The communication between humanity and energy sources reflects the evolution of human society. As the landmark of history, from the awe of the prehistoric man about the fire to the steam's popularization in the industrial revolution, different kinds of energy play an unparalleled role in the development of technology. Up to time now, electricity, the ultimate energy source in existence, powers the development of civilizations. Meanwhile, with the more urgent request for improving the quality of life, higher energy consumption, and more diverse forms of energy storage systems (ESS) are integrated into the world.
Among all the various ESS, the battery received considerable attention because of its high theoretical energy density, the feasibility of current technology, and applicability for different fields. Whereas, the significant issues rest on the sluggish reaction kinetics of electrode materials during fast charging and discharging process. For instance, graphite is the most used anode material for lithium-ion batteries because it has economical production cost and considerable energy density (theoretical capacity: 372 mAh g-1, discharge plateau: ~ 0.1 V vs. Li+/Li). When employed as high-power batteries, however, graphite shows inadequate endurance with rapid fading of its capacity and delivers reduced capacity. The uncontrollable growth of lithium dendrite will also cause safety problems, particularly at high charging current density.
In order to fill the vacancy of the desired high-rate anode candidate, titanium-based materials acquire numerous studies. The most representative candidate is Li4Ti5O12 (LTO), which has been widely studied and commercialized. In respect that LTO has “zero strain” physical property, its cycling stability is exceptionally long, compared with other carbon-based anodic materials. However, low electronic conductivity (10-8 S cm-1), slow ionic diffusivity, low theoretical capacity (175 mAh g-1), and high operational voltage (1.55 V vs. Li+/Li) limit its performance and further development in the matter of power batteries. By contrast, another member from titanium-based materials, lithium titanium silicate (Li2TiSiO5, LTSO), offers an intriguing theoretical capacity of 308 mAh g˗1 and a low potential of 0.28 V vs. Li+/Li.
Nevertheless, inherent properties like low electric and ionic conductivity are still tangling the growth of such materials in power lithium-ion batteries. For the improvement of these drawbacks, using carbon composites and nanocrystallizations is simple and effective. Nevertheless, they bring new issues like low tap density, low initial coulombic efficiency, and complex synthesis. In brief, there is a conflict between the total amount of carbon materials (for conductivity enhancement), particle size (for ionic diffusivity improvement), and tap density (effect on volumetric energy density), initial coulombic efficiency (related to specific surface area).
In this dissertation, we first designed a chemical vapor deposition (CVD)-assisted synthetic strategy to achieve conductive carbon-coating over the surface of fumed-silica and transform the catalysts to LTSO for high-rate anode material of lithium-ion batteries. The application of CVD allows the steerable carbon content and uniform surface carbon growing to compensate for the low conductivity. With optimized carbon content (2.35 wt.%), the obtained LTSO carbon composite could deliver desirable high-rate performance (~ 100 mAh g-1 at 15 C, where 1 C = 300 mA g-1), which is comparable with commercialized LTO.
In consideration of using nanosized particles (fumed-silica, with 20-30 nm primary structure), the current state needs to advance regarding the tap density and initial coulombic efficiency. Thereupon we proposed a novel tactic for CVD-assisted in-situ graphitic carbon-coating by employing the dual functional material, LTSO, with both catalytic and electrochemical activity. The catalytic ability of LTSO render the hierarchical structural design with microsphere particulate, which guarantees the tap density at 1.3 g ml-1. The optimized thin layer (15 nm) conductive carbon-coating, with only 3.5 wt.%, dramatically improves the conductivity form ~ 10-7 S m˗1 to ~ 103 S m˗1. After incorporating all the advantages, carbon-coated LTSO reveals a superior graphite-like volumetric capacity of 441.1 mAh cm-3 and Li4Ti5O12-like rate capability (120.1 mAh cm-3 at 4.5 A g-1).
Based on the attained understanding of LTSO, we then started the exploration of relative titanium-based silicate in sodium-ion batteries (Na2TiSiO5, NTSO) as anode candidate. By using inexpensive materials (fumed-silica and P-25) and facilities (ball milling), the obtained NTSO is very practicable for large-scale production. Benefited from the similar catalytic ability as it of LTSO, NTSO carbon composite shows promising performance with excellent cycling stability (100 mAh g-1 at 100 mA g-1 and 90% capacity retention after 3000 cycles).
