Citation

Long-Wei Liang, Ke Du, Zhong-Dong Peng, Yan-Bing Cao, Guo-Rong Hu.Synthesis and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ as a concentration-gradient cathode material for lithium batteries[J]. Chin. Chem. Lett., 2014,25(06): 883-886  

Long-Wei Liang, Ke Du , Zhong-Dong Peng, Yan-Bing Cao, Guo-Rong Hu    

LiCoO₂ has been widely used as commercial cathode materials for lithium ion batteries,but still has some limitations due to its relatively high cost and the toxicity of cobalt. Research and development of new alternative cathode materials for lithium ion batteries are ongoing,such as layered LiNiO₂,LiMniO₂ and spinel LiMn₂O₄ ^{[1, 2, 3, 4]}. Unfortunately,these materials still have significant drawbacks. LiNi_1-x-yCo_xMn_yO₂ series,which integrate the features of LiCoO₂,LiNiO₂ and LiMniO₂,have attracted much attention for their high capacity,low cost,good cycling stability and safety. It is well known that Ni provides high capacity but poor thermal stability,while Mn maintains excellent cycling performance and safety. And Co offers increased electronic conductivity resulting in an excellent rate capability. In most cases,only Ni and Co are electroactive,whereas Mn has the nominal oxidation state of +4 and is electrochemically inert. Hence,the intrinsic properties of LiNi_1-x-yCo_xMn_yO₂ materials strongly depend on the amount of the metal ions (Ni,Co,Mn) ^{[5, 6, 7]}.

Concentration-gradient structured materials have been studied as cathode material for lithium ion batteries. Sun et al. prepared a LiNi_0.75Co_0.10Mn_0.1502 cathode material where the nickel concentration decreases linearly whereas the manganese concentration increases linearly from the centre to the outer layer of each particle. Huang et al. synthesized spherical LiNixCo1-2xMnxO₂ (x = 0.333,0.4,0.416,0.45) precursors with Co concentrationgradient. Koenig et al. prepared a Li_1.2(Mn_0.62Ni_0.38)_0.802 material, with the transition metal composition at the surface and interior of a particle formed with a continuous gradient in concentration from Ni-enriched centre to Mn-enriched surface ^{[8, 9, 10]}. In this paper, the LiNi_0.6Co_0.2Mn_0.2O₂ material is chosen as the study object. Various methods are applied to synthesize LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials,such as the hydroxide co-precipitation method ^{[11, 12, 13, 14, 15, 16]},the carbonate co-precipitation method ^{[17, 18]},the combustion method ^[19],the solid-state method ^{[6, 20, 21, 22]} and the spray-drying method ^{[20, 22, 23]}. In this research,the Ni_0.6Co_0.2Mn_0.2(OH)2 precursor is prepared by a hydroxide coprecipitated method. In this material,the concentration of nickel decreases gradually from the centre towards the outer layer of the particle,whereas the concentration of manganese increases gradually so that the manganese-rich and nickel-poor outer layer can stabilize the material,especially during high-voltage cycling.

NiSO₄·6H₂O,CoSO₄·7H₂O and MnSO₄·H₂O were used as starting materials. The nickel-rich solution of NiSO₄·6H₂O and CoSO₄·7H₂O (Ni:Co = 0.85:0.15,molar ratio) was named solution A. The manganese-rich aqueous solution of NiSO₄·6H₂O,CoSO₄·7H₂O and MnSO₄·H₂O (Ni:Co:Mn = 1:3:6,molar ratio) was named solution B. Solution A was pumped into solution B in flow rate V. simultaneously,the mixed solution of A and B was agitated vigorously and slowly pumped into a continuous stirred tank reactor (capacity 10 L) in flow rate 2 V. The reactor was under nitrogen atmosphere. The volume of solutions A and B was equal and the two solutions were exhausted at the same time. This protocol allowed the concentration of Ni and Mn fed into the reactor to change continuously,but in opposite direction. At the same time,a 4 mol/L NaOH solution (aq) and an appropriate amount of NH₄OH solution (aq) as a chelating agent were also separately dropped into the reactor. After reacting for a certain time,the precipitate was filtered and washed for several times to remove the residual ions (Na⁺,SO₄ ^2- and other impurity ions). The precipitate was then dried at 80 ℃ overnight. The concentration of the solution,pH value,amount of NH₄OH,stirring speed and temperature in the reactor must be carefully controlled. The obtained spherical precursor and 5% excess LiOH·H₂O powders were mixed thoroughly. The mixture was first calcined at 550 ℃ for 4 h in air,and then heated at 820 ℃ for 12 h in flowing oxygen to obtain the LiNi_0.6Co_0.2Mn_0.2O₂ powders. The synthesized compounds were characterized by XRD (Riguku u/u diffractometer with Cu Ka radiation(r = 1.54056Å ). XRD data were obtained at 2u = 10-808,with a scan speed of 28/min. The particle morphology of the powders was observed using a scanning electron microscopy equipped with an EDXS energy disperse X-ray spectrometer (SEM, Hitachi X-650). The chemical composition of the resulting compounds was analyzed by atomic absorption spectroscopy (AA spectrophotometer,Zhimadu 6650). The charge/discharge tests were carried out by assembling 2025-type coin cells with a lithium metal anode,working cathode and Celgard 2400 microporous membrane. The working cathode was fabricated by 80 wt% LiNi_0.6Co_0.2Mn_0.2O₂,10 wt% acetylene black and 10 wt% PVDF binder. A 1 mol/Lsolution of LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate(DMC) (1:1,v/v) was employed as the electrolyte. The charge-discharge tests were set to be charged and discharged between 2.8 and 4.3 V at 0.1 C rate in the first 2 cycles and then at 1 C rate in the following 100 cycles at 25 ℃ or 60 ℃. Cyclic voltammograms were measured by a Solartron 1287 electrochemical interface between 2.8 and 4.5 V at a scan rate of 0.1 mV/s.

Fig. 1 displays the XRD pattern of the Li[Ni_0.6Co_0.2Mn_0.2]O₂ material. The XRD pattern shows a clear split between the 006/012 and 108/110 peaks,which means that the materials have a better hexagonal structure. All diffraction peaks are indexed on the basis of the α-NaFeO₂ structure (R3m) and no extra diffraction peaks from related secondary phases or impurities exist. Unit cell parameters are calculated and summarized in Fig. 1. The lattice parameters a and c are given as 2.8688 and 14.2198,which are similar to the values reported in the literature references [12- 14,16]. The intensity ratio of I(0 0 3)/I(1 0 4) is a sensitive parameter for determining the cation distribution in the lattice,where a value lower than 1.2 indicates a high degree of cation mixing,due primarily to the occupancy of other ions in the lithium region,and the higher the ratio is,the lower the level of cation mixing and the more beneficial to the lithium-ion transfer ^[22]. The intensity ratio I(0 0 3)/I(1 0 4) of this material is 1.485,which is larger than 1.2,implying that no obvious cation mixing is present in the LiNi_0.6Co_0.2Mn_0.2O₂ sample. In addition,the c/a ratio of this material is greater than 4.9,a value that generally represents a material with layered characteristics. Thus good electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ material can be expected.

Fig. 1

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Fig. 1. XRD pattern of concentration-gradient LiNi0.6Co0.2Mn0.2O2.

The SEM images of the concentration-gradient precursor and lithiated oxides are shown in Fig. 2(a) and (b). The morphology of the powders is nearly spherical and the estimated particle size is about 10 mm in diameter. Furthermore,the spherical morphology remains even after the high temperature calcinations. The crosssection of concentration-gradient precursor particles is also tested by EDXS. As shown in Fig. 2(c),the points collected in the EDXS spectra are labelled as 1,2,3,4 and 5 from outside. The concentration of Ni and Mn distribution tested by EDXS is shown in Fig. 2(d). It is obviously observed that the concentration of nickel decreases gradually from the centre towards the outer layer of the particle,whereas the concentration of manganese increases gradually,consistent with the experimental design. Atomic absorption spectroscopy is employed to measure the final elemental ratio in the sample. The atomic ratio of Li:Ni:Co:Mn in the LiNi_0.6Co_0.2Mn_0.2O₂ material is determined to be 1.03:0.602:0.196:0.202,which is nearly equal to the determined stoichiometric ratio.

Fig. 2

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Fig. 2. SEM images of concentration-gradient Ni0.6Co0.2Mn0.2(OH)2 (a),LiNi0.6Co0.2Mn0.2O2 powders (b),the cross-section of precursor particle (c) and the concentration of Ni, Co and Mn in particle by EDXS testing (d).

Fig. 3(a) exhibits the initial charge-discharge curves and rate cyclability of the LiNi_0.6Co_0.2Mn_0.2O₂ cells from 0.1 C to 10 C in the voltage of 2.8-4.3 V at 25 ℃. The cells are first cycled at 0.1 C (18 mA/g) and then at 0.2 C,0.5 C,1 C,2 C,5 C and 10 C for every five cycles. The cells deliver an initial discharge capacity of 191.5, 185.5,180.2,174.3,168.7,158.8 and 148.5 mAh/g from 0.1 C to 10 C,respectively. Excellent cycling performance can also be seen from Fig. 3(b). Even when discharged at 10 C,a capacity as high as 148.5 mAh/g is achieved,indicating that the LiNi_0.6Co_0.2Mn_0.2O₂ materials synthesized under the optimal conditions have superior rate capacities that have not been reported in the previous research ^{[11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]}. Fig. 3(c) illustrates the charge-discharge profiles of assynthesized Li[Ni_0.6Co_0.2Mn_0.2]O₂ cathode at 1 C rate in the voltage range of 2.8-4.3,4.4 and 4.5 V,with the initial discharge capacities of 174.3,181.7 and 198.6 mAh/g,respectively. For elevated temperature test (60 ℃),the material delivers an initial discharge capacity of 196.9 mAh/g. It is very impressive that the first reversible discharge capacity at 1 C rate in the voltage range of 2.8- 4.3 V is as high as 174.3 mAh/g,which is higher than that of the similar solid solutions containing the same amount of Ni reported before ^{[12, 13, 16]}. The discharge capacity as a function of cycle number for the Li[Ni_0.6Co_0.2Mn_0.2]O₂ cathode cycled at 1 C rate in the voltage of 2.8-4.3,4.4 and 4.5 V is given in Fig. 3(d). In the voltage of 2.8-4.3 V,the discharge capacity gradually decreases from 174.3 mAh/g at the initial cycle to 162.3 mAh/g at the end of 100cycles,with a capacity retention of 93.1%. Between 2.8 and 4.4, 4.5 V,the discharge capacity decreases from 181.7,198.6 mAh/g to 162.9,169.2 mAh/g,respectively,after the same cycles. The corresponding capacity retentions are 89.6% and 85.2%,respectively. In the elevated temperature test (60 ℃),more than 89.7% of the initial discharge capacity is retained after 100 cycles. Therefore, the material exhibits the remarkable electrochemical performance especially the long cyclability,which is superior to the previously reported research results ^{[12, 16]}. The main reason for the outstanding cycle performance is largely attributed to the stable outer layer having lower Ni content and higher Mn content compared to core,which can reduce the cation mixing in the Nidepleted outer shell layer and dissolution of metal ions in the active material and stabilizes the surface structure of the oxide during the charge and discharge process ^[8].

Fig. 3

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Fig. 3. The initial charge-discharge curves (a) and Rate cyclability (b) of LiNi0.6Co0.2Mn0.2O2 cells from 0.1 C to 10 C in the voltage of 2.8-4.3 V and The initial charge-discharge curves at a 1 C rate with raised upper cut-off voltage limit (c) and Cycling performance of LiNi0.6Co0.2Mn0.2O2 at 25 ℃ or 60 ℃ (d).

Cyclic voltammetry of the sample between 2.8 V and 4.5 V at a scan rate of 0.1 mV/s for the first,second and 40th cycle were carried out to investigate the structural stability of Li[Ni_0.6Co_0.2Mn_0.2]O₂ and the results are shown in Fig. 4. It can be seen that only one redox couple exhibits in the C-V curves, illustrating that no structural transitions exist from hexagonal to monoclinic in the voltage range of 2.8-4.5 V. It is clearly shown in Fig. 4 that the major anodic peak shifts from 3.781 V at the first cycle to 3.774 V and 3.768 V at the second and 40th cycle, respectively,while their corresponding redox reaction gaps are 82, 75 and 69 mV. However,there is no obvious difference observed in the cathodic peaks (3.699 V). Furthermore,there is no significant intensity fading among the first,second and 40th cycle, demonstrating the good reversibility of the Li+ ions in the compound during the intercalating and de-intercalating processes.

Fig. 4

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Fig. 4. Cyclic voltammograms of LiNi0.6Co0.2Mn0.2O2 cell at a scan rate of 0.1 mV/s.

In summary,the spherical Ni_0.6Co_0.2Mn_0.2(OH)2 precursor with Ni and Mn concentration-gradient was successfully prepared by a hydroxide co-precipitated method. Well-ordered layered Li[Ni_0.6Co_0.2Mn_0.2]O₂ powders were successfully synthesized by mixing the as-prepared precursor and 5% excess LiOH·H₂O. The sample delivered an initial discharge capacity of 174.3 mAh/g and a capacity retention of 93.1% after 100 cycles when cycled at 1 C between 2.8 V and 4.3 V,as well as excellent rate capability,high cut-off voltage and temperature performance. Therefore,the concentration-gradient structured LiNi_0.6Co_0.2Mn_0.2O₂ cathode material prepared by co-precipitation is a promising cathode material for lithium batteries,which is better than the similar samples containing the same amount of Ni reported before.