I've heard that warp drives need negative energy in order to contract space-time in the front of a spacecraft. But, space-time expands when you have charge, doesn't it? In the Ressner-Nordstrom solution, the metric is given by

[latex]ds^2=-\left(1-\frac{2GM}{rc^2}+\frac{Q^2G}{4\pi \epsilon _0 r^2 c^4}\right)dt^2+\left(1-\frac{2GM}{rc^2}+\frac{Q^2G}{4\pi \epsilon _0 r^2 c^4}\right)^{-1} dr^2 +r^2(d\theta ^2 + r^2sin^2\theta d\phi ^2)[/latex]

Which means space-time is contracted by charge. Why is it that warp drives are not able to use charge instead of negative energy?

IIRC the negative energy of exotic matter is needed to keep wormholes open and stable.