Electrochemistry of Graphite in Li and Na Salt Codissolving Electrolyte for Rechargeable Batteries.

Electrochemical reduction and oxidation of a graphite electrode were carried out in nonaqueous electrolytes that consisted of ethylene carbonate/diethyl carbonate (1:1) solvents where LiClO₄ and NaClO₄ were codissolved. Reversible lithium intercalation into graphite occurred accompanied with no remarkable side reactions in the potential range between 1.5 and 0.02 V vs Li/( 1 mol dm^-3 Li⁺), that is, no sodium deposition and no sodium intercalation were observed. Comparing to results in a sodium- free- electrolyte, i.e., a conventional electrolyte in Li-ion batteries, the initial irreversibility was noticeably suppressed by codis-solving sodium ions, and kinetics of the lithium intercalation were enhanced with better capacity retention during cycling. Additionally, the redox behavior of graphite for lithium-ion batteries was improved by sodium ion Coexistence, with no inferior influences on the negative and LiMn₂O₄ positive electrodes and electrolyte. When the sodium ion concentration increased to 0.22 mol dm^-3, the superior reversibility of lithium intercalation was observed in comparison with that in a sodium-free system; however, the higher concentration of sodium ions deteriorated the lithium intercalation properties. We found that a surface layer formed on graphite electrode got less resistive as a lithium-ion conductor by entrapping sodium during the initial electroreduction with optimal Na⁺ concentration. [ABSTRACT FROM AUTHOR]