Lerp isn't actually for easing, it's for getting a position between two points. But I get what you're saying and my guess is you're using keep in an event like this.
---Sprite: set x to lerp(self.x, 100, 0.5)
Which gives the effect of moving fast to slow. What it's doing is just moving half the distance per frame.
You'll need to add a variable for a reverse effect.
To start it set the variable to say o.
Set speed to 0
Then every tick set a new x and increase the speed
--- set x to min(self.x + speed, 100)
--- add 1 to speed
The min() is to keep from overshooting. Also note this only works for a positive direction. For a negative direction do this:
--- set x to max(self.x - speed, -100)
--- subtract 1 from speed
Also you can fix it up to use dt so it behaves the same regardless of the framerate.