For all ions, insertion with partial solvation shells decreases the maximum ion capacity that can be achieved (Fig. ). Water molecules either occupy or crowd out crystallographic sites that could otherwise host an inserted cation. If water molecules cannot easily exit the tunnels to make way for new ions, then the maximum achievable energy density will be limited. Therefore, polymorphs with smaller tunnels that can't accommodate water or with layers that can swell to host more inserted ions may yield better performance.