Dependence of structure and temperature for lithium-rich layered-spinel microspheres cathode material of lithium ion batteries

<jats:title>Abstract</jats:title><jats:p>Homogeneous lithium-rich layered-spinel 0.5Li<jats:sub>2</jats:sub>MnO<jats:sub>3</jats:sub>·0.5LiMn<jats:sub>1/3</jats:sub>Ni<jats:sub>1/3</jats:sub>Co<jats:sub>1/3</jats:sub>O<jats:sub>2</jats:sub> microspheres (~1 μm) are successfully prepared by a solvothermal method and subsequent high-temperature calcinations process. The effects of temperature on the structure and performance of the as-prepared cathode material are systemically studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), galvanostatical charge/discharge and electrochemical impedance spectra. The results show that a spinel Li<jats:sub>4</jats:sub>Mn<jats:sub>5</jats:sub>O<jats:sub>12</jats:sub> component can be controllably introduced into the lithium-rich layered material at 750°C. Besides, it has been found that the obtained layered-spinel cathode material represents excellent electrochemical characteristics. For example, it can deliver a high initial discharge capacity of 289.6 mAh g<jats:sup>−1</jats:sup> between 2.0 V and 4.6 V at a rate of 0.1 C at room temperature and a discharge capacity of 144.9 mAh g<jats:sup>−1</jats:sup> at 5 C and 122.8 mAh g<jats:sup>−1</jats:sup> even at 10 C. In addition, the retention of the capacity is still as high as 88% after 200 cycles, while only 79.9% for the single-phase layered material. The excellent electrochemical performance of the as-prepared cathode material can probably be attributed to the hybrid structures combining a fast Li-ion diffusion rate of 3D spinel Li<jats:sub>4</jats:sub>Mn<jats:sub>5</jats:sub>O<jats:sub>12</jats:sub> phase and a high capacity of the layered Li-Mn-Ni-Co-O component.</jats:p>

section_content: greatly improved first-cycle efficiency (up to 90%) compared with that of the layered material (71%). Besides, Wu 11 also reported lithium-rich layered cathode material encapsulated with nano-sized spinel LiNi x Mn 22x O 4 or Li 11x MnO 4 membrane for high-energy and high-power lithium-ion batteries. Obviously, lithium-rich cathode materials with layered-spinel hybrid structures are currently becoming the research hotspots. However, it is worth thinking how to integrate the controlled spinel components into the layered materials and what kind of spinel materials will maximally improve the comprehensive performance of composite materials. 

Recently, our group has designed and synthesized lithium-rich layered materials with improved electrochemical performances [12] [13] [14]. Based on our previous works, herein, we report a homogeneous lithium-rich layered Li-Mn-Ni-Co-O microsphere combined with a new spinel Li 4 Mn 5 O 12 component via a solvothermal process and subsequent simple heat treatment. As the the Li 4 Mn 5 O 12 spinel has been prepared at a low temperature 15, 16 and it has a variable structure 17 which will show a change of structure from Li 4 Mn 5 O 12 phase to Li 2 MnO 3 phase when the temperature was raised above 700uC 18. Therefore this lithium-rich spinel Li 4 Mn 5 O 12, which has a three-dimensional (3D) interstitial space permiting fast diffusion of lithium ions and stable tetravalent manganese 19, was controllable introduced into the lithium-rich layered cathode material for the first time. We hope that this hybrid cathode material can maximize the high capacity of the layered component and inherent advantages of the 3D Li 1 insertion/extraction framework of the spinel component. However, we have found that during the calcinations process the temperature of solid phase reaction will dramatically influence the performance of the material. Thus, in this paper, the effects of preparation temperature on the structure and performance of the asprepared cathode materials were systemically studied.

section_content: The typical surfaces and morphologies of the carbonate precursor and corresponding cathode materials prepared under the different calcination temperatures are examined by SEM, as shown in Fig. 1 and Fig. 2, respectively. Fig. 1 shows that the carbonate precursor particles obtained from the solvothermal process are spherical morphology, and the shape of the particle is regular and the size is homogeneous. In addition, the average diameter of the precursor particles is about 1 mm. Besides, as shown in Fig. 2, after the carbonate precursors were calcined at high temperatures with Li 2 CO 3, the resultant cathode materials still maintained a good spherical morphology like precursor particles. In addition, each of the spherical cathode particles was made up of a large number of primary grains. A close observation shows that the primary grains of the samples are strongly influenced by the calcinations temperatures. It can be seen from Fig. 2 (a) that the sample calcined at 700uC (denoted by D700 ) has a clear contour of cubic primary particles with about 250 nm in size which accumulate closely on the surface. When the calcinations temperature was above 700uC, the obtained materials displayed a peeling pomegranate appearance and became obviously smaller size than D700 sample. In addition, the granular shapes of the primary particles slightly protrude toward the outside of the secondary structure of particles along with the temperature increasing. 

The X-ray diffraction (XRD) patterns of as-prepared spherical cathode materials calcined at various temperatures are shown in Fig. 3. All diffraction peaks are quite narrow and sharp, which indicates a high crystallinity of the cathode materials. It can be seen from the Fig. 3 the XRD patterns of the samples calcined at 750uC and 800uC present a layered (R3m and C2/m) and spinel hybrid structure. The spinel peaks can be well indexed to cubic Li 4 Mn 5 O 12 with space group of Fd3m (PDF#46-0810) marked as plum blossom pattern in Fig. 3. Additionally, it can be clearly seen that the diffraction peaks of the monoclinic Li 2 MnO 3 (C2/m) between 20u and 25u become more clear accompanied with the increasing amounts of the layered phase (from D750 to D800). When the calcinations temperature increases, the D900 and D850 samples show a typical a-NaFeO 2 layered structure (space group R3m), excepting the monoclinic Li 2 MnO 3 character (space group C2/m) with some weak peaks between 20u and 25u. Particularly, the clear splitting of the (006)/(012) and (018)/ (110) peaks indicates that the two samples have a well-organized layered structure 20. The XRD patterns of the D900 and D850 samples are also consistent with other lithium-rich layered cathode materials 21. When the calcination temperature further decreased to 700uC, the obtained D700 sample shows a hybrid structure between spinel Li 4 Mn 5 O 12 (Fd3m) and layered phases (R3m). Surprisingly, the weak peaks of the Li 2 MnO 3 (C2/m) between 20u and 25u disappear for the D700 sample. The above results demonstrate that the annealing of Li 4 Mn 5 O 12 at the temperature above 700uC leads to existence of monoclinic Li 2 MnO 3, which is in consistence with the previous reports [16] [17] [18] [19]. In other words, it suggests that the formation of spinel phase can be simply controlled by calcinations temperature, which provides an effective approach for design and development of highperformance cathode materials. As both of D750 and D800 have the two phases (layered and spinel), the ratio of the two phases has been calculated by XRD analysis approach to distinguish the two phases, and the result is showed in Table 1. It can be found that when the temperature is controlled at 700uC, there is no layered phases. When the temperature rises from 700 to 750uC, 16. 9% of spinel phase will be replaced by the layered phase, and while temperature rises from 750 to 800uC, 58. 91% of the spinel phase will be replaced by the layered phase. Further, when the temperature increases to 850 or 900uC, the spinel phase will disappear and the samples show a typical a-NaFeO 2 layered structure with monoclinic Li 2 MnO 3 structure (C2/m). 

The microstructures of the samples are further characterized by high-resolution transmission electron microscopy (HRTEM). The HRTEM images and corresponding fast Fourier transformation (FFT) of the D700 sample is shown in Fig. 4(a), and it can be seen As a consequence, when the calcinations temperature is controlled at 750uC and 800uC, the as-prepared materials show a combination structure of lithium-rich layered and spinel Li 4 Mn 5 O 12 phases. However, when the samples are calcined below 750uC, the as-prepared materials show only a spinel Li 4 Mn 5 O 12 structure. Moreover, the as-prepared materials just show a pure lithium-rich layered structure when calcined above 800uC. These consequences are in good agreement with the XRD results in Fig. 3. 

To compare the electrochemical performance of the as-prepared materials, coin cell tests are performed using the D700, D750, D800, D850 and D900 samples as the cathode materials and Li metal as a counter electrode. while the long plateau,4. 5 V is consistent with the removal of lithium ions and oxygen from the Li 2 MnO 3 component 23, 24. In addition, the D700 sample also shows a short plateau around 4. 5 V, which indicates that a small amount of Li 4 Mn 5 O 12 had been decomposed into monoclinic Li 2 MnO 3 when calcined at 700uC 22. As shown in Fig. 5 The rate capabilities of these cathode materials are shown in Fig. 5(b). Compared D700 with D750, it can be found that the rate performance is well improved when the calcinations temperature increased to 750uC. The D750 cell delivers a discharge capacity of 254. 8 mAh g 21 at 0. 5 C, 144. 9 mAh g 21 at 5 C and 122. 8 mAh g 21 even at 10 C. However, the discharge capacity of D700 material at 0. 5 C, 5 C and 10 C are only 180. 2 mAh g 21, 60. 7 mAh g 21 and 33. 2 mAh g 21, respectively. Besides, the D800, D850 and D900 have a better rate performance than D700 but doesn't appear as much improvement as D750 materials. Although the D750 and D800 both have the lithium-rich layered and spinel Li 4 Mn 5 O 12 phases, the rate performance is improved more obviously when the calcinations temperature is controlled at 750uC. 

The cycling performance of the five samples is shown in Fig. 6(a). The spinel samples of D700 exhibits the highest capacity retention of 88. 9% with the discharge capacity of 165 mAh g 21 at 0. 5 C after 200 cycles. Meanwhile, the discharge capacity of D750 reaches 223. 6 mAh g 21 at 0. 5 C after 200 cycles between 2. 0 and 4. 6 V with a capacity retention of 88%, which has been significantly improved with the introduction of spinel structure 25. The discharge capacity of D800 decreases to 200. 9 mAh g 21, and the capacity retentions is 86. 5% after 200 cycles. Whereas, the discharge capacities of D850 and D900 are just 172. 1 and 159. 7 mAh g 21 with the capacity retentions 79. 9% and 77. 9%,respectively. Table 2 summarizes the battery performance of the samples prepared at different temperatures. It can be clearly seen that the mixed phase structure materials (D750 and D800) show relatively high cycling performance compared with those (D850 and D900) with layered structure, which is attributed to the introduction of the spinel structure. 

To explain the different electrochemical properties of the samples prepared at different temperatures, electrochemical impedance spectroscopy (EIS) is carried out after being charged to 4. 6 V. The corresponding Nyquist plots are given in Fig. 6 (b ). It can be seen that each plot exhibits two semicircles and one slope line. Usually, the first semicircle in the high frequency region is associated with the resistance (R sf ) due to lithium ion diffusion in the surface layer (including SEI layer and surface modification layer) 26, the second semicircle in the medium-to-low frequency region is related to the charge transfer reaction between the surface film and the active cathode mass 27, and the slop line in the low frequency region corresponds to lithium ion diffusion in the material 28. As the intercept of the first semicircle with the real axis (Z9) refers to the ohmic resistance and the diameter of the second semicircle represents the charge transfer resistance (R ct ) 29, it can be found that the five samples show approximately ohmic resistance but different charge transfer resistances. The charge transfer resistances of D700, D750 and D800 get somewhat lower compared with D900 and D850. In addition, it can be seen from Fig. 7 that the samples of D800 and D750 shows a anodic peak at about 2. 8 V probably identified as Mn 41 /Mn 31 redox couple, while the cathode peaks at about 2. 6 V match along with Mn 41 /Mn 31 redox. The reaction at,2. 8 V is associated with the transformation of cubic to tetragonal phase during the extraction of lithium ions from empty 16c octahedral site of the cubic spinel structure 30. Both of D850 and D900 show an irreversible oxidation peak at,4. 5 V which is correspond to the net loss of Li 2 O and another peak at,4. 0 V which is roughly correspond to the expected value for Ni 21 /Ni 41 reaction. The peak at,3. 3 V indicates that the lithium ion will insert into the MnO 2 which is derived from Li 2 MnO 3 at 3. 3 V (Mn 41 to Mn 31 ) 20. 

GITT is considered to be a reliable method to determine the diffusion coefficient of lithium ions (D Li ) with greater accuracy for compounds with varying composition or voltage, which has been extensively used to calculate the value of D Li in electrode materials 31, 32. The lithium ion diffusion coefficient can be calculated by using the following equation 33. 

where V m is the molar volume of Li 1. 5 Ni 0. 2 Co 0. 2 Mn 0. 6 O 2, which is 20. 42 cm 3 mol 21 deduced from the crystallographic data. S is the active surface area of the electrode, and L is the thickness of the electrode 34. To relax the cell voltage to the steady state, a small current flux and a long time interval should be used for the GITT experiment. In the five samples, both D750 and D800 show the layerspinel structure, so it is necessary to compare the GITT test results of the two samples. Fig. 8 (a) and (b) show the GITT curves of D750 and D800 samples at room temperature, the cell was subjected to charge-discharge at 0. 05 C in the voltage window of 2. 0-4. 6 V. Then the cell was charged at a constant current C/10 rate (1 C was taken as 200 mAh g 21 ) for an interval t of 10 min followed by an open circuit stand for 1 h to allow the cell voltage to relax to its steady-state value, E s. The procedure was repeated for the voltage window of operation 2. 0-4. 6 V. It can be seen that the cell voltage stabilizes to a stable value after the 1 h opencircuit stand after each current flux. At the same time, it can be found that the discharging time of D750 is much longer than the others, which indicates that the voltage drops more slowly during the discharge process and this is accord with the result of charge-discharge test. From Fig. 8(c ) it can be seen that variation of dE/dx about the two samples following the x is obvious, in the process of de-insertion the lithium, along with the increasing of x the value of dE/dx gradually rise, but the dE/dx of D800 reaches a maximum and then drops with x 5 0. 63, this phenomenon occurs more early than the samples of D750, this is a strong evidence that the samples can release more capacity during the discharge. Fig. 8(d) shows the D Li values of D750 and D800 samples, it is found that the D Li values of D750 and D800 are from 4. 0 3 10 211 to 8. 0 3 10 211 cm 2 s 21 and from 2. 0 3 10 211 to 4. 0 3 10 211 cm 2 s 21, respectively. It is obvious that the the D Li values of D750 is slightly higher than that of D800 sample, therefore the improvement of the lithium-ion diffusion coefficient can also be an explanation of the greatly improved electrochemistry performance. 

Finally, it should be also pointed out that despite of many studies on this cathode material have been conducting so far, further intensive studies are still necessary. In this paper, the uniform microspheres with layered-spinel composite structure have been synthesized and show an excellent electrochemical performance. Wang' Group 35 has also reported an layered Li[Li 0. 2 Mn 0. 54 Ni 0. 13 Co 0. 13 ]O 2 -spinel LiMn 1. 5 Ti 0. 5 O 4 composite cathode material, but the particle-size of the samples was not uniform and the initial discharge capacity is just 220 mAh g 21, which is apparently much lower than the herein 289. 6 mAh g 21. In addition, a spinel-layered Li-rich Li-Mn-Co-O material synthesized by Li' Group 36 showed a uniform microspheres morphology, however the capacity retention at 50 cycles was reported. 

In summary, lithium-rich layered-spinel composite cathode materials were synthesized via a solvothermal process followed by calcinations at 750uC. The calcination temperature will critically influence the crystal structure of the obtained cathode material. When the calcination temperature is equal to 700uC, the obtained material shows only a spinel structure; while the calcination temperature is above 850uC, the as-prepared material shows just a lithium-rich layered structure. Besides, the introduction of the spinel structure into the layered lithium-rich cathode materials can effectively improve the electrochemical performance of lithium-rich layered cathode material. Both of the materials obtained at 800 and 750uC show the two phases with different ratios, and we further compared the electrochemical performance of the two samples. The hybrid structure cathode material obtained at 750uC can deliver an initial discharge capacity of 289. 6 mAh g 21 between 2. 0 V and Next, the sealed Teflon ware was put into a stainless autoclave, which was hold at 200uC for 20 h, followed by cooling at the room temperature to obtain purple precipitates. The precipitates were centrifugalized and washed by ethanol several times, the residue was dried at 60uC. Finally, the as-produced carbonate precursor was preheated at 500uC for 6 h, and mixed with Li 2 CO 3, then calcined at 700uC, 750uC, 800uC, 850uC, 900uC for 10 h in air respectively, to form the spherical Li-rich cathode materials. To distinguish the products preferably, five kinds of compounds have been named as D700, D750, D800, D850, and D900, which correspond to reaction temperatures of 700uC, 750uC, 800uC, 850uC, and 900uC, respectively. 

Characterizations and Measurements. The structures of samples were characterized by X-ray diffraction on a Rigaku D/MAX-2500 powder diffractometer at 40 kV and 100 mA using a graphite monochromatic and Cu-Ka radiation (k 5 0. 15418 nm) operated with a scan rate of 2u min 21 in the 2h range of 10u-80u. The morphologies were investigated by a JEOL JSM-6610LV scanning electron microscope. 

Transmission electron microscope JEOL JEM-2100F with accelerating voltage of 200 kV was used in order to obtain TEM images. The cathodes for testing cells were fabricated by mixing the cathode materials, carbon black, and polyvinylidene fluoride (PVDF) binder with a weight ratio of 80510510 in N-methyl pyrrolidinone, which were then pasted on aluminum foil followed by drying under vacuum at 110uC for 24 h. The testing cells were assembled with the cathodes thus fabricated, metallic lithium anode, Celgard 2300 film separator, and 1 M LiPF 6 in 151 ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. The assembly of the cells was carried out in an argon-filled glove box, where water and oxygen concentration were kept less than 5 ppm. All the cells were allowed to age for 12 h before testing. Charge-discharge measurements were performed at room temperature under different rates in a voltage range of 2. 0-4. 6 V on the Neware battery test system. 

The galvanostatic intermittent titration technique (GITT) was performed at room temperature under a small current flux of 0. 1 C and a long time interval of 60 min in a voltage range of 2. 0-4. 6 V on the Neware battery test system.
section_name: Results and Discussion

section_name: Acknowledgments

section_content: This work was financially funded by the National Natural Science Foundation of China under project No. 51272221, Transformation Fund of Major Scientific and Technological Achievements of Hunan Province under project No. 2012CK1006, Key Project of Strategic New Industry of Hunan Province under project No. 2013GK4018, and Industrialization Cultivation Project of Colleges and Universities in Hunan Province under project no. 13CY004.
section_name: Acknowledgments

section_name: Author contributions

section_content: Competing financial interests: The authors declare no competing financial interests.
section_name: Additional information