Research method

Improvement of Capacity of Nickel-Metal HydrideBattery

X-ray diffraction/
scattering

Clarification of the optimal composition of the electrode

Role of SPring-8

Achievements

Clarification of the relationship between the crystal structure of electrodematerials used in nickel-metal hydride batteries* and changes in the discharge
capacity with repeated charging and discharging
Optimization of the composition of the alloys in the electrodes, and developmentof a product with at least 20% increased discharge capacity
R&D facility: GS Yuasa Corporation*Nickel-metal hydride battery: A type of secondary battery that can be reused after charging. This type of battery is characterized by the use ofenvironmentally friendly hydrogen storage alloys (metal hydride that stores hydrogen atoms in the interstitial position of its crystals) without theuse of lead or cadmium. Nickel-metal hydride batteries are used in various devices, such as cameras, PCs, and mobile phones, because their energy
density is higher than that of Ni-Cd batteries.

BackgroundResults

The discharge capacity of nickel-metal hydride
batteries depends on the reaction rate ofhydrogen storage by the anode alloy and on theamount of hydrogen reversely released from thealloy (amount of hydrogen storage). Therefore,hydrogen storage alloys based on various newelements are being actively studied. In particular,
La-Mg-Ni-type alloys are drawing much attention.
However, La-Mg-Ni-type alloys, which can store avery large amount of hydrogen, have a disadvantage
in the sense that their performance deteriorates withrepeated charging and discharging. To optimize
their composition, it is necessary to clarify theircrystal structure, but conventional methods wereunsatisfactory to observe the crystal structure,
which contains various stacking structural phases.

By observing La-Mg-Ni-type alloys by X-ray
diffraction using the synchrotron radiation ofSPring-8, it was clarified how the various crystal
structures of the alloy, which contains various
elements, change depending on the percentage ofthe metals added (manganese and aluminum).
Moreover, we analyzed the discharge capacity andchanges in the performance due to repeatedcharging and discharging for different crystal
structures. We found the optimal crystal structurethat can maintain high capacity in a stable mannerfrom among the various possible phases.
From the above result, we successfully obtained
guidelines for designing electrode materials.

Publication: T. Ozaki et al.; Journal of Alloys and Compounds 446-447, 620-624 (2007) T. Ozaki et al.; ITE Letters 8(4), B24 (2007)

Principle of nickel-metal
hydride battery

Change in energy density in AA-sized
nickel-metal hydride battery

Phase structure ofLa0.8Mg0.2Ni3.4-xCo0.3(MnAl)x alloy

Relationship between number of repetitions ofcharging and discharging and discharge capacity

Nickel hydroxide and a hydrogen storage alloy
are used as the cathode and anode,respectively. Hydrogen atoms move from thehydrogen storage alloy towards the nickelhydroxide cathode during discharging.

Battery performance depends on the quality of the hydrogenstorage alloy used as the anode material, and its improvement
has been promoted. The discharge capacity of a battery thatuses a rare earth-nickel-based alloy (AB5-type alloy) as theanode material is at least 85% of the theoretical value;
therefore, the further improvement of the capacity is difficult.

The crystal structures of alloys are dependent on the percentage ofmanganese and aluminum added, (MnAl)x. In particular, at x=0.15, Mn
and Al substitute specific Ni sites that are sandwiched between
rare-earth elements, leading to the stabilization of a 5:19 phase.

The discharge capacity of the alloy that contains a large
amount of the 5:19 phases (x=0.15) hardly decreases evenwith repeated charging and discharging. This is because thedegree of expansion and contraction due to the absorption
and release of hydrogen, respectively, is small in the 5:19phase.

Electrolyte28Hydrogen atom

Composition

Discharging

Energy density (Wh/I)

Charging

Metal atomNew
La-Mg-Ni-typealloy

CathodeNickel hydroxide

AB5-type alloy

Year

Discharge capacity (mAh/g)

Anode
Hydrogen storage alloy

Number of repetitions of charging and discharging

29