It goes without saying that a wide vocabulary is an essential requirement for academic IELTS. That is the bad news. The good news is that the words you need to know are relatively predictable. What this means is that there are certain topics (the environment, transport, education etc) that are very frequently examined, so it only makes sense to focus on these areas. Most candidates are aware of this.
What many students are unaware of is the Academic Word List. This is a list of the 570 words that are most useful for students preparing for university in whatever subject. They are an excellent resource for IELTS as they have been selected from academic texts very like those in the reading paper and also needed for the writing paper.
The question is how to learn them. Certainly not all at once. Rather this is a little but often exercise, one of those you try and do every day if you can. Two points to bear in mind are:
1. Most teachers would agree that between 6 -8 new words at once was enough.
2. To learn a word you need to use several times: once is not enough.