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Improved high rate capability of Li[Li0.2Mn0.534Co0.133Ni0.133]O2 cathode material by surface modification with Co3O4
Journal of Alloys and Compounds 783 (2019) 349e356
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Improved high rate capability of Li[Li0.2Mn0.534Co0.133Ni0.133]O2 cathode material by surface modification with Co3O4 Yu Li, Hui Huang**, Jiage Yu, Yang Xia, Chu Liang, Yongping Gan, Jun Zhang, Wenkui Zhang* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2018 Received in revised form 24 December 2018 Accepted 30 December 2018 Available online 31 December 2018
High capacity Li- and Mn-rich layered oxides are of particular interest as cathode materials for lithiumion batteries, but large initial irreversible capacity, inferior poor cycle stability and rate capability greatly limited large-scale commercial application. In this work, we propose an effective strategy via surface modification of Co3O4 on surface of Li[Li0.2Mn0.534Co0.133Ni0.133]O2 (LMCNO) to improve the electrochemical performance. In comparison with pristine LMCNO, all the Co3O4-modified LMCNO composites show higher capacity, better cycling stability and rate capability. Among them, 1.5 wt% Co3O4-modified LMCNO shows high initial capacity of 253.3 mA h g1, good capacity retention of 73.3% after 80 cycles at 0.2C and much improved rate capability especially at high rates. The enhanced electrochemical properties can be attributed to protecting LMCNO bulk materials from the electrolyte attack by Co3O4 surface layer as well as its high ionic/electronic conductivity. Electrochemical impedance spectroscopy reveals that the Co3O4-modified LMCNO sample shows much smaller charge transfer resistance than pristine sample. © 2019 Elsevier B.V. All rights reserved.
Keywords: Li- and Mn-Rich layered oxide Co3O4 modification Surface modification Electrochemical performance Li-ion batteries
1. Introduction Lithium-ion batteries (LIBs) are considered to be a new generation of power source for their high output voltage, high specific capacity, long cycle life and low self-discharge rate [1e8]. In recent years, extensive researches have been conducted on LIBs to further improve the energy density and cycling capability. The main driving force is that the current LIBs technology cannot fully meet the rapid rising market demands for electric vehicles (EVs) and hybrid electric vehicles (HEVs) [9e11]. Cathode materials are particularly in need of an advance in technology because of much lower capacities relative to the commercial Si/C anode materials. Among several candidates capable of offering a high capacity, Liand Mn-rich layered oxides materials with the chemical formula xLi2MnO3$(1-x)LiMO2 (0 < x < 1, M ¼ Co, Ni, Mn, etc.) have gained special attention by virtue of high specific capacity over 250 mA h g1, originating from transition metal redox reaction and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Zhang).
(H.
Huang),
https://doi.org/10.1016/j.jallcom.2018.12.357 0925-8388/© 2019 Elsevier B.V. All rights reserved.
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unusual oxygen anion redox reaction. The pioneering work of Zhou [12] and Thackeray [13,14] demonstrated that Li- and Mn-rich layered oxides are nanocomposites, composed of rhombohedral LiMO2 phase (space group: R-3m) and monoclinic Li2MnO3-like phase (space group: C2/m). However, Li- and Mn-rich layered oxides suffers from intractable drawbacks such as low initial coulombic efficiency, inferior rate capability, fast voltage and capacity fading when charged over 4.5 V vs Li/Liþ compared with other LiCoO2 and Layered Ni-rich cathode materials such as LiNi0.6Co0.2Mn0.2O2 [15e17], LiNi0.8Co0.1Mn0.1O2 [18e20] and LiNi0.8Co0.15Al0.05O2 [21e23]. It is well known that the low initial columbic efficiency of the Liand Mn-rich layered oxides originates from the irreversible oxygen loss occurring around 4.5 V during the first charge process. The fast voltage and capacity decay arises from the migration of transition metal into the Li layer, which results in a phase transformation from layered to spinel-like structure and the dissolution of metal elements into the electrolyte. In order to solve these issues, several methods have been adopted to improve the electrochemical performance of Li- and Mn-rich layered oxides, such as surface modification [24] and ion doping/substitution [25]. Surface modification has been proved an effective way to improve the electrochemical performance. A proper modification layer can protect the
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surface from directly contact with the electrolyte and suppress the side reactions. Meanwhile, the modification layer with high ionic/ electronic conductivity can also improve the rate capacity by facilitating the charge transfer at the surface of particles. Metal oxides (Al2O3 [26], MgO [27], ZrO2 [28]), phosphates (AlPO4 [29], Li3PO4 [30], FePO4 [31]), and fluorides (AlF3 [32], CaF2 [33] and ZrF4 [34]) have been employed as the modification materials to enhance the electrochemical performance of layered cathode materials. Regrettably, these modification materials are electrochemically inert or ionic insulators, which mainly act as a protection layer to alleviate the side reactions between electrode and electrolyte and the layered-to-spinel transformation. Electrochemically active materials like V2O5 [35] have been reported to effectively improve the cycling stability and storage characteristics of pristine LiNi0.8Co0.1Mn0.1O2 material [36]. Co3O4 has been also introduced to ameliorate the lithium storage property of LiNi0.6Co0.2Mn0.2O2, Li1.1Mn1.9O4 and LiNi0.8Co0.15Al0.05O2 by acting as HF scavenger and protecting electrode materials from electrolyte attack. Yan et al. [37] discovered that Co3O4 could react with electrolyte and serve as a HF scavenger during the cycling process of NCA, thus reduce the acidity and relieve the corrosion of NCA surface in the acidic electrolyte. Through TOF-SIMS analysis, Lee et al. [38] found that the Co3O4 coating layer would react with the generated HF, which was expected to result in the formation of Co-F layer on the outermost surface eventually. The special interest for Co3O4 modification material is related to its high ionic/electronic conductivity, which can achieve superior cycling capability and rate capability at a high cutoff voltage. To our knowledge, Co3O4 as modification materials to improve the electrochemical properties of Li- and Mn-rich cathode materials has rarely been reported. In this work, Co3O4 were introduced to modify Li- and Mn-rich layered oxides (Li [Li0.2Mn0.534Co0.133Ni0.133]O2, referred to as LMCNO hereafter) by simple wet chemical method. The effects of Co3O4 modification on the electrochemical performance of LMCNO samples are discussed in detail in this work.
schematically illustrated in Fig. 1. The pristine LMCNO was prepared by a co-precipitation method. Firstly, Mn(Ac)2$4H2O, Ni(Ac)2$4H2O and Co(Ac)2$4H2O with a chemical stoichiometric ratio of 4:1:1 were dissolved in distilled water. Secondly, the mixed solution of NaOH and ammonia were added dropwise into the above mixtures with violently stirring. The co-precipitation reaction was carried out under N2 atmosphere for 24 h and the pH value was kept at 11. Afterwards, the hydroxide precursor was filtered, washed until pH ¼ 7 and dried at 80 C for 12 h. Finally, the hydroxide precursor was thoroughly mixed with lithium carbonate with a required amount (5% rich to offset Li loss during the sintering) by planetary ball milling, calcined at 500 C for 5 h and at 900 C for 12 h in air to get LMCNO sample. The Co3O4-LMCNO samples were prepared by a wet-chemical method. For modified with Co3O4, the LMCNO powders were slowly added into ethanol. Then the temperature was slowly increased to 40 C with violent stirring. Subsequently, a certain amount of Co(NO3)2$6H2O, which corresponds to the Co3O4 content of about 1 wt %, 1.5 wt % or 2 wt % in sequence, was dissolved in ethanol. Then the Co(NO3)2 solution was added dropwise into the above LMCNO suspension. The mixed solution was stirred at 40 C continuously until the ethanol was evaporated. Subsequently, the mixtures were dried at 80 C for 10 h and calcined at 600 C for 5 h in air. 2.2. Materials characterizations The structure and crystalline phase of the pristine LMCNO and Co3O4-LMCNO samples were confirmed by the X-ray diffraction with a Cu-Ka radiation (l ¼ 1.5418 Å). The surface morphology of the samples was investigated by a scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, FEI Tecnai G2 F30) with an energy dispersive spectroscopy (EDS) detector. 2.3. Electrochemical measurements
2. Experimental 2.1. Preparation of materials All the chemical reagents were used directly without purification. The synthesis processes of LMCNO and Co3O4-LMCNO are
For the electrode fabrication, the active materials (LMCNO and Co3O4-LMCNO), Super P and polyvinylidene fluoride (PVDF) binder were mixed at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidinone (NMP) to obtain a slurry. Then, the slurry was coated onto an Al foil current collector, dried at 80 C in a vacuum oven overnight. The
Fig. 1. Schematic illustration of the synthesis of Co3O4-modified LMCNO samples.
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mass loading of the active material was near 3 mg cm2 for each electrode. The CR2025 coin cells were assembled in an argon-filled glove box (H2O < 0.5 ppm, O2 < 0.5 ppm), using a lithium foil as the counter electrode and a Celgard 2500 membrane as the separator. The electrolyte solution was composed of 1 mol L1 LiPF6 dissolved in ethylene carbonate (EC), ethyl-methyl carbonates (EMC) and diethyl carbonate (DEC) with a 1:1:1 vol ratio. Galvanostatically discharge/charge experiments was conducted on a Neware battery test system in the voltage range of 2.0e4.8 V. The cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were performed on a CHI650B electrochemical workstation. The CV measurement was tested at a scan rate of 0.1 mV s1 in the voltage range of 2.0e4.8 V and EIS was tested in the frequency range between 0.1 Hz and 1 MHz with a 5 mV amplitude. 3. Results and discussion Fig. 2 shows XRD patterns of the pristine LMCNO and 1 wt%, 1.5 wt% or 2 wt% Co3O4-LMCNO composites. All the samples reveal a typical layered hexagonal structure of a-NaFeO2 with the space group R-3m. The clear splitting of (006)/(012) and (108)/(110) peaks indicates a well-ordered layered structure. The weak peak at 2q ¼ 2123 corresponds to a little Li and Mn in the transition metal layers, indicating the presence of Li2MnO3 phase (C2/m space group). Moreover, there is no obvious change in the XRD patterns of LMCNO and Co3O4-LMCNO samples, which implies that the introduction of Co3O4 does not change the structure of LMCNO. Meanwhile, there is no characteristic diffraction peak of Co3O4 observed in the XRD patterns, this may be resulted from the low content of Co3O4 modification. Fig. 3 shows the SEM images of LMCNO and Co3O4-LMCNO samples. All the samples show similar particle morphology with the size distribution of approximately 100e500 nm. As shown in Fig. 3, pristine LMCNO particles shows a clean and smooth surface, while Co3O4-LMCNO samples exhibit rough surface morphology, and Co3O4 nanoparticles are distributed uniformly on the surface of LMCNO. Meanwhile, the number of nanoparticles increases gradually with the increase of Co3O4 from 1 to 2 wt%. To further confirm the presence of Co3O4 on the surface of bulk particles, the highmagnification TEM images of LMCNO and 1.5 wt% Co3O4-LMCNO and 2 wt% Co3O4-LMCNO samples are presented in Fig. 4. As shown in Fig. 4a and b, the pristine LMCNO displays clear lattice fringes (003) with a d-spacing of 0.47 nm, and no impurity layer can be observed on its surface. In contrast, a distinguishable surface layer can be observed on the surface of 1.5 wt% Co3O4-LMCNO (Fig. 4c
Fig. 3. SEM images of the pristine LMCNO and Co3O4-LMCNO samples: (a) pristine LMCNO; (b) 1 wt% Co3O4-LMCNO; (c) 1.5 wt% Co3O4-LMCNO; (d) 2 wt% Co3O4-LMCNO.
and d) and 2 wt% Co3O4-LMCNO (Fig. 4e and f). From Fig. 4c and e, we can see that the thickness of Co3O4 layer is about 15 nm. The lattice spacing of 0.24 nm is attributed to the (311) plane of Co3O4 crystallite. The chemical composition of surface layer can be further confirmed by the STEM image and its EDS elemental mapping profiles in Fig. 4g. As can be seen in the red dashed region, the Co content shows a significant increase as compared to Mn and Ni elements. The result implies the effective modification of Co3O4 on the surface of LMCNO. To study the effects of the Co3O4 surface modification on the electrochemical properties, the pristine LMCNO and Co3O4-modified samples were evaluated by using 2025 coin-type half cells with Li anodes. The charge/discharge curves at a 0.1C rate (30 mA h g1) in the voltage range of 2.0e4.8 V are shown in Fig. 5. In the first charging process, all the samples exhibit a typical two-step characteristic of Li-rich layered oxides, including a smooth voltage ramp below 4.5 V which can be ascribed to the lithium ions extraction from the LiMO2 component and a potential plateau at 4.5 V, corresponding to the simultaneous removal of oxygen from the Li2MnO3 component. However, this long plateau vanishes in the subsequent charging process. During the discharging process, the samples exhibit almost the same profiles with a long voltage ramp. The first charge and discharge capacity of pristine LMCNO is
Fig. 2. XRD patterns of the pristine LMCNO and Co3O4-LMCNO samples.
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Fig. 4. (aeb) TEM and HRTEM images of pristine LMCNO; (ced) TEM and HRTEM images of 1.5 wt% Co3O4-LMCNO; (eef) TEM and HRTEM images of 2 wt% Co3O4-LMCNO sample. (g) HAADF-STEM image and element mappings of 1.5 wt% Co3O4-LMCNO.
371.95 mA h g1 and 271.09 mA h g1, respectively. The capacity loss reaches as high as 27%. After the Co3O4 modification, the capacity loss of the Co3O4-LMCNO samples is dramatically decreased to 20e21%. It is believed that the surface layer of Co3O4 can increase ion channels and prevent the electrode surface from directly contact with the electrolyte, thereby slowing the surface structural rearrangement and the oxygen release process [39,40]. According to previous works [40e42], the dissolution of metal elements and the formation of oxygen vacancies are the main causes of voltage fading. After 80 cycles, the Co3O4-LMCNO samples also show smaller voltage fading than pristine LMCNO, indicating that the surface modification of Co3O4 can inhibit the side reactions between cathode material and electrolyte. Fig. 6 indicates the CV curves of LMCNO and Co3O4-LMCNO for the initial three cycles and the 80th cycle in the voltage range of
2e4.8 V. Each sample shows a similar CV profile for the initial three cycles. One oxidation peak at 4.0 V is associated with the extraction of Liþ from the lithium layer and the oxidation of Ni2þ to Ni4þ and Co3þ to Co4þ. This oxidation peak slightly shifts to the low potential of 3.8 V in the subsequent cycling. Correspondingly, the reduction peak at 3.2 V is ascribed to the reduction from Ni4þ to Ni2þand Co4þ to Co3þ. Another sharp oxidation peak at around 4.6 V reveals the irreversible removal of Li2O from the activated phase of Li2MnO3 to form MnO2, which only appears in the first cycle leading to a low initial Coulombic efficiency [43,44]. After 80 cycles, the redox peak shifts outward and the peak potential of electrode decreased significantly, which are related to the degradation of active particles on the surface of materials [45]. However, the potential interval between the anode and cathode peaks of Co3O4-LMCNO is smaller than that of LMCNO, and the intensity of redox peak in Co3O4-
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Fig. 5. Charge/discharge curves of pristine LMCNO: (a) 1 wt% Co3O4@LMCNO; (b) 1.5 wt% Co3O4@LMCNO; (c) 2 wt% Co3O4@LMCNO; (d) at a current density of 0.1C.
Fig. 6. CV curves of pristine LMCNO and Co3O4@LMCNO samples.
LMCNO decreases more slowly. Results above indicate that the surface modification of Co3O4 improves the reaction kinetics of LMCNO, resulting in better rate performance and cycle stability. Fig. 7 compares the cycling performance of pristine LMCNO and Co3O4-LMCNO samples in the voltage range of 2e4.8 V. At a low rate of 0.2C (60 mA g1), all the coated samples show better cycle stability than pristine LMCNO. After 80 cycles, the discharge capacity of pristine LMCNO drops from 247.0 mA h g1 to
158.8 mA h g1 with the capacity retention of 64.3%. In contrast, the 1.5 wt% Co3O4-LMCNO sample shows the highest capacity retention of 73.4% among all samples. Although the 2 wt% Co3O4-LMCNO sample just delivers a capacity of 170.0 mA h g1 after 80 cycles, the capacity retention of 69.7% is still higher than that of pristine LMCNO. The cycle performance at a high current density of 5C (1500 mA g1) are also shown in Fig. 7b. It can be clearly seen that the cycling capability of the LMCNO sample has been significantly
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Fig. 7. (aec) Cycling performance of pristine LMCNO and Co3O4@LMCNO (0.2C, 5C and 10C); (d) rate performance of pristine LMCNO and Co3O4@LMCNO samples.
improved by the surface modification of Co3O4 at a high rate. The 1.5 wt % Co3O4-LMCNO sample delivers the highest capacity, which is 94.3 mA h g1 after 400 cycles at 5C, while pristine LMCNO only maintains 66.7 mA h g1 under the same conditions, corresponding to the capacity retention of 71.2% and 58.3%, respectively. In Fig. 7c, similar results are obtained when the rate rises to 10C (3000 mA g1), the high rate capability of the Co3O4-LMCNO samples is obviously superior to pristine LMCNO. The results reveal that the Co3O4 surface modification layer with high ionic/electronic conductivity can effectively improve the rate capability since it facilitates the charge transfer at the surface of particles. To further evaluate the rate performance, all samples were tested by successively increased the rate from 0.1 to 5C. It can be seen that the specific capacities of pristine LMCNO and Co3O4-LMCNO samples are very close at 0.1C. With the increasing of current density, 1.5 wt % Co3O4-LMCNO shows the best rate capability. The reversible capacities at 0.1, 0.2, 0.5, 1, 2, and 5C after each 10 cycles are 269.7, 229.7, 185.5, 154.8, 130.1, 95.0 mA h g1, respectively. Additionally, when the current density returned to 0.1C, the delivered capacity still maintains 245.1 mA h g1. It is believed that the geometrical reason is important to achieve high-performance Li-rich layered
oxides materials. In this work, Co3O4 nanoparticles exhibited discontinuous distribution on the surface of LMCNO samples. With the concentration of Co3O4 precursor increased, some Co3O4 nanoparticles will prefer to grow on the surface of Co3O4 rather than the surface of LMCNO. As a result, Co3O4 particles will selfaggregate into large particle size with a poor spatial distribution. So, even if the content of Co3O4 was increased, the electrochemical performance of LMCNO might be not greatly improved. This geometrical reason is why the 1.5 wt% Co3O4-LMCNO exhibits the best rate performance. Fig. 8 shows the Nyquist plots of pristine LMCNO and Co3O4LMCNO samples, and the inset shows the equivalent circuit model. All the samples exhibit a depressed semicircle in the high frequency and a slope line in the low frequency. Generally, the intercept of the 0 high frequency semicircle on the Z axis is attributed to the electrolyte resistance (Re), the depressed semicircle in the high frequency corresponds to the charge transfer resistance (Rct), and the slope line in the low frequency is related to the Warburg impedance (Zw) of Liþ diffusion into electrode. It is clear that the semicircle size of Co3O4-LMCNO is much smaller as compared to pristine LMCNO. The EIS data can be obtained basing on the equivalent circuit model
Fig. 8. (a) Nyquist plots of pristine LMCNO and Co3O4@LMCNO. The inset is the corresponding equivalent circuit. (b) relationship between Z0 and u1/2 at low frequency region.
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in the inset of Fig. 8. The Re values of LMCNO, 1 wt%, 1.5 wt% and 2 wt% Co3O4-LMCNO are 2.2, 2.7, 2.3 and 2.3 U, the Rct values are 101.4, 51.3, 58.7 and 82.3 U, respectively. The small Rct value indicates that the presence of Co3O4 on the surface of LMCNO is beneficial to achieve good electrochemical kinetics and high rate capability. By calculating the Warburg coefficient (s) derived from the low frequency region, we found that the Co3O4-LMCNO samples exhibit higher Liþ diffusion coefficient than the pristine LMCNO, corresponding to higher reaction kinetics, as shown in Fig. 8b. 4. Conclusions In conclusion, LMCNO samples have been synthesized through co-precipitation and following calcination in air. Wet-chemical method was used to modify LMCNO with Co3O4. Materials characterizations demonstrate that Co3O4 modifier does not change the structure of LMCNO, just make the surface rough. The introduction of Co3O4 can improve the reaction reversibility, cycling stability, high rate performance. In all samples, 1.5 wt% Co3O4-modified LMCNO shows high initial capacity of 253.3 mA h g1, good capacity retention of 73.3% after 80 cycles at 0.2C and much improved rate capability especially at high rates. Results above indicate that the appropriate content of Co3O4 modifier can improve the electrochemical properties of LMCNO efficiently. This idea may provide a new way to develop lithium-rich layered oxides cathode materials with outstanding lithium storage performance. Acknowledgements The authors thank financial support from the National Natural Science Foundation of China (Grant no. 21403196, 51572240, 51677170 and 51777194), Natural Science Foundation of Zhejiang Province (Grant no. LY18B030008, LY17E020010 and LY16E070004). References [1] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2010) 587e603. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [3] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angew. Chem. 51 (2012) 9994e10024. [4] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682. [5] B. Scrosati, J. Hassoun, Y.-K. Sun, Lithium-ion batteries. A look into the future, Energy Environ. Sci. 4 (2011) 3287. [6] L. Liang, X. Sun, C. Wu, L. Hou, J. Sun, X. Zhang, C. Yuan, Nasicon-type surface functional modification in core shell LiNi0.5Mn0.3Co0.2O2@NaTi2(PO4)3 cathode enhances its high-voltage cycling stability and rate capacity toward Li-ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 5498e5510. [7] H. Yang, K. Du, G. Hu, Z. Peng, Y. Cao, K. Wu, Y. Lu, X. Qi, K. Mu, J. Wu, Graphene@TiO2 co-modified LiNi0.6Co0.2Mn0.2O2 cathode materials with enhanced electrochemical performance under harsh conditions, Electrochim. Acta 289 (2018) 149e157. [8] K. Wu, K. Du, G. Hu, Red-blood-cell-like (NH4)[Fe2(OH)(PO4)2] 2H2O particles: fabrication and application in high-performance LiFePO4 cathode materials, J. Mater. Chem. A 6 (2018) 1057e1066. [9] J. Wang, X. Sun, Understanding and recent development of carbon coating on LiFePO4cathode materials for lithium-ion batteries, Energy Environ. Sci. 5 (2012) 5163e5185. [10] J. Liu, F. Liu, K. Gao, J. Wu, D. Xue, Recent developments in the chemical synthesis of inorganic porous capsules, J. Mater. Chem. 19 (2009) 6073. [11] D. Aurbach, B. Markovsky, G. Salitra, E. Markevich, Y. Talyossef, M. Koltypin, L. Nazar, B. Ellis, D. Kovacheva, Review on electrodeeelectrolyte solution interactions, related to cathode materials for Li-ion batteries, J. Power Sources 165 (2007) 491e499. [12] H. Yu, H. Kim, Y. Wang, P. He, D. Asakura, Y. Nakamura, H. Zhou, High-energy 'composite' layered manganese-rich cathode materials via controlling Li2MnO3 phase activation for lithium-ion batteries, Phys. Chem. Chem. Phys. :
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Zhao, Improving the electrochemical properties of high-voltage lithium nickel manganese oxide by surface coating with vanadium oxides for lithium ion batteries, J. Power Sources 280 (2015) 114e124. [36] X. Xiong, Z. Wang, H. Guo, Q. Zhang, X. Li, Enhanced electrochemical properties of lithium-reactive V2O5 coated on the LiNi0.8Co0.1Mn0.1O2 cathode material for lithium ion batteries at 60 C, J. Mater. Chem. A 1 (2013) 1284e1288. [37] X. Yan, L. Chen, S.A. Shah, J. Liang, Z. Liu, The effect of Co3O4 & LiCoO2 cladding layer on the high rate and storage property of high nickel material LiNi0.8Co0.15Al0.05O2 by simple one-step wet coating method, Electrochim. Acta 249 (2017) 179e188. [38] K.-S. Lee, S.-T. Myung, H. Bang, K. Amine, D.-W. Kim, Y.-K. Sun, Effect of protecting metal oxide (Co3O4) layer on electrochemical properties of spinel Li1.1Mn1.9O4 as a cathode material for lithium battery applications, J. Power Sources 189 (2009) 494e498.
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Improved high rate capability of Li[Li0.2Mn0.534Co0.133Ni0.133]O2 cathode material by surface modification with Co3O4 Yu Li, Hui Huang**, Jiage Yu, Yang Xia, Chu Liang, Yongping Gan, Jun Zhang, Wenkui Zhang* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2018 Received in revised form 24 December 2018 Accepted 30 December 2018 Available online 31 December 2018
High capacity Li- and Mn-rich layered oxides are of particular interest as cathode materials for lithiumion batteries, but large initial irreversible capacity, inferior poor cycle stability and rate capability greatly limited large-scale commercial application. In this work, we propose an effective strategy via surface modification of Co3O4 on surface of Li[Li0.2Mn0.534Co0.133Ni0.133]O2 (LMCNO) to improve the electrochemical performance. In comparison with pristine LMCNO, all the Co3O4-modified LMCNO composites show higher capacity, better cycling stability and rate capability. Among them, 1.5 wt% Co3O4-modified LMCNO shows high initial capacity of 253.3 mA h g1, good capacity retention of 73.3% after 80 cycles at 0.2C and much improved rate capability especially at high rates. The enhanced electrochemical properties can be attributed to protecting LMCNO bulk materials from the electrolyte attack by Co3O4 surface layer as well as its high ionic/electronic conductivity. Electrochemical impedance spectroscopy reveals that the Co3O4-modified LMCNO sample shows much smaller charge transfer resistance than pristine sample. © 2019 Elsevier B.V. All rights reserved.
Keywords: Li- and Mn-Rich layered oxide Co3O4 modification Surface modification Electrochemical performance Li-ion batteries
1. Introduction Lithium-ion batteries (LIBs) are considered to be a new generation of power source for their high output voltage, high specific capacity, long cycle life and low self-discharge rate [1e8]. In recent years, extensive researches have been conducted on LIBs to further improve the energy density and cycling capability. The main driving force is that the current LIBs technology cannot fully meet the rapid rising market demands for electric vehicles (EVs) and hybrid electric vehicles (HEVs) [9e11]. Cathode materials are particularly in need of an advance in technology because of much lower capacities relative to the commercial Si/C anode materials. Among several candidates capable of offering a high capacity, Liand Mn-rich layered oxides materials with the chemical formula xLi2MnO3$(1-x)LiMO2 (0 < x < 1, M ¼ Co, Ni, Mn, etc.) have gained special attention by virtue of high specific capacity over 250 mA h g1, originating from transition metal redox reaction and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Zhang).
(H.
Huang),
https://doi.org/10.1016/j.jallcom.2018.12.357 0925-8388/© 2019 Elsevier B.V. All rights reserved.
[email protected]
unusual oxygen anion redox reaction. The pioneering work of Zhou [12] and Thackeray [13,14] demonstrated that Li- and Mn-rich layered oxides are nanocomposites, composed of rhombohedral LiMO2 phase (space group: R-3m) and monoclinic Li2MnO3-like phase (space group: C2/m). However, Li- and Mn-rich layered oxides suffers from intractable drawbacks such as low initial coulombic efficiency, inferior rate capability, fast voltage and capacity fading when charged over 4.5 V vs Li/Liþ compared with other LiCoO2 and Layered Ni-rich cathode materials such as LiNi0.6Co0.2Mn0.2O2 [15e17], LiNi0.8Co0.1Mn0.1O2 [18e20] and LiNi0.8Co0.15Al0.05O2 [21e23]. It is well known that the low initial columbic efficiency of the Liand Mn-rich layered oxides originates from the irreversible oxygen loss occurring around 4.5 V during the first charge process. The fast voltage and capacity decay arises from the migration of transition metal into the Li layer, which results in a phase transformation from layered to spinel-like structure and the dissolution of metal elements into the electrolyte. In order to solve these issues, several methods have been adopted to improve the electrochemical performance of Li- and Mn-rich layered oxides, such as surface modification [24] and ion doping/substitution [25]. Surface modification has been proved an effective way to improve the electrochemical performance. A proper modification layer can protect the
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surface from directly contact with the electrolyte and suppress the side reactions. Meanwhile, the modification layer with high ionic/ electronic conductivity can also improve the rate capacity by facilitating the charge transfer at the surface of particles. Metal oxides (Al2O3 [26], MgO [27], ZrO2 [28]), phosphates (AlPO4 [29], Li3PO4 [30], FePO4 [31]), and fluorides (AlF3 [32], CaF2 [33] and ZrF4 [34]) have been employed as the modification materials to enhance the electrochemical performance of layered cathode materials. Regrettably, these modification materials are electrochemically inert or ionic insulators, which mainly act as a protection layer to alleviate the side reactions between electrode and electrolyte and the layered-to-spinel transformation. Electrochemically active materials like V2O5 [35] have been reported to effectively improve the cycling stability and storage characteristics of pristine LiNi0.8Co0.1Mn0.1O2 material [36]. Co3O4 has been also introduced to ameliorate the lithium storage property of LiNi0.6Co0.2Mn0.2O2, Li1.1Mn1.9O4 and LiNi0.8Co0.15Al0.05O2 by acting as HF scavenger and protecting electrode materials from electrolyte attack. Yan et al. [37] discovered that Co3O4 could react with electrolyte and serve as a HF scavenger during the cycling process of NCA, thus reduce the acidity and relieve the corrosion of NCA surface in the acidic electrolyte. Through TOF-SIMS analysis, Lee et al. [38] found that the Co3O4 coating layer would react with the generated HF, which was expected to result in the formation of Co-F layer on the outermost surface eventually. The special interest for Co3O4 modification material is related to its high ionic/electronic conductivity, which can achieve superior cycling capability and rate capability at a high cutoff voltage. To our knowledge, Co3O4 as modification materials to improve the electrochemical properties of Li- and Mn-rich cathode materials has rarely been reported. In this work, Co3O4 were introduced to modify Li- and Mn-rich layered oxides (Li [Li0.2Mn0.534Co0.133Ni0.133]O2, referred to as LMCNO hereafter) by simple wet chemical method. The effects of Co3O4 modification on the electrochemical performance of LMCNO samples are discussed in detail in this work.
schematically illustrated in Fig. 1. The pristine LMCNO was prepared by a co-precipitation method. Firstly, Mn(Ac)2$4H2O, Ni(Ac)2$4H2O and Co(Ac)2$4H2O with a chemical stoichiometric ratio of 4:1:1 were dissolved in distilled water. Secondly, the mixed solution of NaOH and ammonia were added dropwise into the above mixtures with violently stirring. The co-precipitation reaction was carried out under N2 atmosphere for 24 h and the pH value was kept at 11. Afterwards, the hydroxide precursor was filtered, washed until pH ¼ 7 and dried at 80 C for 12 h. Finally, the hydroxide precursor was thoroughly mixed with lithium carbonate with a required amount (5% rich to offset Li loss during the sintering) by planetary ball milling, calcined at 500 C for 5 h and at 900 C for 12 h in air to get LMCNO sample. The Co3O4-LMCNO samples were prepared by a wet-chemical method. For modified with Co3O4, the LMCNO powders were slowly added into ethanol. Then the temperature was slowly increased to 40 C with violent stirring. Subsequently, a certain amount of Co(NO3)2$6H2O, which corresponds to the Co3O4 content of about 1 wt %, 1.5 wt % or 2 wt % in sequence, was dissolved in ethanol. Then the Co(NO3)2 solution was added dropwise into the above LMCNO suspension. The mixed solution was stirred at 40 C continuously until the ethanol was evaporated. Subsequently, the mixtures were dried at 80 C for 10 h and calcined at 600 C for 5 h in air. 2.2. Materials characterizations The structure and crystalline phase of the pristine LMCNO and Co3O4-LMCNO samples were confirmed by the X-ray diffraction with a Cu-Ka radiation (l ¼ 1.5418 Å). The surface morphology of the samples was investigated by a scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, FEI Tecnai G2 F30) with an energy dispersive spectroscopy (EDS) detector. 2.3. Electrochemical measurements
2. Experimental 2.1. Preparation of materials All the chemical reagents were used directly without purification. The synthesis processes of LMCNO and Co3O4-LMCNO are
For the electrode fabrication, the active materials (LMCNO and Co3O4-LMCNO), Super P and polyvinylidene fluoride (PVDF) binder were mixed at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidinone (NMP) to obtain a slurry. Then, the slurry was coated onto an Al foil current collector, dried at 80 C in a vacuum oven overnight. The
Fig. 1. Schematic illustration of the synthesis of Co3O4-modified LMCNO samples.
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mass loading of the active material was near 3 mg cm2 for each electrode. The CR2025 coin cells were assembled in an argon-filled glove box (H2O < 0.5 ppm, O2 < 0.5 ppm), using a lithium foil as the counter electrode and a Celgard 2500 membrane as the separator. The electrolyte solution was composed of 1 mol L1 LiPF6 dissolved in ethylene carbonate (EC), ethyl-methyl carbonates (EMC) and diethyl carbonate (DEC) with a 1:1:1 vol ratio. Galvanostatically discharge/charge experiments was conducted on a Neware battery test system in the voltage range of 2.0e4.8 V. The cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were performed on a CHI650B electrochemical workstation. The CV measurement was tested at a scan rate of 0.1 mV s1 in the voltage range of 2.0e4.8 V and EIS was tested in the frequency range between 0.1 Hz and 1 MHz with a 5 mV amplitude. 3. Results and discussion Fig. 2 shows XRD patterns of the pristine LMCNO and 1 wt%, 1.5 wt% or 2 wt% Co3O4-LMCNO composites. All the samples reveal a typical layered hexagonal structure of a-NaFeO2 with the space group R-3m. The clear splitting of (006)/(012) and (108)/(110) peaks indicates a well-ordered layered structure. The weak peak at 2q ¼ 2123 corresponds to a little Li and Mn in the transition metal layers, indicating the presence of Li2MnO3 phase (C2/m space group). Moreover, there is no obvious change in the XRD patterns of LMCNO and Co3O4-LMCNO samples, which implies that the introduction of Co3O4 does not change the structure of LMCNO. Meanwhile, there is no characteristic diffraction peak of Co3O4 observed in the XRD patterns, this may be resulted from the low content of Co3O4 modification. Fig. 3 shows the SEM images of LMCNO and Co3O4-LMCNO samples. All the samples show similar particle morphology with the size distribution of approximately 100e500 nm. As shown in Fig. 3, pristine LMCNO particles shows a clean and smooth surface, while Co3O4-LMCNO samples exhibit rough surface morphology, and Co3O4 nanoparticles are distributed uniformly on the surface of LMCNO. Meanwhile, the number of nanoparticles increases gradually with the increase of Co3O4 from 1 to 2 wt%. To further confirm the presence of Co3O4 on the surface of bulk particles, the highmagnification TEM images of LMCNO and 1.5 wt% Co3O4-LMCNO and 2 wt% Co3O4-LMCNO samples are presented in Fig. 4. As shown in Fig. 4a and b, the pristine LMCNO displays clear lattice fringes (003) with a d-spacing of 0.47 nm, and no impurity layer can be observed on its surface. In contrast, a distinguishable surface layer can be observed on the surface of 1.5 wt% Co3O4-LMCNO (Fig. 4c
Fig. 3. SEM images of the pristine LMCNO and Co3O4-LMCNO samples: (a) pristine LMCNO; (b) 1 wt% Co3O4-LMCNO; (c) 1.5 wt% Co3O4-LMCNO; (d) 2 wt% Co3O4-LMCNO.
and d) and 2 wt% Co3O4-LMCNO (Fig. 4e and f). From Fig. 4c and e, we can see that the thickness of Co3O4 layer is about 15 nm. The lattice spacing of 0.24 nm is attributed to the (311) plane of Co3O4 crystallite. The chemical composition of surface layer can be further confirmed by the STEM image and its EDS elemental mapping profiles in Fig. 4g. As can be seen in the red dashed region, the Co content shows a significant increase as compared to Mn and Ni elements. The result implies the effective modification of Co3O4 on the surface of LMCNO. To study the effects of the Co3O4 surface modification on the electrochemical properties, the pristine LMCNO and Co3O4-modified samples were evaluated by using 2025 coin-type half cells with Li anodes. The charge/discharge curves at a 0.1C rate (30 mA h g1) in the voltage range of 2.0e4.8 V are shown in Fig. 5. In the first charging process, all the samples exhibit a typical two-step characteristic of Li-rich layered oxides, including a smooth voltage ramp below 4.5 V which can be ascribed to the lithium ions extraction from the LiMO2 component and a potential plateau at 4.5 V, corresponding to the simultaneous removal of oxygen from the Li2MnO3 component. However, this long plateau vanishes in the subsequent charging process. During the discharging process, the samples exhibit almost the same profiles with a long voltage ramp. The first charge and discharge capacity of pristine LMCNO is
Fig. 2. XRD patterns of the pristine LMCNO and Co3O4-LMCNO samples.
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Fig. 4. (aeb) TEM and HRTEM images of pristine LMCNO; (ced) TEM and HRTEM images of 1.5 wt% Co3O4-LMCNO; (eef) TEM and HRTEM images of 2 wt% Co3O4-LMCNO sample. (g) HAADF-STEM image and element mappings of 1.5 wt% Co3O4-LMCNO.
371.95 mA h g1 and 271.09 mA h g1, respectively. The capacity loss reaches as high as 27%. After the Co3O4 modification, the capacity loss of the Co3O4-LMCNO samples is dramatically decreased to 20e21%. It is believed that the surface layer of Co3O4 can increase ion channels and prevent the electrode surface from directly contact with the electrolyte, thereby slowing the surface structural rearrangement and the oxygen release process [39,40]. According to previous works [40e42], the dissolution of metal elements and the formation of oxygen vacancies are the main causes of voltage fading. After 80 cycles, the Co3O4-LMCNO samples also show smaller voltage fading than pristine LMCNO, indicating that the surface modification of Co3O4 can inhibit the side reactions between cathode material and electrolyte. Fig. 6 indicates the CV curves of LMCNO and Co3O4-LMCNO for the initial three cycles and the 80th cycle in the voltage range of
2e4.8 V. Each sample shows a similar CV profile for the initial three cycles. One oxidation peak at 4.0 V is associated with the extraction of Liþ from the lithium layer and the oxidation of Ni2þ to Ni4þ and Co3þ to Co4þ. This oxidation peak slightly shifts to the low potential of 3.8 V in the subsequent cycling. Correspondingly, the reduction peak at 3.2 V is ascribed to the reduction from Ni4þ to Ni2þand Co4þ to Co3þ. Another sharp oxidation peak at around 4.6 V reveals the irreversible removal of Li2O from the activated phase of Li2MnO3 to form MnO2, which only appears in the first cycle leading to a low initial Coulombic efficiency [43,44]. After 80 cycles, the redox peak shifts outward and the peak potential of electrode decreased significantly, which are related to the degradation of active particles on the surface of materials [45]. However, the potential interval between the anode and cathode peaks of Co3O4-LMCNO is smaller than that of LMCNO, and the intensity of redox peak in Co3O4-
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Fig. 5. Charge/discharge curves of pristine LMCNO: (a) 1 wt% Co3O4@LMCNO; (b) 1.5 wt% Co3O4@LMCNO; (c) 2 wt% Co3O4@LMCNO; (d) at a current density of 0.1C.
Fig. 6. CV curves of pristine LMCNO and Co3O4@LMCNO samples.
LMCNO decreases more slowly. Results above indicate that the surface modification of Co3O4 improves the reaction kinetics of LMCNO, resulting in better rate performance and cycle stability. Fig. 7 compares the cycling performance of pristine LMCNO and Co3O4-LMCNO samples in the voltage range of 2e4.8 V. At a low rate of 0.2C (60 mA g1), all the coated samples show better cycle stability than pristine LMCNO. After 80 cycles, the discharge capacity of pristine LMCNO drops from 247.0 mA h g1 to
158.8 mA h g1 with the capacity retention of 64.3%. In contrast, the 1.5 wt% Co3O4-LMCNO sample shows the highest capacity retention of 73.4% among all samples. Although the 2 wt% Co3O4-LMCNO sample just delivers a capacity of 170.0 mA h g1 after 80 cycles, the capacity retention of 69.7% is still higher than that of pristine LMCNO. The cycle performance at a high current density of 5C (1500 mA g1) are also shown in Fig. 7b. It can be clearly seen that the cycling capability of the LMCNO sample has been significantly
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Fig. 7. (aec) Cycling performance of pristine LMCNO and Co3O4@LMCNO (0.2C, 5C and 10C); (d) rate performance of pristine LMCNO and Co3O4@LMCNO samples.
improved by the surface modification of Co3O4 at a high rate. The 1.5 wt % Co3O4-LMCNO sample delivers the highest capacity, which is 94.3 mA h g1 after 400 cycles at 5C, while pristine LMCNO only maintains 66.7 mA h g1 under the same conditions, corresponding to the capacity retention of 71.2% and 58.3%, respectively. In Fig. 7c, similar results are obtained when the rate rises to 10C (3000 mA g1), the high rate capability of the Co3O4-LMCNO samples is obviously superior to pristine LMCNO. The results reveal that the Co3O4 surface modification layer with high ionic/electronic conductivity can effectively improve the rate capability since it facilitates the charge transfer at the surface of particles. To further evaluate the rate performance, all samples were tested by successively increased the rate from 0.1 to 5C. It can be seen that the specific capacities of pristine LMCNO and Co3O4-LMCNO samples are very close at 0.1C. With the increasing of current density, 1.5 wt % Co3O4-LMCNO shows the best rate capability. The reversible capacities at 0.1, 0.2, 0.5, 1, 2, and 5C after each 10 cycles are 269.7, 229.7, 185.5, 154.8, 130.1, 95.0 mA h g1, respectively. Additionally, when the current density returned to 0.1C, the delivered capacity still maintains 245.1 mA h g1. It is believed that the geometrical reason is important to achieve high-performance Li-rich layered
oxides materials. In this work, Co3O4 nanoparticles exhibited discontinuous distribution on the surface of LMCNO samples. With the concentration of Co3O4 precursor increased, some Co3O4 nanoparticles will prefer to grow on the surface of Co3O4 rather than the surface of LMCNO. As a result, Co3O4 particles will selfaggregate into large particle size with a poor spatial distribution. So, even if the content of Co3O4 was increased, the electrochemical performance of LMCNO might be not greatly improved. This geometrical reason is why the 1.5 wt% Co3O4-LMCNO exhibits the best rate performance. Fig. 8 shows the Nyquist plots of pristine LMCNO and Co3O4LMCNO samples, and the inset shows the equivalent circuit model. All the samples exhibit a depressed semicircle in the high frequency and a slope line in the low frequency. Generally, the intercept of the 0 high frequency semicircle on the Z axis is attributed to the electrolyte resistance (Re), the depressed semicircle in the high frequency corresponds to the charge transfer resistance (Rct), and the slope line in the low frequency is related to the Warburg impedance (Zw) of Liþ diffusion into electrode. It is clear that the semicircle size of Co3O4-LMCNO is much smaller as compared to pristine LMCNO. The EIS data can be obtained basing on the equivalent circuit model
Fig. 8. (a) Nyquist plots of pristine LMCNO and Co3O4@LMCNO. The inset is the corresponding equivalent circuit. (b) relationship between Z0 and u1/2 at low frequency region.
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in the inset of Fig. 8. The Re values of LMCNO, 1 wt%, 1.5 wt% and 2 wt% Co3O4-LMCNO are 2.2, 2.7, 2.3 and 2.3 U, the Rct values are 101.4, 51.3, 58.7 and 82.3 U, respectively. The small Rct value indicates that the presence of Co3O4 on the surface of LMCNO is beneficial to achieve good electrochemical kinetics and high rate capability. By calculating the Warburg coefficient (s) derived from the low frequency region, we found that the Co3O4-LMCNO samples exhibit higher Liþ diffusion coefficient than the pristine LMCNO, corresponding to higher reaction kinetics, as shown in Fig. 8b. 4. Conclusions In conclusion, LMCNO samples have been synthesized through co-precipitation and following calcination in air. Wet-chemical method was used to modify LMCNO with Co3O4. Materials characterizations demonstrate that Co3O4 modifier does not change the structure of LMCNO, just make the surface rough. The introduction of Co3O4 can improve the reaction reversibility, cycling stability, high rate performance. In all samples, 1.5 wt% Co3O4-modified LMCNO shows high initial capacity of 253.3 mA h g1, good capacity retention of 73.3% after 80 cycles at 0.2C and much improved rate capability especially at high rates. Results above indicate that the appropriate content of Co3O4 modifier can improve the electrochemical properties of LMCNO efficiently. This idea may provide a new way to develop lithium-rich layered oxides cathode materials with outstanding lithium storage performance. Acknowledgements The authors thank financial support from the National Natural Science Foundation of China (Grant no. 21403196, 51572240, 51677170 and 51777194), Natural Science Foundation of Zhejiang Province (Grant no. LY18B030008, LY17E020010 and LY16E070004). References [1] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2010) 587e603. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [3] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angew. Chem. 51 (2012) 9994e10024. [4] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682. [5] B. Scrosati, J. Hassoun, Y.-K. Sun, Lithium-ion batteries. A look into the future, Energy Environ. Sci. 4 (2011) 3287. [6] L. Liang, X. Sun, C. Wu, L. Hou, J. Sun, X. Zhang, C. Yuan, Nasicon-type surface functional modification in core shell LiNi0.5Mn0.3Co0.2O2@NaTi2(PO4)3 cathode enhances its high-voltage cycling stability and rate capacity toward Li-ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 5498e5510. [7] H. Yang, K. Du, G. Hu, Z. Peng, Y. Cao, K. Wu, Y. Lu, X. Qi, K. Mu, J. Wu, Graphene@TiO2 co-modified LiNi0.6Co0.2Mn0.2O2 cathode materials with enhanced electrochemical performance under harsh conditions, Electrochim. Acta 289 (2018) 149e157. [8] K. Wu, K. Du, G. Hu, Red-blood-cell-like (NH4)[Fe2(OH)(PO4)2] 2H2O particles: fabrication and application in high-performance LiFePO4 cathode materials, J. Mater. Chem. A 6 (2018) 1057e1066. [9] J. Wang, X. Sun, Understanding and recent development of carbon coating on LiFePO4cathode materials for lithium-ion batteries, Energy Environ. Sci. 5 (2012) 5163e5185. [10] J. Liu, F. Liu, K. Gao, J. Wu, D. Xue, Recent developments in the chemical synthesis of inorganic porous capsules, J. Mater. Chem. 19 (2009) 6073. [11] D. Aurbach, B. Markovsky, G. Salitra, E. Markevich, Y. Talyossef, M. Koltypin, L. Nazar, B. Ellis, D. Kovacheva, Review on electrodeeelectrolyte solution interactions, related to cathode materials for Li-ion batteries, J. Power Sources 165 (2007) 491e499. [12] H. Yu, H. Kim, Y. Wang, P. He, D. Asakura, Y. Nakamura, H. Zhou, High-energy 'composite' layered manganese-rich cathode materials via controlling Li2MnO3 phase activation for lithium-ion batteries, Phys. Chem. Chem. Phys. :
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