A promising high-entropy doping strategy has been developed to create an ultrahigh-Ni, cobalt-free, single-crystalline cathode for high-energy lithium-ion batteries. The material, LiNi₀.₈₈Mn₀.₀₃Mg₀.₀₂Fe₀.₀₂Ti₀.₀₂Mo₀.₀₂Nb₀.₀₁O₂ (HE-SC-N88), combines single-crystalline (SC) design with in situ high-entropy (HE) doping to achieve a grain-boundary-free, stable structure with minimal lattice strain. This structure prevents mechanical degradation, reduces surface parasitic reactions, and mitigates oxygen loss. HE-SC-N88 exhibits exceptional electrochemical performance, including prolonged cycling stability under harsh conditions and delayed oxygen loss-induced phase transitions upon heating. The design of HE doping in redefining ultrahigh-Ni, cobalt-free SC cathodes represents a significant advancement toward industrial application of next-generation lithium-ion batteries. The HE-SC-N88 cathode was synthesized through in situ compositionally complex doping during hydroxide coprecipitation, ensuring close atomic contact and enhanced homogeneity. The material features a rhombohedral α-NaFeO₂-type structure with a high crystallinity and a stable lattice that resists chemomechanical breakdown even during prolonged cycling across a broad voltage range (4.3 to 4.7 V versus Li/Li⁺). This enables exceptional electrochemical behaviors, including delayed phase transitions, enhanced wide-temperature capacity, prolonged cycling stability, and robust high-rate capabilities in both coin cells and 4.3-V full pouch batteries. The pillaring effect from multiple dopants endows HE-SC-N88 with remarkable thermal durability, significantly reducing oxygen loss and related phase transitions during heating. Electrochemical evaluations show that HE-SC-N88 exhibits superior performance compared to conventional cathodes, maintaining high capacity retention and cycling stability even at high voltages and temperatures. The HE-SC-N88 cathode demonstrates exceptional cycling stability, retaining over 90% capacity after 3500 cycles at 1 C and 25°C, and over 91% after 1000 cycles at 1 C and 4.5 V. It also shows excellent high-temperature endurance, retaining 78.2% capacity after 2000 deep cycles at 55°C. The HE-SC-N88 cathode exhibits a significantly lower rate of capacity loss and improved electrochemical reversibility compared to conventional cathodes. Structural and mechanical analyses reveal that HE-SC-N88 has a highly stable lattice structure with minimal lattice strain and no significant intergranular or intragranular microcracks. The HE-SC-N88 cathode shows a significantly lower rate of lattice oxygen loss and related phase transitions compared to conventionalA promising high-entropy doping strategy has been developed to create an ultrahigh-Ni, cobalt-free, single-crystalline cathode for high-energy lithium-ion batteries. The material, LiNi₀.₈₈Mn₀.₀₃Mg₀.₀₂Fe₀.₀₂Ti₀.₀₂Mo₀.₀₂Nb₀.₀₁O₂ (HE-SC-N88), combines single-crystalline (SC) design with in situ high-entropy (HE) doping to achieve a grain-boundary-free, stable structure with minimal lattice strain. This structure prevents mechanical degradation, reduces surface parasitic reactions, and mitigates oxygen loss. HE-SC-N88 exhibits exceptional electrochemical performance, including prolonged cycling stability under harsh conditions and delayed oxygen loss-induced phase transitions upon heating. The design of HE doping in redefining ultrahigh-Ni, cobalt-free SC cathodes represents a significant advancement toward industrial application of next-generation lithium-ion batteries. The HE-SC-N88 cathode was synthesized through in situ compositionally complex doping during hydroxide coprecipitation, ensuring close atomic contact and enhanced homogeneity. The material features a rhombohedral α-NaFeO₂-type structure with a high crystallinity and a stable lattice that resists chemomechanical breakdown even during prolonged cycling across a broad voltage range (4.3 to 4.7 V versus Li/Li⁺). This enables exceptional electrochemical behaviors, including delayed phase transitions, enhanced wide-temperature capacity, prolonged cycling stability, and robust high-rate capabilities in both coin cells and 4.3-V full pouch batteries. The pillaring effect from multiple dopants endows HE-SC-N88 with remarkable thermal durability, significantly reducing oxygen loss and related phase transitions during heating. Electrochemical evaluations show that HE-SC-N88 exhibits superior performance compared to conventional cathodes, maintaining high capacity retention and cycling stability even at high voltages and temperatures. The HE-SC-N88 cathode demonstrates exceptional cycling stability, retaining over 90% capacity after 3500 cycles at 1 C and 25°C, and over 91% after 1000 cycles at 1 C and 4.5 V. It also shows excellent high-temperature endurance, retaining 78.2% capacity after 2000 deep cycles at 55°C. The HE-SC-N88 cathode exhibits a significantly lower rate of capacity loss and improved electrochemical reversibility compared to conventional cathodes. Structural and mechanical analyses reveal that HE-SC-N88 has a highly stable lattice structure with minimal lattice strain and no significant intergranular or intragranular microcracks. The HE-SC-N88 cathode shows a significantly lower rate of lattice oxygen loss and related phase transitions compared to conventional