S hitrim razvojem družbe je danes potreba po uporabi prenosnih elektronskih naprav z učinkovitejšimi akumulatorji velikih zmogljivosti očitna. Litij-žveplovi akumulatorji sodijo med najobetavnejše kandidate za prevzem primata Li-ion akumulatorju, predvsem zaradi veliko višje teoretične gostote energije v primerjavi s komercialnega Li-ion akumulatorja, višje teoretične specifične kapacitete, nizke cene aktivnih materialov (bogate naravne rezerve žvepla na Zemlji) in majhnega vpliva na okolje. Posledično litij-žveplovi akumulatorji v zadnjem desetletju sodijo med ene najpomembnejših in zelo obširno raziskanih akumulatorjev. Čeprav ima ta sistem številne prednosti, ima tudi nekaj bistvenih pomanjkljivosti, med katerimi so neprevodnost aktivnega katodnega materiala, odtapljanje litijevih polisulfidov, redoks prenos naboja prek litijevih polisulfidov[1] oziroma mehanizem »shuttle« (angl. Shuttle efect) in dendritska rast litija [2].
V sklopu raziskovalnega dela magistrskega dela so bile opravljene elektrokemijske meritve litij-žveplovih akumulatorjev, sestavljenih iz katodnih materialov različne sestave. V raziskavi uporabljene katode so narejene tako, da je žveplo impregnirano v grafen oksidne (GO) strukture v različnih razmerjih C in S. Izvedba akumulatorske celice, uporabljene v raziskavi, je t. i. vrečasta celica »coffee-bag«. Sestavljanje akumulatorja se je izvedlo v suhi komori z argonovo atmosfero, brez prisotnosti kisika in vode. Celice so bile testirane na določeno število ciklov polnjenja/praznjenja, podatki pa so bili obdelani z uporabo programa Origin. Postopki priprave materialov, sestavljanje akumulatorske celice, uporabljene kemikalije in oprema so predstavljeni v poglavju Eksperimentalni del.
V teoretičnem delu magistrskega dela so predstavljeni osnove elektrokemije in karakteristike elektrokemijskih celic, princip delovanja litij-ionskih akumulatorjev, princip delovanja litij-žveplovih akumulatorjev ter delovanje katodnega materiala, elektrolita in separatorja.
Rezultati meritev in z njimi povezane raziskave katodnih materialov v litij-žveplovih akumulatorjih so povzeti z vidika izboljšanja elektrokemijskega obnašanja akumulatorja, iskanja optimalne sestave katode med pripravljenimi kompozitnimi materiali in primerjave z akumulatorji, pripravljenimi na osnovi infiltracije žvepla v komercialne ogljikove materiale. Končni rezultat je bil naslednji: preiskovane katode so se izkazale kot neprevodne oziroma žveplo ni sodelovalo v elektrokemijski reakciji. Razlog, zakaj so bile katode neprevodne, je najverjetneje nepotečena reakcija disproporcionacije ter parcialna redukcija grafen oksida.With such a fast-paced society development, a need for portable devices with more efficient batteries is quite obvious. Li- S batteries are among the most favourable candidates for primacy takeover from the Li-ion batteries, above all because of the significantly higher theoretical energy density in comparison with commercially available Li-ion batteries. They also have a higher theoretical specific capacity, a lower cost of active materials due to the abundance of naturally occurring Sulphur reserves on Earth as well as a smaller impact on the environment.[2]
Consequently, Li-S batteries will be enlisted as one of the most important batteries to research in the future as well as ones on whom is already done extensive research. Although this type of battery has many advantages, there are some key disadvantages that need referring to such as low Sulphur conductivity, the shuttle mechanism and dendritic lithium growth.
As a part of my master’s thesis research, I did electrochemical measurements of Li-Sulphur batteries composed of different composition cathode materials. The cathodes used in the research were made by impregnating Sulphur in a graphene oxide structure with different C:S ratios. The battery cell execution was a ˝Coffee-bag˝ type. Battery assembly was done in a dry chamber with an Argon atmosphere, without the presence of air or water. I tested all battery cells at a predetermined number of charge/discharge cycles, all data was processed using the Origin program. All preparation procedures such as materials preparation, battery cell assembly, used chemicals and equipment I introduced in the ˝Experimental chapter˝.
In the Theoretical chapter I explained the basics of electrochemistry and electrochemical cell characteristics, working principles of Li-ion batteries, working principles of Li-Sulphur batteries with emphasis on cathode material working principles, electrolytes and separators used in those systems.
I reviewed the results of the measurements and with them connected researched cathode materials from the following point of view: improving of the electrochemical behaviour of the battery cell, finding the best working compositional cathode among all tested cathodes as well as comparison between efficiency of the batteries comprised of the new cathodes and those made with commercially available carbon materials
