Spinel LiMn 2 O 4 Cathode Materials in Wide Voltage Window: Single-Crystalline versus Polycrystalline

: Single-crystal (SC) layered oxides as cathodes for Li-ion batteries have demonstrated better cycle stability than their polycrystalline (PC) counterparts due to the restrained intergranular cracking formation. However, there are rare reports on comparisons between single-crystal LiMn 2 O 4 (SC-LMO) and polycrystalline LiMn 2 O 4 (PC-LMO) spinel cathodes for Li-ion storage. In this work, the Li-ion storage properties of spinel LiMn 2 O 4 single-crystalline and polycrystalline with similar particle sizes were investigated in a wide voltage window of 2–4.8 V vs. Li/Li + . The SC-LMO cathode exhibited a speciﬁc discharge capacity of 178 mA · h · g − 1 , which was a bit larger than that of the PC-LMO cathode. This is mainly because the SC-LMO cathode showed much higher speciﬁc capacity in the 3 V region (Li-ion storage at octahedral sites with cubic to tetragonal phase transition) than the PC-LMO cathode. However, unlike layered-oxide cathodes, the PC-LMO cathode displayed better cycle stability than the SC-LMO cathode. Our studies for the ﬁrst time demonstrate that the phase transition-induced Mn(II) ion dissolution in the 3 V region rather than cracking formation is the limiting factor for the cycle performance of spinel LiMn 2 O 4 in the wide voltage window.


Introduction
Lithium-ion batteries (LIBs) have been the most successful power sources for portable electronics over the last four decades. Recently, LIBs are also emerging as promising energy storage device for electric vehicles (EVs). Nevertheless, the requirement of higher specific energy and improved safety for automotive LIBs is far from over due to the range and safety concerns of EVs [1][2][3]. A natural pathway to improve the energy densities of automotive LIBs is to increase the reversible capacities of the cathode materials. Compared to currently prevailing Co-and Ni-based layered-oxide cathode materials (such as LiCoO 2 and Ni-rich LiNi 1−x−y Co x Mn y O 2 ), the Mn-based spinel LiMn 2 O 4 cathode possesses unique characteristics of low toxicity, abundant source, superior safety, and environmental friendliness [4,5]. The 4 V electrochemistry of the LiMn 2 O 4 cathode has been popularly explored in LIBs [4][5][6][7][8][9][10][11][12]. Unfortunately, practical specific capacities of spinel LiMn 2 O 4 cathodes in the 4 V region are limited to~80-120 mA·h·g −1 , which are lower than layered-oxide cathode materials. The low specific capacity of the spinel LiMn 2 O 4 cathode originates from the lithiation/delithiation in spinel frameworks in the 4 V region occurring at Li-ion occupied tetrahedral (8a) sites only involving one-electron redox reaction. Interestingly, extending the discharge voltage of spinel LiMn 2 O 4 cathode to 2 V vs. Li/Li + can double its specific capacity. There is another voltage plateau at around 2.9 V vs. Li/Li + (labeled as 3 V region) with extra Li-ion intercalation into the unoccupied octahedral sites (16c) sites of the spinel structure. However, upon discharging to the 3 V region, severe phase transition from cubic phase to tetragonal phase may appear because the reduction in average Mn valence state close to trivalence easily induces serious Jahn-Teller distortion and Mn(III) disproportionation reactions [13]. Both Jahn-Teller distortion and Mn(III) disproportionation reactions would lead to the severe capacity fading of spinel LiMn 2 O 4 cathodes. Thus, most previous efforts in spinel LiMn 2 O 4 cathodes were mainly focused on electrochemical studies in the 4 V region [4][5][6][7][8][9][10][11][12].
To suppress the capacity fading of cathodes, one effective strategy is to design a singlecrystal morphology. Currently, single-crystal layered-oxide cathodes have demonstrated better structural integrity than their polycrystalline counterparts due to the reduced phase boundaries and limited surface reactivity [14][15][16][17][18][19][20][21][22][23]. Typically, the polycrystalline cathodes are composed of smaller primary particles agglomerated into large secondary particles. During cycles, the expansion/contraction of the small primary particles in nonuniform directions usually leads to cracking propagation in large secondary particles. On the one hand, the cracking propagation causes a severe loss of electrical contact between the active cathode material and the current collector. On the other hand, the formed cracking exacerbates the parasitic reactions between the electrolyte and freshly exposed particle surfaces [16]. Thus, cracking formation is regarded as one of the main reasons for the capacity loss and structural collapse in layered-oxide cathodes. It has been revealed that singlecrystal morphology can avoid the formation of particle cracking because of the uniform lattice expansion/contraction and fewer phase boundaries. For example, Dahn et al. [22] demonstrated that LiNi 0.5 Mn 0.3 Co 0.5 O 2 cathodes in pouch cells exhibited long-term stability up to 4700 cycles. Zhou et al. [23] reported that the serious oxygen release was effectively restrained within a single-crystal Li-rich Li 1.2 Ni 0.2 Mn 0.6 O 2 electrode, whereas the polycrystalline sample displayed severe irreversible capacity during the initial cycle and capacity fading. In general, the research on single-crystalline and polycrystalline cathodes is still mainly focused on Ni-rich and Li-rich layered-oxide cathodes [14,[17][18][19]23]. There are rare reports on comparisons between single-crystalline and polycrystalline LiMn 2 O 4 spinel cathodes.
In this work, we synthesized single-crystalline LiMn 2 O 4 (SC-LMO) and polycrystalline LiMn 2 O 4 (PC-LMO) cathode materials with similar particle sizes. The differences in electrochemical properties of SC-LMO and PC-LMO cathodes were investigated in a wide voltage window of 2-4.8 V vs. Li/Li + . The SC-LMO cathode exhibited higher Li-ion storage capacity than the PC cathode due to the large contribution at the 3 V voltage plateau. In contrast to what is reported on Ni-rich and Li-rich layered-oxide cathodes, PC-LMO showed superior cycling stability to SC-LMO.

Synthesis of SC-LMO and PC-LMO
The PC-LMO sample was synthesized using Mn 2 O 3 microspheres as the template [24]. First, the MnCO 3 microspheres were prepared via a precipitation method [25]. NaHCO 3 (8.4 g) was dissolved in 700 mL of distilled water. Then, ethanol (70 mL) and the NaHCO 3 solution were added to the MnSO 4 ·H 2 O solution (1.6 g in 700 mL distilled water) in sequence under stirring. The mixture was kept under stirring for 3 h at room temperature. After centrifugation, filtration, and drying, MnCO 3 powders were obtained. Then, the as-obtained MnCO 3 powders were calcined at 700 • C for 10 h in the air to synthesize the Mn 2 O 3 sample. Finally, the Mn 2 O 3 sample was thoroughly mixed with LiOH·H 2 O in a molar ratio of Mn 2 O 3 :LiOH = 1:1.05 and heat-treated at 650 • C for 10 h in a muffle furnace.
SC-LMO was synthesized using a molten salt method. Stoichiometric amounts of LiOH·H 2 O and commercial Mn 2 O 3 (molar ratio 1.05:2) were thoroughly mixed as lithium and nickel precursors. Then, the mixed precursors were ground with a large excess of CsCl salt (melting point: 645 • C). The molar ratio between the CsCl flux and precursors (referred to as the R ratio) was 4. The mixtures of CsCl salt and precursors were put in alumina crucibles and annealed at 850 • C for 10 h. Afterward, the product was washed with deionized water to remove the residual CsCl and then filtered, before finally drying at 100 • C in an oven for 12 h.

Material Characterizations
Powder X-ray diffraction (XRD) patterns were collected using a BrukerD4 X-ray diffractometer (Bruker, Dresden, Germany) with λ (Cu Kα) = 0.154 nm (40 kV, 30 mA). All the obtained XRD patterns are shown with background (without any smoothing). The morphologies of the prepared materials were observed by scanning electron microscopy (Philip XL30, Dresden, Germany) at 15 kV. Elemental analysis was carried out on an inductively coupled plasma optical emission spectrometry (ICP-OES). Dissolution of Mn ions in the electrolyte was quantified using a two-electrode glass device with 10 mL of electrolyte after 10 cycles. The electrolyte was treated under vacuum-drying and then under a small amount of concentrated HNO 3 , before finally diluting to a 50 mL solution using deionized water for ICP-OES tests.

Electrochemical Measurements
The working electrode was prepared by mixing the as-prepared active material (SC-LMO or PC-LMO), conductive agent (acetylene black), and binder (polyvinylidene fluoride, PVDF) with the help of N-methyl-2-pyrrolidone (NMP). The mass ratios of the LMO cathodes, acetylene black, and PVDF were 80 wt.%, 10 wt.%, and 10 wt.%, respectively. The slurry was coated onto an aluminum foil current collector and dried at 120 • C in a vacuum oven overnight. Then, the dried foil was cut into circular electrode with an area of 1.0 cm 2 . The areal mass loading of cathode is in the range of 1.2-1.5 mg·cm −2 . Half-cell assembly was carried out in a glovebox with moisture and oxygen concentrations of 0.1 ppm. A solution of 1.0 M LiPF 6 in ethylene carbonate/ethyl methyl carbonate (EC/EMC, 3:7) solvents was used as electrolyte. Li metal was used as both the counter and reference electrodes for half-cell tests, and all voltages are reported versus the Li/Li + redox couple. Cyclic voltammetry tests were performed using a CHI660 electrochemical workstation between 2.0 V and 5.0 V at 0.1 mV·s −1 . The galvanostatic charge/discharge measurement was carried out by a Land tester (CT2001A) between 2.0 and 4.8 V vs. Li/Li + . All electrochemical tests were performed at room temperature. The specific capacities were obtained as a function of the mass of active cathode materials not including the mass of electrolyte and the Li metal anode.

Results
The crystallographic structure and crystallinity of the SC-LMO and PC-LMO were characterized by XRD patterns. The diffraction peaks (Figure 1a) of these two samples could be well indexed to the cubic structure in a space group Fd3m (standard card, PDF#53-1237) [13], where Li, O and Mn ions occupy the 8a tetrahedral, 32e octahedral, and 16d octahedral sites (Figure 1b). The Li-ion can deintercalate from the 8a tetrahedral sites, and the 8a tetrahedral sites together with 16c octahedral sites form a 3D pathway for Li-ion diffusion. The spinel structure has a three-dimensional network of channels for fast Li-ion conduction. Moreover, the Li-ion can intercalate into the empty 16c octahedral sites in an over-lithiation state, which may cause the transition of crystal structure from cubic LiMn 2 O 4 into tetragonal Li 2 Mn 2 O 4 [26]. Notably, the SC-LMO showed relatively stronger diffraction peaks than PC-LMO, indicating a higher degree of crystallinity in SC-LMO than PC-LMO.
The morphology of the as-synthesized LiMn 2 O 4 samples was examined by SEM. As exhibited in Figure 2a, the morphology of PC-LiMn 2 O 4 was microspherical with a rough surface. The diameter of the microsphere was in the range of 2.0-2.5 µm. The enlarged SEM image indicated that the spherical particle was densely composed of numerous small primary particles with sizes in the range of 100-200 nm (Figure 2b). In contrast to the spherical-shaped polycrystalline particles consisting of small grains, SC-LMO had a relatively regular polyhedral shape with a smooth surface and clear edges. SC-LMO had an average particle size of~2.0 µm.    Li/Li + . The first discharge capacities of SC-LMO and PC-LMO cathodes at a current density of 50 mA·g −1 were 178 and 172 mA·h·g −1 , respectively. Clearly, the SC-LMO had a much higher specific capacity (88 mA·h·g −1 ) than PC-LMO (56 mA·h·g −1 ) in the 3 V region, which is consistent with the CV tests. This indicates that extra Li-ion storage at the 16c octahedral vacancies in spinel LiMn 2 O 4 [13] was more easily activated in single-crystalline particles than in polycrystalline secondary particles.
In the voltage window of 2.0 and 4.8 V vs. Li/Li + , the SC-LMO cathode delivered a discharge capacity of 114 mA·h·g −1 with~67% of its original capacity after 50 cycles, which was much better than the PC-LMO cathode with a capacity retention of~48% after 50 cycles. Better cycling performance was achieved in the polycrystalline cathode than in the single-crystalline cathode. This is quite different from previous Ni-rich NMC and Li-rich cathodes [14,18,19,21].
To further understand the relationship between the cycle stability and crystallinity in the LiMn 2 O 4 cathode, we carried out post-cycling structural characterizations of SC-LMO and PC-LMO electrodes using SEM, XRD, and ICP analysis. Figure 4a,b compare the morphology evolution of PC-LMO and SC-LMO cathodes after 10 cycles. The PC-LMO particles presented micro-cracks (intergranular fracture), whereas the SC-LMO particles showed no sign of grain boundaries after cycles. Regarding the cycle performance of polycrystalline LiMn 2 O 4 cathodes, there was a slight gradual increase in discharge capacity during the 5th-20th cycles. In contrast, the increase in discharge capacity was not observed for the single-crystalline LiMn 2 O 4 cathode (free of cracks after 10 cycles). Recent work from Jürgen Janek et al. [30] reported that the cracking of secondary polycrystalline particles leads to liquid electrolyte infiltration in the cathode active materials, lowering the chargetransfer resistance and increasing the Li-ion diffusion coefficient by more than one order of magnitude. This was also the case in our polycrystalline LiMn 2 O 4 cathode. The slight increase in discharge capacities of PC-LMO might have resulted from the gradual electrolyte wetting due to the cracking formation. Furthermore, post-XRD characterization (Figure 4c) suggested that more impurity phases (tetragonal Li 2 Mn 2 O 4 phase and orthorhombic LiMnO 2 phase) [31] appeared in the single-crystalline LiMn 2 O 4 electrode than in the polycrystalline LiMn 2 O 4 electrode after 10 cycles. Results from elemental analysis by ICP show that the dissolved Mn-ion amount in the electrolyte from the SC-LMO cathode was 1.5 times higher than that from PC-LMO (Figure 4d). The detected Mn ions were dissoluble Mn(II) ions, resulting from the disproportion reaction of Mn(III) before dissolving into the electrolyte. The loss of Mn at the LiMn 2 O 4 cathode not only caused a gradual loss of active cathode, but also deteriorated the stable cathode-electrolyte interface, thereby leading to severe capacity decay during cycles [32]. It is well known that Mn(II) dissolution is also associated with the atomic arrangement of Mn-ions on the crystal facets of LiMn 2 O 4 . A large portion of Mn(II) ions exist at the {110} facet of LiMn 2 O 4 , leading to the {110} facet being more vulnerable to the Mn dissolution [33]. Compared to densely packed particles in polycrystalline LiMn 2 O 4 , it is probable that {110} facets are more exposed in single-crystalline LiMn 2 O 4 . This may be another reason for the more severe Mn(II) ion dissolution and worse cycling stability of single-crystalline LiMn 2 O 4 . Accurate crystal facet/orientation analysis is required for further characterization based on transmission electron microscopy (TEM) with a focused ion beam (FIB) in the future. Moreover, the high number of Mn(III) ions produced from SC-LMO would induce Jahn-Teller distortion due to the valence electronic configuration of t 2g 3 e g 1 , which may lead to the structural degradation of spinel phases [34]. Consequently, in spite of cracking formation, the PC-LMO cathode had better cycle stability than the SC-LMO cathode due to the low capacity contribution in the 3 V region with cubic/tetragonal phase transition. In general, these studies indicate that the phase transition-induced Mn(II) ion dissolution in the 3V region instead of cracking formation is the limiting factor for the cycle performance of the spinel LiMn 2 O 4 cathode operating in a wide voltage window.

Conclusions
In summary, we investigated the electrochemical properties of single-crystal LiMn 2 O 4 particles and polycrystalline LiMn 2 O 4 particles with similar sizes as cathodes for Li-ion storage. Our studies suggest that, while single-crystalline LiMn 2 O 4 cathodes might have the advantage of enhancing specific capacity, polycrystalline LiMn 2 O 4 cathodes have less cubic-tetragonal phase transition in the 3 V region, thus presenting better cycle stability. Unlike layered Ni-rich cathodes, the single-crystalline design does not work well for stable spinel LiMn 2 O 4 cathodes. Post-cycling structural characterizations confirmed that the limiting factor for the cycle performance of the spinel LiMn 2 O 4 cathode in the wide voltage window was Mn(II) ion dissolution rather than intergranular cracking formation.
Author Contributions: Conceptualization, F.W. and P.Z.; methodology, H.L.; validation, C.G. and Y.W.; data analysis, W.B. and J.L.; original draft preparation, F.Y.; writing-review and editing, F.Y., F.W. and P.Z. All authors have read and agreed to the published version of the manuscript.