JP2014067629A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which the cycle characteristics are improved by adding a second positive electrode active material, when using a spinel type nickel manganese composite oxide as the positive electrode active material.SOLUTION: The positive electrode active material of a nonaqueous electrolyte secondary battery contains a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is composed of a lithium manganese nickel composite oxide represented by a general formula LiMnNiMO(0.9≤a≤1.3, 0<x≤1.6, 0.4≤y<2, M is at least one kind of element selected from a group consisting of Fe, Mg, Zn, Co, Al, B, Nb, Mo, Cu and Ti). The second positive electrode active material is composed of a lithium transition metal composite oxide having an initial charge capacity exceeding 200 mAh/g when charged until the positive electrode potential becomes 2.0-4.0 V(vs.Li/Li).

Description

  The present invention relates to a nonaqueous electrolyte secondary battery using a spinel type lithium nickel manganese composite oxide as a main positive electrode active material.

  In recent years, non-aqueous electrolyte secondary batteries represented by lithium secondary batteries are not limited to mobile applications such as mobile phones, portable computers, PDAs, portable music players, power tools such as electric tools, electric bicycles, The use of medium-sized to large-sized batteries ranging from electric vehicles (EV) and hybrid electric vehicles (HEV, PHEV) to power sources, as well as storage power sources such as backup power sources and electric power storages, is being developed, and the applications are diversifying. Have been doing.

Non-aqueous electrolyte secondary batteries mainly use metals, alloys, metalloids, or carbon materials that can occlude and release lithium ions as negative electrode active materials, and include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and manganic acid. A lithium transition metal composite oxide such as lithium (LiMn ₂ O ₄ ) is used as the positive electrode active material. Among these positive electrode active materials composed of these lithium transition metal composite oxides, lithium nickelate is characterized by high capacity, but is inferior to lithium cobaltate in terms of safety and performance (for example, high overvoltage). A problem exists. In addition, lithium manganate is rich in resources and inexpensive, but has a problem of low energy density and dissolution of manganese at high temperatures. For this reason, at present, the use of lithium cobalt oxide as the lithium-containing transition metal composite oxide has become the mainstream.

  However, cobalt is expensive and has a small abundance as a resource. Recently, development of large-sized non-aqueous electrolyte secondary batteries for power storage such as EV, HEV, PHEV, etc., and power storage such as backup power supply and power storage has been accelerated. Such a large non-aqueous electrolyte battery is required to be small in size and light in weight and low in cost. For this reason, positive electrode materials having a high capacity density and a low cost that can replace lithium cobalt oxide have been actively developed.

Among them, the spinel type nickel manganese composite oxide has a high charge / discharge potential of around 4.75 V (vs. Li / Li ⁺ ) and does not contain cobalt in the raw material, so that low cost and high capacity density can be expected. . However, the spinel-type nickel manganese composite oxide has a problem that the capacity gradually decreases as the charge / discharge cycle proceeds. Such a decrease in capacity is believed to be based on the fact that the crystal structure becomes unstable due to the absence of lithium from the positive electrode at the end of charging of the nonaqueous electrolyte secondary battery.

With respect to such a problem, in the lithium secondary battery disclosed in Patent Document 1 below, as a positive electrode active material, a mixture of a lithium manganese composite oxide having a spinel structure and a lithium nickel composite oxide having a layered structure is used. By using this, a decrease in capacity when the charge / discharge cycle is repeated is suppressed. Further, in the lithium secondary battery disclosed in Patent Document 2 below, nV class positive electrode materials (for example, Li ₄ Mn ₅ O ₁₂ , LiV ₂ O ₅ , LiMnO _2, etc.) and (n + 1) are used as the positive electrode active material. By using a material composed of a mixture with a V-class positive electrode material (for example, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO _2, etc.), crystal breakage that may occur when lithium is released is suppressed.

Japanese Patent Laid-Open No. 10-092430 JP 2001-043859 A

  On the other hand, conventionally, there has been known a method of adding vinylene carbonate (VC) as an additive to a non-aqueous electrolyte in order to suppress reductive decomposition of a non-aqueous solvent in a non-aqueous electrolyte of a non-aqueous electrolyte secondary battery. Yes. In order to prevent the negative electrode active material from directly reacting with the non-aqueous solvent, this additive forms a negative electrode surface film (SEI: Solid Electrolyte Interface. Hereinafter referred to as “SEI”), which is also referred to as a passivation layer. Is. As described above, when a non-aqueous electrolytic solution to which VC is added is used, SEI is formed on the negative electrode active material by causing VC to undergo reductive decomposition on the negative electrode surface before lithium is inserted into the negative electrode by the first charge. It is known to prevent unnecessary reductive decomposition of the nonaqueous electrolytic solution on the negative electrode.

A positive electrode using a spinel nickel-manganese composite oxide as a positive electrode active material has a potential profile that suddenly rises to a high potential of 4.5 V (vs. Li ^/ Li ⁺ ) or more immediately after the start of charging. On the surface of the material, oxidative decomposition of the non-aqueous electrolyte and concomitant elution of Mn from the positive electrode active material are likely to occur. For this reason, in the initial charging process, before an appropriate SEI is formed on the surface of the negative electrode active material of the counter electrode, the Mn eluate from the positive electrode comes into contact with the negative electrode, and Mn precipitates on the surface of the negative electrode active material. As a result, deposition of a reductive decomposition reaction product of the non-aqueous electrolyte tends to occur. When the deposition of the reductive decomposition product of the nonaqueous electrolytic solution accompanying the precipitation of Mn occurs from the initial stage of charging, the appropriate SEI formation reaction on the negative electrode is inhibited in the subsequent initial charging process. It becomes difficult to prevent the reductive decomposition of the unnecessary nonaqueous electrolytic solution on the negative electrode.

The reductive decomposition of the unnecessary nonaqueous electrolyte solution at the negative electrode increases the negative electrode potential by incorporating Li appropriately stored in the negative electrode into the reduced decomposition product, and the battery voltage decreases as the negative electrode potential increases. This is a big problem. Further, when the battery voltage drop occurs, the negative electrode potential becomes higher than the dissolution potential of copper in the negative electrode current collector (around 3.45 V (vs. Li / Li ⁺ )), so that the copper is gradually contained in the non-aqueous electrolyte. There is a problem in that the copper that is eluted and eluted in the non-aqueous electrolyte is electrodeposited on the negative electrode to inhibit the battery characteristics.

  In one aspect of the present invention, when a spinel nickel manganese composite oxide having a high charge / discharge potential is used as the positive electrode active material, the cycle characteristics are improved by adding the second positive electrode active material.

A nonaqueous electrolyte secondary battery according to one aspect of the present invention is a nonaqueous electrolyte secondary battery including a positive electrode including a first positive electrode active material and a second positive electrode active material, a negative electrode, and a nonaqueous electrolyte. ,
The first positive electrode active material is represented by the general formula _{Li a Mn x Ni y M 2} -x-y O 4 (0.9 ≦ a ≦ 1.3,0 <x ≦ 1.6,0.4 ≦ y <2 , M is composed of a lithium manganese nickel composite oxide represented by Fe, Mg, Zn, Co, Al, B, Nb, Mo, Cu, and Ti).
The second positive electrode active material is a lithium transition metal composite oxide having an initial charge capacity exceeding 200 mAh / g when charged until the positive electrode potential becomes 2.0 to 4.0 V (vs. Li / Li ⁺ ). It is characterized by comprising.

Formula _{Li a Mn x Ni y M 2} -x-y O 4 (0.9 ≦ a ≦ 1.3,0 <x ≦ 1.6,0.4 ≦ y <2 is a first positive electrode active material, The lithium manganese nickel composite oxide represented by M is at least one element selected from the group consisting of Fe, Mg, Zn, Co, Al, B, Nb, Mo, Cu and Ti is a spinel nickel manganese It is a compound also called a complex oxide, and has a high charge / discharge potential close to 5V. By mixing this first positive electrode active material with a second positive electrode active material having a charging potential as low as 2.0 to 4.0 V and a large initial charging capacity of 200 mAh / g, At the time of charging, the charging reaction of only the second positive electrode active material occurs first at a low potential where the charging reaction of the first positive electrode active material hardly occurs. Therefore, during the charging reaction with the second positive electrode active material, an appropriate SEI formation reaction is caused on the negative electrode active material surface without being adversely affected by the Mn eluate from the first positive electrode active material. Is possible.

  As described above, in the nonaqueous electrolyte secondary battery according to one aspect of the present invention, appropriate SEI formation is performed before the precipitation of the Mn eluate from the first positive electrode active material on the negative electrode during the first charge. Can do. As a result, even if the subsequent charging reaction of the first positive electrode active material occurs at a high potential and Mn elution is likely to be generated, generation of unnecessary reduction reaction on the surface of the negative electrode active material is prevented. It becomes possible to suppress. Therefore, according to the non-aqueous electrolyte secondary battery of the present invention, it is possible to suppress problems such as a decrease in battery voltage in the charging process, and the negative current collector component ( For example, since the dissolution of copper) is suppressed, the problem of the inhibition of battery characteristics due to the dissolution and precipitation of the components of the negative electrode current collector can be solved.

Further, the higher the initial charge capacity of the second positive electrode active material, the larger the relative amount of the first positive electrode active material that can be accommodated in the unit volume, and the higher the capacity after the initial charge. It becomes like this. Therefore, the initial charge capacity of the second positive electrode active material is 250 mAh / g or more when the positive electrode potential is charged from 2.0 V (vs. Li / Li ⁺ ) to 4.0 V (vs. Li / Li ⁺ ). If it is preferable, it is more preferable if it is 300 mAh / g or more.

In addition, as a negative electrode active material which can be used in the nonaqueous electrolyte secondary battery of the present invention, carbon raw materials such as lithium metal, lithium alloy, graphite, non-graphitizable carbon and graphitizable carbon, LiTiO ₂ and TiO _{2 are used.} Examples thereof include titanium oxides such as, metalloid elements such as silicon and tin, silicon oxide (SiOx, 0.5 ≦ x <1.6), and Sn—Co alloys.

  Nonaqueous solvents that can be used in the nonaqueous electrolyte secondary battery of the present invention include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and fluorinated cyclic carbonates. Esters, cyclic carboxylic acid esters such as γ-butyrolactone (BL) and γ-valerolactone (VL), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), dibutyl Chain carbonates such as carbonate (DBC), fluorinated chain carbonates, chain carboxylates such as methyl pivalate, ethyl pivalate, methyl isobutyrate, methyl propionate, N, N′— Dimethylformamide, N-me Amide compounds such as oxazolidinone, sulfur compounds such as sulfolane, etc. ambient temperature molten salt such as tetrafluoroboric acid 1-ethyl-3-methylimidazolium can be exemplified. It is desirable to use a mixture of two or more of these. Among these, cyclic carbonates and chain carbonates having a particularly high dielectric constant and a high ionic conductivity of the nonaqueous electrolytic solution are preferable.

In addition, as the electrolyte salt dissolved in the non-aqueous solvent used in the non-aqueous electrolyte secondary battery of the present invention, a lithium salt generally used as an electrolyte salt in the non-aqueous electrolyte secondary battery can be used. Such lithium salts include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , and mixtures thereof Illustrated. Among these, LiPF ₆ (lithium hexafluorophosphate) is particularly preferable. The amount of electrolyte salt dissolved in the non-aqueous solvent is preferably 0.8 to 2.0 mol / L.

  Further, in the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery of the present invention, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), succinic anhydride (SUCAH), maleic anhydride (MAAH), At least one kind of glycolic anhydride, ethylene sulfite (ES), divinyl sulfone (VS), vinyl acetate (VA), vinyl pivalate (VP), catechol carbonate or the like may be added.

  In the nonaqueous electrolyte secondary battery of the present invention, it is preferable that a mass ratio of the first positive electrode active material to the second positive electrode active material is 90:10 to 97: 3.

  The amount of the second positive electrode active material can be adjusted according to the initial charge capacity of the second positive electrode active material used. However, if the content ratio of the second positive electrode active material is less than 3% by mass with respect to the total amount of the positive electrode active material, charging of the first positive electrode active material starts before SEI is satisfactorily formed during the first charge. This is not preferable because there is a possibility of being lost. In addition, if the content ratio of the second positive electrode active material exceeds 10% by mass with respect to the total amount of the positive electrode active material, charging of the first positive electrode active material does not start even after SEI is formed during the first charge, Since the amount of the first positive electrode active material corresponding to the excess amount of the second positive electrode active material contained per unit volume is reduced, the average charge / discharge voltage of the nonaqueous electrolyte secondary battery is lowered, which is not preferable.

In the non-aqueous electrolyte secondary battery of the present invention, the second positive electrode active material may be Li _X AO _Y (X / Y> 0.6, A is Fe, Mn, Co, Ni, Mo, and Cu. And at least one element selected from the group consisting of at least one element selected from the group consisting of lithium transition metal composite oxides. In this case, the second positive electrode active material is preferably at least one selected from Li ₂ MoO ₃ , Li ₅ FeO ₄ , Li ₆ MnO ₄ and Li ₆ CoO ₄ .

Lithium transition represented by Li _X AO _Y (X / Y> 0.6, A is at least one element selected from the group consisting of Fe, Mn, Co, Ni, Mo, and Cu) as the second positive electrode active material When at least one metal composite oxide is used, an initial charge capacity of 200 mAh / g or more when easily charged until the positive electrode potential becomes 2.0 to 4.0 V (vs. Li / Li ⁺ ) is easily obtained. Can be obtained. In particular, when at least one selected from Li ₂ MoO ₃ , Li ₅ FeO ₄ , Li ₆ MnO ₄ and Li ₆ CoO ₄ is used as the second positive electrode active material, oxygen is released during the initial charging process. The energy density per unit mass of the subsequent positive electrode active material is increased, and a material having a capacity of 250 mAh / g or more can be easily obtained.

In the nonaqueous electrolyte secondary battery of the present invention, it is preferable that the first positive electrode active material is LiMn _1.5 Ni _0.5 O ₄ .

Since LiMn _1.5 Ni _0.5 O ₄ as the first positive electrode active material can be charged / discharged at a positive electrode potential of around 5.0 V (vs. Li / Li ⁺ ), it is a non-aqueous electrolyte with a high voltage and a high energy density. A secondary battery is obtained.

In the nonaqueous electrolyte secondary battery of the present invention, the charge termination potential of the positive electrode can be 4.85 V (vs. Li / Li ⁺ ) or more.

  The higher the end-of-charge voltage of the positive electrode, the higher the energy density of the nonaqueous electrolyte secondary battery can be obtained, but the cycle life of the battery will decrease. According to the nonaqueous electrolyte secondary battery of the present invention, even when the charge end potential of the positive electrode is 4.85 V or higher, a nonaqueous electrolyte secondary battery having a longer cycle life than the conventional nonaqueous electrolyte secondary battery is obtained. It is done.

It shows the initial charging characteristics of LiNi _0.5 Mn _1.5 O _4. It shows the initial charging characteristics of Li ₂ MoO _3. It shows the initial charging characteristics of Li ₅ FeO _4. Is a diagram showing the initial discharge energy density when changing the content of _Li 2 MoO ₃ for LiNi _0.5 Mn _1.5 O _4.

  Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the following embodiment is illustrated for the purpose of understanding the technical idea of the present invention, and is not intended to specify the present invention as the embodiment, and the present invention is not limited to the scope of the claims. The present invention can equally be applied to those in which various modifications are made without departing from the technical idea shown in.

[Measurement of initial charge characteristics of each positive electrode active material]
First, initial charge characteristics of a test cell using LiNi _0.5 Mn _1.5 O ₄ corresponding to the first positive electrode active material of the present invention, Li ₂ MoO ₃ and Li ₅ FeO ₄ corresponding to the second positive electrode active material. Was measured as follows.

[Production of positive electrode]
A positive electrode using LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material was produced as follows. 90 parts by mass of LiNi _0.5 Mn _1.5 O ₄ , 5 parts by mass of acetylene black as a conductive agent, and 5 parts by mass of polyvinylidene fluoride (PVdF) as a binder were weighed, and the dispersion medium Was mixed with N-methyl 2-pyrrolidone (NMP) as a positive electrode mixture slurry. This positive electrode mixture slurry is applied to the surface of a 30 μm thick aluminum foil as a positive electrode conductor by a die coater, then dried to remove NMP as an organic solvent, and compressed to a predetermined thickness by a roll press. Then, by cutting out to a predetermined size, a positive electrode using LiNi _0.5 Mn _1.5 O ₄ as a positive electrode active material was obtained.

In the same manner as described above except that 70 parts by mass of Li ₂ MoO ₃ to Li ₅ FeO ₄ , 20 parts by mass of acetylene black, and 10 parts by mass of polytetrafluoroethylene (PTFE) were added. Thus, a positive electrode using Li ₂ MoO ₃ as the positive electrode active material and a positive electrode using Li ₅ FeO ₄ as the positive electrode active material were prepared.

(Production of negative electrode)
The negative electrode was produced by attaching a lithium metal thin film to the surface of a copper foil as a negative electrode current collector and cutting it out to a predetermined size.

(Preparation of electrolyte)
The non-aqueous electrolyte is hexafluoro as an electrolyte salt in a non-aqueous solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio (1 atm, 25 ° C.) in a ratio of 3: 7. lithium phosphate (LiPF ₆₎ was used dissolved in a proportion of 1.0 mol / L.

[Production of test cell]
An electrode body in which lead terminals (not shown) are attached and stacked on each of the positive electrode and the negative electrode manufactured as described above is inserted into an aluminum laminate battery outer body, and a non-aqueous electrolyte is injected into the battery outer body. After sealing, three types of test cells using LiNi _0.5 Mn _1.5 O ₄ , Li ₂ MoO ₃ and Li ₅ FeO ₄ as the positive electrode active material were produced.

[Initial charging test]
The initial charge test of the test cell using LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material was measured at a charge end rate of 4.9 V at a charge rate of 0.1 It. The results are shown in FIG.

In addition, an initial charge test of a test cell using Li ₂ MoO ₃ as a positive electrode active material was measured at a charge end rate of 4.7 V at a charge rate of 0.04 It. The results are shown in FIG.

Furthermore, the initial charge test of the test cell using Li ₅ FeO ₄ as the positive electrode active material was measured at a charge end rate of 4.0 V at a charge rate of 0.04 It. The results are shown in FIG.

The following can be understood from the results shown in FIGS. That is, from the result shown in FIG. 1, in the test cell using LiNi _0.5 Mn _1.5 O ₄ corresponding to the first positive electrode active material, 4.8 V (vs. Li ^/ Li ⁺ ) is close to the initial charge. The potential suddenly rises up to and then remains at a high potential of 4.8V. Further, from the results shown in FIG. 2, in the test cell using Li ₂ MoO ₃ corresponding to the second positive electrode active material, the charging potential of 4 V (vs. Li / Li ⁺ ) or less from the initial charging stage to the final charging stage. The charge capacity exceeds 200 mAh / g. Furthermore, from the results shown in FIG. 3, in the test cell using Li ₅ FeO ₄ corresponding to the second positive electrode active material, a charging potential of 4 V (vs. Li / Li ⁺ ) or less from the initial charging stage to the final charging stage. And the charging capacity exceeds 250 mAh / g. In addition, when considering FIG. 1 to FIG. 3 collectively, LiNi _0.5 Mn _1.5 O ₄ and the second positive electrode active material corresponding to the first positive electrode active material are included in the range where the charging potential at the initial stage of charging is extremely low. The charging of the corresponding Li ₂ MoO ₃ to Li ₅ FeO ₄ competes, but then the charging potential immediately rises. Therefore, if the charging potential is at least 2.0 V (vs. Li / Li ⁺ ), the second no _Li 2 MoO ₃ corresponding to the positive electrode active material to be seen to proceed to charge the _Li 5 FeO _4.

From the above, a mixture of LiNi _0.5 Mn _1.5 O ₄ as the first positive electrode active material and at least Li ₂ MoO ₃ to Li ₅ FeO ₄ as the second positive electrode active material is used. It can be confirmed that only the second positive electrode active material is charged when the positive electrode potential at the initial stage of charging is in the range of 2.0 to 4.0 V (vs. Li / Li ⁺ ). In addition, since the charging potential of LiNi _0.5 Mn _1.5 O ₄ as the first positive electrode active material is higher than the charging potential of Li ₂ MoO ₃ to Li ₅ FeO ₄ as the second positive electrode active material, the second positive electrode It is obvious that LiNi _0.5 Mn _1.5 O ₄ as the first positive electrode active material is charged after all of the active material is charged.

Here, initial charge characteristics are shown when LiNi _0.5 Mn _1.5 O _{4 is} used as the first positive electrode active material and Li ₂ MoO ₃ and Li ₅ FeO ₄ are used as the second positive electrode active material. However, the first active material formula _{Li a Mn x Ni y M 2} -x-y O 4 (0.9 ≦ a ≦ 1.3,0 <x ≦ 1.6,0.4 ≦ y <2, M is substantially a lithium manganese nickel composite oxide represented by Fe, Mg, Zn, Co, Al, B, Nb, Mo, Cu and Ti. Shows initial charge characteristics having the same tendency as in the case of LiNi _0.5 Mn _1.5 O ₄ . The second active material is represented by the general formula Li _X AO _Y (X / Y> 0.6, A is at least one element selected from the group consisting of Fe, Mn, Co, Ni, Mo, and Cu). If the lithium transition metal composite oxide is used, the initial charge capacity when charged until the positive electrode potential is substantially 2.0 to 4.0 V (vs. Li / Li ⁺ ) exceeds 200 mAh / g Is obtained.

[Preparation of non-aqueous electrolyte secondary battery]
<Example 1>
[Production of positive electrode]
The positive electrode active material uses LiNi _0.5 Mn _1.5 O ₄ as the first positive electrode active material, Li ₂ MoO ₃ as the second positive electrode active material, and LiNi _0.5 Mn _1.5 O ₄ and Li ₂ MoO _3. Were mixed so that the mass ratio was 98: 2. The positive electrode active material was weighed to 90 parts by mass, 5 parts by mass of acetylene black as a conductive agent, and 5 parts by mass of polyvinylidene fluoride (PVdF) as a binder, and N-methyl 2 as a dispersion medium. A positive electrode mixture slurry was prepared by mixing with -pyrrolidone (NMP). This positive electrode mixture slurry is applied to the surface of a 30 μm thick aluminum foil as a positive electrode conductor by a die coater, then dried to remove NMP as an organic solvent, and compressed to a predetermined thickness by a roll press. And the positive electrode was obtained by cutting out to predetermined size.

(Production of negative electrode)
Carbon material (graphite) as a negative electrode active material, sodium carboxymethylcellulose (CMC) as a thickener, and styrene butadiene rubber (SBR) as a binder are each in a mass ratio of 98: 1: 1. And were dispersed in water to prepare a negative electrode mixture slurry. This negative electrode mixture slurry is applied to the surface of a copper foil as a negative electrode current collector by a die coater, dried to form a negative electrode active material mixture layer on both sides of the negative electrode current collector, and then using a compression roller The negative electrode was obtained by compressing to a predetermined thickness and cutting out to a predetermined dimension.

(Preparation of non-aqueous electrolyte)
The non-aqueous electrolyte is hexafluoro as an electrolyte salt in a non-aqueous solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio (1 atm, 25 ° C.) in a ratio of 3: 7. lithium phosphate (LiPF ₆₎ was used dissolved in a proportion of 1.0 mol / L.

[Production of cell]
An electrode body in which lead terminals (not shown) are attached and stacked on each of the positive electrode and the negative electrode manufactured as described above is inserted into an aluminum laminate battery outer body, and a non-aqueous electrolyte is injected into the battery outer body. After that, the nonaqueous electrolyte secondary battery of Example 1 was produced by sealing.

(Initial charge / discharge test)
The nonaqueous electrolyte secondary battery of Example 1 was precharged to a charge depth (SOC: State of Charge) of 10% with a constant current of 0.1 It and then left at 60 ° C. for 16 hours. The battery voltage and battery voltage drop ΔV at this time were determined, and the results are shown in Table 1. Thereafter, the battery was charged at a constant current of 0.1 It until the battery voltage reached 4.95 V, and after 10 minutes of rest, the battery voltage was discharged at a constant current of 0.1 It until the battery voltage reached 3.0 V. The initial discharge energy density at this time was determined, and the results are shown in Table 1.
The initial discharge energy density was calculated using the following formula.
Initial discharge energy density [Wh / L]
= Positive electrode active material packing density [g / cc] × initial discharge capacity [mAh / g] × initial average discharge voltage [V]
However,
Positive electrode packing density [g / cc]
= (Positive electrode plate weight−Al foil weight [mg]) × Positive electrode active material mixing ratio (0.9) / (Positive electrode plate area [cm ² ] × (Electrode plate thickness−Al foil thickness [μm])

[Cycle test]
The capacity retention rate after 8 cycles was measured as follows. Using the non-aqueous electrolyte secondary battery subjected to the initial charge / discharge test as described above, the battery voltage was 4.95 V (positive electrode potential was 5.05 V (vs. Li / Li ⁺ )) at a constant current of 0.1 It. Charged until Next, the battery was discharged at a constant current of 0.1 It until the battery voltage reached 3.0 V, and the discharge capacity at the first cycle was determined. This charge / discharge operation was repeated, the discharge capacity at the eighth cycle was determined, the capacity retention rate (%) at the eighth cycle was determined by the following calculation formula, and the results are shown in Table 1.
Capacity maintenance rate (%)
= 8th cycle discharge capacity [mAh] / 1st cycle discharge capacity [mAh] × 100

<Example 2>
Example ₂ is the same as Example 1 except that the content ratio of the positive electrode active materials LiNi _0.5 Mn _1.5 O ₄ and Li ₂ MoO ₃ used in Example 1 is 95: 5 by mass ratio. A water electrolyte secondary battery was prepared, and the battery voltage, the battery voltage drop ΔV, the initial discharge energy density, and the capacity retention ratio at the eighth cycle were measured in the same manner as in Example 1. The results are collectively shown in Table 1, and the relationship between the initial energy density and the mixing ratio of Li ₂ MoO ₃ is also shown in FIG.

<Example 3>
Example ₃ is the same as Example 1 except that the content ratios of the positive electrode active materials LiNi _0.5 Mn _1.5 O ₄ and Li ₂ MoO _{3 used} in Example 1 are 90:10 by mass ratio. A water electrolyte secondary battery was prepared, and the battery voltage, the battery voltage drop ΔV, the initial discharge energy density, and the capacity retention ratio at the eighth cycle were measured in the same manner as in Example 1. The results are collectively shown in Table 1, and the relationship between the initial energy density and the mixing ratio of Li ₂ MoO ₃ is also shown in FIG.

<Example 4>
Except for the positive electrode active materials LiNi _0.5 Mn _1.5 O ₄ and Li ₂ MoO ₃ used in Example 1, the mass ratio was 85:15. A water electrolyte secondary battery was prepared, and the battery voltage, the battery voltage drop ΔV, the initial discharge energy density, and the capacity retention ratio at the eighth cycle were measured in the same manner as in Example 1. The results are collectively shown in Table 1, and the relationship between the initial energy density and the mixing ratio of Li ₂ MoO ₃ is also shown in FIG.

<Comparative Example 1>
A nonaqueous electrolyte secondary battery of Example 4 was produced in the same manner as in Example 1 except that LiNi _0.5 Mn _1.5 O ₄ was used as the positive electrode active material. Then, the battery voltage, the battery voltage drop ΔV, the initial discharge energy density, and the capacity retention rate at the eighth cycle were measured. The results are collectively shown in Table 1, and the relationship between the initial energy density and the mixing ratio of Li ₂ MoO ₃ is also shown in FIG.

The following can be understood from the measurement results of the batteries of Examples 1 to 4 and Comparative Example shown in Table 1. That is, in the case of the batteries of Examples 1 to 4 using Li ₂ MoO ₃ as the second positive electrode active material, the battery voltage drop ΔV after being left at a high temperature is a battery of a comparative example that does not contain Li ₂ MoO ₃ . It was confirmed that it was lower than the case. Similarly, as in the batteries of Examples 1 to 4, the positive electrode active material is LiNi _0.5 Mn _1.5 O ₄ and the positive electrode potential is 2.0 to 4.0 V (vs. It was confirmed that a drop in battery voltage after leaving at high temperature can be suppressed by combining with a second positive electrode active material having an initial charge capacity of more than 200 mAh / g when charged until .Li / Li ⁺ ). This means that when a positive electrode active material including the first positive electrode active material and the second positive electrode active material having the above-described composition is used, good SEI is formed on the surface of the negative electrode active material at the initial stage of charging. That means.

Similarly, in the case of the batteries of Examples 1 to 4 using Li ₂ MoO ₃ as the second positive electrode active material, the capacity retention rate at the eighth cycle is a battery of a comparative example that does not contain Li ₂ MoO _3. It turned out to be higher than that. Accordingly, if the content ratio of the second positive electrode active material is 2% by mass or more with respect to the total amount of the positive electrode active material, and more preferably 5% by mass or more, Mn is easily eluted from the first positive electrode active material. Even in the situation, it is possible to suppress the occurrence of unnecessary reduction reaction on the surface of the negative electrode active material, the dissolution and precipitation of copper as the negative electrode current collector is suppressed, and the battery resulting from the dissolution and precipitation of copper It was found that inhibition of properties is less likely to occur. If the content ratio of the second positive electrode active material is 15% by mass with respect to the total amount of the positive electrode active material, the initial discharge energy density is greatly reduced. It turned out that 5-10 wt% is preferable.

Claims (6)

A non-aqueous electrolyte secondary battery comprising a positive electrode including a first positive electrode active material and a second positive electrode active material, a negative electrode, and a non-aqueous electrolyte,
The first positive electrode active material is represented by the general formula _{Li a Mn x Ni y M 2} -x-y O 4 (0.9 ≦ a ≦ 1.3,0 <x ≦ 1.6,0.4 ≦ y <2 , M is composed of a lithium manganese nickel composite oxide represented by Fe, Mg, Zn, Co, Al, B, Nb, Mo, Cu, and Ti).
The second positive electrode active material is a lithium transition metal composite oxide having an initial charge capacity exceeding 200 mAh / g when charged until the positive electrode potential becomes 2.0 to 4.0 V (vs. Li / Li ⁺ ). A non-aqueous electrolyte secondary battery comprising:   The non-aqueous electrolyte secondary battery according to claim 1, wherein a mass ratio of the first positive electrode active material to the second positive electrode active material is 90:10 to 97: 3. The second positive electrode active material is represented by the general formula Li _X AO _Y (X / Y> 0.6, A is at least one element selected from the group consisting of Fe, Mn, Co, Ni, Mo, and Cu). The nonaqueous electrolyte secondary battery according to claim 1 or 2, comprising at least one lithium transition metal composite oxide represented. 4. The non-aqueous electrolyte according to claim 3, wherein the second positive electrode active material is at least one selected from Li ₂ MoO ₃ , Li ₅ FeO ₄ , Li ₆ MnO ₄ and Li ₆ CoO _4. Secondary battery. The non-aqueous electrolyte secondary battery according to claim 1, wherein the first active material is LiMn _1.5 Ni _0.5 O ₄ . The non-aqueous electrolyte secondary battery according to claim 1, wherein a charge end potential of the positive electrode is 4.85 V (vs. Li / Li ⁺ ) or more.

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