High-pressure synthesis and electrochemical properties of tetragonal LiMnO2

Tetragonal structured LiMnO2 (t-LiMnO2) samples were synthesized under pressures above 8 GPa and investigated as a positive electrode material for lithium-ion batteries. Rietveld analyses based on X-ray diffraction measurements indicated that t-LiMnO2 belongs to a γ-LiFeO2-type crystal structure with the I41/amd space group. The charge capacity during the initial cycle was 37 mA h g−1 at 25 °C, but improved to 185 mA h g−1 at 40 °C with an average voltage of 4.56 V vs. Li+/Li. This demonstrated the superiority of t-LiMnO2 over other lithium manganese oxides in terms of energy density. The X-ray diffraction measurements and Raman spectroscopy of cycled t-LiMnO2 indicated an irreversible transformation from the γ-LiFeO2-type structure into a LixMn2O4 spinel structure by the displacement of 25% of the Mn ions to vacant octahedral sites through adjacent octahedral sites.


Introduction
Lithium manganese oxides (LMOs) have been extensively studied as a potential positive electrode material for lithium-ion batteries (LIBs), due to their low cost and environmental friendliness. [1][2][3][4][5][6] Since such properties are crucial for large-scale applications for LIBs such as electric vehicles and grid energy storage systems, 7 research efforts have been devoted to the development of LMOs with high energy densities (W s ). Table 1 summarizes the structure types, synthesis methods, and electrochemical properties of previously reported LMOs, and their crystal structures are shown issn Fig. 1a-g. Of these LMOs, a cubic spinel-structured LiMn 2 O 4 ( Fig. 1a) is widely accepted as the most practical positive electrode material. 1,2,8 LiMn 2 O 4 exhibits a rechargeable capacity (Q recha ) of approximately 120 mA h g À1 with an average voltage (E ave ) of 4.1 V vs. Li + /Li, and its W approaches the theoretical limit value due to the moderate theoretical capacity (Q theo ) of 148 mA h g À1 .
In terms of the Q theo , LiMnO 2 and its derivatives are appealing, and their structural and electrochemical properties vary depending on their composition and method of synthesis. A monoclinic layered Li 2 MnO 3 , also written as Li[Li 1/3 Mn 2/3 ]O 2 ( Fig. 1b), has a large Q theo of 458 mA h g À1 . However, Li 2 MnO 3 is essentially electrochemically inactive because of the difficulty in further oxidizing the Mn 4+ species, 9 unless a proton exchange 10 and/or an oxygen loss 11 in its lattice proceed during the initial charge reaction. An orthorhombic LiMnO 2 (o-LiMnO 2 ) 12-18 and a monoclinic LiMnO 2 (m-LiMnO 2 ) 4, 19 have a formal oxidation state of Mn 3+ (Fig. 1c and d) and their charge capacities (Q cha s) reach a maximum of 230 mA h g À1 for o-LiMnO 2 and 270 mA h g À1 for m-LiMnO 2 . However, delithiated o-and m-LiMnO 2 can be irreversibly and spontaneously transformed into a Li x Mn 2 O 4 spinel; thus, the Q recha values for both of these compounds decrease in a manner similar to that seen in LiMn 2 O 4 as the changes in the charge and discharge curves echo those of Li x Mn 2 O 4 . 12,[15][16][17][18][19] In contrast, O2-type layered Li 2/3 [Li 1/ In the initial cycle, the Q cha ranges between 20 and 50 mA h g À1 , and is smaller than the Q recha due to lithium deciency in these compounds. This triggers a decrease in the Q recha whenever these compounds are used in full cells with negative electrode materials that do not contain residual lithium, e.g. graphite and silicon.
Sugiyama et al. synthesized a tetragonal LiMnO 2 (t-LiMnO 2 ) from o-LiMnO 2 by a high-pressure and high-temperature method, with pressures between 4 and 6 GPa and temperatures between 900 and 1200 C. 22 Rietveld analyses based on Xray diffraction (XRD) measurements showed that t-LiMnO 2 has a g-LiFeO 2 -type structure with I4 1 /amd space group (Fig. 1g). In this structure, MnO 6 octahedra form a three-dimensional framework by sharing their edges, resulting in a straight channel of Li + ions along the [111] direction. This conguration between MnO 6 octahedra and Li + ions is clearly different from the framework composed of two-dimensional MnO 6 layers in the LiMnO 2 -based compounds mentioned above (see Fig. 1b-f); rather, the crystal structure of t-LiMnO 2 is comparable to that of LiMn 2 O 4 . Therefore, in addition to the high Q theo (285 mA h g À1 ), t-LiMnO 2 is also a desirable material for understanding the relationships between the structural and electrochemical properties in a group of LMOs.
Despite its unique crystallographic character, the electrochemical properties of t-LiMnO 2 remain unclear. This is likely due to the difficulty in conducting large-scale synthesis via the high-pressure method because a tiny sample container is generally used to generate high pressure (>10 GPa). 23 However, recent technological developments, including the use of belttype and multi-anvil-type equipment, enabled us to evaluate a variety of functional materials. [24][25][26][27] In the current study, we synthesized t-LiMnO 2 under high pressures up to 12 GPa and reported its electrochemical performance as a positive electrode material for LIBs for the rst time, in order to clarify the relationship between the structural and electrochemical properties of t-LiMnO 2 . The Q cha of t-LiMnO 2 reached 185 mA h g À1 with an E ave of 4.56 V upon increasing the operating temperature to 40 C, which is superior to other LMOs. This information will be helpful in designing advanced LMOs with high W in terms of their structure and composition.

Experimental
Sample preparations t-LiMnO 2 was synthesized from o-LiMnO 2 by the high-pressure and high-temperature method using a Walker-type apparatus. 28 o-LiMnO 2 was rst synthesized via solid-state reactions. Stoichiometric amounts of LiOH$H 2 O (Wako Pure Chemical Industries Ltd.) and Mn 2 O 3 (Wako Pure Chemical Industries Ltd.) were mixed in ethanol and the mixture was dried and pressed into a 5 mm-thick pellet with a diameter of 15 mm. The pellet was heated at 1000 C for 12 h under argon gas ow at a heating rate of 200 C h À1 and a cooling rate of 1 C min À1 . The obtained o-LiMnO 2 was crushed and re-pressed into a 5 mm-thick pellet with a diameter of 2.8 mm before being packed into a platinum capsule. The capsule was then placed in a BN insulation sleeve, which was placed in a cylindrical graphite heater. The assembled sample was placed in a (Mg, Co)O octahedral pressure medium with side lengths of 14 mm. The (Mg, Co)O octahedra were slowly compressed to 5, 8, or 12 GPa by eight tungsten carbide truncated 8 mm edges. The compressed samples were then heated at 1000 C for 30 min and subsequently quenched to room temperature, and the pressure was slowly released until an ambient pressure was achieved. Hereaer, the o-LiMnO 2 samples treated at 5, 8, and 12 GPa are represented as LMO (5 GPa), LMO (8 GPa), and LMO (12 GPa), respectively, to avoid a misunderstanding of the actual phase purity. A powder sample of LiMn 2 O 4 was also synthesized by heating a mixture of LiOH$H 2 O and MnO 2 (Kojundo Chemical Laboratory Co., Ltd.) at 1000 C for 12 h under oxygen gas ow.

Characterization of the samples
The powder samples of o-LiMnO 2 , LMO (5 GPa), LMO (8 GPa), and LMO (12 GPa) were characterized by scanning electron microscopy (SEM), synchrotron XRD measurements, and Raman spectroscopy. The SEM images were recorded using an S-3600N (Hitachi High-Technologies, Company Ltd.) at an accelerating voltage of 15 kV. The XRD measurements were conducted at a BL5S2 beamline available at the Aichi Synchrotron Radiation Center. The samples were packed into borosilicate glass capillary tubes with a diameter of 0.3 mm (WJM-Glas Müller GmbH). The XRD patterns were collected using a two-dimensional detector, PILATUS 100K (Dectris Ltd., Baden-Daettwil), over a 2q range between 5 and 95 . The incident X-ray wavelength (l) was determined to be 0.799323(2) or 0.799670(2)Å from the XRD patterns of silicon powders (NIST 640d). Rietveld analyses and drawings of crystal structures were carried out using the RIETAN-FP 29 and VESTA sowares, 30 respectively. The Raman spectra were collected on an NRS-3300 (Jasco Co. Ltd.) using an excitation laser wavelength of 532 nm and a laser power of 0.1 mW. The duration of exposure was 600 s.

Electrochemical properties
The electrochemical reactivities of the o-LiMnO 2 , LMO (5 GPa), LMO (8 GPa), and LMO (12 GPa) samples were examined with sandwich-type two-electrode cells. The powdered sample, acetylene black (AB, Denka Co., Ltd.), and polytetrauoroethylene (PTFE, Daikin Industries Ltd.) were combined in a ratio of 70 : 20 : 10 to give a viscoelastic mixture, which was then pressed onto an aluminum mesh current collector with a diameter of 16 mm. The mixture functioned as a working electrode. Lithium metal pressed onto 19 mm-wide stainless steel was used as a counter electrode. The electrolyte was made from a solution of 1 M LiPF 6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) at an EC : DMC ratio of 3 : 7 v/v (Kishida Chemical Company Ltd.) and the separator was constructed from two sheets of porous polyethylene membrane (Tonen-General Sekiyu K. K.). The cells were assembled in an argon-lled glove box and operated at a current density of 0.025 mA cm À2 . The voltage ranged from 1.8 to 4.8 V and the operating temperature was set at 25 or 40 C. Aer cycling test at 25 C, crystal structures of the samples were investigated using synchrotron XRD and Raman measurements to clarify both macroscopic and microscopic structural changes during the cycling. The cells were charged and discharged twenty times at 25 C. The cycled samples were then taken from the working electrodes, which had been thoroughly rinsed with a diethyl carbonate solution. Each of the samples was packed into a capillary tube with a diameter of 0.7 mm and put into a quartz glass cell (GL Sciences Inc.) for the XRD measurements and for the Raman spectroscopy, respectively. All the procedures were conducted in the argon-lled glove box, so as not to contact with the atmosphere. The laser power of the Raman spectroscopy was set at 1.0 mW in consideration of absorbance of the quartz cell. The Raman spectra of the charged LMO (12 GPa) sample, LiMn 2 O 4 , PTFE, and AB were also measured.

Results and discussion
Morphological and structural characterization ake-like shapes with widths between 2-10 mm and thicknesses less than 2 mm. The particle shapes appear to round out and become uniform in size with increasing applied pressure, i.e., rough shapes with a dispersive size of 1-10 mm at 5 GPa and smooth morphologies with a dominant size of $8 mm at 8 GPa and 12 GPa were observed. This morphological change is likely due to a phase transformation, as will be discussed below.  assigned as an orthorhombic b-NaMnO 2 -type structure with Pmmn space group; that is, a zigzag layered structure in which Li and Mn atoms occupy each of the octahedral 2b sites (Fig. 1c). However, the o-LiMnO 2 sample is not in a single phase but contained some impurity phases as shown by asterisk marks at around 2q ¼ 10 and 16 , and these were found to be hausmannite (Mn 3 O 4 with I4 1 /amd space group) and spinel LiMn 2 O 4 by examining the possible crystal structures of LMOs and manganese oxides. Thus, we performed Rietveld analyses with three coexisting phases, in which a Mn under-stoichiometric Li 1+d Mn 1Àd O 2 is adopted to rene the crystal structure of o-LiMnO 2 . Here, d represents the Mn deciency due to impurities. Moreover, we considered Li and Mn atoms to randomly occupy each of the 2b sites, as a cation mixing of Li and transition metal ions occasionally occurs in lithium insertion materials. 31 The structure parameters of the o-LiMnO 2 sample are listed in Table  2. The lattice parameters of o-LiMnO 2 were calculated to be For the LMO (5 GPa) sample, the diffraction line at 2q ¼ 12.8 decreases, but the diffraction line at 2q ¼ 12.3 clearly generates, due to the formation of the t-LiMnO 2 phase (see the inset of Fig. 3b). As listed in Table S1, † the mass fractions of o-LiMnO 2 and t-LiMnO 2 were found to be 86.6 and 8.6 wt%, respectively. For the LMO (8 GPa) and LMO (12 GPa) samples,  the diffraction line at 2q ¼ 12.8 disappears, and almost all diffraction lines can be assigned as the tetragonal g-LiFeO 2 -type structure with I4 1 /amd space group (Fig. 3c and d). The hausmannite impurity also transformed into a CaMn 2 O 4 -type structure (Pbcm) with an applied pressure up to 8 GPa. 32 Table 3 shows the structure parameters of the LMO (12 GPa) sample as determined by Rietveld analyses under identical assumptions as the o-LiMnO 2 sample. The tetragonal lattice parameters, a t and c t , were determined to be 4.18278(2) and 8.22922(6)Å, respectively. Note that the actual composition of t-LiMnO 2 , Li 1.012 Mn 0.988 O 2 (d ¼ 0.012), was almost identical to that of o-LiMnO 2 . From the occupancy factor, g, it was shown that Mn ions occupy 0.8% of the Li (4a) sites. As shown in Table S3, † there were no signicant differences in the structural parameters, including the a t , c t , composition, atomic coordination, and g between the LMO (8 GPa) and LMO (12 GPa) samples.
The sample, however, the intensities of the Raman bands at $411, 556, and 657 cm À1 become weak compared with its original Raman spectrum. Furthermore, new and weak Raman bands are observed at 406, 510, and 617 cm À1 , and these Raman bands are clearly observed in the Raman spectra of the LMO (8 GPa) and LMO (12 GPa) samples. There are at least ten Raman bands in the Raman spectrum of the LMO (12 GPa) sample, although the theoretical calculation predicted eight Raman bands. This difference is probably due to the nonstoichiometry and/or impurity phases in the LMO (12 GPa) sample. Anyway Raman spectroscopy claried that the transformation from o-LiMnO 2 to t-LiMnO 2 is achieved under pressures above 8 GPa, which is consistent with the results of the XRD measurements.

Electrochemical properties
V) before plateauing at $3.5 V when the Q cha reaches 120 mA h g À1 . Then, the E climbs to 4.8 V with a gentle gradient during the initial charge. In the discharge curve, however, the E drops sharply to 3.5 V, and gradually decreases to $3.0 V without any clear voltage plateaus. The Q cha is 160 mA h g À1 , whereas discharge capacity (Q dis ) is only 70 mA h g À1 . New voltage plateaus appeared at $3.0 and 4.0 V in subsequent charge and discharge curves. The differences in charge and discharge curves between 1 st and subsequent cycles is caused by an irreversible structural transformation to the Li x Mn 2 O 4 spinel during the initial cycle, as previously reported. 12,18,37 Fig. 6a and b show charge and discharge curves of the lithium cells with the o-LiMnO 2 and LMO (12 GPa) samples, respectively, operated at temperature of 40 C. The initial Q cha of o-LiMnO 2 is approximately 200 mA h g À1 , which is larger than the value (¼160 mA h g À1 ) obtained at 25 C. This originates from the appearance of a moderate voltage plateau over 4.0 V with a Q cha of $50 mA h g À1 , as reported for an o-LiMnO 2 /Li cell at 55 C by Cho. 38 Subsequent charge and discharge curves reveal the increases in Q cha from 200 mA h g À1 to 225 mA h g À1 and in Q dis from 155 mA h g À1 to 220 mA h g À1 . The irreversible transformation into the Li x Mn 2 O 4 spinel is thought to be accelerated, compared to the measurements at 25 C.  The E of LMO (12 GPa) slowly increases from an open circuit voltage of approximately 3.0 V to $4.4 V with a Q cha of $40 mA h g À1 during the initial charge. Aerwards, the E remains at until nally reaching the cut-off voltage of 4.8 V. In contrast to the situation observed at 25 C, the Q cha increases dramatically to 185 mA h g À1 , in comparison to the Q theo of 65%. The redox reaction at approximately 4.4 V is not observed in LMO (5 GPa), whereas it is clearly seen in the LMO (8 GPa) (see Fig. S2a and b †). According to the rst-principle calculations and numerical simulations reported by Ceder's group, [39][40][41][42] the Li + ion diffusion barrier for g-LiFeO 2 -type structured materials is higher than that for other positive electrode materials such as LiCoO 2 with an a-NaFeO 2 -type structure and LiMn 2 O 4 with a spinel-type structure. Moreover, the Mn ions that occupied 0.8% of the 4a lithium sites (Table 3) would block the lithium diffusion path at 25 C, as reported for LiFePO 4 , which has a one-dimensional lithium ion path along the b axis. 43 Therefore, by increasing the temperature up to 40 C, Li + ions could be kinetically removed from the lattice during the initial charge reaction. Moreover, the initial charge curve exhibited an E ave of 4.56 V, indicating that the Mn 3+ /Mn 4+ redox potential in t-LiMnO 2 was superior to that of o-LiMnO 2 (E ave ¼ 3.86 V). Since the W for positive electrode materials is calculated as a product of the Q recha and E ave , the W for t-LiMnO 2 during the initial charge was estimated to be 844 mW h g À1 using a Q cha ¼ 185 mA h g À1 and E ave ¼ 4.56 V. This value was much larger than the W for LiMn 2 O 4 ($500 mW h g À1 ) which is already commercially available as the positive electrode material. 8 A high E ave is favorable for the potential application of t-LiMnO 2 in this eld.
The initial discharge curve, by contrast, could not maintain this E ave and exhibited a Q dis of 87 mA h g À1 , resulting in a W value of 273 mW h g À1 . This was due to the type of polarization, which was similar to that seen in the results obtained at 25 C. With an increasing cycle number, the Q cha and Q dis improved to approximately 210 mA h g À1 . Nevertheless, the E ave was about 2.9 V at discharge and 4.0 V at charge over the course of more than two cycles. The W in the charge and discharge curves during the h cycle was 815 mW h g À1 and 618 mW h g À1 , respectively.
Crystal structure change upon electrochemical cycling Ex situ XRD measurements and Raman spectroscopy were conducted on the cycled electrodes to understand the phase transformation of t-LiMnO 2 during cycling. The XRD patterns of the cycled o-LiMnO 2 and LMO (12 GPa) samples are shown in Fig. 7a and b, respectively. The XRD pattern of o-LiMnO 2 is assigned as a mixture of the Li x Mn 2 O 4 spinel (Fd 3m) with x < 1, lithiated Li y Mn 2 O 4 tetragonal (I4 1 /amd) phase with y > 1, and PTFE originated from the binder in the electrode. The lattice parameters, which were calculated using the least squares method with more than ve non-overlapping diffraction lines, are found to be a c ¼ 8.224(9)Å for Li x Mn 2 O 4 , and a t ¼ 5.666(6)Å and c t ¼ 9.151(9)Å for Li y Mn 2 O 4 . Since these lattice parameters correspond to those for Li 0.98 Mn 2 O 4 (a c ¼ 8.230Å) and Li 1.82 Mn 2 O 4 (a t ¼ 5.654Å and c t ¼ 9.202Å), 44 the x and y values are estimated to be 0.98 and 1.82, respectively.
As seen in Fig. 7b, the XRD pattern of the cycled LMO (12 GPa) sample is assigned as a mixture of the Li x Mn 2 O 4 spinel, the Li y Mn 2 O 4 tetragonal, t-Li z MnO 2 , and PTFE. The evolution of the Li x Mn 2 O 4 spinel and Li y Mn 2 O 4 tetragonal phases conrms the transformation to the spinel structure from t-LiMnO 2 during cycling, as in the case for o-LiMnO 2 . The lattice parameters were calculated to be a c ¼ 8.252(4)Å for Li x Mn 2 O 4 , a t ¼ 5.657(4)Å and c t ¼ 9.173(7)Å for Li y Mn 2 O 4 , and a t ¼ 4.1854(8)Å and c t ¼ 8.230(2)Å for t-Li z MnO 2 . Thus, the x, y, and z values were estimated to be 0.98, 1.82, and $1, respectively.
As seen in Fig. S3, † the Raman spectrum of the charged LMO (12 GPa) sample is similar with that of the pristine LMO (12 GPa) sample, except for the Raman band at 654 cm À1 . This indicates that the local structure of LMO (12 GPa) is maintained during the initial charge reaction. However, Fig. 8 claries that the extended twenty cycle test converts the t-LiMnO 2 structure into the spinel structure. That is, there are only three broad Raman bands at $650, 620, and 490 cm À1 in the cycled o-LiMnO 2 and LMO (12 GPa) samples, and these Raman spectra are similar with the Raman spectrum of the pristine LiMn 2 O 4 sample. The Raman bands at $650, 620, and 490 cm À1 are also observed in the cycled LMO (5 GPa) and LMO (8 GPa) samples (see Fig. S4 †). It should be noted that the Raman spectra of the cycled (or charged) o-LiMnO 2 , LMO (5 GPa), LMO (8 GPa), and LMO (12 GPa) samples contain contributions of PTFE and AB, which were used for preparing the working electrodes. As seen in Fig. S5, † the Raman spectrum of PTFE shows three major Raman bands at 733, 385, and 290 cm À1 , while that of AB is featureless. The contributions of PTFE and AB are, hence, negligibly small to the Raman spectra of the cycled (or charged) LMO samples.  Fig. 9. Thus, there are two ways for the transformation into the spinel from o-LiMnO 2 or t-LiMnO 2 . In the case of o-LiMnO 2 , the transformation is caused by the displacement of half of the Mn ions to adjacent vacant octahedral sites. [15][16][17] By contrast, the transformation of t-LiMnO 2 is achieved by the displacement of a quarter of the Mn ions in the 4b sites into the vacant octahedral sites (4a) via adjacent octahedral sites without a rearrangement of the ABCABC oxygen packing (Fig. 9b). In this transformation, t-LiMnO 2 has 50% lower amounts of migrated Mn ions, resulting in approximately twice longer in routes of Mn ions, compared to the case for o-LiMnO 2.
The transformation mechanism of t-LiMnO 2 shows that a suppression of the Mn displacement at the charged state is essential for realizing electrochemical properties such as the Q recha , E ave , and W in the initial charge state over several cycles.   Fig. 5a and d. Diffraction lines represented by red triangles, green squares, blue circles, and magenta diamonds, originate from those from Li x Mn 2 O 4 with the spinel structure, Li y Mn 2 O 4 with tetragonal structure, t-Li z MnO 2 , and PTFE, respectively. The Bragg positions for each of these phases are also shown in the bottom using identically colored vertical lines. Raman spectra of (a) and (b) were taken at the discharged state, as indicated by the red arrows in Fig. 5a and d. 178.8 mA h g À1 , respectively. The E prole is similar to the discharge curve of the Li x Mn 2 O 4 spinel, although the crystal structure of Li 0.35 MnO 2 is maintained even aer twenty cycles. 45 Since the electrochemical properties and structural stabilities of t-LiMnO 2 differ from those of Li 0.35 MnO 2 (ref. 45) (or Li 0.59 MnO 2 ), 47 both Li content (x) and particle size also play important roles in stabilizing structure during cycling.

Conclusions
We rst investigated the electrochemical performances of t-LiMnO 2 to obtain structural and electrochemical information regarding a series of LiMnO 2 compounds with potential for the development of LMOs with high W. The t-LiMnO 2 samples with g-LiFeO 2 -type structure (I4 1 /amd) were prepared from o-LiMnO 2 under pressures up to 8 GPa and at a temperature of 1000 C, as evident by XRD measurements and Raman spectroscopy. Rietveld analyses indicated that the actual composition of t-LiMnO 2 is Li 1.012 Mn 0.988 O 2 , in which Mn ions partially occupy the 4a lithium sites. The Q cha of t-LiMnO 2 rose to 185 mA h g À1 at 40 C during the initial charge, even though it was only 37 mA h g À1 at 25 C. Furthermore, the initial charge curve showed that the E ave (4.56 V) with Mn 3+ /Mn 4+ redox was the highest among LMOs such as o-LiMnO 2 and LiMn 2 O 4 . However, the initial discharge curve did not maintain the E ave , and exhibited a Q dis of 87 mA h g À1 due to an irreversible phase transformation into the Li x Mn 2 O 4 spinel during the initial charge. Substitution of different cations and optimization of Li content and particle size will be necessary to maintain the electrochemical properties of t-LiMnO 2 throughout long cycles.

Conflicts of interest
There are no conicts to declare.