L-shaped battery in the iPhone X actually materialized, though not to the extent initially thought. The usage of stacked system board in the X allowed Apple to fit all chippery in half the space the motherboard is taking in, say, the iPhone 8 Plus, leaving plenty of room for another battery cell underneath it. As a result, the iPhone X, which in size is not much larger than the iPhone 8, has a much higher, 2716 mAh battery capacity.
This year, however, Apple is allegedly planning to bend fully in L-shape, and equip the upcoming heirs of the iPhone X with one-cell L-shaped batteries, which means there will be no need for a separator, making the battery lighter, and with a higher capacity than what we have now. Apple has reportedly partnered with LG Chem, which has experience producing such shape-shifting battery packs, and has built a factory in Nanjing, China, dedicated entirely to iPhone battery production.
Apple is said to be currently inspecting the facilities to give it the green light for producing those higher-capacity batteries for the iPhone Xs (XI) and Xs Plus (XI Plus), or whatever Apple decides to name them.
The rumored affordable iPhone 9, however, with its 6.1" LCD display, will arguably be shipped with a regular rectangular battery pack, as famed analyst Ming-Chi Kuo tipped the other day that it won't have the stacked system board of its upcoming OLED brethren. The same analyst said way back in December that if Apple goes with the one-cell L battery, the iPhone Xs will sport a 3000 mAh pack, which should bode well for its endurance on a charge.
source: ETNews (translated)