Li3PO4 Layer Research Articles

High-capacity phase formation by surface modification of Li3PO4 on nanosized Li2RuO3 electrode for lithium batteries

Effects of modifying the surface of lithium excess layered rock-salt type electrodes by Li3PO4 is investigated using epitaxial Li2RuO3 model electrodes. A 3.6-nm-thick amorphous Li3PO4 layer is deposited on a 25.5-nm-thick Li2RuO3 film by pulsed laser deposition. X-ray absorption near edge structure reveals that the modified Li2RuO3 surface had different electronic states of Ru from the unmodified Li2RuO3 surface, indicating that Li3PO4 deposition changes the structure of the Li2RuO3 surface. Li3PO4-modified Li2RuO3 has a much higher first discharge capacity (296mAhg−1) between 2.8 and 4.2V than unmodified Li2RuO3 (190mAhg−1). The modified and unmodified Li2RuO3 have irreversible capacities in the first charge/discharge process of 22 and 148mAhg−1, respectively. The surface modification induced by Li3PO4 deposition enhances the structural stability of the Li2RuO3 surface during the initial charging process.

Interface properties of a Li3PO4/Al cathode in organic light emitting diodes

Recently Li3PO4/Al has been introduced as an alternative cathode for the commonly used LiF/Al system for organic light emitting diodes (OLEDs) due to its competitive electron injection properties. In the present article the interfaces of the organic semiconductor with the Li3PO4/Al bilayer cathode are investigated using photoelectron spectroscopy to elucidate the origin behind the efficient electron injection. Therefore, a thick Li3PO4 layer was vacuum deposited onto an indium tin oxide substrate and characterized in order to learn about the stoichiometry of evaporated Li3PO4. During evaporation Li3PO4 decomposes, forming a layer consisting of P2O5 and LiPO3. In a second step the interface between Li3PO4 and Alq3 [tris(8-hydroxyquinoline) aluminum] was investigated, whereupon Li3PO4 coverage Alq3 molecules decompose, forming aluminum oxide or aluminum phosphate leaving 8-quinolinol molecules behind. A similar reaction occurs at the Li3PO4/Al interface where again an oxidation of the metallic aluminum points toward the formation of aluminum oxide or phosphate. A work function lowering of up to 180 meV observed once Al was covered by Li3PO4 is likely to be caused by the polar nature of all generated or deposited species. A simple estimate of the surface potential drop caused by those dipoles can account for the work function lowering and explain the highly efficient electron injection property of the Li3PO4/Al cathode in an OLED.

The Li3PO4/Al bilayer: An efficient cathode for organic light emitting devices

In this contribution an efficient cathode material for organic light emitting diodes (OLEDs) is introduced consisting of a thin layer of the metal salt lithium phosphate (Li3PO4) deposited between the organic semiconductor and an Al cathode. The bilayer cathode Li3PO4/Al enables a device performance of small molecule based OLEDs competitive to the benchmark cathode LiF/Al. While current densities and luminances of both systems are alike, the use of Li3PO4 substantially increases the device lifetime. It will be shown that the improved device characteristics can be ascribed to a stably enhanced electron injection. We demonstrate that neither a field enhancement across the Li3PO4 layer due to accumulated holes nor a possible charge transfer doping by the Li3PO4 is the reason for the improved electron injection. Investigation of the Li3PO4/Al interface by Kelvin probe techniques disclosed a work function lowering of the cathode that facilitates electron injection and finally explains the improvement in performance of respective organic light emitting diodes.

Stability of LiFePO4 in water and consequence on the Li battery behaviour

The stability of LiFePO4 in water was investigated. Changes upon exposure to water can have several important implications for storage conditions of LiFePO4, aqueous processing of LiFePO4-based composite electrodes, and eventually for utilisation in aqueous lithium batteries. A Li3PO4 layer of a few nanometers thick was characterised at the LiFePO4 grains surface after immersion in water, accompanied by an increase of FeIII percentage in the grains. For first charge–discharge cycles in a lithium battery, no effect was observed on electrochemical performances for a sample of LiFePO4 immersed for 24 h at a concentration of 50 g L−1 without any pH modification. To limit the aging of LiFePO4 during aqueous electrode processing, it is advised to reduce the immersion duration, to concentrate the LiFePO4 suspensions, and not to modify the pH. In addition, since immersion in water mimics an accelerated exposure to air humidity, LiFePO4 should be stored in a dry atmosphere.