Electrical and electrochemical properties of lithium solvated electron solutions derived from 1,3,5-triphenylbenzenes

Abstract

A series of 1,3,5-triphenylbenzenes (TPBs) bearing various substituents have been made and their ability to form lithium solvated electron solutions (LiSESs) in tetrahydrofuran (THF) solution have been studied. It was found that the TPBs bearing electron withdrawing substituents were of too low solubility in THF to allow formation of LiSESs, whereas the unsubstituted TPB and TPBs bearing solubilizing alkyl and alkoxy substituents were able to form LiSESs in THF upon addition of lithium. The highest conductivity values for LISESs in THF were obtained from the parent TPB, despite its lower solubility in THF compared to the alkyl- and alkoxy-substituted TPBs. The LiSES from the methoxy-substituted TPB showed a much lower conductivity, which was attributed to the electron donating effect of the alkoxy substituent making the TPB less willing to accept an electron from the lithium and thus significantly reducing the number of charge carriers generated. The alkyl-substituted TPB LiSES showed a slightly lower conductivity than the parent TPB, which is attributed to the insulating effects of the alkyl substituents. The conductivities of the LiSESs were found to be highest for a Li : TPB ratio of 2 : 1, suggesting that the materials act like substituted biphenyls. All the SESs showed a metal-like decrease in conductivity with rising temperature. Studies of open circuit voltage versus temperature for the LiSESs made from unsubstituted TPB indicated that the entropy change (ΔS) values of these solutions during discharge were higher than for SESs made using biphenyl or naphthalene and much higher than for solid lithium ion battery anode materials.