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Register a new userConcentration Polarization and Metal Dendrite Initiation in Isolated Electrolyte Microchannels
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Lithium metal penetrations through the liquid-electrolyte-wetted porous separator and solid electrolytes are a major safety concern of next-generation rechargeable metal batteries. The penetrations were frequently discovered to occur through only a few isolated channels, as revealed by black spots on both sides of the separator or electrolyte, which manifest a highly localized ionic flux or current density. Predictions of the penetration time have been infeasible due to the hidden and unclear dynamics in these penetration channels. Here, using the glass capillary cells, we investigate for the first time the unexpectedly sensitive influence of channel geometry on the concentration polarization and dendrite initiation processes. The characteristic time for the complete depletion of salt concentration on the surface of the advancing electrode, i.e. Sands time, exhibits a nonlinear dependence on the curvature of the channel walls along the axial direction. While a positively deviated Sands time scaling exponent can be used to infer a converging penetration area through the electrolyte, a negatively deviated scaling exponent suggests that diffusion limitation can be avoided in expanding channels, such that the fast-advancing tip-growing dendrites will not be initiated. The safety design of rechargeable metal batteries will benefit from considering the true local current densities and the conduction structures.
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A new class of high-performance pyrrolidinium cation based ionanofluid electrolytes with higher lithium salt concentration are developed. The electrolytes are formed by dispersing imidazolium ionic liquid functionalized TiO2 nanoparticles in low conducting, 0.6 M lithium salt doped N-alkyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI) ionic liquid (IL) hosted electrolyte. Viscosity, ionic conductivity and thermal properties of these electrolytes are compared with well studied 0.2 M salt doped Pyr14TFSI IL-based electrolyte. The highly crystalline 0.6 M lithium salt dissolved IL-based electrolytes gradually become amorphous with the increasing dispersion of surface functionalized nanoparticles within it. The ionic conductivity of the electrolytes shows unusual viscosity decoupled characteristics and at the 5.0 wt% nanoparticle dispersion it attains a maximum value, higher than that of pure IL host. As compared to pure IL-based electrolytes, the ionanofluid electrolyte also possesses a significantly higher value of lithium ion transference number. The Li/LiMn2O4 cell with the best conducting ionanofluid electrolyte delivers a discharge capacity of about 131 mAh g-1 at 25 degree C at a current density of 24 mA g-1, much higher than that obtained in 0.2 M Li salt dissociated Pyr14TFSI electrolyte (87 mAh g-1). Superior interfacial compatibility between ionanofluid electrolyte and electrodes as indicated by the excellent rate performance with outstanding capacity retention of the cell as compared to pure IL-based analogue, further establish great application potentiality of this optimized newly developed electrolyte for safer LMBs.
This work presents an ab initio exploration of fundamental mechanisms with direct relevance to dendrite formation at lithium-electrolyte interfaces. Specifically, we explore surface diffusion barriers and solvated surface energies of typical solid-electrolyte interphase layers of lithium metal electrodes. Our results indicate that surface diffusion is an important mechanism for understanding the recently observed dendrite suppression from lithium-halide passivating layers, which were motivated by our previous work. Our results uncover possible mechanisms underlying a new pathway for mitigating dendridic electrodeposition of lithium on metal and thereby contribute to the ongoing efforts to develop stable lithium metal anodes for rechargeable battery systems.
Ionic transports in nanopores hold the key to unlocking the full potential of bi-continuous nanoporous (NP) metals as advanced electrodes in electrochemical devices. The precise control of the uniform NP metal structures also provides us a unique opportunity to understand how complex structures determine transports at nanoscales. For NP Au from the dealloying of a Ag-Au alloy, we can tune the pore size in the range of 13 nm to 2.4 microns and the porosity between 38% and 69% via isothermal coarsening. For NP Ag from the reduction-induced decomposition of AgCl, we can control additionally its structural hierarchy and pore orientation. We measure the effective ionic conductivities of 1 M NaClO4 through these NP metals as membranes, which range from 7% to 44% of that of a free solution, corresponding to calculated pore tortuosities between 2.7 and 1.3. The tortuosity of NP Au displays weak dependences on both the pore size and the porosity, consistent with the observed self-similarity in the coarsening, except for those of pores < 25 nm, which we consider deviating from the well-coarsened pore geometry. For NP Ag, the low tortuosity of the hierarchical structure can be explained with the Maxwell-Garnett equation and that of the oriented structure underlines the random orientation as the cause of slow transport in other NP metals. At last, we achieve high current densities of CO2 reduction with these two low-tortuosity NP Ags, demonstrating the significance of the structure-transport relationships for designing functional NP metals.
Injection from metallic electrodes serves as a main channel of charge generation in organic semiconducting devices and the quantum effect is normally regarded to be essential. We develop a dynamic approach based upon the surface hopping (SH) algorithm and classical device modeling, by which both quantum tunneling and thermionic emission of charge carrier injection at metal/organic interfaces are concurrently investigated. The injected charges from metallic electrode are observed to quickly spread onto the organic molecules following by an accumulation close to the interface induced by the built-in electric field, exhibiting a transition from delocalization to localization. We compare the Ehrenfest dynamics on mean-field level and the SH algorithm by simulating the temperature dependence of charge injection dynamics, and it is found that the former one leads to an improper result that the injection efficiency decreases with increasing temperature at room-temperature regime while SH results are credible. The relationship between injected charges and the applied bias voltage suggests it is the quantum tunneling that dominates the low-threshold injection characteristics in molecular crystals, which is further supported by the calculation results of small entropy change during the injection processes. An optimum interfacial width for charge injection efficiency at the interface is also quantified and can be utilized to understand the role of interfacial buffer layer in practical devices.
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