Lithiophilicity Chemistry of Heteroatom-Doped Carbon to Guide Uniform Lithium Nucleation in Lithium Metal Anodes
Authors: Chen Xiang , Chen Xiaoru , Zhang Qiang 
Published: May 2020

With the ever-increasing energy demands from the current world, batteries with high energy densities are highly required. Lithium (Li) metal anodes, with an ultrahigh theoretical specific capacity (3810 mAh g−1) and the lowest negative electrochemical potential (−3.040 V versus the standard hydrogen electrode), are strongly considered as the ultimate anode choice for Li batteries. However, the practical application of Li metal anodes faces huge challenges, such as the growth of Li dendrites, the low Coulombic efficiency, and the infinite relative volume change during cycling. In particular, the uncontrollable growth of Li dendrites, which is considered the most fundamental issue facing Li metal anodes and has attracted much more attention, not only induces the “dead Li” with severe capacity loss but also can penetrate through the separator, inducing an internal short circuit and other potential safety hazards.

To address the challenging issue of Li dendrites, a dedicated design of lithiophilic conductive framework is strongly considered as one effective method. Up to now, various lithiophilic conductive frameworks, especially carbon hosts, are used to guide uniform Li nucleation and thus deliver a dendrite-free composite anode. Although a variety of lithiophilic hosts has been proposed and indicate superior performance in suppressing dendritic Li growth, the exact working mechanism between Li and the lithiophilic hosts is tightly sealed and the lithiophilic nature of these carbon hosts is far from clear. If a fundamental understanding of the lithiophilicity chemistry is built, then the rational design of lithiophilic frameworks for dendrite-free Li metal anodes can be achieved.

In this contribution, the lithiophilicity chemistry of heteroatom-doped carbon to guide uniform heterogeneous Li nucleation is proved through both first principles calculations and experimental verifications to guide uniform Li nucleation. Three key factors (electronegativity, local dipole, and charge transfer) are proposed to reveal the lithiophilicity origin of heteroatom doping sites in carbon materials. O doping exhibits the best performance among all single-doping strategies, which was further validated by Li nucleation overpotential tests and transmission electron microscopy (TEM) characterizations. In addition, O/B co-doping is predicted to achieve better lithiophilicity than any single doping due to its large local dipole. Inspired from the Li bond theory, 7Li nuclearmagnetic resonance (NMR) spectroscopy can determine the lithiophilicity of hostmaterials as the linear relationship between Li binding energy and 7Li chemical shift. This work uncovers the lithiophilicity chemistry of heteroatom-doped carbons and affords a mechanistic guidance to Li metal anode frameworks for safe rechargeable batteries. This understanding is also applicable to sodium and potassium metal batteries.

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