Patent Application
20080081263
The present invention is below described in more detail by way of Examples. It will be recognized that the following examples merely illustrate the present invention and are not intended to be limiting thereof. Suitable changes can be effected without departing from the scope of the present invention.
(Fabrication of Positive Electrode)
Lithium cobaltate as a positive-electrode active material, ketchen black as a conductive agent, and a fluorine resin as a binder at a ratio by weight of 90:5:5 were mixed. The mixture was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a paste.
This paste was uniformly applied onto both sides of a 15 μm thick aluminum foil by a doctor blade process. The resultant was subsequently placed in a heated drier where it was subjected to a vacuum heat treatment at a temperature of 100-150° C. to remove NMP, and then rolled by a roll pressing machine to a thickness of 0.13 mm to fabricate a positive electrode for a flat-type laminated battery.
(Fabrication of Negative Electrode)
A negative-electrode active material comprised of graphite, a styrene-butadiene rubber binder and a carboxymethylcellulose viscosity control agent at a ratio by mass of 96:2:2 were mixed. The mixture was dissolved in water to prepare a paste.
This paste was uniformly applied onto both sides of a metal core (10 μm thick copper foil) by a doctor blade process. The resultant was subsequently placed in a heated drier where it was heat treated at a temperature of 100-150° C. to remove water, and then rolled by a roll pressing machine to a thickness of 0.12 mm to fabricate a negative electrode for a flat-type laminated battery.
(Preparation of Electrolyte Solution)
LiPF6 was dissolved in a solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) at a 3:7 ratio by volume to a concentration of 1 mole/liter to prepare an electrolyte solution.
Further, vinylene carbonate (VC) or fluoroethylene carbonate (FEC: 4-fluoro-1,3-dioxolane-2-one) was added to the above electrolyte solution in the amount of 2% by mass to prepare an electrolyte solution.
(Construction of Secondary Battery)
The above-fabricated positive and negative electrodes were each cut into a predetermined size. A current collector tab was attached to each of cores of those electrodes. The positive electrode and negative electrode were rolled up with a 20 μm thick, polyolefin microporous film separator between them and fastened by a tape at the outermost periphery to provide a spirally wound electrode assembly which was subsequently pressed in a flat plate configuration.
This spirally wound electrode assembly was inserted into an outer casing comprised of a laminated material of PET, aluminum and others such that an end of each electrode tab extended outwardly from the assembly. After introduction of the above-prepared electrolyte solution, the outer casing was sealed to build a secondary battery.
Before completion of the battery, the electrode assembly was placed in a closed container and suction dried to remove water from the active materials and the separator. Also, the ratio, (negative electrode capacity)/(positive electrode capacity), was set at 1.10. The constructed battery exhibited a discharge capacity of 700 mAh in all cases where the different electrolyte solutions were used.
According to the above procedure, the batteries of Example 1 and Comparative Examples 1 and 2 were constructed, as shown in Table 1.
(Evaluation of Charge-Discharge Characteristics)
The batteries of Example 1 and Comparative Examples 1 and 2 were each charged at a charge current of 700 mA to a battery voltage of 4.38 V, further charged at a constant voltage of 4.38 V to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V. This charge-discharge unit cycling was repeated.
A capacity retention of each battery after 300 charge-discharge cycles under the above conditions is shown in the following Table 2. The capacity retention, as used herein, refers to a value given by (300th cycle discharge capacity)/(1st cycle discharge capacity).
The potential of the positive electrode in the fully charged battery was about 4.48 V versus metallic lithium, for all the batteries. The Li content of the positive-electrode active material in the fully charged condition was determined according to the following procedure.
LiCoO2, serving as the positive-electrode active material, is 1/97.82 mol per gram. Thus, a theoretical capacity when Li in the positive-electrode active material is totally released is given by
In order to determine a charge capacity of the positive-electrode active material in the fully charged condition, a three-electrode test cell using the positive electrode was built. The charge capacity was determined by charging the cell until the potential of the positive electrode reached 4.48 V. As a result, the charge capacity per gram of the active material was 194 mAh/g and the amount of Li remaining in the positive-electrode active material was 80 mAh/g in terms of charge capacity. Accordingly, the amount of Li remaining in the positive-electrode active material in the fully charged condition is (80/274)×100=29.1% of the initial amount of Li. Therefore, the Li content of the positive-electrode active material in the fully charged condition was confirmed to be about 30% of the initial Li content.
(Evaluation of Storage Characteristics in Charged State)
Also, the batteries of Example 1 and Comparative Examples 1 and 2 were each evaluated for storage characteristics in a charged state. Each battery was charged at a current of 700 mA to a battery voltage of 4.38 V, further charged at a constant voltage of 4.38 V to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V to confirm a discharge capacity. Thereafter, the battery was fully charged under the same conditions as above. Each battery in the fully charged condition was stored in a constant temperature chamber at 60° C. for 15 days. After storage, a thickness increase of each battery was measured. The measurement results are shown in Table 2.
As shown in Table 2, the battery of Comparative Example 1, which contains no additives in its electrolyte solution, is less susceptible to thickness increase due to gas generation during storage in the charged state but shows inferior charge-discharge cycle characteristics. Also, the battery of Comparative Example 2, which uses VC as an additive, exhibits good charge-discharge cycle characteristics but shows a marked thickness increase due to gas generation during storage in the charged state. To the contrary, the battery of Example 1, which uses FEC as an additive according to the present invention, has been found to be less susceptible to thickness increase due to gas generation during storage in the charged state and also show superior cycle characteristics.
The following is believed to explain the difference in action and effect between VC and FEC.
That is, the use of VC as an additive results in the formation of an SEI (Solid Electrolyte Interface) on the negative electrode and makes it possible to obtain good cycle characteristics. However, VC readily decomposes oxidatively at the positive electrode side and generates a large amount of gas particularly when the positive electrode is in the high potential condition. FEC, if used as an additive, not only forms a good film on the negative electrode in a manner similar to VC to improve cycle characteristics, but also retards a reaction at the positive electrode side to reduce the amount of gas generated due to oxidative decomposition at the positive electrode.
The battery, when stored in the charged condition at high temperature, generates a larger amount of gas than when stored in the non-charged condition. Also, analysis of a surface of the negative electrode after high-temperature storage resulted in detection of a reaction product due to oxidative composition at the positive electrode. These are believed to suggest that, in a nonaqueous electrolyte secondary battery having a high-potential positive electrode and a negative electrode, two reactions, i.e., an oxidative decomposition reaction at the positive electrode side and a secondary reaction wherein a side reaction product associated with the positive electrode reacts at a surface of the negative electrode, provide a significant effect on deterioration of characteristics of the battery while stored in the charged condition.
For a comparative purpose, a battery was built such that its end-of-charge voltage was 4.2 V, i.e., it was brought to a fully charged condition where a potential of the positive electrode was 4.3 V. Cycle characteristics and storage characteristics in a charged state were evaluated in the same manner as described above. The Li content in the fully charged condition is about 45% of the initial Li content. In either case where VC or FEC was added, this battery showed superior cycle characteristics and storage characteristics in a charged state. However, because of a discharge capacity decline of about 15%, a marked reduction of energy density occurs for the battery.
The stability of the electrolyte solution at a high potential depends essentially on the oxidation resistance of the solvent used for the electrolyte solution. However, the decomposition reaction of the electrolyte solution starts to occur at a lower potential when LiCoO2 or the like is used than when a low active platinum electrode or the like is used. This is because the reactivity of the electrode as a reaction field strongly influences the oxidative decomposition reaction.
Therefore, even in the case where the battery is charged to the same potential, the positive electrode becomes more reactive with the electrolyte solution when a transition metal is in a higher valence state or when a higher proportion of a transition metal is in a high valence state. Accordingly, gas generation due to oxidative decomposition of the electrolyte solution proceeds more remarkably when a positive electrode material capable of releasing more Li, such as LiCoO2 or LiNi1/3Mn1/3Co1/3O2, is used as the positive-electrode active material. The use of a low reactive halogenated ethylene carbonate derivative, in accordance with the present invention, results in obtaining good storage characteristics while maintaining superior cycle characteristics.
The procedure of Example 1 was followed, except that the additives and electrolyte salts specified in Table 3 were used, to build batteries of Examples 2 and 3. Table 3 also shows the corresponding components used in the battery of Example 1.
(Evaluation of Storage Characteristics in Charged State)
Each of the batteries built in Examples 1-3 was charged at a current of 700 mA to a battery voltage of 4.38 V, further charged at a constant voltage of 4.38 V to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V to confirm a discharge capacity. Thereafter, the battery was fully charged under the same conditions as used above. The battery in the fully charged condition was stored in a constant temperature chamber at 60° C. for 20 days. After storage, the battery was discharged under the same conditions as above, followed by a cycle of charge and discharge. A capacity restoration of each battery after 20 days' storage was shown in the following Table 4. The capacity restoration, as used herein, is a value given by (discharge capacity in the charge-discharge cycle subsequent to storage in the charged state)/(discharge capacity in the charge-discharge cycle prior to storage).
As can be clearly seen from comparison between the battery of Example 1 which do not contain LiBF4 in its electrolyte solution and the batteries of Examples 2 and 3 which contains LiBF4 in their electrolyte solutions, the inclusion of LiBF4 in the electrolyte solution improves the capacity restoration of the battery after storage in the charged state. The detailed reason why the capacity restoration of the battery after storage in the charged state is improved is not clear. It is however assumed that LiBF4 in an initial condition decomposes at a surface of the positive-electrode active material to change a surface condition of the positive-electrode active material in such a way as to lower its reactivity with the electrolyte solution.
The procedure of Example 1 was followed, except that the negative-electrode active materials, electrolyte salts and additives specified in Table 5 were used, to build batteries of Examples 4-8 and Comparative Example 3. Table 5 also shows the corresponding components used in the batteries of Examples 2 and 3. The amorphous carbon-coated graphite was made using graphite and a pitch. The coated amorphous carbon was 1% by weight.
(Evaluation of Charge-Discharge Cycles)
The batteries of Example 2-8 and Comparative Example 3 were each charged at a charge current of 700 mA to a battery voltage of 4.38 V, further charged at a constant voltage of 4.38 V to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V. This charge-discharge unit cycling was repeated. The potential of the positive electrode in each battery in the fully charged condition was confirmed to be about 4.48 V versus metallic lithium.
A capacity retention of each battery after 200 charge-discharge cycles under the above conditions is shown in the following Table 6. The charge retention, as used herein, is a value given by (200th cycle discharge capacity)/(1st cycle discharge capacity).
(Evaluation of Storage Characteristics in Charged State)
Each of the batteries of Example 2-8 and Comparative Example 3 was charged at a current of 700 mA to a battery voltage of 4.38 V, further charged a constant voltage of 4.38 V to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V to confirm a discharge capacity. Thereafter, the battery was fully charged under the same conditions as described above. Each battery in the fully charged condition was stored in a constant temperature chamber at 60° C. for 15 days. A thickness increase of each battery after 15 days' storage is shown in the following Table 6.
As shown in Table 6, with regard to a capacity retention after 200 cycles, all the batteries similarly show good characteristics. However, the battery of Comparative Example 3, which uses graphite as the negative-electrode active material and VC as the additive, shows a marked thickness increase during storage in the charged condition. Also, the batteries of Examples 2-4, which use FEC as the additive, show relatively large values for thickness increase.
To the contrary, the batteries of Examples 5-7, which use a amorphous carbon-coated graphite as the negative-electrode active material and 2% FEC as the additive, each show a smaller thickness increase, compared to the batteries of Examples 2 and 3 which use a graphite positive electrode. Also, the battery of Example 8, in which FEC is increased to 5%, shows a smaller thickness increase, compared to the battery of Example 4.
The use of FEC as the additive improves charge-discharge cycle characteristics and suppresses oxidative decomposition at the positive electrode side during charge and discharge. However, the use of the graphite material having a surface coated with amorphous carbon as the negative-electrode active material is believed to have retarded a secondary reaction of the side reaction product formed as a result of oxidative decomposition at the positive electrode side and accordingly enabled a marked improvement of storage characteristics. Also, the degree of oxidative decomposition at the positive electrode side increases in dependence on the potential of the positive electrode and the depth of charge of the positive-electrode active material. Accordingly, the above effect becomes greater particularly in the battery in which the positive electrode is capable of high potential.
Also, the negative electrode using the active material consisting solely of amorphous carbon has a higher potential in the charged condition, relative to using graphite, and thus lowers a battery voltage. This, as a result, lowers an energy density of the battery. The use of the graphite coated with amorphous carbon enables provision of a nonaqueous electrolyte secondary battery which shows superior cycle characteristics and good storage characteristics in the charged state without experiencing an energy density decline due to working voltage drop. The amorphous carbon is preferably contained in the amorphous carbon-coated graphite material in the amount of 0.05-5% by weight.
A cylindrical battery was constructed according to the following procedures.
(Fabrication of Positive Electrode)
Instead of using lithium cobaltate alone, a mixture of layered nickel manganese lithium cobaltate (LiNi1/3Mn1/3Co1/3O2) and lithium cobaltate at a 1:9 ratio by mass was used as the positive-electrode active material. The paste was rolled to a thickness of 0.14 mm. Otherwise, the procedure used above to fabricate the positive electrode for a flat laminated battery was followed to fabricate a positive electrode for a cylindrical battery.
(Fabrication of Negative Electrode)
The coating weight of the paste was suitably altered such that the ratio in initial charge capacity per unit area of the negative electrode to the opposing positive electrode was brought to 1.10. Also, the thickness of the paste was suitably adjusted such that the loading density of the active material remained unchanged. Otherwise, the procedure used above to fabricate the negative electrode for a flat laminated battery was followed to fabricate a negative electrode for a cylindrical battery.
(Preparation of Electrolyte Solution)
Ethylene carbonate (EC), dimethyl carbonate (DMC) and methylethyl carbonate (MEC) in the ratio by volume of 20/40/40 were blended to prepare a solvent. This solvent was used as a “FEC-free” electrolyte solution. Also, a blend in the FEC/DMC/MEC volume ratio=20/40/40 was prepared and designated as a “20% FEC” electrolyte solution. Accordingly, the “20% FEC” means 20% by volume of FEC. Further, a blend in the FEC/DMC/MEC volume ratio=40/30/30 was prepared and designated as a “40% FEC” electrolyte solution. In the preparation of each electrolyte solution, LiPF6 as an electrolyte salt was dissolved in the blend to a concentration of 1 mol/liter.
(Construction of Secondary Battery)
The above-fabricated positive and negative electrodes were each cut into a predetermined size. A current collector tab was attached to each of cores thereof. These electrodes were rolled up with a 18 μm thick, polyolefin microporous film separator between them to make an electrode assembly. This electrode assembly, together with an insulation plate, were then inserted in an exterior can. Thereafter, a negative current collector tab was welded to a bottom of the exterior can.
Subsequently, an explosion-proof valve, a PTC element and a terminal cap were caulkingly fastened through an inner gasket to a sealing plate to make a sealed interior. Thereafter, a positive current collector tab was welded to a top sealing plate. After the electrolyte solution prepared according to the above procedure was poured in the exterior can, the top sealing plate was caulk-fastened through an outer gasket to an open end of the exterior can to complete construction of a battery. The battery when charged to and at 4.35 V exhibited a discharge capacity of 2,800 mAh in all cases where different electrolyte solutions was used.
In the manner as described above, the batteries of Examples 9 and 10 and Comparative Examples 4-6, as shown in Table 7, were constructed.
(Evaluation of Charge-Discharge Cycles)
The batteries of Examples 9 and 10 and Comparative Examples 4-6 were each charged at a charge current of 1,000 mA to a battery voltage of 4.35 V or 4.20 V, further charged at a constant voltage of 4.35 V or 4.20 V to a current value of 54 mA and then discharged at a current of 2,700 mA to a battery voltage of 2.75 V. This charge-discharge unit cycling was repeated to achieve 300 cycles. A capacity retention of each battery after 300 cycles was shown in Table 8.
The Li content of the positive-electrode active material in a fully charged condition was 32% in case of the battery voltage of 4.35 V (positive electrode potential of 4.45 V) or 41% in case of the battery voltage of 4.20 V (positive electrode potential of 4.30 V), each based on the initial Li content.
As shown in Table 8, between the batteries of Comparative Examples 5 and 6 which were both charged to and at 4.20 V, the FEC-containing battery of Comparative Example 6 showed inferior cycle characteristics. However, among the batteries of Comparative Example 4 and Examples 9 and 10 which were all charged to and at 4.35 V, the FEC-containing batteries of Examples 9 and 10 showed superior charge-discharge cycle characteristics to the battery of Comparative Example 4. However, between the 20% FEC battery of Example 9 and the 40% FEC battery of Example 10, the 40% FEC battery of Example 10 showed inferior cycle characteristics. The detail of what caused the above results is not clear, but the following is assumed to explain it.
That is, in case of a low positive electrode potential (low battery voltage), Li is consumed due to a reaction of FEC. The battery then shows inferior cycle characteristics, relative to the FEC-free battery. On the other hand, in case of a high positive electrode potential, oxidative decomposition of the electrolyte solution at the positive electrode side occurs increasingly with charge-discharge cycling. In addition, a decomposition product formed at the positive electrode side moves toward the negative electrode side where it undergoes a side reaction. These deteriorate battery characteristics.
In the case where FEC is added to the electrolyte solution, while Li consumption occurs due to decomposition of FEC, not only oxidative decomposition of the electrolyte solution at the positive electrode but also the side reaction at the negative electrode are suppressed to retard progress of deterioration. As a result, a marked improvement of cycle characteristics can be achieved.
However, if FEC is added in an excessively large amount, a viscosity of the electrolyte solution increases remarkably. This is believed to cancel the cycle improvement effect of FEC and result in the failure to obtain the cycle improvement effect sufficiently.
The electrolyte solution specified in Table 9 was used. Also, a coating weight of the positive paste was adjusted such that the amount of Li released from the positive electrode remained unchanged when the battery was charged to the voltage specified in Table 6. Otherwise, the procedure of Example 1 was followed to construct batteries of Example 11 and Comparative Examples 7-9.
As shown in Table 9, the positive electrode potential is about 4.30 V when the battery voltage is 4.2 V and about 4.50 V when the battery voltage is 4.4 V.
Each of the batteries of Example 11 and Comparative Examples 7-9 was charged at a current of 700 mA to the battery voltage specified in Table 9, further charged at the same voltage to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V. The battery was thereafter fully charged under the same conditions as described above. The battery in the fully charged condition was disassembled under an argon (Ar) atmosphere. Only the positive electrode was removed to measure its thickness. This positive electrode was again encapsulated in a laminated aluminum case, stored in a constant-temperature tank at 60° C. for 10 days and then measured for thickness. The thickness increase of the positive electrode in this storage test is shown in Table 9.
As shown in Table 9 and
To the contrary, the FEC-containing battery of Example 11 generated a smaller amount of gas and showed smaller thickness increase of the positive electrode, compared to the battery of Comparative Example 8.
As described above, when charging was carried out in the positive electrode potential range of higher than 4.4 V versus metallic lithium, the positive electrode increase reactivity with the electrolyte solution and generates a large amount of gas, even in the case of using ethylene carbonate which, at a low voltage, little generated a gas via a reaction with the positive electrode. However, the inclusion of halogenated ethylene carbonate, in accordance with the present invention, has been found to assure superior storage characteristics even in the case where the positive electrode is used in a high potential condition.
The electrolyte solution specified in Table 10 was used. Also, a coating weight of the positive paste was adjusted such that the amount of Li released from the positive electrode remained unchanged when the battery was charged to the voltage specified in Table 10. Otherwise, the procedure of Example 1 was followed to build batteries of Examples 13 and 14 and Comparative Example 10.
Here, the positive electrode potential is about 4.30 V when the battery voltage is 4.2 V and about 4.50 V when the battery voltage is 4.4 V.
Each of the batteries of Examples 13 and 14 and Comparative Example 10 was charged at a current of 700 mA to the battery voltage specified in Table 10, further charged at the same voltage to a current value of 35 mA and then discharged at a current of 700 mA to a battery voltage of 2.75 V. The battery was thereafter fully charged under the same conditions as described above.
For each of those batteries in the fully charged condition, two sample batteries were prepared. One sample battery was disassembled under an argon (Ar) atmosphere and only the negative electrode was removed. This negative electrode was again encapsulated in a laminated aluminum case. This negative electrode while encapsulated in the laminated aluminum case, together with the other sample battery in the fully charged condition, were subjected to a storage test at 60° C. for 10 days. Specifically, the negative electrode alone while encapsulated in the laminated aluminum case and the battery in the fully charged condition were both stored in a constant-temperature chamber at 60° C. for 10 days. A thickness increase of the battery before and after storage, as well as a thickness increase of the negative electrode before and after storage, were measured. The measurement results are shown in Table 10.
As shown in Table 10 and
The battery of Example 14, which contains LiBF4 in its electrolyte solution, shows a smaller thickness increase of the negative electrode than the battery of Example 13 and shows marked reduction in amount of a gas generated at the negative electrode side. As a result, it shows a battery thickness increase comparable in value to the battery of Comparative Example 10.
It is understood from the foregoing that when the battery in the fully charged condition is stored under a constant-temperature atmosphere, a reaction accompanied by gas generation takes place at both of the positive and negative electrodes, resulting in the thickness increase of the battery. At the positive electrode side, gas generation is believed to occur as a result of a reaction of the high-potential positive electrode with the electrolyte solution that causes oxidative decomposition of the electrolyte solution to proceed. At the negative electrode side, gas generation is believed to occur due to decomposition under a high-temperature environment of a film initially formed on a surface of the negative electrode.
It is also understood from the foregoing that when the battery is charged in a condition where a positive electrode potential is high, the use of halogenated ethylene carbonate retards a reaction involving gas generation at the positive electrode side, relative to using no halogenated ethylene carbonate. However, even in the case where halogenated ethylene carbonate is used, the amount of gas generated at the positive electrode side increases with build-up of the positive electrode potential, resulting in increasing a battery thickness. It is believed that LiBF4, when added to the electrolyte solution, decomposes at a surface of the negative electrode in an initial stage of charge-discharge to thereby suppress decomposition of halogenated ethylene carbonate at the negative electrode side and accordingly reduce the amount of gas generated due to the decomposition thereof. This is believed to reduce the amount of gas generated in the battery as a whole and accordingly suppress increase in amount of a gas generated due to voltage build-up.
| Number | Date | Country | Kind |
|---|---|---|---|
| 267253/2006 | Sep 2006 | JP | national |
| 075040/2007 | Mar 2007 | JP | national |
