Stable nickel-substituted spinel cathode material (LiMn 1.9 Ni 0.1 O 4 ) for lithium-ion batteries obtained by using a low temperature aqueous reduction technique

a A nickel substituted spinel cathode material (LiMn 1.9 Ni 0.1 O 4 ) with enhanced electrochemical performance was successfully synthesized by using a locally-sourced, low-cost manganese precursor, electrolytic manganese dioxide (EMD), and NiSO 4 $ 6H 2 O as a nickel source by means of a low temperature aqueous reduction synthesis technique. This synthesis protocol is convenient to scale up the production of the spinel cathode material, with minimal nickel content (Ni ¼ 0.1) in the structure, for lithium-ion battery applications. Ni-ions substituting Mn-ions was con ﬁ rmed using XRD, EDS, XPS and electrochemical performance studies. LiMn 1.9 Ni 0.1 O 4 materials showed an octahedral shape with clearly exposed (111) facets that enhanced the Li-ion kinetics and improved the cycling performance compared to the pristine spinel sample (LiMn 2 O 4 ). The LiMn 1.9 Ni 0.1 O 4 sample exhibited superior capacity retention by retaining 84% of its initial capacity (128 mA h g (cid:2) 1 ) whereas pristine LiMn 2 O 4 retained only 52% of its initial capacity (137 mA h g (cid:2) 1 ). XPS con ﬁ rmed that the Mn 3+ /Mn 4+ ratio changed with nickel substitution and favored the suppression of capacity fading. The study clearly suggests that the integration of small amounts of Ni into the spinel structure is able to eliminate the disadvantageous Jahn – Teller e ﬀ ects in the LiMn 2 O 4 .


Introduction
Lithium-ion battery (LIB) technology is well-developed for portable electronic devices (like cellphones, laptops, iPads, etc.) which have been widely used. However, to implement LIBs for large-scale high-power systems such as plug-in hybrid electric vehicles (PHEVs) or plug-in electric vehicles (PEVs), there is a great need to increase the energy and power capabilities of these batteries. [1][2][3][4][5] Nickel-substituted LiMn 2 O 4 (i.e., LiMn 2Àx -Ni x O 4 ) has emerged as one of the promising spinel cathode materials for lithium-ion batteries. A member of the family is the high-voltage spinel LiMn 1.5 Ni 0.5 O 4 (LMNO) is considered as one of the most promising cathode materials for Li-ion batteries. 6,7 In comparison with the commercial LiCoO 2 positive electrode, LiMn 1.5 Ni 0.5 O 4 has been shown to intercalatedeintercalate Li + ions at very high potential (E ¼ 4.7 V vs. Li + / Li). 8 It has a large high intrinsic rate capability offered by the 3dimensional lithium-ion diffusion in the spinel lattice. Besides, it is much safer, low-cost, and greener. 9 There is a continued need to reduce the cost of the LiMn 1.5 Ni 0.5 O 4 by the use of lowcost synthesis method, the use of low-cost manganese precursor (such as the electrolytic manganese oxide, EMD) as well as drastic reduction in the amount of the expensive nickel in the structure (Ni < 0.5), without compromising its advantageous properties.
In this work, the preparation and electrochemical properties of LiMn 1.9 Ni 0.1 O 4 cathode materials containing very small amount of nickel (x ¼ 0.1) and using EMD precursor have been investigated. The spinel cathode material was chosen due to its low toxicity, abundant material source and its high specic capacity of 148 mA h g À1 . The commercial spinel cathode material (LiMn 2 O 4 ) is a well-studied cathode system for LIB with the potential to serve as an alternative to the toxic and expensive LiCoO 2 . However, the main challenge with LiMn 2 O 4 is the capacity fading due to Jahn-Teller distortion [10][11][12] in the 3 V region, 13 which is due to the generation of new phases during cycling and disproportionation reaction. In order to overcome this limitation, we have adapted a Ni-doping strategy. Literature reports have shown that doping with a small amount of Cr 3+ , Ni 2+ and Al 3+ can stabilize the spinel structure of LiMn 2 O 4 and provides high operating voltage above 4.7 V, suppress the Jahn-Teller effect, and improve the cycling properties. [14][15][16] Although the use of small amount of nickel in the structure (i.e., Ni < 0.5) has rarely been studied, it has been established that Ni ¼ 0.1 provides the best electrochemistry. 12 Therefore, there is a need to further explore the performance of this spinel using new lowcost synthetic routes.
Various synthetic routes have been followed to synthesize different spinel cathode materials, including solid state, 17 combustion, 15 co-precipitation, 18 sol-gel method 19 and modi-ed pechini. 20 Unfortunately these methods require elevated temperatures as high as 700-900 C. Further, LiMn 2Àx Ni x O 4 synthesized by the solid-state method is oen accompanied by the formation of Li x Ni x O impurity phases which causes capacity fading. The crystallinity of the materials is also poor and leads to the dissolution of crystal faces by an electrolyte which deteriorates the rate capability. The techniques based on the processes of co-precipitation can give single phase LiMn 2Àx -Ni x O 4 at lower temperatures. However, these methods involve the use of expensive reagents with complex process. 21 In this work, for the rst time, we opted for a low temperature aqueous reduction method 22 to synthesize LiMn 1.9 Ni 0.1 O 4 cathode materials. We have used NiSO 4 $6(H 2 O) as the nickel source and locally-produced low-cost EMD as the Mn source. This synthesis method not only has the advantage of using a locally-produced low-cost EMD but can also be a viable replacement to co-precipitation technique.

Materials and preparation
Electrolytic manganese dioxide (EMD) from a South African supplier (Delta EMD Pty Ltd) and LiOH$H 2 O, NiSO 4 $6(H 2 O), glucose from Sigma Aldrich were used for the synthesis of spinel LiMn 2Àx Ni x O 4 (x ¼ 0 and 0.1) cathode materials.
Both LiMn 1.9 Ni 0.1 O 4 and its pristine material, LiMn 2 O 4 (for comparison) were prepared using a facile and low temperature aqueous reduction synthesis method by employing electrolytic manganese dioxide (EMD), LiOH$H 2 O, NiSO 4 $6(H 2 O) (for the Ni-doped sample) and glucose as a reducing agent. Briey, a stoichiometric amount of LiOH$H 2 O, EMD and NiSO 4 $6(H 2 O) (for the LiMn 1.9 Ni 0.1 O 4 sample) was dissolved in 60 mL of double-distilled water by continuous stirring at a temperature of 80 C. Aer 1 h, the appropriate amount of glucose dissolved in 20 mL of double-distilled water was added to the mixture. The stirring was continued for a further 8 h at 80 C until the reaction was complete. The slurry was allowed to cool and settle for 12 h. Aer decanting, the product was washed several times with distilled water and dried at 120 C. The resultant powder was calcined at 780 C for 20 h in air and then cooled to room temperature naturally in the furnace. The purpose of further calcination at 780 C to 20 h is to generate the required phase structure and composition in the LiMn 2Àx Ni x O 4 product (x ¼ 0 and 0.1). In both samples, the same method of synthesis was adopted.

Equipment and procedure
The morphology of the samples LiMn 2Àx Ni x O 4 (x ¼ 0 and 0.1) were obtained using a high resolution scanning electron microscope (JEOL, JSM-7600F), operating at an accelerating voltage of 5 kV. The EDS facility attached to the SEM gave the elemental data on the samples. The structural properties of the samples were investigated by X-ray diffraction analysis using a PANalytical X'Pert PRO PW3040/60 X-ray diffractometer with a Ni ltered Cu-Ka (l ¼ 0.154 nm) monochromated radiation source. Data were collected in the 2q range of 10-90 at a scan rate of 2 min À1 . The XPS data were analyzed using the XPS Peak 4.1 program.

Cell fabrication and electrochemical analysis
Electrochemical cells were fabricated as follows: coin cells of 2032 were assembled using lithium metal as anode, Celgard 2400 as separator and a 1 M solution of LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) (1 : 1 : 1, by volume) the electrolyte. The cathode was made from a slurry using a coating procedure from a mix containing active material powder, conducting black and poly(vinylidene uoride) binder in N-methyl-2-pyrrolidone in the proportion 80 : 10 : 10, respectively. The slurry was coated over aluminium foil and dried at 110 C overnight for 12 h. The 18 mm diameter slurry-coated aluminium foil electrodes were punched out and used as cathode. Coin cells were assembled in an argon lled glove box (MBraun, Germany) with moisture and oxygen levels maintained at less than 1 ppm. The chargedischarge cycles of the cells were carried out between 3.5-4.8 V at 0.2C rate with respect to their corresponding theoretical capacities of LiMn 2 O 4 and LiMn 1.9 Ni 0.1 O 4 using a Maccor 4000 series 96-channel battery tester. The electrochemical impedance spectroscopy studies were carried out using a Bio-Logic VMP 3 Potentiostat/Galvanostat controlled by EC-Lab v10.40 soware. EIS data were collected aer ageing the fabricated lithium-ion cell for 24 h. Nyquist plots of the charged and discharged electrodes were recorded aer allowing 1 h of stabilization.

Results and discussion
Morphological and EDS elemental analysis  Fig. 1b shows that the LiMn 1.9 Ni 0.1 O 4 cathode materials have octahedral shape with clearly exposed (111) facets. The (111) facets are known to allow the formation of a thinner solid electrolyte interphase (SEI) than other facets thereby enhancing the Li-ion kinetics and cycling performance. 23 The estimated particle size distribution 24 of the compositions LiMn 2 O 4 , and LiMn 1.9 Ni 0.1 O 4 is graphically presented in Fig. 1. Fig. 1c and d indicates that the particle sizes of the cathode materials are in the range of 0. 30 Energy-dispersive X-ray spectroscopy (EDS) elemental analysis was carried out in order to conrm the doping of Ni-ions.

Structural characterisation
The X-ray diffraction patterns to analyse the crystallographic structure and the impurity phases of the doped compounds synthesized by the aqueous reduction process are shown in  Fig. 2b. The XRD peaks shied slightly towards le with respect to EMD precursor reection peaks, indicating structural change due to aqueous reduction reaction. Fig. 3a and b shows the detailed XPS spectra of the Mn 2p 3/2 peaks of the LiMn 2 O 4 , and LiMn 1.9 Ni 0.1 O 4 samples, respectively. There is a broad peak width for both the materials, which indicates that the Mn exist in more than one oxidation state.     and cation distribution are summarised in

Electrochemical performance
Galvanostatic charge/discharge experiments. The main objective of Ni substitution into a spinel LiMn 2 O 4 cathode using small amounts of Ni is to achieve an improved electrochemical performance. The successful doping with nickel which was conrmed from the EDS, XRD and XPS data as described above, is indeed reaffirmed by an improvement in the electrochemical performance.
The electrochemical activity role is played by Mn 3+ in the pristine LiMn 2 O 4 . In the synthesized spinel, the Mn oxidation state is 3.5+ since an equal number of Mn 3+ and Mn 4+ are assumed to be present before charging. During charging, all Mn 3+ convert ideally to Mn 4+ by driving all Li + ions into the anode electrode. The dissolution of manganese into the electrolyte is generated by the occurrence of the disproportion reaction 2Mn 3+ (solid) / Mn 4+ (solid) + Mn 2+ (solution) in 10,11 the 4 V region. As a result the electrochemical active Mn 3+ will diminish accordingly the discharge capacity will start to fade.
The galvanostatic charge/discharge capacity performance of the cathode materials was carried out at 0.2C rates with respect to their corresponding theoretical capacities. The representative 1 st , 2 nd , and 40 th cycle charge/discharge capacities of LiMn 2Àx -Ni x O 4 (x ¼ 0 and 0.1) are displayed in Fig. 4. During the rst cycle the as-synthesized cathode materials LiMn 2 O 4 , and LiMn 1.9 Ni 0.1 O 4 respectively delivered discharge capacities of 137 mA h g À1 and 128 mA h g À1 . The result shows that the initial discharge capacity decreases for the nickel-doped sample as expected. This trend of decrease in capacity is as a result of a reduction in the reversibly extractable Li + ions from 1 for pristine LiMn 2 O 4 to 1 À x for Ni-substituted lithium manganese oxides upon substitution of the electrochemically active Mn 3+ ions. 28 The discharge capacities of the as-synthesized cathode materials are comparable to experimentally reported values. 29,30    Despite that nickel insertion into EMD is difficult, it is interesting to see that our synthesis protocol was able to insert some amount that could successfully suppress the capacity fading.
The capacity contribution at high voltage 4.5-4.7 V due to Ni 2+ / Ni 4+ is very little, most of the electrochemical capacity is at the 4.1 V due to the Mn 3+ /Mn 4+ redox couple. In addition, the rst cycle charge-discharge reversibility for the samples LiMn 2 O 4 and LiMn 1.9 Ni 0.1 O 4 is 86.9% and 69.7%, respectively. By considering the 2 nd and 40 th cycle capacity, it is noted that LiMn 1.9 Ni 0.1 O 4 shows gain in charge-discharge reversibility of 87% and 96% at the 2 nd and 40 th cycle, respectively. The improvement in reversibility arises from structural stability; at the 1-2 cycles, the spinel cathode material is not properly equilibrated with the electrolyte, but this should expected to improve upon repetitive cycling, and hence the improved reversibility.
Cycling stability and rate capability. To compare the performance of the two spinel materials, we rst examined their cycling performance at 100 cycles at constant rate (0.2C). As evident in Fig. 5a, the nickel-substituted sample, LiMn 1.9 Ni 0.1 -O 4 , exhibited high cycling performance compared to its pristine counterpart. The capacity retention of LiMn 1.9 Ni 0.1 O 4 is about 84% compared to the 52% capacity retention recorded for the pristine LiMn 2 O 4 aer the 100 repetitive cycling at room temperature.
Next, we look at the rate capability of the two spinel materials by performing experiments at different high rates, from 0.4 to 3C (Fig. 5b). Upon completion of the rate capability experiments and the initial rate of 0.4C was repeated, LiMn 1.9 (Fig. 6a) and LiMn 1.9 Ni 0.1 O 4 (Fig. 6b) samples aer 100 cycles, it is interesting to observe that the morphology of the LiMn 1.9 Ni 0.1 O 4 showed a microporous but inter-connected network structures compared to the morphology of the LiMn 2 O 4 that showed huge agglomeration of the starting nanoparticles. The morphology of the LiMn 1.9 Ni 0.1 O 4 should allow for a more facile electrochemistry (in terms of stability and kinetics) than that of the agglomerated. From the above experimental ndings, we can conclude that the high electrochemical performance of the LiMn 1.9 Ni 0.1 O 4 over its pristine counterpart LiMn 2 O 4 can be related to a considerable decrease in the Jahn-Teller distortion spinel. 15,31   Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy is a powerful technique to study the kinetics of lithium intercalation/de-intercalation processes. EIS was carried out to examine the electrode resistance changes for LiMn 2 O 4 and nickel substituted LiMn 1.9 Ni 0.1 O 4 samples synthesized using nickel sulphate as nickel source. The Nyquist plots of pristine LiMn 2 O 4 and LiMn 1.9 Ni 0.1 O 4 are presented in Fig. 7 and the equivalent circuit used is shown in Fig. 7b inset. The intercept at the real (Z 0 ) axis in high frequency corresponds to the series resistance due to anode-separator-electrolytecathode (R s ). The R f and CPE f are the surface lm resistance and lm capacitance. The semicircle in the middle frequency range indicates the charge transfer resistance (R ct ) and CPE dl is the double layer capacitance at the electrolyte-electrode interface. The inclined straight line relates to the Warburg impedance (Z w ) 32 and represents the diffusion impedance. The parameters of the equivalent circuit obtained from computer simulations for the as-synthesized LiMn 2 O 4 and LiMn 1.9 Ni 0.1 O 4 is shown in Table 3. Using the tting, the R ct value of the LiMn 2 O 4 and LiMn 1.9 Ni 0.1 O 4 samples were found to be 373 and 235 U (before 100 cycles), 1105 and 431 U (aer 100 cycles), respectively. These results conrm that the nickel substitution suppressed the charge transfer resistance, which contributed to a higher discharge capacity and better capacity retention aer 100 cycles compared to the pristine LiMn 2 O 4 sample. Molecules with smaller particles are expected to give better electrochemical kinetics. It is surprising therefore to observe that LiMn 1.9 Ni 0.1 O 4 with larger average particle size (1.332 mm) gave an enhanced kinetics compared to the LiMn 2 O 4 (0.405 mm). The interpretation may be found from the SEM images of the two electrodes where the morphology of LiMn 1.9 Ni 0.1 O 4 showed porous and inter-connected networks that allow for electrochemistry to occur more effectively than an agglomerated and bulky morphology.
Plots of ÀZ 0 vs. u À1/2 for LiMn 2Àx Ni x O 4 (x ¼ 0, 0.1) is shown in Fig. 8. The diffusion coefficients for LiMn 2 O 4 and LiMn 1.9 -Ni 0.1 O 4 cathode materials are 6.4 Â 10 À12 and 6.89 Â 10 À11 cm 2 s À1 at room temperature, respectively. The result conrms that nickel substitution has signicantly enhanced the Li + ion diffusion, which is a magnitude higher for nickel substituted LiMn 1.9 Ni 0.1 O 4 than the pristine LiMn 2 O 4 diffusion coefficient.

Conclusions
In summary, we employed low-cost manganese precursor electrolytic manganese dioxide (EMD) and a low temperature aqueous reduction synthesis technique to successfully prepare nickel substituted spinel LiMn 2Àx Ni x O 4 (x ¼ 0 and 0.1) cathode for lithium-ion battery by using NiSO 4 $6H 2 O as nickel source. We have conrmed that the Ni-ions substituted the Mn-ions using XRD, EDS, XPS and electrochemical performance studies. The nickel-substituted sample LiMn 1.9 Ni 0.1 O 4 exhibited superior capacity retention as compared to pristine LiMn 2 O 4 ; LiMn 1.9 Ni 0.1 O 4 retained 84% of its initial capacity whereas pristine LiMn 2 O 4 retained only 52% of its initial capacity.
The study shows that the use of a small amount of Ni to eliminate the Jahn-Teller effects of the LiMn 2 O 4 . In addition, this synthesis protocol has a great potential to be deployed for upscale-up production of pristine and Ni-doped spinel LiMn 2 O 4 cathode materials for lithium-ion battery applications from locally-sourced and low-cost manganese precursor.