Sol-Gel Lithium Silicate Electrolyte Thin Films
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high electronic resistivity
*
high ionic conductivity (>10-6 (ohm-cm)- 1 at room temperature)
*
no defects (pinholes).
The absence of defects is critical to the reliability of any device incorporating this electrolyte. Concurrently, much work on other aspects of lithium batteries has proceeded elsewhere, because the reality of the electric car is not far away [2-4]. In particular, lithium batteries are interesting because they are lightweight, have high energy storage capacity per kilogram and potentially have high energy densities (270 Watt hour/kg). There are, however, material challenges in the assembly of such batteries and the use of inorganic solid electrolytes. One such challenge is the thin film battery where sol-gel processing may have an advantage [4]. Recall for a moment, the basic galvanic cell sketched in Figure 1. Both a rechargeable lithium battery and an electrochromic device operate on the principle of the galvanic cell. The cell consists of an anode, a cathode and an electrolyte. In a rechargeable battery, the cathode material (e.g. LixCoO2) is an intercalation compound with a layered or framework structure [5]. On charging the Li is oxidized (Li -> Li+ + e') at the cathode and reduced at the anode (Li+ + e' -> Li). The cell is reversible in order to be rechargeable. The mechanism for the rechargeable battery is referred to as the "rocking-chair cell". In this case, both the anode and cathode are intercalation compounds [6]. 189 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Charge +
Li->Li + e'
+
Li + e'->Li
0
0%--Pi 0 0/ *
Co
0
Li
Lithium Metal Solid Electrolyte
or Graphite
0- - 0o" 10
2650 C. This temperature corresponds to the melting temperature of crystalline lithium nitrate. This is a new finding in sol-gel prepared lithium silicates, and is an exciting similarity in mechanism to crystalline composites. Apparently, at low temperature, the electrostatic (dipole-dipole) interaction between lithium nitrate and the silica network is strong, and the activation energy is large. The very high value of activation energy -1.70 eV suggests that this is not associated with the "normal" ionic diffusion (hopping) mechanism usually observed in fast ionically conducting solids (
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