He Zhao, Jianzhong Li,*, Haoyuan Xu, Xuanwen Gao, Junjie Shi, Kai Yu, Xueyong Ding

1 School of Metallurgy, Northeastern University, Shenyang 110819, China

2 Key Laboratory for Ecological Metallurgy of Multimetallic Ores (Ministry of Education), Shenyang 110819, China

Keywords:Sodium ion battery Na2FeP2O7/C B-doping Solid phase method

A B S T R A C T In recent years, the composite materials based on polyanionic frameworks as secondary sodium ion battery electrode material have been developed in large-scale energy storage applications due to its safety and stability. The Na2FeP2O7/C (theoretical capacity 97 mA·h·g-1) is recognized as optimum Na-storage cathode materials with a trade-off between electrode performance and cost. In the present work, The Na2FeP2O7/C and boron-doped Na2FeP2-xBxO7/C composites were synthesized via a novel method of liquid phase combined with high temperature solid phase.The non-metallic element B doping not only had positive influence on the crystal structure stability,Na+diffusion and electrical conductivity of Na2FeP2O7/C, but also contributed to the high-value recycling of B element in waste borax. The structure and electrochemical properties of the cathode material were investigated via X-ray diffraction(XRD),scanning electron microscopy(SEM),The X-ray photoelectron spectroscopy(XPS),electrochemical impedance spectroscopy(EIS),cyclic voltammetry(CV),and charge/discharge cycling.The results showed that different amounts of boron doping had positive effects on the structure and electrochemical properties of the material.The initial charge/discharge performances of born doped materials were improved in comparison to the bare Na2FeP2O7/C.The cycle performance of the Na2FeP1.95B0.05O7/C showed an initial reversible capacity of 74.8 mA·h·g-1 and the high capacity retention of 91.8% after 100 cycles at 1.0 C,while the initial reversible capacity of the bare Na2FeP2O7/C was only 66.2 mA·h·g-1. The improvement of apparent Na+ diffusion and electrical conductivity due to B doping were verified by the EIS test and CVs at various scan rate. The experimental results from present work is useful for opening new insight into the contrivance and creation of applicable sodium polyanionic cathode materials for high-performance.

1. Introduction

In recent years, the limited lithium metal resources consumed rapidly lead to the rising costs of raw materials for lithium ion batteries(LIBs)[1].As a promising alternative to LIBs,sodium-ion battery, which possesses the similar ‘rock chair’ principle, has attracted increasing attention due to the abundance and low-cost of sodium resources [2,3]. The safety and stability of composite materials based on polyanionic frameworks as secondary battery electrode material have attracted the attention of researchers [4].Na2FeP2O7/C composite is a suitable choice as positive electrode material for sodium ion batteries, due to its suitable theoretical specific capacity (～97 mA·h·g-1), excellent cycle performance,low preparation cost [5]. Nevertheless, the lower efficiency of charging and discharging for the initial cycle and the terrible rate performance limit its further application.

Actually, the rate performance of Na2FeP2O7was influenced by the discontinuous electrons pathways which could attribute to the discontinuity between the metal octahedrons in Na2FeP2O7crystal structure and then lead to the poor electronic conductivity[6].The surface coating, nano-sizing materials and bulk doping are effective methods to modify the materials. The surface coating can improve the electronic conductivity of cathode materials, for example, Longoniet al.[7] reported a facile, easily scalable and effective synthetic route for a Na2FeP2O7/MWCNT composite,which shows an excellent stability and high capacity retention through cycles. Chenet al.[8] prepared the Na2FeP2O7cathode through a low cost, soft-chemistry synthetic approach, which is proposed as a viable preparation process for Na2FeP2O7/MWCNT composites. Moreover, the further exhaustive electro-chemical characterization has been conducted to legitimate the material mentioned above as a valid cathode material for aprotic sodium ion secondary batteries.For the nano-sizing materials,such as Na2-FeP2O7@C nanocomposite and Na2FeP2O7/carbon nanotubes[9,10],which can obtain a large surface area and shorten the Na+diffusion path distance in Na2FeP2O7/C particles [11]. For doping modification, the research of Na2FeP2O7/C materials are almost focus on the polyanionic doping and Fe-site doping, which can change the structure and morphology of the materials, lead to stabilize the crystal structure and improve the electrochemical properties of Na2FeP2O7/C particles [12-16]. Honmaet al.[17,18] fabricated Na2-xFe1+x/2P2O7/C composite by glass-ceramics method and developed the glass-ceramic cathodes as sodium ion batteries due to the favorable cathode performance. Nevertheless, there are limited studies about non-metallic element doping with boron.Shenet al.[19]adopt boron doping approach to modify the carbon coated Na3V2(PO4)3electrode materials, which exhibits a remarkable improvement in Na storage performance. Therefore, it is necessary to study the influence of boron doping on the properties of Na2FeP2O7/C cathode materials for the high-value recycling of boron in waste borax.

Herein, the Na2FeP2-xBxO7/C (x= 0, 0.025, 0.05 and 0.075,respectively)materials were preparedviaa novel method of liquid phase combined with high temperature solid phase. The Na2-FeP1.95B0.05O7/C cathode material exhibited an initial reversible capacity of 87.5 mA·h·g-1at 0.1 C, and a preferable cycle performance with 91.8% capacity retention after 100 cycles at 1.0 C. In addition, the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry(CV)at various scan rate showed the improvement of apparent Na+diffusion and electrical conductivity due to B doping generated the improved electrochemical performance of Na2FeP2O7/C. It can be an effective approach to achieve the highvalue recycling of B element in waste borax due to the influence of non-metallic element B doping on the crystal structure stability,Na+diffusion and electrical conductivity of Na2FeP2O7/C.

2. Materials and Methods

2.1. Synthesis of Na2FeP2-xBxO7/C

The Na2FeP2O7/C and Na2FeP2-xBxO7/C materials were preparedviaa novel liquid phase combined with high temperature solid phase method, the raw materials of which were of Na2CO3(99%pure, Aldrich), FeC2O4(99% pure, Aldrich), H3PO4(85% pure,Aldrich), ethanol (99% pure, Macklin) and H3BO3(99% pure,Aldrich). Sucrose (99% pure, Aldrich) was used as the carbon sources in this work. The reaction of NaCO3with the pure H3PO4and H3BO3in ethanol solution to formed NaH2PO4and Na2B4O7vianeutralization method due to the low solubility of sodium dihydrogen phosphate and sodium borate in ethanol. In accordance with the stoichiometric ratio of 0.01 mol Na2FeP2-xBxO7/C, weigh a certain amount of pure H3PO4, FeC2O4, H3BO3and anhydrous ethanol in the reactor fully stirred to get 100 ml mixed solution.Then, 0.02 mol NaCO3powder (excessively 3%(mass)) was added and stirred for 6 h.After the solution was aged for 12 h,the mixture was washedviacentrifuge with absolute ethanol,dried at 120°C to obtain precursor. The precursor was mixed with appropriate amounts of sucrose (the mass ratio of sucrose to precursor was 5: 95) in a high-energy ball mill for 8 h. The mixture was pressed into a diameter of 25 mm, thickness of 3 mm round piece at 10 MPa. Subsequently, the piece was transferred into the tube furnace equipped with a program temperature controller pre-pass (5% H2+ 95% Ar) argon-hydrogen mixture for 30 min,then calcined at 220 °C for 4 h, after cooled to room temperature,ground and calcined at 650°C for 10 h to form the Na2FeP2-xBxO7/C materials. For comparison, the bare Na2FeP2O7/C composites material is prepared using the same method with boron free.

2.2. Material characterizations

The X-ray diffraction (XRD, D8-advance, Bruker) with working parameters of CuKα radiation (λ = 0.154056 nm, 40 kV, 200 mA)from 10°to 60°with a step size of 0.02°was performed to identify the crystal phase structures of the powders. The structural refinement was employed by the Rietveld analysisviathe software of Jade 5.0. The morphologies of the samples were investigated by the Scanning Electron Microscope (SEM, Gemini SEM 300, ZEISS)with working parameters of EHT = 15 kV andIprobe= 600 pA. The X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM) with an AlKa(l= 0.83 nm,hn= 1486.7 eV) X-ray source operated at 15 kV and 10 mA were performed to identify the bonding environment of elements.

2.3. Electrochemical measurements

The cathode electrodes were prepared by mixing 80% of active material and 10%acetylene black,then a solution made by dissolving 10% polyvinylidene fluoride (PVDF) inN-methylpyrrolidone(NMP) with a mass ratio of 5% was added to this mixture to form a homogeneous slurry. The slurry was uniformly coated on Al foil and dried for 12 h in the vacuum drying oven, then the obtained electrodes were cut into wafers with a diameter of 13 mm used as the cathode electrode.The electrochemical properties of all samples were studied in the CR2025 button cells, which were assembled in an argon-filled glove box. Sodium metal was used as the anode.The electrolyte was 1.0 mol·L-1NaClO4dissolved in a solution composed of ethylene carbonate and diethyl carbonate (DEC,EC:DEC = 1:1 with volume ratio). A glass fiber diaphragm (GFWhatman) was used as the separator. The charge/discharge of the materials were tested at the current densities of 0.1-2.0 C (1 C = 97 mA·g-1) within voltage of 2.0-4.2 V (vs.Na/Na+) by LAND-2001A test systems (Wuhan, China). The cyclic characterization of the materials was tested at the current densities of 0.5 C and 1.0 C, respectively within voltage of 2.0-4.2 V (vs.Na/Na+). Cyclic voltammetry was conducted by PARSTAT 2273 (American Princeton Company) in the potential range of 2.0-4.2 V (vs.Na/Na+) at scan rates of 0.1 mV·s-1, 0.2 mV·s-1, 0.5 mV·s-1, 1.0 mV·s-1,2.0 mV·s-1and 5.0 mV·s-1,respectively.The electrochemical impedance spectrum measurement was carried out between 0.1 Hz and 100 kHz frequency.The cathode electrodes for cyclic characterization,CV test and EIS test were prepared by mixing the active material,acetylene black,and binder PVDF at a weight ratio of 70/20/10.All the electrochemical measurements were performed under room temperature conditions.

3. Results and Discussion

3.1. Structure and morphologies

As shown by the XRD patterns in Fig. 1a, the peaks of the final Na2FeP2O7/C and Na2FeP2-xBxO7/C, were identified as monoclinic Na2FeP2O7and matched well with the standard patterns(PDF#80-2409) [20]. There were no other peaks of impurities found in the all patterns, which indicate that the samples have better purity in crystalline phase.In addition,the peaks of graphitic carbon could not be detected in the patterns, due to the carbon in Na2FeP2O7/C is amorphous [21]. The crystal structure of the Na2FeP2O7as shown in Fig. 1b. The crystal structure provide a 3-dimensonal framework for sodium ions to migrate efficiently for the infinite arrays of[P2O7]and[FeO6]octahedra[4].Table 1 gives the refined lattice parameters of Na2FeP2-xBxO7/C obtained from the XRD spectra based on the calculation of Jade 5.0 soft. As can be seen, the Na2FeP1.95B0.05O7/C showed the minimum of the lattice parameters ‘b’ and ‘c’, but the maximum value of parameter‘a(chǎn)’. Compared to that of the undoped material, the lattice volume corresponding to the amount of B doped (0.025, 0.05, and 0.075,respectively) were reduced to 0.66%, 0.95%, and 0.88%. The a-axis is favorable for the diffusion of sodium ions,and doping B will lead to the increase of the spacing between the a-axis planes, which is conducive to the migration of sodium ions.Meanwhile,the smaller unit cell volume caused by B doping is beneficial to shorten the distance for the de-insertion of sodium ions [22]. When the doping amount of B increases from 0.025 to 0.05,the parameteraincrease due to the more Fe enter FeO6 occupancy,which presents the better crystallinity of the B doping materials(the typical crystal plane(-201) as shown in Fig. 1a). Correspondingly, the decrease of unit cell volume is due to the shorter B-O bond length.When the B doping amount exceeds 0.05, the crystallinity of the doping materials got worse brought by the excess boron. The decrease of lattice parameteraand increase of cell volume attributed to the FeO6 vacancy caused by the smaller radius of B3+(0.027 nm) than that of P5+(0.038 nm) [23]. It could be recognized as stable facets due to B doping that prevented the loss of oxygen on the surface of particles which attributed to the larger bond energy of B-O(187 kcal·mol-1,1 kcal=4.186 kJ)than that of P-O(141 kcal·mol-1)and Fe-O ((95 ± 3) kcal·mol-1), respectively [24].

Table 1 Lattice parameters of Na2Fe(P2-xBxO7)2/C

Fig. 1. (a) XRD patterns of Na2Fe(P2-xBxO7)2/C; (b) Crystal structure of the Na2FeP2O7.

The morphologies of the Na2FeP2O7/C and Na2FeP2-xBxO7/C were examined by SEM, and the corresponding images are presented in Fig. 2(a-d). The synthetic route used in the above experiment can obtain micron-sized materials.In comparison to pristine Na2FeP2O7/C, the main difference of the B doped materials is the lower average size of the material particles. Meanwhile, the secondary particles of all samples are assembled by a large amount of primary crystal. The average size of the Na2FeP1.95B0.05O7/C material particles is relatively small and the bulk aggregates with a relatively smooth surface can be observed with more than 1 μm of the cathode material.It can be concluded that boron doping may reduce grain size and improve grain uniformity,due to the plenty of the H-O bond in H3BO3escapes during the hightemperature sintering process of the material, which reducing the agglomeration of the material [25]. The high-magnification SEM images of Na2FeP2-xBxO7/C as shown in Fig. S1 (see Supplementary Material), all sample showed the structure of carbon layer which was the pyrolysis production of sucrose since the carbon-coating layers play a role in constraining the crystal growth of Na2FeP2-xBxO7/C particles [21]. Meanwhile, the large particle size of pristine Na2FeP2O7/C imposed a negative effect on the fast ion transportation during the charge and discharge process, which was confirmed by the EIS test. Fig. 2(e-j) shows the EDS images to present the distribution of elements in the Na2FeP2-xBxO7/C. The detected elements, such as P, Fe, Na, B and O in the samples were homogeneous distribution throughout the detected area.

3.2. Electrochemical characterization

The electrochemical charge-discharge tests were first conducted to evaluate the influence of boron on pristine Na2FeP2O7/C for SIBs. The initial charge and discharge curves of Na2FeP2-xBx-O7/C at different rates were shown in Fig. 3(a-d). All sample showed two typical flat voltage plateaus during the charge/discharge process [26]. For Na2FeP1.95B0.05O7/C materials, the first charge-discharge platform is about 3.0 V, and the second charge-discharge platform is about 2.5 V. Nevertheless, compared to the non-doped Na2FeP2O7/C sample,the B doped materials present a wider voltage plateau with smaller polarization part, suggesting that the B doped samples have better reaction kinetics[9]. In addition, Na2FeP1.95B0.05O7/C sample exhibited a significant improvement of the initial charge-discharge capacity with discharge capacities of 87.5, 81.8, 68.2, 60.2 and 46.4 mA·h·g-1at 0.1 C,0.2 C,0.5 C,1.0 C,and 2.0 C,respectively,while the bare Na2-FeP2O7/C sample delivered a poor discharge capacities of 79.2,64.1,50.3,41.0 and 30.8 mA·h·g-1.The results demonstrated that B doping had positive effects on the discharge capacities of Na2FeP2O7/C at different current rates.It could attributed to the improved properties of Na2FeP1.95B0.05O7/C samples by introducing B element,such as homogeneous and smaller particle sizes, which cause a shorter distance for fast Na-ion diffusion in the redox reaction process. Similar phenomena are found in other studies using positive ion doping[13].It is worth noting that the initial charge-discharge capacity of Na2FeP1.925B0.075O7/C is lower than Na2FeP1.95B0.05O7/C.It could be the excess of B element contributed to form smaller particle sizes,and large lattice parameters‘b’and‘c’,which leading to lower electrochemical activity. Fig. 3e presents the highly reversible discharge capacity of the pristine and Na2FeP2-xBxO7/C at different rates. The cells were first discharged at 0.1 C, 0.2 C, 0.5 C,1.0 C, 2.0 C rates and then discharged at and 0.2 C rates, respectively.The discharge capacity of bare Na2FeP2O7/C and Na2FeP1.95-B0.05O7/C is 53.6 and 71.6 mA·h·g-1when the rate restores to 0.2 C after rate performance testes. It is demonstrated that after the proper B-doping, the specific capacity of the material can be increased and discharged at high magnification, indicating that the material has good structural stability. Fig. 3 (f, g) shows that the cycling performance of Na2FeP2-xBxO7/C cathode materials at current density of 0.5 C and 1.0 C, respectively. The discharge capacity of the positive electrode material was 55.0 and 37.7 mA·h·g-1, 54.9 and 47.2 mA·h·g-1, 73.0 and 70.9 mA·h·g-1, 68.6 and 66.0 mA·h·g-1,respectively,and the corresponding capacity retention rate was 68.5%, 86.0%, 97.1% and 96.2%, respectively. Meanwhile, there was a great improvement of cycle performance at a rate of 1.0 C that the discharge capacity of Na2FeP2O7/C and Na2-FeP1.95B0.05O7/C cathode material over 100th cycles were decreased from 74.8 to 68.7 mA·h·g-1and 66.2 to 61.3 mA·h·g-1,respectively. The corresponding capacity retention rate are 91.8%and 92.5%, respectively. Compared with the undoped material,the proper B doping improves the magnification performance of the material and improves the polarization phenomenon of the material during the charging and discharging process. The favorable cycling retention can be attributed to the strong binding force of B-O elements presented a clearly steady cycling property and the improved conductivity of bare Na2FeP2O7/C with more acetylene black, which confirmed that the efficiency improvement of the insertion/extraction of Na+in the crystal lattice[16].The comprehensive electrode performances of various sodium polyanionic cathode materials are compared in Table 2. It can be clearly seen that the initial capacity of Na2FeP1.95B0.05O7/C is higher than most of the known sodium polyanionic cathode materials,and the satisfactory high-rate performance of the Na2FeP1.95B0.05O7/C also exceeds that of many known sodium polyanionic cathode materials, indicating its promising for SIBs applications.

Table 2 Rate capability of Na2FeP1.95B0.05O7/C in this work in comparison with previous reported cathodes

Table 3 Na+ diffusion coefficient calculated from the different peaks of CV curves for bare Na2FeP2O7/C and Na2FeP1.95B0.05O7/C

Fig. 2. SEM diagrams of Na2FeP2-xBxO7/C: (a) x = 0; (b) x = 0.025; (c) x = 0.05; (d) x = 0.075. (e-j) the elements distribution of Na2FeP1.95B0.05O7/C.

Fig. 3. (a-d) The first charge and discharge curves of Na2FeP2-xBxO7/C at different rates: (a) Na2FeP2O7/C; (b) Na2FeP1.975B0.025O7/C; (c) Na2FeP1.95B0.05O7/C; (d) Na2FeP1.925-B0.075O7/C; (e) The cycle property of Na2FeP2-xBxO7/C at different rates: 0.1 C, 0.2 C, 0.5 C, 1.0 C, 2.0 C and then discharged at and 0.2 C rates, respectively; (f) cycling performance of Na2FeP2-xBxO7/C during 100 cycles at 0.5 C; (g) cycling performance of Na2FeP2O7 and Na2FeP1.95B0.05O7 electrodes during 100 cycles at 1.0 C.

3.3. XPS analysis

XPS analysis was conducted in order to gain insight into the oxidation states of the B, Fe, and P ions of the B doped materials. The high resolution XPS spectra obtained for Na2FeP1.95B0.05O7/C were displayed in Fig. 4. The peak at 192.5 eV demonstrates the existence of B in the 1s oxidation state as shown in Fig. 4a [29]. The XPS results of P 2p peak, B 1s and Fe 2p for Na2FeP1.95B0.05O7/C were carefully analysed in Fig. 4(b-d). The only P 2p peak at 133 eV corresponds to the P-O bond in the metal phosphate [11].The B1s spectra of Na2FeP1.95B0.05O7/C sample was composed of four main contributions:in ascending order,the peaks at the binding energy ～187.9 eV, ～190.3 eV, 191.3 and 192.2 are correspond to B-B type bond,BxO type bond,metal borate(B-O)type bond and B2O3bond, respectively [30,31]. These results were certified that the boron have been successfully doped into Na2FeP1.95B0.05O7/C sample. The peak at the binding energy ～709.6 eV and～722.8 eV belongs to Fe2+2p3/2and Fe2+2p1/2,respectively.Meanwhile,the Fe 2p3/2peak at 710.9 eV corresponds to Fe in the+3 oxidation state. The two pair satellite peak in lower binding energy were assigned to Fe2+2p and Fe3+2p [28]. This can prove that the Fe in the Na2FeP1.95B0.05O7/C is composed of two valence states,which is consistent with the redox during the charge and discharge process of the material.

3.4. Na ions diffusion

As shown in Fig.5(a,b),The CV curves of Na2FeP2O7/C and Na2-FeP1.95B0.05O7/C were carried out at scan rates of 0.1, 0.2, 0.5, 1.0,2.0 and 5.0 mV·s-1between 2.0-4.2 V (vs.Na/Na+), respectively.The apparent chemical diffusion coefficient of Na2FeP2O7/C and Na2FeP1.95B0.05O7/C cathode material were measured by the cyclic voltammograms method,due to the diffusion process is the control step and the electrode is a reversible system.The CV curves of both samples present two pair oxidation/reduction peaks, corresponding to the redox reaction of Fe2+/Fe3+[4]. In detail, the first oxidation peak located at around 2.5 V is relate to the deintercalation process of sodium from Na2FeP2O7to Na1.75FeP2O7.The peaks associated with the major platforms at around 3.1 V in the constant current loop could divided into three different peaks (at 2.97,3.11 and 3.25 V), which are consistent with a series of different reaction mechanisms that occur phase transfer during the of sodium ions from Na1.75FeP2O7to Na1FeP2O7[15]. The cyclic voltammogram with different B-doping amount have also been investigated at a scanning rate of 0.2 mV·s-1as shown in Fig. S2.The CV curves of Na2FeP1.95B0.05O7/C also show higher voltage of oxidation peaks and smaller potential deviation (△V1= 0.06 V,△V2=0.10 V)compared to bare Na2FeP2O7/C,which means higher discharge plateaus consistent with the charge-discharge curves in Fig.3.It can be attributed to the B doping facilitate the reduction of the activation energy required for the phase transfer occurring during the deintercalation of sodium ions. Fig. 5(c, d) showed the cathodic and anodic peak currentIpof both materials are linear with the square root of scan rate ν1/2.The chemical diffusion coefficient (DNa+) of Na2FeP2O7and Na2FeP1.95B0.05O7were calculated with the formula (1) and (2) [32,33]:

Fig. 4. High resolution XPS spectra obtained for (A) Full spectrum; (B) P; (C) B and (D) Fe of Na2FeP1.95B0.05O7/C.

WhereIp(A) is the magnitude of the peak current,nis the number of electrons per reaction species,A(cm2)represents the area of the electrode immersed in the solution,F(C·mol-1) is the Faraday constant,DNa(cm2·s-1) strands for the Na diffusion coefficient in the electrode, ν (V·s-1) represents the scan rate, and ΔC0(mol·cm-3)represents the value of the concentration change during the reaction. The slopes of the straight lines fitted from different peaks marked with letters was shown in Table 3 including the corresponding apparent Na+diffusion coefficient calculated from formula(2). There is a visible improvement of the mobility of Na+diffusion at peak a and a′in B doped material(DNa+of 1.5447×10-8cm2·s-1and 9.2065×10-11cm2·s-1,respectively)compared with the B free sample (DNa+of 3.2164 × 10-9cm2·s-1and 1.18 × 10-11cm2·s-1),which demonstrates the promoting Na+migration due to B doping.

In order to detect the influence of B doping on kinetics of Na+diffusion, the EIS date and methods reported elsewhere were performed to calculate the Na+diffusion coefficient[34].All tests performed after each battery was subjected to a complete cyclic voltammetry test. The results of electrochemical impedance spectra for Na2FeP2O7/C and Na2FeP1-xBxO7/C with corresponding equivalent circuit model insert are shown in Fig. 5e. The impedance spectra of both samples were similarly with one capacitive loop at high frequency and inclined line at low frequency correlated to Warburg impedance(Zw).The resistance of charge transfer(Rct) and the constant phase element with non-ideal capacitors(CPE1) correspond to the low frequency semicircle. The intersection of impedance curve and Z’ axis are correlated with the resistance of the electrode and the electrolyte (Rs) [35]. The Z view software was employed to calculate the fitting values of parametersRs,RctandZw,as listed in Table S1. The lower resistance of charge transfer of Na2FeP1.95B0.05O7/C remarkably enhance electronic conductivity and the charge transfer process. The reason for this is that the conducting network structure of Na2FeP1.95B0.05-O7/C provides more wide channels for the transfer of charges in the cathode electrode, which leading to solvation ability and polarizability improvement[36].Simultaneously,the diffusion coefficient(DNa+) of the sodium-ions were calculated with equations (1) and(2) [37,38]:

Where ω (Hz) represents the frequency, the Warburg impedance coefficient (δω) is acquired by the linear fitting ofZ′-ω-0.5plot as shown in Fig. 5f,R(J·mol-1·K-1) is the constant of ideal gas andF(C·mol-1) represents the Faraday constant.T(K) is the absolute temperature andA(cm2) is the surface area of the active material,C(mol·cm-3) represents the density of Na+in the active electrode material related to the state of sodium insertion. The Na2FeP1.95-B0.05O7/C showed a higherDNa+of 1.374 × 10-11cm2·s-1than the as-prepared B free material withDNa+of 1.172 × 10-12cm2·s-1,coinciding with the results measured by the CV tests.It proves once again that the improvement of the Na+mobility diffusion by B doping. As a consequence, the improvement of the kinetic behavior of insertion/extraction of Na+and electronic conductivity interpret the better electrochemical performance of B doping modified materials as previously mentioned.

Fig.5. (a)Cyclic voltammograms of the Na2FeP2O7/C sample for various sweep rates;(b)Cyclic voltammograms of the Na2FeP1.975B0.025O7/C sample for various sweep rates;(c)Peak current Ip as a function of square root of scan rate ν1/2:Na2FeP2O7/C.(d)Peak current Ip as a function of square root of scan rate ν1/2:Na2FeP1.95B0.05O7/C.(e)Nyquist plot of the Na2FeP2-xBxO7/C samples after cyclic voltammograms for various sweep rates.(f)The relationship between Z′ and ω-1/2,Z′′ and ω-1/2 in the low frequency region.

4. Conclusions

In this work,boron doped Na2FeP2O7/C cathode materials were prepared by a novel liquid phase combined with high temperature solid phase method,which could serve as an effective approach to achieve the high-value recycling of B element from waste borax.The effects of B doping on electrochemical properties and the structure were investigated to improve the properties of Na2FeP2-O7/C cathode materials by using XRD, SEM and electrochemical tests. The results showed that B doping improved the dischargespecific capability of cathode materials. The Na2FeP1.95B0.05O7/C material showed homogeneous particles with a smaller size of cathode material and exhibits superior electrochemical performance with specific capacities of 87.5, 81.8, 68.2, 60.2, and 46.4 mA·h·g-1at 0.1 C, 0.2 C, 0.5 C, 1.0 C, and 2.0 C, respectively. The CV and EIS tests analysis showed that the Na2FeP1.95B0.05O7/C material presents smaller polarization and charge transfer resistance in comparison to the pristine Na2FeP2O7/C material.In addition,the existence of B stabilized the crystal structure and reducing the size of homogeneous particles,as well as improved ionic diffusion during both the charge and discharge processes.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2019YFE0123900) and the National Natural Science Foundation of China (51974069)

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.09.011.