Hard carbon is the most popular anode material for Na-ion batteries, but its long-term reliability has still to be further improved. We herein examine fluoroethylene carbonate and succinic anhydride as electrolyte additives for improving the cycle performance of hard carbon anodes. For a proper evaluation, hard carbon symmetric cells as well as hard carbon/Na half-cells are employed to discriminate the influence of the additives on the hard carbon and Na electrodes. We confirm that fluoroethylene carbonate hardly improves the cyclability of hard carbon symmetric cells, but it does enable far better cycle behavior of Na metal, and thus improves the cycle performance of hard carbon/Na half-cells by mitigating the degradation of the hard carbon anodes caused by harmful products derived from Na metal. In contrast, succinic anhydride markedly improves the performance of hard carbon symmetric cells in the cycle and storage tests at 60 degrees C. The superior thermal stability of succinic anhydride is attributed to a Na2CO3/sodium alkyl carbonates-rich solid-electrolyte interphase layer formed on the hard carbon surface, as evidenced by X-ray photoelectron spectroscopy measurements.