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.