Cathode materials such as LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) are of great interest for Li-ion battery (LIB) systems. By increasing the Ni content, the price decreases since Co is an expensive transition metal. In addition, NMC811 offers higher capacity and operating at high voltages, compared to other cathode materials (i.e., LCO, LMO, etc.).[1] However, this cathode material suffers degradation from different sources. One of the main detrimental processes is oxygen loss from the lattice, which can cause decomposition of electrolyte components such as cyclic carbonates and further generate undesirable gassing in LIB systems.[2] Several different organosilicon (OS) compounds have been reported to decrease gassing during storage with NMC622/Gr multi-layer pouch cells.[3] Besides reducing gassing, OS compounds have been reported to increase the thermal stability of PF₆^- and reduce HF, which is detrimental for LIB systems.[4] Currently, the exact mechanism for how OS additives alleviate these undesired electrolyte degradation reactions is not well known.

In this study we performed a mechanistic investigation on the effects of OS3 (organosilicon additive) during Li the intercalation/deintercalation processes on NMC811. First, we explored how OS3 affects the electron transfer kinetics by performing variable scan rate cyclic voltammetry. Second, we studied the mass accumulated on NMC811 surfaces during the first cycle as a function of applied potential, by modifying a commercial electrochemical quartz crystal microbalance (EQCM). Finally, we analyzed the chemical composition of the cathode surface layer and how it evolves as a function of potential using ex situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) at stepwise points during the first cycle. Together, these techniques provide a comprehensive study of the reactions occurring at the NMC811 electrode surface during the first cycle, including lithium intercalation kinetics, mass deposition and dissolution, and the chemical composition of surface decomposition species as a function of potential, it was found that OS3 delays the onset of mass accumulation and therefore delays electrolyte degradation during the first cycle on NMC811 cathode materials as confirmed by the presence of new absorbance features on ex situ ATR-FTIR. These studies further provide new insights into the mechanisms of performance enhancement observed with OS additives.

[1] Zhang, N.; Li, J.; Li, H.; Liu, A.; Huang, Q.; Ma, L.; Li. Y.; Dahn, J.R.; Chem. Mater. 2018, 30, 8852-8860.

[2] Renfrew, S.E.; McCloskey, B. D.; ACS Appl. Energy Mater. 2019, 2, 3762-3772.

[3] Guillot, S.L.; Usrey, M. L.; Peña-Hueso, A.; Kerber, B.M.; Zhou, L.; Du. P.; Johnson, T.; J. Electrochem. Soc. 2021, 168, 030533.

[4] Guillot, S.L.; Peña-Hueso, A.; Usrey, M.L.; Hamers, R.J.; J. Electrochem. Soc. 2017, 164, A1907.