It is desirable to develop alternative electrochemical devices with comparable performance but lower cost to substitute for lithium-ion batteries. Sodium-ion batteries show very similar electrochemical mechanism to lithium-ion batteries. The abundant sodium resource can considerably reduce the cost of energy storage devices as compared with lithium-ion batteries. In this work, a new derivative of sodium iron sulfates, Na₆Fe₅(SO₄)₈ (NFS), is developed as cathode material for sodium-ion batteries. The NFS is synthesized from sodium carbonate, sodium sulfate and iron sulfate raw materials, and it shows a high working voltage of 3.7 V vs. Na⁺/Na. When combined with carbon nanotube (CNT), the NSF@CNT composite demonstrates increased electronic conductivity and superior electrochemical performance. A 3.6 V sodium-ion full battery is constructed based on NFS@CNT cathode and hard carbon (HC) anode materials. Such a full NFS@CNT//HC cell can deliver an energy density towards 350 Wh kg^-1 and cycling stability over prolonged 1000 cycles at 2 C. This work offers a low-cost sodium-ion full battery with an impressive high working voltage and energy/power densities for possible stationary energy storage applications.