JP2012043637A - Positive electrode active material for nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material which is inexpensive and is improved in charge discharge capacity and output characteristics.SOLUTION: A lithium transition metal composite oxide represented by general formula LiNiMnWO(wherein a, b, and c satisfy 1<a<1.2, 0.5≤b≤0.7, and 0<c≤0.02 respectively) is used as the positive electrode active material.

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery.

  In recent years, portable devices such as video cameras, mobile phones, and notebook personal computers have become widespread and downsized, and non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used for power supplies. Furthermore, it has been attracting attention as a power battery for electric vehicles and the like due to recent environmental problems.

As a positive electrode active material for a lithium secondary battery, LiCoO ₂ (lithium cobaltate) is generally widely adopted as being capable of constituting a 4V class secondary battery. When LiCoO ₂ is used as the positive electrode active material, it has been put to practical use with a discharge capacity of about 160 mA / g.

Cobalt, which is a raw material for LiCoO ₂ , is a scarce resource and is unevenly distributed, and thus costs are high and anxiety arises regarding the supply of raw materials.

In response to such circumstances, LiNiO ₂ (lithium nickelate) has also been studied. LiNiO ₂ can practically realize a secondary battery with a discharge capacity of about 200 mA / g with a 4V class. However, the stability of the crystal structure of the positive electrode active material during charge / discharge is difficult.

Therefore, research has been conducted to replace the nickel atom of LiNiO ₂ with another element to improve the stability of the crystal structure and realize a discharge capacity similar to LiCoO ₂ at a low cost. For example, a positive electrode active material having nickel and manganese as essential elements, such as LiNi _0.5 Mn _0.5 O ₂ , has a discharge capacity of about 160 mA / g.

  In addition, the nickel atom is replaced with another element so that the charge / discharge capacity can be maintained even in a more severe situation, for example, when charging at 4.3 V higher than before, or using at a higher discharge current. Examples also exist (see Patent Documents 1 and 2).

JP 2005-235628 A JP 2007-073425 A

  On the other hand, further improvements in the charge / discharge capacity and output characteristics of the positive electrode active material are required for mounting on larger application products, mounting on more sophisticated devices, and further downsizing of the mounted secondary battery. is necessary. However, the conventional technology such as the above technology has not yet achieved both low cost and improvement in charge / discharge capacity and output characteristics.

  The present invention has been made in view of such circumstances. An object of the present invention is to provide a positive electrode active material having improved charge / discharge capacity and output characteristics at a lower cost than conventional ones.

  In order to achieve the above object, the inventors of the present application have made extensive studies and have completed the present invention. The present inventors have found that the charge / discharge capacity and the output characteristics can be improved by adding tungsten to the lithium transition metal composite oxide having a specific composition.

The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention has a general formula Li _a Ni _b Mn _1-b W _c O ₂ (where 1 <a <1.2, 0.5 ≦ b ≦ 0.7, 0 <c ≦ 0.02).

  The crystallite diameter of the lithium transition metal composite oxide is preferably 340 mm or more when 0.5 ≦ b ≦ 0.55, and 400 mm or more when 0.55 <b ≦ 0.7.

  Furthermore, the lithium transition metal composite oxide is preferably composed of secondary particles in which primary particles are aggregated, and the center particle size of the secondary particles is preferably 20 μm or less.

  Since the positive electrode active material of the present invention has the above characteristics, the charge / discharge capacity and the output characteristics can be improved without using costly cobalt. Therefore, a nonaqueous electrolyte secondary battery having high charge / discharge capacity and high output characteristics can be obtained at low cost by using the positive electrode active material of the present invention for the positive electrode.

  Hereinafter, the positive electrode active material of the present invention will be described in detail using embodiments and examples. However, the present invention is not limited to these embodiments and examples.

The composition of the lithium transition metal composite oxide is represented by Li _a Ni _b Mn _1-b W _c O ₂ , 1 <a <1.2, 0.5 ≦ b ≦ 0.7, 0 <c ≦ 0. 02 needs to be satisfied. When a is 1 or less, the charge / discharge capacity and output characteristics are lowered, which is not preferable. Moreover, since it is easy to raise | generate sintering when it is 1.2 or more, it is unpreferable. If b is less than the lower limit, the discharge capacity and output characteristics are lowered, which is not preferable. On the other hand, exceeding the upper limit is not preferable in terms of reduction in discharge capacity, increase in production costs, and generation of subphases. Regarding c, if c = 0, it is not preferable from the viewpoint of output characteristic deterioration. Furthermore, compared to the case of c> 0, there is a tendency to sinter at a lower temperature, and it is difficult to control the crystallite diameter and the center particle diameter. Moreover, when it exceeds the upper limit, it is not preferable in terms of reduction of charge / discharge capacity and output characteristics. Preferred ranges of these a, b and c are 1.12 ≦ a ≦ 1.18, 0.58 ≦ b ≦ 0.62 and 0.007 ≦ c ≦ 0.011, and the charge / discharge capacity and output characteristics are This is advantageous.

  The crystallite size of the lithium transition metal composite oxide is preferably larger than a certain level. The preferred range of the crystallite diameter depends on the composition of the lithium transition metal composite oxide, but is preferably 340 mm or more if 0.5 ≦ b ≦ 0.55, or 400 mm or more if 0.55 <b ≦ 0.7. If the crystallite diameter is so large, the output characteristics are particularly improved. Although there is no upper limit, the effect is remarkable up to about 600cm. More preferred is 550 mm or less.

The structure of the positive electrode active material of the present application is a trigonal system represented by the space group R-3m using the Herman-Morgan symbol, and the crystallite diameter is obtained on the (104) plane obtained by the X-ray diffraction method. A value calculated from the following formula (1) (Scherrer formula) is used based on the resulting diffraction peak. The X-ray diffraction method can be performed, for example, under conditions of a tube current of 40 mA and a tube voltage of 40 kV.
Dc = Kλ / (βcos θ) (1)
(Dc: crystallite diameter, K: Scherrer constant (sintered Si for optical system adjustment (manufactured by Rigaku Corporation) is used, and the value that the crystallite diameter based on the diffraction peak due to the (220) plane is 1000 mm is used) ), Λ: wavelength of the X-ray source (λ = 1.540562 in the case of CuKα1), β: half-width of the diffraction line based on the sample (calculated from β = By by the arc degree method (B: observed half-width (arc degree) Y = 0.9991-0.019505b-2.8205b ² + 2.878b ³ -1.0366b ⁴ (b: half-value width based on device system (according to arc degree method)))), θ: diffraction angle)

  The lithium transition metal composite oxide is composed of secondary particles in which primary particles are aggregated, and the center particle size of the secondary particles is preferably 20 μm or less. Within this range, the output characteristics are particularly improved. A more preferable range is 11 μm or less, and a further preferable range is 3 μm or more and 6 μm or less.

  A value obtained by a laser diffraction method is adopted as the center particle size.

[Production of positive electrode active material]
Next, a production example of the positive electrode active material will be described.

  First, a lithium compound, a nickel compound, a manganese compound, and a tungsten compound are weighed and mixed according to a target composition. As a mixing method, the raw material powders may be mixed with a mixer, or a coprecipitation salt may be formed in the solution by adjusting pH, using a complexing agent, and the like, followed by filtration and drying. Alternatively, a slurry or emulsion may be prepared and a mixture may be obtained by spray drying or the like.

  As the lithium compound, the nickel compound, the manganese compound, and the tungsten compound, any of oxides, hydroxides, carbonates, nitrates, and the like that can be thermally decomposed to become oxides can be used. For tungsten, metallic tungsten may be used.

  Since the central particle size of nickel compounds, manganese compounds, or composite compounds containing these influences the central particle size of the positive electrode active material, the central particle size of the raw material is appropriately selected according to the central particle size of the target positive electrode active material Or adjust.

  Next, the raw material mixture is fired. If the firing temperature is too low, the reaction with lithium will be insufficient, or a sufficient crystallite size will not be obtained, and if it is too high, lithium will volatilize or sintering will occur. is required. Although it depends on the b value of the target composition, it is generally preferably 800 ° C. or higher and 1100 ° C. or lower. More preferably, it is 900 degreeC or more and 1050 degrees C or less. It is sufficient that the firing time is 9 hours or more as the time for maintaining the maximum temperature. An air atmosphere or an oxygen atmosphere can be used as the firing atmosphere. From the viewpoint of cost, atmospheric firing is more preferable. The crystallite diameter is adjusted by changing the firing temperature. As described above, if the temperature is 800 ° C. or higher, a sufficiently large crystallite diameter can be obtained. In general, the higher the firing temperature, the larger the crystallite diameter.

  After firing, if necessary, treatments such as crushing, pulverization, and dry sieving are performed to obtain the positive electrode active material of the present invention.

  Hereinafter, more specific examples will be described in Examples.

A mixture of 0.545 mol of lithium carbonate, 1.00 mol of nickel manganese composite oxide (Ni (II) / Mn (II) = 6/4) with a central particle size of 5 μm, and 0.01 mol of tungsten (VI) oxide in the atmosphere. Bake at 1020 ° C. for 9 hours. Post-baking treatment is performed to obtain a positive electrode active material having the composition Li _1.09 Ni _0.60 Mn _0.40 W _0.01 O ₂ .

A positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 W _0.01 O ₂ is obtained in the same manner as in Example 1 except that the lithium carbonate to be mixed is 0.59 mol.

1.18 mol of lithium hydroxide, 1.00 mol of nickel manganese composite oxide (Ni (II) / Mn (II) = 7/3) with a central particle size of 5 μm, 0.01 mol of tungsten oxide (VI) are mixed, and oxygen atmosphere Bake at 920 ° C for 9 hours. After firing, the _{mixture is} pulverized to obtain a positive electrode active material having the composition Li _1.18 Ni _0.70 Mn _0.30 W _0.01 O ₂ .

A mixture of 0.59 mol of lithium carbonate, 1.00 mol of nickel manganese composite oxide (Ni (II) / Mn = 5/5) and 0.01 mol of tungsten oxide (VI) with a central particle size of 5 μm was mixed at 1050 ° C. in an air atmosphere. Bake for 9 hours. After firing, the _{mixture is} pulverized to obtain a positive electrode active material having the composition Li _1.18 Ni _0.50 Mn _0.50 W _0.01 O ₂ .

A positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 W _0.02 O ₂ is obtained in the same manner as in Example 2 except that the tungsten oxide (VI) to be mixed is 0.02 mol.

A positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 W _0.01 O ₂ is obtained in the same manner as in Example 2 except that the firing temperature is 960 ° C.

A positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 W _0.01 O ₂ is obtained in the same manner as in Example 2 except that the firing temperature is 930 ° C.

A positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 W _0.01 O ₂ is obtained in the same manner as in Example 2 except that the firing temperature is 890 ° C.

Composition Li _1.18 Ni _0.60 Mn _0.40 W in the same manner as in Example 2 except that nickel manganese composite oxide (Ni (II) / Mn (II) = 6/4) having a center particle size of 10 μm was used. A positive electrode active material of _0.01 O ₂ is obtained.

Composition Li _1.18 Ni _0.60 Mn _0.40 W in the same manner as in Example 2 except that nickel manganese composite oxide (Ni (II) / Mn (II) = 6/4) having a center particle diameter of 8 μm was used. A positive electrode active material of _0.01 O ₂ is obtained.

[Comparative Example 1]
A positive electrode active material having the composition Li _0.96 Ni _0.60 Mn _0.40 W _0.01 O ₂ is obtained in the same manner as in Example 1 except that the lithium carbonate to be mixed is 0.48 mol.

[Comparative Example 2]
A positive electrode active material having the composition Li _1.24 Ni _0.60 Mn _0.40 W _0.01 O ₂ is obtained in the same manner as in Example 1 except that the lithium carbonate to be mixed is 0.62 mol. Even if this positive electrode active material is pulverized, it does not pass through a sieve and cannot be treated.

[Comparative Example 3]
A positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 O ₂ is obtained in the same manner as in Example 2 except that tungsten oxide (VI) is not mixed and the firing temperature is 890 ° C.

[Comparative Example 4]
1.18 mol of lithium hydroxide and 1.00 mol of nickel manganese composite oxide (Ni (II) / Mn (II) = 6/4) having a central particle size of 10 μm are mixed and baked at 890 ° C. for 9 hours in an air atmosphere. Post-baking treatment is performed to obtain a positive electrode active material having the composition Li _1.18 Ni _0.60 Mn _0.40 O ₂ .

[Comparative Example 5]
The composition Li _1.18 Ni _0.60 Mn _0.40 was obtained in the same manner as in Comparative Example 5 except that nickel manganese composite oxide (Ni (II) / Mn (II) = 6/4) having a center particle diameter of 8 μm was used. A positive electrode active material of O ₂ is obtained.

[Comparative Example 6]
The composition Li _1.18 Ni _0.60 Mn _0.40 was obtained in the same manner as in Comparative Example 5 except that nickel manganese composite oxide (Ni (II) / Mn (II) = 6/4) having a center particle diameter of 5 μm was used. A positive electrode active material of O ₂ is obtained.

  In Examples 1 to 10 and Comparative Examples 1 to 6, it can be confirmed by a scanning electron microscope that secondary particles in which primary particles are aggregated are formed.

[Evaluation of positive electrode active material]
[Crystallite diameter]
An X-ray diffraction method is performed on the obtained positive electrode active material. The X-ray diffraction method is performed using an X-ray diffractometer (Ultima, manufactured by Rigaku Corporation), using CuKα1 as an X-ray source, under conditions of a tube current of 40 mA and a tube voltage of 40 kV. Based on the X-ray diffraction pattern obtained by the X-ray diffraction method, the crystallite diameter Dc of the positive electrode active material is obtained from the Scherrer equation represented by the above equation (1).

[Particle size of secondary particles]
About the obtained positive electrode active material, the center particle diameter D50 of a secondary particle is measured with a laser diffraction method.

[Evaluation of battery characteristics]
[Production of secondary battery]
A secondary battery is prepared in the following manner and used for various evaluations.

[For battery resistance evaluation]
A paste is prepared by dispersing and dissolving 90% by weight of a positive electrode active material powder, 5% by weight of carbon powder as a conductive material, and 5% by weight of polyvinylidene fluoride (PVDF) in normal methylpyrrolidone (NMP) and kneading. To do. This is applied to a current collector made of aluminum foil, dried, and rolled into a positive electrode plate.

  A graphite material is used as the negative electrode active material. A paste is prepared by dispersing 97.5 wt% of the negative electrode active material powder, 1.5 wt% of carboxymethyl cellulose (CMC), and 1.0 wt% of styrene butadiene rubber (SBR) in water and kneading. This is applied to a current collector made of copper foil, dried, and rolled into a negative electrode plate.

Ethylene carbonate (EC) and methyl ethyl carbonate (MEC) are mixed at a volume ratio of 3: 7. Lithium hexafluorophosphate (LiPF ₆ ) is dissolved as an electrolyte in the resulting mixed solvent to prepare a nonaqueous electrolytic solution having a concentration of 1 mol / l.

  A porous polyethylene film is used as the separator.

  A lead electrode is attached to the positive electrode plate and the negative electrode plate, and the positive electrode, the separator, and the negative electrode are stacked in this order. These are stored in a laminate pack, an electrolyte solution is injected, and the laminate pack is sealed to obtain a laminate type secondary battery. This is used for battery resistance evaluation.

[For charge / discharge capacity evaluation]
Metal lithium is used as the negative electrode active material and is molded into a thin sheet to form a negative electrode plate. The positive electrode plate and separator are the same as those used for battery resistance evaluation.

  A lead electrode is attached to the positive electrode plate, and the negative electrode, the separator, and the positive electrode are sequentially stored in the container. The negative electrode is electrically connected to a stainless steel container bottom, and the container bottom serves as a negative electrode terminal. The separator is fixed by the side of the container made of Teflon (registered trademark). The tip of the positive lead electrode is led out of the container and becomes a positive terminal. The positive and negative terminals are electrically insulated by the container side. After storage, the electrolyte is injected and sealed with a stainless steel container lid to obtain a sealed test battery. This is used for evaluation of charge / discharge capacity.

[Battery resistance]
Measure current and potential as follows to determine battery resistance.

  At a measurement temperature of 25 ° C., a full charge voltage is set to 4.2 V, and constant current charging is performed up to a charge depth of 50%. After applying the pulse for 10 seconds, the discharge and charging are repeated sequentially in 3 minutes. The pulse discharge / charge current value i is 0.04A, 0.08A, 0.12A, 0.16A and 0.20A. The current value i is plotted on the horizontal axis of the graph, the voltage value V after 10 seconds of pulse discharge is plotted on the vertical axis of the graph, and the absolute value of the slope is obtained in the current range in which linear linearity is maintained in the i-V plot. Let R (25).

  At a measurement temperature of −25 ° C., the full charge voltage is set to 4.2 V, and constant current charging is performed to a charge depth of 50%, and then pulse discharge is performed at a specific current value. The pulse is discharged only for 10 minutes after the application for 10 seconds. The pulse discharge current value i is 0.04A, 0.06A, 0.08A, and 0.10A. Hereinafter, the battery resistance is obtained in the same manner as R (25) and is set to R (−25). Low R means high output characteristics.

[Initial charge capacity]
Charge at constant current and constant voltage with a full charge voltage of 4.3V and a charge load of 0.2C (1C: current value that completes the discharge in 1 hour from the fully charged state), and the charge accumulated up to the full charge voltage is the initial charge capacity. _Let Q _c0 .

[Initial discharge capacity]
A constant current discharge is performed at a full charge voltage of 4.3 V, a discharge voltage of 2.75 V, and a discharge load of 0.2 C, and the charge released up to the discharge voltage is defined as an initial discharge capacity _Qd0 .

[Load discharge capacity]
Charging and discharging are performed in the order of a charging potential of 4.3 V, a discharging potential of 2.75 V, and discharging loads of 0.2 C, 1 C, and 3 C, respectively. The discharge capacity at 3C is defined as a load discharge capacity Q ′.

[Initial efficiency]
The ratio of the initial discharge capacity to the initial charge capacity (≡Q _d0 / Q _c0 ) is defined as the initial efficiency E ₀ .

[Load efficiency]
The ratio (≡Q ′ / Q _d0 ) of the load discharge capacity to the initial discharge capacity is defined as load efficiency E ′. High Q ′ and E ′ mean that the load characteristics are good.

  For Examples 1 to 10 and Comparative Examples 1 to 6, Table 1 shows the relationship between the a value and various properties, Table 2 shows the relationship between the b value and various properties, and Table 3 shows the relationship between the c value and various properties. Table 4 shows the relationship between the firing temperature, the crystallite diameter Dc, and various properties, and Table 5 shows the relationship between the central particle diameter D50 of the secondary particles and the various properties.

  From Table 1, it can be seen that the output characteristics and the charge / discharge capacity are improved when 0 <a <1.2. Further, from Tables 2 and 3, it is understood that the output characteristics are improved by satisfying 0.5 ≦ b ≦ 0.7 and 0 <c ≦ 0.2. Table 4 also shows that the output characteristics and charge / discharge capacity are improved as the crystallite diameter increases, and that the crystallite diameter is preferably 400 mm or more in consideration of the load characteristics. Further, it can be seen from Table 5 that the output characteristics are improved when D50 is small, and the output characteristics are improved when tungsten is present at the same D50.

  By using the positive electrode active material of the present invention, a non-aqueous electrolyte secondary battery with high output characteristics that can be used with a large current can be realized at a lower cost than in the past. The non-aqueous electrolyte secondary battery realized in this way can be particularly suitably used for high output applications such as medium-sized devices such as electric tools and electric assist bicycles, large-sized devices such as electric vehicles and hybrid electric vehicles.

Claims (3)

Formula _{Li a Ni b Mn 1-b} W c O 2
(However, 1 <a <1.2, 0.5 ≦ b ≦ 0.7, 0 <c ≦ 0.02)
A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide represented by:   The non-aqueous electrolyte secondary electrode in which the crystallite diameter of the lithium transition metal composite oxide is 340 mm or more when 0.5 ≦ b ≦ 0.55, and 400 mm or more when 0.55 <b ≦ 0.7. Positive electrode active material for batteries.   A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the lithium transition metal composite oxide is composed of secondary particles in which primary particles are aggregated, and the center particle size of the secondary particles is 20 μm or less.