Composite electrolytes ceramic Li7La3Zr2O12/glassy Li2O-Y2O3-SiO2
Graphical abstract
Introduction
Solid electrolytes are widely used in electrochemical devices. Among these materials, electrolytes with the lithium-ion conductivity necessary for new lithium power sources with superior energy capacities are special [1], [2], [3]. One of the most promising solid electrolytes for lithium power sources is Li7La3Zr2O12 (LLZ), which was synthesized by Weppner [4]. LLZ has two structural modifications: cubic (total conductivity ∼10−4 S cm−1 at 25 °C) and tetragonal (∼10−7 S cm−1 at 25 °C). Its conductivity values are comparable at 300 °C [5], [6]. However, it was established that only the addition of a dopant can stabilize the cubic modification of Li7La3Zr2O12 [7], [8], [9].
Researchers have introduced various sintering additives, including glass, into cubic LLZ to reduce the temperature and time of sintering and to increase the total conductivity of the electrolytes [10], [11], [12], [13], [14]. For example, the addition of 1 wt% Li3PO4 glass to Li6.75La3Zr1.75Ta0.25O12 reduces the sintering time from 36 to 6 h and increases the total conductivity of the electrolyte to 7.2 × 10−4 S cm−1 at room temperature [14]. The addition of Li4SiO4 glass to Li6.16Al0.28La3Zr2O12 results in the maximal total conductivity (6.1 × 10−4 S cm−1 at 33 °C) and a relative density of 96% [10].
Previously, we studied the effect of lithium borosilicate glass (65Li2O·27B2O3·8SiO2) on the properties of Li7La3Zr2O12 with tetragonal structure [15]. We showed that the addition of 3 wt% glass increased the total conductivity of the samples by two orders of magnitude from 2.6 × 10−7 to 1.5 × 10−5 S cm−1 at 25 °C. Lithium yttrium-silicate glass with the composition 40.2Li2O·5.7Y2O3·54.1SiO2 (LYS) has been selected for further investigations [16]. The total conductivity of this glass at room temperature is comparable to that of tetragonal LLZ (∼10−7 S cm−1). The advantage of this glass is its chemical stability when it is in contact with molten lithium and air. Here, we examine the effect of adding 40.2Li2O·5.7Y2O3·54.1SiO2 glass on the density and electrical conductivity of the tetragonal modification of LLZ.
Section snippets
Experimental
Li2CO3, La2O3 and an aqueous solution of ZrO(NO3)2 were used as starting components for citrate-nitrate synthesis of tetragonal Li7La3Zr2O12. The reagents were mixed in the stoichiometric ratio, except Li2CO3, which was used with a 10 wt% excess, as in Murugan et al. [4]. The synthesis has been described in detail in the literature [15]. Lithium carbonate (Li2CO3), yttria (Y2O3) and silica (SiO2) were used as starting materials to obtain the glass. Appropriate amounts of the starting materials
The effect of sintering temperature
We studied the glass that we obtained using XRD to investigate its amorphous structure. The composites LLZ – 1 wt% LYS were produced at different sintering temperatures (900–1200 °C). Fig. 1 shows the XRD patterns of the obtained material after calcination. The reflections on the XRD patterns of the composites annealed at 900 and 1000 °C correspond to the tetragonal modification of LLZ. The transition of LLZ from tetragonal to cubic structure occurs as the sintering temperature increases. However,
Conclusions
We have synthesized composites in the Li7La3Zr2O12-x wt% glass 40.2Li2O·5.7Y2O3·54.1SiO2 (x = 1–10) system. The composites with x = 1 were obtained at different sintering temperatures (900–1200 °C). A temperature of 1200 °C was selected for the subsequent annealing of the samples because at this temperature the maximal relative density of the samples (78%) and the maximum total conductivity (3.2 × 10−6 S cm−1) at 25 °C were achieved. With further addition of the glass, the relative density of the samples
Acknowledgment
The research has been carried out with the equipment of the Shared Access Center “Composition of Compounds” of the Institute of High-Temperature Electrochemistry.
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