Asymmetric electrolyte design for high-energy lithium-ion batteries with micro-sized alloying anodes

Micro-sized alloying anodes offer lower cost and higher capacity than graphite in Li-ion batteries. However, they suffer from fast capacity decay and low Coulombic efficiency in carbonate electrolytes because the organic solid electrolyte interphase (SEI) strongly bonds to the alloys, leading to cracks of both SEI and alloying particles, which allows electrolyte penetration and forms new SEI during lithiation–delithiation cycles. Using nano-sized alloying anodes can enhance the cell cycle life but also reduces the battery calendar life and increases the manufacturing costs. Here we significantly improved the cycle performance of micro-sized Si, Al, Sn and Bi anodes by developing asymmetric electrolytes (solvent-free ionic liquids and molecular solvent) to form LiF-rich inorganic SEI, enabling 90 mAh μSi||LiNi0.8Mn0.1Co0.1O2 and 70 mAh Li3.75Si||SPAN pouch cells (areal capacity of 4.5 mAh cm−2; N/P of 1.4) to achieve >400 cycles with a high capacity retention of >85%. The asymmetric electrolyte design forms LiF-rich interphases that enable high-capacity anodes and high-energy cathodes to achieve a long cycle life and provide a general solution for high-energy Li-ion batteries.