Lithium ion secondary battery and production method thereof

A production method of a lithium ion secondary battery including:

• a positive electrode having a positive electrode active material layer on a surface of a positive electrode collector;
• a negative electrode having a negative electrode active material layer on a surface of a negative electrode collector;
• and a nonaqueous electrolyte, the positive electrode, the negative electrode, and the nonaqueous electrolyte are accommodated in a battery case, the production method comprising:
• obtaining a battery assembly by accommodating the positive electrode and the negative electrode in the battery case and further encapsulating the nonaqueous electrolyte into the battery case and sealing the battery case, the nonaqueous electrolyte being obtained by dissolving a lithium salt in a nonaqueous solvent containing γ-butyrolactone as a main component and containing vinylene carbonate and lithium bis(oxalate)borate, and
• subjecting the battery assembly to an initial charge process performed, thereby forming a coat derived from the vinylene carbonate on the surface of the negative electrode active material layer and forming, on the surface of the positive electrode active material layer, a coat derived from bis(oxalate)borate ions generated by ionization of the lithium bis(oxalate)borate.