Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery.

Since the lithium ion secondary battery has a high energy density, it is widely used, for example, as a power source for small electronic devices and a power source for driving a vehicle.

As one such lithium ion secondary battery, Patent Document 1 discloses a lithium ion secondary battery using an olivine-type lithium phosphate as a positive electrode active material. In this Patent Document 1, in view of the poor electron conductivity of the olivine-type lithium phosphorus oxide, a carbon composite of the olivine-type lithium phosphorus oxide is used as the positive electrode active material, and the thickness of the positive electrode active material layer is used. It is described that the load characteristics and the battery capacity are compatible with each other by optimizing the load characteristics.

Further, in Patent Document 2, in order to improve the conductivity of the olivine-type lithium phosphorus oxide, a lithium ion battery in which a conductive path made of carbon is incorporated inside the positive electrode active material particles containing the olivine-type lithium phosphorus oxide is incorporated. The following batteries are listed.

<p><patcit num="1"><text>Japanese Patent No. 3997702</text></patcit><patcit num="2"><text>Japanese Patent Application Laid-Open No. 2003-203628</text></patcit></p>

<p>However, unlike the lithium ion secondary battery described in Patent Document 1, the conductivity of each positive electrode active material particle does not change only by optimizing the thickness of the positive electrode active material layer, so that the load characteristics can be improved. There is room.</p><p>Further, since Patent Document 2 does not describe the detailed state of carbon constituting the conductive path, it is unclear whether the conductive path is sufficient. Therefore, if the conductive path incorporated inside the positive electrode active material particles is insufficient, it cannot be said that the load characteristics are sufficiently improved.</p><p>Further, it is known that when the wettability of the non-aqueous electrolyte with respect to the positive electrode active material is poor, the load characteristics are lowered. However, Patent Documents 1 and 2 do not consider the wettability of the non-aqueous electrolyte with respect to the positive electrode active material at all, and satisfying the requirements shown in Patent Documents 1 and 2 does not necessarily provide sufficient load characteristics. It is considered that it may not be obtained.</p><p>The present invention has been made to solve the above problems, and an object of the present invention is to provide a lithium ion secondary battery capable of improving load characteristics.</p>

<p>The lithium ion secondary battery of the present invention has a positive electrode having a positive electrode active material capable of storing and releasing lithium ions, a negative electrode having a negative electrode active material capable of storing and releasing lithium ions, and the positive electrode. A separator interposed between the negative electrode and the non-aqueous electrolyte containing a polar solvent having a specific dielectric constant of 20 or more is provided, and the surface of the positive electrode active material particles constituting the positive electrode active material contains a carbon compound. A coating layer is formed, and the coating layer contains an acidic functional group, and the amount of the acidic functional group in the complex composed of the positive electrode active material and the coating layer is 0.004 mmol / g or more. Less than 0.0062 mmol / g<u style="Single">the law of nature,</u><u style="Single"> The ratio of the coating layer to the complex is 0.1% by weight or more and 5% by weight or less.</u>It is characterized by that.</p><p>The specific surface area of the complex is 9.0 m.²It may be / g or more.</p><p>The positive electrode active material has a chemical formula of Li._{x x}M_{y y}P_zO_4-δ(However, 0.5 &lt;x / y &lt;1.5, y / z&gt; 1, δ is the oxygen deficiency amount, and M contains at least one of Fe, Mn, Co, and Ni). It may be a lithium acid compound.</p>

<p>According to the present invention, a coating layer containing a carbon compound is formed on the surface of the positive electrode active material particles, and the coating layer contains an acidic functional group to form a complex composed of the positive electrode active material and the coating layer. When the amount of acidic functional group occupies 0.004 mmol / g or more and 0.0062 mmol / g or less<u style="Single">, The ratio of the coating layer to the complex is 0.1% by weight or more and 5% by weight or less.</u>When the non-aqueous electrolyte contains a polar solvent having a relative permittivity of 20 or more, it is possible to achieve both improvement of electron conductivity by the coating layer containing a carbon compound and improvement of wettability of the non-aqueous electrolyte with respect to the positive electrode active material. .. This makes it possible to improve the load characteristics of the lithium ion secondary battery.</p>

<figref num="1">It is sectional drawing of the lithium ion secondary battery in one Embodiment of this invention.</figref>

Hereinafter, embodiments of the present invention will be shown, and the features of the present invention will be described in more detail.

In the following, a lithium ion secondary battery having a structure in which a laminate formed by alternately laminating a plurality of positive electrodes and negative electrodes via a separator and a non-aqueous electrolyte are housed in an exterior body will be described as an example.

FIG. 1 is a cross-sectional view of a lithium ion secondary battery 100 according to an embodiment of the present invention. In the lithium ion secondary battery 100, a laminate 10 formed by alternately stacking a plurality of positive electrodes 11 and 12 via a separator 13 and a non-aqueous electrolyte 14 are housed in a laminate case 20. Has a structure. That is, the lithium ion secondary battery 100 includes a positive electrode 11, a negative electrode 12, a separator 13, and a non-aqueous electrolyte 14.

The laminated case 20 which is an exterior body is formed by thermocompression bonding the peripheral portions of a pair of laminated films 20a and 20b to each other.

The non-aqueous electrolyte 14 housed in the laminated case 20 contains a polar solvent having a relative permittivity of 20 or more. As the non-aqueous electrolyte 14 containing a polar solvent having a relative permittivity of 20 or more, for example, a non-aqueous electrolyte solution containing ethylene carbonate having a relative permittivity of 90 can be used.

The positive electrode terminal 16a is led out to the outside from one end side of the laminated case 20, and the negative electrode terminal 16b is led out to the outside from the other end side. The plurality of positive electrodes 11 are connected to the positive electrode terminals 16a via the lead wire 15a. Further, the plurality of negative electrodes 12 are connected to the negative electrode terminals 16b via the lead wire 15b.

The positive electrode 11 has a positive electrode active material capable of storing and releasing lithium ions. More specifically, the positive electrode 11 has a positive electrode mixture layer containing the positive electrode active material and a positive electrode current collector. The positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode current collector is, for example, a metal leaf such as aluminum.

The positive electrode active material is, for example, the chemical formula Li._{x x}M_{y y}P_zO_4-δ(However, 0.5 &lt;x / y &lt;1.5, y / z&gt; 1, δ is the oxygen deficiency amount, and M contains at least one of Fe, Mn, Co, and Ni). It is a lithium acid compound. As such an olivine-type lithium phosphate compound, for example, lithium iron phosphate (LiFePO)₄(Also called LFP)) can be used. However, LiFePO₄A portion of the Fe site of may be substituted with at least one element selected from the group consisting of Mg, Ca, Ti, Cr, Zr, Zn, and Nb. Further, a part of the Li site may be replaced with Na, or a part of the P site may be replaced with Si.

The positive electrode mixture layer may contain a conductive auxiliary agent and a binder in addition to the positive electrode active material. The positive electrode active material is preferably contained in the positive electrode mixture layer in a proportion of 50% by weight or more and 99% by weight or less.

In the present embodiment, a coating layer containing a carbon compound is formed on the surface of the positive electrode active material particles constituting the positive electrode active material. The coating layer contains acidic functional groups. Then, a complex of the positive electrode active material (hereinafter, simply referred to as a complex) is formed by the positive electrode active material and the coating layer.

However, it is not necessary that the entire surface of the positive electrode active material particles is covered with the coating layer. Further, the coating layer may contain, for example, carbon having no acidic functional group.

The complex can be produced, for example, by heat-treating a mixture obtained by mixing a constituent material of the positive electrode active material and a reducing carbohydrate in an inert atmosphere. However, when the mixture is heat-treated, the heat treatment atmosphere, heating temperature, heating time and the like are adjusted so that a carbon compound containing an acidic functional group can be obtained after the heat treatment. As the reducing carbohydrate, for example, saccharides such as glucose, fructose, sucrose, and dextrin, and those having a reducing functional group such as cellulose, an aldehyde group, and a ketone group can be used.

The type of the acidic functional group is not particularly limited, but is preferably at least one selected from the group consisting of, for example, a carboxyl group, a hydroxyl group, a carbonyl group, and a quinone group.

In the present embodiment, the amount of acidic functional groups in the complex composed of the positive electrode active material and the coating layer is 0.004 mmol / g or more and 0.0062 mmol / g or less. By configuring the non-aqueous electrolyte 14 to contain a polar solvent having a relative dielectric constant of 20 or more while the amount of the acidic functional group satisfies the above requirements, the coating layer containing the carbon compound improves the electron conductivity. By improving the wettability of the non-aqueous electrolyte 14 with respect to the positive electrode active material, it is possible to promote the storage and release of lithium ions at the same time. As a result, the load characteristics of the lithium ion secondary battery 100 in the present embodiment are improved as compared with the conventional lithium ion secondary battery that does not have the above configuration.

Here, a case where a coating layer made of carbon containing no acidic functional group, that is, a coating layer not satisfying the requirements of the present invention is formed on the surface of the positive electrode active material particles will be considered. In this case, as compared with the configuration in which carbon is not attached to the positive electrode active material particles, the electron conductivity is improved due to the carbon attached, but the positive electrode active material particles are formed by the carbon containing no acidic functional group. It is considered that the cover does not allow the positive electrode active material to smoothly store and release lithium ions.

However, according to the lithium ion secondary battery 100 in the present embodiment, a coating layer containing a carbon compound having an acidic functional group is formed on the surface of the positive electrode active material particles, and the content of the acidic functional group is set to the above amount. By setting the non-aqueous electrolyte 14 to contain a polar solvent having a relative dielectric constant of 20 or more, it is possible to achieve both improvement of electron conductivity and improvement of wettability of the non-aqueous electrolyte 14 with respect to the positive electrode active material. ..

The proportion of the coating layer in the complex is preferably 0.1% by weight or more and 5% by weight or less. By setting the ratio of the coating layer to the composite to 0.1% by weight or more and 5% by weight or less, an electron conduction network via the coating layer is efficiently formed, and the lithium diffusion distance inside the positive electrode active material having high resistance is reduced. Therefore, the load characteristics of the lithium ion secondary battery 100 can be improved. In order to further reduce the lithium diffusion distance inside the positive electrode active material and further improve the load characteristics, the ratio of the coating layer to the composite is more preferably 1% by weight or more and 2% by weight or less.

The specific surface area of the complex is 9.0m²It is preferably / g or more. The specific surface area of the complex is 9.0 m²By setting / g or more, the lithium diffusion distance inside the positive electrode active material can be reduced, and the load characteristics can be improved. In order to further improve the load characteristics by reducing the lithium diffusion distance inside the positive electrode active material, the specific surface area of the complex is 10 m.²/ g or more 13m²It is more preferably less than / g.

The negative electrode 12 has a negative electrode active material capable of storing and releasing lithium ions. More specifically, the negative electrode 12 has a negative electrode mixture layer containing the negative electrode active material and a negative electrode current collector. The negative electrode mixture layer is formed on both sides of the negative electrode current collector.

There are no particular restrictions on the type of negative electrode active material, but for example, carbon-based compounds such as graphite, hard carbon, and soft carbon, lithium titanate, and MO._{x x}(However, M is at least one element selected from the group consisting of Ti, Si, Sn, Cr, Fe, and Mo, and x is a numerical value within the range of 0.9 x 2.0.) An alloy system containing at least one selected from the group consisting of metal oxides, silicon, silicon oxides, silicon-containing alloys, silicon-containing compounds, tin, tin oxides, tin-containing alloys, and tin-containing compounds having the same composition. Substances can be used.

The negative electrode current collector is, for example, a metal leaf such as copper. The negative electrode mixture layer may contain a conductive auxiliary agent and a binder in addition to the negative electrode active material.

The separator 13 is interposed between the positive electrode 11 and the negative electrode 12. As the separator 13, various separators that can be used in a lithium ion secondary battery can be used without particular limitation. The separator 13 shown in FIG. 1 has a bag-like shape, but may have a sheet-like shape or a zigzag shape.

&lt;Example&gt; (Measurement of Amount of Acidic Functional Group) As a positive electrode active material, a powder of lithium iron phosphate having a coating layer containing a carbon compound having an acidic functional group formed on the surface of lithium iron phosphate particles (hereinafter referred to as "lithium iron phosphate powder"). LFP powder) was prepared. The average particle size D50 of lithium iron phosphate particles, which is a complex having a coating layer formed on the surface, is 1 μm or more and 11 μm or less, and the specific surface area is 10 m.²/ g or more 12m²It was less than / g.

For the prepared LFP powder, the amount of acidic functional groups contained in the coating layer was measured by the back titration method. In the back titration method, LFP powder is mixed with a basic solution of a known concentration, and the LFP powder and an unreacted basic solution are titrated with an acidic solution to determine the amount of the reacted basic solution, and the amount of the reacted bases. It is a method of obtaining the amount of acidic functional groups from.

Specifically, 3 g of LFP powder is weighed in a container, stabilized in a constant temperature and humidity chamber at a temperature of 25 ° C and a humidity of 50% for 6 hours or more, and then pyridine to methyl isobutyl ketone (MIBK) as a basic solution. 30 ml of a mixed solution of 0.01 mol / L was added, the mixture was sealed, and ultrasonically dispersed with an ultrasonic cleaner for 1 hour. The dispersion was centrifuged, 10 ml of the separated supernatant was diluted with MIBK, and titrated with a MIBK solution of 0.01 mol / L perchloric acid. From the amount of pyridine consumed by the acidic functional groups on the surface of the LFP, the amount of acidic functional groups (acid point amount) in the complex and the amount of acidic functional groups (acid point amount) in the coating layer were determined.

From the amount of lithium iron phosphate when producing the LFP powder as a complex, the amount of the positive electrode active material in the complex and the amount of the coating layer in the complex can be determined in advance. .. That is, if the amount of acidic functional groups in the complex can be determined, the amount of acidic functional groups in the coating layer can also be determined.

Here, as shown in Table 1 described later, seven types of LFP powders having different amounts of acidic functional groups in the complex and carbon compounds in the complex are prepared, and the load characteristics are determined by the method described later. Seven types of batteries were prepared for evaluation.

(Preparation of positive electrode) Next, in addition to the above-mentioned LFP powder, acetylene black (AB) was prepared as a conductive auxiliary agent, and polyvinylidene fluoride (PVdF) was prepared as a binder, and they were used in a weight ratio of LFP powder: AB :. A positive electrode slurry was prepared by dispersing it in N-methyl-2-pyrrolidone (NMP) so that PVdF was 85: 10: 5.

This positive electrode slurry was uniformly applied to both sides of a strip-shaped aluminum foil having a thickness of 20 μm using a die coater, and dried. Then, using a roll press machine, the density is 2.0 g / cm.³After compactioning so as to be, a positive electrode was prepared by cutting to a size of 50 mm × 50 mm. The thickness of the positive electrode mixture layer was set to about 25 μm.

(Preparation of negative electrode) Next, prepare natural graphite as the negative electrode active material and PVdF as the binder, and N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone) so that the weight ratio of natural graphite: PVdF is 95: 5. It was dispersed in NMP) to prepare a negative electrode slurry.

The prepared negative electrode slurry was uniformly applied to both sides of a copper foil having a thickness of 15 μm and a strip shape using a die coater, and dried. The amount of the negative electrode slurry applied was adjusted so that the negative electrode capacity was 1.8 times the positive electrode capacity. Then, using a roll press machine, the density is 1.3 g / cm.³After compactioning so as to be, a negative electrode was prepared by cutting to a size of 52 mm × 52 mm. The thickness of the negative electrode mixture layer was set to about 25 μm.

(Preparation of electrolytic solution) Next, a mixed solvent having a volume ratio of ethylene carbonate (EC) with a relative permittivity of 90 and ethylmethyl carbonate (EMC) with a relative permittivity of 2.8 is EC: EMC = 25: 75. I prepared it. Then, in the prepared mixed solvent, 1 mol of lithium hexafluoride phosphate (LiPF) per liter of the solvent was added.₆) Was dissolved, and vinylene carbonate (VC) was further added to prepare a non-aqueous electrolyte solution. The amount of vinylene carbonate added was adjusted so that the ratio of the prepared non-aqueous electrolytic solution to the whole was 1.0% by weight.

(Manufacturing of Battery) A laminated battery element was manufactured by alternately stacking a plurality of positive electrodes and negative electrodes prepared by the above method via a separator. A microporous polypropylene film having a thickness of 20 μm was used as the separator.

Subsequently, a current collecting lead for connecting to an external connection line was ultrasonically welded to the manufactured battery element. Then, the battery element was housed in a bag-shaped exterior body made of aluminum laminate, which was produced by heat welding on three sides. At this time, the current collector lead was made to protrude to the outside of the exterior body.

Finally, 1.6 g of the non-aqueous electrolytic solution prepared by the above method was injected into the bag-shaped outer body, and then the open side was closed and sealed to prepare a battery.

As described above, seven types of batteries shown in Table 1 described later were prepared using seven types of LFP powders having different amounts of acidic functional groups in the complex and carbon compounds in the complex.

(Initial charge / discharge condition) Each manufactured battery is charged with a constant current of 0.2CA under a temperature condition of 25 ° C until the battery voltage reaches 3.8V, and then 10 at a constant voltage of 3.8V. Charged for hours.

Then, under a temperature condition of 25 ° C, the battery was discharged at a constant current of 1 CA until the battery voltage reached 2.5 V.

(Aging treatment) Each manufactured battery is charged under a temperature condition of 25 ° C until the battery voltage reaches 3.8V with a constant current of 1CA, and the current is 1 / at a constant voltage of 3.8V. It was charged until it attenuated to 50CA. Then, under the temperature condition of 55 ° C, the aging treatment was carried out for one week.

In addition, when gas was generated inside the exterior during the initial charge / discharge and aging treatment, a degas treatment was performed to release the gas to the outside of the exterior.

(Output characteristic test) &lt;Capacity inspection&gt; Under the temperature condition of 25 ° C, each manufactured battery is charged with a constant current of 1CA until the battery voltage reaches 3.5V, and then a constant voltage of 3.5V. Then, the battery was charged until the current decreased to 1/50 CA.

Then, under a temperature condition of 55 ° C., the battery was discharged at a constant current of 1 CA until the battery voltage reached 2.5 V, and the discharge capacity was determined. This discharge capacity was taken as the battery capacity. Table 1 shows the capacity of each battery.

&lt;Load characteristic inspection&gt; Each manufactured battery is charged under a temperature condition of 25 ° C until the charging depth (SOC) reaches 50%, and then under a temperature condition of 0 ° C, 20 at a constant current of 10 C. The DC resistivity was calculated from the ultimate voltage when charging for a second. Then, the value obtained by normalizing the calculated DC resistivity according to the total area of the positive electrode was used as an index of the load characteristic. Table 1 shows the direct current resistivity (DCR) standardized by the total area of the positive electrode for each battery.

<tables><img file="JP7078112B2_D0001.tif" /></tables>

Table 1 shows the amount of acidic functional groups in the complex, the amount of carbon compounds in the complex, and the DC resistance of the batteries for the seven types of batteries with different amounts of acidic functional groups and carbon compounds in the complex. (DCR), resistance rating, and battery capacity are shown. The samples of sample numbers 1 to 5 are evaluation cells satisfying the requirement of the present invention that the amount of acidic functional groups in the complex is 0.004 mmol / g or more and 0.0062 mmol / g or less. On the other hand, the samples of sample numbers 6 and 7 marked with * are comparative cells that do not meet the above requirements of the present invention.

As shown in Table 1, the DC resistivity of the comparison cell of sample number 6 that does not meet the requirements of the present invention is 87 Ωcm.²The DC resistivity of the comparison cell of sample number 7 is 72Ωcm.²However, the DC resistivity of the evaluation cells of sample numbers 1 to 5 satisfying the requirements of the present invention is 68 Ωcm.²It was as follows. Therefore, the resistance evaluation of the evaluation cells of sample numbers 1 to 5 satisfying the requirements of the present invention is marked with "", and the resistance evaluation of the comparison cells of sample numbers 6 and 7 not satisfying the requirements of the present invention is "". × ".

That is, a coating layer containing a carbon compound having an acidic functional group is formed on the surface of the positive electrode active material particles, and the amount of the acidic functional group in the composite is 0.004 mmol / g or more and 0.0062 mmol / g or less. A lithium ion secondary battery that meets the requirements and contains a polar solvent with a relative dielectric constant of 20 or more in a non-aqueous electrolyte has a DC resistance rate as compared with a conventional lithium ion secondary battery that does not meet the above requirements. Is lowered and the load characteristics are improved.

Although not shown in Table 1, in a battery in which the non-aqueous electrolyte does not contain a solvent having a relative permittivity of 20 or more, the amount of acidic functional groups in the complex is 0.004 mmol / g or more and 0.0062 mmol /. It was found that even when the requirements of g or less were satisfied, the wettability of the electrolytic solution to the positive electrode active material deteriorated and the load characteristics were low.

That is, the carbon compound is satisfied by satisfying the condition that the amount of the acidic functional group in the complex is 0.004 mmol / g or more and 0.0062 mmol / g or less, and the non-aqueous electrolyte contains a polar solvent having a specific dielectric constant of 20 or more. It is possible to achieve both the improvement of electron conductivity by the coating layer containing the above and the promotion of storage and release of lithium ions by improving the wettability of the non-aqueous electrolyte with respect to the positive electrode active material, and the load characteristics of the lithium ion secondary battery are improved. improves.

The present invention is not limited to the above embodiment, and various applications and modifications can be added within the scope of the present invention.

In the above-described embodiment, a lithium ion secondary battery having a structure in which a plurality of positive electrodes and negative electrodes are alternately laminated via a separator and a non-aqueous electrolyte is housed in the exterior body will be described as an example. However, the structure of the lithium ion secondary battery according to the present invention is not limited to the above structure. For example, the lithium ion secondary battery may have a structure in which a winding body formed by winding a positive electrode and a negative electrode laminated via a separator and a non-aqueous electrolyte are housed in an exterior body. Further, the exterior body may be a metal can instead of a laminated case.

Although the description has been given with an example of using an olivine-type lithium phosphate compound as the positive electrode active material, the positive electrode active material is not limited to the olivine-type lithium phosphate compound. However, when the present invention is applied using an olivine-type lithium phosphate compound as a positive electrode active material, it is effective by compensating for the drawbacks of the olivine-type lithium phosphate compound having high lithium diffusion resistance and low electron conductivity in particles. The load characteristics can be improved. That is, it is possible to improve the electron conductivity by adhering the carbon compound to the surface of the olivine-type lithium phosphate compound. At this time, the electron conductivity can be improved by the configuration as in the present invention. It is possible to effectively improve the load characteristics by achieving both the improvement of the above and the improvement of the wettability of the non-aqueous electrolyte with respect to the positive electrode active material.

The amount of the acidic functional group in the complex can be obtained not only from the complex used at the time of manufacturing the battery but also from the positive electrode of the manufactured battery. For example, it can be obtained by the following method.

First, the positive electrode mixture layer was peeled off from the positive electrode of the battery to collect the powdered positive electrode powder, and the collected positive electrode powder was added to N-methyl-2-pyrrolidone (NMP) to dissolve PVdF, which is a binder. Remove. NMP is further added to the powder from which PVdF has been removed and centrifugation is performed to separate the conductive auxiliary agent and the complex, and the amount of the acidic functional group is determined using the complex obtained by separation. The amount of acidic functional group can be determined by the above-mentioned back titration method.

Ten Laminate

11 11 Positive electrode

12 Negative electrode

13 Separator

14 Non-aqueous electrolyte

20 Laminate case

100 Lithium ion secondary battery