Optimization of Ratio and Amount of Ta Substitution in Li7La3Zr2O12 with Incorporation of Ca for Lithium Sulfur Battery

Abstract

Inorganic solid-state electrolytes are considered as the ideal electrolyte candidates for next-generation lithium battery due to their high safety without potential risks of flammability and explosion. Garnet-like structural inorganic solid Li+ conductive oxide Li7La3Zr2O12 (LLZO) received considerable attention in recent times because of its superior conductivity (~10-4 S cm-1) at room temperature and excellent stability with metallic lithium. Ta substitution for Zr is considered as the effective method to increase the ionic conductivity of LLZO. Recently, Calcium was adopted to enhance the ionic conductivity by incorporating with Ta, indicating a promising approach to higher ionic conductivity. This work investigated the effect of ratio and amount of doped Ta on the ionic conductivity of LLZO as well as the influence of Ca incorporation on Ta-doped LLZO. A modified solution method was applied to synthesis LLZO. The optimized LLZO was used in lithium sulfur battery as separator to suppress the common shuttle effect in Li-S battery. Li2CO3, La2O3, and Zr(CH3COO)4 were carefully weighed and then dissolved in acetic acid with magnetically stirring, producing homogeneous solution. Different amount of excess Li2CO3 were used to make up for the loss of Li in post heat treatment. To obtain Ta doped Li7La3Zr2O12, different amounts of Ta2O5 were added into the solution according to the stoichiometric ratio. To obtain Ta-doped Li7La3Zr2O12 with Ca incorporation, CaCO3 was dissolved into acetic acid with certain amount. The mixture was stirred and heated until fine precursor powders were precipitated. The obtained powders were heated and pressed into a thin pellet. The Li-S cells were assembled in CR2032 coin cells and pure sublimed sulfur acted as active cathode materials. The total ionic conductivity of Li7−xLa3Zr2−xTaxO12 (0 ≤ x ≤ 2) series are shown in Fig.1a. The calculated results are 7.83×10-6 S cm-1, 6.54×10-5 S cm-1, 7.85×10-5 S cm-1, 1.95×10-4 S cm-1, 8.30×10-5 S cm-1, 7.18×10-7 S cm-1, for x=0, 0.2, 0.6, 1.0 and 2.0 respectively. Although, increasing Ta content in Ta doped LLZO helps to increase the lithium vacancy concentration, which enhances the Li+ conductivity, the excessive Ta substitution will largely reduce lithium content in LLZO, resulting in a decline of ionic conductivity after peaking at 1.95×10-4 S cm-1. The optimized Ta substitution amount is 0.6 Ta per LLZO, corresponding to a formula of Li6.4La3Zr1.4Ta0.6O12. Fig. 1b exhibits the impedance plots of Ca-doped (and Ta-doped) LLZO. For comparison, corresponding patterns from Li7−xLa3Zr2−xTaxO12 (x = 0.6 and 1) are also displayed. Compared with undoped Li7La3Zr2O12, doping Ca alone is found to have little effect on ionic conductivity. Ca with selected amount (0.05 in stoichiometric ratio) is doped together with Ta (x=0.6 and 1.0) to investigate the influence on ionic conductivity. The total conductivity of Li7−x+yCayLa3-yZr2−xTaxO12 is 2.84×10-4 S/cm when x=0.6 and y=0.05, and is 1.29×10-4 S/cm when x=1.0 and y=0.05, respectively. We attributed this improvement to the increase of Li content brought by low valence Ca2+ substitution (compared to La3+), which compensates the loss of Li content caused by doping Ta. Apparently, Ta and Ca plays synergistic effect on the ionic conductivity of LLZO and best ratio from this work is represented as Li6.45Ca0.05La2.95Ta0.6Zr1.4O12 (LCLTZO). Fig.1c and Fig.1d exhibit the cycling performance of Li-S cell with and without LCLTZO. Although the capacity retention of cell with LCLTZO and the cell without solid electrolyte are similar, the coulombic efficiency of cell without solid electrolyte is extremely poor due to severe shuttle effect (Fig.1d). High coulombic efficiency (nearly 100%) is achieved by cell with LCLTZO as shown in Fig.1c. It is legitimate to deduce that LCLTZO with high ionic conductivity suppresses the shuttle effect in Li-S cell to deliver favorable cycling performance and coulombic efficiency. Figure 1