In this work we report the use of the aprotic ionic liquid (AIL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr(14)TFSI) and the protic ionic liquid (PIL) 1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr(H4)TFSI) in view of the realization of potassium ion batteries. Physicochemical characterization of electrolytes reveals features comparable to those obtained for lithium and sodium ion technologies. Electrochemical performance is evaluated in combination with olive pits derived hard carbon electrodes with and without a film-forming additive at room temperature and at 60 degrees C. Similar to our previous study, the use of PyrH(4)TFSI is not applicable due to the lack of electrochemical stability below 2 V vs. K+/K, while reversible K+ insertion/deinsertion is demonstrated for Pyr(14)TFSI. Electrochemical impedance spectroscopy and post-mortem analysis reveal an unstable formation of the solid electrolyte interphase within the HC electrode. (C) 2019 The Electrochemical Society.