Stabilizing Reversible Oxygen Redox Chemistry in Layered Oxides for Sodium‐Ion Batteries

Abstract

premise of guaranteeing electrochemical stability upon cycling. [2] Not rigidly limited to conventional redox reactions based on transition metal (TM) centers, redox activity centered at the oxide anions is emergently viewed as a promising strategy to effectively increase the energy density of SIBs. [3] When conventional cationic and novel anionic activities are triggered simultaneously, several cathodes delivering boosted capacities break the shackle of reversible TM redox couples. [4] As a representative example, Mn-based layered cathodes Na x [M y Mn 1−y ]O 2 (M = Li, Mg and Zn) deliver considerable capacity in the initial discharge process by employing the Mn-based and oxygen-related redox reactions simultaneously. [5] Based on the experience of Li-rich layered cathode materials, the utilization of these attractive anionic redox capacities would accompany harmful irreversible processes induced by excessive oxygen oxidation during charging process. [6] More importantly, irreversible lattice oxygen loss inevitably triggers deleterious migration of TM and structural distortion upon cycling, resulting in capacity decay, fading of the output potential and sluggish kinetics. [7] Therefore, the foremost challenge would focus on not only triggering unconventional anionic redox capacity but also stabilizing reversible these oxygen-related reactions. [5d,8] However, puzzled by foggy charge compensation mechanism between TM based and oxygenrelated redox processes, it is difficult to identify the reversible and irreversible anionic redox reactions. [9] Specifically, based on typically obtained "S-shape" discharge voltage profiles, no distinct boundary between TM and oxygen reduction reactions can be observed. [5b,10] More importantly, despite the existence of undesirable lattice oxygen loss during charging, high discharge capacities can still be obtained, which are actually caused by a reduction of TM couples beyond the pristine state. [9,11] In this case, comprehensive and in-depth characterizations including the valence change of TM and oxygen, O 2 evolution, surface/ bulk oxygen states, among others, are required to make a clear assignment of TM-based and oxygen-related redox capacity, which is essentially helpful for the development of anionic redox activity and inform subsequent materials selection. [3c,5a,12] Furthermore, in sodium-ion batteries, the trigger of anionic redox reactions is typically by the introduction of NaOM configuration, in which M is defined as the Li, Na, Mg and Zn within TM layer. [4c,5a,8,12c,13] Meanwhile, the Tarascon group has proved that lowering the energy of the transition metal d Triggering oxygen-related activity is demonstrated as a promising strategy to effectively boost energy density of layered cathodes for sodium-ion batteries. However, irreversible lattice oxygen loss will induce detrimental structure distortion, resulting in voltage decay and cycle degradation. Herein, a layered structure P2-type Na 0.66 Li 0.22 Ru 0.78 O 2 cathode is designed, delivering reversible ...

A New Approach to Stable Cationic and Anionic Redox Activity in O3‐Layered Cathode for Sodium‐Ion Batteries

“…Therefore, it is concluded that the average oxidation state of Ru may vary from 4+ to 5+ depending on structure and chemical composition, and Ru can be electroactive and nonelectroactive regardless of its initial oxidation state (Table S2, Supporting Information). [34,42,43,[46][47][48][49] Turning to the oxygen redox activity in this material, the O K-edge spectrum was measured and XPS analysis was performed for Na[Ni 2 ). In addition, the peroxo-related peak was still visible during discharge to 3 V and disappeared at 2 V, meaning that the oxidized lattice oxygen was reduced to O 2− in the Na[Ni 2/3 Ru 1/3 ]O 2 .…”

“…[22] For SIBs, the anionic redox reaction is available for not only Na-rich materials but also Na-deficient P2/P3-type compounds. [23][24][25][26][27][28][29][30][31][32][33][34][35] For example, manganese-based P2-type sodiumdeficient materials with low-valence substituents, such as Li, [26,30,31] Zn, [23,29] and Mg, [27,28] have demonstrated oxygenredox activity with good structural stability. It is generally accepted that a NaOA (A = Li, Na, Mg, Zn, or vacancy) local coordination environment is the essential requirement for oxygen redox activation in P2/O3 type layered Na x [ATM] O 2 (TM: transition metals).…”

Pseudo‐Bonding and Electric‐Field Harmony for Li‐Rich Mn‐Based Oxide Cathode

“…[ 2 ] However, the irreversible loss of lattice oxygen inevitably triggers undesirable migration of TM and structure distortion when cycling, leading to a capacity decay with faded output potential. [ 3 ] Meanwhile, the highly reactive Ni 4+ produced at high voltage would induce parasitic side reactions (SR) at the interface of electrode/electrolyte, generating cathode electrolyte interphase (CEI) derived from the electrolyte decomposition, TM migration and dissolution. [ 4 ] The SR, coupled with the irreversible oxygen loss, further induces the capacity loss and voltage fading.…”

Al‐substituted stable‐layered P2‐Na 0 . 6 Li 0 . 15 Al 0 . 15 Mn

“…The powder X‐ray diffraction (PXRD) and inductively coupled plasma (ICP, Table S3) were collected to characterize the crystal structure of the as‐synthesized P2‐NLAMO (Figure 2A). All the diffraction peaks of P2‐NLAMO are highly consistent with typical P2 phase (space group: P6 3 /mmc) 24‐27 . The crystal structure of P2‐NLAMO was further verified by XRD Rietveld refinement.…”