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The Mi 6 by Xiaomi can make use of USB Power Delivery or Qualcomm Quick Charge for fast charging over its USB Type-C connector, which should be based on Qualcomm Quick Charge 4+ technology. The phone is shipped with a Qualcomm Quick Charge 3.0 charger accessory supporting 15 watts (5 volts @ 3 amps), 18 watts  (9 volts @ 2 amps), and 18 watts (12 volts @ 1.5 amps) based charging.

Just in case you want to use a different charger with the Mi 6, we try connecting it to the Apple 87W USB-C Power Adapter that originally came with  the Apple 15" MacBook Pro (Late 2016). Here we see how well the Apple charger works over USB Power Delivery with the non Apple based Mi 6  phone when directly connected with a USB Type-C to Type-C cable.

Looking at the Granite River Labs GRL-USB-PD-A1 test software, the Apple 87W USB-C Power Adapter initiates USB Power Delivery communications with the Mi 6 by constantly showing that it supports 12 watts (5 volts @ 2.4 amps) power profile.

We can see after a while that the Mi 6 finally starts to request 12 watts (5 volts @ 2.4 amps) while on a fully depleted battery to then get accepted by the charger.

After several Vendor Defined messages to adjust voltage and current, the Apple 87W USB-C Power Adapter follows up to add on 27 watts (9 volts @ 3 amps) and 86 watts (20 volts @ 4.3 amps) power profiles to the initial 12 watts (5 volts @ 2.4 amps).

We however observe the Mi 6 still settles on 12 watts (5 volts @ 2.4 amps) as initially established with the Apple charger, despite the available higher power.

Only after exchanging Discover Identity and SVIDs can the Mi 6 be seen to request an eventual 27 watts (9 volts @ 3 amps) to charge faster which the charger also complies.  

When plugged in, the Mi 6 starts pulling only about 0.5 amps at 5 volts as the fully drained battery gets its initial charge while the phone is switched off. As the first 12 watts power is requested, the current pulled drops to zero but quickly goes back up to draw around 12 watts (5 volts @ 2.4 amps). When the phone is turned on, it pulls lower current at about 1.6 amps @ 5 volts. The current then briefly spikes downwards as the voltage rises up to 9 volts during the 2nd power request of 27 watts, but quickly bounces up again to about 1.6 amps. The phone then goes on to draw around 15.3 watts (9 volts @ 1.7 amps) towards 60 seconds of charging.    

It's good to see that the Mi 6 supports USB Power Delivery with an Apple based charger like the Apple 87W USB-C Power Adapter.