Effect of Metal ( Mn , Ti ) Doping on NCA Cathode Materials for Lithium Ion Batteries

NCA (LiNi0.85Co0.10Al0.05−x MxO2, M=Mn or Ti, x < 0.01) cathode materials are prepared by a hydrothermal reaction at 170 C and doped with Mn and Ti to improve their electrochemical properties. The crystalline phases and morphologies of various NCA cathode materials are characterized by XRD, FE-SEM, and particle size distribution analysis. The CV, EIS, and galvanostatic charge/discharge test are employed to determine the electrochemical properties of the cathodematerials.Mn andTi doping resulted in cell volume expansion. This larger volume also improved the electrochemical properties of the cathode materials because Mn and Ti were introduced into the octahedral lattice space occupied by the Li-ions to expand the Li layer spacing and, thereby, improved the lithium diffusion kinetics. As a result, the NCA-Ti electrode exhibited superior performance with a high discharge capacity of 179.6mAh g after the first cycle, almost 23mAh g higher than that obtained with the undoped NCA electrode, and 166.7mAh g after 30 cycles. A good coulombic efficiency of 88.6% for the NCA-Ti electrode is observed based on calculations in the first charge and discharge capacities. In addition, the NCA-Ti cathode material exhibited the best cycling stability of 93% up to 30 cycles.


Introduction
Currently, there is an increasing consumer demand for Liion batteries (LIBs) with long life, high energy, and high power density.LIBs can be used in many devices, from portable electronics to electric vehicles.However, there are many issues that need to be addressed when using LIBs in electric vehicles, including those related to cost, safety, and energy density considerations [1,2].
Recently, transition metal-layered oxide LiMO 2 (M=Ni, Mn, Co) materials have generated interest as suitable solutions to the aforementioned issues, including specific capacity, energy density, safety, and cost [3,4].Among these materials, LiNiO 2 is of low cost, has a large theoretical capacity (275 mAh g −1 ), and is more environmentally friendly than LiCoO 2 [5][6][7][8].However, several limitations such as a difficult synthesis protocol for stoichiometric LiNiO 2 , partially reversible phase for LiNiO 2 , and Li/Ni cation mixing have to be overcome before LiNiO 2 can be commercially used as a cathode material for LIBs.To resolve these issues, different metals such as Co, Al, and Fe were studied as dopants/partial substitutes for Ni, which demonstrated promising but limited performance.Among these, Co and Al codoped LiNi 1−x−y Co x Al y O 2 (NCA, 0.05 ≤  ≤ 0.15, 0.01 ≤  ≤ 0.10) cathode materials exhibit the improved electrochemical properties and the thermal safety because cation substitution of Co and Al increases the stability of the structure [5,9].However, a few drawbacks such as degradation during charge-discharge cycles and thermal runaway are yet to be resolved.Moreover, higher Ni concentrations result in highly unstable Ni 3+ and Ni 4+ ions remaining in the layered structure, and their reaction with the electrolyte speeds up degradation of the materials and batteries [10,11].In order to solve these problems, the small amounts of dopants at the cationic sites have been considered; for example, it has been reported that the addition of small amounts of Cl, Mg, F, and Na improves the capacity and cycle performance [12][13][14][15][16][17].
In this work, the NCA cathode material was prepared by one step of hydrothermal reaction at 170 ∘ C and doped with either Mn or Ti.This is because Mn and Ti have similar atomic radii as that of Ni and, in coordination with oxygen, can replace the transition metal atoms in the octahedral interstitial sites.The aforesaid process enhances thermal and structural stability and, in some cases, decreases electronic and/or ionic resistance [18,19].Herein, we report the effects of Mn or Ti doping on the crystalline structure and electrochemical properties of NCA.

Experimental
The NCA cathode materials were prepared via one step of hydrothermal reaction.First, 0.85 M of nickel(II) sulfate hexahydrate (NiSO 4 ⋅6H 2 O, 99%, Sigma-Aldrich) and 0.15 M of cobalt(II) sulfate heptahydrate (CoSO 4 ⋅7H 2 O, ≥99%, Sigma-Aldrich) were dissolved in 2.5 M of NaOH solution.After stirring vigorously for 1 h, 0.05 M of aluminum hydroxide hexahydrate (Al(OH) 3 ⋅6H 2 O, 98%, Sigma-Aldrich) and ammonia solution were added sequentially, and the mixture was stirred for another half hour.Then, the resultant solution was transferred to a 100-mL Teflon-lined stainless-steel autoclave, which was placed in a laboratory electric oven and heated at 170 ∘ C, for 18 h.After the hydrothermal reaction, the autoclave was cooled to room temperature.The obtained dark-green Ni 0.85 Co 0.10 Al 0.05 (OH) 2 powder was washed with deionized water and ethanol and centrifuged several times, followed by vacuum drying overnight at 120 ∘ C. Next, the Ni 0.85 Co 0.10 Al 0.05 (OH) 2 powder was calcined at 500 ∘ C for 5 h in air to obtain Ni 0.85 Co 0.10 Al 0.05 O 2 , which was mixed with lithium hydroxide (LiOH⋅H 2 O, 99%, Sigma-Aldrich) in a molar ratio of 1 : 1.05 by hand grinding, and then, the mixture was calcined again at 700 ∘ C for 10 h under an O 2 atmosphere (O 2 flow: 3 mL min −1 ) to obtain the Li 1.05 Ni 0.85 Co 0.10 Al 0.05 O 2 (NCA) cathode material.The NCA cathode materials were doped with Mn and Ti to improve their electrochemical properties.For the metal-doping process, 0.01 mol of titanium (IV) isopropoxide (C 12 H 28 O 4 Ti, ≥97%, Aldrich) or 0.01 mol of manganese(II) sulfate monohydrate (MnSO 4 ⋅H 2 O, 98%, Samchun) was added during the addition of the Al source.The obtained pristine NCA and Mn-and Ti-doped NCA cathode materials were denoted as NCA, NCA-Mn, and NCA-Ti, respectively.
For electrode fabrication, the cathode material (asprepared NCA, NCA-Mn, or NCA-Ti), super-p carbon black, and polyvinylidene fluoride (PVdF,   : ∼400000, Sigma-Aldrich) in the weight ratio of 94 : 3 : 3 were ground with Nmethyl-2-pyrrolidone (NMP, Samchun Pure Chemical) as a solvent.The slurry was coated on Al foil by doctor blade method, and then, the electrode was dried for 1 h at 80 ∘ C. The coated electrode was 20 m thick and it was pressed to 80% of the coating thickness.After drying under vacuum at 110 ∘ C for 24 h, the electrode was cut into a round plate (Φ = 15.958mm).The CR2032 coin-type cell was assembled in an argon-filled glove box with oxygen and moisture levels <1 ppm.Lithium foil, polyethylene (PE, W-SCOPE), and 1 M LiPF 6 /EC-DMC (3 : 7, v/v) were used as the counter electrode, separator, and electrolyte, respectively.
The crystalline phases of various NCA cathode materials were identified using X-ray diffraction (XRD, Dmax/1200, Rigaku) using CuK radiation ( = 1.5406 nm, 40 kV, 40 mA) over the range 2 = 10-80 ∘ .The morphologies of various NCA cathode materials were characterized by field-emissionscanning electron microscopy (FE-SEM, LEO-1530, Carl Zeiss).The electrical conductivities and particle size distribution of the NCA cathode materials were analyzed by a powder resistivity measurement system (HPRM-M2, HAN TECH) and a particle size analyzer (Mastersizer 3000, Marvern).Cyclic voltammetry was carried out in an automated charge-discharge equipment (WBCS 3000L, WonATech) at a scanning rate of 0.1 mV s −1 in the voltage range 2.8-4.5 V.The charge-discharge test (WonATech, WBCS 3000L) was performed at potentials ranging from 3.0 to 4.3 V at 0.1C.The electrochemical impedance measurements were performed using electrochemical impedance spectroscopy (EIS, PGSTAT302N, Metrohm Autolab B.V.) in the frequency range 0.01-100 kHz at an amplitude of 10 mV.All electrochemical measurements were performed at 25 ∘ C.

Results and Discussion
Figure 1 shows the XRD patterns of pristine NCA, NCA-Mn, and NCA-Ti powders in the 2 range 10-80 ∘ .All the peaks can be indexed to a hexagonal -NaFeO 2 structure (space group, R3m).The sharp and intense peaks at 18.8 ∘ , 36.7 ∘ , 44.5 ∘ , 48.2 ∘ , and 58.3 ∘ were representative of the (003), ( 101), ( 104), (015), and (107) diffraction planes, respectively [20].Furthermore, the splitting of the two peaks (006)/(102) and (108)/(110) was observed clearly at 37.7-38.7 ∘ and 63.6-65.4∘ , respectively, confirming that a typical layered structure formed in the  crystal lattice [21].In particular, the (003) peak of NCA-Mn and NCA-Ti shifted slightly toward the lower-angle region (inset of Figure 2) because Mn 4+ and Ti 4+ ions were introduced into the octahedral lattice space occupied by the Li-ions to expand the Li layer spacing and, thereby, improve the lithium diffusion kinetics [22].The variations in the lattice parameters were calculated by the Rietveld refinement method; the results are provided in Table 1.The integrated intensity ratio I (003) /I (104) has been used as an indicator of Li + /Ni 2+ cation mixing (Ni 2+ at the 3a site and Li + at the 3b site in the space group R3m): values smaller than 1.3 indicate a high degree of cation mixing because of the presence of other ions in the lithium interslab region.Thus, the reversible capacity of the cathode material tends to decrease when the ratio is <1.2 [23][24][25][26][27].As shown in Table 1, the I (003) /I (104) diffraction peaks are higher for NCA-Ti, which could be greatly affected by the presence of Ti-metal ions in the lithium layer, thus leading to better electrochemical properties.
The typical morphology of the NCA, NCA-Mn, and NCA-Ti powders is shown in Figure 2. Here, the small primary particles are organized into larger agglomerates as secondary particles.These three NCA cathode materials comprise almost spherical particles.
To investigate the doping results, element mapping and energy dispersive X-ray spectrometry (EDS) analyses were performed on the NCA, NCA-Mn, and NCA-Ti cathode materials.Figure 3 shows the Al, Co, Ni, Mn, and Ti element mapping results for NCA (Figure 3(a)), NCA-Mn (Figure 3(b)), and NCA-Ti (Figure 3(c)).The signals from Mn and Ti (bright yellow and green spots, respectively) clearly demonstrate that Mn and Ti were homogeneously distributed throughout the NCA-Mn and NCA-Ti particles.The Mn and Ti contents in the NCA cathode materials were analyzed by EDS.The weight percent of Mn and Ti was 0.8385% and 0.7049%, and their atomic percent was 0.8557% and 0.8529%, respectively.
Figure 4(a) shows the particle size distribution of the NCA, NCA-Mn, and NCA-Ti powders.As shown in Figure 4, three kinds of powders exhibited a bimodal distribution of particle sizes such as with small particle distribution at 3-5 m and large particle at 10-30 m.The NCA powder has an almost 1 : 1 volume density of small and large particle; however, the large range (10-30 m) of volume density was increased after doping of Mn or Ti into the NCA.In results, the NCA-Mn has a higher volume density of large particle size than others.The electrical conductivities of the NCA, NCA-Mn, and NCA-Ti powders were measured, and the results are depicted in Figure 4(b).The electrical conductivity () was measured at room temperature in the compression pressure range 5-23 MPa.The sample weight of the powder was approximately 1.0 g, and the diameter of the measuring die was 1.1 cm.The electrical conductivity was determined by the equation  = ℎ/(⋅), where  is the electrical resistance (Ω),  is the area of the pellet surface (m 2 ), and ℎ is the distance (m) from the bottom to the top of the sample.As shown in Figure 4(b), an increase in pressure increases the electrical conductivity for all the cathode materials considered in this study.The electrical conductivity of the metal-doped NCA powder exceeds that of pristine NCA under all pressure conditions, possibly because of the metal ions in the lithium layer.The electrical conductivities of the NCA, NCA-Mn, and NCA-Ti powders at 10 MPa are 2.92, 3.65, and 3.62 S m −1 , respectively.
The electrochemical properties of different NCA cathodes for LIBs were investigated by cyclic voltammetry (CV) using a coin-type cell with a potential window of 2.8-4.5 V at scan rate of 0.1 mV s −1 , as shown in Figure 5.In the CV curve, the peak currents represent the electrochemical properties of the material and express the phase transitions that occur during the intercalation/deintercalation of lithium ions [28,29].During the charge and discharge process, three pairs of peaks can be observed (I-I  , II-II  , and III-III  ) in the CV diagram, which are attributed to hexagonal to monoclinic, monoclinic to hexagonal, and hexagonal to hexagonal phase transitions, respectively [30,31] In the case of NCA, the oxidation peaks around 3.84, 4.02, and 4.20 V denote I, II, and III, respectively.The corresponding reduction peaks around 3.66, 3.93, and 4.12 V denote I  , II  , and III  .The NCA-Mn and NCA-Ti electrodes exhibit a broad oxidation peak with shoulders near 3.94 and 3.79 V, respectively, which may result from the lower intensity of lithium clustering due to metal doping [30].The initial charge-discharge curves of the NCA, NCA-Mn, and NCA-Ti cathodes are shown in Figure 6(a).The charge-discharge test is performed in the voltage range 3.0-4.3V at a rate of 0.1C.The first discharge capacities of NCA, NCA-Mn, and NCA-Ti are 156.5, 171.4,and 179.6 mAh g −1 , and the first charge capacities are 197.1,209.9, and 202.6 mAh g −1 , with initial coulombic efficiencies of 79.4%, 81.7%, and 88.6%, respectively.Studies have shown that the coulombic efficiency of NCA cathode is closely related to the interfacial resistance, which can be reduced significantly by metal doping [30,31].The cycle performances of NCA, NCA-Mn, and NCA-Ti are shown in Figure 6(b).The discharge capacity of NCA is lower than that of metal-doped NCA-Mn or NCA-Ti.The best discharge capacity is found for the NCA-Ti because Ti 4+ is introduced into the octahedral lattice space occupied by Li + to expand the Li layer spacing; thereby the Li + diffusion kinetics are improved.The discharge capacities of NCA, NCA-Mn, and NCA-Ti at the end of 30 cycles are 150.1,162.9, and 166.7 mAh g −1 , respectively.Better electrochemical performance of NCA-Ti is closely related to its excellent crystallization, layered structure, and electrical conductivity from the aforementioned analysis.
To investigate the variation in surface morphology of the different electrodes after 10 cycles, cross-sectional FE-SEM analysis was performed, as depicted in Figure 7.   respectively.The insertion figures in Figure 7 show the enlarged images for the rectangular part of the cross-sectional images of the electrodes.As can be seen in the crosssectional FE-SEM images, the electrodes show clear change after cycling when compared with the fresh electrodes.The larger primary particle size of NCA is found after 10th cycle discharged compared to before cycling.However, the NCA-Mn and NCA-Ti are almost unchanged even after cycling.
This suggests that the NCA-Mn and NCA-Ti electrodes have a stable structure with much less structural change during cycling.Thus, the electrochemical properties of NCA can be improved by doping with Mn or Ti.Nyquist plots of the fresh NCA, NCA-Mn, and NCA-Ti electrodes and the electrodes after the 10th cycle are shown in Figure 8 [32][33][34][35][36].The Nyquist plots exhibit identical electrochemical behaviors with semicircles in the high-frequency region and a straight sloping line at low frequencies.The semicircle in the high-to-medium frequency region is related to the solid/electrolyte interphase (SEI) layer resistance ( 1 ), and the semicircle at medium frequency is related to the charge transfer resistance (  =  2 +  3 ).The EIS data for various electrodes used in LIBs are listed in Table 2.   for all electrodes are increased after 10th discharging because of continuous SEI formation on the surface of the particles [34,35].The  ct values of the LIBs with the fresh NCA, NCA-Mn, and NCA-Ti electrodes are 191.9,159.2, and 124.2 Ω, respectively, while the corresponding values after 10 cycles are 357.9,300.8, and 238.4 Ω.Moreover, it can be observed that the NCA-Ti electrode has the smallest SEI film resistance ( 1 ).This suggests that the NCA-Ti electrode has superior electrochemical properties and allows for rapid electron transport during the electrochemical Li + insertion/extraction reaction [32,33].The EIS results agree well with the electrical conductivity (Figure 4(b)) and discharge behavior (Figure 6).

Conclusions
NCA cathode materials were successfully prepared by a hydrothermal reaction, and their electrochemical properties were improved by doping with the transition metals Mn and Ti.In particular, the Ti-doped cathode material (NCA-Ti) had a good crystalline structure and showed higher electrical conductivity than the other sample materials.In addition, the NCA-Ti electrode exhibited enhanced electrochemical performance.NCA-Ti showed the best discharge capacity of 179.6 mAh g −1 after the first cycle and an initial coulombic efficiency of 88.6%.

Figure 6 :
Figure 6: (a) Charge-discharge profiles at first cycle and (b) cycling performance of NCA, NCA-Mn, and NCA-Ti electrodes in a voltage range of 3.0 to 4.3 V at 0.1C.
. Experimental results shown in solid lines are fitted by the equivalent circuit in the inset of Figure8

Table 1 :
Lattice parameters of the NCA, NCA-Mn, and NCA-Ti powders.

Table 2 :
EIS results of lithium ion batteries with NCA, NCA-Mn, and NCA-Ti electrodes.