Yes, you absolutely need to use copper wire or braid for the series connections, not just for current, but also for flexibility.
Also, don’t have the series connections start right at the end of one p-group and end at the next cell over like you are showing in that picture. That will result in uneven current through the nickel because some cells in the p-group are way farther from the series connection than others, and this will result in hot spots.
Check out the gallery in ZachTetra’s vendor thread here for some examples of how to do series connections right.
Here is another excellent example from will_manners as well.
