Roles of film-forming additives in diluted and concentrated electrolytes for lithium metal batteries: A density functional theory-based approach

Long-Lifespan 522 Wh kg-1 Lithium Metal Pouch Cell Enabled by Compound Additives Engineering

Matching high-voltage cathodes with lithium metal anodes represents the most viable technological approach for developing secondary batteries with ultra-high energy density exceeding 500 Wh kg-1. Nevertheless, the instability of electrolyte/electrode interface film and commercial electrolytes with cut-off voltage above 4.3 V is still a major concern. Herein, we present that excellent cycling stability with an ultra-high cut-off voltage of up to 5.0 V can be obtained by using three-component additives containing fluoroethylene carbonate (FEC), hexadecyl trimethylammonium chloride (CTAC), and tri(pentafluorophenyl)borane (TPFPB). Excellent ionic conductivity, robust interfacial films on both electrodes, and long-lasting uniform Li+ regulation ability can be obtained in the modifying electrolyte. Consequently, using a high plating/stripping capacity of 3 mAh cm-2 under the current density of 1 mA cm-2, lithium symmetric cells demonstrate stable cycling performance exceeding 800 hours. Meanwhile, the 7.3 Ah-class Li[NixCoyMn1-x-y]O2 (x=0.83, NCM83)|Li pouch cells are assembled, which show a high energy density of 522 Wh kg-1 and present excellent stability over 178 cycles with a high initial coulombic efficiency (CE) of 98.0 %.

Binding FSI- to Construct a Self-Healing SEI Film for Li-Metal Batteries by In situ Crosslinking Vinyl Ionic Liquid

The solid electrolyte interphase (SEI) membrane on the Li metal anode tends to breakdown and undergo reconstruction during operation, causing Li metal batteries to experience accelerated decay. Notably, an SEI membrane with self-healing characteristics can help considerably in stabilizing the Li-electrolyte interface; however, uniformly fixing the repairing agent onto the anode remains a challenging task. By leveraging the noteworthy film-forming attributes of bis(fluorosulfonyl)imide (FSI-) anions and the photopolymerization property of the vinyl group, the ionic liquid 1-vinyl-3-methylimidazolium bis(fluorosulfonyl)imide (VMI-FSI) was crosslinked with polyethylene oxide (PEO) in this study to form a self-healing film fixing FSI- groups as the repairing agent. When they encounter lithium metal, the FSI- groups are chemically decomposed into LiF & Li3N, which assist forming SEI membrane on lithium sheet and repairing SEI membrane in the cracks lacerated by lithium dendrite. Furthermore, the FSI- anions exchanged from film are electrochemically decomposed to generate inorganic salts to strengthen the SEI membrane. Benefiting from the self-healing behavior of the film, Li/LiCoO2 cells with the loading of 16.3 mg cm-2 exhibit the initial discharge capacities of 183.0 mAh ⋅ g-1 and are stably operated for 500 cycles with the retention rates of 81.4 % and the average coulombic efficiency of 99.97 %, operated between 3.0-4.5 V vs. Li+/Li. This study presents a new design approach for self-healing Li metal anodes and durable lithium metal battery.

In-Depth Insight into a Passive Film through Hydrogen-Bonding Network in an Aqueous Zinc Battery

Electrochemical stability and interfacial reactions are crucial for rechargeable aqueous zinc batteries. Electrolyte engineering with low-cost aqueous electrolytes is highly required to stabilize their interfacial reactions. Herein, we propose a design strategy using glutamic additive and its derivatives with modification of hydrogen-bonding network to enable Zn aqueous battery at a low concentration (2 m ZnSO₄ + 1 m Li₂SO₄). Computational, in situ/ex situ spectroscopic, and electrochemical studies suggest that additives with moderate interactions, such as 0.1 mol % glutamic additive (G1), preferentially absorb on the Zn surface to homogenize Zn²⁺ plating and favorably interact with Zn²⁺ in bulk to weaken the interaction between H₂O and Zn²⁺. As a result, uniform deposition and stable electrochemical performance are realized. The Zn||Cu half-cell lasts for more than 200 cycles with an average Coulombic efficiency (CE) of >99.32% and the Zn||Zn symmetrical cells for 1400 h with a low and stable overpotential under a current density of 0.5 mA cm^-2, which is better than the reported results. Moreover, adding 0.1 mol % G1 to the Zn||LFP full cell improves its electrochemical performance with stable cycling and achieves a remarkable capacity of 147.25 mAh g^-1 with a CE of 99.79% after 200 cycles.

Non-fluorinated non-solvating cosolvent enabling superior performance of lithium metal negative electrode battery

The growth of dendrites on lithium metal electrodes is problematic because it causes irreversible capacity loss and safety hazards. Localised high-concentration electrolytes (LHCEs) can form a mechanically stable solid-electrolyte interphase and prevent uneven growth of lithium metal. However, the optimal physicochemical properties of LHCEs have not been clearly determined which limits the choice to fluorinated non-solvating cosolvents (FNSCs). Also, FNSCs in LHCEs raise environmental concerns, are costly, and may cause low cathodic stability owing to their low lowest unoccupied molecular orbital level, leading to unsatisfactory cycle life. Here, we spectroscopically measured the Li⁺ solvation ability and miscibility of candidate non-fluorinated non-solvating cosolvents (NFNSCs) and identified the suitable physicochemical properties for non-solvating cosolvents. Using our design principle, we proposed NFNSCs that deliver a coulombic efficiency up to 99.0% over 1400 cycles. NMR spectra revealed that the designed NFNSCs were highly stable in electrolytes during extended cycles. In addition, solvation structure analysis by Raman spectroscopy and theoretical calculation of Li⁺ binding energy suggested that the low ability of these NFNSCs to solvate Li⁺ originates from the aromatic ring that allows delocalisation of electron pairs on the oxygen atom.

Antioxidation Mechanism of Highly Concentrated Electrolytes at High Voltage

It has been researched that highly concentrated electrolytes (HCEs) can solve the problem of the excessive decomposition of dilute electrolytes at a high voltage, but the mechanism is not clear. In this work, the antioxidation mechanism of HCE at a high voltage was investigated by in situ electrochemical tests and theoretical calculations from the perspective of the solvation structure and physicochemical property. The results indicate that compared with the dilute electrolyte, the change of solvation structures in HCE makes more PF₆^- anions easier to be oxidized prior to the dimethyl carbonate solvents, resulting in a more stable cathode-electrolyte interphase (CEI) film. First, the lower oxidation potential of the solvation structure with more PF₆^- anions inhibits the side effects of the electrolyte effectively. Second, the CEI film, consisted of LiF and Li_xPO_yF_z generated from the oxidation of PF₆^- and Li₃PO₄ generated from the hydrolysis of LiPF₆ via the soluble PO₂F₂^- intermediate, can reduce the interface impedance and improve the conductivity. Intriguingly, the high viscosity of HCEs and the hydrolysis of LiPF₆ are proven to play a positive role in enhancing the interfacial stability of the electrolyte/electrode at a high voltage. This study builds a deep understanding of the bulk and interface properties of HCEs and provides theoretical support for their large-scale application in high-voltage battery materials.