Double-Enhanced Core-Shell-Shell Sb₂S₃/Sb@TiO₂@C Nanorod Composites for Lithium- and Sodium-Ion Batteries

For most alloying- and conversion-type anode materials, a huge volume expansion and structure degradation of the electrodes always hinder their applications. In this work, a novel core-shell-shell Sb₂S₃/Sb@TiO₂@C nanorod composite has been designed layer by layer, which includes an inner Sb₂S₃/Sb heterostructure core protected by an oxygen-deficient TiO₂ shell and a conductive carbon shell. It is interesting to observe that, during the carbothermic reduction process, the previous Sb₂S₃ nanorod cores are partially reduced into a metallic Sb phase and the reduced TiO₂ also creates many oxygen vacancies, which can greatly enhance the conductivity of the semiconductor Sb₂S₃. Thanks to the double effects of the TiO₂ middle shell and carbon outer shell, the unique double-shelled structure design creates an enhanced dual protection, which can better accommodate the volume-expansive deformation and preserve the structural integrity of the active Sb₂S₃/Sb core. Especially, the TiO₂ middle layer is self-assembled by numerous nanoparticles acting as a nanopillar backbone, which supports between the nanorod core and outer carbon shell to better buffer the volume changes. As a result, the core-shell-shell Sb₂S₃/Sb@TiO₂@C anode shows lithium and sodium storage performances superior to those of the pristine Sb₂S₃ and core-shell Sb₂S₃@TiO₂ electrodes. For lithium-ion batteries, the Sb₂S₃/Sb@TiO₂@C nanorod composite achieves an initial discharge/recharge capacity of 1244.9/1005.1 mAh g^-1 with an initial Coulombic efficiency of about 80.7%, an enhanced rate capability with a capacity of 593.2 mA h g^-1 at 5.0 A g^-1, and prolonged cycling life for 500 cycles with a reversible capacity of 495.8 mAh g^-1 at 0.5 A g^-1. For sodium-ion batteries, the nanorodalso exhibits an improved performance with an initial discharge/recharge capacity of 781.4/574.0 mAh g^-1 (initial Coulombic efficiency of about 73.46%) and cycling for 400 cycles with a reversible capacity of 422.6 mAh g^-1 at 0.8 A g^-1. This research sheds light upon double-shell structure designs with an effective middle shell to enhance the energy storage performance of electrode materials.