Nanomechanical measurements shed light on solid-state battery degradation
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Nanomechanical measurements shed light on solid-state battery degradation Matthew T. McDowell Submitted April 29, 2020; Accepted May 6, 2020.
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ithium-ion batteries have enabled the widespread use of portable electronic devices and are propelling the growing electric vehicle market, but new battery technologies with improved performance are necessary for emerging applications such as electric aircraft. The solid-state battery is one such technology that could exhibit enhanced safety and higher energy density compared to conventional lithium-ion batteries. The use of a pure lithium metal anode within solidstate batteries is key for higher energy density (Figure 1a), and it is thought that using solid-state electrolytes instead of conventional liquids could increase the chemical and structural stability of lithium metal.1 Despite continued progress in the development of new inorganic solid-state electrolyte materials, however, a persistent problem has emerged: lithium metal tends to grow as filaments during charging instead of as a flat film, and these filaments can penetrate and fracture the stiff solid-state electrolyte to short circuit the cell (Figure 1b).2–4 To prevent this chemo-mechanical degradation process and enable filament-free charging, it is critical to understand the mechanical properties of lithium metal, which have been elusive because of the highly reactive nature of lithium. Citrin et al. report an important advance in measuring the mechanical properties of nanoscale lithium filaments grown directly from solid-state batteries.5 They find that the strength of lithium is strongly dependent on filament size, which could help explain why nanoscale lithium filaments can penetrate stiff inorganic electrolytes, causing solid-state batteries to fail. Observations of fracture in solid-state electrolytes driven by lithium filament penetration have become commonplace in the development of solid-state batteries.3,4 While the governing mechanisms are not entirely understood, recent work has suggested that lithium electrodeposition into flaws at the lithium/ solid electrolyte interface could cause stress accumulation and crack growth.6 Thus, understanding the size-dependent mechanical properties of lithium is a key piece of the puzzle, but this has proven challenging with conventional methods because of to the presence of surface contamination layers on lithium that can affect mechanical deformation. Experiments on bulk polycrystalline lithium under inert environments have shown sub-MPa yield stress and deformation that is largely
governed by creep.7,8 Nanoindentation experiments probing much smaller micron-scale regions of films have shown significantly higher yield stresses than the bulk,9 and recent in situ transmission electron microscopy (TEM) experiments on Li2CO3-coated lithium filaments also showed high yield stress.10 Although these studies have provided important information, it has remained a challenge to measure the properties of filaments with the size, structure, and surface properties found in
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