Li- and Mn-rich layered oxide (LLO) with a Co-free composition is a promising candidate for next-generation cathodes in low-cost and high-energy-density lithium-ion batteries. Despite their potential, the commercialization of Co-free LLO encounters several electrochemical challenges, such as low activity and initial Coulombic efficiency of the first activation cycle, and compromised cycle retention, which are primarily attributed to the poor phase integrity between LiTMO2 and Li2MnO3 domains. In this study, we identified that the compromised phase integrity in Co-free LLOs can be driven by the sticking Ni2+ compositional design, which induces Li-Ni site-exchange defects in the LiTMO2 domain, leading to severe TMO2 slab mismatches between phases and resulting in a penalty in enthalpy mixing energy. To address this, we proposed a rational off-stoichiometric compositional design. By introducing a slight excess of Li, the Ni valence state shifts slightly from 2+ to 3+, reducing the superexchange interaction and significantly suppressing site exchange formation. The off-stoichiometric Co-free LLO shows highly integrated domains, markedly improving all electrochemical parameters, including Coulombic efficiency, cycle stability, and voltage decay. These findings deepen understanding of designing domain structures to enhance the redox chemistry of the LLO cathode class.