Ionomer Binders Can Improve Discharge Rate Capability in Lithium-Ion Battery Cathodes

A lithium-ion form of a perfluorosulfonate ionomer was used as a binder in LiFePO4-based lithium-ion battery cathodes. Carbon-coated LiFePO4 and acetylene carbon black were blended with ionomer to prepare composite cathodes having a composition 60% LiFePO4, 20% acetylene carbon black, and 20% binder by weight. Cathodes were tested against Li4Ti5O12 anodes using 1.0 M and 0.1 M LiPF6-ethylene carbonate/diethyl carbonate (EC/DEC) electrolytes. Comparison was made with cathodes prepared using poly (vinylidene) difluoride (PVDF) as binder. At low discharge rates (e.g., C/5) both cathode types exhibited similar charge-discharge capacities and voltage profiles. However, under higher rate discharge conditions (e.g., > 1C, up to 5C) cathodes prepared using ionomer binder showed better discharge rate capability than cathodes having PVDF binder. This phenomenon was more pronounced when the salt concentration in the electrolyte was low (e.g., 0.1 M LiPF6-EC/DEC). These findings suggest that use of ionic binders can help to compensate for electrolyte depletion from the electrode porous space as lithium ions are intercalated into lithium-deficient LiFePO4 particles during rapid discharging. Potential uses for electrodes having ionomer binders in enabling lower cost battery electrolytes (because of the reduced need for salt) and in developing high rate cathodes that are nonporous or have low porosity are discussed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3526598] All rights reserved.