Revealing the Critical Role of Titanium in Layered Manganese‐Based Oxides toward Advanced Sodium‐Ion Batteries via a Combined Experimental and Theoretical Study

Abstract

interest on accounts of its low cost and abundance of sodium resource. [3][4][5] Additionally, the replacement of Cu with Al as the anode current collector can further reduce the cost and simplify the manufacturing process. [6] The high-performance cathode materials of sodium-ion batteries have been widely studied, [7] including layered oxides, [8,9] Prussian blue analogues [10,11] and polyanion materials. [12,13] Owing to simple manufacturing methods and reversible insertion/extraction, layered oxide Na x MeO 2 (Me = 3d transition metal, such as Co, Mn, Ni, Ti, Cu) [14][15][16][17][18] materials are considered as one of the most promising candidates. Specifically, layered oxides can be mainly classified into two different types: P2-type and O3-type, in accordance with the coordination of sodium ions in the layered structure and the oxygen stacking sequence. In contrast to O3-type, P2-type exhibits more open framework and direct Na-ion diffusion path associated with superior rate and cycling performance for sodium storage. [19][20][21] Manganese-based materials meet the demands for relatively cheap electrodes and different oxidation states of manganese make the voltage ranges flexible. [6] Additionally, as previously reported, the high electrochemical activity of layered manganese oxides has been demonstrated by the primitive test in 1980s. Therefore, the layered P2-type manganese-based oxide materials can be the appealing candidate as cathodes of sodium-ion batteries.However, owing to the Jahn-Teller active Mn 3+ ions followed by the dissolution of Mn ions into the electrolyte, the electrochemical performance of manganese-based oxides should be further improved via effectively suppressing the Jahn-Teller effect. [22,23] Many researchers tried various methods to optimize the composition of electrodes to improve the cyclic stability and rate performance, consisting of (1) surface coating, [22,24] and (2) substitution with some inactive elements. [25,26] Local atomic environment can be changed by doping with some inactive elements to improve the electrochemical performance, such as Mg, [25] Al, [26] etc. However, up to now, a general strategy toward robust sodium storage performance in layered Mn-based cathodes is still insufficient. In this work, we propose a strategy of Sodium-ion batteries are one of the most promising candidates for largescale energy storage. Manganese-based layered oxides are extensively studied as a cathode of sodium-ion batteries due to the low cost and high electrochemical activity. However, these layered cathodes usually suffer from the severe manganese dissolution originated from Jahn-Teller distortion, thereby leading to severe capacity fading and structural deterioration. Herein, it is demonstrated via a combined experimental and theoretical study, titanium substitution in layered manganese-based oxides can weaken the Jahn-Teller effect, minimize the relative dissolution, and thus enable robust sodium storage during long-term operation. Results reveal that Ti-doping can restrain shrinka...

Rational Design of a P2-Type Spherical Layered Oxide Cathode for High-Performance Sodium-Ion Batteries

“…Meanwhile, sodium ions can be divided into Na e and Na f due to their located positions, which share the edges and faces with the transition metal octahedral, respectively. 29,39 For s-NaLiMMCO, transition metals locate in oxygen layers while Li occupies part of the sodium position. 38 The valence states of the transition metals in s-NaLiMMCO obtained by XPS measurement are depicted in Figure 2b–d.…”

“…This material only shows very low capacity decay after long cycling. 29 Li et al reported a P2-Na 0.67 Mn 0.65 Ni 0.2 Co 0.15 O 2 material, which exhibited a superior capacity of 155 mA h g –1 when cycled at 12 mA g –1 and an excellent rate performance even when cycled at high rate density. 30 Many previous research results indicate that doping with Li, Zn, Mg, Al, and so on can greatly ameliorate the electrochemical properties of the original cathodes.…”

Manganese‐Based Materials for Rechargeable Batteries beyond Lithium‐Ion

“…For example, Ti-doping can restrain the shrinkage or elongation of the MnO octahedral structure, thus minimize the Jahn-Teller distortion for Mn atoms surrounded by titanium. [244] The low valent Mg 2+ doping can also suppress structural distortion by increasing the average charge state of Mn-ion to close to 4+, [245] and the Mn 4+ is Jahn-Teller inactive. However, recent studies show that the impact of Jahn-Teller distortion on Mn-based materials is not all negative.…”

“…[248] Recent studies further show that vacancy defects in graphene coating layer can bond with Mn-ions to prevent them dissolve into electrolyte. [222,249] Elemental doping was reported to be able to suppress Mn-dissolution in oxide materials for SIBs [244] and ZIBs. [250] Some studies ascribed it to the changed local atomic environment that derived from doping mitigates Jahn-Teller distortion, and thus suppress Mn-dissolution.…”

Chemical Modulation of Local Transition Metal Environment Enables Reversible Oxygen Redox in Mn-Based Layered Cathodes

“…[17][18][19] Ti doping can suppress Jahn-Teller distortion through reducing the bond length variation along the axial and equatorial directions. 15,20 Ti dopant is also known to enhance oxygen retention in layered oxides due to its strong chemical bonding with oxygen. 21,22 Such structural stabilization through doping chemistry may also be beneficial towards the reversibility of the oxygen redox process.…”