Improving Linear Alkyl Carbonate Electrolytes with Electrolyte Additives

Abstract

The effectiveness of some selected electrolyte additives were studied in ethylene carbonate-free linear alkyl carbonate electrolyte mixtures containing 1 M LiPF 6 /ethylmethyl carbonate: vinylene carbonate (98:2 w:w). The electrolytes were used in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch type Li-ion cells tested to 4.5 V at elevated temperature. The results showed that cells with linear alkyl carbonate electrolytes outperformed cells with EC-based electrolytes having state-of-the-art additive blends and cells with fluorinated electrolytes during short-term and long-term cycling at elevated temperatures. The addition of triallyl phosphate or pyridine phosphorus pentafluoride as electrolyte additives to the EC-free electrolyte provided cells with better capacity retention during long-term cycling than cells containing vinylene carbonate alone. These results suggest that small quantities of electrolyte additives can be used to create EC-free, linear alkyl carbonate electrolytes that perform well at high voltages. Energy-dense, long-lasting lithium-ion cells are in high demand for electric vehicles, portable electronics and many other important technologies. The easiest way to increase the energy density of a conventional lithium-ion cell is to increase its upper charge cutoff potential when certain layered oxide positive electrode materials are used. However, increasing the upper cutoff potential normally decreases cell lifetime because the rate of unwanted parasitic reactions like electrolyte oxidation increase with potential. Parasitic reactions involving ethylene carbonate (EC)-a ubiquitous co-solvent in lithiumion electrolytes-at high potential has proven to be one of the most important factors to overcome. 1Until recently, EC has been regarded as an essential component of lithium-ion electrolytes and it has been used in nearly all commercial Li-ion cells produced today. EC was first considered as an electrolyte co-solvent by Elliot in 1964 2 and was used as an electrolyte component in cells with lithiated graphite electrodes by Fong et al. in 1990. 3 EC was found to preferentially react with Li + and electrons at the surface of the graphite electrode prior to lithium intercalation, forming a solid electrolyte interface (SEI) on the surface of the graphite electrode that then allowed the reversible intercalation and de-intercalation of lithium ions into the graphite electrode at very low potentials (<0.20 V).3-5 It was generally believed that the graphite electrode could not be electrochemically lithiated in most common solvents, including ethers, esters, butyrolactones and propylene carbonate, without the sacrificial reaction of EC to form the SEI. 6While the passivation of graphite is virtually complete during the first few cycles, the remaining EC in the electrolyte can be oxidized at high voltages, causing the performance of cells containing EC to diminish as the upper charge voltage increases above 4.4 V.7,8 Electrolyte oxidation at the positive electrode at high potentials can lead to salt consum...

Succinic Anhydride as an Enabler in Ethylene Carbonate-Free Linear Alkyl Carbonate Electrolytes for High Voltage Li-Ion Cells

“…Our previous work 25 showed that co-additives such as triallyl phosphate or phosphorous pentafluoride limit impedance growth during high voltage cycling of cells with EC-free-EMC-based electrolytes using VC as the graphite passivating agent. It may very well be that these same co-additives could help limit impedance growth when SA is used as enabler.…”

Investigation of Dimethyl Carbonate and Propylene Carbonate Mixtures for LiNi0.6Mn0.2Co0.2O2‐Li4Ti5O12 Cells

“…Unfortunately, it also has a high melting point and high viscosity leading to poor ionic conductivity. Furthermore, it has recently been suggested that EC is unstable at higher potentials, where it can be oxidized at the positive electrode . For example, parasitic reactions originating from decomposition of EC at high potential lead to capacity fading in NMC based cathodes .…”

“…Attempts to solve these problems have been made by replacing EC with other carbonate‐based solvents such as ethyl methyl carbonate (EMC) . These electrolytes do, however, not form as stable SEI layers as EC does, and various SEI‐forming electrolytes and additives have therefore been investigated to develop EC‐free electrolytes …”

“…Furthermore, it has recently been suggested that EC is unstable at higher potentials, where it can be oxidized at the positive electrode. [2] For example, parasitic reactions originating from decomposition of EC at high potential lead to capacity fading in NMC based cathodes. [3,4] This puts the use of EC in electrolytes intended for use with high-voltage LIB electrodes into question.…”

“…Attempts to solve these problems have been made by replacing EC with other carbonate-based solvents such as ethyl methyl carbonate (EMC). [2,3] These electrolytes do, however, not form as stable SEI layers as EC does, and various SEI-forming electrolytes and additives have therefore been investigated to develop EC-free electrolytes. [5][6][7][8] Propylene carbonate (PC) has a far higher dielectric constant than the common carbonate solvents used in LIB electrolytes, and is therefore ideal as a replacement for EC.…”