Anionic redox chemistry in Na-rich Na2Ru1 − ySnyO3 positive electrode material for Na-ion batteries

• Electrochemical performances of Na-rich type compounds are investigated.
• Na2(Ru,Sn)O3 phase exhibits capacity exceeding the exchange of 1 electron/TM.
• Investigation using XRD demonstrates reversible structural evolution upon cycling.
• Investigation using XPS demonstrates anionic redox activity.

The synthesis and Na- electrochemical activity of Na-rich layered Na2Ru1−ySnyO3 compounds is reported. Like their Li-analogue, Na2Ru1−ySnyO3 shows capacities that exceed theoretical capacity calculated from the cationic redox species. The high capacity was found, by means of XPS analysis, to be associated to the accumulation of both cationic (Ru4 +/Ru5 +) and anionic (O2 −/O2n −) redox processes. The structural evolutions during cycling have been followed and found to be associated with the cation disordering and loss of crystallinity on cycling.