Abstract
Polymer electrolytes containing Li-ion conducting fillers are among the extensively investigated materials for the development of solid-state Li metal batteries. The practical realization of these electrolytes is, however, impeded by their low Li-ion conductivity, which is related to the filler and the interplay between the filler and the polymer. Therefore, we performed an in-depth analysis on the influence of the filler content (0, 10, and 20 wt%) and filler morphology (particles and nanowires) on the electrical and electrochemical properties of the PEO-based composite electrolyte using a wide spectrum of characterization techniques, such as 3D micro-X-ray computed tomography, cross-sectional scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry, impedance spectroscopy, and galvanostatic cycling. The studies reveal that the filler materials are well distributed within the membranes, without any indications for the formation of agglomerates. For 10 wt% filler, a decrease in the crystallinity compared to PEO was observed, in contrast to 20 wt% filler showing an increase in crystallinity. Impedance spectroscopic studies on the Li-ion conductivity of the membranes have shown that the change in the Li-ion conductivity is solely related to the change in the crystallinity, rather than to the participation of LLZO as an active transport mediator. The PEO membranes containing 10 wt% LLZO have been tested in terms of their rate capability in symmetrical Li cells by galvanostatic cycling. A critical current density of up to 1 mA cm−2 at 60°C was observed.
Highlights
With the increasing sophistication of portable electronic and electric vehicles, there have been continuous developments in energy storage devices, and in the past few years, there have been significant changes in the conventional Li-ion battery system (Li et al, 2020a)
The loss of Li is considerably faster for nwLLZO due to their large surface area compared to pLLZO
Despite the presence of the intermediate pyrochlore phase and the absence of a doping element, nwLLZO was present in the cubic phase, which is related to the high surface energy of nanoparticles due to their high curvatures
Summary
With the increasing sophistication of portable electronic and electric vehicles, there have been continuous developments in energy storage devices, and in the past few years, there have been significant changes in the conventional Li-ion battery system (Li et al, 2020a). In order to make a percolating network, several groups focused on the design of membranes composed of LLZO nanowires Those studies reported a significant increase in the room temperature Li-ion conductivity by two orders of magnitude by adding only 5 to 10 wt% of LLZO (Yang et al, 2017; Wan et al, 2019). To better understand the role of filler morphology (particles vs nanowires) in PEO on the Li-ion transport, we synthesized Li6.4Ga0.2La3Zr2O12 (pLLZO) particles and Li7La3Zr2O12 (nwLLZO) by ceramic sintering and electrospinning, respectively. The homogeneous distribution of ceramic filler allowed reversible cycling of Li with rates as high as 1 mA cm−2 at 60°C for the PEO membranes containing 10 wt% nwLLZO. To prepare the pLLZO powder for the polymer membranes, high Li-ion conductive pellets were crushed and ball-milled following a similar procedure as described above. Symmetrical cells with disc-shaped Li electrodes and CE were assembled into Swagelok-type cells in an argon-filled glove box (O2 and H2O < 0.1 ppm) at room temperature
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