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Published August 12, 2020 | Supplemental Material
Journal Article Open

Potassium Difluorophosphate as an Electrolyte Additive for Potassium Ion Batteries


The limited cyclability and inferior Coulombic efficiency of graphite negative electrodes have been major impediments to their practical utilization in potassium-ion batteries (PIBs). Herein, for the first time, potassium difluorophosphate (KDFP) electrolyte additive is demonstrated as a viable solution to these bottlenecks by facilitating the formation of a stable and K⁺-conducting solid–electrolyte interphase (SEI) on graphite. The addition of 0.2 wt % KDFP to the electrolyte results in significant improvements in the (de)potassiation kinetics, capacity retention (76.8% after 400 cycles with KDFP vs 27.4% after 100 cycles without KDFP), and average Coulombic efficiency (∼99.9% during 400 cycles) of the graphite electrode. Moreover, the KDFP-containing electrolyte also enables durable cycling of the K/K symmetric cell at higher efficiencies and lower interfacial resistance as opposed to the electrolyte without KDFP. X-ray diffraction and Raman spectroscopy analyses have confirmed the reversible formation of a phase-pure stage-1 potassium–graphite intercalation compound (KC₈) with the aid of KDFP. The enhanced electrochemical performance by the KDFP addition is discussed based on the analysis of the SEI layer on graphite and K metal electrodes by X-ray photoelectron spectroscopy.

Additional Information

© 2020 American Chemical Society. Received: May 25, 2020; Accepted: July 21, 2020; Published: July 21, 2020. This study was partly supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) program "Elements Strategy Initiative to Form Core Research Center" (JPMXP0112101003) and Japan Society for the Promotion of Science (JSPS, KAKENHI Grant Number 19H04695). One of the authors, H.Y., thanks to the China Scholarship Council (CSC) for the financial support. The authors declare no competing financial interest.

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