You are looking at this wrongly. Energy density is not an issue for lithium ion battery, power density is! The hybrid system needs to be charged in a very short period of time.
http://ev.sae.org/article/10599 gives the following numbers for the A123 battery pack in the electric Drayson Lola.
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The car’s 300-kg (661-lb) battery pack, which was manufactured by Mavizen of Gerrards Cross, U.K., consists of two inline-mounted modules containing lithium-ion nanophosphate cells from A123 Systems of Waltham, MA. The battery units, which are embedded in the structure itself to improve weight efficiency, reportedly deliver 60 kW·h of energy at 700 V (maximum).
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That means an energy density of 60 kWh * 3.6 / 300 kg =
0.7 MJ/kg for that battery technology.
Again using the Drayson battery pack as example.
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The company's 4X2-640 electric drivetrain, which comprises four asynchronous pancake motors affixed to the rear axle, produces 850 peak hp (633 kW). The axial-flux motors were supplied by YASA Motors, a start-up firm that was spun out of nearby Oxford University.
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That means a power density of 633 kW / 300 kg =
2.11 kW/kg for that battery technology.
Power density is the main reason why Toyota went for supercapacitors. The electric motor in the TS030 is 300 hp = 221 kW. A lithium ion battery would have to weigh more than 100 kg.