1,721,159 research outputs found
Electrolyte and electrode designs for energy-dense lithium batteries
No full textLi-ion batteries (LIBs) have become the pivotal nexus for energy storage in the modern society. With the rapid development of electric vehicles (EVs), the fast growth of grid-scale energy storage, and a surge of novel applications, batteries with higher specific energies are urgently demanded. Unfortunately, the state-of-the-art LIBs offer energy densities below 250 Wh kg-1, unable to support application scenarios like ultralong-range EVs, drones, or flying cars, which require batteries with a specific energy of above 350 Wh kg-1. Silicon anodes or lithium-metal anodes are the two most promising high-capacity anodes for the next-generation battery design due to extremely high lithium-storage capacities and low working potentials. Pairing these two anodes with high-voltage cathodes promises high specific energies. Nevertheless, the dendrite issue of lithium metal, the large volume variation of silicon anodes, the oxidation of electrolytes, the crack and dissolution of cathodes seriously block the commercialization of silicon-based LIBs or rechargeable lithium-metal batteries (LMBs). To fully tackle the abovementioned issues, designing advanced electrolytes with desired physicochemical properties that simultaneously stabilize the high-capacity anodes and high-voltage cathodes is indispensable. In addition, appropriate structure designs further enhance the anode stability, thereby increasing the cycling lifespan of batteries. We begin with modulating solid electrolyte interphases (SEIs) of silicon and lithium-metal anodes by a localized high concentration electrolyte. An entirely new lightweight and low-cost fluoride, benzotrifluoride, is proposed as diluent for electrolytes. The benzotrifluoride-diluted high-concentration electrolyte passivates the anodes by forming thin yet dense SEIs, promoting the reversibility of lithiation-delithiation process and avoiding the loss of active materials. Besides, this electrolyte enables stable cathode operation under 4.6 V. Pragmatic pouch cells based on silicon and lithium-metal demonstrate high specific energies of 257.4 and 349.4 Wh kg-1, respectively. To acquire high-performance LMBs with medium salt concentrations, we propose to utilize ethylene glycol dibutyl ether (EGDE) for preparing weakly solvating electrolytes. The EGDE with medium salt concentrations create rich contact ion pairs and ionic aggregates, enabling the stable operation of the Li-metal and high-voltage cathodes. The 4.4 V-class lithium-metal║NCM622 cell with the 3.5 mAh cm-2 cathode and an anode-to-cathode ratio (N/P ratio) of 2.85 operates steadily for 180 cycles. To realize ultrahigh voltage LMBs (> 4.5 V) with medium-to-low salt-to-solvent ratios, we developed a solvent molecule reconstruction strategy by highly controllable polymerization of 1,3-dioxolane (DOL). The polymerization process eradicates free solvents and transforms them into oxidation-proof polymerized DOL. The tailored solvation structures of electrolyte render the LiF-rich anode SEI and cathode electrolyte interphase (CEI), leading to the highly reversible lithium-metal anode, crack-free cathodes, and the thus longevity of full cells. Based on the designed electrolyte, the pouch LMB operating with the 4.6 V cutoff voltage delivers a high energy density of 346.6 Wh kg-1. To support the stable operation of the micron-sized silicon anode that is less costly and more pragmatic for commercialization compared to nanosized silicon anode. LiPF6-in-EGDE electrolytes are designed to construct fluoride-rich and stratified SEIs. With this electrolyte, the micron-sized silicon anode stably operates at 0.5 A g-1, maintaining a high capacity of 1901 mAh g-1 after 500 cycles and showing the high operating Coulombic efficiency (CE) of 99.92%. To further enhance the macroscopic structural stability and electrochemical performance of silicon anodes, the electrostatic self-assembly strategy is proposed for facile, efficient, and large-scale synthesis of silicon-based anodes. The two-dimensional MXene (Ti3C2Xn, X= F, O, OH, etc.) automatically assembles with silicon particles in aqueous solutions, forming honeycomb-like composites that demonstrate substantially improved rate and cycling performances. Key words: Li-ion batteries; rechargeable Li-metal batteries; high concentration electrolyte; weakly solvating electrolyte; silicon anodes.</p
Development of high-performance solid-state lithium metal batteries
No full textSolid-state lithium metal batteries (SSLMBs) that use a lithium metal anode and nonflammable solid-state electrolyte (SSE) promise to offer considerably higher energy density and safety than state-of-the-art lithium-ion batteries do, thus showing great potential for next-generation energy storage applications. However, the development of SSLMBs is hindered by several critical issues, including poor interfacial contact between SSEs and electrodes, low ionic conductivities, large thicknesses, and narrow electrochemical windows of SSEs, which result in poor rate performance and short cycle life. This thesis aims to address these core challenges and create high-performance SSLMBs. We begin with overcoming the poor contact between the lithium metal anode and SSE by magnetron sputtering a SnO2 nanolayer onto the garnet-type Li6.4La3Zr1.4Ta0.6O12 (LLZTO). A highly Li+-conducting interlayer is automatically formed between the garnet and lithium metal after lithiation, thus significantly reducing the interfacial resistance from 1019 to 153 Ω cm2. To reduce the production cost, we further develop a facile, scalable, and cost-effective spinning coating strategy to form a silica nanofilm on the LLZTO surface, which reduces the interfacial resistance to be as low as 49 Ω cm2. To enhance the mechanical strength of SSEs, we design and fabricate a composite solid electrolyte that incorporates a 3D perovskite Li0.33La0.557TiO3 (LLTO) nanofiber network and polyethylene oxide (PEO)-LiTFSI matrix, which inherits both the high shear modulus of perovskite and the flexibility of PEO polymer. A thin PEO layer is meticulously prepared on either side of the PEO-perovskite composite to simultaneously improve the electrolyte/electrodes interfaces. This novel sandwich design enables a full Li/LiFePO4 battery to deliver a high capacity of 135.0 mAh g-1 at 2 C at 60 ℃ with a high capacity retention of 79.0% after 300 cycles. To improve the ionic conductivities of composite electrolytes, we further develop a vertically-aligned LLTO framework by the ice-templating method, which provides fast, continuous, and the shortest pathways for Li+ transport, thus boosting the ionic conductivity from 0.038 to 0.13 mS cm-1. As a result, a Li/LiFePO4 full battery assembled with the developed electrolyte delivers a specific discharge capacity of 144.6 mAh g-1 at 1 C at 60 ℃ with a high capacity retention of 96.0% after 100 cycles. To broaden the electrochemical windows of SSEs and promote the pairing of lithium metal anodes with high-voltage cathodes to achieve high energy density SSLMBs, a poly(acrylonitrile) (PAN)-LiClO4-boron nitride nanoflake (BNNF) composite electrolyte modified by a BNNF layer (PBCEB) is developed. It is demonstrated that the PAN-LiClO4-BNNF composite can sustain an oxidation voltage up to 4.5 V vs. Li/Li+, while the BNNF modifying layer prevents the PAN-LiClO4-BNNF from reduction reaction with lithium metal anode. Thus, a Li/LiNi0.8Co0.1Mn0.1O2 full battery can deliver a high capacity of 173.6 mAh g-1 at 0.2 C and be stably cycled for over 350 cycles at 1 C with a capacity retention of 68.1%. Finally, we rationally design and fabricate a Janus-faced, 3D perovskite LLTO nanofiber framework reinforced composite electrolyte (JPCE) to simultaneously achieve a wide electrochemical window (0 ~ 4.5 V vs. Li/Li+), high ionic conductivity (0.1 mS cm- 1), and small thickness (24 μm). Excitingly, when paired with a lithium metal anode and a LiNi0.8Co0.1Mn0.1O2 cathode, the SSLMB delivers a reversible discharge capacity of as high as 176.1 mAh g-1 at 0.2 C at room temperature, displaying the great promise of the JPCE for high-voltage SSLMBs applications. Keywords: Solid-state electrolyte; solid-state lithium metal battery; surface modification; interfacial resistance; ionic conductivity; electrochemical window; high-voltage cathode.</p
Towards high-performance lithium-sulfur batteries by structure engineering and electrode/electrolyte interphase manipulating
No full textLithium-sulfur (Li-S) batteries are regarded as promising candidates to replace lithium-ion batteries (LIBs) for electric vehicles and portable electronic devices due to the high theoretical energy density, low cost, and environmental benignity. However, the practical applications of Li-S batteries are still hindered by several issues including the sharp capacity decay caused by the lithium polysulfides (LiPs) shuttle effect, sluggish electrochemical redox kinetics, and detrimental Li dendrite growth. In this thesis, the primary objective is to enable practical high energy and long-cycle life Li-S battery by nanostructured electrode engineering and novel electrolyte design to address the above-mentioned issues. The thesis begins with sulfur cathode engineering to relieve the polysulfide shuttle effect and enhance sluggish electrochemical redox kinetics. A polar yolk-shell Co-Fe mixed metal sulfide (FeCoS2) sulfur host with good electric conductivity is developed to improve the LiPs affinity of the host material. In addition, an ordered macroporous host material is developed to enhance ion transportation under high sulfur content and facilitate the growth of Li2S-electrolyte-carbon triple-phase boundary to further boost the sulfur redox kinetics. Meanwhile, double-end binding site composed of polar material and single-atom catalyst is introduced into the ordered macroporous host to relieve the shuttle effect and catalyze the sulfur redox to improve the cycling and rate performance. An Ah-level pouch cell with the engineered cathode delivers high specific energy (>300 Wh kg-1) with a high capacity retention rate of 74% for 80 cycles. To modify the electrode electrochemical behavior during cycling, thus simultaneously enabling a robust solid electrolyte interphase (SEI) formation and relieving LiPs shuttling, this thesis then focuses on electrolyte design for Li-S batteries. A highly fluorinated ether-based electrolyte is proposed to enable the formation of robust LiF-rich SEI on the surface of Li metal anode to modify the Li striping/plating processes and avoid the formation of dendritic Li during cycling. Meanwhile, the low solubility of polysulfide in the fluorinated ether can effectively prevent the flooding of polysulfide, which can relieve the capacity decay caused by polysulfide shuttling. By applying this electrolyte, a Li-S cell with high S loading (4.5 mg cm-2) and low electrolyte/sulfur ratio (10 μL mg-1) can deliver a high areal capacity (> 3 mAh cm-2) and high coulombic efficiency (>99%) for 100 cycles. To control the negative-to-positive capacity ratio (N/P ratio) and improve the anode cycling stability of the battery, nanostructured Si-based materials are proposed as lithium storage anodes. In this thesis, a carbon encapsulated Si nanoparticle is developed to improve the electric conductivity, control the SEI growth, and enhance the structural stability of Si-based anode. Moreover, an integrated microsize Si anode is developed to improve the tap density of Si-based anode. The as-prepared Si anode delivers a high specific capacity (> 1500 mAh g-1) and a high capacity retention rate (88.43%) for 100 cycles. Meanwhile, under high Si loading (2.5 mg cm-2), the nanostructured Si anode can deliver a high areal capacity (> 3.0 mAh cm-2) with a high capacity retention rate of 79.5% for 100 cycles. Keywords: lithium-sulfur batteries; pouch cells; solid electrolyte interphase; lithium dendrite; fluorinated electrolyte.</p
Design of porous electrodes for redox flow batteries using machine learning and multi-objective genetic algorithm
No full textA coupled machine learning (ML) and multi-objective genetic algorithm (MOGA) approach to the design of porous electrode structures for redox flow batteries (RFBs) is developed in this thesis, and over 700 new electrodes that have up to 80% enhancement of specific surface area and 50% enhancement of hydraulic permeability compared with commercial graphite felt electrodes are successfully designed. In order to fasten the commercialization of RFBs, further optimization of the batteries to reduce the cost, increase the power density and electrolyte utilization are desired. An important route is to optimize the porous electrode structures. For RFB electrodes, large specific surface area and high hydraulic permeability are very desirable properties. Considering that traditional trial-and-error approaches are hindered by limited human intuition, here in this thesis, a coupled ML and MOGA strategy is developed for the design of porous RFB electrodes. First, a dataset containing over 2,000 porous electrode structures is generated by a stochastic reconstruction program and the two properties are computed through numerical simulations using morphological algorithm and lattice Boltzmann method (LBM), respectively. Based on the dataset, ML models are trained to learn the underlying relationship between electrode structures and the two properties. The best models, which are trained with artificial neural network (ANN) algorithm, achieve the lowest fitting errors. Based on the ANN models, the MOGA non-dominated sorting genetic algorithm II (NSGA-II) is adopted to design new electrode structures for RFBs. As a result, more than 700 promising candidates are successfully identified. The methodology proposed here is also applicable to other material structure design problems.</p
Toward high-performance solid-state lithium batteries by tailoring electrolytes and electrode/electrolyte interfaces
No full textSolid-state lithium batteries (SSLBs) that use thermally stable solid electrolytes and high-capacity anodes (e.g., Li and Si) hold the most potential to outperform today’s lithium-ion batteries in energy density and safety. However, several critical issues, including poor interfaces between solid electrolytes and electrodes, unstable high-capacity anodes, low ionic conductivity and large thickness of solid electrolytes, hinder the commercialization of SSLBs. In this thesis, we judiciously tailor electrolytes and electrode/electrolyte interfaces to tackle the core challenges and eventually create high-performance SSLBs with high potential for practical applications. We begin with overcoming the interface issues between the solid electrolyte and Li metal anode. First, an in-situ solidified, robust and elastic ionogel interlayer is introduced between a ceramic Li1.3Al0.3Ti1.7(PO4)3 electrolyte and Li metal to preclude adverse reactions and improve the interface contact. Consequently, the cycling life of a Li/Li symmetric cell is dramatically extended from 10 to 300 h. To enable better processibility, we propose to coat an Al/Li dual-salt thin layer onto a polyethylene oxide-based solid polymer electrolyte, which in-situ forms a lithiophilic-lithiophobic gradient interphase that can simultaneously improve the interface adhesion and suppress the Li dendrite. As a result, the Li/Li symmetrical cell with the dual-salt coated electrolyte can stably cycle for over 1000 h without short circuits. Additionally, a robust boron nitride coating layer can also enhance the stability of the Li metal/solid polymer electrolyte interface. To enhance the ionic conductivity and dendrite suppression capability of solid electrolytes, we develop a novel composite solid electrolyte with an asymmetric dual-layer ceramic framework. The vertically-aligned porous layer of the framework provides expressways for Li+ ion conduction, endowing the electrolyte with a high ionic conductivity of 0.101 mS cm-1 at 25 °C, while the thin dense layer homogenizes the ion distribution at the interface facing Li metal anode, allowing uniform Li plating and stripping. As a result, the assembled all-solid-state Li/LiFePO4 battery achieves a high capacity of 143.5 mAh g-1 at 1 C without obvious decay even after 500 cycles. To take a step toward the practical realization of high-capacity and stable all-solid-state batteries, we further develop a novel integrated cathode/solid electrolyte for scalable manufacturing. The integrated design considerably reduces the interfacial resistance. Meanwhile, the strong fiber network endows the solid electrolyte with an ultrasmall thickness of 16 μm and superior dendrite suppression capability. As a result, the all-solid-state battery achieves a high capacity of 155.2 mAh g-1 at 0.5 C with a capacity retention of 84.3% after 500 cycles. Moreover, a pouch cell with this design displays good performance and safety, showing great promise for practical applications. To radically eliminate the risk of dendrite formation while maintaining high energy, Li metal is replaced with a micro-Si (mSi) that has a high theoretical capacity exceeding 3500 mAh g-1. A quasi-solid-electrolyte with polymer matrix modified with H-bonding groups is designed to simultaneously accommodate the volume change of mSi particles and construct self-regulated interphases between the mSi anode and electrolyte. It is demonstrated that the mSi/Li cell can deliver a specific capacity of as high as 2198 mAh g-1 after 150 cycles at 1 A g-1. More impressively, the mSi/NCM622 full cell can stably cycle for over 120 cycles even without pre-lithiating the anode. Finally, the quasi-solid-state mSi/NCM622 pouch cell without stack pressure can still exhibit good electrochemical performance at room temperature, showing great potential for practical applications. Keywords: Solid-state electrolyte; solid-state lithium battery; Li metal anode; Si anode; interface stability; interface resistance; ionic conductivity; battery manufacturing.</p
Development of highly reversible zinc anodes for rechargeable aqueous batteries
No full textRechargeable aqueous zinc batteries offer a safe, economical, and scalable solution for grid-scale storage of renewable energy. However, low coulombic efficiency and short cycle life resulting from the notorious dendrite formation and parasitic reactions of zinc anodes remain a grand challenge for the practical applications of this type of battery. The primary objective of this thesis is to tackle the aforementioned issues, thereby creating highly reversible zinc anodes for high-performance rechargeable aqueous batteries. We begin with developing a hierarchical porous framework for zinc anodes by electroless plating a conformal nanoporous tin (Sn) layer on a copper mesh (NSH). The NSH can reduce the local current density, provide abundant nucleation sites for zinc deposition, homogenize the ion flux and electric field at the electrode surface, and suppress the hydrogen evolution side reaction. As a result, the asymmetric Zn SH cell achieves a coulombic efficiency of 99.0% for over 200 cycles at 2 mA cm−2. Nevertheless, the electrically conductive skeleton induces preferential zinc deposition near the electrode/separator interface, resulting in a low operating areal capacity. Hence, a non-conductive polybenzimidazole (PBI) nanofibers layer is electrospun onto a copper substrate (PBI-Cu). The PBI nanofiber framework with abundant polar groups and uniform microporous structure can uniformize and facilitate the transport of Zn2+ ions at the electrode surface, enabling bottom-up and dendrite-free zinc deposition. Consequently, a symmetric cell with Zn@PBI-Cu electrodes can stably cycle under a large current density (20 mA cm-2) and high areal capacity (5 mAh cm-2). To suppress the side reactions of zinc with electrolyte, an ultrathin and dense Zn2+-conductive sulfonated poly(ether ether ketone) (SPEEK) polymer film is homogeneously coated onto the zinc surface via facile spin-coating. This artificial protective layer simultaneously blocks the water molecules and anions, uniformizes the ion flux, and facilitates the desolvation process of Zn2+ ions, thereby considerably enhancing the stability and reversibility of zinc anodes. In addition to electrodes, electrolyte modulation also represents an effective approach to improving the electrochemical performance of zinc anodes. In this regard, we first report a NH4Br electrolyte additive, of which the cations (i.e., NH4+) preferentially absorb on the tips/protrusions and repel upcoming Zn2+ ions to the vicinity for deposition. The electrostatic shielding effect effectively mitigate the zinc dendrite formation and extend the cyclability of zinc anodes. To reduce active solvated water molecules near the zinc surface and thus suppress side reactions, we explore dimethylacetamide (DMAc) as a water blocker. It is revealed that DMAc in the electrolyte impedes the formation of hydrated Zn2+ solvation while facilitating the association of Zn2+ and SO42−, thereby effectively mitigating side reactions and dendrite growth. Finally, we formulate a new electrolyte to boost the reversibility of zinc anodes. With the guidance of theoretical calculations, dimethyl sulfoxide (DMSO) is added into a Zn(TFSI)2 electrolyte, which effectively introduces TFSI- anions into the solvation sheath of Zn2+ and enables preferable reduction of TFSI- anions prior to zinc deposition, thus in-situ constructing a ZnF2-rich interphase on the zinc surface. The resultant interphase not only regulates the uniform Zn2+ ion transport, thus suppressing the dendrite formation, but also effectively prevents the zinc anode from side reactions with the electrolyte. As a result, the newly formulated electrolyte enables a zinc symmetric cell to achieve a long cycle life of over 2,000 h. More excitingly, full cells with diverse cathodes and the new electrolyte all display impressive cycling performance, showing great promise for practical applications. Keywords: Aqueous zinc batteries; zinc dendrite; interface modification; solid electrolyte interphase; electrolyte modulation</p
Exploration of organic redox-active species for organic redox flow batteries
No full textRedox flow batteries (RFBs) hold great potential for large-scale renewable energy storage as they offer distinctive merits of decoupled energy and power, flexible scalability, great safety, and long cycle life. In recent years, researchers are making increasing efforts in developing organic RFBs via designing organic redox-active species which are considered to be highly chemically and physically tunable, abundant, and sustainable. However, the wide application of organic RFBs is hindered by issues including cross-contamination of electrolyte, low battery energy density, and high material cost. The primary objective of this thesis is to address these challenges via exploring and developing redox-active materials for cross-contamination eliminated, high energy density, and cost-effective organic RFBs. We begin with addressing the cross-contamination issue in aqueous organic RFBs by designing the bipolar organic salt [(bpy-(CH2)3NMe3)]I2 (2.3), which serves as catholyte, anolyte, and charge carrier in the battery test. Such a compound can display superior electrochemical properties with a high theoretical energy density of 32.5 Wh L-1 and good cycling stability with no observable chemical decomposition over 290 cycles. We have also designed the dialkoxybenzene-based artificial bipolar compound (2,5-dimethyl-1,4-dialkoxybenzene/viologen, 3.5) for non-aqueous organic RFB. This compound exhibits an enhanced solubility of 0.66 M in MeCN and delivers capacity retention rate of 99.5 % per cycle within 35 cycles in the charge/discharge cycling test. We then report a highly water-soluble 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO2Na-TEMPO, 4.3) molecule to optimize the energy density of TEMPO-based RFBs. When paired with 1,10-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SPr)2V (anolyte), the resultant RFB operating through a cation-exchange membrane achieved an open circuit voltage of 1.19 V and a high energy density of 14.7 Wh L-1. In the long-term cycling study, this flow battery features stable capacity retention rate of 99.94% per cycle over 400 cycles with nearly 100% CE. Organic RFBs utilizing non-aqueous solvents with wider electrochemical window (up to 5 V) can offer a new pathway to realize high energy density. We explore new classes of compounds including nitrobenzenes and boron-based compounds as anolytes in non-aqueous RFBs. The cost-effective nitrobenzene (NB, 5.1) and its derivatives are well-demonstrated to be promising anolytes for non-aqueous RFBs. Notably, NB shows a low redox potential of -1.47 V and extremely high solubility of 6.5 M in salt-containing acetonitrile solution. The NB/DBMMB RFB, which has a high theoretical energy density of 195 Wh L-1, is successfully operated for more than 100 cycles with a capacity retention of 99.5% per cycle. In-depth electrolyte analysis reveals that azobenzene is a major decomposed species in the cycling test. Besides, we develop a RFB based on a boron-based tert-butyl diketonate (tBuBF2, 6.6), which shows a high solubility of 2.0 M in MeCN and a low reversible redox couple at -1.83 V. The assembled tBuBF2-based RFB exhibits an open circuit cell voltage of 2.57 V and shows good capacity retention over 80 cycles. Finally, to maximize the energy density of non-aqueous RFBs, we design and develop a series of [M(tpy-4OMe)2]2+ (M = Mn, Fe, Co, Ni, Cr) metal coordination complexes. These terpyridine-based complexes are designed to enable multi-electron redox reactions, higher redox potential, and enhanced solubility. [Ni(tpy-4OMe)2](TFSI)2 exhibits a remarkably high solubility of 0.72 M in MeCN, cathodic redox potential as high as 1.2 V versus Ag/Ag+, and low multiple electron transfers redox reaction at the range of -1.6 to -2.0 V. The fabricated symmetric flow battery delivers an extremely high cell voltage of 2.8 V and shows good capacity retention rate of 99.6% per cycle over 100 charge-discharge cycles. Keywords: Organic redox flow battery; Bipolar; Cross-contamination; Energy density; Metal coordination complex.</p
Enabling high-performance lithiated silicon-sulfur batteries by designing electrode structures and optimizing solid-electrolyte interphases
No full textLithiated silicon-sulfur (Si-S) battery is a promising next-generation energy-storage technology due to the raw materials’ high theoretical capacities, earth-abundance, and environmental benignity. Nonetheless, their overall electrochemical performance cannot meet the requirements for large-scale implementation, mainly caused by the internal polysulfide shuttle effect and inferior solid-electrolyte interphase (SEI) stability. Therefore, the primary objective of this thesis is to tackle these issues by electrode structure design and SEI optimization through electrolyte manipulation. The thesis begins with structure engineering of host materials to address the polysulfide migration and volume change of Si. Hollow carbon nanoshells are proposed to accommodate the S and Si nanoparticles, which function as physical barriers and provide sufficient conductive pathways to improve the reaction kinetics. More compact pomegranate-like micro-clusters are then developed for both S cathode and Si anode to further improve the full cell performance. The S is encapsulated in titanium nitride-carbon dual-layer hollow nanospheres assembled as micro-clusters, wherein the inner titanium nitride layer anchors the polysulfides and catalyzes their conversion reaction, while the outer carbon layer facilitates the electron transfer. Additionally, the multiple-layer barriers of the cluster further prevent the polysulfides escape, thus mitigating the side reactions and material loss. Si nanoparticles are hosted in pomegranate carbon clusters with internal void spaces to buffer the volume change, enabling stable SEI formation on the outmost layer. As a result, the lithiated Si-S full cell with the proposed structures achieves a high reversible capacity of 940 mAh g-1 at 0.3 A g-1 and a high areal capacity of 3.5 mAh cm-2. To further alleviate the side reactions and improve the cycling reversibility of lithiated Si-S batteries, the thesis then focuses on optimizing the solid-electrolyte interphase (SEI) by electrolyte engineering. A fluorinated ether electrolyte is proposed, which renders the formation of a robust lithium fluoride-rich SEI on both the anode and cathode. The SEI not only buffers the volume variation of Si microparticles over repeated cycles, but also renders the direct quasi-solid-state conversion of S, drastically reducing the polysulfides generation. The newly developed electrolyte endows the full cell with superior cycling performance to the conventional one and a high areal capacity of 4 mA h cm-2 with a low electrolyte/S ratio of 7.4 μL mg-1. Finally, to enhance the safety level, all-solid-state batteries are developed using polyethylene oxide (PEO)-based composite electrolytes. To tackle the polysulfide dissolution in the PEO, an artificial SEI is constructed on the S cathode by pre-cycling it in the concentrated liquid electrolyte. The compact SEI isolates the S from direct contact with PEO, leading to the one-step solid-solid transition of S without generating dissolved polysulfide. The Li-half cell with the artificial SEI exhibits much better capacity retention than the pristine one (85% vs. 42%). However, the inferior dendrite suppression capability of PEO limits the actual areal capacity of Li metal, so a metal-organic framework (MOF) hosted Si anode is then fabricated to replace the metallic Li, which shows excellent interfacial stability toward the PEO-based composite electrolyte for over 1000 h and achieves a high reversible areal capacity of 3 mAh cm-2. At last, an all-solid-state full battery is assembled using the proposed S@SEI and Si@MOF electrodes, which achieves a high capacity of 850 mAh g1 at 0.1 A g-1. Keywords: lithiated silicon-sulfur battery; polysulfide shuttle effect; fluorinated ether electrolyte; solidelectrolyte interphase; polyethylene oxide</p
Modeling and optimization of vanadium redox flow batteries
No full textVanadium redox flow batteries are a promising technology for large-scale energy storage to eliminate the mismatch between renewable energy sources and power consumption due to their merits of high safety, long lifespan, and flexible design. However, the widespread application of this technology is hindered by the high capital cost. Improving the battery performance is regarded as the solution to reduction in the cost, which requires minimizing the activation overpotential, ohmic overpotential, and concentration overpotential. To achieve this, in addition to optimizing the key components and operating conditions, accurate battery simulations are also crucial to guide the optimization process. The primary objective of the thesis is to develop highly precise numerical models and high-performance vanadium redox flow batteries by optimizing the electrodes and flow fields, as well as increasing the operating temperature. The thesis begins with the reduction in the activation overpotential by in-situ electrodepositing uniform and dense bismuth particles (58 nm) onto anodes, which is achieved by a new in-situ electrodeposition strategy that uses a catholyte with a low concentration of vanadium ions (33 mM V3+). Compared with the conventional method using a catholyte with a high concentration of vanadium ions (1700 mM VO2+), the new strategy renders Bi nanoparticles not being oxidized by VO2+ and VO2+ transported across the membrane from the catholyte. As a result, the battery with the anode treated by the new strategy achieves an energy efficiency of 76.3% even at a current density of 300 mA cm−2, which is higher than that of the battery with an anode treated by the conventional method (74.9%), and the untreated anode (73.3%). The concentration overpotential of VRFBs is then reduced by the optimization of ribs of serpentine flow fields, which is inspired by the simulation results of a newly developed model. This model incorporates solid mechanics to precisely simulate the electrochemical reaction in electrodes with uneven compression caused by ribs of fields. From the simulation results, we found that there are minor increases in ohmic resistance but an obvious enhancement of convection mass transfer when decreasing the compression ratio from 68% to 30%. Thus, we change the flat ribs of the serpentine flow field to ramped ribs, leading to enhanced convection in the electrodes of the under-rib region. As a result, the energy efficiency of the battery with ramped ribs reaches 80.3%, which is higher than the battery with conventional flat ribs (77.6%) at the current density of 200 mA cm-2. Finally, activation overpotential, ohmic overpotential, and concentration overpotential are simultaneously reduced by increasing the temperature of electrolytes. To accurately control the temperature of the electrolytes, an electrochemical-thermal coupled model is built to predict the temperature of different components of the battery system. The simulation results show that the temperature of electrolytes and graphite bipolar plates is at least 3.0 °C higher than that of other key components. Accordingly, the temperature of electrolytes in the battery system is accurately increased from 25 to 50 °C by a temperature controller, leading to the energy efficiency of the battery system being increased from 82.1% to 84.8% at 200 mA cm-2. Keywords: Vanadium Redox flow battery; Bismuth electrodeposition; Floe field design; Operation temperature; Thermal model; Electrode deformation.</p
Numerical and experimental investigations of mass transport in proton exchange membrane fuel cells
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