AMP_04_May_June_2021_Digital_Edition

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 2 3 SOLID ELECTROLYTES The battery of the future replaces the flammable liquid electrolyte with a ceramic or composite lithium-ion conductor referred to as a solid elec- trolyte (SE). Within the SE, Li ions mi- grate through atomic scale vacancies or through couple motion with atomic vi- brations/rotations. Some solid electro- lytes yield Li conductivities approaching those of liquid electrolytes (~10 -3 S-cm), providing a viable alternative to unsta- ble liquid electrolytes. There are at least a dozen different base solid electrolytes including crystalline oxides of garnet, perovskite, antiperovskites, b”-Al 2 O 3 , and phosphates of sodium and lithium (also known as Nasicon and Lisicon), as well as sulfides and germanates [3] . Beyond crystalline ceramics there are amorphous solids including lithium phosphorous oxynitride (Lipon) [4] , and polysulfides [5] . A second class of disor- dered ionic conductors are the poly- mer electrolytes where Li + or Na + ions coordinate with the polymer backbone, though these are not mechanically ro- bust and have low ionic conductivi- ties [6] . Certain solid electrolytes have larger electrochemical windows than liquid electrolytes (~6V vs 4.3V), making them stable against lithium metal an- odes and high voltage cathodes [7] . Com- bining the use of lithium metal anodes, high voltage cathodes, and liquid free electrolytes yields next-generation bat- teries with energy densities twice that of today’s technologies and lifetimes of 50 years. The grand challenge of solid elec- trolytes, and opportunity for materi- als science, is being able to direct and control the synthesis and processing of solid-state battery materials and their ensembles to obtain maximal ion transport (for power), interfacial side reactions (for energy capacity), and or- phaning of active components during cycling (for longevity). Better ion trans- port produces improved power and cy- cle rates needed for transportation or intermittent power supplies and is es- sential to produce a reliable device. One such area is graphically summarized in Fig. 2. The center image shows a scan- ning transmission electron microscopy image of the cu- bic Li 7 La 3 Zr 2 O 12 garnet sol- id electrolyte [8] with three potential orientations of a layered LiCoO 2 -like cath- ode material along with the probable ion conduct- ing pathways. One could imagine the differences in lithium transport across these three hypothetical in- terfaces with a poor trans- port of Li from the (003) oriented material and sig- nificantly better from the (104) and (101) aligned structures. Compounding this challenge is the need to move ions upwards of 300 mm in a few hours during charging and dis- charging. In a traditional lithium-ion battery, the liq- uid electrolyte compen- sates for misalignment of interfaces by providing a fluid interface layer and the resulting three-dimensional trans- port. The final transport challenge in- volves the movement of electrons. Cathodes are generally poor elec- tronic conductors requiring the addi- tion of graphite to provide electri- cal connections. From a processing perspective, how could this crystal ori- entation to promote ion motion be accomplished with high fidelity and re- producibility on the scale of billions of units while introducing electrical con- ductivity pathways? Volume expansion during battery cycling poses another problem. As lith- ium is removed from the anode and returned to the cathode, volume ex- pansions and contractions occur, lead- ing to cracking. This cracking can lead to inaccessible regions, or electroactive material that becomes orphaned due to mechanical degradation. Polymer elec- trolytes can act as a bridge between particles and, therefore, can deal with the volume expansion. Unfortunately, transport across these dissimilar mate- rials is restricted due to poor polymer ionic conductivity and high interfacial resistances. Operation at higher tem- peratures (>50 o C) solves this problem but is not practical for devices that need to operate at -60 o C, such as vehicles. Thus, the fundamental challenge be- comes how do we design new interfaces and architectures to facilitate ion trans- port while accommodating volume changes. One is tempted to draw inspi- ration from biomaterials where mem- brane channels rapidly, and selectively, transmit ligand bound mono- and di- valent cations at body temperature but has not yet found similar success in en- ergy storage [9] . The third major challenge in- volves the stripping and plating of lithium metal. The use of lithium met- al enables a 50% increase in energy density over traditional anodes like graphite [10] . In an all-solid-state bat- tery upwards of 50 mm of lithium met- al needs to be reversibly and reliably deposited and removed with charging and discharging. The challenges of vol- ume expansion at the cathode (few percent) pale in comparison to the chal- lenge of maintaining cell integrity at Fig. 2 — Schematic representation of potential crystallo- graphic orientations of a layered cathode material adopting the LiCoO 2 structure oriented with a cubic garnet solid electrolyte. Lithium (green circles) diffusion paths are represented by the yellow arrows.