JP6551878B2 - Method for producing positive electrode material for lithium ion battery and electrode material produced by this method - Google Patents

Description

  The present invention relates to a method for producing a positive electrode material for a lithium ion battery and an electrode material produced by this method.

  Conventionally, in a lithium ion battery, a composite positive electrode in which a ferroelectric is supported on the particle surface of an active material and sintered is used as a positive electrode.

As such a composite positive electrode, a particle size of 100 nm to 5 μm or less in an additive amount range of 0.1 mol% to 5 mol% of barium titanate BaTiO ₃ which is a ferroelectric substance on the particle surface of LiCoO ₂ -based positive electrode powder. A composite positive electrode supported in a range and sintered in a temperature range of 400 ° C. to 750 ° C. has been proposed (see, for example, Patent Document 1). The purpose of this composite positive electrode is to improve the output characteristics at low temperature. Specifically, 1 mol% of BaTiO ₃ is mechanically added at 700 ° C. to Li _1.1 Ni _0.5 Co _0.2 Mn _0.3 O ₂ particles which are active materials. There is a report that the output characteristics at a low temperature of -30.degree. C. are improved by 139.5% with respect to the untreated product by using the composite positive electrode supported by mixing and making the composite positive electrode into a supported composite.

The above composite positive electrode uses a non-aqueous electrolyte as an electrolyte of a lithium ion battery, but is a composite positive electrode of an all solid lithium ion battery in which the electrolyte is a sulfide solid, and the amount is 1 wt% with respect to the active material. There has also been proposed a composite positive electrode in which a ferroelectric barium titanate BaTiO ₃ having an average particle diameter of 50 nm is combined in a liquid phase (see, for example, Patent Document 2). This composite positive electrode is intended to improve the discharge capacity, and it has been reported that the resistance at the interface between the solid electrolyte and the active material can be reduced by about 20% and the discharge capacity can be improved by about 5%.

JP, 2011-210694, A JP, 2014-116129, A

  For lithium-ion batteries, improvement of output characteristics at low temperatures and improvement of discharge capacity are important performance improvement items, but improvement of capacity characteristics at high-speed charge / discharge rates is also an important item, but conventional composite positive electrodes Thus, improvement in capacity characteristics at a high-speed charge / discharge rate could not be expected.

  In the positive electrode of a lithium ion battery, an insertion / release reaction of Li ions occurs at the interface between the active material constituting the positive electrode and the electrolytic solution, but a composite in which a ferroelectric material is supported on the particle surface of the active material. The positive electrode aims at the effect of promoting the intercalation / desorption reaction of Li ions by utilizing the polarization of the ferroelectric substance, and the ferroelectric substance is supported on the active material in the form of fine particles as much as possible. There is a possibility that improvement in capacity characteristics can be expected.

  However, in the conventional method of mechanically coating an active material with a ferroelectric substance and performing heat treatment, the grain growth of the ferroelectric substance occurs as it is heated, so that the ferroelectric substance is supported on the active material as fine particles as possible. It was difficult.

  In view of such a current situation, the present inventors have studied to improve the capacity characteristics at a high-speed charge and discharge rate by using a composite positive electrode in which the active material is supported on the active material in fine particles as much as possible. The present invention has been achieved.

The method for producing a positive electrode material for a lithium ion battery according to the present invention comprises an active material raw material in powder form of an active material constituting a positive electrode of a lithium ion battery, and a ferroelectric containing a first element and a second element. A dispersion preparation step for preparing a dispersion by dispersing a powdered first element material containing a first element and a powdered second element material containing a second element in a solution And a heating step of forming a dispersion liquid into a gel, and a heating step of heating the gel to cause the active material material to carry a ferroelectric. In particular, the active material raw material is lithium cobaltate, the first element raw material is barium acetate, the second element raw material is titanium butoxide, and the active material constituting the positive electrode of the lithium ion battery is loaded with barium titanate to form a gelation process. In this case, the dispersion is gelled by drying while being stirred at 70 ° C. for 6 hours, and in the heating step, it is heated at 600 ° C. for 20 hours.

Furthermore, in the method for producing a positive electrode material for a lithium ion battery of the present invention, the first element material and the second element material are 5 mol% or less with respect to the active material material when the first element material and the second element material become a ferroelectric in the heating process. It also has a feature.

  According to the present invention, it is possible to synthesize and adhere a ferroelectric substance in the form of nanoparticles to the active material surface by a soft chemical liquid phase method, and to enable homogenization and uniform film thickness of the ferroelectric substance. In addition, the insertion and release of Li ions into the active material can occur more smoothly, and the capacity characteristics at a high-speed charge / discharge rate can be improved.

It is the STEM image and STEM-EDS image of the powder after a heating process. It is a XRD pattern of the powder after a heating process. It is a STEM image of the sample heat-processed at 600 degreeC by the heating process. It is a graph of the capacity | capacitance characteristic in a high speed charging / discharging rate. It is the XRD pattern of the powder after a heating process (600 degreeC). It is the STEM image and STEM-EDS image after a heating process (600 degreeC). It is a graph of the capacity | capacitance characteristic in a high speed charging / discharging rate.

  In the present invention, the ferroelectric particles are synthesized and attached to the active material surface as nano-particles by the soft chemical liquid phase method, rather than attaching the ferroelectric particles to the active material surface by a mechanical method. It is.

  That is, the method for producing a positive electrode material for a lithium ion battery according to the present invention includes a ferroelectric material containing an active material raw material in powder form of an active material constituting a positive electrode of a lithium ion battery, and a first element and a second element. Preparation of a dispersion by dispersing a first element material in powder form containing the first element and a second element material in powder form containing the second element in a solution It has a process, a gelation process which makes a dispersion liquid a gel, and a heating process which heats a gel and makes an active material raw material carry a ferroelectric.

  Hereinafter, specific examples will be described with reference to examples.

<Manufacture of positive electrode material>
The active material used was lithium cobaltate (LiCoO ₂ ) that has been put to practical use in lithium ion batteries. Lithium cobaltate was powdered and had an average particle size of 3 μm. The ferroelectric substance supported on lithium cobaltate was barium titanate (BaTiO ₃ ), the first element source was barium acetate as a Ba source, and the second element source was titanium butoxide as a Ti source.

  Lithium cobaltate is a lithium cobaltate solution dispersed in ethanol, barium acetate is an acetic acid solution dissolved in acetic acid, titanium butoxide is a methoxyethanol solution dissolved in 2-methoxyethanol, lithium cobaltate solution A predetermined amount of an acetic acid solution and a methoxyethanol solution were added to the solution and ultrasonically dispersed to obtain a dispersion. This is the dispersion preparation process. Here, the lithium cobaltate solution is a solution of 5 g of lithium cobaltate dispersed in 40 mL of ethanol, the acetic acid solution is a solution of 1.31 g of barium acetate in 20 ml of acetic acid, and the methoxyethanol solution is A solution in which 1.76 g of titanium butoxide was dissolved in 20 ml of 2-methoxyethanol was adjusted so that barium titanate was 10 mol% with respect to lithium cobaltate.

  The dispersion was stirred and dried at 70 ° C. for 6 hours to obtain a gel. This is the gelation process.

The obtained gel was heat-treated at 400 ° C, 500 ° C, 600 ° C, 700 ° C and 800 ° C for 20 hours, respectively. This is the heating process. The STEM image and STEM-EDS image of each powder after a heating process are shown in FIG. In Fig. 1, "LC-BT-400" is 400 ° C, "LC-BT-500" is 500 ° C, "LC-BT-600" is 600 ° C, "LC-BT-700" In the case of 700 ° C, “LC-BT-800” shows the case of 800 ° C. The same applies to FIGS. 2 and 4 described later. “Bare LC” is the state of untreated LC, ie, only LiCoO ₂ not carrying BaTiO ₃ .

The XRD pattern of the obtained powder is shown in FIG. According to FIG. 2, when the heat treatment temperature is 500 ° C. or less, BaTiO ₃ (BT) is not sufficiently formed, and BaCO ₃ (BC) is a main component, and at 600 ° C. or more, BaTiO ₃ (BT) as a main component It can be seen that

  In particular, as can be seen from the STEM-EDS image in FIG. 1, it was confirmed that a two-phase composite structure of LC phase and BT phase (or BC phase) nanoparticles was obtained at all heat treatment temperatures.

Furthermore, BaTiO ₃ particle growth was observed as the heat treatment temperature increased. Among those in which the BaTiO ₃ particles were formed, the one with the smallest particle size was the sample heat-treated at 600 ° C. The STEM image is shown in FIG. As can be seen from FIG. 3, the surface of the active material is coated with nanoparticles having a uniform particle diameter. When the particle size of the BaTiO ₃ particles was confirmed, the average particle size was 65 nm.

<Performance evaluation method of positive electrode material>
In order to evaluate whether the composite powder manufactured by the above method has the performance as a positive electrode material, the performance evaluation was performed by the following method.

  First, a composite sheet, a conductive additive (acetylene black), and a binder polymer (PVDF) were mixed at a mass ratio of 7: 2: 1 to prepare a positive electrode sheet.

The counter electrode was Li metal, and the electrolyte was 1 mol / L lithium fluorophosphate LiPF _{6 in} which ethylene carbonate and diethyl carbonate were dissolved in a solvent having a volume ratio of 3: 7. Then, a charge / discharge test was performed in a potential range of 3.3 V to 4.5 V using a 2023 type coin cell. The charge / discharge rate was controlled by stepping up the rate in 5 cycles for each rate in the range of current density from 0.1C to 5C or 10C [1C = 160 mA / g (LC theoretical capacity conversion)].

The result of the capacity characteristic at the high speed charge and discharge rate is shown in FIG. For the samples heat-treated at 400 ° C. and 500 ° C., the discharge capacity at the initial capacity and the high-speed charge and discharge rate was extremely lower than that of lithium cobaltate alone (untreated product). This is probably because BaCO ₃ (BC) is an impurity phase that is not a ferroelectric substance. On the other hand, when the heat treatment temperature is 600 ° C. or higher, almost single-phase BaTiO ₃ particles are formed, so the effect of BaTiO ₃ particles can be expected.

The “capacity of 0.1 C-1 cycle” in the above table is defined as “initial capacity”, and “capacity retention ratio to initial capacity in 10 C-5 cycle (total 35 cycles)” is calculated.
In addition, “the improvement ratio to the untreated product” is the comparison of the untreated product volume and the 600 ° C. heated product volume in the “5C-5th cycle (total 30th cycle) capacity” and the untreated product It is a comparison of the capacity and the capacity of 800 ° C heating goods.

As shown in Table 1, the initial capacities of the 600 ° C and 800 ° C heated products were both slightly lower than the untreated products, but this was due to the effect of the mass fraction of BaTiO ₃ that did not contribute to the capacity. is there.

  The capacity at the high-speed charge / discharge rate shows excellent capacity (122 mAh / g) with a product heated at 600 ° C after 30 cycles of 5C-5th cycle, and heating at 800 ° C with an average particle size of 158 nm It can be seen that there is a significant improvement compared to the product. In particular, the value of “122 mAh / g” largely reverses the discharge capacity (78 mAh / g) of the untreated product, and the improvement ratio is 158%. From this, it is understood that the characteristics of the high-speed charge and discharge rate are further improved by micronizing the particle diameter of the ferroelectric to less than 100 nm.

Thus, in Example 1, it was confirmed that the particle diameter of BaTiO ₃ particles can be minimized by setting the optimum heat treatment temperature to 600 ° C. However, it is necessary to study the optimum amount of BaTiO ₃ particles for LiCoO ₂ . The following Example 2 was carried out.

When the temperature of the heating step is set to 600 ° C. and it becomes BaTiO _{3 in} <Production of positive electrode material> described above, 1 mol%, 2.5 mol%, 5 mol%, 10 mol%, 15 mol% of BaTiO _{3 with} respect to LiCoO ₂ Each powder was prepared by adjusting the dispersion liquid so as to cause. Here, the acetic acid solution and the methoxyethanol solution to be added to the lithium cobaltate solution do not simply adjust the addition amount to the lithium cobaltate solution, but appropriately adjust the barium concentration of the acetic acid solution and the titanium concentration of the methoxyethanol solution, Addition of acetic acid and 2-methoxyethanol more than necessary to the lithium cobaltate solution was suppressed. Specifically, the acetic acid solution is a solution of 0.13 g to 1.96 g of barium acetate dissolved in 2 ml to 20 ml of acetic acid, and the methoxyethanol solution is 0.18 g to 2.63 g of 2 ml to 20 ml of 2-methoxyethanol. A solution in which titanium butoxide was dissolved was used.

The XRD pattern of the obtained powder is shown in FIG. Although the peak intensity of BaTiO ₃ (BT) decreases with the decrease of the amount of BaTiO ₃ contained in the powder, this is considered to be the effect of the decrease of the relative amount to LiCoO ₂ , in which case Even so, it can be considered that BaTiO ₃ is formed.

6 shows a sample case of BaTiO ₃ (BT) 1mol%, the STEM-EDS image of the sample in the case of BaTiO ₃ (BT) 5mol%. It can be confirmed from FIG. 6 that the BT phase-LC phase has a two-phase composite structure. Although not shown, it was confirmed that all other samples had a two-phase composite structure of BT phase and LC phase.

Figure 7, BaTiO ₃ (BT) and 1 mol% in the case of a sample, BaTiO ₃ (BT) 5 mol% of the cases the samples, and LiCoO ₂ only in samples (untreated), the capacity characteristics in a high-speed charge and discharge rate The results are shown. Based on this capacity characteristic, the comparison of various characteristics is shown in the following table.

The “capacity of 0.1 C-1 cycle” in the above table is defined as “initial capacity”, and “capacity retention ratio to initial capacity in 10 C-5 cycle (total 35 cycles)” is calculated.
“Improvement ratio relative to untreated products” is the comparison between the capacity of untreated products and the capacity of BT1 mol% products in “capacity of 10C-5th cycle (35th cycle in total)”, and the capacity of untreated products And the volume of the BT 5 mol% product.

As shown in Table 2, the initial capacity decreased with an increase in the added amount of BaTiO ₃ , which is an effect of the mass fraction of the ferroelectric BaTiO ₃ that does not contribute to the capacity.

Further, as shown in Table 2, significantly impact on the capacity characteristics in a high-speed charge and discharge rate of the BaTiO ₃ additive amount, BaTiO ₃ added amount is more effective than 5 mol% is more of 1 mol%, in particular BaTiO ₃ When the addition amount is 1 mol%, after 35 cycles (5th cycle at 10C), the untreated product to be compared, that is, LiCoO ₂ alone has a capacity of 62 mAh / g, whereas 146 mAh / g and 238% improvement. Furthermore, regarding the capacity retention rate at the high-speed charge / discharge rate, when the addition amount of BaTiO ₃ is 1 mol%, the initial capacity value is 78% at the 35th cycle, whereas only the untreated product, that is, LiCoO ₂ It is 33%, and it has been confirmed that it improves significantly.

Claims (2)

An active material raw material in powder form of an active material constituting a positive electrode of a lithium ion battery, and a first powder form containing a first element for constituting a ferroelectric containing a first element and a second element A dispersion liquid producing step of producing a dispersion liquid by dispersing a primary element raw material and a powdered second element raw material containing a second element in a solution;
A gelation step using the dispersion as a gel;
And a heating step of heating the gel to support the ferroelectric material on the active material material .
The active material raw material is lithium cobaltate, the first element raw material is barium acetate, the second element raw material is titanium butoxide, and the active material constituting the positive electrode of the lithium ion battery is supported with barium titanate,
In the gelation step, the dispersion is gelled by drying with stirring at 70 ° C. for 6 hours, and in the heating step, the positive electrode material for a lithium ion battery is heated at 600 ° C. for 20 hours .   2. The positive electrode of the lithium ion battery according to claim 1, wherein the first element material and the second element material are 5 mol% or less with respect to the active material material when the ferroelectric material is formed in the heating step. Method of manufacturing material.