Aqueous Rechargeable Battery Based on Zinc ... - Wiley Online Library

DOI: 10.1002/celc.201500033

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Aqueous Rechargeable Battery Based on Zinc and a Composite of LiNi1/3Co1/3Mn1/3O2 Faxing Wang,[a, b] Yu Liu,[c] Xiaowei Wang,[b] Zheng Chang,[b] Yuping Wu,*[a, b] and Rudolf Holze*[c] A composite of rod-shaped LiNi1/3Co1/3Mn1/3O2 with graphene nanosheets and multiwall carbon nanotubes (MWCNTs) is assembled by using a hydrothermal method. In the composite, LiNi1/3Co1/3Mn1/3O2 and MWCNTs are wrapped into graphene nanosheets. It exhibits better rate capability in comparison with plain LiNi1/3Co1/3Mn1/3O2 as a positive electrode for an aqueous rechargeable battery, using zinc as the negative elec-

trode. The average charge and discharge voltages of this aqueous rechargeable battery are 1.80 and 1.65 V, respectively. Based on the total weight of the electrode materials of the composite and Zn, its energy density can reach 154 Wh kg1, which is comparable with that of Ni-MH batteries. Its cycling behavior is satisfactory.

1. Introduction The vast consumption of fossil fuels has brought serious environmental problems; therefore, researchers worldwide are seeking renewable and clean energy sources. Lead acid and nickel–metal hydride (Ni-MH) rechargeable batteries are commercially available, but have limited cycling capabilities; they also suffer from serious environmental problems because of their inventory of highly toxic heavy metals (lead and nickel), and acid or alkaline solutions. Lithium/sodium–sulfur, metal– oxygen, and redox-flow batteries usually operate at low rates and have low energy efficiencies.[1–3] Although lithium-ion batteries have become one of the most important energy-storage sources, in particular for mobile and portable applications, they are associated with high cost and inherent safety risks, owing to the use of flammable organic electrolyte solutions. Aqueous rechargeable batteries have been suggested and have received wide interest in recent years.[4–10] They are inherently safe by avoiding flammable organic electrolyte solutions. In addition, aqueous electrolyte solutions are not expensive; the assembly process of the batteries is also easy. Among the known systems, batteries based on a negative zinc electrodes and a-MnO2[8a] or zinc hexacyanoferrate[9] positive electrodes have been demonstrated; they exhibit high operation voltages

and specific energy densities. In our laboratory, an aqueous rechargeable zinc–sodium battery was successfully assembled by using metallic zinc and Na0.95MnO2 as the negative and positive electrodes, respectively.[10] All of these aqueous batteries based on zinc as the negative electrode show that zinc can be considered as a highly promising negative electrode candidate in mildly acidic aqueous electrolyte solution. Zinc has a relatively low potential (0.78 V versus the standard hydrogen electrode) and a large theoretical capacity (Qtheor. = 825 mAh g1). Moreover, zinc is a low-cost and non-toxic material produced on a large scale of approximately 12 million tonnes per year.[9] In recent years, the development of energy-storage materials has focused on the use of graphene, owing to its excellent electrical properties, chemical stability, and high specific surface area. Extensive efforts have been devoted to preparing graphene-based composites and hybrid materials.[11–16] The chemical reduction of exfoliated graphene oxide (GO) represents a promising pathway for the mass production of graphene-based materials. In our previous studies, we wrapped the electrode materials (Li3VO4[16a] and LiFePO4[16b]) with graphene nanosheets that were prepared in situ by using onepot, template-free methods. However, freshly prepared graphene nanosheets tend to aggregate or restack, owing to the strong van der Waals interactions between them. Consequently, some of the unique properties that individual sheets possess, such as high specific surface area and peculiar electron transport, are significantly compromised or even lost. Fortunately, it has been found that the introduction of nanomaterials, especially carbon nanotubes (CNTs), could effectively prevent graphene from restacking and even reinforce their electrical conductivity.[17] Recently, many conventional electrode materials (such as LiCoO2,[18a, b] LiMn2O4,[18c–e] and LiFePO4)[18f, g] were prepared with one-dimensional (1D), rod-shaped morphologies. These 1D, rod-shaped materials provide very good electrochemical per-

[a] F. Wang, Prof. Y. Wu College of Energy, Nanjing Tech University Nanjing 211816, Jiangsu Province (China) E-mail: [email protected] [b] F. Wang, X. Wang, Z. Chang, Prof. Y. Wu Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (China) [c] Y. Liu, Prof. R. Holze Institut fr Chemie, Technische Universitt Chemnitz Straße der Nationen62,09107 Chemnitz (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/celc.201500033.

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Articles formance, including high reversible capacity and stable cycling. However, to our knowledge, there is no report on 1D, rodshaped LiNi1/3Co1/3Mn1/3O2, so far. In this paper, we report a facile method to prepare rodshaped LiNi1/3Co1/3Mn1/3O2, which we used subsequently to prepare a composite of LiNi1/3Co1/3Mn1/3O2@graphene@MWCNTs (NCM@G@CNTs) by using a hydrothermal method. When used as the positive electrode material for an aqueous rechargeable lithium battery with zinc as the negative electrode, the battery exhibits a high working voltage, high energy density, and stable cycling behavior.

2. Results and Discussion The synthesis of the composite of graphene nanosheets with LiNi1/3Co1/3Mn1/3O2 and CNTs is schematically presented in Scheme 1. At first, a-MnO2 was prepared through a slightly modified hydrothermal method.[6d] Next, it was immersed into an aqueous solution of LiNO3, Ni(NO3)2, and Co(NO3)2. The suspension was heated to 100 8C and kept under reflux for 12 h before the mixture was dried at 80 8C in air. The dried mixture was transferred to a furnace and heat-treated at 400–1000 8C for a period of time. Finally, the composite of as-prepared LiNi1/3Co1/3Mn1/3O2 with graphene nanosheets and MWCNTs was assembled by using a hydrothermal method. The XRD patterns of the prepared LiNi1/3Co1/3Mn1/3O2 rods are shown in Figure 1 a. No detectable impurity peaks were observed in the sample prepared at 600 8C. When the temperature was increased above 700 8C, some peaks from impurities were found and identified as ternary metal oxides[19] formed through side reactions between the manganese oxide and metal ions (Ni2 + and Co2 + ) at high calcination temperatures. Their SEM images (Figure 2 a–g) show that the length of the LiNi1/3Co1/3Mn1/3O2 rods became shorter with the increasing temperature; thus, a suitable preparation temperature is fixed at 600 8C. Samples annealed for different times from 12 to 36 h at 600 8C in air are obtained, their data are shown in Figure 1 b and Figure 2 h–j. The XRD patterns of all samples are consistent with a single phase of a-NaFeO2 with the space group R-3m (JCPDS 82-1495).[20] The ratios of the intensities of 003 and 104 peaks in the XRD pattern are found to be large (> 1.2), indicating less cation mixing or disordering. Moreover,

Figure 1. XRD patterns of a) LiNi1/3Co1/3Mn1/3O2 rods obtained at various preparation temperatures over 6 h, and b) rod-shaped LiNi1/3Co1/3Mn1/3O2 annealed for different times from 12 to 36 h at 600 8C.

the splitting of (006)/(102) and (108)/(110) indexed peaks in the XRD patterns are more pronounced when the reaction time becomes longer, which suggests an increasingly ordered layered structure.[20] The ratios of I003/I104 for the samples heated at 600 8C for longer times (such as 48 h) are similar to those of the sample heated for 36 h. Thus, a heating time of 36 h is selected to obtain a well-crystallized phase. So, the most suitable temperature and preparation time for the LiNi1/3Co1/3Mn1/3O2 rods are 600 8C and 36 h, respectively. The reduction of GO and assembly of the composite of graphene with LiNi1/3Co1/3Mn1/3O2 rods and MWCNTs are achieved by using a hydrothermal method in the presence of N2H4. As shown in Figure 3 a, the XRD pattern of the composite is similar to that of the plain NCM material. The graphene (002) peak intensity is very weak,

Scheme 1. Preparation process for the NCM@G@CNT composite.

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Articles materials, that is, the smaller the ID/IG ratio, the higher the degree of ordering in the carbon material. Here, the ID/IG values of the composite are 1.654, indicating that mostly graphene nanosheets together with CNTs were obtained instead of GO. According to the field-emission SEM (FESEM) and TEM images of the composite (Figure 3 c and Figure 3 d), the surfaces of the LiNi1/3Co1/3Mn1/3O2 Figure 2. SEM images of preparations at all temperature over 6 h: a) 400, b) 500, c) 600, d) 700, e) 800, f) 900, and rods and g) 1000 8C. SEM images of the as-prepared LiNi1/3Co1/3Mn1/3O2 rod annealed at 600 8C for h) 12, i) 24, and j) 36 h. MWCNTs are wrapped in graphene nanosheets. The distribution of MWCNTs in the graphene is even, as shown in Figure S1 a. MWCNTs with a size of 10– 15 nm are sparsely distributed on the graphene nanosheets, thus producing an electronically conductive network with a large available surface area. EDS analysis of the composite (Figure S1 c and d) also confirms the presence of graphene; the Ni/Co/Mn ratio is about 0.30:0.30:0.33. The BET specific surface area (SBET) of the composites is 97 m2 g1, which is much larger than that of rod-shaped LiNi1/3Co1/3Mn1/3O2 (SBET = 10.3 m2 g1). XPS measurements were performed and the corresponding spectra are presented in Figure S2. The Co 2p3/2 and Mn 2p3/2 peaks with binding energies of 779.5 and 642.2 eV, respectively, correspond to Co3 + and Mn4 + . In the Ni 2p3/2 spectra, the symFigure 3. a) XRD patterns of the virginal NCM and the composite of NCM, graphene, and CNTs; b) Raman spectra metrical peak at a binding of the NCM and NCM@G@CNTs; and c) FESEM and d) TEM images of NCM@G@CNTs. energy of 854.0 eV corresponds to Ni2 + in LiNi1/3Co1/3Mn1/3O2, which agrees with a previous report.[21] The observed C 1s peak at 284.7 eV in the composite owing to the low content. Raman spectra (Figure 3 b) indicate the existence of LiNi1/3Co1/3Mn1/3O2 and graphene in the commainly represents graphitic carbon. The peak of C=O (287 eV) is also seen in the C 1s spectra of the composites, whereas the posite. The spectra of LiNi1/3Co1/3Mn1/3O2 (black line in Figintensity of C=O is dramatically reduced compared to that reure 3 b) and the composite (red line in Figure 3 b) clearly show ported previously.[16] two bands at 500 and 580 cm1, which are assigned to Ramanactive Eg and A1g modes of the transition-metal oxygen arThe electrochemical properties of the composite and the rangements in the layered lithium-metal oxide with rhombohepristine NCM are shown in Figure 4. The polished zinc elecdral R-3m symmetry. The bands in the ranges of 1200– trode has good reversibility in this aqueous electrolyte (Fig1460 cm1 and 1470–1730 cm1 are attributed to the D band ure 4 a). It loses electrons (oxidation) and liberates Zn2 + during (K-point phonons of A1g symmetry) and G band (E2g phonons the discharge process; then it gains electrons and zinc is deposited during the charge process.[10] Both the composite and of Csp2 atoms) of graphene or CNTs.[16] The peak intensity ratio 1 between the 1330 and 1583 cm peaks (ID/IG) generally prothe plain NCM electrodes exhibit one couple of redox peaks at about 1.85 and 1.65 V, respectively, corresponding to the devides a useful index of the degree of crystallinity of the carbon ChemElectroChem 0000, 00, 0 – 0



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Figure 4. a) Cyclic voltammograms of the negative zinc electrode (brown line), NCM (red line), and the composite (blue line). The initial charge/discharge profiles of b) NCM and c) NCM@G@CNTs. d) Cycling performance of NCM and NCM@G@CNTs.

an initial discharge capacity of 115 mAh g1 is achieved, which is higher than that recently reported for LiNi0.1Mn1.9O4 (85 mAh g1 at 150 mA g1),[22a] LiFePO4 (106 mAh g1 at [22b] 0.33 C), LiMn0.6Fe0.4PO4 (112 mAh g1 at 0.1 C),[22c] LiNi1/3Co1/3Mn1/3PO4 (60 mAh g1 at low current density),[23a] and nanoporous LiMn2O4 (below 60 mAh g1 at low current density)[23b] in an aqueous electrolyte solution. Of course, compared to the capacity of LiNi1/3Co1/3Mn1/3O2 in organic electrolyte, the specific capacity of rod-shaped LiNi1/3Co1/3Mn1/3O2 is relatively low, which could be attributed to the different cutoff potential range in aqueous electrolyte solutions. When the current density increases to 5 C, the discharge capacity dramatically decreases to 15 mAh g1, indicating the poor rate capability of the plain NCM. On the contrary, the composite delivers initial dis-

intercalation/intercalation of Li + . A closer examination reveals that the peak separation for the composite is narrower compared to that of the virginal NCM (inset in Figure 4 a). As the potential difference between Zn and NMC or its composite is above 1.5 V, they can be assembled into a rechargeable battery, whose reaction mechanism is shown in Scheme 2, where the cations can be balanced during the charge and discharge processes. The charge and discharge reactions are shown in Equations (1)–(3): Negative electrode reaction: Zn2þ þ 2e G

Charge

ð1Þ

HZn

Discharge

Scheme 2. Schematic illustration of our aqueous rechargeable battery, using Zn as the negative electrode, NCM@G@CNTs as the positive electrode, and a Li2SO4 and Zn(CH3COO)2 aqueous solution as electrolyte.

Positive electrode reaction: Li Ni1=3 Co1=3 Mn1=3 O2 Charge þ G HLi1x Ni1=3 Co1=3 Mn1=3 O2 þ xLi þ xe

ð2Þ charge capacities of about 105, 93, 78, and 68 mAh g1 at 0.5, 1, 3, and 5 C, respectively (Figure 4 c). Electrochemical impedance spectroscopy (EIS) studies of LiNi1/3Co1/3Mn1/3O2 rods and its composites are also carried out; results are shown in Figure S3. Both show similar EIS profiles, with a semicircle in the high-frequency region and a straight line in the low-frequency region. The charge-transfer resistance (Rct) is estimated to be 18 W for Zn//NCM@G@CNTs, which is noticeably lower than 100 W for Zn//NCM. Contributions from the zinc electrode

Discharge

The cell reaction: x= Zn2þ þ Li Ni1=3 Co1=3 Mn1=3 O2 2 Charge þ G HLi1x Ni1=3 Co1=3 Mn1=3 O2 þ xLi þ x=2 Zn

ð3Þ

Discharge

Figure 4 b shows the initial charge/discharge profiles of the plain NCM electrodes at different rates from 0.5 to 5 C. At 0.5 C,

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Articles could be assumed to be without influence in this frequency range. The improved rate capability of the composite, compared to the plain NCM, can be attributed to the following factors: 1) graphene in the composite acts not only as a support for the LiNi1/3Co1/3Mn1/3O2 rods and MWCNTs, but also provides electronic conductive channels during the charge/discharge process; 2) it has been reported that chemical reduction of GO prepared by Hummers method with hydrazine inevitably leaves some functional groups on the graphene, which disrupt the long range p-conjugation of graphene sheets, resulting in the decreased conductivity of graphene sheets. However, the existence of pristine MWCNTs can bridge these defects for electron transport, especially at high current densities;[12a] 3) the presence of the MWCNTs also prevents the face-to-face agglomeration of graphene as a smart spacer, which further enhances the amount of accessible surface area of the composite. The discharge capacity of the composite is lower than that of the NCM at a lower current density (0.5 C), and the main reason is that graphene and MWCNTs are part of the weight of the active electrode without providing storage capacity. The average charge and discharge voltages for Zn//NCM (or the composite) are 1.8 and 1.65 V, respectively. If the capacity of Zn is estimated at 825 mAh g1, the energy density of these aqueous rechargeable batteries can reach 154 Wh kg1 based on the total weight of the composite and Zn; this is comparable to those of Ni-MH rechargeable batteries. The cycling behavior of the battery system, Zn//NCM (or the composite), is shown in Figure 4 d. Both NCM and the composite present satisfactory cycling behavior, and their capacity retentions are nearly 99 % after 40 cycles, which is much better than that of the reported systems LiV3O8/LiNO3/polypyrrole-coated LiNi1/3Co1/3Mn1/3O2[24a] and LiV2.9Ni0.05Mn0.05O8/LiNO3/spherical LiNi1/3Co1/3Mn1/3O2[24b] in aqueous electrolytes. Interestingly, the discharge capacity of NCM@G@CNTs gradually increases during the initial 10 cycles, which is ascribed to the wetting process of the graphene surface with the electrolyte solution.[25, 26] To date, composites of graphene with LiNi1/3Co1/3Mn1/3O2 micro- or nanoparticles as positive electrodes for lithium-ion batteries have been obtained through ultrasonication[27a] or ball milling.[27b] However, it is difficult to achieve a microscopically homogeneous distribution of the two components with such mechanical mixing. For comparison, the composites (LiNi1/3Co1/3Mn1/3O2@graphene and LiNi1/3Co1/3Mn1/3O2@CNTs) were also prepared by using the same method. The total graphene or CNT content was about 10 wt %. However, both show disappointing discharge capacities, especially at high current densities (Figure S4 a). In the case of LiNi1/3Co1/3Mn1/3O2@graphene, the graphene nanosheets in the composite tend to aggregate and the LiNi1/3Co1/3Mn1/3O2 rods are separated from aggregated graphene nanosheets, as shown in the SEM image in Figure S4 b. Although the discharge capacity of LiNi1/3Co1/3Mn1/3O2@CNTs is slightly higher than that of LiNi1/3Co1/3Mn1/3O2@graphene, its discharge capacity is still less at 50 mAh g1. After all, the surface of the CNTs is not as high as that of CNTs@graphene, which cannot provide good contact between micron-sized LiNi1/3Co1/3Mn1/3O2 rods ChemElectroChem 0000, 00, 0 – 0



and CNTs (Figure S4 c). In this study, a simple and facile hydrothermal route, using a mixed suspension of GO nanosheets, MWCNTs, and lithium-metal oxide, is used to obtain a homogeneous distribution of three components. Furthermore, an aqueous rechargeable battery has been constructed with this composite as the positive electrode and zinc as the negative electrode. In this system, the average charge and discharge voltages are 1.8 and 1.65 V, respectively, which exceed those of the recently reported aqueous rechargeable batteries of MoO3/Li2SO4/LiMn2O4 (average charge and discharge voltages are 1.40 and 1.22 V),[28a] Zn/Zn(NO3)2/MnO2 (average charge and discharge voltages 1.7 V and 1.3 V),[8a] FeOx/KOH/Ni(OH)2 (average charge and discharge voltages are 1.4 and 1.1 V),[28b] Na0.22MnO2/Li2SO4 + Na2SO4/LiMn2O4 (average charge and discharge voltages are 0.7 and 0.6 V),[28c] and KCuFe(CN)6/ K2HPO4 + KH2PO4/PPy@AC (average charge and discharge voltages are 1.4 and 1.1 V).[28d]

3. Conclusions A composite of 1D, rod-shaped LiNi1/3Co1/3Mn1/3O2 with graphene and MWCNTs was prepared. An aqueous rechargeable battery, providing a high working voltage based on this composite as the positive electrode and zinc as the negative electrode, has been successfully developed. The average charge and discharge voltages are 1.8 and 1.65 V, respectively. Compared to plain 1D, rod-shaped LiNi1/3Co1/3Mn1/3O2, the composite exhibits an improved rate capability. Both also deliver good cycling performance and their capacity retentions are nearly 99 % after 40 cycles. More importantly, owing to the high operating voltage, a specific energy density of 154 Wh kg1 based on the total mass of active electrode materials can be achieved, which makes it a promising energy-storage alternative for large-scale applications.

Experimental Section Materials Synthesis At first, a-MnO2 was prepared by using a hydrothermal method[6d] with minor modification. MnCl2·4 H2O (0.3562 g) was dissolved in distilled water (40 mL), and then KMnO4 (0.1896 g) was added to the solution under constant stirring. Finally, the mixed solution was transferred into a Teflon-lined stainless-steel autoclave with a volume of 50 mL, and maintained at 180 8C for 24 h. After the reaction was complete, the resulting solid product was filtered off, washed with distilled water and ethanol, and then dried at 80 8C in air. Then, LiNO3 (0.964 g), Ni(NO3)2·6 H2O (1.355 g), and Co(NO3)2·6 H2O (1.356 g) were dissolved in distilled water (40 mL). Next, the as-prepared a-MnO2 (0.405 g) was added to the solution. The suspension was heated to 100 8C and kept under reflux for 12 h, and then the mixture was dried at 80 8C in air. The dried mixture was transferred into a furnace and heat-treated at 400– 1000 8C for a period of time. GO was prepared by using a modified Hummers method.[16] MWCNTs, synthesized by chemical vapor deposition, with outer diameter inner diameter length: 30–40 nm 10–15 nm 0.1– 10 mm, purity: 95 %, purchased from Chengdu Organic Chemicals Co. Ltd.), were purified by refluxing them in nitric acid at 25 8C for

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Articles 4 h to remove the catalyst impurities. For the synthesis of the graphene composite with LiNi1/3Co1/3Mn1/3O2 and MMWCNTs, the asprepared GO (0.01 g) was exfoliated by sonication in water (40 mL) for more than 3 h. MWCNTs (0.01 g) and LiNi1/3Co1/3Mn1/3O2 rods (0.2 g) were then added in GO water solution. After further sonication for about 30 min, N2H4·H2O was added and the solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept at 140 8C for 12 h. After the reduction and assembly process were completed, the resulting solid product was filtered off, washed with distilled water and ethanol, and finally dried at 60 8C to get the composite.

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Characterization The XRD patterns were collected by using a Rigaku D/MAX-IIA X-ray diffractometer with CuKa radiation. SEM images were obtained with a Philips XL30 microscope operated at 25 kV, whereas FESEM images were obtained with a FE-SEM-4800–1. TEM images were obtained by using a JEOL JEM-2010 transmission electron microscope. Raman analysis was performed by using a Raman spectrometer (Renishaw inVia). Surface electronic states were investigated by using XPS (PerkinElmer PHI 5000C ESCA, using Al KR radiation).

Electrochemical Measurements The working electrodes were prepared by thoroughly mixing the samples, acetylene black, and polytetrafluoroethylene (PTFE) in a weight ratio of 8:1:1. The mixture was pressed into a film and then dried at 120 8C overnight. After drying, the film was cut into disks of about 2 mg weight (0.36 cm2 in area and 0.4 mm in thickness). These disks were pressed onto a Ni grid at a pressure of 15 MPa to be used as the working electrodes. Cyclic voltammetry, electrochemical impedance, and galvanostatic charge and discharge tests were performed in a solution of 0.25 mol L1 Li2SO4 and 0.125 mol L1 Zn(CH3COO)2 with a two-electrode cell, where Zn was used as the counter and reference electrode. The electrodes in the glass cell were separated by a nonwoven cloth. The rate behavior was studied at different rates from 0.5 C (50 mA g1) to 5 C in the voltage range 1–1.9 V. All tests were carried out at room temperature.

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Received: January 27, 2015 Revised: March 10, 2015 Published online on && &&, 2015

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ARTICLES F. Wang, Y. Liu, X. Wang, Z. Chang, Y. Wu,* R. Holze*

Kicking up a zinc: An aqueous rechargeable battery is assembled by using metallic zinc and a 1D, rodshaped LiNi1/3Co1/3Mn1/3O2@graphene@MWCNT composite as the negative and positive electrodes, respectively. The battery has a high working voltage, satisfactory rate capability, as well as good cycling performance.

&& – && Aqueous Rechargeable Battery Based on Zinc and a Composite of LiNi1/3Co1/3Mn1/3O2

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