Multiple Charge-discharge Cycles Research Articles

Performance of solid particles as thermal storage media in thermal power flexibility retrofits: Effects of charging and discharging flow rates on single piece stacking bed

An innovative, efficient, and cost-effective energy storage method was introduced for retrofitting thermal power plants, which can be connected in series to scale up quickly (like a battery) and aimed to replace expensive, hazardous molten salts. Charging/discharging processes among steam and solid particles were investigated using energy storage devices with capacities in the tens of kilowatts. Results of the study confirm the excellent performance of steel waste as a thermal energy storage medium. Key findings include that the time required for complete charging decreases from 75 to 300 kg/h, taking 173, 128, 106, and 105 min, respectively. The most efficient charging occurs when the bed temperature is most inhomogeneous, taking 68, 42, 34, and 28 min for 75–300 kg/h. The highest average charging power and heat transfer coefficient are 5 kW and 210 W/(m2·K) at 300 kg/h. In addition, 145 kg/h achieves optimal speed and quantity coupling, resulting in the highest charging efficiency of approximately 84 %. Time-optimized and efficiency-optimized charging flows are given. Real-time efficiency shows that the poorest temperature uniformity corresponds to the highest charging efficiency, with 145 kg/h exhibiting the strongest thermal inertia and 75 kg/h the weakest. Higher flow rates improve internal heat transfer and reduce stratification, facilitating multiple charge-discharge cycles. At discharge rates of 50, 75, and 100 kg/h, steam production is sustained at approximately 30, 40, and 50 kg/h for 11, 7, and 7 min, respectively. Discharge efficiency increases with flow rate, rising from 30 % to 95 %. Phase transition region migrates at approximately 10∼20 mm/s across different discharge rates, with 75 kg/h showing the fastest discharge state. As the discharge rate increases, the phase transition region's total area proportion rises, reaching about 70 % at 100 kg/h. Research demonstrates the potential of this new energy storage medium for efficient and safe retrofitting of thermal power plants.

Probing layer interface behavior in Lithium-ion batteries via concurrent ultrasonic and modal measurements

Lithium-ion batteries are pivotal in various technological applications, from powering electric vehicles to supporting renewable energy storage systems. Understanding and monitoring the intricate chemo-mechanics within lithium-ion cells is imperative for ensuring their reliability and performance over time. Previous research has shown that both ultrasonic and vibrational measurements provide a measure of a cell’s state of charge (SOC) and state of health (SOH) and provide indications of existing or previous thermal or electrical abuse. Recent work suggests that ultrasonic and modal analysis may provide complementary insights into the evolution of layer interfaces during early-life aging due to charge-discharge cycling [J. Acoust. Soc. Am., 154, A284 (2023)]. This work presents the results of concurrent ultrasonic and modal measurements over multiple charge-discharge cycles. Namely, we monitor concurrent changes in the ultrasonic time-of-flight and signal amplitude and the resonance frequency of a clamped-clamped 10 Ah Nickel–Manganese–Cobalt pouch cell as a function of electrical cycling. The evolution of these metrics paired with analytical and numerical models of the cell will be used to understand changes in the material properties at layer interfaces and potential structural alterations within the battery which may be important indicators of SOC and early-life aging mechanisms.

Tracking Sodium Cluster Dynamics in Hard Carbon with a Low Specific Surface Area for Sodium‐Ion Batteries

AbstractHere, the sodium storage mechanism in commercial grade hard carbon with a low surface area is comprehensively investigated using electrochemical impedance spectroscopy (EIS), the galvanostatic intermittent titration technique, and in situ Raman spectroscopy for fresh and cycled electrodes. The reversible shift of the carbon G‐band peak on Raman spectra and substantial change of the charge‐transfer resistance in the sloping region of the voltage profile indicates the intercalation of sodium ions into hard carbon, whereas the low‐voltage plateau is associated with the pore‐filling process. In situ Raman analysis at low frequencies reveals that the pore filling is progressed via formation of small sodium clusters in closed pores. Prolonged cycling demonstrates that intercalation is stable and consistent throughout multiple charge–discharge cycles. The transition from intercalation to pore filling strongly affects the diffusion behavior, leading to considerably slower diffusivity at low voltage. The EIS effectively differentiates the contribution of adsorption to charge storage. The gradual growth of the solid‐electrolyte interphase layer affects the rise of the interfacial resistance as cycling progresses. In combination with the slower diffusivity, the low‐voltage plateau region strictly impedes fast de/sodiation and eventually causes capacity fade.

Origin of electrochemical activation on vanadium hexacyanoferrate cathode for aqueous zinc-ion battery

Herein, we investigate the phase evolution and reactions of vanadium hexacyanoferrate (VHCF) cathode in aqueous Zn-ion battery. X-ray diffraction, in situ Raman spectroscopy, scanning electron microscopy, time-of-flight secondary-ion mass spectrometry, and electrochemical methods reveal electrochemical activation of VHCF electrode in multiple charge–discharge cycles that eventually converts VHCF into zinc hexacyanoferrate and vanadium oxides (VOx). The emerging VOx redox behavior enhances the specific capacity of the electrode from 77 to 165 mAh g−1. Operando UV–vis absorption spectra of the electrolyte in the vicinity of the electrode during electrochemical reaction verifies the presence of decavanadate anion — the intermediate of VHCF transformation into the electrochemically active vanadium oxide. Analysis of the electrodes under various electrochemical conditions and experiments in aqueous and non-aqueous media demonstrate that vanadium of VHCF is redox inactive in the pristine material but products of VHCF transformation start to contribute to the specific capacity of the electrode upon the formation of vanadium oxide.

Recent advances in synthesis of MXene-based electrodes for flexible all-solid-state supercapacitors

Various energy storage sources have been developed so far, among which supercapacitors are more important for the forthcoming generations due to their small size and portability. Supercapacitors as good alternatives to batteries have recently attracted more attention because they have higher power and excellent charging-discharging rate which is considered as a challenging issue that limits the use of batteries. Supercapacitors also have other advantages over batteries, including higher reversibility and cycle life, lower maintenance costs, and safer electrode materials. MXenes have emerged as a new class of 2D composites in electrode materials for supercapacitors as low-cost and environment-friendly carbides and nitrides. MXenes are suitable inorganic compounds with excellent electrochemical properties and mechanical integrity to improve supercapacitor energy density at a new interval. This review presents new synthesis strategies to prevent the self-accumulation of MXene layers. First, the fundamental working theories of different supercapacitors are outlined. Next, an overview of the electrode material based on MXenes is outlined, and the latest solutions for increasing the active sites and improving the ion transfer rate have been collected. Hybridization and doping of MXenes change the properties of the composite, leading to a transformation in the structure and an increase in the capacitance. Furthermore, the utilization of double-transition metal MXenes solves challenges such as structural destruction and short life spans in multiple charge-discharge cycles. Then evaluation of the new MXene-based electrode materials in all-solid-state supercapacitors has been summarized. Finally, an overview of the latest developments in the creation of all-solid-state flexible supercapacitors as well as our predictions for future lines of inquiry is provided.

Operando spectral imaging of the lithium ion battery’s solid-electrolyte interphase

The lithium-ion battery is currently the preferred power source for applications ranging from smart phones to electric vehicles. Imaging the chemical reactions governing its function as they happen, with nanoscale spatial resolution and chemical specificity, is a long-standing open problem. Here, we demonstrate operando spectrum imaging of a Li-ion battery anode over multiple charge-discharge cycles using electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Using ultrathin Li-ion cells, we acquire reference EELS spectra for the various constituents of the solid-electrolyte interphase (SEI) layer and then apply these “chemical fingerprints” to high-resolution, real-space mapping of the corresponding physical structures. We observe the growth of Li and LiH dendrites in the SEI and fingerprint the SEI itself. High spatial- and spectral-resolution operando imaging of the air-sensitive liquid chemistries of the Li-ion cell opens a direct route to understanding the complex, dynamic mechanisms that affect battery safety, capacity, and lifetime.

Binder-Free MnO2/MWCNT/Al Electrodes for Supercapacitors.

Recently, significant progress has been made in the performance of supercapacitors through the development of composite electrodes that combine various charge storage mechanisms. A new method for preparing composite binder-free MnO2/MWCNT/Al electrodes for supercapacitors is proposed. The method is based on the original technique of direct growth of layers of multi-walled carbon nanotubes (MWCNTs) on aluminum foil by the catalytic pyrolysis of ethanol vapor. Binder-free MnO2/MWCNT/Al electrodes for electrochemical supercapacitors were obtained by simply treating MWCNT/Al samples with an aqueous solution of KMnO4 under mild conditions. The optimal conditions for the preparation of MnO2/MWCNT/Al electrodes were found. The treatment of MWCNT/Al samples in a 1% KMnO4 aqueous solution for 40 min increased the specific capacitance of the active material of the samples by a factor of 3, up to 100–120 F/g. At the same time, excellent adhesion and electrical contact of the working material to the aluminum substrate were maintained. The properties of the MnO2/MWCNT/Al samples were studied by electron probe microanalysis (EPMA), Raman spectroscopy, cyclic voltammetry (CV), and impedance spectroscopy. Excellent charge/discharge characteristics of composite electrodes were demonstrated. The obtained MnO2/MWCNT/Al electrodes maintained excellent stability to multiple charge-discharge cycles. After 60,000 CVs, the capacitance loss was less than 20%. Thus, this work opens up new possibilities for using the MWCNT/Al material obtained by direct deposition of carbon nanotubes on aluminum foil for the fabrication of composite binder-free electrodes of supercapacitors.

Intelligent body armor: Advanced Kevlar based integrated energy devices for personal safeguarding

Despite the rapid advances in wearable electronics, there are still challenges in developing wearable energy devices with excellent mechanical properties, let alone resisting bullets. In this work, a triboelectric nanogenerator (TENG) based on Kevlar and epoxy is developed (KE-TENG). It not only can protect users, but also collects the impact energy and store. Interestingly, impact signals can also be transmitted remotely via Bluetooth module and act as an alarm, showing its potential in wearable protective clothing. Epoxy not only improves the triboelectric properties of KE-TENG, but also greatly enhances the robustness under impact. Furthermore, a fiber shaped supercapacitor (F-SC) has been prepared in this work. F-SC has stable energy storage capacity, which does not deteriorate under different bending angles or multiple charge–discharge cycles. Most importantly, it can be assembled with KE-TENG via rectifiers, realizing an integrated system for energy collection and storage.

Synthesis and electrochemical performance of flexible LiFePO4/TinO2n − 1/reduced graphene oxide cathode material for lithium-ion batteries

In this work, the flexible LiFePO4/TinO2n − 1/rGO composite film was prepared as a cathode material for lithium-ion batteries. The LiFePO4/TinO2n − 1 composite was firstly synthesized by improved hydrothermal method, and then the flexible composite film could be prepared through facile vacuum filtration. The resultant self-supporting LiFePO4/TinO2n − 1/rGO film delivers excellent electrochemical performance. It presents a high capacity of 151.2–155.5 mAh g−1 at 0.2 C, and stable capacity retention during multiple charge–discharge cycles. The good electrochemical performance of LiFePO4/TinO2n − 1/rGO film could be contributed to TinO2n − 1 and reduces graphene which can improve the poor electronic and ionic conductivity of LiFePO4.

Mapping and Metastability of Heterogeneity in LiMn2O4 Battery Electrodes with High Energy Density

The hierarchical nature of the cathode in Li-ion batteries can result in phenomena determining electrochemical performance occurring at different length-scales, from individual atoms to the whole electrode. In architectures designed for high density of charge storage, transport limitations can emerge at the microscale that compromise effective utilization and accelerate degradation. These limitations manifest as chemical heterogeneity within the electrode. Micro-focused diffraction mapping using a laboratory X-ray source provides maps with sub-mm resolution of a whole electrode composed of commercial LiMn2O4. Evidence of disparate local utilization both laterally and along the depth of the electrode was obtained, especially at high rates and after multiple charge-discharge cycles. As a model to study the persistence of heterogeneity due to transport limitations, lateral gradients to lithium transport were introduced by cycling against Li anodes of small diameter. The resulting maps revealed the effects of anisotropic electric migration and diffusion to be separated, confirming that diffusion is the primary limitation for long-range kinetics. Tracking of subsequent relaxation revealed that the heterogeneity was metastable despite a strong thermodynamic driving force, maintained by poor lithium transport through the solid electrode matrix. This study enriches our understanding of transport across thick electrode architectures and the imposition of unique frustrated states, away from equilibrium.

Dithiolene Complexes of First-Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries.

Five metal complexes of the dithiolene ligand maleonitriledithiolate (mnt2−) with M=V, Fe, Co, Ni, Cu were studied as redox‐active materials for nonaqueous redox flow batteries (RFBs). All five complexes exhibit at least two redox processes, making them applicable to symmetric RFBs as single‐species electrolytes, that is, as both negolyte and posolyte. Charge–discharge cycling in a small‐scale RFB gave modest performances for [(tea)2Vmnt], [(tea)2Comnt], and [(tea)2Cumnt] whereas [(tea)Femnt] and [(tea)2Nimnt] (tea=tetraethylammonium) failed to hold any significant capacity, indicating poor stability. Independent negolyte‐ and posolyte‐only battery cycling of a single redox couple, as well as UV/Vis spectroscopy, showed that for [(tea)2Vmnt] the negolyte is stable whereas the posolyte is unstable over multiple charge–discharge cycles; for [(tea)2Comnt], [(tea)2Nimnt], and [(tea)2Cumnt], the negolyte suffers rapid capacity fading although the posolyte is more robust. Identifying a means to stabilize Vmnt 3−/2− as a negolyte, and Comnt 2−/1−, Nimnt 2−/1−, and Cumnt 2−/1− as posolytes could lead to their use in asymmetric RFBs.

Combining density functional theory and 23Na NMR to characterize Na2FePO4F as a potential sodium ion battery cathode

Sodium ion batteries offer an inexpensive alternative to lithium ion batteries, particularly for large-scale applications such as grid storage that do not require fast charging rates and high power output. Moreover, the use of polyanionic structures as cathode materials afford incredibly high structural stability relative to layered transition metal oxides that can undergo a structural collapse upon full removal of the charge carrying ions. Sodium iron fluorophosphate, Na2FePO4F, has demonstrated its viability as a potential cathode material for sodium ion batteries, having a robust framework even after multiple charge-discharge cycles. Although solid-state NMR has traditionally been an excellent method for the determination of local structure and dynamic properties of cathode materials during the electrochemical cycling process, reliable assignment of the 23Na chemical shifts resulting from the paramagnetic hyperfine interaction can be difficult when using only empirical rules. Here we present the use of density functional theory calculations to assign the experimentally observed NMR shifts to the crystallographic sites in Na2FePO4F, where it is found that the results do not agree with the previously reported assignment based upon simple geometry arguments. Furthermore, we report the justification of the proposed desodiation mechanism in Na2FePO4F on the basis of theoretical arguments, in good agreement with experimental NMR results reported previously.

Operando Grazing Incidence Small-Angle X-ray Scattering/X-ray Diffraction of Model Ordered Mesoporous Lithium-Ion Battery Anodes.

Emergent lithium-ion (Li+) batteries commonly rely on nanostructuring of the active electrode materials to decrease the Li+ ion diffusion path length and to accommodate the strains associated with the insertion and de-insertion of Li+, but in many cases these nanostructures evolve during electrochemical charging-discharging. This change in the nanostructure can adversely impact performance, and challenges remain regarding how to control these changes from the perspective of morphological design. In order to address these questions, operando grazing-incidence small-angle X-ray scattering and X-ray diffraction (GISAXS/GIXD) were used to assess the structural evolution of a family of model ordered mesoporous NiCo2O4 anode films during battery operation. The pore dimensions were systematically varied and appear to impact the stability of the ordered nanostructure during the cycling. For the anodes with small mesopores (≈9 nm), the ordered nanostructure collapses during the first two charge-discharge cycles, as determined from GISAXS. This collapse is accompanied by irreversible Li-ion insertion within the oxide framework, determined from GIXD and irreversible capacity loss. Conversely, anodes with larger ordered mesopores (17-28 nm) mostly maintained their nanostructure through the first two cycles with reversible Li-ion insertion. During the second cycle, there was a small additional deformation of the mesostructure. This preservation of the ordered structure lead to significant improvement in capacity retention during these first two cycles; however, a gradual loss in the ordered nanostructure from continuing deformation of the ordered structure during additional charge-discharge cycles leads to capacity decay in battery performance. These multiscale operando measurements provide insight into how changes at the atomic scale (lithium insertion and de-insertion) are translated to the nanostructure during battery operation. Moreover, small changes in the nanostructure can build up to significant morphological transformations that adversely impact battery performance through multiple charge-discharge cycles.

Performance of a Non-Aqueous Vanadium Acetylacetonate Prototype Redox Flow Battery: Examination of Separators and Capacity Decay

Non-aqueous electrolytes are stable over wide electrochemical potential windows, generally >2 V, and therefore hold promise for enabling higher energy and power density redox flow batteries (RFBs). Nevertheless, the development of non-aqueous RFBs is at an early stage and significant research efforts are needed to demonstrate non-aqueous RFBs with performance characteristics that exceed those of aqueous RFBs. The membrane or separator is a critical component that to a great extent determines the performance of RFB systems for practical applications. In this work, the performance of a RFB was evaluated with Nafion 1035 membranes and Daramic 175 SLI microporous separators. The non-aqueous electrolyte was based on vanadium (III) acetylacetonate. This chemistry possesses two couples over ∼2.2 V. Charge-discharge cycles were performed in a RFB at a current density of 10 mA cm−2. Coulombic and energy efficiencies of 91% and 80% were achieved using the Nafion membrane. A similar RFB using the Daramic microporous separator achieved columbic and energy efficiencies of 73% and 68%, respectively. The source of capacity decay during multiple charge-discharge cycles was also investigated. The loss in the capacity was related to the poor chemical stability of the vanadium acetylacetonate in the positive electrolyte during battery cycling.

Characterization of Bulk and Surface Chemical States on Electrochemically Cycled LiFePO4: A Solid State NMR Study

Bulk and surface chemical states were both investigated for electrochemically delithiated LixFePO4 using 7Li and 31P MAS NMR spectroscopy. The quantitative lithium extraction/insertion from LiFePO4 and the reversible two-phase reaction behavior between LiFePO4 and FePO4 were confirmed on electrochemical operation. The 7Li and 31P NMR spectra of the fully charged LixFePO4 evidenced a single Li-poor phase Li0.05FePO4 instead of a biphasic mixture of 0.05LiFePO4 and 0.95FePO4. Simultaneous growth of LixPOyFz was also shown as a surface film component on the charged electrodes, which dynamically increased and decreased in intensity during charging and discharging reactions, respectively. The electrode surface was further characterized with XPS to discuss the surface film formation during electrochemical cycles. Combined chemical state analyzes by NMR and XPS spectroscopies suggested that the degradation of LiPF6 salt occurred from the initial redox cycles, but that of solvent occurred after multiple cycling. The deposition of carbonaceous species would be related to a small capacity fading observed after the multiple charge-discharge cycles.