Elucidating Influence of Mg‐ and Cu‐Doping on Electrochemical Properties of O3‐Na_x[Fe,Mn]O₂ for Na‐Ion Batteries

Abstract

Although O3‐NaFe1/2Mn1/2O2 delivers a large capacity of over 150 mAh g−1 in an aprotic Na cell, its moist‐air stability and cycle stability are unsatisfactory for practical use. Slightly Na‐deficient O3‐Na5/6Fe1/2Mn1/2O2 (O3‐Na5/6FeMn) and O3‐Na5/6Fe1/3Mn1/2Me1/6O2 (Me = Mg or Cu, O3‐FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist‐air stability. O3‐Na5/6FeMn, O3‐FeMnCu, and O3‐FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g−1, respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3‐FeMnMg participate in the redox reaction more apparently than O3‐Na5/6FeMn. X‐ray diffraction studies unveil the formation of a P‐O intergrowth phase during charging up to >4.0 V.

Origin of Fast Capacity Decay in Fe‐Mn Based Sodium Layered Oxides

“…[30] However, after a short P3 single-phase solid solution process, the P3(003) diffraction peak splits at about 4.0 V. The original P3(003) diffraction peak still shifts to lower angle, showing a weakening trend, while the new diffraction peaks gradually shifted to high angles, which also appeared in the charging process of O3-Na x Fe 2/3 Mn 1/3 O 2 [30] and O3-Na x Fe 1/3 Mn 1/2 Mg 1/6 O 2 . [18] As the weak and broadened diffraction peak intensity of this new phase disable further refinements, it is also marked as "X" phase here in consistence with previous reports. According to the phase evolution regularity of P-type and O-type materials, it is speculated that "X" phase is likely an intermediate phase produced during the P-O phase transition.…”

“…This is followed by the discharge capacities ranging from 124 to 186 mAh g -1 , equaling to 0.50 to 0.75 mol Na + intercalation. The maximum discharge capacity of the obtained Na x Fe 0.52 Mn 0.48 O 2 is quite close to the reported P2-Na 2/3 Fe 1/2 Mn 1/2 O 2 (190 mAh g -1 , 4.3-1.5 V) [5] and O3-Na 5/6 Fe 1/2 Mn 1/2 O 2 (193 mAh g -1 , 4.3-1.5 V) , [18] indicating the similar electrochemical properties independent with the stacking structure. In addition to the increased capacity, larger voltage polarizations are also observed when elevating the charge-off voltage above 4.0 V, which is generally associated with the phase transition, ion migration and/or oxygen redox.…”

“…Previous research has revealed the OP2 stacking features of the fully charged O3-Na x Fe 1/2 Mn 1/2 O 2 (4.5 V) whereas the detailed structural evolution is still absent. [5,18] Therefore, in situ and ex situ XRD are carried out to comprehensively reveal the phase evolution of the asobtained O3-Na x Fe 0.52 Mn 0.48 O 2 . Figure 4a shows the contour map of in situ XRD collected during initial one-and-a-half cycle in the voltage range of 4.5-1.5 V (original patterns are displayed in Figure S4, Supporting Information).…”

Impact of Mg and Ti doping in O3 type NaNi1/2Mn1/2O2 on reversibility and phase transition during electrochemical Na intercalation

“…However, the observed 00l reections are very broad, indicating a disordered stacking sequence of P and O e layers as stacking faults. According to previous reports, 6,28,32 we simulated XRD patterns of the stacking-faulted layered structures using the FAULTS program 18 as shown in Fig. 6b.…”

“…4 In particular, O3 type (a-NaFeO 2 type) layered oxides are promising candidates as practically feasible high-capacity positive electrode materials because of the high capacity obtainable in the moderately high voltage range to avoid severe anodic electrolyte decomposition 5 and almost the stoichiometric Na amount in the formula to deliver a balanced initial discharge capacity with a charge capacity. 6 Among the O3 type layered oxides, O3 type Na[Ni 1/2 Mn 1/2 ]O 2 is very attractive owing to the large reversible capacity of ca. 200 mA h g À1 .…”