Ion Transport Mechanisms in Single Ion Polymer Electrolytes for Application in Lithium Ion Batteries
Authors: Borzutzki Kristina , Brunklaus Gunther , Kruteva Margarita , Biehl Ralf , Winter Martin 
Published: February 2020

In order to increase the energy density of lithium ion batteries, lithium metal is an attractive anode material based on its high specific capacity and low electrochemical potential. However, a major drawback of lithium metal anodes comprises dendrite growth due to unstable solid electrolyte interphase (SEI) resulting in severe safety hazards. Polymer electrolytes constitute a viable alternative to commonly utilized liquid electrolytes and are able to suppress or even avoid dendrite growth thereby providing increased safety. In particular, single ion conducting polymer electrolytes are promising for application in lithium ion or lithium metal batteries. In contrast to common dual ion conducting electrolytes, single ion conductors afford high transference numbers, hence significantly reducing polarization effects as only lithium ions are mobile whereas anions are bound at the polymer backbone.

In this contribution we present various single ion conducting polymers composed of AB-type alternating block copolymers in which the lithium ions are bound to the backbone (e.g., bis(4-carboxyl benzene sulfonyl)imide moieties). Systematic variation of the constituents can control the achievable properties including the morphology, electrical and electrochemical properties of the resulting polymers.

The electrolyte membranes fabricated for application in lithium meatal batteries are blends of single ion conducting aromatic polymers and a flexible linear polymer such as poly(vinylidene fluoride-co -hexafluoropropylene (PVDF-HFP). The appropriate ratio of the polymer blends is evaluated to yield membranes (swollen with thermally stable solvent solutions of ethylene carbonate and propylene carbonate (EC: PC, 1:1, v/v)), with increased ionic conductivity, reduced solvent uptake as well as sufficient flexibility and mechanical stability as well as reduced solvent uptake. We present optimized routes for the synthesis of single ion conducting polymers and suitable membrane compositions.

Furthermore, feasible relations of both structural and physicochemical properties of the polymer membranes are discussed, particularly with respect to underlying ion transport properties or transport mechanisms, in this way potentially enabling controlled modification or adjustment of either the chemical structures or electrochemical properties of the polymer membranes. A morphology analysis is performed using small angle X-ray scattering (SAXS), while nuclear magnetic resonance spectroscopy (NMR) and electrochemical data including impedance as well as dielectric loss spectra are combined to unravel the major ion transport mechanisms and ion mobility in addition to ionic conductivity, complex permittivity, self-diffusion coefficients and transference numbers. Both the oxidative and reductive stability as well as cycling performance in NMC/ lithium metal cells are also presented.

Profound understanding of the ion transport mechanisms and property relations in single ion conducting polymer electrolyte membranes is essential and will allow to design future electrolyte membranes having desired properties. Thereby, the requirement for different applications can be met and the application of safe and highly performing lithium metal batteries can be enabled.