A Novel Biphasic O3/P2 Layered Na-Transition Metal Oxide Based Cathode with Earth Abundant T_M-Ions with Long-Term Cyclic Stability for Na-Ion Batteries

Layered transition metal oxides cathodes are ideal candidates for high energy density commercially deployable Na-ion batteries. While O3-type layered Na_xT_MO₂s (T_M => transition metal ion) with high starting Na-content (of ~1 per formula unit; x) offers high specific capacity, they suffer from cycling instability. On the other hand, P2-type layered Na_xT_MO₂s exhibits better electrochemical cycling and environmental stability but have lower Na-content, which would hinder their specific capacity in Na-ion full cells. Combining the synergistic effects of both, the present work aims to design and develop a high Na-content Bi-phasic O3/P2 Na_xT_MO₂ composition with different phase fractions by varying the calcination temperature. A novel biphasic O3/P2 Fe-Mn based Na_xT_MO₂been successfully synthesized via solid state, exhibited the first discharge capacity of ~178 mAh/g with an exceptional cycling stability of ~90% capacity retention after 100 cycles at C/10 in a voltage window of 1.5-4.2 V vs. Na/Na⁺. Furthermore, the exceptional cycling stability exhibited by this material is also supported by negligible phase transformations and volume change during electrochemical cycling as observed in in-situ synchrotron XRD studies. The electrochemical performance of the as-developed cathode material was also investigated in Na-ion ‘full’ cell configuration by coupling with commercial hard carbon based anode. The Na-ion ‘full’ cells have exhibited a 1^st cycle reversible capacity of ~160 mAh/g with an average voltage of ~3 V, almost comparable to that of LFP/graphite ‘full’ cells (~150 mAh/g; 3.2 V), which is a significant step forward towards the commercialization of this material. In summary, this work demonstrates the successful development of Fe-Mn based O3/P2 biphasic cathode with an appropriate phase fractions exhibiting exceptional cycling performance both in Na-ion ‘half’ and ‘full’ cells.