This study addresses the challenge of eliminating cobalt from lithium-ion battery cathodes, which are crucial for decarbonizing transportation and power grids. Traditional high-energy layered Li(NiMnCo)O₂ cathodes rely on expensive and scarce cobalt, making their supply chain and sustainability a concern. The researchers report on the synthesis of Li-deficient composite-structured LiNi₀.₉₅Mn₀.₀₅O₂, which combines layered and rocksalt phases. Through multiscale experimental characterization and computational modeling, they reveal that Li-deficiency suppresses the rocksalt-to-layered phase transformation and crystal growth, leading to small-sized composites with reduced anisotropic lattice expansion/contraction during charging and discharging. As a result, the Li-deficient LiNi₀.₉₅Mn₀.₀₅O₂ exhibits 90% first-cycle Coulombic efficiency, 90% capacity retention, and close-to-zero voltage fade after 100 deep cycles, demonstrating its potential as a cobalt-free cathode for sustainable lithium-ion batteries. The study provides a new route to stabilizing cobalt-free cathodes through rational control of lithium stoichiometry, offering a cost-effective and commercially viable alternative to cobalt-reliant cathodes.This study addresses the challenge of eliminating cobalt from lithium-ion battery cathodes, which are crucial for decarbonizing transportation and power grids. Traditional high-energy layered Li(NiMnCo)O₂ cathodes rely on expensive and scarce cobalt, making their supply chain and sustainability a concern. The researchers report on the synthesis of Li-deficient composite-structured LiNi₀.₉₅Mn₀.₀₅O₂, which combines layered and rocksalt phases. Through multiscale experimental characterization and computational modeling, they reveal that Li-deficiency suppresses the rocksalt-to-layered phase transformation and crystal growth, leading to small-sized composites with reduced anisotropic lattice expansion/contraction during charging and discharging. As a result, the Li-deficient LiNi₀.₉₅Mn₀.₀₅O₂ exhibits 90% first-cycle Coulombic efficiency, 90% capacity retention, and close-to-zero voltage fade after 100 deep cycles, demonstrating its potential as a cobalt-free cathode for sustainable lithium-ion batteries. The study provides a new route to stabilizing cobalt-free cathodes through rational control of lithium stoichiometry, offering a cost-effective and commercially viable alternative to cobalt-reliant cathodes.