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High energy lithium sulphur cells and batteries

Periodic Reporting for period 3 - HELIS (High energy lithium sulphur cells and batteries)

Reporting period: 2018-02-01 to 2019-05-31

LSBs are viable candidate for the commercialisation among all post Li-ion battery technologies due to their high theoretical energy density and cost-effectiveness. Despite many efforts, there are still remaining issues that need to be solved and this will provide final direction of lithium sulphur batteries technological development. Some of the technological aspects, like the development of host matrices, interactions of host matrix with polysulphides (PS) and interactions between sulphur and electrolyte have been successfully developed within FP7 project EUROLIS ( porosity of the cathode, interactions between host matrices and PS and proper solvatation of PS are requirements for the complete utilisation of sulphur. A possible direction to improve cycling properties is an effective separation between electrodes. Additionally, lithium metal protection and reduction of electrolyte quantity in the final cell configuration are required if we want to meet objectives connected with long life and high energy density.

The HELIS project has been addressing remaining issues connected with a stability of lithium anode during cycling, with the engineering of complete cell and with questions about lithium sulphur batteries cells implementation into commercial products (aging, safety, recycling, battery packs).Instability of lithium metal in most of conventional electrolytes and formation of dendrites due to uneven distribution of lithium upon the deposition cause several difficulties. Safety problems connected with dendrites and low Coulombic efficiency with a constant increase of inner resistance due to electrolyte degradation represent main technological challenges. From this point of view, stabilisation of lithium metal will have an impact on safety issues. The stabilized interface layer is important from the view of engineering of cathode composite and separator porosity since this is an important parameter for electrolyte accommodation and volume expansion adjustment.Finally the mechanism of lithium sulphur batteries aging can determine the practical applicability of LSB in different applications, but the project is aiming towards the development of three different series of Li-S cell prototypes, all of which will be tested according to specifications for automotive use.

The Li-S battery technology in HELIS project is based on low-cost cathode materials. The cost for electrolyte, binder, and separator costs are estimated to be similar to those in the current Li-ion battery technology. The use of a much cheaper cathode will be maintaining low cost of Li-S cells and the final cell price is estimated to be below 150 €/kWh. By achieving 500 Wh/kg energy density this will substantially improve the driving range, thus having a positive impact on EV acceptance.
The changed chemistry in 3rd RP is based on difficulties with previous generations of prototype cells. Namely, working with electrolytes that enable polysulfide solubility showed a promising direction in the lab-scale but was not possible to be extended to the prototype cells. By using ultramicroporous carbons, already studied in the Eurolis project, sulphur confinement within pores can be completely achieved. The difference between carbon used in the Eurolis project and pre-commercial carbon used for GEN3 of prototype cells is in the availability of large quantities and in the pore size/pore volume of the sample. The pre-commercial sample has much larger pore volume and pore size is close to 1nm, which enables sulphur confinement during conversion into polysulfides and Li2S. With carbon host matrix, we can use carbonate based solvents that are less aggressive to metallic lithium and that prolong cycle life of the anode. However, no protection has been developed for using directly on the prototype line. We used in GEN2 electrolyte quantity of 6.5L/mgS by using microporous carbons and blend of carbonate solvents. We were able to run cells with electrolyte quantity lower than 3 L/mgS. We have reached energy density close to 300 Wh/kg and higher number can be obtained once full optimisation will be done. Safety tests show that the proposed chemistry is suitable, no H2S or any other poisoning gasses were detected. Price-performance remained very similar compared to GEN2 since again we are using fluorinated based solvents and the same salt. Since electrolyte quantity is lower, this can be beneficial for the cost calculations, but probably ultramicroporous carbon will be more expensive compared to mesoporous carbon. This abruptive change in the chemistry of Li-S prototype cells resulted in some deviation of the work and part of the work planned in the WP6 has not been done due to needed more manpower in other tasks. By complete stopping of polysulfide solubility we have managed to cycle Li-S cells with a considerable low amount of electrolyte, but due to constant degradation of metallic lithium, long term cycling can be obtained only with a larger amount of electrolyte initially added to the cells. A small part of electrolyte is used for the passivation of the sulphur cathode which stopes polysulphide dissolution and most of the electrolyte is used for passivation of the fresh surface formed on the metallic lithium during deposition process. At the moment we are capable to show several hundredths of cycles but with a large excess of electrolyte and lithium metal which significantly affect the energy density of prototype cells. Materials transfer in the last period followed pathway developed in the Eurolis project.

Patent application co-owned by NIC and SAFT is in the process of filling. We accumulated also additional foreground on the field of lithium protection, electrolyte formulation, separator fabrication, binders, recycling and modelling which will be used in the future for further optimization of this or any other battery technology.
Li-S cells are currently not considered as potential batteries which will be implemented into electric cars in the near future. However, concerns about the shortage of cobalt and nickel which are currently most used transition metals in Li-ion cells technology keep Li-S batteries as attractive next-generation cells with considerably lower prices compared to the prediction for Li-ion cells where the price of cobalt and nickel can be the major reason for Li-ion cells slow penetration to the automotive sector. Although it looks like that impact of Li-S batteries on the automotive sector has decreased with less interest, we strongly believe that future diverse technologies in the automotive sector will be able to accommodate also Li-S batteries, especially due to reduced availability of some other Li-ion battery technologies. But at the same time, Li-S battery technology is attractive for space applications due to low weight and with the implementation of Li-S cells into satellites, drones and potentially to electric aircrafts, this technology will have a significant contribution to the reduction of greenhouse gas emission and to dependency on crude oil products.

The abruptive change in the selection of the materials (cathode composite and electrolyte composition) enabled us to obtain cells with high energy density which can be cycled with high Coloumbic efficiency and have no safety issues. After the development of industrial viable lithium metal protection, this approach will be ready for further development to higher TRL’s.
Overview of the R&D process flow across partners