Li-ion Pouch Cells Research Articles

Waterborne LiNi0.5Mn1.5O4 Cathode Formulation Optimization through Design of Experiments and Upscaling to 1 Ah Li-Ion Pouch Cells

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is a promising candidate as a lithium-ion battery cathode material to fulfill the high-energy density demands of the electric vehicle industry. In this work, the design of the experiment’s methodology has been used to analyze the influence of the ratio of the different components in the electrode preparation feasibility of laboratory-scale coatings and their electrochemical response. Different outputs were defined to evaluate the formulations studied, and Derringer–Suich’s methodology was applied to obtain an equation that is usable to predict the desirability of the electrodes depending on the selected formulation. Afterward, Solver’s method was used to figure out the formulation that provides the highest desirability. This formulation was validated at a laboratory scale and upscaled to a semi-industrial coating line. High-voltage 1 Ah lithium-ion pouch cells were assembled with LNMO cathodes and graphite-based anodes and subjected to rate-capability tests and galvanostatic cycling. 1 C was determined as the highest C-rate usable with these cells, and 321 and 181 cycles above 80% SOH were obtained in galvanostatic cycling tests performed at 0.5 C and 1 C, respectively. Furthermore, it was observed that the LNMO cathode required an activation period to become fully electrochemically active, which was shorter when cycled at a lower C-rate.

Low pressure cycling of solid state Li-ion pouch cells based on NMC – Sulfide – Nanosilicon chemistry

Bringing solid-state lithium batteries to market is a challenge; in part because their fabrication requires a technically difficult step of solid-electrolyte separator assembly, and because thus far, solid-state cells have only cycled under high pressure. Here, we address both these bottlenecks by assembling lithium-ion batteries based on the argyrodite Li6PS5Cl as the electrolyte using standard coating processes. Moreover, we upscaled to 10 cm2 pouch cells in a dry room by a double casting technique, thanks to the use of fluorine-free polyisobutene as the binder and nanosilicon and LiNi0.6Mn0.2Co0.2O2 (NMC622) respectively as the anode and cathode active materials. Electrode casting is shown to significantly improve the cycling stability when compared to the conventional pelletizing process. The all-coated pouch cells, cycled at a pressure as low as 1 MPa over 160 cycles, show a better capacity retention than the pelletized cells cycling under >100 MPa. The higher performance of the sheet-type cell is thought to originate in the better densification and homogeneity of the cell, attested by its low resistance.

Thermal runaway in commercial lithium-ion cells under overheating condition and the safety assessment method: Effects of SoCs, cathode materials and packaging forms

Overheating, a common field failure of lithium-ion (Li-ion) batteries, can lead to thermal runaway and catastrophic results. Here, overheating behaviors and thermal runaway features of Li-ion cells with different states of charge (SoCs), cathode materials (LiFePO4(LFP), Li[Ni0.5Co0.2Mn0.3]O2 (NCM523)), and packaging forms (pouch and prismatic) are investigated. The overheat-to-thermal runaway process of Li-ion cells can generally be divided into four stages, and the overheating behaviors of the Li-ion cell at each stage are monitored and analyzed. The results show that, under the same overheating condition, with the increase of SoC, the tolerance of LFP Li-ion pouch cell to high-temperature becomes poorer, while the mass loss ratio (i.e. severity of thermal runaway reactions) and the maximum temperature (i.e. thermal runaway hazards) increase. When thermal runaway occurs, the NCM Li-ion cell catches fire and much smoke and sparks are ejected from the rupture, while the LFP Li-ion cells just release a lot of white smoke; the NCM Li-ion cell is more hazardous and exhibits more severe thermal runaway reactions. With respect to the prismatic Li-ion cell, it takes a long time from the opening of the relief valve to the occurrence of thermal runaway although an internal short circuit has occurred; the designed pressure relief valve significantly delays the occurrence of thermal runaway and reduces the thermal runaway risk. In addition, postmortem and SEM analyses are performed on the electrodes harvested from the dead cells after the overheating tests. According to the overheating behaviors and the evolution of characteristic parameters, a modified safety assessment method is proposed. This study can provide important guidance for the safety management of Li-ion cells with different SoCs, cathode materials and packaging forms under the condition of overheating in automotive applications.

How Cyclic Aging at Different Temperatures Affects Safety of Li-Ion Pouch Cells

Li-ion battery safety behavior is critical and should be examined carefully. However, safety testing is typically performed only on pristine cells. This neglects the significant effect of aging conditions, such as ambient temperature, on safety behavior. For example, aging at low temperatures results in Li plating and thus drastically reduces safety [1–3]. Cells aged at higher temperatures with a pronounced solid-electrolyte-interface (SEI) layer on the anode show an increased onset of self-heating leading to improved safety [3,4].The temperature dependent cyclic aging behavior of commercial Li-ion pouch cells is evaluated using an Arrhenius plot. Further Post-Mortem analyses of aged cells from both branches of the V-shaped Arrhenius plot confirm two dominant aging mechanisms: Li plating and SEI growth for the low and high temperature ranges, respectively.Heat-wait-seek (HWS) tests under quasi-adiabatic conditions in an accelerated rate calorimeter (ARC) are used to evaluate safety behavior and identify critical temperatures such as the onset of self-heating. Multiple sensors (voltage, resistance, temperature, strain, and ultrasonic transmission) are placed on the pouch cell to provide a better understanding of the processes taking place during HWS tests. These sensors show that some critical events, such as the onset of swelling, venting, or separator melting, are only slightly affected by aging. In contrast, the onset of self-heating depends on the aging mechanism and state of health (SOH) of the cell, as a more pronounced aging has a greater impact on the change in safety. This knowledge can be used, for example, by cell manufacturers to estimate the minimum safety of aged cells based on tests of new cells.

Real-Time, Non-Invasive Magnetic Field Imaging for Diagnosing Current Flow Patterns inside Fault-Simulated Li-Ion Batteries

With the growing popularity of Li-ion batteries in electric-powered mobility, securing battery safety has become a common goal of the battery community. Battery failures are commonly caused by small, undetectable defects that can form during manufacturing and operation, ranging from materials to the cell level. Although the small errors inside the cells trigger catastrophic failures, tracing them and distinguishing cell failure modes without cell anatomy remains challenging. Given that several defects within the cell could cause abnormal current flow compared to the normal cells, visualization of the current inside the battery is informative to find and trace the invisible defects that cannot be detected using imaging tools for the internal structure of fully packaged commercial cells.This study presents a real-time, non-invasive magnetic field imaging (MFI) method that can signal the magnetic field induced by battery current flow inside the Li-ion pouch cell during cycling (Figure 1). MFI can offer high-speed scanning in a few minutes (~100 mm/min) and spatially high resolution (0.161 mm2) and immediately measure the magnetic field with no need for shielding commonly required for MRI measurement. Based on the Biot-Savart law, we validated magnetic field-current correlation through the MFI of current-carrying 1D straight wires and 2D planes. Then, we found the optimal pouch cell structure for visualizing the current flow patterns using MFI and successfully converted the MFI contour map of the pouch cells into the normalized current distribution. To demonstrate the efficacy of current pattern analysis, we have collected MFI results of fault-simulated batteries (FSBs) to distinguish failure modes that intentionally possess cell manufacturing faults, such as lead-tab contact failure, electrode alignment, and stacking faults. Magnetic field offset using countercurrent-carrying, pouch cell-shaped 2D conductor underneath can selectively detect the failure spots where the current abnormally flows. It is believed that direct visualization of current distribution patterns and figuring out the pattern changes during operation can expedite the non-destructive, immediate diagnosis of commercial cells and troubleshoot safety-related challenges. Figure 1. Single-layer pouch cells and in operando MFI analysis during charging. (a) Scheme for internal structure of single-layer pouch cell (60 × 90 mm2) with counter-side, center-aligned tabs (CS-C cell). (b) Actual image of CS-C cell and MFI scan and applied current directions during the cell charging. MFI mapping images for (c) Bx , (d) By and (e) Bz vector fields at the cell. White arrows within each MFI contour map indicate the current flow prediction. Applying current density: 5.09 mA cm−2, the cell was situated 6 mm below the sensor (z-distance: 2.5 mm, scanned side: Cathode-side up). The total number of scanned data points are 1699 (11 cm) and 64 (16 cm) in y- and x-directions, respectively. Figure 1

Safety Evaluation of Temperature-Dependent Aging Mechanisms of Li-Ion Batteries Using ARC Tests with Multiple Online Sensors

Compared to many other characteristics of Li-ion batteries, the safety behavior is a critical property and should be examined carefully. Especially manufacturers and most studies only focus on the safety of pristine cells. However, the safety behavior can drastically change dependent on the prior aging conditions such as ambient temperature. Especially low temperature aged cells with Li plating show a drastically decreased safety behavior [1]. Moreover, cells aged at higher temperatures with a distinct solid-electrolyte-interface (SEI) layer on the anode show an increased onset of self-heating leading to improved safety [2]. Post-Mortem analysis of aged cells confirmed the two dominating aging mechanisms: Li plating and SEI growth for low and high temperature range, respectively.Heat-wait-seek (HWS) tests under quasi-adiabatic conditions in an accelerated rate calorimeter (ARC) are a well-known method to evaluate the safety behavior of Li-ion batteries and can reveal critical temperatures such as the onset of self-heating. HWS tests on cells aged at different temperatures until different state-of-health (SOH) reveal the trend that a more pronounced aging increases or decreases the onset of self-heating depending on whether SEI growth or Li plating is the dominating aging mechanism.In addition to the temperature measurement, multiple sensors for voltage, resistance, strain, and ultrasonic transmission and reflection were placed on the pouch cell for a better understanding of the ongoing processes while the HWS tests. The benefit of these sensors is that they are cheap, allowing them to be widely implemented. Furthermore, an externally coupled mass spectrometer provides information about the gases produced and released after venting of the cell [3]. By Correlating different sensors signals, critical events on the way to the thermal runaway can be reliably identified or even predict in an early stage.Hence, the HWS tests on Li-ion batteries aged at different temperatures in combination with the multi sensor study provide a comprehensive understanding of the influence of temperature dependent aging state on the safety behavior of Li-ion pouch cells. Acknowledgment We gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) for the financial support of the projects AnaLiBa (03XP0347C) and MiCha (03XP0317C) within the AQua cluster.

An efficient detailed layer model for prediction of separator damage in a Li-Ion pouch cell exposed to transverse compression

Li-ion pouch cells can be understood as thin-layer laminates. Pressure loads normal to flat electrodes can cause failure or thermal runaway. However, detailed models for investigating transverse pressure loads in which each layer is resolved are computationally intensive. This paper outlines two approaches for time-efficient yet accurate detailed explicit finite element models of a cell. Both approaches are characterised by the fact that they are divided into in-plane and out-of-plane behaviour. The modelling of interlaminar contacts is completely omitted, to prevent parasitic contact compliance and efforts for contact handling. A discrete element formulation is used for a computationally efficient simulation of transverse loads. This prevents a reduction of the critical time step, which can result from transversal deformations. The difference between the two modelling approaches lies in the modelling of the out-of-plane behaviour. In the first approach, the behaviour in the out-of-plane direction is generated by characterising each component separately. The second approach assumes homogenous behaviour in the thickness direction. Both approaches rely on characterization data derived from component and cell tests. The cell models are validated against cell tests. Numerical results obtained with the two modelling approaches differ only slightly from the validation data.

An investigation into the effects of state of charge and heating rate on propagating thermal runaway in Li-ion batteries with experiments and simulations

As large systems of Li-ion batteries are being increasingly deployed, the safety of such systems must be assessed. Due to the high cost of testing large systems, it is important to extract key safety information from any available experiments. Developing validated predictive models that can be exercised at larger scales offers an opportunity to augment experimental data In this work, experiments were conducted on packs of three Li-ion pouch cells with different heating rates and states of charge (SOC) to assess the propagation behavior of a module undergoing thermal runaway. The variable heating rates represent slow or fast heating that a module may experience in a system. As the SOC decreases, propagation slows down and eventually becomes mitigated. It was found that the SOC boundary between propagation and mitigation was higher at a heating rate of 50 °C/min than at 10 °C/min for these cells. However, due to increased pre-heating at the lower heating rate, the propagation speed increased. Simulations were conducted with a new intra-particle diffusion-limited reaction model for a range of anode particle sizes. Propagation speeds and onset times were generally well predicted, and the variability in the propagation/mitigation boundary highlighted the need for greater uncertainty quantification of the predictions.

Unraveling SEI formation and cycling behavior of commercial Ni-rich NMC Li-ion pouch cells through operando optical characterization

Lithium-ion batteries, relying on Ni-rich NMC (LiNixMnyCo1−x−yO2) phases, are widely used in various applications due to their high energy density and extended cycle life. However, further improvements are possible with a deeper understanding of their internal chemistry in real-world conditions. Here, using optical fiber Bragg grating sensors placed inside commercial pouch cells, we demonstrate the feasibility of monitoring the heat release during the formation cycles and real-driving cycling. By collecting temperature data for Li-ion cells using different types of NMCs, we were able to extract the contribution of the anode and the impact of additives on overall heat generation. We also present a comparison of the heat generation of NMC-based batteries under varying power and voltage levels, which provides insights into battery safety in dynamic, real-world environments. Our study reveals the importance of continuously monitoring batteries heat release throughout its lifespan to enhance our understanding of the interconnected materials surfaces and electrolyte reactions.

Thermal behaviour of Li-ion battery: An improved electrothermal model considering the effects of depth of discharge and temperature

The current paper presents a two-dimensional electrothermal model that evaluates the thermal and voltage behaviour of a LiFePO4-20 Ah Li-ion pouch cell. The electrothermal model considers the impact of depth of discharge (DoD) and temperature on heat generated inside Li-ion cells by precisely simulating the internal equivalent resistance and entropy change utilizing experimental data. The proposed electrothermal model is validated utilizing experimental data. The results for the temperature estimation utilizing the electrothermal model show a similar result for all discharge current rates. The temperature prediction for all the discharge current rates (C-rate) shows slightly more accurate and precise results toward the end of discharge of Li-ion pouch cell. Overall, the absolute maximum relative error for the temperature prediction is around 6.30 % at 4C discharge rate. Similarly, the voltage estimation shows a high level of accuracy compared to the experimental data, and the simulated voltage has slightly higher values than the experimental voltage data, but the voltage trend is similar to the experimental data. Overall, the simulated results obtained from the proposed electrothermal model exhibit similar trends and high accuracy when compared to the experimental data.

Safety assessment of thermal runaway behavior of lithium-ion cells with actual installed state

To quantitatively evaluate the overcharge-to-thermal runaway (OC-to-TR) behaviors of lithium-ion (Li-ion) cells in practice, here reported are overcharge tests on Li-ion pouch cells with NCM811 cathode under actual restricted condition. Such a restriction results from two splints during the overcharging process to simulate their actual installed state in the battery pack of electric vehicles (EVs). From the monitoring of the cell voltage, temperature, and expansion force, a unique feature that is discovered in this work is that the OC-to-TR process is strongly correlated to the evolution of expansion force, which can further be divided into five stages with distinctly different features. Moreover, this unique correlation depends critically on the current rate (C-rates) and preload. It is concluded that the Li-ion cells were more hazardous at higher C-rates; when a larger preload was applied to the cell, it exhibited better tolerance to overcharge, but the TR reactions became more hazardous (i.e., higher temperature and more severe TR reactions). These findings allow us to propose a safety evaluation method to evaluate the TR risks and TR hazards for Li-ion cells under different restricted conditions. In addition, a safety management method was proposed based on the evolution of several key parameters. The research results can provide some useful information to the safety design of Li-ion pouch cells and the enhancement of the safety function of battery management system.

Change of safety by main aging mechanism – A multi-sensor accelerating rate calorimetry study with commercial Li-ion pouch cells

The safety of Li-ion batteries is the most critical of many important characteristics and must be carefully considered. However, depending on the aging conditions, safety of batteries can change drastically. We had recently introduced ARC-MS as a new tool for safety investigations, which is an updated accelerating rate calorimeter (ARC) combined with a mass spectrometer (MS). Additionally, our ARC-MS setup is equipped by seven more low cost sensors (temperature, voltage, resistance, audio, strain, and transmitted and reflected ultrasound). In this paper, the dependence of safety on the dominant aging mechanism as a function of aging temperature, and state-of-health (SOH) is investigated on the example of 3.3 Ah commercial pouch cells. The ambient temperature during aging has a strong influence on the dominant aging mechanism (Li plating or SEI growth) of this cell as shown by Arrhenius plots of the aging rate and Post-Mortem analyses (ICP-OES, SEM, EDX, Hg porosimetry, STA) of aged cells. Our in-depth evaluation shows that different sensors can provide indications of safety relevant events (onset of swelling, onset of self-heating, cell venting, stable exothermic reactions, separator melting, and thermal runaway) in the cells independently. Combining these low-cost sensors might be a solution for detection of unsafe cells in applications.

A smart risk-responding polymer membrane for safer batteries.

Safety concerns related to the abuse operation and thermal runaway are impeding the large-scale employment of high-energy-density rechargeable lithium batteries. Here, we report that by incorporating phosphorus-contained functional groups into a hydrocarbon-based polymer, a smart risk-responding polymer is prepared for effective mitigation of battery thermal runaway. At room temperature, the polymer is (electro)chemically compatible with electrodes, ensuring the stable battery operation. Upon thermal accumulation, the phosphorus-containing radicals spontaneously dissociate from the polymer skeleton and scavenge hydrogen and hydroxyl radicals to terminate the exothermic chain reaction, suppressing thermal generation at an early stage. With the smart risk-responding strategy, we demonstrate extending the time before thermal runaway for a 1.8-Ah Li-ion pouch cell by 100% (~9hours) compared with common cells, creating a critical time window for safety management. The temperature-triggered automatic safety-responding strategy will improve high-energy-density battery tolerance against thermal abuse risk and pave the way to safer rechargeable batteries.

Safety Assessment of High Dynamic Pre-Loaded Lithium Ion Pouch Cells

The knowledge of the influence of high dynamic loads on the electrical and mechanical behavior of lithium-ion cells is of high importance to ensure a safe use of batteries over the lifetime in electric vehicles. For the first time, the behavior of six commercial Li-Ion pouch cells after a constrained short-time acceleration (300 g over 6 ms) with a resulting cell surface pressure of 9.37 MPa was investigated. At this load, two out of six cells suffered from an internal short circuit, showing several damaged separator layers across the thickness in the area of the cell tabs. For the cells that remained intact, a range of measurement techniques (e.g., inner resistance measurement, electrochemical impedance spectroscopy (EIS), or thermal imaging) was used to reveal changes in the electrical property resulting from the load. The cells without short circuit show an increase of internal resistance (average of 0.89%) after the dynamic pre-load. The electric circuit model based on the EIS measurement indicates a decrease of the resistance R1 up to 30.8%. Additionally, mechanical properties of the cells in an abuse test subsequent to the dynamic pre-load were significantly influenced. The pre-loaded cell could sustain an 18% higher intrusion depth before electrical failure occurred as compared to a fresh cell in an indentation test. The results of this study revealed that a high acceleration pulse under realistic boundary conditions can lead to critical changes in a battery cell’s properties and needs to be taken into account for future safety assessments.

Characterization and modelling of Al and Cu busbar during charging and discharging of Li-ion battery for electric vehicles

• The simulated temperature of the Al-Cu busbar attached to the Li-ion pouch cell is validated with the experimental result. • The temperature generation in the Li-ion cell is influenced by the electrical contact resistance of the weld joint. • The joint resistance of a busbar is mainly dependent on intermetallic formation and grain size at the weld joint of a busbar. The electrical components such as bimetallic busbar joints of the lithium-ion (Li-ion) batteries should be able to withstand high voltages during charge and discharge processes. The busbar is an essential component that transmits high power to electrify the vehicle. The present study describes the sustainability of friction stir welded (FSW) busbar at different C-rates by simulating a Li-ion battery attached to a busbar, then correlating the heat generation of simulation results with an experimental result at 1, 1.5, and 2C-rates. The change in process parameters of FSW samples varies with electrical conductivity at the weld interface. The variation in electrical conductivity with different busbars is due to the formation of various intermetallic and changes in the grain size of the Al and Cu joints. However, the busbar with Cu-rich intermetallic exhibits smaller electrical resistivity. The specific electrical contact resistance of a busbar is obtained from simulation by validating the heat generated during constant time charge-discharge cycles. The temperature rises due to contact resistance in the Al-Cu busbar which can lead to thermal runaway and, eventually, short circuits in the Li-ion battery pack. Based on previous simulation parameters, the Li-ion cells are simulated at 5 and 10C-rates to understand thermal runaway behaviour.

Lithium Difluoro(dioxalato) Phosphate as an Electrolyte Additive for NMC811/Graphite Li-ion Pouch Cells

Lithium difluoro(dioxalato)phosphate (LiDFDOP) has been systemically studied as an electrolyte additive singly and in combination with co-additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) in LiNi0.8Mn0.1Co0.1O2 (NMC811)/artificial graphite (AG) pouch cells. Long-term cycling tests at room and elevated temperatures (20 °C, 40 °C, and 55 °C) with different upper cutoff voltages (4.06 V and 4.20 V) were performed. These results were combined with ultra-high precision coulometry (UHPC), ex-situ gas measurements, and automatic cell storage tests to reveal multiple aspects of cell performance. A density functional theory (DFT) calculation has also been performed and compared to formation data to reveal the mechanistic aspects of LiDFDOP reduction. Radar plots and a figure-of-merit (FOM) approach were further utilized to summarize results and rank additive and additive combination performance for the NMC811/AG cells. This work highlights an effective additive and suitable co-additives for use in NMC811/graphite cells and gives important insights for future electrolyte additive studies.

Unveiling the relationship between micro characteristics of particles and electrode performance in a 60 Ah high-energy-density Li-ion pouch cell

In order to get a super comprehensive performance, electrodes in high energy density batteries always consist of particles with different features. Thus, a deep insight into the interaction between particles can help to further improve the electrode performance. In this paper, an extended pseudo-2-dimension (P2D) model based on a 60 Ah high-energy-density (HED) Li-ion pouch cell (≥280 Wh kg−1) with Si-based anode/Ni-rich cathode couple is established and the internal micro characteristics within electrodes are investigated. A competitive relationship in the direction between particle radial and electrode thickness is observed, and therefore electrodes composed of more large particles have a higher utilization at aluminum (Al) foil current collector. There is a big difference in the average state-of-charge (SOC) between the two particles. Fortunately, the huge difference between particles can be balanced out during the constant voltage process or rest period. According to the model, electrodes with big particles besides the separator and small particles besides the current collector may exhibit the best overall performance and an appropriate kinetic matching between cathode and anode is more beneficial to high-rate charging, rather than a superior positive electrode alone, taking the lithium plating risk into account.

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