Structural Stabilization of P2‐type Sodium Iron Manganese Oxides by Electrochemically Inactive Mg Substitution: Insights of Redox Behavior and Voltage Decay

Abstract

Layered P2-type Na 0.8 Mn 0.5 Fe 0.5 O 2 cathode material is a promising candidate for next-generation sodium-ion batteries due to the economical and environmentally benign characteristics of Mn and Fe. The poor cycling stability of the material, however, is still a problem that must be solved. To address the problem, electrochemically inactive Mg 2 + was introduced into the structure by substituting some of the Fe ions. It was shown that Mg substitution led to a smoother voltage profile with improved cycling performance and rate capability. These observations were attributed to the suppressed structural changes during electrochemical processes. Detailed redox mechanisms, associated local structural changes, and phase transitions were investigated by X-ray absorption spectroscopy and X-ray diffraction. From the detailed analysis of electrochemical behaviors, it was also identified how the redox reactions and structural disordering occurred in the high-and low-voltage regions and how Mg substitution stabilized the structure.

Thermally Activated P2‐O3 Mixed Layered Cathodes toward Synergistic Electrochemical Enhancement for Na Ion Batteries

“…Notably, activated NMO exhibits a phase transition to O2 phase whose characteristic peak is located at ≈17 ° together with the disappearance of P2 (002) peak and O3 (003) peak as well as a significant decrease of (101) peak assigned to both P2 and O3 phases at the end of charging. [34,35] The drastic change of c lattice parameter with O2 phase evolution was indicative of a huge structural change. The structural change of activated NMO with several evolving peaks between 35 ° and 39 ° is also well consistent with its stepwise voltage profile which results from couple of phase transitions as we expected from changes in voltage profile decay of activated NMO.…”

Metal Substitution versus Oxygen-Storage Modifier to Regulate the Oxygen Redox Reactions in Sodium-Deficient Three-Layered Oxides

“…The stable curve profile is achieved after five cycles. Based on our previous study on P3-Na2/3Ni1/2Mn1/2O2, the redox peaks in the range of 3.2-3.8 V are due to the partial sodium extraction/insertion inducing a reversible monoclinic distortion of the layered structure [35,38]. The sodium charges are compensated through the redox activity of nickel ions (i.e., Ni 2+ /Ni 3+ /Ni 4+ ).…”

“…For the unsubstituted oxide P3-Na 2/3 Ni 1/2 Mn 1/2 O 2 , the charges of Na+ are compensated at the expense of the Ni ions (i.e., they adopt simultaneously the oxidation states of +2 and +3), while manganese ions are stabilized in an oxidation state of +4. [31,35]. The insertion of Mg 2+ into P3-Na 2/3 Ni 1/2 Mn 1/2 O 2 is accomplished through the replacement of Ni 2+ , which has an ionic radius close to that of Mg 2+ (0.70 versus 0.72 Å).…”

Iron‐Based Layered Cathodes for Sodium‐Ion Batteries

“…Other elements such as Mg, Al, and Nb have also been explored as dopants to overcome the capacity fading issues of FeÀ Mn binary cathodes. [135][136][137][138][139] Al-doping could slightly increase the electrical conductivity and Na-ion diffusivity of O3-type NaFe 0.5 Mn 0.5 O 2 . However, negligible improvements on the kinetics and stabilities of the electrochemical reaction were observed.…”