Lithium-ion batteries (LiBs) are currently the technology of choice for applications ranging from portable electronics to the expanding global electronic vehicles market. However, current LiBs are getting close to their theoretical limit and it will be unable to meet the rapidly-growing demand for denser energy storage needs in these areas. One of the most promising pathways for improvement is to use silicon as the anode material in lieu of the conventionally-used graphite. Silicon has a substantially higher theoretical capacity compared to graphite, 3580 mAh/g vs 372 mAh/g, respectively.¹ A major drawback of silicon is that it undergoes a massive volume change during lithiation and de-lithiation, up to 400%, and this can lead to large mechanical stresses that induce cracking and eventual breakdown of silicon.² Additionally, as silicon cracks, new surface area is exposed, where chemical and electrochemical reactions between the silicon and liquid electrolyte yield growing passivation layers that are detrimental to transport of Li⁺ ions and stability of the electrode and electrolyte. Thus, to enable silicon anode use, it is imperative to fully understand the complex dynamic formation and evolution of the silicon-electrolyte interphase (SiEI) during battery operation. Here, we employ operando Raman microscopy to probe the SiEI. We observe the electrolyte solution structure (ion solvation and ionic association) and look for molecular signatures of the SiEI components across the interface during cycling. Furthermore, we explore how the nature of the initial silicon surface, e.g. native or thermally-grown SiO₂, as well as the moisture content in the electrolyte affect mechanisms leading to SiEI formation and stability.

On the other side of the battery, new cathode development is focused on increasing the energy densities (up to ~800 Wh/kg_oxide or more) of transition metal oxides with low Co content (due to limited supply and expected high price of Co), such as LiNi_aMn_bCo_cO₂ (NMC-622 or -811).³ Such energy densities are only feasible at higher operational voltages, 4.2-4.8 V (vs. Li/Li⁺), but at these high voltages, there can be cathode instability and safety problems, such as oxygen loss, dissolution of some of the transition metals (particularly Mn), surface reconstructions, reactions with organic electrolytes, corrosion of the current collector and thermal runaway.⁴ Thus, stabilization of cathode surfaces is essential if high-voltage and low-Co NMC-based cathodes are to be realized in application. To this end, we used operando Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR) to study the formation, reactivity and evolution of the electrolyte-cathode interphase during cycling. This technique elucidates our understanding of critical interfacial processes by combining electrochemical (potentiometric or amperometric) information with characterization of molecules, adsorbates, and reaction intermediates involved at the interface. Utilizing a novel ATR-FTIR-compatible cell configuration, we observe irreversible capacity loss at higher voltages, correlated to spectroscopically-detected interfacial ion dynamics. Operando monitoring of the surface chemistry and evolution of the cathode-electrolyte interphase and the interfacial interactions is essential to unlocking better performance of the LiBs from the perspective of the cathode side of the cell.

References:

1. Feng, K.; Li, M.; Liu, W.; Kashkooli, A. G.; Xiao, X.; Cai, M.; Chen, Z., Silicon‐Based Anodes for Lithium‐Ion Batteries: From Fundamentals to Practical Applications. Small 2018, 14 (8), 1702737.
2. Chan, C. K.; Ruffo, R.; Hong, S. S.; Huggins, R. A.; Cui, Y., Structural and electrochemical study of the reaction of lithium with silicon nanowires. Journal of Power Sources 2009, 189 (1), 34-39.
3. Croy, J. R.; Abouimrane, A.; Zhang, Z., Next-generation lithium-ion batteries: The promise of near-term advancements. MRS bulletin 2014, 39 (5), 407-415.
4. Wen, J.; Yu, Y.; Chen, C., A review on lithium-ion batteries safety issues: existing problems and possible solutions. Materials express 2012, 2 (3), 197-212.