The simple answer is that there’s no such word as ‘i’ in Modern English. This wasn’t always the case, though: in Old English we had ic and ih, in Middle English ik, ich and then i.
At first, the personal pronoun ‘I’ was ‘lower cased’, but the slim little letter on its own was difficult to read, especially in handwritten manuscripts. In The Canterbury Tales, written by Geoffrey Chaucer in the 1300s, the personal pronoun ‘I’ was written slightly taller, presumably to make it easier to see.
If you’re under time pressure at work, or tapping out emails on the go, you might be tempted not to capitalise ‘I’. If you do this, you run the risk of seeming uneducated, juvenile or lazy – not a good look when you’re climbing the corporate ladder.
Perhaps the Middle English ‘i’ will make a comeback and we’ll dispense with the upper case. Until then, we recommend you use the grammatically correct ‘I’.