ZrO₂ coated Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathode has enhanced electrochemical performance and stability compared with the pristine. The discharge capacity of both samples have no significant difference. However, the 1.77 wt% ZrO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂ material delivers enhanced charge-discharge cycling (capacity retention of 97.1 % after 100 cycles at 0.5 C) while the capacity retention of pristine is 94.8 % under the same condition. Based on these, we can infer that the ZrO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂ material shows great cyclability, originated from suppressing undesirable side reaction and decrease effectively electrolyte decomposition reaction. Therefore, the ZrO₂ coated Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathode can be considered as an effective strategy for long-life Li-ion batteries.

Keywords: ZrO₂, LiNi_0.8Co_0.1Mn_0.1O₂, electrochemical performances

Li-ion batteries (LIBs) have attracted a lot of attention for electric vehicles (EVs), mobile devices, plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs) and residential energy storage applications. This is because LIBs have an advantage of high operating voltage and energy densities, power performance, low price, long lifetime and good stability compared to traditional batteries [1]. However, LIBs suffer from severe potential deposition and capacity fading when charge-discharge cycling. The energy density, lifetime, material cost and stability are essential criteria for assessing if they can be used in practical application in LIBs. Because the energy density of the LIBs is extremely dependent on the cathode material, thus, significant efforts have been conducted for high-performance cathode material [2].
In Particular, the layered nickel-rich oxide LiNi_1-x-y    Co_xMn_yO₂ (x > 0.8, NCM) cathodes have drawn substantial interest [3]. Among various cathode materials, Nickel-rich NCM shows good electrochemical performance and it is considered as one of most effective cathode materials owing to low price and elevated electrochemical performances [4-5]. The superior energy density, excellent thermal stability and cyclability of Ni-rich NCM made it an ideal candidate for next-generation cathode material. However, high Ni content also results in irreversible capacity and poor cycling performance owing to the similar ionic radius of Ni²⁺ and Li⁺ [6]. Especially, at higher temperature, the structural instability can lead to rapid performance fading of Ni-rich NCM [7]. To overcome these problems, various strategies such as TiO₂, SiO₂ and carbon coating and/or V, B and Mo doping [8-13] have been performed.
Among them, in this work, we synthesized well-crystallized ZrO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) in order to stabilize the structure and improve electrochemical performances. Consequently, the ZrO₂ coated NCM811 can be considered for advanced cathode material.

Co-precipitation method was used to synthesize the Ni-rich NCM811 powders. the Ni-rich NCM811 powders were synthesized by using co-precipitation method NiSO₄-6H₂O, CoSO₄-7H₂O and MnSO₄-H₂O were prepared to assemble Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor. The chelating agent were comprised of NaOH and NH₄OH solution. The as-prepared spherical Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor was used to mix with LiOH-H₂O at a molar ratio 1.05 : 1. After that, the mixture was fired at 480 ^oC for 5 h. Then the mixture was cooled down at 750 ^oC in air for 15 h. For surface coating, all samples of ZrO₂ coated Ni-rich NCM811 were synthesized exploiting different ratios of ZrO₂: Ni-rich NCM811 and then heated at 500 ^oC in air for 5 h.
The cathodes were prepared by mixing Ni-rich NCM811 powder, conductive carbon black binder and polyvinylidene fluoride in the weight ratio of 96 wt%: 2 wt%: 2 wt%. Afterwards, N-Methyl pyrrolidinone (NMP) solvent was included in the cathodes. The 2032 coin cells were compounded with Lithium metal disc in argon-gas-filled glove box as an anode. The electrolyte was consisted of 1 M LiPF₆ in ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (EC:DMC:EMC 1:1:1 in volume). The polyethylene (PE, 20 μm) was used as a separator.
The XRD (Philips, X-pert PRO MPD) was used to measure the crystalline phase of samples. The FE-SEM (Hitachi S-4800) was prepared to observe the morphology of the pristine and ZrO₂ coated samples. The electron diffraction spectroscopy (EDS) was used in order to identify the component elements on the surface of particles. The electrochemical performances were mea-  sured with an equipment (TOSCAT-3100, Toyo system). The Electrochemical Impedance Spectroscopy (EIS) was measured with frequency range from 1 MHz to 100 mHz and the amplitude of the AC signal of 10 mV.

Fig. 1 shows the X-ray diffraction (XRD) spectra of the pristine and ZrO₂ coated NCM811. The shape of all samples is identical without secondary peaks since ZrO₂ coating do not affect the crystal structure of NCM811 materials. Overall, main diffraction peaks are indexed as a layered oxide structure in the hexagonal α-NaFeO₂ structure with a space group of R-3m [14]. Both obvious splitting of the (018)/(110) and (006)/(102) peaks suggests well-ordered layered structure. Also, the ratio of the intensities of (003) and (104) peaks (I(003)/I(104)) are regarded as index of cation mixing [15]. The cation ordering is resulted from the similar radius of Ni²⁺ (0.69 Å) and Li⁺ (0.76 Å) at the 3a site [16]. The I(003)/I(104) of all samples are above 1.2, demonstrating superior cation ordering, which is regarded as an one of the important factor for good electrochemical performance.
Fig. 2 shows the FESEM images of (a-b) pristine and (c-d) ZrO₂ coated NCM811 cathode materials. The pristine and ZrO₂ coated NCM811 sample have similar the morphology (particles size and shape) [17]. All samples have spherical morphology secondary particles with the average size of 15 ~ 20 μm, which is composed of numerous primary particles about 300 ~ 500 nm in diameter. The benefit of spherical secondary particle can achieve both high tap density and energy density. The surface of ZrO₂ coated NCM811 shows slightly rough as compared to pristine NCM811.
EDS (Energy Dispersive X-ray Spectroscopy) spectrum analysis of ZrO₂ coated NCM811 sample are conducted, as shown in Fig. 3. It can be confirmed that the Ni, Co, Mn and Zr are equally distributed and the amount of ZrO₂ is 1.77 wt%. The ZrO₂ coating can decrease the contact area between the cathode and the electrolyte, resulting in suppressing the electrolyte degradation while charge-discharge is cycling [18].
In order to investigate the long-term performance, Fig. 4 presents the cycle performance of the pristine and ZrO₂ coated NCM811 with vinylene carbonate (VC) and propane sultone (PS) additives. The cycle performances are measured at 0.5 C in the voltage between 3.0 and 4.3 V. It has been proved that additives can increase the cyclability for both samples since additive is designed to form the passivating layer on the surface, which can deactivate the active “catalytic” centers [19]. The ZrO₂ coated sample exhibits lower capacity fading than pristine sample after 100 charge-discharge cycles. This is because ZrO₂ coating could stabilize the interface between NCM811 and electrolyte. The ZrO₂ coated and pristine NCM811 have capacity retentions of 97.1% and 94.8%, respectively, after 100 cycles. We can confirm that the ZrO₂ coating effectively delay the capacity fading. In other words, it is demonstrated that the ZrO₂ coating on the Ni-rich NCM811 leads to superior cycle performance. This is because the bond dissociation energies of Zr-O, Ni-O, Co-O and Mn-O have 760, 391.6, 368 and 402 kJ·mol^-1 at 25 ^oC. The excellent cyclability of ZrO₂ coated Ni-rich NCM811 is closely related to the stable crystal structure, resulting from stronger iono-covalent of Zr-O [20].
Electrochemical Impedance Spectroscopy (EIS) measurements are used in order to investigate the charge-transfer kinetics of the electrodes. Nyquist plots of the pristine and the ZrO₂ coated NCM811 are measured after first cycle in the voltage range 3.0 – 4.3 V. Usually, Nyquist plot is composed of semicircle and one slope. Fig. 5 shows the EIS curves of the pristine and ZrO₂ coated NCM811. All samples have same electrolyte resistance (R_e) in the high-frequency because we used same electrolyte. The medium-frequency semicircle is contributed to the resistance of R_ct at the interface between the surface of the particles and the electrolyte [21]. The straight line of low frequency region is the Warburg impedance (W). There is a gap between pristine and ZrO₂ coated Ni-rich NCM811 for R_ct. The R_ct of ZrO₂ coated NCM811 is lower than the R_ct of pristine. After first cycle, the R_ct of pristine is 211.5 Ω, while the R_ct of ZrO₂ coated NCM811 is 203.7 Ω. This is because the ZrO₂ coating layer suppresses the formation of SEI due to accelerating the Li⁺ movement at the surface of cathode [21]. Based on this, we can infer that ZrO₂ coating can reduce side reaction, cell impedance increases and R_ct. Therefore, the better cycle performance of ZrO₂ coating can be explained by lower R_ct.

In this work, we prepare the ZrO₂ coating on the surface of Ni-rich NCM811 cathode in order to enhance the electrochemical performance. The ZrO₂ coating has no effect on the crystal structure, particle size and shape. It is proved that ZrO₂ can not only stabilize the NCM811 structure but also improve the electrochemical performances due to the deceasing side reaction and the increasing electrochemical reaction kinetics of lithium ion. These factors can enable good cycling performance. Therefore, the ZrO₂ coating lead to being considered as an efficient way for Ni-rich NCM811 cathode.

This research was supported by the Daejeon University fund (2020)