Manipulating interfaces for enabling fast charging of alkali-ion batteries

Abstract

Li-ion batteries (LIBs) have been dominating the market of portable electronics since their commercialization in 1991. Nowadays, LIBs are also poised to revolutionize the transportation industry thereby playing a sizeable role in the reduction of CO2 emission through the electrification of vehicles. However, their long charging time of around one hour hinders the mass adoption of electric vehicles. Therefore, new technologies for fast-charging batteries must be developed.Carbons, graphite and hard carbon, are the most commonly used anode materials for rechargeable alkali-ion batteries. Graphite that is used in LIBs as anode however suffers from slow ion-diffusion kinetics. Also, all carbon-based anode materials experience severe ion-transport limitations through the solid electrolyte interphase (SEI). It is known that charging rates can be vastly improved by optimisation of the electrode architecture. Reducing particle size promotes shorter diffusion lengths for electrolyte ions. Increasing electronic conductivity enhances cell performance.This PhD thesis project aimed to identify the fundamental limitations on high-rate performance of carbon anode materials and develop methods to enable fast-charging batteries based on carbon electrodes. Fast cycling is enabled by promoting fast transport of charges: fast transport of ions at the carbon-electrolyte interface and inside the carbon particle bulk, and fast transport of electrons through the current collector-carbon and the particle-to-particle interfaces. To achieve this, a novel thin-film carbon electrode architecture with excellent electronic and ionic charge transport properties has been developed. Also, carbon nanoparticles are used as active material.However, nanosizing particles has the direct effect of increasing the production of SEI layers at the carbon-electrolyte interface. Interestingly, the ion-transport properties of SEI layers can be manipulated through the electron current density during its formation. Therefore, in addition of using carbon nanoparticles, the formation of the SEI layers on the carbon anode under high current densities was investigated.One of the main contributions of this thesis project is the demonstration of the electrochemical cycling performance of carbon nanoparticles for storing alkali metal ions (Li+, Na+ and K+) at high current rates (as high as 100 A/g). At current rate of 100 A/g, a hard carbon nanoparticle electrode with 2.5 wt% carbon nanotubes delivered Na+ capacities up to 144 mAh/g. When cycled against lithium and potassium, hard carbon and graphite nanoparticles featured additional charge storage in defects iniiaddition to intercalation. The total charge storage capacity of the hard carbon nanoparticles follows Li+ > K+ > Na+. At the very high current rate of 100 A/g, charge storage is predominantly occurring at defective sites and on pore surfaces. The specific capacities for Li+ in graphite and hard carbon nanoparticles of 50 nm are about 690 and 564 mAh/g at 1 A/g, respectively. These values far exceed the theoretical specific capacity of commercial graphite (372 mAh/g). Finally, under 5-min charging cycle (4.5 A/g), 50-nm graphite particles were able to retain 184 mAh/g of Li+ capacity compared to 70 mAh/g only for 20-μm graphite particles.The other key contribution of this thesis project is the demonstration of the critical role played by SEI layers in the cycling performance of batteries. It was demonstrated that, through the optimization of the current collector-carbon interface, forming SEI layers at high current densities (100 A/g) is an effective approach to significantly improve battery performance. It was found that SEI layers formed at 1 A/g exhibits 239% higher ionic impedance compared to SEI layers formed at 100 A/g. In comparison with the thin-film electrode architecture, an electrode prepared using the traditional slurry method exhibits lower electronic and ionic conductivities, and requires SEI layers to be formed at very low current rates (19 to 38 mA/g), which are currently commonly used to ensure stable SEI layers. Further, optimizing the SEI layers through high current densities is shown to be applicable to any carbon anodes, regardless of the alkali cation (Na+, K+ or Li+) and particle size. In addition to carbon nanoparticle-based electrodes, this thesis project also investigated micrometre-sized graphite particles of sizes up to tens of micrometres that are used in the LIB industry. Experimental results showed that graphite particles up to 23-μm in size with SEI layers formed at 37.2 A/g (100C) retain stable Li+ intercalation characteristics at 4.5 A/g (12C). While the same electrodes with SEI layers formed at 37.2 mA/g (0.1C) suffered from metal plating, further confirming the crucial importance of SEI layers formed under high current densities in fast-charging applications.In short, this PhD thesis work has demonstrated the importance of controlling electrochemically the formation of functional SEI layers on carbon anodes for fast-charging alkali metal ion batteries, including lithium-ion batteries, sodium-ion batteries and potassium ion batteries. It has also shown that carbon nanoparticles with a SEI layer formed at high current rates can outperform micrometre-sized carbons in terms of charge transport kinetics, indicating the possibility of using nanomaterials for fast-charging alkali-ion batteries.