Abstract
With the growing demand for mobile electronic devices, the energy density requirements for energy storage batteries are significantly increasing especially in the electric vehicle area. This can result in today's commercial Lithium-Ion Batteries (LIBs) eventually not being able to meet the commercial high energy density requirements. Therefore, the development of high-energy-density storage devices has received widespread attention. Compared to commercial graphite anodes, lithium metal anodes (LMA) have lower electrochemical potential (-3.04 V vs. SHE), lighter volume density (0.65 g cm-3), and a higher theoretical capacity (3860 mAh g-1). Hence, the LMA is regarded as the "holy grail" of anode materials. However, Li metal is subject to severe volumetric expansion and uncontrollable dendrite growth during the charging and discharging process, leading to a constant loss of capacity, and increasing the risk of short-circuiting and explosion, which seriously hinders the practical application of LMA. The primary issue limiting battery energy density is the composition of the cathode and anode materials, which make up the lithium battery system's core. The development of cathode materials is relatively modest in comparison to the quickly evolving and diverse cathode materials. Based on the difficulties, this thesis focuses on a variety of crucial parameters, such as low cost, extended cycle life, and stable full cell. Different lithium anodes that are based on lithiophilic Ag have been designed to effectively inhibit dendrite development while also enhancing cycle life and charge/discharge performance. In this thesis, we propose the use of the lithiophilic metallic material––silver (Ag). First, the sophisticated technological 2D materials GO and CNT were chosen as carbon conductive substrates. Ag and Ag-S were introduced to improve the conductivity and stability of the electrodes. Next, smaller size silver single atoms were used and added to the two-dimensional material MXene, and their iv mechanism of action was investigated. Finally, based on the findings of the previous studies, a threedimensional MOF skeleton was built with p-type silver. small size silver particles were introduced to improve the current and capacity of battery operation and achieve fast charging and discharging. The main contributions are summarized below: 1. Metals such as Cu, Zn, Ag, Au, etc. are regarded as the lithiophilic materials for Li, which could guide the deposition of Li. In our first work, a lithiophilic rGO-Ag-S-CNT membrane was proposed for use as a Li metal anode host by connecting the rGO-Ag with S-CNT through a strong Ag-S covalent bond. Due to the large energy gap between Ag and S, the Ag on the rGO would self-assemble with the sulfur-doped CNT to form a robust 3D framework (rGO-Ag-S-CNT). The Ag-S bonds could be served as “grippers” to strongly connect the CNT and rGO layers to boost structural stability. Besides, the CNT inside the rGO layers not only contributes to rapid electronic migration, but it also balances the electron density over the different rGO layers, leading to improved electrochemical kinetics. Furthermore, the Ag nanoparticles could improve the lithiophilic performance of the carbon composite. As a result, the Ag-S bond shows a small overpotential of 7.3 mV, indicating the excellent lithiophilicity of the electrode. Besides, the robust structure induced by the Ag-S bond effectively suppresses Li dendrite growth, enhancing the longterm cycling stability of the batteries (over 500 cycles at 2 mA cm-2). This work effectively solves the instability issue of 3D LMAs by developing a robust Ag-S bond to stabilize the structure. 2. It is well known that lithiophilic materials can induce Li deposition. However, most of these lithiophilic material are metals that range from nano to micrometers in scale. While it is possible to minimize size deviations by controlling the synthesis process of nano or microscale materials, it is often difficult to prevent such deviations to high precision. Hence, when such nano or microscale materials are used, there exists a variation of lithiophilic capacities, ultimately v leading to non-uniform Li deposition. Here, we proposed to introduce lithiophilic Ag single atoms (AgSAs) into the Ti vacancies of two-dimensional titanium carbide MXenes, forming unique Ag- C bonds as the lithiophilic sites to modulate its current density, as well as to induce homogeneous spherical Li metal deposition. Unlike conventional lithiophilic nanomaterials such as Ag, Zn, and copper nanoparticles, the AgSAs have a homogeneous atomic size, reducing variations in lithiophilic capacities. In addition, as most of the AgSAs reported so far are based on the Ag-O and Ag-N systems, the Ag-C system which we have synthesized has been rarely reported to the best of our knowledge. Hence, this special Ag-C structure in Ti vacancies may provide enhanced modulation of the charge density and homogeneous Li deposition. Moreover, as AgSAs are precisely deposited in Ti vacancies, precise lithiophilic sites are provided on the MXene sheets. Therefore, dense and uniform spherical Li metal is formed in the initial deposition stage, which effectively inhibits the growth of Li dendrites despite increasing the depositing capacity to excessive amounts. As a result, the AgSAs-modified MXene electrodes show a low overpotential and high CE of more than 800 cycles, as well as deep plating-stripping capacities of up to 40 mAh cm-2. This desirable electrochemical performance is derived from the key role of Ag atoms in regulating the charge density, resulting in a very uniform electric field on the surface of the MXene-AgSAs electrode, as well as the distribution of Li+ ions. The COMSOL Multiphysics simulations also demonstrate the more uniform distribution of Li+ ions and electrons on MXene- AgSAs electrode compared to MXene and Cu, indicating the significant role AgSAs play in inhibiting the growth of Li dendrites. Furthermore, the full cell of MXene-AgSAs@Li and LiFePO4 also shows a stable cycle life of up to 400 cycles at 1C. This work provides a new route for depositing highly compact and uniform spherical Li, unveiling the role of lithophilic single vi atoms for spherical Li and guiding the design of future strategies to achieve dendrite-free Li deposition. 3. In addition to exploring the effect of Ag-modified 2D materials (i.e., GO and MXene) on lithium metal, we also investigated the effect of different Ag particle sizes on lithium deposition after modification of the 3D MOF skeleton. Hence, we designed a silver-modified vertical cobalt MOF porous nanoarray by the gentle reduction reaction of Ag+ ions in ethanol (Ag+ + Co2+? Ag + Co3+), where the Co position in the MOF was replaced by Ag atoms. This one-for-one replacement process makes the distribution of the lithiophilic Ag homogeneous in the individual MOF nanosheets, which is beneficial for improving the performance of the lithium anode. The Ag atoms can well induce lithium deposition inside these arrays, which highly improves the precise nucleation in the nanoarray. In addition, the holes in the nanosheets after annealing are also very effective in providing a large amount of space for lithium deposition and Li+ ion shuttling, making them particularly useful in regulating the large volume and high current cycling. Furthermore, the large space between carbon fibers also provides effective space for lithium deposition. Based on the above design advantages, the assembled CP-Co-Ag@Li||CP-Co-Ag@Li symmetric cell can achieve an ultra-long cycle life of 1000 h at 4 mA cm-2/4 mAh cm-2. More importantly, this symmetric cell demonstrates an excellent long cycling life of 1000 h and 800 h at ultra-high currents and capacities of 10 mA cm-2/10 mAh cm-2 and 20 mA cm-2/20 mAh cm-2 respectively.