U.S. patent application number 10/942178 was filed with the patent office on 2006-03-16 for hydrogen storage alloys having reduced pct hysteresis.
Invention is credited to Michael A. Fetcenko, Feng Li, Taihei Ouchi, Stanford R. Ovshinsky, Melanie Reinhout, Kwo Young.
Application Number | 20060057019 10/942178 |
Document ID | / |
Family ID | 36034186 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060057019 |
Kind Code |
A1 |
Young; Kwo ; et al. |
March 16, 2006 |
Hydrogen storage alloys having reduced PCT hysteresis
Abstract
A modified A.sub.2B.sub.7 type hydrogen storage alloy having
reduced hysteresis. The alloy consists of a base A.sub.xB.sub.y
hydrogen storage alloy, where A includes at least one rare earth
element and also includes magnesium, B includes at least nickel,
and the atomic ratio of x to y is between 1:2 and 1:5. The base
alloy is modified by the addition of at least one modifier element
which has an atomic volume less than about 8 cm.sup.3/mole, and is
added to the base alloy in an amount sufficient to reduce the
absorption/desorption hysteresis of the alloy by at least 10% when
compared with the base alloy.
Inventors: |
Young; Kwo; (Troy, MI)
; Fetcenko; Michael A.; (Rochester, MI) ;
Ovshinsky; Stanford R.; (Bloomfield Hills, MI) ;
Ouchi; Taihei; (Rochester, MI) ; Li; Feng;
(Troy, MI) ; Reinhout; Melanie; (Shelby Twp,
MI) |
Correspondence
Address: |
ENERGY CONVERSION DEVICES, INC.
2956 WATERVIEW DRIVE
ROCHESTER HILLS
MI
48309
US
|
Family ID: |
36034186 |
Appl. No.: |
10/942178 |
Filed: |
September 16, 2004 |
Current U.S.
Class: |
420/455 ;
420/900 |
Current CPC
Class: |
C01B 3/0078 20130101;
H01M 10/345 20130101; Y02E 60/10 20130101; H01M 4/385 20130101;
C01B 3/0057 20130101; H01M 4/383 20130101; Y02E 60/32 20130101 |
Class at
Publication: |
420/455 ;
420/900 |
International
Class: |
C22C 19/03 20060101
C22C019/03 |
Claims
1. A modified A.sub.xB.sub.y hydrogen storage alloy having reduced
hysteresis wherein said alloy includes: a) a base A.sub.xB.sub.y
hydrogen storage alloy wherein: A includes at least one rare earth
element and also includes magnesium, B includes at least nickel,
and the atomic ratio of x to y is between 1:2 and 1:5; and b) at
least one modifier element wherein: said modifier element has an
atomic volume less than about 8 cm.sup.3/mole, and said modifier
element is added to said base alloy in an amount sufficient to
reduce the absorption/desorption hysteresis of the alloy by at
least 10% when compared with the base alloy.
2. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said atomic ratio of x to y is between 1:3 and
1:4.
3. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 2, wherein said atomic ratio of x to y is between 1:3.3 and
1:3.6.
4. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said modifier element is added to said base alloy
in an amount sufficient to reduce the absorption/desorption
hysteresis of the alloy by at least 20% when compared with the base
alloy.
5. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 4, wherein said modifier element is added to said base alloy
in an amount sufficient to reduce the absorption/desorption
hysteresis of the alloy by at least 40% when compared with the base
alloy.
6. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said modifier element is at least one element
selected from the group consisting of B, Co, Cu, Fe, Cr, and
Mn.
7. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 6, wherein said modifier element includes at least B.
8. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 6, wherein said modifier element includes at least Mn.
9. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein the atomic ratio of nickel to modifier element is
50:1 to 200:1.
10. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 9, wherein the atomic ratio of nickel to modifier element is
75:1 to 150:1.
11. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said at least one rare earth element includes at
least one element selected from the group consisting of lanthanum,
cerium, neodymium, and praseodymium.
12. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said at least one rare earth element misch
metal.
13. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein the atomic ratio of rare earth elements to
magnesium in the A elements is 5:1 to 6:1.
14. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 13, wherein the atomic ratio of rare earth elements to
magnesium in the A elements is 5.5:1 to 5.7:1.
15. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein B further includes aluminum.
16. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 15, wherein the atomic ratio of nickel to aluminum in the B
elements is 30:1 to 40:1.
17. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 16, wherein the atomic ratio of nickel to aluminum in the B
elements is 33:1 to 35:1.
18. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said alloy is
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1B.sub.0.03.
19. The reduced hysteresis A.sub.xB.sub.y hydrogen storage alloy of
claim 1, wherein said alloy is
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1Mn.sub.0.03.
20. An A.sub.xB.sub.y hydrogen storage alloy wherein: A includes at
least one rare earth element and also includes magnesium, B
includes at least nickel, the atomic ratio of x to y is between 1:2
and 1:5; and the surface of said alloy comprising catalytic
metallic regions supported in a highly porous oxide support
matrix.
21. The A.sub.xB.sub.y hydrogen storage alloy of claim 20, wherein
said catalytic metallic regions comprise nickel or nickel
alloy.
22. The A.sub.xB.sub.y hydrogen storage alloy of claim 21, wherein
said catalytic metallic regions comprise nickel.
23. The A.sub.xB.sub.y hydrogen storage alloy of claim 20, wherein
said catalytic metallic regions are 50-70 .ANG. in diameter and are
distributed throughout the oxide interface and varying in proximity
from 2-300 .ANG. from region to region.
24. The A.sub.xB.sub.y hydrogen storage alloy of claim 23, wherein
said catalytic metallic regions are 50-70 .ANG. in diameter and are
distributed throughout the oxide interface and vary in proximity
from 50-100 .ANG., from region to region.
25. The A.sub.xB.sub.y hydrogen storage alloy of claim 20, wherein
said catalytic metallic regions are 10-50 .ANG. in diameter in
size.
26. The A.sub.xB.sub.y hydrogen storage alloy of claim 25, wherein
said catalytic metallic regions are 10-40 .ANG. in diameter.
27. The A.sub.xB.sub.y hydrogen storage alloy of claim 26, wherein
said catalytic metallic regions are 10-30 .ANG. in diameter.
28. The A.sub.xB.sub.y hydrogen storage alloy of claim 27, wherein
said catalytic metallic regions are 10-20 .ANG. in diameter and
vary in proximity from 10-20 .ANG., from region to region.
28. The A.sub.xB.sub.y hydrogen storage alloy of claim 20, wherein
said alloy further contains a microstructure tuning element such as
Cu, Fe, or Zn.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to hydrogen storage
alloys mainly of the either Ce.sub.2Ni.sub.7 or Nd.sub.2Ni.sub.7 or
mixture of both crystal structures. More specifically the present
invention relates to A.sub.2B.sub.7 hydrogen storage alloys having
increased capacity and cycle life, as well as reduced pulverization
via reduced PCT hysteresis. This is achieved by adding one or
combinations of several elemental modifiers having atomic volume of
less than about 8 cm.sup.3/mole.
BACKGROUND OF THE INVENTION
[0002] Consumer and industrial applications continue to drive
demand for new and efficient batteries for use as energy sources.
Important goals include obtaining ever more power from increasingly
smaller battery packages in an environmentally respectful fashion.
Envisioned applications for batteries include everything from
mobile electronics to electric and hybrid electric vehicles.
Portability, rechargeability over a large number of cycles, low
cost, high power, lightweight and consistent performance over
widely varying loads and temperatures are among the key attributes
required for batteries. The specific combination of battery
performance requirements varies widely with the intended
application and the battery components and materials are typically
optimized accordingly. Portable electric devices such as digital
cameras demand higher and higher energy to extend run-time and
therefore higher capacity hydride alloys provide higher energy
Ni-MH batteries.
[0003] An important developing application area for rechargeable
batteries is electric vehicles (EV) and hybrid electric vehicles
(HEV). In these applications, the battery must have the ability to
provide high currents in short time periods in order to achieve
effective acceleration. High discharge rates are therefore
necessary. High battery power over extended time periods are also
needed so that vehicles of reasonable size and weight can be
maintained in motion for reasonable time intervals without
recharging. Rapid recharging over many cycles should also be
possible using readily available electrical power sources. The
preferred cycle life profile also requires a high number of
charge/discharge cycles at a shallow depth of charge/discharge.
Progress has been made in the development of batteries for HEV
applications and a few HEV automobiles have recently been made
available to the U.S. public. Nonetheless, the batteries used in
these automobiles represent compromises and trade offs in relevant
performance parameters and new developments are needed to further
the capabilities of HEV and EV products.
[0004] Nickel metal hydride batteries have emerged as the leading
class of rechargeable batteries and are replacing earlier
generation nickel cadmium batteries in many applications. Relative
to nickel cadmium batteries, nickel metal hydride batteries avoid
significant environmental problems (due to the toxicity of cadmium)
while providing higher energy densities. HEV and EV products are
examples of applications that utilize the high energy and power
available from nickel metal hydride batteries and are also
applications viewed to be impractical for nickel cadmium due to the
disposal problems associated with cadmium. Expanded performance of
HEV and EV products and the future extension of rechargeable
batteries to new applications in the future will greatly depend on
improvements in the capabilities of nickel metal hydride
batteries.
[0005] Nickel metal hydride batteries typically include a nickel
hydroxide positive electrode, a negative electrode that
incorporates a metal containing hydrogen storage alloy, a separator
made from nylon, polypropylene or other polymers, and an aqueous
alkaline electrolyte. The positive and negative electrodes are
housed in adjoining battery compartments that are typically
separated by a non woven, felled, nylon or polypropylene separator.
Several batteries may also be combined in parallel or series to
form larger battery packs capable of providing higher powers,
voltages or discharge rates.
[0006] The charging and discharging reactions of nickel metal
hydride batteries have been discussed in the art and may be
summarized as shown below:
[0007] Charging: positive electrode:
Ni(OH).sub.2+OH.sup.-.revreaction.NiOOH+H.sub.2O+e.sup.- negative
electrode: M+H.sub.2O+e.sup.-.revreaction.MH+OH.sup.-
[0008] Discharging positive electrode:
NiOOH+H.sub.2O+e.sup.-.revreaction.Ni(OH).sub.2+OH.sup.- negative
electrode: MH+OH.sup.-.revreaction.M+H.sub.2O+e.sup.-
[0009] Much work has been completed over the past decade to improve
the performance of nickel metal hydride batteries. Optimization of
the batteries ultimately depends on controlling the rate, extent
and efficiency of the charging and discharging reactions. Factors
relevant to battery performance include the physical state,
chemical composition, catalytic activity and other properties of
the positive and negative electrode materials, the composition and
concentration of the electrolyte, the separator, the operating
conditions; and external environmental factors. Various factors
related to the performance of the positive nickel hydroxide
electrode have been considered, for example, in U.S. Pat. Nos.
5,348,822; 5,637,423; 5,905,003; 5,948,564; and 6,228,535 by the
instant assignee, the disclosures of which are hereby incorporated
by reference.
[0010] Work on suitable negative electrode materials has focused on
intermetallic compounds as hydrogen storage alloys since the late
1950's when it was determined that the compound TiNi reversibly
absorbed and desorbed hydrogen. Subsequent work has shown that
intermetallic compounds having the general formulas AB, AB2 A2B and
AB5, where A is a hydride forming element and B is a weak or non
hydride forming element, are able to reversibly absorb and desorb
hydrogen.
[0011] More recently a new type of alloy has been introduced for
use in electrochemical cells. This alloy material has an
A.sub.2B.sub.7 microstructure. A typical alloy having this
microstructure contains misch metal, magnesium and nickel. This
microstructure is formed by simple stacking of AB.sub.5 unit cells
and AB.sub.2 unit cells. The AB.sub.5 cells contain misch metal and
nickel, while the AB.sub.2 cells also contain magnesium. Examples
of A.sub.2B.sub.7 alloys can be found in JP 2002-69554; JP
2002-83593; JP 2002-105564; JP 2002-105563; and JP 2002-164045.
[0012] Multi-component, multi-phase, electrochemical hydrogen
storage alloys incorporating magnesium were first disclosed in U.S.
Pat. Nos. 5,506,069 ('069), 5,554,456 ('456), and 5,616,432
('432).
[0013] In the '069 patent, the invention used MgNi as the basis to
develop a new family of negative electrode materials. This work
required an analytical approach on different levels. First, the
inventors sought multi-orbital modifiers, for example transition
elements, that would provide a greatly increased number of storage
sites due to the various bonding configurations available in order
to produce an increase in energy density. Second, the inventors had
to look for modifiers and methods that would stabilize Mg as well
as provide sufficient balance to the passivation/corrosion
characteristics of the resulting alloy. This is because
unrestrained corrosion lead to poor cycle life and passivation
resulted in low capacity, poor discharge rate performance, and poor
cycle life.
[0014] Modification of MgNi materials is complicated because Mg
does not have the tolerance for substitution that transition metals
have. Further, MgNi based materials do not tolerate the wide
latitude of precipitated phases formed during alloy solidification.
In other words, alloys of the V--Ti--Zr--Ni or LaNi.sub.5 type may
precipitate as a multitude of crystallographic phases during
solidification and still result in efficiently operating alloys
capable of operating in an alkaline battery environment. This is
problematic with MgNi based materials.
[0015] The MgNi host matrix materials of the '069 patent were, more
specifically, high specific capacity electrochemical hydrogen
storage alloys composed of a Base Alloy comprising a MgNi host
matrix. This MgNi host matrix is an alloy of Mg and Ni in a
preferred ratio of about 1:1. The Base Alloy of the '069 patent was
modified by at least one modifier element chosen from the group
consisting of Co, Mn, Al, Fe, Cu, Mo, W, Cr, V, Ti, Zr, Sn, La, Mm,
and Ca where the total mass of the at least one modifier element
was greater than 0.5, preferably 2.5, atomic percent and less than
30 atomic percent of the final composition.
[0016] The '432 patent advances the work of the '069 patent by
including a substantial volume fraction of an amorphous,
nanocrystalline and/or microcrystalline microstructure into the
MgNi alloy. The '432 patent refers to that microstructure by the
term "intermediate range order." It also describes the material by
the nano-crystallites within the bulk of the alloy material which
are typically about 10-50 Angstroms in size and more specifically
20-50 Angstroms in size. The '432 patent discloses that these
nano-crystallites display special characteristics due to the unique
topology, surface area to bulk ratio, unusual bonding
configurations and enhanced number of active sites. Attributes of
the electrochemical hydrogen storage alloys in the '069 and '432
patents are exceptionally high storage capacity (greater than 700
mAh/g) versus conventional alloys used in Ni-MH batteries (about
300-400 mAh/g). These alloys also demonstrated good rate
capabilities. However, while improved cycle life over first
generation Mg--Ni alloys was observed, cycle life advancements were
still required.
[0017] In the '456 patent the invention of the '069 patent was
further enhanced to provide non-uniform heterogeneous powder
particles for the negative electrode of electrochemical cells.
These powder particles included at least two separate and distinct
hydrogen storage alloy systems which were distinguished by their
respective composition and were preferably either be layered or
encapsulated. The '456 patent built on the work of the '069 patent
and described a new concept of combining at least two separate and
distinct hydrogen storage alloys to produce non-uniform
heterogeneous powder particles. The strategy of combining distinct
hydrogen storage alloys permitted the formulation of negative
electrode materials having a degree of passivation/corrosion
optimization (and thus increases in performance) that was
significantly greater than any previously formulated metal hydride
negative electrode materials. The most preferred heterogeneous
powder particles were formed from Ovonic MgNi based alloy (as
described in the '069 patent) and at least one Ovonic TiNi type or
LaNi.sub.5 type hydrogen storage alloy.
[0018] The '069, '432, and '456 patents were strongly focused on
"protecting the magnesium from oxidation" by various methods
including alloying, structural modification, encapsulation and the
close proximity to another phase acting as a storage phase/catalyst
phase funnel for hydrogen. Despite the excellent overall
performance of these magnesium-based alloys and heterogeneous
composite powders, reduced cycle life, due to the strong affinity
for magnesium oxidation, has still been a concern.
[0019] The new A.sub.2B.sub.7 alloys have found a way to add
magnesium to conventional hydrogen storage alloys which raises the
capacity of the conventional alloy and yet protect the magnesium
from oxidation. Unfortunately, the prior art A.sub.2B.sub.7 alloys
suffer from excessive hydrogen absorption/desorption hysteresis.
The instant inventors believe that hysteresis effects are
detrimental to the long term cycling stability of metal hydride
materials and that materials that exhibit large hysteresis effects
are more susceptible to premature failing on repeated cycling. The
reasoning underlying the instant inventors belief that hysteresis
is detrimental can be described with reference to FIG. 1, which
shows schematic examples of PCT curves exhibiting small and large
hysteresis. Beneath each PCT curve is a depiction of the
distribution of hydrogen in the metal hydride material, where
surface and absorbed hydrogen are shown as dark circles. The
depiction of the absorbed hydrogen distribution shown in FIG. 1 for
small and large hysteresis materials is used as the basis for the
following discussion.
[0020] In small hysteresis materials, the absorption and desorption
isotherms are close to coinciding. Low hysteresis materials are
characterized by an internal distribution of absorbed hydrogen that
is nearly uniform throughout the material. The absorbed hydrogen
concentration near the surface is close to the absorbed hydrogen
concentration in the bulk (interior) of the material. In a large
hysteresis material, in contrast, the concentration of absorbed
hydrogen is less uniform, with the hydrogen concentration in the
vicinity of the surface being greater than the hydrogen
concentration in the bulk. In terms of concentration gradients, a
large hysteresis material exhibits a large gradient in the
concentration of absorbed hydrogen, while a small hysteresis
material exhibits a small gradient in the concentration of absorbed
hydrogen.
[0021] The large concentration gradient associated with a large
hysteresis material is a manifestation of the greater difficulty
associated with absorbing hydrogen into the material. Hydrogen
absorption and desorption are governed by both thermodynamic and
kinetic processes. Thermodynamics controls the equilibrium hydrogen
concentration that can be stored in a metal hydride and determines
the plateau pressure of a hydrogen storage alloy.
Phenomenologically, thermodynamics is concerned with the free
energy difference between the absorbed and unabsorbed states of the
hydrogen storage material. Kinetics, on the other hand, is
concerned with the activation barriers associated with the
absorption (or desorption) of hydrogen. In order for hydrogen to
become absorbed, hydrogen gas or an electrochemical hydrogen
bearing species such as water must adsorb on the surface and
dissociate into hydrogen atoms or ions that diffuse or migrate into
the metal hydride material to occupy hydrogen storage sites. The
diffusion or migration process involves motion of hydrogen within
open regions (e.g. hopping among interstitial sites) of the metal
hydride structure. Such motion is inhibited by the placement of
lattice atoms of the metal hydride and is necessarily accompanied
by an energy barrier. The initial penetration of hydrogen from the
surface into the near surface portion of the interior of the metal
hydride also has an energy barrier associated with it.
[0022] Large energy barriers inhibit the motion of hydrogen and act
to reduce the uniformity of the absorbed hydrogen concentration
within the metal hydride material. As more hydrogen is accumulated
in the near surface region, the concentration gradient between the
near surface region and the interior of the metal hydride
increases, thereby providing a greater diffusive driving force for
the motion of hydrogen toward the interior of the metal hydride.
The establishment of a large concentration gradient thus
facilitates an overcoming of the kinetic barriers to hydrogen
motion. Large hysteresis materials possess greater activation
barriers to hydrogen motion than small hysteresis materials and
thus exhibit larger concentration gradients in absorbed
hydrogen.
[0023] In terms of cycle life stability and long cycle life, large
hysteresis is undesirable because large gradients in the absorbed
hydrogen concentration tend to promote pulverization or spalling
effects upon repeated cycling. It is known in the art that the
absorption of hydrogen in a metal hydride material is accompanied
by an expansion of the metal hydride lattice volume as the atoms of
the metal hydride are repositioned to accommodate absorbed
hydrogen. The magnitude of the lattice expansion depends on the
alloy composition and ranges from small to large. (See, for
example, the article entitled "The correlation between composition
and electrochemical properties of metal hydride electrodes" by J.
J. Reilly et al. appearing in the Journal of Alloys and Compounds,
vol. 293 295, p. 569 582 (1999).)
[0024] The effect of lattice expansion on the cycle life
characteristics is expected to differ for small and large
hysteresis metal hydride materials. In small hysteresis materials,
the absorbed hydrogen is more nearly uniformly distributed
throughout the lattice of the metal hydride material so that
lattice expansion occurs substantially uniformly throughout the
material. In large hysteresis materials, the absorbed hydrogen
exhibits a significant concentration gradient so that the
concentration of hydrogen in some portions of the material is much
different than the concentration in other portions of the material.
As a result, lattice expansion effects occur non uniformly as
regions of high absorbed hydrogen concentration expand to a greater
degree than regions of low absorbed hydrogen concentration. A
differential lattice expansion occurs due to the concentration
gradient as high concentration regions expand more than low
concentration regions.
[0025] Differential lattice expansion is detrimental to cycle life
because it introduces internal stresses into the metal hydride
lattice. A difference in lattice constants between adjacent regions
of a metal hydride material causes a stress to form at the
interface between those regions. The stress is due to a mismatch in
lattice parameters as the preferred atomic positions in the
neighboring regions differ due to a difference in absorbed hydrogen
concentration. The greater is the difference in lattice constants,
the greater the stress is. A metal hydride material is able to
support stresses up to a maximum value that is characteristic of
the material. The material accommodates or relieves the stress
through distortions in atomic positions from their
thermodynamically preferred locations. If the stress exceeds this
stress limit, the metal hydride material is unable to support the
stress and relieves the stress by fracturing. The instant inventors
believe that this fracturing is the cause of the particle size
pulverization that occurs in large hysteresis metal hydride
materials upon cycling. Repeated cycling causes the repeated
creation and elimination of internal stresses that, over time,
fatigue the metal hydride material and progressively degrade the
average particle size.
[0026] Therefore, although A.sub.2B.sub.7 Mg mischmetal based
alloys show higher storage capacity than commercially viable
AB.sub.5 alloys, the PCT hysteresis of prior art A.sub.2B.sub.7
alloys is too high for battery applications, taking into
consideration both the mid point voltage and cycle life
performance. Thus, there is a need in the art for A.sub.2B.sub.7
alloys with reduced hydrogen absorption/desorption hysteresis to
increase the cycle life thereof.
SUMMARY OF THE INVENTION
[0027] The present invention is a modified A.sub.2B.sub.7 type
hydrogen storage alloy having reduced PCT absorption/desorption
hysteresis. The alloy consists of a base A.sub.xB.sub.y hydrogen
storage alloy, where A includes at least one rare earth element and
also includes magnesium, B includes at least nickel, and the atomic
ratio of x to y is between 1:2 and 1:5 and preferably is between
1:3 and 1:4. The base alloy is modified by the addition of at least
one modifier element which has an atomic volume less than about 8
cm.sup.3/mole, and is added to the base alloy in an amount
sufficient to reduce the absorption/desorption hysteresis of the
alloy by at least 10% when compared with the base alloy. More
preferably the modifier element is added to said base alloy in an
amount sufficient to reduce the absorption/desorption hysteresis of
the alloy by at least 20% and most preferably by 40% when compared
with the base alloy.
[0028] The modifier element may be least one element selected from
the group consisting of B, Co, Cu, Fe, Cr, and Mn, and is
preferably B or Mn. The atomic ratio of nickel to modifier element
may be 50:1 to 200:1, more preferably 75:1 to 150:1. The at least
one rare earth element may include at least one element selected
from the group consisting of lanthanum, cerium, neodymium, and
praseodymium, and may be misch metal. The atomic ratio of rare
earth elements to magnesium in-the A elements may be 5:1 to 6:1,
more preferably 5.5:1 to 5.7:1. The B elements may further include
aluminum, and the atomic ratio of nickel to aluminum in the B
elements may be 30:1 to 40:1, more preferably 33:1 to 35:1.
[0029] Additionally the invention includes An A.sub.xB.sub.y
hydrogen storage alloy wherein: A includes at least one rare earth
element and also includes magnesium, B includes at least nickel,
the atomic ratio of x to y is between 1:2 and 1:5; and the surface
of said alloy comprises catalytic metallic regions supported in a
highly porous oxide support matrix. The catalytic metallic regions
may be nickel or nickel alloy. The catalytic metallic regions may
average about 50-70 .ANG. in diameter and distributed throughout
the oxide interface varying in proximity from 2-300 .ANG. from
region to region. More preferably, the catalytic metallic regions
average about 50-70 .ANG. in diameter and are distributed
throughout the oxide interface varying in proximity from 50-100
.ANG., from region to region.
[0030] Alternatively, the catalytic metallic regions are 10-50
.ANG. preferably 10-40 .ANG. and more preferably 10-30 .ANG. in
diameter. Most preferably the catalytic metallic regions are 10-20
.ANG. in diameter and vary in proximity from 10-20 .ANG., from
region to region. The alloy may further contain a microstructure
tuning element such as Cu, Fe, or Zn.
BRIEF DESCRIPTION OF THE FIGURES
[0031] FIG. 1 depicts the absorbed hydrogen distribution for small
and large hysteresis materials;
[0032] FIG. 2, is a plot of the XRD spectra of prior art samples A1
(curve a), A2 (curve b) and inventive sample A3 (curve c), the
spectra specifically show that all three samples contain mainly
A.sub.2B.sub.7 phases (including both Ce.sub.2Ni.sub.7 and
La.sub.2Ni.sub.7 structures); and
[0033] FIGS. 3a-3c are PCT isotherms taken at 30 C for prior art
samples A1 (FIG. 3a), A2 (FIG. 3b) and inventive sample A3 (FIG.
3c), the number inserted in each graph is the PCT hysteresis (In
(P.sub.a/P.sub.d)) at 0.5% hydrogen storage.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present inventors have found that basic A.sub.2B.sub.7
alloys can be modified to have reduced hysteresis by the addition
of elements which have a relatively small atomic volumes. That is,
modifier elements that have larger atomic volumes while increasing
the degree of disorder also block the hydrogen diffusion path
within the alloy. Modifier elements with smaller atomic volume,
increase both the degree of disorder and hydrogen diffusion
capability.
[0035] The base alloy which is modified by the present invention is
an A.sub.xB.sub.y hydrogen storage alloy. The "A" elements include
both of 1) at least one rare earth element; and 2) magnesium. The
at least one rare earth element may comprise misch metal. The "B"
elements include at least nickel. The "B" elements may further
include aluminum. The at least one rare earth element may include
misch metal. Preferred rare earth elements may include lanthanum,
cerium, neodymium, and praseodymium. The atomic ratio of x to y may
be between 1:2 and 1:5 and preferably between 1:3 and 1:4. Most
preferably, the atomic ratio of x to y may be between 1:3.3 and
1:3.6. The atomic ratio of rare earth elements to magnesium in the
A elements may be 5:1 to 6:1 and more preferably may be 5.5:1 to
5.7:1. If aluminum is present, the atomic ratio of nickel to
aluminum in the B elements may be 30:1 to 40:1 and more preferably
may be 33:1 to 35:1.
[0036] Table 1 is a list of common modifier elements showing their
atomic volumes. The inventors have found that elements such as B,
Co, Cu, Fe, Cr, and Mn (i.e. elements having an atomic volume less
than 8 cm.sup.3/mole) are useful as the hysteresis reducing
modifier elements for A.sub.2B.sub.7 alloys their relatively small
size. The modifier element(s) should be added in a sufficient
quantity to significantly reduce the hysteresis of the modified
base alloy when compared to the unmodified alloy. By "significantly
reduce", the inventors mean by at least 10% over the unmodified
base alloy. More preferably the inventors mean by at least 20% and
most preferably the inventors mean by at least 40%. In the alloy
structure, the modifier element(s) are "B" elements. The atomic
ratio of nickel to modifier element may be in the range of 50:1 to
200:1 and more preferably may be 75:1 to 150:1. TABLE-US-00001
TABLE 1 Modifier Element Atomic Volume (cm.sup.3/mole) Boron 4.6
Cobalt 6.7 Copper 7.1 Iron 7.1 Chromium 7.23 Manganese 7.49 Zinc
9.2 Aluminum 10.0 Silicon 12.1 Zirconium 14.1
[0037] The present inventors have prepared materials corresponding
to the prior art, comparative examples and examples according to
the present invention. The alloy designation, category, and
composition of the alloys is presented in Table 2. TABLE-US-00002
TABLE 2 Alloy No. Category Composition A1 Prior Art
La.sub.0.85Mg.sub.0.15Ni.sub.3.34Al.sub.0.1 A2 Prior Art
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1 A3 Inventive
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1B.sub.0.03 A4 Inventive
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1Mn.sub.0.03 A5 Comparative
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1Si.sub.0.03 A6 Comparative
La.sub.0.21Ce.sub.0.03Pr.sub.0.15Nd.sub.0.46Mg.sub.0.15Ni.sub.3.34Al.sub.-
0.1Zr.sub.0.03
[0038] Prior art composition A1 is one of the control samples and
shows a large PCT hysteresis (see Table 3). Prior art composition
A2, the second control sample, shows that replacing La with misch
metal (La--Ce--Pr--Nd alloy) does improve the PCT hysteresis, but
not enough. The inventive alloys, A3 (with B modifier) and A4 (with
Mn modifier) significantly reduce the hysteresis. The comparative
examples A5 (with Si modifier) and A6 (with Zr modifier) indicate
that show that if the modifier atoms occupied too much volume, the
PCT hysteresis will not be reduce to a satisfactory level, if at
all.
Sample Preparation
[0039] Industry grade raw materials having the nominal compositions
indicated in Table 2 were mixed together and melted in an induction
furnace under argon atmosphere. Once the entire mixture was melted,
the melt was held at that temperature for 2 minutes for better
homogeneity. The melt was cooled down to a lower temperature and
poured into a carbon steel pancake mold. The sample was then
annealed in a tube furnace at 1000 C for 5 hours to homogenize the
composition. Small piece of the cooled ingots were taken for
examination by scanning electron microscopy (SEM), x ray
diffraction analysis (XRD), and gas phase pressure concentration
isotherm study (PCT).
The XRD Results
[0040] From XRD analysis, all samples are mainly A.sub.2B.sub.7,
which includes both Ce.sub.2Ni.sub.7 and La.sub.2Ni.sub.7
structures. A clear peak between 32 and 34 2 theta angles shows the
existence of A.sub.2B.sub.7 phase. The representative XRD spectra
of prior art samples A1 (curve a), A2 (curve b) and inventive
sample A3 (curve c) are plotted in FIG. 2. The spectra specifically
show that all three samples contain mainly A.sub.2B.sub.7 phases
(including La.sub.2Ni.sub.7 structures for A1 and both
Ce.sub.2Ni.sub.7 and La.sub.2Ni.sub.7 structures for A2 and
A3).
PCT Measurement
[0041] A PCT isotherm for each alloy was measured at 30 C and the
absorption/desorption hysteresis was calculated from In
(P.sub.a/P.sub.d), where P.sub.a, P.sub.d are the hydrogen
absorption, desorption equilibrium pressure at a hydrogen
concentration of 0.5 wt. %, respectively. The PCT hysteresis for
each alloy is listed in Table 3. The prior art A1 sample shows a
hysteresis of 0.62 which is very large and will yield a low power
and low cycle life battery. The prior art A2 sample shows a
hysteresis of 0.53, which is an improvement due to the replacement
of La with misch metal (a combination of four rare earth elements)
which increases the materials degree of disorder. Inventive samples
A3 and A4 show reduced PCT hysteresis of 0.3 and 0.42,
respectively. The Inventors have noted that as the atomic volume of
the modifier increases, the PCT hysteresis increases as well, which
was confirmed by comparative samples A5 and A6. FIGS. 3a-3c are PCT
isotherms taken at 30 C for prior art samples A1 (FIG. 3a), A2
(FIG. 3b) and inventive sample A3 (FIG. 3c), the number inserted in
each graph is the PCT hysteresis (In (P.sub.a/P.sub.d)) at 0.5%
hydrogen storage. TABLE-US-00003 TABLE 3 PCT Hysteresis Alloy No.
Category In(P.sub.a/P.sub.d) .sup.@ 0.5% H.sub.2 A1 Prior Art 0.62
A2 Prior Art 0.53 A3 Inventive 0.30 A4 Inventive 0.42 A5
Comparative 0.50 A6 Comparative 0.88
Magnetic Susceptibility Measurement
[0042] It is known that for an electrochemical hydrogen storage
alloy to have good rate capability, the powder particles of the
storage alloy must have a highly catalytic surface region. The
earliest form of this catalytic surface was described in U.S. Pat.
No. 4,716,088 ('088). It was found that metal oxides at the
electrode surface can decrease charging efficiency and promote
hydrogen evolution. Overcoming the effects of metal oxides formed
during electrode fabrication is crucial to the successful operation
of metal hydride electrodes in sealed cell applications. The metal
oxides are detrimental to sealed cell performance. First, oxides at
the surface have been found to decrease charging efficiency and
promote hydrogen evolution.
[0043] Another detrimental effect of metal oxides is the hindrance
of new surface area formation. Upon successive charging and
discharging cycles, the surface area of a metal hydride electrode
can increase tremendously from the initial surface area after
fabrication. The degree of surface area increase is related to the
composition of the active material, but excessive levels of metal
oxide can hinder surface area increase almost completely. Thus, the
effects of initial surface oxide are especially important during
the initial stages of cell activation. Besides lowering cell
pressure by affecting current density, maximized surface area is
also important for discharge rate capability and promoting
electrode cycle life.
[0044] The inventors of the '088 patent found that even under
careful fabrication conditions, such as described in U.S. Pat. No.
4,551,400, the hydrogen storage alloy metals are so sensitive to
oxidation that metal oxide formation can be minimized but not
easily eliminated. It was discovered that without any other
treatment, electrodes fabricated under standard processing
conditions, as previously described, have a surface oxide. The
composition, thickness, and oxidation state of the surface oxide is
variable. Factors which can influence the degree of oxidation
include: the active material composition, the type of process used
to prepare powder for electrodes prior to compaction, the particle
size and surface area of the initial active material, the method of
compacting the powder, and the method used to sinter the compacted
powder. The degree of oxidation will generally increase with longer
duration of atmospheric exposure. Generally, the higher the
temperature during processing, the greater the likelihood of metal
oxide formation. The invention of the '088 provided methods to
overcome the effect of the initial oxidation resulting from
material processing or fabrication. This method includes, prior to
placing the negative electrode in a sealed cell, exposing the
electrode to an alkaline solution to alter the nature of the
oxides. This process, referred to as etching, alters the surface
condition of the metal hydride electrode.
[0045] The etching partially removes surface oxides. It is believed
that oxides which are formed during fabrication are relatively
thin, but dense and extremely impermeable to hydrogen diffusion. By
removing some of the soluble components of the surface oxide, it is
believed that hydrogen diffusion is promoted, allowing improved
electrochemical hydrogen transfer and charge acceptance. The
surface oxide after etching can be thicker than that of the initial
electrode, but by removal of the soluble components is more porous
than oxides formed during fabrication. It may also be possible that
oxides formed during etching form hydroxide complexes with the
metals of the active material, rather than the less permeable
oxides. Also, by selectively removing only a portion of the oxide
layer, etching provides catalytic sites of nickel or nickel alloy
metal, which are resistant to oxidation and very insoluble in
potassium hydroxide electrolyte. It is believed that in addition to
providing catalytic surfaces for the discharge reaction, the nickel
being present in the metallic form provides a conductive element to
the surface oxide. In effect, the nickel acts to balance the
insulating qualities of oxides.
[0046] The catalytic surface of electrochemical hydrogen storage
alloys were further improved with the advent of techniques and
materials described in U.S. Pat. No. 5,536,591 ('591). The '591
invention provides hydrogen storage alloy materials having a
significant increase in the frequency of occurrence of the nickel
catalytic regions, as well as a more pronounced localization of
these regions. More specifically, the materials of the present
invention have enriched nickel regions of 50-70 .ANG. in diameter
distributed throughout the oxide interface and varying in proximity
from 2-300 .ANG., preferably 50-100 .ANG., from region to region.
This is illustrated in FIG. 1 of the '591 patent, where the nickel
regions 1 are shown as what appear as grains on the surface of the
oxide interface 2 at 178,000 X. As a result of the increase in the
frequency of occurrence of these nickel regions, the materials of
the '591 patent exhibited significantly increased catalysis and
conductivity.
[0047] In U.S. Pat. Nos. 6,270,719 and 6,740,448, the surface
catalytic region was further enhanced and modified such that
superior catalysis and high rate discharge performance could be
achieved by one or more of the following: [0048] 1) the catalytic
metallic sites of the alloys are formed from a nickel alloy such as
NiMnCoTi rather than just Ni; [0049] 2) the catalytic metallic
sites of the alloys are converted by elemental substitution to an
FCC structure from the BCC structure of the prior art Ni sites;
[0050] 3) the catalytic metallic sites of the alloys are much
smaller in size (10-50, preferably 10-40, most preferably 10-30
Angstroms) than the Ni sites of the prior art alloys (50-70
Angstroms) and have a finer distribution (closer proximity); [0051]
4) the catalytic metallic sites of the alloys are surrounded by an
oxide of a multivalent material (containing MnO.sub.x) which is
believed to possibly be catalytic as well, as opposed to the ZrTi
oxide which surrounded the prior art Ni sites; [0052] 5) the oxide
could be multiphase with very small (10-20 Angstrom) Ni particles
finely distributed in a MnCoTi oxide matrix; [0053] 6) the oxide
may be a mix of fine and coarse grained oxides with finely
dispersed catalytic metallic sites; [0054] 7) alloy modification
with aluminum may suppress nucleation of large (50-70 Angstrom)
catalytic metallic sites (at 100 Angstrom proximity) into a more
desirable "catalytic cloud" (10-20 Angstroms in size and 10-20
Angstroms proximity); and [0055] 8) NiMn oxide is the predominant
microcrystalline phase in the oxide and the catalytic metallic
sites may be coated with NiMn oxide.
[0056] Further enhancement of the surface catalytic oxide interface
region was achieved by the addition of a microstructure tuning
element such as Cu, Fe, or Zn in copending U.S. patent application
Ser. No. 10/405,008 filed Apr. 1, 2003. The microstructure tuning
element provides preferentially etched alloy particles with
interface regions that are highly porous and that include catalytic
metallic particles. The microstructure of the surface region
includes a large volume fraction of voids having spherical or
channel-like shapes and are sufficiently open structurally to
facilitate greater mobility of reactive species within the
microstructure and in the vicinity of catalytic metallic particles.
Greater accessibility to reactive sites accordingly results. The
greater mobility of reactive species and/or the greater density of
catalytic particles lead to faster kinetics and improved
performance (e.g. higher power), especially at low operating
temperatures.
[0057] It should be noted that magnetic susceptibility measurements
can be used to detect the presence and size of the catalytic
metallic particles in the surface catalytic regions of the etched
hydrogen storage alloy particles and as such is a simple test to
characterize such materials.
[0058] Ingots of the samples were pulverized by a hydride/dehydride
process without mechanical grinding as described in U.S. Pat. No.
6,120,936, entitled A METHOD FOR POWDER FORMATION OF A HYDROGEN
STORAGE MATERIAL, herein incorporation by reference. The powder was
activated at 100.degree. C. in a 60% KOH alkaline bath for 2
hours.
[0059] The magnetic susceptibility of the samples was measured by a
PMC alternating gradient magnetometer and the free metallic nickel
size was estimated from the magnetic susceptibility (Ms) vs.
applied magnetic field (H) plot. The results are summarized in
Table 4. All of the inventive samples are similar in both the
amount of metallic Ni and also the size of Ni cluster. This
indicates that the amount of modifier that was added to the alloy
does not affect its surface catalytic property and therefore the
inventive samples maintain the high rate discharge capability of
the prior art materials. TABLE-US-00004 TABLE 4 Alloy No. Category
Signal (memu/g) Ni cluster size (.ANG.) A3 Inventive 6278 17 A4
Inventive 4238 21 A5 Comparative 6710 17 A6 Comparative 4998 16
Electrochemical Measurement
[0060] Powder of 200 mesh or smaller of the samples was pressed
onto a Ni mesh substrates without other conducting metal powder or
inorganic additives. The electrochemical capacity of the alloy was
determined by constructing a flooded full cell using grafted PE/PP
separators, partially pre charged Ni(OH).sub.2 counter electrodes,
and 30% KOH aqueous electrolyte. The cells were charged at a 0.15 C
rate for 10 hours and then discharge to 0.8 volts at a 0.15 C
current plus a 0.015 C pull current. Results are summarized in
Table 5. The alloys of the present invention, A3 and A4, have the
highest capacities. The cycle life of the inventive samples are
second only to sample A6 (Zr modifier) due to the fact that Zr
forms oxides which are insoluble in the alkaline electrolyte
solution and which protect the magnesium underneath from being
oxidized. TABLE-US-00005 TABLE 5 Discharge capacity Cycle
degradation Alloy No. Category (mAh/g) C(10)/C(1) A1 Prior Art 308
0.7987 A2 Prior Art 343 0.8367 A3 Inventive 356 0.882 A4 Inventive
359 0.8607 A5 Comparative 348 0.8132 A6 Comparative 346 0.9249
[0061] The disclosure and discussion set forth herein is
illustrative and not intended to limit the practice of the instant
invention. Numerous equivalents and foreseeable variations thereof
are envisioned to be within the scope of the instant invention. It
is the following claims, including all equivalents, in combination
with the foregoing disclosure, which define the scope of the
instant invention.
* * * * *