1. Field of the Invention
The present invention relates to a cylindrical lithium-ion battery, and in particular relates to a cylindrical lithium-ion battery which is provided with an electrode winding group where a strip-shaped positive electrode having a positive collector applied with positive electrode active-material from/in which lithium ions can be released/occluded through charging/discharging and a strip-shaped negative collector applied with negative electrode active material in/from which lithium ions can be occluded/released through charging/discharging are wound through a strip-shaped separator through which lithium ions can pass, a cylindrical battery container in which the electrode winding group is accommodated, and a lid plate for sealing the battery container having an internal pressure reducing mechanism for discharging gas according to an increase in internal pressure of the battery container.
2. Description of the Related Art
Attention has been paid to a lithium-ion secondary battery as a power supply source for an EV (Electric Vehicle) because of its high power (output) and high energy density. The lithium-ion secondary batteries can be classified into cylindrical shape ones and rectangular shape ones according to their outer shapes. The interior of the cylindrical battery has a winding type structure where each of a positive electrode and a negative electrode comprises a metal foil applied with active material and is formed in a strip shape, and the positive and negative electrodes are wound spirally through a separator(s) so as not to directly come in contact with each other to form an electrode winding group. The electrode winding group is accommodated in a cylindrical can or container which is a battery container, and after the battery container is filled with electrolytic solution, it is sealed and then by initial charge the cylindrical battery is given a function as a battery.
In order to improve energy density, it is preferable to fill up more active material in the battery container, and for this reason, the electrode winding group is inserted into the battery container in a relatively dense manner. However, in a lithium-ion secondary battery where lithium cobaltate and carbon material of graphite system are used as positive active material and negative active material respectively, the active materials of both the positive and negative electrodes cause volume expansion at a time of the initial charge, consequently various drawbacks occur in the electrodes. As a result, the battery causes capacity reduction and life shortening in some cases. For solving the problems, a lithium-ion secondary battery for a civilian use with capacity of about 2 Ah or less, which is nowadays housed in a notebook-type personal computer, a portable telephone or the like, has a structure where a slight space is formed between the electrode winding group and the battery container within the range in which remarkable capacity reduction and energy density lowering can be suppressed. Also, in a case where such a battery is industrially manufactured in a continuous automatic production manner, since inserting easiness of an electrode winding group to a battery container is improved, it is preferable to provide the above space.
Further, in a lithium-ion secondary battery with approximate capacity of 3.0 Ah or more, which is suitable for the EV power supply source, the power becomes necessarily high. Therefore, explosion or firing must completely be prevented from occurring even at an abnormal time when the battery is overcharged, when it is broken or collapsed in a pressurized manner, etc. However, such a lithium-ion secondary battery with high capacity and high power must be increased in length and diameter. If the above strip-shaped electrode comprising the metal foil applied with the active material is made thicker by increasing an application amount of the active material, a layer of the active material peels off or falls off from the foil. As a result, a normal shape of the electrode can not be maintained. For this reason, a diameter of the electrode winding group is made large by making the application thickness of the active material thin and making the strip-shaped electrode long to increase the number of windings.
However, in the electrode winding group where the electrodes are made long and the number of windings is increased, there is a problem that gas generated by decomposition of electrolytic solution or the like at the abnormal time tends to stay inside the electrode winding group and the gas can not be smoothly discharged from an internal pressure reduction mechanism such as a safety valve or the like, thereby causing abnormal heat generation and remarkable deformation of the battery container.
The present inventors have eagerly studied and researched safety about a cylindrical lithium-ion secondary battery. As a result, it has been found that, for securing safety in the cylindrical lithium-ion battery with high capacity and high power, it is necessary to define a space between an electrode winding group and a battery container. That is, the space must be defined for the purpose of securing safety, which is different from the space defined in the lithium-ion secondary battery for the civilian use with relatively small capacity.
In view of the above circumstances, an object of the present invention is to provide a cylindrical lithium-ion battery where abnormal heat generation and remarkable deformation do not occur even at an abnormal time and which has an excellent safety.
In order to solve the above problems, according to the present invention, there is provided a cylindrical lithium-ion battery, comprising: an electrode winding group where a strip-shaped positive electrode having a positive collector applied with positive electrode active material from/in which lithium ions can be released/occluded through charging/discharging and a strip-shaped negative collector applied with negative electrode active material in/from which lithium ions can be occluded/released through charging/discharging are wound through a strip-shaped separator through which lithium ions can pass; a cylindrical battery container in which the electrode winding group is accommodated; and a lid plate for sealing the battery container having an internal pressure reducing mechanism for discharging gas according to an increase in internal pressure of the battery container, wherein, when an average diameter of the electrode winding group is A mm, an inner diameter of the battery container is B mm, a longitudinal length of the electrode winding group is H mm, and the number of windings of the electrode winding group is W, a calculation value K calculated in accordance with the following Formula (1) is 0.89 or more:
K=(Bxe2x88x92A)xc3x9710000/(Wxc3x97H)xe2x80x83xe2x80x83Formula (1)
In the present invention, when the average diameter of the electrode winding group is A mm, the inner diameter of the electrode winding group is B mm, the longitudinal length of the electrode winding group is H mm, and the number of windings of the electrode winding group is W, safety of the cylindrical lithium battery is secured even at the abnormal time or state by setting the calculation value K calculated in accordance with the Formula (1) to 0.89 or more. The Formula (1) means that a gap (space) (Bxe2x88x92A) between an outer periphery of the electrode winding group and an inner periphery of the battery container must be defined larger in order to exhaust gas generated inside the electrode winding group by expansion of the electrode winding group and to discharge the gas outside the battery through the internal pressure reducing mechanism smoothly, since the gas generated inside the electrode winding group at the abnormal time tends to stay inside the electrode winding group as the number of windings W is increased and/or the longitudinal length of the electrode winding group H is made longer. According to the present invention, since the gap (Bxe2x88x92A) which allows the electrode winding group to expand is defined properly between the outer periphery of the electrode winding group and the inner periphery of the battery container by setting the calculation value k given by the Formula (1) to 0.89 or more, the gas generated inside the electrode winding group at the abnormal time expands the electrode winding group to be exhausted smoothly outside the electrode winding group and further discharged outside the battery through the internal pressure reducing mechanism. Thus, safety of the cylindrical lithium-ion battery can be secured. Here, by transforming the Formula (1) and the gap (Bxe2x88x92A) can be obtained in accordance with the following Formula (2):
(Bxe2x88x92A)xe2x89xa78.9xc3x9710xe2x88x925xc3x97(Wxc3x97H)xe2x80x83xe2x80x83Formula (2)
In order to discharge the gas generated inside the electrode winding group at the abnormal time outside the battery smoothly, it is preferable to use lithium-manganese complex oxide as the positive active material and amorphous carbon as the negative active material. It is more preferable that the present invention is applied to a battery where the number of windings W of the electrode winding group is 40 or more and discharge capacity of the battery is 3.0 Ah or more. The operations and effects according to the present invention will become more obvious with reference to the preferred embodiments explained below.