Improvement of the High-Performance Al-Doped LiNi1/3Co1/3Mn1/3O2 Cathode Material for New Electro-Optical Conversion Devices

The ternary cathode material LiNi1/3Co1/3Mn1/3O2 has been extensively focused on as the power sources for new electro-optical conversion devices and lithium-ion batteries. To improve the electrochemical performance, Al doping is one of the effective strategies. Based on the density functional theory of first-principles, the band gap, volume, partial density of states, lithiation formation energy, electron density difference, and electrons’ potential energy of Li1.0-x Al x Ni1/3Co1/3Mn1/3O2 were simulated and analyzed with Materials Studio, Nanodcal and Matlab. Results show that Li0.9Al0.1Ni1/3Co1/3Mn1/3O2 has a better conductivity and cycling capability. The potential energy maps of Li1.0-x Al x Ni1/3Co1/3Mn1/3O2 simulated in Matlab indicate that the rate capability of LiNi1/3Co1/3Mn1/3O2 is promoted after Al doping. Our theoretical advice could be an important choice for the power application of new optoelectronic devices. In addition, our methods could provide some theoretical guidance for the subsequent electrochemical performance investigations on doping of optoelectronic devices or lithium-ion battery materials.


INTRODUCTION
In recent years, rechargeable lithium-ion batteries (LIBs) are the leading power sources for new electro-optical conversion devices, portable electronic devices, electric vehicles and hybrid electric vehicles for their less pollution, good cycle property, no memory effect, high energy density, and high specific capacity at high voltage (4.5 V) [1]. In fact, the specific capacity of commercial cathode materials of LIBs is far lower than that of the anode. With the growing demand for the increasing energy and power densities, conventional cathode materials such as LiCO 2 and spinel LiMn 2 O 4 are not satisfied with the new generation power sources. Moreover, the cathode's cost is much higher than that of the anode. Therefore, it is a major challenge to pursuit the appropriate cathode material for the power module of electro-optical conversion devices or LIBs.
Nowadays, the layered ternary lithium nickel-cobalt-manganese oxide has been well studied and widely applied into LIBs and the power module of optoelectronic devices for their lower price, good cycle performance and high thermal stability [2]. Among various ternary cathode materials, LiNi 1/3 -Co 1/3 Mn 1/3 O 2 , whose structure likes LiCoO 2 with α-NaFeO 2 -type, is extensively investigated owing to high reversible capacity, low cost and enhanced thermal stability [3], and it is considered as an attractive candidate of cathode material for LIBs. Its precursors are synthesized by solid state, co-precipitation, sol-gel, hydrothermal synthesis, combustion, and chemical solution [4]. However, its drawbacks, such as low electronic conductivity, poor cycling performance at the high rate, and phase deterioration during the charging/discharging process, have hindered seriously its practical application [5].
To improve the electrochemical properties of LiNi 1/3 Co 1/3 Mn 1/3 O 2 , many useful strategies, for instance, novel synthesis method [6,7], morphology control [8,9], composite cathode [10,11], surface modification [12,13], and doping [14][15][16], had been carried out experimentally. Shao Z C et al [14] reported LiNi 1/3 Co 1/3-x Mn 1/3 O 2 doped with Al 2 O 3 has the enhanced electrochemical properties when x was 5%. Also, Mg-doped (Zhu JP et al [15]) and Na-doped (Li YH et al [16]) cathode materials can keep the crystal structure stable with least capacity loss, their cycling stability and conductivity can be improved much comparison with their pristines. Al 3+ has the similar outer shell and ionic radius as Mg 2+ and Na + , hence, Al doping has aroused more attention to improve the electrochemical performance of LiNi 1/3 Co 1/3 Mn 1/3 O 2 . Kim S et al [17] found that residual Al in Li [Ni 1/3 Mn 1/3 Co 1/3 ]Al x O 2 has an adverse effect on capacity and cycle ability when x > 0.05%. Zhang ZH et al [18] [20] prepared the LiNi 1/3 Co 1/3-x Al x Mn 1/3 O 2 and employed hollow 3D-birdnest-shaped MnO 2 to provide a large amount of free space, measurements revealed this Al-doped cathode material has an outstanding cyclic performance and capacity. Accordingly, the right doping amount of Al 3+ in LiNi 1/3 Co 1/3 Mn 1/3 O 2 can effectively ameliorate the stability of materials during charging/discharging and enhance the electrochemical performance.
To investigate the physical diffusion mechanics of electrons and Li-ions in the crystal lattice, the density functional theory (DFT) based on first-principles is widely employed [21][22][23]. In this work, Al 3+ as the doping ion has substituted for Li in LiNi 1/3 Co 1/3 Mn 1/3 O 2 , and Li 1-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 had been theoretically simulated and calculated based on DFT by Materials Studio, Nanodcal and Matlab. The more details about the first-principles and DFT were introduced in my previous work [24,25]. The simulations and calculations indicate that Al-doped LiNi 1/3 Co 1/3 Mn 1/3 O 2 has a better electrochemical performance. Our findings can give some theoretical advice about studies of the power module for new electro-optical conversion devices and investigations on LIBs; methods we presented can shorten greatly the whole period of experiments or investigations and reduce the experimental cost [26].

METHODS AND MODEL
Using the exchange-correlation potentials with the generalized gradient approximation [27] of the Perdew-Burke-Ernzerhof [28], calculations about the electronic conductivity of Al-doped LiNi 1/3 Co 1/3 Mn 1/3 O 2 were carried out by CAmbridge Serial Total Energy Package (CASTEP) of Materials Studio 8.0, which the plane wave pseudopotential method is used. The interaction between electrons and ions is described by the projector-augmented-wave method [29]. The ultrasoft pseudopotential is used to depict the Coulombic attraction potential between the inner layer electrons around the nucleus and those of the outer layer. All parameters involved in the calculations, including a plane wave cutoff, k-points in the Monkhorst-Pack scheme, the self-consistency energy tolerance, the maximum stress tolerance, the maximum displacement tolerance, and the average force on every atom, were set as same as those in our previous work [24,25]. The structure geometry should be optimized firstly before calculations.

Band Gap and Partial Density of States
The conductivity is determined by the band gap of materials, the wider band gap means the worst conductivity. The band gaps of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 were calculated when Al doping amount x 0.01, 0.02, 0.03, ......, 0.13 mol. All band gap values at every x mol are plotted in Figure 2. After Al doping, the band structure of Al-doped LiNi 1/3 Co 1/3 Mn 1/3 O 2 can keep stable. The substitution of bigger Al atoms can widen Li-O layers which provides many tunnels to help electrons to immigrate more easily, thus, band gap values have decreased obviously, which means band gaps are narrower. According to Figure 2, between x 0.04 mol and x 0.05 mol, the decreasing tendency pauses, and Factually, from x 0.10 mol to x 0.12 mol, the band gap remains lowly, and the conductivity keeps excellent. The peak of the partial density of state (PDOS) reflects electrons at this level, which directly demonstrates the conductivity. Herein, PDOS of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 was implemented. Figure 3 shows its PDOS when x 0, 0.05, 0.10 and 0.13 mol, respectively. The colored lines in Figure 3 represent the density of different orbital states. In Figure 3A, the peak of PDOS is about 168 eV, clearly describing the bonding and density of states near the Fermi level. When 0 < x < 0.05 mol, the peak of PDOS increases continually, and the conductivity has been enhanced substantially. In Figure 3B, at x 0.05 mol, the peak goes up to 519 eV which is several times higher than that of the pristine, and the conductivity has been enhanced dramatically. When x 0.06-0.09 mol, the peak of PDOS increases slightly, the conductivity has been enhanced further with the increasing x. When x 0.10 mol (shown in Figure 3C), the peak of PDOS is a maximum around 644 eV, which shows the best conductivity. When x > 0.10 mol, the peak goes down quickly. At x 0.13 mol (shown in Figure 3D), the peak goes closely that of the pristine, which indicates too much Al-doping amount will not be useful to the high-performance of conductivity. Considering the results of PDOS, the right Aldoping amount should be controlled within x 0.06-0.10 mol.   When x < 0.12 mol, the volumes can keep stable basically, which is consistent to the band gap. At x 0.04 mol, the slight disorder of Ni 2+ /Li + cation mixing perhaps lead to a slight volume expanding. When x > 0.12 mol, the volume has expanded distinctly, and the structure is instable which is caused by structure transition from layer-to-spinel. In other word, when x < 0.12 mol, Al doping can stabilize the layered crystal structure and keep good cycling performances.

Cell Volume and Lithiation Formation Energy
In general, the difficulty of the lithiation/delithiation process can be reflected by the formation energy. If the formation energy of metal oxide is low, atoms can be separated easily from the crystal lattice. The equation of lithiation formation energy E is the same as referred in my previous work [25]. Figure 4 plots the formation energy E of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 varying with different x. According to Figure 4, E goes down straight when x < 0.12 mol, and there is minimum at x 0.11 mol which the rate capability of material is best; when x > 0.12 mol, the curve increases gradually, electrons and Li-ions will be apart difficultly from the lattice crystal. Hence, analyses of Figure 4 states explicitly that too much Al doping will be harmful to the lithiation/delithiation process, the proper Al doping amount is within x < 0.11 mol.

Electron Density Difference
To investigate electrons' distribution near local atoms, we simulated the electron density difference of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 . Figure 5 shows the simulations at x 0, 0.06, 0.10, and 0.11 mol. Some blocks with heavy colors are reflections of atoms. In comparison with that of the pristine (shown in Figure 5A), when 0 < x < 0.06 mol, the coverage and color of the electron cloud have changed a little, which means electrons near local atoms have not increased much; when x 0.06 mol (shown in Figure 5B), its coverage has distinguished from the before, but its color has still remained; when x > 0.08 mol, its color has turned into orange which means electrons have increased enormously, and its coverage has expanded further; especially, at x 0.10 mol (shown in Figure 5C), its color is still orange, and its coverage is biggest, which exhibits that there are abundant of free electrons around atoms, and Li 0.9 Al 0.1 Ni 1/3 Co 1/3 Mn 1/3 O 2 has superior conductivity; when x 0.11 mol (shown in Figure 5D), its coverage has shrunk greatly, free electrons near local atoms have decreased significantly, and its conductivity has become poor. Therefore, the appropriate doping amount is x 0.08-0.10 mol which Al doping can notably boost the conductivity.

Potential Energy of Electrons
To study the electrons' transfer and rate capability of LiNi 1/3 Co 1/3 -Mn 1/3 O 2 after Al doping, their electrons' potential energy had been FIGURE 4 | Diagram about the relationship between E and x. The curve decreases until x < 0.12 mol, which demonstrates the lithiation/delithiation process can benefit from the appropriate Al doping amount. From x 0.11 mol, E begins to get bigger slightly. The small error bars mean our highaccuracy calculations. 0.06 mol, the color has not changed, and the coverage of electron cloud has expanded significantly, which suggests the conductivity has been enhanced due to more free electrons around atoms. (C) When x 0.10 mol, the color of electron cloud gets orange, and the electron cloud's coverage is biggest, which indicates its conductivity is promoted extremely. (D) When x 0.11 mol, its color has not changed, and its coverage has shrunk much.
Frontiers in Physics | www.frontiersin.org August 2021 | Volume 9 | Article 731851 mapped. If electrons in a potential well can obtain some external energy, they can transport freely from the potential well. Figure 6 shows the 3D potential energy map of Li 0.9 Al 0.1 Ni 1/3 Co 1/3 Mn 1/3 O 2 . From Figure 6, the potential barrier and well are regularly in turn, which indicates layered Li 0.9 Al 0.1 Ni 1/3 Co 1/3 Mn 1/3 O 2 has not occurred the phase transition during the charging/discharging process. The right Al-doping amount can remain its layered structure invariantly.
To analyze the transfer of electrons in potential well after Al doping, diffusion paths were implemented in 2D potential energy image. Figure 7 shows the electrons' diffusion paths of Li 0.9 Al 0.1 Ni 1/3 Co 1/3 Mn 1/3 O 2 . Electrons will immigrate freely along the route marked blue "*", where is the minimum potential energy and numerous channels to diffuse are offered. And the energy barrier of Li-ion insertion/extraction is reduced in the crystal lattice. Consequently, electrons and Li-ions can be  removed and transfer to other places with lower energy barrier. In Figure 7, the potential energy of Li 0.9 Al 0.1 Ni 1/3 Co 1/3 Mn 1/3 O 2 is from around 80 to 0 eV. When x < 0.11 mol, the minimum potential energy of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 decreases with rising x, and electrons can be apart from the potential well more facilely. Thus, this new material has excellent rate capacity and electrochemical performances when x < 0.11 mol.

CONCLUSION
The physical mechanism of enhanced electrochemical properties for Al-doped LiNi 1/3 Co 1/3 Mn 1/3 O 2 was investigated by DFT. After Al doping, Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 has a layered structural stability when x < 0.12 mol; the band gap has a minimum at x 0.11 mol, and the conductivity is best; the peak of PDOS remains highly within x 0.06-0.10 mol, which electrons are multiplied than the pristine, and its conductivity is enhanced dramatically; the lithiation formation energy E is lowest at x 0.11 mol, and electrons and Li-ions can be separated easily within x < 0.12 mol; based on the simulations of the electron density difference, Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 has a better conductivity when x 0.08 -0.10 mol; and electrons' potential barrier is decreasing with rising x, electrons and Liions can be removed and diffused quickly, which means its rate capability is improved effectively. Considering all above calculations and analyses, the electrochemical performance of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 is best at x 0.10 mol. Up to now, it is few reported about the experimental investigations on Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 . We believe that samples of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 can be prepared experimentally by traditional syntheses, and suggest that its superior electrochemical performances of Li 0.9 Al 0.1 Ni 1/3 Co 1/3 Mn 1/3 O 2 will be verified experimentally by physical and chemical tests. Moreover, the electrochemical performance of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 can be improved further combining with other modifications. This study provides an insight to understand the physical improvement mechanism of Al-doped LiNi 1/3 Co 1/3 Mn 1/3 O 2 . Our results and theoretical advice based on DFT could be important for the investigations of Li 1.0-x Al x Ni 1/3 Co 1/3 Mn 1/3 O 2 , doping materials studies about the power sources of new electrooptical conversion devices, and applications in LIBs. Our simulations and calculations have concerned only on the conductivity, cycling and rate capability. Certainly, Al-doped LiNi 1/3 Co 1/3 Mn 1/3 O 2 can be further improved its energy density and reversible charge capacity by structural optimization, coating, and composite etc.

DATA AVAILABILITY STATEMENT
The original contribution presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
YG designed models, analyzed results, and wrote the manuscript. WY carried out calculations. XD gave some proposals.