Discharge State of Layered P2-Type Cathode Reveals Unsafe than Charge Condition in Thermal Runaway Event for Sodium-Ion Batteries

A sol–gel process followed by heat treatment derived a layered P2-type NaCoO₂ cathode, which depicted unit cell parameters values of a = 2.8389 Å, c = 10.9899 Å, and V = 76.71 ų in powder X-ray diffraction pattern. The synthesized cathode exhibited hexagonal, 2D platelets with an ∼300 nm thickness. During the anodic and cathodic sweeps, the cyclic voltammograms revealed multiple redox peaks with the same current densities, shapes, and peak positions, associated with the highly reversible phase transition mechanism of the layered P2-type NaCoO₂ cathode. The sodium cells yielded the capacities of 93/92 mAh g^–1 at 0.5 C and 87/87 mAh g^–1 at 1 C for the 50th charge–discharge cycles. The in situ multimode calorimetry (MMC) studies of sodium cells demonstrated a thermal explosion event, which occurred by sodium melting, short-circuit, electrode decomposition reaction, gas generation, exothermic reaction, released heat energy ,and cell gasket melting. Ultimately, the calculated released total heat energies of ∼550/740 J g^–1 for in situ MMC studies and ∼312/594 J g^–1 for ex situ DSC analyses (charge state at 4 V and discharge state at 2 V) show that the discharged state of sodiated layered P2-type NaCoO₂ cathode material is more unsafe than the charge state. Furthermore, the ex situ differential scanning calorimetry (DSC) spectrum of a discharge state at 2 V of layered P2-type NaCoO₂ revealed a decreased onset temperature (DOT) at 141 °C with two pronounced exothermic peaks at 197 and 266 °C with a released higher total heat energy of 594 J g^–1 than the charge state heat energy at 312 J g^–1, attributed to the higher charge onset temperature (COT) at 191 °C. Thus, the observed higher heat energy and decreased onset temperature for the discharge state at 2 V is associated with the higher Na⁺ ion in the discharge state of the layered P2-type Na_xCoO₂ cathode than that of the pristine cathode, showcasing that the layered P2-type NaCoO₂ cathode is unsafe at the discharged condition for sodium-ion batteries.