Electrochemical impedance spectroscopy of a Li–S battery: Part 2. Influence of separator chemistry on the lithium electrode/electrolyte interface

Improvement in the Electrochemical Properties of Lithium Metal by Heat Treatment: Changes in the Chemical Composition of Native and Solid Electrolyte Interphase Films

This study aims to improve the electrochemical properties of lithium metal for application as a negative electrode in high-energy-density batteries. Lithium metal was heat-treated at varying temperatures to modify the native and solid electrolyte interphase (SEI) films, which decreased the interfacial resistance between the lithium electrode and electrolyte, thereby improving the cycling performance. Moreover, the influence of the native and SEI films on lithium metals depended on the heat-treatment temperature. Accordingly, X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical composition of the native and SEI films on the heat-treated lithium metals before and after immersion in an organic electrolyte solution. The XPS results revealed the high dependence of the chemical composition of the outer layer of the native and SEI films on the heat-treatment temperature, implying that the native and SEI films can be effectively modified by heat treatment.

Flexible Quasi-Solid-State Composite Electrolyte of Poly (Propylene Glycol)-co-Pentaerythritol Triacry-Late/Li1.5Al0.5Ge1.5(PO4)3 for High-Performance Lithium-Sulfur Battery

With a higher theoretical specific capacity (1675 mAh g^-1) and energy density (2600 Wh kg^-1), the lithium-sulfur (Li-S) battery is considered as a promising candidate for a next-generation energy storage device. However, the shuttle effect of polysulfides as well as the large interfacial impedance between brittle solid electrolyte and electrodes lead to the capacity of the Li-S battery decaying rapidly, which limits the practical commercial applications of the Li-S battery. Herein, we reported a facile in situ ultraviolet (UV) curing method to prepare a flexible quasi-solid-state composite electrolyte (QSSCE) of poly(propylene glycol)-co-pentaerythritol triacrylate/Li_1.5Al_0.5Ge_1.5(PO₄)₃ (PPG-co-PETA/LAGP). By combining the high Li-ion conductivity and mechanical strength of inorganic NASICON-structure LAGP and good flexibility of the crosslinked PPG-co-PETA with nanopore structure, the flexible QSSCE with 66.85 wt% LAGP exhibited high Li-ion conductivity of 5.95 × 10^-3 S cm^-1 at 25 °C, Li-ion transference number of 0.83 and wide electrochemical window of ~5.0 V (vs. Li/Li⁺). In addition, the application of QSSCE in the Li-S battery could suppress the shuttle effect of polysulfides effectively, thus the Li-S battery possessed the excellent electrochemical cyclic performance, showing the first-cycle discharge-specific capacity of 1508.1 mAh g^-1, the capacity retention of 73.6% after 200 cycles with 0.25 C at 25 °C and good rate performance.

Electrochemical Impedance Spectroscopy and X-ray Photoelectron Spectroscopy Study of Lithium Metal Surface Aging in Imidazolium-Based Ionic Liquid Electrolytes Performed at Open-Circuit Voltage

Lithium reactivity toward an electrolytic media and dendrite growth phenomenon constitutes the main drawback for its use as an anode material for the lithium battery technology. Ionic liquids (ILs) were pointed out as promising electrolyte solvent candidates to prevent thermal runaway in a lithium battery system. However, the reactivity of lithium toward such a kind of an electrolyte is still under debate. In this study, the interaction between lithium metal and imidazolium-based ILs, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C₁C₆ImTFSI) and 1-hexyl-3-methylimidazolium bis(fluorosulfonyl)imide (C₁C₆ImFSI), has been investigated based on the nondestructive methodology coupling electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) in coin cells aged several days at open-circuit voltage. The main components detected by XPS in the bulk separator and at the surface of the lithium metal are the byproducts of cation and anion degradation. Similarities and differences were noticed depending on the anion nature of bis(trifluoromethylsulfonyl)imide versus bis(fluorosulfonyl)imide. The role of lithium salt addition (LiTFSI) was also pointed, giving rise to the stability improvement of the electrolytic solution toward the lithium anode. A direct correlation between the resistance of the bulk electrolyte and of the interface electrolyte/lithium and chemical composition changes were established based on a detailed EIS and XPS combined study.

Applications of Metal-Organic-Framework-Derived Carbon Materials

Carbon materials derived from metal-organic frameworks (MOFs) have attracted much attention in the field of scientific research in recent years because of their advantages of excellent electron conductivity, high porosity, and diverse applications. Tremendous efforts are devoted to improving their chemical and physical properties, including optimizing the morphology and structure of the carbon materials, compositing them with other materials, and so on. Here, many kinds of carbon materials derived from metal-organic frameworks are introduced with a particular focus on their promising applications in batteries (lithium-ion batteries, lithium-sulfur batteries, and sodium-ion batteries), supercapacitors (metal oxide/carbon and metal sulfide/carbon), electrocatalytic reactions (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction), water treatment (MOF-derived carbon and other techniques), and other possible fields. To close, some existing problem and corresponding possible solutions are proposed based on academic knowledge from the reported literature, along with a great deal of experimental experience.

Free-Standing Selenium Impregnated Carbonized Leaf Cathodes for High-Performance Sodium-Selenium Batteries

A novel approach of carbonizing leaves by thermal pyrolysis with melt diffusion followed by selenium vapor deposition is developed to prepare the carbon-selenium composite cathodes for sodium-selenium batteries. The carbonized leaf possesses internal hierarchical porosity and high mass loading; therefore, the composite is applied as a binder- and current collector-free cathode, exhibiting an excellent rate capability and a high reversible specific capacity of 520 mA h g^-1 at 100 mA g^-1 after 120 cycles and 300 mA h g^-1 even at 2 A g^-1 after 500 cycles without any capacity loss. Moreover, the unique natural three-dimensional structure and moderate graphitization degree of leaf-based carbon facilitate Na⁺/e^- transport to activate selenium which can guarantee a high utilization of the selenium during discharge/charge process, demonstrating a promising strategy to fabricate advanced electrodes toward the sodium-selenium batteries.

Nanopore separator of cross-linked poly(propylene glycol)-co-pentaerythritol triacrylate for effectively suppressing polysulfide shuttling in Li–S batteries

A nanopore polymer separator blocks the polysulfide migration more efficiently than the Celgard separator, endowing a Li–S battery with a much better discharge performance.

How reliable is the Na metal as a counter electrode in Na-ion half cells?

Despite the extensive research on Na-ion batteries little is known about the stability of the Na-metal counter electrode in a half-cell configuration.

Two-Dimensional CeO2/RGO Composite-Modified Separator for Lithium/Sulfur Batteries

In this work, a modified separator coated with a functional layer of reduced graphene oxide (RGO) anchored by cerium oxide (CeO₂) nanoparticles was developed. The superior conductivity of RGO and chemical immobilization of high-ordered sulfur-related species (mainly Li₂S_n 4 ≤ n ≤ 8) of CeO₂ yielded batteries with enhanced characteristics. A remarkable original capacity of 1136 mAh g^-1 was obtained at 0.1 C with capacity retention ratio of 75.7% after 100 charge/discharge cycles. Overall, these data indicate that the separator with CeO₂/RGO composite is promising to suppress the shuttling of polysulfides for better utilization of the active material.

Assessment on the Self-Discharge Behavior of Lithium-Sulfur Batteries with LiNO3-Possessing Electrolytes

It is generally understood that the reduction of nitrate on the metallic Li surface aids in the formation of a solid-electrolyte interphase. LiNO₃ is, therefore, frequently used as an electrolyte additive to help suppress the polysulfide redox shuttle in lithium-sulfur (Li-S) batteries. Although LiNO₃ enables cycling of cells with considerably improved Coulombic efficiency and cyclic performance, the self-discharge behavior has largely been neglected. We present in this work a basic but systematic study to assess self-discharge of Li-S batteries with electrolytes possessing LiNO₃. Comparative electrochemical tests and interfacial analysis reveal that the redox shuttle is fast enough to cause cells to self-discharge at a relatively rapid rate with limited concentration of the LiNO₃ additive. Despite the capacity loss of a full-charged cell under rest for one day can be controlled to 2% with LiNO₃ concentration as high as 0.5 M, the development of a practically viable Li-S technology looks like a daunting challenge. Further increasing LiNO₃ would potentially cause more irreversible reduction of LiNO₃ on the cathode during the first discharge. Therefore, a possible pathway for a long shelf life and low self-discharge is offered as well by the synergic protection of the separator and stabilization of the Li anode surface. The cell using a nanosized Al₂O₃-coated microporous membrane and a LiNO₃-possessing electrolyte exhibits an extremely suppressed self-discharge, providing an alternative perspective for the practical use of Li-S batteries.