A study on electrochemical properties of P2-type Na–Mn–Co–Cr–O cathodes for sodium-ion batteries

Yanzhi Wang*ab, Jiantao Tanga, Xiduo Yanga and Weiwei Huang*a
aHebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, Hebei 066004, China. E-mail: hhwyz@ysu.edu.cn; huangweiwei@ysu.edu.cn; Fax: +86 335 8061569; Tel: +86335 8061569
bState Key Laboratory of Metastable Material Science and Technology, Yanshan University, Qinhuangdao, 066004, China

Received 9th December 2017 , Accepted 28th December 2017

First published on 2nd January 2018


Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) oxides were prepared by a conventional solid-state reaction. The electrochemical properties of P2-type Na–Mn–Co–Cr–O cathodes for sodium-ion batteries have been investigated systematically. X-ray diffractometry and field emission scanning electron microscopy show that Cr ions are successfully incorporated into the lattice of the Na–Co–Mn–O oxide, and a P2-type layered structure with the P63/mmc space group remains unchanged after Cr-doping. The electrochemical measurements show that the maximum discharge capacity and the cycling stability of the matrix material is significantly enhanced by Cr-doping, which may be ascribed to the expanded a-axis and lattice volume, and the improvement of the rate capability is attributed to the significant decrease of the charge transfer resistance and the increase of the apparent Na+ diffusion coefficient. In the potential region of 2.0–4.0 V at 0.12C, Na0.67Co0.25Mn0.65Cr0.10O2 delivers the maximum discharge capacity of 128.1 mA h g−1, which is 10.31% higher than that of the un-doped sample, and the capacity retention of the sample is 66.35% after 100 cycles, which is 39.09% higher than that of the un-doped sample. It is suggested that the optimal Cr content can effectively improve the electrochemical properties of P2-type Na–Mn–Co–O cathodes for sodium ion batteries.


1. Introduction

Rechargeable lithium-ion batteries (LIBs) are well-known and have been widely used to the portable electronic devices, and have the potential to be used in electric vehicles, hybrid electric vehicles, and smart grids.1–4 However, lithium as a minor-metal is unevenly distributed in the earth's crust, which has led to the substantial growth in the price of lithium carbonate during the past decade, the availability of lithium resources, cost and safety issues of LIBs are now becoming the main challenges that impede their widespread applications.5,9 Therefore, room-temperature sodium-ion batteries (SIBs) have attracted great research interest, especially for large-scale electrical energy storage applications.6–19 Nowadays, the development of advanced cathode materials for SIBs is still a challenge.8 Among various cathode materials, layered sodium transition metal oxides with the composition of NaxTMO2 (0 < x ≤ 1, TM = transition metals) have been widely studied for rechargeable SIBs.11–19 These layered materials can be classified into two main groups of O3-type and P2-type, in which the sodium ions are coordinated in octahedral and prismatic environments, respectively.20 It is widely believed that P2-type materials are more suitable for SIBs in comparison with O3-type materials due to their large interlayer spacing and high structural stability.21,22 P2-type NaxMnO2 cathode materials, providing large-sized tunnels for sodium ions, are regarded as one of the most promising candidates because of their high capacity during slow discharge, and the high earth abundance and low toxicity of Mn.21–23 However, this kind of material exhibits poor cycling stability and rate performance, and low average working potential of less than 3.0 V due to Jahn–Teller effects and phase transformation at high voltage.24 Therefore, numerous efforts such as particle size reduction,23,24 elemental substitution,21–27 and layer coating19,28,29 have been made to improve its electrochemical properties in recent years. For example, Pang et al.30 reported that the initial discharge capacity of the P2-Nax(Fe1/2Mn1/2)O2 cathode was only ∼90 mA h g−1, and the cyclability of the cathode was the best between 2.0–4.0 V, poor capacity retentions between 1.5–4.0 V and 2.0–4.2 V were observed after 80 cycles, respectively, which was attributed to the formation of the Cmcm and disordered P63 phases. Bai et al.22 synthesized the P2-type Na2/3Fe1/2Mn1/2O2 sample via a chelating agent assisted route, and the initial discharge specific capacities of the sample were 132.2, 120.1, 119.9, and 97.8 mA h g−1 at different current densities of 26, 52, 130, and 260 mA g−1 in 1.5–4.0 V (vs. Na+/Na), respectively. Although the P2-type Na–Fe–Mn system is the promising candidate as high capacity cathode materials for SIBs, the system's hygroscopic character hinders its handling in moist air.31 The P2-type Na0.67Cu0.15Ni0.20Mn0.65O2 layered oxide was synthesized by a co-precipitation method, the discharge–charge plateaus were above 3.0 V. The sample delivered a capacity of 87 and 78 mA h g−1 at 10 mA g−1 and 1000 mA g−1, respectively.32 The Na0.67Ni0.23Mg0.1Mn0.67O2 compound, which is prepared by a sol–gel method, delivered an initial reversible capacity of 105 mA h g−1, and the capacity could remain at approximately 84.9 mA h g−1 after 100 cycles in 2.0–4.5 V at a current density of 48 mA g−1.33 The Na2/3[Ni1/3Mn2/3]O2 sample presented the initial discharge capacity of 164 mA h g−1 within the 2.5–4.3 V window, and the capacity retention was only 26.8% at the 300th cycle. Al2O3-coated Na2/3[Ni1/3Mn2/3]O2 also presented a similar initial capacity, but the capacity retention was up to 73.2% after 300 cycles.28 Another P2-Na2/3Ni1/3Mn2/3O2 plate, formed at 800, 900, and 1000 °C, delivered the initial discharge capacities of 69, 86, and 80 mA h g−1, and the discharge capacities are 68, 80, and 76 mA h g−1 at 0.1C after 200 cycles, respectively.34 P2-Na2/3Ni1/3Mn5/9Al1/9O2, which is synthesized via a liquid-state method, delivered the discharge capacities of 116.7, 84.4 and 24.6 mA h g−1 at 0.1, 1 and 5C rates, respectively, and its electrochemical properties could be further enhanced by connecting with the reduced graphene oxide.19 It is suggested that the higher cycling stability of P2-type NaxMnO2 doped with ∼10% Co was attributed to the suppression of a Jahn–Teller effects, ordering processes of Na+, and increased Na+ kinetics.23 It is found that the Co-substituted P2-type Na0.44Mn0.89Co0.11O2 gave smooth charge–discharge profiles, and allowed more Na+ intercalation into the inter-layer spacing, and resulted in a discharge capacity of 220 mA h g−1 at a low rate in the initial cycle and 180 mA h g−1 after 40 cycles, and displayed a good rate performance with 104 mA h g−1 at 600 mA g−1 in a 2.0–4.2 V window.35 The P2-Na0.67Mn0.65Ni0.2Co0.15O2 cathode material showed reversible capacities of 155, 137, and 126 mA h g−1 between 1.5–4.2 V at different current densities of 12, 48, and 240 mA g−1, respectively, and it can still deliver capacities of 117, 93 and 70 mA h g−1 at 2C, 5C, and 8C, respectively.36

Recently, Song and coworkers37 reported that the Cr-doped Na2TiO3 layered oxide delivered a reversible capacity of 336 mA h g−1 at a current density of 18.9 mA g−1 with a capacity retention of 74% after 1000 cycles at a current density of 378 mA g−1. Meanwhile, although the initial reversible capacity of O3 NaMnO2 is slightly smaller than that of NaCrO2, much better capacity retention can be observed for O3 NaCrO2 in Na cells.38 Furthermore, the Cr element has been doped into LiCr0.2Ni0.4Mn1.4O4[thin space (1/6-em)]39 and LiNi0.5Mn1.5O4[thin space (1/6-em)]40 to stabilize the structure during cycling and to eliminate the factors which promote the electrolyte oxidation and continuous degradation at a high operating voltage, based on the fact that the bonding energy of Cr–O is higher than Ni–O and Mn–O. In addition, chromium is more abundant with a lower cost compared with common transition metals used in cathodes. Moreover, the elemental substitution has been a promising approach for optimizing electrode material properties, and yielded superior electrochemical performance for SIBs. To the best of our knowledge, the Cr-doped P2-type NaxMnO2 material has not been reported. Herein, we designed a new Na–Mn–Co–Cr–O cathode material to improve the electrochemical performance of the P2-type Mn-based cathode.

2. Experimental section

2.1. Materials synthesis

Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) materials were synthesized by a conventional solid-state reaction. Stoichiometric amounts of sodium acetate (NaAc·3H2O) and cobalt acetate [Co(Ac)2·4H2O] (Tianjin Kermel Chemical Reagent Co. Ltd), and manganese acetate [Mn(Ac)2·4H2O] and chromium oxide (Cr2O5) (Sinopharm Chemical Reagent Co. Ltd) were ground with ethanol by using a ball mill for 5 h, followed by calcination in air at 900 °C in a muffle furnace for 12 h and cooled naturally in the furnace to form the product, respectively.

2.2. Materials characterization

The active materials were characterized by X-ray diffractometry (XRD) (SmartLab), field emission scanning electron microscopy (FESEM) (S-4800) attached with energy dispersive X-ray analysis (EDX elemental mapping), Raman spectroscopy (HORIBA), and X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi).

2.3. Electrochemical measurements

The electrochemical measurements of the active materials were evaluated in Swagelok type cells using a Na disk as the counter electrode and 1.0 M NaClO4 in propylene carbonated (PC) with 3 vol.% of fluoroethylene carbonate (FEC) solution as the electrolyte. The working electrode was produced by dispersing 80 wt% active material, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidione (NMP) solvent to form a uniform slurry. The slurry was coated on Al foil and dried under vacuum at 120 °C for 12 h. Celgard 2400 was used as a separator. The cells were assembled in an argon-filled glovebox. Electrochemical performances were tested on a BTS-5 V10 mA system in the voltage range of 2.0–4.0 V vs. Na+/Na at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) measurements were conducted on a CHI 660E electrochemical workstation.

3. Results and discussion

3.1. Structure characteristics

The XRD patterns and Raman spectra of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) samples are depicted in Fig. 1. As shown in Fig. 1a, it is clearly seen that the P2-type layered structure of the matrix material with the P63/mmc space group has not been changed by Cr-doping, which is consistent with the standard pattern of JCPDS 54-0894.22,33,34,41,42 However, impurity peaks attributed to Na2CrO4 can be observed on increasing x. The lattice parameters are listed in Table 1, which are close to the reported values of a = b = 2.898 Å and c = 11.210 Å for Na2/3Fe1/3Mn2/3O2,41 and a = b = 2.886 Å and c = 11.179 Å for Na0.67Ni0.23Mg0.1Mn0.67O2,33 and a = b = 2.889 Å and c = 11.225 Å for Na2/3Fe1/2Mn1/2O2-700.22 Obviously, the lattice parameters of a (or b) and cell volumes increase with increasing x, which is attributed to the larger radius of the Cr-ion than that of the Mn-ion. On the other hand, the larger in-plane atomic distance is accompanied by the lower electrostatic repulsion between O–O atoms, leading to the better stability,22,42 the Cr-doped sample shows the longer length of the a-axis than that of the un-doped sample, which is beneficial for the stability of the P2 layered structure. In addition, the lattice parameter c increases from 11.295 Å (x = 0.05) to 11.367 Å (x = 0.15), which is usually attributed to the overall increase of electrostatic repulsion along the c axis between the ions of transition metal layers by Cr-doping. This is beneficial for the Na-ion transport during the Na+ insertion/extraction process, which is significant for improving high rate performance of Na ion batteries. As shown in Fig. 1b, the four samples show two distinct Raman bands at ∼578 and ∼466 cm−1, corresponding to A1g and Eg from the symmetrical stretching and deformation of the metal–oxygen bond in layered transition metal oxides, respectively. This indicates that the four samples have the layered structure.32 In addition, the band at 346 cm−1 was a characteristic peak of Cr2O3, the bands at 846 and 897 cm−1 match with that of a chromium oxide dimer, the band at 930 cm−1 shows polymeric Cr6+ species, which is consistent with the result of XRD.
image file: c7qi00778g-f1.tif
Fig. 1 The XRD patterns (a) and Raman spectra (b) of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) samples.
Table 1 The lattice parameters of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) samples
Sample a (Å) b (Å) c (Å) V3)
x = 0 2.861 2.861 11.295 80.064
x = 0.05 2.865 2.865 11.294 80.281
x = 0.10 2.870 2.870 11.296 80.576
x = 0.15 2.874 2.874 11.367 81.309


3.2. Morphology and elemental analysis

The FESEM images of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) samples are presented in Fig. 2. As can be seen, the particles of the four samples exhibit typical flake shapes, and the particle sizes are not uniform, and have an in-plane extension in a range of several hundred nanometers to several micrometers, indicating that Cr-doping does not obviously change the morphology of the matrix material. EDX elemental mapping images of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) samples are presented in Fig. 3. It is found that elements of Na, Co, Mn, and O are uniformly distributed throughout the flakes, while the Cr element is revealed for Cr-doped samples.
image file: c7qi00778g-f2.tif
Fig. 2 FESEM images of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) samples, (a) x = 0, (b) x = 0.05, (c) x = 0.10 and (d) x = 0.15.

image file: c7qi00778g-f3.tif
Fig. 3 EDX elemental mapping images of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) samples, (a) x = 0, (b) x = 0.05, (c) x = 0.10 and (d) x = 0.15.

3.3. XPS analysis

Fig. 4 displays the XPS patterns of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) samples. It is found that no obvious changes in the binding energy of the Na 1s (Fig. 4a), O 1s (Fig. 4b), Co 2p (Fig. 4c), and Mn 2p (Fig. 4d) were detected with increasing x, indicating that the valence states of Na, Mn, Co, and O ions remain almost the same after Cr-doping. The main peak of O 1s spectra at ∼529.7 eV illustrates that oxide ions (O2−) exist in the four samples, and the binding energy of the Na 1s at ∼1070.4 eV indicates that the valence state of sodium cations (Na1+) is not affected during Cr-doping,38 the binding energy of Mn 2p1/2 and Mn 2p3/2 are ∼653.9 eV and ∼642.4 eV, respectively, illustrating the presence of a typical Mn4+ oxidation state in all the samples,32,33 and the existence of Co3+ in the four samples can be testified by the Co 2p1/2 peak at ∼795.6 eV and Co 2P3/2 peak at ∼780.3 eV,43 respectively. The Cr 2p3/2 peak at ∼577.0 eV and its satellite peak can be ascribed to Cr3+,37 and the Cr 2p3/2 peak at ∼579.6 eV and its satellite peak can be assigned to Cr6+ on the surface of the powder,42 which demonstrates the coexistence of Cr3+ and Cr6+ in the Cr-doped samples. The relative emission intensity of Cr6+ to Cr3+ increases, indicating that more and more Cr3+ are converted to Cr6+ with the increase of Cr amount as well. It has previously been shown the formation of Cr6+ in cathode materials of LiMn2−yCryO4[thin space (1/6-em)]44 and LiCr0.2Ni0.4Mn1.4O4.39 As shown in the line marked annealed in Fig. 4e, Cr6+ could be converted to Cr3+ by proper annealing39,40 in order to avoid environmental pollution before using the Cr-doped material, and the details will be described in the other report.
image file: c7qi00778g-f4.tif
Fig. 4 XPS patterns of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) samples (a) Na 1s scan, (b) O 1s scan, (c) Co 2p scan, (d) Mn 2p scan, and (e) Cr 2p scan.

3.4. Charge/discharge profile

Fig. 5 exhibits galvanostatic charge/discharge profiles of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes in the initial 3 cycles in a voltage range of 2.0–4.0 V at 0.12C rate, respectively. The charge curves of the Na0.67Co0.25Mn0.75O2 cathode exhibit plateau-like features at ∼3.5 V, whereas the Cr-doped cathodes exhibit plateau-like features at ∼3.5 V in the initial charge curves, and subsequently every charge curve exhibits two plateau-like features at ∼2.4 V and ∼3.5 V, respectively, which can be explained as follows, because the oxidation state of the Mn-ion is +4 and the Co-ion +3 from the above XPS results, the Na+ extraction in the charge process must be concomitant with the oxidation reaction from Co3+ to Co4+ at ∼3.5 V. But the Na+ insertion in the discharge process might be accompanied by the reduction reactions from Co4+ to Co3+ at ∼3.4 V, and Mn4+ to Mn3+ and (or) Co3+ to Co2+ at ∼2.2 V, respectively. Therefore, the Na+ extraction in the following charge process might be concomitant with the oxidation reactions from Mn3+ to Mn4+, and (or) Co2+ to Co3+ at a charge plateau of ∼2.4 V, and Co3+ to Co4+ at ∼3.5 V, respectively. The charge compensation, which is achieved by the stabilization of Co3+ and Mn4+ along Na+ extraction with oxidation of Co3+ to Co4+, is also observed for P2-Na0.67Mn0.65Ni0.2Co0.15O2[thin space (1/6-em)]36 and P2-Na2/3Co2/3Mn1/3O2.45 On the other hand, all the discharge curves of Cr-doped cathodes show similar plateau-like features at ∼2.2 V compared to those of the Na0.67Co0.25Mn0.75O2 cathode, which illustrates that the extracted Na ions can be inserted back into the crystal structure again without obvious structure transition. In addition, the coulombic efficiency of the Cr-doped sample in the first cycle is significantly higher than that of the un-doped sample, which is not achievable in a full cell. This indicates that Cr-doping may induce more Na-vacancies in materials.46 Such abnormal efficiency (>100%) measured in the narrow voltage range of 2.0–4.0 V would become normal when the voltage range is enlarged to 1.5–4.2 V or 1.5–4.4 V.47
image file: c7qi00778g-f5.tif
Fig. 5 Galvanostatic charge/discharge profiles of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes at 0.12C rate in the initial 3 cycles, (a) x = 0, (b) x = 0.05, (c) x = 0.10 and (d) x = 0.15.

3.5. Cyclic voltammetry

CV curves of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes, corresponding to the maximum discharge capacity in the voltage ranges of 2.0–4.0 V at a scan rate of 0.1 mV s−1, are shown in Fig. 6a. As can be seen, the peaks at 3.0–4.0 V relate to the redox reactions of the Co4+/Co3+ ionic pair, and the peaks at 2.0–3.0 V should be associated with the redox reactions of Mn4+/Mn3+ and (or) Co3+/Co2+ ionic pairs,48 which agrees with the above-mentioned charge/discharge plateaus. In addition, the CV curve area of the Na0.67Co0.25Mn0.65Cr0.10O2 cathode is the largest in the four CV curves, indicating that the optimized Cr amount is x = 0.10 under the experimental conditions.
image file: c7qi00778g-f6.tif
Fig. 6 The CV curves (a) at a scan rate of 0.1 mV s−1 in the cycles corresponding to the maximum discharge capacity, cyclic performances (b) at 0.12C and rate capabilities (c) of Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes.

3.6. Cyclic performance

Cyclic performances of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes at 0.12C are presented in Fig. 6b. Firstly, the maximum discharge capacity of the cathodes increases from 110.6 mA h g−1 (x = 0) to 128.1 mA h g−1 (x = 0.10), which may be attributed to the expanded a-axis and lattice volume by Cr-doping, and then decreases to 102.2 mA h g−1 (x = 0.15), which may be ascribed to the formation of impurity phase Na2CrO4. The maximum discharge capacity (128.1 mA h g−1) of the Na0.67Co0.25Mn0.65Cr0.10O2 cathode is evidently higher than 102 mA h g−1 for Na0.50Cu0.15Ni0.20Mn0.65O2 and 87 mA h g−1 for Na0.67Cu0.15Ni0.20Mn0.65O2 at 10 mA g−1 in the voltage range of 2.0–4.2 V,32 and 69, 86, and 80 mA h g−1 for P2-Na2/3Ni1/3Mn2/3O2 plates formed at 800, 900, and 1000 °C,34 respectively, and 116.7 mA h g−1 for P2-Na2/3Ni1/3Mn5/9Al1/9O2 at 0.1C synthesized via a liquid-state method.19 Secondly, it is calculated that the capacity retentions of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes after 100 cycles are 27.26%, 65.85%, 66.35% and 65.14%, respectively, which indicates that the cycling stability of the sample increases with increasing Cr content. The capacity retention 66.35% of the Na0.67Co0.25Mn0.65Cr0.10O2 cathode is significantly higher than ∼55% of the P2-type Na2/3(Fe1/2Mn1/2)O2 cathode after the 80th cycle at 0.1C,30 and the cyclability of the Na0.67Co0.25Mn0.65Cr0.10O2 cathode is superior to that of P2-Na0.67[Ni0.4Co0.2Mn0.4]O2 synthesized by a one-step solid-state method and cooled naturally in the furnace.45 This may be ascribed to the expanded a-axis by Cr-doping, which has the larger in-plane atomic distance accompanying the lower electrostatic repulsion between O–O atoms leading to the better stability. This may also be attributed to the protective effect of the chromium oxide against the dissolution of manganese oxides in electrolytes. In addition, as mentioned above, the Na+ insertion in the discharge process can be accompanied by the reduction reactions from Mn4+ to Mn3+, therefore the capacity fading of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes in the initial 20 cycles should be mainly caused by a Mn3+-induced Jahn–Teller distortion formed during repeated cycling, the dissolution of manganese into the electrolyte due to the reactions with HF, and the oxidation of the organic electrolyte on the electrode.23,24 Thirdly, interesting phenomena are that the activation processes of 2 cycles are observed for the Cr-doped cathodes, and there are no such phenomena for the un-doped cathode.

3.7. Rate capability

Fig. 6c indicates rate capabilities of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes as the current density increases from 0.12C to 2.94C and returns to 0.12C. It shows that the rate capabilities of the four cathodes first increase and then decrease with increasing x, which suggests that optimal Cr-doping is helpful for improving the rate capability of the matrix material. For example, the Na0.67Co0.25Mn0.75O2 cathode delivers a mean discharge capacity of 103.94, 71.24, 52.79, 35.32 and 13.55 mA h g−1 at 0.12C, 0.29C, 0.59C, 1.18C and 2.94C, respectively, whereas the Na0.67Co0.25Mn0.65Cr0.10O2 cathode displays mean discharge capacities of 123.74, 108.85, 86.61, 67.35 and 49.41 mA h g−1, respectively. The lower specific capacity at higher current densities should be attributed to a larger polarization for both cathodes. When the current density turns back to 0.12C after high rate cycling, Na0.67Co0.25Mn0.75O2 and Na0.67Co0.25Mn0.65Cr0.10O2 cathodes can also deliver the discharge capacity of 65.43 and 124.84 mA h g−1, respectively, indicating that the rate capability of the Na0.67Co0.25Mn0.65Cr0.10O2 cathode is significantly superior to that of the Na0.67Co0.25Mn0.75O2 cathode. The discharge capacity of 67.35 mA h g−1 at 1.18C for the Na0.67Co0.25Mn0.65Cr0.10O2 cathode is obviously higher than 63 mA h g−1 at 1C for P2-Na2/3Ni1/3Mn2/3O2 plates post-treated at 800 °C.34 The role of Cr-doping may be as follows: Cr-doping enhances the electronic conductivity, which is favorable for the rate capability; the aliovalent Cr-doping can be beneficial to Na+ conduction because it increases disorder in the transition metal layers and can prevent Na+/vacancy ordering, which further enhances the rate capability.43 The enhancements of rate capabilities can also be explained by the results of EIS analyses.

3.8. EIS measurement and analysis

Fig. 7a shows the EIS measurements of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes by using a frequency range of 0.01 Hz–100 kHz at open circuit potential before charge/discharge tests. It is easy to see that the electrochemical impedance spectrum consists of two semicircles at high–medium frequency and a straight line at low frequency. The high-frequency intercept at the Zreal axis corresponds to the ohmic resistance of the cell setup combined with electrolyte solution resistance and electric contact resistance (Rs), and the intercept of the semicircle at high frequency with the Zreal axis is attributed to the resistance of the SEI film (Rf), and the intercept of the semicircle at medium frequency with the Zreal axis corresponds to the charge transfer resistance (Rct) at the electrode/electrolyte interface, and an inclined line at low frequency correlates to Warburg impedance (Zw) associated with Na+ diffusion in the active particles. According to the corresponding equivalent circuit in Fig. 7c, the impedance parameters can be calculated and are tabulated in Table 2. It can be seen that the values of Rs for the four cathodes change slightly between 5 and 29 Ω, however, the values of Rct of the cathodes decrease significantly from 1404 (x = 0) to 44 Ω (x = 0.10) and then increase to 670 Ω (x = 0.15) with increasing x. This shows that the Cr-doping significantly decreases Rct of the Na0.67Co0.25Mn0.75O2 cathode, indicating the improved electrochemical reaction kinetics of the material. On the other hand, the values of Rf for the four cathodes decrease significantly from 1419 (x = 0) to 462 Ω (x = 0.10) and then increase to 9334 Ω (x = 0.15) with increasing x. In general, the increases of the Rf values demonstrate the continual decomposition of the electrolyte and the formation of the unfavorable SEI film during the cycling process. It is suggested that the optimal Cr-doping can suppress the electrolyte decomposition and alleviate the dissolution of Mn2+ from the matrix material.
image file: c7qi00778g-f7.tif
Fig. 7 (a) EIS measurements at open circuit potential, and (b) their linear fitting of the Zreal versus ω−1/2 for Na0.67Mn0.75−xCo0.25CrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes, and (c) the corresponding equivalent circuit.
Table 2 Electrochemical kinetic parameters of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) cathodes
Sample Impedance parameters (Ω) Na+ diffusion coefficient (×10−13, cm2 s−1)
Rs Rf Rct
x = 0 5 1419 1404 0.14
x = 0.05 14 1353 1127 4.07
x = 0.10 29 462 44 11.33
x = 0.15 15 9334 670 6.15


According to the inclined lines in the low frequency region, an apparent Na+ diffusion coefficient (D) can be evaluated by the following equation:19

 
D = R2T2/(2A2n4F4C2σ2) (1)
where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons per reaction, F is the Faraday constant, C is the molar concentration of Na+ in the lattice, and σ is the Warburg coefficient which is related to Zreal in the following equation:
 
Zreal = Rs + Rct + σω−1/2 (2)

In this equation, ω is the angular frequency in the low frequency region, and both Rs and Rct are kinetics parameters independent of frequency. The linear fitting of the four samples is shown in Fig. 7b. The apparent Na+ diffusion coefficients of Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) samples calculated from the linear fitting of the Zreal versus ω−1/2 are also listed in Table 2. It shows that the values of D for the four samples first increase from 0.14 × 10−13 cm2 s−1 (x = 0) to 11.33 × 10−13 cm2 s−1 (x = 0.10) and then decrease to 6.15 × 10−13 cm2 s−1 (x = 0.15), which indicates the apparent Na+ diffusion coefficient in the matrix material has been evidently increased by Cr-doping. This should be attributed to the stable structure and a wider track of Na+ migration in the Cr-doped sample, which is evidence that Cr-doping promotes Na+ and electronic migration and then improves electrochemical properties.22

4. Conclusions

In summary, P2-type Na0.67Co0.25Mn0.75−xCrxO2 (x = 0, 0.05, 0.10, 0.15) oxides were prepared through a solid state method. Cr-doping does not change the P2-type structure and morphology of the matrix material. The lattice parameters of a and cell volumes increase with increasing Cr content. In the potential region of 2.0–4.0 V at 0.12C rate, the Na0.67Co0.25Mn0.65Cr0.10O2 oxide shows the highest discharge capacity among the four samples, and the capacity retentions of the cathodes after 100 cycles increase with the increase of Cr content, which may be ascribed to the expanded a-axis and lattice volume after Cr-doping. Meanwhile, the rate capabilities of the four cathodes first increase and then decrease on increasing the dopant, which is attributed to the significant changes of the charge transfer resistance and the apparent Na+ diffusion coefficient. The optimal Cr-doping strategy should be effective for the Na–Mn–O cathode of SIBs to improve the electrochemical performance and to promote their practical application.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21403187) and the Natural Science Foundation of Hebei Province of China (Grant No. B2015203124).

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