  Recent advances on wireless energy transfer (WET) make it a promising
solution for powering future Internet of Things (IoT) devices enabled by the
upcoming sixth generation (6G) era. The main architectures, challenges and
techniques for efficient and scalable wireless powering are overviewed in this
paper. Candidates enablers such as energy beamforming (EB), distributed antenna
systems (DAS), advances on devices' hardware and programmable medium, new
spectrum opportunities, resource scheduling and distributed ledger technology
are outlined. Special emphasis is placed on discussing the suitability of
channel state information (CSI)-limited/free strategies when powering
simultaneously a massive number of devices. The benefits from combining DAS and
EB, and from using average CSI whenever available, are numerically illustrated.
The pros and cons of the state-of-the-art CSI-free WET techniques in ultra-low
power setups are thoroughly revised, and some possible future enhancements are
outlined. Finally, key research directions towards realizing WET-enabled
massive IoT networks in the 6G era are identified and discussed in detail.
