Active material for a negative electrode of an alkaline accumulator of the nickel-metal hydrid type

Abstract

A hydridable alloy of formula R 1-x-y Mg x M y Ni s-a B a wherein R is selected from the group consisting in rare earths, yttrium and a mixture thereof; M represents Zr and/or Ti; B is selected from the group consisting in Mn, Al, Co, Fe and a mixture thereof; 0.1<x<0.4; 0≦y<0.1; 3<s<4.5 and 0≦a<1; at least 5% of the volume of which consists of a stack of sequences of a pattern of the A 2 B 4 type and of n patterns of the CaCu 5 type randomly distributed along one direction, n being an integer comprised between 1 and 10 and representing the number of patterns of the CaCu 5 type per pattern of the A 2 B 4 type. A method for making a hydridable alloy comprising steps for compression and applying a current through a mixture comprising Mg 2 Ni and a compound comprising nickel and one or several elements selected from the group consisting in rare earths and yttrium.

1 . A hydridable alloy of formula R 1-x-y Mg x M y Ni s-a B a  wherein
 R is selected from the group consisting in rare earths, yttrium and a mixture thereof;   M represents Zr and/or Ti;   B is selected from the group consisting in Mn, Al, Co, Fe and a mixture thereof;   
       0.1<x<0.4; 0≦y<0.1; 3<s<4.5 and 0≦a<1; 
       at least 5% of the volume of which consists of a stack of sequences with a pattern of the A 2 B 4  type and n patterns of the CaCu 5  type randomly distributed along one direction, n being an integer comprised between 1 and 10 and representing the number of patterns of the CaCu 5  type per pattern of the A 2 B 4  type. 
     
     
         2 . The alloy according to  claim 1 , wherein the volume consisting of the stack of sequences with a pattern of the A 2 B 4  type and n patterns of the CaCu 5  type randomly distributed represents at least 10% by volume of the alloy, preferably at least 20%. 
     
     
         3 . The alloy according to  claim 1 , wherein the volume consisting of the stack of sequences with a pattern of the A 2 B 4  type and n patterns of the CaCu 5  type randomly distributed, represents less than 90% by volume of the alloy, preferably less than 70%. 
     
     
         4 . The alloy according to  claim 1 , wherein n is less than or equal to 8, preferably less than or equal to 6, still preferably less than or equal to 4. 
     
     
         5 . The alloy according to  claim 1 , wherein x is comprised between 0.1 and 0.3, preferably between 0.15 and 0.25. 
     
     
         6 . The alloy according to  claim 1 , wherein a is less than 0.3, preferably less than 0.15. 
     
     
         7 . The alloy according to  claim 1 , wherein s is comprised between 3 and 4, preferably between 3.5 and 4. 
     
     
         8 . The alloy according to  claim 1 , comprising Nd and Pr, wherein the R″/R′ molar ratio is less than 0.5, R″ designating the sum of the number of moles of Nd and of Pr, and R′ designating the sum of numbers of moles of rare earths, Y, Zr and Ti. 
     
     
         9 . A method for making a hydridable alloy comprising the steps:
 a) mixing Mg 2 Ni with a compound comprising:
 i) nickel, 
 ii) one or several elements selected from the group consisting in rare earths and yttrium, with optionally Ti and/or Zr, 
 iii) optionally an element selected from the group consisting in Mn, Al, Co, Fe and a mixture thereof; 
   b) milling the mixture;   c) sintering the mixture by compressing the mixture and applying current through the mixture.   
     
     
         10 . The method according to  claim 9 , wherein step c) is carried out by the flash sintering technique. 
     
     
         11 . The method according to  claim 9 , wherein the compound of step a) has the formula R′Ni y  with y comprised between 4 and 5, R′ designating the sum of the numbers of moles of rare earths, Y, Zr and Ti. 
     
     
         12 . The method according to  claim 9 , wherein the compression of step c) is accomplished under a pressure comprised between 40 and 80 MPa. 
     
     
         13 . The method according to  claim 9 , wherein step c) is accomplished at a temperature comprised between 700 and 900° C. 
     
     
         14 . The method according to  claim 9 , wherein the hydridable alloy has the formula R 1-x-y Mg x M y Ni s-a B a  wherein
 R is selected from the group consisting in rare earths, yttrium and a mixture thereof;   M represents Zr and/or Ti;   B is selected from the group consisting in Mn, Al, Co, Fe and a mixture thereof;   
       0.1<x<0.4; 0≦y<0.1; 3<s<4.5 and 0≦a<1. 
     
     
         15 . The method according to  claim 14 , wherein x is comprised between 0.1 and 0.3, preferably between 0.15 and 0.25. 
     
     
         16 . The method according to  claim 14 , wherein a is less than 0.3, and preferably less than 0.15. 
     
     
         17 . The method according to  claim 14 , wherein s is comprised between 3 and 4, preferably 3.5 and 4. 
     
     
         18 . The method according to  claim 14 , wherein the alloy comprises Nd and Pr and the R″/R′ molar ratio is less than 0.5, R″ designating the sum of the numbers of moles of Nd and of Pr, R′ designating the sum of the numbers of moles of rare earths, Y, Zr and Ti. 
     
     
         19 . An alloy which may be obtained by the method according to  claim 9 . 
     
     
         20 . A negative electrode of an alkaline accumulator of the nickel metal hydride type, comprising an alloy according to  claim 1 . 
     
     
         21 - 22 . (canceled) 
     
     
         23 . A negative electrode of an alkaline accumulator of the nickel metal hydride type, comprising an alloy according to  claim 19 . 
     
     
         24 . The electrode according to  claim 20  comprising from 0.4 to 1% by weight of yttrium oxide and/or from 1 to 2% by weight of manganese oxide. 
     
     
         25 . An alkaline accumulator of the nickel metal hydride type comprising an electrode according to  claim 20 . 
     
     
         26 . An alkaline accumulator of the nickel metal hydride type comprising an electrode according to claim  21 .