JP4496366B2 - Negative electrode material for polymer solid electrolyte lithium secondary battery and method for producing the same - Google Patents

Description

The present invention relates to a graphite-based material as an intercalation negative electrode of a polymer solid electrolyte lithium ion secondary battery using polyethylene oxide (PEO) as an electrolyte material.

Since the introduction of lithium ion batteries in 1991, much attention has been paid to their high operating voltage and high energy density.
The lithium ion battery is widely used as a power source for mobile phones, notebook computers, video cameras, and other digital products. Commercially available lithium ion batteries use a graphite-based material having a low operating potential and excellent cycleability as a negative electrode, a transition metal oxide containing lithium as a positive electrode, and a solvent mainly composed of a carbonate-based organic compound. A lithium salt such as LiPF ₆ dissolved therein is used as the electrolyte.

However, a lithium ion battery using an organic liquid as an electrolyte always has a safety problem due to leakage of the electrolyte and thermal stability. This problem is an obstacle to the application of large scale lithium ion batteries to electric vehicles or hybrid vehicles (EV / HEV).
In the last 20 years of research, a solid PEO electrolyte in which a lithium salt is dissolved in polyethylene oxide (PEO) has a temperature higher than room temperature, but has an electrical conductivity of 10 ⁻³ Scm ⁻¹ . The polymer solid electrolyte lithium secondary battery using this PEO electrolyte may be able to solve the problems that occur in the organic liquid electrolyte.

By the way, the lithium polymer battery developed so far uses lithium (Li) metal as a negative electrode, and has a safety problem due to the formation of lithium dendrite. In order to solve this problem, it is effective to employ a so-called intercalation compound.
For example, it is desirable to employ a Li—M—O material that does not generate strain due to charge / discharge, such as spinel-structured Li _1.33 Ti _1.67 O ₄ , and exhibits excellent cycle characteristics. There is a drawback that the voltage is small and high. On the other hand, a method using a Li alloy having a larger capacity is conceivable, but these are difficult to employ because a large volume change occurs due to a charge / discharge cycle and electrode deterioration cannot be solved.
After all, a graphitic material that is flat and exhibits a low potential and exhibits excellent cycle characteristics in an organic electrolyte system is considered to be the optimum negative electrode.

However, a layered graphite material that exhibits excellent performance in a commercially available lithium ion battery has a poor compatibility with the electrolyte / electrode interface in a PEO electrolyte, exhibits low initial efficiency and inferior cycle characteristics, and cannot be applied at present. From the above points, in order to improve the electrochemical performance of the graphite negative electrode as the PEO electrolyte, an emphasis is placed on the optimum design of the interface between the PEO and the carbon negative electrode (for example, Patent Document 1), but the results are not always satisfactory. Not shown.

Japanese Patent Application No.6-292744

In addition, Tsumura et al. Have used a method in which natural graphite is coated with carbon and brought into contact with PEO (Non-patent Document 1).
The graphite negative electrode thus obtained is a PEO electrolyte at 60 ° C. and has a capacity of about 300 mAhg ⁻¹ up to the fourth cycle. However, a large initial irreversible capacity remains.

T. Tsumura, Solid State Ionics, 135 (2000) 209-212

It is a first object of the present invention to improve the electrochemical performance of graphite as a negative electrode for a polymer solid electrolyte lithium secondary battery using a combination of PEO and a lithium salt as an electrolyte.

The present invention has succeeded in greatly improving the characteristics by optimizing the contact interface between the PEO electrolyte and the graphitic carbon, more specifically by improving the surface structure of the graphitic carbon. Specifically, the problem is achieved by providing a negative electrode material for a polymer solid electrolyte lithium secondary battery in which a compound film composed of lithium and carbon is formed on the surface of graphite. Here, the compound film composed of lithium and carbon refers to a film composed of a compound represented by the chemical formula of LiC _x . And it is desirable that the surface layer portion of the compound film made of lithium and carbon formed on the graphite surface portion is at least in an amorphous state. FIG. 1 shows a schematic structure of a compound film made of graphite and lithium and carbon formed on the surface of graphite. Here, it is considered that the surface layer portion of the compound film made of lithium and carbon is in an amorphous state.

  Further, the solid polymer electrolyte is obtained by dissolving a lithium salt in polyethylene oxide, and it is more preferable that the polyethylene oxide contains a ceramic filler. This is because the inclusion of the ceramic filler increases the strength of PEO and improves the electrochemical characteristics.

And this invention is the manufacturing method which performs a ball milling process to the mixture comprised with graphite, metallic lithium, and the organic solvent whose boiling point is 190 to 260 degreeC, and forms the compound film which consists of lithium and carbon in a graphite surface part Involved. If the boiling point of the organic solvent is lower than 190 ° C., the organic solvent is in a gaseous state at room temperature. Conversely, if the boiling point is higher than 260 ° C., the organic solvent is solidified around 10 ° C., and the characteristics as the organic solvent cannot be fully exhibited. is there. Accordingly, dodecane (CH ₃ (CH ₂ ) ₁₀ CH ₃ ) (boiling point 216 ° C., freezing point −10 ° C.) is suitable as the organic solvent.
Further, in the above-mentioned mixture, the graphite / lithium weight mixing ratio is 0.01 to 0.8 graphite with respect to lithium 1, and the organic solvent / lithium volume mixing ratio is 1 to 40 ml organic solvent with respect to 1 g lithium. It is characterized by that. If the ratio is outside the above ratio, the formation of a compound film composed of lithium and carbon on the graphite surface becomes inefficient.

  According to the present invention, it has become possible to provide a polymer electrolyte lithium secondary battery having high initial charge / discharge efficiency and cycle stability, and it has become possible to develop applications for which safety is important.

In the present invention, high energy mechanical milling (HEMM) is performed in a suitable organic solvent for lithium metal and graphite-based carbon. The basic method is as follows. First, after a specified amount of graphitic material is dried for a certain period of time in a high-temperature vacuum, the graphitic material, metallic lithium and a specific organic solvent are further mixed in an inert gas. Here, the organic solvent is used because it is effective for improving the dispersion of metallic lithium and graphitic carbon to prevent the collective adhesion between the component materials and to form a uniform and stable film on the surface. .

Next, the mixture is subjected to high energy mechanical milling (HEMM) for a certain period of time. As a result, an amorphous structure is formed at the edge portion of the graphitic carbon, and certain anions such as O ²⁻ and electrolyte components are simultaneously inserted into the graphite layer together with Li ⁺ due to the amorphous structure. It is prevented. Furthermore, it becomes possible to form a stable lithium and carbon compound (LiC _x ) having high Li ion conductivity on the surface of the graphitic material particles. This is because such a compound prevents direct contact between the PEO electrolyte and the graphitic material. The HEMM-treated mixture is further dried at an elevated temperature to remove residual organic solvent. The graphitic carbon material obtained by processing in this way exhibits excellent electrochemical characteristics exhibited by high initial charge / discharge efficiency, high reversible capacity, and stable cycle characteristics with respect to the PEO electrolyte.

Examples of the present invention will be described below, but the technical scope of the present invention is not limited by the examples, and various modifications can be made without changing the gist thereof. Further, the technical scope of the present invention extends to an equivalent range.

Microbead mesocarbon (MCMB) having a particle size of 20-25 μm is dried at 80 ° C. in vacuum for 20 hours. A certain amount of MCMB treated in this way is mixed with lithium metal and dodecane {CH ₃ (CH ₂ ) ₁₀ CH ₃ ). The ratio of graphite material to metallic lithium was 20.3: 1 (weight ratio). The ratio of the graphite material to dodecane was 1 g of graphite per 1 ml of dodecane. The mixture is placed in a ball mill pot (40 cc) with an appropriate amount of balls (1 cm in diameter). Here, 12 balls were used for 1 g of the mixture. Next, the pot was filled with argon gas, the mixture was subjected to HEMM treatment for 15 hours at a rotation speed of 600 rpm, and then the mixture was dried in vacuum at 90 ° C. for 10 hours.

The production of the PEO solid electrolyte is as follows. A certain amount of LiN (CF ₃ SO ₂ ) ₂ and polyethylene oxide (MW = 6 × 10 ⁵ ) (O / Li = 18) are dissolved in anhydrous acetonitrile solvent. BaTiO ₃ (about 0.1 μ) is uniformly dispersed as an inorganic filler. The obtained viscous composite solution is cast on a fluororesin plate. Next, the acetonitrile solvent is slowly evaporated in a nitrogen stream, and then dried in a vacuum at 90 ° C. for 10 hours. The thickness of the obtained film was 300 μm. The conductivity of the composite polymer electrolyte was 1.7 × 10 ⁻³ Scm ⁻¹ at 80 ° C. and 0.82 × 10 ⁻³ Scm ⁻¹ at 65 ° C.

The composite electrode was made in a glove box. The constituent materials of the electrode were PEO, LiN (CF ₃ SO ₂ ) ₂ , acetylene black, and graphitic material particles, which were mixed in a mortar in the presence of hexane and pressed against a 300 mesh stainless steel net. Stainless steel acted as a current collector, and the electrode area was 0.55 cm ² and the thickness was 100 to 160 μm.
In order to investigate the electrochemical characteristics of the electrode, lithium metal was used as a counter electrode, a PEO film was used as an electrolyte and separator, and the layers were stacked in a 2025 coin-type battery cell. Charging / discharging was performed at a current density of 0.05 mAcm ⁻² , a voltage range of 2.0 / 0 V vs. Li / Li +, and a temperature of 70 ° C. Prior to measurement, the cell was held at 70 ° C. for 2 hours. The capacity of the electrode was calculated per active material weight.
An electron micrograph of the compound film composed of lithium and carbon obtained as described above is shown in FIG. It can be seen from FIG. 2 that the LiC _x crystal layer is formed on the surface portion of the graphite crystal and the LiC _x amorphous layer is formed on the surface layer portion by the treatment method of the present invention.

FIG. 3 is a graph comparing the charge / discharge characteristics of the PEO electrolyte system when the treatment according to the present invention is performed with respect to graphite having a particle size of 20-25 μm. The initial charge / discharge efficiency increased from 37.6% to 63.7%, and the reversible capacity increased from 120 mAhg ⁻¹ to 260 mAhg ⁻¹ , showing a marked improvement. Moreover, the capacity retention performance by the cycle of the treated graphite was also excellent.

  Natural graphite having a particle diameter of 1 μm was dried in a vacuum at 80 ° C. for 20 hours, and a certain amount of dried graphite, metallic lithium and dodecane were mixed. The graphite to lithium ratio was 10.3: 1 by weight, and the graphite to dodecane ratio was 2 ml of dodecane per 1 g of graphite. The mixture was put into a ball mill pot (40 cc) together with an appropriate amount of balls (1 cm in diameter). At this time, 16 balls were added to 1 g of the mixture. The pot was filled with argon gas, and the mixture was subjected to HEMM treatment for 20 hours at 600 rpm. At this time, a pause of 5 minutes was taken every 1.5 hours. The mixture was then dried in vacuo at 90 ° C. for 10 hours. The subsequent creation conditions were the same as in the example. The results are shown in FIG.

Here, FIG. 4 compares the characteristics in the PEO electrolyte system when the treatment according to the present invention is applied to the graphite having a particle diameter of 1 μm and when the treatment is not performed. The charge / discharge efficiency increased from 28.1% to 75.4%, and the reversible capacity increased from 110 mAhg ⁻¹ to 220 mAhg ⁻¹ , indicating a marked improvement. The capacity retention performance by the cycle of the treated graphite was also excellent.

FIG. 5 shows a cyclic voltammogram (70 ° C.) in a PEO electrolyte system with and without the treatment of graphite having a particle diameter of 20-25 μm. When Li is inserted into normal graphite for the first time, peaks of 0.2, 0.9, 1.35, and 2.3 V appearing due to an irreversible reaction between the PEO and the electrode occur in the treated graphite. It disappeared completely.

Table 1 compares the initial charge and discharge efficiency and the reversible capacity of the graphite treated with the present invention and the untreated graphite in different electrolytes. The untreated graphite material, unlike the combination with the liquid electrolyte, showed clearly inferior properties to the PEO electrolyte. However, the graphite material treated according to the present invention showed a dramatic improvement in both initial efficiency and reversible capacity.

It is a schematic diagram at the time of forming LiC _x on the graphite surface by HEMM treatment. It shows a transmission electron micrograph showing a LiC _x formed on the graphite surface. Here, Photo [1] is a diagram showing a case where the processing of the present invention is not performed, and Photo [2] is a diagram showing a case where processing is performed. It is a figure which shows charging / discharging performance at the time of using graphite with a particle size of 20-25 micrometers for negative electrode material. When the modification according to the present invention is applied, the characteristics are considerably improved as compared to without modification. It is a figure which shows charging / discharging performance at the time of using graphite with a particle size of 1 micrometer for negative electrode material. When the modification according to the present invention is applied, the characteristics are considerably improved as compared to without modification. It is a figure which shows the cyclic voltammogram at the time of using graphite with a particle size of 20-25 micrometers for negative electrode material.

Claims (7)

  A negative electrode material for a polymer solid electrolyte lithium secondary battery, wherein a compound film comprising lithium and carbon is formed on a surface portion of graphite. 2. The negative electrode material for a solid polymer electrolyte lithium secondary battery according to claim 1, wherein the surface layer portion of the compound film composed of lithium and carbon formed on the graphite surface portion is at least in an amorphous state. .
  3. The negative electrode material for a solid polymer electrolyte lithium secondary battery according to claim 1, wherein the solid polymer electrolyte is obtained by dissolving a lithium salt in polyethylene oxide.   The negative electrode material for a polymer solid electrolyte lithium secondary battery according to claim 3, wherein the polyethylene oxide contains a ceramic filler.   A polymer solid characterized by subjecting a mixture composed of graphite, metallic lithium and an organic solvent having a boiling point of 190 ° C. to 260 ° C. to ball milling to form a compound film composed of lithium and carbon on the surface of the graphite A method for producing a negative electrode material for an electrolyte lithium secondary battery. In the above mixture, the weight mixing ratio of graphite / lithium is 0.01-0.8 of graphite with respect to lithium 1, and the volume mixing ratio of organic solvent / lithium is 1-40 ml of organic solvent with respect to 1 g of lithium. The manufacturing method of the negative electrode material for polymer solid electrolyte lithium secondary batteries of Claim 5 characterized by the above-mentioned .
Manufacturing method of the organic solvent dodecane _{(CH 3 (CH 2) 10} CH 3) negative electrode material for solid polymer electrolyte lithium secondary battery according to claim 5 or 6, characterized in that a.

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