Lithium metal is considered as the “ultimate anode material”, because it is the most electropositive element vs. standard hydrogen electrode, and can provide a capacity of 3800 mAh g-1. However, there are several issues that need to be addressed before lithium metal can be applied as an anode material in practice, which includes: 1) Lithium dendrite growth problem, which may bring the potential hazard of shorting the batteries; 2) Lithium anode suffers severe volume change during intercalation-deintercalation process of cycling, which will deteriorate the cyclability of the battery; 3) The poor solid-solid contact problem of lithium anode with ceramic electrolyte can induce significant interfacial resistance, thus further decreasing the capacity; 4) Lithium metal is prone to react with organic impurities at high potential/temperature, thus decreasing the cyclability and capacity by forming an unstable SEI layer at the interface.
The lithium metal anode creates highly demanding requirements for an ideal electrolyte: 1) High conductivity; 2) high mechanical strength to stop lithium dendrite; 3) intimate contact condition at lithium anode/electrolyte; 4) adjustment to lithium volume change during cycling. The existing electrolytes, including polymer, gel-polymer and ceramic electrolyte may be superior to the exigent requirements in one aspect, but cannot satisfy all. Based on the existing research, our multilayer composite solid electrolyte may provide a versatile electrolyte, which has optimized comprehensive electrochemical properties, to address the above issues.
Figure 1 shows two types of configurations for the proposed multilayer composite electrolyte.
Figure 1: Schematic of the proposed multilayer composite electrolyte: A) strategy 1; B) strategy 2
The electrolyte in Figure 1A is composed of a thin ceramic/glass ceramic electrolyte and with an ultra-thin layer of polymer electrolyte dip coated on both sides. The ceramic/glass-ceramic electrolyte provides sufficient mechanical strength to stop the lithium dendrite growth. The dip coated polymer electrolyte can alleviate the volume change issues, and improve the solid-solid contact condition at electrode/electrolyte interface. The ceramic electrolyte may be fabricated with a porous composite structure, which can provide a skeleton of three dimensional architecture of ion transportation network. The glass-ceramic electrolyte can be fabricated as very thin sheets (< 30 µm) without losing the mechanical hardness for blocking lithium dendrite growth, and thus significantly improve its conductivity.
The electrolyte in figure 1B demonstrates another proposed design strategy for the electrolyte optimization design. The electrolyte is composed of polymer electrolyte, with an ultra-thin ceramic layer spray coated on its surface. LLZO is a good candidates considering its high stability with lithium metal, even at high temperature. The ceramic layer was closely attached on the sticky polymer electrolyte surface without affecting its flexibility. The purpose of the ceramic layer is to stabilize the lithium metal anode with electrolyte by isolating the direct contact of lithium metal with organic components. A favorable stable interface can 1) improve the cyclability of the battery by suppressing the SEI layer growth; 2) unify the intercalation-deintercalation process, thus alleviating the lithium dendrite formation issue.