O3-type NaNi1/3Fe1/3Mn1/3O2 layered cathode for Na-ion batteries: Structural evolution and redox mechanism upon Na (de) intercalation

Efficient Stabilization of Na Storage Reversibility by Ti Integration into O′3-Type NaMnO 2

“…This phase is regarded as the intergrown layered material of P3-and O1-type structures. The interlayer distance of y =~0 in Na y Ni 1/3-Fe 1/3 Mn 1/3 O 2 is observed to be 5.05 Å[46], and this value is also close to that of Na 0.05 Mn 0.8 Ti 0.2 O 2 .…”

“…Broad diffraction lines observed for the O1 phase suggest the presence of stacking faults along the c-axis direction. For other O3 phases, e.g., Na y Ni 1/3-Fe 1/3 Mn 1/3 O 2[46] and Na y Ti 0.25 Fe 0.25 Co 0.25 Ni 0.25 O 2[47], the formation of an OP2 phase (a sequence of ABCA), which contains alternating octahedral and prismatic sites for sodium ion layers, is evidenced. This phase is regarded as the intergrown layered material of P3-and O1-type structures.…”

Probing the charged state of layered positive electrodes in sodium-ion batteries: reaction pathways, stability and opportunities

“…The material utilises the Co 2+/3+ redox couple to raise the average oxidation state of Mn and suppress the structural instability caused by Jahn-Teller distortion of Mn 3+ , thereby achieving a reasonable reversible capacity of 120 mA h g À1 . 138 The oxidation state of Ti 4+ is retained during cycling to further enhance structural stability at the charged state, up to 4.3 V. The combination of Mn with Fe and Ni has been suggested to enable additional reversible capacity when charging beyond 4 V. In studies of O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 , it was found that both the Ni 2+/4+ and Fe 3+/4+ redox couples contribute to capacity below 4 V, however the Fe redox couple remains dominant above 4 V. 139 Despite stabilization of the Mn 4+ state during cycling, Fe migration into the Na layer remains an issue at high voltages. High voltage stability has been observed in P2-Na x Co 2/ instead achieved by oxidation of both the Co and Ni ions to a 4+ oxidation state, accompanied by anionic redox activity from oxygen atoms as depicted in Fig.…”

Direct Observation of Alternating Octahedral and Prismatic Sodium Layers in O3‐Type Transition Metal Oxides

“…This gliding leads to a much more complex set of phase transformations for NaTMO2 than that for the Li compounds. [7][8][9][10] While most of these transformations appear reversible upon Na extraction and insertion, the hysteresis, inhomogeneity, and stresses associated with the more complex phase transitions in layered Na cathodes can lead to long term stability issues. Understanding these phases is particularly important as Na cathodes need to be charged to a relatively higher level of alkali extraction than Li compounds to make them competitive in light of their lower cell voltage.…”