To stop the app from compressing your music into lower bitrates, follow these steps:

Spotify uses the format for mobile and desktop, and AAC for its web player. Here is how the bitrates break down: Low: 24 kbps (Great for saving data on a weak connection). Normal: 96 kbps (The standard default). High: 160 kbps (Good balance for most listeners).

This blog post provides a guide on how to maximize your Spotify audio quality.