Electrochemical kinetics of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ ‘composite’ layered cathode material for lithium-ion batteries

Abstract

The ‘composite’ layered material of 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ has been successfully prepared by the solid state reaction method, and was characterized by XRD and SEM methods. The kinetics of the electrochemical insertion and extraction of lithium ions during the first three cycles in this material was investigated in detail by the open-circuit voltage (OCV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS) methods. The activation energies (E_a) of interfacial lithium ion transfer at various oxidation–reduction reactions were evaluated from the temperature-dependence of lithium ion transfer resistance. The results show that the electrochemical kinetics of the lithium ion extraction and insertion reactions in the first three cycles of this ‘composite’ material is mainly controlled by the Li₂MnO₃ and Li₂MnO₃-related components in this material. The lithium ion extraction processes from the Li₂MnO₃ component and LiMnO₂ component (after the 1^st cycle) are kinetically limited as compared with that from the LiMn_0.42Ni_0.42Co_0.16O₂ component, and the lithium ion insertion processes into the MnO₂ (after the 1^st cycle) component are kinetically limited as compared with that into the Mn_0.42Ni_0.42Co_0.16O₂ component. In addition, the interface reaction of the lithium ion into the Mn_0.42Ni_0.42Co_0.16O₂ component is also easier than that of the lithium ion into the MnO₂ component originated from the Li₂MnO₃ component.

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1. Introduction

Recently, ‘composite’ layered materials between Li[Li_1/3Mn_2/3]O₂ (generally designated as Li₂MnO₃) and LiMO₂ (M=Mn, Ni, Co, etc.) have received pronounced attention, and have been considered as promising cathode materials for the next generation lithium-ion batteries owing to their high discharge capacities (∼250 mA h g⁻¹) in excess of that of conventional cathode materials when operated above 4.6 V.^1–10

The charge and discharge mechanisms of these materials are very different from the conventional lithium ion battery cathode materials such as LiCoO₂, LiNiO₂ and LiFeO₄, and many scientists in the world are still working on the reaction mechanism especially the 1^st charge process of these materials.^1,2,4,5,10 We have previously reported the reaction process of these materials during the 1^st and subsequent cycles, and found that the electrochemical properties of these materials are kinetically controlled, the cycle stability and rate capability still need to be improved for practical utilization.^4,5 In order to further understand the kinetically controlled charge and discharge processes of these cathode materials and improve their electrochemical properties, we should study the lithium ion electrochemical insertion and extraction processes of these cathode materials in detail.

For lithium ion battery cathode materials, several processes play a role in lithium ion and electron transfer during charge and discharge processes, which are shown as follows.^11–13 Firstly, the electron transport occurs at the cathode material and the interface between the current collector and the electrode. Secondly, the lithium ions diffuse in the cathode materials. Thirdly, the lithium ions transfer at the interface between the electrode and electrolyte. Finally, the lithium ions transport in the electrolyte. Among these processes, the lithium ion diffusion in the active material and lithium ion transfer at the electrode/electrolyte interface are essential processes for lithium ion charge and discharge in cathode material. Thus, it is very important to investigate these two processes of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ ‘composite’ layered material in detail.

In this work, the ‘composite’ layered material 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ is prepared by the solid state reaction method, which can also be written as 0.5Li₂MnO₃·0.25LiMn_0.5Ni_0.5O₂·0.25LiMn_0.33Ni_0.33Co_0.33O₂ and Li_1.2Mn_0.567Ni_0.167Co_0.067O₂. The kinetics of the electrochemical insertion and extraction of lithium ions during the first three cycles in this material are investigated in detail by the open-circuit voltage (OCV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The activation energies (E_a) of interfacial lithium ion transfer with different oxidation-reduction potentials are evaluated from the temperature-dependence of lithium ion transfer resistance.

2. Experimental

2.1 Materials synthesis

The ‘composite’ layered manganese-rich cathode material used in this work was prepared by a solid state reaction technology. In a typical synthesis, firstly, required amounts of the transition metal acetates Mn(CH₃COO)₂·4H₂O (Wako), Ni(CH₃COO)₂·4H₂O (Wako), Co(CH₃COO)₂·4H₂O (Wako) were mixed for 3 h in pulverisette 6 machine with 200 r/min rotating speed. Secondly, the stoichiometric LiOH·H₂O (Wako) (5% excess) was mixed with the transition metal mixture together, then calcined in the furnace at 500 °C for 5 h, and then the powder was pressed into pellets and calcined in the furnace at 900 °C for 15 h. Finally, the ‘composite’ layered manganese-rich cathode material of 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ was prepared.

2.2 Characterization

The crystal structures of the pristine powder (0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂) and the cathodes of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells before and after the 1^st cycle with different cut-off voltages were obtained using powder X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer using Cu K_α radiation. The diffraction data were recorded in the 2θ range of 10–90° with a step of 0.02° and a count time of 1 s. The cathode electrode materials of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells were obtained from the coin cells and washed with ethylene carbonate/dimethyl carbonate (EC/DEC, 1 : 1(v/v)) in an argon-filled glove box. The morphology and size of the prepared powders were also observed using scanning electron microscopy (SEM, TOPCON DS-720 instrument).

2.3 Electrochemical test

The charge/discharge tests were carried out using the CR2032 coin-type cells, consisting of a cathode and lithium metal anode separated by a Celgard 2400 porous polypropylene film. The cathode electrodes were prepared with a weight to weight ratio of 80% of active material, 15% of teflonized Acetylene Black (AB), and 5% of polytetrafluoroethylene. The cathode electrodes were pressed onto the aluminium screen and dried at 100 °C for 5 h. The cells were assembled in a glove box filled with dried argon gas. The electrolyte was 1M LiPF₆ in ethylene carbonate/dimethyl carbonate (EC/DEC, 1 : 1(v/v)). The cells were charged and discharged in the voltage range of 2.0–4.4 V, 2.0–4.6 V and 2.0–4.8 V at a constant current density of 5 mA g⁻¹ at 25 °C for the first cycle. The open-circuit voltage (OCV), galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and activation energies (E_a) were performed on the Solartron 1253B Frequency Response Analyzer.

3. Results and discussion

Fig. 1 (a)(b) shows scanning electron microscopic images of the ‘composite’ layered materials (0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂) prepared by a solid state reaction method. It can be seen from Fig. 1(a) that the synthesized ‘composite’ layered material consists of sub-micrometer particles with irregular shape and uniform particle size. Fig. 1(b) shows an enlarged view of this ‘composite’ layered material. It is clear that the sub-micrometer sized particle is in the range of 200–400 nm. Generally, the particle size of the materials prepared by a solid state reaction method is much larger for synthesising at longer times and high temperature, however in our work, the particle size is uniform and small, which is attributable to the ball milling process in this preparation method. The detailed structure information of this material has been discussed in the paper by the synchrotron X-ray powder diffraction (SXRD) method.⁴

Fig. 1 SEM images of the ‘composite’ layered material (0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂) prepared by a solid state reaction method: (a) ×20.69 K; (b) ×100 K.

Fig. 2 shows the initial charge and discharge profiles of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells with different cut-off voltages. The current density is 5 mA g⁻¹, and cut-off voltages are 4.4, 4.6 and 4.8 V, respectively. It is clear that the initial charge and discharge capacities are affected largely by the cut-off voltage. With adding the cut-off voltage, the discharge capacities increase largely, especially when the cut-off voltage is larger than 4.4 V. In the case when the cut-off voltage is 4.4 V, the discharge capacity of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells is 96 mA h g⁻¹. However, when the cut-off voltage increases to 4.6 V, the discharge capacity is 232 mA h g⁻¹. Moreover, all the charge profiles of this electrode material can be divided into two distinguishing charge regions during the initial charge process. It is reported that different charge regions relate to the lithium extraction processes with different electrochemical or chemical reactions.^3,4,14,15 The first charge regions below 4.4 V originate from the lithium ion extraction process from the LiMn_0.42Ni_0.42Co_0.16O₂ component,^3,4,15 while the second charge region above 4.4 V is mainly due to the removal of lithium ions associated with loss of oxygen from the Li₂MnO₃ component.^3,4,16 With the cut-off voltage increasing from 4.6 to 4.8 V, the second charge region becomes longer, and the discharge capacity increases to 272 mA h g⁻¹. The increased capacity during this process indicates that more lithium ions associated with oxygen are extracted from the cathode materials.

Fig. 2 Initial charge and discharge profiles of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells with different cut-off voltages. The charge and discharge curves with 4.8 V cut-off voltage were taken from ref. 4 for comparison.

The electrochemical properties including charge/discharge capacity, irreversible capacity and columbic efficiency of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells with different cut-off voltages are summarized in Table 1. With the cut-off voltage increasing, especially from 4.4 to 4.6 V, the irreversible capacity (IRC) loss increases largely, while the columbic efficiency decreases. Therefore, the irreversible capacity of these ‘composite’ layered materials is induced mostly by the second charge regions after 4.4 V, which is consistent with the research results of Thackeray.³Fig. 3 shows the powder X-ray diffraction patterns of the pristine electrodes and electrodes after being charged/discharged with different cut-off voltages. Highlighted X-ray diffraction patterns related to the 2θ range of 19–23° are shown in the inserts of Fig. 3. It is obvious that most Bragg diffraction peaks of all electrode materials with different cut-off voltages are indexed to the LiNiO₂ unit cell with space group symmetry R3̄m. With the cut-off voltage increasing, all peaks shift to a lower angle, indicating that the lattice volumes enlarge. Moreover, the Bragg diffraction peaks in the 2θ range of 19–23° (the inserts of Fig. 3) are significantly weak and essentially disappear, especially when the cut-off voltage is 4.8 V.

Fig. 3 XRD patterns of the pristine and discharged states cathode electrodes of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells with different cut-off voltages.

Table 1 Initial charge and discharge properties of the ‘composite’ layered material (0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂) with different cut-off voltages

Cut off voltage/V	The 1^st charge capacity/mA h g^−1	The 1^st discharge capacity/mA h g^−1	IRC loss of the 1^st cycle/mA h g^−1	Coulombic efficiency of the 1^st cycle /%
2.0–4.4	112	96	16	86
2.0–4.6	302	232	70	77
2.0–4.8^4	352	272	80	77

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These results suggest that reactions above 4.4 V are no longer the reversible solid-state redox reactions accompanied by lithium ion insertion/extraction into/from the crystal lattice, and the oxygen removal from the bulk of the electrode materials leads to structural reconstruction.⁴ These phenomena are consistent with the research results by Naoaki Yabuuchi.¹ Differently, there is no evidence of the existence of Li₂CO₃ after the cells discharged to 2.0 V from the 4.8 V charged state in Fig. 3, while some sharp and symmetrical peaks as the diffraction peaks of Li₂CO₃ appeared in the reported synchrotron X-ray diffraction profiles of the 0.5Li₂MnO₃–0.5LiCo_1/3Ni_1/3Mn_1/3O₂ electrode.¹ These differences may be due to the different material composition and light strength of the X-ray source.

In order to understand the structure changes of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ ‘composite’ layered material with different cut-off voltages, the lattice parameters of the pristine electrodes and electrodes with discharged states are calculated through the least squares method using higher six peaks of XRD patterns, as shown in Fig. 4. In general, the lattice parameters, a and c, and the lattice volume V of the electrodes with discharged state increase with the cut-off voltage increasing. Changes of these values are little when the cut-off voltages are 4.4 V, while large as in the case when the cut-off voltages are 4.6 V. The increase in the value of a, c and V is attributed to the increasing Mn³⁺ ion content because trivalent manganese has a larger ionic radius (0.65 Å for high-spin, and 0.58 Å for low-spin) relative to Mn⁴⁺ (0.53 Å). In addition, these results also give evidence that Li₂MnO₃ components are activated when the cut-off voltage is larger than 4.4 V.

Fig. 4 Lattice parameters of the cathode electrode materials of the Li/0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ cells with different discharged states. The lattice parameters were calculated through the least squares method using higher six peaks of XRD patterns.

3.1 OCV analysis

In our previous work, we have confirmed that the electrochemical properties of these materials are kinetically controlled, the charge and discharge capacities of these materials are controlled by the current density, and the Li₂MnO₃ components are activated not only in the first cycle but also in the subsequent cycles.⁴ In order to understand their kinetically controlled process during the 1^st cycle and the following cycles, we need to study the lithium ion electrochemical insertion and extraction processes in detail during different cycles.

The cell in this work was first charged at a constant current density (20 mA g⁻¹) for an interval of 1 h followed by an open-circuit stand for 2 h to allow the cell voltage to relax to a steady-state value. In the first charge process, this procedure was repeated 15 times calculated by the 1^st charging capacity (302 mA h g⁻¹/20 mA g⁻¹ ≈ 15 h).⁴ In the first discharge process, this procedure was repeated 11 times calculated by the 1^st discharging capacity (224 mA h g⁻¹/20 mA g⁻¹ ≈ 11 h).⁴ In the 2^nd and 3^rd cycles, this procedure was repeated 11 times. Fig. 5(a)(b) show the charge/discharge and OCV curves of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material in the 1^st, 2^nd and 3^rd charge and discharge processes. There are about five main reactions during the charge and discharge processes of these ‘composite’ layered materials, which have been researched in detail in our previous work.⁴

Fig. 5 GITT and OCV curves of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material in the first, second and third charges (a) and discharges (b) with a 20 mA g⁻¹ charge/discharge current density and a time interval of 1 h.

In the 1^st charge process of Fig. 5(a), there is almost no overpotential observed until point 6 (OCV = 4.35 V) is reached, and the main kinetic process of lithium during this stage is associated with the lithium ion extraction reaction from the LiMn_0.42Ni_0.42Co_0.16O₂ component. Above this point, there is an obvious plateau of the charge and OCV curves, and the over-potential increases largely from point 6 (OCV = 4.35 V) to point 15 (OCV = 4.46 V), which implies that the main reaction associated with the lithium ion extraction process from the Li₂MnO₃ component during this stage are kinetically limited.⁴ In the 2^nd and 3^rd charge processes, there are obvious phenomena that the over-potentials decrease firstly, keep stable and then increase quickly, which implies that the electrode polarization at the beginning and end of these charge processes is large. In ref. 4, the oxidation potentials associated with the lithium ion extraction reactions from the LiMnO₂ component and the Li₂MnO₃ component during the 2^nd and 3^rd charge process are about 3.5 and 4.5 V, respectively, which is well consistent with the potentials associated with the highest over-potentials in Fig. 5(a). Thus, the high over-potentials at the beginning of these charge processes are probably associated with the lithium ion extraction reaction polarization from the LiMnO₂ component originated from the activated Li₂MnO₃ component, while the high over-potentials at the end of these charge processes are probably associated with the electrochemical polarization of the lithium ion extraction process from the unactivated Li₂MnO₃ component and the concentration polarization induced by the lithium ion concentration reduction of the electrode materials. Besides, the low over-potentials in the middle of the 2^nd and 3^rd charge processes are probably contributed to the low electrode polarization associated with the lithium ion extraction reaction from the LiMn_0.42Ni_0.42Co_0.16O₂ component.

In the 1^st, 2^nd and 3^rd discharge processes (Fig. 5 (b)), there is one same phenomenon that the over-potentials are very small before point 6 (OCV = 3.6 V), and increase with the discharge voltage decreasing from 3.6 V (OCV of point 6) to 3.2 V (OCV of point 11). We showed that the insertion reaction of a lithium ion into the activated MnO₂ after the 1^st charge activation process occurred below 3.5 V during the discharge process,⁴ which is very close to the discharge voltage at the end of the fifth discharge curve of the 1^st discharge process in Fig. 5(b). Therefore, the over-potential variation trend during the discharge process reveals that the insertion reaction of a lithium ion into the MnO₂ component is kinetically limited as compared with that into the Mn_0.42Ni_0.42Co_0.16O₂ component. In addition, there is a very obvious phenomenon that all of the discharge potential plateaus below point 6 increase with the cycle number increasing. This phenomenon means that the amount of activated MnO₂ component originated from the Li₂MnO₃ component increases with the cycle number increasing, which is also a good evidence of the continued activation reaction of the Li₂MnO₃ component in this material and consistent with our previous conclusion.⁴ In addition, the high over-potentials at the end of the discharge processes are probably associated with the side-reactions induced from the concentration polarization.

Consequently, on the basis of the analysis of OCV curves during charge and discharge processes, the lithium ion extraction processes from the LiMnO₂ component and Li₂MnO₃ component are kinetically limited as compared with those from the LiMn_0.42Ni_0.42Co_0.16O₂ component, and the lithium ion insertion processes into the MnO₂ component are kinetically limited as compared with those into the Mn_0.42Ni_0.42Co_0.16O₂ component. These results are consistent with the analysis below and testified by activation energy analysis.

3.2 GITT analysis

In order to discuss the kinetic processes of the lithium ion extraction and insertion reaction in the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material in detail and quantize the lithium ion transfer speed inside this ‘composite’ material. The chemical diffusion coefficient of a lithium ion (D_Li+) in the active material of this ‘composite’ material was introduced in this work. The chemical diffusion coefficient of a lithium ion (D_Li+) is an important kinetic parameter for lithium ion insertion and extraction reactions in intercalation materials, which can be got from the electrochemical techniques such as galvanostatic intermittent titration (GITT), potentiostatic intermittent titration (PITT), EIS and CV.^17–23 Among these techniques, GITT based on chronopotentiometry at nearly thermodynamic equilibrium conditions is a reliable technique to determine the chemical diffusion coefficient of lithium ion in intercalation materials.^17–23 Thus, firstly, the lithium ion diffusion coefficient in this ‘composite’ material during a charge and discharge process is studied in detail by the GITT technique.

On the basis of the GITT measurement, the diffusion coefficient of lithium ions in the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ compound can be determined by solving Fick’s second law of diffusion and calculated by the following eqn (1):^20–23

where m_B and M_B are the molecular weight and mass of the cathode material, V_M is the molar volume of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ compound, which is 19.84 cm³mol⁻¹ deduced from the crystallographic data. S is the active surface area of the electrode, which is 2.67 m²g⁻¹ deduced from the BET testing, and L is the thickness of the electrode.

For a single titration at 4.26 V, the applied current flux and the resulting voltage profile during the 1^st charge process for 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material is shown in Fig. 6(a), labeled with different parameters, E₀, E_s, τ, t₀, t₀+τ, E_τ and E_s. If the relationship of the cell voltage during the time period τ versus τ^1/2 shows straight line behavior, as shown in Fig. 6(b), then eqn (1) can be further simplified as eqn (2):^20–23

Fig. 6 (a) Single titration at 4.26 V during GITT measurement of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material with representation of different parameters. (b) Relationship of the cell voltage for the above titration and τ^1/2.

Fig. 7(a) shows the variation of the lithium ion diffusion coefficient (D_Li+) as a function of OCV in the 1^st, 2^nd and 3^rd charge processes. It is clear that the D_Li+ in the 1^st charge process is about 10⁻¹⁴ cm² s⁻¹ when the OCV is lower than 4.2 V, and then decreases to 10⁻¹⁷ cm² s⁻¹ with the OCV increasing to about 4.5 V, associating with the plateau charge process. This phenomenon shows clearly that the reaction kinetics during the 1^st charge process includes two regions, and the electrochemical reaction in the last region is severely sluggish. The sluggish reaction maybe attributes to the high kinetic barriers associated with the lithium ion diffusion in the Li₂MnO₃ component and concentration polarization at the end of the charge process. The D_Li+ of the 2^nd and 3^rd charge processes also show a similar variation trend to that of the 1^st charge process. However, the D_Li+ before 4.2 V (OCV) of the 2^nd and 3^rd charge processes are a little smaller than that of the 1^st charge process, which means that the lithium ion diffusion speed in the active material before 4.2 V (OCV) decreases after the 1^st cycle, which is very different from the conventional cathode material such as LiCoO₂^24,25 and LiMn₂O₄.^24,26–28 This may be attributed to the sluggish reaction associated with the lithium ion extraction from the LiMnO₂ component after the 1^st cycle, which is well consistent with the OCV analysis. In addition, after the 1^st cycle, the smallest D_Li+ during the 2^nd and 3^rd charge processes is about 10⁻¹⁶ cm² s⁻¹ (OCV is about 4.5 V), and is still much smaller than 10⁻¹⁴ cm² s⁻¹ (the D_Li+ when the OCV is smaller than 4.2 V). This phenomenon indicates that the unactivated Li₂MnO₃ component in this material after the 1^st cycle still exists and can be activated in subsequent charge processes. These research results are well consistent with our findings of these ‘composite’ materials⁴, but different from the research results by Zhe Li et al., who indicate that all of the Li₂MnO₃ components can be activated after the 1^st cycle.¹⁸ The difference may be attributed to the different local structures of these materials induced from the different element compositions and preparation methods.

Fig. 7 Li⁺ diffusion coefficients calculated from the GITT curves for the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material as a function of the cell voltage (OCV) during the charge process.

Fig. 7(b) shows the variation of the lithium ion diffusion in the 1^st, 2^nd and 3^rd discharge processes. It shows that there are obvious three regions with the OCV decreasing. Based on the OCV analysis in Fig. 5, the first plateau between about 4.5 V (OCV) and 3.9 V (OCV) may be attributed to the insertion reaction of a lithium ion into the Mn_0.42Ni_0.42Co_0.16O₂ component, and the second plateau from about 3.8 V (OCV) to 3.5 V (OCV) may be attributed to the lithium ion insertion reaction in the MnO₂ component originated from the Li₂MnO₃ component. The immediate decrease D_Li+ from 3.5 V (OCV) to 3.3 V (OCV) may be attributed to the large concentration polarization at the end of the discharge processes.

3.3 EIS analysis

In order to analyze the kinetic process of the lithium ion at the electrode/electrolyte interface, EIS technique is also introduced in this work. Electrochemical impedance measurements of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode materials during the 1^st, 2^nd and 3^rd charge and discharge process were carried out with different states after the cell rests for 2 h.

Fig. 8 shows the Nyquist plots of this material during the 1^st charge and discharge process. All of the Nyquist plots except for that of the fresh cell show two semicircles at high and middle frequencies, and a slope line at low frequency. Based on the previous research on this type layered cathode materials,^18,20,23 the semicircles in the high frequency can be attributed to the lithium ion diffusion through the SEI film, the semicircle in the middle frequency is associated with the charge transfer reaction, and the slope line appearing in the low frequency is attributed to the lithium ion diffusion (Warburg diffusion) in the electrode material. During the 1^st charge process, the variation of the Nyquist plot with different states shows obvious two stages. When the OCV is lower than 4.068 V, the semicircle radiuses in the high and middle frequencies decrease with the OCV increasing (shown in Fig. 8(a)), while the semicircle radius in the middle frequency increases with the OCV increasing from 4.198 V to the end of charge process (Fig. 8(b)). In the discharge process, as shown in Fig. 8(c), the Nyquist plots also show two semicircles and a slope line. The semicircle radius in the middle frequency decreases firstly and then increases with the OCV decreasing from 4.457 V to the end of the discharge process.

Fig. 8 Plots for the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material at different OCV during the first charge and discharge processes.

The Nyquist plots during the 2^nd and 3^rd charge/discharge processes are shown in Fig. 9. The semicircle radiuses in the middle frequency decrease firstly and then increase with the OCV increasing from around 3.3 V to 4.5 V during the 2^nd and 3^rd charge processes. During the discharge process, the variation trend of the Nyquist plots also presents a similar phenomenon, the contribution from the lithium ion transfer in the electrode/electrolyte interface decreases firstly and then increases with the OCV decreasing from about 4.5 V to 3.2 V.

Fig. 9 Nyquist plots for the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material at different stages during the second and third charge and discharge processes.

The EIS data can be analyzed by the equivalent circuit given in the inset of Fig. 10, and the respective circuit elements are deduced by fitting the experimental data points with this equivalent circuit. In the equivalent circuit, R_e represents the resistive contribution from the electrolyte resistance and cell components, R_s and C_s represent the resistance and associated capacitance, respectively, associating with the high frequency semicircle, R_ct and C_dl represent the charge transfer resistance and its associated double-layer capacitance, respectively, associating with the low frequency semicircle, and W is the Warburg impedance related to the lithium ion diffusion in the electrode material.

Fig. 10 Typical Nyquist and fitting plots using the equivalent circuit.

All of the fitted impedance parameters (R_e, R_s and R_ct) of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material during the 1^st, 2^nd and 3^rd charge/discharge processes with different OCV are presented in Fig. 11. Obviously, the main impedance of this material is contributed to the charge transfer resistance during the first three charge and discharge processes. In the 1^st charge process, shown in Fig. 11(a), there are clearly three stages of the R_ct curve with the OCV increasing, and described as follows. Firstly, the R_ct keeps stable at about 100–200 Ω when the OCV is located between 3.841 V (point 2 in Fig. 5(a)) and 4.198 V (point 5 in Fig. 5(a)), associating with the lithium ion extraction process at the interface between the LiMn_0.42Ni_0.42Co_0.16O₂ component and the electrolyte. And then, R_ct increases slowly from 487 to 901 Ω with OCV increasing from 4.353 V (point 6 in Fig. 5(a)) to 4.434 V (point 10 in Fig. 5(a)), associating with the lithium ion extraction process at the interface between the Li₂MnO₃ component and the electrolyte. Finally, the R_ct increases sharply from 1128 to 4468 Ω with the charge process from 4.441 V (point 11 in Fig. 5(a)) to 4.459 V (point 15 in Fig. 5(a)), which is contributed to the high concentration polarization at the end of charge process.

Fig. 11 R_e, R_s and R_ct obtained by fitting the impedance data for the 1^st, 2^nd and 3^rd cycles in Fig. 8 and Fig. 9 during the charge and discharge process, with the equivalent circuit shown in Fig. 10.

Based on the EIS analysis of the 1^st charge process of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material, it appears that the lithium ion transfers at the interface between the electrode and electrolyte during the Li₂MnO₃ component activation stage are very large compared to those during the LiMn_0.42Ni_0.42Co_0.16O₂ component electrochemical reaction stage.

Fig. 11(c) and Fig. 11(e) show the variation trend of the R_e, R_s and R_ctversus OCV during the 2^nd and 3^rd charge processes with varying OCV. It is obvious that the R_ct decreases when the OCV increases from about 3.3 to 3.8 V, remains stable from about 3.8 to 4.2 V, and increases from about 4.2 to 4.5 V. The high R_ct before 3.8 V (OCV) maybe attribute to the lithium ion transfer resistance at the interface between the LiMnO₂ component electrode in local region and electrolyte. And, the increased R_ct between 4.2 V (OCV) and 4.5 V (OCV) are maybe associated with the lithium ion charge transfer resistance at the interface between the unactivated Li₂MnO₃ component and electrolyte and the concentration polarization.

Based on the OCV analysis in Fig. 5(b), during the discharge processes of the 1^st, 2^nd and 3^rd cycles in Fig. 11 (b)(d)(f), the R_ct between 4.2 V (OCV) and 3.7 V (OCV) associated with the lithium ion insertion into the Mn_0.42Ni_0.42Co_0.16O₂ component is a little smaller than that between 3.7 V (OCV) and 3.5 V (OCV) associated with the lithium ion insertion into the MnO₂ component. Therefore, the lithium ion transfer resistance at the electrode/electrolyte of the MnO₂ component activated from Li₂MnO₃ component is a little larger than that of the Mn_0.42Ni_0.42Co_0.16O₂ component. Besides, the high R_ct at the end of the discharge process maybe contribute to the large concentration polarization induced by the increased lithium ion concentration in the active material.

3.4 Activation energy analysis

Activation energies of the interface reactions during varying charge and discharge voltages are also introduced in this work to investigate the kinetics of the charge-transfer reaction of the 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ electrode material. The study on the activation energy is useful for clear understanding of the kinetic trend of this material with different components in different stages because the activation energy value indicates the essential kinetics of the reaction without the influence of effective surface area, activity of reactants and temperature.¹¹ In this work, for the detailed activation energy calculation and discussion, four research points during the 1^st charge and discharge process, and three research points during the 2^nd charge process are chosen based on the redox-reaction and dQ/dV curves of these ‘composite’ materials. All of these points are marked in the insert figures of Fig. 12. The dQ/dV (Q is the capacity and V is the voltage of the cell) curves are calculated from numerical data in the 1^st charge/discharge and 2^nd charge curve of this ‘composite’ material with 20 mA g⁻¹ charge/discharge current density.

Fig. 12 Activation energy of the 1^st charge/discharge processes and 2^nd charge process.

The charge/discharge activation energy can be calculated using the following equation.¹¹

1/Rct = Aexp(−Ea/RT) (3)

In which, the symbols A, Ea, R and T denote frequency factor, activation energy, gas constant and absolute temperature, respectively.

Fig. 12(a) shows the Arrhenius plots of the interfacial conductivity (1/R_ct) versus different temperature during the 1^st charge process. Two potentials associated with the lithium ion extraction reactions from the LiMn_0.42Ni_0.42Co_0.16O₂ component (4.0 V in dQ/dV curve) and the Li₂MnO₃ component (4.5 V in dQ/dV curve) are chosen for discussion.⁴ It is obvious that the interface reaction activation energy of the lithium ion extracting from the Li₂MnO₃ component is about 35 kJ mol ⁻¹, which is much larger than 20 kJ mol ⁻¹ associated with the lithium ion extracting from the LiMn_0.42Ni_0.42Co_0.16O₂ component. These results are well consistent with the OCV and EIS analysis results. Besides, it can be clearly seen from Fig. 12(a) that the interface reaction of the lithium ion extracting from the Li₂MnO₃ component is more sensitive to temperature than that of the lithium ion extracting from the LiMn_0.42Ni_0.42Co_0.16O₂ component. This phenomenon means that temperature would affect the activation process of the Li₂MnO₃ component, which will be discussed in detail in the future.

The Arrhenius plots of the interfacial conductivity (1/R_ct) versus varying temperature during the 1^st discharge process are shown in Fig. 12(b). The reaction associated with 3.75 V is mainly contributed to the lithium ion insertion process into the Mn_0.42Ni_0.42Co_0.16O₂ component, while the reaction of 3.35 V is mainly associated with the lithium ion insertion process into the MnO₂ component.⁴ Therefore, Fig. 12(b) reveals that the interface reaction of the lithium ion into the Mn_0.42Ni_0.42Co_0.16O₂ component is a little easier than that of the lithium ion into the MnO₂ component originated from the Li₂MnO₃ component. In addition, the interface reaction activation energies during the 2^nd charge process are also presented in Fig. 12(c), and the researched potentials of 3.5 and 3.85 V are associated with the lithium ion extraction processes from the novel LiMnO₂ component and the LiMn_0.42Ni_0.42Co_0.16O₂ component.⁴ Therefore, it is clear that the interface reaction activation energy (32 kJ mol ⁻¹) of the lithium ion extracted from the novel LiMnO₂ component is much larger than that (20 kJ mol⁻¹) of lithium ion extracted from the LiMn_0.42Ni_0.42Co_0.16O₂ component. As for the largest interface reaction activation energy (40 kJ mol ⁻¹) associated with the 4.5 V potential, it is most probably contributed to the large transfer barrier of lithium ion at the interface of the unactivated Li₂MnO₃ component. Besides, other reactions at the 4.5 V potential maybe also exist and need to be confirmed in the future. In this work, all of the interface reaction activation energy research results can well support the OCV, GITT and EIS analysis results.

4. Conclusions

The ‘composite’ layered material of 0.5Li₂MnO₃·0.5LiMn_0.42Ni_0.42Co_0.16O₂ had been prepared by the solid state reaction method, and was characterized by XRD and SEM methods. The kinetics of the electrochemical insertion and extraction of lithium ions during the first three cycles in this material were investigated in detail by the open circuit voltage (OCV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS) methods. The activation energies of interfacial lithium ion transfer in different oxidation-reduction reactions were evaluated from the temperature-dependence of lithium ion transfer resistance. The results show that the electrochemical kinetics of the lithium ion extraction and insertion reactions in the first three cycles are very different from that of conventional cathode materials as the case of this material is charged to larger than 4.4 V. The rate properties of this ‘composite’ material are mainly controlled by the Li₂MnO₃ component composited in this material. Even though this component can be activated after the 1^st charge process, the novel MnO₂ and LiMnO₂ component still have lower lithium ion diffusion coefficient and higher interface reaction barriers. Thus, in order to improve the rate performance of this composite material, some methods are introduced based on this work. To decrease the proportion of the Li₂MnO₃ component, reducing the distance of lithium ion diffusion and coat material associated with low interface reaction barrier on the surface of these ‘composite’ materials may improve their rate performance.

Acknowledgements

This work was supported by the Innovative Basic Research Toward Creation of High-performance Battery in the Funding Program for World-leading Innovative R&D on Science and Technology.

References

1. N. Yabuuchi, K. Yoshii, S. T. Myung, I. Nakai and S. Komaba, J. Am. Chem. Soc., 2011, 133, 4404–4419.
2. T. Ohzuku, M. Nagayama, K. Tsuji and K. Ariyoshi, J. Mater. Chem., 2011, 21, 10179–10188.
3. M. M. Thackeray, S. H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem., 2007, 17, 3112–3125.
4. H. Yu, H. Kim, Y. Wang, P. He, D. Asakura, Y. Nakamura and H. Zhou, Phys. Chem. Chem. Phys., 2012, 14, 6584–6595.
5. P. He, H. J. Yu, D. Li and H. S. Zhou, J. Mater. Chem., 2012, 22, 3680–3695.
6. H. S. Kim, J. H. Jeong, B. S. Jin, W. S. Kim and G. X. Wang, J. Power Sources, 2011, 196, 3439–3442.
7. A. Ito, Y. Sato, T. Sanada, M. Hatano, H. Horie and Y. Ohsawa, J. Power Sources, 2011, 196, 6828–6834.
8. S. H. Kang, V. G. Pol, I. Belharouak and M. M. Thackeray, J. Electrochem. Soc., 2010, 157, A267–a271.
9. Z. H. Lu and J. R. Dahn, J. Electrochem. Soc., 2002, 149, A815–a822.
10. D. Y. W. Yu and K. Yanagida, J. Electrochem. Soc., 2011, 158, A1015–a1022.
11. Y. Yamada, Y. Iriyama, T. Abe and Z. Ogumi, J. Electrochem. Soc., 2010, 157, A26–a30.
12. Y. Yamada, F. Sagane, Y. Iriyama, T. Abe and Z. Ogumi, J. Phys. Chem. C, 2009, 113, 14528–14532.
13. Y. Yamada, Y. Iriyama, T. Abe and Z. Ogumi, Langmuir, 2009, 25, 12766–12770.
14. S. H. Kang, P. Kempgens, S. Greenbaum, A. J. Kropf, K. Amine and M. M. Thackeray, J. Mater. Chem., 2007, 17, 2069–2077.
15. C. S. Johnson, N. C. Li, C. Lefief, J. T. Vaughey and M. M. Thackeray, Chem. Mater., 2008, 20, 6095–6106.
16. A. R. Armstrong, M. Holzapfel, P. Novak, C. S. Johnson, S. H. Kang, M. M. Thackeray and P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 8694–8698.
17. K. Tang, X. Q. Yu, J. P. Sun, H. Li and X. J. Huang, Electrochim. Acta, 2011, 56, 4869–4875.
18. Z. Li, F. Du, X. F. Bie, D. Zhang, Y. M. Cai, X. R. Cui, C. Z. Wang, G. Chen and Y. J. Wei, J. Phys. Chem. C, 2010, 114, 22751–22757.
19. D. W. Dees, S. Kawauchi, D. P. Abraham and J. Prakash, J. Power Sources, 2009, 189, 263–268.
20. K. M. Shaju, G. V. S. Rao and B. V. R. Chowdari, Electrochim Acta, 2004, 49, 1565–1576.
21. K. M. Shaju, G. V. S. Rao and B. V. R. Chowdari, J. Mater. Chem., 2003, 13, 106–113.
22. K. M. Shaju, G. V. S. Rao and B. V. R. Chowdari, J. Electrochem. Soc., 2003, 150, A1–a13.
23. K. M. Shaju, G. V. S. Rao and B. V. R. Chowdari, Electrochim. Acta, 2003, 48, 2691–2703.
24. J. Xie, N. Imanishi, T. Matsumura, A. Hirano, Y. Takeda and O. Yamamoto, Solid State Ionics, 2008, 179, 362–370.
25. H. Xia, L. Lu and G. Ceder, J. Power Sources, 2006, 159, 1422–1427.
26. M. Y. Saidi, J. Barker and R. Koksbang, J. Solid State Chem., 1996, 122, 195–199.
27. K. Dokko, M. Mohamedi, M. Umeda and I. Uchida, J. Electrochem. Soc., 2003, 150, A425–a429.
28. J. Xie, T. Tanaka, N. Imanishi, T. Matsumura, A. Hirano, Y. Takeda and O. Yamamoto, J. Power Sources, 2008, 180, 576–581.

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