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.

Imaging morphological changes that occur during the lifetime of rechargeable batteries is necessary to understand how these devices fail. Since the advent of lithium-ion batteries, researchers have known that the lithium metal anode has the highest theoretical energy density of any anode material. However, rechargeable batteries containing a lithium metal anode are not widely used in consumer products because the growth of lithium dendrites from the anode upon charging of the battery causes premature cell failure by short circuit. Lithium dendrites can also form in commercial lithium-ion batteries with graphite anodes if they are improperly charged. We demonstrate that lithium dendrite growth can be studied using synchrotron-based hard X-ray microtomography. This non-destructive imaging technique allows researchers to study the growth of lithium dendrites, in addition to other morphological changes inside batteries, and subsequently develop methods to extend battery life.

Enabling the use of lithium metal anodes is a critical step required to dramatically increase the energy density of rechargeable batteries. However, dendrite growth in lithium metal batteries, and a lack of fundamental understanding of the factors governing this growth, is a limiting factor preventing their adoption. Herein we present the effect of battery cycling temperature, ranging from 90 to 120°C, on dendrite growth through a polystyrene-block-poly(ethylene oxide)-based electrolyte. This temperature range encompasses the glass transition temperature of polystyrene (107°C). A slight increase in the cycling temperature of symmetric lithium-polymer-lithium cells from 90 to 105°C results in a factor of five decrease in the amount of charge that can be passed before short circuit. Synchrotron hard X-ray microtomography experiments reveal a shift in dendrite location from primarily within the lithium electrode at 90°C, to primarily within the electrolyte at 105°C. Rheological measurements show a large change in mechanical properties over this temperature window. Time-temperature superposition was used to interpret the rheological data. Dendrite growth characteristics and cell lifetimes correlate with the temperature-dependent shift factors used for time-temperature superposition. Our work represents a step toward understanding the factors that govern lithium dendrite growth in viscoelastic electrolytes.

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.

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.

The uncontrollable nonplanar electrodeposition of lithium is a significant barrier to the widespread adoption of high energy density rechargeable batteries with a lithium metal anode. A promising approach for preventing the growth of lithium dendrites is the use of solid polymer electrolytes with a high shear modulus. Current density is the key variable in the electrodeposition of lithium. The present study is the first attempt at quantifying the effect of current density on the geometry and density of dendrites and other protrusions during electrodeposition through a solid polymer electrolyte. The geometry and density of defects formed on the lithium electrode were determined by X-ray microtomography. The tomograms revealed protrusions on the electrodeposited lithium electrodes that were either globular or dendritic, or void defects. The range of current densities over which stable, planar deposition was observed is quantified. At higher current densities, globular protrusions were observed. At the highest current density, both globular and dendritic protrusions were observed. The areal density of protrusion defects increased sharply with current density, while the overall defect density is a weak function of current density. Our work enables comparisons between the experimentally determined onset of nonplanar electrodeposition and prevailing theoretical predictions with no adjustable parameters.

Lithium metal is a high-energy-density battery electrode material, but the largely irreversible growth of lithium protrusions on an initially planar electrode during cycling makes it unsuitable for incorporation into a commercial battery. In this study, a lithium electrode with globular protrusions was stripped electrochemically, and the local morphology of the electrode as a function of time was determined by hard X-ray tomography. We demonstrate that globules are preferentially stripped compared to a planar electrode in our system, which incorporates a nanostructured block copolymer electrolyte. We report current density at the electrode as a function of micron-scale position and time. The local current density during the electrode healing process calculated from a reference frame at the electrode/electrolyte interface provides insight into the driving forces responsible for selective stripping of the globule. These results imply the possibility of discharging protocols that may return a lithium electrode to its initial planar state.

Solid block polymer electrolytes are promising candidates for the development of high energy density rechargeable lithium metal based batteries. All solid-state batteries comprising lithiummetal negative electrode and lithium iron phosphate (LiFePO4) composite positive electrode were assembled. A polystyrene-b-poly(ethylene oxide) (SEO) copolymer doped with a lithium salt was used as the electrolyte. After cycling the batteries, the reason for capacity fade and failure was determined by imaging the batteries using synchrotron hard X-ray microtomography. These experiments revealed partial delamination of the lithium foil and the block copolymer electrolyte layer. The void volume between the foil and electrolyte layer obtained after 40 to 90 cycles is comparable to volume change in the battery during one cycle. A simple model to account for the effect of delamination on current density in the battery is presented. Capacity fade and battery failures observed in our experiments are consistent with this model. No evidence of lithium dendrite formation was found. In contrast, cycled lithium-lithium symmetric cells with the same polymer electrolyte at the same current density failed due to dendrite formation. No evidence of delamination was found in these cells.