Fluorinated alkoxyborate- and alkoxyaluminate-based electrolytes and their use in multivalent batteries

Pavčnik, Tjaša (author), Bitenc, Jan (mentor)

Magnesium and calcium batteries are considered promising post-Li battery technologies due to the low redox potentials of Mg and Ca metals (–2.37 V and –2.87 V vs. SHE, respectively), high gravimetric and volumetric densities of metal anodes, and natural abundance of Mg and Ca elements, which is an important factor in terms of price and sustainability. The main challenge is the development of suitable electrolytes that enable reversible plating/stripping of Mg/Ca with high Coulombic efficiency in a wide potential window, while also being compatible with the metal anode and cathode materials. The focus of the thesis is to explore fluorinated alkoxyborate and alkoxyaluminate electrolytes for Mg and Ca batteries. Starting with Mg alkoxyborates, we prepare a series of electrolytes with different degrees of fluorination and investigate the correlations between anion structure and the physicochemical and electrochemical properties of the electrolytes. Enhanced electrochemical performance is observed when boron is replaced by aluminium. In the second part, MgAlhfip as a state-of-the-art electrolyte comes under the spotlight. We investigate the influence of the MgAlhfip synthesis procedure on electrolyte purity and its electrochemical performance. To improve the purity of underperforming electrolytes, we introduce two approaches: an additional step in the synthesis procedure and the use of scavenging additives. The potential of MgAlhfip electrolyte for practical applications is examined in terms of its tolerance towards water contamination. To mimic conditions in practical applications, we study the effects of additives under aggravated cycling conditions. In the last part, we introduce a novel CaAlhfip electrolyte for Ca batteries. We benchmark its physicochemical and electrochemical properties with a model CaBhfip electrolyte. Additionally, we expose the decomposition of solvent on the Ca metal anode as a key issue of Ca electrolytes.

• Language: English
• Keywords: Mg batteries, synthesis, fluorinated alkoxyborate electrolytes, fluorinated alkoxyaluminate electrolytes, Ca batteries
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2023
• PID: 20.500.12556/RUL-150783
• COBISS.SI-ID: 169070339
• Publication date in RUL: 22.09.2023

Secondary language

• Language: Slovenian
• Title: Elektroliti na osnovi fluoriranih alkoksiboratov in alkoksialuminatov in njihova uporaba v multivalentnih akumulatorjih

Abstract

Magnezijevi in kalcijevi akumulatorji veljajo za obetavne akumulatorje prihodnosti zaradi nizkega redoks potenciala Mg in Ca kovin (–2.37 V oziroma –2.87 V proti SHE), visoke gravimetrične in volumetrične gostote kovinskih anod in pogostosti Mg in Ca v naravi, kar je pomemben dejavnik tako z vidika stroškov kot trajnosti. Glavni izziv predstavlja razvoj ustreznih elektrolitov, ki omogočajo reverzibilno odlaganje/odtapljanje Mg oziroma Ca z visoko Kulonsko učinkovitostjo v širokem napetostnem oknu in so združljivi s kovinsko anodo in katodnimi materiali. V tem delu raziskujemo elektrolite na osnovi fluoriranih alkoksiboratov in alkoksialuminatov za Mg in Ca akumulatorje. V prvem delu preiskujemo različne elektrolite na osnovi Mg alkoksiboratov z različno stopnjo fluoriranja in iščemo povezave med strukturo aniona ter fizikalno-kemijskimi in elektrokemijskimi lastnostmi elektrolita. Izboljšano elektrokemijsko delovanje elektrolita opazimo, ko bor zamenjamo z aluminijem. V drugem delu se raziskave osredotočajo na najsodobnejši MgAlhfip elektrolit, kjer raziskujemo vpliv izbire sinteze MgAlhfip na čistost elektrolita in njegovo elektrokemijsko delovanje. Za izboljšanje čistosti slabše delujočih elektrolitov predstavimo dva pristopa: dodaten korak v postopku sinteze in aditive, ki opravljajo funkcijo čistilcev. Potencial MgAlhfip za praktično uporabo raziskujemo z vidika občutljivosti elektrolita na prisotnost vode. Za posnemanje pogojev v praktičnih aplikacijah učinek aditivov raziskujemo z uporabo kompleksnejših protokolov odlaganja in odtapljanja Mg. V zadnjem delu predstavljamo nov CaAlhfip elektrolit za Ca akumulatorje. Njegove fizikalno-kemijske in elektrokemijske lastnosti primerjamo z referenčnim CaBhfip elektrolitom. Kot ključna težava Ca elektrolitov je identificirana razgradnja topila na kovinski Ca anodi.

• Keywords: Mg akumulatorji, sinteza, elektroliti na osnovi fluoriranih alkoksiboratov, elektroliti na osnovi fluoriranih alkoksialuminatov, Ca akumulatorji