A3036 Journal of The Electrochemical Society, 160 (5) A3036-A3040 (2013)0013-4651/2013/160(5)/A3036/5/$31.00 © The Electrochemical Society

JES FOCUS ISSUE ON INTERCALATION COMPOUNDS FOR RECHARGEABLE BATTERIES

A Strategy to Improve Cyclic Performance of LiNi0.5Mn1.5O4in a Wide Voltage Region by Ti-DopingM. Lin,a S. H. Wang,b Z. L. Gong,a,z X. K. Huang,c and Y. Yanga,b,∗,z

aSchool of Energy Research, Xiamen University, Xiamen 361005, ChinabState Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistryand Chemical Engineering, Xiamen University, Xiamen 361005, ChinacDepartment of Chemistry, Fujian Normal University, Fuzhou 35007, China

A strategy to improve cyclic performance of 5 V spinel LiNi0.5Mn1.5O4 in a wide voltage region (4.95–2.0 V) by Ti-doping has beenproposed and demonstrated for the first time. The effects of Ti substitution for Mn on the structure and electrochemical properties ofthe LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6) during cycling between 4.95 and 2.0 V were investigated. X-ray diffraction (XRD) resultsshow that all of these materials are cubic spinel structure with a space group of Fd3 m, where their unit cell parameter a increaseswith the substituting amount of Mn by Ti, which is in agreement with the fact that the ionic radius of Ti4+ (74.5 pm) is much largerthan that of Mn4+ (67 pm). Electrochemical test results show that proper amount of Ti substitution for Mn is able to significantlyimprove the cyclic performance of LiNi0.5Mn1.5−xTixO4 spinel during cycling at 4.95–2.0 V, especially for the capacity retention ofthe ca. 2.7 V plateau. Take LiNi0.5Mn1.0Ti0.5O4 as an example, the capacity retention in the upper (4.95–3.0 V) and lower voltage(3.0–2.5 V) regions after 100 cycles are 57.1% and 79.8%, respectively, while they are only 47.0% and 24.5% for un-substitutedLiNi0.5Mn1.5O4 sample, respectively.© 2013 The Electrochemical Society. [DOI: 10.1149/2.004305jes] All rights reserved.

Manuscript submitted December 30, 2012; revised manuscript received February 5, 2013. Published March 2, 2013. This paper ispart of the JES Focus Issue on Intercalation Compounds for Rechargeable Batteries.

High energy density and environmental friendly lithium-ion bat-teries have attracted a lot of interests in recent years, especially asdevices for electrical vehicles (EVs) and hybrid electrical vehicles(HEVs).1–3 Operating voltage and capacity of the cathode materialare two major factors influencing the energy density of lithium-ionbatteries.4–6 Thus, recently many researchers have focused their ef-forts on new cathodes with higher voltage plateau or higher capacity.Spinel LiNi0.5Mn1.5O4 is one of the most promising and attractivecathode materials due to various advantages, such as its low cost,7

unique 3-dimensional lithium-ion diffusion channels, and relativelyhigh working voltage (ca. 4.7 V).8–13 However, the theoretical capacityof LiNi0.5Mn1.5O4 is only 147 mAh g−1 when cycled between 5.0 and3.0 V. The relatively low capacity will hinder the application of thismaterial. Theoretically, when cycled between 5.0 and 2.0 V, thereare two lithium ions can be inserted/extracted into/from thespinel structure, and the theoretical capacity can be as high asca. 294 mAh g−1.

Park and co-workers14 employed ultrasonic spray pyrolysismethod to prepare two different structures (Fd3 m and P4332)of LiNi0.5Mn1.5O4-δ cathode materials and studied their charg-ing/discharging performance between 5.0 and 2.0 V, but they didn’tshow the long-term cycling stability of these materials. Lee et al.15

employed a co-precipitation method to synthesize the metal hydrox-ide precursor Mn0.75Ni0.25(OH)2. By controlling the heat-treatmentcondition, they obtained four LiNi0.5Mn1.5O4 spinel cathodes withdifferent degrees of transition-metal ion ordering. Their results showthat all samples suffer from significant capacity fading when the cy-cling voltage window was extended to 5.0–2.0 V. After 50 cycles, thecapacity retention is only 63% and 43% for the highly ordered spinelmaterial (P4332) and the highly disordered spinel material (Fd3 m),respectively. Where the significantly capacity fading was attributed tothe severe structural change from cubic to tetragonal phase.

One possible approach for improving the cyclic performance ofthe spinel LiNi0.5Mn1.5O4 is the partial substitution of manganeseor nickel with other transition metals such as Co,16,17 Al,18,19 Fe,20

Cr,21 Cu,22 Mg23 and Ti.24–27 Ti substitution was found to be ableto improve the cyclic performance as well as the rate capability ofLiNi0.5TixMn1.5−xO4 spinel at high voltages (>3.5 V). For examples,

∗Electrochemical Society Active Member.zE-mail: zlgong@xmu.edu.cn; yyang@xmu.edu.cn

Alcantara and co-workers24 prepared LiNi0.5Mn1.5−xTixO4 (0.05 ≤ x≤ 0.6) spinel oxide materials by a co-precipitation method, where thepresence of small amounts of Ti (x = 0.05 and 0.1) yielded a slightimprovement of the electrochemical performance. Kim et al.25 stud-ied the effects of Ti substitution for Mn on the structure and electro-chemical properties of LiNi0.5Mn1.5−xTixO4. The LiNi0.5Mn1.5−xTixO4

(x = 0–1.0) was synthesized at 900◦C by the solid-state reaction, fol-lowed by a post-annealing process at 700◦C in air. Electrochemicaldata shows that Ti substitution for Mn increases the cycling perfor-mance as well as the rate capability of the spinel materials. How-ever, they did not show any electrochemical results of the Ti-dopedLiNi0.5Mn1.5−xTixO4 down to lower voltage region.

In this work, Ti-substituted LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6)spinel materials were synthesized by a facial solid-state reactionmethod. The effects of Ti substitution for Mn on the structure andelectrochemical properties of LiNi0.5Mn1.5−xTixO4 when cycled be-tween 4.95 V and 2.0 V were studied.

Experimental

LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6) powders were synthesized by afacial solid-state reaction method. Stoichiometric amounts of Li2CO3,NiCO3, MnCO3, and TiO2 (Li excessive 10%) were thoroughly mixedusing ball milling for 10 h. The mixed slurry was dried in an ovenat 80◦C for 5 h. The resulting powder was pressed into pellets andcalcined at 750◦C for 20 h in air, and cool down to room temperature,where the heating and cooling rates above 400◦C were set to be 1◦Cmin−1.

The as-prepared samples were examined by powder X-ray diffrac-tion (XRD) analysis, using a PANalytical X’Pert diffractometer withCu Kα radiation (Philips) operated at 40 kV and 30 mA using a stepsize of 0.0167◦ with a counting time of 10 s per step. Morphologyobservation of samples were studied by scanning electron microscopy(SEM) performed on a Hitachi S-4800.

The electrochemical performance of each sample was evaluatedwith a standard CR2025-type coin cell. The cathode was fabricatedby blending the prepared powders, acetylene black, and polyvinyli-dene fluoride (80:10:10) in N-methyl-2-pyrrolidone. The slurry wasthoroughly mixed by ball milling and then pressed onto a piece ofaluminum foil followed by drying in an oven at 120◦C for 2 h. Thecoin cells were assembled with the prepared cathode, mental lithiumanode, Celgard 2400 polypropylene separator, and 1 M LiPF6 in an

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Journal of The Electrochemical Society, 160 (5) A3036-A3040 (2013) A3037

Figure 1. XRD patterns of the as-prepared LiNi0.5Mn1.5−xTixO4 (0 ≤ x≤ 0.6): (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4, (f) x= 0.5, and (g) x = 0.6.

ethyl carbonate/dimethyl carbonate (1:1, v/v) electrolyte. Cell testingwas carried out between 4.95 and 2 V at 30◦C and 60◦C using a Landbattery test system (Wuhan, China).

Results and Discussion

XRD patterns of the as-prepared LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6)powders are shown in Fig. 1. All patterns exhibit the typical cubicspinel structure with the space group of Fd3 m. Small amounts ofLixNi1−xO impurities could be identified by weak intensity reflectionsat 2θ = 43.7 and 63.6◦, especially for samples with higher Ti dopingcontent.

The changes in unit cell parameter with composition can be usedas an indirect evidence for Ti/Mn substitution.24 The calculated cellparameters of the as-prepared LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6)powders are shown in Fig. 2. It shows that the cell parameters of thespinel powders are nearly proportional to the amount of the substitutedTi. This tendency is in good agreement with the fact that the ionicradius of Ti4+ (74.5 pm) is much larger than that of Mn4+ (67 pm).

The morphologies of the as-prepared LiNi0.5Mn1.5−xTixO4

(0 ≤ x ≤ 0.6) materials are shown in Fig. 3. All samples show simi-lar morphologies with a sub-micron sized primary particles aggregatestructure. Therefore, the electrochemical properties of the as-preparedmaterials with various Ti-substituting amounts will have no relationwith their morphologies.

Charge-discharge curves of the as-prepared LiNi0.5Mn1.5−xTixO4

(0 ≤ x ≤ 0.6) materials during the initial 50 cycles at 30◦C with acurrent density of 75 mA g−1 between 4.95 and 2.0 V are presentedin Fig. 4. It shows that Ti substituted for Mn modifies the charge-

Figure 2. Changes in calculated cubic cell parameter with Ti content (x) forLiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6) samples.

Figure 3. SEM images of the as-prepared LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6)samples: (a) x = 0, (b) x = 0.1, (c) x = 0.3, and (d) x = 0.5.

discharge profiles somewhat. For the un-doped LiNi0.5Mn1.5O4 mate-rial, it shows five distinctive plateaus upon discharging to 2.0 V. Ex-tensive investigations on this material and the electrochemical mecha-nisms for each plateau have been reported in previous works.28–30 Thetwo plateaus at ca. 4.7 V and one plateau at ca. 4.0 V is associated withthe reduction of Ni4+ to Ni2+ and Mn4+ to Mn3+, respectively. Thepresence of these plateaus corresponds to lithium-ion insertion intothe 8a tetrahedral sites of the cubic spinel structure. The two plateausat ca. 2.7 V and 2.1 V are attributed to the reduction of Mn4+ to Mn3+

involving lithium-ion insertion into 16c octahedral sites of the spinelstructure, which will results in a cubic to tetragonal phase transition.

The specific capacity of LiNi0.5Mn1.5−xTixO4 materials decreasewith the increasing of the amount of substituted Ti in the spinel struc-ture, as shown in Fig. 4, where the decrease in capacity is mainlyattributed to the shortening of the plateaus at ca. 4.7, 4.0, and 2.1 V.Kim et al.25 suggested that the substitution of Ti for Mn might blockthe migration pathway of lithium-ion in the spinel octahedral sites andthus decrease the capacity at the plateau of ca. 4.7 V. The capacity ofthe materials at 4.0 V plateau is associated with the amount of Mn3+

in a form of LiMn2O4 in the materials. It is generally believed that thepresence of Mn3+ in the spinel LiMn2O4 is the major reason for itspoor cyclability since Jahn-Teller distortion of the MnO6 octahedronand Mn dissolution resulting from a disproportionation reaction ofMn3+ are the major reasons for the capacity degeneration.31,32 Fig. 4shows that the capacity of ca. 4.0 V plateau decreases with the increaseof Ti content. When doping amount reached 0.4, the plateau at 4.0 V isalmost negligible. The shortening of this plateau indicates that Ti sub-stitution helps minimize the amount of Mn3+ in the pristine samples.According to Lee et al.,15 the plateau at ∼2.1 V is related to the cubic toa new tetragonal phase (T2 phase) transition, which results in 13.75%volume expansion from the original cubic structure. Such a large lat-tice expansion and distortion may accelerate the collapse of the spinelstructure and leads to a poor cyclic performance for LiNi0.5Mn1.5O4

material. It can be seen that Ti substation suppress the evolution ofthe 2.1 V plateau (Fig. 4h), which may alleviate the severe structuralchanges and improve the cycling stability of LiNi0.5Mn1.5−xTixO4.

Fig. 5 shows the dQ/dV plots of the as-preparedLiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6) samples during the firstdischarging process. For un-doped LiNi0.5Mn1.5O4 sample, two dis-tinct peaks at ca. 4.7 V can be distinguished (Fig. 5a). Ti substitutiondecreases the voltage gap between these two peaks and amalgamatesthem nearly into one peak as shown in Figs. 5f and 5g, suggestingthe decrease of ordering in the layer of transition metal ions. As alsocan be seen that the plateau voltage corresponding to the reduction ofNi4+ to Ni2+ at around 4.7 V increases with the increasing amount ofTi substitution. It was proposed that the stronger Ti-O bonding thanMn-O would increase the redox potential of Ni4+/Ni2+.24

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A3038 Journal of The Electrochemical Society, 160 (5) A3036-A3040 (2013)

Figure 4. Charge-discharge curves of the as-prepared LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6)samples at 30◦C with a current density of75 mA g−1 between 4.95 and 2.0 V: (a) x= 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3,(e) x = 0.4, (f) x = 0.5, and (g) x = 0.6,(h) combined of the first charge-dischargecurves of all the samples.

Fig. 6 shows the cyclic performance of as-preparedLi/LiNi0.5Mn1.5−xTixO4 cells cycled at 30 and 60◦C between 4.95 and2.0 V. When cycled at 30◦C, un-doped LiNi0.5Mn1.5O4 sample exhibitsa high initial discharge capacity of 239.1 mAh g−1 but severe capacitydegradation, with capacity retention of only 44.5% after 100 cycles.Small amount of Ti substitution (x ≤ 0.3) shows no improvementon the cycling stability of LiNi0.5Mn1.5−xTixO4, even slightly worse.Further increasing Ti substitution amount to x ≥ 0.4 can obviouslyenhance the cyclic stability of LiNi0.5Mn1.5−xTixO4 with the best cy-cling stability obtained at x = 0.5. LiNi0.5Mn1.0Ti0.5O4 sample showsan initial discharge capacity of 206.5 mAh g−1, with higher capacityretention of 57.6% after 100 cycles, compared with only 44.5% for theun-doped sample. The cyclic performances of LiNi0.5Mn1.5−xTixO4 (0≤ x ≤ 0.6) at 60◦C shows similar trends with that at 30◦C, higher

amount of Ti substitution (x ≥ 0.4) can obviously improve the cyclingstability of LiNi0.5Mn1.5−xTixO4. In general, Mn-based spinel mate-rials have worse cycling stability at elevated temperature due to themore severe dissolution of Mn-ions into the electrolyte.33–35 This phe-nomenon can also be seen on un-substituted LiNi0.5Mn1.5O4, whichshows more severe capacity degradation when cycled at 60◦C than at30◦C. However, it is interesting to note that the cycling stability ofLiNi0.5Mn1.0Ti0.5O4 at 60◦C is even better than that at 30◦C after 100cycles. This indicates that Ti substation maybe able to suppress thedissolution of manganese into the electrolyte at the elevatedtemperature.

The above results indicate that proper amount of Ti substitutionfor Mn (x = 0.5) can obviously improve the cyclic performance ofthe LiNi0.5Mn1.5−xTixO4 spinel materials. As mentioned above, two

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Journal of The Electrochemical Society, 160 (5) A3036-A3040 (2013) A3039

Figure 5. The 1 st discharge dQ/dV plots of the as-preparedLiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6) samples at a current density of 75 mAg−1 between 4.95 and 2.0 V: (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e)x = 0.4, (f) x = 0.5, and (g) x = 0.6.

plateaus at ca. 4.7 V corresponding to Ni4+/Ni2+ redox reaction andone plateau at ca. 2.7 V relate to Mn4+/Mn3+ redox reaction are themajor contribution of the capacity of LiNi0.5Mn1.5−xTixO4 spinel. Todetermine the effects of Ti substitution on the cyclic performance of

these two plateaus separately, capacity retentions at different volt-age regions for LiNi0.5Mn1.5O4 and LiNi0.5Mn1.0Ti0.5O4 are shown inFig. 7. Ti substitution leads to the improvement of the capacity re-tention of both the upper (4.95–3.0 V) and lower (3.0–2.5 V) voltageregion at both 30 and 60◦C, whereas more significant for the lower(3.0–2.5 V) voltage region. In contrast, un-doped LiNi0.5Mn1.5O4 sam-ple suffers from very severe capacity degradation at the whole voltageregion, only 47.0% and 24.5% capacity retention for the upper andlower voltage region, respectively. When Ti substitutes into the struc-ture at x = 0.5, the capacity fade is effectively suppressed, especiallyfor the lower voltage region. The capacity retention for the upper andlower voltage regions are 57.1% and 79.8%, respectively.

Lee et al. proposed that the plateau at ca. 2.7 V is attributed tothe reduction of Mn4+ to Mn3+, involving lithium ion insertion into16c octahedral sites of the spinel structure, in accompany with thestructure transformation from cubic to tetragonal phase.15 They be-lieve that the presence of the tetragonal phase will accelerate thecapacity fading, which is one of the reasons for the poor cyclic per-formance of spinel materials when cycled at a wide voltage region. Inour work, ex situ XRD measurement was employed to investigate thephase transition process of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.0Ti0.5O4

samples throughout the discharge process between 4.95 and 2.0 Vas shown in Fig. 8. It can be seen that, both materials show a cu-bic to tetragonal phase transition below 3 V during discharge. Whenthe un-substituted LiNi0.5Mn1.5O4 was discharged to 3.0 V, tetragonalphase arose as indicated by the developed peaks at 18.6◦ (Fig. 8a).

Figure 6. Cyclic performance of the as-prepared LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6)samples at (a) 30◦C and (b) 60◦C with a cur-rent density of 75 mA g−1 between 4.95 and2.0 V.

Figure 7. Comparison of capacity retention ofLiNi0.5Mn1.5O4 and LiNi0.5Mn1.0Ti0.5O4 cy-cled at a current density of 75 mA g−1 between4.95 and 2.0 V: (a) and (c) Initial 100 cyclesdischarge profiles of two materials at 30◦C and60◦C, respectively. (b) and (d) different plateauscapacity retention of this two materials at 30◦Cand 60◦C, respectively.

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A3040 Journal of The Electrochemical Society, 160 (5) A3036-A3040 (2013)

Figure 8. Ex situ XRD patterns throughout the first dis-charge: (a) LiNi0.5Mn1.5O4 and (b) LiNi0.5Mn1.0Ti0.5O4sample. C and T refer to the cubic and tetragonal phase,respectively.

After discharging to 2.0 V, most of the cubic phase transformed totetragonal phase with only trace amount of cubic phase residual. Bycontrast, the results for LiNi0.5Mn1.0Ti0.5O4 are quite different. Whendischarging to 3.0 V, no obvious tetragonal phase can be observed.Even after discharge to 2.0 V, there is still a large partial of cubicphase remained (Fig. 8b). The above results indicate that Ti substitu-tion for Mn does suppress the phase transition of LiNi0.5Mn1.5−xTixO4

spinel during cycling at a wide voltage region and hence increase thestructural stability of the materials. Although high specific capacity(>200 mAh/g) and improve cycling stability can be obtained fromTi substituted LiNi0.5Mn1.5−xTixO4 spinel. However, there is onlyone lithium ions per formula (corresponding to 147 mAh g−1) canbe extracted from LiNi0.5Mn1.5−xTixO4 during the charging process.When cycled between 2.0–5.0 V, extra lithium should be supplied bythe anode, thus the suitable choice of anode materials coupled withLiNi0.5Mn1.5−xTixO4 also needs further consideration. When high spe-cific capacity is required, the combination of traditional graphite withLiNi0.5Mn1.5−xTixO4 may not be suitable because graphite contains nolithium. For high capacity application, LiNi0.5Mn1.5−xTixO4 may beused as cathode for lithium-polymer battery based on lithium metalanode. Also, lithiated-carbonaceous materials may be another choiceas anode materials to assemble a full-cell. When they match withLiNi0.5Mn1.5−xTixO4, the full-cell may maintain the advantages de-rived from cathode. Of course, further studies on safety issues andsome other questions are necessary in the future.

Conclusions

Ti substituted LiNi0.5Mn1.5−xTixO4 (0 ≤ x ≤ 0.6) (5 V spinel)solid solution has been successfully prepared by solid-state reactionmethod. All the prepared materials exhibit a typical cubic spinel struc-ture with a space group of Fd3 m. The almost linearly increasing inlattice parameter with the amount of Ti substitution is in good agree-ment with the substitution of smaller Mn4+ (67 pm) by larger Ti4+

(74.5 pm). The un-substituted LiNi0.5Mn1.5O4 sample exhibits a highdischarge capacity of 239.1 mAh g−1, however very poor cyclic perfor-mance with only 44.5% and 34.5% capacity retention after 100 cyclesat 30 and 60◦C, respectively. Ti substitution decreases the initial dis-charge capacity, while proper amount of Ti substitution (x = 0.5) canobviously enhance the cycling stability of LiNi0.5Mn1.5−xTixO4 in awide voltage range (4.95–2.0 V), especially at an elevated temperature(such as 60◦C). The improved cycling stability is mainly attributed tothe increase of capacity retention of the lower voltage region (3.0–2.5 V), which is associated with severe structural change. Ex situXRD analysis reveals that Ti substituted for Mn suppresses the struc-ture change during cycling in a wide voltage region and hence increasethe structural stability of the materials.

Acknowledgment

Financial support from National Basic Research Program of China(973 program, grant No. 2011CB935903) and National Natural Sci-

ence Foundation of China grant No. 21233004 and 21021002) aregratefully acknowledged.

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