• Energy Research

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.

Abstract Diagnosing the state-of-health of lithium ion batteries in-operando is becoming increasingly important for multiple applications. We report the application of differential thermal voltammetry (DTV) to lithium iron phosphate (LFP) cells for the first time, and demonstrate that the technique is capable of diagnosing degradation in a similar way to incremental capacity analysis (ICA). DTV has the advantage of not requiring current and works for multiple cells in parallel, and is less sensitive to temperature introducing errors. Cells were aged by holding at 100% SOC or cycling at 1C charge, 6D discharge, both at an elevated temperature of 45 °C under forced air convection. Cells were periodically characterised, measuring capacity fade, resistance increase (power fade), and DTV fingerprints. The DTV results for both cells correlated well with both capacity and power, suggesting they could be used to diagnose SOH in-operando for both charge and discharge. The DTV peak-to-peak capacity correlated well with total capacity fade for the cycled cell, suggesting that it should be possible to estimate SOC and SOH from DTV for incomplete cycles within the voltage hysteresis region of an LFP cell.

Abstract Understanding and tracking battery degradation mechanisms and adapting its operation have become a necessity in order to enhance battery durability. A novel use of differential thermal voltammetry (DTV) is presented as an in-situ state-of-health (SOH) estimator for lithium-ion batteries. Accelerated ageing experiments were carried on 5Ah commercial lithium-ion polymer cells operated and stored at different temperature and loading conditions. The cells were analysed regularly with various existing in-situ diagnosis methods and the novel DTV technique to determine their SOH. The diagnosis results were used collectively to elaborate the degradation mechanisms inside the cells. The DTV spectra were decoupled into individual peaks, which each represent particular phases in the negative and positive electrode combined. The peak parameters were used to quantitatively analyse the battery SOH. A different cell of the same chemistry with unknown degradation history was then analysed to explore how the cell degraded. The DTV technique was able to diagnose the cell degradation without relying on supporting results from other methods nor previous cycling data.

Abstract Accurate diagnosis of lithium ion battery state-of-health (SOH) is of significant value for many applications, to improve performance, extend life and increase safety. However, in-situ or in-operando diagnosis of SOH often requires robust models. There are many models available however these often require expensive-to-measure ex-situ parameters and/or contain unmeasurable parameters that were fitted/assumed. In this work, we have developed a new empirically parameterised physics-informed equivalent circuit model. Its modular construction and low-cost parametrisation requirements allow end users to parameterise cells quickly and easily. The model is accurate to 19.6 mV for dynamic loads without any global fitting/optimisation, only that of the individual elements. The consequences of various degradation mechanisms are simulated, and the impact of a degraded cell on pack performance is explored, validated by comparison with experiment. Results show that an aged cell in a parallel pack does not have a noticeable effect on the available capacity of other cells in the pack. The model shows that cells perform better when electrodes are more porous towards the separator and have a uniform particle size distribution, validated by comparison with published data. The model is provided with this publication for readers to use.

Over the lifetime of a lithium-ion battery, various irreversible degradation mechanisms such as for example excessive Solid Electrolyte Interphase (SEI) layer formation and mechanical fracture of the electrodes can lead to uneven capacity loss in the cathode and anode. This can result in a stoichiometric drift effect whereby the measured cell potential may be within the safe operating range specified by the cell manufacturer but the individual electrodes are at a thermodynamically unstable regime leading to an accelerated capacity fade (Figure 1). Stoichiometric drift can be measured for individual electrodes using a reference electrode however this is not available in commercial cells. Information from Slow Rate Cyclic Voltammetry (SRCV) measurements can be used to observe stoichiometric drift when reference electrode is not present, however, this is a time consuming procedure which can take up to several hours or days to complete depending on the sweeping rate. Differential Thermal Voltammetry (DTV) has been proposed as a novel in-situ diagnosis technique to measure stoichiometric drift as a faster complement to the SRCV. DTV uses the temperature profile of the cell to infer the same data as the SRCV but more applicable to real world systems (Figure 1). The peaks in the DTV curve represent phase transformation and in carefully controlled conditions, it may be possible give an indication of the entropy changes of the individual electrodes. DTV can be completed in minutes, can easily be computed in-situ, only requires voltage and temperature measurements and does not require iso-thermal conditions. Simulations were carried out on a 1D thermally coupled electrochemical battery model with degradation modes. The cell used in this study was a 4.8 Ah lithium-ion polymer cell, with Nickel-Manganese-Cobalt oxide (NMC) based cathode and carbon/graphite anode. Results show that shifts in the location of the SRCV peaks can indicate stoichiometric drift and that the same information can be inferred through the novel in-situ diagnosis technique but at a rate of order of magnitude faster than the SRCV measurements. The simulations were experimentally validated on commercial 4.8 Ah Kokam lithium-ion polymer cells placed through various accelerated aging processes: load cycling and constant voltage storage at high and room temperatures. It can be demonstrated that different operating/storage conditions induce various degradation mechanisms in the battery. For example, a high temperature operation will promote faster SEI layer growth hence loss of cyclable lithium and increased internal resistance associated with a thicker SEI and a high C-rate cycling will induce mechanical fractures of the electrodes resulting in loss of electro-active material. Throughout the experiment, the cells were regularly analysed using Electrochemical Impedance Spectroscopy, SRCV and DTV to characterise the degradation for each type of operating conditions and to check for the validity of the novel methodology. By using the novel diagnosis technique, the stoichiometric drift of the battery can be measured sufficiently fast in order to provide useful information regarding the battery condition and to extend the battery life by reducing the operating voltage window according to stoichiometric drift of battery electrodes. Figure 1: (right) Potentials of a lithium-ion battery against stoichiometric ratios of lithiations for a carbon anode and NMC cathode. (left) Comparison of experimental SRCV and DTV results for a 4.8Ah lithium polymer Kokam cell at various stages of capacity fade.