Hydrogen storage alloys having reduced PCT hysteresis

Abstract

A modified A2B7 type hydrogen storage alloy having reduced hysteresis. The alloy consists of a base AxBy 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 cm3 / 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.

Description

FIELD OF THE INVENTION [0001] The present invention relates generally to hydrogen storage alloys mainly of the either Ce2Ni7 or Nd2Ni7 or mixture of both crystal structures. More specifically the present invention relates to A2B7 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 cm3 / 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 an...

AI Technical Summary

Benefits of technology

[0030] Alternatively, the catalytic metallic regions are 10-50 Å preferably 10-40 Å and more preferably 10-30 Å in diameter. Most preferably the catalytic metallic regio

Problems solved by technology

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.

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.

This is because unrestrained corrosion lead to poor cycle life and passivation resulted in low capacity, poor discharge rate performance, and poor cycle life.

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.

This is problematic with MgNi based materials.

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.

Unfortunately, the prior art A2B7 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 large concentration gradient associated with a large hysteresis material is a manifestation of the greater difficulty associated with absorbing hydrogen into the material.

Large energy barriers inhibit the motion of hydrogen and act to reduce the uniformity of the absorbed hydrogen concentration within the metal hydride material.

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.

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.

Differential lattice expansion is detrimental to cycle life because it introduces internal stresses into the metal hydride lattice.

If the stress exceeds this stress limit, the metal hydride material is unable to support the stress and relieves the stress by fracturing.

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.

Therefore, although A2B7 Mg mischmetal based alloys show higher storage capacity than commercially viable AB5 alloys, the PCT hysteresis of prior art A2B7 alloys is too high for battery applications, taking into consideration both the mid point voltage and cycle life performance.

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