O3-type Na(Mn0.25Fe0.25Co0.25Ni0.25)O2: A quaternary layered cathode compound for rechargeable Na ion batteries

a r t i c l e i n f o Sodium Na-ion battery We report a new layered Na(Mn 0.25 Fe 0.25 Co 0.25 Ni 0.25)O 2 compound with O3 oxygen stacking. It delivers 180 mAh/g initial discharge capacity and 578 Wh/kg specific energy density with good cycling capability at high cutoff voltage. In situ X-ray diffraction (XRD) shows a reversible structure evolution of O3-P3-O3′-O3″ upon Na de-intercalation. The excellent capacity and cycling performance at high cutoff voltage make it an important model system for studying the general issue of capacity fading in layered Na cathode compounds. Layered sodium metal oxides have attracted considerable interest as cathodes for Na-ion batteries partly due to the fact that all seven layered NaTMO 2 with the O3-structure, where TM is a single oxidizable 3d transition metal ion from Ti, V, Cr, Mn, Fe, Co, Ni [1], can be easily synthesized , and show the capability to intercalate Na ions reversibly [2–7]. This is very different from their Li analogues, where only LiCoO 2 and LiNiO 2 reversibly intercalate Li ions [8]. Furthermore, different transition metal ions can be easily mixed in the TM layer to make new NaTMO 2 compounds [9–15]. Among them, O3-Na(Fe 0.5 Co 0.5)O 2 (denoted hereafter as FC) shows a capacity around 160 mAh/g with excellent capacity retention when cycled below 4.0 V and an average voltage of 3.14 V [11]. O3-Na(Ni 0.5 Mn 0.5)O 2 (denoted hereafter as NM) shows an initial discharge capacity of 185 mAh/g and average voltage of 3.22 V, partly due to a long high voltage plateau around 4.0 V [12,16]. But capacity retention of NM is poor if the 4.0 V plateau is included in the galvanostatic cycling. We report here in this communication a new quaternary O3-structured compound with composition Na(Mn 0.25 Fe 0.25 Co 0.25 Ni 0.25)O 2 (denoted hereafter as MFCN), with theoretical capacity of 239 mAh/g and initial discharge capacity of 180 mAh/g over an average discharge voltage of 3.21 V. More importantly, its capacity retention is significantly improved over NM and FC even when cycled with high charge voltage cutoff. In-situ lab X-ray diffraction (XRD) to reveal the structure evolution of MFCN in the first electrochemical cycle shows a reversible O3-P3-O3′-O3″ phase transformation. Our result shows the opportunity to further improve the electrochemical performance of layered NaTMO 2 compounds by designing new combinations of transition metal ions …