A novel cathode interphase formation methodology by preferential adsorption of a borate-based electrolyte additive

ABSTRACT The coupling of high-capacity cathodes and lithium metal anodes promises to be the next generation of high-energy-density batteries. However, the fast-structural degradations of the cathode and anode challenge their practical application. Herein, we synthesize an electrolyte additive, tris(2,2,3,3,3-pentafluoropropyl) borane (TPFPB), for ultra-stable lithium (Li) metal||Ni-rich layered oxide batteries. It can be preferentially adsorbed on the cathode surface to form a stable (B and F)-rich cathode electrolyte interface film, which greatly suppresses the electrolyte-cathode side reactions and improves the stability of the cathode. In addition, the electrophilicity of B atoms in TPFPB enhances the solubility of LiNO3 by 30 times in ester electrolyte to significantly improve the stability of the Li metal anode. Thus, the Li||Ni-rich layered oxide full batteries using TPFPB show high stability and an ultralong cycle life (up to 1500 cycles), which also present excellent performance even under high voltage (4.8 V), high areal mass loading (30 mg cm−2) and wide temperature range (−30∼60°C). The Li||LiNi0.9Co0.05Mn0.05O2 (NCM90) pouch cell using TPFPB with a capacity of 3.1 Ah reaches a high energy density of 420 Wh kg−1 at 0.1 C and presents outstanding cycling performance.


Materials Characterizations.
The X-ray diffraction (XRD) measurements of the samples were carried out on a Rigaku Smartlab with Cu-Kα radiation.The operando XRD test during charging and discharging rate of 18 mA g -1 was performed at 25 °C and diffraction patterns were collected every 16 min.The morphologies and structures of NCM cathode and Li anode were characterized by a scanning electron microscope (SEM, HITACH S4800) and a Cold-Field-Emission Double Cs-corrected TEM (Thermo Fischer Spectra 300).X-ray photoelectron spectroscopy (XPS) measurements were collected on a PHI 5000 VersaProbe Ⅱ instrument.The 19 F, 13 C, 7 B and 1 H nuclear magnetic resonance (NMR) data were collected with NMR spectrometer (ADVANCED Ⅲ 400 MHz, Bruker, Switzerland) with DMSO-d6 solvent.The transition metal (TM) concentration of the Li anode was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES, SpectroArcosⅡ MV, Germany).The cycled LMA was washed by DEC and dissolved in 2 mL 1 M HCl and then diluted to 50 mL solution with deionized H 2 O using a 50 mL volumetric flask, and the TM content on the surface of LMA was calculated by the mass of TM elements in the solution over the mass of active material in the NCM811 cathode.The CEI and SEI component data was collected by time-of-flight secondary ion mass spectrometry (ToF-SIMS, PHI nanoTOF Ⅱ, 30 keV, 2 nA, and the raster size is 60 × 60 μm.).
The cross-sectional NCM cathode was investigated by an ion milling system (IMS, HITACH IM4000 plus).Before analysis, the cells were disassembled in an Ar-filled glove box (Mikrouna) (H 2 O < 0.1 ppm, O 2 < 0.1 ppm) and the electrode surfaces were rinsed with 5 mL of DMC.After drying at 25 °C in glovebox for 10 min, the electrodes were transferred by the transfer sample holder with an Ar-filled to isolate the air.

Fabrication of cells.
Low areal mass loading (~2 mg cm -2 ) NCM811 cathode was prepared by following steps.The mixture of 0.8 g (80 wt%) NCM811 powder, 0.1 g (10 wt%) PVDF5130 and 0.1 g (10 wt%) Super P was manually ground in an agate mortar for 10 mins in an air environment and then dispersed in 2 mL NMP by magnetic stirring for 4 h to form a slurry.Then the slurry was casted on an Al foil and dried at 120 °C for 2 h under vacuum.High areal mass loading (>20 mg cm -2 ) NCM811, NCM90 and LCO cathode was prepared by following steps.The mixture of 4.7 g (94 wt%) NCM811 (NCM90 or LCO) powder, 0.15 g (3 wt%) PVDF5130 and 0.15 g (3 wt%) Super P was manually ground in an agate mortar for 15 mins in an air environment and then dispersed in 4.1 g NMP by grinding mill for 15 min at 2000 rpm to form a slurry.Then the slurry was casted on an Al foil and dried at 120 °C for 2 h under vacuum.The CR2032 Li||NCM811, Li||NCM90 and Li||LCO coin cells were assembled in an Ar-filled glove box (O 2 and H 2 O < 0.1 ppm) using 40 μL BE with/without TPFPB.The thickness of lithium foil is 200 μm and the diameter is 15.6 mm. and the thickness of the separator is 25 μm.For single-layer Li||NCM811 pouch cell, NCM811 cathode (mass loading: ~ 20 mg cm -2 , 4 cm × 3.5 cm) and one single coated anode (4.3 cm × 4 cm) were stacked one by one and separated by Celgard 2500 (4.5 cm × 4.5 cm), the electrolyte volume is 200 μL.We assembled in a dry room whose dew point is -50 º C.

Electrochemical measurements.
The electrochemical impedance spectroscopy (EIS) of Li||NCM811cells was performed from 7 MHz to 10 mHz at an amplitude of 5 mV, 6 data points per decade, and the 3.95 V of charging progress (after charge to 3.95 V, a constant-voltage charging was applied until the specific current is lower than 9 mA g -1 , and rest for 2 h) applied before carrying out the EIS measurements at 25 º C using a VMP3.The Li ion conductivity of BE and BE with 1% TPFPB were test by assembling stainless steel||stainless steel cells under -30~60 °C using a VMP3.Cycling and rate performances of Li||NCM811cells were measured at a temperature range of -30~60 °C on a battery test system (LAND CT-2001A).Most of the batteries were operated at 25±1 º C except part of batteries was tested at 60 º C, -20 º C and -30 º C. The specific current and specific capacity refers to the mass of the active material in the cathode.The electrochemical energy storage tests at various temperatures were carried out in a constant temperature chamber.

Vogel-Tammann-Fulcher function fitting.
The EIS data of Li||Li cells were collected by VMP3 from -30 to 60 º C. The VTF behavior [1], which is more relevant for electrolyte organic solutions, is described by equation ( 1) Here B is the pseudo-activation energy for the conductivity (expressed in units of E a /k), and T 0 is the reference temperature which normally falls 10-50 K below the experimental glass transition, T g .

Calculations.
HOMO and LUMO energy.potentials [2-7]were chosen to describe the ionic cores, and valence electrons were described using plane wave basis set with a kinetic energy cutoff of 500 eV.The electronic energy was converged when the total energy change was smaller than 10 -4 eV.The residual force threshold for the convergence of geometry optimization was set to be 10 -2 eV Å.We model the surface using a symmetric periodic slab, (003) crystal face of NCM811, and a 15 Å -2 vacuum layer was inserted between the slab and its periodic image.The atoms of matrix slab are fixed for reducing calculation only when evaluated adsorption energy.
The binding energy of individual molecules on the NCM811 surface were computed as: Where E NCM811_slab and E Individual molecules are the energy of the bare NCM811 surface and individual molecules, respectively, and E total was the total energy of the configurations of individual molecule on the NCM811 surfaces.(i-j) Li||Cu cell using BE with 1%TPFPB-1%LiNO 3 .# the article doesn't mentioned the capacity retention of this pouch cell.NM present Not mentioned.

3 4 Figure S8 .
Figure S8.Electrochemical impedance spectra of stainless steel||stainless steel cells with (a) BE and (b) 1% TPFPB at different temperature.(c) VTF function fitting of relationship of ionic conductivity of BE and BE with 1% TPFPB at different temperatures.

1 Figure S9 .
Figure S9.Rate performance of Li||NCM811 cells with high cathode loading of 20 mg cm -2 between

1 Figure S10 .
Figure S10.Cycling performance of thin Li||NCM811 coin cells at 25 °C between 3 and 4.8 V.The

Figure S14 .
Figure S14.Cycling performance of Li||NCM90 coin cells between 3 and 4.3 V at 25 °C .The charge/discharge rate is 0.5/1 C, the areal mass-loading of NCM90 is around 20 mg cm -2 , the thickness of Li anode is ~ 200 μm, and the N/P ratio of these cells is ~10.

Figure S15 .Figure S16 .
Figure S15.Cross section image of NCM90 cathode retrieved from (a) Li||NCM90 cell using BE, (b) Li||NCM90 cell using BE with 1% TPFPB and (c) Li||NCM cell using BE with 1% TPFPB and 1% LiNO 3 after 100 cycles between 3 and 4.3 V at 25 °C.The charge/discharge rate is 0.5/1 C, the areal mass-loading of NCM90 is around 4 mA h cm -2 , the thickness of Li anode is ~ 200 μm and the N/P ratio of these cells is ~10.

Figure S17 .Figure S18 . 1 Figure S19 .
Figure S17.Ex situ XPS measurements and analysis of NCM811 cathode retrieved from (a, c) Li||NCM811 cells using BE and (b, d) Li||NCM811 cells using BE with 1% TPFPB after 5 cycles at 0.1 C. The XPS spectra of (a, b) F 1s and (c, d) its corresponding composition and distribution of different constituents.The cells were disassembled at fully discharged state.

1 Figure S20 .
Figure S20.Depth profiling of secondary ion fragments on the NCM811 surface using (a) BE and (b)

7 Table S2 .
Detailed parameters of 3.1 Ah Li||NCM90 pouch cells.Note: the area weight of NCM90 cathode is the total weight of cathode, which is included in Al foil, cathode particle, super P and PVDF.