Tris(trimethylsilyl) Phosphite as an Efficient Electrolyte Additive To Improve the Surface Stability of Graphite Anodes

Tris­(trimethylsilyl) phosphite (TMSP) has received considerable attention as a functional additive for various cathode materials in lithium-ion batteries, but the effect of TMSP on the surface stability of a graphite anode has not been studied. Herein, we demonstrate that TMSP serves as an effective solid electrolyte interphase (SEI)-forming additive for graphite anodes in lithium-ion batteries (LIBs). TMSP forms SEI layers by chemical reactions between TMSP and a reductively decomposed ethylene carbonate (EC) anion, which is strikingly different from the widely known mechanism of the SEI-forming additives. TMSP is stable under cathodic polarization, but it reacts chemically with radical anion intermediates derived from the electrochemical reduction of the carbonate solvents to generate a stable SEI layer. These TMSP-derived SEI layers improve the interfacial stability of the graphite anode, resulting in a retention of 96.8% and a high Coulombic efficiency of 95.2%. We suggest the use of TMSP as a functional additive that effectively stabilizes solid electrolyte interfaces of both the anode and cathode in lithium-ion batteries

Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li–S Battery

The possibility of achieving high-energy, long-life storage batteries has tremendous scientific and technological significance. A prime example is the Li–S cell, which can offer a 3–5-fold increase in energy density compared with conventional Li-ion cells, at lower cost. Despite significant recent advances, there are challenges to its wide-scale implementation. Upon sulfur reduction, intermediate soluble lithium polysulfides readily diffuse into the electrolyte, causing capacity fading and poor Coulombic efficiency in the cell. Herein, we increase the capacity retention and cycle life of the Li–S cell through the use of nanocrystalline and mesoporous titania additives as polysulfide reservoirs and examine the role of surface <i>ad</i>sorption vs pore <i>ab</i>sorption. We find that the soluble lithium polysulfides are preferentially absorbed within the pores of the nanoporous titania at intermediate discharge/charge. This provides the major factor in stabilizing capacity although surface binding (adsorption) also plays a more minor role. A cell containing TiO₂ with a 5 nm pore diameter exhibited a 37% greater discharge capacity retention after 100 cycles than a cell without the titania additive, which was optimum compared to the other titania that were examined

Simultaneous Realization of Multilayer Interphases on a Ni-Rich NCM Cathode and a SiO_<i>x</i> Anode by the Combination of Vinylene Carbonate with Lithium Difluoro(oxalato)borate

Ni-rich NCM and SiOx electrode materials have garnered the most attention for advanced lithium-ion batteries (LIBs); however, severe parasitic reactions occurring at their interfaces are critical bottlenecks in their widespread application. In this study, an effective additive combination (VL) composed of vinylene carbonate (VC) and lithium difluoro(oxalato)borate (LiDFOB) is proposed for both Ni-rich NCM and SiOx electrode materials. The LiDFOB additive individually delivers inorganic-rich cathode–electrolyte interphase (CEI) and solid–electrolyte interphase (SEI) layers in anodic and cathodic polarizations before the VC additive. Subsequently, the VC additive is capable of the formation of additional CEI and SEI layers composed of relatively organic-rich components through an electrochemical reaction; thus, inorganic–organic hybridized CEI and SEI layers are simultaneously formed at the Ni-rich NCM and SiOx electrodes. Accordingly, the VL-assisted electrolyte exhibits remarkably prolonged cycling retention for the Ni-rich NCM cathode (86.5%) and SiOx anode (72.7%), whereas the standard electrolyte shows a substantial decrease in cycling retention for the Ni-rich NCM cathode (59.2%) and SiOx anode (18.1%). Further systematic analyses prove that VL-assisted electrolytes form effective interphases for Ni-rich NCM and SiOx electrodes simultaneously, thereby leading to stable and prolonged cycling behaviors of LIBs that offer high energy densities

Metal–Organic Framework as a Multifunctional Additive for Selectively Trapping Transition-Metal Components in Lithium-Ion Batteries

To improve the interfacial stability of lithium-ion batteries, a metal–organic framework (MOF) was designed and synthesized as an advanced additive for nickel-rich cathodes to trap the transition metal components. Use of the MOF was found to not compromise the specific capacity of the cells, and cells cycled with a nickel-rich layered oxide embedded with a metal–organic framework exhibited considerably improved cycle retention, even at high temperatures. A systematic analysis demonstrated that only negligible amounts of nickel-ion species migrated from the nickel-rich cathode to the anode surface, and the volume of nickel ions trapped inside the porous structure of the MOF could be determined by quantifying the mass change of the electrode. Finally, the surface degradation triggered by the nickel-ion dissolution was seen to be remarkably suppressed because the MOF improved the surface stability of the nickel-rich cathodes

Screening for Superoxide Reactivity in Li-O₂ Batteries: Effect on Li₂O₂/LiOH Crystallization

Unraveling the fundamentals of Li-O₂ battery chemistry is crucial to develop practical cells with energy densities that could approach their high theoretical values. We report here a straightforward chemical approach that probes the outcome of the superoxide O₂^–, thought to initiate the electrochemical processes in the cell. We show that this serves as a good measure of electrolyte and binder stability. Superoxide readily dehydrofluorinates polyvinylidene to give byproducts that react with catalysts to produce LiOH. The Li₂O₂ product morphology is a function of these factors and can affect Li-O₂ cell performance. This methodology is widely applicable as a probe of other potential cell components

Physically Cross-linked Polymer Binder Induced by Reversible Acid–Base Interaction for High-Performance Silicon Composite Anodes

Silicon is greatly promising for high-capacity anode materials in lithium-ion batteries (LIBs) due to their exceptionally high theoretical capacity. However, it has a big challenge of severe volume changes during charge and discharge, resulting in substantial deterioration of the electrode and restricting its practical application. This conflict requires a novel binder system enabling reliable cyclability to hold silicon particles without severe disintegration of the electrode. Here, a physically cross-linked polymer binder induced by reversible acid–base interaction is reported for high performance silicon-anodes. Chemical cross-linking of polymer binders, mainly based on acidic polymers including poly­(acrylic acid) (PAA), have been suggested as effective ways to accommodate the volume expansion of Si-based electrodes. Unlike the common chemical cross-linking, which causes a gradual and nonreversible fracturing of the cross-linked network, a physically cross-linked binder based on PAA–PBI (poly­(benzimidazole)) efficiently holds the Si particles even after the large volume changes due to its ability to reversibly reconstruct ionic bonds. The PBI-containing binder, PAA–PBI-2, exhibited large capacity (1376.7 mAh g^–1), high Coulombic efficiency (99.1%) and excellent cyclability (751.0 mAh g^–1 after 100 cycles). This simple yet efficient method is promising to solve the failures relating with pulverization and isolation from the severe volume changes of the Si electrode, and advance the realization of high-capacity LIBs

Surface Modification of Sulfur Electrodes by Chemically Anchored Cross-Linked Polymer Coating for Lithium–Sulfur Batteries

Lithium–sulfur batteries suffer from severe self-discharge due to polysulfide dissolution into electrolytes. In this work, a chemically anchored polymer-coated (CAPC) sulfur electrode was prepared, through chemical bonding by coordinated Cu ions and cross-linking, to improve cyclability for Li/S batteries. This electrode retained specific capacities greater than 665 mAh g^–1 at high current density of 3.35 A g^–1 (2<i>C</i> rate) after 100 cycles with an excellent Coulombic efficiency of 100%

Magnesium Anode Pretreatment Using a Titanium Complex for Magnesium Battery

Although magnesium batteries have received a great deal of attention as a promising power source, the native oxide layer on the Mg surface significantly impedes practical applications, because of the sluggish kinetic behavior of Mg-ion deposition and dissolution. Here, a new approach to improve electrochemical reactivity of Mg anode is proposed, based on chemical pretreatment of the Mg anode using a titanium complex, Ti­(TFSI)₂Cl₂, that effectively removes the native oxide layer on the Mg anode surface. The pretreatment of the Mg anode by Ti­(TFSI)₂Cl₂ remarkably decreases the binding affinity between Mg and O via the formation of a multicoordinate complex (Mg–O–Ti). Thereafter, a series of chemical reactions cleave the Mg–O bonds, resulting in a fresh Mg surface. This creates a cell comprised of the Ti­(TFSI)₂Cl₂-pretreated Mg anode, glyme-based electrolytes, and cathode material that exhibits reversible electrochemical behavior at the electrode/electrolyte interface, resulting in practical applicability and good electrochemical performance

Two-Dimensional Phosphorene-Derived Protective Layers on a Lithium Metal Anode for Lithium-Oxygen Batteries

Lithium-oxygen (Li-O₂) batteries are desirable for electric vehicles because of their high energy density. Li dendrite growth and severe electrolyte decomposition on Li metal are, however, challenging issues for the practical application of these batteries. In this connection, an electrochemically active two-dimensional phosphorene-derived lithium phosphide is introduced as a Li metal protective layer, where the nanosized protective layer on Li metal suppresses electrolyte decomposition and Li dendrite growth. This suppression is attributed to thermodynamic properties of the electrochemically active lithium phosphide protective layer. The electrolyte decomposition is suppressed on the protective layer because the redox potential of lithium phosphide layer is higher than that of electrolyte decomposition. Li plating is thermodynamically unfavorable on lithium phosphide layers, which hinders Li dendrite growth during cycling. As a result, the nanosized lithium phosphide protective layer improves the cycle performance of Li symmetric cells and Li-O₂ batteries with various electrolytes including lithium bis­(trifluoromethanesulfonyl)­imide in <i>N,N</i>-dimethylacetamide. A variety of <i>ex situ</i> analyses and theoretical calculations support these behaviors of the phosphorene-derived lithium phosphide protective layer