JP4039918B2 - Gel electrolyte secondary battery and manufacturing method thereof - Google Patents

Description

【００１４】
（Ｒ₁はフッ素原子あるいはフッ素化置換低級アルキル基を示し、Ｒ₂は水素原子、フッ素原子、低級アルキル基あるいはフッ素化置換低級アルキル基を示し、Ｒ₃は水素原子、低級アルキル基あるいはフッ素化置換低級アルキル基を示す。）
上記エステル化合物は良好な電池特性をもたらし、特に難燃化の改良効果がより著しい。上記定義中、低級アルキル基は、炭素数１〜４の基を使用することが好ましい。アルキル基の炭素数が５以上になると不燃性の低下が著しく、二次電池の難燃化の改良効果が少なくなり、所望の熱安定性が発現しにくいので好ましくない。また、アルキル基にＦ原子を増やすことは生産コストが高くなるので、あまり好ましくない。
【００１５】
上記エステル化合物としては、ジフルオロ酢酸メチル、ジフルオロ酢酸エチル、５Ｈ−オクタフルオロペンタノ酸エチル、７Ｈ−ドデカフルオロヘプタン酸エチル、９Ｈ−ヘキサデカフルオロノナノ酸エチル、２−トリフルオロメチル−３，３，３−トリフルオロプロピオン酸メチル、１−トリフルオロ−２−トリフルオロメチル−プロピオン酸メチル等が好ましいが、これらに限定されない。
【００１６】
上記以外のフッ素化合物としては、以下の構造式（ＩＩ）に示されるフッ素化エーテル類、構造式（ＩＩＩ）に示されるフッ素化エステル類、構造式（ＩＶ）に示されるフッ素化炭酸エステル（カーボネート）類が挙げられる。これらは単独でも又は２種類以上混合しても用いることができる。なお、以下に挙げるフッ素化合物の定義中、低級アルキル基は、炭素数１〜４の基を使用することが好ましい。
【００１７】
Ｒ₄−Ｏ−Ｒ₅ （ＩＩ）
（Ｒ₄とＲ₅は、同一又は異なって、低級アルキル基あるいはフッ素化低級アルキル基を示す。但し、一つはフッ素化低級アルキル基である。）
フッ素化エーテル類としては、具体的にはメチルノナフルオロブチルエーテル、１−フルオロメトキシ−１−メトキシメタン、１−ジフルオロメトキシ−１−メトキシメタン、１−トリフルオロメトキシ−１−メトキシメタン、１，１−ジフルオロメトキシメタン、１−フルオロメトキシ−２−メトキシエタン、１−ジフルオロメトキシ−２−メトキシエタン、１−トリフルオロメトキシ−２−メトキシエタン、１，１−ジフルオロメトキシエタン、１−フルオロエトキシ−２−メトキシエタン、１−ジフルオロエトキシ−２−メトキシエタン、１−トリフルオロエトキシ−２−メトキシエタン、１−フルオロエトキシ−２−フルオロメトキシエタン、１−フルオロエトキシ−２−エトキシエタン、１−ジフルオロエトキシ−２−エトキシエタン、１，２−ジフルオロエトキシエタン等が好ましいが、これらに限定されない。
【００１８】
Ｒ₆−ＣＯＯ−Ｒ₇ （ＩＩＩ）
（Ｒ₆とＲ₇は、同一又は異なって、低級アルキル基あるいはフッ素化低級アルキル基を示す。但し、一つはフッ素化低級アルキル基である。）
フッ素化エステル類としては、具体的には酢酸フルオロエチル、酢酸トリフルオロエチル、プロピオン酸フルオロメチル、プロピオン酸ジフルオロメチル、プロピオン酸トリフルオロメチル、プロピオン酸フルオロエチル、プロピオン酸ジフルオロエチル、プロピオン酸トリフルオロエチル、トリフルオロ酢酸メチル、トリフルオロ酢酸エチル等が好ましいが、これらに限定されない。
【００１９】
Ｒ₈−ＯＣＯＯ−Ｒ₉ （ＩＶ）
（Ｒ₈とＲ₉は、同一又は異なって、低級アルキル基あるいはフッ素化低級アルキル基を示す。但し、一つはフッ素化低級アルキル基である。）
フッ素化カーボネート類としては、具体的には３−フルオロプロピレンカーボネート、４−フルオロプロピレンカーボネート、３−ジフルオロプロピレンカーボネート、３−フルオロ−４−フルオロプロピレンカーボネート、３−ジフルオロ−４−フルオロプロピレンカーボネート、４−トリフルオロメチル−エチレンカーボネート、４−トリフルオロメチル−３−フルオロエチレンカーボネート、４−トリフルオロメチル−４−フルオロエチレンカーボネート、４−トリフルオロメチル−３−ジフルオロエチレンカーボネート、４−トリフルオロメチル−３−フルオロ−４−フルオロエチレンカーボネート等が好ましいが、これらに限定されない。
【００２０】
フッ素化合物としては、９０℃以上２００℃以下で不燃性ガスを生成するフッ素化合物が好ましい。９０℃未満で不燃性ガスを生成するフッ素化合物であると、電池製造時（特に架橋時）や通常使用時に不燃性ガスが生じてしまうので好ましくない。また、２００℃より高い温度で不燃性ガスを生成するフッ素化合物であると、電池の熱暴走に追従できずに熱安定性の確保ができないため好ましくない。
難燃性物質であるリン化合物を前記フッ素化合物と併用してもよい。例えば、メチルエチレンフォスフェート、エチルエチレンフォスフェート、メチルネオペンチルフォスフェート等の環状リン酸エステル、トリメチルフォスフェート、トリエチルフォスフェート等の非環状リン酸エステルが挙げられる。
【００２１】
上記のフッ素化合物、リン化合物の電気化学的特性は、カソード限界が２．０Ｖ ｖｓ．Ｌｉ／Ｌｉ⁺以下でアノード限界が４．２Ｖ ｖｓ．Ｌｉ／Ｌｉ⁺以上であることが望ましい。更に、これらの条件を満たせば、上記以外のフッ素化合物、リン化合物でも使用できる。
特に好ましいフッ素化合物は、ジフルオロ酢酸メチル、ジフルオロ酢酸エチル、２−トリフルオロメチル−３，３，３−トリフルオロプロピオン酸メチル、メチルノナフルオロブチルエーテルである。
【００２２】
ゲル電解質には、ゲル状物質にリチウム電池用電解液を吸収させている固体状のゲル電解質、例えば、公知のリチウムポリマー電池のゲル電解質を使用できる。前記電解液としては公知のリチウムイオン電池の電解液を使用できる。ゲル電解質の代表的形態としては、化学ゲルと物理ゲルに大別される。
化学ゲルとしてはマクロモノマー（高分子量重合体；高分子であるが架橋してゲル電解質のマトリックスを形成する）の架橋体に電解液を吸収させているものである。
【００２３】
例えば、２個以上のビニル基を有するエチレンオキシド（ＥＯ）、プロピレンオキシド（ＰＯ）等のアルキレンオキシドやビニル基とエポキシ基との組み合わせであるグリシジルメタクリレート等から得られるポリマーが挙げられる。これらの樹脂の混合体は１種又は２種類以上を組み合わせて使用してもよい。
物理ゲルとしてはポリアクリルニトリル（ＰＡＮ）、ポリフッ化ビリニデン（ＰＶｄＦ）とヘキサフルオロピレン（HFP）の共重合体等が用いられ、電解液が可塑剤として加えられる。これらの高分子を１種又は２種類以上を組み合わせて使用してもよい。
電解液はゲル電解質中に重量比率５０〜９９％の量で存在する。５０重量％より少ないとリチウムイオン伝導性が阻害され、９９重量％より多いと固体化が難しいので好ましくない。
【００２４】
ゲル電解質は、上記架橋体とリチウム電池用電解液とフッ素化合物とからなる。本明細書においてゲル電解質中のフッ素化合物の濃度とは、フッ素化合物のみの濃度のことで、フッ素化合物中のフッ素の濃度を意味するのではない。更に、マクロモノマーと前記電解液に含まれるフッ素濃度、正極もしくは負極の結着剤に含まれるフッ素の濃度は含まれない。なお、本発明の発明者等は、例えば、ＣＨＦ₂ＣＯＯＣ₂Ｈ₅を２０重量％添加するのと、Ｃ₄Ｆ₉ＯＣＨ₃を１０重量％添加するのとでは、前者の方が効果を有することを見い出している。このことは、ゲル電解質中のフッ素化合物由来のフッ素の濃度を多くするよりも、フッ素化合物の濃度を単純に多くする方がより効果を奏することを意味している。ゲル電解質中のフッ素化合物の存在の有無の判別、フッ素化合物の濃度はＩＣＰ（誘導結合プラズマ発光分光分析）や元素分析を用いて求めることができる。
【００２５】
正極（＝活物質＋導電剤＋結着剤）に含まれるゲル電解質中のフッ素化合物の濃度は、ゲル電解質中の電解液に対して重量比率１０〜５０％の範囲であることが好ましく、２０〜５０％の範囲で含まれていることがより好ましい。フッ素化合物の含有量が１０重量％未満だと難燃性の効果が発現し難くなり、逆に５０重量％より多いとイオン伝導性が低下し、電池特性が低下するため好ましくない。
【００２６】
負極（＝活物質＋結着剤）に含まれるゲル電解質中のフッ素化合物を含む場合、その濃度は、ゲル電解質中の電解液に対して重量比率０〜１０％の範囲であることが好ましく、１〜５％の範囲であることがより好ましい。フッ素化合物の含有量が０重量％に近くなるほど難燃性の効果を発現し難くなる。ゆえに、正極と非電解質層に含まれるゲル電解質中のフッ素化合物の濃度を高くすることになる。しかし、高くし過ぎると、ゲル電解質のイオン伝導性が低下し、電池特性が低下する可能性がでてくる。特に、１０重量％より多い場合、充放電効率が低下し、電池特性が低下するため好ましくない。なお、フッ素化合物を含まない場合は、負極はゲル電解質を含まない構成も採用できる。
なお、ゲル電解質とセパレータからなる非電解質層にもフッ素化合物が含まれていてもよいが、このフッ素化合物の濃度はゲル電解質中の電解液に対して重量比率０〜５０％の範囲で含まれていることが好ましい。より好ましい濃度は、負極に含まれるゲル電解質のフッ素化合物の濃度以上、正極に含まれるゲル電解質のフッ素化合物の濃度以下である。
【００２７】
負極活物質としては、黒鉛粒子の表面に非晶質炭素を付着させた炭素材料（以後、表面非晶質黒鉛と記す）を用いることが好ましい。表面非晶質黒鉛を使用するとゲル電解質、特にフッ素化合物の分解を著しく防ぐことができる。具体的には、ゲル電解質に含まれる電解液より発生するエチレンガス、炭酸ガス等の分解生成ガス発生に伴う電池の内圧上昇による破裂、外部への液漏れ、更には電池の発火を防止でき、長期信頼性及び安全性を更に高めることができる。加えて、ゲル電解質に含まれるフッ素化合物の分解を防ぐ効果がある。そのため、ゲル電解質中にフッ素化合物をより多く含有させることが可能となる。このことは電池の発火を防止、長期信頼性及び安全性を助長することにつながる。
【００２８】
表面非晶質黒鉛とは、高結晶性の黒鉛材料を芯材として、公知の気相法、液相法、固相法等の手法により、該黒鉛粒子の表面に非晶質炭素を付着させて得ることができる。表面非晶質黒鉛は、ＢＥＴ法により測定される比表面積に関わる細孔が、非晶質炭素の付着によってある程度塞がれていることが好ましく、比表面積が１〜５ｍ²／ｇの範囲が好ましい。
比表面積が５ｍ²／ｇより大きくなると、ゲル電解質に含まれる電解液やフッ素化合物との接触面積も大きくなるので、それらの分解反応の悪影響が大きくなるため好ましくない。更に、前駆体溶液中の重合開始剤の負極表面への吸着量が増えるため、前駆体溶液の架橋を阻害したり、初回の充放電効率を低下させたりする等好ましくない。比表面積が１ｍ²／ｇより小さくなると、ゲル電解質との接触面積が小さくなるため、電極反応速度が遅くなり、電池の負荷特性が低くなるので好ましくない。
【００２９】
芯材に用いる高結晶性の黒鉛材料としては、公知のものを使用することができる。ここで、芯材となる高結晶性の黒鉛材料としては、Ｘ線広角回折法による（００２）面の平均面間隔（ｄ₀₀₂）が０．３３５〜０．３４０ｎｍ、あるいは、Ｌｃ、Ｌａが１０ｎｍ以上のものを使用することが好ましい。
ｄ₀₀₂が０．３４０ｎｍより大きい場合、あるいは、Ｌｃ、Ｌａが１０ｎｍより小さい場合には、芯材としての結晶性が充分でないので、これを用いて表面非晶質黒鉛を作製した際には、リチウムの溶解析出に近い電位部分（Ｌｉの電位基準で０〜３００ｍＶ）の容量が十分ではなくなるので好ましくない。
【００３０】
なお、Ｘ線広角回折法による結晶子の大きさ（Ｌｃ、Ｌａ）を判定する方法としては、公知の方法、例えば“炭素材料実験技術１，ｐ．５５〜６３，炭素材料学会編（科学技術社）”に記載されている方法を適用することが可能である。
具体的には、試料が粉末の場合はそのままで、微小片状の場合にはメノウ乳鉢で粉末化し、試料に対して約１５ｗｔ％のＸ線標準用高純度シリコン粉末を内部標準物質として加え混合し、試料をセルにつめ、グラファイトモノクロメーターで単色化したＣｕＫα線を線源とし、反射式ディフラクトメーター法によって広角Ｘ線回折曲線を測定する。曲線の補正には、いわゆるローレンツ、偏向因子、吸収因子、原子散乱因子等に関する補正は行なわず次の簡便法を用いる。
【００３１】
すなわち、（００２）回折に相当する曲線のベースラインを引き、（００２）の面の補正回折曲線を得る。そして、補正回折曲線において、ピーク高さの半分の位置におけるいわゆる半価値βを用いてＣ軸方向の結晶子大きさＬｃをＬｃ＝（Ｋ・λ）／（β・ｃｏｓθ）で求める。ここで、λは１．５４１８Aであり、θは回折角である。同様にＬａも測定することが可能である。
また、アルゴンレーザーラマンによる１５８０ｃｍ^-1付近のピーク強度比に対する１３６０ｃｍ^-1付近のピーク強度比（以後Ｒ値と記す）が０．５以下（より好ましくは０．４以下）であることが好ましい。Ｒ値が０．５を超える場合には、芯材としての結晶性が充分ではなく、表面非晶質黒鉛を作製した際にリチウムの溶解析出に近い電位部分の容量が十分ではなくなるので好ましくない。
【００３２】
なお、付着部分の結晶性については特に限定はされないが、基本的に芯材に比べて結晶性の低いもの、つまりｄ₀₀₂、Ｒ値等が大きいものを採用することにより、表面非晶質黒鉛としての効果が得られる。Ｘ線回折では、その材料のバルクの性質が規定されるため、表面層が薄い場合には大きな差となって表れないこともあるが、例えば、この場合、表面の物性を測定できるラマン測定に測定されるＲ値を有効に用いることができる。より好ましくは低結晶性の炭素材料はｄ₀₀₂が０．３４ｎｍより大きく、Ｒ値は０．５より大きい（より好ましくは０．４より大きい）ものであり、これらは表面に付着させる炭素材料のＣＶＤ条件や種々原料の焼成条件を同じにして擬似的に表面の炭素材料のみを作製し、その物性を測定することによって間接的に規定することができる。
【００３３】
上記第１のゲル電解質二次電池中の正極及び負極にフッ素化合物を含む場合、正極用前駆体溶液中のフッ素化合物濃度が、負極用前駆体溶液中のフッ素化合物濃度より高くするように調整し、それらを各々正極と負極に含浸させてから架橋して、正極と負極との間に非電解質層を介在させることにより製造することができる。
正極用前駆体溶液中のフッ素化合物濃度としては、電解液に対して重量比率１０〜５０％の範囲であることが好ましく、２０〜５０％の範囲であることがより好ましい。フッ素化合物濃度が１０重量％未満だとゲル電解質形成後、難燃性の効果が発現し難くなり、逆に５０重量％より多いとイオン伝導性が低下し、電池特性が低下するので好ましくない。
【００３４】
負極用前駆体溶液中のフッ素化合物濃度としては、電解液に対して重量比率０〜１０％の範囲であることが好ましく、１〜５％の範囲であることがより好ましい。フッ素化合物濃度が０重量％に近くなるほど難燃性の効果を発現し難くなる。ゆえに、正極用前駆体溶液中のフッ素化合物の濃度を高くすることになるが、高くしすぎるとゲル電解質のイオン伝導性が低下し、電池特性が低下する可能性がでてくる。逆に１０重量％より多いと充放電効率が低下し、電池特性が低下するので好ましくない。
【００３５】
更に、本発明では、リチウムイオンを挿入／脱離しうる活物質を含む正極と負極、該正極と該負極との間に非水溶媒とリチウム塩を含むゲル状物質が配置された非水ゲル電解質層を備え、ゲル電解質層が、上記構造式（Ｉ）で示されるフッ素化合物を含むことを特徴とする第２のゲル電解質二次電池も提供される。
第２のゲル電解質二次電池は、非水ゲル電解質層に上記構造式（Ｉ）で示されるフッ素化合物を含むこと以外は、上記第１のゲル電解質二次電池の構成をそのまま使用することができる。
【００３６】
非水ゲル電解質層中のフッ素化合物は、ゲル状物質に対して重量比率１０〜５０％の範囲で含まれていることが好ましい。フッ素化合物濃度が１０重量％未満だとゲル電解質形成後、難燃性の効果が発現し難くなり、逆に５０重量％より多いとイオン伝導性が低下し、電池特性が低下するので好ましくない。
より好ましい非水ゲル電解質層中のフッ素化合物は、ジフルオロ酢酸メチル、ジフルオロ酢酸エチル、２−トリフルオロメチル−３，３，３−トリフルオロプロピオン酸メチル、メチルノナフルオロブチルエーテルである。
以下、本発明をより具体的に説明するが、本発明はこれらに限定されるものではない。
【００３７】
本発明に係わる二次電池の一例である非水電解質としてゲル電解質（イオン伝導性高分子）を具備する二次電池を図１（Ａ）及び（Ｂ）に示す。図１（Ａ）及び（Ｂ）の二次電池は、集電体１上に形成した正極活物質２（導電剤、結着剤は図示省略）とゲル電解質３を含む正極４と、集電体５上に負極活物質６（結着剤は図示省略）とゲル電解質７を含む負極８の間に、ゲル電解質３を含むセパレータからなる非水電解質層９を配置した構造である。
【００３８】
図１（Ａ）は、正極４と非水電解質層９に含まれるフッ素化合物の濃度が負極８に含まれるフッ素化合物の濃度よりも高い例である。また、図１（Ｂ）に示すように、正極４のフッ素化合物の濃度が非水電解質層９と負極８よりも高くても本発明を実施できる。なお、負極８にリチウム金属を用いても本発明の効果が得られる。
【００３９】
イオン伝導体がリチウムイオン以外の場合でも、本発明を実施できる。負極にリチウムイオン以外のアルカリ金属のイオン、例えば、電気化学的にナトリウムイオンを挿入／脱離し得る材料、電気化学的にナトリウムを析出／溶解し得るナトリウム金属や、アルカリ土類金属のイオン、例えば、電気化学的にマグネシウムイオンやカルシウムイオンを挿入／脱離し得る材料、電気化学的にマグネシウムやカルシウムを析出／溶解し得るマグネシウム金属やカルシウム金属を用いることもできる。
その場合、正極には電気化学的にナトリウムイオンを挿入／脱離し得るＦｅ（ＳＯ₄）₂やＮａＦｅＯ₂等や、アルカリ土類金属のイオン、例えば、電気化学的にマグネシウムイオンを挿入／脱離し得る酸化バナジウム等を用いることもできる。
以下に、二次電池の製造工程と評価方法の一例を示す。
【００４０】
ａ）負極の作製
負極の作製方法を以下に記載する。負極は、活物質、結着剤等を混合して形成できる。
具体的には、結着剤を乳鉢中で溶剤に溶かして、負極の炭素材料を分散させる。分散処理には混練機、ボールミル等が用いられ、炭素材料、結着剤が均一に分散する状態にペーストを調節する。このペーストを集電体の金属箔に塗布し、これを４０〜１００℃で仮乾燥する。その後、１５０℃程度で熱処理をし、所定の活物質密度にするため、プレス機を用いて圧縮成形する。圧縮成形には通常ローラープレス機が用いられ、これらプレス機を適用する場合のプレス面の材質、回転方法、温度、雰囲気等は特に限定しない。その後、電極の無塗工部にリードを溶接し、水分除去のために１５０℃程度で減圧乾燥したものを負極として用いる。
【００４１】
負極活物質である炭素材料としては、公知のリチウムイオン電池の負極材料を使用できるが、表面非晶質黒鉛を用いることが好ましい。炭素材料の粒径分布は０．１〜１５０μｍ程度であることが好ましい。結着剤としてはＰＶｄＦ、ポリテトラフルオロエチレン（ＰＴＦＥ）等を使用できるが、これらに限定されるものではない。混合比は活物質１００重量部に対して、結着剤を１〜３０重量部とすることが好ましい。高エネルギー密度の電池を作製するためには、負極の活物質密度は１．４ｇ／ｃｍ³以上が好ましい。なお、負極作製において結着性を上げるために結着剤の融点前後の温度で熱処理を行なうことが好ましい。
【００４２】
ｂ）正極の作製
正極の作製方法を以下に記載する。正極は、活物質に導電剤、結着剤等を混合して形成できる。
具体的には、結着剤を乳鉢中で溶剤に溶かし、活物質と導電剤を分散させる。分散処理には通常混練機、ボールミル等が用いられ、活物質、導電剤、結着剤が均一分散する状態にペーストを調節する。このペーストを集電体の金属箔に塗布し、これを４０〜１００℃で仮乾燥する。その後、１５０℃程度で熱処理をし、所定の活物質密度にするため、プレス機を用いて圧縮成形する。圧縮成形には通常ローラープレス機が用いられ、これらプレス機を適用する場合のプレス面の材質、回転方法、温度、雰囲気等は特に限定しない。その後、電極の無塗工部にリードを溶接し、水分除去のために１５０℃程度で減圧乾燥したものを正極として用いる。
【００４３】
正極活物質としては、ＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎＯ₂、ＬｉＦｅＯ₂や、この系列のＬｉＡ_1-xＴ_xＯ₂（ここでＡはＦｅ、Ｃｏ、Ｎｉ、Ｍｎのいずれかであり、Ｔは遷移金属、４Ｂ族、あるいは５Ｂ族の金属を表す。０＜Ｘ≦１）、ＬｉＭｎ₂Ｏ₄等、公知のリチウムイオン電池の正極材料を使用できる。
導電剤としてはアセチレンブラック等の炭素類や、グラファイト粉末等を使用できるが、これらに限定されるものではない。
結着剤としてはＰＶｄＦ、ＰＴＦＥ等を使用できるが、これらに限定されるものではない。
【００４４】
混合比は、活物質１００重量部に対して、導電剤を１〜５０重量部、結着剤を１〜３０重量部とすることが好ましい。高エネルギー密度の電池を作製するためには、正極の活物質密度は２．８ｇ／ｃｍ³以上であることが好ましく、更には３．０ｇ／ｃｍ³以上がより好ましい。正極作製において結着性を上げるために、結着剤の融点前後の温度で熱処理を行なうことが好ましい。
【００４５】
正極、負極は基本的には正極、負極活物質を結着剤にて固定化した各々の活物質を集電体となる金属箔上に形成したものである。前記集電体の材質・形状は限定されず、正極、負極活物質、及び電解液に対して、化学的、電気化学的に安定性のある導体を使用することができる。金属箔の材料としては、アルミニウム、ステンレス、銅、ニッケル等が挙げられる。電気化学的安定性、延伸性及び経済性を考慮すると、正極にはアルミニウム箔、負極には銅箔が好ましい。なお、正極、負極集電体の形態は金属箔以外にも、メッシュ、エキスパンドメタル等の形態であってもよい。
【００４６】
ｃ）ゲル電解質の前駆体溶液の調製とゲル電解質の形成方法
ゲル電解質の代表的形態としては、化学ゲルと物理ゲルに大別される。
化学ゲルの形成方法としては、マクロモノマーと、電解液と、フッ素化合物と、重合開始剤からなるゲル電解質の前駆体溶液を調製し、それを架橋反応を含む重合反応を行ない固体化する方法が挙げられる。
正極用前駆体溶液中のフッ素化合物濃度は、電解液に対して好ましくは重量比率１０〜５０％の範囲、より好ましくは２０〜５０％の範囲とする。負極用前駆体溶液中のフッ素化合物濃度としては、電解液に対して好ましくは重量比率０〜３０％の範囲、より好ましくは１〜１０％の範囲とする。
【００４７】
マクロモノマーとしてはエチレンオキシド（ＥＯ）、プロピレンオキシド（ＰＯ）、グリシジルメタクリエート等が挙げられる。これらのマクロモノマーは１種又は２種類以上を組み合わせて使用してもよい。
マクロモノマーの電解液に対する量は、少なすぎると固体化が難しく、多すぎるとリチウムイオン伝導性が阻害されるので、重量比率１〜５０％が好ましい。架橋方法としては、紫外線、電子線、可視光等の光エネルギーを用いる方法、熱を用いる方法が挙げられる。架橋反応あるいは重合反応を促進させるために重合開始剤を添加してもよい。特に、紫外線あるいは加熱による架橋方法においては、重合開始剤を電解液に対して数％以下加えることが好ましい。
【００４８】
紫外線用の重合開始剤としては、アゾイソブチロニトリル（ＡＩＢＮ）、ベンゾイルパーオキシド（ＢＰＯ）、２，２−ジメトキシ−２−フェニルアセトフェノン（ＤＭＰＡ）等の市販品を用いることができる。これら開始剤は１種又は２種類以上を組み合わせて使用してもよい。紫外線の波長は２５０〜３６０ｎｍが適当である。
加熱用の重合開始剤としては、１０時間半減期温度が４０℃以上９０℃以下であるものが好ましい。加熱温度は４０〜８０℃が適当である。
【００４９】
物理ゲルの形成方法としては、ＰＶｄＦ、ＨＥＰ、ポリメタクリル酸メチル、ポリ塩化ビニル等のポリマーを１種又は２種類以上混合し、テトラヒドロフラン（ＴＨＦ）、Ｎ−メチル−２−ピロリドン（ＮＭＰ）等の溶剤に溶解させて、キャストし乾燥等により溶剤を除去したものに、フッ素化合物を含む電解液を含浸させることにより作製する方法が挙げられる。
正極のフッ素化合物濃度は、電解液に対して好ましくは重量比率１０〜５０％の範囲、より好ましくは２０〜５０％の範囲とする。負極のフッ素化合物濃度としては、電解液に対して好ましくは重量比率０〜３０％の範囲、より好ましくは１〜１０％の範囲とする。
【００５０】
電解液の非水溶媒としては、炭酸エチレン（ＥＣ）、炭酸プロピレン（ＰＣ）、炭酸ブチレン（ＢＣ）等の環状炭酸エステル類や、ＤＭＣ、ＤＥＣ、炭酸メチルエチル（ＭＥＣ）等の鎖状炭酸エステル類や、γ−ブチロラクトン（γ−ＢＬ）等の環状カルボン酸エステルを好適に用いることができる。
【００５１】
炭素材料を負極に用いる場合、ゲル電解質の分解を少なくするために、ＥＣを含むことが好ましい。非水溶媒中におけるＥＣの含有量としては体積比率０〜８０％であることが好ましい。低温特性を向上させるためには、少なくともγ−ＢＬを含有していることが望ましい。ゲル電解質の前駆体溶液の電極活物質内部、あるいはセパレータ基材内部への浸透性を向上させるためには、ＤＭＣ、ＤＥＣ、ＭＥＣ等を非水溶媒全体に対して体積比率０〜５０％添加することが好ましい。これらの１種あるいは２種以上の混合溶媒として使用される。また、ビニレンカーボネート（ＶＣ）や、エチレンサルファイト（ＥＳ）を該非水溶媒の総重量に対して、重量比率１〜１０％程度添加してもよい。
【００５２】
電解液のリチウム塩としては、過塩素酸リチウム、４フッ化ホウ酸リチウム、６フッ化リン酸リチウム、６フッ化砒素リチウム等の公知リチウム塩が挙げられ、これらの１種或いは２種以上を混合して使用できる。非水溶媒にリチウム塩を溶解することにより、リチウム電池用電解液を調製する。
リチウム塩濃度は非水溶媒全体に対して０．８〜２．５ｍｏｌ／ｌであることが好ましい。０．８ｍｏｌ／ｌより塩濃度が低くなると高負荷時の放電特性を得るのに必要なイオン伝導率を得られず、２．５ｍｏｌ／ｌより塩濃度が高くなるとリチウム塩のコストが高くなるだけでなく、粘度が高くなるので電極内に染み込み難くなる。更に、リチウム塩を溶解するのに非常に長い時間を必要とするので、工業的に不適であるので好ましくない。
【００５３】
なお、電解液を調製するのに使用する非水溶媒、リチウム塩は上記に掲げたものに限定されない。非水溶媒の代わりに常温型溶融塩（イオン性液体）、例えば、ＥＭＩＢＦ₄（１−エチル−３―メチルイミダゾリウムテトラフルオロボレート）にＬｉＢＦ₄などのリチウム塩を共存させたもの、アミドアニオンを有するもの、環状アンモニウムカチオンを有するもの等を用いることができる。また、ＡｌＣｌ₃−ＥＭＩＣ（１−エチル−３―メチルイミダゾリウムクロリド）−ＬｉＣｌ、ＡｌＣｌ₃−ＥＭＩＣ−ＬｉＣｌ−ＳＯＣｌ₂等の常温型溶融塩（イオン性液体）も電解液の代わりに適用することができる。
【００５４】
ｄ）電池の組み立て
本発明のゲル電解質二次電池の製造方法としては、例えば以下のように行なうことができる。
まず、ａ）で作製した負極と、ｂ）で作製した正極と、セパレータとに、ｃ）で調整したゲル電解質の前駆体溶液を染み込ませて、光を照射もしくは加熱して架橋する。この時、正極用前駆体溶液のフッ素化合物の濃度を負極用前駆体の濃度よりも高くしておく。架橋後、図１（Ａ）と（Ｂ）に示すように、ゲル電解質と一体化した正極４と負極８との間に非水電解質層９を介在させる。そして、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み、熱融着させることによりシート形の電池を作製する。また、架橋して固体化後、正極(セパレータ(負極を複数個重ねて、外装材の袋に挿入してもよい。この後、この電池を５０〜７０℃にて、６〜２４時間放置してもよい。
【００５５】
ゲル電解質を保持するためのセパレータとしては、電気絶縁性の合成樹脂繊維、ガラス繊維、天然繊維等の不織布あるいは織布等が挙げられる。中でもポリ塩化ビニリデン、ポリエチレン、ポリプロピレン等の不織布が品質の安定性等の点から好ましい。
これら合成樹脂の不織布では電池が異常発熱した場合に、セパレータが熱により溶解し、正負極間を遮断する機能を付加したものもあり、安全性の観点からこれらも好適に使用することができる。
【００５６】
セパレータの厚みは特に限定はないが、必要量の液を保持することが可能で、かつ正極と負極との短絡を防ぐ厚さがあればよく、通常０．０１〜１ｍｍ程度のものを用いることができ、好ましくは０．０２〜０．０５ｍｍ程度である。これらセパレータは透気度が１〜５００ｓｅｃ／ｃｍ³であることが、低い電池内部抵抗を維持しつつ、電池内部短絡を防ぐだけの強度を有しているため好ましい。電池の形状は上記に示したラミネート型以外にも、円筒形、角形、コイン形、ボタン形、シート形等、種々の形状に適用できる。
【００５７】
例えば、円筒形や角形電池では、主にシート電極を缶に挿入し、缶とシート電極を電気的に接続する。該前駆体溶液を注入し、絶縁パッキンを介して封口板を封口、あるいはハーメチックシールにより封口板と缶を絶縁して封口し、加熱して電池を作製する。このとき、安全素子を備え付けた安全弁を封口板として用いることができる。
安全素子には、例えば、過電流防止素子として、ヒューズ、バイメタル、ＰＴＣ素子等がある。また、安全弁の他に電池缶の内圧上昇の対策として、ガスケットに亀裂を入れる方法、封口板に亀裂を入れる方法、電池缶に切れ込みを入れる方法等を用いる。また、過充電や過放電対策を組み込んだ外部回路を用いてもよい。また、コイン形やボタン形電池の場合は、正極や負極はペレット状に形成し、これを缶中に入れ、前駆体溶液を注入し、絶縁パッキンを介して蓋をかしめて、加熱して電池を作製する。セパレータには合成樹脂系の不織布等を用いる。
【００５８】
ｅ）電池特性の評価
充放電作動試験は一定電流値で電池電圧が４．１〜４．３Ｖに到達するまで充電する。電池電圧が４．１〜４．３Ｖに到達後は時間制御で充電する。放電は電池電圧が２．７〜３．０Ｖになるまで一定電流値で行なう。
なお、電極特性及び電池評価は全て不活性ガス雰囲気下のグローブボックス中にて行われる。不活性ガスとしては通常アルゴン、窒素等が好適に用いられる。
【００５９】
ｆ）ゲル電解質中のフッ素化合物濃度の分析
水と有機溶媒のどちらにも溶ける溶媒、例えば、ＴＨＦ等を用いて、ゲル電解質から電解液とフッ素化合物を抽出する（抽出した電解液とフッ素化合物の混合液を抽出液と呼ぶ）。このときに、フッ化物イオンが水素イオンと反応してフッ化水素として揮発しないように、予め弱塩基性にしておくと定量性の信頼向上に効果的である。
【００６０】
そして、ＩＣＰを用いて、上記で得られた抽出液中のフッ化物イオンを定量分析する。なお、電解液中のリチウム塩のフッ素も同時に定量されるので、差し引いておく必要がある。例えば、ＬｉＢＦ₄に関してはフッ素４ｍｏｌに対して、リチウム１ｍｏｌ、ホウ素１ｍｏｌのような組成になっているので、他の元素の定量分析も同時に行なうことにより、リチウム塩のフッ素濃度を差し引くことができる。以上のように、ゲル電解質中のフッ素化合物の濃度は抽出液中のフッ化物イオンを分析することにより求めることができる。
また、元素分析を用いれば、ゲル電解液中のフッ素濃度を分析できる。その場合も、リチウム塩中のフッ素濃度を差し引く必要があるので、リチウム塩を構成しそうな元素、例えば、リチウム、ホウ素、リンの濃度も同時に求め、ＬｉＢＦ₄、ＬｉＰＦ₆の化学式よりＦの量を算出して、ゲル電解質中のフッ素化合物の濃度を分析する。
【００６１】
【実施例】
以下、本発明について実施例及び比較例を示して、その効果を具体的に説明するが、本発明は下記の実施例に制限されるものではない。
（実施例１）
フッ素化合物として構造式（Ｉ）において、Ｒ₁とＲ₂がＣＦ₃、Ｒ₃がＣＨ₃である（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃を用いて、以下の工程にて実施例１の電池を作製した。
【００６２】
ａ）負極の作製
炭素材料には表面非晶質黒鉛（平均粒径１８μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．５、比表面積１〜２ｍ²／ｇ）を用いた。結着剤ＰＶｄＦを乳鉢中で溶剤ＮＭＰに溶かして、表面非晶質黒鉛を分散させた。分散処理には２軸遊星方式の混合混錬機を使用し、炭素材料、結着剤が均一に分散する状態にペーストを調節した。負極の組成は炭素材料１００重量部、ＰＶｄＦ１０重量部とした。このペーストを約２０μｍの銅箔に塗布し、これを５０〜７０℃で仮乾燥した。その後、約１５０℃で１２時間熱処理をし、活物質密度１．５ｇ／ｃｍ³程度になるまで、大気中にてローラープレス機を用いて圧縮成形した。電極サイズを３．０×６．５ｃｍ（塗工部３．０×６．０ｃｍ）とし、無塗工部にニッケル箔（５０(ｍ）のリードを溶接した。その後、水分除去のために約１５０℃にて１２時間減圧乾燥したものを負極として用いた。
【００６３】
なお、Ｘ線広角回折法による平均面間隔（ｄ₀₀₂）及び結晶子の大きさ（Ｌｃ、Ｌａ）を測定する方法としては、公知の方法、例えば"炭素材料実験技術１，ｐ．５５〜６３，炭素材料学会編（科学技術社）"や特開昭６１−１１１９０７に記載されている方法を適用した。結晶子の大きさを求める形状因子Ｋ（＝Ｌｃ・β・ｃｏｓθ／λ；β：半価幅、θ：ｄ₀₀₂の角度、λ：Ｘ線の波長）は０．９を用いた。また、粒径はレーザー回折式粒度分布計（島津社製ＳＡＬＤ１１００）を用いて測定を行ない、粒度分布においてピークをもつ粒径として求めた。
【００６４】
ｂ）正極の作製
正極活物質にはコバルト酸リチウムＬｉＣｏＯ₂（平均粒径１０μｍ）を使用した。ＬｉＣｏＯ₂は公知の方法で合成を行った。Ｘ線源としてターゲットＣｕの封入管からの出力２ｋＷのＣｕＫα線を使用したＸ線回折測定、ヨードメトリー法によるコバルトの価数分析及びＩＣＰによる元素分析の結果から得られた試料はＬｉＣｏＯ₂であることが確認された。ＰＶｄＦを乳鉢中でＮＭＰに溶かし、上記正極活物質と導電剤アセチレンブラックを分散させた。分散処理には２軸遊星方式の混合混錬機を使用し、正極活物質、導電剤、結着剤が均一分散する状態にペーストを調節した。正極の組成はＬｉＣｏＯ₂１００重量部、アセチレンブラック５重量部、ＰＶｄＦ５重量部とした。
このペーストを約２０μｍのアルミニウム箔上に塗布し、これを５０〜７０℃で仮乾燥した。その後、約１５０℃で１２時間熱処理をし、活物質密度３．０ｇ／ｃｍ³程度になるまで、大気中にてローラープレス機を用いて圧縮成形した。電極サイズを３．０×６．５ｃｍ（塗工部３．０×６．０ｃｍ）とし、無塗工部にニッケル箔（５０μｍ）のリードを溶接した。その後、水分除去のために約１５０℃にて１２時間減圧乾燥したものを正極として用いた。
【００６５】
ｃ）ゲル電解質の前駆体溶液の調製
正極とセパレータに含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとγ−ＢＬとＭＥＣ（体積比率２４：５６：２０）の混合溶媒に、ＬｉＢＦ₄を２．２ｍｏｌ／ｌとなるように溶解して、更に、ＶＣを３ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液と（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃とを重量比４：１（フッ素化合物の濃度２０重量％）で混合した。次に、この混合液と分子量７，５００〜９，０００の三官能性アクリレートの重合体もしくは共重合体（ＴＡ）、分子量２，８００〜３，０００の単官能性アクリレートの重合体もしくは共重合体（ＭＡ）とを、重量比率９７：２．４：０．６で混合した。更に、重合開始剤１，０００ｐｐｍを前記混合液に添加して、正極と非水電解質層の前駆体溶液を得た。
【００６６】
負極に含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとγ−ＢＬとＭＥＣ（体積比率２４：５６：２０）の混合溶媒に、ＬｉＢＦ₄を１．０ｍｏｌ／ｌとなるように溶解して、更に、ＶＣを３ｗｔ％を添加してリチウム電池用電解液を得た。次に、前記電解液と（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃とを重量比９：１（フッ素化合物の濃度１０重量％）で混合した。次に、この混合液とＴＡとＭＡとを重量比率９７：２．４：０．６で混合した。更に、重合開始剤２，０００ｐｐｍを前記混合液に添加して、負極用前駆体溶液を得た。
【００６７】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層（ゲル電解質と含んだセパレータ）に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００６８】
（実施例２）
フッ素化合物として構造式（Ｉ）において、Ｒ₁とＲ₂がＣＦ₃、Ｒ₃がＣＨ₃である（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃を用いて、以下の工程にて実施例２の電池を作製した。
【００６９】
ａ）負極の作製
炭素材料に表面非晶質黒鉛（平均粒径２５μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．２５、比表面積１〜２ｍ²／ｇ）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
【００７０】
ｃ）ゲル電解質の前駆体溶液の調製
正極とセパレータに含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとγ−ＢＬとＭＥＣ（体積比率２４：５６：２０）の混合溶媒に、ＬｉＢＦ₄２．４ｍｏｌ／ｌを溶解して、更に、ＶＣを５ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液と（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃とを重量比４：１（フッ素化合物濃度２０重量％）で混合した。次に、この混合液と分子量７，５００〜９，０００のＴＡと、分子量２，８００〜３，０００のＭＡとを、重量比率９７：２．４：０．６で混合した。更に、重合開始剤１，０００ｐｐｍを前記混合液に添加して、正極用と非水電解質層用の前駆体溶液を得た。
【００７１】
負極に含浸させるフッ素化合物を含まない（フッ素化合物の濃度０重量％）ゲル電解質の前駆体溶液を以下のように調整した。ＥＣとγ−ＢＬとＭＥＣ（体積比率２４：５６：２０）の混合溶媒に、ＬｉＢＦ₄を１．０ｍｏｌ／ｌとなるように溶解して、更に、ＶＣを５ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、この混合液とＴＡとＭＡとを重量比率９７：２．４：０．６で混合した。更に、重合開始剤２，０００ｐｐｍを前記混合液に添加して、負極用の前駆体溶液を得た。
【００７２】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００７３】
（実施例３）
フッ素化合物として構造式（Ｉ）において、Ｒ₁とＲ₂がＣＦ₃、Ｒ₃がＣＨ₃である（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃を用いて、以下の工程にて実施例３の電池を作製した。
【００７４】
ａ）負極の作製
炭素材料に表面非晶質黒鉛（平均粒径１２μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．３５、比表面積１〜２ｍ²／ｇ）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
ｃ）ゲル電解質の前駆体溶液の調製
実施例２と同様の操作を繰り返して、フッ素化合物を含む正極用前駆体溶液と、フッ素化合物を含まない非水電解質層用及び負極用前駆体溶液を得た。
【００７５】
ｄ）電池の組み立て
ａ）で作製した負極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極に、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた負極と正極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００７６】
（実施例４）
フッ素化合物として構造式において、Ｒ₁とＲ₂がＣＦ₃、Ｒ₃がＣＨ₃である（ＣＦ₃）₂ＣＨＣＯＯＣＨ₃を用いて、以下の工程にて実施例４の電池を作製した。
ａ）負極の作製
炭素材料に人造黒鉛（ＫＳ−２５）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
ｃ）ゲル電解質の前駆体溶液の調製
実施例３と同様の操作を繰り返して、正極用前駆体溶液と、負極用前駆体溶液を得た。
【００７７】
ｄ）電池の組み立て
ａ）で作製した負極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極に、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた負極と正極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００７８】
（実施例５）
フッ素化合物として構造式（Ｉ）において、Ｒ₁とＲ₂がＦ、Ｒ₃がＣＨ₂Ｃ
Ｈ₃であるＣＨＦ₂ＣＯＯＣＨ₂ＣＨ₃を用いて、以下の工程にて実施例５の電池を作製した。
【００７９】
ａ）負極の作製
炭素材料に表面非晶質黒鉛（平均粒径２５μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．２５、比表面積１〜２ｍ²／ｇ）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
【００８０】
ｃ）ゲル電解質の前駆体溶液の調製
正極とセパレータに含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとＤＭＣとＭＥＣ（体積比率２５：５５：２０）の混合溶媒に、ＬｉＰＦ₆を１．５ｍｏｌ／ｌとなるように溶解してリチウム電池用電解液を得た。次に、前記電解液とＣＨＦ₂ＣＯＯＣＨ₂ＣＨ₃とを重量比４：１（フッ素化合物の濃度２０重量％）で混合した。次に、この混合液と分子量７，５００〜９，０００のＴＡ、分子量２，８００〜３，０００のＭＡとを、重量比率９７：２．４：０．６で混合した。更に、重合開始剤１，０００ｐｐｍを前記混合液に添加して、正極用と非水電解質層用前駆体溶液を得た。
【００８１】
負極に含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとＤＭＣとＭＥＣ（体積比率２５：５５：２０）の混合溶媒に、ＬｉＰＦ₆を１．０ｍｏｌ／ｌとなるように溶解して、更に、ＶＣを３ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液とＣＨＦ₂ＣＯＯＣＨ₂ＣＨ₃とを重量比９：１（フッ素化合物の濃度１０重量％）で混合した。次に、この混合液とＴＡとＭＡとを重量比率９７：２．４：０．６で混合した。更に、重合開始剤２，０００ｐｐｍを前記混合液に添加して、負極用前駆体溶液を得た。
【００８２】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００８３】
（実施例６）
フッ素化合物として構造式（ＩＩ）において、Ｒ₄がＣ₄Ｆ₉、Ｒ₅がＣＨ₃であるＣ₄Ｆ₉ＯＣＨ₃（メチルノナフルオロブチルエーテル）を用いて、以下の工程にて実施例６の電池を作製した。
【００８４】
ａ）負極の作製
炭素材料に表面非晶質黒鉛（平均粒径２５μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．２５、比表面積１〜２ｍ²／ｇ）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
【００８５】
ｃ）ゲル電解質の前駆体溶液の調製
正極とセパレータに含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとＥＭＣ（体積比率４０：６０）の混合溶媒に、ＬｉＢＥＴＩを１．５ｍｏｌ／ｌとなるように溶解して、リチウム電池用電解液を得た。次に、前記電解液とＣ₄Ｆ₉ＯＣＨ₃とを重量比４：１（フッ素化合物の濃度２０重量％）で混合した。次に、この混合液と分子量７，５００〜９，０００のＴＡと、分子量２，８００〜３，０００のＭＡとを、重量比率９７：２．４：０．６で混合した。更に、重合開始剤１，０００ｐｐｍを前記混合液に添加して、正極用と非水電解質層用前駆体溶液を得た。
【００８６】
負極に含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとＥＭＣとＣ₄Ｆ₉ＯＣＨ₃（体積比率４０：６０）の混合溶媒に、ＬｉＢＥＴＩを０．９ｍｏｌ／ｌになるように溶解して、更に、ＶＣを３ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液とＣ₄Ｆ₉ＯＣＨ₃とを重量比４：１（フッ素化合物の濃度１０重量％）で混合した。次に、この混合液とＴＡとＭＡとを重量比率９７：２．４：０．６で混合した。更に、重合開始剤２，０００ｐｐｍを前記混合液に添加して、負極用前駆体溶液を得た。
【００８７】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００８８】
（実施例７）
フッ素化合物として構造式（ＩＩ）において、Ｒ₄がＣＦ₃ＣＦ₂ＣＨ₂、Ｒ₅がＣＨ₃であるＣＦ₃ＣＦ₂ＣＨ₂ＯＣＨ₃を用いて、以下の工程にて実施例７の電池を作製した。
【００８９】
ａ）負極の作製
炭素材料に表面非晶質黒鉛（平均粒径２５μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．２５、比表面積１〜２ｍ²／ｇ）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
【００９０】
ｃ）ゲル電解質の前駆体溶液の調製
正極とセパレータに含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとγ−ＢＬとＤＥＣ（体積比率２５：５５：２０）の混合溶媒に、ＬｉＢＦ₄を２．１ｍｏｌ／ｌになるように溶解して、更に、ＶＣを６ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液とＣＦ₃ＣＦ₂ＣＨ₂ＯＣＨ₃とを重量比４：１（フッ素化合物の濃度２０重量％）で混合した。次に、この混合液と分子量７，５００〜９，０００のＴＡ、分子量２，８００〜３，０００のＭＡとを、重量比率９７：２．４：０．６で混合した。更に、重合開始剤１，０００ｐｐｍを前記混合液に添加して、正極用と非水電解質層用前駆体溶液を得た。
【００９１】
負極に含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとγ−ＢＬとＭＥＣ（体積比率２５：５５：２０）の混合溶媒に、ＬｉＢＦ₄を１．０ｍｏｌ／ｌになるように溶解して、更に、ＶＣを６ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液とＣＦ₃ＣＦ₂ＣＨ₂ＯＣＨ₃とを重量比９：１（フッ素化合物の濃度１０重量％）で混合した。次に、前記電解液とＴＡとＭＡとを重量比率９７：２．４：０．６で混合した。更に、重合開始剤２，０００ｐｐｍを前記混合液に添加して、負極用前駆体溶液を得た。
【００９２】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００９３】
（実施例８）
フッ素化合物として構造式（ＩＩＩ）において、Ｒ₆がＣＨＦ₂、Ｒ₇がＣＨ₃であるＣＨＦ₂ＣＯＯＣＨ₃を用いて、以下の工程にて実施例８の電池を作製した。
【００９４】
ａ）負極の作製
炭素材料に表面非晶質黒鉛（平均粒径２５μｍ、ｄ₀₀₂＝０．３３６ｎｍ、Ｒ値＝０．２５、比表面積１〜２ｍ²／ｇ）を用いたこと以外は、実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
【００９５】
ｃ）ゲル電解質の前駆体溶液の調製
正極とセパレータに含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとＤＭＣとＭＥＣ（体積比率２５：５５：２０）の混合溶媒に、ＬｉＰＦ₆を２．０ｍｏｌ／ｌになるように溶解してリチウム電池用電解液を得た。次に、前記電解液とＣＨＦ₂ＣＯＯＣＨ₂ＣＨ₃とを重量比４：１（フッ素化合物の濃度２０重量％）で混合した。次に、この混合液と分子量７，５００〜９，０００のＴＡ、分子量２，８００〜３，０００のＭＡとを、重量比率９７：２．４：０．６で混合した。更に、重合開始剤１，０００ｐｐｍを前記混合液に添加して、正極用と非水電解質層用前駆体溶液を得た。
【００９６】
負極に含浸させるフッ素化合物を含むゲル電解質の前駆体溶液を以下のように調整した。ＥＣとＤＭＣとＭＥＣ（体積比率２５：５５：２０）の混合溶媒に、ＬｉＰＦ₆を１．０ｍｏｌ／ｌになるように溶解して、更に、ＶＣを３ｗｔ％となるように添加してリチウム電池用電解液を得た。次に、前記電解液とＣＨＦ₂ＣＯＯＣＨ₂ＣＨ₃とを重量比率９：１（フッ素化合物の濃度１０重量％）で混合した。次に、前記電解液とＴＡとＭＡとを重量比率９７：２．４：０．６で混合した。更に、重合開始剤２，０００ｐｐｍを前記混合液に添加して、負極用前駆体溶液を得た。
【００９７】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製した負極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製した正極用前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【００９８】
（比較例１）
以下の工程にて比較例１の電池を作製した。
ａ）負極の作製
実施例１と同様の操作を繰り返して負極を得た。
ｂ）正極の作製
実施例１と同様の操作を繰り返して正極を得た。
ｃ）ゲル電解質の前駆体溶液の調製
実施例１と同様の操作を繰り返して、フッ素化合物を含まない前駆体溶液のみを調製した。
【００９９】
ｄ）電池の組み立て
ａ）で作製した負極に、ｃ）で調製したゲル電解質の前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、ｂ）で作製した正極の表面を２０μｍの厚さの不織布（セパレータ）で覆い、ｃ）で調製したゲル電解質の前駆体溶液を染み込ませて、紫外線照射によって架橋させた。次に、非水電解質層に覆われた正極と負極を重ねて、二枚のアルミニウムラミネート樹脂フィルムの間に挟み込み熱融着させることによりシート状の電池を作製した。
【０１００】
▲１▼電池特性の評価
フッ素化合物を含むゲル電解質の電池特性に及ぼす影響を検討した。実施例及び比較例の電池容量は全て６００ｍＡｈ程度になるように正極と負極の活物質量を設定した。
これら電池を一定電流値（０．２Ｃ）で電池電圧が４．１Ｖになるまで充電し、４．１Ｖに到達後は一定電圧で総充電時間が１２時間になるまで充電した。放電は電池電圧が２．７５Ｖになるまで一定電流値（０．２Ｃ）で行なった。なお、電池評価は全て不活性ガス雰囲気下のグローブボックス中、２０℃にて行なった。
【０１０１】
▲２▼前躯体溶液とゲル電解質中のフッ素化合物濃度の分析
ゲル電解質二次電池を解体して、図１の正極に含まれるゲル電解質３と、負極に含まれるゲル電解質７と、非水電解質層９に含まれるゲル電解質を採取した。次に、ＴＨＦを用いて、各々のゲル電解質から電解液とフッ素化合物を抽出した。そして、ＩＣＰを用いて、上記で得られた抽出液中のフッ化物イオンを定量分析した。前躯体溶液に関してはそのまま定量分析した。なお、リチウム塩のフッ素濃度を差し引いた。
表１に電池の初回の放電容量とフッ素化合物の濃度を示す。
【０１０２】
【表１】

【０１０３】
負極にフッ素化合物を含まない実施例２〜４は、フッ素化合物を１０重量％程度含む実施例１、５〜８よりも放電容量が高かった。このことは負極とフッ素化合物の相性のよくないため、放電容量の低下に反映していると考えられる。
ゆえに、実施例１〜８のように、正極もしくは非水電解質層に含まれるフッ素化合物の濃度が、負極に含まれるフッ素化合物の濃度よりも高くても、比較例２と同等の電池特性を維持できることが分かった。
一方で、実施例４の電池の放電容量は、実施例２と３の電池のそれよりも低いものとなった。黒鉛系炭素材料を用いると電解液が分解するという報告があるように、実施例４ではゲル電解質の分解反応が顕著に起こり、充電が困難になっている。よって、本発明の黒鉛粒子の表面に非晶質炭素を付着させた炭素材料を用いることにより、高容量のゲル電解質二次電池を提供できる。
【０１０４】
▲３▼過充電試験による安全性の評価
フッ素化合物を含むゲル電解質を用いた電池の安全性に及ぼす影響を検討した。実施例及び比較例の電池容量は全て６００ｍＡｈになるように正極と負極の活物質量を設定した。
実施例１〜８及び比較例１の電池を各々１０個作製した。試験に用いた全ての電池を断熱材ガラスウールで包んだ。これら電池を一定電流値（３Ｃ）で電池電圧が４．８Ｖになるまで充電し、４．８Ｖに到達後は一定電圧で総充電時間が４時間になるまで充電した。そして、充電を開始してから終了するまで、電池の状態を観察した。なお、電池評価は全て大気中、室温にて行なった。過充電試験結果を表２に示す。
【０１０５】
【表２】

【０１０６】
表２に示すように、実施例１〜８の電池は、過充電状態でも破裂しただけで、比較例１のように全焼した電池がなく、熱安定性が著しく向上したといえる。また、破裂した温度は実施例１〜８は１３０〜１５０℃、比較例１は２００℃付近であった。このことはフッ素化合物が温度上昇と共に気化して、不燃性ガスを生成し、その反応が吸熱反応であったため、電池が破裂したものの発火を防いだものと考えられる。
【０１０７】
本実施例では正極に含まれるゲル電解質中のフッ素化合物の濃度２０重量％、負極に含まれるゲル電解質中のフッ素化合物の濃度１０重量％を用いたが、正極に含まれるゲル電解質中のフッ素化合物の濃度１０〜５０重量％、負極に含まれるゲル電解質中のフッ素化合物の濃度０〜１０重量％のものを用いても同様な結果が得られた。
また、本実施例では正極及び負極に含まれるゲル電解質中のフッ素化合物が同じものを用いたが、正極及び負極に含まれるゲル電解質中のフッ素化合物が異なっていても同様な結果が得られた。
以上、実施例１〜８に示すように、正極もしくは非水電解質層に含まれるフッ素化合物の濃度が負極中に含まれるフッ素化合物の濃度よりも高くすることにより、電池特性を維持しつつ、過充電を行なっても二次電池の発火を防ぐことが可能となった。
【０１０８】
【発明の効果】
本発明によれば、ゲル電解質二次電池の少なくとも正極がフッ素化合物を含むので、過充電により電池温度が異常高温になった時、フッ素化合物が熱暴走反応の発生する温度よりも低い温度で不燃性ガスを生成するので、発火の危険が回避された安全性の高いゲル電解質二次電池及びその製造方法を提供することができる。
更に、正極に含まれるゲル電解質中のフッ素化合物の濃度が、負極に含まれるゲル電解質中のフッ素化合物の濃度よりも高い構成、正極のみフッ素化合物を含む構成又は特定のフッ素化合物を含むゲル電解質層を備えていれば、電池特性を損なうこともない。
ゆえに、本発明の産業的意義は大である。
【図面の簡単な説明】
【図１】本発明に係わる二次電池の一例である正極、負極、非水電解質層の断面構造を表す図である。
【符号の説明】
１ 集電体
２ 正極活物質
３ ゲル電解質（イオン伝導性高分子）
４ 正極
５ 集電体
６ 負極活物質
７ ゲル電解質（イオン伝導性高分子）
８ 負極
９ 非水電解質層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gel electrolyte secondary battery and a method for manufacturing the same. More specifically, the present invention relates to a gel electrolyte secondary battery using a gel electrolyte containing a fluorine compound and a method for manufacturing the same. The obtained gel electrolyte secondary battery has high safety against overcharging, heating, and the like.
[0002]
[Prior art]
At present, commercially available non-aqueous electrolytes for lithium primary batteries and lithium secondary batteries use lithium salt dissolved in an organic solvent (lithium battery electrolyte). However, since the organic solvent is likely to leak to the outside of the battery, volatilization, etc., problems such as poor long-term reliability and scattering of the electrolyte in the sealing process remain.
Therefore, in order to improve leakage resistance, safety, and long-term storage, the use of gel electrolyte (ion-conducting polymer) as a non-aqueous electrolyte has attracted attention as one means for solving the above problems. Has been. For example, as represented by JP-A-58-75779, JP-A-59-149601, etc., as a non-aqueous electrolyte, a gel electrolyte in a solid state in which an electrolyte is absorbed in a gel substance is used. Has been proposed.
[0003]
However, since most of the gel electrolyte is composed of a combustible substance mainly composed of an electrolytic solution, the gel electrolyte does not satisfy sufficient thermal safety during overcharging or heating. That is, when overcharged or near there is a fire, the temperature in the battery rises to an abnormally high temperature, which may rupture or ignite. In many cases, current protection circuits and protection elements cannot prevent these problems. Therefore, the battery system itself must withstand these problems.
[0004]
Since the driving voltage of the lithium secondary battery is 4 V or more, the gel electrolyte using water as a solvent cannot be used because its withstand voltage is insufficient. Therefore, an organic solvent that does not decompose even at a driving voltage of 4 V or higher is used for the gel electrolyte. However, the disadvantage of organic solvents is their high flammability, and there are concerns about ignition and combustion when the battery is exposed to high temperatures. A chain carbonate ester (carbonate) having a very low flash point such as dimethyl carbonate (DMC; flash point 17 ° C.), diethyl carbonate (DEC; flash point 33 ° C.), or the like is used as the electrolyte. Therefore, there is a possibility of firing when an external short circuit or an internal short circuit occurs due to overcharge or for some reason.
[0005]
As one means for solving these problems, it has been proposed to make a battery flame-retardant by mixing a fluorine compound into an electrolytic solution. For example, Japanese Patent Application Laid-Open No. 7-6786 provides an electrolytic solution having a high flash point and excellent thermal stability using a chain carbonate having a fluorine atom-substituted alkyl group. Similarly, Japanese Patent Application Laid-Open No. 8-298134 proposes to improve the safety and reliability of a battery by using a chain fluorinated ester. Other than the ester compound, Japanese Patent Laid-Open No. 8-37024 proposes a secondary battery in which an electrolytic solution contains a chain or cyclic fluorinated ether. In Japanese Patent Laid-Open No. 10-12272, by mixing fluorinated alkane or chain fluorinated ether within a range (0.5 to 30% by weight) that does not impair battery characteristics, the flash point of the electrolyte is set to 50. A technique for setting the temperature to ℃ or higher is disclosed. Since these use a fluorine compound as a liquid non-aqueous electrolyte, the concentration is constant inside the battery.
[0006]
In addition, it has been studied to make a secondary battery flame-retardant by adding a fluorine compound to the gel electrolyte. JP-A-11-172096 describes the use of one or more fluorine compounds selected from the group of fluorinated carbonates, fluorinated ethers and fluorinated esters. In this publication, it is said that a gel electrolyte and a solid electrochemical device that can maintain a sufficiently high ionic conductivity, exhibit flame retardancy, and have excellent safety against heat generation and the like can be obtained. However, the concentration distribution of the fluorine compound in the gel electrolyte is not clearly described, and the manufacturing method thereof is the same as the conventional one.
[0007]
[Problems to be solved by the invention]
When a fluorine compound is added to the electrolyte solution for a lithium battery, battery characteristics such as charge / discharge efficiency and cycle characteristics deteriorate. This is because the fluorine compound tends to cause a reduction reaction on the surface of the carbon material, so that a side reaction occurs mainly at the negative electrode during charging of the battery. Furthermore, since the ionic conductivity of the electrolytic solution is lowered, high load characteristics cannot be obtained. Thus, it is considered that many fluorine compounds have poor compatibility with the positive and negative electrode materials, particularly the negative electrode carbon material. However, as long as the fluorine compound is used in a liquid state, it is extremely difficult to avoid contact with the electrode material. Therefore, it can be said that it is difficult to use the fluorine compound in the electrolytic solution.
In addition, even if a gel electrolyte (ion-conducting polymer) in which an electrolyte solution is absorbed in a gel substance contains a fluorine compound, contact with the electrode material is unavoidable. It is difficult to obtain good battery characteristics for the same reason as described above.
[0008]
As described above, even if the fluorine compound is used to improve the flame retardancy of the battery, the battery characteristics are deteriorated. Moreover, even if a fluorine compound is included in the gel electrolyte to such an extent that good battery characteristics are maintained, it is difficult to make the battery flame-retardant. Therefore, it is desired to find a fluorine compound suitable for battery flame retardancy and battery characteristics, and to provide a structure and manufacturing method for a secondary battery using the same.
[0009]
[Means for Solving the Problems]
  In view of the above problems, as a result of intensive studies aimed at preventing ignition when the battery temperature of a secondary battery having a gel electrolyte (ion conductive polymer) becomes abnormally high due to overcharging or the like, at least It has been found that if the fluorine electrolyte is contained in the gel electrolyte contained in the positive electrode or if a specific fluorine compound is contained in the electrolyte layer, the flame retardancy is improved. Furthermore, by making the concentration of the fluorine compound in the gel electrolyte contained in the positive electrode higher than the concentration of the fluorine compound in the gel electrolyte contained in the negative electrode, a gel electrolyte secondary battery that simultaneously satisfies good battery characteristics and flame retardancy Has also found that can be provided.
  Thus, according to the present invention, a positive electrode having a gel electrolyte containing an active material capable of inserting / extracting lithium ions and an electrolyte, a negative electrode, and a gel containing a nonaqueous solvent and a lithium salt between the positive electrode and the negative electrode A non-aqueous gel electrolyte layer in which a substance is disposed, wherein the positive electrode contains a fluorinated ester compound and / or a fluorinated ether compound, and the negative electrode does not contain a fluorinated ester compound and / or a fluorinated ether compound, or Provided is a gel electrolyte secondary battery comprising a lower concentration than the positive electrodeThe
[0011]
  FurtherFurthermore, according to the present invention, there is provided a method for producing the gel electrolyte secondary battery, wherein the fluorine compound concentration in the positive electrode forming precursor solution containing a solvent and a fluorinated ester compound and / or a fluorinated ether compound is determined. The concentration of the fluorine compound in the precursor solution for forming a negative electrode containing a solvent and a fluorinated ester compound and / or a fluorinated ether compound is made higher, and the positive electrode and negative electrode containing the active material are impregnated with the precursor solution, respectively. Thus, a gel electrolyte is obtained, and then a non-aqueous gel electrolyte layer is interposed between the positive electrode and the negative electrode.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the first gel electrolyte secondary battery of the present invention, a fluorine compound is contained in at least the gel electrolyte in the positive electrode. Furthermore, it is more preferable that the concentration of the fluorine compound in the gel electrolyte contained in the positive electrode is higher than the concentration of the fluorine compound in the gel electrolyte contained in the negative electrode, or only the positive electrode contains the fluorine compound. The reaction when the fluorine compound is vaporized to generate a nonflammable gas is an endothermic reaction. Therefore, it can vaporize at the time of abnormal heating of a battery and can reduce the temperature in a battery.
Examples of the fluorine compound include ester compounds represented by the following structural formula (I).
[0013]
[Chemical Formula 3]

[0014]
(R₁Represents a fluorine atom or a fluorinated substituted lower alkyl group, and R₂Represents a hydrogen atom, a fluorine atom, a lower alkyl group or a fluorinated substituted lower alkyl group, and R_ThreeRepresents a hydrogen atom, a lower alkyl group or a fluorinated substituted lower alkyl group. )
The ester compound provides good battery characteristics, and in particular, the flame retardant improvement effect is more remarkable. In the above definition, the lower alkyl group is preferably a group having 1 to 4 carbon atoms. If the alkyl group has 5 or more carbon atoms, the nonflammability is remarkably lowered, the effect of improving the flame retardancy of the secondary battery is reduced, and the desired thermal stability is hardly exhibited, which is not preferable. Further, increasing the number of F atoms in the alkyl group is not preferable because the production cost increases.
[0015]
Examples of the ester compound include methyl difluoroacetate, ethyl difluoroacetate, ethyl 5H-octafluoropentanoate, ethyl 7H-dodecafluoroheptanoate, ethyl 9H-hexadecafluorononanoate, 2-trifluoromethyl-3,3, Preferable examples include methyl 3-trifluoropropionate and methyl 1-trifluoro-2-trifluoromethyl-propionate, but are not limited thereto.
[0016]
Other fluorine compounds include fluorinated ethers represented by the following structural formula (II), fluorinated esters represented by the structural formula (III), and fluorinated carbonates (carbonates represented by the structural formula (IV)). ). These may be used alone or in combination of two or more. In addition, it is preferable to use a C1-C4 group for a lower alkyl group in the definition of the fluorine compound mentioned below.
[0017]
R_Four-O-R_Five    (II)
(R_FourAnd R_FiveAre the same or different and each represents a lower alkyl group or a fluorinated lower alkyl group. However, one is a fluorinated lower alkyl group. )
Specific examples of the fluorinated ethers include methyl nonafluorobutyl ether, 1-fluoromethoxy-1-methoxymethane, 1-difluoromethoxy-1-methoxymethane, 1-trifluoromethoxy-1-methoxymethane, 1,1 -Difluoromethoxymethane, 1-fluoromethoxy-2-methoxyethane, 1-difluoromethoxy-2-methoxyethane, 1-trifluoromethoxy-2-methoxyethane, 1,1-difluoromethoxyethane, 1-fluoroethoxy-2 -Methoxyethane, 1-difluoroethoxy-2-methoxyethane, 1-trifluoroethoxy-2-methoxyethane, 1-fluoroethoxy-2-fluoromethoxyethane, 1-fluoroethoxy-2-ethoxyethane, 1-difluoroethoxy -2-Ethoxye Emissions, 1,2 although difluoro diethoxyethane are preferable, but are not limited to.
[0018]
R₆-COO-R₇    (III)
(R₆And R₇Are the same or different and each represents a lower alkyl group or a fluorinated lower alkyl group. However, one is a fluorinated lower alkyl group. )
Specific examples of the fluorinated esters include fluoroethyl acetate, trifluoroethyl acetate, fluoromethyl propionate, difluoromethyl propionate, trifluoromethyl propionate, fluoroethyl propionate, difluoroethyl propionate, trifluoropropionate. Ethyl, methyl trifluoroacetate, ethyl trifluoroacetate and the like are preferred, but not limited thereto.
[0019]
R₈-OCOO-R₉    (IV)
(R₈And R₉Are the same or different and each represents a lower alkyl group or a fluorinated lower alkyl group. However, one is a fluorinated lower alkyl group. )
Specific examples of the fluorinated carbonates include 3-fluoropropylene carbonate, 4-fluoropropylene carbonate, 3-difluoropropylene carbonate, 3-fluoro-4-fluoropropylene carbonate, 3-difluoro-4-fluoropropylene carbonate, 4 -Trifluoromethyl-ethylene carbonate, 4-trifluoromethyl-3-fluoroethylene carbonate, 4-trifluoromethyl-4-fluoroethylene carbonate, 4-trifluoromethyl-3-difluoroethylene carbonate, 4-trifluoromethyl- 3-Fluoro-4-fluoroethylene carbonate and the like are preferable, but not limited thereto.
[0020]
As a fluorine compound, the fluorine compound which produces | generates a nonflammable gas at 90 to 200 degreeC is preferable. A fluorine compound that generates a nonflammable gas at a temperature lower than 90 ° C. is not preferable because a nonflammable gas is generated during battery production (particularly during crosslinking) or during normal use. Further, a fluorine compound that generates a nonflammable gas at a temperature higher than 200 ° C. is not preferable because it cannot follow the thermal runaway of the battery and cannot secure thermal stability.
A phosphorus compound which is a flame retardant substance may be used in combination with the fluorine compound. Examples thereof include cyclic phosphate esters such as methyl ethylene phosphate, ethyl ethylene phosphate, and methyl neopentyl phosphate, and acyclic phosphate esters such as trimethyl phosphate and triethyl phosphate.
[0021]
The electrochemical characteristics of the above fluorine compounds and phosphorus compounds have a cathode limit of 2.0 V vs. Li / Li⁺Below the anode limit is 4.2 V vs. Li / Li⁺The above is desirable. Furthermore, if these conditions are satisfied, other fluorine compounds and phosphorus compounds can be used.
Particularly preferred fluorine compounds are methyl difluoroacetate, ethyl difluoroacetate, methyl 2-trifluoromethyl-3,3,3-trifluoropropionate, and methyl nonafluorobutyl ether.
[0022]
As the gel electrolyte, a solid gel electrolyte in which an electrolyte solution for a lithium battery is absorbed in a gel material, for example, a gel electrolyte of a known lithium polymer battery can be used. As the electrolytic solution, a known lithium ion battery electrolytic solution can be used. Typical forms of the gel electrolyte are roughly classified into chemical gels and physical gels.
As a chemical gel, a macromonomer (a high molecular weight polymer; a high molecular weight polymer that is cross-linked to form a matrix of a gel electrolyte) has an electrolytic solution absorbed.
[0023]
Examples thereof include polymers obtained from alkylene oxides such as ethylene oxide (EO) and propylene oxide (PO) having two or more vinyl groups, and glycidyl methacrylate which is a combination of vinyl groups and epoxy groups. A mixture of these resins may be used alone or in combination of two or more.
As the physical gel, polyacrylonitrile (PAN), a copolymer of polyvinylidene fluoride (PVdF) and hexafluoropyrene (HFP) or the like is used, and an electrolytic solution is added as a plasticizer. These polymers may be used alone or in combination of two or more.
The electrolyte is present in the gel electrolyte in an amount of 50 to 99% by weight. If it is less than 50% by weight, lithium ion conductivity is inhibited, and if it exceeds 99% by weight, solidification is difficult, which is not preferable.
[0024]
The gel electrolyte is composed of the cross-linked body, an electrolytic solution for a lithium battery, and a fluorine compound. In this specification, the concentration of the fluorine compound in the gel electrolyte refers to the concentration of only the fluorine compound, and does not mean the concentration of fluorine in the fluorine compound. Further, the fluorine concentration contained in the macromonomer and the electrolyte solution, and the fluorine concentration contained in the positive or negative electrode binder are not included. In addition, the inventors of the present invention, for example, CHF₂COOC₂H_Five20% by weight of C and C_FourF₉OCH_ThreeIt is found that the former is more effective when 10% by weight is added. This means that it is more effective to simply increase the concentration of the fluorine compound than to increase the concentration of fluorine derived from the fluorine compound in the gel electrolyte. The presence or absence of a fluorine compound in the gel electrolyte and the concentration of the fluorine compound can be determined using ICP (inductively coupled plasma emission spectroscopy) or elemental analysis.
[0025]
The concentration of the fluorine compound in the gel electrolyte contained in the positive electrode (= active material + conductive agent + binder) is preferably in the range of 10 to 50% by weight with respect to the electrolyte solution in the gel electrolyte. More preferably, it is contained in the range of ˜50%. If the content of the fluorine compound is less than 10% by weight, the effect of flame retardancy is hardly exhibited. Conversely, if it exceeds 50% by weight, the ionic conductivity is lowered and the battery characteristics are lowered, which is not preferable.
[0026]
When the fluorine compound in the gel electrolyte contained in the negative electrode (= active material + binder) is included, the concentration is preferably in the range of 0 to 10% by weight with respect to the electrolyte solution in the gel electrolyte, A range of 1 to 5% is more preferable. The closer the content of the fluorine compound is to 0% by weight, the more difficult it is to exhibit the effect of flame retardancy. Therefore, the concentration of the fluorine compound in the gel electrolyte contained in the positive electrode and the non-electrolyte layer is increased. However, if it is too high, the ion conductivity of the gel electrolyte is lowered, and the battery characteristics may be lowered. In particular, when it is more than 10% by weight, the charge / discharge efficiency is lowered and the battery characteristics are lowered, which is not preferable. In addition, when a fluorine compound is not included, the structure which does not contain a gel electrolyte can also be employ | adopted for a negative electrode.
The non-electrolyte layer composed of the gel electrolyte and the separator may also contain a fluorine compound, but the concentration of the fluorine compound is contained within a range of 0 to 50% by weight with respect to the electrolyte solution in the gel electrolyte. It is preferable. A more preferable concentration is not less than the concentration of the fluorine compound of the gel electrolyte contained in the negative electrode and not more than the concentration of the fluorine compound of the gel electrolyte contained in the positive electrode.
[0027]
As the negative electrode active material, it is preferable to use a carbon material in which amorphous carbon is attached to the surface of graphite particles (hereinafter referred to as surface amorphous graphite). If surface amorphous graphite is used, the decomposition of the gel electrolyte, particularly the fluorine compound, can be remarkably prevented. Specifically, it can prevent rupture due to an increase in the internal pressure of the battery accompanying the generation of decomposition gas such as ethylene gas and carbon dioxide generated from the electrolyte contained in the gel electrolyte, liquid leakage to the outside, and further ignition of the battery, Long-term reliability and safety can be further improved. In addition, there is an effect of preventing the decomposition of the fluorine compound contained in the gel electrolyte. Therefore, it becomes possible to contain more fluorine compounds in the gel electrolyte. This prevents battery ignition and promotes long-term reliability and safety.
[0028]
Surface amorphous graphite is a material in which amorphous carbon is attached to the surface of graphite particles using a highly crystalline graphite material as a core material by a known gas phase method, liquid phase method, solid phase method or the like. Can be obtained. In the surface amorphous graphite, pores related to the specific surface area measured by the BET method are preferably blocked to some extent by the adhesion of amorphous carbon, and the specific surface area is 1 to 5 m.²A range of / g is preferred.
Specific surface area is 5m²When it is larger than / g, the contact area with the electrolytic solution and the fluorine compound contained in the gel electrolyte also increases, so the adverse effect of the decomposition reaction increases, which is not preferable. Furthermore, since the amount of adsorption of the polymerization initiator in the precursor solution to the negative electrode surface increases, it is not preferable such as inhibiting the crosslinking of the precursor solution or reducing the initial charge / discharge efficiency. Specific surface area is 1m²If it is smaller than / g, the contact area with the gel electrolyte is reduced, so that the electrode reaction rate is reduced and the load characteristics of the battery are lowered, which is not preferable.
[0029]
As the highly crystalline graphite material used for the core material, known materials can be used. Here, as a highly crystalline graphite material used as a core material, an average interplanar spacing (d) of (002) plane by the X-ray wide angle diffraction method (d₀₀₂) Of 0.335 to 0.340 nm, or Lc and La of 10 nm or more are preferably used.
d₀₀₂Is larger than 0.340 nm, or when Lc and La are smaller than 10 nm, the crystallinity as a core material is not sufficient. Since the capacity of the potential portion close to dissolution precipitation (0 to 300 mV on the basis of the Li potential) is not sufficient, it is not preferable.
[0030]
In addition, as a method for determining the crystallite size (Lc, La) by the X-ray wide angle diffraction method, a known method, for example, “Carbon Materials Experimental Technology 1, p. It is possible to apply the method described in “
Specifically, if the sample is a powder, it is left as it is. If it is in the form of a fine piece, it is pulverized in an agate mortar, and about 15 wt% of high-purity silicon powder for X-ray standard is added to the sample as an internal standard substance and mixed Then, the sample is put in a cell, and a wide angle X-ray diffraction curve is measured by a reflective diffractometer method using CuKα rays monochromatized by a graphite monochromator as a radiation source. For the correction of the curve, the following simple method is used without correcting the so-called Lorentz, deflection factor, absorption factor, atomic scattering factor and the like.
[0031]
That is, a baseline of a curve corresponding to (002) diffraction is drawn to obtain a corrected diffraction curve of the (002) plane. Then, in the corrected diffraction curve, a crystallite size Lc in the C-axis direction is obtained by Lc = (K · λ) / (β · cos θ) using a so-called half value β at a position half the peak height. Here, λ is 1.5418A, and θ is the diffraction angle. Similarly, La can be measured.
Also, 1580cm by argon laser Raman^-11360cm to the peak intensity ratio in the vicinity^-1The peak intensity ratio in the vicinity (hereinafter referred to as R value) is preferably 0.5 or less (more preferably 0.4 or less). When the R value exceeds 0.5, the crystallinity as a core material is not sufficient, and when the surface amorphous graphite is produced, the capacity of the potential portion close to the dissolution and precipitation of lithium is not sufficient, which is not preferable. .
[0032]
The crystallinity of the adhering portion is not particularly limited, but basically has a lower crystallinity than the core, that is, d₀₀₂By adopting a material having a large R value or the like, the effect as surface amorphous graphite can be obtained. In X-ray diffraction, the bulk properties of the material are defined, so if the surface layer is thin, it may not appear as a large difference, but in this case, for example, in the Raman measurement that can measure the physical properties of the surface The measured R value can be used effectively. More preferably, the low crystalline carbon material is d.₀₀₂Is greater than 0.34 nm, and the R value is greater than 0.5 (more preferably greater than 0.4). These are the same CVD conditions for carbon materials to be deposited on the surface and firing conditions for various raw materials. It can be defined indirectly by producing only a surface carbon material in a pseudo manner and measuring its physical properties.
[0033]
When the positive electrode and the negative electrode in the first gel electrolyte secondary battery contain a fluorine compound, the fluorine compound concentration in the positive electrode precursor solution is adjusted to be higher than the fluorine compound concentration in the negative electrode precursor solution. They can be produced by impregnating the positive electrode and the negative electrode, respectively, cross-linking them, and interposing a non-electrolyte layer between the positive electrode and the negative electrode.
The concentration of the fluorine compound in the positive electrode precursor solution is preferably in the range of 10 to 50% by weight with respect to the electrolytic solution, and more preferably in the range of 20 to 50%. If the fluorine compound concentration is less than 10% by weight, the effect of flame retardancy is hardly exhibited after formation of the gel electrolyte. Conversely, if the concentration is more than 50% by weight, the ion conductivity is lowered and the battery characteristics are lowered.
[0034]
The concentration of the fluorine compound in the negative electrode precursor solution is preferably in the range of 0 to 10% by weight with respect to the electrolytic solution, and more preferably in the range of 1 to 5%. The closer the fluorine compound concentration is to 0% by weight, the more difficult it is to exhibit the effect of flame retardancy. Therefore, the concentration of the fluorine compound in the positive electrode precursor solution is increased. However, if the concentration is too high, the ion conductivity of the gel electrolyte is lowered, and the battery characteristics may be lowered. On the other hand, if it exceeds 10% by weight, the charge / discharge efficiency is lowered, and the battery characteristics are lowered, which is not preferable.
[0035]
Furthermore, in the present invention, a non-aqueous gel electrolyte in which a positive electrode and a negative electrode containing an active material capable of inserting / extracting lithium ions, and a gel-like substance containing a non-aqueous solvent and a lithium salt are disposed between the positive electrode and the negative electrode There is also provided a second gel electrolyte secondary battery comprising a layer, wherein the gel electrolyte layer contains a fluorine compound represented by the structural formula (I).
The second gel electrolyte secondary battery may use the configuration of the first gel electrolyte secondary battery as it is, except that the non-aqueous gel electrolyte layer contains the fluorine compound represented by the structural formula (I). it can.
[0036]
The fluorine compound in the non-aqueous gel electrolyte layer is preferably contained in a weight ratio of 10 to 50% with respect to the gel substance. If the fluorine compound concentration is less than 10% by weight, the effect of flame retardancy is hardly exhibited after formation of the gel electrolyte. Conversely, if the concentration is more than 50% by weight, the ion conductivity is lowered and the battery characteristics are lowered.
More preferred fluorine compounds in the non-aqueous gel electrolyte layer are methyl difluoroacetate, ethyl difluoroacetate, methyl 2-trifluoromethyl-3,3,3-trifluoropropionate, and methyl nonafluorobutyl ether.
Hereinafter, the present invention will be described more specifically, but the present invention is not limited thereto.
[0037]
1A and 1B show a secondary battery including a gel electrolyte (ion-conducting polymer) as a nonaqueous electrolyte which is an example of a secondary battery according to the present invention. 1A and 1B includes a positive electrode active material 2 formed on a current collector 1 (a conductive agent and a binder are not shown), a positive electrode 4 including a gel electrolyte 3, and a current collector. In this structure, a non-aqueous electrolyte layer 9 made of a separator containing the gel electrolyte 3 is disposed between the negative electrode active material 6 (the binder is not shown) and the negative electrode 8 containing the gel electrolyte 7 on the body 5.
[0038]
FIG. 1A is an example in which the concentration of the fluorine compound contained in the positive electrode 4 and the nonaqueous electrolyte layer 9 is higher than the concentration of the fluorine compound contained in the negative electrode 8. Further, as shown in FIG. 1B, the present invention can be implemented even when the concentration of the fluorine compound of the positive electrode 4 is higher than that of the nonaqueous electrolyte layer 9 and the negative electrode 8. Even if lithium metal is used for the negative electrode 8, the effect of the present invention can be obtained.
[0039]
Even when the ion conductor is other than lithium ion, the present invention can be carried out. Ions of alkali metals other than lithium ions, for example, materials capable of electrochemically inserting / extracting sodium ions, sodium metals capable of electrochemically depositing / dissolving sodium, ions of alkaline earth metals, A material capable of electrochemically inserting / extracting magnesium ions or calcium ions, or a magnesium metal or calcium metal capable of electrochemically depositing / dissolving magnesium or calcium can also be used.
In that case, Fe (SO_Four)₂And NaFeO₂Or alkaline earth metal ions, for example, vanadium oxide capable of electrochemically inserting / extracting magnesium ions can also be used.
Below, an example of the manufacturing process and evaluation method of a secondary battery is shown.
[0040]
a) Preparation of negative electrode
A method for producing the negative electrode is described below. The negative electrode can be formed by mixing an active material, a binder and the like.
Specifically, the binder is dissolved in a solvent in a mortar to disperse the carbon material of the negative electrode. A kneading machine, a ball mill, or the like is used for the dispersion treatment, and the paste is adjusted so that the carbon material and the binder are uniformly dispersed. This paste is applied to a metal foil of a current collector, and this is temporarily dried at 40 to 100 ° C. Then, in order to heat-process at about 150 degreeC and to make a predetermined | prescribed active material density, it compression-molds using a press. A roller press is usually used for compression molding, and the material, rotation method, temperature, atmosphere, and the like of the press surface when these presses are applied are not particularly limited. Thereafter, a lead is welded to the uncoated portion of the electrode, and the product dried under reduced pressure at about 150 ° C. to remove moisture is used as the negative electrode.
[0041]
As a carbon material which is a negative electrode active material, a known negative electrode material of a lithium ion battery can be used, but it is preferable to use surface amorphous graphite. The particle size distribution of the carbon material is preferably about 0.1 to 150 μm. As the binder, PVdF, polytetrafluoroethylene (PTFE) or the like can be used, but is not limited thereto. The mixing ratio is preferably 1 to 30 parts by weight of the binder with respect to 100 parts by weight of the active material. In order to produce a high energy density battery, the active material density of the negative electrode is 1.4 g / cm.^ThreeThe above is preferable. In addition, it is preferable to perform heat treatment at a temperature around the melting point of the binder in order to improve the binding property in the production of the negative electrode.
[0042]
b) Preparation of positive electrode
A method for producing the positive electrode is described below. The positive electrode can be formed by mixing an active material with a conductive agent, a binder, or the like.
Specifically, the binder is dissolved in a solvent in a mortar, and the active material and the conductive agent are dispersed. For the dispersion treatment, a kneader, a ball mill, or the like is usually used, and the paste is adjusted so that the active material, the conductive agent, and the binder are uniformly dispersed. This paste is applied to a metal foil of a current collector, and this is temporarily dried at 40 to 100 ° C. Then, in order to heat-process at about 150 degreeC and to make a predetermined | prescribed active material density, it compacts using a press machine. A roller press is usually used for compression molding, and the material, rotation method, temperature, atmosphere, and the like of the press surface when these presses are applied are not particularly limited. Thereafter, a lead is welded to the uncoated portion of the electrode, and the product dried under reduced pressure at about 150 ° C. to remove moisture is used as the positive electrode.
[0043]
As the positive electrode active material, LiCoO₂LiNiO₂LiMnO₂LiFeO₂And this series of LiA_1-xT_xO₂(Where A is any of Fe, Co, Ni, and Mn, T represents a transition metal, a group 4B or 5B metal, 0 <X ≦ 1), LiMn₂O_FourFor example, a known positive electrode material for a lithium ion battery can be used.
Examples of the conductive agent include carbons such as acetylene black, graphite powder, and the like, but are not limited thereto.
PVdF, PTFE, etc. can be used as the binder, but are not limited thereto.
[0044]
The mixing ratio is preferably 1 to 50 parts by weight of the conductive agent and 1 to 30 parts by weight of the binder with respect to 100 parts by weight of the active material. In order to produce a high energy density battery, the active material density of the positive electrode is 2.8 g / cm.^ThreeIt is preferable that it is above, and further 3.0 g / cm^ThreeThe above is more preferable. In order to improve the binding property in the production of the positive electrode, it is preferable to perform heat treatment at a temperature around the melting point of the binder.
[0045]
The positive electrode and the negative electrode are basically formed by forming each active material obtained by fixing the positive electrode and the negative electrode active material with a binder on a metal foil as a current collector. The material and shape of the current collector are not limited, and a conductor that is chemically and electrochemically stable with respect to the positive electrode, the negative electrode active material, and the electrolytic solution can be used. Examples of the metal foil material include aluminum, stainless steel, copper, and nickel. In view of electrochemical stability, stretchability and economy, an aluminum foil is preferable for the positive electrode and a copper foil is preferable for the negative electrode. The form of the positive electrode and the negative electrode current collector may be a mesh, expanded metal, or the like in addition to the metal foil.
[0046]
c) Preparation of precursor solution of gel electrolyte and formation method of gel electrolyte
Typical forms of the gel electrolyte are roughly classified into chemical gels and physical gels.
As a method of forming a chemical gel, there is a method of preparing a gel electrolyte precursor solution composed of a macromonomer, an electrolytic solution, a fluorine compound, and a polymerization initiator, and solidifying it by performing a polymerization reaction including a crosslinking reaction. Can be mentioned.
The fluorine compound concentration in the positive electrode precursor solution is preferably in the range of 10 to 50% by weight, more preferably in the range of 20 to 50% with respect to the electrolytic solution. The concentration of the fluorine compound in the negative electrode precursor solution is preferably in the range of 0 to 30% by weight, more preferably in the range of 1 to 10% with respect to the electrolytic solution.
[0047]
Examples of the macromonomer include ethylene oxide (EO), propylene oxide (PO), and glycidyl methacrylate. These macromonomers may be used alone or in combination of two or more.
If the amount of the macromonomer with respect to the electrolytic solution is too small, solidification is difficult, and if it is too large, lithium ion conductivity is inhibited. Therefore, the weight ratio is preferably 1 to 50%. Examples of the crosslinking method include a method using light energy such as ultraviolet rays, electron beams, and visible light, and a method using heat. A polymerization initiator may be added to accelerate the crosslinking reaction or the polymerization reaction. In particular, in the crosslinking method by ultraviolet rays or heating, it is preferable to add a polymerization initiator to the electrolytic solution by several% or less.
[0048]
Commercially available products such as azoisobutyronitrile (AIBN), benzoyl peroxide (BPO), 2,2-dimethoxy-2-phenylacetophenone (DMPA) can be used as the polymerization initiator for ultraviolet rays. These initiators may be used alone or in combination of two or more. The wavelength of ultraviolet light is suitably 250 to 360 nm.
As the polymerization initiator for heating, those having a 10-hour half-life temperature of 40 ° C. or more and 90 ° C. or less are preferable. The heating temperature is suitably 40-80 ° C.
[0049]
As a method of forming a physical gel, one or more polymers such as PVdF, HEP, polymethyl methacrylate, and polyvinyl chloride are mixed, and tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), etc. There is a method in which a solution prepared by dissolving in a solvent, casting and removing the solvent by drying or the like is impregnated with an electrolytic solution containing a fluorine compound.
The fluorine compound concentration of the positive electrode is preferably in the range of 10 to 50% by weight, more preferably in the range of 20 to 50% with respect to the electrolytic solution. The concentration of the fluorine compound in the negative electrode is preferably in the range of 0 to 30% by weight, more preferably in the range of 1 to 10% with respect to the electrolytic solution.
[0050]
Nonaqueous solvents for the electrolyte include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and chain carbonates such as DMC, DEC, and methyl ethyl carbonate (MEC). And cyclic carboxylic acid esters such as γ-butyrolactone (γ-BL) can be preferably used.
[0051]
When a carbon material is used for the negative electrode, it is preferable to include EC in order to reduce the decomposition of the gel electrolyte. The EC content in the non-aqueous solvent is preferably 0 to 80% by volume. In order to improve low temperature characteristics, it is desirable to contain at least γ-BL. In order to improve the permeability of the gel electrolyte precursor solution into the electrode active material or the separator substrate, DMC, DEC, MEC, etc. are added in a volume ratio of 0 to 50% with respect to the whole non-aqueous solvent. It is preferable. These are used as one or a mixed solvent of two or more. Further, vinylene carbonate (VC) or ethylene sulfite (ES) may be added in a weight ratio of about 1 to 10% with respect to the total weight of the non-aqueous solvent.
[0052]
Examples of the lithium salt of the electrolyte include known lithium salts such as lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenide, and one or more of these may be used. Can be mixed and used. An electrolytic solution for a lithium battery is prepared by dissolving a lithium salt in a non-aqueous solvent.
The lithium salt concentration is preferably 0.8 to 2.5 mol / l with respect to the entire non-aqueous solvent. If the salt concentration is lower than 0.8 mol / l, the ionic conductivity necessary to obtain the discharge characteristics under high load cannot be obtained, and if the salt concentration is higher than 2.5 mol / l, the cost of the lithium salt only increases. In addition, since the viscosity increases, it is difficult to penetrate into the electrode. Furthermore, since it takes a very long time to dissolve the lithium salt, it is not preferable because it is industrially unsuitable.
[0053]
The non-aqueous solvent and lithium salt used for preparing the electrolytic solution are not limited to those listed above. Normal temperature molten salt (ionic liquid) instead of non-aqueous solvent, for example, EMIBF_Four(1-ethyl-3-methylimidazolium tetrafluoroborate) to LiBF_FourThose having a lithium salt such as those having an amide anion, those having a cyclic ammonium cation, and the like can be used. AlCl_Three-EMIC (1-ethyl-3-methylimidazolium chloride) -LiCl, AlCl_Three-EMIC-LiCl-SOCl₂A room-temperature molten salt (ionic liquid) such as can also be used instead of the electrolytic solution.
[0054]
d) Battery assembly
As a manufacturing method of the gel electrolyte secondary battery of this invention, it can carry out as follows, for example.
First, the negative electrode produced in a), the positive electrode produced in b), and the separator are impregnated with the precursor solution of the gel electrolyte prepared in c), and light is irradiated or heated to crosslink. At this time, the concentration of the fluorine compound in the positive electrode precursor solution is set higher than the concentration of the negative electrode precursor. After crosslinking, as shown in FIGS. 1A and 1B, a nonaqueous electrolyte layer 9 is interposed between the positive electrode 4 and the negative electrode 8 integrated with the gel electrolyte. And a sheet-like battery is produced by sandwiching between two aluminum laminate resin films and heat-sealing. Further, after crosslinking and solidification, a positive electrode (separator (a plurality of negative electrodes may be stacked and inserted into a bag of an exterior material. After that, the battery is left at 50 to 70 ° C. for 6 to 24 hours). May be.
[0055]
Examples of the separator for holding the gel electrolyte include nonwoven fabrics or woven fabrics such as electrically insulating synthetic resin fibers, glass fibers, and natural fibers. Among these, non-woven fabrics such as polyvinylidene chloride, polyethylene, and polypropylene are preferable from the viewpoint of quality stability.
Some of these synthetic resin non-woven fabrics have a function in which when the battery abnormally generates heat, the separator is melted by heat to block between the positive and negative electrodes, and these can be suitably used from the viewpoint of safety.
[0056]
The thickness of the separator is not particularly limited, but it is sufficient that the separator can hold a necessary amount of liquid and has a thickness that prevents a short circuit between the positive electrode and the negative electrode. Usually, a thickness of about 0.01 to 1 mm is used. Preferably, it is about 0.02-0.05 mm. These separators have an air permeability of 1 to 500 sec / cm.^ThreeIt is preferable because it has a strength sufficient to prevent a battery internal short circuit while maintaining a low battery internal resistance. The shape of the battery can be applied to various shapes such as a cylindrical shape, a square shape, a coin shape, a button shape, and a sheet shape in addition to the laminate type shown above.
[0057]
For example, in a cylindrical or rectangular battery, a sheet electrode is mainly inserted into a can and the can and the sheet electrode are electrically connected. The precursor solution is injected, the sealing plate is sealed through an insulating packing, or the sealing plate and the can are insulated and sealed with a hermetic seal, and heated to produce a battery. At this time, a safety valve equipped with a safety element can be used as a sealing plate.
Examples of the safety element include a fuse, a bimetal, and a PTC element as an overcurrent prevention element. In addition to the safety valve, as a countermeasure against the increase in the internal pressure of the battery can, a method of making a crack in the gasket, a method of making a crack in the sealing plate, a method of making a cut in the battery can, and the like are used. Further, an external circuit incorporating an overcharge or overdischarge countermeasure may be used. In the case of a coin-type or button-type battery, the positive electrode and negative electrode are formed into pellets, placed in a can, injected with a precursor solution, caulked with an insulating packing, and heated to produce a battery. Is made. A synthetic resin nonwoven fabric or the like is used for the separator.
[0058]
e) Evaluation of battery characteristics
In the charge / discharge operation test, charging is performed at a constant current value until the battery voltage reaches 4.1 to 4.3V. After the battery voltage reaches 4.1 to 4.3 V, it is charged by time control. Discharging is performed at a constant current value until the battery voltage reaches 2.7 to 3.0V.
The electrode characteristics and battery evaluation are all performed in a glove box under an inert gas atmosphere. Usually, argon, nitrogen, etc. are used suitably as an inert gas.
[0059]
f) Analysis of fluorine compound concentration in gel electrolyte
An electrolytic solution and a fluorine compound are extracted from the gel electrolyte using a solvent that is soluble in both water and an organic solvent, for example, THF (the extracted mixed solution of the electrolytic solution and the fluorine compound is referred to as an extract). At this time, if it is weakly basic in advance so that fluoride ions do not react with hydrogen ions and volatilize as hydrogen fluoride, it is effective in improving the reliability of quantitativeness.
[0060]
And the fluoride ion in the extract obtained above is quantitatively analyzed using ICP. In addition, since the fluorine of the lithium salt in electrolyte solution is also quantified simultaneously, it is necessary to subtract. For example, LiBF_FourSince the composition is such as 1 mol of lithium and 1 mol of boron with respect to 4 mol of fluorine, the fluorine concentration of the lithium salt can be subtracted by simultaneously performing quantitative analysis of other elements. As described above, the concentration of the fluorine compound in the gel electrolyte can be obtained by analyzing the fluoride ion in the extract.
Further, if elemental analysis is used, the fluorine concentration in the gel electrolyte can be analyzed. Also in that case, since it is necessary to subtract the fluorine concentration in the lithium salt, the concentrations of elements that are likely to constitute the lithium salt, such as lithium, boron, and phosphorus, are also obtained at the same time._Four, LiPF₆The amount of F is calculated from the chemical formula, and the concentration of the fluorine compound in the gel electrolyte is analyzed.
[0061]
【Example】
EXAMPLES Hereinafter, although an Example and a comparative example are shown about this invention and the effect is demonstrated concretely, this invention is not restrict | limited to the following Example.
Example 1
In the structural formula (I) as a fluorine compound, R₁And R₂Is CF_Three, R_ThreeIs CH_Three(CF_Three)₂CHCOOCH_ThreeUsing Example 1, the battery of Example 1 was fabricated through the following steps.
[0062]
a) Preparation of negative electrode
Carbon materials include surface amorphous graphite (average particle size 18 μm, d₀₀₂= 0.336 nm, R value = 0.5, specific surface area 1 to 2 m²/ G) was used. The binder PVdF was dissolved in the solvent NMP in a mortar to disperse the surface amorphous graphite. For the dispersion treatment, a biaxial planetary kneader was used to adjust the paste so that the carbon material and the binder were uniformly dispersed. The composition of the negative electrode was 100 parts by weight of carbon material and 10 parts by weight of PVdF. This paste was applied to a copper foil of about 20 μm and temporarily dried at 50 to 70 ° C. Thereafter, heat treatment is performed at about 150 ° C. for 12 hours, and the active material density is 1.5 g / cm.^ThreeUntil it is about, it was compression molded using a roller press in the atmosphere. The electrode size was set to 3.0 × 6.5 cm (coating part 3.0 × 6.0 cm), and a nickel foil (50 (m) lead was welded to the non-coating part. What was dried under reduced pressure at 150 ° C. for 12 hours was used as the negative electrode.
[0063]
Note that the average interplanar spacing (d₀₀₂) And crystallite size (Lc, La) can be measured by known methods such as “Carbon Materials Experimental Technology 1, p. The method described in Sho 61-1111907 was applied. Form factor K for obtaining crystallite size (= Lc · β · cos θ / λ; β: half-value width, θ: d₀₀₂The angle of λ: X-ray wavelength was 0.9. The particle size was measured using a laser diffraction particle size distribution analyzer (SALD1100 manufactured by Shimadzu Corporation), and was determined as a particle size having a peak in the particle size distribution.
[0064]
b) Preparation of positive electrode
For the positive electrode active material, lithium cobalt oxide LiCoO₂(Average particle size 10 μm) was used. LiCoO₂Was synthesized by a known method. The sample obtained from the results of X-ray diffraction measurement using CuKα rays with an output of 2 kW from the target Cu enclosure as the X-ray source, cobalt valence analysis by iodometry, and elemental analysis by ICP is LiCoO.₂It was confirmed that. PVdF was dissolved in NMP in a mortar, and the positive electrode active material and the conductive agent acetylene black were dispersed. A biaxial planetary kneader was used for the dispersion treatment, and the paste was adjusted so that the positive electrode active material, the conductive agent, and the binder were uniformly dispersed. The composition of the positive electrode is LiCoO₂100 parts by weight, 5 parts by weight of acetylene black, and 5 parts by weight of PVdF were used.
This paste was applied onto an aluminum foil having a thickness of about 20 μm and temporarily dried at 50 to 70 ° C. Thereafter, heat treatment is performed at about 150 ° C. for 12 hours, and the active material density is 3.0 g / cm.^ThreeUntil it is about, it was compression molded using a roller press in the atmosphere. The electrode size was 3.0 × 6.5 cm (coated portion 3.0 × 6.0 cm), and a nickel foil (50 μm) lead was welded to the uncoated portion. Then, what was dried under reduced pressure at about 150 ° C. for 12 hours to remove moisture was used as the positive electrode.
[0065]
c) Preparation of gel electrolyte precursor solution
A gel electrolyte precursor solution containing a fluorine compound impregnated in the positive electrode and the separator was prepared as follows. In a mixed solvent of EC, γ-BL and MEC (volume ratio 24:56:20), LiBF_FourWas dissolved to 2.2 mol / l, and VC was further added to 3 wt% to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and (CF_Three)₂CHCOOCH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration 20% by weight). Next, this mixed liquid and a polymer or copolymer (TA) of a trifunctional acrylate having a molecular weight of 7,500 to 9,000, a polymer or copolymer of a monofunctional acrylate having a molecular weight of 2,800 to 3,000. The union (MA) was mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 1,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for the positive electrode and the nonaqueous electrolyte layer.
[0066]
A gel electrolyte precursor solution containing a fluorine compound to be impregnated into the negative electrode was prepared as follows. In a mixed solvent of EC, γ-BL and MEC (volume ratio 24:56:20), LiBF_FourWas dissolved to 1.0 mol / l, and 3 wt% of VC was further added to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and (CF_Three)₂CHCOOCH_ThreeWere mixed at a weight ratio of 9: 1 (concentration of fluorine compound: 10% by weight). Next, this mixed solution, TA and MA were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 2,000 ppm of a polymerization initiator was added to the mixed solution to obtain a negative electrode precursor solution.
[0067]
d) Battery assembly
The negative electrode prepared in a) was impregnated with the negative electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode prepared in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the positive electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, a positive electrode and a negative electrode covered with a non-aqueous electrolyte layer (a separator including a gel electrolyte) were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery. .
[0068]
(Example 2)
In the structural formula (I) as a fluorine compound, R₁And R₂Is CF_Three, R_ThreeIs CH_Three(CF_Three)₂CHCOOCH_ThreeThe battery of Example 2 was fabricated through the following steps.
[0069]
a) Preparation of negative electrode
Surface amorphous graphite (average particle size 25 μm, d₀₀₂= 0.336 nm, R value = 0.25, specific surface area 1 to 2 m²/ G) was used, and the same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
[0070]
c) Preparation of gel electrolyte precursor solution
A gel electrolyte precursor solution containing a fluorine compound impregnated in the positive electrode and the separator was prepared as follows. In a mixed solvent of EC, γ-BL and MEC (volume ratio 24:56:20), LiBF_Four2.4 mol / l was melt | dissolved, and also VC was added so that it might become 5 wt%, and the electrolyte solution for lithium batteries was obtained. Next, the electrolyte and (CF_Three)₂CHCOOCH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration 20% by weight). Next, this mixed solution, TA having a molecular weight of 7,500 to 9,000, and MA having a molecular weight of 2,800 to 3,000 were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 1,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for a positive electrode and a nonaqueous electrolyte layer.
[0071]
The precursor solution of the gel electrolyte not containing the fluorine compound impregnated in the negative electrode (fluorine compound concentration 0% by weight) was prepared as follows. In a mixed solvent of EC, γ-BL and MEC (volume ratio 24:56:20), LiBF_FourWas dissolved to 1.0 mol / l, and VC was further added to 5 wt% to obtain an electrolytic solution for a lithium battery. Next, this mixed solution, TA and MA were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 2,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for a negative electrode.
[0072]
d) Battery assembly
The negative electrode prepared in a) was impregnated with the negative electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode prepared in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the positive electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode and the negative electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0073]
(Example 3)
In the structural formula (I) as a fluorine compound, R₁And R₂Is CF_Three, R_ThreeIs CH_Three(CF_Three)₂CHCOOCH_ThreeUsing Example 1, the battery of Example 3 was fabricated in the following steps.
[0074]
a) Preparation of negative electrode
Surface amorphous graphite (average particle size 12 μm, d₀₀₂= 0.336 nm, R value = 0.35, specific surface area 1 to 2 m²/ G) was used, and the same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
c) Preparation of gel electrolyte precursor solution
The same operation as in Example 2 was repeated to obtain a positive electrode precursor solution containing a fluorine compound and a non-aqueous electrolyte layer negative electrode precursor solution and a negative electrode precursor solution containing no fluorine compound.
[0075]
d) Battery assembly
The surface of the negative electrode produced in a) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the negative electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode prepared in b) was impregnated with the positive electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the negative electrode and the positive electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0076]
Example 4
In the structural formula as a fluorine compound, R₁And R₂Is CF_Three, R_ThreeIs CH_Three(CF_Three)₂CHCOOCH_ThreeThe battery of Example 4 was fabricated through the following steps.
a) Preparation of negative electrode
A negative electrode was obtained by repeating the same operation as in Example 1 except that artificial graphite (KS-25) was used as the carbon material.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
c) Preparation of gel electrolyte precursor solution
The same operation as in Example 3 was repeated to obtain a positive electrode precursor solution and a negative electrode precursor solution.
[0077]
d) Battery assembly
The surface of the negative electrode produced in a) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the negative electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode prepared in b) was impregnated with the positive electrode precursor solution prepared in c) and cross-linked by ultraviolet irradiation. Next, the negative electrode and the positive electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0078]
(Example 5)
In the structural formula (I) as a fluorine compound, R₁And R₂Is F, R_ThreeIs CH₂C
H_ThreeCHF₂COOCH₂CH_ThreeA battery of Example 5 was produced by the following steps.
[0079]
a) Preparation of negative electrode
Surface amorphous graphite (average particle size 25 μm, d₀₀₂= 0.336 nm, R value = 0.25, specific surface area 1 to 2 m²/ G) was used, and the same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
[0080]
c) Preparation of gel electrolyte precursor solution
A gel electrolyte precursor solution containing a fluorine compound impregnated in the positive electrode and the separator was prepared as follows. In a mixed solvent of EC, DMC and MEC (volume ratio 25:55:20), LiPF₆Was dissolved to 1.5 mol / l to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and CHF₂COOCH₂CH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration 20% by weight). Next, this mixed solution was mixed with TA having a molecular weight of 7,500 to 9,000 and MA having a molecular weight of 2,800 to 3,000 at a weight ratio of 97: 2.4: 0.6. Furthermore, 1,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for a positive electrode and a nonaqueous electrolyte layer.
[0081]
A gel electrolyte precursor solution containing a fluorine compound to be impregnated into the negative electrode was prepared as follows. In a mixed solvent of EC, DMC and MEC (volume ratio 25:55:20), LiPF₆Was dissolved to 1.0 mol / l, and VC was further added to 3 wt% to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and CHF₂COOCH₂CH_ThreeWere mixed at a weight ratio of 9: 1 (concentration of fluorine compound: 10% by weight). Next, this mixed solution, TA and MA were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 2,000 ppm of a polymerization initiator was added to the mixed solution to obtain a negative electrode precursor solution.
[0082]
d) Battery assembly
The negative electrode prepared in a) was impregnated with the negative electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode prepared in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the positive electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode and the negative electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0083]
(Example 6)
In the structural formula (II) as a fluorine compound, R_FourIs C_FourF₉, R_FiveIs CH_ThreeC_FourF₉OCH_ThreeUsing (methyl nonafluorobutyl ether), a battery of Example 6 was produced through the following steps.
[0084]
a) Preparation of negative electrode
Surface amorphous graphite (average particle size 25 μm, d₀₀₂= 0.336 nm, R value = 0.25, specific surface area 1 to 2 m²/ G) was used, and the same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
[0085]
c) Preparation of gel electrolyte precursor solution
A gel electrolyte precursor solution containing a fluorine compound impregnated in the positive electrode and the separator was prepared as follows. LiBETI was dissolved in a mixed solvent of EC and EMC (volume ratio 40:60) so as to be 1.5 mol / l to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and C_FourF₉OCH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration 20% by weight). Next, this mixed solution, TA having a molecular weight of 7,500 to 9,000, and MA having a molecular weight of 2,800 to 3,000 were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 1,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for a positive electrode and a nonaqueous electrolyte layer.
[0086]
A gel electrolyte precursor solution containing a fluorine compound to be impregnated into the negative electrode was prepared as follows. EC, EMC and C_FourF₉OCH_ThreeLiBETI was dissolved in a mixed solvent of (volume ratio 40:60) so as to be 0.9 mol / l, and VC was further added so as to be 3 wt% to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and C_FourF₉OCH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration: 10% by weight). Next, this mixed solution, TA and MA were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 2,000 ppm of a polymerization initiator was added to the mixed solution to obtain a negative electrode precursor solution.
[0087]
d) Battery assembly
The negative electrode prepared in a) was impregnated with the negative electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode prepared in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the positive electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode and the negative electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0088]
(Example 7)
In the structural formula (II) as a fluorine compound, R_FourIs CF_ThreeCF₂CH₂, R_FiveIs CH_ThreeCF which is_ThreeCF₂CH₂OCH_ThreeA battery of Example 7 was produced by the following steps.
[0089]
a) Preparation of negative electrode
Surface amorphous graphite (average particle size 25 μm, d₀₀₂= 0.336 nm, R value = 0.25, specific surface area 1 to 2 m²/ G) was used, and the same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
[0090]
c) Preparation of gel electrolyte precursor solution
A gel electrolyte precursor solution containing a fluorine compound impregnated in the positive electrode and the separator was prepared as follows. In a mixed solvent of EC, γ-BL and DEC (volume ratio 25:55:20), LiBF_FourWas dissolved to 2.1 mol / l, and VC was further added to 6 wt% to obtain an electrolyte for a lithium battery. Next, the electrolyte and CF_ThreeCF₂CH₂OCH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration 20% by weight). Next, this mixed solution was mixed with TA having a molecular weight of 7,500 to 9,000 and MA having a molecular weight of 2,800 to 3,000 at a weight ratio of 97: 2.4: 0.6. Furthermore, 1,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for a positive electrode and a nonaqueous electrolyte layer.
[0091]
A gel electrolyte precursor solution containing a fluorine compound to be impregnated into the negative electrode was prepared as follows. In a mixed solvent of EC, γ-BL and MEC (volume ratio 25:55:20), LiBF_FourWas dissolved to 1.0 mol / l, and VC was further added to 6 wt% to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and CF_ThreeCF₂CH₂OCH_ThreeWere mixed at a weight ratio of 9: 1 (concentration of fluorine compound: 10% by weight). Next, the electrolyte solution, TA, and MA were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 2,000 ppm of a polymerization initiator was added to the mixed solution to obtain a negative electrode precursor solution.
[0092]
d) Battery assembly
The negative electrode prepared in a) was impregnated with the negative electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode prepared in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the positive electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode and the negative electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0093]
(Example 8)
In the structural formula (III) as a fluorine compound, R₆Is CHF₂, R₇Is CH_ThreeCHF₂COOCH_ThreeA battery of Example 8 was produced by the following steps.
[0094]
a) Preparation of negative electrode
Surface amorphous graphite (average particle size 25 μm, d₀₀₂= 0.336 nm, R value = 0.25, specific surface area 1 to 2 m²/ G) was used, and the same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
[0095]
c) Preparation of gel electrolyte precursor solution
A gel electrolyte precursor solution containing a fluorine compound impregnated in the positive electrode and the separator was prepared as follows. In a mixed solvent of EC, DMC and MEC (volume ratio 25:55:20), LiPF₆Was dissolved to 2.0 mol / l to obtain an electrolytic solution for a lithium battery. Next, the electrolyte and CHF₂COOCH₂CH_ThreeWere mixed at a weight ratio of 4: 1 (fluorine compound concentration 20% by weight). Next, this mixed solution was mixed with TA having a molecular weight of 7,500 to 9,000 and MA having a molecular weight of 2,800 to 3,000 at a weight ratio of 97: 2.4: 0.6. Furthermore, 1,000 ppm of a polymerization initiator was added to the mixed solution to obtain a precursor solution for a positive electrode and a nonaqueous electrolyte layer.
[0096]
A gel electrolyte precursor solution containing a fluorine compound to be impregnated into the negative electrode was prepared as follows. In a mixed solvent of EC, DMC and MEC (volume ratio 25:55:20), LiPF₆Was dissolved to 1.0 mol / l, and VC was further added to 3 wt% to obtain an electrolyte for a lithium battery. Next, the electrolyte and CHF₂COOCH₂CH_ThreeWere mixed at a weight ratio of 9: 1 (concentration of fluorine compound: 10% by weight). Next, the electrolyte solution, TA, and MA were mixed at a weight ratio of 97: 2.4: 0.6. Furthermore, 2,000 ppm of a polymerization initiator was added to the mixed solution to obtain a negative electrode precursor solution.
[0097]
d) Battery assembly
The negative electrode prepared in a) was impregnated with the negative electrode precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode prepared in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the positive electrode precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode and the negative electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0098]
(Comparative Example 1)
The battery of Comparative Example 1 was produced through the following steps.
a) Preparation of negative electrode
The same operation as in Example 1 was repeated to obtain a negative electrode.
b) Preparation of positive electrode
The same operation as in Example 1 was repeated to obtain a positive electrode.
c) Preparation of gel electrolyte precursor solution
The same operation as in Example 1 was repeated to prepare only a precursor solution containing no fluorine compound.
[0099]
d) Battery assembly
The negative electrode produced in a) was impregnated with the gel electrolyte precursor solution prepared in c) and crosslinked by ultraviolet irradiation. Next, the surface of the positive electrode produced in b) was covered with a nonwoven fabric (separator) having a thickness of 20 μm, and the gel electrolyte precursor solution prepared in c) was soaked and crosslinked by ultraviolet irradiation. Next, the positive electrode and the negative electrode covered with the non-aqueous electrolyte layer were overlapped, and sandwiched between two aluminum laminate resin films, and heat-sealed to produce a sheet-like battery.
[0100]
(1) Evaluation of battery characteristics
The effects of gel electrolytes containing fluorine compounds on battery characteristics were investigated. The active material amounts of the positive electrode and the negative electrode were set so that the battery capacities of the examples and comparative examples were all about 600 mAh.
These batteries were charged at a constant current value (0.2 C) until the battery voltage reached 4.1 V, and after reaching 4.1 V, they were charged at a constant voltage until the total charging time reached 12 hours. Discharging was performed at a constant current value (0.2 C) until the battery voltage reached 2.75V. All battery evaluations were performed at 20 ° C. in a glove box under an inert gas atmosphere.
[0101]
(2) Analysis of fluorine compound concentration in precursor solution and gel electrolyte
The gel electrolyte secondary battery was disassembled, and the gel electrolyte 3 contained in the positive electrode of FIG. 1, the gel electrolyte 7 contained in the negative electrode, and the gel electrolyte contained in the nonaqueous electrolyte layer 9 were collected. Next, the electrolytic solution and the fluorine compound were extracted from each gel electrolyte using THF. And the fluoride ion in the extract obtained above was quantitatively analyzed using ICP. The precursor solution was quantitatively analyzed as it was. The fluorine concentration of the lithium salt was subtracted.
Table 1 shows the initial discharge capacity of the battery and the concentration of the fluorine compound.
[0102]
[Table 1]

[0103]
Examples 2 to 4 in which the negative electrode did not contain a fluorine compound had a higher discharge capacity than Examples 1 and 5 to 8 containing about 10% by weight of a fluorine compound. This is considered to be reflected in a decrease in discharge capacity because the compatibility between the negative electrode and the fluorine compound is not good.
Therefore, as in Examples 1 to 8, even when the concentration of the fluorine compound contained in the positive electrode or the nonaqueous electrolyte layer is higher than the concentration of the fluorine compound contained in the negative electrode, the battery characteristics equivalent to those of Comparative Example 2 are maintained. I understood that I could do it.
On the other hand, the discharge capacity of the battery of Example 4 was lower than that of the batteries of Examples 2 and 3. As described in the report that the electrolytic solution is decomposed when the graphite-based carbon material is used, in Example 4, the decomposition reaction of the gel electrolyte occurs remarkably, and charging is difficult. Therefore, a high-capacity gel electrolyte secondary battery can be provided by using a carbon material in which amorphous carbon is attached to the surface of the graphite particles of the present invention.
[0104]
(3) Safety evaluation by overcharge test
The effect on the safety of batteries using gel electrolytes containing fluorine compounds was investigated. The active material amounts of the positive electrode and the negative electrode were set so that the battery capacities of the examples and comparative examples were all 600 mAh.
Ten batteries of Examples 1 to 8 and Comparative Example 1 were produced. All the batteries used in the test were wrapped with thermal insulation glass wool. These batteries were charged at a constant current value (3C) until the battery voltage reached 4.8V, and after reaching 4.8V, they were charged at a constant voltage until the total charging time reached 4 hours. The state of the battery was observed from the start to the end of charging. All battery evaluations were performed at room temperature in the air. Table 2 shows the overcharge test results.
[0105]
[Table 2]

[0106]
As shown in Table 2, it can be said that the batteries of Examples 1 to 8 were ruptured even in an overcharged state, and there was no battery that was completely burnt like Comparative Example 1, and the thermal stability was remarkably improved. Moreover, the temperature which burst was 130-150 degreeC in Examples 1-8, and the comparative example 1 was 200 degreeC vicinity. This is considered to be because the fluorine compound was vaporized as the temperature increased to generate a nonflammable gas, and the reaction was an endothermic reaction, which prevented the battery from igniting but igniting.
[0107]
In this example, the concentration of the fluorine compound in the gel electrolyte contained in the positive electrode was 20% by weight and the concentration of the fluorine compound in the gel electrolyte contained in the negative electrode was 10% by weight, but the fluorine compound in the gel electrolyte contained in the positive electrode was used. The same results were obtained even when the concentration of 10 to 50% by weight and the concentration of the fluorine compound in the gel electrolyte contained in the negative electrode was 0 to 10% by weight.
Further, in this example, the same fluorine compound in the gel electrolyte contained in the positive electrode and the negative electrode was used, but similar results were obtained even if the fluorine compounds in the gel electrolyte contained in the positive electrode and the negative electrode were different. .
As described above, as shown in Examples 1 to 8, the concentration of the fluorine compound contained in the positive electrode or the non-aqueous electrolyte layer is set higher than the concentration of the fluorine compound contained in the negative electrode, while maintaining the battery characteristics. Even if the battery is charged, the secondary battery can be prevented from firing.
[0108]
【The invention's effect】
According to the present invention, since at least the positive electrode of the gel electrolyte secondary battery contains a fluorine compound, when the battery temperature becomes abnormally high due to overcharging, the fluorine compound does not burn at a temperature lower than the temperature at which the thermal runaway reaction occurs. Therefore, it is possible to provide a highly safe gel electrolyte secondary battery in which the risk of ignition is avoided and a method for manufacturing the same.
Furthermore, the structure in which the concentration of the fluorine compound in the gel electrolyte contained in the positive electrode is higher than the concentration of the fluorine compound in the gel electrolyte contained in the negative electrode, the structure containing only the fluorine compound in the positive electrode, or the gel electrolyte layer containing a specific fluorine compound If it is provided, battery characteristics will not be impaired.
Therefore, the industrial significance of the present invention is great.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a cross-sectional structure of a positive electrode, a negative electrode, and a non-aqueous electrolyte layer that are examples of a secondary battery according to the present invention.
[Explanation of symbols]
1 Current collector
2 Positive electrode active material
3 Gel electrolyte (ion-conducting polymer)
4 Positive electrode
5 Current collector
6 Negative electrode active material
7 Gel electrolyte (ion conductive polymer)
8 Negative electrode
9 Non-aqueous electrolyte layer

Claims (6)

  A non-aqueous solution in which a positive electrode provided with a gel electrolyte containing an active material capable of inserting / extracting lithium ions and an electrolyte, a negative electrode, and a gel-like material containing a non-aqueous solvent and a lithium salt are disposed between the positive electrode and the negative electrode A gel electrolyte layer, wherein the positive electrode includes a fluorinated ester compound and / or a fluorinated ether compound, and the negative electrode does not include a fluorinated ester compound and / or a fluorinated ether compound, or includes a lower concentration than the positive electrode. Gel electrolyte secondary battery characterized by the above.   The fluorine compound in the positive electrode is included in the range of 10 to 50% by weight with respect to the electrolytic solution in the positive electrode, and the fluorine compound in the negative electrode is in the range of 0 to 10% by weight with respect to the electrolytic solution in the negative electrode. The gel electrolyte secondary battery according to claim 1, which is contained in The fluorine compound has the following structural formula (I)

(R ₁ represents a fluorine atom or a fluorinated substituted lower alkyl group, R ₂ represents a hydrogen atom, a fluorine atom, a lower alkyl group or a fluorinated substituted lower alkyl group, and R ₃ represents a hydrogen atom, a lower alkyl group or a fluorinated group. The gel electrolyte secondary battery according to claim 1, which represents a substituted lower alkyl group. The non-aqueous gel electrolyte layer, a gel electrolyte secondary cell according to claim 3 including a fluorine compound represented by the previous SL structural formula (I).   The gel electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode includes a compound in which amorphous carbon is attached to the surface of graphite particles.   It is a manufacturing method of the gel electrolyte secondary battery as described in any one of Claims 1-3, Comprising: The fluorine in the precursor solution for positive electrode formation containing a solvent and a fluorinated ester compound and / or a fluorinated ether compound The compound concentration is made higher than the fluorine compound concentration in the negative electrode forming precursor solution containing a solvent and a fluorinated ester compound and / or a fluorinated ether compound, and the positive electrode and negative electrode each containing the active material are impregnated with the precursor solution, respectively. A method for producing a gel electrolyte secondary battery comprising: cross-linking and obtaining a gel electrolyte, and then interposing a non-aqueous gel electrolyte layer between the positive electrode and the negative electrode.

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