JP5165515B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

There is a lithium ion secondary battery as a secondary battery that is used as a power source for electronic devices and is expected to be reduced in size and weight. As the positive electrode active material of these lithium ion secondary batteries, metal oxides containing Li such as lithium cobaltate (LiCoO ₂ ) and lithium manganate (LiMn ₂ O ₄ ) have been studied and put into practical use.

  However, in recent years, with increasing demand for higher capacity of batteries, such as adding force, the usage conditions that exceed the allowable range of reliability and safety, that is, to suppress the heat generation of the battery at the time of abuse Technology development is required.

For this reason, as a positive electrode material, a phosphate compound represented by LiFePO ₄ containing Fe has been proposed because it has a feature that it has a strong PO bond and is thermally stable.

  The phosphate compound containing Fe has a problem in terms of increasing the energy density because it has a low electronic conductivity compared to lithium cobaltate and the like, and has a low potential of 3.3 V with respect to Li metal.

On the other hand, the compound represented by LiMnPO ₄ using Mn instead of Fe has a potential with respect to Li metal of around 4.2 V, and the operating potential of the battery can be made equivalent to that of lithium cobalt oxide. However, there is a problem that the electron conductivity is lower than that of the phosphate compound using Fe.

In order to improve the charge / discharge characteristics of LiMnPO ₄ , various studies have been made so far.

  In Patent Document 1 and Patent Document 2, a method is proposed in which a part of Mn is substituted with a different element and then hydrothermal synthesis is performed at a temperature of 350 ° C. or lower.

In Patent Document 1, Li _x A _y D _z PO _{4 is used} for the purpose of realizing high discharge capacity, stable charge / discharge cycle characteristics, high fillability and high output by using inexpensive and resource-rich elements. (However, A is one selected from Co, Ni, Mn, Fe, Cu, Cr, D is Mg, Ca, Fe, Ni, Co, Mn, Zn, Ge, Cu, Cr, Ti, Sr, Ba , Sc, Y, Al, Ga, In, Si, B, one or more selected from rare earth elements and different from the above A). Then, a lithium component, a phosphorus component, the A component, the D component, and an organic acid soluble in water are added to a solvent containing water as a main component, and then the solution is heated under pressure so that the Li lithium battery that generates _x a _y D _z PO ₄ Method of manufacturing an active material, and a positive electrode active material for a lithium battery obtained was disclosed by this production method.

Patent Document 2, for the purpose of using in abundance as a resource element at a low cost, high discharge capacity, stable charge and discharge cycle characteristics, to achieve high filling property and high _{output, Li x Al y A z PO} 4 (Where A is one or more selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, x + 3y + 2z = 3, x, y, and z are positive numbers). A method for producing an electrode material comprising: a Li source, an A source (where A is one or two selected from the group of Co, Mn, Ni, Fe, Cu, Cr) A method for producing an electrode material is disclosed, in which an Al source, a PO ₄ source and an organic acid are added to form a solution, and then the solution is reacted under high temperature and high pressure.

Patent Document 3 discloses a method for producing a carbon-coated Li-containing meteorite or NASICON powder that forms a Li-containing meteorite or NASICON crystal phase by a predetermined process for the purpose of providing a carbon-coated Li-containing powder and a method for producing the same. Is disclosed. Further, in Patent Document 4, the crystal phase is Li _u M _v (XO ₄ ) _w [wherein u = 1, 2, or 3, v = 1 or 2, w = 1 or 3, M is Ti _{a V b Cr c Mn d Fe e} Co f Ni g Sc h Nb i ( where, a + b + c + d + e + f + g + h + i = 1) _represents, X represents a _{P x-1 S x (0} ≦ x ≦ 1). ] Is disclosed.

Patent Document 4 discloses a method for producing electrode material powder, an electrode material powder and an electrode, and a lithium battery that can easily and inexpensively produce nanoparticles having a uniform composition and high purity even when inexpensive LiOH is used. Li _x A _y B _z PO ₄ (where A is one or more selected from the group consisting of Fe, Co, Mn, Ni, Cr, and Cu, and B is Mg, Ca , Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, Y, and one or more selected from the group of rare earth elements) A method for producing a powder, comprising lithium hydroxide, A source and / or B source, phosphoric acid and / or phosphate, lithium hydroxide and phosphoric acid and / or phosphate phosphoric acid Reaction inhibition that inhibits reaction with groups And a precursor containing a solution, dispersion or suspension containing the A source and a reaction retarder that retards the reaction between the phosphoric acid and / or the phosphate group of the phosphate in a high-temperature atmosphere. A method for producing electrode material powder is disclosed, characterized in that the precursor is heat treated.

However, the examples of Patent Documents 1 and 3 only describe LiFePO ₄ , and Patent Document 2 describes Li _x Al _y Fe _z PO ₄ or Li _x Al _y Fe _z1 Mg _z2 PO ₄ and Only the substances that can be represented are described, and none of Patent Documents 1 to 3 discloses an example of LiMnPO ₄ .

However, LiMnPO ₄ is described in the example of Patent Document 4. LiMnPO ₄ in this example is described as having a uniform composition composed of a single phase of LiMnPO _{4 as} a result of powder X-ray diffraction (XRD) as a spherical shape with an average particle size of secondary particles of 0.5 to 3 μm. Yes.

  According to these methods, since the synthesis is performed at a low temperature, the particle size of the obtained compound is reduced, the reaction surface area is increased, and Li insertion / extraction is facilitated. However, when different kinds of elements are included, there is a risk that the potential curve at the time of discharge will have a multi-stage plateau, and the control system for charging and discharging the battery will be complicated.

  In Non-Patent Document 1, attempts are made to refine crystal grains by synthesizing in an organic solvent such as ethylene glycol.

  Non-Patent Document 1 discusses that phosphate compounds using Fe, Ce, and Ni can form fine and uniform particles. However, in the phosphate compound using Mn, the crystal particles become needle-like crystals, and it is difficult to increase the density of the electrodes.

JP 2005-276474 A JP 2006-261060 A JP 2005-530676 Gazette JP 2005-116393 A J. et al. Yang: J.A. Electrochem. Soc. 153 (4) A716-723 (2006)

In order to apply LiMnPO ₄ as a positive electrode active material of a lithium ion secondary battery, the present invention has a potential with respect to Li metal of around 4.2 V, and the operating potential of the battery can be equivalent to that of lithium cobalt oxide. An object of the present invention is to improve the electron conductivity and achieve both high energy density and high safety as a lithium ion secondary battery.

The positive electrode active material of the present invention is a positive electrode active material formed of LiMnPO ₄ , and this LiMnPO ₄ is characterized in that the primary particles are in close contact and the secondary particles have fine irregularities formed by the primary particles. And

According to the present invention, it is possible to improves the positive electrode active material in which the electron conductivity of LiMnPO ₄ of the lithium ion secondary batteries, satisfying both high energy density and high safety as a lithium ion secondary battery.

  In addition, according to the present invention, it is possible to provide a lithium ion secondary battery that is thermally stable even during abuse and can realize high energy density.

  The present invention relates to a positive electrode active material for a lithium ion secondary battery excellent in safety.

  Examples of the present invention are shown below. The present invention is not limited to these examples.

  Lithium acetate dihydrate (0.025 mol) is added to 25 ml of distilled water, and an aqueous solution (lithium acetate aqueous solution) is prepared while stirring. Thereafter, manganese acetate tetrahydrate (0.025 mol) is added. Thereafter, ammonium dihydrogen phosphate (0.025 mol) was added. The steps so far are referred to as a step of mixing lithium, manganese and phosphoric acid, that is, a lithium / manganese / phosphoric acid mixing step or a raw material mixing step.

Thereafter, citric acid (0.075 mol) is added and mixed as a chelating agent. Thereafter, the water is evaporated while heating and stirring. After evaporating the water, the remaining material was recovered and used as a precursor, and this precursor was heat-treated at 800 ° C. (called a firing atmosphere) using an atmosphere furnace (argon gas stream) to produce LiMnPO ₄ . did. The heat treatment performed here may generally be called heat treatment in a predetermined firing atmosphere.

  In this example, citric acid was added as described above, but other organic acids such as malic acid, tartaric acid, succinic acid, and the like can be used instead of citric acid. The organic acid may be a mixture of a plurality of organic acids among citric acid, malic acid, tartaric acid, succinic acid, and the like.

  The calcined sample was pulverized for 1 hour using a meteor type ball mill (manufactured by FRITSCH: Planetary micro mill pulse set 7), and coarse particles of 45 μm or more were removed by sieving. The resistivity was measured by weighing 1 g of the sample and using a powder resistance evaluation apparatus (Mitsubishi Chemical: Lorester GP). In this evaluation, the resistivity when a load of 40 MPa was applied by hydraulic pressure was used as a standard.

  The XRD evaluation method is shown below. In so-called 2θ / θ measurement using an automatic X-ray diffractometer (manufactured by Rigaku Corporation: RINT-UltimaIII), X-ray source: CuKα, output: 40 kV × 40 mA, 2θ = 15-80 ° sampling angle 0.01 The X-ray diffraction profile was measured under the conditions of a divergence slit of 0.5 °, a scanning slit of 0.5 °, and a light receiving slit of 0.15 mm at a scan rate of 0.1 ° / sec. The obtained diffraction profile is processed in the order of smoothing, background removal, and Kα2 removal, and then collated with an ICDD (International Center for Diffraction Data) card (PDF-2), and the diffraction angle 2θ is around 20.5 °. (011) and I (131) peak intensity around 2θ = 35.1 ° were measured.

  An example of a method for producing a lithium secondary battery is as follows.

  A positive electrode active material is mixed with a conductive material of carbon material powder and a binder such as polyvinylidene fluoride to prepare a slurry. The mixing ratio of the conductive material to the positive electrode active material is preferably 3 to 10% by weight. The mixing ratio of the binder to the positive electrode active material is preferably 2 to 10% by weight. At this time, in order to uniformly disperse the positive electrode active material in the slurry, it is preferable to perform sufficient kneading using a kneader. The obtained slurry is applied onto an aluminum foil having a thickness of 15 to 25 μm by, for example, a roll transfer machine. After coating, the positive electrode plate is formed by drying and pressing. As for the thickness of the mixture part which mixed the positive electrode active material, the electrically conductive material, and the binder, 200-250 micrometers is desirable.

  Hereinafter, preparation of the positive electrode will be described.

  A positive electrode was produced using the obtained positive electrode active material. The positive electrode active material, the carbon-based conductive material, and the binder previously dissolved in the solvent N-methyl-2-pyrrolidone (NMP) are expressed in mass% and are 89.0: 5.5: 5. 5 was mixed, and the mixed slurry was applied to an aluminum current collector having a thickness of 20 μm.

Then, it dried at 120 degreeC and compression-molded so that the electrode density might be set to about 1 g / cm < ³ > with the press. After compression molding, a positive electrode was produced by punching into a disk shape having a diameter of 15 mm using a punched metal fitting.

  Hereinafter, production of a test battery will be described.

Using the produced positive electrode, a metal lithium was used as a negative electrode, and a microporous film such as a separator made of polyethylene (PE) or polypropylene (PP) was interposed between the positive electrode and the negative electrode to suppress short circuit. As the electrolytic solution, a solution obtained by dissolving 1.0 mol of LiPF ₆ in a mixed solvent of EC (ethylene carbonate) and EMC (ethyl methyl carbonate) was used.

  Hereinafter, characteristic evaluation of the positive electrode will be described.

  Here, the discharge capacity characteristics of the positive electrode were evaluated by the following procedure. Using a test battery, the battery was charged at a constant current / constant voltage to 4.2 V at a charge rate of 1/50 C, and then discharged to 2.7 V at a discharge rate of 1/50 C. This was taken as one cycle and repeated three cycles. The discharge capacity at the third cycle was evaluated as the discharge capacity of the present invention.

  The firing was performed under the same conditions as in Example 1 except that the firing atmosphere was an argon gas stream mixed with 3% hydrogen gas.

  In the raw material mixing step, the same conditions as in Example 1 were used except that lithium dihydrogen phosphate was used instead of lithium acetate dihydrate and ammonium dihydrogen phosphate. In this case, manganese acetate tetrahydrate may be added to the lithium dihydrogen phosphate aqueous solution, or lithium dihydrogen phosphate may be added to the manganese acetate aqueous solution in which manganese acetate tetrahydrate is dissolved in distilled water. This step may be called a step of mixing lithium dihydrogen phosphate and manganese acetate tetrahydrate.

  As in Example 2, the firing atmosphere was an argon gas stream mixed with 3% hydrogen gas, and in the same manner as in Example 3, instead of lithium acetate dihydrate and ammonium dihydrogen phosphate in the raw material mixing step. The same conditions as in Example 1 were used except that lithium dihydrogen phosphate was used.

(Comparative Example 1)
Lithium carbonate (0.025 mol), manganese carbonate (0.025 mol) and ammonium dihydrogen phosphate (0.025 mol) are put in a zirconia pot, 0.9 g of carboxymethylcellulose (CMC) is added, and a meteor type Mix for 1 h (1 hour) using a ball mill. The recovered mixed material was heat-treated at 800 ° C. for 4 hours using an atmosphere furnace (argon gas stream).

  Sample evaluation after firing was performed under the conditions described in Example 1.

(Comparative Example 2)
It carried out according to the comparative example 1 except having added 0.3g of carbon black instead of carboxymethylcellulose.

(Comparative Example 3)
Lithium acetate dihydrate (0.025 mol) is added to 25 ml of ethylene glycol and a solution is prepared with stirring. Thereafter, manganese acetate tetrahydrate (0.025 mol) is added. Then phosphoric anhydride (0.025 mol) was added. Thereafter, the precursor recovered by filtration was heat-treated at 800 ° C. for 4 hours using an atmosphere furnace (argon gas stream).

  This comparative example is based on the manufacturing method of Non-Patent Document 1.

  Sample evaluation after firing was performed under the conditions described in Example 1.

(Comparative Example 4)
It carried out according to the comparative example 3 except having added 2.5g of diethanolamines to the ethylene glycol solution.

"Evaluation results of resistivity and XRD peak intensity"
The resistivity evaluation results of Examples 1 to 4 and Comparative Examples 1 to 4 are shown in FIG. The horizontal axis represents the strength ratio {I (011) / I (131)} between the (011) plane and the (131) plane, and the vertical axis represents the resistivity.

  From this figure, the materials of Examples 1 to 4 in which the value of the intensity ratio {I (011) / I (131)} between the (011) plane and the (131) plane is 0.713 to 0.762 are The rate is 10 Ω · cm or less, indicating that the electrical conductivity is excellent. On the other hand, it can be seen that the materials of Comparative Examples 1 to 4 have a resistivity of 100 to 600 kΩ · cm and are not so high in conductivity.

  2A to 4B respectively show electron micrographs of the materials obtained in Example 1 and Comparative Examples 1 and 3, and schematic diagrams of crystals appearing in the electron micrographs.

  As shown in FIGS. 2A and 2B, the material obtained in Example 1 is in a state where secondary particles or tertiary particles are formed by aggregation of spherical primary particles of several tens of nm (microcrystalline particles 1 in FIG. 2B). is there. Here, the primary particles are the fine crystal particles 1 of the smallest unit, and the primary particles are aggregated in close contact to form secondary particles. Further, if the particle size of the secondary particles is regarded as several hundred nm to 1 μm, it can be considered that the secondary particles are aggregated in close contact to form tertiary particles. From the electron micrograph of FIG. 2A, it can be seen that the grain boundaries of the primary particles are so close that they cannot be observed. As a result, the contact resistance between the primary particles is lowered, and the overall resistivity is considered to be lowered. Reference numeral 51 in FIG. 2B indicates a concave portion on the surface of the tertiary particle.

  As shown in FIGS. 3A and 3B, it can be seen that the material obtained in Comparative Example 1 is in a state where large primary particles (crystal particles 2 in FIG. 3B) of several μm are aggregated. Further, in the electron micrograph of FIG. 3A, the grain boundaries of the primary particles are clear. This seems to suggest that the contact resistance at this grain boundary may be relatively high.

  As shown in FIGS. 4A and 4B, the material obtained in Comparative Example 3 is an aggregate of acicular primary particles (acicular crystals 3 in FIG. 4B) having a minor axis of several tens of nanometers and a major axis of several hundreds of nanometers. I understand that. This shape is very similar to the particle structure shown in Non-Patent Document 1.

  Further, when Example 1 is compared with Comparative Example 1 or 3, Example 1 is more advantageous in contact between secondary particles. That is, it is considered that the secondary particles of Example 1 have fine unevenness formed by fine spherical primary particles, and the contact area with the adjacent secondary particles is increased. Here, the fine irregularities are irregularities on the surface of secondary particles formed by agglomeration of spherical primary particles of several tens of nanometers, and the surface of the secondary particles is formed by adjacent primary particles adhering to each other. It is the aggregate | assembly of the convex part (mountain) and recessed part (valley) formed in this. The interval between the protrusions and the protrusions (period of unevenness) is equal to the interparticle distance of the primary particles. When the convex part of the secondary particle having fine irregularities is buried in the concave part of the adjacent secondary particle, the secondary particles have many contact points (wide contact area) like a gear meshing well. I think so.

  On the other hand, in the case of the comparative example 1, since the primary particles are relatively large, the number of contact points per unit dimension (unit length or unit area) with the adjacent particles is smaller than in the first example. In the case of the comparative example 3, since the needle-like crystal protrudes in various directions on the surface of the secondary particle, it is considered that the contact probability with the adjacent secondary particle is lowered.

  From the above results, by controlling the X-ray diffraction peak intensity ratio {I (011) / I (131)}, which is a feature of the present invention, to an appropriate range, the anisotropy of the crystal grains is eliminated, and the electric conduction It is inferred that the performance has improved.

"Evaluation results of discharge capacity"
The evaluation result of the discharge capacity of Examples 1-4 and Comparative Examples 1-4 is shown in FIG. The horizontal axis represents the intensity ratio {I (011) / I (131)} between the (011) plane and the (131) plane, and the vertical axis represents the discharge capacity.

  Also in the discharge capacity of the model battery, the value of the peak intensity ratio {I (011) / I (131)} between the (011) plane and the (131) plane is 0.713 to 0.762, as in FIG. The materials of certain Examples 1 to 4 had a discharge capacity of 20 mAh / g or more, which was higher than those of Comparative Examples 1 to 4.

  As described above, the material obtained in the example according to the present invention has a reduced energy loss due to the close contact of the primary particles and the increased electrical conductivity of the material, and as shown in FIGS. In addition, it is presumed that the formation of fine primary particles aggregates eliminates the anisotropy of the crystal particles and improves the diffusion of Li ions.

  Considering the rise of the curve shown in FIG. 5 or the variation of the material of the example, the value of the peak intensity ratio {I (011) / I (131)} between the (011) plane and the (131) plane is 0. If it is the range of 705-0.780, the high discharge capacity which is the effect of this invention will be obtained. Further, among the examples, the range where the discharge capacity is particularly high is a region including Examples 3 and 4, that is, the peak intensity ratio {I (011) / I (131) between the (011) plane and the (131) plane. } Is in the range of 0.725 to 0.760. The most desirable range is a region including Example 3, that is, the value of the peak intensity ratio {I (011) / I (131)} between the (011) plane and the (131) plane is 0.730 to 0.750. It is a range.

Since the positive electrode active material obtained in the present invention is thermally stable as compared with lithium cobalt oxide (LiCoO ₂ ) and the like that have been conventionally used, a large-sized lithium ion secondary battery excellent in safety can be obtained. It can be applied to the power sources for mobiles and stationary power storage that are required.

It is a graph which shows the evaluation result of the resistivity and XRD peak intensity of the Example by this invention. It is an electron micrograph of the positive electrode active material of Example 1 by this invention. It is a schematic diagram which shows the microcrystal particle | grains of the positive electrode active material of Example 1 by this invention. 2 is an electron micrograph of a positive electrode active material of Comparative Example 1. 3 is a schematic diagram showing crystal particles of a positive electrode active material of Comparative Example 1. FIG. 4 is an electron micrograph of a positive electrode active material of Comparative Example 3. 6 is a schematic diagram showing crystal particles of a positive electrode active material of Comparative Example 3. FIG. It is a graph which shows the evaluation result of the discharge capacity and XRD peak intensity of the Example by this invention.

Explanation of symbols

  1: fine crystal particles, 2: crystal particles, 3: acicular crystals.

Claims (4)

A positive electrode active material formed of LiMnPO ₄ , wherein the LiMnPO ₄ aggregates in a state where primary particles, which are the smallest unit crystal particles, are in close contact with each other to form secondary particles, and the secondary particles are the primary particles. have a fine unevenness formed by, measured by X-ray diffraction method (011) plane and the (131) plane and the peak intensity ratio of (I (011) / I (131)) is from 0.705 to 0 780, A positive electrode active material.   The peak intensity ratio (I (011) / I (131)) between the (011) plane and the (131) plane measured by the X-ray diffraction method is 0.713 to 0.762. The positive electrode active material according to 1. A lithium ion secondary battery including a positive electrode active material formed of LiMnPO ₄ , wherein the LiMnPO ₄ aggregates in a state where primary particles that are crystal units of the smallest unit are in close contact to form secondary particles, the secondary particles have a fine unevenness formed by the primary particles, the LiMnPO ₄ is measured by X-ray diffraction method (011) plane and the (131) plane and the peak intensity ratio of (I (011 ) / I (131)) is 0.705 to 0.780 . In the LiMnPO ₄ , the peak intensity ratio (I (011) / I (131)) between the (011) plane and the (131) plane measured by the X-ray diffraction method is 0.713 to 0.762. The lithium ion secondary battery according to claim 3 .

Images