Enhanced electrochemical performance of RuO2 doped LiNi1/3Mn1/3Co1/3O2 cathode material for Lithium-ion battery

LiNi 1/3 Mn 1/3 Co 1/3 O 2 as a promising cathode material for lithium-ion batteries was synthesized by a sol-gel method using nitrate precursor calcined at 800°C for 10 hours. The crystallite nature of samples is conrmed from X-ray diffraction analysis. SEM and TEM analyses were used to investigate the surface morphology of the prepared samples. It was found that, highly crystalline polyhedral RuO 2 nanoparticles are well doped on the surface of pristine LiNi 1/3 Mn 1/3 Co 1/3 O 2 with a size of about approximately 200 nm. The chemical composition of the prepared samples was characterized by EDX and XPS analyses. The electrochemical performance of the proposed material was studied by cyclic voltammetry and charge/discharge analyses. The electrode kinetics of the samples was studied by electrochemical impedance spectroscopy. The developed RuO 2 doping may provide an effective strategy to design and synthesize the advanced electrode materials for lithium ion batteries. The doping strategy has dramatically increased the capacity retention from 74 % to 90% with a high discharge capacity of 251.2 mAhg − 1 . 3 % RuO 2 -doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathode materials have showed the similar characteristics of two potential plateaus obtained at 2.8 and 4.2 V compared with un doped electrode cathode material. These results revealed the enhanced performance of RuO 2 - doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 during insertion and extraction of lithium ions compared to pristine material.


Introduction
Increasing demand for portable electronics and electric vehicles has attracted immense interest in Li-ion batteries research as it is considered as a promising electric storage technology for upcoming electric vehicles (EV) and renewable energy power stations owing to its high capacity and low cost [1][2][3][4][5]. Previously, LiCoO 2 has ruled over the lithium ion market due to its ease of fabrication and cycling performance. However, the safety concerns of cobalt and requirements of high potentials stimulated the research towards the alternative positive materials [6]. At present, olivine LiFePO 4 , spinel LiMn 2 O 4 , Li-Mnrich layered oxides has been widely used in lithium ion batteries. But, the low voltage and low capacity of the above materials and high cost impedes its wide usage. [7,8]. Hence the layered LiNi 0.3 Mn 0.3 Co 0.3 O 2 cathode materials gained more focus and are more attractive due to the high theoretical speci c capacity, relatively low cost and better thermal stability [9,10]. The successful application of these materials can increase the energy density of Li-ion batteries, such as LiNi 0.3 Mn 0.3 Co 0.3 O 2 (LNMC), delivered a higher capacity more than 250 mAhg − 1 when they are charged at higher potentials greater than 4.2 V [11]. Many researchers have reported the improved performance of this cathode material by adopting various strategies [12][13][14][15][16]. Some of them are surface coating, ion doping, wet chemical synthesis, particle size reducing and so on. Among them, metal ion doping is a facile method to improve the electrochemical properties of cathode material. The electrochemical performance needs to be improved, especially for the development of electric vehicles. On the other hand, the radius of the lithium ion (0.76 nm) is close to the radius of the nickel ion (0.69 nm). Therefore, the cation disorder tends to happen between the nickel ions and lithium ions [17]. The higher cation disorder would make it more di cult for lithium ions to deintercalate from the layered structure, resulting in a loss of electrochemical performance. So the content of Ni can affect the electrochemical performance of the layered lithium-nickel-manganese-cobalt oxide materials dramatically. Therefore, the modi cation on nickel may improve structural ordering and electrochemical performance.
The doped compounds or elements into the cathode materials, such as Sm, Y, Al 2 O 3 , Nb, W etc [18][19][20][21][22][23] has been studied by many people. Doping can change both structure and/or morphology, as the doped elements enter into the crystal lattice of the cathode materials. The lattice parameters may also be changed after the crystal lattice has had more kinds of elements. S. C. Yin et al studied the X-ray/Neutron diffraction and lithium De/Re intercalation in Li 1-x Co 1/3 Ni 1/3 Mn 1/3 O 2 [24]. The electrochemical properties may improve after modi cation, and the electrochemical properties will also then be improved. Recently, the applications of RuO 2 in LIBs have engendered substantial interest owing to their high surface area and electronic conductivity. Cathode materials doped by RuO 2 haven't studied in detail. Here, we expect The LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathode material was synthesized by the citric acid-assisted sol-gel method and the preparation procedure is schematically illustrated in Fig.1. The stoichiometric amount of LiNO 3 , and equimolar concentration (0.3 M) of (Ni(NO 3 ) 2 , Co(NO 3 ) 2. 6H 2 O, and Mn(NO 3 ) 2 were dissolved in 100 ml deionized water and then 1M of citric acid solution (50 ml) was added drop wise into the above solution and stirred well by using the magnetic stirrer at 80 o C. Citric acid was used as a chelating agent in the reaction system. Ammonium hydroxide (NH 4 OH) was added to adjust the pH to 9. Then the reaction continued to form a viscous gel. The resulting gel was dried in hot air oven at 120 o C for 8 h to obtain pristine LiNi 1/3 Mn 1/3 Co 1/3 O 2 in powder form. The obtained powder was calcined at 800 o C for 10 h under air atmosphere in a mu e furnace and then allowed to cool naturally to room temperature. The nal product was ground into ne powder using a mortar and kept in a desiccator for further use.
For doping RuO 2 with the pristine material , previously synthesized pristine LiNi 1/3 Mn 1/3 Co 1/3 O 2 was mixed with RuCl 3 in different ratios such as 1, 2 and 3% were dispersed in ethanol and kept for stirring. 100 ml of ammonium hydroxide (NH 4 OH) was added drop wise to start the precipitation process of ruthenium hydroxide on to the surface of pristine material. After the completion of precipitation, the obtained product was washed with copious amount of water to remove the unreacted chloride ions.
Resultant powder was dried at 180°C and further subjected to heat treatment at 600°C for 12 hours to get the RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2.

Electrode preparation and Coin cell assembly
To examine the electrochemical properties of RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 , standard 2032 Coin type cells were assembled under argon atmosphere in a glove box. In a typical procedure, doctor blade method was used to prepare the proposed cathode material. The slurry was prepared with a proportion of 80 weight % of active materials 10 weight % of conductive acetylene black and 10 weight % of PVDF binder dissolved in N-methyl 2-Pyrrolidone (NMP) solvent. The paste was then applied on the aluminum foil current collector and then dried at 120 o C for 12 h in a hot air oven. The working electrodes were prepared by loading the cutting disc lms with a diameter of 1.0 cm into the cleaned and polished aluminum meshes, and then they were pressed under the pressure of 10 MPa for 1 minute to fabricate the cathode material. Lithium sheet (China Energy Lithium Co., Ltd) was served as the negative electrode, and commercial polyethylene (PE) micro porous lm (ND420H129-100, Asahi Kasei Chemical Co.) was used as a separator. The electrolyte solution was 1 mol dm −3 LiPF 6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume).The galvanostatic charge/discharge cycles were carried out at a current density of 20 mA g −1 in the voltage range from 2.5 to 4.2 V on a battery testing system (CT2001A, Wuhan LAND Electronics Co., Ltd) at room temperature. The electrochemical studies were carried out at room temperature on an Autolab PGSTAT30 Potentiostat/Galvanostat electrochemical workstation (EcoChemi, Netherlands) at a scan rate of 0.1 mV s −1 in the potential range between 2.5 and 4.2 V.

Characterization
The surface morphology and the crystallite size were observed from scanning electron microscopy (SEM) using FEI Quanta 250 (FEI Corporation, Japan) instrument, and transmission electron microscopy (TEM) (200kV FEI Tecnai F20). The crystal planes and their structure of the proposed battery material were studied from X-ray diffraction patterns using Rigaku Ultima IV (USA) fully automatic high resolution X-ray diffractometer by employing Cu-K α (λ=1.54Å) radiation. The elemental composition was studied by using XPS (ESCALAB 250xi, Thermo Scienti c) and EDX analysis associated with SEM. The charge transfer resistance R ct values were derived from Electrochemical Impedance spectroscopy. The discharge capacity of LiNi 1/3 Mn 1/3 Co 1/3 O 2 and RuO 2 doped samples was studied using charge discharge curves.
Results And Discussion X-ray diffraction analysis For all the samples, the diffraction peaks were sharp and well de nite revealed the good crystallinity and the crystal structure was recognized as a hexagonal α-NaFeO 2 structure with R3m space group [25,26] that approves the occurrence of layers of Li, Ni, Mn, and Co in a single-phase layered structure. 3wt % RuO 2 doped sample exhibited higher intensity crystal planes than that of other samples indicating the importance of RuO 2 and good crystallinity.  Fig 4c. Lattice fringes are also well agreed with the crystal planes and are shown as Fig. 4d. The wider fringe spacing is 0.47 nm for the (003) planes of the layered structure, and the slightly ner lattice fringe is 0.30 nm for the (111) planes of the rutile structured RuO 2 . However, the fringe spacing of RuO 2 is lower than that of pure RuO 2 crystals (0.31 nm), which is ascribed to the doped Ru element.

XPS analysis
The XPS technique was used to investigate the elemental composition of the as-prepared material and the valence states of elements (Fig 6). However, it can be found that the orbital binding energy of Ni 2p 3/2 is located at 851 eV in Fig. 6a which signi cantly shifts to higher binding energy positions (856 eV) indicating that a part of Ni 2+ turns into higher valence state. According to this result, it could be concluded that partial Ru 4+ ions enter the crystalline lattice of the LiNi 1/3 Mn 1/3 Co 1/3 O 2 material and replace a part of Ni 2+ ions. From the XPS spectra of Ru 3p, it can be seen that the main peaks at about 465.3, 462.8 eV correspond to Ru 3p 5/2 and Ru 3p 3/2 signals of Ru 4+ and Ru 3+ respectively [28] with no signi cant chemical shift. The addition of Ru promotes the generation of more number of oxygen vacancies. In addition, it also facilitates the redox reaction between Ru 4+ and Ru 3+ of RuO 2 and further enhances its oxygen storage capacity. The two main peaks centered at 781.1 eV and 797.2 eV are due to the 2p 3/2 , 2p 1/2 spin orbit splitting of cobalt respectively [29]. The more oxygen vacancies were also veri ed using O 1s spectra of the as-prepared materials. In view of the analysis, the peaks located at 529.2 and 531.2 eV were assigned to lattice oxygen, oxygen vacancies, and chemisorbed oxygen, respectively [30]. Obviously, the content of oxygen vacancies is further increased after the incorporation of Ru. It is clear that Ru doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 possesses more oxygen vacancies, which is favorable for promoting the activation of LiNi 1/3 Mn 1/3 Co 1/3 O 2 . From the above results, it can be concluded that Ru 4+ ions incorporated into the pristine material was expected to enhance the electrochemical performance of the material.
Electrochemical Impedance spectroscopy Electrochemical Impedance spectroscopy is used to study the kinetics during lithium intercalation/deintercalation process. consists of a semicircle in the intermediate frequency ranges followed by an inclined straight line at the low frequency range. In general, the semicircle in the high to medium frequency region is related to the charge transfer resistance (R ct ) [31,32] and in the low frequency region represents the Warburg impedance (Z w ), which is ascribed to Li-ion diffusion in the solid phase state of the electrode material.
According to Fig 7, the R ct value of the RuO 2 doped electrode is smaller than that of the un doped pristine electrode. The diameter of the semicircle for the un doped electrode is 226.79 Ω and that of the doped electrode is 205.47 Ω. Since the diameter decreases for the RuO 2 doped electrode has con rmed the enhancement of conductivity. As a consequence, the electrochemical properties get improved. The equivalent circuit is shown as inset in Fig 7. Electrochemical performance of RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathode material The electrochemical performance of the proposed battery material was investigated by galvanostatic charge/discharge curves at 0.1 C rates in the voltage range between 2.8 to 4.2 V. As seen in Fig 8, the charge/discharge curve plateaus meets at around 3.9 V and the potential drop was observed at 3.6 V warrants the higher energy density. In general, the wider charge/discharge pro le suggests the better stability of the electrode. The discharge capacity for the pristine material was estimated as 194.9 mAhg -1 whereas for 1, 2 and 3 wt % of RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2, the discharge capacity values are calculated as 214.9, 242.9, and 251.2 mAhg -1 respectively. As expected, 3 wt % RuO 2 doped sample exhibits better cyclic performance compared to other electrodes which is well agreed with the literature [33]. The embedded RuO 2 particles suppress the metal ion dissolution and unwanted side reaction between the electrode and electrolyte leads to the enhanced Li transportation. Consequently, rate performance of the proposed cathode material was improved. After 100 cycles, 3.2 % capacity fade was observed for 3% RuO 2 doped sample which may be due to the dissolution of transition metal ions at higher voltage or the electrolyte decomposition.
In order to test the electrode stability and capacity retention, galvanostatic charge /discharge test was carried out (Fig 9). All the electrodes were charged and discharged at 0.1 C rates between 2.8 to 4.2 V for 100 cycles. For the pristine LiNi 1/3 Mn 1/3 Co 1/3 O 2, the initial discharge capacity is 194.9 mAhg -1 , and after 100 cycles the discharge capacity is reduced to 143.27mAhg -1 and 74 % of capacity was retained. In the case of 1% of RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2, 75 % capacity retention was obtained whereas for 2% RuO 2 doped material capacity remains as 217 mAhg -1 and the retention is 89 %. 3% RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 , shown better capacity retention and long lasting stability. The initial discharge capacity is 251.2 mAhg -1 and it was slightly declined to 226 mAhg -1 after 100 cycles indicating the superior performance of the proposed cathode material. The doping strategy revealed the better performance than the previous reports [34]. The capacity retention was determined as 90 %. The addition of RuO 2 facilitates the transportation of Li 2+ ions in the electrode-electrolyte interface. The existence of Ru in crystal lattice could enhance the ability of Li diffusion, more Li ions can be easily extracted from Li layers. Consequently, higher extent of phase transformation from layered Li 2 MnO 3 to certain spinel-like regions could be expected. As a consequence, capacity caused by reinsertion of Li ions into local transformed spinel-like regions during discharge will be increased. Another reason for the higher capacity contribution from spinel-like regions is that the appropriate amount of Ru (x = 0.01) gives rise to a certain impact on transferred spinel-like lattice to enhance Li diffusivity in these regions [35]. It is worth mentioning that, on increasing the cycle numbers the speci c capacity also increases owing to the good activation of the electrodes.
Cyclic Voltammetry is a useful method for determining the structural changes in Lithium intercalation/deintercalation process. To get further information about the electrochemical characteristics, Cyclic Voltammetry experiments were performed in the scan range of 2.8 to 4.2 V at a scan rate of 1 mVs -1 . Cyclic voltammograms of rst six cycles of RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 is shown in Fig 10. A double layer capacitive behavior was observed rather than the typical redox behavior of RuO 2 .

Conclusions
Herein we report the facile synthesis of LiNi 1/3 Mn 1/3 Co 1/3 O 2 and RuO 2 doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 with different weight percentage and demonstrated their improved battery performance for Lithium ion batteries. The proposed samples were characterized by different characterization techniques. The structural elucidation was done by XRD analysis. The surface morphology and chemical composition was studied by TEM and XPS analyses respectively. The stability and the capacity retention were investigated by using galvanostatic charge/discharge curves. About 226 mAhg − 1 discharge capacity was retained after 100 cycles suggested the high speci c capacitance and good cyclic stability of the proposed electrode material. This may be attributed to the presence of RuO 2 doped layer which reduce the barrier for lithium transfer between the electrode-electrolyte interfaces. In addition, the abundant oxygen vacancies in the doped layer signi cantly facilitate the activation the electrode during the initial charging process. Besides, the reduced oxygen loss can effectively inhibit the decomposition of the electrolyte and protect the active substances from being dissolved, thus forming a thin and stable lm. Therefore, the newly developed 3% RuO 2 doped on pristine LiNi 1/3 Mn 1/3 Co 1/3 O 2 may provide an effective strategy to design and synthesize advanced electrode materials for future energy storage devices Declarations