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Adding 200 miles of driving range in 10 min by charging at 400 kW, the so-called extreme fast charging (XFC), is considered as the key to mainstream adoption of electric vehicles (EVs). No EVs today, however, can withstand 400-kW charging power due to the risk of Li plating, which severely deteriorates battery life. Recently, our group reported an asymmetric temperature modulation (ATM) method that enables 10 min charging of high-energy EV cells with remarkable cycle life. [1] The key concept of the ATM method is to charge a Li-ion cell at an elevated temperature of 60oC and discharge (and stores) the cell at the cool ambient temperature (Figure 1). The high charging temperature effectively eliminates Li plating thanks to the enhanced kinetics and transport, and the limited time of exposure to 60oC (~10 min per cycle, or 0.1% of the lifetime of an EV) prevents severe SEI growth. We demonstrated that a high-energy cell with a specific energy of 209 Wh/kg sustained 2500 cycles of 10-minute XFC with only 8.3% capacity loss, far exceeding the US DOE target. In this talk, we will give a comprehensive discussion of the ATM method. Also, we will share our latest progress on the XFC of cells with much higher specific energy (260 Wh/kg). References [1] X.G. Yang, T. Liu, Y. Gao, S. Ge, Y. Leng, D. Wang, C.Y. Wang, Joule, 3 (2019) 3002-3019. Figure 1

Fast charging technology is widely recognized as a critical factor in bolstering consumer appeal of battery electric vehicles (BEVs). The U.S. Department of Energy (DOE) has set a goal of developing extreme fast charging (XFC) battery technology that can recharge a 200-mile BEV in 10 minutes. A key barrier to XFC of automotive Li-ion batteries is the issue of lithium plating, which drastically deteriorates battery life and even induces hazardous consequences. Fundamentally, lithium plating is affected by the rate capability of ion transport in the electrolyte, intercalation reaction at graphite surface, and solid-state diffusion in graphite particles. Research in the literature, accordingly, has been focusing on improving electrolyte transport properties, enhancing charge transfer kinetics, or utilizing smaller particles. Li-ion battery is well known for its trade-off nature. Improving one property without sacrificing others is always challenging. An alternative approach to suppressing lithium plating is to elevate charge temperature. Indeed, an increase in temperature can boost the rate capability of the above three processes simultaneously. However, an elevated temperature, on the other hand, accelerates the solid-electrolyte-interphase (SEI) growth. Owing to the interplay between lithium plating and SEI growth, it is universally believed that Li-ion batteries have an optimal life at room temperature. [1] In 2018, our group presented a numerical study revealing that the optimal temperature of a Li-ion battery, in contrary to the conventional wisdom, actually increases with an increase of charge rate and/or energy density.[2] Enlightened by this finding, we proposed a heated-charge method [2, 3] to charge a Li-ion cell at an elevated temperature of 40-60oC to enable XFC without Li plating. Very recently, Tesla has applied this concept in its V3 superchargers. Using an ‘On-route Battery Warmup’ strategy, Tesla vehicles are designed to arrive at a fast charging station at a temperature of ~40oC, demonstrating the commercial viability of charging a BEV at an elevated temperature. Nevertheless, a potential issue of Tesla’s on-route warmup strategy is the slow heating speed, making a battery stay at a high temperature for a long period of time and hence incurring excessive SEI-induced degradation. Currently, we are developing an XFC battery based on the heated-charge method. With a novel structure, our battery has >100x faster heating than conventional external heating methods, thereby limiting the duration of heating to a matter of seconds. More profoundly, we will present in this talk that our battery, having an energy density of 210 Wh/kg, could sustain 2,000 cycles of 10-min (6C) charge to 80% state of charge with only 7% capacity loss. References [1] T. Waldmann, M. Wilka, M. Kasper, M. Fleischhammer, M. Wohlfahrt-Mehrens, J. Power Sources, 262 (2014) 129-135. [2] X.G. Yang, C.Y. Wang, J. Power Sources, 402 (2018) 489-498. [3] X.G. Yang, G. Zhang, S. Ge, C.Y. Wang, Proceedings of the National Academy of Sciences, 115 (2018) 7266-7271.

Fast charging is the enabler of mainstream adoption of electric vehicles (EVs). There is a race worldwide for building fast charging stations with the power up to 350kW, which are able to charge a 200-mile-range EV at 6C (10 minutes). State-of-the-art EV batteries (cell-level energy density >200 Wh/kg) could only withstand 2C (30 minutes) charging due to the issue of lithium plating. Even worse, the charge rate must be dramatically reduced at low temperatures to prevent lithium plating. Most recently, our group presented a controllable cell structure that enables lithium plating free (LPF) fast charging at all temperatures. [1] The key is to charge a cell always above a temperature that can prevent lithium plating. A rapid heating step will be performed before charging if the cell temperature is low. It is demonstrated that a 9.5Ah pouch cell with an energy density of 170 Wh/kg could be charged by 80% state of charge in 15 minutes even at -50oC, an extremely low temperature at which all conventional Li-ion cells would cease to work. Furthermore, the LPF cell sustained 4500 cycles of 15-min fast charging at 0oC with 12 years and >280,000 miles of EV lifetime, whereas a baseline conventional cell at the same condition only survived 50 cycles. Based on the LPF cell structure, we proposed an approach to enable 10-min extreme fast charging (XFC) of high energy EV cells. By numerical analysis, we revealed that the optimal temperature for Li-ion battery charging increases with the increase of charge rate and energy density. [2] Thus, we are now partnering with the U.S. Department of Energy (DOE) to develop an XFC battery that is charged at high temperatures (>60oC). The target is to achieve 6C charging of >225 Wh/kg cells with a life of >1000 cycles at In this talk, we will give a comprehensive discussion of the LPF cell structure and operating strategy. We will also share our latest progress on the development of XFC cells. References [1] X.G. Yang, G. Zhang, S. Ge, C.Y. Wang, Proc. Natl. Acad. Sci. U.S.A. 115 (2018) 7266-7271. [2] X.G. Yang, C.Y. Wang, J. Power Sources 402 (2018) 489-498.

Range anxiety is one of the most significant barriers to mainstream adoption of electric vehicles (EVs). An effective solution to range anxiety is increasing energy density of EV batteries combined with fast charging. Energy density of EV batteries has advanced significantly over the past few years, from 150 Wh/kg in 2012 to ~220 Wh/kg of state-of-the-art cells, and is targeting >300 Wh/kg by 2020. A practical approach to higher energy density of Li-ion cells is to increase areal loading of electrode active materials. Two critical challenges, however, arise. One is that cycle life of Li-ion cells drops significantly with increasing loading amount. The other is that the ability of fast charging is drastically limited with increasing electrode loading. Gallagher et al. (1) tested a sets of graphite/NMC622 cells with different areal loadings, and found that cells with higher areal loadings have much poorer cycle life. Furthermore, though the cells were cycled at room temperature with only 1C charge rate, a significant amount of lithium deposit was observed in cells with higher anode loading, indicating serious lithium plating in these high energy cells. These results differ greatly from those in the literature based on high power cells. Aging of high power cells is typically believed to be dominated by growth of solid-electrolyte interphase (SEI), and lithium plating is expected to occur only at harsh charge conditions (e.g. low temperature, high charge rate). To date, research on aging behavior of high energy Li-ion cells is still rather limited (2). In this talk, we will give a comprehensive discussion on the aging characteristics of high energy Li-ion batteries for next generation EVs, with special focus on fast charging conditions @2-3C rate. An aging model will be introduced which incorporates both SEI growth and lithium plating. As such, cell aging associated with SEI growth and with lithium plating can be distinguished. The model is applied to predict the cycle life of Li-ion cells at various operating conditions. Specially, we will focus on the effects of a) areal loading density, b) operating temperature, and c) charge rate on the cycle life of Li-ion cells. References: 1. K. G. Gallagher, S. E. Trask, C. Bauer, T. Woehrle, S. F. Lux, M. Tschech, P. Lamp, B. J. Polzin, S. Ha, B. Long, Q. Wu, W. Lu, D. W. Dees and A. N. Jansen, J. Electrochem. Soc., 163, A138 (2016). 2. Y. Leng, S. Ge, D. Marple, X.G. Yang, C. Bauer, P. Lamp and C.Y. Wang, J. Electrochem. Soc., 164, A1037 (2017).

Cycle life is critically important for applications of lithium-ion batteries (LIBs). Numerous models have been proposed in the literature to predict the life time of LIBs (1, 2). Most of these models focused on growth of solid-electrolyte-interphase (SEI), which leads to capacity fade that is proportional to the square root of time, and the aging rate increases with cell temperature by following the Arrhenius law. Several recent experimental studies, however, reported aging behavior that cannot be explained by, at least not solely by, SEI growth. For instance, Waldmann et al. (3) cycled a set of 1.5Ah graphite/NMC cells at different temperatures, and found that the cell at 25oC had the longest life. Plotting cell aging rate with reciprocal temperature, they observed a transition of activation energy from a positive value at T>25oC to a negative value at ToC, indicating a transition of dominating aging mechanisms. The main aging mechanism at T>25oC is SEI growth, and at ToC is believed to be lithium plating, i.e. deposition of metallic lithium on graphite surfaces. The lithium deposition reaction directly reduces cell capacity by consuming active lithium. In worst-case scenarios, the plated lithium can pierce the separator, induce internal short circuit and cause catastrophic consequences. Research efforts on the effects of lithium plating over cell aging are rather limited in the literature, especially in terms of numerical modeling. In the present study, we present a comprehensive LIB aging model with incorporation of both SEI growth and lithium plating. The model can well capture the recently discovered LIB aging behaviors associated with lithium plating. In addition, the relative contribution to cell aging by SEI growth and by lithium plating can be quantified for cells at different operating conditions with different design parameters. Figure 1 shows the variation of cell capacity with cycle number for a plug-in electric-vehicle (PHEV) LIB with energy density of 170 Wh/kg cycled at 25oC with 1C charge and 2C discharge. At this moderate operating condition it is usually believed that lithium plating is not a concern. However, we found in our experiment that cell capacity dropped abruptly after ~3000 cycles, which is in consistence with the recently reported nonlinear aging behavior by Schuster et al. (4). The model presented in this study well captures this nonlinear behavior, as shown in Fig. 1a, and this abrupt drop in cell capacity is attributed to the rapid rise of lithium plating rate after extended cycling as shown in Fig. 1b. The model is then applied to predict the cycle life of the above PHEV cell at different ambient temperatures, as shown in Fig. 2a. It is found that the cell at 20oC has the longest cycle life. Either increasing or lower the temperature would accelerate cell aging. These results are in accordance with the recent experiment findings by Waldmann et al (3). The effects of electrode thickness on the cycle life are also investigated, as shown in Fig. 2b. These cells are cycled at 25oC with 1C charge and 1C discharge. Even at this moderate operating condition, cells with thicker electrode have much poorer life as shown in Fig. 2b, which is because lithium plating is more prone to occur with the increase of electrode thickness. This result has a profound implication on high-energy, thick-electrode cells for all electric vehicles (EV). Details about the model development, and about the effects of cell design parameters and operating conditions on cell aging due to lithium plating will be given in this presentation. Finally, we shall propose some novel strategies to significantly extend the cycle life and increase fast rechargeability of energy-dense EV batteries. References: 1. M. Safari, M. Morcrette, A. Teyssot and C. Delacourt, J. Electrochem. Soc., 156, A145 (2009). 2. M. B. Pinson and M. Z. Bazant, J. Electrochem. Soc., 160, A243 (2013). 3. T. Waldmann, M. Wilka, M. Kasper, M. Fleischhammer and M. Wohlfahrt-Mehrens, J. Power Sources, 262, 129 (2014). 4. S. F. Schuster, T. Bach, E. Fleder, J. Müller, M. Brand, G. Sextl and A. Jossen, J. Energy Storage, 1, 44 (2015). Figure 1

Large temperature difference arises in big-size Li-ion cells (e.g. 94Ah Samsung cells) due to low thermal conductivity of battery materials, especially under extreme conditions like high C rate operation at low temperatures or abuse conditions [1]. Previous research [2,3] has attempted to embed micro thermocouples in 18650-format cylindrical cells (1.6 Ah) and demonstrated that internal temperature sensing is more useful than surface temperature sensing in early detection of abnormal thermal behaviors for safety enhancement. For practical applications of this technique, internal temperature sensors must be robust and durable to survive battery life cycle and must be low cost. In this study we developed a new resistance temperature device (RTD) based on nickel foil that meets both criteria. Embedded in 10Ah pouch cells, these in-cell temperature sensors still function well after the cells experienced 2,500 cycles. It is believed that low-cost, durable and reliable in-cell temperature sensors have finally arrived to enable substantial improvement in the Li-ion battery’s high-temperature safety and thermal management. References: X.G. Yang, T. Liu and C.Y. Wang, Innovative heating of large-size automotive Li-ion cells, Journal of Power Sources, accepted for publication, 2017. G.S. Zhang, L. Cao, S. Ge, C.Y. Wang, C. E. Shaffer and C. D. Rahn. In situ measurement of radial temperature distributions in cylindrical Li-ion cells, Journal of The Electrochemical Society, 2014, 161: A1499-A1507 G.S. Zhang, L. Cao, S. Ge, C. Y. Wang, C. E. Shaffer and C. D. Rahn. Reaction temperature sensing (RTS)-based control for Li-ion battery safety, Scientific Reports, 2015, 5: 18237

Ford has invested $2.1 million to open a new joint Li-ion battery fabrication and characterization laboratory at the Univeristy of Michigan. This prototyping laboratory enables multiple scale cell manufacturing and high process quality that is feasible to scale up, and evaluation of battery chemistries and electrode design while reducing the risk and cost. State-of-the-art manufacturing methods will be used to build surrogate cells that replicate the performance of full-scale production batteries, allowing for faster implementation in future production electrified vehicles. More details will be presented at the meeting.

Increased utilization of Lithium-ion batteries for a variety of applications is driving the need for advanced simulation tools that can predict the combined structural, electrical, electrochemical, and thermal response to abuse conditions. If such simulation tools are integrated into the product development process, the resultant data has the potential to inform decisions during development and create highly optimized designs. Academic and national laboratory researchers have pioneered these models1-3, but there is currently no consensus on the required methods for multi-physics coupling or acceptable degree of homogenization. If coupling is present between the mechanical and electrical or thermal solvers, it is typically one-way and largely manually applied, which requires highly advanced users to implement due to mesh and element compatibility issues. In order to initiate a much larger investigation into the abuse response of Li-ion cells, modules, and packs, a coupled structural, electrical, and thermal simulation framework has been developed within the commercially available LS-DYNA software. The finite element model leverages a three-dimensional mesh structure that fully resolves the unit cell components. Depending on the time scale of the mechanical crush, coupling with the mechanical solver can be one-way for impact loading conditions that occur in the sub-second time scale, or two-way for continuous crush in the tens to hundreds of seconds time scale. Measurements of the resistance between unit cell layers as a function of applied pressure are reported and used to parameterize the coupling between the structural, electrical, and thermal solvers. A spatially distributed equivalent circuit model predicts the electrical response with minimal computational complexity. The thermal model provides information to schedule the electrical model parameters, while simultaneously accepting irreversible and reversible sources of heat generation. The spatially distributed models of the electrical and thermal dynamics allow for the localization of current density and corresponding temperature response to provide the necessary tool structure for predicting the onset of thermal runaway. Future work will focus on the structural material models and failure conditions required to quantitatively predict the battery crush response to a variety of loading conditions. Plans for additional multi-physics functionality within the simulation tool will also be highlighted. References: E. Sahraei, R. Hill, and T. Wierzbicki, J. Power Sources, 201, 307 – 321 (2012). E. Sahraei, M. Kahn, J. Meier, and T. Wierzbicki, RSC Adv., 5, 80369 – 80380 (2015). C. Zhang, S. Santhanagopalan, M. A. Sprague, and A. A. Pesaran, J. Power Sources, 290, 102 – 113 (2015).

Fast charging is an enabling technology for electric vehicles (EV). Unfortunately lithium-ion batteries are notoriously incapable of fast charging at subzero temperatures,1-3 thereby hindering EVs from dependable and convenient operation under all weather conditions. In this work we describe a novel battery structure, called all-climate battery (ACB), which adds a resistor sheet inside the electrochemical cell to modulate the cell’s internal resistance according to temperature. Based on ACB technology, we have built 12Ah pouch cells using graphite anode, NMC cathode and 1M LiPF6 in EC:EMC (3:7 by wt) + 2% VC and showed that such ACB cells are able to be charged from 20% to 90% state of charge (SOC) in -30oC environment in ~22 minutes, while keeping the cell voltage never exceeding 4.2V (a necessary condition to avoid Li plating). Figure 1 displays the experimental results of fast charging a 12Ah ACB cell from -30oC with a charge protocol of CV (4.2V) and limited by CC (3C or 36A). It is seen that the charge current under 4.2V charge voltage starts only at ~7A; however, it rapidly increases to 36A (3C limit) in around 400 seconds. Simultaneously the cell outer surface temperature increases from the initial temperature of -30oC to above the freezing point. Thereafter, the ACB cell undergoes 3C constant current charging with the cell voltage dips below 4.2V between 400 and 1100 seconds. During this period, the cell SOC rapidly increases from the initial SOC of 20%, eventually reaching 90% in ~22 min. Figure 2 compares two charge experiments under the same charge protocols of a 12Ah ACB cell and its baseline cell without ACB technology. It is clearly shown that ACB technology has dramatically improved the fast charge capability of Li-ion batteries. More efforts are ongoing to accelerate the fast charge process from subzero temperatures and to further reduce the charge time from 22 min to within 15 min. These advanced improvements will be elaborated in this presentation. References: 1. S. S. Zhang, K. Xu, and T. R. Jow, Electrochem. Commun., 4, 928 (2002). 2. M. C. Smart, J. F. Whitacre, B. V. Ratnakumar, and K. Amine, J. Power Sources, 168, 501 (2007). 3. Y. Ji, Y. Zhang, and C.-Y. Wang, J. Electrochem. Soc., 160, A636 (2013).

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Direct methanol fuel cells (DMFCs) has been considered as one of the most promising power sources for portable applications due to its unique features such as high energy density of methanol, facile fuel storage, fast refueling capability and simple system structure. Nevertheless, there are still several challenging issues which need to be resolved before DMFCs are ready for widespread commercialization. Since the voltage of a single DMFC is typically below 0.6 V, DMFCs are usually connected in series and stacked together to meet the power demands of various applications. The increased size, voltage and power of a stack, however, brings several additional issues, among which the maldistribution of air flow through different individual cells is extremely detrimental. Since most fuel cell stacks utilize fully or at least partially parallel flow configurations, even a slight deviation of flow resistance, e.g. formation of water slugs in the channels, can lead to significant variations of air flow within the stack [1]. A great deal of efforts have been made in literature to investigate the issue of flow maldistribution, as reviewed by Wang et al.. [2] It has been revealed that the nonuniformity of air flow rate (AFR) through different cells is exacerbated with the increase of stack size, and this air maldistribution can result in variations of single cell voltages across the stack. In some extreme cases, the voltage of certain cells can even become negative, which seriously affects the performance and durability of the stack.[3] Therefore, individual cell voltages within a stack are usually measured in order to monitor the single cell behavior inside the stack. However, the voltage signals alone cannot give any insight into the internal status of each cell, as voltage variations could be attributed to various reasons besides air maldistribution. Therefore, a deep insight into the fundamental relationship between cell voltage and the AFR is pretty important. In the present work, we measured the voltages of a single DMFC operating at a constant current density of 20 mA/cm with different AFRs in the cathode. In the meantime, a two dimensional model based on our previous work [4] is developed to explore the detailed behaviors inside the cell at various AFRs. One of the most important features of our model is that we account for hydrogen evolution reaction (HER) not only in the anode but also in the cathode. Fig. 1 shows the experimental and simulation results of cell voltages at various AFRs. It can be seen that the modeling results agree quite well with the experimental data. Meanwhile, it can also be noted that the cell voltage varies significantly with AFR. Two critical points should be addressed in this figure. One is point A, where the cell voltage starts to decrease with AFR. The other is point F, where the voltage approaches zero. It has been found that these two critical points are closely related to the HER either in the anode or in the cathode, as shown in Fig. 2, where the HER rates at various AFRs are plotted. From Figs. 1 and 2 we can note that the onset of HER in the anode is the major reason for the drop of voltage at point A. In the meantime, at point F where the voltage reaches zero, a transition from HER in the anode to HER in the cathode occurs. This HER along with the current and potential distributions at each AFR will be addressed in detail within this work.

Lithium-ion battery electrodes utilizing mixtures of active materials have seen increased attention in recent years. These cells have merit for electric vehicle applications due to the potential for improved cycle life and increased performance. An aging campaign has been conducted with 15 Ah automotive pouch cells to quantify the relationship between a set of severity factors and the aging process for cells using composite electrodes, where aging is represented by capacity fade. A simplified electrochemical model which postulates that the materials within the composite electrode operate as parallel paths for current is used to interpret trends in the aging data. Using the model, loss of active material from individual components of the composite positive electrode can be simulated, as well as loss of cyclable lithium. Comparison with differential capacity data allows an assessment of the most likely degradation mechanism for the initial capacity fade.

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