Inorganic cathode materials for magnesium batteries

Robba, Ana (author), Dominko, Robert (mentor)

High-capacity battery systems are needed for further development of portable electronics and electric vehicles. Current lithium-ion batteries are getting close to their theoretical limitations, and this research is focused on new alternatives. One of them is the use of magnesium anode, desirable for its high volumetric capacity, relative safety, availability and price. Theoretically, pairing magnesium anode with an inorganic insertion cathode offers high voltages, while conversion cathodes boast high specific capacities. Realizing these theoretical promises is not simple,and a better understanding of the basic mechanisms is needed. In the presented work, we have researched two manganese oxide polymorphs as potential insertion cathodes. Magnesium insertion into spinel and birnessite structure was investigated in aqueous and organic electrolytes. Structural changes were analysed with transmission electronmicroscopy. We confirmed the successful insertion of Mg into both structures. Severe structural degradation and transformation were detected in samples, influencing electrochemical responses of the cells. Our work on conversion materials was focused on the Mg-S system. First, we investigated the proposed mechanism of sulfur reduction and determined the final discharge product. With operando techniques, we showed that the sulfur reduction proceeds through polysulfide formation during high-voltage plateau and the precipitation of the MgS as the final product in the low-voltage plateau. Precipitated MgS was found to be amorphous with tetrahedral coordination of Mg, resembling the wurtzite structure. With obtained information and understanding, we tried to improve high polarisation and fast capacity fade of the system. The addition of Se to the S cathode did not significantly improve polarisation or capacity fading. Concentrated electrolytes, used to lower polysulfide solubility, only partially improved cycling stability. Finally, we evaluated the influence of Cu current collector on the electrochemical properties of the Mg-S system. We confirmed that the presence of Cu decreases the polarisation and improves the stability by actively participating in redox reactions. With that, the energy density of such a cell is unattractive for commercialization. With the presented research, we deepened our understanding of magnesium batteries and their fundamental issues. Hopefully, this insight will help us solve the remaining challenges preventing the practical application of the system.

• Language: English
• Keywords: magnesium battery, cathode, Mg insertion, Mg-S battery, mechanism, polysulfides, Mg anode, current collector
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2020
• PID: 20.500.12556/RUL-120962
• COBISS.SI-ID: 32429059
• Publication date in RUL: 28.09.2020

Secondary language

• Language: Slovenian
• Title: Anorganski katodni materiali za magnezijeve akumulatorje

Abstract

Razvoj prenosne elektronike in električnih vozil zahteva razvoj novih shranjevalnikov električne energije z visoko energijsko gostoto. Trenutni litij-ionski akumulatorji se približujejo teoretičnim omejitvam in raziskave se počasi osredotočajo na iskanje alternativnih rešitev. Ena od teh je uporaba magnezijeve anode, ki je privlačna predvsem zaradi visoke volumetrične kapacitete, relativne varnosti, dostopnosti in seveda cene. Kombinacija magnezijeve anode z anorganskimi insercijskimi katodami v teoriji obljublja visoke napetosti, medtem ko je glavna prednost konverzijskih materialov njihova visoka specifična kapaciteta. Razumevanje temeljnih zakonitosti delovanja tovrstnih sistemov pa je nujno, da se bomo sploh lahko približali teoretičnim vrednostim. V predstavljeni doktorski disertaciji smo med mogočimi insercijskimi katodami raziskovali dva polimorfa manganovega oksida. Vgradnjo magnezija v spinelno in birnesitno strukturo smo proučevali v vodnem in organskem elektrolitu. Strukturne spremembe smo analizirali z uporabo transmisijskega elektronskega miksroskopa. Reverzibilno vgradnjo magnezija smo potrdili v obeh materialih, v vzorcih smo zaznali večje strukturne spremembein transformacije, ki so vplivale na elektrokemijsko aktivnost celice. Naše raziskave na področju konverzijskih katod so bile usmerjene v razumevanje Mg-S sistema. Začeli smo s potrditvijo mehanizma redukcije žvepla in določitvijo končnega produkta. Z operando tehnikami smo pokazali, da redukcija žvepla v Mg-S akumulatorjih poteka prek nastanka polisulfidov na višjem platoju do formacije MgS kot končnega produkta na nižjem platoju. Ugotovili smo, da je nastali MgS amorfen, s tetraedrično koordinacijo, ki je podobna vurcitni strukturi. Z zbranimi informacijami smo želeli izboljšati visoko polarizacijo in hiter padec kapacitete v Mg-S akumulatorjih. Dodatek Se k žveplovi katodi ni bistveno vplival na zmanjšanje polarizacije. Koncentrirani elektroliti, ki smo jih uporabili za znižanje topnosti polisulfidov, so le delno izboljšali stabilnost delovanja celice. Na koncu smo analizirali še vpliv uporabe bakrovega tokovnega nosilca na elektrokemijske značilnosti sistema. Potrdili smo, da lahko prisotnost Cu zniža polarizacijo in izboljša stabilnost, tako da aktivno sodeluje v elektrokemijskih reakcijah. S tem pa se močno zniža energijska gostota sistema, kar zmanjša njegovo potencialno uporabnost. S predstavljenim delom smo poglobili razumevanje magnezijevih akumulatorjev in njihovih glavnih slabosti. S pridobljenim znanjem bomo lahko uspešneje pristopali k reševanju izzivov, ki ovirajo praktično uporabo tovrstnih akumulatorjev.

• Keywords: magnezijev akumulator, katoda, insercija magnezija, Mg-S akumulator, mehanizem, polisulfidi, magnezijeva anoda, tokovni nosilec