Electrochemical properties of chromium oxy ﬂ uoride CrO 2 − x F x with 0 ≤ x ≤ 0.3 †

To overcome the limitations of graphite as a negative electrode material for lithium-ion batteries (LIBs), transition metal oxy ﬂ uorides are under active development. In this study, chromium oxy ﬂ uorides CrO 2 − x F x with 0 ≤ x ≤ 0.3 were synthesized under a high-pressure/high-temperature (HP/HT) environment, and their electrochemical properties were examined in a nonaqueous lithium cell. The HP/HT-treated CrO 2 maintained a rutile structure and exhibited a rechargeable capacity ( Q recha ) of over 400 mA h g − 1 at 298 K. The replacement of O 2 − ions with F − ions in CrO 2 was con ﬁ rmed by linear changes in the tetragonal lattice parameters, weaker ferromagnetic interactions between Cr 4+ ions, and elemental mappings of F − ions. The Q recha values of the x > 0 samples at 298 K decreased to 150 – 300 mA h g − 1 because of low electric conductivity in CrO 2 − x F x . However, the Q recha values at 318 K increased to 600 – 700 mA h g − 1 , and the cycle performance over 30 cycles was better than that of the HP/HT-treated CrO 2 sample with no F − substitution. Hence, CrO 2 − x F x was found to be a promising negative electrode material for LIBs, although its cycle stability should be further improved.

Furthermore, O 2− ions in CrO 2 have easily been substituted with F − ions under HP environments above 6 GPa, 2,3 providing dense CrO 2−x F x compounds.0][31][32] To the best of our knowledge, the electrochemical properties of CrO 2−x F x including CrF 2 have never been reported, except for amorphous CrO 2−x compounds with 0 ≤ x ≤ 0.5. 33Here, the CrO 2−x F x samples demonstrated a Q recha of 100-400 mA h g −1 at 298 K and a Q recha of 600-700 mA h g −1 at 318 K.The structural, magnetic, and electrochemical properties as a function of x in CrO 2−x F x are discussed in detail.The HP/HT synthesis and experimental setup were described in detail elsewhere. 34-37

Characterization
The particle morphologies of the synthesized samples were investigated using a scanning electron microscope (SEM; S-3600 N, Hitachi High-Technologies Co., Ltd).Approximately 1 mg of sample, which was attached onto the sample holder with carbon tape, was coated with electrically conducting Au particles (IB-3, Eiko Co., Ltd).To clarify the temperature at which the rutile structure converts to corundum, thermogravimetric/differential thermal analysis (TG/DTA) was conducted on pristine CrO 2 powder (Thermo plus EVO2, TG-DTA 8122, Rigaku Co., Ltd).Specifically, the sample was heated to 1273 K at 5 K min −1 under air flowing at 100 mL min −1 .X-ray diffraction (XRD) measurements were performed at the BL5S2 beamline of the Aichi Synchrotron radiation center.Each sample was inserted into a glass capillary tube with a diameter of 0.3 mm (WHM-Glas Müller Bmbh) to eliminate the effects of selective orientation.XRD patterns of the samples were recorded in the 2θ range between 5 and 90°using Pilatus 100 K detectors.The X-ray wavelength was determined to be 0.799420(3) Å using a silicon standard (NIST 640d).Rietveld analyses were conducted using RIEATN-FP software, 38 and crystal structures were drawn using VESTA software. 39he magnetic susceptibilities (χ) were examined using a superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum Design).χ was recorded in fieldcooling (FC) mode with a magnetic field (H) of 10 kOe while the temperature decreased from 400 K to 5 K. Magnetization (M) vs. H curves were also recorded at 5 K in the H range between −55 and 55 kOe.
For only the x = 0.2 sample, the distribution of Cr, O, and F atoms in the particles were examined using an electron probe microanalyzer (EPMA; JXA-8500F, JEOL Ltd) with an accelerating voltage of 15 kV.This is because as described later, the particle size of CrO 2−x F x drastically changed at x ≥ 0.2.The sample was embedded in epoxy resin and then cut into a rectangle form using a cross-sectional polisher (SM-09010, JEOL Ltd) equipped with Ar + ions.The experimental procedure was similar to the one we used for our recent EPMA analysis of LiCo 0.64 Mn 0.36 O 2 . 37

Electrochemical measurements
Discharge and charge profiles were recorded using a nonaqueous lithium cell.The electrolyte was 1 M LiPF 6 dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) (EC/ DEC = 1/1 by volume, Kishida Chemical Co., Ltd).A mixed electrode consisting of 70 wt% active material, 20 wt% conducting carbon (acetylene black, HS-100, Denka Co., Ltd), and 10 wt% polytetrafluoroethylene (PTFE) was used as the working electrode (diameter = 10 mm), while lithium metal pressed onto a stainless steel plate (diameter = 19 mm) was used as the counter electrode.After being fabricating in the argon-filled glove-box, the lithium cells were operated at a current of 0.1 or 0.5 mA (≃0.1 or 0.5 mA cm −2 ) between 0.02 and 3.0 V.The cells were tested at both 298 and 318 K.The lithium cells with the x = 0 and 0.2 samples were cycled over 30 times at 298 and 318 K, and the rest of lithium cells were cycled over 5 cycles at 298 K. Above cycle tests were performed with a current of 0.1 mA.In addition, the lithium cells with x = 0.05, 0.15, 0.2, and 0.3 were cycled over 50 cycles at 318 K with a current of 0.5 mA.
To clarify the reaction mechanism, ex situ XRD was performed at the BL5S2 beamline of the Aichi Synchrotron radiation center.The lithium cells with the x = 0 and 0.2 samples were discharged down to 0.02 V, and then, in the argon-filled glove-box, the electrodes were removed.Each sample was packed into a glass capillary tube with a diameter of 0.7 mm (WHM-Glas Müller Bmbh), and XRD data were then recorded as for the initial samples.XRD patterns were also taken after the 30-cycle test (x = 0 and 0.2).The X-ray wavelength was determined to be 0.799436(2) Å.The experimental conditions are described in further detail in the results and discussion section.with a smooth surface.The average size of the primary particles, which are aggregates of pristine CrO 2 particles, is approximately 20 μm.

Results and discussion
Fig. 1c shows the TG/DTA curve of the pristine CrO 2 sample to clarify its thermal stability under ambient pressure.The weight of CrO 2 rapidly decreases starting at 766 K, and levels off to an almost constant value above 1073 K.The change in weight (Δw) above 1073 K was ∼−10.0%, which was consistent with the calculated Δw value (=−9.5%)based on the transformation of CrO 2 with a rutile structure into Cr 2 O 3 with a corundum structure: Despite the oxygen loss under ambient pressure, the HP environment stabilized the crystal structure of CrO 2 .Fig. 1d  and e show XRD patterns of the pristine CrO 2 and HP/HTtreated CrO 2 samples, respectively.The pristine CrO 2 sample contains only the rutile phase with lattice parameters of a t = 4.4224(1) Å and c t = 2.9160(1) Å.The corresponding Rietveld analysis results are shown in Fig. S2a.† The crystal structure of the HP/HT-treated CrO 2 sample is also assigned to the rutile structure, although a small amount of β-CrO 2 phase coexists in the sample (indicated by * in Fig. 1e).β-CrO 2 , which considered a HP-form of CrO 2 , adopts a distorted rutile structure with the Pnnm space group. 29According to the Rietveld analysis results shown in Fig. S2b, † the weight fraction of the β-CrO 2 phase was estimated to be 6.5%.The lattice parameters of the major CrO 2 phase were determined to be a t = 4.4182(2) Å and c t = 2.9199(1) Å, which were comparable to those of the pristine CrO 2 sample.discarded to ignore the contributions of the decomposition reaction of the PTFE binder.The voltage of the pristine CrO 2 rapidly drops to ∼0.4 V at the beginning of the discharge reaction and then maintains a constant value at ∼0.2 V.The discharge capacity (Q dis ) reaches 377 mA h g −1 .The subsequent charge curve is quite different from the discharge curve, indicating a large voltage hysteresis of ∼0.8 V.This is a typical characteristic of the conversion reaction, which occurs in various M a O b F c 6-22 and MO 23,24 compounds.
The discharge and charge curves of the HP/HT-treated CrO 2 sample were similar to those for the pristine CrO 2 sample; however, the values of Q dis and charge capacity (Q cha ) differed.Specifically, the maximum Q cha values for the pristine and HP/ HT-treated CrO 2 samples were 360.1 and 427.0 mA h g −1 , respectively.Moreover, as shown in Fig. 2c, the cycling performance of the HP/HT-treated CrO 2 was superior to that of the pristine CrO 2 .In addition, the pristine CrO 2 retained 27.6% of its capacity over 30 cycles, and the HP/HT-treated CrO 2 retained 55.6%.Note that this cycling test lasted for over one month because of the low current density of 0.1 mA cm −2 .This is a severe electrochemical test as the cells were exposed to low voltages below 0.5 V for a long time.The cycle test at such low current densities has also been applied to the preliminary evaluation of LiNi 1/2 Mn 1/2 O 2 40 and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 . 41he complete conversion reaction is represented by where Q theo is calculated to be 1276.3mA h g −1 .Multiple electrons also involve in the electrochemical reactions of other oxides 42 and sulfur compounds. 43,44The observed Q recha of the HP/HT-treated CrO 2 was limited to approximately one-third of Q theo .To clarify the origins of such a significant decrease in capacity, ex situ XRD measurements were conducted in the fully discharged state.Fig. 2d-f show the XRD patterns in the initial state, fully discharged state down to 0.1 V, and after cycling over 30 times, respectively.In the fully discharged state, broad diffraction lines appear at around 2θ = 22.4,32.2, 39.4, and 51.7°, which are each assigned to a diffraction line of CrO with Fm3 ˉm space group (PDF-4+01-073-9520). 45 The cubic lattice parameter (a c ) of the CrO phase was calculated to be ∼4.10Å.The diffraction line at 2θ = 17.3°(d ≃ 2.66 Å) is indexed to the 111 diffraction line of Li 2 O (PDF-4+04-008-3420). 45 Continuous cycling strengthens the intensities of the CrO and Li 2 O phase peaks, thus demonstrating that the initial CrO 2 phase transforms into an amorphous-like phase with continued cycling.The actual electrochemical reaction of CrO 2 is thus represented by where Q theo is calculated to be 638.2mA h g −1 .The formation of the CrO phase instead of the Cr metal is caused by the low operating voltage (∼0.2 V) during the discharge reaction; RuO 2 , for instance, exhibited an operating voltage of ∼0.8 V when the Ru metal was formed in the particles. 232.CrO 2−x F x Fig. 3a-e show the XRD patterns of the CrO 2−x F x samples with x = 0.05, 0.1, 0.15, 0.2, and 0.3.All the XRD patterns were assigned to the rutile structure, similar to the x = 0 samples.The Cr 2 O 3 impurity was observed in the x = 0.2 and 0.3 samples.The a t and c t values, which were determined via Rietveld analyses, are shown in Fig. 3f and g, respectively, which also show reported a t and c t values.3 The lattice parameters a t and c t in this study increase almost monotonically with x, indicating that as expected, O 2− ions are substituted by F − ions.The lattice volume (V) obtained by a t 2 × c t are shown in Fig. S3.† However, these lattice parameters were slightly lower than those of the reported values, 3 particularly at x ≥ 0.2.Recently, anion ordering between O 2− and F − ions appeared in the electron diffraction patterns of CrO 2−x F x with x = 0.1, 0.12, and 0.14.31 In contrast, because the X-ray scattering factors of O 2− and F − ions are similar to each other in this study, such anion ordering is not observed in the XRD measurements.
The SEM images in the inset of Fig. 3a-e clearly show that substituting F − for O 2− altered the particle sizes of CrO 2−x F x .The average size of primary particles is approximately 10 μm for the x ≤ 0.15 samples, whereas it is less than 3 μm for the x = 0.2 and 0.3 samples.The drastic change in the particle size was also observed on the HP/HT studies on Li[Li 1/3 Ti 5/3 ]O 4 . 34nlarged SEM images for the x = 0.2 and 0.3 samples are shown in Fig. S4.† Since CrO 2 exhibited ferromagnetic behavior with a Curie temperature (T C ) of ∼390 K, 3,[29][30][31][32] the effects of substituting F − for O 2− ions were investigated from the aspect of magnetism.Fig. 4a and b show the temperature dependence of χ and χ −1 measured in the FC mode with H = 10 kOe.As the temperature decreases from 400 K, χ of the pristine CrO 2 sample increases from ∼0.5 emu mol −1 to ∼0.9 emu mol −1 and then maintains an almost constant value (∼1.0 emu mol −1 ) below 200 K. Similar behavior is also observed in the χ (or χ −1 ) vs. temperature curves of the x = 0, 0.05, 0.1, 0.15, and 0.2 samples.However, the x = 0.3 sample indicates a slightly different trend, especially below 100 K, wherein χ rapidly drops to ∼0.45 emu mol −1 at 5 K.This decrease in χ is attributed to partial antiferromagnetic Cr 4+ -Cr 3+ interactions produced by the F − ion substitution. 29,30Antiferromagnetic interactions also appear in the M-H curves at 5 K (Fig. 3c).The pristine CrO 2 sample shows soft ferromagnetic behavior with a maximum (saturation) M (M max ) of ∼11 900 emu mol −1 .This M max value corresponds to ∼2.1μ B , which is consistent with the spin-only calculated magnetic moment (=2μ B ) assuming that Cr 4+ ions are in the t 2g 2 (S = 1) state.Substituting F − ions for O 2− ions decreases the M max value, except for the x = 0.2 sample (Fig. 4d).Moreover, the M-H curve for x = 0.3 is a competitive consequence of ferromagnetic interactions between Cr 4+ ions and antiferromagnetic interactions between Cr 4+ and Cr 3+ ions.
To clarify the distributions of O and F atoms in CrO 2−x F x particles, Fig. 5 shows the EPMA mapping results from the x = 0.2 sample for O and F atoms.O and F atoms are distributed throughout the entire particles, but F is slightly localized near the surface of the particles.This means that the F − ions are inhomogeneously distributed inside the particles, although the lattice parameters a t and c t increased almost linearly with x.Cr, O, and F atoms are mapped at the 1 μm scale in Fig. S5.† Fig. 6a-e show the discharge and charge curves of the x = 0.05, x = 0.1, x = 0.15, x = 0.2, and x = 0.3 samples operated at 298 K.For comparison, the discharge and charge curves of CrF 2 are also shown in Fig. 6f.For the x = 0.05 sample, the Q dis and Q cha values during the initial cycle are 399.0 and 364.4 mA h g −1 , respectively, which are slightly lower than those for the HP/HT-treated CrO 2 (x = 0) sample.[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Since the operating voltage of the discharge reaction was close to the discharge cut-off voltage ( = 0.02 V), the discharge reaction of the CrO 2−x F x compounds finished before storing a   large amount of Li ions.Note that CrF 2 exhibits a Q recha of ∼100 mA h g −1 and a capacitor-like voltage profile.
The lithium cells with the x = 0 and 0.2 samples were also cycled at 318 K to clarify the cycle stability at high temperatures.Fig. 6g shows the discharge and charge curves of the x = 0.2 sample at 318 K with a current of 0.1 mA, and Fig. 6h shows the corresponding cycle performance over 30 cycles.The Q dis and Q cha values during the initial cycle are 773.1 and 667.3 mA h g −1 , respectively, which are approximately three times larger than those at 298 K.Although the Q cha value decreases with cycling, it remains at 338.8 mA h g −1 after 30 cycles, thus providing a capacity retention of 50.8%.Note that this capacity retention is superior to that of the HP/HT-treated CrO 2 at 318 K, wherein 40.0% of the capacity was retained (see Fig. S6 †).Furthermore, as shown in Fig. S7, † the capacity fading at 318 K was suppressed by the substitution of F ions, especially at the latter of 50 cycles.According to the ex situ XRD measurements, the CrO phase with a c = 4.09 Å was produced in the fully discharged state (Fig. S8b †).Furthermore, the diffraction line at 2θ ≃ 17.3°was hardly observed, indicating that the substitution of F − ions suppresses the formation of the Li 2 O phase.This is probably due to the strong ionic interaction between the Cr 4+ and F − ions.Since the initial rutile structure of CrO 2−x F x transformed into an amorphous phase after the cycling test (Fig. S8c †), the discharge and charge reactions of CrO 2−x F x proceed between the CrO and amorphous phases, unlike the electrochemical reaction described in eqn (1).
Finally, we discuss the significance of the CrO 2−x F x materials as a negative electrode for LIBs.The operating voltage in the discharge reaction was close to 0.2 V, which is the lowest operating voltage observed for oxyfluorides (Table 1) and is similar to that of the graphite.Furthermore, the theoretical density of CrO 2−x F x according to the XRD data is ∼4.89 g cm −3 , which is two times larger than that of the graphite (≃2.2 g cm −3 ).Since LIBs are the device that functions in a limited space, CrO 2−x F x is a promising negative electrode material for LIBs in terms of volumetric energy density.However, even with the x = 0.2 sample, the cycle stability of CrO 2−x F x is not sufficient for practical LIB applications.According to the electrochemical measurements after the cycle test at 318 K, both Q dis and Q cha values recovered to ∼87% of their initial values at the low current density of ∼0.03 mA cm −2 (Fig. 6g).Hence, the capacity fading in CrO 2−x F x is caused not by a destruction of the core material but by a failure of contact between the active material and the conducting carbon.Further optimizing the electrode mixture and the type of conducting carbon and/or binders could improve the cycle performance of CrO 2−x F x .

Conclusion
The HP/HT-treated CrO 2 maintained the rutile structure even after heating at 1273 K and exhibited a Q recha of 400 mA h g −1 at 298 K.The operating voltage of the discharge reaction was close to 0.2 V, which is the lowest reported operating voltage among oxyfluoride materials such as FeO x F 2−x and BiO x F 3-2x .F − ions successfully replaced the O 2− ions in CrO 2 under the HP/HT environment, providing a linear increase in the lattice parameters a t and c t and weakening the ferromagnetic interaction between Cr 4+ ions.The Q recha values of the x > 0 samples decreased to 150-300 mA h g −1 at 298 K but increased to more than 600 mA h g −1 at 318 K.The cycle performance of x = 0.2 at 318 K was superior to that of the HP/HT-treated CrO 2 sample, suggesting that the substitution of F − ions stabilized the crystal lattice during the discharge and charge reactions at high temperatures.The electrochemical properties of CrO 2−x F x are promising as an alternative of graphite, the most popular negative electrode material in LIBs.Furthermore, the present results provide a new direction for synthesizing (lithium) oxyfluoride materials with a large volumetric energy density.

3. 1 .
Fig. 1a and b show SEM images of pristine CrO 2 and HP/HTtreated CrO 2 samples.Particles of the pristine CrO 2 exhibit a flake-like morphology with a lateral size of 5-50 μm and a thickness of ∼2 μm.These flake-like particles consisted of numerous nanoscale particles, as shown in Fig. S1.† Particles of HP/HT-treated CrO 2 exhibit a non-uniform morphology Fig. 2a and b show the discharge and charge curves of the pristine CrO 2 and HP/HT-treated CrO 2 samples, respectively, operating at 298 K.Each discharge curve for the first cycle was

Fig. 1
Fig. 1 SEM images of the (a) pristine CrO 2 and (b) HP/HT-treated CrO 2 samples.(c) The TG/DTA curve of the pristine CrO 2 sample.Synchrotron XRD patterns of the (d) pristine CrO 2 and (e) HP/HT-treated CrO 2 samples.The * symbol indicates the diffraction line from the β-CrO 2 phase.

Fig. 2
Fig. 2 Discharge and charge curves of the (a) pristine CrO 2 and (b) HP/ HT-treated CrO 2 samples operated at 0.1 mA and 298 K. (c) Cycling performance of the two CrO 2 samples at 298 K. Ex situ XRD patterns of the HP/HT-treated CrO 2 samples: (d) initial state, (e) fully discharged state at 0.1 V, and (f ) after cycling over 30 times.Inset: SEM image of the sample after cycling.

Fig. 3
Fig. 3 Synchrotron XRD patterns of the CrO 2−x F x samples with (a) x = 0.05, (b) x = 0.1, (c) x = 0.15, (d) x = 0.2, and (e) x = 0.3.An SEM image of each sample is shown in the inset.Diffraction lines from the Cr 2 O 3 phase are indicated by circles.Lattice parameters (f ) a t and (g) c t as a function of x in CrO 2−x F x , including the a t and c t values reported in ref. 2.

Fig. 5
Fig. 5 EPMA results from the x = 0.2 sample: mappings of (a) O and (b) F and (c) overlapping mapping of O and F atoms.(d) Corresponding SEM image.Scale bars: 2 μm scale.

Fig. 6
Fig. 6 Discharge and charge curves of the CrO 2−x F x samples with (a) x = 0.05, (b) x = 0.1, (c) x = 0.15, (d) x = 0.2, (e) x = 0.3, and (f ) CrF 2 operated at 298 K. (g) Discharge and charge curves of the x = 0.2 sample operated at 318 K. (h) Cycling performance of the x = 0.2 sample at 318 K.The blue lines in (c) indicate the discharge and charge curves at a low current density of ∼0.03 mA cm −2 after the cycle test.