Electrochemical lithium recovery with lithium iron phosphate: what causes performance degradation and how can we improve the stability?

Abstract

Electrochemical processes enable fast lithium extraction, for example, from brines, with high energy efficiency and stability. Lithium iron phosphate (LiFePO₄) and manganese oxide (λ-MnO₂) have usually been employed as the lithium gathering electrode material. Compared with λ-MnO₂, LiFePO₄ has a higher theoretical capacity and lower lithium insertion potential but suffers from low performance stability. Therefore, exploring the reason for capacity fading and putting forward an effective approach to address this issue is important. In this work, we studied the effect of additional present cations and dissolved oxygen on the stability of LiFePO₄, using a rocking chair cell configuration to eliminate the effect of the other electrode. We found that adding Ca²⁺ to the solution and dissolved oxygen aggravate the capacity fading of LiFePO₄, whereas Na⁺ and Mg²⁺ do not show an obvious influence on the stability of LiFePO₄. By continuous nitrogen-flushing of the electrolyte and carbon coating of the electrode material, the stability of LiFePO₄ was significantly enhanced. The lithium extraction capacity of LiFePO₄/C is 21 mg_Li g_electrode⁻¹ with an energy consumption of 3.03 ± 0.5 W h mol_Li⁻¹ and capacity retention of 82% in 10 cycles in 5 mM LiCl + 50 mM NaCl solution at a cell voltage range of −0.5 V to +0.5 V.