Li7La3Zr2O12 Protonation as a Means to Generate Porous/Dense/Porous-Structured Electrolytes for All-Solid-State Lithium-Metal Batteries.

Ceramic Li7La3Zr2O12 (LLZO) represents a promising candidate electrolyte for next-generation all-solid-state lithium-metal batteries. However, lithium-metal batteries are prone to dendrite formation upon fast charging. Porous/dense and porous/dense/porous LLZO structures were proposed as a solution to avoid or at least delay the formation of lithium-metal dendrites by increasing the electrode/electrolyte contact area and thus lowering the local current density at the interface. In this work, we show the feasibility of producing porous/dense/porous LLZO by a new and scalable method. The method consists of LLZO chemical deep protonation in a protic or acidic solvent, followed by thermal deprotonation at high temperatures to create the porous structure by water and lithium oxide elimination. We demonstrate that the produced structure extends the lifetime of Li/LLZO/Li symmetric cells by a factor of 8 compared to a flat LLZO at a current density of 0.1 mA/cm2 and with a capacity of 1 mAh/cm2 per half-cycle. We also show clear improvement of the Li/LLZO/LiFePO4 full cell performance with a thermally deprotonated LLZO.

Impact of Protonation on the Electrochemical Performance of Li7La3Zr2O12 Garnets.

Li7La3Zr 2O12 (LLZO) garnet ceramics are promising electrolytes for all-solid-state lithium-metal batteries with high energy density. However, these electrolytes are prone to Li+/H+ exchange, that is, protonation, resulting in degradation of their Li-ion conductivity. Here, we identify how common processing steps, such as surface cleaning in alcohol or acetone, trigger LLZO partial protonation. We deconvolute the contributions to the electrochemical impedance spectra of both the protonated LLZO phase (HLLZO) and its decomposition products forming upon annealing. While the mixed conduction of H+/Li+ ions in HLLZO decreases the contribution of the electrolyte to the overall impedance, it deteriorates the transport of Li+ ions across the LLZO/Li interface. This is also the case after thermal decomposition of HLLZO, which occurs at significantly lower temperature than that for pristine LLZO. As a result, symmetric Li/LLZO/Li cells suffer from inhomogeneous lithium electrodeposition within the first three cycles when stripping and plating a capacity of 1 mA·h/cm2 per half-cycle at 0.1 mA/cm2. We demonstrate that LLZO protonation is avoided when applying solvents with very low acidity, such as hexane. Such Li/LLZO/Li cells provide stable cycling over more than 300 h, demonstrating the importance of selecting an appropriate solvent for LLZO processing to prevent dendrites formation.

Thermomechanical Polymer Binder Reactivity with Positive Active Materials for Li Metal Polymer and Li-Ion Batteries: An XPS and XPS Imaging Study.

The lithium and lithium-ion battery electrode chemical stability in the pristine state has rarely been considered as a function of the binder choice and the electrode processing. In this work, X-ray photoelectron spectroscopy (XPS) and XPS imaging analyses associated with complementary Mössbauer spectroscopy are used in order to study the chemical stability of two pristine positive electrodes: (i) an extruded LiFePO4-based electrode formulated with different polymer matrices [polyethylene oxide and a polyvinylidene difluoride (PVdF)] and processed at different temperatures (90 and 130 °C, respectively) and (ii) a Li[Ni0.5Mn0.3Co0.2]O2 (NMC)-based electrode processed by tape-casting, followed by a mild or heavy calendering treatment. These analyses have allowed the identification of reactivity mechanisms at the interface of the active material and the polymer in the case of PVdF-based electrodes.

New airtight transfer box for SEM experiments: Application to lithium and sodium metals observation and analyses.

The surface of some materials reacts very quickly on contact with air, either because it is oxidized or because it gets humidity from the air. For the sake of original surface observation by scanning electron microscopy (SEM), we conceived an airtight transfer box to keep the samples under vacuum from the place of manufacturing to the SEM chamber. This object is designed to fit in all the models of SEM including those provided with an airlock chamber. The design is voluntarily simplified to allow the manufacturing of the object by a standard mechanical workshop. The transfer box can be easily opened by gravity inside the SEM and allows the preservation of the best vacuum inside, before opening. SEM images and energy dispersive spectroscopy (EDX) analyses of metallic lithium and sodium samples are presented prior and after exposure to the air. X-ray Photoelectron Spectroscopy (XPS) analyses of all samples are also discussed in order to investigate the chemical environments of the detected elements.