Batterymate (1889)

“He [Morgan Murphy] will go with the club on its southern trip today. Henry Burns, his battery mate, still holds out.” The Boston Herald, April 26, 1889 p5

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1909. “[Orval] Overall drove his battery mate home with his long smash over the right field fence” (Ring W. Lardner, Chicago Tribune, July 9; Peter Morris).