I did some back of the envelope calculations on the energy density of my setup compared to lead-acid AGM batteries often used in UPS applications.
a 4S10P battery made from junk cells each having around 800 mAh gives me 14.8 V x 8.315 Ah nominal capacity of 123.062 Wh (calculated by https://www.repackr.com/ battery pack builder). As the battery size is 27 cm x 10 cm x 4 cm this gives a volume of 1080 cm^3 and a volumetric energy density of 0.11 Wh/cm^3.
A random "flat" AGM battery (CSB HRL 634W F2) used in a 1U rack UPS I found laying around in the hackerspace has a nominal capacity of 6 V x 9Ah = 54 Wh with the size of 15.1 cm x 3.4 cm x 9.4 cm without leads. This means that the volume is 482.596 cm^3 and the volumetric energy density of also 0.11 Wh/cm^3.
Not bad considering my lithium cells are essentially on the level of junk not reusable for anything else.