Using Mixed Salt Electrolytes to Stabilize Silicon Anodes for Lithium-Ion Batteries via In situ Formation of Li-M-Si Ternaries (M=Mg, Zn, Al, Ca).

Abstract

Replacing traditional graphite anode by Si anode can greatly improve the energy density of lithium-ion batteries. However, the large volume expansion and the formation of highly reactive lithium silicides during charging cause the continuous lithium and electrolyte consumption as well as the fast decay of Si anodes. In this work, by adding 0.1 M M(TFSI)x (M = Mg, Zn, and Al, and Ca) as a second salt into the electrolyte, we stabilize the anode chemistry through in situ formation of Li-M-Si ternary phases during the charging process. First, lithium silicides and magnesium lithium silicides were synthesized as model compounds to investigate the influence of metal doping on the reactivity of lithiated Si. Using solid-state nuclear magnetic resonance spectroscopy, we show that Mg doping can dramatically suppresses the chemical reactions between the lithium silicide compounds and common electrolyte solvents. New mixed-salt electrolytes were prepared containing M(TFSI)x as a second salt to LiPF6 and tested in commercially relevant electrodes, which show higher capacity, superior cyclability, and higher coulombic efficiencies in both half-cell and full-cell configurations (except for Zn) when compared with standard electrolytes. Post-electrochemistry characterizations demonstrate that adding M salts leads to the co-insertion of M cations along with Li into Si during the lithiation process, stabilizing silicon anions by forming more stable Li-M-Si ternaries, which fundamentally changes the traditional Li-Si binary chemistry while minimally affecting silicon electrochemical profiles and theoretical capacities. This study opens a new and simple way to stabilize silicon anodes to enable widespread application of Si anodes for lithium-ion batteries.