Nonaqueous electrolyte type secondary battery with a film casing

#The present invention relates to a nonaqueous electrolyte type secondary battery including an electricity generating element accommodated in a film casing and more particularly to a nonaqueous electrolyte type secondary battery configured to swell little.

#A secondary battery of the type described is conventional and includes an electricity generating element and a casing accommodating the electricity generating element. The casing is implemented by two aluminum laminate films each consisting or aluminum foil and a thermo-bonding resin film formed on the aluminum foil. Metallic leads protrude from the inside to the outside of the casing via the bonded portion of the casing. Today, there is an increasing demand for a thin, light battery configuration capable of implementing a thin, light electric apparatus. In this sense, the battery accommodated in the film casing is advantageous over a battery accommodated in a hard metallic casing.

#The battery with the film casing needs more strict safety implementations than the battery with the hard metallic casing. For example, the battery should undergo a minimum of deformation in contour. Of course, the battery with the film casing should have an ability generally required of a battery, e.g., high energy density (high charge-discharge capacity), a high cycle characteristic, and a storage capacity characteristic despite self-discharge.

#The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

#FIG. 1 is an isometric view showing a nonaqueous electrolyte type secondary battery embodying the present invention;

#FIG. 2 is a sectional view of the secondary battery shown in FIG. 1;

#The principle of the present invention will be described first. In accordance with the present invention, in a nonaqueous electrolyte type secondary battery including an electricity generating element, which includes at least a positive electrode implemented by a lithium-manganese compound oxide, a negative electrode, an electrolyte and a separator, and films encasing the electricity generating element, a composition causing the electrolyte to easily react with water to thereby produce hydrogen ions is provided. A hydrogen ion uptaking agent is so positioned as to contact the electrolyte existing in the secondary battery.

#In accordance with the present invention, the electrolyte contains a supporting electrolyte that is either one of LiPF6 and LiBF4.

#We reached the present invention as a result of extended research and experiments directed toward the reduction of Mn to be eluted from the lithium-manganese compound oxide. Assume a nonaqueous electrolyte type secondary battery using a lithium-manganese compound oxide as a positive electrode active substance. Then, it is possible to determine the deterioration of the cycle characteristic by using a Mn ion concentration in an electrolyte as an index. Also, it is possible to determine the deterioration of the storage characteristic on the basis of a change in a Li ion concentration in the electrolyte. Gases causing the battery to swell are ascribable partly to the decomposition of the positive electrode active agent (elution of Mn and release of oxygen) and partly to Mn precipitated on the surface of a negative electrode and causing the electrolysis of the electrolyte. The generation of such gases can therefore be determined by use of a Mn ion concentration in the electrolyte.