Deinsertion and Concomitant Na+

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Reversible K+ Insertion/Deinsertion and Concomitant Na+ Redistribution in P’3-Na0.52CrO2 for High-Performance Potassium-Ion Battery Cathodes Nirmalesh Naveen, Woon Bae Park, Su Cheol Han, Satendra Pal Singh, Young Hwa Jung, Docheon Ahn, Kee-Sun Sohn, and Myoungho Pyo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05329 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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Chemistry of Materials

Reversible K+ Insertion/Deinsertion and Concomitant Na+ Redistribution in P’3-Na0.52CrO2 for High-Performance Potassium-Ion Battery Cathodes Nirmalesh Naveen a Woon Bae Park,b Su Cheol Han, a Satendra Pal Singh, b Young Hwa Jung,c Docheon Ahn, c Kee-Sun Sohn,*, b and Myoungho Pyo*, a a

Department of Printed Electronics Engineering, Sunchon National University, Chonnam 57922, Republic of Korea

b

Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Republic of Korea c

Beamline Division, Pohang Accelerator Laboratory (PAL), Pohang 37673, Republic of Korea

ABSTRACT: P’3-type Na0.52CrO2 is proposed as a viable cathode material for potassium-ion batteries (KIBs). In-situ generated title compound during the first charge of O3-NaCrO2 in K+-containing electrolytes can reversibly accommodate 0.35 K+-ions with no interference with Na+. In addition to the sequential interlayer slippage that occurs with Na+-insertion, K+insertion into Na0.52CrO2 induces a sudden phase separation, which ultimately results in a biphasic structure when fully discharged (K+-free O3-NaCrO2 and K+-rich P3-K0.6Na0.17CrO2). A reversible transition between monophasic (Na0.52CrO2) and biphasic states during repeated K+-insertion/deinsertion is also maintained, which contributes to superior electrochemical properties of the title compound when used as a KIB cathode. Na0.52CrO2 delivers a specific capacity of 88 mAh·g-1 with an average discharge potential of 2.95 V vs. K/K+. This high level of energy density (260 Wh·kg-1 at 0.05 C) is not substantially decreased at fast C-rates (195 Wh·kg-1 at 5C). When cycled at 2C, the 1st reversible capacity of 77 mAh·g-1 gradually decreases to 52 mAhg-1 during initial 20 cycles, but no further capacity fading is observed for subsequent cycles (51 mAhg-1 after 200 cycles). Density-functional-theory computation reveals that the rearrangement of Na+ is an energetically favored process rather than a homogeneous distribution of Na+/K+.

INTRODUCTION Potassium-ion batteries (KIBs) have recently begun to receive attention from the battery community because sodium-ion batteries (SIBs), which have been a part of the intense quest for next-generation batteries over the past several years, now face the difficulty of commercialization.[1-3] The main problem is a lack of reliable anodes such as graphite for use in lithium-ion batteries (LIBs).[4,5] In contrast to the limited degree of Na+-intercalation into graphite,[6,7] however, reversible intercalation/deintercalation of K+ into/out of graphite has recently been demonstrated.[8-11] The weaker solvation and/or the higher binding energy of K+, compared with that of Na+, were considered to produce the graphite intercalation compound KC8. The availability of a reliable graphite anode in KIBs provides a significant advantage to the realization of KIBs operating via a K+ shuttle-cock mechanism. Despite a relatively short period of research, various organic/inorganic materials have already been proposed as possible KIB cathodes. These include Prussian blue homo-

logues,[12,13] amorphous FeSO4,[14] organic polycyclic compounds,[15,16] and crystalline compounds (FeSO4F,[17] KTi2(PO4)3,[18] K3V2(PO4)3,[19] KxMnO2,[20,21] [22] [23] K0.7Fe1/2Mn1/2O2, and KxCoO2 ). Most of these have shown reasonable capacities of between 60 and 100 mAh·g-1 with an average potential of ca. 2.0 ~ 3.0 V vs. K/K+. The reversible insertion/deinsertion of relatively large K+ ions, however, seemed to deteriorate the electrochemical stability and the high-rate performance, which became more distinct in crystalline inorganic materials. For example, Ceder et al. have reported the use of P3K0.5MnO2 as a KIB cathode, which delivers a reversible capacity of ca. 95 mAh·g-1 at a current density of 20 mA·g-1 (0.08C).[21] This value, however, was continuously decreased to ca. 65 mAh·g-1 after 50 charge/discharge (C/D) cycles and sharply dropped to 38 mAh·g-1 at high C-rates (300 mA·g-1, 1.2C). The relatively inferior cyclability and rate-capability of KIB cathodes, by comparison with LIB and SIB cathodes, could likely be ascribed to the requirement of large dimensional changes with the inser-

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tion/deinsertion of K+-ions, which is more difficult in a robust crystalline framework. Electrochemical K+-insertion into a SIB cathode was also studied by Barpanda et al.[24] P2-Na0.84CoO2 was first charged for de-sodiation in K+-electrolytes and followed by a reversible potassiation/de-potassiation process. After multiple C/D cycles, a complete phase transition from Na0.84CoO2 to K0.5CoO2 was claimed. The compound delivered a reversible capacity of ca. 80 mAh·g-1 at 0.1C, which was decreased to ca. 55 mAh·g-1 after 50 C/D cycles. The capacity was also substantially decreased to ca. 52 mAh·g-1 at 1C. Although the electrochemical properties of P2-Na0.84CoO2 as a KIB cathode were not comparable to those of P2-NaxCoO2 as a SIB cathode,[25] this work first revealed that SIB cathodes can be utilized for reversible K+-insertion/deinsertion without interfering with the Na+ions released during the 1st charge.

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insertion also induces a decrease in the diffusion length of K+ in a solid state, which contributes to the high rateperformance. These interesting behaviors are fully characterized, and the excellent electrochemical properties are described. The presence of two phases is clearly evidenced by electrochemical observations and ensuing structural analysis. In addition, density-functional-theory (DFT) computation is performed to validate that the formation of two phases is a thermodynamically favored process.

EXPERIMENTAL SECTION Materials and Characterization: O3-type NaCrO2 was prepared via a conventional solid state method. Asreceived Na2CO3 (5 wt.% excess, anhydrous 99.99%, Sigma-Aldrich) and Cr2O3 (99.9%, Sigma-Aldrich) were mixed and homogenized using agate mortar. The mixture was pelletized and transferred to an alumina tube furnace using an alumina crucible. The temperature was raised to 900 °C at a 5 °C·min-1 ramping rate and held for 5 h under a continuous flow of Ar. The sample was allowed to naturally cool and was immediately transferred to an Ar-filled glove box for storage until further analysis.

Figure 1. Schematic diagram depicting two possible routes + for reversible K -insertion/deinsertion in P’3-type Na0.52CrO2. + + Rather than the homogeneous Na /K -distribution (route A), + + K -insertion into Na0.52CrO2 results in the formation of Na + replenished and K -rich phases (route B), which contributes to the excellent rate-capability and stability of Na0.52CrO2 + during repeated K -insertion/deinsertion. Small spheres + (blue) represent Na ions. Arrows indicate the direction of + contraction/expansion along the c-axis with K -insertion.

The crystal structure of as-made and C/D-cycled samples were analyzed using an X-ray powder diffractometer (Rigaku ULTIMA 4) equipped with Cu Kα radiation (λ = 1.5406 Å). The elemental compositions were determined via ICP-AES (Varian 720ES) and EDX combined with FESEM (JEOL JSM-7100F). The chemical state and composition of the samples was examined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher X-ray photoelectron spectrometer, Al Kα X-ray source). The SAED patterns were obtained using high-resolution transmission electron microscopy (JEOL JEM-2100F) operated at 200 kV. For the diffraction patterns of a single-crystalline feature, the SAED were explicitly acquired along the periphery (~20 nm) of the particles. Structural evolution during C/D was monitored by collecting in-situ XRD patterns from a synchrotron X-ray source at the Pohang Accelerator Laboratory (beamline 3D, Korea). The data were collected on an MAR345 image plate with an incident wavelength of 1.0332 Å (12 keV).

Herein, we describe the K+-insertion/deinsertion behavior in P’3-Na0.52CrO2 as a promising KIB cathode. The reversible phase transition and electrochemical properties of P’3Na0.52CrO2 during C/D in K+-electrolytes are described. In contrast to the previous work,[24] P’3-Na0.52CrO2 possesses unique behavior in Na+/K+ distribution, which is believed to contribute to an excellent level of both rate-capability and cyclic stability. Rather than the formation of a monophasic structure with homogeneous Na+/K+ distribution, K+-insertion into Na0.52CrO2 induces the formation of two phases (Figure 1). The Na+-replenished domain, which does not undergo relatively large dimensional changes by K+-insertion/deinsertion, provides structural stability to Na0.52CrO2, which translates to electrochemical stability. The simultaneous migration of Na+ with K+-

Electrochemical Measurements: Electrodes were prepared by mixing as-synthesized NaCrO2 (80 wt.%), AB (10 wt.%) and PVdF (10 wt.%) in an Ar-filled glove box. An electrode film coated on Al foil was punched, weighed (ca. 2 mg·cm-2), and assembled into a 2032 coin-cell using a glass filter (Whatman, USA) and an appropriate metallic anode for a 2-electrode half-cell configuration. For lithium and sodium cells, LiPF6 (1 M) and NaPF6 (1 M) in EC/DEC (1:1, v/v) were used as electrolytes, respectively. For potassium cells, an electrolyte containing KFSI (1 M, Solvionic, France) in EC/DEC was utilized. The cell for insitu XRD measurements was specifically constructed with an aperture of 3 mm for beam passage and sealed with a Kapton tape. The electrochemical tests were performed on a battery testing station (WBCS 3000, WonATech)

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within a potential range of 2.0 ~ 3.6 V vs. K/K+. GITT was performed at a current pulse of 5.6 mA·g-1 for 30 min with subsequent relaxation for 2 h. The capacities were normalized using the weight percentage of NaCrO2 and the C-rate was calculated by equating 1C = 250 mA·h·g-1. DFT Calculation: A generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE)[40-43] was employed as an exchange correlation potential in the Vienna ab initio simulation package (VASP5.3).[44,45] A projector augmented wave (PAW) potential[46] was adopted along with a cutoff energy of 500 eV and a total energy accuracy of 10-5 eV. The DFT+U method[47-49] was adopted to reasonably deal with the localized 3d electrons of Cr with the on-site interaction and exchange parameter (Ueff = U − J) of 3.5 eV.[48] The positions of all atoms and lattice parameters were fully relaxed until the atomic forces converged to 0.02 eV·Å−1. Spin polarizations were considered in the structural optimization. The supercell models, 2×2×1 for O3, 1×2×3 for O’3, 2×2×1 for P3, and 1×2×3 for P’3, was chosen for model sizes and shapes that were similar. The k-mesh was set to 6×6×2 for all structures using the Monkhorst–Pack scheme.[50]

originate from the oxidation of Cr3+ and concomitant Na+ extraction. Hereafter, the composition after the 1st charge will be denoted as Na0.52CrO2 and the composition analysis for validation is provided below. The subsequent discharge, however, showed an immediate potential drop, suggesting that Li+-insertion led to a contraction of the interlayer spacing, and, thereby, to Cr6+ migration to an interstitial tetrahedral site in the lithium layer (i.e., hindrance of Li+-diffusion).

RESULTS AND DISCUSSION Unique Behaviors of P’3-Na0.52CrO2 during K+insertion/deinsertion. ACrO2 (A = alkaline metal) is the only transition metal oxide known for all Li+, Na+, and K+, m (O3-type).[26] Howwith the identical space group of R ever, the slight difference in lattice dimensions that is due to the different ionic sizes of alkaline metals imposes a great impact on electrochemical properties and/or ambient stability. O3-LiCrO2 is well-known to be electrochemically inactive for Li+-insertion/deinsertion.[27-29] The Cr6+ ions formed via a disproportionation reaction (3Cr4+ → 2Cr3+ + Cr6+) migrate to the interstitial tetrahedral site in the lithium layer,[30] which results in significant suppression of Li+-deinsertion from the 1st charge and negligible capacities during the subsequent C/D cycles.[27] A limited degree of Li+-extraction implies that O3-LiCrO2 is an intrinsically inappropriate template for reversible K+insertion/deinsertion. The direct use of O3-KCrO2 as a KIB cathode could avoid this problem. O3-KCrO2, however, requires stringent synthetic conditions, which complicates scale-up.[31,32] Furthermore, as-made KCrO2 is extremely moisture-sensitive and easily converted to K0.5CrO2 and K2CrO4.[33,34] Based on these factors, the utilization of O3-NaCrO2 for reversible K+insertion/deinsertion appears to be the most plausible approach for the development of a viable KIB cathode in ACrO2. O3-NaCrO2 was first redox-switched in ethylene carbonate:diethyl carbonate (EC:DEC) of 1.0 M LiPF6 between 2.0 and 3.8 V vs. Li/Li+ (Figure 2A). NaCrO2 delivered a specific capacity of ca. 119 mAh·g-1 during the 1st charge, corresponding to the formation of Na0.52CrO2, which was the case when all the charge was assumed to

Figure 2. C/D profiles of O3-NaCrO2 in EC:DEC of 1.0 M (A) LiPF6, (B) NaPF6, and (C) KFSI at 0.05C. Insets show the st st dQ/dV curves for the 1 charge-discharge in (A) and the 1 nd discharge-2 charge in (B) and (C). From the dQ/dV curves, + + it is evident that the insertion/deinsertion of Na and K in Na0.52CrO2 induces different phase transitions.

Figure 2A also indicates that the involvement of Na+-ions was negligible, because the abrupt decrease in the 1st discharge capacity could be alleviated by the co-insertion of Na+-ions released during the 1st charge. On the other hand, NaCrO2 in EC:DEC of 1.0 M NaPF6 showed reversible Na+insertion/deinsertion behaviors as expected (Figure 2B). The 1st charge capacity was similar to that in LiPF6 (117 mAh·g-1), and subsequent C/D showed typical potential profiles that corresponded to a reversible phase transition:

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monoclinic P’3 ⇄ monoclinic O’3 ⇄ rhombohedral O3.[2729] When cycled in EC:DEC of 1.0 M potassium bis(fluorosulfonyl)imide (KFSI), NaCrO2 also revealed reversible K+-insertion/deinsertion behaviors (Figure 2C). The formation of Na0.52CrO2 after the 1st charge (121 mAh·g-1) in KFSI was invariant, but the subsequent C/D profiles were different from those in NaPF6. The potential decrease during the initial discharge was somewhat steeper, and the following plateau region was noticeably shorter. Furthermore, the discharge profile during the later discharge stage showed a multiple potential step and finally reached the discharge capacity of 88 mAh·g-1.

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(poly(vinylidene fluoride), PVdF) and conducting carbon (acetylene black, AB). The XRD pattern of as-made NaCrO2 powder corresponded to that of JCPDS 088-0720 and showed two characteristic peaks of O3-type layered transition metal oxides at 2θ of 16.63 and 41.69o ((003)hex and (104)hex peaks, respectively). After the 1st charge, the (003)hex peak moved to (001)mon located at lower 2θ and the (104)hex peak intensity was substantially decreased to (111)mon, indicating the formation of P’3-Na0.52CrO2.[28,35,36]

The K+-insertion/deinsertion with no involvement of Na+ can be confirmed by comparing the C/D profiles of Na0.52CrO2 that was cycled in fresh KFSI electrolyte (Figure S1, Supporting Information). The C/D profile was identical to that shown in Figure 2C with only marginal differences in capacity values. The K-metal counter electrode after full charge also showed a negligible level of Na content on the surface in energy dispersive X-ray (EDX) spectroscopic study, verifying again that the Na+ions in KFSI did not significantly affect the K+insertion/deinsertion behavior in Na0.52CrO2 (Figure S2, Supporting Information). The chemical compositions after K+-insertion into Na0.52CrO2 can be inferred from a discharge capacity of 88 mAh·g-1 (Figure 2C). Since only K+-ions are inserted into Na0.52CrO2, the total composition after discharge is believed to be K0.35Na0.52CrO2. The field emission scanning microscope (FESEM) images and corresponding composition analysis via energy dispersive X-ray spectroscopy (EDX) also revealed the formation of K0.35Na0.52CrO2 (Figure S3, Supporting Information). The relative atomic percentages of K, Na, and Cr were 18.2, 29.3, and 52.5 %, respectively (K0.35Na0.56CrO2), which approximated that of K0.35Na0.52CrO2. The elemental compositions obtained from Inductively-coupled plasma/atomic emission spectroscopy (ICP-AES) also coincided with the above results, showing K/Na/Cr at% of 0.32/0.51/1.00. The total composition after discharge of Na0.52CrO2 in KFSI, therefore, will be denoted as K0.35Na0.52CrO2. The EDX results for as-made and 1st charged compounds also approximated NaCrO2 and Na0.52CrO2 (Na1.04CrO2 and Na0.51CrO2, respectively). The chemical compositions obtained from XPS survey scans for pristine, charged and discharged samples also agreed well with the ICP and EDX results (Figure S4A, Supporting Information). In addition, high-resolution XPS spectra on Cr 2p of the corresponding samples disclosed the typical reversible transition between Cr4+ and Cr3+ states during C/D (Figure S4B, Supporting Information).[28,35,36] In order to understand the structural change caused by K+-insertion/deinsertion in Na0.52CrO2, ex-situ X-ray diffraction (XRD) patterns of charged and discharged samples were compared (Figure 3A). Except for as-made NaCrO2 powder, the diffraction patterns were obtained for thin films (ca. 20 μm) on Al, which contained binder

Figure 3. (A) XRD patterns of as-made NaCrO2 powder and C/D-cycled films. (B) Expanded view of low-angle peaks in comparison with the standard peaks of P3-K0.6CrO2 (JCPDS 028-0745), O3-KCrO2 (JCPDS 028-0743), P’3-Na0.52CrO2 (ICSD 291601), and O3-NaCrO2 (JCPDS 088-0720). The 1st + discharge-a indicates the K -inserted Na0.52CrO2 film in a fresh KFSI electrolyte. Asterisks in (A) are the peaks matched with P3-K0.6CrO2. SAED patterns along with the indices of the diffraction spots near the center spot obtained after in m space group, (C2) dexing with (C1) NaCrO2 in the R Na0.52CrO2 in the C2/m space group, and (C3) NaCrO2 and m and R3m space groups, respecKxCrO2 (x = 0.6) in the R tively. The presence of two phases gives rise to diffuse diffraction spots in C3. The spots due to NaCrO2 (marked by blue arrow) and KxCrO2 (marked by pink arrow) are close, but the latter lies slightly farther from the center spot.

On the other hand, it was interesting that the XRD pattern of a subsequently discharged sample indicated the formation of two phases after K+-insertion. While the two prominent (003)hex and (104)hex peaks of NaCrO2 were revived at the same 2θ positions, a series of peaks newly appeared, as indicated by the asterisks in Figure 3A. The two phases present in the discharged state were merged into monophasic Na0.52CrO2 after the 2nd charge, and the

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phase transitions between monophasic and biphasic states were maintained during repeated C/D cycling, implying that these unique structural evolutions are not a transient phenomenon (see 2nd charge, 10th discharge, and 11th charge in Figure 3A). Note also that XRD and XPS measurements on a discharged sample after 100 C/D also showed the preservation of monophasic/biphasic transitions and a high concentration of Na+ (Figure S5, Supporting Information).

K0.6CrO2. Rather, it was composed of a small amount of Na+ within the K0.6CrO2 domain, which contributed to the additional contraction of an interslab distance.

To identify the new phase, the peak positions corresponding to an interplanar spacing were closely examined using the standard patterns (Figure 3B). It was clear that, while the peak position located at high 2θ (16.68o) in a discharged sample coincided with the (003)hex peak position of O3-NaCrO2, the peak position located at low 2θ (14.1o) approximated that of the (003)hex of P3-K0.6CrO2. All the other new peaks (denoted by asterisks in Figure 3A) were also reasonably matched with the diffraction pattern of P3-K0.6CrO2 (JCPDS 028-0745, isostructural with Na0.6CoO2 in R3m symmetry (ICSD 9992)). The new phase, therefore, is likely a K+-rich phase. The formation of O3-NaCrO2 after discharge was also interesting because, as confirmed above, once extracted during the 1st charge of O3-NaCrO2, the involvement of Na+-ions during subsequent C/D processes was negligible. XRD patterns of the sample prepared by discharging Na0.52CrO2 in a fresh KFSI electrolyte also revealed a biphasic formation (see 1st discharge-a in Figure 3B), which again proved that the formation of O3-NaCrO2 could not be ascribed to Na+-insertion. The formation of a biphasic structure in a discharged state was further confirmed via selected area electron diffraction (SAED). While the diffraction patterns of asmade NaCrO2 and the 1st-charged sample (Figure 3C1 and 3C2, respectively) corresponded to an O3-type with a m and to a P’3-type with a space group space group of R of C2/m, the discharged sample showed a conspicuously different feature (Figure 3C3). The dual diffraction pattern, in which each spot was slightly overlapped, was obvious. Though overlapped, however, the patterns matched well with those of O3-NaCrO2 and P3-K0.6CrO2. Le Bail refinement (Figure 4) was applied to the XRD patterns in Figure 3 to evaluate the lattice parameters of as-made NaCrO2, Na0.52CrO2, and a biphasic mixture (Table 1). The XRD patterns were fitted well with O3-NaCrO2, P’3-Na0.52CrO2, and a mixture of O3-NaCrO2 and P3K0.6CrO2 for as-made, 1st charged, and 1st discharged compounds, respectively. Only a slight change from the cell parameters in the standard database was observed with the exception in P3-K0.6CrO2. For example, while the caxis length of NaCrO2 (15.954 Å, JCPDS 088-0720) was marginally changed to 15.925 Å in a biphasic mixture, the c-axis length of K0.6CrO2 (18.84 Å, JCPDS 028-0745) was noticeably contracted to 18.73 Å in a biphasic mixture after refinement. This was understandable because the exact composition of a K+-rich phase could not be

Figure 4. Full pattern Le Bail fits obtained after using (A) the m space group for NaCrO2, (B) the C2/m space group for R m and R3m together for NaCrO2 Na0.52CrO2, and (C) the R and KxCrO2 (x = 0.6), respectively. Experimental, calculated, and difference profiles are shown by the black dots, red line, and blue line, respectively. The vertical tick marks above the difference profile denote the position of Bragg reflections. The peaks marked with an asterisk (*) and a delta (∆) represent the traces of Al and the PVdF/AB phases, respectively.

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Table 1. Lattice parameters of various phases obtained after Le Bail refinement. space group

a

b

c

β

unit cell volume

Rp

Rwp

χ

R3m

2.97169(3)

2.97169(3)

15.9451(3)

-

121.94

20.3

13.1

3.99

C2/m

5.0025(7)

2.8808(3)

5.8523(6)

105.55(1)

81.11

20.4

17.0

8.95

NaCrO2

R3m

2.97160(7)

2.97160(7)

15.9247(6)

-

121.78

13.8

11.3

2.97

K0.6CrO2

R3m

2.9130(2)

2.9130(2)

18.734(2)

-

137.67

13.8

11.3

2.97

compound NaCrO2 (Pristine) Na0.52CrO2 st

(1 charged)

2

biphasic st

(1 discharged)

From the formation of NaCrO2, it is possible to determine the exact composition of a K+-rich phase. Since the discharged sample has a total composition of K0.35Na0.52CrO2 and contains a NaCrO2 phase, the K+-rich phase cannot be defined as K0.6CrO2 with no Na+ incorporated. This phase seems to contain 0.17 Na+ (K0.6Na0.17CrO2), and the biphasic mixture is likely composed of 42 % NaCrO2 and 58 % K0.6Na0.17CrO2. The surge of K+-ions into P’3-Na0.52CrO2 during discharge, therefore, seemed to stimulate the migration of Na+-ions and to induce the biphasic formation of O3-NaCrO2 and K+-rich P3K0.6Na0.17CrO2. This unique K+ insertion/deinsertion mechanism could be beneficial in terms of cyclability and rate-performance. K+-insertion makes only a certain fraction of Na0.52CrO2 severely expand (Δcinterslab = +0.61 Å for Na0.52CrO2 → K0.6Na0.17CrO2, based on the cell parameters in Table 1), and the remainder is subjected to the contraction (Δcinterslab = -0.33 Å for Na0.52CrO2 → NaCrO2, based on the cell parameters in Table 1). Since the homogeneous K+/Na+ distribution in K0.35Na0.52CrO2 may induce an overall increase in Δcinterslab throughout the entire crystalline domain, the Na+replenished region is expected to provide cyclic stability to Na0.52CrO2. The rearrangement of Na+ also decreases the diffusion length of K+ in a solid state, which can result in an increase of rate-performance. Superior Electrochemical Properties of P’3-Na0.52CrO2 as KIB Cathodes. As expected, the rate capability of Na0.52CrO2 for K+-insertion was excellent when compared with the previously reported inorganic KIB cathodes (Figure 5A).[18-24] When charged at a constant rate of 0.05C, the shape of the discharge curves were identical, but the potentials were shifted downward with increases in the C-rates, which indicates that ‘IR drop’ is the domi-

Figure 5. Electrochemical performance of P’3-Na0.52CrO2 in KFSI. (A) C/D profiles at different discharge rates. The cells were newly prepared for each experiment. The charge was performed at a fixed rate of 0.05C. (B) Ragone plot showing the maintenance of high energy densities at high C-rates. (C) Decrease of capacities with an increase of the C/D rates (same C/D rates). (D) Capacity retention and Coulombic efficiency during 200 cycles at 2C. (E) GITT curve and chemical diffusion coefficients during discharge. Dotted line indi-1 cates sequential current pulses of 5.6 mA·g for 30 min and subsequent rest periods for 2 hours.

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nating factor in a downward shift, rather than the sluggishness of either charge transfer or diffusion.). A discharge capacity of 88 mAh·g-1 at 0.05C was reduced to 72 mAh·g-1 at 5C, showing only an 18% decrease for a 100fold increase in the C-rate. It is also worth noting that the discharge profiles were obtained using different coin-cells, which suggested the reproducibility of the electrochemical properties in Figure 5A. As a result, the high levels of the energy-density (ED) features of Na0.52CrO2 when used as a KIB cathode were retained during a fast C/D cycle (Figure 5B). The greatest ED of 258 Wh·kg-1 at a power density (PD) of 36.6 W·kg-1 (0.05C) was decreased to 194 Wh·kg-1 at 3311 W·kg-1 (5C), indicating thatNa0.52CrO2 can be a promising KIB cathode possessing high levels of both ED and PD. A fast charge also did not aggravate the high ED/PD feature of Na0.52CrO2 (Figure 5C). When the C/D was performed at the same C-rates, the discharge capacities were stepwise reduced with an increase in the C-rates, but maintained a substantial level of 55 and 50 mAh·g-1 at 2 and 5C, respectively. These reversible capacities during a fast C/D cycle were relatively high, given the consideration that Na0.52CrO2 delivered a discharge capacity of 65 mAh·g-1 at 0.05C after reaching a steady-state (the capacity value after returning to 0.05C, as shown in Figure 5C).The cyclic stability of Na0.52CrO2 was also superior to other KIB cathodes reported thus far (Figure 5D). Though a substantial decrease was observed during the initial 20 cycles at 2C, the capacities became stable afterwards, delivering 55 mAh·g-1 for the remaining 180 C/D cycles. Coulombic efficiencies of ca. 100 % were also retained after the initial 20 C/D cycles. The preservation of the similar voltage profiles during 200 cycles was also evident, indicating again the retention of Na+/K+ rearrangement mechanisms during prolonged C/D (Figure S6, Supporting Information). The noticeable drop in capacities during initial 20 C/D appeared to imply the existence of a certain irreversible process during initial cycles. This could be partial K+-trapping and relatively slow K+diffusion under high C rates, and/or surface side reactions during initial charge.

as-prepared NaCrO2 has a platelet shape and a broad particle size distribution (a few tens of nm to a few μm), the results shown in Figure 5E will be meaningful only in estimating the variation in Ds with the depth of discharge (DOD). With the exception of a slight increase in the initial discharge states, the D values continued to decrease with the DOD until a constant potential region at ca. 3.0 V was reached. The low Ds of between 0.14 × 10-14 and 0.45 × 10-14 cm2·s-1 were maintained during discharge of a constant potential. Further discharge, however, resulted in a sharp increase of Ds. Ds that were higher by nearly one order of magnitude (ca. 2.2 × 10-14 cm2·s-1) were obtained in the later stages of discharge. This behavior of Ds according to the DOD is contrasted with the featureless fluctuation of the Ds in K0.5MnO2,[21] which implies that the unique phase transition behavior in Na0.52CrO2 can affect K+-diffusion kinetics. As mentioned above, the excellent electrochemical properties of Na0.52CrO2 as a KIB cathode (rate capability and cyclability in particular) are believed due to the unique phase transition behaviors during K++ insertion/deinsertion. The formation of K -rich K0.6Na0.17CrO2 and NaCrO2 phases at a fully discharged state appears to alleviate the abrupt dimensional change of an entire crystal domain and to decrease the diffusion length of K+, which leads to both excellent cyclic stability and high rate-performance, as shown in Figure 5.

The variation in diffusional kinetics during discharge was also examined via galvanostatic intermittent titration technique (GITT). A current pulse of 5.6 mA·g-1 (0.022C) was applied for 30 min, which was then followed by a rest period of 2 hours. Chemical diffusion coefficients (Ds) were calculated using the following equation D=

∆ ∆

(1)

where τ is the duration of a current pulse (1800 s), nm and S are the number of moles (1.35 × 10-15 mol) and the surface area (5.02 × 10-9 cm2) of a single particle, respectively, Vm is the molar volume of Na0.52CrO2 (24.72 cm3·mol-1), ΔEs is the change in steady-state voltages after a current pulse, and ΔEt is the voltage change during a current pulse.[37] Note that we assumed a spherical particle with a radius of 200 nm for the calculations of nm and S. Because

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Figure 6. In-situ XRD patterns revealing the structural evost nd lution during (A) the 1 discharge and (B) the 2 charge. Inset XRD pattern shows a magnified view of 2θ ranges corresponding to interslab distances. The C/D profiles are also shown as insets with the sequential numbers denoting the points where XRD was measured. For the variation of XRD st patterns during the 1 charge of O3-NaCrO2, see Figure S7 in the Supporting Information.

Structural Evolution during C/D. The continuous structural changes between monophasic Na0.52CrO2 and biphasic K0.6Na0.17CrO2/NaCrO2 during C/D were monitored via in-situ XRD. During the 1st charge in KFSI, O3NaCrO2 showed well-known phase transition behaviors with Na+-deinsertion (Figure S7, Supporting Information).[28,35,36] The continuous structural changes from O3 to O’3+O3, O’3, P’3+O’3, and P’3 were obvious from the variations in the positions and intensities of the characteristic peaks (see the caption of Figure S7). As expected by the ex-situ XRD (Figure 3), Na0.52CrO2 also showed a sequential phase transition to a biphasic structure when discharged in KFSI (Figure 6A). During the initial potential-decline (No. 1 ~ No. 7, black color), the compound retained its P’3-type, with simultaneous contraction along the c-axis and expansion along the abplane. Note a gradual shift of the (001)mon peak to a higher 2θ and of the serial peaks within 36o and 38o ((110)mon, (201)mon, (220)mon, and (11-1)mon) to a lower 2θ with K+insertion. Further discharge induced a transformation of the P’3 phase to an O’3 phase, which resulted in a twophase mixture (constant potential region between large and small potential drops, No. 8 ~ No. 13, red color). The peak positions did not vary within this range. Only the relative intensity of O’3 to P’3 was continuously enhanced, which signified a simple transformation of P’3 to O’3 with K+-insertion. After passing the point of a small potentialdrop (between No. 13 and No. 14), the diffractogram began to show the characteristic peaks of O3-NaCrO2 and P3-K+-rich phases. As a result, the three phases coexisted at this potential plateau (No. 14 ~ No. 16, green color), but the intensities of O’3 gradually diminished with discharge. Note that all the peak positions were also invariant, again signifying the transformation of O’3 to O3 (NaCrO2) and P3 (K+-rich) with K+-insertion. Immediately after the potential plateau region, the O’3-phase completely disappeared and only O3 (NaCrO2) and P3 (K+-rich) remained. The invariance in the peak positions of O3-NaCrO2 during the multi-step potential-decrease (No. 17 ~ No. 20, blue color) was interesting, because it implied that fully Na+occupied NaCrO2 was formed via a sudden phase separation rather than via gradual Na+-accumulation. In contrast, the peaks of a K+-rich phase were continuously moved to a higher 2θ with discharge, which indicated a continuous increase in the K+-content up to K0.6Na0.17CrO2. The in-situ XRD patterns during the subsequent charge showed an exact reversal of behaviors (Figure 6B). A biphasic structure returned to P’3-Na0.52CrO2 through a three-phase region (O3-NaCrO2, P3-K+-rich, and O’3) and

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a two-phase region (O’3 and P’3). No distinctive difference was observed except for the presence of a threephase region for a relatively longer duration (No. 16 ~ No. 10, green color). Theoretical Validation via DFT Computations. DFT calculation was employed to compare the formation energy (Ef) value of a biphasic structure with those of four hypothetical structures consisting of homogeneously dispersed K+/Na+. The biphasic structure was constructed with 42% O3-NaCrO2 and 58% P3-K7/12 Na2/12CrO2 (i.e., K0.58Na0.17CrO2), which approximated the experimentally determined compositions (42% O3-NaCrO2 + 58% P3K0.6Na0.17CrO2 with a total composition of K0.35Na0.52CrO2).The four hypothetical monophasic structures would be plausible after discharge, but have not been observed experimentally: O3-K0.33Na0.5CrO2 (R3m), O’3-K0.33Na0.5CrO2 (C2/m), P3- K0.33Na0.5CrO2 (R3m), and P’3- K0.33Na0.5CrO2 (C2/m). All these hypothetical structures were reasonably relaxed during the course of DFT calculation. The configuration diversity for the random distribution of K+/Na+ ions in the hypothetical structures caused difficulty with calculating the Ef of the entire cases. For example, the possible number of configurations for P’3 amounted to 8,632,720. Therefore, 10 configurations per each hypothetical structure were randomly pinpointed for the calculation of Ef, and the average value was adopted as a representative Ef for each hypothetical structure. Likewise, 10 configurations for P3-K7/12 Na2/12CrO2 in a biphasic structure were randomly chosen and the average Ef value (-4.93 ± 0.07 eV) was combined with the Ef (-6.08 eV) of O3-NaCrO2 to obtain the Ef (-5.41 ± 0.04 eV) of a biphasic structure. Details of the chosen configurations are presented in Table S1.

Figure 7. Calculated values for formation energy per formula unit of a biphasic mixture and four hypothetical monophasic structures. O3/P3: 42% O3-NaCrO2 + 58% P3-K7/12Na2/12CrO2; O3: K4/12Na6/12CrO2 (R3m); O’3: K4/12Na6/12CrO2 (C2/m), P3: K4/12Na6/12CrO2 (R3m); P’3: K4/12Na6/12CrO2 (C2/m). Inset models show the four hypothetical structures with a random dis-

8


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Chemistry of Materials +

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+

tribution of K /Na : (yellowish green) sodium; (violet) potassium; (red) oxygen; (blue) chromium. o

In the DFT calculation, a total energy at 0 K for each constituent element was used as the chemical potential to obtain the formation energy. The calculated chemical potentials were in good agreement with the values in the literatures.[38,39] The calculated Ef values for the four hypothetical monophasic structures and the biphasic structure are compared in Figure 7. All the hypothetical structures exhibited an average Ef within a reasonable range below zero. Figure 7 also shows that, despite configuration diversity, the variance in the calculated Ef was not so great that the selection of 10 configurations for each phase was sufficient to be representative. The Ef for each hypothetical structure (-5.29 ± 0.04, -5.27 ± 0.05, -5.22 ± 0.04, and 5.24 ± 0.03 eV for O3, O’3, P3, and P’3, respectively.) was certainly greater than that of the biphasic structure (-5.41 ± 0.04 eV). This coincided with the experimental observations, which indicated that the biphasic structure (i.e., phase separation during discharge) is energetically more favorable than the hypothetical monophasic structures with a random distribution of K+/Na+ ions.

CONCLUSION In conclusion, we report that P’3-Na0.52CrO2 can be used as a promising cathode in KIBs. When discharged, it accommodates 0.35 K+-ions, which results in K0.35Na0.52CrO2 (88 mAh·g-1). However, the more interesting feature of Na0.52CrO2 as a KIB cathode is found in the formation of a biphasic structure with K+-insertion. The reversible phase transition between biphasic NaCrO2/K0.6Na0.17CrO2 in the discharged state and monophasic Na0.52CrO2 in the charged state reduces the diffusion length of K+ and mitigates the overall dimensional change, which eventually contributes to high-rate performance and cyclic stability, respectively. As a result, P’3-Na0.52CrO2 retains a high ED of 194 Wh·kg-1 at 5C in comparison to an ED of 258 Wh·kg-1 at 0.05C. The repeated C/D cycles shows virtually no capacity loss after the initial 20 cycles. DFT calculation also supports the formation of a biphasic structure as an energetically favored process.

ASSOCIATED CONTENT The supporting information is available free of charge on the ACS publications website at DOI: XXXXX. + CD profiles of Na0.52CrO2 in fresh K -electrolyte, SEM and EDX images of fresh and cycled potassium metal, SEM images with elemental analysis, XPS measurements of O3+ NaCrO2, Na0.52CrO2 and K inserted Na0.52CrO2, XRD and XPS survey spectra of an electrode cycled after 100 C/D, voltage profiles recorded at various cycle numbers, In-situ diffraction patterns of partially desodiated NaCrO2 and configurations selected for theoretical calculations.

AUTHOR INFORMATION Corresponding Author

Author Contributions N. Naveen and W. B. Park equally contributed to this work.

Funding Sources The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future (2015M3D1A1069710). This research was also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A6A1030419).

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*E-mail: [email protected]; [email protected]

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Table of contents P’3-Na0.52CrO2 as a promising KIB cathode is presented. The compound undergoes phase separation to NaCrO2 + + and a K -rich domain (K0.6Na0.17CrO2) when fully discharged in K -electrolytes. Subsequent charge recovers the pris+ + + tine Na0.52CrO2, with no noticeable involvement of Na in K -insertion/deinsertion. The Na -replenished domain + contributes to the excellent cyclability, and the shortened K -diffusion length enhances the rate-performance.


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Deinsertion and Concomitant Na+

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