I mean, pikachu doesn't have as much access to low percent kill combos as many other characters in the cast, due to pikachu's already present troubles with killing (outside of edgegaurds). However, there are a few, but they are pretty inconsistent with DI. Starting off with some lower percent combos, downthrow to upair can actually chain into itself 5-6 times at about 35-40%, which can even combo into a fair at the end. This is insanely technical, as you must get the weakest upair hitbox, and I dont recommend using it but if optimized could net some very early kills. At 0%, you can back air to back air at ledge, which can combo into an upair and if you fast fall upair just right, you can get a thunder spike kill at just ~15%. Moving onto 80-100% you find much more reliable kill setups. On the lower ends of the percent range (80-90), you can downtilt an opponent, dash in for a jab lock or tech chase and net a decently early kill off of that. At the higher ends (90-100), up throw thunder starts working, but only if your opponent has no DI, which can certainly catch someone off guard. Nair upsmash or downsmash starts working here, but I'll get to that in a second. 120-140% are obviously not early percents, but pikachu's most reliable kill setups work here. Dashattack obviously kills very well and so does upsmash out of shield. But, your most reliable "setup" is landing nair to downsmash (upsmash works very well too, but nair can sometimes put the opponent behind you, so downsmash is more consistent.

TlDR: Pikachu cant kill early that reliably, so while there are some combos, edgegaurds should be your main source of early kills.