Modulation of Local Charge Distribution Stabilized the Anionic Redox Process in Mn-Based P2-Type Layered Oxides

Abstract

An anionic redox reaction is an extraordinary method for obtaining high-energy-density cathode materials for sodium-ion batteries (SIBs). The commonly used inactive-element-doped strategies can effectively trigger the O redox activity in several layered cathode materials. However, the anionic redox reaction process is usually accompanied by unfavorable structural changes, large voltage hysteresis, and irreversible O2 loss, which hinders its practical application to a large extent. In the present work, we take the doping of Li elements into Mn-based oxide as an example and reveal the local charge trap around the Li dopant will severely impede O charge transfer upon cycling. To overcome this obstacle, additional Zn2+ codoping is introduced into the system. Theoretical and experimental studies show that Zn2+ doping can effectively release the charge around Li+ and homogeneously distribute it on Mn and O atoms, thus reducing the overoxidation of O and improving the stability of the structure. Furthermore, this change in the microstructure makes the phase transition more reversible. This study aimed to provide a theoretical framework for further improve the electrochemical performance of similar anionic redox systems and provide insights into the activation mechanism of the anionic redox reaction.

Coupling the Electronic Distribution and Oxygen Redox Potential via Cu Substitution of Layered Oxide Cathodes for Sodium-Ion Batteries

“…Previous research disclosed with sole Li doping, the electrons are mainly grabbed by Li, thus leading the p-band center of O shifted toward the Fermi level, making it more prone to oxidation in the lower voltage region. 15 As can be seen in Figure S12, Na 0.7 Li 0.3 Mn 0.7 O 2 displayed the lowest O redox overpotential due to over oxidation according to the calculation results. However, this over-reaction also leads to Nevertheless, Na 0.7 Li 0.1 Cu 0.2 Mn 0.7 O 2 showed the highest capacity retention, i.e., 81.63% after 100 cycles at 1C, which is much higher than all of the solely Li-doped samples.…”

“…To elucidate the effect of Cu substitution, Na 0.7 Li 0.3 Mn 0.7 O 2 and Na 0.3 L 0.1 Mn 0.9 O 2 were also synthesized where Na 0.7 Li 0.3 Mn 0.7 O 2 showed a pure P2 phase while Na 0.3 L 0.1 Mn 0.9 O 2 exhibited obviously impurity phase (see Figure S11). Previous research disclosed with sole Li doping, the electrons are mainly grabbed by Li, thus leading the p-band center of O shifted toward the Fermi level, making it more prone to oxidation in the lower voltage region . As can be seen in Figure S12, Na 0.7 Li 0.3 Mn 0.7 O 2 displayed the lowest O redox overpotential due to over oxidation according to the calculation results.…”

“…As displayed in Figure 4a, the main diffraction peaks showed a continuous shift within the P2 phase, indicating a complete solid solution behavior of LC0.2. 34 No notable sign of the formation of O2, 35 OP4, 15 or Z 36 phase can be observed. Specifically, during the charging process, the peaks (002) and (004), associated with lattice parameter c, moved continuously to lower angles, while the peaks (100) and (012) linked to lattice parameter a moved to higher angles.…”

“…However, its ultrahigh voltage plateau of O has a detrimental effect on its cycle performance, leading to a specific capacity decay of 160 mAh g –1 after 30 cycles. Li plays a crucial role in triggering anionic redox and enhancing specific capacity, yet its poor oxygen stability and cycling performance limit its application . Conversely, Cu demonstrates notable advantages in both oxygen stability and cycling performance.…”

“…Li plays a crucial role in triggering anionic redox and enhancing specific capacity, yet its poor oxygen stability and cycling performance limit its application. 15 Conversely, Cu demonstrates notable advantages in both oxygen stability and cycling performance. In this regard, Chen et al Herein, we presented a systematical theoretical investigation of the charge distribution of Na y Li 0.1 Cu x Mn 0.9−x O 2 through the adjustment of the proportion of Cu elements replacing Mn elements.…”