WO2014109406A1 - リチウムイオン二次電池 - Google Patents
リチウムイオン二次電池 Download PDFInfo
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- WO2014109406A1 WO2014109406A1 PCT/JP2014/050377 JP2014050377W WO2014109406A1 WO 2014109406 A1 WO2014109406 A1 WO 2014109406A1 JP 2014050377 W JP2014050377 W JP 2014050377W WO 2014109406 A1 WO2014109406 A1 WO 2014109406A1
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- Y10T29/49108—Electric battery cell making
- Y10T29/49112—Electric battery cell making including laminating of indefinite length material
Definitions
- the present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.
- lithium ion secondary batteries are expanding as a power source for electric vehicles (EV), hybrid vehicles (HEV), and plug-in hivets (PHEV) due to the growing environmental awareness on a global scale. Since the accident at the Fukushima Daiichi Nuclear Power Station in March 2011, lithium-ion batteries using large laminate cells have been used as large storage batteries for large-scale power storage systems for offices and power storage systems such as next-generation smart houses. The spread of secondary batteries is also expected. Large-sized lithium ion secondary batteries have longer life characteristics required than small power supplies for mobile phones and mobile devices. In particular, large-sized lithium ion secondary batteries for in-vehicle use and power storage systems are required to have long-term life characteristics of at least 15 years.
- lithium-ion secondary batteries have a small capacity deterioration rate with respect to the number of charge / discharge cycles, and in order to further increase the safety of large batteries etc., sudden capacity deterioration occurs while driving or operating storage batteries. There is a demand for battery characteristics that do not cause the phenomenon, the so-called rapid fade phenomenon.
- a lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator.
- the positive electrode active material used for the positive electrode lithium cobaltate (LiCoO 2 ), manganese spinel (LiMn 2 O 4 ) or the like is mainly used. Since the positive electrode active material has high electric resistance, the electric resistance of the positive electrode is lowered using a carbon-based conductive additive.
- the binder for example, a polymer such as styrene / butadiene rubber, fluororubber, synthetic rubber, polyvinylidene fluoride, acrylic resin, or the like is used.
- the negative electrode active material natural graphite, artificial graphite obtained by heat treatment of coal / petroleum pitch, etc. at high temperature, amorphous carbon obtained by heat treatment of coal, petroleum pitch coke, acetylene pitch coke, etc., metallic lithium or AlLi Lithium alloys such as are used.
- a carbon-based conductive additive may be used for the negative electrode.
- the electrolytic solution a nonaqueous electrolytic solution in which an electrolyte such as a lithium salt is dissolved is used.
- an electrolyte such as a lithium salt
- a separator is comprised with the film which isolate
- Patent Document 1 Japanese Patent Application Laid-Open No. 2005-142004 discloses a technique of adding a conductive additive of carbon black in order to reduce negative electrode resistance. Since carbon black is composed of primary particles of the order of several tens of nanometers, it tends to agglomerate and forms secondary particles to bridge between active materials, which is effective in ensuring the conductivity of the initial charge / discharge cycle. .
- Patent Document 2 describes a technique in which the surface of graphite particles is coated with another carbon material in a lithium ion battery using a negative electrode in which a plurality of negative electrode active materials such as natural graphite and artificial graphite are mixed.
- a lithium battery having a relatively high capacity and high coulomb efficiency can be obtained.
- it does not meet the demand for a highly safe lithium ion secondary battery characterized in that a precipitate containing metallic lithium does not occur on the negative electrode surface.
- Patent Document 3 discloses a technique characterized in that the ratio of graphite is 50% by mass or less, and a film containing sulfur and nitrogen is present on the negative electrode surface.
- the technology having such a coating is an effective means for suppressing battery swelling as the battery cycle progresses. However, it has not yet satisfied the long-term charge / discharge characteristics without causing rapid fade.
- Patent Document 4 a sulfur compound is present on the surface of the negative electrode, and sulfur having no oxygen is present in the first proximity where a peak is observed at 162.9 to 164.0 eV by XPS analysis.
- a technology for obtaining a film with high ion conductivity is disclosed. Although this technique has an effect of suppressing decomposition of the electrolytic solution, there is room for improvement in order to improve the safety of a lithium ion secondary battery mainly composed of a graphite-based negative electrode.
- Patent Documents 5 and 6 disclose a nonaqueous electrolyte technique characterized by containing a cyclic disulfonic acid ester. However, there is much room for improvement in terms of high safety that does not cause metallic lithium deposition even at the end of discharge.
- Patent Document 7 discloses a battery technology in which spherical natural graphite particles and 3% Chinese scale graphite fine powder are mixed. However, the battery described in Patent Document 7 also has room for improvement in order to realize low resistance, high cycle characteristics, and sufficient safety.
- the surface functional group amount O / C of the carbon material used for the lithium ion battery is 1% or more and 4% or less, and the surface functional group amount Cl / C + S 165 / C is 0.05% or more, 0
- a lithium battery that reduces gas generation during the initial cycle and during high-temperature storage is described, characterized in that it is less than 0.5%.
- XPS is not suitable for analyzing surface functional groups of several thousand ppm or less. Therefore, a battery having excellent cycle characteristics and high safety has not been sufficiently studied.
- the carbon black reacts with the electrolyte while the charge / discharge cycle is repeated, and the primary particles are gasified and further etched. May disappear and the conductive network of secondary particles may be divided.
- the resistance of the lithium ion battery suddenly increases, which may cause rapid capacity deterioration (rapid fade).
- such a lithium battery with severe capacity deterioration may cause precipitates containing metal Li on the surface of the negative electrode, and the problem is that the safety of the battery decreases.
- Carbon black which is generally widely used as a conductive additive, has a role of adhering to the surface of the electrode active material and filling the gap between the active materials to increase conductivity. Carbon black forms a conductive network in the state of presenting secondary particles and maintains conductivity.
- the size of the secondary particles of carbon black depends on the shearing force at the time of kneading in the slurry preparation process, and there is a problem that the size of the secondary particles changes.
- the conductive network that often fills the gap between the active material and the volume expansion of about 11% accompanying the charge / discharge cycle. It becomes easy to stop following. In addition, cell resistance is increased, and the electrolytic solution is further decomposed, thereby causing rapid rapid fade.
- An object of the present invention is to provide a lithium ion battery that has excellent long-term life characteristics and excellent charge / discharge characteristics that do not cause rapid capacity deterioration.
- the present embodiment includes a negative electrode active material containing first carbon and second carbon,
- the first carbon is spherical graphite;
- the second carbon is massive graphite;
- the present invention relates to a lithium ion secondary battery including the same, and a method for manufacturing the same.
- the present invention it is possible to provide a lithium ion secondary battery excellent in safety that does not cause rapid capacity deterioration (rapid fade) particularly during charging and discharging in a high temperature environment.
- FIG. 6 is a diagram showing capacity change curves at battery cell voltages of 2.0 to 3.4 V of batteries of Examples 1 and 9 and Reference Examples 3 and 4.
- FIG. 1 shows an example of a schematic cross-sectional view for explaining the structure of the lithium ion battery of the present embodiment.
- the lithium ion battery has a spherical natural graphite 2 (first carbon), massive non-graphitizable carbon 3 (second carbon A), and massive artificial graphite (second Carbon B), and negative electrode 1 having a layer containing plate-like graphite 5 as a conductive additive, and positive electrode 12 having a layer containing positive electrode active material 13 on positive electrode current collector 15 sandwich separator 11.
- the separator 11 is impregnated in the electrolytic solution 8.
- the electrolytic solution 8 includes an additive 9.
- the additive 9 is formed on the surfaces of the first carbon 2, the second carbon 3 and 4 and the conductive additive 5 as the negative electrode active material at the time of initial charging. A film is formed.
- the negative electrode includes a negative electrode active material containing first carbon and second carbon,
- the first carbon is spherical graphite;
- the second carbon is massive graphite;
- the sulfur concentration (Sx) in the first carbon and the sulfur concentration (Sy) in the second carbon are each independently 0 ppm or more and 300 ppm or less.
- the negative electrode includes a negative electrode active material containing first carbon and second carbon, a conductive additive, and a binder.
- the first carbon is spherical graphite;
- the second carbon is massive non-graphitizable carbon and / or massive artificial graphite;
- the conductive additive is plate-like graphite,
- the sulfur concentration (Sx) in the first carbon and the sulfur concentration (Sy) in the second carbon are each independently 0 ppm or more and 300 ppm or less.
- ratio coefficient (carboxyl group / phenolic hydroxyl group) between the carboxyl group which is the surface functional group of the first carbon and the phenolic hydroxyl group is GM (sf)
- GM (sf) is 0.1 to 1 .1
- ratio coefficient (carboxyl group / phenolic hydroxyl group) between the carboxyl group which is the surface functional group of the second carbon and the phenolic hydroxyl group is GV (sf)
- GV (sf) is 0.1 to 1.1. It may be.
- ppm means “mass ppm”.
- Sx, Sy, and Sz represent the sulfur concentration in the first carbon, the second carbon, and the conductive additive, respectively, and these indicate the sulfur concentration before charging of the lithium ion secondary battery.
- ratio coefficient between carboxyl group and phenolic hydroxyl group represents (number of carboxyl groups / number of phenolic hydroxyl groups).
- the negative electrode active material includes a negative electrode active material including first carbon and second carbon, the first carbon is spherical graphite, and the second carbon is massive graphite. is there.
- the negative electrode active material includes first carbon and second carbon, the first carbon is spherical graphite, and the second carbon is massive non-graphitizable carbon and / or massive. It is preferable to use artificial graphite.
- the one with the higher content (mass) in the negative electrode active material is the “main material”, and the one with the lower content (mass) is the “subsidiary”.
- the main material the one with the higher content (mass) in the negative electrode active material
- the one with the lower content (mass) is the “subsidiary”.
- material Sometimes referred to as “material”.
- the first carbon contained in the negative electrode active material is spherical graphite.
- Spherical graphite is manufactured using scaly graphite as a raw material, and has a structure in which scaly graphite is folded into a spherical shape. For this reason, a cut is observed in the spherical graphite, and it has a cabbage-like appearance in which the cut is directed in various directions. In addition, voids are observed on the fracture surface of the spherical graphite. Since the first carbon contained in the negative electrode active material is spherical, the orientation of the crystal is directed in various directions even after the rolling process at the time of electrode preparation, so that lithium ions can be easily moved between the electrodes. . Furthermore, by using spherical graphite, voids suitable for holding the electrolyte solution can be obtained between the negative electrode active materials, so that a lithium secondary battery excellent in high output characteristics can be obtained.
- the short axis direction (the length in the shortest direction) and the long axis direction (the length in the longest direction)
- the ratio of the lengths of (minor axis) / (major axis) is larger than 0.2, it can be determined as a spherical shape.
- the (short axis) / (major axis) of the first carbon of the negative electrode active material is preferably 0.3 or more, more preferably 0.5 or more.
- the sulfur concentration (Sx) in the first carbon is 0 ppm or more and 300 ppm or less, preferably 0 ppm or more and 250 ppm or less, more preferably 0 ppm. It is 100 ppm or less. If the content of the sulfur component contained in the negative electrode active material in advance is too large, a film having high resistance is formed by the sulfur component. However, when Sx is within the above range, a good SEI film is formed on the negative electrode. Can be formed.
- the sulfur concentration in the first carbon can be measured by, for example, fluorescent X-ray analysis.
- the first carbon has a GM (sf) of 0 when the ratio coefficient (carboxyl group / phenolic hydroxyl group) of the carboxyl group and phenolic hydroxyl group of the surface functional group is GM (sf). 0.1 to 1.1, preferably 0.3 to 1.0. If GM (sf) is too large, the number of active points on the graphite surface will increase, which will cause excessive side reactions with the electrolyte and gas generation. On the other hand, if GM (sf) is too small, the active sites may become too small and the effect of the additive in the charge liquid may be diminished.
- the measurement of the surface functional group of 1st carbon can be implemented by the following neutralization titration methods, for example. 10 g of the sample is weighed, and 30 ml of 0.05 mol / L aqueous solution of NaOH (sodium hydroxide), Na 2 CO 3 (sodium carbonate) and NaHCO 3 (sodium bicarbonate) is added to the sample bottle. And it stirs in the glove box of nitrogen atmosphere, and it leaves still at room temperature, a sample is settled, and neutralization titration is performed for the supernatant with 0.05 mol / L hydrochloric acid. For titration, an automatic titrator AT-410WIN can be used.
- the total acidic functional group amount corresponds to the consumption amount of sodium hydroxide
- the strongly acidic carboxy amount corresponds to the amount of sodium bicarbonate
- the phenolic hydroxyl group corresponds to the amount obtained by subtracting the consumption amount of sodium carbonate from sodium hydroxide.
- the first carbon in the negative electrode active material is natural graphite as long as it is a spherical graphite capable of occluding and releasing cations and having the above characteristics with respect to Sx and preferably GM (sf).
- spherical natural graphite is preferable.
- spherical graphite may be used individually by 1 type, and may use multiple types together. Spherical natural graphite can be produced in large quantities at a low price, and therefore, its industrial utilization is particularly high with the spread and expansion of large-sized lithium battery applications.
- the spherical graphite as the first carbon may be coated with amorphous carbon or the like, or may be uncoated.
- a negative electrode active material having a surface coated with amorphous carbon such as natural graphite
- the reactivity with the electrolytic solution is excellent, while sulfur contained in the carbon source used for the surface coating.
- the SEI film having a high charge transfer resistance may be formed depending on the amount and surface functional groups.
- Natural graphite or the like whose surface is coated with amorphous carbon may have many hydrophilic surface functional groups such as phenolic hydroxyl groups and carboxyl groups formed on the surface of amorphous carbon. In this case, when the additive in the electrolytic solution undergoes reductive decomposition, the moisture adsorbed on the hydrophilic group will decompose the additive, which may make it difficult to form a good SEI film.
- the core material of natural graphite particles is very soft, whereas amorphous carbon Since the surface layer of this coating layer is hard, when the particles are deformed by pressing, the amorphous carbon coating layer and the natural graphite particle layer may be crushed unevenly. In this case, the SEI film is thick and non-uniform so that a partially high-resistance film is formed, resulting in capacity degradation. Therefore, in this embodiment, it may be more preferable to use spherical natural graphite whose surface is not coated with amorphous carbon as the negative electrode active material.
- the physical properties of the first carbon graphite are as follows: the acid treatment in the impurity removal process of the raw natural graphite, the spheroidization treatment from the flake graphite, the subsequent carbonization treatment, etc. Depends on. Therefore, the spherical graphite which is the first carbon of this embodiment can be manufactured by changing the production conditions of the spherical graphite.
- the content of the first carbon in the total mass of the negative electrode mixture is preferably 50% by mass or more, 55 More preferably, it is more preferably not less than 60% by mass, still more preferably not less than 60% by mass, preferably not more than 90% by mass, more preferably not more than 87% by mass, and further preferably not more than 85% by mass.
- the second carbon contained in the negative electrode active material is massive graphite.
- the second carbon contained in the negative electrode active material may be massive non-graphitizable carbon and / or massive artificial graphite. In the massive non-graphitizable carbon or the massive artificial graphite, no reason is observed and it has a homogeneous shape.
- the shape of the second carbon contained in the negative electrode active material can also be confirmed by SEM (scanning microscope) observation.
- SEM scanning microscope
- the short axis direction the length in the shortest direction
- the long axis direction the length in the longest direction
- the (minor axis) / (major axis) of the negative electrode active material is preferably 0.3 or more, more preferably 0.5 or more.
- the negative electrode contains the massive second carbon
- voids are formed between the particles of the first carbon
- the conductive additive is dispersed there
- the electrolyte is evenly distributed
- the electrolyte contains the additive
- a good SEI film is formed by the additive.
- non-graphitizable carbon and artificial graphite used as the second carbon are harder than natural graphite, when natural graphite is used as the first carbon, deformation of the first carbon during electrode pressing is prevented. can do.
- the sulfur concentration (Sy) in the second carbon is 0 ppm or more and 300 ppm or less, preferably 10 ppm or more and 250 ppm or less, more preferably 10 ppm or more and 200 ppm. It is as follows. When Sy is within the above range, a good SEI film can be formed on the negative electrode.
- the second carbon of the negative electrode active material is GV (sf), where GV (sf) is the ratio coefficient (carboxyl group / phenolic hydroxyl group) of the carboxyl group and phenolic hydroxyl group of the surface functional group. May be 0.1 to 1.1. Preferably, it is 0.1 to 0.5. If GV (sf) is too large, the number of active points on the graphite surface will increase, causing excessive side reactions with the electrolyte and gas generation. On the other hand, if GV (sf) is too small, the active sites may become too small, and the effect of the additive in the electrolytic solution may be diminished.
- the sulfur concentration in the second carbon, and the ratio coefficient between the carboxyl group and phenolic hydroxyl group of the surface functional group of the second carbon are the same methods as in the case of the first carbon. Can be measured.
- one kind of massive non-graphitizable carbon and massive artificial graphite may be used alone, or a plurality of kinds may be used in combination.
- the content of the second carbon in the total mass of the negative electrode mixture is preferably 5% by mass or more, and more preferably 8% by mass or more. Preferably, 10 mass% or more is more preferable, 50 mass% or less is preferable, 40 mass% or less is more preferable, and 30 mass% or less is further more preferable. If the second carbon content is too small, the number of second carbons inserted between the first carbons is insufficient, and a sufficient gap for inserting plate-like graphite is created as described later. It becomes difficult. For this reason, a sufficient conductive network cannot be formed, and the effect of improving conductivity may not be sufficiently exhibited.
- the second carbon when used as the secondary material, if the content of the second carbon is too small, the deformation preventing effect of the main material during electrode pressing may be reduced. On the other hand, if the content of the second carbon is too large, the second carbon enters more than necessary between the negative electrode active materials of the first carbon, so the probability that the particles of the second carbon overlap each other increases. It may be difficult to form a uniform SEI film on the carbon.
- Examples of the method for producing the second carbon lump-like hardly graphitized carbon include a method of firing at 1600 to 2600 ° C. using petroleum pitch, phenol resin or the like by-produced during crude oil decomposition.
- a method for producing the second carbon-like artificial graphite for example, when coal pitch is used as a raw material, a method of mixing with petroleum pitch or coal tar pitch and firing at 2000 to 3000 ° C. can be mentioned. It is done.
- the first carbon has a larger content (mass) than the second carbon, that is, the first carbon is the main material, and the second carbon Carbon is preferably a secondary material.
- a negative electrode contains the graphite which has plate shape as a conductive support material.
- the shape of the conductive additive can also be confirmed by SEM (scanning microscope) observation. In the SEM image of the conductive additive, if the ratio (minor axis; c-axis direction length) / (major axis; a-axis direction length) is 0.2 or less, it can be determined as a plate-like shape. it can.
- the (minor axis; length in the c-axis direction) / (long axis; length in the a-axis direction) of the conductive additive is preferably 0.1 or less, more preferably 0.05 or less. .
- the sulfur concentration (Sz) in the conductive additive is preferably 0 ppm or more and 300 ppm or less, more preferably 10 ppm or more and 250 ppm or less.
- Sz is within the above range, a good SEI film can be formed on the negative electrode.
- the conductive auxiliary material of the negative electrode active material has GA (sf) of GA (sf) when the ratio coefficient (carboxyl group / phenolic hydroxyl group) of the carboxyl group and phenolic hydroxyl group of the surface functional group is GA (sf). , Preferably 0.1 to 1.1, more preferably 0.5 to 1.1. If GA (sf) is too large, the number of active points on the graphite surface will increase, causing excessive side reactions with the electrolyte and gas generation. On the other hand, if GA (sf) is too small, the active sites may become too small, and the effect of the additive in the charge liquid may be diminished.
- the sulfur concentration in the conductive additive and the ratio coefficient (carboxyl group / phenolic hydroxyl group) of the carboxyl group and phenolic hydroxyl group of the surface functional group of the conductive additive were measured by the same method as in the case of the first carbon. can do.
- the plate-like graphite conductive additive 5 has a part of its edge surface in contact with the surface of the negative electrode active material 2 of spherical graphite, the surface of the non-graphitizable carbon 3 or the artificial graphite 4. Preferably it is.
- the electrical resistance in the a-axis direction of the graphite structure is c-axis direction.
- the conductivity is particularly excellent. Therefore, as shown in FIG. 1, the resistance of the negative electrode can be most effectively reduced by bringing both end surfaces (edge surfaces) of the plate-like graphite conductive additive into contact with the surface of the negative electrode active material.
- the SEI film at the contact portion is not easily broken during the charge / discharge cycle. This is because, when the additive is reduced and decomposed, a good quality SEI film is also formed on the surface of the negative electrode active material through the contact portion of the negative electrode active material or the auxiliary material of plate-like graphite, so that a strong bond can be achieved. Conceivable. Thus, when the SEI film having a low resistance is generated satisfactorily, it is maintained even after repeated charge and discharge, so that the life characteristics of the lithium ion battery can be greatly improved.
- the content of the plate-like graphite conductive additive in the total mass of the negative electrode mixture is 2.0% by mass or more and 10% by mass.
- the following is preferable. If the content of the plate-like graphite conductive aid is less than 2.0% by mass, the number of conductive aids inserted between the main material and the secondary material is insufficient, and a sufficient conductive network cannot be formed, In some cases, the conductivity improvement effect cannot be fully exhibited. On the other hand, if there is too much conductive aid, the conductive aid will enter more than necessary between the main material and the secondary material, creating a gap, which may increase the initial cell thickness of the lithium ion battery. is there.
- the increase in cell thickness due to the addition of a conductive additive is designed to be 10% or less. Is considered necessary. Therefore, by increasing the content to 10% by mass or less, the cell thickness increase rate may be 10% or less.
- the thickness of the plate-like graphite conductive additive is preferably 0.01 ⁇ m or more, more preferably 0.05 ⁇ m or more, and preferably 0.5 ⁇ m or less. If the thickness of the plate-like graphite conductive aid is larger than 0.5 ⁇ m, the side surface reaction with the electrolyte increases due to an increase in the graphite edge surface of the plate-like graphite conductive aid. In some cases, the gaps that increase permeability are blocked. On the other hand, if the thickness of the plate-like graphite conductive additive is less than 0.01 ⁇ m, the mechanical strength against pressing during electrode production may not be maintained.
- the thickness of the plate-like graphite conductive additive can be calculated as the average thickness of 100 plate-like graphite conductive aids obtained by SEM (scanning microscope) observation.
- the specific surface area of the plate-like graphite conductive additive is preferably 8 m 2 / g or more and 40 m 2 / g or less. If the specific surface area exceeds 40 m 2 / g, side reactions with the electrolyte increase and gas is generated, which may deteriorate the life characteristics of the battery. On the other hand, when the specific surface area is less than 8 m 2 / g, the particle size of the plate-like graphite conductive aid becomes large, and the gap between the negative electrode active materials may not be brought into contact efficiently.
- the specific surface area of the conductive auxiliary material according to the related technology is, for example, 800 m 2 / g to 1300 m 2 / g for ketjen black, which is an order of magnitude larger than the plate-like graphite used in this embodiment, and is acetylene black, carbon black. Is in the range of 50 m 2 / g to 100 m 2 / g. For this reason, generally the specific surface area of the conductive support material of related technology is 2 to 4 times larger than the specific surface area of the plate-shaped graphite conductive support material of this invention.
- the plate-like graphite conductive aid used in the present invention has a relatively small specific surface area, and its particle shape is plate-like, so that it has extremely good characteristics as a conductive aid.
- the specific surface area of the plate-like graphite conductive additive can be measured by the BET method.
- the negative electrode active material of graphite contributes to charging / discharging of the lithium ion secondary battery
- the specific surface area is preferably 0.5 m 2 / g or more and 8 m 2 / g or less, more preferably 0.5 m 2 / g or more and 8 m or less. Whereas it is less than 2 / g, the conductive additive of graphite improves the conductivity in the negative electrode, and the specific surface area is 8 m 2 / g or more.
- the plate-like graphite conductive additive is preferably artificial graphite having moderately developed crystallinity, but is not limited thereto. Even if it is natural graphite type, it is preferable if it has crystallinity equivalent to artificial graphite.
- the crystallinity of the surface of the conductive additive can be evaluated by Raman spectroscopy. As the Raman band of graphite, a G band corresponding to the in-plane vibration mode (around 1580 to 1600 cm ⁇ 1 ) and a D band derived from in-plane defects (around 1360 cm ⁇ 1 ) are observed. If these peak intensities are respectively IG and ID , it means that the higher the peak intensity ratio IG / ID , the higher the degree of graphitization.
- I G / ID is a value smaller than 2
- the film forming effect on the electrode surface by the additive may be deteriorated.
- I G / ID is larger than 10, reaction with the electrolytic solution may occur. And the life characteristics may deteriorate.
- plate-like graphite conductive additives depend on the firing temperature and the type and pressure of the atmosphere gas at the time of production, and various plate-like graphite conductive aids can be created by changing the production conditions.
- the raw material is natural graphite, it is an acid treatment in the impurity removal process and a step of pulverizing the flake graphite into a plate shape.
- artificial graphite when coal pitch is used as a raw material, a method of mixing with petroleum pitch or coal tar pitch, etc., crushing and adjusting the particle size, and firing at 2000 to 3000 ° C. can be mentioned.
- the first carbon that is spherical graphite, the second non-graphitizable carbon and / or the second carbon that is massive artificial graphite, and the plate-like graphite as a conductive additive are used.
- the shape of these particles is different, so that the massive second carbon and the plate-like conductive additive are dispersed in the gaps between the spherical graphite particles.
- an appropriate gap is maintained, the flow path of the electrolytic solution is secured, and the conductivity of the particles is secured.
- the spherical graphite as the first carbon, the first carbon and the second carbon, the first carbon and the current collector, and the electronic conductivity between the second carbon and the current collector are improved.
- the additive added to the electrolytic solution can uniformly react in the negative electrode, and a high-quality SEI film with low resistance can be formed. Therefore, the lithium ion battery of this embodiment can suppress an increase in electronic resistance and can greatly improve the life characteristics.
- the sulfur concentration (Sx) in the first carbon and the sulfur concentration (Sy) in the second carbon are each independently 0 ppm or more and 300 ppm or less, preferably the sulfur concentration (Sz) in the conductive additive. Is 0 ppm or more and 300 ppm or less, it is possible to form a thin, high-quality SEI film having a uniform resistance and a low resistance without forming a locally high-resistance film due to the sulfur component remaining on the electrode surface.
- the ratio coefficient (carboxyl group / phenolic hydroxyl group) between the carboxyl group and the phenolic hydroxyl group, which are surface functional groups of the first carbon, the second carbon, and the conductive additive is GM (sf), GV (sf).
- GA (sf), GM (sf) and GV (sf) are each independently 0.1 to 1.1, preferably GA (sf) is 0.1 to 1.1. Deterioration due to the reaction between the moisture of the additive and the surface functional group is less likely to occur, and a high-quality SEI film can be formed.
- the sulfur concentration in the carbon of the first carbon and the second carbon that is contained in the negative electrode active material is 100 ppm or less. It is more preferable that the sulfur concentration in any carbon of the second carbon is 100 ppm or less.
- the above Sx, Sy and Sz are Sx / Sy ⁇ 3, Sx / Sz ⁇ 3 and Sy / Sz ⁇ 3 It is more preferable that at least one of the relational expressions is satisfied. If these ratios are too large, locally high resistance SEI films may occur. Therefore, when Sx, Sy, and Sz satisfy the above relationship, it is possible to prevent the occurrence of a patchy uneven SEI film due to the additive to the electrolytic solution, and the charge transfer resistance can be further reduced.
- the average particle diameter D 50m of the first carbon is not particularly limited, but is preferably 5 to 80 ⁇ m, for example, and the average particle diameter D 50v of the second carbon is preferably 5 to 40 ⁇ m, for example.
- the average particle diameter D 50a of the conductive additive is preferably 2 to 20 ⁇ m, for example.
- the secondary particles are gasified by oxidation, and the conductive network of the secondary particles is divided by etching, and due to a sudden increase in resistance, charging cannot be performed and capacity is reduced.
- fine particles such as carbon black sometimes fill the gaps between the negative electrode active materials while agglomerating, which has been a factor that hinders the penetration of the electrolyte.
- the massive non-graphitizable carbon, the massive artificial graphite, and the platy graphite conductive additive that are the second carbon of the negative electrode active material have an appropriate average particle diameter. And it is excellent in the uniform dispersibility at the time of slurry preparation, and since a conductive network is rarely parted also during a charging / discharging cycle, a rapid increase in resistance and a decrease in capacity are suppressed. Moreover, as above-mentioned, a moderate clearance gap can be made between negative electrode active materials by using a plate-shaped graphite conductive support material.
- the electrolyte flow path is easily formed, and not only the movement of lithium ions is facilitated, but also functions as a liquid pool of the electrolyte, so that it is possible to suppress electrolyte depletion during the charge / discharge cycle, and rapid capacity deterioration Can be suppressed.
- the ratio of the average particle diameter D 50m of the first carbon of the negative electrode active material to the average particle diameter D 50v of the second carbon, D 50m / D 50v is preferably 1 or more and 8 or less, and is 1 or more and 6 or less. More preferably, it is 1.5 or more and 3 or less. When D 50m / D 50v is within this range, the characteristics of the lithium ion battery can be significantly improved.
- the average particle diameter D 50 m of the first carbon of the negative electrode active material, D 50m / D 50a is the average ratio of the particle diameter D 50a of the plate-like graphite conductive agent is preferably at 1 to 11, It is more preferably 1 or more and 10 or less, and further preferably 2 or more and 5 or less. When D 50m / D 50a is within this range, the characteristics of the lithium ion battery can be significantly improved.
- D 50m / D 50v When D 50m / D 50v is smaller than 1, that is, when the average particle diameter of the second carbon is relatively large, the gap between the first carbon and the second carbon becomes too narrow, so that the plate shape There is a tendency that the tendency of contacting the negative electrode active material between the edge surfaces of the conductive material becomes small. That is, the negative electrode active material particles tend to be in a state where they are separated on the upper and lower basal surfaces of the plate-like graphite conductive additive. In addition, when D 50m / D 50v is too large, that is, when the average particle diameter of the second carbon is relatively small, the gap between the first carbon and the second carbon becomes too wide, and the conductivity is increased.
- the auxiliary material comes into contact with the particles of the negative electrode active material on the basal surface.
- the plate-like graphite conductive aid is difficult to connect between the negative electrode active material particles at the edge surface, so that not only the effect of improving the conductivity is reduced, but also the insertion and absorption of lithium ions into the negative electrode active material particles. Disturbs release.
- a plate-like graphite conductive aid having a size comparable to the gap between the negative electrode active materials. 50 m 2 / D 50a is preferably within the above range. When the average particle size ratio is satisfied, the battery characteristics can be greatly improved as described above.
- the average particle diameter D 50m of the first carbon, the average particle diameter D 50v of the second carbon , and the average particle diameter D 50a of the plate-like graphite conductive aid detect the particle diameter from laser light scattering. It was determined from the particle size distribution measurement based on volume using a laser diffraction / scattering particle size / particle size distribution apparatus.
- the negative electrode may include a negative electrode active material other than the first carbon and the second carbon, and a conductive additive other than the plate-like graphite (for example, carbon black).
- a copper foil or the like can be used as the negative electrode current collector 7.
- the positive electrode active material 13 is not particularly limited as long as it absorbs cations at the time of discharge, and is a lithium / transition metal composite oxide, for example, lithium / cobalt composite oxide (LiCoO 2 , LiCoAlO 2 , LiCoMnO 2, etc.), lithium Nickel composite oxide (LiNiO 2 , LiNiCoO 2 , LiNiMnO 2 , LiNiCoMnO 2 etc.), lithium-manganese composite oxide (LiMnO 2 , LiMn 2 O 4 , LiMnMgO 4 , Li 2 MnO 3 etc.), olivine phosphate A metal oxide such as LiFePO 4 can be used. An aluminum foil or the like can be used as the positive electrode current collector 15.
- LiCoO 2 , LiCoAlO 2 , LiCoMnO 2, etc. lithium Nickel composite oxide (LiNiO 2 , LiNiCoO 2 , LiNiMnO 2 , LiNiCoMn
- the binder can be used in the layer containing the negative electrode active material in the negative electrode 1, and in some cases, can be used in the layer containing the positive electrode active material in the positive electrode 12.
- the binder in the negative electrode 1, is composed of particles of the first carbon 2 and the second carbon (3,4) of the negative electrode active material, the negative electrode active material and the conductive additive 5, and the negative electrode active material.
- the substance and the negative electrode current collector 7 are bonded.
- a polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), an acrylic polymer, etc. are suitable.
- NMP N-methyl-2pyrrolidone
- ion exchange water is mainly used as a solvent, and a thickener such as carboxymethyl cellulose (CMC) can be used in combination. If the amount of the binder is too small, sufficient adhesion strength (peeling strength) cannot be obtained. If the amount is too large, the binder inhibits free entry and exit of lithium ions, resulting in increased charge transfer resistance. Battery capacity is also reduced.
- the ratio of the binder to the negative electrode mixture is preferably 1% by mass to 10% by mass, and more preferably 2% by mass to 5% by mass.
- the solvent for the electrolytic solution 8 is selected from cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, ⁇ -lactones, cyclic ethers, chain ethers, and organic solvents of these fluorinated derivatives. Further, at least one organic solvent can be used.
- Cyclic carbonates propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and their derivative chain carbonates: dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), Dipropyl carbonate (DPC) and their derivatives
- DMC dimethyl carbonate
- DEC diethyl carbonate
- EMC ethyl methyl carbonate
- DPC Dipropyl carbonate
- Aliphatic carboxylic acid esters methyl formate, methyl acetate, ethyl propionate, and their derivatives ⁇ -lactones: ⁇ -butyrolactone
- cyclic ethers tetrahydrofuran , 2-methyltetrahydrofuran, and their derivative chain ethers: 1,2-diethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, and derivatives thereof
- DEE 1,2-diethoxye
- alkali metals such as Li, K, and Na, ClO 4 ⁇ , BF 4 ⁇ , PF 6 ⁇ , CF 3 SO 3 ⁇ , (CF 3 SO 2 ) 2 N ⁇ , (C A salt composed of an anion of a compound containing a halogen such as 2 F 5 SO 2 ) 2 N ⁇ , (CF 3 SO 2 ) 3 C ⁇ , or (C 2 F 5 SO 2 ) 3 C ⁇ can be dissolved.
- the solvent and electrolyte salt which consist of these basic solvents can also be used individually or in combination.
- a gel electrolyte in which an electrolytic solution is contained in a polymer gel may be used.
- the electrolytic solution preferably further contains an additive that undergoes reductive decomposition at a voltage lower than the reduction voltage of the solvent.
- This additive undergoes reductive decomposition prior to the solvent during charge and discharge to form a high-quality SEI film on the negative electrode surface. Further, even when charging / discharging is repeated, the SEI film can be stably maintained on the negative electrode surface.
- This SEI film has a role of suppressing the decomposition reaction of the electrolyte solution on the surface of the negative electrode, performing a desolvation reaction accompanying the insertion / desorption of the lithium ion battery, and suppressing physical structural deterioration of the negative electrode active material. .
- a cyclic sulfonate ester represented by the following general formula (1) having two sulfonyl groups.
- Q represents an oxygen atom, a methylene group or a single bond
- A represents a substituted or unsubstituted alkylene group having 1 to 5 carbon atoms, a carbonyl group, a sulfinyl group, a substituted or unsubstituted carbon.
- B is a substituted or unsubstituted alkylene group, substituted or unsubstituted A fluoroalkylene group or an oxygen atom.
- A is an alkylene group having 1 to 5 carbon atoms, from the viewpoints of stability of the compound, ease of synthesis of the compound, solubility in a solvent, cost, and the like.
- a 1 to 5 fluoroalkylene group and a divalent group having 2 to 6 carbon atoms in which an alkylene unit or a fluoroalkylene unit is bonded via an ether bond are preferred.
- B is preferably an alkylene group having 1 to 5 carbon atoms.
- a chain sulfonic acid ester represented by the following general formula (2) having two sulfonyl groups can also be used.
- X represents an alkylene group having 1 to 6 carbon atoms
- R represents an alkyl group having 1 to 6 carbon atoms
- two Rs may be the same group or different groups. good.
- ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate is used as a solvent
- vinylene carbonate (VC) or propane sultone (PS) is used as an additive for reductive decomposition at a voltage lower than the reduction voltage of the solvent.
- the content of the additive in the electrolytic solution is not particularly limited, but is preferably 0.5% by mass or more and 7% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less in the electrolytic solution. If it is less than 0.5% by mass, a sufficient effect may not be exhibited in forming a film by an electrochemical reaction on the electrode surface. If the content of the additive is too large, the viscosity of the electrolyte may be increased.
- the present invention will be described with reference to examples, but the present invention is not limited to the examples.
- the one with the larger content (content mass) is the “main material” and the one with the smaller content is the “substituent”.
- the main material is the “main material” and the one with the smaller content.
- Example 1 ⁇ Manufacture of positive electrode> 92 parts by mass of manganese spinel (LiMn 2 O 4 ) powder having an average particle size of 10 ⁇ m as a positive electrode active material, 4 parts by mass of a binder, 4 parts by mass of carbon black as a conductive additive, and uniformly dispersed in NMP A slurry for the positive electrode was prepared. PVDF was used as the binder.
- carbon black is used as the conductive additive for the positive electrode.
- the positive electrode has almost no volume expansion or contraction due to repeated charge and discharge as in the negative electrode, and the potential is different. Moreover, it is because there is no loss
- the positive electrode was produced by uniformly applying a slurry for a positive electrode to a 20 ⁇ m thick aluminum foil as a positive electrode current collector using a coater, and then evaporating NMP. After drying one side, a positive electrode was produced on the back side in the same manner as a double-sided coated electrode. After drying, the positive electrode density was adjusted with a roll press. The amount of positive electrode mixture per unit area was 48 mg / cm 2 .
- PVDF was used as the binder.
- the negative electrode was prepared by uniformly applying a negative electrode slurry on a copper foil having a thickness of 10 ⁇ m as a negative electrode current collector using a coater, and then evaporating NMP. After drying, the negative electrode density was adjusted with a roll press. The amount of the negative electrode mixture after drying was 10 mg / cm 2 .
- the composition in the negative electrode mixture was 10% by mass of massive non-graphitizable carbon A, 3% by mass of plate-like graphite a, 6% by mass of PVDF, and 81% by mass of spherical natural graphite A.
- spherical natural graphite A having a sulfur concentration of 0 ppm and a ratio coefficient GM of carboxyl groups and phenolic hydroxyl groups of acidic surface functional groups of 0.5 was used.
- the sulfur concentration of the main material, the secondary material, and the conductive material was measured using a fluorescent X-ray analysis method (ZSX Primus II manufactured by Rigaku Corporation).
- D 50m / D 50v which is the ratio of the average particle diameter D 50m of the spherical natural graphite A and the average particle diameter D 50v of the massive non-graphitizable carbon A, is 1.8, and the average particle diameter of the spherical natural graphite A is 1.8
- D 50 m which is the ratio of the average particle diameter D 50a of the plate-like graphite a (conductive additive)
- D 50m / D 50a was 6.7.
- D 50m, D 50v, D 50a was measured by a laser diffraction particle size distribution analyzer.
- a medium-sized laminate cell battery was produced.
- a method for producing a medium-sized laminate cell will be described below.
- the positive electrode described above was cut into 8.0 cm ⁇ 4.8 cm, and the negative electrode was cut into 9.0 cm ⁇ 5.6 cm. Among these, 8.0 cm ⁇ 1.0 cm on one side of the positive electrode and 9.0 cm ⁇ 1.0 cm on one side of the negative electrode were left as uncoated portions for connecting the tabs.
- An aluminum positive electrode tab having a width of 7 mm, a length of 12 cm, and a thickness of 0.1 mm was welded to the uncoated portion of the positive electrode.
- a negative electrode tab made of nickel having the same shape was welded to the negative electrode uncoated portion.
- the separator used was 10 cm ⁇ 7.0 cm polypropylene. This separator covered both surfaces of the positive electrode, and further, the negative electrode was disposed so as to face the positive electrode from both surfaces, thereby producing an electrode laminate.
- the electrode laminate is sandwiched between two aluminum laminate films of 16 cm ⁇ 10 cm, three sides excluding one side of the long side are heat sealed with a width of 8 mm, the electrolyte is injected, and the remaining one side is heated.
- the battery was sealed to produce a medium-sized laminate cell battery.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 30 ppm of 3% by mass of plate-like graphite a and 6% by mass of PVDF were used.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 3% by mass of plate-like graphite a having a ratio coefficient GA 0.6, and 6% by mass of PVDF.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 1 mol / L LiPF 6 was dissolved as an electrolyte, and 5.0 mass% of a cyclic disulfonic acid ester of the formula (4) was mixed as an additive.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 1 mol / L LiPF 6 was dissolved as an electrolyte, and 2.0 mass% of vinylene carbonate (VC) was mixed as an additive.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 1 mol / L LiPF 6 was dissolved as an electrolyte, and 2.0 mass% of propane sultone (PS) was mixed as an additive.
- PS propane sultone
- 1 mol / L LiPF 6 was dissolved as an electrolyte, and as an additive, 1% by mass of the compound of the formula (3) and 1.0% by mass of the cyclic disulfonic acid ester of the formula (4) were mixed.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 1 mol / L LiPF 6 is dissolved as an electrolyte in the electrolytic solution, and 1% by mass of the compound of the formula (3) and 1.0% by mass of the cyclic disulfonic acid ester of the formula (4) are mixed as additives.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 1 mol / L LiPF 6 was dissolved as an electrolyte, and as an additive, 1% by mass of the compound of the formula (3) and 1.0% by mass of the cyclic disulfonic acid ester of the formula (4) were mixed.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- the sulfur concentration is 330 ppm
- GV 1.2 of the carboxyl group and the phenolic hydroxyl group
- sulfur concentration as a conductive additive.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- the material was additive-free, a sulfur concentration of 30 ppm as a conductive additive, 3% by weight of plate-like graphite a having a ratio coefficient GA of carboxyl group to phenolic hydroxyl group of 0.6, and 6% by weight of PVDF.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- 1 mol / L LiPF 6 was dissolved as an electrolyte, and 4.0% by mass of the cyclic disulfonic acid ester of the formula (4) was mixed.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- a battery was fabricated in the same manner as in Example 1 except for the above.
- the life test was carried out by repeating charging and discharging in a thermostatic chamber. Specifically, the lithium ion batteries produced in the above Examples, Reference Examples and Comparative Examples are charged to a maximum voltage of 4.2 V under a constant current charging condition of 1 C, and then a constant voltage charging at 4.2 V is performed. The battery was charged for a total charging time of 2.5 hours. The discharge was performed at a constant current of 1C up to 2.5V. This charging / discharging was made into 1 cycle and repeated 500 cycles. The ratio of the discharge capacity after 500 cycles to the initial discharge capacity was defined as the capacity retention rate.
- the temperature of the thermostatic chamber was set to a temperature higher than the room temperature of 45 ° C. because the deterioration of the temperature was accelerated and the life characteristics could be determined at an early stage. Further, the battery was disassembled after 500 cycles at 45 ° C., and the negative electrode surface was observed with an SEM (scanning electron microscope) and a stereomicroscope.
- Table 1 shows the main physical properties of non-graphitizable carbon A and artificial graphite A used as the second carbon (secondary material) in the negative electrode of each lithium ion battery.
- Table 2 shows the characteristics of the plate-like graphite a used as a conductive additive,
- Tables 3 to 5 show the structure of the negative electrode of each lithium ion battery, measurement results of capacity retention after 500 cycles, and observation of the negative electrode surface after battery decomposition. Results are shown.
- the lithium ion battery produced in the example could be charged and discharged without significant capacity deterioration even in a high temperature environment of 45 ° C.
- a high-quality SEI film was formed by adding an additive that reduces and decomposes at a voltage lower than the reduction voltage of the solvent to the electrolytic solution.
- any one of the compound represented by the formula (3), the compound represented by the formula (4), VC, and PS is used alone as an additive for the electrolytic solution. It was shown that high temperature cycle characteristics at 45 ° C. can be obtained even when combined.
- Reference Examples 3 and 4 are lithium ion batteries using a negative electrode active material in which the GM (sf) of the first carbon (spherical natural graphite) is not within the range of 0.1 to 1.1. The capacity degradation was great.
- FIG. 2 shows the dQ / dV values with respect to the voltage on the horizontal axis for Examples 1 and 9 and Reference Examples 3 and 4.
- the reaction rate of the reductive decomposition reaction of the additive to the electrolytic solution can be estimated from the capacity change curve at 2.2 to 2.8 V of the lithium ion battery. That is, it can be obtained from the dQ / dV value shown in FIG. 2 and the area surrounded by the horizontal axis voltage (2.2 to 2.8 V).
- the decomposition of the additive in the electrolytic solution is promoted, and the surface of the main material, secondary material, and conductive additive of the negative electrode active material is strong. It was confirmed that a film was formed, and as a result, excellent cycle characteristics at high temperatures were obtained. In addition, it was also confirmed that more excellent cycle characteristics were obtained by using a cyclic sulfonate ester or a chain sulfonate ester having two sulfonyl groups as an additive. Such excellent cycle characteristics at high temperatures are extremely important effects for lithium ion batteries for in-vehicle use and power storage systems that are supposed to be used in a high temperature environment near the equator.
- Negative electrode 2 First carbon (spherical graphite) 3 Second carbon A (bulk non-graphitizable carbon) 4 Second carbon B (bulk artificial graphite) 5 Conductive aid (plate-like graphite) 6 SEI 7 Negative Electrode Current Collector 8 Electrolyte 9 Additive 11 Separator 12 Positive Electrode 13 Positive Electrode Active Material 14 Carbon Black 15 Positive Electrode Current Collector
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Abstract
Description
前記第一の炭素が球状の黒鉛であり、
前記第二の炭素が塊状の黒鉛であり、
前記第一の炭素中の硫黄濃度(Sx)および前記第二の炭素中の硫黄濃度(Sy)が、それぞれ独立に、0ppm以上300ppm以下であることを特徴とするリチウムイオン二次電池用負極、それを含むリチウムイオン二次電池、およびこれらの製造方法に関する。
本実施形態において、負極は、第一の炭素と第二の炭素とを含む負極活物質を含み、
前記第一の炭素が球状の黒鉛であり、
前記第二の炭素が塊状の黒鉛であり、
前記第一の炭素中の硫黄濃度(Sx)および前記第二の炭素中の硫黄濃度(Sy)が、それぞれ独立に、0ppm以上300ppm以下である。
前記第一の炭素が、球状の黒鉛であり、
前記第二の炭素が塊状の難黒鉛化炭素および/または塊状の人造黒鉛であり、
前記導電助材が、板状の黒鉛であり、
前記第一の炭素中の硫黄濃度(Sx)および前記第二の炭素中の硫黄濃度(Sy)が、それぞれ独立に、0ppm以上300ppm以下である。
前記第二の炭素の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)をGV(sf)としたとき、GV(sf)が0.1~1.1であっても良い。
本実施形態において、負極活物質は、第一の炭素と第二の炭素とを含む負極活物質を含み、前記第一の炭素が球状の黒鉛であり、前記第二の炭素が塊状の黒鉛である。
本実施形態において、負極は、導電助材として板状の形状を有する黒鉛を含むことが好ましい。導電助材の形状もSEM(走査型顕微鏡)観察により確認することができる。導電助材のSEM画像において、その比(短軸;c軸方向の長さ)/(長軸;a軸方向の長さ)が0.2以下の場合は板状の形状と判断することができる。なお、導電助材の(短軸;c軸方向の長さ)/(長軸;a軸方向の長さ)は、好ましくは0.1以下、より好ましくは0.05以下であるのが良い。
Sx/Sy<3、
Sx/Sz<3、および
Sy/Sz<3
の関係式のうち、少なくとも1つを満たすことがより好ましい。これらの比が大きすぎると、局部的に抵抗の高いSEI皮膜が生じてしまうことがある。よって、Sx、SyおよびSzが上記関係を満たすことにより、電解液への添加剤による斑状の不均一なSEI皮膜が生じることを防ぐことができ、電荷移動抵抗をより小さくすることができる。
正極活物質13としては、放電時にカチオンを吸収するものであれば特に限定されず、リチウム・遷移金属複合酸化物、例えばリチウム・コバルト複合酸化物(LiCoO2、LiCoAlO2,LiCoMnO2等)、リチウム・ニッケル複合酸化物(LiNiO2、LiNiCoO2,LiNiMnO2、LiNiCoMnO2等)、リチウム・マンガン複合酸化物(LiMnO2、LiMn2O4、LiMnMgO4、Li2MnO3等)、オリビン型リン酸塩(LiFePO4等)の金属酸化物を使用できる。正極集電体15としては、アルミニウム箔などを使用できる。
結着剤は負極1中の負極活物質を含有する層中に使用し、場合によっては正極12中の正極活物質を含有する層中に使用することができる。例えば、図1において、負極1では結着剤は、負極活物質の第一の炭素2および第二の炭素(3,4)の各粒子どうし、負極活物質と導電助材5、さらに負極活物質と負極集電体7とを接着させている。結着剤としては、特に限定はされないが、ポリフッ化ビニリデン(PVDF)やスチレンブタジエンゴム(SBR)、アクリル系ポリマーなどが好適である。有機系の結着剤を用いる場合は、溶媒としてN-メチル-2ピロリドン(NMP)が最適である。
電解液8用の溶媒としては、環状カーボネート類、鎖状カーボネート類、脂肪族カルボン酸エステル類、γ-ラクトン類、環状エーテル類、鎖状エーテル類およびこれらのフッ化誘導体の有機溶媒から選ばれた少なくとも1種類の有機溶媒を用いることができる。
環状カーボネート類:プロピレンカーボネート(PC)、エチレンカーボネート(EC)、ブチレンカーボネート(BC)、およびこれらの誘導体
鎖状カーボネート類:ジメチルカーボネート(DMC)、ジエチルカーボネート(DEC)、エチルメチルカーボネート(EMC)、ジプロピルカーボネート(DPC)、およびこれらの誘導体
脂肪族カルボン酸エステル類:ギ酸メチル、酢酸メチル、プロピオン酸エチル、およびこれらの誘導体
γ-ラクトン類:γ-ブチロラクトン、およびこれらの誘導体
環状エーテル類:テトラヒドロフラン、2-メチルテトラヒドロフラン、およびこれらの誘導体
鎖状エーテル類:1,2-ジエトキシエタン(DEE)、エトキシメトキシエタン(EME)、ジエチルエーテル、およびこれらの誘導体
その他:ジメチルスルホキシド、1,3-ジオキソラン、ホルムアミド、アセトアミド、ジメチルホルムアミド、アセトニトリル、プロピオニトリル、ニトロメタン、エチルモノグライム、リン酸トリエステル、トリメトキシメタン、ジオキソラン誘導体、メチルスルホラン、1,3-ジメチル-2-イミダゾリジノン、3-メチル-2-オキサゾリジノン、アニソール、N-メチルピロリドン、フッ素化カルボン酸エステル
これらを一種又は二種以上を混合して使用することができる。
<正極の製造>
正極活物質として平均粒子径10μmのマンガンスピネル(LiMn2O4)粉末を92質量部、結着剤を4質量部、導電助材としてカーボンブラックを4質量部、NMP中に均一に分散させて、正極用のスラリーを作製した。結着剤としては、PVDFを使用した。
NMP中に、副材として、10質量部の塊状の難黒鉛化炭素A[平均粒径(体積基準)D50v=11μm、比表面積=5.5m2/g、ラマン分光法によるG/D比(IG/ID)=1.0]、導電助材として、3質量部の板状の黒鉛a[平均粒径(体積基準)D50a=3μm、平均板厚=0.1μm、比表面積=15m2/g、ラマン分光法によるG/D比(IG/ID)=2.8]、6質量部の結着剤、主材として81質量部の球状の天然黒鉛A(平均粒径D50m=20μm)を添加して、負極用のスラリーを作製した。結着剤としては、PVDFを使用した。負極は、負極集電体として厚さ10μmの銅箔にコーターを用いて均一に負極用のスラリーを塗布し、その後、NMPを蒸発させることによって作製した。乾燥後、ロールプレスにて負極密度を調整した。乾燥後の負極合剤量は、10mg/cm2とした。負極合剤中の組成は、10質量%の塊状の難黒鉛化炭素A、3質量%の板状の黒鉛a、6質量%のPVDF、及び81質量%の球状の天然黒鉛Aとなった。主材として、硫黄濃度は0ppm、酸性表面官能基のカルボキシル基とフェノール系水酸基の比率係数GMは0.5の球状天然黒鉛Aを用いた。副材として、硫黄濃度は30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の難黒鉛化炭素Aを用いた。導電助材として硫黄濃度が、30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の板状黒鉛aを用いた。主材、副材、導電材の硫黄濃度は、蛍光X線分析法(理学電機工業製ZSX PrimusII)を用いて測定した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が160ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛C(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が230ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛D(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B(平均粒径D50m=20μm)、副材として硫黄濃度を100ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.3の10質量%の塊状の難黒鉛化炭素B(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B、副材として硫黄濃度を200ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.6の10質量%の塊状の難黒鉛化炭素C(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B(平均粒径D50m=20μm)、副材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が100ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.7の3質量%の板状の黒鉛b、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B(平均粒径D50m=20μm)、副材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が200ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.8の3質量%の板状の黒鉛c、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.9の81質量%の球状の天然黒鉛E(平均粒径D50m=20μm)、副材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppmの3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の71質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の20質量%の塊状の難黒鉛化炭素A、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の61質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の30質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDF、とした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の61質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の30質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の8質量%の板状の黒鉛a、6質量%のPVDF、とした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の78質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛aと、さらに導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=1.2の3質量%のカーボンブラック、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。なお、表4中、Sz、GAおよび導電助材添加量は、導電助材のうち、板状黒鉛aについて記載した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の76質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の8質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(4)の環状ジスルホン酸エステルを5.0質量%、混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(3)の化合物を1質量%、式(4)の環状ジスルホン酸エステルを1.0質量%、混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、ビニレンカーボネート(VC)を2.0質量%混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、プロパンスルトン(PS)を2.0質量%混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(4)の環状ジスルホン酸エステルを1.0質量%とプロパンスルトン(PS)を1.0質量%混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(4)の環状ジスルホン酸エステルを1.0質量%とビニレンカーボネート(VC)を1.0質量%、混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.3の10質量%の塊状の人造黒鉛A(平均粒径D50v=13μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(3)の化合物を1質量%、式(4)の環状ジスルホン酸エステルを1.0質量%混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の71質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.3の20質量%の塊状の人造黒鉛A(平均粒径D50v=13μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(3)の化合物を1質量%、式(4)の環状ジスルホン酸エステルを1.0質量%、混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の人造黒鉛A(平均粒径D50m=23μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.3の10質量%の塊状の人造黒鉛A(平均粒径D50v=13μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状黒鉛a、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、式(3)の化合物を1質量%、式(4)の環状ジスルホン酸エステルを1.0質量%混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の69質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の15質量%の板状の黒鉛a、6質量%のPVDF、とした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が350ppm、カルボキシル基とフェノール系水酸基の比率係数GA=1.2の3質量%の板状の黒鉛d、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が320ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛F(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛B(平均粒径D50m=20μm)、副材として硫黄濃度が330ppm、カルボキシル基とフェノール系水酸基の比率係数GV=1.2の10質量%の塊状の難黒鉛化炭素D(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.09の81質量%の球状の天然黒鉛G(平均粒径D50m=20μm)、副材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=1.2の81質量%の球状の天然黒鉛H(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の91質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材は無添加、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の84質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材は無添加、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=1.2の3質量%のカーボンブラック、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、式(4)の環状ジスルホン酸エステルを4.0質量%、混合したものを用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の81質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材として硫黄濃度を30ppm、カルボキシル基とフェノール系水酸基の比率係数GA=0.6の3質量%の板状の黒鉛a、6質量%のPVDF、電解液の添加剤は無添加とした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の84質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材は無添加、6質量%のPVDFとした。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の84質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GV=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材は無添加とし、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、ビニレンカーボネート(VC)を2.0質量%を用いた。これ以外は、実施例1と同様にして電池を作製した。
負極合剤の組成を、主材として硫黄濃度が0ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.5の84質量%の球状の天然黒鉛A(平均粒径D50m=20μm)、副材として硫黄濃度が30ppm、カルボキシル基とフェノール系水酸基の比率係数GM=0.2の10質量%の塊状の難黒鉛化炭素A(平均粒径D50v=11μm)、導電助材は無添加、6質量%のPVDFとした。電解液中、電解質として1mol/LのLiPF6を溶解し、添加剤として、プロパンスルトン(PS)を2.0質量%用いた。これ以外は、実施例1と同様にして電池を作製した。
SEM(走査型顕微鏡)観察により、実施例1~23、参考例1~11、および比較例1~2の負極活物質の主材(第一の炭素)が球状の形状を有する点、副材(第二の炭素)が塊状の形状を有する点、および導電助材が板状の形状を有する点を確認した。さらに、実施例1~23および参考例1において、負極活物質の主材および副材の表面に導電助材のエッジ面の一部が接する点を確認した。同様に、比較例1、2、参考例3~5および8においても、負極活物質の主材および/または副材の表面に導電助材のエッジ面の一部が接する点を確認した。
寿命試験は、恒温槽内にて充放電を繰り返すことによって実施した。具体的には、上記各実施例、参考例および比較例で作製したリチウムイオン電池を1Cの定電流充電条件で上限電圧4.2Vまで充電し、続いて4.2Vでの定電圧充電を行い、総充電時間2.5時間の充電を行うものとした。放電は、1Cで定電流放電を2.5Vまで行うものとした。この充放電を1サイクルとし、500サイクル繰り返した。そして、500サイクル後の放電容量の初期放電容量に対する比を、容量維持率とした。なお、恒温槽の温度は、劣化が加速されるので、早期に寿命特性を見極めることができるため、45℃という常温よりもより高い温度に設定した。さらに、45℃、500サイクル後に電池を分解し、負極表面をSEM(走査型電子顕微鏡)と実体顕微鏡にて観察した。
Sx:第一の炭素中の硫黄濃度(ppm)
Sy:第二の炭素中の硫黄濃度(ppm)
Sz:導電助材中の硫黄濃度(ppm)
GM(sf):第一の炭素の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)
GV(sf):第二の炭素の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)
GA(sf):導電助材の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)
副材添加量:負極合剤全質量(負極活物質と導電助材と負極結着剤との合計質量)中の副材の割合(質量%)
導電助材添加量:負極合剤全質量(負極活物質と導電助材と負極結着剤との合計質量)中の導電助材の割合(質量%)
式(3):式(3)で表される化合物(添加剤)
式(4):式(4)で表される化合物(添加剤)
添加剤の含量:電解液の全質量中の添加剤の割合(質量%)
容量維持率:45℃、500サイクル後の容量維持率(%)
ラピッドフェード:サイクルの途中で急激に容量劣化したため、所定のサイクル数の測定ができなかった。
負極表面の状態の評価:
○:良好。負極表面の析出物が観察されなかった。
△:少量の析出物が観察された。
×:析出物が観察された。
参考例3および4は、第一の炭素(球状天然黒鉛)のGM(sf)が0.1~1.1の範囲内にない負極活物質を用いたリチウムイオン電池であり、実施例に比べて容量劣化が大きかった。図2に、実施例1および9、ならびに参考例3および4について、横軸の電圧に対するdQ/dV値を示す。ここで、電解液への添加剤の還元分解反応の反応率は、リチウムイオン電池の2.2~2.8Vにおける容量変化曲線から見積もることができる。すなわち、図2に示すdQ/dV値と横軸の電圧(2.2~2.8V)で囲まれた面積とから求めることができる。実施例1および9のリチウムイオン電池については2.3V付近(矢印参照)に還元反応を示すピークが観測された。これに対して、参考例4では、2.3V付近のピークは見られず、2.6~2.7V付近にピークが見られた。よって、詳しいメカニズムは不明ではあるが、負極活物質の第一の炭素の黒鉛の表面官能基係数GM(sf)と添加剤の還元反応には相関関係があることが示唆された。
2 第一の炭素(球状黒鉛)
3 第二の炭素A(塊状難黒鉛化炭素)
4 第二の炭素B(塊状人造黒鉛)
5 導電助材(板状黒鉛)
6 SEI
7 負極集電体
8 電解液
9 添加剤
11 セパレータ
12 正極
13 正極活物質
14 カーボンブラック
15 正極集電体
Claims (14)
- 第一の炭素と第二の炭素とを含む負極活物質を含み、
前記第一の炭素が球状の黒鉛であり、
前記第二の炭素が塊状の黒鉛であり、
前記第一の炭素中の硫黄濃度(Sx)および前記第二の炭素中の硫黄濃度(Sy)が、それぞれ独立に、0ppm以上300ppm以下であることを特徴とするリチウムイオン二次電池用負極。 - 導電助材と、結着剤とを更に含み、
前記第二の炭素が難黒鉛化炭素および/または人造黒鉛であり、
前記導電助材が板状の黒鉛であり、
前記第一の炭素の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)をGM(sf)としたとき、GM(sf)が0.1~1.1であり、かつ、
前記第二の炭素の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)をGV(sf)としたとき、GV(sf)が0.1~1.1であることを特徴とする請求項1に記載のリチウムイオン二次電池用負極。 - 前記導電助材中の硫黄濃度(Sz)が、0ppm以上300ppm以下であり、
前記導電助材の表面官能基であるカルボキシル基とフェノール系水酸基との比率係数(カルボキシル基/フェノール系水酸基)をGA(sf)としたとき、GA(sf)は、0.1~1.1であることを特徴とする請求項2に記載のリチウムイオン二次電池用負極。 - 前記導電助材の厚さは0.01μm以上、0.5μm以下であることを特徴とする請求項2または3に記載のリチウムイオン二次電池用負極。
- 前記第一の炭素と前記第二の炭素のうち、負極活物質中の含有質量が大きい方の炭素中の硫黄濃度が100ppm以下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。
- 前記第一の炭素中の硫黄濃度(Sx)が100ppm以下であ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。
- 前記第二の炭素中の硫黄濃度(Sy)が100ppm以下であることを特徴とする請求項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。
- 請求項1~7のいずれか一項に記載のリチウムイオン二次電池用負極と、
添加剤を含む非水電解液と、
リチウムを吸蔵、放出することが可能な正極活物質を含む正極と、を備えたリチウムイオン二次電池。 - 前記非水電解液は、添加剤として、スルホニル基を2個有する環状スルホン酸エステルを含有することを特徴とする請求項8に記載のリチウムイオン二次電池。
- 前記第一の炭素の平均粒子径D50mと、前記第二の炭素の平均粒子径D50vの比であるD50m/D50vは、1以上8以下であることを特徴とする請求項8~10のいずれか1項に記載のリチウムイオン電池。
- 前記第一の炭素の平均粒子径D50mと、前記導電助材である前記板状の黒鉛の平均粒子径D50aの比であるD50m/D50aは、1以上11以下であることを特徴とする請求項8~11のいずれか1項に記載のリチウムイオン二次電池。
- 前記負極に含まれる負極活物質と、導電助材と、結着剤との合計重量に対する第二の炭素の含有量は、5質量%以上、30質量%以下であることを特徴とする請求項8~12のいずれか1項に記載のリチウムイオン二次電池。
- 電極素子と電解液と外装体を有するリチウムイオン二次電池の製造方法であって、
第一の炭素と第二の炭素とを含む負極活物質を混合した負極合剤を用いて負極を製造する工程と、
正極と、負極と、を対向配置して電極素子を作製する工程と、
前記電極素子と、電解液と、を外装体の中に封入する工程と、
を含み、
前記第一の炭素が球状の黒鉛であり、
前記第二の炭素が塊状の黒鉛であり、
前記第一の炭素中の硫黄濃度(Sx)および前記第二の炭素中の硫黄濃度(Sy)が、それぞれ独立に、0ppm以上300ppm以下であることを特徴とする、リチウムイオン二次電池の製造方法。
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US20150349332A1 (en) | 2015-12-03 |
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