In this Perspective, we highlight recent progress and challenges related to the integration of lithium metal anodes in solid-state batteries. While prior reports have suggested that solid electrolytes may be impermeable to lithium metal, this hypothesis has been disproven under a variety of electrolyte compositions and cycling conditions. Herein, we describe the mechanistic origins and importance of lithium filament growth and interphase formation in inorganic and organic solid electrolytes. Multimodal techniques that combine real and reciprocal space imaging and modeling will be necessary to fully understand nonequilibrium dynamics at these buried interfaces. Currently, most studies on lithium electrode kinetics at solid electrolyte interfaces are completed in symmetric Li–Li configurations. To fully understand the challenges and opportunities afforded by Li-metal anodes, full-cell experiments are necessary. Finally, the impacts of operating conditions on solid-state batteries are largely unknown with respect to pressure, geometry, and break-in protocols. Given the rapid growth of this community and the diverse portfolio of solid electrolytes, we highlight the need for detailed reporting of experimental conditions and standardization of protocols across the community.

We explore the relationship between the morphology and ionic conductivity of block copolymer electrolytes over a wide range of salt concentrations for the system polystyrene-blockpoly(ethylene oxide) (PS-b-PEO, SEO) mixed with lithium bis-(trifluoromethanesulfonyl)imide salt (LiTFSI). Two SEO polymers were studied, SEO(16−16) and SEO(4.9−5.5), over the salt concentration range r = 0.03−0.55. The numbers x and y in SEO(x−y) are the molecular weights of the blocks in kg mol −1 , and the r value is the molar ratio of salt to ethylene oxide moieties. Smallangle X-ray scattering was used to characterize morphology and grain size at 120°C, differential scanning calorimetry was used to study the crystallinity and the glass transition temperature of the PEO-rich microphase, and ac impedance spectroscopy was used to measure ionic conductivity as a function of temperature. The most surprising observation of our study is that ionic conductivity in the concentration regime 0.11 ≤ r ≤ 0.21 increases in SEO electrolytes but decreases in PEO electrolytes. The maximum in ionic conductivity with salt concentration occurs at about twice the salt concentration in SEO (r = 0.21) as in PEO (r = 0.11). We propose that these observations are due to the effect of salt concentration on the grain structure in SEO electrolytes.

A systematic study of the dependence of ionic conductivity on the grain size of a lamellar block copolymer electrolyte was performed. A freeze-dried mixture of poly(styrene)-block-poly(ethylene oxide) and lithium bis-(trifluoromethylsulfonyl)imide salt was heated in steps from 29 to 116°C and then cooled back to 29°C with an annealing time ranging from 30 to 60 min at each temperature. Grain structure and ionic conductivity during these steps were quantified by in situ small-angle X-ray scattering and ac impedance spectroscopy, respectively. Conductivity depends both on grain structure and temperature. A normalization scheme to decouple the dependence of conductivity on temperature and grain structure is described. Ionic conductivity at a given temperature was found to decrease by a factor of 5.2 ± 0.9 as the SAXS measure of grain size increased from 13 to 88 nm. The fact that in the system studied, large, well-formed lamellar grains are less conducting than poorly defined, small grains suggests a new approach for optimizing the transport properties of block copolymer electrolytes. Further work is necessary to confirm the generality of this finding.

A significant limitation of rechargeable lithiumion batteries arises because most of the ionic current is carried by the anion, the ion that does not participate in energyproducing reactions. Single-ion-conducting block copolymer electrolytes, wherein all of the current is carried by the lithium cations, have the potential to dramatically improve battery performance. The relationship between ionic conductivity and morphology of single-ion-conducting poly(ethylene oxide)-bpolystyrenesulfonyllithium(trifluoromethylsulfonyl)imide (PEO−PSLiTFSI) diblock copolymers was studied by smallangle X-ray scattering and ac impedance spectroscopy. At low temperatures, an ordered lamellar phase is obtained, and the "mobile" lithium ions are trapped in the form of ionic clusters in the glassy polystyrene-rich microphase. An increase in temperature results in a thermodynamic transition to a disordered phase. Above this transition temperature, the lithium ions are released from the clusters, and ionic conductivity increases by several orders of magnitude. This morphology−conductivity relationship is very different from all previously published data on published electrolytes. The ability to design electrolytes wherein most of the current is carried by the lithium ions, to sequester them in nonconducting domains and release them when necessary, has the potential to enable new strategies for controlling the charge−discharge characteristics of rechargeable lithium batteries.

Interfaces between a solid electrolyte and electrodes in ASSBs need to be carefully engineered to enable high performance. This study leverages advanced characterization techniques (atomic force microscopy and synchrotron X-ray transmission microscopy) as well as computational tools to understand the mechanical response of electrode|electrolyte interfaces in model hybrid electrolyte systems. The impact of mechanical properties of the extrinsic interfaces on electrochemical performance is evaluated. Active control of interfacial properties is identified as a potential route to engineer high-performance solidstate batteries.

Hybrid nanostructured materials comprising block copolymers, nanoparticles, and lithium salts have the potential to serve as electrolytes in non-flammable rechargeable lithium batteries. Here we show that the addition of functionalized nanoparticles, at an optimized concentration, into lamellar block copolymer electrolytes, results in an increase in ionic conductivity. This is due to the occurrence of a lamellar-to-bicontinuous phase transition, driven by the addition of nanoparticles. The magnitude of the increase in conductivity is consistent with a simple model that accounts for the morphology of the conducting channels. The conductivity of the optimized hybrid electrolyte is only 6% lower than that of an idealized nanostructured electrolyte with perfectly connected conducting pathways and no dead ends.

Dynamic crosslinking of commodity thermoplastics with multiple interfaces enables remarkably tough reversible adhesion.

We report on the synthesis and characterization of a series of microphase-separated, single-ion-conducting block copolymer electrolytes. Salty nanoparticles comprising silsesquioxane cores with covalently bound polystyrenesulfonyllithium (trifluoromethylsulfonyl)imide (PSLiTFSI) chains were synthesized by nitroxidemediated polymerization. Hybrid electrolytes were obtained by mixing the salty nanoparticles into a microphase-separated polystyrene-b-poly(ethylene oxide) (SEO) block copolymer. Miscibility of PSLiTFSI and poly(ethylene oxide) (PEO) results in localization of the nanoparticles in the PEO-rich microphase. The morphology of hybrid electrolytes was determined by scanning transmission electron microscopy. We explore the relationship between the morphology and ionic conductivity of the hybrid. The transference number of the electrolyte with the highest ionic conductivity was measured by dc polarization to confirm the single-ion-conducting character of the electrolyte. Discharge curves obtained from lithium metal−hybrid electrolyte− FePO 4 batteries are compared to the data obtained from the batteries with a conventional block copolymer electrolyte.