Synthesis, Structure, and Electrochemical Properties of the Layered Sodium Insertion Cathode Material: NaNi_¹/₃Mn_¹/₃Co_¹/₃O₂

Abstract

A layered phase, NaNi_¹/₃Mn_¹/₃Co_¹/₃O₂ (NaNMC), isostructural to NaCoO₂ has been synthesized. Stoichiometric NaNMC crystallizes in a rhombohedral R3̅m space group where Na is in an octahedral environment (O3-Type). Galvanostatic cycling on NaNMC vs Na cell indicated a reversible intercalation of 0.5 Na, leading to a capacity of 120 mAh·g^–1 in the voltage range of 2–3.75 V and indicating its possible application in Na-ion batteries. The electrochemically driven Na insertion/deinsertion in NaNMC is associated with several phase transitions and solid solution regimes which are studied by <i>in situ</i> X-ray diffraction. Sodium deinsertion in Na_<i>x</i>NMC resulted in sequential phase transitions composed of biphasic and monophasic domains. The composition driven structural evolution in Na_<i>x</i>NMC follows the sequence O3 ⇒ O1 ⇒ P3 ⇒ P1 phases with an increased ‘<i>c</i>’ parameter, while the ‘<i>a</i>’ parameter remains almost unchanged