The pair has been writing songs in Palm Springs, California and Charli has insisted their work is the "best music ever."
"@tovelo and i have been out in Palm Springs writing songs together for a week (tested and safe) and we been making the best music ever!!!!!" she wrote on Instagram. "Excited for u guys to hear things, but in the meantime, checkout my updated motherf**king future playlist at the link in my bio and make sure u play it start to finish SUPER LOUD!!!! (sic)"
Tove Lo shared the message on her page and wrote, "Yes. Love writing with you."
The collaboration comes after Charli recently admitted she feels like an "outsider" in the music industry.
"I feel like outside of my fans I don't really provide people with the opportunity to feel much ownership over me or my decisions, which is maybe why i have a sort of outsider status within pop music and also experience rejection from the 'industry' side of music sometimes (sic)," she tweeted.
And she added, "I always give my collaborators credit. my producers/mixers/artists I collaborate with etc. i have never once pretended I am solely responsible for the songs I release - but I also recognise that none of my art would be the same or even possible without my own vision & talent (sic)."