Analyzing the Mechanism of Functional Groups in Phosphate Additives on the Interface of LiNi0.8Co0.15Al0.05O2 Cathode Materials

Abstract

The design of a functional electrolyte system that is compatible with the LiNi0.8Co0.15Al0.05O2 (LNCA) cathode is of great importance for advanced lithium-ion batteries (LIBs). In this work, chelated lithium salts of lithium difluoro(bisoxalato) phosphate (LiDFBOP) and lithium tetrafluoro(oxalate) phosphate (LTFOP) are synthesized by a facile and general method. Then, the complexes of LiDFBOP, LTFOP, and lithium difluorophosphate (LiDFP), all of which have a central phosphorus atom, were selected as the salt-type additives for the LiPF6-based electrolyte to improve the electrochemical performances of LNCA/Li half-batteries, respectively. The results of electrochemical tests, quantum chemistry calculations, potential-resolved in situ electrochemical impedance (PRIs-EIS) measurements, and surface analyses show that the interface property and the battery performance are closely associated with molecular structures of phosphorus-centered complex additives. It indicates that LiDFP with the P═O bond can significantly reduce the interfacial impedance of LNCA/Li half-batteries due to the increase of Li3PO4 and the decrease of Li2CO3 in the cathode electrolyte interface (CEI). While in LiDFBOP, according to the calculated vertical ionization potential (VIP), the two oxalate-chelated ligands bring about a bidirectional cross-linking reaction, which makes it preferential to be oxidized. This process is self-healing and can form a dense and stretched CEI, which is favorable to the cycle performance at the late stage. In contrast, the polymerization reaction will occur in one direction for LTFOP due to its lone oxalate ligand. Additionally, an unfavorable side reaction between LTFOP and EC has been proposed by the aid of Gibbs free energy calculation. This is a good explanation for the formation of the uneven and unstable CEI, as well as the continuous decomposition of the electrolyte in PRIs-EIS measurement. This work has an extensive applicability and practical significance not only for molecular designing of novel lithium salts, but also for the construction of a functional electrolyte system that is compatible with different electrode materials.