Tailoring Stress and Ion-Transport Kinetics via a Molecular Layer Deposition-Induced Artificial Solid Electrolyte Interphase for Durable Silicon Composite Anodes

Reversible silicon anodes enabled by fluorinated inorganic-organic hybrid coating

Silicon anodes deliver batteries with energy densities much higher than those based on today's dominant graphite anodes. However, they commonly exhibit huge volume variations and unfavorable interface stability, causing a gradually diminishing capacity on extended cycling. Most Si-based batteries consisting Si/C composites in industry can only use a very limited amount of Si (<30 % by weight). Exploiting molecular layer deposition (MLD) technique, a fluorine-rich flexible inorganic-organic hybrid alucone (AlFHQ) shell is controllably deposited onto Si electrodes. Employing ex situ XPS and AFM, the AlFHQ film presents reversible electrochemical evolutions in terms of composition and morphology. The interactions the between Li+ and -O-2,3,5,6-fluorobenzene functional groups help to construct a mechanically-chemically robust LiF-rich hybrid solid electrolyte interphase (SEI) on Si anode, delivering enhanced interfacial stability and integrity of electrode. An optimized coating thickness (≈5 nm) for interfacial stabilization and Li+ transport kinetics is demonstrated, namely, Si@AlFHQ-20. The reported fluorine-rich hybrid modification technique endows (a), stable cycling of Si anode (≈3.8 mAh cm-2) with an ultrahigh initial Coulombic efficiency (ICE) of 92.3 %; (b), enhanced rate capability of 1468 mAh/g at 2.0 A g-1 and good cycling performance; and (c), overall cell (Si@AlFHQ-20//LiCoO2@Al2O3) operational stability for more than 100 cycles under stringent cathode conditions (2.68 mAh cm-2, high cutoff voltage at 4.55 V).

Well-Dispersed Bi nanoparticles for promoting the lithium storage performance of Si Anode: Effect of the bridging Bi nanoparticles

Silicon (Si) is considered a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical specific capacity of up to 4200 mAh/g. However, the poor cycling and rate performances of Si induced by the low intrinsic electronic conductivity and large volume expansion during the lithiation/delithiation process limit its practical application. Herein, a novel silicon/bismuth@nitrogen-doped carbon (Si/Bi@NC) composite with nanovoids was synthesized and investigated as an advanced anode material for LIBs. In such a structure, ultrafine bismuth nanoparticles coupled with an N-doped carbon layer were introduced to modify the surface of Si nanoparticles. Subsequently, the lithiated LixBi has excellent high ionic conductivity and acts as a fast transport bridge for lithium ions. The introduced carbon coating layer and nanovoids can buffer the volume expansion of Si during the lithiation/delithiation process, thus maintaining structural stability during the cycling process. As a result, the Si/Bi@NC composite exhibits excellent electrochemical performance, providing a relatively high capacity of 955.8 mAh/g at 0.5 A/g after 450 cycles and excellent rate performance with a high capacity of 477.8 mAh/g even at 10.0 A/g. Furthermore, the assembled full cell with LiFePO4 as cathode and pre-lithium Si/Bi@NC as anode can provide a high capacity of 138.8 mAh/g at 1C after 90 cycles, exhibiting outstanding cycling performance.

Ion-Solvent Interplay in Concentrated Electrolytes Enables Subzero Temperature Li-Ion Battery Operations

Despite the essential role of ethylene carbonate (EC) in solid electrolyte interphase (SEI) formation, the high Li⁺ desolvation barrier and melting point (36 °C) of EC impede lithium-ion battery operation at low temperatures and induce sluggish Li⁺ reaction kinetics. Here, we demonstrate an EC-free high salt concentration electrolyte (HSCE) composed of lithium bis(fluorosulfonyl)imide salt and tetrahydrofuran solvent with enhanced subzero temperature operation originating from unusually rapid low-temperature Li⁺ transport. Experimental and theoretical characterizations reveal the dominance of intra-aggregate ion transport in the HSCE that enables efficient low-temperature transport by increasing the exchange rate of solvating counterions relative to that of solvent molecules. This electrolyte also produces a <5 nm thick anion-derived LiF-rich SEI layer with excellent graphite electrode compatibility and electrochemical performance at subzero temperature in half-cells. Full cells based on LiNi_0.6Co_0.2Mn_0.2O₂||graphite with tailored HSCE electrolytes outperform state-of-the-art cells comprising conventional EC electrolytes during charge-discharge operation at an extreme temperature of -40 °C. These results demonstrate the opportunities for creating intrinsically robust low-temperature Li⁺ technology.