Recently, the portable and cordless trend is rapid among consumer electronic appliances. Hitherto, the role as the driving power source of such electronic appliances has been played by the nickel-cadmium battery, nickel-hydrogen storage battery, or enclosed type small lead storage battery. However, as these appliances are promoted in reduction in size and weight and sophistication in function, there is a stronger demand for higher energy density, smaller size and lighter weight in the secondary battery as the driving power source. In this background, various positive electrode active substances have been proposed. Among others, principal ones include lithium compound transition metal oxide showing a high charging and discharging voltage such as LiCoO.sub.2 (for example, Japanese Patent Publication No. 63-59507), LiNiO.sub.2 aiming at higher capacity (for example, U.S. Pat. No. 4,302,518), compound oxide of plural metal elements such as Li.sub.z Ni.sub.y CO.sub.1-y O.sub.2 with lithium (Japanese Laid-open Patent No. 63-299056), and LixMyNzO2 (M: at least one material selected from Fe, Co, Ni; N: at least one selected from Ti, V, Cr, Mn) (Japanese Laid-open Patent No. 4-267053). Using them as positive electrode active substances, various nonaqueous electrolyte solution secondary batteries employing carbon materials capable of occluding and releasing lithium ions in the negative electrode have been proposed. Moreover, batteries using LiCoO.sub.2 in the positive electrode and carbon in the negative electrode have been developed. LiNiO.sub.2 is a stable source of Ni as the raw material, and low cost and high capacity are expected. Thus, LiNiO.sub.2 has been intensively researched and developed as a hopeful active substance.
However, in the batteries using the hitherto reported positive electrode active substances (in particular, LiNi.sub.y M.sub.1-y O.sub.2, where M is at least one of Co, Mn, Cr, Fe, V and Al; 1y0.5), in the potential range commonly used as the battery (4.3 V to 2 V to Li), it is known that there is a considerable difference in charging and discharging capacity between the first cycle charging (lithium releasing reaction) and discharging (lithium occluding reaction) (for example, A. Rougier et al., Solid State Ionice 90, 83, 1996). The lithium ion (Li.sup.+) corresponding to this charging and discharging capacity difference (positive electrode irreversible capacity) is irreversible, and therefore this lithium ion is released from the positive electrode in charging, but is not occluded in discharging. Generally, the percentage of the Li.sup.+ occlusion amount by discharge to the release amount of Li.sup.+ by first cycle charge is called the charging and discharging rate of the positive electrode active substance. In particular, the charging and discharging efficiency is low in the case of LiNi.sub.y M.sub.1-y O.sub.2.
In the case of negative electrode material, the percentage of the Li.sup.+ release amount by discharge to the Li.sup.+ occlusion amount by first cycle charge is called the charging and discharging efficiency. As such negative electrode material of nonaqueous electrolyte solution secondary battery, a carbon material showing a high charging and discharging efficiency of about 90% or more such as graphite is used.
As shown in FIG. 3, in the conventional secondary battery using a positive electrode of which charging and discharging efficiency is lower than that of a negative electrode, Li.sup.+ released from the positive electrode by first cycle charging is occluded in the negative electrode, and, by discharge, Li.sup.+ corresponding to the positive electrode reversible capacity is released from the negative electrode, and the Li.sup.+ corresponding to the positive electrode irreversible capacity is left over in the negative electrode even after completion of discharge. The Li.sup.+ left over on the negative electrode includes ions corresponding to the reversible capacity (capacity indicated by A in FIG. 3) left over on the negative electrode in the undischarged state even after completion of discharge of the battery, because the reversible capacity of the positive electrode is smaller than that of the negative electrode although it is possible to discharge completely by nature, and ions corresponding to the capacity (negative electrode irreversible capacity) that cannot be released intrinsically by discharge reaction, as remaining fixed on the negative electrode.
There is a limit in the reversible Li occlusion amount of the carbon material of the negative electrode, that is, in the reversible charging capacity. For example, when graphite is used in the negative electrode, the limit is the charging capacity 372 mAh/g corresponding C.sub.6 Li. When an amorphous carbon material other than graphite is used, there is a material showing a larger limit amount. If attempted to charge beyond this limit, Li.sup.+ is reduced, and metal lithium electrochemical deposits on the surface of the negative electrode. This electrochemical depositing metal lithium is likely to react chemically with the electrolyte solution, and is inert electrochemically, and it is desorbed from the negative electrode main body. Thus, the charging and discharging efficiency drops, and hence the cycle characteristic of the battery may be lowered significantly.
That is, if the Li.sup.+ corresponding to the residual reversible capacity not responsible for charging and discharging indicated by A in FIG. 3 is held in the negative electrode carbon, the reversible electric capacity of the negative electrode that can be charged in the second and subsequent cycles becomes smaller. Accordingly, the electric capacity capable of charging and discharging in the battery decreases, and an electric quantity over the limit of reversible electric capacity is likely to be passed when charging, and the metal lithium is likely to electrochemical deposit on the negative electrode surface. To solve such a problem, it may be considered to increase the carbon amount to be used in the negative electrode in order to have a sufficient extra capacity in the occlusion capacity of the negative electrode even in the state of occluding all Li.sup.+ released from the positive electrode by the first cycle charge in the negative electrode. As a result, electrochemical deposition of metal lithium by charging is suppressed, but a wider space is occupied by the increment of carbon, and the filling volume of the active substance decreases accordingly. It hence decreases the battery capacity.
USPTO Backgrounds
USPTO Backgrounds