ABSTRACT
Electrolyte materials and properties are key determinants of battery performance. Yet research into developing new electrolyte materials has received only a fraction of the attention devoted to cathode, and more recently anode, materials. This may be due, in part, to the historically long struggle required to identify an electrolyte composition which enabled the reversible cycling of the graphite anode and LiCoO2 cathode. Although lithium graphite intercalation compounds were discovered in the 1950s, it took decades to develop the mixture of ethylene carbonate (EC), a linear carbonate such as diethyl carbonate (DEC) and lithium hexaœuorophosphate (LiPF6) which enabled the commercialization of Li-ion batteries by Sony in 1991.[1] Current state-of-theart electrolytes have a very similar composition: EC plus a linear carbonate, mixed with LiPF6 and select additives (Figure 4.1) to optimize the properties of the solid-electrolyte interface (SEI).[1-3]
The energy of a battery is largely determined by the type and amount of the electrode active materials, but the electrolyte can also have a signiƒcant
CONTENTS
4.1 Trends in Electrolytes for Lithium-Ion Batteries: An Overview ......... 147 4.2 Organic Solvent-Based Liquid Electrolytes for Lithium-Ion
Batteries ................................................................................................... 149 4.2.1 Solvents ............................................................................................ 149 4.2.2 Salts .................................................................................................. 157 4.2.3 Additives ......................................................................................... 162
4.3 Ionic Liquids for Lithium-Ion Batteries .................................................. 169 4.4 Polymer Electrolytes for Lithium-Ion Batteries ..................................... 174 4.5 Aqueous Electrolytes for Lithium-Ion Batteries .................................... 178 4.6 Glass and Ceramic Electrolytes for Lithium-Ion Batteries .................. 180 4.7 Conclusions ................................................................................................. 183 References ............................................................................................................. 184
impact (i.e., by reacting with the electrode active material to form an SEI, by limiting accessibility of ions to active material, etc.). The electrolyte and electrode/electrolyte interfaces, however, are also a central factor in the battery power, cost, life, and safety. The full battery reaction requires that both electrons and Li+ cations be transported from one electrode to another. The limited mobility (ionic conductivity) of the Li+ cations through the SEI and bulk electrolyte is the biggest contributor to limited battery power. The electrolyte and porous polymer separator are a signiƒcant fraction of the material costs, especially when thinner electrodes are utilized due to the mass transport limitations of thick electrodes. The battery lifetime is often limited by pernicious side reactions of the electrolyte with cell components. Finally, safety-the most crucial parameter for traction batteries for vehicles-can be compromised by short circuits (due to manufacturing impurities, dendrite formation, battery abuse, etc.), leading to localized heating, or thermal abuse, which increases the reactivity of the components while simultaneously generating increased internal pressure due to volatilization of the œammable solvents, resulting in isolated cases of battery œaming or explosions. Such failure modes are unacceptable for vehicle batteries.
