Theoretical study of processes on the solid electrolyte interface of rechargeable magnesium batteries

Kopač Lautar, Anja (author), Rejec, Tomaž (mentor), Doublet, Marie-Liesse (co-mentor)

The research of battery materials is becoming an increasingly important scientific field due to the growing demand for the electric energy. Magnesium batteries represent one of the promising multivalent battery architectures. To optimize the Mg battery for commercial use, a potential dependent interfacial processes should be understood. Yet, due to great complexity of the interface, there is a lack of theoretical approaches that would enable facing this challenge. A computationally affordable, fully unparameterized, and widely applicable theoretical methodology based on density functional theory and grand canonical approach is extended to investigate the electrochemical stability of Mg-metal/electrolyte interfaces, and to predict their thermodynamic behaviour. The calculated Mg$^{2+}$/Mg$^{0}$ redox potential differs by less than 3\% from the experimental value, demonstrating that the methodology provides physically meaningful and reliable results. The methodology is used to study two different Mg electrolytes, based on ethylene carbonate (EC) and dimethylether (DME) solvents. Experiments have shown that the Mg battery fails with the EC electrolyte, while it works fairly well with the DME electrolyte. Our results successfully elucidate atomistic mechanisms that explain the experimental observations. Moreover, our theoretical insights provides valuable guidelines for designing electrolytes with favourable properties. To broaden the theoretical understanding from atomistic to meso-scale, the dependence of morphology evolution on surface orientation is investigated. We found that the surface with the highest area fraction is not necessarily the one with the lowest surface energy, which is usually the only one considered in literature. Morphology evolution should thus be studied on all commonly present surface orientations. Our results show that diffusion of Mg atoms on the Mg anode is slow on some commonly present Mg surface orientations, indicating that Mg could exhibit uneven deposition.

• Language: English
• Keywords: interfaces, double layer, surface science, batteries, magnesium batteries, electrochemistry
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2019
• PID: 20.500.12556/RUL-112049
• COBISS.SI-ID: 3374692
• Publication date in RUL: 21.10.2019

Secondary language

• Language: Slovenian
• Title: Teoretična študija procesov na medfazni površini med elektrolitom in elektrodo v magnezijevih akumulatorjih

Abstract

Zaradi naraščajoče potrebe po električni energiji postaja raziskovanje baterijskih materialov vedno bolj pomembno znanstveno področje. Magnezijevi akumulatorji predstavljajo obetaven sistem na področju multivalentnih akumulatorjev, vendar je za njihovo optimizacijo in komercializacijo potrebno razumeti procese na medfazni površini elektroda/elektrolit v odvisnosti od napetosti. Kompleksnost teh procesov je privedla v pomanjkanje teoretičnih pristopov, ki bi omogočili njihovo razumevanje. Računsko dostopno, popolnoma neparametrizirano in široko uporabno teoretično metodologijo, ki temelji na teoriji gostotnega funkcionala in velekanonskem ansamblu, smo razširili z namenom raziskovanja elektrokemijske stabilnosti in termodinamskega obnašanja medfazne površine med elektrolitom in Mg elektrodo. Izračunani Mg$^{2+}$/Mg$^0$ redoks potentical se razlikuje za manj kot 3\% od eksperimentalne vrednosti, kar dokazuje, da metodologija zagotavlja fizikalno smiselne in zanesljive rezultate. Metodologija je uporabljena za študij dveh različnih elektrolitov baziranih na etilen karbonatu (EC) in dimetil etru (DME). Eksperimenti so pokazali, da Mg akumulator z EC elektrolitom ne deluje, med tem ko z DME elektrolitom deluje dokaj dobro. Naši rezultati pojasnijo atomistične mehanizme, ki teoretično razložijo eksperimentalno opaženo razliko v obnašanju sistemov. Pridobljeni teoretični vpogledi prispevajo tudi smernice za načrtovanje elektrolitov z optimiziranimi lastnostmi. Da bi razširili razumevanje iz atomistične na mezo-skalo, smo raziskali spreminjanje morfologije različnih Mg površin. Pokazali smo, da najbolj zastopana površina ni nujno najbolj stabilna, čeprav je ponavadi edina obravnavana v literaturi. Spreminjanje morfologije mora zato biti raziskano na vseh pogosto prisotnih orientacijah površine. Naši rezultati so pokazali, da je difuzija Mg atomov na Mg anodi počasna na nekaterih pogosto prisotnih orientacijah površine, kar kaže na možnost neenakomernega odlaganja magnezija.

• Keywords: medfazne površine, dvojna plast, znanost površin, baterije, magnezijevi akumulatorji, elektrokemija