JP3927085B2 - Method for analyzing positive electrode active material for secondary battery - Google Patents

Description

【００３０】
上記した(1)式は部分分級効率曲線として示され、この曲線の傾きが鋭い（大きい）ほど分級精度が高いことを意味する。分級精度指数κは分級精度を定量化した値であり、部分分級効率が25％のときの粒径（Ｄ_p25（μm））と部分分級効率が75％のときの粒径（Ｄ_p75（μm））との値から、下記の(2)式により求められる値である。
κ＝Ｄ_p25／Ｄ_p75 …(2)
この場合、分級精度指数κは1より小さい値となり、1に近づくほど分級精度が高いことを意味する。
【００３１】
正極活物質（ＬｉＣｏＯ₂）を実際に分級した結果の一例（後述する実施例１の試料1の分級結果）を、表１、表２、図１、図２および図３に示す。正極活物質の分級は、気流式分級装置としてターボクラシファイアを用いて実施し、粗粉と微粉の割合が2:98（粗粉の分級比＝2％）となるように分級条件を調整した。この際、風量は分散力を高めるために装置の最大風量を使用し、分級比はロータ回転数で調整した。
【００３２】
【表１】

【００３３】
【表２】

【００３４】
表１は分級前粉体（正極活物質）、分級粗粉、分級微粉、および粗粉＋微粉の質量累積頻度（質量％）を示している。なお、(1)式の部分分級効率η（ｄ）の計算式では、原料の質量累積粒度Ｒ₀（ｄi）、Ｒ₀（ｄ i-1）を用いているが、分級時における一部粒子の損失などを考慮して、(1)式に基づいて部分分級効率η（ｄ）を計算する際には、Ｒ₀（ｄi）、Ｒ₀（ｄ i-1）は粗粉＋微粉の質量累積頻度（質量％）を便宜的に使用するものとする。表１の部分分級効率はこのようにして求めたものである。図１は表１による各累積頻度（質量％）および部分分級効率（質量％）をグラフ化したものである。なお、表２は分級前粉体（正極活物質）、分級粗粉、分級微粉、および粗粉＋微粉の粒度分布（各粒子区画の頻度（質量％）を中心粒径の質量に置き換えたときの頻度（質量％））であり、図２は各粉体の粒子サイズと頻度に基づく粒度分布グラフである。
【００３５】
この分級例の分級精度指数κを上述した(2)式に基づいて算出したところ、分級精度指数κは0.90であり、粗粉が精度よく分級されていることが分かる。分級精度が高いことは図１および図２からも明らかである。なお、分級精度指数κを算出するにあたっては、まず図３に示すように、部分分級効率（％）と粒子サイズとの関係をグラフ化し、このグラフからＤ_p25とＤ_p75を求める。この分級例のＤ_p25は17.9μm、Ｄ_p75は19.8μmである。従って、分級精度指数κはＤ_p25／Ｄ_p75＝17.9／19.8＝0.90となる。
【００３６】
なお、本発明における分級前粉体や分級粉体などの粒度分布は、粒子にレーザ光を当てたときに起こる光の散乱現象を利用した、LEEDS ＆ NORTHRUP社製のMICROTRAC II PARTICLE-SIZE ANALYZERを用いて測定した値を指すものとする。また、各粉体中の不純物金属元素の含有率は、測定試料を塩酸などにより溶解した後、ＩＣＰ分析法により測定した値を示すものとする。
【００３７】
この実施形態の二次電池用正極活物質は、上述したような条件に基づいて正極活物質の分級を行った際に、正極活物質（分級前の粉体状複合金属酸化物全体）中の不純物金属元素の含有率Ａに対して、分級により得た粗粉中の不純物金属元素の含有率Ｂの比（Ｂ／Ａ）が0.9 以上 1.4以下であることを特徴とするものである。なお、本発明の正極活物質はその製造方法に限定されるものではなく、上述した分析・評価法に基づく不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下であれば本発明の構成を満足するものである。
【００３８】
上述した分析・評価法に基づく不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下であるということは、正極活物質中の粒子状金属不純物の含有量が十分に低減されていることを意味する。従って、そのような正極活物質を用いて非水電解液二次電池を作製することによって、初期充電時の不純物金属イオンの析出に起因するマイクロショートの発生などを抑制することが可能となる。上述した条件で分析・評価した際の正極活物質の不純物含有率の比（Ｂ／Ａ比）は1.1以下であることがより好ましい。
【００３９】
分級前の正極活物質と分級により得た粗粉との間で含有率を比較する不純物金属元素には、非水電解液二次電池の動作や特性などに悪影響を及ぼす金属元素が選択される。正極活物質を構成する金属元素以外の金属元素を含む粒子状金属不純物は、二次電池を初期充電した際に金属イオンとして溶出し、この溶出した金属イオンが負極側で還元されて析出し、この析出物がセパレータを貫通して正極と接触することによって、マイクロショートなどを引き起こすことになる。従って、各種の金属元素が比較の対象となるが、特に不純物イオンとなりやすい金属元素、具体的にはＭｇ、Ｃａ、Ｂａ、Ｓｒ、Ｓｃ、Ｙ、Ｔｉ、Ｚｒ、Ｈｆ、Ｖ、Ｃｒ、Ｎｂ、Ｍｏ、Ｔａ、Ｗ、Ｍｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｃｕ、Ｚｎ、Ｇａ、Ｇｅ、Ｔｃ、Ｒｕ、Ｒｈ、Ｐｄ、Ａｇ、Ｃｄ、Ｉｎ、Ｓｎ、Ｓｂ、Ｒｅ、Ｏｓ、Ｉｒ、Ｔｌ、Ｐｂ、およびＢｉから選ばれる少なくとも1種の元素（ただし、正極活物質を構成する金属元素を除く）について比較することが好ましい。
【００４０】
なお、上記した不純物金属元素の含有率Ａ、Ｂは、それぞれ各単体元素について求め、これら各単体金属の含有率Ａ、ＢからそれぞれＢ／Ａ比を算出するものとする。すなわち、対象とする不純物金属元素について、分級前の正極活物質中の含有率Ａと粗粉中の含有率Ｂを求め、これら各含有率Ａ、ＢからＢ／Ａ比を算出し、このＢ／Ａ比が0.9 以上 1.4以下である場合に、本発明の二次電池用正極活物質を構成するものである。
【００４１】
不純物金属元素の含有率Ａ、Ｂの測定および比較は、上記した全ての金属元素に対して行い、それら全ての金属元素についてＢ／Ａ比が0.9 以上 1.4以下を満足させることが最も望ましいが、少なくとも有意に含有している元素について、上記した条件を満足させれば電池特性および製造歩留りの向上を図ることができる。さらに、簡便には粉砕機の摩耗粉などの製造工程で混入すると考えられる元素、例えばＦｅ、Ｃｒ、Ｃｕ、Ｚｎ、Ｍｇ、Ｃａが上記した条件を満足していれば、電池特性および製造歩留りの向上を図ることができる。
【００４２】
上述したように、粒子状金属不純物は二次電池の初期充電時における電圧低下不良などの原因となるため、本発明においては不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下であることを基準として、粒子状金属不純物の含有量が十分に低減されている正極活物質を提供するものである。このような正極活物質を用いて非水電解液二次電池を作製することによって、初期充電時の不純物金属イオンの析出に起因するマイクロショートの発生などを抑制することができるため、電池特性および製造歩留りの向上を図ることが可能となる。すなわち、非水電解液二次電池の製造不良や初期不良の発生が大幅に削減され、さらには電池性能の向上を図ることができる。
【００４３】
上述した不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下の二次電池用正極活物質は、例えば粗大な粒子状金属不純物の含有率が低い原料を用いると共に、製造工程で粒子状金属不純物の混入を抑制するプロセスを採用したり、あるいは最終工程で混入した粗大な不純物粒子を除去するなどによって得ることができる。ただし、前述したように本発明の正極活物質の製造方法は特に限定されるものではなく、あくまでも前述した分析・評価法に基づく不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下であれば本発明の正極活物質として用いることができるものである。
【００４４】
正極活物質の製造方法の一例を示すと、まず通常の焼成法にしたがってＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎ₂Ｏ₄などのＬｉ含有複合金属酸化物を合成する。Ｌｉ含有複合金属酸化物の合成工程は、リチウムの化合物とコバルト、ニッケル、マンガンなどの化合物とを原料として用い、これらを所定の比率で混合した後、例えば大気中にて650〜950℃の温度で焼成することにより実施される。原料としての化合物には、酸化物、炭酸塩、硫酸塩、硝酸塩、水酸化物などが用いられる。これらは金属性不純物量が少ないものを使用することが好ましい。焼成に先立って、金属不純物を除去するための精製などを実施してもよい。さらに、酸化コバルト粉末と炭酸リチウムなどの各原料粉を所定の割合で混合する前に、使用原料を100℃以上の温度で1時間以上乾燥させることも有効である。これにより原料粉の流動性が向上して、混合装置の磨耗による不純物粒子の混入を抑制することができる。
【００４５】
合成工程により得られたＬｉ含有複合金属酸化物（焼成物）は粉砕機を用いて、例えば平均粒径が0.5〜15μm、より好ましくは1〜10μmとなるように粉砕される。この際、粒子状金属性不純物の混入を防止するように、粉砕機の粒子接触部に磨耗が極めて少ない材料や磨耗しても不良を起こさない材料を使用することが好ましい。具体的には、セラミックスや樹脂などで粒子接触部をコーティングすることは有効である。全ての部品を上記したような材料でコーティングする必要はなく、粒子が高速で接触する部分や金属同士がぶつかり合う部分のみに上記したようなコーティングを施すだけでも効果がある。
【００４６】
焼成物の粉砕にあたっては、ジェットミルのような粒子同士の衝突で粉砕するような粉砕機を用いることが有効である。さらに、粉砕並びに粒子制御のための篩分けなどを行った後に、粒子状金属不純物を含む粗大粒子を取り除くために、分級や酸などによる溶解除去を実施することも有効である。この後、上述した分析・評価法に基づいて粒子状金属不純物の含有量を調べる。そして、粒子状金属不純物の分析・評価法における不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下の二次電池用正極活物質は、次いで導電剤と混合された後、さらに結着剤および溶媒が加えられてスラリー化される。このスラリーを集電体（金属箔など）上に塗布し、加熱乾燥して薄板状にした後、所定の寸法に切断することによって、非水電解液二次電池の正極が得られる。
【００４７】
上述したように、正極活物質を製造する際の代表的な工程としては、原料の混合工程、混合物の焼成工程、焼成物の粉砕工程、粉砕物の分級工程の4つが挙げられる。不純物の混入を防ぐための1つの手段として、各製造装置の粒子接触部を非金属材料で構成することが挙げられる。ただし、全ての粒子接触部を非金属製または非金属コーティングとすることは、製造装置のコスト増などを招くことから、工業的には正極活物質の製造コストの増加要因となる。また、金属が露出した部分が多少なりとも残るため、金属不純物の混入を完全に防ぐことは難しい。特に、正極活物質の代表的な製造工程においては、ステンレス（ＳＵＳ）などの金属材料を用いた製造装置が使用されており、Ｆｅなどの金属不純物が混入しやすい環境にある。
【００４８】
本発明の二次電池用正極活物質は、基本的には前述した分析・評価法に基づく不純物含有率の比（Ｂ／Ａ比）が0.9 以上 1.4以下のものを選択することにより得ることができる。ただし、積極的に不純物含有率の比（Ｂ／Ａ比）を0.9 以上 1.4以下とする上で、以下に示すような製造工程が好ましい例として挙げられる。まず第１に、前述した原料粉の乾燥工程を実施することが好ましい。乾燥温度は100℃以上であればよいが、あまり温度が高いと乾燥設備に負担がかかることから、乾燥温度は100〜300℃程度とすることが好ましい。乾燥時間も1時間以上であればよく、あまり長いと乾燥設備に負担がかかるため、乾燥時間は1〜10時間程度とすることが好ましい。
【００４９】
第２に、製造装置の粒子接触部に非金属コーティングを施すことが挙げられる。非金属コーティング材には、ガラス、窒化物、酸化物、炭化物などのセラミックス材料、ウレタン樹脂、フッ素系樹脂、エポキシ樹脂、液晶樹脂などの樹脂材料などを用いることが好ましい。特に、樹脂コーティングは好適である。正極活物質は硬度が高く、さらに粉砕工程や分級工程などといった正極活物質が激しくぶつかる工程が存在する。このため、セラミックス材料などの弾性力が低い高硬度のコーティング材では磨耗が激しく、直ぐに削り取られてしまうおそれが大きい。コーティング材が磨耗して金属部品が露出すると、そこから不純物金属が正極活物質に混入するだけでなく、削れたセラミックスコーティング材も混入して悪影響を及ぼすおそれがある。このようなことから、粒子接触部のコーティングには、セラミックスコーティングよりも弾性力のある樹脂コーティングの方が好ましい。さらに、製造工程によっては100℃以上の高温下で行われることもあるため、コーティング材は耐熱性を有していることが好ましい。
【００５０】
なお、上述した本発明の分析方法は、二次電池用正極活物質の分析・評価に効果を発揮するものであるが、必ずしも二次電池用正極活物質に限られるものではなく、各種の粉体材料（例えば二次電池用正極活物質や蛍光体粉末などを含む粉体状電子機能材料）中に含まれる粒子状金属不純物の分析・評価に適用することが可能である。
【００５１】
次に、参考例としての非水電解液二次電池について説明する。図４はリチウムイオン二次電池の構造例を一部断面で示す図である。同図において、１は例えばステンレスからなる電池容器（電池缶）である。この電池容器１の底部には、絶縁体２が配置されている。電池容器１の形状としては、例えば有底円筒状や有底角筒状などが適用される。本発明は円筒形二次電池および角型二次電池のいずれにも適用可能である。電池容器１は負極端子を兼ねている。電池容器１内には発電要素として電極群３が収納されている。
【００５２】
電極群３は、正極４、セパレータ５および負極６をこの順序で積層した帯状物を、負極６が外側に位置するように、例えば渦巻き状に巻回した構造を有している。電極群３は渦巻き型に限らず、正極４、セパレータ５および負極６をこの順序で複数積層したものであってもよい。電極群３が収納された電池容器１内には、非水電解液が充填されている。電池容器１内の電極群３の上方には、中央部が開口された絶縁紙７が載置されている。電池容器１の上方開口部には絶縁封口板８が配置されている。絶縁封口板８は、電池容器１の上端部付近を内側にかしめ加工することによって、電池容器１に対して液密に固定されている。
【００５３】
絶縁封口板８の中央部には、正極端子９が嵌合されている。正極端子９には正極リード１０の一端が安全弁１１を介して接続されている。正極リード１０の他端は、正極４に接続されている。負極６は図示しない負極リードを介して、負極端子である電池容器１に接続されている。これらによって、非水電解液二次電池としてのリチウムイオン二次電池１２が構成されている。
【００５４】
次に、電極群３を構成する正極４、セパレータ５および負極６と非水電解液について、さらに詳しく述べる。まず、正極４は本発明の二次電池用正極活物質、導電剤および結着剤を適当な溶媒に懸濁させ、この懸濁物を集電体上に塗布、乾燥して薄板状にすることにより作製される。
【００５５】
正極活物質に混合される導電剤や結着剤としては、従来から非水電解液二次電池に用いられている、種々の材料を使用することができる。導電剤としてはアセチレンブラック、カーボンブラック、黒鉛などが用いられる。結着剤としては、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶＤＦ）、エチレン−プロピレン−ジエン共重合体（ＥＰＤＭ）、スチレン−ブタジエンゴム（ＳＢＲ）などが用いられる。正極活物質、導電剤および結着剤の配合割合は、正極活物質80〜95質量％、導電剤3〜20質量％、結着剤2〜7質量％の範囲とすることが好ましい。正極活物質、導電剤および結着剤を含む懸濁物を塗布する集電体としては、例えばアルミニウム箔、ステンレス箔、ニッケル箔などが用いられる。
【００５６】
セパレータ５、負極６、非水電解液などの他の電池構成要素についても、従来から非水電解液二次電池に用いられている種々の材料や構成を適用することができる。例えば、セパレータ５としては合成樹脂製不織布、ポリエチレン製多孔質フィルム、ポリプロピレン製多孔質フィルムなどが用いられる。負極６は負極活物質と結着剤とを適当な溶媒に懸濁し、この懸濁液を集電体上に塗布、乾燥して薄板状とすることにより作製される。
【００５７】
負極活物質としては、リチウムイオンを吸蔵・放出することが可能な、熱分解炭素類、ピッチ・コークス類、グラファイト類、ガラス状炭素類、フェノール樹脂やフラン樹脂のような有機高分子化合物の焼成体、炭素繊維、活性炭などの炭素材料、あるいは金属リチウム、Ｌｉ−Ａｌ合金のようなリチウム合金、ポリアセチレンやポリピロールのようなポリマーなどが用いられる。結着剤には正極５と同様なものが用いられる。負極活物質と結着剤の配合割合は、負極活物質90〜95質量％、結着剤2〜10質量％の範囲とすることが好ましい。負極活物質および結着剤を含む懸濁物を塗布する集電体としては、例えば銅、ステンレス、ニッケルなどの箔、メッシュ、パンチドメタル、ラスメタルなどが用いられる。
【００５８】
さらに、非水電解液は非水溶媒に電解質を溶解することにより調製される。非水溶媒としては、例えばリチウムイオン二次電池の溶媒として公知の各種非水溶媒を用いることができる。非水電解液用の非水溶媒は特に限定されるものではないが、例えばプロピレンカーボネート、エチレンカーボネートなどと、ジメチルカーボネート、メチルエチルカーボネート、ジエチルカーボネート、γ−ブチロラクトン、1,2-ジメトキシエタン、1,2-ジエトキシエタン、エトキシメトキシエタンなどとの混合溶媒などが用いられる。電解質としては、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＡｓＦ₆、ＬｉＣＦ₃ＳＯ₃などのリチウム塩が例示される。このような電解質の非水溶媒に対する溶解量は0.5〜1.5mol/L（リットル）の範囲とすることが好ましい。
【００５９】
上述したようなリチウムイオン二次電池１２によれば、正極活物質中の粒子状金属不純物量が低減されていることから、初期充電時のマイクロショートの発生などを有効に抑制することができる。従って、リチウムイオン二次電池１２の製造歩留りを大幅に高めることが可能となる。さらに、粒子状金属不純物は電池性能の低下要因ともなるため、その含有量が低減されているリチウムイオン二次電池１２は優れた電池性能を発揮するものである。
【００６０】
【実施例】
次に、本発明の具体的な実施例およびその評価結果について述べる。
【００６１】
実施例１、比較例１
まず、酸化コバルト粉末と炭酸リチウムとを所定の割合で混合し、空気中にて900℃で4時間焼成した。この焼成物を適当な粉砕機を用いて粉砕した後、塊状の粗大粒子や微細粒子などを除くために篩分けを行った。このようにして、平均粒径（Ｄ50）が1〜20μmのＬｉＣｏＯ₂粉末をそれぞれ正極活物質（試料1〜3）として得た。これらの正極活物質粉末はその製造にあたって、粉砕機、成形機、混合機、分級機などの材質、これら製造に用いる装置の運転条件などを調整することで、粒子状金属不純物量の低減を図ったものである。
【００６２】
次に、上記した各正極活物質中に含まれる粒子状金属不純物量を分析・評価するために、前述した方法並びに条件にしたがって正極活物質をそれぞれ分級した。各正極活物質の分級は、粗粉の分級比がそれぞれ2％となるように、また分級精度指数κが0.9程度となるように条件を調整した。分級は前述したようにターボクラシファイアを用いて実施した。
【００６３】
次いで、各正極活物質の分級前粉体、分級粗粉および分級微粉の粒度分布を以下のようにして測定した。まず、各試料を0.5g採取し、これを100mlの水中に投入して撹拌した。さらに超音波分散を100W,3minの条件で行った後、LEEDS ＆NORTHRUP社製のMICROTRAC II PARTICLE-SIZE ANALYZER TYPE7997-10を使用して粒度分布を測定した。これらの粒度分布から各正極活物質の分級精度指数κを求めた。各試料の具体的な分級精度指数κを表３に示す。なお、実施例１の試料1の具体的な分級結果は前述したように、表１、表２、図１、図２および図３に示した通りである。
【００６４】
そして、これら各正極活物質（試料1〜3）について、分級前の正極活物質中のＦｅ含有率と分級粗粉中のＦｅ含有率を、それぞれ不純物金属元素の含有率Ａ、Ｂとして測定した。不純物金属元素の含有率は、前述した方法に基づいて測定した。これらの不純物金属元素の含有率Ａ、Ｂと、それらの値からＢ／Ａ比を求めた。各正極活物質の不純物含有率Ａ、Ｂ、およびＢ／Ａ比はそれぞれ表３に示す通りである。
【００６５】
一方、本発明との比較例１として、焼成後の粉砕条件や篩分け条件を変更する以外は、実施例１と同様にして、ＬｉＣｏＯ₂粉末からなる正極活物質（試料4〜5）を作製した。これらについても、実施例１と同様にして粒子状金属不純物量を分析・評価を行った。その結果は表３に示す通りである。
【００６６】
上述した実施例１および比較例１による各正極活物質を使用して、リチウムイオン二次電池をそれぞれ作製した。なお、リチウムイオン二次電池の作製にあたっては、分級前粉体（粗粉＋微粉）をそれぞれ正極活物質として用いた。まず、正極活物質90質量％と導電剤としてグラファイト6質量％と結着剤としてポリフッ化ビニリデン4質量％とを混合して正極合剤を調製した。この正極合剤をN-メチル-2-ピロリドンに分散させてスラリー状とし、これをアルミニウム箔に塗布、乾燥させた後、ローラープレス機で圧縮成形した。これを所定のサイズに裁断することによって、シート状の正極を得た。
【００６７】
一方、炭素材料93質量％と結合剤としてのポリフッ化ビニリデン7質量％とを混合して負極合剤を調製した。この負極合剤を用いる以外は、正極と同様にしてシート状の負極を作製した。そして、シート状の正極と微孔性ポリエチレンフィルムからなるセパレータとシート状の負極をこの順序で積層し、この積層物を負極が外側に位置するように渦巻き状に巻回することにより電極群を作製した。この電極群にリードを取り付けて有底円筒状の容器（電池缶）に収容し、さらに非水電解液を封入することにより、円筒形リチウムイオン二次電池を組立てた。なお、非水電解液はエチレンカーボネートとメチルエチルカーボネートの1:1混合溶媒に1mol/Lの濃度でＬｉＰＦ₆を溶解して調製した。
【００６８】
このようにして作製した実施例１および比較例１の各円筒形リチウムイオン二次電池の特性を以下のようにして測定、評価した。まず、組立てた電池の初充電として、20℃の環境の下で0.6Aの電流制限を設けて、4.2Vの定電圧充電を8時間行った。これを10日間室温で保存した後に電圧を測定した。この10日間放置後の電圧をそれぞれ表３に示す。
【００６９】
【表３】

【００７０】
表３から明らかなように、実施例１および比較例１による各正極活物質は、分級前粉体のＦｅ含有率がほとんど同等の値を示している。それにもかかわらず、そのような各正極活物質を用いて作製したリチウムイオン二次電池については、実施例１では10日間放置後の電圧低下が僅かであるのに対して、比較例１では大幅に電圧が低下していることが分かる。これは、実施例１の各正極活物質はいずれもＢ／Ａ比が0.9 以上 1.4以下であり、粒子状金属不純物の含有量が少ないのに対して、比較例１の各正極活物質はいずれもＢ／Ａ比が1.4を超えており、粒子状金属不純物を多く含んでいることに起因するものである。
【００７１】
このように、本発明の分析・評価法を適用することによって、通常の分析方法では検出することが困難な粒子状金属不純物を粗粉側に濃縮することができる。その結果として、分級粗粉中の不純物含有率Ｂと分級前の正極活物質中の不純物含有率Ａから求められるＢ／Ａ比によって、正極活物質全体としての粒子状金属不純物の含有量を有効に評価することができる。そして、評価結果としてのＢ／Ａ比が0.9 以上 1.4以下の正極活物質を使用することによって、初期不良を抑制したリチウムイオン二次電池を再現性よく得ることが可能となる。
【００７２】
実施例２〜６、比較例２〜６
上記した実施例１と同様にして、実施例２〜６の各試料として正極活物質（ＬｉＣｏＯ₂粉末）をそれぞれ作製した。この際、粉砕機、成形機、混合機、分級機などの材質、これら製造に用いる装置の運転条件などを調整することで、それぞれ粒子状金属不純物量の低減を図った。
【００７３】
上記した各正極活物質中に含まれる粒子状金属不純物量の分析・評価を、実施例１と同様にして行った。なお、分析・評価用の分級についても実施例１と同様な条件下で行った。分級前粉体と分級粗粉中の不純物金属元素の含有率Ａ、Ｂは、実施例２ではＣｕ含有率、実施例３ではＺｎ含有率、実施例４ではＣｒ含有率、実施例５ではＣａ含有率、実施例６ではＭｇ含有率をそれぞれ測定した。各実施例の測定結果は表４〜表８にそれぞれ示す通りである。
【００７４】
一方、本発明との比較例２〜６として、焼成後の粉砕条件や篩分け条件を変更する以外は、実施例２〜６と同様にして、ＬｉＣｏＯ₂粉末からなる正極活物質を作製した。これらについても、実施例２〜６と同様にして粒子状金属不純物量を分析・評価を行った。その結果は表４〜表８にそれぞれ示す通りである。
【００７５】
このような実施例２〜６および比較例２〜６による各正極活物質を使用して、実施例１と同様にしてリチウムイオン二次電池をそれぞれ作製した。これら各リチウムイオン二次電池を実施例１と同一条件で充電し、同様に10日間放置後の電圧を測定した。それらの測定結果を表４〜表８にそれぞれ示す。
【００７６】
【表４】

【００７７】
【表５】

【００７８】
【表６】

【００７９】
【表７】

【００８０】
【表８】

【００８１】
表４〜表８から明らかなように、実施例２〜６によるＢ／Ａ比が0.9 以上 1.4以下の各正極活物質を用いて作製したリチウムイオン二次電池は、いずれも10日間放置後の電圧低下が僅かであることが分かる。このように、不純物金属元素としてのＣｕ、Ｚｎ、Ｃｒ、Ｃａ、ＭｇなどのＢ／Ａ比が0.9 以上 1.4以下の正極活物質を用いた場合においても、初期不良を抑制したリチウムイオン二次電池を再現性よく得ることができる。
【００８２】
なお、上記した実施例１〜６では不純物金属元素としてＦｅ、Ｃｕ、Ｚｎ、Ｃｒ、Ｃａ、Ｍｇを取り上げた。これは前述したように、正極活物質の製造装置がステンレスなどの鉄合金で形成されていることが多いため、特に混入しやすい不純物金属元素として上記した元素を示したものである。他の金属元素についても、不純物金属元素の含有率の比（Ｂ／Ａ比）を0.9 以上 1.4以下にすることによって同様な効果を得ることができる。
【００８３】
実施例７、比較例７
上記した実施例１および比較例１と同様にして、実施例７および比較例７の各試料として正極活物質（ＬｉＣｏＯ₂粉末）をそれぞれ作製した。これら各正極活物質中に含まれる粒子状金属不純物量の分析・評価を、実施例１と同様にして行った。なお、分級については分級精度指数κが0.8程度となるように調整した。分級前粉体と分級粗粉中の不純物金属元素の含有率Ａ、Ｂは、実施例１と同様にＦｅ含有率を測定した。実施例７および比較例７の測定結果を表９示す。
【００８４】
このような実施例７および比較例７による各正極活物質を使用して、実施例１と同様にしてリチウムイオン二次電池をそれぞれ作製した。これら各リチウムイオン二次電池を実施例１と同一条件で充電し、同様に10日間放置後の電圧を測定した。それらの測定結果を表９に示す。
【００８５】
【表９】

【００８６】
表９から明らかなように、分級精度指数κが0.7以上となるように分級し、その結果に基づいて求めたＢ／Ａ比が0.9 以上 1.4以下の正極活物質を用いることによって、リチウムイオン二次電池の初期不良を抑制することができる。なお、実施例７および比較例７では、分級精度指数κを実施例１より若干低く設定しているため、分級粗粉中のＦｅ含有率が若干低いことが分かる。これは分級粗粉中への粒子状金属不純物の濃縮の程度が若干低下しているためである。ただし、分級精度指数κが0.7以上の分級に基づくＢ／Ａ比が0.9 以上 1.4以下の正極活物質によれば、リチウムイオン二次電池の初期不良を確実に抑制できることが分かる。
【００８７】
実施例８、比較例８
水酸化ニッケル粉末と水酸化リチウム粉末とを所定の割合で混合し、空気中にて700℃で6時間焼成した。この焼成物を適当な粉砕機を用いて粉砕した後、塊状の粗大粒子や微細粒子などを除くために篩分けを行った。このようにして、ＬｉＮｉＯ₂粉末からなる正極活物質を得た。この正極活物質の製造にあたって、粉砕機、成形機、混合機、分級機などの材質、これら製造に用いる装置の運転条件などを調整することによって、粒子状金属不純物量の低減を図った。
【００８８】
上記した正極活物質中に含まれる粒子状金属不純物量の分析・評価を、実施例１と同様にして行った。なお、分析・評価用の分級についても実施例１と同様な条件下で行った。分級前粉体と分級粗粉中の不純物金属元素の含有率Ａ、Ｂは、実施例１と同様にＦｅについて求めた。その結果は表１０に示す通りである。
【００８９】
一方、本発明との比較例８として、焼成後の粉砕条件や篩分け条件を変更する以外は、実施例８と同様にしてＬｉＮｉＯ₂粉末からなる正極活物質を作製した。この正極活物質についても、実施例８と同様にして粒子状金属不純物量を分析・評価した。その結果は表１０に示す通りである。
【００９０】
このような実施例８および比較例８による各正極活物質（ＬｉＮｉＯ₂粉末）を使用して、実施例１と同様にしてリチウムイオン二次電池をそれぞれ作製した。これら各リチウムイオン二次電池を実施例１と同一条件で充電し、同様に10日間放置後の電圧を測定した。それらの測定結果を表１０にそれぞれ示す。
【００９１】
【表１０】

【００９２】
表１０から明らかなように、ＬｉＮｉＯ₂からなる正極活物質を用いる場合においても、ＬｉＮｉＯ₂粉末のＢ／Ａ比が0.9 以上 1.4以下であれば、リチウムイオン二次電池の初期不良を抑制し得ることが分かる。
【００９３】
実施例９、比較例９
二酸化マンガン粉末と炭酸リチウムとを所定の割合で混合し、空気中にて800℃で6時間焼成した。この焼成物を適当な粉砕機を用いて粉砕した後、塊状の粗大粒子や微細粒子などを除くために篩分けを行った。このようにして、ＬｉＭｎ₂Ｏ₄粉末からなる正極活物質を得た。この正極活物質の製造にあたって、粉砕機、成形機、混合機、分級機などの材質、これら製造に用いる装置の運転条件などを調整することによって、粒子状金属不純物量の低減を図った。
【００９４】
上記した正極活物質中に含まれる粒子状金属不純物量の分析・評価を、実施例１と同様にして行った。なお、分析・評価用の分級についても実施例１と同様な条件下で行った。分級前粉体と分級粗粉中の不純物金属元素の含有率Ａ、Ｂは、実施例１と同様にＦｅについて求めた。その結果は表１１に示す通りである。
【００９５】
一方、本発明との比較例９として、焼成後の粉砕条件や篩分け条件を変更する以外は、実施例９と同様にしてＬｉＭｎ₂Ｏ₄粉末からなる正極活物質を作製した。この正極活物質についても、実施例９と同様にして粒子状金属不純物量を分析・評価した。その結果は表１１に示す通りである。
【００９６】
このような実施例９および比較例９による各正極活物質（ＬｉＭｎ₂Ｏ₄粉末）を使用して、実施例１と同様にしてリチウムイオン二次電池をそれぞれ作製した。これら各リチウムイオン二次電池を実施例１と同一条件で充電し、同様に10日間放置後の電圧を測定した。それらの測定結果を表１１にそれぞれ示す。
【００９７】
【表１１】

【００９８】
表１１から明らかなように、ＬｉＭｎ₂Ｏ₄からなる正極活物質を用いる場合においても、ＬｉＭｎ₂Ｏ₄粉末のＢ／Ａ比が0.9 以上 1.4以下であれば、リチウムイオン二次電池の初期不良を抑制し得ることが分かる。
【００９９】
なお、上記した各実施例においては、分級前の正極活物質を用いてリチウムイオン二次電池を作製したが、分級により得た粗粉（正極活物質から分級粗粉を除いたもの）を用いてリチウムイオン二次電池を作製しても、当然ながら同様な結果が得られるものである。
【０１００】
【発明の効果】
以上説明したように、本発明では電池性能や製造歩留りを低下させる要因（粒子状金属不純物など）の分析・評価法を確立し、この分析・評価法に基づいて正極活物質中の粒子状金属不純物量を評価している。従って、本発明の条件を満足する二次電池用正極活物質を用いることによって、非水電解液二次電池の製造歩留りや電池性能を再現性よく向上させることが可能となる。
【図面の簡単な説明】
【図１】 本発明の実施例１による正極活物質（試料1）の分級評価の前後における各粉体の累積頻度と部分分級効率を示す図である。
【図２】 本発明の実施例１による正極活物質（試料1）の分級評価の前後における各粉体の粒度分布を示す図である。
【図３】 本発明の実施例１による正極活物質（試料1）の分級評価における分級精度指数を求めるために作製した部分分級効率と粒子サイズとの関係を示す図である。
【図４】 本発明の一実施形態による非水電解液二次電池の構成を示す断面図である。
【符号の説明】
１……電池容器，４……正極，５……セパレータ，６……負極，１２……非水電解液二次電池[0001]
BACKGROUND OF THE INVENTION
  The present inventionTwoThe present invention relates to a method for analyzing a positive electrode active material for a secondary battery.
[0002]
[Prior art]
In recent years, portable electronic devices such as notebook personal computers, personal digital assistants (PDAs), mobile phones, and video cameras are rapidly spreading. Along with this, secondary batteries used as power sources for portable electronic devices are strongly required to be small and have a high capacity and a long cycle life. As a secondary battery satisfying such a requirement, for example, a lithium ion secondary battery using a non-aqueous electrolyte containing a lithium salt is known. For lithium ion secondary batteries, LiCoO₂, LiNiO₂, LiMn₂O_FourLi-containing transition metal composite oxides such as are used as positive electrode active materials. A carbon-based material is used for the negative electrode, and LiPF is used in a non-aqueous solvent.₆And LiBF_FourA non-aqueous electrolyte solution in which a lithium salt is dissolved is used.
[0003]
Lithium-ion secondary batteries have higher energy density than conventional Ni-Cd batteries and Ni-hydrogen secondary batteries, and are safer than secondary batteries using lithium metal. It has the feature that it is remarkably superior. For these reasons, lithium ion secondary batteries are used in large quantities as power sources for portable electronic devices.
[0004]
By the way, LiCoO₂And LiNiO₂A positive electrode using a positive electrode active material such as, for example, is produced as follows. First, a mixture of cobalt oxide or nickel hydroxide, lithium carbonate, lithium hydroxide, or the like is fired at a temperature of about 600 to 1000 ° C. in the atmosphere or oxygen to obtain a composite oxide. The massive complex oxide is pulverized to a size of about several μm to several tens of μm, and further classified by sieving as necessary. The composite metal oxide powder thus obtained is suspended in a suitable solvent together with a conductive agent and a binder to form a slurry, and this slurry is applied onto a current collector (metal foil), dried, and thinned. Thus, a positive electrode is produced (see JP-A-11-135119, JP-A-11-149925, etc.).
[0005]
[Problems to be solved by the invention]
However, the lithium ion secondary battery using the conventional positive electrode as described above has a problem that a voltage drop failure is liable to occur at the time of initial charging, which leads to a decrease in manufacturing yield and a decrease in battery performance. It is. Examination of such a phenomenon revealed that particulate metal impurities are often mixed in the positive electrode active material produced by the conventional manufacturing method, and these have caused problems. The amount of particulate metal impurities mixed in is very small, and it is an amount that does not pose a problem even if the content of impurity metal elements is analyzed as a whole of the positive electrode active material, so it was overlooked in the conventional manufacturing process and analysis method Is.
[0006]
  The present invention has been made to address such problems., ElectricAn object of the present invention is to provide a method for analyzing a positive electrode active material for a secondary battery that analyzes and evaluates factors (such as particulate metal impurities) that reduce pond performance and production yield.
[0007]
[Means for Solving the Problems]
The present inventors investigated and examined the relationship between the particulate metal impurities mixed in the positive electrode active material described above and the defective rate (especially the initial defective rate) of the secondary battery. Since the amount is extremely small when viewed as the whole positive electrode active material, no correlation can be found between the content of impurity metal elements analyzed for the whole positive electrode active material and the defective rate of the secondary battery. It has been found.
[0008]
In contrast, when the coarse powder is accurately separated from the positive electrode active material and the amount of the impurity metal element contained in the coarse powder is analyzed, the content of the impurity metal element in the coarse powder becomes a defective rate. It turned out to be closely related. In other words, the particulate metal impurities are concentrated in the coarse powder by classifying the positive electrode active material with high accuracy using the difference in particle diameter and density of the constituent particles. The inventors have found that the content of the concentrated metal impurity element (content of the impurity metal element in the coarse powder) is closely related to the defective rate of the secondary battery.
[0009]
  The present invention has been made based on such knowledge, and the method for analyzing a positive electrode active material for a secondary battery according to the present invention is a positive electrode comprising a powdered metal oxide used in a non-aqueous electrolyte secondary battery. A method for analyzing an active material, wherein the powdered metal oxide is classified using a difference in particle size and density of its constituent particles, and the classification ratio isBy mass ratioA step of obtaining coarse powder within a range of 0.1 to 5%, a content A of impurity metal elements in the powdered metal oxide before classification, and inclusion of impurity metal elements in the coarse powder obtained by classification Included in the powdered metal oxide before classification based on the step of measuring the ratio B and the ratio (B / A) of the impurity metal element content B to the impurity metal element content A The amount of particulate metal impuritiesAnd the B / A ratio is 0.9 more than 1.4 When it is in the following range, it is determined that the amount of the particulate metal impurity is a powdered metal oxide.And a process.
[0010]
As described above, for example, metal impurities are mixed in the positive electrode active material for the secondary battery, which causes a problem. In particular, particulate metal impurities having a relatively large particle size (for example, high-density particles) are likely to elute due to a high positive electrode potential when the secondary battery is initially charged. When the eluted metal ions are reduced and deposited on the negative electrode side, the precipitate breaks through the separator and causes a micro short circuit with the positive electrode side.
[0011]
Particulate metal impurities that are likely to be dissolved and precipitated in the electrolytic solution cause defects when contained in the positive electrode, even if the content is on the order of several ppm. However, in the current method for analyzing metal impurities, since most analysis errors are on the order of several ppm, impurities that are originally uniformly contained in the raw material of the positive electrode material such as Co raw material (for example, in the crystal at the atomic level) Impurities that have been incorporated into the background) are buried in the background, and particulate metal impurities cannot be detected. Conversely, even if the total content of impurity metal elements is large, defects may not occur unless coarse impurity particles (particulate metal impurities) are included.
[0012]
In the present invention, a positive electrode active material (powder metal oxide) is classified with high accuracy as a method for analyzing and evaluating the content of particulate metal impurities that are difficult to detect by ordinary analysis methods. The content ratio B of impurity metal elements (metal elements that adversely affect the operation and characteristics of the secondary battery) in the obtained coarse powder is compared with the content ratio A of impurity metal elements in the positive electrode active material before classification Has been applied. That is, the particulate metal impurities are concentrated on the coarse powder side by classifying the positive electrode active material with high accuracy. Therefore, by comparing the impurity content B in the coarse powder enriched with the particulate metal impurities with the impurity content (impurity content before classification) A as the whole positive electrode active material, particles in the positive electrode active material are obtained. The content of the metal metal impurity can be evaluated.
[0013]
  Specifically, the ratio (B / A) of the impurity content B in the coarse powder to the impurity content (impurity content before classification) A as the whole positive electrode active material is0.9 more than 1.4If it is below, the manufacturing yield and battery performance of the nonaqueous electrolyte secondary battery produced using such a positive electrode active material can be improved. That is, the content ratio B / A of the impurity metal element described above is0.9 more than 1.4The following means that the content of the particulate metal impurity in the positive electrode active material is sufficiently reduced. Therefore, by producing a non-aqueous electrolyte secondary battery using such a positive electrode active material, it is possible to suppress the occurrence of micro-shorts due to the precipitation of impurity metal ions during initial charging. As a result, it is possible to provide a non-aqueous electrolyte secondary battery having excellent battery performance and high production yield with good reproducibility.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, modes for carrying out the present invention will be described.
The positive electrode active material for a secondary battery of the present invention is used for a positive electrode of a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. As such a positive electrode active material, for example, a metal oxide such as a composite metal oxide containing lithium is used. Examples of the lithium-containing composite metal oxide include a lithium-cobalt composite oxide, a lithium-nickel composite oxide, a lithium-manganese composite oxide, and an oxide obtained by further combining or mixing these.
[0019]
Lithium-cobalt composite oxide and lithium-nickel composite oxide are basically LiAO.₂(A is at least one element selected from Co, Ni, and Mn), but the ratio of Li to Co, Ni, etc. may deviate from the stoichiometric composition, and the amount of oxygen is also It is not limited to stoichiometric composition. A part of the A element such as Co or Ni (for example, 10 atomic% or less) can be replaced with a transition metal element such as Sn, Al, V, Cr, or Fe. Lithium-manganese composite oxide is basically LiB₂O_Four(B is an element containing at least Mn selected from Mn, Co and Ni), but the ratio of Li and Mn may be slightly deviated from the stoichiometric composition, and the oxygen content is also stoichiometric. The composition is not limited. A part of the B element such as Mn (for example, 10 atomic% or less) may be substituted with a transition metal element such as Fe, Sn, Al, V, Cr, or Ni.
[0020]
That is, as the positive electrode active material,
General formula: LiA_aO_x
(In the formula, A represents at least one element selected from Co, Ni, and Mn, and a and x are numbers in the range of 0.8 ≦ a ≦ 1.1 and 1.6 ≦ x ≦ 2.4. (Atom% or less can be replaced by transition metal elements such as Sn, Al, V, Cr, Fe)
and
General formula: LiB_bO_y
(In the formula, B represents an element containing at least Mn selected from Mn, Co, and Ni, and b and y are numbers in the range of 1.5 ≦ b ≦ 2.1 and 3.6 ≦ y ≦ 4.4. (Atom% or less can be replaced with transition metal elements such as Fe, Sn, Al, V, Cr, Ni)
It is preferable to use at least one composite metal oxide selected from the group consisting of:
[0021]
  The composite metal oxide as described above is powdered and used as a positive electrode active material. In the positive electrode active material composed of powdered composite metal oxide (composite metal oxide powder), the positive electrode active material for secondary battery of this embodiment is based on the analysis and evaluation method described in detail later. When content is evaluated, the impurity content shown below is satisfied. That is, as a result of the analysis / evaluation according to the present invention, the content A of the impurity metal element in the positive electrode active material (the entire powdered composite metal oxide) before classification and a coarse ratio of 0.1 to 5% When the impurity metal element content B in the powder is compared, the ratio (B / A) of these impurity contents is0.9 more than 1.4It is a positive electrode active material comprising a powdery composite metal oxide as follows.
[0022]
A method for analyzing and evaluating particulate metal impurities in the positive electrode active material in the present invention will be described in detail. In the analysis method of the present invention, first, a positive electrode active material composed of a powdered composite metal oxide is obtained so as to obtain a coarse powder having a classification ratio in the range of 0.1 to 5%. Classify using the difference. This classification is performed only to analyze and evaluate the content of particulate metal impurities, and has been conventionally performed for adjusting the particle diameter of the positive electrode active material, that is, the positive electrode active after pulverization. This is different from classification for removing coarse powder and fine powder from a substance. The positive electrode active material is composed of the entire powdered composite metal oxide before classification. In other words, the positive electrode active material of this embodiment is constituted by the whole powder including classified coarse powder and other particles (formally called fine powder).
[0023]
In the analysis / evaluation method of the positive electrode active material, a classification method capable of classifying powder with high accuracy is applied. That is, simple classification includes sieving, but fine powder materials having an average particle size of about several μm to several tens of μm used for the positive electrode active material cannot be classified with high accuracy. Therefore, in the analysis / evaluation method of the present invention, classification is performed by utilizing the fact that the resistance force of particles differs depending on the particle size and density against physical forces such as gravity, inertial force, and centrifugal force. . Specifically, it is preferable to apply airflow classification that performs classification based on a balance between gravity, inertial force, centrifugal force, and the like and fluid resistance force.
[0024]
Airflow classification can process a large amount of powder and is industrially suitable, and classification can be performed based on density (mass) together with particle diameter. Therefore, it is preferably used for the analysis / evaluation method of particulate metal impurities in the present invention. Here, representative examples of airflow classification include classification using a balance between centrifugal force due to free vortex and fluid resistance force, classification using a balance between centrifugal force due to forced vortex and fluid resistance force, and the like. Among these, a classification device using a centrifugal force generated by a free vortex such as a cyclone has a low dispersion force, and there is a possibility that the classification accuracy cannot be sufficiently increased. In contrast, classifiers that use the balance between centrifugal force due to forced vortices and fluid resistance, such as micron separators, turboplexes, Accucuts, and turboclassifiers, have strong dispersion power. In addition, since the damage to the particles is small and the classification accuracy is excellent, it is particularly suitable as a classification device used in the analysis / evaluation method for particulate metal impurities of the present invention.
[0025]
In the analysis / evaluation method of the positive electrode active material, for example, the classification accuracy index κ is 0.7 or more by using a classification device as described above (particularly, a classification device using a balance between centrifugal force due to forced vortex and fluid resistance). The positive electrode active material is classified so that a coarse powder having a classification ratio (mass ratio) of 0.1 to 5% is obtained. By obtaining coarse powder having a classification ratio in the range of 0.1 to 5%, particulate metal impurities can be effectively and practically concentrated on the coarse powder side. By comparing the content of the metal impurity element between the coarse powder enriched with the particulate metal impurity and the positive electrode active material before classification, the content of the particulate metal impurity as the entire positive electrode active material can be accurately determined. It becomes possible to evaluate practically.
[0026]
That is, particulate metal impurities that are likely to be dissolved and precipitated in the electrolyte solution cause defects when contained in the positive electrode even if the content is on the order of several ppm. However, since the analysis error (quantitative analysis) of a normal metal impurity by the ICP method or the like has an analysis error on the order of several ppm, it is not possible to accurately detect only the amount of particulate metal impurity. Conversely, even if the total content of the impurity metal elements is large, there is a possibility that no defect will occur if no particulate metal impurities are contained. On the other hand, by classifying the positive electrode active material so as to obtain a coarse powder having a classification ratio in the range of 0.1 to 5%, it is difficult to detect particulate metal impurities in the classified coarse powder by ordinary analysis methods. Can be concentrated. Therefore, by comparing the content B of the metal impurity element in the classified coarse powder with the content A of the metal impurity element in the positive electrode active material before classification, the content of the particulate metal impurity as the whole positive electrode active material Can be evaluated.
[0027]
In the above-described classification of the positive electrode active material, the coarse metal powder having a classification ratio exceeding 5% cannot sufficiently concentrate the particulate metal impurities because the amount of fine powder mixed therein increases. Therefore, even if the content ratio B of the metal impurity element in the coarse powder and the content ratio A of the metal impurity element as the whole positive electrode active material are compared, the impurity content closely related to the defective rate of the secondary battery The ratio of the rates (B / A ratio) cannot be obtained. On the other hand, if the classification ratio is less than 0.1%, a measurement sample for obtaining a ratio (B / A ratio) of impurity content closely related to the defective rate of the secondary battery, that is, measurement of an amount necessary for composition analysis This is not preferable because a large amount of classification treatment is required to obtain the sample. In order to improve the practicality of the comparison between the impurity content B in the coarse powder and the impurity content A as the whole positive electrode active material, the classification ratio of the coarse powder is more preferably in the range of 1 to 3%.
[0028]
The classification of the positive electrode active material is preferably performed so that the classification accuracy index κ is 0.7 or more. A classification accuracy index κ of less than 0.7 means that the separation accuracy of the coarse powder is low, and the particle size distribution of the coarse powder becomes broad. Such a coarse powder cannot sufficiently concentrate the particulate metal impurities, so that the correlation between the impurity content ratio (B / A ratio) and the defective rate of the secondary battery is weakened. Therefore, even if it is a positive electrode active material which made B / A ratio in the predetermined range, there exists a possibility that the defective rate of a secondary battery cannot fully be reduced. The classification accuracy index κ is more preferably 0.8 or more. Since the classification accuracy index κ is high, the particulate metal impurities are in a highly concentrated state, so the lower the impurity content in such a highly concentrated state, the more the amount of particulate metal impurities in the positive electrode active material as a whole. Means less.
[0029]
  Here, the classification accuracy index κ is obtained as follows. That is, first, the partial classification efficiency η (d) is obtained based on the following equation (1). The partial classification efficiency η (d) is obtained by dividing the continuously changing particle diameter into several sections and indicating the recovery rate in the sections, and is obtained from the particle size distribution of each powder.
[Expression 2]

[0030]
The above equation (1) is shown as a partial classification efficiency curve, and the sharper (larger) the slope of this curve means the higher the classification accuracy. The classification accuracy index κ is a value obtained by quantifying the classification accuracy. The particle size (D) when the partial classification efficiency is 25%._p25(Μm)) and the particle size when the partial classification efficiency is 75% (D_p75(Μm)) and the value obtained from the following equation (2).
κ = D_p25/ D_p75    … (2)
In this case, the classification accuracy index κ is a value smaller than 1, meaning that the closer to 1, the higher the classification accuracy.
[0031]
Positive electrode active material (LiCoO₂Table 1 and Table 2, FIG. 1, FIG. 2 and FIG. 3 show an example of the result of actually classifying ()) (classification result of sample 1 of Example 1 described later). The positive electrode active material was classified using a turbo classifier as an airflow classifier, and the classification conditions were adjusted so that the ratio of coarse powder to fine powder was 2:98 (classification ratio of coarse powder = 2%). At this time, the maximum air volume of the device was used to increase the dispersion force, and the classification ratio was adjusted by the rotor speed.
[0032]
[Table 1]

[0033]
[Table 2]

[0034]
  Table 1 shows the cumulative mass frequency of the pre-classification powder (positive electrode active material), classification coarse powder, classification fine powder, and coarse powder + fine powder (mass%). In the calculation formula of partial classification efficiency η (d) in equation (1), the mass cumulative particle size R of the raw material₀(Di), R₀(d i-1), But when calculating the partial classification efficiency η (d) based on the equation (1) in consideration of the loss of some particles at the time of classification, R₀(Di), R₀(d i-1) Is the mass cumulative frequency of coarse powder + fine powder (mass%) Shall be used for convenience. The partial classification efficiency in Table 1 was obtained in this way. FIG. 1 shows each cumulative frequency according to Table 1 (mass%) And partial classification efficiency (mass%) Is graphed. Table 2 shows the pre-classification powder (positive electrode active material), classification coarse powder, classification fine powder, and particle size distribution of coarse powder + fine powder (each particleWhen the frequency (mass%) of the compartment is replaced with the mass of the center particle sizeFrequency (massFIG. 2 is a particle size distribution graph based on the particle size and frequency of each powder.
[0035]
When the classification accuracy index κ of this classification example is calculated based on the above-described equation (2), the classification accuracy index κ is 0.90, indicating that the coarse powder is classified with high accuracy. It is clear from FIGS. 1 and 2 that the classification accuracy is high. In calculating the classification accuracy index κ, first, as shown in FIG. 3, the relationship between the partial classification efficiency (%) and the particle size is graphed._p25And D_p75Ask for. D of this classification example_p25Is 17.9μm, D_p75Is 19.8 μm. Therefore, the classification accuracy index κ is D_p25/ D_p75= 17.9 / 19.8 = 0.90.
[0036]
In addition, the particle size distribution of the pre-classification powder and classification powder in the present invention uses MICROTRAC II PARTICLE-SIZE ANALYZER manufactured by LEEDS & NORTHRUP, which utilizes the light scattering phenomenon that occurs when laser light is applied to the particles. It shall refer to the value measured using. Moreover, the content rate of the impurity metal element in each powder shall show the value measured by the ICP analysis method, after melt | dissolving a measurement sample with hydrochloric acid etc.
[0037]
  When the positive electrode active material for secondary batteries of this embodiment is classified based on the conditions as described above, the positive electrode active material (the whole powdery composite metal oxide before classification) in the positive electrode active material Ratio (B / A) of impurity metal element content B in the coarse powder obtained by classification with respect to impurity metal element content A0.9 more than 1.4It is characterized by the following. The positive electrode active material of the present invention is not limited to the production method thereof, and the impurity content ratio (B / A ratio) based on the analysis / evaluation method described above is0.9 more than 1.4The following configuration satisfies the configuration of the present invention.
[0038]
  Impurity content ratio (B / A ratio) based on the analysis / evaluation method described above is0.9 more than 1.4The following means that the content of the particulate metal impurity in the positive electrode active material is sufficiently reduced. Therefore, by producing a non-aqueous electrolyte secondary battery using such a positive electrode active material, it is possible to suppress the occurrence of micro-shorts due to precipitation of impurity metal ions during initial charging. The impurity content ratio (B / A ratio) of the positive electrode active material when analyzed and evaluated under the above-described conditions is more preferably 1.1 or less.
[0039]
As the impurity metal element for comparing the content ratio between the positive electrode active material before classification and the coarse powder obtained by classification, a metal element that adversely affects the operation and characteristics of the nonaqueous electrolyte secondary battery is selected. . Particulate metal impurities including metal elements other than the metal elements constituting the positive electrode active material are eluted as metal ions when the secondary battery is initially charged, and the eluted metal ions are reduced and deposited on the negative electrode side, This precipitate penetrates the separator and comes into contact with the positive electrode, thereby causing a micro short circuit or the like. Therefore, various metal elements are objects of comparison, but metal elements that are particularly likely to be impurity ions, specifically, Mg, Ca, Ba, Sr, Sc, Y, Ti, Zr, Hf, V, Cr, Nb, Mo, Ta, W, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Re, Os, Ir, Tl, Pb, It is preferable to compare at least one element selected from Bi and Bi (excluding the metal element constituting the positive electrode active material).
[0040]
  The content rates A and B of the impurity metal elements described above are obtained for each single element, and the B / A ratio is calculated from the content ratios A and B of each single metal. That is, for the target impurity metal element, the content rate A in the positive electrode active material before classification and the content rate B in the coarse powder are obtained, and the B / A ratio is calculated from these content rates A and B. / A ratio is0.9 more than 1.4In the following cases, the positive electrode active material for a secondary battery of the present invention is constituted.
[0041]
  The measurement and comparison of the impurity metal element contents A and B are performed for all the metal elements described above, and the B / A ratio for all the metal elements is0.9 more than 1.4Although it is most desirable to satisfy the following, it is possible to improve battery characteristics and production yield if at least the elements contained significantly satisfy the above-mentioned conditions. Furthermore, if the elements considered to be easily mixed in the manufacturing process such as the abrasion powder of the pulverizer, such as Fe, Cr, Cu, Zn, Mg, and Ca, satisfy the above-described conditions, the battery characteristics and the manufacturing yield can be improved. Improvements can be made.
[0042]
  As described above, since the particulate metal impurities cause a voltage drop failure at the time of initial charge of the secondary battery, the ratio of impurity content (B / A ratio) is set in the present invention.0.9 more than 1.4On the basis of the following, the present invention provides a positive electrode active material in which the content of particulate metal impurities is sufficiently reduced. By producing a non-aqueous electrolyte secondary battery using such a positive electrode active material, it is possible to suppress the occurrence of micro-shorts due to the precipitation of impurity metal ions during initial charging. The production yield can be improved. That is, the production failure and initial failure of the non-aqueous electrolyte secondary battery are greatly reduced, and further the battery performance can be improved.
[0043]
  The impurity content ratio (B / A ratio) described above is0.9 more than 1.4The following positive electrode active material for a secondary battery uses, for example, a raw material with a low content of coarse particulate metal impurities, and adopts a process that suppresses mixing of particulate metal impurities in the manufacturing process, or in the final process. It can be obtained, for example, by removing the mixed coarse impurity particles. However, as described above, the method for producing the positive electrode active material of the present invention is not particularly limited, and the impurity content ratio (B / A ratio) based on the analysis / evaluation method described above is merely used.0.9 more than 1.4The following can be used as the positive electrode active material of the present invention.
[0044]
An example of a method for producing a positive electrode active material is as follows. First, according to a normal firing method, LiCoO₂, LiNiO₂, LiMn₂O_FourA Li-containing composite metal oxide such as is synthesized. The synthesis step of the Li-containing composite metal oxide uses a lithium compound and a compound such as cobalt, nickel, manganese, etc. as raw materials, and after mixing them at a predetermined ratio, for example, a temperature of 650 to 950 ° C. in the atmosphere This is performed by baking. Oxides, carbonates, sulfates, nitrates, hydroxides and the like are used as the raw material compounds. It is preferable to use those having a small amount of metallic impurities. Prior to firing, purification or the like for removing metal impurities may be performed. Furthermore, it is also effective to dry the used raw material at a temperature of 100 ° C. or higher for 1 hour or longer before mixing each raw material powder such as cobalt oxide powder and lithium carbonate at a predetermined ratio. Thereby, the fluidity | liquidity of raw material powder improves and it can suppress mixing of the impurity particle by abrasion of a mixing apparatus.
[0045]
The Li-containing composite metal oxide (baked product) obtained by the synthesis step is pulverized by using a pulverizer such that the average particle size is 0.5 to 15 μm, more preferably 1 to 10 μm. At this time, it is preferable to use a material with very little wear or a material that does not cause a defect even if it is worn at the particle contact portion of the grinder so as to prevent mixing of particulate metallic impurities. Specifically, it is effective to coat the particle contact portion with ceramics or resin. It is not necessary to coat all the parts with the material as described above, and it is effective to apply the coating as described above only to the portion where the particles contact at high speed or the portion where the metals collide with each other.
[0046]
  In crushing the fired product, it is effective to use a crusher such as a jet mill that crushes particles by collision. Furthermore, after removing the coarse particles containing the particulate metal impurities after carrying out pulverization and sieving for particle control, it is also effective to carry out dissolution removal with classification or acid. Thereafter, the content of the particulate metal impurity is examined based on the analysis / evaluation method described above. And the ratio of impurity content (B / A ratio) in the analysis / evaluation method of particulate metal impurities is0.9 more than 1.4The following positive electrode active material for a secondary battery is then mixed with a conductive agent, and further added with a binder and a solvent to form a slurry. The slurry is applied on a current collector (such as a metal foil), heated and dried to form a thin plate, and then cut into predetermined dimensions to obtain a positive electrode of a non-aqueous electrolyte secondary battery.
[0047]
As described above, there are four typical steps for producing the positive electrode active material: a raw material mixing step, a mixture firing step, a fired product grinding step, and a ground product classification step. One means for preventing the contamination of impurities is to configure the particle contact portion of each manufacturing apparatus with a non-metallic material. However, making all the particle contact portions non-metallic or non-metallic coating causes an increase in the cost of the manufacturing apparatus, and thus industrially increases the manufacturing cost of the positive electrode active material. Further, since the exposed portion of the metal remains, it is difficult to completely prevent the metal impurities from being mixed. In particular, in a typical manufacturing process of a positive electrode active material, a manufacturing apparatus using a metal material such as stainless steel (SUS) is used, and an environment in which metal impurities such as Fe are likely to be mixed is present.
[0048]
  The positive electrode active material for a secondary battery of the present invention basically has an impurity content ratio (B / A ratio) based on the analysis / evaluation method described above.0.9 more than 1.4It can be obtained by selecting: However, the ratio of impurity content (B / A ratio) is positively0.9 more than 1.4In the following, the following manufacturing steps are preferred examples. First, it is preferable to carry out the above-described raw material powder drying step. The drying temperature may be 100 ° C. or higher. However, if the temperature is too high, the drying equipment is burdened. Therefore, the drying temperature is preferably about 100 to 300 ° C. The drying time may be 1 hour or more, and if it is too long, the drying equipment is burdened. Therefore, the drying time is preferably about 1 to 10 hours.
[0049]
Secondly, a non-metallic coating is applied to the particle contact portion of the production apparatus. As the non-metallic coating material, it is preferable to use ceramic materials such as glass, nitride, oxide, and carbide, and resin materials such as urethane resin, fluorine resin, epoxy resin, and liquid crystal resin. In particular, resin coating is suitable. The positive electrode active material has high hardness, and there is a process in which the positive electrode active material collides violently, such as a pulverization process and a classification process. For this reason, a high-hardness coating material having a low elastic force such as a ceramic material is highly worn, and is likely to be scraped off immediately. When the coating material is worn out and the metal part is exposed, not only the impurity metal is mixed into the positive electrode active material from there, but also a shaved ceramic coating material may be mixed and have an adverse effect. For this reason, a resin coating having elasticity is more preferable than a ceramic coating for coating the particle contact portion. Furthermore, depending on the manufacturing process, the coating material is preferably heat resistant because it may be performed at a high temperature of 100 ° C. or higher.
[0050]
The analysis method of the present invention described above is effective for analyzing and evaluating the positive electrode active material for secondary batteries, but is not necessarily limited to the positive electrode active material for secondary batteries. The present invention can be applied to the analysis and evaluation of particulate metal impurities contained in body materials (for example, powdered electronic functional materials including positive electrode active materials for secondary batteries and phosphor powders).
[0051]
  next,As a reference exampleNon-aqueous electrolyte secondary batteryIn the pondexplain about. FIG.IsTitanium ion secondary batteryofConstructionExampleFIG. In the figure, reference numeral 1 denotes a battery container (battery can) made of, for example, stainless steel. An insulator 2 is disposed at the bottom of the battery container 1. As the shape of the battery case 1, for example, a bottomed cylindrical shape or a bottomed rectangular tube shape is applied. The present invention is applicable to both cylindrical secondary batteries and prismatic secondary batteries. The battery container 1 also serves as a negative electrode terminal. An electrode group 3 is housed in the battery container 1 as a power generation element.
[0052]
The electrode group 3 has a structure in which, for example, a spiral shape is formed by winding a strip in which the positive electrode 4, the separator 5, and the negative electrode 6 are laminated in this order so that the negative electrode 6 is located outside. The electrode group 3 is not limited to the spiral type, and a plurality of positive electrodes 4, separators 5 and negative electrodes 6 may be laminated in this order. The battery container 1 in which the electrode group 3 is accommodated is filled with a non-aqueous electrolyte. Above the electrode group 3 in the battery container 1, an insulating paper 7 having a central portion opened is placed. An insulating sealing plate 8 is disposed in the upper opening of the battery container 1. The insulating sealing plate 8 is liquid-tightly fixed to the battery container 1 by caulking the vicinity of the upper end portion of the battery container 1 inward.
[0053]
A positive electrode terminal 9 is fitted in the central portion of the insulating sealing plate 8. One end of a positive electrode lead 10 is connected to the positive electrode terminal 9 via a safety valve 11. The other end of the positive electrode lead 10 is connected to the positive electrode 4. The negative electrode 6 is connected to the battery container 1 which is a negative electrode terminal via a negative electrode lead (not shown). These constitute a lithium ion secondary battery 12 as a non-aqueous electrolyte secondary battery.
[0054]
Next, the positive electrode 4, the separator 5, the negative electrode 6, and the nonaqueous electrolytic solution constituting the electrode group 3 will be described in more detail. First, the positive electrode 4 is obtained by suspending the positive electrode active material for a secondary battery of the present invention, a conductive agent and a binder in an appropriate solvent, and applying this suspension on a current collector and drying to form a thin plate. It is produced by this.
[0055]
As the conductive agent and binder mixed with the positive electrode active material, various materials conventionally used in non-aqueous electrolyte secondary batteries can be used. As the conductive agent, acetylene black, carbon black, graphite or the like is used. As the binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), or the like is used. The mixing ratio of the positive electrode active material, the conductive agent, and the binder is preferably in the range of 80 to 95% by mass of the positive electrode active material, 3 to 20% by mass of the conductive agent, and 2 to 7% by mass of the binder. As the current collector for applying the suspension containing the positive electrode active material, the conductive agent and the binder, for example, an aluminum foil, a stainless steel foil, a nickel foil or the like is used.
[0056]
Various materials and configurations conventionally used in non-aqueous electrolyte secondary batteries can also be applied to other battery components such as the separator 5, the negative electrode 6, and the non-aqueous electrolyte. For example, as the separator 5, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like is used. The negative electrode 6 is produced by suspending a negative electrode active material and a binder in a suitable solvent, and applying this suspension on a current collector and drying to form a thin plate.
[0057]
As the negative electrode active material, firing of organic polymer compounds such as pyrolytic carbons, pitch cokes, graphites, glassy carbons, phenolic resins and furan resins capable of inserting and extracting lithium ions Body, carbon fiber, carbon material such as activated carbon, lithium alloy such as metallic lithium and Li-Al alloy, polymer such as polyacetylene and polypyrrole, and the like are used. The same binder as that of the positive electrode 5 is used. The mixing ratio of the negative electrode active material and the binder is preferably in the range of 90 to 95% by mass of the negative electrode active material and 2 to 10% by mass of the binder. As the current collector for applying the suspension containing the negative electrode active material and the binder, for example, foil such as copper, stainless steel and nickel, mesh, punched metal, lath metal and the like are used.
[0058]
Further, the nonaqueous electrolytic solution is prepared by dissolving an electrolyte in a nonaqueous solvent. As the non-aqueous solvent, for example, various known non-aqueous solvents can be used as a solvent for a lithium ion secondary battery. The non-aqueous solvent for the non-aqueous electrolyte is not particularly limited. For example, propylene carbonate, ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1 , 2-diethoxyethane, a mixed solvent with ethoxymethoxyethane, or the like is used. As an electrolyte, LiPF₆, LiBF_FourLiClO_Four, LiAsF₆, LiCF_ThreeSO_ThreeAnd lithium salts such as The amount of the electrolyte dissolved in the nonaqueous solvent is preferably in the range of 0.5 to 1.5 mol / L (liter).
[0059]
  As mentioned aboveNariAccording to the lithium ion secondary battery 12, since the amount of particulate metal impurities in the positive electrode active material is reduced, it is possible to effectively suppress the occurrence of micro-shorts during initial charging. Therefore, the production yield of the lithium ion secondary battery 12 can be significantly increased. Furthermore, since the particulate metal impurities also cause a decrease in battery performance, the lithium ion secondary battery 12 whose content is reduced exhibits excellent battery performance.
[0060]
【Example】
Next, specific examples of the present invention and evaluation results thereof will be described.
[0061]
Example 1 and Comparative Example 1
First, cobalt oxide powder and lithium carbonate were mixed at a predetermined ratio and fired at 900 ° C. for 4 hours in air. The fired product was pulverized using an appropriate pulverizer and then sieved to remove massive coarse particles and fine particles. In this way, LiCoO having an average particle size (D50) of 1 to 20 μm₂Each powder was obtained as a positive electrode active material (samples 1 to 3). In the production of these positive electrode active material powders, the amount of particulate metal impurities is reduced by adjusting the materials of the pulverizer, molding machine, mixer, classifier, etc., and the operating conditions of the equipment used for the production. It is a thing.
[0062]
Next, in order to analyze and evaluate the amount of particulate metal impurities contained in each positive electrode active material, the positive electrode active material was classified according to the method and conditions described above. The classification of each positive electrode active material was adjusted so that the classification ratio of the coarse powder was 2%, respectively, and the classification accuracy index κ was about 0.9. Classification was performed using a turbo classifier as described above.
[0063]
Subsequently, the particle size distribution of each positive electrode active material before classification, classified coarse powder, and classified fine powder was measured as follows. First, 0.5 g of each sample was collected, and the sample was put into 100 ml of water and stirred. Furthermore, after ultrasonic dispersion was performed under conditions of 100 W and 3 min, the particle size distribution was measured using a MICROTRAC II PARTICLE-SIZE ANALYZER TYPE 7997-10 manufactured by LEEDS & NORTHUP. From these particle size distributions, the classification accuracy index κ of each positive electrode active material was determined. Specific classification accuracy index κ of each sample is shown in Table 3. In addition, the specific classification result of the sample 1 of Example 1 is as having shown in Table 1, Table 2, FIG. 1, FIG. 2, and FIG. 3 as mentioned above.
[0064]
For each of these positive electrode active materials (samples 1 to 3), the Fe content in the positive electrode active material before classification and the Fe content in the classified coarse powder were measured as impurity metal element contents A and B, respectively. . The content of the impurity metal element was measured based on the method described above. B / A ratio was calculated | required from content rate A, B of these impurity metal elements, and those values. Table 3 shows the impurity contents A, B, and B / A ratio of each positive electrode active material.
[0065]
On the other hand, as Comparative Example 1 with the present invention, LiCoO was performed in the same manner as in Example 1 except that the pulverization conditions and sieving conditions after firing were changed.₂A positive electrode active material (samples 4 to 5) made of powder was prepared. Also for these, the amount of particulate metal impurities was analyzed and evaluated in the same manner as in Example 1. The results are as shown in Table 3.
[0066]
Using the positive electrode active materials according to Example 1 and Comparative Example 1 described above, lithium ion secondary batteries were produced. In the production of the lithium ion secondary battery, pre-classified powder (coarse powder + fine powder) was used as the positive electrode active material. First, 90% by mass of the positive electrode active material, 6% by mass of graphite as a conductive agent, and 4% by mass of polyvinylidene fluoride as a binder were mixed to prepare a positive electrode mixture. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to form a slurry, which was applied to an aluminum foil and dried, followed by compression molding with a roller press. By cutting this into a predetermined size, a sheet-like positive electrode was obtained.
[0067]
On the other hand, 93% by mass of the carbon material and 7% by mass of polyvinylidene fluoride as a binder were mixed to prepare a negative electrode mixture. A sheet-like negative electrode was produced in the same manner as the positive electrode except that this negative electrode mixture was used. Then, a separator composed of a sheet-like positive electrode and a microporous polyethylene film and a sheet-like negative electrode are laminated in this order, and this laminate is wound in a spiral shape so that the negative electrode is located on the outside. Produced. A lead was attached to this electrode group, accommodated in a bottomed cylindrical container (battery can), and a non-aqueous electrolyte was further sealed to assemble a cylindrical lithium ion secondary battery. The non-aqueous electrolyte is LiPF at a concentration of 1 mol / L in a 1: 1 mixed solvent of ethylene carbonate and methyl ethyl carbonate.₆Was prepared by dissolving.
[0068]
The characteristics of the cylindrical lithium ion secondary batteries of Example 1 and Comparative Example 1 produced in this way were measured and evaluated as follows. First, as a first charge of the assembled battery, a current limit of 0.6 A was provided under an environment of 20 ° C., and a constant voltage charge of 4.2 V was performed for 8 hours. This was stored for 10 days at room temperature and then the voltage was measured. Table 3 shows the voltage after standing for 10 days.
[0069]
[Table 3]

[0070]
  As is clear from Table 3, each of the positive electrode active materials according to Example 1 and Comparative Example 1 has almost the same value of the Fe content of the powder before classification. Nevertheless, with respect to the lithium ion secondary battery produced using each of the positive electrode active materials, the voltage drop after standing for 10 days in Example 1 is slight, whereas in Comparative Example 1, it is greatly reduced. It can be seen that the voltage has dropped. This is because each positive electrode active material of Example 1 has a B / A ratio.0.9 more than 1.4The positive electrode active material of Comparative Example 1 has a B / A ratio, whereas the content of the particulate metal impurities is small.1.4This is because it contains a large amount of particulate metal impurities.
[0071]
  Thus, by applying the analysis / evaluation method of the present invention, it is possible to concentrate particulate metal impurities that are difficult to detect by a normal analysis method to the coarse powder side. As a result, the content of the particulate metal impurities as the whole positive electrode active material is effectively determined by the B / A ratio obtained from the impurity content B in the classified coarse powder and the impurity content A in the positive electrode active material before classification. Can be evaluated. And the B / A ratio as an evaluation result is0.9 more than 1.4By using the following positive electrode active material, it is possible to obtain a lithium ion secondary battery in which initial defects are suppressed with good reproducibility.
[0072]
Examples 2-6, Comparative Examples 2-6
In the same manner as in Example 1, the positive electrode active material (LiCoO₂Powder) was prepared. At this time, the amount of the particulate metal impurities was reduced by adjusting the materials of the pulverizer, the molding machine, the mixer, the classifier, etc., and the operating conditions of the apparatus used for the production.
[0073]
Analysis and evaluation of the amount of particulate metal impurities contained in each of the positive electrode active materials described above were performed in the same manner as in Example 1. The analysis / evaluation classification was performed under the same conditions as in Example 1. Content A and B of impurity metal elements in the pre-classification powder and the classification coarse powder are Cu content in Example 2, Zn content in Example 3, Cr content in Example 4, and Ca content in Example 5. The content rate, in Example 6, the Mg content rate was measured. The measurement results of each example are as shown in Tables 4 to 8, respectively.
[0074]
On the other hand, as Comparative Examples 2 to 6 with the present invention, LiCoO was carried out in the same manner as in Examples 2 to 6 except that the pulverization conditions and sieving conditions after firing were changed.₂A positive electrode active material made of powder was prepared. Also about these, it carried out similarly to Examples 2-6, and analyzed and evaluated the amount of particulate metal impurities. The results are as shown in Tables 4 to 8, respectively.
[0075]
Using each positive electrode active material according to Examples 2-6 and Comparative Examples 2-6, lithium ion secondary batteries were produced in the same manner as in Example 1. Each of these lithium ion secondary batteries was charged under the same conditions as in Example 1, and the voltage after standing for 10 days was measured in the same manner. The measurement results are shown in Tables 4 to 8, respectively.
[0076]
[Table 4]

[0077]
[Table 5]

[0078]
[Table 6]

[0079]
[Table 7]

[0080]
[Table 8]

[0081]
  As is clear from Tables 4 to 8, the B / A ratios according to Examples 2 to 6 are0.9 more than 1.4It can be seen that all of the lithium ion secondary batteries produced using the following positive electrode active materials have a slight voltage drop after standing for 10 days. Thus, the B / A ratio of Cu, Zn, Cr, Ca, Mg, etc. as impurity metal elements is0.9 more than 1.4Even when the following positive electrode active material is used, a lithium ion secondary battery in which initial defects are suppressed can be obtained with good reproducibility.
[0082]
  In Examples 1 to 6 described above, Fe, Cu, Zn, Cr, Ca, and Mg were taken up as impurity metal elements. As described above, since the positive electrode active material manufacturing apparatus is often formed of an iron alloy such as stainless steel, the elements described above are shown as impurity metal elements that are particularly likely to be mixed. For other metal elements, the impurity metal element content ratio (B / A ratio)0.9 more than 1.4Similar effects can be obtained by the following.
[0083]
Example 7 and Comparative Example 7
In the same manner as in Example 1 and Comparative Example 1 described above, the positive electrode active material (LiCoO) was used as each sample of Example 7 and Comparative Example 7.₂Powder) was prepared. Analysis and evaluation of the amount of particulate metal impurities contained in each of these positive electrode active materials was performed in the same manner as in Example 1. Note that the classification accuracy index κ was adjusted to about 0.8. The content rates A and B of the impurity metal elements in the pre-classification powder and the classification coarse powder were measured in the same manner as in Example 1. Table 9 shows the measurement results of Example 7 and Comparative Example 7.
[0084]
Using each positive electrode active material according to Example 7 and Comparative Example 7, lithium ion secondary batteries were produced in the same manner as in Example 1. Each of these lithium ion secondary batteries was charged under the same conditions as in Example 1, and the voltage after standing for 10 days was measured in the same manner. Table 9 shows the measurement results.
[0085]
[Table 9]

[0086]
  As is clear from Table 9, the classification accuracy index κ is classified so as to be 0.7 or more, and the B / A ratio obtained based on the result is0.9 more than 1.4By using the following positive electrode active material, the initial failure of the lithium ion secondary battery can be suppressed. In Example 7 and Comparative Example 7, the classification accuracy index κ is set slightly lower than that in Example 1, so that it can be seen that the Fe content in the classified coarse powder is slightly lower. This is because the degree of concentration of particulate metal impurities in the classified coarse powder is slightly reduced. However, the B / A ratio based on classification with a classification accuracy index κ of 0.7 or more is0.9 more than 1.4According to the following positive electrode active material, it turns out that the initial defect of a lithium ion secondary battery can be suppressed reliably.
[0087]
Example 8 and Comparative Example 8
Nickel hydroxide powder and lithium hydroxide powder were mixed at a predetermined ratio and calcined in air at 700 ° C. for 6 hours. The fired product was pulverized using an appropriate pulverizer and then sieved to remove massive coarse particles and fine particles. In this way, LiNiO₂A positive electrode active material made of powder was obtained. In the production of this positive electrode active material, the amount of particulate metal impurities was reduced by adjusting the materials of pulverizers, molding machines, mixers, classifiers, etc. and the operating conditions of the apparatuses used for these productions.
[0088]
Analysis and evaluation of the amount of particulate metal impurities contained in the positive electrode active material described above were performed in the same manner as in Example 1. The analysis / evaluation classification was performed under the same conditions as in Example 1. The contents A and B of the impurity metal elements in the pre-classification powder and the classification coarse powder were obtained for Fe in the same manner as in Example 1. The results are as shown in Table 10.
[0089]
On the other hand, as Comparative Example 8 with the present invention, LiNiO was performed in the same manner as in Example 8 except that the pulverization conditions and sieving conditions after firing were changed.₂A positive electrode active material made of powder was prepared. For this positive electrode active material, the amount of particulate metal impurities was analyzed and evaluated in the same manner as in Example 8. The results are as shown in Table 10.
[0090]
Each positive electrode active material (LiNiO) according to Example 8 and Comparative Example 8 as described above.₂In the same manner as in Example 1, lithium ion secondary batteries were respectively produced. Each of these lithium ion secondary batteries was charged under the same conditions as in Example 1, and the voltage after standing for 10 days was measured in the same manner. The measurement results are shown in Table 10, respectively.
[0091]
[Table 10]

[0092]
  As is clear from Table 10, LiNiO₂Even in the case of using a positive electrode active material comprising LiNiO₂The B / A ratio of the powder is0.9 more than 1.4If it is below, it turns out that the initial defect of a lithium ion secondary battery can be suppressed.
[0093]
Example 9 and Comparative Example 9
Manganese dioxide powder and lithium carbonate were mixed at a predetermined ratio and fired in air at 800 ° C. for 6 hours. The fired product was pulverized using an appropriate pulverizer and then sieved to remove massive coarse particles and fine particles. In this way, LiMn₂O_FourA positive electrode active material made of powder was obtained. In the production of this positive electrode active material, the amount of particulate metal impurities was reduced by adjusting the materials of pulverizers, molding machines, mixers, classifiers, etc. and the operating conditions of the apparatuses used for these productions.
[0094]
Analysis and evaluation of the amount of particulate metal impurities contained in the positive electrode active material described above were performed in the same manner as in Example 1. The analysis / evaluation classification was performed under the same conditions as in Example 1. The contents A and B of the impurity metal elements in the pre-classification powder and the classification coarse powder were obtained for Fe in the same manner as in Example 1. The results are as shown in Table 11.
[0095]
On the other hand, as Comparative Example 9 with the present invention, LiMn was performed in the same manner as in Example 9 except that the pulverization conditions and sieving conditions after firing were changed.₂O_FourA positive electrode active material made of powder was prepared. For this positive electrode active material, the amount of particulate metal impurities was analyzed and evaluated in the same manner as in Example 9. The results are as shown in Table 11.
[0096]
Such positive electrode active materials (LiMn) according to Example 9 and Comparative Example 9₂O_FourIn the same manner as in Example 1, lithium ion secondary batteries were respectively produced. Each of these lithium ion secondary batteries was charged under the same conditions as in Example 1, and the voltage after standing for 10 days was measured in the same manner. The measurement results are shown in Table 11, respectively.
[0097]
[Table 11]

[0098]
  As is clear from Table 11, LiMn₂O_FourEven in the case of using a positive electrode active material comprising LiMn₂O_FourThe B / A ratio of the powder is0.9 more than 1.4If it is below, it turns out that the initial defect of a lithium ion secondary battery can be suppressed.
[0099]
In each of the above-described examples, a lithium ion secondary battery was prepared using the positive electrode active material before classification. However, coarse powder obtained by classification (excluding classification coarse powder from the positive electrode active material) was used. Even if a lithium ion secondary battery is manufactured, the same result can be obtained.
[0100]
【The invention's effect】
As described above, in the present invention, an analysis / evaluation method for factors (particulate metal impurities, etc.) that lower battery performance and production yield is established, and based on this analysis / evaluation method, the particulate metal in the positive electrode active material is established. The amount of impurities is evaluated. Therefore, by using a positive electrode active material for a secondary battery that satisfies the conditions of the present invention, the production yield and battery performance of a non-aqueous electrolyte secondary battery can be improved with good reproducibility.
[Brief description of the drawings]
FIG. 1 is a diagram showing the cumulative frequency and partial classification efficiency of each powder before and after classification evaluation of a positive electrode active material (sample 1) according to Example 1 of the present invention.
FIG. 2 is a graph showing the particle size distribution of each powder before and after classification evaluation of a positive electrode active material (sample 1) according to Example 1 of the present invention.
FIG. 3 is a graph showing the relationship between the partial classification efficiency and the particle size prepared for obtaining the classification accuracy index in the classification evaluation of the positive electrode active material (sample 1) according to Example 1 of the present invention.
FIG. 4 is a cross-sectional view showing a configuration of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Battery container, 4 ... Positive electrode, 5 ... Separator, 6 ... Negative electrode, 12 ... Nonaqueous electrolyte secondary battery

Claims (3)

A method for analyzing a positive electrode active material comprising a powdered metal oxide used in a non-aqueous electrolyte secondary battery,
Classifying the powdered metal oxide using the difference in particle size and density of the constituent particles to obtain a coarse powder having a classification ratio of 0.1 to 5% by mass ratio ;
A step of measuring the content A of the impurity metal element in the powdered metal oxide before classification and the content B of the impurity metal element in the coarse powder obtained by the classification;
Based on the ratio of content B of the impurity metal element to the content A of the impurity metal element (B / A), to assess the particulate metal impurity content in the classification before the powdery metal oxide , for a secondary battery B / a ratio and having a step of determining that the powdery metal oxide, wherein the particulate metal impurity amount is reduced when the range of 0.9 to 1.4 Method for analyzing positive electrode active material. In the analysis method of the positive electrode active material for secondary batteries of Claim 1,
The classification of the powdered metal oxide is:

The relationship between the partial classification efficiency η (d) and the particle size d obtained based on the above is defined as a partial classification efficiency curve, and the particle size (D _p25 ) and partial classification when the partial classification efficiency is 25 % from the partial classification efficiency curve When the value of the particle size (D _p75 ) when the efficiency is 75 % is obtained, the classification accuracy index κ represented by the ratio of these values (D _p25 / D _p75 ) is 0.7 or more. A method for analyzing a positive electrode active material for a secondary battery. In the analysis method of the positive electrode active material for secondary batteries of Claim 1 or Claim 2,
The method for analyzing a positive electrode active material for a secondary battery, wherein the classification of the powdered metal oxide is performed using a classification device based on a balance between centrifugal force and fluid resistance force by forced vortex.

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