Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow
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ncy J. Dudney Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
P. Sudharshan Phani International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana– 500005, India (Received 23 December 2017; accepted 29 March 2018)
Nanoindentation experiments performed in 5 and 18 lm thick vapor deposited polycrystalline lithium films at 31 °C reveal the mean pressure lithium can support is strongly dependent on length scale and strain rate. At the smallest length scales (indentation depths of 40 nm), the mean pressure lithium can support increases from ;23 to 175 MPa as the indentation strain rate increases from 0.195 to 1.364 s1. Furthermore, these pressures are ;46–350 times higher than the nominal yield strength of bulk polycrystalline lithium. The length scale and strain rate dependent hardness is rationalized using slightly modified forms of the Nabarro–Herring and Harper–Dorn creep mechanisms. Load–displacement curves suggest a stress and length-scale dependent transition from diffusion to dislocation-mediated flow. Collectively, these experimental observations shed significant new light on the mechanical behavior of lithium at the length scale of defects existing at the lithium/solid electrolyte interface.
I. INTRODUCTION
Understanding the mechanical properties of metallic lithium may elucidate why it is difficult to form and maintain good, low resistance interfaces that are stable for extended cycling of lithium in rechargeable batteries. Of particular interest is understanding how coupling between the electrochemical and mechanical behavior of Li and defects at the anode/solid electrolyte (SE) interface enable the formation and growth of lithium dendrites originating at the interface and growing across the SE separator. Among the critical gaps in knowledge is a comprehensive understanding of how the plastic properties of Li change with key variables such as, but not limited to, length scale, strain rate, temperature, crystallographic orientation, and electrochemical cycling. Knowing how these factors limit the performance of next generation energy storage devices will directly enable the development of robust methods to create stable interfaces that promote safe, long-term, high-rate cycling performance by mitigating the anode’s
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Address all correspondence to this author. e-mail: [email protected] Corresponding Editor: Erik Herbert This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2018.84
gradual loss of Li over the cycle life of the battery and by eliminating or greatly reducing the risk of metallic short-circuiting. Recent research activity underscores the need for new knowledge to fill critical gaps in understanding the Li/SE interface.1,2 Experimental evidence provided by Cheng et al.1 clearly shows that upon cycling, metallic Li can short circuit an aluminum-doped polycrystalline ceramic electrolyte known as LLZO by selectively infiltrating its grain boundaries. This phenomenon occurs despite the shea
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