JP4739780B2 - Non-aqueous electrolyte battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte battery including a positive electrode and a negative electrode capable of inserting and extracting lithium, and a non-aqueous electrolyte containing gamma-butyrolactone, and particularly to improve storage characteristics by improving the positive electrode. The present invention relates to a nonaqueous electrolyte battery that can be produced.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries as drive power sources are required to have higher capacities. A non-aqueous electrolyte battery that performs charge / discharge by moving lithium ions between the positive and negative electrodes along with charge / discharge has a high energy density and high capacity. As widely used. In recent years, the use of such features has led to the development of not only mobile applications such as mobile phones, but also medium to large battery applications ranging from electric tools, electric vehicles, and hybrid vehicles.

  Here, a cobalt oxide containing lithium is used as the positive electrode of the non-aqueous electrolyte battery. Among these, a lithium-containing transition metal oxide having a layered rock-salt structure represented by lithium cobaltate is used alone. For example, as shown in Patent Document 1 below, a lithium-containing transition metal oxide having a layered rock salt structure when exposed to a high temperature atmosphere in a charged state or when continuously charged by abnormal charging, etc. There is a problem in that oxygen is difficult to desorb and may cause an exothermic reaction with the electrolyte.

  Therefore, in preparation for such an abnormality as described above, at present, a protection circuit for maintaining safety is provided inside the battery pack, and voltage and current are precisely controlled. In addition, the battery can itself has a PTC (Positive Temperature Coefficient) element that plays a role in preventing abnormal heat generation when an excessive current flows, and a gas discharge valve with a current blocking function in case of a gas pressure increase in the battery Many safeguards such as the above are provided, and battery safety measures are sufficiently implemented. However, in recent years, it has been necessary to suppress the reaction between the positive electrode active material and the electrolytic solution from the viewpoint of simplifying the protective function and reducing the cost.

In view of the above, a nonaqueous electrolyte battery using gamma-butyrolactone having a high boiling point and thermally stable as a solvent has been proposed.
However, a non-aqueous electrolyte battery using gamma-butyrolactone as an electrolyte has not been able to obtain sufficient charge storage characteristics compared to a non-aqueous electrolyte battery using a chain carbonate typified by diethyl carbonate as an electrolyte. As the cause, it is considered that gamma-butyrolactone is decomposed on the positive and negative electrodes.

  Therefore, in the following Patent Document 2, in a non-aqueous electrolyte battery containing gamma butyrolactone, by optimizing the content of gamma butyrolactone, the thickness of the exterior material, etc., gas generation when stored at high temperature is suppressed, and Techniques for improving large current discharge characteristics and charge / discharge cycle characteristics have been proposed.

Japanese Patent Laid-Open No. 11-16666

JP 2000-235868 A

However, only by optimizing the content of gamma-butyrolactone and the thickness of the outer packaging, gamma-butyrolactone undergoes side reactions on the positive electrode surface during high-temperature storage, which is considered to be the main cause of deterioration. Has a problem that it cannot be sufficiently improved.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a nonaqueous electrolyte battery capable of improving the charge storage characteristics at high temperatures without reducing the initial discharge capacity.

  In order to achieve the above object, the invention according to claim 1 of the present invention comprises a negative electrode capable of occluding and releasing lithium, and a positive electrode containing a positive electrode active material comprising lithium cobaltate having a layered rock salt structure. A non-aqueous electrolyte battery comprising a non-aqueous electrolyte containing a solvent and a solute, and wherein the solvent contains gamma-butyrolactone, wherein the positive electrode active material contains lithium cobalt oxide in a low temperature phase It is characterized by.

If the positive electrode active material contains lithium cobalt oxide in a low temperature phase as in the above configuration, the charge storage characteristics can be improved without reducing the initial discharge capacity of the battery. This is considered to be due to the following reasons.
That is, if the positive electrode active material contains lithium cobalt oxide in the low temperature phase, the side reaction between the electrolyte solution (gamma butyrolactone) and the positive electrode active material during high temperature storage is first performed between the lithium cobalt oxide in the low temperature phase. Since it occurs (that is, it concentrates with lithium cobaltate in a low temperature phase), it can be suppressed from occurring with lithium cobaltate having a layered rock salt structure. Therefore, since deterioration of lithium cobaltate having a layered rock salt type structure is suppressed, it is considered that a decrease in charge storage characteristics is suppressed.

In addition, lithium cobaltate in the low temperature phase has a smaller charge / discharge capacity than lithium cobaltate having a layered rock salt structure, but lithium can be inserted and desorbed (the discharge capacity of lithium cobaltate in the low temperature phase is layered) It is about 40 to 50% of the discharge capacity of lithium cobaltate having a rock salt structure). Therefore, the initial discharge capacity of the battery is not significantly reduced by containing the low-temperature phase lithium cobalt oxide.
Therefore, the charge storage characteristics can be improved without reducing the initial discharge capacity of the battery.

Here, the low-temperature phase lithium cobaltate in the above configuration means lithium cobaltate obtained when a lithium compound and a cobalt compound are heat-treated in a temperature range of 300 to 600 ° C. It has a discharge capacity in the vicinity of 3 to 3.9V.
On the other hand, lithium cobaltate having a layered rock salt structure is obtained at a heat treatment temperature higher than that of low-temperature phase lithium cobaltate, and has a discharge capacity in the vicinity of 3.8 to 4.3 V as a potential relative to lithium metal.
In addition, in order to improve the structural stability and electrochemical characteristics of the low-temperature phase lithium cobalt oxide in the present invention, elements such as Ni and Mn can be appropriately added.

The invention according to claim 2 is characterized in that, in the invention according to claim 1, the amount of the gamma butyrolactone relative to the total amount of the solvent is regulated to 50% by volume or more.
The reason for this regulation is that when the amount of gamma-butyrolactone relative to the total amount of the solvent is 50% by volume or more, side reactions between gamma-butyrolactone and the positive electrode active material often occur. This is because it is further demonstrated.

According to a third aspect of the present invention, in the first or second aspect of the present invention, the low-temperature phase lithium cobalt oxide has a spinel structure or a structure similar thereto.
The low-temperature phase lithium cobalt oxide in the present invention includes, for example, Non-Patent Document 1 (Materials Research Bulletin, 28, pp. 235-246, 1992) and Non-Patent Document 2 (Solid State lonics, 62, pp. 53-60, 1993) has a structure similar to the spinel structure. Therefore, in order to specify the crystal structure of the low-temperature phase lithium cobalt oxide based on the above document, it is defined as a spinel structure or a similar structure as described above.

  However, the above two documents are reports on the crystal structure of lithium cobaltate when heat-treated at 400 ° C. When heat-treated at other temperatures (eg, 600 ° C), other crystal structures can be obtained. Is also possible. Accordingly, the crystal structure of the low-temperature phase lithium cobalt oxide in the present invention includes all the crystal structures of lithium cobalt oxide obtained when heat-treated in a temperature range of 300 to 600 ° C.

  According to a fourth aspect of the present invention, in the first to third aspects of the present invention, the ratio of the lithium cobaltate in the low temperature phase to the lithium cobaltate having the layered rock salt structure is regulated to 0.01 to 3 mass%. It is characterized by that.

  In this way, if the ratio of the low-temperature phase lithium cobaltate is less than 0.01% by mass, the effect of adding the low-temperature phase lithium cobaltate is not sufficiently exhibited, while the low-temperature phase lithium cobaltate When the ratio exceeds 3% by mass, the lithium cobaltate in the low temperature phase has a smaller discharge capacity than the lithium cobaltate having a layered rock salt structure as described above, so that the discharge capacity of the battery is reduced.

  According to the present invention, since deterioration of lithium cobalt oxide having a layered rock salt structure is suppressed, it is possible to dramatically improve the charge storage characteristics at high temperatures without reducing the initial discharge capacity of the battery. Excellent effect.

  Hereinafter, the present invention will be described in more detail. However, the present invention is not limited to the following best modes, and can be appropriately modified and implemented without departing from the scope of the present invention.

(Preparation of positive electrode)
First, Li ₂ CO ₃ and Co ₃ O ₄ are mixed in an Ishikawa type mortar so that the molar ratio of Li: Co is 1.03: 1, and 20 ° C. at 850 ° C. in an air atmosphere. After heat treatment for a time, pulverization gave lithium cobaltate (Li _1.03 CoO ₂ ) having a layered rock salt structure.
Next, the lithium cobaltate (Li _1.03 CoO ₂ ) and CoCO ₃ were weighed and mixed so that the molar ratio of Li _1.03 CoO ₂ and CoCO ₃ was 1: 0.03. Next, the obtained powder was heat-treated at 400 ° C. for 20 hours, so that lithium cobaltate (LT) containing 3% by mass of lithium cobaltate (LT) in a low temperature phase with respect to lithium cobaltate having a layered rock salt structure was contained. A positive electrode active material comprising LiCoO ₂ containing) was obtained.

  Thereafter, the positive electrode active material and the carbon powder as the conductive agent were mixed at a mass ratio of 90: 5 to obtain a positive electrode mixture. Thereafter, a slurry is prepared by adding a solution prepared by dissolving fluororesin powder (polyvinylidene fluoride) as a binder in N-methyl-2-pyrrolidone to the above positive electrode mixture (with the positive electrode mixture in the slurry). The mass ratio with polyvinylidene fluoride is 95: 5). Further, this slurry is applied to one side of a positive electrode current collector made of aluminum foil by a doctor blade method, dried, rolled, and cut into a disk having a diameter of 20 mm. Thus, a positive electrode was produced.

(Preparation of negative electrode)
A negative electrode was produced by punching a 20 mm diameter disc from a lithium rolled plate having a predetermined thickness.

(Preparation of electrolyte)
In a mixed solvent obtained by mixing ethylene carbonate (EC) and gamma butyrolactone (γ-BL) at a volume ratio of 30:70, lithium tetrafluoroborate (LiBF ₄ ) as a solute was added at a concentration of 1.2 mol / The electrolyte is prepared by adding 2 parts by weight of vinylene carbonate and 2 parts by weight of trioctyl phosphate as a surfactant to 100 parts by weight of the mixed solvent. did.

(Battery assembly)
As shown in FIG. 1, a separator 3 made of a polyethylene microporous film was sandwiched between a positive electrode (working electrode) 2 and a negative electrode (counter electrode) 1 produced as described above. Next, the positive electrode current collector 2 a was brought into contact with the upper lid 4 b of the battery can 4 of the test cell, and the negative electrode 1 was brought into contact with the bottom of the battery can 4. These were accommodated in the battery can 4, and the said upper cover 4b and the bottom part 4a were electrically insulated with the insulating packing 5, and the nonaqueous electrolyte battery which concerns on this invention was produced.

[Example]
Example 1
As Example 1, the nonaqueous electrolyte battery shown in the best mode for carrying out the invention was used.
The battery thus produced is hereinafter referred to as the present invention battery A1.

(Example 2)
A non-aqueous electrolyte battery is produced in the same manner as in Example 1 except that the positive electrode active material is prepared so that the ratio of the low-temperature phase lithium cobaltate (LT) to the lithium cobaltate having a layered rock salt structure is 4% by mass. Was made.
Specifically, lithium cobaltate (Li _1.04 CoO ₂ ) is prepared by using Li ₂ CO ₃ and Co ₃ O _{4 so} that the molar ratio of Li: Co is 1.04: 1. A battery was fabricated in the same manner as in Example 1 except that lithium acid and CoCO ₃ were used so that the molar ratio of Li _1.04 CoO ₂ and CoCO ₃ was 1: 0.04.
The battery thus produced is hereinafter referred to as the present invention battery A2.

(Comparative example)
A nonaqueous electrolyte battery was produced in the same manner as in Example 1 except that LiCoO ₂ having a layered rock salt structure not containing low-temperature phase lithium cobaltate was used as the positive electrode active material.
Specifically, Li ₂ CO ₃ and Co ₃ O ₄ are mixed in an Ishikawa type mortar so that the molar ratio of Li: Co is 1: 1, and at 850 ° C. in an air atmosphere. After heat treatment for 20 hours, pulverization produces lithium cobaltate (LiCoO ₂ ) having a layered rock salt type structure, and this is used as a positive electrode active material (that is, after this lithium cobaltate and CoCO ₃ A battery was fabricated in the same manner as in Example 1 except that the step of mixing and heat treating at 400 ° C. was not performed.
The battery thus produced is hereinafter referred to as comparative battery X.

(Experiment)
About this invention battery A1, A2 and the comparison battery X, charging / discharging and a preservation | save are performed on the following conditions, and it is displaying by the discharge capacity before a storage (however, in Table 1, it is shown by the discharge capacity ratio before the preservation | save with respect to the comparison battery X). ), Initial charge / discharge efficiency, and capacity recovery rate were measured, and the results are shown in Table 1.

Test conditions [Charge / discharge conditions in initial charge / discharge efficiency measurement]
-Charging conditions After charging to a charge end voltage of 4.3 V at a charge current of 9.3 mA, the battery is charged to a charge end voltage of 4.3 V at a charge current of 3.1 mA. The temperature during charging is 25 ° C.
-Discharge condition Discharge to discharge end voltage 2.75V with discharge current 3.1mA. The temperature during discharge is 25 ° C.
And the charge capacity and the discharge capacity were measured at the time of the said charge / discharge. And the said discharge capacity was made into the initial stage discharge capacity.
[Calculation of initial charge / discharge efficiency]
From the initial charge capacity and discharge capacity, the initial charge / discharge efficiency was calculated according to the following equation (1).
Initial charge / discharge efficiency (%) = {(initial discharge capacity) ÷ (initial charge capacity)} × 100 (1)

[Charging / discharging conditions in discharge capacity measurement before storage]
After the initial charge / discharge was completed, charge / discharge was performed twice under the same conditions as the charge / discharge conditions, and the second discharge capacity was defined as the discharge capacity before storage.
[Charging and storage conditions for charging and storage]
-Charging conditions After the charge / discharge before the storage is completed, charging is performed under the same conditions as the above charging conditions.
-Storage conditions After carrying out the above charging, stored for 10 days at a temperature of 60 ° C.

[Charging / discharging conditions for discharge capacity measurement after storage]
After the storage is completed, discharging, charging, and re-discharging are performed under the same conditions as the charging / discharging conditions in the discharge capacity measurement before the storage.
And the discharge capacity was measured at the time of another discharge, and this was made into the discharge capacity after a preservation | save.

[Calculation of capacity recovery rate]
From the discharge capacity before storage and the discharge capacity at the time of re-discharge after storage, the capacity recovery rate was calculated according to the following equation (2).
Capacity recovery rate (%) = {(discharge capacity at the time of re-discharge after storage) / (discharge capacity before storage)} × 100 (2)
In addition, it shows that a charge storage characteristic is excellent, so that this capacity | capacitance return rate is large.

As is clear from Table 1, the batteries A1 and A2 of the present invention using LT-containing LiCoO ₂ as the positive electrode active material had a capacity recovery rate as compared with the comparative battery X using LiCoO ₂ containing no LT as the positive electrode active material. It is recognized that it is growing.

Therefore, in a battery using an electrolytic solution containing γ-BL, the use of lithium cobaltate having a layered rock-salt structure containing lithium cobaltate in a low temperature phase as a positive electrode active material provides high-temperature charge storage characteristics. It turns out that it improves.
This is because deterioration of lithium cobaltate having a layered rock-salt structure occurs because γ-BL and lithium cobaltate in the low temperature phase first cause a side reaction when the positive electrode active material contains lithium cobaltate in the low temperature phase. It is considered that the decrease in charge storage characteristics was suppressed.
However, the present invention battery A2 in which the proportion of the low-temperature phase lithium cobaltate (LT) is 4% by mass with respect to the lithium cobaltate having a layered rock salt structure is compared with the present invention battery A1 in which the ratio is 3% by mass. It can be seen that the initial discharge capacity and the initial charge / discharge efficiency are reduced.

[Reference example]
In this reference example, the effect of the present invention was performed in order to confirm that the effect was specific to a nonaqueous electrolyte battery using a solvent containing γ-BL. The contents will be described below.
(Reference Example 1)
As a solvent for the electrolytic solution, a mixed solvent of EC and ethyl methyl carbonate (EMC) (volume ratio is 30:70) is used instead of a mixed solvent of EC and γ-BL, and phosphorus as a surfactant is used. A nonaqueous electrolyte battery was produced in the same manner as in Example 1 of this example except that trioctyl acid was not added.
The battery thus produced is hereinafter referred to as reference battery Y1.

(Reference Example 2)
As a solvent for the electrolytic solution, a mixed solvent of EC and ethyl methyl carbonate (EMC) (volume ratio is 30:70) is used instead of a mixed solvent of EC and γ-BL, and phosphorus as a surfactant is used. A nonaqueous electrolyte battery was produced in the same manner as in the comparative example of this example, except that trioctyl acid was not added.
The battery thus produced is hereinafter referred to as reference battery Y2.

(Experiment)
The reference batteries Y1 and Y2 are charged and discharged and stored, and the discharge capacity before storage (in Table 2, indicated by the ratio of discharge capacity before storage to the reference battery Y2), initial charge and discharge efficiency, The capacity recovery rate was measured, and the results are shown in Table 2. The experimental conditions are the same as those in the experiment of the present embodiment.

As is clear from Table 2, no difference is observed in the capacity recovery rate between the reference batteries Y1 and Y2. That is, in a battery using an electrolyte solution that does not contain γ-BL, the presence or absence of lithium cobaltate in the low-temperature phase in the positive electrode does not affect the charge storage characteristics at high temperatures.
From this, it can be seen that the superiority of the charge storage characteristics due to the inclusion of the low-temperature phase lithium cobaltate is manifested only when an electrolytic solution containing γ-BL is used.

[Other matters]
(1) The negative electrode active material is not limited to the above metal lithium, and various negative electrode materials conventionally used in non-aqueous electrolyte batteries, such as lithium-aluminum alloys, lithium-lead alloys, lithium- Use lithium alloys such as silicon alloys, lithium-tin alloys, carbon materials such as graphite, coke, and organic fired bodies, and metal oxides with a lower potential than Sn active materials such as SnO ₂ , SnO, and TiO _2. Can do.

(2) The solvent that can be mixed with gamma-butyrolactone is not limited to the above-mentioned ethylene carbonate, but solvents conventionally used in non-aqueous electrolyte batteries, such as propylene carbonate, 1,2-butylene carbonate, 2, Cyclic carbonates such as 3-butylene carbonate, cyclic esters such as propane sultone, chain carbonates such as ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether And chain ethers such as ethyl methyl ether, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, acetonitrile and the like. It is. Among them, ethylene carbonate used in the examples is desirable for improving various characteristics of the battery.

(3) The electrolyte salt used in the present invention is not limited to the above LiBF ₄ , but LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (l, m is an integer of 0 or more), LiC (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂ ) (p, q , R is an integer of 0 or more), Li [(C ₂ O ₄ ) ₂ B], LiBF ₂ (C ₂ O ₄ ), LiPF ₄ (C ₂ O ₄ ), LiPF ₂ (C ₂ O ₄ ) _2, etc. It may be present, or a combination of these may be used. Among these, particularly when gamma-butyrolactone is used as a solvent, it is desirable to contain at least LiBF ₄ that forms a stable film. These electrolyte salts are used by dissolving in the non-aqueous electrolyte solution at a concentration of 0.1 to 1.5M, preferably 0.5 to 1.5M.

(4) It is desirable that the electrolytic solution contains one or more cyclic carbonate compounds having a C═C unsaturated bond. This is because if a cyclic carbonate compound having a C═C unsaturated bond is contained, a stable film excellent in lithium ion permeability is formed on the negative electrode, so that the decomposition of γ-butyrolactone is suppressed. Because you can.
Among the cyclic carbonate compounds having a C═C unsaturated bond, a cyclic carbonate having a C═C unsaturated bond is particularly preferred. As the cyclic carbonate having a C═C unsaturated bond, Vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4,5-diethyl vinylene carbonate, 4,5-dipropyl vinylene carbonate, 4-ethyl-5-methyl vinylene carbonate, 4-ethyl-5-propyl vinylene carbonate, 4 -Ethyl-5-propyl vinylene carbonate, 4-methyl-5-methyl vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate and the like are exemplified.

Among these cyclic carbonates, vinylene carbonate and vinyl ethylene carbonate are more preferable because the above effects are further exhibited.
In addition, the ratio of the cyclic carbonate compound having a C═C unsaturated compound in the non-aqueous electrolyte is 1 to 15 parts by mass, more preferably 2 to 10 parts by mass with respect to the total amount of the non-aqueous electrolyte. This is because, if the content of the cyclic carbonate compound having a C═C unsaturated compound is too small, the effect of improving the charge / discharge characteristics may not be sufficiently obtained, while if the content is too large, it is formed on the negative electrode surface. This is because the applied film becomes too thick and the reaction resistance of the negative electrode increases, and as a result, the charge / discharge characteristics may deteriorate.

(5) It is desirable that a surfactant be added to the electrolytic solution. This is because if the surfactant is added in this way, the wettability to the separator is improved, so that various characteristics of the battery can be further improved. Examples of the surfactant include trioctyl phosphate and ester, and the addition amount of the surfactant is about 0.5 to 5 parts by mass with respect to 100 parts by mass of the non-aqueous electrolyte. Is preferred.

(6) The shape of the battery is not limited to the above flat type, and the present invention is not particularly limited to a cylindrical shape, a square shape, etc., and can be applied to non-aqueous electrolyte batteries having various shapes. Of course. Further, there are no particular limitations on battery constituent members such as a current collector that holds a separator, a battery case, and an active material while also collecting current, and various known members may be selectively used.

(7) The present invention is not limited to a liquid battery, but can be applied to a gel polymer battery. Examples of the polymer material in this case include polyether solid polymer, polycarbonate solid polymer, polyacrylonitrile solid polymer, oxetane polymer, epoxy polymer, a copolymer composed of two or more of these, or a crosslinked polymer. A molecule or PVDF is exemplified, and a solid electrolyte in which this polymer material, a lithium salt, and an electrolyte are combined into a gel can be used.

  The present invention can be applied not only to a driving power source of a mobile information terminal such as a mobile phone, a notebook computer, and a PDA, but also to a large battery such as an in-vehicle power source of an electric vehicle or a hybrid vehicle.

It is sectional drawing which shows an example of this invention battery.

Explanation of symbols

1 Negative electrode 2 Positive electrode 3 Separator

Claims (3)

A negative electrode capable of occluding and releasing lithium; a positive electrode containing a positive electrode active material made of lithium cobaltate having a layered rock salt structure; and a non-aqueous electrolyte containing a solvent and a solute; In a non-aqueous electrolyte battery containing gamma butyrolactone,
The positive electrode active material contains a low-temperature phase lithium cobalt oxide ,
The nonaqueous electrolyte battery , wherein a ratio of the lithium cobaltate in the low-temperature phase to the lithium cobaltate having a layered rock salt structure is regulated to 0.01 to 3% by mass .   The nonaqueous electrolyte battery according to claim 1, wherein the amount of the gamma butyrolactone relative to the total amount of the solvent is regulated to 50% by volume or more.   The nonaqueous electrolyte battery according to claim 1, wherein the low-temperature phase lithium cobalt oxide has a spinel structure or a structure similar thereto.