regarding to your answer I am looking for both I guess. I am specifically writing about the Libero in volleyball and I need to find out which the most important psychological skills are he needs to fulfill his position on the highes level. the problem is that I couldnt find something comparable in the field of volleyball.
This looks like a good place to start: Shoenfelt, E., & Griffith, A. (2008). Evaluation of a Mental Skills Program for Serving for an Intercollegiate Volleyball Team. Perceptual and Motor Skills, 107(1), 293-306.
Also, from the perspective of sport and performance psychology, elite levels of sport may require similar mental skills despite requiring different physical skills. The skills sport psychologists work with largely depends on their "Theory of Performance Excellence": Aoyagi, Poczwardowski, Balague, Bauman, Gordin, Haberl, . . . Yukelson, David P. (2012). Expert approaches to sport psychology : Applied theories of performance excellence / Mark W. Aoyagi & Artur Poczwardowski, editors. Morgantown, WV: Fitness Information Technology.
Less important is the specific sport for psychological skills, but the type of sport (endurance vs sprint, team vs solo, scored vs rated, etc) is relevant enough that widening your scope for papers could include sports LIKE volleyball as well: Beauchamp, M., & Eys, M. (2014). Group Dynamics in Exercise and Sport Psychology. (2nd ed.).