Li2ZrO3-Coated Monocrystalline LiAl0.06Mn1.94O4 Particles as Cathode Materials for Lithium-Ion Batteries

Li2ZrO3-coated and Al-doped micro-sized monocrystalline LiMn2O4 powder is synthesized through solid-state reaction, and the electrochemical performance is investigated as cathode materials for lithium-ion batteries. It is found that Li2ZrO3-coated LiAl0.06Mn1.94O4 delivers a discharge capacity of 110.90 mAhg−1 with 94% capacity retention after 200 cycles at room temperature and a discharge capacity of 104.4 mAhg−1 with a capacity retention of 87.8% after 100 cycles at 55 °C. Moreover, Li2ZrO3-coated LiAl0.06Mn1.94O4 could retain 87.5% of its initial capacity at 5C rate. This superior cycling and rate performance can be greatly contributed to the synergistic effect of Al-doping and Li2ZrO3-coating.


Introduction
Spinel LiMn 2 O 4 (LMO) has been recognized as one of the promising cathode materials for the lithium-ion batteries due to the environmental compatibility, low cost, and high specific capacity [1][2][3][4]. However, the poor rate performance caused by the low Li + conductivity and the rapid capacity fading induced by the Jahn-Teller distortion have severely impeded its commercial applications [5][6][7].
Nanocrystallization has been proved to be an effective way to improve the electrochemical kinetics by decreasing the diffusion path of Li + and enlarging contact area between electrode and electrolyte [8,9]. Nevertheless, the enlarged specific surface area will inevitably increase the side reactions. Although the ion-doping technique could alter the electrochemical properties to some extent, the capacity decay still occurs due to the Mn ions dissolution, especially under high temperatures [10][11][12][13]. Alternatively, surface coating is often adopted to improve the cycling performance by inhibiting the side reactions between LMO and electrolyte [14,15]. Unfortunately, the coating strategy cannot improve the transport kinetics of Li + . Therefore, the combination of ion-doping and coating should be considered to make full advantages.
Except for the side reactions, the nanocrystallization will lead to flawed crystallinity which is harmful for the Li + ion transport, however LMO has a unique three-dimensional tunnel structure in which the Li + could transfer through the 8a-16c-8a pathway [16]. Therefore, the monocrystal size should not be the determining factor to restrain the Li + transport. Moreover, the micron-sized monocrystalline LMO modified with ion-doping and surface coating may simultaneously decrease the contact area with electrolyte and improve the electrochemical performance. H 3 BO 3 + Li 2 CO 3 → Li 3 BO 3 (1) The detailed ingredient of the micron-sized monocrystalline LiAl 0.06 Mn 1.94 O 4 was listed in Table 1. As a comparison, the LiMn 2 O 4 samples without Al-doping, Li 3 BO 3 additive or Li 2 ZrO 3 -coating were also prepared through the same sintering process. Structural characterization of the samples was analyzed by X-ray diffraction (XRD, Rigaku SmartLab diffractometer) with Cu Kα radiation in the 2θ range of 10-70 • . The particle size, surface morphology, and elemental composition were observed by scanning electron microscopy (SEM, Hitachi S-3400N) equipped with an energy dispersive spectrometer (EDS, Oxford 7426). The microstructure was detected with transmission electron microscopy (TEM, FEI Tecnai G2 F20). The samples were smashed before the TEM analysis.
Synthesized LiMn 2 O 4 particle was mixed with carbon black and polyvinylidene fluoride (PVDF) with the mass ratio of 90:5:5 in N-methyl pyrrolidinone (NMP). Then, the slurry was coated on the aluminum foil and vacuum-dried at 100 • C. The electrode laminate was roll-pressed and punched to be discs with a diameter of 14 mm. The loading density of active material is approximately 5-8 mg cm −2 . The electrolyte was 1 M LiPF 6 in dimethyl carbonate (DMC)/ethyl carbonate (EMC)/ethylene carbonate (EC) (1:1: The 2032-type coin cells using Li metal as anode were prepared in an argon-filled glove box and tested in the voltage range of 3.0-4.3 V at several rates (1C = 148 mAhg −1 ) on a battery test system (Neware). The cells were charged/discharged at 0.2C for 3 times first and then cycled at 0.5C for 200 times at 25 • C. Meanwhile, same cells were charged/discharged at 0.2C for 3 times and at 0.5C for 100 times at 55 • C, successively. Moreover, to evaluate the Nanomaterials 2021, 11, 3223 3 of 9 rate performance, the cells were also charged at 0.5C and discharged at 0.5C, 1C, 2C, 3C, and 5C, respectively.
The electrochemical impedance spectroscopy (EIS) of the cells was performed on electrochemical workstation (CHI600E) with the frequency range of 10 5 to 10 −2 Hz. The EIS tests were performed after the 1st and 200th cycle, respectively.

Li 3 BO 3 Additive Promote the Grain Growth
The XRD patterns of the samples are shown in Figure S1. All the diffraction peaks match well with LiMn 2 O 4 (JCPDF 35-0782). It proves that the Al-doping Li 3 BO 3 additive or the Li 2 ZrO 3 -coating has no effect on the spinel cubic structure with the spatial group Fd3m. While focusing on the effect of the Li 3 BO 3 additive, it could be found that the full width at half maximum (FWHM) of the LMO-B is much broader than that of the LMO (Table S1). As shown in Figure 1a (Figure 1b,c). The grain size of the LMO is less than 2 µm, while that of the LMO-B sample is approximately 2-8 µm according to the particle size distribution analysis ( Figure S2). Both the XRD and SEM results prove that the Li 3 BO 3 additive could promote the grain growth of monocrystal LiMn 2 O 4 effectively. and then cycled at 0.5C for 200 times at 25 °C. Meanwhile, same cells were charged/discharged at 0.2C for 3 times and at 0.5C for 100 times at 55 °C, successively. Moreover, to evaluate the rate performance, the cells were also charged at 0.5C and discharged at 0.5C, 1C, 2C, 3C, and 5C, respectively. The electrochemical impedance spectroscopy (EIS) of the cells was performed on electrochemical workstation (CHI600E) with the frequency range of 10 5 to 10 −2 Hz. The EIS tests were performed after the 1st and 200th cycle, respectively.

Li3BO3 Additive Promote the Grain Growth
The XRD patterns of the samples are shown in Figure S1. All the diffraction peaks match well with LiMn2O4 (JCPDF 35-0782). It proves that the Al-doping Li3BO3 additive or the Li2ZrO3-coating has no effect on the spinel cubic structure with the spatial group Fd3m. While focusing on the effect of the Li3BO3 additive, it could be found that the full width at half maximum (FWHM) of the LMO-B is much broader than that of the LMO (Table S1). As shown in Figure 1a . The grain size of the LMO is less than 2 μm, while that of the LMO-B sample is approximately 2-8 μm according to the particle size distribution analysis ( Figure S2). Both the XRD and SEM results prove that the Li3BO3 additive could promote the grain growth of monocrystal LiMn2O4 effectively.

Effect of the Al-Doping
With the Al-doping in LiMn2O4, the morphology of the LAMO-B (Figure 2a) has no significate change in comparison with the LMO-B sample ( Figure 1c). While the systematic right shift of diffraction peaks could be observed, compared with the LMO-B sample, the main diffraction peaks of the LAMO-B sample shifted toward to the high angles as shown in Figure 2b. Meanwhile, the calculated lattice parameter of the LMO-B and LAMO-B is 8.2235 and 8.2090, respectively. It means that the Al atom with smaller radius has occupied the Mn site in the monocrystal LiMn2O4 lattice structure.

Li 2 ZrO 3 -Coating on the Monocrystal LiMn 2 O 4
To analyze the influence of the Li 2 ZrO 3 -coating amount, the morphologies of the four samples with Al-doping and Li 2 ZrO 3 coating were compared as shown in Figure 3. It is found that the micro-sized large grains are surrounded by smaller ones in all four samples and all the secondary particles show a diameter as large as~10 mm. It proves that the Li 2 ZrO 3 coating could not affect the size distribution.

Li2ZrO3-Coating on the Monocrystal LiMn2O4
To analyze the influence of the Li2ZrO3-coating amount, the morphologies of the four samples with Al-doping and Li2ZrO3 coating were compared as shown in Figure 3. It is found that the micro-sized large grains are surrounded by smaller ones in all four samples and all the secondary particles show a diameter as large as ~10 mm. It proves that the Li2ZrO3 coating could not affect the size distribution.

Li2ZrO3-Coating on the Monocrystal LiMn2O4
To analyze the influence of the Li2ZrO3-coating amount, the morphologies of the four samples with Al-doping and Li2ZrO3 coating were compared as shown in Figure 3. It is found that the micro-sized large grains are surrounded by smaller ones in all four samples and all the secondary particles show a diameter as large as ~10 mm. It proves that the Li2ZrO3 coating could not affect the size distribution. Meanwhile, it is found that the Al-doping and Li2ZrO3 coating distribute uniformly in the final samples. Taking the LAMO-B-Zr2 sample for example, the EDS mapping results exhibit uniform distribution of the Mn, Al, and Zr elements in the secondary particles as shown in Figure 4. Meanwhile, it is found that the Al-doping and Li 2 ZrO 3 coating distribute uniformly in the final samples. Taking the LAMO-B-Zr2 sample for example, the EDS mapping results exhibit uniform distribution of the Mn, Al, and Zr elements in the secondary particles as shown in Figure 4.   To further observe the microstructure of the samples, TEM analysis and corresponding selected area electron diffraction (SAED) of the LMO-B are shown in Figure 5a. The diffraction spots can be indexed to (220), (111), (311), and (400) planes of cubic spinel LiMn2O4 (JCPDS 35-0782). Compared with the pristine surface of the LMO-B sample, a coating layer with a thickness of ~5.2 nm could be found in the LAMO-B-Zr2 sample (Figure 5b). Moreover, the coating layer should be brought from the Li2ZrO3 additive.

Electrochemical Performances
The cycling and rating performance of the cells with the Al-doping and Li2ZrO3-coating samples was tested at the same conditions ( Figure 6). The initial discharge capacity of the cells at 25 °C and 0.2C rate decreased steadily with the increscent Li2ZrO3-coating as shown in Figure 6a. It is reasonable, as the Li2ZrO3 provides no capacity contribution, the more coating, the less proportion of the active materials in the cell. Nevertheless, Li2ZrO3 aimed at reducing the side reactions serving as a protect layer. The effect of the Li2ZrO3coating amount on the cycling performance at 25 °C and 55 °C is shown in Figure 6b,c, respectively. It was found that the capacity retention of the four samples is 93.0%, 93.9%, 90.2%, and 82.3% at 25 °C after 200 cycles with the increasing Li2ZrO3-coating amount. Moreover, at 55 °C, the corresponding capacity retention is 81.0%, 87.5%, 79.2%, and 79.5%, respectively. It is obvious that the LAMO-B-Zr2 could decrease the capacity loss and provide the optimal protection at both temperatures. Moreover, as shown in Figure 6d, the rate performance of LAMO-B-Zr2 is also better than the other three samples, especially at higher rates. Although the discharge capacity of LAMO-B-Zr2 at 0.5C is less than the LAMO-B-Zr1, the capacity at 1, 2, 3, and 5C rates is larger than the other three samples. It proves that the deficiency of Li2ZrO3 could not provide overall protection from the side reactions, while the excess of Li2ZrO3 would impede the migration of lithium ion. LAMO-

Electrochemical Performances
The cycling and rating performance of the cells with the Al-doping and Li 2 ZrO 3coating samples was tested at the same conditions ( Figure 6). The initial discharge capacity of the cells at 25 • C and 0.2C rate decreased steadily with the increscent Li 2 ZrO 3 -coating as shown in Figure 6a. It is reasonable, as the Li 2 ZrO 3 provides no capacity contribution, the more coating, the less proportion of the active materials in the cell. Nevertheless, Li 2 ZrO 3 aimed at reducing the side reactions serving as a protect layer. The effect of the Li 2 ZrO 3coating amount on the cycling performance at 25 • C and 55 • C is shown in Figure 6b,c, respectively. It was found that the capacity retention of the four samples is 93.0%, 93.9%, 90.2%, and 82.3% at 25 • C after 200 cycles with the increasing Li 2 ZrO 3 -coating amount. Moreover, at 55 • C, the corresponding capacity retention is 81.0%, 87.5%, 79.2%, and 79.5%, respectively. It is obvious that the LAMO-B-Zr2 could decrease the capacity loss and provide the optimal protection at both temperatures. Moreover, as shown in Figure 6d, the rate performance of LAMO-B-Zr2 is also better than the other three samples, especially at higher rates. Although the discharge capacity of LAMO-B-Zr2 at 0.5C is less than the LAMO-B-Zr1, the capacity at 1, 2, 3, and 5C rates is larger than the other three samples. It proves that the deficiency of Li 2 ZrO 3 could not provide overall protection from the side reactions, while the excess of Li 2 ZrO 3 would impede the migration of lithium ion. LAMO-B-Zr2 exhibited the best performance in a series of Al-doping and Li 2 ZrO 3 coating samples.
To explore the influence of the Al-doping and the Li 2 ZrO 3 coating, the electrochemical performance of the LMO-B, LAMO-B, and LAMO-B-Zr2 samples were characterized in parallel. Figure 7a presents the first charge-discharge curves obtained at 0.2C and 25 • C between 3.0 and 4.3 V (vs. Li/Li + ). There are two voltage plateaus near 4.0 V and 4.1 V, which indicates the two-stage processes of Li + ion insertion/extraction reactions for a typical characterization of a well-crystallized spinel LiMn 2 O 4 [17,18]. The initial discharge specific capacities of the as-prepared samples show a decline trend with the Al-doping and the Li 2 ZrO 3 coating. This result is comprehensible because the percentage of Mn 3+ is partially occupied by the Al-doping or lowered by the Li 2 ZrO 3 coating.
B-Zr2 exhibited the best performance in a series of Al-doping and Li2ZrO3 coating samples. To explore the influence of the Al-doping and the Li2ZrO3 coating, the electrochemical performance of the LMO-B, LAMO-B, and LAMO-B-Zr2 samples were characterized in parallel. Figure 7a presents the first charge-discharge curves obtained at 0.2C and 25 °C between 3.0 and 4.3 V (vs. Li/Li + ). There are two voltage plateaus near 4.0 V and 4.1 V, which indicates the two-stage processes of Li + ion insertion/extraction reactions for a typical characterization of a well-crystallized spinel LiMn2O4 [17,18]. The initial discharge specific capacities of the as-prepared samples show a decline trend with the Al-doping and the Li2ZrO3 coating. This result is comprehensible because the percentage of Mn 3+ is partially occupied by the Al-doping or lowered by the Li2ZrO3 coating.
However, according to the cycling stability at room temperature and 55 °C shown in Figure 7b,c, respectively, the capacity retention of the LAMO-B-Zr2 is much higher than that of the LMO-B and LAMO-B ( Table 2). The possible reason may lie in that the Aldoping could stable the crystal structure during the cycling. Analogous results could be found in other literature reports [19,20]. On the other hand, with the help of the Li2ZrO3 coating, the capacity retention rate of the LAMO-B-Zr2 could go further to 93.9% after 200 cycles at 25 °C and keep at 87.8% after 100 cycles at 55 °C. It turns out that the Li2ZrO3coating could suppress the side-reactions effectively.  Otherwise, the rate performance of the cells between 3.0 and 4.3 V (vs. Li/Li + ) is shown in Figure 7d. All the cells exhibit reduced discharge capacities as the increased Crates. Although the capacity of LAMO-B-Zr2 at 0.5C is inferior to the other cells as the lowered proportion of active substances, the discharge capacities themselves or the capacity retention of the LAMO-B-Zr2 at 1C, 2C, 3C, and 5C rates are much higher than the other control samples. Moreover, the discharge capacity of LAMO-B-Zr2 could reach up to 103.41 mAhg −1 at 5C.
The specific discharge capacity and the capacity retention of the LAMO-B-Zr2 sample at both room temperature and 55 °C are better than those of some reported LiMn2O4 single-crystalline and Al-doped LiMn2O4. The detailed comparisons of the electrochemical performance are listed in Table S2.  Figure 8a,b, respectively. All the cells are discharged to 4.0 V before testing. All the Nyquist plots comprises two depressed semicircles in the high and middle frequency region and a straight line in the low frequency region. The intersection at the high frequency shows the Ohmic resistance (Ro), the diameter of the semicircle at the high frequency means the resistance (Rs) of solid electrolyte interface (SEI), and the diameter of the semicircle at low frequency suggests the resistance of the charge transfer (Rct). The right intersection of the semicircle with the horizontal axis in the low frequency represent the total internal resistance (Rt = Ro + Rs + Rct) [21,22]. The EIS plots were fitted with equivalent circuit shown in Figure 8a by Zview software. Moreover, the corresponding fitting results of the impedance are listed in Table 3. It is found that the Al- However, according to the cycling stability at room temperature and 55 • C shown in Figure 7b,c, respectively, the capacity retention of the LAMO-B-Zr2 is much higher than that of the LMO-B and LAMO-B ( Table 2). The possible reason may lie in that the Al-doping could stable the crystal structure during the cycling. Analogous results could be found in other literature reports [19,20]. On the other hand, with the help of the Li 2 ZrO 3 coating, the capacity retention rate of the LAMO-B-Zr2 could go further to 93.9% after 200 cycles at 25 • C and keep at 87.8% after 100 cycles at 55 • C. It turns out that the Li 2 ZrO 3 -coating could suppress the side-reactions effectively. Otherwise, the rate performance of the cells between 3.0 and 4.3 V (vs. Li/Li + ) is shown in Figure 7d. All the cells exhibit reduced discharge capacities as the increased C-rates. Although the capacity of LAMO-B-Zr2 at 0.5C is inferior to the other cells as the lowered proportion of active substances, the discharge capacities themselves or the capacity retention of the LAMO-B-Zr2 at 1C, 2C, 3C, and 5C rates are much higher than the other control samples. Moreover, the discharge capacity of LAMO-B-Zr2 could reach up to 103.41 mAhg −1 at 5C.
The specific discharge capacity and the capacity retention of the LAMO-B-Zr2 sample at both room temperature and 55 • C are better than those of some reported LiMn 2 O 4 single-crystalline and Al-doped LiMn 2 O 4 . The detailed comparisons of the electrochemical performance are listed in Table S2.
Furthermore, the EIS results of the LMO-B, LAMO-B, and LAMO-B-Zr2 cells after the 1st and 200th cycle is shown in Figure 8a,b, respectively. All the cells are discharged to 4.0 V before testing. All the Nyquist plots comprises two depressed semicircles in the high and middle frequency region and a straight line in the low frequency region. The intersection at the high frequency shows the Ohmic resistance (R o ), the diameter of the semicircle at the high frequency means the resistance (R s ) of solid electrolyte interface (SEI), and the diameter of the semicircle at low frequency suggests the resistance of the charge transfer (R ct ). The right intersection of the semicircle with the horizontal axis in the low frequency represent the total internal resistance (R t = R o + R s + R ct ) [21,22]. The EIS plots were fitted with equivalent circuit shown in Figure 8a by Zview software. Moreover, the corresponding fitting results of the impedance are listed in Table 3. It is found that the Al-doping could decrease the impedance compared with the pristine LMO-B and the LAMO-B samples. It is suggested that the coating strategy is an efficient method of decreasing impedance.

Conclusions
The micron-sized monocrystalline LiAl 0.06 Mn 1.94 O 4 with a grain size of 2-8 µm was prepared in this work. With the synergistic modification of Al-doping and Li 2 ZrO 3 coating, the cycling stability and rating performance have been significantly improved. Meanwhile, the optimal Li 2 ZrO 3 coating amount was investigated. The LAMO-B-Zr2 with 2 mol% Li 2 ZrO 3 addition possesses capacity retention of 93.9% after 200 cycles at room temperature and 87.8% after 100 cycles at 55 • C. This work provides appreciable point of view into the practical application of the LMO cathode in lithium-ion batteries.