Self-healable dynamic poly(urea-urethane) gel electrolyte for lithium batteries

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Abstract

Hindered urea bonds are introduced as self-healing units in a polymer electrolyte for Li-metal batteries. Differently from standard commercial separators, the poly(urea-urethane) system works for hundreds of cycles after several damage/healing steps.

Abstract

Self-healing materials are actively studied in order to extend the lifetime and performance of batteries. Dynamic covalent networks have recently emerged as one of the best self-healable materials which allow thermosets to be reprocessed and recycled. Among all the different exchangeable bonds studied over the last few years, hindered urea bonds appear to be one of the most feasible options to create self-healable materials due to their exchange activation at low temperatures. Although this chemistry is very popular in composite and coating applications, it has not been considered for designing self-healable materials for batteries. In this work we synthesize a membrane containing dynamic hindered urea crosslinking points by reacting tris[2-(isopropylamino)ethyl]amine with hexamethylene diisocyanate, followed by the addition of polyethylene glycol. It is proved that this newly designed material possesses self-healable properties and higher ionic conductivity than the commercially available liquid electrolyte embedded in a porous Celgard® 2500 separator. The polyurethane gel electrolyte shows very homogeneous Li plating and stripping in Li symmetrical cells and is also compatible with Li-mediated electrochemical ammonia synthesis approaches. Most importantly, after severely mechanically damaging the gel membrane, the polymer electrolyte shows great recovery of the electrochemical properties, experiencing more than 100 charge/discharge cycles (after cutting) at C/5 rate.

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Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy

Kevin Wood, Eric Kazyak, Alexander Chadwick … (2016)

Enabling ultra-high energy density rechargeable Li batteries would have widespread impact on society. However the critical challenges of Li metal anodes (most notably cycle life and safety) remain unsolved. This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. Herein, we present a comprehensive understanding of the voltage variations observed during Li metal cycling, which is directly correlated to morphology evolution through the use of operando video microscopy. A custom-designed visualization cell was developed to enable operando synchronized observation of Li metal electrode morphology and electrochemical behavior during cycling. A mechanistic understanding of the complex behavior of these electrodes is gained through correlation with continuum-scale modeling, which provides insight into the dominant surface kinetics. This work provides a detailed explanation of (1) when dendrite nucleation occurs, (2) how those dendrites evolve as a function of time, (3) when surface pitting occurs during Li electrodissolution, (4) kinetic parameters that dictate overpotential as the electrode morphology evolves, and (5) how this understanding can be applied to evaluate electrode performance in a variety of electrolytes. The results provide detailed insight into the interplay between morphology and the dominant electrochemical processes occurring on the Li electrode surface through an improved understanding of changes in cell voltage, which represents a powerful new platform for analysis.

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Dead lithium: mass transport effects on voltage, capacity, and failure of lithium metal anodes

Kuan-Hung Chen, Kevin Wood, Eric Kazyak … (2017)

A mechanistic analysis of voltage shape changes in lithium metal anodes explains how dead lithium causes capacity fade and failure. Improvement of the performance of Li metal anodes is critical to enable high energy density rechargeable battery systems beyond Li-ion. However, a complete mechanistic understanding of electrode overpotential variations that occur during extended cycling of Li metal is lacking. Herein, we demonstrate that when using a Li metal electrode, the dynamic changes in voltage during extended cycles can be increasingly attributed to mass transport. It is shown that these mass transport effects arise as a result of dead Li accumulation at the Li metal electrode, which introduces a tortuous pathway for Li-ion transport. In Li–Li symmetric cells, mass transport effects cause the shape of the galvanostatic voltage response to change from “peaking” to “arcing”, along with an increase in total electrode overpotential. The continued accumulation of dead Li is also conclusively shown to directly cause capacity fade and rapid “failure” of Li–LCO full cells containing Li metal anodes. This work provides detailed insights into the coupled relationships between cycling, interphase morphology, mass transport and the overall cell performance. Furthermore, this work helps underscore the potential of Li–Li symmetric cells as a powerful analytical tool for understanding the effects of Li metal electrodes in full cell batteries.

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Dynamic urea bond for the design of reversible and self-healing polymers

Hanze Ying, Yanfeng Zhang, Jianjun Cheng (2014)

Polymers bearing dynamic covalent bonds may exhibit dynamic properties, such as self-healing, shape memory and environmental adaptation. However, most dynamic covalent chemistries developed so far require either catalyst or change of environmental conditions to facilitate bond reversion and dynamic property change in bulk materials. Here we report the rational design of hindered urea bonds (urea with bulky substituent attached to its nitrogen) and the use of them to make polyureas and poly(urethane-ureas) capable of catalyst-free dynamic property change and autonomous repairing at low temperature. Given the simplicity of the hindered urea bond chemistry (reaction of a bulky amine with an isocyanate), incorporation of the catalyst-free dynamic covalent urea bonds to conventional polyurea or urea-containing polymers that typically have stable bulk properties may further broaden the scope of applications of these widely used materials.