Gradational anionic redox enabling high-energy P2-type Na-layered oxide cathode

Highlights

• Gradational anionic redox enabling cathode material provides a promising strategy.

• Theoretical and various experimental results verified the redox-active mechanism.

• P2-Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ can inhibit the undesirable structural distortion.

• This study represents an advanced way for developing cathode materials for NIBs.

Abstract

Anionic-redox-based layered oxide materials are considered promising cathodes for Na-ion batteries because of their high energy densities. However, the anionic redox reaction at high voltage results in structural instability of the layered oxides, leading to not only poor electrochemical properties but also structural degradation after prolonged cycling. Herein, through combined studies using first-principles calculation and various experimental techniques, we investigate the role of the combination of earth-abundant Mn, Fe, and Mg in enabling a stable and gradational anionic redox reaction in a P2-type Na-layered oxide cathode during charge/discharge, resulting in outstanding electrochemical performance. At 10 mA g⁻¹, P2-type Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ delivers a large specific capacity of ∼207 mAh g⁻¹, corresponding to ∼0.8 mol Na⁺ de/intercalation via both cationic and anionic redox reactions. The outstanding cycle performance, well-retained crystal structure, and morphology after prolonged cycling indicate that the anionic redox reaction of O²⁻/O⁻ stably occurred in the P2-type Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ structure despite the charging process in the high-voltage region. Furthermore, the use of earth-abundant Mn, Fe, and Mg is beneficial in terms of the economic feasibility for low-cost and high-energy Na-ion batteries. These intensive investigations provide key knowledge for understanding anionic-redox-based cathode materials with high structural stability for Na-ion batteries.

Introduction

The importance of sustainable energy storage systems is ever-increasing in an effort to reduce environmental impacts and address the exhaustion of limited energy resources [1], [2], [3]. Li-ion batteries (LIBs) have been widely used in portable electronic devices, electric vehicles (EVs), and large-scale energy storage systems (ESSs) owing to their high energy density and long cycle life [4], [5], [6]. However, conventional cathode materials, such as LiCoO₂ and Li[Ni_1–x–yCo_xMn_y]O₂, are greatly reliant on certain transition-metal resources, such as Co and Ni, causing the price of LIBs to be affected by the current political climate [7]. In addition, given the recent rise in lithium prices, concerns about the economic feasibility of LIBs are growing. To prepare for these inevitable factors, researchers have aimed to develop alternative energy sources with low production costs and high energy densities as replacements for LIBs.

Recently, Na-ion batteries (NIBs) have received extensive attention as one of the most promising alternatives to LIBs, owing to their use of earth-abundant Na resources and a monovalent-ion-based reaction chemistry similar to that of LIBs [8]. Various promising cathode active materials for NIBs have been introduced, including layered-type transition-metal oxides (Na_x[TM]O₂) ([TM]: transition metal) [9], [10], polyanion-type compounds [11], [12], and Prussian blue analogues [13], [14]. The layered-type Na_x[TM]O₂ cathode group has been especially highlighted because of the high reversible specific capacity and power capability based on the fast two-dimensional guest-ion diffusion pathways. Unlike LIBs, the layered oxide cathodes of NIBs can drive a battery reaction reversibly using inexpensive 3d transition metals, which is incomparably advantageous in terms of production cost. More specifically, P2-type Na_y[Mn_xFe_1-x]O₂ (P2-Na_y[Mn_xFe_1–x]O₂) cathode materials composed of earth-abundant elements have been introduced as a superior low-cost and non-toxic cathode. However, most researches on P2-Na_y[Mn_xFe_1–x]O₂ cathode materials have reported their poor electrochemical performance stemming from structural and chemical instability [15]. For example, Tang and co-workers reported on the poor cycle retention of a P2-Na_0.67[Mn_0.5Fe_0.5]O₂ cathode, which exhibited a capacity retention of ∼56.9 % after 50 cycles even at a low current density of 20 mA g⁻¹ [16]. This undesirable cycle performance was associated with the large volume change from the large Na⁺ ionic size (1.02 Å) and Jahn–Teller distortion from the Mn³⁺/Mn⁴⁺ redox reaction [17], [18]. Moreover, to increase the practical gravimetric capacities of the layered oxide group, several studies on the development of novel cathodes based on an anionic redox reaction (O²⁻/O⁻) were recently reported [19], [20], [21]. However, the anionic redox reaction is accompanied by structural instability, disordering, and sluggish kinetics, resulting in poor electrochemical performance as well as structural and morphological degradation after prolonged cycling [19], [20]. Therefore, to achieve outstanding power capability and cycle performance of anionic-redox-based P2-Na_y[Mn_xFe_1–x]O₂ cathode materials, enhancement of the structural stability is needed. Various researches on doping of other TM cations in P2-Na_y[Mn_xFe_1–x]O₂ were reported to improve the electrochemical properties [22], [23], [24], [25], [26], [27]. However, it was known that anionic redox reaction of O²⁻/O⁻ in Mn-based layered oxide cathode can be occurred when the following conditions are satisfied [28], [29]; (1) oxidation state of Mn⁴⁺, (2) existence of Na⁺ in the structure even after oxidation to Mn⁴⁺. In terms of the doped TM cations in P2-Na_y[Mn_xFe_1–x]O₂, they can be oxidized during charge and provide the electrons for Na⁺ deintercalation, like the Mn and Fe cations. Thus, it is difficult to prepare the anionic-redox-based P2-Na_y[Mn_xFe_1–x]O₂ through the simple doping of TM cations.

Since the redox inactive Mg ion maintains redox state as +2 regardless of Na⁺ de/intercalation process, thus the bonding interaction between Mg cation and O anion is less varied during Na⁺ de/intercalation than that of between transition metal (TM) cation and O anion, which implies that existence of Mg cations is capable of suppressing Jahn-Teller distortion of Mn based active materials. Thus, we speculated that the structural and electrochemical stabilities of anionic-redox-based P2-Na_y[Mn_xFe_1-x]O₂ could be successfully enhanced by substitution of a considerably large amount of the Mg²⁺ ions instead of Fe and Mn in [TM] sites. Herein, a novel anionic-redox-based P2-Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ cathode is shown to exhibit a large specific capacity of ∼207 mAh g⁻¹ in the voltage range of 1.5–4.4 V (vs Na⁺/Na) and a stable O²⁻/O⁻ redox reaction during prolonged cycling. For 150 cycles at 100 mA g⁻¹, up to ∼73 % of the initial capacity was retained with a high coulombic efficiency of > 99 %. Furthermore, after prolonged cycling, the crystal structure and morphologies of P2-Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ were also well maintained without severe degradation, despite the anionic redox reaction in the high-voltage region. Through combined first-principles calculation and various structural analyses, we revealed that the presence of ∼0.22 mol Mg²⁺ ions in [TM] sites can result in the high structural stability of P2-Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ such as slightly changed local environments of [Mg, Fe, Mn]O₆ octahedra before and after anionic redox reaction by gradationally and selectively oxidizing oxygen anions.