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Improvement the electrochemical performance of Cr doped layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 with Li4Ti5O12 coating
Ceramics International xxx (xxxx) xxx–xxx
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Improvement the electrochemical performance of Cr doped layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 with Li4Ti5O12 coating Yunjian Liua,b, Shengquan Zhenga, Qiliang Wanga, Yanbao Fub, Huafeng Wana, Aichun Doua, ⁎ Vincent S. Battagliab, Mingru Sua, a b
School of Material Science and Technology Jiangsu University, Zhenjiang 212013, China Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
A R T I C L E I N F O
A BS T RAC T
Keywords: Lithium cathode material Layered-spinel composite Cr doped Surface coating Electrochemical performance
The Cr doped layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 was synthesized and coated with different content of Li4Ti5O12 by a sol–gel method. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of Li4Ti5O12 coatings on the electrochemical performance of the pristine material was evaluated from charge/discharge cycles, rate performance, and electrochemical impedance spectroscopy (EIS). The XRD results show that the lattice crystal and the content of spinel phase have been increased in the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials after Li4Ti5O12 coating. The results from TEM and selected area electron diffraction (SAED) indicate that the Li4Ti5O12 coating assumes a spinel structure on the Li1.1Ni0.235Mn0.735Cr0.03O2.3. The discharge capacities, cycling and rate performances of the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials in the first cycle are improved with the addition of Li4Ti5O12. Li1.1Ni0.235Mn0.735Cr0.03O2.3 coated with 3 wt% Li4Ti5O12 shows the highest discharge capacity (271.7 mA h g−1), highest capacity retention (99.4% for 100 cycles), and best rate capability (132 mA h g−1 at 10 C). EIS result indicates that the resistance of Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrode decreases with the addition of Li4Ti5O12. The enhanced electrochemical performance can be ascribed to the increased spinel content, lower resistance and the enhanced lithium-ion diffusion kinetics.
1. Introduction Lithium-ion batteries are being intensively pursued as a power source for vehicle applications as they offer much higher energy density compared to other rechargeable systems like the Ni-MH batteries. However, the energy density of the current lithium-ion technology is limited by the cathode capacity, and there is immense interest to develop new cathodes with higher capacity or higher operating voltages [1–3]. In this regard, lithium rich layered oxides [4–7] and lithium rich layered-spinel composite oxides [8–10] have become attractive since they can exhibit high capacity of ~250 mA h g−1 when charged above 4.5 V. Unfortunately, the phenomena of voltage decay have been found in the lithium rich layered oxides and lithium rich layered-spinel composite oxides, because of the transitions to spinel-like cation arrangements during the electrochemical cycling [11]. The main issue with voltage fading is that it continuously alters the cell capacity associated ⁎
with a given state of charge (SOC) and, hence, cannot satisfy constant power and energy requirements during operation. Depression of the voltage profile results in the decreases the specific energy density and output power of LIBs which eventually leads to device failure. Furthermore, the poor rate performance of lithium rich layered oxides and lithium rich layered-spinel composite oxides are also need to be improved to meet the ever-increasing demand of high-power and high capacity for hybrid electric vehicles [12]. Extensive efforts, such as surface modification [13,14] and cation doping [15,16], have been made to understand and mitigate the voltage fade in Li-rich layered solid solution oxides. Recently, many researchers have paid more attention on the synthesis and performance of the layer-spinel composite cathode materials [8–10,17–22]. The layer-spinel composite cathode materials have both layer and spinel phase, and show higher first coulomb efficiency, better rate performance. However, there are few papers focus on the improvement of layer-spinel composite materials. So, it is
Corresponding author. E-mail address: [email protected] (M. Su).
http://dx.doi.org/10.1016/j.ceramint.2017.04.011 Received 18 March 2017; Received in revised form 29 March 2017; Accepted 3 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Y., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.04.011
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Fig. 1. XRD patterns of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.5 powders.
material Li1.1Ni0.235Mn0.735Cr0.03O2.3. Li4Ti5O12 has been reported as a kind of fast lithium-ion conductor because of its high mobility of lithium ions in the structure. In addition, it is also a zero-strain material, which means that there is no structural change during the insertion/extraction process of lithium-ion. It has been proved as a novel coating material for lithium cathode materials [28,29]. In this paper, Li1.1Ni0.235Mn0.735Cr0.03O2.3 was coated with Li4Ti5O12 using sol–gel method with different contents (1, 3, 5 wt%) successfully. The influence of the Li4Ti5O12 coating layer on the physical and electrochemical performance of the Li1.1Ni0.235Mn0.735 Cr0.03O2.3 material was investigated in detail.
necessary and important to study the improvement of the layer-spinel composite materials. Our group have studied the layer-spinel composite materials for a few years [23,24]. In our previous paper [24], the voltage fade phenomena has been mitigated after Cr doping, and the middle voltage of discharge profiles of Li1.1Ni0.235Mn0.735Cr0.03O2.3 has been increased compared with those of the pristine Li1.1Ni0.25Mn0.75 O2.3. However, the rate performance and mitigation of the voltage decay of layer-spinel composite Li1.1Ni0.235Mn0.735Cr0.03O2.3 is still needed to be enhanced. Coating of lithium-ion conductive oxides on the lithium rich layer cathode materials has been reported as an effective modification technique, such as LiAlO2 [25] LiVO3 [26] and Li2ZrO3 [27]. Inspired by the modification of lithium rich layered oxides, we would like to modify the Li1.1Ni0.235Mn0.735Cr0.03O2.3 with fast lithium-ion conductor in order to make further efforts to improve the rate performance and further more mitigate the voltage decay of cathode
2. Experimental The layered-spinel composite cathode Li1.1Ni0.235Mn0.735Cr0.03O2.3 powders were all synthesized by a co-precipitation method through 2
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Fig. 2. SEM images of synthesized and EDS results of pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 powder.
550 °C for 4 h and 850 °C for 10 h, then cooled in liquid nitrogen. The sample was indicated as 'a'. The Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials with different content were prepared via a sol–gel method. Firstly, a certain quantity of Ti(C4H9O)4 and CH3COOLi·2H2O were dissolved in ethanol and distilled water with a cationic ratio of Li:Ti=4:5 to form a clear solution. Secondly, a certain quantity of citric acid was added into the above solution by strong stirring to get a clear yellow sol. Then the prepared Li1.1Ni0.235Mn0.735Cr0.03O2.3 powders were dispersed uniformly in this sol. After stirring for 6 h, a viscous black gel was obtained. Finally, the gel was dried at 120 °C for 1 h to get a black
oxalate [24]. Required amounts of the transition metal nitrates were dissolved in deionized water and the concentration of transition metal was controlled at 0.1 M. Then the transition metal solution was added drop by drop into a 0.1 M NaOH solution to form the coprecipitation hydrate of Ni, Mn and Cr. The pH of solution was controlled at 11 by adding ammonia and the temperature was controlled at 70 °C during the precipitation progress. The coprecipitation precursor were washed for several times with deionized water and then dried overnight at 120 °C in an air-oven. To synthesis the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials, the dried co-precipitation precursor mixed with a desired stoichiometric amount of lithium carbonate were sintered in air at 3
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Fig. 3. TEM images of pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 powder.
precursor, and calcinated in air at 600 °C for 8 h to yield the final powder. The content of Li4Ti5O12 in the final product was 1, 3, 5 wt% (indicated as ‘b’, ‘c’, ‘d’, respectively). X-ray diffraction (XRD) patterns of these samples were performed on a Panalytical X′Pert diffractometer (Holland) with Cu Kα radiation operated at 40 kV and 30 mA. Data were collected in 2θ range of 10– 90° at 2° min−1. The particle size and morphology of the samples were examined by SEM (FEI, Quanta200F) and TEM (Hitachi, 7650). Electrochemical measurements were carried out using R2025 cointype cells. The positive electrodes were prepared by coating a slurry containing 80% active materials, 10% acetylene black, 10% poly (vinylidene fluoride) binder on circular Al current collector foils followed by drying at 120 °C for 1 h. Electrochemical cells were assembled with the positive electrodes as-prepared, metallic lithium foil as counter electrode, Cellgard 2400 as separator, and 1 M LiPF6 dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume) as electrolyte in an argon-filled glove box. Charge-discharge experiments were performed galvanostatically with 0.1 C (1 C=240 mA h g−1) between 2.0 and 4.8 V on battery testers (Neware CT3008).
3. Results and discussion Fig. 1 shows the XRD patterns of synthesized Li1.1Ni0.235Mn0.735 Cr0.03O2.3 and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3. The selected segments (33–50°), (17.5–19.5°), (35–38°), (43–46°) of XRD patterns have been shown in Fig. 1B–E. As shown in Fig. 1A, features are present in the 20–25° region for all samples, indicating the presence of the Li2MnO3-type C2/m phase. XRD peaks at ~36.5° and ~44° can be attributed to the presence of LiNi0.5Mn1.5O4 type spinel phase within the material [30]. The results indicate that all the synthesized materials show both layered and spinel phase. The results suggest that Li4Ti5O12 is only modified the surface of the active material without changing the crystal structure. As details shown in Fig. 1B–E, it can be seen that the diffraction patterns of the Li4Ti5O12coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 shift toward lower angles compared to that of pure Li1.1Ni0.235Mn0.735Cr0.03O2.3, indicating that the lattice constants increase. This is caused by the slightly larger ionic radius of Ti4+ (0.0605 nm) compared to that of Mn4+ (0.053 nm) and Li+ (0.059 nm) [31]. This result indicates that Li4Ti5O12 may be incorporated with the structure, thus forming a solid solution. As 4
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reported [32], the expansion of the lattice crystal could provide more lattice space for lithium intercalation and deintercalation and reduce the tendency for cracking with lithium intercalation. Furthermore, as shown in Fig. 1C and D in detail, with the content of coating Li4Ti5O12 increasing, the intensity of (111) and (311) for spinel phase increases. These results indicate that the content of spinel phase may be increased in the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials compared with pristine Li1.1Ni0.25Mn0.75O2.3. As reported in previous work, the capacity was increased and the rate performance was enhanced with an increasing over lithiation of the spinel component in layer-spinel composite materials [33,34]. Fig. 2 shows the scanning electron microscopy (SEM) images and EDS results of pristine and Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 powder. The SEM images in Fig. 2 show that the powders contain the aggregates of primary particles. The primary particles are homogenous and have a size between 300 and 500 nm. As seen in the EDS results, the Ti element has been detected on the surface of Li4Ti5O12-coated samples while the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 does not exhibit any Ti element. Therefore, it can be speculated that the surface of the prepared Li1.1Ni0.235Mn0.735Cr0.03O2.3 is covered with small Li4Ti5O12 particles. It can be expected that the coated Li4Ti5O12 will decrease the direct contact area between the cathode and electrolyte, and thus suppress the surface side reaction between Li1.1Ni0.235Mn0.735Cr0.03O2.3 and electrolyte, which may result in better electrochemical performance. To confirm the microstructure of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials, the TEM analysis has been tested and the results have been shown in Fig. 3. As seen in Fig. 3a, it is clear that the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 sample has no visible fringes, which means that the crystals of Li1.1Ni0.235Mn0.735 Cr0.03O2.3 grow very well and have good crystal. Compared with Fig. 3a, Li1.1Ni0.235Mn0.735Cr0.03O2.3 particles are coated by Li4Ti5O12 which can be seen in Fig. 3b–d and coated more effectively with the amounts of Li4Ti5O12 increasing. The selected area electron diffraction (SAED) of the coated layer (Fig. 3e) is shown in Fig. 3f. The result in Fig. 3f indicates the crystalline phase characteristics of the coating layers and suggests that surface coating with the Li4Ti5O12 has been successfully achieved on the cathode powders. Fig. 4 shows the first charge and discharge curves of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes at 0.1 C between 2.0 and 4.8 V with constant current-constant voltage (CC-CV) method. As shown in Fig. 4, all the prepared samples have two distinguished voltage regions during the initial charge process. The appearance of a charge plateau at 4.5 V has been caused by the removal of Li2O from the structure of Li2MnO3 [35], which means that the Li4Ti5O12-coating does not change the intrinsic lithium de/intercalation properties of Li1.1Ni0.235Mn0.735Cr0.03O2.3, such as charge/discharge behavior. There
Table 1 Electrochemical cell data collected at 0.1 C rate and 2.0–4.8 V of the synthesized and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3. Samples
Charge capacity (mA h g−1)
Discharge capacity (mA h g−1)
Irreversible capacity loss (mA h g−1)
Coulomb efficiency (%)
a b c d
281.1 283.6 287.8 277.7
261.9 264.3 271.7 265.5
19.2 19.3 16.1 12.2
93.17 93.19 94.41 95.61
are plateaus at 4.8 V in the charge curves, which is the constant voltage plateau. Furthermore, there are another three obvious plateaus at 4.7, 2.7 and 2.2 V in the discharge curves. To our knowledge, the 4.7 V plateau originates from the Ni2+/Ni4+ redox couple and the ordering of lithium at 8a sites with 50% filling in the LiNi0.5Mn1.5O4 phase [36]. The other two reaction plateaus under 3.0 V (about 2.7 and 2.2 V) are originated from lithium-ion insertion into the empty 16c octahedral sites of the cubic spinel structure through the reduction of Mn4+ to Mn3+, which is associated with a cubic to tetragonal phase transition (2.7 V) [35] and then transformed to tetragonal (I41/amd) phase (2.2 V) at fully discharged state [37]. The first charge and discharge capacities, ICL values and coulomb efficiencies of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials are listed in Table 1. As seen in Table 1, the discharge capacity and first coulomb efficiency of Li1.1Ni0.235Mn0.735Cr0.03O2.3 is enhanced after Li4Ti5O12 coating. Among the coated samples, the 3 wt% Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 (271.7 mA h g−1) shows a higher capacity compared to the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 (261.9 mA h g−1). The result shows that Li4Ti5O12 coating can lead to the enhanced reversible properties for Li-rich cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 effectively. It is because that the coating layer can suppress the oxidation of the electrolyte, the dissolution of the transition metals, and then restrain the simultaneous removal of Li+ and O2 during the first cycle. Fig. 5A compares the cycling performance of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes at 0.2 C in the voltage range of 2.0–4.8 V at room temperature. After 100 cycles at 0.2 C, the capacity retentions of 1, 3 and 5 wt% Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 is 97.8%, 98.9%, 99.4% and 97.3%, respectively. All the samples show excellent cyclic performance. As reported in previous paper [38], the lithium-rich layered oxide phase in the composite cathodes undergoes phase transformation during cycling and the 5 V spinel phase in the composite cathode exhibits remarkable cyclability in a wide voltage range of 2–5 V despite of the occurrence of Jahn-Teller (cubic to tetragonal) distortion. However, the cyclic performance is still improved after Li4Ti5O12 coated. The reason can be ascribed to the Li4Ti5O12 coating layer, which not only avoids the direct contact between the electrode and electrolyte to prevent the Li1.1Ni0.235Mn0.735Cr0.03O2.3 from dissolving into the electrolyte, but also reduces the oxidation of the electrolyte on the surface of cathode at charge state [39]. The rate performance of the samples was measured at different rates (Fig. 5B). The cells were sequentially charged at 0.1 C and discharged in the range of 2.0–4.8 V at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 C for five cycles. As the applied current density increases, all the samples show gradual decreases of discharge capacities. However, the Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 samples show the relatively moderate rate capacity fading compared with pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 except of the 5% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3. And the 3% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 shows the best rate performance. In comparison, the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 sample displays poor rate capability with increasing current density, the value remaining around 113 mA h g−1 at 10.0 C, while 3% Li4Ti5O12 coated sample
Fig. 4. First charge and discharge curves of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes.
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associated with Mn4+/Mn3+ redox related to Li occupation within octahedral sites; R4 is related to Li occupation within octahedral sites accompanied by Ni+4/+3/+2 redox couples; R5 is ascribed to Li occupation within tetrahedral sites. As seen, the R3 for sample 'a' and 'c' shift continuously toward lower potential upon cycling, which indicates transitions to spinellike cation arrangements in the host structure. However, the R3 for 3 wt% Li4Ti5O12 coated sample 'c' shifts more slowly than that of sample 'a'. Furthermore, the R3 for spinel phase of LiNi0.5Mn1.5O4 in the sample 'a' and 'c' are disappearing with cycling. And the R3 in the sample 'c' is more remarkable than that of sample 'c' after 55 cycles (Fig. 6E). These results indicate that the layer and spinel structure stability of LiNi0.235Mn0.735 Cr0.03O2.3 has been improved after Li4Ti5O12 coating. As reported the gradually-formed Li2CrO4 nano-domains may play a critical role in inhibiting migration of transition metals and diffusion of vacancy site during electrochemical (de)lithiation, thus restraining the growth of spinellike structure inside particles [42]. Besides, a substitutional compound Li1.1NixMnyCrzTi1−x-y-zO2.3 has formed on the surface of coated LiNi0.235Mn0.735Cr0.03O2.3 through the interaction of the coated oxide with the substrate, which improves the stability of the LiNi0.235Mn0.735Cr0.03O2.3 because of the strong bond of Ti–O. It can be concluded that the voltage decay phenomenon can be depressed restrained by doping and surface coating. EIS provides information about the kinetics of electrochemical reactions of electrodes. To better understand the improvement of electrochemical performance, the samples are examined by EIS. The measurements are carried out in the charged state of 4.4 V after 50 cycles tested at 0.1 C with three electrodes system. The measured impedance spectra are presented in Fig. 7A. A high frequency semicircle and a low-frequency tail are observed. Generally, an intercept at the Z real-axis in high frequency region corresponded to ohm resistance (Rs). The high-frequency semicircle is related to charge transfer resistance (Rct). The low frequency tail is associated with Lit ion diffusion process in the solid phase of electrode. Each impedance spectrum is fitted with suggested equivalent circuit model (Fig. 7B) to give simulation of the ohm resistance (Rs) and charge transfer resistance (Rct), as summarized in Table 3. As seen in Table 3, a rapid decrease of the surface charge transfer resistance and ohm resistance has been observed in the Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 except of the 5 wt% coated sample. The 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 cathode shows the lowest charge transfer resistance (Rct). The surface coating decreases Rct by reducing the SEI layer thickness due to a suppressed interaction between the cathode surface and electrolyte while maintaining a micro-porous structure allowing lithium-ions to diffuse through [43]. Furthermore, as reported in previous papers [44,45], the excellent lithium-ion conductivity of Li4Ti5O12 should be another reason. Furthermore, based on the Warburg diffusion supported by the low-frequency region, the diffusion coefficient (DLi+) of lithium ions is calculated based on the following equations
Fig. 5. Cycling (A) and rate (B) performance of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes.
delivers a discharge capacity of 132 mA h g−1. The results show that the rate capability of Li1.1Ni0.235Mn0.735Cr0.03O2.3 has been improved with Li4Ti5O12 coating effectively. It is well known that the faster ionic diffusion ability of Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 particles contributed to the better rate capability [40,41]. The midpoint voltages (MPVs) of discharge at different cycles of pristine and 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials are summarized in Table 2. Middle voltage refers to the voltage when the discharge specific capacity is 50%. As seen in Table 2, the midpoint voltages of every cycle for coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 are higher than that of pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3. Further compared with the pristine Li1.1Ni0.25Mn0.75O2.3 [24], the midpoint voltages (MPVs) of discharge of 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 is enhanced remarkably. The differential capacity vs. voltage (dQ/dV) curves of Li`Ni0.235Mn0.735Cr0.03O2.3 and 3 wt% Li4Ti5O12 coated electrodes at different cycles are shown in Fig. 6. As shown in Fig. 6, in the discharging curves, there are mainly five electrochemical processes, noted as R1 at 2.2 V, R2 at 2.7 V, R3 at 3.2 V, R4 at 3.7 V and R5 around 4.5 V. It is documented that R1is related with transformation to tetragonal (I41/amd) phase; R2 is associated with spinel phase of LiNi0.5Mn1.5O4; R3 is
1
15
30
45
55
a c
3.544 3.552
3.416 3.425
3.322 3.348
3.254 3.287
3.220 3.258
(1)
Z′ = Rs + Rct +σw·ω−0.5
(2)
As shown in Eq. (1), R is the ideal gas constant, T is the absolute temperature, n is the number of electrons per molecule during charge/ discharge process (for this reaction, it is 1), F is the Faraday constant, C0 is the concentration of Li+ in pre unit cell (for this cathode it is 4.67×10−2 mol cm−3), A is surface area of the electrode in cm2 (for this electrode it is 1.44 cm2), σ is the Warburg factor which has relationship with Z′ (shown in Eq. (2)). Fig. 8 shows the linearity of the Z′ and ω−1/2 in the low-frequency region. On the basis of the above information, the lithium-ion diffusion coefficients of pristine and 3 wt% coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 cathodes after 50 cycles are calculated. The results are listed in Table 4. As shown in Table 4, the lithium
Table 2 Middle voltage of pristine and 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrode at different discharge cycle. Sample
⎞2 ⎛ RT ⎟⎟ D = 0.5⎜⎜ 2 2 ⎝ An F σwC0 ⎠
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Fig. 6. Differential capacity vs. voltage (dQ/dV) curves of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes at different cycles 1(A), 15(B), 30(C), 45(D), 55(E). Table 3 Impedance parameters of equipment circuit. Electrodes
RS (Ω)
Rct (Ω)
a b c d
11.22 9.16 4.07 12.18
359.9 295.6 202.7 667.2
Fig. 7. AC impedance of the pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes (A) and equivalent circuit (B).
Fig. 8. Relationships between Z′ and ω−1/2.
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Table 4 Kinetic parameters of the pristine and 3 wt% Li4Ti5O12-coated samples.
[11]
Sample
RS (Ω)
Rct (Ω)
σW (Ω cm2 s−0.5)
D (cm2 s−1)
0% 3%
11.22 4.07
359.9 202.7
37.007 19.127
4.256×10–12 1.432×10–11
[12]
[13]
diffusion coefficients of pristine and 3 wt% coated Li1.1Ni0.235Mn0.735 Cr0.03O2.3 cathodes are 4.256×10–12 and 1.432×10–11 cm2 s−1. It can be found that the lithium diffusion coefficient is increased after 3 wt% Li4Ti5O12 coated. This reveals that the appropriate amount of Li4Ti5O12 modification has a positive effect on the electrochemical performance of Li1.1Ni0.235Mn0.735Cr0.03O2.3 with increase of lithium ion diffusivity.
[14]
[15]
[16]
4. Conclusion [17]
The Cr doped layer-spinel composite cathode material Li1.1Ni0.235 Mn0.735Cr0.03O2.3 was synthesized and coated with fast ion conductor Li4Ti5O12. After coating, the 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735 Cr0.03O2.3 shows the best electrochemical performances, especially the rate performance. The discharge capacity of Li1.1Ni0.235Mn0.735Cr0.03 O2.3 at a 10 C rate increases from 113 to 132 mA h g−1 upon coating with Li4Ti5O12. Furthermore, the layer and spinel structure stability of LiNi0.235Mn0.735Cr0.03O2.3 has been enhanced after Li4Ti5O12 coating, which results in improved midpoint voltages. EIS results show that the Rct of pristine LiNi0.235Mn0.735Cr0.03O2.3 electrodes after 50 cycles is decreased with the addition of the Li4Ti5O12 coating. The lithium diffusion coefficient is increased from 4.256×10–12 to 1.432×10– 11 cm2 s−1 after 3 wt% Li4Ti5O12 coating. Compared with Li1.1Ni0.25 Mn0.75O2.3 [22], this study shows that the electrochemical performance of layer-spinel composite cathode material can be improved by Cr doping and Li4Ti5O12 coating.
[18]
[19]
[20]
[21]
[22]
Acknowledgments
[23]
The authors gratefully acknowledge the National Natural Science Foundation of China (51304081) (51604124) (51604125) and Natural Science Foundation of Jiangsu Province (BK20140581) (BK20150506) (BK20140558).
[24]
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Improvement the electrochemical performance of Cr doped layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 with Li4Ti5O12 coating Yunjian Liua,b, Shengquan Zhenga, Qiliang Wanga, Yanbao Fub, Huafeng Wana, Aichun Doua, ⁎ Vincent S. Battagliab, Mingru Sua, a b
School of Material Science and Technology Jiangsu University, Zhenjiang 212013, China Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
A R T I C L E I N F O
A BS T RAC T
Keywords: Lithium cathode material Layered-spinel composite Cr doped Surface coating Electrochemical performance
The Cr doped layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 was synthesized and coated with different content of Li4Ti5O12 by a sol–gel method. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of Li4Ti5O12 coatings on the electrochemical performance of the pristine material was evaluated from charge/discharge cycles, rate performance, and electrochemical impedance spectroscopy (EIS). The XRD results show that the lattice crystal and the content of spinel phase have been increased in the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials after Li4Ti5O12 coating. The results from TEM and selected area electron diffraction (SAED) indicate that the Li4Ti5O12 coating assumes a spinel structure on the Li1.1Ni0.235Mn0.735Cr0.03O2.3. The discharge capacities, cycling and rate performances of the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials in the first cycle are improved with the addition of Li4Ti5O12. Li1.1Ni0.235Mn0.735Cr0.03O2.3 coated with 3 wt% Li4Ti5O12 shows the highest discharge capacity (271.7 mA h g−1), highest capacity retention (99.4% for 100 cycles), and best rate capability (132 mA h g−1 at 10 C). EIS result indicates that the resistance of Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrode decreases with the addition of Li4Ti5O12. The enhanced electrochemical performance can be ascribed to the increased spinel content, lower resistance and the enhanced lithium-ion diffusion kinetics.
1. Introduction Lithium-ion batteries are being intensively pursued as a power source for vehicle applications as they offer much higher energy density compared to other rechargeable systems like the Ni-MH batteries. However, the energy density of the current lithium-ion technology is limited by the cathode capacity, and there is immense interest to develop new cathodes with higher capacity or higher operating voltages [1–3]. In this regard, lithium rich layered oxides [4–7] and lithium rich layered-spinel composite oxides [8–10] have become attractive since they can exhibit high capacity of ~250 mA h g−1 when charged above 4.5 V. Unfortunately, the phenomena of voltage decay have been found in the lithium rich layered oxides and lithium rich layered-spinel composite oxides, because of the transitions to spinel-like cation arrangements during the electrochemical cycling [11]. The main issue with voltage fading is that it continuously alters the cell capacity associated ⁎
with a given state of charge (SOC) and, hence, cannot satisfy constant power and energy requirements during operation. Depression of the voltage profile results in the decreases the specific energy density and output power of LIBs which eventually leads to device failure. Furthermore, the poor rate performance of lithium rich layered oxides and lithium rich layered-spinel composite oxides are also need to be improved to meet the ever-increasing demand of high-power and high capacity for hybrid electric vehicles [12]. Extensive efforts, such as surface modification [13,14] and cation doping [15,16], have been made to understand and mitigate the voltage fade in Li-rich layered solid solution oxides. Recently, many researchers have paid more attention on the synthesis and performance of the layer-spinel composite cathode materials [8–10,17–22]. The layer-spinel composite cathode materials have both layer and spinel phase, and show higher first coulomb efficiency, better rate performance. However, there are few papers focus on the improvement of layer-spinel composite materials. So, it is
Corresponding author. E-mail address: [email protected] (M. Su).
http://dx.doi.org/10.1016/j.ceramint.2017.04.011 Received 18 March 2017; Received in revised form 29 March 2017; Accepted 3 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Y., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.04.011
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Fig. 1. XRD patterns of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.5 powders.
material Li1.1Ni0.235Mn0.735Cr0.03O2.3. Li4Ti5O12 has been reported as a kind of fast lithium-ion conductor because of its high mobility of lithium ions in the structure. In addition, it is also a zero-strain material, which means that there is no structural change during the insertion/extraction process of lithium-ion. It has been proved as a novel coating material for lithium cathode materials [28,29]. In this paper, Li1.1Ni0.235Mn0.735Cr0.03O2.3 was coated with Li4Ti5O12 using sol–gel method with different contents (1, 3, 5 wt%) successfully. The influence of the Li4Ti5O12 coating layer on the physical and electrochemical performance of the Li1.1Ni0.235Mn0.735 Cr0.03O2.3 material was investigated in detail.
necessary and important to study the improvement of the layer-spinel composite materials. Our group have studied the layer-spinel composite materials for a few years [23,24]. In our previous paper [24], the voltage fade phenomena has been mitigated after Cr doping, and the middle voltage of discharge profiles of Li1.1Ni0.235Mn0.735Cr0.03O2.3 has been increased compared with those of the pristine Li1.1Ni0.25Mn0.75 O2.3. However, the rate performance and mitigation of the voltage decay of layer-spinel composite Li1.1Ni0.235Mn0.735Cr0.03O2.3 is still needed to be enhanced. Coating of lithium-ion conductive oxides on the lithium rich layer cathode materials has been reported as an effective modification technique, such as LiAlO2 [25] LiVO3 [26] and Li2ZrO3 [27]. Inspired by the modification of lithium rich layered oxides, we would like to modify the Li1.1Ni0.235Mn0.735Cr0.03O2.3 with fast lithium-ion conductor in order to make further efforts to improve the rate performance and further more mitigate the voltage decay of cathode
2. Experimental The layered-spinel composite cathode Li1.1Ni0.235Mn0.735Cr0.03O2.3 powders were all synthesized by a co-precipitation method through 2
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Fig. 2. SEM images of synthesized and EDS results of pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 powder.
550 °C for 4 h and 850 °C for 10 h, then cooled in liquid nitrogen. The sample was indicated as 'a'. The Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials with different content were prepared via a sol–gel method. Firstly, a certain quantity of Ti(C4H9O)4 and CH3COOLi·2H2O were dissolved in ethanol and distilled water with a cationic ratio of Li:Ti=4:5 to form a clear solution. Secondly, a certain quantity of citric acid was added into the above solution by strong stirring to get a clear yellow sol. Then the prepared Li1.1Ni0.235Mn0.735Cr0.03O2.3 powders were dispersed uniformly in this sol. After stirring for 6 h, a viscous black gel was obtained. Finally, the gel was dried at 120 °C for 1 h to get a black
oxalate [24]. Required amounts of the transition metal nitrates were dissolved in deionized water and the concentration of transition metal was controlled at 0.1 M. Then the transition metal solution was added drop by drop into a 0.1 M NaOH solution to form the coprecipitation hydrate of Ni, Mn and Cr. The pH of solution was controlled at 11 by adding ammonia and the temperature was controlled at 70 °C during the precipitation progress. The coprecipitation precursor were washed for several times with deionized water and then dried overnight at 120 °C in an air-oven. To synthesis the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials, the dried co-precipitation precursor mixed with a desired stoichiometric amount of lithium carbonate were sintered in air at 3
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Fig. 3. TEM images of pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 powder.
precursor, and calcinated in air at 600 °C for 8 h to yield the final powder. The content of Li4Ti5O12 in the final product was 1, 3, 5 wt% (indicated as ‘b’, ‘c’, ‘d’, respectively). X-ray diffraction (XRD) patterns of these samples were performed on a Panalytical X′Pert diffractometer (Holland) with Cu Kα radiation operated at 40 kV and 30 mA. Data were collected in 2θ range of 10– 90° at 2° min−1. The particle size and morphology of the samples were examined by SEM (FEI, Quanta200F) and TEM (Hitachi, 7650). Electrochemical measurements were carried out using R2025 cointype cells. The positive electrodes were prepared by coating a slurry containing 80% active materials, 10% acetylene black, 10% poly (vinylidene fluoride) binder on circular Al current collector foils followed by drying at 120 °C for 1 h. Electrochemical cells were assembled with the positive electrodes as-prepared, metallic lithium foil as counter electrode, Cellgard 2400 as separator, and 1 M LiPF6 dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume) as electrolyte in an argon-filled glove box. Charge-discharge experiments were performed galvanostatically with 0.1 C (1 C=240 mA h g−1) between 2.0 and 4.8 V on battery testers (Neware CT3008).
3. Results and discussion Fig. 1 shows the XRD patterns of synthesized Li1.1Ni0.235Mn0.735 Cr0.03O2.3 and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3. The selected segments (33–50°), (17.5–19.5°), (35–38°), (43–46°) of XRD patterns have been shown in Fig. 1B–E. As shown in Fig. 1A, features are present in the 20–25° region for all samples, indicating the presence of the Li2MnO3-type C2/m phase. XRD peaks at ~36.5° and ~44° can be attributed to the presence of LiNi0.5Mn1.5O4 type spinel phase within the material [30]. The results indicate that all the synthesized materials show both layered and spinel phase. The results suggest that Li4Ti5O12 is only modified the surface of the active material without changing the crystal structure. As details shown in Fig. 1B–E, it can be seen that the diffraction patterns of the Li4Ti5O12coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 shift toward lower angles compared to that of pure Li1.1Ni0.235Mn0.735Cr0.03O2.3, indicating that the lattice constants increase. This is caused by the slightly larger ionic radius of Ti4+ (0.0605 nm) compared to that of Mn4+ (0.053 nm) and Li+ (0.059 nm) [31]. This result indicates that Li4Ti5O12 may be incorporated with the structure, thus forming a solid solution. As 4
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reported [32], the expansion of the lattice crystal could provide more lattice space for lithium intercalation and deintercalation and reduce the tendency for cracking with lithium intercalation. Furthermore, as shown in Fig. 1C and D in detail, with the content of coating Li4Ti5O12 increasing, the intensity of (111) and (311) for spinel phase increases. These results indicate that the content of spinel phase may be increased in the Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials compared with pristine Li1.1Ni0.25Mn0.75O2.3. As reported in previous work, the capacity was increased and the rate performance was enhanced with an increasing over lithiation of the spinel component in layer-spinel composite materials [33,34]. Fig. 2 shows the scanning electron microscopy (SEM) images and EDS results of pristine and Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 powder. The SEM images in Fig. 2 show that the powders contain the aggregates of primary particles. The primary particles are homogenous and have a size between 300 and 500 nm. As seen in the EDS results, the Ti element has been detected on the surface of Li4Ti5O12-coated samples while the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 does not exhibit any Ti element. Therefore, it can be speculated that the surface of the prepared Li1.1Ni0.235Mn0.735Cr0.03O2.3 is covered with small Li4Ti5O12 particles. It can be expected that the coated Li4Ti5O12 will decrease the direct contact area between the cathode and electrolyte, and thus suppress the surface side reaction between Li1.1Ni0.235Mn0.735Cr0.03O2.3 and electrolyte, which may result in better electrochemical performance. To confirm the microstructure of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials, the TEM analysis has been tested and the results have been shown in Fig. 3. As seen in Fig. 3a, it is clear that the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 sample has no visible fringes, which means that the crystals of Li1.1Ni0.235Mn0.735 Cr0.03O2.3 grow very well and have good crystal. Compared with Fig. 3a, Li1.1Ni0.235Mn0.735Cr0.03O2.3 particles are coated by Li4Ti5O12 which can be seen in Fig. 3b–d and coated more effectively with the amounts of Li4Ti5O12 increasing. The selected area electron diffraction (SAED) of the coated layer (Fig. 3e) is shown in Fig. 3f. The result in Fig. 3f indicates the crystalline phase characteristics of the coating layers and suggests that surface coating with the Li4Ti5O12 has been successfully achieved on the cathode powders. Fig. 4 shows the first charge and discharge curves of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes at 0.1 C between 2.0 and 4.8 V with constant current-constant voltage (CC-CV) method. As shown in Fig. 4, all the prepared samples have two distinguished voltage regions during the initial charge process. The appearance of a charge plateau at 4.5 V has been caused by the removal of Li2O from the structure of Li2MnO3 [35], which means that the Li4Ti5O12-coating does not change the intrinsic lithium de/intercalation properties of Li1.1Ni0.235Mn0.735Cr0.03O2.3, such as charge/discharge behavior. There
Table 1 Electrochemical cell data collected at 0.1 C rate and 2.0–4.8 V of the synthesized and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3. Samples
Charge capacity (mA h g−1)
Discharge capacity (mA h g−1)
Irreversible capacity loss (mA h g−1)
Coulomb efficiency (%)
a b c d
281.1 283.6 287.8 277.7
261.9 264.3 271.7 265.5
19.2 19.3 16.1 12.2
93.17 93.19 94.41 95.61
are plateaus at 4.8 V in the charge curves, which is the constant voltage plateau. Furthermore, there are another three obvious plateaus at 4.7, 2.7 and 2.2 V in the discharge curves. To our knowledge, the 4.7 V plateau originates from the Ni2+/Ni4+ redox couple and the ordering of lithium at 8a sites with 50% filling in the LiNi0.5Mn1.5O4 phase [36]. The other two reaction plateaus under 3.0 V (about 2.7 and 2.2 V) are originated from lithium-ion insertion into the empty 16c octahedral sites of the cubic spinel structure through the reduction of Mn4+ to Mn3+, which is associated with a cubic to tetragonal phase transition (2.7 V) [35] and then transformed to tetragonal (I41/amd) phase (2.2 V) at fully discharged state [37]. The first charge and discharge capacities, ICL values and coulomb efficiencies of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials are listed in Table 1. As seen in Table 1, the discharge capacity and first coulomb efficiency of Li1.1Ni0.235Mn0.735Cr0.03O2.3 is enhanced after Li4Ti5O12 coating. Among the coated samples, the 3 wt% Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 (271.7 mA h g−1) shows a higher capacity compared to the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 (261.9 mA h g−1). The result shows that Li4Ti5O12 coating can lead to the enhanced reversible properties for Li-rich cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 effectively. It is because that the coating layer can suppress the oxidation of the electrolyte, the dissolution of the transition metals, and then restrain the simultaneous removal of Li+ and O2 during the first cycle. Fig. 5A compares the cycling performance of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes at 0.2 C in the voltage range of 2.0–4.8 V at room temperature. After 100 cycles at 0.2 C, the capacity retentions of 1, 3 and 5 wt% Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 is 97.8%, 98.9%, 99.4% and 97.3%, respectively. All the samples show excellent cyclic performance. As reported in previous paper [38], the lithium-rich layered oxide phase in the composite cathodes undergoes phase transformation during cycling and the 5 V spinel phase in the composite cathode exhibits remarkable cyclability in a wide voltage range of 2–5 V despite of the occurrence of Jahn-Teller (cubic to tetragonal) distortion. However, the cyclic performance is still improved after Li4Ti5O12 coated. The reason can be ascribed to the Li4Ti5O12 coating layer, which not only avoids the direct contact between the electrode and electrolyte to prevent the Li1.1Ni0.235Mn0.735Cr0.03O2.3 from dissolving into the electrolyte, but also reduces the oxidation of the electrolyte on the surface of cathode at charge state [39]. The rate performance of the samples was measured at different rates (Fig. 5B). The cells were sequentially charged at 0.1 C and discharged in the range of 2.0–4.8 V at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 C for five cycles. As the applied current density increases, all the samples show gradual decreases of discharge capacities. However, the Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 samples show the relatively moderate rate capacity fading compared with pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 except of the 5% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3. And the 3% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 shows the best rate performance. In comparison, the pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3 sample displays poor rate capability with increasing current density, the value remaining around 113 mA h g−1 at 10.0 C, while 3% Li4Ti5O12 coated sample
Fig. 4. First charge and discharge curves of pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes.
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associated with Mn4+/Mn3+ redox related to Li occupation within octahedral sites; R4 is related to Li occupation within octahedral sites accompanied by Ni+4/+3/+2 redox couples; R5 is ascribed to Li occupation within tetrahedral sites. As seen, the R3 for sample 'a' and 'c' shift continuously toward lower potential upon cycling, which indicates transitions to spinellike cation arrangements in the host structure. However, the R3 for 3 wt% Li4Ti5O12 coated sample 'c' shifts more slowly than that of sample 'a'. Furthermore, the R3 for spinel phase of LiNi0.5Mn1.5O4 in the sample 'a' and 'c' are disappearing with cycling. And the R3 in the sample 'c' is more remarkable than that of sample 'c' after 55 cycles (Fig. 6E). These results indicate that the layer and spinel structure stability of LiNi0.235Mn0.735 Cr0.03O2.3 has been improved after Li4Ti5O12 coating. As reported the gradually-formed Li2CrO4 nano-domains may play a critical role in inhibiting migration of transition metals and diffusion of vacancy site during electrochemical (de)lithiation, thus restraining the growth of spinellike structure inside particles [42]. Besides, a substitutional compound Li1.1NixMnyCrzTi1−x-y-zO2.3 has formed on the surface of coated LiNi0.235Mn0.735Cr0.03O2.3 through the interaction of the coated oxide with the substrate, which improves the stability of the LiNi0.235Mn0.735Cr0.03O2.3 because of the strong bond of Ti–O. It can be concluded that the voltage decay phenomenon can be depressed restrained by doping and surface coating. EIS provides information about the kinetics of electrochemical reactions of electrodes. To better understand the improvement of electrochemical performance, the samples are examined by EIS. The measurements are carried out in the charged state of 4.4 V after 50 cycles tested at 0.1 C with three electrodes system. The measured impedance spectra are presented in Fig. 7A. A high frequency semicircle and a low-frequency tail are observed. Generally, an intercept at the Z real-axis in high frequency region corresponded to ohm resistance (Rs). The high-frequency semicircle is related to charge transfer resistance (Rct). The low frequency tail is associated with Lit ion diffusion process in the solid phase of electrode. Each impedance spectrum is fitted with suggested equivalent circuit model (Fig. 7B) to give simulation of the ohm resistance (Rs) and charge transfer resistance (Rct), as summarized in Table 3. As seen in Table 3, a rapid decrease of the surface charge transfer resistance and ohm resistance has been observed in the Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 except of the 5 wt% coated sample. The 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 cathode shows the lowest charge transfer resistance (Rct). The surface coating decreases Rct by reducing the SEI layer thickness due to a suppressed interaction between the cathode surface and electrolyte while maintaining a micro-porous structure allowing lithium-ions to diffuse through [43]. Furthermore, as reported in previous papers [44,45], the excellent lithium-ion conductivity of Li4Ti5O12 should be another reason. Furthermore, based on the Warburg diffusion supported by the low-frequency region, the diffusion coefficient (DLi+) of lithium ions is calculated based on the following equations
Fig. 5. Cycling (A) and rate (B) performance of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes.
delivers a discharge capacity of 132 mA h g−1. The results show that the rate capability of Li1.1Ni0.235Mn0.735Cr0.03O2.3 has been improved with Li4Ti5O12 coating effectively. It is well known that the faster ionic diffusion ability of Li4Ti5O12-coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 particles contributed to the better rate capability [40,41]. The midpoint voltages (MPVs) of discharge at different cycles of pristine and 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 materials are summarized in Table 2. Middle voltage refers to the voltage when the discharge specific capacity is 50%. As seen in Table 2, the midpoint voltages of every cycle for coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 are higher than that of pristine Li1.1Ni0.235Mn0.735Cr0.03O2.3. Further compared with the pristine Li1.1Ni0.25Mn0.75O2.3 [24], the midpoint voltages (MPVs) of discharge of 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 is enhanced remarkably. The differential capacity vs. voltage (dQ/dV) curves of Li`Ni0.235Mn0.735Cr0.03O2.3 and 3 wt% Li4Ti5O12 coated electrodes at different cycles are shown in Fig. 6. As shown in Fig. 6, in the discharging curves, there are mainly five electrochemical processes, noted as R1 at 2.2 V, R2 at 2.7 V, R3 at 3.2 V, R4 at 3.7 V and R5 around 4.5 V. It is documented that R1is related with transformation to tetragonal (I41/amd) phase; R2 is associated with spinel phase of LiNi0.5Mn1.5O4; R3 is
1
15
30
45
55
a c
3.544 3.552
3.416 3.425
3.322 3.348
3.254 3.287
3.220 3.258
(1)
Z′ = Rs + Rct +σw·ω−0.5
(2)
As shown in Eq. (1), R is the ideal gas constant, T is the absolute temperature, n is the number of electrons per molecule during charge/ discharge process (for this reaction, it is 1), F is the Faraday constant, C0 is the concentration of Li+ in pre unit cell (for this cathode it is 4.67×10−2 mol cm−3), A is surface area of the electrode in cm2 (for this electrode it is 1.44 cm2), σ is the Warburg factor which has relationship with Z′ (shown in Eq. (2)). Fig. 8 shows the linearity of the Z′ and ω−1/2 in the low-frequency region. On the basis of the above information, the lithium-ion diffusion coefficients of pristine and 3 wt% coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 cathodes after 50 cycles are calculated. The results are listed in Table 4. As shown in Table 4, the lithium
Table 2 Middle voltage of pristine and 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrode at different discharge cycle. Sample
⎞2 ⎛ RT ⎟⎟ D = 0.5⎜⎜ 2 2 ⎝ An F σwC0 ⎠
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Fig. 6. Differential capacity vs. voltage (dQ/dV) curves of the pristine and Li4Ti5O12 coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes at different cycles 1(A), 15(B), 30(C), 45(D), 55(E). Table 3 Impedance parameters of equipment circuit. Electrodes
RS (Ω)
Rct (Ω)
a b c d
11.22 9.16 4.07 12.18
359.9 295.6 202.7 667.2
Fig. 7. AC impedance of the pristine and coated Li1.1Ni0.235Mn0.735Cr0.03O2.3 electrodes (A) and equivalent circuit (B).
Fig. 8. Relationships between Z′ and ω−1/2.
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Table 4 Kinetic parameters of the pristine and 3 wt% Li4Ti5O12-coated samples.
[11]
Sample
RS (Ω)
Rct (Ω)
σW (Ω cm2 s−0.5)
D (cm2 s−1)
0% 3%
11.22 4.07
359.9 202.7
37.007 19.127
4.256×10–12 1.432×10–11
[12]
[13]
diffusion coefficients of pristine and 3 wt% coated Li1.1Ni0.235Mn0.735 Cr0.03O2.3 cathodes are 4.256×10–12 and 1.432×10–11 cm2 s−1. It can be found that the lithium diffusion coefficient is increased after 3 wt% Li4Ti5O12 coated. This reveals that the appropriate amount of Li4Ti5O12 modification has a positive effect on the electrochemical performance of Li1.1Ni0.235Mn0.735Cr0.03O2.3 with increase of lithium ion diffusivity.
[14]
[15]
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4. Conclusion [17]
The Cr doped layer-spinel composite cathode material Li1.1Ni0.235 Mn0.735Cr0.03O2.3 was synthesized and coated with fast ion conductor Li4Ti5O12. After coating, the 3 wt% Li4Ti5O12 coated Li1.1Ni0.235Mn0.735 Cr0.03O2.3 shows the best electrochemical performances, especially the rate performance. The discharge capacity of Li1.1Ni0.235Mn0.735Cr0.03 O2.3 at a 10 C rate increases from 113 to 132 mA h g−1 upon coating with Li4Ti5O12. Furthermore, the layer and spinel structure stability of LiNi0.235Mn0.735Cr0.03O2.3 has been enhanced after Li4Ti5O12 coating, which results in improved midpoint voltages. EIS results show that the Rct of pristine LiNi0.235Mn0.735Cr0.03O2.3 electrodes after 50 cycles is decreased with the addition of the Li4Ti5O12 coating. The lithium diffusion coefficient is increased from 4.256×10–12 to 1.432×10– 11 cm2 s−1 after 3 wt% Li4Ti5O12 coating. Compared with Li1.1Ni0.25 Mn0.75O2.3 [22], this study shows that the electrochemical performance of layer-spinel composite cathode material can be improved by Cr doping and Li4Ti5O12 coating.
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[19]
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Acknowledgments
[23]
The authors gratefully acknowledge the National Natural Science Foundation of China (51304081) (51604124) (51604125) and Natural Science Foundation of Jiangsu Province (BK20140581) (BK20150506) (BK20140558).
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