Characterization of Li⁺ Transport through the Organic-Inorganic Interface by using Electrochemical Impedance Spectroscopy

Abstract

Understanding Li+ transport at polymer||inorganic interfaces is crucial for developing composite electrolytes in solid-state batteries. In our investigation, we employed impedance spectroscopy and established a multilayer methodology for assessing Li+ transport at this interface. The inorganic phase chosen was Li6.25Al0.25La3Zr2O12 (AlLLZO), and the organic phase comprised a Poly(ethylene oxide) (PEO) network with dangling chains. Li+ incorporation in the polymer, as a free either salt or associated with anion grafting onto the PEO network, was explored. Additionally, the PEO network was either pressure-adhered to the inorganic surface (ex-situ configuration) or synthesized onto the AlLLZO surfaces (in-situ configuration) to investigate processing effects. Using a transmission line model for impedance data analysis, our study identified two key elements governing Li+ transport at the interface: Ri, representing resistance along the ionic pathway, and Rt and Ct, describing distributed resistance and capacitance within the interface. We observed that Ri is influenced by the polymerization process in the presence of AlLLZO ceramic, while Rt remains constant regardless of the synthesis method. This suggests varying Li+ concentrations at the interphase in the in-situ configuration, while interface/interphase heterogeneity remains consistent across configurations. The estimated activation energy indicates more energetically favorable direct Li+ transport in the in-situ configuration.

Harnessing Cold Sintering to Fabricate Composite Polymer Electrolytes - A Paradigm Shift in Organic-Inorganic Material Assembly

“…Among the fitting parameters, f c is the characteristic frequency of the semi-circle which is intrinsically characteristic of the materials involved in the Li-ion transport. 23 The f c value of Al-LLZO/PEO-based network CSP pellet is at 8 × 10 5 Hz, differing from that of the PEO-based network (9 × 10 2 Hz), and the Al-LLZO CSP pellet (2 × 10 5 Hz). This trend is similarly observed for the Al-LLZO/Si-SPE CSP pellet, where a f c of 8 × 10 5 Hz is obtained compared to 2 × 10 4 Hz for Si-SPE.…”

“…5. 23 This circuit comprises Rswagelock + CPE // (R + DX + CPE BE ), where the R swagelock denotes the resistance of the setup (cell, cables), and CPE and R elements stand for the percolated ionic conducting phase. The TLM (represented by DX in the equivalent circuit) characterizes the complex Al-LLZO||PEO network interface, while CPE BE represents the capacitive response of the ion blocking electrodes.…”

“…Finally, based on ionic conductivity and E a values' of various composites, we demonstrated that the transport of Li + occurs primarily at the LLZO||polymer interface, and it is a weakly thermally activated process. Interestingly, the DMF in the cold sintering allows the dissolution of LLZO surface, increasing locally 23 the Li + concentration at this interface. Actually, DMF should also help in the Li + transport by modifying the organisation of PEO close to the LLZO surface.…”

“…Using LiTFSI as salt in the PEO-based network, the resulting solid polymer electrolyte (SPE) exhibits a conductivity of 1 × 10 −5 S.cm −1 at room temperature, whereas a conductivity of 1 × 10 −7 S.cm −1 is achieved when the TFSI group is grafted onto the PEO-based network, resulting in a single-ion conducting solid polymer electrolyte (referred to as Si-SPE). In our previous studies involving PEO-based network/LLZO/ PEO-based network multilayers, 23 we hypothesized that in situ polymerization induces the formation of an interphase at the PEO|| LLZO interface. This hypothesis was supported by the analysis of electrical equivalent circuits using the Transmission Line Model (TLM).…”

“…Figure 5. (a) Equivalent circuit to fit impedance data of CSP pellet and (b) representation of TLM (DX element in the equivalent circuit) 23. …”