Anode-free lithium metal batteries are the most promising candidate to outperform lithium metal batteries due to higher energy density and reduced safety hazards with the absence of metallic lithium anode during initial cell fabrication. In general, researchers report capacity retention, reversible capacity, or rate capability of the cells to study the electrochemical performance of anode-free lithium metal batteries. However, evaluating the behavior of batteries from limited aspects may easily overlook other information hidden deep inside the meretricious results or even lead to misguided data interpretation. In this work, we present an integrated protocol combining different types of cell configuration to determine various sources of irreversible coulombic efficiency in anode-free lithium metal cells. The decrypted information from the protocol provides an insightful understanding of the behaviors of LMBs and AFLMBs, which promotes their development for practical applications.

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Anode-free batteries (AFBs) are impressive and recent phenomena in the era of energy storage devices due to their high energy density and relative ease of production compared to the traditional Lithium metal batteries (LMBs). However, dendrite formation during plating and stripping and low coulombic efficiency (CE) are the main challenges that impede practical implementation of these batteries. Here we report an extremely stable dual-salt electrolyte, 2M LiFSI+1M LiTFSI (2FSI+1TFSI)) in DME/DOL (1:1, v/v), system in comparison to the single salt 3M LiTFSI (3TFSI) in DME/DOL (1:1, v/v), to effectively stabilize AFB composed of LiFePO 4 cathode and bare Cu-foil anode for the first time. The electrolyte stabilized anode-free cell with the configuration Cu||LiFePO 4 via reductive decomposition of its anions and enabled the cell to be cycled with CE of 98.9% for 100 cycles. This results from the formation of stable, ion conductive and electrically insulating inorganic components rich Solid Electrolyte Interface (SEI) layer on the surface of in-situ deposited Li-metal that blocks the undesirable parasitic reaction between the deposited Li and the electrolyte. Thus, aforesaid SEI mitigates formation of dead lithium and dissolution of the in-situ deposited Li surface during repeated cycling and prolongs cycle life of the battery.

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Recently, metallic zinc (Zn) is becoming a promising ideal anode material for rechargeable aqueous batteries by providing high theoretical capacity (820 mA h/g) with divalent reaction, environmental friendliness, earthy abundance, low cost, low toxicity, higher water compatibility, and low electrochemical potential (−0.762 V vs SHE). However, intensive growth of zinc dendrites while plating/stripping lowers its coulombic efficiency and shortens the cycle life of the rechargeable devices. Here, we report a concentrated aqueous electrolyte (4.2 M ZnSO4 + 0.1 M MnSO4) with improved cycling stability of zinc metal anode achieving an average coulombic efficiency (ACE) ∼99.21% cycling for more than 1000 h at 0.2 mA/cm2 current density using a Zn||Cu cell. However, a frequently used diluted electrolyte (2 M ZnSO4 + 0.1 M MnSO4) only produces ACE ≈ 97.54% with a relatively short life cycle. The developed concentrated electrolyte with strongly aggregated ion pairs shows the synergetic effects of the enhanced solvation/desolvation process, electrostatic shielding, and Le Chatelier’s principle. Consequently, the additives simultaneously suppress Zn dendrites and dissolution of Mn2+ ions from the MnO2 cathode. A highly stable and reversible Zn||MnO2 cell retaining about 88.37% retention capacity was obtained after cycling for more than 1200 cycles at 938 mA/g current density.