Hierarchical Carbon Structures for Advanced High Areal Capacity Lithium-Ion Battery Electrodes
The ever-growing energy demand from modern portable electronics and electric vehicles calls for a matching development in lithium-ion batteries (LIBs) that offer high power density, energy density, and long cycle life. However, LIBs based on traditional layered metal oxide cathodes and graphite anodes are quickly approaching their specific energy limits. The exploration of advanced electrode architectures provides a novel route to improving the energy density of LIBs and is often overlooked. This PhD thesis investigates the design, fabrication, and testing of structurally engineered and high areal capacity LIB electrodes based on hierarchical carbon structures such as carbon nanotubes (CNTs). Firstly, (surface-modified) direct-spun CNT mats are proposed as conductive scaffolds for lithium metal anodes (LMAs), which are desirable for next-generation batteries because of their ultra-high energy density. However, the practical applications of LMAs are hindered by the inhomogeneous Li dendrite formation during cycling which leads to poor efficiency and safety issues. The proposed electrode provides ample Li nucleation sites, reduces the local current densities, and promotes uniform Li plating/stripping at selective locations, thus achieving coulombic efficiencies (CEs) of 98-99% for over 130 cycles in CNT|Li half-cells (lithiation capacity: 2.0 mA h cm-2, current density: 1.0 mA cm-2). Next, a freestanding carbon fibre paper with carbon fillers (CFP) electrode is proposed with a novel dual charge storage mechanism – it acts both as a host for Li intercalation and as a conductive, porous, and lithiophilic 3D scaffold for Li plating/stripping. The CFP electrode exhibits excellent long-term cycling stability, as evidenced by CEs of over 99.5% on the 250th cycle in CFP|Li half-cells (lithiation capacity: 1.5 mA h cm-2, current density: 0.5 mA cm-2). Notably, both the CNT mat and CFP electrodes can be manufactured at an industrial scale, showing real promises for practical applications. Lastly, thick electrodes can help improve the energy density of LIBs by increasing the ratio of electrochemically active materials in a battery cell, but they often suffer from poor rate performances that is caused by the increase of various internal resistance terms associated with respective charge storage mechanisms, as well as manufacturing challenges. Here, ultra-thick lithium titanate (LTO) electrodes based on nanostructured CNT honeycomb conductive backbones (LCHB) are proposed – the hierarchical CNT honeycombs help improve Li-ion diffusion, provide more electrically conductive pathways, increase electrochemically active surface area, and offer mechanical support. High aspect ratio (over 1000) CNT honeycomb backbones of over 1 mm tall are successfully manufactured, and subsequently LCHB electrodes with areal loadings of over 25 mg cm-2 are successfully fabricated. The LCHB electrodes demonstrated superior rate performance (over 50% at 2 C as compared to less than 10% for planar electrode with similar areal loading) and longer cycle life (over 230 cycles until 10% capacity is lost as compared to less than 15 cycles for planar electrodes). Furthermore, EIS results revealed LCHB electrodes with low frequency resistance term (consists of charge transfer and Li ion transfer resistances) an order of magnitude lower than planar electrodes.
3D electrode, Lithium-ion battery, Lithium-metal anode, nanomanufacturing, ultra-thick electrode
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