Electricity: battery or capacitor charging or discharging – Diverse charging or discharging rates for plural batteries
Reexamination Certificate
2002-10-22
2004-06-29
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Diverse charging or discharging rates for plural batteries
C320S130000
Reexamination Certificate
active
06756767
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for charging and discharging a nonaqueous electrolyte secondary battery. Particularly, the present invention relates to a method of charging and discharging a nonaqueous electrolyte secondary battery which provides excellent cyclic life characteristics.
Rechargeable secondary batteries, such as Nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni—MH) batteries, and lithium-ion (Li-ion) batteries, are known batteries which are mounted on portable electronic equipment and which generate power driving the equipment. Such secondary batteries deteriorate through repeated charging and discharging and reduce the battery capacity thereof. These batteries have a cyclic life in which the battery capacity is substantially lost through repeated uses for a long period of time. For example, in Ni—MH batteries, repeated charging and discharging (or charging/discharging cycle) causes erosion or deterioration of the negative electrode or positive electrode, thus producing non-uniformity of the electrolysis solution. This results in a cyclic life in which only the charging capacity of substantially a half of the initial full charging capacity of a battery cell can be obtained.
Such a cyclic life can be predicted when examination is made, for example, by limiting the temperature and current in charging/discharging to predetermined conditions. However, because actual batteries are used in various situations, the charging/discharging cycles described on product catalogs cannot be applied to all batteries. Particularly, over-charging or over-discharging often causes a shortage in the cyclic life of the battery. For that reason, in the design and development of the secondary batteries, the problem is to fabricate batteries that adapt to working conditions and environments and that have sufficient cyclic operational life.
By the way, the secondary batteries having the above-mentioned cyclic life are used for electronic equipment, including information equipment (such as digital cameras, personal computers, and the like) and communications equipment (such as personal telephones). However, the time period for which the equipment can be used through one-time charging is shortened with degradation of the battery capacity.
When the battery reaches the end of its cyclic life, the electronic equipment itself does not work substantially. In such a case, the user cannot simply determine whether the equipment is in failure or the battery is dead or the battery has reached the end of its cyclic life. In such a situation, it is necessary to detect the cyclic life of the secondary battery on the side of the electronic measuring equipment and tell the user the detected information.
There a method of predicting a battery capacity by measuring the internal resistance inherent in a battery and by detecting the rate of increase with the charging/discharging cycles or by integrating the capacity until the battery becomes an empty state through discharging from a full charging state.
However, the internal resistance measuring method requires a high-precision measuring device. Moreover, in the method of integrating changes in battery capacity due to discharging, because the voltage across a detection resistor through which the current flows is amplified and detected, power is wasted. Moreover, this method requires a high-precision amplifier.
It may be considered that the count value of charging/discharging cycles is stored in a memory disposed in the battery. However, such a memory leads to increasing the fabrication costs. Moreover, the problem is that some use conditions of a battery result in the charging/discharging count of an initial promised cyclic life not matching with the actual charging/discharging count.
In recent years, nonaqueous electrolyte secondary batteries have been made of an alkali metal such as lithium acting as the negative electrode active substance, and an electrolysis solution in which LiClO4, LiBF4, iAsF6, LiPF6, or LiCF3SO3 is dissolved in an organic solvent. The organic solvent is, for example, propylene carbonate, &ggr;-butyrolactone, dimethoxyethane, tetrahydrafuran, or dioxane. These nonaqueous electrolyte batteries, having a high energy density, have been broadly used for small-size electronic devices such as electronic watches, cameras, and others. One of problems in making it possible to charge a nonaqueous electrolyte battery of that type is suppressing an arboreous, fibrillar, or acerate alkaline metal separated out on the negative electrode in the course of charging process; that is, the so-called dendrite. When the dendrite grows excessively, the operational life of the battery may be instantaneously terminated because of an internal shorted-circuit between the negative electrode and the positive electrode. Even if it is tried to dissolve the dendrite in the course of the post-discharging, the dendrite is locally dissolved and part thereof is electrically alienated from the polar plate. For that reason, all dendrites cannot be dissolved. That is, this phenomenon decreases a discharging (dissolution) amount to a charging (precipitation) amount, thus deteriorating the charging/discharging efficiency.
Two methods have been conventionally proposed to suppress the formation of dendrite in the course of the charging/discharging. In one method charging is carried out under such moderate conditions that the negative electrode potential is maintained at more than −50 mVvs.Li+/Li and that the alkali metal (lithium) to be educed is maintained at 0.3 mAh/cm
2
. In the other method, the charging/discharging efficiency is simultaneously improved by combining the electrolysis solution and a solvent of a high dielectric constant and of a low viscosity. These approaches are based on the idea of uniformly a separation reaction of the alkali metal on the electrode surface and thus suppressing the separation of dendrite.
However, even if the above-mentioned methods are employed, it is still difficult to fabricate a nonaqueous electrolyte secondary battery which has a cyclic life of 500 cycles or more, practically required as a secondary battery, while a high energy density is maintained. This difficulty comes from the fact that minute heterogeneous reactions repeated with charging/discharging cycles and integration of dendrite lead to changes to the heterogeneous reaction on electrodes. The minute uniformity results from the formation of a passive film through the reaction between an alkali metal acting as an active substance and an electrolysis solution or from differences in internal pressure applied to the respective electrodes in the winding-type battery. Moreover, the unevenness is triggered by the formation of dendrite and a decrease in charging/discharging efficiency.
In secondary batteries, a high voltage caused in charging may induce a dangerous state such as smoking or burning. In such a case, an irreversible chemical reaction may occur inside the battery, thus remarkably reducing the battery performance. Moreover, a low voltage caused in discharging may also induce an irreversible chemical reaction inside a battery, thus remarkably reducing the battery performance. For secondary batteries, manufacturers specify the upper limit voltage and the lower limit voltage, to be obeyed by users, to maintain batteries in safe state and to suppress drastic decreases in the performance thereof.
Generally, a secondary battery has a tendency of both the chargeable capacity and the dischargeable capacity decreasing as charging and discharging are repeated. The capacity decrease is called cyclic deterioration, or merely, deterioration. This is a matter of the operational life of a battery. There are several kinds of definitions of the chargeable/dischargeable capacity of a secondary battery. Typically, the chargeable/dischargeable capacity is defined as the product of an output power or output current when a battery charged to the upper voltage has been discharged to the lower voltage, multiplied by a discharging time period.
McGinn & Gibb PLLC
NEC Corporation
Tso Edward H.
LandOfFree
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