Unravelling the ion transport and the interphase properties of a mixed olivine cathode for Na-ion battery

The replacement of Li by Na in an analogue battery to the commercial Li-ion one appears a sustainable strategy to overcome the several concerns triggered by the increased demand for the electrochemical energy storage.However,the apparently simple change of the alkali metal represents a challenging step which requires notable and dedicated studies.Therefore,we investigate herein the features of a NaFe_(0.6)Mn_(0.4)PO_(4)(NFMP)cathode with triphylite structure achieved from the conversion of a LiFe_(0.6)Mn_(0.4)PO_(4)(LFMP)olivine for application in Na-ion battery.The work initially characterizes the structure,morphology and performances in sodium cell of NFMP,achieving a maximum capacity exceeding 100 mAh g^(−1)at a temperature of 55℃,adequate rate capability,and suitable retention confirmed by ex-situ measurements.Subsequently,the study compares in parallel key parameters of the NFMP and LFMP such as Na^(+)/Li^(+)ions diffusion,interfacial characteristics,and reaction mechanism in Na/Li cells using various electrochemical techniques.The data reveal that relatively limited modifications of NFMP chemistry,structure and morphology compared to LFMP greatly impact the reaction mechanism,kinetics and electrochemical features.These changes are ascribed to the different physical and chemical features of the two compounds,the slower mobility of Na^(+)with respect to Li^(+),and a more resistive electrode/electrolyte interphase of sodium compared with lithium.Relevantly,the study reveals analogue trends of the charge transfer resistance and the ion diffusion coefficient in NFMP and LFMP during the electrochemical process in half-cell.Hence,the NFMP achieved herein is suggested as a possible candidate for application in a low-cost,efficient,and environmentally friendly Na-ion battery.

Current collectors based on multiwalled carbon-nanotubes and fewlayer graphene for enhancing the conversion process in scalable lithium-sulfur battery

We investigated herein the morphological,structural,and electrochemical features of electrodes using a sulfur(S)-super P carbon(SPC)composite(i.e.,S@SPC-73),and including few-layer graphene(FLG),multiwalled carbon nanotubes(MWCNTs),or a mixture of them within the current collector design.Furthermore,we studied the effect of two different electron-conducting agents,that is,SPC and FLG,used in the slurry for the electrode preparation.The supports have high structural crystallinity,while their morphologies are dependent on the type of material used.Cyclic voltammetry(CV)shows a reversible and stable conversion reaction between Li and S with an activation process upon the first cycle leading to the decrease of cell polarization.This activation process is verified by electrochemical impedance spectroscopy(EIS)with a decrease of the resistance after the first CV scan.Furthermore,CV at increasing scan rates indicates a Li+diffusion coefficient(D)ranging between 10^(−9) and 10^(−7) cm^(2)·s^(−1)in the various states of charge of the cell,and the highest D value for the electrodes using FLG as electron-conducting agent.Galvanostatic tests performed at constant current of C/5(1 C=1675 mA·g_(s)^(−1))show high initial specific capacity values,which decrease during the initial cycles due to a partial loss of the active material,and subsequently increase due to the activation process.All the electrodes show a Coulombic efficiency higher than 97%upon the initial cycles,and a retention strongly dependent on the electrode formulation.Therefore,this study suggests a careful control of the electrode in terms of current collector design and slurry composition to achieve good electrode morphology,mechanical stability,and promising electrochemical performance in practical Li-S cells.