Chem. Rev. 2004, 104, 4303−4417 4303 Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries Kang Xu Electrochemistry Branch, Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783-1197 Received November 3, 2003 Contents 8. Novel Electrolyte Systems 4362 8.1. Problems Facing State-of-the-Art Electrolytes 4362 1. Introduction and Scope 4303 8.2. Functional Electrolytes: Additives 4363 1.1. Fundamentals of Battery Electrolytes 4303 8.2.1. Bulk Electrolyte: Ion Transport 4364 1.2. The Attraction of “Lithium” and Its Challenge 4304 8.2.2. Anode: SEI Modification 4366 1.3. From “Lithium” to “Lithium Ion” 4305 8.2.3. Cathode: Overcharge Protection 4372 1.4. Scope of This Review 4306 8.3. New Electrolyte Components 4378 2. Electrolyte Components: History and State of the 4306 8.3.1. Nonaqueous Solvents 4378 Art 8.3.2. Lithium Salts 4383 2.1. Solvents 4307 8.4. Novel Electrolytes with a Wide Temperature 4390 2.1.1. Propylene Carbonate (PC) 4308 Range 2.1.2. Ethers 4308 8.4.1. Low-Temperature Performance 4390 2.1.3. Ethylene Carbonate (EC) 4309 8.4.2. High-Temperature Performance 4399 2.1.4. Linear Dialkyl Carbonates 4310 8.5. Electrolytes of Low Flammability 4400 2.2. Lithium Salts 4310 8.6. Polymer and Polymer Gel Electrolytes 4405 2.2.1. Lithium Perchlorate (LiClO4) 4311 8.6.1. Solid Polymer Electrolyte 4406 2.2.2. Lithium Hexafluoroarsenate (LiAsF6) 4312 8.6.2. Gel Polymer Electrolyte 4408 2.2.3. Lithium Tetrafluoroborate (LiBF4) 4312 9. Concluding Remarks 4410 2.2.4. Lithium Trifluoromethanesulfonate (LiTf) 4312 10. Acknowledgments 4410 2.2.5. Lithium Bis(trifluoromethanesulfonyl)imide 4313 11. References and Notes 4411 (LiIm) and Its Derivatives 2.2.6. Lithium Hexafluorophosphate (LiPF6) 4314 2.3. Brief Summary 4315 1. Introduction and Scope 3. Liquid Range of Electrolyte Solutions 4315 4. Ion Transport Properties 4318 1.1. Fundamentals of Battery Electrolytes 5. Electrochemical Stability: on Inert Electrodes 4322 Electrolytes are ubiquitous and indispensable in all 5.1. Anion of Lithium Salts 4323 electrochemical devices, and their basic function is 5.2. Solvents 4325 independent of the much diversified chemistries and 6. Electrochemical Stability: on Active Electrodes 4326 applications of these devices. In this sense, the role 6.1. Passivation on Lithium Anode 4326 of electrolytes in electrolytic cells, capacitors, fuel 6.2. Electrolyte/Carbonaceous Anode Interface: 4329 cells, or batteries would remain the same: to serve SEI as the medium for the transfer of charges, which are 6.2.1. Exfoliation and Irreversible Capacities on 4329 in the form of ions, between a pair of electrodes. The a Carbonaceous Anode vast majority of the electrolytes are electrolytic 6.2.2. Mechanism of SEI Formation 4331 solution-types that consist of salts (also called “elec- 6.2.3. Characterization of Surface Chemistry 4337 trolyte solutes”) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are 6.3. Electrolyte/Cathode Interface 4341 in a liquid state in the service-temperature range. 6.3.1. Passivation Film on a Cathode 4342 [Although “nonaqueous” has been used overwhelm- 6.3.2. Characterization of Surface Chemistry 4344 ingly in the literature, “aprotic” would be a more 6.3.3. Breakdown of Surface Layer 4346 precise term. Either anhydrous ammonia or ethanol 6.3.4. Passivation of Current Collector 4347 qualifies as a “nonaqueous solvent” but is unstable 6.4. A Few Words on Surface Characterizations 4351 with lithium because of the active protons. Neverthe- 7. Chemical and Thermal Stability/Safety of 4352 less, this review will conform to the convention and Electrolytes use “nonaqueous” in place of “aprotic”.] 7.1. Long-Term Stability of Electrolytes at 4352 Because of its physical location in the electrochemi- Elevated Temperatures cal devices, that is, being sandwiched between posi- 7.2. Stability of the SEI or Surface Layer at 4354 tive and negative electrodes, the electrolyte is in close Elevated Temperatures interaction with both electrodes; therefore, when new 7.3. Thermal Safety of Electrolytes against Abuse 4357 electrode materials come into use, the need for 10.1021/cr030203g CCC: $48.50 © 2004 American Chemical Society Published on Web 09/16/2004 4304 Chemical Reviews, 2004, Vol. 104, No. 10 Xu candidate electrode materials, and thus constantly requests improvements in electrolyte stability. Ad hoc surface chemistry is often necessary for the kinetic stability of these new electrolyte/electrode interfaces. While the potencies of electrode materials are usually quantified by the redox potential in volts against some certain reference potential,3 the stability of an electrolyte can also be quantified by the range in volts between its oxidative and reductive decomposition limits, which is known as the “electrochemical win- dow”. Obviously, the redox potential of both electrode materials must fall within this electrochemical win- dow to enable a rechargeable battery operation. Certainly, electrochemical stability is only one of the requirements that an electrolyte should meet. A Kang Xu was born in Chengdu, China, and received his B.S. degree in generalized list of these minimal requirements should Chemistry from Southwest Normal University in Chongqing, China, in 1985 and M.S. in Polymer Chemistry from Lanzhou Institute of Chemical Physics, include the following: (1) It should be a good ionic Academy of Sciences, in 1988. After working on polymer electrolyte conductor and electronic insulator, so that ion trans- materials from 1988 to 1992 at Chengdu Institute of Organic Chemistry, port can be facile and self-discharge can be kept to a Academy of Sciences, he went to Arizona State University and received minimum. (2) It should have a wide electrochemical a Ph.D. degree in Chemistry in 1996 under the tutelage of C. Austen window, so that electrolyte degradation would not Angell. From 1997 to 2002, he was awarded the National Research Council occur within the range of the working potentials of Research Associate Fellowship and the American Society for Engineer Education Postdoctoral Fellowship, respectively, and he served during both the cathode and the anode. (3) It should also be the tenures as a guest researcher at U.S. Army Research Laboratory inert to other cell components such as cell separators, with T. Richard Jow as academic advisor. He was employed by the U.S. electrode substrates, and cell packaging materials. Army Research Laboratory in 2002. His research interests concern (4) It should be robust against various abuses, such materials development for electrochemical energy storage applications, as electrical, mechanical, or thermal ones. (5) Its which include lithium or lithium ion batteries and electrochemical capacitors. components should be environmentally friendly. He won R&D Achievement Awards from the Department of the Army in 1999, 2001, and 2002 for his work on electrolyte materials. He authored over 60 research publications and 11 patents and is a member of the 1.2. The Attraction of “Lithium” and Its Challenge Electrochemical Society. Lithium has long received much attention as a promising anode material. The interest in this alkali compatible electrolytes usually arises. The interfaces metal has arisen from the combination of its two between the electrolyte and the two electrodes often unique properties: (1) it is the most electronegative dictate the performance of the devices. In fact, these metal (∼ -3.0 V vs SHE), and (2) it is the lightest electrified interfaces have been the focus of interest metal (0.534 g cm-3).3 The former confers upon it a since the dawn of modern electrochemistry1 and negative potential that translates into high cell remain so in the contemporary lithium-based re- voltage when matched with certain cathodes, and the chargeable battery technologies. latter makes it an anode of high specific capacity In a battery,2 the chemical nature of positive and (3.86Ahg-1). In the 1950s lithium metal was found negative electrodes (also called cathode and anode, to be stable in a number of nonaqueous solvents respectively, although by a more strict definition, this despite its reactivity,4 and this stabilization was convention is only correct during discharge) decides attributed to the formation of a passivation film on the energy output, while the electrolyte, in most the lithium surface, which prevents it from having a situations, defines how fast the energy could be sustained reaction with electrolytes. Intensified re- released by controlling the rate of mass flow within search activities resulted in the commercialization the battery. Conceptually, the electrolyte should of a series of lithium-based primary cells in the 1960s undergo no net chemical changes during the opera- and 1970s, and the electrolyte solvents ranged from tion of the battery, and all Faradaic processes are organic (propylene carbonate) to inorganic (thionyl expected to occur within the electrodes. Therefore, chloride and sulfur dioxide).5-8 in an oversimplified expression, an electrolyte could The continued efforts to expand lithium chemistry be viewed as the inert component in the battery, and into rechargeable technology, however, encountered it must demonstrate stability against both cathode severe difficulties in terms of the cycle life and and anode surfaces. This electrochemical stability of safety.9,10 Soon it was realized that the source of the the electrolyte, which in actual devices is usually problems was the morphology of the lithium crystals realized in a kinetic (passivation) rather than ther- newly deposited from the electrolytes upon re- modynamic manner, is of especial importance to charge.11,12 Needlelike lithium crystals (called “den- rechargeable battery systems, but it is often chal- drite”) grow on the anode upon charge and, during lenged by the strong oxidizing and reducing nature the subsequent discharge, become electrically isolated of the cathode and the anode, respectively. The from the substrate due to nonuniform dissolution severity of this challenge is ever increasing with the rates at different sites of the dendrite.