Apple may be shifting to a copper Qi coil in the 2018 iPhones, which could improve charging efficiency.
According supply chain reports, Apple will be switching from thinner FPC coils with higher resistance to thicker copper wire coils with lower resistance for the wireless charging receiver built into iPhones.
The latest iPhones support wireless charging at up to 7.5W, which actually isn’t much faster than wired charging with a 5W power adapter, due to the efficiency limitations of the FPC coil.
FPC is a mixture of iron, at least one other metal, and plastic, whereas copper is a more pure material. Given the same charging wattage, FPC has the advantage of being thinner than a copper coil. Thermodynamics are complex, but given all other factors equal, increasing the wattage of the coil also increases the heat generated in the circuit roughly exponentially with that linear increase in wattage, necessitating a better thermal dissipation solution. The thicker, and homogenous, copper coil could provide that solution.
The switch to copper would yield improvements, assuming that Apple found a way to fit the thicker coil in its next iPhones.