The luminescence in the second near-infrared (NIR-II) spectral region (1,000–1,700 nm) has recently attracted great attention for emerging biological applications owing to its merit of deep tissue bioimaging and high spatiotemporal resolution. However, it still remains a challenge to achieve the highly efficient NIR-II emissions of lanthanides in nanomaterials. Herein, we report an ideal design of sensitizing lithium sublattice core–shell nanocrystals for efficient NIR-II emission properties from a set of lanthanide emitters including Er³⁺, Tm³⁺, Ho³⁺, Pr³⁺, and Nd³⁺. In particular, the typical NIR-II emission of Er³⁺ at 1.5 μm was greatly enhanced by further manipulating the energy transfer via Er³⁺–Ce³⁺ cross-relaxation, and the quantum yield can reach up to 35.74% under 980 nm excitation (12.5 W·cm⁻²), which is the highest value to the best of our knowledge. The 808 nm responsive efficient NIR-II emission was also enabled at the single-particle level through rational core–shell–shell structure design. Moreover, the lithium-sublattice provides an obvious spectral Stark-splitting feature, which can be used in the ultrasensitive NIR-II nanothermometer with relative sensitivity of 0.248% K⁻¹ and excellent thermal cycling stability. These results open a door to the research of new kinds of efficient NIR-II luminescent materials, showing great promise in various frontier fields such as deep tissue nanothermometry and in vivo bioimaging.