In Situ and Ex Situ Studies of Interfacial Processes on Intermetallic Li-ion Anodes
Authors: Kostecki Robert , Hasa Ivana , Zhang Liang , Ross Philip N 
Published: February 2020

Intermetallic (Si, Sn, Sb etc.) electrodes offer significantly higher volumetric and gravimetric energy density compared to the widely used graphite-based electrodes, which make them promising candidates for next generation Li-ion cells for transportation applications. [1] However, numerous studies have demonstrated that the inherently non-passivating behavior of intermetallic electrodes in standard organic carbonate-based Li-ion electrolytes [2] is aggravated by significant volume changes during the charging and discharging processes. Mechanical stresses from volume change lead to particle decrepitation, resulting in electronic isolation of particles, loss of mechanical integrity of composite electrodes and interfacial instability. Particle factures expose fresh electrode surface to the electrolyte during cycling, leading to formation of a thick film of electrolyte reduction products, causing impedance rise, capacity loss and lithium inventory shift in the cell. [3]
The solid electrolyte interphase (SEI) layer, which forms at the electrode/electrolyte interface during the initial charge/discharge cycles, is the key component that determines the long-term stability and cycling behavior of negative Li-ion battery electrodes. [4-6] Electrolyte reduction and SEI layer formation on these electrodes usually take place at potentials below 1.8V vs. Li/Li+ and accompany the formation of Li-Me phases, the so-called “Me-Li alloying” process at E <0.8 V vs. Li/Li+. The exact mechanism of the SEI formation processes on Si, Sn, Sb and their alloys with lithium, the SEI composition and the effect on the electrode electrochemical cycling performance is not well understood. Interestingly, model studies on Sn single crystal electrodes in organic carbonate electrolytes revealed a strong correlation between the crystal surface orientation and the SEI composition. [7,8] A similar study of the composition of the SEI on a silicon monocrystal electrode showed strong effects of different SEI formation protocols, presence/absence of intrinsic SiO2, electrolyte composition and impurities e.g., HF. On the other hand, SEI-forming electrolyte additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are known to alter the composition and properties of the SEI on Si and improve the electrode electrochemical performance.
This study involves rigorous charcaterization studies of tin, silicon, Si-based alloys, and composite Si-based model electrodes to determine and understand the key thermodynamic and kinetic parameters, which enable their function and operation in Li-ion battery systems. Advanced diagnostic tools are used to gain fundamental insight into mechanism of intermatallic-based electrodes failure. An emphasis is placed on in situ methods that use multiple techniques at the same time e.g., imaging with spectroscopy. The diagnostic experimental strategies involve evaluations of model silicon and other intermetallic model composite electrodes as well as studies of the properties of the individual components and their interfaces in a carefully designed and well defined experimental systems. The interfacial instability of the electrolyte and the uncertainty associated with the formation of a stable solid electrolyte interphase (SEI) are the key problems being addressed.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Applied Battery Research for Transportation (ABR) Program and Award Number DE-EE0006443.

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