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Some thoughts:

The voltage is just above that typical of a production EV, which doesn't narrow the choice at all but opens up lots of options.

The current seems too high for a Leaf pack, but no problem for Tesla (Model S or X, as Model 3 wouldn't be readily available yet).

Yes, anything using 18650 or other small cylindrical cells will have an insane number of cells to manually assemble, but this doesn't matter if you use complete production EV modules... and it would be substantial convenience to use ready-built modules, of any cell format.

The Ampahaulic Sleeper cells are typically used by drag racers - the 90 seconds needed may be too long for these to be optimal, although you could use their "continuous" rating, so run 2 or 3 or even more in parallel... it's still a featherweight pack compared to production EV cells near their production ratings.
 

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So, I missed that 1000 kg is the mass target for the entire vehicle. Yeah, at 400V the stock modules of any production EV will be too large

You might look at modules from plug-in hybrids, since they are smaller and yet some run high voltages; the lower total energy storage doesn't matter, so the issue will be whether they can stand the very high discharge for the short period. Otherwise, to use production EV cells you need to rewire them from the stock module configuration to put fewer in parallel.
 

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Does anyone have details on the Hitachi pouch cells used in the 2016+ Chevy Malibu ECO?
...
Hitachi Automotive Systems cells spec:

Item Specification
Size (mm) 120×80×12
Weight (kg) 0.24
Average Voltage (V) 3.7
Capacity (Ah) 5.2
Output Density (W/kg) 5,000
Energy Density (Wh/kg) 80

1.5kwh, 115 volt, 32 cells in total pack
The count of 32 cells is from the 2013 Malibu with a BAS mild hybrid system and the cylindrical cells, not the 2016 Malibu with the Voltec system and these prismatic cells. The 2016 Malibu with 1.5 kWh capacity of 19 Wh cells must have about 78 cells; from a photo it appears to have eight groups of 10 cells (so 80 total). They appear to be all in series, which would imply 296 volts (nominal), which makes sense since it needs to work with the same Voltec hybrid system as the Chevrolet Volt.

Non-plug-in hybrids need high power density, not high energy density, so this type of vehicle does look like a reasonable potential cell source.

110 in series by 4 in parallel of these cells (for nominal 407 V and nominally 8.4 kWh) would be 106 kg and 51 litres of volume... plus wiring and packaging. 800 amps total or 200 amps per cell would be 38C. That doesn't sound bad, but it's five and half Malibu packs to find, tear down, and reassemble.
 

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Our cells ...
I would suggest 4P108S about 128lbs about 1.5 cubic feet.
That's 58 kg and 43 litres.

110 in series by 4 in parallel of these cells (for nominal 407 V and nominally 8.4 kWh) would be 106 kg and 51 litres of volume... plus wiring and packaging. 800 amps total or 200 amps per cell would be 38C. That doesn't sound bad, but it's five and half Malibu packs to find, tear down, and reassemble... and I have only considered nominal voltage, not what it sags to in use as John explained.
These two options are essentially the same configuration, mass, and volume.
 
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