Stabilization of NCM811 cathode interface through macromolecular compound protective film formed by 2,5-bis(2,2,2-trifluoroethoxy)-benzoic acid additive in lithium metal batteries

Lithium metal batteries (LMBs) offer substantial promise for next-generation energy storage owing to lithium metal's low reduction potential (−3.045 V vs. the standard hydrogen electrode) and its high specific capacity of 3860 mA h g−1. Among various cathode materials in LMBs, LiNi0.8Co0.1Mn0.1O2 (NCM811) is extensively employed because of its notably high specific capacity (over 200 mA h g−1) and comparatively lower cost. However, structural stress, nickel ions migration, and uneven Li+ deposition in NCM811 particles lead to cracking, irreversible decomposition of active substances, and the growth of mossy Li dendrites, causing severe capacity decline and low Coulomb efficiency in LMBs. In this study, we introduce an effective ethoxyl additive, 2,5-bis(2,2,2-trifluoroethoxy)-benzoic acid (2,5BTBA), directly into the carbonate electrolyte. This additive forms a dense and conductive macromolecular protective film on the NCM811 cathode and lithium metal anode during initial cycles, preventing electrode contact with the electrolyte. Consequently, it safeguards the cathode's structural integrity and enables dense lithium deposition. Adding 3 wt% 2,5BTBA, the Li/NCM811 battery retains a high capacity of 150.60 mA h g−1 and 89.41% retention after 700 cycles at 0.5C, maintaining an average Coulomb efficiency of 99.13%. This study presents an efficient and straightforward strategy to enhance the capacity retention of LMBs.

All chemical reagents were analytical grade.

Preparation of electrodes
For the fabrication of NCM811 cathode, NCM811 (0.6 g), conductive carbon (0.075 g), and binder (PVDF-5130, 0.075 g) were mixed with a mass radio of 8:1:1 in N-methyl pyrrolidone (2.5 mL) and stirred for 24 hours to obtain a uniform slurry.The slurry was evenly coated on aluminum foil for cathode and dried in vacuum at 80 °C for 12 h.The resulted cathode electrodes were cut into circular sheets with a diameter of 12 mm and the regular loading density of the NMC811 was 1.81-2.37mg/cm 2 .The preparation of LFP cathode is the same as that of NCM811 cathode.
Lithium metal anodes were made of lithium foil (provided by Guangdong Canrd New Energy Technology Co., Ltd.) that cut into 14 mm in argon atmosphere.

Materials characterizations
The surface morphologies of the samples were performed by the scanning electron microscope (SEM, Zeiss Supra, Germany).The valence states of C, F, and Ni elements after cycling were carried out by X-ray photoelectron spectroscopy (XPS, Thermo-Fisher, ESCALAB 250Xi, U.S.A.).Battery disassembling and electrode cutting were carried out and then sealed in the glove box.Then, the sealed sample was transferred into the XPS chamber, taken out rapidly in the chamber and vacuumed immediately to avoid exposure and oxidation in air.All profiles were calibrated at 284.6 eV.X-ray diffractometer (XRD, BRUKER, D8, Germany) and Atomic Force Microscope (AFM, Bruker Dimension Icon, Germany) was used to study the structure of the samples.The material and structure information were analyzed by Raman spectroscopy (Raman, Renishaw inVia, Renishaw, England).

Electrochemical measurements
All electrochemical tests in this work were carried out in a CR2025-type coin cell.
The assembling processes were performed in an argon filled glove box (H 2 O, O 2 < 0.01 ppm).The separators of the cell were commercial Celgard 2500.The amount of the electrolyte was 80 μL.The galvanostatic measurements of LMBs were performed on the battery testing system (NEWARE, CT-3008), within the voltage range of 2.7-4.3V (vs.Li + /Li) at a constant temperature of 25 ℃.In the first two cycles, all batteries were activated with a low current of 1/15 C, while the following cycles were cycled at a rate of 0.5 C (1 C = 188 mAg -1 vs. Li + /Li).Disregarding the first two cycles of activation, the capacity was calculated on the basis of the third cycle.Linear sweep voltammetry (LSV) was tested between 3.0 V and 10.0 V (Shanghai Chenhua, CHI 760e, China).
On the same workstation, the Electrochemical Impedance Spectroscopy (EIS) of LMBs at different cycles of 50 and 100 were measured at a frequency of 10 mV (frequency ranging from 100 kHz to 0.1 Hz).
The Li + transference numbers for electrolytes were studied with AC impedance and DC polarization analysis.The polarization currents, referred to the initial current (I 0 ) and steady-state current (I S ) of the Li/Li cell, were obtained with a polarization potential (ΔV) at 10 mV.In addition, the initial and steady-state interfacial resistances (R O and R S ) of Li/electrolyte were determined by impedance measurements before and after potentiostatic polarization.The impedance measurement is performed at an open circuit potential in the frequency range from 0.10 Hz to 1.0 MHz.t Li+ was calculated based on Bruce Vincent Evans equation (Eq 1): (1) (∆ -    ) The ionic conductivity was calculated based on the equation (Eq.2): (2) where L separator is the thickness of the separator, S stands for the area of the electrode and R electrolyte represents the Ohmic resistance of the electrolyte, which was tested at an open-circuit potential in a frequency range from 0.10 Hz to 1.0 MHz.
The improved value of interface ion conductivity was calculated based on the equation (Eqs.3-5): (3) where R electrolyte , R electrode and interface and R represent the resistance of electrolyte, the resistance of electrode as well as interface and total resistance, respectively.

Figure S2 .
Figure S2.Conductivity before and after polarization and polarization curve with the

Figure S6 .
Figure S6.SEM images of cathodes of Li/NCM811(single crystal) batteries without

Figure S7 .
Figure S7.Equivalent circuit models used for EIS spectra in Figure 3e and 3f.The

Figure S8 .
Figure S8.The cycling performance of batteries with different content of additives

Figure S11 .
Figure S11.Rate performance of Li/NCM811 batteries with different content of

Table S1 .
Fitted results of EIS plot in Figure3eand 3f.