Regulating the donor number of solvents for long-cycle anode-free lithium metal batteries

Anode-free lithium-metal batteries (AFLMBs) are one of the up-and-coming high-energy-density battery systems because they eliminate the need for conventional graphite electrodes or excess lithium metal anodes (LMAs). However, AFLMBs have serious problems of low coulombic efficiencies (CEs) and poor cycle stability. Herein, this study proposes a local high-concentration electrolyte (LHCE) based on donor number (DN) modulation, which achieves stable dissolution of LiNO₃ and an anion-enhanced solvation structure through the synergistic action of a high DN solvent (tetraglyme, G4) and an ultra-low DN diluent (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, TTE). The design addresses the core challenges of low CEs and poor cycling stability in AFLMBs due to the lack of a lithium source. Computational and experimental results show that regulating the DN value of the solvent significantly reduces the Li⁺ desolvation energy and forms an inorganic-rich solid electrolyte interphase (SEI), which guides the uniform deposition of Li. The electrolyte achieves a high CE (about 99.0%) and long cycle life (1400 h). Based on this, the AFLMB retains 86.3% capacity after 50 cycles, far exceeding the 25-cycle life of conventional carbonate electrolytes (CCEs).