Extend wireless Qi charging with a coil in the middle
It's hard to say without doing a very detailed analysis. The magnetic field falls off with the distance cubed, but a coil in the middle with have resistive losses associated with the coil. The coils are also most likely tuned to certain frequency and adding in extra inductance may also change the resonance of the charger and also create loss. I would think that for a few mm, adding in an extra coil will create more losses than efficiency gains .