JP5958964B2 - Positive electrode mixture and method for producing the same, non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a positive electrode mixture, a method for producing a positive electrode mixture, and a non-aqueous electrolyte secondary battery.

  One non-aqueous electrolyte secondary battery is a lithium ion secondary battery. A lithium ion secondary battery is a secondary battery that can be charged and discharged by moving lithium ions in a non-aqueous electrolyte between a positive electrode and a negative electrode that occlude and release lithium ions.

Patent Document 1 discloses a lithium manganese nickel-based composite oxide (positive electrode) represented by Li _x Mn _1.5 Ni _0.5 O _4-w (where 0 <x <2, 0 ≦ w <2). A technique for improving the cycle characteristics of a lithium ion secondary battery by coating the surface of the active material) with a metal oxide containing at least one metal element selected from Mg, Al, Ti, Zr and Zn. It is disclosed.

JP 2006-36545 A

  As described in the background art, in the technology disclosed in Patent Document 1, manganese ions contained in the positive electrode active material are eluted into the electrolyte solution by coating the surface of the positive electrode active material with a metal oxide. And the cycle characteristics of the lithium ion secondary battery are improved.

However, the technique disclosed in Patent Document 1 has a problem that the cycle characteristics of the lithium ion secondary battery are not sufficiently improved. That is, in the technique disclosed in Patent Document 1, the surface of the positive electrode active material is coated with a metal oxide. Here, since the metal oxide is an amphoteric oxide that reacts with an acid and a base, the metal oxide contained in the positive electrode elutes into the electrolyte and deposits on the negative electrode, deteriorating the cycle characteristics of the lithium ion secondary battery. There is a problem of making it. For example, when zinc oxide (ZnO) is used as the metal oxide, ZnO becomes Zn ²⁺ ions and elutes in the electrolyte under acidic conditions. Under basic conditions, Zn (OH) ₄ ^2- ion is eluted into the electrolyte. There is a problem that zinc ionized in this way is deposited on the negative electrode by an electrochemical reaction and deteriorates the cycle characteristics of the lithium ion secondary battery. Therefore, the technique disclosed in Patent Document 1 is insufficient in improving the cycle characteristics of the lithium ion secondary battery.

  In view of the above problems, an object of the present invention is to provide a positive electrode mixture, a method for manufacturing a positive electrode mixture, and a nonaqueous electrolyte secondary battery that can improve the cycle characteristics of the nonaqueous electrolyte secondary battery. is there.

A positive electrode mixture according to one embodiment of the present invention includes a positive electrode active material and an additive that covers at least a part of the surface of the positive electrode active material, and the additive has a cristobalite structure at least in part. LiMPO ₄ (M is a typical element).

In the positive electrode mixture, the additive may contain LiZnPO ₄ .

In the positive electrode mixture, the additive may include LiMgPO ₄ .

In the positive electrode mixture, the additive may comprise a mixed phase of olivine structure derived from the cristobalite structure and LiMgPO ₄ derived from LiZnPO _4.

  In the positive electrode mixture, the positive electrode active material may contain lithium nickel manganate.

  The positive electrode mixture may further contain a conductive agent.

  A nonaqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode having the positive electrode mixture, a negative electrode having a negative electrode mixture containing a negative electrode active material, a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent. And having.

A method for producing a positive electrode mixture according to one aspect of the present invention includes a step of mixing a raw material containing Li ₂ O, P ₂ O ₅ and an oxide of a typical element to prepare a mixed powder; A step of forming an additive having a cristobalite structure by mechanical milling, and a step of mixing the additive and the positive electrode active material to form a positive electrode mixture.

In the method for producing a positive electrode mixture, the oxide of the typical element may be ZnO, and the additive may contain LiZnPO ₄ .

  In the method for producing the positive electrode mixture, when the additive is formed, mechanical milling treatment may be performed at a base plate rotation speed of 300 to 550 rpm for 20 to 50 hours.

  In the manufacturing method of the positive electrode mixture, when the additive is formed, mechanical milling may be performed at a base plate rotation speed of 450 to 550 rpm.

In the manufacturing method of the positive electrode mixture, MgO may be further added when preparing the mixed powder, and the mixed powder may be mechanically milled to form an additive containing LiZnPO ₄ and LiMgPO ₄ .

  ADVANTAGE OF THE INVENTION By this invention, the positive electrode mixture which can improve the cycling characteristics of a nonaqueous electrolyte secondary battery, the manufacturing method of a positive electrode mixture, and a nonaqueous electrolyte secondary battery can be provided.

It is a figure which shows the positive mix which concerns on embodiment. It is a flowchart which shows the preparation procedures of the additive contained in a positive mix. It is a figure which shows the X-ray-diffraction pattern of an additive. It is a flowchart which shows the preparation procedures of a positive electrode. It is a figure which shows the X-ray-diffraction pattern of an additive. It is a table | surface which shows the positive electrode active material of each produced sample, an additive, the preparation conditions of an additive, and the ratio of a positive electrode active material and an additive. It is a flowchart which shows the flow of the high temperature storage test of the produced sample. It is a charging / discharging curve (1st cycle) of the produced sample. It is a charging / discharging curve (180th cycle) of the produced sample. It is a charging / discharging curve (after 3 days storage at 60 degreeC) of the produced sample. It is a figure which shows the relationship between the cycle number of the produced sample, and discharge capacity (after storing at 60 degreeC for 3 days). It is a figure which shows the impedance measurement result before the high temperature preservation | save of the sample concerning Example 1. FIG. It is a figure which shows the impedance measurement result before the high temperature preservation | save of the sample concerning Example 2. FIG. It is a figure which shows the impedance measurement result before the high temperature preservation | save of the sample concerning the comparative example 2. FIG. It is a figure which shows the impedance measurement result after the high temperature preservation | save of the sample concerning Example 1. FIG. It is a figure which shows the impedance measurement result after the high temperature preservation | save of the sample concerning Example 2. FIG. It is a figure which shows the impedance measurement result after the high temperature preservation | save of the sample concerning the comparative example 2. FIG. It is a table | surface which shows the additive of each sample, charging / discharging efficiency, and cycle stability. It is a table | surface which shows the additive of each sample, charging / discharging efficiency, the capacity | capacitance after high temperature storage, cycle stability, and resistance increase rate. 2 is a scanning electron micrograph of a sample surface according to Example 1; 3 is a scanning electron micrograph of a sample surface according to Example 2.

  Embodiments of the present invention will be described below. Below, the positive mix concerning this Embodiment and the nonaqueous electrolyte secondary battery (henceforth a lithium ion secondary battery is demonstrated as an example) using this positive mix are demonstrated.

<Positive electrode (positive electrode mixture)>
FIG. 1 is a diagram illustrating a positive electrode mixture according to an embodiment. As shown in FIG. 1, the positive electrode mixture according to the present embodiment includes a positive electrode active material 1 and an additive 2 that covers at least a part of the surface of the positive electrode active material 1. Furthermore, the positive electrode mixture according to the present exemplary embodiment may include a conductive agent 3.

The positive electrode active material 1 is a material capable of inserting and extracting lithium. For example, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium nickelate (LiNiO ₂ ), nickel that is a mixture thereof Lithium manganate, nickel cobalt lithium manganate, or the like can be used. Lithium nickel manganate has a spinel structure, and examples of the composition include LiNi _0.5 Mn _1.5 O ₄ . As the composition of the lithium nickel cobalt manganese oxide, for example, include _{LiNi 1/3 Co 1/3 Mn 1/3 O 2} .

Additive 2 contains LiMPO ₄ having a cristobalite structure at least in part. Here, M is a typical element, for example, M = B, Mg, Al, Si, Ca, Zn, Ga, Ge, As, Sr, Cd, In, Sn, Sb, Ba, Hg, Tl, Pb, It is at least one element selected from Bi and Po. That is, the additive 2 is a compound composed of lithium and a phosphate of a typical element. For example, LiZnPO ₄ having a cristobalite structure can be used as the additive 2. Further, as the additive 2, a material containing LiZnPO ₄ and LiMgPO ₄ may be used. In this case, the additive 2 comprises a mixed phase of olivine structure derived from the cristobalite structure and LiMgPO ₄ derived from LiZnPO _4.

When preparing the additive 2 (hereinafter, LiZnPO ₄ will be described as an example), for example, Li ₂ O, ZnO, and P ₂ O ₅ that are raw materials are weighed in a proportion represented by the composition formula of LiZnPO _4. Put into a closed crushing container. These raw materials are mechanically mixed and ground using a mechanical milling method.

  The mechanical milling method is a method in which a mechanical energy such as pulverization, impact, friction, or the like is applied to a solid material to activate the material surface to cause the material to react or change its structure. Examples of the mechanical milling method include a method using a ball mill apparatus.

The sealed crushing container containing the raw material is attached to a planetary ball mill apparatus, and mechanical milling is performed at a base plate rotation speed of 300 to 550 rpm, more preferably 450 to 550 rpm for 20 to 50 hours. For example, the material of the closed crushing container is stainless steel, and the material of the ball is zirconia, plastic polyamide, silicon nitride, tungsten carbide, alumina, chrome steel, or the like. Thus, LiZnPO ₄ having a cristobalite structure can be produced by mixing the raw materials Li ₂ O, ZnO and P ₂ O ₅ and performing mechanical milling treatment. The crystal structure of LiZnPO ₄ can be confirmed using an X-ray diffraction method.

  In the mechanical milling process, if the rotation speed of the base plate is less than 300 rpm, it is difficult to form crystals, and if the rotation speed of the base plate is higher than 550 rpm, sintering tends to occur. 300 to 550 rpm is a preferable range.

Then, the positive electrode active material 1 and the additive 2 (LiZnPO ₄ ) are mixed at a predetermined ratio, and further, the conductive agent 3, the solvent, and the binder (binder) are added and kneaded to produce a positive electrode mixture. can do. A positive electrode of a lithium ion secondary battery can be manufactured by applying the positive electrode mixture thus prepared to a positive electrode current collector and drying it.

  Here, as the conductive agent 3, for example, carbon black such as acetylene black (AB) and ketjen black, and graphite (graphite) can be used. As the solvent, for example, an NMP (N-methyl-2-pyrrolidone) solution can be used. As the binder, for example, polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), or the like can be used. As the positive electrode current collector, aluminum or an alloy containing aluminum as a main component can be used.

<Negative electrode>
The negative electrode of the lithium ion secondary battery has a negative electrode active material. The negative electrode active material is a material capable of inserting and extracting lithium, for example, a powdery carbon material made of graphite or the like, an amorphous carbon-coated natural graphite in which natural graphite is coated with amorphous carbon, or the like Can be used. Then, similarly to the positive electrode, the negative electrode can be produced by kneading the negative electrode active material, the solvent, and the binder, applying the kneaded negative electrode mixture to the negative electrode current collector, and drying. Here, as the negative electrode current collector, for example, copper, nickel, or an alloy thereof can be used.

<Non-aqueous electrolyte>
The nonaqueous electrolytic solution is a composition in which a supporting salt is contained in a nonaqueous solvent. Here, as the non-aqueous solvent, one or two selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. More than one type of material can be used. The supporting salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiI 1 type, or 2 or more types of lithium compounds (lithium salt) selected from these etc. can be used.

<Separator>
The lithium ion secondary battery according to the present embodiment may include a separator. As the separator, a porous polymer film such as a porous polyethylene film, a porous polyolefin film, and a porous polyvinyl chloride film, or a lithium ion or ion conductive polymer electrolyte film may be used alone or in combination. it can.

<Lithium ion secondary battery>
Hereinafter, a lithium ion secondary battery provided with a wound electrode body will be described as an example. The wound electrode body is obtained by laminating a long positive electrode sheet (positive electrode) and a long negative electrode sheet (negative electrode) through a long separator, winding the laminate, and obtaining the wound It is formed by crushing the body from the side. Here, the positive electrode sheet has a structure in which a positive electrode mixture layer containing a positive electrode active material is held on both surfaces of a foil-like positive electrode current collector. Similarly to the positive electrode sheet, the negative electrode sheet has a structure in which a negative electrode mixture layer containing a negative electrode active material is held on both surfaces of a foil-like negative electrode current collector.

  A container of a lithium ion secondary battery includes a flat rectangular parallelepiped container body having an open upper end, and a lid that closes the opening. The material constituting the container is preferably a metal material such as aluminum or steel. Or the container which shape | molded resin materials, such as polyphenylene sulfide resin (PPS) and a polyimide resin, may be sufficient. On the upper surface (that is, the lid) of the container, a positive electrode terminal electrically connected to the positive electrode of the wound electrode body and a negative electrode terminal electrically connected to the negative electrode of the wound electrode body are provided.

  And the positive electrode lead terminal and the negative electrode lead terminal are respectively provided in the portion where the positive electrode sheet and the negative electrode sheet at both ends of the wound electrode body are exposed (the portion where the positive electrode mixture layer and the negative electrode mixture layer are not present). And electrically connected to the negative terminal. The wound electrode body produced in this way is accommodated in the container body, and the opening of the container body is sealed using the lid. Thereafter, the non-aqueous electrolyte is injected from the injection hole provided in the lid, and the injection hole is closed with a sealing cap, whereby the lithium ion secondary battery according to this embodiment can be manufactured. .

<Conditioning process>
Conditioning treatment is performed on the lithium ion secondary battery manufactured by the above method. The conditioning process can be performed by repeating the charging and discharging of the lithium ion secondary battery a predetermined number of times. The charge rate, the discharge rate, and the charge / discharge set voltage at the time of performing the conditioning process can be arbitrarily set.

  As described in the background art, in the technology disclosed in Patent Document 1, manganese ions contained in the positive electrode active material are eluted into the electrolyte solution by coating the surface of the positive electrode active material with a metal oxide. And the cycle characteristics of the lithium ion secondary battery were improved.

However, the technique disclosed in Patent Document 1 has a problem that the cycle characteristics of the lithium ion secondary battery are insufficiently improved. That is, in the technique disclosed in Patent Document 1, the surface of the positive electrode active material is coated with a metal oxide. Here, since the metal oxide is an amphoteric oxide that reacts with an acid and a base, the metal oxide contained in the positive electrode elutes into the electrolyte and deposits on the negative electrode, deteriorating the cycle characteristics of the lithium ion secondary battery. There was a problem of letting. For example, when zinc oxide (ZnO) is used as the metal oxide, ZnO becomes Zn ²⁺ ions and elutes in the electrolyte under acidic conditions. Under basic conditions, Zn (OH) ₄ ^2- ion is eluted into the electrolyte. There is a problem that zinc ionized in this way is deposited on the negative electrode by an electrochemical reaction and deteriorates the cycle characteristics of the lithium ion secondary battery. Therefore, in the technique disclosed in Patent Document 1, improvement of the cycle characteristics of the lithium ion secondary battery has been insufficient.

Therefore, in the invention according to the present embodiment, an additive containing LiMPO ₄ (M is a typical element) having a cristobalite structure at least partially is added to the positive electrode mixture. By adding the additive in this manner, at least a part of the surface of the positive electrode active material can be coated with the additive, and the reaction between the positive electrode active material and the electrolytic solution can be suppressed under a high voltage. it can. In other words, the activity of the surface of the positive electrode active material can be reduced by coating at least part of the surface of the positive electrode active material with the additive. For this reason, when the surface of a positive electrode active material and electrolyte solution contact, it can suppress that electrolyte solution decomposes | disassembles. In the invention according to the present embodiment, the above effect can be obtained even when the upper limit of the operating voltage of the lithium ion secondary battery is 4.2 V or higher, more preferably 4.5 V or higher. This is also apparent from the results of charge / discharge tests of lithium ion secondary batteries conducted in the voltage range of 3.0 to 5.0 (V vs. Li ^/ Li ⁺ ), which will be described in the following examples.

  In particular, in the invention according to the present embodiment, a phosphate having a cristobalite structure with a stable crystal structure is used as an additive. Phosphate is a compound that is stable with respect to acids and bases, and therefore, it is possible to prevent the additive from eluting into the electrolyte. Therefore, the cycle characteristics of the lithium ion secondary battery can be improved.

  Furthermore, in the invention according to the present embodiment, since a material containing lithium is used as the additive, lithium contained in the additive functions as a lithium ion conductor. Therefore, it is possible to suppress a decrease in rate characteristics of the lithium ion secondary battery and an increase in battery resistance due to high temperature storage.

  The invention according to the present embodiment described above provides a positive electrode mixture, a method for manufacturing the positive electrode mixture, and a non-aqueous electrolyte secondary battery that can improve the cycle characteristics of the non-aqueous electrolyte secondary battery. can do.

  Next, examples of the present invention will be described.

<Preparation of additive>
First, a method for producing the additive will be described. FIG. 2 is a flowchart for explaining the procedure for producing the additive. In order to produce LiZnPO ₄ as an additive, first, Li ₂ O, ZnO and P ₂ O ₅ as raw materials for the additive were prepared (step S1). Then, in a glove box filled with high-purity argon, Li ₂ O, ZnO, and P ₂ O ₅ as raw materials are weighed to a ratio shown by the composition formula of LiZnPO ₄ and put in a closed crushing container and mixed. (Step S2). Then, an airtight milling vessel was attached to a planetary ball mill device (Fritsch Japan, P-7), and an additive (LiZnPO ₄ ) was produced by performing mechanical milling treatment at a platform rotation speed of 450 rpm for 20 hours. (Step S3). The material of the closed crushing container used at this time was stainless steel, and the material of the ball was zirconia.

Further, an additive (LiZnPO ₄ ) was produced by performing mechanical milling treatment at a mechanical milling condition different from the above, that is, a base plate rotation speed of 550 rpm for 50 hours (Example 2).

The crystal structure of the additive (LiZnPO ₄ ) thus prepared was examined using an X-ray diffractometer (manufactured by Rigaku, Mini Flex). A sample for X-ray diffraction measurement was prepared by uniformly packing the powder after mechanical milling in air in a sample folder of an X-ray diffractometer. The X-ray source was CuKa line, the voltage of the X-ray tube was 30 kV, and the current was 15 mA. The measurement was performed under the conditions of diffraction angle 5 ° ≦ 2θ ≦ 80 °, scan speed 2 ° / min, and sampling width 0.01 °.

FIG. 3 is a diagram showing an X-ray diffraction pattern of the additive (LiZnPO ₄ ). As shown in FIG. 3, X-ray diffraction pattern of LiZnPO ₄ (450rpm, 20h: Example 1), LiZnPO ₄ and X-ray diffraction pattern shown in JCPDS card (cristobalite structure) (PDF # 051-1662) Matched. Similarly, an X-ray diffraction pattern (550 rpm, 50 h: Example 2) of LiZnPO ₄ produced under different mechanical milling conditions is also used in the X-ray diffraction pattern (PDF # 051-) shown on the JCPDS card of LiZnPO ₄ (cristobalite type structure). 1662). Therefore, it was confirmed that these additives (LiZnPO ₄ ) have a cristobalite structure.

<Preparation of positive electrode>
Next, a method for manufacturing the positive electrode will be described. FIG. 4 is a flowchart for explaining the procedure for producing the positive electrode. First, LiNi _0.5 Mn _1.5 O _{4 as} a positive electrode active material and LiZnPO ₄ as an additive were mixed at a ratio of 99: 1 (weight ratio) to prepare a positive electrode mixed powder (step S11). Next, acetylene black (AB) as a conductive agent was mixed with the prepared positive electrode mixed powder. Further, an NMP (N-methyl-2-pyrrolidone) solution in which polyvinylidene fluoride (PVdF) as a binder was dissolved was mixed with the mixed powder (step S12). And this slurry was stirred for 2 hours and mixed, and the slurry was produced (step S13). The mixing ratio of the positive electrode mixed powder (positive electrode active material + additive), conductive agent (AB), and binder (PVdF) at this time was 85: 5: 10 (weight ratio). The slurry thus obtained is a positive electrode mixture.

Thereafter, the obtained positive electrode mixture was applied to an Al foil (15 μm thickness) as a positive electrode current collector by a doctor blade method (step S14). Then, the Al foil coated with the positive electrode mixture was dried in air at 80 ° C. for 2 hours to remove the NMP solution (step S15). Furthermore, it vacuum-dried at 120 degreeC for 10 hours (step S16). After vacuum drying, the positive electrode mixture was molded and pressed to press-bond the positive electrode mixture to the positive electrode current collector (step S17). Thereafter, vacuum drying was further performed at 120 ° C. for 10 hours (step S18). The area of the produced positive electrode was 1.77 cm ² (circular shape with a diameter of 1.5 cm).

<Production of lithium ion secondary battery>
A CR2032-type two-pole coin cell was produced using the positive electrode produced as described above. At this time, metallic lithium was used as the negative electrode. Further, as an electrolytic solution, a mixed solvent obtained by mixing EC (ethylene carbon) and EMC (ethyl methyl carbonate) at a volume ratio of 3: 7, and lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt was added at a concentration of 1 mol / was prepared by dissolving such that dm ^3.

  FIG. 6 is a table showing the positive electrode active material, the additive, the preparation conditions of the additive, and the ratio of the positive electrode active material and the additive for each sample prepared. In this example, a sample to which an additive prepared by performing mechanical milling treatment at 450 rpm for 20 hours was added as Example 1, and produced by performing mechanical milling treatment at 550 rpm for 50 hours for Example 1. A sample to which the additive was added was designated as Example 2. The difference between Example 1 and Example 2 is only the condition of the mechanical milling process, and the other points are the same.

<Comparative Example 1>
As Comparative Example 1, a sample using ZnO as an additive was prepared. When producing the positive electrode of Comparative Example 1, the positive electrode active material LiNi _0.5 Mn _1.5 O ₄ and the additive ZnO were mixed at a ratio of 99: 1 (weight ratio), respectively, and the positive electrode mixed powder Was prepared. The subsequent manufacturing procedure is the same as the positive electrode manufacturing procedure of Examples 1 and 2 (see step S12 and subsequent steps in FIG. 4), and thus a duplicate description is omitted. In the same manner as described above, a CR2032-type bipolar coin cell was produced using the produced positive electrode. At this time, metallic lithium was used as the negative electrode. Further, as an electrolytic solution, a mixed solvent obtained by mixing EC (ethylene carbon) and EMC (ethyl methyl carbonate) at a volume ratio of 3: 7, and lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt was added at a concentration of 1 mol / was prepared by dissolving such that dm ^3.

<Comparative example 2>
As Comparative Example 2, a sample to which no additive was added was produced. Making the positive electrode of Comparative Example 2 was mixed acetylene black (AB) as a conductive agent in _LiNi 0.5 _Mn 1.5 _{O 4} as a positive electrode active material. Furthermore, this mixed powder was mixed with an NMP (N-methyl-2-pyrrolidone) solution in which polyvinylidene fluoride (PVdF) as a binder was dissolved. At this time, the mixing ratio of the positive electrode active material, AB, and the binder was 85: 5: 10 (weight ratio). The subsequent manufacturing procedures are the same as the positive electrode manufacturing procedures of Examples 1 and 2 (see step S13 and subsequent steps in FIG. 4), and thus redundant description is omitted. In the same manner as described above, a CR2032-type bipolar coin cell was produced using the produced positive electrode. At this time, metallic lithium was used as the negative electrode. Further, as an electrolytic solution, a mixed solvent obtained by mixing EC (ethylene carbon) and EMC (ethyl methyl carbonate) at a volume ratio of 3: 7, and lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt was added at a concentration of 1 mol / was prepared by dissolving such that dm ^3.

<Comparative Example 3>
As Comparative Example 3, a sample using LiMgPO ₄ as an additive was prepared. The production method of LiMgPO ₄ as an additive is basically the same as the production procedure shown in FIG. First, Li ₂ O, MgO and P ₂ O ₅ which are raw materials for additives were prepared. Then, in a glove box filled with high-purity argon, Li ₂ O, MgO, and P ₂ O ₅ as raw materials are weighed to a ratio represented by the composition formula of LiMgPO ₄ and put in a closed crushing container and mixed. did. Then, a sealed pulverization container was attached to a planetary ball mill apparatus (P-7, manufactured by Fritsch Japan), and a mechanical milling process was performed at a platform rotation speed of 450 rpm for 20 hours. The material of the closed crushing container used at this time was stainless steel, and the material of the ball was zirconia.

The crystal structure of the additive (LiMgPO ₄ ) thus prepared was examined using an X-ray diffractometer (manufactured by Rigaku, Mini Flex). A sample for X-ray diffraction measurement was prepared by uniformly packing the powder after mechanical milling in air in a sample folder of an X-ray diffractometer. The X-ray source was CuKa line, the voltage of the X-ray tube was 30 kV, and the current was 15 mA. The measurement was performed under the conditions of diffraction angle 5 ° ≦ 2θ ≦ 80 °, scan speed 2 ° / min, and sampling width 0.01 °.

FIG. 5 is a diagram showing an X-ray diffraction pattern of the additive (LiMgPO ₄ ). As shown in FIG. 5, X-ray diffraction pattern of LiMgPO ₄ is consistent with LiMgPO ₄ (olivine) X-ray diffraction pattern shown in JCPDS card (PDF # 00-032-0574). Therefore, it was confirmed that this additive (LiMgPO ₄ ) has an olivine structure.

When preparing the positive electrode of Comparative Example 3, the positive electrode active material LiNi _0.5 Mn _1.5 O ₄ and the additive LiMgPO ₄ were mixed in a ratio of 99: 1 (weight ratio), respectively, and mixed with the positive electrode. A powder was prepared. The subsequent manufacturing procedure is the same as the positive electrode manufacturing procedure of Examples 1 and 2 (see step S12 and subsequent steps in FIG. 4), and thus a duplicate description is omitted. In the same manner as described above, a CR2032-type bipolar coin cell was produced using the produced positive electrode. At this time, metallic lithium was used as the negative electrode. Further, as an electrolytic solution, a mixed solvent obtained by mixing EC (ethylene carbon) and EMC (ethyl methyl carbonate) at a volume ratio of 3: 7, and lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt was added at a concentration of 1 mol / was prepared by dissolving such that dm ^3.

  FIG. 6 shows positive electrode active materials, additives, preparation conditions for the samples, and ratios of the positive electrode active material and the additives according to Examples 1 and 2 and Comparative Examples 1 to 3.

  For each sample produced as described above, a charge / discharge test, impedance measurement, and high-temperature storage test were performed. The test methods and test results are shown below.

<Charge / discharge test>
When the test electrode was a positive electrode and metal lithium was the negative electrode, the process of desorbing lithium ions from the positive electrode was “charge”, and the process of inserting lithium ions into the positive electrode was “discharge”. A charge / discharge test device (Hokuto Denko Co., Ltd .: HJ-1001 SM8A) was used as the measurement device. The measurement conditions were such that the current density during charge / discharge in the first cycle was 0.2 mA / cm ² , the current density during charge / discharge after the second cycle was 0.5 mA / cm ² , and the voltage range was 3.0- 5.0 (V vs. Li ^/ Li ⁺ ), and the measurement temperature was 25 ° C.

<Impedance measurement>
In order to measure the resistance (AC impedance) of the test electrode (positive electrode), EIS (Electrochemical Impedance Spectroscopy) measurement was performed. An electrochemical measuring device (manufactured by IVIUM, Netherlands: potentiostat / galvanostat) was used as the measuring device. The measurement conditions were an AC amplitude of 10 mV and a frequency range of 10 mHz to 100 kHz.

<High temperature storage test>
A high temperature storage test was performed on each of the prepared samples. The flow of the high temperature storage test is shown in the flowchart of FIG. First, charge and discharge in the first cycle were performed on each manufactured sample at a current density of 0.2 mA / cm ² (room temperature) (step S21). After charge and discharge in the first cycle, each sample was charged at a current density of 0.5 mA / cm ² (room temperature) (charge in the second cycle: step S22). Thereafter, the impedance of each sample at room temperature was measured (step S23). The impedance measured at this time is the impedance before high-temperature storage.

Then, each sample was preserve | saved for 3 days in a 60 degreeC environment, and the high temperature preservation | save test was implemented (step S24). After high temperature storage, each sample was left in a room temperature environment, and the impedance of each sample at room temperature was measured (step S25). The impedance measured at this time is the impedance after high-temperature storage. Then, after discharging each sample at a current density of 0.5 mA / cm ² (discharge in the second cycle), a charge / discharge test in the third and subsequent cycles was performed (step S26).

<Test result 1 (charge / discharge test result at room temperature)>
FIG. 8A shows charge / discharge curves (first cycle) of samples according to Example 1 and Comparative Examples 1 and 2. FIG. FIG. 8B shows charge / discharge curves (180th cycle) of the samples according to Example 1 and Comparative Examples 1 and 2. FIG. Here, Example 1 is a sample to which LiZnPO ₄ (mechanical milling treatment conditions: 20 hours at 450 rpm on the base plate) is added as an additive for the positive electrode, Comparative Example 1 is a sample to which ZnO is added as an additive for the positive electrode, comparison Example 2 is a sample in which no additive was added to the positive electrode.

As shown in FIG. 8A, in the charge / discharge test in the first cycle, the charge / discharge capacity and the charge / discharge voltage of each sample were approximately the same. On the other hand, as shown in FIG. 8B, in the charge / discharge test at the 180th cycle, the capacity of each sample decreased. At this time, in the sample of Example 1, a decrease in capacity was suppressed as compared to the samples of Comparative Examples 1 and 2. That is, in Example 1 in which LiZnPO ₄ was added as an additive for the positive electrode, the electrode characteristics at room temperature were improved.

<Test result 2 (result of high-temperature storage test)>
FIG. 9A shows charge / discharge curves (after storage at 60 ° C. for 3 days) of samples according to Example 2 and Comparative Examples 2 and 3. FIG. The charge / discharge curve shown in FIG. 9A is the charge / discharge curve of the third cycle (see step S26 in FIG. 7). Here, Example 2 is a sample to which LiZnPO ₄ (mechanical milling treatment condition: 50 hours at a base plate rotation speed of 550 rpm) is added as an additive for the positive electrode, and Comparative Example 2 is a sample to which no additive is added to the positive electrode. Example 3 is a sample to which LiMgPO ₄ was added as an additive for the positive electrode.

As shown in FIG. 9A, in Example 2 in which LiZnPO ₄ was added as an additive for the positive electrode, a decrease in capacity could be suppressed more than in Comparative Examples 2 and 3. Further, comparing Comparative Examples 2 and 3, in Comparative Example 3 in which LiMgPO ₄ was added as an additive for the positive electrode, it was possible to suppress a decrease in capacity compared to Comparative Example 2 in which no additive was added to the positive electrode. . Therefore, it was possible to suppress a decrease in capacity by adding an additive to the positive electrode. In particular, when LiZnPO ₄ was used as an additive for the positive electrode, the decrease in capacity could be most suppressed.

FIG. 9B shows the relationship between the number of cycles and the discharge capacity after the samples according to Example 2 and Comparative Examples 2 and 3 were stored at 60 ° C. for 3 days. The test result shown in FIG. 9B is the result of the charge / discharge cycle test in step S26 of FIG. As shown in FIG. 9B, Example 2 in which LiZnPO ₄ was added as an additive for the positive electrode showed excellent cycle stability.

<Test result 3 (impedance measurement result)>
FIGS. 10A to 10C show the impedance measurement results of the samples according to Example 1, Example 2, and Comparative Example 2, respectively, before high-temperature storage. Moreover, the impedance measurement result after the high temperature preservation | save of the sample concerning Example 1, Example 2, and the comparative example 2 is shown to FIG. 11A-FIG. 11C, respectively. Here, Example 1 is a sample to which LiZnPO ₄ (mechanical milling treatment conditions: 20 hours at 450 rpm) is added as a positive electrode additive, and Example 2 is LiZnPO ₄ (mechanical milling treatment conditions) as a positive electrode additive. : Sample added with base plate rotation speed of 550 rpm for 50 hours), Comparative Example 2 is a sample with no additive added to the positive electrode.

As shown in FIGS. 10A to 10C, it was found that even when an additive (LiZnPO ₄ ) was added to the positive electrode, the impedance did not increase. Further, as shown in FIG 11A~ Figure 11C, by adding positive electrode additive (LiZnPO _4), it was possible to suppress an increase in impedance after high temperature storage. That is, in Comparative Example 2, the impedance significantly increased after high-temperature storage (see FIGS. 10C and 11C), but in Examples 1 and 2, an increase in impedance after high-temperature storage could be suppressed (FIGS. 10A and B). And FIGS. 11A and 11B). This is because by using a material containing lithium as an additive, the lithium contained in the additive functions as a lithium ion conductor, thereby suppressing an increase in impedance after high-temperature storage. Conceivable.

<Test result 4>
FIG. 12 shows the additive, charge / discharge efficiency, and cycle stability of the samples according to Example 1 and Comparative Examples 1 and 2.
The charge / discharge efficiency (average of 100 cycles) is the charge / discharge efficiency at the nth cycle = (discharge amount at the nth cycle / charge amount at the nth cycle) × 100, and 100 cycles from the first cycle using this equation. It calculated | required by calculating | requiring the average charge / discharge efficiency of each cycle which calculated | required each charge / discharge efficiency to eyes.

  The cycle stability (after 180 cycles) is the ratio (%) of the capacity of each sample in the 180th cycle (see FIG. 8B) to the capacity of the first cycle in each sample (see FIG. 8A). That is, in the case of Example 1, the capacity at the first cycle is 137 mAh / g and the capacity at the 180th cycle is 56 mAh / g, so that the cycle stability (after 180 cycles) is 41% (= 56/137). × 100).

As shown in FIG. 12, in Example 1 in which LiZnPO ₄ was added as an additive for the positive electrode, charge / discharge efficiency and cycle stability (after 180 cycles) were better than those in Comparative Examples 1 and 2. Therefore, when LiZnPO ₄ was added as a positive electrode additive, the characteristics of the electrode were improved.

<Test result 5>
FIG. 13 shows the additive, charge / discharge efficiency, capacity after high-temperature storage, cycle stability, and resistance increase rate of the samples according to Example 1 and Comparative Examples 2 and 3. Here, the calculation method of charging / discharging efficiency is the same as that of FIG. The capacity after high temperature storage corresponds to the capacity shown in FIG. 9A. The cycle stability (after 100 cycles) is the cycle stability after high temperature storage, and is the ratio of the capacity at the 100th cycle to the capacity at the first cycle after high temperature storage. That is, the higher the cycle stability, the more the battery capacity is maintained.

  The resistance increase rate is a value obtained by dividing the resistance value of each sample after high-temperature storage by the initial resistance value. The initial resistance value of Example 1 was 18Ω (see FIG. 10A), the resistance value after high-temperature storage was 75Ω (see FIG. 11A), and the resistance increase rate was 4.1 times. The initial resistance value of Comparative Example 2 was 23Ω (see FIG. 10C), the resistance value after high-temperature storage was 200Ω (see FIG. 11C), and the resistance increase rate was 8.7 times. The initial resistance value of Comparative Example 3 was 16Ω, the resistance value after high-temperature storage was 89Ω, and the resistance increase rate was 5.5 times.

As shown in FIG. 13, in Example 1 in which LiZnPO ₄ was added as an additive for the positive electrode, charge / discharge efficiency and cycle stability (after 100 cycles) were better than those in Comparative Examples 2 and 3. In Example 1, the capacity after storage at high temperature was the largest, and the resistance increase rate was the lowest. Therefore, when LiZnPO ₄ was added as a positive electrode additive, the characteristics of the electrode were improved.

Moreover, in Comparative Example 3 in which LiMgPO ₄ was added as an additive for the positive electrode, charge / discharge efficiency and cycle stability (after 100 cycles) were better than in Comparative Example 2 in which no additive was added to the positive electrode. . Further, Comparative Example 3 had a larger capacity after high-temperature storage than Comparative Example 2, and further had a low resistance increase rate. Therefore, the characteristics of the electrode were improved when LiMgPO ₄ was added to the positive electrode, compared to when no additive was added to the positive electrode. However, comparing the test results of Example 1 and Comparative Example 3, the characteristics of the electrode were most improved when LiZnPO ₄ was added as the positive electrode additive.

14 and 15 are scanning electron micrographs of the sample surfaces according to Example 1 and Example 2, respectively. As shown in FIGS. 14 and 15, a part of the surface of LiNi _0.5 Mn _1.5 O ₄ that is a positive electrode active material is covered with LiZnPO ₄ that is an additive (a portion surrounded by a circle). I understand that. The average particle diameter of 20 positive electrode active materials was about 780 nm (minimum particle diameter 400 nm, maximum particle diameter 1.2 μm).

  The present invention has been described with reference to the above-described embodiment and examples. However, the present invention is not limited only to the configurations of the above-described embodiment and examples. It goes without saying that various modifications, corrections, and combinations that can be made by those skilled in the art within the scope of the invention are included.

1 Positive electrode active material 2 Additive 3 Conductive agent

Claims (10)

A positive electrode active material;
An additive covering at least a part of the surface of the positive electrode active material,
The additive includes LiZnPO ₄ having a cristobalite structure at least in part.
Positive electrode mixture. The positive electrode mixture according to claim 1 , wherein the additive further contains LiMgPO ₄ . The additive comprises a mixed phase of olivine structure derived from the cristobalite structure and LiMgPO ₄ derived from LiZnPO _4, cathode mix according to claim 2. The positive electrode mixture according to any one of claims 1 to 3 , wherein the positive electrode active material contains lithium nickel manganate. The positive electrode mixture according to any one of claims 1 to 4 , wherein the positive electrode mixture further contains a conductive agent. A positive electrode having the positive electrode mixture according to any one of claims 1 to 5 ,
A negative electrode having a negative electrode mixture containing a negative electrode active material;
A non-aqueous electrolyte containing a lithium salt and a non-aqueous solvent;
A non-aqueous electrolyte secondary battery. Mixing a raw material containing Li ₂ O, P ₂ O ₅ and ZnO to prepare a mixed powder;
Performing a mechanical milling process on the mixed powder to form an additive containing LiZnPO ₄ having a cristobalite structure;
A step of mixing the additive and the positive electrode active material to form a positive electrode mixture,
A method for producing a positive electrode mixture. The method for producing a positive electrode mixture according to claim 7 , wherein when forming the additive, mechanical milling is performed at a base plate rotation speed of 300 to 550 rpm for 20 to 50 hours. The method for producing a positive electrode mixture according to claim 8 , wherein when forming the additive, mechanical milling is performed at a base plate rotation speed of 450 to 550 rpm. When preparing the mixed powder, MgO is further added,
Performing mechanical milling on the mixed powder to form an additive containing LiZnPO ₄ and LiMgPO ₄ ;
The manufacturing method of the positive mix as described in any one of Claims 7 thru | or 9 .