Unravelling the Nature of the Intrinsic Complex Structure of Binary‐Phase Na‐Layered Oxides

Abstract

The layered sodium transition metal oxide, NaTMO2 (TM = transition metal), with a binary or ternary phases has displayed outstanding electrochemical performance as a new class of strategy cathode materials for sodium‐ion batteries (SIBs). Herein, an in‐depth phase analysis of developed Na1−xTMO2 cathode materials, Na0.76Ni0.20Fe0.40Mn0.40O2 with P2‐ and O3‐type phases (NFMO‐P2/O3) is offered. Structural visualization on an atomic scale is also provided and the following findings are unveiled: i) the existence of a mixed‐phase intergrowth layer distribution and unequal distribution of P2 and O3 phases along two different crystal plane indices and ii) a complete reversible charge/discharge process for the initial two cycles that displays a simple phase transformation, which is unprecedented. Moreover, first‐principles calculations support the evidence of the formation of a binary NFMO‐P2/O3 compound, over the proposed hypothetical monophasic structures (O3, P3, O′3, and P2 phases). As a result, the synergetic effect of the simultaneous existence of P‐ and O‐type phases with their unique structures allows an extraordinary level of capacity retention in a wide range of voltage (1.5–4.5 V). It is believed that the insightful understanding of the proposed materials can introduce new perspectives for the development of high‐voltage cathode materials for SIBs.

Sodium Composite Oxide Cathode Materials:Phase Regulation, Electrochemical Performance and Reaction Mechanism

“…Changing the sodium content has a significant effect on the crystal structure. [105,106] As the sodium content increases, the crystal structure of Na x Ni 0.20 Fe 0.40 Mn 0.40 O 2 changes from a pure P2 phase (x = 0.60) to a composite structure of P2 and O3 phases (x = 0.74, 0.76 and 0.78) and eventually becomes a pure O3 phase (x = 0.84) (Figure 5a). [105] The results of HRPD and NDP determine that Na 5b), compared to 118.5 mAh g À 1 and 78 % between 2.0-4.2 V. A series of Na 0.9 À x Ni 0.45 Mn 0.55 O 2 (x = 0.02, 0.04 and 0.08) with different sodium content were synthesized using the sol-gel method.…”

“…[105,106] As the sodium content increases, the crystal structure of Na x Ni 0.20 Fe 0.40 Mn 0.40 O 2 changes from a pure P2 phase (x = 0.60) to a composite structure of P2 and O3 phases (x = 0.74, 0.76 and 0.78) and eventually becomes a pure O3 phase (x = 0.84) (Figure 5a). [105] The results of HRPD and NDP determine that Na 5b), compared to 118.5 mAh g À 1 and 78 % between 2.0-4.2 V. A series of Na 0.9 À x Ni 0.45 Mn 0.55 O 2 (x = 0.02, 0.04 and 0.08) with different sodium content were synthesized using the sol-gel method. [106] Compared to Na 0.9 Ni 0.45 Mn 0.55 O 2 with pure O3 phase, XRD Rietveld refinement result of Na 0.88 Ni 0.45 Mn 0.55 O 2 suggests that the ratio of O3 and P2 phase is 88.4 %: 11.6 % in the intergrowth structure.…”

“…Similarly, some studies have compared the performance of composite oxide cathode materials and single-phase cathode materials. [49,58,105,108,128,149] For example, the single-phase materials P2-Na 2/3 Ni 1/3 Mn 0.57 Ti 0.1 O 2 and O3-NaNi 1/3 Fe 1/3 Mn 1/3 O 2 were prepared by Wang et al for comparison with P2/O3-Na 0.766 Ni 0.33 Mn 0.5 Fe 0.1 Ti 0.07 O 2 . [49] Another example is that different crystal structure Na 0.7 Li 0.06 Mg 0.06 Ni 0.22 Mn 0.67 O 2 can be obtained by controlling the calcination temperature.…”

Modulation of Local Charge Distribution Stabilized the Anionic Redox Process in Mn-Based P2-Type Layered Oxides

“…During the past decades, cathode materials, which decide the SIB performance, have been extensively studied by many researchers. , Among these studied materials, several-layered structure cathode materials that can host sodium within the open structure have attracted considerable interest and have been extensively investigated owing to their high theoretical capacity, multiple compositions, and simple synthesis method. While a lot of effort has been made, such as the interface modification, , multiple-element doping, , and manipulating heterostructures, , layered structure SIB materials still do not meet the current requirement of wide application. An obstacle to the practical usage of SIB materials is the limited capacity of layered oxide compounds to conduct sodium solely through the transition metal (TM) redox reactions.…”

Defect-type AlOxnanointerface boosting layered Mn-based oxide cathode for wide-temperature sodium-ion battery

“…can be categorized into O3, P2, O2 and P3 types etc. [9][10][11][12][13][14][15][16] Apparently, the O3-type cathodes show relatively higher reversible capacities due to their larger initial Na contents than those of P-type materials. [17][18][19][20][21] Nevertheless, the fragile interface induced by air sensitivity and the possible dissolution of TM ions in O3-type cathodes usually cause severe degradation of the layered structures and rapid fading of reversible capacity at room temperature.…”