Transport Properties and Local Ions Dynamics in LATP-Based Hybrid Solid Electrolytes

Hybrid solid electrolytes (HSEs), namely mixtures of polymer and inorganic electrolytes, have supposedly improved properties with respect to inorganic and polymer electrolytes. In practice, HSEs often show ionic conductivity below expectations, as the high interface resistance limits the contribution of inorganic electrolyte particles to the charge transport process. In this study, the transport properties of a series of HSEs containing Li(1+ x ) Alx Ti(2- x ) (PO4 )3 (LATP) as Li+ -conducting filler are analyzed. The occurrence of Li+ exchange across the two phases is proved by isotope exchange experiment, coupled with 6 Li/7 Li nuclear magnetic resonance (NMR), and by 2D 6 Li exchange spectroscopy (EXSY), which gives a time constant for Li+ exchange of about 50 ms at 60 °C. Electrochemical impedance spectroscopy (EIS) distinguishes a short-range and a long-range conductivity, the latter decreasing with LATP concentration. LATP particles contribute to the overall conductivity only at high temperatures and at high LATP concentrations. Pulsed field gradient (PFG)-NMR suggests a selective decrease of the anions' diffusivity at high temperatures, translating into a marginal increase of the Li+ transference number. Although the transport properties are only marginally affected, addition of moderate amounts of LATP to polymer electrolytes enhances their mechanical properties, thus improving the plating/stripping performance and processability.

Synergistic theoretical and experimental study on the ion dynamics of bis(trifluoromethanesulfonyl)imide-based alkali metal salts for solid polymer electrolytes

Model validation of a well-known class of solid polymer electrolyte (SPE) is utilized to predict the ionic structure and ion dynamics of alternative alkali metal ions, leading to advancements in Na-, K-, and Cs-based SPEs for solid-state alkali metal batteries. A comprehensive study based on molecular dynamics (MD) is conducted to simulate ion coordination and the ion transport properties of poly(ethylene oxide) (PEO) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt across various LiTFSI concentrations. Through validation of the MD simulation results with experimental techniques, we gain a deeper understanding of the ionic structure and dynamics in the PEO/LiTFSI system. This computational approach is then extended to predict ion coordination and transport properties of alternative alkali metal ions. The ionic structure in PEO/LiTFSI is significantly influenced by the LiTFSI concentration, resulting in different lithium-ion transport mechanisms for highly concentrated or diluted systems. Substituting lithium with sodium, potassium, and cesium reveals a weaker cation-PEO coordination for the larger cesium-ion. However, sodium-ion based SPEs exhibit the highest cation transport number, indicating the crucial interplay between salt dissociation and cation-PEO coordination for achieving optimal performance in alkali metal SPEs.

Molecular-Level Insight into Charge Carrier Transport and Speciation in Solid Polymer Electrolytes by Chemically Tuning Both Polymer and Lithium Salt

The advent of Li-metal batteries has seen progress toward studies focused on the chemical modification of solid polymer electrolytes, involving tuning either polymer or Li salt properties to enhance the overall cell performance. This study encompasses chemically modifying simultaneously both polymer matrix and lithium salt by assessing ion coordination environments, ion transport mechanisms, and molecular speciation. First, commercially used lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is taken as a reference, where F atoms become partially substituted by one or two H atoms in the -CF₃ moieties of LiTFSI. These substitutions lead to the formation of lithium(difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) salts. Both lithium salts promote anion immobilization and increase the lithium transference number. Second, we show that exchanging archetypal poly(ethylene oxide) (PEO) with poly(ε-caprolactone) (PCL) significantly changes charge carrier speciation. Studying the ionic structures of these polymer/Li salt combinations (LiTFSI, LiDFTFSI or LiDFSI with PEO or PCL) by combining molecular dynamics simulations and a range of experimental techniques, we provide atomistic insights to understand the solvation structure and synergistic effects that impact macroscopic properties, such as Li⁺ conductivity and transference number.

New insights on the origin of chemical instabilities between poly(carbonate)-based polymer and Li-containing inorganic materials

Composites electrolytes, owing to their potential to combine both polymeric and ceramic properties, are promising candidates for Solid-State-Batteries (SSBs). Here, we assessed the effect of ceramic fillers (Li1+xAlxTi2-xP3O12, Li6.55Ga0.15La3Zr2O12, Al2O3) in a poly(ethylene oxide carbonate)-LiTFSI. First, the role of filler chemistry on thermal and electrochemical properties is evaluated: the polymer crystallinity is reduced, resulting in a gain of ionic conductivity at low temperatures; and the ionic conductivity at low temperature (<30 °C) is boosted for LLZO filler particles. This behaviour is commonly attributed to new conduction pathways generated within the fillers; however, here we demonstrate that a polymer degradation induced by the filler chemistry modifies the polymer chemistry in poly(ethylene glycol), initiated by LiOH that can be found on the LLZO surface. The electrolyte containing LATP or Al2O3 does not under any degradation. Hence, special attention must be paid to surface impurities, as instability/degradation may occur.

Stable non-corrosive sulfonimide salt for 4-V-class lithium metal batteries

Rechargeable lithium metal (Li⁰) batteries (RLMBs) are considered attractive for improving Li-ion batteries. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) has been extensively used as a conducting salt for RLMBs due to its advantageous stability and innocuity. However, LiTFSI-based electrolytes are corrosive towards aluminium (Al⁰) current collectors at low potentials (>3.8 V versus Li/Li⁺), thereby excluding their application in 4-V-class RLMBs. Herein, we report on a non-corrosive sulfonimide salt, lithium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI), that remarkably suppresses the anodic dissolution of the Al⁰ current collector at high potentials (>4.2 V versus Li/Li⁺) and significantly improves the cycling performance of Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC111) cells. In addition, this sulfonimide salt results in the growth of an advantageous solid electrolyte interphase on the Li⁰ electrode. The replacement of either LiTFSI or LiPF₆ with LiDFTFSI endows a Li⁰||NMC111 cell with superior cycling stability and capacity retention (87% at cycle 200), demonstrating the decisive role of the salt anion in dictating the electrochemical performance of RLMBs.

Flame retardant polyphosphoester copolymers as solid polymer electrolyte for lithium batteries

Solid-state lithium batteries are considered one of the most promising battery systems due to their high volumetric energy density, in this work a flame retarded polymer electrolyte is proposed.

Effect of Chemical Structure and Salt Concentration on the Crystallization and Ionic Conductivity of Aliphatic Polyethers

Poly(ethylene oxide) (PEO) is the most widely used polymer in the field of solid polymer electrolytes for batteries. It is well known that the crystallinity of polymer electrolytes strongly affects the ionic conductivity and its electrochemical performance. Nowadays, alternatives to PEO are actively researched in the battery community, showing higher ionic conductivity, electrochemical window, or working temperature range. In this work, we investigated polymer electrolytes based on aliphatic polyethers with a number of methylene units ranging from 2 to 12. Thus, the effect of the lithium bis(trifluoromethanesulfone) imide (LiTFSI) concentration on the crystallization behavior of the new aliphatic polyethers and their ionic conductivity was investigated. In all the cases, the degree of crystallinity and the overall crystallization rate of the polymers decreased drastically with 30 wt % LiTFSI addition. The salt acted as a low molecular diluent to the polyethers according to the expectation of the Flory–Huggins theory for polymer–diluent mixtures. By fitting our results to this theory, the value of the interaction energy density (B) between the polyether and the LiTFSI was calculated, and we show that the value of B must be small to obtain high ionic conductivity electrolytes.