Delocalized Metal–Oxygen π-Redox Is the Origin of Anomalous Nonhysteretic Capacity in Li-Ion and Na-Ion Cathode Materials

Abstract

The anomalous capacity of Li-excess cathode materials has ignited a vigorous debate over the nature of the underlying redox mechanism, which promises to substantially increase the energy density of rechargeable batteries. Unfortunately, nearly all materials exhibiting this anomalous capacity suffer from irreversible structural changes and voltage hysteresis. Non-hysteretic excess capacity has been demonstrated in Na 2 Mn 3 O 7 and Li 2 IrO 3 , making these materials key to understanding the electronic, chemical and structural properties that are necessary to achieve reversible excess capacity. Here, we use high-fidelity random-phase-approximation (RPA) electronic structure calculations and group theory to derive the first fully consistent mechanism of non-hysteretic oxidation beyond the transition metal limit, explaining the electrochemical and structural evolution of the Na 2 Mn 3 O 7 and Li 2 IrO 3 model materials. We show that the source of anomalous non-hysteretic capacity is a network of π-bonded metal-d and O-p orbitals, whose activity is enabled by a unique resistance to transition metal migration. The π-network forms a collective, delocalized redox center. We show that the voltage, accessible capacity, and structural evolution upon oxidation are collective properties of 1 the π-network rather than that of any local bonding environment. Our results establish the first rigorous framework linking anomalous capacity to transition metal chemistry and long-range structure, laying the groundwork for engineering materials that exhibit truly reversible capacity exceeding that of transition metal redox.

Recent Advances in Electrode Materials with Anion Redox Chemistry for Sodium-Ion Batteries

“…The existence of thermodynamically favorable O −• over the peroxide-like O 2 2− dimers was predicted by computations and showed that hole stabilization occurred through a (σ + π) multiorbital Mn-O bond. Similar predictions were reported in the work of Kitchaev et al, where a π-bonded Mn-d and O-p orbital network formed a collective deloca-lized redox center [81]. Therefore, the authors concluded that such a π-network rather than any local bonding environment was responsible for the two-step voltage profile with a lowvoltage hysteresis and structural evolution observed in Na 2 Mn 3 O 7 .…”

“…However, the overall stacking sequence of oxygen ions was barely changed during both steps of Na extraction/insertion, which differs from the behavior in P2/P3 cathodes, where extraction/insertion of Na lead to the formation of O-P phases with simultaneous decrease of the interlayer distance. [81]. Therefore, the authors concluded that such a π-network rather than any local bonding environment was responsible for the two-step voltage profile with a lowvoltage hysteresis and structural evolution observed in Na 2 Mn 3 O 7 .…”

“…An important alternative to improve upon the reversibility of oxygen-redox reactions is the use of vacancy-containing materials, as shown for the P2-□ Mg material in Section 2.1.2. Herein, another P-type sodium-deficient cathode Na 2 Mn 3 O 7 (Na 4/7 [□ 1/7 Mn 6/7 ]O 2 ) with □ Mn vacancies is presented[73][74][75][76][77][78][79][80][81]. The layered structure of Na 2 Mn 3 O 7 with every seven Mn site vacancies (…”

Correlating ligand-to-metal charge transfer with voltage hysteresis in a Li-rich rock-salt compound exhibiting anionic redox

“…43 ) phases. Apparently, in such ribbon or mesh superstructures, oxygen redox could proceed by involving O 2p localized holes or delocalized π states 44 that are free of the reductive coupling process, like those having redox from 'TM-O' bonding states (for example LiCoO 2 (ref. 1 ) and Na x [Ni 1/3 Mn 2/3 ]O 2 (ref.…”

Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

“…As discussed previously, [22] a possible strategy to suppress the voltage hysteresis in O-redox cathodes is to prevent O 2 formation by inhibiting TM migration; i.e., kinetically trapping the charged structures in a metastable state with bulk localised, 'split-polaron' or delocalised Oholes. [16,58,66] We suggest that this strategy is unlikely to be effective in Li 2 MnO 2 F because of relatively easy Mn migration. This is a two-fold effect, related to the high Li:TM ratio in Li 2 MnO 2 F. Firstly, a high concentration of Li will simply allow many possible vacant cation sites for Mn to migrate into on Li extraction.…”