Study of a High-Voltage NMC Interphase in the Presence of a Thiophene Additive Realized by Operando SHINERS

Abstract

Improving the electrochemical properties and cycle life of high-voltage cathodes in lithium-ion batteries requires a deep understanding of the structural properties and failure mechanisms at the cathode electrolyte interphase (CEI). We present a study implementing an advanced Raman spectroscopy technique to specifically address the compositional features of interphase during cell operation. Our operando technique, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), provides a reliable platform to investigate the dynamics of the interphase structure and elucidate the compositional changes near the cathode surface. To improve the CEI properties, thiophene was introduced and investigated as an effective, high-voltage film-forming additive by largely diminishing the capacity fading triggered at high potentials in LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathodes. While the cells without thiophene show severe capacity fading, cells with an optimized concentration of thiophene exhibit a significant performance improvement. Operando SHINERS detects the presence of a stable CEI. The results suggest that the composition of the CEI is dominated by polythiophene and copolymerization products of ethylene carbonate with thiophene, which protects the electrolyte components from further decomposition. The formation mechanism of the polymeric film was modeled using quantum chemistry calculations, which shows good agreement with the experimental data.

Quadrupled Cycle Life of High‐Voltage Nickel‐Rich Cathodes: Understanding the Effective Thiophene‐Boronic Acid‐Based CEI via Operando SHINERS

“…ΔV A is a tool to estimate the internal resistance of LIB cells. [ 29,39 ] As shown in Figure 1b, both cells containing the considered electrolyte formulations showed a continuous increase in resistance with ongoing galvanostatic cycling. However, it was observed that the resistance of the cells containing the baseline electrolyte increased significantly faster with ongoing galvanostatic cycling, reaching a ΔV of 850 mV after 500 cycles.…”

“…The first one (A) is similar to the polymerization mechanism of thiophene, which was considered in our previous work, where the oxidized additive is deprotonated by a PF 6 − anion. [ 29 ] Additionally, a second mechanism (B), where the proton is intramolecularly transferred to the semi‐deprotonated boronic acid group instead of the deprotonation via an external anion, was suggested. Please note that the calculated energies of the reaction mechanisms in Figure 3 in electron volts (eVs) are equivalent to potentials in volts, as only one electron is transferred per elementary reaction step.…”

“…In our previous work, we already discussed the possibility of forming an EC‐thiophene copolymer using theoretical modeling. [ 29 ]…”

“…[ 22,24,27,28 ] Our previous study already showed that SHINERS is a powerful tool for investigating the CEI in high‐voltage LIB cells. [ 29 ] In addition, several other studies have realized the potential of this technique to characterize the interphase on different anode materials. [ 22,23,29–32 ]…”

“…[ 29 ] In addition, several other studies have realized the potential of this technique to characterize the interphase on different anode materials. [ 22,23,29–32 ]…”