Sodium and Manganese Stoichiometry of P2‐Type Na_2/3MnO₂

Abstract

To realize a reversible solid-state Mn(III/IV) redox couple in layered oxides, co-operative Jahn-Teller distortion (CJTD) of six-coordinate Mn(III) (t2g (3) -eg (1) ) is a key factor in terms of structural and physical properties. We develop a single-phase synthesis route for two polymorphs, namely distorted and undistorted P2-type Na2/3 MnO2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na2/3 MnO2 delivers 216 mAh g(-1) as a 3 V class positive electrode, reaching 590 Wh (kg oxide)(-1) with excellent cycle stability in a non-aqueous Na cell and demonstrates better electrochemical behavior compared to undistorted Na2/3 MnO2 . Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na2/3 MnO2 , providing a new insight into utilization of the Mn(III/IV) redox couple for positive electrodes of Na-ion batteries.

Ultralow‐Strain Zn‐Substituted Layered Oxide Cathode with Suppressed P2–O2 Transition for Stable Sodium Ion Storage

“…In the continuous charge process, anion redox could occur according to the similar phenomenon in other P2 analogues32–34 and a total amount of 0.56 Na + are expected to be extracted (corresponding to Na 0.07 ZnMnO 2 ). On the other hand, the charge plateau observed at high potential is a fingerprint of anion redox reaction30–34 that contributes to most of the detected capacity 27,29–34. In the first discharge process, a discharge capacity of 202.4 mAh g −1 can be achieved, corresponding to 0.79 Na + insertion into the NZM host leading to a formula of Na 0.86 MnO 2 .…”

Highly Reversible Oxygen‐Redox Chemistry at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn Vacancy)

“…Owing to the redox activity of both Mn and O, Na 2 Mn 3 O 7 delivers a large reversible capacity of ≈200 mAh g −1 with the capacity retention of 85% after 20 cycles (Figure 6b). [35] [35] …”

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

“…The electrochemical mechanisms of distorted and undistorted P2‐type Na 2/3 MnO 2 were studied by Komaba's group. It was found that the distorted (P′2‐type) Na 2/3 MnO 2 delivered a high capacity of 216 mAh g −1 as a 3 V class cathode material, with excellent cycling stability and better electrochemical behavior compared to the undistorted Na 2/3 MnO 2 , as presented in Figure a,b . Monoclinic α‐NaMnO 2 (C2/m) shows an initial capacity of 185 mAh g −1 when cycling between 3.8 and 2.0 V. It also experiences fast capacity decay, however, in subsequent cycles …”

“…Charge–discharge profiles of different single metal oxide cathodes for SIBs: a) P2‐Na 2/3 MnO 2 and b) P′2‐Na 2/3 MnO 2 . Reproduced with permission . Copyright 2016, Wiley‐VCH.…”