Fluorination Effect on Lithium- and Manganese-Rich Layered Oxide Cathodes

Lithium- and manganese-rich (LMR) layered oxides are promising high-energy cathodes for next-generation lithium-ion batteries, yet their commercialization has been hindered by a number of performance issues. While fluorination has been explored as a mitigating approach, results from polycrystalline-particle-based studies are inconsistent and the mechanism for improvement in some reports remains unclear. In the present study, we develop an in situ fluorination method that leads to fluorinated LMR with no apparent impurities. Using well-defined single-crystal Li1.2Ni0.2Mn0.6O2 (LNMO) as a platform, we show that a high fluorination level leads to decreased oxygen activities, reduced side reactions at high voltages, and a broadly improved cathode performance. Detailed characterization reveals a particle-level Mn3+ concentration gradient from the surface to the bulk of fluorinated-LNMO crystals, ascribed to the formation of a Ni-rich LizNixMn2–xO4–yFy (x > 0.5) spinel phase on the surface and a “spinel-layered” coherent structure in the bulk where domains of a LiNi0.5Mn1.5O4 high-voltage spinel phase are integrated into the native layered framework. This work provides fundamental understanding of the fluorination effect on LMR and key insights for future development of high-energy Mn-based cathodes with an intergrown/composite crystal structure.


Synthesis
LNMO and F-LNMO crystals were prepared by a modified molten-salt method. 1,2 ichiometric amounts of Li 2 CO 3 , Ni(CH 3 COO) 2 •4H 2 O and Mn(CH 3 COO) 2 •4H 2 O (Sigma-Aldrich) were used as lithium, nickel and manganese precursors, respectively.Specifically, 3.9 mmol Li 2 CO 3 (30% excess), 1 mmol Ni(CH 3 COO) 2 •4H 2 O and 3 mmol Mn(CH 3 COO) 2 •4H 2 O, were mixed with various amounts of KF/LiF (mole ratio 49:51) through ball milling.KCl was used as the molten salt for the reactions.The mole ratio between the transition-metal salts and the KCl flux was kept at 8. The amounts of the fluoride salts were set to be 0, 0.05, 0.125 and 0.25 mmol for LNMO, LNMO-F1, LNMO-F2.5 and LNMO-F5, respectively.The mixed powders were annealed at 450 °C for 6 h and further heated at 900 °C for 12 h.The heating rate was 10 °C min - 1 .After calcination, the furnace was cooled down to room temperature naturally.The obtained samples were washed with deionized water to remove soluble salts by centrifugation and then filtered, and finally dried at 100 °C in an oven for 12 h.

Electrochemical measurements
The composite cathodes were prepared by mixing the as-prepared LNMO or F-LNMO sample, a polyvinylidene fluoride (PVDF) binder and a conductive carbon black (mass ratio 8:1:1) in an N-methyl-2-pyrrolidone (NMP) solvent.The resulting slurry was then cast on an aluminum foil and dried at 100 °C for 12 h in a vacuum oven.Electrode discs with a size of 1.6 cm 2 and an average active mass loading of ~3 mg cm -2 were cut out and used as the working electrodes in cell testing.The 2032-type coin cells were assembled in an argon-filled glovebox, using Li foil (Alfa-Aesar) as both counter and reference electrodes.Celgard 2400 membrane and 1.2 M LiPF 6 in EC/EMC (volume ratio 3:7) were used as the separator and electrolyte, respectively.The assembled half-cells were galvanostatically cycled using a VMP3 multichannel potentiostat/galvanostat.All electrochemical measurements were performed at room temperature with a current density of 20 mA g -1 .

Characterization
Morphologies were evaluated by using an scanning electron microscope (JEOL JSM-7500F field emission) at a 10 kV accelerating voltage.The crystallinity and phase purity of LNMO and F-LNMO samples were analyzed by using synchrotron X-ray diffraction patterns collected at beam line 11-3 (λ = 1.54 Å) at the Stanford Synchrotron Radiation Lightsource (SSRL) of the SLAC National Accelerator Laboratory.LaB 6 crystals were used for calibration.Soft X-ray absorption spectroscopy analysis for the powder samples and cycled electrodes was conducted at SSRL beam line 10-1 and 8-2, using a 1000 l mm −1 spherical grating monochromator with 20 mm entrance/exit slits, a 0.2 eV energy resolution, and a 1 mm 2 beam spot.The XPS measurements were performed using the K-Alpha XPS System from Thermo Scientific.The photon source was a monochromatized Al K α line (hν = 1486.6eV).The spectra were acquired using a spot size of 400 µm and constant pass energy.The spectra were obtained using a combined low energy electron/ion flood source for charge neutralization and a dual monoatomic and gas cluster argon ion source for depth profiling and sample cleaning.The STEM/EELS data were collected using JEOL JEM-ARM200CF microscope (200 kV) with a probe spherical aberration corrector, Gantan Quantum EELS system and a JEOL SDD-detector with a 100 mm 2 X-ray sensor at Pacific Northwest National Laboratory.The inner and outer collection angles of annular dark field detector were set at 68 and 280 mrad for STEM-HAADF/BAF imaging.FIB was used to section single-crystal particles before analysis which were mapped using STEM to analyze the surface facets.The atomic structures were captured along the [010] direction of the layered structure.
Operando DEMS measurements were performed in a custom-made Swagelok cell in Ar-filled glovebox as described in our previous publication. 3The Swagelok cells were assembled with a LNMO or LNMO-F5 cathode and a Li metal anode in 80 μl of 1 M LiPF 6 in 3:7 v/v of EC:DEC electrolyte.The assembled Swagelok cells were cycled at a current density of 20 mA g −1 between 2 and 4.8 V using a Bio-Logic VSP-series potentiostat under a positive Ar pressure (~ 1.2 bar).

Figure S6 .
Figure S6.a) STEM-HAADF image of LNMO-F5 at (102) surface and b) pixel-by-pixel point acquisition at (102)/(003) surfaces and bulk of LNMO-F5 for the EELS spectra.c) The corresponding Mn L-edge EELS profiles of LNMO and LNMO-F5 collected at the different points in c).d) Overlapping of F K-edge and Mn L-edge in STEM-EDX analysis.

Figure
Figure S7.a) Coulombic efficiencies, b) specific capacity retention and c) energy density retention of LNMO and F-LNMO cathodes cycled at 0.1C in the voltage range of 2 -4.8 V.

Figure S8 .Figure S9 .
Figure S8.Synchrotron XRD patterns of pristine and recovered electrodes after various cycles: a) LNMO and b) LNMO-F5.Right panels show the expanded views.

Figure S10 .
Figure S10.Operando DEMS analysis of a) LNMO and b) LNMO-F5 with voltage profiles and the corresponding O 2 /CO 2 gas evolution during the initial 2 cycles.

Table S1 .
Properties of F-containing salts used for in situ fluorination of LNMO crystals.

Table S2 .
Chemical compositions of pristine LNMO and F-LNMO samples determined by ICP analysis.