Impact of TM-O Bonding Covalency on the Structure and Performance of Li-Rich Layered Oxide Positive Electrodes for Li-Ion Batteries

Abstract

The application of positive electrode (cathode) materials with anionic redox activity is hoped to improve the energy density of future Li-ion batteries, and thus facilitate the mileage issue in rapidly growing electric transport industry. Such positive expectations are dictated by promising application potential of the Li4/3-xNi2+ xMn4+ 2/3-xCo3+ xO2 layered oxides demonstrating outstanding reversible discharge capacity exceeding 250 mAh/g and specific energy density of > 1000 Wh/kg. Such outstanding values are the result of joint participance of cationic (Ni2+→Ni3+→Ni4+, Co3+→Co4+) redox transitions and processes of O2 n-/2O2- units formation in charge compensation mechanism upon Li (de)intercalation.Nevertheless, oxygen redox chemistry in battery materials is a «double-edged sword», since it`s associated with practical drawbacks like sluggish kinetics, voltage hysteresis, voltage fade and safety worries alongside greatly improved energy density. At the same time, the exact nature of partially oxidized oxygen species is still raising intensive debates as well as the role of TM-O bonding in oxygen oxidation reversibility and interplay between anionic redox and accompanying bulk and local structure transformations and their accumulation during prolonged cycling.In our work, we probed the anionic redox properties as a function of electronic structure and chemical bonding substituting a small fraction of 3d-metals with Ru in a parent Li1.2Ni0.2Mn0.6O2 according to xLi2RuO3-(1-x)Li1.2Ni0.2Mn0.6O2 solid solution system (x is up to 0.1). This approach allowed us to gently tune the ratio between the contributions of the cationic and anionic redox whereas employing the same mixed Ni-Mn carbonate precursor excludes the impact of different sample morphology on the electrochemical behavior, providing a legitimate justification to attribute all the observed effects solely to crystal structure and TM-O bonding character. Both experimental results and theoretical calculations demonstrated that gradual increasing of Ru content drastically changes electrochemical behavior of the Li-rich layered oxides improving the reversibility of the oxygen redox thus suppressing irreversible oxygen oxidation at the first charge. In turn, diminished gaseous O2 evolution led to the retardation of “structural densification” as well as concomitant mitigation of Mn redox activity and changes in spatial distribution of the reduced Mn species. Moreover, despite the discharge voltage fade did not differ much for compounds with different Ru concentration, Ru doping surely decreased the charge voltage fade and, surprisingly, total discharge capacity likely due to inhibited Mn4+/3+/2+ redox activity in the Ru-doped compounds. In our report we will present the whole set of observations on xLi2RuO3-(1-x)Li1.2Ni0.2Mn0.6O2 model system aimed to trace the whole chain of events from increasing the covalency of the TM-O bond, suppressing irreversible oxygen oxidation and appearance of the reduced Mn species to retarding the structure densification in the Li-rich layered oxides.