Improved electrochemical performance of LiNi_0.4Ti_0.1Mn_1.5O₄ as cathode of lithium ion battery by carbon-coating

Abstract

The effects of Ti substitution for Ni, carbon coating on the structure and electrochemical properties of LiMn_1.5Ni_0.5O₄ are studied. LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ cathode materials have been synthesized by a solid-state reaction using industrial raw materials in bulk scale. X-ray diffraction clearly shows that LiMn_1.5Ni_0.5O₄ has higher crystallinity after Ti doping. Scanning electron microscopy clearly exhibits that Ti doping does not change the basic spinel structure, as well as coated carbon layer covers the surfaces of the LiNi_0.4Ti_0.1Mn_1.5O₄ particles. In addition, charge–discharge tests indicate that LiNi_0.4Ti_0.1Mn_1.5O₄ sample has higher discharge capacities at the rates of 0.5, 1 and 3 C at 25 °C. It should be noted that carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ shows higher discharge capacities at the rates of 5, 7 and 10 C at 25 °C as well as various rates for 55 °C. Cyclic performances developed at 25 and 55 °C demonstrate that the capacity retention is remarkably improved compared to the two uncoated samples. The influence of the Ti-doping and carbon-coating on the coulombic efficiency at high temperature (55 °C) has also been investigated. Among the various samples investigated, surface modification with carbon gives an improved coulombic efficiency. The remarkably enhanced electrochemical properties of the carbon-coated sample may be because of the suppression of the solid electrolyte interfacial (SEI) layer development and faster kinetics of both the Li⁺ diffusion, as well as the charge transfer reaction.

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1. Introduction

High energy density and environmentally friendly lithium-ion batteries have attracted significant interest in recent years, especially as devices for electrical vehicles (EVs) and hybrid electrical vehicles (HEVs).^1–3 Operating voltage and capacity of the cathode material are two major factors influencing the energy density of lithium-ion batteries.^4–6 Therefore, a series of transition metal-substituted spinel lithium manganese oxides (LiM_xMn_2−xO₄; M = Cr, Co, Fe, Ni and Cu) have been synthesized,^7–11 which have high-voltage plateaus greater than 4.5 V. Among these doped materials, LiNi_0.5Mn_1.5O₄ has attracted considerable attention because of its good cyclic properties and relatively high capacity, with a plateau at around 4.7 V.^10,12–15 More importantly, nickel has been found to be present as Ni²⁺ in LiNi_0.5Mn_1.5O₄,¹⁶ which would suggest the absence of Mn³⁺, and thereby one would expect the entire capacity to occur in the 5 V region because of the Ni²⁺/Ni⁴⁺ redox couple. However, in practice, the low available capacity at high temperatures¹⁷ and poor cycle life at high current densities restrict its commercial applications.¹⁸ In recent work,¹⁹ it has been reported that the degradation of LiNi_0.5Mn_1.5O₄ in electrochemical performance is mainly because of the decomposition of electrolyte under high potential and side reactions of the active material particles with the electrolyte. The other reason is its intrinsically low electronic conductivity. Based on these studies, it is imperative to overcome these problems to meet the requirement for practical applications in high specific energy density lithium-ion batteries. In recent years, studies on LiNi_0.5Mn_1.5O₄ have been predominantly focused on the improvement in its electrochemical performance using various methods, such as lattice doping with Ru,²⁰ Cr^21,22 and Zn²³; and surface modification with ZnO^24,25 and Al₂O₃.²⁶

As has been reported,²⁷ Ti⁴⁺ doping improves the cyclic performance and discharge capacity of the pristine LiNi_0.5Mn_1.5O₄, because of the Ti–O chemical bond being stronger than Ni–O. Accordingly, it is reasonable to deduce that doping element Ti in LiNi_0.5Mn_1.5O₄ can improve its structural and chemical stabilities. In addition, conductive carbon is characterized by fast electron transport and regarded as an excellent coating material for LiFePO₄ in terms of the suppression of SEI formation and the enhancement of electronic conductivity.^28,29 Inspired by these works, the structural and electrochemical performance of LiNi_0.5−xTi_xMn_1.5O₄ (x = 0, 0.1) and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ materials has been investigated in the paper, which has not been previously reported.

In the present paper, the aim of this study is to investigate the rate capability and elevated-temperature performances of LiNi_0.5Mn_1.5O₄ synthesized by industrial raw materials (Li₂CO₃, Ni₂O₃, electrolytic MnO₂, and TiO₂). PEG4000 is selected as the carbon source.

2. Experimental

2.1. Material preparation

LiNi_0.5−xTi_xMn_1.5O₄ (x = 0, 0.1) powders were synthesized by the solid-state reaction method. Stoichiometric amounts of Li₂CO₃, Ni₂O₃, electrolytic MnO₂ and TiO₂ were used as starting materials without further purification. Next, the precursors were ground in a ball-milling machine with anhydrous ethanol as dispersant for 1 h to ensure a complete reaction, followed by drying at 50 °C for 12 h. In order to compensate for the loss of Li at high reaction temperature, approximately additional 5% of Li⁺ was made up. In the following step, the resulting powders were preheated at 500 °C for 5 h, and then sintered at 850 °C for 12 h to obtain the final LiNi_0.5Mn_1.5O₄ and LiNi_0.4Ti_0.1Mn_1.5O₄ particles, in which the cooling rate was set to be 0.5 °C min⁻¹ from 850 °C to 580 °C in flowing air atmosphere. For preparing the carbon-coated sample, the predetermined PEG4000 was mixed with the as-prepared LiNi_0.4Ti_0.1Mn_1.5O₄ (part of the above obtained sample) in a solvent mixture of distilled water and ethanol (1 : 3 by volume). The amount of PEG4000 used corresponded to 3 wt% of the LiNi_0.4Ti_0.1Mn_1.5O₄ powders. These mixtures were treated with sonication for 1 h, and stirred by a magnetic stirrer for 4 h. The resulting solvent was evaporated at 50 °C overnight, and then was allowed to dry at 90 °C until a dry powder was obtained. In order to ensure complete adhesion of the carbon particles with the core material LiNi_0.4Ti_0.1Mn_1.5O₄, the formed powders were annealed at 350 °C for 1 h under argon to avoid the oxidation of carbon.

2.2. Characterization of material

The crystal structures of the products were characterized by X-ray diffraction (XRD) using a Cu target and recorded with a step of 0.05°. The diffraction patterns were recorded at room temperature in the 2θ range from 10° to 80°. The morphology observations of samples were studied by scanning electron microscopy (SEM) performed on a Quanta-200T.

2.3. Electrochemical properties

The electrochemical performances of each sample were evaluated with a standard CR2025-type coin cell. The cathode was fabricated by blending the prepared powders (80 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (10 wt%) in N-methyl-2-pyrrolidone (NMP). The electrodes were dried under vacuum oven at 120 °C for 12 h, and then punched and weighed. The electrochemical cells were assembled in a glove box under a dry and high purity argon atmosphere. The complete coin cell comprises a cathode, a Celgard 2300 as the separator and a lithium foil anode. To ensure the reliability of the test results of high voltage spinel, the electrolyte was 1 mol L⁻¹ LiPF₆ in a mixture of ethylene carbonate (EC)–dimethyl carbonate (DMC) (1 : 1, v/v). The charge–discharge testing was carried out between 3.50 and 4.95 V using a NEWARE BTS-5 battery test system at 25 and 55 °C, respectively. Rate capability tests were conducted by charging at a constant voltage after the galvanometric charge (0.5, 1, 3, 5, 7 and 10 C rates), and then discharging at the corresponding rate. The cycling performance tests were conducted at a 1 C/5 C (25 °C) and 1 C/1 C (55 °C) charge–discharge rate (1 C = 146.7 mA g⁻¹). Before the electrochemical tests, all of the coin cells were pre-cycled for 3 cycles at 0.5 C to achieve a stable charge state. Cycle voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out using the above cells on a CHI650D electrochemical workstation. Cyclic voltammetry (CV) was conducted in the voltage range from 3.5 V to 5.0 V (versus Li/Li⁺) at the scanning rate of 0.1 mV s⁻¹. In EIS measurement, the excitation voltage applied to the cells was 5 mV sinusoidal perturbation in the frequency range from 100 kHz to 0.01 Hz at room temperature.

3. Results and discussion

3.1. Structural and morphological characterization

Fig. 1(a) compares the XRD patterns of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples. All the diffraction peaks can be indexed as a cubic spinel structure and Fd3m space group. The LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ material have sharper and higher diffraction peaks compared to LiMn_1.5Ni_0.5O₄, indicating that Ti⁴⁺ doping endows better crystallinity. However, it can also be observed that there are small peaks at 43.6° in the pattern of products, illustrating that there are secondary phases Li_xNi_1−xO or NiO^30,31 indicated by the asterisk. It has been reported that an impurity phase (Li_xNi_1−xO) can be formed at annealing temperatures above 750 °C in LiMn_1.5Ni_0.5O₄.³² Moreover, Amine et al.³³ have reported that LiMn_1.5Ni_0.5O₄ loses oxygen and becomes disproportionate to a spinel and rock salt NiO when it is heated above 600 °C. No evidence of diffraction peaks from carbon is observed because of its low quantity and/or amorphous nature (the calcination temperature is as low as 350 °C³⁴). In addition, Ti impurities phase is not observed in our LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples. These results indicate that the spinel structure is not affected after doping the LiNi_0.5Mn_1.5O₄ powder with Ti.

Fig. 1 XRD patterns of (a) LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples; (b) the enlarged intensity of the (111) peaks.

From the enlargement of the XRD patterns in Fig. 1(b), it can be observed that the diffraction patterns of the LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ shift toward higher angles compared to that of pure LiMn_1.5Ni_0.5O₄, showing that the lattice constants decreases. The reason is that Ti⁴⁺ ions enter the crystal lattice of LiMn_1.5Ni_0.5O₄ during the doping process. Hence, it can be concluded that a substitutional compound LiNi_0.4Ti_0.1Mn_1.5O₄ has been formed by the interaction of the doped oxide. These variations are attributed to the ionic radius differences among Li⁺ (0.59 Å), Ni²⁺ (0.69 Å), Ti⁴⁺ (0.605 Å), and Mn⁴⁺ (0.53 Å),³⁵ which reveal the incorporation of Ti atoms onto the Ni sites, indicating Ti is successfully introduced into the LiNi_0.5Mn_1.5O₄ matrix structure.

The surface morphologies of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ powders are given in Fig. 2. The LiNi_0.5−xTi_xMn_1.5O₄ (x = 0, 0.1) and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ particles prepared in this work showed primitively good octahedral particles structure, in which Li⁺ migration is possible through the pathway from tetrahedral to empty octahedral site, satisfying the insertion/extraction of Li into/from LiNi_0.5−xTi_xMn_1.5O₄. Moreover, the samples consist of particles mainly in the range of 0.3–3.0 μm. Fig. 2(a) and (b) shows that LiMn_1.5Ni_0.5O₄ and LiNi_0.4Ti_0.1Mn_1.5O₄ particles have smooth surface facets. The difference of the two samples is that the particle size of LiNi_0.4Ti_0.1Mn_1.5O₄ is better-distributed compared to LiMn_1.5Ni_0.5O₄. On the other hand, the surface of the coated LiMn_1.5Ni_0.5O₄ is covered with small particles that mainly consist of carbon, as showed in Fig. 2(c). The conductive carbon layer can not only avoid the direct contact between the active cathode material and the electrolyte, but provide pathways for electron transfer.

Fig. 2 SEM images of samples: (a) LiMn_1.5Ni_0.5O₄; (b) LiNi_0.4Ti_0.1Mn_1.5O₄; (c) carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄.

3.2. Electrochemical characterization

The charge–discharge profiles of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples at 25 °C are presented in Fig. 3. All the curves show a flat voltage plateau at around 4.7 V corresponding to Ni²⁺/Ni⁴⁺ redox process and a small plateau at around 4.0 V corresponding to Mn³⁺/Mn⁴⁺ redox process. The presence of the plateau corresponds to lithium-ion insertion into the 8a tetrahedral sites of the cubic spinel structure. The plateau at around 4.0 V is attributed to the reduction of Mn⁴⁺ to Mn³⁺ involving lithium-ion insertion into 16c octahedral sites of the spinel structure, which will result in a cubic to tetragonal phase transition.

Fig. 3 Charge and discharge profiles with various charge rates (0.5, 1, 3, 5, 7 and 10 C) followed by a constant voltage charge and discharging at the corresponding rate (25 °C): (a) LiMn_1.5Ni_0.5O₄; (b) LiNi_0.4Ti_0.1Mn_1.5O₄; (c) carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄.

From Fig. 3, the discharge specific capacity of Ti-free LiMn_1.5Ni_0.5O₄ can reach 125.7, 125.0, 118.7, 114.6, 107.8 and 98.6 mA h g⁻¹ at the rates of 0.5, 1, 3, 5, 7 and 10 C, respectively (Fig. 3(a)). Moreover, the discharge specific capacity of LiNi_0.4Ti_0.1Mn_1.5O₄ could deliver 135.3, 134.8, 130.2, 126.9, 122.3 and 116.6 mA h g⁻¹ at the rates of 0.5, 1, 3, 5, 7 and 10 C, respectively (Fig. 3(b)). For the sample carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄, the discharge specific capacity is 134.0, 132.5, 129.8, 128.1, 124.4 and 119.4 mA h g⁻¹ at the rates of 0.5, 1, 3, 5, 7 and 10 C, respectively (Fig. 3(c)). The carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ sample shows an improved discharge capacity compared to carbon-coated LiMn_1.5Ni_0.5O₄ found in the report,³⁶ which suggests that the present synthesis process is effective. Therefore, a great enhancement in rate performance can be obtained through the carbon coating LiNi_0.4Ti_0.1Mn_1.5O₄ method.

In comparison of Fig. 3(a) with Fig. 3(b) and (c), the discharge specific capacity increases after the Ti⁴⁺ doping at various discharge rates, meaning that the polarization is notably reduced by the Ti⁴⁺ doping, which is beneficial for high discharge capacity. This behavior can be understood as follows: the presence of appropriate amounts of Ti atoms favors crystal growth, probably by releasing microstrains caused by lattice defects. In addition, the Ti substitution leads to a progressive broadening and, hence, a decrease in the average domain size. The latter effect can be explained similarly by the difference between the Ti⁴⁺ and Ni⁴⁺ ionic radii.

Furthermore, it can be determined that the discharge specific capacity of LiNi_0.4Ti_0.1Mn_1.5O₄ is higher than that of carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ at relatively low rates (0.5, 1 and 3 C). It is worth noting that the carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ sample has a higher discharge specific capacity compared to LiNi_0.4Ti_0.1Mn_1.5O₄ at the rates of 5, 7 and 10 C. The improved discharge capacities at high rates can be attributed to the higher electronic conductivity of carbon compared to that of LiNi_0.4Ti_0.1Mn_1.5O₄. The carbon phase provides faster pathways for electron transfer, facilitating the electronic connection among LiNi_0.4Ti_0.1Mn_1.5O₄ particles and thereby improving the high rate capability.

In order to examine the effectiveness of Ti⁴⁺ doping and carbon coating on improving the rate capability of the cathode material at high temperature (55 °C), the discharge specific capacity at different discharge rates is also investigated and represented in Fig. 4.

Fig. 4 Charge and discharge profiles with various charge rates (0.5, 1, 3, 5, 7 and 10 C) followed by a constant voltage charge and discharging at the corresponding rate: (a) LiMn_1.5Ni_0.5O₄ at 55 °C; (b) LiNi_0.4Ti_0.1Mn_1.5O₄ at 55 °C; (c) carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ at 55 °C.

As shown in Fig. 4, it can be clearly seen that the discharge capacities of LiMn_1.5Ni_0.5O₄ are enhanced after Ti⁴⁺ doping at high temperature (55 °C). It is considered that the faster Li⁺ ion diffusion in the LiNi_0.4Ti_0.1Mn_1.5O₄ electrode result in the improvement of the rate capability. More importantly, the discharge specific capacities (131.8, 131.2, 129.1, 125.0, 117.6 and 105.6 mA h g⁻¹) of carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ sample are higher than those of LiNi_0.4Ti_0.1Mn_1.5O₄ sample (131.0, 130.6, 127.7, 121.8, 113.5 and 104.7 mA h g⁻¹) at corresponding discharge rates (0.5, 1, 3, 5, 7 and 10 C), demonstrating that the surface modification with carbon can make the electrochemical insertion/extraction process of Li⁺ ion more reversible at the high temperature (55 °C).

Fig. 5 shows the cycling performances of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ cells at 5 C. The initial discharge capacities of the three samples are 121.7, 124.2 and 127.1 mA h g⁻¹, respectively. After 1000 cycles, the capacity retention of the samples at 5 C is 80.6%, 87.2% and 91.5% of their initial discharge special capacity, respectively. It is demonstrated that the cycling capability of LiMn_1.5Ni_0.5O₄ is remarkably improved after Ti element doping, which is associated with the fact that the chemical bond of Ti–O is stronger than that of Ni–O, leading the structure to be stable between charge and discharge cycles process. Furthermore, carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ displays relatively remarkable cycling stability compared to the fade seen with the LiNi_0.4Ti_0.1Mn_1.5O₄ sample, particularly at the discharge later stage, which can be attributed to the carbon coating layer preventing the dissolution of LiNi_0.4Ti_0.1Mn_1.5O₄ materials from the electrolyte.

Fig. 5 Cycling performances of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples charged/discharged with a constant current of 1 C/5 C at 25 °C.

The cycle performances of LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ cells at 55 °C are shown in Fig. 6. As illustrated, the initial discharge specific capacity of the three samples is 128.2, 130.5 and 132.1 mA h g⁻¹ at 1 C, respectively, indicating that the capacity differences are small among the three samples. Nevertheless, with increasing the cycle number, the pristine LiMn_1.5Ni_0.5O₄ and LiNi_0.4Ti_0.1Mn_1.5O₄ suffer from significant polarization increase and capacity fading. The increased polarization may be due to the possible dissolution of the spinel into the electrolyte and electrolyte decomposition at high temperature. Furthermore, the decomposition products can coat on the surface of the electrodes and hinder lithium transport, leading to increased polarization.

Fig. 6 Cycling performances of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples charged/discharged with a constant current of 1 C/1 C at 55 °C.

On the contrary, the carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ electrode displays a greatly improved cyclic performance with a discharge capacity of 126.9 (96.1% of its initial capacity) after 100 cycles. The improvement in the cyclic performance should be related to increase in conductivity and the decrease in electrolyte oxidation caused by the carbon coating.

Coulombic efficiency is a key index for evaluating the stability of the electrolyte that is working with the LNMO cathode. To investigate the possible decomposition of the electrolyte at high temperature (55 °C), the coulombic efficiency values of the three samples with cycle number are plotted in Fig. 7. Clearly, the efficiency value changes in the following order: LiMn_1.5Ni_0.5O₄ ≈ LiNi_0.4Ti_0.1Mn_1.5O₄ < carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄. This indicates that the oxidation of the electrolyte in the LiMn_1.5Ni_0.5O₄/Li and LiNi_0.4Ti_0.1Mn_1.5O₄/Li cells is considerably more severe compared to the carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄/Li cell. From this result, it is considered that carbon coating is an effective method to improve the coulombic efficiency of LNMO materials. It is noted that there is no significant influence of the Ti doped LiMn_1.5Ni_0.5O₄ on coulombic efficiency at 55 °C.

Fig. 7 Coulombic efficiencies of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ samples charged/discharged with a constant current of 1 C/1 C at 55 °C.

The CV curves of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ after 1000 cycles at 5 C, conducted at a rate of 0.1 mV s⁻¹, are shown in Fig. 8. The peaks in the CV curves from three samples correspond to the oxidation of Mn³⁺, Ni²⁺ and Ni³⁺. It can be seen that sample carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ has greater peak current densities compared to LiMn_1.5Ni_0.5O₄ and LiNi_0.4Ti_0.1Mn_1.5O₄, indicating its superior performance at high discharge current rate. This result is consistent with that of our above-mentioned cycling performance experiments (Fig. 5). Moreover, the difference (ΔE_p) in peak potentials between the redox peaks of sample carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ (0.17 V) is considerably smaller compared to samples LiMn_1.5Ni_0.5O₄ (0.22 V) and LiNi_0.4Ti_0.1Mn_1.5O₄ (0.20 V), which indicates faster lithium insertion/extraction kinetics and higher reversibility of the electrode reaction in the former.³⁷

Fig. 8 Cyclic voltammograms of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ electrodes recorded after 1000 cycles at 25 °C.

Fig. 9 shows the A.C. impedance spectra of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ electrodes cells before and after 1000 cycles at 5 C. A potentiostatic step of 1–3 h is performed to stabilize the potential after activating or cycling, followed by the impedance measurement that is carried out at open circuit. As shown in Fig. 9, the EIS of the cell consists of two parts, a semicircle at high and medium frequency, as well as a sloping line at low frequency. The depressed semicircle reflects the interface resistance, including surface layer, as well as charge transfer reaction; the sloping line at low frequency range is related to the solid-state diffusion of Li⁺ ions in the active materials.³⁸

Fig. 9 Electrochemical impendence spectroscope curves of the LiMn_1.5Ni_0.5O₄, LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ electrodes recorded before and after 1000 cycles at 25 °C.

As can be seen in the figure, the impedance of sample carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ (20 Ω, 62 Ω) is smaller compared to LiMn_1.5Ni_0.5O₄ (41 Ω, 114 Ω) and LiNi_0.4Ti_0.1Mn_1.5O₄ (37 Ω, 113 Ω) before and after 1000 cycles at 5 C, especially after 1000 cycles. In addition, the Warburg parameters are summarized in Table 1, presenting that the samples LiNi_0.4Ti_0.1Mn_1.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ have smaller Warburg values before and after 1000 cycles. Accordingly, it can be inferred that the two samples have lower lithium diffusion resistance compared to LiNi_0.5Mn_1.5O₄, which is beneficial to the insertion/removal of lithium in the lattice of the spinel oxide, thereby improving the electrochemical performance.

Table 1 Warburg parameters calculated from equivalent circuit

Samples	Before cycling	After 1000 cycles
LiNi0.5Mn1.5O4	0.391(Ω)	0.496(Ω)
LiNi0.4Ti0.1Mn1.5O4	0.344(Ω)	0.444(Ω)
Carbon-coated LiNi0.4Ti0.1Mn1.5O4	0.333(Ω)	0.428(Ω)

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According to the analysis of Manthiram,³⁹ the variations in the electrolyte resistance among all three electrodes can be neglected. Consequently, the results indicate the suppression of the development of the SEI layer and reduction of the charge transfer resistance of LiNi_0.4Ti_0.1Mn_1.5O₄ after the surface modification with carbon protecting layer. However, the impedance differences between LiMn_1.5Ni_0.5O₄ and LiNi_0.4Ti_0.1Mn_1.5O₄ are similar before and after 1000 cycles, and demonstrate that there is a little influence in the electrode resistance after Ti-doping. In addition, a significant increase of interface resistance for the two samples after 1000 cyclings may be relative to the electrolyte decomposing and the formation of a more resistive surface film.

4. Conclusions

Doping the spinel LiMn_1.5Ni_0.5O₄ and coating LiNi_0.4Ti_0.1Mn_1.5O₄ cathode material with a small amount of Ti⁴⁺ and conductive carbon have been successfully realized with TiO₂ and carbonization of PEG4000, respectively. The Ti-doped LiMn_1.5Ni_0.5O₄ and carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ electrodes exhibit considerably enhanced rate capability and rate cycling performance compared to the Ti-free LiMn_1.5Ni_0.5O₄ at 25 and 55 °C. Furthermore, the discharge capacities of LiNi_0.4Ti_0.1Mn_1.5O₄ are higher compared to carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ at relatively low rates (0.5, 1 and 3 C) at 25 °C. Interestingly, the carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ sample has higher discharge capacities compared to LiNi_0.4Ti_0.1Mn_1.5O₄ at the rates of 5, 7 and 10 C rates at 25 °C. In addition, the carbon-coated LiNi_0.4Ti_0.1Mn_1.5O₄ exhibits considerably more excellent cycling stability, higher discharge capacities and coulombic efficiency compared to the LiMn_1.5Ni_0.5O₄ and LiNi_0.4Ti_0.1Mn_1.5O₄ samples at high temperature (55 °C). The improvement in the high rate capability and cycle performance of the carbon coated LiNi_0.4Ti_0.1Mn_1.5O₄ can be attributed to the acceleration of electron transfer and the suppression of side reactions by the carbon surface modification.

Acknowledgements

We acknowledge the Natural Science Foundation of Heilongjiang Province of China (B201201), the National Natural Science Foundation of China (21203040), China postdoctoral science foundation (Grant no. 2014M561357), the Fundamental Research Funds for the Central Universities (HEUCF201403018), the National Natural Science Foundation of China (Grant no. 21273058), China postdoctoral science foundation (Grant no. 2012M520731), Heilongjiang post-doctoral financial assistance (LBH-Z12089) for their financial support.

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