Selective lithium extraction with LiMn2O4: Impossible triangle and reconcile via in-situ growth on flexible 3D substrate

The electrochemically switched ion exchange (ESIX) with LiMn2O4, which can selectively extract lithium from salt-lake brines, has attracted intensive attention in recent years owing to the rapid development of the electric vehicle industry. However, the balancing of the membrane electrode's impossible triangle, i.e., high load capacity, high porosity, and high conductivity, is something that has long puzzled scientists. In this study, one-step in-situ hydrothermal synthesis was utilized to prepare LiMn2O4 one-dimensional nano-rod (ODNR, 10 ∼ 40 nm and a length-diameter ratio of 18–220) on the flexible carbon cloth substrate (LiMn2O4@CC), thus reconciling the impossible triangle. The exchange capacity (37.73 mg/g), dissolution ratio (<0.05 %), kinetics (88 % capacity with 1.5 h), and specific energy consumption (5.04 Wh/mol·Li) of ODNR LiMn2O4@CC flexible electrode are significantly superior to the LiMn2O4 on rigid titanium electrode (LiMn2O4@RTE) ascribing to the 3 times higher conductivity, 2 to 4 orders of magnitude higher internal Li+ diffusion coefficient, and the intrinsic 3D porous nature of the flexible electrode. Electrochemical analyses verified the redox of ODNR LiMn2O4/λ-MnO2 is a one-electron transfer electrochemical reversible process with a fast electron transfer process, which may be induced by the in-situ growth ODNR and the 3D porous structure of the flexible substrate. The separation coefficients for Li+/Mg2+ and Li+/Na+ were 804.6 and 513.2, respectively. And the capacity retention ratio is > 92 % even after 50 cycles. This work sheds light on the reconciling of the impossible triangle for membrane electrode fabrication.