Degradation of lithium-ion batteries under automotive-like conditions: aging tests, capacity loss and q-OCP interpretation

Battery electric vehicles are spreading worldwide as a relevant solution for the decarbonization of the transportation sector, ensuring high volume and weight-based energy density, high efficiency and low cost. Nevertheless, batteries are known to age in a rather complex and conditions-dependent way. This work aims at investigating battery aging resulting from close-to-real world conditions, highlighting single stressors role. Hence, aiming at representativeness for automotive application, an extensive literature review is performed, identifying a wide set of representative conditions together with their specific variations to be investigated. Realistic driving schedules like WLTP is identified and continuously applied in cycling on commercial samples, investigating the capacity loss from a q-OCP perspective with an equilibrium model. In general, loss of lithium inventory is detected as the main degradation parameter, likely related to SEI growth. Recharge C-rate and load profile appear as poorly-affecting degradation, while a dominant role is associated with operating temperature. Interestingly, temperature and cycling-related degradation appears to be independent and their effects can be effectively superimposed. Loss of active positive electrode material seems particularly affected by cycling depth of discharge, likely having mechanical origin as particle cracking

Gli impianti nelle strutture sanitarie campali

Il crescente numero di catastrofi naturali e conflitti armati nel mondo comporta un drammatico delle emergenze umanitarie. In tali contesti, è necessaria una distribuzione capillare dei servizi sanitari, in modo da poter limitare i danni alle persone e ridurre per quanto possibile i decessi. La distruzione degli ospedali locali o l'assenza di strutture sanitarie, o ancora l'inadeguatezza delle stesse a ricevere molti feriti enfatizzano l'importanza degli ospedali da campo. Nati e sviluppatisi nel 1700 - 1800 circa in ambito militare, essi hanno assunto nel corso degli anni un ruolo chiave nella risposta alle emergenze anche in ambito civile. Portare il soccorso dove esso sia necessario in tempi rapidi è lo scopo dei field hospital per i quali è perciò indispensabile un'estrema mobilità, in termini di trasportabilità e di modularità, caratteristica quest’ultima che consente di ottenere configurazioni diverse in base alla conformazione del territorio o alle strategie militari grazie a differenti disposizioni dei moduli. Gli ospedali da campo devono essere inoltre di rapida messa in funzione e adatti a fornire assistenza a numerosi pazienti. Essi possono essere costituti da vari tipi di tende o da shelter o essere una combinazione dei due. Tali soluzioni modulari vengono impiegate anche in contesti diversi da quelli di immediata emergenza; si inseriscono per esempio a supporto di ospedali tradizionali per sostituire ambulatori o piastre operatorie nei periodi di ristrutturazione, oppure si usano come 'distretti sanitari mobili' per portare cure in territori privi di assistenza o come laboratori mobili di analisi chimico-fisiche. Stante il vasto ambito di applicazione, si evince l'importanza di equiparare l'efficienza e l'asepsi di queste strutture a quelle delle strutture tradizionali, con un adeguamento ad hoc dell'impiantistica ad esse dedicata in modo da poterle considerare non soltanto soluzioni di emergenza, ma moduli tecnologicamente avanzati e tali dunque da poter essere usati anche in sostituzione permanente di presidi in muratura. Il locale di maggiore criticità per quanto riguarda gli impianti è la sala operatoria. Le operating room (o.r.) di nuova generazione in cui si svolgono interventi cosiddetti "puliti" prevedono l'immissione dell'aria a flusso laminare attraverso plafoni dotati di filtri assoluti in modo da raggiungere una classe di contaminazione particellare ISO 5 e ridurre il numero di infezioni post-operatorie. Nel progetto di seguito illustrato si è cercato di assimilare a questa tipologia di o.r. la sala operatoria mobile in esame considerando condizioni climatiche esterne tipiche del Medio Oriente e ponendo attenzione ad alcune criticità proprie della struttura. Lo studio dell'inserimento della sala in un layout possibile di blocco operatorio campale realizzato in shelter e tensostrutture si è incentrato sulla separazione ottimale tra i percorsi sporchi da quelli puliti. Per rendere indipendenti nel corso della missione la piastra chirurgica e, più in generale, l'intero ospedale per ciò che riguarda la fornitura di ossigeno medicinale, si è valutato il vantaggio economico e logistico dell'uso di un produttore on-site basato sulla tecnologia PSA (Pressure Swing Adsorption). Infine, si sono stimati i consumi energetici del blocco operatorio

In operando investigation of anode overpotential dynamics in direct methanol fuel cells

This work illustrates the application to direct methanol fuel cell technology of an innovative reference electrode setup with a through-plate configuration, which enables localised measurement of electrode potential in an operating cell. The utility of the technique is demonstrated by monitoring the evolution of anode overpotential at two different locations in the cell over different time scales, ranging from minutes to hundreds of hours. The measurements provide valuable insight into critical degradation phenomena, identifying localised hydrogen evolution on the anode during short term operation and highlighting the contribution of anode temporary degradation to the overall performance decay during long term operation. This novel approach can be used as a diagnostic tool to improve operational protocols, such as refresh cycles, for direct methanol fuel cells

Fast and reliable calibration of thermal-physical model of lithium-ion battery: a sensitivity-based method

Physical simulation of lithium-ion battery is crucial to consolidate the understanding of its operating mechanisms and, potentially, its state of health; nevertheless, a reliable model calibration is complex due to the large number of physical parameters involved. Here, a thorough sensitivity analysis is performed on the simulation of discharge, relaxation and impedance spectroscopy tests, to highlight the response of the Doyle-Fuller-Newman model output, implemented with a thermal model to compute heat transfer effects, to a variation of 28 model parameters as a function of ⁓160 combinations of temperature, battery state of charge and C-rate. The analysis highlights how up to 14 parameters can be regarded as insensitive, reasonably excludable from model calibration, while other parameters show a strongly miscellaneous response, possible to be maximized adopting specific conditions. Therefore, an innovative method is proposed by experimentally exploiting two temperature levels and combining the three techniques, demonstrated to be highly complementary for a fast and reliable model calibration. As a case study, it is applied on a commercial battery sample, enabling a repeatable and physically sound calibration of the model parameters, as successfully demonstrated over a set of full discharges in 12 combinations of temperatures and C-rate. The comparison with a standard discharge-based calibration process highlights the strength of the proposed protocol

In operando measurement of localised cathode potential to mitigate DMFC temporary degradation

An innovative external reference electrode technique has been applied to the cathode of an operating DMFC in order to identify variations in electrode potential across the active area of the cell. The evolution of cathode potential at two different locations in the cell was monitored during operation, with the primary focus on studying the potential dynamics during the temporary degradation recovery procedure, the so-called refresh cycle. The results highlight for the first time a non-uniform local recovery of temporary degradation at the cathode during refresh cycles, associated with varying rates of platinum oxide reduction across the cell, which could lead to current density redistribution and contribute to an uneven degradation of the components. The technique shows great promise for the improvement of long term DMFC performance via optimisation of refresh cycle protocols

Novel macro-Segmented Fuel Cell approach to investigation of localized degradation in PEMFCs

Here we present a novel concept of a segmented fuel cell, developed to fully and independently characterize four macro- segments of a single cell without introducing any modification to the MEA structure itself. This permits complete characterization of local electrochemical performance (using polarization curves, impedance spectra, cyclic and linear voltammetry) and mass composition (via gas-chromatography) during durability tests. Moreover, each of the four segments is fitted with an innovative reference electrode using an external, through-plane array configuration, enabling separation of anode and cathode contributions and their spatial evolution. This setup has been firstly applied to generate unprecedented insights into the evolution of local performance during DMFC operation. The variation of anode and cathode potential as a function of location within the cell and the correlation with heterogeneous current density distribution are discussed

On the actual cathode mixed potential in direct methanol fuel cells

Methanol crossover is one of the most critical issues hindering commercialization of direct methanol fuel cells since it leads to waste of fuel and significantly affects cathode potential, forming a so-called mixed potential. Unfortunately, due to the sluggish anode kinetics, it is not possible to obtain a reliable estimation of cathode potential by simply measuring the cell voltage. In this work we address this limitation, quantifying the mixed potential by means of innovative open circuit voltage (OCV) tests with a methanol-hydrogen mixture fed to the anode. Over a wide range of operating conditions, the resulting cathode overpotential is between 250 and 430 mV and is strongly influenced by methanol crossover. We show using combined experimental and modelling analysis of cathode impedance that the methanol oxidation at the cathode mainly follows an electrochemical pathway. Finally, reference electrode measurements at both cathode inlet and outlet provide a local measurement of cathode potential, confirming the reliability of the innovative OCV tests and permitting the evaluation of cathode potential up to typical operating current. At 0.25 A cm−2 the operating cathode potential is around 0.85 V and the Ohmic drop through the catalyst layer is almost 50 mV, which is comparable to that in the membrane