Confining Ultrafine MoO₂ in a Carbon Matrix Enables Hybrid Li Ion and Li Metal Storage

Poor cycle and rate performance caused by volume effects and sluggish kinetics is the main bottleneck for most lithium-ion battery (LIB) anode materials run on the conversion reaction. Although nanostructure engineering has shown to be an effective method to reduce the undesirable volume effects, cycling instability usually remains in nanostructured electrodes owning to particle aggregation in discharge and loss of active materials in charge. Here, to make these kinds of materials practical, we have developed a structure of ultrafine MoO₂ nanoparticles (<3 nm) confined by a conductive carbon nanosheet matrix (MoO₂/C). Instead of running on the conversion mechanism, the Li storage in the MoO₂/C composite is through a two-step mechanism in discharge: intercalation followed by the formation of metallic Li, acting as a hybrid host for both Li ion intercalation and metallic Li plating. The Li-storage mechanism has been revealed by in situ X-ray diffraction analysis and in situ scanning transmission electron microscopy with corresponding electron energy loss spectrum analysis, which explains the natural origin of such high capacity along with good cyclability. This unique MoO₂/C structure exhibits an excellent discharge capacity (810 mAh g^-1 at 200 mA g^-1) and cyclability (75% capacity retention over 1000 cycles). The carbon sheet plays a vital role in both a conductive network and a structure supporter with a robust confining effect that keeps the size of MoO₂ uniformly under 3 nm even after high-temperature calcination. Our finding provides insights for the design of next-generation LIB anode materials with high capacity and longevity.