Electrochemistry in the Large Tunnels of Lithium Postspinel Compounds

Lithium spinels (LiMM’O₄) are an important class of mixed-cation materials that have found uses in batteries, catalysis, and optics. Postspinels are a series of related framework structures with the AMM’O₄ host composition that are formed with larger A-site cations, typically under high pressure. Postspinels have one-dimensional tunnel structures with pores that are larger than those in spinel and triangular in cross-section, but they are relatively unexplored as intercalation electrodes. While lithium postspinels have been previously found to be thermodynamically stable only at high pressures, we have identified a synthetic pathway that produces the lithium-containing materials at ambient pressure using an ion-exchange process from the corresponding sodium postspinels. Here, we report the synthesis and a survey of the electrochemical properties of 10 new lithium CaFe₂O₄-type postspinel compounds where M = Mn³⁺, V³⁺, Cr³⁺, Rh³⁺, Fe²⁺, Mg²⁺, Co²⁺ and M’ = Ti⁴⁺ and/or Sn⁴⁺. Although complete delithiation is not achieved during electrochemical cycling, many of the lithium postspinels have substantial charge storage capacity in Li battery cells owing to the ability of the large framework tunnels to accommodate more than one lithium ion per formula unit. Multiple redox couples are accessed for LiMnSnO₄, Li_0.96Mn_0.96Sn_1.04-xTi_xO₄, Li_0.96V_0.96Ti_1.04O₄, Li_0.96Cr_0.96Ti_1.04O₄, and LiFe_0.5Ti_1.5O₄. Compositions with moderate or poor lithium cyclability are also discussed for comparison. Redox mechanisms and trends are identified by comparing this new redox-active framework to related spinels, ramsdellites, and ‘Na_0.44MnO₂’ structures, and from density functional theory (DFT) electronic structures. Operando diffraction shows complex structural responses to lithium insertion and extraction in this postspinel framework. A DFT framework was proposed to identify promising lithium postspinel phases that could be accessed metastably under ambient pressure conditions and to assess their stability to lithium insertion and extraction. This work suggests that CaFe₂O₄-type hosts are a promising new class of lithium-ion energy storage materials.