A Selective Interaction between Cation and Separator Membrane in Lithium Secondary Batteries

Abstract

The presence of a specific Coulombic interaction between the lithium cation and the separator membrane in lithium secondary batteries was proven in this study, through evaluating the mobilities and microviscosities of the mobile ions in electrolyte solutions outside and within the separator membrane. The magnitude of the interaction depends on the solvation structure of the lithium cation, whose net charge is affected by the type and number of solvating species due to their shielding effect. Lithium cations with larger or more strongly coordinated solvating species are less attracted to the membrane; therefore, they display higher mobility compared to weakly solvated cations that are more strongly attracted to the membrane. We confirmed that the mobility of lithium cations in the separator membrane is controlled by their solvated structure in the electrolyte, as well as by the surface charge of the separator membrane. This knowledge could lead to the systematic design of battery performance appropriate for the battery loading system.

Ion Transport in Solid Medium—Evaluation of Ionic Mobility for Design of Ion Transport Pathways in Separator and Gel Electrolyte

“…Figure 5 represents the estimated results of ionic mobilities, D cation and D anion , and the microviscosities, η, α, and β of the electrolyte solution, L1, which was identified in Figure 4 , in polyethylene (PE) separator membranes as a function of membrane porosity [ 24 , 30 ]. PE membranes were prepared by the conventional wet method with stretching in biaxial direction and have the randomly arranged pathways [ 3 , 31 ].…”

“…η; microviscosity attributed to the van der Waals interaction between the ions (Li(EC) 3 + , TFSI − ) and all surrounding species in the solution (yellow region), α; microviscosity from the Coulombic interaction between the closest cations and anions (which is a dominant element from the Coulombic interactions), β ca ; microviscosity reflecting the Coulombic interaction between the cations and inside wall of pores (brown barrier) that holds the solution. Reprinted with permission from [ 24 ]. Copyright 2017 ACS Publications.…”

“… Temperature dependences of ( a ) D cation and D anion , ( b ) η and α, and ( c ) r cation and x of L1; 1 M LiPF 6 /EC+DEC (EC:DEC = 1:1) and L3; 1 M LiPF 6 /EC+DEC (EC:DEC = 1:6.9) solutions without the separator membrane. Reprinted with permission from [ 24 ]. Copyright 2017 ACS Publications.…”

“…Hallow marks at 55% represent the polypropylene (PP) membrane and at 100% represent the free electrolyte solution. Reprinted with permission from [ 24 , 30 ]. Copyright 2017, 2018 ACS Publications.…”

Effect of the Stretching Process of Polyethylene Separators on Rate Capability of Lithium-Ion Batteries

“…The separator is one of the components in LIBs, and it plays important roles such as preventing short circuits and providing ion transport pathways during charge/discharge processes. − Although the separator does not contribute directly to the electrochemical reactions in LIBs, it affects the ionic mobility, and consequently, the battery performance. , Therefore, designing porous morphology for efficient ion transport in the separator enhances the battery performance. In recent years, research on the rational structure design (morphology) of separators has attracted significant attention. − In our previous studies, using a combination of conductivity analysis and pulse-gradient spin-echo nuclear magnetic resonance spectroscopy (PGSE-NMR), we have explained the relationship between the pathway morphology and the ion mobility of the electrolyte in the separator. − …”