Synthesis of Li3BO3 Based Glass-Ceramic Electrolytes and Their Application of All-Solid-State Batteries

Abstract

Lithium ion batteries are widely used in many electronic devices such as cellular phone and personal computer. Recently, lithium ion batteries have been used for large scale energy storage systems and electronic vehicles. Safety and reliability of batteries are important factors in large scale batteries. Flammable organic liquid electrolytes are used in conventional lithium ion batteries, which have risks of leakage, fire and explosion. All-solid-state lithium batteries using nonflammable inorganic solid electrolytes are one of the solutions for improving safety of rechargeable energy sources. In order to realize all-solid-state batteries with high energy density, development of active materials and solid electrolytes are needed. Sulfide electrolytes showed high lithium ion conductivities of around 10-4-10-2 S cm-1 at room temperature[1]. However, sulfide electrolytes are not stable under ambient atmosphere with generating hydrogen sulfide from the electrolytes. On the other hand, oxide electrolytes are attractive for application in batteries with safety. In general, it is difficult to use oxide electrolytes in all-solid-state batteries due to their poor deformability at room temperature, which results in a huge interfacial resistance between electrolytes and electrodes [2]. Among oxide electrolytes, glass electrolytes with low melting point such as Li3BO3 glass are promising materials for obtaining good contact between electrolytes and electrodes. The Li3BO3 glass showed the relative packing density of 71 % and the conductivity of 3.4×10-7 S cm-1 by pressing at room temperature [3]. Lithium ion conducting glasses based on Li3BO3 were prepared by rapid melt-quenching and mechanochemical techniques [4, 5]. In the binary system Li3BO3-Li2SO4, the 90Li3BO3·10Li2SO4 (mol%) glass exhibited a packing density of 85%. By heating this glass, a solid solution based on a high temperature phase of Li3BO3 was precipitated. The glass-ceramic electrolyte showed the conductivity of 9.4×10-6 S cm-1 at room temperature. Moreover, the Li3BO3-Li2CO3 glasses were also prepared by mechanical milling. In this system, a solid solution based on Li2CO3 was precipitated by heating the milled 20Li3BO3·80Li2CO3 electrolyte at 500oC, and this electrolyte showed the conductivity of 1.2×10-6 S cm-1at room temperature. In this study, glasses in the ternary system Li3BO3-Li2SO4-Li2CO3 were prepared by mechanochemical techniques, and their glass-ceramics were obtained by heat treatment. Especially, the 33Li3BO3·33Li2SO4·33Li2CO3 (mol%) glass has a packing density of 90%, which is comparable to that of sulfide electrolytes such as Li3PS4 glass [2]. By heating this glass, thermodynamically metastable phase was obtained and this glass-ceramic electrolyte showed the conductivity of 1.8×10-6 S cm-1 at room temperature. Although the conductivity of the ternary glass-ceramic was lower than that of 90Li3BO3·10Li2SO4 glass-ceramic electrolyte, the formability of the glass was higher in the ternary one. All-solid-state (In / LiNi1/3Mn1/3Co1/3O2) batteries using 90Li3BO3·10Li2SO4 or 33Li3BO3·33Li2SO4·33Li2CO3 glass-ceramic electrolytes were fabricated. These cells operated as secondary batteries at 100oC. The cell with the 33Li3BO3·33Li2SO4·33Li2CO3 electrolyte showed the initial discharge capacity of around 150 mAh g-1, which is larger than that of the cell with 90Li3BO3·10Li2SO4 electrolyte (100 mAh g-1). The higher capacity would be achieved by better formability of the ternary electrolyte. It is noted that good formability of the electrolytes is effective in increasing contact areas between active materials and electrolytes was obtained.