Highly Stable Na3Fe2(PO4)3@Hard Carbon Sodium-Ion Full Cell for Low-Cost Energy Storage

Abundant flake-porous Na3Fe2(PO4)3 has been prepared via a simple spray drying method. As a cathode material in sodium-ion batteries (SIBs), the galvanostatic charge/discharge test results indicate that the initial reversible discharge specific capacity of the flake-porous Na3Fe2(PO4)3 electrode can reach to 100.8 mAh g–1 (about 93% of the theoretical capacity of 105 mAh g–1) under a current density of 10 mA g–1 (0.1 C) and the high rate capability at 500 mA g–1 (5 C) is up to 60 mAh g–1 after 1100 cycles. The in situ X-ray diffraction pattern and ex situ X-ray photoelectron spectroscopy results indicate that the charge/discharge processes of this cathode material go through a reversible electrochemical reaction of Na3Fe2(PO4)3/Na5Fe2(PO4)3. The outstanding electrochemical performance of Na3Fe2(PO4)3 is attributed to its [Fe2(PO4)3] “lantern unit”-stacked NASICON-type structure and two-dimensional (2D) porous-sheet morphology. The flake-porous Na3Fe2(PO4)3 cathode with a commercial hard carbon anode full cell shows an energy density of 76 Wh kg–1 and the maximum power density of up to 760 W kg–1. The full cell also shows excellent low-temperature performance even at −20 °C (40 mAh g–1 at 100 mA g–1). The outstanding electrochemical and low-temperature performances prove that this full cell is an ideal device for large-scale electrical energy storage (EES).