Understanding the Role of Al(TFSI)3 Additive at the Solid Electrolyte Interphase (SEI) for Improved Lithium-Ion Batteries with Silicon Anodes Via Solid-State NMR

Introducing multivalent cation salts (e.g., Ca 2+ , Mg 2+ , and Al 3+ ) has been proven to improve electrochemical performance such as capacity, cyclability, Coulombic efficiency, and calendar life of Si anode by the in situ formation of inert Li-Metal-Si ternary phases. 1–3 Among these ions Al 3+ is of great interest with large positive charge and small radius, providing potential new chemistry. In this work, Al(TFSI) 3 was synthesized and purified and 0.1 M was added as a second salt into the Gen2 (1.2 M LiPF 6 in 30 wt % EC + 70 wt % EMC) electrolyte with 3 wt % FEC. Then NMC532/commercially relevant Si anode full-cells were assembled, stabilized with three formation cycles at a C/10 rate and divided into two groups, one with extended cycling and the other held at 4.1 V for one month to investigate calendar life. Meanwhile, non-Al(TFSI) 3 cells were also prepared and tested from the same batch as baselines for the study. Multi-nuclei solid-state MAS-NMR including 1 H, 7 Li, 13 C, 19 F, 27 Al, and 29 Si of the Si powder collected from those samples after electrochemical testing at both 300 MHz and 500 MHz showed detailed information of different species formed. It was confirmed that Al additive stabilized Si species at the SEI and suppressed side reactions with the electrolyte by forming Li-Al-Si Zintl phase. Specifically, cross-polarization (CP) experiments such as 13 C{ 1 H} and 29 Si{ 1 H} allowed the selective investigation and detection of the species in close proximity to protons at the SEI. This additive strategy can be combined with the novel electrolyte, binder, and Si anode material to pave new avenues towards the commercialization of Si-based lithium-ion batteries. Acknowledgement This work was part of the Silicon Consortium Program (SCP) and was done at Argonne supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-AC02-06CH11357. References B. Han et al., ACS Appl. Mater. Interfaces , 11 , 29780–29790 (2019). X. Li et al., Chem. Mater. , 33 , 4960–4970 (2021). Y. Zhang et al., Advanced Energy Materials , 11 , 2101820 (2021).