Differential Capacitance Curves Research Articles

Addressing Irreversibility and Structural Distortion in WS2 Inorganic Fullerene-Like Nanoparticles: Effects of Voltage Cutoff Experiments in Beyond Li+-Ion Storage Applications.

Large interlayer spacing beneficially allows Na+- and K+-ion storage in transition-metal dichalcogenide (TMD)-based electrodes, but side reactions and volume change, which pulverize the TMD crystalline structure, are persistent challenges for the utilization of these materials in next-generation devices. This study first determines whether irreversibility due to structural distortion, which results in poor cycling stability, is also apparent in the case of inorganic fullerene-like (IF) tungsten disulfide (WS2) nanocages (WS2IF). To address these problems, this study proposes upper and lower voltage cutoff experiments to limit specific reactions in Na+/WS2IF and K+/WS2IF half-cells. Three-dimensional (3D) differential capacity curves and derived surface plots highlight the continuation of reversible reactions when a high upper cutoff technique is applied, thereby indirectly suggesting restricted structural dissolution. This resulted in improved capacity retention with stable performance and a higher Coulombic efficiency, laying the ground for the use of TMD-based materials beyond Li+-ion storage devices.

Probing the interface of choline chloride-based deep eutectic solvent ethaline with gold surfaces: A molecular dynamics simulation study

Technologies involving a solvent|surface interface, such as nanotechnology, electrochemistry, and energy storage applications, are actively pursuing ecologically responsible and sustainable development practices. In response to this pressing need, deep eutectic solvents have emerged as a promising solution to bridge the gap between technological requirements and environmental concerns. In this work, we present the results of a molecular dynamics simulation study of the interface between a monocrystalline gold surface and the deep eutectic solvent ethaline, where a molar ratio of 1:2 choline chloride:ethylene glycol was used for ethaline. The simulations covered a range of temperatures from 313 K to 343 K and applied charge values ranging from 0 to ± 24 μC cm‒2. Several key interfacial properties were thoroughly analyzed, including among others, charge density profiles, radial distribution functions, hydrogen bond close contacts, and molecular orientation. Additionally, we examined how the differential capacitance varied upon the applied potential. Our findings reveal that, at neutral surfaces, all components of the solvent are present in the innermost layer, with ethylene glycol molecules being the most prevalent, followed by choline cations and a residual amount of chloride anions. For lower applied charges, this mixed composition at the boundary layer persists, despite the growing accumulation of ionic species with charges opposite to that of the electrode. As surface polarization increases, unique innermost boundary layers composed exclusively of one of the ionic species and the hydrogen bond donor molecules are observed, forming a multilayer structure, with subsequent layers enriched of paired counterions. Interestingly, even at higher applied charges, choline cations and ethylene glycol molecules tended to orient themselves in a parallel fashion toward the electrodes. Differential capacitance curves exhibited a camel-shaped behavior, suggesting a complex interplay of electrochemical processes at the DES|Au(100) interface. In summary, our study provides valuable insights into the interfacial properties of deep eutectic solvents on gold surfaces and their response to changes in temperature and potential, which are crucial for understanding and optimizing deep eutectic solvent-based electrochemical systems.

Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor.

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

Short-term and High-performance Recycling of Cathode Material from Spent Lithium Ion Batteries

In the industrial practice of spent lithium-ion battery recycling, mechanical separation, pyrometallurgy and chemical dissolution technologies are often used to achieve the separation and recovery of cathode materials, but there are problems such as low material dissociation rate, low recovery rate, high energy consumption, serious lithium loss, and impurity ion embedding. In this paper, supercritical CO2 fluid was used to strengthen the separation of the cathode active material from the aluminum foil in the spent lithium-ion battery to achieve direct and efficient recovery of the active substance. The analysis of charge and discharge, AC impedance and differential capacitance curves show that the electrode of supercritical recovered material has higher ion diffusion coefficient and better redox reversibility, so it has higher charge and discharge capacity and better cycle stability than the traditional high-temperature calcined recovered material.

(Invited) Electrochemical and Postmortem Analyses of 18650-Type Li-Ion Secondary Battery Degraded at High and Low Temperature Environments

Lithium-ion secondary batteries have been widely used as small-size and lightweight energy sources for applications of laptops, robots and electric vehicles. For realizing a high performance, the energy sources are required to have high energy density with high reliability. The batteries are charged and discharged repeatedly based on anode and cathode reactions, which both accompany side reactions to a greater or less extent. The battery degradation phenomena are well explained by kinetics of chemical reactions evaluated by non-destructive examination.1 To understand the battery degradation mechanism for developing a durable one, it is indispensable to clarify the side reactions occurred to cause the degradation. For that purpose, differential capacity curves (dQ/dV vs. V) and electrochemical impedance spectroscopy (EIS) were employed as electrochemical measurements.2 To support the mechanism estimated by the measurements, postmortem analyses were conducted using SEM, XPS and XRD. These investigations were performed to the commercially available 18650 cell consisting of Li(Ni, M)O2 (M: metal elements to partially replace Ni) as the cathode material and graphite as the anode material. The employed loads were calendar deterioration and charge/discharge cycle deterioration.3,4 Firstly, the calendar deterioration behavior of the 18650-type battery was evaluated by storing them at 80 °C for different durations. The results indicate that the battery capacity decreased with the increasing number of storage days in a high-temperature environment. From the differential capacity curves, we found that the change in the overvoltage, which was caused by the electrode reaction, that appeared at approximately 4.2 V was most significant. To analyze this phenomenon, EIS of the deteriorated battery was measured at 4.2 V, and it was confirmed that the cathode-based resistance of the battery increased significantly at this potential. Furthermore, postmortem analysis revealed that the cathode active material of the tested batteries clearly deteriorated; however, no significant change in the active material in the anode was observed. Therefore, it is considered that the cathode materials of nickel-based lithium-ion secondary batteries deteriorate by storing at 80 °C, thereby resulting in reduced battery capacity and increased cathode component resistance after storing at high temperatures.Secondly, cycle deterioration behaviors of the lithium-ion batteries in different operating temperature were studied. Compared with the deterioration in the high-temperature range, the degree of decrease in battery capacity at the low temperature was greater, and the more severe the deterioration was shown at temperatures far from the safe temperature range. From the differential capacity analysis, the peaks corresponding to the anode was significantly reduced at the voltage of the initial charging stage after cycle deterioration at low temperature. This means that the lithium ions intercalated into/deintercalated from the graphite layer decreased. In addition, EIS of the batteries before and after cycle deterioration was measured at each deteriorated temperature. Based on these, postmortem analysis was conducted with a major focus on the anode after 20-cycle deterioration at low temperature. From the SEM observation, the anode can be known to grow thicker with generating cracks and gas pockets. According to the XPS measurements, it is found that Li2CO3 and LiF are generated by decomposition of electrolytic solution at the anode surface.Overall, it was clarified that the deterioration at high temperature affected the cathode and that at low temperature affected the anode.

Integrability and dynamics of the Poisson–Boltzmann equation in simple geometries

In this article, we conduct a systematic study of the integrability and dynamics of the PB equation in plane, cylinder and sphere geometries. Specifically, for the PB equation with single species and zero permanent charges, explicit solutions are available in the plane and cylinder geometries, while non-integrability in the sphere geometry is demonstrated by the differential Galoisian approach. Furthermore, we establish a stable manifold result on a noncompact straight-line solution of the PB equation, providing a unified dynamical system perspective on the problem of single charged membrane. Notably, in the case of plane geometry, our results reveal that the concave-upward profile of the differential capacitance curve arises from the interplay between anions and cations present in the electrolyte, as opposed to the saddle structure of the PB equation.

GeO2 Nanoparticles Decorated in Amorphous Carbon Nanofiber Framework as Highly Reversible Lithium Storage Anode.

Germanium oxide (GeO2) is a high theoretical capacity electrode material due to its alloying and conversion reaction. However, the actual cycling capacity is rather poor on account of suffering low electron/ion conductivity, enormous volume change and agglomeration in the repeated lithiation/delithiation process, which renders quite a low reversible electrochemical lithium storage reaction. In this work, highly amorphous GeO2 particles are uniformly distributed in the carbon nanofiber framework, and the amorphous carbon nanofiber not only improves the conduction and buffers the volume changes but also prevents active material agglomeration. As a result, the present GeO2 and carbon composite electrode exhibits highly reversible alloying and conversion processes during the whole cycling process. The two reversible electrochemical reactions are verified by differential capacity curves and cyclic voltammetry measurements during the whole cycling process. The corresponding reversible capacity is 747 mAh g-1 after 300 cycles at a current density of 0.3 A g-1. The related reversible capacities are 933, 672, 487 and 302 mAh g-1 at current densities of 0.2, 0.4, 0.8 and 1.6 A g-1, respectively. The simple strategy for the design of amorphous GeO2/carbon composites enables potential application for high-performance LIBs.

Understanding the Effects of Electrode Material, Single Crystal Facet, and Electrolyte Ion on the Helmholtz Capacitance of Metal/Aqueous Solution Interfaces.

The comprehensive interpretation of the measured differential Helmholtz capacitance curve is vital for advancing our understanding of the interfacial structure. While several possible physical effects contributing to the Helmholtz capacitance have been proposed theoretically, combining those factors to explain the experimentally observed potential-dependent capacitance profile remains a significant challenge. In this study, we employ ab initio molecular dynamics simulations to model various metal/solution interfaces. Our investigation primarily emphasizes the substantial effect of water chemisorption on the potential-dependent behavior of the Helmholtz capacitance. Additionally, we identify other critical factors that profoundly impact the Helmholtz capacitance: (1) Ions with low hydration energy hinder the availability of surface sites for water adsorption, resulting in a diminished enhancement of capacitance from water chemisorption. (2) Using large-sized ions leads to an expansion of the Helmholtz layer, causing a decrease in the Helmholtz capacitance. (3) Metal surfaces with higher affinity for water attract water adsorption at lower potentials, resulting in a lower peak potential for the differential Helmholtz capacitance curve.

The impurity effect on the capacitance of ionic liquid-based supercapacitors: A classical density functional theory study of ionic liquids inside the spherical cavity of the porous electrode

The effect of impurities on the capacitance curve has been investigated for ionic liquid-based supercapacitors. The coarse-grained model in the framework of classical density functional theory (CDFT) was used to predict the electric double layer (EDL) formed at the concave wall of the spherical cavity of a porous electrode. Our results demonstrate that the adsorption and desorption of impurities by the cavity have a significant impact on the shape of the capacitance curve. In some cases, a bell-camel shape transition in the capacitance curve occurs due to the adsorption of impurities. By increasing the impurity concentration and its attraction with the cavity, the position of the observed maximums in the camel shape of the capacitance curves shifts toward a higher electrode potential. In addition, we have shown that impurity adsorption affects the value of the zeta potential by reducing the total net ion charges stored inside the cavity. Increasing the electrode potential to a specific value causes the total desorption of impurities; hereinafter, we call that the potential of zero impurity adsorption, ΨZIA. The amount of this potential depends on the concentration of impurities, cavity size, impurity-cavity attraction, and the shape of ions in ionic liquids. As we expected, the impurity has no effect on the amount of capacitance for electrode potentials higher than ΨZIA. Finally, the differential capacitance curves for symmetric impure ionic liquid in spherical cavities have been compared to those for slit pores. Results show the same behavior for both of them, but the effect of impurity can be ignored at lower electrode potential in the case of spherical cavities due to the more confinement effect.

Electric double layer contribution to sum frequency generation signal from Au electrode.

Understanding the electric double layer (EDL) of the metal electrode-electrolyte interface is essential to electrochemistry and relevant disciplines. In this study, potential-dependent electrode Sum Frequency Generation (SFG) intensities of polycrystalline gold electrodes in HClO4 and H2SO4 electrolytes were thoroughly analyzed. The potential of zero charges (PZC) of the electrodes was -0.06 and 0.38V in HClO4 and H2SO4, respectively, determined from differential capacity curves. Without specific adsorption, the total SFG intensity was dominated by the contribution from the Au surface and increased similar to that of the visible (VIS) wavelength scanning, which pushed the SFG process closer to the double resonant condition in HClO4. However, the EDL contributed about 30% SFG signal with specific adsorption in H2SO4. Below PZC, the total SFG intensity was dominated by the Au surface contribution and increased with potential at a similar slope in these two electrolytes. Around PZC, as the EDL structure became less ordered and the electric field changed direction, there would be no EDL SFG contribution. Above PZC, the total SFG intensity increased much more rapidly with potential in H2SO4 than in HClO4, which suggested that the EDL SFG contribution kept increasing with more specific adsorbed surface ions from H2SO4.

The nanofluidic capacitor: Differential capacitance in the absence of reservoirs.

Within the framework of the classical, mean-field Poisson-Boltzmann (PB) theory, we carry out direct numerical simulations to determine the differential capacitance of a closed nanochannel of a circular cross section, embedded in a polymeric host with charged walls and sealed at both ends by metal electrodes under an external potential bias. Our approach employs the modified PB equation, which accounts for the finite size of ions and the dependency of the electrolyte's relative permittivity on the local electric field. In view of the absence of reservoirs, the modified PB equation becomes subject to global algebraic constraints, without prior knowledge of a bulk electrolyte concentration. Equilibrium ion distributions and differential capacitance curves are investigated as functions of electrolyte properties and the surface charge density modulation. This modulation leads to asymmetric differential capacitance curves that can be tuned. More generally, our approach provides a transparent numerical framework for accurately simulating confined nanofluidic systems with new physical properties that may be exploited in novel iontronic circuit elements.

Efficient recovery and utilization of electrolytic manganese anode mud- preparation of high-performance Li1.2Ni0.2Mn0.6O2 cathode material for lithium-ion batteries

The booming electrolytic manganese industry has brought a huge amount of electrolytic manganese anode mud (EMAM). The management and value-added recovery of EMAM were very important for environmental improvement and economic benefits. In this study, EMAM is used as a cost-effective raw material. Firstly, the manganese-containing solution was obtained by reducing acid leaching with EMAM. Then, Li1.2Ni0.2Mn0.6O2 (Z-LNMO) cathode materials were prepared from a manganese-containing solution by co-precipitation process. Finally, to enhance the performance of the Z-LNMO materials, surfactants (C12H25SO4Na, C10H14N2Na2O8, C18H29NaO3S, C19H42BrN) were introduced in the preparation of precursor materials to obtain the material with higher crystallinity. ICP results showed that S/Fe ions of EMAM were retained in Z-LNMO materials. Moreover, the Z-LNMO sample shows a stoichiometric similar element content to Li1.2Ni0.18Fe0.02Mn0.6O1.96S0.04. EDS results show that S/Fe ions were uniformly distributed in Z-LNMO materials. The shift of XRD peak further indicates that the use of EMAM waste is expected to directly obtain S/Fe ions doped lithium-rich manganese-based cathode materials. The first-principle calculation shows that the S/Fe ions as the doping source make the transfer of excited electrons from the valence band to the conduction band easier, thereby enhancing the conductivity of the materials. The CV curve and differential capacity (dQ/dV) curve show that the Z-LNMO material prepared by EMAM inhibits the excessive oxidation of Li2MnO3 to a certain extent, thus providing excellent reversible capacity and stabilizing the material structure. Electrochemical evaluation reveals that the Z-LNMO materials prepared with the EMAM exhibit superior cycling stability (112% after 100 cycles at 0.1C). Moreover, the introduction of C10H14N2Na2O8 (EDTA-2Na) and C12H25SO4Na (SDS) in the preparation of precursor materials can improve the first discharge-specific capacity and stability of materials by modifying the morphology of precursor materials (refine particles). Among them, the materials modified by EDTA-2Na show the best performance, and the particle size is about 200 nm. Compared with the original materials, the improved first discharge-specific capacity (218.6 mAh g−1 at 0.1C) and capacity retention (245.8 mAh g−1 after 100 cycles) of the materials modified with EDTA-2Na were improved. The EIS results also confirm that the introduction of EDTA-2Na effectively decreases the charge transfer resistance and facilitates the Li diffusion of the as-prepared cathode materials. This work confirmed the possibility of extracting manganese from EMAM to prepare lithium-rich manganese-based cathode materials, and provided a new promising way for the efficient value-added utilization of EMAM.

Fructus cannabis protein extract powder as a green and high effective corrosion inhibitor for Q235 carbon steel in 1 M HCl solution

The Fructus cannabis protein extract powder (FP), was firstly used as a green and high effective corrosion inhibitor through a simple water-extraction method. The composition and surface property of FP were characterized by FTIR, LC/MS, UV, XPS, water contact angle and AFM force-curve measurements. Results indicate that FP contains multiply functional groups, such as NH, CO, CN, CO, etc. The adsorption of FP on the carbon steel surface makes it higher hydrophobicity and adhesion force. The corrosion inhibition performance of FP was researched by electrochemical impedance, polarization curve and differential capacitance curve. Moreover, the inhibitive stability of FP, and the effects of temperature and chloride ion on its inhibition property were also investigated. The above results indicate that the FP exhibits excellent corrosion inhibition efficiency (~98 %), and possesses certain long-term inhibitive stability with inhibition efficiency higher than 90 % after 240 h immersion in 1 M HCl solution. The high temperature brings about the FP desorption on the carbon steel surface, while high concentration of chloride ion facilitates the FP adsorption. The adsorption mechanism of FP follows the Langmuir isotherm adsorption. This work will provide an insight for protein as a green corrosion inhibitor.

Hierarchical porous Co-CoO@NC hollow microspheres with capacity growth by reactivation of solid-electrolyte interface films

Transition metal oxide (TMO)-based electrodes exhibit increased capacities, yet the mechanism behind the true cause of capacity in such materials remains unclear. Herein, hierarchical porous and hollow Co-CoO@NC spheres assembled by nanorods with refined nanoparticles and amorphous carbon have been synthesized by a two-step annealing approach. A temperature gradient-driven mechanism is revealed for the evolution of the hollow structure. Compared with the solid CoO@NC spheres, the novel hierarchical of Co-CoO@NC can fully utilize the interior active material by exposing both ends of each nanorod into electrolyte. The hollow interior provides extra space for the volume variation, leading to an up-trend capacity of 919.3 mAh g−1 at 200 mA g−1 over 200 cycles. Differential capacity curves disclose that solid electrolyte interface (SEI) films reactivation partly contributes to increasing reversible capacity. The introduction of nanosized Co particles benefits the process by participating in the transformation of SEI components. This study provides a guide for constructing anodic material with exceptional electrochemical performance.

Oxygen activity excited thiophene-diketopyrrolopyrrole-based organic molecules as anode for lithium-ion batteries with enhanced interfacial storage capacity and long cycling capability

In this work, we synthesized various thiophene-diketopyrrolopyrrole-based (DPP-2T-based) molecules by Knoevenagel condensation and halogenation reaction, which applied DPP-2T-based small molecules as anode for lithium-ion batteries (LIBs) for the first time. We studied the different substituents on battery performance and explored the charge–discharge mechanism for DPP-2T-based molecules. DPP-2T-based molecules exhibit different electrochemical properties by substituting various groups at nitrogen positions, where DPP-2T-TA (tert-butyl acetate) electrode possesses better capacity and long-term cycle performance than that of DPP-2T-H (no substitution) and DPP-2T-C6 (hexane) electrode. DPP-2T-TA electrode present an unusual upward tendency for its discharge specific capacity during cycling, which displays that of 530 mAh/g (1000 mA g−1) after 2000 cycles and the capacity retention reaches up to 142.5 %. It is better than those of DPP-2T-H (439 mAh/g, 108.1 %) and DPP-2T-C6 (423 mAh/g, 113.1 %) electrode, which is ascribed to oxygen activity excitation in tert-butyl acetate. This moment, the energy density and power density of DPP-2T-TA electrode are 346 Wh kg−1 and 699 W kg−1, respectively. To further explain this phenomenon, we used many ex-situ measurgin methods such as differential specific capacity (dQ/dV) curves, scanning electron microscope (SEM), Fourier transform infrared (FT-IR) spectrometer, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and natural bond orbital (NBO) analysis to characterize the detailed lithiation/delithiation process of DPP-2T-TA electrode. The results suggest that the unusual capacity amplification for DPP-2T-TA electrode attributed to lithium-ions inserted into unsaturated carbons and increases its interfacial storage capacity. We believe that the synthesis of DPP-2T-based organic molecules can provide a novel strategy for the development of organic electrode materials for LIBs.

Soybean extract firstly used as a green corrosion inhibitor with high efficacy and yield for carbon steel in acidic medium

The soybean extract (SE) was firstly investigated for corrosion inhibition characteristics on carbon steel in sulfuric medium. The weight loss and electrochemical methods were used to research the inhibition performance and adsorption properties of SE in combination with halide ions. Results showed the SE + I - inhibitors possessed outstanding corrosion inhibition efficiency, ~98%. Furthermore, the effective compound of SE was identified to be responsible for the corrosion inhibition effect. The AFM , water contact angle and XPS measurements confirm the formation of a corrosion inhibitor film on carbon steel. Combining differential capacitance curves, corrosion inhibition mechanism was further interpreted. Good inhibition efficiency, green extract solvent, facile extraction procedure, high yield and environment sustainability, endow SE as novel corrosion inhibitor with a promising future for industrial application. ● Soybean extract (SE) was firstly utilized as an eco-friendly, promising corrosion inhibitor. ● SE exhibited excellent inhibition efficiency (~98%) in combination with I - for carbon steel in sulfuric solution. ● Characteristic adsorption of I - would make for protonated SE adsorption on positively charged carbon steel surface.

New crowding states of ionic liquid induced by configuration change of ion adsorption on charged electrode

In ionic liquid based electrical double layer, crowding state is used to explain the unique shape of the differential capacitance curve. However, the differential capacitance values are still significantly underestimated and an impractically high electrode potential is required to observe the crowding state in simulations. In this study, based on the electrode charge distribution obtained from joint density functional theory calculations, we successfully predict the differential capacitance curve with both shape and magnitude correct. At the same time, we find a new type of ionic liquid crowding state at low potentials within the electrochemical window, which is induced by adsorption state transition of organic ions. These results provide new insights in understanding ionic liquid at electrified interfaces.

Operando Thermo‐Electrochemical Diagnostics with Au, TiO2, and LiFePO4 as Reference Electrodes in Li‐Ion Pouch Cells

Thermo‐electrochemical monitoring of state‐of‐the‐art rechargeable Li‐ion batteries during operation is critical for safety and reliability, yet it remains quite limited, relying on conventional full cell measurements with in situ techniques lagging behind. Here, the feasibility of incorporating Au, LiFePO4, and TiO2 electrochemical sensors into commercially available pouch cells, in conjunction with thermocouples to track the cell behavior during galvanostatic cycling at varying operating temperatures is demonstrated. It is found that Au provides the best in operando based electrochemical diagnostics for stable thermo‐electrochemical performance via long‐term cyclability (>1000 h), differential capacity curves, and at 25, 40, and 60 °C, followed by TiO2, and LiFePO4. Furthermore, cell temperature changes are analyzed alongside reference electrode readings, demonstrating the advantages of the complementary nature of these techniques for cell thermodynamics. Collectively, these findings offer deep insight into an improved battery management approach for next‐generation lithium‐ion energy storage systems.

Electrochemical investigation of ion-beam sputter-deposited carbon thin films for Li-ion batteries

The C-rate capability of 230 nm- and 16 nm-thin ion-beam sputter-deposited amorphous carbon films, an interesting class of carbonaceous material for lithium-ion batteries, was investigated up to Li-platting. Stepwise ascending and descending constant Li+ currents after each fifth cycle, followed by hundreds of cycles with the highest current were applied. The carbon films show similar cycling with irreversible losses during the first five cycles, followed by reversible cycling with a capacity close to that of graphite. The capacity is significantly lower at high currents; however, it is restored for subsequent cycling again at low currents. Differential charge and differential capacity curves reveal three Li+ uptake and three Li+ release peaks located between 0 and 3 V. Irreversible as well as reversible Li bonding can be associated with all these peaks. Irreversibly bonded Li can be found at the surface (solid electrolyte interphase) and in the bulk of the carbon films (Li trapping). Reversible Li bonding might be possible inside the carbon films in graphite-like nano-domains and at defects. The thinner film reveals a more pseudo-capacitive cycling behavior, pointing to enhanced Li kinetics.Graphical abstract

The effects of Fe/Al co-modified Ni-rich Li[Ni1-x-yCoxMny]O2 for enhancing electrochemical performance

Various impurities such as Cu, Al, and Fe can be incorporated in the leachate of spent lithium-ion batteries (LIB) after a conventional acidic leaching process. Here, we synthesize Fe/Al co-modified Ni-rich Li[Ni1-x-yCoxMny]O2 (NCM) cathode materials via a coprecipitation reaction for Fe doping and the subsequent calcination process for additional Al doping. The purpose of this work is to seek a potential of utilizing the positive effects of impurity elements for improving the LIB performance of Ni-rich NCM. The physicochemical properties of cathode materials are compared by scanning electron microscopy, energy dispersive spectroscopy, inductively coupled plasma optical emission spectroscopy and X-ray diffraction analysis. Electrochemical analyses including galvanostatic intermittent titration technique, differential capacity curves from cycling data are systematically performed to assess the effects of the sequential doping processes. It is found that an Al doping content of 1% in Fe-doped NCM is optimal in terms of rate and cycle performances. This study proposes a rational design for upgrading the resynthesized Ni-rich NCM cathode materials from the leachate of spent LIBs.