Nonaqueous Li-O2 batteries are an intensively studied future energy storage technology because of their high theoretical energy density. However, a number of barriers prevent a practical application, and one of the major challenges is the reduction of the high charge overpotential: Whereas lithium peroxide (Li2O2) is formed during discharge at around 2.7 V (vs Li(+)/Li), its electrochemical decomposition during the charge process requires potentials up to 4.5 V. This high potential gap leads to a low round-trip efficiency of the cell, and more importantly, the high charge potential causes electrochemical decomposition of other cell constituents. Dissolved oxidation catalysts can act as mobile redox mediators (RM), which enable the oxidation of Li2O2 particles even without a direct electric contact to the positive electrode. Herein we show that the addition of 10 mM TEMPO (2,2,6,6-tetramethylpiperidinyloxyl), homogeneously dissolved in the electrolyte, provides a distinct reduction of the charging potentials by 500 mV. Moreover, TEMPO enables a significant enhancement of the cycling stability leading to a doubling of the cycle life. The efficiency of the TEMPO mediated catalysis was further investigated by a parallel monitoring of the cell pressure, which excludes a considerable contribution of a parasitic shuttle (i.e., internal ionic short circuit) to the anode during cycling. We prove the suitability of TEMPO by a systematic study of the relevant physical and chemical properties, i.e., its (electro)chemical stability, redox potential, diffusion coefficient and the influence on the oxygen solubility. Furthermore, the charging mechanisms of Li-O2 cells with and without TEMPO were compared by combining different electrochemical and analytical techniques.

Li 7 La 3 Zr 2 O 12 (LLZO)-based garnet materials are recently being investigated as suitable electrolytes for solid-state batteries with lithium-metal electrodes. Unfortunately, lithium-metal penetration through polycrystalline garnet-type electrolytes limits the electric current density during cell charging. In this study, we introduce an electrochemical operando approach that is well suited to get insights into the early stage of lithium-metal penetration that was yet only accessible with very elaborate neutron measurements. Combined with in situ as well as ex situ electron microscopic techniques, we investigate the morphological instability of the lithium-metal anode on garnet-type solid electrolytes under cathodic load and demonstrate the inter-relationship between microkinetic aspects and lithium-penetration susceptibility.

Lithium peroxide (Li2O2), the solid and intrinsically electronic insulating discharge product of Li-O2 batteries strongly influences the discharge and charge kinetics. In a series of experiments, we investigated the growth of Li2O2 upon discharge and the corresponding reduction and oxidation processes by varying the depth of discharge. The results indicate that insulating Li2O2 particles with a disc-like shape were formed during the initial discharge stage. Afterward, the nucleation and growth of Li2O2 resulted in the formation of conducting Li2O2 shells. When the discharge voltage dropped below 2.65 V, the Li2O2 discs evolved to toroid-shaped particles and defective superoxide-like phase presumably with high conductivity was formed on the rims of Li2O2 toroids. Both Li2O2 and the superoxide-like phase are unstable in ether-based electrolytes resulting in the degradation of the corresponding cells. Nevertheless, by controlling the growth of Li2O2, the chemical reactivity of the discharge product can be suppressed to improve the reversibility of Li-O2 batteries.

To significantly increase the energy density of lithium-based batteries, the use of lithium metal as an anode is an option despite all of the associated challenges. Due to its high reactivity, lithium is covered with a passivation layer that may affect cell performance and reproducibility of electrochemical characterization. In most studies, this is ignored and lithium metal is used without considering the passivation layer and carrying out a proper characterization of the surface. Against this background, we systematically characterized various lithium samples with X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and complementary energy-dispersive X-ray spectroscopy (EDX), resulting in a complete three-dimensional chemical picture of the surface passivation layer. On all analyzed lithium samples, our measurements indicate a nanometer-thick inorganic passivation layer consisting of an outer lithium hydroxide and carbonate layer and an inner lithium oxide-rich region. The specific thickness and composition of the passivation layer depend on the treatment before use and the storage and transport conditions. Besides, we offer guidelines for experimental design and data interpretation to ensure reliable and comparable experimental conditions and results. Lithium plating through electron beam exposure on electrically contacted samples, the reactivity of freshly formed lithium metal even under ultrahigh-vacuum (UHV) conditions, and the decomposition of lithium compounds by argon sputtering are identified as serious pitfalls for reliable lithium surface characterization.

The development of aprotic lithium-oxygen (Li-O2) batteries suffers from high charging overvoltages. Dissolved redox mediators, like nitroxides, providing increased energy efficiency and longer lifetime are promising tools to overcome this challenge. Since this auspicious concept is still in its infancy, the underlying chemical reactions as well as the impact of the different (electro)chemical parameters are poorly understood. Herein, we derive an electrochemical model for the charging reactions, which is validated by potentiostatic measurements. The model elucidates the impact of the major factors including basic cell parameters and the chemical properties of the redox mediator. The model is applied to the promising class of nitroxides, which is systematically investigated by using derivatives of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), AZADO (2-azaadamantane-N-oxyl), and an azaphenalene based nitroxide. The nitroxides are electrochemically characterized by cyclic voltammetry and their performance as redox mediators is studied in Li-O2 batteries with an ether-based electrolyte. Based on the presented model, the charging profiles of the different nitroxide redox mediators are correlated with their molecular structures.