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Predicting thermal safety events of lithium-ion (Li-ion) batteries is significant in optimizing electrochemical systems with high thermal tolerance. The safety performances of Li-ion batteries are dictated by the thermal stability of their component materials both individually and collectively due to intricate exothermic reactions. Although the heat release of individual battery material has been thoroughly investigated, the safety hazards of inter-electrode chemical crosstalk under thermal abuse scenarios remain elusive and thus need a fundamental understanding. This study carries out a comprehensive thermal analysis of various material samples harvested from a commercial Li-ion cell using differential scanning calorimetry (DSC), complemented with full-cell accelerating rate calorimetry (ARC) and computational modeling. Reaction kinetics of electrolyte, wet cathode, wet anode and DSC-full cell samples imitating cell layered architectures are delineated to reveal substantial thermal interactions between electrodes. High-resolution kinetic parameters of reaction mechanisms are estimated using a synergy of Kissinger's method and mechanism-driven non-linear optimization strategies. A thermal abuse model is built based on the extracted kinetic parameters to simulate the cell-level thermal runaway phenomenon and compared with experimental observations, indicating how interlayer crosstalk effects significantly impact the thermal safety characteristics of Li-ion cell chemistries.

High temperature gases released through the safety vent of a lithium-ion cell during a thermal runaway event contain flammable components that, if ignited, can increase the risk of thermal runaway propagation to other cells in a multi-cell pack configuration. Computational fluid dynamics (CFD) simulations of flow through detailed geometric models of four vent-activated commercial 18650 lithium-ion cell caps were conducted using two turbulence modeling approaches: Reynolds-averaged Navier-Stokes (RANS) and scale-resolving simulations (SRS). The RANS method was compared with independent experiments of discharge coefficient through the cap across a range of pressure ratios and then used to investigate the ensemble-averaged flow field for the four caps. At high pressure ratios, choked flow occurs either at the current collector plate when flow through the current collector plate is more restrictive or the positive terminal vent holes when flow through the current collector plate is less restrictive. Turbulent mixing occurred within the vent cap assembly, in the jets emerging from the vent holes, and in recirculating zones directly above the vent cap assembly. The global maximum turbulent viscosity ratio (μT/μ) of the MTI, LG MJ1, K2, and LG M36 caps at pressure ratio ofP1/P2= 7 were 4,575, 3,360, 3,855, and 2,993, respectively. SRS and RANS simulations showed that both velocity magnitude and fluctuating velocity magnitude were lower for vent holes which are obstructed by the burst disk. SRS showed high levels of fluctuating velocity in the jets, up to 48.5% of the global maximum velocity. The present CFD models and the resulting insights provide the groundwork for future studies to investigate how jet structure and turbulence levels influence combustion and heat transfer in propagating thermal runaway scenarios.

A pouch cell method for retesting double-sided electrodes harvested from commercial lithium-ion batteries in a 3-electrode cell arrangement has been developed. By relying on pressure from restraint plates to make tab electrical connections, this method (1) requires no welding, (2) does not require a dry room, (3) does not require precision sealing equipment inside of an inert-gas glove box, and (4) does not require removal of composite material off of one side of the electrode which may compromise the composite to be tested on the other side. Lithium chips pressed onto copper mesh serve as the reference and counter electrodes and the electrolyte used was 1.0 M LiPF6 1:1 ethylene carbonate (EC):Diethyl carbonate (DEC) v/v. Electrochemical cycling of electrodes from a commercial 3.6 Ah 18650 lithium-ion cell demonstrated cell function and showed stable capacity and potential charge/discharge profiles after two cycles for the cathode and four cycles for the anode. The areal capacity of the anode and cathode was determined to be 5.50 ± 0.31 and 5.50 ± 0.30 mAh/cm2, respectively, based on a potential range of 0.005–1.5 V versus Li/Li+ for the anode and 3.0–4.25 V versus Li/Li+ for the cathode. High frequency, 500 kHz impedance measurements of the anode and cathode cells shows a real impedance of 1.55 Ohms and 2.17 Ohms, respectively, which is similar to prior studies on pouch cells with continuous tabs of similar capacity.

In this work, two methods were investigated for determining the composition of carbonate solvent systems used in lithium-ion (Li-ion) battery electrolytes. One method was based on comprehensive two-dimensional gas chromatography with electron ionization time-of-flight mass spectrometry (GC×GC/EI TOF MS), which often enables unknown compound identification by their electron ionization (EI) mass spectra. The other method was based on comprehensive two-dimensional gas chromatography with flame ionization detection (GC×GC/FID). Both methods were used to determine the concentrations of six different commonly used carbonates in Li-ion battery electrolytes (i.e., ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC) in model compound mixtures (MCMs), single-blind samples (SBS), and a commercially obtained electrolyte solution (COES). Both methods were found to be precise (uncertainty < 5%), accurate (error < 5%), and sensitive (limit of detection

Physics-based aging models are critical for understanding capacity degradation mechanisms in Li-ion batteries. This paper presents a technique for aging analysis of a Li-ion cell due to growth of the solid electrolyte interphase (SEI) layer driven by a solvent decomposition reaction at the electrode surface. The model employs an iterative technique based on the analytical solutions of the underlying conservation equations. The single-particle model describing Li-ion intercalation and de-intercalation processes is solved analytically using Green's function technique. The SEI formation problem is solved using the integral balance method. An iterative technique that combines these analytical solutions is shown to result in a converged result within a few iterations. The model is shown to agree well with results from past studies, as well as a numerical simulation. The capacity fade of Li-ion batteries is investigated under different operating conditions and different regimes, including both cycling and storage. The present model offers much faster computation time than numerical models for modeling the degradation of Li-ion cells. Further, the iterative technique described here may serve as a framework for semi-analytical solutions for other, more complicated problems. This work contributes towards improving the performance and reliability of electrochemical energy conversion and storage systems.

Lithium batteries have a tendency to fail violently under adverse conditions leading to the rapid venting of gas. Overcharge, thermal heating, and a combination of the two conditions are applied here to investigate the gas venting process. A test chamber has been constructed with data recordings including chamber pressure and temperature, battery voltage, current, and surface temperature as functions of time throughout the charging and failure processes. High-speed imaging and schlieren flow visualization are used to visualize the gas venting process. A direct comparison between lithium iron phosphate based K2 26650 and lithium nickel manganese cobalt oxide LG 18650 cells is made through a test series of the three failure methods. Failure under thermal, overcharge, and thermal-overcharge conditions are generally similar in terms of the gas venting process, but are observed to have increasingly energetic failures. The thermal-overcharge abuse condition demonstrates an ability to reconnect via internal short circuit even after an initial electrical failure seen as the refusal to accept charge. This reconnection is associated with a secondary, more energetic failure which can produce weak shock pressure waves.

The current interrupt device (CID) and vent mechanism in the cap of 18650 lithium-ion cells decrease thermal runaway risks by electrically isolating the cell upon internal pressure increase and relieving internal pressure before case rupture, respectively. Geometry analysis, CID- and vent-activation pressure measurement, and finite element analysis mechanical simulation of the vent mechanism were performed for MTI and LG MJ1 18650 caps. The MTI cap used a spot-weld and the LG MJ1 cap used a notch-groove connection for the CID. The vent-activation mechanism was a notch-grooved disk for both caps. The CID-activation pressures were 1.058 ± 0.053 and 1.293 ± 0.119 MPa at ambient temperature and 0.920 ± 0.076 and 1.066 ± 0.068 MPa at 100°C for the MTI and LG MJ1 caps, respectively. The vent-activation pressures were 2.308 ± 0.196 and 2.202 ± 0.083 MPa at ambient temperature and 1.919 ± 0.132 and 1.866 ± 0.084 MPa at 100°C for the MTI and LG MJ1 caps, respectively. Mechanical simulations predicted vent-activation pressures in agreement with experiments and showed decrease maximum stress sensitivity to applied pressure near the vent-activation pressure, consistent with observed sample-to-sample vent-activation pressure variation. Mechanical simulation also predicted vent-activation pressures of 0.448 and 0.400 MPa for the MTI and LG MJ1 caps, respectively, at 300°C.

Measurement and modeling of thermal phenomena in Li-ion cells is a critical research challenge that directly affects both performance and safety. Even though the operation of a Li-ion cell is in most cases a transient phenomenon, most available thermal models for Li-ion cells predict only steady-state temperature fields. This paper presents the derivation, experimental validation and application of an analytical model to predict the transient temperature field in a cylindrical Li-ion cell in response to time-varying heat generation within the cell. The derivation is based on Laplace transformation of governing energy equations, and accounts for anisotropic thermal conduction within the cell. Model predictions are in excellent agreement with experimental measurements on a thermal test cell. The effects of various thermophysical properties and parameters on transient thermal characteristics of the cell are analyzed. The effect of pulse width and cooling time for pulsed operation is quantified. The thermal response to multiple cycles of discharge and charge is computed, and cell-level trade-offs for this process are identified. The results presented in this paper may help understand thermal phenomena in Li-ion cells, and may contribute towards thermal design and optimization tools for energy conversion and storage systems based on Li-ion cells.

While Lithium-ion batteries have the potential to serve as an excellent means of energy storage, they suffer from several operational safety concerns. Temperature excursion beyond a specified limit for a Lithium-ion battery triggers a sequence of decomposition and release, which can preclude thermal runaway events and catastrophic failure. To optimize liquid or air-based convective cooling approaches, it is important to accurately model the thermal response of Lithium-ion cells to convective cooling, particularly in high-rate discharge applications where significant heat generation is expected. This paper presents closed-form analytical solutions for the steady-state temperature profile in a convectively cooled cylindrical Lithium-ion cell. These models account for the strongly anisotropic thermal conductivity of cylindrical Lithium-ion batteries due to the spirally wound electrode assembly. Model results are in excellent agreement with experimentally measured temperature rise in a thermal test cell. Results indicate that improvements in radial thermal conductivity and axial convective heat transfer coefficient may result in significant peak temperature reduction. Battery sizing optimization using the analytical model is discussed, indicating the dependence of thermal performance of the cell on its size and aspect ratio. Results presented in this paper may aid in accurate thermal design and thermal management of Lithium-ion batteries.

Cylindrical Li-ion cells have demonstrated among the highest power density of all Li-ion cell types and typically employ a spiral electrode assembly. This spiral assembly is expected to cause large anisotropy in thermal conductance between the radial and axial directions due to the large number of interfaces between electrode and electrolyte layers in the radial conduction path, which are absent in the axial direction. This paper describes a novel experimental technique to measure the anisotropic thermal conductivity and heat capacity of Li-ion cells using adiabatic unsteady heating. Analytical modeling of the method is presented and is shown to agree well with finite-element simulation models. Experimental measurements indicate that radial thermal conductivity is two orders of magnitude lower than axial thermal conductivity for cylindrical 26650 and 18650 LiFePO 4 cells. Due to the strong influence of temperature on cell performance and behavior, accounting for this strong anisotropy is critical when modeling battery behavior and designing battery cooling systems. This work improves the understanding of thermal transport in Li-ion cells, and presents a simple method for measuring anisotropic thermal transport properties in cylindrical cells.