Unveiling Oxygen Redox Activity in P2-Type Na_xNi_0.25Mn_0.68O₂ High-Energy Cathode for Na-Ion Batteries

Abstract

Na-ion batteries are emerging as convenient energy-storage devices for large-scale applications. Enhanced energy density and cycling stability are key in the optimization of functional cathode materials such as P2-type layered transition metal oxides. High operating voltage can be achieved by enabling anionic reactions, but irreversibility of O2–/O2 n–/O2 evolution still limits this chance, leading to extra capacity at first cycle that is not fully recovered. Here, we dissect this intriguing oxygen redox activity in Mn-deficient Na x Ni0.25Mn0.68O2 from first-principles, by analyzing the formation of oxygen vacancies and dioxygen complexes at different stages of sodiation. We identify low-energy intermediates that release molecular O2 at high voltage, and we show how to improve the overall cathode stability by partial substitution of Ni with Fe. These new atomistic insights on O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials.

Hysteresis‐Suppressed Reversible Oxygen‐Redox Cathodes for Sodium‐Ion Batteries

“…Returning to P2-type materials, in order to increase operation voltage and suppress voltage hysteresis, our group circumvented the low operation voltage of P2-Na x [Zn y Mn 1−y ]O 2 by substituting half of Zn 2+ with Ni 2+ , resulting in a high average voltage of approximately 3.5 V on discharge. [34] P2-Na 0.67 [Ni 0.33 Mn 4+ 0.67 ]O 2 [43][44][45][46][47] and P2-Na 0.67 [Li 0.22 Mn 4+ 0.78 ]O 2 [14,17,18,29,30,36,39] are the most studied sodium layered materials. The former compound has high operation voltage and lower voltage hysteresis, while the P2-O2 phase transformation with severe volume change is inevitable on charge, which leads to poor cycling performance.…”

Ab initio determination of a simultaneous dual-ion charging mechanism for Ni0.25Mn0.75O2 through redox reactions of Ni2+/Ni4+ and O2−/O−

“…7c presents the electronic structure of the fully perchlorated structure. From the claim of Li et al, 25 and others in literature, 46,47,50,[106][107][108][109] it was expected to see further redox activity of O(p) states. Comparing Fig.…”

“…This shi to higher energies is indicative of O atoms compensating for the charge removed during cation deintercalation, as previously reported. 46,49,50,[106][107][108][109][110] Concurrent with this change, Ni(d) states have emptied and shied to the conduction band (around 1 to 2 eV), indicating Ni atoms are compensating for charge removal during cation deintercalation. 46,47,50,108,110,111 Lastly, Fig.…”

“…One must follow ligand eld theory 100 (the more accurate modication of crystal eld theory 112,113 ) to take this spin moment and determine the oxidation state it corresponds to. 49,50,[106][107][108]111 A description of this process is given in Section S7, † where electronic congurations for Ni and Mn are presented in Fig. S8 and S9, † respectively.…”

Replacement of Cobalt in Lithium-Rich Layered Oxides by n-Doping: A DFT Study

“…To model transition metal disorder in multicomponent TM layers in the LNM supercell, we have adopted the special quasi-random structure (SQS) approach [50,51]: this method allows for the modeling of a random solid solution in a supercell of the desired size by mimicking random correlation functions up through nearest-neighbor, next-nearestneighbor interactions, and so on. The SQS method relies on the cluster expansion (CE) formalism proposed by Mayer [52].…”