Cathode materials in lithium-ion batteries exhibit distinct behaviors under high-temperature conditions, with thermal stability being a critical factor for safety and performance. Among the most studied cathodes are lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt oxide (LCO), and lithium nickel cobalt aluminum oxide (NCA). Each demonstrates unique degradation mechanisms, phase transitions, and oxygen release characteristics when exposed to elevated temperatures.
NMC cathodes, particularly those with high nickel content (e.g., NMC811), are known for their high energy density but exhibit lower thermal stability compared to LFP. At temperatures above 200°C, NMC undergoes phase transitions from a layered structure to a spinel and eventually to a rock-salt phase, accompanied by oxygen release. The onset of oxygen release typically occurs between 200°C and 250°C, depending on the nickel content. Higher nickel concentrations accelerate oxygen loss due to the reduced binding energy of oxygen in the lattice. This exothermic reaction contributes to thermal runaway risks. Metal dissolution, particularly manganese, further destabilizes the cathode-electrolyte interface, leading to capacity fade.
LFP cathodes demonstrate superior thermal stability, with no significant oxygen release below 300°C. The olivine structure remains stable up to higher temperatures due to strong P-O covalent bonds, which inhibit oxygen loss. Phase transitions in LFP are minimal, with decomposition occurring only above 400°C. The absence of transition metals prone to dissolution (e.g., cobalt or nickel) also reduces side reactions at high temperatures. However, LFP’s lower energy density limits its use in applications requiring compact energy storage.
LCO cathodes, while offering high volumetric energy density, exhibit poor thermal stability. Oxygen release begins around 150°C–200°C, with a rapid phase transition from layered to spinel and rock-salt structures. The cobalt dissolution rate increases significantly above 100°C, accelerating electrolyte decomposition and solid-electrolyte interphase (SEI) growth. Surface coatings such as aluminum oxide (Al₂O₃) or magnesium oxide (MgO) have been shown to delay oxygen evolution by stabilizing the surface structure.
NCA cathodes share similarities with high-nickel NMC in terms of thermal degradation. Oxygen release starts near 180°C–220°C, with aluminum doping providing marginal improvements in structural stability. However, nickel remains the dominant factor in oxygen loss, and the cathode is prone to microcracking at high temperatures, exposing fresh surfaces to electrolyte decomposition.
Crystal structure changes under heat are closely tied to the cathode’s transition metal composition. Layered oxides (NMC, LCO, NCA) experience gradual collapse into disordered spinel and rock-salt phases, while LFP retains its olivine framework until much higher temperatures. The following table summarizes key thermal stability parameters:
| Cathode | Oxygen Release Onset (°C) | Phase Transition Sequence | Metal Dissolution Risk |
|---------|---------------------------|---------------------------|------------------------|
| NMC811 | 200–250 | Layered → Spinel → Rock-salt | High (Mn, Ni) |
| LFP | >300 | Olivine remains stable | Low |
| LCO | 150–200 | Layered → Spinel → Rock-salt | High (Co) |
| NCA | 180–220 | Layered → Spinel → Rock-salt | High (Ni) |
Surface coatings are a primary strategy to mitigate high-temperature degradation. Alumina (Al₂O₃) coatings on NMC and NCA cathodes reduce direct electrolyte contact and suppress oxygen release. Phosphate-based coatings (e.g., Li₃PO₄) on LCO enhance thermal stability by forming a protective barrier against cobalt dissolution. For LFP, carbon coatings improve electronic conductivity without compromising thermal resilience.
Recent research has explored dopants to stabilize cathode structures at high temperatures. Magnesium doping in NMC reduces cation mixing and delays phase transitions. Zirconium-doped NCA shows improved resistance to microcracking, while titanium doping in LCO enhances oxygen retention.
In conclusion, cathode selection for high-temperature applications requires balancing energy density and thermal stability. LFP remains the most stable but sacrifices energy density, while high-nickel NMC and NCA offer greater capacity at the cost of higher thermal risks. Surface modifications and dopants continue to be critical in advancing high-temperature performance. Future developments may focus on hybrid cathodes or novel coatings to further bridge the gap between stability and energy density.