Designing better electrolytes

Electrolytes and the associated interphases constitute the critical components to support the emerging battery chemistries that promise tantalizing energy but involve drastic phase and structure complications. Designing better electrolytes and interphases holds the key to the success of these batteries. As the only component that interfaces with every other component in the device, an electrolyte must satisfy multiple criteria simultaneously. These include transporting ions while insulating electrons between the electrodes and maintaining stability against electrodes of extreme chemical natures: the strongly oxidative cathode and the strongly reductive anode. In most advanced batteries, the two electrodes operate at potentials far beyond the thermodynamic stability limits of electrolytes, so the stability therein has to be realized kinetically through an interphase formed from the sacrificial reactions between electrolyte and electrodes. Description Progress and challenges for electrolytes Compared with the development of new cathodes and anodes, there has been less of a focus on the development of electrolytes. However, it is the electrolyte that controls the flow of ions and charges, and it is the only component in intimate contact with all the others. With the push toward higher energy and power densities, electrolytes are also involved in kinetically formed interphases that aid in the stability of a battery but can also hamper its operation. In a review, Meng et al. captures a number of trends that have emerged in the development of advanced battery electrolytes. —MSL A review discusses progress in designing better electrolytes for electrochemical devices. BACKGROUND The electrolyte is an indispensable component in every electrochemical device, including lithium-ion batteries (LIBs). It physically segregates two electrodes from direct electron transfer while allowing working ions to transport both charges and masses across the cell so that the cell reactions can proceed sustainably. Whether powering our phones, driving our vehicles, or harvesting the intermittent energy from solar and wind farms, electrolytes in these LIBs determine how fast and how many times our devices can be recharged or how efficiently energy can be captured and stored over the grid. Occasionally, when an LIB is pushed away from the designed electrochemistry pathways by various factors such as excessive heat, mechanical mutilation, or internal short circuits induced under extreme charging conditions, electrolytes are also responsible for the fire and explosion accidents that we read about in the news. The electrolyte is the most unique component in a battery. Because it must physically interface with every other component, it is obligated to satisfy numerous constraints simultaneously, including rapidly transporting ions and masses, effectively insulating electrons, and maintaining stability toward the strongly oxidative cathode and strongly reductive anode. Historically, the electrolyte-anode interfacing was the last piece of the puzzle to complete modern LIB chemistry. ADVANCES The commercial success of LIBs has attracted intense interest and investments in electrolyte research, which led to the identification of interphases as the key component responsible for the stable and reversible operations of cathode and anode materials far beyond the thermodynamic stability limits of any known electrolyte. These interphases, often with nanometer thickness, are formed by electrolytes in a self-limiting decomposition process, and they ensure fast rates of charging and discharging, maximum voltage, and reversibility of LIBs. In the past three decades, the chemistry, morphology, and formation mechanisms of interphases have been thoroughly investigated. Researchers have learned how such interphases are structured and what key ingredients they comprise and, most importantly, how to tailor them using electrolyte engineering. Today, it is widely accepted that designing better electrolytes also implies designing the associated interphases for the electrode materials. Although the accurate prediction of interphasial chemistry remains difficult, and key fundamental properties of interphases such as the rate and mechanism of ion transport across interphases are still unknown, the structure of the ion solvation sheath has been identified as an effective tool that directs the formation process of interphases. Such knowledge has been driving a series of new electrolyte concepts for emerging battery chemistries. OUTLOOK Efforts are being made to develop battery chemistries that promise high energy density, rapid charging, low cost, high sustainability, and independence from elements or materials of high geopolitical or ethical risks. Each individual chemistry may demand a unique electrolyte and corresponding interphase, but a few universal trends emerge: (i) a super-concentration of salts is used to leverage unusual properties arising from the altered ion-solvation structures; (ii) both polymeric and inorganic materials are used to solidify electrolytes so that the aggressive lithium-metal anode can be harnessed with higher safety; (iii) efforts are made to identify the most effective interphasial ingredients so that an interphase of singular composition can be designed and artificially applied; (iv) liquefied gaseous components are used to expand the low-temperature limits of conventional electrolytes; and (v) unusual electrochemical behaviors are explored by confining ion-solvation sheaths in nano- or sub-nano environments. Electrolytes and the associated interphases play the central role in supporting diversified battery chemistries. On the anode side (left), the electrolyte must form an interphase that prevents graphitic anode from exfoliation, tolerates the drastic volume changes of a silicon electrode, and suppresses the growth of a dendritic form of lithium metal. On the cathode side (right), an interphase is critical in preventing the irreversible reactions with electrolytes, maintaining the lattice structure of transition metal oxides, suppressing the cross-cell shuttling of polysulfide species, and assisting the complicated triphasial reactions of an air-cathode. In all of these scenarios, interphases must enable ionic transport while insulating electronic transport.