High-energy–density carbon-coated bismuth nanodots on hierarchically porous molybdenum carbide for superior lithium storage

• Hierarchically porous Mo 2 C (PMC) was used as a support for Bi nanodot deposition. • Ultrasmall Bi (6.4 nm) nanodots offered short transport length for the Li + -ions diffusion. • Highly-conductive PMC support could hinder the volume expansion of Bi upon cycling. • High capacity (422 mAh g −1 ) and stability (0.002 mAh g −1 decay/cycle) resulted. • High energy density (352 Wh kg −1 , 563 Wh L –1 ) could be expected in full-cell design. The use of carbon-based supports, such as graphene and porous carbon, is a well-established approach to overcome the rapid capacity fading issues associated with alloy-based anode materials in lithium-ion batteries (LIBs). However, adopting carbonaceous materials that typically exhibit a low density eventually diminishes the primary purpose of alloys as high-energy–density anode materials. In this study, we introduce three-dimensional hierarchically porous molybdenum carbide (PMC) with high energy density, robust mechanical strength, and high electronic conductivity, which make it a promising alternative support for suppressing the huge volume expansion of alloying-based materials. Carbon-coated, ultrasmall Bi nanodots with an average size of 6.4 nm are uniformly embedded on the PMC surface (denoted as C-Bi/PMC) by facilitating heterogeneous nucleation. When tested as an anode in an LIB, the C-Bi/PMC electrode exhibits a high reversible capacity of 422 mAh g −1 at 50 mA g −1 , high-rate capacity of 268 mAh g −1 at 1000 mA g −1 , and long-term stability of 400 mAh g −1 at 250 mA g −1 over 500 cycles followed by 0.002 mAh g −1 decay per cycle at 5000 mA g −1 over subsequent 1000 cycles. When paired with LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode as full-cell LIBs, the C-Bi/PMC anode deliver high gravimetric and volumetric energy densities of 352 Wh kg −1 and 563 Wh L –1 , respectively. In-situ X-ray diffraction patterns captured during cycling reveal that the Li + -ion insertion mechanism in the voltage plateau region at 0.7–1.0 V consists of the intercalation between Bi layers followed by the formation of triclinic LiBi phase and the subsequent transition of triclinic LiBi to cubic Li 3 Bi phase.