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Combining Calorimetry and Electrochemical Methods to Gain Insight Into the Charging Mechanisms of Electrochemical Capacitors and Batteries


Increased demand for electrical energy storage to support the deployment of renewable energy sources and to decarbonize the transportation sector has established electrochemical capacitor (EC) and rechargeable batteries as the current frontier of science. Accurate understanding of the phenomena occurring during cycling of ECs and batteries can assist in determining their limitations, in optimizing their design and the material selection and in developing novel electrode materials. Recently, isothermal operando calorimetry has been successfully developed to identify the charging mechanisms occurring in ECs and batteries. It has identified the thermal signatures of (i) Joule heating, (ii) EDL formation/dissolution, (iii) redox reactions, (iv) overscreening effect, (v) electrolyte decomposition, (vi) irreversible ion intercalation into the electrode, and (vii) insulator to semiconductor transition.

Heat generation rates in hybrid supercapacitors consisting of positive α-MnO2 cryptomelane electrodes and AC counter electrodes with different aqueous electrolytes were measured experimentally via isothermal operando calorimetry under galvanostatic cycling at 20 �C. First, two devices with 0.5 M K2SO4 or Cs2SO4 aqueous electrolytes were investigated for their different solvation shell thickness and bare ion size. The measured heat generation rate at the AC electrode was attributed to irreversible Joule heating and reversible EDL formation/dissolution. On the other hand, the heat generation rate at the α-MnO2 electrode was caused by Joule heating and redox reactions. Moreover, for large potential windows, an endothermic dip was observed at the α-MnO2 at the end of the charging step and was attributed to the onset of hydrolysis. Hydrolysis was observed for potential window of 2 V in 0.5 M Cs2SO4 aqueous electrolyte and for 1.8 V for 0.5 M K2SO4. The wider potential window than the theoretical 1.23 V was attributed to the thinner solvation shell of Cs+ compared to K+ which limited the amount of water present near the electrode.

Heat generation rate measured in hybrid supercapacitors is often dominated by ion adsorption/desorption and redox reactions. However, during adsorption/desorption and redox reactions, ions fully or partially shed or form their solvation shells. Here, hybrid supercapacitors consisting of an α-MnO2 and an AC electrode with MgSO4 aqueous electrolytes with different concentrations were investigated via isothermal operando calorimetry. MgSO4 salt was chosen for its high solubility in water (≤ 2.9 M at 20 �C) near neutral pH and its large enthalpy of solvation compared to previously considered K2SO4 and Cs2SO4. The measured heat generation rates at the AC electrodes were similar to those previously measured in devices with K2SO4 and Cs2SO4 aqueous electrolytes. However, the heat generation rate measured at the α-MnO2 electrodes was significantly different from those observed for K2SO4 and Cs2SO4 aqueous electrolytes. This was attributed to the heat generation due to solvation/desolvation of Mg2+ cations. However, the thermal signature of solvation was not observed at the AC electrode as the solvated Mg2+ cations were small enough to enter the pores in the AC electrodes without becoming partially desolvated. This interpretation was confirmed by a simple thermal model.

Ion size can also affect the charging mechanisms and capacity of hybrid supercapacitors. To assess the effect of ion size on the charging mechanisms and heat generation in hybrid supercapacitors three devices were tested and consisted of an α-MnO2 and an AC electrode with either (i) 0.5 M Li2SO4, (ii) 0.5 M Na2SO4, or (iii) 0.5 M Cs2SO4 aqueous electrolyte. The thermal signatures measured at the AC electrodes in 0.5 M Na2SO4 and 0.5 M Cs2SO4 aqueous electrolyte were qualitatively similar and attributed to EDL formation/dissolution. However, they were different from the thermal signature measured in 0.5 M Li2SO4 aqueous electrolyte when Li+ possibly participated in surface redox reactions with the AC electrode due to its small size and high electronegativity. Then, the heat generation rate was endothermic during charging due to the non-spontaneous Li+ surface redox reactions and exothermic during discharging. Interestingly, Na+ and Cs+ cations did not participate in surface redox reactions into AC as their compounds with carbon are not stable.

The irreversible heat generation in the redox active α-MnO2 electrode of hybrid supercapacitors exceeded Joule heating due to the contributions of polarization heating and the hysteretic ion concentration profile evolution at the electrode surface. This dissertation formulates expressions and performs numerical simulations of the irreversible heat generation rates associated with charge and mass transfer resistances based on the modified Poisson-Nernst-Planck model. These contributions to the total heat generation rate were expressed as resistive losses through the charge transfer or the mass transfer resistances by analogy with Joule heating. These resistances were not constant during cycling and instead depended on the state of charge of the electrode.

Finally, while batteries feature high energy density, their power density is often limited. LixNa1.5-xVOPV4F0.5 (LNVOPF) has been identified as a promising high rate cathode material whose rate performance can rival that of pseudocapacitive electrodes such as α-MnO2. This dissertation investigates the structural evolution of LNVOPF during lithiation and delithiation to elucidate the origin of its excellent rate performance. Open circuit voltage and entropic potential were measured in three different coin cells with LNVOPF cathodes and Li metal anodes in 1 M LiPF6 in EC:DMC electrolyte. The cathodes consisted of either LNVOPF micronbricks or nanoparticles and were manufactured with either P3HT or PVDF as binders. The evolutions of open circuit voltage and entropic potential indicated that LNVOPF exhibits solid solution behavior with ion ordering. The device with the LNVOPF nanoparticles featured faster kinetics and larger apparent diffusion coefficient of Li+ than that with LNVOPF micronbricks. This was attributed to the larger particle size and the large electrode resistance of the coin cell with LNVOPF micronbricks which retained more Na than LNVOPF nanoparticles after Li exchange. This interpretation was corroborated with thermodynamic calculation of the entropic potential evolution.

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