Tetraethylammonium tetrafluoroborate (TEABF(4)) is investigated as a functional additive to 1.0 M LiPF6/ethylene carbonate-dimethyl carbonate-ethyl methyl carbonate (1:1:1 w/w/w) electrolyte for LiFePO4/artificial graphite (AG) lithium-ion battery. TEABF(4)-containing electrolytes have lower ionic conductivity than TEABF(4)-free electrolyte, but the interface impedance of graphite/electrolyte is reduced by adding TEABF(4). The rate capability and cycle stability of LiFePO4/AG cells are improved significantly by adding a certain concentration of TEABF(4) to the electrolyte, especially 0.1 M. Therefore, TEABF(4) is a promising additive for high performance lithium-ion batteries. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3439642] All rights reserved.