Kromatografsko določanje polisulfidnih zvrsti v Mg-S akumulatorjih

Povh, Matija (author), Kralj Cigić, Irena (mentor), Drvarič Talian, Sara (co-mentor)

Mg-S akumulatorji predstavljajo obetavno alternativo Li-ionskim in Li-S akumulatorjem. Mg je v primerjavi z Li veliko bolj pogosta kovina v zemeljski skorji, kaže manjšo tendenco po tvorbi dendritov ter ima v kombinaciji z žveplovo katodo visoko teoretično volumetrično kapaciteto. Med največjimi izzivi v razvoju Mg-S akumulatorjev sta pomanjkanje elektrolitov, kompatibilnih z Mg anodo in žveplovo katodo, ter prehajanje polisulfidnih zvrsti, ki nastajajo med elektrokemijskim praznjenjem akumulatorja, stran od katode in posledično izguba aktivnega materiala. Da bi dobili boljši vpogled v delovanje Mg-S akumulatorjev, smo primerjali delovanje akumulatorskih celic z dvema različnima elektrolitoma s stališča elektrokemijskih lastnosti in nastanka ter topnosti polisulfidnih zvrsti. Pri akumulatorskih celicah z elektrolitom na osnovi Mg(TFSI)2 smo opazili en napetostni nivo in nižjo kapaciteto, medtem ko sta se v akumulatorskih celicah z elektrolitom na osnovi Mg[Al(hfip)4]2 pojavila dva napetostna nivoja, ki ustrezata nastanku polisulfidnih zvrsti in končnih trdnih produktov (MgS2 in MgS). Akumulatorske celice z elektrolitom na osnovi Mg[Al(hfip)4]2 so imele višjo kapaciteto. Z analizo vzorcev iz katode in separatorja s tehniko HPLC smo določili koncentracijski profil nastajanja polisulfidnih zvrsti med elektrokemijskim praznjenjem v akumulatorskih celicah. V celicah z elektrolitom na osnovi Mg(TFSI)2 je koncentracija polisulfidnih zvrsti naraščala tekom celotnega praznjenja, v celicah z elektrolitom na osnovi Mg[Al(hfip)4]2 pa naraščala v prvem napetostnem nivoju in se zniževala v drugem. Na koncu praznjenja v teh celicah polisulfidnih zvrsti praktično ni bilo več. Pri obeh elektrolitih najprej začnejo nastajati polisulfidne zvrsti z daljšimi verigami, nato še s krajšimi. Akumulatorske celice z elektrolitom na osnovi Mg[Al(hfip)4]2 so delovale bolje – dosegle so višjo kapaciteto, pri njih sta se pojavila oba napetostna nivoja. Vzrok za odsotnost drugega napetostnega nivoja v celicah z elektrolitom na osnovi Mg(TFSI)2 je velika začetna prenapetost, ki je najverjetneje posledica pasivizacije anode zaradi nečistoč v elektrolitu ali nestabilnosti elektrolita na površini anode.

• Language: Slovenian
• Keywords: Mg-S akumulatorji, polisulfidne zvrsti, Mg elektroliti, HPLC
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2023
• PID: 20.500.12556/RUL-151027
• COBISS.SI-ID: 173245699
• Publication date in RUL: 27.09.2023

Secondary language

• Language: English
• Title: Chromatographic determination of polysulfide species in Mg-S batteries

Abstract

Mg-S batteries are promising candidates in replacing Li-ion and Li-S batteries. Mg is more abundant in the earth’s crust than Li, shows lower tendency to form dendrites and has higher theoretical volumetric capacity when coupled with sulphur cathode. The biggest challenges in Mg-S batteries development are lack of electrolytes compatible with Mg anode and sulphur cathode and diffusion of polysulfide species that are formed during discharge away from the cathode. To be able to better understand the working mechanics of Mg-S batteries, we compared battery cells with two different electrolytes regarding their electrochemistry, polysulfide species formation and their solubility. In battery cells containing electrolyte based on Mg(TFSI)2 we observed one voltage plateau and lower capacity than in battery cells containing electrolyte based on Mg[Al(hfip)4]2, where we observed higher cell capacity and two voltage plateaus that represent formation of polysulfide species and final discharge products (MgS2 and MgS), respectively. We determined concentration profile of polysulfide species during discharge with HPLC via the analysis of samples obtained from cathode and separator. In cells containing electrolyte based on Mg(TFSI)2 the concentration of polysulfide species is rising throughout the discharge while in the cells containing electrolyte based on Mg[Al(hfip)4]2 the concentration of polysulfide species is rising in the first voltage plateau and decreasing in second plateau. In the end these cells practically did not contain any polysulfide species. We observed that in both electrolytes longer polysulfide species are formed first while shorter polysulfide species are formed later. Battery cells containing electrolyte based on Mg[Al(hfip)4]2 showed better performance with higher capacity, both voltage plateaus were also visible. The cause for absence of second voltage plateau in cells containing electrolyte based on Mg(TFSI)2 is large overcharge in the beginning of discharge, which is most likely caused by passivation of anode due to the presence of impurities in electrolyte or instability of electrolyte on the anode surface.

• Keywords: Mg-S batteries, polysulfide species, Mg electrolytes, HPLC