Tailoring Anionic Redox Activity in a P2-Type Sodium Layered Oxide Cathode via Cu Substitution

Abstract

Na-ion cathode materials cycling at high voltages with long cycling life and high capacity are of imminent need for developing future high-energy Na-ion batteries. However, the irreversible anionic redox activity of Na-ion layered cathode materials results in structural distortion and poor capacity retention upon cycling. Herein, we develop a facile doping strategy by incorporating copper into the layered cathode material lattice to relieve the irreversible oxygen oxidation at high voltages. On the basis of a comprehensive comparison with the Cu-free material, both the overoxidation of O 2− to trapped molecular O 2 and Mn-related Jahn−Teller distortion have been effectively inhibited by restraining both the oxygen activity and participation of Mn 4+ /Mn 3+ redox activity. Not limited to discovering stable cycling behavior at high voltages after Cu substitution, our findings also highlight an effective strategy to stabilize the anionic redox activity and elucidate the stabilization mechanism of Cu substitution, thus paving the way for further improvement of layered oxide cathode materials for high-energy Na-ion batteries.

Correlating Mg Displacement with Topologically Regulated Lattice Oxygen Redox in Na-Ion Layered Oxide Cathodes

“…Such an impressive feature has also been reported for Na 2 Mn 3 O 7 with native Mn vacancies, whose anomalous nonhysteretic behavior on LOR can be understood by its stable electronic structure upon (de)­sodiation . Both P2- and P3-NMMO show conspicuous intensity damping of Mn 4+ O during the subsequent discharging process to 2.0 V (“FD”), which could be explained by the formation of a small quantity of Mn 3+ since Mn 3+ –O–Mn 4+ exhibits much weaker superexchange coupling in comparison with Mn 4+ –O–Mn 4+ . ,, It is also possible that some Mn 4+ begins to be reduced before oxidized (O 2 ) n − is entirely reduced to O 2– . , …”

“…50 Both P2-and P3-NMMO show conspicuous intensity damping of Mn 4+ �O during the subsequent discharging process to 2.0 V ("FD"), which could be explained by the formation of a small quantity of Mn 3+ since Mn 3+ −O−Mn 4+ exhibits much weaker superexchange coupling in comparison with Mn 4+ −O−Mn 4+ . 49,51,52 It is also possible that some Mn 4+ begins to be reduced before oxidized (O 2 ) n− is entirely reduced to O 2− . 4,53 Most LOR-active cathodes suffer from O loss from the lattice during the initial charging and even the subsequent charging process, 54−57 which is detrimental to the structural integrity.…”

Review on Cathode Materials for Sodium‐ and Potassium‐Ion Batteries: Structural Design with Electrochemical Properties

“…The average voltage, specific capacity, and energy density of recently reported cathode materials are summarized in Figure 2 [46–108] . In the NIB system, layered oxide cathode was found to be more advantageous in overall electrochemical properties, including the gravimetric capacity and energy density.…”

“…Hu et al introduced Li/Cu co-substitution into the P2-type layered structure to achieve reversible anionic redox reaction in the high-voltage range. [50] The P2À Na 0.73 Li 0.21 Mn 0.74 Cu 0.05 O 2 has Na-OÀ Li configurations in crystal structure, which activates oxygen redox through O 2p non-bonding orbital. [116] Also, the Cu dopant plays a key role to stabilize this anion redox reaction through the strong covalent bonding with oxygen.…”

Improving Electrochemical Activity of P2‐type Na2/3Mn2/3Ni1/3O2 by Controlling its Crystallinity

“…[4,5] To mitigate these structural changes, different metal substitutions have been investigated and were found to improve the cycling performance of P2-Na 2/3 (Mn 2/3 Ni 1/3 )O 2 . [6][7][8] Moreover, some of the substituents, such as Mg, Li, and the introduction of the vacancy at the TM sites facilitate the oxygen redox activity in layered Na x MO 2 materials [9][10][11][12][13][14][15][16][17] by generating the nonbonding oxygen orbits along Na-OÀ X configurations, where X represents Li, Mg or vacancy.…”