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HELIS Report Summary

Project ID: 666221
Funded under: H2020-EU.

Periodic Reporting for period 1 - HELIS (High energy lithium sulphur cells and batteries)

Reporting period: 2015-06-01 to 2016-09-30

Summary of the context and overall objectives of the project

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 remaining issues that need to be solved and this will provide final direction of lithium sulphur batteries technological development. Some of technological aspects, like 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 ( Open 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.

The HELIS project will be addressing remaining issues connected with a stability of lithium anode during cycling, with engineering of complete cell and with questions about lithium sulphur batteries cells implementation into commercial products (ageing, 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. Stabilised interface layer is important from view of engineering of cathode composite and separator porosity since this is important parameter for electrolyte accommodation and volume expansion adjustment. Finally the mechanism of lithium sulphur batteries ageing can determine the practical applicability of LSB in different applications, but 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 the 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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Project is a continuation of the EUROLIS project (FP7 project No. 314515). During the first reporting period activities were connected with preparation of active materials for the first prototypes which will be used for ageing study. Within this generation of prototype cells, we are using knowhow of the materials that gave the best configuration within EUROLIS. At the same time we have started activities with formulation of new carbon host matrices which will enable decrease of electrolyte/sulphur ratio and enable high areal capacities (utilization of more than 1000 mAh/g capacity with loadings higher than 4 mg of sulphur per cm2. This work is supported by modelling work package. First results from modelling were implemented into the positive electrode formulation and experimental results are in the agreement with modelling results. Electrolyte used in the first set of prototypes is based on formulation within EUROLIS project. A lot of activities in the first reporting period were performed in order to determine the optimal quantity of electrolyte which will than determine the thickness and porosity of the separator. New solvents and combination of different solvents are under testing, including some new types of glymes which are not cancerogenic. Activities are focused on solvents with low solubility of PS in order to have soluble PS as redox mediators and at the same time to prevent electrolyte saturation with PS and related transport problems. These activities are supported by modelling part, where interactions between PS and different solvents are studied. In the first generation of prototype cells protection of lithium surface will be performed only through addition of LiNO3 into electrolyte, however activities for the next generation of prototypes have started. Preliminary studies using ALD (atomic layer deposition) coating and polymer coatings show promising direction which in the combination of presence of low concentration of PS can prolong cycle life of lithium. Another promised strategy is protection of lithium with graphene based materials either chemically modified or within the composite in the polymer matrix. Separator development is based on the achievements within EUROLIS project and additional activities are devoted to the tests of commercial separators coated with inorganic layers by ALD technique. Aim of the first generation of prototypes is also to check the problem related with safety and potential evolution of H2S gasses. We have performed controlled thermal runaway of one Li-S cell and we have checked gaseous products formed during the thermal runaway. Both experiments are showing that further research is required in order to stabilize Li-S battery. For that reason also some additional safety rules were required at the production line for prototypes and as well in the partner’s laboratories where electrochemical tests will be done. Work performed in the work package dealing with recycling is on time and some additional activities are planned with cells that were prepared and characterised within EUROLIS project.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Project in the first reporting period has been focused on the preparation of prototype cells which are going to be used for study of ageing mechanisms (chemical and electrochemical). The material selection was based on the last prepared configuration in the EUROLIS project however shift from cylindrical cells with configuration 18650 to D-sized cells has been made within HELIS project. The shift to another type of the cells is also connected with a change of cell producing plan since the latter case prototype production line at SAFT is already in the use. Along with shifting to the new cells with higher energy we also have to adopt safety since cell in development stage can have unpredicted behaviour, the formation of H2S is our biggest concern. In terms of materials production we have moved to higher quantities and with a help of the industrial partners from advisory board we have implemented their materials into the first series of prototype cells. Important step has been made with a help of modelling part of the project since the results from calculations have been directly implemented into the electrode formulation which helped us to obtain cathode electrodes with higher areal loading and consequently with higher capacity. Electrolyte – sulphur ratio is still not optimal and remains an open issue for the next reporting period. First protection of lithium surface has been done within the first set of prototype cells while further research is required to have more efficient protection of lithium negative electrode.

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