JP4724911B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

【００５０】
合成原料であるリン酸第一鉄八水和物には、その合成工程上、ある程度のＦｅ^３＋が含まれている。合成原料にＦｅ^３＋が残存すると、焼成により３価のＦｅ化合物が生成されるため、ＬｉＦｅＰＯ_４炭素複合体の単相合成が妨げられてしまう。このため、焼成前の合成原料に還元剤を添加し、焼成時に合成原料中に含まれているＦｅ^３＋をＦｅ^２＋に還元する必要がある。
【００５１】
しかし、還元剤によるＦｅ^３＋のＦｅ^２＋への還元能力には限界があり、合成原料中のＦｅ^３＋の含有率が多すぎる場合、Ｆｅ^３＋が還元されきれずにＬｉＦｅＰＯ_４炭素複合体中に残存してしまうことがある。
【００５２】
そこで、リン酸第一鉄八水和物中の鉄総量に対するＦｅ^３＋の含有率を、６１質量％以下とすることが好ましい。合成原料であるリン酸第一鉄八水和物中の鉄総量に対するＦｅ^３＋の含有率を６１質量％以下と予め制限することにより、焼成時においてＦｅ^３＋を残存させることなく、すなわちＦｅ^３＋に起因する不純物を生成させることなく、ＬｉＦｅＰＯ_４炭素複合体の単相合成を確実に行うことができる。
【００５３】
なお、リン酸第一鉄八水和物を生成する際の放置時間が長いほど、生成物中のＦｅ^３＋の含有率が多くなるので、放置時間を所定の時間に制御することにより、任意のＦｅ^３＋の含有率を有するリン酸第一鉄八水和物を生成させることができる。また、リン酸第一鉄八水和物中の鉄総量に対するＦｅ^３＋の含有率は、メスバウア測定法により測定することができる。
【００５４】
また、合成原料に添加される炭素材料は、合成原料のリン酸第一鉄八水和物中に含まれるＦｅ^２＋が大気中の酸素や焼成等によりＦｅ^３＋に酸化されたとしても、焼成時にＦｅ^３＋をＦｅ^２＋に還元する還元剤として働く。したがって、合成原料にＦｅ^３＋が残存していたとしても、不純物の生成が防止され、ＬｉＦｅＰＯ_４炭素複合体の単相合成が可能となる。さらに、炭素材料は、合成原料に含まれるＦｅ^２＋のＦｅ^３＋への酸化を防止する酸化防止剤として働く。すなわち、炭素材料は、焼成前又は焼成時において大気中及び焼成炉内に存在する酸素により、Ｆｅ^２＋がＦｅ^３＋へ酸化されてしまうことを防止する。
【００５５】
すなわち、炭素材料は、上述したように正極活物質の電子伝導性を向上させる導電材としての働きをするとともに、還元剤及び酸化防止剤として働く。なお、この炭素材料は、ＬｉＦｅＰＯ_４炭素複合体の構成要素となるので、ＬｉＦｅＰＯ_４炭素複合体の合成後に除去する必要がない。したがって、ＬｉＦｅＰＯ_４炭素複合体の製造が効率化される。
【００５６】
なお、ＬｉＦｅＰＯ_４炭素複合体の単位質量あたりの炭素含有量は、３質量％以上とすることが好ましい。ＬｉＦｅＰＯ_４炭素複合体の単位質量あたりの炭素含有量を３質量％以上とすることにより、ＬｉＦｅＰＯ_４が本来有する容量及びサイクル特性を十分に引き出すことが可能となる。
【００５７】
ミリング工程では、混合工程で得られた混合物に、粉砕・混合を同時に行うミリングを施す。ここで、ミリングとは、ボールミルを用いた強力な粉砕・混合をいう。また、ボールミルとしては、例えば遊星型ボールミル、シェイカー型ボールミル、メカノフュージョン等を好適に用いることができる。
【００５８】
混合工程で得られた混合物にミリングを施すことにより、合成原料及び炭素材料を均一に混合することができる。また、ミリングを施すことにより合成原料を微細化すると、合成原料の比表面積を増大させることができる。したがって、原料同士の接触点が増大し、引き続く焼成工程における合成反応を速やかに進行させることが可能となる。
【００５９】
合成原料を含有する混合物にミリングを施すに際しては、粒子径３μｍ以上の粒子の粒度分布が体積基準の積算頻度にして２２％以下となるようにすることが好ましい。合成原料の粒度分布を上記範囲とすることにより、合成原料は、表面積として、合成反応に十分な表面活性を得ることができる広さを有することとなる。これにより、焼成温度が例えば６００℃という合成原料の融点以下という低い温度であっても、反応効率が良好であり、ＬｉＦｅＰＯ_４炭素複合体の単相合成を確実に行うことができる。
【００６０】
また、ＬｉＦｅＰＯ_４炭素複合体の粉体密度が２．２ｇ／ｃｍ^３以上となるように、ミリングを施すことが好ましい。上記粉体密度となるように合成原料を微小化することにより、ＬｉＦｅＰＯ_４の比表面積を大きくすることができる。これにより、ＬｉＦｅＰＯ_４と炭素材料との接触面積を大きくすることができ、正極活物質の電子伝導性を向上させることが可能となる。
【００６１】
したがって、合成原料を含有する混合物にミリングを施すことにより、高容量である非水電解液電池１を実現する正極活物質を製造することができる。
【００６２】
焼成工程では、ミリング工程でミリングを施した混合物を焼成する。混合物を焼成することにより、リン酸リチウムとリン酸第一鉄八水和物とを反応させ、ＬｉＦｅＰＯ_４を合成する。
【００６３】
ＬｉＦｅＰＯ_４の合成反応は、下記化２に示す反応式で表される。なお、下記化２に示す反応式においては、Ｌｉ_３ＰＯ_４と、Ｆｅ_３（ＰＯ_４）_２又はその水和物であるＦｅ_３（ＰＯ_４）_２・ｎＨ_２Ｏ（ただし、ｎは水和数である。）とを反応させた場合を示す。
【００６４】
【化２】

【００６５】
上記化２に示す反応式から明らかなように、合成原料としてＦｅ_３（ＰＯ_４）_２を用いた場合、副生成物が生じない。また、Ｆｅ_３（ＰＯ_４）_２・ｎＨ_２Ｏを用いた場合、副生成物として無毒である水のみが生じる。
【００６６】
ところで、従来は合成原料として炭酸リチウム、リン酸二水素アンモニウム及び酢酸鉄（ＩＩ）を所定比で混合し、焼成し、下記化３に示す反応によってＬｉＦｅＰＯ_４を合成していた。
【００６７】
【化３】

【００６８】
上記化３に示す反応式から明らかなように、従来のＬｉＦｅＰＯ_４の合成方法では、焼成時に有毒なアンモニアや酢酸等の副生成物が生じていた。このため、これら有毒な副生成物を処理するための大規模な集気装置等の設備が必要となり、製造コスト上昇の原因となっていた。また、これらの副生成物が大量に生じるため、ＬｉＦｅＰＯ_４の収率が低下していた。
【００６９】
しかしながら、合成原料としてＬｉ_３ＰＯ_４と、Ｆｅ_３（ＰＯ_４）_２又はその水和物であるＦｅ_３（ＰＯ_４）_２・ｎＨ_２Ｏ（ただし、ｎは水和数である。）とを用いることにより、有毒な副生成物を生じることなく、目的物質であるＬｉＦｅＰＯ_４を得られる。言い換えると、従来の製造方法に比べて、焼成時における安全性が著しく向上する。また、従来では有毒な副生成物を処理するために大規模な処理設備が必要だったが、上述した製造方法では、副生成物が無毒である水なので、処理工程を大幅に簡略化でき、処理設備を縮小できる。したがって、従来の副生成物であるアンモニア等を処理する際に比べて、製造コストを大幅に削減することができる。さらにまた、上記化２に示す反応式から明らかなように、副生成物の生成が少量であるので、ＬｉＦｅＰＯ_４の収率を大幅に向上させることができる。
【００７０】
混合物の焼成を行う際の焼成温度は、上記の合成方法により４００℃〜９００℃とすることが可能であるが、電池性能を考慮すると、６００℃程度とすることが好ましい。焼成温度が４００℃未満であると、化学反応及び結晶化が十分に進まず、合成原料であるＬｉ_３ＰＯ_４等の不純物相が存在し、均一なＬｉＦｅＰＯ_４を得られない虞がある。一方、焼成温度が９００℃を上回ると、結晶化が過剰に進行してＬｉＦｅＰＯ_４の粒子が大きくなり、ＬｉＦｅＰＯ_４と炭素材料との接触面積が減少し、電子伝導性が下がるため、十分な放電容量を得られない虞がある。
【００７１】
焼成時において、合成されたＬｉＦｅＰＯ_４炭素複合体中のＦｅは２価の状態である。このため、合成温度である６００℃程度の温度においては、ＬｉＦｅＰＯ_４炭素複合体中のＦｅは、焼成雰囲気中の酸素によって下記化４に示す反応式によりＦｅ^３＋に速やかに酸化されてしまう。これに起因して、３価のＦｅ化合物等の不純物が生成され、ＬｉＦｅＰＯ_４炭素複合体の単相合成が妨げられてしまう。
【００７２】
【化４】

【００７３】
そこで、焼成雰囲気として窒素、アルゴン等の不活性ガス又は水素や一酸化炭素等の還元性ガスを用いるとともに、焼成雰囲気中の酸素濃度を、ＬｉＦｅＰＯ_４炭素複合体中のＦｅが酸化されない範囲、すなわち１０１２ｐｐｍ（体積）以下とすることが好ましい。焼成雰囲気中の酸素濃度を、１０１２ｐｐｍ（体積）以下とすることにより、６００℃程度の合成温度においてもＦｅの酸化を防止し、ＬｉＦｅＰＯ_４炭素複合体の単相合成を確実に行うことが可能となる。
【００７４】
焼成雰囲気中の酸素濃度が１０１２ｐｐｍ（体積）よりも高い場合には、焼成雰囲気中の酸素量が多すぎるため、ＬｉＦｅＰＯ_４炭素複合体中のＦｅがＦｅ^３＋に酸化されてしまい、これに起因して不純物が生成してしまうため、ＬｉＦｅＰＯ_４炭素複合体の単相合成が妨げられてしまう虞がある。
【００７５】
焼成後のＬｉＦｅＰＯ_４炭素複合体の取り出しについては、焼成後のＬｉＦｅＰＯ_４炭素複合体の取り出し温度、すなわちＬｉＦｅＰＯ_４炭素複合体を大気中に暴露する際のＬｉＦｅＰＯ_４炭素複合体の温度は３０５℃以下とすることが好ましい。また、焼成後のＬｉＦｅＰＯ_４炭素複合体の取り出し温度を２０４℃以下とすることがより好ましい。ＬｉＦｅＰＯ_４炭素複合体の取り出し温度を３０５℃以下とすることにより、焼成後のＬｉＦｅＰＯ_４炭素複合体中のＦｅが大気中の酸素により酸化され、不純物が生成されることを防止できる。
【００７６】
焼成後にＬｉＦｅＰＯ_４炭素複合体を十分に冷却しない状態で取り出した場合、ＬｉＦｅＰＯ_４炭素複合体中のＦｅが大気中の酸素により酸化され、不純物が生成される虞がある。しかしながら、余り低い温度までＬｉＦｅＰＯ_４炭素複合体を冷却したのでは、作業効率の低下を招く虞がある。
【００７７】
したがって、焼成後のＬｉＦｅＰＯ_４炭素複合体の取り出し温度を３０５℃以下とすることにより、焼成後のＬｉＦｅＰＯ_４炭素複合体中のＦｅが大気中の酸素により酸化されて不純物が生成されることを防止するとともに、作業効率も維持することが可能となり、電池特性として好ましい特性を有するＬｉＦｅＰＯ_４炭素複合体を効率よく合成することができる。
【００７８】
なお、焼成後のＬｉＦｅＰＯ_４炭素複合体の冷却は焼成炉内で行うが、このときの冷却方法は、自然冷却でも良く、また、強制冷却でも良い。ただし、冷却時間の短縮、すなわち、作業効率を考慮した場合には、強制冷却することが好ましい。そして、強制冷却する場合には、焼成炉内を上述した酸素濃度、すなわち１０１２ｐｐｍ（体積）以下とするように酸素と不活性ガスとの混合ガス、又は不活性ガスのみを焼成炉内に供給すれば良い。
【００７９】
上記においては、ミリングを施す前に炭素材料の添加を行っているが、炭素材料の添加は、ミリング後又は焼成後に行うことも可能である。
【００８０】
ただし、炭素材料を焼成後に添加する場合、焼成時の還元効果、及び酸化防止効果を得ることはできず、導電性向上効果のみのために用いるという条件が付く。したがって、炭素材料を焼成後に添加する場合、他の手段によりＦｅ^３＋の残存を防止することが必要となる。
【００８１】
また、炭素材料を焼成後に添加する場合、焼成により合成された生成物はＬｉＦｅＰＯ_４炭素複合体ではなく、ＬｉＦｅＰＯ_４である。そこで、焼成により合成されたＬｉＦｅＰＯ_４に炭素材料を添加した後、再度ミリングを施す。ミリングを再度行うことにより、添加した炭素材料は微細化され、ＬｉＦｅＰＯ_４の表面に付着しやすくなる。また、ミリングを再度行うことにより、ＬｉＦｅＰＯ_４と炭素材料とが十分に混合されるので、微細化された炭素材料をＬｉＦｅＰＯ_４の表面に均一に付着させることができる。したがって、焼成後に炭素材料を添加した場合においても、ミリングを施す前に炭素材料を添加した場合と同様の生成物、すなわちＬｉＦｅＰＯ_４炭素複合体を得ることが可能であり、また、上述した同様の効果を得ることが可能である。
【００８２】
上述のようにして得られたＬｉＦｅＰＯ_４炭素複合体を正極活物質として用いた非水電解液電池１は、例えば次のようにして製造される。
【００８３】
負極２としては、まず、負極活物質と結着剤とを溶媒中に分散させてスラリーの負極合剤を調製する。次に、得られた負極合剤を集電体上に均一に塗布、乾燥して負極活物質層を形成することにより負極２が作製される。上記負極合剤の結着剤としては、公知の結着剤を用いることができる他、上記負極合剤に公知の添加剤等を添加することができる。また、負極活物質となる金属リチウムをそのまま負極２として用いることもできる。
【００８４】
正極４としては、まず、正極活物質となるＬｉＦｅＰＯ_４炭素複合体と、炭酸リチウムと、結着剤とを混合して正極合剤を調製し、溶媒中に分散させてスラリー状とする。すなわち、この非水電解液電池１を製造するにおいては、正極活物質中には予め炭酸リチウムやＣＯ_３ ^２−イオンは含有されておらず、正極合剤を調製する際に炭酸リチウムを正極活物質と混合する。ここで、炭酸リチウムを、正極合剤中における炭酸リチウムの含有率が０．３質量％〜８．０質量％となるように混合する。すなわち、正極活物質層中における炭酸リチウムの含有率が０．３質量％〜８．０質量％となるように炭酸リチウムを混合する。正極合剤中における炭酸リチウムの含有率を０．３質量％〜８．０質量％とすることにより、当該正極合剤を用いて構成した非水電解液電池１を高温において充放電サイクルを繰り返した場合においても、容量劣化を抑制することができる。したがって、非水電解液電池１の高温使用における充放電サイクル特性を改善することが可能となる。
【００８５】
次に、得られたスラリー状の正極合剤を集電体上に均一に塗布、乾燥して正極活物質層を形成することにより正極４が作製される。上記正極合剤の結着剤としては、公知の結着剤を用いることができる他、上記正極合剤に公知の添加剤等を添加することができる。
【００８６】
非水電解液は、電解質塩を非水溶媒中に溶解することにより調製される。
【００８７】
そして、負極２を負極缶３に収容し、正極４を正極缶５に収容し、負極２と正極４との間に、ポリプロピレン製多孔質膜等からなるセパレータ６を配する。負極缶３及び正極缶５内に非水電解液を注入し、絶縁ガスケット７を介して負極缶３と正極缶５とをかしめて固定することにより、コイン型の非水電解液電池１が完成する。
【００８８】
以上のようにして製造された非水電解液電池１は、正極合剤中に炭酸リチウムを０．３質量％〜８．０質量％の範囲で含有するため、高温において充放電サイクルを繰り返した場合においても容量劣化を抑制することができる。したがって、この非水電解液電池１は、高温使用における充放電サイクル特性に優れた非水電解質二次電池とされる。
【００８９】
また、上記においては、上述したようにＬｉＦｅＰＯ_４の合成原料と炭素材料とを混合して混合物とする混合工程と、混合工程で得られた混合物にミリングを施すミリング工程と、ミリング工程でミリングを施した混合物を焼成する焼成工程とを経て正極活物質を合成している。しかし、本発明においては、正極活物質の製造方法はこれに限定されることなく、例えば上記の方法において炭素材料を添加せず、またミリング工程を除いて正極活物質を合成しても良い。
【００９０】
なお、上述したような本実施の形態に係る非水電解液電池１は、円筒型、角型、コイン型、ボタン型等、その形状については特に限定されることはなく、また、薄型、大型等の種々の大きさにすることができる。
【００９１】
【実施例】
以下、本発明を具体的な実験結果に基づいて説明する。ここでは、本発明の効果を調べるべく、ＬｉＦｅＰＯ_４炭素複合体を合成し、得られたＬｉＦｅＰＯ_４炭素複合体を正極活物質として用いて非水電解質電池を作製し、その特性を評価した。
【００９２】
＜実施例１＞
正極活物質としてＬｉＦｅＰＯ_４複合体を合成した。この正極活物質の製造方法を以下に示す。
【００９３】
まず、Ｌｉ_３ＰＯ_４とＦｅ_３（ＰＯ_４）_２・８Ｈ_２Ｏとを、リチウムと鉄との元素比率が１：１となるように混合し、さらに様々な大きさの粒径を有するアセチレンブラック粉末を焼成物全体の１０質量％となるように添加して混合物とした。次に、混合物及び直径１０ｍｍのアルミナ製ボールを、質量比で混合物：アルミナ製ボール＝１：２として直径１００ｍｍのアルミナ製ポットに投入し、遊星型ボールミルを用いてこの混合物にミリングを施した。なお、遊星型ボールミルとして、実験用遊星回転ポットミル「ＬＡ−ＰＯ_４」（伊藤製作所製）を使用し、下記に示す条件としてミリングを施した。
【００９４】
遊星型ボールミルミリング条件
公転半径：２００ｍｍ
公転回転数：２５０ｒｐｍ
自転回転数：２５０ｒｐｍ
運転時間：１０ｈ
次に、ミリングを施した後の混合物をセラミックるつぼに入れ、窒素雰囲気中の電気炉にて６００℃の温度で５時間焼成することによりＬｉＦｅＰＯ_４炭素複合体を得た。
【００９５】
このようにして得たＬｉＦｅＰＯ_４炭素複合体を用い、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９９．７：０．３として得られる混合品を９１質量部、導電剤としてグラファイトを６質量部、結着剤としてポリフッ化ビニリデン３質量部を混合して正極合剤を調製し、これをＮ−メチル−２−ピロリドンに分散させてスラリー状とした。
【００９６】
次に、正極集電体として厚さ２０μｍの帯状のアルミニウム箔の両面に上記正極合剤のスラリーを均一に塗布し、乾燥後ロールプレス機で圧縮成型して帯状正極を作製した。
【００９７】
負極活物質として、出発原料に石油ピッチを用い、これに酸素を含む官能基を１０〜２０％導入、いわゆる酸素架橋した後、不活性ガス気流中１０００℃で焼成してガラス状炭素に近い性質の難黒鉛炭素材料を得た。この材料について、Ｘ線回折測定を行った結果、（００２）面の面間隔は３．７６Åで、真比重は１．５８であった。
【００９８】
このようにして得た炭素材料を９０質量部と、結着剤としてポリフッ化ビニリデン１０質量部とを混合して負極合剤を作製し、これをＮ−メチル−２−ピロリドンに分散させてスラリー状とした。
【００９９】
次に、負極集電体として厚さ１０μｍの帯状の銅箔を用意し、その両面に上記負極合剤のスラリーを均一に塗布し、乾燥後ロールプレス機で圧縮成型して帯状の負極を作製した。
【０１００】
セパレータとしては、厚さ２５μｍの微孔性ポリプロピレンフィルムを使用し、負極、正極、セパレータを渦巻型に巻回し、電極素子を作製した。
【０１０１】
このようにして得た電極素子をニッケルメッキを施した鉄製の缶に収納した。そして、渦巻式電極素子の上下両面に絶縁板を配置し、アルミニウム製正極リードを正極集電体から導出して電池蓋に、ニッケル製負極リードを負極集電体から導出して電池缶に溶接した。
【０１０２】
次に、この電池缶の中にプロピレンカーボネート５０容量％と１，２−ジメトキシエタン５０容量％との混合溶媒にＬｉＰＦ_６を１モル／ｌ溶解させた電解液を注入した。
【０１０３】
最後にアスファルトを塗布した絶縁ガスケットを介して上記電池缶と電池蓋をかしめて封口し、直径２０ｍｍ、高さ５０ｍｍの円筒型電池を作製した。
【０１０４】
＜実施例２＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９９．５：０．５としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１０５】
＜実施例３＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９９：１としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１０６】
＜実施例４＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９７：３としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１０７】
＜実施例５＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９５：５としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１０８】
＜実施例６＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９２：８としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１０９】
＜比較例１＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を１００：０とした、すなわち炭酸リチウムを混合しなかったこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１１０】
＜比較例２＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９９．８：０．２としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１１１】
＜比較例３＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９０：１０としたこと以外は、実施例１と同様にして円筒型電池を作製した。
【０１１２】
以上のようにして作製した実施例１乃至実施例３及び比較例１乃至比較例３の円筒型電池について、以下のようにして充放電サイクル特性試験を行い、初期放電容量及び５０サイクル後の放電容量維持率を求めた。
【０１１３】
＜充放電サイクル特性試験＞
充放電サイクル特性は、充放電を繰り返した後の容量維持率により評価した。
【０１１４】
まず、円筒型電池に対して、２３℃雰囲気内で１Ａの定電流定電圧充電を上限４．１Ｖまで３時間行い、次に、６．２Ωの定抵抗放電を終止電圧２．７５Ｖまで行った。そして、このときの放電容量を初期放電容量とした。さらに４５℃雰囲気内で同様の充放電条件で充放電を５０サイクル行い、５０サイクル目における放電容量を求めた。そして、初期放電容量（Ｃ１）に対する、５０サイクル目の放電容量（Ｃ２）の比率（（Ｃ２／Ｃ１）×１００）を放電容量維持率として求めた。その結果を表１に示す。なお、表１における電池評価は、初期放電容量が１０００ｍＡｈ以上、且つ５０サイクル目の放電容量維持率が９５％以上のものを合格として○を記し、初期放電容量が１０００ｍＡｈ以下、もしくは５０サイクル目の放電容量維持率が９５％未満のものを不合格として×を記した。ここで、初期放電容量が１０００ｍＡｈ以上、且つ５０サイクル目の放電容量維持率が９５％以上とは、電池特性として好ましい値である。
【０１１５】
【表１】

【０１１６】
表１から判るように、正極活物質層中における炭酸リチウムの含有率が０．３質量％〜８．０質量％である実施例１乃至実施例６では、実用レベルの電池特性として望ましい初期放電容量１０００ｍＡｈを大きく越えた値を示しており、また、５０サイクル目の放電容量維持率も実用レベルの電池特性として望ましい９５％を大きく越えており、優れた充放電サイクル特性を示している。これは、正極活物質層中に炭酸リチウムが適正な量だけ含有されているため、４５℃という高温において充放電サイクルを繰り返した場合においても容量劣化が抑制されたためであると考えられる。
【０１１７】
一方、正極活物質層中における炭酸リチウムの含有率が０．３質量％未満である比較例１及び比較例２では、初期放電容量に関しては、実用レベルの電池特性として望ましい１０００ｍＡｈを越えているが、５０サイクル目の放電容量維持率は実用レベルの電池特性として望ましい９５％を下回った値となっている。これは、正極活物質層中における炭酸リチウムの含有量が少なすぎる、もしくは、正極活物質層中に炭酸リチウムが含有されていないため、４５℃という高温において充放電サイクルを繰り返した場合における容量劣化を十分に抑制することができなかったためであると考えられる。
【０１１８】
また、正極活物質層中における炭酸リチウムの含有率が８．０質量％を越えている比較例３では、初期放電容量が実用レベルの電池特性として望ましい１０００ｍＡｈを下回っている。これは、正極活物質層中における炭酸リチウムの含有量が多すぎるため、正極活物質層中の正極活物質の含有量が少なくなってしまったためであると考えられる。
【０１１９】
以上のことより、正極活物質層中に炭酸リチウムを０．３質量％〜８．０質量％の範囲で含有することにより、４５℃という高温において充放電サイクルを繰り返した場合においても容量劣化を抑制することができ、非水電解質二次電池の高温使用における充放電サイクル特性を改善する効果を得られることが確認された。
【０１２０】
次に、正極活物質としてＬｉＦｅＰＯ_４炭素複合体以外のＬｉ_ｘＦｅ_１−ｙＭ_ｙＰＯ_４炭素複合体を合成し、これを用いた電池の特性評価を行った。
【０１２１】
＜実施例７＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｍｎ_０．８ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２２】
＜実施例８＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｃｒ_０．８ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２３】
＜実施例９＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｃｏ_０．８ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２４】
＜実施例１０＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｃｕ_０．８ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２５】
＜実施例１１＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｎｉ_０．８ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２６】
＜実施例１２＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｖ_０．７５ＰＯ_４ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２７】
＜実施例１３＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｍｏ_０．７５ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２８】
＜実施例１４＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｔｉ_０．７５ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１２９】
＜実施例１５＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．３Ｚｎ_０．７ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１３０】
＜実施例１６＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．３Ａｌ_０．７ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１３１】
＜実施例１７＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．３Ｇａ_０．７ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１３２】
＜実施例１８＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｍｇ_０．７５ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１３３】
＜実施例１９＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｂ_０．７５ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１３４】
＜実施例２０＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｎｂ_０．７５ＰＯ_４を用いたこと以外は実施例１と同様にして円筒型電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例１乃至実施例６と同様の効果が確認された。
【０１３５】
以上のことより、実施例７乃至実施例２０でも４５℃という高温において充放電サイクルを繰り返した場合における容量劣化を抑制することができ、非水電解質二次電池の高温使用における充放電サイクル特性を改善する効果が確認された。
【０１３６】
したがって、一般式Ｌｉ_ｘＦｅ_１−ｙＭ_ｙＰＯ_４炭素複合体を含有する正極活物質層中に炭酸リチウムを０．３質量％〜８．０質量％の範囲で含有することにより、４５℃という高温において充放電サイクルを繰り返した場合における容量劣化を抑制することができ、非水電解質二次電池の高温使用における充放電サイクル特性を改善することができるといえる。
【０１３７】
次に、ポリマー電池を作製し、特性を評価した。
【０１３８】
＜実施例２１＞
まず、ゲル状電解質を以下に示すようにして作製した。まず、ヘキサフルオロプロピレンが６．９質量％の割合で共重合されたポリフッ化ビニリデンと、非水電解液と、ジメチルカーボネートとを混合し、撹拌、溶解させ、ゾル状の電解質溶液を調製した。次いで、ゾル状の電解質溶液に、ビニレンカーボネート（ＶＣ）を０．５質量％の割合で添加してゲル状電解質溶液とした。なお、非水電解液として、エチレンカーボネート（ＥＣ）と、プロピレンカーボネート（ＰＣ）とを体積比で６：４の割合で混合した混合溶媒にＬｉＰＦ_６を０．８５ｍｏｌ／ｋｇの割合で溶解させたものを使用した。
【０１３９】
次いで、正極を以下に示すようにして作製した。まず、実施例１で作製したＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９９．７：０．３として得られる混合品を８５質量部、導電剤としてグラファイトを１０質量部、結着剤としてフッ素樹脂粉末であるポリ（ビニリデンフルオロライド）５質量部とを混合して正極合剤を調製した後、Ｎ−メチルピロリドンを加えてスラリー状にしたものを準備した。次に、このスラリーを厚み２０μｍのアルミ箔に塗布、加熱乾燥後、加圧工程を経て正極塗布箔を作製した。次に、この正極塗布箔の片面にゲル状電解質溶液を塗布後、乾燥して溶剤を除去した後、セルの径に準じて直径１５ｍｍの円形に打ち抜き、正極電極とした。
【０１４０】
次いで、負極を以下に示すようにして作製した。まず、黒鉛粉末にバインダーとしてフッ素樹脂粉末を１０質量％混合し、Ｎ−メチルピロリドンを加えてスラリー状にしたものを準備した。次に、このスラリーを銅箔に塗布、加熱乾燥後、加圧工程を経てセルの大きさに準じて直径１６．５ｍｍの円形に打ち抜き、負極電極とした。
【０１４１】
以上のようにして得られた正極を正極缶に収容し、負極を負極缶に収容し、正極と負極との間にセパレータを配した。そして、正極缶と負極缶とをかしめて固定することにより、直径２０ｍｍ、厚み１．６ｍｍの２０１６型のコイン型リチウムポリマー電池を作製した。
【０１４２】
＜実施例２２＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９２：８としたこと以外は、実施例２１と同様にしてコイン型リチウムポリマー電池を作製した。
【０１４３】
＜比較例４＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９９．８：０．２としたこと以外は、実施例２１と同様にしてコイン型リチウムポリマー電池を作製した。
【０１４４】
＜比較例５＞
正極合剤を調製する際に、ＬｉＦｅＰＯ_４炭素複合体と炭酸リチウムＬｉ_２ＣＯ_３との混合質量比を９０：１０としたこと以外は、実施例２１と同様にしてコイン型リチウムポリマー電池を作製した。
【０１４５】
以上のようにして作製した実施例２１、実施例２２、比較例４及び比較例５のコイン型リチウムポリマー電池について、上記と同様にして充放電サイクル特性試験を行い、初期放電容量密度及び３０サイクル後の放電容量維持率を求めた。その結果を表２に示す。なお、表１における電池評価は、初期放電容量が３．５ｍＡｈ以上、且つ３０サイクル目の放電容量維持率が９５％以上のものを合格として○を記し、初期放電容量が３．５ｍＡｈ以下、もしくは３０サイクル目の放電容量維持率が９５％未満のものを不合格として×を記した。ここで、初期放電容量が３．５ｍＡｈ以上、且つ３０サイクル目の放電容量維持率が９５％以上とは、電池特性として好ましい値である。
【０１４６】
【表２】

【０１４７】
表２から判るように、正極活物質層中における炭酸リチウムの含有率が０．３質量％〜８．０質量％である実施例２１及び実施例２２では、実用レベルの電池特性として望ましい初期放電容量３．５ｍＡｈを超えた値を示しており、また、３０サイクル目の放電容量維持率も実用レベルの電池特性として望ましい９５％を大きく越えており、優れた充放電サイクル特性を示している。これは、正極活物質層中に炭酸リチウムが適正な量だけ含有されているため、４５℃という高温において充放電サイクルを繰り返した場合においても容量劣化が抑制されたためであると考えられる。
【０１４８】
一方、正極活物質層中における炭酸リチウムの含有率が０．３質量％未満である比較例４では、初期放電容量に関しては、実用レベルの電池特性として望ましい３．５ｍＡｈを越えているが、５０サイクル目の放電容量維持率は実用レベルの電池特性として望ましい９５％を下回った値となっている。これは、正極活物質層中における炭酸リチウムの含有量が少なすぎるため、４５℃という高温において充放電サイクルを繰り返した場合における容量劣化を十分に抑制することができなかったためであると考えられる。
【０１４９】
また、正極活物質層中における炭酸リチウムの含有率が８．０質量％を越えている比較例５では、初期放電容量が実用レベルの電池特性として望ましい３．５ｍＡｈを下回っている。これは、正極活物質層中における炭酸リチウムの含有量が多すぎるため、正極活物質層中の正極活物質の含有量が少なくなってしまったためであると考えられる。
【０１５０】
以上のことより、ポリマー電池の場合においても正極活物質層中に炭酸リチウムを０．３質量％〜８．０質量％の範囲で含有することにより、４５℃という高温において充放電サイクルを繰り返した場合においても容量劣化を抑制することができ、非水電解質二次電池の高温使用における充放電サイクル特性を改善する効果を得られることが確認された。
【０１５１】
次に、正極活物質としてＬｉＦｅＰＯ_４炭素複合体以外のＬｉ_ｘＦｅ_１−ｙＭ_ｙＰＯ_４炭素複合体を合成し、これを用いたポリマー電池の特性評価を行った。
【０１５２】
＜実施例２３＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｍｎ_０．８ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５３】
＜実施例２４＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｃｒ_０．８ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５４】
＜実施例２５＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｃｏ_０．８ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５５】
＜実施例２６＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｃｕ_０．８ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５６】
＜実施例２７＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２Ｎｉ_０．８ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５７】
＜実施例２８＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｖ_０．７５ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５８】
＜実施例２９＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｍｏ_０．７５ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１５９】
＜実施例３０＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｔｉ_０．７５ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１６０】
＜実施例３１＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．３Ｚｎ_０．７ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１６１】
＜実施例３２＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．３Ａｌ_０．７ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１６２】
＜実施例３３＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．３Ｇａ_０．７ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１６３】
＜実施例３４＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｍｇ_０．７５ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１６４】
＜実施例３５＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｂ_０．７５ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、上述した実施例２１及び実施例２２と同様の効果が確認された。
【０１６５】
＜実施例３６＞
正極活物質としてＬｉＦｅＰＯ_４炭素複合体の代わりに、ＬｉＦｅ_０．２５Ｎｂ_０．７５ＰＯ_４炭素複合体を用いたこと以外は実施例１と同様にしてコイン型リチウムポリマー電池を作製し、上記と同様にして充放電サイクル試験を行ったところ、以上のことより、実施例２３乃至実施例３３でも４５℃という高温において充放電サイクルを繰り返した場合における容量劣化を抑制することができ、非水電解質二次電池の高温使用における充放電サイクル特性を改善する効果が確認された。
【０１６６】
したがって、ポリマー電池の場合においても、一般式Ｌｉ_ｘＦｅ_１−ｙＭ_ｙＰＯ_４炭素複合体を含有する正極活物質層中に炭酸リチウムを０．３質量％〜８．０質量％の範囲で含有することにより、４５℃という高温において充放電サイクルを繰り返した場合における容量劣化を抑制することができ、非水電解質二次電池の高温使用における充放電サイクル特性を改善することができるといえる。
【０１６７】
【発明の効果】
本発明に係る非水電解質二次電池は、一般式Ｌｉ_ｘＦｅ_１−ｙＭ_ｙＰＯ_４（ただし、Ｍは、Ｍｎ、Ｃｒ、Ｃｏ、Ｃｕ、Ｎｉ、Ｖ、Ｍｏ、Ｔｉ、Ｚｎ、Ａｌ、Ｇａ、Ｍｇ、Ｂ及びＮｂからなる群より選ばれた少なくとも１種を表す。また、０．０５≦ｘ≦１．２、０≦ｙ≦０．８である。）で表される化合物を含有する正極活物質層を有する正極と、リチウムをドープ及び脱ドープし得る負極と、非水電解質とを備えてなり、上記正極活物質層は、炭酸リチウムを０．３質量％〜８．０質量％含有してなるものである。
【０１６８】
以上のような本発明に係る非水電解質二次電池は、正極活物質層中に炭酸リチウムを０．３質量％〜８．０質量％の範囲で含有するため、高温で充放電サイクルを繰り返した場合における容量劣化を抑制することが可能である。
【０１６９】
これにより、非水電解質二次電池の高温使用における充放電サイクル特性を改善することが可能となる。
【０１７０】
したがって、本発明によれば、高エネルギー密度を有し、高温における充放電サイクル特性に優れた非水電解質二次電池を提供することが可能となり、その工業的価値は大きい。
【図面の簡単な説明】
【図１】 本発明を適用した非水電解質二次電池の一構成例を示す縦断面図である。
【図２】 炭素材料のラマンスペクトルピークを示す特性図である。
【符号の説明】
１ 非水電解液電池、２ 負極、３ 負極缶、４ 正極、５ 正極缶、６ セパレータ、７ 絶縁ガスケット[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a non-aqueous electrolyte secondary battery using a positive electrode active material capable of reversibly doping and dedoping lithium.
[0002]
[Prior art]
  In recent years, with the rapid progress of various electronic devices, research on a rechargeable secondary battery as a power source that can be used conveniently and economically for a long time is underway. As typical secondary batteries, lead storage batteries, alkaline storage batteries, non-aqueous electrolyte secondary batteries, and the like are known.
[0003]
  Among the secondary batteries as described above, a lithium ion secondary battery that is a non-aqueous electrolyte secondary battery has advantages such as high output and high energy density. The lithium ion secondary battery includes at least a positive electrode and a negative electrode having an active material capable of reversibly removing and inserting lithium ions, and a nonaqueous electrolyte.
[0004]
  Here, as the negative electrode active material, generally lithium metal such as lithium metal, Li-Al alloy, conductive polymer doped with lithium such as polyacetylene and polypyrrole, intercalation compound or carbon material in which lithium ions are incorporated in the crystal, etc. Is used. As the electrolytic solution, a solution in which a lithium salt is dissolved in an aprotic organic solvent is used.
[0005]
  On the other hand, a metal oxide, a metal sulfide, or a polymer is used as the positive electrode active material, for example, TiS.₂, MoS₂, NbSe₂, V₂O₅Etc. are known. The discharge reaction of the non-aqueous electrolyte secondary battery using these materials proceeds by lithium ions eluting into the electrolytic solution at the negative electrode and intercalation of lithium ions between the layers of the positive electrode active material at the positive electrode. Conversely, when charging, the reverse reaction proceeds and lithium intercalates at the positive electrode. That is, charge / discharge can be repeated by repeating the reaction in which lithium ions from the negative electrode enter and exit the positive electrode active material.
[0006]
  Currently, as a positive electrode active material of a lithium ion secondary battery, it has high energy density, high voltage, etc.₂, LiNiO₂, LiMn₂O₄Etc. are used. However, since these positive electrode active materials have a metal element having a low Clark number in their composition, they are expensive and have a problem that stable supply is difficult. Moreover, since these positive electrode active materials are comparatively high in toxicity and have a great influence on the environment, new positive electrode active materials that can replace them are required.
[0007]
  On the other hand, it has been proposed to use a compound having an olivine structure as a positive electrode active material of a lithium ion secondary battery. For example, LiFePO which is a compound having an olivine structure₄Has a volume density of 3.6 g / cm³A high potential of 3.4 V is generated and the theoretical capacity is as large as 170 mAh / g. LiFePO₄Is a promising material as a positive electrode active material of a lithium ion secondary battery because it contains one electrochemically dedopeable Li per Fe atom in the initial state. Moreover, LiFePO₄Has the above-mentioned LiCoO because it has iron in its composition, which is abundant and inexpensive in resources.₂, LiNiO₂, LiMn₂O₄Compared to the above, the cost is low, and since the toxicity is low, the influence on the environment is small.
[0008]
[Problems to be solved by the invention]
  By the way, high-temperature cycle characteristics are among the performance required for secondary batteries. In particular, portable devices such as camera-integrated VTRs, cellular phones, and laptop computers can be left in the car interior or charged, and one of the important battery performances is cycle life at high temperatures. Positioned.
[0009]
  However, when a non-aqueous electrolyte secondary battery having a compound having an olivine structure as a positive electrode active material and a material capable of doping and undoping lithium as a negative electrode active material is configured, the battery repeats a charge / discharge cycle at a high temperature. There is a defect that causes a significant capacity deterioration, and it has been confirmed that even when a charge / discharge cycle is performed in an atmosphere at 45 ° C., the life becomes about ¼ or less of room temperature.
[0010]
  Therefore, the present invention has been proposed in view of such conventional circumstances, and includes a positive electrode having a positive electrode active material layer containing a compound having an olivine structure, and a non-aqueous electrode that can be doped and dedoped with lithium. The purpose is to improve the charge / discharge cycle characteristics of the electrolyte secondary battery when used at high temperatures.
[0011]
[Means for Solving the Problems]
  In order to achieve the above object, the non-aqueous electrolyte secondary battery according to the present invention has a general formula Li._xFe_1-yM_yPO₄(However, M represents at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb. 05 ≦ x ≦ 1.2, 0 ≦ y ≦ 0.8), a positive electrode having a positive electrode active material layer containing a compound represented by the following formula, a negative electrode capable of doping and dedoping lithium, and a non-aqueous electrolyte The positive electrode active material layer is made of 0.3% lithium carbonate.mass% To 8.0mass% Content.
[0012]
  The non-aqueous electrolyte secondary battery configured as described above has 0.3% lithium carbonate in the positive electrode active material layer.mass% To 8.0massTherefore, capacity deterioration when the charge / discharge cycle is repeated at a high temperature is suppressed.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
  Embodiments of the present invention will be described below. In addition, this invention is not limited to the following description, In the range which does not deviate from this invention, it can change suitably.
[0014]
  As shown in FIG. 1, a nonaqueous electrolyte battery 1 manufactured by applying the present invention includes a negative electrode 2, a negative electrode can 3 that contains the negative electrode 2, a positive electrode 4, and a positive electrode can 5 that contains the positive electrode 4. And a separator 6 disposed between the positive electrode 4 and the negative electrode 2, and an insulating gasket 7. The negative electrode can 3 and the positive electrode can 5 are filled with a non-aqueous electrolyte.
[0015]
  The negative electrode 2 is made of, for example, a metal lithium foil serving as a negative electrode active material. When a material capable of doping and undoping lithium is used as the negative electrode active material, the negative electrode 2 has a negative electrode active material layer containing the negative electrode active material formed on the negative electrode current collector. For example, a nickel foil or the like is used as the negative electrode current collector.
[0016]
  As a negative electrode active material that can be doped or dedoped with lithium, a layered compound such as metal lithium, a lithium alloy, a conductive polymer doped with lithium, a carbon material, or a metal oxide can be used.
[0017]
  As a binder contained in a negative electrode active material layer, the well-known resin material etc. which are normally used as a binder of a negative electrode active material layer in this kind of nonaqueous electrolyte battery can be used.
[0018]
  The negative electrode can 3 accommodates the negative electrode 2 and serves as an external negative electrode of the nonaqueous electrolyte battery 1.
[0019]
  The positive electrode 4 is a positive electrode active material containing a positive electrode active material capable of electrochemically releasing lithium and reversibly absorbing on a positive electrode current collector made of, for example, an aluminum foil. A material layer is formed. Here, the positive electrode active material layer is mainly composed of the positive electrode active material, and includes a binder, a conductive material, and the like as necessary. Furthermore, in this non-aqueous electrolyte battery 1, lithium carbonate is added to the positive electrode active material layer by 0.3%.mass% To 8.0mass%contains. This non-aqueous electrolyte battery 1 has 0.3% lithium carbonate in the positive electrode active material layer.mass% To 8.0mass%, The deterioration of capacity can be suppressed even when the charge / discharge cycle is repeated at a high temperature. Therefore, the non-aqueous electrolyte battery 1 is a non-aqueous electrolyte secondary battery with improved charge / discharge cycle characteristics at high temperature use and excellent charge / discharge cycle characteristics at high temperature use.
[0020]
  As a positive electrode active material, although a detailed manufacturing method is mentioned later, it has an olivine structure and has general formula Li_xFe_1-yM_yPO₄(However, M represents at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb. 05 ≦ x ≦ 1.2 and 0 ≦ y ≦ 0.8), or a composite of these compounds and a carbon material, that is, Li_xFe_1-yM_yPO₄A carbon composite is used.
[0021]
  Hereinafter, Li_xFe_1-yM_yPO₄LiFePO as₄And a composite comprising this and a carbon material, that is, LiFePO₄The case where a carbon composite is used as the positive electrode active material will be described.
[0022]
  LiFePO₄The carbon composite is LiFePO₄On the surface of the particles, the LiFePO₄A large number of particles of a carbon material having a particle size that is extremely smaller than the particle size of the particles are adhered. Since the carbon material has conductivity, the carbon material and LiFePO₄LiFePO composed of₄The carbon composite is, for example, LiFePO₄Compared with, it is excellent in electronic conductivity. That is, LiFePO₄The carbon composite is LiFePO₄Since the carbon particles adhering to the surface of the particles improve the electron conductivity, LiFePO₄The original capacity is drawn out sufficiently. Therefore, LiFePO as a positive electrode active material₄By using the carbon composite, the non-aqueous electrolyte battery 1 having a high capacity can be realized.
[0023]
  LiFePO₄Units in carbon compositesmassThe carbon content per unit is 3mass% Or more is preferable. LiFePO₄Units in carbon compositesmassCarbon content per unit is 3massIf it is less than%, LiFePO₄Since the amount of carbon particles adhering to the surface of the particles is not sufficient, there is a possibility that the effect of improving the electronic conductivity cannot be obtained sufficiently.
[0024]
  LiFePO₄As a carbon material constituting the carbon composite, in Raman spectroscopy, the wave number of the Raman spectrum of graphite (hereinafter referred to as Gr) is 1340 to 1360 cm.^-1Wave number from 1570 to 1590 cm for diffraction lines appearing in^-1Those having an intensity area ratio A (D / G) of diffraction lines of 0.3 or more can be suitably used.
[0025]
  Here, the intensity area ratio A (D / G) is a wave number of 1570 to 1590 cm measured by Raman spectroscopy as shown in FIG.^-1G peak and wave number 1340-1360cm^-1It is defined as a Raman spectrum intensity area ratio A (D / G) that does not include a background with the D peak appearing in FIG. Further, not including the background means not including the noise portion.
[0026]
  As mentioned above, wave numbers 1570 to 1590 cm are among the numerous peaks of the Raman spectrum of Gr.^-1G peak and wave number 1340-1360cm^-1Two peaks called D peaks appearing at are observed. Among these, the D peak is not originally a peak derived from the G peak, but a Raman inactive peak that appears when the structure is distorted and the symmetry of the structure is lowered. Therefore, the D peak is a measure of the distorted structure of Gr, and the intensity area A (D / G) between the D peak and the G peak has a linear relationship with the reciprocal of the a-axis direction crystallite size La of Gr. It is known.
[0027]
  Specifically, an amorphous carbon material such as acetylene black can be preferably used as such a carbon material.
[0028]
  In addition, the carbon material having the strength area ratio A (D / G) of 0.3 or more as described above can be obtained by performing a treatment such as pulverization with a pulverizer. And the carbon material which has arbitrary A (D / G) can be easily obtained by controlling a grinding | pulverization time.
[0029]
  For example, graphite, which is a crystalline carbon material, is crushed using a powerful pulverizer such as a planetary ball mill, so that the structure is easily destroyed and becomes amorphous. Accordingly, the strength area ratio A (D / G) increases. That is, it becomes possible to easily obtain a carbon material having an arbitrary A (D / G), that is, 0.3 or more by controlling the operation time of the pulverizer. Therefore, a crystalline carbon-based material or the like can be preferably used as the carbon material by pulverizing.
[0030]
  LiFePO₄The powder density of the carbon composite is 2.2 g / cm.³The above is preferable. LiFePO₄The carbon composite has a powder density of 2.2 g / cm.³When the synthetic raw material is milled to such an extent as described above, it becomes sufficiently fine. Therefore, the filling rate of the positive electrode active material is improved, and the nonaqueous electrolyte battery 1 having a high capacity can be realized. LiFePO₄Since the carbon composite is miniaturized to satisfy the above powder density, LiFePO₄It can be said that the specific surface area of is also increasing. That is, LiFePO₄A sufficient contact area between the carbon material and the carbon material can be ensured, and the electron conductivity can be improved.
[0031]
  LiFePO₄The powder density of the carbon composite is 2.2 g / cm³If less than LiFePO₄Since the carbon composite is not sufficiently compressed, the active material filling rate in the positive electrode 4 may not be improved.
[0032]
  LiFePO₄Brunauer Emmett Teller (hereinafter referred to as BET) specific surface area of carbon composite is 10.3 m²/ G or more is preferable. LiFePO₄BET specific surface area of carbon composite is 10.3m²/ G or moremassLiFePO₄The specific surface area of LiFePO can be made sufficiently large.₄The contact area between the carbon material and the carbon material can be increased. Therefore, the electronic conductivity of the positive electrode active material can be reliably improved.
[0033]
  In addition, LiFePO₄The primary particle size of the carbon composite is preferably 3.1 μm or less. LiFePO₄By setting the primary particle size of the carbon composite to 3.1 μm or less, the unitmassLiFePO₄The specific surface area of LiFePO can be made sufficiently large.₄The contact area between the carbon material and the carbon material can be increased. Therefore, the electronic conductivity of the positive electrode active material can be reliably improved.
[0034]
  As a binder contained in a positive electrode active material layer, the well-known resin material etc. which are normally used as a binder of a positive electrode active material layer in this kind of nonaqueous electrolyte battery can be used.
[0035]
  The positive electrode can 5 accommodates the positive electrode 4 and serves as an external positive electrode of the non-aqueous electrolyte battery 1.
[0036]
  The separator 6 separates the positive electrode 4 and the negative electrode 2, and a known material that is usually used as a separator for this type of nonaqueous electrolyte battery can be used. For example, a polymer film such as polypropylene is used. Is used. Moreover, the thickness of a separator needs to be as thin as possible from the relationship between lithium ion conductivity and energy density. Specifically, the thickness of the separator is suitably 50 μm or less, for example.
[0037]
  The insulating gasket 7 is incorporated in and integrated with the negative electrode can 3. The insulating gasket 7 is for preventing leakage of the non-aqueous electrolyte filled in the negative electrode can 3 and the positive electrode can 5.
[0038]
  As the non-aqueous electrolyte, a solution in which an electrolyte is dissolved in an aprotic non-aqueous solvent is used.
[0039]
  Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyllactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl- 1,3-dioxolane, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate and the like can be used. In particular, from the viewpoint of voltage stability, it is preferable to use cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate and vinylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate. Moreover, such a non-aqueous solvent may be used individually by 1 type, and 2 or more types may be mixed and used for it.
[0040]
  Moreover, as an electrolyte dissolved in a non-aqueous solvent, for example, LiPF₆LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃, LiN (CF₃SO₂)₂Lithium salts such as can be used. Among these lithium salts, especially LiPF₆, LiBF₄Is preferably used.
[0041]
  In addition, although the non-aqueous electrolyte battery 1 using the non-aqueous electrolyte was described as an example as the non-aqueous electrolyte battery to which the present invention is applied, the present invention is not limited to this, and the non-aqueous electrolyte is not limited thereto. As described above, the present invention is also applicable when a solid electrolyte is used. Here, as the solid electrolyte, any solid polymer electrolyte such as an inorganic solid electrolyte and a gel electrolyte can be used as long as the material has lithium ion conductivity. Here, examples of the inorganic solid electrolyte include lithium nitride and lithium iodide. The polymer solid electrolyte is composed of an electrolyte salt and a polymer compound that dissolves the electrolyte salt. The polymer compound includes poly (ethylene oxide), an ether polymer such as the cross-linked product, and a poly (methacrylate) ester. -Based polymers, acrylate-based polymers and the like can be used alone, or can be copolymerized or mixed in the molecule. In this case, for example, as the matrix of the gel electrolyte, various polymer materials can be used as long as they can gel by absorbing the non-aqueous electrolyte. Examples of such a polymer material include poly (vinylidene fluoride), fluorine-based polymers such as poly (vinylidene fluoride-CO-hexafluoropropylene), poly (ethylene oxide), and the same crosslinked product. Ether polymers, poly (acrylonitrile), and the like can be used. Among these, it is particularly preferable to use a fluorine-based polymer from the viewpoint of redox stability.
[0042]
  A method for manufacturing the non-aqueous electrolyte battery 1 configured as described above will be described below.
[0043]
  First, Li as a positive electrode active material_xFePO₄A composite of carbon and a carbon material is synthesized according to the production method shown below.
[0044]
  To synthesize this positive electrode active material, Li_xFePO₄These synthetic raw materials are mixed, milled, fired, and a carbon material is added at any of the above points. Li_xFePO₄As a synthetic raw material, Li₃PO₄And Fe₃(PO₄)₂Or its hydrate Fe₃(PO₄)₂・ NH₂O (where n is a hydration number).
[0045]
  Hereinafter, lithium phosphate (Li₃PO₄) And ferrous phosphate octahydrate (Fe) synthesized as shown below₃(PO₄)₂・ 8H₂O) and various steps after adding the carbon material to this synthetic raw material, LiFePO₄The case where a carbon composite is synthesized will be described.
[0046]
  First, LiFePO₄A mixing step is performed by mixing the synthetic raw material and the carbon material into a mixture. Subsequently, the milling process which mills the mixture obtained at the mixing process is performed. Subsequently, the baking process which bakes the mixture which gave the milling process is performed.
[0047]
  In the mixing step, lithium phosphate and ferrous phosphate octahydrate are mixed as a synthetic raw material at a predetermined ratio, and a carbon material is further added to form a mixture.
[0048]
  Ferrous phosphate octahydrate used as a synthetic raw material is iron sulfate heptahydrate (FeSO₄・ 7H₂O) is dissolved in water to an aqueous solution of disodium hydrogen phosphate monodihydrate (2Na₂HPO₄・ 12H₂It is synthesized by adding O) and leaving it for a predetermined time. The synthetic reaction of ferrous phosphate octahydrate is represented by the reaction formula shown in Chemical Formula 1 below.
[0049]
[Chemical 1]

[0050]
  The synthetic ferrous phosphate octahydrate has a certain amount of Fe in its synthesis process.³⁺It is included. Fe as synthetic raw material³⁺If a residual amount remains, a trivalent Fe compound is generated by firing, so LiFePO₄Single-phase synthesis of carbon composites will be hindered. For this reason, a reducing agent is added to the synthetic raw material before firing, and Fe contained in the synthetic raw material at the time of firing.³⁺Fe²⁺It is necessary to reduce it.
[0051]
  However, Fe by reducing agent³⁺Fe²⁺There is a limit to the ability to reduce to³⁺If there is too much content of Fe, Fe³⁺LiFePO cannot be reduced₄It may remain in the carbon composite.
[0052]
  Therefore, Fe relative to the total amount of iron in ferrous phosphate octahydrate³⁺The content of 61mass% Or less is preferable. Fe relative to total iron in ferrous phosphate octahydrate as a synthetic raw material³⁺The content of 61mass% Or less in advance, Fe is reduced during firing.³⁺Without remaining, that is, Fe³⁺LiFePO without producing impurities due to₄A single-phase synthesis of the carbon composite can be reliably performed.
[0053]
  In addition, the longer the standing time when producing ferrous phosphate octahydrate, the more Fe³⁺Since the content ratio of Fe increases, by controlling the standing time to a predetermined time, any Fe³⁺Ferrous phosphate octahydrate having the following content can be produced. In addition, Fe relative to the total amount of iron in ferrous phosphate octahydrate³⁺The content of can be measured by a Mossbauer method.
[0054]
  The carbon material added to the synthetic raw material is Fe contained in the synthetic raw material ferrous phosphate octahydrate.²⁺Due to atmospheric oxygen or firing³⁺Even when oxidized to Fe³⁺Fe²⁺Acts as a reducing agent that reduces to Therefore, Fe³⁺Even if it remains, the generation of impurities is prevented and LiFePO₄Single phase synthesis of carbon composites is possible. Furthermore, the carbon material is Fe contained in the synthetic raw material.²⁺Fe³⁺Acts as an antioxidant to prevent oxidation That is, the carbon material is made of Fe by oxygen present in the atmosphere and in the firing furnace before or during firing.²⁺Is Fe³⁺It is prevented from being oxidized.
[0055]
  That is, as described above, the carbon material functions as a conductive material that improves the electronic conductivity of the positive electrode active material, and also functions as a reducing agent and an antioxidant. This carbon material is LiFePO₄Since it becomes a component of the carbon composite, LiFePO₄There is no need to remove it after the synthesis of the carbon composite. Therefore, LiFePO₄The production of the carbon composite is made efficient.
[0056]
  LiFePO₄Unit of carbon complexmassThe carbon content per unit is 3mass% Or more is preferable. LiFePO₄Unit of carbon complexmassCarbon content per unit is 3mass% Or more, LiFePO₄It is possible to sufficiently extract the capacity and cycle characteristics originally possessed by the.
[0057]
  In the milling step, the mixture obtained in the mixing step is subjected to milling for simultaneous pulverization and mixing. Here, milling refers to powerful crushing and mixing using a ball mill. As the ball mill, for example, a planetary ball mill, a shaker type ball mill, a mechano-fusion, or the like can be suitably used.
[0058]
  By subjecting the mixture obtained in the mixing step to milling, the synthetic raw material and the carbon material can be uniformly mixed. Moreover, if the synthetic raw material is refined by milling, the specific surface area of the synthetic raw material can be increased. Therefore, the contact point between the raw materials increases, and the synthesis reaction in the subsequent firing step can be rapidly advanced.
[0059]
  When milling a mixture containing a synthetic raw material, it is preferable that the particle size distribution of particles having a particle diameter of 3 μm or more be 22% or less in terms of volume-based integration frequency. By setting the particle size distribution of the synthetic raw material in the above range, the synthetic raw material has a surface area sufficient to obtain a surface activity sufficient for the synthetic reaction. Thereby, even when the firing temperature is as low as the melting point of the synthetic raw material of, for example, 600 ° C., the reaction efficiency is good, and LiFePO 4₄A single-phase synthesis of the carbon composite can be reliably performed.
[0060]
  LiFePO₄The powder density of the carbon composite is 2.2 g / cm³Milling is preferably performed to achieve the above. By miniaturizing the synthetic raw material so as to achieve the above powder density, LiFePO₄The specific surface area can be increased. As a result, LiFePO₄The contact area between the carbon material and the carbon material can be increased, and the electron conductivity of the positive electrode active material can be improved.
[0061]
  Therefore, the positive electrode active material which implement | achieves the nonaqueous electrolyte battery 1 which is high capacity | capacitance can be manufactured by milling the mixture containing a synthetic raw material.
[0062]
  In the firing step, the mixture that has been milled in the milling step is fired. By baking the mixture, lithium phosphate and ferrous phosphate octahydrate are reacted, and LiFePO₄Is synthesized.
[0063]
  LiFePO₄The synthesis reaction of is represented by the reaction formula shown in Chemical Formula 2 below. The following2In the reaction formula shown in FIG.₃PO₄And Fe₃(PO₄)₂Or its hydrate Fe₃(PO₄)₂・ NH₂The case where O (where n is a hydration number) is reacted is shown.
[0064]
[Chemical 2]

[0065]
  As is apparent from the reaction formula shown in the chemical formula 2, Fe as a synthesis raw material₃(PO₄)₂By-products are not produced. Fe₃(PO₄)₂・ NH₂When O is used, only non-toxic water is produced as a by-product.
[0066]
  By the way, conventionally, lithium carbonate, ammonium dihydrogen phosphate, and iron (II) acetate are mixed as a synthesis raw material at a predetermined ratio, calcined, and LiFePOPO4 by a reaction shown in the following chemical formula 3.₄Was synthesized.
[0067]
[Chemical Formula 3]

[0068]
  As is clear from the reaction formula shown in Chemical Formula 3, the conventional LiFePO 4₄In this synthesis method, toxic by-products such as ammonia and acetic acid were produced during firing. For this reason, facilities, such as a large-scale air collector for processing these toxic by-products, were required, which caused an increase in manufacturing costs. Moreover, since these by-products are produced in large quantities, LiFePO₄The yield of was reduced.
[0069]
  However, as a synthetic raw material, Li₃PO₄And Fe₃(PO₄)₂Or its hydrate Fe₃(PO₄)₂・ NH₂By using O (where n is a hydration number), the target substance LiFePO is produced without producing toxic by-products.₄Can be obtained. In other words, the safety during firing is significantly improved as compared with the conventional manufacturing method. In addition, in the past, a large-scale processing facility was required to process toxic by-products, but in the manufacturing method described above, since the by-products are non-toxic water, the processing steps can be greatly simplified. The processing equipment can be reduced. Therefore, the manufacturing cost can be greatly reduced as compared with the case of processing ammonia or the like which is a conventional by-product. Furthermore, as is apparent from the reaction formula shown in the chemical formula 2 above, since a small amount of by-products are produced, LiFePO 4₄The yield of can be greatly improved.
[0070]
  The firing temperature when firing the mixture can be set to 400 ° C. to 900 ° C. by the above synthesis method, but is preferably about 600 ° C. in consideration of battery performance. When the firing temperature is lower than 400 ° C., the chemical reaction and crystallization do not proceed sufficiently, and Li, which is a synthetic raw material, is used.₃PO₄Etc., and there is a homogeneous LiFePO₄You may not be able to get it. On the other hand, when the firing temperature exceeds 900 ° C., crystallization proceeds excessively and LiFePO 4₄The particles of LiFePO₄The contact area between the carbon material and the carbon material is reduced, and the electron conductivity is lowered. Therefore, there is a possibility that a sufficient discharge capacity cannot be obtained.
[0071]
  LiFePO synthesized during firing₄Fe in the carbon composite is in a divalent state. For this reason, at a synthesis temperature of about 600 ° C., LiFePO₄Fe in the carbon composite is expressed by the reaction formula shown in the following chemical formula 4 by oxygen in the firing atmosphere.³⁺Oxidized quickly. Due to this, impurities such as a trivalent Fe compound are generated, and LiFePO₄Single-phase synthesis of carbon composites will be hindered.
[0072]
[Formula 4]

[0073]
  Therefore, an inert gas such as nitrogen or argon or a reducing gas such as hydrogen or carbon monoxide is used as the firing atmosphere, and the oxygen concentration in the firing atmosphere is set to LiFePO 4.₄The range in which Fe in the carbon composite is not oxidized, that is, 1012 ppm (volume) or less is preferable. By setting the oxygen concentration in the firing atmosphere to 1012 ppm (volume) or less, oxidation of Fe is prevented even at a synthesis temperature of about 600 ° C., and LiFePO 4₄A single-phase synthesis of the carbon composite can be reliably performed.
[0074]
  When the oxygen concentration in the firing atmosphere is higher than 1012 ppm (volume), the amount of oxygen in the firing atmosphere is too large.₄Fe in the carbon composite is Fe³⁺As a result, the impurities are generated due to the oxidation of LiFePO.₄There is a possibility that the single-phase synthesis of the carbon composite may be hindered.
[0075]
  LiFePO after firing₄For removal of the carbon composite, LiFePO after firing₄Removal temperature of carbon composite, ie, LiFePO₄LiFePO when carbon composite is exposed to air₄The temperature of the carbon composite is preferably 305 ° C. or lower. Moreover, LiFePO after firing₄More preferably, the carbon composite take-out temperature is 204 ° C. or lower. LiFePO₄LiFePO after firing by setting the carbon composite take-out temperature to 305 ° C. or lower₄It is possible to prevent Fe in the carbon composite from being oxidized by oxygen in the atmosphere and generating impurities.
[0076]
  LiFePO after firing₄When the carbon composite is taken out without being sufficiently cooled, LiFePO₄There is a possibility that Fe in the carbon composite is oxidized by oxygen in the atmosphere and impurities are generated. However, LiFePO to very low temperatures₄If the carbon composite is cooled, the working efficiency may be reduced.
[0077]
  Therefore, LiFePO after firing₄LiFePO after firing by setting the carbon composite take-out temperature to 305 ° C. or lower₄LiFePO that prevents Fe from being oxidized by oxygen in the air to produce impurities and maintains work efficiency, and has favorable characteristics as battery characteristics.₄A carbon composite can be efficiently synthesized.
[0078]
  In addition, LiFePO after firing₄The carbon composite is cooled in a firing furnace. The cooling method at this time may be natural cooling or forced cooling. However, forced cooling is preferable when the cooling time is shortened, that is, when working efficiency is considered. In the case of forced cooling, only a mixed gas of oxygen and inert gas or only an inert gas is supplied into the firing furnace so that the inside of the firing furnace has the above-described oxygen concentration, that is, 1012 ppm (volume) or less. It ’s fine.
[0079]
  In the above, the carbon material is added before milling. However, the carbon material can be added after milling or after firing.
[0080]
  However, when adding a carbon material after baking, the reduction effect at the time of baking and the antioxidant effect cannot be acquired, but the conditions of using only for an electroconductive improvement effect are attached. Therefore, when adding the carbon material after firing, Fe means by other means³⁺It is necessary to prevent the remaining.
[0081]
  In addition, when the carbon material is added after firing, the product synthesized by firing is LiFePO 4.₄LiFePO, not carbon composite₄It is. Therefore, LiFePO synthesized by firing₄After adding the carbon material, milling is performed again. By performing milling again, the added carbon material is refined, and LiFePO₄It becomes easy to adhere to the surface. Also, by re-milling, LiFePO₄And the carbon material are sufficiently mixed, so that the refined carbon material is LiFePO₄It can be made to adhere uniformly to the surface. Therefore, even when the carbon material is added after firing, the same product as that when the carbon material is added before milling, that is, LiFePO₄A carbon composite can be obtained, and the same effect as described above can be obtained.
[0082]
  LiFePO obtained as described above₄The nonaqueous electrolyte battery 1 using a carbon composite as a positive electrode active material is manufactured as follows, for example.
[0083]
  As the negative electrode 2, first, a negative electrode active material and a binder are dispersed in a solvent to prepare a slurry negative electrode mixture. Next, the negative electrode 2 is produced by uniformly coating the obtained negative electrode mixture on a current collector and drying to form a negative electrode active material layer. As the binder of the negative electrode mixture, a known binder can be used, and a known additive or the like can be added to the negative electrode mixture. Moreover, the metallic lithium used as a negative electrode active material can also be used as the negative electrode 2 as it is.
[0084]
  As the positive electrode 4, first, LiFePO used as a positive electrode active material.₄A carbon composite, lithium carbonate, and a binder are mixed to prepare a positive electrode mixture, which is dispersed in a solvent to form a slurry. That is, in manufacturing the non-aqueous electrolyte battery 1, lithium carbonate or CO 2 is previously contained in the positive electrode active material.₃ ^2-No ions are contained, and lithium carbonate is mixed with the positive electrode active material when preparing the positive electrode mixture. Here, lithium carbonate has a lithium carbonate content of 0.3 in the positive electrode mixture.mass% To 8.0mass% To mix. That is, the lithium carbonate content in the positive electrode active material layer is 0.3.mass% To 8.0mass% Lithium carbonate is mixed. The content of lithium carbonate in the positive electrode mixture is 0.3mass% To 8.0massWhen the non-aqueous electrolyte battery 1 configured using the positive electrode mixture is repeated at a high temperature, the capacity deterioration can be suppressed. Therefore, it becomes possible to improve the charge / discharge cycle characteristics of the nonaqueous electrolyte battery 1 when used at a high temperature.
[0085]
  Next, the obtained positive electrode mixture in the form of a slurry is uniformly coated on a current collector and dried to form a positive electrode active material layer, whereby the positive electrode 4 is produced. As the binder of the positive electrode mixture, a known binder can be used, and a known additive or the like can be added to the positive electrode mixture.
[0086]
  The nonaqueous electrolytic solution is prepared by dissolving an electrolyte salt in a nonaqueous solvent.
[0087]
  Then, the negative electrode 2 is accommodated in the negative electrode can 3, the positive electrode 4 is accommodated in the positive electrode can 5, and a separator 6 made of a polypropylene porous film or the like is disposed between the negative electrode 2 and the positive electrode 4. A coin-type nonaqueous electrolyte battery 1 is completed by injecting a nonaqueous electrolyte into the negative electrode can 3 and the positive electrode can 5 and caulking and fixing the negative electrode can 3 and the positive electrode can 5 via the insulating gasket 7. To do.
[0088]
  The nonaqueous electrolyte battery 1 manufactured as described above contains 0.3% of lithium carbonate in the positive electrode mixture.mass% To 8.0mass%, The deterioration of capacity can be suppressed even when the charge / discharge cycle is repeated at a high temperature. Therefore, the nonaqueous electrolyte battery 1 is a nonaqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics at high temperature use.
[0089]
  In the above, as described above, LiFePO₄The positive electrode through a mixing step of mixing a synthetic raw material and a carbon material into a mixture, a milling step of milling the mixture obtained in the mixing step, and a firing step of firing the mixture that has been milled in the milling step Active material is synthesized. However, in the present invention, the method for producing the positive electrode active material is not limited to this. For example, the carbon material may not be added in the above method, and the positive electrode active material may be synthesized without the milling step.
[0090]
  The non-aqueous electrolyte battery 1 according to the present embodiment as described above is not particularly limited with respect to its shape, such as a cylindrical shape, a square shape, a coin shape, a button shape, etc., and is thin and large. The size can be various.
[0091]
【Example】
  Hereinafter, the present invention will be described based on specific experimental results. Here, in order to investigate the effect of the present invention, LiFePO 4₄LiFePO obtained by synthesizing a carbon composite₄A non-aqueous electrolyte battery was produced using the carbon composite as the positive electrode active material, and its characteristics were evaluated.
[0092]
  <Example 1>
  LiFePO as positive electrode active material₄A complex was synthesized. The manufacturing method of this positive electrode active material is shown below.
[0093]
  First, Li₃PO₄And Fe₃(PO₄)₂・ 8H₂O is mixed so that the element ratio of lithium and iron is 1: 1, and acetylene black powder having various sizes of particle sizes is added to 10% of the entire fired product.mass% To give a mixture. Next, the mixture and the alumina balls having a diameter of 10 mm were put into an alumina pot having a diameter of 100 mm in a mass ratio of mixture: alumina balls = 1: 2, and the mixture was milled using a planetary ball mill. As a planetary ball mill, an experimental planetary rotary pot mill “LA-PO”₄(Made by Ito Seisakusho) and milling was performed as the conditions shown below.
[0094]
  Planetary ball mill milling conditions
  Revolving radius: 200mm
  Revolving speed: 250rpm
  Rotational speed: 250rpm
  Driving time: 10h
  Next, the mixture after milling is placed in a ceramic crucible and baked at a temperature of 600 ° C. for 5 hours in an electric furnace in a nitrogen atmosphere to thereby form LiFePO 4.₄A carbon composite was obtained.
[0095]
  LiFePO thus obtained.₄LiFePO using carbon composite₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmass91 blends obtained with a 99.7: 0.3 ratiomassPart, 6 graphite as a conductive agentmassPolyvinylidene fluoride 3 as bindermassParts were mixed to prepare a positive electrode mixture, which was dispersed in N-methyl-2-pyrrolidone to form a slurry.
[0096]
  Next, the positive electrode mixture slurry was uniformly applied to both surfaces of a 20 μm-thick strip-shaped aluminum foil as a positive electrode current collector, and after drying, compression-molded with a roll press to produce a strip-shaped positive electrode.
[0097]
  As a negative electrode active material, petroleum pitch is used as a starting material, and a functional group containing oxygen is introduced into this material by 10 to 20%, so-called oxygen crosslinking, and then fired at 1000 ° C. in an inert gas stream to be close to glassy carbon The non-graphite carbon material was obtained. As a result of X-ray diffraction measurement of this material, the (002) plane spacing was 3.76 mm and the true specific gravity was 1.58.
[0098]
  The carbon material obtained in this way is 90%.massAnd polyvinylidene fluoride 10 as a bindermassPart was mixed to prepare a negative electrode mixture, which was dispersed in N-methyl-2-pyrrolidone to form a slurry.
[0099]
  Next, a 10 μm-thick strip-shaped copper foil is prepared as a negative electrode current collector, and the slurry of the negative electrode mixture is uniformly applied on both sides thereof, and after drying, compression-molded with a roll press to produce a strip-shaped negative electrode did.
[0100]
  As the separator, a microporous polypropylene film having a thickness of 25 μm was used, and the negative electrode, the positive electrode, and the separator were wound in a spiral shape to produce an electrode element.
[0101]
  The electrode element thus obtained was stored in a nickel-plated iron can. Insulating plates are arranged on both the upper and lower surfaces of the spiral electrode element, the aluminum positive electrode lead is led out from the positive electrode current collector, and the nickel negative electrode lead is led out from the negative electrode current collector and welded to the battery can. did.
[0102]
  Next, in this battery can, LiPF was mixed with a mixed solvent of 50% by volume of propylene carbonate and 50% by volume of 1,2-dimethoxyethane.₆An electrolyte solution in which 1 mol / l was dissolved was injected.
[0103]
  Finally, the battery can and the battery lid were crimped and sealed through an insulating gasket coated with asphalt to produce a cylindrical battery having a diameter of 20 mm and a height of 50 mm.
[0104]
  <Example 2>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 99.5: 0.5.
[0105]
  <Example 3>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 99: 1.
[0106]
  <Example 4>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 97: 3.
[0107]
  <Example 5>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 95: 5.
[0108]
  <Example 6>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 92: 8.
[0109]
  <Comparative Example 1>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was fabricated in the same manner as in Example 1 except that the ratio was 100: 0, that is, lithium carbonate was not mixed.
[0110]
  <Comparative example 2>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 99.8: 0.2.
[0111]
  <Comparative Example 3>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA cylindrical battery was produced in the same manner as in Example 1 except that the ratio was 90:10.
[0112]
  The cylindrical batteries of Examples 1 to 3 and Comparative Examples 1 to 3 manufactured as described above were subjected to a charge / discharge cycle characteristic test as follows, and the initial discharge capacity and the discharge after 50 cycles. The capacity maintenance rate was obtained.
[0113]
  <Charge / discharge cycle characteristics test>
  The charge / discharge cycle characteristics were evaluated by the capacity retention rate after repeated charge / discharge.
[0114]
  First, for a cylindrical battery, 1 A constant current constant voltage charging was performed for 3 hours up to an upper limit of 4.1 V in a 23 ° C. atmosphere, and then 6.2 Ω constant resistance discharge was performed to a final voltage of 2.75 V . The discharge capacity at this time was defined as the initial discharge capacity. Furthermore, 50 cycles of charge / discharge were performed in a 45 ° C. atmosphere under the same charge / discharge conditions, and the discharge capacity at the 50th cycle was determined. Then, the ratio ((C2 / C1) × 100) of the discharge capacity (C2) at the 50th cycle to the initial discharge capacity (C1) was determined as the discharge capacity retention rate. The results are shown in Table 1. In addition, the battery evaluation in Table 1 indicates that the initial discharge capacity is 1000 mAh or more and the discharge capacity maintenance rate of the 50th cycle is 95% or more is marked as “good”, and the initial discharge capacity is 1000 mAh or less, or the 50th cycle. The case where the discharge capacity retention rate was less than 95% was rejected, and x was marked. Here, an initial discharge capacity of 1000 mAh or more and a discharge capacity maintenance ratio at the 50th cycle of 95% or more are preferable values as battery characteristics.
[0115]
[Table 1]

[0116]
  As can be seen from Table 1, the content of lithium carbonate in the positive electrode active material layer was 0.3mass% To 8.0massIn Examples 1 to 6, which are%, a value far exceeding the initial discharge capacity of 1000 mAh, which is desirable as a battery characteristic at a practical level, is shown, and the discharge capacity maintenance rate at the 50th cycle is also a battery characteristic at a practical level. It greatly exceeds the desirable 95% and shows excellent charge / discharge cycle characteristics. This is presumably because the lithium carbonate contained in the positive electrode active material layer in an appropriate amount suppresses capacity deterioration even when the charge / discharge cycle is repeated at a high temperature of 45 ° C.
[0117]
  On the other hand, the lithium carbonate content in the positive electrode active material layer is 0.3.massIn Comparative Example 1 and Comparative Example 2 that are less than%, the initial discharge capacity exceeds 1000 mAh, which is desirable as a battery characteristic at a practical level, but the discharge capacity retention ratio at the 50th cycle is desirable as a battery characteristic at a practical level 95. The value is less than%. This is because the content of lithium carbonate in the positive electrode active material layer is too small, or since lithium carbonate is not contained in the positive electrode active material layer, the capacity deteriorates when the charge / discharge cycle is repeated at a high temperature of 45 ° C. It is thought that this is because the above could not be sufficiently suppressed.
[0118]
  Moreover, the content rate of lithium carbonate in the positive electrode active material layer is 8.0.massIn Comparative Example 3 exceeding%, the initial discharge capacity is less than 1000 mAh, which is desirable as battery characteristics at a practical level. This is presumably because the content of the positive electrode active material in the positive electrode active material layer has decreased because the content of lithium carbonate in the positive electrode active material layer is too large.
[0119]
  From the above, 0.3% of lithium carbonate is contained in the positive electrode active material layer.mass% To 8.0mass%, The deterioration of capacity can be suppressed even when the charge / discharge cycle is repeated at a high temperature of 45 ° C., and the effect of improving the charge / discharge cycle characteristics when the non-aqueous electrolyte secondary battery is used at a high temperature. It was confirmed that
[0120]
  Next, LiFePO as a positive electrode active material₄Li other than carbon composite_xFe_1-yM_yPO₄A carbon composite was synthesized, and the characteristics of the battery using the carbon composite were evaluated.
[0121]
  <Example 7>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Mn_0.8PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0122]
  <Example 8>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Cr_0.8PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0123]
  <Example 9>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Co_0.8PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0124]
  <Example 10>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Cu_0.8PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0125]
  <Example 11>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Ni_0.8PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0126]
  <Example 12>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25V_0.75PO₄PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0127]
  <Example 13>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Mo_0.75PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0128]
  <Example 14>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Ti_0.75PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0129]
  <Example 15>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.3Zn_0.7PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0130]
  <Example 16>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.3Al_0.7PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0131]
  <Example 17>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.3Ga_0.7PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0132]
  <Example 18>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Mg_0.75PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0133]
  <Example 19>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25B_0.75PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0134]
  <Example 20>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Nb_0.75PO₄A cylindrical battery was produced in the same manner as in Example 1 except that the above was used, and a charge / discharge cycle test was performed in the same manner as described above. As a result, the same effects as in Examples 1 to 6 were confirmed. It was.
[0135]
  From the above, Example 7 to Example 20 can also suppress the capacity deterioration when the charge / discharge cycle is repeated at a high temperature of 45 ° C., and the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery at high temperature use The improvement effect was confirmed.
[0136]
  Therefore, the general formula Li_xFe_1-yM_yPO₄In the positive electrode active material layer containing the carbon composite, 0.3% of lithium carbonate was added.mass% To 8.0mass%, It is possible to suppress the capacity deterioration when the charge / discharge cycle is repeated at a high temperature of 45 ° C., and to improve the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery when used at a high temperature. I can say that.
[0137]
  Next, a polymer battery was prepared and the characteristics were evaluated.
[0138]
  <Example 21>
  First, a gel electrolyte was prepared as shown below. First, hexafluoropropylene is 6.9.mass% Polyvinylidene fluoride copolymerized at a ratio of%, a non-aqueous electrolyte, and dimethyl carbonate were mixed, stirred, and dissolved to prepare a sol electrolyte solution. Next, vinylene carbonate (VC) is added to the sol-like electrolyte solution in an amount of 0.5.mass% To obtain a gel electrolyte solution. As a non-aqueous electrolyte, LiPF was mixed with a mixed solvent in which ethylene carbonate (EC) and propylene carbonate (PC) were mixed at a volume ratio of 6: 4.₆Was dissolved at a rate of 0.85 mol / kg.
[0139]
  Subsequently, the positive electrode was produced as follows. First, the LiFePO produced in Example 1₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmass85 blends obtained with a 99.7: 0.3 ratiomassPart, 10 graphite as a conductive agentmassPart, poly (vinylidene fluoride) 5 which is a fluororesin powder as a bindermassAfter preparing a positive electrode mixture by mixing the parts, a slurry was prepared by adding N-methylpyrrolidone. Next, this slurry was applied to an aluminum foil having a thickness of 20 μm, heated and dried, and then subjected to a pressing step to prepare a positive electrode coated foil. Next, after applying a gel electrolyte solution to one surface of this positive electrode coated foil and drying to remove the solvent, it was punched into a circle having a diameter of 15 mm according to the cell diameter to obtain a positive electrode.
[0140]
  Next, a negative electrode was produced as shown below. First, a fluororesin powder is used as a binder in graphite powder.mass% And N-methylpyrrolidone was added to prepare a slurry. Next, this slurry was applied to a copper foil, dried by heating, and then punched into a circle having a diameter of 16.5 mm in accordance with the size of the cell through a pressurizing process to obtain a negative electrode.
[0141]
  The positive electrode obtained as described above was accommodated in a positive electrode can, the negative electrode was accommodated in a negative electrode can, and a separator was disposed between the positive electrode and the negative electrode. Then, the positive electrode can and the negative electrode can were caulked and fixed to produce a 2016 coin-type lithium polymer battery having a diameter of 20 mm and a thickness of 1.6 mm.
[0142]
  <Example 22>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA coin-type lithium polymer battery was produced in the same manner as in Example 21 except that the ratio was 92: 8.
[0143]
  <Comparative example 4>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA coin-type lithium polymer battery was produced in the same manner as in Example 21 except that the ratio was 99.8: 0.2.
[0144]
  <Comparative Example 5>
  When preparing the positive electrode mixture, LiFePO₄Carbon composite and lithium carbonate Li₂CO₃Mixed withmassA coin-type lithium polymer battery was produced in the same manner as in Example 21 except that the ratio was 90:10.
[0145]
  The coin-type lithium polymer batteries of Example 21, Example 22, Comparative Example 4 and Comparative Example 5 produced as described above were subjected to charge / discharge cycle characteristic tests in the same manner as described above, and the initial discharge capacity density and 30 cycles were measured. The subsequent discharge capacity retention rate was determined. The results are shown in Table 2. The battery evaluation in Table 1 indicates that the initial discharge capacity is 3.5 mAh or more and the discharge capacity maintenance rate at the 30th cycle is 95% or more, and is marked as ◯, and the initial discharge capacity is 3.5 mAh or less, or The case where the discharge capacity maintenance rate at the 30th cycle was less than 95% was rejected and x was marked. Here, the initial discharge capacity is 3.5 mAh or more, and the discharge capacity maintenance ratio at the 30th cycle is 95% or more, which is a preferable value as battery characteristics.
[0146]
[Table 2]

[0147]
  As can be seen from Table 2, the content of lithium carbonate in the positive electrode active material layer was 0.3mass% To 8.0massIn Example 21 and Example 22, which are%, a value exceeding the initial discharge capacity of 3.5 mAh, which is desirable as a battery characteristic at a practical level, is shown, and the discharge capacity retention rate at the 30th cycle is also a battery characteristic at a practical level. The desirable 95% is greatly exceeded, and excellent charge / discharge cycle characteristics are exhibited. This is presumably because the lithium carbonate contained in the positive electrode active material layer in an appropriate amount suppresses capacity deterioration even when the charge / discharge cycle is repeated at a high temperature of 45 ° C.
[0148]
  On the other hand, the lithium carbonate content in the positive electrode active material layer is 0.3.massIn Comparative Example 4, which is less than%, the initial discharge capacity exceeds 3.5 mAh, which is desirable for practical battery characteristics, but the discharge capacity retention rate at the 50th cycle is 95%, which is desirable for practical battery characteristics. The value is lower. This is presumably because the content of lithium carbonate in the positive electrode active material layer was too small, and thus capacity deterioration when the charge / discharge cycle was repeated at a high temperature of 45 ° C. could not be sufficiently suppressed.
[0149]
  Moreover, the content rate of lithium carbonate in the positive electrode active material layer is 8.0.massIn Comparative Example 5, which exceeds 50%, the initial discharge capacity is less than 3.5 mAh, which is desirable as battery characteristics at a practical level. This is presumably because the content of the positive electrode active material in the positive electrode active material layer has decreased because the content of lithium carbonate in the positive electrode active material layer is too large.
[0150]
  From the above, even in the case of a polymer battery, 0.3% of lithium carbonate is contained in the cathode active material layer.mass% To 8.0mass%, The deterioration of capacity can be suppressed even when the charge / discharge cycle is repeated at a high temperature of 45 ° C., and the effect of improving the charge / discharge cycle characteristics when the non-aqueous electrolyte secondary battery is used at a high temperature. It was confirmed that
[0151]
  Next, LiFePO as a positive electrode active material₄Li other than carbon composite_xFe_1-yM_yPO₄A carbon composite was synthesized and the characteristics of a polymer battery using the carbon composite were evaluated.
[0152]
  <Example 23>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Mn_0.8PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0153]
  <Example 24>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Cr_0.8PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0154]
  <Example 25>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Co_0.8PO₄A coin-type lithium polymer battery was prepared in the same manner as in Example 1 except that the carbon composite was used, and a charge / discharge cycle test was performed in the same manner as described above. The effect of was confirmed.
[0155]
  <Example 26>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Cu_0.8PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0156]
  <Example 27>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.2Ni_0.8PO₄A coin-type lithium polymer battery was prepared in the same manner as in Example 1 except that the carbon composite was used, and a charge / discharge cycle test was performed in the same manner as described above. The effect of was confirmed.
[0157]
  <Example 28>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25V_0.75PO₄A coin-type lithium polymer battery was prepared in the same manner as in Example 1 except that the carbon composite was used, and a charge / discharge cycle test was performed in the same manner as described above. The effect of was confirmed.
[0158]
  <Example 29>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Mo_0.75PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0159]
  <Example 30>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Ti_0.75PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0160]
  <Example 31>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.3Zn_0.7PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0161]
  <Example 32>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.3Al_0.7PO₄A coin-type lithium polymer battery was prepared in the same manner as in Example 1 except that the carbon composite was used, and a charge / discharge cycle test was performed in the same manner as described above. The effect of was confirmed.
[0162]
  <Example 33>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.3Ga_0.7PO₄A coin-type lithium polymer battery was prepared in the same manner as in Example 1 except that the carbon composite was used, and a charge / discharge cycle test was performed in the same manner as described above. The effect of was confirmed.
[0163]
  <Example 34>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Mg_0.75PO₄A coin-type lithium polymer battery was prepared in the same manner as in Example 1 except that the carbon composite was used, and a charge / discharge cycle test was performed in the same manner as described above. The effect of was confirmed.
[0164]
  <Example 35>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25B_0.75PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. As in the above-described Examples 21 and 22, The effect of was confirmed.
[0165]
  <Example 36>
  LiFePO as positive electrode active material₄LiFe instead of carbon composite_0.25Nb_0.75PO₄A coin-type lithium polymer battery was produced in the same manner as in Example 1 except that a carbon composite was used, and a charge / discharge cycle test was conducted in the same manner as described above. From the above, Examples 23 to Even when the charge / discharge cycle was repeated at a high temperature of 45 ° C. even in No. 33, it was possible to suppress the capacity deterioration, and the effect of improving the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery at high temperature was confirmed.
[0166]
  Therefore, even in the case of a polymer battery, the general formula Li_xFe_1-yM_yPO₄In the positive electrode active material layer containing the carbon composite, 0.3% of lithium carbonate was added.mass% To 8.0mass%, It is possible to suppress the capacity deterioration when the charge / discharge cycle is repeated at a high temperature of 45 ° C., and to improve the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery when used at a high temperature. I can say that.
[0167]
【The invention's effect】
  The nonaqueous electrolyte secondary battery according to the present invention has a general formula Li_xFe_1-yM_yPO₄(However, M represents at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb. 05 ≦ x ≦ 1.2, 0 ≦ y ≦ 0.8), a positive electrode having a positive electrode active material layer containing a compound represented by the following formula, a negative electrode capable of doping and dedoping lithium, and a non-aqueous electrolyte And the positive electrode active material layer comprises 0.3% of lithium carbonate.mass% To 8.0mass% Content.
[0168]
  The non-aqueous electrolyte secondary battery according to the present invention as described above has 0.3% lithium carbonate in the positive electrode active material layer.mass% To 8.0mass%, It is possible to suppress capacity deterioration when the charge / discharge cycle is repeated at a high temperature.
[0169]
  Thereby, it becomes possible to improve the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery when used at a high temperature.
[0170]
  Therefore, according to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery having a high energy density and excellent charge / discharge cycle characteristics at high temperatures, and its industrial value is great.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing a configuration example of a nonaqueous electrolyte secondary battery to which the present invention is applied.
FIG. 2 is a characteristic diagram showing a Raman spectrum peak of a carbon material.
[Explanation of symbols]
  1 nonaqueous electrolyte battery, 2 negative electrode, 3 negative electrode can, 4 positive electrode, 5 positive electrode can, 6 separator, 7 insulating gasket

Claims (3)

Formula _{Li x Fe 1-y M y} PO 4 ( provided that, M is chosen Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, from the group consisting of B and Nb A positive electrode having a positive electrode active material layer containing a compound represented by: 0.05 ≦ x ≦ 1.2 and 0 ≦ y ≦ 0.8, and lithium. Comprising a negative electrode that can be doped and dedoped, and a non-aqueous electrolyte;
The positive active material layer is a non-aqueous electrolyte secondary battery you lithium carbonate 0.3 wt% to 8.0 wt%. The non-aqueous electrolyte, a non-aqueous electrolyte secondary battery 請 Motomeko 1 wherein Ru liquid based electrolytes der. The non-aqueous electrolyte, a non-aqueous electrolyte secondary battery 請 Motomeko 1 wherein Ru der polymer electrolyte.

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