Yunke Wang,Yongjia Li,Yenan Zhang,Guozheng Zha,Feng Liang,Yongnian Dai,Yaochun Yao,*

1 The National Engineering Laboratory for Vacuum Metallurgy,Kunming University of Science and Technology,Kunming 650093,China

2 Engineering Laboratory for Advanced Battery and Materials of Yunnan Province,Kunming University of Science and Technology,Kunming 650093,China

3 Faculty of Metallurgy and Mining,Kunming Metallurgy College,Kunming 650033,China

Keywords:NCA@Cr2O5 composite Cathode material Synergistic effect Electrochemical properties Low voltage

ABSTRACT LiNi0.8Co0.15Al0.05O2 (NCA)@Cr2O5 composite electrode combines the high rate-capability characteristics of NCA with the stability of Cr2O5,playing a synergistic role in improving the cyclic stability,initial discharge capacity and the security of low cut-off voltage(2.0 V).When the mass ratio of Cr2O5 in NCA is 45%(mass),the capacity retention rate increases from 58.5%without Cr2O5 to 69.3%in the range of 2.0–4.3 V.The initial discharge capacity of NCA@Cr2O5 composite material is 211.4 mA∙h∙g-1,its first coulombic efficiency is 94.2%,and the charging capacity remains approximately constant when mixed with 15%(mass)Cr2O5.The reason for the improvement of the initial charge–discharge efficiency (ICDE) was explained.Impedance and cyclic voltammetry analysis reveal more detailed reasons of the observed improvements.Compared with NCA cathode material,the NCA@Cr2O5 composite material can provide not only additional stable sites and channels for Li+ insertion/extraction to make up for the loss of active Li+ sites and prevent the accumulation of Li+ in the circulation process,but also protect the NCA electrode from the corrosion of the electrolyte decomposition by the Cr2O5 nanoparticles adhering to NCA interface.

1.Introduction

To improve the energy density,lifespan and safety property of lithium-ion batteries (LIBs) is of great importance for electric/hybrid vehicles (EVs/HEVs) power.Although Ni-rich ternary layered cathode materials LiNi0.8Co0.15Al0.05O2(NCA) have a similar high theoretical specific capacity of 279 mA∙h∙g-1and a layered structural characteristic to LiCoO2,NCA is cheaper compared to LiCoO2and more stable than LiNiO2,which resulting in making it a hot spot among many researches in energy storage.Nevertheless,the application challenge of the NCA cells is the vulnerability of their chemical stability against operation under abnormal conditions.Reliable safety requires that it can withstand repeated discharge at a low cut-off voltage (LCV).Therefore,it is significant to study the repetitive charge and discharge performance of NCA at LCV for the protection of lithium-ion batteries [1].

In addition,the cycling performance of these Ni-rich cathode materials is poor,the storage capacity and power decay are fast,and the initial charge–discharge efficiency (ICDE) on the layered structure is low[2,3].Great efforts have been devoted to overcome the disadvantages of the capacity loss for NCA materials and to obtain high energy density of the cell.Most researchers focus on lattice doping [4,5] to optimize the structure of the materials,so as to enhance the overall performance;through coating and surface modification[6–8]to prevent particle corrosion and side reaction with the electrolyte,which is crucial to the performance of LIBs;or employing various conductive additive [9,10],which constructs a conductive percolation network to enhance electronic conductivity.However,for the issue of the originally irreversible capacity loss (about 20%) has been neglected in NCA cells.This problem was caused by the formation of antisite defects from occupation of Ni2+in the Li+sites and the generation of phase transition from degeneration of ordered transition layer in the first cycling.In some sense,lower ICDE is almost inevitable and still presents a huge difficulty for their applications in the future [11].Some high valence oxides of Group VIB metals,such as Cr3O8,MoO3and WO3have been received considerable attention because of their high working potential (～3 V vs.Li).They have been used for cathode materials as lithium intercalated reversibly from their structure during charging-discharging as a result of the reversible formation of ternary non-stoichiometric phase.In particular,chromium oxide is considered as a promising cathode material for its high specific energy of～270 W∙h∙kg-1and energy density of～675 W∙h∙L-1at low drain rates [12–14].Yang J et al.improved the initial discharge capacity properties of LiNi0.5Co0.2Mn0.3O2by coating Cr8O21on the surface of it after planetary ball milling[15].Feng X Y et al.[16,17] prepared pure Cr2O5at 350–400 °C.In the potential range 2.0–4.5 V (vs.Li),the Cr2O5had been used in lithium batteries,the samples showed the very high specific capacity.When the Cr2O5had been used in sodium-ion batteries,it delivered a high capacity of more than 300 mA∙h∙g-1.A mixture of Cr2O5and Li-rich cathode was designed by Ding et al.[18] to increase the initial coulombic efficiency whether it was applied to the full battery or half battery.

We are going to use the unique advantages of every material to make the operation requirements of the battery used efficiently,because many types of electronic connections are needed between the active material and the conducting substrate.In this study,Cr2O5was reported to blending cathode of Ni-rich material NCA to improve the electrochemical performance,and the potential range of NCA@Cr2O5composite material was adequately determined,the different low cut-off voltages(LCV)for Cr2O5,NCA and NCA@xCr2O5materials were discussed on the base of their electrochemical properties.The mixing of different cathode materials could minimize the defects of the materials,make the blends have higher energy or power density,enhance the stability and reduce the cost.We first synthesized Cr2O5powders,and mixed with NCA to prepare NCA@xCr2O5composites by a simple mechanical milling method.Then,their electrochemical properties were conducted.This work provides a promising direction for the preparation of cathode materials with high ICDE and stable cycle at LCV in the future.The composite material can be used to facilitate the research of overdischarging protection for lithium-ion batteries,and also is meaningful to deeper understanding of the two cathode material.

2.Experimental

2.1.Synthesis of the materials

Fig.1 shows the synthesis of composite materials.Refer to previous literature,the CrO3(Aldrich,99.9%) was placed in the heating zone (350 °C) of the box furnace for 2 h to obtain Cr2O5powders[16],the product was ground into fine powder in an agate mortar.For NCA material,commercially available NCA powders,which had sphere-like morphology with an average particle size of about 10 μm,were used without any further treatment.The NCA@xCr2O5composite was obtained by grinding with a mortar for 30 min and guaranty the homogeneity.The electrode only containing NCA is labeled NCA,the electrode only containing Cr2O5is labeled Cr2O5,and Cr2O5with different mass ratio of 15%,30%and 45%in the NCA material are labeled as NCAC1,NCAC2 and NCAC3,respectively.It is useful to provide a table to compare the different composite cathode material in a different type of mass ratio for better understanding and references.Some certain blended cathode materials based on literature and related sources are listed in Table 1.

Table 1Summary of composite cathodes with different type of mass ratio

2.2.Characterization of electrode materials

X-Ray diffraction (XRD) studies were performed on the phase composition and structure of the samples (RIGAKUT/RIII-18KW)using Cu Kα radiation (λ=0.154 nm).XRD data were obtained at 10°–80° with a scan rate of 3° per minute at room temperature.The surface morphologies and microstructures of powders were observed using a scanning electron microscopy with field emission(SEM,FEI Quanta FEG 400) operated at 20 kV.The specific surface area was calculated with the Brunauer-Emment-Teller test (BET,Quant achrome) model and the pore size distribution was determined using the Barrett-Joyner-Halenda(BJH)method.The electrical conductivity of the Cr2O5and NCA sample were measured by a four-probe resistivity tester (ST2722-SD) equipped with a high resistance meter (ST2255).

2.3.Electrochemical measurements

Fig.1.Schematic representations synthesis of NCA@Cr2O5 mixed electrode.

Electrochemical evaluations coin-type (CR2025) were used to investigate the electrochemical performances of all half-cells.The cathode electrodes were prepared by dissolving NCA (or Cr2O5or NCA@Cr2O5) active material,conductive carbon black and agglutinating polyvinylidene difluoride(PVDF)in the solvent N-methyl-2-pyrrolidinone (NMP) at the mass ratio of 8:1:1.The cathode electrode material was coated on the Al foil current collector and dried at 80 °C for 24 h in the vacuum oven.The testing batteries were assembled using an argon-filled glove box,disks of 1.53 cm2were punched out of the Al foil as cathode,mass loading of electrode on each disk were set at a similar level about 3.0–3.3 mg.A microporous Celgard 2400 polypropylene membrane was selected as the separator,soaked in the electrolyte,which was a 1.0 mol∙L-1solution of LiPF6salt dissolved in a 1:1 by volume mixture of ethylene carbonate and dimethyl carbonate (EC/DMC),and lithium metal was choosed as counter electrode.Cycling and rate performances tests of coin cells were gradually carried out under constant current and constant voltage protocol(CC-CV)using a battery test system (XWJ Neware Tech.Co.,BTS3000,China) with upper cut-off voltages (UCV) of 4.3 V,and different LCV of 2.0,2.8 and 3.0 V.The initial charge–discharge capacity was measured at 0.1C rate cycling (1C=200 mA∙g-1).The cycle life test was carried out by performing 50 cycles at 0.2 C rate.Cyclic voltammetry (CV) was conducted at a scan rate of 0.1 mV∙s-1.The electrochemical ac impedance(EIS)was measured after cycling tests for 3 cycles with a frequency range of 102kHz to 10 MHz and an amplitude voltage of 5 mV by an electrochemical workstation(Ametek,PMC-1000dc,America) at 30 °C.

3.Results and Discussion

3.1.Phase analysis

The crystal structure of the Cr2O5,NCA and a mixture of the two are characterized as shown in Fig.2.The XRD results indicated that the diffraction peaks of pure Cr2O5are consistent with JCPDS card:PDF#28-0370(Fig.2a).The obvious diffraction peaks splits located in(0 0 6)/(1 0 2)and(1 0 8)/(1 1 0)of NCA@Cr2O5composites comforming to a well crystallized layered hexagonal α-NaFeO2structure with a space group of R3m (Fig.2b).It can be inferring that the structure of NCA and Cr2O5pristine materials are not changed by mechanical physical mixing.With an increase in the addition content of Cr2O5,the lattice reflections peaks of Cr2O5in NCA@Cr2-O5composite materials present in 10°–15° and 25°–30° become stronger,but the major reflections of the composites still belong to the pure NCA.

3.2.Surface analysis

The morphologies of the pristine NCA,Cr2O5,and composite NCAC3 samples are analyzed by the amplificatory picture of SEM,magnified 20,000 and 100,000 times,the grain of samples,big aggregates and the relation between single grain and pore are observed,the results are shown in Fig.3a,b,and c,respectively.Pristine NCA particles with spherical shape and size of about 10 μm are consisted by the agglomeration of multiple prismatic Nano-sized primary particles with a smooth surfaces of～500 nm.From the higher magnification SEM image,a slit hole formed by the accumulation of prismatic particles,which offer larger interfaces between particle and electrolyte.In contrast,after the reduction of CrO3,the Cr2O5particles become lamellar plates with size of range from 100 nm to 2 μm,those flake particles exhibiting smooth surfaces without any other flaws.Compared with the pristine NCA particles surface are clean and smooth without any attachment,the spherical morphology of NCA particles does not change in NCA@Cr2O5composite after physical mixing of certain quality Cr2O5,but the texture of the NCA@Cr2O5composite surface is changed,Cr2O5micro particles are enriched on the surface and in the gap of NCA particles to form more framework for Li+diffusion.

With the aim of further prove the Cr2O5can enrich in the pores of NCA,analysis of BET of Cr2O5,NCA and the composite material were taken into account.It can be seen from their adsorption curves in the Fig.4a–e that these materials have strong interaction with nitrogen at low pressure (0.0–0.1),and present typical Langmuir type IV curves.Except for Cr2O5,NCA and the three composites all exhibit H3-type hysteresis loops,which agrees well with the SEM,and are considered to be slit holes formed by particle accumulation.From the pore size distribution curves,the pore size(3.592 nm)and BET surface area(6.677 m2∙g-1)of Cr2O5are larger than that of NCA (3.191 nm and 1.583 m2∙g-1),this is consistent with the adsorption curve.After 15% (mass) Cr2O5is mixed into NCA,the pore size of NCAC1 increases(4.950 nm)and the BET surface area decreases greatly(0.497 m2∙g-1)due to the small amount of Cr2O5covering and accumulating on the surface and gap of NCA particles to blocking the pores of the NCA.With the Cr2O5piles up gradually (30%),the pore size continues to increase to 5.675 nm.However,because Cr2O5has a large BET surface area,the BET surface area of NCAC2 increases slightly to 0.517 m2∙g-1.When the content of Cr2O5increases to 45%,the surface and gap of NCA particles are filled by Cr2O5basically.Therefore,the pore size of NCAC3 becomes almost the same as that of Cr2O5(3.545 nm),and the BET surface area of NCAC3 increases to 3.225 m2∙g-1.

The above phase and surface analysis observations demonstrate that the pure Cr2O5was obtained by thermal decomposition of CrO3at atmospheric pressure are optionally distributed around NCA spherical particles,and a coating/blending structure is constructed successfully.The motivation for blending these two compounds is to make the NCA can be ‘‘protected” by Cr2O5materials to improve the ICDE and cycle stability,and the Cr2O5materials can achieve high rate performance under the premise of maintaining their respective high specific capacity.These two kinds of particles are bound together by physical adsorption,so their structure is intact.Cr2O5not only has a specific capacity greater than 236 mA∙h∙g-1,but also can serve as a host to insert the extracted lithium which cannot be inserted back into the layered NCA.

3.3.Conductivity analysis

The resistivity of Cr2O5and NCA is provided to verify the main conductive part of the conductive network formed by NCA and Cr2O5particles,and to clarify their Li+and electronic transport characteristics.

The result of measurements in air at 25 °C are shown in Fig.5,which shows that the prepared Cr2O5in this work is basically an electronic insulator,with an electrical resistivity of 412.33 MΩ∙cm at 20 MPa,while the resistivity of NCA material is 90.00 Ω∙cm and decreases with the increase of pressure.When the pressure increases to 30 MPa,the resistivity decreases to 57.11 Ω∙cm.Combining with the electrochemical analysis from the EIS test results,instead of being an electronic conductor,Cr2O5is a good conductor of Li+.During the discharge process,Li+pass from the anode through the separator and return to the cathode and react with Cr2O5to form the compound LixCr2O5,and the compound LixCr2O5is also a good conductor of Li+.In the subsequent charge and discharge process,the LixCr2O5undergoes a stable redox reaction to provide capacity of the battery.In addition,the EIS test also showed that the composite NCA@Cr2O5has a higher lithium ion diffusion coefficient (DLi+) than the pristine material.The conductive net skeleton composed of NCA and the Cr2O5that distributed on the surface and gap of NCA effectively avoids the corrosion of NCA,and improved the electrochemical properties of both substances.

Fig.2.X-Ray diffraction patterns of powder:(a) Cr2O5 and (b) NCA and NCA@Cr2O5 composite with different mass ratios.

3.4.Electrochemical performances of materials

To reasonably fabricate the NCA@Cr2O5composite cathode cells,the electrochemical performance of the cathode electrode active material Cr2O5and NCA were investigated by cells respectively in the different voltages (LCV) ranges of 3.0–4.3,2.8–4.3 and 2.0–4.3 V at 30 °C (Fig.6).

Initial charge and discharge curves and cycle life of Cr2O5at a rate of 0.2 C(40 mA∙h∙g-1)with different discharge cutoff voltages of 2.8 and 2.0 V were recorded.The initial charge–discharge capacity profile of Cr2O5electrode exhibits typical electrochemical process (Fig.6a) [16].It delivers a highest discharge capacity of 245.7 mA∙h∙g-1at 2.0–4.3 V.Therefore,at the LCV of 3.0 V,it is too close to the discharge platform voltage of Cr2O5material,which limits the capacity performance.The cycling performance curves indicate that the non-stoichiometric LixCr2O5material shows a capacity retention of 72.8% after 50 cycles at 2.0–4.3 V at a rate of 0.2 C(Fig.6b).As shown in Fig.6d,there is no apparent variation in the first charge–discharge curves of the NCA cathode when they cycled to 3.0–4.3 V and 2.0–4.3 V,the first discharge capacity is 192.2 mA∙h∙g-1with 83.3% ICDE at 3.0–4.3 V and 194.3 mA∙h∙g-1with 87.5% ICDE at 2.0–4.3 V due to the LCV.The cycling performance measured at a rate of 0.2C presents a capacity retention of 70.6% (136.4 mA∙h∙g-1) at LCV of 3.0 V,however,it only presents a capacity retention of 59.0% when it was over-discharged at LCV of 2.0 V(115.0 mA∙h∙g-1)after 50 cycles(Fig.6e).From the voltage and the corresponding specific capacity curves profile,it can be seen that during discharging,the voltage changes rapidly below 3.0 V,it is due to the electrode rapid polarization overvoltage,therefore,the operation performance of the battery is not stable.Results indicate that the cycle in the LCV between 2.0 and 4.3 V can lead to permanent capacity loss and adversely affect cycle life of NCA material under normal operating conditions,especially after 35 cycles,the capacity decays very fast.

The first charge–discharge curves of NCA@Cr2O5composites with different Cr2O5contents at constant current of 0.1 C(20 mA∙h∙g-1) and voltage range of 2.0–4.3 V were compared.As seen from Fig.7,the charge curves of all the composites are a sloping region,corresponding to the extraction of Li+from the lithium layer of the cathode material,which is accompanied with the oxidation of Ni2+to Ni3+to Ni4+[27,28].Therefore,the charge capacity decreases while the discharge capacity increases with an increase in Cr2O5content and a decrease in NCA content.This is consistent with the CV test results.For the NCA@Cr2O5composite,there is an obvious discharge area below 3.0 V,which conforms to the discharge characteristics of pure Cr2O5.

The second charge–discharge capacity and coulombic efficiency of Cr2O5cell,the first charge–discharge capacity and ICDE for NCA,NCAC1,NCAC2 and NCAC3 cells in the voltage range of 2.0 to 4.3 V were presented in Fig.8.We can see visually exciting from the histogram that,NCA is able to transmit charge–discharge capacities of 222.1 and 194.3 mA∙h∙g-1with the ICDE increases from 87.5% to 94.2%when the mass ratio of Cr2O5is 15%.The discharge capacity increases to 211.4 mA∙h∙g-1while the charge capacity has almost no change (224.4 mA∙h∙g-1) and the irreversible capacity loss decreases.When the mass ratio of Cr2O5increases to 30%,the charge capacity is reduced to 41.3 mA∙h∙g-1,which is due to the formation of lithium phase LixCr2O5and the less Ni2+/Ni3+and Ni3+/Ni4+redox reaction in the first discharge step.When the amount of Cr2O5reached 45%,the charging capacity does not change compared with that of 30%,but the discharge capacity increases to 230 mA∙h∙g-1.However,the ICDE exceeding 100% is misunderstanding and not meaningful for the half cell of the NCA@Cr2O5composite cathode and lithium anode.Therefore,in this work,when a small amount of Cr2O5with excellent lithium storage performance is added into the NCA,the study of ICDE is meaningful without affecting the charging capacity.

Fig.3.SEM measurements of structures on the surface of NCA (a);Cr2O5 (b) and NCAC3 (c).Right:magnified SEM micrographs.

The cycling and rate stability of the Cr2O5,layered NCA and NCA@Cr2O5composites were compared between 2.0 and 4.3 V in Fig.9.The electrochemical results demonstrate that the pure Cr2O5exhibits a better cycling performance at the low and same charge–discharge current density except for the first five cycles(Fig.9a).It is due to the loss of active Li+in the first several electrochemical discharge process of Cr2O5by forming the reversible LixCr2O5,and the formation of SEI layer on the surface of cathode material [29].However,the reversible capacities of Cr2O5material are only 57 mA∙h∙g-1and 3.3 mA∙h∙g-1at higher current densities of 5 C (1000 mA∙g-1) and 10 C (2000 mA∙g-1),respectively.The addition of NCA material improves the charge–discharge performance of Cr2O5material at high rate,and NCAC2 delivers the discharge capacity of 144 mA∙h∙g-1and 117 mA∙h∙g-1at 5 C and 10 C,respectively.Compared with NCA cathode,the composite samples NCAC1 NCAC2 and NCAC3 exhibit higher initial discharge capacity,better cycle stability with the increase of Cr2O5.Their capacity retentions are remarkably improved from 58.5% to 61.8%,63.1%and 69.3%after 50 cycles,respectively.It was attributed to the stability of the Cr2O5material,and at the same time,the Cr2O5is attached to the surface of the NCA,protecting the NCA from the corrosion products of electrolyte decomposition.Meanwhile,electrochemical test results show that the operation voltage window of NCA was widened.Under the voltage window of 2.0–4.3 V,the unblended sample NCA undergoes apparent attenuation after the cycle to 35.In contrast,the composite samples show better cyclic stability under the same voltage range,suggesting that the Cr2O5is indeed responsible for the charge–discharge steadily under low voltage of 2.0–3.0 V,when the battery was chargeddischarged at higher voltage (>3.0 V),NCA provide the capacity mainly,Cr2O5covers the surface of NCA particles and preferentially reacts with the electrolyte to form a stable SEI film,which protects NCA from being attacked by HF and ensures the structural stability and integrity of NCA.Therefore,to sum up,the addition of Cr2O5broadens the working voltage window of NCA material and improves its cyclic stability in a wider voltage range.

Fig.4.Nitrogen adsorption–desorption of samples: (a) NCA; (b) Cr2O5; (c) NCAC1; (d) NCAC2 and (e) NCAC3. The inset is pore size distribution curve.

Fig.5.Electrical resistivity of (a) Cr2O5 and (b) NCA samples.

The corresponding charging-discharging profile of NCAC1 NCAC2 and NCAC3 are shown in Fig.10,which shows a distinct plateau at 2.8 V for the discharge processes in the NCA@Cr2O5samples.It is ascribed to a redox process in the Cr2O5materials,as evidenced by the cyclic voltammetry’s of Fig.12.,Fig.10 displayed that the addition of Cr2O5inhibits the rapid potential attenuation and the performance of cyclic stability under low voltage of all composite samples is improved compared with that of NCA(Fig.6),hence,the synergistic effect can be stabilizing the discharge protection and improving the energy density of NCA materials under LCV.

Fig.6.The electrochemical properties of Cr2O5 and NCA cathode materials in the different cutoff voltages at 0.2 C:the first charge–discharge potential curves of(a)Cr2O5 and(d) NCA,the cycling performances of (b) Cr2O5 and (e) NCA,the selected charge–discharge curves of (c) Cr2O5 and f NCA.

Fig.7.The first charge(blue curves)-discharge(yellow curves)profiles of NCA@Cr2-O5 composite with different Cr2O5 contents.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Fig.8.Variations of the second charge–discharge capacity of Cr2O5 sample and coulombic efficiency,the first charge–discharge capacity of the rest of samples and ICDE.

In order to further clarify the reason for the improvement of the electrochemical performance,the effect of NCA@Cr2O5composites synergistic effect on the electrochemical resistance has been evaluated by EIS measurements.The Nyquist representations of impedance spectrum of all samples at the state of charge (SOC) of 50%after 3 cycles are shown in Fig.11a.Each of the impedance spectrum composes of the two semicircular arcs at high frequency and intermediate frequency and one slope line at low frequency,correspond to the solution resistance(Rs),the resistance of the surface film on the electrode(Rsf),the charge transfers resistance(Rct),and a Warburg impedance which is reflecting the Li+diffusion in the bulk(Ws),respectively[30,31].It can be seen from Fig.11a that Rsfand Rctof NCA@xCr2O5composites electrode are smaller than for separate electrode NCA and Cr2O5.Based on the equivalent circuit model,the fitting values of resistances are listed in Table 2,which displays the Rs,Rsfand Rctvalues of NCA@xCr2O5composites decrease first and then increase with the increase of Cr2O5,and the NCAC2 electrode shows the minimumresistance.The liner relation ofZrevs.ω-1/2is present in Fig.11b.The diffusioncoefficient(D)of Li+was calculated byutilizedthe following equation [32]:

Table 2Electrochemical impedance parameters for all samples

The smaller σ value implies that the lithium diffusivity(DLi+)of NCAC2 sample is larger than that of other materials,which have low lithium transport rate hinder their performance at higher discharge rates [11].Thus,NCAC2 has better cycle stability and rate performance compared with the commercial NCA and Cr2O5(Fig.9b).This result can be due to the appropriate amount of NCA and Cr2O5composite have synergistic effect,in which Cr2O5micro particles were filled between the NCA particles and enriched on the NCA surface to provide a dense channel for Li+transport.

Fig.9.(a)The cycling performance of the Cr2O5,NCA,and NCA@Cr2O5 composites at 0.2 C,(b)the rate performances of Cr2O5,NCA,and NCA@Cr2O5 composites at different charge–discharge rates.

Fig.10.Corresponding charge/discharge curves of (a) NCAC1,(b) NCAC2 and (c) NCAC3 composites in the 1st,5th,10th,20th and 50th cycle at the rate of 0.2 C.

Fig.11.(a) EIS and (b) Zre vs.ω-1/2 curve of Cr2O5,NCA,and NCA@Cr2O5 composites.

Fig.12.CVs of NCA@Cr2O5 composites at different cycles (a) 1st cycle,(b) 2nd cycle and (c) 3rd cycle and (d) Cr2O5.

Cyclic voltammetry (CV) was employed to further investigate the reduction and oxidation property of charge–discharge process of materials.As shown in Fig.12,Oxidation/reduction peaks assigned to the Ni3+/Ni4+redox couple is located at 3.9 V and 3.6 V respectively during Li+intercalation/ (de)intercalation.Compared with the original NCA (3.88 V),the anodic peak positions move to the higher voltage direction,and stronger intensities of two sharp peaks,indicating that the more stable electrochemical reaction for NCAC1 NCAC2 and NCAC3,these materials are corresponding to the voltage of 4.07,3.96 and 3.98 V respectively.Three pairs of redox peaks were observed in Ni-rich material[33].In the first cycle,there was an anodic peak at 3.7 V and two cathodic peaks at 3.3 V and 2.8 V,both corresponding to the Cr2O5.During the second cycle and following charging-discharging process,the peak at 2.8 V is disappear,which is consistent with the phase transformation between Cr2O5and LixCr2O5.Hence,it can be confirmed that LixCr2O5phase available to react via an intercalation reaction mechanism to continuous increase of the specific charge upon further charge–discharge cycles.Table 3 summarizes the positions of oxidation and reduction peak and potential intervals ΔV for the NCA@Cr2O5composites of the 1st,2nd and 3rd cycles.From the first to the second cycle,the peaks of NCA@Cr2O5composite shifted to lower potential,it is because of the first cycle is the activation of material to form SEI film,the material reduced the potential about 200 V,and in subsequent cycle,the position of the peak is almost overlapped,indicating NCA@Cr2O5composite material remarkably enhanced structural stability.

Table 3The potentials of the redox peaks and the intervals (ΔV/V) for the varying contents of Cr2O5 in NCA electrodes at a sweep rate of 0.1 mV∙s-1.

4.Conclusions

Pure Cr2O5was prepared by a simple thermal decomposition of CrO3in the box-type furnace at 350 °C,then mixed with LiNi0.8-Co0.15Al0.05O2to form NCA@Cr2O5composite.Charge-discharge tests show that a small amount of Cr2O5material enhances the ICDE and the stability of discharging at LCV of NCA material.The results show that NCAC2,mixed with 30% (mass) Cr2O5,has the best cycling and rate performance,while NCAC1 exhibits the greatest ICDE,it increases from 87.5%to 94.2%,while the charge capacity hardly changes.The capacity retention is improved with increasing the amount of Cr2O5after 50 cycles in the voltage range from 2.0 to 4.3 V.On the other hand,NCA material improves the rate capability of Cr2O5material at large current density.EIS and CV results indicate that NCA material combination with the lithium-free insertion host Cr2O5,which facilitates lithium diffusivity,and protects NCA from being attacked by HF and ensures the structural stability and integrity of NCA upon cycling.This work will facilitate our fundamental understanding on the composite cathodes with lithium-free additives and Ni-rich layered cathode oxides,and provide valuable guidance for the design and synthesis of cathode materials with high performance.

Declaration of Competing Interest

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 financially supported by the National Natural Science Foundation of China (52064031),the Program for Innovative Research Team in the University of Ministry of Education of China (IRT_17R48) and National Natural Science Foundation of China (51674129).The authors sincerely acknowledge the anonymous reviewers for their insights and comments to further improve the quality of the manuscript.