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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP of chemicals that enhance SEI formation? What about allowing a variable ion conductivity to control charge and discharge rates? Such concepts have been proposed previously,18-20 indicating that the separator could potentially be considered as a repository containing a large number of functional elements. High Capacity Battery High Rate Pseudocapacitor Capacitor Voltage Figure 2.1.1. Idealized representation of differential capacity (dC/dV) profiles for three basic charge storage mechanisms. Reprinted with permission from Ref. 15. Copyright (2013) American Chemical Society. New Functionality from Electrochemical Storage Structures - Multifunctional Binders: Fully unlocking the chemical versatility of polymers would be a potentially effective way to impart additional functionality to the binder in a battery electrode. In principle, this could reduce the number of battery components and simplify electrode fabrication, while potentially providing gains in energy density. As an example, electronically conductive binders could provide a means to eliminate the electronically conducting additive, typically carbon (Figure 2.1.2a). If combined with mechanical stability (Figure 2.1.2b), such enhanced binders could be a crucial component of electrodes involving active materials with large volume expansion, such as Si with Li (Figure 2.1.2c).21,22 Mechanical stability could also be achieved indirectly with binders that are self- healing (Figure 2.1.2c).23 Evidence exists that polymers with mixed conductivity could be used to eliminate both electronic additives and porosity.24 Multi-functionality in polymers is likely limited only by the chemists’ imagination, but guidance by high throughput computations based on screening critical descriptors23 is likely to be a crucial component of discovery. Polymer chemists striving to achieve this goal will benefit from further advances in computational methods that accelerate the design of effective and efficient synthetic routes.25 New Functionality from Electrochemical Storage Structures - Multifunctional Electrode Particles: Almost all battery electrodes, Li-ion and other battery chemistries, are manufactured from a slurry containing particles of the electrochemically active material. The resulting porosity of the electrode thus provides a large contact area between the electrolyte and these particles, though the electrochemical instability of the resulting electrode- electrolyte interfaces creates a significant voltage limitation for operation.26 Insulating products and/or electrode corrosion result, thereby increasing the resistance to charge transfer even to the point of passivating the electrode against the desired Li+ insertion/deinsertion reactions. A demonstrated strategy to avoid this outcome is to form a thin, preferably ion-permeable barrier between the two components that serves as a corrosion passivant.27-29 Two general issues arise with this approach. First, this treatment is typically performed post-synthesis, i.e., on active material powder where particles are highly agglomerated. Buried interfaces are difficult to access and coat under these conditions, which creates a challenge to completely passivate the interfaces, especially considering that material shuffling during cycling can modify their exposure to the electrolyte with respect to the pristine state.30 Atomic layer deposition holds promise in its ability to produce controlled conformal coatings, but further advances in chemical versatility while providing convincing evidence of homogeneity are still needed.31,32 Second, ionically and/or electronically insulating coating phases may create high interfacial resistances, producing additional demands on the multifunctionality required. Core-shell or graded-composition structures could be the means to optimize the design of multifunctional particle architectures, e.g., where the core provides maximum storage capacity 12 PRIORITY RESEARCH DIRECTION – 1 dC/dVPDF Image | Next Generation Electrical Energy Storage
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