Solid-state electrolytes such as Li2S-P2S5 compounds are promising materials that could enable Li metal anodes. However, many solid-state electrolytes are unstable against metallic lithium, and little is known about the chemical evolution of these interfaces during cycling, hindering the rational design of these materials. In this work, operando X-ray photoelectron spectroscopy and real-time in situ Auger electron spectroscopy mapping are developed to probe the formation and evolution of the Li/Li2S-P2S5 solid-electrolyte interphase during electrochemical cycling, and to measure individual overpotentials associated with specific interphase constituents. Results for the Li/Li2S-P2S5 system reveal that electrochemically driving Li+ to the surface leads to phase decomposition into Li2S and Li3P. Additionally, oxygen contamination within the Li2S-P2S5 leads initially to Li3PO4 phase segregation, and subsequently to Li2O formation. The spatially non-uniform distribution of these phases, coupled with differences in their ionic conductivities, have important implications for the overall properties and performance of the solid-electrolyte interphase.

Silicon (Si) has been well recognized as a promising candidate to replace graphite because of its earth abundance and high‐capacity storage, but its large volume changes upon lithiation/delithiation and the consequential material fracturing, loss of electrical contact, and over‐consumption of the electrolyte prevent its full application. As a countermeasure for rapid capacity decay, a composite electrode of graphite and Si has been adopted by accommodating Si nanoparticles in a graphite matrix. Such an approach, which involves two materials that interact electrochemically with lithium in the electrode, necessitates an analytical methodology to determine the individual electrochemical behavior of each active material. In this work, a methodology comprising differential plots and integral calculus is established to analyze the complicated interplay among the two active batteries and investigate the failure mechanism underlying capacity fade in the blend electrode. To address performance deficiencies identified by this methodology, an aluminum alkoxide (alucone) surface‐modification strategy is demonstrated to stabilize the structure and electrochemical performance of the graphite‐Si composite electrode. The integrated approach established in this work is of great importance to the design and diagnostics of a multi‐component composite electrode, which is expected to be high interest to other next‐generation battery system.

In order to meet the critical energy-storage challenges of the future, a next-generation lithium-ion battery will need to achieve a higher energy density and longer cycle life. While increasing the nickel content in layered LiMO 2 (M = Ni, Mn, Co) significantly improves the capacity of the material, nickel-rich cathodes cycled in conventional organic electrolytes commonly suffer from crystallographic phase transformation and the growth of a resistive interfacial layer, both of which result in voltage fade and capacity degradation during cycling. However, pairing a nickel-rich cathode with an appropriate ionic liquid (IL) electrolyte enables exceptional cycling stability and energy retention. This work demonstrates how a pyrrolidinium-based IL electrolyte not only allows for cycling to higher voltages but shows a 95% energy retention and average discharge capacity of 189 mAh g −1 over 150 cycles between 3 and 4.5 V vs. Li/Li + with a nickel-rich layered cathode. Based on electrochemical and crystallographic analyses, the exceptional performance of the cells cycled in IL is attributed to the stability of the electrode-electrolyte interfacial layer formed by the IL which protects the active material and suppresses the structural degradation commonly observed in nickel-rich cathodes.

A new concept of pairing an active material and a mixed conductor is explored as a solid‐state battery electrode. By imbedding nano‐FeS2 domains into an amorphous LiTiS2 matrix, a hybrid power‐energy system is achieved while additionally improving upon many common solid electrode design flaws. High‐resolution transmission electron microscopy is used to probe the active material/mixed conductor interface over the course of cycling. Arguably the most beneficial development is enhancement of charge transfer, manifesting in a significantly increased exchange current as captured in a Tafel analysis. By developing a solution to active material isolation and creating a more homogenous electrode design, cycling at a high rate of C/2 for 500 cycles is obtained. Additionally, the electrode can recover full capacity simply by reducing system rate. Capacity recovery implicates a lack of active material isolation, a common problem in solid‐state batteries.