Nickel–Cadmium

Nickel–cadmium (Ni–Cd) technology refers to a class of rechargeable electrochemical cells and batteries that utilise nickel oxyhydroxide (NiOOH) and cadmium (Cd) as the primary active materials. Introduced in the early 20th century, Ni–Cd batteries became one of the most influential and widely used rechargeable battery systems before the advent of nickel–metal hydride (Ni–MH) and lithium-ion technologies. Their ruggedness, long cycle life, and reliable performance in a wide range of temperatures made them indispensable in portable electronics, aviation, emergency power systems, and industrial applications. This article provides a comprehensive, 360-degree overview of nickel–cadmium — covering its chemistry, development, construction, performance, applications, safety, and environmental aspects.

Historical Background

The nickel–cadmium battery was invented in 1899 by the Swedish engineer Waldemar Jungner, who experimented with nickel and iron electrodes. He discovered that cadmium produced a more stable and efficient rechargeable cell. However, due to high material costs, early Ni–Cd batteries were not commercially widespread.
By the 1930s, technological refinements — particularly in electrode design and electrolyte control — led to the production of vented Ni–Cd batteries, which found applications in aviation and industrial backup systems. The sealed Ni–Cd cell, developed in the 1940s–50s, revolutionised portable electronics, paving the way for use in power tools, cameras, and early cordless phones.
Although nickel–cadmium technology has largely been supplanted by nickel–metal hydride and lithium-ion systems in consumer products, it remains relevant in specific professional and high-reliability sectors.

Chemical Composition and Cell Chemistry

The basic nickel–cadmium cell consists of two electrodes immersed in an alkaline electrolyte (usually potassium hydroxide, KOH).

• Positive electrode (cathode): Nickel oxyhydroxide (NiOOH)
• Negative electrode (anode): Cadmium (Cd)
• Electrolyte: Concentrated aqueous potassium hydroxide (20–30% solution)

Electrochemical reactions:
During discharge, the following reactions occur:

• At the cathode:

  2NiOOH+2H2O+2e−→2Ni(OH)2+2OH−2NiOOH + 2H₂O + 2e⁻ → 2Ni(OH)₂ + 2OH⁻2NiOOH+2H2​O+2e−→2Ni(OH)2​+2OH−

• At the anode:

  Cd+2OH−→Cd(OH)2+2e−Cd + 2OH⁻ → Cd(OH)₂ + 2e⁻Cd+2OH−→Cd(OH)2​+2e−

The overall cell reaction on discharge is:
2NiOOH+Cd+2H2O→2Ni(OH)2+Cd(OH)2 2NiOOH + Cd + 2H₂O → 2Ni(OH)₂ + Cd(OH)₂2NiOOH+Cd+2H2​O→2Ni(OH)2​+Cd(OH)2​
On charging, the reactions are reversed, regenerating NiOOH at the positive electrode and metallic Cd at the negative electrode. The nominal cell voltage is approximately 1.2 volts, remaining relatively constant during discharge.

Construction and Design Types

Nickel–cadmium batteries exist in vented (flooded) and sealed designs, differing primarily in gas management and electrolyte retention.

1. Vented Ni–Cd Batteries:
   • Used in industrial, aviation, and railway systems.
   • Contain liquid electrolyte that can be replenished.
   • Fitted with vents to release gases generated during overcharge.
   • Typically designed for deep-cycle and long-life applications, with service lives exceeding 20 years.
2. Sealed Ni–Cd Batteries:
   • Common in portable electronic devices.
   • Use limited electrolyte immobilised in separators (often nylon or polypropylene).
   • Incorporate oxygen recombination technology to minimise gas escape during charge cycles.
   • Compact and maintenance-free, though more sensitive to overcharging.

Electrode structure:

• Sintered plate electrodes — fine nickel powder sintered on a metallic substrate, offering high surface area and rapid charge acceptance.
• Pasted plate electrodes — porous nickel substrate coated with active material paste, cheaper but less robust.
• Pocket plate electrodes — active materials enclosed in perforated steel pockets; rugged but heavier.

Cells are assembled into battery packs using series or parallel connections to achieve desired voltages and capacities.

Electrical Characteristics and Performance

Nickel–cadmium batteries display several distinctive performance attributes:

• Nominal Voltage: 1.2 V per cell.
• Energy Density: Typically 40–60 Wh/kg — lower than modern lithium-ion, but higher than lead–acid.
• Power Density: High; capable of delivering large currents rapidly.
• Operating Temperature Range: −20 °C to +60 °C, making them suitable for harsh environments.
• Cycle Life: 1000–2000 charge–discharge cycles (often more with proper management).
• Self-Discharge Rate: Around 10–20% per month at room temperature.
• Internal Resistance: Low, enabling efficient high-rate discharge.

The discharge curve remains nearly flat, providing a stable output voltage until near the end of discharge. This reliability under load was one of the reasons for their extensive use in professional equipment and emergency backup systems.

Applications

Nickel–cadmium batteries have served an extensive range of applications across different industries.

1. Aviation and Aerospace:
   • Used for starting aircraft engines, onboard power, and emergency systems.
   • Resistant to extreme temperatures and pressure changes.
2. Railway and Industrial Backup Systems:
   • Reliable source of DC power for signalling, lighting, and control circuits.
   • Long service life and tolerance to deep discharges.
3. Portable Tools and Equipment:
   • Early cordless drills, cameras, and two-way radios utilised sealed Ni–Cd cells for their robustness and rapid recharge capabilities.
4. Medical Devices:
   • Power supply for portable diagnostic and therapeutic equipment where reliability is critical.
5. Emergency Lighting and UPS Systems:
   • Maintains charge over extended standby periods and delivers instant energy when required.
6. Military and Marine Applications:
   • Favoured for their mechanical strength, safety, and performance under extreme environmental conditions.

Although Ni–Cd batteries have been largely replaced in consumer electronics by nickel–metal hydride (Ni–MH) and lithium-ion batteries, they remain irreplaceable in certain heavy-duty and mission-critical roles.

Advantages

Nickel–cadmium batteries possess several inherent strengths that have sustained their use for decades:

• High Cycle Durability: Capable of thousands of charge–discharge cycles with minimal capacity loss.
• Broad Temperature Tolerance: Operate efficiently under sub-zero or high-temperature conditions.
• High Discharge Rate Capability: Suitable for applications demanding bursts of power.
• Overcharge and Over-discharge Resistance: More tolerant to abuse compared with lithium-ion cells.
• Low Internal Resistance: Ensures good voltage stability even at high current loads.
• Long Shelf Life and Storage Stability: Maintain charge for long periods without deterioration.
• Mechanical Ruggedness: Withstand vibration, impact, and pressure variations.

These attributes make Ni–Cd batteries particularly valuable in industrial, military, and emergency backup systems where reliability outweighs considerations of weight or energy density.

Disadvantages and Limitations

Despite their advantages, nickel–cadmium batteries present several drawbacks that have limited their modern use:

• Cadmium Toxicity: Cadmium is a heavy metal with severe environmental and health hazards; improper disposal can lead to soil and water contamination.
• Memory Effect: Repeated partial discharges can cause apparent capacity reduction if not periodically fully discharged.
• Moderate Energy Density: Significantly lower than nickel–metal hydride and lithium-ion systems.
• Self-Discharge: Relatively high, leading to energy loss during storage.
• Maintenance (for vented types): Requires periodic topping up of electrolyte.
• Cost: Higher initial cost compared with lead–acid batteries.

These limitations have prompted regulatory restrictions and the progressive phasing out of Ni–Cd batteries from consumer markets.

Safety and Handling

Nickel–cadmium batteries are generally robust, but improper handling can pose risks:

• Overcharging: Prolonged overcharging may lead to gas evolution (hydrogen and oxygen), pressure build-up, and electrolyte leakage.
• Short Circuits: Can result in high currents causing heating or rupture.
• Electrolyte Exposure: Potassium hydroxide is caustic and can cause chemical burns; protective gloves and goggles should be used.
• Thermal Runaway: Though rare, excessive charging current under high temperatures can trigger overheating.

For safety, cells are equipped with pressure vents, temperature cut-offs, and current-limiting circuits in battery packs. Proper charging protocols and smart chargers prevent over-voltage and over-temperature conditions.

Environmental Impact and Recycling

The presence of cadmium, a toxic heavy metal, makes nickel–cadmium batteries a major environmental concern. Cadmium can accumulate in biological systems and is classified as a carcinogen. Consequently, many countries have imposed stringent regulations on Ni–Cd battery disposal and production.
Recycling plays a vital role in mitigating these effects. The recycling process typically involves:

1. Mechanical separation of battery casings.
2. Thermal treatment or smelting to recover cadmium and nickel metals.
3. Electrolyte neutralisation and safe waste management.

Recovered cadmium and nickel are reused in new batteries or metallurgical processes, minimising the need for virgin materials. The European Union’s Battery Directive and similar regulations worldwide promote the collection and recycling of all Ni–Cd cells to prevent environmental contamination.

Modern Alternatives and Future Outlook

Advances in rechargeable battery technology have largely replaced Ni–Cd batteries in mainstream applications.

• Nickel–Metal Hydride (Ni–MH): Offers higher energy density and eliminates cadmium toxicity.
• Lithium-Ion: Provides superior energy-to-weight ratio, longer shelf life, and minimal memory effect.

Nevertheless, Ni–Cd batteries persist in aerospace, rail, and emergency power systems, where operational reliability and high discharge currents remain critical. Ongoing improvements in electrode design, separator technology, and charging control systems continue to extend their service life and environmental performance.
Research also explores hybrid Ni–Cd systems with partial replacement of cadmium or novel alkaline chemistries to reduce toxicity while maintaining performance characteristics.