How much energy is required to charge a battery to 50% vs 100%?
The batteries you're talking about are lithium ion. They have specific charging procedure where you put constant current until 80% and then you do constant voltage from there to 100%. That's why it takes longer above 80%. Constant voltage means there's less and less potential difference between the resting battery voltage and the charging voltage so it'll get slower and slower. Up to 80% it's basically linear. After that it'd be effectively asymptotic since the charge voltage is effectively the same as the final resting battery voltage.

You could charge your batteries differently than this industry standard, but if you did so, you may blow up the batteries.