Abstract
The state-of-the-art Li-ion battery has energy density plateauing at ~300 Wh/kg. Replacing the graphite-based anode with Li metal is the easiest way to increase energy density. However, a lithium metal anode is prone to non-uniform plating/striping that leads to capacity decay and dendrite formation. Dendrites trigger short-circuiting and possible explosions as the liquid electrolytes that are used in Li-ion batteries are flammable. Solid-state batteries (SSBs) have the potential to enable Li-metal anodes as they are typically less reactive and nonflammable. Additionally, SSBs exhibit greater mechanical stability and can prevent dendritic growth [1,2]. Furthermore, solid electrolytes show much higher thermal stability, are non-toxic, and have high energy density, making the solid-state battery one of the best choices for the next generation of energy storage devices.
Solid polymer electrolytes are an important class of materials for making solid-state batteries commercially viable. These have the potential to increase energy density and decrease contact resistance between anode and separator by formation of a suitable solid-electrolyte-interphase (SEI) [1]. However, this technology still has major hurdles to overcome, like lower Li-ion conductivity when compared to state-of-art ceramic separators. In recent years, garnet-type lithium oxide perovskites have gained attractiveness as state-of-art ceramic separators for SSBs. LLZTO is one such ceramic electrolyte that is being thoroughly investigated by researchers as it shows very high Li-ion conductivity at room temperature [2]. However, these materials suffer from poor interfacial contact. Recently, Yang, et al., [3] combined the best of both worlds with a new type of solid polymer separator which has better physical contact between separator and lithium and good li-ion conductivity at room temperature.
In this work, we investigate the transport of Li-ions across both a solid polymer electrolyte and LLZTO solid electrolyte using a symmetric Li-cell configuration. Fig. 1 (a) and (c) show the Li plating/stripping cycling performance in a symmetric cell. The cell voltage measured during plating and stripping is to be very high for LLZTO compared to polymer electrolyte. A possible explanation may be due to high interfacial resistance arising between solid ceramic and lithium metal. Impedance spectroscopy was performed on both LLZTO and polymer separators after each current density step (24 h) and shown in Fig. 1 (b) and (d) respectively. The impedance increased with cycling for the LLZTO separator but decreased with cycling for polymer electrolyte. This may indicate that better interfacial contact between Li and polymer exists and that these connections may become more established while cycling. Furthermore, the transport of Li-ions across the separators will be analyzed using the transference number calculated using the Bruce-Vincent method. The influence of temperature and separator thickness on the transference number will also be used to characterize the nature of ion transport across such solid electrolyte separators. Such deep understanding of the transport mechanism is needed to minimize the different losses in SSBs and make it commercially viable.
Figure 1: Li plating/stripping cycling performance of the (a) LLZTO electrolyte, and (c) PEO polymer electrolyte at different current density, with 12 minutes for each plating/stripping half cycle, for a total of 72 h at 70 ℃ temperature and their corresponding impedance are shown in (b) and (d) respectively.
References
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