Novel Lithium Imides; Effects of -F, -CF3, and -C≡N Substituents on Lithium Battery Salt Stability and Dissociation

Abstract

New lithium imide salts have been studied using computational chemistry methods. Intrinsic anion oxidation potentials and ion pair dissociation energies are presented for six lithium sulfonyl imides (R-O 2 S-N-SO 2 -R) and six lithium phosphoryl imides (R 2 -OP-N-PO-R 2 ), as a function of -F, -CF 3 , and -C7N substitution. The modelled properties are used to estimate the electrochemical oxidation stability of the anions and the relative ease of charge carrier creation in lithium battery electrolytes. The results show that both properties are improved with cyanosubstitution, which in part is corroborated when comparing with other classes of lithium salts. However, the comparison also shows ambiguous oxidation stability results for cyano-substituted reference salts of the type PF x (CN) 6−x − and BF x (CN) 4−x − , using two different approaches -we present a tentative explanation for this. For the imide anions and PF 6 − , the bond dissociation energy is introduced as a third property, to gauge the thermal stability of the imide anions. The results suggest that the C-S and C-P bonds are the most liable to break and that the thermal stability is inversely related to the ion pair dissociation energy.

Design of an electrolyte composition for stable and rapid charging–discharging of a graphite negative electrode in a bis(fluorosulfonyl)imide-based ionic liquid

“…Conversely, the Coulombic efficiency for the 0.43 mol dm À3 LiFSI/EMImFSI electrolyte during the first cycle was estimated to be 79.6%, which is significantly lower than that for the 0.43 mol dm À3 LiTFSI/EMImFSI electrolyte (90.4%). These results imply that a sufficient amount of Li þ and FSI À in the electrolyte in addition to the slightly weak coordination between them [41,42] provides more suitable electrode/electrolyte interface on the negatively polarized graphite electrode, leading to good reversibility for the insertion/extraction of lithium into/from the graphite electrode. In contrast, the significantly low Coulombic efficiency at the first cycle for 0.43 mol dm À3 LiFSI/EMImFSI is probably due to a large amount of EMIm þ neighboring the electrode and a rapid EMIm þ supply from the low-viscous bulk electrolyte.…”

“…In the case of high concentration of LiTFSI/EMImFSI, most of Li þ in the electrolyte might interact and form complex with TFSI À because of the stronger coordination between Li þ and TFSI À . In contrast, high concentration of FSI À in the medium makes Li þ transfer easily, because of the relatively weak interaction between Li þ and FSI À [41,42]. Such an improvement in ionic conductivity resulting from using LiFSI rather than LiTFSI as the lithium source has been widely reported for all electrolytes, from typical organic solvent-based to ionic liquid-based electrolytes [20,22,43,44].…”

“…In contrast, the significantly low Coulombic efficiency at the first cycle for 0.43 mol dm À3 LiFSI/EMImFSI is probably due to a large amount of EMIm þ neighboring the electrode and a rapid EMIm þ supply from the low-viscous bulk electrolyte. Moreover, the EMImFSI electrolyte with a high concentration of LiTFSI destabilizes the interface, presumably caused by the stronger coordination between Li þ and TFSI À [41,42] preventing the formation of the interface layer, which results in the decomposition of the ionic liquid. Furthermore, the graphite negative electrode exhibits good cycle durability in the 1.46 mol dm À3 LiFSI/EMImFSI electrolyte, achieving a discharge capacity of 345e340 mA h g À1 with a Coulombic efficiency over 99.9% during 20 cycles; however, the discharge capacity for LiTFSI/EMImFSI significantly decreased toward 275 mAh g À1 due to the unstable electrode/electrolyte interface mentioned above, which gradually provides the decomposition of the ionic liquid and finally leads to internal shorts as observed after the 10th cycle.…”

Exploring the Liquid Structure and Ion Formation in Magnesium Borohydride Electrolyte Using Density Functional Theory

“…The M0X family of density functionals have been successfully applied in the study of lithium based electrolytes. [37][38][39] As explained in the following section, M06/6-31+G(d) was found to predict ionic dissociation energies and geometries similar to those calculated at the MP2/6-311++G(d,p) level of theory. Unless otherwise stated, the M06/6-31+G(d) method was used for all calculations in this study.…”

Lithium salts for advanced lithium batteries: Li–metal, Li–O2, and Li–S

“…in which anion solvation occur by hydrogen bond formation, in aprotic (often just quoted as "non-aqueous") solvents, the absence of acidic protons implies that the dissolution of a Li-salt primarily is by solvent-Li + interactions. [62][63][64] Two recent representative examples using this strategy for distinctly different families of new WCAs aimed at battery usage are the studies by Carboni et al 65 and Scheers et al 66 Given the parameters above, the Li-salt solubility is usually the first consideration when investigating new Li-salts. Besides the importance of solvent properties like polarity, viscosity, etc., the strength of the Li + -anion interaction is critical.…”