Voltage Decay Research Articles

Controllable Synthesis and Surface Modifications of a Metastable O2-Type Li-Rich Cathode Material

Li-rich materials have become one of the most promising cathode candidates for next-generation lithium-ion battery systems due to their high capacity and operating voltage. Conventional O3-type Li-rich materials undergo a structural transition from a layered to a spinel phase during cycling, leading to the degradation in their electrochemical performance, especially in terms of their voltage decay. The oxygen atoms comprising the structure of O2-type Li-rich materials are stacked in the ABAC configuration, which can effectively suppress these harmful phase transitions. However, O2-type Li-rich materials are metastable structures and can only be synthesized via the means of complex ion exchange methods. In addition, the surface of the material is susceptible to side reactions with the electrolyte when charged to high voltages. Here, we explored the optimal conditions for the synthesis of O2-type Li[Li0.25Ni0.1Co0.05Mn0.6]O2 (LLNCM) in more detail by preparing the precursors using the sol-gel method. Meanwhile, the modification of the material’s surface via low-temperature hydrolysis of aluminum isopropoxide has been proposed for the first time in this study to avoid the damage of metastable materials by the high-temperature coating process. The surface-modified materials prepared under optimal conditions exhibited an excellent electrochemical performance, indicating that a highly stable O2-type bulk phase structure with effective surface modification is a potential way to promote the commercial applications of Li-rich cathode materials.

Ge-Regulated Ordered Phase in Pseudosphere-Structured LiNi0.5Mn1.5O4 Spinel Effectively Inhibits Mn Dissolution.

Due to the higher energy density, high thermal stability, and low cost, LiNi0.5Mn1.5O4 (LNMO) spinel, with a large voltage operating window, has been one of the most promising cathode materials for lithium-ion batteries (LIBs). However, the interfacial reaction between the cathode and electrolyte and the two-phase reaction within the bulk of LNMO would destroy the original structure and lead to capacity deterioration, posing a significant challenge. Therefore, the way to suppress the transition-metal (TM) dissolution in LNMO has attracted much attention. However, the ordered/disordered phase regulation by metal atom doping to prohibit TM dissolution has not been extensively explored. Herein, a Ge-doping strategy is proposed to adjust the ratio of disordered/ordered phases in LNMO, resulting in exceptional structural stability. For the modified spinel cathode, there is almost no voltage drop and the capacity retention is up to 92.2% over 1000 cycles at 1C. These results demonstrate that incorporating Ge into LNMO forms a robust structure that effectively increases the amount of Mn4+ while blocking the diffusion of TM ions. In addition, Ge-doping also protects the bulk from further reactions with electrolytes, significantly enhancing the interfacial stability and relieving voltage decay in cycling. This approach can also be applied to design other high-stability cathodes through ordered/disordered phase regulation.

Conversion of Induced Polarization Data and Their Uncertainty from Time Domain to Frequency Domain Using Debye Decomposition

The time-domain (TD) induced polarization (IP) method is used as an extension of direct current (DC) resistivity measurements to capture information on the ability of the subsurface to develop electrical polarization. In the TD, the transient voltage decay is measured after the termination of the current injection. To invert tomographic TD IP data sets into frequency-domain (FD) models of complex electrical resistivity, a suitable approach for converting TD IP transients and their corresponding uncertainties into the FD is essential. To apply existing FD inversion algorithms to TD IP measurements, a conversion scheme must transform the measured decay curves into FD impedances and also propagate the corresponding measurement uncertainty from the TD to the FD. Here, we present such an approach based on a Debye decomposition (DD) of the decay curve into a relaxation-time distribution and the calculation of the equivalent spectrum. The corresponding FD data error can be obtained by applying error propagation through all of these steps. To accomplish the DD we implement a non-linear Gauss–Newton inversion scheme. We test the conversion scheme in a synthetic study and demonstrate its application to field data on a tomographic TD IP data set measured on the Maletoyvaemskoie ore field (Kamchatka, Russia). The proposed conversion scheme yields accurate impedance data for relaxation processes, which are resolved by the TD measurements. The error propagation scheme provides a reasonable FD uncertainty estimate, as confirmed by a Monte Carlo analysis of the underlying parameter distributions.

Experimental Characterization of a Novel Fluidized‐Bed Zn–Air Fuel Cell

AbstractZn–air fuel cells are promising energy storage devices for renewable energy and power sources, as they are cost‐effective and have high energy density. However, limited charge and discharge cycles and low round‐trip efficiency have long been obstacles to large‐scale market deployment. Herein, a new fluidized‐bed zinc–air fuel cell is designed, constructed, tested, and characterized. The integration of the fluidized bed in the zinc–air fuel cell leads to key advantages such as: erosion of the anode passivation layer, which plays a key role in the rapid voltage decay and keeping the products of reactions away from the proximity of the electrodes, reducing the concentration overpotential, due to the concentration gradient of electrode species in the diffusion boundary layer between the electrode–electrolyte interface and the electrolyte bulk.

A Li-rich layered oxide cathode with negligible voltage decay

A Li-rich layered oxide cathode with negligible voltage decay

BaSnO3-SnO2 heterojunction mesoporous photoanode for quantum dot-sensitized solar cells

Mesoporous photoanodes, as quantum dot adsorption framework and electron transfer paths, are the key components in efficient quantum dot-sensitized solar cells (QDSSCs). Binary heterojunction mesoporous photoanodes hold promise for facilitating electron transport due to the heterojunction potential, when compared to conventional mono-component mesoporous photoanodes. In this work, we construct a binary mesoporous photoanode with enhanced mesoporous and electron transport using a BaSnO3-SnO2 heterojunction, and then fabricated CdSe/CdS co-sensitized QDSSCs. The charge extraction, open-circuit voltage decay, electrochemical impedance spectroscopy, and intensity-modulated voltage/photocurrent spectroscopy measurements demonstrated that, in contrast to the BaSnO3 mesoporous photoanode, the BaSnO3-SnO2 mesoporous photoanode could effectively reduce charge recombination and extend the electron lifetime, thus promoting charge collection in the QDSSCs. Based on these merits, the BaSnO3-SnO2 mesoporous photoanode-based solar cells yielded a power conversion efficiency of 3.14%, resulting in a ca. 11% increase in power conversion efficiency, compared to the reference BaSnO3 solar cells.

Cycling stability of LiNi0.80Co0.15Al0.05O2 cathode modified by solid-state electrolyte film

Ni-rich LiNi0.80Co0.15Al0.05 (NCA) cathode material with high energy density always suffers from structural degradation during cycles, resulting in rapid capacity fading and severe voltage decay. In this study, a solid-state electrolyte film of Li1.8Sc0.8Ti1.2(PO4)3 (LSTP) is constructed on the surface of NCA cathode via a chemical route to enhance the cycling stability of NCA cathode. The Ex-situ XRD, EIS, dQ/dV, and SEM analyses show that the structural degradation of NCA derives mainly from the corrosion of electrolyte rather than H2-H3 phase transition, which gives rise to the extremely weak phase transitions of H1-M−H2−H3 after cycles. Besides the isolation role of LSTP film, it is found that Ti4+ ions in LSTP incorporate into the subsurface structure of NCA, which can accelerate the Li+ diffusion, reduce the Li+/Ni2+ disorder, reinforce the crystal structure, and enlarge the inter-slab spacing of NCA. Consequently, the coated NCA-2 %LSTP cathode reveals well structural integrity and evident phase transition behavior, delivering stable cycling performance with a superior capacity retention of 96.8 % at 1.0C after 100 cycles. This study not only deeply analyzes the mechanism of the structural degradation of NCA, but also demonstrates an efficient improving approach through LSTP modification.

Dye-sensitized solar cells with aluminum-doped zinc oxide nanoparticles-loaded electrolyte: A breakthrough solution for recombination and hysteresis suppression

Dye-sensitized solar cells (DSCs) suffer from reduced power conversion efficiency due to electron recombination with triiodide (I3−) ions at the photoanode/electrolyte interface. Organic additives are conventionally used in electrolyte to reduce recombination and improve the efficiency of the device. These additives react covalently with iodine (I2) in the electrolyte, leading to a reduction in the concentration of I3− ions. Other inorganic oxides, such as silica (SiO2), titania (TiO2), and magnesia (MgO), have been shown to improve the efficiency of the solar cell. These oxides can adsorb heavy ions to some extent. In this study, we investigated aluminum-doped zinc oxide nanoparticles (AZO NPs) as an inorganic additive with ionic adsorption properties towards I− and I3− anions in the electrolyte. We confirmed the ionic adsorption of AZO NPs through zeta potential and electrochemical impedance measurements. We observed a significant decrease in recombination between photogenerated electrons and electrolyte ions after adding AZO NPs, which resulted in an increase in open circuit voltage (VOC) and fill factor (FF) values. Furthermore, the incorporation of AZO NPs reduced the compelled hysteresis caused by the mismatch in current density-voltage curves for the forward and reverse scan direction. Mott-Schottky analysis confirmed the increase in VOC due to the negative shift in the Fermi level of the TiO2 photoanode. Additionally, open circuit voltage decay measurements indicated a longer electron lifetime at the TiO2/electrolyte interface in AZO NPs loaded electrolyte. Our findings suggest that AZO NPs have potential as an effective inorganic additive for improving the performance of DSCs.

An online sensor-less methanol concentration monitoring method for direct methanol fuel cell based on the voltage decay rate during the rest cycle

Monitoring the methanol concentration is critical for direct methanol fuel cell (DMFC) reliability. However, monitoring the concentration is expensive due to the high cost of concentration sensors. This paper describes an advanced sensor-less method for monitoring methanol concentration during the rest cycle of a DMFC using the voltage decay rate (kvt). Based on the dynamic behavior of methanol mass transfer and reaction in DMFC during the rest cycle, a feedback equation for methanol concentration is constructed in this method. The rationality of the feedback equation is then confirmed through a series of sensitivity analysis experiments at various methanol concentrations, cell temperatures, and load currents. In addition, 1,900 experiments under random conditions simulating practical applications are used to assess the accuracy of the feedback equation. According to the results, 89.6% of the 1,900 experiments have concentration prediction errors of less than 0.05 mol L−1. The mean error and standard deviation of error are found to be 0.005 mol L−1 and 0.031 mol L−1, respectively. These findings point to the desired accuracy and stability of the sensor-less method, as well as promising applications in a variety of scenarios.

Correlating Rate-Dependent Transition Metal Dissolution between Structure Degradation in Li-Rich Layered Oxides.

Understanding the mechanism of the rate-dependent electrochemical performance degradation in cathodes is crucial to developing fast charging/discharging cathodes for Li-ion batteries. Here, taking Li-rich layered oxide Li1.2 Ni0.13 Co0.13 Mn0.54 O2 as the model cathode, the mechanisms of performance degradation at low and high rates are comparatively investigated from two aspects, the transition metal (TM) dissolution and the structure change. Quantitative analyses combining spatial-resolved synchrotron X-ray fluorescence (XRF) imaging, synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques reveal that low-rate cycling leads to gradient TM dissolution and severe bulk structure degradation within the individual secondary particles, and especially the latter causes lots of microcracks within secondary particles, and becomes the main reason for the fast capacity and voltage decay. In contrast, high-rate cycling leads to more TM dissolution than low-rate cycling, which concentrates at the particle surface and directly induces the more severe surface structure degradation to the electrochemically inactive rock-salt phase, eventually causing a faster capacity and voltage decay than low-rate cycling. These findings highlight the protection of the surface structure for developing fast charging/discharging cathodes for Li-ion batteries.

Enabling Long Cycling with Excellent Structure Stability for High-Nickel Layered Cathodes in Lithium Metal Batteries

Rechargeable Li metal batteries (LMBs) constructed from Li metal and high-Ni NMCs have attracted much attention because of their high energy density in electrical vehicles. The choice of electrolyte plays a crucial role in determining the performance of Li||high-Ni NMC LMBs. In this work, we investigated the electrochemical performance and microstructure change of Li||0.7Nb-NMC 9055 LMBs using ether-based electrolyte (LiFSI–1.2DME–3TTE) and carbonate electrolyte (1 M LiPF6 in EC/DMC). The results show that ether-based electrolyte can significantly enhance the cycling life of Li||0.7Nb-NMC 9055 battery, providing a capacity of 203 mAh/g vs 179 mAh/g (carbonate electrolyte) for 150 cycles without voltage decay at C/5 charge and C/2 discharge rates. The excellent electrochemical performance (capacity and voltage) originates from the perfect structural stability confirmed by the dQ/dV versus V curves, which was further supported by electrochemical impedance spectroscopy test, showing nearly no surface charge transfer resistance increase in ether-based electrolyte, versus continuous growth in carbonate electrolyte.

Enhancing Ionic Transport and Structural Stability of Lithium-Rich Layered Oxide Cathodes via Local Structure Regulation.

Lithium-rich manganese-based layered oxides (LRM) have garnered considerable attention as cathode materials due to their superior performance. However, the inherent structural degradation and obstruction of ion transport during cycling lead to capacity and voltage decay, impeding their practical applications. Herein, an Sb-doped LRM material with local spinel phase is reported, which has good compatibility with the layered structure and provides 3D Li+ diffusion channels to accelerate Li+ transport. Additionally, the strong Sb-O bond enhances the stability of the layered structure. Differential electrochemical mass spectrometry indicates that highly electronegative Sb doping effectively suppresses the release of oxygen in the crystal structure and mitigates successive electrolyte decomposition, thereby reducing structural degradation of the material. As a result of this dual-functional design, the 0.5 Sb-doped material with local spinel phases exhibits favorable cycling stability, retaining 81.7% capacity after 300 cycles at 1C, and an average discharge voltage of 1.87mV per cycle, which is far superior to untreated material with retention values of 28.8% and 3.43mV, respectively. This study systematically introduces Sb doping and regulates local spinel phases to facilitate ion transport and alleviate structural degradation of LRM, thereby suppressing capacity and voltage fading, and improving the electrochemical performance of batteries.

Multi-scale design of three-dimensional highly-conductive spherical Li-rich layered manganese oxide toward long-term stable cyclability

The improvement of the cyclability and voltage decay of Li-rich layered manganese oxides (LLMOs) is crucial for developing Li-rich-based high energy density Li-ion batteries. However, the electron/ion transport mismatch and constant corrosion induced by the electrolyte remain huge challenges. In this work, we design a 3D highly conductive spherical structure using COMSOL simulations and synthesize a lithium silicate and graphene cocoated LLMO with high tap density by using liquid polysiloxane as the silicon source. Their synergistic effects reduce the activation energy in the lithiation/delithiation process, decrease the polarization, improve the thermal stability, and hinder the dissolution of transition metal ions in the material, thereby enhancing the electrochemical performance of the composite. After 200 cycles, the discharge capacity of the cocoated material is 212.9 mAh g−1 (91.3% capacity retention), the discharge specific energy reaches 713.5 Wh kg−1, and the discharge middle voltage increases by 0.1 V relative to the pristine sample after 200 cycles. The 3D structural design and multiscale modifications described herein greatly facilitate the commercialization of lithium-rich layered manganese oxides as cathode materials for next-generation Li-ion batteries with high energy density.

Performance degradation decomposition-ensemble prediction of PEMFC using CEEMDAN and dual data-driven model

Proton exchange membrane fuel cells (PEMFCs) are essential modern sustainable energy generation devices. Since such an electrochemical system has a limited lifetime, accurately estimating its performance degradation is critical for practical applications. When a large amount of measurement data is available, many nonlinear forecasting methods can be used to predict the performance degradation of a PEMFC system, and the prediction accuracy can be improved by optimizing the structure and parameters of the algorithm. However, the voltage recovery phenomenon would pose a challenge to the classical data-driven methods. In this work, we propose a novel hybrid data-driven PEMFC performance prediction framework by exploring the extensive degradation information buried in the voltage decay data. With complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN), the raw voltage data are first decomposed into sequences of multiple time scales. Then, the linear and nonlinear components in the decomposed sequences are predicted by autoregressive integrated moving average (ARIMA) and the attention-based gated recurrent unit (GRU), respectively. Comparative studies show that the proposed method can improve the prediction performance by 42.6%–84.2% on FC1 and 35.0%–90.6% on FC2, compared to state-of-the-art algorithms on the basis of an open-source dataset of PEMFCs.

Confining bulk molecular O2 by inhibiting charge transfer on surface anions toward stable redox electrochemistry in layered oxide cathodes

Molecular O2 has been clarified as an important O-oxidation model in anionic redox, the charge compensation provided by which pushes energy-density limits of layered oxide cathodes. However, how to confine the bulk-formed molecular O2 and retard its conversation to free gaseous O2 (↑), an origin of unstable redox electrochemistry, remains open questions. Here, we propose a strategy via tuning relative values of Mott–Hubbard U and charge-transfer energy Δ to suppress charge transfer on surface anions to confine the bulk molecular O2. Supported by theoretical calculations, Nb5+ without 4d electrons which can enable U < < Δ is selected as a charge-transfer insulator. The thus Nb5+-surface-tailored model compound Na0.67Fe0.5Mn0.5O2 shows an oxygen-redox-inactive surface and well-confined bulk molecular O2 as directly uncovered by soft X-ray absorption spectroscopy and 50 K-electron paramagnetic resonance results respectively. Meanwhile, a stable redox electrochemistry with enhanced cycling stability, inhibited voltage decay and eliminated P2-O2 phase transitions is observed because of the well-caged bulk O2. More broadly, this work presents a versatile access to stabilize the important O-oxidation model, enriching approaches of stabilizing the anionic redox electrochemistry.

Stabilized Anionic Redox by Rational Structural Design from Surface to Bulk for Long-Life Fast-Charging Li-Rich Oxide Cathodes.

On account of high capacity and high voltage resulting from anionic redox, Li-rich layered oxides (LLOs) have become the most promising cathode candidate for the next-generation high-energy-density lithium-ion batteries (LIBs). Unfortunately, the participation of oxygen anion in charge compensation causes lattice oxygen evolution and accompanying structural degradation, voltage decay, capacity attenuation, low initial columbic efficiency, poor kinetics, and other problems. To resolve these challenges, a rational structural design strategy from surface to bulk by a facile pretreatment method for LLOs is provided to stabilize oxygen redox. On the surface, an integrated structure is constructed to suppress oxygen release, electrolyte attack, and consequent transition metals dissolution, accelerate lithium ions transport on the cathode-electrolyte interface, and alleviate the undesired phase transformation. While in the bulk, B doping into Li and Mn layer tetrahedron is introduced to increase the formation energy of O vacancy and decrease the lithium ions immigration barrier energy, bringing about the high stability of surrounding lattice oxygen and outstanding ions transport ability. Benefiting from the specific structure, the designed material with the enhanced structural integrity and stabilized anionic redox performs an excellent electrochemical performance and fast-charging property..

A hybrid analytical and numerical model for cross-over and performance decay in a unit cell vanadium redox flow battery

Developing an accurate and efficient model for cross-over is critical for improving the long-term performance of redox flow batteries (RFBs). This work presents a hybrid analytical and numerical model that combines a two-dimensional analytical solution to the active species, a one-dimensional analytical model for cross-over mechanisms, and a zero-dimensional numerical model for outlet concentrations of reactants. By comparing with experiment over 41 cycles (ca. 144 h), the model reported a mean voltage difference of 0.0089 V, mean time difference of 14 s per cycle (of 3.5 h), maximum relative difference for capacity and energy of 1.34% and 1.63%, respectively. The predicted mean concentrations for V2+, V3+, and VO2+ in membrane are 65% ∼ 77% of measured values from the literature. Upon validation, the model reproduced behaviors in electrolyte imbalance similar to those observed in experiments and numerical models, and revealed the control of cross-over, self-discharge, stoichiometry of side reactions, and Coulombic efficiency on electrolyte imbalance. The model also demonstrates excellent computational efficiency for simulating 41 cycles (around 37,000 points) within 3 ∼ 4 s. The demonstrated efficiency and accuracy for predicting cross-over and its impacts on voltage, capacity&energy decay, membrane concentrations, and electrolyte imbalance makes it a reliable tool for optimizing RFBs’ long-term performance.

Voltage Decay of Li‐Rich Layered Oxides: Mechanism, Modification Strategies, and Perspectives (Adv. Funct. Mater. 25/2023)

Voltage Decay In article number 2213260, Zhaoping Liu, Quanbing Liu, Bao Qiu, and co-workers review the mechanism of voltage decay generation in Li-rich layered oxides, which is caused by a combination of phase transition, microscopic defects, and oxygen redox. The review also provides critical insight into the root of voltage decay, as well as predictions for potential solutions.

Design of a simple and low-cost solid-state ultra-fast high-voltage switch.

Ultra-fast high-voltage switches (UFHVSs) are a core component of time-of-flight mass spectrometers for realizing high accuracy ion acceleration, deceleration, and temporal focusing. The desirable features of high performance UFHVSs include a large range of adjustability of pulse width, a high maximum output amplitude, and minute rising and falling times. Besides the simplicity of the driver circuit, the total cost of the whole device is also critical to its practical applications. In this work, we present a low-cost and easy-fabrication 5000V bipolar solid-state UFHVS for a high-resolution mass spectrometer. A double-pulse transformer isolates the circuit's high- and low-voltage sides and synchronously drives series-connected cascode SiC FETs to form its push-pull topology. This scheme allows transmitting drive signals with long widths but without the magnetic saturation of the transformer. Testing results show that output pulses reach a maximum voltage of 5000V and a width of 150 µs, with rising and falling times of 8.5 and 18.3ns, respectively. More importantly, they have nearly no voltage decay.

Understanding Potential Decay during OCV Hold via Dry Recovery Process

Nearly ∼100 mV of open circuit voltage (OCV) decay is observed in the first 24 h of a typical OCV hold test for assessing the chemical durability of a polymer electrolyte. Significant recovery of this OCV loss has been achieved by subjecting the cell to so-called wet recovery process by lowering the cell potential often under over-humidified conditions, thereby confounding the effect of potential reduction and catalyst/ionomer interfacial reorganization due to humidification. In this work by applying a dry recovery protocol (30% RH) comprising 30 min of lower potential hold in the H2/N2 environment followed by the H2/Air environment, OCV recovery of 82%, 73%, 62% and 58 was achieved by holding potentials at 0.13, 0.2, 0.6, and 0.8 V, respectively. Small changes in electrochemically active surface area and hydrogen cross-over rate observed 48 h cannot explain the nearly 100 mV OCV. The wet recovery process (low potential and 100% RH) resulted in 85% recovery of ∼100 mV of lost OCV implying oxide coverage increase as the dominant factor for sharp OCV decay. Further, it is shown that cross-over hydrogen is not fully oxidized at OCV-like conditions (>0.6 V), highlighting the need to quantify potential-dependent crossover current for OCV decay models.