- Lithium nickel oxide (LiNiO₂) is an inorganic compound composed of lithium (Li), nickel (Ni), and oxygen (O), and is categorized as a layered transition metal oxide. It is a pale green to gray crystalline solid that crystallizes in a rhombohedral structure similar to that of α-NaFeO₂, featuring alternating layers of lithium ions and edge-sharing NiO₆ octahedra.
- LiNiO₂ is a key material in electrochemistry, particularly in the field of lithium-ion batteries, where it serves as a cathode material due to its high theoretical capacity and ability to reversibly intercalate and de-intercalate lithium ions.
- LiNiO₂ is typically synthesized through a solid-state reaction between lithium hydroxide (LiOH) or lithium carbonate (Li₂CO₃) and nickel oxide (NiO) at high temperatures (typically 600–800 °C) under controlled oxygen conditions. Precise stoichiometric control is essential, as deviations can lead to the formation of undesirable phases or nickel-rich compounds that degrade battery performance. A common issue in synthesis is the partial reduction of Ni³⁺ to Ni²⁺, which can lead to lithium/nickel site mixing, reducing the efficiency of lithium diffusion and deteriorating the material’s electrochemical performance.
- Electrochemically, LiNiO₂ exhibits a high specific capacity (up to ~200 mAh/g), which arises from the redox reaction of nickel ions (Ni³⁺/Ni⁴⁺) during charge and discharge cycles. When lithium is removed during charging, nickel is oxidized to higher valence states, and the structure remains relatively intact. However, with deep delithiation or prolonged cycling, structural instability and capacity fading can occur due to phase transitions and oxygen loss, which makes pure LiNiO₂ less stable than its mixed-metal counterparts. Additionally, LiNiO₂ is sensitive to moisture and CO₂ in air, which can degrade its surface and reduce its lifespan.
- To improve stability and performance, LiNiO₂ is often modified or used as a precursor in nickel-rich layered oxides, such as LiNi₁₋ₓCoₓO₂ (LNO/NC) or LiNi₁₋ₓ₋yCoₓMnᵧO₂ (NCM/NCA), where cobalt and manganese help suppress cation mixing and improve structural integrity. These derivatives are widely used in commercial lithium-ion batteries for electric vehicles and portable electronics, striking a balance between high energy density, cycle life, and safety.
- In terms of practical applications, LiNiO₂ and its derivatives are central to the development of high-energy lithium-ion batteries, particularly where high energy density and low cobalt content are desired. The push for cobalt-free or cobalt-reduced cathodes has renewed interest in optimizing LiNiO₂ itself, as nickel is more abundant and less ethically problematic than cobalt. Researchers are investigating advanced synthesis techniques, surface coatings, and dopants to enhance the structural and electrochemical stability of LiNiO₂.
- From a safety and handling perspective, LiNiO₂ must be processed in dry, inert environments due to its air and moisture sensitivity. Exposure to humid air can lead to the formation of surface lithium hydroxide or lithium carbonate, which impairs electrochemical performance and increases the risk of thermal instability. Additionally, under abusive conditions such as overcharging or short-circuiting, LiNiO₂ can release oxygen and contribute to thermal runaway, highlighting the importance of proper battery management systems.
