Optimization of nanoporous silicon anodes with high ionic conductivity argyrodite solid electrolytes in solid state batteries

Abstract

We are currently undergoing an energy transition, in which batteries are playing a vital role. Their cost is quickly decreasing and they provide a way of storage for electricity generated by renewable energy sources such as wind and solar power generation. With the increase in demand for battery capacity comes the demand for safer, more energy dense, higher power output and faster charging batteries. To meet these demands researchers are now focusing on solid state batteries. And in these batteries on the anode side on a specif material called silicon (Si). With its abundance in the earth’s crust and a gravimetric energy density of more then 10 times that of graphite, which is currently most used as the anode in lithium ion batteries, this material seems very promising. But the silicon material expands up to 300-400% when fully charged/lithiated. This of course brings challenges for using silicon in solid state batteries, where the expansion could destroy the battery from within. Multiple solutions have been tried to deal with this volume expansion, and one of the solutions is to add nanopores into the silicon material so the material expands into the pores and not outwards. Such a material is provided by a company called E-magy, they have already shown this material to be effective in liquid lithium-ion batteries. But this thesis will focus on how this material performs in solid state batteries by comparing the galvanostatic battery cycling and electrochemical impedance spectroscopy data, to solid state-batteries that use silicon particles of the same size without the nanopores (called Com Si). For the solid electrolyte (SE) in these batteries a Cl rich version of lithium argyrodite is chosen with the chemical formula Li5, 5PS4, 5Cl1, 5. This material combines properties of halides and sulfides resulting in a deformable high ionic conductivity solid electrolyte that can form a stable interface in the battery operating potentials and accommodate the volume expansion of the silicon. Regular lithium argyrodite ( Li6PS5Cl ) and the Cl rich variant where synthesized with respective ionic conductivities of 2,9 and 7,5 mS/cm, to compare there performance in the Si solid state batteries. Si/SE/CNF (Carbon nano fiber to increase the electronic conductivity) composite powder mixture electrodes where made with varying compositions, where the 6/3/1 composition showed the best performance with a initial discharge capacity of 2410 mAh/g with a current density of 0,229 mA/cm2 for Com Si . After the formation cycles this battery showed a capacity retention of 73% going from 1680 mAh/g to 1230 mAh/g at a current density of 0,458 mA/cm over the span of 45 cycles. The CNF was shown to not have a positive and possibly a negative impact on the battery performance and with the results from the Si/SE/CNF batteries there was not a clear difference between the performance of porous and non porous silicon. The switch was made to using Si deposited on Cu electrodes with a PVDF binder instead of the Si/SE/CNF composite electrodes. The best battery performance was achieved with a non porous 1,2 mg Si electrode showcasing a initial discharge capacity of 2700 mAh/g, and a capacity retention of 81,3% after the formation cycles going from 2250 to 1830 mAh/g with the same current densities as before. This test was done with both SE’s showcasing a clear improvement in capacity retention with the Li5, 5PS4, 5Cl1, 5 but still no clear difference between the porous and non porous material. Then, to better preserve the nano porous Si structure, the Si deposited on Cu electrodes where only charged to a capacity of 1000, 1500 and 2000 mAh/g by using time constraints. This showed clear improvements for the nanoporous Si material over the regular Si for capacities of 1000 and 1500 mAh/g over 50 cycles. The 1000 mAh/g E-magy Si battery showed a coulombic efficiency of 99% and a stable minimum negative potential of -0,5 V vs In/LiIn for 50 cycles. Using Li5, 5PS4, 5Cl1, 5 of Li6PS5Cl lead to more stable battery performance and less of an increase in the required potentials to reach the same capacities each cycle. This study provides a insight into how the battery performance of Si solid state batteries is influenced by: Nanopores in the Si material, different Si electrode preparation methods, Si composite electrode composition ratio’s, choice of the olid electrolyte, and the utilized capacity of the Si electrode. And thereby makes a contribution to the large amount of research that is currently being done to allow for Si anodes to successfully be utilized in solid state batteries.