Li-ion batteries have become an attractive key component in energy storage systems as the worldwide energy infrastructure shifts towards being more environmentally concerned. The development of Li-ion batteries has also been pressured from the rapid growth of modern technologies such as mobile electronic device and an electric vehicle based transportation society. However, unlike the fast pace development of other aspects of these technologies, the development of better Li-ion batteries has remained a limiting factor. Therefore, today’s technology market demands the appearance of new Li-ion battery system with higher energy density. To satisfy this demand, Li-ion batteries must advance beyond intercalation cathode materials. Transition metal chloride cathode conversion materials show promise of a new paradigm in energy storage system due to their relatively high energy density. However, disintegration of metal chlorides remains a critical issue in application to a battery.

In this thesis, highly concentrated electrolytes that consist of 0.1M LiCl/6M LiTFSI and 0.1M LiCl/0.1M LiNO3/6M LiTFSI in TEGDME solvent is introduced to effectively suppress the dissolution of cathode materials. Among metal chloride materials, CoCl2 and NiCl2 show promise with high capacities of 160 mAh g-1 and 330 mAh g-1, up to 40% and 80% of their theoretical capacities respectively, in a superconcentrated electrolyte with LiNO3 additive introduced to protect the lithium surface. NiCl2 in a properly treated electrolyte (0.1M LiCl/0.1M LiNO3/6M LiTFSI in TEGDME) showed the most promising behavior with stable cycling and low hysteresis ~ 0.5 V as a new positive electrode material to replace intercalation based rechargeable battery.

In realization of carbon neutral and sustainable society, it is an imperative duty to develop giant energy storage systems for solar/wind power plants and for electrified automotive vehicles. Rechargeable lithium (Li) metal battery, once abandoned three decades ago due to the safety issues, has been gaining increasing attention from the battery scientists again to surpass the energy density of the contemporary Li-ion cells. One approach to stabilize the interface between Li metal and organic liquid electrolytes is to passivate the Li surface by protective coatings, which can reduce permeation rate of the liquid electrolyte and improve the morphology of electroplated Li deposits. In Chapter 1, recent progress of the protective coatings is summarized, and the material properties are categorized according to the proposed protection mechanisms. Ion conduction through the inorganic coatings is determined by the intrinsic ion conductivity of the material itself and independent from the liquid electrolytes. In contrast, the ion conductivity of polymer coatings is dependent on the swelling behavior in the liquid electrolytes. However, there is a dearth of understanding on factors controlling the swelling ratio of polymer coatings and the influence on ion-transport properties. To answer these questions, we carry out quantitative analysis on the effect of solvent polarity and cross-link density to the ion-transport and rheological properties of a polymer coating in Chapter 2. In Chapter 3, we develop a gel polymer electrolyte for Li metal battery, which can stabilize the surface of Li metal by in-situ formation of robust passivation layer. The electrolyte also possesses a safety feature which can shut-down the battery cycling under the condition of abusive thermal runaway. In addition to the energy storage systems, we also study an electrochemical method to recover energy from waste heat resources. In Chapter 4, we report the method of using vaporization of a volatile redox couple to achieve the highest conversion ratio of voltage (Seebeck coefficient) generated from the temperature gradient. This dissertation contributes to the development of high-density energy storage system and waste heat recovery by leveraging the multifunctional electrochemistry and through the molecular designs of electrolyte materials.

Lithium-ion batteries are one of the most promising energy storage devices for their light weight and superior cycling stability. However, the state-of-the-art lithium-ion batteries cannot satisfy the ever-increasing market demand of high energy density electrochemical energy devices. Advanced lithium batteries based on novel electrode materials could provide higher energy density thus become a hot research topic.This dissertation will discuss the designs and applications of novel electrode materials to address the performance challenges for different types of energy storage devices. Chapter 2 provides a new strategy to fabricate a “lithium-free” all-solid-state battery. The 3D hybrid anode design improves the cycling stability of all-solid-state batteries by overcoming the commonly observed cell failure due to the electrode volume change and lithium dendrite growth. This design provides a promising approach towards a high energy density, long life, and low-cost all-solid-state battery technology. In Chapter 3, a concentrated ether-based electrolyte with LiTFSI and LiNO3 as co-salts is proposed, which enables stable cycling of a Li-SPAN battery. In addition to providing excellent protection for lithium metal anodes by forming the solid electrolyte interface (SEI), the electrolyte promotes the formation of a crystalline cathode electrolyte interface (CEI) on the SPAN surface composed of LiF and LiNO2. The CEI effectively prevents the formation of soluble polysulfide species and enables stable cycling of the Li-SPAN batteries. In Chapter 4, a V2O5-Si multi-layer composite anode is proposed and fabricated. The mixed conductive V2O5 layer effectively confines the volume change of Si layer and prevents the parasitic reactions between Si and electrolyte. This strategy enables the anode a long cycle life as well as a long calendar life while maintaining high energy density.

As one of the most promising energy storage devices in the future, all solid-state lithium metal battery has attracted extensive attention. However, the Li dendrite growth and large volume change during cycling have limited the practical application of Li metal anode. Here, we apply a freestanding interlayer (IL) in between solid-state electrolyte (SSE) and anode to suppress the dendrite growth, and a freestanding 3D anode is designed coupling with excellent ion conductor inside to fabricated a 3D anode with both good ionic conductivity and electronic conductivity. In this work, large MCMB (graphite) particles are utilized as 3D anode host, which provide with free space for Li metal deposition, and KB is used as ion conducting agent, which drive Li metal away from the interface of SSE and make Li deposited in 3D anode. The testing result of this 3D MCMB/KB anode design shows an improved critical current density of 5 mA cm-2. The symmetric cells with both MCMB and MCMB/KB anode show very stable cycling performance with a capacity of 2 mAh cm-2 at 0.5 mA cm-2 for over 1,000h. However, when the capacity is increased to 4 mAh cm-2, the symmetric cell with MCMB anode is shorted in 23 cycles, while the cell with MCMB/KB anode runs stably for over 50 cycles. This 3D anode design offers a promising approach for achieving high energy density and long cycle life solid state Li metal anode cells.

Enhancing the performance of secondary batteries is vital for the widespread adoption of renewable energy technologies such as electric transport, grid storage, and portable electronics. To increase the energy density of these systems, extensive efforts have focused on replacing the graphite anode in conventional Li-ion batteries with lithium metal, which boasts a high capacity of 3860 mAh g−1 and a low reduction potential of −3.04 V vs standard hydrogen electrode, making it an ideal choice. However, lithium metal batteries (LMBs) face a formidable challenge: limited cycle life caused by low reversibility and the pulverization of Li due to poor Li plating-stripping behavior. Despite advances in electrolyte design that have improved Li anode coulombic efficiencies to over 99.5%, further pushing up the value remains challenging. Hence, a comprehensive understanding of the mechanisms behind dense Li electrodeposition is crucial to maintain a low-porosity Li anode throughout the cycling process, extending the battery cycle life to facilitate the successful integration of renewable energy technologies.This dissertation outlines our efforts to comprehend and mitigate the pulverization process of the lithium metal anode during plating-stripping cycles, aiming to achieve long-cycle-life lithium metal batteries. In the first project, we develop a 3D carbon host for lithium deposition, where we discover that the pore size of the 3D host significantly influences the locations of lithium electrodeposition, vital for long-term stability. In the second project, we design a carbon-based substrate and observe that faceted Li seeds with a hexagonal shape can be uniformly grown on carbon-polymer composite films. Our investigation unveils the crucial role of carbon defects as nucleation sites for their formation. The presence of uniformly distributed crystalline seeds facilitates low-porosity Li deposition, effectively reducing Li pulverization during cycling, and enabling fast-charging capabilities for Li metal batteries. In the third project, we identify that non-uniform stripping of lithium leads to subsequent non-uniform, dendritic deposition, initiating a vicious cycle that results in lithium pulverization, cell shorting, or capacity degradation. To address this challenge, we engineer a lithium-carbon composite, which exhibits more uniform stripping and subsequent deposition on carbon particles. Our approaches address challenges during both stripping and plating processes, providing valuable insights for future designs of high areal capacity lithium metal anodes.

The rechargeable Li ion batteries are approaching their energy density limitation, while the prosperous growth of electric vehicle market is demanding cheaper and more sustainable batteries with higher energy density.To meet this goal, new battery material is needed to replace the current battery cathode, namely the LiCoO2 and LiNixMnyCo1-x-yO2 (NMC), which both contains the increasingly expensive transition metal, cobalt. One way to limit the cobalt usage is to increase the nickel substitution, as Ni is cheaper and more abundant compared to Co. Additionally, high Ni NMC delivers more capacity than their low Ni counterparts. However, transition metal substituent introduced an unexpected problem, i.e., the 1st cycle capacity loss. With electrochemical characterization and synchrotron X-ray diffraction, we have identified the sluggish Li intercalation at the end of discharge is the root-cause of this problem, which provided guidance for future improvement on these materials. In addition to optimizing the NMC cathode material, designing new cathode chemistry is another promising approach. Sulfur is a good cathode candidate for next generation energy storage system, due to its high capacity (~1675 mAh cm-2, 8 times as high as NMC), low price, and abundance in earth’s crust. However, elemental sulfur cathode suffers from its insulating nature and polysulfide dissolution problem. Sulfurized polyacrylonitrile (SPAN) is a sulfur based conductive polymer, which prevents sulfur dissolution by forming covalent bonding with sulfur and provides electron pathway by the chemical backbone. Although SPAN typically shows extraordinary stable cycling performance due to its unique structure and high specific capacity (~700 mAh cm-2), the Li-SPAN batteries reported in literature are yet to satisfy the industry demand due to its low areal capacity and incompatibility with ether electrolyte, which is commonly used in Li metal batteries. We discovered that LiNO3 as an electrolyte additive, enables SPAN to stably cycle in ether electrolyte, by forming a LiF-rich CEI layer. Its reaction mechanism in different electrolytes was investigated by X-ray absorption spectroscopy, where Li2Sx dissolution was observed in ether electrolyte without additive. Besides the electrolyte optimization, we replaced the traditionally used PVdF binder with mechanically robust CMC binder, which prevents the mechanical disintegration of the high areal loading cathode (> 6 mAh cm-2) and enables its stable cycling with reduced porosity (30%). When it comes to the anode, Li metal is the ultimate choice of rechargeable battery anode material due to its highest gravimetric capacity (3862 mAh cm-2) and lowest electrochemical potential (-3.04 V vs SHE.). However, the irregular morphology of electrochemically deposited Li leads to lots of problems, such as parasitic reactions, electrochemically isolated “dead” Li formation, and dendrite shorting. Many approaches have been developed to suppress the dendritic lithium formation and increase the lithium metal stripping/plating efficiency to > 99.0%. However, the porosity of lithium anode increases upon long cycling is a real challenge, which causes electrolyte depletion, increases cell impedance, and ultimately dictates the end of cell life. We demonstrated a bottom-up approach that an Fe/LiF nanocomposite substrate promotes the nucleation and growth of hexagonal single crystal Li at the initial stage of Li deposition, inducing dense Li deposition on top of the nuclei. Leveraging the low porosity Li, we have shown >1000 (Coulombic efficiency (CE) = 99.17%) and >600 (CE=99.06%) cycle in half cells under exceptionally high current density, 3 and 5 mA cm-2. Further, the full cell tests using NMC811 cathode with practical areal capacity of > 3 mAh cm-2, 1-fold excess of Li, lean electrolyte (3 g Ah-1), and cycled at high current density of 3 mA cm-2 retains > 80% cell capacity for more than 130 cycles, which is a 550% improvement over the baseline cells. We believe that through proper design and optimization of cathode and anode materials, the commercialization route for rechargeable Li metal battery with high energy density will be realized in the coming years.

Driven by the flourishing of renewable energy sources and increasing demands of portable electronics and electric vehicles, high-performance energy storage devices are required for the applications at different scales. Benefiting from the small size, high surface area and hierarchical structures, nanostructured materials have been playing critical roles for the development of advanced energy storage devices.

This dissertation will discuss the applications of nanostructured materials to resolve the unique problems for different types of energy storage devices. Chapter 2 provides a new strategy to fabricate a cation exchange membrane features a dense, crack-free tungsten oxide coating layer on Nafion that also penetrates into the Nafion’s hydrophilic, ionic cluster regions. The hierarchical structural designs overcome the inherent tradeoff between conductivity and permeability of ion exchange membranes for redox flow batteries. In Chapter 3, a facile and scalable method is demonstrated to fabricate a 3D lithium metal anode with lithium nitrate, polyvinylidene difluoride, and nano-sized carbon black. The multi-functional 3D electrodes enable dendrite-free lithium metal cycling with high coulombic efficiency. In Chapter 4, a non-toxic, free-standing and flexible cathode is developed by grafting polydopamine on carbon nanotubes for aqueous zinc-ion battery. Cross-linked highly uniform active materials and the efficient conducting network overcome the long term cycling stability issue of aqueous zinc-ion batteries.

A scalable, room-temperature synthesis process named conversion reaction synthesis was developed that can spontaneously produce nanoporous metal structures from simple ionic precursor compounds. This process involves reacting an organolithium reducing agent (specifically n-butyllithium) with a metal halide to produce a metal/lithium halide nanocomposite comprised of nanocrystalline phases of the constituent compounds. The lithium halide is then dissolved with a polar solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a pure metal nanoporous structure. This approach is highly versatile and can produce nanoporous metals made from both more-noble metals (Au, Ag, Cu) and less-noble transition metals (Co, Fe, Ni) with pore sizes ranging from 2 to 50 nm with specific surface areas from 1.0 m2/g to 160 m2/g. These nanoporous microstructures can be designed to have a specific pore size by engineering the precursor or by annealing the nanocomposite phase prior to purification. Annealing the metal-lithium halide nanocomposites allowed for controlled growth of the metal phase, the rate of which depends on diffusivity of the metal. Conversion reaction synthesis can also form hybrid nanoporous structures between different metals and between metals and non-metals from co-precipitated nanocomposite precursors. Numerous nanoporous morphologies prepared with conversion reaction synthesis were analyzed primarily using a combination of electron microscopy, nitrogen adsorption analysis and x-ray diffraction to investigate the nanoporous metals’ formation mechanism and to demonstrate the wide capabilities of this method. Conversion reaction synthesis expands our access to additional compositions and microstructures of nanoporous metals.

A great challenge in solid-state synthesis is the ability to predict and design reaction pathways to synthesize new materials. In order to rationally plan synthesis procedures, one must understand the reaction mechanism involving thermodynamic and kinetic forces such as diffusion, interfacial reactions, activation energies and metastable states. This work focuses on studying conversion reactions as a platform to investigate solid-state reaction kinetics, nanomaterial synthesis and energy storage applications. Conversion reactions are essentially displacement reactions in which the cations of two ionic compounds are exchanged to form a two-phase nanocomposite. They may be used for scaling synthesis procedures by lowering the activation energy of a reaction and can also serve as high-capacity battery materials due to the multiple electron transfers that occur per redox center.This work involves three parts: 1) developing an in-situ method of tracking a solid-state conversion reaction using AC impedance, 2) applying a conversion reaction to synthesize 1D nanomaterials from bulk precursors and 3) exploring the kinetics behind conversion reactions in all solid-state batteries. The first part leverages the sensitivity of AC impedance to be used as a characterization technique to help detect the onset of a reaction by measuring conductivity and using the data to provide insights into the kinetics of the reaction. The second and third parts apply conversion reactions to help tune nanomaterial synthesis and compare reaction mechanisms of iron halide conversion batteries.

To meet the demand for applications ranging from cell phones to electric vehicles, battery energy density continues to rise. With the use of highly energetic materials such as Li-metal anodes coupled with a reduction of the inactive components generally tasked with safety, failure events from misuse or manufacturing defects will inevitably increase. Furthermore, catastrophic battery failures due to internal short are extremely difficult to detect and can occur even under normal working conditions. To enable next generation Li-metal batteries, an inexpensive “fail safe” mechanism for internal shorting that does not sacrifice energy density is highly desirable. In this dissertation two novel battery separator designs and an easily implementable cathode design modification have been developed to improve battery safety, control, and lifetimes by approaching these problems from both the ionic and the electronic pathways. Firstly, an iongate separator was developed to increase battery calendar life and improve inherent safety by using a rapid and reversible battery shut-off mechanism enabled by a 10x increase in internal ionic resistance. Secondly, a nano-composite Janus separator was implemented to intercept dendrites with a high-resistance interlayer, now controlling the internal electronic resistance of the cell. This separator provides protection and early failure detection, nearly completely eliminating short circuit current and the accompanying cell temperature rise. Lastly, this concept was simplified by using a gradient-conductivity cathode that directly utilizes the inherent resistive properties of the battery active material to create the protection mechanism, halving short circuit current and cell temperature during shorting events. This simplified approach is broadly applicable and results in a particularly inexpensive protection scheme without incurring penalty to energy density. In summary, this dissertation introduces three novel approaches to improve battery safety by controlling both the internal ionic and electronic pathways that are at fault for often catastrophic battery failures.