Electrode Thickness Research Articles

Synthesis and performance of binder-free porous carbon electrodes in electrochemical capacitors.

Porous binder-free carbon electrodes were obtained by impregnating cellulose filter papers with phenolic resin through soft-salt template synthesis and thermal pyrolysis. These self-standing electrodes were used directly in a supercapacitor device. To understand the impacts of filter paper (FP) thickness on carbon filter paper (CFP) morphology, porosity, and surface functionalities, five different materials were examined. The CFP electrode thickness was adjusted linearly with the FP thickness to produce electrodes ranging from 100 to ∼800 μm. As the thickness of the CFP increased, there was an increase in the specific surface area and oxygen-based functionalities. Electrochemical testing in a 1 M KOH aqueous electrolyte demonstrated that electrode thickness played a key role in electrochemical capacitor (EC) performance, i.e., capacitance enhances linearly when the electrode becomes thinner. For low thicknesses (<280 μm), the capacitance and rate capability decreased slightly with increasing thickness; for high thicknesses, the performance drastically degraded despite the high specific surface area and oxygen surface functionalities. This effect was amplified at high regimes, indicating that high electrode thicknesses and fiber diameters limited electrolyte diffusion in the applied synthesis conditions. Thus, the high material porosity was inaccessible to ion adsorption. Consequently, thinner electrodes showed the highest capacitance (197 F g-1 at 0.1 A g-1) and rate capability (81%) values, exceeding those of their traditional binder-electrode counterparts prepared under similar conditions. Finally, for alkali metal hydroxide solutions, 1 M KOH exhibited a cation match with the salt template (KCl), while CsOH achieved additional capacitance retention (88% at 10 A g-1). Complex ion adsorption mechanisms were observed through a quartz crystal microbalance.

Design and fracture mechanics of lithium-ion batteries

Fracture mechanics plays a crucial role among the mechanisms causing damage, meant as capacity fade, in lithium-ion batteries. Mechanical stresses arise in the electrode active material particles because of the interaction of lithium ions with electrode microstructure during battery operation. The stresses lead to fractures growth in the electrode, which accelerates detrimental chemical reactions. In this work, a modelling approach is presented to assess the fracture level in the electrode microstructure, evaluating the influence of the current delivered by the battery, and electrode design characteristics, such as the electrode thickness, the electrode active material fraction and the size of the electrode micro-particles. The results show that stress intensity factor linearly increase with the current delivered by the battery. Furthermore, thicker electrodes, greater active material fraction and greater electrode micro-particles represent a more detrimental condition from the fracture mechanics point of view. The results provide a practical electrode design guideline for electrode manufacturing, especially for choosing the right particle size in the electrode powder, the electrode thickness and its composition to limit fracture according to the current expected to be delivered by the battery.

Hetero Acoustic Layer Surface Acoustic Wave Resonator Composed of LiNbO3 and Quartz.

In this work, a shear-horizontal (SH) mode surface acoustic wave (SAW) resonator based on LiNbO3 (LN)/Quartz (Qz) hetero acoustic layer (HAL) structure was studied by simulation and experiment. By this HAL structure, the displacement and electric displacement are well confined in the piezoelectric layer. A lower mechanical loss of Qz than that of lossy amorphous SiO2 further enhances the quality ( Q ) factor. In addition, a negative temperature coefficient of frequency (TCF) of LN is compensated by selecting the crystalline orientation of Qz with a positive TCF. According to simulation results, the Euler angles of (0°, 101°, and 0°) and the normalized thickness of 0.2-0.3 λ (wavelength) for LN are selected to obtain a higher impedance ratio ( Z -ratio) and bandwidth (BW). The Euler angles of (0°, 160°, and 90°) for Qz are selected to obtain the positive maximum TCF. The fabricated resonator exhibits a Z -ratio of 95 dB and a BW of 15.9% in the 700 MHz range. The fit figure of merit (FoM) reaches 410, which is the best level ever reported for an LN-based resonator. The TCF of the resonator is -77 ppm/°C at anti-resonance frequency. A group of resonators composed of LN and LN/Qz with thin and thick electrodes were fabricated to further illustrate the good performance of LN/Qz. The LN/Qz HAL SAW resonator demonstrated in this work exhibits a high Z -ratio, low TCF, and wideband, which has the potential for high-performance wideband filters with steep passband and good temperature characteristics.

A strategy to build high-performance thick electrodes for lithium-ion batteries with enhanced compressive modulus and regulated tortuosity in the phase-inversion process

By tuning the composition of the non-solvent bath used in the non-solvent induced phase inversion process for fabricating thick and low-tortuosity battery electrodes, optimal electrochemical performances and compressive modulus were achieved.

Influence of electrode characteristics on the laser cutting of lithium-ion battery anodes

Laser cutting of lithium-ion battery (LIB) electrodes is a highly flexible process receiving wider industry adoption. However, the manufacturing of LIB electrodes involves numerous steps, where variations can impact the electrode cutting process. Additionally, material chemistry is identified as one of the influential parameters affecting laser cutting. There are numerous combinations of state-of-the-art laser sources and anode materials. Conducting extensive examinations would be necessary to choose the optimized laser source for electrode cutting. On the other hand, understanding the influence of thickness and chemistry of anode materials on the processability is important. This study examines methodologically the characteristics of the anodes and their impact on laser cutting performance. The effects of electrode thickness and active material chemistry on the laser cutting performance using a ps-pulsed laser with burst mode capability were studied. Examinations were conducted on four various combinations of LIB anodes, encompassing four different thicknesses and two distinct chemistries.

Monolithic porous carbon membrane-based hybrid electrodes with ultrahigh mass loading carbon-encapsulated Co nanoparticles for high-performance supercapacitors

Monolithic carbon membrane-based hybrid electrodes with an ultrahigh active mass loading are developed by carbonization of Co-adsorbed Pleurotus eryngii (PE) for directly assembling a supercapacitor with high energy density and cycling stability.

Unleashing the high energy potential of zinc-iodide batteries: high-loaded thick electrodes designed with zinc iodide as the cathode.

The realization of high energy is of great importance to unlock the practical potential of zinc-iodine batteries. However, significant challenges, such as low iodine loading (mostly less than 50 wt%), restricted iodine reutilization, and severe structural pulverization during cycling, compromise its intrinsic features. This study introduces an optimized, fully zincified zinc iodide loaded onto a hierarchical carbon scaffold with high active component loading and content (82 wt%) to prepare a thick cathode for enabling high-energy Zn-I2 batteries. The synergistic interactions between nitrogen heteroatoms and cobalt nanocrystals within the porous matrix not only provide forceful chemisorption to lock polyiodide intermediates but also invoke the electrocatalytic effects to manipulate efficient iodine conversion. The ZnI2 cathode could effectively alleviate continuous volumetric expansion and maximize the utilization of active species. The electrochemical examinations confirm the thickness-independent battery performance of assembled Zn-I2 cells due to the ensemble effect of composite electrodes. Accordingly, with a thickness of 300 μm and ZnI2 loading of up to 20.5 mg cm-2, the cathode delivers a specific capacity of 92 mA h gcathode-1 after 2000 cycles at 1C. Moreover, the Zn-I2 pouch cell with ZnI2 cathode has an energy density of 145 W h kgcathode-1 as well as a stable long cycle life.

In situ formation of GeSe nanocrystals in carbon nanofiber network supports for ultra-thick sodium ion storage anodes via enhanced carrier transport

Metallic selenides are considered as a promising class of alternative anodes for sodium ion batteries due to the high capacity conferred by their hybrid sodium storage mechanism. However, the low reversible volume expansion induced by larger size sodium ion storage greatly limits their cycle life, theoretical capacity mining and electrode thickness enhancement. Herein, a germanium selenide/carbon nanofibers (GeSe/CNFs) sodium storage anode is first reported by taking advantage of the encapsulation and dispersion of electrostatic spinning technology, i.e., GeO2 is first mixed with polyacrylonitrile and electrospun to form a wrapped structure, followed by in situ pre-oxidation, carbonization, and selenization. For the optimized electrode, it delivers a high capacity up to 567 mAh g−1 based on the mass of GeSe (320 mAh g−1 based on GeSe/CNFs), reaching > 80% of its theoretical capacity, with a > 2300 cycles cycling stability at 1.0 A g−1. In addition, its capacity barely decays after 700 cycles at near commercial electrode thickness (115 µm, the capacity retention > 47–63% even at the ultrahigh thickness of 187–219 µm). This study opens up new possibilities for the development of Se-based sodium storage anode materials with long life, high capacity and ultra-thick electrode that are expected to meet practical needs.

Architecting the Microenvironment Skeleton of Active Materials in High-Capacity Electrodes by Self-Assembled Nano-Building Blocks.

In analogy to the cell microenvironment in biology, understanding and controlling the active-material microenvironment (ME@AM) microstructures in battery electrodes is essential to the successes of energy storage devices. However, this is extremely difficult for especially high-capacity active materials(AMs) like sulfur, due to the poor controlling on the electrode microstructures. To conquer this challenge, here, a semi-dry strategy based on self-assembled nano-building blocks is reported to construct nest-like robust ME@AM skeleton in a solvent-and-stress-less way. To do that, poly(vinylidene difluoride) nanoparticle binder is coated onto carbon-nanofibers (NB@CNF) via the nanostorm technology developed in the lab, to form self-assembled nano-building blocks in the dry slurry. After compressed into an electrode prototype, the self-assembled dry-slurry is then bonded by in-situ nanobinder solvation. With this strategy, mechanically strong thick sulfur electrodes are successfully fabricated without cracking and exhibit high capacity and good C-rate performance even at a high AM loading (25.0mgcm-2 by 90wt% in the whole electrode). This study may not only bring a promising solution to dry manufacturing of batteries, but also uncover the ME@AM structuring mechanism with nano-binder for guiding the design and control on electrode microstructures.

Stretchable Zn‐Ion Hybrid Capacitor with Hydrogel Encapsulated 3D Interdigital Structure

AbstractTo enhance the areal energy density of current flexible energy storage devices, hybrid capacitors combining the advantages of supercapacitors and batteries are proposed and further enhanced by incorporating the 3D interdigital structure design. However, uneven electric field distribution and hindered ion diffusion kinetics due to the non‐electroactive components in these devices limit the enhancement of areal electrochemical performances when expanding the electrodes longitudinally. Herein, hydrogels with high ionic conductivity and high mechanical stability are designed to accommodate Zn2+‐containing electrolytes and integrated with Ti3C2Tx‐MXene electrodes to assemble flexible Zn‐ion hybrid capacitors (ZIHCs). Fully encapsulated by ionic conductive hydrogels, 3D interdigital electrodes enable omnidirectional ion transport and unimpeded ionic accessibility, facilitating adequate electrode reactions, rapid energy storage, and uniform energy distribution. Hence, the all‐hydrogel‐encapsulated ZIHC achieved a 50‐fold increase in capacitance with a quadrupled electrode thickness, exhibiting a large areal capacitance of 1432 mF cm−2 and an energy density of 389.7 µWh cm−2 without sacrificing power density and rate performance. Finite element simulations further illustrate the uniform distribution of potential, electric field intensity, and energy density in this structure. In addition, the device shows great stability under deformation, excellent adhesion, and underwater workability, demonstrating great promise for next‐generation wearable energy storage devices.

Study on morphology and N-doping effects of carbon cathodes for zinc-ion hybrid supercapacitors

Aqueous zinc-ion hybrid supercapacitors (ZHSCs) are spotlighted as next-generation energy storage devices; however, the carbon cathodes usually dictate their performance. Herein, we aim to optimize the electrochemical performance of the carbon cathodes through particle morphology control along with porosity and surface-active site control. N-doped spherical mesocelluler carbon foam (S-MCF-N) is synthesized using a mesoporous silica template, and spherical mesocelluler carbon foam (S-MCF) and irregularly shaped mesocellular carbon foam (MSUF-C) are synthesized as control samples. Consequently, S-MCF-N shows the best ion storage capability with its large surface area, with active sites that cause redox reactions and high electrical conductivity. In particular, the spherical morphology of S-MCF-N achieves a higher tap density and can reduce the electrode thickness, which decreases the diffusion length for electrolyte migration and electrode resistance. Therefore, S-MCF-N as the cathode materials deliver a specific capacitance of 208 F g−1 at 0.2 A g−1 and superb cycling stability of up to 7000 cycles at 5 A g−1 without capacity decay. Furthermore, benefiting from the reduced electrode thickness, S-MCF-N has a 112 % higher volumetric energy density than MSUF-C while showing a 42 % higher gravimetric energy density.

Understanding the Role of Electrode Thickness on Redox Flow Cell Performance**

AbstractThe electrode thickness is a critical design parameter to engineer high performance redox flow cells by impacting the available surface area for reactions, current and potential distributions, and required pumping power. To date, redox flow cell assemblies employ repurposed off‐the‐shelf fibrous electrodes which feature a broad range of thicknesses. However, comprehensive guidelines to select the optimal electrode thickness for a given reactor architecture remain elusive. Here, we investigate the effect of the electrode thickness in the range of 200–1100 μm on the cell performance by stacking electrode layers in four different flow cell configurations – Freudenberg paper and ELAT cloth electrodes combined with flow‐through and interdigitated flow fields. We employ a suite of polarization, electrochemical impedance spectroscopy and pressure drop measurements together with pore network modeling simulations to correlate the electrode thickness for various reactor designs to the electrochemical and hydraulic performance. We find that thicker electrodes (420 μm paper electrodes and 812 μm cloth electrodes) are beneficial in combination with flow‐through flow fields, whereas when using interdigitated flow fields, thinner electrodes (210 μm paper electrodes and 406 μm cloth electrodes) result in a better current density and pressure drop trade‐off. We hope our findings will aid researchers and technology practitioners in designing their electrochemical flow cells under convective operation.

O, N‐Codoped, Self‐Activated, Holey Carbon Sheets for Low‐Cost And High‐Loading Zinc‐Ion Supercapacitors

AbstractLow‐cost and high‐loading cathodes are crucial for practical application of zinc‐ion supercapacitors (ZICs) but achieving optimal performance in high‐loading electrodes faces challenges due to sluggish ion transport, increased resistance, and unstable structure. Guided by theoretical calculations, high‐loading carbon cathodes based on holey activated carbon sheets (HACS) are fabricated from a carefully chosen molecule. A simple pyrolysis‐leaching treatment transformed the molecule into HACS with large surface area, hierarchical porous structure, and electroactive oxygen/nitrogen dopants. When combined with an aqueous binder, the optimized HACS‐based high‐loading electrode (16.1 mg cm−2) exhibits high‐capacitance (454 F g−1 ) and fast‐rate (1 A g−1) characteristics under lean electrolyte (6.2 µL mg−1). More impressively, HACS is dry‐pressed into free‐standing thick electrodes up to 35.4 mg cm−2 and corresponding practical ZIC under limited Zn and low N/P ratio demonstrates ultrahigh areal capacitance (9 F cm−2) and energy density (3.47 mWh cm−2). The outstanding performance can be attributed to fast ion transport enabled by through‐plane pores of HACS, as well as abundant double‐layer and redox‐active surfaces from favorable heteroatom‐doped porous nanosheets. With its cost‐effectiveness, elemental abundance, and structural tunability, this molecular carbon strategy offers a platform for making self‐activated carbon electrodes at the molecular level towards practical supercapacitors.

NMC811 Electrodes with High Mass Loadings Enabled By Non-Solvent Induced Phase Inversion

Higher energy density Li-ion batteries that can enable longer driving ranges are currently subject to intensive research interest. Increasing the thickness of electrodes and reducing the proportion of inactive components per cell is one way to achieve this. In thick electrodes, a higher electronic and ionic overpotential and mechanical failure (cracking, delamination etc.) induced by binder migration during the drying process leads to sluggish (dis)charge performance or even cell failure respectively. Here we report non-solvent induced phase inversion as a scalable, effective method to arrive at thick NMC811 electrodes.By tuning the non-solvent properties and other processing conditions to be compatible with Ni-rich electrodes, it was possible to obtain NMC811 electrodes with mass loadings up to 11 mAh/cm2 (single sided) which outperform their conventional processed counterparts in Li/NMC half cells. This improvement could be attributed to the altered carbon-binder structure, which improves the pore connectivity and lowers the electrode tortuosity factor. The rapid solvent removal also reduces the long-range binder migration during drying. With imaging and porosimetry techniques we show the structural change which leads to the observed improved electrochemical performance. The method also shows good compatibility with double sided slot-die electrode coating procedure, making this a technique with potentially high industrial feasibility towards making thicker NMC811 and other electrodes.

Printed for Performance: Harnessing the Electrochemical Potential of Prussian White Thick Electrodes Via 3D Printing

The development of Prussian White (PW) water-based inks for thick, structured electrodes will be presented. Thicker electrodes (>100 µm) have the potential to have both high power and higher energy, however there is a trade-off between both. When increasing the thickness of a standard two-dimensional (2D) electrode this lengthens the ionic pathways, limiting our rate of ionic diffusion. To overcome these restraints, we can introduce a 3D architecture within the electrode microstructure to maintain favourable, short ionic pathways, whilst increasing the active surface area of the electrode.Here, we are investigating a high capacity, Na-ion material that is aqueous processed, to fabricate thick electrodes that can provide a high-capacity cell. PW is a Na-ion hexacyanoferrate crystal (NaxFe[Fe(CN)6]), where the redox properties of the iron atoms can be visualised in the voltage profile. PW has potential to be used for applications that Li-ion may not be necessarily required for, as it can reach a comparable capacity of 160 mAh/g, similar to that of LFP (Lithium iron phosphate). Moreover, this Na-ion material can overcome the supply and ethical issues that are currently associated with Lithium. Thus, PW here has the capability to become part of the next generation of batteries.To fully understand the PW electrochemical behaviour, the material underwent a baseline process. From our initial research, the moisture sensitivity of PW was observed clearly in the electrochemistry where sloping voltages and poor cell capacities were seen. Therefore, an effective drying process was developed to prevent the leaching of water molecules, thus allowing the full usage of the PW and now achieving the theoretical capacity of 160 mAh/g.PW was also examined via different mass loadings from 1-3 mAh/cm2, where we successfully demonstrated how thicker electrodes can provide a comparable cell capacity throughout, as shown by our electrochemical data. However, the limitations of thicker electrodes (3 mAh/cm2) were detected in the rate test data, where the PW experiences no capacity retention at high C rates, due to the limits of the elongated diffusion pathways in a 2D electrode. The GITT (galvanostatic intermittent titration technique) data visualised the diffusion mechanism of the thick PW electrodes, whilst from the EIS (electrochemical impedance spectroscopy) current density values were obtained.Thus, PW is being investigated here to see whether it has potential to be 3D printed via extrusion to create our desired structured electrodes. To understand how effectively an electrode ink will be printed, the rheology properties of the ink is also characterised here. The different parameters of achieving a high viscous ink at low shear rates, will also be discussed to allow the fabrication of thick, structured PW electrodes.With the initial 3D printed electrodes, the rate test data seen in Figure 1., the 3D printed electrodes hold a comparable capacity at low C rates, however, it is limited electrochemically at high C rates. It is clear that further optimisation of the ink rheology is needed and this can be done with the introduction of CNT (carbon nano tubes), higher solid content or using a secondary solvent to create an ink that is printable. Once structured electrodes are achieved through tuning the ink rheology and printing parameters, these electrodes will then be compared electrochemically against our standard 2D electrode, with hopes to overcome the ionic limits of thick 2D electrodes.The wider impact is that 3D printing via extrusion can be used for other materials, once a rheological checklist is passed. Thus, 3D printing has the potential to produce a cell with both high energy and power. The combination of Prussian White and 3D printing offers a promising method to developing novel electrode materials with enhanced performance for a range of electrochemical applications. Figure 1. Discharge capacities of PW9 (doctor blade) & 14.1 (3D printed) at different thicknesses, Na-half cell cycled between 1.3-3.8 V and discharged at C/5, C/2, C, 2C & 5C. Figure 1

Influence of Binders on Printability and Battery Performance of Thick 3D-Printed Positive Electrode for Li-Ion Batteries

Nowadays, lithium-ion batteries have been considered to be the superior commercial batteries in the market, thanks to their high energy density (until ca. 250 Wh/kg at cell level) and low fabrication cost (actually < 0.3 Euro/Wh at cell level). However, due to the recent global push for electrification, there is a constant need to further improve battery performance while minimalizing the cost. Besides the development of new battery materials, engineering the electrodes could be a valid solution to meet such demand. By increasing the electrode thickness to 200 µm, the active material loading in the electrode is expected to increase to over 30 wt%, hence enhancing the energy density and limiting the cost at the cell level. Nevertheless, thick electrodes fabricated by conventional coating methods suffer severely from weak mechanical strength and delamination as well as long and highly-tortuous Li-ion diffusion path, sluggish electrochemical kinetics, and poor charge/discharge rate performance. Recently, great attention has gone to 3D printing (additive manufacturing) with the promise to overcome these limitations, thanks to the porous structures of the 3D- printed electrodes.In this work, we present 450 µm thick lithium iron phosphate (LFP) electrodes produced by 3D micro-extrusion with a superior LFP loading of ca. 30 mg/cm2 and a high areal capacity of ca. 4.2 mAh/cm2. Two commercial polymeric binders were employed, including carboxymethyl cellulose (CMC) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). It is concluded that both CMC and PEDOT:PSS are essential to obtain well-printed 3D electrodes. While CMC contributes significantly to the mechanical strength of the 3D-printed LFP electrodes, it is of great importance to add PEDOT:PSS to avoid intra-fiber cracks. Furthermore, the electrodes containing both binders have shown significantly improved specific capacity at different C-rates compared to those formulated from either CMC or PEDOT:PSS. Further analyses give insights into charge transfer resistance, tortuosity and electrical conductivity of the investigated electrodes. Additionally, we highlight the difference in the battery performance of 3D-printed electrodes and their standard doctor-bladed counterparts.Reference(1) Kuang, Y.; Chen, C.; Kirsch, D.; Hu, L. Thick Electrode Batteries: Principles, Opportunities, and Challenges. Adv. Energy Mater. 2019, 9 (33), 1901457. https://doi.org/10.1002/aenm.201901457.

Contribution of numerical modelling to design oxygen electrode for micro-solid oxide cells: A case study of high-performance nano-columnar La2NiO4 thin films

This article presents a numerical study that aims to guide the development of nano-columnar La2NiO4 thin films as high performing oxygen electrodes for micro-Solid Oxide Cell (μSOC). La2NiO4 thin films were deposited by Pulsed Injection Metal Organic Chemical Vapor Deposition (PI-MOCVD) and their performance is characterized by Electrochemical Impedance Spectroscopy (EIS) and Electrical Conductivity Relaxation (ECR). Excellent performance is obtained for 1 μm thick film with an Area Specific Resistance (ASR) of 0.12 Ω cm2 at 600 °C, making them suitable for low-temperature μSOCs. A Finite Element Method (FEM) model has been implemented and used to predict the ASR and impedance spectra in order to guide further optimization of the film thickness and morphology. After experimental validation of the model using various electrode thicknesses, the optimal value is estimated to be between 3 and 4 μm for the temperature range of 450–650 °C. For each temperature and thickness, the model can clearly predict whether the electrode is limited by the surface exchange reactions or the bulk diffusion. The simulations revealed that the gas diffusion process is not rate-limiting when the electrode thickness is below 5 μm and the temperature is below 650 °C. Furthermore, a geometrical parameters analysis study allowed to determine the ideal nano-columnar morphology for this promising electrode composition.

Fabrication and Model-Guided Design of Thick-Format Lithium Ion Electrodes

To enable the use of renewable energy in place of fossil fuels for grid-scale electricity generation, low-cost, long-duration (3 hr charge, 12 hr discharge) energy storage systems must be developed to bridge the intermittency of these resources. This present work explores the adoption of thick-format lithium-ion electrodes to minimize inactive material cost and achieve <$100/kWh of storage. Thick lithium iron phosphate (LFP) pellets were fabricated using a dry pressing process, and the effects of composition, thickness/porosity balance, and electrolyte choice on the performance of the LFP half cells were explored. LFP electrodes with thicknesses up to 1 mm and capacities up to ~15 mAh/cm2 exhibited good rate performance (~90% utilization at a C/10 rate) and stable cycling over 100 cycles. A physics-based model was used to study transport within these thick electrodes. As thickness increases, large concentration gradients form across the electrode, with salt depletion at the current collector and salt accumulation at the separator exceeding the solubility limit of conventional electrolytes. Unlike fast-charging thin electrodes where kinetic overpotentials dominate inefficiencies, thick electrode performance is diffusion-limited, suggesting that methods for minimizing tortuosity and increasing ionic conductivity will be key in enabling low-cost thick electrodes. Figure 1

3D Morphological Evolution of Dead and Active Lithium at the Graphite-Separator-Electrolyte Interface during Fast Charging

Lithium-ion batteries (LIBs) have profoundly advanced the development of electric vehicles (EV). However, one of the remaining bottlenecks in the widespread adoption of EVs is the long charging time required for commercial LIBs. There is a global push towards extreme fast charging (XFC) to reduce charging times to 10-15 minutes.1 However, XFC results in rapid degradation in the electrochemical performance of batteries, mainly due to Li plating. This plating refers to the loss of active Li, where it either becomes dead meaning electronically disconnected after plating on the anode, or it becomes inactive due to the irreversible reaction of Li with the electrolyte to form a solid electrolyte interphase. Dead Li contributes significantly to capacity loss during XFC;3 thus, differentiating dead and inactive from active Li is extremely important to unravel their link with battery capacity fade. However, owing to the complexity of these competing electrochemical reactions in addition to the intricacy of detecting Li at the buried graphite-separator-electrolyte interface, a comprehensive understanding of Li plating remains a challenge.In this work, we conducted non-destructive, in-situ three-dimensional (3D) characterization of dead and active Li on energy-dense graphite electrodes during XFC using high-resolution neutron micro-computed tomography (µCT). Thick or industry-relevant (negative and positive electrodes ~ 130 μm) graphite electrodes are promising battery materials because they store higher amounts of energy. However, they plate Li faster and at lower XFC cycle numbers, and hence experience higher loss in battery capacity due to Li plating.To characterize Li plating on thick graphite electrodes, we performed high-resolution neutron µCT4 (pixel size: ~ 5.74 µm; effective spatial resolution: ~10-15 µm) at the ICON beamline5 at the Swiss Spallation Neutron Source at the Paul Scherrer Institute. Neutron µCT was used because it provides high relative neutron contrast between Li and C (~ 71.87: 5.55 barns6) for in-situ 3D imaging. Therefore, to leverage high-resolution neutron µCT for in-situ Li detection, we designed a neutron-friendly LIB. Our designed battery enabled excellent neutron transmission at the graphite-separator-electrolyte interface while exhibiting similar XFC performance at a 6C charging-rate between 3.0 and 4.1V. We imaged three batteries in the charged and discharged states under the following conditions: 4 cycles at 1C charging; 4 and 6 cycles at 6C charging. Here, we note that since inactive Li was below the spatial resolution of the ICON beamline, only dead, active, and plated Li were investigated in this work.Our results reveal changes in dead Li morphologies from isolated deposits after 4 cycles at 1C charging to increasingly mossy-like dense deposits covering the entire circumference of the graphite electrode after 6 cycles at 6C charging. Furthermore, as the XFC cycle number increased from four to six at 6C charging, the amount of dead Li increased significantly. After 6 cycles at 6C charging, we also observed dead Li outgrowths in a circular geometry onto the separator around the graphite electrode. Using our imaging data, we determined a quantitative link between the amounts of dead and active Li with the capacity loss of the battery. Our analysis suggests that the amounts of plated and dead Li increase with the charging rate and cycle number. Finally, we developed quantitative relationships in 3D between the amounts of dead, active, and plated Li, and XFC-related capacity loss of the battery.In conclusion, this work contributed to the advanced understanding of the morphological evolution of dead, active, and plated Li and their relationship with XFC-related capacity loss on thick graphite electrodes. We believe these insights will help inform rational 3D designs of thick graphite electrodes that will minimize Li plating in fast-charged LIBs.

State-of-Charge Dependent Change in the Microporous Structure of Graphite Electrodes

Regarding the negative electrode, the most used electrode material in commercially available lithium-ion batteries (LIBs) up to the present day is graphite.1 Upon battery operation, an irreversible volume change in the microporous structure of the graphite electrode is detected. It is associated with the formation of the solid-electrolyte interphase (SEI) within the first cycles and an irreversible thickness increase of the electrode. Consequently, one observes a change in the electrode’s porosity after the initial formation cycles.2,3 Next to irreversible microstructure changes, one observes reversible changes in the microstructure associated with the intercalation of Li-ions into the host structure of the graphite particles upon charging the battery. The electrode particles expand during this process, increasing particle volume by ~13% when fully lithiated.4 In commercial LIBs, electrode materials, separators, and current collectors are tightly packed for a high volumetric energy density. This restricts the thickness expansion of the electrodes.5 Consequently, the graphite particles must expand into the internal pores of the electrodes, resulting in a decrease in the void volume of the porous network. Since the porous electrodes and the separator are filled with the electrolyte and the particle expansion of graphite is more significant than that of the active material in the positive electrode, one observes electrolyte motion in commercial LIBs during charge and discharge. It can be tracked by techniques requiring neutron beam facilities or expensive devices such as X-ray CT instruments.2,6 Recently, Aiken et al. developed a relatively simple apparatus to track the electrolyte motion by monitoring the changing moment of inertia of cylindrical cells during operation and simplifying the approach to study the discussed effect.In this study, we developed a method to track the reversible change of the microporous structure of an artificial graphite (MAGE-5, Hitachi, Japan) with an LP572 electrolyte (1 M LiPF6 in EC:EMC 7:3 w:w with 2 wt% VC, Gotion) during a charge and discharge cycle after formation employing electrochemical impedance spectroscopy (EIS) and operando dilatometry with both cell setups having a 1.5 bar cell compression Since the electrode’s microstructure is connected to the macroscopic properties using the MacMullin number of:NM=τ/ε=RIon·A·κ/dit correlates the porosity , electrodes thickness , ionic pore resistance , area of the electrode , conductivity of the electrolyte , and the tortuosity .8 Thus, we analyzed the change of the ionic pore resistance of graphite electrodes employing EIS in a MAGE//Li half-cell setup with a micro-reference electrode and a free-standing graphite electrode.Due to the particle expansion, we monitor an increase of the ionic pore resistance of up to 40% during charging, i.e., lithiation, of the graphite electrode.Next, we determined the SOC-dependent increase in thickness of the graphite electrode revealing a reversible thickness increase of 3%. Knowing the relation of porosity and tortuosity in a generalized Bruggeman form and using Equation 1 as described above with a constant electrode area and assuming a constant electrolyte conductivity , allowed us to extract the relative SOC-dependent decrease in porosity and void volume of the electrodes. At the end of the charge, both values decreased by approx. 10% and 9%, respectively, confirming the electrolyte repression from the graphite electrode.