Doping small amount of multivalent cation salts (e.g., Ca²⁺, Mg²⁺, and Al³⁺) in the electrolyte for Li-ion battery with Si anode has a profound impact on the electrochemical performance such as capacity, cyclability, Coulombic efficiency, and calendar aging of Si anode due to the formation of inert Li-Metal-Si ternary phases. Among these ions, Al³⁺ is of particular interest because of its large positive charge and small radius, which provides the potential for new chemistry.
Al(TFSI)₃ was first synthesized, purified and 0.05/0.1 M was added as a second salt into the Gen2 (1.2 M LiPF₆ in 30 wt % EC + 70 wt % EMC) electrolyte with 3 wt % FEC. NMC811/commercially relevant Si anode full-cells were assembled and stabilized with three formation cycles at a C/10 rate. The cells were cycled extendedly between 3.0 V and 4.1 V at a C/3 rate or held at 4.1 V for one month to investigate calendar aging. Meanwhile, non-Al(TFSI)₃ cells were also prepared and tested from the same batch as baselines for the study.
The results showed that 0.05 M Al(TFSI)₃ was beneficial while the 0.1 M concentration was not. Post-analysis with SEM, high-resolution XRD, and magic-angle-spinning (MAS) NMR including ¹H, ⁷Li, ¹³C, ¹⁹F, ²⁷Al, and ²⁹Si provided detailed information on the interface morphology and different species formed in both bulk Si and Solid Electrolyte Interphase (SEI). It was confirmed that Al additive stabilized Si species at the SEI and suppressed side reactions with the electrolyte by forming a Li-Al-Si Zintl phase, while extra AlF₃ formed at the SEI and the Lewis acidity of the Al³⁺ bearing electrolyte formulation may have adversely affected cell performance which remains to be addressed.
This trivalent additive strategy may be combined with novel electrolytes, binders, Si anode materials, and cell architecture to aid in the commercialization of Si-based lithium-ion batteries.

Haoyu Liu

Appearances