Nickel and cobalt play critical yet distinct roles in the cathode chemistry of nickel-based battery systems. These two transition metals contribute differently to the performance, cost, and stability of the battery, making their interplay a key consideration in cathode design. While both elements belong to the same group in the periodic table, their electrochemical behaviors and material properties lead to trade-offs that influence the overall viability of nickel-based batteries.

In nickel-based cathodes, such as those found in nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) batteries, nickel serves as the primary active material. The nickel oxyhydroxide (NiOOH) compound forms the basis of the cathode’s electrochemical activity, undergoing reduction during discharge and oxidation during charging. Nickel’s high theoretical capacity and ability to cycle repeatedly make it indispensable for energy storage. However, its performance is heavily influenced by the presence of cobalt, which is often added in small quantities to enhance structural stability and electronic conductivity.

Cobalt’s role in nickel-based cathodes is primarily as a stabilizer. In Ni-Cd and Ni-MH systems, cobalt is typically incorporated into the nickel hydroxide active material as a dopant or as a surface coating. Even at low concentrations, usually between 2% to 10%, cobalt significantly improves the reversibility of the nickel redox reaction. Without cobalt, nickel hydroxide tends to form less conductive phases during cycling, leading to increased internal resistance and reduced cycle life. Cobalt also suppresses the formation of gamma-phase nickel oxyhydroxide, a less desirable form that can cause mechanical stress and electrode swelling.

The cost implications of using cobalt in nickel-based cathodes are substantial. Cobalt is significantly more expensive than nickel, with its price historically fluctuating due to supply chain constraints and geopolitical factors. In contrast, nickel is relatively abundant and cheaper, making it the dominant material by weight in the cathode. However, the small but critical amount of cobalt required still contributes to the overall material cost. Manufacturers must balance the performance benefits of cobalt against its economic impact, particularly in applications where cost sensitivity is high.

Energy density is another area where nickel and cobalt exhibit differing influences. Nickel’s high capacity is the primary driver of energy density in these systems, but cobalt’s role in improving charge efficiency and cycle stability indirectly supports sustained energy delivery over time. In Ni-MH batteries, for example, the inclusion of cobalt in the nickel hydroxide cathode helps maintain higher discharge voltages and reduces capacity fade, leading to better long-term energy retention. While cobalt itself does not contribute directly to energy storage, its stabilizing effect allows nickel to perform closer to its theoretical limits.

Stability and cycle life are perhaps the most pronounced contrasts between nickel and cobalt in these cathodes. Pure nickel hydroxide cathodes suffer from poor conductivity and phase instability, leading to rapid degradation. Cobalt addresses these issues by promoting the formation of more conductive beta-phase nickel oxyhydroxide and reducing particle agglomeration during cycling. This results in fewer structural defects and a more uniform charge distribution across the electrode. As a result, cobalt-containing nickel-based cathodes can achieve thousands of cycles with minimal capacity loss, whereas cobalt-free versions degrade much faster.

Thermal stability is also affected by the presence of cobalt. Nickel-based batteries are generally less prone to thermal runaway compared to lithium-ion systems, but the addition of cobalt further enhances their safety profile. Cobalt-doped cathodes exhibit lower heat generation during overcharge scenarios, reducing the risk of electrolyte decomposition and gas evolution. This makes cobalt-containing nickel-based batteries more suitable for applications where safety is paramount, such as in medical devices and aerospace systems.

Environmental and sourcing considerations further differentiate nickel and cobalt. Nickel is mined in large quantities worldwide, with major producers including Indonesia, the Philippines, and Russia. Its supply chain is more stable and less prone to disruptions. Cobalt, on the other hand, is heavily concentrated in the Democratic Republic of Congo, raising concerns over ethical mining practices and supply chain transparency. While nickel-based batteries use far less cobalt than lithium-ion systems, the reliance on cobalt still poses sustainability challenges.

Manufacturing processes for nickel-based cathodes also reflect the differing roles of these metals. Nickel hydroxide is typically prepared through precipitation methods, with cobalt introduced either as a co-precipitated dopant or as a post-coating. The inclusion of cobalt requires additional processing steps, increasing production complexity. However, these steps are justified by the performance gains, particularly in high-rate applications where conductivity and cycle life are critical.

In summary, nickel serves as the workhorse of nickel-based battery cathodes, providing the bulk of the energy storage capacity and electrochemical activity. Cobalt, though used in smaller quantities, plays an outsized role in enhancing stability, conductivity, and cycle life. The cost disparity between the two metals necessitates careful optimization to balance performance and economics. While efforts to reduce cobalt dependency exist in other battery chemistries, nickel-based systems continue to rely on its beneficial properties to achieve the required performance benchmarks. The interplay between these two metals remains a defining feature of nickel-based battery technology, influencing everything from material selection to end-use applications.