The Nickel-Iron Battery

The Ni-Fe system developed by Edison in 1901 was the predominant commercial secondary battery till the early 1920s. Past applications have been to railcar lighting, mine lamps, mine vehicles, lift trucks, etc. 

The electrochemical reactions (limited in the battery to a two-electron redox process on iron) can be represented by 

The capacity loss due to self-discharge is considerable. It can amount, in a fully charged battery, to 2% during the first 20 min of open-circuit standtime and exceeds 5%, after 4h.  

As opposed to the positive electrode, the redox process at iron does not proceed in the solid phase but via a dissolution-precipitation mechanism simplified below to 

In spite of their low solubility ( 5 x 10 M litre), HFeOJ ions diffuse to the positive electrode and are oxidized to solid FeOOH causing further dissolution of iron and its continous transfer to the positive electrode. The process is irreversible, the potential of the nickel electrode being too positive, even during discharge, for the reduction of trivalent iron. Further decrease of capacity is caused by the lowering of oxygen overpotential on the nickel oxide in the presence of FeOOH. The self-discharge and iron transfer processes are somewhat inhibited by additives to the electrode (sulfur) or electrolyte (e.g., lithium and sulfide ions, or hydrazine sulfate). 

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Electrochemistry and Kinetics. The electrochemistry of the nickel—iron battery and the crystal stmctures of the active materials depends on the method of preparation of the material, degree of discharge, the age (Life cycle), concentration of electrolyte, and type and degree of additives, particularly the presence of lithium and cobalt. A simplified equation representing the charge—discharge cycle can be given as ... 

Probably the best-known battery system using an iron anode is called the nickel/iron battery. It should be written (-) Fe / KOH / NiO(OH) (+), having its merits as a heavy-duty accumulator [7], By... 

The nickel-iron battery has an iron anode, an NiO(OH) cathode, and a KOH electrolyte. This battery uses the following half-reactions and has an E° value of 1.37 V at 25°C ... 

Nickel-Iron Battery In 1901, Thomas Edison invented the nickel-iron battery. The following reaction takes place in the battery. 

Both the positive and negative tubular and pocket current collectors are made of perforated nickel-plated steel. They are very robust and are virtually indestructible. The low energy density, poor charge retention, and poor low temperature performance, along with high cost of manufacture, have led to a decUne in use of the nickel-iron battery system. The negative electrode, or anode, is iron and the positive... 

The nickel-iron battery cell fabrication process is essentially unchanged in over 50 years. Special attention must be paid to use high purity materials and particle size characteristics of the active materials. The iron negative active material is made from pure iron that is dissolved in sulfuric acid. The resulting Fe(S04>2 is recrystallized and dried. This is washed free of sulfuric acid and roasted at 915°C to form a mixture of FeaOs and Fe metal and is, then, blended with small amotmts of FeS, sulfur, and HgO for use in the negative plate assembly. 

For instance, the nickel-iron battery, invented almost at the same time (Edison, 1901) as the nickel-cadmium battery, has a poor charge efficiency, which causes excessive heating and hydrogen release. Another example is nickel-zinc technology, for which further study seems necessary, because it is subject to the formation of dendrites which limit its lifetime. 

The development of nickel/cadmium batteries started in the beginning of the twentieth century in parallel to that of the nickel/iron battery. The latter played an important role mainly as a sturdy traction battery that reached many charge/ discharge cycles. But after World War II it gradually lost its market, mainly because of the high hydrogen evolution rate and comparatively low power efficiency. The nickel/cadmium battery, however, still has a strong market position, mainly in its sealed version as a portable power source, but also as a flooded battery in traction and stationary applications. 

The lead-acid battery system is by far the least costly of the secondary batteries, particularly the SLI type. The lead-acid traction and stationary batteries, having more expensive constmctional features and not as broad a production base, are several times more costly, but are still less expensive than the other secondary batteries. The nickel-cadmium and the rechargeable zinc/manganese dioxide batteries are next lowest in cost, followed by the nickel/metal hydride battery. The cost is very dependent on the cell size or capacity, the smaller button cells being considerably more expensive than the larger cylindrical and prismatic cells. The nickel-iron battery is more expensive and, for this reason among others, lost out to the less expensive battery system. 

The active materials of the nickel-iron battery are metallic iron for the negative electrode, nickel oxide for the positive, and a potassium hydroxide solution with lithium hydroxide for the electrolyte. The nickel-iron battery is unique in many respects. The overall electrode reactions result in the transfer of oxygen from one electrode to the other. The exact details of the reaction can be very complex and include many species of transitory existence.The electrolyte apparently plays no part in the overall reaction, as noted in the following reactions ... 

Capacity. The capacity of the nickel-iron battery is limited by the capacity of the positive electrode and, hence, is determined by the length and number of positive tubes in each plate. The diameter of the tubes generally is held constant by each manufacturer. The 5-h discharge rate is commonly used as the reference for rating its capacity. 

Discharge Characteristics. The nickel-iron battery may be discharged at any current rate it will deliver, but the discharge should not be continued beyond the point where the battery nears exhaustion. It is best adapted to low or moderate rates of discharge (1- to 8-h rate). Figure 25.6 shows the discharge curves at different rates of discharge at 25°C. 

Self-Discharge. The self-discharge rate, eharge retention, or stand characteristic of the nickel-iron battery is poor. At 25 C a eell win lose 15% of its capacity in the first 10 days and 20 to 40% in a month. At lower temperatures the self-discharge rate is lower. For example, at 0°C the losses are less than one-half of those experienced at 25T. 

The battery is less damaged by repeated deep discharge than any other battery system. In practice, an operator will drive a battery-operated vehicle until it stalls, at which point the battery voltage is a fraction of a volt per cell (some cells may be in reverse). This has a minimal effect on the nickel-iron battery in comparison with other systems. 

R. Hudson and E. BrogUo, Development of the Nickel-Iron Battery System for Electric Vehicle Propulsion, Proc. 29th Power Sources Conf., Electrochemical Society, Pennington, N.J., 1980. 

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