The initial stages of Li_(2)O_(2) formation during oxygen reduction reaction in Li-O_(2) batteries:The significance of Li_(2)O_(2) in charge-transfer reactions within devices

Lithium-oxygen batteries are a promising technology because they can greatly surpass the energy density of lithium-ion batteries.However,this theoretical characteristic has not yet been converted into a real device with high cyclability.Problems with air contamination,metallic lithium reactivity,and complex discharge and charge reactions are the main issues for this technology.A fast and reversible oxygen reduction reaction(ORR)is crucial for good performance of secondary batteries',but the partial knowledge of its mechanisms,especially when devices are concerned,hinders further development.From this perspective,the present work uses operando Raman experiments and electrochemical impedance spectroscopy(EIS)to assess the first stages of the discharge processes in porous carbon electrodes,following their changes cycle by cycle at initial operation.A growth kinetic formation of the discharge product signal(Li_(2)O_(2))was observed with operando Raman,indicating a first-order reaction and enabling an analysis by a microkinetic model.The solution mechanism in the evaluated system was ascribed for an equivalent circuit with three time constants.While the time constant for the anode interface reveals to remain relatively constant after the first discharge,its surface seemed to be more non-uniform.The model indicated that the reaction occurs at the Li_(2)O_(2) surface,decreasing the associated resistance during the initial discharge phase.Furthermore,the growth of Li_(2)O_(2) forms a hetero-phase between Li_(2)O_(2)/electrolyte,while creating a more compact and homogeneous on the Li_(2)O_(2)/cathode surface.The methodology here described thus offers a way of directly probing changes in surface chemistry evolution during cycling from a device through EIS analysis.

Operando FTIR study on water additive in lithium-sulfur batteries to mitigate shuttle effect

Additives in the electrolytes of Li-S batteries aim to increase overall capacity,improve Li ion conductivity,enhance cyclability,and mitigate the shuttle effect,which is one of the major issues of this system.Here,the use of water as an additive in the commonly used electrolyte,1.0 M LiTFSI/1.0%(w/w) LiNO_(3) and a 1:1 mixture of 1,3-dioxolane(DOL) and 1,2-dimethoxyethane(DME) was investigated.We used Co_(2)Mn_(0.5)Al_(0.5)O_(4)(CMA) as an electrocatalyst anchored on an activated carbon(AC) electrode with added sulfur via a melt-diffusion process.The structural analysis of CMA via Rietveld refinement showed interatomic spaces that can promote ionic conductivity,facilitating Li^(+) ion migration.Electrochemical tests determined 1600 ppm as the optimal water concentration,significantly reducing the shuttle effect.Post-mortem XPS analysis focused on the lithium metal anode revealed the formation of Li_(2)O layers in dry samples and LiOH in wet samples.Better capacity was observed in wet samples,which can be attributed to the superior ionic conductivity of LiOH at the electrode/electrolyte interface,surpassing that of Li_(2)O by 12 times.Finally,Operando FTIR experiments provided real-time insights into electrolyte degradation and SEI formation,elucidating the activity mechanisms of water and Li_(2)CO_(3) over the cycles.This work presents results that could aid future advancements in Li-S battery technology,offering possibilities to mitigate its challenges with inexpensive and scalable additives.

Raman probing carbon&aqueous electrolytes interfaces and molecular dynamics simulations towards understanding electrochemical properties under polarization conditions in supercapacitors

Raman probing of carbon electrode and electrolyte under dynamic conditions is performed here using different aqueous electrolytes to elucidate the fundamental events occurring in electrochemical supercapacitor during charge–discharge processes.The areal capacitance ranges from 1.54 to 2.31μF cm^(-2)μm and it is determined using different techniques.These findings indicate that the Helmholtz capacitance governs the overall charge-storage process instead of the space charge(quantum)capacitance commonly verified for HOPG electrodes in the range of~3 to 7μF cm^(-2).Molecular dynamics simulations are employed to elucidate the origin of the reversible Raman spectral changes during the charge–discharge processes.A correlation is verified between the reversible Raman shift and the surface excesses of the different ionic species.A theoretical framework is presented to relate the effect of the applied potential on the Raman shift and its correlation with the surface ionic charge.It is proposed that the Raman shift is governed by the interaction of solvated cations with graphite promoted by polarization conditions.It is the first time that a comparative study on different aqueous electrolyte p H and cation ion size has been performed tracking the Raman spectra change under dynamic polarization conditions and contrasting with comprehensive electrochemistry and dynamic molecular simulations studies.This study shines lights onto the charge-storage mechanism with evidence of Kohn anomaly reduction in the carbon electrode during the reversible adsorption/desorption and insertion/extraction of ionic species.