Electrical Double Layer Formation at Intercalation Cathode-Organic Electrolyte Interfaces During Initial Lithium-Ion Battery Reactions

Information on the cathode/organic-electrolyte interface structure provides clues regarding the rate and reversibility of lithium intercalation reactions in lithium-ion batteries. Herein, structural changes within the LiCoO2 electrode, throughout the interphase region, and in the LiPF6/propylene carbonate electrolyte are observed concurrently by in situ neutron reflectometry. The formation of an electrical double layer (EDL) during the early stages of charging and discharging is investigated and compared with that at an intercalation-inactive Nb:SrTiO3 electrode. At the intercalation-inactive interface between Nb:SrTiO3 and the electrolyte, a voltage-dependent ionic distribution corresponding to the EDL forms on the electrolyte side without the formation of a cathode/electrolyte interphase (CEI) layer. In contrast, at the intercalation-active LiCoO2/electrolyte interface, a CEI layer forms immediately after cell construction, and the ionic distribution in the electrolyte formed outside the CEI layer scarcely changes upon voltage application. The CEI/electrolyte interface is shielded from potential changes by the electronically insulating CEI; therefore, structural changes in the EDL are restricted. This supports the prevailing understanding that the CEI layer defines the rates of solvation/de-solvation and adsorption/desorption reactions of lithium ions. In situ neutron reflectometry studies demonstrate that the electric double layer (EDL) structure formed at a Nb:SrTiO3/LiPF6 propylene carbonate (PC) interface lacking a cathode electrolyte interphase (CEI) layer undergoes changes with electrode potential. In contrast, a LiCoO2/LiPF6 PC interface covered with a CEI layer has minimal changes in the EDL structure. This finding proves the feasibility of controlling lithium-intercalation properties by tailoring EDL structures via specific CEI compositions.image