Tuning the surface stabilization of LiNiO2 cathode via mixed conductive carbon nanotube/lithium polyacrylate coatings – Electrochemical performance and operando gas evolution analysis

LiNiO₂ (LNO), one of the most promising Ni-rich cathode materials for Li-ion batteries is limited in its practical applicability due to structural and surface degradation. Protective surface coatings are a viable strategy to create a stable interface. In this work, the surface modification of LNO cathode using mixed electron/ion conductive composite coatings based on carboxyl-functionalized multi-walled carbon nanotubes (oMWCNTs) and polyacrylic acid (PAA) is presented, aiming at an optimal balance of electronic and ionic conductivity, respectively. In-situ conversion reaction of PAA with the detrimental Li residues (Li₂CO₃, LiOH) on LNO surface into lithium polyacrylate (LiPAA) coating layer is demonstrated to facilitate Li⁺ ion transport. Fine-tuning of the oMWCNT/PAA ratio shows that the electrochemical performance of the LNO cathode is improved when the ionic contribution is increased to 75% of the total coating. Galvanostatic cycling of coated LNO@oMWCNT/PAA (1:3) in a half-cell configuration shows a capacity retention of 92.5% at the end of 100 cycles at 0.2C, while the uncoated cathode retains only 76.7%. In non-optimized LNO//graphite full cells, the capacity retention improves from 68.4 % for the uncoated LNO to 87.5 % for the coated sample. Finally, operando gas evolution analysis of the LNO electrode by OEMS (online electrochemical mass spectrometry) studies shows that the coated electrode produces significantly less amount of gases during the electrochemical cycling, including hindering of oxygen evolution at high voltage compared to the uncoated LNO electrode, proving the positive effect of the oMWCNT/PAA coating.