What matters most is not the anode/cathode material alone. It is the chemical reactivity /degradation / dendrite formation at the interface with the electrolyte.
So yes, Si is cheaper, but if degrades when in contact with the solid electrolyte under non-equilibrium conditions/high currents, than you only end up with a prototype that can do only a few cycles/low current, i.e. doesn't meet automotive requirements. I believe Goliath is at this stage. The most important and difficult to achieve is a single cell that hits automotive requirements. QS is there, Ilinka's EV baterry is not there yet.
That being said I still invested in Ilinka because: - the price is relatively decent. if Ilinka's EV battery prototype reaches the stage of a working cell that meets automotive requirements, the price would jump more than x10 the actual price. - the price recently increased without news; I deduce this is not a pump and dump, otherwise they would have released some PR before the pump. - the stock must have been recently bought by a hedge fund, idk which one though. I see however that another fund decreased accordingly its stake in Ilinka, see: https://www.londonstockexchange.com/stock/IKA/ilika-plc/analysis?lang=en