JP6331822B2 - Positive electrode active material, positive electrode and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material, a positive electrode, and a lithium ion secondary battery.

Conventionally, layered compounds such as LiCoO ₂ and LiNi _1/3 Mn _1/3 Co _1/3 O ₂ and spinel compounds such as LiMn ₂ O ₄ have been used as positive electrode active materials for lithium ion secondary batteries. In recent years, compounds having an olivine type structure typified by LiFePO ₄ have attracted attention. It is known that a positive electrode material having an olivine structure has high thermal stability at high temperatures and high safety. However, a lithium ion secondary battery using LiFePO ₄ has a disadvantage that its average discharge voltage is as low as about 3.5 V and the energy density is low. Therefore, LiCoPO ₄ , LiNiPO _{4, and the} like have been proposed as phosphoric acid-based positive electrode materials that can realize a high charge / discharge voltage. However, the present situation is that a sufficient capacity is not obtained even in lithium ion secondary batteries using these positive electrode materials.

Further, as a positive electrode active material similar to LiMPO ₄ type olivine phosphate, an active material such as Li ₂ CoSiO ₄ having _two lithium atoms in a unit cell has been proposed (for example, see Patent Document 1).

Japanese Patent Application No. 2006-167960

  However, although it is said that the battery proposed in the above publication can freely design the capacity between the average discharge voltages of 3.5 V and 2.5 V, the discharge capacity is not yet satisfactory and the discharge capacity is even larger. Development of a positive electrode active material exhibiting the above has been desired. Therefore, an object of the present invention is to provide a lithium ion secondary battery having a high average discharge voltage and a sufficient capacity.

  The present invention has been made in view of the above-described problems of the prior art, and aims to provide a positive electrode active material, a positive electrode, and a lithium ion secondary battery that have a high average discharge voltage and a sufficient capacity. To do.

Positive electrode active material in accordance with the present invention in order to achieve the above object, the general formula _{Li 2-x V 1-y} M y O (Si 1-z Ge z) O 4 ( wherein, M is Mo, Nb, It is at least one element selected from the group consisting of Ta and Ti, and x, y, and z are ranges of 0 ≦ x ≦ 2, 0.05 ≦ y ≦ 0.5, and 0 ≦ z ≦ 0.3, respectively. A lithium-vanadium-silicate compound represented by

  According to the present invention, a positive electrode active material, a positive electrode, and a lithium ion secondary battery that have a high average discharge voltage and a sufficient capacity can be realized. Although the reason for this is not necessarily clear, it can be considered as follows.

  Since the positive electrode active material according to the present invention can contain 2 mol of Li with respect to 1 mol of V and the transition metal (M), the theoretical value of the capacity is increased, and further, tetravalent and tetracoordinated. At least selected from the group consisting of Mo (0.65 Å), Nb (0.68 Å), Ta (0.68 Å) and Ti (0.61 Å) having an ionic radius greater than the ionic radius of V (0.58 Å) It is considered that by substituting V partially with one kind of element, an interstitial distance suitable for insertion and extraction of Li is obtained, an average discharge voltage is high, and a sufficient capacity can be obtained. Similarly, by partially replacing Si with Ge (0.39Å) having an ion radius larger than that of tetravalent and tetracoordinated Si (0.26Å), it is suitable for insertion and extraction of Li. It is considered that the interstitial distance is high, the average discharge voltage is high, and a sufficient capacity can be obtained.

  The range of x is 0 ≦ x ≦ 2. Usually, a compound with x = 2 is synthesized to have an initial composition. Further, in the case of x = 2, immediately after the secondary battery is assembled, it can be started from discharge, so that there is an advantage that charging is unnecessary. Further, the valence of the transition metal can be adjusted by adjusting the value of x.

  The range of y is 0.05 ≦ y ≦ 0.5. From the viewpoint of discharge capacity, 0.1 ≦ y ≦ 0.4 is more preferable. A preferable range of z is 0 ≦ z ≦ 0.3.

  Further, the peak intensity ratio A between the diffraction peak intensity A of (200) plane near 2θ = 28 ° and the diffraction peak intensity B of (001) plane near 2θ = 20 ° by X-ray diffraction using CuKα ray. When / B is 0.87 or more and 1.32 or less, even if Li is inserted or desorbed during charge / discharge, the crystal structure is stable and crystallinity is high to some extent, and damage due to the active material crushing process or the like tends to be suppressed Therefore, it is considered that the average discharge voltage is high and sufficient capacity can be obtained.

  In addition, the positive electrode according to the present invention includes a current collector and the positive electrode active material, and includes a positive electrode active material layer provided on the current collector, thereby providing a high average discharge voltage and sufficient capacity. Can be obtained.

  Moreover, the lithium ion secondary battery which concerns on this invention can obtain a sufficient capacity | capacitance with a high average discharge voltage by providing the said positive electrode.

  According to the present invention, it is possible to provide a positive electrode active material, a positive electrode, and a lithium ion secondary battery that have a high average discharge voltage and a sufficient capacity.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment. 2 is an X-ray diffraction pattern of a mixed powder of a positive electrode active material and a conductive additive after pulverization in Example 1. FIG.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

<Lithium ion secondary battery>
The lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.

  The laminated body 30 is configured such that the positive electrode 10 and the negative electrode 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

<Positive electrode active material>
Then, the positive electrode active material which concerns on this embodiment is demonstrated.

Positive electrode active material according to the present _embodiment, the lithium is represented by _{Li 2-x V 1-y} M y O (Si 1-z Ge z) O 4 - characterized in that it is a silicate compound - vanadium. M is at least one element selected from the group consisting of Mo, Nb, Ta, and Ti, and has an ionic radius larger than that of tetravalent and tetracoordinate V. Similarly, Si may be partially substituted by Ge having an ion radius larger than that of tetravalent and tetracoordinate Si. Further, x, y, and z are preferably in the ranges of 0 ≦ x ≦ 2, 0.05 ≦ y ≦ 0.5, and 0 ≦ z ≦ 0.3, respectively. By using the lithium-vanadium-silicate compound as described above, it is considered that the interstitial distance is suitable for insertion and extraction of Li, the average discharge voltage is high, and a sufficient capacity can be obtained.

  Further, the peak intensity ratio A / B between the diffraction peak intensity A of the (200) plane near 2θ = 28 ° and the diffraction peak intensity B of the (001) plane near 2θ = 20 ° according to the X-ray diffraction method is 0.87. More preferably, it is 1.32 or less. When the peak intensity ratio A / B is 0.87 or more and 1.32 or less, the crystal structure is stable to a certain degree even if Li is inserted and desorbed during charge and discharge, and the crystallinity is high to a certain extent. Since damage tends to be suppressed, it is considered that the average discharge voltage is high and sufficient capacity can be obtained.

<Method for producing positive electrode active material>
The method for producing the positive electrode active material is not particularly limited, but can be synthesized by a solid phase reaction synthesis method, a hydrothermal synthesis method, a carbothermal reduction method, or the like. Below, the manufacturing method of the positive electrode active material using the solid-phase reaction synthesis method which concerns on this embodiment is demonstrated. The solid phase reaction synthesis method includes a raw material preparation step and a firing step. You may implement a grinding | pulverization process and a classification process as needed after a baking process.

  In the raw material preparation step, raw materials such as a lithium source, a transition metal source, a silicon source, and a germanium source are weighed and mixed to prepare. Mixing may be performed using, for example, a mortar, ball mill, bead mill, vibrating ball mill, planetary ball mill, or the like. Furthermore, in order to adjust the valence of the transition metal source, a reducing agent such as carbon may be added.

  The heat treatment method in the firing step is arbitrary, and for example, a box furnace, a tubular furnace, a tunnel furnace, a rotary kiln, or the like can be used. The heat treatment is usually divided into three parts of temperature rise, maximum temperature hold, and temperature drop, and the steps of temperature rise, maximum temperature hold, and temperature drop may be repeated twice or more. Further, a crushing step that means eliminating aggregation to such an extent that the secondary particles are not destroyed may be sandwiched between the heat treatments.

  Although the atmosphere of a baking process is not specifically limited, In argon atmosphere, oxygen atmosphere, nitrogen atmosphere, or those mixed atmosphere is preferable.

  In the pulverization step, for example, a planetary ball mill or a jet mill can be used as a pulverization method. By the pulverization, the primary particles or the secondary particles are micronized. Moreover, you may grind | pulverize the mixture of a positive electrode active material and a conductive support agent in a grinding | pulverization process. In the pulverization step, the positive electrode active material layer 14 of the lithium ion secondary battery is produced, that is, the slurry prepared from the active material, the conductive auxiliary agent, the binder, the solvent, and the like is subjected to a pulverization treatment to obtain a positive electrode current collector. The positive electrode active material layer 14 may be formed by applying on the body 12 and drying.

  In the classification step, coarse particles and fine particles can be removed by classifying the product obtained in the firing step or the product obtained in the pulverization step. The classification has advantages such as that drying is not necessary and scale-up is easy, and the dry method is preferable to the wet method. Examples of the dry classifier include a vibration sieve machine, an ultrasonic vibration sieve machine, a cyclone, a micron, a separator, and a high-precision airflow classifier. Moreover, you may repeat a classification process in multiple times as needed.

The calculation method of the diffraction peak intensity of the positive electrode active material can be obtained by an X-ray diffraction method. The crystal structure is identified from the X-ray diffraction pattern, and the peak is determined from the diffraction peak of (200) plane near 2θ = 28 ° and the diffraction peak of (100) plane near 2θ = 20 ° of the lithium-vanadium-silicate compound. The strength can be determined.
<Positive electrode>
Subsequently, the positive electrode 10 according to the present embodiment will be described.

  As the positive electrode current collector 12 of the positive electrode 10, for example, an aluminum foil or the like can be used. The positive electrode active material layer 14 contains at least the positive electrode active material according to the present embodiment and a conductive additive. The positive electrode active material layer 14 may include a binder that binds the positive electrode active material and the conductive additive.

  Examples of the conductive aid include carbon materials such as carbon blacks, metal powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal powders, and conductive oxides such as ITO.

  The binder is not particularly limited as long as the positive electrode active material and the conductive additive can be bound to the positive electrode current collector 12, and a known binder can be used. Examples thereof include fluororesins such as polyvinylidene fluoride (hereinafter referred to as PVDF), polytetrafluoroethylene (PTFE), and a vinylidene fluoride-hexafluoropropylene copolymer.

  The ratio of the positive electrode active material, the conductive additive, and the binder of the positive electrode active material layer 14 is not particularly limited, but the electrode density tends to decrease when the ratio of the positive electrode active material is small, and the ratio of the positive electrode active material is 80% by weight or more. Is preferred.

  Such a positive electrode 10 is prepared by a known method, for example, a positive electrode active material, a conductive additive and a binder, and a solvent corresponding to the type thereof, for example, N-methyl-2-pyrrolidone (hereinafter referred to as N-methylpyrrolidone in the case of PVDF). ), A slurry added to a solvent such as N, N-dimethylformamide is applied to the surface of the positive electrode current collector 12 and dried.

<Negative electrode>
As the negative electrode current collector 22, a copper foil or the like can be used. Moreover, as the negative electrode active material layer 24, the thing containing a negative electrode active material, a conductive support agent, and a binder can be used. It does not specifically limit as a conductive support agent, A carbon material, a metal powder, etc. can be used. As the binder used for the negative electrode, fluororesins such as PVDF, polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP) and the like can be used.

As the negative electrode active material, carbon materials such as graphite and non-graphitizable carbon, metals that can be combined with lithium such as Al, Si and Sn, and amorphous materials mainly composed of oxides such as SiO ₂ and SnO ₂ Examples thereof include particles containing a compound, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

  The negative electrode 20 may be manufactured by adjusting the slurry and applying it to the negative electrode current collector 22 in the same manner as the positive electrode 10.

<Electrolyte>
The electrolytic solution is not particularly limited. For example, in the present embodiment, an electrolytic solution containing a lithium salt in an organic solvent can be used. Examples of the lithium _salt, LiPF _6, LiClO 4, salts of LiBF ₄ or the like can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Preferable examples of the organic solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. These may be used alone or in combination of two or more at any ratio.

  The separator 18 is at least one component selected from the group consisting of a monolayer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or a group consisting of cellulose, polyester and polypropylene. The fiber nonwoven fabric which consists of can be used.

  The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, a metal laminate film can be used. .

  The leads 60 and 62 are made of a conductive material such as aluminum.

  This positive electrode active material can also be used as an electrode material for electrochemical elements other than lithium ion secondary batteries. As such an electrochemical element, a lithium ion secondary battery other than a lithium ion secondary battery such as a metal lithium secondary battery (an electrode including the positive electrode active material particles of the present invention as a positive electrode and metal lithium as a negative electrode) is used. Examples thereof include secondary batteries and electrochemical capacitors such as lithium capacitors.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
[Preparation of positive electrode active material]
The VO ₂ 3.69 g, and TiO ₂ 0.89 _g, and Li 2 SiO ₃ and 5.00g weighed and mixed using a mortar. The mixed powder was put in an alumina crucible and heat-treated in an argon atmosphere at 800 ° C. for 12 hours using an electric furnace. When it was taken out after cooling, a purple powder was obtained.

The obtained positive electrode active material was subjected to composition analysis by an inductively coupled plasma method (hereinafter, ICP method), and as a result, it was confirmed that the composition was Li ₂ V _0.8 Ti _0.2 SiO ₅ .

  The obtained positive electrode active material, acetylene black as a conductive auxiliary agent, and ketjen black were weighed at a weight ratio of 80: 5: 5, put in a container, and pulverized by a planetary ball mill at 500 rpm. For 10 minutes.

For the mixed powder of the positive electrode active material and the conductive additive after the pulverization treatment, the crystal structure was identified and the diffraction peak intensity was determined by X-ray diffraction. Using “UltimaIV” manufactured by Rigaku Corporation as an X-ray diffractometer, the measurement was performed under the following measurement conditions.
Measurement conditions Filter: Ni
Target: Cu Kα 1.54060Å
X-ray output setting: 40kV-40mA
Slit: Divergence 1/2 °, scattering 1/2 °, light receiving 0.15mm
Scanning speed: 2 ° / min
Sampling width: 0.02 °
The measurement results are shown in FIG. It was confirmed that the crystal structure almost coincided with the diffraction pattern of tetragonal Li ₂ VOSiO ₄ , and (200) plane diffraction peak intensity A near 2θ = 28 ° and (001) plane near 2θ = 20 °. The peak intensity ratio A / B with the diffraction peak intensity B of 1.16 was 1.16.

[Production of evaluation cell]
A slurry obtained by dispersing a mixed powder of the positive electrode active material and conductive additive after the pulverization treatment of Example 1 and PVDF as a binder in a weight ratio of 90:10 in N methylpyrrolidone as a solvent. Was prepared. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to produce a positive electrode on which a positive electrode active material layer was formed.

  Next, artificial graphite (FSR manufactured by BTR) and a 5 wt% N-methylpyrrolidone solution of PVdF were mixed as a negative electrode in a ratio of artificial graphite: PVDF = 93: 7 to prepare a slurry paint. The negative electrode was produced by apply | coating a coating material to the copper foil which is a collector, and drying and rolling.

A positive electrode and a negative electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (element body). This laminate was placed in an aluminum laminate pack. As the electrolyte, ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved as a supporting salt at 1 mol / L.

  The above electrolyte was poured into an aluminum laminate pack containing the laminate, and then vacuum-sealed to produce an evaluation cell of Example 1.

[Measurement of battery characteristics]
The evaluation cell of Example 1 was charged at a constant current of up to 4.4 V at 25 ° C. and a current value of 18 mA / g, and then discharged to 2.5 V at a current value of 18 mA / g. At this time, the discharge capacity of Example 1 was 168 mAh / g, and the average discharge voltage was 3.47V.

(Examples 2-6, Comparative Examples 1 and 2)
In Examples 2 to 6 and Comparative Examples 1 and 2, the synthesis of the positive electrode active material and the positive electrode active material were carried out in the same manner as in Example 1 except that the raw materials were weighed so as to have the active material compositions shown in Table 1. A mixed powder of a conductive additive and a conductive auxiliary was prepared, and the peak intensity ratio A / B was determined from the identification of the crystal structure and the diffraction peak intensity by the X-ray diffraction method. Further, evaluation cells were prepared and battery characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Examples 7 to 9)
In Examples 6 to 8, the synthesis of the positive electrode active material and the positive electrode active material and the conductive auxiliary agent were carried out in the same manner as in Example 1 except that the rotation speed of the grinding treatment by the planetary ball mill was 480 rpm, 530 rpm and 550 rpm, respectively. The peak intensity ratio A / B was determined from the identification of the crystal structure and the diffraction peak intensity by the X-ray diffraction method. Further, evaluation cells were prepared and battery characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Examples 10 and 11, Comparative Example 3)
Examples 10 and 11 and Comparative Example 3 were the same as Example 1 except that VO ₂ , TiO ₂ , Li ₂ SiO ₃ and Li ₂ GeO ₃ were weighed so as to have the active material composition described in Table 1. The positive electrode active material was synthesized by the method described above and a mixed powder of the positive electrode active material and the conductive additive was prepared, and the peak intensity ratio A / B was obtained from the identification of the crystal structure and the diffraction peak intensity by the X-ray diffraction method. Further, evaluation cells were prepared and battery characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 1.

  As is apparent from Table 1, it was confirmed that the discharge capacity of the evaluation cells of Examples 1 to 11 was 160 mAh / g or more and the average discharge voltage was 3.41 V or more. Moreover, it was confirmed that the peak intensity ratio A / B of Examples 1-4, 6, 8, 10, and 11 was 0.87 or more and 1.32 or less, and the discharge capacity was 164 mAh / g or more.

(Examples 12 to 18, Comparative Examples 4 and 5)
In Examples 12 to 18 and Comparative Examples 4 and 5, except that VO ₂ , TiO ₂ , MoO ₂ , Li ₂ SiO ₃ and Li ₂ GeO ₃ were weighed so as to have the active material composition shown in Table 2, Synthesis of the positive electrode active material and a mixed powder of the positive electrode active material and the conductive additive were prepared in the same manner as in Example 1, and the peak intensity ratio A / B was determined from the identification of the crystal structure and the diffraction peak intensity by the X-ray diffraction method. Asked. Further, evaluation cells were prepared and battery characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 2.

  As is apparent from Table 2, it was confirmed that the discharge capacity of the evaluation cells of Examples 12 to 18 was 160 mAh / g or more and the average discharge voltage was 3.42 V or more. Moreover, the peak intensity ratio A / B of Examples 12 to 17 was 1.01 to 1.24, and the discharge capacity was confirmed to be 164 mAh / g or more.

(Examples 19 to 31, Comparative Examples 6 to 8)
In Examples 19 to 31 and Comparative Examples 6 to 8, VO ₂ , TiO ₂ , MoO ₂ , Nb ₂ O ₅ , Ta ₂ O ₅ , Li ₂ SiO ₃ and the active material compositions shown in Table 3 were used. Synthesis of positive electrode active material and positive electrode active material in the same manner as in Example 1 except that Li ₂ GeO ₃ was weighed and 50% mol of acetylene black was weighed with respect to Nb ₂ O ₅ and Ta ₂ O ₅ . A mixed powder of a conductive additive and a conductive auxiliary was prepared, and the peak intensity ratio A / B was determined from the identification of the crystal structure and the diffraction peak intensity by the X-ray diffraction method. Further, evaluation cells were prepared and battery characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 3.

  As is apparent from Table 3, it was confirmed that the discharge capacity of the evaluation cells of Examples 19 to 31 was 160 mAh / g or more and the average discharge voltage was 3.41 V or more. Moreover, the peak intensity ratio A / B of Examples 19 to 29 was 0.97 or more and 1.19 or less, and the discharge capacity was confirmed to be 164 mAh / g or more.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 60 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (4)

A positive electrode active material, which is a lithium-vanadium-silicate compound represented by the general formula (1).
_{Li 2-x V 1-y} M y O (Si 1-z Ge z) O 4 (1)
(In the formula, M is at least one element selected from the group consisting of Mo, Nb, Ta and Ti, and x, y and z are 0 ≦ x ≦ 2, 0.05 ≦ y ≦ 0. 5 and 0 ≦ z ≦ 0.3.)   The peak intensity ratio A / B between the diffraction peak intensity A on the (200) plane and the diffraction peak intensity B on the (001) plane according to the X-ray diffraction method of the lithium-vanadium-silicate compound is 0.87 or more and 1.32 or less. The positive electrode active material according to claim 1, wherein:   A positive electrode comprising a current collector and a positive electrode active material layer comprising the positive electrode active material according to claim 1 or 2 and provided on the current collector.   A lithium ion secondary battery comprising the positive electrode according to claim 3.

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