The present invention relates to a non-aqueous electrolyte secondary battery having a high energy density and excellent cycle characteristics.
With the recent advancement of portable appliances and cordless appliances, there has been a demand for batteries having a high energy density in order to realize drive for long period. To meet this demand, attentions have been focused on such batteries as lithium ion secondary batteries and nickel-metal hydride storage batteries which use a hydrogen storage alloy in the negative electrode. Among them, those using metallic lithium in the negative electrode are promising in the viewpoint of a further higher capacity. However, a secondary battery using a multivalent cation of Mg, Al or the like is considered as a more promising battery system.
For example, a battery using magnesium in the negative electrode is expected to have a higher theoretical volume energy density than one using metallic Li, because two electrons transfer when 1 M of magnesium reacts in the negative electrode. Since magnesium is abundant in natural resources, low-priced and environmentally friendly, it is highly desired as a negative electrode material.
However, there has been a problem that the use of an aqueous solution as the electrolyte in a secondary battery containing magnesium makes the dissolution and precipitation of the magnesium difficult. This is because electrolysis of water occurs during charge owing to low hydrogen overvoltage. On the other hand, the use of a non-aqueous solvent as the electrolyte facilitates the dissolution and precipitation of the magnesium since it expands the range of the usable voltage (e.g., Japanese Laid-Open Patent Publication Nos. Sho 62-211861, Hei 1-95469 and Hei 4-28172; and Journal of Applied Electrochemistry, Vol. 27, 221-225, (1997)).
The cause that prevents the dissolution and precipitation of magnesium as described above is probably because a number of ions, which constitute a solvent, are present in the form of counter ions around a magnesium ion, which is a multivalent cation. Thus, the decomposition reaction of the electrolyte concurrently occurs, so that the dissolution and precipitation of magnesium does not occur in the amount corresponding to the amount of the current being passed.
In this respect, it has been reported that a highly efficient charge/discharge is possible only when the dissolution and precipitation of magnesium is attempted by the use of a tetrahydrofuran solution containing ethyl magnesium bromide dissolved therein (Journal of Applied Electrochemistry, Vol. 27, 221-225, (1997)).
However, as a result of study on a battery in which the above negative electrode was combined with a positive electrode, it was found that a bromine ion as a solute was oxidized on the positive electrode to generate a bromine gas, while the current is passed. Additionally, when tetrahydrofuran was used singly as a solvent, tetrahydrofuran was oxidized and decomposed on the positive electrode with the progress of charge/discharge cycles, thereby causing a significant decrease in the battery capacity.
In view of the above-described prior arts, an object of the present invention is to provide a non-aqueous electrolyte secondary battery having a high capacity and excellent charge/discharge cycle characteristics.
The non-aqueous electrolyte secondary battery in accordance with the present invention comprising a rechargeable positive electrode, a non-aqueous electrolyte and a rechargeable negative electrode is characterized in that the non-aqueous electrolyte is added with a halogen-containing organic magnesium compound represented by the formula (1):
RMgX,xe2x80x83xe2x80x83(1) 
where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, X is fluorine atom, chlorine atom, bromine atom or iodine atom; or the formula (2):
RMgY,xe2x80x83xe2x80x83(2) 
where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, Y is xe2x80x94ClO4xe2x88x92, xe2x80x94BF4xe2x88x92, xe2x80x94PF6xe2x88x92 or xe2x80x94CF3SO3xe2x88x92.
It is effective that the non-aqueous electrolyte contains at least one solvent selected from the group consisting of an ester type solvent, an amine type solvent, a nitrile type solvent, an amide type solvent, a sulfur type solvent and a linear ether type solvent; and a halogen-containing organic magnesium compound represented by the formula (1): RMgX, where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, X is fluorine atom, chlorine atom, bromine atom or iodine atom.
The non-aqueous electrolyte preferably further contains a cyclic ether type solvent.
The ester type solvent is preferably propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate or xcex3-butyrolactone.
Further, the amine type solvent is preferably pyridine.
The nitrile type solvent is preferably acetonitrile.
The amide type solvent is preferably N-methylformamide or dimethylformamide.
The sulfur type solvent is preferably dimethylsulfoxide or sulfolane.
Further, the linear ether type solvent is preferably dimethoxyethane or diethyl ether.
The cyclic ether type solvent is preferably tetrahydrofuran or 2-methyltetrahydrofuran.
Further, in the present invention, it is effective that the non-aqueous electrolyte contains a halogen-containing organic magnesium compound represented by the formula (2): RMgY, where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, Y is xe2x80x94ClO4xe2x88x92, xe2x80x94BF4xe2x88x92, xe2x80x94PF6xe2x88x92 or xe2x80x94CF3SO3xe2x88x92.
In the formulae (1) and (2), the aliphatic hydrocarbon group R preferably has 1 to 4 carbon atoms.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.