Three-Dimensional Freestanding Frameworks as Host Materials for Highly Stable Lithium Metal Anodes

Abstract

The increasing demand for high-performance energy storage in electric vehicles and large-scale applications has highlighted the inadequacies of graphite anodes in lithium (Li) batteries and showcased the advantages of Li metal anodes, which offer higher energy densities and lower electrochemical potentials. To address the challenges associated with Li metal anodes, such as dendrite growth, recent studies have focused on innovative approaches, including novel electrolyte additives, solid-state electrolytes, artificial interphases, and three-dimensional (3D) anode hosts, to improve their safety and efficacy. Among these strategies, the design of 3D hosts for Li metal anodes emerges as particularly promising. These hosts provide a substantial electrode-electrolyte contact area, effectively mitigating local current densities and accommodating volume changes during Li plating/stripping. Among these, 3D carbon-based hosts, characterized by their lower density, high electrical conductivity, and adjustable physicochemical properties, are particularly notable. However, conventional carbon-based hosts often suffer from poor lithiophilicity, leading to non-uniform Li deposition and reduced cycle life. Therefore, leveraging the tunable properties of carbon precursors, the construction of lithiophilic species to decorate carbon hosts. According to the kinetics theory, the accumulation of Li+ at a single location, leading to dendrite formation, occurs due to inadequate solid-phase diffusion of Li metal. Consequently, in the most recent developments, the lithiophilic modification of the carbon substrate surface has emerged as an effective strategy to optimize Li plating/stripping behavior. Research indicates that the lithiophilic surface tends to reduce the nucleation barrier, promoting uniform Li nucleation and growth in the initial stages of Li plating. This approach is vital in controlling the morphology evolution of Li deposition. The first work in this thesis is focus on constructing a 3D Li host by adopting the lithiophilicity to regulate the interfacial lithium deposition. The matrix structure is composed of ~20 nm Cu3P/CoP heterostructural nanobubbles embedded within ~300 nm carbon nanoboxes, where in-situ grown carbon nanotubes (CNTs) on the carbon nanoboxes are intertwined with commercial CNTs, effectively confining Li. The Cu3P/CoP@C/CNT matrix offers 3D porous interspace and strong chemical bonding with Li. After plating Li, the Li@Cu3P/CoP@C/CNT anode achieves 94.6% efficiency over 220 cycles and operates cycling stability for 400 h with low voltage hysteresis. LiFePO4|Li@Cu3P/CoP@C/CNT cells cycle 300 times with minimal capacity decay, promoting Li metal anode development. This work is the first step attempt for following works. The second study further demonstrates the role of a lithiophilicity-enhanced 3D lithium host, taking into account lithium diffusivity. Flexible 3D N-doped carbon nanofibers (NCNF) coated with 2D NiCo2S4 nanosheets (CNCS) were designed. With the physicochemical dual effects, CNCS provides limited surface Li diffusivity and higher Li affinity, leading to uniform Li nucleation and reduced random Li accumulation, as shown by molecular dynamics simulations. This structure reduces exchange current density and contains metallic Li, preventing dendritic growth. The Li/CNCS anode shows a lifespan of almost 1200 h with low overpotential, and LiFePO4|Li/CNCS full cells exhibit enhanced performance compared to bare Li anodes at a low N/P ratio (capacity ratio of negative and positive electrode) of 2.45. This study builds on the initial work by combining investigations of Li affinity and lithium diffusivity with experimental characterization results. The last work presents a more detailed investigation in the interfacial Li diffusion and adsorption on a 3D lithium host. This study introduces a novel approach to stabilize Li metal anodes using a valence gradient in iron nanoparticles, created through an annealing-assisted drop-coating method on a 3D carbon cloth substrate (CFg-T). The valence gradient, featuring Fe0, Fe2+, and Fe3+ from the inner to the exterior part, regulates Li+ diffusion and adsorption. Fe0 ensures steady Li atom supply, while Fe2+ and Fe3+ facilitate strong Li adsorption and slow diffusion, promoting uniform deposition and reducing dendrite growth. The CFg-550 framework shows optimal performance, with minimal hysteresis voltage after 1200 h in symmetric cells. Integrated with LiFePO4, the full cell demonstrates excellent cycling stability for 950 cycles at 1C with a low N/P ratio of 1.19. This valence gradient design enhances the stability of high-energy-density Li metal batteries. It is the first to use an Fe-valence gradient to modulate Li+ diffusion and deposition, establishing a guiding principle for transition-metal valence gradient host design in controlling Li+ diffusivity and affinity.