Directionally assembled FeCO₃ nanoparticles wrapped with rGO enabled by the synergy of tartaric and ascorbic acids for lithium/sodium storage

Ferrous carbonate (FeCO₃) particles synthesized by conventional routes generally possess large-size bulk structures with the drawbacks of low conductivity and easy pulverization, thereby limiting their promising application as high-capacity anodes for lithium/sodium ion batteries (LIBs/SIBs). Herein, a robust strategy via the synergy of tartaric and ascorbic acids (TA/AA) is developed to craft FeCO₃ dumbbells that are directionally assembled by ultrafine nanoparticles and uniformly encapsulated in reduced graphene oxide matrixes (denoted FeCO₃ NADs-rGO). It is noteworthy that TA serves as a capping agent to inhibit the overgrowth of FeCO₃ nanoparticles and guide their directional assembly into dumbbells. Concurrently, AA functions as a reductant to prevent the oxidation of Fe²⁺ and initiate the reduction of GO. Remarkably, benefiting from the efficient synergy of hierarchical architecture and rGO encapsulation, FeCO₃ NADs-rGO delivers highly-reversible lithium and sodium storage capacities of 1853 and 641 mAh/g in the 1200th and 100th cycles at 100 and 20 mA g⁻¹, respectively, which are 2.1 and 3.9-fold over FeCO₃ micron peanut kernels-rGO (884 and 163 mAh/g) fabricated without introducing TA. As revealed by experimental investigations and theoretical calculations, FeCO₃ nanoparticles shorten the ion transfer distance and rGO accelerates the electron conduction to boost the electrochemical kinetics for high-rate capability. More importantly, the ample mesopores of FeCO₃ NADs accommodate the inward stress and the rGO buffer the outward expansion of lithiated/sodiated FeCO₃ NADs to strengthen the electrode durability. As such, FeCO₃ NADs-rGO may stand out as a superior LIB/SIB anode, and the implementation of facile capping and reductant co-regulators (e.g., TA and AA) could synergistically render the creation of various rGO-based nanocomposites with outstanding mechanical flexibility and electrochemical activity for advanced energy storage.