Nickel Metal Hydride Battery

Nickel Metal Hydride Battery

A nickel metal hydride battery, NiMH, is a rechargeable battery with a positive electrode made of nickel hydroxide and a negative electrode made of a metal hydride (a hydrogen-absorbing alloy). The NiMH battery was commercially introduced in 1989 and was mainly used as a power source in portable personal computers. Since then, the NiMH battery system has become very popular in electric hybrid vehicles and makes up 10% of the total market for rechargeable batteries. Compared to the NiCd battery, the NiMH provides 40 percent higher specific energy resulting in about two times higher capacity. NiMH batteries are also less affected by voltage depression, but the main advantage is the absence of toxic cadmium. The memory effect of NiMH batteries is much less than nickel-cadmium batteries.

Compared to alkaline batteries, the internal resistance of NiMH batteries is much lower. Because of this, they have the advantage that a higher voltage can be maintained under high load. 

With high specific energy (up to 100Wh/kg) and energy density (double of lead-acid and 40% more than nickel-cadmium), high cycle life, low cost, recyclability (cadmium is hazardous), and other qualities made it the most used battery and still nowadays is one of the most used. Its applications are much diverse: battery cells AA or AAA, cameras, old mobile phones, electric razors, medical instruments and equipment, and high-power static applications. Compared to the lead-acid battery, the drawback is that it is more expensive than lead acid per kWh. 

NiMH batteries are (re)charged by applying electric current, which reverses the chemical reactions that occur during discharge/use. Devices to supply the appropriate current are called chargers. The electronics in charging systems and control circuits for NiMHs are simple and inexpensive, and the battery is considered safe. The NiMH battery also has high self-discharge and can lose up to 20 % of its charge during the first 24 hours and thereafter 10 % per month.

Like NiCd batteries, they have a nominal voltage of 1.2V per cell with a typical end-of-discharge voltage of 1V. 

The total voltage of the redox reaction is E⁰ = 0.49V – ( – 0.83V) = 1.32V.

Nevertheless, they are widely used as a replacement for alkaline batteries, which have 1.5V nominal voltage per cell, in many applications and are commonly found in standard battery designs. In some applications, however, the lower nominal voltage can be a disadvantage. For example, unregulated lights that are designed for 1.5V operation tend to shine less brightly.

Chemistry of Nickel Metal Hydride Battery – How it works

A typical NiMH cell contains:

• Cathode: a nickel(III) oxide-hydroxide positive electrode plate
• Anode: a metal hydride (a hydrogen-absorbing alloy)
• Separator
• Electrolyte: an alkaline electrolyte (potassium hydroxide).

The metal M in the negative electrode of a NiMH cell is an intermetallic compound. Many different compounds have been developed for this application. The research on materials for negative electrodes for the Ni/MH battery has involved several alloy families: AB5, AB2, and AB. In this notation, A represents a metallic element with a strong affinity to hydrogen (typically rare-earth element like lanthanum, cerium, neodymium, or praseodymium), whereas B is a metallic element with weak hydrogen affinity (typically transition metal like nickel, cobalt, manganese, or aluminum). Today the hydride-forming materials based on the AB5 intermetallics are the most versatile and commercially important family of reversible hydriding alloys. The optimal composition for electrode purposes has been found for an alloy with the following composition: 

MmNi3.55Mn0.4Al0.3Co0.75

where Mm is a Mischmetal, which is an alloy of rare-earth elements. It is also called cerium mischmetal or rare-earth mischmetal. A typical composition includes approximately 55% cerium, 25% lanthanum, and 15~18% neodymium, with traces of other rare earth metals.

The layer structure of a NiMH round cell can be seen in the figure. An outer perforated foil serves as a carrier for the metal hydride powder, which forms the negative electrode. During discharge, the hydrogen bound in the metal hydride MH is oxidized to a proton, and metal of 0 oxidation state is formed:

MH +OH^– → M+H₂O+e^–

The resulting protons react with the OH^– hydroxide ions of the KOH solution to form water. The redox potential is approximately -0.83 V. The separator holds the electrolyte, a 20% KOH solution, and prevents direct contact with the positive electrode. The cathode consists of a sheet of nickel hydroxide and nickel oxide hydrate. Here, nickel of oxidation state +III is reduced to nickel of oxidation state +II in the reaction:

NiO(OH) + H₂0 + e^–→ Ni(OH)₂ + OH^–

The overall reaction during discharge is:

NiO(OH) + MH → Ni(OH)₂ +  M

During this process, free electrons are bound so that this pole becomes the positive electrode. The redox voltage of the reduction is approximately 0.49 V. The total voltage of the redox reaction is thus E⁰ = 0.49V – ( – 0.83V) = 1.32V.

The specific energy of a NiMH cell is about 80 Wh/kg, which is almost as high as that of an alkaline cell and more than twice as high as that of a NiCd battery. NiMH batteries are sensitive to overcharging, overheating, incorrect polarity, and also to deep discharge.

Advantages and Disadvantages of Nickel Metal Hydride Batteries

Characteristics of Nickel Metal Hydride Batteries

To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.

Why are alkaline batteries (AAA or AA) made to be 1.5V while rechargeables are 1.2V?

In general, batteries convert stored chemical energy into electrical energy through an electrochemical process. This then provides a source of electromotive force to enable currents to flow in electric and electronic circuits.

• Primary (single-use or alkaline) batteries use cells that have 1.5V open circuit voltage when fresh. 
• Secondary (rechargeable) batteries use cells from NiMh or NiCd, which have 1.2V open circuit voltage.

In practice, alkaline batteries and rechargeable batteries can be used interchangeably in sets. They have only different voltage characteristics. It is given by their different chemistry. Primary cells gradually drop in voltage from use. They start at 1.5 volts, drop to 1.2 and continue to 1.0 where the appliance stops working. Secondary cells operate more uniformly, even with only 1.2 volts. They have flat discharge where they pretty much stay at 1.2 volts until depleted and then drop off very quickly to below 1.0 volts.

Since electronic devices are usually made to run from cell voltages of 1.0 to 1.5 volts, alkaline batteries and rechargeable batteries perform similarly. In fact, it’s generally considered that secondary 1.2 V cells work better than alkalines having lower output impedance and more consistent voltage from start to finish of a charge.

Other Types of Batteries

The following list summarizes notable electric battery types composed of one or more electrochemical cells. Four lists are provided in the table. The first list is a battery classification by size and format. Then, the primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry. The third list is a list of battery applications. The final list is a list of different battery voltages.