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The large overpotential of nonaqueous Li−O 2 batteries when charging causes low round-trip efficiency and decomposition of the electrode materials and electrolyte. The origins of this overpotential have been enthusiastically explored to date; however, a full understanding has not yet been reached because of the complexity of multistep reaction mechanisms. Here, we applied structural and electrochemical analysis techniques to investigate the reaction step that results in the increase of the overpotential when charging. Rietveld refinement of ex situ powder X-ray diffraction showed that a Li-deficient phase of Li 2 O 2 , Li 2−x O 2 , formed when discharging and was present over the course of charging. The galvanostatic intermittent titration technique revealed that the rate-determining process in the first step of charging was a solid−solution type of delithiation. The chemical diffusion coefficient of Li + ions in Li 2−x O 2 , D Li , decreases as the cell voltage increases, which in turn leads to a decrease in the oxidation rate of Li 2−x O 2 . Under galvanostatic conditions, the deceleration of oxidation induces further increase of the cell voltage; therefore, an intrinsic mechanism of positive feedback to increase the cell voltage occurs in the first step. The results demonstrate that the continuity of the first step can be extended by the suppression of changes in any of the elements of the positive feedback loop, i.e., the oxidation rate, cell voltage, or D Li .

A novel indirect charging system that uses a redox mediator was demonstrated for Li-O batteries. 4-Methoxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl (MeO-TEMPO) was applied as a mediator to enable the oxidation of LiO, even though LiO is electrochemically isolated. This system promotes the oxidation of LiO without parasitic reactions attributed to electrochemical charging and reduces the charging time.

By increasing the pore and particle size of porous TiO 2 films, the photovoltaic properties of solid-state dye-sensitized solar cells were dramatically improved. We analyzed the relation between the photovoltaic property and the microstructure of TiO 2 /Dye/CuI measured by scanning electron microscopy of the cross section of the cells. The larger pore size of the TiO 2 films improved the CuI filling and the high degree of filling improved the short-circuit current density. Multilayered TiO 2 films consisting of a TiO 2 layer with small TiO 2 particles and a second layer with larger TiO 2 particles gave a power conversion efficiency under 1 sun of 6.0%. #

In non-aqueous lithium-oxygen batteries, the one-electron reduction of oxygen and subsequent lithium oxide formation both occur during discharge. This lithium oxide can be converted to insulating lithium peroxide via two different pathways: a second reduction at the cathode surface or disproportionation in solution. The latter process is known to be advantageous with regard to increasing the discharge capacity and is promoted by a high donor number electrolyte because of the stability of lithium oxide in media of this type. Herein, we report that the cathodic oxygen reduction reaction during discharge typically exhibits negative differential resistance. Importantly, the magnitude of negative differential resistance, which varies with the system component, and the position of the cathode potential relative to the negative differential resistance determined the reaction pathway and the discharge capacity. This result implies that the stability of lithium oxide on the cathode also contributes to the determination of the reaction pathway.