Electrochemical quartz crystal microbalance study of lithium-ion dynamics in LiMn2O4/λ-MnO2 for ion-selective capacitive deionization

Lithium manganese oxide (LMO) has been widely utilized in capacitive deionization (CDI) for selective lithium extraction from salt-lake brines, owing to its excellent lithium intercalation capacity, selectivity , and recovery efficiency. However, due to the lack of an in-situ detection method, the LMO electrode kinetics in aqueous solution is mainly based on deductions and inferences. Here, electrochemical workstation-quartz crystal microbalance is utilized to study the lithium dynamics in LiMn 2 O 4 / λ -MnO 2 in aqueous system for the first time. In-situ electrochemical analysis verifies that the lithium intercalation and deintercalation during the redox of LiMn 2 O 4 / λ -MnO 2 is a one-electron transfer quasi-reversible process. The intermediate phase, i.e., Li 0.5 Mn 3+ 0.5 Mn 4+ 1.5 O 4 with P 2 1 3 space group and layered structure, is first identified in between LiMn 2 O 4 and λ -MnO 2 with Fd 3 ¯ m space group and spinel structures in the aqueous system. Li + occupies the 4 a and 8 a sites, respectively, in Li 0.5 Mn 3+ 0.5 Mn 4+ 1.5 O 4 and LiMn 2 O 4 . Also, the spontaneous lithiation of λ -MnO 2 and faster lithiation kinetics than that of delithiation is clarified with electrochemical analyses. This research utilized the in-situ research means to interpret the lithium dynamics during the lithiation and delithiation process, which sheds light on the issues on which there is no consensus.