If you don't need much range (a couple feet, say), you can use ambient backscatter which doesn't need to store up energy for bursts.

Here's a video about it from 2013 [1] showing some test devices. Here's an article about it [2].

A bit later they extended this to make it work with off-the-shelf WiFi devices, so you could have a battery-free device that can communicate to ordinary WiFi devices [3]. Still short range, but now you could make it so the reader is a commodity WiFi device like a smartphone, instead of a specialty device.

Here are some other researchers who demonstrated a backscatter tag that diddled with Bluetooth Low Energy (BLE) instead of WiFi, and was correctly received by an iPad at over 9 m using the existing iOS Bluetooth stack with no modifications [4].

And here is a battery-fee phone that communicates with a powered base station [5]. It works up to 30 ft away. The phone uses energy from ambient RF and small photodiodes.

[1] https://www.youtube.com/watch?v=gX9cbxLSOkE

[2] https://www.washington.edu/news/2013/08/13/wireless-devices-...

[3] https://iotwifi.cs.washington.edu/files/wifiBackscatter.pdf

[4] https://www.washington.edu/news/2017/07/05/first-battery-fre...