Nickel-based transition metal oxides with layered structures viz. (LiNixMnyCozO2 (NMC, x+y+z=1; where, x≥0.5) or LiNixCozAlyO2 (NCA, x+y+z=1; where, x≥0.85) are among the most promising materials for use in the next generation LIBs, due to their ability to deliver high capacities with competitive prices. However, they possess problems such as surface side reactions and chemical instabilities at the highly de-lithiated stages (> 4.3 V vs. Li+/Li) lead to safety issue and structural instabilities. The degradation results in both capacity loss/performance deterioration. To overcome the aforementioned issues and to achieve long-term performance of Ni-rich materials, surface coating is considered as an efficient strategy. Hence, in the present thesis, a carbon coating was produced on LiNi0.6Mn0.2Co0.2O2 (NMC622) material. Carbon coatings with three different precursors were performed and compared, namely furfuryl alcohol based, oxalic acid based, and resorcinol formaldehyde-based ones. The physicochemical and electrochemical characterisations of the coating were performed; owing to better electrochemical performance, ease of process, ability to form carbon layers having good mechanical properties and chemical inertness towards corrosive species such as HF, furfuryl alcohol-based coating was considered for the further detailed study. The furfuryl alcohol-based coating was carried out via polymerisation of furfuryl alcohol, followed by a calcination step. The effect of calcination on the structure of polymerised NMC622 and its electrochemical behaviour are in detail investigated and discussed using various physical and electrochemical analytics. The coating with ~15-20 nm thick amorphous layer of carbon on top of NMC622 particles, results in increased capacity retention of ~10 % compared to uncoated counterparts after the completion of 400 cycles, and ~50 % of improvement in capacity at 10C discharge rate. Nevertheless, to understand the effect of calcination on the performance improvement of the NMC622, samples treated at elevated temperature, but without carbon coating were tested under the same conditions and compared with the pristine untreated samples, where the uncoated calcined samples found to improve their performance because of improvement in the crystallinity and removal of the surface impurities through heating. Furthermore, in the recent times the surface deposition techniques are advanced such that, precise deposition of coatings up to angstrom level on to the electrode is possible by maintaining its excellent uniformity. Hence, to investigate this process and its effect in our NMC622, surface coating of the electrodes was performed, that is surface covering of prepared NMC622 electrode carried out with different thickness such as 4 nm, 8 nm, 16 nm, and 32 nm via physical vapour deposition technique. Electrochemical analysis showed an increase in the performance of the coated samples compared to uncoated pristine electrode. Especially, the 16 nm carbon coating showed better performances in long-term as well as C-rate analysis. The high demand for batteries needs better electrode engineering and process technology. The traditional cathode fabrication with polyvinylidenefluoride (PVDF) based organic binder involves toxic N-methyl-pyrrolidone (NMP) solvent for the electrode processing. Which increases the cost, safety concerns, health hazards and affects the eco-friendliness of the production processes. To overcome these issues, uncoated/coated NMC622 cathodes aqueous slurry was prepared using sodium carboxymethyl cellulose (Na-CMC) and polyacrylic acid (PAA) as binders. Phosphoric acid was added during slurry mixing to adjust the pH values. The fabricated electrodes from the latter process do not show any physical or mechanical instabilities. The electrochemical tests such as long-term charge-discharge and C-rate performance showed improvement in the carbon coated samples when compared with the uncoated ones. Furthermore, an isothermal pseudo-2D framework model based on the Doyle-Fuller-Newman model was constructed using coated/uncoated NMC 622 (LiNi0.6Mn0.2Co0.2O2) with a liquid electrolyte (1M LiPF6 in ethylene carbonate (EC)): ethyl methyl carbonate (EMC) = 3:7 (w/w), and 2 wt.% vinylene carbonate (VC)) and an anode (lithium metal). The model simulates the isothermal electrochemical behaviour of the cell during charge-discharge at a defined C-rate. The results show an improvement in capacity for the surface-coated NMC622. Further, the full cells at coin cell level, consisting of uncoated/coated NMC622 cathodes vs. graphite anodes were fabricated to investigate its performance in the Li-ion battery configuration. The results showed better electrochemical performance with excellent capacity retention in the coated material compared to uncoated/untreated material full cells at 25 oC and 60 oC. Additionally, to transfer the gained knowledge from the coin cells to a market near LIBs, laboratory prototype pouch cells of the coated and uncoated NMCs were fabricated and found to show good performance. The promising results demonstrate the ability of the commercialization of the technology; however, further optimizations of the process are required to make it industrially accepted technology. In a way, the present thesis work shows a complete cycle of surface stabilization of Ni-rich NMC622 from the material level to the lab prototype study.
|Betreuer/-in / Berater/-in|
|Datum der Bewilligung||22 Juni 2022|
|Publikationsstatus||Veröffentlicht - 2022|
- Battery Materials Development and Characterisation