Investigation of Li-Ion Solvation in Carbonate Based Electrolytes Using Near Ambient Pressure Photoemission

The near ambient pressure photoemission (NAPP) technique equipped with a liquid jet (LJ) is used for the first time to explore the electronic structure of the most commonly employed carbonate based Li-ion battery electrolytes. Experiments were performed at the SIM beamline of the Swiss Light Source (SLS) with the purpose of monitoring the Li-ion (Li+, Li 1s) solvation of 1M LiClO4 in 1:1 EC:DMC, both anhydrous and with the addition of 5 % H2O, and in DMSO. These electrolytes have high vapor pressures that prevent their study by traditional XPS and therefore necessitate the use of NAPP. Our measurements show differences in binding energies between the Cl 2p and Li 1s core levels (Delta E = Cl 2p(3/2)-Li 1s) between different solvents, in particularly between the EC:DMC and the DMSO. The addition of only 5 % H2O clearly influences the electronic structure in DMC:EC, but to a lesser extent than completely changing the solvent. Density functional theory (DFT) calculations of solvated Li+ structures within the solvent-separated ion pair (SSIP) model provide support to our experimental findings by revealing that the observed Delta E between solvents is directly related to the change in the electronic structure of the Li+ cation and ClO4 (-) anion due to the modification of the solvation shell. This study establishes LJ NAPP as a powerful analytical method for the study of Li+ solvation that will prove complementary to the more established approaches of FTIR and NMR, but at the same time will allow for new experiments that cannot yet be realized by FTIR and NMR.