Effect of glass former composition on the conductivity and network structure of lithium borophosphate glass electrolyte for solid state thin film battery

Abstract

During the last two decades, lithium-based glasses have been studied extensively as solid electrolytes for solid electrolytes for solid-state secondary batteries. However, solid-state lithium batteries have not been widely used because solid electrolyte has not completely satisfied the requirements for practical application. [1] The major limitation of glass electrolyte is the low lithium ion mobility while the electrochemical stability and the chemical inertness against the active materials employed lithium battery are regarded as major advantages. The application of the glass electrolyte to conventional bulk type battery, hence, is impractical in spite of its merits and the researches on this group of materials are diminishing recently. However, if the lithium ion resistance is reduced to the reasonable level, the glass electrolyte can be a good candidate for solid state electrolyte. The reduction of resistance can be achieved by reducing the thickness of electrolyte to a few micrometers range, which can be easily achieved by thin film fabrication technologies and is necessary for the thin film battery. Among various inorganic glasses, the lithium borophosphate oxide glass system is regarded as the most suitable thin film electrolyte due to its relatively high ionic conductivity, stable chemical properties, and the ease of thin film fabrication. Although the lithium borophosphate oxide glass system has been studied for long time as a candidate for solid state electrolyte, the detailed and rigorous study on the conductivity in wide range of composition is not found in literature when one is intended to use as a basic engineering data. In this study, the conductivity of the lithium borophosphate oxide glass system was studied in full range of composition of glass former and modifier to prepare the basic engineering data prior to the fabrication of thin film electrolyte of the same composition. Also, the structural variation of the oxide amorphous electrolytes of Li2O-B2O3-P2O5 system was investigated by Raman spectroscopy to find out the correlation with the electrical conductivity and to seek the best composition among the various compositions of this material system, for the deposition of thin film electrolytes. xLi2O-(1-x)(yB2O3-(1-y)P2O5) glasses were prepared by melt quenching method. The starting materials were mixed in proportions appropriate to from 20g batches. Each batch was melted at 1000 oC for 6h in Pt crucibles in an electrically heated furnace. AC conductivity was measured by complex impedance method. The correlation between the structural modification of glass network and conductivity was characterized by FTIR (IR Spectroscopy: Magna-IR 760 Spectrometer) and Raman spectroscopy as a function of the chemical composition in the range from 400 to 4000 cm-1. The Raman spectra of the glasses were performed using a laser Raman Spectrophotometer (JASCO NRS-3100) and a conventional counting system in 400-1800cm-1 range under air atmosphere. Electrical properties of the as-quenched glasses were characterized using complex impedance technique. In Fig.1, the lithium-ion conductivities of xLi2O-(1-x)(yB2O3-(1-y)P2O5) glasses as functions of molar ratio of network formers (B2O3 and P2O5) is shown. The ionic conductivity exhibited the maximum at B2O3 : P2O5 = 1:1. Generally, the conductivity was enhanced as network modifier contents increased due to an increase in both lithium ion concentration and non-bridging oxygen (NBO) generating the electrostatic attraction to lithium ions. However, when the B/P ratio was 2 at 40~47 mol% Li2O concentration, this glass system exhibited precipitous drop of conductivity, probably due to structural change of glass. Raman study revealed that (B-O-P)- groups containing a non-bridging oxygen may be unstable and the concentration is decreased. Instead, the characteristic peaks due to (PO3) nn- chains appears at this composition, which suggests that the tendency to depolymerization or devitrification of glass network is increased. [2]