Improving Ionic Conductivity with Bimodal-Sized Li7La3Zr2O12 Fillers for Composite Polymer Electrolytes.

Ceramic-polymer composite electrolytes (CPEs) are being explored to achieve both high ionic conductivity and mechanical flexibility. Here, we show that, by incorporating 10 wt % (3 vol %) mixed-sized fillers of Li7La3Zr2O12 (LLZO) doped with Nb/Al, the room-temperature ionic conductivity of a polyvinylidene fluoride (PVDF)-LiClO4-based composite can be as high as 2.6 × 10-4 S/cm, which is 1 order of magnitude higher than that with nano- or micrometer-sized LLZO particles as fillers. The CPE also shows a high lithium-ion transference number of 0.682, a stable and low Li/CPE interfacial resistance, and good mechanical properties favorable for all-solid-state lithium-ion battery applications. X-ray photoelectron spectroscopy and Raman analysis demonstrate that the LLZO fillers of all sizes interact with PVDF and LiClO4. High packing density (i.e., lower porosity) and long conducting pathways are believed responsible for the excellent performance of the composite electrolyte filled with mixed-sized ionically conducting ceramic particles.

Effects of Cobalt Deficiency on Nickel-Rich Layered LiNi0.8Co0.1Mn0.1O2 Positive Electrode Materials for Lithium-Ion Batteries.

The role of cobalt deficiency on the crystal structure, surface chemistry, and electrochemical performance of Ni-rich layered LiNi0.8Co0.1Mn0.1O2 (NCM811) positive electrode materials was experimentally studied possibly for the first time. We synthesized pristine and cobalt-deficient NCM811 samples by solid-state reaction. Using a variety of characterization techniques and electrochemical measurements, we show that cobalt nonstoichiometry can suppress Ni2+/Li+ cation mixing, but can simultaneously promote the formation of oxygen vacancies, leading to rapid capacity fade and inferior rate capability compared to pristine NCM811. This study provides new insights into the effects of cobalt deficiency on the cation mixing and electrochemical performance of a Ni-rich layered NCM compound.

Linking lignin source with structural and electrochemical properties of lignin-derived carbon materials.

Valorization of lignin to high-value chemicals and products along with biofuel production is generally acknowledged as a technology platform that could significantly improve the economic viability of biorefinery operations. With a growing demand for electrical energy storage materials, lignin-derived activated carbon (AC) materials have received increasing attention in recent years. However, there is an apparent gap in our understanding of the impact of the lignin precursors (i.e., lignin structure, composition and inter-unit linkages) on the structural and electrochemical properties of the derived ACs. In the present study, lignin-derived ACs were prepared under identical conditions from two different lignin sources: alkaline pretreated poplar and pine. The lignin precursors were characterized using composition analysis, size exclusion chromatography, and 2D HSQC nuclear magnetic resonance (NMR). Distinctive distributions of numerous micro-, meso- and macro-porous channels were observed in the two lignin-derived ACs. Poplar lignin-derived ACs exhibited a larger BET surface area and total mesopore volume than pine lignin-derived AC, which contributed to a larger electrochemical capacitance over a range of scan rates. X-ray photoelectron spectroscopic analysis (XPS) results revealed the presence of oxygen-containing functional groups in all lignin-derived ACs, which participated in redox reactions and thus contributed to an additional pseudo-capacitance. A possible process mechanism was proposed to explain the effects of lignin structure and composition on lignin-derived AC pore structure during thermochemical conversion. This study provides insight into how the lignin composition and structure affect the derived ACs for energy storage applications.

Rapid Characterization of Local Shape Memory Properties through Indentation.

ABSTARCT: Shape memory alloys (SMAs) have the ability to show large recoverable shape changes upon temperature, stress or magnetic field cycling. Their shape memory, material and magnetic properties (e.g. transformation temperatures, strain, saturation magnetization and strength) determine their prospects for applications from small-scale microelectromechanical systems to large scale aerospace and biomedical systems. It should be noted that properties of SMAs are highly temperature dependent. Generally, the conventional mechanical characterization methods (e.g, tension, compression, and torsion) are used on bulk samples of SMAs to determine those properties. In this article, it will be shown that indentation technique can be used as an alternative rapid method to determine some of the important shape memory properties of SMAs. Indentation response of a high-temperature NiTiHf alloy was determined as a function of temperature. A clear relationship between the work recoverable ratio and transformation temperatures, superelastic and plastic behavior was observed. This work shows that indentation response can be used to measure local superelasticity response, determine phase transformation temperatures and reveal the temperature intervals of the deformation mechanisms of shape memory alloys.

Systematic Investigation of the Alucone-Coating Enhancement on Silicon Anodes.

Polyvinylidene fluoride (PVDF) is the most popular binder in commercial lithium-ion batteries but is incompatible with a silicon (Si) anode because it fails to maintain the mechanical integrity of the Si electrode upon cycling. Herein, an alucone coating synthesized by molecular layer deposition has been applied on the laminated electrode fabricated with PVDF to systematically study the sole impact of the surface modification on the electrochemical and mechanical properties of the Si electrode, without the interference of other functional polymer binders. The enhanced mechanical properties of the coated electrodes, confirmed by mechanical characterization, can help accommodate the repeated volume fluctuations, preserve the electrode structure during electrochemical reactions, and thereby, leading to a remarkable improvement of the electrochemical performance. Owing to the alucone coating, the Si electrodes achieve highly reversible cycling performance with a specific capacity of 1490 mA h g-1 (0.90 mA h cm-2) as compared to 550 mA h g-1 (0.19 mA h cm-2) observed in the uncoated Si electrode. This research elucidates the important role of surface modification in stabilizing the cycling performance and enabling a high level of material utilization at high mass loading. It also provides insights for the future development of Si anodes.

Unveiling the Critical Role of Polymeric Binders for Silicon Negative Electrodes in Lithium-Ion Full Cells.

Because of its natural abundance and high theoretical specific capacity (3579 mAh g-1, based on Li15Si4), silicon and its composites have been extensively studied as the negative electrode for future high energy density lithium-ion batteries. While rapid failure due to the significant volumetric strain of lithium-silicon reactions makes bulk silicon unsuitable for practical applications, silicon nanoparticles can sustain the large volume changes without fracturing. However, polymeric binders are usually required to maintain the structural integrity of electrodes made of particles. Recent lithium-ion half-cell tests have shown that lithium ion-exchanged Nafion (designated as Li-Nafion) and sodium alginate are highly promising binders for nanoparticle silicon electrodes. Nevertheless, there is scant information on the performance and durability of these electrodes in full cell tests which are likely to reveal the role of binders under more realistic conditions. This work focuses on understanding the role of various binders in lithium-ion full cells consisting of Si negative electrode and LiNi1/3Mn1/3Co1/3O2 positive electrode. This study demonstrates, possibly for the first time, that silicon nanoparticles with either Li-Nafion or sodium alginate as binder can maintain a constant capacity of 1200 mAh g-1 for more than 100 cycles. In addition, during deep charge/discharge cycling, silicon electrodes containing Li-Nafion, Nafion, and sodium alginate can exhibit better capacity retention and higher specific capacity than that of silicon electrodes using polyvinylidene fluoride (PVDF) as a binder.

Low-Temperature Treated Lignin as Both Binder and Conductive Additive for Silicon Nanoparticle Composite Electrodes in Lithium-Ion Batteries.

This work demonstrates a high-performance and durable silicon nanoparticle-based negative electrode in which conventional polymer binder and carbon black additive are replaced with lignin. The mixture of silicon nanoparticles and lignin, a low cost, renewable, and widely available biopolymer, was coated on a copper substrate using the conventional slurry mixing and coating method and subsequently heat-treated to form the composite electrode. The composite electrode showed excellent electrochemical performance with an initial discharge capacity of up to 3086 mAh g-1 and retaining 2378 mAh g-1 after 100 cycles at 1 A g-1. Even at a relatively high areal loading of ∼1 mg cm-2, an areal capacity of ∼2 mAh cm-2 was achieved. The composite electrode also displayed excellent rate capability and performance in a full-cell setup. Through synergistic analysis of X-ray photoelectron spectroscopy, Raman, and nanoindentation experiment results, we attribute the amazing properties of Si/lignin electrodes to the judicious choice of heat treatment temperature at 600 °C. At this temperature, lignin undergoes complex compositional change during which a balance between development of conductivity and retaining of polymer flexibility is realized. We hope this work could lead to practicable silicon-based negative electrodes and stimulate the interest in the utilization of biorenewable resources in advanced energy applications.

Synergetic Effects of Inorganic Components in Solid Electrolyte Interphase on High Cycle Efficiency of Lithium Ion Batteries.

The solid electrolyte interphase (SEI), a passivation layer formed on electrodes, is critical to battery performance and durability. The inorganic components in SEI, including lithium carbonate (Li2CO3) and lithium fluoride (LiF), provide both mechanical and chemical protection, meanwhile control lithium ion transport. Although both Li2CO3 and LiF have relatively low ionic conductivity, we found, surprisingly, that the contact between Li2CO3 and LiF can promote space charge accumulation along their interfaces, which generates a higher ionic carrier concentration and significantly improves lithium ion transport and reduces electron leakage. The synergetic effect of the two inorganic components leads to high current efficiency and long cycle stability.

Design of Nanostructured Heterogeneous Solid Ionic Coatings through a Multiscale Defect Model.

Understanding of the electrical conduction, that is, ionic and electronic conduction, through the solid electrolyte interphase (SEI) is critical to the design of durable lithium-ion batteries (LIBs) with high rate capability and long life. It is believed that an ideal SEI should not only be an ionic conductor, but also an electronic insulator. In this study, we present a theoretical design of an artificial SEI consisting of lithium fluoride (LiF) and lithium carbonate (Li2CO3) on a LIB anode based on a newly developed density functional theory (DFT) informed space charge model. We demonstrate that the migration of lattice Li ions from LiF phase to form Li interstitials in Li2CO3 is energetically favorable near the LiF/Li2CO3 interface. At equilibrium, this interfacial defect reaction establishes a space charge potential across the interface, which causes the accumulation of ionic carriers but the depletion of electronic carriers near the LiF/Li2CO3 interface. To utilize this space charge effect, we propose a computationally designed, nanostructured artificial SEI structure with high density of interfaces of LiF and Li2CO3 perpendicular to the electrode. On the basis of this structure, the influences of grain size and volume ratio of the two phases were studied. Our results reveal that reducing the grain size of Li2CO3 in the nanostructured composite can promote ionic carriers and increase the ionic conductivity through the composite SEI by orders of magnitude. At the same time, the electronic conductivity is reduced due to electron depletion near the LiF/Li2CO3 interface. Furthermore, an optimal volume fraction that ensures high ionic and low electronic conduction was predicted.

Voltage hysteresis of lithium ion batteries caused by mechanical stress.

The crucial role of mechanical stress in voltage hysteresis of lithium ion batteries in charge-discharge cycles is investigated theoretically and experimentally. A modified Butler-Volmer equation of electrochemical kinetics is proposed to account for the influence of mechanical stresses on electrochemical reactions in lithium ion battery electrodes. It is found that the compressive stress in the surface layer of active materials impedes lithium intercalation, and therefore, an extra electrical overpotential is needed to overcome the reaction barrier induced by the stress. The theoretical formulation has produced a linear dependence of the height of voltage hysteresis on the hydrostatic stress difference between lithiation and delithiation, under both open-circuit conditions and galvanostatic operation. Predictions of the electrical overpotential from theoretical equations agree well with the experimental data for thin film silicon electrodes.

Unravelling the Impact of Reaction Paths on Mechanical Degradation of Intercalation Cathodes for Lithium-Ion Batteries.

The intercalation compounds are generally considered as ideal electrode materials for lithium-ion batteries thanks to their minimum volume expansion and fast lithium ion diffusion. However, cracking still occurs in those compounds and has been identified as one of the critical issues responsible for their capacity decay and short cycle life, although the diffusion-induced stress and volume expansion are much smaller than those in alloying-type electrodes. Here, we designed a thin-film model system that enables us to tailor the cation ordering in LiNi(0.5)Mn(1.5)O4 spinels and correlate the stress patterns, phase evolution, and cycle performances. Surprisingly, we found that distinct reaction paths cause negligible difference in the overall stress patterns but significantly different cracking behaviors and cycling performances: 95% capacity retention for disordered LiNi(0.5)Mn(1.5)O4 and 48% capacity retention for ordered LiNi(0.5)Mn(1.5)O4 after 2000 cycles. We were able to pinpoint that the extended solid-solution region with suppressed phase transformation attributed to the superior electrochemical performance of disordered spinel. This work envisions a strategy for rationally designing stable cathodes for lithium-ion batteries through engineering the atomic structure that extends the solid-solution region and suppresses phase transformation.

Revealing triple-shape memory effect by polymer bilayers.

Bilayer polymers that consist of two epoxy dual-shape memory polymers of well-separated glass transition temperatures have been synthesized. These bilayer epoxy samples exhibit a triple-shape memory effect (TSME) with shape fixities tailorable by changing the ratio between the two layers. The triple-shape fixities of the bilayer epoxy polymers can be explained by the balance of stress between the two layers. Based on this work, it is believed that the following three molecular design criterions should be considered in designing triple-shape memory polymers with optimum TSME: 1) well-separated thermal transitions, 2) a strong interface, and 3) an appropriate balance of moduli and relative ratios between the layers (or microphases).

Nonlinear analysis of oscillatory indentation in elastic and viscoelastic solids.

Determining the mechanical properties at micro- and nanometer length scales using nanoindentation or atomic force microscopy is important to many areas of science and engineering. Here we establish equations for obtaining storage and loss modulus from oscillatory indentations by performing a nonlinear analysis of conical and spherical indentation in elastic and viscoelastic solids. We show that, when the conical indenter is driven by a sinusoidal force, the square of displacement is a sinusoidal function of time, not the displacement itself, which is commonly assumed. Similar conclusions hold for spherical indentations. Well-known difficulties associated with measuring contact area and correcting thermal drift may be circumvented using the newly derived equations. These results may help improve methods of using oscillatory indentation for determining elastic and viscoelastic properties of solids.

Nanoscale wear and machining behavior of nanolayer interfaces.

An atomic force microscope was used to subnanometer incise a nanomultilayer to consequently expose individual nanolayers and interfaces on which sliding and scanning nanowear/machining have been performed. The letter reports the first observation on the nanoscale where (i) atomic debris forms in a collective manner, most-likely by deformation and rupture of atomic bonds, and (ii) the nanolayer interfaces possess a much higher wear resistance (desired for nanomachines) or lower machinability (not desired for nanomachining) than the layers.