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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP Electrolytic solution: e– + and - ion transport properties Solvation/solubility Ion pairing, clustering Gassing V stability windows Current collectors: Reactivity/corrosion Dissolution Separator: Thickness and porosity Wettability, ohmic resistance Dendrite resistance e– Composite electrode: e– and +ion transport Mechanical strain Particle fracture/ disconnection Interfacial reactions Surface reactions SEI formation Charge transfer Cell: Equivalent series resistance Electrode cross-talk Manufacturing defects e– Solid state device: Single ion conductor (e.g., glass) Cation, e–, h transport properties V stability window Interfacial contact e– Monolithic electrode: Grain orientations Grain boundaries e– and +ion transport Mechanical strain/fracture Stripping and plating (metals) Double layer charging Figure 2.5.1. Schematics of generic electrochemical devices. Although they consist of only a few components, the phenomena that govern their behavior are complex and can contribute to many failure modes. From F. Lin et al., Synchrotron X-ray techniques for studying materials eIectrochemistry in rechargeable batteries, Chem. Rev., 2017, DOI: 10.1021/acs.chemrev.7b00007. Longer-term degradation: Batteries often exhibit degradation modes as gradual loss of capacity with charge/ discharge cycling, during periods of rest, or periods at higher power/current. Behavior is also influenced by states of charge and thermal environments. While kinetics typically plays a significant role, degradation mechanisms that occur during times of rest or open circuit can result from thermodynamic instabilities of the cell components.3 Degradation phenomena include self-discharge, increasing interfacial impedance, and an increasing fraction of inactive material phases. Interaction between the cathode, anode, and electrolyte can all contribute to the mechanisms and the kinetics underlying the degradation mode, making it considerably more challenging to study and identify the mechanisms. The diversity of degradation scenarios is exemplified for a graphite anode in Li-ion cells in Figure 2.5.2. Large differences in polarization resistance are found when cathodes are aged at different states of charge at elevated temperatures.3 Simply immersing a cathode material into a non-aqueous electrolyte for a period of time can result in surface reconstruction,4 although the role this plays in degradation at different temperatures and states of charge remains unclear. Catastrophic failure: Catastrophic failure by “sudden death” degradation mechanisms can be more dramatic. For example, dendrites formed on pure metal anodes can grow through a liquid or solid electrolyte, produce a short circuit to the cathode, and cause immediate failure and the risk of fire. Thermal runaway is a second form of sudden death, triggered by heating above a threshold temperature that decomposes the cathode (~150°C for cobalt-based cathodes in Li-ion batteries, higher for FePO4 cathodes), releasing oxygen that reacts with the flammable electrolyte. The reaction is exothermic, so the heat it releases raises the temperature and accelerates the rate of reaction in a positive feedback loop. Thermal runaway reactions are hard to stop, often running until the reactants are exhausted and the battery fully destroyed. This sudden death failure mode presents a major safety hazard. Battery Chemistries: Despite the pervasive presence of Li-ion batteries in use today, fundamental understanding of degradation and failure is rather rudimentary, without benefit of controlled experiments to prove/disprove hypotheses. Yet, the battery world is increasingly pursuing “beyond Li ion” technologies to achieve the higher performance demanded by large-scale applications. Clearly, mechanisms for degradation and failure are even less understood in these domains, as illustrated in the examples below. 66 PRIORITY RESEARCH DIRECTION – 5PDF Image | Next Generation Electrical Energy Storage
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