above graphitizable carbon can be obtained by adjusting to 30 μm.

  The material exhibiting the above-mentioned graphitizability is not particularly limited, and examples thereof include thermoplastic resins, naphthalene, anthracene, phenanthrolene, coal tar, tar pitch and the like, and preferably coal coal tar and It is petroleum-based tar.

  Here, in the graphitizable carbon, the interplanar spacing d002 value in the C-axis direction obtained by X-ray diffraction method is preferably 0.34 nm or more and less than 0.36 nm, and is 0.341 nm or more and 0.355 nm or less Is more preferably 0.342 nm or more and 0.350 nm or less.

  The above-mentioned graphitizable carbon can be used as it is, but depending on the pulverizing conditions, the specific surface area may become large and desired properties may not be exhibited. Therefore, it is desirable to form a carbon coating layer on the surface of the above-mentioned graphitizable carbon.

  The above-mentioned carbon coating layer can be formed, for example, by attaching an organic compound (carbon precursor) which leaves a carbonaceous substance by heat treatment to the surface of the above-described graphitizable carbon and then baking it. The method for attaching the organic compound to the surface of the graphitizable carbon is not particularly limited. For example, after the graphitizable carbon serving as the core is dispersed and mixed in a solution in which the organic compound is dissolved in a solvent, A wet process to remove the is desirable from the viewpoint of uniformity.

  As baking conditions at the time of covering carbon, it is desirable to heat-treat at 700-1400 ° C under non-oxidative atmosphere. When the calcination temperature is lower than 700 ° C., the initial irreversible capacity tends to be large, and when heated above 1400 ° C., there is little change in the electrochemical performance, causing an increase in processing cost.

  As the conductive material, graphite such as natural graphite or artificial graphite, carbon black such as acetylene black, or amorphous carbon such as needle coke can be used. One of these may be used alone, or two or more of them may be used in combination. By adding the conductive material, an effect such as reducing the resistance of the electrode can be obtained.

  The range of the content of the conductive material with respect to the mass of the negative electrode mixture is preferably 1 to 40 mass% from the viewpoint of improving the conductivity and reducing the irreversible capacity.

  The material and shape of the current collector for the negative electrode are not particularly limited, but copper foil is preferable from the viewpoint of processing easiness and cost. The copper foil includes a rolled copper foil formed by a rolling method and an electrolytic copper foil formed by an electrolytic method, but both can be used as a current collector.

  The binder for the negative electrode active material is not particularly limited as long as it is a material stable to the non-aqueous electrolyte solution and the dispersion solvent used in forming the electrode, and is the same as the binder used for the positive electrode active material. Can be used.

  As a dispersion solvent for forming a slurry, as long as it can dissolve or disperse a negative electrode active material, a binder, a conductive material used as needed, a thickener, etc., there is no limitation on the kind thereof. Either an aqueous solvent or an organic solvent may be used. In particular, when using an aqueous solvent, it is preferable to use a thickener. A dispersing agent etc. are added according to this thickener, and it slurries using latex, such as SBR.

  The content of the binder relative to the mass of the negative electrode mixture is preferably in the range of 0.1 to 20% by mass. When the content of the binder is 0.1% by mass or more, the negative electrode active material can be sufficiently bonded, and sufficient mechanical strength of the negative electrode mixture layer can be obtained. If the content is 20% by mass or less, sufficient battery capacity and conductivity can be obtained.

  In particular, when a rubber-like polymer represented by SBR is used as the main component as the binder, the range of the content of the binder with respect to the mass of the negative electrode composite is 0.1 to 5.0 mass%. Is preferred.

  When a fluorine-based polymer typified by polyvinylidene fluoride is used as the main component of the binder, the content of the binder relative to the mass of the negative electrode mixture is preferably 0 to 15% by mass.

  Thickeners are used to adjust the slurry viscosity. The thickening agent is not particularly limited, and specific examples include carboxymethylcellulose and methylcellulose.

  When using a thickener, it is preferable that content of the thickener with respect to the mass of negative electrode compound material is the range of 0.1-5 mass%.

3) Electrolyte Solution The electrolyte solution is composed of a lithium salt (electrolyte), a non-aqueous solvent that dissolves it, and an additive.

The lithium salt is not particularly limited as long as it is a lithium salt that can be used as an electrolyte for lithium ion secondary batteries, but if it is comprehensively judged solubility in a solvent, charge / discharge characteristics in the case of batteries, etc. Lithium fluorophosphate (LiPF ₆ ) is preferred.

  Although there is no restriction | limiting in particular about the density | concentration of the electrolyte in electrolyte solution, It is desirable that the density | concentration range of electrolyte is 0.5-2.0 mol / L. If the concentration is less than 0.5 mol / L, sufficient ionic conductivity of the electrolytic solution can not be obtained, and if it is higher than 2.0 mol / L, the viscosity becomes high, and there is a risk that process control such as pouring becomes difficult There is.

  The non-aqueous solvent is not particularly limited as long as it can be used as an electrolyte solvent for lithium ion secondary batteries, but cyclic carbonates, linear carbonates, linear esters, cyclic ethers, linear Ether etc. are mentioned. For example, ethylene carbonate, propylene carbonate, butylene carbonate, dialkyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl acetate, tetrahydrofuran, dimethoxyethane and the like can be mentioned.

  These may be used alone or in combination of two or more kinds, but it is preferable to use a mixed solvent in which two or more kinds of compounds are used in combination, a high dielectric constant solvent of cyclic carbonates, chain carbonates, It is preferable to use a low viscosity solvent such as a chain ester in combination.

4) Separator While the separator electrically insulates between the positive electrode and the negative electrode, it has ion permeability, and in particular, it has oxidation property on the positive electrode side and durability to reducibility on the negative electrode. There is no limit. Resin, an inorganic substance, glass fiber etc. are used as a material of the separator which satisfy | fills such a characteristic.

  As the resin, an olefin polymer, a fluorine polymer, a cellulose polymer, a polyimide, a nylon or the like is used. It is preferable to select from materials which are stable to the electrolytic solution and excellent in liquid retention, and it is preferable to use a porous sheet or non-woven fabric made of polyolefin such as polyethylene and polypropylene as a raw material.

  Hereinafter, description will be made using an example. However, the present invention is not limited to these examples.

  The preparation of the positive electrode was performed as follows.

As a positive electrode active material, lithium-nickel-manganese-cobalt composite oxide (NMC) having a layered crystal structure was used. In this example, a positive electrode active material represented by a composition formula LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used. The positive electrode active material has a BET specific surface area of 0.4 m ² / g and a median diameter of 6.5 μm.

  As the above-mentioned positive electrode active material, flaky graphite (average particle diameter: 20 μm) and acetylene black (trade name: HS-100, average particle diameter 48 nm (electrochemical industry catalog value), electric chemical industry Co., Ltd.) as a conductive material And polyvinylidene fluoride were sequentially added as a binder and mixed to obtain a positive electrode mixture. The mass ratio was set as active material: conductive material: binder = 90: 5: 5.

Further, N-methyl-2-pyrrolidone (NMP), which is a dispersion solvent, was added to the above mixture, and the mixture was kneaded to prepare a slurry. This slurry was uniformly and uniformly applied to both sides of a 20 μm thick aluminum foil as a current collector for a positive electrode. Thereafter, it was subjected to a drying treatment, and compacted to a predetermined density by a press. The density of the positive electrode mixture was 2.8 g / cm ^3, and the single-side coating amount of the positive electrode mixture was 140 g / m ² .

  The preparation of the negative electrode was performed as follows.

  As the negative electrode active material, graphitizable carbon whose surface was coated with 2% carbon was used. Here, the proportion (2%) of carbon on the surface is calculated with the mass of the entire negative electrode active material containing this carbon as 100%.

  The firing temperature at the time of coating the graphitizable carbon with carbon is 900.degree. As a result of performing thermogravimetric analysis (TG) on the carbon-coated graphitizable carbon after firing as a result, the mass ratio to the mass (initial mass) at 25 ° C. is 95% at 550 ° C., It was 5% at 650 ° C.

The negative electrode active material produced had a specific surface area of 3.1 m ² / g as measured by a BET method, and an average particle diameter of 15 μm.

Polyvinylidene fluoride was added to the negative electrode active material as a binder. The mass ratio was active material: binder = 92: 8. To this, N-methyl-2-pyrrolidone (NMP), which is a dispersion solvent, was added and kneaded to prepare a slurry. This slurry was uniformly and uniformly applied to both sides of a 10 μm-thick rolled copper foil as a current collector for a negative electrode. The density of the mixture of the negative electrode was increased to 1.15 g / cm ³ by a press, and the single-sided coating amount of the negative electrode mixture was 73 g / m ² .

  In the preparation of the battery, the positive electrode and the negative electrode were wound with a 30 μm-thick polyethylene separator interposed therebetween so as not to be in direct contact with each other. At this time, the tab lead pieces of the positive electrode and the tab lead pieces of the negative electrode were respectively positioned on opposite end surfaces of the winding group. In addition, the lengths of the positive electrode, the negative electrode and the separator were adjusted, and the diameter of the winding group was 17.8 ± 0.1 mm.

  FIG. 1 shows a schematic configuration of a battery manufactured using the above-mentioned wound group.

  In the figure, the battery 1 has a configuration in which the positive electrode 2, the negative electrode 3, and the separator 4 sandwiched therebetween are sealed in the battery can 5. The positive electrode 2 is electrically connected to the battery lid 8 by a positive electrode tab lead 6. The negative electrode 3 is electrically connected to the battery can 5 by a negative electrode tab lead 7.

  The wound group (including the positive electrode 2, the negative electrode 3 and the separator 4) manufactured as described above was incorporated into the battery 1 as follows.

  As shown in the drawing, the negative electrode tab lead 7 led from the negative electrode 3 was welded to the bottom of the battery can 5 from the cut of the polypropylene insulating ring 10. Thereafter, the positive electrode tab lead 6 led from the positive electrode 2 was drawn upward from the cut of the insulating ring 10. Thereafter, a gasket 9 made of polypropylene was placed in the opening of the battery can 5, and the positive electrode tab lead 6 was welded to the battery lid 8. Thereafter, a predetermined amount of electrolytic solution was injected from the upper portion of the battery can 5 and the upper end portion of the battery can 5 was bent by a caulking machine to complete the 18650 type battery.

As an electrolytic solution, 1.2 mol / L of lithium hexafluorophosphate (LiPF ₆ ) is dissolved in a solution in which ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate are mixed at a volume ratio of 2: 3: 2, respectively. What added 0.2 mass% of 1,3,2- dioxa thiolane 2, 2- dioxide (DTD) was used. Here, DTD is one in which all of R ¹ and R ² are hydrogen atoms among the additives represented by the chemical formula (1).

  The evaluation of the battery characteristics was performed by the method shown below. The evaluation of discharge capacity performed the measurement using a constant current constant voltage (CC-CV) charge system in a 25 degreeC environment (it abbreviates as the following "CC-CV charge"). The CC-CV charge is a method of charging at a constant current value (constant current), switching to constant voltage charging when the voltage value reaches a specified value, and continuing charging for a predetermined time.

  Next, the initialization conditions of the battery are shown.

  First, the battery after injection is allowed to stand at 25 ° C. for 10 hours to make the electrolyte compatible with the separator and the electrode (positive electrode and negative electrode). Thereafter, charging was started at a current value of 0.2 CA, and when reaching 4.1 V, charging was continued for 5 hours so as to maintain 4.1 V, and the process was ended. After charging, after 30 minutes of rest, 0.2 CA constant current discharge was performed, and the operation was finished when it reached 2.7 V. After three cycles of this, charging was performed to 3.7 V with a constant current of 0.2 CA. Thereafter, aging treatment was performed for 5 days under a 25 ° C. environment.

  The evaluation method of the capacity characteristics of the battery and the direct current resistance (hereinafter abbreviated as "DCR") is shown below.

  First, charging was started at a current value of 0.2 CA, and when reaching 4.1 V, charging was continued for 5 hours so as to maintain 4.1 V, and the process was ended. After charging, after 30 minutes of rest, 0.2 CA constant current discharge was performed, and the operation was finished when it reached 2.7 V. After three cycles of this, CC-CV charging is performed to 4.1V. After that, it was paused for 30 minutes. Here, the discharge capacity at the third cycle is taken as the capacity of the battery.

  After that, discharge for 10 seconds is performed with a current value of 0.1 CA, and a battery voltage at 10 seconds is measured. Thereafter, CC-CV charging is performed again to 4.1 V, discharge for 10 seconds is performed with a current value of 0.2 CA, and a battery voltage at 10 seconds is measured. Furthermore, after performing CC-CV charge to 4.1V by the same method, discharge for 10 seconds is carried out by the electric current value of 0.5 CA, and the voltage in 10 second is measured. Then, DCR was calculated from the slope of the current-voltage graph using the voltage difference before the discharge start, the voltage difference at 10 seconds, and the current value at each current value (0.1 CA, 0.2 CA, 0.5 CA).

  The cycle test of the battery was performed by the method shown below.

  In the thermostat at 25 ° C., CC-CV charging of 0.5 CA was performed as the charging condition, and charging was continued for 3 hours after reaching 4.1 V, and the process was terminated. Thereafter, a 30-minute rest was performed, a 0.5 CA constant current discharge was performed to 2.7 V, and a 30-minute rest was performed. Assuming that such charge-discharge is one cycle, the above-mentioned direct current resistance is evaluated at an arbitrary number of cycles, and the cycle characteristics (discharge capacity and DCR) are evaluated.

  Evaluation of the battery was performed according to Example 1 except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution was 0.5% by mass.

  Evaluation of the battery was performed according to Example 1 except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution was 1.0 mass%.

  Evaluation of the battery was performed according to Example 1 except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution was set to 1.5% by mass.

  Evaluation of the battery was carried out according to Example 1 except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution was 2.0 mass%.

  Except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution is 0.5% by mass, and the amount of lithium difluoro (oxalato) borate (LDFOB) is 1.0% by mass The evaluation of the battery was carried out according to Example 1.

  Except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution is 1.0% by mass, and the amount of lithium difluoro (oxalato) borate (LDFOB) is 0.5% by mass The evaluation of the battery was carried out according to Example 1.

(Comparative example 1)
The battery was evaluated according to Example 1 except that no additive was used in the electrolytic solution.

(Comparative example 2)
Evaluation of the battery was performed according to Example 1 except that only 1% by mass of vinylene carbonate (VC) was added to the electrolytic solution.

(Comparative example 3)
The battery was evaluated according to Example 1 except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution was 0.1 mass%.

(Comparative example 4)
According to Example 1 except that the used negative electrode active material is artificial graphite and the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution is 0.5% by mass. The battery was evaluated.

(Comparative example 5)
Evaluation of the battery was carried out according to Example 1 except that the amount of 1,3,2-dioxathiolane 2,2-dioxide (DTD) added to the electrolytic solution was 2.5 mass%.

  Table 1 summarizes the additives of Examples 1 to 7 and Comparative Examples 1 to 5.

(Initial battery characteristics evaluation)
Table 2 shows the results of measuring the battery capacity and the direct current resistance before carrying out the charge and discharge cycle.

  From this table, it can be seen that the discharge capacity before the cycle is 0.535 Ah or more in Examples 1 to 7 and 0.530 Ah or less in Comparative Examples 1 to 5. Further, it can be seen that the DCR before the cycle is 41.8 mΩ or less in Examples 1 to 7 and 42.6 mΩ or more in Comparative Examples 1 to 5.

(Evaluation of charge and discharge cycle life)
FIG. 2 shows the discharge capacities of the batteries of Examples 1 to 7 and Comparative Examples 1 to 5 after 300 cycles of charge and discharge. The abscissa represents the amount of added DTD, and the ordinate represents the discharge capacity.

  It can be seen from this figure that the batteries of Examples 1 to 7 maintain a high discharge capacity even after charge and discharge cycles, as compared with the batteries of Comparative Examples 1 to 5.

  FIG. 3 shows direct current resistance (DCR) after carrying out charge and discharge for 300 cycles for the batteries of Examples 1 to 7 and Comparative Examples 1 to 5. The horizontal axis is the amount of added DTD, and the vertical axis is the direct current resistance.

  It can be seen from this figure that the batteries of Examples 1 to 7 maintain a smaller direct current resistance even after the charge and discharge cycles, as compared with the batteries of Comparative Examples 1 to 5.

  Therefore, it is desirable that the concentration of DTD in the electrolyte be 0.2 mass% or more and 2.0 mass% or less.

  When the electrolytic solution contains lithium difluoro (oxalato) borate in addition to DTD, the total concentration of these additives in the electrolytic solution is preferably 0.2% by mass or more and 2.0% by mass or less.

  The lithium ion secondary battery of the present invention is required to have a large-sized lithium ion secondary battery excellent in life characteristics because it can suppress a decrease in discharge capacity and an increase in direct current resistance accompanying charge and discharge cycles by the action of additives. It can be applied to power sources for mobile and stationary power storage.

  1: Battery, 2: Positive electrode, 3: Negative electrode, 4: Separator, 5: Battery can, 6: Positive electrode tab lead, 7: Negative electrode tab lead, 8: Battery lid, 9: Gasket, 10: Insulating ring.

Claims (5)

A lithium ion secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolytic solution,
The negative electrode active material contains graphitizable carbon,
The said electrolyte solution is a non-aqueous electrolyte solution, and the lithium ion secondary battery containing the additive represented by following Chemical formula (1).

(Wherein, R ¹ and R ² each independently represent a functional group selected from the group consisting of a hydrogen atom and a hydrocarbon group having 1 to 5 carbon atoms.) The lithium ion secondary battery according to claim 1, wherein all of R ¹ and R ² are hydrogen atoms.   The lithium ion secondary battery according to claim 2, wherein a concentration of the additive represented by the chemical formula (1) in the electrolytic solution is 0.2 mass% or more and 2.0 mass% or less. The lithium ion secondary battery according to claim 1, wherein the electrolytic solution contains an additive represented by the following chemical formula (2).

The electrolytic solution contains an additive represented by the following chemical formula (2),
The total concentration of the additive represented by the chemical formula (1) and the additive represented by the chemical formula (2) in the electrolytic solution is 0.2% by mass or more and 2.0% by mass or less. The lithium ion secondary battery according to Item 2.

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