Nickel Metal Hydride Battery Technology
The pioneering work on Nickel Metal Hydride Battery was performed at the Battelle Geneva Research Centre starting after its invention in 1967 as a derivative of bot Ni-Cd and The Ni-H2 cells used in satellites. The key motivations for the Ni-MH studies were the environmental advantages associated with higher energy, lower pressure, and cost of Ni-MH compared to Ni-Cd.: The development work was sponsored over nearly 2 decades by Daimler-Benz Comp./Stuttgart and by Volkswagen AG within the framework of Deutsche Automobilgesellschaft. The batteries showed high energy and power densities up to 50 Wh/kg, 1000 W/kg and reasonable cycle life of 500 cycles [https://en.wikipedia.org/wiki/Cobasys]
Nickel Metal Hydride Battery Technology for Hybrid Vehicles:
In 1992, under a cooperative agreement with DOE, the USABC initiated the development of nickel-metal hydride (Ni-MH) battery technology.
DOE funding through that cooperative agreement was instrumental to the development of Ni-MH technology at two manufacturers, Energy Conversion Devices, Inc. (ECD Ovonics) and SAFT America. ECD Ovonics’ Ni-MH technology is now manufactured at COBASYS, LLC, its 50-50 manufacturing joint venture with Chevron Technology Ventures, LLC. ECD is also licensing its technology to Sanyo, which supplies Ni-MH batteries for the Ford Escape, Cmax, and Fusion hybrid vehicles; to Honda, for its hybrid vehicles; and to Panasonic, which supplies batteries for Toyota hybrid vehicles. Under the terms of the original ECD contract, a small fraction of these licensing fees have been remitted to DOE and the USABC.
In 2008, the Nickel Metal Hydride Battery market had a share of 10% of the total rechargeable battery industry. The important reasons for the rapid growth of Ni-MH have been the growth of HEVs and the development of Ni-MH cells as direct replacements for alkaline primary cells.
Nickel-metal hydride system is similar in many ways to Ni-Cd cells. In the oxygen recombination reaction also, the system is similar to VRLA cells in the design of oxygen diffusion from PAM to NAM and starved electrolyte design.
The advantages Nickel Metal Hydride battery are:
Low cost, versatile cell size, excellent performance characteristics (including high charging current absorption), a wide range of operational temperature (-30 to 70ºC), the safety of operation at higher voltages, (350 + V), simplicity of controlling the charging process, etc. Moreover, it is environmentally friendly (compared with nickel-cadmium cells).
Of course, there are disadvantages also: higher cost in comparison with lead-acid cells lower energy specifics when compared with Li-ion cells.
Energy producing electrochemical reactions in Nickel Metal Hydride battery
There is a lot of similarity between Ni-Cd and Ni-MH cells, except the negative electrode. As in the case of Ni-Cd cells, during discharge, the positive active material (PAM), nickel oxy hydroxide, is reduced to nickel hydroxide. (Thus the positive electrode behaves as a cathode):
NiOOH + H2O + e → Ni(OH)2 + OH– Eo = 0.52 V
The negative active material (NAM), metal hydride (MH), is oxidized to the metal alloy (M). (Thus the negative electrode behaves as an anode):
MH + OH– → M + H2O + e Eo= 0.83 V
That is, desorption of hydrogen occurs during discharge and the hydrogen combines with a hydroxyl ion to form water while also contributing an electron to the circuit.
The overall reaction on discharge is
MH + NiOOH Discharge↔charge M + Ni(OH)2 Eo = 1.35 V
Please remember that
Cell voltage = VPositive – VNegative
0.52 – (-0.83) = 1.35 V
Here it is to be noted that water molecules shown in the half cell reactions do not appear in the overall or total cell reaction. This is due to the electrolyte (aqueous potassium hydroxide solution) not participating in the energy producing reaction and it is there only for conductivity purposes.
Also note that the aqueous solution of sulphuric acid used as an electrolyte in the lead acid cells actually is participating in the reaction as shown below:
PbO2 + Pb + 2H2SO4 Discharge↔charge 2PbSO4 + 2H2O
This is an important difference between lead acid cells and alkaline cells.
The process is reversed during charge
The sealed nickel-metal hydride cell uses an oxygen-recombination reaction similar to the one occurring in valve regulated lead acid (VRLA) cells, thus preventing the build up of pressure that may result from the generation of gases towards the end of the charge and particularly during overcharge.
During charge PAM reaches full charge before the NAM and so the positive electrode begins to evolve oxygen.
2OH– → H2O + ½O2 + 2e–
The oxygen gas diffuses through the pores of the separator to the negative electrode facilitated by the starved electrolyte design and the use of an appropriate separator.
At the NAM, the oxygen reacts with the metal hydride electrode to produce water, thus preventing pressure build-up inside the battery. Even so, there is a safety valve in cases of extended overcharge or charger malfunction; it is possible that oxygen, and hydrogen, will be generated faster than it can be recombined. In such instances, the safety vent will open to reduce the pressure and prevent battery rupture. The vent reseals once the pressure is relieved. The exit of gas through the re-sealable vent can carry electrolyte droplets, which may form crystals or rust once deposited on the can. (https://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf)
4MH + O2 → 4M + 2H2O
Moreover, by virtue of design, the NAM does not become fully charged, which prevents the generation of hydrogen. This is true for the early stages of cycling where the only gas found inside the cell is oxygen. However, on continued cycling, hydrogen gas begins to evolve, and a significant rise in the proportional hydrogen within is observed. Hence it is very important to control charge voltage at the end of charge and during overcharge to limit the generation of oxygen to below the rate of recombination to prevent the build-up of gases and pressure.
A design factor referred to earlier in the design of Ni-MH cells is NAM to PAM ratio. It is based on
use of more NAM than the PAM.
The ratio depends on the applications and is in the range of 1.3 to 2 (NAM/PAM), the lower values are employed where higher specific energy is important while higher values are used in high power and long cycle life design cells.
Fabrication of Nickel Metal Hydride Battery cells
The Ni-MH cells are sealed cells with a safety device and with metallic cases and tops, both of which are insulated from each other by a gasket. The case bottom is the negative terminal and the top serves as the positive terminal.
In all the design types, whether cylindrical or prismatic or button cells, the cathode is either sintered type or pasted type.
The positive electrode in the cylindrical Ni-MH cell is a porous sintered substrate or foam-based nickel substrate over which nickel compounds are impregnated or pasted, and converted into the active material by electro-deposition.
The substrate serves as mechanical support for the sintered structure acts as a current collector for the electrochemical reactions that occur throughout the porous plates. It also provides mechanical strength and continuity during the manufacturing processes. Either perforated nickel-plated steel or pure nickel strip in continuous lengths, or woven screens of nickel or nickel-plated steel wire is used. A common perforated type maybe 0.1 mm thick with 2 mm holes and a void area of about 40%. Expanded metals and perforated sheets are of lower cost, but they have poor high-rate capability. Sintered structures are much more expensive but suitable for high discharge performance.
Foams have generally replaced sintered plaque electrodes.
Similarly, the negative electrode is also a highly porous structure using a perforated nickel foil or grid over which the plastic bonded active hydrogen storage alloy is coated. The electrodes are separated with a synthetic nonwoven material, which serves as an insulator between the two electrodes and as a medium for absorbing the electrolyte.
Nickel Metal Hydride Battery Positive active material (cathode material)
Similar to Ni-Cd cells, the positive electrodes in Ni-MH cells, whether cylindrical or prismatic, use the sintered or pasted type. The nickel hydroxide for use in Ni-MH cells is basically the same as that used in Ni-Cd. Today’s high-performance nickel hydroxide is more advanced in, capacity, coefficient of utilization, power and discharge rate capability, cycle life, high temperature charging efficiency, and cost.
High-density nickel hydroxide with spherical particles is most commonly employed in pasted positive electrodes. /the said material is prepared in precipitation chambers where nickel sulphate (along with some additives like cobalt and zinc salts to improve performance aspects ) is reacted with sodium hydroxide mixed with a little ammonia.
The more common pasted positive plate is typically produced by mechanically pasting high-density spherical nickel hydroxide into the pores of a foam metal substrate, which in turn is produced by coating polyurethane foam (PUF) with a layer of nickel either by electroplating or by chemical vapour deposition. This is followed by a heat treatment process to remove the base polyurethane. The pore size and density of the foam can also be adjusted to improve performance characteristics.
The foam is then loaded with nickel hydroxide in a paste containing conductive cobalt oxides, which form a conductive network between the nickel hydroxide and the metal current collector. Just as lead sulphate in a lead-acid cell, the nickel hydroxide is a poor conductor. Now the foam plate is ready for the next step.
The other type of electrode is the sintered one. This type has better power capability but at the cost of lower capacity and higher cost.
Sintered positives begin with the pasting of filamentary nickel onto a substrate such as perforated foil, where the nickel fibres are then sintered under a high-temperature annealing furnace in a reducing atmosphere using nitrogen/hydrogen. In the process binders from the pasting process are burned away, leaving a conductive skeleton of nickel.
Nickel hydroxide is then precipitated into the pores of the sintered skeleton using either a chemical
or electrochemical impregnation process. The impregnated electrodes are then formed or pre-activated
in an electrochemical charge/discharge cycling process. Now the sintered plate is ready for the next step.
Metal Hydride Alloy for negative electrodes (anode material)
Ni-MH cells use metal hydride active material in the form of a hydrogen-absorbing alloy. There are several different compositions for the alloy. They are:
1. AB5 alloy
2. AB2 alloy
3. A2B7 alloy
These are engineered alloys made up of rare earth metals in varying proportions. It is beyond the scope of this article to describe the production and properties of these alloys. Readers are requested to refer to relevant publications on these alloys and specialised books on Ni-MH batteries.
The negative electrode is again a highly porous structure using a perforated nickel foil or grid onto which the plastic bonded active hydrogen storage alloy is coated and processed.
Nickel metal hydride battery electrolyte
As in Ni-Cd cells, the electrolyte in Ni-MH cells is an aqueous solution of about 30% potassium hydroxide, providing high conductivity over a wide temperature range. Lithium hydroxide (LiOH) is invariably an additive at a concentration of about 17 grammes per litre (GPL). This helps in improving the charging efficiency at the positive electrode by suppressing oxygen evolution reaction, which is a competing reaction lowering charge acceptance.
As in the case of VRLA and Ni-Cd cells, Ni-MH cells are also of the sealed, starved electrolyte design. The plates are almost saturated with electrolyte. The separator is only partially saturated to allow rapid gas diffusion for efficient gas recombination reaction. Adding NaOH assists in improving high-temperature charge efficiency, but at a cost of reduced life as a result of elevated corrosion of the NAM.
Nickel metal hydride battery separator
The function of the separator is to prevent electrical contact between the positive and negative electrodes while retaining electrolyte necessary for ionic transport. The first generation separators for Ni-MH cells were standard Ni-Cd and NiH2 separator materials of nonwoven polyamide (nylon) cloth separator. However, Ni-MH cells proved to be more sensitive to self-discharge, especially when such separators were used. The presence of oxygen and hydrogen gas causes the polyamide materials in the nylon separator to decompose.
The corrosion products (nitrite ions) from this decomposition allowed for the poisoning of the nickel hydroxide, promoting premature oxygen evolution and also forming compounds capable of redox shuttle between the two electrodes, which further increases the rate of self-discharge. Therefore this type of separator is not used nowadays. Instead, polyolefin separators are employed in nextgen cells. The “permanently wettable polypropylene” is now widely used. The improved separator is a composite of PP and PE with special treatments. Self-discharge rate and cycle life are affected appreciably with texture, wettability and gas permeability.