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Battery thermal management systems are critical for high performance electric vehicles, where the ability to remove heat and homogenise temperature distributions in single cells and packs are key considerations. Immersion cooling, which submerges the battery in a dielectric fluid, has the potential of increasing the rate of heat transfer by 10,000 times relative to passive air cooling. In 2-phase systems, this performance increase is achieved through the latent heat of evaporation of the liquid-to-gas phase transition and the resulting turbulent 2-phase fluid flow. However, 2-phase systems require additional system complexity, and single-phase direct contact immersion cooling can still offer up to 1,000 times improvements in heat transfer over air cooled systems. Fluids which have been considered include: hydrofluoroethers, mineral oils, esters and water-glycol mixtures. This review therefore presents the current state-of-the-art in immersion cooling of lithium-ion batteries, discussing the performance implications of immersion cooling but also identifying gaps in the literature which include a lack of studies considering the lifetime, fluid stability, material compatibility, understanding around sustainability and use of immersion for battery safety. Insights from this review will therefore help researchers and developers, from academia and industry, towards creating higher power, safer and more durable electric vehicles.

The surface cooling technology of power battery pack has led to undesired temperature gradient across the cell during thermal management and the tab cooling has been proposed as a promising solution. This paper investigates the feasibility of applying tab cooling in large-format lithium-ion pouch cells using the Cell Cooling Coefficient (CCC). A fundamental problem with tab cooling is highlighted, the CCC for tab cooling decreases as capacity increases. Coupling low CCCs with greater heat generation leads to significant temperature gradients across the cell. Here, the “bottleneck” that limits heat rejection through the tabs is evaluated. The thermal resistance of the physical tabs is identified to be the main contributor towards the poor heat rejection pathway. A numerical thermal model is used to explore the effect of increased tab thickness and results showed that the cell-wide temperature gradients could be significantly reduced. At the negative tab, increasing from 0.2 mm to 2 mm led to a 100% increase in CCCneg whilst increasing the positive tab from 0.45 mm to 2 mm led to an 82% increasing in CCCpos. Together, this is shown to contribute to a 51% reduction in temperature gradient across the cell in any instance of operation.

Lithium ion batteries experience volume deformation in service, leading to a large internal stress in modules and potential safety issues. Therefore, understanding the mechanism of volume deformation of a lithium ion battery is critical to ensuring the long-term safety of electric vehicles. In this work, the irreversible and reversible deformation of a large-format lithium ion battery under four degradation paths, including cycling at −5°C/1 C, 55°C/1 C and 25°C/4 C, and storage at 55°C/100% state of charge, are investigated using laser scanning. The reversible deformation decreases while the irreversible deformation increases as batteries age, following a linear trend with the state of health. The mechanism behind irreversible deformation is investigated using incremental capacity analysis and scanning electron microscopy. The irreversible deformation of the battery cycled at 25°C/4 C and stored at 55°C becomes extremely large below 80% state of health, mainly because of the additional deposit layers on the anode and increased gas production, respectively. Mechanical calculations show the huge stress in the aged modules. Proper spacers between batteries are suggested to reduce such damage. This study is valuable for understanding the mechanical safety of battery modules.

Heterogeneous degradation is a key challenge faced in the production of large format lithium-ion battery (LIB) cells, and is difficult to evaluate non-destructively. This study demonstrates that reversible strain heterogeneity has the potential of becoming a useful non-destructive tool for local degradation analysis of large format LIB cells. A commercial 59.5 Ah LIB cell with a Li[Ni0.6Co0.2Mn0.2]O2 (NCM622) cathode and graphite anode was degraded at 1.3C current with and without external constraint. The aged unconstrained cells experienced a sudden capacity drop and abnormal expansion at certain locations during discharging at 87% SOH (state of health), which was not observed for the constrained ones. Detailed post-mortem analysis was carried out to understand the capacity drop mechanism. The abnormal expansion was ascribed to the gas bubbles produced by localized severe side reactions between the graphite particles and electrolyte, and the significant heat in certain regions with high impedance. The quick spread of the defective regions was responsible for the sudden capacity drop. This work confirms that the reversible strain distribution contains useful information inside the battery and can help monitor battery degradation and capacity drop.

In the recent years, lithium-ion batteries have become the battery technology of choice for portable devices, electric vehicles and grid storage. While increasing numbers of car manufacturers are introducing electrified models into their offering, range anxiety and the length of time required to recharge the batteries are still a common concern. The high currents needed to accelerate the charging process have been known to reduce energy efficiency and cause accelerated capacity and power fade. Fast charging is a multiscale problem, therefore insights from atomic to system level are required to understand and improve fast charging performance. The present paper reviews the literature on the physical phenomena that limit battery charging speeds, the degradation mechanisms that commonly result from charging at high currents, and the approaches that have been proposed to address these issues. Special attention is paid to low temperature charging. Alternative fast charging protocols are presented and critically assessed. Safety implications are explored, including the potential influence of fast charging on thermal runaway characteristics. Finally, knowledge gaps are identified and recommendations are made for the direction of future research. The need to develop reliable onboard methods to detect lithium plating and mechanical degradation is highlighted. Robust model-based charging optimisation strategies are identified as key to enabling fast charging in all conditions. Thermal management strategies to both cool batteries during charging and preheat them in cold weather are acknowledged as critical, with a particular focus on techniques capable of achieving high speeds and good temperature homogeneities.

Reference electrodes (REs) implanted in lithium-ion batteries are essential indicators in the fields of health monitoring and safety management. The non-destructive charging profiles, for example, are usually determined by electrode potential measurements performed with the RE. However, errors in RE potential measurements, resulting in seriously flawed conclusions, are seldom discussed in real lithium-ion batteries. This study investigates the reliability of anode potentials measured with a Li/Cu RE implanted in commercial cells. Artefacts are advanced in RE measurements based on the inconsistency of measured anode potentials and lithium plating behaviors, and further validated by the excess anode overpotential while charging to high state of charge at high rates. Furthermore, artefact phenomenon is reflected in the electrochemical model highlighting the RE blocking of the Li-ion flow. Inhomogeneous lithium intercalation currents at the anode-separator interface are revealed to bring out the excess anode overpotential in RE measurements. Finally, the impact of critical parameters on potential artefacts is examined, and proper RE sizes and battery operating conditions are proposed to ensure the reliability of potential measurements. This work emphasizes the existence of artefacts in RE potential measurements, and provides a useful guide on eliminating errors and improving accuracy of RE in real lithium-ion batteries.

Nickel-rich layered lithium transition metal oxides, LiNixCoyMn1-x-yO2, are key cathode materials for high-energy lithium-ion batteries owing to their high specific capacity. However, the commercial deployment of nickel-rich oxides has been hampered by their poor thermostability and insufficient cycle life. Here full batteries with uncoated and TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathodes and graphite anodes are compared in terms of electrochemical performance and safety behavior. The battery using a TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathode exhibited better cyclic performance at high cutoff voltage. Electrochemical impedance spectroscopy analysis indicated that the TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathode gave the battery a more stable charge transfer resistance. Transmission electron microscopy demonstrated that TiO2 coating reduced accumulation of the cathode electrolyte interface layer on the particle surface. Time-of-flight secondary ion mass spectrometry demonstrated that TiO2 coating markedly enhanced the interface stability of the cathode particle and protected the particle from serious etching by the electrolyte. Accelerating rate calorimetry revealed that the trigger temperature of thermal runaway for the battery using TiO2-coated LiNi0.5Co0.2Mn0.3O2 as cathode material was 257 °C, which was higher than that of the battery with the uncoated LiNi0.5Co0.2Mn0.3O2 cathode (251 °C). In situ X-ray diffraction during heating demonstrated that this enhanced safety can be attributed to the suppressed phase evolution of the coated cathode material.

Over the past decade, major progress in diagnosis of battery degradation has had a substantial effect on the development of electric vehicles. However, despite recent advances, most studies suffer from fatal flaws in how the data are processed caused by discrete sampling levels and associated noise, requiring smoothing algorithms that are not reliable or reproducible. We report the realization of an accurate and reproducible approach, as “Level Evaluation ANalysis” or LEAN method, to diagnose the battery degradation based on counting the number of points at each sampling level, of which the accuracy and reproducibility is proven by mathematical arguments. Its reliability is verified to be consistent with previously published data from four laboratories around the world. The simple code, exact fitting, consistent outcome, computational availability and reliability make the LEAN method promising for vehicular application in both the big data analysis on the cloud and the online battery monitoring, supporting the intelligent management of power sources for autonomous vehicles.

Thermal runaway is a major concern for the large-scale application of lithium-ion batteries. The thermal runaway performance of lithium-ion batteries not only depends on materials and cell design, but also changes with degradation. This paper presents a comparative investigation of the aging effects on the thermal runaway behavior of a large format lithium-ion battery. The batteries are first degraded under four different aging paths. The aging mechanisms are then investigated through post-mortem analysis on the battery at the end of life, by comparing the electrochemical properties, morphology and composition of the fresh and degraded electrodes. The thermal stabilities of the fresh and degraded electrodes are also evaluated using differential scanning calorimetry. Adiabatic thermal runaway tests are performed on the batteries at different states of health using accelerating rate calorimetry to reveal the evolution of battery thermal runaway performance under the four degradation paths. Finally, the correlations between the aging mechanism and the changes in battery thermal runaway behavior are summarized. The results show that the thermal stability of the anode+electrolyte thermodynamic system exhibits obvious changes, which contribute to the evolution of battery thermal runaway performance, while the thermal stability of the cathode remained unchanged. Lithium plating turns out to be the key reason for the deterioration of battery thermal runaway performance during aging process.

Lithium-ion batteries have been widely used as an important countermeasure to alleviate the energy crisis and environmental pollution, especially in automobile field. However, batteries still suffer from safety issues which limits wider application and sometimes arises public concern. A robust battery management system (BMS) is critical to detect possible safety problem of the battery pack. Many researches have been tried different ways to improve the accuracy and robustness of BMS by introducing mechanical signals like force and swelling. Most models reported in articles related SOC or concentration of lithium ions in electrodes with swelling or force through a look-up table. However, it’s not clear what C rate is appropriate for the look up table. Also most of the models are validated only for discharge process. Actually, the charge process is also important since there have been various thermal runaway accidents of automobiles during charging. In this paper, a modified phenomenological force model is proposed. The deviation of the predicted force and detected ones is shown. Then the effectiveness of utilizing swelling-SOC look-up table at different C-rates is discussed. Finally, it’s pointed out that lithium plating is responsible for the force error at end of charge, which implies that the force signal has the potential to detect lithium plating which is critical for battery aging and safety issues. The phenomenological force model is built as followed: firstly, swelling at free state and force at constrained state of cells at different C rates (1/6C, 1/3C, 1C, CCCV protocol) of commercial 30.5Ah NCM/C pouch cells is obtained separately. Then force and swelling is plotted together through their relation with SOC, as can be seen is Figure 1 (a) and (b). Finally, cells are regarded as linear springs and a specific experiment is chosen to identify the stiffness as 0.0151kN/µm for force at 1C and 0.0130kN/µm for force at 1/3C. The pure thermal swelling is investigated by 2C pulse test (charge and discharge continuously for 1440s). It’s found that the cell only expands for 1.5µm when temperature rises 8°C and thus thermal swelling for this types of cells will be ignored in further analysis. For force at 1/3C charging, by substituting swelling at 1/6C with 1/3C, RMSE can be reduced from 0.06713kN to 0.02607kN (accuracy of force sensor is 0.045kN). Similar result can be seen for other situation. Such differences originate from middle SOC which relates to the phase transition of graphite. At high SOC which corresponds to the end of Figure(a), force experiences a downward trend for force at 1C, which can be seen for discharge process or low C rates. This can be explained by the mechanism shown in Figure 1 (c) and (d). For low C rates, graphite particles can absorb all the li-ions in time. Thus, no extra li-ions will accumulate outside those particles. However, for high C rates charging, the number of Li-ions intercalate into a graphite particle per second is much higher. Here we assume that green line in Figure 1(c) indicates the capacity that a graphite particle can absorb per second, which is driven by voltage. Thus for stage ①, Li concentration will increase monotonously and precipitate out at certain point, which occupy larger volume. During constant voltage period (stage ②), as current drops, the precipitated Li will be dissolved and intercalated back to graphite particles which will reduce overall volume of cells. This downward trend can’t be seen for discharge process because during Li-ions will migrate from graphite anode to NCM cathode and won’t accumulated outside graphite particles. Moreover, the volume change of NCM cathode is much smaller. To conclude, it’s found that by utilizing swelling detected at same C rate as force, the nonlinear effect at middle SOC range arisen from phase transition of graphite can be eliminated. The downward trend of force at high C rates charge process is attributed to Li plating. Further work will focus on further exploiting force signal to identify Li plating and assess safety issue of cells. Figure 1. A phenomenological force model. (a) Force vs swelling for different C rates during charge, 1/6C-1/3C means swelling is detected at 1/6 and force is detected at 1/3C; (b) Force vs swelling for different C rates during discharge; (c) Force and current during charge; (d) mechanism of Li plating and absorbing for high C rate during charge. Figure 1