Electrochemical Performance of Large-Grained NaCrO2 Cathode

Jun 18, 2019 - (10−13) Besides, this material also shows excellent thermal stability. Dahn's early work reported that Na0.5CrO2 obtained in a charge...
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Electrochemical Performance of Large-Grained NaCrO2 Cathode Materials for Na-Ion Batteries Synthesized by Decomposition of Na2Cr2O7·2H2O Yong Wang,†,§ Wei Li,†,§ Guorong Hu,† Zhongdong Peng,† Yanbing Cao,† Hongcai Gao,‡ Ke Du,*,† and John B. Goodenough*,‡ †

School of Metallurgy and Environment, Central South University, Changsha 410083, P. R. China Texas Materials Institute and Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States

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‡

S Supporting Information *

ABSTRACT: The solid-state reaction has been widely employed as the standard procedure to prepare oxide cathode materials for sodium-ion batteries. However, it involves multiple steps and consumes much energy. In this work, we report a facile method to synthesize a large-grained O3−NaCrO2 cathode by directly reducing sodium dichromate dihydrate (Na2Cr2O7· 2H2O) under a hydrogen atmosphere. Owing to its unique large particle morphology, the as-prepared NaCrO2 exhibits a high tap density of 2.55 g cm−3. The compact NaCrO2 shows excellent electrochemical performance with a high reversible capacity of 123 mAh g−1 at 0.1C, a high capacity retention of 88.2% after 500 cycles at 2C, and an outstanding rate capability of 68 mAh g−1 at 20C. The performance is attributed to a stable structure from the distinctive morphology with small specific surface area to suppress interfacial side reactions and rapid Na-ion diffusion channels with a highly (110)-oriented crystal structure. Ex situ X-ray diffraction and cyclic voltammetry tests demonstrate the consecutive and reversible phase transition mechanism with facile Na+ migration. Importantly, the obtained cathode material exhibits an excellent performance in sodium-ion full cells with hard carbon as the anode.

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cathode materials of lithium batteries, LiCoO2 and LiFePO4,14 demonstrating the safety performance of the cathode material for SIBs. Nevertheless, the NaCrO2 electrodes always suffer from rapid capacity fading during cycling.15 To overcome this obstacle, some accomplishments have been made including cation substitution,16,17 carbon coating,18,19 and modification of the electrolyte.11 Hagiwara’s research group found that an extraordinary cycling stability with high Coulombic efficiency and outstanding rate capability can be easily achieved with an ionic liquid as the electrolyte.11 However, the ionic liquid is expensive and unsuitable for large-scale applications. Our latest work demonstrated that Ti substitution could delay the undesired O3−P3 phase transition and enlarge the Na-ion migration channels, thus significantly enhancing the cycling and rate performance of NaCrO2.16 However, the cycling performance of Ti-doped NaCrO 2 still needs to be improved.16,17 Carbon coating has been reported as an effective strategy to improve the electrochemical properties of the NaCrO2 cathode since carbon coating can improve the

INTRODUCTION Due to a high energy density and long cycle life, lithium-ion batteries (LIBs) have achieved great success in the fields of portable equipment, electric vehicles, and power grid storage.1−3 However, the limited resources and the increasing cost of lithium will make it difficult to meet the growing demand for energy storage, especially for low-cost and largescale energy storage systems.4,5 On this account, there is a potential opportunity for the application of sodium-ion batteries (SIBs) due to the abundance and wide distribution of sodium resources.6 SIBs have been extensively studied since 2010, especially for layered oxide cathode materials.7−9 Among a multitude of layered oxides NaxMO2 (x ≤ 1, M = Ni, Co, Mn, Fe, V, Ti, Cr, Cu), O3−NaCrO2 is a promising cathode that can exhibit a reversible capacity of 120 mAh g−1 (Na1−xCrO2, 0 ≤ x ≤ 0.48) with a nearly flat voltage profile at 3 V (vs Na+/Na), through a series of consecutive phase transitions (from original hexagonal O3 to monoclinic O′3 and finally to monoclinic P3).10−13 Besides, this material also shows excellent thermal stability. Dahn’s early work reported that Na0.5CrO2 obtained in a charged state with a Na-based nonaqueous electrolyte exhibited better thermal stability (∼350 °C) than commercial © XXXX American Chemical Society

Received: April 12, 2019 Revised: June 18, 2019 Published: June 18, 2019 A

DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. (a) XRD patterns of lg-NCO synthesized in various temperatures. (b) Cr 2p XPS spectra of lg-NCO synthesized at 900 °C. (c) Results of the Rietveld refinement on a powder X-ray diffraction pattern of lg-NCO synthesized at 900 °C. Schematic of the O3−NaCrO2 crystal structure is shown in the inset. (d) Comparison of XRD patterns of lg-NCO and s-NCO.

unique large NaCrO2 particles is as high as 2.55 g cm3, which is comparable to that of commercial lithium cobalt oxide.

electrical conductivity and suppress the side reactions effectively. Ding’s group studied a carbon-coated NaCrO2 synthesized by the solid-state reaction with citric acid as the source of carbon; the discharge capacity only decreased from 118 to 110 mAh g−1 after 40 cycles at a current density of 5 mA g−1.18 Yu et al. reported a carbon-coated NaCrO2 via carbonization of pitch that exhibited improved cycle performance (90% capacity retention after 300 cycles at a charge/ discharge rate of 20 mA g−1) and excellent rate capability (99 mAh g−1, 150C).19 Nevertheless, carbon coating introduces an electrochemically inactive component with low density and decreases the particle size of NaCrO2, which reduces the volume energy density of the battery. Herein, we demonstrate a novel strategy to synthesize NaCrO2 cathode material by using sodium dichromate dihydrate (Na2Cr2O7·2H2O) as a raw material and H2 as a reducing agent. Ex situ X-ray diffraction (XRD) was used to characterize the reduction mechanism of sodium dichromate at different temperatures (350−950 °C). Combined with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) techniques, the morphology, particle size, and microstructure were systematically investigated. As comparison, NaCrO2 synthesized by traditional solid-state reaction was also studied. Due to the stable interface with large particle size and favorable Na-diffusion crystal structure with (110) preferred orientation, the as-prepared NaCrO2 cathode material prepared by the facile new method exhibited excellent capacity retention and rate capability compared to those of a typical sample prepared by the conventional method. In addition, the tap density of the

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EXPERIMENTAL SECTION

Material Synthesis. The NaCrO2 (NCO) powder was synthesized by a simple decomposition reaction. A certain amount of Na2Cr2O7·2H2O was placed in an alumina crucible and calcined at different temperatures between 350 and 950 °C for 10 h with a ramping rate of 2 °C min−1 in an Ar/H2 (95:5% vol %) atmosphere. The produced NaCrO2 (hereafter denoted as lg-NCO) powder was obtained by slowly cooling to room temperature and then transferred immediately into an Ar-filled glovebox. For comparison, NaCrO2 was also synthesized via a traditional solid-state reaction and denoted as sNCO (the detailed synthesis process is shown in the Supporting Information). Material Characterization. To verify the purity of the products, powder X-ray diffraction patterns were collected with a Rigaku’s Minflex diffractometer with Cu Kα radiation (λ = 1.54056 Å) in the 2θ scale between 10 and 80° at a scan rate of 10°/min. The X-ray diffraction pattern used for cell refinement was collected by a stepscan mode in the same range with a step width of 0.02° and a step time of 2 s and then fitted by the GSAS software. A laser particle size analyzer (BT-9300S) was used to determine particle size distribution. The morphology and element distribution of the products were characterized by SEM and energy-dispersive spectrometry (EDS) (JSM-6360LV) with an operating voltage of 20 kV. Detailed structural information was further obtained by TEM and HRTEM performed on a field-emission transmission electron microscope (Tecnai G2 F20 S-TWIN, FEI, 200 kV). The electrical conductivity of NaCrO2 was carried out by a four-probe method. Electrochemical Characterization. The cathode film was made by mixing NCO powder, acetylene black, and poly(vinylidene fluoride) with the weight ratio of 8:1:1 in N-methylpyrrolidone. The obtained slurry was coated onto Al foils and vacuum-dried for 12 B

DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials h at 120 °C. Electrochemical properties were evaluated with CR2025 coin half-cells composed of the NCO cathode and a metallic-sodium anode with 1 M NaClO4 in PC/FEC (95:5% volume ratio) and glass fiber (GF/D) as an electrolyte and a separator. The cells were assembled in an Ar-filled glovebox. Charge/discharge measurements were performed in the voltage window of 2.3−3.6 V at room temperature with a Land BTI-10 battery test system. The active material loading amount was 2.2−2.5 mg cm−2. The cyclic voltammetry (CV) experiment was carried out at different scanning speeds between 2.3 and 3.6 V with a three-electrode system on a CHI660D electrochemical working station (Shanghai Chen Hua). The electrochemical impedance spectroscopy (EIS) was measured on a CHI660D electrochemical workstation with the frequency range from 100 kHz to 0.005 Hz and a disturbance voltage of 5 mV.

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space group R3̅m (JCPDS card no. 25-0819), and the values of the unit cell parameters a and c are 2.97334 and 15.97563 Å, respectively, and V is 122.35 Å3, which are in good agreement with those in a previous report.15 The inset of Figure 1c shows a typical O3−NaCrO2 crystal structure composed of a CrO2 slab layer and interslab NaO2 layer that are alternatively stacked along the c direction. This layered structure ensures a rapid migration of Na ions in both the a- and b-axis directions. For comparison, the XRD pattern of s-NCO is also analyzed (Figure S1a). The results demonstrate that the O3-type layered structure is also successfully synthesized by the solidstate reaction, and the detailed cell parameters are displayed in Table S1 (Supporting Information). Figure 1d shows the XRD reflections of both materials. It can be seen that the peak intensity of lg-NCO is much stronger than that of s-NCO, demonstrating a better crystallinity of lg-NCO. In addition, the peak intensity of the (003) plane is evidently greater than that of the (104) plane for lg-NCO, while the (003) peak is even weaker than the (104) peak for s-NCO. The calculated ratio of I(003)/I(104) of lg-NCO is 1.37, which is significantly higher than 0.75 of s-NCO. This increased ratio means that the layered structure of lg-NCO is more obvious, so the intercalation/deintercalation of Na+ is easier. Moreover, similar trends occur in the peak intensity of 110 and 018 planes of both materials. A lg-NCO sample shows the larger ratio of I(110)/I(018) (2.00) than that for s-NCO (0.93). The enhanced ratio of the lg-NCO sample suggests a remarkable growth of the 110 plane during crystallization. As reported, the increasing number of 110 planes means more {010} active facets (such as (010) and (100), perpendicular to the c-axis) are exposed in the crystal structure (Figure 3a).22,23 These active planes can provide more paths for Na-ion transportation in the layered structure (Figure 1c). Such characteristics will facilitate Na-ion diffusion and greatly improve the rate capacity of the material. SEM and TEM observations were performed to get the surface and microstructure information of the as-prepared samples. The SEM image (Figure 2a) shows that the lg-NCO sample has well-crystallized platelike particles of irregular shape, with a wide size range of 1−80 μm. The image at a

RESULTS AND DISCUSSION

To investigate the mechanism of the reduction process during sintering, XRD patterns of the reduced products of Na2Cr2O7· 2H2O at different temperatures were collected and are displayed in Figure 1a. It can be seen that lg-NCO with low crystallinity gradually forms between 350 and 450 °C, accompanied by some impurities, such as Cr2O3. These impurities are probably an intermediate product of incomplete reduction reactions resulting from an insufficient driving force at the relatively low temperature. With the elevation of the calcination temperature, the degree of crystallinity of lg-NCO is gradually enhanced with disappearance of the impurities, indicating a more complete reaction at higher temperatures. A pure NaCrO2 with high crystallinity forms when the temperature rises to 900 °C. However, the impurity in Cr2O3 reappears at 950 °C, which is probably caused by the volatilization of sodium at high temperatures. The X-ray photoelectron spectroscopy (XPS) of Cr 2p of lg-NCO obtained at 900 °C is displayed in Figure 1b. The Cr 2p3/2 main peak at 577.2 eV confirms that all Cr6+ (579.0 eV) are reduced to Cr3+ in the final product.20 As expected, the chemical reaction can be described as Na2Cr2O7·2H2O + 3H2 = 2NaCrO2 + 5H2O. It is obvious that the temperature plays an important role in the reduction reaction. To prepare the oxide as the cathode material for SIBs, an appropriate temperature is essential according to the above-mentioned analytical result. Compared to the traditional solid-state reaction by mixing Na2CO3 and Cr2O3,18,21 this facile synthesis method shows two definite advantages. On the one hand, lower cost and energy-efficiency can be guaranteed since Na2Cr2O7·2H2O has the advantage of lower cost than other chromium compounds (e.g., Cr2O3, Cr(NO3)3·9H2O, usually prepared from sodium dichromate) and the process involves only one step of decomposition reaction without the traditional mixing process with at least two kinds of raw materials. Moreover, cations of sodium and chromium are already distributed homogeneously at the atomic level in Na2Cr2O7·2H2O, avoiding heterogeneity caused by the mixing process of different raw materials, such as Na2CO3 and Cr2O3. We can see that the synthesis process is very simple and energy efficient. Moreover, the reduced products have the characteristic of high purity and crystallinity, which is favorable for electrochemical performance. To get more details on the structural information of the reduced product, a typical XRD pattern of as-prepared lg-NCO at 900 °C and its refinement results with a satisfactory Rwp value of 8.60% by the Rietveld method are presented in Figure 1c and Table S1 (Supporting Information), respectively. All of the peaks could be well-indexed to the pure O3 phase with

Figure 2. (a) SEM images at different magnifications and (b−e) EDS maps of l-NCO powder. C

DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX

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confirmed in the subsequent TEM and HRTEM analyses. As expected, this special morphology can obviously provide more active facets for Na-ion intercalation/deintercalation.24 In contrast, as shown in Figure S1, the s-NCO sample displays agglomerated spheroid morphology with the primary platelike particles (shown in the magnified picture) and the sizes are generally on the micrometer scale. The comparison of particle size, specific area, and tap density of lg-NCO and s-NCO samples is summarized in Table S2. It can be seen that the D50 value for lg-NCO is 22.10 μm, more than 10 times that for sNCO. The Brunauer−Emmett−Teller (BET) surface area measured by nitrogen adsorption/desorption analysis is dramatically reduced from 13.69 m2 g−1 for s-NCO to 1.65 m2 g−1 for lg-NCO. Owing to the high sensitivity to H2O and CO2, the Na-containing oxides are commonly isolated from air and thus will greatly increase the cost for storage. Thus, the decreased specific surface area of cathode material can play a vital role in improving processability in practical application, such as electrode slurry preparation. To investigate the structural stability of the two samples in the air, lg-NCO and s-NCO were exposed to ambient air over 24 h. The results show that the weight growth proportion of s-NCO is more pronounced than that of lg-NCO due to the larger BET surface area (Figure S2a). The XRD measurement demonstrates that the change in weight is related to the formation of NaOH on the surface (Figure S2b); lg-NCO with the small specific area can effectively slow down this harmful reaction. Besides, the tap density of the lg-NCO sample is as high as 2.55 g cm−3, which can definitely increase the volume energy density when used in a full cell. From the EDS mapping on the lg-NCO particles (Figure 2b−e), it can be seen that the elements sodium, chromium, and oxygen are uniformly distributed on the surface of the lg-NCO particles, coinciding with our

higher magnification exhibits that the large-grained particles consist of multiple closely stacked sheets, which agrees well with the characteristics of layered oxides. It is worth noting that these particles can be deduced as (010) plates formed by the accumulation of the (001) plates, as illustrated in Figure 3a. The preferred orientation of (010) will be further

Figure 3. (a) Schematic illustration of two kinds of nanoplates of NaCrO2. Typical TEM images of (b) lg-NCO and (c) s-NCO samples and HRTEM images of (d) lg-NCO and (e) s-NCO paticles. The insets are their corresponding fast Fourier transform (FFT) partterns.

Figure 4. (a) Typical initial charge/discharge curves of lg-NCO. (b) Cycling performance of lg-NCO and s-NCO at 0.1C (1C = 100 mA g−1) from 2.3 to 3.6 V. (c) Rate capability of lg-NCO and s-NCO. (d) Cycling performance of lg-NCO at 2C. D

DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 5. Cyclic voltammogram curves of (a) lg-NCO and (b) s-NCO between 2.3 and 3.6 V at different rates. (c, d) Corresponding Ip−v0.5 patterns in sodiation and desodiation processes.

and 113 mAh g−1 is maintained after 100 cycles, corresponding to an excellent capacity retention of 91.9% and an average capacity loss of 0.081% per cycle. The Coulombic efficiency of the material remains approximately 100% during cycling (Figure 4b). The improved discharge capacity and capacity retention can be ascribed to the high purity and crystallinity of the unique structure as mentioned above. What is more, as shown in Figure 4c, lg-NCO exhibits excellent rate performance with 110 mAh g−1 at 1C, 100 mAh g−1 at 5C, and 51 mAh g−1 at 30C and then the discharge capacity (118 mAh g−1 at 0.1C) could be recovered after cycling at high current densities, revealing the excellent rate capability of lg-NCO. Moreover, lg-NCO exhibits a rate performance superior to sNCO, even though the particles of the s-NCO sample are much smaller. The outstanding rate performance of lg-NCO can be attributed to its special morphology with significantly enhanced growth of {010} active facets to provide fast transport channels for sodium ions in the charge/discharge process, which is distinguished from the previously reported bare NaCrO2 materials and even is comparable to that of NaCrO2 with carbon coating (Table S3).11,15,16,18,19,21 The good cycle stability is also investigated and displayed in Figure 4d. As for lg-NCO, a specific capacity of 107 mAh g−1 can be achieved at 2C and it remains 94 mAh g−1 after 500 cycles with a capacity retention of 88.2%. However, an inferior performance is obtained in s-NCO with an initial capacity of 101 mAh g−1 and a lower capacity retention of 70.37% after 500 cycles with 2C rate. As we know, the battery’s cycling performance is related to the structural stability of electrode materials and the side reactions happening in the battery. The side reactions include dissolution of the active transition metal element, electrolyte decomposition catalyzed by metal ions with high valence, and impurity reactions on the surface of the active material. Here, our large-grained NaCrO2 with well-developed

prediction before the experiment for the even element distribution of the pristine material (Na2Cr2O7·2H2O) and no element segregation after the reduction reaction at high temperatures. The TEM and HRTEM images of lg-NCO and s-NCO samples are displayed in Figure 3b−e. It can be seen that typical large particles of lg-NCO are comprised of several stacking sheets as confirmed by the HRTEM image observation in Figure 3d. For the s-NCO sample, the submicron-sized and flakelike particle with clear edges and corners can be observed in Figure 3c and this primary particle shows the tendency to high aggregation. The above findings agree well with the results of the SEM analysis. The lattice fringe spacings of both materials (2.57 and 2.55 nm) in Figure 3d,e are very close to the (01̅0) and (100) lattice spacing of the NaCrO2 material, which suggests that the frontal planes of both samples are perpendicular to the [001] direction and the side planes parallel to the [001] direction are the well-known {010} active facets. It should be noted that the total area of the exposed active planes of lg-NCO is obviously larger than that of s-NCO in consideration of the thickness and the degree of agglomeration of the particles based on the above SEM and TEM analyses, which is in good accordance with the XRD results. The electrochemical performance of the lg-NCO and sNCO particles was evaluated in Na-half-cells, as shown in Figure 4. The initial charge/discharge curves of both electrodes exhibit a typical long plateau at ∼3.0 V at 0.1C with several short higher voltage plateaus (Figure 4a), which are consistent with a previous report15 The s-NCO electrode delivers a capacity of 114.8 mAh g−1 at 0.1C in the first cycle and only 93.3 mAh g−1 remained after 100 cycles with a low capacity retention of 81.3% (Figure 4b). To our surprise, the initial discharge capacity of lg-NCO can reach 123 mAh g−1 at 0.1C E

DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX

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10−11 cm2 s−1, which is roughly the same order of magnitude as some other O3-type multimetal oxides (Table S4).3,16,28−31 It indicates that the sodium-ion diffusion coefficient of lg-NCO with special morphology is not greatly affected by a large particle size. To investigate the interrelation between the superior electrochemical performance and the structural stability, electrochemical impedance spectroscopic (EIS) tests were conducted on the electrodes. Figure 6 shows the EIS curves of lg-NCO and s-NCO half-cells in the charged state of 3.6 V after 10 and 100 cycles at 0.1C. All of the Nyquist plots are composed of two semicircles and a sloped line, representing the resistance of the surface film (Rf), the charge transfer resistance (Rct), and the Warburg impedance (Zw). The intercept of the Nyquist plots and the Z′-axis refers to the impedance of Na+ migration in the electrolyte (Rs).16 The EIS data were fitted by Z-View2 software, and the detailed fitting data is summarized in Table 2. It can be seen that the Rct value

crystallization, unique large laminated particle morphology, and large amount of active facets expose can effectively stabilize its layered structure during cycling and significantly reduce the side reactions. A series of CV measurements were carried out to investigate the kinetics of Na+ intercalation/deintercalation at the electrode/electrolyte interface. The CV curves of the lgNCO and s-NCO samples at the different scan speeds of 0.1, 0.2, 0.3, 0.4, and 0.5 mV s−1 in the voltage range of 2.3−3.6 V vs Na+/Na are shown in Figure 5a,c, respectively. Several pairs of redox peaks can be observed that are consistent with the charge/discharge profile in Figure 4a. With increasing scan rates, cathodic and anodic peaks of both samples move to lower and higher potentials, respectively. As shown in Figure 5b,d, the peak current (Ip) is linear to the square root of the scan rate (ν), which confirms that the electrochemical activity of NaCrO2 is controlled by diffusion. Moreover, the electric conductivity is 1.06 × 10−4 S cm−1 for lg-NCO, which is close to the value of 1.37 × 10−4 S cm−1 for s-NCO. In these semiinfinite and finite diffusion cases, the apparent diffusion coefficient of Na ions (DNa+) can be calculated from the Randles−Sevcik equation25−27

Table 2. EIS Parameters of Equivalent Circuits after Different Cycles after 10 cycles

Ip = 2.69′105n3/2AC0D1/2ν1/2

where n is the number of electrons per reaction species, A is the effective area of the electrode, and C0 represents the concentration of sodium ions. The apparent diffusion coefficients (DNa+) of anodic peaks (a, b) and cathodic peaks (c, d, e, f) are listed in Table 1. The results demonstrate that

state charge discharge

peaks

lg-NCO

s-NCO

a b c d e f

0.1421 0.0905 −0.0497 −0.0662 −0.0403 −0.0550

0.0656 0.0477 −0.0255 −0.0295 −0.0193 −0.0227

DNa+ (cm2 s−1) lg-NCO 7.09 2.87 8.67 1.54 5.70 1.06

× × × × × ×

10−11 10−11 10−12 10−11 10−12 10−11

s-NCO 1.51 7.99 2.28 3.05 1.31 1.81

× × × × × ×

Rs (Ω)

Rf (Ω)

Rct (Ω)

Rs (Ω)

Rf (Ω)

Rct (Ω)

lg-NCO s-NCO

10.49 10.71

46.29 9.945

30.37 34.01

10.94 10.93

54.84 8.801

17.40 77.41

of lg-NCO reduces as the cycle proceeds, while this value is remarkably increased for s-NCO. We believe that the smaller Rct may be attributed to limited interfacial reactions on lgNCO that benefits the Na-ion intercalation/deintercalation process during cycling. The structural changes of the lg-NCO electrode were investigated by ex situ XRD measurements to study further the mechanism of the charge and discharge processes. Figure 7 shows an ex situ XRD pattern of lg-NCO electrodes at various potential states during the first cycle. Al foil was used as a current collector, and its peaks were marked with asterisks. The diffraction peaks of the hexagonal O3 phase are observed for the pristine lg-NCO. During the charging process, both the (003) and (006) peaks gradually shift to a lower angle and new peaks (at 16.18 and 32.8°, respectively) on the left shoulders grow with the decrease of the original (003) and (006) peak intensity, suggesting the formation of a monoclinic O′3 phase.11,13 When the electrode is charged to 3.2 V, the

Table 1. Calculated Na+ Diffusion Coefficients for lg-NCO and s-NCO slope

after 100 cycles

sample

10−11 10−12 10−12 10−12 10−12 10−12

the DNa+ of lg-NCO is nearly 1 order of magnitude larger than that of s-NCO, which can explain the superior rate capability of lg-NCO. The calculated average DNa+ of lg-NCO is 2.33 ×

Figure 6. Nyquist plots of (a) lg-NCO and (b) s-NCO electrodes at charge state after different cycles. The inset is the corresponding equivalent circuit model diagram. F

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Figure 7. Ex situ XRD for lg-NCO at different charge and discharge states.

Figure 8. XRD patterns of the (a) lg-NCO and (b) s-NCO for fresh electrodes and the electrodes after 100 cycles and the corresponding SEM images of the pristine (c, e) and cycled (d, f) samples.

Figures 8 and 9, respectively. From the results of XRD (Figure 8a,b), after repeating Na-ion intercalation/deintercalation multiple times, it can be seen that the diffraction peaks of both electrodes decrease in intensity and the (003) peak shifts to a lower angle, indicating the inevitable destruction of the layered structure. As expected, the main phase of the lg-NCO electrode is well preserved (only 0.04° shift) and a distinct peak shift (0.18°) of the s-NCO electrode is observed. Meanwhile, for the lg-NCO electrode, there are no obvious changes in the morphology of the particles and the integrity of the large grain shape with adherent planes is also well maintained, as shown in SEM photos in Figure 8c,d. As for the s-NCO electrode, severe fragmentation of flaky particles occurs after long cycles (seen in Figure 8e,f). The microstructure of the cycled cathodes was also analyzed in depth by TEM and HRTEM characterization. Obviously, we can see that the cycled lg-NCO electrode in Figure 9a is composed of the closely stacked plate-type structure and this unique micro-

(003) and (006) peaks of the O′3 phase further shift to a lower angle and increase the intensity continuously, while the peaks of the original O3 phase disappear. From 3.2 to 3.4 V during the charge process, new peaks on the left side of the (003) and (006) peaks of the O′3 phase start to form; they represent the P3 phase and are enhanced with further charging at the expense of those for the O′3 phase, indicating a structural transformation from the monoclinic O′3 to the monoclinic P3 phase. In addition, the diminishing of the 104 peak and the appearance of the 015 peak also imply the O′3−P3 phase transition. When the electrode is charged to 3.5 V, the O′3 phase almost disappears and the P3 phase is the dominate phase. In the discharging process, the P3 phase transforms back to the original O3 phase, which indicates a high reversibility of the phase transition during Na+ (de)insertion. To further examine the stability of the cathodes, XRD patterns and SEM and TEM images of the lg-NCO and sNCO were collected after different cycles and are illustrated in G

DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 9. TEM and HRTEM images of the (a) fresh and (c) cycled lg-NCO electrodes and of the (b) fresh and (d) cycled s-NCO electrodes. The corresponding FFT patterns are displayed in (c) and (d).

Figure 10. Electrochemical performances of the lg-NCO/HC full cell. (a) Galvanostatic charge/discharge curves of different cycles, and the inset shows galvanostatic charge/discharge curves of HC. (b) Cycle performance at 10 mA g−1.

HC anodes were cycled 3 times to achieve a relatively stable state before fabricating full cells. As shown in Figure 10a, the lg-NCO/HC full cell can deliver a discharge capacity of 112.6 mAh g−1 with an average operation voltage of 2.97 V at 0.1C in the first cycle. The reversible capacity of the full cell remained 91.65% after 50 cycles at 0.1C (Figure 10b). The full cell performance further demonstrates that lg-NCO is a promising cathode material for practical use.

structure is well maintained on cycling compared to the fresh lg-NCO in Figure 3b,d, further confirming what is observed in the SEM images (Figure 8c,d) and indicating a stability of the special morphology. Moreover, the HRTEM image (Figure 9c) demonstrates that the O3 structure of lg-NCO remains unchanged. However, obvious cracks and disordered (003) lattice fringes appear on the edges of the cycled s-NCO particle, as shown in Figure 9b,d. All of the results above from XRD, SEM, TEM, and HRTEM have further proved the structural stability of the lg-NCO cathodes. The strong robustness of the structure facilitates the reversible process of Na+ intercalation/deintercalation and improves the electrochemical properties of the electrode, especially for long cycle stability. The electrochemical performance of a full cell was also evaluated to probe the practical application of lg-NCO material. Full cells were assembled using lg-NCO as cathodes and hard carbon (HC) as anodes; the corresponding charge/ discharge profiles are displayed in the inset of Figure 10a. The

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CONCLUSIONS Large-grained NaCrO2 with high crystallization was successfully synthesized by a novel simple strategy with sodium dichromate dehydrate (Na2Cr2O7·2H2O) as a raw material and H2 as a reducing agent. The as-prepared NaCrO2 electrode delivered a high reversible capacity of 123 mAh g−1 at 0.1C and 51 mAh g−1 at 30C and exhibited an outstanding cycling performance keeping 88.2% of the initial capacity after 500 cycles at 2C. The excellent electrochemical performance of lgNCO is attributed to a uniquely large laminated particle H

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morphology with a large number of (010) active plates and the remarkable structural stability of lg-NaCrO2. Highly reversible phase transition and facile Na-ion diffusion in the charge/ discharge process were demonstrated by ex situ X-ray diffraction and CV analysis. The facile synthesis strategy paves the way toward the practical application of NaCrO2 in SIBs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b01456. Synthesis of s-NaCrO2, XRD refinement, and SEM; comparison of storage performance, crystalline parameter, and physical characteristics of s-NaCrO2 and lgNaCrO2; and electrochemical performance of NaCrO2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.D.). *E-mail: [email protected] (J.B.G.). ORCID

Hongcai Gao: 0000-0002-3671-8765 John B. Goodenough: 0000-0001-9350-3034 Author Contributions §

Y.W. and W.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 51772333 and 51874358), the Natural Science Foundation of Hunan Province (Grant 2015JJ3152), Central South University Research Foundation of Teacher (2014JSJJ005), and Shiyanjia Lab for TEM test. J.B.G. acknowledges the support of the Robert A. Well Foundation of Houston, Texas (F-1066).

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DOI: 10.1021/acs.chemmater.9b01456 Chem. Mater. XXXX, XXX, XXX−XXX