Modification and Influence of Screen-Printed Graphene Oxide on LiNi

Abstract

Electrodes consisting of LiNi_1/3Co_1/3Mn_1/3O₂, aluminum foil, conductive additives, and polyvinylidene fluoride were coated with a thin graphene oxide layer via a simple screen-printing method. The cycle performance and rate capability were tested at a cut-off voltage of 4.3 V. Results show that the capacity deceases whereas the polarization increases during the galvanostatic charge-discharge tests for primary electrodes. For the graphene-oxide-modified electrodes, the capacity decrement reduces and polarization increment rate evidently slows down. As a result, the cycle stability and rate capability are improved because the graphene oxide coating suppresses the side reactions between the LiNi_1/3Co_1/3Mn_1/3O₂ electrodes and electrolyte. The research provides an ecofriendly and highly effective strategy to improve the performance of LiNi_1/3Co_1/3Mn_1/3O₂ electrodes.

Keywords

LiNi_1/3Co_1/3Mn_1/3O₂; cathode; graphene oxide; screen-printing; lithium ion battery

Science Press

Layered lithium metal oxides LiMO₂ (M=Ni, Co, Mn, Al) are considered as the most promising cathodes due to their high energy density^[1-3]. Among the oxides, LiNi_xMn_xCo_1-2xO₂ (x≤0.5) shows high capacity, excellent thermal stability, low cost, and high safety^[4,5]. Significant improvement has been achieved for the rate capability and cycle performance^[6-10]. However, the instability is still a key problem. Many researches reveal that the thickening of solid electrolyte interphase (SEI) film caused by the transition metals escaping from the active material is an unstable factor^[11-15].

Surface coating is widely used to retard the dissolution of transition metals into the interface of the active material and electrolyte^[7,16-18], thereby improving the stability of SEI film process. In recent researches, the transition metal oxide particles are coated with thin films, which are usually poor conductors of electronics or lithium ions. Another way to stabilize SEI film is to deposit a thin film on the finished electrodes. This method has been employed to modify cathode and anode electrodes, such as LiCoO₂^[19], Li₄Ti₅O₁₂^[20,21], and LiFePO₄^[22]. The coating layers are usually deposited by radio frequency (RF) magnetron sputtering and atomic layer deposition method. Few literatures are available about the modification of LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) electrodes.

In this research, the graphene oxide (GO) thin film was uniformly deposited on the traditionally prepared NCM electrodes by a simple screen-printing method. The results of galvanostatic charge-discharge test, cyclic voltammetry test, and electrochemical impedance spectra indicated that the rate capability and cycle stability were enhanced.

1 Experiment

The NCM powder (Shenzhen, N8-L), acetylene black (average grain size of 40 nm), polyvinylidene fluoride (PVDF, AR), and the aqueous solution of GO (Suzhou, Tanfeng) were used as the primary materials. Firstly, the uniform paste of NCM powder, acetylene black, graphite KS-6, and PVDF were spread on the aluminum foil and air-dried in oven for 6~8 h, as mentioned in Ref.[23], with the mass ratio of 8:0.6:0.4:1. For the screen-printing process, the nylon screen mesh with wire of 20 μm in diameter and 20 μm in aperture was selected. The dry NCM electrode was placed on the vacuum metal surface of the automatic coating machine. Then, the mesh was placed on the electrode; 6 mL GO solution was extracted with the concentration of 2 mg·mL^-1 using the pipette, and dropped evenly on the nylon wire. The electrode was scraped back and forth twice with the scraper. Finally, the coated electrodes were dried at 120 °C for 10 h. The fully dried electrodes were cut into small round pieces with 14 mm in diameter for battery assembling afterwards. The average area density of GO was 0.032 mg·cm^-2.

For battery testing, two-electrode coin cells (CR2032) were assembled in an argon-filled glovebox with water index less than 0.735 mg·m^-3. The primary and modified electrodes were used as cathodes with lithium foil as anodes, namely NCM and GO@NCM electrodes. Then 1 mol·L^-1 LiPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) solution (volume ratio of 1:1) was adopted as the electrolyte. The structure and morphology of primary and modified electrodes were characterized by X-ray diffraction (XRD, Bruker, D2-Phaser, Cu Kα) and scanning electron microscope (SEM, Hitachi, S-4800). Micro-Raman spectroscopy was performed on the primary and modified electrodes using a micro-Raman spectrometer from Renishaw inVia equipped with a laser of 532 nm, four charge-coupled-device (CCD) cameras, and an optical microscope (OM, Leica). The 50× objective lens was used to focus the incident beam and grating. The galvanostatic charge-discharge tests were condu-cted on CT2001A (Wuhan LAND electronics Co., Ltd). The reference voltage in this research was Li/Li⁺. The potential was set within a range of 2.5~4.3 V. Cyclic voltammetry (CV, 2.5~4.3 V, 0.1 and 0.5 mV·s^-1) and electrochemical impedance spectra (EIS, 3.6 V, 0.1~10 MHz, 10 mV) information were collected by an electrochemical workstation (CHI660E).

2 Results and Discussion

XRD patterns of the primary and GO modified NCM electrodes are shown in Fig.1. The XRD patterns of modified electrodes have similar characteristics with those of primary electrodes. The 2θ at 18.0°, 36.0°, 37.6°, 43.7°, 48.0°, 58.0°, 63.7°, 64.3°, and 67.6° is referred as the plane of (003), (101), (006), (104), (105), (107), (108), (110), and (113), respectively. These peaks are indexed as the space group R-3m. The peak at 2θ=25.9° is caused by the (002) planes of graphite KS-6, which is a component of electrical additives. The XRD patterns of GO cannot be detected because the amount of GO in the modified electrodes is very small.

The formation of GO on the electrodes is confirmed by Raman spectroscopy. As shown in Fig.2, from the Raman spectrum of GO modified NCM electrodes, the peaks observed at 479 and 589 cm^-1 are attributed to the E_g and A_1g bands of NCM, respectively, indicating that the NCM have the R-3m symmetry. The relation between the peak index in the XRD patterns and the R-3m symmetry of NCM is proved by the Raman spectrum^[24]. The first-order scattering under E_2g mode in graphene and that under breathing mode in aromatic rings (very intense and broad) suggest that the typical broad bands at 1348 and 1596 cm^-1 are D and G bands^[25,26], respectively. Furthermore, the weak overtone regions of signals are also distinguished at 2696 and 2945 cm^{-1 [}25-27]. The result confirms that the NCM are well coated by GO and the GO coating layer shows no detectable impact on the physical characteristics of NCM electrodes.

The SEM image of primary electrode is shown in Fig.3a. The secondary particles with diameter of 0.1~20 μm are NCM. The flocculent material is carbon black, namely acetylene black. The flakes among the NCM particles and carbon black are graphite KS-6, which agrees with the XRD patterns (Fig.1). For the GO modified specimens, the NCM secondary particles, carbon black, and graphite KS-6 flakes are all well covered by a thin GO layer, as shown in Fig.3b.

Fig.3 SEM images of primary (a) and GO modified (b) NCM electrodes

The corresponding batteries are charged and discharged at the rate of 31 mA·g^-1 for 5 cycles. The reversible specific capacities of primary and GO modified NCM batteries are 161 and 163 mAh·g^-1, respectively. The capacities of two batteries change to 145 and 144 mAh·g^-1 when the charge-discharge current changes to 154 mA·g^-1, as shown in Fig.4. The capa-city of all specimens decreases during the cycle test, whereas the decrease rate of GO modified electrodes is much slower than that of the primary electrodes. At 105^th cycle, the capacity of the GO modified electrodes retains a high value of 103 mAh·g^-1, while the lower capacity of 80 mAh·g^-1 is obtained for the primary electrodes.

To further illustrate the enhanced cycle performance, the typical charge-discharge curves and the corresponding dQ/dV profiles of 6^th and 80^th cycle of the primary and GO modified electrodes are shown in Fig.5. Obviously, the voltage redu-ction of primary NCM electrodes during cycling is obviously restrained by GO modification, as shown in Fig.5a. The polarization of the primary and GO modified NCM electrodes at 6^th cycle is 0.0335 and 0.1296 V, increasing to 0.2049 and 0.3445 V at 80^th cycle, respectively. The polarization of the primary electrodes is raised due to a large magnitude by more than 6 times after the charge-discharge tests, which is caused by the rapid degradation of the primary electrodes. For the GO modified electrodes, the polarization at 80^th cycle is 2.6 times higher than that at 6^th cycle, indicating a smaller polarization increment. Therefore, the polarization increment speed is effectively suppressed by the GO layer fabricated by screen-printing method. The suppressed voltage reduction and the lower polarization increment speed suggest a higher structure stability and enhanced cycle performance.

Fig.5 Charge-discharge curves (a) and corresponding dQ/dV profiles (b) of primary and GO modified NCM electrodes at 6^th and 80^th cycles

The discharge capacity of primary electrodes was tested at the current density of 31, 77, 154, 308, and 462 mA·g^-1, whose value is 161, 150, 132, 97, and 37 mAh·g^-1, respectively, as shown in Fig.6a. For the GO modified electrodes, the discharge capacities improve to 163, 152, 134, 109, and 72 mAh·g^-1 at different current densities. The capacity difference becomes larger with increasing the current density. The cyclic voltammetry tests were carried out at scan rates of 0.1 and 0.5 mV·s^-1. As shown in Fig.6b, the obtained CV curves explain the reason for the improved rate capability. The potential shifting of oxidation and reduction peak of primary NCM electrodes is 0.166 V at the scan rate of 0.1 mV·s^-1, which is raised to 0.459 V when the scan rate is 0.5 mV·s^-1. The potential difference increases by ~1.8 times. However, the potential of GO modified electrodes only increases by 1.1 times. The potential shift values of the GO modified electrodes are 0.245 and 0.522 V at 0.1 and 0.5 mV·s^-1, respectively. For both primary and GO modified NCM electrodes, the peaks current increases with the increase of scanning rate. In addition, the oxidation and reduction peaks shift to higher and lower potential positions, respectively, suggesting the diffusion controlled behavior^[28]. For diffusion controlled process, the Li⁺ diffusion coefficient is computed by Randles-Sevcik formula^[29,30] as follows:

$$I_{\mathrm{p}}=\mathrm{2.68}\times {\mathrm{10}}^{\mathrm{5}}{n}^{\mathrm{3}/\mathrm{2}}A{D}^{\mathrm{1}/\mathrm{2}}C{\nu }^{\mathrm{1}/\mathrm{2}}$$

(1)

where I_p is the peak current intensity, n is the number of electrons per reaction species (n=1), A is the surface area of the electrode, D represents the diffusion coefficient, C is the molar concentration of Li in NCM (~4.943×10^-2 mol·cm^-3), and v is the scan rate. According to Eq.(1), the plot of the reduction peak current intensity I_p versus the square root of the scan rate (v^1/2) should be a straight line, and the slope can be estimated as the diffusion coefficient, as shown in Fig.7.

Obviously, the slope of relationship line of GO@NCM electrodes is larger, indicating that the diffusion process is faster. Besides, for GO@NCM and NCM electrodes, the Li⁺ diffusion coefficient ($D_{\mathrm{L}{\mathrm{i}}^{+}}$) is 3.69×10^-10 and 2.82×10^-10 cm²·s^-1, respectively. The GO@NCM electrodes have higher diffusion coefficient than the NCM electrodes do. Based on the above results, it can be concluded that the GO modified NCM electrodes have a faster transfer process for lithium ions and a better reversibility of the electrode reaction.

The electrochemical impedance plots of primary and GO modified NCM electrodes are shown in Fig.8. As shown in Fig.8a, the high frequency semicircle is ascribed to the charge transfer behavior in the interface of the electrode and electrolyte (R_ct). The R_ct of GO modified electrodes is greater than that of primary electrodes by 200 Ω for newly assembled batteries, as shown in Table 1. After screen-printing, the GO thin film with high resistance towards the aggressive environment forms on the electrode surface during the secondary drying process^[31,32]. The thin film reduces the direct contact between the NCM electrodes and the electrolyte. The coating layer of GO suppresses the side reaction between NCM electrodes and electrolyte, showing a marked increase in the charge transfer resistance. There is no obvious semi-circle arising from SEI film for the newly assembled batteries. The most likely reason is that SEI film barely forms during the test. The Nyquist plots of the cycled batteries consist of more than one semicircle, as shown in Fig.8b. There is one new semicircle for cycled primary electrodes, which is ascri-bed to the SEI film formation after CV tests. Meanwhile, two

Fig.8 EIS plots of primary and GO modified electrodes of freshly assembled (a) and cycled (b) batteries

Table 1 Fitted data of primary and GO modified electrodes (Ω)

Battery state	Electrode	RGO	Rf	Rct
Freshly assembled	NCM	-	-	272
	GO@NCM	-	-	472
Cycled	NCM	-	91	105
	GO@NCM	36	65	229

new semicircles appear for the cycled GO modified elec-trodes, referring to the resistances of GO (R_GO) and SEI film.

The fitted R_ct of the GO modified electrodes is 229 Ω, as shown in Table 1, which is larger than that of the primary electrodes. The increasing R_ct suggests a larger polarization, which agrees with the charge-discharge test results. The reduction of SEI film resistance (R_f) of GO modified

electrodes indicates the better cycle performance and rate capability.

3 Conclusions

1) The uniform graphene oxide (GO) coating was fabricated by the screen-printing method on LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) electrodes.

2) The GO coating layer shows no detectable impact on the physical characteristics of NCM electrodes. GO modified NCM electrodes exhibit elevated rate capability and cycle performance. The performance improvement is ascribed to the faster transfer of lithium ions.

3) The GO coating layer suppresses the degradation of the NCM electrodes by restraining the polarization increment du-ring the galvanostatic charge-discharge tests and restricts the potential shifting between the redox peaks at high scan rate.