Stabilization of O–O Bonds by d ⁰ Cations in Li _{4+ x} Ni _{1– x} WO ₆ (0 ≤ x ≤ 0.25) Rock Salt Oxides as the Origin of Large Voltage Hysteresis

Abstract

Multinary lithium oxides with the rock salt structure are of technological importance as cathode materials in rechargeable lithium ion batteries. Current state-of-the-art cathodes such as LiNi _1/3Mn _1/3Co _1/3O ₂ rely on redox cycling of earth-abundant transition-metal cations to provide charge capacity. Recently, the possibility of using the oxide anion as a redox center in Li-rich rock salt oxides has been established as a new paradigm in the design of cathode materials with enhanced capacities (>200 mAh/g). To increase the lithium content and access electrons from oxygen-derived states, these materials typically require transition metals in high oxidation states, which can be easily achieved using d ⁰ cations. However, Li-rich rock salt oxides with high valent d ⁰ cations such as Nb ⁵⁺ and Mo ⁶⁺ show strikingly high voltage hysteresis between charge and discharge, the origin of which is uninvestigated. In this work, we study a series of Li-rich compounds, Li _{4+ x} Ni _{1– x} WO ₆ (0 ≤ x ≤ 0.25) adopting two new and distinct cation-ordered variants of the rock salt structure. The Li _4.15Ni _0.85WO ₆ (x = 0.15) phase has a large reversible capacity of 200 mAh/g, without accessing the Ni ³⁺/Ni ⁴⁺ redox couple, implying that more than two-thirds of the capacity is due to anionic redox, with good cyclability. The presence of the 5d ⁰ W ⁶⁺ cation affords extensive (>2 V) voltage hysteresis associated with the anionic redox. We present experimental evidence for the formation of strongly stabilized localized O–O single bonds that explain the energy penalty required to reduce the material upon discharge. The high valent d ⁰ cation associates localized anion–anion bonding with the anion redox capacity.