Preparation of Ni-Doped Li2tio3 Using an Inorganic Precipitation–Peptization Method

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Article Preparation of Ni-Doped Li2TiO3 Using an Inorganic Precipitation–Peptization Method

Li-Yuan Zhang 1,2 , Yi Shui 1, Ling-Ling Zhao 1, Ping Zhu 1, Wen-Yong Xu 1 and Yao-Hui You 1,2,*

1 College of Chemistry and Chemical Engineering, Neijiang Normal University, Neijiang 641112, China; [email protected] (L.-Y.Z.); [email protected] (Y.S.); [email protected] (L.-L.Z.); [email protected] (P.Z.); [email protected] (W.-Y.X.) 2 Key Laboratory of Fruit Waste Treatment and Resource Recycling of the Sichuan Provincial College, Neijiang 641112, China * Correspondence: [email protected]; Tel.: +86-83-2234-1577

Received: 25 September 2019; Accepted: 23 October 2019; Published: 26 October 2019

Abstract: The precursor for a lithium-ion sieve is prepared using an inorganic precipitation-peptization method with titanium sulfate as the titanium source and lithium acetate as the lithium source. The effects of Ni2+ (Nickel ions) doping on the stability of the sol, crystal morphology and interplanar 2+ spacing of Li2TiO3 are investigated. The results indicate that, after Ni doping with varying fractions, the stability of the precursor sol first increases then decreases, and the maximum stabilization time of the precursor sol doped with 1% Ni2+ is 87 h. When doped with 1% Ni2+, the sol performance is most stable, the porous Li2TiO3 is obtained, and the specific surface area of Li2TiO3 increases by up to 1.349 m2/g from 0.911 m2/g. Accompanying the increase in calcination temperature, the inhibition of Ni2+ doping on the growth and crystallization of grains decreases. When the temperature is lower than 750 ◦C, Ni atoms replace the Ti atoms that are substituted for Li atoms in the original pure Li layer, forming lattice defects, resulting in the disappearance of (002) and ( 131) diffraction peaks for − Li2TiO3, the reduced ordering of crystal structure, a decrease in the interplanar spacing of the (002) plane, lattice expansion and an increase in the particle size to 100–200 nm. When the temperature exceeds 750 ◦C, with the increase of calcination temperature, the influence of Ni doping on the growth and crystallinity of grains decreases, and the (002) crystal surface starts to grow again.

Keywords: Li2TiO3 precursor sol; Ni doping; precipitation-peptization; crystal defect

1. Introduction As a new energy metal, lithium has the advantages of cleanliness and high eﬃciency compared to traditional energy metals. Lithium is widely used, therefore, in lithium batteries [1], energy research [2] and other related industries; moreover, the demand for lithium in the market is increasing., Most lithium resources in China are distributed in ores, salt lake brine and seawater [3]. Since lithium reserves in salt lake brine and seawater are so rich, the extraction of lithium from such sources is the focus of further development. Presently, the main methods for extracting lithium from salt lake brine are the precipitation method, extraction method [4–7], ion exchange adsorption method [8,9], calcination method [10,11], and salting method [12]. Among these methods, the simple ion exchange adsorption method is suitable for most salt lake brines. The key to this method lies in the synthesis of an ion-exchange agent for selective absorption of lithium ions [13]. As a result, lithium-ion sieves have attracted extensive attention from chemical researchers around the world. Currently, the research on lithium-ion sieves mainly includes manganese and titanium lithium-ion sieves. Xu et al. [14] synthesized Li4Mn5O12 (Spinel Manganese Series lithium ion sieve precursor) using an ethylenediamine tetraacetic acid (EDTA)-citric acid complexation method, followed by pickling to obtain a manganese-based

Coatings 2019, 9, 701; doi:10.3390/coatings9110701 www.mdpi.com/journal/coatings Coatings 2019, 9, 701 2 of 17 lithium-ion sieve with a maximum absorption capacity of 43.1 mg/g. The eluent, however, is liable to cause the dissolution of manganese and erosion of the manganese skeleton [15], which results in a low recycling efficiency and unsuitability for industrial lithium extraction. To address this problem, some researchers [16] propose that the dissolution rate can be controlled via doping, which can reduce the absorption capacity to a certain extent. Compared with manganese-based lithium-ion sieves, titanium-based lithium-ion sieves have the advantages of simple preparation, large absorption capacity [17], and a low dissolution rate. Titanium-based lithium-ion sieves mainly are prepared by pickling the Li2TiO3 precursor with eluent. The main preparation methods for Li2TiO3 include solid-phase reaction [18], the hydrothermal method [19], the sol–gel process [20], precipitation–peptization [21]. Due to the difficulties of solid–liquid separation and poor fluidity, however, the efficiency of powdered lithium-ion sieves prepared by the solid-phase reaction and hydrothermal method is so slow that it is not feasible for column-loading operation. To solve the above issue, in our previous work [22], an H2TiO3-lithium absorbent, prepared using the sol–gel process, was loaded onto ceramic foams. The pickling and adsorption performances were investigated with an ion-exchange column. It was demonstrated that H2TiO3-lithium absorbent supported on ceramic foams had a good absorption performance [23]., The cost of the raw material, tetrabutyl titanate, is too high, however. During another work of our group [24], lithium absorbent also was synthesized, with lithium acetate as a lithium source and titanium sulfate as a titanium source, using the Xu precipitation–peptization method. The sol prepared by the traditional precipitation–peptization method has limited stability [24,25], however, which results in poor reusability of the sol. To improve the stability of the sol, doping metals such as Ni, Al, and La can be used. It has been reported [26] that a precursor manganese lithium-ion sieve, doped with aluminum, shows remarkable improvement in the stability and dissolution rate. Transition-metal lanthanum doping [27] also has a significant effect on improving the stability of samples, reducing the dissolution rate and improving the recycling ability. As a transition metal with moderate cost, good formability and strong corrosion resistance, nickel also can enhance [28] the stability and reduce the dissolution rate of samples after doping. Following doping treatment with nickel, the precursors of a series of manganese samples showed X-ray diffraction patterns that are quite different from those of the standard cards, indicating that most of the nickel atoms enter the lattice of the lithium manganese spinel [29]. As a result, both the dissolution rate of manganese and its saturated absorption capacity were reduced [30]. Based on a series of previous studies on Ni-doped Mn-Li ion sieve precursors, innovated in this work, a Ni-doped Ti-Li ion sieve precursor is prepared to explore the influence of Ni-doping on colloidal stability, surface structure, the crystal size and crystal plane growth, and other aspects of an Li2TiO3 crystal. This work aims to prolong the stable existence time of sol, improve the reusability of sol, and obtain a kind of Ti-Li ion sieve precursor with a porous structure.

2. Experimental Using the traditional high temperature solid-state method, the contact between a lithium source and solid metal oxide is not enough and the mixing is uneven, resulting in poor crystallinity and defects in the crystal structure. Regarding the sol–gel process, the reactants are mixed uniformly, however, the reaction time is long, the operation requirements are high, and the cost of the organic titanium source is high. The preparation of a Li-ion sieve by an inorganic precipitation–peptization method can make up for the shortcomings of the previous method. During this method, Ti4+ is transformed into 2 precipitate by a suitable precipitant, the excess SO4 − is removed by centrifugal washing, followed by adding a proper amount of complexing agent into the fresh precipitate to make the precipitate disperse automatically, forming sol. Compared to the former methods, this method is more operable and can reduce the cost greatly. Thus, in this work, the precursor of a lithium-ion sieve was prepared by doping transition metal nickel combined with an inorganic precipitation–peptization method. The speciﬁc operations are as follows. Coatings 2019, 9, 701 3 of 17

2.1. Preparation of Ni-Doped Li2TiO3 Sol

Employing a water bath at 60 ◦C, 10.82 g Ti(SO4)2 and 6.61 g CH3COOLi were dissolved, respectively, using 200 mL of distilled water to form the transparent solution. NiCl 6H O solution 2· 2 (Ni/Ti = 1%, 2%, 3%) was added if necessary. An ammonia solution was added to the Ti(SO4)2 solution at a speed of one drop per ten seconds under magnetic stirring until the pH value was 8.5. The obtained precipitate was centrifuged and washed with distilled water four times, then placed in a beaker, followed by the addition of 200 mL distilled water. The mixture was stirred while the lithium source was introduced at a uniform speed. Finally, 30% H2O2 (25 mL) was added dropwise into the mixing solution at a speed that was maintained at approximately 3 seconds/drop. Subsequently, the transparent yellow sol was obtained by stirring at room temperature for 1 h.

2.2. Preparation of Li2TiO3 Powders

Following aging at room temperature, the colloids were dried in an oven at 80 ◦C to obtain the dry gel. Then, the dry gel was ground evenly and calcined at diﬀerent temperatures in a muﬄe furnace. The calcination temperature was set to 650–900 ◦C with a holding period of 2 h.

2.3. Characterization The viscosity of the colloidal sol was measured by a digital viscometer (NDJ-5S, Shanghai Jingtian Electronic Instrument Co., Ltd., Shanghai, China). Thermal properties of the samples were estimated by Thermogravimetric-Differential Scanning Calorimetry (TG-DSC TA-Q600, TA Company, New Castle, DE, USA), the specimen was heated at a ramp rate of 10 C min 1 to 1000 C in dry air ◦ · − ◦ conditions. The microscopic structures of the samples were characterized by scanning electronic microscopy (SEM, VEGA3, TESCAN, Brno, Czech Republic) and transmission electron microscopy (TEM, JEM 2010, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan). The crystalline structure of the samples was investigated with an X-ray diffractometer (XRD, DX-2700, Dandong Haoyuan Instrument Co., Ltd., Liaoning, China) using Cu Kα radiation at a scanning rate of 0.05 s 1 and a ◦· − working voltage/current of 40 kV/40 mA. The pore size and specific surface area were measured on a surface area analyzer (BET, Autosorb iQ2, Quantachrome, Boynton Beach, FL, USA). The chemical composition and elemental chemical status were obtained by X-ray photoelectron spectroscopy (XPS, 250 Xi, Thermo Fisher Scientific, Waltham, MA, USA). The functional groups of the samples were revealed by Escalab Fourier-transform infrared spectroscopy (FT-IR, WQF-510A, Beijing Beifen–Ruili Analytical Instrument Co., Ltd., Beijing, China).

3. Results and Discussion

3.1. Viscosity Test The stability of sol is related directly to its viscosity. To investigate the influence of Ni doping on the stability of sol, the viscosity value of sol was measured. Figure1 shows the viscosity values for a sol doped with different ratios of nickel at 87 h. During this test, a lower viscosity indicates a better stability of the sol. The results shown in Figure1 suggest the sol viscosity first decreases and then increases with an increasing nickel doping amount. This is because the characteristic adsorption of Ni2+ on the surface of the sol particle reaches a saturated state with an increasing doping amount. Excessive Ni2+ causes the spatial distribution of the sol particles to become denser. Additionally, the probability of interaction between the particles increases, while the intermolecular potential decreases. Thus, the flocculation of the sol system occurs [31], which results in the increased viscosity value. Previously, in our other work, it was demonstrated that sol could be loaded effectively onto the matrix when the maximum viscosity of the sol did not exceed 15 MPa s. According to the results given in · Figure1, only the 2% nickel-doped sol showed a viscosity value lower than 15 MPa s when the aging · time was 87 h, and the viscosity value for the 1% nickel-doped sol was close to 15 MPa s. · Coatings 2019, 9, 701x FOR PEER REVIEW 44 of of 16 17

24 s) ⋅ 22

Sol viscosity (MPa 12

10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Doping ratio of Ni (%)

Figure 1. TheThe sol sol viscosity viscosity of of sol sol doped doped with with different diﬀerent mass ratios of Ni at 87 h.

The changes in colloidal viscosity with time, after doping with varying mass ratios of nickel, are The changes in colloidal viscosity with time, after doping with varying mass ratios of nickel, are listed in Table1. It is evident that the viscosity of the sol with nickel was much lower compared to the listed in Table 1. It is evident that the viscosity of the sol with nickel was much lower compared to undoped case, which is consistent with the above results. Considering 75–87 h, the sol viscosity of the the undoped case, which is consistent with the above results. Considering 75–87 h, the sol viscosity nickel-doped precursors increased dramatically. The sol viscosity of nickel-doped precursors with 1% of the nickel-doped precursors increased dramatically. The sol viscosity of nickel-doped precursors nickel doping was close to 15 MPa s at 87 h, while that of precursors doped with 2% nickel was lower with 1% nickel doping was close to· 15 MPa∙s at 87 h, while that of precursors doped with 2% nickel than 15 MPa s. The sol viscosity change was less stable than that of the 1% nickel-doped sol during the was lower than· 15 MPa∙s. The sol viscosity change was less stable than that of the 1% nickel-doped ﬁrst 75 h. Additionally, the low viscosity of the sol leads to weak adhesion on the substrate [21], which sol during the first 75 h. Additionally, the low viscosity of the sol leads to weak adhesion on the is not beneﬁcial for loading. Therefore, the precursor sol doped with 1% nickel is more suitable for the substrate [21], which is not beneficial for loading. Therefore, the precursor sol doped with 1% nickel loading of the substrate. is more suitable for the loading of the substrate.

Table 1. Variation of sol viscosity of Ni with varying mass percentage over time. Table 1. Variation of sol viscosity of Ni with varying mass percentage over time. Viscosity /MPa s 1 18 23 27 39 48 51 63 69 75 87 Viscosity /MPa∙s · 1 18 23 27 39 48 51 63 69 75 87 0.55% Ni% 2.897 2.010 2.087 2.157 1.923 2.320 2.260 5.630 10.20 12.767 16.167 0.55% Ni% 2.897 2.010 2.087 2.157 1.923 2.320 2.260 5.630 10.20 12.767 16.167 1% Ni% 2.783 2.650 2.627 2.840 2.497 2.620 2.783 2.903 2.767 3.860 15.30 1%2% Ni% Ni% 2.783 3.190 2.650 1.923 2.627 2.423 2.840 1.853 2.497 1.903 2.620 1.973 2.783 2.050 2.903 6.693 3.7872.767 7.847 3.860 10.467 15.30 2%3% Ni% Ni% 3.190 3.230 1.923 2.137 2.423 2.263 1.853 2.170 1.903 1.583 1.973 1.877 2.050 1.940 6.693 2.743 1.8303.787 2.947 7.847 18.9010.467 3% Ni% 3.230 2.137 2.263 2.170 1.583 1.877 1.940 2.743 1.830 2.947 18.90 3.2. Performance of the Precursor Sol 3.2. Performance of the Precursor Sol One of the obvious characteristics of colloids is the Tyndall effect. The stability of sol can be qualitativelyOne of the reflected obvious by characteristics the Tyndall eff ofect colloids produced is the by Tyndall laser irradiation. effect. The The stability performance of sol can of thebe qualitativelyprecursor sol reflected as a function by the of Tyndall aging timeeffect is produced shown in by Figure laser 2irradiation.. The new The sols performance were transparent, of the precursororange-yellow sol as anda function showed of anaging obvious time is Tyndall shown e inffect. Figure As 2. the The aging new time sols waswere extended transparent, to 39 orange- h, the yellowundoped and sol showed remained an obvious yellow, butTyndall became effect. opaque, As the while aging the time 1% was Ni-doped extended sol remainedto 39 h, the yellow undoped and soltransparent. remained Both yellow, samples but still became showed opaque, the Tyndall while e fftheect. When1% Ni-doped the aging sol time remained was further yellow extended and transparent.to 63 h, the undoped Both samples sol showed still showed a tendency the Tyndall for flocculation effect. When and the precipitation aging time andwas thefurther Tyndall extended effect towas 63 no h, longerthe undoped observed, sol showed while the a 1%tendency Ni-doped for flocculation sol became yellowand precipitation and opaque, and but the still Tyndall showed effect the wasTyndall no longer effect. observed, This indicates while that the nickel1% Ni-doped doping isso beneficiall became yellow to improve and opaque, the stability but still of the showed colloidal the Tyndallstructure effect. of the This sol. Weindicates hypothesize that nickel that thedoping main is reason beneficial for this to improve effect might the stability be as follows: of the The colloidal small structureamount of of NiCl the sol.6H WeO hypothesize doped into thethat solthe is main dissolved reason in for water this effect and formsmight abe conductive as follows: electrolyte The small 2· 2 amountsolution. of Ni NiCl2+ can2∙6H interact2O doped with into the chargedthe sol is particles dissolved in thein water sol and and form forms a characteristic a conductive adsorption electrolyte on solution.the surface Ni to2+ formcan interact a sol particle with the with charged a certain particles size [32 in], whichthe sol leads and toform a change a characteristic in the electric adsorption double onlayer the structure surface to in form the sol a system.sol particle Thus, with the a repulsion certain size between [32], which the colloidal leads to particles a change is increasedin the electric and doublethe surface layer potential structure is enhanced,in the sol leading system. to Thus, the dispersion the repulsion of the colloidalbetween particlesthe colloidal and an particles enhanced is increasedstability of and the the sol. surface potential is enhanced, leading to the dispersion of the colloidal particles and an enhanced stability of the sol.

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(a) (b)

(e) (f)

(g) (h)

Figure 2. TheThe state state of of the the sol sol doped doped with with Ni Ni aged for varying times. ( (aa)) 0% 0% Ni, Ni, 1 1 h; h; ( (bb)) 0% 0% Ni, Ni, 18 18 h; h; ( (cc)) 0% Ni, 39 h; ( d) 0% Ni, 63 h; ( e) 1% Ni, 1 h; (f) 1% Ni, 18 h; (g) 1% Ni, 39 h; ((h) 1%1% Ni,Ni, 6363 h.h.

3.3. TG–DSC TG–DSC Analysis Analysis

Following drying, the gel was obtained. The gel needed to be converted into a Ni-Li22TiO3 crystalcrystal by calcination andand thethe determinationdetermination of of the the calcination calcination temperature temperature depended depended on on TG–DSC TG–DSC analysis. analysis. To Tostudy study the thermalthe thermal stability stability of Ni-doped of Ni-doped xerogel xerogel precursors precursors and the and sample the compositionsample composition by changing by changingtemperature temperature at the same at time,the same TG–DSC time, analysis TG–DSC was analysis carried was out. carried Figure out.3 shows Figure the 3 TG–DSC shows the curves TG– DSCfor the curves 1% Ni-doped for the 1% precursor. Ni-doped Viewing precursor. Figure Viewin3, the totalg Figure weight 3, the loss total of xerogel weight was loss 5.04% of xerogel during was the 5.04%whole during heat treatment. the whole Three heat obvioustreatment. exothermic Three ob peaksvious areexothermic observed peaks on the are DSC observed Curve, on and the the DSC TG Curve,Curve showsand the a TG downward Curve shows trend. a downward Among them, trend. the Among ﬁrst mass them, loss the (0.31%) first mass occurs loss at (0.31%) 171 ◦C, occurs which atcorresponds 171 °C, which to thecorresponds removal of to evaporatedthe removal free of evaporated water in the free precursor water in andthe precursor desorption and of desorption acetic acid. ofThe acetic second acid. mass The loss second occurs mass at 453loss◦ occursC, the weightat 453 °C, loss the percentage weight loss of thepercentage sample isof approximately the sample is approximately0.22%, which may 0.22%, be relatedwhich may to the be partial related decomposition to the partial decomposition of ammonium sulfate.of ammonium When thesulfate. calcination When the calcination temperature is 701 °C, the TG curve shows that the weight loss percentage for the

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Coatings 2019, 9, x FOR PEER REVIEW 6 of 16 temperature is 701 ◦C, the TG curve shows that the weight loss percentage for the sample is 4.51% and samplethe step is change4.51% and at approximately the step change 701 at◦ Capproximat may be dueely to701 the °C change may be in due speciﬁc to the heat change capacity in specific after the heatincrease capacity in crystallizationafter the increase of in the crystallization lithium titanate, of the which lithium leads titanate, to a baselinewhich leads drift to [ 21a baseline]. drift [21].

TG (%) DSC (W/g) Exothermic 2.0 100 - 0.31% - 0.22%

1.5

1.0 - 4.51% 111℃ 0.25W/g 96 0.5 101℃ 0.69W/g 453℃ 0.51W/g 94 0.0 0 200 400 600 800 1000 Temperature (℃)

Figure 3. TG–DSC curves for a 1% Ni-doped sample. Figure 3. TG–DSC curves for a 1% Ni-doped sample. 3.4. SEM Analysis

3.4. SEMTo studyAnalysis the effect of nickel doping on the surface morphology, particle size of Li2TiO3 crystal, and determine the optimum amount of Ni-doping in the precursor, samples were characterized by To study the effect of nickel doping on the surface morphology, particle size of Li2TiO3 crystal, SEM. Figure4 shows the SEM images of the precursor of the lithium-ion sieve with di fferent nickel and determine the optimum amount of Ni-doping in the precursor, samples were characterized by doping amounts. Some particles in the undoped sample are ellipsoid while the others are irregular SEM. Figure 4 shows the SEM images of the precursor of the lithium-ion sieve with different nickel with a smooth and dense surface and a compact and orderly arrangement, which is not conducive to doping amounts. Some particles in the undoped sample are ellipsoid while the others are irregular the subsequent eluting transition process. Accompanying the increasing nickel doping amount, the with a smooth and dense surface and a compact and orderly arrangement, which is not conducive to roughness of the particle surface also increases. Compared with the pristine sample, the particle size of the subsequent eluting transition process. Accompanying the increasing nickel doping amount, the the sample with 1% Ni doping in Figure4b increases to approximately 100–200 nm. The morphology roughness of the particle surface also increases. Compared with the pristine sample, the particle size of the sample is no longer ellipsoidal but granular. The results are consistent with the results of lattice of the sample with 1% Ni doping in Figure 4b increases to approximately 100–200 nm. The distortion and the expansion effect in XRD analysis. Moreover, the lattice distribution of Li TiO is morphology of the sample is no longer ellipsoidal but granular. The results are consistent with2 the3 changed by the nickel addition, thus affecting the formation and growth of the crystal nucleus and the results of lattice distortion and the expansion effect in XRD analysis. Moreover, the lattice distribution formation of obvious particle accumulation pores, which is consistent with the following BET analysis. of Li2TiO3 is changed by the nickel addition, thus affecting the formation and growth of the crystal Due to the increased extent of the mesoporous structure and the neat arrangement of mesopores, nucleus and the formation of obvious particle accumulation pores, which is consistent with the the surface presents a porous structure. Therefore, the sample doped with 1% nickel is conducive to following BET analysis. elution in the pickling process and can achieve elution equilibrium in a shorter time [23]. Additionally, Due to the increased extent of the mesoporous structure and the neat arrangement of mesopores, a porous structure is helpful to improve the absorption capacity of the adsorbent. The particle size the surface presents a porous structure. Therefore, the sample doped with 1% nickel is conducive to of the sample with 2% nickel also is increased and contains some particle accumulation pores. The elution in the pickling process and can achieve elution equilibrium in a shorter time [23]. particle size and pore size are not uniform. Seen in Figure4d, the sample completely loses the pore Additionally, a porous structure is helpful to improve the absorption capacity of the adsorbent. The structure due to the excessive amount of nickel doping, which is not conducive to the subsequent particle size of the sample with 2% nickel also is increased and contains some particle accumulation elution transition process for Li TiO . The above phenomenon indicates that the nickel doping amount pores. The particle size and pore2 size3 are not uniform. Seen in Figure 4d, the sample completely loses has a direct effect on the morphology of the crystal. An appropriate amount of Ni doping (1% in this the pore structure due to the excessive amount of nickel doping, which is not conducive to the work) can form a surface with an obvious porous structure, which is desired for the elution transition subsequent elution transition process for Li2TiO3. The above phenomenon indicates that the nickel process. The physical adsorption capacity of the titanium lithium ion sieve also is improved. doping amount has a direct effect on the morphology of the crystal. An appropriate amount of Ni doping (1% in this work) can form a surface with an obvious porous structure, which is desired for the elution transition process. The physical adsorption capacity of the titanium lithium ion sieve also is improved.

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(a) (b)

Figure 4. SEM images of Li2TiO3 doped with different amounts of nickel at 750 °C. (a) 0% Ni; (b) 1% Figure 4. SEM images of Li2TiO3 doped with diﬀerent amounts of nickel at 750 ◦C. (a) 0% Ni; (b) 1% Ni; (c) 2% Ni; (d) 3% Ni.

3.5. BET Analysis The above SEM test shows that both the pure sample and the Ni-doped sample have a porous structure. To To further further understand understand the the specific specific situ situationation of of the the pore structure of the samples, BET characterization is used.used. Figure5 5 shows shows the the N N22 adsorption–desorption isothermal curvescurves forfor LiLi22TiO33 doped with and without 1% nickel doping. Displayed in Figure 55,, thethe absorptionabsorption capacitycapacity increasesincreases steeply with increasingincreasing pressurepressure whenwhen thethe relativerelative pressurepressure PP/P/P00 << 0.4.0.4. The absorption capacity increases sharply with increasingincreasing pressurepressure whenwhen thethe relativerelative pressurepressure PP/P/P00 >> 0.9. Both the samples show the characteristic type-IVtype-Ⅳ adsorption–desorption isotherms, which is related to the fact that the sample contains a certain amount of mesoporousmesoporous structure [[33].33]. Additionally, slit holes or particle stacking holes also may exist [[34].34]. The relative pressure required for the appearance of a hysteresis ring for for a a 1% 1% nickel-doped nickel-doped sample sample is smaller is smaller than than that that for the for pure the puresample, sample, with its with larger its largerarea, larger area, largernumber number of mesoporous of mesoporous structures, structures, and andgreater greater N2 Nabsorption2 absorption capacity. capacity. This This confirms confirms that that the absorption capacity of the sample has a certain relationshiprelationship with its specificspecific surface area and pore volume [[35].35]. The parameters for the pore structures of the samples are listed in Table 2. The pore diameter of the pure sample and 1% Ni-doped sample is approximately 3.8 nm, while the pore volume of the sample doped with 1% Ni is increased significantly by approximately twice that for the pure sample, which is because nickel enters the lattice of the lithium titanate, resulting in structural changes in the lithium titanate and the narrower orifice. Looking at the aperture distribution of the sample given in Figure 5, it also can be known that the maximum cumulative pore volume of the nickel doping samples is twice as that of the pure sample. Moreover, the presence of the mesoporous orifice can increase the specific surface area. Thus, the specific surface area and pore volume of Li2TiO3 are improved greatly [36] after Ni doping.

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Table 2. Pore structures of samples.

Sample Pore Diameter (nm) Pore Volume (cm3/g) Surface Area (m2/g) 1%Ni-doped 3.834 0.316 1.349 Coatings 2019Pure, 9, 701sample 3.837 0.165 0.911 8 of 17

5.5 5.5 120 1800 3500

250 /g) 1600 /g) 3 5.0 3 5.0 3000 100 cm cm 1400 -4 -4

) 200 2500 4.5 80 1200 4.5 /g) /g/nm) /g 2 3 /g/nm) 2 3 2000 150 1000 cm 5 4.0 cm 60 4.0 5 － 1500 － cm

800

( 100 3.5 1000 3.5 40 600 50 500 400

3.0 (×10 dV/dD 3.0 20 dV/dD (×10 200 0 0 2.5 -500 2.5 0 0 0 50 100 150 200 250 300 350 Cumulative pore volume (×10

Cumulative pore volume (×10 -200 2.0 Pore diameter (nm) 2.0 0 50 100 150 200 Pore diameter (nm) 1.5 1.5 1.0 1.0 Quantity adsorbed Quantity adsorbed (cm 2 0.5 1%Ni-Li TiO 2 Li TiO N 2 3 0.5 2 3 N 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P ) 0 Relative Pressure (P/P ) 0 (a) (b)

Figure 5. Nitrogen adsorption–desorption isotherm and pore size distribution for Li TiO .(a) Li TiO ; Figure 5. Nitrogen adsorption–desorption isotherm and pore size distribution for Li2TiO2 3.3 (a) Li2TiO2 3; 3 (b) 1% Ni-Li TiO . (b) 1% Ni-Li2TiO2 3. 3 The parameters for the pore structures of the samples are listed in Table2. The pore diameter 3.6. XRD Analysis of the pure sample and 1% Ni-doped sample is approximately 3.8 nm, while the pore volume of the sampleTo determine doped with the 1% crystal Ni is phases increased of the significantly samples and by approximately investigate the twice influe thatnce forof the purecalcination sample, temperaturewhich is because and nickel nickel doping entersthe amount lattice on of thethe lithiumcrystal titanate,size and resulting crystal plane in structural growth changes of Li2TiO in the3, sampleslithium are titanate analyzed and theby narrowerXRD. According orifice. Lookingto the XRD at the patterns aperture for distribution undoped samples of the sample at different given in calcinationFigure5, it temperatures also can be known (Figure that 6), the the maximum XRD peaks cumulative for the undoped pore volume samples of the at nickeldifferent doping calcination samples temperaturesis twice as that are ofcoincident the pure with sample. the standard Moreover, card the, and presence no heteropeaks of the mesoporous appear, indicating orifice can that increase the Lithe2TiO specific3 product surface formed area. has Thus, a relatively the specific high purity. surface Peak area division and pore is volume observed of for Li2 TiOthe 3(−are131) improved crystal planegreatly at 650–800 [36] after °C. Ni This doping. is because the X-ray used in the test is the characteristic Ka emission from the metal target, and the Ka ray actually is composed of Ka1 and Ka2 rays with very similar wavelengths. Since a special monochromatorTable 2. Pore is structures not used of in samples. the test, the collected XRD pattern is actually a superimposed spectrum of two kinds of diffraction patterns and, thus, the data appear to Sample Pore Diameter (nm) Pore Volume (cm3/g) Surface Area (m2/g) show two split peaks. A splitting in the other peaks is not obvious due to peak broadening. It also can be seen1%Ni-doped from Figure 6a that3.834 the diffraction peak intensity 0.316 for lithium titanate 1.349 obtained at a low Pure sample 3.837 0.165 0.911 calcination temperature (550–600 °C) is low, which indicates that the obtained crystal has a low crystallinity and small grain size. As the calcination temperature increases, however, the intensity of the3.6. characteristic XRD Analysis diffraction peak increases correspondingly and the peak becomes sharper, illustratingTo determine the growing the crystalgrain size phases and of the the more samples complete and investigate crystal structure. the influence The diffraction of the calcination peak intensitytemperature of the and crystal nickel plane doping (−133) amount is generally on the crystal high sizeat 650–800 and crystal °C, indicating plane growth that of the Li2 lithiumTiO3, samples and titaniumare analyzed ions in by the XRD. interstitial According plane to (Li2 the layer) XRD are patterns arranged for undopedin order. As samples the calcination at different temperature calcination increases,temperatures the ratio (Figure I(002)6/I),(−133) the for XRD the peaks Li2TiO for3 powders the undoped is increased samples gradually, at di fferent indicating calcination that temperatures the crystal supercellularare coincident structure with the is standard developed card, gradually and no heteropeaksand the particle appear, uniformity indicating of that the thepowder Li2TiO is3 better.product However,formed has this a relativelyis not conducive high purity. to the Peak subsequent division is elution observed and for absorption the ( 131) reactions crystal plane of the at 650–800 samplesC. − ◦ dueThis to isthe because large grain the X-ray size and used the in correspondingly the test is the characteristic reduced specific Ka emission surface fromarea. the metal target, and theAccording Ka ray actually to the is XRD composed patterns of Ka1for the and sample Ka2 rays at withdifferent very calcination similar wavelengths. temperatures Since with aspecial a Ni dopingmonochromator amount of is 1% not (Figure used in 6b) the and test, the the standard collected card, XRD patternit can be is actuallyseen that, a superimposed when the calcination spectrum temperatureof two kinds is oflower diffraction than 750 patterns °C, the and, characteristic thus, the datapeaks appear due to to the show (002) two and split (−131) peaks. crystal A splitting faces of in thethe obtained other peaks Li2TiO is3 notpowders obvious do duenot appear. to peak This broadening. may be due It also to the can different be seen fromionic Figureradii of6 aNi that2+ and the 4+ 2+ 2+ Tidi ff(rractionNi²⁺ = 0.069 peak nm, intensity rTi⁴⁺ = 0.0605 for lithium nm). When titanate a small obtained amount at a lowof Ni calcination is doped, temperature Ni replaces (550–600 a part of◦C) Tiis4+ low,in the which lattice indicates to form that a substitutional the obtained point crystal de hasfect, a causing low crystallinity lattice distortion. and small Moreover, grain size. it Asalso the calcination temperature increases, however, the intensity of the characteristic diffraction peak increases

correspondingly and the peak becomes sharper, illustrating the growing grain size and the more complete crystal structure. The diﬀraction peak intensity of the crystal plane ( 133) is generally high at − 650–800 ◦C, indicating that the lithium and titanium ions in the interstitial plane (Li2 layer) are arranged in order. As the calcination temperature increases, the ratio I(002)/I( 133) for the Li2TiO3 powders is − Coatings 2019, 9, x FOR PEER REVIEW 9 of 16 leads to lattice expansion and reduces the order of the original crystal structure. Thus, the intensity for the corresponding crystal plane diffraction peak of the product is weakened or even completely suppressed [37]. When the calcination temperature is 750 °C, the diffraction peaks due to the (002) and (−131) planes appear in Li2TiO3 with a Ni doping amount of 1%, but the diffraction peak intensity is quite weak, indicating that the crystal grain development is incomplete. However, compared with the undoped samples, the grain size of the samples shows a growing trend, which is consistent with the SEM analysis results. The intensity of the characteristic diffraction peak increases correspondingly as the calcination temperature increases; furthermore, the peak becomes sharp, which indicates increased grain size and grain growth integrity. The grain sizes of undoped and doped 1% Ni samples at different calcination temperatures are calculated according to the Scherrer formula (Equation (1)) and listed in Tables 3 and 4, which shows that the grain size of the sample increases with temperature. This is because the atomic thermal activation energy and the diffusion coefficient of the atoms both increase with increasing temperature, which makes it easier for the atoms to migrate from the grain boundary to another crystal cell and form the state of atomic aggregation, thus making the overall energy of the crystal more stable [38]. When the calcination temperature is too high, however, the pore size and pore volume of the mesoporous crystal are reduced significantly, the skeletal density of the crystal increases accordingly, and the surface of the sample becomes dense, which is not conducive to ion intercalation and is drawn-out [39]. Shown in Table 4, when the calcination temperature is 750 °C, the crystal plane growth is incomplete. Following the addition of a small amount of NiCl2∙6H2O dispersant, however, when calcined at 750 °C, the growth speed of the nucleus is greater than that of crystal nucleation [40], which results in the largest grain size of the crystal at this temperature. Accompanying the increase of the calcination temperature to 800 °C, however, the grain size of the crystal decreases. This could be because, with the increase of the calcination temperature, the cell parameters a, b and c increase, resulting in the decrease of grain size (crystal grain miniaturization) and the increase of lattice distortion [41]. Alongside the further increase of the calcination temperature to 850 and 900 ℃, the crystal grain size increases to a certain extent and then tends to be stable, which may be due to the distribution of fine Ni oxide particles in the crystal. The grain boundary is pinned by the oxide particles, and the grain growth is restrained. Thus, the grain size tends to be stable after growing to a certain extent. Viewing Tables 3 and 4, it can be determined that the grain size of the sample after doping with Ni is larger than that of the undoped sample at the same calcination temperature. This probably is due to the fact Coatings 2019, 9, 701 9 of 17 that the Ni doping not only increases the specific surface area and pore volume of the mesoporous sample but also reduces the unit cell shrinkage [42]. According to previous report [43], the unit cell shrinkageincreased gradually,increases as indicating the grain thatsize thedecreases. crystal Thus supercellular, the unit structure cell shrinkage is developed decreases gradually with increasing and the grainparticle size. uniformity of the powder is better. However, this is not conducive to the subsequent elution and absorption reactions of the samples due to the large grain size and the correspondingly reduced speciﬁc surface area. D = Kλ/(βcosθ) (1) )

(002)

(202) o

(-133) 900 C -131 (-206) ( 800oC

850oC

o 150 C 800oC

o 100 C o 150 C

o (a.u.) Intensity

650 C o

Intensity (a.u.) Intensity 100 C 600oC o 550oC 650 C

10 20 30 40 50 60 70 Li TiO :ICDD PDF#33-0831 Li TiO :ICDD PDF#33-0831 2 3 2 3 10 20 30 40 50 60 10 10 20 30 40 50 60 10 2 Theta (Degree) 2 Theta (Degree) (a) (b)

Figure 6. XRD patterns for samples at different calcination temperatures. (a) Li2TiO3;(b) Li2TiO3 doped with 1% Ni. According to the XRD patterns for the sample at different calcination temperatures with a Ni doping amount of 1% (Figure6b) and the standard card, it can be seen that, when the calcination temperature is lower than 750 ◦C, the characteristic peaks due to the (002) and ( 131) crystal faces of − 2+ the obtained Li2TiO3 powders do not appear. This may be due to the different ionic radii of Ni and 4+ 2+ 2+ Ti (rNi2+ = 0.069 nm, rTi4+ = 0.0605 nm). When a small amount of Ni is doped, Ni replaces a part of Ti4+ in the lattice to form a substitutional point defect, causing lattice distortion. Moreover, it also leads to lattice expansion and reduces the order of the original crystal structure. Thus, the intensity for the corresponding crystal plane diffraction peak of the product is weakened or even completely suppressed [37]. When the calcination temperature is 750 ◦C, the diffraction peaks due to the (002) and ( 131) planes appear in Li TiO with a Ni doping amount of 1%, but the diffraction peak intensity is − 2 3 quite weak, indicating that the crystal grain development is incomplete. However, compared with the undoped samples, the grain size of the samples shows a growing trend, which is consistent with the SEM analysis results. The intensity of the characteristic diffraction peak increases correspondingly as the calcination temperature increases; furthermore, the peak becomes sharp, which indicates increased grain size and grain growth integrity. The grain sizes of undoped and doped 1% Ni samples at different calcination temperatures are calculated according to the Scherrer formula (Equation (1)) and listed in Tables3 and4, which shows that the grain size of the sample increases with temperature. This is because the atomic thermal activation energy and the diffusion coefficient of the atoms both increase with increasing temperature, which makes it easier for the atoms to migrate from the grain boundary to another crystal cell and form the state of atomic aggregation, thus making the overall energy of the crystal more stable [38]. When the calcination temperature is too high, however, the pore size and pore volume of the mesoporous crystal are reduced significantly, the skeletal density of the crystal increases accordingly, and the surface of the sample becomes dense, which is not conducive to ion intercalation and is drawn-out [39]. Shown in Table4, when the calcination temperature is 750 ◦C, the crystal plane growth is incomplete. Following the addition of a small amount of NiCl 6H O dispersant, 2· 2 however, when calcined at 750 ◦C, the growth speed of the nucleus is greater than that of crystal nucleation [40], which results in the largest grain size of the crystal at this temperature. Accompanying the increase of the calcination temperature to 800 ◦C, however, the grain size of the crystal decreases. This could be because, with the increase of the calcination temperature, the cell parameters a, b and c increase, resulting in the decrease of grain size (crystal grain miniaturization) and the increase of Coatings 2019, 9, 701 10 of 17 lattice distortion [41]. Alongside the further increase of the calcination temperature to 850 and 900 °C, the crystal grain size increases to a certain extent and then tends to be stable, which may be due to the distribution of fine Ni oxide particles in the crystal. The grain boundary is pinned by the oxide particles, and the grain growth is restrained. Thus, the grain size tends to be stable after growing to a certain extent. Viewing Tables3 and4, it can be determined that the grain size of the sample after doping with Ni is larger than that of the undoped sample at the same calcination temperature. This probably is due to the fact that the Ni doping not only increases the specific surface area and pore volume of the mesoporous sample but also reduces the unit cell shrinkage [42]. According to previous report [43], the unit cell shrinkage increases as the grain size decreases. Thus, the unit cell shrinkage decreases with increasing grain size.

D = Kλ/(βcosθ) (1)

Table 3. Cell parameters for pure samples at diﬀerent calcination temperatures.

Samples I(002)/I( 133) Crystalline Size: ( 133) nm − − 550 ◦C 0.648 13.47 600 ◦C 0.960 16.30 650 ◦C 1.077 20.73 700 ◦C 1.199 23.82 750 ◦C 1.375 25.39 800 ◦C 1.363 29.47

Table 4. Lattice parameters for 1% Ni-doped samples at diﬀerent calcination temperatures.

Samples Crystalline Size: ( 133) nm − 650 ◦C 20.79 700 ◦C 31.93 750 ◦C 38.46 800 ◦C 33.71 850 ◦C 37.78 900 ◦C 37.94

3.7. TEM Analysis To further determine the influence of nickel doping on atom arrangement and interplanar spacing, TEM characterization is carried out. TEM images and an electron diffraction map for the Li2TiO3 after doping 1% Ni are shown in Figure7. It is evident that the doped sample has a block structure with a particle size of 100–200 nm (Figure7a, High Voltage: 200 KV, Magnification: 40,000x, Ruler: 200 nm). According to the electron diffraction map, the Ni-doped sample has a polycrystalline structure in which the electron diffraction ring corresponds to the (002) crystal surface, indicating that the atoms in the pure Li layer of the doped sample are arranged in order after calcination at 750 ◦C[19]. While the bright spot strength is quite low, which indicates that the grain development is not complete, the atoms in the pure Li layer are arranged with low order. This is consistent with the weak peak strength of the (002) crystal surface at 750 C in the XRD pattern. Looking at Figure7b (Magnification: 800,000 , High ◦ × Voltage: 200 KV, Ruler: 5 nm), the layered structure of the constituency corresponds to the LiTi2 layer along the C-axis direction and there are no periodic conversion spots in the vertical direction of the LiTi2 layer in the selected SADE graph, which is consistent with the work by Yu et al. [19]. Periodic bright spots in the direction of 110◦ appear, however, which is 8◦ larger than that reported by Yu et al. [19]. It is suggested that Ni atoms replace the Ti atoms that are substituted for the Li atom in the original pure Li layer. Following calcination at 750 ◦C, the Li atom forms a lattice defect, resulting in a low arrangement order of atoms in the pure Li layer. Figure7c (Ruler: 5 nm) demonstrates that the (002) interplanar spacing is 0.4672 nm, which is smaller than the theoretical value of (002) (0.4800 nm). This is Coatings 2019, 9, 701 11 of 17 due to the lattice distortion caused by the Ni2+ replacing Ti4+ in the lattice due to the larger ion radius,

Coatingscausing 2019 a change, 9, x FOR in PEER the REVIEW cell parameters and, eventually, leading to a decreased interplanar spacing.11 of 16

(a) (b)

(c)

Figure 7. TEMTEM images images of of samples samples after adding 1% Ni. ( (aa)) Sample Sample Magnified Magniﬁed 40,000×; 40,000 ;( (bb)) Selected Selected area area × electron diffraction diﬀraction pattern; (c) (002) CrystalCrystal planeplane spacing.spacing.

3.8. FT–IR FT–IR Analysis To determine the influence influence of nickel doping on the functional group group structure structure of of the the samples, samples, FT–IR analysis is used. Figure Figure 88aa showsshows thethe infraredinfrared spectraspectra forfor thethe samplesample calcinedcalcined atat 750750 °C.◦C. Curve (1) is pure Li TiO and Curve (2) corresponds to the sample doped with 1% Ni. Seen in Curve (1), (1) is pure Li22TiO3 3and Curve (2) corresponds to the sample doped with 1% Ni. Seen in Curve (1), the the characteristic peaks at 3700–3200, 1600, 1430, 1030 and 900 cm 1 disappear because H O, SO 2 , characteristic peaks at 3700–3200, 1600, 1430, 1030 and 900 cm−1 disappear− because H2O, SO2 42−, NH4 −4+ + andNH 4oxygenand oxygengroups groupsare removed are removed with an increasing with an increasing calcination calcination temperature. temperature. Compared Compared to Curve (1), to 1 theCurve characteristic (1), the characteristic peaks at 3700–3200, peaks at 3700–3200, 1030 and 900 1030 cm and−1 in 900 Curve cm− (2)in also Curve disappear, (2) also disappear, which is due which to 1 theis due increase to the in increase calcination in calcination temperature. temperature. The negative The peaks negative at 1600 peaks cm at−1 1600in Curve cm− (2)in are Curve caused (2) areby 1 thecaused decrease by the in decrease water content in water in the content enviro innment. the environment. The characteristic The characteristic peak at 1430 peak cm− at1 is 1430 because cm− sixis hydratedbecause six nickel hydrated chloride nickel is coated chloride on is the coated surfac one theof the surface sample, of the resulting sample, in resulting a portion in of a portionNH4+ not of 4+ 1 beingNH removednot being completely. removed completely. The wide peaks The wide at 3700–3200 peaks at 3700–3200cm−1 in Curves cm− (3)in Curvesand (4) are (3) andthe stretching (4) are the vibrationstretching peaks vibration of O–H peaks in oftheir O–H association in their association with water. with The water. absorption The absorption peaks near peaks 1600 near cm−1 1600 are 1 causedcm− are by caused the bending by the vibration bending vibrationof water molecules of water molecules and the absorption and the absorption peaks near peaks 1430 near cm−1 1430 are cm 1 are caused by the telescopic vibration of the N–H bond in NH + ions. Additionally, the small caused− by the telescopic vibration of the N–H bond in NH4+ ions. Additionally,4 the small absorption absorption peaks at approximately 1030 cm 1 are caused by SO 2 , indicating that SO 2 and NH + peaks at approximately 1030 cm−1 are caused− by SO42−, indicating 4that− SO42− and NH4+ are4 not− removed4 1 completelyare not removed during completely the preparation during of thethe preparationsample. The of peak the at sample. 900 cm The−1 belongs peak atto 900the cmcharacteristic− belongs absorptionto the characteristic peak of –O–O– absorption (peroxide peak group), of –O–O– which (peroxide indicates group), that a whichperoxide indicates bond exists that ain peroxide the sol. Thebond unstable exists in hydrogen the sol. Theperoxide unstable basically hydrogen is decomposed peroxide basically completely is decomposed in the process completely of drying, in and the theprocess influence of drying, of the and peroxide the influence group ofin thethe peroxidehydrogen group peroxide in the can hydrogen be ruled peroxide out, suggesting can be ruled that out,the solsuggesting system thatis composed the sol system of peroxide is composed titanium of peroxide complexes. titanium Curve complexes. (4), however, Curve (4), shows however, a sharper shows 1 1 characteristica sharper characteristic peak at 1600 peak cm at 1600−1 and cm 1430− and cm 1430−1 compared cm− compared to Curve to Curve(3), which (3), which indicates indicates that thatthe structurethe structure of the of sample the sample surface surface is affected is affected by the by thenickel nickel chloride chloride hexahydrate hexahydrate cladding cladding layer. layer.

Coatings 2019, 9, 701 12 of 17 Coatings 2019, 9, x FOR PEER REVIEW 12 of 16

900cm-1 a b 1430cm-1 (3) Li TiO (1) Li TiO 2 3 2 3

-1 1030cm 1600cm-1 1600cm-1 -1 3100-3200cm (2) 1%Ni-Li TiO 2 3

-1 Transmittane/a.u.

Transmittane/a.u. -1 1430cm -1 (4)1%Ni-Li TiO 1600cm 900cm 2 3 1430cm-1 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 -1 Wave number/cm Wave number/cm -1

FigureFigure 8. 8.InfraredInfrared spectra spectra for for Li Li2TiO2TiO3 prepared3 prepared under under diffe diﬀrenterent conditions. conditions. (a) ( aAfter) After calcination; calcination; (b ()b ) BeforeBefore calcination. calcination.

3.9.3.9. XPS XPS Analysis Analysis XPSXPS characterization characterization is is used toto determinedetermine the th surfacee surface element element composition composition and and the relativethe relative atomic atomicconcentrations, concentrations, the corresponding the corresponding chemical chemical bond, andbond, valence and valence state of state the pure of the sample pure andsample Ni-doped and Ni-dopedsample at sample 750 ◦C. at Thereby, 750 °C. the Thereby, effect of the Ni dopingeffect of on Ni the doping chemical onenvironment the chemical of environment the original elements of the originalin the sampleelements can in bethe investigated. sample can be The investigated. X-ray source The is X-ray a monochromatic source is a monochromatic Al Kaa (hvn = 1486.6 Al Kaa eV) with a power of 150 W. The vacuum degree of the chamber is better than 1 10 7 Pa, the fixed (hvn = 1486.6 eV) with a power of 150 W. The vacuum degree of the chamber× is better− than 1 × µ 10transmission−7 Pa, the fixed energy transmission of the energy energy analyzer of the is 30 energy eV, and analyzer the size ofis the30 beameV, and spot the is 500size ofm. the The beam charge spotcorrection is 500 ofμm. the The sample charge spectrum correction is carried of the out sample using the spectrum carbon pollutantis carried C1s out with using a binding the carbon energy of 284.60–284.80 eV. The results are shown in Figure9. The XPS full spectrum of the undoped sample pollutant C1s with a binding energy of 284.60–284.80 eV. The results are shown in Figure 9. The (Curve 1) and the Ni-doped sample (Curve 2) are given in Figure9a. It can be known that the undoped XPS full spectrum of the undoped sample (Curve 1) and the Ni-doped sample (Curve 2) are given in sample mainly is composed of Li, O, and Ti. Following Ni doping, the sample is mainly composed of Figure 9a. It can be known that the undoped sample mainly is composed of Li, O, and Ti. Following Li, O, Ti and Ni, wherein element C is introduced by the pretreatment process before the sample test. Ni doping, the sample is mainly composed of Li, O, Ti and Ni, wherein element C is introduced by The two peaks with binding energies of 464.08 eV and 458.28 eV in Figure9b correspond to Ti 2p the pretreatment process before the sample test. The two peaks with binding energies of 464.08 eV1 /2 and Ti 2p3/2, respectively. As seen in Figure9b, the peak signal at 458.28 eV is stronger, indicating that and 458.28 eV in Figure 9b correspond to Ti 2p1/2 and Ti 2p3/2, respectively. As seen in Figure 9b, the Ti4+ loading is increased in the sample. Regarding the Ni-doped sample, almost no significant shift in peak signal at 458.28 eV is stronger, indicating that Ti4+ loading is increased in the sample. Regarding the two peaks is observed compared to the undoped sample. It preliminarily can be concluded that the the Ni-doped sample, almost no significant shift in the two peaks is observed compared to the incorporation of Ni does not change the chemical environment of Ti in Li TiO calcined at 750 C and undoped sample. It preliminarily can be concluded that the incorporation2 of Ni3 does not change◦ the that the outer electrons of the titanium atom do not undergo significant transfer. This indicates that the chemical environment of Ti in Li2TiO3 calcined at 750 °C and that the outer electrons of the titanium outer electron density and chemical state of the titanium atom are not changed [44]. The intensities of atom do not undergo significant transfer. This indicates that the outer electron density and chemical the Ti 2p1/2 and Ti 2p3/2 peaks for the undoped samples are 7935.44 s and 22302.7 s, respectively, and state of the titanium atom are not changed [44]. The intensities of the Ti 2p1/2 and Ti 2p3/2 peaks for they are reduced to 5020.69 s and 12708.8 s, respectively, which show that the content of Ti decreases the undoped samples are 7935.44 s and 22302.7 s, respectively, and they are reduced to 5020.69 s and after Ni-doping. The Ni2p spectrum, and the peak at 857.08 eV corresponding to the Ni–O bond, are 12708.8 s, respectively, which show that the content of Ti decreases after Ni-doping. The Ni2p given in Figure9e. spectrum, and the peak at 857.08 eV corresponding to the Ni–O bond, are given in Figure 9e. The figure shows that, after the incorporation of nickel, a part of the Ni combines with O to form a Ni-O bond, which replaces the Ti–O bond in Li2TiO3. Thus, the Ti content decreases after Ni doping, and both the intensities of the Ti2p1/2 and Ti2p3/2 peaks are reduced. Figure 9c represents the O1s component, in which the peak signal of the undoped sample at 529.28 eV represents the Ti–O binding energy. Subsequent to 1% Ni-doping, the corresponding binding energy of the peak is 529.58 eV, which shifts toward higher energy by 0.30 eV.

Coatings 2019, 9, 701 13 of 17 Coatings 2019, 9, x FOR PEER REVIEW 13 of 16

1:Li TiO Ti2p 2 3 1:Li TiO 2/3 2:1% Ni-Li TiO 2 3 2 3 O1s 2:1% Ni-Li2TiO3 Ti2p 1/2 2 2 Ni2p Ti2p C 1s Ti 3p and 3s Intensity/a.u. Intensity/a.u. 1 1

1200 1000 800 600 400 200 0 415 410 465 460 455 450 Binding energy/eV Binding energy/eV

(a) (b) 1:Li TiO 2 3 1:Li TiO 2:1% Ni-Li TiO 2 3 2 3 Li1s 2:1% Ni-Li TiO O1s 2 3

1 Intensity/a.u. Intensity/a.u.

544 542 540 538 536 534 532 530 528 526 Binding energy/eV 62 60 58 56 54 52 50 Binding energy/eV (c) (d)

Ni2p 1% Ni-Li TiO 2 3 Intensity/a.u.

890 880 810 860 850 Binding energy/eV (e)

Figure 9.9. XPS forfor samplessamples beforebefore and and after after Ni Ni doping. doping. ( a()a Survey;) Survey; (b ()b Ti) Ti 2p 2p peaks; peaks; (c )(c O) 1sO 1s peaks; peaks; (d) ( Lid) 1sLi peaks;1s peaks; (e) ( Nie) Ni 2p 2p peaks. peaks.

TheThis ﬁgure result shows shows that, the afterimpacts the incorporation of the Ni doping of nickel, on the a part oxygen of the in Ni titanium combines oxide—the with O to form outer a Ni-Oelectron bond, density which of replacesit decreases, the Ti–O its shielding bond in Lieffect2TiO decreases,3. Thus, the and Ti contentthe inner decreases electron afterbinding Ni doping,energy andincreases. both theOverall, intensities the chemical of the Ti2p state1/ 2ofand oxygen Ti2p ch3/2angespeaks accordingly are reduced. [45]. Figure According9c represents to the Curve the O1s (2) component,in Figure 9c, in the which O1s thepeak peak in the signal figure of thecontains undoped two samplelow-energy at 529.28 peaks eV [45]. represents The peak the at Ti–O Eb = binding 529.58 eV belongs to lattice oxygen in Ti–O, while that at 531.48 eV is due to oxygen adsorption including surface hydroxyls and adsorbed water, most of which is due to surface hydroxyls [46]. Figure 9d

Coatings 2019, 9, 701 14 of 17 energy. Subsequent to 1% Ni-doping, the corresponding binding energy of the peak is 529.58 eV, which shifts toward higher energy by 0.30 eV. This result shows the impacts of the Ni doping on the oxygen in titanium oxide—the outer electron density of it decreases, its shielding effect decreases, and the inner electron binding energy increases. Overall, the chemical state of oxygen changes accordingly [45]. According to the Curve (2) in Figure9c, the O1s peak in the figure contains two low-energy peaks [45]. The peak at Eb = 529.58 eV belongs to lattice oxygen in Ti–O, while that at 531.48 eV is due to oxygen adsorption including surface hydroxyls and adsorbed water, most of which is due to surface hydroxyls [46]. Figure9d shows the XPS spectra of the Li1s orbital component. The spectra show that, after doping with 1% Ni, the Li1s peak has an obvious chemical shift, the binding energy increases from 54.28 to 54.78 eV, moving 0.50 eV toward a higher binding energy. This shows that, after Ni-doping, the cell volume increases, and the diffusion of lithium ions are promoted because the radius of the nickel ion is larger than that of the titanium ion. Therefore, the outer electron density of lithium in lithium titanate decreases and the chemical state of lithium changes, which also is consistent with the previous TEM analysis. Table5 quantitatively shows the relative atomic concentrations of C 1s, O 1s, Li 1s, Ti 2p and Ni 2p in Li2TiO3 and 1% Ni–Li2TiO3 calcined at 750 ◦C. Viewing Table5, the relative ratio of the atomic concentration of Li to Ti in Li2TiO3 is about 2:1, which is in accordance with the theoretical stoichiometric ratio, however, that in the sample 1% Ni-Li2TiO3 is greater than 2:1, indicating that the Ti content is reduced. Ni atoms account for 1.21 at %, however, which indicates that, after doping, Ni enters the crystal structure of Li2TiO3 and replaces part of Ti, which is consistent with the results of XRD and TEM analysis.

Table 5. The relative atomic concentrations of C, O, Li, Ti and Ni before and after Ni doping.

Sample Calcination Temperature/◦CC/at % O/at % Li/at % Ti/at % Ni/at %

Li2TiO3 750 19.24 43.74 22.2 14.82 0.0 1%Ni-Li2TiO3 750 21.86 39.37 29.91 7.65 1.21

The relative atomic concentrations of C 1s and O 1s changes before and after Ni doping. The reason for the change of C is there is a certain C content range due to the residual orbital composition in the pretreatment process before the test of the sample. The decrease of the relative atomic concentration of O is because the positive charge in the crystal is reduced after the replacement of Ti by Ni. To balance the positive charge, oxygen vacancy is produced and, thus, the relative atomic concentration of O is decreased after Ni doping.

4. Conclusions Accompanying an increasing amount of Ni doping, the stability of the viscosity of the precursor sol first increased then decreased, and the maximum stabilization time of the precursor sol doped with 1% Ni was 87 h. Following doping with 1% Ni, the number of mesopores increased and the specific surface area increased by up to 1.349 m2/g. Along with the increase of calcination temperature, the inhibition of Ni doping on the growth and crystallization of grains decreased. When the calcination temperature was lower than 750 ◦C, Ni doping caused lattice defects, inhibited crystal plane growth, and reduced the ordering of crystal structure. When the calcination temperature was 750 ◦C, the grain size was the largest. Increasing the calcination temperature further, the effects of Ni doping on the growth and crystallization degree of crystal gradually were reduced.

Author Contributions: Performed the experiments, data collection and analysis, L.-Y.Z.; wrote the original draft, Y.S.; reviewed and edited the paper, L.-L.Z.; language editing, P.Z.; drawed the ﬁgures with software, W.-Y.X.; study design, Y.-H.Y. Funding: This work was ﬁnancially supported by Sichuan Science and Technology Program (Grant No. 2019YJ0383) and The Undergraduate Innovative Project of Neijiang Normal University (X2018075 and X2018025). Coatings 2019, 9, 701 15 of 17

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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