Multifunctional fluorinated phosphonate-based localized high concentration electrolytes for safer and high-performance lithium-based batteries

Abstract

This study introduces a novel fluorinated phosphonate-based additive, Bis(2,2,2-trifluoroethyl)(methoxycarbonylmethyl)phosphonate (BTCMP), in a localized high-concentration electrolyte (LHCE) to address key challenges in lithium metal batteries (LMBs), such as dendritic lithium growth and porous electrode morphology. The pristine LHCE forms a fluorine-rich solid electrolyte interphase (SEI) layer, primarily composed of lithium fluoride (LiF). However, the pristine LHCE suffers from severe electrolyte decomposition, forming a thicker and more resistive cathode electrolyte interphase (CEI), leading to increased impedance and reduced cyclability. Interestingly, Density Functional Theory (DFT) investigations revealed that the BTCMP inclusion modifies the solvation structure by dispersing the aggregates into smaller fragments, facilitating easier Li⁺ migration than the pristine LHCE. The addition of BTCMP suppresses solvent decomposition as confirmed by an increasing trend in the lowest unoccupied molecular orbital (LUMO) of corresponding LHCE molecules. Further, the presence of BTCMP results in a thinner and more stable CEI, reducing electrolyte decomposition, maintaining better ion transport, and preserving the cathode's structural integrity, as supported by TEM and XPS analysis. The BTCMP-based F-rich LHCE retained 82 % of its initial capacity while maintaining a coulombic efficiency of 99.5 % after 200 cycles in a Li||LNMO cell while the pristine LHCE only retained 47.2 % of initial capacity post-150 cycles. Additionally, the flame-retardant properties of BTCMP-based LHCE highlight its safety compared to commercial electrolytes.