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Block copolymer electrolytes for lithium batteries


Increasing interest in renewable energy technologies has recently brought compact and cost-effective energy storage into the spotlight. A wide variety of applications could benefit from an appropriate high-energy storage medium capable of efficiently collecting and releasing electrical energy. In this work, the lithium metal battery is introduced as one of the most exciting candidates with the potential to fill this need. In chapter one, today's leading battery solution is explored and compared to the lithium metal cell, and challenges to cycle and calendar life in each system are explained. In particular, the advantages and limitations of the state-of-the-art Li-ion chemistry, including its graphite-based negative electrode, are discussed. Specific challenges to the implementation of metallic lithium - the negative electrode with the highest possible specific energy - are also presented, with the pervasive growth of catastrophic lithium dendrites being the most significant obstacle to its success. An active body of investigation into the formation of these dendritic microstructures in lithium metal cells and various strategies toward eliminating them are introduced. Finally, with support from recent research, we propose that the hard-soft, nanostructured block copolymer electrolyte, poly(styrene-block-ethylene oxide), (PS-b-PEO), represents a fundamentally new approach toward stopping lithium dendrite failure and, in so doing, realizing a metallic lithium anode as part of a stable higher-energy rechargeable battery.

The second chapter presents a complete set of fundamental transport measurements on the solid electrolyte, PS-b-PEO containing the lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt. The LiTFSI mutual diffusion coefficient, measured by restricted diffusion in symmetric lithium cells, is reported along with the ionic conductivity, measured by potentioelectrochemical impedance spectroscopy, for a wide range of salt concentrations at 80 °C. A comparison between these results and those for the homopolymer-PEO system are also discussed. In addition, a straightforward approach toward measuring the lithium transference number in solid electrolyte samples is reported and compared to various existing methods. The transference measurement reported herein depends on an experimental determination of the limiting current, which is undertaken for a range of salt concentrations, also in lithium symmetric cells at 80 °C.

In the third chapter, the focus turns to practical batteries containing lithium metal as a negative electrode and lithium iron phosphate (LiFePO4) as a positive electrode. An extensive electrochemical characterization on batteries with these electrodes and the solid electrolyte PS-b-PEO containing LiTFSI is first reported. In cells with high specific energy, exceptional electrochemical and high-temperature stability is demonstrated over months of repeated charge and discharge cycling. Data collected at charge/discharge rates in the appropriate range for electric vehicle applications (i.e., C/2, which is defined as the current necessary to fully charge or discharge a cell in 2 hours) are compared to data collected on comparable homopolymer-based cells. In order to project ultimate cycle and calendar life limitations, coulombic and energy efficiency measurements are taken for each system, and electron micrographs demonstrate the unprecedented reversibility of the metallic lithium electrochemical reaction in all-solid-state batteries containing the block copolymer electrolyte.

Finally, chapter four describes high-resolution in situ concentration mapping of dissolved LiTFSI in working lithium symmetric cells containing the same block copolymer electrolyte. By synchrotron scanning transmission X-ray microscopy (STXM), performed on all-solid-state batteries, real-time ion composition data, generated by quantitative X-ray absorption measurements, are reported under galvanostatic charge and discharge conditions. In particular, nanometer-resolution fluorine 1s absorbance data within working batteries are converted to Li+ concentration maps to elucidate the evolution of ion composition changes in cells during cycling. Furthermore, a general approach toward accurate, in situ, fundamental transport measurements, including a representative measurement of the lithium transference number for LiTFSI in PS-b-PEO, is reported. We believe this technique represents a seminal effort toward a general method of in situ, nanoscale, soft-X-ray characterization of lithium-based batteries that can be extended to the investigation of other electrolytes as well as to a wide variety of electrode materials for electrochemical systems of all types.

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