A mixed iron-manganese based pyrophosphate cathode, NaFeMnPO, for rechargeable sodium ion batteries

The development of secondary batteries based on abundant and cheap elements is vital. Among various alternatives to conventional lithium-ion batteries, sodium-ion batteries (SIBs) are promising due to the abundant resources and low cost of sodium. While there are many challenges associated with the SIB system, cathode is an important factor in determining the electrochemical performance of this battery system. Accordingly, ongoing research in the field of SIBs is inclined towards the development of safe, cost effective cathode materials having improved performance. In particular, pyrophosphate cathodes have recently demonstrated decent electrochemical performance and thermal stability. Herein, we report the synthesis, electrochemical properties, and thermal behavior of a novel Na 2 Fe 0.5 Mn 0.5 P 2 O 7 cathode for SIBs. The material was synthesized through a solid state process. The structural analysis reveals that the mixed substitution of manganese and iron has resulted in a triclinic crystal structure ( P 1&cmb.macr; space group). Galvanostatic charge/discharge measurements indicate that Na 2 Fe 0.5 Mn 0.5 P 2 O 7 is electrochemically active with a reversible capacity of ∼80 mA h g −1 at a C/20 rate with an average redox potential of 3.2 V. ( vs. Na/Na + ). It is noticed that 84% of initial capacity is preserved over 90 cycles showing promising cyclability. It is also noticed that the rate capability of Na 2 Fe 0.5 Mn 0.5 P 2 O 7 is better than Na 2 MnP 2 O 7 . Ex situ and CV analyses indicate that Na 2 Fe 0.5 Mn 0.5 P 2 O 7 undergoes a single phase reaction rather than a biphasic reaction due to different Na coordination environment and different Na site occupancy when compared to other pyrophosphate materials (Na 2 FeP 2 O 7 and Na 2 MnP 2 O 7 ). Thermogravimetric analysis (25-550 °C) confirms good thermal stability of Na 2 Fe 0.5 Mn 0.5 P 2 O 7 with only 2% weight loss. Owing to promising electrochemical properties and decent thermal stability, Na 2 Fe 0.5 Mn 0.5 P 2 O 7 , can be an attractive cathode for SIBs. The development of secondary batteries based on abundant and cheap elements is vital.