All‐solid‐state batteries with an alkali metal anode have the potential to achieve high energy density. However, the onset of dendrite formation limits the maximum plating current density across the solid electrolyte and prevents fast charging. It is shown that the maximum plating current density is related to the interfacial resistance between the solid electrolyte and the metal anode. Due to their high ionic conductivity, low electronic conductivity, and stability against sodium metal, Na‐β″‐alumina ceramics are excellent candidates as electrolytes for room‐temperature all‐solid‐state batteries. Here, it is demonstrated that a heat treatment of Na‐β″‐alumina ceramics in argon atmosphere enables an interfacial resistance <10 Ω cm2 and current densities up to 12 mA cm−2 at room temperature. The current density obtained for Na‐β″‐alumina is ten times higher than that measured on a garnet‐type Li7La3Zr2O12 electrolyte under equivalent conditions. X‐ray photoelectron spectroscopy shows that eliminating hydroxyl groups and carbon contaminations at the interface between Na‐β″‐alumina and sodium metal is key to reach such values. By comparing the temperature‐dependent stripping/plating behavior of Na‐β″‐alumina and Li7La3Zr2O12, the role of the alkali metal in governing interface kinetics is discussed. This study provides new insights into dendrite formation and paves the way for fast‐charging all‐solid‐state batteries.

Water‐in‐salt electrolytes have enabled the development of novel high‐voltage aqueous lithium‐ion batteries. This study explores the reasons why analogous sodium electrolytes have struggled to reach the same level of electrochemical stability. Solution structure and electrochemical stability are compared for 11 sodium salts, selected among the major classes of salts proposed for highly concentrated electrolytes. The water environment established for each anion is related to its position in the Hofmeister series and a surprisingly strong correlation between the chaotropicity of the anion and the resulting electrochemical stability of the electrolyte is found. The search for suitable sodium salts is complicated by the fact that higher salt concentrations are needed than for their lithium equivalents. Reaching such a high concentration of >25 mol kg−1 with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. Hence, alternative approaches such as multisolvent systems should be explored. The water solubility of NaTFSI can be increased from 8 to 30 mol kg−1 in the presence of ionic liquids. Such a ternary electrolyte enables stable cycling of a 2 V class sodium‐ion battery based on the NaTi2(PO4)3/Na2Mn[Fe(CN)6] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.

Using galvanostatic techniques, an oxidative stability up to 4.6 V versus Li/Li+ and beyond has been reported for the prototypical polymer electrolyte consisting of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in poly(ethylene oxide) (PEO). However, no long‐term cycling of a battery with this high cut‐off voltage has been demonstrated. Electrochemical and spectroscopic/spectrometric methods are employed to critically reinvestigate the electrochemical oxidation mechanisms of PEO electrolytes. It is found that the onset of PEO oxidation occurs at much lower voltage of around 3.2 V versus Li/Li+, at which the terminal OH group is deprotonated. At 3.6 V, the chain of the PEO is oxidized. Both processes result in the formation of the strong acid HTFSI, which in turn chemically attacks the PEO to form methanol and 2‐methoxyethanol. A stable cycling of a solid‐state lithium‐metal battery with a high‐energy LiNi0.8Mn0.1Co0.1O2 (NMC811) posititve electrode to an upper cut‐off voltage of 3.6 V versus Li/Li+ is demonstrated, however, resulting in enhanced capacity fading when increasing the upper cut‐off voltage to 3.8 V versus Li/Li+ or higher. Thus, operating PEO electrolytes beyond 3.6 V versus Li/Li+ requires protective layers at the positive electrode‐electrolyte interface to prevent PEO oxidation.

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Understanding the solid electrolyte interphase (SEI) in lithium batteries is very important to face the major safety issue of lithium dendritic growth during battery charge. The aim of this work is to study the thickness and the chemical nature of the SEI by XPS, as well as their influence on the electrochemical performance of the battery for different liquid organic electrolytes. XPS imaging is also used in this work to get a chemical mapping of the SEI layer components formed on the metallic lithium electrode surface cycled in different conditions. Data processing based on the principal component analysis (PCA) method has been conducted in order to illustrate the SEI layer heterogeneities. The obtained results are compared with energy-dispersive X-ray spectroscopy (EDX) mapping. Thereby, the benefits and the precision of the XPS imaging technique to identify chemical compounds distribution have been highlighted. These different analyses have led to a better knowledge of the redox processes occurring at the top surface of lithium metal electrodes cycled in different liquid electrolytes.

the low thermal stability and flammability of liquid electrolytes employed in traditional lithium-ion batteries. Solid electrolytes also promise to enable the use of lithium metal as negative electrode, which exhibits very high specific capacity (3860 mAh g −1 ) and the lowest electrochemical potential (−3.04 V) of all chemistries, offering a pathway to batteries with energy densities of >400 Wh kg −1 and >1200 Wh L −1 on cell level when paired with a high-energy positive electrode. [2][3][4][5][6][7][8][9][10] However, the electrochemical stability window of most solid electrolytes is typically not wide enough to enable stable cycling of lithium metal versus a 4 V-class positive electrode required to reach such high energy density. [11][12][13] Instead, most solid electrolytes tend to either chemically or electrochemically reduce at the interface to lithium metal and/or oxidize at the interface to the positive electrode resulting in poor cycling performance and poor cycle life, if no passivating interphase is formed. [14,15] Furthermore, lithium metal anodes are prone to the formation of socalled lithium metal dendrites upon charging, which can penetrate into the solid electrolyte and can cause the cell to shortcircuit. Voids forming at the interface between lithium metal and solid electrolytes upon battery discharge cause current constrictions and were demonstrated to promote dendrite formation. [16][17][18][19][20] Therefore, a solid electrolyte needs to not only form a stable interface to lithium metal and the positive electrode, but must also enable stable plating and stripping of lithium metal.Compared to inorganic solid electrolytes, polymer solid electrolytes are typically more flexible and able to maintain a much more intimate contact with the electrodes during cycling, which alleviates voids formation at the interface between lithium metal and solid electrolytes. However, the low room-temperature ionic conductivity and narrow electrochemical stability window hinder their application. [21][22][23] Incorporating plasticizers into polymer solid electrolytes helps to boost the ionic conductivity at room temperature while retaining the polymer's flexibility. [24][25][26] Polymer matrices, including poly(ethylene oxide) (PEO), [26][27][28][29] polyacrylonitrile (PAN), [30,31] poly(methyl methacrylate) (PMMA), [32,33] and poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP), [34][35][36][37] and plasticizers, such as carbonates, [38][39][40] Polymer solid electrolytes for solid-state batteries typically suffer from low ionic conductivity and low oxidative stability. Herein, a polymer electrolyte based on a polymerized ionic liquid and an ionic liquid plasticizer offering simultaneously a high room-temperature ionic conductivity of 0.8 mS cm −1 and a high oxidative stability >5.0 V versus Li + /Li, is reported. The electrolyte is compatible with lithium metal and non-flammable upon direct flame exposure. In symmetric lithium metal cells, the electrolyte enables stable lithium plating and stripping at 0.1...

Li 7 La 3 Zr 2 O 12 (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/cm 2 per half-cycle at 0.1 mA/cm 2 . 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.

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.