PROGRAM | Mechanical Engineering

By: Rownak Jahan Mou Chair: Koffi Pierre Yao

ABSTRACT

The cycling of high-capacity silicon (Si) anodes capable of 3579 mAh·g-1 in next-generation lithium-ion batteries is greatly hindered by the instability of the solid electrolyte interphase (SEI). The large ~350% volume expansion of Si during (de)lithiation causes continuous cracking of the SEI and its reconstruction, leading to loss of lithium inventory and extensive consumption of electrolyte. The challenge lies in establishing a stable SEI for Si anode that can withstand the drastic volume changes and offers protection of the active material from electrolyte in extensive cycling and resting conditions. Consecutive studies are conducted to understand the dynamics of Si SEI between delithiated and lithiated states and its evolution during aging that affects battery lifetime. Impact of Fluoroethylene carbonate (FEC), a well-known additive for improving the cycling performance of Si, on the growth and stability of SEI is investigated in detail. Later based on the fundamental understanding, a pathway for ex-situ development of synthetic polymeric SEIs is proposed that can enhance performance and longevity of Si-based lithium-ion batteries.

The first study focuses on unraveling the SEI dynamic morphology and chemistry evolution from delithiated to lithiated states with and without FEC additive which is crucial for engineering a stable Si anode. Complementary ATR-FTIR, AFM, tip IR, and XPS probing reveal the presence of an elastomeric polycarbonate-like matrix in the FEC-generated SEI which is absent from the FEC-free SEI. Adding FEC to the baseline 1 M LiPF6 in EC:EMC (1:1) electrolyte promotes formation of a thinner and more conformal SEI, and subdues morphology and chemistry changes between consecutive half-cycles. The study concludes that the enhanced mechanical compliance, chemical invariance, and reduced Li inventory consumption of the FEC-SEI are logically the key features underlying the Si cycling enhancement by FEC.

The second study explores the effect of FEC on the aging of Si-based lithium-ion batteries, investigating both cycle aging and calendar aging. Comparative study of Si||LiFePO4 full cells with Gen2 electrolyte (EC:EMC 3:7 w/w + 1.2M LiPF6), with and without 3 wt.% FEC, reveals that the FEC-containing cell has improved capacity retention after both cycle aging (100 cycles, 1C) and calendar aging (Open Circuit Voltage, 180 hours) conditions. Morphological analysis via SEM indicates fewer visual changes in Si electrode thickness after calendar aging. XPS analysis reveals that FEC reduces electrolyte decomposition on Si anode surface during OCV, keeps the active Si material more accessible after calendar aging, and thus, improves capacity retention post-calendar aging. This study highlights that FEC-generated SEI passivates Si surface and provides protection against excess parasitic reaction during rest which is essential for the longevity and cyclability of Si-based Lithium-ion battery after aging.

Above studies indicate that a polymeric SEI provides greater flexibility to withstand Si volume expansion and can passivate Si surface preserving Li inventory and electrolyte in the battery.  Finally, the last study proposes a pathway to ex-situ formation of artificial polymer SEI using electrophoretic deposition (EPD) process to address Si SEI instability in cycling. The study includes comparison of synthetic SEIs of chitosan obtained without (Si/Chit) and with CH3COOLi (Si/Chit+CH3COOLi) added during EPD and demonstrates a facile route to tuning of the polymer SEI chemistry. Adding CH3COOLi during EPD enables conformal deposition of the synthetic SEI. Electrochemical cycling shows that the chitosan+CH3COOLi coating nearly doubles capacity retention compared to a bare Si thin film. XPS and FTIR spectroscopy reveal that CH3COOLi caps the -NH2 groups of chitosan through amidation during EPD, which suppresses catalytic reduction of the electrolyte. This approach validates EPD as a low-capital method, highlights the potential of the process for achieving and tuning synthetic SEIs on Si electrodes. It offers a promising avenue for controlled and tailored polymeric SEIs on conversion-type electrodes with high volumetric expansion.

Together, these studies provide a understanding of the dynamics of SEI on silicon, role of an additive like FEC on addressing the challenges of Si-SEI instability, and an economic pathway for engineering artificial stable SEI on Si anodes for lithium-ion batteries.

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