Improving the Electrochemical Performance of LiNi0.80Co0 ... - MDPI

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5 Mar 2018 - Qiwei Tang, Jinzhao Lv, Xianjun Du and Yanmeng Dou. Shandong ... wg.wang@yhnewenergy.com (W.W); [email protected] (H.P.); ...
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Improving the Electrochemical Performance of LiNi0.80Co0.15Al0.05O2 in Lithium Ion Batteries by LiAlO2 Surface Modification Chunhua Song ID , Wenge Wang, Huili Peng, Ying Wang *, Chenglong Zhao *, Huibin Zhang, Qiwei Tang, Jinzhao Lv, Xianjun Du and Yanmeng Dou Shandong YuHuang New Energy Technology Co. Ltd., Heze 274000, China; [email protected] (C.S.); [email protected] (W.W); [email protected] (H.P.); [email protected] (H.Z.); [email protected] (Q.T.); [email protected] (J.L.); [email protected] (X.D.); [email protected] (Y.D.) * Correspondence: [email protected] (Y.W.); [email protected] (C.Z.) Received: 27 October 2017; Accepted: 13 February 2018; Published: 5 March 2018

Featured Application: LiNi0.80Co0.15Al0.05O2 can be used to fabricate high energy density lithium-ion batteries for uses including powering electric vehicles, energy storage (including communication base stations, wind power generation, solar power generation), and digital electronics. Abstract: LiNi0.80 Co0.15 Al0.05 O2 (NCA) as a lithium ion battery cathode material has received attention for its highly specific capacity and excellent low temperature performance. However, the disadvantages of its high surface lithium compound residues and high pH value have influenced its processing performance and limited its application. This paper uses a facile method to modify NCA through LiAlO2 coating. The results showed that when the molar ratio of Al(NO3 )3 ·9H2 O and lithium compound residues at the surface of NCA cathode material was 0.25:1, the pH of the cathode material decreased from 12.70 to 11.80 and the surface lithium compound residues decreased from 3.99% to 1.48%. The NCA cell was charged and discharged for 100 cycles at 1 C in the voltage range of 3.0–4.3 V, to test the capacity retention of NCA. It was found to be as high as 94.67%, which was 5.36% higher than the control NCA cell. The discharge capacity of NCA-0.25-500 ◦ C was 139.8 mAh/g even at 8 C rate, which was 15% higher than the raw NCA. Further research indicated that Al(NO3 )3 ·9H2 O reacted with the surface lithium compound residues of NCA and generated LiAlO2 , which improved the NCA electrochemical performance. Keywords: LiNi0.80 Co0.15 Al0.05 O2 ; LiAlO2 coating; lithium compound residues; pH value; capacity retention

1. Introduction Lithium ion batteries (LIBs) are the next generation of power sources for technology such as electric vehicles, mobile phones, and notebook computers, due to its higher energy density, longer cyclic life, faster charge/discharge rates, and better safety. The performance of cathode materials is one of the most important factors that determine the performance of a battery. Therefore, developing a cathode material with high energy density, long cyclic life, and high security is essential [1]. Nickel-rich cathode material Li(Nix M1−x )O2 is one of the most applicable cathode materials for advanced LIBs, due to its high energy, power density, and lower cost [2]. Intense research efforts have found that Co and Al co-doping could stabilise the layered structure of LiNiO2 . To date, a Co and Al co-doped NCA has been successfully designed, and it shows excellent thermodynamic stability and electrochemical reactivity with a specific capacity as high as 200 mAh/g. Therefore, NCA shows promise as a candidate for lithium ion batteries, particularly for use in electric vehicles. Appl. Sci. 2018, 8, 378; doi:10.3390/app8030378

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However, some aspects of NCA remain problematic such as poor cycle stability, low electronic conductivity, and the disordered spinel structure in the charged states due to its inherent shortcomings with instability structure at elevated voltages [3–5]. NCA has been synthesized by various methods including Co-precipitation which is the most common method. To compensate for the loss of lithium at high temperature, excessive lithium is needed in the preparation, however it does not completely evaporate during the high temperature solid phase reaction. Some lithium oxide (Li2 O) remains on the surface of the cathode materials which can react directly with CO2 and atmospheric H2 O to generate Li2 CO3 and LiOH [6], as in the reaction shown at Equation (1). In the coating process of battery fabrication, the slurry readily forms a gel and loses its binding ability, which is detrimental to the foil coating. Furthermore, Li2 CO3 may react with HF and generate CO2 , resulting in high temperature inflatable and cyclic performance degradation [4]. Additionally, under the highly delithiated state, the reduction of Ni4+ to Ni2+ occurred on the NCA surface and led to the release of O2 , and the consequent thermal instability and safety problems. LiNi0.80 Co0.15 Al0.05 O2 + 12 xO2 + 12 xCO2 + 12 xH2 O ⇒ Li1−2x Ni0.80 Co0.15 Al0.05 O2 + 21 xLi2 CO3 + xLiOH

(1)

To reduce the residual lithium compound and the pH of NCA, intensive methods have been developed recently: (i.)

Controlling the residual lithium compound content by reducing the proportion of lithium salt or by extending the time of the solid phase reaction at high temperature. (ii.) Washing the cathode materials several times in water, carbonate, organic acids, or organic solvents. (iii.) Enhancing the capacity and electronic conductivity by coating the electroactive materials; TiO2 , AlF3 , Al2 O3 , SiO2 , and AlPO4 have been suggested as suitable coating materials for the layered cathodes [7–16]. The coating material mainly acts as an isolation layer to prevent side reactions and reduce the pH, which improved thermal stability and cycling stability of the cathode materials. In this study, we improved the performance of Ni-rich cathode material NCA using a facile method by modifying LiAlO2 . The results revealed that the residual lithium compound and the pH of NCA was successfully reduced by coating it with LiAlO2 . Furthermore, the SEM images of NCA indicated that the layer was distributed uniformly on the cathode material, which could enhance process ability, safety, and the electrochemical performance of lithium ion batteries. 2. Experiment 2.1. Synthesis of NCA All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used without further purification. NiSO4 ·6H2 O, CoSO4 ·7H2 O, and Al2 (SO4 )3 ·18H2 O were mixed at a molar ratio of Ni2+ :Co2+ :Al3+ = 80:15:5. This mixed solution was pumped slowly into a continuously stirred tank reactor. At same time, a NaOH and NH4 OH composite solution was also pumped into the reactor and the pH value controlled at 11.4. After the reaction, the precipitate was filtered and washed with distilled water to remove the residual ions. The precipitation was then dried in a vacuum oven at 120 ◦ C for 12 h to obtain the (Ni0.80 Co0.15 Al0.05 )(OH)2 precursor. The (Ni0.80 Co0.15 Al0.05 )(OH)2 precursor and LiOH·H2 O were mixed thoroughly with LiOH·H2 O 5% excess in a molar ratio. The mixture was heated at 450 ◦ C for 6 h, then calcined at 780 ◦ C for 12 h in oxygen at a heating rate of 2 ◦ C/min. NCA was prepared after crushing and sieving.

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2.2. LiAlO2 Modified NCA The cathode material 2.2. LiAlO NCA obtained was mixed thoroughly with Al(NO3)3·9H2O by milling in an agate 2 Modified mortar for 30 min. The mixture was first handled at 75 °C for 2 h in air, and then calcined at different The cathode material obtained was mixed thoroughly with Al(NO ) ·9H2 O by milling in an agate temperatures for 2 h with a heating rate of 5 °C/min.◦ NCA with the3 3molar ratio of Al(NO3)3 and mortar for 30 min. The mixture was first handled at 75 C for 2 h in air, and then calcined at different residual Li2CO3 of the cathode surface being 0.25 were calcined at 400 °C, 500 °C and 600 °C for 2 h, temperatures for 2 h with a heating rate of 5 ◦ C/min. NCA with the molar ratio of Al(NO3 )3 and which are expressed as NCA-0.25-400 °C, NCA-0.25-500 °C and NCA-0.25-600 °C, respectively. residual Li2 CO3 of the cathode surface being 0.25 were calcined at 400 ◦ C, 500 ◦ C and 600 ◦ C for Products calcined at 500 °C for 2 h are labeled as NCA-2, NCA-0.5 and NCA-0.25 with the molar ratio 2 h, which are expressed as NCA-0.25-400 ◦ C, NCA-0.25-500 ◦ C and NCA-0.25-600 ◦ C, respectively. of Al(NO3)3 and excess Li◦2CO3 at the surface of NCA being 2, 0.5 and 0.25, respectively. Products calcined at 500 C for 2 h are labeled as NCA-2, NCA-0.5 and NCA-0.25 with the molar ratio of Al(NO and excess Li2 CO3 at the surface of NCA being 2, 0.5 and 0.25, respectively. 3 )3 Characterizations 2.3. Material 2.3. Material Characterizations The phase of all samples were identified by X-ray diffraction (XRD, D2 Phaser, Bruker, Karlsruhe, Germany) using Cu Kα radiation at 40 kV and 40 mA. The scanning range of the diffraction angle The phase of all samples were identified by X-ray diffraction (XRD, D2 Phaser, Bruker, Karlsruhe, (2θ) was 10–90°. The morphologies of products were determined by scanning electron microscope Germany) using Cu Kα radiation at 40 kV and 40 mA. The scanning range of the diffraction angle (2θ) (Ultra plus, Zeiss, Oberkochen, Germany). The electrochemical impedance spectroscopy (EIS) was 10–90◦ . The morphologies of products were determined by scanning electron microscope (Ultra measures were performed using an impedance analyzer (CHI660E, CH, Shanghai, China) over the plus, Zeiss, Oberkochen, Germany). The electrochemical impedance spectroscopy (EIS) measures were frequency range of 0.002–1000 Hz with AC voltage amplitude of 5 mV. performed using an impedance analyzer (CHI660E, CH, Shanghai, China) over the frequency range of 0.002–1000 Hz with AC voltage amplitude of 5 mV. 2.4. Electrochemical Measurement 2.4. Electrochemical Measurement CR2032 coin-type cells were used for the electrochemical test. The cathode electrode consisted of 88 CR2032 wt % active material, wt % used acetylene black (Super P), 3 wt % conductive (ks-6) and 6 coin-type cells3 were for the electrochemical test. The cathodegraphite electrode consisted wt % PVDF binder. A lithium metal foil was used the anode. (Cellgard of 88 wt % active material, 3 wt % acetylene blackas(Super P), 3A wtpolypropylene % conductivefilm graphite (ks-6)2400) and was used as the separator. LiPF 6 in a 1:1:1 (v/v/v) mixture of ethylene carbonate (EC), dimethyl 6 wt % PVDF binder. A lithium metal foil was used as the anode. A polypropylene film (Cellgard carbonate (DMC), andseparator. ethyl methyl usedofasethylene the electrolyte. Assembly of the 2400) was used as the LiPF6carbonate in a 1:1:1 (EMC) (v/v/v)was mixture carbonate (EC), dimethyl cells was carried out in a dry Ar-filled glove box (UNILab, MBRAUN, Garching, Germany). carbonate (DMC), and ethyl methyl carbonate (EMC) was used as the electrolyte. Assembly of the cells Galvanostatic performedMBRAUN, at a voltage range of 3.0–4.3 VGalvanostatic on a battery was carried outcharge-discharge in a dry Ar-filledtests glovewere box (UNILab, Garching, Germany). testing system (CT2001A, LANHE, Wuhan, charge-discharge tests were performed at aChina). voltage range of 3.0–4.3 V on a battery testing system (CT2001A, LANHE, Wuhan, China). 3. Results and Discussion 3. Results Discussion Figureand 1 shows the XRD diffraction profiles of the NCA with different coating contents of LiAlO 2 . As can be seen, the diffraction of the all samples aredifferent well-indexed the hexagonal α-2 . Figure 1 shows the XRD diffractionpatterns profiles of NCA with coatingtocontents of LiAlO As can 2be seen, the diffraction samples aredisplay well-indexed to the hexagonal α-NaFeO structure with a space patterns group R 3ofm.allAll samples clear diffraction peaks without any2 NaFeO structure with a space group R3m. All samples display clear diffraction peaks without any discernable discernable secondary phase. The clear splitting of (006)/(012) and (018)/(110) peak pairs for NCA, secondaryand phase. The clear splitting of (006)/(012) and (018)/(110) peak pairs for NCA, NCA-0.25 NCA-0.25 NCA-0.5 suggests the formation of highly ordered hexagonal layered structure [1,17]. and NCA-0.5 suggests the formation of highly ordered hexagonal layered structure [1,17]. The results The results suggest that the crystal structure of NCA is not affected by a small amount of LiAlO 2. As suggest thatdiffraction the crystalpatterns structureofofNCA-2, NCA isthe notseparation affected byofa(006)/(012) small amount LiAlO2 . As seen in the seen in the andof (018)/(110) peak pairs is diffraction patterns of NCA-2, the separation of (006)/(012) and (018)/(110) peak pairs is not clear, not clear, indicating that increasing amounts of Al(NO3)3·9H2O will influence the crystalline structure indicating of NCA. that increasing amounts of Al(NO3 )3 ·9H2 O will influence the crystalline structure of NCA.

Figure 1. XRD patterns of four samples, NCA, NCA-0.25, NCA-0.5 and NCA-2. Figure 1. XRD patterns of four samples, NCA, NCA-0.25, NCA-0.5 and NCA-2.

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Table 1 presents the values of a, c, c/a, I003 /I104 calculated from XRD patterns of the samples. Tableradius 1 presents the(rvalues of a,isc,close c/a, to I003Li /I104 calculated XRD patterns the samples. The + (r The ionic of Ni2+ = 70 pm) = 74 pm),from which results in theofmigration of some 2+ + ionic of Nito Li (r += sites. 70 pm)The is close to of Li cation (r = 74mixing pm), which resultsevaluated in the migration of some ofratio the of theradius Ni2+ ions extent is usually by the intensity 2+ ions to Li+ sites. The extent of cation mixing is usually evaluated by the intensity ratio of Ni of (003)/(104). Higher values of I003 /I104 correspond to a low extent of cation mixing. It can be seen (003)/(104). values I003I /I104/Icorrespond to a low extent of cation mixing. It can be seen from from Table 1Higher that the valueofof 003 104 of all samples are higher than 1.2, which indicates that the Table 1 that value of I003of /I104 of allmixing samples areIthigher than that the samples samples havethe a low degree cation [17]. is noted that1.2, thewhich latticeindicates parameter increases with have a low degree of cation mixing [17]. It is noted that the lattice parameter increases the the addition of Al(NO3 )3 ·9H2 O, but the value of c/a is not reduced. Larger ratios of c/a bringwith greater addition of Al(NO 3)3·9H2O, but the value of c/a is not reduced. Larger ratios of c/a bring greater degrees of crystallinity and stability of the NCA with layered structure [18]. degrees of crystallinity and stability of the NCA with layered structure [18]. Table 1. Lattice parameters and the values of I003 /I104 for NCA, NCA-0.25, NCA-0.5 and NCA-2. Table 1. Lattice parameters and the values of I003/I104 for NCA, NCA-0.25, NCA-0.5 and NCA-2. Sample Å) Å) c c(± 0.0004 Å)Å) Sample a (± a 0.0003 (±0.0003 (±0.0004 NCA NCA 2.8576 14.0508 2.8576 14.0508 NCA-0.25 2.8601 14.0340 NCA-0.25 2.8601 14.0340 NCA-0.5 2.8677 14.2335 NCA-0.5 2.8677 14.2335 NCA-2 2.8648 14.0131

NCA-2

2.8648

14.0131

c/a c/a 4.92 4.92 4.91 4.91 4.96 4.96 4.89 4.89

I003 /I104 I003/I104 2.352.35 2.192.19 1.90 1.902.31 2.31

The SEM images images of of NCA, NCA,NCA-0.25, NCA-0.25,NCA-0.5, NCA-0.5,and andNCA-2 NCA-2are are shown Figure 2 and it can The SEM shown in in Figure 2 and it can be be seen that the surface of the pristine NCA is smooth. After LiAlO modification, a layer appears 2 seen that the surface of the pristine NCA is smooth. After LiAlO2 modification, a layer appears on the on the surface NCA the becomes surface becomes rough,increases which increases with increasing amount surface of NCAofand theand surface rough, which with increasing amount of Al(NO3of )3 Al(NO ) [1,18]. From Figure 2b, it can be seen that there are some floccules on the particle surface 3 3 [1,18]. From Figure 2b, it can be seen that there are some floccules on the particle surface and between and betweenclearances. the particleWhen clearances. When the of molar ratio of Al(NO3 )3residual ·9H2 O and residual Li CO3 the particle the molar ratio Al(NO 3)3·9H2O and Li2CO 3 of the 2NCA of the NCA surface is 0.5:1, more floccules are present between the particles and some floccules are surface is 0.5:1, more floccules are present between the particles and some floccules are agglomerated. agglomerated. Evenratio at theofmolar ratio of Al(NO3 )3residual ·9H2 O and Li2 CO is 2:1, the agglomerated Even at the molar Al(NO 3)3·9H2O and Li2residual CO3 is 2:1, the3 agglomerated floccules floccules appear on the surface of NCA. In combination with the XRD, the layered of optimal NCA is appear on the surface of NCA. In combination with the XRD, the layered structurestructure of NCA is optimal at a molar ratio of Al(NO ) · 9H O and residual Li CO of 0.25. 3 residual 2 2 3 Li2CO3 of 0.25. at a molar ratio of Al(NO3)3·9H2O3and

Figure 2. SEM images of the as-prepared samples (a) NCA, (b) NCA-0.25, (c) NCA-0.5, (d) NCA-2. Figure 2. SEM images of the as-prepared samples (a) NCA, (b) NCA-0.25, (c) NCA-0.5, (d) NCA-2.

Figure 3 displays the XRD patterns of NCA-0.25 under different calcination temperatures. The Figure 3 displays the XRD patterns to of the NCA-0.25 under different calcination temperatures. diffraction patterns of samples correspond hexagonal α-NaFeO 2 structure with a space group The diffraction patterns of samples correspond to the hexagonal α-NaFeO2 structure with a space R 3 m. When the calcination temperature is 400 °C (Figure 3b), the position of all peaks shifts slightly group R3m. When the calcination temperature is 400 ◦ C (Figure 3b), the position of all peaks shifts toward the lower degree. At the calcination temperature of 500 °C and 600 °C, there are no peaks of slightly toward the lower degree. At the calcination temperature of 500 ◦ C and 600 ◦ C, there are no the impurity phase, indicating that the structure of the sample is not destroyed. peaks of the impurity phase, indicating that the structure of the sample is not destroyed.

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Figure 3. (a) XRD patterns of the pristine NCA sample and NCA-0.25 samples at different calcination Figure 3. (a) XRD patterns of the pristine NCA sample and NCA-0.25 samples at different calcination temperature from from 400 400 ◦°C to 600 600 ◦°C, patterns of of (a) (a) between between 33 33°◦ and temperature C to C, and and (b) (b) the the magnified magnified patterns and 50°. 50◦ . Figure 3. (a) XRD patterns of the pristine NCA sample and NCA-0.25 samples at different calcination

from pH 400 °C to 600 °C, and (b) the magnified patterns of atmospheric (a) between 33° and NCAtemperature with higher values reacts readily with CO 2 and H2O50°.and results in a NCA with higher pH values reacts readily with CO2 and atmospheric H2 O and results in a degraded electrochemical performance. Therefore, unmodified NCA is detrimental to long term with higher pH values reactsTherefore, readily withunmodified CO2 and atmospheric H2O and results in a term degradedNCA electrochemical performance. NCA is detrimental to long storage. Figure 4 shows the cycle performance of the pristine NCA sample and the NCA-0.25 samples degraded electrochemical performance. Therefore, unmodified NCA is detrimental to long term storage. Figure 4 shows the cycle performance of the pristine NCA sample and the NCA-0.25 samples at different calcination temperatures from 400 of °Cthe to pristine 600◦ °C.NCA Table 2 shows that when thesamples molar ratio storage.calcination Figure 4 shows the cycle performance thewhen NCA-0.25 at different temperatures from 400 ◦ C to 600 C. Table 2sample showsand that the molar ratio of of Al(NO 3)3·9H2O and residual Li2CO3 of the NCA surface is 0.25, the pH value and residual alkaline at different calcination temperatures from 400 °C to 600 °C. Table 2 shows that when the molar ratio Al(NO Liis2 CO NCA surface is 0.25, theLiAlO pH value and residual alkaline 3 )3 ·9H2 O and residual 3 of theto of materials beneficial storage performance. 2 modified NCA shows no of Al(NOis3)3reduced, ·9H2O andwhich residual Li 2CO3 of the NCA surface is 0.25, the pH value and residual alkaline of materials is reduced, which is beneficial to storage performance. LiAlO modified NCA shows no 2 improvements discharge of materialsin is first reduced, which capacity. is beneficial to storage performance. LiAlO2 modified NCA shows no improvements in discharge capacity. improvements in first discharge capacity. However, thefirst discharge capacity of LiAlO2 modified NCA is significantly higher than the raw However, the discharge capacity ofofLiAlO modified NCA significantly the raw 22 modified theFurthermore, discharge capacity LiAlO NCA is is significantly thanthan the raw materialsHowever, at 1 C rate. capacity retention of LiAlO 2 modified NCAhigher ishigher higher than the raw materials at 1 C rate. Furthermore, capacity retention of LiAlO modified NCA is higher than the materials 1 C rate.indicate Furthermore, capacity retention of LiAlO2 2modified NCA is higher than the raw raw materials. Theatresults that LiAlO 2 modification can improve the electrochemical properties materials. The results indicate canimprove improve electrochemical properties materials. The results indicatethat thatLiAlO LiAlO22 modification modification can thethe properties and physical properties of the cathode materials. The NCA-0.25-400 °Celectrochemical exhibits an initial discharge and physical properties of the cathodematerials. materials. The NCA-0.25-400 °C◦exhibits an initial discharge and physical properties of the cathode The NCA-0.25-400 C exhibits an initial discharge capacity of 166.3 mAh/g at 1 C and capacity retention of 88.03% after 100 cycles. When the calcination capacity of 166.3 mAh/g at 1 Cand andcapacity capacity retention retention of 88.03% after 100100 cycles. When the calcination capacity of 166.3 mAh/g at 1 C of 88.03% after cycles. When the calcination temperature of NCA-0.25 is 500 °C and 600 °C, the initialdischarge discharge capacity172.3 is 172.3 and 168.6 mAh/g temperature of NCA-0.25 is 500 and 600 °C,the theinitial initialdischarge capacity is and 168.6 mAh/g ◦ C °C ◦ C, temperature of NCA-0.25 is 500 and 600 capacity is 172.3 and 168.6 mAh/g at at 1 C, its capacity retention 92.88%,respectively. respectively. Based analysis, at 1and C, and its capacity retentionisis94.67% 94.67% and and 92.88%, Based on on XRDXRD analysis, the the 1optimum C, and itselectrochemical capacity retention is 94.67% and 92.88%, respectively. Based on XRD analysis, the optimum performance calcinationtemperature temperature of NCA-0.25 is°C. 500At°C. At optimum electrochemical performanceisiswhen when the the calcination of NCA-0.25 is 500 ◦ C. electrochemical performance is when the calcination temperature of NCA-0.25 is 500 At the the same time, in in order to toavoid of the thedata datacaused caused accident ofsame one the same time, order avoidthe the uncertainty uncertainty of by by the the accident of one time, in order towe avoid the of theof data caused bytest thethree accident ofthe one measurement, we take the measurement, we take the averagevalue value of each parallel times, results are appropriate, measurement, take theuncertainty average each parallel test three times, the results are appropriate, average value ofand each parallel seeing Table S1 and Figure S1. three times, the results are appropriate, seeing Table S1 and Figure S1. seeing Table S1 Figure S1.test

Figure 4. Cyclic performance of the pristine NCA sample and NCA-0.25 samples at different

Figure 4. Cyclic performance of the pristine NCA sample and NCA-0.25 samples at different calcination calcination temperatures from 400 °C to 600 °C. ◦ C. temperatures fromperformance 400 ◦ C to 600of Figure 4. Cyclic the pristine NCA sample and NCA-0.25 samples at different

calcination temperatures from 400 °C to 600 °C.

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Table 2. Physical and chemical properties of the pristine NCA sample and NCA-0.25 samples at ◦ C. different temperatures from 400 ◦of C to 600 Table 2. calcination Physical and chemical properties the pristine NCA sample and NCA-0.25 samples at

different calcination temperatures from 400 °C to 600 °C.

110 Cycles Discharge First Discharge Residual Alkaline First Discharge Capacity Capacity at Capacity at 1 C (Li2 CO3 % + Discharge 110 Cycles Efficiency (%) Residual LiOH%) Alkaline First 0.2 Discharge Capacity First Discharge (%) (mAh/g) C (mAh/g) pH Capacity at 1 C Retention Capacity (Li2CO3% + LiOH%) at 0.2 C (mAh/g) Efficiency (%) (mAh/g) Retention (%) NCA 12.70 3.99% 183.1 88.6% 173.0 89.31% NCA 3.99% 183.1 NCA-0.25-400 °C12.7011.86 1.78% 181.1 83.1% 88.6% 166.3 173.0 88.03%89.31% NCA-0.25-500 1.48% 184.5 83.8% 83.1% 172.3 166.3 94.67%88.03% NCA-0.25-400 ℃ °C 11.8611.80 1.78% 181.1 NCA-0.25-600 1.38% 184.1 84.3% 83.8% 168.6 172.3 92.88%94.67% NCA-0.25-500 ℃ °C 11.8011.82 1.48% 184.5 NCA-0.25-600 ℃ 11.82 1.38% 184.1 84.3% 168.6 92.88% Sample Sample

pH

The TEM and EDS images of pristine NCA and NCA-0.25 are shown in Figure 5. The TEM images The TEM and EDS images of pristine NCA and NCA-0.25 are shown in Figure 5. The TEM show that there is a significant difference in the particle surface before and after coating with a uniform images show that there is a significant difference in the particle surface before and after coating with coating layer of about 5.3 nm after LiAlO2 modification (Figure 5b). The results indicate that the a uniform coating layer of about 5.3 nm after LiAlO2 modification (Figure 5b). The results indicate Al(NO3 )3 ·9H2 O reacted with the surface lithium compound residues and generated LiAlO2 , which that the Al(NO3)3·9H2O reacted with the surface lithium compound residues and generated LiAlO2, can suppress the reactions between the electrode and the electrolyte, and facilitated the charge transfer which can suppress the reactions between the electrode and the electrolyte, and facilitated the charge process. The composition of NCA was examined by EDS, and it was found that NCA had typical transfer process. The composition of NCA was examined by EDS, and it was found that NCA had diffraction peaks of O, Ni, Co, Al. The molar ratio of Ni, Co and Al was 82.4:14.8:2.8, which is close to typical diffraction peaks of O, Ni, Co, Al. The molar ratio of Ni, Co and Al was 82.4:14.8:2.8, which is the NCA molecular formula. Because Al(OH)3 is a bisexual compound, it is difficult to co-precipitate close to the NCA molecular formula. Because Al(OH)3 is a bisexual compound, it is difficult to coAl with Ni and Co, and the actual Al content is smaller than 5%. Furthermore, the content of aluminum precipitate Al with Ni and Co, and the actual Al content is smaller than 5%. Furthermore, the content on the particle surface of NCA increased to 3.8% after LiAlO2 modification [11]. of aluminum on the particle surface of NCA increased to 3.8% after LiAlO2 modification [11].

Figure 5. TEM micrographs of the pristine NCA (a) and NCA-0.25 (b), EDS pattern of the pristine Figure 5. TEM micrographs of the pristine NCA (a) and NCA-0.25 (b), EDS pattern of the pristine NCA NCA (c) and NCA-0.25 (d). (c) and NCA-0.25 (d).

Electrochemical measurements were taken to investigate the influence of LiAlO2 on the Electrochemical measurements were taken to investigate influence of LiAlO onNCA the electrochemical performances of NCA. Figure 6a shows the initialthe charge/discharge curves2 of electrochemical performances of NCA. Figure 6a shows the initial charge/discharge curves of NCA and NCA-0.25-500 °C at 0.2 C. The initial coulombic efficiencies of NCA and NCA-0.25-500 °C are ◦ C at 0.2 C. The initial coulombic efficiencies of NCA and NCA-0.25-500 ◦ C are and NCA-0.25-500 88.6% and 83.8%, respectively. Despite the initial coulombic efficiencies, they declined after LiAlO2 88.6% and 83.8%, respectively. Despite is thesmall. initialWhen coulombic declined 2 modification, however the difference cells efficiencies, were cycledthey for 100 cyclesafter at 1LiAlO C, the modification, however the difference is small. When cells were cycled for 100 cycles at 1 C, the discharge discharge capacities of raw NCA and NCA-0.25-500 °C were 154.5 and 163.1 mAh/g (Figure 6b), ◦ C were 154.5 and 163.1 mAh/g (Figure 6b), respectively, capacities of raw NCA and respectively, indicating thatNCA-0.25-500 LiAlO2 can improve the cycling stability of NCA [19]. indicating that LiAlO2 can improve the cycling stability of NCA [19].

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Figure at at 0.20.2 CC (a)(a) and at the 110th cycle at 1atC1(b) Figure 6. 6. Charge Chargeand anddischarge dischargeprofiles profilesatatthe thefirst firstcycle cycle and at the 110th cycle C of the as-prepared samples; the cycle performances at 1 C (c) and rate capability (d) of the as-prepared (b) of the as-prepared samples; the cycle performances at 1 C (c) and rate capability (d) of the samples. as-prepared samples.

Figure 6c shows the cycling performances for 100 cycles at 1C in the voltage range of 3.0–4.3 V Figure 6c shows the cycling performances for 100 cycles at 1C in the voltage range of 3.0–4.3 V and it shows that NCA exhibits a rapid discharge capacity fading after 100 cycles and a low capacity and it shows that NCA exhibits a rapid discharge capacity fading after 100 cycles and a low capacity retention of 89.31%. The lower capacity retention suggests that electrode materials may react with retention of 89.31%. The lower capacity retention suggests that electrode materials may react with electrolyte and generate particles on the surface, which hinder the lithium ions intercalation. In electrolyte and generate particles on the surface, which hinder the lithium ions intercalation. In contrast, contrast, NCA-0.25-500 °C shows a better cyclic performance with a lower discharge fading after 100 NCA-0.25-500 ◦ C shows a better cyclic performance with a lower discharge fading after 100 cycles and cycles and a relatively high capacity retention of 94.67%. This indicates that LiAlO2 modification can a relatively high capacity retention of 94.67%. This indicates that LiAlO2 modification can restrain the restrain the capacity degradation effectively. To examine the rate capabilities of raw NCA and NCAcapacity degradation effectively. To examine the rate capabilities of raw NCA and NCA-0.25-500 ◦ C, 0.25-500 °C, discharge capabilities were taken at 0.2 C, 1 C, 2 C, 5 C and 8 C. As shown in Figure 6d, discharge capabilities were taken at 0.2 C, 1 C, 2 C, 5 C and 8 C. As shown in Figure 6d, compared compared to NCA, NCA-0.25-500 °C show better discharge capacity and cycle stability as the to NCA, NCA-0.25-500 ◦ C show better discharge capacity and cycle stability as the discharge rate discharge rate increased. This is especially so when the discharge capacities of◦ NCA-0.25-500 °C is increased. This is especially so when the discharge capacities of NCA-0.25-500 C is 139.8 mAh/g at 139.8 mAh/g at 8 C rate, which is 15% higher than NCA. This is mainly due to the more stable 8 C rate, which is 15% higher than NCA. This is mainly due to the more stable structure during cycling structure during cycling of NCA with the coating layer, which can protect the particle from the side of NCA with the coating layer, which can protect the particle from the side reactions on the surface. reactions on the surface. Therefore, LiAlO2 modification can improve the discharge capacities and Therefore, LiAlO2 modification can improve the discharge capacities and cyclic stability. cyclic stability. The electrochemical impedance spectra (EIS) of the samples are shown in Figure 7. Figure 7a The electrochemical impedance spectra (EIS) of the samples are shown in Figure 7. Figure 7a shows electrochemical impedance spectroscopy and equivalent circuit of samples before discharge [20]. shows electrochemical impedance spectroscopy and equivalent circuit of samples before discharge Figure 7b shows electrochemical impedance spectroscopy and equivalent circuit of samples after 25 [20]. Figure 7b shows electrochemical impedance spectroscopy and equivalent circuit of samples after charge–discharge cycles at 1 C. The circuit elements of Rs, Rf, Rct, CPE, Ws and Wo represent the 25 charge–discharge cycles at 1 C. The circuit elements of Rs, Rf, Rct, CPE, Ws and Wo represent the internal resistance of the cell, the surface film resistance (the resistance of SEI film), the charge-transfer internal resistance of the cell, the surface film resistance (the resistance of SEI film), the chargeresistance, constant phase element, the Li+ diffusion in finite thin layer impedance, and the Warburg transfer resistance, constant phase element, the Li+ diffusion in finite thin layer impedance, and the electrode bulk, respectively [21]. Warburg electrode bulk, respectively [21]. As can be seen from Figure 7a, the assembled cell displays a single semicircle at the higher As can be seen from Figure 7a, the assembled cell displays a single semicircle at the higher frequency and a straight line at low frequency, which corresponds to the surface film resistance and frequency and a straight line at low frequency, which corresponds to the surface film resistance and Warburg resistance. It can be seen from Figure 7a that there is no charge-transfer resistance due to Warburg resistance. It can be seen from Figure 7a that there is no charge-transfer resistance due to the fact that the anode has no electrochemical reaction before charge-discharge [22]. Table 3 shows the fact that the anode has no electrochemical reaction before charge-discharge [22]. Table 3 shows the impedances of LiAlO2 modified samples drastically decreased compared to the raw material. the impedances of LiAlO2 modified samples drastically decreased compared to the raw material. This This phenomenon illustrates that LiAlO2 modification is beneficial to charge migration on the electrode phenomenon illustrates that LiAlO2 modification is beneficial to charge migration on the electrode interface and could decrease the battery internal impedance. interface and could decrease the battery internal impedance. After 25 cycles, it can be seen that in EIS the charge-transfer resistance is formed. The EIS spectra of raw material consists of two semicircles and a line. The EIS spectra of Al(NO3)3·9H2O modified

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After 25 cycles, it can be seen that in EIS the charge-transfer resistance is formed. The EIS spectra of raw material two semicircles a line. EIS spectra of Al(NO 3 )3 ·9H2 O modified samples consistconsists of threeof semicircles and and a line. TheThe second semicircle is well-known to have samples consist three semicircles and a line. The second semicircle is well-known to have originated originated fromof the surface layer resistance, which is ascribed to the formation of a film on the particle from the surface layer resistance, which is ascribed to the formation of a film on the particle surface after LiAlO2 modification. Table 3 shows the electrode resistance increases after surface LiAlO2 after LiAlO2 modification. the electrode resistance increases after LiAlOthe 2 modification, modification, at the same Table time, 3it shows can protect the cathode material from contacting electrolyte. at the samefinite time,thin it can protect the cathode material from contacting the electrolyte. Moreover, finite Moreover, layer impedance and the charge-transfer resistance of LiAlO2 modified samples thin layer impedance and the charge-transfer resistance of LiAlO modified samples were lower than 2 2 can improve the electrochemical were lower than that of the raw material, indicating that LiAlO that of the rawofmaterial, performance NCA. indicating that LiAlO2 can improve the electrochemical performance of NCA.

Figure 7. Electrochemical impedance spectra (EIS) of the lithium ion cells assembled with different Figure 7. Electrochemical impedance spectra (EIS) of the lithium ion cells assembled with different electrodes, which were obtained before discharge (a) and after 25 cycles at 1 C rate (b). All impedance electrodes, which were obtained before discharge (a) and after 25 cycles at 1 C rate (b). All impedance data were obtained at 4.3 V. data were obtained at 4.3 V. Table 3. Independent resistive components of the pristine NCA sample and NCA-0.25 samples Table 3. Independent resistive components of the pristine NCA sample and NCA-0.25 samples analysed analysed using an equivalent circuit. using an equivalent circuit. Sample Sample NCA NCA-0.25 NCA NCA-0.25

After 25 Cycles Original After Rs Original Rf Wo Rs Rct 25 Cycles Rf Ws Wo 8.28 327.01 372.60 9.16 200.20 13.62 160.90 Rs Rf Wo Rs Rct Rf Ws Wo ----8.71 159.70 82.43 9.93 9.54 32.06 11.57 29.65 8.28 327.01 372.60 9.16 200.20 13.62 160.90 —– 8.71 159.70 82.43 9.93 9.54 32.06 11.57 29.65

4. Conclusions 4. Conclusions NCA has been successfully prepared by the co-precipitation method. After LiAlO2 modification, the surface lithium compound residues and pH of the cathode material was reduced. When the molar NCA has been successfully prepared by the co-precipitation method. After LiAlO2 modification, ratio of Al(NO3)3·9H2O and residual Li2CO3 of the NCA surface is 0.25:1, the pH is 11.80 and the the surface lithium compound residues and pH of the cathode material was reduced. When the molar surface lithium residue is 1.48%, which could improve its processing performance. Although the ratio of Al(NO3 )3 ·9H2 O and residual Li2 CO3 of the NCA surface is 0.25:1, the pH is 11.80 and the initial discharge specific capacity slightly decreased after LiAlO2 modification, the high-rate ability surface lithium residue is 1.48%, which could improve its processing performance. Although the and the cycling stability improved. Furthermore, the impedances of cathode material decreased and initial discharge specific capacity slightly decreased after LiAlO2 modification, the high-rate ability the reversibility of lithium emergence/insertion increased after LiAlO2 coating Therefore, LiAlO2 modification could improve NCA electrochemical performance. The capacity retention increased from 89.31% to 94.67% at 1 C between 3.0 and 4.3 V at 1 C after 100 cycles.

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and the cycling stability improved. Furthermore, the impedances of cathode material decreased and the reversibility of lithium emergence/insertion increased after LiAlO2 coating Therefore, LiAlO2 modification could improve NCA electrochemical performance. The capacity retention increased from 89.31% to 94.67% at 1 C between 3.0 and 4.3 V at 1 C after 100 cycles. Supplementary Materials: The following are available online at www.mdpi.com/2076-3417/8/3/378/s1, Figure S1: The mean and standard deviation diagrams of the three discharge of NCA series(NCA, NCA-0.25-400 ◦ C, NCA-0.25-500 ◦ C and NCA-0.25-600 ◦ C): blue, green and grey histograms correspond to discharge at 0.2 C, discharge at 1 C and discharge at 1 C after 110 cycles, respectively., Table S1: The each, mean and standard deviation of the three discharge of NCA, NCA-0.25-400 ◦ C, NCA-0.25-500 ◦ C and NCA-0.25-600 ◦ C, respectively. Acknowledgments: We gratefully acknowledge financial supports from the Department of Science and Technology of Shandong Province (2017CXGC0503, 2016PT07), Shan Dong Economic and Information Technology Committee(201731917004). Author Contributions: Chunhua Song, Ying Wang and Chenglong Zhao conceived and designed the experiments; Wenge Wang, Huibin Zhang and Qiwei Tang performed the experiments; Chunhua Song, Ying Wang, Chenglong Zhao and Wenge Wang analyzed the data; Jinzhao Lv, Xianjun Du and Yanmeng Dou contributed reagents/materials/analysis tools; Chunhua Song, Wenge Wang and Huili Peng wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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