Engineering topics
Chen, Xilin
Publications and source records attributed to Chen, Xilin.
Lithium compensation for full cell operation
Disclosed herein are embodiments of a lithium-ion battery system comprising an anode, an anode current collector, and a layer of lithium metal in contact with the current collector, but not in contact with the anode. The lithium compensation layer dissolves into the electrolyte to compensate for the loss of lithium ions during usage of the full cell. The specific placement of the lithium compensation layer, such that there is no direct physical contact between the lithium compensation layer and the anode, provides certain advantages.
Methods and energy storage devices utilizing electrolytes having surface-smoothing additives
Electrodeposition and energy storage devices utilizing an electrolyte having a surface-smoothing additive can result in self-healing, instead of self-amplification, of initial protuberant tips that give rise to roughness and/or dendrite formation on the substrate and anode surface. For electrodeposition of a first metal (M1) on a substrate or anode from one or more cations of M1 in an electrolyte solution, the electrolyte solution is characterized by a surface-smoothing additive containing cations of a second metal (M2), wherein cations of M2 have an effective electrochemical reduction potential in the solution lower than that of the cations of M1.
Enhanced Performance of Li|LiFePO4 Cells Using CsPF6 as an Electrolyte Additive
The practical application of lithium (Li) metal anode in rechargeable Li batteries is hindered by both the growth of Li dendrites and the low Coulombic efficiency (CE) during repeated charge/discharge cycles. Recently, we have discovered that CsPF6 as an electrolyte additive can significantly suppress Li dendrite growth and lead to highly compacted and well aligned Li nanorod structure during Li deposition on copper substrates. In this paper, the effect of CsPF6 additive on the performance of rechargeable Li metal batteries with lithium iron phosphate (LFP) cathode was further studied. Li|LFP coin cells with CsPF6 additive in electrolytes show well protected Li anode surface, decreased resistance, enhanced rate capability and extended cycling stability. In Li|LFP cells, the electrolyte with CsPF6 additive shows excellent long-term cycling stability (at least 500 cycles) at a charge current density of 0.5 mA cm-2 without internal short circuit. At high charge current densities, the effect of CsPF6 additive becomes less significant. Future work needs to be done to protect Li metal anode, especially at high charge current densities and for long cycle life.
Methods and electrolytes for electrodeposition of smooth films
Electrodeposition involving an electrolyte having a surface-smoothing additive can result in self-healing, instead of self-amplification, of initial protuberant tips that give rise to roughness and/or dendrite formation on the substrate and/or film surface. For electrodeposition of a first conductive material (C1) on a substrate from one or more reactants in an electrolyte solution, the electrolyte solution is characterized by a surface-smoothing additive containing cations of a second conductive material (C2), wherein cations of C2 have an effective electrochemical reduction potential in the solution lower than that of the reactants.
Dendrite-Free Lithium Deposition with Self-Aligned Nanorod Structure
Suppressing lithium (Li) dendrite growth is one of the most critical challenges for the development of Li metal batteries. We recently proposed a novel self-healing electrostatic shield (SHES) mechanism which can fundamentally change the Li deposition behavior and lead to the growth of dendrite-free Li films. Here, we report for the first time that the as-deposited dendrite-free Li films grown with assistance of SHES additive are actually composed of highly-aligned and compacted Li nanorods with hemispherical tips. Both surface and cross sectional morphology evolution of the Li films during repeated Li deposition/stripping processes were systematically investigated. A new model has been established to explain the formation and evolution of the Li nanorods. A fundamental understanding on the internal structure and evolution of Li metal films may lead to new approaches to stabilize the long term cycling stability of Li metal anode.
Functioning Mechanism of AlF 3 Coating on the Li- and Mn-Rich Cathode Materials
We report systematic studies of the microstructural changes of uncoated and AlF3-coated Li-rich Mn-rich (LMR) cathode materials (Li1.2Ni0.15Co0.10Mn0.55O2) before and after cycling using a combination of aberration-corrected scanning/transmission electron microscopy (S/TEM) and electron energy loss spectroscopy (EELS). TEM coupled with EELS provides detailed information about the crystallographic and electronic structure changes that occur after cycling, thus revealing the fundamental improvement mechanism of surface coating. The results demonstrate that the surface coating reduces oxidation of the electrolyte at high voltage, suppressing the accumulation of a thick solid electrolyte interface (SEI) layer on electrode particle surface. Surface coating significantly enhances the stability of the surface structure and protects the electrode from severe etching/corrosion by the acidic species in the electrolyte, reducing the formation of etched surfaces and corrosion pits. Moreover, surface coating alleviates the undesirable voltage fade by mitigating layered to spinel-like phase transformation in the bulk region of the material. These fundamental findings may also be widely applied to explain the functioning mechanisms of other surface coatings used in a broad range of electrode materials.
An Electrically Switchable Metal-Organic Framework
Crystalline metal organic framework (MOF) materials containing interconnected porosity can be chemically modified to promote stimulus-driven (light, magnetic or electric fields) structural transformations that can be used in a number of devices. Innovative research strategies are now focused on understanding the role of chemical bond manipulation to reversibly alter the free volume in such structures of critical importance for electro-catalysis, molecular electronics, energy storage technologies, sensor devices and smart membranes. In this letter, we study the mechanism for which an electrically switchable MOF composed of Cu(TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane) transitions from a high-resistance state to a conducting state in a reversible fashion by an applied potential. The actual mechanism for this reversible electrical switching is still not understood even though a number of reports are available describing the application of electric-field-induced switching of Cu(TCNQ) in device fabrication.
Mesoporous Silicon Sponge as an Anti-Pulverization Structure for High-Performance Lithium-ion Battery Anodes
Nanostructured silicon is a promising anode material for high performance lithium-ion batteries, yet scalable synthesis of such materials, and retaining good cycling stability in high loading electrode remain significant challenges. Here, we combine in-situ transmission electron microscopy and continuum media mechanical calculations to demonstrate that large (>20 micron) mesoporous silicon sponge (MSS) prepared by the scalable anodization method can eliminate the pulverization of the conventional bulk silicon and limit particle volume expansion at full lithiation to ~30% instead of ~300% as observed in bulk silicon particles. The MSS can deliver a capacity of ~750 mAh/g based on the total electrode weight with >80% capacity retention over 1000 cycles. The first-cycle irreversible capacity loss of pre-lithiated MSS based anode is only <5%. The insight obtained from MSS also provides guidance for the design of other materials that may experience large volume variation during operations.
Dendrite-Suppression Electrolytes for Lithium-Metal and Lithium-Ion Batteries
Lithium (Li) dendrite formation is one of the critical challenges for rechargeable Li-metal batteries and Li-ion batteries, which has not been effectively solved by traditional methods. In this paper, we report the systematic investigation of a novel self-healing electrostatic shield (SHES) mechanism to change the Li deposition behavior by using low-concentration electrolyte additives whose redox potentials are close to that of Li+. The SHES additive cations force Li to be deposited in the region away from protuberant tips due to their preferentially accumulation on the active sites of anode surface. Typical adsorption behavior rather than electrochemical reduction of the additive cation is observed. The existence of Cs+ cations in the electrolytes does not change the components or structure of Li surface films, which is consistent with what the SHES mechanism predicts. The morphologies of deposited Li films on copper substrates are smooth and uniform during repeated deposition-stripping cycles and at various current densities (from 0.1 to 1.0 mA cm-2) by adding only a small amount of Cs+ additive in the electrolytes. Cs+-additive also suppresses Li dendrite growth on carbon anode surface during overcharging, which should improve the safety of Li-ion batteries during abuse conditions. It does not affect the rate capability but improves the long-term cycling stability of Li metal anode, carbon anode and conventional Li-ion cathode materials.
Optimized Operating Range for Large-Format LiFePO4/Graphite Batteries
e investigated the long-term cycling performance of large format 20Ah LiFePO4/graphite batteries when they are cycled in various state-of-charge (SOC) ranges. It is found that batteries cycled in the medium SOC range (ca. 20~80% SOC) exhibit superior cycling stability than batteries cycled at both ends (0-20% or 80-100%) of the SOC even though the capcity utilized in the medium SOC range is three times as large as those cycled at both ends of the SOC. Several non-destructive techniques, including a voltage interruption approach, model-based parameter identification, electrode impedance spectra analysis, ΔQ/ΔV analysis, and entropy change test, were used to investigate the performance of LiFePO4/graphite batteries within different SOC ranges. The results reveal that batteries at the ends of SOC exhibit much higher polarization impedance than those at the medium SOC range. These results can be attributed to the significant structural change of cathode and anode materials as revealed by the large entropy change within these ranges. The direct correlation between the polarization impedance and the cycle life of the batteries provides an effective methodology for battery management systems to control and prolong the cycle life of LiFePO4/graphite and other batteries.