Jianfeng Cheng,Meixuan Li,Yutong Wang,Jiexiang Li,Jiawei Wen,Chunxia Wang,Guoyong Huang,

1 College of New Energy and Materials, China University of Petroleum (Beijing), Beijing 102249, China

2 State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (Beijing), Beijing 102249, China

Keywords:High voltage spinel Al/Co doping High temperature cycling stability

ABSTRACT The poor structural stability and capacity retention of the high-voltage spinel-type LiNi0.5Mn1.5O4(LNMO) limits their further application.Herein,Al and Co were doped in LNMO materials for a more stable structure and capacity.The LNMO,LiNi0.45Al0.05Mn1.5O4 (LNAMO) and LiNi0.45Co0.05Mn1.5O4(LNCMO) were synthesized by calcination at 900 °C for 8 h,which was called as solid-phase method and applied universally in industry.XRD,FT-IR and CV test results showed the synthesized samples have cation disordering Fd-3m space group structures.Moreover,the incorporation of Al and Co increased the cation disordering of LNMO,thereby increasing the transfer rate of Li+.The SEM results showed that the doped samples performed more regular and ortho-octahedral.The EDS elemental analysis confirmed the uniform distribution of each metal element in the samples.Moreover,the doped samples showed better electrochemical properties than undoped LNMO.The LNAMO and LNCMO samples were discharged with specific capacities of 116.3 mA·h·g-1 and 122.8 mA·h·g-1 at 1 C charge/discharge rate with good capacity retention of 95.8% and 94.8% after 200 cycles at room temperature,respectively.The capacity fading phenomenon of the doped samples at 50 °C and 1 C rate was significantly improved.Further,cations doping also enhanced the rate performance,especially for the LNCMO,the discharge specific capacity of 117.9 mA·h·g-1 can be obtained at a rate of 5 C.

1.Introduction

Along with the iterative development of portable battery products,as well as the increasing consumer demand for these products,the research of lithium-ion battery technology has been paid much attention.At present,lithium-ion batteries are widely used in aerospace,automotive applications and considered to have huge potential in grid energy storage[1-3].The cathode materials play an essential role in lithium-ion batteries.Currently,the common cathode materials in commercially-used lithium-ion batteries mainly include layered LiCoO2[4],LiNi1-x-yCoyMnxO2[5-7],olivine-structured LiFePO4[8],Ni-Co-Al ternary materials (NCA)[9],LiMn2O4[10,11],LiNi0.5Mn1.5O4(LNMO) [6] and LiMnO2[12].LiCoO2involves the scarce and expensive Co element.Besides,the safety of the batteries needs to be improved [13].LiFePO4shows low-cost and raw material-rich features,but the further development is limited by the low vibrational density and electronic conductivity [14,15].The spinel-type LNMO has an energy density of 650 Wh·kg-1,a theoretical specific capacity of about 146.7 mA·h·g-1,and operating voltage as high as 4.7 V.The Mn resources used in LNMO are much cheaper and more environmentally friendly [10-12,16-18].LNMO has been considered as a promising electrode material for lithium-ion batteries,especially for the application in electric vehicles.

Despite the advantages of spinel-type LNMO,some defects,such as inferior structural stability and severe capacity degradation,hinder its commercial applications[19-22].At present,spinel LNMO is mainly limited by three issues.Firstly,the Jahn-Teller effect of Mn3+causes lattice distortion,followed by the increasing internal stress of the particles,which leads to the collapse of the structure.Secondly,during the charging/discharging process,the valence of Mn shifts into 2+and 4+.And Mn2+dissolves into the electrolyte and reacts with Li+,resulting the consumption of Li+and the collapse of electrode materials.In addition,under the high voltage environment of～4.7 V,the electrolyte can be decomposed and HF can be formed easily in the electrolyte,and these side reactions aggravate the collapse of the structure,followed by capacity attenuation [23,24].

Nowadays,metal-ion doping is considered to be one of the effective methods to optimize the structure and electrochemical performance of electrode materials.The doping of metal cations affects the structure and properties of LNMO materials.The doped ions mainly include Cr [25,26],Ti [7,27],Fe [28],Cu [29],Mg[30,31],Zn[32,33],Nd[34],Co[35],etc.Taliket al.[36]synthesized LiMn2O4by sol-gel method and doped Al,Mg,Ni and Fe into LMO.The importance of metal cation doping for cathode material modification was confirmed by their research.Wanget al.[37]prepared Cr-doped LNMO through the method of sol-gel.Cr doping could inhibit the formation of impurity phases and promote the growth of less-defect crystal and reduce the phenomenon of metal ions dissolution.After 1000 cycles,the capacity retention of Cr-doped LNMO is 82.75% at a 1 C rate.Mg element was also selective site doped on LNMO materials [38].The doping sites were tetrahedral and octahedral sites of Fd-3m structure.The sample synthesized by this work was subjected to 1500 cycles at a concentration of 1 C,and the capacity retention rate was 86.3%.Leeet al.[39] used the sol-gel method to synthesize LNMO cathode materials and investigated the effect of calcination temperature on the morphology and properties of LNMO.The prepared LNMO had a large particle size with Fd-3m space group and stable electrochemical performance when the calcination temperature was 900 °C.In addition,the LNMO-900 electrode sample exhibited relatively low dissolution of Ni and Mn and good corrosion resistance,thus demonstrating stable electrochemical property.These conclusions provided highly informative for the selection of calcination temperature in this experiment.

The electrochemical reaction equation of LNMO is as follows:LiNi0.5Mn1.5O4?LixNi0.5Mn1.5O4+(1-x) Li++(1-x) e-.Al and transition metal Co have abundant resources and low cost.The reason for choosing Al and Co as dopants is that they are close to Ni and Mn in the periodic table.Significant distortion of spinel LNMO structure can be avoided by doping Al and Co.The main effect of Al doping on the material is to stabilize the structure and increase the disorder of the material,thus improving the recycling performance and rate performance.In addition,the ionic radius of Al3+is 5.35×10-2nm,smaller than Ni2+(6.90×10-2nm) and Mn3+(6.45×10-2nm),thus Al3+can diffuse into the crystal structure of LNMO easily.The ionic radius of Co3+is 6.10×10-2nm,similar to that of Ni2+and Mn3+,and makes little impact on the crystal structure of LNMO.Co doping can effectively inhibit the cation mixing between lithium ion and nickel ion.Herein,Al and Co were used as doping element in this paper.Solid phase method was adopted,the raw materials were mixed and calcined at 900 °C for 8 h to synthesize LiNi0.5Mn1.5O4(LNMO),LiNi0.45Al0.05Mn1.5O4(LNAMO) and LiNi0.45Co0.05Mn1.5O4(LNCMO) materials.The morphology of the three samples and the influence of doped cations on the lattice constant of LNMO were observed.The effects of doped cations on the cycle stability and rate performance of the batteries were compared.

2.Experimental

2.1.Synthesis procedure

The synthesis flow chart is shown in Fig.S1.LNMO,LNAMO and LNCMO were prepared by the solid phase method.To prepare 0.02 mol LNMO,LNAMO and LNCMO were used as raw materials.To synthesis 0.02 mol LNMO,LNAMO and LNCMO respectively,Li2CO3(analytically pure,99.9%),NiCO3(analytically pure,98%),MnCO3(analytically pure,99.95%),γ-Al2O3(nano powder,analytically pure,98%),and Co3O4(analytically pure,98%) were weighed according to the stoichiometric ratios.In detail,stoichiometric Li2CO3,NiCO3,MnCO3,γ-Al2O3,and Co3O4were used as raw materials,of which the lithium sources were added with the excess of 3% (mass).Ethanol was used as the dispersant,and the raw materials were placed in a 25 ml ball mill tank and mixed by a ball mill(JX-2G,Shanghai,China).The ball-milling parameters were set as follows: the rotating rate was 500 r·min-1,and the milling time was 10 h.After that,the ground raw materials were dried at 90 °C for 12 h.And then the powders were calcinated at 900 °C for 8 h in air,with the ramping rate of 5 °C·min-1.Ultimately,the as-prepared samples were left to cool naturally in the furnace.

2.2.Structural characterization

The Bruker D8 Advance diffractometer was used for phase purity analysis and structure characterization of the samples.The X-ray powder diffractometer (XRD) performed continuous scanning under the condition of 2θ=10°-70°.Fourier transform infrared spectroscopy (FT-IR) was performed on a V80 instrument (Bruker) to study the structural properties of different products.Morphology was examined using a Quanta 200F field emission scanning electron microscope (SEM) and F20 field emission transmission electron microscope (HR-TEM).The elemental compositions were determined by energy dispersive spectroscopy.

2.3.Electrochemical tests

The cathode consisted of LNMO,acetylene black (ATB),and polyvinylidene fluoride (PVDF) with a mass ratio of LNMO/ATB/P VDF=8:1:1.The lithium metal was used as the anode.The aluminum foil coated with the cathode slurry was dried in a vacuum at 90°C for 16 h,and then cut into electrode discs with a diameter of 12 mm.The mass range of the active materials on the cathode sheet is 1.5-2.0 mg.The cathode and anode were assembled in CR2032 type coin cells.The electrolyte used in the cells was 1 mol·L-1LiPF6in 1:1 ethylene carbonate (EC)/ diethyl carbonate(DEC).The celgard 2400 was chosen as the diaphragm.The long cycle performance and rate performance tests were carried out in LAND CT2001A.The cyclic voltammetry test (CV,voltage range of 3-5 V,scanning rate of 0.1 mV·s-1) and electrochemical impedance spectroscopy test (EIS,amplitude of 0.005 V,frequency range of 100000-0.01 Hz)were carried out using CHI760E electrochemical workstation.

3.Results and Discussion

3.1.X-ray diffraction

Fig.1 shows the XRD patterns of LNMO,LNAMO and LNCMO.According to the XRD results of the samples,the diffraction peaks of each sample are narrow and sharp,which indicates the high crystallinity of each sample.The diffraction peaks of the three samples are well matched with the JCPDS file 80-2162 of the Fd-3m space group.The crystal plane information corresponding to the diffraction peaks at various angles has been marked in Fig.1(a).The XRD diffraction peaks of all samples show the Fd-3m disordered space group.The diffraction peaks at 18.64°,36.14°,37.80°,43.94°,48.12°,58.15° and 63.88° correspond to (1 1 1),(3 1 1),(2 2 2),(4 0 0),(3 3 1),(5 1 1) and (4 4 0) crystal planes,respectively.The indices of crystal planes have been marked in Fig.1(a).We selected the diffraction patterns of 2θ=37°-44.25° in Fig.1(a) and enlarged the area in Fig.1(b).It can be seen from Fig.1(b) that the weak response peaks in LNMO diffraction patterns appear in approximately 37.5° and 43.5°,which could be ascribed to NiO phase,revealing the existence of NiO impurities.

Fig.1.(a) XRD patterns of the samples of LNMO,LNAMO and LNCMO.(b) Localized magnification of (a) between 37° and 44.25°.

The patterns of LNAMO and LNCMO present no impurities.The results show that Al and Co doping inhibited the formation of NiO impurity phases during high-temperature synthesis,without changing the spinel structure of LNMO[31].Table 1 shows the lattice information of the three samples.The lattice parameters of LNMO,LNAMO and LNCMO samples were 8.179×10-3nm,8.164×10-3nm and 8.178×10-3nm,respectively.And the lattice volumes were 54.71 nm,54.42 nm and 54.70 nm,respectively.It can be seen that the LNAMO had the smallest lattice parameter and lattice volume.The main reason lies in that the ionic radius of Al is 5.35×10-2nm,which is much smaller than the ionic radius of Ni and Mn.Thus,when Al is doped into the lattice structure,the lattice parameters are reduced.The lattice parameter and lattice volume of the LNCMO are also slightly smaller than those of the LNMO,and the ionic radius of Co is 6.10×10-2nm,which is slightly smaller than that of Ni.

Table 1 Lattice information of three samples

According to Debye-Scherrer equation,the average grain size of each sample was calculated.We substituted the half-width of the diffraction peak of the (1 1 1) crystal plane in the vicinity of 2θ=18.8°for each sample into the Debye-Scherrer equation to calculate the average grain size of each sample [32].The results show that the grain size of the LNMO is about 55.80 nm.The crystal particle size of LNAMO is approximately 62.30 nm,and that of LNCMO is approximately 61.90 nm.The Al and Co doping enhanced the Ni redox reaction potential by shortening the energy band overlap between Ni and O.In addition,the O lattice could be stabilized and the side reactions could be hindered,the doped Al3+and Co2+can reduce side reactions and stabilize the O lattice,thereby improving the cycling stability of the cathode material [40].

3.2.FT-IR spectroscopy

There are two types of crystal structures of high-voltage LiNi0.5Mn1.5O4cathode materials,including the cubicP4332 type with Ni/Mn ordered distribution and the face-centered cubic Fd-3m type with disordered distribution,of which the Fd-3m type LNMO is shown to exhibit superior lithium diffusion coefficient and electronic conductivity.Since the scattering coefficients of Ni and Mn are relatively similar,it is not desirable to just refer to the analytical results of XRD technique,while FT-IR test can sensitively identify the local vibration of crystal symmetry and can be used as an effective test to identify Fd-3m andP4332 spinel type LNMO,to further determine the structural symmetry of the fabricated samples.The FT-IR spectra of the three samples are shown in Fig.2.It can be found from the FT-IR spectra of the LNMO samples that the characteristic peaks of the Fd-3m space group appear at approximately 625,585,557 and 507 cm-1,where the IR peaks at 625 cm-1and 557 cm-1are related to the vibration of Mn-O,while the peaks at 585 cm-1and 507 cm-1are ascribed to the vibration of Ni-O [34,41].The ratio of the IR peak intensities at 625 cm-1and 585 cm-1reflects the symmetry of the space group,and the larger the ratio is,the more obvious the structural disorder feature.It can be seen that the cation disordering is more pronounced for the LNAMO and LNCMO samples,where the LNCMO sample shows the high degree of asymmetry.

Fig.2.FT-IR spectra of the three samples.

3.3.Morphology analysis

To observe the effect of Al and Co doping on the morphology and particle size,electron scanning microscopy experiments were performed on the samples,as shown in Fig.3(a)-(f).The orthooctahedral morphology of the LNMO sample particles is not obvious.The LNMO sample particles fail to show obvious orthooctahedral morphology,and the particles agglomerate seriously,in addition,the particle size scale distribution is uneven.In contrast,the LNAMO and LNCMO samples present very distinctive ortho-octahedral morphology with sharp and clear edges and complete grain growth.The grain size of the LNCMO sample is more uniform.TEM was used to obverse the morphology of the samples,and it is found to be in general agreement with the results obtained by scanning electron microscopy.Therefore,it can be indicated that the crystal growth could be promoted by the addition of Al and Co during the calcination of LNMO.And the intact crystal structure ensure that the samples have superior electrochemical properties.

Fig.3.SEM and TEM images of the three samples: (a),(d) LNMO;(b),(e) LNAMO;(c),(f) LNCMO.HRTEM images of (g) LNMO;(h) LNAMO;(i) LNCMO.

The microstructure of the samples was investigated by HR-TEM(Fig.3(g)-(i)).HR-TEM results show that the labeled spot lattice fringe spacing in LNMO sample labeling region is 0.2532 nm,corresponding to the (3 1 1) crystal plane.The labeled spot lattice fringe spacing in the labeled region of LNAMO sample is 0.4855 nm,corresponding to the (1 1 1) crystal plane.The labeled spot lattice fringe spacing in the labeled region of LNCMO sample is 0.3011 nm,corresponding to the (4 0 0) crystal plane [37].

The results of STEM mapping pictures are shown in Fig.4.The results show that Al,Co,Ni,Mn and O are uniformly distributed in the LNAMO and LNCMO samples.The EDS test results (Fig.S2)further confirm the existence of Al and Co elements.The elemental content obtained in the EDS test shows that the content of each element is basically the same as the pre-set ratio (the molar ratio of LNMO sample is Li: Ni: Mn: O=1.03:5:1.5:4,that of LNAMO sample is Li: Ni: Al: Mn:O=1.03:0:05:05:5:4,and that of LNCMO sample is Li: Ni: Co: Mn: O=1.03:0:05:05:5:4).In addition,it can be seen from the Fig.4 that the distribution of Al and Co elements in all samples is relatively uniform,and they are effectively combined with Ni and Mn elements [42].

Fig.4.STEM Mapping images of the distribution of Ni,Mn,O,Al and Co elements in LNMO samples: (a) LNAMO;(b) LNCMO.

3.4.Electrochemical performance

Fig.5(a) shows the initial charge/discharge curve of each sample cycled at 1 C after three activation cycles at a 0.1 C rate.Before the electrochemical test,the assembled batteries need to be cyclically activated,for the reason that the electrode surfaces of the newly assembled batteries have the oxide films,which hinder the insertion and extraction of lithium ions during the operation,resulting in the batteries to be in the state of low reactivity,thus they need to be electrochemically activated [2,23].The charge and discharge specific capacity of the LNMO sample are 124.3 mA·h·g-1and 113.9 mA·h·g-1,respectively,with a coulombic efficiency of 91.63%.The charge and discharge specific capacity of the LNAMO sample are 122.4 mA·h·g-1and 116.3 mA·h·g-1,respectively,with a coulombic efficiency of 95.02%.The charge and discharge specific capacity of the sample with LNCMO are 129.1 mA·h·g-1and 122.8 mA·h·g-1,respectively,with a coulombic efficiency of 95.12%.The discharge specific capacity of the cationdoped samples is slightly higher than the LNMO sample,with the highest discharge specific capacity of 122.8 mA·h·g-1for the LNCMO sample.Furthermore,the coulombic efficiency of the doped samples is basically 95%,which is nearly 4% higher than the LNMO samples.The higher discharge specific capacity and coulombic efficiencies of LNAMO and LNCMO samples may benefit from the strong bonding of Al-O and Co-O.The LixNi1-xO impurity in LNMO may lead to its low electrochemical performance [43].

Fig.5.(a) Initial charge/discharge curve of each sample cycled at 1 C after three activation cycles at 0.1 C magnification;(b) CV curves of each sample.

Furthermore,three samples exhibit three voltage plateaus at～4.0 V as well as～4.7 V.The～4.0 V voltage plateau corresponds to the redox reactions of Mn3+/Mn4+,and the～4.65 V and～4.75 V voltage plateaus are caused by Ni2+/Ni3+and Ni3+/Ni4+redox reactions.Hence,this phenomenon indirectly indicates that all three types of samples are Fd-3m type space group structures.Due to the doping of Al3+and Co2+into LNCM,Mn3+is introduced into the material.According to the valence balance,the valence of Ni element increases,consequently the Ni2+/Ni3+and Ni3+/Ni4+platforms corresponding to～4.65 V and～4.75 V slightly reduced[44].

Fig.5(b) presents the CV curves of the three samples.It can be clearly seen that all of the three samples have redox peaks near 4.0 V,4.65 V and 4.75 V,which are consistent with the information reflected by the charge and discharge curves(Fig.5(a)).By observing the reduction peak curve at 4.7 V,the peak area of LNAMO and LNCMO is smaller than that of LNMO,indicating that the highvoltage discharge platform decreases,because the content of Ni in the material decreases due to the doping of Al and Co.The inner graph is an enlarged graph of the redox peak at 4.0 V.The pair of redox peaks at 4 V for the three samples further proved the presence of Mn3+in the samples.Through the doping of Al and Co,the peak current and area at 4.0 V increase,which indicates that the Mn3+content in the sample increases.Among them,the LNCMO sample has the largest peak current and area at 4.0 V,which is consistent with the results obtained from XRD and FT-IR spectroscopy.After doping,the redox peak potential difference of the material was reduced.According to Haridaset al.[45],Mn3+can be used as a carrier for charge transport in the electrode material,which makes the conductivity better and reduces the polarization effect.In addition,Kunduraciet al.[46] found that it is precisely because Mn3+could improve the electronic conductivity of the material,therefore the rate and cycle performance of the material would be improved.In addition,as shown by the dashed box,the potential difference between the redox peaks of Ni2+/Ni3+and Ni3+/Ni4+pairs of doped LNMO samples at 4.65 V and 4.75 V has increased.According to Guet al.[47]found that Fd-3m potential difference between the two pairs of redox peaks at 4.8 V is larger than that of theP4332 type LNMO.It can be seen that the doping of Al3+and Co3+made the LNMO tend to the cation disordering Fd-3m type.

Fig.S3 shows the XPS plots of LNAMO and LNCMO.the XPS results are used to illustrate the coexistence of Mn3+and Mn4+in the LNMO materials doped with Al and Co.Besides,some of the Mn4+in the LNMO material becomes Mn3+on the basis of the addition of Al and Co elements.And the presence of Mn3+is favorable to improve the electronic conductivity and ionic conductivity of LNMO.Meanwhile,this further implies that the LNMO samples doped with Al and Co are all Fd-3m type structures.Fig.S3(c)shows the presence of Ni in the samples in the form of Ni2+and Ni3+.From Fig.S3 (d) and (e),the signal peaks of Al and Co were detected in the LNAMO and LNCMO samples,showing Al3+and Co3+,respectively [48].

To further evaluate the electrochemical performance of LNAMO and LNCMO samples,galvanostatic charge-discharge tests were carried out in the voltage range of 3.0-5.0 V.Fig.6(a) shows the cycle curves of LNMO,LNAMO and LNCMO samples at 25 °C and 1 C rate.In general,the samples shown a small increase in discharge specific capacity in the first few cycles because the active material on the electrode is activated by the penetration of the electrolyte.The specific discharge capacities of the LNMO,LNAMO,and LNCMO samples after 200 cycles were 102.5,111.4 and 116.4 mA·h·g-1,and the capacity retention rates were 90%,95.8% and 94.8%,respectively.The cation-doped LNMO cathode material has excellent long-term cycling stability,which indicates that the introduction of Al and Co particles can increase the stability of the crystal structure and inhibit the structural collapse during the working process.The improved performance is mainly attributed to the formation of Al-O and Co-O with high bonding energy after the introduction of metal ions.

Fig.6.(a)Cycling performance of the samples at 25°C and 1 C rate;(b)Cycling performance of the samples at 50°C and 1 C rate;(c)Specific capacity of the three samples at different rates;(d) Nyquist plots before three sample cycles;(e) Nyquist plots of three samples after 100 cycles.

Serious capacity degradation occurred in LNMO material when the cell was cycled under high temperature conditions (50-60 °C).Fig.6(b) is the cycle performance curve of three samples at 50 °C and 1 C rate.After 200 cycles,the specific capacity of the LNMO sample decayed from 146.3 to 122.6 mA·h·g-1,and the rate of capacity retention was 83.8%.Compared with LNMO,the specific capacity of the LNAMO sample decreased from 128.8 mA·h·g-1to 118.9 mA·h·g-1,and the rate of retention was 92.3%.The specific capacity of the LNCMO sample decreased from 137.7 mA·h·g-1to 129 mA·h·g-1,and the retention rate was 93.7%.The capacity fading phenomenon of the doped sample at the high temperature of 50 °C was significantly inhibited.

Fig.6(c) presents the rate cycling performance of the three samples in 3-5 V.The samples doped with Al and Co exhibit excellent rate performance.Especially for LNCMO samples,the discharge specific capacity reaches 117.9 mA·h·g-1at 5 C.The discharge specific capacity of the LNAMO sample at 5 C is 104.1 mA·h·g-1.However,the specific capacity of the LNMO sample is only 86.9 mA·h·g-1,and the impurity phase in the sample may affect the performance.In addition,the increase in Mn3+content in the sample after doping enhance the conductivity and Li+diffusion coefficient of the sample,thereby further ameliorating the rate performance of the samples [38,49].

Fig.6(d) shows the EIS results of the cells assembled with three samples before electrochemical cycling.The Nyquist spectrum usually consists of a semicircular curve in the high-frequency region and a diagonal line in the low-frequency region.Among them,high-frequency zone information is considered to reflect the interface charge transfer resistance (Rct).TheRctvalues of the undoped,LNAMO and LNCMO samples are 72.87,33.73,and 29.54 Ω,respectively.TheRctfor the doped samples is significantly reduced,especially theRctfor the sample doped with Co.This shows that the transmission barriers between Li+and electrons could be reduced effectively by doping,and the rate performance could be improved siginificantly.On the other hand,the result also shows that the higher Mn3+content in the doped LNMO samples,and the LNCMO samples had the maximum content of Mn3+,which is consistent with the results obtained from XRD,FT-IR and CV test.

Fig.6(e) shows the EIS spectra of the three samples after 100 cycles at 1 C after activation.All samples are fitted according to the circuit presented in the Fig.6(e),and the fitting parameters are listed in Table 2.It can be seen from the spectrum that each curve has two semi-arcs and a straight line.The semicircle in the high frequency region reflects the migration impedance (Rf)between Li+and the interface film,the semicircle in the middle frequency region reflects the charge transfer resistance (Rct),and the slope of the line in the low frequency region reflects the Warburg impedance (W1) when Li+diffuses in solid materials,where,the intercept with the horizontal axis before the beginning of each curve is the electrode ohm resistance (Rs).Through the equivalent circuit fitting,it is found that all the batteries present lowRsvalues,and theRfandRctvalues of the battery with the doping cathode decline,which indicates that the surface film impedance can be improved,and the charge transfer resistance can be reduced by Al3+and Co3+,leading to the improved rate performance.The fitting resistance values of the LNCMO sample are smaller.

Table 2 EIS fitting parameters of samples after 100 cycles

To further observe the structural changes of the material after electrochemical cycling,the batteries after 200 times at roomtemperature were disassembled,and XRD and SEM analysis of the cathode was performed.Fig.7(a) shows the XRD patterns of the three samples before cycling,and Fig.7(b)shows the XRD patterns of the three samples after cycling at 25 °C and 1 C.The diffraction peaks of the Al foil current collector was marked in the figure.The samples on the electrodes after the cycle test are all cubic spinel phases.Due to certain ion dissolution and structural collapse of the active material during the cycle,the intensity of the diffraction peak of the cathode piece decreases after the cycle.Fig.8 visually shows the morphological changes of the cathode pieces of LNMO,LNAMO and LNCMO before and after the cycle test.It can be learnt that these three samples are all in the shape of a regular octahedron.However,the surface of the LNMO sample was severely decomposed.

Fig.7.(a) XRD patterns of three samples before cycle;(b) XRD patterns of the three samples after 200 cycles at 25 °C and 1 C.

Fig.8.SEM images of the electrodes of the LNMO samples before and after 200 cycles at 25 °C.(a1,2) LNMO;(b1,2) LNAMO;(c1,2) LNCMO.

Metal ion doping is an effective means to improve the capacity attenuation.For the selection of doping elements,Al and Co have also been studied.Table 3 compares the results of our work with the results of other similar studies,and the results are shown in Table 3.

Table 3 Summary of doping Al and Co strategies to modify LiNi0.5Mn1.5O4

4.Conclusions

In this paper,LNMO,LNAMO and LNCMO samples were prepared by the solid phase method with simple steps and industrial versatility.The influence of Al and Co cation doping on the electrochemical performances was studied and compared.The results show the synthesized samples has a disordered Fd-3m spinel structure,and the cation-doped sample exhibits more regular octahedral morphology and more uniform particle size.LNCMO shows optimum electrochemical performance.The data in this work shows that the incorporation of Al and Co elements into the LNMO material can effectively enhance the structural stability and electrochemical activity.The Co element can significantly increasethe cation disordering of the LNMO material and reduce the polarization of the LNMO,thereby increasing the diffusion rate of Li+and improving the rate performance.After activation,the first discharge specific capacity of LNAMO and LNCMO reached 116.3 mA·h·g-1and 122.8 mA·h·g-1,respectively,and the capacity retention rate after 200 cycles could reach more than 95%.The capacity retention rate after 200 cycles at 1 C and 50 °C of LNAMO and LNCMO could reach 92.3% and 93.7%,respectively.The Mn3+content in the sample doped with Al and Co increased to a certain extent,which improved the conductivity and Li+diffusion rate of the LNMO materials and enable the samples to obtain superior rate performance.

Data Availability

Data will be made available on request.

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 supported by the National Natural Science Foundation of China (52022109 and 51834008),Beijing Municipal Natural Science Foundation(2202047),National Key Research and Development Program of China (2021YFC2901100),Science Foundation of China University of Petroleum,Beijing(2462021QNX2010,2462020YXZZ019,2462020YXZZ016,and 2462022QZDX008).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.02.020.