Unveiling the (de-)lithiation mechanism of nano-sized LiMn2O4 allows the design of a cycling protocol for achieving long-term cycling stability

Abstract

While nano-sized LiMn₂O₄ spinel cathode materials have demonstrated enhanced electrochemical performance, (de-)intercalation processes and their impact on structure stability are still not fully understood. Consequently, it is not obvious how to further improve their long-term cycling stability. Herein, we report an in-depth investigation of the local atomic geometry, electronic and crystallographic structure evolution using operando XAS and XRD to shed light on the (de-)lithiation and Jahn–Teller distortion mechanisms when cycled in a wide voltage range of 2.0 to 4.3 V vs. Li⁺/Li. The results from operando XAS indicated that it is easier to intercalate Li⁺ into the structure of the nano-sized spinel particles than into the micro-sized spinel particles, as confirmed by operando XRD exemplifying the reversible formation of the Jahn–Teller distorted tetragonal phase. Utilizing a wide voltage range with a discharge until 2.0 V vs. Li⁺/Li, the nano-sized spinel presents more defined plateaus in the high voltage galvanostatic charge/discharge profiles indicating improved Li⁺ diffusivity. Leveraging on these findings, a novel electrochemical cycling protocol, with periodic deep discharge to 2 V, yields superior electrochemical performance for the nano-sized spinel cathode cycled in the range of 3.3 to 4.3 V vs. Li⁺/Li exhibiting an excellent structure cyclability and an unprecedented increase in the specific capacity upon long-term cycling.