A bank might not actually charge savers a negative rate, but it might levy a "storage fee" that exceeds any positive interest earned, according to Professor of Economics Dennis Hoffman.

In this story published May 31, 2020, in The Arizona Republic:

This scenario would represent a penalty for holding cash. By pushing rates ever lower, economic policymakers would be encouraging you to rid yourself of cash by spending money and hence stimulating economic activity.

– Professor of Economics Dennis Hoffman, who's the director of the L. Seidman Research Institute