Neue Elektrolyt- und Anoden-Konzepte für die Lithium-Schwefel-Batterie

The accelerating global warming makes it essential to reduce emissions of greenhouse gases such as CO2. In this regard, batteries are considered to play a key role, as they can store regenerative energy and make it available for a wide range of applications. A promising next-generation battery in this context is the lithium-sulfur (Li-S) battery due to the high theoretical specific capacity of sulfur (1672 mAh gS-1) and the high abundance (low cost) of sulfur as active material. However, today’s Li-S batteries still suffer from issues like poor cycle life, low coulombic efficiency (CE) and fast self-discharge which hinder their broad commercialization. The two main reasons for the above-described limitations are the so-called polysulfide shuttle and the instability of the lithium metal anode. The main objective of this work is to develop Li-S batteries with high cycle life. Stabilizing the lithium metal anode is the greatest challenge to tackle, in order to achieve long-lasting Li-S cells. For that purpose, two different strategies are evaluated in this work: 1) Stabilization of the solid electrolyte interface (SEI) by adding suitable electrolyte additives and 2) coating of the lithium anode with an artificial SEI. Furthermore, graphite is studied as a possible replacement for lithium as anode material. In the first part of this work, the following electrolyte additives are studied: Fluoroethylene carbonate, vinylene carbonate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate (LiDFOB), P2S5, triethyl phosphate and 1,3-propane sultone. At first, their compatibility with polysulfides is investigated. Only P2S5 shows complete and LiDFOB shows temporary stability against polysulfides, while the other additives react strongly with polysulfides. By the use of the two additives, a certain stabilization of the lithium anode can be achieved. Nevertheless, the additives do not lead to a better-performing Li-S battery under realistic conditions. The experiments also reveal a positive influence of polysulfides on the stability of the lithium anode. For this reason, the influence of polysulfides is investigated further. To increase the specific energy of a Li-S battery, its electrolyte quantity must be minimized, presumably leading to a polysulfide-saturated state during cycling. This work reveals that especially in electrolytes with high polysulfide solubility the viscosity increases strongly and the ionic conductivity declines strongly in the polysulfide-saturated state. This leads to kinetic inhibitions in the sulfur cathode and hence limited rate capability of the Li-S cell. Furthermore, the higher polysulfide solubility enhances the polysulfide shuttle, resulting in low CE and poor charging efficiency. At the same time, polysulfides added to the electrolyte suppress the increase of the overpotential in Li vs. Li cells during cycling and thus have a positive impact on the stability of the lithium anode. The addition of polysulfides leads to a smoother deposition of lithium with a lower surface area and, on the other hand, a slightly higher fraction of LiF can be identified in the SEI. In sum, for real-world Li-S cells, it is advantageous to use an electrolyte with low polysulfide solubility, since then, small amounts of polysulfides are always available to stabilize the anode, but at the same time the negative side effects mentioned above are prevented.
As a second strategy, lithium anodes with various artificial protective layers are evaluated. The protective coatings are based on treatment with Tris(n,n-tetramethylene)phosphoric acid triamide, indium (III) acetylacetonate or 1,4-bis(trifluoromethyl)benzene. None of the investigated protective layers can completely suppress dendrite growth. In all treated electrodes, lithium is deposited above the protective layer during cycling, leading to short circuits or drying out of the cell. Therefore, no significant improvement in the Li-S cell performance can be achieved by using the protective layers.
As an alternative strategy, the lithium anode is replaced by a graphite anode. Graphite anodes exhibit only small volume changes during cycling and tremendous CE due to low parasitic side reactions. In the first step, various electrolytes are investigated for their compatibility with graphite. The best results can be obtained with hexyl methyl ether based electrolytes. In the second step, various battery parameters such as capacity balancing of anode and cathode and the voltage window are adjusted to realize the best possible results. With these improvements, a Li-S cell with an initial specific capacity of 960 mAh g-1 and capacity retention of 53 % after 150 cycles can be achieved. Furthermore, two promising advancements of the concept are investigated. Firstly, silicon/graphite composites are integrated into Li-S cells, which offer higher specific energy with comparable performance. Secondly, lithium-coated graphite electrodes are evaluated, which are produced by facile, scalable and cost-effective lithium melt deposition technique. Lithium melt-coated graphite electrodes with low prelithiation capacities perform comparably well to electrochemically-prelithiated electrodes. Nevertheless, considering higher prelithiation capacities, electrochemically prelithiated electrodes still outperform the melt-coated graphite electrodes since the melt-coating is currently not sufficiently homogeneous. In summary, a high cycle life of Li-S batteries with graphite anode is demonstrated which proofs them to be a promising new battery concept for low-cost stationary storage.