Development of borophene reinforced cathodes for solid-state lithium sulfur batteries

Elemental sulfur (S) is an exciting rising star for the cathode part of batteries and shows promising results for next-generation battery technologies. The non-toxic and environmentally friendly character of sulfur, as well as its abundance in nature and low cost, makes sulfur more attractive for next-generation technologies and paves the way even more. With a theoretical capacity of 1675 mAh g-1 and an energy density of 2567 Wh kg-1, lithium-sulfur batteries are 5 times higher than the specific energy density of today's known commercial Li-ion batteries. Because of that, lithium-sulfur batteries are considered an excellent alternative to lithium-ion batteries in the future. There is no significant difference between the working mechanisms with conventional battery technologies. However, commercial implementation still has some difficulties when using their active materials in liquid electrolytes ambiance. Most importantly, the shuttle effect is caused by the dissolution of the undesired lithium polysulfide intermediates into the lithium anode through the electrolyte and separator, resulting in low capacity and rapid capacity fading. Recently, solid-state batteries have been recommended to overcome these problems. The difference between solid-state lithium-sulfur batteries and other batteries is that they only use a single solid electrolyte with high ionic conductivity instead of a liquid electrolyte and separator together. However, interface problems may arise between the electrolyte and the electrodes in this case. This may affect the ionic and electrical conductivity and reduces the cell's capacity and stability. This thesis study has investigated the effect of 2-dimensional beta borophene as a cathode additive for all-solid-state lithium-sulfur batteries. The comparisons have been carried out regarding the impact on ionic and electrical conductivity. Although the studies of borophene's contributions in the literature on the anode component, this thesis focuses on the cathode contribution for the first time. While MoS2 has been selected as the cathode active material, carbon black has been selected as the electrical conductor, and Li7P3S11 solid electrolyte has been selected as an ionic conductor in lithium-sulfur cells. Borophene has been synthesized from boron powder by the exfoliation method. Li7P3S11 solid electrolytes were obtained by using Li2S and P2S5 ingredients and have been reported as one of the most efficient solid electrolytes for lithium-sulfur batteries in the literature. However, due to some interface problems of LPS electrolyte and consequent performance losses in ionic conductivity, one of the investigations has been focussed on the effects of borophene additive (partial replacement with solid electrolyte) on the stability of electrochemical cells in this thesis. As a cathode-active material, MoS2, containing sulfur, and its two-dimensional (2D) material nature, eliminates many of the disadvantages that sulfur exhibits when used alone. Thanks to its hollow sandwich structure, it has been widely reported that MoS2 positively contributes not only to the ionic conductivity but also to the electrochemical stability in lithium-sulfur cells. In order to examine the effect of borophene on ionic conductivity in all-solid-state lithium-sulfur batteries, multicomponent composite cathodes were prepared in (MoS2 / Conductive Carbon / Li7P3S11 + Borophene) overall compositions. 30% MoS2 active material and 30% carbon black conductive carbon amount of the cathode combinations were kept constant to investigate the effect of borophene on the stability of solid electrolytes. The remaining 40% is Li7P3S11 + Borophene mixture. Solid electrolytes and borophene were mixed in different amounts and added to the total cathode amounts. In this way, it was observed more clearly how borophene affects the ionic conductivity of lithium-sulfur all-solid-state batteries. According to the results obtained in the thesis, the specific capacity of the cells was affected negatively when the borophene amount increased. However, the stability and cycle life of the lithium-sulfur cells was improved. When LPS electrolyte is used 100% amount, the capacity of the cells increases up to the 10th cycle then a decrease is observed. This decrement continued gradually until the 50th cycle and completely faded its capacity at the end. When borophene is used in equal amounts with LPS, a continuous increase in capacity is observed from the 1st to the 50th cycle. The increase in the specific capacity of the cells is directly related to the gradual activation. After the 50th cycle, the cell protected its stability until the 100th cycle. This means that the activation process is completed and reflects the actual capacity of the cell after then. Electrochemical stability is also observed when borophene is completely substituted for the solid electrolyte. However, the capacity is observed as too low. It can be said that the contribution of full borophene to ionic conductivity is limited. According to the experimental results, equally using solid electrolytes and borophene contributes more positively to conserving capacity among all three combinations. The cathodes (MoS2 / Li7P3S11 / Conductive Carbon + Borophene) prepared by the same method for comparison the electrical conductivity and were kept constant with 30% active material and 40% solid electrolyte. In the remaining 30% of amount, carbon black and borophene were mixed in different compositions to see how borophene affects the electrical conductivity of all solid state lithium sulfur cells more clearly. The borophene's contribution to electrical conductivity in lithium-sulfur cells was investigated in MoS2 / Li7P3S11 / conductive carbon + borophene cathode combination by preparing three different conductive carbon + borophene compositions. Three different compositions with conductive carbon dominant, borophene dominant, and equally mixed versions were prepared, and the results were discussed. According to the results of the experimental studies, when the borophene amount increased while the carbon black amount decreased, it has been observed negative effect on the electrical conductivity of the lithium-sulfur cell. When the results are compared in terms of both ionic conductivity and electrical conductivity; it was concluded that borophene did not have a positive contribution to the electrical conductivity in lithium-sulfur battery cathodes. However, the same thing cannot be said about ionic conductivity. The stability of the cell increased with the increase of the borophene amount. However, the specific capacity has significantly decreasedBased on these overall results, it is not recommended using of borophene as a substitute for conductive carbon in all-solid-state lithium-sulfur batteries. This thesis used borophene in high amounts to contribute to the solid electrolyte, resulting in a severe decrease in the specific capacity of the cell. For this reason, it is recommended to add borophene to the solid electrolyte in lithium-sulfur cathodes at lower rates to avoid reducing the cell's capacity and to obtain a positive contribution to the stability of the cell. It can be said that the method developed within the scope of this thesis study are easy, practical, and applicable. For this reason, the studies are expected to shed light on the new generation of electrochemical energy storage systems and be a guide for commercialization.