Challenges and Opportunities for Second-life Batteries: A Review of Key Technologies and Economy

Due to the increasing volume of Electric Vehicles in automotive markets and the limited lifetime of onboard lithium-ion batteries (LIBs), the large-scale retirement of LIBs is imminent. The battery packs retired from Electric Vehicles still own 70%-80% of the initial capacity, thus having the potential to be utilized in scenarios with lower energy and power requirements to maximize the value of LIBs. However, spent batteries are commonly less reliable than fresh batteries due to their degraded performance, thereby necessitating a comprehensive assessment from safety and economic perspectives before further utilization. To this end, this paper reviews the key technological and economic aspects of second-life batteries (SLBs). Firstly, we introduce various degradation models for first-life batteries and identify an opportunity to combine physics-based theories with data-driven methods to establish explainable models with physical laws that can be generalized. However, degradation models specifically tailored to SLBs are currently absent. Therefore, we analyze the applicability of existing battery degradation models developed for first-life batteries in SLB applications. Secondly, we investigate fast screening and regrouping techniques and discuss the regrouping standards for the first time to guide the classification procedure and enhance the performance and safety of SLBs. Thirdly, we scrutinize the economic analysis of SLBs and summarize the potentially profitable applications. Finally, we comprehensively examine and compare power electronics technologies that can substantially improve the performance of SLBs, including high-efficiency energy transformation technologies, active equalization technologies, and technologies to improve reliability and safety

Mechanical failure of lithium-ion batteries

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 223-244).The commercialization of lithium-ion batteries has accelerated the electrification process of vehicles. In the past decade, one could see great advances in the life span, cost, performance, specific energy, and specific power of batteries. At the same time, the safety of batteries has not been adequately addressed by most stakeholders in the Electric Vehicle market. The present thesis systematically investigates the deformation mechanisms of the multi-layered structure of lithium-ion battery cells subjected to various loading conditions with particular emphasis on predicting the onset of the electrical short circuit. It starts with a comprehensive testing and modeling study of all the components of the cell, including the current collectors, the separator, the pouch/shell casing, and particularly, the coatings of electrodes.A detailed computational model for quasi-static loading is subsequently established in Abaqus/explicit, which is very effective to predict the load-displacement response, peak load, displacement to fracture and short circuit, as well as the shear fracture phenomenon. The computational model is then extended to cover the effect of strain rate dependence by introducing the poro-mechanical theory. Darcy's law is used to describe the flow of the electrolyte inside the granular structure of the coating, and the Kozeny-Carman equation is adapted to calculate the permeability of the porous media of the battery cell. The model is shown to accurately predict the strengthening effect of the battery cell under low-speed dynamic loading, observed in experiments. The effect of mechanical deformations of a battery cell on its electrochemical performance is investigated next through a series of control tests on the coin-cell type batteries made of deformed electrodes.The batteries are tested with ten cycles of charge-discharge, and a clear capacity fade in the damaged cells compared with the undamaged ones is observed. Electrochemical impedance spectroscopy tests are then performed, and the possible mechanism of the capacity fade is proposed. In the last part of the thesis, two applications of the developed computational modeling strategy are exhibited. One is the axial deformation of the 18650 cylindrical cells, and the other is the protective structural design of EV battery pack subjected to a "ground impact".by Juner Zhu.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineerin

Prediction of shear crack formation of lithium-ion batteries under rod indentation : Comparison of seven failure criteria

One of the major catastrophic events in the accidents involving electric vehicles is the electric short circuit, leading to the thermal runaway, and possible fire and explosion. The short circuit is a result of the development of local or through-thickness fracture inside a cell. Fracture of the discrete layered structure of lithium-ion batteries is a complex problem involving six materials with completely different deformation and fracture properties. The homogenized model of the deformation of batteries has emerged as the best compromise between simplicity and accuracy, and therefore the well-established Deshpande–Fleck model of crushable foams is used in the present analysis to describe the deformation behavior. For the fracture description of battery cells, the authors follow the experience accumulated over the decades in predicting failure of metals and geomaterials. The constitutive description of these classes of materials gained in complexity, requiring elaborated experimental techniques for the determination of material parameters. However, the choice of possible tests for pouch or cylindrical cells is very limited, which hinders the complexity in fracture models. Those tests are typically in-plane and out-of-plane compression tests because it is almost impossible to subject an individual pouch battery to tensile or shear loading. Therefore, out of vast literature in the field of fracture, only the simplest models are considered. In this study, the seven simplest fracture models, involving only one or maximum two material constants, are coupled with the homogenized Deshpande–Fleck model to study the fracture behavior of pouch cells. Their performance is critically evaluated in terms of the capability of predicting the initiation and propagation of crack under mechanical transverse loading.Peer reviewe

Recurrent neural network modeling of the large deformation of lithium-ion battery cells

As the automotive industry transitions from combustion to electric motors, there is a growing demand for efficient computational models that can describe the homogenized large deformation response of Li-ion batteries. Here, a detailed three-dimensional unit cell model with periodic boundary conditions is developed to describe the large deformation response of a typical anode-separator-cathode lay-up of a pouch cell. The model makes use of a Deshpande-Fleck foam model for the porous polymer separator and Drucker-Prager cap models of the granular cathode and anode coatings. Using the unit cell model, the stress-strain response of a battery cell is computed for 20’000 random loading paths in the six-dimensional strain space. Based on this data, a recurrent neural network (RNN) model is trained, validated and tested. It is found that an RNN model composed of two gated recurrent units in series with a deep fully connected network is capable to describe the large deformation response with a high level of accuracy. As a byproduct, it is shown that advanced conventional constitutive models such as the anisotropic Deshpande-Fleck model cannot provide any predictions of satisfactory accuracy.ISSN:0749-6419ISSN:1879-215

Mechanical Deformation of Lithium-Ion Pouch Cells under In-Plane Loads-Part I

During an accident of an electric vehicle, the battery pack can be damaged by the intrusion of an external object, causing large mechanical deformation of its lithium-ion battery cells, which may result in an electrical short circuit and subsequently the possible thermal runaway, fire, and even explosion. In reality, the external objects can come in different directions, for example, an out-of-plane indentation that perpendicularly punches the large surface of the pouch cell and an in-plane loading that compresses the thin edge of the cell. In this study, the mechanical deformation of a large-format lithium-ion pouch cell under in-plane loads is investigated via three different types of tests - in-plane compression of fully constrained cells, in-plane compression of cells sandwiched by foams, and in-plane indentation by a round punch. A special apparatus is designed to apply different boundary conditions on the cell, and the deformation history, especially the formation of the buckles of the cells, are monitored by two digital cameras. Post-testing structural analysis is carried out by a cross-sectional cutting and polishing procedure, which gives clear evidence of buckling of all the component layers.Peer reviewe

Mechanical Deformation of Lithium-Ion Pouch Cells under in-plane Loads-Part II

Based on the experimental observation, pouch cells can withstand severe deformation during fully confined in-plane compression with flat punches without any risks of a short circuit. During the deformation, the structuralbehavior is characterized by regular kinks, buckles, and shear bands. This study aims to provide a modeling approach for the in-plane compression on lithium-ion pouch batteries in a fully confined case with a flat punch. To capture the right mechanism of buckling while maintaining a satisfactory computational efficiency, two approaches are proposed: a homogenized model with imperfections and an enhanced homogenized model with equivalent layers of metal foils. The first approach introduces periodic geometrical imperfections with a wavelength as observed in the experiments. The second one creates a model in between the homogenized model and detailed model with equivalent properties of coating materials and metal foils. It is concluded that the introduction of imperfections could not correctly capture the folding mechanism, while with the latter hybrid approach, it is possible to capture the right progressive folding pattern of the battery cells during the in-plane compression test. Different potential approaches of the simulation model are investigated for obtaining a better agreement of the prediction and the measured experimental load-displacement response.Peer reviewe

Nondestructively Visualizing and Understanding "Soft Short" and Li Creeping in All-solid-state Lithium-Metal Batteries

All-solid-state Li-metal batteries (ASLMBs) have the potential to outperform conventional Li-ion batteries in terms of high energy density and safety; however, their applications are challenged by the dendrite-related short circuit. Therefore, it is imperative to understand the mechano-electro-chemo behavior of Li. For the first time, we nondestructively visualize the Li behavior in ASLMBs through operando neutron imaging and X-ray computed tomography (XCT). The 2D neutron radiography tracks the real-time Li evolutions before and after the "soft short". The 3D neutron tomography evidences the Li-metal deformation, and XCT provides 3D views of the battery after the "soft short". Despite "soft short", our observation indicates that Faradaic processes persist. Meanwhile, the coupling of stacking pressure and plating-induced stress triggers the Li-metal deformation toward the current collector and the SE, which results in the "soft short". This work inspires future research on stabilizing the Li metal in ASLMBs from the mechano-electro-chemo aspect

Unveiling the Mechanical and Electrochemical Evolution of Nano Silicon Composite Anodes in Sulfide based All-solid-state Batteries

The utilization of silicon (Si) anodes in all-solid-state lithium batteries (ASLBs) provides the potential for high energy density. However, the compatibility of sulfide solid-state electrolytes (SEs) with Si and carbon is often questioned due to potential decomposition. To investigate this, operando X-ray absorption near-edge structure (XANES) spectroscopy, ex-situ scanning electron microscopy (SEM) and ex-situ X-ray nano-tomography (XnT) were utilized to study the chemistry and structure evolution of nano Si composite anodes. Results from XANES demonstrated a partial decomposition of SEs during the first lithiation stage, which was further accelerated by the presence of carbon. But the performance of first three cycles in Si-SE-C was stable, which proved the generated media is ionically conductive. XnT and SEM results showed that the addition of SEs and carbon improved the structural stability of the anode with fewer pores and voids. A chemo-elasto-plastic model revealed that SEs and carbon buffered the volume expansion of Si, thus enhancing mechanical stability. The balance between the pros and cons of SEs and carbon in enhancing reaction kinetics and structural stability enabled the Si composite anode to demonstrate the highest Si utilization with higher specific capacities and better rate than pure Si and Si composite anodes with only SEs