WO2014097819A1 - リチウムイオン二次電池用負極材及びその評価方法 - Google Patents
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- H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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Definitions
- the present invention provides a negative electrode material for a lithium ion secondary battery that can have a high charge / discharge capacity and excellent cycle characteristics when used as a negative electrode material for a lithium ion secondary battery, a negative electrode material evaluation method, and The present invention relates to a lithium ion secondary battery including the negative electrode material.
- lithium ion batteries that are lightweight and have a large charge capacity are widely used as secondary batteries. Further, in order to be used for future next-generation high-performance electronic device terminals and electric vehicles that can replace gasoline vehicles, further increase in capacity is indispensable.
- negative electrode materials Si-based negative electrodes with a large capacity per unit weight are expected from conventional carbon-based materials (graphite carbon, hard carbon, etc.), and since Si-based materials are abundant in the future, This is advantageous from the viewpoint of cost.
- the negative electrode active material when these materials are used as the negative electrode active material, a large volume change occurs when the charge / discharge cycle is repeated. As a result, the active material is miniaturized, cracks are generated on the electrode surface, the active material and the electrode are peeled off, and as a result, the conductivity is lowered, and sufficient charge / discharge cycle characteristics cannot be obtained. Is an issue.
- Patent Document 1 a method of carbonizing SiO after graphite and mechanical alloying for the purpose of imparting conductivity and suppressing deterioration due to volume change (Patent Document 1), carbon on the surface of silicon oxide particles (Patent Document 2), a method of fusing the surface of silicon particles with silicon carbide (Patent Document 3), and introducing a metal compound into silicon and silicon-containing particles (Patent Document 4).
- Patent Document 5 describes that a compound containing Si and O used as a negative electrode as constituent elements may contain a microcrystalline phase or an amorphous phase of Si. There is no description regarding the state of time and the density or size of the microcrystalline or amorphous phase of Si.
- JP 2000-243396 A Japanese Patent Laid-Open No. 2002-42806 Japanese Patent No. 4450192 JP 2010-135336 A JP 2007-242590 A
- One aspect of the present invention is characterized in that a Li x Si compound is present inside a Li oxide in a charged and discharged state, and the Li x Si compound is dispersed inside the Li oxide.
- the present invention relates to a negative electrode material for a lithium ion secondary battery.
- a different aspect of the present invention is a lithium ion secondary battery having a structure in which a Li x Si compound is present inside a Li oxide in a charged and discharged state and the Li x Si compound is dispersed inside the Li oxide.
- a method for evaluating a negative electrode material wherein the size, density, and interparticle distance of Li x Si in the Li oxide are measured by an X-ray small angle scattering method in a state where the negative electrode material is charged and discharged. It is related with the evaluation method of the negative electrode material for lithium ion secondary batteries characterized by measuring at least 1 and evaluating battery performance.
- a negative electrode material for a lithium ion secondary battery in which volume change associated with charge / discharge is suppressed. Furthermore, according to a different aspect of the present invention, it is possible to provide an evaluation method for selecting a negative electrode material having a small volume change accompanying charge / discharge and having excellent performance.
- FIG. 1 is a schematic view showing an example of the negative electrode material of the present embodiment.
- the negative electrode material is in the form of particles, and shows a state in which the Li x Si compound 2 is dispersed in the Li oxide 1 after charging.
- the Li x Si compound means a compound represented by the formula Li x Si, and may be simply referred to as Li x Si hereinafter.
- the surface of the negative electrode material particles is the carbon film 3. It is not essential to be covered with.
- the active material Li x Si exists as islands (particles) dispersed inside the Li oxide with an average diameter a (nm), and the distance between the islands of Li x Si is b (nm). Existing. At this time, Li x Si has a lower density than the surrounding Li oxide and can be uniformly dispersed in the Li oxide.
- a and b can take arbitrary values, but a is preferably in the range of 0.5 nm to 15 nm, more preferably in the range of 1 nm to 10 nm, taking into account the change in size during charging and discharging.
- B are preferably in the range of 1-20 nm, more preferably in the range of 3-15 nm.
- the particle diameter of the negative electrode material particles is usually 10 nm to 100 ⁇ m, preferably 100 nm to 100 ⁇ m, more preferably 100 nm to 50 ⁇ m. If the thickness is smaller than 100 nm, the edge structure is increased, and therefore, deterioration is likely to occur. On the other hand, when the size is 50 ⁇ m or more, the film thickness of the electrode is increased, Li diffusion during charge / discharge is easily hindered, and the practicality may be insufficient.
- the Li oxide density can be 1.8 to 3.0 g / cm 3
- the Li x Si compound density (when charging) can be 0.5 to 1.7 g / cm 3. It is.
- An arbitrary value can be taken within this range, but the density of Li oxide is more preferably in the range of 2.0 to 2.5 g / cm 3 , and the density of Li x Si is more preferable. Is in the range of 1.0 to 1.4 g / cm 3 . In this range, the difference in density is an appropriate size, and deterioration due to a size change during charging and discharging can be effectively prevented.
- the density difference is an appropriate size, and the size, density and interparticle size of the Li x Si are measured by an X-ray small angle scattering method as described later. It is possible to accurately evaluate the change in structure due to distance and the like and charging and discharging. For example, by evaluating the negative electrode material at the time of charging and discharging, it is possible to predict relaxation of the volume change due to charge / discharge, separation from the current collector due to generation of cracks on the electrode surface accompanying the volume change, and the like. Moreover, the fluctuation
- the negative electrode material of this embodiment enables accurate evaluation of the structure because the Li oxide and Li x Si have the above-described density, and the battery is manufactured based on the knowledge obtained from the evaluation and prediction. It can be said that it has the effect of enabling implementation of measures for improvement (that is, measures that are possible not only for electrode materials but also for the entire battery).
- the density can be reduced by increasing the proportion of Li in the Li x Si.
- the active material Li x Si can be used between 0 ⁇ x ⁇ 4.4. That is, during charging, charging is performed in a range where the upper limit of x is up to 4.4. Further, it is used within a range satisfying 0 ⁇ x even during discharge.
- Li oxide Li 2 O, LiOH, and Li x SiO y can be used.
- Li x SiO y 0 ⁇ x ⁇ 4 and 0 ⁇ y ⁇ 4 can be used.
- SiO x (0 ⁇ x ⁇ 2) is heat-treated at an appropriate temperature in a vacuum, in a nitrogen gas, an inert gas atmosphere, or a hydrogen atmosphere, and Si particles are contained in the SiO x. precipitated, after a lithium-ion battery, by charging, it can be Li x Si compound in the interior of the Li oxide, and the negative electrode material was dispersed.
- silicon particles and silicon oxide can be distributed inside the particles by a reaction of SiO 2 ⁇ Si + SiO 2 .
- the heat treatment temperature is generally 600 to 1500 ° C., but preferably 700 to 1100 ° C.
- the heat treatment temperature is generally 600 to 1500 ° C., but preferably 700 to 1100 ° C.
- the temperature is 700 ° C. or lower, Si particles are difficult to form, which is not effective.
- it is 1100 degreeC or more, since oxidation is accelerated
- a carbon film may be formed on the surface of the negative electrode material, and this carbon film can be formed by sputtering, arc vapor deposition, chemical vapor deposition, or the like.
- the chemical vapor deposition method which is chemical vapor deposition is preferable because the vapor deposition temperature and vapor deposition atmosphere can be easily controlled.
- This CVD method can be carried out by placing the nanocarbon mixture in an alumina or quartz boat or the like, or floating or transporting it in a gas.
- any substance that generates carbon by thermal decomposition as a carbon source compound can be appropriately selected depending on the experimental environment.
- hydrocarbon compounds such as methane, ethane, ethylene, acetylene, and benzene
- organic solvents such as methanol, ethanol, toluene, and xylene, CO, and the like
- the atmospheric gas it can be used by heating to a temperature of 400 to 1200 ° C. in the presence of an inert gas such as argon or nitrogen, or a mixed gas of these and hydrogen.
- the flow rate of the carbon source and the atmospheric gas can be appropriately used as long as they are in the range of 1 mL / min to 10 L / min. More preferably, the carbon source can be coated more uniformly in the range of 10 mL / min to 500 mL / min. In the atmosphere gas, the range of 100 mL / min to 1000 mL / min is more preferable.
- the pressure can be used in the range of 10 to 10000 Torr, but more preferably 400 to 850 Torr.
- the thickness of the carbon film is preferably in the range of 1 nm to 100 nm, more preferably in the range of 5 nm to 30 nm. Sufficient conductivity can be imparted by setting the thickness of the carbon film within the above range. If the thickness of the coating is too small, the conductivity is not sufficient, and if it is too thick, the volume becomes large and it is difficult to obtain a sufficient capacity.
- a lithium secondary battery can be produced by forming a negative electrode from the negative electrode material thus obtained (precisely, the precursor of the negative electrode material of the present invention) and using a positive electrode, an electrolyte, and a separator.
- a non-aqueous solution containing a lithium salt such as LiPF 6 , LiClO 4 , LiBF 4 , LiAlO 4 , LiAlCl 4 , LiSbF 6 , LiSCN, LiCl, LiCF 3 SO 3 is used. What mixed the above can be used.
- non-aqueous solvent for the electrolytic solution one or more of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like are used in combination.
- a known lithium-containing transition metal oxide can be used as the positive electrode active material. Specifically, LiCoO 2, LiNiO 2, LiMn 2 O 4, LiFePO 4, LiFeSiO 4, LiFeBO 3, Li 3 V 2 (PO 4) 3, etc. Li 2 FeP 2 O 7 and the like.
- the negative electrode material of this embodiment can be formed by performing at least one charge.
- the negative electrode material formed as described above has a Li x Si compound in the Li oxide in the charged and discharged state, and the Li x Si compound is in the form of particles in the Li oxide. It is a dispersed negative electrode material.
- the structure of the negative electrode material after charging, after discharging, after charging and discharging, etc. can be evaluated for particle size, size distribution, interparticle distance, and the like by the X-ray small angle scattering method. Since the negative electrode material of the present embodiment has a density difference between Li x Si and Li oxide, the size, density, interparticle distance, etc. of Li x Si, and the structure by charge and discharge are measured by the X-ray small angle scattering method. Can be accurately evaluated.
- a method for evaluating battery performance by measuring at least one of the size, density, and interparticle distance of Li x Si in Li oxide is also provided.
- the size of Li x Si can be obtained by converting the result obtained by the X-ray small angle scattering method into a particle size distribution by curve fitting.
- the density difference from the Li oxide of the parent phase is reduced, the peak intensity is lowered, so that the density of Li x Si can be evaluated from the density difference from the Li oxide.
- the density of Li oxide is obtained from the volume and weight of Li oxide thinned, while the density of Li x Si is obtained by comparing the obtained small angle scattering spectrum with Li oxide of the parent phase and Li oxide. it can be determined by curve fitting assuming a density of x Si.
- the interparticle distance and volume fraction can also be obtained by curve fitting the results obtained by the X-ray small angle scattering method.
- the interparticle distance and the volume fraction can be calculated by curve fitting using Rigaku Nano-solver (version 3.4).
- the size of the Li x Si by measuring at least one of the density and the distance between particles, Li x Si aptitude
- the negative electrode material can be evaluated by determining whether it has a proper size, density or interparticle distance.
- the high performance negative electrode material has a Li x Si size in the range of 0.5 to 15 nm, preferably in the range of 1 to 10 nm, and a density in the range of 0.5 to 1.7 g / cm 3 , preferably Since the range of 1 to 1.5 g / cm 3 and the interparticle distance are in the range of 1 nm to 20 nm, preferably in the range of 3 to 15 nm, at least one of these, preferably two or more, more preferably three are selected.
- the negative electrode material to be filled can be selected as a negative electrode material having excellent performance.
- the negative electrode material can be selected.
- One in which the variation rate of at least one, preferably 2 or more, more preferably three, of the size, density and interparticle distance of Li x Si is small (that is, within a predetermined variation rate set in accordance with the purpose) can be selected as a negative electrode material having excellent cycle characteristics.
- a material having a fluctuation rate of usually 30% or less, preferably 10% or less is selected as a negative electrode material having excellent performance in cycle characteristics. it can.
- the negative electrode material having excellent performance can be selected by evaluating the structure of Li x Si in the Li oxide, and the design of the negative electrode and the entire battery configuration Can be useful.
- Example 1 (Production of negative electrode and battery) A SiO was heat-treated at 1000 ° C. in Ar, 15 wt% of polyimide was added to a carbon-coated sample (85 wt%) by CVD, N-methyl-2-pyrrolidinone was further mixed, and the mixture was sufficiently stirred to prepare a paste.
- the obtained paste was applied to a copper foil for a current collector at a thickness of 80 ⁇ m.
- the electrode was pressure-molded with the roller press. Further, this electrode was fired at 350 ° C. for 1 hour in a nitrogen atmosphere, punched out to 2 cm 2 and used as a negative electrode. Li foil was used for the counter electrode.
- the electrolyte was LiPF 6 mixed at a volume ratio of 1: 7 with 3: 7 ethylene carbonate and diethyl carbonate.
- a lithium ion secondary battery cell for evaluation was produced using a 30 ⁇ m polyethylene porous film.
- the obtained cell was set in a charge / discharge tester, charged at a constant current of 0.2 mA / cm 2 until the voltage reached 0.02 V, and charged by reducing the current at a state of 0.02 V. .
- the charging was terminated when the current value reached 60 ⁇ A / cm 2 .
- Discharging was performed at a constant current of 0.2 mA / cm 2 and terminated when the cell voltage reached 2.0 V, and the discharge capacity was determined.
- the initial charge capacity and initial discharge capacity were 2330 mAh / g and 1650 mAh / g, respectively, per active material, and the charge / discharge efficiency was 71%.
- FIG. 2 shows the results of the WAXS measurement.
- the analysis results are shown in Table 1.
- the average particle size was changed from 6.7 nm to 7.8 nm, the closest interparticle distance was changed from 9.4 nm to 11.0 nm, and the volume fraction was changed from 56.8 to 57.3 nm.
- the average particle size distribution of 6.7 nm is Li x Si (Li 15 Si 4 , Li 7 Si 3, etc.). It can be seen that the Li x Si particle size is slightly increased by performing 30 cycles. Since the volume fraction is the same, Li x Si seems to be slightly enlarged and the distance between each particle is widened. In addition, the volume fraction decreased after discharging, but the particle size and interparticle distance were almost the same as after charging.
- Example 2 Samples were prepared by changing the heat treatment conditions in Experimental Example 1 and evaluated using a coin cell.
- the heat treatment temperature was set to 0 ° C. (untreated), 600 ° C., 700 ° C., 800 ° C., 1100 ° C., 1200 ° C., and the carbon film was coated by a CVD method.
- samples for charging after 30 cycles 1000 mAh / g were prepared, and SAXS measurement and WAXS measurement were performed.
- the difference in particle size after charge and after cycle (charge) was compared with heat treatment at 1000 ° C., the change in particle size was small and excellent cycle characteristics were exhibited in heat treatment between 700 and 1100 ° C.
Abstract
Description
(負極および電池の作製)
SiOをAr中で1000℃で熱処理し、CVD法により炭素でコートした試料(85wt%)にポリイミドを15wt%加え、さらにNメチル-2-ピロリジノンを混ぜ十分に攪拌し、ペーストを作製した。ここで、得られたペーストを集電体用の銅箔に厚さ80μmで塗布した。その後、120℃で1時間乾燥させた後、ローラプレスにより電極を加圧成形した。さらに、この電極を350℃で1時間窒素雰囲気下で焼成し、2cm2に打ち抜き負極とした。対極は、Li箔を使用した。電解液はLiPF6を体積比で3:7のエチレンカーボネートとジエチルカーボネートに1Mで混合した。セパレータは、30μmのポリエチレン製多孔質フィルムを用いて、評価用のリチウムイオン二次電池セルを作製した。
図2は、WAXS測定の結果を示す。2θ=20°、40°付近のブロードなピークはLi15Si4のピークである。また、2θ=30°~35°のブロードなピークは、1000mAh/g放電により消失していることから、可逆的なSiの化合物であると予想され、Li7Si3、Li7Si12のような相である。また、30サイクルの充放電後、ピークが同じ形状であり構造劣化はほとんどない。
3.4)を使ったカーブフィッティングにより、粒子サイズ分布に変換したものが図4である。
充電後の試料、初期充電後1000Ah/g放電の試料、30サイクル(1000mAh/g)後充電の試料を、集束イオンビーム(FIB)により加工し、透過電子顕微鏡で各々を観察した。その結果、黒いコントラストの付いたLixSiの粒子を観察することができた。この時、平均粒子サイズは1~10nm、最近接粒子間距離は、3~15nm程度であり、平均はそれぞれ7nm、8nmであった。
実験例1の熱処理条件を変えて試料を作製、コインセルにより評価した。熱処理温度は、0℃(未処理)、600℃、700℃、800℃、1100℃、1200℃に設定し、また、CVD法により炭素膜をコートした。充電後、30サイクル(1000mAh/g)後充電の試料を作製し、SAXS測定とWAXS測定を行った。充電後とサイクル後(充電)の粒子サイズの違いを1000℃の熱処理と比較したところ、700~1100℃の間の熱処理において、粒子サイズの変化が小さく、且つ、優れたサイクル特性を示した。
LixSiの密度を変えるため、実験例1の条件で作製したコインセルを充電の量を変化させて、サイクル評価を行った。充電量を400~1400mAh/gの間で制御することで、LiSi、Li12Si7、Li7Si3、Li13Si4、Li15Si4、Li21Si5、Li22Si5の相を作製した。その結果、Li2Si5の組成は、サイクル特性において、充放電維持率の低下が他の組成に比べて大きかった。
2 LixSi化合物
3 炭素膜
Claims (10)
- 充電及び放電状態においてLi酸化物の内部にLixSi化合物が存在し、且つ、LixSi化合物がLi酸化物内部に分散している構造を有することを特徴とするリチウムイオン二次電池用負極材。
- 前記Li酸化物の密度が、1.8~3.0g/cm3であり、且つ、前記LixSi化合物の充電時の密度が0.5~1.7g/cm3であることを特徴とする請求項1記載のリチウムイオン二次電池用負極材。
- 前記LixSi化合物において、0<x≦4.4であることを特徴とする請求項1または2に記載のリチウムイオン二次電池用負極材。
- 前記Li酸化物は、Li2OまたはLixSiOy(0<x≦4、0<y≦4)であることを特徴とする請求項1~3のいずれか1項に記載のリチウムイオン二次電池用負極材。
- 前記LixSi化合物のサイズが0.5nm~15nmの範囲であり、LixSi間の距離が1~20nmの範囲で、且つ、前記Li酸化物のサイズが100nm~100μmのサイズであることを特徴とする請求項1~4のいずれか1項に記載のリチウムイオン二次電池用負極材。
- 前記リチウムイオン二次電池用負極材が、炭素により被覆されていることを特徴とする請求項1~5のいずれか1項に記載のリチウムイオン二次電池用負極材。
- シリコン酸化物を不活性ガス雰囲気下または水素雰囲気下で、700~1100℃で熱処理を行って、前記シリコン酸化物中にシリコン粒子が分散された構造を形成する工程と、
前記熱処理後のシリコン酸化物を負極として、リチウムイオンの存在下に充電を行って、充電及び放電状態においてLi酸化物の内部にLixSi化合物が存在し、且つ、LixSi化合物がLi酸化物内部に分散している構造を有する負極を製造することを特徴とするリチウムイオン二次電池用負極材の製造方法。 - 充電及び放電状態においてLi酸化物の内部にLixSi化合物が存在し、且つ、LixSi化合物がLi酸化物内部に分散している構造を有するリチウムイオン二次電池用負極材の評価方法であって、
前記負極材が充電された状態及び放電された状態において、X線小角散乱法により、前記Li酸化物中のLixSi化合物のサイズ、密度および粒子間距離の少なくとも1つを測定し、電池性能を評価することを特徴とするリチウムイオン二次電池用負極材の評価方法。 - 前記負極材の充放電サイクル試験を行い、該充放電サイクル試験前後において、X線小角散乱法により前記Li酸化物中のLixSiのサイズを測定し、その変化の大小に基づいて電池性能を評価することを特徴とする請求項8記載のリチウムイオン二次電池用負極材の評価方法。
- 請求項1~6のいずれか1項に記載のリチウムイオン二次電池用負極材を備えるリチウムイオン二次電池。
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