Effects of Sodium Substitution on the Rate Performances of Spherical LiMn 2 O 4 Cathode for Lithium Ion Batteries

LiMn2O4 Cathode for Lithium Ion Batteries Jianbing Jiang,Wei Li, Haojie Deng and Lijian Xu* College of Packaging and Material Engineering, Hunan University of Technology, Zhuzhou, 412007, China Hunan Key Laboratory of Biomedical Nanomaterials and Devices, College of Life Sciences and Chemistry, Hunan University of Technology, Zhuzhou 412007, P. R. China Foshan University, School of Materials Science and Energy Engineering, Foshan 528000, P. R. China * Correspondence: xljl235@hut.edu.cn; xlj235@163.com Abstract Sodium substitution Li1-xNaxMn2O4 cathodes were synthesized by a solid-state reaction method. The morphologies and crystal structures of Li1-xNaxMn2O4 were characterized


INTRODUCTION
The serious energy crisis has promoted the research and development of chemical stored [1][2][3].Lithium ion batteries provide four times the energy, half the weight, and longer life compared to lead acid battery systems.It makes a great contribution on small portable electronic devices, hybrid electric vehicles (HEV) and electric vehicles (EV) [4][5][6][7][8].Among all the cathode materials, the spinel LiMn2O4 is considered as one of the most promising cathode materials for rechargeable LIBs due to its low cost, abundance, fast charge-discharge reactions, high coulombic efficiency and nontoxicity [9][10][11][12][13][14].
However, the large capacity loss of LiMn2O4 during the cycling process discourages further commercial use of the material.[15][16][17][18] The capacity fading during cycling is attributed to various factors, such as manganese dissolution in the electrolyte [19][20][21], Jahn-Teller distortion [22][23][24][25], and the formation of an oxygen deficiency [26].To overcome these problems, many efforts have been devoted to modify the problems originating from the intrinsic characteristics of LiMn2O4.Cation doping is considered to be an effective strategy to alleviate the problems of LiMn2O4 by means of a series of transition metal substitution [21,[27][28][29][30][31][32].
LiMn2O4 is the cheapest lithium ion cathode, but its price is also many times that of lead acid battery, which limits its application in some fields.The high price of LiMn2O4 is mainly due to its raw materials.Electrolytic manganese dioxide (EMD) has been widely used as manganese compound precursors for synthesis of LiMn2O4 powders [33][34][35].However, there is a mass of Na in EMD because alkali was adopted to neutralize during the progress of its progress.Li2CO3 prepared from salt lake or spodumene was another main material for producing LiMn2O4.However, there are also large amount of sodium in Li2CO3 because there are a mass of sodium in the salt lake.Many companies pay a lot of money to wash sodium in EMD or deal with sodium in salt lake, which results in the increasing price of LiMn2O4.
The effects of sodium substitution on the performances of LiMn2O4 must be studied in order to reduce its cost.In this work, a series of Li1-xNaxMn2O4 were prepared from Mn3O4, Li2CO3 and Na2CO3 by a solid state method.The effects of sodium substitution on the crystal structures, morphologies and electrochemical properties were investigated.

Synthesis
Mn3O4 was prepared by controlled crystallization process with MnSO4 and O2 in water for about 12 h at 65-75℃.During the progress, ammonia solution was dropped to control the pH between 8.5 and 9.5.After the reaction was completed, the precipitate was filtered and washed by distilled water for three times and dried at 120C overnight.
Li1-xNaxMn2O4 was prepared by a solid state synthesis from Li2CO3, Na2CO3 and the as-prepared Mn3O4.The mixture of Li2CO3, Na2CO3 and the as-prepared Mn3O4 ((Li+Na)/Mn = 1.03:2) was mixed with a mortar and pestle, then ground thoroughly and calcined at 650℃ for 8 h and 800℃ for 10 h in air.After cooling in furnace, the final black powder (Li1-xNaxMn2O4) was gotten.

Characterization
In order to identify the crystal structure, X-ray diffraction (XRD) analysis was performed using Philips X'Pert MPD (Philips) instrument with Cu Kα radiation.
Scanning electronic microscopy (SEM) was performed using a JSM-5600LV (JEOL) instrument to detect surface morphology and analyze the size of the particles.

Electrochemical characterization
Electrochemical performance for the cathode was evaluated by assembling 2025-type coin cells with working cathode, lithium metal anode and celgard 2340 microporous membrane.The working cathode was fabricated using 80 wt% Li1-xNaxMn2O4 powders, 10 wt% carbon black and 10 wt% poly(vinyl difluoride) PVDF binder.1M solution of LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in wt%) and obtained from Ferro Corporation was employed as electrolyte.The test cells were assembled into a glove box with an excellent environment control and the concentrations for both H2O and O2 were below 1 ppm.The charge-discharge performance for the cells was conducted on a battery test system (Land CT 2001A, Land Co. China).In the calculation of C-rate, 148 mA/g is assumed to be 1C.

Electrochemical impedance spectroscopy (EIS) analysis were carried out under open
circuit condition in the frequency range from 100 kHz to 0.005 Hz with 10 mV ac amplitude, conducted at CHI660D (CH Instruments).

Morphology and Structure of Mn3O4
Fig. 1 shows the SEM of the precursor prepared by the controlled crystallization method.
It is obvious that Mn3O4 powders have helical spherical morphology in secondary particles, while the primary particles are similar-sphere and they are densely agglomerated in secondary forms.This kind of morphology of helical spheres obviously comes from the formation of coordination compound of manganese ammoniate during the progress, which might plays a role in controlling not only the releasing velocity of manganese-ions, but also the growing direction of Mn3O4.

Morphology and Structure of Li1-xNaxMn2O4
The XRD patterns of Li1-xNaxMn2O4 are shown in Fig. 2. No obvious changes in the peak positions and intensities can be observed compared to the standard sample (JCPDS, card no.35-0782).It indicated that the prepared Li1-xNaxMn2O4 samples possessed the cubic spinel structure with space group of Fd-3m, suggesting that the sodium substitution did not change the intrinsic cubic spinel structure, where Li + ions resided in the tetrahedral (8a) sites and Mn 3+ /Mn 4+ ions occupied the octahedral (16d) sites.The lattice constant was calculated according to the eight diffraction lines (as shown in Fig. 2) by a Least Squares Method, and the results were shown in Fig. 3.Because the ionic radius of Na + is bigger than that of Li + , the lattice parameter increased linearly with the increase of sodium substitution content.The SEM images of all the synthesized Li1-xNaxMn2O4 powders were shown in Figure 4.As seen in Fig. 4, after firing, the sphere shape becomes more uniform and ordered.All the powder particles consisted of potatoshaped secondary particles.The first particle on the surface was changed from sphere to nanometer exfoliate after sodium substituted.It becomes more obvious when more sodium substituted lithium.However, some rod-like particles also appeared as the substitution of sodium increases more than 8%, which must be avoided because it will impale the microporous membrane and result in the battery short circuit.

Electrochemical Properties
Fig. 5 shows the initial charge-discharge voltage profiles of sodium substituted Li1-xNaxMn2O4 at 0.1 C rate in the voltage range of 3.0-4.3V.For all samples, two welldefined potential plateaus can be observed, which correspond to that of a two-stage intercalation/de-intercalation process of lithium in spinel LiMn2O4.The detailed data of the charge-discharge capacity was summarized in Table 1.With the increase of sodium substitution, both of the initial charge and discharge specific capacity decreased.
The decrease in capacities of sodium substituted electrodes may be due to the decrease in the concentration of Li + .Therefore, further increase of sodium substitution is not preferred as it is detrimental to the high specific capacity.2. As shown in Fig. 6, the discharge capacity decreases as the current density increases in all samples, but the discharge capacity decreases more slightly with increasing substitution ratio.This result confirms that the sodium substitution effectively enhances the rate capability.Cycle performance of Li1-xNaxMn2O4 samples was studied between 3.0 and 4.3V at room temperature at different specific current.The results that discharge at 2 C rate and 5 C rate were illustrated in Fig. 7 and Fig. 8, respectively.The corresponding data are summarized in Table 3.As shown in Fig. 7, as the substitution of sodium increase from 2% to 10%, the initial discharge capacity of the cathode powders decrease slightly from 115.6mAh/g to 108.1mAh/g, but the capacity retention increased from 86.68% to 89.64% after 100 cycles.It illustrates that sodium substitution enhances the performance of LiMn2O4 at low current density.However, the results were reversed when further increase the discharge current to 5 C rate.As shown in Fig. 8, as the substitution content of sodium increase from 2% to 6%, the initial discharge capacity of the cathode powders increases from 94.5mAh/g to 114.2mAh/g, but the capacity retention decreases from 85.82% to 81.52% after 300 cycles.Thus, the sodium substitution may enhance the discharge capacity but decrease the cycle performance at large current density.There are two reasons for the high capacity of cathode substituted by sodium.The first reason is the formation of nanometer exfoliate particle (as shown in Fig. 4) on the surface of cathode, which results in the short ion diffusion distance.The second reason is that the unit cell parameter (as shown in Fig. 3) of Li1-xNaxMn2O4 was enlarged when lithium was substituted by sodium, lithium ion transference become easier which result in the high discharge capacity.However, the Jahn-teller become severer when the unit cell parameter becomes bigger, which result in the poor cycle performance of Li1-xNaxMn2O4 cathode.High temperature performance is an important parameter to estimate the electrochemical performance of a kind of electrode material.After comparing the results above, it can be found the rate capacity and cycle performance of Li0.94Na0.06Mn2O4are very good.Thus, it was further study in the voltage range of 3.0-4.3V at 55℃.Fig. 9 shows the charge-discharge voltage profiles at various current densities.As the current density increases from 0.1 to 0.5, 1, 5, 10 and 15 C, the discharge capacity decreases slightly from 125.1 to 123.3, 119.6, 116.4,102.3 and 93.2 mAh/g, respectively.Fig. 10 shows the cycle performance curve at a constant specific current of 5C.The initial charge and discharge capacities are 114 mAh/g and 112.1 mAh/g, respectively.After 100 cycles, the charge and discharge capacities are 84.3 mAh/g and 82.9 mAh/g, the discharge capacity retention of Li1.94Na0.06Mn2O4remains at 73.95%.Comparing with its cycle performance at room temperature, both the first discharge capacity and the discharge capacity retention decreased slightly.The poor cycle stability can be attributed to the dissolution of Mn ion at high temperature.Thus the cycle performance of sodium substituted Li1-xNaxMn2O4 must be improved by other ways, such as doping with other element or coating.
Electrochemical impedance spectroscopy analysis is an effective means to study the kinetics of the electrode reaction [36,37].In order to further elucidate the decreases from 424.7Ω to 227.9 with the increase of sodium substitution from 2% to 10%, confirming the role of sodium substation enabled high rate performance reported above.

CONCLUSION
The effects of sodium substitution on the performances of spherical LiMn2O4 cathode were studied in this paper.The sodium substitution does not change the basic spinel structure.But it affects the rate performance of LiMn2O4.Sodium substitution enhances its initial discharge capacity at large current density.As the molar ratio of sodium to lithium increase from 0.02 to 0.06, the initial discharge capacity increase from 94.5 to 114.2 mAh/g, and the capacity retention decreases from 85.82% to 81.52% after 300 cycles at a current density of 740mA/g at room temperature.

Fig. 6
Fig.6 compares the discharge capacity of Li1-xNaxMn2O4 electrodes at various current electrochemical properties of the Li1-xNaxMn2-xO4 (x=0.02-0.1),EIS analysis of the composite electrodes at charged (4.3 V) state was been carried out.The measured impedance of the electrochemical cell is a collective response of the kinetic process occurring in the electrode.Fig.11 displays the electrochemical impedance spectra of Li1-xNaxMn2-xO4 electrodes.The equivalent circuit model and the resistance values obtained from fitting are given in the inset of the Fig.11.In the equivalent circuit, RΩ is the ohmic resistance of the battery, including the total resistance of electrolyte, separator, conductive material, etc.; Rct represents the charge transfer resistance; CPE (Constant phase element) is used to replace the capacitor in order to fit the experimental data appropriately; CPE1 corresponds to the surface film capacitance in high-frequency semicircle; and CPE2 corresponds to double layer capacitance in the low-frequency line.As can be seen in Fig.11, the charge transfer resistance of Li1-xNaxMn2-xO4

Table 2
The detailed data of specific initial discharge capacity of Li1-xNaxMn2O4 (x=0-0.1)at various current densities at room temperature (values of the first cycle).

Table 3 .
The detailed data of initial discharge capacity and capacity retention after