Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of lithium-ion (Li-ion) batteries. However, a number of setbacks prevent their integration into commercial devices. The main limiting factor is due to nanoscale phenomena occurring at the electrode/electrolyte interfaces, ultimately leading to degradation of battery operation. These key problems are highly challenging to observe and characterize as these batteries contain multiple buried interfaces. One approach for direct observation of interfacial phenomena in thin film batteries is through the fabrication of electrochemically active nanobatteries by a focused ion beam (FIB). As such, a reliable technique to fabricate nanobatteries was developed and demonstrated in recent work. Herein, a detailed protocol with a step-by-step process is presented to enable the reproduction of this nanobattery fabrication process. In particular, this technique was applied to a thin film battery consisting of LiCoO2/LiPON/a-Si, and has further been previously demonstrated by in situ cycling within a transmission electron microscope.

Lithium lanthanum titanate (LLTO) is a promising solid state electrolyte for solid state batteries due to its demonstrated high bulk ionic conductivity. However, crystalline LLTO has a relatively low grain boundary conductivity, limiting the overall material conductivity. In this work, we investigate amorphous LLTO (a-LLTO) thin films grown by pulsed laser deposition (PLD). By controlling the background pressure and temperature we are able to optimize the ionic conductivity to 3 × 10-4 S/cm and electronic conductivity to 5 × 10-11 S/cm. XRD, TEM, and STEM/EELS analysis confirm that the films are amorphous and indicate that oxygen background gas is necessary during the PLD process to decrease the oxygen vacancy concentration, decreasing the electrical conductivity. Amorphous LLTO is deposited onto high voltage LiNi0.5Mn1.5O4 (LNMO) spinel cathode thin films and cycled up to 4.8 V vs. Li showing excellent capacity retention. These results demonstrate that a-LLTO has the potential to be integrated into high voltage thin film batteries.

Lithium metal is viewed as the ultimate battery anode because of its high theoretical capacity and low electrode potential, but its implementation has been limited by low Coulombic efficiency and dendrite formation above a critical current density. Determining the fundamental properties dictating lithium metal plating-stripping behavior is challenging because characterization techniques are limited by the sensitivity of lithium metal to damage by external probes, which regularly results in altered morphology and chemistry. Motivated by recent application of cryogenic transmission electron microscopy (cryo-TEM) to characterize lithium metal at the atomic scale, we explore the cryogenic focused ion beam (cryo-FIB) method as a quantitative tool for characterizing the bulk morphology of electrochemically deposited lithium and as a technique that enables TEM observation of Li-metal/solid-state electrolyte interfaces. This work highlights the importance of cryo-FIB methodology for preparing sensitive battery materials and elucidates the impact of electrolyte selection on the density and morphology of plated lithium.

Among the several challenges to enable next-generation batteries is the development of an electrolyte which compatible with both lithium (Li) metal anode and high-voltage cathode at wide temperature range. Liquefied gas electrolytes with a new cosolvent and higher salt concentration show improved ionic conductivity of > 4 mS/cm at wide temperature range from -80 to +70 °C. With a new solvation structure, the liquefied gas electrolytes demonstrated high-temperature operation of Li-metal batteries at 55°C, which is the operation above the electrolytes’ critical point for the first time. The electrolytes enable improved Li metal stability and coulombic efficiency at aggressive current and capacity of 3 mA·cm-2 and 3 mAh·cm-2 with average coulombic efficiency of 99%. The use of liquefied gas electrolytes presents stable cycling of Li/NMC (4.4 V) cell at all-temperature range between -60 and 55°C. This study opens up a promising avenue toward the applications of all-temperature high energy density Li-metal batteries.

In this work, we investigated the interface between the sodium anode and the sulfide-based solid electrolytes Na₃SbS₄ (NAS), Na₃PS₄ (NPS), and Cl-doped NPS (NPSC) in all-solid-state-batteries (ASSBs). Even though these electrolytes have demonstrated high ionic conductivities in the range of 1 mS cm^-1 at ambient temperatures, sulfide sold-state electrolytes (SSEs) are known to be unstable with Na metal, though the exact reaction mechanism and kinetics of the reaction remain unclear. We demonstrate that the primary cause of capacity fade and cell failure is a chemical reaction spurred on by electrochemical cycling that takes place at the interface between the Na anode and the SSEs. To investigate the properties of the Na-solid electrolyte interphase (SSEI) and its effect on cell performance, the SSEI was predicted computationally to be composed of Na₂S and Na₃Sb for NAS and identified experimentally via X-ray photoelectron spectroscopy (XPS). These two compounds give the SSEI mixed ionic- and electronic-conducting properties, which promotes continued SSEI growth, which increases the cell impedance at the expense of cell performance and cycle life. The SSEI for NPS was similarly found to be comprised of Na₂S and Na₃P, but XPS analysis of Cl-doped NPS (NPSC) showed the presence of an additional compound at the SSEI, NaCl, which was found to mitigate the decomposition of NPS. The methodologies presented in this work can be used to predict and optimize the electrochemical behavior of an all-solid-state cell. Such joint computational and experimental efforts can inform strategies for engineering a stable electrolyte and SSEI to avoid such reactions. Through this work, we call for more emphasis on SSE compatibility with both anodes and cathodes, essential for improving the electrochemical properties, longevity, and practicality of Na-based ASSBs.

The electrochemical performance and mechanistic effects of incorporating two salts in an ether electrolyte in Li-metal cells were investigated experimentally and via molecular scale modeling. Improvements in efficiency and cycling stability over baseline electrolytes in lithium (Li) versus copper (Cu) cells were directly correlated to the initial Li-nucleation process, as observed via cryogenic-focused ion beam (cryo-FIB) prepared cross sections, which revealed that Li films deposited with the bisalt electrolyte were significantly thinner and denser than those from the baseline. This behavior was further traced back to the initial nucleation process via cryogenic transmission electron microscopy (cryo-TEM), which shows stark differences in the morphology and conformality of deposited Li as a function of electrolyte chemistry. X-ray photoelectron spectroscopy (XPS) indicated that the solid electrolyte interphase (SEI) formed from the bisalt electrolyte primarily consists of larger anion fragments, suggesting that the anion chemistry strongly influences the extent of reduction and resulting surface chemistry. Ab initio DFT-based and force field-based molecular dynamics calculations revealed the complex interplay (positioning, orientation, reactivity, kinetics) between the FSI - and TFSI - anions in the double layer at both electrode-electrolyte interfaces and the ramifications for stabilizing these interfaces. As a result of the unique interphasial chemistry brought by this bisalt system, this electrolyte supports an unprecedented (for an ether-based electrolyte) capacity retention (>88%) after 300 cycles (∼2 months of cycling) in a 4.4 V NMC622-Li cell, and a >30% improvement in capacity retention over baseline after 50 cycles in 4.4 V NMC622-Cu "anode-free" cells. Altogether, these results provide new insight into how the bisalt effect can be leveraged for regulating the timescale, chemistry, and extent of interfacial reactions. When balanced properly, this promotes efficient plating/deplating of Li, and potentially supports widespread implementation of high nickel content NMC cell configurations with limited or no excess lithium.

Development of safe electrolytes that are compatible with both lithium metal anodes and high-voltage cathodes that can operate in a wide-temperature range is a formidable, yet important challenge. Recently, a new class of electrolytes based on liquefied gas solvents has shown promise in addressing this issue. Concerns, however, have been raised on the pressure, flammability and low maximum operating temperature of these systems. Here, we endeavor to mitigate safety and practicality concerns by demonstrating an enhanced safety feature inherent in liquefied gas electrolytes and by showing the viability of using difluoromethane as a liquefied gas solvent which has lower pressure, lower flammability, and improved maximum operation temperature characteristics compared with fluoromethane. We create a custom-built setup to enable liquefied gas electrolyte characterization through Raman spectroscopy and supplement this with molecular dynamics (MD) simulations. The electrolyte shows good conductivity through a wide temperature range and compatibility with both the lithium metal anode and 4 V class cathodes. The demonstrated use of such alternative liquefied gas solvents opens a path towards the further development of high-energy and safe batteries that can operate in a wide-temperature range.