Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium-Based Compounds
Authors: Kim Hee Jae , Myung Seung-Taek 
Published: February 2020

Inducing anionic redox reaction of transition metal oxides in sodium ion batteries is one of the most reliable way to enhance capacity. Unfortunately, although the delivered capacity obtained from the transition-metal redox and oxygen redox is one of highest among sodium cathodes, the system suffered from not only serious capacity fading but also poor rate capability because of the sluggish kinetics of the oxygen redox. Given the overlapping in the density of states (DOS) between the oxygen 2p and cobalt 3d (Co4+/3+) orbitals in LiCoO2, which is one of the most representative materials showing good cyclability and acceptable rate performances, we applied this phenomenon to P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2 (x = 0.0–0.2).

The P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2 (x = 0.0–0.2) compounds were synthesized using a spray pyrolysis method. NaNO3, Mn(NO3)2·4H2O, Mg(NO3)2·6H2O, Co(NO3)2·6H2O, citric acid, and sucrose were used as starting materials for the spray pyrolysis. Calcination was conducted at 870 °C in air to obtain the P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2 (x = 0.0–0.2). The compounds were stored in an Ar-filled glove box to minimize the moisture contact during storage.

Powder X-ray diffraction (XRD; PANalytical, Empyrean) using Cu Kα radiation was employed to identify the crystalline phases of the as-prepared P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2 (x = 0.0–0.2) powders. The produced powders were examined using scanning electron microscopy (SEM; SU-8010, Hitachi) to analyze morphology of P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2 (x = 0.0–0.2) powders. Differential scanning calorimetry (DSC; Pyris 1, Perkin–Elmer) measurements were performed to analyze the thermal stability of desodiated electrodes.

R2032 coin-type cells using Na metal as the negative electrode were assembled for electrochemical cell tests. The electrolyte solution was 0.5 M NaPF6 in a mixed solvent of propylene carbonate (PC) and fluoroethylene carbonate (FEC) in a volume ratio of 98:2. The fabricated cells were charged and then discharged between 1.5 and 4.6 V at a rate of 0.1C (26 mA g−
1), and the current was further raised to 5C (1.3 A g−
1) for a long-term cycle test at 25 °C.

X-ray absorption near edge structure (XANES) spectroscopy was employed to understand the variation in the oxidation states of the transition metals and oxygen. Transmission electron microscopy (TEM; JEM-F200, JEOL) was used to analyze structural factors of the pristine P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 electrode and the electrode after 1000 cycles.

Density functional theory (DFT) calculations were performed using the projector augmented wave method implemented in the Vienna ab initio simulation package (VASP).

Among studied compounds P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2 (x = 0.0–0.2), P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 electrode delivered high discharge capacity of about 214 mAh g−
1, which is attributed to the reactions of Co4+/3+, O1
−
/2
−, and Mn4+/3+, with approximately 87% (186 mAh g−
1) of the capacity retention over 100 cycles at a rate of 0.1 C (26 mA g-1). More importantly, the present P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 was able to deliver discharge capacities of approximately 108 mAh g−
1 at a rate of 5C (1.3 A g−
1), showing capacity retention of over 72% (78 mAh g−
1) after 1000 cycles (Figure 1). This outstanding electrode performance was rationalized by theoretical calculations; namely, the incorporation of Co in the crystal structure was shown to dramatically reduce the band-gap energies. Detailed information will be presented at the conference.

Figure 1. Long–term cyclability at 5 C (1.3 A g−
1) for x = 0.2 in P2 Na0.6[Mg0.2Mn0.8
−
xCox]O2.

Figure 1