Nano-micron composite lithium-rich cathode materials prepared by oxalic acid one-step method

Lithium-rich material (SOP) with special nano-micron composite structure is synthesized by simple one-step oxalic acid method. The SOP sample is made of micron particles (1–3 μm) and nano-particles (~ 100 nm). Compared with the co-precipitation prepared lithium-rich materials (COP), SOP has good rate performance and cycle stability. The specific discharge capacity of SOP sample reaches 104.2 mAh/g at 10 C, while that of COP sample is only 68.5 mAh/g. After 300 cycles, SOP still has a specific discharge capacity of 201 mAh/g and the capacity retention rate is 89.6%, while COP only has a specific discharge capacity of 135.8 mAh/g and the capacity retention rate is 58%. Structure allocation with nano-micron particles is conducive to electrochemical performance improvement. This study provides a new idea for the synthesis of lithium-rich material with better performance.


Introduction
In recent years, environmental pollution problems and the rising oil price trend have stimulated the rapid development of small electric vehicles and plug-in rechargeable hybrid electric vehicles [1][2][3].Lithium ion batteries (LIBs) have broad application prospects in portable electronic devices, electric vehicles, energy storage batteries, and other fields, due to their high energy density, long cycle life, and other advantages [4][5][6].As the main power battery of new energy vehicles, the quality of LIBs significantly affects its comprehensive performance and determines the quality of new energy vehicles.Traditional LIBs, such as LiCoO 2 , LiMn 2 O 4 , and LiFePO 4 , are faced with low energy density, which cannot meet the demand for high energy density in new energy industry.Researchers all over the world are committed to the development of high-performance LIBs for transportation applications.LIBs are widely considered as one of the most promising high-performance battery materials, with specific capacities up to 250 mAh/g [7][8][9][10].However, due to electrolyte breakdown caused by lithium ions and stress-induced structure collapse, serious surface damage can occur, leading to the decrease of voltage in the cycling process.It seriously restricts the application of Lirich layered oxides as the cathode material of LIBs [11,12].
In order to improve the cycle stability of lithium-rich layered oxide cathode materials, most research groups are committed to modifying the particle surface with metal oxide (ZnO), metal fluoride (AlF 3 ), and metal phosphate (AlPO 4 ) [13] present in the electrolyte during the cycle to limit the dissolution of metal ions on the surface.However, all of these efforts can only improve performance to some extent [14,15].However, serious surface damage can occur due to electrolyte decomposition and stress-induced structural collapse caused by lithium ions.This phenomenon is widespread, resulting in a gradual drop in voltage and a steady decline in capacity during the cycle, which seriously restricts the application of lithium-rich layered oxides as cathode materials [12].It is well known that crystal structure and morphology are two key factors affecting the electrochemical performance of cathode materials [16].During the cycling process, changes in structure and morphology lead to the degradation of electrochemical properties, especially the cycling stability.
Therefore, a Li-rich cathode material with nano-micron composite structure was prepared by simple oxalic acid one-step method for the first time, which can effectively avoid the above problems.The purpose of this paper is to synthesize Li-rich ternary cathode material with higher specific capacity, more stable cycling performance and better rate performance.

Experimental procedure
In this work, we synthesized a lithium-rich cathode material (SOP) with nano-micron composite structure using oxalic acid in one step.Commercially available reagents were used as raw materials, including lithium nitrate (LiNO  and 126 mmol of LiNO 3 , 54 mmol of Mn(NO 3 ) 2 , 50% W/W aqueous solution, and 13 mmol of Ni(NO 3 ) 2 •6H 2 O were dissolved in 60 g of absolute ethyl alcohol, recorded as solution (B).Then, the two solutions were slowly injected into the reactor, while the ultrasonic rod was inserted into the reactor, adjusted the ultrasonic power to 500 W, and then stirred with magnetic force until the two solutions were completely mixed at room temperature.Subsequently, the oxalate precursor with uniform distribution of lithium, nickel, cobalt, and manganese was obtained by heating to 75 °C and the pH was kept at about 7.5 with NH 3 •H 2 O in the reaction process.After that, the oxalate precursor was repeatedly ground into a powder.It was placed in a muffle furnace to preheat at 450 °C for 5 h, and then ground again after natural cooling.Finally, the obtained powder was placed in a tubular furnace which was heated from room temperature to 850 °C and held for 12 h followed by furnace cooling.The final product was obtained and recorded as SOP.As a contrast, coprecipitation method was also used to prepare lithium-rich materials with the same chemical composition, which were recorded as COP [17].
In this experiment, the main phase of the prepared sample was analyzed by X-ray diffraction (XRD) produced by RIGAKU company of Japan, using Cu target as radiation source.The working voltage and current were, respectively, set as 40 kV and 150 mA, and the scattering angle (2θ) was 10°-90°.Scanning electron microscope (SEM, JSM-5600LV) was used to observe the surface morphology, particle size and agglomeration of the prepared samples.High-resolution transmission electron microscopy (HRTEM, FEI G2) equipped with selected area electron diffraction (SAED) was employed to analyze the phase morphology and structure.
In order to study the charge and discharge capacity, cycle performance, and capacity retention of the materials at different charge and discharge rates, the assembled coin cell was tested on LAND CT 2001 A (Wuhan Blue Electronic Equipment Co., LTD.) and the charge and discharge range were 2.0-4.8V.The detailed assemble process of tested battery was as follows: the assembly model used in this experiment was CR2032.With lithium sheet as the negative electrode, Celgard 2400 porous polypropylene membrane as the diaphragm, and l.0-mol/L LiPF 6 /(EC/EMC/DMC V:V:V = 1:1:1) as the electrolyte, the battery assembly was carried out in a protective Ar gas atmosphere.Assembling the battery from bottom to top was based on the sequence as shown in Fig. 1.Subsequently, the assembled battery was sealed using a hydraulic sealing machine.

XRD analysis
Figure 2 shows a comparison of XRD patterns of the two samples.First of all, it can be seen from Fig. 2 that the peak shapes on XRD patterns of the two samples are complete and sharp.In addition to the superlattice diffraction peaks in the range of 20°-25°, the other diffraction peaks are corresponding to the layered α-NaFeO 2 structure belonging to the hexagonal system with R-3 m space group [18][19][20].There are weak diffraction peaks in the dotted box, which is detected due to the ordered arrangement of Li and Mn in the layered LiMnO 2 component in a small range to form a superlattice structure [21,22].Besides, XRD can be also used to indicate the structural characteristics of materials.For lithium-manganese-rich based cathode materials, obvious splitting of (006)/(012) and (018)/(110) is an important feature of layered structure.Many literatures reported that the more obvious the splitting degree is, the better the layered structure is [23][24][25].As shown in Fig. 2, the splitting of (006)/(012) and ( 018)/(110) of SOP is different with that of COP, and SOP has a better layered structure.In addition, the ratio of c/a can also be used to measure the layered structure of the material.If the value of c/a is greater than 4.9, it demonstrates that the material has a cubic closed-packing structure.The larger the c/a ratios are, the larger the inner layer space of the crystal cell will be, and the material will have a wider lithium ion deblocking channel.In general, the peak intensity ratio of I(003)/I(104) can be used to measure the cation mixing degree of the material.It is generally believed that when I(003)/I(104) is greater than 1.2, the cation mixing degree of the material is lower, and the electrochemical performance will be better [26].Cell parameters of the two materials were calculated by the least square refinement method.It is shown in Table 1 that SOP has larger values of I(003)/I(104) and c/a, so SOP has better crystal structure and electrochemical performance compared with COP.   with a diameter of about 5 μm composed of primary particles of about 100-200 nm, while SOP is a nearly spherical secondary particle with a diameter of about 30 μm, which is composed of primary particles of about 1-3 μm and nanoprimary particles of about 100 nm.The structural shape of primary particles in the two samples has great difference, as shown in Fig. 3b and d.SOP with a special nano-micron composite structure has a great influence on its electrochemical performance.On the one hand, larger micron particles can overcome the stress-induced structural collapse caused by lithium ions, reduce the dissolution of Mn ions, and improve the cycle performance.On the other hand, smaller nanoparticles can shorten the diffusion path of lithium ion in the de-intercalation process, facilitate the rapid de-intercalation of lithium ions in the cycle process, and improve its rate performance [27].Therefore, SOP can obtain excellent performance on the whole.In addition, it is observed that the surface of SOP sample is smoother than that of COP, which can improve the contact between conductive agent and active substance and then promote charge transfer.Due to the employment of ultrasound in the preparation process, the surface energy of SOP material is changed, which results in the formation of lithium-rich cathode material with special nano-micron composite structure.

TEM analysis
For further observation of the crystal structure and morphology, TEM analysis was carried out.Figure 4a shows HRTEM image of SOP sample.It can be seen that the width of lattice fringes of SOP sample is 0.47 nm, which is consistent with the lattice spacing of (003) of LiMO 2 phase in lithium-rich materials.Two groups of light and dark diffraction spots can be seen from Fig. 4b, in which the dark spots correspond to Li 2 MnO 3 and the bright spots correspond to LiMO 2 .This indicates that SOP sample has a good twophase layered structure.

Electrochemical properties analysis
Figure 5 shows the capacity changes of the two lithium-rich materials at rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C. It can be observed that with the increasing of the rate, the discharge capacity of the two materials has a significant downward trend.Compared with COP, SOP has better rate performance.The specific discharge capacity of SOP is 104.2 mAh/g at 10 C rate, while that of COP is only 68.5 mAh/g, with a difference value of 35.7 mAh/g (increased by 52.1%).The results show that SOP sample has a higher discharge specific capacity at a larger discharge rate, which indicates that the nano-micron composite structure materials synthesized by oxalic acid one-step method are conducive to the rapid de-intercalation of lithium ions.The main reason is that the nanoparticles are small in size and uniform distributed, which greatly reduces the diffusion path of lithium ion in the de-intercalation process [28,29].In addition, the material has good crystal form and smooth surface, which makes good contact between conductive agent and active material, promotes charge transfer, and improves the rate performance of the material.In order to compare the cycle performance of SOP and COP, a battery made of the above-mentioned two materials was tested at a charge-discharge rate of 0.1 C. As shown in Fig. 6, the first discharge specific capacity of COP is 234 mAh/g.After 300 cycles, the discharge specific capacity is 135.8 mAh/g, and the capacity retention rate is 58%.Compared with COP, SOP has higher discharge specific capacity and better cycle stability.The first discharge specific capacity of SOP is 223.4 mAh/g at 0.1-C discharge rate.After 300 cycles, the specific discharge capacity is still over 200 mAh/g and the capacity retention rate is 89.6%.This is mainly attributed to the following reasons.SOP materials have nano-micron composite structure, in which larger micron particles overcome the stress-induced structural collapse caused by lithium ions, reduce the dissolution of Mn ions, and improve its cycle performance.Meanwhile, the nanoscale particles shorten the transmission path of lithium ion, which is conducive to the rapid de-intercalation of lithium ion in the crystal.It also alleviates the impact and damage of lithium ion on the crystal lattice, improves the crystal structure stability, and thus enhances the cycle stability of the materials.
In general, capacitance differential curve is used to study the electrochemical reaction of lithium-manganese-based cathode electrode materials during charging and discharging.Figure 7a shows the capacitance differential capacity peaks after specific cycle of SOP and COP.During the first cycle of charging, there are two relatively obvious oxidation peaks at 4.0 V (peak one) and 4.5 V (peak two), respectively.Peak one corresponds to the first platform for initial charging, mainly due to the release of lithium ions from transition metal oxides, accompanying with the oxidation of Ni 2+ [30][31][32].The stronger the peak one is, the more obvious the oxygen release is.It can be seen from Fig. 7a and c that the intensity of the differential capacity peaks of SOP is obviously lower than that of COP, which is consistent with the relatively long curve platform of the first charging.Peak two corresponds to the second platform of the first charging curve, due to the release of oxygen in the crystal lattice of Li 2 MnO 3 phase and the formation of new active MnO 2 -layered substances after the release of Li 2 O.The absence of oxidation peak in the subsequent cycles suggests that the activation of Li 2 MnO 3 is irreversible.
There are three reduction peaks in the first-cycle discharge process, which are at 3.4 V, 3.8 V, and 4.2 V, respectively.Peak at 4.2 V corresponds to the position where lithium ions are embedded in the lithium layer, while peaks at 3.4 and 3.8 V are corresponding to the position where lithium ions are embedded in the transition metal layer of the crystal material.It can be seen from Fig. 7 that the peak at 4.2 V of SOP is stronger than that of COP, indicating that SOP has a stronger impetus to enter into the lithium layer.With the increase of the cycle number, the redox peak of COP material has a relatively obvious shift, while that of SOP is basically unchanged.It can be concluded that SOP has strong anti-polarization ability and better stability.
It has been reported that the AC impedance diagram of lithium ion battery electrode materials is mainly composed of three parts.The first part is the semi-circular arc in highfrequency region, which reflects the transfer resistance of lithium battery through surface film layer, the capacitance of surface film layer, and the semicircle of interface film impedance.The second part is a semi-circular arc in midfrequency region, which reflects resistance that transmits electric charge.The third part is a straight line located in low-frequency region, which reflects the diffusion ability of lithium ion battery in solid electrode materials, namely Warburg impedance [33].Figure 8 shows the AC impedance spectra of the two materials.The charge-discharge current density is 300 mA/g.The equivalent circuit diagram is constructed for impedance map by z-view simulation analysis software fitting, in which R s is the SEI membrane impedance, R ct is the load transfer impedance, and ZW is the Warburg impedance [34].
It can be seen from Table 2 that the two materials have similar R s and R ct values of both the materials increase slightly, but the difference is not significant.However, the value of R ct changes greatly, and the change degree of COP is much greater than that of SOP.This well explains that SOP has better cycle performance and discharge specific capacity than COP.

Conclusion
In this study, one-step oxalic acid method was used to synthesize the lithium-rich material Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 .Special nano-micron composite structure of the lithium-rich cathode material was obtained.Compared with traditional co-precipitation method, the positive electrode prepared by oxalic acid one-step method has good rate performance and cycle stability due to its special structure.Larger micron particles (1-3 μm) overcome the stress-induced structure collapse caused by lithium ions, reduce the dissolution of Mn ions, and improve its cycle performance.Smaller nanoparticles (~ 100 nm) shorten the diffusion path of lithium ions in the de-intercalation process, which facilitates the rapid de-intercalation of lithium ions in the cycle process and improves its multiplier performance and thus obtained excellent electrochemical performance.The specific discharge capacity of SOP sample reaches 104.2 mAh/g at 10 C, while that of COP sample is only 68.5 mAh/g.After 300 cycles, SOP still has a specific discharge capacity of 201 mAh/g and the capacity retention rate is 89.6%, while COP only has a specific discharge capacity of 135.8 mAh/g and the capacity retention rate is 58%.Therefore, this study provides a new idea for the synthesis and performance optimization of such materials.

FigureFig. 1 Fig. 2
Figure3a-f shows powder morphology of SOP and COP samples.It can be clearly seen that there are secondary particles composed of different primary particles.As shown in Fig.3a and b, COP is a nearly spherical secondary particle

Fig. 3
Fig. 3 SEM images and element distribution of sample SOP (a, b, e) and COP (c, d, f)

Fig. 4 aFig. 5
Fig. 4 a HRTEM image of sample SOP and b corresponding SAED image

Table 1
Structure parameters of samples cop and sop obtained from XRD patterns Samples I(003)/I(104) a (Å) c (Å) c/a

Table 2
Impedance parameters of COP and SOP under different cycles Cycle number 0th 200th Sample R s (ohm) R ct (ohm) R s (ohm) R f (ohm) R ct (ohm)