Nonpolarizing oxygen-redox capacity without O-O dimerization in Na2Mn3O7

Abstract

Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis >0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O22− dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na2-xMn3O7, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O−• was identified in Na2-xMn3O7. Computations reveal that O−• is thermodynamically favorable over the peroxide-like O22− dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.

Addressing voltage decay in Li-rich cathodes by broadening the gap between metallic and anionic bands

“…Specifically, the oxidation products of the lattice oxygen in LSLR project more oxygen anions with an electron hole than LR. In a latest study, this effect was also achieved in a cathode material for sodium-ion batteries 37 . The underlying fine structure could be revealed by the vibration near the elastic peak at 0.1–0.9 eV (Fig.…”

Recent Advances in Electrode Materials with Anion Redox Chemistry for Sodium-Ion Batteries

“…In a recent work, Tsuchimoto et al confirmed a unique behavior of O in Na 2 Mn 3 O 7 using DFT calculations and magnetic and spectroscopic measurements (Figure 6(f)) [78]. The existence of thermodynamically favorable O −• over the peroxide-like O 2 2− dimers was predicted by computations and showed that hole stabilization occurred through a (σ + π) multiorbital Mn-O bond.…”

“…Despite the promising low-voltage hysteresis and high capacity, Na 2 Mn 3 O 7 still suffers from degradation during cycling. Furthermore, the mechanism of the anion redox reaction with such a small hysteresis is still under debate [78,79].…”

“…An important alternative to improve upon the reversibility of oxygen-redox reactions is the use of vacancy-containing materials, as shown for the P2-□ Mg material in Section 2.1.2. Herein, another P-type sodium-deficient cathode Na 2 Mn 3 O 7 (Na 4/7 [□ 1/7 Mn 6/7 ]O 2 ) with □ Mn vacancies is presented[73][74][75][76][77][78][79][80][81]. The layered structure of Na 2 Mn 3 O 7 with every seven Mn site vacancies (…”

Engineered Three-Electrode Cells for Improving Solid State Batteries

“…After the 1 st cycle, the charge starts with a decrease in RCT which matches well the initial voltage slope between 1.5 and 1.9 V vs. LiIn:In, followed by a constant and lowest value of RCT corresponding to the plateau around 2 V. Thus, RCT displays a strong asymmetry between charge and discharge, and its variations appear to match well the voltage variations of LTFS. The strong asymmetry suggests that it might be related with the differing reaction pathways between charge and discharge for processes owing to the anionic redox process pertaining to the Li-rich layered sulphide LTFS as well documented 26,27 .…”