RESUMEN
Spinel-type lithium manganese oxide (LiMn2O4) cathodes suffer from severe manganese dissolution in the electrolyte, compromising the cyclic stability of LMO-based Li-ion batteries (LIBs). In addition to causing structural and morphological deterioration to the cathode, dissolved Mn ions can migrate through the electrolyte to deposit on the anode, accelerating capacity fade. Here, we examine single-crystal epitaxial LiMn2O4 (111) thin-films using synchrotron in situ X-ray diffraction and reflectivity to study the structural and interfacial evolution during cycling. Cyclic voltammetry is performed in a wide range (2.5-4.3 V vs Li/Li+) to promote Mn3+ formation, which enhances dissolution, for two different electrolyte systems: an imidazolium ionic liquid containing lithium bis-(trifluoromethylsulfonyl)imide (LiTFSI) and a conventional carbonate liquid electrolyte containing lithium hexafluorophosphate (LiPF6). We find exceptional stability in this voltage range for the ionic liquid electrolyte compared to the conventional electrolyte, which is attributed to the absence of Mn dissolution in the ionic liquid. X-ray reflectivity shows a negligible loss of cathode material for the films cycled in the ionic liquid electrolyte, further confirmed by inductively coupled plasma mass spectrometry and transmission electron microscopy. Conversely, a substantial loss of Mn is found when the film is cycled in the conventional electrolyte. These findings show the significant advantages of ionic liquids in suppressing Mn dissolution in LiMn2O4 LIB cathodes.
RESUMEN
LiNiO2 (LNO) is a promising cathode material for next-generation Li-ion batteries due to its exceptionally high capacity and cobalt-free composition that enables more sustainable and ethical large-scale manufacturing. However, its poor cycle life at high operating voltages over 4.1 V impedes its practical use, thus motivating efforts to elucidate and mitigate LiNiO2 degradation mechanisms at high states of charge. Here, a multiscale exploration of high-voltage degradation cascades associated with oxygen stacking chemistry in cobalt-free LiNiO2 , is presented. Lattice oxygen loss is found to play a critical role in the local O3-O1 stacking transition at high states of charge, which subsequently leads to Ni-ion migration and irreversible stacking faults during cycling. This undesirable atomic-scale structural evolution accelerates microscale electrochemical creep, cracking, and even bending of layers, ultimately resulting in macroscopic mechanical degradation of LNO particles. By employing a graphene-based hermetic surface coating, oxygen loss is attenuated in LNO at high states of charge, which suppresses the initiation of the degradation cascade and thus substantially improves the high-voltage capacity retention of LNO. Overall, this study provides mechanistic insight into the high-voltage degradation of LNO, which will inform ongoing efforts to employ cobalt-free cathodes in Li-ion battery technology.
RESUMEN
Ionogel electrolytes based on ionic liquids and gelling matrices offer several advantages for solid-state lithium-ion batteries, including nonflammability, wide processing compatibility, and favorable electrochemical and thermal properties. However, the absence of ionic liquids that are concurrently stable at low and high potentials constrains the electrochemical windows of ionogel electrolytes and thus their high-energy-density applications. Here, ionogel electrolytes with a layered heterostructure are introduced, combining high-potential (anodic stability: >5 V vs Li/Li+ ) and low-potential (cathodic stability: <0 V vs Li/Li+ ) imidazolium ionic liquids in a hexagonal boron nitride nanoplatelet matrix. These layered heterostructure ionogel electrolytes lead to extended electrochemical windows, while preserving high ionic conductivity (>1 mS cm-1 at room temperature). Using the layered heterostructure ionogel electrolytes, full-cell solid-state lithium-ion batteries with a nickel manganese cobalt oxide cathode and a graphite anode are demonstrated, exhibiting voltages that are unachievable with either the high-potential or low-potential ionic liquid alone. Compared to ionogel electrolytes based on mixed ionic liquids, the layered heterostructure ionogel electrolytes enable higher stability operation of full-cell lithium-ion batteries, resulting in significantly enhanced cycling performance.
RESUMEN
By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.
RESUMEN
Two-dimensional covalent organic frameworks often π stack into crystalline solids that allow precise spatial positioning of molecular building blocks. Inspired by the hydrogen-bonded G-quadruplexes found frequently in guanine-rich DNA, here we show that this structural motif can be exploited to guide the self-assembly of naphthalene diimide and perylene diimide electron acceptors end-capped with two guanine electron donors into crystalline G-quadruplex-based organic frameworks, wherein the electron donors and acceptors form ordered, segregated π-stacked arrays. Time-resolved optical and electron paramagnetic resonance spectroscopies show that photogenerated holes and electrons in the frameworks have long lifetimes and display recombination kinetics typical of dissociated charge carriers. Moreover, the reduced acceptors form polarons in which the electron is shared over several molecules. The G-quadruplex frameworks also demonstrate potential as cathode materials in Li-ion batteries because of the favourable electron- and Li-ion-transporting capacity provided by the ordered rylene diimide arrays and G-quadruplex structures, respectively.
RESUMEN
Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer electronics, electrified transportation, and a smart grid accommodating intermittent renewable energy sources. Nanostructured electrode materials present compelling opportunities for high-performance lithium-ion batteries, but inherent problems related to the high surface area to volume ratios at the nanometer-scale have impeded their adoption for commercial applications. Here, we demonstrate a materials and processing platform that realizes high-performance nanostructured lithium manganese oxide (nano-LMO) spinel cathodes with conformal graphene coatings as a conductive additive. The resulting nanostructured composite cathodes concurrently resolve multiple problems that have plagued nanoparticle-based lithium-ion battery electrodes including low packing density, high additive content, and poor cycling stability. Moreover, this strategy enhances the intrinsic advantages of nano-LMO, resulting in extraordinary rate capability and low temperature performance. With 75% capacity retention at a 20C cycling rate at room temperature and nearly full capacity retention at -20 °C, this work advances lithium-ion battery technology into unprecedented regimes of operation.
