In Situ X-Ray Absorption Spectroscopy for Batteries: Discovery of New Mechanisms and Materials

Development of transformational electrochemical energy storage technologies is an imperative for enabling sustainable technologies such as vehicle electrification and renewable energy generation. Major advances in these technologies are made possible by developing the ability to predict how structural changes correlate with the functionality of new materials. Characterization by x-ray absorption spectroscopy (XAS) is an important tool in developing a better understanding of the structure-function relationship in battery materials, including structural changes upon cycling and aging mechanisms. XAS is commonly used to study electrochemical reactions in situ and is particularly valuable when working with nanomaterials or highly disordered or doped crystalline systems. A successful in situ XAS experiment on battery materials requires a careful choice of experimental conditions including design of the in situ cell, choice of beamline, energy scan mode, and detectors. An overview of these considerations for various types of battery materials will be presented along with results which illustrate the pitfalls inherent in these experiments. Results will be presented on a variety of Li-ion battery cathode and anode materials, including metals and metal oxides where there is a high capacity but rapid aging due to severe swelling of the anode material upon lithiation. Our results have identified the structural signature of metal-Li bonds and their persistence upon discharge, which imply a loss of electrical contact as the primary failure mode of these materials. In situ results on aqueous battery materials such as Fe 2 O 3 and NiOOH will also be presented and discussed with particular emphasis on the comparison between nanoparticles in solid state electrodes and in nanofluid suspension electrodes known as nanoelectrofuel (NEF). NEF are stable dispersions of battery active nanoparticles in electrolyte that effectively charge/discharge as they are pumped through custom-designed flow cell(s) and represent a high-energy-density rechargeable, renewable, and recyclable electrochemical fuel.