High C-rate Research Articles

Fluorine-doped carbon coating of LiFe0.5Mn0.5PO4 enabling high-rate and long-lifespan cathode for lithium-ion batteries

The limited electron and ionic conductivity, along with the sluggish kinetics caused by the Jahn-Teller effect of Mn3+, impose constraints on the electrochemical performance of LiFexMn1-xPO4. Herein, the surface of LiFe0.5Mn0.5PO4 (LFMP) is modified with a F-doped carbon using the solvothermal and calcination methods. The incorporation of F-doped carbon coating, along with the formation of interfacial F-Li, F-Fe and F-Mn bonds between the carbon layer and LFMP nanoparticles, significantly mitigates charge transfer resistance, facilitates rapid electron transfer, as well as enhances Li+ diffusion kinetics. The LFMP@C-F2 cathode prepared in this study exhibits an unexceptionable capacity retention of 90.5 % after 300 cycles at a low rate of 0.2C and a capacity retention of 78.8 % over 1000 cycles at a high rate of 1C. When incorporated into the solid battery configuration (Li/PEO-LATP CSE/LFMP@C-F2), it exhibits an initial discharge specific capacity of 148 mAh g−1 and maintains a capacity retention of 85.8 % after 60 cycles at 0.1C, thereby offering an innovative approach to enhance the performance of LFMP in terms of cycling stability and rate capacity in lithium-ion batteries, as well as to apply LFMP into solid-state lithium batteries.

Selective oxidative leaching and restoration of FePO4 from spent LiFePO4 powder for regeneration into LiFePO4 cathode

With the large-scale commercial use of LiFePO4(LFP), the resource regeneration of retired LFP is one of the crucial issues in the EV industry. More attention is being paid to efficient recovery of Fe and P resources and the high quality of FePO4 precursor through process flows for developing recycling methods. The selective extraction process of lithium from spent LFP powder in H3PO4 system was studied, and the distribution and migration behavior of each element during the leaching process were analyzed. Under the optimized leaching conditions, the leaching rates of each element in spent LFP powder are Li: 96.1 %, Fe: 0.28 % and Al: 23.3 %, respectively, showing high efficiency of Li extraction. At the same time, the leaching kinetics of the oxidation leaching process was studied, and the apparent activation energy was calculated to be 53.45 kJ mol−1, indicating that the oxidation leaching process is controlled by chemical reaction. The FePO4 precursor was directly reconstructed from iron-phosphorus slag using a short H3PO4 purification process. The capacity retention of the regenerated LFP using the H3PO4-purified FePO4 precursor is 99.6 % after 300 cycles at 1C, and its specific capacity for reversible discharge at a high rate of 5C is 140.2 mAh g−1, demonstrating excellent electrochemical performance. The selective leaching in H3PO4 system shows that the regeneration route of FePO4 precursors is promising for scalable fabrication.

Anti-swelling gel polymer electrolyte membrane for high-performance lithium-ion battery

Gel polymer electrolyte (GPE) represents an effective and advanced substitution of polyolefin separator in high-rate lithium-ion rechargeable batteries. We here develop a novel pyridine ionic liquid (IL)-based GPE membrane for that purpose via a one-step reactive vapor-induced phase separation (RVIPS) method. Casting mixture solution of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) and 1-butylpyridinium hexafluorophosphate ([C4Py]PF6) to evaporate solvent in ammonia water vapor, the IL-based membrane was successfully prepared in one step. The effects of the IL content on the membrane microstructure were investigated in detail. Our results demonstrate that the pyridine IL plays a role as plasticizer to suppress crystallization of the polymer, while the RVIPS process can induce dehydrofluorination-crosslinking of PVDF-HFP to greatly reduce swelling of the membrane in liquid electrolyte. Based on those, the GPE from the membrane containing 10 % [C4Py]PF6 exhibits the best electrochemical properties (ionic conductivity of 1.48 mS cm−1, Li+ transference number of 0.66, and electrochemical stable window of 5.2V), though it uptakes a moderate amount of liquid electrolyte. Moreover, the long-term interfacial stability of the GPE with the metal anodes was checked to indicate its suitability as excellent GPE. According to the battery tests, the GPE exhibits superior discharge capacities at high C-rates, achieving 129.7 mAh g−1 at 3C and 115.5 mAh g−1 at 5C, while remains a low capacity decay of 0.014 % per cycle over 300 cycles at 3C. Therefore, our work provides a facile strategy of one-step RVIPS to prepare pyridine ionic liquid-based GPE, which demonstrates promising potential for high-performance rechargeable lithium-ion battery.

Boosting Li-S batteries through the synergistic effect of recycled ferrites and external magnetic induction

Despite being considered one of the most promising energy storage technologies, lithium-sulfur batteries (LSBs) are limited in terms of commercialization by the shuttle effect and slow reaction kinetics. In this work, we demonstrate for the first time that the use of recycled ferrite in conjunction with an external magnetic field generated by a permanent magnet can enhance the reaction kinetics and the adsorption of polysulfides (LiPSs), and hence the electrochemical stability. An in-depth kinetic study shows that under the effect of an external magnetic field, the electrode has lower polarization, a higher Li+ diffusion coefficient and a lower activation energy between electrochemical stages. The electrode also has a capacity retention up to 40 % higher and half the capacity loss per cycle at a high rate of 1C. At an ultra-high rate of 10C, the electrode has a capacity of 507 mAh g−1 after 150 cycles and an areal capacity of up to 3 mAh cm−2 at an ultra-high loading of 13 mg cm−2. In addition to the promising results observed in electrochemical terms, our approach is also more sustainable due to the use of a recycled electronic material obtained via dry milling, thereby avoiding the use of fossil carbons.

Stable Long‐Term Cycling of Room‐Temperature Sodium‐Sulfur Batteries Based on Non‐Complex Sulfurised Polyacrylonitrile Cathodes

AbstractThe cost‐effectiveness and high theoretical energy density make room‐temperature sodium‐sulfur batteries (RT Na−S batteries) an attractive technology for large‐scale applications. However, these batteries suffer from slow kinetics and polysulfide dissolution, resulting in poor electrochemical performance. The sulfurised polyacrylonitrile (SPAN) cathode is postulated as a suitable material due to the retention of sulfur through covalent bonds and the increase in the conductivity of sulfur. Furthermore, in this work the synthesis of SPAN has been carried out using simple synthesis methods, making it scalable, economical, and without the use of toxic compounds. The incorporation of the SPAN material helps mitigate the shuttle effect, reducing the capacity loss and improving both the efficiency and lifespan of Na−S batteries. The SPAN‐based cathode demonstrates that this RT Na−S battery configuration shows high stability, reaching 1000 cycles with a capacity loss per cycle of 0.11 % and a satisfactory specific capacity of 400 mAh/gs at a high rate of 2 C. This study demonstrates that the utilisation of SPAN derived from a non‐complex synthesis can be a viable alternative for enhancing the future of Na−S batteries technology.

Multi-objective optimization of a novel hybrid battery thermal management system using response surface method

This study evaluates the thermal performance of a Z-type battery thermal management system (BTMS) designed for nine lithium-ion batteries discharged at a high rate of 5C, using Computational Fluid Dynamics (CFD) simulations. The investigation employs Response Surface Methodology (RSM) to optimize two critical thermal performance parameters: the maximum battery temperature (Tmax) and the maximum temperature difference between cells (ΔTmax). Various cooling strategies are explored to comprehensively assess the BTMS, including natural convection, forced convection, cooling fins, phase change material (PCM), and composite PCM. These methods are analyzed to determine their effectiveness in controlling the thermal behavior of the battery pack. The simulation results indicate that integrating different cooling techniques can significantly lower Tmax from 352.38 K to 309.14 K and reduce ΔTmax from 14.6 K to 3.31 K, depending on the method used. Under critical conditions, such as the failure of the active cooling system, the BTMS still maintained a Tmax of 310.64 K and a ΔTmax of 0.95 K, demonstrating its robustness and reliability. Further optimization identified the ideal configuration for the system, including an inlet air speed of 1.2 m/s, an inlet temperature of 297.15 K, and a PCM thickness of 3.8 mm, achieving optimal thermal performance with a Tmax of 303.97 K and ΔTmax of 3.17 K. This study offers valuable insights into the design and optimization of effective BTMS for enhanced battery safety and longevity.

Effect of Temperature on Intermediate Phases of Na3V2(PO4)3 during Cycling by Operando X-ray Diffraction.

Na3V2(PO4)3 (NVP) has gained a lot of attention due to its remarkable properties, such as its robust crystal structure, cycle life, rate capabilities, and so on. Nevertheless, NVP undergoes a substantial decrease in its rate capability at low temperatures, which limits its practical applications. In this study, the performance of NVP at low, room, and high temperatures during cycling is thoroughly investigated using synchrotron operando X-ray diffraction. The (de)insertion of two sodium ions from Na3V2(PO4)3 to Na1V2(PO4)3 appeared to occur via two intermediate phases (Na2V2(PO4)3 and Na1.64V2(PO4)3). The Na1.64V2(PO4)3 phase which is observed for the first-time during operando XRD measurements of NVP, exhibited limited stability at high temperatures. The increase in the quantity of these intermediate phases from high to low temperatures, especially at high C-rates, could be anticipated to be one of the contributing factors of poor rate capabilities of NVP at low temperatures. This study encourages the exploration of suitable strategies to enhance the performance of NVP at low temperatures and high C-rates.

An Alternative Polymer Material to PVDF Binder and Carbon Additive in Li-Ion Battery Positive Electrode.

Li-ion battery performance relies fundamentally on modulation at the microstructure and interface levels of the composite electrodes. Correspondingly, the binder is a crucial component for mechanical integrity of the electrode, serving to interconnect the active material and conductive additive and to firmly attach this composite to the current collector. However, the commonly used poly(vinylidenefluoride) (PVDF) binder presents several limitations, including the use of toxic solvent during processing, a low electrical conductivity which for compensation requires the addition of carbon black, and weak interactions with active materials and collectors. This study investigates Poly(3,4-ethylenedioxythiophene):poly[(4-styrenesulfonyl) (trifluoromethylsulfonyl) imide] (PEDOT:PSSTFSI) as an alternative binder and conductive additive, in replacement of both PVDF and carbon black, in Li-ion batteries with LiFe0.4Mn0.6PO4 at the positive electrode. Complex PEDOT:PSSTFSI significantly improves the electronic conductivity and lithium diffusion coefficient within the electrode, in comparison to standard PVDF binder and carbon black. This enhances significantly the electrochemical performance at high C-rates and for high active mass loading electrodes. Furthermore, an excellent long-range cyclability is achieved.

In Operando Health Monitoring for Lithium-Ion Batteries in Electric Propulsion Using Deep Learning

Battery management systems (BMSs) play a vital role in understanding battery performance under extreme conditions such as high C-rate testing, where rapid charge or discharge is applied to batteries. This study presents a novel BMS tailored for continuous monitoring, transmission, and storage of essential parameters such as voltage, current, and temperature in an NCA 18650 4S lithium-ion battery (LIB) pack during high C-rate testing. By incorporating deep learning, our BMS monitors external battery parameters and predicts LIB’s health in terms of discharge capacity. Two experiments were conducted: a static experiment to validate the functionality of BMS, and an in operando experiment on an electrically propelled vehicle to assess real-world performance under high C-rate abuse testing with vibration. It was found that the external surface temperatures peaked at 55 °C during in operando flight, which was higher than that during static testing. During testing, the deep learning capacity estimation algorithm detected a mean capacity deviation of 0.04 Ah, showing an accurate state of health (SOH) by predicting the capacity of the battery. Our BMS demonstrated effective data collection and predictive capabilities, mirroring real-world conditions during abuse testing.

Nano-organic polymers with rich redox sites as anode materials for dual-ion batteries

Organic electrode materials (OEMs) are considered one of the potential electroactive materials used in rechargeable batteries due to their high availability, low price, and sustainability. Nevertheless, the high solubility of their organic small molecules and their low electrical conductivity limit their practical applications. In this study, porous nanorods organic conjugated polymer were successfully prepared by a solvothermal method using 1,4,5,8-naphthalenetetracarboxylic anhydride and azodicarbonamide as precursors for dual-ion battery anodes. The successful synthesis of polymer extends the π-conjugated structure, effectively reducing its solubility in organic electrolytes. Moreover, the azo group (NN) in the polymer was easily conjugated with the adjacent aromatic ring, increasing the utilization of the active site, thereby improving the conductivity of the material so that the polymer has a high discharge capacity (90.5 mAh g−1 at 2 C, up to 96 % capacity retention). Even with a high rate of 10 C, the polymer still exhibits excellent electrochemical performance (50.0 mAh g−1 at 270 cycles). Moreover, reaction kinetics tests show that the electrode material was based on a pseudocapacitive storage mechanism influenced by diffusion and double-layer control. Therefore, the design strategy of such a material paves the way for the application of dual-ion batteries.

Impact of applied and preceding pressure on performance and reversible swelling of lithium-ion pouch cells with varying microporous separators

During the compression of battery cells in battery pack production, high forces occur temporarily due to the viscoelastic behavior of the cell components. The magnitude of these forces depends on the compression speed. Shortening the production time and, thus, increasing the compression speed is desirable for cost reduction. However, it must be ensured that the cells are not negatively affected by increased forces during production. The impact of these preceding forces, acting on the cell before operation, and the applied force during operation on the performance and the reversible swelling of 5Ah NMC811/graphite lithium-ion pouch cells are investigated in this study. To this end, a universal compression testing machine was adapted to enable high-precision thickness measurements under defined constant pressure and temperature conditions. In addition, the effects of different microporous separators manufactured via dry process and wet process in an otherwise identical cell configuration were investigated. In the test procedure, the pressure was incrementally increased from 0.01MPa to 2.5MPa. Each measurement was performed at the distinct constant pressure levels, followed by a subsequent measurement at a reference pressure of 0.2MPa to evaluate the impact of the preceding step. The results revealed that elevated preceding pressures on the cell can reduce the discharge rate capability on higher C-rates by up to 11%. The discharge rate capability of the cells with the wet-processed separator was more significantly influenced by increasing applied and preceding pressure than for those with the dry-processed separator. It was found that the reversible swelling of the cell was reduced by applying higher pressures, whereas elevated preceding pressure increased the reversible swelling. The results of this study suggest that cells should not be subjected to excessive forces before operation, for instance, during battery pack production, as this can negatively affect performance and swelling behavior. Furthermore, the mechanical stability of the separator during the pack production process should also be considered when designing a cell.

Fluorinated aggregated nanocarbon with high discharge voltage as cathode materials for alkali-metal primary batteries.

Due to its exceptionally high theoretical energy density, fluorinated carbon has been recognized as a strong contender for the cathode material in lithium primary batteries particularly valued in aerospace and related industries. However, CF x cathode with high F/C ratio, which enables higher energy density, often suffer from inadequate rate capability and are unable to satisfy escalating demand. Furthermore, their intrinsic low discharge voltage imposes constraints on their applicability. In this study, a novel and high F/C ratio fluorinated carbon nanomaterials (FNC) enriched with semi-ionic C-F bonds is synthesized at a lower fluorination temperature, using aggregated nanocarbon as the precursor. The increased presence semi-ionic C-F bonds of the FNC enhances conductivity, thereby ameliorating ohmic polarization effects during initial discharge. In addition, the spherical shape and aggregated configuration of FNC facilitate the diffusion of Li+ to abundant active sites through continuous paths. Consequently, the FNC exhibits high discharge voltage of 3.15V at 0.01C and superior rate capability in lithium primary batteries. At a high rate of 20C, power density of 33,694Wkg-1 and energy density of 1,250Wh kg-1 are achieved. Moreover, FNC also demonstrates notable electrochemical performance in sodium/potassium-CF x primary batteries. This new-type alkali-metal/CF x primary batteries exhibit outstanding rate capability, rendering them with vast potential in high-power applications.

Quantifying the Aging of Lithium-Ion Pouch Cells Using Pressure Sensors

Understanding the behavior of pressure increases in lithium-ion (Li-ion) cells is essential for prolonging the lifespan of Li-ion battery cells and minimizing the safety risks associated with cell aging. This work investigates the effects of C-rates and temperature on pressure behavior in commercial lithium cobalt oxide (LCO)/graphite pouch cells. The battery is volumetrically constrained, and the mechanical pressure response is measured using a force gauge as the battery is cycled. The effect of the C-rate (1C, 2C, and 3C) and ambient temperature (10 °C, 25 °C, and 40 °C) on the increase in battery pressure is investigated. By analyzing the change in the minimum, maximum, and pressure difference per cycle, we identify and discuss the effects of different factors (i.e., SEI layer damage, electrolyte decomposition, lithium plating) on the pressure behavior. Operating at high C-rates or low temperatures rapidly increases the residual pressure as the battery is cycled. The results suggest that lithium plating is predominantly responsible for battery expansion and pressure increase during the cycle aging of Li-ion cells rather than electrolyte decomposition. Electrochemical impedance spectroscopy (EIS) measurements can support our conclusions. Postmortem analysis of the aged cells was performed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to confirm the occurrence of lithium plating and film growth on the anodes of the aged cells. This study demonstrates that pressure measurements can provide insights into the aging mechanisms of Li-ion batteries and can be used as a reliable predictor of battery degradation.

Non-equilibrium thermal models of lithium batteries

Temperature fluctuations impact battery performance, safety, and health. Industry-standard cell-level models of these phenomena ignore thermal gradients within the electrodes’ active material, i.e., assume the latter to be in “thermal equilibrium”. We present a “non-equilibrium” thermal model that explicitly accounts for spatial variability of temperature with the active material (and the carbon-binder domain). We investigate the conditions, expressed in terms of the heat-generation rate and the thermal properties of a cell’s liquid (electrolyte) and solid (active material and CBD) phases, under which the thermal equilibrium assumption breaks down and our model should be used instead. The differences between these two thermal models are investigated further by coupling them with an industry-standard electrochemical model. The resulting thermal–electrochemical model demonstrates the importance of thermal gradients within the active material at high C-rates (discharge current densities) and for large grain sizes. Under these conditions, the equilibrium assumption underestimates internal temperature by as much as 50%. These two thermal models are then applied to a commercial NMC battery with multiple units. Our non-equilibrium model predicts the battery surface temperature that is in good agreement with measurements, while the equilibrium model underestimates the observed temperature.

Co-axially electrospun Li-rich layered Oxide@Spinel core-shell heterostructure nanofibers for enhanced stability and electrochemical performance

Lithium-rich layered oxides (LLO) possessing high specific capacity has the limitation of voltage fade, poor high C-rate performance and low conductivity. Spinel coating on LLO surface is known to mitigate the voltage fade, however, enabling high C-rate performance remains challenging. In this study, LLO nanofiber core with composition 0.6Li2MnO3·0.4LiMn0.25Ni0.38Co0.37O2 is prepared by electrospinning process and the same is coated with spinel LiMn1.5Ni0.5O4 shell to obtain LLO/spinel (LLO/S) core-shell heterostructure. The spinel shell coating is accomplished (i) by a novel co-axial (CA) electrospinning process and (ii) by wet chemical (WC) approach. The CA electrospinning process provides a uniform core-shell structure compared to the WC process. The electron microscopy studies reveal the fibrous microstructure in LLO/S heterostructures with energy dispersive spectroscopy compositional mapping showing the spinel composition on the surface. Higher concentration (>10 wt.%) of spinel coating are shown to break the fibers into particulates. X-ray diffraction and high-resolution transmission electron microscopy analysis confirm the spinel structure formation along with layered structure. While the capacity is slightly compromised compared to the pristine LLO nanofiber, the LLO/S heterostructure with 10 wt.% spinel coating exhibits a high reversible capacity of 268 mAhg-1 with minimal capacity loss during the 1st cycle (with coulombic efficiency 83.5%) and an excellent high C-rate capability (88 mAhg-1 at 10C-rate and 55 mAhg-1 at 20C-rate). The electrochemical studies also demonstrate the importance of optimal spinel content in the coating and the method of spinel coating on the layered material surface. The encapsulation of LLO with spinel layer which has 3D-diffusion pathways for Li-ion transport facilitates high ionic and electronic conductivity and hence leads to enhanced electrochemical performance of LLO/S heterostructured cathodes.

Cycle performance analysis of hybrid battery thermal management system coupling phase change material with liquid cooling for lithium-ion battery module operated at high C-rates

High performance battery thermal management system (BTMS) is urgently needed to satisfy the severe cooling demand of battery modules operated at high C-rates during continuous charging-discharging cycles. Hybrid BTMS coupling phase change material (PCM) with liquid cooling is a promising solution which attracts great attention in recent years. Here, cycle performance of the hybrid system is analyzed in detail via comparison with the pure PCM system to examine its reliability and demonstrate its superiority during continuous 3C-charging and 4C-discharging cycles. Effects of PCM melting temperature are studied and RT44HC is found a suitable PCM choice with appropriate melting temperature. Analysis of the first three cycles is proved important and basically sufficient as system thermal behavior stabilizes afterwards. Further, interacting mechanisms of expanded graphite (EG) content and battery distance on system cooling performance are elucidated and their possible combinations satisfying temperature requirements of battery module in both temperature rise and temperature uniformity are presented. Optimal combination of EG content and battery distance is 3 % and 3 mm. Effects of coolant velocity are also investigated and suitable velocity for the optimized hybrid system is set at 0.01 ms−1 to achieve balance between cooling performance and auxiliary energy cost.

Interleaved Electrode Structures for Enhanced Electrolyte Mass Transport in Li-Ion Batteries : A Numerical Study

The enhancement of energy and power density in Li-ion batteries remains a focal point in battery research. When Li-ion batteries are operated at high C-rates, mass transport limitations cause large concentration gradients of Lithium ions to form in the electrolyte. The resultant concentration overpotentials and underutilization of active material effectively imply a lower capacity for higher current rates, which creates a tradeoff between energy and power density. The integration of 3D interleaved electrode structures has been proposed to augment power density while preserving energy density. This work comprehensively analyzes the capacity improvement achieved by utilizing a wavy interleaved structure using electrochemical simulations. A pseudo-3D (P3D) model based on the Doyle-Fuller-Newman framework is employed to analyze the performance of various depths and frequencies of the wavy interleaved structure. Galvanostatic simulations are performed for NMC/Graphite-SiOx and LMO/Graphite cells at different C-rates, and the resultant voltage is systematically divided into component overpotentials to delineate the contribution of activation, ohmic, and mass-transfer overpotentials in both solid electrode and electrolyte. With the same amount of active material, simulations indicate a performance improvement of 7% in capacity at high C-rates. It is observed that using a wavy structure significantly reduces electrolyte concentration heterogeneities and resulting concentration gradients when compared to a conventional flat structure. The overpotential breakdown confirms that the improvement in voltage is indeed contributed precisely by the concentration overpotential. Concentration overpotential decreases with deeper interleaved structures as well as for more spatially frequent structures.Additionally, the impact of the interleaved electrode structure on cell degradation was also assessed. Lithium plating, which is the most prominent side reaction in the anode, was modeled using a Butler-Volmer kinetic equation, with deposition occurring locally when the solid phase potential drops below that of the electrolyte potential. The interleaved structure alleviates plating significantly due to higher uniformity in Li-ion concentration. With this work, we computationally demonstrate that interleaved structure works well for extracting higher capacity with the same active material, and also aids in suppressing Lithium plating.Figure Caption : Computed Li-ion concentration profile in the solid active material of an interleaved electrode Li-ion cell as a function of location (x, y) and distance from particle center (z) Figure 1

Investigation of Dry-Made NMC 811 Electrodes with Varying Binder and the Conductive Agent Contents

Lithium-ion battery electrode materials consist of active materials (AM), conductive agents, and binders. Although conductive agents and polymeric binders occupy a small portion of the electrode, they play an indispensable role. Conductive agents are needed to improve the electronic conductivity of the electrode, while binders ensure mechanical integrity by providing particle/particle cohesion and electrode-film/substrate adhesion. Optimizing the interactions of active materials, conductive agents, and binders is crucial for achieving the desired electrode performance and durability. Although multiple studies have explored the impact of electrode composition for slurry-based electrodes, it is still unclear how composition affects the electrodes made by dry processes. In this study, we performed a detailed investigation of the effects of the different electrode compositions by changing the ratio of polyvinylidene difluoride (PVDF) binder and carbon black (CB) for dry-made LiNi0.8Mn0.1Co0.1O2 (NMC 811) electrodes. The electrostatic spray deposition method was used as a dry electrode fabrication process due to its many advantages, such as reduced energy consumption and inexpensive processing requirements. We investigated electrodes of four different compositions, 96:3:1, 96:2:2, 90:7.5:2.5, and 90:5:5 (AM:PVDF: CB), corresponding to PVDF/CB mass ratios of 1:1 and 3:1.We found that electrodes with a high PVDF/CB ratio of 3:1 have larger impedance than electrodes with a low PVDF/CB ratio of 1:1 due to the insulating aggregates of PVDF alone, causing deteriorated electrochemical performance. At the PVDF/CB ratio of 1:1, an even distribution of PVDF/CB on the AM particle surfaces and in the pores between the AM particles was observed. For both PVDF/CB ratios of 1:1 and 3:1, increasing the total amount of PVDF and CB (e.g., decreasing the AM amount) in the composite electrode enhanced both the C-rate and cycling performance. However, electrodes with the PVDF/CB ratio of 1:1 at high AM content (96%) still showed a comparable C-rate performance to that with lower AM content (90%). Thus, dry-made electrodes with high AM content can achieve a good high C-rate performance, especially for high-energy-density applications.This study provides a better understanding of the effects of CB/PVDF, which is key to finding an optimal structure and composition of the dry-made electrodes.

A synergistic modification strategy for enhancing the cycling stability and rate capacity of single-crystal nickel-rich cathode materials

Ni-rich layered LiNi0.8Co0.1Mn0.1O2 (NCM811) is considered to be one of the most promising cathode materials due to its advantages of high specific capacity, lower cost and environmental friendliness. However, the relatively high nickel content in the LiNi0.8Co0.1Mn0.1O2 layered cathode materials leads to the instability of the internal structure and surface of the materials during long-term cycling, which results in the deterioration of electrochemical performance. In this study, single-crystal NCM811 (SNCM811) was prepared using a convenient solid-state method. By doping Al into SNCM811 (SNCMAl) and purposefully low-temperature surface treatment with a small amount of LiPF6, a Ni-rich single-crystal NCM811 modified by both Al bulk doping and LiF surface coating (SNCMAl@LiF) was obtained. The reversible capacity of SNCMAl@LiF with a cutoff voltage of 2.7–4.3 V at 0.1C is 200.3 mAh·g−1. And the capacity retention of the single-crystal cathode materials is 92.4 % with capacity of 171.07 mAh·g−1 after 100 cycles at 1C. Even at high rate of 10C, the discharge specific capacity maintains 148.8 mAh·g−1, demonstrating excellent electrochemical performance. The dual-modified single-crystal material has the lowest potential difference and electrochemical impedance without intragranular cracks and crystal slips after 100 cycles. Stronger AlO bond in single crystals reduced the degree of cation mixing and maintained the stability of the layered structure. LiF-modified surface can promote the transfer of Li+, maintain the stability of the electrode/electrolyte interface and then improve the electrochemical performance of cathode materials. Al-doping and LiF-coating provides a guidance on how to enhance the electrochemical performance of Ni-rich single-crystal cathode materials.

Effects of Non-Uniform Temperature Distribution on the Degradation of Liquid-Cooled Parallel-Connected Lithium-Ion Cells

Our previous work on an air-cooled stack of five pouch-format lithium-ion (Li-ion) cells showed that non-uniform temperature can cause accelerated degradation, especially of the middle cell. In this work, a stack of five similar cells was cycled at a higher C-rate and water-cooled to create a larger temperature gradient for comparison with the air-cooled stack. It was hypothesized that the larger temperature gradient in the water-cooled stack would exacerbate the degradation of the middle cell. However, the results showed that the middle cell degraded slightly slower than the side cells in the water-cooled stack. This trend is opposite to that in the air-cooled stack. This difference could be attributed to the combined effects of a smaller temperature rise and larger temperature gradient in the water-cooled stack than in the air-cooled stack. Post-mortem analysis of cycled cells and a fresh cell showed that the degradation mainly came from the anode. Increased lithium plating and decreased porosity in the side cells are possible mechanisms for the faster degradation compared with the middle cell. It was also found that all the cells in the water-cooled stack experienced a phenomenon of capacity drop and recovery after a low C-rate reference performance test and extended rest. This phenomenon can be attributed to lithium diffusion between the anode active area and the anode overhang area.