"i" in english is always "I". Is there a preference to autocorrect that auto corrects small "i" to capital "I" in macOS?
More specifically if you just want to capitalize the i as a word, in System Preferences > Keyboard > Text... add i to the replace column and I (capitalized) to the With column. It will capitalize i written by itself, but leaves it if it's within a word.
(This only works in Sierra. El Capitan and I think earlier versions require 2 characters to create a text shortcut.)