JP6032180B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  Lithium ion secondary batteries are known to have increased internal resistance with charge / discharge cycles and long-term storage. This causes a reduction in battery capacity and output. Conventionally, various attempts have been made to suppress an increase in internal resistance of a battery and improve durability. For example, Japanese Unexamined Patent Application Publication No. 2012-253010 (Patent Document 1) discloses a power storage element in which a non-aqueous electrolyte solution contains lithium difluorobisoxalate phosphate.

JP 2012-253010 A

In order to suppress an increase in internal resistance associated with charge / discharge cycles in a lithium ion secondary battery, it is effective to form a film that transmits lithium ions (Li ⁺ ) and does not have electron conductivity on the negative electrode surface. It is. Such a coating is also called SEI (Solid Electrolyte Interface), and is mainly composed of lithium carbonate (ROCOLi (R is an alkyl group)), lithium fluoride (LiF), etc., which are decomposition products of the nonaqueous electrolyte. Yes. Since the negative electrode surface on which SEI is formed is inactive to the reduction of the nonaqueous electrolyte, the decomposition of the nonaqueous electrolyte accompanying the charge / discharge cycle is suppressed, and as a result, the cycle durability is improved.

Lithium difluorobisoxalate phosphate (hereinafter sometimes abbreviated as “LiFOP”) disclosed in Patent Document 1 is a compound represented by a composition formula of LiPF ₂ (C ₂ O ₄ ) ₂ , In contrast, it undergoes reductive decomposition at a potential of about 2.0V. Therefore, when LiFOP is added to the non-aqueous electrolyte of a lithium ion secondary battery, it is considered that it is reduced and decomposed during the first charge, and the decomposition product adheres to the negative electrode surface and becomes a part of SEI. Conventionally, it has been considered that the cycle durability is improved by LiFOP by such a mechanism. For example, as disclosed in Patent Document 1, the amount added is in consideration of the amount of gas generated during reductive decomposition. Has been optimized.

  However, further improvement in cycle durability is desired for lithium ion secondary batteries. As described above, the addition of LiFOP to the non-aqueous electrolyte is effective in improving cycle durability, but the effect is not sufficient at present.

  The present invention has been made in view of the above situation, and an object of the present invention is to provide a lithium ion secondary battery having excellent cycle durability.

  (1) A lithium ion secondary battery of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte, and the positive electrode active material contains 0.25 mol% of zirconium (Zr). Including the above, lithium difluorobisoxalate phosphate is added to the nonaqueous electrolyte in the range of 0.25 mass% to 1.00 mass%.

  As described above, in the lithium ion secondary battery of the present invention, the positive electrode active material contains Zr in a specific range, and LiPOF is added to the nonaqueous electrolyte in a specific range. Thus, by using Zr and LiFOP together, the cycle durability is remarkably improved beyond the range expected from the conventional LiPOF addition effect. The reason can be considered as follows.

LiPOF not only forms SEI on the negative electrode surface, but also POF ⁻ ions generated by LiPOF itself or Li ⁺ desolvation adhere to the positive electrode surface to form a film on the positive electrode surface.

  On the other hand, Zr has been used as an additive to the positive electrode active material. When the positive electrode active material contains Zr, the decomposition reaction of the nonaqueous electrolyte in the positive electrode is suppressed, and improvement in cycle durability is expected. However, it is known that Zr elutes from the positive electrode when the charge / discharge cycle is repeated, and the effect does not last for a long time.

  As described above, the present invention uses LiPOF and Zr together in a specific range. Thereby, elution of Zr from the positive electrode is suppressed, and an increase in reaction resistance can be continuously suppressed over a long period of time. That is, a film formed by attaching LiPOF or the like to the positive electrode acts as a protective film and suppresses elution of Zr from the positive electrode. And the effect is remarkable when content of Zr in a positive electrode active material is 0.25 mol% or more, and the addition amount of LiPOF to a nonaqueous electrolyte is 0.25 mass% or more and 1.00 mass% or less. Can be increased.

  (2) The amount of LiPOF added is preferably 0.25 mass% or more and 0.75 mass% or less, and more preferably 0.38 mass% or more and 0.75 mass% or less. When the addition amount of LiFOP occupies this range, cycle durability can be further improved.

  (3) A lithium ion secondary battery according to another aspect of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte, and the positive electrode active material contains 0.25 mol of zirconium. The positive electrode and the negative electrode have a coating film containing a substance derived from lithium difluorobisoxalate phosphate on the surface.

  As described above, by forming a coating derived from LiPOF on both the positive and negative electrodes, the negative electrode has the effect of suppressing the decomposition of the nonaqueous electrolyte by SEI, and the positive electrode has the effect of suppressing the decomposition of the nonaqueous electrolyte by Zr over a long period of time. Together, these can significantly improve cycle durability.

  The lithium ion secondary battery of the present invention has excellent cycle durability.

It is a graph which shows the relationship between Zr content of the positive electrode active material in the lithium ion secondary battery concerning one Embodiment of this invention, and the capacity | capacitance maintenance factor after a cycle durability test. Relationship between the Zr content of the positive electrode active material and the addition amount of lithium difluorobisoxalate phosphate to the nonaqueous electrolyte in the lithium ion secondary battery according to one embodiment of the present invention and the capacity retention rate after the cycle durability test It is a graph which shows. Relationship between the Zr content of the positive electrode active material and the addition amount of lithium difluorobisoxalate phosphate to the nonaqueous electrolyte in the lithium ion secondary battery according to one embodiment of the present invention and the rate of increase in resistance after the cycle durability test It is a graph which shows. It is a graph which shows the relationship between the combination of the positive electrode active material and the additive seed | species to a nonaqueous electrolyte in the lithium ion secondary battery concerning one Embodiment of this invention, and the capacity | capacitance maintenance factor after a cycle endurance test. It is a graph which shows the relationship between the amount of Zr detected from the negative electrode in the lithium ion secondary battery concerning one Embodiment of this invention, and the number of charging / discharging cycles.

  Hereinafter, embodiments of the present invention (hereinafter referred to as “present embodiments”) will be described in more detail, but the present invention is not limited thereto.

<Lithium ion secondary battery>
The present inventor conducted intensive research to solve the above problems, and LiPOF not only forms SEI on the negative electrode surface but also forms a film on the positive electrode surface to suppress elution of additive elements contained in the positive electrode. As a result, the present invention has been completed by further research based on the knowledge.

  That is, the lithium ion secondary battery of this embodiment includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte, and the positive electrode active material includes 0.25 mol% or more of zirconium, Lithium difluorobisoxalate phosphate is added to the non-aqueous electrolyte in the range of 0.25 mass% to 1.00 mass%.

<Battery structure>
In the present embodiment, the battery structure is not particularly limited. For example, a current collector tab is welded to a sheet-like positive electrode and a negative electrode, and a winding-type electrode body is obtained by winding the sheet-like separator so that the positive electrode and the negative electrode face each other. The electrode body is inserted into an outer can having a predetermined shape, one current collecting tab and the outer can are welded, and the other current collecting tab and the cap are welded. It is possible to employ a structure in which an electrolyte is injected and then the outer can and the cap are sealed by a predetermined means through a gasket.

  Examples of the shape of the battery outer package include a cylindrical shape and a rectangular shape, and the battery outer package usually includes an outer can and a cap. The cap is provided with a positive or negative terminal portion, and the terminal portion is insulated from the counter electrode by a resin material, for example. The material of the battery outer package may be appropriately selected from various metals or alloy materials in consideration of withstand voltage and strength. For example, aluminum and its alloys, iron (Fe), stainless steel, etc. can be used.

Hereinafter, each part which comprises the lithium ion secondary battery of this embodiment is demonstrated.
<Positive electrode>
The positive electrode of the present embodiment includes a positive electrode active material, and is typically a sheet-like member in which a positive electrode mixture including a positive electrode, a conductive additive, and a binder is fixed on a current collector core material. It is. Such a positive electrode is made of, for example, a positive electrode mixture slurry obtained by kneading a positive electrode active material, a conductive additive, a binder, and an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)). It can be produced by coating and drying on a current collector core material to form a positive electrode mixture layer on the current collector core material, and then compressing the positive electrode mixture layer to a predetermined thickness. When compressing the positive electrode mixture layer, in order to prevent the positive electrode mixture from falling off, the thickness of the positive electrode mixture layer is set to the density of the positive electrode mixture layer (mass of the positive electrode mixture layer / volume of the positive electrode mixture layer). ) Is preferably adjusted to about 2.0 to 4.0 g / cm ³ .

As the positive electrode active material of this embodiment, a lithium-containing transition metal oxide capable of electrochemically inserting and extracting Li ⁺ can be used. Examples of such lithium-containing transition metal oxides include LiCoO ₂ , LiNiO ₂ , LiNi _a Co _b O ₂ (a + b = 1, 0 <a <1, 0 <b <1), LiMnO ₂ , LiMn ₂ O. ₄ , LiNi _a Co _b Mn _c O ₂ (a + b + c = 1, 0 <a <1, 0 <b <1, 0 <c <1), LiFePO _{4 and the} like. Among these, as the positive electrode active material of the present embodiment, LiNi _a Co _b Mn _c O ₂ is preferable, and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is more preferable from the viewpoint of output characteristics and the like. In addition, the positive electrode of this embodiment may contain 2 or more types of positive electrode active materials.

The positive electrode active material as described above may be secondary particles in which primary particles having a small particle diameter are aggregated. In such a case, the average particle diameter of the primary particles of the positive electrode active material can be, for example, about 1 μm to 10 μm, and the average particle diameter of the secondary particles can be, for example, about 5 to 20 μm. Here, the average particle diameter means a 50% volume average particle diameter (that is, d50) by a laser diffraction / scattering method. Cathode active specific surface area of the material (by BET method) may be a 0.5m ² /g~3.0m ² / g approximately, in view of reactivity with the non-aqueous electrolyte and the positive electrode, 2.0 m ² / G or less is preferable. The content rate of the positive electrode active material in the positive electrode mixture is, for example, about 80 to 99% by mass, and preferably about 90 to 99% by mass from the viewpoint of the energy density of the battery.

A method for producing a positive electrode active material such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is not particularly limited. For example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is a coprecipitated hydroxide (precursor) represented by the composition formula of Ni _1/3 Co _1/3 Mn _1/3 (OH) ₂ ; after mixing the Li source (LiOH, Li ₂ CO _3, etc.), it can be prepared by calcining at a temperature of about 500 ° C. to 1000 ° C..

  The positive electrode active material of this embodiment contains 0.25 mol% or more of Zr. This is because if the content of Zr in the positive electrode active material is less than 0.25 mol%, the effect of suppressing the decomposition of the nonaqueous electrolyte may not be sufficiently obtained. The Zr content is more preferably 0.4 mol% or more. This is because when the Zr content occupies this range, the cycle durability tends to be further improved. Further, the upper limit of the Zr content is not particularly limited, but even if the Zr content exceeds 1.0 mol%, it is not economically advantageous since a significant performance improvement cannot be expected. Therefore, the Zr content is preferably 1.0 mol% or less.

  The presence form of Zr in the positive electrode active material is not particularly limited. For example, it may exist as an oxide on the surface of the secondary particle or the grain boundary of the primary particle, or a part or all of it may be dissolved in the positive electrode active material.

  The positive electrode active material of the present embodiment may contain other elements as long as it contains Zr within the above range, and even if it contains other elements, it does not depart from the scope of the present invention. Here, as other elements, for example, sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), potassium (K), calcium (Ca), titanium (Ti), vanadium (V) , Chromium (Cr), iron (Fe), gallium (Ga), germanium (Ge), rubidium (Rb), strontium (Sr), niobium (Nb), molybdenum (Mo), tungsten (W) and the like. it can.

  The content of Zr in the positive electrode active material is, for example, an energy dispersive X-ray spectrometer (EDX) or an inductively coupled plasma-atomic emission spectrometer (ICP-AES). It can be measured using an inductively coupled plasma mass spectrometer (ICP-MS).

The method for incorporating Zr into the positive electrode active material is not particularly limited. For example, when a positive electrode active material precursor is obtained by a coprecipitation method, a Zr compound is added to a solution, and Zr is crystallized together with the precursor to obtain a precursor containing Zr. For example, a positive electrode active material containing Zr can be obtained by firing together with LiOH, Li ₂ CO ₃ or the like. Further, Zr may be fired as a form of oxide (ZrO ₂ ) or Li composite oxide (for example, Li ₂ ZrO ₃ ) after being mixed with a positive electrode active material by a dry or wet powder mixer. .

  The conductive additive contained in the positive electrode is for assisting electrical conduction between the positive electrode active materials and between the positive electrode active material and the current collector core material, and a carbon material having high conductivity is preferable. As the carbon material, for example, acetylene black (AB), ketjen black (KB), graphite, vapor growth carbon fiber (VGCF), or the like can be used. The content of the conductive additive in the positive electrode mixture is, for example, about 1 to 10% by mass, and preferably about 1 to 5% by mass from the viewpoint of the energy density of the battery.

  The binder contained in the positive electrode is for fixing the positive electrode active materials to each other and fixing the positive electrode active material and the current collector core. For example, polyvinylidene fluoride (PVdF), polychlorinated chloride A vinylidene (PVDC: Polyvinylidene Chloride), a polytetrafluoroethylene (PTFE: Polytetrafluoroethylene), a polyethylene oxide (PEO: Polyethylene Oxide) etc. can be mentioned. Of these, PVdF is preferred from the viewpoint of coatability. The content of the binder in the positive electrode composite is, for example, about 1 to 10% by mass, and preferably about 1 to 5% by mass from the viewpoint of the energy density of the battery.

  As the current collector core material for the positive electrode, a metal foil having high conductivity and high corrosion resistance can be used. As such a metal foil, for example, an aluminum foil or an aluminum alloy foil is preferable. When using aluminum foil, it is preferable that the thickness of foil is about 10 micrometers-30 micrometers from a viewpoint of intensity | strength and electroconductivity.

<Negative electrode>
The negative electrode of the present embodiment includes a negative electrode active material, and is typically a sheet-like member in which a negative electrode mixture containing a negative electrode active material and a binder is fixed to a negative electrode current collector core material. . Such a negative electrode is obtained by, for example, applying a negative electrode mixture slurry obtained by kneading a negative electrode active material, a binder, and a solvent (for example, water) onto a current collector core and drying the negative electrode mixture layer Can be prepared by compressing the negative electrode mixture layer to a predetermined thickness. When compressing the negative electrode mixture layer, in order to prevent the negative electrode mixture from falling off, the thickness of the negative electrode mixture layer is set to the density of the negative electrode mixture layer (the mass of the negative electrode mixture layer / the negative electrode mixture layer). The volume is preferably adjusted to be about 0.5 to 2.5 g / cm ³ .

  Examples of the negative electrode active material of the present embodiment include materials that can occlude and release lithium ions (for example, carbon materials such as graphite and coke), metallic lithium, and materials that can be alloyed with lithium (for example, silicon and tin oxide). Can be used. Of these, graphite materials are preferred from the viewpoint of discharge characteristics and durability. Furthermore, natural graphite (NG: Natural Graphite) is more preferable from the viewpoint of production cost. The content rate of the negative electrode active material in the negative electrode mixture is, for example, about 90 to 99% by mass, and preferably about 95 to 99% by mass from the viewpoint of the energy density of the battery.

  The binder contained in the negative electrode is for fixing the negative electrode active materials to each other and fixing the negative electrode active material and the current collector core. For example, carboxymethylcellulose (CMC), PVdF, PTFE, Styrene-butadiene rubber (SBR) can be used. Among these, it is particularly preferable to use CMC and SBR in combination from the viewpoint of coatability. The content of the binder in the negative electrode mixture is, for example, about 1 to 10% by mass, and preferably about 1 to 5% by mass from the viewpoint of the energy density of the battery.

  As the current collector core material for the negative electrode, a metal foil having high conductivity and high chemical and electrochemical stability can be used. As such a metal foil, for example, a copper (Cu) foil or a copper alloy foil is preferable. When using copper foil, it is preferable that the thickness of foil is about 5-20 micrometers from a viewpoint of intensity | strength and electroconductivity.

<Nonaqueous electrolyte>
The nonaqueous electrolyte of this embodiment is typically a liquid electrolyte in which a solute (lithium salt) is dissolved in an aprotic solvent. Here, examples of the aprotic solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (GBL), and gamma-butyrolactone (GBL). Cyclic carbonates such as vinylene carbonate (VC), chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) Can be used. These aprotic solvents can be used in an appropriate combination of two or more from the viewpoint of electrical conductivity and electrochemical stability. In particular, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate, and the volume ratio of the cyclic carbonate to the chain carbonate is preferably about 1: 9 to 5: 5. If a specific example is given, 3 types, EC, DMC, and EMC, can be mixed and used, for example. Note that the nonaqueous electrolyte of the present embodiment may be in the form of a gel or a solid.

Examples of the solute lithium salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and bis (trifluoro). Lomomethanesulfonyl) imidolithium (Li (CF ₃ SO ₂ ) ₂ N), lithium trifluoromethanesulfonate (Li (CF ₃ SO ₃ )), or the like can be used. Moreover, you may use 2 or more types together about these solutes. The concentration of the solute in the nonaqueous electrolyte is not particularly limited, but is preferably about 0.5 to 2.0 mol / L from the viewpoint of discharge characteristics and storage characteristics.

  LiFOP is added to the nonaqueous electrolyte of the present embodiment in the range of 0.25 mass% to 1.00 mass%. As a result, high-quality SEI is formed on the negative electrode surface, and a protective film is also formed on the positive electrode surface, so that elution of Zr from the positive electrode can be suppressed. If the amount of LiFOP added is less than 0.25% by mass, the elution of Zr may not be sufficiently suppressed. If the amount of LiFOP added exceeds 1.00% by mass, the SEI of the negative electrode and the protective film of the positive electrode are excessively thick. In some cases, the internal resistance increases with charge / discharge cycles.

  As described above, LiPOF added to the non-aqueous electrolyte is reduced and decomposed on the negative electrode side to form SEI on the surface of the negative electrode and to form a protective film on the surface of the positive electrode. Therefore, even if the amount added to the non-aqueous electrolyte is in the range of 0.25 mass% or more and 1.00 mass% or less, when the battery is disassembled and the non-aqueous electrolyte is extracted, the extracted non-aqueous electrolyte The amount of LiPOF included can be less than 0.25% by weight. Even in such a case, it does not depart from the scope of the present invention.

In addition, as long as LiFOP is added in the above range, the nonaqueous electrolyte of the present embodiment may contain SEI on the negative electrode or a substance capable of forming a protective film on the positive electrode. For example, 1,3-propane sultone (PS: 1,3-Propanesultone), 1,3-propene sultone, 1,3-butane sultone (BS), 1,4-butane sultone, 2,4 -Sultone compounds such as butane sultone, lithium tetrafluorooxalate phosphate (LiPF ₄ (C ₂ O ₄ )), lithium bisoxalate borate (LiB (C ₂ O ₄ ) ₂ , hereinafter abbreviated as “LiBOB” And a lithium salt having an oxalate complex such as lithium difluorooxalate borate (LiBF ₂ (C ₂ O ₄ )) as an anion.

<Separator>
The separator in this embodiment is for preventing Li ⁺ permeation and preventing electrical contact between the positive electrode and the negative electrode. As such a separator, a microporous film made of a polyolefin-based material is preferable from the viewpoint of mechanical strength and chemical stability. Here, polyethylene (PE: Polyethylene), polypropylene (PP: Polypropylene), etc. can be used as a polyolefin-type material, and these can also be used in combination. Further, a plurality of microporous membranes may be laminated and used. The thickness of the separator can be, for example, about 5 to 40 μm. What is necessary is just to adjust suitably the hole diameter and porosity of a separator so that the air permeability of a separator may become a desired value.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to these.

<Production of lithium ion secondary battery>
(Preparation of positive electrode)
A nickel sulfate aqueous solution, a cobalt sulfate aqueous solution, and a manganese sulfate aqueous solution were added to the reaction vessel while adjusting so that Ni: Co: Mn = 1: 1: 1, thereby obtaining a mixed solution. Next, a sodium hydroxide aqueous solution was dropped into the mixed solution to obtain a coprecipitated hydroxide represented by the composition formula of Ni _1/3 Co _1/3 Mn _1/3 (OH) ₂ . This coprecipitated hydroxide, ZrO ₂ , and LiOH are mixed and fired in air at 1000 ° C. for 24 hours, thereby being represented by the composition formula of LiNi _1/3 Co _1/3 Mn _1/3 O _2. A positive electrode active material was obtained. Furthermore, the positive electrode active material powder was obtained by pulverizing the positive electrode active material. It was 0.25 mol% when content of Zr in this positive electrode active material was measured using ICP-AES (product name "iCAP6300", the product made by Thermo Fisher Scientific).

  A positive electrode active material, AB as a conductive additive, and a solution obtained by dissolving PVdF as a binder in NMP are mixed so that the positive electrode active material: AB: PVdF = 90: 5: 5 by mass ratio. Further, by kneading, a positive electrode mixture slurry was obtained.

  Next, the positive electrode mixture slurry was applied to both surfaces of an Al foil (thickness: 15 μm) as a current collector core material and dried to form a positive electrode mixture layer on the Al foil. Subsequently, the positive electrode mixture layer and the Al foil were rolled using a roll mill. Thus, a sheet-like positive electrode containing 7.84 g of the positive electrode active material was obtained.

(Preparation of negative electrode)
First, natural graphite was prepared as a negative electrode active material. Next, the negative electrode active material, CMC as the binder (thickening material), and SBR as the binder are mixed so that the negative electrode active material: CMC: SBR = 98: 1: 1 by mass ratio. A negative electrode mixture slurry was obtained by kneading in water.

  Next, the negative electrode mixture slurry was applied to both surfaces of a Cu foil (thickness 10 μm) as a current collector core material and dried to form a negative electrode mixture layer on the Cu foil. Subsequently, the negative electrode active material layer and the Cu foil were rolled using a roll mill. As described above, a negative electrode containing 5.32 g of a negative electrode active material was obtained.

(Nonaqueous electrolyte adjustment)
EC, DMC, and EMC were mixed at a volume ratio of EC: DMC: EMC = 3: 4: 3 to obtain an aprotic solvent. Next, LiPF ₆ (1.1 mol / L) was dissolved as a solute in the aprotic solvent to obtain a liquid nonaqueous electrolyte.

(assembly)
The positive electrode current collecting tab was welded to the positive electrode obtained as described above, and the negative electrode current collecting tab was welded to the negative electrode. Subsequently, the electrode body was obtained by winding up so that a positive electrode and a negative electrode might face each other on both sides of the polypropylene separator.

  Next, this electrode body was inserted into a bottomed outer can, the negative electrode current collecting tab was welded to a predetermined position on the bottom of the outer can, and the positive electrode current collecting tab was welded to the terminal portion of the cap. Subsequently, 8 g of nonaqueous electrolyte was injected from the opening of the outer can. Furthermore, a gasket was disposed between the cap and the outer can, and the opening of the outer can was bent (that is, caulked) toward the cap so as to compress the gasket, thereby sealing the opening.

  As described above, a cylindrical lithium ion secondary battery (standard size: 18650 (diameter: 18 mm, height: 65 mm), rated capacity: 1000 mAh) was obtained.

<Experiment 1: Evaluation Regarding Zr Content in Positive Electrode Active Material>
In Experiment 1, the influence of the Zr content in the positive electrode active material on the cycle durability was investigated. First, in the “preparation of positive electrode” described above, seven types of positive electrode active materials having different Zr contents were prepared. And 7 types of test batteries were obtained like the above except using those positive electrode active materials.

(Cycle durability test)
Next, a cycle durability test of each test battery was performed. That is, in the thermostat set to 60 degreeC, the charge / discharge cycle which makes charge and discharge of the following conditions 1 cycle (cyc) was performed 1000 cyc.
(1) Charging conditions CC charging (current value: 2 It), cut-off voltage: 4.1 V.
(2) Discharge conditions CC discharge (current value: 2 It), cut-off voltage: 3.0V.

  Here, “CC (Constant Current)” indicates “constant current”, and “CV (Constant Voltage)” indicates “constant voltage”. The unit of current value “It” indicates a current value for discharging the rated capacity of the battery in one hour.

(Battery capacity measurement conditions)
The battery capacity before and after the cycle durability test was measured under the following conditions. Then, the capacity retention rate (%) was calculated by dividing the battery capacity after the cycle endurance test (after 1000 cyc) by the battery capacity before the cycle endurance test (initial capacity). The ambient environment temperature at the time of measuring the capacity was 25 ° C. (room temperature).
(1) Charging conditions The battery was charged by CC-CV charging. That is, the battery was charged at a constant current of 1 It until the battery voltage reached 4.1 V (CC charging), and after reaching 4.1 V, charging was performed while maintaining the voltage of 4.1 V while the current was attenuated ( CV charging). The charging was terminated when the total charging time (CC charging time + CV charging time) reached 3 hours.
(2) Discharge condition After the charge was completed, the battery was left for 10 minutes and then discharged. The discharge was performed in two steps as follows. First, discharge was performed at a current value of 0.33 It as a first stage discharge until the battery voltage reached 3.0 V (CC discharge). Thereafter, the battery was allowed to stand for 10 minutes, and then discharged as a second stage discharge at a current value of 0.33 It until the battery voltage reached 3.0 V (CC discharge). The sum of the first stage discharge capacity and the second stage discharge capacity was defined as the battery capacity.

(Consideration of results)
The relationship between the Zr content of the positive electrode active material obtained as described above and the capacity retention rate of the battery is shown in FIG. FIG. 1 is a graph showing the relationship between the content of Zr in the positive electrode active material and the capacity retention rate. The horizontal axis of FIG. 1 shows the Zr content in the positive electrode active material used for each battery, and the vertical axis shows the capacity retention rate of the battery after the cycle durability test. As shown in FIG. 1, when the Zr content was between 0 and 0.25 mol%, it was confirmed that the capacity retention rate of the battery improved as the Zr content increased. And when content of Zr became 0.25 mol% or more, the effect was saturated and the tendency for a capacity | capacitance maintenance factor to remain substantially constant was confirmed. Therefore, from this result, it is considered that the Zr content of the positive electrode active material is required to be 0.25 mol% or more.

<Experiment 2: Evaluation of LiPOF Addition in Nonaqueous Electrolyte>
In Experiment 2, the influence of the added amount of LiPOF in the nonaqueous electrolyte on the cycle durability was evaluated. First, in “Preparation of positive electrode” described above, various positive electrode active materials having a Zr content shown in Table 1 were prepared. Next, in the “adjustment of non-aqueous electrolyte” described above, various non-aqueous electrolytes to which LiPOF was added in the amounts shown in Table 1 were prepared. These are combined as shown in Table 1 to obtain a test battery No. 1-12 were produced. Of these, No. 5-10 correspond to the lithium ion secondary battery of an Example, and other than that corresponds to the lithium ion secondary battery of a comparative example.

(Cycle durability test)
The cycle durability test of each test battery was performed in the same manner as described above. And the capacity | capacitance maintenance factor after 1000 cyc was computed. The results are shown in Table 1 and FIG. 2A.

(Measurement of resistance increase rate)
In Experiment 2, the rate of increase in reaction resistance was also evaluated along with the capacity maintenance rate. That is, the reaction resistance was measured before and after the cycle durability test, and the resistance increase rate was calculated by dividing the reaction resistance after the cycle durability test by the reaction resistance before the cycle durability test.

(Reaction resistance measurement conditions)
The reaction resistance of the battery was measured as follows. That is, after the charge amount of each test battery was adjusted to 40%, it was put into a thermostat set to −30 ° C. and AC impedance measurement was performed. For this measurement, a frequency response analyzer (FRA: Frequency Response Analyzer) (model “1255B”) sold by Toyo Technica Co., Ltd. and a potentio / galvanostat (model “1287A”) are combined. Used. Moreover, the measurement was started from the high frequency side, and the range of the measurement frequency was 100,000 to 0.001 Hz.

  The measurement result of the AC impedance obtained in this way was plotted on the coordinates with the real part of the frequency response on the horizontal axis and the imaginary part on the vertical axis, and a Nyquist diagram was created. And the real part of the impedance which makes the diameter of the semicircle drawn in this coordinate was considered as the reaction resistance of the test battery. Such calculation of reaction resistance can also be performed by measurement / analysis software (for example, “ZPlot”) attached to FRA (model “1255B”).

  The resistance increase rate obtained as described above is shown in Table 1 and FIG. 2B. In Table 1 and FIG. 2B, for example, a resistance increase rate of 1.11 indicates that the reaction resistance after the cycle endurance test is 1.11 times that of the initial reaction resistance.

(Consideration of results)
FIG. 2A is a graph showing the relationship between the amount of LiPOF added and the capacity retention rate of the battery. The horizontal axis of FIG. 2A shows the amount of LiPOF added to the nonaqueous electrolyte used in each battery, and the vertical axis shows the capacity retention rate of the battery. FIG. 2A also shows the results of using Zr alone and LiPOF alone for comparison.

  As shown in FIG. 2A, when Zr and LiPOF were used in combination, the capacity retention ratio was improved beyond the range expected from the case of using Zr alone and the case of using LiPOF alone. In particular, the effect is remarkable when the Zr content of the positive electrode active material is 0.25 mol% or more and the LiPOF addition amount to the nonaqueous electrolyte is 0.25 mass% or more and 1.00 mass% or less. It was. Further, it has been clarified that when the amount of LiPOF added to the non-aqueous electrolyte is in the range of 0.38% by mass or more and 0.75% by mass or less, a higher effect is obtained and the capacity retention rate is improved by about 8.3%. . The reason why such a result was obtained is considered to be that when Zr and LiPOF are used in combination, they act synergistically on the capacity retention rate of the battery. That is, LiPOF not only improves the capacity retention rate by forming SEI on the negative electrode surface, but also forms a protective film on the positive electrode surface to suppress elution of Zr from the positive electrode. Then, it is considered that the cycle durability is remarkably improved by the combination of the non-aqueous electrolyte decomposition suppression effect by SEI in the negative electrode and the non-aqueous electrolyte decomposition suppression effect by Zr in the positive electrode.

  On the other hand, FIG. 2B is a graph showing the relationship between the resistance increase rate after the cycle endurance test and the amount of LiPOF added. The horizontal axis of FIG. 2B shows the amount of LiPOF added to the nonaqueous electrolyte used in each battery, and the vertical axis shows the rate of increase in resistance after the cycle durability test. As can be seen from Table 1, FIG. 2A, and FIG. 2B, a high correlation can be confirmed between the resistance increase rate shown in FIG. 2B and the capacity retention rate shown in FIG. 2A. That is, the resistance increase rate is remarkably high when the Zr content of the positive electrode active material is 0.25 mol% or more and the LiPOF addition amount to the non-aqueous electrolyte is 0.25 mass% or more and 1.00 mass% or less. Further, the rate of increase in resistance is further reduced when the amount of LiPOF added to the nonaqueous electrolyte is in the range of 0.38 mass% to 0.75 mass%. The reason why such a result was obtained is thought to be that the amount of Zr elution from the positive electrode was reduced corresponding to the amount of LiPOF added, and the effect of suppressing the increase in reaction resistance by Zr was sustained over a long period of time. .

<Experiment 3: Evaluation of Additive Type to Nonaqueous Electrolyte>
In Experiment 3, cycle durability was evaluated when an additive other than LiPOF and Zr addition to the positive electrode active material were used in combination. In Experiment 3, LiBOB, LiBF ₄ and PS were employed as additives considered to act on the positive electrode in the same manner as LiPOF. In the “adjustment of non-aqueous electrolyte”, various non-aqueous electrolytes were prepared in the same manner as above except that 0.5% by mass of each additive was added to the non-aqueous electrolyte. Next, as shown in FIG. 3, a positive electrode active material not containing Zr and a positive electrode active material containing 0.25 mol% of Zr were prepared, and various positive electrodes were obtained using these.

  As shown in FIG. 3, various test batteries were prepared by combining various nonaqueous electrolytes and various positive electrodes. The cycle durability test of the various test batteries thus obtained was performed in the same manner as described above, and the capacity retention rate was calculated. The results are shown in FIG.

(Consideration of results)
FIG. 3 is a graph showing the capacity retention ratio after a cycle durability test in the case of combining a positive electrode active material containing 0.25 mol% of Zr and a non-aqueous electrolyte containing various additives. Among the combinations shown in FIG. 3, “Zr: 0.25 mol% + LiPF ₂ (C ₂ O ₄ ) ₂ : 0.5 mass%” corresponds to the lithium ion secondary battery of the example. FIG. 3 also shows the results when Zr is used alone and LiPOF is used alone as a comparison.

As is clear from FIG. 3, a significantly high capacity retention rate was confirmed when Zr addition to the positive electrode active material and LiPOF addition to the non-aqueous electrolyte were used in combination. On the other hand, all of the test batteries to which LiBOB, LiBF ₄ and PS were added did not reach the capacity maintenance rate of the test battery to which LiPOF was added, and these three types of capacity maintenance rates were almost the same. From these results, it is presumed that the protective film formed on the positive electrode by LiPOF is a film having a particularly high elution suppressing effect on Zr.

<Experiment 4: Evaluation of Zr Elution Amount>
In Experiment 4, the Zr elution suppression effect by LiPOF was verified. First, a plurality of test batteries according to Examples using a positive electrode active material containing 0.25 mol% of Zr and a nonaqueous electrolyte containing 0.50 mass% of LiPOF were produced in the same manner as described above. In addition, a plurality of test batteries according to comparative examples using a positive electrode active material containing 0.25 mol% of Zr and a non-aqueous electrolyte not containing LiPOF were produced.

  These batteries were subjected to a cycle durability test in the same manner as described above, and the batteries were disassembled after the elapse of 0 cyc, 300 cyc, 500 cyc, and 1000 cyc, and the negative electrode was collected. Then, the negative electrode mixture was scraped from the collected negative electrode, and the amount of Zr contained in the negative electrode mixture was measured by the ICP-AES. The result is shown in FIG.

(Consideration of results)
FIG. 4 is a graph showing the relationship between the number of charge / discharge cycles performed on the test battery and the amount of Zr in the negative electrode mixture. When Zr is eluted from the positive electrode active material, it is considered that the eluted Zr moves to the negative electrode side by electrophoresis and precipitates on the negative electrode surface. Therefore, the amount of Zr in the negative electrode mixture in Experiment 4 can be regarded as the amount of Zr eluted from the positive electrode active material to the nonaqueous electrolyte.

  As is clear from FIG. 4, in the test battery of the comparative example that does not contain LiPOF, the amount of Zr detected from the negative electrode increases with the number of cycles. On the other hand, in the test battery of the example containing 0.50% by mass of LiPOF, the amount of Zr slightly increased after 300 cyc compared to the initial (0 cyc), but thereafter the amount of Zr remained almost flat. . Comparing the example after the elapse of 1000 cyc and the comparative example, it can be seen that the amount of Zr is suppressed to ¼ or less in the example. From this result, it is clear that LiPOF suppresses the elution of Zr from the positive electrode active material.

From the results of the above Experiments 1 to 4, a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte are included, and the positive electrode active material includes 0.25 mol% or more of zirconium (Zr). The lithium ion secondary battery in which lithium difluorobisoxalate phosphate (LiPF ₂ (C ₂ O ₄ ) ₂ ) is added to the non-aqueous electrolyte in the range of 0.25 mass% to 1.00 mass%, It was confirmed that the cycle durability was superior to that of the prior art.

  The lithium ion secondary battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte. The positive electrode active material includes 0.25 mol% or more of zirconium, and the positive electrode and the positive electrode The negative electrode has a coating film containing a substance derived from lithium difluorobisoxalate phosphate on its surface.

  Although the embodiments and examples of the present invention have been described as described above, it is also planned from the beginning to appropriately combine the configurations of the above-described embodiments and examples.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

Claims (1)

A positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte,
The positive electrode active material contains 0.25 mol% or more of zirconium,
A lithium ion secondary battery in which lithium difluorobisoxalate phosphate is added to the nonaqueous electrolyte in a range of 0.25 mass% to 1.00 mass%.

Images