Is there anything functionally equivalent to a turbo for electric motors?
Obviously, I am not asking for something that reuses exhaust gases to force in more air, because there are no exhaust gases nor the need for air.
What I am asking is if there is something that can use "waste energy" to give an IMMEDIATE POWER BOOST to an electric motor, which is what the turbo of combustion engines do, in functional terms.
The "take some of the output force to reuse it as input" can be interpreted as regenerative braking, but the big differences are:
If you see a turbo as a supplementary power to the max power output of the engine I would see supercapacitors as something close too. Supercapacitors can deliver a high current to the engine (thus high power) that the batteries can not deliver for a short time, thus making the car go faster for a short period (more akin to what a nitrous oxide injection would do) at expense of overheating, decreased efficiency, and otherwise reducing the lifetime of the electric engine.
regenerative braking
This question and answer regarding the subject matter has some very good information in it as well the answer reveals a mathematical paradox with regenerative braking
This Q&A is a bit off your topic but has breadcrumbs regarding recapturing lost energy through a turbo to charge a batter and kinetic energy recovery through braking in Formula 1
No, there isn't any equivalent. A turbo is used because combustion engines are inherently inefficient: they convert chemical energy into mechanical energy, using an awkward detour via heat. Unfortunately, heat is pretty much the worst possible way to store energy: by the laws of thermodynamics, you can only convert it to other forms of energy if you also increase entropy. If you do the physics, you find out that the maximum efficiency is the Carnot efficiency
η = 1 − TC/TH
where TC and TH are the cold and hot temperature points of the engine cycle, i.e. surrounding air vs. combustion temperature. Note that the fraction approaches zero as TH rises*, i.e. the loss can be made quite small by letting the combustion happen at high temperature. But you can't make the temperature infinitely high, and therefore some energy is inevitably lost.
You can consider the turbo as a device that retrieves back some part of the lost energy† or, more to the point, you can just see it as a means of increasing the operation pressure and thus temperature, and thereby reducing the loss somewhat. At any rate, a turbo is just a means to adress the problem that a combustion engine is not efficient. (In practice, you won't find any engine with efficiency better than 30%.)
There's no need to do that for an electric motor – because these are efficient! They convert electric energy to mechanical via magnetic fields, and that process is far better controlled. You can approach 100% efficiency without needing to have any temperature or so approach infinity.
Sure, there are some small losses in the copper windings' electrical resistivity, in eddy currents and bearing friction, but these can be made very small by precision design.
If anything, it might make sense to search for a turbo-analogue for batteries, because these are actually the weak part of an electric car, efficiency-wise. Perhaps it would make sense to tack on some kind of waste-heat retrieval to these.
*In case you remark that the loss also vanishes if TC becomes zero: correct, but there's not much you can do about TC. Cooling the air below ambient temperature would require a giant fridge, which would of course overall just waste even more energy. Cooling after a supercharger does make sense though, because the temperature is already higher than ambient here, i.e. this can be done passively.
†In the end, “retrieving waste energy” is moot: you can always consider the motor and turbo together as one thermodynamic engine, and its total efficiency can not be better than Carnot.
You could capture the heat from the electric motor, and convert that into more energy using a thermoelectric device. University of Florida research
There are some good answers here already that pretty much entirely cover the topic. One thing that wasn't mentioned however is KERS - kinetic energy recovery systems. Effectively you have a large mass (flywheel) that spins up as the vehicle is in motion. Generally under brakes or vac (no throttle) the driveline feeds energy into this flywheel. When needed, the flywheel then engages via a clutch and can feed that energy back into the driveline.
While not strictly an EV technology (and, in fact, I'm not sure if any EV's use KERS), it's another possible avenue.
Most Electric traction systems Buck down the voltage that goes to the electric motor from the battery .For example a 48V electric cart motor controller will give the motor Up to 48V but no more .Summarising and boiling down electric motor theory for this stack voltage is speed and current is torque .The jargon term TURBO is used when the controller is modified to sometimes act as a Voltage Amplifier giving more volts than the battery makes .This gives more speed but not more torque .This is how electric carts can be and are modded to go faster while keeping the same motor and the same battery pack .You have to know what you are doing otherwise you will blow up the motor just like adding turbo to a petrol engine .The degree of boost is the amount of voltage amplification that the controller is set up for .25% is a ball park reasonable figure.
