Li 2 NiO 2 F a New Oxyfluoride Disordered Rocksalt Cathode Material

Lithium-rich disordered rocksalts such as Li 1.3 Nb 0.3 Mn 0.4 O 2 and Li 2 MnO 2 F are being investigated as high energy density cathodes for next generation Li-ion batteries. They can support the (de) lithiation of lithium ions over large compositional ranges while preserving the same overall structure. Here, we present a new Ni-rich oxy ﬂ uoride cathode, Li 2 NiO 2 F, with a disordered rocksalt structure. Li 2 NiO 2 F and can deliver a discharge capacity of 200 mAh g − 1 at an average voltage of 3.2 V. © 2021 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access


Supplementary material for this article is available online
Finding new, high energy density cathode materials which are free from Co is crucial for the development of next generation batteries. In the race to eliminate Co from Li-ion batteries, there is a great effort underway to commercialize Ni-rich layered cathodes based on the composition LiNiO 2 , however, these materials suffer from layer stacking changes and structural instability at high voltages. Recently, researchers have been looking beyond conventional layered cathodes towards disordered rocksalt materials which are able to retain crystal structure stability over large composition ranges. Cation disordered oxides such as Li 4 2 3 are all capable of delivering reversible capacities in excess of 300 mAh g −1 . Despite the random distribution of transition metal (TM) and lithium ions among the cation sites, a large fraction of the Li-ions can still diffuse readily through the structure. 4,5 Beyond the oxide compounds, mixed anion oxyfluorides are also being investigated. The first reports of Li 2 VO 2 F in 2015 made by ball-milling 6 have since inspired others to synthesise a range of disordered rocksalt oxyfluoride cathodes materials with similar compositions, including Li 2 MnO 2 F, 7,8 Li 2 FeO 2 F, 9 Li 2 TiO 2 F, 10 Li 2 MoO 2 F 11 and those with two or more transition metals such as Li 1 15,16 Li 2 NiO 2 F, which is free from inactive d 0 transition metals, offers the possibility to access high capacities supported by the Ni 3+/4+ redox couple which is generally higher voltage than Fe 3+/4+ , V 3+/4+ or Mn 3+/4+ making it a suitable choice to target high energy densities. This compound is also Li-rich allowing access to the O-redox couple which may provide even higher capacities than LiNiO 2 .
Here, we report the synthesis and electrochemical investigation of Li 2 NiO 2 F, with a disordered rocksalt structure and Ni in the +3 oxidation state. Capacity on the first charge to 4.8 V is 300 mAh g −1 , of which 200 mAh g −1 is recovered on subsequent discharge to 2.0 V. The average voltage on discharge is 3.2 V. Oxygen loss occurs on charge and Ni is reduced beyond 3+ on discharge. This study extends the family of Li 2 TMO 2 F oxyfluoride compounds to include TM = Ni 3+ .

Experimental Methods
Synthesis.-Li 2 NiO 2 F was prepared by grinding together singlephase LiNiO 2 and commercial LiF (Aldrich, 99.99%) in the molar ratio 1:1 in an argon-filled glovebox. The mixture was then placed in a sealed zirconia jar of 20 cm 3 internal volume and ball milled at 750 rpm for 12 h in a Fritsch Pulverisette 7 planetary ball mill. The mass ratio between the powder and the balls was 1:30. The precursor LiNiO 2 was synthesised by mixing LiNO 3 (Aldrich, 99.99%) and NiCO 3 (Aldrich, 99.9%) in the molar ratio 1:1 in the glove box. A cold pressed pellet of the ground starting materials was then contained in an alumina boat inside a predried silica tube. The tube was heated to 650°C under O 2 atmosphere at a rate of 2°C min −1 and then held at 650°C for 15 h before ramping back to room temperature at a rate of 2°C min −1 . All samples were stored in an Ar-filled glove box (O 2 /H 2 O < 0.1 ppm) until further characterization.
Material characterization.-Powder X-ray diffraction (XRD) was carried out using a Rigaku SmartLab X-ray powder diffractometer equipped with a 9 kW Cu anode. GSAS Suite software was used to perform Rietveld refinements on powder X-ray diffraction data.
Elemental ratio of Li:Ni was determined through the analysis of Inductively Coupled Plasma -Optical Emission Spectroscopy (ICP-OES) using a Perkin-Elmer Optima 8000 ICP-OES. The ball-milled target product was first dissolved into 2% HNO 3 . It was diluted to the concentration (1:200) that can be interpolated from available standard solutions with known molarity. The diluted samples were then loaded into the ICP instrument for analysis. For cycled materials, 10 wt% Al 2 O 3 was added to the electrode as a constant internal reference and the Ni/Al ratio was measured to determine the Ni content in Li 2 NiO 2 F.
The morphology of the sample was characterized by a Zeiss Merlin analytical scanning electron microscope (SEM). Annulardark field scanning transmission electron microscopy (ADF-STEM) z E-mail: peter.bruce@materials.ox.ac.uk *Electrochemical Society Member.
images were collected with JEOL ARM 200 F operated at a voltage of 200 kV.
Operando electrochemical mass spectrometry (OEMS) was used in this work to probe evolution of O 2 gas from the Li 2 NiO 2 F cathode. A custom-made electrochemical cell with gas inlet and outlet ports was connected to a Thermo Fischer quadrupole mass spectrometer via sealed tubing. Turbomolecular pump (Pfeiffer Vacuum) and mass-flow controllers (Bronkhorst) were used to control the flowing rate of Ar gas in the cell.
Ni K-edge hard X-ray absorption near edge structure (XANES) experiments were performed at beamline B18 at the Diamond Light Source, Harwell, U.K. This beamline has a wide energy range (2.05 −35 keV) and is equipped with a double-crystal monochromator containing two pairs of crystals, Si (111) and Si (311), optimized for quick EXAFS measurements. 17 The data were collected in transmission mode and the intensities of the incident and transmitted beams were measured via gas filled ionization chambers. Two or three scans were collected for each sample and they were summed, calibrated, background subtracted, and normalized using Athena. 18 Electrochemical measurement.-Electrodes were prepared by making slurries containing 80 wt% active cathode material, 10 wt% polyvinylidene fluoride (PVDF) binder, and 10 wt% Carbon Super P. All the preparation was carried out in an Ar-filled glovebox and these electrodes were then incorporated into 2032 electrochemical cells using a Li metal as counter electrode, the Whatman glass fibre as separators and 1 M LiPF6 in 1:1 w/w ethylene carbonatedimethyl carbonate as electrolyte. Electrochemical measurements were performed using a Maccor battery cycler at room temperature. The charge and discharge experiments were carried out galvanostatically at a constant current over a voltage range 2.0 to 4.3 V and 2.0 to 4.8 V.

Results and Discussion
Structural and morphological characterization.-Li 2 NiO 2 F was synthesized as described in the Experimental section. Powder X-ray Diffraction (PXRD), Supplementary Fig. 1 (available online at stacks.iop.org/JES/168/080521/mmedia), confirmed LiNiO 2 was a single phase precursor and with a stoichiometry Li 0.98 Ni 1.02 O 2 (molar volume = 33.86(1) A 3 ) according to the interrelation between composition and the Rhombohedral unit cell volume, Table SI. 19 The ball-milling reaction was performed under Argon atmosphere in hermetically sealed jars and no pressurisation of the jar from O 2 release during milling was observed implying a lack of change in Ni oxidation state from LiNiO 2 to Li 2 NiO 2 F. The Li:Ni atomic ratio in the as-prepared Li 2 NiO 2 F was also measured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), and is in close agreement Supplementary Table SI. Energy-dispersive spectroscopy (EDS) mapping images, presented in Supplementary  Fig. 2 confirm the uniform distribution of Ni, O and F throughout the particles.
PXRD data were collected on the pristine Li 2 NiO 2 F, Fig. 1, and can be indexed on the rocksalt structure, space group Fm-3m, without the presence of crystalline impurity phases. Li 2 NiO 2 F exhibits very broad peaks, common to rocksalts prepared by ballmilling. Reitveld refinement was carried out yielding a lattice parameter of 4.0748(2) Å, which is smaller than for the Mn-analogue Li 2 MnO 2 F (4.1176(5) Å) in line with the differences in the ionic radii of Ni 3+ and Mn 3+ . 7 The lattice and structural parameters are presented in Supplementary Table SIII. The morphology of the ball-milled particles was examined using scanning electron microscopy (SEM), Supplementary Fig. 3, revealing secondary particle sizes in the range 50-200 nm. Selected area electron diffraction (SAED) data, Fig. 2a, show reflections that can be indexed on a cubic Fm-3m cation-disordered rock-salt. Highresolution transmission electron microscopy (HRTEM) images, Fig. 2b, show that the secondary particles are composed of smaller primary particles about 5-10 nm in size, in line with the broad X-ray diffraction peaks seen in Fig. 1. The lattice fringes with interplanar spacing 0.20 nm are also shown corresponding to the (200) plane of Li 2 NiO 2 F rocksalt structure. 6 Li and 19 F solid-state magic angle spinning (MAS) NMR spectroscopy were performed to investigate lithium and fluorine within Li 2 NiO 2 F, Fig. 3. The 6 Li spectrum for LiNiO 2 shows a single resonance centred at ∼710 ppm corresponding to Li in the alkali layer, consistent with previous reports and confirming the purity of the precursor. 20 In the Li 2 NiO 2 F disordered rocksalt, the main resonance is at a significantly lower chemical shift, with a centre of gravity closer to ∼30 ppm. This is due partly to the increased lithium content which dilutes the number of neighbouring paramagnetic Ni centres but also indicates a substantial degree of disorder. The asymmetric tail arises from different local Li environments with varying proximity to Ni, characteristic of disordered rocksalts. 21 The environments at higher chemical shifts are surrounded by a greater number of paramagnetic Ni 3+ centres. Likewise, a similar peak asymmetry is observed in the 19 F NMR spectrum of Li 2 NiO 2 F, which also shows a tail away from the LiF isotropic chemical shift (−204 ppm). These observations indicate successful bulk incorporation of Li and F into the disordered rock salt lattice.
Electrochemical characterization.-First-cycle charge and discharge curves of Li 2 NiO 2 F between 2−4.8 V are shown in Fig. 4a. Li 2 NiO 2 F exhibits a first charge capacity of 300 mAh g −1 (equivalent to 1.38 li per formula unit), 80 mAh g −1 beyond the theoretical capacity of 220 mAh g −1 based on oxidation of Ni 3+ to Ni 4+ . However, this is not completely recovered on discharge and almost 100 mAh g −1 of irreversible capacity is observed. After the initial charge-discharge, the material exhibits decreasing capacity over subsequent cycles ( Fig. 4b and Supplementary Fig. S5). Limiting the upper voltage cut-off to 4.3 V avoids the irreversible capacity loss and improves the subsequent capacity decay, consistent with the charge stored above this voltage limit being irreversible in nature, Figs. 4c, 4d and Supplementary Fig. S5.
On discharging from 4.3 V to 2.0 V, there is evidence of a small amount of excess capacity which most likely arises from lithiation of cation vacancy defects. To check for cation vacancies in the pristine material, we discharged the pristine material to 2 V and observed 50 mAh g −1 capacity (equivalent to 0.25 li), Supplementary Fig. S6. The low voltage of this capacity is consistent with the accompanying reduction of Ni from +3 to +2, implying there must be some anion as well as cation vacancies in pristine Li 2 NiO 2 F.
Charge compensation in Li 2 NiO 2 F.-Oxygen loss is a common mechanism of irreversible capacity in cathode materials at high voltages. To quantify the amount of oxygen loss from Li 2 NiO 2 F, operando electrochemical mass spectrometry (OEMS) was employed. The OEMS results are presented in Fig. 5 and show a significant amount of O 2 and CO 2 released during the initial charge. Quantification of the evolved gases reveals up to 120 mAh g −1 of capacity can be attributed to O loss through direct O 2 evolution and CO 2 from oxygen releases from the lattice reacting with the electrolyte. Analysis of the latter followed the previously described procedure. 22 After the first cycle, the amount of O loss is much diminished and correspondingly there is improved capacity retention. Similarly, by limiting the first cycle voltage cut-off to 4.3 V,  which is before most of the O loss is observed, there is significantly less irreversible capacity.
To investigate the redox changes on Ni accompanying the first cycle, K-edge XANES data were collected, Supplementary Fig. S4. On charge, there is an increase in the main edge energy indicating Ni oxidises towards +4. On discharge, the main edge shifts to a lower energy than that of the pristine indicating Ni is reduced below the initial +3 state. After discharging to 2 V, the XANES data appear to show a greater degree of Ni reduction than that expected electrochemically accounting for the irreversible capacity from O loss (200 mAh g −1 discharge capacity and 300-120 = 180 mAh g −1 reversible charge capacity). This implies that some of the oxygen loss arises from Ni reduction at the surface (i.e. through reductive elimination 2Ni 4+ -O 2− → 2Ni 2+ + O 2 ) rather than direct oxidation of the oxide ions.  23 To assess the role of metal dissolution in Li 2 NiO 2 F, we performed elemental analysis on the cathode after cycling using ICP-OES. The results show a loss of 2.5% of the Ni from the cathode after 100 cycles, Supplementary Fig. S7, indicating dissolution does contribute in part to the gradual capacity decay that is seen. This degree of dissolution is less pronounced than the Mn rocksalt analogue where we detected 8% loss of Mn after 100 cycles, which is in line with broader understanding that Mn dissolution tends to be more severe than Ni in other cathodes, such as the layered oxides and Li-rich materials. 24 Given the extent of the capacity decay we observe for Li 2 NiO 2 F (almost 50% loss by 50 cycles) we conclude that this small amount  of dissolution is unlikely to be the dominant process responsible here and instead more complex surface instability or interphase formation are likely to be the cause. Future work targeted at understanding and inhibiting these surface reactions which affect all disordered rocksalts, will help improve the performance of these new materials.

Conclusions
We report the synthesis and characterisation of a new disordered rocksalt oxyfluoride compound, Li 2 NiO 2 F, the first based on Ni. The structure and morphology of Li 2 NiO 2 F conforms to other rocksalt materials prepared by mechanochemical ball-milling with 5-10 nm primary crystallite sizes. Li 2 NiO 2 F can store 300 mAh g −1 capacity on charge up to 4.8 V of which 120 mAh g −1 is attributed to irreversible O-loss leading to a discharge capacity of 200 mAh g −1 at an average voltage of 3.2 V. Li 2 NiO 2 F would be a promising Cofree cathode if strategies to suppress O loss and capacity fade over cycling could be successfully implemented.