Failure caused by dendrite growth in high-energy-density, rechargeable batteries with lithium metal anodes has prevented their widespread use in applications ranging from consumer electronics to electric vehicles. Efforts to solve the lithium dendrite problem have focused on preventing the growth of protrusions from the anode surface. Synchrotron hard X-ray microtomography experiments on symmetric lithium-polymer-lithium cells cycled at 90 °C show that during the early stage of dendrite development, the bulk of the dendritic structure lies within the electrode, underneath the polymer/electrode interface. Furthermore, we observed crystalline impurities, present in the uncycled lithium anodes, at the base of the subsurface dendritic structures. The portion of the dendrite protruding into the electrolyte increases on cycling until it spans the electrolyte thickness, causing a short circuit. Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface structures in the lithium electrode.

Understanding and controlling the electrochemical deposition of lithium is imperative for the safe use of rechargeable batteries with a lithium metal anode. Solid block copolymer electrolyte membranes are known to enhance the stability of lithium metal anodes by mechanically suppressing the formation of lithium protrusions during battery charging. Time-resolved hard X-ray microtomography was used to monitor the internal structure of a symmetric lithium-polymer cell during galvanostatic polarization. The microtomography images were used to determine the local rate of lithium deposition, i.e. local current density, in the vicinity of a lithium globule growing through the electrolyte. Measurements of electrolyte displacement enabled estimation of local stresses in the electrolyte. At early times, the current density was maximized at the globule tip, as expected from simple current distribution arguments. At later times, the current density was maximized at the globule perimeter. We show that this phenomenon is related to the local stress fields that arise as the electrolyte is deformed. The local current density, normalized for the radius of curvature, decreases with increasing compressive stresses at the lithium-polymer interface. To our knowledge, our study provides the first direct measurement showing the influence of local mechanical stresses on the deposition kinetics at lithium metal electrodes. There is increasing interest in the transport of ions at lithium metal electrodes due to the current focus on increasing the energy density of rechargeable lithium batteries.1 In theory, replacing a graphite electrode with lithium metal in a lithium-ion battery will result in a 40% increase in gravimetric energy density.2 Battery chemistries with energy densities that are substantially larger than that of the lithium-ion chemistry, such as lithium-sulfur and lithium-air, rely on the availability of a rechargeable lithium metal anode. Electrodeposition of metallic films is also an integral step in the manufacture and use of a broad range of devices spanning consumer electronics to energy storage.3-5 Conventionally, in both batteries and electrochemical processing, metals are electrodeposited from liquid electrolytes. [6][7][8] However, recent advances in polymer and ceramic electrolytes have allowed for the deposition (and stripping) of metals from electrolytes with a high modulus.9-11 These stiff electrolyte materials influence the mechanism of metallic electrodeposition. Notably, stiff polymer electrolytes are known to suppress the growth of dendrites in batteries containing a lithium metal anode.12,13 Suppressing the growth of protruding metallic lithium structures, like dendrites and globules, is imperative for the safe and reliable use of high energy density, rechargeable batteries with metallic anodes. 14,15 Numerous experimental studies have addressed the issue of dendrite growth in lithium batteries. 8,[16][17][18][19][20][21][22][23][24][25] While the increase in current density in the vicinity of a dendrite o...

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...