Advanced O3-Type Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 Cathode for Sodium Ion Batteries

Abstract

Previous studies have investigated O3-type layer sodium compounds because of several advantages, including the intercalation ability of Na+ into a layer structure like LiMO2 (M: transition metals), the ease of synthesis, and the lack of cation mixing, which perturbs ion diffusion in the Na layers due to the larger ionic radius of Na+ compared to Li+. To date, detailed structural and electrochemical studies are limited because of the instability of the material structure at high voltages, especially for cathode materials containing Fe transition metals. For O3-type cathodes, in addition to an early report by Yamamoto et al.,[1] Okada’s group recently revisited α-NaFeO2. Since then, Komaba and co-workers studied NaFeO2, which delivers a discharge capacity of about 80 mAhg−1 with a limited charge cutoff voltage up to 3.3 V vs Na/Na+, above which they hypothesized that structural deterioration by Fe migration caused severe capacity fade within several cycles.[2] These prior works are milestone studies since an inexpensive Fe3+/4+ redox reaction produces a moderate capacity in an acceptable operating voltage range. Komaba et al. also suggested that partial replacement of Fe by Co to yield NaFe0.5Co0.5O2 generated favorable results by suppressing Fe migration and thereby yielding a discharge capacity of about 160 mA h g−1 (12 mA g−1) in the operating voltage range of 2.5−4.0 V. Compared to NaFeO2, this capacity is higher by a factor of 2, and the operating voltage was increased as well. Furthermore, Komaba et al. looked at solid solution NaFeO2 NaNi0.5Mn0.5O2 with a molar ratio of 4:6, NaFe0.4M0.6O2 (M =Ni0.5Mn0.5), which exhibited an available capacity of approximately 130 mA h g−1 at a 0.05 C-rate. This result is improved relative to NaFeO2 but is inferior to Na[Ni0.5Mn0.5]O2, as previously published. Johnson et al.[3] reported a new layer compound, Na(NiII 1/3FeIII 1/3MnIV 1/3)O2, which demonstrated similar charge−discharge behavior as a Li(Ni1/3Co1/3Mn1/3)O2 cathode with a delivered first-discharge capacity of about 130 mA h g−1 at a 0.1 C-rate up to 4.0 V. Recently, O3-type Na[Ni0.25Fe0.5Mn0.25]O2 cathode materials were reported by our group which show stable cycle performance and rate capability, but it still shows the limited discharge capacity of 140 mA h g−1 because of voltage limitation (3.9 V). From the above, it is clear that the discharge capacity of NaFeO2can be enhanced by replacing Fe with other transition metals to prevent iron migration at higher voltages. However, for all of the cases mentioned, the upper cutoff voltage was limited to 4.0 V to avoid decomposition of the electrolyte. We postulate that the reversible capacity can be further increased provided that the available upper cutoff voltage of the O3-type NaFexM1−xO2 (M = transition metals) can be increased. NaMnO2 is stable for cycling up to 4 V based on the Mn3+/4+ redox reaction, although it suffers from a poor rate capability.[4] Also, according to our and other prior reports, with partial substitution of transition metals by Li in the transition metal layer, Li[Li0.06(Ni0.5Mn0.5)0.94]O2, Na0.85Li0.17Ni0.21Mn0.64O2, and P2 type Na0.8[Li0.12Ni0.22Mn0.66]O2, Na0.83[Li0.07Ni0.31Mn0.62]O2 were effective in stabilizing the crystal structure during cycling and C-rate testing.[5] Utilizing the above-mentioned concept, namely, Li and Mn adaption into the α-NaFeO2 structure, we produced Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 via oxalate coprecipitation.