• Energy Research

Abstract During operation of large prismatic lithium-ion batteries, temperature heterogeneities are aggravated which affect the performance, lifetime and safety of the cells and packs. Therefore, an accurate model to predict the evolution of temperature profiles in a cell is essential for effective thermal management. In this paper, a pseudo 3D coupled multi-node electro-thermal model is presented for real-time prediction of the heterogeneous temperature field evolution on the surface and inside the battery. The model consists of two parts: a heat generation model based on a second-order equivalent-circuit model and a multi-node heat transfer model based on the battery geometry. Three types of nodes are adopted to describe the thermal characteristics of various components of the cell. Simulation results show that the proposed model has a great consistency with finite element method, and its computational cost is reduced by 90%. The validity of the coupled electrical and thermal model is also demonstrated experimentally for a 105 Ah prismatic cell applying wide ranges of temperature and SOC. The maximum error is less than 2 K throughout the cycles. The proposed model holds a great potential for online temperature estimation in advanced lithium-ion battery thermal management system design.

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Scenarios with rapid energy conversion for lithium-ion batteries are increasingly relevant, due to the desire for more powerful electric tools or faster charging electric vehicles. However, higher power means higher cooling requirements, affecting the battery temperature and its thermal gradients. In turn, temperature is a key quantity influencing battery performance, safety and lifetime. Therefore, thermal models are increasingly important for the design and operation of battery systems. Key parameters are specific heat capacity and thermal conductivity. For these parameters, this paper presents a comprehensive review of the experimental results in the literature, where the median values and corresponding uncertainties are summarized. Whenever available, data is analyzed from component to cell level with the discussion of dependencies on temperature, state of charge (SOC) and state of health (SOH). This meta-analysis reveals gaps in knowledge and research needs. For instance, we uncover inconsistencies between the specific heat capacity of electrode-separator stacks and full-cells. For the thermal conductivity, we found that thermal contact resistance and dependencies on battery states have been poorly studied. There is also a lack of measurements at high temperatures, which are required for safety studies. Overall, this study serves as a valuable reference material for both modellers and experimenters.

Abstract Modern applications of lithium-ion batteries such as smartphones, hybrid & electric vehicles and grid scale electricity storage demand long lifetime and high performance which typically makes them the limiting factor in a system. Understanding the state-of-health during operation is important in order to optimise for long term durability and performance. However, this requires accurate in-operando diagnostic techniques that are cost effective and practical. We present a novel diagnosis method based upon differential thermal voltammetry demonstrated on a battery pack made from commercial lithium-ion cells where one cell was deliberately aged prior to experiment. The cells were in parallel whilst being thermally managed with forced air convection. We show for the first time, a diagnosis method capable of quantitatively determining the state-of-health of four cells simultaneously by only using temperature and voltage readings for both charge and discharge. Measurements are achieved using low-cost thermocouples and a single voltage measurement at a frequency of 1 Hz, demonstrating the feasibility of implementing this approach on real world battery management systems. The technique could be particularly useful under charge when constant current or constant power is common, this therefore should be of significant interest to all lithium-ion battery users.

Lithium ion capacitors (LICs) store energy using double layer capacitance at the positive electrode and intercalation at the negative electrode. LICs offer the optimum power and energy density with longer cycle life for applications requiring short pulses of high power. However, the effect of electrode balancing and pre-lithiation on usable energy is rarely studied. In this work, a set of guidelines for optimum design of LICs with activated carbon (AC) as positive electrode and lithium titanium oxide (LTO) as negative electrode was proposed. A physics-based model has been developed and used to study the relationship between usable energy at different effective C rates and the mass ratio of the electrodes. The model was validated against experimental data from literature. The model was then extended to analyze the need for pre-lithiation of LTO. The limits for pre-lithiation in LTO and use of negative polarization of the AC electrode to improve the cell capacity have been analyzed using the model. Furthermore, the model was used to relate the electrolyte depletion effects to poorer power performance in a cell with higher mass ratio. The open-source model can be re-parameterised for other LIC electrode combinations, and should be of interest to cell designers.

Minigrids are expected to play a key role in achieving universal access to electricity. The International Energy Agency estimates that 25% of people gaining access to electricity by 2030 will do so through a mini-grid1. Generation is often paired with a battery, primarily lead acid or li-ion in this context. Battery storage can help to displace the use of diesel generators, through providing solutions to the mismatch between daytime solar photovoltaic (PV) generation and evening demand. Batteries are a key cost component of mini-grids. However, how to economically choose and operate batteries in an off-grid context is often unclear2. Factors such as ambient temperature and high State of Charge (SOC) have been shown to significantly increase the rate of capacity fade in li-ion batteries3. A good understanding of battery degradation is required to maximise lifetime and reduce costs. Derating strategies, informed by degradation modelling, limit charging/discharging currents under conditions that will accelerate capacity fade. These methods can be implemented for little cost, having been shown to be able to effectively manage factors such as battery temperature and SOC4,5. Through limiting current, derating will have implications for system reliability. Therefore, it is important to fully understand the impact of these strategies in off-grid systems. This study uses a mini-grid simulation and optimisation tool (Continuous Lifetime Optimisation of Variable Electricity Resources, CLOVER)6,7 and coupled electrothermal and semi-empirical degradation models for a Lithium Iron Phosphate cell (Full Battery System Model, FBSM) to model capacity fade for two locations in the global south. The first location has a milder climate more suitable for li-ion operation (Gitaraga, Rwanda), whereas the second exhibits more extreme temperatures (Bhinjpur, India). CLOVER was used to optimally size solar mini-grid systems and to calculate financial and environmental variables. FBSM was used to model battery aging, considering factors such as cell self-heating, observing the impact of calendar aging and 3 separate cycle aging mechanisms. The model was also used to evaluate the effect of derating strategies on battery degradation, in terms of capacity fade. In the first instance, these are calculated with no derating, and in the second with simple derating strategies based on limiting SOC and battery temperature operating windows. Important factors for mini-grid operation such as battery lifetime, unmet energy fraction, and Levelised Cost of Used Electricity (LCUE) are considered. This study finds calendar aging to be the most significant degradation mechanism for all scenarios investigated, causing a minimum of 87% of total capacity loss. Calendar aging is primarily a function of ambient temperature and SOC. Extreme highs and lows in ambient temperature are found to accelerate degradation. In the case study, PV and battery size was relatively high with respect to the load, resulting in relatively low battery currents, and a tendency of the system towards high SOC levels. Low currents exhibited mean that the significance of joule losses is found to be very small, thus the influence of cell self-heating was found to be negligible. SOC derating to achieve an average SOC of 50% is found to increase battery lifetime by over 11 years through limiting time spent at high SOC, without impacting on the system reliability. Temperature derating is able to increase battery lifetime for a maximum of 2.3 years. Derating led to increases in the unmet energy fraction and frequency of blackouts. However, these increases did not result in any change to the emissions intensity of electricity or the LCUE. Capacity fade observed for one of the systems investigated in both Gitaraga and Bhinjpur has been shown in Figure 1. Therefore, it is shown that derating strategies are a low-cost method that can increase battery lifetime, without increasing the cost of electricity or emissions intensity. Thus, improving the financial viability of mini-grids. Insights from degradation modelling enable better decision making regarding the operation of li-ion batteries in this context. Acknowledgements This work was kindly supported by the EPSRC Faraday Institution Multi-Scale Modelling Project (EP/S003053/1, grant number FIRG003). References IEA, Energy Access Outlook, (2017). S. Few, O. Schmidt, and A. Gambhir, Energy Sustain. Dev., 48, 1–10 (2019) https://doi.org/10.1016/j.esd.2018.09.008. M. Schimpe et al., J. Electrochem. Soc., 165, A181–A193 (2018). J. V. Barreras, T. Raj, and D. A. Howey, in Proceedings: IECON 2018 - 44th Annual Conference of the IEEE Industrial Electronics Society,, p. 4956–4961, IEEE (2018) https://ieeexplore.ieee.org/document/8592901/. M. Schimpe, J. V. Barreras, B. Wu, and G. J. Offer, in PRiME 2020/238th ECS Meeting,, Honolulu, HI, USA. P. Sandwell, (2018). P. Sandwell, N. Ekins-Daukes, and J. Nelson, Energy Procedia, 130, 139–146 (2017). Figure 1

Hybridizing a lead–acid battery energy storage system (ESS) with supercapacitors is a promising solution to cope with the increased battery degradation in standalone microgrids that suffer from irregular electricity profiles. There are many studies in the literature on such hybrid energy storage systems (HESS), usually examining the various hybridization aspects separately. This paper provides a holistic look at the design of an HESS. A new control scheme is proposed that applies power filtering to smooth out the battery profile, while strictly adhering to the supercapacitors’ voltage limits. A new lead–acid battery model is introduced, which accounts for the combined effects of a microcycle’s depth of discharge (DoD) and battery temperature, usually considered separately in the literature. Furthermore, a sensitivity analysis on the thermal parameters and an economic analysis were performed using a 90-day electricity profile from an actual DC microgrid in India to infer the hybridization benefit. The results show that the hybridization is beneficial mainly at poor thermal conditions and highlight the need for a battery degradation model that considers both the DoD effect with microcycle resolution and temperate impact to accurately assess the gain from such a hybridization.