Non-aqueous electrolyte power storage element and production method for same

Abstract

A non-aqueous electrolyte power storage element according to one aspect of the present invention comprises: a positive electrode that contains positive electrode active material particles that include a compound that has an olivine crystal structure; and a non-aqueous electrolyte that contains a salt that includes an oxalate complex anion. The D50 particle diameter of the positive electrode active material particles is no more than 3 μm, and the BET specific surface area of the positive electrode active material particles is no more than 10 m2 / g.

Description

Nonaqueous electrolyte storage element and method for manufacturing same

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element and a method for producing the same.

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring charge-transporting ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte energy storage elements.

[0003] Compounds having an olivine crystal structure, such as lithium iron phosphate, are known as positive electrode active materials used in non-aqueous electrolyte storage elements. Patent Document 1 describes a non-aqueous electrolyte secondary battery including a positive electrode containing lithium iron phosphate as the positive electrode active material and a negative electrode containing graphite as the negative electrode active material.

[0004] Japanese Patent Application Laid-Open No. 2007-213961

[0005] It is desirable for non-aqueous electrolyte storage elements to have low resistance in terms of output performance, etc. In particular, non-aqueous electrolyte storage elements that use lithium iron phosphate as the positive electrode active material and graphite as the negative electrode active material have an operating voltage equivalent to that of lead-acid batteries, and are therefore sometimes used as replacements for automotive lead-acid batteries. For non-aqueous electrolyte storage elements that replace automotive lead-acid batteries, a high CCA (Cold Cranking Ampere) value is important, and therefore reduced resistance is desirable.

[0006] An object of the present invention is to provide a nonaqueous electrolyte electricity storage element that uses a compound having an olivine-type crystal structure and has low resistance, and a method for producing such a nonaqueous electrolyte electricity storage element.

[0007] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode containing positive electrode active material particles including a compound having an olivine crystal structure, and a non-aqueous electrolyte containing a salt having an oxalato complex anion, wherein the positive electrode active material particles have a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２ / g or less.

[0008] A method for producing a non-aqueous electrolyte storage element according to another aspect of the present invention includes crushing secondary particles of a positive electrode active material containing a compound having an olivine crystal structure to produce a positive electrode active material having a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２ / g or less, preparing a positive electrode containing the positive electrode active material particles, and preparing a non-aqueous electrolyte containing a salt having an oxalato complex anion.

[0009] According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte storage element that uses a compound having an olivine-type crystal structure and has low resistance, and a method for manufacturing such a nonaqueous electrolyte storage element.

[0010] Fig. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element, and Fig. 2 is a schematic view showing one embodiment of an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements.

[0011] First, an outline of the nonaqueous electrolyte electricity storage element and the method for manufacturing the same disclosed in this specification will be described.

[0012] [1] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode containing positive electrode active material particles including a compound having an olivine crystal structure, and a nonaqueous electrolyte containing a salt having an oxalate complex anion, wherein the positive electrode active material particles have a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２ / g or less.

[0013] The nonaqueous electrolyte storage element described in [1] above uses a compound having an olivine crystal structure and exhibits low resistance. While the reason for this is unclear, the following reasons are presumed. Generally, charge-transporting ions (such as lithium ions) in compounds having an olivine crystal structure migrate only along the crystal axes of the olivine crystal structure. Therefore, when positive electrode active material particles are in the form of secondary particles formed by agglomeration of numerous primary particles, the linear migration distance of the charge-transporting ions is shortened, which is thought to increase ion transport resistance. Therefore, it is desirable to use a compound having an olivine crystal structure in a form as close to primary particles as possible (i.e., each particle is formed from as few primary particles as possible). Here, it is thought that there is a negative correlation between the primary particle size and BET specific surface area of ​​positive electrode active material particles. Therefore, a small D50 particle size and a small BET specific surface area of ​​positive electrode active material particles means that the secondary particle size is small, the primary particle size is large, and each particle (usually a secondary particle) is formed from a relatively small number of primary particles. In the nonaqueous electrolyte storage element described in [1] above, the positive electrode active material particles have a D50 particle size of 3 μm or less, and a BET specific surface area of ​​10 m ２ / g or less, each particle is formed from a relatively small number of primary particles, which is thought to reduce ion transport resistance. Furthermore, in the nonaqueous electrolyte storage element described in [1] above, the nonaqueous electrolyte contains a salt having an oxalate complex anion. In the nonaqueous electrolyte storage element described in [1] above, the formation of a good coating derived from the salt on the surface of the positive electrode active material particles is thought to further reduce resistance. In particular, positive electrode active material particles containing a compound having an olivine-type crystal structure are generally coated on the surface with a carbon material to enhance electronic conductivity. Furthermore, positive electrode active material particles formed from a relatively small number of primary particles can be suitably obtained by a crushing process, as described in detail below. However, when positive electrode active material particles are formed by crushing, portions of the compound having an olivine-type crystal structure that are not coated with the carbon material are likely to be exposed. Therefore, it is thought that the resistance is reduced by covering these exposed portions with a coating derived from the salt. As described above, in the nonaqueous electrolyte storage element described in [1] above, the positive electrode active material particles have a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２ / g or less, the ion transport resistance of the positive electrode active material particles is reduced, and a good coating film derived from a salt having an oxalato complex anion is formed on the surface of the positive electrode active material particles, thereby reducing the resistance.

[0014] The salt contained in the non-aqueous electrolyte is qualitatively and quantitatively analyzed by ion chromatography (IC). The IC analysis is performed using a Thermo Fisher Scientific "Dionex ICS-5000+" analyzer. Water is used as the eluent. Specifically, the analysis is performed as follows. A series of measurements are performed continuously under the same conditions. First, the non-aqueous electrolyte storage element is disassembled to remove the non-aqueous electrolyte. If the non-aqueous electrolyte cannot be removed, the non-aqueous electrolyte storage element is centrifuged to remove the non-aqueous electrolyte. If the non-aqueous electrolyte cannot be removed even after centrifugation, an appropriate solvent (e.g., acetonitrile) is injected into the non-aqueous electrolyte storage element, and the non-aqueous electrolyte diluted with the extraction solvent is removed. The non-aqueous electrolyte may be completely dissolved using an appropriate solvent (e.g., acetonitrile). (Qualitative Analysis) The measurement sample (non-aqueous electrolyte) is subjected to IC analysis. The components contained in the measurement sample are predicted from the peak positions of each peak in the obtained ion chromatogram. A known sample of the predicted component (hereinafter referred to as "predicted component") is subjected to IC analysis. The retention time of the peak corresponding to the predicted component in each peak of the measured sample is compared with the retention time of the known sample, and if they match, the prediction is assumed to be correct. (Quantitative analysis) Quantitative analysis is performed using the calibration curve method. First, a known sample of the predicted component with a known concentration is subjected to IC measurement, and the peak area is determined to create a calibration curve. The calibration curve is calculated using the coefficient of determination (r ２ The calibration curve is created so that the difference (ratio) is between 0.99 and 1. The content of the predicted component in the measurement sample is determined from the calibration curve and the peak area of ​​the predicted component in the measurement sample. The above procedure is performed for all peaks detected in the IC analysis of the measurement sample, and the content of each predicted component is determined.

[0015] The "D50 particle size" (the same applies to the "D90 particle size" and "D10 particle size" described below) and "BET specific surface area" of the positive electrode active material particles are values ​​measured on positive electrode active material particles removed from a nonaqueous electrolyte storage element and treated according to the following procedure. First, the nonaqueous electrolyte storage element is discharged at a constant current of 0.1 C to the discharge cut-off voltage during normal use. Here, "normal use" refers to the use of the nonaqueous electrolyte storage element under the charge / discharge conditions recommended or specified for the nonaqueous electrolyte storage element. The nonaqueous electrolyte storage element in this state is disassembled, the positive electrode is removed, and components (such as the nonaqueous electrolyte) adhering to the positive electrode are thoroughly washed with dimethyl carbonate. The positive electrode is then dried under reduced pressure at room temperature for 24 hours. Next, a powder of the positive electrode active material layer is collected from the positive electrode. After removing optional components such as conductive agents mixed in the powder of the positive electrode active material layer using air classification or the like, the resulting mixture is washed and filtered using a solvent in which the binder and thickener are soluble, thereby removing the binder and thickener, thereby obtaining positive electrode active material particles. Finally, the obtained positive electrode active material particles are heated and dried to obtain a sample to be measured. The heating temperature may be any temperature that can remove the solvent, and can be, for example, 60°C to 100°C. The process from disassembly of the nonaqueous electrolyte storage element to collection of the sample to be measured can be carried out in an argon atmosphere with a dew point of −60°C or lower, as necessary.

[0016] The "D50 particle size" of the positive electrode active material particles is the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by a laser diffraction / scattering method for a diluted solution obtained by diluting the particles with a solvent in accordance with JIS-Z-8825 (2013). The "D90 particle size" of the positive electrode active material particles is the value at which the volume-based cumulative distribution calculated in the same manner is 90%. The "D10 particle size" of the positive electrode active material particles is the value at which the volume-based cumulative distribution calculated in the same manner is 10%.

[0017] The "BET specific surface area" of the positive electrode active material particles is a value obtained by measurement according to the following procedure. 1.00 g of the positive electrode active material particles is placed in a measurement sample tube and dried under reduced pressure at 120°C for 12 hours. Next, an adsorption isotherm is measured by a nitrogen gas adsorption method using liquid nitrogen within a relative pressure P / P0 (P0 = approximately 770 mmHg) range of 0 to 1. Five points are extracted from the region of P / P0 = 0.05 to 0.3 of the obtained adsorption isotherm, and a BET plot is performed. The BET specific surface area is calculated from the y-intercept and slope of the line.

[0018] [2] In the nonaqueous electrolyte storage element according to [1] above, the positive electrode active material particles may have a D90 particle size of 8 μm or less.

[0019] The nonaqueous electrolyte storage element described in [2] above has a lower resistance because it contains fewer positive electrode active material particles with large secondary particle diameters formed from a large number of primary particles.

[0020] [3] In the nonaqueous electrolyte storage element according to the above [1] or [2], the positive electrode active material particles may contain a carbon material present on at least a portion of the surface thereof.

[0021] The nonaqueous electrolyte storage element described in the above [3] has lower resistance because the electron conductivity of the positive electrode active material particles is increased by the carbon material.

[0022] [4] In the nonaqueous electrolyte storage element according to any one of [1] to [3] above, the compound having an olivine crystal structure may be lithium iron phosphate.

[0023] The nonaqueous electrolyte electricity storage element described in [4] above is a preferred embodiment of the present invention.

[0024] [5] In the nonaqueous electrolyte storage element according to any one of [1] to [4] above, the salt having an oxalato complex anion may be lithium difluorobis(oxalato)phosphate.

[0025] The nonaqueous electrolyte electricity storage element described in [5] above is a preferred embodiment of the present invention.

[0026] [6] In the nonaqueous electrolyte storage element according to any one of [1] to [5] above, the positive electrode active material particles may be crushed.

[0027] [7] In the nonaqueous electrolyte storage element according to any one of [1] to [6] above, the nonaqueous electrolyte may further contain at least one selected from the group consisting of unsaturated cyclic carbonates and imide salts.

[0028] [8] In the nonaqueous electrolyte storage element according to any one of [1] to [6] above, the nonaqueous electrolyte may further contain at least one selected from the group consisting of vinylene carbonate and lithium difluorophosphonyl fluorosulfonylimide.

[0029] [9] The nonaqueous electrolyte storage element according to any one of [1] to [8] above may further include a negative electrode containing a carbon material.

[0030]

[10] The nonaqueous electrolyte storage element according to any one of [1] to [8] above may further include a negative electrode containing graphite.

[0031] The nonaqueous electrolyte storage elements described in the above [6] to

[10] are also suitable embodiments of the present invention.

[0032]

[11] A method for producing a nonaqueous electrolyte storage element according to another aspect of the present invention includes crushing secondary particles of a positive electrode active material containing a compound having an olivine crystal structure, thereby producing a positive electrode active material having a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２ / g or less, preparing a positive electrode containing the positive electrode active material particles, and preparing a non-aqueous electrolyte containing a salt having an oxalato complex anion.

[0033] According to the method for producing a nonaqueous electrolyte storage element described in

[11] above, it is possible to obtain a nonaqueous electrolyte storage element that uses a compound having an olivine crystal structure and has low resistance.

[0034] A nonaqueous electrolyte electricity storage element, an electricity storage device, a method for manufacturing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, and other embodiments will be described in detail below. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0035] <Non-aqueous electrolyte storage element> A non-aqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container that accommodates the electrode assembly and the non-aqueous electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. At least a portion of the non-aqueous electrolyte exists in a state in which it permeates the voids between the positive electrode, the negative electrode, and the separator. As an example of a non-aqueous electrolyte storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0036] (Positive Electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0037] The positive electrode substrate has electrical conductivity. Whether or not it has electrical conductivity is determined by whether or not the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 －２ The resistance is determined using Ω cm as a threshold value. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0038] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density of the nonaqueous electrolyte storage element.

[0039] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0040] The positive electrode active material layer contains positive electrode active material particles and, as needed, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0041] The positive electrode active material particles contain a compound having an olivine-type crystal structure. The compound having an olivine-type crystal structure has a crystal structure that can be assigned to the space group Pnma. The crystal structure that can be assigned to the space group Pnma means that the compound has a peak that can be assigned to the space group Pnma in an X-ray diffraction diagram.

[0042] The compound having an olivine-type crystal structure may be a compound containing a lithium element and a transition metal element. The compound having an olivine-type crystal structure may be a compound containing an oxo acid anion (PO ４ ３－ , S.O. ４ ２－ , SiO ４ ４－ , B.O. ３ ３－ , V.O. ４ ３－ Examples of suitable oxo acid anions include compounds containing lithium ions, lithium ions, and transition metal ions. Compounds having an olivine crystal structure may further contain other elements (e.g., halogen elements). Examples of suitable transition metal elements contained in compounds having an olivine crystal structure include iron, manganese, nickel, and cobalt, with iron being more preferred. Examples of suitable oxo acid anions contained in compounds having an olivine crystal structure include phosphate anions (PO ４ ３－) is preferred.

[0043] The compound having an olivine-type crystal structure is preferably a compound represented by the following formula (1): Li ａ M ｂ (A.O. ｃ ) ｄ X ｅ ... (1) In formula (1), M is at least one transition metal element. A is at least one selected from B, Al, Si, P, S, Cl, Ti, V, Cr, Mo, and W. X is at least one halogen element. a, b, c, d, and e are numbers satisfying 0<a≦3, 0<b≦2, 2≦c≦4, 1≦d≦3, and 0≦e≦1. a, b, c, d, and e may all be integers or decimals.

[0044] As M in formula (1), any one of Fe, Mn, Ni, and Co, or a combination of any two of them, is preferable. As M, Fe, Mn, or a combination thereof is more preferable, and Fe is more preferable. Furthermore, the content of Fe in M ​​is preferably 50 mol% or more, more preferably 70 mol% or more, 90 mol% or more, or 99 mol% or more. As A, P is preferable. As X, F is preferable. In one embodiment, a = 1, b = 1, c = 4, d = 1, and e = 0 may be preferable.

[0045] Specific examples of compounds having an olivine crystal structure include LiFePO ４ , LiCoPO ４ , LiFe ｘ Co １－ｘ P.O. ４ (0<x<1), LiMnPO ４ , LiNiPO ４ , LiFe ｘ Mn １－ｘ P.O. ４ (0<x<1), LiCrPO ４ , LiFeVO ４ , Li ２ FeSiO ４ , Li ２ Fe ２ (SO ４ ) ３ , LiFeBO３ , LiFePO ３．９ F ０．２ , Li ２ MnSiO ４ , Li ２ CoPO ４ F, etc. The atoms or polyanions in these compounds having an olivine-type crystal structure may be partially substituted with other atoms or anion species. Examples of compounds having an olivine-type crystal structure include lithium iron phosphate (LiFePO ４ ) is preferred. The lithium iron phosphate may be one in which some of the atoms or polyanions constituting the lithium iron phosphate are substituted with other atoms or anion species. The compound having an olivine crystal structure may be used alone or in combination of two or more.

[0046] The content of the compound having an olivine crystal structure in the positive electrode active material particles is, for example, preferably 80% by mass or more and 99.9% by mass or less, and more preferably 90% by mass or more and 99% by mass or less.

[0047] Positive electrode active material particles containing a compound having an olivine crystal structure are usually secondary particles formed by agglomeration of primary particles of the compound having an olivine crystal structure. The positive electrode active material particles may be single particles in which multiple primary particles exist independently without agglomeration, or may be a mixture of single particles and secondary particles. Note that "primary particles" refer to particles in which no grain boundaries are observed externally when observed with a scanning electron microscope (SEM). "Secondary particles" refer to particles formed by agglomeration of multiple primary particles.

[0048] The positive electrode active material particles preferably contain a carbon material present on at least a portion of their surfaces. A portion of the carbon material may be present inside (between primary particles of) secondary particles of the compound having an olivine crystal structure. The positive electrode active material particles may have portions that are not covered with the carbon material (typically, portions where the compound having a polyanion structure is exposed). The presence of a carbon material on at least a portion of the surface of the positive electrode active material particles containing the compound having a polyanion structure allows the positive electrode active material particles to exhibit sufficient electron conductivity between particles.

[0049] A carbon material refers to a material containing carbon as the main constituent element. A main constituent element refers to an element that is contained in the largest amount by mass. For example, the carbon content in the carbon material may be 80% by mass or more, 90% by mass or more, 95% by mass, 99% by mass, or 99.9% by mass or more. The carbon material is preferably a carbon material other than an uncarbonized polymer compound. Examples of elements other than carbon that may be contained in the carbon material include oxygen, hydrogen, and nitrogen. Examples of carbon materials include graphite and non-graphitic carbon.

[0050] The content of the carbon material in the positive electrode active material particles is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.2% by mass or more and 10% by mass or less, and even more preferably 0.3% by mass or more and 5% by mass or less. When the content of the carbon material in the positive electrode active material particles is equal to or greater than the lower limit, it is possible to improve electronic conductivity, etc. When the content of the carbon material in the positive electrode active material particles is equal to or less than the upper limit, it is possible to increase the content of the compound having a polyanion structure, and it is possible to increase, for example, the discharge capacity per volume of the positive electrode active material layer, etc.

[0051] The total content of the compound having a polyanion structure and the carbon material in the positive electrode active material particles is preferably 90% by mass or more and 100% by mass or less, and may be 95% by mass or more, 98% by mass or more, 99% by mass or more, or 99.9% by mass or more.

[0052] The upper limit of the D50 particle size of the positive electrode active material particles is 3 μm, preferably 2.5 μm, more preferably 2.0 μm, and even more preferably 1.5 μm. When the D50 particle size of the positive electrode active material particles is equal to or less than the above upper limit, the resistance can be reduced. The lower limit of the D50 particle size is preferably 0.1 μm, more preferably 0.3 μm, and even more preferably 0.7 μm. When the D50 particle size of the positive electrode active material particles is equal to or greater than the above lower limit, the positive electrode active material particles can be easily manufactured or handled. The D50 particle size of the positive electrode active material particles may be within a range that combines any of the above lower limits and any of the above upper limits.

[0053] The upper limit of the D90 particle size of the positive electrode active material particles is preferably 8 μm, more preferably 6 μm, and even more preferably 4 μm. When the D90 particle size of the positive electrode active material particles is equal to or less than the above upper limit, the resistance can be further reduced. The lower limit of the D90 particle size is preferably 0.5 μm, more preferably 1 μm, and even more preferably 2 μm. When the D90 particle size of the positive electrode active material particles is equal to or greater than the above lower limit, the positive electrode active material particles can be easily manufactured or handled. The D90 particle size of the positive electrode active material particles may be within a range that combines any of the above lower limits and any of the above upper limits.

[0054] The upper limit of the D10 particle size of the positive electrode active material particles is preferably 2 μm, more preferably 1.5 μm, and even more preferably 1.0 μm. When the D10 particle size of the positive electrode active material particles is equal to or less than the above upper limit, the resistance can be further reduced. The lower limit of the D10 particle size is preferably 0.05 μm, more preferably 0.1 μm, and even more preferably 0.2 μm. When the D10 particle size of the positive electrode active material particles is equal to or greater than the above lower limit, the positive electrode active material particles can be easily manufactured or handled. The D10 particle size of the positive electrode active material particles may be within a range that combines any of the above lower limits and any of the above upper limits.

[0055] In order to obtain positive electrode active material particles with a predetermined particle size (such as D50 particle size), a crusher (pulverizer), a classifier, or the like is used. In particular, as will be described in detail later, by crushing secondary particles of a positive electrode active material containing a compound having an olivine crystal structure, positive electrode active material particles with a D50 particle size of 3 μm or less can be effectively obtained. In other words, the positive electrode active material particles are preferably crushed.

[0056] The upper limit of the BET specific surface area of ​​the positive electrode active material particles is 10 m ２ / g, and 9m ２ / g is preferred, and 8m ２ When the BET specific surface area of ​​the positive electrode active material particles is equal to or less than the upper limit and the D50 particle size of the positive electrode active material particles is equal to or less than the upper limit, each positive electrode active material particle is formed from a relatively small number of primary particles, and ion transport resistance is reduced. The lower limit of the BET specific surface area is 3 m ２ / g is preferred, and 5m ２ / g is more preferred, and 7m ２ / g is even more preferable. When the BET specific surface area of ​​the positive electrode active material particles is equal to or greater than the above lower limit, the contact area between the positive electrode active material particles and the non-aqueous electrolyte increases, and output performance and the like tend to be improved. The BET specific surface area of ​​the positive electrode active material particles may be within a range that combines any of the above lower limits and any of the above upper limits.

[0057] The positive electrode active material particles containing a compound having an olivine-type crystal structure can be produced, for example, by the following method: ２ P.O. ４ and a lithium salt of a phosphate such as ammonium hydroxide, etc., and heated to obtain a compound having an olivine-type crystal structure. Examples of the precursor include hydroxides of transition metal elements (hydroxide precursors), sulfate compounds of transition metal elements, and phosphate compounds of transition metal elements. The hydroxide precursor can be obtained, for example, by a precipitation reaction between transition metal ions and hydroxide ions in water.

[0058] The obtained compound having an olivine-type crystal structure and a carbon source are mixed in water to form a slurry. The slurry is pulverized using a bead mill or the like and dried. The primary particle size of the obtained positive electrode active material particles can be adjusted by adjusting the pulverization conditions. For example, the primary particle size of the positive electrode active material particles tends to become smaller by reducing the diameter of the beads used in the bead mill or by lengthening the processing time. The dried mixture of the compound having an olivine-type crystal structure and the carbon source is fired under a reducing atmosphere or an inert atmosphere to obtain particles in which secondary particles of the positive electrode active material containing the compound having an olivine-type crystal structure are coated with a carbon material. Alternatively, a precursor and LiH ２ P.O. ４ By mixing a lithium salt of phosphate such as ammonium phosphate with a carbon source and calcining under a reducing or inert atmosphere, secondary particles of a positive electrode active material containing a compound having an olivine crystal structure can be obtained, in which the secondary particles are coated with a carbon material. Organic substances such as sucrose, lactose, maltose, sucrose, polyvinyl alcohol, and ascorbic acid can be used as the carbon source. The calcination temperature is preferably in the range of 500°C to 1000°C. By setting the calcination temperature at 500°C or higher, the carbonization of the carbon source is particularly sufficient, resulting in positive electrode active material particles with sufficient electronic conductivity. By setting the calcination temperature at 1000°C or lower, the sublimation of lithium and other elements in the compound having an olivine crystal structure is suppressed, thereby preventing deviations in the elemental composition ratio and suppressing particle growth, thereby improving charge / discharge performance. Furthermore, when producing a positive electrode active material that does not contain a carbon material, the mixing with the carbon source can be omitted.

[0059] The resulting particles can be crushed using a jet mill or the like to obtain positive electrode active material particles containing a compound having an olivine crystal structure. The degree of crushing can adjust the D50 particle size and other properties of the resulting positive electrode active material particles. Furthermore, the BET specific surface area of ​​the resulting positive electrode active material particles can be adjusted by adjusting the firing temperature, firing time, and other parameters. For example, increasing the firing temperature or lengthening the firing time tends to increase the primary particle size of the resulting positive electrode active material particles and decrease the BET specific surface area. Additionally, the BET specific surface area of ​​the resulting positive electrode active material particles can be adjusted by adjusting the precursor production conditions (e.g., pH during the precipitation reaction).

[0060] The content of the positive electrode active material particles in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material particles within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0061] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbon materials, metals, and conductive ceramics. Examples of carbon materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes, and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and carbon nanotubes may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0062] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the nonaqueous electrolyte storage element can be increased. Note that the carbon material contained in the positive electrode active material particles is not considered to be a conductive agent.

[0063] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0064] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 9% by mass, so that the positive electrode active material particles can be stably held.

[0065] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. In one embodiment of the present invention, the positive electrode active material layer may not contain a thickener.

[0066] The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof. In one embodiment of the present invention, the positive electrode active material layer may not contain a filler.

[0067] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material particles, conductive agent, binder, thickener, and filler.

[0068] The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0069] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0070] The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate and to increase the energy density of the nonaqueous electrolyte storage element.

[0071] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0072] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0073] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for a lithium ion secondary battery. Examples of the negative electrode active material include metallic lithium; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; Li ４ Ti ５ O １２ , LiTiO ２、 TiNb ２ O ７ Examples of the material include titanium-containing oxides such as titanium dioxide, polyphosphate compounds, silicon carbide, and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, carbon materials are preferred, graphite or non-graphitic carbon is more preferred, and graphite is even more preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0074] "Graphite" refers to a graphite material that has an average lattice spacing (d ００２ ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0075] "Non-graphitic carbon" refers to a carbon material that has an average lattice spacing (d ００２ ) is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitizable carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitizable carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0076] Here, the "discharged state" of the carbon material refers to a state in which the carbon material, which is the negative electrode active material, is discharged so that lithium ions that can be absorbed and released during charging and discharging are sufficiently released from the carbon material. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic lithium as the counter electrode is 0.7 V or higher.

[0077] "Non-graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less.

[0078] "Graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having a particle size of 0.34 nm or more and less than 0.36 nm.

[0079] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the production or handling of the negative electrode active material becomes easier. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the negative electrode active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metallic lithium, the negative electrode active material layer may be in the form of a foil.

[0080] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0081] When the negative electrode active material layer contains a conductive agent, the content of the conductive agent in the negative electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. The content of the conductive agent in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. In one embodiment of the present invention, the negative electrode active material layer may not contain a conductive agent.

[0082] When the negative electrode active material layer contains a binder, the content of the binder in the negative electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 8% by mass.

[0083] When the negative electrode active material layer contains a thickener, the content of the thickener in the negative electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 0.5% by mass to 8% by mass, and may be 5% by mass or less, or may be 2% by mass or less.

[0084] When the negative electrode active material layer contains a filler, the content of the filler in the negative electrode active material layer can be 0.1 mass % or more and 8 mass % or less, and usually 5 mass % or less is preferable, and 2 mass % or less is more preferable. In one embodiment of the present invention, the negative electrode active material layer does not necessarily contain a filler.

[0085] (Separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0086] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C under an air atmosphere at 1 atmosphere, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of safety of the nonaqueous electrolyte storage element.

[0087] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0088] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0089] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a salt having an oxalate complex anion (hereinafter also referred to as "salt X"). The non-aqueous electrolyte usually further contains a salt other than salt X as an electrolyte salt. Salt X may function as the electrolyte salt. A non-aqueous electrolyte solution may be used as the non-aqueous electrolyte. The non-aqueous electrolyte solution contains a non-aqueous solvent, in which salt X and an electrolyte salt are dissolved.

[0090] The salt X is composed of an oxalato complex anion and a counter cation. The oxalato complex anion contains at least one oxalate ion (C ２ O ４ ２－ ) is the anion of a complex formed by a coordinate bond with a central element. Examples of the central element include nonmetallic elements such as boron, phosphorus, and silicon, with phosphorus being preferred. Examples of the counter cation include alkali metal ions such as lithium ion, sodium ion, and potassium ion, with lithium ion being preferred. In other words, salt X is preferably a lithium salt. Salt X may be used singly or in combination of two or more.

[0091] The salt X is a compound having a boron (B) element as a central element and at least one oxalate ion (C ２ O ４ ２－ salts having a tetracoordinated oxalato complex anion in which lithium bis(oxalato)borate (Li[B(C ２ O ４ ) ２ ]; LiBOB), lithium difluorooxalatoborate (Li[BF ２ (C ２ O ４ )]; LiFOB), lithium bis(trifluoroethoxy)oxalatoborate (Li[B(CF ３ CH ２ O) ２ (C ２ O ４ ) )]), a compound having phosphorus (P) as the central element and at least one oxalate ion (C ２ O ４ ２－), for example, lithium tris(oxalato)phosphate (Li[P(C ２ O ４ ) ３ ]), lithium difluorobis(oxalato)phosphate (Li[PF ２ (C ２ O ４ ) ２ ]; LiFOP), lithium tetrafluorooxalatophosphate (Li[PF ４ (C ２ O ４ Among these, from the viewpoint of forming a better coating, phosphorus (P) as the central element and at least one oxalate ion (C ２ O ４ ２－ ) is preferably a salt having a hexacoordinated oxalato complex anion, and LiFOP is more preferred.

[0092] The content of salt X in the nonaqueous electrolyte is preferably 0.01% by mass to 5% by mass, more preferably 0.05% by mass to 3% by mass, even more preferably 0.1% by mass to 2% by mass, and even more preferably 0.2% by mass to 1% by mass. By having the content of salt X in the above range, the resistance of the nonaqueous electrolyte storage element can be further reduced.

[0093] The electrolyte salt (salt other than salt X) can be appropriately selected from known electrolyte salts. Examples of the electrolyte salt include lithium salt, sodium salt, potassium salt, magnesium salt, and onium salt. Among these, lithium salt is preferred.

[0094] The lithium salt is LiPF ６ , LiPO ２ F ２ , LiBF ４ , LiClO ４ , LiN(SO ２ F) ２ Inorganic lithium salts such as LiSO ３ CF ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) (SO ２ C ４ F ９ ), LiC(SO ２ CF ３ ) ３ , LiC(SO ２ C ２ F ５ ) ３ Among these, inorganic lithium salts are preferred, and LiPF ６ is more preferred.

[0095] The content of the electrolyte salt in the non-aqueous electrolyte or non-aqueous electrolytic solution is 0.1 mol / dm ３ 2.5mol / dm or more ３ It is preferably 0.3 mol / dm or less. ３ 2.0mol / dm or more ３ It is more preferable that it is 0.5 mol / dm or less. ３ More than 1.7mol / dm ３ It is more preferable that it is 0.7 mol / dm or less. ３ 1.5mol / dm or more ３ It is particularly preferable that the content of the electrolyte salt is within the above range. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte or non-aqueous electrolytic solution can be increased. Note that the content of the electrolyte salt in the non-aqueous electrolyte or non-aqueous electrolytic solution is the sum of the content of salt X and the content of salts other than salt X.

[0096] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0097] Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), etc. The cyclic carbonate is preferably a saturated cyclic carbonate, and more preferably EC.

[0098] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0099] As the non-aqueous solvent, it is preferable to use at least one of a cyclic carbonate and a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0100] The non-aqueous electrolyte or non-aqueous electrolytic solution may contain additives in addition to the salt X, the electrolyte salt, and the non-aqueous solvent. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); unsaturated cyclic carbonates such as vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, and vinylethylene carbonate; imide salts such as lithium bis(fluorosulfonyl)imide and lithium difluorophosphonylfluorosulfonylimide; aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; and halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole. Succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3, 2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, and lithium difluorophosphate.Among these, it is preferable to contain at least one selected from the group consisting of unsaturated cyclic carbonates and imide salts, and it is more preferable to contain both unsaturated cyclic carbonates and imide salts. As the unsaturated cyclic carbonate, vinylene carbonate is preferable. As the imide salt, lithium difluorophosphonyl fluorosulfonylimide is preferable. These additives may be used alone or in combination of two or more.

[0101] The content of the additive contained in the non-aqueous electrolyte or non-aqueous electrolytic solution is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, even more preferably 0.2% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and 3% by mass or less, relative to the total mass of the non-aqueous electrolyte or non-aqueous electrolytic solution. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and to further improve safety.

[0102] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0103] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc. and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

[0104] Examples of sulfide solid electrolytes include Li ２ S-P ２ S ５ , LiI-Li ２ S-P ２ S ５ , Li １０ Ge-P ２ S １２ etc.

[0105] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0106] Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0107] <Electricity Storage Device> The nonaqueous electrolyte energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), etc., a power source for electronic devices such as a personal computer or a communication terminal, or a power source for power storage, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit.

[0108] 2 shows an example of an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

[0109] <Method for manufacturing nonaqueous electrolyte storage element> The method for manufacturing the nonaqueous electrolyte storage element of this embodiment can be appropriately selected from known methods, but the following method is preferred. That is, in the method for manufacturing a nonaqueous electrolyte storage element according to one embodiment of the present invention, secondary particles of a positive electrode active material containing a compound having an olivine crystal structure are crushed to produce a nonaqueous electrolyte storage element having a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２The method for producing a nonaqueous electrolyte storage element of the present embodiment may further include preparing an electrode assembly, and housing the electrode assembly and the nonaqueous electrolyte in a container.

[0110] The secondary particles of the positive electrode active material containing a compound having an olivine crystal structure, which are subjected to crushing to obtain positive electrode active material particles, may contain a carbon material present on at least a portion of their surface. The secondary particles of the positive electrode active material subjected to crushing can be obtained, for example, by firing a plurality of raw material compounds, as described above. Crushing of the secondary particles of the positive electrode active material can be performed using a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow jet mill, or the like. Crushing can be performed by either a dry method or a wet method. The secondary particles of the positive electrode active material after crushing may be classified. A sieve, an air classifier, or the like can be used as a classification method.

[0111] The positive electrode can be fabricated, for example, by applying a positive electrode mixture paste to a positive electrode substrate directly or via an intermediate layer, and then drying. The positive electrode mixture paste contains components constituting the positive electrode active material layer, such as positive electrode active material particles, and a dispersion medium. After drying, the applied positive electrode mixture paste may be pressed or the like. The specific and preferred forms of the fabricated positive electrode are the same as the specific and preferred forms of the positive electrode provided in the nonaqueous electrolyte storage element according to one embodiment of the present invention.

[0112] Preparing a nonaqueous electrolyte may mean preparing a nonaqueous electrolyte. The preparation of the nonaqueous electrolyte can be carried out, for example, by mixing the components of the nonaqueous electrolyte. Specific and preferred forms of the prepared nonaqueous electrolyte are the same as those of the nonaqueous electrolyte included in the nonaqueous electrolyte storage element according to one embodiment of the present invention.

[0113] The manufacturing method may further include preparing a negative electrode, preparing an electrode assembly, and housing the electrode assembly and the non-aqueous electrolyte in a container.

[0114] Preparing a negative electrode may also mean fabricating a negative electrode. Fabrication of a negative electrode can be performed, for example, by applying a negative electrode mixture paste to a negative electrode substrate directly or via an intermediate layer, and then drying. The negative electrode mixture paste contains components constituting a negative electrode active material layer, such as a negative electrode active material, and a dispersion medium. After drying, the applied negative electrode mixture paste may be pressed or the like. Instead of applying a negative electrode mixture paste, a negative electrode active material layer may also be provided by laminating metal foil. Specific and preferred forms of the prepared negative electrode are the same as those of the negative electrode provided in the nonaqueous electrolyte storage element according to one embodiment of the present invention.

[0115] Preparing the electrode body may mean fabricating the electrode body. The electrode body can be fabricated, for example, by stacking or winding a positive electrode and a negative electrode with a separator interposed therebetween. The method for housing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be injected through an injection port formed in the container, and then the injection port may be sealed.

[0116] <Other Embodiments> The nonaqueous electrolyte storage element of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, or part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0117] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The present invention can also be applied to various secondary batteries.

[0118] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode.

[0119] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0120] Example 1 (Preparation of Positive Electrode Active Material Particles) Positive electrode active material particles composed of lithium iron phosphate secondary particles coated with a carbon material were obtained by the following procedure. Lithium hydroxide monohydrate, diammonium hydrogen phosphate, and iron sulfate heptahydrate were weighed out so that the molar ratio of Li:Fe:P was 3:1:1, and mixed in ion-exchanged water to prepare a solution. Next, this solution was transferred to a polytetrafluoroethylene container and placed in a pressure-resistant hydrothermal reaction vessel. The hydrothermal reaction vessel was thoroughly purged with nitrogen gas, sealed, and then heated at 170°C for 15 hours to perform hydrothermal synthesis. After completion of the hydrothermal synthesis reaction, the product was allowed to cool to room temperature. The resulting product was thoroughly washed with ion-exchanged water and then dried under reduced pressure at 120°C for 6 hours to obtain lithium iron phosphate (LFP). The resulting LFP was mixed with sucrose powder as a carbon source in water to obtain a slurry. The resulting slurry was pulverized in a bead mill using zirconia beads and dried by spray drying. A mixture of dried LFP and scroll powder was fired at 650°C under a nitrogen atmosphere to obtain particles in which secondary particles of the LFP-containing positive electrode active material were coated with a carbon material. The resulting particles were crushed using a jet mill to obtain crushed positive electrode active material particles. The bead mill conditions (bead diameter and processing time), firing conditions (firing temperature and firing time), and crushing conditions (crushing time) were adjusted to obtain the desired physical properties. The BET specific surface area, D10 particle size, D50 particle size, and D90 particle size of the resulting positive electrode active material particles were measured. The results are shown in Table 1.

[0121] (Preparation of Positive Electrode) A positive electrode mixture paste was prepared using the obtained positive electrode active material particles, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material particles, AB, and PVDF was 90:5:5 in terms of solid content. This positive electrode mixture paste was applied to aluminum foil as a positive electrode substrate, dried, and roll-pressed to form a positive electrode active material layer, thereby obtaining a positive electrode.

[0122] (Preparation of Negative Electrode) A negative electrode mixture paste was prepared by mixing graphite as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of graphite to SBR to CMC was 98.0:1.0:1.0 in terms of solid content. This negative electrode mixture paste was applied to copper foil as a negative electrode substrate, dried, and roll-pressed to form a negative electrode active material layer, thereby obtaining a negative electrode.

[0123] (Preparation of non-aqueous electrolyte) LiPF was dissolved in a solvent containing ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate in a volume ratio of 30:35:35. ６ to 1.0 mol / dm ３ A solution was obtained by dissolving 0.7 mass % of vinylene carbonate, 2.5 mass % of lithium difluorophosphonyl fluorosulfonylimide, and 0.5 mass % of lithium difluorobis(oxalato)phosphate (LiFOP) in this solution to obtain a nonaqueous electrolyte.

[0124] (Separator) A polyethylene microporous film was used as the separator.

[0125] (Assembly of non-aqueous electrolyte storage element) The positive electrode, the negative electrode, and the separator were stacked to prepare an electrode assembly. The obtained electrode assembly was placed in a container, and the non-aqueous electrolyte was then poured into the container and sealed, thereby obtaining the non-aqueous electrolyte storage element of Example 1.

[0126] [Comparative Examples 1 to 7] The nonaqueous electrolyte storage elements of Comparative Examples 1 to 7 were obtained in the same manner as in Example 1, except that the positive electrode active material particles used were positive electrode active material particles in which secondary particles of a positive electrode active material containing LFP were coated with a carbon material, and the positive electrode active material particles had the BET specific surface area and particle sizes (D10 particle size, D50 particle size, and D90 particle size) shown in Table 1, and the content of lithium difluorobis(oxalato)phosphate (LiFOP) in the nonaqueous electrolyte was set as shown in Table 1. The positive electrode active material particles of Comparative Examples 4 and 6 differed in BET specific surface area and particle size from the positive electrode active material particles of Example 1 by changing the firing conditions and by sieving the obtained particles in which secondary particles of a positive electrode active material containing LFP were coated with a carbon material instead of crushing them with a jet mill. The positive electrode active material particles of Comparative Examples 1 and 3 were obtained by the following procedure. Iron phosphate hydrate and lithium hydroxide monohydrate were weighed out so that the molar ratio of Li:Fe:P was 2:1:1, and mixed in ion-exchanged water containing polyvinylpyrrolidone and fructose to prepare a solution. The resulting solution was dried by spray drying to prepare a powdery precursor. The prepared precursor was fired under a nitrogen atmosphere to obtain particles in which secondary particles of a positive electrode active material containing LFP were coated with a carbon material. The obtained particles were crushed using a jet mill to obtain crushed positive electrode active material particles. The spray drying conditions (method, solution concentration, nozzle diameter, disk shape, rotation speed, etc.), firing conditions (sintering temperature and firing time), and crushing conditions (crushing time) were adjusted to obtain predetermined physical properties. The positive electrode active material particles of Comparative Examples 5 and 7 were obtained by the following procedure. Phosphoric acid and iron powder were reacted in ion-exchanged water. The resulting solution was dried by spray drying to prepare a powdery precursor. The prepared precursor was fired under a nitrogen atmosphere to obtain Fe. ７ (P.O. ４ ) ６ The obtained Fe ７ (P.O. ４ ) ６A solution was prepared by mixing phosphoric acid, lithium carbonate, and fructose in ion-exchanged water. The obtained solution was dried by spray drying to prepare a powdery precursor. The prepared precursor was fired under a nitrogen atmosphere to obtain particles in which secondary particles of a positive electrode active material containing LFP were coated with a carbon material. The spray drying conditions (method, solution concentration, nozzle diameter, disk shape, rotation speed, etc.) and firing conditions (firing temperature and firing time) were adjusted to obtain predetermined physical property values, and sieving was performed. In addition, "-" in Table 1 indicates that LiFOP was not contained.

[0127] [Evaluation] (Initial Charge / Discharge) Each nonaqueous electrolyte storage element of the Examples and Comparative Examples was initially charged and discharged under the following conditions. In a thermostatic chamber at 25°C, constant-current charging was performed with a charging current of 0.2 C and a charge cut-off voltage of 3.5 V, followed by constant-voltage charging at 3.5 V. The charge was terminated when the charging current decayed to 0.01 C. A 10-minute rest period was then provided. A constant-current discharge was then performed with a discharging current of 0.2 C and a discharge cut-off voltage of 2.0 V. (Initial Capacity Confirmation Test) Next, an initial capacity confirmation test was performed on each nonaqueous electrolyte storage element at 25°C as follows: Constant-current charging was performed with a charging current of 1.0 C and a charge cut-off voltage of 3.5 V, followed by constant-voltage charging at 3.5 V. The charge was terminated when the charging current decayed to 0.01 C. A 10-minute rest period was then provided. Thereafter, constant current discharge was performed with a discharge current of 1.0 C and a discharge cut-off voltage of 2.0 V. The discharge capacity at this time was designated the "initial discharge capacity." The fully charged state based on the initial discharge capacity was designated as 100% SOC (state of charge). (DC Resistance Measurement) After the above initial charge and discharge, the DC resistance (DCR) was measured as follows. At 25 ° C, constant current charging was performed at a current of 1.0 C until an SOC equivalent to 50% was reached. After storing in a thermostatic chamber at -10 ° C for 4 hours, the battery was discharged for 30 seconds at a current of 0.2 C, 0.5 C, or 1.0 C. After each discharge, constant current charging was performed at a current of 1.0 C to bring the SOC to 50%. The relationship between the current and the voltage 10 seconds after the start of discharge in each discharge was plotted, and the DC resistance (DCR) was calculated from the slope of the straight line obtained from the plot of the three points. The determined direct current resistance (DCR) is shown in Table 1.

[0128]

[0129] As shown in Table 1, the D50 particle size is 3 μm or less and the BET specific surface area is 10 m ２ The nonaqueous electrolyte storage element of Example 1, in which positive electrode active material particles having a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m or less were used and the nonaqueous electrolyte contained LiFOP, a salt having an oxalate complex anion, had a low direct current resistance (DCR) of 300 mΩ or less. ２ / g or less, and the nonaqueous electrolyte did not contain a salt having an oxalate complex anion. All of the nonaqueous electrolyte storage elements of Comparative Examples 1 to 7 that did not satisfy one or more of the requirements had a direct current resistance (DCR) of more than 300 mΩ. When sieving was performed on particles in which secondary particles of a positive electrode active material containing LFP were coated with a carbon material, as in the positive electrode active material particles used in Comparative Examples 4 to 7, the resulting positive electrode active material particles had large particle sizes (D10 particle size, D50 particle size, and D90 particle size). This is thought to be due to the aggregation of particles in which secondary particles of a positive electrode active material containing LFP were coated with a carbon material after sieving.

[0130] The present invention can be applied to nonaqueous electrolyte storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

[0131] REFERENCE SIGNS LIST 1 nonaqueous electrolyte energy storage element 2 electrode body 3 container 4 positive electrode terminal 41 positive electrode lead 5 negative electrode terminal 51 negative electrode lead 20 energy storage unit 30 energy storage device

Claims

1. A positive electrode containing positive electrode active material particles including a compound having an olivine crystal structure, and a non-aqueous electrolyte containing a salt having an oxalato complex anion, wherein the positive electrode active material particles have a D50 particle size of 3 μm or less, and the positive electrode active material particles have a BET specific surface area of ​​10 m ２ / g or less.

2. The nonaqueous electrolyte storage element according to claim 1, wherein the positive electrode active material particles have a D90 particle size of 8 μm or less.

3. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the positive electrode active material particles contain a carbon material present on at least a portion of the surface thereof.

4. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the compound having an olivine type crystal structure is lithium iron phosphate.

5. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the salt having an oxalato complex anion is lithium difluorobis(oxalato)phosphate.

6. By crushing the secondary particles of the positive electrode active material containing a compound having an olivine type crystal structure, a positive electrode active material having a D50 particle size of 3 μm or less and a BET specific surface area of ​​10 m ２ / g or less; producing a positive electrode containing the positive electrode active material particles; and preparing a nonaqueous electrolyte containing a salt having an oxalato complex anion.

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