Xueyi Guo, Chenlin Yang, Jinxiu Chen, Qinghua Tian, Hongmei Zhang, Guoyong Huang,,*

1 School of Metallurgy and Environment, Central South University, Changsha 410083, China

2 College of New Energy and Materials, China University of Petroleum-Beijing, Beijing 102249, China

3 State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum-Beijing, Beijing 102249, China

4 State Key Laboratory of Advanced Chemical Poer Sources, Guizhou Meiling Power Sources Co. Ltd., Zunyi 563003, China

Keywords:LiMn2O4 LiNi0.5Mn1.5O4 Novel morphology Li-ion battery Electrochemistry Preparation Nanomaterials

A B S T R A C T LiNi0.5Mn1.5O4 and LiMn2O4 with novel spinel morphology were synthesized by a hydrothermal and postcalcination process. The synthesized LiMn2O4 particles (5-10 μm) are uniform hexahedron, while the LiNi0.5Mn1.5O4 has spindle-like morphology with the long axis 10-15 μm, short axis 5-8 μm. Both LiMn2O4 and LiNi0.5Mn1.5O4 show high capacity when used as cathode materials for Li-ion batteries. In the voltage range of 2.5-5.5 V at room temperature, the LiNi0.5Mn1.5O4 has a high discharge capacity of 135.04 mA·h·g-1 at 20 mA·g-1, which is close to 147 mA·h·g-1 (theoretical capacity of LiNi0.5Mn1.5O4). The discharge capacity of LiMn2O4 is 131.08 mA·h·g-1 at 20 mA·g-1. Moreover, the LiNi0.5Mn1.5O4 shows a higher capacity retention(76%)compared to that of LiMn2O4(61%)after 50 cycles.The morphology and structure of LiMn2O4 and LiNi0.5Mn1.5O4 are well kept even after cycling as demonstrated by SEM and XRD on cycled LiMn2O4 and LiNi0.5Mn1.5O4 electrodes.

1. Introduction

Lithium-ion batteries (LIBs) swept the world since the early 1990s[1,2].And it obtained the 2019 Nobel Prize in Chemistry,further acknowledged the success of LIBs commercialization [3,4].Compared to other battery systems, LIBs have wide working temperature, low self-discharge and no memory effect [5-8]. But the energy density of LIBs is still far from satisfactory.Rapid increasing demand of LIB for portable electronic devices, electric vehicles(EVs),and hybrid electric vehicles(HEVs)urgently requires cathode materials with high thermal stability,high energy density and long lifetime[9-12].Traditional cathode materials are difficult to satisfy these requirements for emerging electric vehicles due to the low voltage, high cost and low safety [13]. In order to cope with this demand, a mass of researchers focus on developing new electrode materials,such as Li-rich cathode materials,Ni-rich cathode materials and other anode materials [14]. Li-rich cathode materials were introduced by Dahn and Thackeray [15-18]. The coin cells with this cathode have an amazing capacity (＞250 mA·h·g-1), but their average operating voltage (3.5 V) is lower than those traditional cathode materials (3.8 V). And the discharge voltage would continue to decay during the operation of the battery [14].Obviously, the decrease of energy density is inevitable because of the energy density is related to the voltage and capacity.Therefore,the development of electrode materials with high voltage and high capacity is an important measure to improve the energy density[19,20].

Among the large number of cathode materials, the Co-free spinel LNMO shed a light on design of high-energy density for the next-generation LIBs than that of LiFePO4and LiCoO2due to its high operating voltage and capacity [5,21]. Although LiFePO4has excellent safety features,it can no longer meet the demand of large power batteries with small size. LiCoO2has poor safety features and is expensive. In contrast, LNMO has no pollution to the environment and is the most promising cathode. Moreover, the highvoltage LNMO has high ionic conductivity for Li+to diffuse during the working period.This is also the main reason for his high capacity.But the rapid capacity fade is a very distressing problem for the spinel LNMO.This may be related to the increase of the impedance of the CEI(Cathode Electrolyte Interface)during battery operation.Therefore, it has a wide field of application with good prospects and attracts a large number of researchers. This promoted a lot of prominent articles published include some excellent reviews.The prepared LNMO delivered a considerable cycle performance about 126 mA·h·g-1between 3.0 V and 4.95 V at 0.1C, and the capacity retention ratio about 71%after 70 cycles.X.Qinet al.synthesized a hollow hierarchical LNMO cathode material via a ureaassisted synthesized a hollow hierarchical LNMO cathode material via a urea-assisted hydrothermal method [22]. The cell matching this cathode have an amazing electrochemical performance that capacity after 100 cycles at 1C is still 96.8% of the initial capacity.The spinel LNMO may be as one of prospective cathode materials in future.

Large quantities of researchers believe that the electrochemical performance of materials has a great relationship with its morphology and structures [23-30]. However, the key factor of different morphology and structures is the difference of synthesis process and calcination conditions.Heren,we synthesized LiMn2O4and LiNi0.5Mn1.5O4with novel spinel morphology by a hydrothermal and post-calcination process. The synthesized LiMn2O4particles (5-10 μm) with uniform hexahedron have an excellent discharge capacity about 131.08 mA·h·g-1at 20 mA·g-1. The LiNi0.5Mn1.5O4particles with spindle-like morphology have the long axis 10-15 μm and short axis 5-8 μm.When the LiNi0.5Mn1.5-O4used as cathode materials for Li-ion batteries, it also showed incredible performance. And it has a high discharge capacity of 135.04 mA·h·g-1at 20 mA·g-1.Moreover,the LiNi0.5Mn1.5O4shows a higher capacity retention (76%) compared to that of LiMn2O4(61%) after 50 cycles. The morphology and structure of LiMn2O4and LiNi0.5Mn1.5O4are well kept even after cycling as demonstrated by SEM and XRD on cycled LiMn2O4and LiNi0.5Mn1.5O4electrodes.

2. Experimental

2.1. Materials preparation

The spinel LMO and LNMO powders were synthesized as follows.All reagents are analytical grade,without further purification.Typically, 3.6763 g Mn(CH3COO)2·4H2O (15 mmol), 1.2442 g Ni(CH3COO)2·4H2O(5 mmol),2 g CO(NH2)2and 6 g polyvinyl pyrrolidone (PVP) were dissolved in 150 ml deionized water and stirred magnetically for 10 min to form a homogeneous solution. Then the solution was transferred to a 200 ml Teflon-lined stainlesssteel autoclave. The autoclave was sealed and kept in an oven at 120 °C for 6 h and then cooled naturally to room temperature.The precipitates were collected and washed three times by deionized water and ethyl alcohol,and then dried at 60°C until 12 h.The obtained precursor powders were transferred to a mortar to ground with LiOH·H2O and ethanol for 10 min. Then, the obtained mixture was calcined at 800 °C for 6 h in air, followed by another 3 h dwell time at 600 °C to obtain the LNMO samples. The LMO samples were also synthesized in a similar process.

2.2. Materials characterization

The phase of the as-synthesized LMO and LNMO samples were analyzed by powder X-ray diffraction(XRD,Rigaku 2000,Cu Kα)in the2θ range between 10° and 80°. The morphology of LMO and LNMO materials were characterized by a scanning electron microscope (SEM, JSM-6360LV JEOL). Thermogravimetric (TG) analyses of the samples were conducted with a synchronous thermal analyzer (SDTQ600 TA) at a heating rate of 10 °C·min-1in air from room temperature to 1000 °C. The Brunauer Emmett Teller (BET)surface areas of the materials were measured using an automatic specific surface area analyzer (ASAP 2020, MICROMETER). The chemical states of samples were studied by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi).

2.3. Electrochemical measurements

The CR2032 coin-type cells were composed to evaluate the electrochemical performances of LNMO and LMO cathode materials as the previous works [30,31]. The electrochemical performances of the cells were conducted on a multi-channel battery cycler(LAND,CT2001A,Wuhan)with a voltage range of 2.5-5.5 V(vsLi/Li+).The cyclic voltammograms(CV)were performed on an electrochemical station(Autolab,PGSTAT302 N)between 2.5-5.5 V(vsLi/Li+).Electrochemical impedance spectroscopic (EIS) data were collected on an electrochemical station (Autolab, PGSTAT302 N) with an ac amplitude of 5 mV in the frequency range of 100 kHz to 0.01 Hz.All the test were performed at room temperature.

3. Results and Discussion

The TG-DTA curves of the LiMn2O4and LiNi0.5Mn1.5O4precursors were conducted in order to understand the evolution of LiMn2O4and LiNi0.5Mn1.5O4during calcination (Fig. 1). The first mass losses of LiMn2O4and LiNi0.5Mn1.5O4precursors were 2.03%and 5.16%, respectively. This can be ascribed to the evaporation of moisture and solvent remained in the precursors under 285 °C. The second mass losses of both samples around 300-600°C were about 30%,which was attributed to the decomposition of precursors (MnCO3, Ni0.25Mn0.75CO3) and LiOH. The final mass losses after 600°C were much smaller than the previous two mass losses.This may correspond to the volatilization of extra Li salts at high temperatures, and some phase transitions also occurred at high temperatures and improved the crystallinity. The total mass losses were 31.12% and 38.82%, respectively, which is in perfect accord with the theoretical values to form LiMn2O4and LiNi0.5Mn1.5O4.

Surface properties are key factors affecting the electrochemical performance of materials. Fig. 2 shows the specific surface areas and pore size distributions of the synthesized LiMn2O4and LiNi0.5Mn1.5O4powders analyzed. The specific surface areas of the as-prepared LiMn2O4and LiNi0.5Mn1.5O4were 4.493 m2·g-1and 2.197 m2·g-1, respectively (Fig. 2a). The specific area of the LiMn2O4particles was twice that of the LiNi0.5Mn1.5O4particles,which may be due to the smaller particle size of the LiMn2O4.Meanwhile, the BJH pore-size distributions (Fig. 2b) shows that the average pore sizes of the as-prepared LiMn2O4and LiNi0.5Mn1.5O4cathode materials were 10.5300 nm and 9.6211 nm, respectively. The average pore size of the as-prepared LiMn2O4was also slightly larger than that of the LiNi0.5Mn1.5O4cathode materials. The larger specific areas and better pore distributions are both conducive to the contact between cathode materials and electrolyte and the transmission of Li+[31], achieving better performance.

In order to investigate the chemical states of each element in the synthesized LiMn2O4and LiNi0.5Mn1.5O4powders, the XPS results were carried out (Fig. 3 and Fig. 4). The full survey of the LiMn2O4and LiNi0.5Mn1.5O4both demonstrate the presence of lithium (1 s), Oxygen (O1s) and Manganese (Mn2p) (Fig. 3a and Fig. 4a), but the difference is that there are Nickel (Ni2p) exists in the survey spectra of the LiNi0.5Mn1.5O4. The double peaks located at the banding energy of 642.8 eV and 653.8 eV are in good agreement with Mn2p3/2and Mn2p1/2of Mn (IV)species, respectively (Fig. 3b and Fig. 4b). The binding energy of Mn4+for the Mn2p3/2level have been reported to be 642.8-643.5 eV and the lower oxidation states of Mn3+is in the range of 641.2-641.5 eV[32]. The two major peaks at binding energy of 854.8 ev and 871.8 eV are related to Ni2p3/2and Ni2p1/2(Fig.4c),which ascribed to the Ni3+, and the Ni4+make the LiNi0.5Mn1.5O4cathode get a higher discharge plateau [33].

Fig. 1. TG-DTA curves for the precursors of (a) LiMn2O4 and (b) LiNi0.5Mn1.5O4 powders.

Fig. 2. (a) N2 adsorption isotherm and (b) BJH pore-size distributions of the synthesized LiMn2O4 and LiNi0.5Mn1.5O4 powders.

Fig. 3. XPS spectra of LiMn2O4.

The LiMn2O4and LiNi0.5Mn1.5O4powders synthesized by hydrothermal process in this work were employed as cathode materials to evaluate its electrochemical performance. The LiMn2O4electrode delivers similar charge and discharge curves at various current density, and the discharge capacities of the LiMn2O4electrode are 131.08 (20 mA·g-1), 125.96 (50 mA·g-1),124.29 (100 mA·g-1) and 119.66 (200 mA·g-1) mA·h·g-1, respectively (Fig. 5a). There are two voltage plateaus on the discharge curves of the LiMn2O4electrode. The first plateau at about 4 V is related to the Mn3+/Mn4+redox couple, and the second one at 2.75 V is attributed to the surface polarization caused by the side reactions between electrode and electrolyte [34]. The discharge capacities of the LiNi0.5Mn1.5O4electrode are 135.04(20 mA·g-1), 125.10 (50 mA·g-1), 118.03 (100 mA·g-1) and 107.98 (200 mA·g-1) mA·h·g-1(Fig. 5b), and it should be noticed that the capacity at 20 mA·g-1(135.04 mA·h·g-1) is close to the theoretical capacity of LiNi0.5Mn1.5O4(147 mA·h·g-1). The discharge curves of the LiNi0.5Mn1.5O4electrode including three voltage plateaus, the first voltage plateaus at about 4.7 V can be associated with the Ni2+/Ni4+redox couples, and the others at 4.0 V and 2.75 V are similar to those in the LiMn2O4electrode.The first discharge voltage plateau of the LiNi0.5Mn1.5O4is much higher than that of the LiMn2O4, and the LiNi0.5Mn1.5O4has a higher discharge capacity at low current density. However, the LiMn2O4possesses better performance on rate capability. And the cycling performances of the LiMn2O4and LiNi0.5Mn1.5O4cathode materials at the current density of 50 mA·g-1were also compared (Fig. 6). The discharge capacity of the LiNi0.5Mn1.5O4cathode materials after 50 charge-discharge cycles is 94.34 mA·h·g-1, corresponding to a capacity retention of 75.4%,which is higher than that of the LiMn2O4(77.73 mA·h·g-1,61.7%). The distressing cycling stability of the synthesized LiMn2O4and LiNi0.5Mn1.5O4particles is related to the low crystallinity, small particle sizes and non-uniform particle size distribution [35]. Obviously, the side reactions between the electrolyte and active materials were accelerated when the operating voltage reached -5 V. This can attribute to the small LNMO particles which have large specific surface area.

Fig. 4. XPS spectra of LiNi0.5Mn1.5O4.

Fig. 5. Initial charge-discharge curves of (a) LiMn2O4 and (b) LiNi0.5Mn1.5O4 cathode at different current density.

The Cyclic Voltammetry (CV) characterization of LiMn2O4and LiNi0.5Mn1.5O4cathode was conducted at room temperature with a scanning rate of 0.02 mV·s-1. Meanwhile, the voltage range of the LiMn2O4and LiNi0.5Mn1.5O4cathode was 2.5-5.0 V and 2.5-5.5 V, respectively. In, Fig. 7a the redox peaks near 4.0 V was assigned to the Mn3+/Mn4+redox couple[36],and a couple of weak peaks around 3.0 V was also observed, which can be associated with the side reactions between electrode and electrolyte at low potential. The CV curves of the LiNi0.5Mn1.5O4cathode (Fig. 7b)shows a pair of strong redox peaks located in the high voltage region of 4.6-4.8 V, which was assigned to Ni2+/Ni4+redox couple[37]. The minor redox peaks near 4.0 V and the tiny redox peaks near 3.0 V in the low-voltage range can be attributed to the Mn3+/Mn4+redox couple and side reactions. All the redox peaks in both samples agreed well with the voltage-capacity curves,and what surprised us is that the CV curves are well overlapped,indicating excellent electrochemical reversibility and good cycling stability of the cathodic host materials for Li-ion insertion/extraction reactions.To better evaluate the electrochemical performance of the synthesized LiMn2O4and the LiNi0.5Mn1.5O4cathode materials,the EIS measurement was carried out on both samples(Fig.8),and the impedance spectra was simulated with an equivalent circuit (Inset of Fig. 8). All cells were at a stable open-circuit voltage value before the EIS measurements were carried out.The plot consists of a semicircle at high frequency relating to the charge transfer resistance (Rct) and a sloping line in the low frequency region ascribing to the diffusion of lithium ion in the sample. The value ofRctfor the LiMn2O4is smaller than that of the LiNi0.5Mn1.5O4,which indicates that the LiMn2O4has a better ion transport property than the LiNi0.5Mn1.5O4after 100 cycles.

Fig. 6. Cycle performance of LiMn2O4 and LiNi0.5Mn1.5O4 cathode at 50 mA·g-1.

Fig. 7. Cyclic Voltammetry curves of (a) LiMn2O4 and (b) LiNi0.5Mn1.5O4 cathode.

Fig. 8. Electrochemical impedance spectra of the LiMn2O4 and LiNi0.5Mn1.5O4 cathode after 100 cycles at 100 mA· g-1.

To further explore the evolution of the synthesized LiMn2O4and the samples after cycling at 200 mA·g-1, XRD patterns were conducted (Fig. 9a). All diffraction peaks of the synthesized pristine LiMn2O4can be assigned to cubic spinel phase (JCPDS no.88-1026;space group Fd3m(227)),without impurities[38].The main diffraction peaks located at 2θ of 18.64°, 36.14°, 37.80°, 43.94°,48.12°, 58.15°, 63.88°, 67.17°, 75.65° and 76.69° correspond to(111), (311), (222), (400), (331), (511), (440), (531), (533) and(622) crystalline planes, respectively. The diffraction peaks are sharp and clear, demonstrating that the samples are highlycrystallized. In order to determine the phase state after cycling,cells were disassembled after 100 cycles at 200 mA·g-1and the aluminum foils with active material were taken out for testing.In the XRD pattern of the post-cycled sample, there are three diffraction peaks of Al at 2θ of 38.47°,65.09°,and 78.23°,the other diffraction peaks of the sample are the same as those of the pristine LiMn2O4,with slightly lower intensity.The XRD patterns of LiMn2-O4exhibits the synthesized LiMn2O4sample can maintain the original phase after high current cycle and has good structural stability.Similar with the LiMn2O4, the XRD patterns of the synthesized LiNi0.5Mn1.5O4and the samples after cycling at 200 mA·g-1were also conducted (Fig. 9b). All the characteristic diffraction peaks can be indexed to cubic spinel LiNi0.5Mn1.5O4(JCPDS data no. 80-2162; space group Fd3m (227)), and no obvious impurities, such as LixNi1-xO and NiO [39,30], show up in the synthesized LiNi0.5-Mn1.5O4. The main diffraction peaks located at 2θ of 18.79°,36.44°, 38.13°, 44.31°, 48.53°, 58.66°, 64.46°, 67.80°, 76.37° and 77.42° correspond to (111), (311), (222), (400), (331), (511),(440), (531), (533) and (622) crystalline planes, respectively. The diffraction peaks in the post-cycled sample are the same as those in the pristine LiNi0.5Mn1.5O4sample, with lower intensity. And there are three impurity diffraction peaks from Al at2θ of 38.47°,65.09°, and 78.23°. The XRD results demonstrate that the synthesized LiNi0.5Mn1.5O4sample has certain structural stability and can maintain its phase structure after cycling at high current.

Fig. 9. XRD patterns of original and post-cycle samples for (a) LiMn2O4 and (b) LiNi0.5Mn1.5O4.

Fig. 10. SEM images of LiMn2O4, (a) precursors, (b) final product, (c) the samples after 100 cycles at 50 mA·g-1 and (d) 200 mA·g-1.

The precursors of LiMn2O4particles (5-10 μm) are uniformly dispersed with a Hexahedron-like morphology(Fig.10a).The synthesized LiMn2O4inherited the morphology of precursors and generated many micropores on the surface due to the high temperature calcination (Fig. 10b). The cycled particles at 50 mA·g-1maintained the morphology of pristine LiMn2O4(Fig. 10c). However, the surface micropores are covered by PVDF and super P conductive additive.The cycled sample at 200 mA·g-1still exhibits hexahedral morphology (Fig. 10d). However, the surface structure collapses, which may be one of the reasons for the capacity fading in the last cycles. The precursor of LiNi0.5Mn1.5O4has a spindle-like morphology with long axis of 10-15 μm and short axis of 5-8 μm (Fig. 11a). Similar with LiMn2O4, the asprepared LiNi0.5Mn1.5O4particles also retains the morphology of precursor, but the surface of the particles was rougher than the precursors (Fig. 11b). The cycled particles at 50 mA·g-1(Fig. 11c)and 200 mA·g-1(Fig. 11d) still keep the shape of the as-prepared LiNi0.5Mn1.5O4, but the structural collapse is severer than that of LiMn2O4. This also causes the as-prepared LiNi0.5Mn1.5O4has a lower capacity retention rate than LiMn2O4.

Fig. 11. SEM images of LiNi0.5Mn1.5O4, (a) precursors, (b) final product, (c) the samples after 100 cycles at 50 mA·g-1 and (d) 200 mA·g-1.

4. Conclusions

In conclusion, the uniform Hexahedron-like LiMn2O4and spindle-like LiNi0.5Mn1.5O4has been prepared through a hydrothermal and post-calcination process. The XRD and SEM results of the cycled LiMn2O4and LiNi0.5Mn1.5O4show that both materials exhibit good morphology and structure stability during cycling. As cathode materials for Li-ion batteries, the LiNi0.5Mn1.5O4delivered a high discharge capacity of 135.04 mA·h·g-1at the low current density of 20 mA·g-1, which is close to the theoretical capacity of LiNi0.5Mn1.5O4(147 mA·h·g-1).The discharge capacity of LiMn2O4is 131.08 mA·h·g-1at a low current density of 20 mA·g-1. LiNi0.5Mn1.5O4remains a discharge capacity of 94.34 mA·h·g-1after 50 cycles at the low current density of 20 mA g-1, corresponding to a capacity retention of 75.4%. This work provides a guidance for hydrothermal synthesis of high efficiency cathode materials in lithium-ion battery.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Project supported by the National Natural Science Foundation of China (52022109 and 51834008), Beijing Municipal Natural Science Foundation (2202047), Science Foundation of China University of Petroleum, Beijing (2462018YJRC041 and 2462020YXZZ016), and the Opening Project of State Key Laboratory of Advanced Chemical Power Sources (SKL-ACPS-C-20).