Even though hybrid and plug-in hybrid vehicles do possess certain challenges, energy storage systems are something, electric vehicles are facing trouble to be adapted. In conventional batteries, the behavior of pulsed power and energy content are considered to be the best performance.
Two major issues identified in-vehicle applications are life cycle and abuse tolerance. In the present scenario, in most of the HEVs, nickel-metal hydride batteries are used and for PHEVs, lithium-ion batteries.

In some applications, ultra-capacitors are considered to be the major concern with the demand for power. The investigation of the flywheels is also important. The current energy storage is with the limitations of unsatisfied power density and energy density. The life cycles are limited with the high costs and potential safety issues.
In my opinion, the future is not just about the metal design bodies but the main fuel or the energy storage system. A vehicle with an efficient and healthy environmental energy storage system with a long-lasting capability to be operable till the long run is something that needs to be invented or discovered.
Sources:- Hybrid electric vehicles by Mi, Masrur, and Wenzhong Gao.