JP2018181766A - Positive electrode for lithium secondary battery - Google Patents

Abstract

[PROBLEMS] To provide a positive electrode active material having low resistance and excellent thermal stability of a crystal structure.
A formula _{(1): LiNi x Co y} Mn z O 2 (x, y, z is, 1.56 ≦ [(y + z) / x] ≦ 1.86, real number satisfying x + y + z = 1) ; in The positive electrode for lithium secondary batteries which contains the positive electrode active material shown, and whose water content based on the Karl-Fisher method (heating temperature: 300 degreeC) is 2200-2900 ppm per unit mass of the said positive electrode active material.
[Selected figure] Figure 3

Description

  The present invention relates to a positive electrode for a lithium secondary battery.

The positive electrode for a lithium secondary battery includes a positive electrode active material capable of reversibly absorbing and desorbing lithium ions. As such a positive electrode active material, a lithium transition metal complex oxide is widely used (see Patent Documents 1 and 2). For example, Patent Document 1 has the following general formula: Li _t Ni _1-a-b Co _a M _b O ₂ (where M is Mg, Al, Ca, Ti, V, Cr, Mn, Zr, Nb, Lithium which is at least one element selected from the group consisting of Mo and W, and is represented by 0.95 ≦ t ≦ 1.20, 0 ≦ a ≦ 0.22, 0 ≦ b ≦ 0.15); Disclosed is a positive electrode active material having nickel composite oxide particles and a covering layer covering the particles.

JP, 2016-072071, A JP, 2015-207416, A

  In the lithium-nickel composite oxide particles of Patent Document 1, the value of (1-ab) in the above general formula is 0.63 or more. That is, when the whole metal element other than lithium is 100 mol%, nickel will occupy 63 mol% or more. According to the study of the present inventor, it is effective to use the positive electrode active material having a high proportion of nickel in this way from the viewpoint of reducing the battery resistance during normal use and improving the input / output characteristics. However, there is a problem that the thermal stability of the positive electrode active material is lowered and the temperature rise during overcharge becomes remarkable.

  The present invention was created to solve such problems, and its object is to realize a lithium secondary battery for realizing a lithium secondary battery having both battery characteristics in normal use and resistance to overcharge. Providing a positive electrode for the

The present invention, the following equation _{(1): LiNi x Co y} Mn z O 2 ( provided that, x, y, z are, 1.56 ≦ [(y + z) / x] ≦ 1.86, satisfy x + y + z = 1 A lithium secondary battery including the positive electrode active material shown by the above; and the water content based on the Karl Fischer method (heating temperature: 300 ° C.) is 2200 ppm or more and 2900 ppm or less per unit mass of the positive electrode active material A positive electrode for the

  The positive electrode active material represented by the formula (1) is low in resistance and excellent in the thermal stability of the crystal structure. The positive electrode contains the positive electrode active material and water of 2200 to 2900 ppm per unit mass of the positive electrode active material. The lithium secondary battery provided with the positive electrode of such a configuration can exhibit excellent battery characteristics at the time of normal use, and at the time of overcharge, a temperature rise can be suppressed to realize excellent overcharge resistance. .

It is a graph in the case of [(y + z) / x] = 1.38. It is a graph in the case of [(y + z) / x] = 1.56. It is a graph in the case of [(y + z) / x] = 1.63. It is a graph in the case of [(y + z) / x] = 1.86. It is a graph in the case of [(y + z) / x] = 2.00.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings as appropriate. In addition, matters other than matters particularly mentioned in the specification (for example, composition and properties of the positive electrode) and matters necessary for the practice of the present invention (for example, other battery components and the like which do not characterize the present invention) The general manufacturing process of the battery, etc.) can be understood as a design matter of those skilled in the art based on the prior art in the art. The present invention can be implemented based on the contents disclosed in the present specification and common technical knowledge in the field.

[Positive electrode for lithium secondary battery]
The positive electrode of the present embodiment typically includes a positive electrode current collector and a positive electrode active material layer fixed to the surface thereof. As a positive electrode collector, metal foils, such as aluminum, are suitable, for example.

The positive electrode active material layer is, as a positive electrode active material capable of reversibly absorbing and desorbing lithium ions, a lithium nickel cobalt manganese composite oxide having a layered structure (typically, a layered rock salt type structure) represented by the following formula (1) Contains.
LiNi _x Co _y Mn _z O ₂ (1)
However, in the formula (1), x, y and z are real numbers satisfying 1.56 ≦ [(y + z) / x] ≦ 1.86 and x + y + z = 1. By satisfying 1.56 ≦ [(y + z) / x], when the battery voltage exceeds the normal use range and the positive electrode has a high potential (eg, 4.3 V (vs. Li / Li ⁺ ) or more) Also, the crystal structure of the lithium-nickel-cobalt-manganese composite oxide can be stably maintained. In addition, by satisfying [(y + z) / x] ≦ 1.86, the battery resistance can be stably suppressed low.

  Although x, y and z are not particularly limited as long as the above formulas are satisfied, x preferably satisfies 0.35 ≦ x ≦ 0.39. It is preferable that y + z satisfy 0.61 ≦ (y + z) ≦ 0.65. y and z may be y <z, y = z, or z <y. y and z may be approximately equal (for example, the difference between the two may be approximately ± 0.2 or less, typically ± 0.1 or less).

The lithium-nickel-cobalt-manganese composite oxide may contain an oxyhydroxide of the constituent metal element, such as NiOOH or CoOOH, in part of its crystal. The oxyhydroxide decomposes at a temperature of approximately 200 to 300 ° C. to form water. For example, nickel oxyhydroxide causes a reaction of 4NiOOH → 4NiO + 2H ₂ O + O ₂ at around 220 ° C. to 230 ° C. to produce water.

  As described in Patent Document 1, conventionally, a coating layer is physically fixed on the surface of a lithium transition metal complex oxide for the purpose of suppressing the elution of a constituent element or the like. However, the lithium transition metal complex oxide repeatedly expands and contracts with charge and discharge of the battery. For this reason, there is a concern that the physical coating layer may peel off little by little with charge and discharge and the effect of the coating may be lost. In addition, when the ion conductivity and / or the electron conductivity of the coated material is low, there is also a concern that the battery resistance may be increased and the battery characteristics may be degraded. Compared to these conventional methods, the positive electrode of the present embodiment can achieve both battery characteristics in normal use and resistance to overcharge despite the fact that the surface of the positive electrode active material does not have a physical covering layer. It is a thing. Therefore, it can be said that the positive electrode of the present embodiment is more advantageous from the viewpoint of a long-term cycle.

  The positive electrode active material layer may contain an optional component other than the positive electrode active material, for example, a conductive material, a binder or the like. The conductive material is typically exemplified by a carbon material, for example, carbon black such as acetylene black (AB). As a binder, halogenated vinyl resins, such as polyvinylidene fluoride (PVdF), are illustrated, for example.

The positive electrode of the present embodiment has a water content of 2100 ppm to 2900 ppm based on the Karl Fischer method (heating temperature: 300 ° C.).
The water content adjusted positive electrode can be produced, for example, as follows. First, a lithium nickel cobalt manganese composite oxide represented by the above formula (1), a conductive material, and a binder are mixed in a suitable solvent (for example, N-methyl-2-pyrrolidone (NMP)) to form a slurry The composition of Next, a positive electrode current collector is prepared, and the composition prepared above is applied to the surface of the positive electrode current collector. It is dried to remove the solvent in the composition. Next, the positive electrode active material layer is fixed on the positive electrode current collector. This is kept in a constant temperature and humidity chamber with a humidity of 50 to 100% RH, for example, at room temperature (25 ° C.) in an environment where water is supplied. The holding period may be appropriately adjusted so that the above water content is realized, in accordance with the physical properties of the positive electrode active material, the properties of the positive electrode active material layer, and the like. Then, the positive electrode supplied with the water is heated and dried at a predetermined temperature (typically, not higher than the boiling point of water, for example, 50 to 100 ° C.). Thereby, the positive electrode adjusted to the amount of water of 2100 ppm or more and 2900 ppm or less can be obtained.

In the present specification, “the Karl Fischer method (heating temperature: 300 ° C.)” means the amount of water vaporized when the positive electrode is heated at 300 ° C. using a Karl Fischer moisture meter. The value measured by coulometric titration. In general, the positive electrode active material contains two kinds of water, ie, adsorbed water adsorbed on the surface and crystal water contained in crystals. By heating the positive electrode to be measured at 300 ° C., not only the water adsorbed on the surface of the positive electrode active material and the like, but also the crystal water such as the above-mentioned oxyhydroxide can be vaporized, It is possible to appropriately grasp the total amount of water inside.
Further, in the present specification, “water content per positive electrode active material (ppm)” is a mass fraction obtained by dividing the water content (mass) contained in the positive electrode by the mass of the positive electrode active material, that is, ppm (mass / mass) Say

[Lithium secondary battery]
The positive electrode of the present embodiment is used to construct a lithium secondary battery.
The lithium secondary battery of the present embodiment includes an electrode assembly having the above-described positive electrode and negative electrode, and a non-aqueous electrolyte. The electrode body is oppositely disposed in a state in which the positive electrode and the negative electrode are insulated via, for example, a resin separator.

  The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer fixed to the surface thereof. As the negative electrode current collector, for example, a metal foil such as copper is suitable. The negative electrode active material layer contains a negative electrode active material capable of reversibly absorbing and desorbing lithium ions. As a suitable example of a negative electrode active material, carbon materials, such as graphite, are mentioned, for example. The negative electrode active material layer may contain optional components other than the negative electrode active material, such as a binder and a thickener. Examples of the binder include rubbers such as styrene butadiene rubber (SBR). As a thickener, celluloses, such as carboxymethylcellulose (CMC), are illustrated, for example.

The non-aqueous electrolyte is typically a non-aqueous electrolyte that is liquid at room temperature (25 ° C.) and contains a non-aqueous solvent and a lithium salt. However, the non-aqueous electrolyte may be in the form of a polymer (gel). Examples of non-aqueous solvents include non-protic solvents such as carbonates, esters and ethers. Ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) etc. are mentioned as one specific example of carbonates. The lithium salt dissociates in the non-aqueous solvent to form lithium ions. Examples of lithium salts include fluorine-containing lithium salts such as LiPF ₆ and LiBF ₄ . The non-aqueous electrolyte is, in addition to the non-aqueous solvent and the supporting salt, for example, various film forming agents such as lithium bis oxalate borate (LiBOB) and vinylene carbonate (VC), various additives such as a dispersing agent and a thickener, May be included.

In the lithium secondary battery of the present embodiment, the water contained in the positive electrode reacts with the lithium salt contained in the non-aqueous electrolyte to form a film on the surface of the positive electrode active material. For example, when using a phosphorus-containing lithium salt, specifically LiPF ₆ as the lithium salt, the following reaction formula:
LiPF ₆ + 4H ₂ O → LiF + 5HF + H ₃ PO ₄ ;
Reaction to produce phosphoric acid (H ₃ PO ₄ ). By the film of phosphoric acid formed on the surface of the positive electrode active material, the crystal structure can be stably maintained even during overcharge.
Therefore, by setting the water content of the positive electrode to 2200 ppm or more, the thermal stability of the positive electrode active material can be improved to enhance the overcharge resistance.

However, the water contained in the positive electrode and Ni in the lithium-nickel-cobalt-manganese composite oxide simultaneously with the formation of the above-mentioned phosphoric acid and the following reaction formula:
LiNiO ₂ + H ₂ O → NiOOH + LiOH;
Also produces reactions like Lithium hydroxide (LiOH) is produced | generated on the surface of a positive electrode collector by this reaction. When lithium hydroxide reacts with the positive electrode current collector, a high resistance film is formed on the surface of the positive electrode active material. For example, in the case of using an aluminum foil as a positive electrode current collector, the surface of the aluminum foil is corroded to generate aluminum hydroxide. As a result, the battery resistance increases and the battery characteristics in normal use are degraded.
Therefore, by suppressing the water content of the positive electrode to 2900 ppm or less, the battery resistance can be reduced to improve the battery characteristics during normal use.

  As described above, the lithium secondary battery of the present embodiment can balance the excellent battery characteristics at the time of normal use and the excellent overcharge resistance at the time of overcharge at a high level by including the above-mentioned positive electrode. The lithium secondary battery of the present embodiment can be used for various applications, but taking advantage of the features described above, for example, a battery for driving a motor such as a plug-in hybrid car, a hybrid car, an electric car (driving power supply Can be suitably used.

  The following examples illustrate some of the embodiments of the present invention, but are not intended to limit the present invention to these embodiments.

<Fabrication of lithium secondary battery>
First, a nickel source (NiSO ₄ ), a manganese source (MnSO ₄ ), and a cobalt source (CoSO ₄ ) are dissolved in water to have the composition ratio (molar ratio) shown in Table 1, and sodium hydroxide is used. A raw material hydroxide was obtained by stirring while neutralizing. This raw material hydroxide was mixed with lithium carbonate and fired in the air. As a result, five types of lithium nickel manganese cobalt composite oxides ((y + z) / x) = 1.38 to 2.00 different in composition ratio were obtained.

  Next, a positive electrode was produced using each of the five types of lithium nickel manganese cobalt composite oxides as a positive electrode active material. Specifically, first, a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed to prepare a positive electrode slurry. The positive electrode slurry was applied to the surface of an aluminum foil (positive electrode current collector) and dried to prepare five positive electrodes each having a positive electrode active material layer on the positive electrode current collector.

  Next, the water content of the positive electrode was adjusted. Specifically, the positive electrode was stored for 1 to 14 days in a constant temperature and humidity chamber controlled to 25 ° C. and 50% RH, and then dried at 100 ° C. for 3 hours. Thereby, the water content of the positive electrode was made different. Then, a part of the positive electrode active material layer was scraped off, heated at 300 ° C., and the water content in the positive electrode was measured by the Karl Fischer method. Thereby, as shown in Table 1, 5 types (of (y + z) / x) = 1.38 to 2.00) having different composition ratios of the positive electrode active material × 4 types of having different water content of the positive electrode = 20 types A positive electrode was prepared.

  Next, the said positive electrode and negative electrode were laminated | stacked in the state which interposed the separator, and the electrode body (Example 1-Example 20) was produced. As the negative electrode, one having a negative electrode active material layer containing natural graphite as a negative electrode active material on a copper foil (negative electrode current collector) was used. In addition, as a separator, a three-layered structure of PP / PE / PP in which polypropylene (PP) was laminated on both sides of polyethylene (PE) was used.

Next, the electrode body (Examples 1 to 20) and the non-aqueous electrolyte were housed in a battery case, and the battery case was sealed. As a non-aqueous electrolytic solution, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of EC: DMC: EMC = 3: 4: 3, a supporting salt What dissolved LiPF ₆ as a 1 mol / L concentration was used. Thus, a battery assembly (Examples 1 to 20) was constructed.

Next, in a temperature environment of 25 ° C., the battery assembly is charged at a constant current of 1/3 C until the voltage between the positive and negative electrodes reaches 4.1 V, and then charged for 1.5 hours with the constant voltage Done (conditioning process). Next, the SOC of the battery was adjusted to 90%, and held for 20 hours in a temperature environment of 60 ° C. (aging treatment). In addition, "1 C" was taken as the value of the electric current which can fully charge the battery capacity (design capacity) estimated from positive electrode active material mass in 1 hour.
Thus, lithium secondary batteries of Examples 1 to 20 were produced.

<−30 ° C resistance>
The lithium secondary battery was adjusted to a state of SOC 60% under a temperature environment of 25.degree. This battery was moved to a constant temperature bath at -30 ° C, constant current discharge was performed for 10 seconds at a rate of 10 C, voltage change amount at this time was divided by discharge current value, and IV resistance (mΩ) was calculated. . The results are shown in Table 1 and FIGS.

<Overcharge tolerance>
A thermocouple was attached to the wall of the lithium secondary battery, and while monitoring the battery voltage and the battery temperature, charging was performed until the battery shuts down at a constant current of 10 C. Then, the temperature T1 of the lithium secondary battery immediately after the shutdown and the temperature T2 one minute after the shutdown were measured, and the temperature rise (° C.) after the shutdown was calculated by T2-T1. In addition, it can be said that the lower the temperature rise after shutdown, the better the resistance during overcharge. The results are shown in Table 1 and FIGS.

  As shown in Table 1 and FIG. 1, when [(y + z) / x] = 1.38, the thermal stability of the positive electrode active material was relatively low. In addition, even if the water content of the positive electrode was increased, the temperature increase during overcharge was large, and the overcharge resistance was insufficient. On the other hand, as shown in Table 1 and FIG. 5, in the case of [(y + z) / x] = 2.00, the battery resistance was relatively high. In addition, even when the water content of the positive electrode was increased, the improvement of the overcharge resistance was hardly observed.

  In addition, as shown in Table 1 and FIGS. 2 to 4, when the water content of the positive electrode is less than 2200 ppm, the temperature rise width during overcharge is relatively large, and the overcharge resistance is insufficient. It is considered that this is because the amount of water in the positive electrode is small, and the amount of generation of the phosphoric acid film formed by the reaction of the water in the positive electrode and the lithium salt of the non-aqueous electrolyte is insufficient. On the other hand, when the water content of the positive electrode exceeded 2900 ppm, the battery resistance in a low temperature environment was relatively high. It is considered that this is because the aluminum foil is corroded to form a highly resistant aluminum hydroxide film on the surface of the positive electrode active material.

Compared to these comparative examples, as shown in Table 1 and FIGS. 2 to 4, the comparative example contains a positive electrode active material satisfying 1.56 ≦ [(y + z) / x] ≦ 1.86, and the water content is 2200 ppm or more In Examples 6, 7, 10, 11, 14 and 15 having a positive electrode of 2900 ppm or less, the battery resistance at -30 ° C is suppressed to 27.8 mΩ or less, and the temperature rise after shutdown is 22.2 ° C or less It was suppressed. That is, excellent battery characteristics in normal use and excellent overcharge resistance in overcharge were balanced at a high level.
These results show the technical significance of the technology disclosed herein.

  The present invention has been described in detail above, but the above embodiments and examples are merely examples, and the invention disclosed herein includes various modifications and alterations of the specific example described above.

Claims (1)

The following equation _{(1): LiNi x Co y} Mn z O 2 ( provided that, x, y, z are, 1.56 ≦ [(y + z) / x] ≦ 1.86, is a real number satisfying x + y + z = 1. Containing a positive electrode active material represented by
The positive electrode for lithium secondary batteries whose water content based on the Karl-Fisher method (heating temperature: 300 degreeC) is 2200 ppm or more and 2900 ppm or less per unit mass of the said positive electrode active material.

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