Hi @gestoordeend
According to the [https://pcsupport.lenovo.com/hk/en/products/laptops-and-netbooks/thinkbook-series/thinkbook-13s-iwl/20r9/parts/display/compatible?linkTrack=Caps%3ABody_SearchProduct&searchType=4&keyWordSearch=ThinkBook%2013s-IWL%20Laptop%20-%20Type%2020R9|parts list] for the laptop, there a 6 battery models that are compatible with the laptop, (click on ''Select Commodity > Rechargeable Batteries, internal (6)''.
Here's the [https://download.lenovo.com/consumer/mobiles_pub/thinkbook_13s-14s_iwl_hmm_201905.pdf|hardware maintenance manual] for the laptop.
Go to p.31 to view the procedure to remove the battery. The model part number is printed on the battery itself, if you wish to verify that you're getting the same as is installed now.
Search online using the ''part number only'' to find suppliers that suit you best.