Controlling Ionic Transport through the PEO-LiTFSI/LLZTO Interface

Abstract

A highly resistive Polyethylene oxide-LiTFSI(PEO-LiTFSI)/ Lithium Lanthanum Zirconium Oxide (LLZO) interface, with a resistivity of 95 kOhms.cm2 (30°C) is believed to limit the total conductivity of ceramic-polymer composite electrolyte (CPE). To achieve higher ionic conductivity, the interfacial impedance (Rinterface) must be reduced to <~100 Ohms.cm2 to enable cell impedances comparable to Li-ion technology. The goal of this study was to investigate the origin of this high Rinterface. It was hypothesized that LLZO surface impurities and abrupt changes in Li-ion concentration between the PEO-LiTFSI/LLZO electrolytes contribute to the high impedance. By removing surface impurities through heat-treatment, the Rinterface was reduced to 180 Ohms.cm2 at 30°C. Optimization of Li-salt concentration in PEO to 15:1, resulted in reduction of Rinterface from 1.6 kOhms.cm2 to 421 Ohms.cm2. By understanding the underpinning mechanisms that govern the ceramic-polymer interface impedance, we believe it is possible to develop high conductivity CPE that are easy to fabricate and integrate into solid-state batteries.

Enabling “lithium-free” manufacturing of pure lithium metal solid-state batteries through in situ plating

“…The NCA electrode was cast with an active loading of ∼3 mAh cm −2 and an approximate porosity of 35%. The PEO-LiTFSI catholyte was synthesized as described by Gupta et al 30 and then infiltrated into the cathode porosity by heating the PEO and uniaxially pressing into the cathode against the heat-treated LLZO surface under a pressure of 4.2 MPa and held for several hours at a temperature of 80°C.…”

“…To evaluate the performance of the in situ formed Li-metal anode with a relevant model cathode, a Li-metal/LLZO/NCA all-solid-state cell was fabricated. To avoid the need for small quantities of liquid electrolyte to improve the NCA/LLZO interface charge transfer kinetics, an all-solid-state PEO/NCA composite cathode was used, as it has been demonstrated that the PEO/LLZO interface resistance can be reduced to relatively low values (<200 Ω cm 2 at room temperature) 30 . However, as the bulk conductivity of polyethylene oxide/lithium bis(triflouromethanesulfonyl)imide (PEO-LiTFSI) polymers is low at room temperature (∼1 × 10 −6 S cm −1 ), the cell was operated at 60°C.…”

Uncharted Waters: Super-Concentrated Electrolytes

“…Similar concept now is being actively explored in the development of the polymer-electrolyte-in-ceramic hybrid electrolytes with the interfacial region being the key to optimizing the overall ionic transport. 9,10 A decade later, unusual interphasial properties were noticed with a similar venture in liquid non-aqueous electrolyte. According to Jeong et al, 11 the well-known exfoliation of graphite by PC would not happen when some lithium salts are used at a higher-than-usual concentration.…”

From Liquid- to Solid-State Batteries: Ion Transfer Kinetics of Heteroionic Interfaces

“…The influence of surface impurities on the PE/SE interface resistance has hardly been studied. Regarding the interface between PEO 27 :LiTFSI and LLZO:Ta, it was shown that removing (mostly Li 2 CO 3 ) impurities from the LLZO:Ta surface strongly decreases the interface resistance [70]. However, in order to compare different studies, knowledge of surface properties before contacting the PE and the SE is crucial.…”

“…At lower salt concentrations, less lithium ions participate in the lithiumion transport resulting in an increasing interface resistance. However, going to concentrations above 15:1, the authors assumed the precipitation of lithium salt in PEO, also leading to an increasing interface resistance [70].…”