JP4399871B2 - Manufacturing method of non-aqueous secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous secondary battery in which a positive electrode and a negative electrode are composed of a simple substance or a compound capable of occluding and releasing lithium, and a polymer solid electrolyte or a polymer gel electrolyte is used as an electrolyte .
[0002]
[Prior art]
In recent years, with the progress of downsizing and higher functionality of portable devices, there is an increasing demand for smaller, lighter and higher energy batteries for the power source. In response to this demand, lithium secondary batteries have been commercialized. Among them, a secondary battery composed of carbon and an oxide of a transition element such as tin (Sn) or silicon (Si) as a negative electrode, a so-called lithium ion secondary battery is excellent in cycle performance and has a demand as a battery having a high energy density. It is growing.
[0003]
In the lithium ion secondary battery, the positive electrode active material is composed of lithium cobalt oxide (LiCoO ₂ ) or lithium manganate (LiMn ₂ O ₄ ). By charging, Li shifts from the positive electrode to the negative electrode and is occluded by the negative electrode. Conversely, Li shifts from the negative electrode to the positive electrode due to discharge. Generally, both the positive electrode active material and the negative electrode active material are powders. In a conventional battery, these active material powders are mixed with a binder such as polyvinylidene fluoride (PVDF), a polymer solid electrolyte such as polyethylene oxide (PEO) and a conductive agent such as carbon to form a paste. The paste was applied on aluminum foil and copper foil to form a positive electrode and a negative electrode.
[0004]
[Problems to be solved by the invention]
However, in the conventional battery, the capacity decreased with the progress of the charge / discharge cycle, and the charge / discharge cycle performance was not satisfactory. In particular, in the case of a prismatic or flat battery that is difficult to be subjected to tight pressure, the disadvantages are remarkable.
[0005]
The capacity decrease with the progress of this cycle is due to an increase in the impedance of the battery. That is, the positive and negative electrode active materials change the crystal lattice constant when lithium ions enter and exit during charge and discharge. Along with this, the active material powder repeats expansion and contraction. This is thought to be related to a decrease in the electron conductivity of the electrode and an increase in the impedance at the interface between the electrode and the electrolyte. Conventionally, various proposals have been made to improve the adhesion between the negative electrode carbon and the electrolyte. Part 1 relates to surface modification of negative electrode carbon. For example, the plasma treatment disclosed in JP-A-7-105938 and the corona discharge treatment disclosed in JP-A-7-183027. In addition, JP-A-8273659 proposes that Si be interposed on the surface of carbon particles.
[0006]
Part 2 relates to the improvement of the binder, and relates to the improvement of the material of the binder. For example, in JP-A 7-2 No. 201315 has been proposed to cross-link the polyvinylidene fluoride (PVDF). Part 3 relates to the improvement of the electrolyte composition, and many proposals have been made regarding the combination of solvents and additives.
[0007]
As described above, various proposals have been made for the purpose of suppressing the increase in internal impedance of the battery during the charge / discharge cycle. However, the effect is not sufficient, and further improvement has been demanded.
The present invention has been made in view of the drawbacks of the conventional battery described above, and in a non-aqueous secondary battery in which the positive electrode and the negative electrode are made of a material capable of occluding and releasing Li, the discharge characteristics at high load, It is intended to provide a battery with excellent charge / discharge cycle performance.
[0008]
[Means for Solving the Problems]
In the present invention, the positive electrode and the negative electrode are composed of a simple substance or a compound capable of occluding and releasing lithium, and a vinyl compound is graft-polymerized on the surface of at least the simple substance or the compound constituting the negative electrode. This is a method for producing a non-aqueous secondary battery in which graft polymerization of the vinyl compound and curing of the polymer solid electrolyte or polymer gel electrolyte are simultaneously performed using a gel electrolyte.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the battery according to the present invention, the vinyl compound graft-polymerized is disposed on at least the surface of the active material particles of the positive electrode and the negative electrode, and the electron conductivity of the electrode and the interface impedance between the electrode and the electrolyte are excellent when repeatedly charged and discharged. Maintained.
The battery according to the present invention is a nonaqueous secondary battery in which the positive electrode and the negative electrode are composed of a simple substance or a compound capable of occluding and releasing lithium, and a vinyl compound is graft-polymerized on at least the surface of the simple substance or the compound constituting the negative electrode. It is. The molecular weight of the monomer unit of the vinyl compound is 200 or less, more preferably 150 or less.
[0010]
Specific examples include the following. Methyl acrylate (MA), methyl methacrylate (MMA), glycidyl methacrylate (MAG), nitrile methacrylate (MAN), N vinylpyrrolidone (NVP), isoprene (IP), acrylonitrile (AN), allyl acetoacetate, allyl Examples thereof include benzene, allyl ethyl ether, allyl phenyl ether, allyl glycidyl ether, styrene (ST), vinyl pyridine (VP), and vinyl sulfonate (VSL). Furthermore, these copolymers may be used. These vinyl compounds having a molecular weight of 200, preferably 150 or less, have a low viscosity and an excellent affinity for the active material particles. In addition, since the molecular size is small, the density of the graft can be increased and a strong bond can be obtained. Further, these vinyl compounds have good affinity with PC and EC, which are main solvents for the electrolyte solution of non-aqueous secondary batteries and polymer gel electrolytes. Also, it has good affinity with polyethers such as PEO, which are polymers of polymer solid electrolytes, phosphoazenes, siloxane polymers, and the like. From this, the migration of lithium ions between the active material particles grafted with these vinyl compounds and the electrolyte proceeds rapidly, and good electrical characteristics as a battery can be obtained.
[0011]
It is more desirable to use monomers for grafting than to use prepolymerized polymers. By using the monomer, a dense film can be obtained on the surface.
[0012]
These vinyl compounds are firmly bonded by being grafted onto the surface of the active material particles. In addition, it has a good affinity with the electrolytic solution and has a property of taking the electrolytic solution into the inside. For this reason, the contact between the active material particles and the electrolytic solution can be favorably maintained. In particular, when the electrolyte is a solid electrolyte such as a polymer solid electrolyte (SPE), the quality of the battery greatly depends on the contact between the active material particles and the electrolyte solid. The vinyl compound has good affinity with a polymer solid electrolyte made of polyethylene oxide or the like, and can maintain good contact between the active material particles and the electrolyte as in the case of the electrolytic solution.
[0013]
According to the present invention, in the case of a polymer solid electrolyte, the above effect can be effectively achieved by simultaneously grafting the monomer onto the active material particle surface and curing the polymer constituting the polymer solid electrolyte. Played . This is thought to be due to the entanglement between the two polymer chains and chemical bonding, resulting in the formation of a strong contact. It is taken in selecting the ratio of the active material particles and monomer can be controlled generate the amount of coating.
[0014]
In addition, since the active material particles grafted with the vinyl compound on the surface are excellent in dispersibility in the mixture paste, a uniform paste can be obtained, and the paste is soft and excellent in coating properties. Moreover, the packing density of the active material particles is improved, and as a result, the discharge capacity is increased.
[0015]
【Example】
Hereinafter, the battery according to the present invention will be described with reference to examples.
[0016]
Figure 1 is a cross-sectional view of a battery showing an example of a nonaqueous secondary battery according to the embodiment. 1 is a positive electrode made of a mixture of an active material lithium cobalt oxide (LiCoO ₂ ), a conductive agent acetylene black (AB), a binder resin PVDF, and a polymer solid electrolysis, on a positive electrode current collector 3 made of aluminum (Al). It is coated. 2 is a negative electrode made of a mixture of carbon such as graphite or oxides of transition elements such as Si, Sn, Pb, and P, chalcogenite, a binder resin PVDF or a solid polymer electrolyte, and a negative current collector made of copper (Cu) It is coated on the body 4. 5 is a separator made of a microporous film of polyethylene (PE) or polypropylene (PP) or a polymer solid electrolyte film. 6 is composed of a Li salt such as LiPF ₆ or LiBF _{4 in} a mixed solvent such as propylene carbonate (PC), EC, dimethyl carbonate (DMC), or diethyl carbonate (DEC). The solid polymer electrolyte is composed of a polymer such as PEO and the lithium salt. Further, a gel electrolyte composed of a system containing a solvent such as PC or EC as a plasticizer or a polymer such as polyacrylonitrile and an electrolytic solution may be used.
[0017]
Of the positive electrode and the negative electrode, at least the active material particles of the negative electrode have the vinyl compound bonded to the surface by graft polymerization. For example, in order to graft MMA onto the surface of the carbon powder that is the negative electrode active material, MMA monomer is reacted in the presence of normal butyl lithium (nBuLi) in a hexane solvent. Alternatively, grafting is performed by irradiation with an electron beam or the like. When grafting on the surface of carbon particles, the carbon particles are heated in a weakly oxidizing atmosphere or treated with an oxidizing strong acid such as nitric acid to add functional groups such as carboxyl and carbonyl groups to the surface of the carbon particles. carry out. By this treatment, grafting can be performed densely and efficiently.
[0018]
Hereinafter, the battery according to the present invention will be described by taking as an example the case where the negative electrode constituent material is carbon particles and the graft layer on the surface of the carbon particles is PMMA. In FIG. 1, 1 is a positive electrode. The positive electrode 1 is composed of 90 parts by weight of an active material LiCoO ₂ having an average particle size of 10 μm, 5 parts by weight of a conductive material AB, and 5 parts by weight of PVDF. 3 is a positive electrode current collector made of Al mesh. The thickness of the positive electrode 1 is about 200 μm. Reference numeral 2 denotes a negative electrode, which consists of a mixture of 95 parts by weight of carbon particles having an average particle diameter of 10 μm and 5 parts by weight of PVDF. The thickness of the negative electrode 2 is about 200 μm. 4 is a negative electrode current collector made of Cu mesh. 5 is a polyethylene film separator having a thickness of 25 μm and an average pore diameter of about 0.5 μm. 6 is an electrolytic solution in which the supporting electrolyte is a lithium hexafluorophosphate (LiPF ₆ ) and the solvent is a mixed solvent solution in which EC, DEC, and DMC are 1: 1: 1 in a volume ratio.
[0019]
A predetermined amount (about 20 mg of PMMA per 1 g of carbon) of PMMA is grafted on the surface of the carbon particles that are the negative electrode active material in advance. Since the grafted PMMA takes in the electrolytic solution, the contact between the active material particles and the electrolytic solution is improved. Further, the active material particles are firmly bound to each other.
[0020]
In FIG. 2, 1 is a positive electrode. The positive electrode 1 is composed of the same mixture of LiCoO ₂ as in Example 1, 75 parts by weight, 5 parts by weight of graphite powder and 20 parts by weight of polymer gel electrolyte. The polymer gel electrolyte is composed of 6 parts by weight of a 1M LiPF ₆ solution of a mixed solvent of PC and EC having a volume ratio of 0.3: 0.7 and 4 parts by weight of polyethylene oxide (PEO). And gelled. The thickness of the positive electrode 1 is 200 μm. 2 is a negative electrode. The negative electrode 2 is a mixture of 80 parts by weight of carbon particles grafted with PMMA on the surface and 20 parts by weight of a polymer gel electrolyte. The thickness of the negative electrode 2 is 200 μm. 3 is a separator made of a polymer gel electrolyte and has a thickness of 100 μm. 4 is a battery case and 5 is a lid. 6 is a positive electrode current collector made of Al foil, and 7 is a negative electrode current collector made of Cu foil. 8 is a gasket made of polypropylene (PP).
[0021]
Like the above, PMMA is grafted on the surface of LiCoO ₂ or carbon particles. This MMA has good affinity with the polyether constituting the polymer solid electrolyte, and the molecular chains are intertwined with each other or chemically bonded, so that the contact between the active material particles and the electrolyte becomes strong. In particular, when the ether monomer is a liquid having a functional group and is cured after forming a coating film of the mixture with the carbon particles, the molecular chains of PMMA and the polyether are entangled and the bond is sufficiently advanced to achieve an extremely strong bond. Is done.
[0022]
As described above, in FIG. 2, an example of a polymer solid electrolyte is described. However, in the case of a polymer gel electrolyte including a polymer as a matrix and a solvent such as PC or EC, the affinity between PMMA and the polymer matrix is good. The effect similar to that in the case of the solid polymer electrolyte can be obtained by bonding and bonding.
[0023]
However, since an electrically insulating polymer is grafted on the surface of the active material particles, if the ratio of the polymer is too large, the conductivity of the electrode cannot be maintained, and conversely, the characteristics deteriorate. Therefore, there is an appropriate value for the ratio of solid particles to polymer. FIG. 3 shows the relationship between the amount of PMMA grafted on the carbon particles of the negative electrode and the surface thereof, and the discharge capacity of the battery. The amount of PMMA is expressed in g per unit area of carbon particles. The electrolytic solution is a liquid, and the discharge rate is 0.2 C (5 hour rate). As can be seen from FIG. 3, the amount of PMMA is preferably 0.01 to 0.1 g / m ² , more preferably 0.02 to 0.07 g / m ² . Here, the active material particles are carbon and the vinyl compound is PMMA as an example. However, if the amount of MMA is defined as g / m ² and the vinyl compound has a monomer molecular weight of 200 or less, other materials may be used. However, the desired amount of MMA is the same.
[0024]
In the battery according to the present invention, the adhesion between the active material particles constituting the electrode and the electrolyte is good. Also, the active material packing density is high. FIG. 4 is a graph showing the relationship between the discharge rate and the discharge capacity of a battery using a polymer gel electrolyte among the batteries using a polymer solid electrolyte and a polymer gel electrolyte having remarkable effects. The battery 1 is a battery obtained by grafting PMMA on the surface of 0.02 g / m ^{2 for} both the positive electrode active material LiCoO ₂ particles and the negative electrode carbon particles. Battery 2 is a battery in which only negative carbon particles are grafted. Compared to conventional batteries that do not have any batteries also graft polymerized vinyl compounds, even in low-rate discharge at 0.1 C, 1C, high rate of 2C (high load) in the discharge even more, are excellent in discharge characteristics .
[0025]
In the case of a battery having a liquid electrolyte, the effect of providing a graft-polymerized vinyl compound is not as remarkable as that of a battery having a polymer solid electrolyte or a polymer gel electrolyte, but it is still effective.
[0026]
5 according to the battery 2 and also, a battery grafted with PMMA only to carbon particle surface as the active material of the negative electrode, and the present invention battery 3 was conducted grafted with a polymer of the gel electrolyte curing reaction at the same time the battery It is the figure which showed the charging / discharging cycling characteristics in room temperature. In FIG. 5, charging was performed at a constant current charge of 0.2 C and a final voltage of 4.2 v, and discharging was performed at a constant current discharge of 0.2 C and a final voltage of 2.7 v. As can be seen from FIG. 5, any battery provided with a graft-polymerized vinyl compound has a smaller decrease in discharge capacity with the progress of the cycle and superior charge / discharge cycle characteristics as compared with the conventional battery. Among these curing of the graft and the electrolyte was carried out simultaneously, characteristics of the present invention the battery 3 is excellent. This is presumably because PMMA grafted on the surface of the solid particles and the polymer constituting the polymer gel electrolyte are chemically bonded, and the solids of the particles and the electrolyte are in strong contact with each other.
[0027]
In addition, in a battery having a liquid electrolyte as described above, a battery including a graft-polymerized vinyl compound has excellent charge / discharge cycle characteristics as compared with a conventional battery.
[0028]
FIG. 6 is a diagram showing the transition of the discharge capacity when the charge / discharge cycle test at 60 ° C. of the battery 2 , the present invention battery 3 and the conventional battery is carried out. As in the case of room temperature, the charging is a constant current of 0.2 C and a final voltage of 4 and 2 v, the discharging is a constant current of 0.2 C and the final voltage is 2.7 v. As shown in FIG. 6, compared with the conventional battery, any battery provided with the graft-polymerized vinyl compound has improved cycle characteristics, and the effect is even more pronounced at room temperature. This is presumably because the difference in the strength of contact between the solid particles and the electrolyte was accelerated at high temperatures.
[0029]
As described above, the nonaqueous secondary battery according to the present invention is an excellent battery in terms of electrical characteristics.
[0030]
【The invention's effect】
According to the present invention, in a non-aqueous secondary battery using a polymer solid electrolyte, the grafting of the monomer onto the active material particle surface and the curing of the polymer constituting the polymer solid electrolyte are performed simultaneously, thereby Since the contact between the particles and the electrolyte can be maintained satisfactorily, it is possible to provide a battery having excellent repeated charge / discharge operation characteristics at high temperatures.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a battery according to an example .
FIG. 2 is a cross-sectional view of a battery according to an example .
FIG. 3 is a graph showing the relationship between the amount of graft polymer added and the discharge capacity.
FIG. 4 is a diagram showing discharge capacities at various rates of various batteries at room temperature.
FIG. 5 is a diagram showing the results of a charge / discharge cycle test at room temperature.
FIG. 6 is a diagram showing the results of a charge / discharge cycle test at 60 ° C.

Claims (1)

The positive electrode and the negative electrode are composed of a simple substance or a compound capable of occluding and releasing lithium, and a vinyl compound is graft-polymerized on at least the surface of the simple substance or compound constituting the negative electrode, and a polymer solid electrolyte or a polymer gel electrolyte is used as an electrolyte. the graft polymerization of the vinyl compound, and curing the polymer solid electrolyte or polymer gel electrolyte is Ru are carried out simultaneously, the production method of the nonaqueous secondary battery.