Synthesis and Performance Evaluation of Na_(2‐x)Li_xFeP₂O₇ (x=0, 0.6) Hybrid Cathodes

Abstract

This study reports hybrid cathodes formation by cation substitution in which Li+ substitution has been considered for Na+ in the structure of Na(2‐x)LixFeP2O7 (x=0, 0.6) to form Na1.4Li0.6FeP2O7 cathodes. Na(2‐x)LixFeP2O7 (x=0, 0.6) cathodes were synthesized using the solid‐state synthesis technique and characterized by various methods. The structural analysis (XRD, FE‐SEM) indicates that the submicron‐sized, phase pure, and crystalline materials having irregular morphology have been developed. Moreover, Li+ substitution does not alter the triclinic parent structure of Na2FeP2O7. Thermogravimetric analysis (TGA) shows that Li+ substitution into Na2FeP2O7 improves its thermal stability up to 550 °C with only ∼5 % weight loss. The electrochemical performance of Na2FeP2O7 and Na1.4Li0.6FeP2O7 in both lithium (Li) and sodium (Na) half‐cells is investigated using different electrochemical techniques. It is noticed that Na1.4Li0.6FeP2O7 is electrochemically active both in lithium (Li) and sodium (Na) cells with promising cyclability. However, compared with Na2FeP2O7, Na1.4Li0.6FeP2O7 suffers from inferior electrochemical performance, which might be associated with the lattice distortion of Na2FeP2O7 due to Li+ substitution having a lower ionic radius than the Na+. Considering Na2FeP2O7 as a baseline material, a new hybrid Na1.4Li0.6FeP2O7 cathode has been developed, which can be used to synthesize other new cathode materials with improved performance.

Progress on Fe‐Based Polyanionic Oxide Cathodes Materials toward Grid‐Scale Energy Storage for Sodium‐Ion Batteries

“…Reproduced with permission. [ 98 ] Copyright 2020, John Wiley and Sons. e) Crystal structure of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 .…”

“…4 Li 0.6 FeP 2 O 7 , its electrochemical performance is inferior to that of Na 2 FeP 2 O 7 , which may link to the lattice distortion induced by Li-ion's smaller radius (Figure 4d). [98] Zhao and colleagues investigated the influence of B doping, and they discovered that Na 2 Fe 1.95 B 0.05 O 7 /C composite in monoclinic P2 1 /c displayed an initial reversible capacity of 74.8 mAh g -1 , which is much higher than the 66.2 mAh g -1 of Na 2 FeP 2 O 7 /C without B doping. [99] Mg-doped Na 2 FeP 2 O 7 enhanced the cycling stability and rate capability, where the initial reversible capacity of Na 2 FeP 2 O 7 doped with 5% Mg was 90 mAh g -1 at 0.1 C and did not degrade noticeably after 2000 cycles at 9 C. [100] From the synthesis perspective of different sodium sources, Zheng et al found that Na 2 FeP 2 O 7 powder synthesized from sodium dihydrogen phosphate had a better performance, which showed a reversible capacity of 81.0 mAh g -1 at C/10, about 83.5% of the theoretical capacity.…”

“…d) Rate performance of Na 2 FeP 2 O 7 @C/EG, Na 2 FeP 2 O 7 @C, and Na 2 FeP 2 O 7 /BC. Reproduced with permission [98]. Copyright 2020, John Wiley and Sons.…”