Disordered structures contribute to the relaxor ferroelectric properties of the lead-free system Na1/2Bi1/2TiO3-(x)% BaTiO3. Such structures are difficult to characterize with conventional structural analysis techniques. Here, diffuse X-ray scattering was used to analyze the disordered structures from four single-crystals of Na1/2Bi1/2TiO3-(x)%BaTiO3, with compositions of (x)% = 0%, 4%, 9% and 17%. Such a range of compositions allowed a distinction between the structures that occur across the morphotropic phase boundary, where enhanced electromechanical properties have been reported. Throughout the compositional range, the resulting diffuse scattering showed the modification of correlated atomic displacements, transformation of planar defects in the oxygen octahedral tilt-systems, and the presence of modulations of approximately 10 unit cells. The results reported here demonstrate an evolution of the short-range structures across the morphotropic phase boundary that may underpin the electro-mechanical properties of Na1/2Bi1/2TiO3-(x)%BaTiO3. This information will ultimately assist in tailoring disordered structures for next-generation piezoelectric materials.