Understanding the fundamentals of redox mediators in Li-O2 batteries: a case study on nitroxides.

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.

TEMPO: a mobile catalyst for rechargeable Li-O2 batteries.

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.

Strontium and bisphosphonate coated iron foam scaffolds for osteoporotic fracture defect healing.

The purpose of this work was to investigate new bone formation in macroporous iron foams coated with strontium (FeSr) or bisphosphonate (FeBiP) compared to plain iron foam (Fe) and empty defect in a critical size metaphyseal bone defect model in ovariectomized rats. 60 female rats were subjected to bilateral ovariectomy and multi-deficient diet for 3 months. A 4 mm wedge shaped metaphyseal osteotomy was created, fixed with a mini-plate and subsequently filled with Fe, FeSr, FeBiP or left empty. After 6 weeks, µCt analysis revealed a statistically significant increased bone formation at the implant interface in FeSr compared to FeBiP (p = 0.035) and Fe (p = 0.002), respectively. Increased mineralized tissue was also seen within the pores in FeSr (p = 0.023) compared to Fe. Histomorphometry revealed significantly increased bone formation at the implant interface in FeSr (p < 0.001) and FeBiP (p = 0.006) compared to plain Fe with increased osteoblast and decreased osteoclast activity in combination with increased BMP2 and decreased RANKL/OPG in immunohistochemistry. ToF-SIMS analysis showed overlapping Ca signals with Fe for both FeSr and FeBiP thereby indicating tissue in-growth into the scaffolds. In conclusion, iron foam with strontium or bisphosphonate coating are of further interest in metaphyseal fracture defects in osteopenic bone.

Evolution of Li2O2 growth and its effect on kinetics of Li-O2 batteries.

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.