Electrochemical Deposition and Stripping Behavior of Lithium Metal across a Rigid Block Copolymer Electrolyte Membrane

Abstract

Replacing the conventional graphite anode in rechargeable batteries with lithium metal results in a significant increase in energy density. However, growth of electronically conductive structures, like dendrites, from lithium anodes causes premature battery failure by short circuit. Mechanically rigid electrolytes are thought to promote smooth lithium deposition by increasing the energy required for lithium reduction at regions of high local strain, like a dendrite tip. The study reported herein used X-ray microtomography, Focused Ion Beam (FIB) milling, and Scanning Electron Microscopy (SEM) imaging to investigate the electrochemical stripping and deposition behavior of lithium in symmetric lithium -polymer cells using a rigid polystyrene-b-poly(ethylene oxide) membrane as the electrolyte. In situ experiments show the formation of globular lithium structures that grow to puncture the polymer electrolyte membrane. They form on faceted impurity particles that are initially located at the lithium/electrolyte interface. While the impurities are uniformly distributed throughout the lithium foil in initial images, their relative concentration near the electrolyte changes as lithium is stripped from one electrode and deposited on the other. Notably, the deposited lithium is devoid of faceted impurities. This electrolytic refining of lithium could be used to prepare anodic lithium foils for batteries with improved cycle life. Lithium metal is a highly desirable anode material for applications requiring a high energy density battery due to its electropositivity and low atomic mass. Simply replacing the traditional graphite anode with lithium metal in a conventional lithium ion battery results in a significant increase in specific energy. Next generation battery chemistries, like lithium-sulfur and lithium-air, presume the use of a lithium metal anode to achieve theoretical specific energies of 2458 Wh/kg and 5217 Wh/kg respectively.1-3 The theoretical specific energies of the sulfur and air battery chemistries fall to 572 and 939 Wh/kg if a traditional graphite anode is substituted for lithium metal. Given its importance in high energy density battery chemistries, there is strong motivation to understand the redox behavior of lithium metal.Notably, lithium metal tends to form dendrites as lithium ions deposit on the lithium metal foil during battery charging.4-8 Lithium dendrites propagate through the electrolyte layer, and when they reach the cathode, the battery fails by short-circuit.9,10 This failure can be catastrophic if it occurs in the presence of a flammable electrolyte. Consequently, the use of lithium metal anodes with traditional liquid electrolytes is generally considered unsafe.11 Furthermore, liquid electrolytes form a mechanically unstable solid electrolyte interface (SEI) layer with lithium metal. This exacerbates lithium dendrite growth, resulting in premature battery failure.12-14 Polymer electrolytes, like poly(ethylene oxide), form a more stable SEI layer when cycled against lithium metal.15,16 Addi...

Influence of Electrolyte Modulus on the Local Current Density at a Dendrite Tip on a Lithium Metal Electrode

“…The globule volume, V g , increases quadratically as a function of t Li (the curve in Figure 4b represents V g = 33t 2 Li + 915t Li − 4437). One can also measure the area of the globule at the planar interface between the bottom lithium electrode and the electrolyte.…”

“…b As the globule grew, the current delocalized away from the tip of the globule. This map was measured between time points 8.27 C/cm 2 Figure 6d shows the maximum compressive and tensile stresses in the polymer as a function of the charge passed. In early stages of growth, the globule grew mainly in height, and, consequently, both the compressive stress at the globule tip and the tensile stress at the globule perimeter increased substantially between 8.27 C/cm 2 and 16.53 C/cm 2 as shown in Figure 5e.…”

“…A qualitative description of these results was reported previously. 2 The present study seeks to quantify the kinetics of lithium deposition on and near a globule as it grows through a solid polymer electrolyte.…”

Guided Lithium Deposition by Surface Micro‐Patterning of Lithium‐Metal Electrodes

“…Typically, uncontrollable Li electrodeposition on Li‐metal electrodes is attributed to chemical and morphological inhomogeneity of the Li‐metal surface, and it results in the growth of dendritic and mossy Li . During repeated charge/discharge cycles, the formation of Li dendrite and dead Li become more intense, and they may lead to severe safety issues such as thermal runaway, fire, and explosions caused by an internal short circuit . Furthermore, the intrinsic high reactivity of Li‐metal with organic electrolytes causes their rapid consumption and the build‐up of a Solid electrolyte interphase (SEI) to form a resistive porous layer …”

“…[7] During repeated charge/discharge cycles, the formation of Li dendrite and dead Li become more intense, and they may lead to severe safety issues such as thermal runaway, fire, and explosions caused by an internal short circuit. [8][9][10][11] Furthermore, the intrinsic high reactivity of Li-metal with organic electrolytes causes their rapid consumption and the build-up of a Solid electrolyte interphase (SEI) to form a resistive porous layer. [12][13][14] To inhibit Li dendrite formation and stabilize the Li-metal anode, extensive efforts have been made.…”