Regulating Na Occupation in P2‐Type Layered Oxide Cathode for All‐Climate Sodium‐Ion Batteries

Abstract

by researchers because of its abundant sodium reserves and much cheaper price than lithium-ion batteries (LIBs). [1] Moreover, Na ions show better mobility in electrolytes [2] due to the smaller Stokes radius (≈4.6 Å) than that of Li + (≈4.8 Å), resulting in better rate and low-temperature performances. [3] Thus, these merits make SIBs a promising candidate for large-scale energy storage. [4] Layered oxides with high tap density and high theoretical capacity are one of the most promising options [5] for sodium-ion battery cathode. Among the layered oxides, P2-type with wide interlayer spacing for Na-ion diffusion are the ideal choice for developing wide-temperature-range and fast-charge batteries. P2-type Na 2/3 Ni 1/3 Mn 2/3 O 2 (NNMO) is conspicuous due to high specific capacity over 160 mAh g −1 between 2.0-4.4 V related to two-electron redox processes of Ni 2+ /Ni 4+ redox couple. [6] Although the two-electron redox provides a high capacity for NNMO, with an uncontrollable electrochemical process, severe structural evolution and irreversible phase transition at highly desodiation states (from P2 to O2) lead to poor cycle stability and low capacity retention, [7] hindering the application of NNMO.Tremendous efforts have been reported to alleviate the phase transition and capacity decay in NNMO, including elemental doping, [8] coating, [9] and morphology controlling. [10] Nevertheless, the electrochemical performances, especially the rate performances, have rarely been substantially improved. Therefore, some extra modifications or designs are still needed to promote Na + diffusion. In P2-type layered oxide materials, there are two different Na ions occupation sites, Na e (1/3, 2/3, 3/4) in 2d Wyckoff sites and Na f (0, 0, 1/4) in 2b Wyckoff sites. Na e sites are in 2d Wyckoff sites sharing edges with adjacent six TMO 6 octahedrons, and the Na f sites are in 2b Wyckoff sites sharing faces with two TMO 6 octahedrons (the upper and lower). [7] Constrained by the strong electrostatic repulsion between nearest-neighbor Na e and Na f (≈1.67 Å), and the ionic diameter of Na ions (≈2.04 Å), the neighboring two sites cannot be occupied at the same time. [11] Furthermore, the balance of the electrostatic attraction between layers and Na-Na repulsion interaction makes the proportion of sodium ions at different positions (Na e /Na f ) tends to be a fixed value. [11] Since the migration behaviors of Na + at edge and face sites are different, P2-typeNa 2/3 Ni 1/3 Mn 2/3 O 2 (NNMO) has been investigated as one of the promising cathode materials of sodium-ion batteries (SIBs) due to a low-cost and wide-temperature-range adaptability. However, its application faces a number of obstacles because of the poor cycling stability and bad rate capabilities. Herein, by accommodating more Na-ions at the e-site (Na e ) in P2-type NNMO, which is thermodynamically more stable, P2-type layered oxides (Na e /Na f > 1.64) with outstanding electrochemical performance are obtained. Specifically, the Na 0.696 Ni 0.329 Mn 0.671 O 2 (NM-2) exh...

Boosting the Ultrastable High-Na-Content P2-type Layered Cathode Materials with Zero-Strain Cation Storage via a Lithium Dual-Site Substitution Approach

“…With the rise in Na content within the P2 structure, an intriguing observation emergesthe lattice parameter c experiences a gradual decrease (see Tables S2–S4). This phenomenon can be attributed to the enhanced electrostatic attraction between Na + ions and the neighboring TMO 2 plates . Interestingly, in the case of N 0.9 L 0.1 NMO, an intriguing anomaly emergesthe lattice parameter a surpasses that of other substances.…”

“…This phenomenon can be attributed to the enhanced electrostatic attraction between Na + ions and the neighboring TMO 2 plates. 12 Interestingly, in the case of N 0.9 L 0.1 NMO, an intriguing anomaly emerges�the lattice parameter a surpasses that of other substances. This intriguing occurrence can potentially be attributed to the substitution of TM ions by Li ions within the TM layer, where the respective ionic radii follow the order r Li + > r Ni 2+ > r Mn 4+ ; see Figure 1b.…”

“…Sodium ion batteries (SIBs) have been regarded as a prospective alternative to lithium-ion batteries (LIBs) owing to the merits of sodium abundance in the Earth’s crust and the low price of sodium resources. − Exploiting long-life cathode materials with superior reliability plays an utmost role in the commercial application of SIBs. Among most candidates, layered oxides Na x TMO 2 (TM represents transition metals) have gained considerable attention due to high theoretical specific capacity, feasible synthesis, and environmental benignity. − Considering the coordination of the Na-ions [i.e., octahedral (O) or prismatic (P)] and the number of the O-ion layer or TMO 6 layer in a one-unit cell (typically, 1–3), Na x TMO 2 cathode materials are categorized into P2, P3, O2, and O3 phases. ,,, P2-type and O3-type layered oxides are better suited to functioning as cathode materials for SIBs. The P2-type structure offers a distinct advantage by enabling facile Na transport through direct crossover between “open” prismatic sites, whereas the O3-type structure necessitates Na-ions to pass through intermediate tetrahedral sites while transitioning from one octahedral site to another.…”

Wide-temperature-range sodium-metal batteries: from fundamentals and obstacles to optimization

“…The chemical formula for transitionmetal oxides is Na x TMO 2 , where TM stands for transition metal, such as Mn, Ni, Fe, Co, Cr or Ti. 418,419 Based on structural variation, transition-metal oxides are mainly subdivided into tunnelling oxides and layered oxides. Compared with tunnelling oxides, layered oxides are considered as suitable cathodes for SMBs due to high theoretical specific capacity, simple preparation method and flexible crystal structure.…”

Single-Crystal Growth of P2-Type Layered Oxides with Increased Exposure of {010} Planes for High-Performance Sodium-Ion Batteries

“…This effect can be avoided by introducing Ni 2+ into Na x MnO 2 to form P2-type Na 0.66 Ni 0.33 Mn 0.67 O 2 (NNM). Furthermore, as the Ni 4+ /Ni 2+ redox pair can realize two-electron transfer and has high potential, NNM exhibits a high operating voltage and excellent water resistance. − However, a substantial Na + /vacancy ordering effect is observed in the sodiation/desodiation of NNM. The NNM structure undergoes a poorly reversible P2–O2 phase transition when in its highly desodiated state, , which is accompanied by a change of ∼20% in the lattice volume, leading to the generation of intergranular and intragranular cracks. , These unfavorable structural factors result in the rapid capacity decay of the NNM during cycling.…”