JP4308446B2 - Carbonaceous material and lithium secondary battery - Google Patents

Description

【００６７】
「表２」
試料 加熱粉砕後のＳｉ微粒子 容量比(20th/1st)
実施例2 0.3μm 88.0%
比較例3 2.0μm 81.2%
比較例4 − 84.4%
【００６８】
表１に示すように、比較例１及び２の１サイクル目の放電容量が、実施例１〜４とほぼ同等か、若しくは高くなっていることがわかる。
これは、実施例１〜４の場合、Ｓｉ微粒子を炭素製るつぼ中で加熱したことにより、Ｓｉ微粒子のＳｉ相中にＳｉＣ相が析出し、リチウムと合金を形成するＳｉ相の含有量が相対的に減少したためと考えられる。
一方、比較例１及び２は、ジルコニウム製のるつぼを用いたため、Ｓｉ微粒子のＳｉ相中にはＳｉＣ相が析出せず、このためＳｉ相の含有量が相対的に実施例１〜４よりも高くなったたためと考えられる。
【００６９】
更に、実施例１〜３と実施例４とを放電容量で比較すると、実施例４が高い放電容量を示している。
これは、実施例１〜３の場合、Ｓｉ微粒子とＢ₂Ｏ₃を混合して加熱したことにより、Ｂ₂Ｏ₃の酸素原子がＳｉを酸化し、ＳｉＢ₄相の他にＳｉＯ₂相が比較的多く析出したため、Ｓｉ相の含有量が相対的に減少したためと考えられる。
一方、実施例４では、Ｓｉ微粒子とＢとを混合して加熱したため、実施例１〜３に比べて酸素が少ない状況であり、雰囲気中の微小な残存酸素等によりわずかにＳｉＯ₂相が析出するものの、実施例１〜３よりもその量は少なく、このためＳｉ相の含有量が相対的に実施例１〜３よりも高くなったためと考えられる。
【００７０】
更に、実施例１〜３については、Ｂ₂Ｏ₃の添加量が増加するにつれて放電容量が低下している。
これは、Ｂ₂Ｏ₃の添加量が増加するに従ってＳｉＢ₄相が多く析出し、Ｓｉ相の含有量が相対的に減少したためと考えられる。
【００７１】
次に充放電効率については、表１から、実施例１〜４の充放電効率が比較例１及び２よりも高くなっていることがわかる。
これは、Ｓｉ微粒子中にＳｉＣ相、ＳｉＯ₂相及びＳｉＢ₄相が析出し、リチウムと合金を形成するＳｉ相の含有量が相対的に減少したために、Ｓｉ微粒子自体の膨張、収縮が適度に抑制され、これにより複合粒子の黒鉛からの遊離が少なくなって、充放電効率が向上したためと考えられる。
また、ＳｉＣ相、ＳｉＯ₂相及びＳｉＢ₄相の析出により、Ｓｉ相の結晶性が低下し、これによりＳｉ相中におけるリチウムの拡散速度が向上したとも、充放電効率向上の一因と考えられる。
【００７２】
図６には、実施例２の炭素質材料のＳｉ微粒子のＸ線回折パターンを示す。図６から明らかなように、Ｓｉ相の他に、ＳｉＣ相、ＳｉＯ₂相及びＳｉＢ₄相に由来する回折ピークが観察される。
Ｓｉ相の（１１１）面の回折強度をＰ_Siとし、ＳｉＯ₂相の（１１１）面の回折強度をＰ_SiO2とし、ＳｉＣ相の（１１１）面の回折強度をＰ_SiCとし、ＳｉＢ₄相の（１０４）面の回折強度をＰ_SiBとしたとき、図６から、Ｐ_SiO2／Ｐ_Si＝０．０３４であり、Ｐ_SiC／Ｐ_Si＝０．０４４であり、Ｐ_SiB／Ｐ_SiO2＝１．５０であり、Ｐ_SiB／Ｐ_SiC＝１．１６であることがわかる。
【００７３】
次に、表２から明らかなように、実施例２の容量比は、比較例４よりも大幅に向上していることがわかる。
これは、１サイクル目の充放電効率が向上した理由と同様に、Ｓｉ微粒子中にＳｉＣ相、ＳｉＯ₂相及びＳｉＢ₄相が析出し、リチウムと合金を形成するＳｉ相の含有量が相対的に減少したために、Ｓｉ微粒子自体の膨張、収縮が適度に抑制され、これにより複合粒子の黒鉛からの遊離が少なくなって、サイクル特性が向上したためと考えられる。
また、ＳｉＣ相、ＳｉＯ₂相及びＳｉＢ₄相の析出により、Ｓｉ相の結晶性が低下し、これによりＳｉ相中におけるリチウムの拡散速度が向上したとも、サイクル特性向上の一因と考えられる。
また、比較例３の結果より、加熱焼成後のＳｉ微粒子が大きいとＳｉ微粒子の膨脹、収縮を抑制する硬化が薄れることがわかる。
【００７４】
【発明の効果】
以上、詳細に説明したように、本発明の炭素質材料によれば、黒鉛粒子及びＳｉ微粒子がＬｉを吸蔵するので、黒鉛粒子単独の場合よりも充放電容量が向上する。また黒鉛粒子に対して高比抵抗なＳｉ微粒子の周りに導電性炭素材を配置することで、Ｓｉ微粒子の導電性を見かけ上、向上させる。更にＳｉ微粒子を硬質炭素膜で被覆することにより、Ｌｉの吸蔵・放出に伴うＳｉ微粒子の体積膨張・収縮が機械的に抑えられる。更にまた、黒鉛粒子と複合粒子を非晶質炭素膜で覆うことにより、黒鉛粒子が直接に電解液に触れることなく電解液分解が抑制されるとともに、複合粒子が黒鉛粒子から脱落することがなく、更に充電による体積膨張に起因するＳｉ微粒子の微粉化を防止する。更に、結晶質Ｓｉ相中にＳｉＯ₂相、ＳｉＣ相及びＳｉＢ₄相が析出することにより、相対的にＳｉ相の含有量が低減するとともに、Ｓｉ相に歪みを与えて結晶性を低下させ、過度のＬｉ吸蔵が抑制される。これにより、Ｌｉの吸蔵・放出によるＳｉ微粒子の膨張・収縮が適度に抑制される。ＳｉＯ₂相、ＳｉＣ相及びＳｉＢ₄相はＬｉと反応しないためそれ自身は容量をもたないが、Ｌｉイオンの拡散を促進するとともに、Ｓｉ微粒子の体積膨張による微粉化が抑制される。更に、ＳｉＯ₂相、ＳｉＣ相及びＳｉＢ₄相の全てを含むため、上記の機能をより効果的に得ることができる。
以上のことから本発明の炭素質材料では、充放電容量を高くするとともに、Ｓｉ微粒子の体積膨張及び複合粒子の脱落、および充電による体積膨張に起因するＳｉ微粒子の微粉化を抑制して、サイクル特性の低下を防止することができる。
【００７５】
また、本発明のリチウム二次電池によれば、本発明に係る炭素質材料を負極として備えているので、エネルギー密度及びサイクル特性を向上させることができる。
【図面の簡単な説明】
【図１】 本発明の実施形態である炭素質材料の一例を示す断面模式図である。
【図２】 本発明の実施形態である炭素質材料の別の一例を示す断面模式図である。
【図３】 本発明の実施形態である炭素質材料の更に別の一例を示す断面模式図である。
【図４】 本発明の実施形態である炭素質材料の他の一例を示す断面模式図である。
【図５】 本発明の実施形態である炭素質材料に含まれる複合粒子の一例を示す断面模式図である。
【図６】 加熱後のＳｉ微粒子のＸ線回折パターンを示す図である。
【符号の説明】
１ 炭素質材料
２ 黒鉛粒子
３ 複合粒子
４ 非晶質炭素膜
５ Ｓｉ微粒子
６ 導電性炭素材
７ 硬質炭素膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carbonaceous material for a lithium secondary battery and a lithium secondary battery.
[0002]
[Prior art]
In order to meet the needs of portable electronic devices that are becoming smaller, lighter, and higher in performance, there is an urgent need to increase the capacity of lithium secondary batteries.
Incidentally, graphite, which is one of the negative electrode active materials of lithium secondary batteries, has a theoretical electric capacity of 372 mAh / g. However, in order to obtain a higher capacity negative electrode active material, it is amorphous. It is necessary to proceed with the development of carbon materials or new materials that can replace carbon materials.
Conventionally, silicon and its compounds have been studied as a new material to replace graphite. It is known that silicon and its compounds form an alloy with lithium itself and can provide a larger electric capacity than graphite.
Therefore, recently, as a negative electrode material for a lithium secondary battery, (1) a material obtained by simply mixing a silicon compound powder with graphite, or (2) a fine silicon compound on the graphite surface using a silane coupling agent or the like. A chemically fixed material, and (3) a material obtained by bonding or covering a graphite-based carbonaceous material and a metallic material such as Si with an amorphous carbonaceous material have been proposed.
[0003]
[Problems to be solved by the invention]
However, in the material of (1) above, since the graphite and the silicon compound are not necessarily in close contact with each other, the silicon compound is liberated from the graphite when the graphite expands and contracts due to the progress of the charge / discharge cycle. Since the electron conductivity itself is low, the silicon compound is not sufficiently used as the negative electrode active material, and there is a problem that the cycle characteristics of the lithium secondary battery are deteriorated.
[0004]
In the material (2), the charge / discharge cycle is maintained in a state where the silicon compound is in close contact with the graphite during the initial stage. Therefore, the silicon compound functions as a negative electrode active material like graphite, but the charge / discharge cycle proceeds. And the silicon compound itself expands along with the alloy formation with lithium, thereby breaking the bond by the silane coupling agent and releasing the silicon compound from the graphite, the silicon compound is not sufficiently utilized as the negative electrode active material, There existed a subject that the cycling characteristics of a lithium secondary battery fell. Moreover, the silane coupling process performed at the time of manufacture of negative electrode material may not be performed uniformly, but the subject that the negative electrode material of the stable quality was not obtained easily occurred.
[0005]
Further, the same problem as the material (2) occurs in the material (3). That is, as the charge / discharge cycle progresses, the expansion of the metal material itself due to the formation of an alloy with lithium breaks the bond due to the amorphous carbon material and the metal material is released from the graphite-based carbon material, and the metal material is activated by the negative electrode. There is a problem that the cycle characteristics of the lithium secondary battery are deteriorated because the material is not sufficiently used as a substance.
[0006]
The present invention has been made in view of the above circumstances, and provides a carbonaceous material having high charge / discharge capacity and at the same time excellent cycle characteristics, and also provides a lithium secondary battery having this carbonaceous material. With the goal.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention adopts the following configuration.
The carbonaceous material of the present invention contains at least silicon and carbon around graphite particles having a (002) plane spacing d002 of less than 0.337 nm by X-ray wide angle diffraction, and has a particle size smaller than that of the graphite particles. Composite particles are dispersed and the graphite particles and the composite particles are covered with an amorphous carbon film having an interplanar spacing d002 of 0.37 nm or more. The composite particles are made of Si composed of crystalline silicon. A conductive carbon material is disposed around the fine particles, and the Si fine particles and the conductive carbon material are covered with a hard carbon film. The Si fine particles are formed of SiO in the crystalline Si phase.₂Phase, SiC phase and SiB_FourThe phase is precipitated.
[0008]
In the present invention, the meaning of “around” represents the positional relationship of the composite particles with respect to the graphite particles, and means “on or near the surface” of the graphite particles. Further, the meaning of “around” represents the positional relationship of the conductive carbon material with respect to the Si fine particles, and means “on or near the surface” of the Si fine particles.
Further, the meaning of “dispersed and arranged” means a state in which a plurality of composite particles are located on the surface of the graphite particles in a state of being dispersed or dispersed apart from each other without being aggregated.
The meaning of “coating” means a state in which the particles to be coated are bonded together by completely covering the particles to be coated. In this case, the coating target particles do not necessarily have to be in direct contact.
Specifically, covering the graphite particles and the composite particles with the amorphous carbon film means that the graphite particles and the composite particles are completely covered with the amorphous carbon film so that the graphite particles and the composite particles are bonded, or amorphous. This means that the composite particles were embedded in the carbonaceous film and brought close to the surface of the graphite particles.
Similarly, covering the Si fine particles and the conductive carbon material with the hard carbon film means that the Si fine particles and the conductive carbon material are completely covered with the hard carbon film to bond the Si fine particles and the conductive carbon material, It means that a conductive carbon material was embedded in the film and brought close to the surface of the Si fine particles.
Furthermore, the meaning of “precipitation” is a term for explaining the state of the crystal phase, and means a state in which a precipitation phase having a composition different from that of the parent phase is formed in the parent phase. That is, SiO₂Phase, SiC phase and SiB_FourThis means that the phase is inseparably contained in the Si phase, and the Si phase, SiO₂Phase, SiC phase and SiB_FourIt does not mean that the phases are physically separated from each other.
[0009]
In such a carbonaceous material, since the graphite particles and the Si fine particles occlude Li, the charge / discharge capacity is improved as compared with the case of the graphite particles alone.
Further, the conductivity of the Si fine particles is apparently improved by arranging a conductive carbon material around the Si fine particles having a high specific resistance to the graphite particles.
Further, by coating the Si fine particles with the hard carbon film, volume expansion / contraction of the Si fine particles accompanying the insertion / release of Li can be mechanically suppressed.
Furthermore, by covering the graphite particles and composite particles with an amorphous carbon film, the decomposition of the electrolytic solution is suppressed without the graphite particles touching the electrolytic solution directly, and the composite particles are not dropped from the graphite particles. Furthermore, the micronization of Si fine particles due to volume expansion due to charging is prevented.
[0010]
Furthermore, SiO in the crystalline Si phase₂Phase, SiC phase and SiB_FourPrecipitation of the phase relatively reduces the content of the Si phase, strains the Si phase, lowers the crystallinity, and suppresses excessive Li occlusion. Thereby, the expansion / contraction of the Si fine particles due to the insertion / release of Li is moderately suppressed. SiO₂Phase, SiC phase and SiB_FourSince the phase does not react with Li, the phase itself does not have a capacity, but promotes the diffusion of Li ions and suppresses pulverization due to volume expansion of Si fine particles.
Furthermore, SiO₂Phase, SiC phase and SiB_FourSince all of the phases are included, the above functions can be obtained more effectively.
From the above, in the carbonaceous material of the present invention, while increasing the charge / discharge capacity, suppressing the pulverization of the Si fine particles due to the volume expansion of the Si fine particles and the dropping of the composite particles, and the volume expansion due to charging, It becomes possible to prevent deterioration of cycle characteristics.
In particular, it is possible to prevent dissociation from the graphite particles due to the volume expansion of the Si fine particles, and to more effectively prevent a decrease in cycle efficiency. Further, by increasing the diffusion rate of Li ions, it is possible to quickly occlude / release Li ions even in an electrode filled with an active material at a high density, thereby improving charge / discharge efficiency.
[0011]
The carbonaceous material of the present invention is the carbonaceous material described above, wherein the diffraction intensity of the (111) plane of the Si phase by X-ray wide angle diffraction is expressed as P._SiAnd said SiO₂The diffraction intensity of the (111) plane of the phase is P_SiO2And the diffraction intensity of the (111) plane of the SiC phase is P_SiCAnd the SiB_FourThe diffraction intensity of the (104) plane of the phase is P_SiBP_SiO2/ P_SiIs 0.005 or more and 0.1 or less, and P_SiC/ P_SiIs 0.005 or more and 0.1 or less, and P_SiB/ P_SiO2Is 0.1 or more and 5.0 or less, and P_SiB/ P_SiCIs 0.1 or more and 5.0 or less.
[0012]
In such a carbonaceous material, since the diffraction intensity ratio of each phase is in the above range, the Si phase content does not extremely decrease, and the Li occlusion amount does not decrease. In addition, SiO₂Phase, SiC phase and SiB_FourBy optimizing the phase content, the volume expansion / contraction of the Si fine particles is suppressed.
Therefore, the charge / discharge capacity of the carbonaceous material is increased, and further, the dissociation from the graphite particles due to the volume expansion of the Si fine particles and the pulverization of the Si fine particles due to the volume expansion due to the charge are prevented, thereby preventing a decrease in cycle efficiency. Is possible.
[0013]
The carbonaceous material of the present invention is the carbonaceous material described above, wherein the graphite particles have a particle size of 2 μm or more and 70 μm or less, and the composite particles have a particle size of more than 50 nm and 2 μm or less. The thickness of the amorphous carbon film is in the range of 50 nm to 5 μm.
[0014]
If the particle size of the graphite particles is less than 2 μm, the particle size of the graphite particles is relatively smaller than the particle size of the composite particles, and it is difficult to uniformly attach the composite particles to the surface of the graphite particles. When the particle size exceeds 70 μm, the adhesion to the current collector is lowered and the voids in the electrode are increased, which is not preferable.
Also, the particle size of the composite particles is more than 50 nm and not more than 2 μm, preferably more than 50 nm and not more than 500 nm in order to disperse and arrange the composite particles on the surface of the graphite particles. This is because the minimum particle diameter needs to be 2 μm or less, and if the particle diameter is 500 nm or less, the volume change of the composite particles due to expansion / contraction can be reduced. On the other hand, when the particle size is 50 nm or less, the disorder of the crystal structure of the Si fine particles contained in the composite particles becomes large, and the Li occlusion amount decreases, which is not preferable.
Furthermore, if the film thickness of the amorphous carbon film is less than 50 nm, the graphite particles may not be completely covered with the amorphous carbon film, and it will not be possible to prevent the composite particles from falling off the graphite particles, and also prevent the electrolyte from decomposing. If the film thickness exceeds 5 μm, the irreversible capacity due to amorphous carbon is increased, and lithium ions do not reach the graphite particles, so that the Li occlusion amount is reduced and charged. Since discharge capacity falls, it is not preferable.
[0015]
The carbonaceous material of the present invention is the carbonaceous material described above, wherein the Si fine particles have a particle size in the range of 10 nm to less than 2 μm, and the conductive carbon material has a specific resistance of 10^-FourΩ · m or less and the bending strength of the hard carbon film is 500 kg / cm²In addition to the above, the film thickness is 10 nm or more and 1 μm or less.
[0016]
The reason why the particle size of the Si fine particles is 10 nm or more is to prevent the disorder of the crystal structure of the Si fine particles and improve the Li occlusion amount. The reason why the particle size is less than 2 μm is to reduce the particle size of the composite particles. This is to make it smaller than 2 μm, which is the minimum particle size of the graphite particles.
In addition, the specific resistance of the conductive carbon material is 10^-FourThe reason why it is less than Ω · m is to provide sufficient conductivity to the Si fine particles.
Furthermore, the bending strength of the hard carbon film is 500 kg / cm.²The reason for the above is to reduce the volume change by mechanically suppressing the expansion and contraction of the Si fine particles accompanying the insertion and release of Li, and the thickness of the hard carbon film is set to 10 nm or more and 1 μm or less. This is because when the film thickness is less than 10 nm, the binding force between the conductive carbon material and the Si fine particles is reduced and the effect of suppressing the volume expansion of the composite particles is lost. This is because the ions do not reach the Si fine particles and the charge / discharge capacity decreases, which is not preferable.
[0017]
The carbonaceous material for a lithium secondary battery of the present invention is the carbonaceous material described above, wherein the content of the composite particles is 1 wt% or more and 25 wt% or less.
[0018]
If the content of the composite particles is less than 1% by weight, it is not preferable because a sufficient discharge capacity exceeding the case where only the carbon material is used as an active material cannot be obtained.
On the other hand, if the content exceeds 25% by weight, the contribution of the carbon material portion is reduced, and the voltage increases from the initial stage of discharge to near the reaction potential of Si, which is not preferable. This is not preferable because volume expansion / contraction is likely to occur due to Si fine particles, and cycle characteristics deteriorate.
[0019]
Next, a lithium secondary battery of the present invention is characterized by including any of the carbonaceous materials described above.
The lithium secondary battery includes, for example, a positive electrode, an electrolyte, and a negative electrode having the negative electrode material, and has various shapes such as a cylindrical shape, a square shape, a coin shape, and a sheet shape. In addition, the lithium secondary battery of the present invention is not limited to the form described here, and may be composed of other forms.
According to the lithium secondary battery, a lithium secondary battery having high energy density and excellent cycle characteristics can be configured.
[0020]
Next, in the method for producing a carbonaceous material according to the present invention, Si fine particles made of crystalline silicon are mixed with B₂O_ThreeBy firing at 1300 ° C. or higher and 1400 ° C. or lower in a carbon crucible with powder, SiO in the crystalline Si phase is obtained.₂Phase, SiC phase and SiB_FourA step of precipitating a phase, attaching a conductive carbon material to the Si fine particles, forming a polymer material film covering the Si fine particles to form a composite particle precursor, and further firing the composite particle precursor A step of obtaining composite particles by using the polymer film as a hard carbon film; attaching the Si fine particles to graphite particles; and forming a polymer material film covering the graphite particles to form a carbonaceous material precursor; And a step of obtaining a carbonaceous material by firing the carbonaceous material precursor to convert the polymer film into an amorphous carbon film.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 to 4 show schematic sectional views of a carbonaceous material for a lithium secondary battery of the present invention. This carbonaceous material is formed by dispersing composite particles around graphite particles and covering the graphite particles and composite particles with an amorphous carbon film.
[0022]
Here, “around” represents the positional relationship of the composite particles with respect to the graphite particles, and means “on or near the surface” of the graphite particles. That is, it includes a state in which the composite particles are bonded to the surface of the graphite particles and the composite particles are spaced from the surface of the graphite particles and positioned around the graphite particles.
Further, “dispersed and arranged” means a state in which a plurality of composite particles are dispersed on each other and are bonded to the surface of the graphite particles or slightly spaced from the surface. The composite particles may be in contact with each other to the extent that they do not aggregate.
“Coating” means a state in which the particles to be coated are bonded together by completely covering the particles to be coated. In this case, the coating target particles do not necessarily have to be in direct contact.
Specifically, covering the graphite particles and the composite particles with the amorphous carbon film means that the graphite particles and the composite particles are completely covered with the amorphous carbon film so that the graphite particles and the composite particles are bonded, or amorphous. This means that the composite particles were embedded in the carbonaceous film and brought close to the surface of the graphite particles.
Accordingly, the carbonaceous material of the present invention includes various forms as shown below.
[0023]
For example, the carbonaceous material 1 shown in FIG. 1 is bonded to the surface of the graphite particle 2 in a state where a plurality of composite particles 3 are dispersed, and the amorphous carbon film 4 is larger than the particle size of the composite particles 3. The graphite particles 2 and the composite particles 3 are coated with a small and uniform film thickness.
[0024]
2 is bonded to the surface of a plurality of graphite particles 2 in a state where a plurality of composite particles 3 are dispersed with each other, and the amorphous carbon film 4 has a particle size of the composite particles 3. The graphite particles 2 and the composite particles 3 are formed to have a larger and more uniform film thickness, and a plurality of graphite particles 2 are combined by the amorphous carbon film 4. .
2 shows a state in which two or three graphite particles 2 are combined by an amorphous carbon film 4, the present invention is not limited to this, and four or more graphite particles 2. May be combined.
[0025]
Further, the carbonaceous material 1 shown in FIG. 3 is bonded to the surface of the graphite particle 2 in a state where a plurality of composite particles 3 are dispersed with each other, and the amorphous carbon film 4 connects the graphite particles 2 and the composite particles 3. It is configured by coating. The amorphous carbon film 4 shown in FIG. 3 has a non-uniform film thickness. For example, the portion covering only the graphite particles 2 is set larger than the particle size of the composite particles 3. It is set smaller than the particle size of the composite particles 3.
[0026]
Further, the carbonaceous material 1 shown in FIG. 4 is bonded to the surface of the graphite particle 2 in a state where a plurality of composite particles 3 are dispersed with each other, and the amorphous carbon film 4 connects the graphite particles 2 and the composite particles 3. It is configured by coating. The amorphous carbon film 4 shown in FIG. 4 has a non-uniform film thickness. For example, the portion covering only the graphite particles 2 is set larger than the particle diameter of the composite particles 3. It is set smaller than the particle diameter of the composite particles 3. Moreover, the surface of the amorphous carbon film 4 is formed on a smooth surface without irregularities without reflecting the shape of the composite particles 3.
[0027]
The carbonaceous material of the present invention is not limited to those shown in FIGS. 1 to 4 and may be any material as long as the meanings of the above terms are satisfied.
[0028]
As the graphite particles 1 contained in the carbonaceous material, those having a (002) plane spacing d002 of 0.335 nm or more and less than 0.337 nm by X-ray wide angle diffraction are preferably used, and 0.335 nm or more and 0.34 nm are used. The following are more preferable.
When the interplanar spacing d002 is 0.337 nm or more, the crystallinity of the graphite particles is lowered, the initial irreversible capacity is remarkably increased, and the electron conductivity is lowered, which is not preferable.
The particle size of the graphite particles 2 is preferably in the range of 2 μm to 70 μm.
When the particle size of the graphite particles 2 is less than 2 μm, the particle size of the graphite particles 2 is relatively smaller than the particle size of the composite particles 3... And the composite particles 3. This is not preferable because it becomes difficult, and if the particle size exceeds 70 μm, the adhesion to the current collector is lowered and the gap in the electrode is also increased.
[0029]
Next, as shown in FIGS. 1 to 4, the amorphous carbon film 4 covers the graphite particles 2 and the composite particles 3, and has the composite particles 3 attached on the surface of the graphite particles 2. The amorphous carbon film 4 also has an action of bonding the graphite particles 2... As shown in FIG. The amorphous carbon film 4 is formed by heat-treating at least one of a thermoplastic resin, a thermosetting resin, a vinyl resin, a cellulose resin, a phenol resin, a coal pitch material, a petroleum pitch material, and a tar material. In this way, the graphitization is not relatively advanced, it is amorphous, and it has an interplanar spacing d002 of 0.37 nm or more. Since the amorphous carbon film 4 is amorphous, there is no possibility of decomposition even when the organic electrolyte touches the amorphous carbon film 4, and the charge / discharge efficiency of the carbonaceous material 1 can be increased.
If the interplanar spacing d002 of the amorphous carbon film 4 is less than 0.37 nm, the crystallinity of the amorphous carbon film 4 is improved, the graphite structure is approached, and the organic electrolyte may be decomposed. Absent.
[0030]
Further, since the composite particles 3 are arranged on the surface of the graphite particles 2 by the amorphous carbon film 4, the relatively high specific resistance composite particles 3 are prevented from being released from the graphite particles 2, Generation | occurrence | production of the composite particle 3 ... which does not contribute to charging / discharging reaction can be prevented.
The amorphous carbon film 4 is obtained by putting the polymer material in a solvent in which the polymer material is dissolved, precipitating the polymer material on the surface of the graphite particles 2, and further firing it. The entire particle 2 can be completely covered, and the density is relatively low and lithium ions can easily pass therethrough, so that the reaction between the graphite particles 2 and the composite particles 3 and lithium ions is not hindered.
The film thickness of the amorphous carbon film 4 is preferably in the range of 50 nm to 5 μm. If the film thickness is less than 50 nm, the graphite particles 2 are not completely covered, and the composite particles 3... May fall off the graphite particles 2, which is not preferable. If the film thickness exceeds 5 μm, it is caused by amorphous carbon. This is not preferable because the irreversible capacity increases.
[0031]
Next, as shown in FIG. 5, the composite particles 3... Have conductive carbon materials 6 arranged around the Si fine particles 5, and the Si fine particles 5 and the conductive carbon materials 6. It is coated.
Further, the Si fine particles 3 are made of SiO in the crystalline Si phase.₂Phase, SiC phase and SiB_FourThe phase is precipitated.
Here, “around” represents the positional relationship of the conductive carbon material 6 to the Si fine particles 5 and means “on or near the surface” of the Si fine particles 5. That is, it includes a state in which the conductive carbon materials 6 are bonded to the surface of the Si fine particles 5 and the conductive carbon materials 6 are spaced from the surface of the Si fine particles 5 and positioned around the Si fine particles 5.
The Si fine particles 5 and the conductive carbon material 6 are covered with the hard carbon film 7. The Si fine particles 5 and the conductive carbon material 6 are completely covered with the hard carbon film 7 so that the Si fine particles 5 and the conductive carbon material 7 are covered. This includes bonding the raw materials 6... And embedding the conductive carbon materials 6 in the hard carbon film 7 so as to be close to the surface of the Si fine particles 5.
[0032]
Furthermore, “precipitation” is a term that describes the state of the crystal phase, and means a state in which a precipitation phase having a composition different from that of the parent phase is formed in the parent phase. That is, SiO in the Si phase₂Phase, SiC phase and SiB_FourThis means that the phase is inseparably contained, Si phase, SiO₂Phase, SiC phase and SiB_FourIt does not mean that the phases are physically separated from each other.
[0033]
The particle diameter of the composite particle 3 is preferably in the range of more than 50 nm and 2 μm or less, and more preferably in the range of more than 50 nm and 500 nm or less.
The particle size of the composite particles 3 is set to 2 μm or less in order to disperse the composite particles 3 on the surface of the graphite particles 2. The particle size of the composite particles 3 is set to 2 μm or less which is the minimum particle size of the graphite particles 2. This is because if the particle size is 500 nm or less, the volume change due to the expansion and contraction of the Si fine particles 5 accompanying the occlusion and release of lithium can be reduced. Further, the reason why the lower limit of the particle diameter exceeds 50 nm is that when it is 50 nm or less, the crystal structure of the Si fine particles 5 contained in the composite particles 3 is greatly disturbed, the Li occlusion amount is reduced, and the charge / discharge capacity is reduced. This is because there is a possibility that it may decrease.
[0034]
The Si fine particles 5 contain crystalline silicon (Si phase) as a main component, and SiO 2₂Phase, SiC phase and SiB_FourThe phase is precipitated, and the particle size is in the range of 10 nm or more and less than 2 μm.
Silicon is an element that forms an alloy with lithium, and an alloy is formed when lithium ions act on the Si phase composed of silicon. In particular, lithium ions penetrate into the surface of the Si fine particles 5 or voids inside the Si fine particles 5 to form an alloy, whereby the Si fine particles 5 themselves expand.
[0035]
The Si fine particles 5 have SiO₂Phase, SiC phase and SiB_FourPhases are included, and these phases do not react with lithium and thus do not have a capacity per se, but have an effect of promoting the diffusion of lithium ions.
Therefore, SiO in the Si phase₂Phase, SiC phase and SiB_FourWhen the phase is included, the diffusion rate of lithium ions in the Si phase is improved. For example, even in an electrode filled with this carbonaceous material at a high density, it is possible to quickly occlude and release Li ions, and charge and discharge efficiency. Can be improved.
[0036]
In addition, the Si fine particles 5 have SiO 2₂Phase, SiC phase and SiB_FourWhen the phase is contained, the content of the Si phase is relatively lowered, and the Si phase is distorted to lower the crystallinity. As a result, the amount of occlusion of lithium ions slightly decreases, but at the same time, the expansion and contraction of the Si fine particles accompanying the occlusion / release of lithium are moderately suppressed. Thereby, pulverization due to the volume expansion of the Si fine particles is suppressed, and the composite particles are prevented from falling off due to the volume expansion of the Si fine particles, thereby preventing deterioration of cycle characteristics.
[0037]
Specifically, the diffraction intensity of the (111) plane of the Si phase by X-ray wide angle diffraction is expressed as P_SiAnd SiO₂The diffraction intensity of the (111) plane of the phase is P_SiO2And the diffraction intensity of the (111) plane of the SiC phase is P_SiCAnd SiB_FourThe diffraction intensity of the (104) plane of the phase is P_SiBP_SiO2/ P_SiIs 0.005 or more and 0.1 or less, and P_SiC/ P_SiIs 0.005 or more and 0.1 or less, and P_SiB/ P_SiO2Is 0.1 or more and 5.0 or less, and P_SiB/ P_SiCIs preferably 0.1 or more and 5.0 or less.
P_SiO2/ P_SiIs less than 0.005, SiO₂This is not preferable because the phase content is reduced, the expansion and contraction of the Si fine particles 5 cannot be suppressed, and the diffusion rate of lithium ions is reduced. P_SiO2/ P_SiIf it exceeds 0.1, the content of the Si phase in the Si fine particles 5 is lowered, and the charge / discharge capacity is lowered.
P_SiC/ P_SiIs less than 0.005, it is not preferable because the content of the SiC phase is reduced, the expansion and contraction of the Si fine particles 5 cannot be suppressed, and the diffusion rate of lithium ions is reduced._SiC/ P_SiIf it exceeds 0.1, the content of the Si phase in the Si fine particles 5 is lowered, and the charge / discharge capacity is lowered.
[0038]
Furthermore, P_SiB/ P_SiO2If it is less than 0.1, the effect of suppressing the expansion and contraction of the Si fine particles 5 is almost lost. P_SiB/ P_SiO2When 5.0 exceeds 5.0, SiO₂This is not preferable because the phase hinders the effect of promoting the diffusion of lithium ions, and the distortion of the crystal structure of the Si phase becomes too large, resulting in a decrease in discharge capacity.
Furthermore, P_SiB/ P_SiCIf it is less than 0.1, the effect of suppressing the expansion and contraction of the Si fine particles 5 is almost lost. P_SiB/ P_SiCIf it exceeds 5.0, the SiC phase hinders the effect of promoting the diffusion of lithium ions, and the distortion of the crystal structure of the Si phase becomes too large, resulting in a decrease in discharge capacity.
In addition, SiO₂Phase, SiC phase is particularly effective in promoting the diffusion of lithium ions._FourThe phase has a particularly strong effect of suppressing the expansion and contraction of the Si fine particles 5, but each of these phases alone cannot sufficiently exhibit the above-described effects, and all phases coexist, resulting in high efficiency and high capacity. An electrode material exhibiting a maintenance rate can be obtained. Therefore, in the present invention, SiO₂Phase, SiC phase and SiB_FourIt is preferred to always include all of the phases.
[0039]
The reason why the particle size of the Si fine particles 5 is 10 nm or more is to prevent the disorder of the crystal structure of the Si fine particles 5 and improve the Li storage amount. The reason why the particle size is less than 2 μm This is because the particle size of 3 needs to be smaller than 2 μm, which is the minimum particle size of the graphite particles 2.
[0040]
Next, the conductive carbon materials 6 are arranged on or near the surface of the Si fine particles 5, and in FIG. 5, the particulate conductive carbon materials 6 are arranged around the Si fine particles 5. However, the shape of the conductive carbon material 6 is not limited to a particle shape, and may be various forms such as a film shape, a layer shape, and a fiber shape.
The conductive carbon material 6 is located on the surface of the Si fine particles 5 that are semiconductors and imparts apparent conductivity to the Si fine particles 5. The specific resistance of the conductive carbon material 6 is 10^-FourA range of Ω · m or less is preferred. Specific resistance is 10^-FourIf it exceeds Ω · m, the apparent conductivity of the Si fine particles 5 decreases, and the charge / discharge reaction of lithium ions to the Si fine particles 5 does not proceed smoothly, and the charge / discharge capacity of the carbonaceous material can be improved. Since it disappears, it is not preferable.
Examples of the conductive carbon material 6... Include carbon black, ketjen black, vapor grown carbon fiber (VGCF), and the like.
[0041]
The hard carbon film 7 covers the Si fine particles 5 and the conductive carbon materials 6... And the conductive carbon materials 6 are arranged on the surface of the Si fine particles 5. This hard carbon film 7 is obtained by firing polyvinyl alcohol, phenol resin, or the like, and has a bending strength of 500 kg / cm.²In addition to the above, the film thickness is 10 nm or more and 1 μm or less.
[0042]
The hard carbon film 7 is for preventing the release of the composite particles 3 from the graphite particles 2 caused by the expansion and contraction of the Si fine particles 5 accompanying the charge / discharge reaction of lithium ions. Mechanically suppress shrinkage. Therefore, the bending strength of the hard carbon film 7 is 500 kg / cm.²It is preferable to make it above. Bending strength is 500kg / cm²If it is less than the range, the expansion / contraction of the Si fine particles 5 cannot be mechanically suppressed, and the composite particles 3 may be released from the graphite particles 2, which is not preferable.
Moreover, when the film thickness of the hard carbon film 7 is less than 10 nm, the binding force between the conductive carbon material 6... And the Si fine particles 5 is lowered, and the effect of suppressing the volume expansion of the composite particles 3 is lowered. . Further, if the film thickness exceeds 1 μm, the negative reversible capacity is increased due to amorphous carbon, which is not preferable.
[0043]
And it is preferable that content of said composite particle 3 in the carbonaceous material of this invention is 1 to 25 weight%. If the content of the composite particles 3 is less than 1% by weight, it is not preferable because a sufficient discharge capacity that exceeds the case of using only a carbon material as an active material cannot be obtained. On the other hand, if the content exceeds 25% by weight, the contribution of the carbon material portion is reduced, reaching the reaction potential of Si from the beginning of discharge, and the average voltage of the battery is lowered. Narrowing and re-aggregation are likely to cause volume expansion / contraction due to the Si fine particles 5, and cycle characteristics are deteriorated.
[0044]
When the carbonaceous material 1 reacts with lithium ions, the lithium ions are mainly stored in the graphite particles 2 and combine with the Si fine particles 5 to form an alloy. The conductive carbon material 6 is adhered to the surface of the Si fine particles 5 and the conductivity is apparently high, and lithium ions are easily alloyed with the Si fine particles 5.
At this time, the volume of the graphite particles 2 and the Si fine particles 5 expands. However, since the Si fine particles 5 are covered with the hard carbon film 76, the volume expansion is mechanically suppressed, and the composite including the Si fine particles 5. The particles 3 are not dissociated from the graphite particles 2.
Further, the Si fine particles 5 ... have a Si phase and SiO.₂Phase, SiC phase and SiB_FourSince the phase is contained, the occlusion amount of lithium ions is suppressed, and the volume expansion of the Si fine particles 5 is moderately suppressed. Also, the composite particles 3 including the Si fine particles 5 are dissociated from the graphite particles 2. There is nothing.
Therefore, the Si fine particles 5 can always contribute to the charge / discharge reaction, and the charge / discharge capacity of the carbonaceous material 1 does not decrease even if the charge / discharge cycle proceeds.
[0045]
Further, by covering the graphite particles 2 and the composite particles 3 with the amorphous carbon film 4, the graphite particles 2 do not directly touch the organic electrolyte solution, and the decomposition of the organic electrolyte solution is suppressed. Further, the composite particles 3 are not detached from the graphite particles 2, and further, the fine particles of the Si fine particles 5 due to volume expansion due to charging are prevented.
[0046]
Therefore, according to said carbonaceous material 1, while charging / discharging capacity | capacitance is made high, pulverization of Si microparticles 5 ... resulting from the volume expansion of Si microparticles 5 ..., dropping-off of the composite particles 3 ..., and volume expansion accompanying charging. Can be suppressed and deterioration of cycle characteristics can be prevented.
[0047]
Said carbonaceous material can be manufactured as follows, for example.
The production of the carbonaceous material includes a step of producing composite particles and a step of mixing graphite particles with the obtained composite particles and coating them with an amorphous carbon film.
First, in the process of producing composite particles, Si fine particles consisting only of the Si phase and a boron compound such as boron or boron oxide as a boron source are prepared, and the Si fine particle boron or boron compound is put into a carbon crucible and is not used. Heat in an active atmosphere at about 1300-1400 ° C. for 120-300 minutes. By this heating, carbon constituting the crucible and the Si phase react to precipitate a SiC phase in the Si fine particles, and boron contained in the boron source reacts to the Si phase to cause SiB in the Si fine particles._FourThe phase precipitates, and the slightly mixed oxygen reacts with the Si phase to form SiO in the Si fine particles.₂A phase precipitates.
However, when the heating temperature is less than 1300 ° C. and / or the heating time is less than 120 minutes, the SiC phase, SiO₂Phase and SiB_FourThe phase does not sufficiently precipitate, which is not preferable. When the heating temperature exceeds 1400 ° C., Si melts, which is not preferable. When the heating time exceeds 300 minutes, the SiC phase, SiO 2₂Phase and SiB_FourThis is not preferable because the amount of phase precipitation becomes excessive.
[0048]
Further, the mixing ratio of the Si fine particles and the boron source such as boron or boron compound is preferably 10: 1.
If the amount of boron is small relative to Si fine particles, SiB_FourThis is not preferable because the amount of precipitation of the phase decreases, and an excessive amount of boron is not preferable because the crystal structure of the Si phase is excessively distorted and the discharge capacity is reduced.
[0049]
Next, the Si fine particles after heating and the conductive carbon material are mixed by dry mixing or wet mixing. In the case of wet mixing, it is preferable to use a dispersion medium such as isopropyl alcohol, acetone, or water.
[0050]
Next, the polymer material is dissolved in an appropriate solvent, and a mixture of the Si fine particles and the conductive carbon material is mixed into this solution, and then the solvent is removed. By removing the solvent, a composite particle precursor in which a polymer film is coated on Si fine particles and a conductive carbon material is formed.
In addition, it is preferable to use at least 1 sort (s) among a thermoplastic resin, a thermosetting resin, a vinyl-type resin, a cellulose resin, and a phenol-type resin as said polymeric material, and it is preferable to use especially a phenol resin. Coal pitch materials, petroleum pitch materials, tar materials, etc. may be used.
[0051]
Next, the composite particle precursor is heat-treated to carbonize the polymer film to form a hard carbon film.
The heat treatment is preferably performed in a vacuum atmosphere or an inert gas atmosphere, the heat treatment temperature is preferably in the range of 800 ° C. to 1200 ° C., and the heat treatment time is preferably 120 minutes or longer.
When the heat treatment is performed in a vacuum atmosphere or an inert gas atmosphere, the polymer film is prevented from being oxidized and a good hard carbon film can be formed.
When the heat treatment temperature is less than 800 ° C., carbonization is not completely performed, the specific resistance of the hard carbon film is high, and it is difficult to insert and remove lithium ions, and when the heat treatment temperature exceeds 1200 ° C., This is not preferable because the Si fine particles are carbonized to generate excessive SiC, and the graphitization of the carbon film progresses to reduce the strength of the film. Similarly, a heat treatment time of less than 120 minutes is not preferable because a uniform hard carbon film cannot be formed.
In this way, composite particles are obtained.
[0052]
In the next step, graphite particles are mixed into the obtained composite particles by dry mixing or wet mixing. In the case of wet mixing, it is preferable to use a dispersion medium such as ethanol.
[0053]
Next, another polymer material is dissolved in an appropriate solvent, and a mixture of composite particles and graphite particles is mixed with this solution, and then the solvent is removed. By removing the solvent, a carbonaceous material precursor in which the polymer particles are coated with the composite particles and the graphite particles is formed.
In addition, it is preferable to use at least one polymer material such as a thermoplastic resin, a thermosetting resin, a vinyl resin, a cellulose resin, and a phenol resin as the polymer material, and in particular, a phenol resin is used. Is preferred. Coal pitch materials, petroleum pitch materials, tar materials and the like may be used.
[0054]
Next, the carbonaceous material precursor is heat-treated to carbonize the polymer film to form an amorphous carbon film.
The heat treatment is preferably performed in a vacuum atmosphere or an inert gas atmosphere, the heat treatment temperature is preferably in the range of 800 ° C. to 1200 ° C., and the heat treatment time is preferably 120 minutes or longer.
When the heat treatment is performed in a vacuum atmosphere or an inert gas atmosphere, the polymer film is prevented from being oxidized and a good amorphous carbon film can be formed.
Note that if the heat treatment temperature is less than 800 ° C., the carbonization is not performed completely because of the low temperature, the specific resistance of the amorphous carbon film is high, and it is difficult to insert and remove lithium ions. If the temperature exceeds 1200 ° C., the Si fine particles are carbonized to generate excessive SiC, and the graphitization of the polymer film proceeds to reduce the strength of the amorphous carbon film.
Similarly, a heat treatment time of less than 120 minutes is not preferable because a uniform hard carbon film cannot be formed.
In this way, the carbonaceous material according to the present invention is obtained.
[0055]
A lithium secondary battery can be composed of a negative electrode having the above carbonaceous material, a positive electrode capable of inserting and extracting lithium, and an organic electrolyte.
As the positive electrode, for example, LiMn₂O_FourLiCoO₂, LiNiO₂LiFeO₂, V₂O_FiveExamples thereof include a positive electrode material capable of occluding and releasing lithium, such as TiS and MoS, and a positive electrode material such as an organic disulfide compound or an organic polysulfide compound.
As a specific example of the positive electrode or the negative electrode, the above-mentioned positive electrode material or carbonaceous material is mixed with a binder and, if necessary, a conductive additive, and applied to a current collector made of a metal foil or a metal net. What was shape | molded in the sheet form can be illustrated.
[0056]
As the organic electrolyte, for example, an organic electrolytic solution in which a lithium salt is dissolved in an aprotic solvent can be exemplified.
As aprotic solvents, propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethyl Sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate , Diethylene glycol, dimethyl An aprotic solvent such as ether, or a mixed solvent in which two or more of these solvents are mixed can be exemplified. In particular, any one of propylene carbonate, ethylene carbonate, and butylene carbonate must be included, and dimethyl carbonate, methyl ethyl It is preferable that any one of carbonate and diethyl carbonate is necessarily contained.
[0057]
Moreover, as a lithium salt, LiPF₆, LiBF_Four, LiSbF₆, LiAsF₆LiClO_Four, LiCF_ThreeSO_Three, Li (CF_ThreeSO₂)₂N, LiC_FourF₉SO_Three, LiSbF₆, LiAlO_Four, LiAlCl_Four, LiN (C_xF_{2x + 1}SO₂) (C_yF_{2y Ten 1}SO₂) (Where x and y are natural numbers), LiCl, LiI, etc. can be exemplified by mixing one or two or more lithium salts, particularly LiPF.₆, LiBF_FourThose containing any one of these are preferred.
In addition to this, a conventionally known organic electrolyte for a lithium secondary battery may be used.
[0058]
As another example of the organic electrolyte, a polymer such as PEO or PVA mixed with any of the lithium salts described above, or a polymer having a high swellability impregnated with an organic electrolyte, a so-called polymer electrolyte May be used.
Furthermore, the lithium secondary battery of the present invention is not limited to the positive electrode, the negative electrode, and the electrolyte, and may include other members as necessary. For example, the lithium secondary battery may include a separator that separates the positive electrode and the negative electrode. .
[0059]
According to the above lithium secondary battery, since the carbonaceous material 1 is provided, a lithium secondary battery having high energy density and excellent cycle characteristics can be configured.
[0060]
【Example】
[Manufacture of Carbonaceous Material of Examples]
10 g of Si powder having an average particle diameter of 2 μm and 1.4 to 2.8 g of boron oxide or 1 g of boron were put in a carbon crucible having an internal volume of 200 ml and heated at 1400 ° C. for 240 minutes in an argon gas atmosphere. The Si fine particles are hardened after aggregation by this heat treatment, and become larger particles than before the heating. This was pulverized with a ball mill or the like to a particle size of 300 nm.
Next, 2 parts by weight of carbon black was mixed with 1 part by weight of the pulverized Si fine particles. Carbon black has a specific resistance of 10^-FourΩ · m.
Next, a solution in which 10 parts by weight of a phenol resin was dissolved in isopropyl alcohol was prepared, and the mixture of the Si fine particles and carbon black was mixed with this solution and stirred sufficiently, and then the solvent was removed. In this way, a composite particle precursor formed by adhering carbon black and a phenol resin film to the surface of the Si fine particles was formed.
Next, this composite particle precursor was heat-treated at 1000 ° C. for 180 minutes in an argon atmosphere to carbonize the polyvinyl alcohol resin film to form a hard carbon film having a thickness of 0.05 μm.
When the polyvinyl alcohol resin is carbonized alone under the same conditions as described above, the bending strength of the resulting carbide is 800 kg / cm.²Therefore, it is estimated that the bending strength of the hard carbon film is the same.
In this way, composite particles were obtained.
[0061]
Next, 5 parts by weight of the composite particles were added to 95 parts by weight of natural graphite having an average particle diameter of 15 μm, and isopropyl alcohol was further added to perform wet mixing. In addition, the interplanar spacing d002 of the (002) plane by X-ray wide angle diffraction of natural graphite was 0.3355 nm. Next, an isopropyl alcohol solution containing 10 parts by weight of a phenol resin was added to and mixed with the mixture of the natural graphite and the composite particles, and then isopropyl alcohol was evaporated. In this way, a carbonaceous material precursor formed by adhering composite particles and a polyvinyl alcohol resin film to the surface of natural graphite was formed.
[0062]
Next, this carbonaceous material precursor was baked at 1000 ° C. (1273 K) in a vacuum atmosphere to carbonize the phenol resin to obtain an amorphous carbon film having a thickness of 0.05 μm.
When the phenol resin is carbonized alone under the same conditions as described above, the surface spacing d002 of the (002) plane of the resulting carbide is about 0.39 nm. Is estimated to be similar.
Thus, the carbonaceous materials of Examples 1 to 4 were obtained.
[0063]
[Production of Comparative Carbonaceous Material]
The above example except that 10 g of Si powder having an average particle size of 2.0 μm and 2.8 g of boron oxide or 1 g of boron were placed in a zirconium crucible with an internal volume of 100 ml and heated at 1400 ° C. for 180 minutes in an argon gas atmosphere. The carbonaceous materials of Comparative Examples 1 and 2 were obtained in the same manner as in 1-4.
The Si fine particles after heating prepared in Example 2 are pulverized by a ball mill or the like until the average particle size becomes 2 μm again, and then the carbonaceous material of Comparative Example 3 is obtained in the same manner as in Examples 1 to 4. It was.
Furthermore, a carbonaceous material composed only of natural graphite was designated as Comparative Example 4.
[0064]
[Create test cell for charge / discharge test]
Polyvinylidene fluoride was mixed with the carbonaceous materials of Examples 1 to 4 and Comparative Examples 1 to 4, and N-methylpyrrolidone was further added to form a slurry liquid. This slurry solution was applied to a copper foil having a thickness of 14 μm by a doctor blade method and dried in a vacuum atmosphere at 120 ° C. for 24 hours to volatilize N-methylpyrrolidone. In this way, a negative electrode mixture having a thickness of 100 μm was laminated on the copper foil. The content of polyvinylidene fluoride in the negative electrode mixture is 8% by weight, and the density of the negative electrode mixture is 1.5 g / cm.^ThreeThat was all.
And the copper foil which laminated | stacked the negative electrode compound material was punched out to the circle | round | yen of diameter 13mm, and it was set as the negative electrode of Examples 1-4 and Comparative Examples 1-4.
[0065]
The negative electrodes of Examples 1 to 4 and Comparative Examples 1 to 4 were used as working electrodes, the metal lithium foil punched out in a circle was used as a counter electrode, a separator made of a porous polypropylene film was inserted between the working electrode and the counter electrode, and electrolysis was performed. Using a solution in which LiPF6 is dissolved in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) as a solute so as to have a concentration of 1 (mol / L), a coin type A test cell was created.
The charge / discharge current density is 0.2 C, and the charge end voltage is 0 V (L i / L i⁺), And the discharge end voltage is 1.5 V (Li / Li⁺) As a charge / discharge test.
Table 1 shows the discharge capacity and charge / discharge efficiency in the first cycle of Examples 1 to 4 and Comparative Examples 1 and 2.
Table 2 shows the capacity ratio (20th / 1st) of the discharge capacity at the 20th cycle and the discharge capacity at the 1st cycle in Example 2 and Comparative Examples 3 and 4. However, the capacity ratio was measured by 1C discharge.
[0066]
[Table 1]

[0067]
"Table 2"
Sample Volumetric ratio of Si fine particles after heat grinding (20th / 1st)
Example 2 0.3 μm 88.0%
Comparative Example 3 2.0 μm 81.2%
Comparative Example 4-84.4%
[0068]
As shown in Table 1, it can be seen that the discharge capacity in the first cycle of Comparative Examples 1 and 2 is substantially the same as or higher than those of Examples 1 to 4.
In the case of Examples 1 to 4, when the Si fine particles were heated in the carbon crucible, the SiC phase precipitated in the Si phase of the Si fine particles, and the content of the Si phase forming an alloy with lithium was relatively This is thought to be due to the decrease.
On the other hand, since Comparative Examples 1 and 2 used a crucible made of zirconium, the SiC phase did not precipitate in the Si phase of the Si fine particles. It is thought that it became high.
[0069]
Furthermore, when Examples 1-3 and Example 4 are compared by discharge capacity, Example 4 has shown high discharge capacity.
In Examples 1-3, the Si fine particles and B₂O_ThreeBy mixing and heating, B₂O_ThreeOxygen atoms oxidize Si and SiB_FourIn addition to the phase SiO₂This is probably because the Si phase content was relatively reduced because a relatively large number of phases were precipitated.
On the other hand, in Example 4, since Si fine particles and B were mixed and heated, there was less oxygen than in Examples 1 to 3, and a slight amount of SiO due to minute residual oxygen or the like in the atmosphere.₂Although the phase is precipitated, the amount thereof is smaller than those in Examples 1 to 3, which is considered to be because the Si phase content is relatively higher than those in Examples 1 to 3.
[0070]
Furthermore, for Examples 1 to 3, B₂O_ThreeThe discharge capacity decreases as the amount of addition increases.
This is B₂O_ThreeAs the added amount of SiB increases, SiB_FourIt is considered that a large amount of phases were precipitated and the content of the Si phase was relatively reduced.
[0071]
Next, regarding charge / discharge efficiency, it can be seen from Table 1 that the charge / discharge efficiencies of Examples 1 to 4 are higher than those of Comparative Examples 1 and 2.
This is because the SiC phase, SiO₂Phase and SiB_FourSince the phase is precipitated and the content of the Si phase forming an alloy with lithium is relatively reduced, the expansion and contraction of the Si fine particles themselves are moderately suppressed, thereby reducing the release of the composite particles from the graphite. This is probably because the charge / discharge efficiency was improved.
SiC phase, SiO₂Phase and SiB_FourPrecipitation of the phase reduces the crystallinity of the Si phase, and this improves the diffusion rate of lithium in the Si phase.
[0072]
FIG. 6 shows an X-ray diffraction pattern of the Si fine particles of the carbonaceous material of Example 2. As is apparent from FIG. 6, in addition to the Si phase, the SiC phase, SiO₂Phase and SiB_FourA diffraction peak originating from the phase is observed.
The diffraction intensity of the (111) plane of the Si phase is P_SiAnd SiO₂The diffraction intensity of the (111) plane of the phase is P_SiO2And the diffraction intensity of the (111) plane of the SiC phase is P_SiCAnd SiB_FourThe diffraction intensity of the (104) plane of the phase is P_SiB, P from FIG._SiO2/ P_Si= 0.034 and P_SiC/ P_Si= 0.044 and P_SiB/ P_SiO2= 1.50 and P_SiB/ P_SiC= 1.16.
[0073]
Next, as is apparent from Table 2, it can be seen that the capacity ratio of Example 2 is significantly improved as compared with Comparative Example 4.
This is because, similarly to the reason that the charge / discharge efficiency in the first cycle is improved, the SiC phase, SiO 2₂Phase and SiB_FourSince the phase is precipitated and the content of the Si phase forming an alloy with lithium is relatively reduced, the expansion and contraction of the Si fine particles themselves are moderately suppressed, thereby reducing the release of the composite particles from the graphite. This is thought to be due to the improved cycle characteristics.
SiC phase, SiO₂Phase and SiB_FourPrecipitation of the phase reduces the crystallinity of the Si phase, which improves the diffusion rate of lithium in the Si phase.
Moreover, it can be seen from the results of Comparative Example 3 that when the Si fine particles after heating and baking are large, the curing that suppresses the expansion and contraction of the Si fine particles is thinned.
[0074]
【The invention's effect】
As described above in detail, according to the carbonaceous material of the present invention, since the graphite particles and the Si fine particles occlude Li, the charge / discharge capacity is improved as compared with the case of the graphite particles alone. Further, the conductivity of the Si fine particles is apparently improved by arranging a conductive carbon material around the Si fine particles having a high specific resistance to the graphite particles. Further, by coating the Si fine particles with the hard carbon film, volume expansion / contraction of the Si fine particles accompanying the insertion / release of Li can be mechanically suppressed. Furthermore, by covering the graphite particles and composite particles with an amorphous carbon film, the decomposition of the electrolytic solution is suppressed without the graphite particles touching the electrolytic solution directly, and the composite particles are not dropped from the graphite particles. Furthermore, the micronization of Si fine particles due to volume expansion due to charging is prevented. Furthermore, SiO in the crystalline Si phase₂Phase, SiC phase and SiB_FourPrecipitation of the phase relatively reduces the content of the Si phase, strains the Si phase, lowers the crystallinity, and suppresses excessive Li occlusion. Thereby, the expansion / contraction of the Si fine particles due to the insertion / release of Li is moderately suppressed. SiO₂Phase, SiC phase and SiB_FourSince the phase does not react with Li, the phase itself does not have a capacity, but promotes the diffusion of Li ions and suppresses pulverization due to volume expansion of Si fine particles. Furthermore, SiO₂Phase, SiC phase and SiB_FourSince all of the phases are included, the above functions can be obtained more effectively.
From the above, in the carbonaceous material of the present invention, the charge / discharge capacity is increased, the fine expansion of the Si fine particles due to the volume expansion of the Si fine particles and the drop of the composite particles, and the volume expansion due to charging is suppressed, and the cycle It is possible to prevent deterioration of characteristics.
[0075]
Moreover, according to the lithium secondary battery of this invention, since the carbonaceous material which concerns on this invention is provided as a negative electrode, an energy density and cycling characteristics can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing an example of a carbonaceous material that is an embodiment of the present invention.
FIG. 2 is a schematic cross-sectional view showing another example of a carbonaceous material that is an embodiment of the present invention.
FIG. 3 is a schematic cross-sectional view showing still another example of a carbonaceous material that is an embodiment of the present invention.
FIG. 4 is a schematic cross-sectional view showing another example of a carbonaceous material that is an embodiment of the present invention.
FIG. 5 is a schematic cross-sectional view showing an example of composite particles contained in a carbonaceous material according to an embodiment of the present invention.
FIG. 6 is a diagram showing an X-ray diffraction pattern of Si fine particles after heating.
[Explanation of symbols]
1 Carbonaceous material
2 Graphite particles
3 composite particles
4 Amorphous carbon film
5 Si fine particles
6 Conductive carbon material
7 Hard carbon film

Claims (7)

Composite particles containing at least silicon and carbon and having a particle size smaller than that of the graphite particles are dispersed around the graphite particles having a (002) plane spacing d002 of less than 0.337 nm by X-ray wide angle diffraction. And the graphite particles and the composite particles are covered with an amorphous carbon film having an interplanar spacing d002 of 0.37 nm or more,
In the composite particles, a conductive carbon material is disposed around Si fine particles made of crystalline silicon, and the Si fine particles and the conductive carbon material are covered with a hard carbon film,
The Si fine particles are obtained by depositing a SiO ₂ phase, a SiC phase, and a SiB ₄ phase in a crystalline Si phase,
A carbonaceous material for a negative electrode active material of a lithium ion secondary battery, wherein the Si fine particles have a particle size in a range of 10 nm or more and less than 2 μm. The diffraction intensity of the (111) plane of the Si phase by X-ray wide angle diffraction is P _Si , the diffraction intensity of the (111) plane of the SiO ₂ phase is P _SiO2, and the diffraction intensity of the (111) plane of the SiC phase is When it is P _SiC and the diffraction intensity of the (104) plane of the SiB ₄ phase is P _SiB ,
P _SiO2 / _{P Si} is from 0.005 to _{0.1, P} SiC _/ P _Si is from 0.005 to _{0.1, P SiB /} P _SiO2 is located at 0.1 to 5.0 The carbonaceous material for a negative electrode active material of a lithium ion secondary battery according to claim 1, wherein P _SiB / P _SiC is 0.1 or more and 5.0 or less.   The particle size of the graphite particles is in the range of 2 μm or more and 70 μm or less, the particle size of the composite particles is in the range of more than 50 nm and 2 μm or less, and the film thickness of the amorphous carbon film is in the range of 50 nm or more and 5 μm or less. The carbonaceous material for a negative electrode active material of a lithium ion secondary battery according to claim 1 or 2, wherein the carbonaceous material is a lithium ion secondary battery. The specific resistance of the conductive carbon material is 10 ⁻⁴ Ω · m or less, the bending strength of the hard carbon film is 500 kg / cm ² or more, and the film thickness is 10 nm or more and 1 μm or less. The carbonaceous material for the negative electrode active material of the lithium ion secondary battery according to any one of claims 1 to 3.   5. The carbonaceous material for a negative electrode active material of a lithium ion secondary battery according to claim 1, wherein the content of the composite particles is 1 wt% or more and 25 wt% or less.   A lithium secondary battery comprising the carbonaceous material for a negative electrode active material of the lithium ion secondary battery according to any one of claims 1 to 5. A process of precipitating SiO ₂ phase, SiC phase and SiB ₄ phase in crystalline Si phase by firing Si fine particles made of crystalline silicon together with B ₂ O ₃ powder in a carbon crucible at 1300 ° C. or higher and 1400 ° C. or lower. When,
A conductive carbon material is attached to the Si fine particles, and a polymer material film is formed to cover the Si fine particles to form a composite particle precursor, and the composite particle precursor is further baked to thereby convert the polymer film into hard carbon. Obtaining composite particles as a membrane;
With attaching the composite particles to the black lead particles, the polymer film by the carbonaceous material precursor to form a polymeric material coating covering the graphite particles, further calcining the carbonaceous material precursor 0 A method for producing a carbonaceous material for a negative electrode active material of a lithium ion secondary battery, comprising: obtaining a carbonaceous material as an amorphous carbon film having an interplanar spacing d002 of 37 nm or more .

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