On the P2-Na_xCo_1–y(Mn_2/3Ni_1/3)yO₂ Cathode Materials for Sodium-Ion Batteries: Synthesis, Electrochemical Performance, and Redox Processes Occurring during the Electrochemical Cycling

Abstract

P2-type NaMO sodiated layered oxides with mixed transition metals are receiving considerable attention for use as cathodes in sodium-ion batteries. A study on solid solution (1 - y)P2-NaCoO-(y)P2-NaMnNiO (y = 0, 1/3, 1/2, 2/3, 1) reveals that changing the composition of the transition metals affects the resulting structure and the stability of pure P2 phases at various temperatures of calcination. For 0 ≤ y ≤ 1.0, the P2-NaCoMnNiO solid-solution compounds deliver good electrochemical performance when cycled between 2.0 and 4.2 V versus Na/Na with improved capacity stability in long-term cycling, especially for electrode materials with lower Co content (y = 1/2 and 2/3), despite lower discharge capacities being observed. The (1/2)P2-NaCoO-(1/2)P2-NaMnNiO composition delivers a discharge capacity of 101.04 mAh g with a capacity loss of only 3% after 100 cycles and a Coulombic efficiency exceeding 99.2%. Cycling this material to a higher cutoff voltage of 4.5 V versus Na/Na increases the specific discharge capacity to ≈140 mAh g due to the appearance of a well-defined high-voltage plateau, but after only 20 cycles, capacity retention declines to 88% and Coulombic efficiency drops to around 97%. In situ X-ray absorption near-edge structure measurements conducted on composition NaCoMnNiO (y = 1/2) in the two potential windows studied help elucidate the operating potential of each transition metal redox couple. It also reveals that at the high-voltage plateau, all of the transition metals are stable, raising the suspicion of possible contribution of oxygen ions in the high-voltage plateau.

Understanding Challenges of Cathode Materials for Sodium‐Ion Batteries using Synchrotron‐Based X‐Ray Absorption Spectroscopy

“…In the EXAFS spectra, an additional peak around 0.98 Å was observed for the pristine sample, while there was no additional peak for the Zn‐doped sample, indicating that the Jahn‐Teller distortion was suppressed to a certain degree (Figure a–c). Multiple element doping can also reduce the Jahn‐Teller effect . Ma's group employed operando transmission X‐ray microscopy (TXM) techniques and ex‐situ XANES spectra to investigate a proposed nonequilibrium solid solution of Na 1–δ Ni 1/3 Fe 1/3 Mn 1/3 O 2 .…”

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium‐Ion Batteries

“…For Na x (Co, Mn, Ni)O 2 materials, the effects of composition on the electrochemical performance and the stability of the solid solution compounds P2‐[(1− y )Na x CoO 2 ‐ y Na x Mn 2/3 Ni 1/3 O 2 ] ( y = 0, 1/3, 1/2, 2/3 and 1), namely Na x Co 1‐ y [Mn 2/3 Ni 1/3 ] y O 2 , were investigated, as shown in Figure a . When cycled between 2.0 and 4.2 V, the Na x Co 1/2 Mn 1/3 Ni 1/6 O 2 ( y = 1/2) delivered the highest discharge capacity of 101.04 mAh g −1 with 97% capacity retention at the 100th cycle and a coulombic efficiency of >99.2%.…”

“…a) d Q /d V versus potential curves (left) and cycling performance (right) of P2‐Na x Co (1− y ) Mn 2 y /3 Ni y /3 O 2 ( y = 0, 1/3, 2/1, 2/3 and 1) within the voltage range of 2.0–4.2 V. Reproduced with permission . Copyright 2017, American Chemical Society.…”

State-of-the-Art Electrode Materials for Sodium-Ion Batteries

“…This effect has been evidenced in Na 0.7 CoO 2 , where the Mg substitution also increases the rate capability owing to an increase of the Na + diffusion coefficient [45]. In addition, the cationic substitution improves the cycling life due to the suppression of the Na + /vacancy ordering condition, experienced in P2-Na x Co 1-y (Mn 2/3 Ni 1/3 ) y O 2 [46], and also Ca-substitution [45,47]. This is also true with Ni substitution, but only below a solubility limit [48], and Ti substitution, but only for Ti concentration smaller than 10% [49].…”