Anion effect on Li/Na/K hybrid electrolytes for Graphite//NCA (LiNi0.8Co0.15Al0.05O2) Li-ion batteries

Abstract

The electrolyte is an essential component of a battery system since it is responsible for the conduction of ions between the electrodes. In the quest for cheaper alternatives to common organic electrolytes for lithium-ion batteries (LIB), we formulated hybrid electrolytes comprising a mixture of Na, K, and Li alkaline salts with ethylene carbonate (EC), ethyl methyl carbonate (EMC), and lithium hexafluorophosphate (LiPF6), giving a total salt concentration of 1.5 M; we determined their physicochemical properties and investigated their electrochemical behavior on a nickel cobalt aluminum oxide (NCA) cathode and graphite (Gr) anode. The electrolytes demonstrated a melting transition peak (Tm), eutectic behavior, and ionic conductivities (~13 mS cm−1) close to those of a commercial LIB electrolyte (SE, EC/EMC + 1 M LiPF6) and activation energies of ca. 3 kJ mol−1. The half-cell coin cells revealed high coulombic efficiency (99%), specific capacity (175 mAh g−1 at C/10), and capacity retention (92% for NaCF3SO3) for the NCA cathode and a moderate performance (coulombic efficiency of 98% for 20 cycles) on the graphite anode after the formation of the SEI layer. The hybrid electrolytes were cycled at 25 °C in a Gr//NCA cell yielding specific capacities of ca. 225 mAh g−1 at a C/5 rate, corroborating that the anion plays a key role and highlighting their potential for energy storage applications.