JP4240853B2 - Positive electrode active material for lithium ion secondary battery - Google Patents

Description

【００５４】
【発明の効果】
リチウムイオン二次電池の正極活物質として一般式がＬｉ_ｗＣｏ_１−ｘ Ｂａ _ｘＯ_ｙＳ_ｚ０５、０＜ｘ≦０．１０、１≦ｙ≦２．５、０＜ｚ≦０．０１５である。）で表される正極活物質を用いることにより、電池内のガス発生を低減し、電池特性（サイクル特性、高負荷特性）及び熱安定性を向上させることができる。
【図面の簡単な説明】
【図１】正極活物質中のＳ量（ｚ値）と電池の膨張率の関係を示す特性図
【図２】正極活物質の比表面積と電池の膨張率の関係を示す特性図
【図３】正極活物質中のＭｇ量（ｘ値）と容量維持率の関係を示す特性図
【図４】正極活物質中のＳ量（ｚ値）と容量維持率の関係を示す特性図
【図５】正極活物質中のＭｇ量（ｘ値）と高負荷容量の関係を示す特性図
【図６】正極活物質中のＳ量（ｚ値）と高負荷容量の関係を示す特性図
【図７】正極活物質中のＬｉ量（ｗ値）と高負荷容量の関係を示す特性図
【図８】正極活物質中のＬｉ量（ｗ値）と放電容量の関係を示す特性図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode active material used for a lithium ion secondary battery, and more particularly, to a positive electrode active material that generates less gas and has excellent battery characteristics (cycle characteristics, high load characteristics) and thermal stability.
[0002]
[Prior art]
In recent years, lithium-ion secondary batteries having high energy density have been adopted as batteries incorporated in electronic devices such as portable personal computers and video cameras. The lithium ion secondary battery includes a positive electrode plate in which a positive electrode active material such as lithium cobalt composite oxide is held on a positive electrode current collector that is a support, and a negative electrode active material such as lithium metal that is a support. A negative electrode plate held on a current collector, LiPF₆The non-aqueous electrolyte solution which consists of the organic solvent which melt | dissolved lithium salts, such as, and the separator which intervenes between a positive electrode plate and a negative electrode plate, and prevents a short circuit of both electrodes. Among these, a laminated battery stored in a battery case of a metal-laminated resin film or a battery stored in a thin metal case is wound with a positive electrode plate, a negative electrode plate, and a separator formed into a thin sheet shape. Compared with a battery housed in a metal case of a mold, there is a problem that it easily expands due to gas generation in the battery, heat generation or external heating, and the battery pack case storing the battery also expands and deforms.
[0003]
Conventionally, LiCoO as a positive electrode active material of a lithium ion secondary battery₂When the charge voltage is increased for the purpose of improving the discharge capacity, crystal transition of the positive electrode active material or decomposition of the positive electrode active material occurs, releasing oxygen from cobalt acid, and this oxygen is non-aqueous The electrolyte solution was oxidized and decomposed. As a result, gas was generated in the battery, and the above problem occurred in the laminated battery.
[0004]
Similarly, when the charging voltage is increased for the purpose of improving the discharge capacity, the battery characteristics (cycle characteristics, high load characteristics) and thermal stability are also reduced along with the crystal transition or decomposition of the positive electrode active material. Also, the positive electrode active material LiCoO₂Has low conductivity, and therefore, the conductivity is improved by coating the conductive carbon. However, when the contact with the carbon is poor, it causes cycle deterioration.
[0005]
[Problems to be solved by the invention]
The present invention has been made in view of the above-described circumstances, and provides a positive electrode active material that can reduce gas generation of a lithium ion secondary battery and improve battery characteristics (cycle characteristics, high load characteristics) and thermal stability. With the goal.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventor has a general formula of Li as a positive electrode active material of a lithium ion secondary battery._wCo_1-x Ba _xO_yS_z(However, 0.95 ≦ w ≦ 1.05, 0 <x ≦ 0.10, 1 ≦ y ≦ 2.5, 0 <z ≦ 0.015) is used. Thus, the inventors have found that the above problems can be solved and have completed the present invention.
[0007]
That is, the positive electrode active material for a lithium ion secondary battery of the present invention has a general formula of Li_wCo_1-x Ba _xO_yS_z(However, 0.95 ≦ w ≦ 1.05, 0 <x ≦ 0.10, 1 ≦ y ≦ 2.5, 0 <z ≦ 0.015) The Li amount (w value) in the composition affects the discharge capacity and high load capacity of the lithium ion secondary battery, and is preferably in the range of 0.95 ≦ w ≦ 1.05. Further, the amount of M (x value) and the amount of S (z value) in the composition greatly affect the gas generation and battery characteristics (cycle characteristics, high load characteristics) of the lithium ion secondary battery, and 0 <x ≦ 0. .10, 0 <z ≦ 0.015 is preferable, and 0.0001 ≦ x ≦ 0.05 and 0.003 ≦ z ≦ 0.009 are more preferable. The amount of O (y value) in the composition varies depending on the method of introducing the S element into the positive electrode active material, and is in the range of 1 ≦ y ≦ 2.5.
[0008]
The positive electrode active material for a lithium ion secondary battery of the present invention has a specific surface area of 0.2 to 2.0 m.²/ G range. The specific surface area of the positive electrode active material greatly affects the gas generation of the lithium ion secondary battery. In particular, in the case of the positive electrode active material of the present invention represented by the above general formula, the specific surface area is 0.2 to 2.0 m.²Gas generation can be greatly reduced in the range of / g. More preferably 0.4-0.8m²/ G.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The synthesis of the positive electrode active material for a lithium ion secondary battery of the present invention was performed by mixing sulfur or a sulfur compound with a compound containing at least one element selected from the group consisting of a lithium compound, a cobalt compound, and Mg and Ba, as shown below. It is performed by firing and then pulverizing the raw material mixture.
[0010]
As the lithium compound, cobalt compound, and compound containing at least one element of Mg and Ba, oxides, hydroxides, carbonates, nitrates, sulfates, acetates, oxalates, and the like can be used. . Preferably, as the lithium compound, Li₂O, LiOH, Li₂CO₃, LiHCO₃, LiNO₃, Li₂SO₄・ H₂O, Li (CH₃COO), Li₂C₂O₄As a cobalt compound, Co₃O₄, Co₂O₃, Co (OH)₂, CoCO₃, Co (NO₃)₂・ 6H₂O, CoC₂O₄As a compound containing at least one element of Mg, Sr and Ba, MgO, Mg (OH)₂, MgCO₃, Mg (NO₃)₂・ 6H₂O, MgC₂O₄・ 2H₂O, SrO, Sr (OH)₂, SrCO₃, Sr (NO₃)₂, SrC₂O₄・ H₂O, BaO, Ba (OH)₂・ 8H₂O, BaCO₃, Ba (NO₃)₂, BaC₂O₄・ H₂O or the like can be used.
[0011]
As the sulfur compound, oxides, sulfides, sulfates, hydrogen sulfates, pyrosulfates, sulfites, peroxosulfates, thiosulfates, alkyl sulfates, and the like can be used. Preferably, (NH₄)₂S, Li₂SO₄・ H₂O, CoSO₄, (NH₄)₂SO₄, (NH₄)₂S₂O₈Etc. can be used.
[0012]
In mixing these raw materials, the powdery raw materials may be mixed as they are, or may be mixed as a slurry using water or an organic solvent. The slurry mixture is dried to obtain a raw material mixture.
[0013]
The raw material mixture thus obtained is fired in the temperature range of 500 to 1000 ° C. for 1 to 24 hours in air or in a weakly oxidizing atmosphere. Preferably, baking is performed at a temperature range of 800 to 1000 ° C. for 6 to 12 hours. When the firing temperature is less than 500 ° C., unreacted raw materials remain in the positive electrode active material, and the original characteristics of the positive electrode active material cannot be utilized. On the other hand, when the temperature exceeds 1000 ° C., the particle size of the positive electrode active material becomes too large and the battery characteristics deteriorate. When the firing time is less than 1 hour, the diffusion reaction between the raw material particles does not proceed. When 24 hours have elapsed, the diffusion reaction is almost completed, and therefore no further firing is necessary.
[0014]
The fired product obtained by the above firing is pulverized using a cracking machine to have a specific surface area of 0.2 to 2.0 m.²/ G, the positive electrode active material of the present invention having an average particle size in the range of 1.0 to 12.0 μm is obtained.
[0015]
In the lithium ion secondary battery using the positive electrode active material of the present invention, the oxidative decomposition reaction of the electrolytic solution is suppressed, and the amount of gas generated in the battery is reduced. Characteristics, high load characteristics) and thermal stability are also improved.
[0016]
Next, a lithium ion secondary battery is produced using the positive electrode active material of the present invention, and the results of measuring gas generation, battery characteristics (cycle characteristics, high load characteristics) and thermal stability will be described.
[0017]
(Production of lithium ion secondary battery)
90 parts by weight of the positive electrode active material powder and a conductive agent (for example, graphite such as natural graphite, scaly graphite, artificial graphite, expanded graphite, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc. Carbon blacks, conductive fibers such as carbon fibers, metal fibers, etc. are used alone or in combination.) A paste is prepared by kneading 5 parts by weight and 5 parts by weight of polyvinylidene fluoride. A positive electrode current collector is coated and dried to form a positive electrode plate. Further, carbon (for example, natural graphite, artificial graphite, non-graphite carbon, etc.) is used for the negative electrode, a porous propylene film is used for the separator, and LiPF is used as a mixed solvent of ethylene carbonate: diethyl carbonate = 1: 1 (volume ratio) as the electrolyte.₆A lithium ion secondary battery is manufactured using a solution in which is dissolved at a concentration of 1 mol / l. Here, a thin sheet of a positive electrode plate, a negative electrode plate and a separator is wound to produce a laminated battery housed in a battery case of a metal laminated resin film.
[0018]
(Evaluation of gas generation)
The general formula is LiCo_0.999Mg_0.001O₂S_zAnd LiCoO₂S_zA laminate battery is manufactured using various positive electrode active materials represented by the following formula, and is charged at a constant current up to 4.2 V at a charging load of 0.5 C, and then stored at 80 ° C. for 2 days. ) Is obtained from the following equation (where 1C is a current load that completes charging or discharging in one hour).
Expansion coefficient of battery = {(battery thickness after storage at 80 ° C.−battery thickness before measurement) / battery thickness before measurement} × 100
[0019]
FIG. 1 shows the relationship between the amount of S (z value) in the positive electrode active material and the expansion coefficient of the battery. As is apparent from this figure, the positive electrode active material LiCo of the present invention_0.999Mg_0.001O₂S_zThe expansion rate of the battery using the (solid line) is low in the range of z <0 <z ≦ 0.015, particularly very low in the range of 0.003 ≦ z ≦ 0.009. It can be seen that the amount of gas generated is reduced. Further, the positive electrode active material LiCoO containing no Mg element₂S_zIt can be seen that the expansion coefficient is very low compared to the battery using (dotted line). Thus, by including both Mg element and S element in the positive electrode active material, the expansion coefficient of the battery is greatly reduced as compared with the case where only the S element is included.
[0020]
Next, a laminated battery is produced using various positive electrode active materials having different specific surface areas, and the expansion coefficient (%) of the battery is similarly obtained. In FIG. 2, the general formula is LiCo_0.999Mg_0.001O₂S_0.005(Thick solid line), LiCoO₂S_0.005(Thick dotted line), LiCo_0.999Mg_0.001O₂(Thin solid line) and LiCoO₂The relationship between the specific surface area of the positive electrode active material represented by (thin dotted line) and the expansion coefficient of the battery is shown. As is apparent from this figure, the positive electrode active material LiCo of the present invention_0.999Mg_0.001O₂S_0.005The specific surface area is 2.0m²/ G or less, especially 0.8m²/ G or less, the amount of gas generated in the battery is reduced. Specific surface area is 2.0m²If it exceeds / g, the reactivity of the oxidative decomposition reaction of the electrolytic solution occurring on or near the surface of the positive electrode active material is increased, and as a result, the amount of gas generated in the battery is considered to increase. Specific surface area is 0.2m²If it is smaller than / g, the particle size of the positive electrode active material becomes too large and the battery characteristics deteriorate, so the specific surface area is 0.2 to 2.0 m.²/ G is preferred, 0.4 to 0.8 m²The range of / g is more preferable. Also, from this figure, the positive electrode active material LiCo of the present invention_0.999Mg_0.001O₂S_0.005The battery using the positive electrode active material LiCoO containing no Mg element₂S_0.005, Cathode active material LiCo containing no S element_0.999Mg_0.001O₂And positive electrode active material LiCoO containing neither Mg element nor S element₂The expansion coefficient is very low as compared with the battery using, and it can be seen that the expansion coefficient of the battery is greatly reduced by including both the Mg element and the S element in the positive electrode active material.
[0021]
(Evaluation of cycle characteristics)
The general formula is LiCo_1-xMg_xO₂S_0.005LiCo_1-xMg_xO₂LiCo_0.999Mg_0.001O₂S_zAnd LiCoO₂S_zA laminate battery is manufactured using various positive electrode active materials represented by the following, and at a normal temperature (25 ° C.), after being charged with constant current to 4.2 V at a charging load of 0.5 C, it is discharged to 2.75 V at 1.0 C. Charging / discharging is performed 500 cycles, and the capacity retention rate (%) at the 500th cycle is obtained from the following equation.
Capacity retention rate = (discharge capacity at 500th cycle / discharge capacity at the first cycle) × 100
[0022]
In FIG. 3, the general formula is LiCo_1-xMg_xO₂S_0.005(Solid line) and LiCo_1-xMg_xO₂The relationship between the Mg amount (x value) of the positive electrode active material represented by (dotted line) and the capacity retention rate is shown in FIG._0.999Mg_0.001O₂S_z(Solid line) and LiCoO₂S_zThe relationship between the S amount (z value) of the positive electrode active material represented by (dotted line) and the capacity retention rate is shown. From FIG. 3, the positive electrode active material LiCo of the present invention_1-xMg_xO₂S_0.005It can be seen that the capacity retention rate of the battery using the battery is high when the x value is in the range of 0 <x ≦ 0.10, and particularly high in the range of 0.0001 ≦ x ≦ 0.05. Also, from FIG. 4, the positive electrode active material LiCo of the present invention_0.999Mg_0.001O₂S_zThe capacity retention rate of the battery using the battery is high when the z value is in the range of 0 <z ≦ 0.015, and particularly high in the range of 0.003 ≦ z ≦ 0.009. Recognize. As is clear from FIG. 3 and FIG. 4, the battery using the positive electrode active material of the present invention contains both Mg element and S element in the positive electrode active material, and therefore includes only one of the elements. Compared to the above, the capacity retention rate is high and the cycle characteristics are excellent.
[0023]
(Evaluation of high load characteristics)
The general formula is LiCo_1-xMg_xO₂S_0.005LiCo_1-xMg_xO₂LiCo_0.999Mg_0.001O₂S_zAnd LiCoO₂S_zA laminate battery is manufactured using various positive electrode active materials represented by the following, and after a constant current charge to 4.2 V at a charge load of 2.0 C, the discharge capacity when discharged to 2.75 V at 2.0 C is a high load. Obtained as a capacity (mAh / g).
[0024]
In FIG. 5, the general formula is LiCo_1-xMg_xO₂S_0.005(Solid line) and LiCo_1-xMg_xO₂The relationship between the Mg amount (x value) of the positive electrode active material represented by (dotted line) and the high load capacity is shown in FIG._0.999Mg_0.001O₂S_z(Solid line) and LiCoO₂S_zThe relationship between the S amount (z value) of the positive electrode active material represented by (dotted line) and the high load capacity is shown. From FIG. 5, the positive electrode active material LiCo of the present invention_1-xMg_xO₂S_0.005It can be seen that the high load capacity of the battery using the battery is high when the x value is in the range of 0 <x ≦ 0.10, and particularly high in the range of 0.0001 ≦ x ≦ 0.05. Also, from FIG. 6, the positive electrode active material LiCo of the present invention_0.999Mg_0.001O₂S_zThe high load capacity of the battery using the battery is high when the z value is in the range of 0 <z ≦ 0.015, particularly very high in the range of 0.003 ≦ z ≦ 0.009. Recognize. As is clear from FIGS. 5 and 6, the battery using the positive electrode active material of the present invention contains both the Mg element and the S element in the positive electrode active material, and therefore includes only one of the elements. Compared with, high load capacity is high and high load characteristics are excellent.
[0025]
Thus, by including both Mg element and S element in the positive electrode active material, the crystal transition or decomposition of the positive electrode active material is further suppressed as a synergistic effect. As a result, the expansion coefficient of the battery is remarkably reduced, and the battery characteristics ( Cycle characteristics, high load characteristics) are greatly improved. The same effect can be obtained when at least one element of Mg and Ba is introduced instead of Mg element.
[0026]
Similarly, the general formula is Li_wCo_0.999Mg_0.001O₂S_0.005A laminate battery is produced using various positive electrode active materials represented by the following formula, and a high load capacity (mAh / g) is obtained. FIG. 7 shows the relationship between the amount of Li (a value) in the positive electrode active material and the high load capacity. From this figure, it can be seen that the high load capacity decreases when the w value becomes larger than 1.05.
[0027]
FIG. 8 shows the relationship between the Li amount (w value) in the positive electrode active material and the discharge capacity in the case of discharging at a normal current density (0.25 C). From this figure, it can be seen that the discharge capacity decreases when the w value becomes smaller than 0.95.
[0028]
Therefore, in consideration of both the high load capacity and the normal discharge capacity, the w value needs to be set in a range of 0.95 ≦ w ≦ 1.05.
[0029]
Examples of the present invention will be described below, but it goes without saying that the present invention is not limited to specific examples.
[0030]
【Example】
[referenceExample 1] Lithium carbonate (Li₂CO₃), Cobalt trioxide (Co₃O₄), Magnesium carbonate (MgCO₃), Lithium sulfate (Li₂SO₄・ H₂O) is weighed so that w = 1.0, x = 0.001, z = 0.005 and dry mixed. The obtained raw material mixture was baked in the air at 900 ° C. for 10 hours, and then pulverized using a roughing machine to obtain a specific surface area of 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Mg_0.001O₂S_0.005Get.
[0031]
The specific surface area is measured by a constant pressure BET one-point method using nitrogen gas adsorption. The average particle diameter is a value obtained by measuring the specific surface area by the air permeation method and obtaining the average value of the particle diameters of the primary particles, and is measured using a Fisher sub-sieve sizer (F.S.S.S.). The composition analysis is measured by the following method. That is, Li is measured by flame photometry, Co is titrated, and Mg, Ba, and S are measured by ICP emission spectroscopy.
[0032]
[referenceExample 2] Except x = 0.005referenceSimilar to Example 1, the specific surface area is 0.62 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.995Mg_0.005O₂S_0.005Get.
[0033]
[referenceExample 3] Except x = 0.01referenceSimilar to Example 1, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.99Mg_0.01O₂S_0.005Get.
[0034]
[referenceExample 4] Except z = 0.003referenceSimilar to Example 1, the specific surface area is 0.62 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Mg_0.001O₂S_0.003Get.
[0035]
[referenceExample 5] Except z = 0.0099referenceSimilar to Example 1, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.4 μm_0.999Mg_0.001O₂S_0.009Get.
[0036]
[referenceExample 6] Except for z = 0.012.referenceSimilar to Example 1, the specific surface area is 0.64 m.²/ G, positive active material powder LiCo with an average particle size of 3.4 μm_0.999Mg_0.001O₂S_0.012Get.
[0037]
[referenceExample 7] Except z = 0.015referenceSimilar to Example 1, the specific surface area is 0.64 m.²/ G, positive active material powder LiCo with an average particle size of 3.4 μm_0.999Mg_0.001O₂S_0.015Get.
[0038]
[referenceExample 8] Magnesium carbonate (MgCO₃) Instead of magnesium hydroxide (Mg (OH)₂) Except usingreferenceSimilar to Example 1, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Mg_0.001O₂S_0.005Get.
[0039]
[referenceExample 9] Magnesium carbonate (MgCO₃) Instead of magnesium nitrate (Mg (NO₃)₂・ 6H₂Except using O)referenceSimilar to Example 1, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Mg_0.001O₂S_0.005Get.
[0040]
[referenceExample 10] Magnesium carbonate (MgCO₃) Instead of magnesium oxalate (MgC₂O₄・ 2H₂Except using O)referenceSimilar to Example 1, the specific surface area is 0.64 m.²/ G, positive active material powder LiCo with an average particle size of 3.4 μm_0.999Mg_0.001O₂S_0.005Get.
[0041]
[referenceExample 11] Lithium sulfate (Li₂SO₄・ H₂Except that sulfur (S) is used instead of O)referenceSimilar to Example 1, the specific surface area is 0.64 m.²/ G, positive active material powder LiCo with an average particle size of 3.4 μm_0.999Mg_0.001O₂S_0.005Get.
[0042]
[referenceExample 12] Lithium sulfate (Li₂SO₄・ H₂Ammonium sulfide ((NH₄)₂Except using S)referenceSimilar to Example 1, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Mg_0.001O₂S_0.005Get.
[0043]
[referenceExample 13] Lithium sulfate (Li₂SO₄・ H₂Ammonium peroxodisulfate ((NH₄)₂S₂O₈) Except usingreferenceSimilar to Example 1, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Mg_0.001O₂S_0.005Get.
[0044]
[Example 1] Magnesium carbonate (MgCO₃) Instead of barium carbonate (BaCO)₃) Except usingreferenceSimilar to Example 1, the specific surface area is 0.64 m.²/ G, positive active material powder LiCo with an average particle size of 3.4 μm_0.999Ba_0.001O₂S_0.005Get.
[0045]
[Comparative Example 1] Magnesium carbonate (MgCO₃), Lithium sulfate (Li₂SO₄・ H₂Except not using O)referenceSimilar to Example 1, the specific surface area is 0.61 m.²/ G, cathode active material powder LiCoO having an average particle size of 3.6 μm₂Get.
[0046]
[Comparative Example 2] Magnesium carbonate (MgCO₃) Except not usingreferenceSimilar to Example 1, the specific surface area is 0.61 m.²/ G, cathode active material powder LiCoO having an average particle size of 3.6 μm₂S_0.005Get.
[0047]
[Comparative Example 3] Lithium sulfate (Li₂SO₄・ H₂Except not using O)referenceSimilar to Example 1, the specific surface area is 0.61 m.²/ G, cathode active material powder LiCo with an average particle size of 3.6 μm_0.999Mg_0.001O₂Get.
[0048]
[Comparative Example 4] Lithium sulfate (Li₂SO₄・ H₂Except not using O)referenceIn the same manner as in Example 14, the specific surface area is 0.63 m.²/ G, positive active material powder LiCo with an average particle size of 3.5 μm_0.999Ba_0.001O₂Get.
[0049]
(Evaluation)referenceExamples 1-13. Example 1And the laminated battery was produced using the positive electrode active material powder obtained by Comparative Examples 1-4, and the result measured about gas generation, a battery characteristic (cycle characteristic, high load characteristic), and thermal stability is put together in Table 1. The expansion coefficient of the battery, the capacity retention ratio at normal temperature (25 ° C.), and the high load capacity are measured in the same manner as described above. The capacity maintenance rate at high temperature (60 ° C.) is measured in the same manner as the capacity maintenance rate at room temperature (25 ° C.) except that measurement is performed in a 60 ° C. high temperature bath. About thermal stability, differential scanning calorimetry is performed as follows, and it evaluates with heat generation start temperature.
[0050]
(Evaluation of thermal stability)
(1) A demountable cell positive electrode capable of preparing a paste by kneading 90 parts by weight of a positive electrode active material powder, 5 parts by weight of carbon as a conductive agent, and 5 parts by weight of polyvinylidene fluoride, and capable of evaluating this as a single electrode A secondary battery is manufactured by applying to a current collector and using ethylene carbonate as an electrolytic solution.
{Circle around (2)} After charging and discharging with a constant current, the battery is charged under a constant current until the battery voltage becomes 4.3V.
(3) After charging, the positive electrode is taken out from the secondary battery, washed and dried, and the positive electrode active material is scraped off.
(4) 5 mg of the positive electrode active material and 2 mg of ethylene carbonate scraped from the positive electrode are placed in an Al cell, and differential scanning calorimetry is performed to determine the heat generation start temperature.
[0051]
Differential scanning calorimetry is a method for analyzing the thermal characteristics of a sample material by measuring the temperature difference between the reference material and the sample while simultaneously heating them at a constant rate. Then, the differential scanning calorific value does not change in the low temperature part, but the differential scanning calorific value greatly increases above a certain temperature. The temperature at this time is defined as the heat generation start temperature, and it can be said that the higher the temperature, the better the thermal stability.
[0052]
From Table 1, compared with Comparative Examples 1-3,referenceExamples 1 to 13 include both the Mg element and the S element in the positive electrode active material, thereby reducing the expansion coefficient of the battery, increasing the capacity retention rate and the high load capacity, and improving the battery characteristics (cycle characteristics and high load characteristics). It turns out that it is excellent. Regarding the cycle characteristics, not only the cycle characteristics at normal temperature (25 ° C.) but also the cycle characteristics at high temperature (60 ° C.) are excellent, and it can be seen that the battery characteristics are excellent even when the battery is used in a high temperature. Moreover, it can be seen that the heat generation start temperature is higher than that of the comparative example and is excellent in thermal stability. For example, compared with Comparative Example 1 that does not include any of Mg element and S element, and Comparative Examples 2 and 3 that include only one kind of element, all of the two kinds of elements are included.referenceIn the case of Example 1, the expansion coefficient of the battery is low, and the capacity maintenance ratio and the high load capacity are high. Moreover, the heat generation start temperature is also high. Thus, by including both Mg element and S element in the positive electrode active material, the crystal transition or decomposition of the positive electrode active material is further suppressed as a synergistic effect. (Cycle characteristics, high load characteristics) and thermal stability are greatly improved. further,Example 1And from Comparative Example 4, it can be seen that the same effect can be obtained when Ba element is introduced instead of Mg element.
[0053]
[Table 1]

[0054]
【The invention's effect】
The general formula is Li as a positive electrode active material of a lithium ion secondary battery._wCo_1-x Ba _xO_yS_z05, 0 <x ≦ 0.10, 1 ≦ y ≦ 2.5, and 0 <z ≦ 0.015. ), The generation of gas in the battery can be reduced, and battery characteristics (cycle characteristics, high load characteristics) and thermal stability can be improved.
[Brief description of the drawings]
FIG. 1 is a characteristic diagram showing the relationship between the amount of S (z value) in a positive electrode active material and the expansion coefficient of a battery.
FIG. 2 is a characteristic diagram showing the relationship between the specific surface area of the positive electrode active material and the expansion coefficient of the battery.
FIG. 3 is a characteristic diagram showing the relationship between the amount of Mg (x value) in the positive electrode active material and the capacity retention rate.
FIG. 4 is a characteristic diagram showing the relationship between the amount of S (z value) in the positive electrode active material and the capacity retention rate.
FIG. 5 is a characteristic diagram showing the relationship between the amount of Mg (x value) in the positive electrode active material and a high load capacity.
FIG. 6 is a characteristic diagram showing the relationship between the amount of S (z value) in the positive electrode active material and high load capacity.
FIG. 7 is a characteristic diagram showing the relationship between the amount of Li (w value) in the positive electrode active material and the high load capacity.
FIG. 8 is a characteristic diagram showing the relationship between the Li amount (w value) in the positive electrode active material and the discharge capacity.

Claims (2)

The general formula is Li _w Co _1-x Ba _x O _y S _z (where 0.95 ≦ w ≦ 1.05, 0 <x ≦ 0.10, 1 ≦ y ≦ 2.5, 0 <z ≦ 0. 015)), a positive electrode active material for a lithium ion secondary battery. ^{2. The} positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the specific surface area is in the range of 0.2 to 2.0 m ² / g.

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