SHI Jiayuan,WANG Qingjie,XU Xusheng,YANG Qinghua,CHEN Xiaotao,LIU Fuliang,SHI Bin
(State Key Laboratory of Advanced Chemical Power Sources (SKL-ACPS),Guizhou Meiling Power Sources Co.,Ltd,Zunyi 563003,China)
Abstract: LiFePO4 cathode was successfully co-coated by ZrO2 and N-doped carbon layer based on the coprecipitation of Zr species and polydopamine on the LiFePO4 surfaces.The mutual promotion between the hydrolyzation of ZrO2 precursor and the self-polymerization of dopamine was realized in the one-step synthesis.After being used in the coin battery as cathode material,the ZrO2 and N-doped carbon co-coated LiFePO4 displayed improved cycling stability (97.0% retention at 0.2 C after 200 cycles) and enhanced rate performance(130.7 mAh·g−1 at 1 C) due to its higher electrochemical reactivity and reversibility compared with those of commercial LiFePO4.
Key words: LiFePO4;ZrO2;N-doped carbon;cathode;lithium batteries
Rechargeable lithium ion batteries have been widely applied as the electrochemical energy storage devices in portable electronics and electric vehicles.Among various cathode materials,the olivine lithium iron phosphate (LiFePO4,LFP) has been regarded as one of the most promising candidates for the largescale application of lithium batteries.It possesses the theoretical discharge capacity of 170 mAh·g−1and a voltage platform of 3.4 V.Unfortunately,LiFePO4possessed slow ion diffusion (10−13-10−16cm2·s−1) and low electronic conductivity (10−8-10−10S·cm−1),leading to its limitation in commercial applications[1].
In order to solve the above problems,many strategies have been adopted to modify LiFePO4,including conductive layer coating and heteroatom doping[2].Carbon coating of LiFePO4has been well developed through the high temperature treatment of the polydopamine-coated LiFePO4to improve its electron conductivity[3].Dopamine as a natural molecule can form thin films through its selfpolymerization in alkaline conditions[4].The strong affinity and complexation ability of dopamine on material surface allowed continuous deposition of polydopamine as coating layer of LiFePO4[5].In addition,the N species in the dopamine-derived carbon layers resulted in conductivity improvement of the LiFePO4/C materials[6].However,the polydopamine formation was usually carefully controlled in the Trisbuffer,which increased the complexity of LiFePO4modification.
To improve the ionic conductivity of cathode,ZrO2has been coated on the LiFePO4surface[7].The comparison of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)results between the pristine and ZrO2-coated LiNi0.6Co0.2Mn0.2O2indicated the decreased polarization and reduced the interfacial resistance of cathode upon ZrO2modification[8].Moreover,the ZrO2coating layer as the physical barrier can effectively prevent the direct contact and inhibit the side reaction between cathode materials and electrolyte,which enhanced the electrochemical stability of the cathode materials[9].However,the electron conductivity of LiFePO4can not be significantly improved with coating of ZrO2.Therefore,the co-coating of LiFePO4with ZrO2and carbon may be a feasible option for the simultaneous improvements of electronic and ionic conductivity of LiFePO4.
In this work,the coprecipitation of Zr species and polydopamine was realized through mutual promotion between hydrolyzation of ZrO2precursor and self-polymerization of dopamine in the presence of LiFePO4.After being treated at 500 ℃,the LiFePO4was successfully co-coated by ZrO2and N-doped carbon layer,which showed improved cycling stability and enhanced rate performance for lithium batteries.
0.172 g of zirconium propoxide solution (70wt%)and 0.08 g of dopamine hydrochloride were dissolved in 20 mL of isopropyl alcohol and 1 mL of water,respectively.Then,2 g of commercial LiFePO4was added into the diluted zirconium propoxide solution.After being stirred for 30 min,the dopamine solution was added and stirred for another 24 h at room temperature.The precipitate can be obtained through centrifugalization,washing with water and ethanol and vacuum drying of the reaction mixture.The final N-doped carbon and ZrO2co-coated LiFePO4(named as LFP-ZC) can be obtained through calcining the above precipitate at 500 ℃ for 6 h in argon.
The crystal structure of samples was characterized by an X-ray powder diffractometer under Cu Kα radiation ranging from 10° to 80° at a scan rate 6 °/min.The surface morphology and particle size were evaluated using a field-emitting scanning electron microscope (SEM,FEI Quanta 250).Energy dispersive spectroscopy (EDS) was employed in SEM to investigate the distribution of elements.The mass ratio of different elements in samples was measured via inductively coupled plasma optical emission spectroscopy (ICP-OES) (AGILENT ICPOES 730).The X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250Xi spectrometer(ThermoFisher Scientific) with Al-Kα (1486.6 eV) as the X-ray source.Thermogravimetric analysis (TG)was performed with an STA 409 PC Luxx thermal analyzer from room temperature to 800 ℃ at a heating rate of 10 ℃/min in air.
The electrochemical properties were investigated by 2032 coin-type cells assembled in an Ar-filled glovebox.The working electrodes were prepared by mixing 80wt% active material,10wt% super P,and 10wt% polyvinylidenefluoride (PVDF) in an appropriate amount of N-methyl-2-pyrrolidine with the assistance of ultrasound,which was then pasted on aluminum foil and dried at 120 ℃ in vacuum overnight.Lithium foil was used as anode and Celgard 2400 membrane as separator.An electrolyte of 1M LiPF6dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1,v/v) was used.The galvanostatic charge-discharge cycles text was performed at various currents (0.1 C,0.2 C,0.5 C and 1 C) within 2.5-4.2 V(vs.Li/Li+) at 25 ℃.The electrochemical impedance spectroscopy (EIS) was conducted on VersaSTAT 3F electrochemical workstation with a voltage amplitude of 5 mV from 10 mHz to 100 kHz.The cyclicvoltammetry (CV) was performed at 0.2 mA·s-1from 2.0 to 4.5 Vvs.Li+/Li.,and it was carried out on VersaSTAT 3F electrochemical workstation.
To investigate the crystallographic structures of LFP and LFP-ZC samples,their XRD patterns are shown in Fig.1 and compared with the standard XRD spectra of LFP and ZrO2.The pristine LFP and LFPZC samples displayed similar results and both patterns can be indexed to space group Pnma and match well with the orthorhombic LiFePO4structure (JCPDS card number: 40-1499)[10].No peaks related to ZrO2phase can be observable for LFP-ZC sample,indicating that the ZrO2coating was too thin or too small to be detected[11].No other impurity phases can be found in the XRD pattern of LFP-ZC,indicating the negligible influence of co-coating process on the LiFePO4phase.
Fig.1 XRD patterns of (a) pristine LFP,(b) LFP-ZC and the standard spectra of LFP and ZrO2
Fig.2 compares the morphology and structures of LFP and LFP-ZC through SEM images.Both samples were composed by the irregular particles with the size from hundreds of nanometers to a few micrometers.No obvious difference can be distinguished between these two samples.It indicated the negligible effect of the co-coating on the morphology of the LFP sample.The elemental mapping images in the red box of the LFPZC indicated the presence of Fe,P,and O elements from LFP.The homogeneous distribution of C,N and Zr suggested the successful co-coating of ZrO2and N-doped C on the LFP surfaces.
Fig.2 SEM iamges of (a) pristine LFP,(b) LFP-ZC and the elemental mapping of C,O,P,Zr,Fe,and N in the red box of the LFP-ZC SEM image
The thermogravimetric curves of LFP and LFPZC are compared in Fig.3.The weight loss from room temperature to 150 ℃ can be attributed to the escape of physisorbed water from samples.Both LFP and LFPZC showed about 99.4% of weight retention at 150 ℃,indicating the negligible difference of water content in these two samples.The weight increase above 150 ℃was mainly due to the oxidation of LFP to Li3Fe2(PO4)3and Fe2O3[12].Moreover,the oxidation process can be described by formula (1),and the theoretical weight gain of pure LFP was about 5.07% after complete oxidation.
Fig.3 The TG curves of pristine LFP and LFP-ZC samples
However,the weight gain of the commercial LFP used in this work was less than 4%,which may be due to the carbon addition in the commercial LFP,as indicated in its XPS results (Fig.4(a)).Compared with the LFP (103.0%),the lower weight gain of the LFPZC was due to the oxidation of N-doped carbon coating layer at the elevated temperature[13].The weight change of LFP-ZC was 100.4% after oxidation,indicating that the content of N-doped carbon in the LFP-ZC composites was about 2.6%.In addition,the ICP results indicated the ZrO2content of the modified samples was about 2.3%.
XPS spectra were performed to study surface chemical composition and valence states of LFP(Fig.4(a)) and LFP-ZC (Fig.4(b)) samples.As shown in Fig.4(b),the peaks at binding energies of 54,710,133,531,284,and 399 eV were exhibited,corresponding to the elements of Li 1s,Fe 2p,P 2p,O 1s,C 1s,and N 1s,respectively.For the Fe spectrum in Fig.4(c),two peaks at about 710 and 723 eV can be attributed to the characteristic of Fe 2p3/2and Fe 2p1/2for the Fe element.The peaks appeared at 181 and 184 eV can be regarded as Zr 3d5/2and Zr 3d3/2,respectively,indicating the existence of Zr (IV) in LFP-ZC (Fig.4(d))[14].The C 1s and N 1s peaks can be observed near 284 and 399 eV,respectively (Figs.4(e) and 4(f)).Therefore,ZrO2and N-doped C have been successfully coated on LFP surfaces,as indicated by XPS results.
Fig.4 XPS survey spectra of (a) the pristine LFP,(b) the LFP-ZC,and the wide scan spectra of (c) Fe 2p,(d) Zr 3d,(e) C 1s,and (f) N 1s
To compare the electrochemical kinetics characteristics of LFP and LFP-ZC,the EIS and CV measurements were carried out,as shown in Fig.5.Compared with LFP,the EIS curve of LFPZC with smaller semicircle and gentler slope indicated its lower charge-transfer resistance and higher lithium ion diffusion rate during cycling.The corresponding equivalent circuit is illustrated in the inset of Fig.5(a).TheRs,Rct,Zw,andCdwere used as the ohmic resistance,charge-transfer resistance,Warburg impedance and the double layer capacity of the electrode,respectively[15].Thus,the calculatedRctvalues of LFP and LFP-ZC were 238.2 and 158.3 Ω,respectively.
The diffusion coefficient (DLi+) of lithium ions can be calculated through equation (2)[16],whereRis the gas constant,Tis the absolute temperature,Athe electrode area,nthe number of electrons per molecule during oxidation,Fthe Faraday constant,Cthe concentration of lithium ions,andσthe Warburg factor,which can be calculated from the slope of the real axis resistance (Zre)vsthe inverse square root of the angular frequency (ω-1/2) according to Fig.5(b) and equation(3)[17].Accordingly,the values ofDLi+were 4.8×10-14and 3.6×10-15cm2·S-1,respectively,which were comparable to the results in previous reports[18]:
Fig.5 (a) EIS spectra,(b) the relationship between Zre and ω-1/2 at low frequencies and (c) the CV curves of the pristine LFP and LFP-ZC samples
Fig.5(c) shows the CV curves of LFP and LFPZC.Only one pair of redox peaks can be observed for both LFP and LFP-ZC,which were corresponded to the electrochemical reaction of the Fe2+/Fe3+pairs.Compared with LFP,the sharper anodic/cathodic peaks of CV curves indicated the higher electrochemical reactivity of LFP-ZC[19].The potential intervals of the samples between two peaks were 343 and 301 mV for LFP and LFP-ZC,respectively.It indicated the lower polarization and improved reaction kinetics of LFP-ZC electrode[20].Compared with LFP (0.906),the area ratio between anodic and cathodic peaks was higher (0.969) for LFP-ZC,indicating the enhanced electrochemical reversibility upon its N-doped C and ZrO2co-coating[21].Therefore,the lower polarization and enhanced electrochemical reversibility of LFPZC can be confirmed by CV results,which was in agreement with their EIS and galvanostatic charging and discharging tests.
Fig.6(a) displays the initial charge/discharge profiles of two samples.The initial discharge capacities of LFP and LFP-ZC are 158.4 and 155.6 mAh·g−1at 0.1 C,respectively.The curves in the square area were enlarged as the inset of Fig.6(a).The potential intervals (ΔE) between charge and discharge curves were 97 and 47 mV at 0.1 C for LFP and LFP-ZC,respectively (Table 1).The interval decrease of LFPZC can be attributed to the improved electron affinity and enhanced lithium diffusivity upon N-doped carbon and ZrO2co-coating,indicating the accelerated reaction kinetics and decreased polarization of LFP-ZC cathode[22].
Table 1 Polarization between charge and discharge curves of the cathodes containing LFP and LFP-ZC at various rates
Table 2 The average discharge capacities and capacity retention of LFP and LFP-ZC at different C-rates
The cycling performance of pristine LFP and LFP-ZC at 0.2 C is presented in Fig.6(b).The discharge capacity and its capacity retention were 141.3 mAh·g−1and 97.0% for LFP-ZC after 200 cycles,respectively,which were much higher than those of pristine LFP(52.9 mAh·g−1and 42.0%).Such high capacity retention of LFP-ZC was superior to those of the previously reported LFP-based cathode materials,such as the LFP/C obtained by Wang’s group (96.7% after 50 cycles at 0.1 C)[23]and LFP@C reported by Yuan’s group (92.7% after 100 cycles at 0.2 C)[24].For the pristine LFP,the fluctuation of discharging capacity became serious after 120 cycles.It indicated the destroyed electrochemical stability of pristine LFP after long-term cycling,which has not been found in the corresponding results of LFP-ZC after 200 cycles.The changes of coulombic efficiency during 200 electrochemical cycles at 0.2C are shown in Fig.6(c).The average coulombic efficiencies were 95.2% during 120 cycles for pristine LFP and 98.5% during 200 cycles for LFP-ZC.
Figs.6(d) and 6(e) plott the charge/discharge curves of LFP and LFP-ZC samples at various C-rates.The potential differences (ΔE) between charge and discharge curves became larger with the increase of the current intensities for both LFP and LFP-ZC samples(Table 1),indicating the more serious polarization at higher rates.The ΔEvalues of LFP-ZC was 47,69,140,and 281 mV,respectively,which were much smaller than those of LFP.It indicates the improved electrochemical reversibility and enhanced rate performance of LFP-ZC with the N-doped C and ZrO2coating.Fig.6(f) and Table 2 show the rate performance of LFP and LFP-ZC.The discharging capacities of LFP-ZC were 156.2,151.2,141.2,and 130.7 mAh·g−1at 0.1,0.2,0.5 and 1 C,respectively.The capacity retention at 1 C was 86.4% compared with that at 0.2 C.The discharging capacity and capacity retention of LFP-ZC at 1 C in this work were superior to those of the previously reported LFP-based cathode materials,such as Ga-doped LFP@C prepared by Cui’s group(103.6 mAh·g−1,75.9%)[25]and LFP@C obtained by Dong’s group (121.6 mAh·g−1,77.7%)[26].
The co-coating of ZrO2and N-doped carbon layer on the LiFePO4cathode materials was realized through the mutual promotion between ZrO2precursor hydrolyzation and dopamine self-polymerization.Compared with commercial LiFePO4,the ZrO2and N-doped C co-coated LiFePO4showed lower chargetransfer resistance,faster lithium ion diffusion and enhanced electrochemical reversibility of the cocoated LiFePO4.As a result,the ZrO2and N-doped C co-coated LiFePO4displayed higher discharging capacity (141.3 mAh·g−1) and better average coulombic efficiency (98.5%) upon 200 cycles than those of LiFePO4.
