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As bad as that one looks...I would buy a new one tbh
Fill it. Add two caps full of bleach. Get the bleachy water down in the drinking tube. Let it sit for a few minutes. Drain and dry with a paper towel using a wooden spoon to get all the way in.The stuff you get from REI in the packets is basically powdered bleach or bromide.
All the above work great and I have used them before. Another one that works great is the denture cleaning tablet (Efferdent or Polydent) work great and work awesome in your thermos.