Structural Evolution of Solid-State Li2O2 with Reduced Charge Overpotential in Li−O2 Batteries

Abstract

Reducing charge-discharge overpotential is helpful to enhance efficiency and cyclability of Li−O2 batteries. In a common non-aqueous Li−O2 cell, the overpotential is largely related to the formation and decomposition processes of solid-state Li2O2. Although several intrinsic properties of Li2O2 have been considered to contribute to the high overpotential, such as the low electrical conductivity, slow dynamics, large particle aggregations, and side-reaction products, et.al,1-4 a fundamental study of specific relationship between overpotential and electrochemical reactions associating with structural evolution of Li2O2 has yet to be undertaken. Besides, the underlying mechanism of the oxygen evolution reaction on charge remains less understood.To address the above problems, in this work, we proposed a Pt modification strategy to tune cathode architectural to optimize catalytic and electrical properties as well as electrode dynamics. We employed two different approaches of magnetron sputtering and thermal reduction to realize the Pt surface-coating and bulk-doping, respectively. Both could significantly reduce the charge overpotentials of Li−O2 cells with a higher performance from the bulk-doped catalyst. Meanwhile, systematic studies showed an unprecedented relevancy between overpotential and structural evolution of Li2O2, and the Pt nano-composition in the cathode was found to directly affect the formation and decomposition mechanism of Li2O2. Furthermore, we carried out the density functional theory calculations that provided molecular insights into the catalytic role of Pt and Pt3Co nanocrystals (resulting from surface-coating and bulk-coating, respectively) in reducing the charge overpotential. As a result, the nanoscale bulk-doping approach was demonstrated to be a promising strategy to address the insufficient catalytic and electrical properties and sluggish electrode dynamics of oxygen cathodes for Li−O2 batteries. The exclusive insight into structural evolution of Li2O2 with the reduced charge-discharge overpotential could afford favorable theoretical investigates for further explorations on cathode materials. Figure 1 Electrochemical discharge/charge profile and corresponding formation and decomposition mechanism of Li2O2 for the Pt bulk-doping catalyst. References 1 Freunberger S A, et al. Reactions in the rechargeable lithium–O2 battery with alkyl carbonate electrolytes. J. Am. Chem. Soc., 2011, 133, 8040-8047. 2 Thotiyl M M O, et al. A stable cathode for the aprotic Li–O2 battery. Nat. Mater., 2013, 12, 1050-1056. 3 Chen Y, et al. Charging a Li–O2 battery using a redox mediator. Nat. Chem., 2013, 5, 489-494. 4 Lim H-D, et al. Rational design of redox mediators for advanced Li–O2 batteries. Nat. Energy, 2016, 1, 16066. Figure 1