Li-ion Pouch Cells Research Articles

Transition Metal-Doped Ni-Rich Layered Cathode Materials for Sustainable Li-Ion Batteries

Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials. Many studies on various dopants have been reported; however, a general relationship between the dopants and their effect on the stability of the positive electrode upon prolonged cell cycling has yet to be established. Here, we explore the impact of the oxidation states of various dopants (i.e., Mg2+, Al3+, Ti4+, Ta5+, and Mo6+) on the electrochemical, morphological, and structural properties of a Ni-rich cathode material (i.e., Li[Ni0.91Co0.09]O2). Galvanostatic cycling measurements in pouch-type full Li-ion cells show that cathodes featuring dopants with high oxidation states significantly outperform their undoped counterparts and the dopants with low oxidation states. In particular, Li-ion pouch cells with Ta5+- and Mo6+-doped Li[Ni0.91Co0.09]O2 cathodes retain about 81.5% of their initial specific capacity after 3,000 cycles at 200 mAh g-1. Furthermore, physicochemical measurements and analyses suggest substantial differences in the grain geometries and crystal lattice structures of the various cathode materials, which contribute to their widely Myung et al. ACS Energy Lett. 2017, 2, 196-223.Kim et al. Energy Environ. Sci. 2018, 11, 1271-1279.Kim et al. energy. 2020, 5, 860-869.Kim et al. ACS Energy Lett. 2017, 2, 1848-1854.

In Situ Thickness Measurements During Cycling of Li-Ion Pouch Cells with Silicon Negative Electrodes

Cell expansion and contraction are major concerns for battery pack and module developers. This work is an effort to understand the behavior of cell expansion due to formation, lithiation and cycling, with measurements made by a system that uses a Linear Variable Differential Transformer (LVDT) sensor. For a NCM622-Silicon cell, impact of factors such as initial compression pressure and calendaring of electrodes on cell expansion were explored. Reversible expansion during charge/discharge in a cycle is a function of cell capacity and reversible expansion can be reduced with increasing initial compression. Irreversible expansion during cycling grows linearly with the number of cycles and can also be reduced with initial compression of the cell. With initial compression of 45 psi, measurements showed 3% cell expansion during formation, 4% reversible expansion in a charge/discharge cycle with C/5 rate, and 12% irreversible expansion over 220 cycles. From the data, a hypothesis was developed suggesting that SEI growth and plastic deformation of the silicon particles could be the main causes for irreversible expansion of the cells with silicon electrodes, with the assumption that cells show less than 20% capacity fade over life.

Binder-Free Cathode Enabling High Capacity Li-Ion Battery

Li-ion batteries have polymeric binder in their electrodes resulting in high internal resistance and hence long charge times and compromised product lifetime. We have developed Li-ion batteries utilizing binder-free NMC cathodes containing carbon nanotube (CNT). Our proprietary processing of CNT into viscous organic or water-based slurry without surfactant or dispersant reduces or eliminates the need for organic binder and carbon black additives in these cathodes. The benefits of incorporating our processed CNT slurry into NMC cathode electrodes are realized by significant gains in specific energy and power. When the organic binder in the cathode is completely removed, our Li-ion batteries retain superior performance metrics over conventional binder-containing cells, including significantly reduced impedance (-31%) and increased capacity (5%). Li-ion pouch cells containing these binder-free NMC/CNT cathodes show 2.5% higher specific energy. Our cells with binder-free cathode complete 500 cycles with 50% less capacity fade than cells with binder, and display more than 2x improvement in capacity at 10C rate discharge. A processing advantage of eliminating the polymeric binder from the formulation mix was demonstrated by the use of IPA as dispersing solvent in place of the costlier and toxic NMP solvent. Li-ion cells containing binder-free NMC/CNT cathodes from IPA mix preserved their performance advantages over conventional NMP-based binder-containing cells. Figure 1

Calibration of Fiber Bragg Grating - Sensors for Subsequent Temperature and Pressure Measurements in Li-Ion Pouch Cells

The monitoring of temperature during operation of lithium ion batteries is crucial for a stable and safe performance of a battery system. However, a major challenge is to obtain exact internal temperature progression upon charge and discharge processes. Though commonly used resistance temperature sensors can give considerably accurate temperature values, they are limited to the application outside of the cell due to the distortion of their signal through induction effects of the applied cell currents, if installed inside the pouch cell and therefore, they are not suitable for in situ measurements. This poses serious problems especially for big cell stacks, as core temperatures for those cells differ greatly from the outer parts and particularly the surface of the cell encasement. Fiber Bragg Grating (FBG) sensors on the other hand, provide data that are unaffected by induction effects as their operating principle is solely based on the reflection of light of certain wavelengths within a glass fiber. However, a specific difficulty is the distinction between temperature and pressure effects, as the reflected signal is sensitive to both. Often times additional tubing around the sensor is needed to eliminate pressure effects or an additional sensor is placed in close proximity, inside or outside, to the FBG-sensor as a reference. The former, however, can affect the cell stack physically acting as a defect spot with unwanted side effects.In our work, we show that a careful calibration of the FBG-sensor can be accomplished, so that both effects can be separated and the FBG-sensor can then be used to gather information not only about temperature but also about volume changes in an operating cell. Therefore, a temperature calibration took place, with the cell being held at a constant SoC. Subsequently, a calibration of the SoC - dependent pressure-induced mechanical deformation was performed at very low C-rate to minimize thermal effects. The so gathered calibration parameters can then be applied to all performed cycling tests, regardless of the C - rate. A calibration routine and mathematical solution to separate the influences of temperature and pressure to receive exact in situ temperature values for a 10 Ah lithium ion battery cell is presented. The calibration method is applied to cycling and rate tests at various C-rates.The results show, that wavelength shifts upon charging and discharging processes are mainly resulting from volume changes of the active cell components, meaning that the FBG – sensor has a very high sensitivity towards pressure changes. With the before mentioned calibration method the impact of mechanical deformation on the FBG - sensor can be subtracted from the raw data and allow for a direct temperature reading. The temperature data correlates with data gathered from commonly used sensors placed on the outside of the pouch cell. Tests with varying discharge rates confirm these results and show, that the method produces precise temperature data even at different C-rates, even though only slow C-rates were applied during calibration.In detailed comparison, the FBG - sensors show a faster response towards temperature changes compared to conventional external temperature sensors, which can be related to the insulating effect of the pouch foil. In case of high discharge rates, we were able to show that the measured temperatures inside of the pouch cell are higher in comparison to the outside ones. Furthermore, SoC - induced changes in pressure measured by two FBG-sensors, which were located at different positions on the cell stack, are almost identical. This indicates that pressure build-up within a pouch cell occurs evenly through the cell stack.

Simultaneous identification of the heat capacity and the anisotropic thermal conductivities of a Li-ion pouch cell by a non-destructive analytical approach

The specifications and aging of Li-ion batteries highly depends on temperature. Battery thermal models are therefore useful to improve the thermal management of battery packs. This study presents a method to determine the heat capacity and the anisotropic thermal conductivities of a Li-ion pouch cell. A single experimental setup is devised and involves local heating of the battery with a resistance heater. The thermal properties of the cell are then identified by calibrating an analytical thermal model with the experimental temperature data. A three-dimensional analytical model is developed for this purpose. It allows for simultaneous characterization of the cell anisotropy. The thermal properties are identified at 50% state of charge between 0 °C and 40 °C. Models with adiabatic and convection boundary conditions are compared and showed similar results. Overall, the values and tendencies agree with the literature. This non-destructive method can be directly applied to any pouch and prismatic cells, and adapted to cylindrical cell.

Novel additives-package to mitigate the failure modes of high-capacity LiNi0.82Co0.11Mn0.07O2-based lithium-ion battery

Electrolyte additive strongly influences on energy density and cycle-life of state-of-the-art lithium-ion batteries (LIBs). Without additive, inevitable side reactions at nickel-rich cathode-electrolyte and anode-electrolyte interfaces result in non-satisfactory solid electrolyte interphase (SEI) formation and performance failure due to metal-dissolution from cathode particularly under multiply harsh condition of high voltages and temperatures. Herein, we report the design of a novel additives-package comprising of lithium borate, fluorinated anhydride and fluorinated phosphate at ≤1 wt% for a LIB with markedly improved capacity and performance. We show the synergy effects of individual additive in forming robust SEI layers at both 82% nickel (NCM-82) cathode and graphite anode under 4.35 V, 45 °C and 1C rate by compensating one’s lacking role(s) by others’. Failure modes of NCM-82 chemistry LIBs such as high-voltage instability, metal-dissolution and cracks, surface and structural instability are mitigated, enabling high-capacity of 210 mAh g−1 and high capacity retention of 94% after 100 cycles, with respect to reference electrolyte. Moreover, long cycle-life of 300 cycles of single-layer Li-ion pouch cell at 2C and 100 cycles of industrial pouch LIB (45 mAh cm−2) at 1C is achieved. Our design strategy for additives-package offers a promising path to long-cycled high-energy LIBs.

Material Optimization Engineering toward xLiFePO4·yLi3V2(PO4)3 Composites in Application-Oriented Li-Ion Batteries.

The development of LiFePO4 (LFP) in high-power energy storage devices is hampered by its slow Li-ion diffusion kinetics. Constructing the composite electrode materials with vanadium substitution is a scientific endeavor to boost LFP’s power capacity. Herein, a series of xLiFePO4·yLi3V2(PO4)3 (xLFP·yLVP) composites were fabricated using a simple spray-drying approach. We propose that 5LFP·LVP is the optimal choice for Li-ion battery promotion, owning to its excellent Li-ion storage capacity (material energy density of 413.6 W·h·kg−1), strong machining capability (compacted density of 1.82 g·cm−3) and lower raw material cost consumption. Furthermore, the 5LFP·LVP||LTO Li-ion pouch cell also presents prominent energy storage capability. After 300 cycles of a constant current test at 400 mA, 75% of the initial capacity (379.1 mA·h) is achieved, with around 100% of Coulombic efficiency. A capacity retention of 60.3% is displayed for the 300th cycle when discharging at 1200 mA, with the capacity fading by 0.15% per cycle. This prototype provides a valid and scientific attempt to accelerate the development of xLFP·yLVP composites in application-oriented Li-ion batteries.

Equivalent circuit definition and calendar aging analysis of commercial Li(NixMnyCoz)O2/graphite pouch cells

Aging-induced degradation of commercial Li-ion pouch cells with lithium nickel-manganese‑cobalt-oxide-based cathodes and graphite anodes is studied at various operating conditions (temperature and voltage) by galvanostatic measurements and in situ electrochemical impedance spectroscopy (EIS). A detailed equivalent electrical circuit model, capable to fit the measured EIS spectra, is developed and validated. The work also confirms the capacity fade dependence of the calendar-aged cells on stress factors such as temperature and state-of-charge. The retained capacity is found to decrease linearly with t0.5. However, it stands out that the degradation registered for cells held at 95% state-of-charge is lower than that for those at 70% and for the cases when temperatures are between 25 °C and 37.5 °C, which is a not very common singularity. A high performing calendar aging model is introduced accordingly. Both the electrical circuit model defined and the accurate calendar aging model developed provide useful tools for battery management systems in order to monitor and control both the state-of-health and the state-of-charge of these commercial cells.

Explosion behavior investigation and safety assessment of large-format lithium-ion pouch cells

Large-format lithium-ion (Li-ion) batteries with high energy density for electric vehicles are prone to thermal runaway (or even explosion) under abusive conditions. In this study, overcharge induced explosion behaviors of large-format Li-ion pouch cells with Li[Ni0.8Co0.1Mn0.1]O2 cathode at different current rates (C-rates) (0.5C, 1C, 2C) were investigated. The explosion characteristics of the cells were elucidated by discussing the evolution of the cell voltage, the surface temperature and the shock wave pressure. Generally, the whole overcharge process could be divided into four stages according to the evolution of several key parameters and the overcharge behaviors; the overcharge C-rate has a great influence on cells’ thermal behaviors. The experimental results showed that the thermal runaway process of Li-ion cells caused by overcharging consisted of two kinds of explosions, physical explosion and chemical explosion. The existence of observable negative pressure zone in the pressure curves indicated that the Li-ion cells are not a self-supplying oxygen system during the explosion. Further, the explosion dynamics parameters were matched. An explosion TNT-equivalent conversion strategy that depended on the pressure of the shock wave was utilized to evaluate the released energy and its hazards. In addition, with respect to the overcharge of Li-ion pouch cells, a safety assessment method and a safety management method were proposed based on the explosion behaviors. From the perspective of battery safety, this study is of great significance for the safety design of Li-ion cells and can provide guidance for engineers to optimize the safety function of battery packs.

Influence of electrolyte presence on the vibration behavior of a Li-ion pouch cell and on the mechanical characteristics of its incorporated [formula omitted]-coated polyolefin separator

Quality improvement of battery crash simulation models is essential for realistic mechanically induced short-circuit prediction in modern electric vehicles, where the battery is part of the load path. In this paper, the electrolyte influence on the mechanical behavior of a commercial pouch cell and its incorporated Al2O3-coated separator is investigated. To mechanically characterize the separator, both compression and tensile tests were performed. Modal analyses were performed to determine the damping behavior and dynamic stiffness of a normally conditioned cell (wet) and a cell without electrolyte (dry). In the tensile tests, a slight anisotropy and an increase in stiffness, triggered by higher test speeds, were observed. The wet specimens soaked in substitute electrolyte mixture showed longer elongation to failure with a stiffer response than the dry specimens under tensile loading. In the compression tests, exactly the opposite mechanical response of wet and dry specimens was measured. From the modal analyses, a stiffening of the dry cell could be determined. Based on the component tests, a finite element model of the separator was developed in LS-DYNA, enabling prediction of the deformation behavior of the separator under mechanical loads — this can be further used in high-fidelity cell simulation models.

Methods—Spatially Resolved Diffraction Study of the Uniformity of a Li-Ion Pouch Cell

A lab-made, multilayered Li-ion battery pouch cell is investigated using in-operando neutron powder diffraction (NPD) and spatially resolved powder X-ray diffraction (SR-PXRD) with the aim of investigating how to compare the information obtained from the two complementary techniques on a cell type with a complicated geometry for diffraction. The work focusses on the anode and cathode lithiation as obtained from the LiC6/LiC12 weight ratio and the NMC111 c/a-ratio, respectively. Neutron powder diffractograms of a sufficient quality for Rietveld refinement are measured using a rotation stage to minimize geometrical effects. Using SR-PXRD, the cell is shown to be non-uniform in its anode and cathode lithiation, with the edges of the cell being less lithiated/delithiated than the center in the fully charged state. The non-uniformity is more pronounced for high charging current than low charging current. The averaged SR-PXRD data is found to match the bulk NPD data well. This is encouraging as it seems to allow comparisons between studies using either of these complementary techniques. This work will also serve as a benchmark for our future studies on pouch cells with novel non-commercial cathode and/or anode materials.

EIS2MOD: A DRT-Based Modeling Framework for Li-Ion Cells

The correct assessment of battery states is essential to maximize battery pack performances while ensuring reliable and safe operation. This article introduces EIS2MOD, a novel modeling framework for li-ion cells based on distribution of relaxation time. A physically-based electric circuit model is developed starting from electrochemical impedance spectroscopy and open-circuit voltage measurements. Distribution of relaxation time (DRT) is applied to deconvolve the electrochemical phenomena from the EIS. The presented methodology is based on: DRT calculation from EIS; DRT analysis for ECM configuration; and model parameters extraction and fitting. The proposed framework is applied to large format li-ion pouch cells, which are tested over the whole state of charge (SoC) range and a wide temperature range (−10 °C to 35 °C). Different current profiles have been tested to validate the model, showing its high accuracy in reproducing the battery cell behavior (e.g., RMSE on the battery terminals voltage lower than 1.50% for driving cycle simulations at variable temperature and SoC). An additional advantage of EIS2MOD is its light computational load thus offering an attractive framework for battery management system implementation.

In-situ X-ray tomographic imaging study of gas and structural evolution in a commercial Li-ion pouch cell

Gas generation within Li-ion batteries (LIB) can lead to an increase in resistance, thereby, reducing their cycle lifetime. The chance of catastrophic failure via internal gas evolution may increase as a function of cell size and capacity. However, in-situ studies of gas evolution at the cell level are very limited due to limited number of techniques that can effectively probe this. Hence, for the first time, we employed high-energy X-ray tomography to non-destructively observe the structural evolution (gas and electrodes) as a function of cycle numbers for a 400 mAh commercial Li-ion pouch cell. Gas agglomeration led to cell deformation in different areas were observed in 4D (3D + time), the subsequent quantification including the volume fraction, surface area and thickness showed a heterogeneous gas distribution, revealing the degradation mechanism involving the coalescence of gas. This study demonstrates a feasible case of the use of lab-based X-ray to investigate the cell degradation and monitor state of health (SOH) by tracking the thickness in-situ, providing practical guidance for designing safer pouch cells.

How do Depth of Discharge, C-rate and Calendar Age Affect Capacity Retention, Impedance Growth, the Electrodes, and the Electrolyte in Li-Ion Cells?

Lithium-ion cells testing under different state of charge ranges, C-rates and cycling temperature have different degrees of lithium inventory loss, impedance growth and active mass loss. Here, a large matrix of polycrystalline NMC622/natural graphite Li-ion pouch cells were tested with seven different state of charge ranges (0–25, 0–50, 0–75, 0–100, 75–100, 50–100 and 25%–100%), three different C-rates and at two temperatures. First, capacity fade was compared to a model developed by Deshpande and Bernardi. Second, after 2.5 years of cycling, detailed analysis by dV/dQ analysis, lithium-ion differential thermal analysis, volume expansion by Archimedes’ principle, electrode stack growth, ultrasonic transmissivity and X-ray computed tomography were undertaken. These measurements enabled us to develop a complete picture of cell aging for these cells. This then led to an empirical model for cell capacity loss vs SOC range and calendar age. Although these particular cells exhibited substantial positive electrode active mass loss, this did not play a role in capacity retention because the cells were anode-limited during full discharge under all the tests carried out here. However, the positive electrode mass loss was strongly coupled to positive electrode swelling and electrolyte “unwetting” that would eventually cause dramatic failure.

Spatially Resolved Operando Synchrotron-Based X-Ray Diffraction Measurements of Ni-Rich Cathodes for Li-Ion Batteries

Understanding the performance of commercially relevant cathode materials for lithium-ion (Li-ion) batteries is vital to realize the potential of high-capacity materials for automotive applications. Of particular interest is the spatial variation of crystallographic behavior across (what can be) highly inhomogeneous electrodes. In this work, a high-resolution X-ray diffraction technique was used to obtain operando transmission measurements of Li-ion pouch cells to measure the spatial variances in the cell during electrochemical cycling. Through spatially resolved investigations of the crystallographic structures, the distribution of states of charge has been elucidated. A larger portion of the charging is accounted for by the central parts, with the edges and corners delithiating to a lesser extent for a given average electrode voltage. The cells were cycled to different upper cutoff voltages (4.2 and 4.3 V vs. graphite) and C-rates (0.5, 1, and 3C) to study the effect on the structure of the NMC811 cathode. By combining this rapid data collection method with a detailed Rietveld refinement of degraded NMC811, the spatial dependence of the degradation caused by long-term cycling (900 cycles) has also been shown. The variance shown in the pristine measurements is exaggerated in the aged cells with the edges and corners offering an even lower percentage of the charge. Measurements collected at the very edge of the cell have also highlighted the importance of electrode alignment, with a misalignment of less than 0.5 mm leading to significantly reduced electrochemical activity in that area.

Probing heat generation and release in a 57.5 A h high-energy-density Li-ion pouch cell with a nickel-rich cathode and SiOx/graphite anode

This study clarifies the heat generation and release of a 57.5 Ah HED (266.9 W h kg−1) Li-ion cell with a nickel-rich cathode and SiOx/graphite anode. Significant heat accumulation and delayed heat release effects in large-format cells are uncovered.

An Efficient Detailed Layer Model for Prediction of Separator Damage in a Li-Ion Pouch Cell Exposed to Transverse Compression

An Efficient Detailed Layer Model for Prediction of Separator Damage in a Li-Ion Pouch Cell Exposed to Transverse Compression

Transition metal-doped Ni-rich layered cathode materials for durable Li-ion batteries

Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials. Many studies on various dopants have been reported; however, a general relationship between the dopants and their effect on the stability of the positive electrode upon prolonged cell cycling has yet to be established. Here, we explore the impact of the oxidation states of various dopants (i.e., Mg2+, Al3+, Ti4+, Ta5+, and Mo6+) on the electrochemical, morphological, and structural properties of a Ni-rich cathode material (i.e., Li[Ni0.91Co0.09]O2). Galvanostatic cycling measurements in pouch-type Li-ion full cells show that cathodes featuring dopants with high oxidation states significantly outperform their undoped counterparts and the dopants with low oxidation states. In particular, Li-ion pouch cells with Ta5+- and Mo6+-doped Li[Ni0.91Co0.09]O2 cathodes retain about 81.5% of their initial specific capacity after 3000 cycles at 200 mA g−1. Furthermore, physicochemical measurements and analyses suggest substantial differences in the grain geometries and crystal lattice structures of the various cathode materials, which contribute to their widely different battery performances and correlate with the oxidation states of their dopants.

In-built ultraconformal interphases enable high-safety practical lithium batteries

There is an urgent need for high-safety and high-energy lithium-ion batteries to satisfy the rapidly increasing need for energy storage. Nickel-rich layered cathodes have been at the forefront of the revolution for batteries due to their relatively high capacity and low cost. However, with the increase of nickel content, the batteries suffer from severe safety concerns, which caused by thermal runaway. Here we show that the ultraconformal cathode-electrolyte interphase (CEI) protective skin with high inorganic content dramatically enhances the safety of high-energy practical Li-ion pouch cells. We find that the robust CEI skin significantly improves the intrinsic thermal stability, mitigates the evolution of oxygen resulting from phase transition, and effectively suppresses the associated parasitic reactions between the delithiated cathodes and electrolyte. The in-situ CEI engineering strategy is simple and suitable for practical industrial manufacture, and it provides design ideas for aggressive nickel-rich cathodes towards safe and high-energy batteries.

In situ inorganic conductive network formation in high-voltage single-crystal Ni-rich cathodes

High nickel content in LiNixCoyMnzO2 (NCM, x ≥ 0.8, x + y + z = 1) layered cathode material allows high specific energy density in lithium-ion batteries (LIBs). However, Ni-rich NCM cathodes suffer from performance degradation, mechanical and structural instability upon prolonged cell cycling. Although the use of single-crystal Ni-rich NCM can mitigate these drawbacks, the ion-diffusion in large single-crystal particles hamper its rate capability. Herein, we report a strategy to construct an in situ Li1.4Y0.4Ti1.6(PO4)3 (LYTP) ion/electron conductive network which interconnects single-crystal LiNi0.88Co0.09Mn0.03O2 (SC-NCM88) particles. The LYTP network facilitates the lithium-ion transport between SC-NCM88 particles, mitigates mechanical instability and prevents detrimental crystalline phase transformation. When used in combination with a Li metal anode, the LYTP-containing SC-NCM88-based cathode enables a coin cell capacity of 130 mAh g−1 after 500 cycles at 5 C rate in the 2.75-4.4 V range at 25 °C. Tests in Li-ion pouch cell configuration (i.e., graphite used as negative electrode active material) demonstrate capacity retention of 85% after 1000 cycles at 0.5 C in the 2.75-4.4 V range at 25 °C for the LYTP-containing SC-NCM88-based positive electrode.