Optimization of Electrolytes for Si-Containing Full Cell

Introduction The Si/electrolyte interface is increasingly recognized as being a key factor in the successful implementation of Si-based materials in Li-ion batteries. Si-based negative electrode materials undergo massive volume changes during cycling. Pure Si expands by approximately 280% while 3M’s active/inactive Si alloys expand on the order of 135%. The expansion and contraction of Si has important consequences on the surface stability of these materials. This work will focus on the combined impact of material design and electrolyte choices on cell performance. Results and Discussions The 3M Si alloy is compared to pure Si in EC/EMC 3/7 + 10% FEC. Cross section SEM images are taken after 100 cycles in a half cell. While the 3M alloy shows a discernible SEI on distinct particles, the pure Si has dramatically expanded in size and completely lost its particle morphology. The impact of electrolyte choice on the surface morphology of the 3M Si alloy is then explored across a range of electrolytes compositions using cross section SEM images after 100 cycles. Further testing is performed in 200 mAh pouch cells (Lifun), with a LiCoO2 positive electrode, a Si-alloy/graphite negative electrode, and approximately 3 mAh/cm2 of reversible capacity. In order to understand the impact of electrolyte composition as well as failure mechanisms, cells were studied by long term cycling, ultrahigh precision cycling, post-cycling gas chromatography/mass spectroscopy (GC/MS) of the electrolyte, in-situ volume measurements, and electrochemical impedance spectroscopy. Fluorethylene carbonate (FEC) has long been recognized as an important electrolyte component for Si-containing full cells. GC/MS studies are used to study the consumption of FEC with cycle number as well as changes in electrolyte composition. FEC is found to be preferentially consumed relative to other electrolyte solvents and its depletion is found to generally cause sudden failure. A hierarchy of reactivity of electrolyte solvents is established. An electrolyte additive in the new product introduction process at 3M will be presented. This additive is found to help delay sudden failure as well as suppress FEC gassing. Conclusion When implementing well-designed Si-based materials in commercially-relevant full cells, the electrolyte is a key parameter to be optimized. In this work, electrolyte optimizations lead to delayed sudden failure, suppressed gassing and reduced impedance growth. These findings suggest considerable gains are still attainable through further electrolyte optimizations, which will enable deeper penetration of Si-based materials into the marketplace.

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