JP3637676B2 - Lithium ion secondary battery - Google Patents

Description

【００３６】
【表２】

【００３７】
【発明の効果】
本発明のリチウム二次電池は、電極の脱ドープ容量が大きく、原材料内にある低結晶部分を効率的に除去することによって導電性を向上させるとともに、本来炭素質物が有する放電容量を効率よく引き出すことができることが特徴である。そのため、本発明のリチウム二次電池は、高容量で、充放電サイクル特性に優れる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium ion secondary battery, and more particularly to a lithium secondary battery using a carbon electrode material having a high capacity and charge / discharge efficiency defined by a doping-undoping capacity and excellent cycle characteristics.
[0002]
[Prior art]
With the downsizing of electronic devices, high-capacity secondary batteries have become necessary, and lithium-ion secondary batteries with higher energy density have attracted attention compared to nickel-cadmium and nickel-hydrogen batteries. Attempts have been made to use lithium metal as the negative electrode material. However, this material has been found to cause dendritic lithium to grow on the lithium metal surface through repeated charging and discharging, penetrate the separator, eventually reach the positive electrode, and short-circuit, causing a fire accident. In order to improve this, it has been found that a carbonaceous material capable of preventing lithium deposition by performing lithium occlusion in the charge and discharge process between layers is used as the negative electrode. Among them, lower crystallinity and specific gravity, such as graphitic carbon such as polymer carbide, coke, carbon fiber, coal and petroleum pitch fired product, mesocarbon microbead, as disclosed in JP-A-4-237949, Carbonaceous materials defined by Raman spectroscopy, specific surface area and other properties have been proposed. JP-A-57-208079 discloses that graphite having the highest crystallinity is preferably used as the carbonaceous material. However, since graphite uses intercalation of lithium ions into graphite crystals as the principle of charge / discharge, there is a problem that a capacity of 372 mAh / g or more calculated from the maximum lithium-introduced compound LiC ₆ cannot be obtained. It was.
[0003]
On the other hand, it has been reported that a carbonaceous material having a very small amount of crystallized portion fired at 950 ° C. or less exhibits a capacity larger than the theoretical capacity of 372 mAh / g of graphite, and is attracting attention as a capacity increasing method. This carbonaceous material is known to have different capacities, efficiency, irreversible capacity, defined as the difference between doping capacity and dedoping capacity, and potential characteristics during charging / discharging depending on the firing temperature (Dahn et al. Science, 270, 590 (1995)).
[0004]
Thus, the carbonaceous material tends to increase the charge capacity of the first lithium ion as it is fired at a lower temperature in the temperature range up to about 450 ° C. There was a problem that the irreversible capacity defined as the difference also increased. For this reason, the carbonaceous material fired in one stage until the target temperature is reached in a normal low temperature range of 950 ° C. or lower has a large charge capacity, but at the same time, the irreversible capacity also increases. It could not be put into practical use as a battery electrode material.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a lithium secondary battery using a carbonaceous material as an electrode material which has a large dedoping capacity and can efficiently draw out the original discharge capacity of an amorphous carbonaceous material.
That is, an object of the present invention relates to providing a lithium ion secondary battery having high capacity, high charge / discharge efficiency, and excellent cycle characteristics.
[0006]
[Means for Solving the Problems]
As a result of intensive investigations to solve the above problems, the present inventors have conducted a detarring step in a specific temperature range for a long time before firing the material, so that the charge / discharge efficiency is higher than that of conventional carbonaceous materials, In particular, the inventors have found that a carbonaceous material having a greatly improved discharge capacity can be prepared, and have reached the present invention. That is, the gist of the present invention is that a carbonaceous material precursor is prepared by detarring and pitching an organic material at 300 to 500 ° C , and this carbonaceous material precursor is pulverized to a particle size of 1 cm or less and then at 500 to 950 ° C. It exists in the manufacturing method of the negative electrode material characterized by making it the amorphous carbon whose volume resistivity is 10 < ¹ > ohm * cm or more and 10 < ⁷ > ohm * cm or less by baking (however, 20-100 weight per 100 weight part organic substance) Except when adding part of the nitrating agent to remove tar and pitch).
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The raw material for obtaining the carbonaceous material of the present invention will be described in detail below.
As organic substances whose carbonization proceeds in the liquid phase, coal-based heavy oils such as coal tar pitch and dry distillation liquefied oil from soft pitch to hard pitch, DC heavy oils such as atmospheric residual oil, vacuum residual oil, Examples include petroleum heavy oils such as cracked heavy oils such as ethylene tar that are by-produced during thermal cracking of crude oil and naphtha.
[0008]
Furthermore, aromatic hydrocarbons such as acenaphthylene, decacyclene, anthracene, N ring compounds such as phenazine and acridine, S ring compounds such as thiophene, alicyclic rings such as adamantane, biphenyl, terphenyl, etc. And polymers such as polyphenylene, polyvinyl chloride, and polyvinyl alcohol.
[0009]
Organic substances that progress in carbonization in the solid phase include natural polymers such as cellulose and saccharides, thermoplastic resins such as polyphenylene sulfide and polyphenylene oxide, thermosetting resins such as furfuryl alcohol resin, phenol-formaldehyde resin, and imide resin. Resin etc. are mentioned.
Of these, preferred are organic substances that undergo carbonization in the liquid phase, and more preferred are coal-based heavy oils and / or petroleum-based heavy oils.
[0010]
A carbonaceous material precursor is obtained by detarring and pitching these raw materials at 300 to 500 ° C., preferably 1 to 8 hours, preferably 330 to 450 ° C. for 2 to 5 hours. In this case, it is preferably performed in an inert gas atmosphere. After the carbonaceous material precursor is preferably pulverized, the carbonaceous material precursor of the present invention is obtained by further calcining at 500 to 950 ° C, preferably 600 to 800 ° C. The pulverization of the carbonaceous material precursor has a particle size of 1 cm or less, preferably 1 mm or less, more preferably 100 μm or less, and the final carbonaceous material is pulverized, preferably 5 to 100 μm, for use in manufacturing the electrode. It is most preferable to adjust the size of the granular material to the target particle size after firing at the stage of the carbonaceous material precursor before firing.
[0011]
The carbonaceous material has a volume resistivity of 10 ¹ to 10 ⁷ Ω · cm, a specific surface area of 1 m ² / g to 100 m ² / g, and H / C (hydrogen / carbon atom abundance ratio) of 0.05 to Amorphous carbon defined by 0.5 is preferred.
Moreover, it is more preferable that the detarring and pitching step for obtaining the carbonaceous precursor is performed under an inert gas atmosphere such as nitrogen, argon, helium, or under these gas flows.
[0012]
Furthermore, in order to improve the electric potential and the charge / discharge characteristics, the carbonaceous material can be produced by blending a conductive filler with the raw material of the carbonaceous material. Preferable specific examples of the conductive filler include conductive carbon black such as acetylene black and ketjen black, artificial graphite (such as T6, KS6, SFG6 manufactured by TIMAL), natural graphite (NG2, NG7 manufactured by Kansai Thermal Chemical Co., Ltd.) Etc.), carbon powder such as vapor-grown carbon fiber, and metal powder are preferably nickel powder, copper powder, and stainless steel powder because of the negative electrode potential in the battery.
[0013]
In particular, nickel powder is preferable because it has good conductivity and excellent oxidation resistance. Carbonyl nickel powder produced by thermal decomposition of nickel tetracarbonyl has high purity, and spherical particles having spike-like protrusions are formed. Since it has a shape connected to a filament, it is preferable because it has excellent contact between particles and can easily form a conductive path.
In addition, the carbon-based filler is excellent in compatibility at the raw material mixing stage with a carbonaceous material, particularly a heavy oil-based raw material, and it is easy to produce a composite material having a uniform composition. In addition, after firing, it is possible to improve the conductivity of the carbonaceous material and contribute to the electrode capacity due to the lithium ion occlusion and release ability of the filler itself.
[0014]
In general, when a conductive filler is added to an insulating material or a high-resistance material, a so-called percolation phenomenon in which the resistance rapidly decreases at a specific volume fraction is exhibited. Therefore, the ratio of the conductive filler needs to be larger than the percolation threshold. More specifically, the content of the conductive filler and the carbonaceous material is preferably 85 to 50 Vol. %, The conductive filler is 15-50 Vol. %, More preferably carbonaceous material is 85 to 65 Vol. %, The conductive filler is 15 to 35 Vol. %.
[0015]
If the amount of the conductive filler is less than the above range, the potential is lowered and the rapid charge / discharge characteristics are hardly improved. If the amount is more than the above range, the volume energy density and the weight energy density may be lowered. The above range is not the raw material charge ratio but the content at the final carbonaceous material stage. Therefore, at the time of preparation, it is necessary to determine the blending amount of the raw material in consideration of the composition ratio in the final stage.
[0016]
These manufacturing methods will be described next.
The carbonaceous material and the conductive filler are mixed in a mixing machine with a heating means at a charging ratio such that the final composition is within the above range, degassed and detarred at 300 to 500 ° C. for 1 to 8 hours, and solid After obtaining the product, it is made a granular material of 1 cm or less, preferably 1 mm or less, more preferably 100 μm or less. Thereafter, firing is preferably performed at 500 to 950 ° C. for 0.5 to 3 hours to obtain a conductive filler composite electrode material.
[0017]
The amorphous carbonaceous material thus obtained is preferably pulverized to a range of 1 to 100 μm, more preferably an average particle size of 5 to 50 μm, and a binder, a solvent, etc. are added to the pulverized product to form a slurry, The slurry is applied to a substrate of a metal current collector such as a copper foil and dried to form an electrode. Further, the electrode material can be directly formed into the shape of an electrode by a method such as roll molding or compression molding.
[0018]
Binders that can be used for the above purpose include resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, and cellulose, styrene / butadiene rubber, isoprene rubber, butadiene rubber, and ethylene, which are stable to solvents.・ Rubber polymers such as propylene rubber, styrene / butadiene / styrene block copolymers, hydrogenated products thereof, thermoplastic elastomeric polymers such as styrene / isoprene / styrene block copolymers, hydrogenated products thereof, syndiotactic Soft resinous polymers such as tic 12-polybutadiene, ethylene / vinyl acetate copolymer, propylene / α-olefin (carbon number 2 to 12) copolymer, polyvinylidene fluoride, polytetrafluoroethylene, polytetrafluoroethylene Ethylene copolymer, etc. Motokei polymer, an alkali metal ion, the polymeric composition may be mentioned in particular has an ionic conductivity of lithium ions.
[0019]
Examples of the polymer having ion conductivity include polyether polymer compounds such as polyethylene oxide and polypropylene oxide, crosslinked polymers of polyether compounds, polyepichlorohydrin, polyphosphazene, polysiloxane, polyvinylpyrrolidone, and polyvinylidene carbonate. , A system in which a polymer compound such as polyacrylonitrile is combined with a lithium salt or an alkali metal salt mainly composed of lithium, or an organic compound having a high dielectric constant such as propylene carbonate, ethylene carbonate or γ-butyrolactone Can be used. The ionic conductivity at room temperature of such an ion conductive polymer composition is preferably 10 ⁻⁵ S / cm or more, more preferably 10 ⁻³ S / cm or more.
[0020]
As a mixing form of the carbonaceous material used in the present invention and the above binder, various forms can be taken. That is, a form in which both particles are mixed, a form in which a fibrous binder is entangled with carbonaceous particles, or a form in which a binder layer is attached to the surface of carbonaceous particles. The mixing ratio of the carbonaceous material and the binder is preferably 0.1 to 30% by weight, more preferably 0.5 to 10% by weight with respect to the carbonaceous material. Addition of a binder in an amount larger than this increases the internal resistance of the electrode, which is not preferable. If the amount is less than this, the binding property between the current collector and the carbonaceous powder is poor.
[0021]
The secondary battery is configured by combining the negative electrode plate thus prepared, the electrolyte solution described below, and the positive electrode plate with other battery components such as a separator, a gasket, a current collector, a sealing plate, and a cell case. The battery that can be made is not particularly limited, such as a cylindrical shape, a square shape, a coin shape, etc. Basically, a current collector and a negative electrode material are placed on a cell floor plate, and an electrolytic solution and a separator are further placed thereon. A positive electrode is placed so as to face the negative electrode and caulked together with a gasket and a sealing plate to obtain a secondary battery.
[0022]
Nonaqueous solvents that can be used for the electrolyte include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, 1, A single organic solvent such as 3-dioxolane, or a mixture of two or more organic solvents can be used.
[0023]
An electrolyte such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiAsF ₆ or the like of about 0.5 to 2.0 M is dissolved in these solvents.
Further, a solid polymer electrolyte that is a conductor of an alkali metal cation such as lithium ion can also be used.
The material of the positive electrode body is not particularly limited, but is preferably made of a metal chalcogen compound that can occlude and release alkali metal cations such as lithium ions during charge and discharge. Examples of such metal chalcogen compounds include vanadium oxide, vanadium sulfide, molybdenum oxide, molybdenum sulfide, manganese oxide, chromium oxide, titanium oxide, titanium sulfide, and the like. And composite oxides and sulfides. Preferably, Cr ₃ O ₈ , V ₂ O ₅ , V ₅ O ₁₃ , VO ₂ , Cr ₂ O ₅ , MnO ₂ , TiO ₂ , MoV ₂ O ₈ , TiS ₂ V ₂ S ₅ MoS ₂ , MoS ₃ VS ₂ Cr _0.25 V _0.75 S ₂ , Cr _0.5 V _0.5 S _2, etc. In addition, LiMY ₂ (M is a transition metal Y such as Co and Ni is a chalcogen compound such as O and S), LiM ₂ Y ₄ (M is Mn, Y is O), oxide such as WO ₃ , CuS, Fe Sulfides such as _0.25 V _0.75 S ₂ and Na _0.1 CrS ₂ , phosphorus such as NiPS ₃ and FePS ₃ , sulfur compounds, and selenium compounds such as VSe ₂ and NbSe ₃ can also be used. As with the negative electrode material, these are mixed with a binder and applied onto the current collector to form a positive electrode plate.
[0024]
The separator that holds the electrolytic solution is generally a material that has excellent liquid retaining properties. For example, a non-woven polyolefin resin or a porous film is used to impregnate the electrolytic solution.
Among the evaluation contents, measurements of negative electrode charge / discharge capacity, cycle characteristics, potential-capacity curve, and the like were performed as follows.
[0025]
The negative electrode material formed into a pellet form using a binder was made into a half battery with a separator and an electrolytic solution and a counter electrode made of lithium metal, assembled in a 2016 coin cell, and evaluated with a charge / discharge tester.
On the other hand, the resistivity was calculated by measuring the surface resistance of the negative electrode material processed into a sheet shape using a binder by the four-probe method.
[0026]
When the test was conducted under such conditions, IR drop in the carbon negative electrode plate of the present invention was reduced and dedoping capacity was increased.
As described above, the electrode for a lithium ion secondary battery of the present invention is fired by pulverizing and further firing at 500 to 950 ° C. when the raw material heavy oil is detarred, pitched and solidified. The IR drop in the later carbon negative electrode decreased, and a large dedoping capacity was exhibited.
[0027]
The reason for the improvement in electrode performance is that, as can be inferred from the result of resistivity, the crystal part of the very undeveloped carbonaceous material precursor related to the size of the irreversible capacity is due to long-term detarring. It is inferred that it was removed to some extent. Therefore, it is considered that a partially ordered structure composed of larger crystals was formed in the fired material, and that the decrease in IR drop in the material due to the decrease in resistivity resulted in an increase in the dedoping capacity. .
[0028]
【Example】
EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
(Example 1)
2 kg of ethylene heavy end tar (Mitsubishi Chemical Co., Ltd.) obtained at the time of naphtha decomposition is put into a stainless bat with an internal volume of 10 liters, and this is heated in a nitrogen gas atmosphere with an internal temperature kept at 400 ° C And detarring and solid pitching were performed for 4 hours. Thereby, the light fraction of ethylene heavy end tar was removed, and the product which was a block-like solid solution was recovered.
[0029]
The obtained product was pulverized to a maximum diameter of 1 mm and heat-treated in a batch heating furnace from room temperature to 700 ° C. in an inert atmosphere for 2 hours. This was pulverized and adjusted to a particle size of 7 to 20 μm using a vibrating sieve, and then used as a sample. The element was subjected to elemental analysis and H / C was calculated to be 0.27. The BET specific surface area was 11 m ² / g.
[0030]
To 5 g of this electrode material sample, 10% by weight of a dimethylacetamide solution of polyvinylidene fluoride (PVdF) added in terms of solid content was stirred to obtain a slurry. This slurry was applied onto a copper foil and pre-dried at 80 ° C. After further pressure bonding, it was punched into a disk shape having a diameter of 20 mm and dried under reduced pressure at 110 ° C. to obtain an electrode.
[0031]
The slurry was applied on a polyethylene terephthalate thin film and pre-dried at 80 ° C. After removing portions other than the 20 cm × 10 cm rectangle, drying under reduced pressure was performed at 110 ° C. The results of measuring the resistivity of this product are shown in Table 1.
The obtained electrode was sandwiched with a polypropylene separator impregnated with an electrolytic solution to produce a coin cell facing the lithium metal electrode, and a charge / discharge test was performed. As the electrolytic solution, a solution in which lithium perchlorate was dissolved at a rate of 1.0 mol / L in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 was used.
[0032]
The charge and discharge test performed doped to interelectrode potential difference at a current density of 0.16 mA / cm ² is to 0V, and was de-doped at a current density of 0.33 mA / cm ² until the interelectrode potential difference 1.5V.
The capacity value is obtained by conducting a 5-cycle charge / discharge test on each of three coin-type cells, and the initial average doping capacity, initial average dedoping capacity, initial average doping capacity-irreversible capacity expressed by initial average dedoping capacity, The charge / discharge efficiency represented by (initial average dedoping capacity / initial average doping capacity) × 100 (%) was calculated and evaluated. The evaluation results are shown in Table 2.
[0033]
(Example 2)
A sample was obtained in exactly the same manner as in Example 1 except that the holding temperature of the heating oven for obtaining the solid pitch was 350 ° C. The sample was subjected to elemental analysis and H / C was calculated to be 0.30. The BET specific surface area was 17 m ² / g.
The results of the same evaluation as in Example 1 using the sample are shown in Table 1 and Table 2, respectively.
[0034]
(Comparative Example 1)
The same operation as in the example except that the heavy oil as a raw material was heated up to 700 ° C. in 2 hours and held at 700 ° C. for 1 hour in a batch heating furnace, and detarring, pitching and firing were performed in one step. Went.
The resistivity is shown in Table 1, and the evaluation results are shown in Table 2.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
【The invention's effect】
The lithium secondary battery of the present invention has a large electrode dedoping capacity, and improves the conductivity by efficiently removing the low crystal portion in the raw material, and also efficiently draws out the discharge capacity originally possessed by the carbonaceous material. It is a feature that it can be. Therefore, the lithium secondary battery of the present invention has a high capacity and excellent charge / discharge cycle characteristics.

Claims (9)

Tar cutting organics at 300 to 500 ° C., and pitch to create a carbonaceous material precursor, the carbonaceous material precursor, is fired at five hundred to nine hundred and fifty ° C. After grinding to a particle size less than 1cm volume resistivity 10 ^A method for producing a negative electrode material characterized in that amorphous carbon is ¹ Ω · cm or more and 10 ⁷ Ω · cm or less (however, 20-100 parts by weight of a nitrating agent is added per 100 parts by weight of organic matter to remove tar , Except when pitching). Petroleum or coal-based heavy oil is detarred and pitched at 300 to 500 ° C. to prepare a carbonaceous material precursor, and the carbonaceous material precursor is pulverized to a particle size of 1 cm or less and then calcined at 500 to 950 ° C. The volume resistivity is 10 ¹ 10 at Ω · cm or more ⁷ A negative electrode material manufacturing method characterized by amorphous carbon having an Ω · cm or less (however, when 100 to 100 parts by weight of heavy oil is added with 20 to 100 parts by weight of a nitrating agent for detarring and pitching. except). The method for producing a negative electrode material according to claim 1, wherein the carbonaceous material precursor is pulverized to a particle size of 100 μm or less. 4. The method for producing a negative electrode material according to claim 1, wherein detarring and pitching are performed for 1 to 8 hours. The method for producing a negative electrode material according to any one of claims 1 to 3, wherein detarring and pitching are performed at 330 to 450 ° C for 2 to 5 hours. 6. The method for producing a negative electrode material according to claim 1, wherein detarring and pitching are performed in an inert gas atmosphere. Amorphous carbon has a specific surface area of 1m ² / G or more 100m ² / G or less, and H / C (hydrogen / carbon atom abundance ratio) is 0.05-0.5, The manufacturing method of the negative electrode material in any one of Claim 1 thru | or 6 characterized by the above-mentioned. A negative electrode material obtained by the production method according to claim 1. The lithium ion secondary battery which has a negative electrode created using the negative electrode material of Claim 8.