Thin Film Lithium Niobites with Intrinsic High-Rate Performance for Lithium-Ion Batteries

Abstract

Lithium-ion batteries have been broadly favored as one of the best energy storage systems which have various classes of electrode materials being studied to satisfy the demand on improved electrochemical performance. As a similar class to titanium-based oxides, niobium-based oxides continue to be considered as promising electrode materials with multiple pairs of Nb redox couples, which can deliver more capacity than Ti-based oxides depending on their phase and charge/discharge condition. Among several phases of niobium oxides, lithium niobite (LiNbO2) has attracted scientific attention as a mixed ion-electron conductor proposing its use for the optical application as well as memristor and superconductor. Nevertheless, the research on lithium niobites still lacks, especially for the use as energy storage materials in lithium-ion batteries. By sputter deposition, 1.1-micron-thick lithium niobite membranes were fabricated for thin film electrodes. The characterization using XRD and XPS with consecutive etching confirmed the unique (101)-oriented crystalline arrangement of as-synthesized thin film electrodes and elucidated their initial lithium deficiency. Free of conducting agent and binder, the electrodes exhibited high capacity utilization and stable retention over 400 cycles. They also presented remarkable high-rate performance even at 14 A/g of extremely high current density (ca. 70C) and demonstrated a full recovery of capacity at the subsequent moderate rate. It was revealed the unit parameter change originated from the insertion and desertion of lithium cations occupying the octahedral sites between (NbO2)n- layers of edge-shared NbO6 trigonal prisms. None of phase evolution or extinction during the cycling implies that the major capacity contribution of lithium niobite comes from the single-phase intercalation reactions, which was supported by voltage profiles as well. Post-mortem analysis using XPS with Ar ion etching enabled to suggest that besides the main intercalation reaction, from Li0.5NbO2 to LiNbO2, about 9.3% of LiNbO2 undergoes a conversion reaction to Nb.