Improved cycling performance of LiNi_0.8Co_0.15Al_0.05O₂/Al₂O₃ with core-shell structure synthesized by a heterogeneous nucleation-and-growth process

Abstract

Al₂O₃ was successfully coated on LiNi_0.8Co_0.15Al_0.05O₂ cathode material by a heterogeneous nucleation-and-growth process with a core-shell structure for Li-ion battery. X-ray diffraction (XRD) measurements were used to indicate that the crystal structure of LiNi_0.8Co_0.15Al_0.05O₂ had no changes and no impurity phase existed after coating. Scanning electron microscopy (SEM) showed differences of surface morphology between coated and uncoated samples. A thin and bright coating layer was visually observed through transmission electron microscope (TEM). It reveals that the thickness of coating layer is 7 nm approximately. Electrochemical measurements were also carried out. Although the initial discharge capacity of the coated sample decreased, the 1-wt.% Al₂O₃-coated sample showed improved cycling performance at room temperature (25 °C) and elevated temperature (55 °C). It provided higher capacity retention of 71.7 and 70.1 % for 1C at 25 and 55 °C after 100 cycles, in comparison with 55.3 and 55.8 % for the uncoated sample. Meanwhile, interfacial resistance between active material and electrolyte decreased detected by electrochemical impedance spectroscopy (EIS) test. These enhancements in electrochemical characterizations are attributed to the improved stability of interface and the coating layer which served as the physical barriers to protect the active material from electrolyte attack.

Introduction

Nickel-rich cathode material LiNi_0.8Co_0.15Al_0.05O₂ is one of the most applicable cathode materials for advanced lithium ion batteries (LIBs). It has been widely used as power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), due to its high energy, power density, and lower price [1–4]. However, there are some shortcomings, such as rapid capacity fading especially due to chemical instability of abundant Ni³⁺ and Ni⁴⁺ ions and its instability of structure at highly delithiated status and elevated temperature [5–8]. The appearance of abundant Ni⁴⁺ ions aggravates the side reaction between electrode and electrolyte, increases the impedance because of formation of solid electrolyte interface (SEI) layer, and lowers the cycling performance of LIBs [9–11]. In addition, LiOH impurities form on the surface of nickel-rich cathode material, react with LiPF₆ electrolyte, and yield HF which dissolves metal ions [12–14].

Many studies have indicated that it is a promising approach to improve the thermal stability and cycling stability of cathode materials of the nickel (Ni)-rich cathode material by coating of various materials. These coating materials can (i) act as a physical barrier, provide coated materials from HF attack, and thus reduce metal dissolution from the active materials to improve the surface stability of coated material; (ii) reduce side reactions between the electrode and the electrolyte; and (iii) enhance the capacity and electronic conductivity for some active coating materials [15]. For layered cathodes, metal oxides (SiO₂ [16], ZrO₂ [17, 18], and TiO₂ [19, 20]), metal fluorides (AlF₃ [21]), and metal phosphates (AlPO₄ and Co₃(PO4)₂ [22]) are suggested as suitable coating materials. For much cheaper price and more stable physical properties, Al₂O₃ coatings have been reported widely in previous references [12, 23, 24].

In this paper, Al₂O₃ was coated on LiNi_0.8Co_0.15Al_0.05O₂ cathode material via a heterogeneous nucleation-and-growth process [25–28] successfully. It is found that the cycling performance of Al₂O₃-coated cathode material was highly improved at high current intensity. In addition, the physical and electrochemical characterizations of the coated samples are discussed.

Experimental

Preparation of Al₂O₃-coated samples

LiNi_0.8Co_0.15Al_0.05O₂ was synthesized by co-precipitation method [29] as both the base material for coating and uncoated reference material. To prepare Al₂O₃-coated samples, LiNi_0.8Co_0.15Al_0.05O₂ powders were dispersed in aluminum isopropoxide ethanol solution with stirring vigorously for 5 h and evaporated at 60 °C subsequently to remove ethanol. The mixture was exposed in air with hydrolysis of aluminum isopropoxide to form film of gel on the surface of LiNi_0.8Co_0.15Al_0.05O₂ particles. And then particles were heated in air at 500 °C for 3 h in Muffle oven to obtain the Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ materials. Amounts of Al₂O₃ were in the range 0.5 to 2 wt.%, and we denote the coating amounts of 0.5, 1, and 2 wt% as Al₂O₃-05, Al₂O₃-10, and Al₂O₃-20 respectively.

Physical characterization

The crystal structures of the samples were identified by X-ray diffraction (XRD) with Cu Kα radiation (ULTIMA-3, Rigaku, Japan), operated at 40 kV and 40 mA. The morphology and size of the samples was observed by field-emission scanning electron microscopy equipped with an EDXS energy disperse X-ray spectrometer (FESEM, Ultra55, Zeiss, Germany). Transmission electron microscopy (TEM, JEM-2100, JEOL) was taken to observe the coating layer. The quantitative atomic compositions and element valence of Al₂O₃-coated samples were determined by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, UK).

Electrochemical measurements

The cathode materials were mixed with acetylene black and polyvinylidene fluoride (PVDF) with a weight ratio of 80:10:10 in n-methyl-2-pyrrolidone (NMP). The slurry was coated on Al foil and dried in vacuum oven at 120 °C for 8 h. And then, the coin-type cell (CR2032) was assembled in a glove box (MBRAUN, Germany) filled with Ar, using lithium foil as the counter electrode, Celgard 2300 as the separator, and 1 M LiPF₆ in ethylene carbonate and dimethyl carbonate (1:1) as the electrolyte. The electrochemical performances were evaluated by electrochemical test instrument (Maccor S4000, USA) at different current densities in a voltage range of 2.8–4.5 V. The electrochemical impedance spectroscopy (EIS) measurement was carried out using an electrochemical workstation (Solartron 1287, USA) over a frequency range from 10 mHz to 100 kHz with a 5-mV voltage vibration in order to detect the cell resistances at different cycle stages.

Result and discussion

XRD patterns and Miller indices of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ material are shown in Fig. 1. The crystal structure of the samples and all the diffraction peaks are in good agreement with the patterns of LiNiO₂ (JCPDS No. 74-0919) which reveals a single phase of α-NaFeO₂ structure with space group of R-3 m. Calculated by software Jade, the average crystal lattice constants are a = 0.2868 nm, c = 1.4177 nm for the uncoated sample with only a slight increase in the crystal constants, compared with a = 0.2868 nm, c = 1.4186 nm for the coated one. All these results suggest that the crystal structure of LiNi_0.8Co_0.15Al_0.05O₂ is not affected by coating. The splitting of the (006)/ (012) and (108)/ (110) peaks as well as the integrated ratios of I (003)/I (104) > 1.2 indicate the well-ordered α-NaFeO₂ structure with the limited cation mixing for both the bare and coated samples [30]. It can be indicated that the coating layer was nicely formed through a heterogeneous nucleation-and-growth process.

Fig. 1

XRD patterns of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂: uncoated (a), Al₂O₃-05 (b), Al₂O₃-10 (c), and Al₂O₃-20 (d)

In order to clarify the oxidation state of Al in the Al₂O₃-coated samples, XPS analysis was performed at room temperature. XPS analysis of the coated sample (Al₂O₃-10) was carried out, and the Al2p XPS spectrum is given in Fig. 2. In this figure, the observed binding energy of the Al2p is 72.0 eV, which is very close to the values of 72.9 eV for Al₂O₃ (1999 XPS International, Inc.). This result shows that the Al element on the material surface is trivalent.

Fig. 2

XPS spectrum of Al2p taken from the surface of Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ (Al₂O₃-10)

The surface morphologies of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ samples observed by SEM are shown in Fig. 3a, b. There are obvious differences in surface morphologies between the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ samples. Plenty of nanometer primary particles are attached to the surface of secondary particles, as shown in Fig. 3a. After coating Al₂O₃ on the surface, primary particles of LiNi_0.8Co_0.15Al_0.05O₂ vanish with a few Al₂O₃ particles occurring indicating the Al₂O₃-coating layer is formed on the surface of the LiNi_0.8Co_0.15Al_0.05O₂ samples. The average diameter of LiNi_0.8Co_0.15Al_0.05O₂ particles is 7 μm. Also, the distribution of the four elements is revealed by a mapping of a single Al₂O₃-coated particle (Fig. 3c). SEM-EDS spectrum (Fig. 3d) shows O, Ni, Co, and Al characteristic peaks. According to atomic percentages of Ni, Co, and Al, the weight percentages of Al₂O₃ coated on the particle is 0.96 wt%.

Fig. 3

SEM micrograph of the uncoated LiNi_0.8Co_0.15Al_0.05O₂ (a), SEM micrograph of the 1 wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ (b), elemental mapping recorded from 1 wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ with corresponding mappings of different elements (c), and EDS pattern of 1 wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ (d)

Fine particles can provide nucleation centers, and can decrease the kinetic barrier to nucleation of a supersaturated solution. In this experiment, LiNi_0.8Co_0.15Al_0.05O₂ was used as nucleation centers in a supersaturated alumina sol suspension, so alumina sol can form a homogeneous layer on LiNi_0.8Co_0.15Al_0.05O₂ cores by the heterogeneous nucleation-and-growth processing. Transmission electron micrographs of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ powders (Al₂O₃-10) are shown in Fig. 4. By comparing morphology pictures of the uncoated samples with a clean and smooth surface in Fig. 4a, a thin and brighter coating layer is visually observed in Fig. 4b. It reveals that the thickness of coating layer is 7 nm approximately. Based on these facts, we confirm that a uniform surface coating of Al₂O₃ particles was successfully achieved.

Fig. 4

TEM micrographs of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂: uncoated (a) and Al₂O₃-10 (b)

Figure 5a shows initial charge–discharge curves of the Li/uncoated and Li/Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ cells at a current density of 18 mA g⁻¹ (0.1C) between 2.8 and 4.5 V. All the similar charge–discharge curves imply that no change of structure occurred through the process of Al₂O₃ coating. The initial discharge capacities of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ materials (Al₂O₃-05, Al₂O₃-10, and Al₂O₃-20) are 202.83, 199.21, 189.86, and 184.70 mAh g⁻¹, respectively. The capacities of the Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ materials are lower than those of the uncoated material, which are on account of the existence of the inactive Al₂O₃ coated on the surface. Although the inactive Al₂O₃ coated on the surface decreases the initial discharge capacities of cells, the cyclability of 1-wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ cells increases at the current rate of 180 mA g⁻¹ (1C) between 2.8 and 4.5 V at room temperature (25 °C) and elevated temperature (55 °C) as shown in Fig. 5b, c. The discharge capacity at 25 and 55 °C of the uncoated LiNi_0.8Co_0.15Al_0.05O₂ materials decreases from 185.78 to 102.76 mAh g⁻¹ and 215.24 to 120.24 mAh g⁻¹ at the rate of 1C after 100 cycles and shows a capacity retention rate of 55.3 and 55.8 %, respectively; 1-wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ cells provide a higher capacity retention of 71.7 % (173.94 to 124.71 mAh g⁻¹) and 70.1 % (193.06 to 135.34 mAh g⁻¹) at 25 and 55 °C after the same cycles, by contrast; 0.5- and 2-wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ cells show no improvements in electrochemical characters because the amounts of Al₂O₃ are too small and large. Too thin or thick coating layer will have no effect of protecting the anode or impede transmission of ions. This remarkable improvement is mainly due to the dense Al₂O₃ coated on the surface which acts as a physical barrier protecting coated materials from HF attack, reduces side reactions between the electrode and the electrolyte, and provides a more stable electrode/electrolyte interface. Besides, 1-wt.% Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ cells also show better discharge capacity retentions during cycling at higher current densities (1C and 2C).

Fig. 5

Electrochemical properties: initial charge/discharge curves of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ at a current density of 18 mA g⁻¹ (0.1C) over the voltage range of 2.8–4.5 V (a), cycle performance at the current rate of 1C at room temperature (25 °C) (b), cycle performance at the current rate of 1C at elevated temperature (55 °C) (c), and rate capability test of the Li/uncoated and Li/Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ cells as a function of C rate (d)

The electrochemical impedance spectroscopy (EIS) measurement was carried out after 50 charge–discharge cycles at 1C to elucidate changes in electrochemical performance after the Al₂O₃ coating. Nyquist plots of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ electrodes are shown in Fig. 6. The semicircle in the high-to-medium frequency region is ascribed to the surface impedance (Rf) of Li⁺ diffusion in the surface layer; the other in the medium frequency region is assigned to the charge transfer resistance (Rct). [31–33]. Equivalent circuit matches with the curve, and the values of solution resistance (Rs), surface resistance (Rf), and charge transfer resistance (Rct) are as listed in Table 1. The lower Rf and Rct are attributed to a more stable surface protected by Al₂O₃ coating which can suppress the increasing impedance and favor the lithium diffusion of the host oxide during the charge–discharge process [34].

Fig. 6

Nyquist plots of the uncoated and Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ electrodes after 50 charge–discharge cycles

Table 1 Fitting values of the Rs, Rf, and Rct based on the equivalent circuit in Fig. 6

Conclusion

In this paper, Al₂O₃ was successfully coated on layered Ni-rich LiNi_0.8Co_0.15Al_0.05O₂ material. A thin coating layer with 7 nm was formed on the surface. The Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ materials show improved cycling performance at the current rate of 1C over the voltage range of 2.8–4.5 V. This coating layer acts as a physical barrier protecting coated materials from HF attack, reduces side reactions between the electrode and the electrolyte, and forms a more stable SEI layer. Al₂O₃-coated LiNi_0.8Co_0.15Al_0.05O₂ synthesized by a facile method with improved cycling performance presents great potential to be applied in the large-scale energy storage systems (ESSs).