The main reason for the slow pace of battery improvement is chemistry. In conventional lithium-ion batteries the anode is made of carbon in the form of graphite, and graphite has reached its energy density limit.
A new anode material is required to improve battery performance, and that material is silicon. Silicon can store up to 10 times more lithium compared to graphite which enables batteries to have much greater energy. But silicon swells dramatically when it is charged with lithium and that can cause it to crack and stop working.