Cobalt-substituted P2-Na0.67MnO2 and purple basil-derived hard carbon for high-performance sodium-ion battery full cells: insight to ex situ structural analysis

This study explores two energy storage materials: cobalt-doped P2-type Na_0.67MnO₂ (Na_0.67Mn_0.9Co_0.1O₂, NMCO) and hard carbon derived from purple basil (Ocimum basilicum L., HC-based PB) biomass. NMCO was synthesized via a solid-state method involving high-temperature quenching in liquid nitrogen (LN₂). Analytical techniques confirmed a pure P2-type layered structure with reduced lattice volume due to Co³⁺ substitution. FTIR identified Na–O, Mn–O, and Co–O bonds, while XPS revealed reduced Mn³⁺ content, enhancing structural stability by mitigating the Jahn–Teller effect. Electrochemical tests of NMCO showed charge/discharge capacities of 184 mAhg⁻¹ and 185 mAhg⁻¹ with a coulombic efficiency of 99.5%. HC-based PB, exhibiting disordered graphitic structures, demonstrated higher charge and discharge capacities of 231 and 349 mAhg⁻¹, respectively, despite a relatively low efficiency of 66%. Long-term cycling demonstrated capacity fading for both materials after 100 cycles. Ex situ XRD confirmed NMCO’s structural integrity, while HC’s amorphous structure contributed to its stability. These findings provide valuable insights into these materials’ electrochemical performance and durability for energy storage applications.