Thermal and structural instability of sodium-iron carbonophosphate ball milled with carbon

Highlights: • NFPC/C composites were prepared by ball milling NFPC with carbon using SPEX 8000. • Structural disordering and partial decomposition of NFPC occurs upon ball milling. • NFPC/C composites show better high-rate performance due to improved conductivity. • A single-phase mechanism of sodium (de)intercalation is established. -- Abstract: Pristine Na₃FePO₄CO₃ (NFPC) with the monoclinic structure and the P2₁/m space group was prepared by hydrothermal synthesis at 120 °C. To increase the conductivity of NFPC, it was ball milled with carbon using a SPEX 8000 mill. Crystal and local structure, morphology, thermal stability, conductivity and electrochemical properties of NFPC and NFPC/C composites were studied by XRD, DSC/TG, FTIR, Mӧssbauer spectroscopy, ²³NMR spectroscopy, magnetic measurements, SEM, EIS and galvanostatic cycling. It has been shown that the as-prepared NFPC is stable below 500 °C and then decomposes to Fe₃O₄ and Na₃PO₄. Ball milling of NFPC with and without carbon leads to its partial decomposition with the formation of nanosized superparamagnetic Fe₃O₄ particles and a significant structural disordering, though the crystal symmetry maintains unchanged. Due to high sensitivity of NFPC to air, pristine sample contains some portion of the Fe³⁺ ions; it increases after ball milling. As a result, all samples are able to cycle starting both with charge and discharge. NFPC shows high stability upon cycling with the specific discharge capacity close to the theoretical one (96 mA·h·g⁻¹ for one-electron reaction). Though the capacity of the NFPC/C composites is slightly lower at low cycling rate than that of pristine NFPC, they show better high-rate performance due to improved conductivity via the formation of the highly conductive carbon matrix. As-established low lattice volume variation upon (de)intercalation of the sodium ions along with realization of a single-phase mechanism is responsible for a long cycle life of the NFPC/C cathode material.