Bi-concave lenses have a negative focal length and are best used to diverge a converging beam. Similar to a plano-concave lens, a bi-concave lens can diverge a collimated beam to a virtual focus like in a Galilean-type beam expander.
The focal length of each lens can be calculated using the following equation:
f= -(R1*R2)/((n-1)*(R2-R1)).
where n is the index of refraction and R1,R2 is the radius of curvature for each surface of the lens.
They can be also coated with MgF2 to protect the surface or AR coated to increase the transmission.