Synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis-oxalato borate (LiBOB) salts in LiPF6-based electrolyte for high-performance Li-ion batteries

• The effect of Li-salt mixing in LiPF6/EC/EMC electrolyte is investigated.
• The corrosion of Al by LiFSI is successfully suppressed by the addition of LiBOB.
• The high surface impedance arising from LiBOB is reduced by the presence of LiFSI.
• Electrochemical behaviour of the graphite anode and the LiFePO4 cathode is improved by Li-salt mixing.

The effect of Li-salt mixing in Li-ion battery electrolyte based on LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) is investigated. The addition of an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) into the LiPF6-based electrolyte contributes to an electrochemical improvement of the graphite anode. However, the LiFePO4 cathode is difficult to cycle in such an electrolyte due to the severe corrosion of the aluminium current collector by FSI anions. Lithium bis-oxalato borate (LiBOB) is able to passivate Al and suppress the corrosion arising from the FSI anions. An improvement of rate performance and cycling stability for both the LiFePO4 cathode and the graphite anode is obtained in 1.0 mol L−1 LiPF6/EC/EMC electrolyte containing 0.2 mol L−1 LiFSI and 0.2 mol L−1 LiBOB salts.Moreover, an excellent compatibility between the graphite anode and LiFePO4 cathode in the ternary-salt electrolyte system is further confirmed by the full cell tests. The electrochemical performance improvement of the electrolyte resulting from Li-salt mixing provides a new way for optimization of electrolyte for high performance Li-ion batteries.

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