Optical sensors for operando stress monitoring in lithium-based batteries containing solid-state or liquid electrolytes.

The study of chemo-mechanical stress taking place in the electrodes of a battery during cycling is of paramount importance to extend the lifetime of the device. This aspect is particularly relevant for all-solid-state batteries where the stress can be transmitted across the device due to the stiff nature of the solid electrolyte. However, stress monitoring generally relies on sensors located outside of the battery, therefore providing information only at device level and failing to detect local changes. Here, we report a method to investigate the chemo-mechanical stress occurring at both positive and negative electrodes and at the electrode/electrolyte interface during battery operation. To such effect, optical fiber Bragg grating sensors were embedded inside coin and Swagelok cells containing either liquid or solid-state electrolyte. The optical signal was monitored during battery cycling, further translated into stress and correlated with the voltage profile. This work proposes an operando technique for stress monitoring with potential use in cell diagnosis and battery design.

Extending insertion electrochemistry to soluble layered halides with superconcentrated electrolytes.

Insertion compounds provide the fundamental basis of today's commercialized Li-ion batteries. Throughout history, intense research has focused on the design of stellar electrodes mainly relying on layered oxides or sulfides, and leaving aside the corresponding halides because of solubility issues. This is no longer true. In this work, we show the feasibility of reversibly intercalating Li+ electrochemically into VX3 compounds (X = Cl, Br, I) via the use of superconcentrated electrolytes (5 M LiFSI in dimethyl carbonate), hence opening access to a family of LixVX3 phases. Moreover, through an electrolyte engineering approach, we unambiguously prove that the positive attribute of superconcentrated electrolytes against the solubility of inorganic compounds is rooted in a thermodynamic rather than a kinetic effect. The mechanism and corresponding impact of our findings enrich the fundamental understanding of superconcentrated electrolytes and constitute a crucial step in the design of novel insertion compounds with tunable properties for a wide range of applications including Li-ion batteries and beyond.

Li-Rich Layered Sulfide as Cathode Active Materials in All-Solid-State Li-Metal Batteries.

Great hopes are placed on all-solid-state Li-metal batteries (ASSBs) to boost the energy density of the current Li-ion technology. However, these devices still present a number of unresolved issues that keep them far from commercialization; such as interfacial instability, lithium dendrite formation, and lack of mechanical integrity during cycling. To mitigate these limiting aspects, the most advanced ASSB systems presently combine a sulfide- or oxide-based solid electrolyte (SE) with a coated Li-based oxide as the positive electrode and a lithium anode. Through this work, we propose a different twist by switching from layered oxides to layered sulfides as active cathode materials. Herein, we present the performance of a Li-rich layered sulfide of formula Li1.13Ti0.57Fe0.3S2 (LTFS) in room temperature operating all-solid-state batteries, using ß-Li3PS4 as SE and both InLi and Li anode materials. These batteries exhibit good cyclability, small polarization and, in the case of the Li anode, no initial irreversible capacity. We also suggest the possibility of using this Li-rich sulfide mixed with oxide cathode materials as part of the positive electrode in ASSBs in order to improve the cathode/sulfide SE interface. Our proof of concept using LiNi0.6 Mn0.2Co0.2O2 (NMC 622) showed that the addition of a small amount of LTFS had a direct positive impact in the battery performance.

Zinc Porphyrin Metal-Center Exchange at the Solid-Liquid Interface.

Demetalation of zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP) under acidic conditions and ion exchange with Cu(2+) ions at neutral pH are both rapid reactions in the liquid medium. However, for ZnTPP monolayers adsorbed on a Au(111) surface exposed to aqueous solution, we find that, although ion exchange takes place rapidly as expected, demetalation does not occur, even at pH values as low as 0. Based on this, we conclude that metal center exchange on the surface does not proceed through a free-base porphyrin as an intermediate. Furthermore, once formed, CuTPP is stable on the surface and the reverse exchange from CuTPP to ZnTPP in the presence of Zn(2+) ions could not be achieved. The preference for copper is so strong that even an attempt to exchange adsorbed ZnTPP with Ni(2+) ions in the presence of traces of Cu(2+) yielded CuTPP rather than NiTPP.

Surface Porphyrins Metalate with Zn Ions from Solution.

Controlling the metalation of surface porphyrins is a critical process in porphyrin-based devices. Indeed, surface porphyrins are known to metalate in ultrahigh vacuum from codeposited metal atoms or substrate atoms; however, it is not yet known if surface porphyrins could metalate from ions in solution, that is, the most likely environment for porphyrin-based devices. Using X-ray photoelectron spectroscopy we have studied the metalation of monolayers and multilayers of a free-base tetraphenyl porphyrin adsorbed on Au(111) with ions in solution. We found that full metalation with Zn(2+) can be achieved already at room temperature in contrast with the elevated temperatures required for metalation with codeposited metal atoms.