US20120134871A1

US20120134871A1 - Active material for a negative electrode of an alkaline accumulator of the nickel-metal hydrid type

Info

Publication number: US20120134871A1
Application number: US13/305,317
Authority: US
Inventors: Patrick Bernard; Bernard Knosp; Michel Latroche; Junxian ZHANG; Virginie SERIN; Martin Hytch
Original assignee: SAFT Societe des Accumulateurs Fixes et de Traction SA; Centre National de la Recherche Scientifique CNRS; Universite Paris Est Creteil Paris 12
Current assignee: SAFT Societe des Accumulateurs Fixes et de Traction SA; Centre National de la Recherche Scientifique CNRS; Universite Paris Est Creteil Paris 12
Priority date: 2010-11-29
Filing date: 2011-11-28
Publication date: 2012-05-31
Also published as: EP2487270B1; ES2498917T3; EP2487270A1; FR2968015A1; FR2968015B1

Abstract

A hydridable alloy of formula R_1-x-yMg_xM_yNi_s-aB_awherein

- 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 A₂B₄type and of n patterns of the CaCu₅type randomly distributed along one direction, n being an integer comprised between 1 and 10 and representing the number of patterns of the CaCu₅type per pattern of the A₂B₄type.

A method for making a hydridable alloy comprising steps for compression and applying a current through a mixture comprising Mg₂Ni and a compound comprising nickel and one or several elements selected from the group consisting in rare earths and yttrium.

Description

TECHNICAL FIELD

The technical field to which relates the invention is that of hydridable alloys for a negative electrode of an accumulator with an alkaline electrolyte of the nickel-metal hydride type as well as that of methods for preparing such alloys.

STATE OF THE ART

Applications such as electric vehicles (EV for Electric Vehicle), hybrid vehicles (HEV for Hybrid Electric Vehicle), rechargeable hybrid vehicles (PHEV for Plug-in Hybrid Electric Vehicle), safety lighting (ELU for Emergency Lighting Unit) or PhotoVoltaic applications (PV) require increasing needs in terms of bulk energy. An accumulator with an alkaline electrolyte of the nickel metal hydride (Ni-MH) type comprising a negative electrode based on a hydridable alloy of the type AB₅(wherein A represents one or more elements forming stable hydrides under normal conditions of temperature and of pressure, such as for example rare earths, and B one or more elements, for which the hydrides are unstable under the same conditions, such as for example nickel) and a positive electrode based on nickel hydroxide does not satisfactorily meet the development of this energy need.
In an Ni-MH accumulator, the initial capacity is limited by construction (or design) by the capacity of the positive electrode. Thus, increasing the initial capacity of the accumulator requires an increase in the capacity of the positive electrode, therefore of its active material volume, since optimization of the technology of this electrode is at a development stage where the performances are now equivalent to the theoretical yields. Further, the lifetime of an Ni-MH accumulator is limited by the corrosion of the alloy and its consequences, i.e. the reduction in the capacity of the negative electrode and the drying of the bundle consecutive to consumption of water by the corrosion reaction inducing an increase in the impedance of the accumulator. The increase in the initial capacity of the accumulator is therefore accomplished to the detriment of its lifetime since this leads to a limitation either of the volume of the electrolyte, or of that of the alloy. Thus, an increase in the initial capacity of the accumulator can only be obtained to the detriment of its lifetime. Conversely, it is possible to achieve an increase in the lifetime of the accumulator but to the detriment of its initial capacity. Research work is therefore conducted in order to obtain an active material of the negative electrode providing both high capacity and a long lifetime.
In order to increase the bulk capacity, novel negative electrode materials have been contemplated. Mention may be made of alloy families with a stoichiometry of the AB₂type. However, although their initial capacity is much greater than that of a AB₅stoichiometry alloy, their power and lifetime are considerably reduced. Recently, the use of an alloy of composition (R, Mg)B_xwas proposed, wherein R represents one or more elements selected from rare earths, yttrium, Zr and Ti and B represents the nickel element partly substituted with other elements such as Co, Mn, Al or Fe, with x comprised between 3 and 4. These alloys may consist of one or more crystalline phases, such as:
the phase of composition AB₅crystallizing in the hexagonal system of the CaCu₅type;
the phases of composition AB₂, so-called Laves phases, either crystallizing in the cubic system: a so-called “C15” phase of the MgCu₂type, or in the hexagonal system: so-called “C14” phase of the MgZn₂type or so-called “C36” phase of the MgNi₂type;
the phases of composition AB₃either crystallizing in the hexagonal system (H—CeNi₃type) or in the rhombohedral system (R—PuNi₃type);
the phases of composition A₂B₇either crystallizing in the hexagonal system (H—Ce₂Ni₇type), or in the rhombohedral system (R—Gd₂Co₇type);
the phases of composition A₅B₁₉either crystallizing in the hexagonal system (H—Pr₅Co₁₉type), or in the rhombohedral system (R—Ce₅Co₁₉type);
the phase of composition AB₄.
Each of these crystalline phases, except for the phases AB₅and AB₂may be considered as consisting of one or more patterns of the type C of formula AB₅associated with a pattern of type L of composition A₂B₄corresponding to 2 formulae AB₂.
For example:
a crystalline phase of composition AB₃consists of a pattern of the type C and of a pattern of the type L;
a crystalline phase of composition A₂B₇consists of a pattern of type C and of two consecutive patterns of type L;
a crystalline phase of composition A₅B₁₉consists of a pattern of type C and of three consecutive patterns of type L;
a crystalline phase of composition AB₄consists of a pattern of type C and of four consecutive patterns of type L.
The following documents describe hydridable alloys having a good lifetime under charging-discharging cycle conditions.
Document JP 2001-316744 describes a hydridable alloy of formula Ln_1-xMg_x(Ni_1-yT_y)_zwherein:

- Ln is an element selected from rare earths, Ca, Sr, Sc, Y, Ti, Zr and Hf, lanthanum representing 10 to 50 atomic % of Ln,
- T is at least one element selected from Li, V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Al, Ga, Zn, Sn, In, Cu, Si, P and B;

0.05≦x<0.20, 0≦y≦0.5 and 2.8≦z≦3.9.
Document JP 2002-069554 describes a hydridable alloy of formula R_1-aMg_aNi_bCO_cM_dwherein
R represents at least two elements selected from rare earths and Y;
M represents at least one element selected from Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P and B; 0.15<a<0.35; 0≦c≦1.5; 0≦d≦0.2 and 2.9<b+c+d<3.5. It is stated that crystalline phases of the AB₂, AB₃, A₂B₇, AB₅, Mg₂Ni and MgNi₂type may be obtained.
Document EP-A-1 026 764 describes a hydridable alloy of formula AM_xwherein A may be a rare earth, optionally substituted with magnesium; and M may be selected from Cr, Mn, Fe, Co and Ni. x is comprised between 2.7 and 3.8. The average atomic radius r of the atoms of the alloy is comprised between 1.36 and 1.39 Å. x and r satisfy the relationship 1.42≦0.017x+r≦1.44.
Document U.S. Pat. No. 6,214,492 describes a hydridable alloy comprising a crystalline phase comprising at least one unit cell which is a stack of at least one structure of the A₂B₄type and of at least one structure of AB₅type, the ratio between the number of structures of the A₂B₄type and the number of structures of AB₅is comprised between 0.5 and 1. Preferably the unit cell is an ordered stack of the LCLCC type, wherein L represents the structure of type A₂B₄and C represents a structure of type AB₅. The crystalline phase comprises a repetition of stacks of the LCLCC type.
Document US 2004/0134569 describes a hydridable alloy of formula Ln_1-xMg_xNi_y-aAl_awherein Ln is at least one rare earth; 0.05≦x<0.20; 2.8≦y≦3.9 and 0.10≦a≦0.25.
Document US 2004/0146782 describes the hydridable alloy of formula Ln_1-xMg_xNi_y-aM_awherein Ln is at least one rare earth, M is at least one element selected from Al, V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si et P and 0.05≦x<0.20. 2.8≦y≦3.9 and 0.10≦a≦0.50.
Document US 2005/0100789 describes a negative electrode of a nickel metal hydride accumulator, comprising:
a) a hydridable alloy of formula Ln_1-xMg_xNi_yAl_zM_awherein Ln is at least one rare earth, M is an element other than a rare earth, Mg, Ni or Al, and 0.10≦x<0.30. 2.8≦y≦3.6. 0≦z≦0.30 and 3.0≦y+z+a≦3.6 and
b) an amount of manganese of less than 1% based on the weight of the hydridable alloy. This document describes that an alloy which crystallizes in the AB₅structure is easily oxidized. Therefore, with it is not possible to obtain a good lifetime of the electrode during cycling.
Documents US 2005/0175896 and US 2005/0164083 describe a hydridable alloy comprising at least one rare earth, magnesium, nickel and aluminium and mainly crystallizing in a structure of the Ce₂Ni₇type.
Document EP-A-2 096 691 describes a hydridable alloy of formula Ln_1-xMg_xNi_y-a-bAl_aM_bwherein Ln represents at least two elements including lanthanum, M represents at least one element selected from Co, Mn and Zn, 0.1≦x≦0.20, 3.6≦y≦3.9, 0.1≦a≦0.2 and 0≦b≦0.1. This alloy comprises in majority a crystalline phase of the A₅B₁₉type and may comprise A₂B₇and/or AB₅phases. The equilibrium hydrogen pressure at 40° C. is comprised between 0.3 and 1.7 bars for an absorbed hydrogen amount equal to 0.5 H/M.
Document US 2008/0085209 describes a hydridable alloy of formula R_1-uMg_uNi_t-vM_vwherein R represents at least one element selected from La, Ce, Nd and Pr; M represents one or more elements selected from Mn, Fe, Al, Co, Cu, Zr, Sn and M does not contain Cr; 0≦u≦0.25, v≦0.5 and 3.5≦t≦4.5. This alloy comprises in majority a crystalline phase of the A₅B₁₉type and optionally the A₂B₇and/or AB₅phases. The equilibrium hydrogen pressure at 40° C. is less than 1.5 bars for an amount of absorbed hydrogen amount of less than 1%.
Document JP 2010-073424 describes a hydridable alloy containing a rare earth, magnesium and nickel. It comprises a stack of crystalline phases crystallizing in the AB₃, A₂B₇, A₅B₁₉, AB₄and AB₅systems. The phases AB₃, A₂B₇, A₅B₁₉, AB₄each consist of a stack of a pattern of the A₂B₄type with n patterns of the AB₅type, n ranging from 1 to 4. The stacking of the crystalline phases is carried out in an ordered way, i.e., with a gradual variation of the parameter n, therefore of the number of patterns of the AB₅type in the direction of the stacking The following stacking order is indicated AB₃, A₂B₇, A₅B₁₉, AB₄, i.e. a B/A ratio of 3; 3,5; 3,8 and 4 respectively. This gradual increase in the B/A ratio indicates a gradual increase in the amount of nickel based on the total amount of rare earths and of magnesium in the direction of the stacking The number n of patterns of the AB₅type gradually increases in the direction of the stacking
Document JP 09-194971 describes a hydridable alloy of formula R₂(Ni_7-x-y-zMn_xA_yB_z)_nwherein R represents at least one rare earth, A represents at least one element selected from Co, Cr, Fe, Al, Zr, W, Mo, Ti; and B represents at least one element selected from Cu, Nb, V; 0.3≦x≦1.5; 0≦y≦1.0; 0≦z≦1.0; y+z≦1.0 and 0.96≦n≦1.1. This alloy comprises a crystalline phase of the hexagonal Ce₂Ni₇type.
Document EP-A-0 783 040 describes a hydridable alloy of formula (R_1-xL_x)(Ni_1-yM_y)_zwherein R represents La, Ce, Pr or Nd, M represents Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, Si, V, Cr, Nb, Hf, Ta, W, B or C; with 0.05≦x≦0.4; 0≦y≦0.5 and 3.0≦z≦4.5.
Document JP 2004-115870 describes a hydridable alloy of formula Ln_1-xMg_xNi_yM_zwherein Ln represents at least one element selected from Y, Sc and rare earths; M represents one or more elements selected from Co, Mn, Al, Fe, V, Cr, Nb, Ga, Zn, Sn, Cu, Si, P or B; 0.1≦x≦0.5; 2.5≦y≦3.5; 0≦z≦0.5 and 3.0≦y+z≦3.5.
An electrode active material is sought which has a high initial mass capacity as well as a good lifetime during cycling. Such an active material is characterized by an initial mass capacity of at least 310 mAh/g and with a capacity degradation of less than 15% after 100 cycles. A method for preparing such an active material is also sought.

SUMMARY OF THE INVENTION

For this purpose, the object of the invention is a hydridable alloy, of formula R_1-x-yMg_xM_yNi_s-aB_awherein
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 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;
wherein at least 5% of the volume consists of a stack of sequences of a pattern of the A₂B₄type and n patterns of the CaCu₅type randomly distributed along a direction, n being an integer comprised between 1 and 10 and representing the number of patterns of the CaCu₅type per pattern of the A₂B₄type.
According to an embodiment, the volume consisting of the stack of sequences of a pattern of the A₂B₄type and of n patterns of the CaCu₅type randomly distributed, represents at least 10% of the volume of the alloy, preferably at least 20%.
According to an embodiment, the volume consisting of the stack of sequences of a pattern of the A₂B₄type and of n patterns of the CaCu₅type randomly distributed, represents less than 90% of the volume of the alloy, preferably less than 70%.
According to an embodiment, n is less than or equal to 8, preferably less than or equal to 6, still preferably less than or equal to 4.
According to an embodiment x is comprised between 0.1 and 0.3, preferably between 0.15 and 0.25.
According to an embodiment, a is less than 0.3, preferably less than 0.15.
According to an embodiment, s is comprised between 3 and 4, preferably between 3.5 and 4.
According to an embodiment, the alloy comprises Nd and Pr, and 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 the numbers of moles of rare earths Y, Zr and Ti.
The invention also relates to a method for making a hydridable alloy comprising the steps:

- a) mixing Mg₂Ni with a compound comprising:
  - i) nickel,
  - ii) one or more elements selected from the group consisting in rare earths and yttrium, optionally with 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.

According to an embodiment, step c) is achieved by the flash sintering technique.
According to an embodiment, the compound of step a) has the formula R′Ni_ywith y comprised between 4 and 5, R′ designating the sum of the numbers of moles of rare earths, Y, Zr and Ti.
According to an embodiment, the compression of step c) is carried out under a pressure comprised between 40 and 80 MPa.
According to an embodiment, step c) is carried out at a temperature comprised between 700 and 900° C.
According to an embodiment, the hydridable alloy has the formula R_1-x-yMg_xM_yNi_s-aB_awherein
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.
This method may be used for making the alloy according to the invention.
This alloy may be used as a negative electrode active material of an alkaline accumulator of the nickel metal hydride type. The invention therefore also relates to an electrode comprising said alloy as well as to an accumulator of the nickel metal hydride type comprising this electrode.
According to an embodiment, the electrode comprises from 0.4 to 1% by weight of yttrium oxide and/or from 1 to 2% by weight of manganese oxide.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1A shows an image obtained by high resolution transmission electron microscopy (HRTEM) of an alloy of the prior art comprising a single-crystal grain crystallizing in the A₅B₁₉system.

FIG. 1B is a schematic illustration of the analysis of a single-crystal grain crystallizing in the A₅B₁₉system. This grain only consists of a stack of sequences comprising one pattern of type L for three patterns of type C.

FIG. 2A shows an image obtained by high resolution transmission electron microscopy (HRTEM) of a grain of an alloy according to the invention having sequences of patterns of the AB₃, A₂B₇, A₅B₁₉, AB₄and A₇B₂₉type.

FIG. 2B is a schematic illustration of the analysis with a high resolution transmission electron microscope of a grain of an alloy according to the invention having sequences of patterns of the AB₃, A₂B₇, A₅B₁₉, AB₄and A₇B₂₉type. This grain consists of a random stack of sequences of a pattern of the type L for n patterns of the type C, n varying from 1 to 5.

FIG. 3 illustrates an isothermal pressure-composition curve at 40° C. of a hydridable alloy according to the invention (PCT curve).

FIG. 4 illustrates a schematic view of the device used in the flash sintering method (SPS Spark Plasma Sintering).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The alloy according to the invention has the formula R_1-x-yMg_xM_yNi_s-aB_awherein:
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.
In an embodiment, R comprises La, Nd, Pr and Sm.
In an embodiment, R consists of La, Nd, Pr and Sm; B is one or more elements selected from Mn, Al and Co; x is comprised between 0.1 and 0.3; s is comprised between 3 and 4 and a is less than 0.5. The stoichiometry is greater than AB₃since this corresponds to an AB₅pattern for an A₂B₄pattern. The upper limit corresponds to the AB₄phase, which was detected by Ozaki [T. Ozaki and al., J. Alloys and Comp., Vol. 446-447, pp 620-624, 2007] and which corresponds to 4 patterns of the AB₅type for one pattern of the A₂B₄type. Moreover, the Mg level is greater than 0.1 in order to obtain a sufficient capacity. An increase in the Mg level beyond 0.3 tends to promote a mixture of AB₃(or AB₂) and AB₅phases to the detriment of the other phases of the A_n+2B_5n+4type. With partial substitutions of Ni with Mn and Al, it is possible to lower the equilibrium hydrogen pressure while Co and Al promote the stability of the alloy during cycling.
In an embodiment, R consists of La, Nd, Pr and Sm; the R″/R′ ratio is less than 0.5; R″ designating the sum of the numbers of moles of Nd and of Pr; B is one or more elements selected from Al and Co; x is comprised between 0.1 and 0.3; s is comprised between 3.5 and 4; and a is less than 0.5. For values of s of less than 3.5, the AB₃phase proportion, which decomposes during cycling, increases and causes accelerated reduction of the capacity during the cycling. In order to avoid this, the use of stoichiometries greater than or equal to 3.5 and the use of aluminium and cobalt which promote cycling stability are preferred, rather than manganese as substituants for nickel. However, these modifications cause an increase in the equilibrium hydrogen pressure. In order to maintain this pressure within acceptable limits for the application to Ni-MH batteries, the Nd+Pr level is reduced to the benefit of La.
In an embodiment, R consists of La, Nd, Pr, Sm; the R″/R′ ratio is less than 0.5; B is one or more elements selected from Al and Co; x is comprised between 0.15 and 0.25; s is comprised between 3.5 and 4; and a is less than 0.3. Aluminium and cobalt have the effect of improving the cycling stability of the alloy. However, the capacity of the alloy decreases when their level increases beyond a threshold which depends on the magnesium level. This decrease in capacity is limited if the magnesium level is restricted between 0.15 and 0.25 and if the Al+Co level is less than 0.3.
In an embodiment, R consists of La, Nd, Pr, Sm; the R″/R′ ratio is less than 0.5; B is Al; x is comprised between 0.15 and 0.25; s is comprised between 3.5 and 4, and a is less than 0.15. As cobalt is an expensive and speculative element, it seems to be preferable to use an alloy without cobalt and only retain aluminium as a substituent for nickel. However, an aluminium level of more than 0.15 causes a decrease in the equilibrium pressure and therefore in the voltage of the element.
The composition of the alloy may be determined by elementary chemical analysis by the plasma emission spectroscopy method (ICP for Inductively Coupled Plasma), atomic spectroscopy or X-ray fluorescence spectroscopy.
The alloy comprises one or more crystalline phases selected from:
a) the phase AB₅crystallizing in the hexagonal system of the CaCu₅type;
b) the phases AB₂, so-called Laves phases, either crystallizing in the cubic system: a so-called “C15” phase of the MgCu₂type, or in the hexagonal system; so-called “C14” phase of the MgZn₂type or “C36” phase of the MgNi₂type.
c) the phases of the R_m+1MgNi_5m+4type consisting of a periodic stack of sequences including m C patterns (identical with the lattice cell of the CaCu₅type structure) and of one pattern L (a pattern forming AB₂Laves phases), with hexagonal (H) or rhombohedral (R) symmetry with m ranging from 1 to 10.
m=1 corresponds to the AB₃phases either crystallizing in the hexagonal system (H—CeNi₃type), or in the rhombohedral system (R—PuNi₃type);
m=2 corresponds to the A₂B₇phases either crystallizing in the hexagonal system (H—Ce₂Ni₇type), or in the rhombohedral system (R—Gd₂CO₇type);
m=3 corresponds to the A₅B₁₉phases either crystallizing in the hexagonal system (H—Pr₅Co₁₉type), or in the rhombohedral system (R—Ce₅Co₁₉type);
m=4 corresponds to the phase of composition AB₄.
These crystalline phases may be detected by analyzing an X-ray diffraction diagram.
At least 5% by volume of the alloy consists of a so-called ‘crystalline phase with random stacking’, characterized by a stacking of sequences of a pattern of the A₂B₄type and of n patterns of the CaCu₅type randomly distributed along a direction, n being an integer comprised between 1 and 10 and representing the number of patterns of the CaCu₅type per pattern of A₂B₄type.
By “random variation” is meant the variation in which the value assumed by the parameter n for a given sequence is independent of the values assumed by this parameter in adjacent sequences. The so-called ‘random stacking crystalline’ structure may be observed in high resolution transmission electron microscopy (HRTEM). This structure will be better understood by referring to the examples illustrated in FIGS. 1A, 1B and 2A, 2B.
FIG. 1A shows an image obtained by high resolution transmission electron microscopy (HRTEM) of a material domain of an alloy of the prior art and FIG. 1B, its schematic illustration. This domain belongs to a grain, i.e. an area consisting of a same crystal. This grain has a so-called ‘ordered stacking crystalline structure’. Each vertical line illustrates the alignment of a same pattern. The dark lines correspond to an alignment of patterns of type L. The bright lines correspond to an alignment of patterns of type C. In the vertical direction, the pattern is either always a pattern of type C, or always a pattern of type L. By sweeping through the material domain along a horizontal direction, it is observed that the latter may be considered as consisting of a repetition of sequences each consisting of one pattern of type A₂B₄and of 3 patterns of type CaCu₅(1L3C). The number of patterns of the CaCu₅type inserted between 2 patterns of the A₂B₄type is constant and equal to 3. As this number is constant, the distance separating two dark lines is identical on the analyzed material domain. This indicates that the material domain subject to analysis only consists of the crystalline phase of the A₅B₁₉type which consists of sequences of patterns of the 1L3C type. This alloy of the prior art has a periodic structure.
FIG. 2A shows an image obtained by high resolution transmission electron microscopy (HRTEM) of a material domain of an alloy according to the invention belonging to a grain of a so-called “random stacking crystalline” structure and FIG. 2B, its schematic illustration. It shows a succession of vertical lines, some of them appearing as a dark line, others as a bright line. A dark line represents an alignment of patterns of type L. A bright line represents an alignment of patterns of type C. In the vertical direction of FIGS. 2A and 2B, the pattern is either always a pattern of the type C, or always a pattern of type L. Upon covering the material domain along a horizontal direction, it is observed that the latter may be considered as consisting of a stack of sequences each consisting of n patterns of the CaCu₅type for one pattern of the A₂B₄type, n being an integer comprised between 1 and 10 varying randomly along the horizontal direction of the stack. The number n of patterns of type C between two patterns of type L is not constant and varies randomly. This random variation in the number of patterns C between two L patterns in a same grain characterizes the invention. The number of patterns of type C between each pattern of type L may be noted in FIGS. 2A and 2B. Upon covering FIG. 2B from left to right, it is seen that n assumes the following successive values “7, 3, 5, 2, 2, 3, 2, 4, 3, 2, 5”. The so-called ‘random stacking crystalline’ phase according to the invention is characterized by the existence of a short range order (<6 nm) along the direction of the stack and by the existence of a longer range order along the planes normal to the stack, in the whole section of the grain.
According to an embodiment, n varies randomly and not periodically.
According to an embodiment, the value of n in the different sequences is less than or equal to 8, preferably less than or equal to 6, still preferably less than or equal to 4.
According to an embodiment, the volume of said ‘random stacking crystalline’ phase is greater than 10%, still preferably greater than 20% and less than 70% based on the alloy.
According to an embodiment, the alloy has a hydrogen absorption capacity greater than 1% by mass, preferably greater than 1.3%, still preferably greater than 1.45%.
According to an embodiment, the alloy has a hydrogen equilibrium pressure comprised between 0.01 and 5 bars at 40° C. for a hydrogen mass concentration in the alloy of 0.5%. The capacity of hydrogen absorption by the alloy and the equilibrium pressure with hydrogen for a hydrogen mass concentration in the alloy of 0.5% may be determined from an isothermal pressure-composition curve representing the dihydrogen pressure in equilibrium with a hydrogen concentration inserted into the alloy (in % by mass or by moles of H inserted per mole of metal). The H/Metal ratio is the ratio between the number of moles of hydrogen atoms inserted into the alloy and the number of moles of metal atoms of the alloy. By “Metal” is meant the whole of the metals contained in the alloy, i.e. R, Mg, M, Ni and B.
The curve of FIG. 3 represents the dihydrogen pressure versus the hydrogen insertion level (H/metal ratio) in the alloy at 40° C. (PCT curve). It is established after 3 activation cycles (absorption and desorption of hydrogen) of the alloy and degassing of the residual hydrogen at 80° C. This curve has a first low pressure portion, where the pressure varies rapidly with the hydrogen concentration. During this portion, atomic hydrogen is “soluble” in the alloy and gradually diffuses through the latter. This forms a solid (disordered) solution of hydrogen in the alloy. The hydride does not yet form.
In a second portion, the curve has the shape of a slightly tilted plateau. The onset of this plateau marks the onset of the formation of the hydride (in the hydride, atomic hydrogen occupies the insertion sites of the alloy in an ordered way). During the plateau phase, atomic hydrogen gradually occupies all the insertion sites of the alloy. The end of the plateau corresponds to the situation in which practically all the insertion sites of the alloy are occupied by hydrogen.
In a third portion, the hydrogen pressure again increases rapidly with the hydrogen concentration and the amount of hydrogen which the alloy may still insert does not increase very much. The alloy is then only found in the form of the hydride in which excess atomic hydrogen is inserted in a disordered way. Desorption of hydrogen occurs at a plateau pressure below the absorption pressure.
It is possible to determine on the axis of the abscissas of the PCT curve, the position of the H/metal ratio equal to 0.5 and to infer therefrom the value of the equilibrium pressure by reading the ordinate of the corresponding point of the desorption curve. It is also possible to determine the position of the axis of the abscissas of the PCT curve corresponding to a hydrogen mass concentration of 0.5%, by knowing the 1 g molar mass of atomic hydrogen and the molar mass of the alloy used. The value of the equilibrium pressure is inferred therefrom by reading the ordinate of the corresponding point.
The hydrogen plateau pressure is not inherent to the composition of the hydridable alloy but to the composition of the phase which absorbs hydrogen. Now, for a same alloy composition, different elaboration methods, notably different heat treatments, may lead to obtaining a single phase alloy or a multiphase alloy, none of the phases of which having a composition identical with that of the alloy.
In an embodiment, the alloy has an equilibrium pressure with hydrogen H₂at 40° C. for an atomic hydrogen mass concentration in the alloy of 0.5%, comprised between 0.01 and 5 bars, preferably between 0.01 and 1 bar, still preferably between 0.05 and 0.7 bars. It is necessary to adapt the conditions for synthesis of the alloy in a range of temperatures comprised between 700 and 900° C. under a pressure comprised between 40 and 80 MPa according to its composition in order to obtain the desired characteristics.
According to a preferred embodiment, the maximum hydrogen mass concentration in the alloy under pressure of 10 bars is greater than 1.3%, preferably greater than 1.45%. It is necessary to adapt the conditions for synthesis of the alloy in a range of temperatures comprised between 700 and 900° C. under a pressure comprised between 40 and 80 MPa according to its composition in order to obtain the desired characteristics.
The invention also relates to a method for preparing a hydridable alloy with a fast sintering method. This manufacturing method comprises the steps:

- a) mixing Mg₂Ni with a compound comprising:
  - i) nickel,
  - ii) one or more 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 of Mn, Al, Co, Fe and a mixture thereof;
- b) milling the mixture;
- c) sintering the mixture by compressing the mixture and applying a current through the mixture.

Step c) may be carried out by using the rapid sintering technique (SPS Spark Plasma Sintering), which is also known under the acronym of FAST (Field Activated Sintering Technique). By simultaneous application of a compression force and of a square-wave shaped DC current of great intensity, this technique allows complete sintering of powders within only a few minutes. A mixture is milled, comprising Mg₂Ni and a compound comprising nickel and one or more elements selected from the group consisting in rare earths and yttrium. It is optionally possible to add to the mixture, titanium and/or zirconium as well as one or more elements selected from Mn, Al, Co, Fe. The mixture is placed between an anvil and a piston. The mixture is compressed under a force preferably comprised between 40 and 80 MPa and heated by the Joule effect, which causes sintering. Temperatures comprised between 750 and 1,000° C. may be obtained. FIG. 4 illustrates a matrix M containing the mixture of powder inserted between a piston P and an anvil M.
It was discovered that the structure of the ‘random stacking crystalline’ phase of the alloy according to the invention was able to be obtained by applying the method as described to a mixture comprising nickel and one or more elements selected from the group consisting in rare earths and yttrium.
The invention also relates to an anode (negative electrode) comprising the alloy as an electrochemically active material. The anode is made by pasting an electrically conducting support with a paste consisting of an aqueous mixture of the alloy according to the invention with additives and optionally with conductive agents.
The support may be of the foam, felt, planar or three-dimensional perforated sheet type, in nickel or in nickel-plated steel.
The additives are intended to facilitate application or the performances of the anode. They may be thickeners such as carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), polyacrylic acid (PAAc), poly(ethylene oxide) (PEO). They may also be binders such as copolymers of butadiene-styrene (SBR), polystyrene acrylate (PSA), polytetrafluoroethylene (PTFE). They may also be polymeric fibers such as polyamide, polypropylene, polyethylene, etc., for improving the mechanical properties of the electrode.
The conductive agents may be nickel powder, carbon powder or fibers, nanotubes.
The alloy may be mixed with an yttrium compound in order to increase the capacity of the anode. In particular, this effect arises in the case of discharges with a strong current (discharges under 5C rating conditions). The mixture of the alloy with an yttrium compound also has the effect of increasing the cycling lifetime of the anode.
The yttrium compound is selected from a non-exhaustive list comprising an yttrium oxide such as Y₂O₃, and yttrium hydroxide such as Y(OH)₃or an yttrium salt. Preferably the yttrium compound is the yttrium oxide Y₂O₃.
The yttrium compound is mixed with the alloy in such a proportion that the yttrium mass represents from 0.1 to 2% of the mass of the alloy, preferably from 0.2 to 1% of the mass of the alloy, preferably from 0.2 to 0.7% of the mass of the alloy.
According to a preferred embodiment, the alloy is mixed with a compound based on manganese selected from a non-exhaustive list comprising oxides such as MnO, MnO₂, or a hydroxide or salt based on manganese. Preferably, the manganese-based compound is the oxide MnO. The mixture of the alloy with a manganese compound has the effect of preventing or delaying the occurrence of micro short-circuits ascribed to cobalt deposits from the positive electrode or from the negative electrode, in the separator during the cycling of the accumulator. These micro short-circuits actually generate exacerbated self-discharge which is expressed by an acceleration in the reduction of the restored capacity during discharge. The mass proportion of manganese in the negative electrode is comprised between 1.5 and 2.5% of the hydridable alloy mass.
The invention also relates to an accumulator with an alkaline electrolyte comprising at least one anode according to the invention, at least one cathode (positive electrode) in nickel, at least one separator and one alkaline electrolyte.
The cathode consists of the cathode active mass deposited on a support which may be a sintered support, a nickel foam, a planar or three-dimensional perforated sheet in nickel or in nickel-plated steel.
The cathode active mass comprises the cathode active material and additives intended to facilitate its application or its performances. The cathode active material is a nickel hydroxide Ni(OH)₂which may be partly substituted with Co, Mg and Zn. This hydroxide may be partly oxidized and may be coated with a surface layer based on cobalt compounds.
Among the additives, mention may be made, without this list being exhaustive, of carboxymethylcellulose (CMC) hydroxypropylmethylcellulose (HPMC), hydroxypropylcellulose (HPC), hydroxyethylcellulose (HEC), polyacrylic acid (PAAc), polystyrene maleic anhydrides (SMA), copolymers of butadiene-styrene (SBR), optionally carboxylated, a copolymer of acrylonitrile and butadiene (NBR), a copolymer of styrene, ethylene, butylene and styrene (SEBS), a terpolymer of styrene, of butadiene and of vinylpyridine (SBVR), polystyrene acrylate (PSA), polytetrafluoroethylene (PTFE), a fluorinated copolymer of ethylene and propylene (FEP), polyhexafluoropropylene (PPHF), ethylvinyl alcohol (EVA), zinc oxide ZnO, fibers (Ni, C, polymers), powders of cobalt-based compounds such as Co, Co(OH)₂, CoO, Li_xCoO₂, H_xCoO₂, Na_xCoO₂, as well as additives intended to improve the charging efficiency such as Y₂O₃, Yb₂O₃or Nb₂O₅.
The separator generally consists of polyolefin (for example polypropylene) fibers or of non-woven porous polyamide.
The electrolyte is a concentrated alkaline aqueous solution comprising at least one hydroxide (KOH, NaOH, LiOH), in a concentration generally of the order of several times normality.
Conventionally, the pastes are prepared for electrodes, the electrodes are made and then at least one cathode, separator and an anode are superposed in order to form the electrochemical bundle. The electrochemical bundle is introduced into a container cup and is impregnated with the aqueous alkaline electrolyte. The accumulator is then closed.
The invention relates to any format of accumulators: a prismatic format (planar electrodes) or a cylindrical format (spiral or concentric electrodes).
The accumulator of the Ni-MH type according to the invention may be of the open (either open or half-open) type or of the sealed type.
The accumulator according to the invention is particularly well adapted as a source of energy for an electric vehicle or a portable appliance.
Other features and advantages of the present invention will become apparent upon reading the examples.

EXAMPLES

A) The Making of the Alloy

An alloy 1 according to the invention with a composition of La_0.8Mg_0.2Ni_3.67was prepared by the flash sintering technique (SPS for Spark Plasma Sintering) from two precursors: Mg₂Ni elaborated by powder metallurgy and LaNi_y(y=4.3875) elaborated by melting a mixture of the simple elements in an induction oven. These powders were ground into powder for two hours in a planetary milling machine Fritsch Pulverisette 4 with a speed of rotation of 400 revolutions per minute and a Ball/Powder mass ratio equal to 10:1. Subsequently the mixture was sintered with SPS under pressure of 50 MPa for one hour at a temperature of 820° C.
An alloy 2 used as a counter-example, with the same composition as the alloy 1, was prepared with a fast cooling technique (melt spinning), at a speed of 10⁶K/s, from two precursors used for elaborating the alloy 1. The thereby prepared alloy was then annealed for 10 days at a temperature of 800° C. under an inert atmosphere.
An alloy 3 of the prior art with a composition of La_0.57Ce_0.27Nd_0.12Pr_0.04Ni_3.95Mn_0.40Al_0.30CO_0.55was prepared by melting together simple elements in a segmented copper crucible cooled with water and by slow cooling. This alloy was then annealed for 4 days at 1,050° C. under an inert atmosphere.
Table 1 below recalls these compositions.

TABLE 1

Elementary compositions and elaboration conditions of the alloys (SPS = spark
pressure sintering or flash sintering, MS = melt spinning or fast cooling, CF
casting in a cooled crucible; A represents the elements La, Ce, Nd, Pr and Mg, B
represents the elements Ni, Mn, Al and Co).

Alloy

La

Ce

Nd

Pr

Mg

Ni

Mn

Al

Co

B/A

Elaboration

Annealing

1	0.80	0	0	0	0.2	3.67	0	0	0	3.67	SPS 820° C.
2	0.80	0	0	0	0.2	3.67	0	0	0	3.67	MS	800° C.
												10 days
3	0.57	0.27	0.12	0.04	0	3.95	0.4	0.3	0.55	5.20	CF	1,050° C.
												4 days

The thereby obtained alloys were characterized by x-ray diffraction (XRD) by means of a Bruker AXS D8 θ-θ diffractometer (Bragg-Brentano geometry, Cu Kα radiation, 2θ angular domain=20 to 90°, step 0.04°). The analysis of the phases and the structural determination were carried out by means of the program for fitting the whole of the FULLPROF profile, based on Rietveld's method. Local chemical analysis was carried out by means of an electronic microprobe in Wavelength Dispersive Spectroscopy (WDS-EPMA Wavelength Dispersive Spectroscopy-Electron Probe Micro Analysis) CAMEBAX SX-100.
The isothermal hydrogen absorption and desorption curves (PCT) were established at 40° C. by the Sievert volumetric method.
Table 2 recalls the composition of the alloys in terms of phases as well as their PCT characteristics.

TABLE 2

Compositions in terms of phases and PCT characteristics of the alloys
(capacity under a pressure of 10 bars and equilibrium pressure at half capacity).

Phase %

PCT at 40° C.

							Random	Pressure	Capacity
Alloy	AB₃	A₂B₇h	A₂B₇r	A₅B₁₉h	A₅B₁₉r	AB₅	stacking	(bars)	(%)

1	0	6	9	10	50	0	25	0.4	1.53
2	14	23	0	7	15	40	0	1.0	1.48
3	0	0	0	0	0	100	0	0.45	1.15

The samples for examinations with high resolution transmission electron microscopy (HRTEM) were prepared by mechanical polishing and thinning with an ion beam before being immediately transferred into the microscope in order to avoid oxidation of the surface. The observations were carried out by means of an FEI Tecnai microscope under an acceleration voltage of 200 kV. The high resolution transmission electron microscopy photographs show grey vertical lines which appear raised (these are patterns of the type L or Laves phase) relatively to a background consisting of vertical lines which appear brighter (patterns of type C).
In the case of the alloy 2 of the counter-example, made by melt spinning, the observed microstructures are characterized by a number of bright lines (and a gap) between two dark lines which is constant over a long distance. This structure is characteristic of a crystalline phase of the A₅B₁₉type for a number of bright lines between two dark lines equal to 3. On the other hand, in the case of the alloy according to the invention, it is observed that the number of bright lines between two dark lines is not constant and varies randomly.

B) The Making of the Electrodes:

For making the electrodes, the alloy is mechanically powdered down to a grain size of 60 μm (Dv₅₀=60 μm, Dv₅₀is the median volume diameter of the sample). The thereby obtained powder is mixed with 1% of styrene-butadiene polymer (SBR styrene-butadiene rubber) and 0.5% of CMC in an aqueous solution in order to make a paste which was deposited in a nickel foam. The thereby obtained strip was laminated and dried, and then cut out in order to produce negative electrodes.

C) The Making of the Ni-MH Accumulator:

A bundle consisting of a negative electrode framed with two positive electrodes containing nickel hydroxide with excess capacity and separated from each other by a separator consisting of a membrane impervious to oxygen and of two non-woven webs of polyolefin, was mounted and clamped between wedges in a polyethylene tub in order to form an open accumulator, for which the capacity equal to 1 Ah, is limited by the negative electrode. The bundle was then impregnated in vacuo with an aqueous KOH solution with an excess concentration of 8.7 N. The capacity of the alloy was measured in open elements limited by the negative electrode, in foam technology, under the following conditions:
After a formation cycle, the accumulator underwent about 10 activation cycles consisting of:
charging with a current of 35 mA/g for 16 h,
a resting period of 1 h and
discharging carried out with a current of 70 mA/g down to a cut-off voltage of 0.9V.
Next, the accumulator underwent fast cycling ageing consisting of a 48 min discharge with holding at 0.9V and 52 min charging at a rate of 350 mA/g. A cycle for performance measurement is carried out intermittently under the conditions of the activation cycles. The results are shown in Table 3.

TABLE 3

Results of the electrochemical evaluation.

	Capacities (mAh/g) during C/5
	discharging after charging for 16 h at C/10
	and a resting period of 1 h	Stability

State	Initial	Activated	Aged	index (%)
Cycle	1	10	100	S₁₀₀

Alloy	1	318	322	275	86.5
	2	313	275	245	78.3
	3	272	282	241	88.6

The alloy 1 according to the invention, in the activated state, has a capacity of 322 mAh/g, 47 mAh/g greater than that of the alloy 2 (counter-example) and 40 mAh/g greater than that of the alloy 3 of the prior art. This table also shows the available capacity under the same conditions as above (16 h charging at 35 mA/g, rest period 1 h, discharging at 70 mA/g, cut-off voltage of 0.9V) by these elements in cycle 100, i.e. after 90 cycles of ageing during fast cycling under C conditions. The stability of the alloy is estimated by the index S₁₀₀calculated as follows:
S ₁₀₀=(Capacity measured at cycle 100/Capacity at cycle 1)×100.
These results show that the alloy 3 of the prior art has a low initial capacity of 272 mAh/g and a very good cycling stability (S₁₀₀=88.6%). On the other hand, the alloy of the counter-example 2, with the same composition as the alloy 1 according to the invention, but made by the melt spinning technique, has a high initial capacity of 313 mAh/g but a poor cycling stability (S₁₀₀=78.3%). The alloy 1 according to the invention has the advantage of combining a high initial capacity of 318 mAh/g and a cycling stability, the level of which remains high (86.6%), and close to that of the alloy 3 of the prior art.

Claims

1. A hydridable alloy of formula R_1-x-yMg_xM_yNi_s-aB_awherein

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 A₂B₄type and n patterns of the CaCu₅type randomly distributed along one direction, n being an integer comprised between 1 and 10 and representing the number of patterns of the CaCu₅type per pattern of the A₂B₄type.

2. The alloy according to claim 1, wherein the volume consisting of the stack of sequences with a pattern of the A₂B₄type and n patterns of the CaCu₅type 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 A₂B₄type and n patterns of the CaCu₅type 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 Mg₂Ni 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_ywith 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-yMg_xM_yNi_s-aB_awherein

M represents Zr and/or Ti;

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.