Microwave-assisted hydrometallurgical extraction of Li4Ti5O12 and LiFePO4 from ilmenite: effect of PPy-Br2 derived C-coating with N, Br, and Nb5+ Co-doping on electrodes for high-rate energy storage performance

Abstract

Microwave-driven hydrometallurgical (MW-HM) method has been adopted to extract spinel-type Li₄Ti₅O₁₂ (LTO) and orthorhombic-type LiFePO₄ (LFP) from naturally occurring Ilmenite (FeTiO₃) within 2 h unlike the conventional process that requires >30 h. We have successfully demonstrated aliovalent-Nb⁵⁺ doping and carbon coating with N, Br co-doping upon the pyrolysis of a polypyrrole-(PPy)-Br₂ charge-transfer-(CT)-complex via MW-hydrothermal and MW-solid state heating within 30 min. Further, we also investigated the effect of carbon coating and co-doping of LTO and LFP electrodes at high C-rate performance of the lithium battery. XRD, XPS, FTIR, and Raman spectroscopy results confirmed the co-existence of dual-phase Li₄Ti₅O₁₂/rutile-TiO₂ (LTO-RTO) with substitution-induced transition of Ti⁴⁺ → Ti³⁺ in spinel-LTO due to Nb⁵⁺ and N, Br co-doping, which facilitates fast Li⁺ ion and electron transfer at the electrode–electrolyte interface. Conversely, in situ Nb⁵⁺ doped LiFePO₄ combined with ex situ carbon-coating with N, Br co-doping improved the overall electronic conductive behavior. The UV-Visible absorption spectra and Tauc plots further support the decrease in the band gap upon co-doping, thus promoting n-type electronic behavior of the electrodes. A significant enhancement in the discharge-capacities of the carbon-coated N, Br co-doped Li₄Ti_4.97 Nb_0.03O₁₂/rutile-TiO₂ (NBC-LTNO-RTO) and pristine anode in the range of 174–148 mA h g⁻¹ and 167–125 mA h g⁻¹ was exhibited at different rates in the range of 0.2 C–20 C with 97% and 94% capacity retention, respectively. Instead, the carbon-coated N, Br co-doped LiFe_0.99 Nb_0.01PO₄ (NBC-LFNP) and pristine cathode exhibited discharge capacities in the range of 169–73 mA h g⁻¹ and 136–65 mA h g⁻¹ at different rates in the range of 0.2 C–20 C with 92% and 40% capacity retention for 500 cycles, respectively. Hence, this innovative, rapid, and sustainable chemical process for the fabrication of the modified cathode and anode from the earth abundant ilmenite ore as the single source with high-rate capability and ultra-stability can be used for high-power and safer lithium-ion energy storage.