Polymer and composite electrolytes
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Introduction In their prophetic 1980 paper, Mizushima, Jones, Wineman, and Goodenough provided the first evidence of reversible lithium intercalation in a 4 V cathode (LixCoO2).1 They proposed the “use of a solid electrolyte of large breakdown voltage to enable a greater fraction of the potential energy density to be utilized.” Experiments in this work were done using a liquid electrolyte: a mixture of a lithium salt (lithium tetrafluoroborate) and propylene carbonate. Harris and Tobias2 first proposed the possibility of using alkyl carbonates as solvents for lithium salts in batteries. This class of electrolytes also enables reversible intercalation into and out of graphite, as shown by Fong, VonSacken, and Dahn.3 The lithium-ion battery used today is built on these three discoveries. The main motivation that drives the development of solid electrolytes today is the possibility of increasing energy density by replacing the graphite anode with a lithium foil, as Goodenough and co-workers recognized. Two classes of solid electrolytes have emerged: mixtures of salts and organic polymers, and inorganic materials—ceramics and glasses—that contain mobile lithium ions. The field of polymer electrolytes began with the work of Fenton, Parker, and Wright, who discovered that alkali salts dissolve in poly(ethylene oxide) (PEO).4 PEO, which is a semicrystalline solid at room temperature, is only conductive
at temperatures above the melting temperature (60°C).5 Above the melting temperature, PEO is a viscoelastic liquid; the linear chains undergo Brownian motion on a time scale that is dictated by chain length.6 The conventional approach to “solidifying” viscoelastic chains is chemical cross-linking.7 Inorganic solids that conduct lithium ions have been identified for lithium batteries.8–12 In such crystalline solids, lithium ions hop from one unit cell to the next, and the motion of the ions depends on the activation barrier along the transport pathway. The motion of ions through inorganic glasses is similar, except for the fact that the atoms surrounding the mobile ions are not arranged on a well-defined lattice. Some inorganic solids exhibit room-temperature conductivity comparable to that of liquid electrolytes.10 The properties of different solids can be combined in composite electrolytes. Block copolymers, wherein a PEO chain is covalently bonded to a rigid polymer such as polystyrene (PS), are one example. The PS chains are trapped in the glassy domains and the covalent bond prevents Brownian motion of the PEO chains. The presence of rigid nonconducting domains (all known dry lithium-ion-conducting polymers are rubbery) reduces conductivity, but can lead to a dramatic increase in the modulus of the electrolyte. One approach to stabilize the lithium metal anode is through the use of rigid solid electrolytes.13 Building robust rechargeable batteries with brittle
Daniel T. Hallinan Jr., Florida A&M University–Florida State University, USA; [email protected] Irune Villaluenga, Blue Current, USA; [email protected] Nita
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