1.  Field of the Invention
The present invention relates to a lithium secondary cell and, in particular, to a lithium secondary cell having a positive electrode containing an active material and a binder. 
2.  Description of the Related Art
In recent years, lithium cells, which have a high weight energy density, have been widely used as power supplies for electronic appliances such as portable telephones and portable video cameras.  A lithium cell includes a positive electrode containing a positive electrode active material containing lithium and capable of deintercalating lithium as lithium ions at the time of charging and capable of intercalating lithium ions at the time of discharging, a negative electrode having a negative electrode active material and capable of intercalating lithium ions at the time of charging and capable of deintercalating lithium ions at the time of discharging, and a non-aqueous electrolytic solution, including an organic solvent, containing a dissolved electrolyte composed of a supporting salt containing lithium. 
Lithium cells, in order to improve the weight to energy ratio, have positive and negative electrodes formed in the shape of sheets, which are wound or stacked through a sheet-like separator and contained in a case.  The sheet-like positive and negative electrodes have a structure where a composite material layer containing an active material is formed on the surface of a metal foil making up a collector. 
A positive electrode active material for use with the lithium cell generally includes a transition metal oxide or a chalcogen oxide such as LixCoO2, LixNiO2, LixMn2O4, LixFeO2, V2O5, Cr2O5, MnO2, TiS2 or MoS2. 
For lithium cells, LixCoO2, LixNiO2 and LixMn2O4 are known to be especially promising as positive electrode active materials for non-aqueous electrolyte lithium cells of the 4V class.  Among these Li compounds, LixNiO2 has the largest theoretical capacity and is expected to become a positive electrode active material, for the cells, that can be supplied steadily and at low cost.  Further, LixNiO2 can be a compound expressed as LixNi1xe2x88x92yMyO2 with an element M added thereto. 
For converting an electrode having these positive electrode active materials into a sheet electrode, the method disclosed in Japanese Unexamined Patent Publication No.  2-158055 is known.  In this method a powdered active material, a conduction agent, a water solution of calboxylmethylcellulose and an aqueous dispersion of polytetrafluoroethylene are uniformly mixed and prepared as a paste.  The paste is applied to a film-like conductive foil, such as a rolled aluminum foil, dried and rolled. 
In the process, a resin such as polytetrafluoroethylene or carboxymethylcellulose, having a superior resistance to organic solvents is, used as a binder.  These resins are not swollen or dissolved by an organic solvent such as ethylene carbonate (EC), propylene carbonate (PC) or diethyl carbonate (DEC) widely used as a solvent in the electrolytic solution for the lithium cell.  Also, the active material, even if it is expanded or contracted during use in the lithium cell, can be maintained in a strongly bound state.  If the binder is swollen or dissolved by an organic solvent, the binding force thereof would be reduced and the charge-discharge cycle characteristic thereof would be considerably deteriorated.  The use of binders having a superior resistance to organic solvents can realize a superior charge-discharge cycle characteristic of the cell.  This feature is exhibited more conspicuously when the cell is at a high operating temperature. 
From the viewpoint of electrode production, the use of water-dispersive or water-soluble resins as binders eliminates the need of an organic solvent and thus can suppress the cost increase which otherwise could not be avoided if an organic solvent was used.  Specifically, the elimination of the use of an organic solvent reduces the solvent cost which otherwise might be required for the organic solvent itself and eliminates the need of the solvent recovery processing equipment for electrode production, thereby effectively contributing to the reduction of the industrial waste such as the waste liquid of the organic solvent. 
Although LixNiO2 or the like positive electrode active material can increase the capacity of the lithium cell, the use of a water-dispersive or water-soluble resin having a superior resistance to organic solvents as a binder, however, poses the problem of a reduced large current discharge characteristic of the lithium cell due to a reduced cell capacity and an increased charge transfer resistance.  The cell performance is deteriorated by reason of the fact that LixNiO2 or the like positive electrode active material has a high reactivity with water, and therefore an exchange reaction occurs between protons and lithium ions on the surface of the positive electrode active material or a film is formed on the surface of the active material due to the reaction with water during the process of preparing the water-soluble active material paste for producing an electrode. 
To solve this problem by eliminating the effect of water, a vain attempt has been made to prepare a paste with polytetrafluoroethylene dispersed in a non-aqueous organic solvent to produce an electrode.  The failure of this attempt is due to the problem that polytetrafluoroethylene is not well dispersed in a non-aqueous organic solvent, polytetrafluoroethylene particles are entangled with each other to form a gel during the preparation of the paste and an aggregate is generated after electrode production often causing shorts when the cell is assembled. 
With the intention of improving the dispersion of polytetrafluoroethylene in a non-aqueous organic solvent, a method has been proposed to add a compound soluble in a non-aqueous organic solvent.  Japanese Unexamined Patent Publication No.  8-106897, for example, discloses a method of adding a polyvinylidene fluoride (PVDF) or plyvinylidene chloride soluble in a non-aqueous organic solvent, and Japanese Unexamined Patent Publication No.  10-12243 discloses a method of adding polyvinylbutyral for the same purpose. 
These compounds, however, have a resistance to electrolytic solutions inferior to polytetrafluoroethylene, and therefore are swollen or dissolved by an electrolytic solution.  At the same time, the binding force is reduced by the expansion and contraction of the electrode active material due to charge and discharge, resulting in a considerable cycle deterioration. 
Also, in the case where a resin easily swollen or dissolved by an electrolytic solution is used as a binder, the rate of coverage of the positive electrode active material by the binder is reduced, so that the contact area between the electrolytic solution and the positive electrode active material is increased during charge and discharge.  Thus, the reactivity between the electrolytic solution and the positive electrode active material increases.  Once the electrolytic solution reacts with the positive electrode active material, an insulative film of lithium fluoride is formed on the surface of the positive electrode active material by the decomposing reaction of the electrolytic solution.  The insulative film thus formed poses the problem of an increased internal resistance of the cell. 
The present invention has been developed in view of the aforementioned situation and the object thereof is to provide a lithium secondary cell which uses a water-soluble polymer having a superior resistance to organic solvents as a binder, has a high capacity, and a superior charge-discharge cycle, while suppressing an increase in the internal resistance. 
It has been discovered, as a result of extensive research into positive electrode active materials and binders, that the problem described above can be solved in such a way that the area involved in the reaction with water is reduced by limiting the specific surface area of the positive electrode active material, and a film of a water-soluble polymer having a high resistance to electrolytic solutions is formed uniformly on the surface of the positive electrode active material thereby to suppress an increase in the internal resistance accompanying the charge-discharge cycle. 
Specifically, according to this invention, there is provided a lithium secondary cell comprising a positive electrode including a positive electrode active material having a LiNiO2 compound expressed as LixNi1xe2x88x92yMyO2 (M is at least one element selected from a group including Co, Mn, Al, B, Ti, Mg and Fe, 0 less than xxe2x89xa61.2, 0 less than yxe2x89xa60.25) and a binder for binding the positive electrode active material, and an electrolytic solution, wherein the BET specific surface area of the LiNiO2 compound is not more than 0.65 m2/g, the binder is a water-soluble polymer not swollen with the electrolytic solution, and the surface of the positive electrode active material is covered by the water-soluble polymer. 
In the lithium secondary cell according to the invention, the specific surface area of the LiNiO2 compound of the positive electrode active material is reduced thereby to reduce the area involved in the reaction, thus suppressing a decrease in the cell capacity due to the reaction with water during cell production.  Also, the binder is formed of a water-soluble polymer having a superior resistance to organic solvents.  Since this water-soluble polymer uniformly covers the surface of the positive electrode active material, the increase of the internal resistance and the decrease of the discharge capacity are suppressed, thereby producing a cell having a superior charge-discharge cycle characteristic.