Predicting lithium-ion battery degradation is worth billions to the global automotive, aviation and energy storage industries, to improve performance and safety and reduce warranty liabilities. However, very few published models...

Charge-redistribution (CR) in electrochemical double layer capacitors (EDLCs) manifests as voltage drop during open-circuit relaxation (OCR) after galvanostatic (GS) charging, and voltage recovery during OCR after GS discharging. The complex porous structure of electrodes causes a capacitor to charge/discharge non-uniformly which causes charge-redistribution in OCR. In this work, we have used a macro-homogeneous transport model which incorporates electrolyte concentration dependent conductivity and constant capacitance to analyze CR. The model makes a new prediction that CR occurs over two time scales, a short one, of a couple of seconds, driven by potential gradient across an electrode at the termination of the preceding galvanostatic operation, and a long one, of hundreds of seconds, driven by electrolyte concentration gradients inside and outside pores. The extent of CR at two electrodes differs dramatically. The model also modifies other predictions of constant conductivity based models developed earlier in view of depletion/accumulation of electrolyte in pores. The linear variation of cell potential with log(t), used as a test of self-discharge due to faradaic reaction, is predicted by our transport models for CR itself. The scaling is different for short and long time CR, and sensitively depends on electrolyte conductivity in pores.

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.

Electrochemical double layer capacitors (EDLCs) store electrical energy by accumulating ions near charged interfaces in porous electrodes. The available transport models for their characteristics often ignore the associated complexities: electrode-specific capacitance, its dependence on local potential difference and electrolyte concentration, and ionic conductivity in charged pores. As a result, modelling efforts are yet to be supported, supplemented, and validated with measurements. Detailed electrode specific measurements are reported in this work on discharge of porous carbon-H 2 SO 4 EDLCs. A substantially decreased charge recovery at high currents and subsequent prolonged potential recovery in open circuit mode are salient features. These are compared with predictions of a comprehensive transport model, which uses potential dependent electrode capacitances retrieved from measurements. The model, with no fitted parameters and Bruggeman correction for ionic conductivity in pores, fails to capture observed features, unless an order of magnitude decrease beyond Bruggeman correction is effected. The required decrease is larger at higher electrolyte concentrations and for electrode with double layer of larger size ions. The corrected model quantitatively explains the unusual features observed, in addition to matching measured cyclic-voltammograms with substantial chargeredistribution as the cause of lag between current and voltage.

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Lithium ion capacitors are an important energy storage technology, providing the optimum combination of power, energy and cycle life for high power applications. However, there has been minimal work on understanding how they degrade and how this should influence their design. In this work, a 1D electrochemical model of a lithium ion capacitor with activated carbon (AC) as the positive electrode and lithium titanium oxide (LTO) as the negative electrode is used to simulate the consequences of different degradation mechanisms in order to explore how the capacity ratio of the two electrodes affects degradation. The model is used to identify and differentiate capacity loss due to loss of active material (LAM) in the lithiated and de-lithiated state and loss of lithium inventory (LLI). The model shows that, with lower capacity ratios (AC/LTO), LAM in the de-lithiated state cannot be identified as the excess LTO in the cell balances the capacity loss. Cells with balanced electrode capacity ratios are therefore necessary to differentiate LAM in lithiated and de-lithiated states and LLI from each other. We also propose in situ diagnostic techniques which will be useful to optimize a LIC’s design. The model, built in COMSOL, is available online.

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Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors, are energy storage devices. Charge-discharge in EDLCs occurs through electrostatic interactions between electrodes and oppositely charged ions. Self-discharge is the gradual loss of energy when a charged EDLC is left in open circuit state. A comparatively higher rate of self-discharge (SD) than in the batteries prevents the use of EDLCs in stand-alone devices or those charged only infrequently. The process of self-discharge, to the best of our knowledge, is far from being fully understood. It manifests as decrease in open-circuit potential which, depending on the charging protocol, can be dominated by charge-redistribution [1-3] induced rapid decrease in the initial stages. Conway [4] proposed model-based diagnostics to identify activation, diffusion, and leakage-resistance controlled mechanisms from voltage vs. time measurements. We show that for the limited voltage range window available, the method is subjective and ambiguous for real measurements. We have proposed and carried out an alternative set of extensive experiments to gain insights into the self-discharge process for an in-house capacitor with sulfuric acid as electrolyte and activated carbon as electrodes. We varied electrode thickness, the distance between the electrodes, and the state of mixing in the electrolyte. We also measured self-discharge of electrochemically isolated charged half-cells (Figures 1a,b). The results bring out interesting and unambiguous findings, independent of the degree of fit with the expected variations for various controlling mechanisms. They rule out leakage resistance as the cause of self-discharge and disprove the shuttle mechanism [2,3] (Figure 1c,d). The analysis of measurements with electrodes of different thicknesses establishes that activation process controlled self-discharge does not dominate either. And the predicted effect of charge-redistribution is in the opposite direction. The role of the complex interplay of diffusion and electrochemical reactions holds promise. Figure 1