Since a lithium primary battery using a lithium metal or its alloy for the negative electrode active material has a high voltage and high energy density compared with conventional aqueous solution-type batteries, achieving downsizing and weight reduction is easy. Therefore, lithium primary batteries are applied for various usages, such as for a main power source of small electronic devices and for a back-up power source.
For the positive electrode active material of lithium primary batteries, generally, a metal oxide such as manganese dioxide, and graphite fluoride are used. The lithium primary battery using graphite fluoride is especially excellent in long-term storage and stability in high temperature range, and can be used in a wide temperature range, compared with those batteries using manganese dioxide.
However, as electronic devices becoming more multi-functional and small-sized, a further improvement in lithium primary battery performance is demanded. Particularly, in the case of a main power source for electronic devices to be mounted on automobiles and a back-up power source, sufficient discharge performance is required in a temperature range from low temperature to high temperature (about −40° C. to about 125° C.). When discharged at a large current, lithium primary batteries show characteristic drop in voltage in the initial stage of discharge and then gradual increase in voltage. In the lithium primary battery using graphite fluoride, the degree of voltage drop in the initial stage is high especially in the discharge in low temperature range.
One of the factors of the decline in discharge performance at low temperature is changes in viscosity of the organic electrolyte. The viscosity of γ-butyrolactone (γBL) used as the solvent of the electrolyte increases at low temperature. Therefore, ion conductivity of the electrolyte decline at low temperature. There has been proposed a usage of a solvent mixture in which 1,2-dimethoxyethane having a low boiling point and a low viscosity and γBL are mixed in a volume ratio of 1:1. In this case, there is an increase in the discharge voltage at a low temperature range of about −20° C., showing the effects of improvement. However, in such batteries, the degree of gas generation is high when stored at high temperature of about 100° C. Thus, there is a drawback in that batteries expand during storage at high temperature and normal discharge is disabled.
Further, in lithium primary batteries, the internal resistance is increased by storage at high temperature. Thus, when a battery stored at a high temperature of for example about 125° C. is discharged, the voltage drop immediately after the discharge becomes intense.
When the positive electrode polarization and negative electrode polarization are measured by a low temperature discharge −20° C. or less, in the early stage of discharge, the degree of polarization becomes higher in the negative electrode than in the positive electrode. In the batteries after a high temperature storage as well, the degree of polarization in the negative electrode becomes high. Therefore, by reducing the reaction over-voltage of negative electrode, low temperature characteristics and storage characteristics at high temperature can be improved greatly.
In the field of lithium secondary batteries, for an improvement in charge and discharge reaction, researches have been conducted on surface modification of a negative electrode comprising a lithium metal. Particularly, to reduce dendrite generation, there has been proposed a formation of a cover layer comprising carbon on the negative electrode surface (Patent Documents 1 and 2).
However, in primary batteries, which are not charged, the dendrite generation is not a great problem in the first place. Additionally, lithium metal melts from its surface by discharge. Therefore, even though a film or a layer is formed for the surface modification of the negative electrode, these are separated at the time of discharge. Thus, there is substantially no attempt to apply the surface modification techniques in the negative electrode of lithium secondary batteries to primary batteries.
[Patent Document 1]
    Japanese Laid-Open Patent Publication No. Hei 6-168737[Patent Document 2]    Japanese Laid-Open Patent Publication No. Hei 10-172540