Analyzing the Stability and Kinetics of Ceramic Electrolyte/Organic Electrolyte Interfaces for LI Metal Batteries

Abstract

The need for high-energy-density Li-ion batteries has provided the impetus to replace graphite anodes with Li metal anodes. Unfortunately, the liquid electrolytes (LEs) used in state-of-the-art (SOA) Li-ion batteries are unstable with Li metal. Owing to its high ionic conductivity, stability against Li and safety, Lithium Lanthanum Zirconium Tantalum Oxide (LLZTO), a solid ceramic electrolyte (SE), has been suggested as a promising alternative. However, the implementation of the SE in an all-solid-state battery could lead to cycling instabilities due to the formation of a resistive, electrochemically, and mechanically unstable cathode/SE interface. One potential approach to overcome the challenges is by introducing a gel polymer electrolyte (GPE) as a catholyte. In this hybrid electrolyte scheme, the LLZTO protects Li metal and a GPE improves the LLZTO/cathode interfacial stability and kinetics. The success of this approach, however, is reliant on two main assumptions – a) electrochemical properties of the GPE would not be affected by volumetric changes in the cathode during operation and b) the polymer and liquid electrolyte in the GPE will be (electro)chemically stable against LLZTO. The overarching goal of this thesis was to identify potential shortcomings of these assumptions and provide solutions to address them.

To achieve this goal, the thesis was subdivided into three studies - 1) studying the effects of temperature and pressure on electrochemical performance of a model polymer electrolyte 2) understanding the factors controlling the polymer/LLZTO interfacial kinetics 3) evaluating the stability of LLZTO with potential LEs used in the GPE. With the aid of various characterization techniques including SEM, XRD, Raman Spectroscopy, XPS, electrochemical impedance spectroscopy, and galvanostatic cycling and unique experimental designs, several important implications were derived from each study.

First, the roles of temperature and pressure on the electrochemical properties of the PEO-LiTFSI model system were evaluated. The results indicate that both, the bulk ionic conductivity and electrode/electrolyte charge transfer kinetics, are affected by temperature and stack pressure. It was observed that activation energy for Li-ion conduction shows a sharper transition at the melting point of the polymer for bulk conduction than for the electrode/electrolyte interface. It was also observed that a critical stack pressure was required to form an optimized electrode/electrolyte interface.

Second, the factors controlling the kinetics and stability of LLZTO with both constituents of the GPE were analyzed. First, using PEO-LiTFSI the underlying factors that control the LLZTO/polymer interfacial kinetics were studied. It was found that the LLZTO surface impurities and Li-ion concentration gradient between the two electrolytes were responsible for the high interfacial resistance (Rinterface). The fundamental knowledge gained in this study enabled a reduction in the Rinterface from ~95 kOhms.cm2 down to 180 Ohms.cm2.

Lastly, the stability of LLZTO was evaluated with different organic solvents and Li salts present in LEs. It was found that LiPF6-containing LE reacts with LLZTO to form LiF, LaF3, and ZrF4 at the interface leading to an increased SE/LE Rinterface. It was concluded that the chemical instability at LLZTO/LE interface was specific to the Li salt used. Thus, by selecting Li salts that exhibit stable behavior with LLZTO, the potential chemical instabilities can be avoided. Further, the optimization of Li salt concentration resulted in a low Rinterface (~30 Ohms.cm2).

The culmination of the knowledge gained from the studies can be used for the development of hybrid electrolytes for enabling Li metal anodes.