JP5216960B2 - Positive electrode active material for lithium ion secondary battery - Google Patents

Positive electrode active material for lithium ion secondary battery Download PDF

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JP5216960B2
JP5216960B2 JP2006348787A JP2006348787A JP5216960B2 JP 5216960 B2 JP5216960 B2 JP 5216960B2 JP 2006348787 A JP2006348787 A JP 2006348787A JP 2006348787 A JP2006348787 A JP 2006348787A JP 5216960 B2 JP5216960 B2 JP 5216960B2
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裕子 澤木
敦 畠山
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Hitachi Maxell Energy Ltd
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Description

本発明は、オリビン型リン酸鉄リチウム微粒子を有するリチウムイオン二次電池用正極活物質とその製造方法に関するものである。   The present invention relates to a positive electrode active material for lithium ion secondary batteries having olivine type lithium iron phosphate fine particles and a method for producing the same.

リチウムイオン二次電池は、携帯用電子機器やハイブリッド自動車などに用いるための電池として、急速に開発が進められている。この場合の負極活物質としては主に炭素材料が用いられ、正極活物質としては、金属酸化物、金属硫化物、各種ポリマーなどが用いられる。特に、コバルト酸リチウム、ニッケル酸リチウム、マンガン酸リチウムなどのリチウム複合酸化物は、高エネルギー密度で高電圧の電池を実現できることから、現在、リチウムイオン二次電池の活物質として、一般的に用いられている。   Lithium ion secondary batteries are being rapidly developed as batteries for use in portable electronic devices and hybrid vehicles. In this case, a carbon material is mainly used as the negative electrode active material, and metal oxides, metal sulfides, various polymers, and the like are used as the positive electrode active material. In particular, lithium composite oxides such as lithium cobaltate, lithium nickelate, and lithium manganate are generally used as active materials for lithium ion secondary batteries because they can realize batteries with high energy density and high voltage. It has been.

一方、特許文献１や特許文献２にあるように、オリビン構造を持つ遷移金属リン酸化合物を正極活物質として用いたリチウムイオン二次電池の提案もされており、中でも特許文献２には、原料が安価である材料としてリン酸鉄リチウム（ＬｉＦｅＰＯ４）を正極活物質として用いたリチウムイオン二次電池が提案されている。 On the other hand, as disclosed in Patent Document 1 and Patent Document 2, a lithium ion secondary battery using a transition metal phosphate compound having an olivine structure as a positive electrode active material has also been proposed. As an inexpensive material, lithium ion secondary batteries using lithium iron phosphate (LiFePO 4 ) as a positive electrode active material have been proposed.

リン酸鉄リチウムを作製する方法としては、主に次のような方法が挙げられる。
（１）原料となるリチウム化合物、鉄化合物、およびリン酸化合物を、ボールミルあるいは乳鉢などで機械的に粉砕・混合し、固相反応させた後、還元雰囲気下で加熱処理する方法。
（２）原料となるリチウム塩、鉄塩、リン酸を溶解させた溶液を蒸発乾固させた後、還元雰囲気下で加熱処理する方法。
（３）原料となるリチウム塩、鉄塩、リン酸を溶解させた溶液を噴霧乾燥させた後、還元雰囲気下で加熱処理する方法。
（４）原料となるリチウム塩、鉄塩、リン酸を溶解させた溶液に水熱処理を施す方法。 Examples of methods for producing lithium iron phosphate mainly include the following methods.
(1) A method in which a lithium compound, an iron compound, and a phosphoric acid compound as raw materials are mechanically pulverized and mixed in a ball mill or a mortar to cause a solid phase reaction, followed by heat treatment in a reducing atmosphere.
(2) A method in which a solution in which a lithium salt, an iron salt, and phosphoric acid as raw materials are dissolved is evaporated to dryness, followed by heat treatment in a reducing atmosphere.
(3) A method in which a solution in which a lithium salt, an iron salt, and phosphoric acid as raw materials are dissolved is spray-dried and then heat-treated in a reducing atmosphere.
(4) A method in which a hydrothermal treatment is performed on a solution in which lithium salt, iron salt, and phosphoric acid as raw materials are dissolved.

これらの合成法のうち、（１）の方法は最も一般的に行われており、簡便かつ安価にリン酸鉄リチウムを製造することができる。しかしながら、（１）の方法で得られるオリビン型リン酸鉄リチウムは、数〜数十μｍサイズの、小さくともサブミクロンサイズの粒子となり、製法上の特徴から粒子サイズ分布が広く、形状が一定ではない。オリビン型リン酸鉄リチウムを用いたリチウムイオン二次電池としては、例えば、オリビン型リン酸鉄リチウムを導電助剤や結着剤などと混合して正極合剤とし、これにより集電体上に正極合剤層を形成して正極を作製し、該正極を組み込んでリチウムイオン二次電池とされたものが挙げられるが、（１）の方法により製造されるような形態のオリビン型リン酸鉄リチウムでは、導電助剤などと均一に混合することが難しく、このような均一性の低い正極合剤を用いて構成したリチウムイオン二次電池では、特性が劣るものとなる。   Among these synthesis methods, the method (1) is most commonly performed, and lithium iron phosphate can be produced easily and inexpensively. However, the olivine-type lithium iron phosphate obtained by the method (1) is a particle having a size of several to several tens of micrometers, at least a submicron size, and has a wide particle size distribution due to the characteristics of the manufacturing method, and the shape is not constant. Absent. As a lithium ion secondary battery using olivine-type lithium iron phosphate, for example, olivine-type lithium iron phosphate is mixed with a conductive additive or a binder to form a positive electrode mixture. A positive electrode is prepared by forming a positive electrode mixture layer and incorporating the positive electrode into a lithium ion secondary battery. The olivine-type iron phosphate in a form manufactured by the method (1) Lithium is difficult to be uniformly mixed with a conductive additive or the like, and a lithium ion secondary battery configured using such a low-uniformity positive electrode mixture has poor characteristics.

また、（２）や（３）のように溶液を特定の状態で乾燥させる方法を用いた公知例としては、例えば特許文献３や特許文献４があり、いずれも数百ｎｍサイズのオリビン型リン酸鉄リチウム粒子が得られている。このうち特に特許文献４では、噴霧乾燥を行う際に、溶液中に反応遅延剤を加えることにより数十ｎｍのリン酸鉄リチウム粒子の合成が可能とされており、実施例では最小で数百ｎｍサイズの粒子が得られている。そして、これらの粒子は二次凝集体を形成しており、一次粒子に分散した微粒子は得られていない。二次凝集体を形成しているオリビン型リン酸鉄リチウムでは、均一性の高い正極合剤を調製することは難しく、（１）の方法により得られたオリビン型リン酸鉄リチウムを用いて電池を構成した場合と同様の問題が生じてしまう。   Moreover, as a well-known example using the method of drying a solution in a specific state as in (2) or (3), there are, for example, Patent Document 3 and Patent Document 4, both of which are olivine-type phosphorous having a size of several hundred nm. Lithium iron oxide particles are obtained. Among them, particularly in Patent Document 4, it is possible to synthesize lithium iron phosphate particles of several tens of nanometers by adding a reaction retarder to the solution during spray drying. nm-sized particles are obtained. These particles form secondary aggregates, and fine particles dispersed in the primary particles are not obtained. With the olivine-type lithium iron phosphate forming the secondary aggregate, it is difficult to prepare a positive electrode mixture with high uniformity. A battery using the olivine-type lithium iron phosphate obtained by the method (1) is used. The same problem as in the case of configuring is generated.

更に、（４）の製造方法については特許文献５に開示があり、この特許文献５では、結晶成長を抑制させるための有機添加物の存在下で水熱処理を施す例が記載されており、数十ｎｍのリン酸鉄リチウム粒子の合成が可能とされている。また、特許文献５に開示されているような水熱合成法を用いた場合には、より低温での処理により結晶性の良好な粒子を得ることができ、一次粒子に分散した粒子を得ることができるというメリットがある。しかしながら、特許文献５に開示の方法であっても、例えば実施例に記載されているように、実際には最小で約１００ｎｍサイズの粒子しか得ることができておらず、数ｎｍから数十ｎｍ程度の粒子を得ることは困難である。   Further, the production method (4) is disclosed in Patent Document 5, which describes an example in which hydrothermal treatment is performed in the presence of an organic additive for suppressing crystal growth. It is possible to synthesize 10 nm lithium iron phosphate particles. In addition, when the hydrothermal synthesis method disclosed in Patent Document 5 is used, particles having good crystallinity can be obtained by treatment at a lower temperature, and particles dispersed in primary particles can be obtained. There is a merit that you can. However, even with the method disclosed in Patent Document 5, for example, as described in the examples, only particles having a size of about 100 nm can be obtained at a minimum, and several nanometers to several tens of nanometers can be obtained. It is difficult to obtain a degree of particles.

特開平９−１３４７２４号公報JP-A-9-134724 特開平９−１７１８２７号公報Japanese Patent Laid-Open No. 9-171827 特開２００３−１５７８４５号公報JP 2003-157845 A 特開２００５−１１６３９３号公報JP 2005-116393 A 特開２００５−２７６４７６号公報JP 2005-276476 A

正極活物質として使用するリン酸鉄リチウム粒子の粗大化は、すなわち比表面積の低下につながり、電池の充放電時の電流密度を低下させ、その結果、高出力化を妨げる要因となる。かかる理由からリン酸鉄リチウム粒子の更なる微粒子化が望まれているが、現状では高々１００ｎｍ程度の大きさが得られる最小粒径であり、例えば特許文献３、４に記載されている方法を用いた場合であっても、より微粒子化させたオリビン型リン酸鉄リチウム粒子を得ることができなかった。   The coarsening of the lithium iron phosphate particles used as the positive electrode active material leads to a decrease in the specific surface area, which decreases the current density during charge / discharge of the battery, and as a result, hinders an increase in output. For this reason, further refinement of lithium iron phosphate particles is desired, but at present, the minimum particle size is as small as about 100 nm. For example, the methods described in Patent Documents 3 and 4 are used. Even when it was used, it was not possible to obtain olivine-type lithium iron phosphate particles having a finer particle size.

本発明は上記事情に鑑みてなされたものであり、オリビン型リン酸鉄リチウム微粒子を有し、特性の良好なリチウムイオン二次電池を構成可能な正極活物質と、該正極活物質を製造する方法を提供することにある。   The present invention has been made in view of the above circumstances, and has a positive electrode active material having olivine type lithium iron phosphate fine particles and capable of forming a lithium ion secondary battery with good characteristics, and manufacturing the positive electrode active material It is to provide a method.

本発明者らは鋭意検討を重ねた結果、あらかじめ微細な鉄酸化物微粒子を合成し、これを核としてオリビン型リン酸鉄リチウムを結晶化させることにより、微細かつ粒径の分布が狭く、二次凝集体を形成せずに一次粒子が良好に分散したリチウムイオン二次電池用正極活物質を製造できることを見出し、本発明を完成するに至った。   As a result of intensive studies, the present inventors have synthesized fine iron oxide fine particles in advance and crystallized olivine-type lithium iron phosphate using the fine iron oxide fine particles as a nucleus. It has been found that a positive electrode active material for a lithium ion secondary battery in which primary particles are well dispersed without forming a secondary aggregate can be produced, and the present invention has been completed.

すなわち、上記目的を達成し得た本発明のリチウムイオン二次電池用正極活物質は、少なくともオリビン型リン酸鉄リチウム微粒子を有するリチウムイオン二次電池用正極活物質であって、二次凝集体を形成せずに単一粒子として存在しており、上記オリビン型リン酸鉄リチウム微粒子は、平均粒子径が５〜５０ｎｍであり、全個数中９０％以上の微粒子の粒子径が３〜７０ｎｍの範囲内にあることを特徴とするものである。   That is, the positive electrode active material for a lithium ion secondary battery of the present invention capable of achieving the above object is a positive electrode active material for a lithium ion secondary battery having at least olivine-type lithium iron phosphate fine particles, and a secondary aggregate The olivine-type lithium iron phosphate fine particles have an average particle size of 5 to 50 nm, and 90% or more of the total number of fine particles has a particle size of 3 to 70 nm. It is characterized by being within the range.

本発明によれば、オリビン型リン酸鉄リチウム微粒子を有し、特性の良好なリチウムイオン二次電池を構成可能な正極活物質と、該正極活物質を製造する方法を提供できる。本発明のリチウムイオン二次電池用正極活物質を用いて構成されるリチウムイオン二次電池では、より多くの電流が流れ、充放電時に高い電流密度が得られることが期待できる。   ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material which has olivine type lithium iron phosphate microparticles | fine-particles and can comprise a lithium ion secondary battery with a favorable characteristic, and the method of manufacturing this positive electrode active material can be provided. In the lithium ion secondary battery configured using the positive electrode active material for a lithium ion secondary battery of the present invention, it can be expected that a larger amount of current flows and a high current density is obtained during charging and discharging.

本発明のリチウムイオン二次電池用正極活物質（以下、単に「正極活物質」という場合がある）は、二次凝集体を形成せずに単一粒子として分散しており、かつオリビン型リン酸鉄リチウム微粒子を有していて、該オリビン型リン酸鉄リチウム微粒子は、平均粒子径が５〜５０ｎｍであり、全個数中９０％以上の微粒子の粒子径が３〜７０ｎｍの範囲内にあり、微細かつ粒径分布の狭いものである。本発明の正極活物質はこのような形態を有することにより、リチウムイオン二次電池用正極を製造する際に使用する正極合剤層中において、導電性を確保するために使用される導電助剤との分散がより均一になるため、正極合剤層の導電性を高めることができ、この作用によって、特性の良好なリチウムイオン二次電池を構成することができる。また、正極活物質自体が微細であることで、Ｌｉ（リチウム）の拡散速度が大きくなるため、この作用によってもリチウムイオン二次電池の特性を高めることができる。   The positive electrode active material for a lithium ion secondary battery of the present invention (hereinafter sometimes simply referred to as “positive electrode active material”) is dispersed as a single particle without forming a secondary aggregate, and olivine-type phosphorus It has lithium iron oxide fine particles, and the olivine type lithium iron phosphate fine particles have an average particle diameter of 5 to 50 nm, and the particle diameter of 90% or more of the total number is within the range of 3 to 70 nm. It is fine and has a narrow particle size distribution. Since the positive electrode active material of the present invention has such a form, a conductive additive used to ensure conductivity in the positive electrode mixture layer used when manufacturing a positive electrode for a lithium ion secondary battery. Therefore, the conductivity of the positive electrode mixture layer can be increased, and a lithium ion secondary battery with good characteristics can be formed by this action. In addition, since the positive electrode active material itself is fine, the diffusion rate of Li (lithium) increases, so that the characteristics of the lithium ion secondary battery can also be enhanced by this action.

本発明の正極活物質の有するオリビン型リン酸鉄リチウム微粒子の平均粒子径は、大きすぎると上記の作用を発揮できなくなることから、５０ｎｍ以下であり、二次凝集体の形成が阻害され易いことから、２０ｎｍ以下であることが好ましい。また、オリビン型リン酸鉄リチウム微粒子の平均粒子径は、あまり小さすぎると、リン酸鉄リチウムの結晶構造上、格子点の数が少なすぎるために安定な結合が起こらず、オリビン構造を保持し難くなり、正極活物質の製造自体が困難となることから、５ｎｍ以上であり、８ｎｍ以上であることが好ましい。   The average particle size of the olivine-type lithium iron phosphate fine particles possessed by the positive electrode active material of the present invention is 50 nm or less because the above action cannot be exhibited if it is too large, and the formation of secondary aggregates is likely to be hindered. Therefore, it is preferably 20 nm or less. In addition, if the average particle size of the olivine-type lithium iron phosphate particles is too small, the number of lattice points is too small due to the crystal structure of lithium iron phosphate, so that stable bonding does not occur and the olivine structure is maintained. Since it becomes difficult and manufacture of the positive electrode active material itself becomes difficult, it is 5 nm or more, and preferably 8 nm or more.

正極活物質の有するオリビン型リン酸鉄リチウム微粒子は、喩え上記の平均粒子径を満たすものであっても、粒子径の分布が広く、例えば中に粗大な粒子が存在しているような場合には、正極活物質に係るオリビン型リン酸鉄リチウムを微細にすることによる上記作用が損なわれる虞がある。本発明に係るオリビン型リン酸鉄リチウム微粒子は、全個数中の９０％以上の微粒子の粒子径が３〜７０ｎｍの範囲内にあり、極めて微細な粒子や粗大な粒子を含まず、上記作用を十分に発揮できるものである。本発明に係るオリビン型リン酸鉄リチウム微粒子は、全個数中の９０％以上の微粒子が、５ｎｍ以上の粒子径を有していることが好ましく、また、５０ｎｍ以下の粒子径を有していることが好ましい。   Even if the olivine-type lithium iron phosphate fine particles possessed by the positive electrode active material satisfy the above average particle size, the particle size distribution is wide, for example, in the case where coarse particles exist. There is a possibility that the above-mentioned action due to the fine olivine type lithium iron phosphate relating to the positive electrode active material is impaired. The olivine-type lithium iron phosphate fine particles according to the present invention have a particle diameter of 90% or more of the total number in the range of 3 to 70 nm, do not contain extremely fine particles or coarse particles, and have the above-described action. It can be fully demonstrated. In the olivine-type lithium iron phosphate fine particles according to the present invention, 90% or more of the total number of fine particles preferably has a particle size of 5 nm or more, and has a particle size of 50 nm or less. It is preferable.

また、本発明の正極活物質に係るオリビン型リン酸鉄リチウム微粒子の個々の微粒子は、単結晶であることが好ましい。オリビン型リン酸鉄リチウム微粒子が単結晶の場合には、多結晶の場合よりも導電性が良好であるため、これを用いた正極の導電性をより高めて、より特性の良好なリチウムイオン二次電池を構成できるようになる。よって、本発明に係るオリビン型リン酸鉄リチウム微粒子は、その平均結晶子サイズが平均粒子径とほぼ同等であることが好ましく、具体的には、平均結晶子サイズが、５ｎｍ以上、より好ましくは８ｎｍ以上であって、５０ｎｍ以下、より好ましくは２０ｎｍ以下であることが望ましい。なお、正極活物質の有するオリビン型リン酸鉄リチウム微粒子が全て単結晶のときには、実際には、微粒子の最表面付近では結晶構造が曖昧になる場合もあるため、平均結晶子サイズは平均粒子径よりも若干小さいが、ほぼ同等サイズの測定値となる。   Moreover, it is preferable that each fine particle of the olivine type lithium iron phosphate fine particle which concerns on the positive electrode active material of this invention is a single crystal. When the olivine-type lithium iron phosphate fine particles are single crystals, the conductivity is better than that of polycrystals. Therefore, the conductivity of the positive electrode using the olivine-type lithium iron phosphate particles can be further increased, and lithium ion secondary particles with better characteristics can be obtained. Next battery can be configured. Therefore, the olivine-type lithium iron phosphate fine particles according to the present invention preferably have an average crystallite size approximately equal to the average particle diameter, specifically, the average crystallite size is 5 nm or more, more preferably It is desirable that it is 8 nm or more and 50 nm or less, more preferably 20 nm or less. When the olivine-type lithium iron phosphate fine particles of the positive electrode active material are all single crystals, the average crystallite size is actually the average particle size because the crystal structure may be ambiguous near the outermost surface of the fine particles. Although it is slightly smaller than this, it is a measurement value of almost the same size.

本発明の正極活物質は、オリビン型リン酸鉄リチウム微粒子のみで構成されていてもよいが、オリビン型リン酸鉄リチウム微粒子の周囲に、アモルファス状の物質（例えば、ガラス質構造の物質）が存在していてもよく、オリビン型リン酸鉄リチウム微粒子が上記物質により覆われていてもよい。オリビン型リン酸鉄リチウム微粒子の周囲に、このような物質が存在することにより、正極活物質の二次凝集体の形成がより良好に阻害され、単一粒子の分散体となる。オリビン型リン酸鉄リチウム微粒子の周囲に存在する物質としては、アルミニウムまたはイットリウムが好ましい。   The positive electrode active material of the present invention may be composed only of olivine-type lithium iron phosphate fine particles, but an amorphous material (for example, a glassy material) is formed around the olivine-type lithium iron phosphate fine particles. It may be present, and olivine-type lithium iron phosphate fine particles may be covered with the above substance. The presence of such a substance around the olivine-type lithium iron phosphate fine particles more effectively inhibits the formation of secondary aggregates of the positive electrode active material, resulting in a single particle dispersion. As the substance present around the olivine type lithium iron phosphate fine particles, aluminum or yttrium is preferable.

本明細書でいう正極活物質に係るオリビン型リン酸鉄リチウム微粒子の平均粒子径および粒子径分布は、正極活物質を透過型電子顕微鏡（ＴＥＭ）で観察し、オリビン型リン酸鉄リチウム微粒子１００個について求めた値であり、平均粒子径は、上記１００個の粒子の数平均粒子径である。なお、オリビン型リン酸鉄リチウム微粒子の形状が真球状でない場合（ＴＥＭ画像では真円状でない場合）には、個々の微粒子の粒子径は、その長径（最も長い径）と短径（最も短い径）との平均値とする。また、本明細書でいう正極活物質に係るオリビン型リン酸鉄リチウム微粒子の平均結晶子サイズは、正極活物質について、Ｃｕ−Ｋα線を用い、スキャン速度２°／ｍｉｎで測定されるＸ線回折スペクトルにおけるピークの半値幅から求められる値である。なお、Ｘ線回折スペクトルは集中法で測定し、測定範囲は２θ＝２０〜８０°とする。   The average particle size and particle size distribution of the olivine-type lithium iron phosphate fine particles related to the positive electrode active material referred to in this specification are obtained by observing the positive electrode active material with a transmission electron microscope (TEM), and the olivine-type lithium iron phosphate fine particles 100 The average particle size is the number average particle size of the 100 particles. In addition, when the shape of the olivine type lithium iron phosphate fine particles is not a perfect sphere (when the shape is not a perfect circle in the TEM image), the particle diameter of each fine particle is the longest diameter (longest diameter) and the shortest diameter (shortest). Diameter). In addition, the average crystallite size of the olivine-type lithium iron phosphate fine particles related to the positive electrode active material in the present specification is an X-ray measured for the positive electrode active material using a Cu—Kα ray at a scan rate of 2 ° / min. This is a value obtained from the half width of the peak in the diffraction spectrum. The X-ray diffraction spectrum is measured by the concentration method, and the measurement range is 2θ = 20 to 80 °.

なお、ガラス質構造の物質がオリビン型リン酸鉄リチウム微粒子の周囲に存在している場合には、ＴＥＭ写真ではバックグラウンドの濃淡として観測され、また、リン酸鉄リチウム微粒子自体を覆う形でガラス質構造の物質が存在している場合には、ＴＥＭ写真撮影の際にリン酸鉄リチウム粒子の粒子境界がくっきりとした線で観測されないという特徴がある。粉末Ｘ線回折のような構造解析においても、上記物質がガラス質構造（アモルファス状）である場合には回折線が現れない。これらの理由により、ガラス質構造の物質の存在を分析する際には、まず蛍光Ｘ線分析（ＸＲＦ）または誘導結合プラズマ（ＩＣＰ）発光分光分析などの組成分析により相当量のアルミニウムなどの元素の存在を必ず確認し、更に、粉末Ｘ線回折（ＸＲＤ）により、含有元素由来のいずれの構造も現れずアモルファス構造をとっていることを確認した上で、ＴＥＭ観察により状態を確認する必要がある。なお、アモルファス状であってもガラス質構造でない場合には、正極活物質のＴＥＭ観察の際にバックグラウンドの濃淡としてではなく、不定形粒子など可視の物質として観察される。   In addition, when a vitreous material exists around the olivine-type lithium iron phosphate fine particles, it is observed as a background shading in the TEM photograph, and the glass covers the lithium iron phosphate fine particles themselves. In the case where a substance having a texture structure is present, there is a feature that the particle boundary of lithium iron phosphate particles is not observed with a clear line at the time of TEM photography. Even in a structural analysis such as powder X-ray diffraction, no diffraction line appears when the substance has a glassy structure (amorphous). For these reasons, when analyzing the presence of a vitreous material, first a composition analysis such as X-ray fluorescence (XRF) or inductively coupled plasma (ICP) emission spectroscopy is used to analyze a substantial amount of elements such as aluminum. It is necessary to confirm the state by TEM observation after confirming the existence, and further confirming that the structure derived from the contained elements does not appear and has an amorphous structure by powder X-ray diffraction (XRD). . In addition, when it is amorphous but does not have a glassy structure, it is observed as a visible substance such as an amorphous particle, not as a background shade, in TEM observation of the positive electrode active material.

上記のような本発明の正極活物質は、本発明法、すなわち、平均粒子径が５０ｎｍ以下である鉄酸化物微粒子を成長核に用い、該鉄酸化物微粒子とリチウム源およびリン酸源を含む溶液とを混合し、得られた混合物を加熱してリン酸鉄リチウム微粒子を合成する製造方法により製造することができる。以下、「鉄酸化物微粒子の合成」、「リン酸鉄リチウム微粒子の合成」の順に詳細に説明する。   The positive electrode active material of the present invention as described above includes the method of the present invention, that is, iron oxide fine particles having an average particle diameter of 50 nm or less as growth nuclei, and the iron oxide fine particles, a lithium source and a phosphate source. It can manufacture with the manufacturing method which mixes a solution and heats the obtained mixture and synthesize | combines lithium iron phosphate microparticles | fine-particles. Hereinafter, it will be described in detail in the order of “synthesis of iron oxide fine particles” and “synthesis of lithium iron phosphate fine particles”.

＜鉄酸化物微粒子の合成＞
まず、オリビン型リン酸鉄リチウムの成長核として作用する鉄酸化物微粒子を合成する。鉄酸化物としては、Ｆｅ２Ｏ３、ＦｅＯＯＨ、Ｆｅ３Ｏ４などが好ましい。また、これらの鉄酸化物（Ｆｅ２Ｏ３、ＦｅＯＯＨ、Ｆｅ３Ｏ４など）の含有するＦｅの一部を他の元素で置換した一部置換体でもよい。上記鉄酸化物の一部置換体に係る他の元素（置換元素）は、オリビン構造を形成した際に、かかるオリビン構造を破壊することなく、鉄酸化物結晶におけるＦｅサイトを置換し得る金属元素であればよく、例えば、アルミニウムやイットリウムが挙げられる。また、アルミニウムやイットリウムは、焼結防止剤や表面処理剤として一般的に含有されるものであり、このような形態で鉄酸化物微粒子に存在していても構わない。 <Synthesis of iron oxide fine particles>
First, iron oxide fine particles that act as growth nuclei of olivine type lithium iron phosphate are synthesized. As the iron oxide, Fe 2 O 3 , FeOOH, Fe 3 O 4 and the like are preferable. These iron oxides (Fe 2 O 3, FeOOH, Fe 3 O 4 , etc.) may be partially substituted body part containing to Fe was substituted by other elements. The other element (substitution element) related to the partial substitute of the iron oxide is a metal element that can replace the Fe site in the iron oxide crystal without destroying the olivine structure when the olivine structure is formed. For example, aluminum or yttrium may be used. Aluminum and yttrium are generally contained as sintering inhibitors and surface treatment agents, and may be present in the iron oxide fine particles in such a form.

上記のアルミニウムやイットリウムは３価の元素であり、オリビン構造の中に取り込まれ難く、多くは合成されるオリビン型リン酸鉄リチウム微粒子の周囲に析出し、アモルファス状（例えばガラス質構造）の形態を取る。よって、オリビン型リン酸鉄リチウムの周囲に、アモルファス状（例えばガラス質構造）のアルミニウムまたはイットリウムが存在する態様の正極活物質を製造するには、上記の鉄酸化物の一部置換体を用いて正極活物質を製造すればよい。   The above-mentioned aluminum and yttrium are trivalent elements and are not easily incorporated into the olivine structure. Many of them are deposited around the synthesized olivine-type lithium iron phosphate fine particles and are in an amorphous form (for example, a glassy structure). I take the. Therefore, in order to produce a positive electrode active material in which amorphous (for example, glassy structure) aluminum or yttrium is present around olivine-type lithium iron phosphate, a partially substituted iron oxide is used. Thus, a positive electrode active material may be manufactured.

なお、オリビン型リン酸鉄リチウムの周囲にアモルファス状（好ましくはガラス質構造）のアルミニウムやイットリウムが存在する場合には、特にオリビン型リン酸鉄リチウム微粒子の粒子径が小さいときに、その凝集を良好に防止できるが、鉄酸化物の一部置換体中にアルミニウムやイットリウムが多量に存在する場合には、オリビン型リン酸鉄リチウム合成時に不純物として析出することがある。そのため、鉄酸化物の一部置換体においては、アルミニウムまたはイットリウムで置換しているサイトは、全Ｆｅサイトの１０％以下であることが好ましい。他方、鉄酸化物の一部置換体を使用することによるオリビン型リン酸鉄リチウム微粒子の二次凝集防止作用をより有効に発揮させる観点からは、鉄酸化物の一部置換体においてアルミニウムまたはイットリウムで置換しているサイトは、全Ｆｅサイトの３％以上であることが好ましい。   When amorphous (preferably glassy) aluminum or yttrium is present around the olivine-type lithium iron phosphate, the agglomeration occurs particularly when the olivine-type lithium iron phosphate fine particles are small. Although it can be prevented well, when a large amount of aluminum or yttrium is present in the partially substituted iron oxide, it may be precipitated as an impurity during the synthesis of the olivine-type lithium iron phosphate. Therefore, in the partially substituted body of iron oxide, the sites substituted with aluminum or yttrium are preferably 10% or less of the total Fe sites. On the other hand, from the viewpoint of more effectively exerting the secondary aggregation preventing action of the olivine-type lithium iron phosphate fine particles by using a partially substituted iron oxide, aluminum or yttrium in the partially substituted iron oxide is used. It is preferable that the sites substituted with are 3% or more of all Fe sites.

鉄酸化物微粒子の作製方法は、結晶性が高く、平均粒子径が５０ｎｍ以下の微粒子状の鉄酸化物を合成可能な方法であれば、いずれの合成方法でもかまわないが、微粒子を得るにあたっては、水熱合成法が最も好ましい。Ｆｅ３Ｏ４およびＦｅ２Ｏ３については、５０ｎｍ以下の微粒子を作製する既存の方法が多く存在し、そのいずれの方法を用いてもよい。上記の鉄酸化物の中でもＦｅＯＯＨは、反応性が高く最終的にオリビン構造のリン酸鉄リチウム微粒子を得やすいが、代わりに、ＦｅＯＯＨそのものの成長速度が非常に速くて針状に成長しやすく、巨大な針状粒子となる場合が多いため、微粒子を得ることが比較的難しい。そのような事情から、微粒子状のＦｅＯＯＨの合成方法について、以下に詳細に示す。 The method for producing the iron oxide fine particles may be any synthetic method as long as the method is capable of synthesizing fine iron oxide particles having high crystallinity and an average particle diameter of 50 nm or less. The hydrothermal synthesis method is most preferable. For Fe 3 O 4 and Fe 2 O 3 , there are many existing methods for producing fine particles of 50 nm or less, and any of these methods may be used. Among the above iron oxides, FeOOH is highly reactive and easily obtains lithium iron phosphate fine particles having an olivine structure. Instead, the growth rate of FeOOH itself is very fast and easily grows in a needle shape. It is relatively difficult to obtain fine particles because they are often large acicular particles. Under such circumstances, a method for synthesizing fine-particle FeOOH is described in detail below.

第一に、過剰の水酸化ナトリウムを溶解したアルカリ溶液を、硫酸鉄（II）水溶液中に攪拌しながら滴下し、黒緑色の沈殿を生成させる。この際、結晶成長抑制効果のあるものとしてアルミニウムを添加してもよく、その場合には水酸化ナトリウム溶液中にアルミン酸ナトリウムなどのアルミニウム化合物を溶解させて、同様に滴下すればよい。次いで、空気を巻き込むように強攪拌、または空気をバブリングしながら攪拌し、沈殿中のＦｅ２価イオンが全て酸化されて３価になるまで、約２〜３時間の間攪拌を続ける。Ｆｅが完全に３価になると、沈殿の色はオレンジ色に変化する。この際、原料の硫酸鉄が、保存方法が悪いなどの原因で酸化されて一部３価の鉄が混合していると、溶液中で２価から３価に変化する工程を経ることがないため、その後も別の反応経路を辿ることとなり、最終生成物であるＦｅＯＯＨ微粒子に、粒子径の粗大なＦｅ２Ｏ３粒子が混合することとなり好ましくない。 First, an alkaline solution in which excess sodium hydroxide is dissolved is dropped into an aqueous iron (II) sulfate solution with stirring to produce a black-green precipitate. At this time, aluminum may be added as having an effect of suppressing crystal growth. In that case, an aluminum compound such as sodium aluminate may be dissolved in a sodium hydroxide solution and dropped in the same manner. Next, the mixture is stirred vigorously so as to entrain air, or while bubbling air, and the stirring is continued for about 2 to 3 hours until all Fe divalent ions in the precipitate are oxidized and become trivalent. When Fe becomes completely trivalent, the color of the precipitate changes to orange. At this time, if the raw iron sulfate is oxidized due to a poor preservation method or the like and trivalent iron is mixed in part, it does not go through a step of changing from divalent to trivalent in the solution. Therefore, after that, another reaction route is followed, and Fe 2 O 3 particles having a large particle diameter are mixed with FeOOH fine particles as the final product, which is not preferable.

第二に、得られたオレンジ色の沈殿物を１５〜２０時間の間室温で静置し、１３０〜１８０℃の温度で水熱処理を施すことにより、ＦｅＯＯＨ微粒子を得る。この際、室温静置の時間が短すぎれば、最終生成物であるＦｅＯＯＨ微粒子の結晶成長が不十分となり、粒子径分布の悪い粗悪なＦｅＯＯＨ微粒子となるため好ましくない。また、室温静置の時間が長すぎれば、ＦｅＯＯＨ特有の針状成長が始まり、粒子径の大きな針状粒子となってしまうため、好ましくない。水熱処理の温度は１３０℃より低くても圧力のかかる温度であれば構わないが、ＦｅＯＯＨ粒子の結晶性をよくするためにも、１３０℃以上とすることが好ましい。また、１８０℃以上であってもＦｅＯＯＨ微粒子を得ることは可能であるが、粒子サイズが大きくなってしまったり、処理温度が高くなれば圧力も高くなるため、使用する装置に制限が加わることなどを考慮して、１８０℃以下とすることが好ましい。水熱処理後の懸濁液は、よく洗浄した後ろ過・乾燥して、ＦｅＯＯＨ微粒子を得る。   Secondly, the obtained orange precipitate is allowed to stand at room temperature for 15 to 20 hours and subjected to hydrothermal treatment at a temperature of 130 to 180 ° C. to obtain FeOOH fine particles. At this time, if the standing time at room temperature is too short, the crystal growth of the final product FeOOH fine particles becomes insufficient, resulting in poor FeOOH fine particles having a poor particle size distribution. Further, if the standing time at room temperature is too long, needle-like growth unique to FeOOH starts and needle-like particles having a large particle diameter are formed, which is not preferable. The hydrothermal treatment temperature may be lower than 130 ° C. as long as the pressure is applied, but it is preferably 130 ° C. or higher in order to improve the crystallinity of the FeOOH particles. Moreover, although it is possible to obtain FeOOH fine particles even at 180 ° C. or higher, since the particle size becomes large or the pressure increases as the processing temperature increases, the apparatus to be used is restricted. In view of the above, it is preferable to set the temperature to 180 ° C. or lower. The suspension after hydrothermal treatment is thoroughly washed and then filtered and dried to obtain FeOOH fine particles.

上記のような方法により平均粒子径が５０ｎｍ以下の鉄酸化物微粒子が得られるが、本発明法で使用する鉄酸化物微粒子の平均粒子径の下限は、例えば、５ｎｍであることが好ましい。なお、本明細書でいう鉄酸化物微粒子の平均粒子径は、正極活物質に係るオリビン型リン酸鉄リチウム微粒子の平均粒子径と同じ測定方法により求められる値である。   Although iron oxide fine particles having an average particle diameter of 50 nm or less can be obtained by the method as described above, the lower limit of the average particle diameter of the iron oxide fine particles used in the method of the present invention is preferably 5 nm, for example. In addition, the average particle diameter of the iron oxide fine particles referred to in this specification is a value obtained by the same measurement method as the average particle diameter of the olivine-type lithium iron phosphate fine particles related to the positive electrode active material.

＜リン酸鉄リチウム微粒子の合成＞
次に、いわゆる含浸法により、リン酸鉄リチウム微粒子を合成する。まず、多数の鉄酸化物微粒子中に、リチウム源およびリン酸源であるリチウムイオンとリン酸イオンとを含む溶液を含浸させ、鉄酸化物微粒子と上記溶液とを混合する。上記溶液におけるリチウムイオン源およびリン酸イオン源としては、当量を溶解させた場合に沈殿を作らない組み合わせであれば特に限定されるものではなく、例えば炭酸リチウムとリン酸、水酸化リチウムとリン酸、などの組み合わせで溶解させることができる。リチウムイオン源としては、上記の炭酸リチウム、水酸化リチウムの他、硝酸リチウムなど、リン酸を溶解させた酸性水に溶解するリチウム塩であればよい。またリン酸イオン源としては、他種の金属元素が含有されていないリン酸（Ｈ３ＰＯ４）を用いることが最も好ましい。リチウムイオンとリン酸イオンとを含む溶液における溶媒には水を用いることが、最も低コストかつ簡便であり好ましいが、エタノールなどの親水性有機溶媒については、分布を整える効果もあるため、水に加えて溶媒として使用しても構わない。 <Synthesis of lithium iron phosphate fine particles>
Next, lithium iron phosphate fine particles are synthesized by a so-called impregnation method. First, a large number of iron oxide fine particles are impregnated with a solution containing lithium ions and phosphate ions, which are a lithium source and a phosphoric acid source, and the iron oxide fine particles and the above solution are mixed. The lithium ion source and phosphate ion source in the above solution are not particularly limited as long as they are a combination that does not form a precipitate when the equivalent is dissolved. For example, lithium carbonate and phosphoric acid, lithium hydroxide and phosphoric acid , Etc. can be dissolved in combination. The lithium ion source may be any lithium salt that dissolves in acidic water in which phosphoric acid is dissolved, such as lithium nitrate, in addition to the above lithium carbonate and lithium hydroxide. As the phosphate ion source, it is most preferable to use phosphoric acid (H 3 PO 4 ) that does not contain other kinds of metal elements. It is preferable to use water as the solvent in the solution containing lithium ions and phosphate ions because it is the lowest cost, simple and preferable. However, since hydrophilic organic solvents such as ethanol also have an effect of adjusting the distribution, In addition, it may be used as a solvent.

なお、鉄酸化物微粒子と上記溶液との混合処理の後水熱処理を行う場合には、上記溶液におけるリチウムイオンおよびリン酸イオンの濃度は、モル基準で、リチウムとリン酸が当量であればよく、鉄に対して過剰であっても構わない。また、鉄酸化物微粒子と上記溶液との混合処理の後水熱処理を行わずに乾燥加熱処理を行う場合には、リチウムイオン：リン酸イオン：鉄＝１：１：１（モル比）となるように精密に秤量する。以上の組成により、リチウムイオンおよびリン酸イオンを含む溶液を調製し、溶液中に鉄酸化物微粒子を分散させることで両者を混合する。分散させる方法としては、超音波やモータによる攪拌など、鉄酸化物微粒子を良好に分散させ得るものであればいずれの方法を用いても構わない。   In addition, when performing hydrothermal treatment after mixing treatment of iron oxide fine particles and the above solution, the concentration of lithium ions and phosphate ions in the above solution should be equivalent to that of lithium and phosphoric acid on a molar basis. It may be excessive with respect to iron. Further, when the dry heat treatment is performed without performing the hydrothermal treatment after the mixing treatment of the iron oxide fine particles and the above solution, lithium ion: phosphate ion: iron = 1: 1: 1 (molar ratio). Weigh accurately. With the above composition, a solution containing lithium ions and phosphate ions is prepared, and both are mixed by dispersing iron oxide fine particles in the solution. As a dispersion method, any method may be used as long as it can disperse the iron oxide fine particles satisfactorily, such as ultrasonic wave or stirring by a motor.

鉄酸化物微粒子と上記溶液との混合処理の後、得られた混合物に加熱処理を施して、オリビン型リン酸鉄リチウム微粒子を合成し、本発明の正極活物質とする。加熱処理方法としては、リチウム源およびリン酸源の水溶液（リチウムイオンとリン酸鉄イオンとを含む溶液）に係る水が存在する状態で、加圧下で加熱する水熱処理を行う方法；上記水熱処理の後に、更に不活性雰囲気または還元雰囲気中で乾燥加熱処理する方法；上記水熱処理を経ずに、リチウム源およびリン酸源を含浸させた鉄酸化物微粒子から水を乾燥除去し、不活性雰囲気または還元雰囲気中で乾燥加熱処理する方法；が挙げられる。   After the mixing treatment of the iron oxide fine particles and the above solution, the obtained mixture is subjected to a heat treatment to synthesize olivine-type lithium iron phosphate fine particles to obtain the positive electrode active material of the present invention. As the heat treatment method, a hydrothermal treatment in which water is applied in an aqueous state of a lithium source and a phosphate source solution (a solution containing lithium ions and iron phosphate ions) and heated under pressure; the hydrothermal treatment described above Thereafter, a dry heat treatment in an inert atmosphere or a reducing atmosphere; without removing the water from the iron oxide fine particles impregnated with the lithium source and the phosphoric acid source without performing the hydrothermal treatment, an inert atmosphere is obtained. Or a method of drying and heating in a reducing atmosphere.

加熱処理方法として水熱処理のみを行うか、水熱処理を施した後に乾燥加熱処理を施す場合には、鉄酸化物微粒子とリチウム源およびリン酸源を含む溶液との混合物を耐圧容器などに入れて１５０〜２００℃の温度で水熱処理を行い、リン酸鉄リチウム微粒子を得る。   In the case where only hydrothermal treatment is performed as a heat treatment method or dry heat treatment is performed after hydrothermal treatment, a mixture of iron oxide fine particles and a solution containing a lithium source and a phosphoric acid source is put in a pressure vessel or the like. Hydrothermal treatment is performed at a temperature of 150 to 200 ° C. to obtain lithium iron phosphate fine particles.

加熱処理方法として水熱処理を施さない方法を採用する場合には、鉄酸化物微粒子とリチウム源およびリン酸源を含む溶液との混合物から、６０〜１００℃の温度で溶媒を蒸散させ、完全に乾燥させて、リン酸鉄リチウム微粒子の前駆体を得る。   When employing a method that does not perform hydrothermal treatment as the heat treatment method, the solvent is evaporated from the mixture of the iron oxide fine particles and the solution containing the lithium source and the phosphate source at a temperature of 60 to 100 ° C. Drying to obtain a precursor of lithium iron phosphate fine particles.

乾燥加熱処理は、上記の水熱処理により得られたリン酸鉄リチウム微粒子、または、上記の乾燥により得られたリン酸鉄リチウム微粒子の前駆体に対して行う。加熱処理としては、不活性雰囲気または還元雰囲気中で、５００〜７００℃の温度で熱処理を行うことが好ましい。リン酸鉄リチウム微粒子を正極活物質として用いる際には、導電助剤となるカーボン粉末を混合する場合があるが、その際にはカーボン粉末自体の持つ還元性を利用できるため、不活性雰囲気で加熱処理することが好ましい。カーボンの混合量によっては還元雰囲気での加熱処理が好ましい場合もあるが、リン酸鉄リチウムの前駆体は還元が進みすぎると分解が始まり、リン酸鉄リチウムが得られないことがあるため、雰囲気を調整する必要がある。また、カーボン粉末を混合せずに乾燥加熱処理を行う場合には、還元雰囲気中での加熱処理が好ましい。この際には、不活性ガス中での加熱処理では３価のＦｅを含むリチウム、鉄、リン酸の化合物が生成し、オリビン構造にならないため好ましくない。加熱処理の温度は、５００℃以下であっても約４００℃以上であればオリビン構造を形成することは可能であるが、結晶が未発達となったり未反応成分などが残留したりすることがあり、好ましくない。また７００℃以上の高温では、結晶性の高いオリビン構造が得られるものの、得られたリン酸鉄リチウム粒子の粒子径が大きくなってしまい、微粒子化できないため、好ましくない。   The dry heat treatment is performed on the lithium iron phosphate fine particles obtained by the hydrothermal treatment or the precursor of the lithium iron phosphate fine particles obtained by the drying. As the heat treatment, heat treatment is preferably performed at a temperature of 500 to 700 ° C. in an inert atmosphere or a reducing atmosphere. When lithium iron phosphate fine particles are used as the positive electrode active material, carbon powder serving as a conductive auxiliary agent may be mixed. In this case, since the reducing properties of the carbon powder itself can be used, Heat treatment is preferable. Depending on the amount of carbon, heat treatment in a reducing atmosphere may be preferable, but the precursor of lithium iron phosphate starts to decompose if the reduction proceeds too much, and lithium iron phosphate may not be obtained. Need to be adjusted. Moreover, when performing dry heat processing without mixing carbon powder, the heat processing in a reducing atmosphere is preferable. In this case, a heat treatment in an inert gas is not preferable because a compound of lithium, iron, and phosphoric acid containing trivalent Fe is generated and does not have an olivine structure. Even if the temperature of the heat treatment is 500 ° C. or lower, an olivine structure can be formed if it is about 400 ° C. or higher, but crystals may not be developed or unreacted components may remain. Yes, not preferred. High temperatures of 700 ° C. or higher are not preferable because an olivine structure with high crystallinity can be obtained, but the obtained lithium iron phosphate particles have a large particle size and cannot be made into fine particles.

なお、鉄酸化物微粒子として、アルミニウムやイットリウムなどを含有する一部置換体の微粒子を用いた場合には、詳細にどのような機構かは不明であるが、上記加熱処理の段階で、アルミニウムまたはイットリウムなどがリン酸鉄リチウム周辺にガラス質を形成すると考えられ、最終的にガラス質構造のアルミニウムまたはイットリウムなどが周囲に存在するオリビン型リン酸鉄リチウム微粒子が得られる。   In addition, when the partially substituted fine particles containing aluminum, yttrium, or the like are used as the iron oxide fine particles, it is unclear in detail what the mechanism is, but at the stage of the heat treatment, aluminum or It is considered that yttrium and the like form glassy around lithium iron phosphate, and finally, olivine-type lithium iron phosphate fine particles in which aluminum or yttrium having a vitreous structure is present are obtained.

以上のように、あらかじめ結晶成長の核となる結晶性の良好な鉄酸化物の微粒子を作製し、この微粒子をリチウムイオンおよびリン酸イオンを含む溶液中に分散させ、その後に加熱処理を施すことによって、平均粒子径が５０ｎｍ以下というリン酸鉄リチウム微粒子を有する本発明の正極活物質を製造することが可能となる。   As described above, iron oxide fine particles with good crystallinity that are the core of crystal growth are prepared in advance, and the fine particles are dispersed in a solution containing lithium ions and phosphate ions, and then heat-treated. Thus, the positive electrode active material of the present invention having lithium iron phosphate fine particles having an average particle diameter of 50 nm or less can be produced.

このようにして得られた本発明の正極活物質を用い、常法に従って電極とし、更に該電極を正極に用いてリチウムイオン二次電池を構成することができる。本発明の正極活物質は微粒子状のオリビン型リン酸鉄リチウム、すなわち比表面積の大きなオリビン型リン酸鉄リチウムを有しており、これにより、正極の導電性を高めることが期待できることから、本発明の正極活物質を用いて構成されるリチウムイオン二次電池は、優れた特性を有するものとなる。また、本発明の正極活物質は、従来公知のオリビン型リン酸鉄リチウムと同様に、原料が安価であり、しかも安全性の高いものである。   The thus obtained positive electrode active material of the present invention can be used as an electrode according to a conventional method, and further, the electrode can be used as a positive electrode to constitute a lithium ion secondary battery. The positive electrode active material of the present invention has fine olivine-type lithium iron phosphate, that is, olivine-type lithium iron phosphate with a large specific surface area, which can be expected to increase the conductivity of the positive electrode. The lithium ion secondary battery constructed using the positive electrode active material of the invention has excellent characteristics. In addition, the positive electrode active material of the present invention is inexpensive and high in safety, as in the case of the conventionally known olivine type lithium iron phosphate.

以下、実施例に基づいて本発明を詳細に述べる。ただし、下記実施例は本発明を制限するものではなく、前・後記の趣旨を逸脱しない範囲で変更実施をすることは、全て本発明の技術的範囲に包含される。   Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

実施例１
まず、成長核となる水酸化酸化鉄（ＦｅＯＯＨ）微粒子を作製した。硫酸鉄（II）七水和物２２．２３ｇを水４００ｍｌに溶解し、これとは別に１０質量％濃度の水酸化ナトリウム水溶液を調製し、硫酸鉄水溶液に攪拌しながら滴下した。ｐＨ８になるまで滴下を続け、溶液中の鉄２価イオンが全て鉄３価イオンになり、沈殿物の色が黄色になるまで約３時間の間、強攪拌した。その後室温で１９時間静置し、１６０℃で水熱処理を施した後、水洗、ろ過、乾燥し、１０〜２０ｎｍサイズ（平均粒子径１４ｎｍ）のＦｅＯＯＨ微粒子を得た。 Example 1
First, iron hydroxide oxide (FeOOH) fine particles serving as growth nuclei were prepared. Separately, 22.23 g of iron (II) sulfate heptahydrate was dissolved in 400 ml of water. Aside from this, a 10% strength by weight aqueous sodium hydroxide solution was prepared and added dropwise to the aqueous iron sulfate solution with stirring. The dropwise addition was continued until pH 8 was reached, and the mixture was vigorously stirred for about 3 hours until all the iron divalent ions in the solution became iron trivalent ions and the color of the precipitate became yellow. Thereafter, the mixture was allowed to stand at room temperature for 19 hours, hydrothermally treated at 160 ° C., washed with water, filtered, and dried to obtain FeOOH fine particles having a size of 10 to 20 nm (average particle diameter of 14 nm).

次に、水酸化リチウム一水和物０．５３ｇおよび８５％リン酸溶液１．４６ｇを水５０ｍｌに溶解して、リチウムイオンとリン酸イオンとを含む水溶液を調製した。このリチウムイオンとリン酸イオンとを含む水溶液に、先に作製したＦｅＯＯＨ微粒子１．１１ｇを加え、超音波で分散させた後、攪拌し、リチウムイオンとリン酸イオンとを含む水溶液中にＦｅＯＯＨ微粒子をよく分散させた。その後９０℃で乾燥させ、リン酸鉄リチウムの前駆体微粒子を得た。   Next, 0.53 g of lithium hydroxide monohydrate and 1.46 g of 85% phosphoric acid solution were dissolved in 50 ml of water to prepare an aqueous solution containing lithium ions and phosphate ions. To this aqueous solution containing lithium ions and phosphate ions, 1.11 g of the previously prepared FeOOH fine particles are added, dispersed with ultrasonic waves, and then stirred, and FeOOH fine particles are added to the aqueous solution containing lithium ions and phosphate ions. Was well dispersed. Thereafter, drying was performed at 90 ° C. to obtain precursor fine particles of lithium iron phosphate.

次に、上記のリン酸鉄リチウムの前駆体微粒子について、水素１０％−窒素９０％雰囲気中５００℃で２時間の加熱処理を行い、オリビン型リン酸鉄リチウムを合成して、正極活物質を得た。   Next, the precursor fine particles of lithium iron phosphate are subjected to a heat treatment at 500 ° C. for 2 hours in a hydrogen 10% -nitrogen 90% atmosphere to synthesize olivine-type lithium iron phosphate to obtain a positive electrode active material. Obtained.

このようにして得られた正極活物質について、粉末Ｘ線回折スペクトル測定を行い、図１に示すようにオリビン構造の明確なピークが現れていることが確認された。この際、回折ピークの半値幅から求めた正極活物質中のリン酸鉄リチウムの平均結晶子サイズは１８．１ｎｍであった。また、正極活物質のＴＥＭ写真を図２に示す。このＴＥＭ観察により、リン酸鉄リチウム微粒子は、全粒子の粒子径が５〜３０ｎｍの範囲にあり、二次凝集体を形成しておらず、一次粒子の状態で存在することが確認された。このＴＥＭ写真から求めたリン酸鉄リチウム微粒子の平均粒子径は、１９．２ｎｍであった。   The positive electrode active material thus obtained was subjected to powder X-ray diffraction spectrum measurement, and it was confirmed that a clear peak of the olivine structure appeared as shown in FIG. At this time, the average crystallite size of lithium iron phosphate in the positive electrode active material determined from the half width of the diffraction peak was 18.1 nm. A TEM photograph of the positive electrode active material is shown in FIG. From this TEM observation, it was confirmed that the lithium iron phosphate fine particles had a particle diameter of 5 to 30 nm in all particles, did not form secondary aggregates, and existed as primary particles. The average particle diameter of the lithium iron phosphate fine particles determined from this TEM photograph was 19.2 nm.

実施例２
リチウムイオンとリン酸イオンとを含む水溶液を、水酸化リチウム一水和物１．０６ｇおよび８５％リン酸溶液２．９２ｇを水５０ｍｌに溶解して調製したものに変更した以外は、実施例１と同様にして、上記水溶液中にＦｅＯＯＨ微粒子をよく分散させた。その後、上記水溶液とＦｅＯＯＨ微粒子との分散物を耐熱容器に入れ、１８０℃で５時間の水熱処理を施し、得られた懸濁液を水洗、ろ過、乾燥して、リン酸鉄リチウムの前駆体微粒子を得た。 Example 2
Example 1 except that the aqueous solution containing lithium ions and phosphate ions was changed to one prepared by dissolving 1.06 g of lithium hydroxide monohydrate and 2.92 g of 85% phosphoric acid solution in 50 ml of water. Similarly, the FeOOH fine particles were well dispersed in the aqueous solution. Thereafter, the dispersion of the above aqueous solution and FeOOH fine particles is put in a heat-resistant container, subjected to hydrothermal treatment at 180 ° C. for 5 hours, and the resulting suspension is washed with water, filtered and dried to obtain a precursor of lithium iron phosphate. Fine particles were obtained.

次に、上記のリン酸鉄リチウムの前駆体微粒子について、水素１０％−窒素９０％雰囲気中６００℃で２時間の加熱処理を行い、オリビン型リン酸鉄リチウムを合成して、正極活物質を得た。   Next, the precursor fine particles of lithium iron phosphate are subjected to a heat treatment at 600 ° C. for 2 hours in a 10% hydrogen-90% nitrogen atmosphere to synthesize olivine-type lithium iron phosphate to obtain a positive electrode active material. Obtained.

このようにして得られた正極活物質について、粉末Ｘ線回折スペクトル測定を行った結果、実施例１と同様にオリビン構造の明確なピークが現れ、回折ピークの半値幅から求めた平均結晶子サイズは４２．９ｎｍであった。また、ＴＥＭ観察を行った結果、リン酸鉄リチウム微粒子は、全粒子の粒子径が３０〜５０ｎｍの範囲にあり、実施例１と同様に一次粒子の状態で存在することが確認された。なお、ＴＥＭ写真から求めたリン酸鉄リチウム微粒子の平均粒子径は４５．５ｎｍであった。   As a result of measuring the powder X-ray diffraction spectrum of the positive electrode active material thus obtained, a clear peak of the olivine structure appeared as in Example 1, and the average crystallite size obtained from the half-value width of the diffraction peak Was 42.9 nm. Further, as a result of TEM observation, it was confirmed that the lithium iron phosphate fine particles had a particle diameter of 30 to 50 nm in all particles, and were present in the state of primary particles as in Example 1. The average particle diameter of the lithium iron phosphate fine particles determined from the TEM photograph was 45.5 nm.

実施例３
リチウムイオンとリン酸のイオンとを含む水溶液に、ＦｅＯＯＨ微粒子１．１１ｇと共にカーボン粉末１．９ｇを分散させた以外は、実施例１と同様にして、リン酸鉄リチウムの前駆体微粒子を得た。 Example 3
Precursor fine particles of lithium iron phosphate were obtained in the same manner as in Example 1 except that 1.11 g of FeOOH fine particles and 1.9 g of carbon powder were dispersed in an aqueous solution containing lithium ions and phosphoric acid ions. .

次に、上記のリン酸鉄リチウムの前駆体微粒子について、窒素雰囲気中５００℃で２時間の加熱処理を行い、オリビン型リン酸鉄リチウムを合成して、正極活物質を得た。   Next, the precursor fine particles of lithium iron phosphate were subjected to a heat treatment at 500 ° C. for 2 hours in a nitrogen atmosphere to synthesize olivine type lithium iron phosphate to obtain a positive electrode active material.

このようにして得られた正極活物質について、粉末Ｘ線回折スペクトル測定を行った結果、カーボンに起因するブロードなバックグラウンド強度の上にオリビン構造の明確なピークが現れ、回折ピークの半値幅から求めた平均結晶子サイズは２８．６ｎｍであった。また、ＴＥＭ観察を行った結果、カーボン粒子とリン酸鉄リチウム微粒子との混合物が観測され、リン酸鉄リチウム微粒子は、全粒子の粒子径が１５〜４０ｎｍの範囲にあり、実施例１と同様に一次粒子の状態で存在することが確認された。なお、ＴＥＭ写真から求めたリン酸鉄リチウム微粒子の平均粒子サイズは３２．５ｎｍであった。   As a result of performing powder X-ray diffraction spectrum measurement on the positive electrode active material thus obtained, a clear peak of the olivine structure appeared on the broad background intensity caused by carbon, and from the half-value width of the diffraction peak, The obtained average crystallite size was 28.6 nm. Further, as a result of TEM observation, a mixture of carbon particles and lithium iron phosphate fine particles was observed, and the lithium iron phosphate fine particles had a particle diameter in the range of 15 to 40 nm, as in Example 1. In the state of primary particles. The average particle size of the lithium iron phosphate fine particles determined from the TEM photograph was 32.5 nm.

実施例４
ＦｅＯＯＨ微粒子を１．１１ｇ用いる代わりに、Ｆｅの１０原子％をＡｌで置換した（すなわち、Ｆｅサイトの１０％をＡｌで置換した）マグネタイト（Ｆｅ３Ｏ４）粒子（平均粒子径２０ｎｍ）０．９３ｇを用いた以外は、実施例１と同様にして正極活物質を得た。 Example 4
Instead of using 1.11 g of FeOOH fine particles, magnetite (Fe 3 O 4 ) particles (average particle diameter of 20 nm) in which 10 atomic% of Fe was substituted with Al (that is, 10% of Fe sites were substituted with Al) A positive electrode active material was obtained in the same manner as in Example 1 except that 93 g was used.

このようにして得られた正極活物質について、粉末Ｘ線回折スペクトル測定を行った結果、実施例１と同様にオリビン構造の明確なピークが現れ、回折ピークの半値幅から求めた平均結晶子サイズは１２．７ｎｍであった。アルミニウムに起因する粉末Ｘ線回折ピークは現れなかったが、蛍光Ｘ線分析による組成分析の結果、アルミニウムは約８質量％存在することが確認された。また、ＴＥＭ観察を行った結果、リン酸鉄リチウム微粒子は、全粒子の粒子径が８〜２０ｎｍの範囲にあり、その大部分をガラス状構造の物質が覆い、一次粒子の状態で存在することが確認された。ＴＥＭ写真から求めたリン酸鉄リチウム微粒子の平均粒子サイズは１５．９ｎｍであった。このＴＥＭ写真を図３に示す。ガラス状構造の物質が微粒子周辺を覆っている様子が良く分かるように、敢えてその境界部分の写真を示した。写真右上半分では粒子の境界線がはっきりと見えているのに対し、写真左下半分では靄がかかったように覆われていることが分かる。   As a result of measuring the powder X-ray diffraction spectrum of the positive electrode active material thus obtained, a clear peak of the olivine structure appeared as in Example 1, and the average crystallite size obtained from the half-value width of the diffraction peak Was 12.7 nm. Although a powder X-ray diffraction peak due to aluminum did not appear, as a result of compositional analysis by fluorescent X-ray analysis, it was confirmed that about 8% by mass of aluminum was present. In addition, as a result of TEM observation, the lithium iron phosphate fine particles have a particle diameter of 8 to 20 nm in all particles, and most of them are covered with a glassy structure substance and exist in the form of primary particles. Was confirmed. The average particle size of the lithium iron phosphate fine particles determined from the TEM photograph was 15.9 nm. This TEM photograph is shown in FIG. In order to make it easier to see how the glass-like structure covers the periphery of the fine particles, a picture of the boundary is shown. It can be seen that in the upper right half of the photo, the boundary of the particles is clearly visible, whereas in the lower left half of the photo, it is covered like a wrinkle.

比較例１
硫酸鉄（II）七水和物３．４９ｇ、および８５％リン酸溶液１．４６ｇを水３０ｍｌ中に溶解し、Ｎ−メチル−２−ピロリジノン６０ｍｌを加えて、鉄およびリン酸を含む溶液を調製した。これとは別に、水酸化リチウム一水和物１．５９ｇを水５０ｍｌに溶解し、鉄およびリン酸を含む溶液中に攪拌しながら滴下し、懸濁液を調製した。得られた懸濁液に１６０℃の水熱処理を６時間施してリン酸鉄リチウムを合成し、正極活物質を得た。 Comparative Example 1
3.49 g of iron (II) sulfate heptahydrate and 1.46 g of 85% phosphoric acid solution are dissolved in 30 ml of water, 60 ml of N-methyl-2-pyrrolidinone is added, and the solution containing iron and phosphoric acid is dissolved. Prepared. Separately, 1.59 g of lithium hydroxide monohydrate was dissolved in 50 ml of water and added dropwise to a solution containing iron and phosphoric acid with stirring to prepare a suspension. The obtained suspension was subjected to hydrothermal treatment at 160 ° C. for 6 hours to synthesize lithium iron phosphate to obtain a positive electrode active material.

このようにして得られた正極活物質について、粉末Ｘ線回折スペクトル測定を行った結果、実施例１と同様にオリビン構造の明確なピークが現れ、回折ピークの半値幅から求めた平均結晶子サイズは＞１００ｎｍであった。また、ＴＥＭ観察を行った結果、リン酸鉄リチウム微粒子は、全粒子の粒子径が９０〜１５０ｎｍの範囲にあり、一次粒子の状態で存在することが確認された。また、ＴＥＭ写真から求めたリン酸鉄リチウム微粒子の平均粒子径は１２４ｎｍであった。   As a result of measuring the powder X-ray diffraction spectrum of the positive electrode active material thus obtained, a clear peak of the olivine structure appeared as in Example 1, and the average crystallite size obtained from the half-value width of the diffraction peak Was> 100 nm. As a result of TEM observation, it was confirmed that the lithium iron phosphate fine particles had a particle diameter of 90 to 150 nm in all particles and existed in a primary particle state. The average particle diameter of the lithium iron phosphate fine particles determined from the TEM photograph was 124 nm.

比較例２
硝酸鉄（II）七水和物６．９８ｇ、炭酸リチウム１．８７ｇ、およびリン酸ニ水素アンモニウム２．９１ｇを、φ１ｍｍのジルコニアビーズを用いてボールミルにより粉砕・混合し、前駆体を作製した。その後、水素１０％−窒素９０％の雰囲気中６００℃で加熱処理を行ってリン酸鉄リチウムを合成し、正極活物質を得た。 Comparative Example 2
Iron (II) nitrate heptahydrate 6.98 g, lithium carbonate 1.87 g, and ammonium dihydrogen phosphate 2.91 g were pulverized and mixed by a ball mill using zirconia beads having a diameter of 1 mm to prepare a precursor. Thereafter, heat treatment was performed at 600 ° C. in an atmosphere of 10% hydrogen and 90% nitrogen to synthesize lithium iron phosphate to obtain a positive electrode active material.

このようにして得られた正極活物質について、粉末Ｘ線回折スペクトル測定を行った結果、実施例１と同様にオリビン構造の非常に鋭い明確なピークが現れ、回折ピークの半値幅から求めた平均結晶子サイズは＞１００ｎｍであった。また、ＴＥＭ観察を行った結果、リン酸鉄リチウム微粒子の粒子径は数百ｎｍ〜約１０μｍと分布が広く、強固な二次凝集体を形成しており、平均粒子径は３．８μｍであった。   The positive electrode active material thus obtained was subjected to powder X-ray diffraction spectrum measurement. As a result, a very sharp and clear peak with an olivine structure appeared as in Example 1, and the average obtained from the half-value width of the diffraction peak The crystallite size was> 100 nm. Moreover, as a result of TEM observation, the particle diameter of the lithium iron phosphate fine particles has a wide distribution of several hundred nm to about 10 μm and forms a strong secondary aggregate, and the average particle diameter is 3.8 μm. It was.

比較例３
ＦｅＯＯＨ微粒子を用いる代わりに、平均粒子径５００ｎｍのＦｅ２Ｏ３粒子を用いた以外は、実施例１と同様にしてリン酸鉄リチウムを合成し、正極活物質を得た。 Comparative Example 3
Lithium iron phosphate was synthesized in the same manner as in Example 1 except that Fe 2 O 3 particles having an average particle diameter of 500 nm were used instead of the FeOOH fine particles to obtain a positive electrode active material.

このようにして得られたリン酸鉄リチウム粒子について、粉末Ｘ線回折スペクトル測定を行った結果、実施例１と同様にオリビン構造の明確なピークが現れ、回折ピークの半値幅から求めた平均結晶子サイズは＜１００ｎｍであった。また、ＴＥＭ観察を行った結果、リン酸鉄リチウム微粒子の粒子径は１５０〜４００ｎｍであり、粒子の形状は不定形であり、加熱による分解・分裂が起こったと見られる破片状の粒子が観測された。また、リン酸鉄リチウム微粒子の平均粒子径は３５０ｎｍであった。   The lithium iron phosphate particles thus obtained were subjected to powder X-ray diffraction spectrum measurement. As a result, a clear peak of the olivine structure appeared as in Example 1, and the average crystal determined from the half-value width of the diffraction peak The child size was <100 nm. Moreover, as a result of TEM observation, the particle size of the lithium iron phosphate fine particles was 150 to 400 nm, the shape of the particles was indeterminate, and debris-like particles that were considered to have been decomposed or split by heating were observed. It was. The average particle diameter of the lithium iron phosphate fine particles was 350 nm.

表１に、各実施例および比較例の評価結果をまとめて示す。なお、表１に示す「平均粒子径」、「粒子径の範囲」、および「平均結晶子サイズ」は、全て正極活物質に係るリン酸鉄リチウム微粒子に関するものであり、また、「粒子径の範囲」の欄には、各実施例、比較例の正極活物質のＴＥＭ観察により確認された全リン酸鉄リチウム微粒子に関する値を示している。   Table 1 summarizes the evaluation results of each example and comparative example. The “average particle size”, “particle size range”, and “average crystallite size” shown in Table 1 all relate to lithium iron phosphate fine particles related to the positive electrode active material, and “particle size In the “range” column, values relating to all lithium iron phosphate fine particles confirmed by TEM observation of the positive electrode active materials of the respective examples and comparative examples are shown.

Figure 0005216960
Figure 0005216960

表１から明らかなように、実施例１〜４の正極活物質は、オリビン型リン酸鉄リチウム微粒子の平均粒子径が５〜５０ｎｍで、全微粒子の粒子径が３〜７０ｎｍの範囲にあり、また、平均結晶子サイズが平均粒子径と同等程度の大きさの５〜５０ｎｍである。それと共に、平均粒子径が５０ｎｍ以下といった微粒子であるにも関わらず、一次粒子の状態で存在している。   As is clear from Table 1, the positive electrode active materials of Examples 1 to 4 have an average particle size of 5 to 50 nm of olivine-type lithium iron phosphate fine particles and a particle size of all fine particles in the range of 3 to 70 nm. The average crystallite size is 5 to 50 nm, which is about the same as the average particle diameter. At the same time, the particles are present in the state of primary particles although they are fine particles having an average particle diameter of 50 nm or less.

一方、比較例１のように、成長抑制剤となる有機化合物（Ｎ−メチル−２−ピロリジノン）を添加して水熱合成を行った場合には、比較的小さな粒子が得られ、結晶性も良好であるが、これ以上の微粒子を得ることができなかった。また比較例２では、最も一般的に広く行われているメカニカル・アロイによる製法を行っているが、この場合には非常に強固な二次凝集体を形成し、また一次粒子径も数μｍ程度と粗大なものになってしまうことが分かる。更に比較例３では原料として粗大な酸化鉄粒子を用いたために、最終生成物であるリン酸鉄リチウム粒子の粒子径も大きなものとなってしまうことが分かる。   On the other hand, as in Comparative Example 1, when hydrothermal synthesis was performed by adding an organic compound (N-methyl-2-pyrrolidinone) serving as a growth inhibitor, relatively small particles were obtained and the crystallinity was also high. Although it was good, no more fine particles could be obtained. In Comparative Example 2, the most commonly used method of mechanical alloying is performed. In this case, a very strong secondary aggregate is formed, and the primary particle diameter is about several μm. It turns out that it becomes coarse. Furthermore, in Comparative Example 3, since coarse iron oxide particles were used as a raw material, it can be seen that the particle diameter of the lithium iron phosphate particles as the final product would be large.

実施例１の正極活物質の粉末Ｘ線回折スペクトルを示す図である。3 is a graph showing a powder X-ray diffraction spectrum of the positive electrode active material of Example 1. FIG. 実施例１の正極活物質のＴＥＭ写真である。2 is a TEM photograph of the positive electrode active material of Example 1. 実施例４の正極活物質のＴＥＭ写真である。4 is a TEM photograph of a positive electrode active material in Example 4.

Claims (3)

少なくともオリビン型リン酸鉄リチウム微粒子を有するリチウムイオン二次電池用正極活物質であって、
二次凝集体を形成せずに単一粒子として存在しており、
上記オリビン型リン酸鉄リチウム微粒子は、平均粒子径が５〜５０ｎｍであり、全個数中９０％以上の微粒子の粒子径が３〜７０ｎｍの範囲内にあることを特徴とするリチウムイオン二次電池用正極活物質。 A positive electrode active material for a lithium ion secondary battery having at least olivine-type lithium iron phosphate fine particles,
Exist as single particles without forming secondary aggregates,
The olivine-type lithium iron phosphate fine particles have an average particle size of 5 to 50 nm, and the particle size of fine particles of 90% or more of all particles is in the range of 3 to 70 nm. Positive electrode active material. オリビン型リン酸鉄リチウム微粒子の平均結晶子サイズが５〜５０ｎｍである請求項１に記載のリチウムイオン二次電池用正極活物質。   The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the average crystallite size of the olivine-type lithium iron phosphate fine particles is 5 to 50 nm. オリビン型リン酸鉄リチウム微粒子の周囲に、ガラス質構造のアルミニウムまたはイットリウムが存在している請求項１または２に記載のリチウムイオン二次電池用正極活物質。
The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein aluminum or yttrium having a glassy structure is present around the olivine type lithium iron phosphate fine particles.
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