Fictitious phase separation in Li layered oxides driven by electro-autocatalysis.

Layered oxides widely used as lithium-ion battery electrodes are designed to be cycled under conditions that avoid phase transitions. Although the desired single-phase composition ranges are well established near equilibrium, operando diffraction studies on many-particle porous electrodes have suggested phase separation during delithiation. Notably, the separation is not always observed, and never during lithiation. These anomalies have been attributed to irreversible processes during the first delithiation or reversible concentration-dependent diffusion. However, these explanations are not consistent with all experimental observations such as rate and path dependencies and particle-by-particle lithium concentration changes. Here, we show that the apparent phase separation is a dynamical artefact occurring in a many-particle system driven by autocatalytic electrochemical reactions, that is, an interfacial exchange current that increases with the extent of delithiation. We experimentally validate this population-dynamics model using the single-phase material Lix(Ni1/3Mn1/3Co1/3)O2 (0.5 < x < 1) and demonstrate generality with other transition-metal compositions. Operando diffraction and nanoscale oxidation-state mapping unambiguously prove that this fictitious phase separation is a repeatable non-equilibrium effect. We quantitatively confirm the theory with multiple-datastream-driven model extraction. More generally, our study experimentally demonstrates the control of ensemble stability by electro-autocatalysis, highlighting the importance of population dynamics in battery electrodes (even non-phase-separating ones).

Discrete modeling of ionic space charge zones in solids.

The discrete model of space charge zones in solids reveals and remedies a variety of problems with the classic continuous Gouy-Chapman solution that occur for pronounced space charge potentials. Besides inherent problems of internal consistency, it is essentially the extremely steep profile close to the interface which makes this continuum approach questionable. Not only is quasi-1D discrete modeling a sensible approach for large space charge effects, it can also favorably be combined with the continuum description. A particularly useful application is the explicit implementation of crystallographic details and non-idealities close to the interface. This enables us to consider elastic, structural or saturation effects as well as permittivity variations in a simple but realistic way. We address details of the charge carrier profiles, but also overall properties such as space charge capacitance and space charge resistance. In the latter case the difference in the total charge (at identical concentration) is of importance, in the first case it is the inherent difference in the centroid of charge (at identical total charge) that is remarkable. The model is equally applicable for ionic charge carriers and small polarons.

Synergistic, ultrafast mass storage and removal in artificial mixed conductors.

Mixed conductors-single phases that conduct electronically and ionically-enable stoichiometric variations in a material and, therefore, mass storage and redistribution, for example, in battery electrodes. We have considered how such properties may be achieved synergistically in solid two-phase systems, forming artificial mixed conductors. Previously investigated composites suffered from poor kinetics and did not allow for a clear determination of such stoichiometric variations. Here we show, using electrochemical and chemical methods, that a melt-processed composite of the 'super-ionic' conductor RbAg4I5 and the electronic conductor graphite exhibits both a remarkable silver excess and a silver deficiency, similar to those found in single-phase mixed conductors, even though such behaviour is not possible in the individual phases. Furthermore, the kinetics of silver uptake and release is very fast. Evaluating the upper limit of the relaxation time [corrected] set by interfacial ambipolar diffusion reveals chemical diffusion coefficients that are even higher than those achieved for sodium chloride in bulk liquid water. These results could potentially stimulate systematic research into powerful, even mesoscopic, artificial mixed conductors.

Electro-chemo-mechanical charge carrier equilibrium at interfaces.

Electrochemical interfaces involving solids enable charge transfer, electrical transport, and mass storage in energy devices. One central concept that determines the interfacial charge carrier concentration is the space-charge field. The classical theory accounts for electrochemical equilibrium in the absence of mechanical effects; such effects have recently been found critical in many solids, such as materials for lithium-ion and solid-state batteries, perovskite solar cells, and fuel cells. Towards elucidating the interplay between charge carriers and mechanics, we establish a generalized electro-chemo-mechanical space-charge model and categorize the carriers into physically-meaningful four types, based on the signs of the charge number (i.e., polarity) and the partial molar volume (i.e., expansion coefficient). Beyond the electrostatic effects discussed in the literature, our work reveals the importance of elastic effects, as demonstrated by simulations of a composite beam bending experiment. The analysis highlights opportunities to systematically tune the interfacial electrical conductivity and the reaction kinetics of solids through mechanics. Our treatment provides a rational basis for understanding stress-driven phenomena at interfaces in a wide range of solids.

Spring-Like Pseudoelectroelasticity of Monocrystalline Cu2S Nanowire.

Prediction from the dual-phase nature of superionic conductors-both solid and liquid-like-is that mobile ions in the material may experience reversible extraction-reinsertion by an external electric field. However, this type of pseudoelectroelasticity has not been confirmed in situ, and no details on the microscopic mechanism are known. Here, we in situ monitor the pseudoelectroelasticity of monocrystalline Cu2S nanowires (NWs) using transmission electron microscopy (TEM). Specifically, we reveal the atomic scale details including phase transformation, migration and redox reactions of Cu+ ions, nucleation, growth, as well as spontaneous shrinking of Cu protrusion. Caterpillar-diffusion-dominated deformation is confirmed by the high-resolution transmission electron microscopy (HRTEM) observation and  ab initio calculation, which can be driven by either an external electric field or chemical potential difference. The observed spring-like behavior was creatively adopted for electric nanoactuators. Our findings are crucial to elucidate the mechanism of pseudoelectroelasticity and could potentially stimulate in-depth research into electrochemical and nanoelectromechanical systems.

Space charge storage in composites: thermodynamics.

Contacts of two phases, which allow for synergistic dissociative storage of a component in two space charge zones ("job-sharing storage"), are considered from the viewpoint of point defect thermodynamics. The respective relations between charge and component activity (chemical potential of the component) are derived, or - for more complex cases - the recipes for their derivation are given. These relations describe - according to different experimental conditions - the connection between mass storage and outer partial pressure or between mass storage and the cell voltage in a coulometric titration cell. They also reflect theoretical charge/discharge curves in battery cells when job-sharing storage predominates, and thus are also significant for supercapacitors. In addition to explicitly worked-out cases, it is pointed out how more general situations, such as simultaneous storage in bulk and in boundaries, specific adsorption or size effects, are to be treated.

Nanosheets of earth-abundant jarosite as novel anodes for high-rate and long-life lithium-ion batteries.

Nanosheets of earth-abundant jarosite were fabricated via a facile template-engaged redox coprecipitation strategy at room temperature and employed as novel anode materials for lithium-ion batteries (LIBs) for the first time. These 2D materials exhibit high capacities, excellent rate capability, and prolonged cycling performance. As for KFe3(SO4)2(OH)6 jarosite nanosheets (KNSs), the reversible capacities of above 1300 mAh g(-1) at 100 mA g(-1) and 620 mAh g(-1) after 4000 cycles at a very high current density of 10 A g(-1) were achieved, respectively. Moreover, the resulting 2D nanomaterials retain good structural integrity upon cycling. These results reveal great potential of jarosite nanosheets as low-cost and high-performance anode materials for next-generation LIBs.

Phase evolution in single-crystalline LiFePO4 followed by in situ scanning X-ray microscopy of a micrometre-sized battery.

LiFePO4 is one of the most frequently studied positive electrode materials for lithium-ion batteries during the last years. Nevertheless, there is still an extensive debate on the mechanism of phase transformation. On the one hand this is due to the small energetic differences involved and hence the great sensitivity with respect to parameters such as size and morphology. On the other hand this is due to the lack of in situ observations with appreciable space and time resolution. Here we present scanning transmission X-ray microscopy measurements following in situ the phase boundary propagation within a LiFePO4 single crystal along the (010) orientation during electrochemical lithiation/delithiation. We follow, on a battery-relevant timescale, the evolution of a two-phase-front on a micrometre scale with a lateral resolution of 30 nm and with minutes of time resolution. The growth pattern is found to be dominated by elastic effects rather than being transport-controlled.

Ge/C nanowires as high-capacity and long-life anode materials for Li-ion batteries.

Germanium-based materials (Ge and GeOx) have recently demonstrated excellent lithium-ion storage ability and are being considered as the most promising candidates to substitute commercial carbon-based anodes of lithium-ion batteries. Nevertheless, practical implementation of Ge-based materials to lithium-ion batteries is greatly hampered by the poor cyclability that resulted from the huge volume variation during lithiation/delithiation processes. Herein, uniform carbon-encapsulated Ge and GeOx nanowires were synthesized by a one-step controlled pyrolysis of organic-inorganic hybrid GeOx/ethylenediamine (GeOx/EDA) nanowires in H2/Ar and Ar atmospheres, respectively. The as-obtained Ge/C and GeOx/C nanowires possess well-defined 0D-in-1D morphology and homogeneous carbon encapsulation, which exhibit excellent Li storage properties including high specific capacities (approximate 1200 and 1000 mA h g(-1) at 0.2C for Ge/C and GeOx/C, respectively). The Ge/C nanowires, in particular, demonstrate superior rate capability with excellent capacity retention and stability (producing high stable discharge capacities of about 770 mA h g(-1) after 500 cycles at 10C), making them promising candidates for future electrodes for high-power Li-ion batteries. The improved electrochemical performance arises from synergistic effects of 0D-in-1D morphology and uniform carbon coating, which could effectively accommodate the huge volume change of Ge/GeOx during cycling and maintain perfect electrical conductivity throughout the electrode.

Free-standing Ag/C coaxial hybrid electrodes as anodes for Li-ion batteries.

Free-standing coaxially structured Ag/carbon hybrid electrodes were prepared as potential anodes for micro-Li-ion batteries, which show excellent electrochemical performance, being essentially due to the beneficial effect of the unique structure, i.e. the Ag-core enhances the flexibility and electrochemical kinetics, while the carbon shell buffers volumetric change during cycling.