JP2014072012A - Power storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power storage device having high cycle characteristics, even if a lithium metal fluoride phosphate compound where fluorine is deficient is used as a positive electrode active material, while reducing the internal resistance.SOLUTION: A power storage device includes a positive electrode containing a lithium metal fluoride phosphate compound where fluorine is partially deficient from stoichiometric composition as a positive electrode active material, and an electrolyte to which an organic compound containing fluorine in a molecule is added. Since an organic compound containing fluorine in a molecule is added into the electrolyte, deficiency of fluorine is made up even if a positive electrode active material having a large deficiency amount of fluorine in a crystal structure is used, and high cycle characteristics and reduced internal resistance equivalent to those of a positive electrode active material having a small deficiency amount of fluorine can be exhibited.

Description

  The present invention relates to an electricity storage device, and more particularly to an electricity storage device including a lithium metal fluorophosphate compound as a positive electrode active material.

In recent years, lithium ion storage devices such as lithium ion secondary batteries have been used in various fields such as automobiles and portable information communication related devices. In such a lithium ion electricity storage device, a lithium metal fluorophosphate compound (LiMPO ₄ F) is known as a material used for the positive electrode active material. This metal fluorophosphate compound has attracted attention in recent years because it exhibits a high voltage and a high capacity.

For example, Patent Document 1 discloses a method for producing a positive electrode active material represented by Li _1-x M _1-y PO ₄ F as an example of a lithium metal fluorophosphate compound as a positive electrode active material. Yes.

Patent Document 2 discloses Li _1.25 Fe _0.9 Mg _0.1 PO ₄ F _0.25 , which is a lithium metal fluorophosphate compound having an olivine structure.

JP 2009-18189 A Special table 2005-522009 gazette

  However, the present inventors have compared the fluorine content with the theoretical value by a solid phase reaction performed when synthesizing the lithium metal fluorophosphate compound as described in Patent Documents 1 and 2 above. I found it to decrease. This is considered to be caused by, for example, the NASICON structure being destroyed by excessive baking. Therefore, in the synthesized positive electrode active material, fluorine is partially lost with respect to the stoichiometric composition.

  Then, the present inventors have found that when a compound having a large amount of fluorine deficiency is used as the positive electrode active material, the cycle characteristics are deteriorated and the initial internal resistance is increased as compared with a compound having few deficiencies.

The reason why the cycle characteristics are adversely affected in this way is considered to be mainly due to the phase separation of the positive electrode active material into the LiVPO ₄ F phase and the LiVOPO ₄ phase during the charging process. Specifically, referring to the discharge curves of the LiVPO ₄ F phase and the LiVOPO ₄ phase, after the initial charge / discharge, the plateau potential of the battery is generated in two places, and after a predetermined number of cycles, the plateau considered to be derived from the LiVOPO ₄ phase Disappears. That is, this means that the LiVOPO ₄ phase is greatly deteriorated as compared with the LiVPO ₄ F phase, and therefore the active material having a composition deficient in fluorine is compared with the active material not deficient in fluorine. It is considered that the cycle characteristics deteriorate. Furthermore, the reason why the initial internal resistance increases is thought to be because the LiVOPO ₄ phase has lower electron conductivity than the LiVPO ₄ F phase. That is, since the positive electrode active material is phase-separated during the charging process, a LiVOPO ₄ phase having relatively low electronic conductivity is generated, so that the initial internal resistance is increased.

  The present invention has been made in view of the above problems, and the purpose thereof is to have high cycle characteristics even when a lithium metal fluorophosphate compound lacking fluorine is used as a positive electrode active material. An object of the present invention is to provide an electricity storage device with reduced internal resistance.

  In order to achieve the above object, an electricity storage device according to the present invention includes a positive electrode containing a lithium metal fluorophosphate compound in which fluorine is partially lost from the stoichiometric composition as a positive electrode active material, and fluorine in the molecule. And an electrolytic solution to which an organic compound contained is added.

  As described above, when a positive electrode active material having a large amount of fluorine deficiency in the crystal structure is applied to an electricity storage device such as a lithium ion secondary battery, cycle characteristics and the like tend to deteriorate. By adding an organic compound containing fluorine in the inside, even if a positive electrode active material having a large amount of fluorine deficiency in the crystal structure is used, the deficiency of the fluorine is compensated, and it is equivalent to a positive electrode material having a small amount of fluorine deficiency. High cycle characteristics and reduced internal resistance can be exhibited.

  Further, it is generally known that in the positive electrode active material of a lithium metal fluorophosphate compound, the variation in the fluorine content in the crystal structure causes variations in cycle characteristics and adversely affects the cell. However, in the present invention, by using an electrolyte containing an organic compound containing fluorine, the cycle characteristics are stable regardless of the size of fluorine deficiency, and charge / discharge characteristics equivalent to or better than those of positive electrode materials with a small amount of fluorine deficiency are exhibited. can do.

The content of fluorine in the lithium metal fluorophosphate compound is preferably 60% or more with respect to the stoichiometric composition. In particular, the fluorine content in the lithium metal fluorophosphate compound is expressed when the general formula is expressed as LiMPO ₄ F _x (M is a transition metal selected from V, Fe, Mn, Cr, Ti). 0.6 ≦ x <1.0.

  Further, the fluorine-containing organic compound is preferably fluoroethylene carbonate.

As fluoroethylene carbonate, monofluoroethylene carbonate, difluoroethylene carbonate, or trifluoroethylene carbonate can be used, and monofluoroethylene carbonate (C ₃ H ₃ O ₃ F) having one fluorine atom in the molecule is used. Most preferred.

  In the present invention, the organic compound is added to the electrolytic solution so that the fluorine content in the organic compound is equal to or greater than the deficiency of fluorine in the lithium metal fluorophosphate compound. This is because the loss of fluorine in the lithium metal fluorophosphate compound can be reliably compensated.

  Specifically, the organic compound is added to the electrolytic solution so that the number of fluorine atoms contained in the organic compound is equal to or greater than the number of fluorine atoms deficient in the lithium metal fluorophosphate compound.

  According to the present invention, even when a positive electrode active material having a large amount of fluorine deficiency in the crystal structure is used, charge / discharge characteristics equivalent to or better than those of a positive electrode material having a small amount of fluorine deficiency can be exhibited. It is possible to prevent variation in cell characteristics due to the variation between the cells, and improve the reliability of power storage devices such as lithium ion secondary batteries.

It is sectional drawing which showed typically the electrical storage device inside concerning this Embodiment. It is a diagram showing a spectrum by ⁷ Li-NMR of LiVPO ₄ F _0.6 / C. It is a diagram showing a spectrum by ⁷ Li-NMR of LiVOPO _4. (A), the discharge curve after 1 cycle of the power storage device to the LiVPO ₄ F _0.6 / C as a positive electrode active material, (B) is a discharge curve after 2 cycles. (A) is the measurement result of AC impedance performed before the cycle test on the electricity storage device using LiVPO ₄ F _0.6 / C as the positive electrode active material, and (B) is the measurement result of AC impedance performed after the cycle test. is there. (A), the discharge curve after 1 cycle of the power storage device to the LiVPO ₄ F _0.9 / C as a positive electrode active material, (B) is a discharge curve after 2 cycles. (A) is the measurement result of AC impedance performed before the cycle test for the electricity storage device using LiVPO ₄ F _0.9 / C as the positive electrode active material, and (B) is the measurement result of AC impedance performed after the cycle test. is there.

  Embodiments according to the present invention will be described below. In this embodiment, an example in which a lithium ion secondary battery is used as an electricity storage device is given.

(Positive electrode active material layer)
As the positive electrode active material, a lithium metal fluorophosphate compound that can be doped / undoped with lithium ions is used. Here, the “lithium metal fluorinated phosphate compound” means that anion phosphate ion (PO ₄ ³⁻ ) and fluorine ion (F ⁻ ) are cations lithium ion (Li ⁺ ) and metal ion ( M). Examples thereof include Li _x MPO ₄ F _x in which fluorine is bonded to a compound having an olivine structure. In addition, it is preferable that the particle size of a positive electrode active material is 0.1-30 micrometers.

As a method for producing the positive electrode active material, for example, a carbon material such as vanadium pentoxide (V ₂ O ₅ ) and (NH ₄ ) ₂ HPO ₄ as raw materials, and a mixed solvent of 2-propanol and water is used. Etc., and the precursor is obtained by firing the mixture. And after mixing the obtained precursor and fluorides, such as LiF, in a mixed solvent, it is made to solid-phase-react by baking and a positive electrode active material is obtained.

  Here, it is known that the positive electrode active material manufactured as described above has a structure in which fluorine is lost due to a solid-phase reaction. Therefore, the cycle characteristics and the like may be deteriorated by this fluorine-deficient structure, but in this embodiment, the cycle characteristics and the like can be prevented from deteriorating even for the positive electrode active material having a structure in which fluorine is deficient. Is. Therefore, in the following examples, in addition to the naturally occurring fluorine deficiency state, the amount of fluorine deficiency is intentionally increased by adjusting the firing conditions in the mixture of the precursor and the fluoride. The effect is confirmed.

  The positive electrode active material obtained by the above production method is mixed with a binder containing polyvinylidene fluoride (PVDF) or the like and KB or CB as a conductive additive, and N-methyl-2-pyrrolidinone (NMP) or the like as a solvent. A positive electrode active material layer for an electricity storage device containing the positive electrode active material can be obtained by forming and cutting the resulting slurry using

(Negative electrode active material layer)
The negative electrode active material is a substance that can be doped / undoped with lithium ions, and a metal material, a carbon material that can occlude lithium ions, a metal material, an alloy material, an oxide, or a mixture thereof is used. The particle size of the negative electrode active material is preferably 0.1 to 30 μm. Examples of the alloy material include a silicon alloy and a tin alloy. Examples of the oxide include silicon oxide, tin oxide, and titanium oxide. Examples of the carbon material include graphite, non-graphitizable carbon, graphitizable carbon, and polyacene organic semiconductor. You may mix and use these materials.

  Then, a negative electrode mixture layer is formed by applying a negative electrode slurry in which a mixture containing a binder such as the negative electrode active material and polyvinylidene fluoride (PVDF) is dispersed in a solvent, and drying on a negative electrode current collector. Can do.

(Electrolyte)
As the nonaqueous electrolytic solution, an electrolytic solution in which a general lithium salt is used as an electrolyte and this is dissolved in a solvent is used. The electrolyte and the solvent are not particularly limited. For example, as the electrolyte, LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (C ₂ F ₅ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, or a mixture thereof can be used. These electrolytes may be used alone or in combination. LiPF ₆ and LiBF ₄ are particularly preferable. Furthermore, as a solvent for the non-aqueous electrolyte, chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate ( BC), cyclic carbonates such as vinylene carbonate (VC), acetonitrile (AN), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 1,3-sioxolane (DOXL), dimethyl sulfoxide (DMSO), sulfolane (SL), propionitrile (PN), or other relatively low molecular weight solvents, or mixtures thereof can be used. A mixture of a chain carbonate and a cyclic carbonate is particularly preferred. Furthermore, a mixture using two or more types of chain carbonates or two or more types of cyclic carbonates may be used.

In particular, in this embodiment, an organic compound containing fluorine in the molecule is added as a further solvent in addition to the solvent and the non-electrolyte. The organic compound is preferably one represented by C ₃ H _{4 -a} O ₃ F _a , and particularly preferably one containing one fluorine atom in the molecule (a = 1) because it is chemically stable. In particular, as the fluorine-containing organic compound, monofluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, or the like can be used, but monofluoroethylene carbonate (C ₃ H ₃ O ₃ ) having one fluorine atom in the molecule. F) is most preferred.

  In the present embodiment, the organic compound is added to the electrolytic solution so that the fluorine content in the organic compound is equal to or greater than the amount of fluorine deficiency in the lithium metal fluorophosphate compound. In particular, the organic compound is added to the electrolytic solution so that the number of fluorine atoms contained in the organic compound is equal to or greater than the number of fluorine atoms deficient in the lithium metal fluorophosphate compound.

Specifically, for example, when the positive electrode active material is expressed as LiMPO ₄ F _x (M is a transition metal selected from V, Fe, Mn, Cr, Ti, 0.6 ≦ x <1.0), The molecular weight is M (LiMPO ₄ F _x / C), the mass is W (LiMPO ₄ F _x / C), the total mass of the electrolyte is W (electrolyte), the concentration of the organic compound in the electrolyte is α%, and If the molecular weight of the organic compound is M (organic compound), the number of fluorine atoms in the electrolytic solution is [{α × W (electrolytic solution)} / {100 × M (organic compound)}], and LiMPO ₄ F _Since the number of fluorine atoms in _{x 2} / C is [{W (LiMPO ₄ F _x / C) × x} / M (LiMPO ₄ F _x / C)], in order to compensate for the loss of fluorine atoms, [{ W (LiMPO ₄ F _x / C) × (1-x)} / M (LiMPO ₄ becomes F _x / C)]. That is, the following formula 1
[{Α × W (electrolyte)} / {100 × M (organic compound)}] ≧ [{W (LiMPO ₄ F _x / C) × (1-x)} / M (LiMPO ₄ F _x / C) ] (Formula 1)

By adjusting the concentration α (%) of the organic compound so as to satisfy the above condition, a state in which the fluorine content in the organic compound is equal to or greater than the fluorine deficiency in the lithium metal fluorophosphate compound is realized.

(Separator)
There is no restriction | limiting in particular in the separator used by this embodiment, A well-known separator can be used. For example, a porous body having durability against an electrolytic solution, a positive electrode active material, and a negative electrode active material and having continuous air holes and having no electronic conductivity can be preferably used. Examples of such a porous body include woven fabric, non-woven fabric, synthetic resin microporous membrane, and glass fiber. A synthetic resin microporous membrane is preferably used, and a polyolefin microporous membrane such as polyethylene or polypropylene is particularly preferable in terms of thickness, membrane strength, and membrane resistance.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to this embodiment.

Example 1
[Production of positive electrode]
1. Raw material (i) V ₂ O ₅ : 90.94 g (0.5 mol)
(Ii) (NH ₄ ) ₂ HPO ₄ : 132.06 g (1 mol)
(Iii) Carbon black (CB): 16.015 g (carbon 1.5 mol)
2. Mixed solvent Water: Mixed solvent of 18.02 g (1.0 mol) and 3-propanol 316.48 g Step (a): In the mixed solvent, the raw materials V ₂ O ₅ , (NH ₄ ) ₂ HPO ₄ and KB are mixed. Wet mixed for 3 hours.

  Step (b): The obtained mixture was heat-treated at about 300 ° C. for 2 hours in an argon atmosphere (primary firing).

Step (c): Thereafter, the mixture was further subjected to a solid phase reaction by heat treatment at about 800 ° C. for 16 hours in the same manner under argon (secondary firing). A precursor (VPO ₄ / C) was synthesized.

Step (d): Using a ball mill, a 2-propanol solvent as 25.94 g of LiF (that is, the molar ratio of the precursor to LiF is 1: 1) with respect to 160.19 g of the obtained precursor (VPO _{4 /} C) Mixed in for 1 hour.

  Step (e): The obtained mixture was heat-treated at about 670 ° C. for 3 hours under an argon atmosphere to obtain 178.53 g of a positive electrode active material powder.

Step (f): The fluorine content of the obtained positive electrode active material powder was measured using lanthanum-alizarin complexone spectrophotometry according to JIS-K0102 34.1. The measurement is a method for measuring fluoride ions by measuring the absorbance of a blue complex formed by the reaction of a complex of lanthanum and alizarin complexone with fluoride ions. From this measurement, it was found that the fluorine content was deficient with respect to LiVPO ₄ F.

Therefore, considering that the raw materials are V ₂ O ₅ , (NH ₄ ) ₂ HPO ₄ , and carbon black, the positive electrode active materials are LiVPO ₄ F _0.9 / C and LiVPO ₄ F _0.6 / C. It may be a mixture of C or a mixture of LiVPO ₄ F and LiVOPO ₄ . However, since these are known to show the same diffraction pattern in the XRD measurement, they cannot be clearly distinguished in the XRD measurement.

Therefore, in order to specify the local structure around Li in the positive electrode active material in more detail, the local structure around Li was specified by ⁷ Li-NMR. The results are shown in FIG. The measurement conditions are as follows.

Spectrometer: AVANCE300 (manufactured by Bruker), observation nucleus: ⁷ Li (117 MHz), measurement method: MAS method, MAS condition: 40 kHz, measurement temperature: room temperature (actual measurement value 23 ° C.), integration number: 1024 times, reference material: LiCl (-1.19 ppm, equivalent to 0.00 ppm of 1M LiCl aqueous solution)

On the other hand, FIG. 3 shows the NMR spectrum of LiVOPO ₄ . The measurement conditions of the spectrum are as follows.

Spectrometer: AVANCE300 (manufactured by Bruker), observation nucleus: ⁷ Li (117 MHz), measurement method: MAS method, MAS conditions: 12, 10, 8 kHz, measurement temperature: room temperature (actual measurement value 27 ° C.), number of integrations : 128 times, reference material: LiCl (−1.19 ppm, equivalent to 0.00 ppm of 1M LiCl aqueous solution)

From the above measurement for the positive electrode active material according to the present invention, a large peak at 123 ppm was observed (see FIG. 2). Here, it is known that the NMR spectrum of LiVOPO ₄ shows a peak at 81 ppm (see FIG. 3). However, a peak was observed at 123 ppm in the positive electrode active material prepared in the present invention, and a peak was observed at 81 ppm. Therefore, there was no possibility that the positive electrode active material was a mixture of LiVPO ₄ F and LiVOPO ₄ , and it could be determined that LiVPO ₄ F _0.6 / C lacking fluorine in the crystal structure.

Step (g): A binder containing polyvinylidene fluoride (PVDF) so that the powder of the positive electrode active material LiVPO ₄ F _0.6 / C obtained in step (f) has a mass ratio of 90: 5: 5 , And ketjen black as a conductive aid, and a slurry was obtained using N-methyl-2-pyrrolidinone (NMP) as a solvent. Thereafter, the slurry is uniformly applied and molded so as to have a porous density of 2 g / cm ³ , cut into a square of 24 × 36 mm, and a positive electrode active material composed of the above positive electrode active material LiVPO ₄ F _0.6 / C. A positive electrode including a material layer was obtained.

[Production of negative electrode]
Graphite and PVDF (mass ratio 94: 6) as a binder were mixed and a slurry diluted with NMP was prepared. The slurry was applied uniformly on both sides or one side of a copper current collector having through holes so that the mixture density per side was 1.5 mg / cm ^3, and was cut into a 26 × 38 mm square. A negative electrode was obtained.

[Preparation of electrolyte]
31.8% by mass of ethylene carbonate (EC), 25.6% by mass of dimethyl carbonate (DMC), 24.2% by mass of ethyl methyl carbonate (EMC), and 2.0% by mass of monofluoroethylene carbonate (FEC) And 16.4% by mass of LiPF ₆ that is an electrolyte salt were mixed to prepare an electrolyte solution.

[Production of battery]
Twelve positive electrodes A and twelve negative electrodes (two inner-side coatings) were laminated by the above-described process via a polyolefin microporous film as a separator. Two negative electrodes coated on one side were applied to the outermost layer. Further, a lithium electrode in which metallic lithium was bonded to a stainless porous foil was disposed in the outermost layer through a separator, and an electrode laminated unit composed of a positive electrode A, a negative electrode, a lithium electrode, and a separator was produced. The electrode laminate unit was packaged with an aluminum laminate film, and the electrolyte solution was injected. Thus, a lithium ion secondary battery was assembled.

[Investigation of discharge characteristics]
The discharge characteristics were investigated using a lithium ion secondary battery. Discharge characteristics were obtained by measuring changes in voltage with the passage of capacity (time). In particular, in this example, the discharge characteristics in the first cycle and the discharge characteristics in the second cycle were investigated. 4A and 4B show discharge curves of the first cycle and the second cycle of the lithium ion secondary battery, respectively.

[Cycle test]
The lithium ion secondary battery was charged and discharged for 20 cycles at 3V to 4.5V. The capacity retention rate after 20 cycles was 92.7%.

[AC impedance measurement]
Before and after the cycle test, AC impedance measurement was performed using a lithium ion secondary battery. The measurement results of AC impedance before and after the cycle test are shown in FIGS.

(Example 2)
Each step was performed under the same conditions as in Example 1 except that the baking conditions in step (e) of Example 1 were set at about 670 ° C. for 1 hour in an argon atmosphere. As a result of the measurement in the step (f), it was found that the obtained positive electrode active material had a crystal structure of LiVPO ₄ F _0.9 / C. At this time, a peak was observed at 123 ppm in the NMR spectrum.

  Further, after the step (g), the results of the first cycle and the second cycle of the discharge characteristic investigation of the obtained lithium ion battery are shown in FIGS. 6 (A) and 6 (B), respectively. The measurement results are shown in FIGS. Furthermore, the capacity retention rate after 20 cycles was 94.0%.

  Below, a comparative example is demonstrated.

(Comparative Example 1)
In the components of the electrolytic solution, each step was performed under the same conditions as in Example 1 except that vinylene carbonate (VC): 2.0 mass% was mixed instead of monofluoroethylene carbonate (FEC): 2.0 mass%. went.

As a result of the measurement in the above step (f), it was found that the crystal structure of the obtained positive electrode active material was LiVPO ₄ F _0.6 / C.

  Further, after the step (g), the results of the discharge characteristic investigation of the obtained lithium ion battery are shown by broken lines in FIGS. 4A and 4B, and the results of the AC impedance measurement before and after the cycle are shown in FIG. ) And (B). Furthermore, the capacity retention rate after 20 cycles was 90.3%.

(Comparative Example 2)
Each process was performed on the conditions similar to the comparative example 1 except the baking conditions in the process (e) of the comparative example 1 having been 1 hour at about 670 degreeC by argon atmosphere.

It was found that the crystal structure of the obtained positive electrode active material was LiVPO ₄ F _0.9 / C.

  Further, after the step (g), the results of the discharge characteristic investigation of the obtained lithium ion battery are shown in FIGS. 6A and 6B, and the results of the AC impedance measurement are shown in FIG. Furthermore, the capacity retention rate after 20 cycles was 93.0%.

  The conditions and results of the examples and comparative examples are summarized in Table 1.

As understood from the above results, the discharge curves of LiVPO ₄ F _0.9 / C with a small amount of fluorine deficiency, ie, discharges in Example 2 and Comparative Example 2 shown in FIGS. 6 (A) and 6 (B), respectively. As can be understood by referring to the curve, a curve having the same shape is drawn in both the first cycle and the second cycle regardless of the type of the electrolyte. This means that constant discharge characteristics are maintained regardless of the type of the electrolyte because the amount of fluorine deficiency is small. Regarding the capacity maintenance rate, it was found that the comparative example 2 maintained a relatively high level, but Example 2 in which FEC was added to the electrolytic solution showed a more preferable value.

On the other hand, referring to the discharge curves of LiVPO ₄ F _0.6 / C with a large amount of fluorine deficiency, that is, the discharge curves in Example 1 and Comparative Example 1 shown in FIGS. 4 (A) and 4 (B), respectively. As can be seen, in Comparative Example 1 of the electrolyte solution to which FEC was not added, a new plateau appears at a portion of about 3.9 V in the discharge curve of the second cycle, whereas in Example 1 in which FEC was added, The electrolyte has a shape approximating a discharge curve of LiVPO ₄ F _0.9 / C with few defects. This means that, when FEC is added to the electrolyte, the discharge characteristics are prevented from being deteriorated due to fluorine deficiency.

  Moreover, referring to the AC impedance measurement results in FIG. 7 for Comparative Example 2 and Example 2, the electrode resistance of the battery is not substantially changed before the cycle test, regardless of whether FEC is added to the electrolytic solution. It is understood. However, referring to the measurement results after the cycle test, the resistance of the electrode of the cell using the electrolytic solution to which FEC was not added (cell of Comparative Example 2) was increased, whereas FEC was added. The electrode of the cell using the electrolytic solution (cell of Example 2) maintains a low resistance state (see FIG. 7B).

  Furthermore, referring to the AC impedance measurement results in FIG. 5 for Comparative Example 1 and Example 1, even before the cycle test, the cell in which no FEC was added to the electrolyte (the cell of Comparative Example 1) It is understood that the electrode resistance is high. This is considered to be due to the large amount of fluorine deficiency in the positive electrode active material. On the other hand, in the case of a cell in which FEC is added to the electrolytic solution (the cell of Example 1), the cell electrode resistance is clearly reduced. In particular, this effect appears more significantly in the measurement results after the cycle test (see FIG. 5B).

  According to the electricity storage device according to the present embodiment, when a positive electrode active material having a large amount of fluorine deficiency in the crystal structure is applied to an electricity storage device such as a lithium ion secondary battery, cycle characteristics and the like tend to deteriorate. In the present invention, by adding an organic compound containing fluorine in the molecule to the electrolyte solution, even if a positive electrode active material having a large amount of fluorine deficiency in the crystal structure is used, the fluorine deficiency is compensated, and fluorine deficiency is obtained. High cycle characteristics equivalent to a small amount of positive electrode material and reduced internal resistance can be exhibited.

  Further, it is generally known that in the positive electrode active material of a lithium metal fluorophosphate compound, the variation in the fluorine content in the crystal structure causes variations in cycle characteristics and adversely affects the cell. However, in the present invention, by using an electrolyte containing an organic compound containing fluorine, the cycle characteristics are stable regardless of the size of fluorine deficiency, and charge / discharge characteristics equivalent to or better than those of positive electrode materials with a small amount of fluorine deficiency are exhibited. can do.

  In addition, this invention is not limited to the said embodiment, A various change is possible within the range of the summary of invention. For example, the amount of addition of the organic compound to the electrolytic solution is the method described in the above embodiment so that the fluorine content in the organic compound is equal to or greater than the amount of fluorine deficiency in the lithium metal fluorophosphate compound. It is not restricted to what is determined, You may make it determine in consideration of the components of various cells, such as a positive electrode active material, a negative electrode active material, and electrolyte solution.

In addition, the composition of the positive electrode active material is not limited to that described in the above embodiment, and various compositions such as Li _x MPO ₄ F _x in which fluorine is bonded to a compound having an olivine structure can be employed. . Furthermore, the value of x F _x in the composition of the positive electrode active material, i.e., the fluorine amount is not limited to 0.6 and 0.9 above, with respect to the general formula defined in accordance with the composition of the positive electrode active materials Any value that results in a deficiency of fluorine is included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Lithium ion electrical storage device 12 Negative electrode 14 Negative electrode collector 16 Negative electrode active material layer 18 Positive electrode 20 Positive electrode collector 22 Positive electrode active material layer 24 Lead 25 Separator 26 Lead

Claims (7)

A positive electrode comprising, as a positive electrode active material, a lithium metal fluorophosphate compound in which fluorine is partially lost from the stoichiometric composition;
An electrolytic solution to which an organic compound containing fluorine is added in the molecule;
An electricity storage device comprising:   2. The electricity storage device according to claim 1, wherein a content of fluorine in the lithium metal fluorophosphate compound is 60% or more based on a stoichiometric composition. The content of fluorine in the lithium metal fluorophosphate compound is 0 when the general formula is expressed as LiMPO ₄ F _x (M is a transition metal selected from V, Fe, Mn, Cr, Ti). The electric storage device according to claim 1, wherein: 6 ≦ x <1.0.   The electric storage device according to claim 1, wherein the organic compound is fluoroethylene carbonate. The electric storage device according to claim 4, wherein the fluoroethylene carbonate is C ₃ H ₃ O ₃ F having one fluorine atom in a molecule.   2. The organic compound is added to the electrolytic solution so that a fluorine content in the organic compound is equal to or more than a deficiency of fluorine in the lithium metal fluorophosphate compound. The electrical storage device of any one of -5.   The organic compound is added to the electrolytic solution so that the number of fluorine atoms contained in the organic compound is equal to or greater than the number of fluorine atoms deficient in the lithium metal fluorophosphate compound. The power storage device according to any one of claims 1 to 6.

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