Electrochemically Stable, High Transference Number Lithium Bis(malonato)borate Polymer Solution Electrolytes

Implementation of next-generation Li-ion batteries based on high-voltage cathodes requires the development of new electrolytes that are stable over wide electrochemical potential windows. High lithium-ion transference number (tLi+) electrolytes, in which most of the ionic current is carried by Li+, offer additional opportunities to improve the efficiencies of such batteries by minimizing unproductive anion motion. Herein, we report the synthesis and characterization of solution electrolytes based on poly(lithium bis(nonenylmalonato)borate) (P(LiBNMB)) with molecular weights Mn = 2.7–55 kg/mol and dispersities Đ = Mw/Mn = 1.6–3.2. Cyclic voltammetry studies reveal that propylene carbonate (PC) solutions of P(LiBNMB) are electrochemically stable up to 5.2 V (versus Li/Li+). Electrochemical impedance spectroscopy (EIS) studies indicate that the total ionic conductivities of these P(LiBNMB)/PC solution electrolytes with [Li+] ≈ 0.10 M are nearly Mn-invariant with σ = 0.31–0.39 mS/cm. Complementary ion diffusivity measurements by 1H and 7Li NMR diffusion ordered spectroscopy (DOSY) and potentiostatic polarization also reveal that they exhibit high tLi+ = 0.77–0.98. Thus, semidilute solutions of oligomeric salts comprising as few as 21 monomer units unexpectedly display tLi+ > 0.90 with σ = 0.34 mS/cm.