Significance  Lithium serves as a significant strategic metal in emerging industries, and lithium isotopes have become crucial geochemical tracers due to their pronounced mass-dependent fractionation effects. Therefore, lithium minerals in coal-bearing strata have emerged as a priority in recent exploration of strategic metallic mineral resources. Investigating the isotopic composition and variation of lithium in coal-bearing strata helps reveal the sources, migration, and enrichment process of lithium while also providing a theoretical basis for lithium exploration and exploitation in coal-bearing strata.

Advances  This study offers a summary of advances in research on lithium in coal-bearing strata and its isotopes from three aspects: (1) the general characteristics of lithium; (2) the isotopic composition and fractionation mechanisms of lithium, and (3) the test methods, extraction, and separation of lithium, as well as techniques for the tests and analysis of lithium isotopes. The results indicate that lithium is extensively distributed in the mantle and crust, exhibiting strong activity with fluids. Two stable natural isotopes of lithium (i.e., ⁶Li and ⁷Li) exhibit significant fractionation effects due to their differences in the diffusion rate and relative mass, establishing them as critical geochemical tracers. Lithium minerals in coal-bearing strata in China are primarily distributed in the Carboniferous to Permian strata in North China and the Late Permian strata in South China. Lithium element occurs principally in secondary clay minerals, with its enrichment jointly influenced by multiple factors like sedimentary diagenesis, microbial activity, tectonism, magmatic-hydrothermal activity, and groundwater migration. The isotopic fractionation of lithium in coal-bearing strata is primarily affected by factors including temperature, weathering, metamorphism, and the formation of secondary clay minerals. The methods for determining lithium content in samples from coal-bearing strata have been relatively mature, and high-precision techniques for lithium isotope tests have offered a possibility for the extensive application of lithium isotopes. Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has been preliminarily employed to investigate the mechanisms behind the isotopic fractionation of lithium in coal-bearing strata, whereas in-situ microanalytical techniques for lithium isotopes in samples from these strata remain in the exploratory stage. The complex compositions and structures of lithium carriers in coal-bearing strata highlight an urgent need to develop standard samples and test criteria for in-situ analysis of lithium isotopes from coal-bearing strata. To separate and extract lithium resources in these strata, the key is to enhance leaching efficiency and perform purification and recovery of lithium from leachate.

Prospects  Current studies on lithium in coal-bearing strata suffer from several limitations, including a limited understanding of the isotopic fractionation mechanisms, the presence of mass discrimination effects of test methods, a lack of standard samples for in-situ analysis, and limited validation through simulation experiments. The trends in research on lithium in coal-bearing strata and its isotopes will focus on the dynamic migration processes and enrichment mechanisms of lithium in coal-bearing strata, the develop of high-precision techniques for the tests and analysis of lithium isotopes, the coupling mechanisms between lithium isotopic fractionation and the sedimentary and thermal evolution processes, and the separation, extraction, and recovery of lithium resources in coal-bearing strata.