Study of Synergistic Effects of Cu and Fe on P2-Type Na_0.67MnO₂ for High Performance Na-Ion Batteries

Abstract

P2-type Na 0.67 MnO 2 with a stable structure and an open framework can provide numerous channels for fast Na + de/intercalation, for which it is considered to be advantageous in application of the cathode material for Na-ion batteries. However, the complex phase transition occurring during cycling and the lattice distortion triggered by the Jahn−Teller effect severely restrict its development. Herein, the modified Na 0.67 MnO 2 with Cu or Fe single-element doping as well as Cu and Fe double-element doping was synthesized by the sol−gel method, and the effects of doping on the crystal structure and electrochemical performances of Na 0.67 MnO 2 were studied. It was demonstrated that the phase of the material did not change after the introduction of Cu and Fe elements, and the cycling stability and rate performance were greatly improved by Cu and Fe double-doping owing to their synergistic effect. The Na 0.67 Mn 0.92 Fe 0.04 Cu 0.04 O 2 (NMFCO) cathode delivers discharge specific capacities of 110.5 mA h g −1 at 5 C and 91.8 mA h g −1 at 10 C and exhibits the high-capacity retention of 94.35% at 1 C and 90.68% at 5 C after 100 cycles. Overall, this study offers a guiding direction for accelerating the modification of P2-type Na 0.67 MnO 2 as a cathode active material for high performance Na-ion batteries.

New Strategy to Build a High-Performance P′2-Type Cathode Material through Oxygen Vacancies and Mg Substitution for Sodium-Ion Batteries

“…It can be observed that the potential deviation between peak-O1 and peak-R1 of Ovs & Mg-sub was smaller than that of Ovs, indicating less polarization and more reversibility of Ovs & Mg-sub. The peak-O2 and peak-R2 correspond to the phase transformation between P′2 and OP4, providing secondary capacity contribution . It is the reason why Ovs & Mg-sub has excellent specific capacity in the 2.0–4.0 V voltage range.…”

A Review of Degradation Mechanisms and Recent Achievements for Na‐Ion Layered Transition Metal Oxides

“…In contrast, Na 0.67 MnO 2 possesses a high specific discharge capacity, but it is easy to react with H 2 O and CO 2 in the air to generate surface impurities, resulting in layered structure instability and electrochemical performance decline. [82][83][84] In terms of elemental composition, it is the stronger alkalinity of sodium ion that leads to a weaker air stability for sodium ion electrode materials compared to lithium ion electrode materials. Compared with LiOH, NaOH is highly hydrophilic and can absorb large amounts of water.…”

“…By doping small amounts of cationic or anionic atoms into the lattice of sodium ion batteries cathode materials, the air/water stability can be significantly improved. [82,88] This generally includes cationic dopants (Al, Mg, Ca, Ti, Nb, and B) and anionic dopants (F, Cl, and S). Among the above dopants, Al, Mg and Fe are considered to be the most promising elements due to their low cost.…”

“…The typical tunnel‐type oxide Na 0.44 MnO 2 possesses a unique S channel (Figure 7a), which ensures structural stability during cycling and is very stable in both air and water, but its specific discharge capacity is low, [80,81] as show in Figure 7b,c. In contrast, Na 0.67 MnO 2 possesses a high specific discharge capacity, but it is easy to react with H 2 O and CO 2 in the air to generate surface impurities, resulting in layered structure instability and electrochemical performance decline [82–84] . In terms of elemental composition, it is the stronger alkalinity of sodium ion that leads to a weaker air stability for sodium ion electrode materials compared to lithium ion electrode materials.…”

Structural Evolution of Air-Exposed Layered Oxide Cathodes for Sodium-Ion Batteries: An Example of Ni-doped NaxMnO2

“…The severe Jahn–Teller effect associated with the six-coordinated high-spin Mn 3+ cations causes the transformation of the lattice structure from hexagonal (P2) to orthorhombic (P2′). ,, The P2–P2′ transformation induces large lattice strain and Na + /vacancy order, which reflects in reduced sodium cation mobility and relevant capacity fading and further leads to structural collapse during repeated sodiation/desodiation cycles. The most commonly adopted strategy to mitigate cooperative Jahn–Teller distortion, suppress P2–P2′ phase transformation, and enhance structural stability of the oxide and mobility of the Na + ion consists in the substitution of a proper amount (usually 0.05 < y < 0.2 ,,, ) of Mn 3+ Jahn–Teller centers with electrochemically inactive cations (such as Li + , Mg 2+ , Zn 2+ , Al 3+ ) or active cations (such as Fe 3+ , Co 3+ , Ni 2+/3+ , and Cu 2+ ), ,,,− maintaining a single Na x Mn 1– y M y O 2 phase …”