- Email: [email protected]
Stabilizing the high-voltage cycle performance of LiNi0.8Co0.1Mn0.1O2 cathode material by Mg doping
Journal of Power Sources 438 (2019) 227017
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Stabilizing the high-voltage cycle performance of LiNi0.8Co0.1Mn0.1O2 cathode material by Mg doping Xiaolan Liu a, Shuo Wang a, Li Wang a, Ke Wang a, Xiaozhong Wu a, Pengfei Zhou a, b, *, Zhichao Miao a, Jin Zhou a, Yi Zhao a, Shuping Zhuo a, ** a b
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, PR China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, China
H I G H L I G H T S
� LiNi0.8Co0.1Mn0.1O2 cathode material is modified by Mg doping. � The Liþ/Ni2þ mixing of LiNi0.8Co0.1Mn0.1O2 material is reduced by Mg doping. � NCMMg0.03 material exhibits improved cycling retention and thermal stability. � The NCMMg0.03//MCMB full cell delivers an energy density of 595.3 W h kg 1 at 0.5 C. A R T I C L E I N F O
A B S T R A C T
Keywords: Mg doping LiNi0.8Co0.1Mn0.1O2 cathode material High voltage Cycle stability Lithium-ion batteries
LiNi0.8Co0.1Mn0.1O2 is considered as a promising cathode material for lithium ion batteries because of its high capacity and low cost. However, the LiNi0.8Co0.1Mn0.1O2 suffers structural instability and irreversible phase transition during charge/discharge processes, especially under high voltage, resulting in serious capacity fading and thermal runaway. Here, we propose a simple and effective method of modifying LiNi0.8Co0.1Mn0.1O2 by Mg doping. Benefiting from the pillaring effects of inactive Mg in the crystal structure, Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials exhibit low Liþ/Ni2þ cation mixing, high structural stability, and improved cyclic stability in the voltage of 3.0–4.5 V. The optimal Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 achieves a high capacity retention of 81% over 350 cycles at 0.5 C and exhibits enhanced thermal stability at 4.5 V. The promotion mechanism is explored systematically by a combination study of electrochemical characterizations, demonstrating the faster Liþ diffu sion kinetics, higher electronic conductivity, and stronger structure due to the Mg doping. Moreover, the full cell of Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2//mesocarbon microbeads delivers a promising energy density of 595.3 W h kg 1 at 0.5 C (based on the mass of the cathode). The present work demonstrates that moderate Mg doping is a facile yet effective strategy to modify high-performance LiNi0.8Co0.1Mn0.1O2 for high-voltage lithium ion batteries.
1. Introduction Lithium-ion batteries (LIBs) with higher energy density, longer cycle life, more safety performance and lower price are increasingly deman ded [1–5]. Cathode materials play a key role in LIBs and their im provements are crucial for enhancing energy density of LIBs. Nowadays, exploring high energy density, lower production cost and high safety cathode materials for LIBs is still a great challenge. Among the cathode materials, layered lithium nickel cobalt manganese oxides
(LiNixCoyMnzO2, NCM) [6] and their derivatives have attracted partic ular attention due to their high potential capacity, high working po tential, and low production cost [7]. In particular, LiN0.8Co0.1Mn0.1O2 (NCM811) shows extraordinary promise for high energy density LIBs due to their high capacity of above 220 mA h g 1 and the voltage plat form of about 3.7 V. Unfortunately, the practical and large-scale appli cation of NCM811 has been impeded by the serious capacity fading and thermal run-away (especially charge to high cutoff voltage), which re sults from the structural instability caused by the Liþ/Ni2þ cation mixing
* Corresponding author. School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, PR China. ** Corresponding author. E-mail addresses: [email protected] (P. Zhou), [email protected] (S. Zhuo). https://doi.org/10.1016/j.jpowsour.2019.227017 Received 4 June 2019; Received in revised form 25 July 2019; Accepted 13 August 2019 Available online 16 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
in the phase structure [8,9]. During the charging process, the Ni2þ in Li sites is oxidized to smaller Ni3þ/4þ (0.056 and 0.053 nm) [10], which deteriorates the layered structure and leads to irreversible phase trans formation from layered structure to disordered spinel and rolk-salt structure [11–16]. Therefore, it is still a significant challenge to push forward the large-scale application of NCM811 materials. Accordingly, strenuous efforts have been devoted to enhance the electrochemical performance of NCM811 materials, including element doping, electrolyte optimization, and surface coating. In particular, cation doping such as Mg2þ [17–21], Al3þ [22–24], Ti4þ [25], Zr4þ [26, 27], Nb5þ [28,29], and Cr3þ [30,31], has been demonstrated as an effective strategy to improve the electrochemical performance of NCM materials. Among the various substituted cations, electrochemically inactive Mg2þ (0.072 nm) doping is attractive. The doped Mg can distribute in the interlayer Li slabs and suppress Liþ/Ni2þ cation mixing owing to their similar ionic radius. Li et al. [32] found that the Mg doping can stabilize the layered structure, suppress Liþ/Ni2þ cation mixing, and enhance the electrochemical property of LiNi0.90 Co0.07Mg0.03O2 material. By theoretical calculation, Min et al. [33] found that the Mg doping can inhibit the change of lattice parameters and improve the structural stability of LiNi0.8Co0.1Mn0.1O2 material during (de)lithiation processes. Fu et al. [34] and Huang et al. [35,36] reported Mg substitute for each Co, Mn, and Ni in LiNi0.6Co0.2Mn0.2O2, respectively, and the results show that Mg doping can improve the electrochemical performance and structural stability of the LiNi0.6 Co0.2Mn0.2O2 material. In addition, Mg is also used as a doping element in LiNi0.80Co0.15Al0.05O2, LiNi0.4Co0.2Mn0.4O2, Li[Li0.2Ni0.2Mn0.6]O2 [37–39] materials, which also achieved good research results. However, to the best of our knowledge, there is no report about Mg doping in NCM811 materials, leaving much room to develop advanced cathode materials for LIBs. Herein, the effect of Mg doping on the structural and electrochemical performance of NCM811 material has been investigated. The Li (Ni0.8Co0.1Mn0.1)1-xMgxO2 samples with different Mg contents were prepared by a solid-state synthesis process. It was found that the Mg doping does affect the phase structure and electrochemical performance of NCM811 material. Compared with the pure NCM811, the optimal Li (Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 sample possesses a good layered structure with low Liþ/Ni2þ mixing, delivers an impressive high initial reversible capacity with good cycle performance in 3.0–4.5 V, and exhibits admi rable thermal stability. When matched with mesocarbon microbeads (denoted as MCMB), the as-fabricated full cell shows an excellent energy density of 595.3 W h kg 1 at 0.5 C (based on the mass of the cathode). The above findings verify that the proper amount of Mg doping can stabilize the layered structure and improve the electrochemical perfor mance of NCM811 material in the voltage of 3.0–4.5 V.
2.2. Materials characterization The phase structure of all samples was measured by X-ray diffraction (XRD, Rigaku MiniFlex600) using Cu Kα radiation. The morphologies of the samples were characterized by scanning electron microscopy (SEM, JEOL JSM7500F) and transmission electron microscopy (TEM, Philips Tecnai-F20). The particle size distribution of Ni0.8Co0.1Mn0.1(OH)2 was measured by laser particle size analyser (MASTER SIZER 3000). Surface area analysis was carried out by measuring the N2 adsorption-desorption isotherms at 77 K on a BELSORP-mini instrument. The thermal stability of the cathode material at a delithiated state of 4.5 V was examined with a differential scanning calorimetry (DSC, Netzsch STA 449F3 Jupiter thermal analysis system) from 30 to 350 � C at a heating rate of 10 � C min 1. The accurate component of the synthesized samples was char acterized by inductively coupled plasma emission spectrometer (ICPAES PerkinElmer Optima 8300). 2.3. Electrochemical measurement The cathode electrodes were prepared by blending 80 wt% active materials, 10 wt% Super P and 10 wt% poly vinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone (NMP). The mixed slurry was coated onto aluminum foil and dried at 100 � C for 12 h in vacuum. The electrode was cut into pieces with the diameter of 12 mm and the cathode material loading was about 3 mg. For electrochemical tests, the CR2032 coin-type cells were assembled. The cell was consisted of a cathode, Li foil as counter electrode, polypropylene membrane with micropores (Celgard 2400) as the separator, and 1.15 mol L 1 LiPF6 in 1:2:2 (volume) mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) as electrolyte. This process is carried out in an argon-filled glove box (Mikrouna Universal 2440/750) with water and oxygen levels below 1 ppm. The cells were charged/discharged at different rates (1 C ¼ 190 mA g 1) between 3.0 and 4.5 V. Cyclic vol tammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed on an IVIUM electrochemical workstation. The electron conductivity was measured according to standard EIS procedure on pellets (IVIUM electrochemical workstation). The galvanostatic inter mittent titration technique (GITT) was tested with a charging/dis charging constant current of 0.1 C (5 C) for 10 min (1 min), then it was relaxed for 40 min between 3.0 and 4.5 V. For full-cell demonstration, the MCMB was used as anode and the ratio of cathode to anode was about 1.7. All tests were carried out at room temperature 25 � C. 3. Results and discussion The SEM image (Fig. 1a) exhibits that the Ni0.8Co0.1Mn0.1(OH)2 precursor has a typical spherical shape. The microspheres are composed of a plurality of sheet-like primary grains (Fig. 1b) with a BET specific surface area of 129.9 m2 g 1 (Fig. S1). The SEM energy-dispersive spectrometer (EDS) mappings of Ni0.8Co0.1Mn0.1(OH)2 microsphere (Fig. 1c–e) reveal the uniform distributions of Ni, Co and Mn elements along the spherical outline. Moreover, the particle size distribution (Fig. 1f) shows that Ni0.8Co0.1Mn0.1(OH)2 has a relatively single and narrow particle size distribution, and the particle diameter of D10, D50, D90 is 6.39, 11.65, 21.71 μm, respectively. Furthermore, The XRD pattern (Fig. 1g) of Ni0.8Co0.1Mn0.1(OH)2 coincides with the standard Ni (OH)2 (JCPDS No. 14–0117), indicating the formation of a homoge neous ternary hydroxide solid solution and high phase purity. The XRD patterns of Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials are shown in Fig. 2a and b. It can be seen that the NCM811, NCMMg0.01, NCMMg0.03, NCMMg0.05, and NCMMg0.06 have the same characteristic diffraction peaks as the hexagonal LiNiO2 (JCPSD No. 09–0063). The compositions of the prepared samples were measured by ICP-AES and the results are close to those of the designed values (Table S1). For NCM materials, apart from the high phase purity and crystallinity, a low Liþ/ Ni2þ cation mixing degree and an ordered layered structure is also very
2. Experimental 2.1. Material preparation The spherical Ni0.8Co0.1Mn0.1(OH)2 precursor was prepared before hand by co-precipitation method [40]. In order to prepare (Ni0.8C o0.1Mn0.1)1-xMgx(OH)2 precursor, MgSO4 was dissolved into 100 mL deionized water by continuous stirring at a temperature of 30 � C, then the Ni0.8Co0.1Mn0.1(OH)2 was added to the solution and kept stirring for 1 h, followed by the addition of moderate NaOH solution. Then the (Ni0.8Co0.1Mn0.1)1-xMgx(OH)2 precursor and LiOH⋅H2O were thoroughly mixed in an agate mortar and calcinated at 550 � C for 4 h and 780 � C for 12 h under oxygen to obtain the corresponding Li(Ni0.8C o0.1Mn0.1)1-xMgxO2 (x ¼ 0, 0.01, 0.03, 0.05, 0.06) materials, which were named as NCM811, NCMMg0.01, NCMMg0.03, NCMMg0.05 and NCMMg0.06.
2
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 1. (a, b) SEM images, (c-e) EDS mapping images, (f) particle size distribution, and (g) XRD pattern of Ni0.8Co0.1Mn0.1(OH)2.
important. At present, the degree of cation mixing and the integrity of the layered structure of NCM materials can be obtained by analyzing the XRD data [41–43]. The higher ratio of c/a and I003/I104 signifies a lower degree of Liþ/Ni2þ mixing while the smaller value of R factor (referred to as (I006þI012)/I101) and the clear splitting of peaks (006)/(012) and (018)/(110) means the layered structure is more ordered. Therefore, the values of c/a, I003/I104 and R factor for Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials are listed in Table 1. The NCMMg0.03 has the largest value of c/a and I003/I104 with the smallest R factor, indicating that the NCMMg0.03 has a lower degree of Liþ/Ni2þ mixing and a well-ordered layered structure. The values of c/a and I003/I104 gradually decrease with the increasing of Mg content, indicating the mixing arrangement of Liþ/Ni2þ increases. This is because fewer Mg locating the Li sites can suppress Ni2þ in the Li layer and reduce Liþ/Ni2þ mixing, while exces sive Mg occupying the Li sites will generate a force to push Liþ to the Ni layer that results in increased Liþ/Ni2þ mixing. The similar results have been observed in LiNi0.6Co0.2Mn0.2-xMgxO2 materials [35]. Moreover, the Rietveld refinement results of NCM811 and NCMMg0.03 are dis played in Fig. 2c and d and Table S2. It is observed that the occupancy of Ni2þ in Li site for NCMMg0.03 is only about 2.1%, much lower than that of NCM811 (3.2%), further revealing that Mg doping obviously de creases the degree of Liþ/Ni2þ mixing and maintains the well-ordered layer structure. Moreover, about 1.7% Mg2þ was counted to be located in Li sites owing to the similar radius between Mg2þ and Liþ, which is consistent with previous report about LiNi0.90Co0.07Mg0.03O2 [32] and Li[Ni(1/3-z)Co(1/3-z)Mn(1/3-z)Mgz]O2 [44]. The schematic structure of the NCMMg0.03 is shown in Fig. 2e. The doped Mg in Li site working as backbone can enlarge the interlayer and support the layer
structure during the charge/discharge processes [32]. It is indicated that the proper amount of Mg doping can effectively reduce the degree of Liþ/Ni2þ cation mixing and stabilize the layered structure. The unique phase structural merits of the as-prepared NCMMg0.03 might lay the foundation for its superior lithium storage property. Fig. 3a and b and Fig. S2 display the SEM images of as-prepared Li (Ni0.8Co0.1Mn0.1)1-xMgxO2 materials. Obviously, all the samples inherit the spherical morphology of Ni0.8Co0.1Mn0.1(OH)2 precursor. Moreover, the elemental distribution of NCMMg0.03 is analyzed by EDS mapping. The distribution of Mg element (Fig. 3f) is evenly overlapped with the elements of Ni (Fig. 3c), Co (Fig. 3d), and Mn (Fig. 3e), which are all uniformly distributed on the microspheres. As depicted in Fig. 3g and h, the secondary microsphere with dense structure is assembled by numerous primary particles with size of around 80 nm in diameter. The HRTEM image of NCMMg0.03 (Fig. 3i) reveals a clear lattice fringes with an interplanar distance of 0.476 nm, corresponding to the (003) crystal plane of LiNiO2 (rhombohedral, R-3m). The result further indicates the high degree of crystallization and well-developed layered structure of NCMMg0.03 material. The electrochemical performance of the as-prepared Li(Ni0.8C o0.1Mn0.1)1-xMgxO2 materials was first investigated by coin-type Li cells. Fig. 4a shows the first charge/discharge profiles of pristine and Mgdoped NCM811 samples at 0.1 C in the potential range of 3.0–4.5 V. The initial specific discharge capacity of NCM811 (191.8 mA h g 1) is lower than that of NCMMg0.01 (203.6 mA h g 1) and NCMMg0.03 (226.1 mA h g 1). But the specific discharge capacity shows a gradual decline for NCMMg0.05 (181.1 mA h g 1) and NCMMg0.06 1 (168.8 mA h g ). It can be seen that the initial charge capacities of 3
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 2. (a) XRD patterns of Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 and (b) enlarged XRD pattern at 37� –39� and 63� –66� . Rietveld refinements of X-ray diffraction pattern for (c) NCM811 and (d) NCMMg0.03. (e) Schematic structure of NCMMg0.03.
176.6, 162.1, and 144.9 mA h g 1, respectively. The discharge specific capacity of NCMMg0.03 is lower than that of NCM811 at 5 C. The reason for this phenomenon could be that the Mg located in Li sites impedes the fast Liþ diffusion when the rate is increased. This will be discussed later. When the current rate returns to 0.1 C, the discharge capacity of NCMMg0.03 could be recovered to 212.4 mA h g 1, implying a strong structural tolerance to rapid Liþ extraction/insertion of NCMMg0.03. Moreover, the superb cyclic stability of NCMMg0.03 is demonstrated in Fig. 4e, affording an initial discharge capacity of 210.9 mA h g 1 at 0.5 C with a capacity retention of 81% over 350 cycles. By contrast, the ca pacity retention is only 67% for the NCM811 after 350 cycles. In general, NCMMg0.03 exhibits better cyclic stability and rate capability than the other components, indicating that proper content of Mg doping can enhance the electrochemical performance of NCM811. The positive ef fect of Mg doping is attributed to the pillar effect of Mg in the crystal structure that can reduce the degree of Liþ/Ni2þ mixing and stabilize the layered structure. Additionally, the change of morphological structure and phase structure of NCMMg0.03 and NCM811 electrode was examined by SEM and XRD after 350 cycles at 0.5 C (Fig. S3). Some microcracks are observed in NCM811 electrode (Fig. S3a). The microcracks act as new reaction sites that increase parasitic reactions and form thick SEI layer, which might inhibit Liþ diffusion, increase resistance, and deteriorate cycle stability. By contrast, the NCMMg0.03 electrode still sustains its original microspherical structure after the long cycling (Fig. S3b). Moreover, the cation mixing and lattice parameters of NCMMg0.03 and NCM811 were evaluated and compared after 350 cycles by XRD patterns (Fig. S3c). The values of I003/I104 is decreased to 1.737 and 1.988 for NCM811 and NCMMg0.03 after 350 cycles, respectively. In comparison with NCM811, NCMMg0.03 has a lower degree of Liþ/Ni2þ mixing after
Table 1 The unit cell parameters c/a, I(003)/I(104) and R factor of Li(Ni0.8Co0.1Mn0.1)1xMgxO2 materials. Sample
a ¼ b(Å)
c (Å)
c/a
I003/I104
R-factor
NCM811 NCMMg0.01 NCMMg0.03 NCMMg0.05 NCMMg0.06
2.87121 2.86898 2.86885 2.86880 2.87160
14.20621 14.19296 14.21622 14.19893 14.20470
4.948 4.947 4.955 4.949 4.947
2.195 2.367 2.386 2.344 2.296
0.431 0.389 0.374 0.402 0.456
NCMMg0.01 and NCMMg0.03 are higher than that of NCM811. The reason for this phenomenon is that the Ni2þ in the Li slab can be oxidized to Ni3þ/4þ during the charging process, which induces a local collapse of the layered structure and impedes further Liþ deintercalation from the structure. The doped Mg in Li sites can support the layered structure and inhibit the collapse of the structure. This is beneficial to facilitate further Liþ deintercalation at high voltage and result in high capacity. The similar result is also observed in Li(Ni0.8Co0.1Mn0.1)1-xZrxO2 [2] and Li (Ni0.8Co0.15Al0.05)1-xMgxO2 [45]. By comparison, NCMMg0.03 sample delivers the highest specific capacity of 226.1 mA h g 1, which is also much superior to those of the other reported Ni-rich NCM materials (Table S3). These results confirm that the proper Mg doping can increase the reversible capacity owing to the low Liþ/Ni2þ cation mixing and stabilized layered structure. However, the over-doping of inactive Mg can result in low capacity owing to the decreased active Ni content. Furthermore, the comparison of rate performance between NCM811 and NCMMg0.03 at different current rates is demonstrated in Fig. 4b–d. At the rate of 0.1, 0.2, 0.5, 1, 2 and 5 C, the discharge capacities of NCM811 are 208, 201.4, 188.2, 175.9, 164.8, and 148.9 mA h g 1, while the corre sponding discharge capacities of NCMMg0.03 are 226.5, 208.2, 189.6, 4
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 3. (a, b) SEM images, (c-f) EDS mapping images, (g, h) TEM images, (i) HRTEM image of NCMMg0.03.
350 cycles, according with its superior cycle stability. Compared with the unit cell volume of uncycled NCM811 and NCMMg0.03 (Table S2), the unit cell volume of cycled NCM811 and NCMMg0.03 is reduced (Table S4). The unit cell volume variation of NCMMg0.03 (ΔV ¼ 0.29 Å3) is lower than that of NCM811 (ΔV ¼ 0.6 Å3), indicating an effect of Mg doping in relieving volume deformation and enhancing structural sta bility. The results further prove that NCMMg0.03 has a stable morphology and phase structure after 350 cycles in the voltage of 3.0–4.5 V, which ensure its excellent electrochemical performance. To further investigate the electrochemical behavior, CV, GITT, and EIS measurements were conducted on NCM811 and NCMMg0.03. Fig. 5a and b shows the first three CV curves of NCM811 and NCMMg0.03 at 0.1 mV s 1. There are three pairs of redox peaks in both samples, which are belong to the phase transitions of hexagonal phase (H1) to the monoclinic phase (M), M to the new hexagonal phase (H2), and H2 to the other hexagonal phase (H3), respectively [46]. In the first cycle, the voltage difference between the major oxidation peak potential (3.872 V) and reduction peak potential (3.720 V) is 0.152 V for NCMMg0.03, which is much lower than that of NCM811 (0.188 V), suggesting that the Mg doping can decrease the polarization and improve the reversibility of NCM811. Moreover, compared with NCM811, the CV curves of NCMMg0.03 in the subsequent two cycles almost overlap, revealing the superior reversibility during Liþ insertion/extraction. To understand the electrochemical kinetics of NCMMg0.03, the GITT was used to investigate the Liþ diffusion coefficient ðDLiþ Þ. Fig. 5c and Fig. S4a show the GITT curves of NCMMg0.03 and NCM811 at 0.1 C. Obviously, compared with NCMMg0.03, the NCM811 electrode exhibits the higher overpotential, which is especially noticeable at the very end of each charge/discharge process. According to Fick’s second law of diffusion and its simplified calculation formula, the value of DLiþ can be calculated according to eqn (1): � �2 �2 � � � 4 m B VM △ES (1) DLiþ ¼ τ＜＜L2 DLiþ πτ MB A △Eτ
Fig. 4. (a) The initial charge/discharge curves of Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 at 0.1 C. (b-d) Rate performance and (e) Long-life cycling performance of NCM811 and NCMMg0.03.
where mB, MB and VM is the mass, molar mass, and molar volume of the 5
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 5. CVS of (a) NCM811 and (b) NCMMg0.03. (c) GITT curves and (d) DLiþ calculated from GITT curves of NCMMg0.03. EIS curves of (e) NCM811 and (f) NCMMg0.03.
active material, respectively. A is the contact area between the elec trolyte and the positive electrode material. τ, △Es, and △Eτ are demonstrated in Fig. S5. The DLiþ is calculated and plotted as a function of voltage in Fig. 5d and Fig. S4b. The average value of DLiþ for NCMMg0.03 during the charge/discharge process is 8.57 � 10 10 and 8.25 � 10 10 cm2 s 1 at 0.1 C, respectively, which is higher than that of NCM811 (7.892 � 10 10 and 7.828 � 10 10 cm2 s 1). However, when the GITT test was conducted at a high rate of 5 C, the NCMMg0.03 ex hibits a lower DLiþ (1.53 � 10 8 and 1.03 � 10 8 cm2 s 1) than that of NCM811 (1.74 � 10 8 and 1.10 � 10 8 cm2 s 1) (Figs. S4c–f). The re sults indicate that the Mg doping can facilitate Liþ diffusion at a low rate owing to its pillaring effect but hinder fast Liþ diffusion at a high rate owing to its location in Li sites. EIS was used to further study the reaction kinetics and interfacial electrochemistry of NCM811 and NCMMg0.03 electrode. As shown in Fig. 5e and f, the Nyquist plot shows two semicircles followed by a linear curve. The high frequency semicircle can be specified as the passivation film resistance of the material surface (recorded as Rsf). The interme diate frequency semicircle is attributed to the charge transfer resistance (recorded as Rct) at the interface between the electrolyte and electrode. NCM811 exhibits higher Rct than that of NCMMg0.03 during cycling owing to the microcrack formation and structure corruption. After 5th and 350th cycle, the value of Rct for NCM811 is 817.3 and 2790 Ω, respectively. In comparison, the value of Rct for NCMMg0.03 is 677.3 and 2119 Ω, respectively, implying the superior charge transfer rate duing
long-term cycling. Moreover, the electronic conductivity of NCMMg0.03 is about 2.05 � 10 4 S cm 1, which higher than that of NCM811 (6.53 � 10 5 S cm 1). Based on the above results, it can be concluded that the Mg doping can effectively decrease the charge transfer resis tance and increase the electronic conductivity of NCM811. The thermal stability of NCM811 and NCMMg0.03 is examined at a charged state of 4.5 V by DSC method (Fig. 6a). The NCM811 exhibits an exothermal peak about 222.3 � C with a heat generation of 866.5 J g 1. For comparison, the exothermal peak of NCMMg0.03 shifts to a higher temperature of 234.1 � C and the heat generation is reduced to 753.1 J g 1. It clearly shows that the NCMMg0.03 sample processes an improved thermal stability owing to its stable structure. Furthermore, to evaluate the practicability of the as-prepared Li(Ni0.8C o0.1Mn0.1)0.97Mg0.03O2 material, a full Li-ion battery based on MCMB anode and Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 cathode is fabricated (the assemble process is provided in Supplementary). The full cell delivers an initial discharge capacity of 183.6 mA h g 1 at 0.1 C with an operation voltage of 3.73 V, corresponding to an energy density of 684.8 W h kg 1 (based on the mass of the cathode, Fig. 6b). Notably, the as-fabricated full cell delivers a considerable energy density of 595.3 W h kg 1 with retention of ~75% after 100 cycles at 0.5 C (based on the mass of the cathode) (Fig. 6c). These results suggest that the as-prepared Li (Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 material presents excellent electro chemical performance, which enables its practical application as a cathode material in the high energy density LIBs. 6
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 6. (a) DSC profiles of NCM811 and NCMMg0.03. (b) Charge/discharge profiles of NCMMg0.03//MCMB full battery (inset: Schematic illustration of the full cell). (c) Cycle performance of NCMMg0.03//MCMB full battery.
4. Conclusions
[3] J.L. Shi, J.N. Zhang, M. He, X.D. Zhang, Y.X. Yin, H. Li, Y.G. Guo, L. Gu, L.J. Wan, ACS Appl. Mater. Interfaces 8 (2016) 20138–20146. [4] T. Hatsukade, A. Schiele, P. Hartmann, T. Brezesinski, J. Janek, ACS Appl. Mater. Interfaces 10 (2018) 38892–38899. [5] Y.S. Wang, Y.Y. Li, Z.P. Qiu, X.Z. Wu, P.F. Zhou, T. Zhou, J.P. Zhao, Z.C. Miao, J. Zhou, S.P. Zhuo, J. Mater. Chem. 6 (2018) 11189–11197. [6] K.M. Shaju, P.G. Bruce, J. Power Sources 174 (2007) 1201–1205. [7] B.J. Chae, T. Yim, J. Power Sources 360 (2017) 480–487. [8] S. Gao, Y.T. Cheng, M. Shirpour, ACS Appl. Mater. Interfaces 11 (2019) 982–989. [9] M.M. Chen, E.Y. Zhao, D.F. Chen, M.M. Wu, S.B. Han, Q.Z. Huang, L.M. Yang, X. L. Xiao, Z.B. Hu, Inorg. Chem. 56 (2017) 8355–8362. [10] P.F. Zhou, Z. Zhang, H.J. Meng, Y.Y. Lu, J. Cao, F.Y. Cheng, Z.L. Tao, J. Chen, Nanoscale 8 (2016) 19263–19269. [11] A. Manthiram, J.C. Knight, S.T. Myung, S.M. Oh, Y.K. Sun, Adv. Energy Mater. 6 (2016) 1501010. [12] J. Xu, F. Lin, M.M. Doeff, W. Tong, J. Mater. Chem. 5 (2017) 874–901. [13] J.M. Zheng, S. Myeong, W. Cho, P.F. Yan, J. Xiao, C.M. Wang, J. Cho, J.G. Zhang, Adv. Energy Mater. 7 (2017) 1601284. [14] W. Liu, P. Oh, X. Liu, M.J. Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Angew. Chem. Int. Ed. 54 (2015) 4440–4458. [15] P.F. Zhou, H.J. Meng, Z. Zhang, C.C. Chen, Y.Y. Lu, J. Cao, F.Y. Cheng, J. Chen, J. Mater. Chem. 5 (2017) 2724–2731. [16] Y.C. Li, W.M. Zhao, W. Xiang, Z.G. Wu, Z.G. Yang, C.L. Xu, Y.D. Xu, E.H. Wang, C. J. Wu, X.D. Guo, J. Alloy. Comp. 766 (2018) 546–555. [17] A. Choi, J. Lim, H.J. Kim, S.C. Jung, H.W. Lim, H. Kim, M.S. Kwon, Y.K. Han, S. M. Oh, K.T. Lee, Adv. Energy Mater. 8 (2018) 1702514. [18] A. Liu, J. Li, R. Shunmugasundaram, J.R. Dahn, J. Electrochem. Soc. 164 (2017) A1655–A1664. [19] S. Sallard, D. Sheptyakov, C. Villevieille, J. Power Sources 359 (2017) 27–36. [20] G.R. Hu, M.F. Zhang, L.W. Liang, Z.D. Peng, K. Du, Y.B. Cao, Electrochim. Acta 190 (2016) 264–275. [21] Q. Xie, W.D. Li, A. Manthiram, Chem. Mater. 31 (2019) 938–946. [22] Y.C. Li, W. Xiang, Z.G. Wu, C.L. Xu, Y.D. Xu, Y. Xiao, Z.G. Yang, C.J. Wu, G.P. Lv, X. D. Guo, Electrochim. Acta 291 (2018) 84–94. [23] P.Y. Hou, F. Li, Y.Y. Sun, M.L. Pan, X. Wang, M.H. Shao, X.J. Xu, ACS Sustain. Chem. Eng. 6 (2018) 5653–5661. [24] P. Sun, Y. Ma, T.Y. Zhai, H.Q. Li, Electrochim. Acta 191 (2016) 237–246. [25] I.M. Markus, F. Lin, K.C. Kam, M. Asta, M.M. Doeff, J. Phys. Chem. Lett. 5 (2014) 3649–3655. [26] F. Schipper, M. Dixit, D. Kovacheva, M. Talianker, O. Haik, Y. Grinblat, E. M. Erickson, C. Ghanty, D.T. Major, B. Markovsky, D. Aurbach, J. Mater. Chem. 4 (2016) 16073–16084. [27] C.S. Yoon, M.J. Choi, D.W. Jun, Q. Zhang, P. Kaghazchi, K.H. Kim, Y.K. Sun, Chem. Mater. 30 (2018) 1808–1814. [28] K. Wu, G.F. Jia, X.H. Shangguan, G.W. Yang, Z.H. Zhu, Z.J. Peng, Q. Zhuge, F.Q. Li, X.L. Cui, J. Alloy. Comp. 765 (2018) 700–709. [29] S.Y. Liu, X. Chen, J.Y. Zhao, J.M. Su, C.C. Zhang, T. Huang, J.H. Wu, A.S. Yu, J. Power Sources 374 (2018) 149–157.
The effects of Mg doping on the cycling stability of Li(Ni0.8C o0.1Mn0.1)1-xMgxO2 (x ¼ 0, 0.01, 0.03, 0.05, 0.06) are investigated in 3.0–4.5 V. The Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 with the optimal Mg content possesses uniform particle size distribution, low degree of Liþ/ Ni2þ mixing, and well-ordered layered. When used as a cathode material for LIBs, Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 demonstrates the best elec trochemical performance. The delivered high reversible discharge ca pacity is 226.1 mA h g 1 at 0.1 C and capacity retention approached 81% after 350 cycles at 0.5 C. Furthermore, the full cell of Li(Ni0.8C o0.1Mn0.1)0.97Mg0.03O2//MCMB delivers a specific energy density of 595.3 W h kg 1 at 0.5 C (based on the mass of the cathode). The superior electrochemical performance is attributed to the Mg doping, which can expand the interlayer and enhance structural stability. The simple syn thesis method and superior electrochemical performance suggest the widely potential application of Mg doped LiNi0.8Co0.1Mn0.1O2 as a promising cathode material for LIBs. Acknowledgements We are grateful for the financial supports by Key Research and Development Program of Shandong Province (2019GGX103027), Nat ural Science Foundation of Shandong Province (ZR2018BB039, ZR2017JL014), National Natural Science Foundation of China (Grant 21576158, 21576159), Taishan Scholar Foundation (tsqn2018), and 111 Project (B12015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227017. References [1] Y.X. Wang, B. Liu, Q.Y. Li, S. Cartmell, S. Ferrara, Z.Q. Daniel Deng, J. Xiao, J. Power Sources 286 (2015) 330–345. [2] S. Gao, X.W. Zhan, Y.T. Cheng, J. Power Sources 410–411 (2019) 45–52.
7
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
[30] L.J. Li, X.H. Li, Z.X. Wang, H.J. Guo, P. Yue, W. Chen, L. Wu, J. Alloy. Comp. 507 (2010) 172–177. [31] B. Zhang, L.J. Li, J.C. Zheng, J. Alloy. Comp. 520 (2012) 190–194. [32] H. Li, P.F. Zhou, F.M. Liu, H.X. Li, F.Y. Cheng, J. Chen, Chem. Sci. 10 (2019) 1374–1379. [33] K.M. Min, S.W. Seo, Y.Y. Song, H.S. Lee, E. Cho, Phys. Chem. Chem. Phys. 19 (2017) 1762. [34] C.Y. Fu, Z.L. Zhou, Y.H. Liu, Q. Zhang, Y.S. Zheng, G.X. Li, J. Wuhan Univ. Technol.-Materials Sci. Ed. 26 (2011) 211–215. [35] Z.J. Huang, Z.X. Wang, X.B. Zheng, H.J. Guo, X.H. Li, Q. Jing, Z.H. Yang, RSC Adv. 5 (2015) 88773. [36] Z.J. Huang, Z.X. Wang, X.B. Zheng, H.J. Guo, X.H. Li, Q. Jing, Z.H. Yang, Electrochim. Acta 182 (2015) 795–802. [37] A.H. Tavakoli, H. Kondo, Y. Ukyo, A. Navrotsky, J. Electrochem. Soc. 160 (2012) A302–A305.
[38] Q. Zhang, Y.S. Zheng, S.K. Zhong, Key Eng. Mater. 519 (2012) 152–155. [39] D. Wang, Y. Huang, Z.Q. Huo, L. Chen, Electrochim. Acta 107 (2013) 461–466. [40] W.D. Li, X.M. Liu, H. Celio, P. Smith, A. Dolocan, M.F. Chi, A. Manthiram, Adv. Energy Mater. 8 (2018) 1703154. [41] K.J. Park, B.B. Lim, M.H. Choi, H.G. Jung, Y.K. Sun, M. Haro, N. Vicente, J. Bisquert, G. Garcia-Belmonte, J. Mater. Chem. 3 (2015) 22183–22190. [42] Q. Sa, J.A. Heelan, Y. Lu, D. Apelian, Y. Wang, ACS Appl. Mater. Interfaces 7 (2015) 20585–20590. [43] H.B. Xie, G.R. Hu, K. Du, Z.D. Peng, Y.B. Cao, J. Alloy. Comp. 666 (2016) 84–87. [44] G.H. Kim, S.T. Myung, H.S. Kim, Y.K. Sun, Electrochim. Acta 51 (2006) 2447–2453. [45] B. Huang, X.H. Li, Z.X. Wang, H.J. Guo, X.H. Xiong, Ceram. Int. 40 (2014) 13223–13230. [46] R. Robert, C. Bunzli, E.J. Berg, P. Novak, Chem. Mater. 27 (2015) 526–536.
8
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Stabilizing the high-voltage cycle performance of LiNi0.8Co0.1Mn0.1O2 cathode material by Mg doping Xiaolan Liu a, Shuo Wang a, Li Wang a, Ke Wang a, Xiaozhong Wu a, Pengfei Zhou a, b, *, Zhichao Miao a, Jin Zhou a, Yi Zhao a, Shuping Zhuo a, ** a b
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, PR China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, China
H I G H L I G H T S
� LiNi0.8Co0.1Mn0.1O2 cathode material is modified by Mg doping. � The Liþ/Ni2þ mixing of LiNi0.8Co0.1Mn0.1O2 material is reduced by Mg doping. � NCMMg0.03 material exhibits improved cycling retention and thermal stability. � The NCMMg0.03//MCMB full cell delivers an energy density of 595.3 W h kg 1 at 0.5 C. A R T I C L E I N F O
A B S T R A C T
Keywords: Mg doping LiNi0.8Co0.1Mn0.1O2 cathode material High voltage Cycle stability Lithium-ion batteries
LiNi0.8Co0.1Mn0.1O2 is considered as a promising cathode material for lithium ion batteries because of its high capacity and low cost. However, the LiNi0.8Co0.1Mn0.1O2 suffers structural instability and irreversible phase transition during charge/discharge processes, especially under high voltage, resulting in serious capacity fading and thermal runaway. Here, we propose a simple and effective method of modifying LiNi0.8Co0.1Mn0.1O2 by Mg doping. Benefiting from the pillaring effects of inactive Mg in the crystal structure, Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials exhibit low Liþ/Ni2þ cation mixing, high structural stability, and improved cyclic stability in the voltage of 3.0–4.5 V. The optimal Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 achieves a high capacity retention of 81% over 350 cycles at 0.5 C and exhibits enhanced thermal stability at 4.5 V. The promotion mechanism is explored systematically by a combination study of electrochemical characterizations, demonstrating the faster Liþ diffu sion kinetics, higher electronic conductivity, and stronger structure due to the Mg doping. Moreover, the full cell of Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2//mesocarbon microbeads delivers a promising energy density of 595.3 W h kg 1 at 0.5 C (based on the mass of the cathode). The present work demonstrates that moderate Mg doping is a facile yet effective strategy to modify high-performance LiNi0.8Co0.1Mn0.1O2 for high-voltage lithium ion batteries.
1. Introduction Lithium-ion batteries (LIBs) with higher energy density, longer cycle life, more safety performance and lower price are increasingly deman ded [1–5]. Cathode materials play a key role in LIBs and their im provements are crucial for enhancing energy density of LIBs. Nowadays, exploring high energy density, lower production cost and high safety cathode materials for LIBs is still a great challenge. Among the cathode materials, layered lithium nickel cobalt manganese oxides
(LiNixCoyMnzO2, NCM) [6] and their derivatives have attracted partic ular attention due to their high potential capacity, high working po tential, and low production cost [7]. In particular, LiN0.8Co0.1Mn0.1O2 (NCM811) shows extraordinary promise for high energy density LIBs due to their high capacity of above 220 mA h g 1 and the voltage plat form of about 3.7 V. Unfortunately, the practical and large-scale appli cation of NCM811 has been impeded by the serious capacity fading and thermal run-away (especially charge to high cutoff voltage), which re sults from the structural instability caused by the Liþ/Ni2þ cation mixing
* Corresponding author. School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, PR China. ** Corresponding author. E-mail addresses: [email protected] (P. Zhou), [email protected] (S. Zhuo). https://doi.org/10.1016/j.jpowsour.2019.227017 Received 4 June 2019; Received in revised form 25 July 2019; Accepted 13 August 2019 Available online 16 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
in the phase structure [8,9]. During the charging process, the Ni2þ in Li sites is oxidized to smaller Ni3þ/4þ (0.056 and 0.053 nm) [10], which deteriorates the layered structure and leads to irreversible phase trans formation from layered structure to disordered spinel and rolk-salt structure [11–16]. Therefore, it is still a significant challenge to push forward the large-scale application of NCM811 materials. Accordingly, strenuous efforts have been devoted to enhance the electrochemical performance of NCM811 materials, including element doping, electrolyte optimization, and surface coating. In particular, cation doping such as Mg2þ [17–21], Al3þ [22–24], Ti4þ [25], Zr4þ [26, 27], Nb5þ [28,29], and Cr3þ [30,31], has been demonstrated as an effective strategy to improve the electrochemical performance of NCM materials. Among the various substituted cations, electrochemically inactive Mg2þ (0.072 nm) doping is attractive. The doped Mg can distribute in the interlayer Li slabs and suppress Liþ/Ni2þ cation mixing owing to their similar ionic radius. Li et al. [32] found that the Mg doping can stabilize the layered structure, suppress Liþ/Ni2þ cation mixing, and enhance the electrochemical property of LiNi0.90 Co0.07Mg0.03O2 material. By theoretical calculation, Min et al. [33] found that the Mg doping can inhibit the change of lattice parameters and improve the structural stability of LiNi0.8Co0.1Mn0.1O2 material during (de)lithiation processes. Fu et al. [34] and Huang et al. [35,36] reported Mg substitute for each Co, Mn, and Ni in LiNi0.6Co0.2Mn0.2O2, respectively, and the results show that Mg doping can improve the electrochemical performance and structural stability of the LiNi0.6 Co0.2Mn0.2O2 material. In addition, Mg is also used as a doping element in LiNi0.80Co0.15Al0.05O2, LiNi0.4Co0.2Mn0.4O2, Li[Li0.2Ni0.2Mn0.6]O2 [37–39] materials, which also achieved good research results. However, to the best of our knowledge, there is no report about Mg doping in NCM811 materials, leaving much room to develop advanced cathode materials for LIBs. Herein, the effect of Mg doping on the structural and electrochemical performance of NCM811 material has been investigated. The Li (Ni0.8Co0.1Mn0.1)1-xMgxO2 samples with different Mg contents were prepared by a solid-state synthesis process. It was found that the Mg doping does affect the phase structure and electrochemical performance of NCM811 material. Compared with the pure NCM811, the optimal Li (Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 sample possesses a good layered structure with low Liþ/Ni2þ mixing, delivers an impressive high initial reversible capacity with good cycle performance in 3.0–4.5 V, and exhibits admi rable thermal stability. When matched with mesocarbon microbeads (denoted as MCMB), the as-fabricated full cell shows an excellent energy density of 595.3 W h kg 1 at 0.5 C (based on the mass of the cathode). The above findings verify that the proper amount of Mg doping can stabilize the layered structure and improve the electrochemical perfor mance of NCM811 material in the voltage of 3.0–4.5 V.
2.2. Materials characterization The phase structure of all samples was measured by X-ray diffraction (XRD, Rigaku MiniFlex600) using Cu Kα radiation. The morphologies of the samples were characterized by scanning electron microscopy (SEM, JEOL JSM7500F) and transmission electron microscopy (TEM, Philips Tecnai-F20). The particle size distribution of Ni0.8Co0.1Mn0.1(OH)2 was measured by laser particle size analyser (MASTER SIZER 3000). Surface area analysis was carried out by measuring the N2 adsorption-desorption isotherms at 77 K on a BELSORP-mini instrument. The thermal stability of the cathode material at a delithiated state of 4.5 V was examined with a differential scanning calorimetry (DSC, Netzsch STA 449F3 Jupiter thermal analysis system) from 30 to 350 � C at a heating rate of 10 � C min 1. The accurate component of the synthesized samples was char acterized by inductively coupled plasma emission spectrometer (ICPAES PerkinElmer Optima 8300). 2.3. Electrochemical measurement The cathode electrodes were prepared by blending 80 wt% active materials, 10 wt% Super P and 10 wt% poly vinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone (NMP). The mixed slurry was coated onto aluminum foil and dried at 100 � C for 12 h in vacuum. The electrode was cut into pieces with the diameter of 12 mm and the cathode material loading was about 3 mg. For electrochemical tests, the CR2032 coin-type cells were assembled. The cell was consisted of a cathode, Li foil as counter electrode, polypropylene membrane with micropores (Celgard 2400) as the separator, and 1.15 mol L 1 LiPF6 in 1:2:2 (volume) mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) as electrolyte. This process is carried out in an argon-filled glove box (Mikrouna Universal 2440/750) with water and oxygen levels below 1 ppm. The cells were charged/discharged at different rates (1 C ¼ 190 mA g 1) between 3.0 and 4.5 V. Cyclic vol tammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed on an IVIUM electrochemical workstation. The electron conductivity was measured according to standard EIS procedure on pellets (IVIUM electrochemical workstation). The galvanostatic inter mittent titration technique (GITT) was tested with a charging/dis charging constant current of 0.1 C (5 C) for 10 min (1 min), then it was relaxed for 40 min between 3.0 and 4.5 V. For full-cell demonstration, the MCMB was used as anode and the ratio of cathode to anode was about 1.7. All tests were carried out at room temperature 25 � C. 3. Results and discussion The SEM image (Fig. 1a) exhibits that the Ni0.8Co0.1Mn0.1(OH)2 precursor has a typical spherical shape. The microspheres are composed of a plurality of sheet-like primary grains (Fig. 1b) with a BET specific surface area of 129.9 m2 g 1 (Fig. S1). The SEM energy-dispersive spectrometer (EDS) mappings of Ni0.8Co0.1Mn0.1(OH)2 microsphere (Fig. 1c–e) reveal the uniform distributions of Ni, Co and Mn elements along the spherical outline. Moreover, the particle size distribution (Fig. 1f) shows that Ni0.8Co0.1Mn0.1(OH)2 has a relatively single and narrow particle size distribution, and the particle diameter of D10, D50, D90 is 6.39, 11.65, 21.71 μm, respectively. Furthermore, The XRD pattern (Fig. 1g) of Ni0.8Co0.1Mn0.1(OH)2 coincides with the standard Ni (OH)2 (JCPDS No. 14–0117), indicating the formation of a homoge neous ternary hydroxide solid solution and high phase purity. The XRD patterns of Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials are shown in Fig. 2a and b. It can be seen that the NCM811, NCMMg0.01, NCMMg0.03, NCMMg0.05, and NCMMg0.06 have the same characteristic diffraction peaks as the hexagonal LiNiO2 (JCPSD No. 09–0063). The compositions of the prepared samples were measured by ICP-AES and the results are close to those of the designed values (Table S1). For NCM materials, apart from the high phase purity and crystallinity, a low Liþ/ Ni2þ cation mixing degree and an ordered layered structure is also very
2. Experimental 2.1. Material preparation The spherical Ni0.8Co0.1Mn0.1(OH)2 precursor was prepared before hand by co-precipitation method [40]. In order to prepare (Ni0.8C o0.1Mn0.1)1-xMgx(OH)2 precursor, MgSO4 was dissolved into 100 mL deionized water by continuous stirring at a temperature of 30 � C, then the Ni0.8Co0.1Mn0.1(OH)2 was added to the solution and kept stirring for 1 h, followed by the addition of moderate NaOH solution. Then the (Ni0.8Co0.1Mn0.1)1-xMgx(OH)2 precursor and LiOH⋅H2O were thoroughly mixed in an agate mortar and calcinated at 550 � C for 4 h and 780 � C for 12 h under oxygen to obtain the corresponding Li(Ni0.8C o0.1Mn0.1)1-xMgxO2 (x ¼ 0, 0.01, 0.03, 0.05, 0.06) materials, which were named as NCM811, NCMMg0.01, NCMMg0.03, NCMMg0.05 and NCMMg0.06.
2
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 1. (a, b) SEM images, (c-e) EDS mapping images, (f) particle size distribution, and (g) XRD pattern of Ni0.8Co0.1Mn0.1(OH)2.
important. At present, the degree of cation mixing and the integrity of the layered structure of NCM materials can be obtained by analyzing the XRD data [41–43]. The higher ratio of c/a and I003/I104 signifies a lower degree of Liþ/Ni2þ mixing while the smaller value of R factor (referred to as (I006þI012)/I101) and the clear splitting of peaks (006)/(012) and (018)/(110) means the layered structure is more ordered. Therefore, the values of c/a, I003/I104 and R factor for Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials are listed in Table 1. The NCMMg0.03 has the largest value of c/a and I003/I104 with the smallest R factor, indicating that the NCMMg0.03 has a lower degree of Liþ/Ni2þ mixing and a well-ordered layered structure. The values of c/a and I003/I104 gradually decrease with the increasing of Mg content, indicating the mixing arrangement of Liþ/Ni2þ increases. This is because fewer Mg locating the Li sites can suppress Ni2þ in the Li layer and reduce Liþ/Ni2þ mixing, while exces sive Mg occupying the Li sites will generate a force to push Liþ to the Ni layer that results in increased Liþ/Ni2þ mixing. The similar results have been observed in LiNi0.6Co0.2Mn0.2-xMgxO2 materials [35]. Moreover, the Rietveld refinement results of NCM811 and NCMMg0.03 are dis played in Fig. 2c and d and Table S2. It is observed that the occupancy of Ni2þ in Li site for NCMMg0.03 is only about 2.1%, much lower than that of NCM811 (3.2%), further revealing that Mg doping obviously de creases the degree of Liþ/Ni2þ mixing and maintains the well-ordered layer structure. Moreover, about 1.7% Mg2þ was counted to be located in Li sites owing to the similar radius between Mg2þ and Liþ, which is consistent with previous report about LiNi0.90Co0.07Mg0.03O2 [32] and Li[Ni(1/3-z)Co(1/3-z)Mn(1/3-z)Mgz]O2 [44]. The schematic structure of the NCMMg0.03 is shown in Fig. 2e. The doped Mg in Li site working as backbone can enlarge the interlayer and support the layer
structure during the charge/discharge processes [32]. It is indicated that the proper amount of Mg doping can effectively reduce the degree of Liþ/Ni2þ cation mixing and stabilize the layered structure. The unique phase structural merits of the as-prepared NCMMg0.03 might lay the foundation for its superior lithium storage property. Fig. 3a and b and Fig. S2 display the SEM images of as-prepared Li (Ni0.8Co0.1Mn0.1)1-xMgxO2 materials. Obviously, all the samples inherit the spherical morphology of Ni0.8Co0.1Mn0.1(OH)2 precursor. Moreover, the elemental distribution of NCMMg0.03 is analyzed by EDS mapping. The distribution of Mg element (Fig. 3f) is evenly overlapped with the elements of Ni (Fig. 3c), Co (Fig. 3d), and Mn (Fig. 3e), which are all uniformly distributed on the microspheres. As depicted in Fig. 3g and h, the secondary microsphere with dense structure is assembled by numerous primary particles with size of around 80 nm in diameter. The HRTEM image of NCMMg0.03 (Fig. 3i) reveals a clear lattice fringes with an interplanar distance of 0.476 nm, corresponding to the (003) crystal plane of LiNiO2 (rhombohedral, R-3m). The result further indicates the high degree of crystallization and well-developed layered structure of NCMMg0.03 material. The electrochemical performance of the as-prepared Li(Ni0.8C o0.1Mn0.1)1-xMgxO2 materials was first investigated by coin-type Li cells. Fig. 4a shows the first charge/discharge profiles of pristine and Mgdoped NCM811 samples at 0.1 C in the potential range of 3.0–4.5 V. The initial specific discharge capacity of NCM811 (191.8 mA h g 1) is lower than that of NCMMg0.01 (203.6 mA h g 1) and NCMMg0.03 (226.1 mA h g 1). But the specific discharge capacity shows a gradual decline for NCMMg0.05 (181.1 mA h g 1) and NCMMg0.06 1 (168.8 mA h g ). It can be seen that the initial charge capacities of 3
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 2. (a) XRD patterns of Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 and (b) enlarged XRD pattern at 37� –39� and 63� –66� . Rietveld refinements of X-ray diffraction pattern for (c) NCM811 and (d) NCMMg0.03. (e) Schematic structure of NCMMg0.03.
176.6, 162.1, and 144.9 mA h g 1, respectively. The discharge specific capacity of NCMMg0.03 is lower than that of NCM811 at 5 C. The reason for this phenomenon could be that the Mg located in Li sites impedes the fast Liþ diffusion when the rate is increased. This will be discussed later. When the current rate returns to 0.1 C, the discharge capacity of NCMMg0.03 could be recovered to 212.4 mA h g 1, implying a strong structural tolerance to rapid Liþ extraction/insertion of NCMMg0.03. Moreover, the superb cyclic stability of NCMMg0.03 is demonstrated in Fig. 4e, affording an initial discharge capacity of 210.9 mA h g 1 at 0.5 C with a capacity retention of 81% over 350 cycles. By contrast, the ca pacity retention is only 67% for the NCM811 after 350 cycles. In general, NCMMg0.03 exhibits better cyclic stability and rate capability than the other components, indicating that proper content of Mg doping can enhance the electrochemical performance of NCM811. The positive ef fect of Mg doping is attributed to the pillar effect of Mg in the crystal structure that can reduce the degree of Liþ/Ni2þ mixing and stabilize the layered structure. Additionally, the change of morphological structure and phase structure of NCMMg0.03 and NCM811 electrode was examined by SEM and XRD after 350 cycles at 0.5 C (Fig. S3). Some microcracks are observed in NCM811 electrode (Fig. S3a). The microcracks act as new reaction sites that increase parasitic reactions and form thick SEI layer, which might inhibit Liþ diffusion, increase resistance, and deteriorate cycle stability. By contrast, the NCMMg0.03 electrode still sustains its original microspherical structure after the long cycling (Fig. S3b). Moreover, the cation mixing and lattice parameters of NCMMg0.03 and NCM811 were evaluated and compared after 350 cycles by XRD patterns (Fig. S3c). The values of I003/I104 is decreased to 1.737 and 1.988 for NCM811 and NCMMg0.03 after 350 cycles, respectively. In comparison with NCM811, NCMMg0.03 has a lower degree of Liþ/Ni2þ mixing after
Table 1 The unit cell parameters c/a, I(003)/I(104) and R factor of Li(Ni0.8Co0.1Mn0.1)1xMgxO2 materials. Sample
a ¼ b(Å)
c (Å)
c/a
I003/I104
R-factor
NCM811 NCMMg0.01 NCMMg0.03 NCMMg0.05 NCMMg0.06
2.87121 2.86898 2.86885 2.86880 2.87160
14.20621 14.19296 14.21622 14.19893 14.20470
4.948 4.947 4.955 4.949 4.947
2.195 2.367 2.386 2.344 2.296
0.431 0.389 0.374 0.402 0.456
NCMMg0.01 and NCMMg0.03 are higher than that of NCM811. The reason for this phenomenon is that the Ni2þ in the Li slab can be oxidized to Ni3þ/4þ during the charging process, which induces a local collapse of the layered structure and impedes further Liþ deintercalation from the structure. The doped Mg in Li sites can support the layered structure and inhibit the collapse of the structure. This is beneficial to facilitate further Liþ deintercalation at high voltage and result in high capacity. The similar result is also observed in Li(Ni0.8Co0.1Mn0.1)1-xZrxO2 [2] and Li (Ni0.8Co0.15Al0.05)1-xMgxO2 [45]. By comparison, NCMMg0.03 sample delivers the highest specific capacity of 226.1 mA h g 1, which is also much superior to those of the other reported Ni-rich NCM materials (Table S3). These results confirm that the proper Mg doping can increase the reversible capacity owing to the low Liþ/Ni2þ cation mixing and stabilized layered structure. However, the over-doping of inactive Mg can result in low capacity owing to the decreased active Ni content. Furthermore, the comparison of rate performance between NCM811 and NCMMg0.03 at different current rates is demonstrated in Fig. 4b–d. At the rate of 0.1, 0.2, 0.5, 1, 2 and 5 C, the discharge capacities of NCM811 are 208, 201.4, 188.2, 175.9, 164.8, and 148.9 mA h g 1, while the corre sponding discharge capacities of NCMMg0.03 are 226.5, 208.2, 189.6, 4
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 3. (a, b) SEM images, (c-f) EDS mapping images, (g, h) TEM images, (i) HRTEM image of NCMMg0.03.
350 cycles, according with its superior cycle stability. Compared with the unit cell volume of uncycled NCM811 and NCMMg0.03 (Table S2), the unit cell volume of cycled NCM811 and NCMMg0.03 is reduced (Table S4). The unit cell volume variation of NCMMg0.03 (ΔV ¼ 0.29 Å3) is lower than that of NCM811 (ΔV ¼ 0.6 Å3), indicating an effect of Mg doping in relieving volume deformation and enhancing structural sta bility. The results further prove that NCMMg0.03 has a stable morphology and phase structure after 350 cycles in the voltage of 3.0–4.5 V, which ensure its excellent electrochemical performance. To further investigate the electrochemical behavior, CV, GITT, and EIS measurements were conducted on NCM811 and NCMMg0.03. Fig. 5a and b shows the first three CV curves of NCM811 and NCMMg0.03 at 0.1 mV s 1. There are three pairs of redox peaks in both samples, which are belong to the phase transitions of hexagonal phase (H1) to the monoclinic phase (M), M to the new hexagonal phase (H2), and H2 to the other hexagonal phase (H3), respectively [46]. In the first cycle, the voltage difference between the major oxidation peak potential (3.872 V) and reduction peak potential (3.720 V) is 0.152 V for NCMMg0.03, which is much lower than that of NCM811 (0.188 V), suggesting that the Mg doping can decrease the polarization and improve the reversibility of NCM811. Moreover, compared with NCM811, the CV curves of NCMMg0.03 in the subsequent two cycles almost overlap, revealing the superior reversibility during Liþ insertion/extraction. To understand the electrochemical kinetics of NCMMg0.03, the GITT was used to investigate the Liþ diffusion coefficient ðDLiþ Þ. Fig. 5c and Fig. S4a show the GITT curves of NCMMg0.03 and NCM811 at 0.1 C. Obviously, compared with NCMMg0.03, the NCM811 electrode exhibits the higher overpotential, which is especially noticeable at the very end of each charge/discharge process. According to Fick’s second law of diffusion and its simplified calculation formula, the value of DLiþ can be calculated according to eqn (1): � �2 �2 � � � 4 m B VM △ES (1) DLiþ ¼ τ＜＜L2 DLiþ πτ MB A △Eτ
Fig. 4. (a) The initial charge/discharge curves of Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 at 0.1 C. (b-d) Rate performance and (e) Long-life cycling performance of NCM811 and NCMMg0.03.
where mB, MB and VM is the mass, molar mass, and molar volume of the 5
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 5. CVS of (a) NCM811 and (b) NCMMg0.03. (c) GITT curves and (d) DLiþ calculated from GITT curves of NCMMg0.03. EIS curves of (e) NCM811 and (f) NCMMg0.03.
active material, respectively. A is the contact area between the elec trolyte and the positive electrode material. τ, △Es, and △Eτ are demonstrated in Fig. S5. The DLiþ is calculated and plotted as a function of voltage in Fig. 5d and Fig. S4b. The average value of DLiþ for NCMMg0.03 during the charge/discharge process is 8.57 � 10 10 and 8.25 � 10 10 cm2 s 1 at 0.1 C, respectively, which is higher than that of NCM811 (7.892 � 10 10 and 7.828 � 10 10 cm2 s 1). However, when the GITT test was conducted at a high rate of 5 C, the NCMMg0.03 ex hibits a lower DLiþ (1.53 � 10 8 and 1.03 � 10 8 cm2 s 1) than that of NCM811 (1.74 � 10 8 and 1.10 � 10 8 cm2 s 1) (Figs. S4c–f). The re sults indicate that the Mg doping can facilitate Liþ diffusion at a low rate owing to its pillaring effect but hinder fast Liþ diffusion at a high rate owing to its location in Li sites. EIS was used to further study the reaction kinetics and interfacial electrochemistry of NCM811 and NCMMg0.03 electrode. As shown in Fig. 5e and f, the Nyquist plot shows two semicircles followed by a linear curve. The high frequency semicircle can be specified as the passivation film resistance of the material surface (recorded as Rsf). The interme diate frequency semicircle is attributed to the charge transfer resistance (recorded as Rct) at the interface between the electrolyte and electrode. NCM811 exhibits higher Rct than that of NCMMg0.03 during cycling owing to the microcrack formation and structure corruption. After 5th and 350th cycle, the value of Rct for NCM811 is 817.3 and 2790 Ω, respectively. In comparison, the value of Rct for NCMMg0.03 is 677.3 and 2119 Ω, respectively, implying the superior charge transfer rate duing
long-term cycling. Moreover, the electronic conductivity of NCMMg0.03 is about 2.05 � 10 4 S cm 1, which higher than that of NCM811 (6.53 � 10 5 S cm 1). Based on the above results, it can be concluded that the Mg doping can effectively decrease the charge transfer resis tance and increase the electronic conductivity of NCM811. The thermal stability of NCM811 and NCMMg0.03 is examined at a charged state of 4.5 V by DSC method (Fig. 6a). The NCM811 exhibits an exothermal peak about 222.3 � C with a heat generation of 866.5 J g 1. For comparison, the exothermal peak of NCMMg0.03 shifts to a higher temperature of 234.1 � C and the heat generation is reduced to 753.1 J g 1. It clearly shows that the NCMMg0.03 sample processes an improved thermal stability owing to its stable structure. Furthermore, to evaluate the practicability of the as-prepared Li(Ni0.8C o0.1Mn0.1)0.97Mg0.03O2 material, a full Li-ion battery based on MCMB anode and Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 cathode is fabricated (the assemble process is provided in Supplementary). The full cell delivers an initial discharge capacity of 183.6 mA h g 1 at 0.1 C with an operation voltage of 3.73 V, corresponding to an energy density of 684.8 W h kg 1 (based on the mass of the cathode, Fig. 6b). Notably, the as-fabricated full cell delivers a considerable energy density of 595.3 W h kg 1 with retention of ~75% after 100 cycles at 0.5 C (based on the mass of the cathode) (Fig. 6c). These results suggest that the as-prepared Li (Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 material presents excellent electro chemical performance, which enables its practical application as a cathode material in the high energy density LIBs. 6
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
Fig. 6. (a) DSC profiles of NCM811 and NCMMg0.03. (b) Charge/discharge profiles of NCMMg0.03//MCMB full battery (inset: Schematic illustration of the full cell). (c) Cycle performance of NCMMg0.03//MCMB full battery.
4. Conclusions
[3] J.L. Shi, J.N. Zhang, M. He, X.D. Zhang, Y.X. Yin, H. Li, Y.G. Guo, L. Gu, L.J. Wan, ACS Appl. Mater. Interfaces 8 (2016) 20138–20146. [4] T. Hatsukade, A. Schiele, P. Hartmann, T. Brezesinski, J. Janek, ACS Appl. Mater. Interfaces 10 (2018) 38892–38899. [5] Y.S. Wang, Y.Y. Li, Z.P. Qiu, X.Z. Wu, P.F. Zhou, T. Zhou, J.P. Zhao, Z.C. Miao, J. Zhou, S.P. Zhuo, J. Mater. Chem. 6 (2018) 11189–11197. [6] K.M. Shaju, P.G. Bruce, J. Power Sources 174 (2007) 1201–1205. [7] B.J. Chae, T. Yim, J. Power Sources 360 (2017) 480–487. [8] S. Gao, Y.T. Cheng, M. Shirpour, ACS Appl. Mater. Interfaces 11 (2019) 982–989. [9] M.M. Chen, E.Y. Zhao, D.F. Chen, M.M. Wu, S.B. Han, Q.Z. Huang, L.M. Yang, X. L. Xiao, Z.B. Hu, Inorg. Chem. 56 (2017) 8355–8362. [10] P.F. Zhou, Z. Zhang, H.J. Meng, Y.Y. Lu, J. Cao, F.Y. Cheng, Z.L. Tao, J. Chen, Nanoscale 8 (2016) 19263–19269. [11] A. Manthiram, J.C. Knight, S.T. Myung, S.M. Oh, Y.K. Sun, Adv. Energy Mater. 6 (2016) 1501010. [12] J. Xu, F. Lin, M.M. Doeff, W. Tong, J. Mater. Chem. 5 (2017) 874–901. [13] J.M. Zheng, S. Myeong, W. Cho, P.F. Yan, J. Xiao, C.M. Wang, J. Cho, J.G. Zhang, Adv. Energy Mater. 7 (2017) 1601284. [14] W. Liu, P. Oh, X. Liu, M.J. Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Angew. Chem. Int. Ed. 54 (2015) 4440–4458. [15] P.F. Zhou, H.J. Meng, Z. Zhang, C.C. Chen, Y.Y. Lu, J. Cao, F.Y. Cheng, J. Chen, J. Mater. Chem. 5 (2017) 2724–2731. [16] Y.C. Li, W.M. Zhao, W. Xiang, Z.G. Wu, Z.G. Yang, C.L. Xu, Y.D. Xu, E.H. Wang, C. J. Wu, X.D. Guo, J. Alloy. Comp. 766 (2018) 546–555. [17] A. Choi, J. Lim, H.J. Kim, S.C. Jung, H.W. Lim, H. Kim, M.S. Kwon, Y.K. Han, S. M. Oh, K.T. Lee, Adv. Energy Mater. 8 (2018) 1702514. [18] A. Liu, J. Li, R. Shunmugasundaram, J.R. Dahn, J. Electrochem. Soc. 164 (2017) A1655–A1664. [19] S. Sallard, D. Sheptyakov, C. Villevieille, J. Power Sources 359 (2017) 27–36. [20] G.R. Hu, M.F. Zhang, L.W. Liang, Z.D. Peng, K. Du, Y.B. Cao, Electrochim. Acta 190 (2016) 264–275. [21] Q. Xie, W.D. Li, A. Manthiram, Chem. Mater. 31 (2019) 938–946. [22] Y.C. Li, W. Xiang, Z.G. Wu, C.L. Xu, Y.D. Xu, Y. Xiao, Z.G. Yang, C.J. Wu, G.P. Lv, X. D. Guo, Electrochim. Acta 291 (2018) 84–94. [23] P.Y. Hou, F. Li, Y.Y. Sun, M.L. Pan, X. Wang, M.H. Shao, X.J. Xu, ACS Sustain. Chem. Eng. 6 (2018) 5653–5661. [24] P. Sun, Y. Ma, T.Y. Zhai, H.Q. Li, Electrochim. Acta 191 (2016) 237–246. [25] I.M. Markus, F. Lin, K.C. Kam, M. Asta, M.M. Doeff, J. Phys. Chem. Lett. 5 (2014) 3649–3655. [26] F. Schipper, M. Dixit, D. Kovacheva, M. Talianker, O. Haik, Y. Grinblat, E. M. Erickson, C. Ghanty, D.T. Major, B. Markovsky, D. Aurbach, J. Mater. Chem. 4 (2016) 16073–16084. [27] C.S. Yoon, M.J. Choi, D.W. Jun, Q. Zhang, P. Kaghazchi, K.H. Kim, Y.K. Sun, Chem. Mater. 30 (2018) 1808–1814. [28] K. Wu, G.F. Jia, X.H. Shangguan, G.W. Yang, Z.H. Zhu, Z.J. Peng, Q. Zhuge, F.Q. Li, X.L. Cui, J. Alloy. Comp. 765 (2018) 700–709. [29] S.Y. Liu, X. Chen, J.Y. Zhao, J.M. Su, C.C. Zhang, T. Huang, J.H. Wu, A.S. Yu, J. Power Sources 374 (2018) 149–157.
The effects of Mg doping on the cycling stability of Li(Ni0.8C o0.1Mn0.1)1-xMgxO2 (x ¼ 0, 0.01, 0.03, 0.05, 0.06) are investigated in 3.0–4.5 V. The Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 with the optimal Mg content possesses uniform particle size distribution, low degree of Liþ/ Ni2þ mixing, and well-ordered layered. When used as a cathode material for LIBs, Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 demonstrates the best elec trochemical performance. The delivered high reversible discharge ca pacity is 226.1 mA h g 1 at 0.1 C and capacity retention approached 81% after 350 cycles at 0.5 C. Furthermore, the full cell of Li(Ni0.8C o0.1Mn0.1)0.97Mg0.03O2//MCMB delivers a specific energy density of 595.3 W h kg 1 at 0.5 C (based on the mass of the cathode). The superior electrochemical performance is attributed to the Mg doping, which can expand the interlayer and enhance structural stability. The simple syn thesis method and superior electrochemical performance suggest the widely potential application of Mg doped LiNi0.8Co0.1Mn0.1O2 as a promising cathode material for LIBs. Acknowledgements We are grateful for the financial supports by Key Research and Development Program of Shandong Province (2019GGX103027), Nat ural Science Foundation of Shandong Province (ZR2018BB039, ZR2017JL014), National Natural Science Foundation of China (Grant 21576158, 21576159), Taishan Scholar Foundation (tsqn2018), and 111 Project (B12015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227017. References [1] Y.X. Wang, B. Liu, Q.Y. Li, S. Cartmell, S. Ferrara, Z.Q. Daniel Deng, J. Xiao, J. Power Sources 286 (2015) 330–345. [2] S. Gao, X.W. Zhan, Y.T. Cheng, J. Power Sources 410–411 (2019) 45–52.
7
X. Liu et al.
Journal of Power Sources 438 (2019) 227017
[30] L.J. Li, X.H. Li, Z.X. Wang, H.J. Guo, P. Yue, W. Chen, L. Wu, J. Alloy. Comp. 507 (2010) 172–177. [31] B. Zhang, L.J. Li, J.C. Zheng, J. Alloy. Comp. 520 (2012) 190–194. [32] H. Li, P.F. Zhou, F.M. Liu, H.X. Li, F.Y. Cheng, J. Chen, Chem. Sci. 10 (2019) 1374–1379. [33] K.M. Min, S.W. Seo, Y.Y. Song, H.S. Lee, E. Cho, Phys. Chem. Chem. Phys. 19 (2017) 1762. [34] C.Y. Fu, Z.L. Zhou, Y.H. Liu, Q. Zhang, Y.S. Zheng, G.X. Li, J. Wuhan Univ. Technol.-Materials Sci. Ed. 26 (2011) 211–215. [35] Z.J. Huang, Z.X. Wang, X.B. Zheng, H.J. Guo, X.H. Li, Q. Jing, Z.H. Yang, RSC Adv. 5 (2015) 88773. [36] Z.J. Huang, Z.X. Wang, X.B. Zheng, H.J. Guo, X.H. Li, Q. Jing, Z.H. Yang, Electrochim. Acta 182 (2015) 795–802. [37] A.H. Tavakoli, H. Kondo, Y. Ukyo, A. Navrotsky, J. Electrochem. Soc. 160 (2012) A302–A305.
[38] Q. Zhang, Y.S. Zheng, S.K. Zhong, Key Eng. Mater. 519 (2012) 152–155. [39] D. Wang, Y. Huang, Z.Q. Huo, L. Chen, Electrochim. Acta 107 (2013) 461–466. [40] W.D. Li, X.M. Liu, H. Celio, P. Smith, A. Dolocan, M.F. Chi, A. Manthiram, Adv. Energy Mater. 8 (2018) 1703154. [41] K.J. Park, B.B. Lim, M.H. Choi, H.G. Jung, Y.K. Sun, M. Haro, N. Vicente, J. Bisquert, G. Garcia-Belmonte, J. Mater. Chem. 3 (2015) 22183–22190. [42] Q. Sa, J.A. Heelan, Y. Lu, D. Apelian, Y. Wang, ACS Appl. Mater. Interfaces 7 (2015) 20585–20590. [43] H.B. Xie, G.R. Hu, K. Du, Z.D. Peng, Y.B. Cao, J. Alloy. Comp. 666 (2016) 84–87. [44] G.H. Kim, S.T. Myung, H.S. Kim, Y.K. Sun, Electrochim. Acta 51 (2006) 2447–2453. [45] B. Huang, X.H. Li, Z.X. Wang, H.J. Guo, X.H. Xiong, Ceram. Int. 40 (2014) 13223–13230. [46] R. Robert, C. Bunzli, E.J. Berg, P. Novak, Chem. Mater. 27 (2015) 526–536.
8