A mixed iron–manganese based pyrophosphate cathode, Na2Fe0.5Mn0.5P2O7, for rechargeable sodium ion batteries

Abstract

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 Na₂Fe_0.5Mn_0.5P₂O₇ 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 space group). Galvanostatic charge/discharge measurements indicate that Na₂Fe_0.5Mn_0.5P₂O₇ 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 Na₂Fe_0.5Mn_0.5P₂O₇ is better than Na₂MnP₂O₇. Ex situ and CV analyses indicate that Na₂Fe_0.5Mn_0.5P₂O₇ 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 (Na₂FeP₂O₇ and Na₂MnP₂O₇). Thermogravimetric analysis (25–550 °C) confirms good thermal stability of Na₂Fe_0.5Mn_0.5P₂O₇ with only 2% weight loss. Owing to promising electrochemical properties and decent thermal stability, Na₂Fe_0.5Mn_0.5P₂O₇, can be an attractive cathode for SIBs.