JP4785482B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

In recent years, metallic lithium or an alloy capable of inserting and extracting lithium ions, or a carbon material is used as a negative electrode active material, and a lithium transition metal composite oxide represented by a chemical formula: LiMO ₂ (M is a transition metal) is used as a positive electrode material. Nonaqueous electrolyte batteries are attracting attention as batteries having a high energy density.

A typical example of the lithium transition metal composite oxide is lithium cobalt composite oxide (LiCoO ₂ ), which has already been put into practical use as a positive electrode active material for non-aqueous electrolyte secondary batteries. However, those containing Ni or Mn as a transition metal have been studied, and material systems containing all these three kinds of transition metal elements have been actively studied (for example, Patent Documents 1 to 3). 4, Non-Patent Document 1). Among such lithium transition metal oxides containing Ni, Co, and Mn, a material represented by the chemical formula: LiMn _x Ni _x Co _(1-2x) O ₂ having the same composition of Mn and Ni is in a charged state ( It has been reported in Non-Patent Document 2 and the like that specifically shows high thermal stability even in a high oxidation state).

Patent Document 5 reports that a composite oxide in which Ni and Mn are substantially equal has a voltage in the vicinity of 4 V equivalent to LiCoO ₂ , and has a high capacity and excellent charge / discharge efficiency. . Includes such, Ni, Mn, and Co, the lithium transition metal composite oxide having a substantially equal comprising layered structure that Ni and Mn (e.g., _{Formula: Li a Mn s Ni t Co} u O 2 (0 ≦ a ≦ 1.2, s + t + u = 1, 0 <s ≦ 0.5, 0 <t ≦ 0.5, 0.45 ≦ s / (s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ Batteries using a positive electrode whose main material (50% by weight or more satisfies 0.55, u ≧ 0) have a high thermal stability during charging, so the reliability of the battery is dramatically improved. Can be expected to do.

Furthermore, such a lithium transition metal composite oxide is set to be higher than the current charging voltage (for example, 4.5 V (vs. Li ^/ Li ⁺ ) or more at the positive electrode potential) because of its high structural stability. Has also been reported to exhibit better cycle characteristics than LiCoO _{2 and the} like currently used (Non-Patent Document 3).

Currently, in a non-aqueous electrolyte secondary battery using a lithium-containing transition metal oxide (for example, LiCoO ₂ ) as a positive electrode and a carbon material as a negative electrode, the charge end voltage is generally 4.1 V to 4.2 V. However, in this case, only 50 to 60% of the positive electrode is used with respect to the theoretical capacity. Thus, the chemical _{formula: Li a Mn s Ni t Co} u O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0.5,0 <t ≦ 0.5,0.45 ≦ s / ( s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u ≧ 0), and a lithium transition metal composite oxide having a layered structure is used, the charging voltage is set high. However, the capacity of the positive electrode can be used at 70% or more of the theoretical capacity without greatly degrading the thermal stability, and the capacity and energy density of the battery can be increased.

However, the chemical _{formula: Li a Mn s Ni t Co} u O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0.5,0 <t ≦ 0.5,0.45 ≦ s / ( s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u ≧ 0), and a lithium transition metal composite oxide having a layered structure is used as the positive electrode active material However, in a high-potential charge state where the charge potential of the positive electrode is 4.5 V (vs. Li / Li ⁺ ) or higher, the amount of Li desorption increases, and the number of locations where oxygen ions face each other in the crystal structure increases. In addition, the crystal structure becomes unstable, and at the same time, the thermal stability in the coexistence of the electrolytic solution is lowered, so that it is difficult to ensure safety. That is, a battery in which the potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) or more when the battery is fully charged (for example, carbon whose charging potential of the negative electrode is 0.1 V (vs. Li ^/ Li ⁺ )). In the case of using a negative electrode, a battery having a charging voltage of 4.4 V or higher has a problem with respect to battery safety.
Japanese Patent No. 2561556 Japanese Patent No. 3244314 JP-A-10-199525 International Publication No. 02/41419 Pamphlet JP 2002-42813 A Journal of Power Sources 90 (2000) 176-181 Electrochemical and Solid-State Letters, 4 (12) A200-A203 (2001) Chemistry Letters, 2001, P.642-643

An object of the present invention is to provide a positive electrode potential at full charge in a non-aqueous electrolyte secondary battery using a positive electrode containing a lithium transition metal composite oxide having a layered structure and containing Li, Ni, and Mn. The object of the present invention is to provide a non-aqueous electrolyte secondary battery having high thermal stability and excellent safety even when the voltage is 4.5 V (vs. Li ^/ Li ⁺ ) or more.

The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode and a negative electrode using a material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte, and the positive electrode has a potential of 4.5 V (vs. Li ^/ Li ⁺ In the non-aqueous electrolyte secondary battery configured such that the opposite charge capacity ratio between the positive electrode and the negative electrode is 1.0 to 1.15 when charged until reaching a), the main material of the positive electrode active material is a chemical formula: Li _{a Mn s Ni t Co u Mo v} O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0.5,0 <t ≦ 0.5,0.45 ≦ s / (s + t) ≦ 0 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u> 0 , 0.001 ≦ v ≦ 0.01)).

Here, in a conventional battery using LiCoO ₂ for the positive electrode and a carbon material or a Li alloy material for the negative electrode, the charging voltage of the battery is 4.1 V to 4.2 V, and a carbon material is used for the negative electrode Then, the potential of the positive electrode in a charged state is 4.2 to 4.3 V (vs. Li / Li ⁺ ). Further, the counter charge capacity ratio when charged at this voltage is generally designed to be 1.0 to 1.15 (charge amount of positive electrode <charge amount of negative electrode). The reason for this design is that when this counter charge capacity ratio is less than 1.0, metallic lithium is deposited on the negative electrode surface, the cycle characteristics and safety of the battery are significantly reduced, and the counter charge capacity ratio is 1 If it exceeds .15, the excess negative electrode material not involved in the reaction increases, so the energy density of the battery decreases. Therefore, in the non-aqueous electrolyte secondary battery of the present invention, a material capable of occluding and releasing lithium, such as a carbon material or a Li alloy material, is used for the negative electrode, and the positive electrode has a potential of 4.5 V (vs. Li / Li ⁺ ). A typical example is such that the opposite charge capacity ratio when charging until reaching 1.0 is 1.0 to 1.15. For example, the counter charge capacity ratio in the range of 1.0 to 1.15 in a conventional non-aqueous electrolyte secondary battery charged to 4.3 V (vs. Li ^/ Li ⁺ ) is obtained, and the potential of the positive electrode is 4. When converted to the counter charge capacity ratio when charged until reaching 5 V (vs. Li ^/ Li ⁺ ), it is 0.89 to 0.99, which is outside the scope of the present invention.

When the negative electrode active material is a carbon material, the battery voltage when the positive electrode potential is 4.5 V (vs. Li ^/ Li ⁺ ) is 4.4 V. Therefore, in the non-aqueous electrolyte secondary battery of the present invention, when the negative electrode active material is a carbon material, the above-described counter charge capacity ratio is the same as the counter charge capacity ratio when the battery charge voltage is charged to 4.4 V. Become.

Lithium transition metal composite oxide containing molybdenum used in the present invention has the _{formula: Li a Mn s Ni t Co} u Mo v O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0. 5, 0 <t ≦ 0.5, 0.45 ≦ s / (s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u> 0 , 0.001 ≦ v ≦ 0. 01 is satisfied). In addition, the lithium transition metal composite oxide used in the present invention has a specific surface area of 0.1 to 2.0 m ² / g in order to suppress the reaction between the positive electrode active material and the electrolyte under a high potential. It is desirable to be within the range.

As the non-aqueous electrolyte solvent used in the present invention, it is desirable to use a mixed solvent of a cyclic carbonate having a high dielectric constant and a chain carbonate having a low viscosity, but the cyclic carbonate is likely to cause oxidative decomposition at a high potential. Therefore, the mixing ratio is preferably 10 to 30% by volume. Further, as the positive electrode current collector, an aluminum foil having a thickness of 10 to 30 μm is generally used. When such a current collector is used, a high potential (4.5 V (vs. Li ^/ Li ⁺ )) is used. in more), there is a possibility that the corrosion of the foil itself progresses, as such corrosion is the effect of suppressing the (presumably because passive film of aluminum fluoride is formed) of LiPF ₆ supporting salt It is desirable to include. Furthermore, when the positive electrode is at a high potential, the oxidative decomposition of the electrolytic solution on the carbon surface used as the conductive agent is likely to proceed. Therefore, the amount of carbon as the conductive agent contained in the positive electrode is 5% by weight. The following is desirable.

_{Formula: Li a Mn s Ni t Co} u Mo v O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0.5,0 <t ≦ 0.5,0.45 ≦ s / ( s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u> 0 , 0.001 ≦ v ≦ 0.01), and a lithium transition metal composite having a layered structure In an electrode using an oxide as a positive electrode active material, the mechanism by which molybdenum is contained in the positive electrode active material to improve the thermal stability in a high-potential charged state is not clear at this time, but molybdenum is included. Thus, in a high-potential charged state, a) the crystal structure of the active material is stabilized, b) the oxidation state of the transition metal composite oxide is changed, and the catalytic activity for the electrolytic decomposition reaction is reduced, c) Effective for electrolyte decomposition reaction on the surface of positive electrode active material Such film is formed is inferred. In any case, the effect of adding molybdenum to the thermal stability is not well accepted in the state of charge of 4.3V (vs.Li/Li ^+), 4.5V of (vs.Li/Li ⁺⁾ High It appears effectively only in a charged state (see Table 2 and FIGS. 2 and 3).

  The amount of molybdenum contained in the positive electrode active material is 0.1 mol% or more and 1.0 mol% or less with respect to the total weight of transition metal elements other than Mo in the lithium transition metal composite oxide. desirable. If the amount of molybdenum is too small, an effect on thermal stability does not appear. On the other hand, if the amount of molybdenum is too large, the discharge characteristics of the positive electrode when charged at a high potential may be adversely affected.

According to the present invention, the chemical _{formula: Li a Mn s Ni t Co} u Mo v O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0.5,0 <t ≦ 0.5,0 .45 ≦ s / (s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u> 0 , 0.001 ≦ v ≦ 0.01. In a non-aqueous electrolyte secondary battery using a positive electrode mainly composed of a lithium transition metal composite oxide having a thermal stability even when the potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) or higher. It can be set as the nonaqueous electrolyte secondary battery excellent in safety | security.

  Therefore, it is possible to increase the charge voltage to increase the discharge capacity and to provide a non-aqueous electrolyte secondary battery with high thermal safety.

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

  A three-electrode beaker cell was prepared by the method described below, charged, and the thermal stability of the electrode in the charged state was evaluated.

<Experiment 1>
(Example 1)
[Preparation of positive electrode active material]
The molar ratio of LiOH, coprecipitated hydroxide represented by Mn _0.33 Ni _0.33 Co _0.34 (OH) ₂ , MoO ₂ , Li, transition metals other than Mo, and Mo is 1: 1: 0.01. As shown, the mixture was mixed in an Ishikawa type mortar, pulverized after heat treatment at 1000 ° C. for 20 hours in an air atmosphere, and expressed as LiMn _0.33 Ni _0.33 Co _0.34 Mo _0.01 O ₂ having an average particle size of about 10 μm. Lithium transition metal composite oxide was obtained.

(Production of working electrode)
In the positive electrode active material thus obtained, carbon as a conductive agent, polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone as a dispersion medium, the weight of the active material, the conductive agent, and the binder. The mixture was added so that the ratio was 92: 5: 3 and then kneaded to prepare a positive electrode slurry. The produced slurry was applied on an aluminum foil as a current collector, dried, then rolled using a rolling roller, and a current collecting tab was attached to produce a working electrode.

(Preparation of electrolyte)
In a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) is dissolved so as to have a concentration of 1 mol / liter, An electrolytic solution was prepared.

[Production of three-electrode beaker cell]
A three-electrode beaker cell shown in FIG. 1 was produced in a glove box under an Ar (argon) atmosphere. Note that lithium metal was used as the counter electrode and the reference electrode. As shown in FIG. 1, in the manufactured three-electrode beaker cell, the working electrode 1, the counter electrode 2, and the reference electrode 3 are immersed in the electrolytic solution 4.

[Evaluation of initial charge / discharge characteristics]
The prepared three-electrode beaker cell is charged at room temperature at a constant current of 0.75 mA / cm ² (about 0.3 C) until the working electrode potential reaches 4.5 V (vs. Li ^/ Li ⁺ ). Furthermore, after charging until the potential reaches 4.5 V (vs. Li / Li ⁺ ) at a constant current of 0.25 mA / cm ² (about 0.1 C), 0.75 mA / cm ² (about 0 The initial charge and discharge characteristics were evaluated by discharging until the potential reached 2.75 V (vs. Li ^/ Li ⁺ ) at a constant current of .3C).

[Evaluation of thermal stability]
After evaluating the initial charge / discharge characteristics, charging was performed at room temperature at a constant current of 0.75 mA / cm ² (about 0.3 C) until the working electrode potential reached 4.5 V (vs. Li ^/ Li ⁺ ). Further, the battery was charged with a constant current of 0.25 mA / cm ² (about 0.1 C) until the potential reached 4.5 V (vs. Li ^/ Li ⁺ ). After charging, the beaker cell was disassembled, the working electrode was washed in EMC, and then vacuum dried. A DSC sample was prepared by putting 3 mg of a part of this working electrode and 2 mg of EC into an aluminum DSC cell.

  The DSC measurement was performed from room temperature to 350 ° C. at a rate of temperature increase of 5 ° C./min using alumina as a reference.

(Example 2)
In the production of the positive electrode active material, the molar ratio of LiOH, a coprecipitated hydroxide represented by Mn _0.33 Ni _0.33 Co _0.34 (OH) ₂ , MoO ₂ , Li, a transition metal other than Mo, and Mo is 1 A three-electrode beaker cell was prepared in the same manner as in Example 1 except that the mixture was mixed in an Ishikawa-style raid mortar so that the ratio was 1: 0.005.

  Initial charge / discharge characteristics and thermal stability were evaluated in the same manner as in Example 1.

(Comparative Example 1)
In the production of the positive electrode active material, LiOH and a coprecipitated hydroxide represented by Mn _0.33 Ni _0.33 Co _0.34 (OH) ₂ are used so that the molar ratio of Li to the whole transition metal is 1: 1, A three-electrode beaker cell was produced in the same manner as in Example 1 except that the mixture was mixed in a rough mortar.

  Initial charge / discharge characteristics and thermal stability were evaluated in the same manner as in Example 1.

(Comparative Example 2)
A three-electrode beaker cell was produced in the same manner as in Example 1.

The initial charge / discharge characteristics and thermal stability were evaluated in the same manner as in Example 1 except that the charging potential of the working electrode was 4.3 V (vs. Li ^/ Li ⁺ ).

(Comparative Example 3)
A three-electrode beaker cell was produced in the same manner as in Example 2.

The initial charge / discharge characteristics and thermal stability were evaluated in the same manner as in Example 1 except that the charging potential of the working electrode was 4.3 V (vs. Li ^/ Li ⁺ ).

(Comparative Example 4)
A three-electrode beaker cell was produced in the same manner as in Comparative Example 1.

The initial charge / discharge characteristics and thermal stability were evaluated in the same manner as in Example 1 except that the charging potential of the working electrode was 4.3 V (vs. Li ^/ Li ⁺ ).

(Comparative Example 5)
A three-electrode beaker cell was produced in the same manner as in Example 1 except that LiCoO ₂ was used as the positive electrode active material. LiCoO ₂ is prepared by mixing LiOH and Co (OH) ₂ in a Ishikawa type mortar with a molar ratio of Li and Co of 1: 1, and then in an air atmosphere at 1000 ° C. Obtained by pulverization after heat treatment for 20 hours.

The initial charge / discharge characteristics and thermal stability were evaluated in the same manner as in Comparative Example 1 except that the charge potential of the working electrode was 4.3 V (vs. Li ^/ Li ⁺ ).

  The initial charge and discharge characteristics of the three-electrode beaker cells A1 and A2 of Examples 1 and 2 and the three-electrode beaker cells X1 to X5 of Comparative Examples 1 to 5 manufactured as described above, and thermal stability by DSC. The evaluation results are shown in Tables 1 and 2. Table 1 also shows the specific surface areas of the positive electrode materials produced in each example and each comparative example. In addition, the exothermic peak temperature in Table 2 is a temperature when the maximum calorific value is shown in the measurement temperature region. 2 shows DSC measurement results of beaker cells X2 to X5, and FIG. 3 shows DSC measurement results of beaker cells A1 and A2 and beaker cells X1 and X5.

As is apparent from Table 1, the initial discharge capacity is improved by 15% or more by changing the charge end potential from 4.3 V (vs. Li ^/ Li ⁺ ) to 4.5 V (vs. Li ^/ Li ⁺ ). I understand that. Further, the added amount of molybdenum (Mo), but is not observed its effect in the charging cutoff potential 4.3V (vs.Li/Li ^+), the charging end potential 4.5V (vs.Li/Li ^+), A slight decrease in the initial discharge capacity is confirmed as the addition amount increases, and it is considered that the discharge capacity further decreases in the region where the addition amount v is 0.01 <v. Accordingly, it is desirable that the range of the addition amount v satisfies 0.001 ≦ v ≦ 0.01.

Further, as is clear from Table 2 and FIGS. 2 and 3, the thermal stability of LiMn _0.33 Ni _0.33 Co _0.34 O ₂ is increased by adding molybdenum at the end of charging of 4.3 V (vs. Li ^/ Li ⁺ ). Although the exothermic peak intensity decreases, the exothermic peak temperature is shifted to the low temperature side. Patent Document 4 describes that molybdenum (Mo) is added to a Ni—Mn—Co lithium-containing transition metal oxide mainly composed of Ni. In the prior art, as in Comparative Examples 2 to 4, the peak is obtained. Although the strength is decreased, the exothermic peak temperature tends to shift to a low temperature side. Therefore, a battery having a conventional charging voltage of 4.2 V (4.3 V at the positive electrode potential (vs. Li ^/ Li ⁺ )) has not been sufficiently improved in thermal stability. On the other hand, in the battery of the present invention in which the voltage is set to 4.4 V or more, the heat stability as a battery shows a tendency to improve the thermal stability of both the exothermic peak temperature and the exothermic peak intensity as the molybdenum content increases. It was found that the mechanical stability was improved. Thus, the addition of molybdenum to LiMn _0.33 Ni _0.33 Co _0.34 O ₂ is effective only when the charge voltage is higher than the conventional charge voltage (4.5 V (vs. Li ^/ Li ⁺ ) or more at the positive electrode potential). It can be seen that

In the above examples, the positive electrode material of the present invention is evaluated by a beaker cell. However, when the nonaqueous electrolyte secondary battery of the present invention is manufactured, the potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ), The positive electrode capacity and the negative electrode capacity are designed so that the ratio of the opposing charge capacity of the positive electrode to the negative electrode when it is charged to reach 1.0 to 1.15, and a nonaqueous electrolyte secondary battery is manufactured.

The schematic diagram which shows the three-electrode-type beaker cell produced in the Example of this invention. The figure which shows the DSC measurement result of beaker cell X2-X5. The figure which shows the DSC measurement result of beaker cell A1, A2 and beaker cell X1,5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Working electrode 2 ... Counter electrode 3 ... Reference electrode 4 ... Electrolyte

Claims (4)

A positive electrode and a negative electrode using a material capable of inserting and extracting lithium ions, a non-aqueous electrolyte, and a positive electrode when charged until the potential of the positive electrode reaches 4.5 V (vs. Li ^/ Li ⁺ ); In the nonaqueous electrolyte secondary battery configured such that the counter charge capacity ratio of the negative electrode is 1.0 to 1.15,
The positive active main material is the chemical formula of the _{substance: Li a Mn s Ni t Co} u Mo v O 2 (0 ≦ a ≦ 1.2, s + t + u = 1,0 <s ≦ 0.5,0 <t ≦ 0.5 0.45 ≦ s / (s + t) ≦ 0.55, 0.45 ≦ t / (s + t) ≦ 0.55, u> 0 , 0.001 ≦ v ≦ 0.01. A nonaqueous electrolyte secondary battery comprising a lithium transition metal composite oxide. The active material of the negative electrode is a carbon material, and the battery voltage when the potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) is 4.4 V. Non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide has a specific surface area of 0.1 to 2.0 m ² / g.   The positive electrode contains a carbon material as a conductive agent, and the content of the carbon material is 5% by weight or less based on the total of the positive electrode active material, the conductive agent, and the binder. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3.

Images