In a previous blog we addressed a similar topic, but the discussion was limited to aqueous electrolytes. Here we would like to revisit the topic for organic electrolytes. We will briefly talk about the types of organic electrolytes; the specific energy and energy density of batteries operated with such electrolytes, and some limitations and safety concerns associated with organic electrolytes.
As discussed in a previous blog, the role played by the electrolyte in the cell is to close the internal electrical circuit. Ions are charge carriers in the internal circuit of the battery, enabling electrons (i.e., current) to flow through the external circuit, where they can power a load.
In non-aqueous batteries, power is being generated by more sophisticated chemical reactions than in aqueous batteries, where water is involved in the chemistry.
Lithium ion batteries (LIBs) operate at high cell voltage values of 3.2–3.8 V, which would decompose (by electrolysis) the water present in an aqueous electrolyte solution. Therefore, LIBs are made exclusively with organic electrolytes that consist of a dissolved salt, specifically, lithium cation (Li+) associated with a large size anion (including PF6–, LiBF4–, and LiClO4–) and an organic solvent, most commonly ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate, or a mixture or EC and DMC. The solvent mixture offers a particularly high conductivity, low viscosity, and the ability to form a solid electrolyte interphase (SEI). Why is the formation of SEI beneficial? Over the initial charging, the organic solvent present in the electrolyte decomposes at the negative electrode, and forms a SEI, which prevents further decomposition of the electrolyte after the second charge. Thus, EC decomposes at a relatively high voltage, and develops a dense, stable interface.
Typical conductivities of such organic electrolytes at room temperature are of the magnitude of 10 mS/cm (where S is Siemens = reciprocal ohm, Ω-1), increasing by approximately 30–40% at the temperature of a hot summer day (40 °C), and decreasing slightly, on a winter day, when the temperature drops to the freezing point of water. Present LIBs, based on organic liquid electrolytes, show significant limitations toward accomplishing high energy density. While lithium metal represents the anode material with the highest possible gravimetric energy density, it cannot be used with a liquid electrolyte because of insufficient cycle life and severe safety issues. Safety is a must for large-size batteries, intended for stationary applications and sustainable transportation, including electric vehicles.
Thermal properties of the electrolyte are crucial for the safety of LIBs. Unfortunately, the organic carbonate-based electrolytes, utilized in most LIBs on the market, show a poor thermal stability, even at temperatures below 100 °C. Undesired reactions between battery components and the electrolyte can result from unforeseen local overheating or short circuits; these phenomena can produce a rapid increase in battery temperature and, eventually, trigger a fire or explosion. In a recent paper, Steffen Hess and co-workers investigated the flammability of electrolytes, as an important feature of the thermal safety behavior of LIBs (Journal of The Electrochemical Society 2015, 162, pp. A3084-A3097.) Authors determined the flash points and self-extinguishing times of 25 solvents used in LIBs, including carbonates, ethers, esters, and lactones. Over the past years, several strategies have been explored to increase safety, such as changing LIB chemistry and electrolyte, and addition of fire retardants to the electrolyte.
Replacing organic liquid electrolytes by solvent-free solid electrolytes (SEs) prevents the thermal runaway reaction of LIB; hence, it is expected to rejuvenate LIB research. Lithium-ion-conducting SEs can support high-energy battery chemistries, by preventing safety issues of conventional LIBs, operated with liquid electrolytes. It appears as a significant challenge to provide the combination of high ionic conductivity and a broad electrochemical window in solid electrolytes (See: Energy Storage Europe Conference Düsseldorf, Germany, 2017).
Insufficient ionic conductivity is the reason for the limited use of SEs in present LIBs; consequently, these cells cannot meet the current density requirements. An advantage of several lithium-ion-conducting materials is that they are highly compatible with lithium metal; their lasting disadvantage resides in low ionic conductivities, which are a few orders of magnitude lower than that of liquid electrolytes. Their high interfacial resistance represents one additional challenge for SEs.
SEs can be based on a wide variety of polymers, including high molecular weight and low molecular weight poly(oxyethylene), PEO, poly(vinylidene fluoride-hexafluoropropylene) co-polymer, PVDF-HFP, poly(vinyl chloride), PVC, etc. provide a relatively stable interface. In a recent paper Nerea Lago and co-workers disclosed an all-solid state secondary LIB, equipped with a solid polymer electrolyte, which cycled at specific energies of 410–425 Wh kg-1 (CHEMSUSCHEM 2015, 8, pp. 3039-3043.)
Use of room temperature ionic liquids (IL) provides an alternative to limiting the flammability and volatility of organic electrolytes. Advantages of IL-based electrolytes include the favorable physicochemical properties of ILs, such as low vapor pressure, high thermal stability, good ionic conductivity, and a wide electrochemical stability window. ILs are often classified as non-flammable materials.
In an upcoming paper D. J. You and co-workers from the Seoul National University, report on a LIB made with an SE, containing one particular ionic liquid. Authors found that with increasing amounts of IL in the SE, the lithium ion mobility increases, and battery performance improves. Their cell has a high specific capacity equal to 76 Ah/kg, at temperatures, close to 0 oC. This translates into a specific energy of 274–281 Wh kg-1, at the high end of LIB with organic liquid electrolytes. One additional advantage offered by this SE-based cell is its safety in operation.
There are many options for organic electrolytes intended to be used in galvanic cells. The composition of these electrolytes is critical in providing good conductivity and increased safety for batteries. For high power density of the battery, the organic solvents or solvent mixtures present in liquid electrolytes should enable fast ion migration, and by this, greater current densities. Solid-state batteries operated with gel polymer electrolytes show significant promise with regard to power density, as revealed by the performance of high energy density solid-state supercapacitors; one of them was fabricated with composite electrodes and an ionic liquid, and had a specific power of 9.0 kW kg-1 (S. Narayan et al. ECS Transactions 2013, 45, pp. 173-181).