Rare earth element-containing hydrogen storage alloy, method of producing the same, negative electrode using said alloy, and alkaline rechargeable battery using said electrode encapsulated

Abstract

Provided is a rare earth element-containing hydrogen storage alloy which fulfills at least one requirement selected from the requirements that (1) the total content of nonmetal elements present as impurities in the alloy matrix is not higher than 1,200 ppm by weight, (2) the total content of alkaline earth metal elements present as impurities in the alloy matrix is not higher than 100 ppm by weight, (3) the content of Mg present as an impurity in the alloy matrix is not higher than 80 ppm by weight, (4) the content of Cl present as an impurity in the alloy matrix is not higher than 10 ppm by weight, (5) the content of Pb present as an impurity in the alloy matrix is not higher than 100 ppm by weight, (6) the alloy has, in a surface layer covering from the alloy surface to a depth of 20 nm, an oxygen concentration that decreases sharply in the depth direction and is not higher than 10 weight % on the average in the surface layer defined above, (7) the alloy comprises Ni as a constituent element and contains, in a surface layer covering from the alloy surface to a depth of 20 nm, oxygen-combined Ni atoms in a proportion sharply decreased in the depth direction the average of which is not higher than 5 weight % to the total Ni atoms in the surface layer defined above, and (8) the surface hardness of the alloy is at least 600 kg/mm 2 in terms of Vickers hardness. Further, there are described a process of producing the aforesaid hydrogen storage alloy, a negative electrode using the hydrogen storage alloy defined above and an alkaline rechargeable battery using such a negative electrode.

What is claimed is:  
     
         1 . A rare earth element-containing hydrogen storage alloy, wherein at least Mg, Cl or Pb element is present as one of impurities in said alloy matrix in a content of no higher than 80 ppm, no higher than 10 ppm or no higher than 100 ppm, respectively, by weight.  
     
     
         2 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein alkali metal elements are present as impurities in said alloy matrix in a total content of no higher than 10 ppm by weight.  
     
     
         3 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein alkaline earth metal elements, including Mg, are present as impurities in said alloy matrix in a total content of no higher than 100 ppm by weight.  
     
     
         4 . A rare earth element-containing hydrogen storage alloy, wherein at least nonmetal elements or alkaline earth metal elements are present as impurities in said alloy matrix in a total content of no higher than 1,200 ppm or no higher than 100 ppm, respectively, by weight.  
     
     
         5 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein a sulfur element is present as one of impurities in said alloy matrix in a content of no higher than 10 ppm by weight.  
     
     
         6 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein an oxygen element is present as one of impurities in said alloy matrix in a content of no higher than 400 ppm by weight.  
     
     
         7 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein a carbon element is present as one of impurities in said alloy matrix in a content of no higher than 500 ppm by weight.  
     
     
         8 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein a nitrogen element is present as one of impurities in said alloy matrix in a content of no higher than 50 ppm by weight.  
     
     
         9 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein a boron element is present as one of impurities in said alloy matrix in a content of no higher than 10 ppm by weight.  
     
     
         10 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , wherein a fluorine element is present as one of impurities in said alloy matrix in a content of no higher than 10 ppm by weight.  
     
     
         11 . A rare earth element-containing hydrogen storage alloy, said alloy having an oxygen concentration which shows a sharp decrease from the alloy surface towards the inner part in a surface layer covering from the surface to a depth of 20 nm and has a value of no higher than 10 weight % on average in said surface layer.  
     
     
         12 . A rare earth element-containing hydrogen storage alloy, said alloy having a surface hardness of at least 600 kg/mm 2  in terms of Vickers hardness.  
     
     
         13 . A rare earth element-containing hydrogen storage alloy, said alloy comprising a Ni element and showing a rapid decrease in oxygen-combined Ni atom concentration in a surface layer covering from the alloy surface to a depth of 20 nm, wherein the proportion of the oxygen-combined Ni atoms to the total Ni atoms is not greater than 5 weight % on the average in said surface layer.  
     
     
         14 . A rare earth element-containing hydrogen storage alloy according to    claim 1   , which has a composition represented by formula Mm(NiCoM) x  wherein Mm is at least one rare earth element selected from the group consisting of Ce, La, Pr and Nd, M is at least one element selected from the group consisting of Mn, Al, Cu Fe and Ti and x is within the range of 4.0 to 6.0.  
     
     
         15 . A process of producing a rare earth element-containing hydrogen storage alloy having a composition represented by formula Mm(NiCoM) x  wherein Mm is at least one rare earth element selected from the group consisting of Ce, La, Pr and Nd, M is at least one element selected from the group consisting of Mn, Al, Cu, Fe and Ti and x is within the range of 4.0 to 6.0; which comprises a step of melting at least a part of starting materials by the use of a plasma melting method and reduces a content of at least Mg, Cl or Pb element present as one of impurities in said alloy matrix to no higher than 80 ppm, no higher than 10 ppm or no higher than 100 ppm, respectively, by weight.  
     
     
         16 . A process of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , wherein at least one of the starting materials molten by the use of a plasma melting method is a Misch metal.  
     
     
         17 . A process of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , wherein all the starting materials are molten by the use of a plasma melting method.  
     
     
         18 . A process of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , which comprises a step of melting all the starting materials by the use of a high-frequency induced melting method prior to said melting step using a plasma melting method.  
     
     
         19 . A process of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , which comprises a melting step using a high-frequency induced melting method after said melting step using a plasma melting method.  
     
     
         20 . A method of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , wherein the plasma melting method is carried out in an atmosphere of inert gas.  
     
     
         21 . A method of producing a rare earth element-containing hydrogen storage alloy according to    claim 20   , wherein the inert gas is argon or helium.  
     
     
         22 . A method of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , wherein the plasma melting method is performed using a water-cooled metallic hearth for accommodating a melt.  
     
     
         23 . A method of producing a rare earth element-containing hydrogen storage alloy according to    claim 22   , wherein the melt accommodated in the water-cooled metallic hearth is agitated by the high-frequency magnetic field induced by arranging high-frequency coils around said hearth and sending high-frequency electricity thereto.  
     
     
         24 . A method of producing a rare earth element-containing hydrogen storage alloy according to    claim 23   , wherein the water-cooled metallic hearth has a cylindrical crucible-form space provided with segments arranged symmetrically with respect to the vertical axis and holds the melt in a floating condition with a high-frequency magnetic field induced inside thereof by flowing a high-frequency electric current so as to surround the hearth.  
     
     
         25 . A method of producing a rare earth element-containing hydrogen storage alloy according to    claim 15   , wherein the plasma melting method is carried out by setting a positive electrode on the torch side and a negative electrode on the melt side to generate plasma.  
     
     
         26 . A negative electrode for an alkaline rechargeable battery, which has on a conductive support a hydrogen storage alloy according to    claim 1   .  
     
     
         27 . A negative electrode according to    claim 26   , which provides the alkaline rechargeable battery having a discharge capacity of at least 250 mAh/g in at least the third charge-discharge cycle carried out at 20° C.  
     
     
         28 . A negative electrode according to    claim 26   , which provides the alkaline rechargeable battery wherein the discharge capacity in the discharge operation carried out at 20° C. under the high efficienty discharge condition of 3C is not lower than 70% of the discharge capacity in the discharge operation carried out at 20° C. under the discharge condition of 0.2C.  
     
     
         29 . A negative electrode according to    claim 26   , which provides the alkaline rechargeable battery wherein the discharge capacity at −10° C. is not lower than 50% of the discharge capacity at 20° C.  
     
     
         30 . An alkaline rechargeable battery having a negative electrode according to    claim 26   .