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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP Lack of Guided Mitigation Strategies: Current approaches to improve lifetime are often based upon an Edisonian trial-and-error approach because of the lack of fundamental understanding how different mitigation strategies work. For example, electrolyte additives are often used to allow cycling to higher potentials,6 thus improving the energy density of cells. Likewise, surface coatings of various materials, from carbons to polymers to oxides, are used to suppress unwanted reactions between active particles and electrolytic solutions. Significant opportunity exists for the rational design of degradation mitigation strategies that are guided by fundamental understanding. Strong collaboration between synthesis, characterization, and modeling is required to solve this problem. This scientific challenge is to inspire the design of resilient and robust electrodes and other cell components. 3.3.3 IMPACT Successful formulation and mitigation strategies should result in suppression and prevention of unwanted processes. An example of a recent successful strategy is given in Figure 3.3.2.7 In some metal oxide electrodes, repetitive insertion/deinsertion of Li ions cause strain hardening, fatigue, and fracture of the oxide, leading to separation from the current collector and loss of reversible capacity. This leads to a limit in cycle life of 5,000- 10,000 cycles for electrochemical capacitors. This research demonstrated that poly(methyl methacrylate)/ propylene carbonate gel electrolytes extend the cycling stability of MnO2@Au nanowires to >100,000 cycles. Prevention or control of “sudden death” events may also lead to greater safety as well as longer lifetime. It is known, for example, that short circuits do not necessarily always result in catastrophic events. What leads to these slow short circuits, and can they be made reversible? If this can be understood, battery engineers can more aggressively pursue high energy density chemistries without compromising performance or adding expenses at the system level to guarantee safety. Along these lines, solid electrolytes offer potential for improved resistance to lithium dendrite formation in batteries with lithium metal anodes, but lithium metal can deposit at grain boundaries, resulting in device failure. Lithium formation depends on the local electrochemical potentials formed within the cell. Understanding metal deposition, as well as the phases that form at electrode- electrolyte interfaces and grain boundaries, and controlling electrolytes and interfaces would result in more resilient devices, which can fully exploit the high energy density that use of lithium metal promises. To date, the promise of lithium metal electrodes has not been fully realized due to safety concerns. Devices may also fail more gradually due to slow processes in the cells, such as parasitic side reactions or cumulative mechanical strain, as active materials undergo repetitive volume changes during cycling. These effects may take a long time to manifest and may not be adequately captured by accelerated lifetime testing protocols. Successful quantification of such phenomena in the kinetic regime by appropriate models would result in predictive capabilities that reduce the need for extended and time-consuming testing. Characterization techniques with sufficient resolution and precision to deconvolute and decouple the thermal-mechanical- electrochemical phenomena that occur will provide information that allows solutions to be found quickly. These solutions could be in the form of electrolyte additives, coatings on active material particle surfaces, or entirely new materials, electrodes, or cell architectures or other approaches not yet identified. This enhanced design space could result in parallel improvements in storage and lifetime. In addition to the potential for greatly improved electrochemical storage devices, these approaches will provide a needed integrated understanding from the atomic to macroscopic scales in a consistent framework of multi-dimensional spatial and temporal resolutions with temporal correlation to facilitate material-to-system designs. Interfacial processes, including formation of SEIs at anodes and cathode/electrolyte interfaces, can often determine the immediate and long-term electrochemical properties of cells, especially their lifetimes. SEI formations are often very complex and have properties that depend on cycling conditions, specifically current densities and temperature. Good interfacial contact between components in solid-state cells can be difficult to maintain, yet this is critical to their function. Rational design and optimization of these interfaces would result in more resilient devices that are less sensitive to abuse conditions such as temperature excursions. Intelligently designed interfaces may allow battery engineers to incorporate new, high-capacity electrode materials based on (for example) oxygen redox processes more easily. Surface oxygen reactivity, if unmanaged, can result in surface reconstruction and formation of chemical species, which can reduce lifetimes by either depositing on the 112 PANEL 3 REPORTPDF Image | Next Generation Electrical Energy Storage
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