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 (t Li+) 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 M n = 2.7–55 kg/mol and dispersities Đ = M w/M n = 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 M n-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 t Li+ = 0.77–0.98. Thus, semidilute solutions of oligomeric salts comprising as few as 21 monomer units unexpectedly display t Li+ > 0.90 with σ = 0.34 mS/cm.