Understanding the Low-Voltage Hysteresis of Anionic Redox in Na₂Mn₃O₇

Abstract

The large-voltage hysteresis remains one of the biggest barriers to optimizing Li/Na-ion cathodes using lattice anionic redox reaction, despite their very high energy density and relative low cost. Very recently, a layered sodium cathode Na2Mn3O7 (or Na4/7Mn6/7□1/7O2, □ is vacancy) was reported to have reversible lattice oxygen redox with much suppressed voltage hysteresis. However, the structural and electronic structural origin of this small-voltage hysteresis has not been well understood. In this article, through systematic studies using ex situ/in situ electron paramagnetic resonance and X-ray diffraction, we demonstrate that the exceptional small-voltage hysteresis (<50 mV) between charge and discharge curves is rooted in the well-maintained oxygen stacking sequence in the absence of irreversible gliding of oxygen layers and cation migration from the transition-metal layers. In addition, we further identify that the 4.2 V charge/discharge plateau is associated with a zero-strain (de)­intercalation process of Na+ ions from distorted octahedral sites, while the 4.5 V plateau is linked to a reversible shrink/expansion process of the manganese-site vacancy during (de)­intercalation of Na+ ions at distorted prismatic sites. It is expected that these findings will inspire further exploration of new cathode materials that can achieve both high energy density and efficiency by using lattice anionic redox.

Stabilizing Reversible Oxygen Redox Chemistry in Layered Oxides for Sodium‐Ion Batteries

“…[ 24 ] The obvious variation of Ru–O peak at two potential plateaus region is due to the change of O–O distances derived from anionic redox reactions, which also causes slight distortion in the MO 6 octahedral environment at high voltage range meanwhile. [ 25 ] Generally speaking, the slope regions within the initial charge and final discharge stages (blue region, Figure 3c) can be assigned to the Ru‐involved cationic redox reaction processes while the two charge and discharge plateaus (green region, Figure 3c) can be attributed to the Ru‐free redox process, which can be ascribed to the oxygen‐related anionic redox reaction processes.…”

A High‐Performance Primary Nanosheet Heterojunction Cathode Composed of Na0.44MnO2 Tunnels and Layered Na2Mn3O7 for Na‐Ion Batteries

“…Owing to the existence of a large amount of long‐range periodic manganese vacancies (one out of seven Mn are vacant in the Mn‐O layer), two clear potential plateaus appear at 4.2 and 4.5 V with cutoff curves of 4.6 and 4.7 V, respectively; the 4.5 V potential plateau is not obtained for the 4.5 V cutoff curve. The high voltage potential plateaus are ascribed to the shrinkage/expansion of manganese‐site vacancies during the charge/discharge process . As suggested, at the high voltage range, charge compensation is performed by O‐redox (O 2− /(O 2 ) n − , n =4, 3, 2, 1, or 0).…”

“…The high voltage potential plateaus are ascribed to the shrinkage/expansion of manganese‐site vacancies during the charge/discharge process . As suggested, at the high voltage range, charge compensation is performed by O‐redox (O 2− /(O 2 ) n − , n =4, 3, 2, 1, or 0). The higher the charge voltage, the more favorable it is to release oxygen gas ( n =0); thus, the 4.5 V potential plateaus are not stable and tend to release oxygen, as indicated by the initial C.E.…”

“…In the structure, there is one out of seven Mn vacancies in the TM layer, oxygen displays a distorted P 3‐type stack, and Na + occupies both a distorted prism and octahedral sites. The reversible oxygen‐redox for the Na‐deficient cathode results from the reaction between the nonbonding 2p orbitals of the oxygen atoms and the nearby Mn vacancies, which contribute two charge plateaus in the high voltage region . Compared with oxygen‐redox from a noble metal‐based cathode (Na 2 IrO 3 and Na 2 RuO 3 ), the low‐cost Na 2 Mn 3 O 7 cathode is much more attractive .…”

“…Recently, a triclinic structure of Na 2 Mn 3 O 7 (denoted as Na 4/7− x [1/7 Mn 6/7 ]O 2 ) has received intensive research attention because it can be charged to 4.7 V (the charge compensation of the high voltage range is realized by reversible oxygen‐redox), and it has a large capacity (250 mAh g −1 at 1/20 C), which favors a high energy density. These pioneering works analyzed the intrinsic structure, charge compensation mechanism, and low‐voltage hysteresis . In the structure, there is one out of seven Mn vacancies in the TM layer, oxygen displays a distorted P 3‐type stack, and Na + occupies both a distorted prism and octahedral sites.…”