Structure and Electrochemistry of Na_xFe_xMn_1-xO₂(1.0 ≤ x ≤ 0.5) for Na-Ion Battery Positive Electrodes

Abstract

The primary objective of research activities in Na-ion battery development is the implementation of inexpensive, sustainable and environmentally friendly battery materials. Here we report an investigation of Na x Fe x Mn 1-x O 2 electrode materials which contain elements commonly found in the earth's crust, have very low toxicity and cost, and can be prepared in open air atmosphere using simple procedures. Samples with x > 0.65 were found to have the same structure as O3 α-NaFeO 2 under selected synthesis conditions. Samples with x < 0.65 did not have the P2 or O3 structures, but retained a layered structure in which staging likely occurs. Samples having x = 0.5 were found to cycle reversibly at a capacity over 170 mAh/g (corresponding to a theoretical energy density of approximately 460 Wh/kg) with 85% capacity retention after 20 cycles. When the capacity is restricted to 135 mAh/g (340 Wh/kg), capacity retention approaches 95% after 20 cycles.High energy density batteries made from inexpensive sustainable resources would be attractive for a number of applications, including automotive and grid storage. Such applications require batteries capable of excellent coulombic efficiency (>99.99%) and structural stability. Na-ion batteries are a possible candidate technology for these applications. Major research in Na-ion batteries began in the 1980's, but research interest decreased as Li-ion batteries gained prominence. Because of heightened environmental concerns and new markets for secondary batteries in consumer electronics, grid storage and automotive applications, there has been a renewed research interest in Na-ion batteries in recent years, especially those which comprise electrodes made from sustainable inexpensive materials.Iron based cathode materials are of particular interest because of the abundance and low cost of iron. While Fe 3+ and Li + cations tend to mix in lithium iron oxides, 1 this is not the case for Na. NaFeO 2 has two polymorphs. β-NaFeO 2 has an orthorhombic structure and has not been reported to be electrochemically active. α-NaFeO 2 is the prototype structure for the O3 type layered metal oxide phases. Na deintercalation from α-NaFeO 2 was first reported by Kikkawa et al. in 1985, 2 where Na 0.9 FeO 2 was achieved using bromine as an oxidizing agent. The O3 structure was preserved during this process. The first electrochemical study of α-NaFeO 2 was reported in 1994 by Takeda et al., where α-NaFeO 2 was incorporated into Li half cells using LiClO 4 as a lithium salt and a lithium foil counter electrode. 3 A new monoclinic phase of Na 0.5 FeO 2 was reported after electrochemical deintercalation of Na (corresponding to a capacity of 120 mAh/g), with the monoclinic distortion appearing in a two phase reaction during sodium removal. It was argued that the distortion was too small to be due to Jahn-Teller effects, but was more likely caused by structural distortion in response to Na-vacancies. Li could not be effectively intercalated into the structure to form isostructural or other type Li ...

Communication—O3-Type Layered Oxide with a Quaternary Transition Metal Composition for Na-Ion Battery Cathodes: NaTi0.25Fe0.25Co0.25Ni0.25O2

“…8,13,22,23 Fe 4+ in this compound was observed by Yue et al, 18 and other work also showed oxidation of Fe 3+ to Fe 4+ below 4 V. 6,8,13 We found the latter in Figure S1. Thus, the reversible and irreversible plateaus at 2.8 and Figure 3c may originate from the O3-P3 transition and Fe 4+ migration, respectively.…”

“…6 The charged state stability of the layered structure, and thereby their electrochemical properties, 7 can be enhanced by partially substituting Fe into redox-inactive elements and/or other transition metals. [8][9][10][11] Li et al proposed an optimal Fe content (< ∼33%) that promotes Na diffusion at the high state of charge. 12,13 Here, we use Ti 4+ as a structural stabilizer and report an O3-type 14 Na transition metal oxide (O3-NaMO 2 ) composition, NaTi 0.25 Fe 0.25 Co 0.25 Ni 0.25 O 2 (TFCN), for the optimized Fe redox activity.…”

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition, Air Stability, and Performance

“…O3-type Na[Fe 1/2 Mn 1/2 ]O 2 can also be synthesized by controlling the ratio of Na/(Fe and Mn). [158] In contrast with P2 phase, large polarization (>1 V) during charge and discharge processes is observed for O3-type Na[Fe 1/2 Mn 1/2 ]O 2 when the Na cell is cycled in the voltage range of 1.5-4.25 V. [167] Interestingly, the synthesis of P2-and O3-Na 2/3 [Fe 2/3 Mn 1/3 ]O 2 with the same stoichiometry, which not only allowed a direct comparison of two phases for the first time, but also enabled some inferences to be made regarding the effect of phase on electrochemistry. In both phases, no Jahn-Teller structural distortion occurs due to the presence of high-spin octahedrally coordinated Fe 3+ (t 2g 3 e g 2 ) and Mn 4+ (t 2g 3 e g 0 ).…”

Synthesis and Electrochemistry Study of P2- and O3-phase Na2/3Fe1/2Mn1/2O2

“…[7] Obviously, due to the production cost of Li and its limited geographic distribution, the use of Li-ion batteries may be limited to hand held devices or electric vehicles, [8] and therefore, investigation regarding more abundant intercalation elements like Na + is inevitable. [9,10] Among many cathode candidates, Fe and Mn containing layered oxides have been investigated extensively, [11][12][13][14][15] because of advantages including environmental benignity, inexpensiveness, and large reversible capacity. Typical phases investigated are P2-and O3-phase where P2-phase has been synthesized as Na + deficient system (~0.67), and O3-phase has been prepared as fully sodiated system.…”