Size effect of Micro/Nano-spinel LiMn2O4 on Li+ intercalation kinetics and Mg2+/Li+ Separation for electrochemical Lithium extraction from high-Mg/Li brines

Spinel LiMn 2 O 4 is a promising electrode for electrochemically switched ion exchange (ESIX) lithium extraction, but its sluggish solid-phase diffusion and severe Mn dissolution limit its application in high-Mg 2+ /Li + salt lake brines. Herein, we report a clear size effect that regulates Li + intercalation kinetics and Mg 2+ /Li + separation by comparing hydrothermally synthesized nano-sized N-LiMn 2 O 4 (20–100 nm) and commercial micron-sized LiMn 2 O 4 (∼10 μm). Nanosizing induces preferential exposure of thermodynamically stable (111) facets, effectively suppressing surface reconstruction and Mn dissolution. In-situ distribution of relaxation time (DRT) reveals that the rate-limiting step changes from pure solid-phase diffusion in micron LiMn 2 O 4 to surface-diffusion co-control in N-LiMn 2 O 4 , reducing solid-phase diffusion resistance by 75% and charge-transfer resistance by 78.6%. Benefiting from kinetic optimization, the N-LiMn 2 O 4 /C/PVDF electrode maintains a Li + /Mg 2+ separation factor > 100 even at an ultrahigh Mg 2+ /Li + ratio of 500, far exceeding the micron counterpart. After optimization (wet film thickness = 200 μm, adsorption potential = 1.0 V), the electrode retains >90% capacity after 10 cycles in 200 mg·L −1 LiCl solution. This work clarifies the size-dependent kinetic transition mechanism and provides a rational design strategy for high-selectivity electrodes toward efficient lithium extraction from high-Mg/Li brines.