Control of homogeneous lithium deposition governs prospects of advanced cell development and practical applications of high-energy-density lithium metal batteries. Polymer electrolytes are thus explored and employed to mitigate the growth of high-surface-area lithium species while enhancing the reversibility of the lithium reservoir upon cell cycling. Herein, an in-depth understanding of the distribution of membrane properties and lithium deposition behavior affected by the selection of polymer segment species is derived. It is demonstrated that severely localized lithium deposits featuring needle-like morphologies may be readily observed when electrostatic fields (or partial charges) and the amount of Li + coordinators of the primary and secondary polymer segment species appear rather dissimilar, leading to a sudden cell failure at early stages of cell operation. In comparison, employment of optimized copolymer electrolytes enables superior cell performance at 1C even with thicker cathodes (6.3 mg cm −2 ). Additionally, the improvement of cell-cycling stability due to enhancement of similarity of dipole moments and partial charge distributions among copolymer segments are also demonstrated for different polymer systems, contributing to avoidance of undesired lithium protrusions, also reflecting a viable concept for the design of future copolymer electrolytes.
Rechargeable solid-state lithium metal batteries are highly promising systems beyond lithium-ion technology, potentially affording high gravimetric and volumetric energy densities. Despite substantial progress in this field, significant challenges remain, including interfacial changes upon cell operation and achievement of reversible lithium inventory. Here, polymer electrolytes may offer flexible solutions and operational safety, providing compromises among conflicting demands of mechanical robustness and sufficient ionic conductivity, though advancements and tailored design of future polymer materials of functional layers and cell designs require input from insights into interfacial processes, characteristic charge transfer rates and associated resistances at electrode|electrolyte interfaces, respectively.
In this work, electrochemical impedance spectroscopy (EIS) is therefore carefully exploited for the analysis of relevant charge transfer processes in case of introduced polymer electrolytes[1], invoking a distribution of relaxation times (DRT) approach. The obtained complex permittivity and conductivity of the materials upon variation of temperature as well as explicit modification of the thin lithium metal electrodes with artificial polymer layers yield characteristic data of the evolution of interfacial resistances and the corresponding charge transfer reactions, in this way demonstrating the significant diagnostic strength of EIS/DRT analysis for future developments. In addition, operando EIS/DRT analysis is done upon plating of lithium metal, complemented by 7Li solid-state NMR spectra acquired at various states during lithium deposition, thereby revealing occurring species and their microstructures based on the 7Li NMR chemical shifts.
In summary, EIS/DRT analysis is utilized as powerful diagnostic technique, highlighting how to exploit observable trends of characteristic parameters at electrode|electrolyte interfaces for future design of polymer-based materials as well as high performance cell concepts.
[1]Chiou et al., Journal of Power Sources 538 (2022) 231528
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