Over the past three decades, lithium-ion batteries (LIBs) have revolutionized portable electronics and electric vehicles thanks to their high energy density and long cycle life. However, the main concerns about LIBs lie in the growing cost and limited resources of lithium. Sodium-ion batteries (SIBs) have recently attracted extensive attention as an alternative to LIBs because sodium sources do not present geographical issues that lithium sources have. Recent reports on cathode materials for SIBs have demonstrated electrochemical performance comparable to LIBs. The major scientific challenges for a competitive sodium-ion battery technology is the development of anodes with superior long-term cyclic stability and good rate capability. In this thesis, we focus on the group 6 transition me...[ Read more ]

Over the past three decades, lithium-ion batteries (LIBs) have revolutionized portable electronics and electric vehicles thanks to their high energy density and long cycle life. However, the main concerns about LIBs lie in the growing cost and limited resources of lithium. Sodium-ion batteries (SIBs) have recently attracted extensive attention as an alternative to LIBs because sodium sources do not present geographical issues that lithium sources have. Recent reports on cathode materials for SIBs have demonstrated electrochemical performance comparable to LIBs. The major scientific challenges for a competitive sodium-ion battery technology is the development of anodes with superior long-term cyclic stability and good rate capability. In this thesis, we focus on the group 6 transition metal dichalcogenides (e.g., MoS₂ and MoSe₂) and the metallic Na as anodes for Na storage.

Molybdenum disulfide (MoS₂), a typical two-dimensional material, has attracted increasing interest for energy storage applications due to its layered structure, tunable physical and chemical properties, and high capacity. Nevertheless, MoS₂ has several issues: its electronic conductivity is low, and its structure deteriorates rapidly during charge/discharge cycles, leading to poor electrochemical performance. Here, we apply new synthesis strategies, including nanoengineering, carbon modification, phase engineering, and interlayer expansion, to improve the performance of MoS₂-based anodes for sodium storage. First, novel MoS₂/Carbon (MoS₂/C) microspheres with three-dimensional (3D) architecture were developed as anode for SIBs using a facile hydrothermal strategy. Thanks to carbon intercalation and interlayer expansion, the MoS₂/C electrode delivers a reversible capacity of 498 mAh g^-1 at 100 mA g^-1, and maintains a high reversible capacity above 310 mAh g^-1 after 600 cycles at 4 A g^-1, demonstrating superior rate capability and long-term cyclic stability. Quantitative kinetics analysis reveals a pseudocapacitance-dominated Na⁺ storage mechanism, especially at high current densities. Furthermore, density functional theory (DFT) calculations show that the Na transport rates are faster through the MoS₂/C heterointerface, due to a low diffusion energy barrier, than along the MoS₂/MoS₂ bilayers. Also, a dual-phase MoS₂ (DP-MoS₂) is synthesized by combining two distinct 1T (trigonal) and 2H (hexagonal) phases to improve the storage and transportation of Na ions. Compared to the conventional 2H-MoS₂ counterpart, the DP-MoS₂ phase material presents a highly reversible Na⁺ intercalation/ extraction process aided by expanded interlayer spacing along with much higher electronic conductivity and Na ion affinity. MoSe₂ is analogous to MoS₂, having an even wider interlayer distance of 6.4 Å and a narrower bandgap of ~1.1 eV, making it a promising material for high-power SIB anode. As a proof of concept, we develop few-layer MoSe₂ nanosheets that are encapsulated by nitrogen/phosphorus co-doped carbon and reduced graphene oxide (MoSe₂@NPC/rGO) composites. The MoSe₂@NPC/rGO nanosheets effectively shorten the ion diffusion length while the few-layer MoSe₂ nanosheets expose a large surface area for access by the electrolyte. In addition, the rGO sheets intercalated within the MoSe₂@NPC/rGO composite function as channels for fast electron transfer and substrate for rapid surface reactions. The composite anode delivers a high reversible capacity of ~100 mAh g^-1 even at an extremely high current density of 50 A g^-1, demonstrating an outstanding rate capability.

Although MoS₂ and MoSe₂ are promising anode materials for SIBs, their average voltages are above 1 V vs. Na/Na⁺, which means the assembled SIB full cells will have limited energy densities. In pursuit of high-energy SIBs, Na metal has gained many interests thanks to its low electrochemical potential of -2.714 V versus standard hydrogen electrode and high theoretical specific capacity of 1166 mAh g^-1. To date, however, Na metal anode is not applicable because of many reasons, including dendrite growth, uncontrolled interfacial reactions and degradation, and large volumetric changes during Na stripping and deposition. To surmount these challenges, we formulate a new non-flammable electrolyte consisting of sodium trifluoro-methanesulfonimide (NaTFSI) in a mixture of trimethyl phosphate (TMP) and fluoroethylene carbonate (FEC) for protecting the Na metal anode. The ab initio molecular dynamics simulations and surface analysis reveal the formation of NaF-rich solid electrolyte interphase (SEI) films, which suppress the growth of Na dendrites on the anode, enhancing the electrochemical performance of the RT Na-S batteries.

We also developed a novel 3D porous carbon host with a sodiophilic gradient across its thickness by sputtering gold nanoparticles on the bottom part of carbon foam. The Au/CF scaffold allows the “bottom-up growth” mode where the Au coating serves as the sodiophilic sites, which in turn facilitates uniform Na deposition through the whole Au/CF thickness. The Na-predeposited Au/CF (Na@Au/CF) composite anodes can run for 1000 h with a steady overpotential of ~20 mV at a current density of 2 mA cm^-2 in symmetrical cells, illustrating great potential in stabilizing Na deposition. When coupled the Na@Au/CF anode with Na₃V₂(PO₄)₂F₃ and sulfurized polyacrylonitrile cathodes, the assembled full batteries exhibit highly improved electrochemical performance.