JP2014197462A - Electrode active material for lithium secondary batteries, lithium secondary battery, and method for manufacturing electrode active material for lithium secondary batteries - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a practical electrode active material for lithium secondary batteries arranged by use of LFPO-based active material.SOLUTION: A electrode active material for lithium secondary batteries comprises: a powdery compound expressed by the chemical formula, LiFeMPO, where M is Ni or Co, or both; and a coating film of carbon formed thereon. More preferably, 1.0-10.0 wt% of the carbon is included to the compound. The electrode active material for lithium secondary batteries may be arranged so that the compound has an average particle diameter of 2.0 μm or smaller.

Description

  The present invention relates to a technique for improving characteristics of an electrode active material for a lithium secondary battery.

Examples of electrode active materials for lithium secondary batteries that have been put into practical use include LiCoO ₂ and LiMn ₂ O ₄ , and these electrode active materials are used as positive electrode materials. LiCoO ₂ has a high energy density and a real capacity of 140 mAh / g. However, expensive Co is included as an essential composition, and it is difficult to reduce the price of the lithium secondary battery. Co is toxic, and it is desirable to replace it with a safer active material from the viewpoint of environmental problems. On the other hand, LiMn ₂ O ₄ has a substantial capacity of 110 mAh / g, which is lower than LiCoO ₂ . There is also a problem that Mn elutes at 60 ° C. or higher and capacity characteristics deteriorate. Therefore, it is necessary to realize an electrode active material for a lithium secondary battery that has a higher capacity, higher safety, and more inexpensive.

By the way, in recent years, a material represented by a chemical formula of Li ₂ Fe _(1-x) M _x P ₂ O ₇ has attracted attention as an electrode active material for a lithium secondary battery (M is one of Ni and Co, Or both). This material (hereinafter sometimes referred to as “LFPO-based active material”) has a phosphoric acid skeleton with a stable crystal structure and little deterioration in properties at high temperatures. In addition, the main transition metal in the composition is Fe and does not require a large amount of expensive and toxic Co like LiCoO ₂ . If Co is not included in the above chemical formula, it is not necessary to consider toxicity. Lithium iron pyrophosphate (hereinafter referred to as LFPO) in which x = 0 in the above chemical formula can be composed of inexpensive and safe iron (Fe) and phosphorus (P) other than lithium.

  In the LFPO-based active material containing LFPO, two lithium ions per chemical formula can contribute to charging / discharging, and a high capacity of 220 mAh / g is theoretically obtained. Currently, research is being conducted in various directions to put this LFPO-based active material into practical use. The outline of the LFPO-based active material is described in Non-Patent Documents 1 and 2 below.

State-of-the-art R & D support program, “Innovative basic research for the creation of high-performance power storage devices, details of implementation in FY2010”, [online], [Search March 5, 2013], Internet <URL: http: //www.first-mizuno.com/pdf/2010_situation_of_execution.pdf> The University of Tokyo, “Press release“ Lithium-ion battery, the University of Tokyo discovers a new iron-based electrode material, double the capacity ””, [online], [Search March 6, 2013], Internet <URL: http://www.tu-tokyo.ac.jp/tpage/public/pdf/release_20100929.pdf>

  As described above, the LFPO-based active material is composed of inexpensive and safe Fe and P as a main component, and has a stable crystal structure and a high capacity, and is currently in practical use for lithium secondary batteries. It has the potential to replace electrode active materials. However, the LFPO-based active material has poor electronic conductivity, and even if incorporated in a lithium ion battery as an electrode material as it is, the overvoltage is large and sufficient performance as a battery cannot be obtained. In addition, the LFPO-based active material also has a problem that when the electrode material is formed by the same method as that for a conventional lithium secondary battery, the ion conductivity is poor and a sufficient capacity cannot be obtained. In other words, even if the theoretical capacity is high, the capacity cannot be expressed efficiently.

  Accordingly, an object of the present invention is to provide a practical electrode active material for a lithium secondary battery using an LFPO-based active material. Another object of the present invention is to provide a lithium secondary battery using the electrode active material and a method for producing an electrode active material for a lithium secondary battery.

In order to achieve the above object, the present invention provides a powdery compound represented by the chemical formula Li ₂ Fe _(1-x) M _x P ₂ O ₇ where M is either Ni or Co, or both. The electrode active material for a lithium ion secondary battery is characterized in that a film is formed.

Further, it is preferable to use an electrode active material for a lithium ion secondary battery characterized in that the carbon is contained in an amount of 1.0 wt% to 10.0 wt% with respect to the compound. More preferably, the average particle size of the compound is 2.0 μm or less. Then, the compound may be Li ₂ FeP ₂ lithium ion secondary battery electrode active material represented by the chemical formula O _7.

The present invention also provides a lithium secondary battery using the electrode active material for a lithium ion secondary battery as described above as a positive electrode active material or a negative electrode active material, and a method for producing an electrode active material for a lithium ion secondary battery It reaches to. And the invention related to the manufacturing method is
A raw material mixing step of mixing a carbon raw material with a raw material of a compound represented by the chemical formula Li ₂ Fe _(1-x) M _x P ₂ O ₇ ;
After pressing the mixture obtained by the raw material mixing step to form a pellet, the pellet is heated at a temperature lower than the firing temperature to obtain a primary fired product,
After the powder obtained by pulverizing the primary fired product is pressed into a pellet, the main firing step of firing the pellet to obtain a secondary fired product,
A particle size adjusting step of pulverizing the secondary fired product to have a predetermined average particle size;
A method for producing an electrode active material for a lithium ion secondary battery.

  According to the present invention, an electrode active material for a lithium secondary battery having high capacity and high safety and chemical stability can be provided at low cost. In addition, an inexpensive and high-capacity lithium secondary battery can be provided.

It is a figure which shows the manufacturing method of the electrode active material for lithium secondary batteries which concerns on the Example of this invention. It is a figure which shows the assembly method of the lithium secondary battery using the electrode active material for lithium secondary batteries which concerns on the said Example. It is a figure which shows the charging / discharging characteristic of the lithium secondary battery using the electrode active material for lithium secondary batteries which concerns on the Example of this invention.

=== About LFPO-based active material ===
LiCoO ₂ and LiMn ₂ O _{4, which} are conventional electrode active materials for lithium secondary batteries, can move one lithium ion per chemical formula, whereas Li ₂ Fe _(1-x) M _x P ₂ O There are two LFPO-based active materials represented by the chemical formula ( ₇ ). And since the capacity | capacitance per 1 lithium ion is 110 mAh / g, theoretically a high capacity | capacitance of 220 mAh / g may be obtained. However, as described above, the LFPO-based active material has a problem of low electron conductivity, and a lithium secondary battery using the LFPO-based active material as an electrode active material has not yet been put into practical use.

=== About Example of the Invention ===
The electrode active material according to the embodiment of the present invention is LFPO with improved electron conduction characteristics and ion conduction characteristics. Specifically, in the chemical formula of the LFPO-based active material represented by Li ₂ Fe _(1-x) M _x P ₂ O ₇ , a carbon coating is applied to LFPO represented by Li ₂ FeP ₂ O ₇ where x = 0. The electrode active material is in the form of fine particles, and the electron conductivity of the electrode active material itself is improved. In addition, when an electrode active material is used in a lithium secondary battery, a conductive material and a binder are mixed with the electrode active material to form an electrode material, so that lithium ions are reversibly and smoothly applied to the electrode material. An electrode active material whose average particle size is appropriately adjusted so that it can be inserted and removed is also an example of the present invention.

=== About Samples ===
In order to evaluate the characteristics of the electrode active material of this example, or to define a preferable average particle size in the amount of carbon coated on LFPO and the electrode active material, various electrode active materials having different production conditions were prepared, A lithium secondary battery using the electrode active material was produced as a sample. Below, the manufacturing method of an electrode active material and the assembly method of a lithium secondary battery using the electrode active material are demonstrated.

<Method for producing electrode active material>
FIG. 1 shows a method for producing an electrode active material containing LFPO as a main component. First, (NH ₄ ) ₂ HPO ₄ , Li ₂ CO ₃ , FeC ₂ O ₄ .2H ₂ O as raw materials for LFPO were weighed in a stoichiometric ratio (s1), and the weighed raw materials were mixed in a magnetic mortar. (S2 → s4). Alternatively, ascorbic acid was mixed with LFPO as a carbon material for increasing the electron conductivity of the electrode active material according to the sample (s2 → s3 → s4). The amount of ascorbic acid mixed was adjusted according to the sample.

Next, a mixture of electrode active material raw materials is pressed at a pressure of 6 t / cm ² to form a flat cylindrical pellet having a diameter of 21.5 mm (s5). The pellet is placed in an alumina crucible, and Ar Temporary baking was performed by heating at a temperature of 300 ° C. for 1 hour in an atmosphere to produce a primary fired product (s6).

  The primary fired product was then pulverized in an agate mortar (s7), and the powder obtained by the pulverization was pressed again under the same conditions as described above to form pellets (s8). The pellets were put into an alumina crucible, and this time, heated for 2 hours at a temperature of 600 ° C. in an Ar atmosphere (fired) to obtain a secondary fired product (s9).

  Finally, the secondary fired product was pulverized so as to have an average particle size according to the sample. In addition, grinding | pulverization was performed in the different procedure according to the average particle diameter. When obtaining a powder having an average particle size of 5.0 μm or less, it was pulverized in an alcohol medium using a ball mill (s10 → s11). In the pulverization by this ball mill, electrodes having average particles of 0.01 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1.0 μm, 2.0 μm, and 5.0 μm are adjusted by adjusting the pulverization time and the type of medium. An active material was obtained. When obtaining a powder having an average particle size larger than 5 μm, it was pulverized with an agate mortar (s10 → s12). Thereby, an electrode active material having an average particle diameter of 10.0 μm was obtained.

<Method for producing lithium secondary battery>
FIG. 2 shows a method for assembling a lithium secondary battery using the electrode active material manufactured by the procedure shown in FIG. The electrode active material manufactured by the method described above is used as a positive electrode active material, and the electrode active material (hereinafter also referred to as a positive electrode active material), ketjen black (KB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder. (N-methylpyrrolidone) NMP was mixed as a solvent to obtain a slurry paste-like positive electrode material (s21). Note that the ratios of the positive electrode active material, KB, and PVDF are 73 wt%, 14 wt%, and 13 wt%. And drying what applied the said positive electrode material uniformly on 20 micrometers aluminum foil which is a positive electrode electrical power collector (s22), cut out the aluminum foil with which the said positive electrode material was coated into a 20 cm square rectangular shape, This was used as a positive electrode (s23).

  The negative electrode uses metal Li as the negative electrode active material, and the metal lithium (Li) is pasted on a 20 μm copper foil as a current collector (s25), and the copper foil on which the metal Li is pasted is 20 cm square. It was produced by cutting out into a rectangular shape (s26). And the lead wire used as an electrode terminal was connected to the collector of a positive electrode and a negative electrode (s24, s27), and the positive electrode and the negative electrode were laminated | stacked through the paper-made separator (s28). A glass plate was further laminated on the outside of the structure in which the positive electrode and the negative electrode were laminated (hereinafter referred to as an electrode laminate) via this separator (s29).

Next, the lead wire is led out to the outside of the bag while the electrode structure with the glass plate sandwiched is placed in the bag-like laminate film (s30). Then, an electrolyte was poured into the laminate film (s31), defoamed in vacuum, and the opening of the laminate film was sealed by a method such as thermocompression bonding, thereby completing the assembly of the lithium secondary battery. (S32, s33). The electrolyte used was a solution in which 1M LiPF ₆ was dissolved as a supporting salt in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a ratio of 30 wt% and 70 wt%.

=== First Embodiment ===
The electrode active material according to the embodiment of the present invention is based on the technical idea of improving the electronic conductivity by coating LFPO, which has a low electronic conductivity with a single substance, with carbon. Therefore, as a first example of the present invention, a positive electrode active material made of LFPO coated on carbon was cited, and characteristics of the positive electrode active material were examined. In the examination, a lithium secondary battery was assembled using a positive electrode active material in which the amount of carbon was changed by changing the amount of ascorbic acid mixed in the mixing step in FIG. Using the lithium secondary battery as a sample, the discharge capacity characteristics of each sample were evaluated.

  Specifically, charging and discharging are performed so that the voltage between the positive and negative electrodes is 4.5 V and 2.0 V at a low current of 40 μA while maintaining 25 ° C. in the thermostatic bath, and the charging time and discharging at that time Evaluation was made based on the capacity (mAh / g) obtained from the time. Moreover, the crystal structure of the positive electrode active material used for each sample was evaluated by x-ray diffraction measurement. The crystal structure of LFPO is determined by comparing with the result of X-ray diffraction measurement (Rietveld analysis) for a single LFPO described in, for example, page 5 of Patent Document 1 above. Evaluation was made by confirming whether or not there was a crystal phase having a phase different from that of LFPO by coating a single LFPO with carbon.

  Table 1 shows the carbon amount, discharge capacity characteristics, and X-ray diffraction measurement results for each sample.

表１において、サンプル１はカーボン被膜がないＬＦＰＯ自体を正極活物質としたものである。そしてこのサンプル１（以下、比較例１とも言う）に対し、ＬＦＰＯにカーボンを被膜した正極活物質を用いたサンプル２〜９ではその全てにおいて放電容量特性が向上した。さらに、カーボン量が１．０ｗｔ％以上１０．０ｗｔ％以下とすると、従来の正極活物質にも置換可能な実測値で９０ｍＡｈ／ｇ以上の容量が得られた。なお、カーボン量を１５．０ｗｔ％以上とすると結晶構造に異相となる構造が出現した。

In Table 1, Sample 1 uses LFPO itself having no carbon coating as the positive electrode active material. In comparison with Sample 1 (hereinafter, also referred to as Comparative Example 1), discharge capacity characteristics were improved in all of Samples 2 to 9 using a positive electrode active material in which carbon was coated on LFPO. Furthermore, when the carbon amount was 1.0 wt% or more and 10.0 wt% or less, a capacity of 90 mAh / g or more was obtained as an actually measured value that could be substituted for a conventional positive electrode active material. When the carbon amount was 15.0 wt% or more, a structure having a different phase in the crystal structure appeared.

  From the above, it was confirmed that by applying a carbon film to LFPO, the electronic conductivity was improved and the capacity was surely increased. Furthermore, it has also been found that better discharge capacity characteristics can be obtained by mixing carbon in the electrode active material in an amount of 1.0 wt% to 10.0 wt% in order to form the carbon coating.

  In Samples 2 and 3 where the amount of carbon is less than 1.0 wt%, since the amount of carbon is insufficient, the electron conductivity decreases and the overvoltage increases, so that the discharge capacity characteristics of Samples 4 to 7 are obtained. It is thought that it was not possible. In Samples 8 and 9 having a carbon amount of more than 10.0 wt%, it is considered that LFPO, which is a supply source of lithium ions in the electrode active material, was relatively reduced, so that high discharge capacity characteristics were not obtained. . In Samples 8 and 9 having a large amount of carbon, it was also confirmed that a crystal structure having a phase different from that of LFPO appeared by coating carbon on LFPO. In any case, it is a fact that LFPO coated with carbon has improved electronic conductivity and increased capacity as compared with LFPO alone.

=== Second Embodiment ===
As described above, the electrode material is not composed of a single electrode active material, but is a mixture of a conductive material and a binder in the electrode active material. The mixture is further formed into a predetermined shape. For example, in the sample produced here, the electrode active material was contained as a positive electrode active material in a paste-like positive electrode material, and the positive electrode material was coated on a metal foil and formed into a sheet shape.

  In a lithium secondary battery, improving ion conductivity is to allow lithium ions to be more smoothly inserted and removed from an electrode material containing an electrode active material. And improving ionic conductivity leads to improving discharge capacity characteristics. Therefore, as a second embodiment, a positive electrode active material made of LFPO, which has a carbon coating and is adjusted so that the average particle diameter is in an appropriate numerical range, is given. Then, in order to define a more preferable average particle diameter for the positive electrode active material, a lithium secondary battery using various positive electrode active materials having different average particle diameters while keeping the carbon amount constant is assembled as a sample, and the discharge capacity of each sample Characteristics were evaluated. As for the carbon amount, 5 wt% in the sample 6 having excellent discharge capacity characteristics from the results shown in Table 1 was adopted. In addition, a lithium secondary battery using a positive electrode active material having no carbon coating and an average particle size of 10.0 μm was also assembled as a sample.

  Table 2 shows the average particle diameter and discharge capacity characteristics of the positive electrode active material for each sample.

表２において、サンプル１８はカーボン被膜がない正極活物質を用いたサンプル（以下、比較例２とも言う）である。またサンプル１４は括弧内に番号「６」を付記したように表１におけるサンプル６と同じである。そしてこの表２より、正極活物質の平均粒径を２．０μｍ以下としたサンプル１０〜１５では１００ｍＡｈ／ｇ以上の高い容量が得られた。平均粒径が５．０μｍ以上のサンプル１６と１７では、放電容量特性がそれぞれ７０ｍＡｈ／ｇと６０ｍＡｈ／ｇとサンプル１０〜１５と比較すると低かった。これは、粒径が大きいために粒子内のリチウムイオン伝導の抵抗が高くなり、過電圧も大きくなり放電容量特性が低下したものと考えられる。しかし、平均粒径がサンプル１７と同じ１０．０μｍでカーボン被膜がないＬＦＰＯを正極活物質として用いた比較例２では、放電容量特性がサンプル１７の１／２であったことから、平均粒径が１０．０μｍと大きな場合であってもＬＦＰＯにカーボン被膜を形成することによって放電容量特性が向上することが確認できた。

In Table 2, Sample 18 is a sample using a positive electrode active material without a carbon coating (hereinafter also referred to as Comparative Example 2). Sample 14 is the same as Sample 6 in Table 1 as indicated by the number “6” in parentheses. From Table 2, a high capacity of 100 mAh / g or more was obtained in Samples 10 to 15 in which the average particle diameter of the positive electrode active material was 2.0 μm or less. Samples 16 and 17 having an average particle size of 5.0 μm or more had discharge capacity characteristics of 70 mAh / g and 60 mAh / g, respectively, which were lower than those of samples 10-15. This is presumably because the resistance of lithium ion conduction in the particles is increased due to the large particle size, the overvoltage is increased, and the discharge capacity characteristics are lowered. However, in Comparative Example 2 in which LFPO having an average particle diameter of 10.0 μm, which is the same as that of Sample 17 and having no carbon coating, was used as the positive electrode active material, the discharge capacity characteristic was ½ that of Sample 17, It was confirmed that the discharge capacity characteristics were improved by forming a carbon film on LFPO even when the value was as large as 10.0 μm.

  In Samples 10 to 14 in which the average particle diameter of the positive electrode active material is 1.0 μm or less, 110 mAh / g, which is a theoretical capacity when one lithium ion contributes to charge / discharge, is obtained. If it is 0.0 μm or less, it is not necessary to reduce the average particle diameter of the electrode active material to a dark cloud. If the average particle size is excessively small, a large amount of binder may be required when forming the slurry-like electrode material. Therefore, the electrode active material has an appropriate average particle size in consideration of manufacturability. What is necessary is just to adopt.

  From the above, it is essential that the electrode active material according to the first embodiment of the present invention has a carbon film formed on LFPO, and more preferably, the amount of carbon is 1.0 to LFPO. It is 10.0 wt%. The electrode active material according to the second embodiment is LFPO on which a carbon film is formed, and has an average particle size of 2.0 μm or less. And as a lithium secondary battery using the electrode active material which concerns on the Example of this invention in FIG. 3, the charging / discharging characteristic of the sample 6 in Table 1 (sample 14 in Table 2) and the comparative examples 1 and 2 was shown. . 3A and 3B are graphs showing the charge / discharge characteristics of Comparative Example 2 and Comparative Example 1, respectively, and FIG. 3C is a graph showing the charge / discharge characteristics of Sample 6 (Sample 14). is there. These graphs show the relationship between the capacity and the voltage between the positive and negative electrodes when charging and discharging are repeated three times.

  First, the charge / discharge characteristic graph 101 of Comparative Example 2 shown in FIG. 3A shows a characteristic curve during charging because LFPO having a carbon film and no average particle size of 10.0 μm is used as the positive electrode active material. 101a and the characteristic curve 101b at the time of discharge deviate, and it can be confirmed that the charge / discharge characteristics are greatly deteriorated due to overvoltage. In the charge / discharge characteristic graph 102 of Comparative Example 1 shown in FIG. 3 (B), the charge / discharge characteristics are slightly improved as compared with Comparative Example 2 because the average particle size is 1.0 μm, but the charge is still during charging. The characteristic curve 102a is different from the characteristic curve 102b during discharge. That is, it is impossible to fundamentally solve the deterioration of charge / discharge characteristics due to overvoltage only by making the electrode active material fine particles. When the charge / discharge characteristic graph 103 of sample 6 (sample 14) shown in FIG. 3C corresponding to the embodiment of the present invention is seen, the characteristic curve 103a during charging and the characteristic curve 103b during discharge have a wide capacity. It can be seen that they are close in range and maintained at a constant voltage, and the effect of suppressing overvoltage by coating LFPO with carbon can be confirmed.

=== Other Examples etc. ===
<About mixing time of carbon raw materials>
The electrode active materials used for the above samples are all manufactured based on the procedure shown in FIG. However, as a matter of course, the electrode active material coated with the carbon film has no precedent, and there is no clear standard for the timing of mixing the carbon raw material with respect to the conventional LFPO manufacturing procedure. In the above embodiment, carbon was mixed with the LFPO raw material in order to suppress the grain growth of LFPO and to form a uniform carbon film. The later is considered. Therefore, in order to confirm the tendency of the superiority or inferiority of the discharge capacity characteristics due to the mixing timing of the carbon raw material, a lithium secondary battery using LFPO coated with carbon mixed at different mixing timings as a positive electrode active material was prepared as a sample, The discharge capacity characteristics of each sample were evaluated. The mixing amount of the carbon raw material was uniformly 5 wt% for all the samples.

  Table 3 shows the relationship between the preparation conditions of the positive electrode active material and the discharge capacity characteristics.

表３に示したように、各サンプルは、正極活物質の製造条件としてカーボン原料の混合時期と焼成条件が異なっている。混合時期としてはＬＦＰＯの原料とともにカーボン原料を混合する場合（原料混合時）、仮焼成後に混合する場合（仮焼成後）および本焼成後に混合する場合（本焼成後）があり、焼成条件としては、本焼成後の二次焼成品を粉砕したものを正極活物質として用いる場合（再焼成なし）と、本焼成後にカーボン原料を混合した場合において、二次焼成品を粉砕したものを再度本焼成と同じ条件で再度焼成（再焼成）するとともに、その再焼成によって得た粉体（再焼成品）を粉砕したものを正極活物質とする場合（再焼成あり）とがある。

As shown in Table 3, each sample has different carbon raw material mixing times and firing conditions as production conditions for the positive electrode active material. The mixing timing includes mixing the carbon raw material together with the LFPO raw material (at the time of raw material mixing), mixing after the pre-firing (after the pre-firing), and mixing after the main firing (after the main firing). In the case of using the pulverized secondary fired product after the main firing as the positive electrode active material (without re-firing) and mixing the carbon raw material after the main firing, the secondary fired product is ground again. And firing again (refired) under the same conditions as above, and pulverizing the powder (refired product) obtained by the refire is used as the positive electrode active material (with refired).

  In the above firing conditions, re-firing was employed only when the average particle size of the secondary fired product was as small as 1.0 μm. This is because it is difficult to pulverize to a desired average particle size even if a carbon raw material is added to a secondary baked product containing no carbon and pulverized. In Sample 20 that was refired, the average particle size of the secondary fired product was 1.0 μm, and the refired product was pulverized so that the average particle size was 2.0 μm. Samples 19, 21, and 22 in Table 3 are the same as Samples 14, 15, and 17 in Table 2. In Table 3, the sample numbers in Table 2 are shown in parentheses.

  Here, looking at the discharge capacity characteristics of each sample shown in Table 3, the samples 19 and 20 have the same average particle diameter of 1.0 μm when pulverized after the secondary firing, but the mixing timing of the carbon raw material is the same. Is different. Samples 22 to 24 have the same average particle size of 10.0 μm when pulverized after secondary firing, but the mixing timing of the carbon raw materials is also different. When samples 19 and 20 are compared, or when samples 22 to 24 are compared, in either case, sample 19 or sample 22 in which the carbon raw material is mixed together with the LFPO raw material has the same average particle size and carbon raw material. The discharge capacity characteristics were superior to those of other samples with different mixing times. In Samples 20 and 21, the final positive electrode active material had the same average particle diameter of 2.0 μm. In this case, Sample 20 in which the carbon raw material is mixed with the LFPO raw material has better discharge capacity characteristics. It was.

  As described above, when the average particle size is large as in Samples 22 to 24, when the carbon raw material is mixed together with the LFPO raw material, the preliminary firing and the main firing are performed in a state where the carbon raw material is mixed. As a result, carbon is coated on LFPO, grain growth is suppressed even when the average particle size of LFPO is large, and the carbon film is also formed uniformly. As a result, sample 22 is obtained by mixing the carbon raw material together with the LFPO raw material. It is considered that the discharge capacity characteristics of the sample were improved over those of the samples 23 and 24.

  On the other hand, in the case where the average particle size is small as in Samples 19 to 21, a part of the carbon coating covering the periphery of LFPO is dropped by mixing carbon together with the LFPO raw material and performing pulverization after secondary firing. It can be considered that the ionic conductivity is improved, and as a result, the discharge capacity characteristics are also improved. In Sample 20, which was re-fired by adding a carbon raw material to the secondary fired product, the final electrode active material itself was coated with carbon to ensure electronic conductivity. It is thought that it was obstructed.

<About positive electrode and negative electrode>
In each of the above samples, the produced electrode active material was incorporated into the lithium secondary battery as the positive electrode active material because of the potential difference from the metal Li used as the negative electrode active material, but the electrode active material having a higher potential than LFPO was used as the negative electrode. If used, it is theoretically possible to use the electrode active material of each of the above embodiments as the negative electrode active material.

<About the composition of LFPO>
In each of the above samples, LFPO having x = 0 in the chemical formula of Li ₂ Fe _(1-x) M _x P ₂ O ₇ was used as the electrode active material, but part of Fe was changed to Co or Ni, which are the same transition metals. It can be easily imagined that the characteristics of the lithium secondary battery using the electrode active material can be approximated to the above samples.

<Embodiment of lithium secondary battery>
A lithium secondary battery using LFPO is known to operate at a voltage of 5.3 V when two lithium ions contribute to charge and discharge, and an extremely high energy density is obtained. However, in the lithium secondary battery using the electrolytic solution, since the electrolytic solution is decomposed at 5 V or more, each sample is operated at 2.5 V to 4.5 V using the electrolytic solution. On the other hand, in the case where two lithium ions are allowed to contribute to charging / discharging, it can be applied to an all-solid battery using a solid electrolyte instead of the electrolytic solution. That is, the lithium ion battery according to the present invention extends not only to a lithium secondary battery using an electrolytic solution, but also to an electrode active material applicable to an all-solid battery and a solid battery using the electrode active material.

101-103 Lithium secondary battery charge / discharge characteristics graph,
s1 FLPO raw material weighing step, s3 carbon raw material (ascorbic acid) weighing step,
s4 electrode active material raw material mixing step, s6 temporary baking step, s9 main baking step,
s11, s12 crushing (particle size adjustment) process

Claims (6)

A carbon film is formed on a powdery compound represented by the chemical formula Li ₂ Fe _(1-x) M _x P ₂ O ₇ where M is either Ni or Co, or both. Electrode active material for lithium ion secondary batteries.   2. The electrode active material for a lithium ion secondary battery according to claim 1, wherein the carbon is contained in an amount of 1.0 wt% to 10.0 wt% with respect to the compound.   3. The electrode active material for a lithium ion secondary battery according to claim 1, wherein the compound has an average particle size of 2.0 μm or less. The electrode active material for a lithium ion secondary battery according to claim 1, wherein the compound is represented by a chemical formula of Li ₂ FeP ₂ O ₇ .   A lithium secondary battery using the electrode active material for a lithium ion secondary battery according to claim 1 as a positive electrode active material or a negative electrode active material. A method for producing an electrode active material for a lithium ion secondary battery, comprising:
A raw material mixing step of mixing a carbon raw material with a raw material of a compound represented by the chemical formula Li ₂ Fe _(1-x) M _x P ₂ O ₇ ;
After pressing the mixture obtained by the raw material mixing step to form a pellet, the pellet is heated at a temperature lower than the firing temperature to obtain a primary fired product,
After the powder obtained by pulverizing the primary fired product is pressed into a pellet, the main firing step of firing the pellet to obtain a secondary fired product,
A particle size adjusting step of pulverizing the secondary fired product to have a predetermined average particle size;
The manufacturing method of the electrode active material for lithium ion secondary batteries characterized by including this.

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