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Tuning electrochemical properties of Li-rich layered oxide cathode by adjusting Co/Ni ratio and mechanism investigation using in situ XRD and OEMS Shou-Yu Shen, Yu-Hao Hong, Fuchun Zhu, Zhenming Cao, Yu-Yang Li, FuSheng Ke, Jing-Jing Fan, Li-Li Zhou, Li-Na Wu, Peng Dai, Ming-Zhi Cai, Ling Huang, Zhi-You Zhou, Jun-Tao Li, Qi-Hui Wu, and Shi-Gang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00919 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Applied Materials & Interfaces

Tuning electrochemical properties of Li-rich layered oxide cathode by adjusting Co/Ni ratio and mechanism investigation using in situ XRD and OEMS ShouYu Shen,1 YuHao Hong,1 FuChun Zhu,1 ZhenMing Cao,1 YuYang Li,1 FuSheng Ke,2 JingJing Fan,1 LiLi Zhou,1 LiNa Wu,1 Peng Dai,1 MingZhi Cai,1 Ling Huang,*,1 ZhiYou Zhou,1 JunTao Li,3 QiHui Wu,4 ShiGang Sun*,1,3 1. College of Chemistry and Chemical Engineering, Xiamen University, Fujian, 361005, China. 2. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China 3. College of Energy and School of Energy Research, Xiamen University, Fujian, 361005, China 4. Department of Materials Chemistry, School of Chemical Engineering and Materials Science Quanzhou Normal University, Quanzhou, 362000, China

KEYWORDS: voltage decay, Li-rich material, in situ XRD, OEMS, electrochemistry

ABSTRACT: Owing to high specific capacity of ~250 mAh g-1, lithium-rich layered oxide cathode materials (Li1+xNiyCozMn(3-x-2y-3z)/4O2) have been considered as one of the most promising candidates for the next generation cathode materials of lithium ion batteries. However, the commercialization of this kind of cathode materials is seriously restricted by voltage decay

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upon cycling though Li-rich materials with high cobalt content have been widely studied and show good capacity. This research successfully suppresses voltage decay upon cycling while maintaining high specific capacity with low Co/Ni ratio in Li-rich cathode materials. Online continuous flow differential electrochemical mass spectrometry (OEMS) and in situ XRD techniques have been applied to investigate the structure transformation of Li-rich layered oxide materials during charge-discharge process. The results of OEMS revealed that low Co/Ni ratio lithium-rich layered oxide cathode materials released no lattice oxygen at the first charge process, which will lead to the suppression of the voltage decay upon cycling. The in situ XRD results displayed the structure transition of lithium-rich layered oxide cathode materials during the charge-discharge process. The Li1.13Ni0.275Mn0.580O2 cathode material exhibited high initial medium discharge voltage of 3.710 V and 3.586 V medium discharge voltage with the lower voltage decay of 0.124 V after 100 cycles.

1. INTRODUCTION Featuring high discharge capacity over 250 mAh g-1

1-4

, Li-rich layered oxide cathode

(referred as high energy NCM materials, HE-NCM) has been selected as one of the most promising cathode materials for it meets the requirements of energy density in electric vehicles and hybrid electric vehicles. HE-NCM cathodes, unfortunately, are challenged by low initial Coulombic efficiency, low tap density, poor rate capability, fast capacity decay, low discharge voltage plateau and voltage decay of discharge plateau upon cycling. Although surface coating57

, lattice doping8-10, binder11-12 and electrolyte additive13 have been developed to address these

problems, low discharge voltage plateau and voltage decay of the discharge plateau upon cycling


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are still not effectively improved. Cobalt and nickel are the most two important components in HE-NCM cathodes, and their content has been found to dramatically affect the electrochemical performance of the cathodes14. HE-NCM cathodes with a certain amount of cobalt ratio that show super specific capacity even at high rate have been widely studied. But high cobalt content materials always present relative low discharge voltage plateau and suffer serious voltage decay upon cycling, which hinders its commercialization. Recently, Manthiram et al. investigated the effect of the plateau region at 4.5 V on the voltage decay of HE-NCM by varying the cobalt content15. However, the results were limited in charge-discharge and dQ/dV profiles, the suppressing mechanism of the voltage decay still leaves unclear. Also the ratios of Mn/(Co+Ni) varied from 1.5 to 2.08 in that study, which is of little significance because the range is still limited. What’s more, the study neglected the dramatic change of initial medium discharge voltage caused by the varying Co and Ni contents. Yet, the study is worth attention because improving initial medium discharge voltage is also an effective way to enhance the energy density of lithium-ion battery16. In this research, four HE-NCM materials were prepared with various Co/Ni ratio of 0.0, 0.5, 1.0, 2.0. Therefore these HE-NCM cathodes can display different initial medium discharge voltage and voltage decay upon cycling. The corresponding chemical formulas being Li1.13Ni0.275Mn0.580O2,

Li1.13Ni0.181Co0.089Mn0.560O2,

Li1.13Ni0.130Co0.135Mn0.551O2,

and

Li1.13Ni0.0860Co0.174Mn0.544O2, respectively, in which the Mn/(Ni+Co) ratio was fixed at 2:1. The results demonstrated that the electrodes with low Co/Ni ratios showed lower voltage decay than those with high Co/Ni ratios. In order to investigate the mechanism behind initial medium discharge voltage and voltage decay upon cycling, online continuous flow differential electrochemical mass spectrometry (OEMS) and in situ XRD were employed. The OEMS results


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indicated that the HE-NCM materials with high cobalt released more oxygen in the first charging process and suffer from seriously voltage decay upon cycling while these with no cobalt released no oxygen and delivered relatively higher initial medium discharge voltage, lower voltage decay and relatively high specific capacity. Particularly, the Li1.13Ni0.275Mn0.580O2 material exhibited the highest initial medium discharge voltage of 3.710 V and the lowest voltage decay of 0.124 V after 100 cycles.

2. EXPERIMENTAL SECTIONS Preparing Samples: all samples were prepared by evenly mixing the co-precipitated carbonates precursor of Ni, Co, Mn with Li2CO3 in 70 mL ethanol with a 200 mL beaker. After the ethanol went evaporated at 70 °C with magnetic stirring, the mixtures were preheated at 450 °C for 6 h and then calcined at 850 °C for 12 h in the air with 5°/min heating rates. The co-precipitated carbonates were prepared with following steps. Firstly, 2 M metal-ions containing solution of MnSO4·H2O (99.0%, Sinopharm Chemical Reagent Co., Ltd), CoSO4·7H2O (99.5%, Sinopharm Chemical Reagent Co., Ltd), NiSO4·6H2O (98.5%, Sinopharm Chemical Reagent Co., Ltd) (Mn : Co : Ni = 0.667 : x : (0.333-x) , molar ratio), 2 M Na2CO3 (≥ 99.8%, Guangdong Guanghua Sci-Tech Co., Ltd.) solution, 0.56 M NH4HCO3 (21.0%~11.0%, only NH3 was counted), Sinopharm Chemical Reagent Co., Ltd) solution, and 1 M NaOH (96.0%, Guangdong Guanghua Sci-Tech Co., Ltd.) solution were prepared. Secondly, the metal-ion containing solution, Na2CO3 solution and NH4HCO3 solution were pumped into a self-designed CSTR at the rate of 0.6 mL/min with 800 rpm continuous stirring at 55 °C. Thirdly, the reactor was filled with 1.5 L deionized water before reaction and the pH, with the help of a pH controller, was kept 8.00 by adding NaOH solution. Fourthly, the carbonate precursors were obtained after 20 h of reaction


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and were separated from the CSTR by Buchner funnel. Finally, after being washed by deionized water for several times, the precursors were dried in vacuum at 100 °C over one night. Electrodes Preparation and Electrochemical Tests: CR2025 coin cells were used to test the electrochemical performance of the samples. The cathode electrodes consist of 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF). The active materials and acetylene black were first mixed and then manually ground before being mixed with the PVDF in N-methyl-2-pyrrolidinone and stirred for 5 h. The slurry was spread onto 0.02 mm Al foil current collector by using a simplified manual coater device and dried at 110 °C for 10 h in vacuum. The dried films were punched into circular disks whose diameter was 16 mm. The cells were assembled in an argon glove box(water < 0.1 ppm, O2 < 1 ppm) with Li metal as the anode, Celgard 2400 membrane as the separator and LB-111 (main components: 1 M LiPF6 in ethylene carbonate(EC)–dimethyl carbonate(DMC) mixture (1:1 by volume), DoDoChem) as electrolyte. The constant current charge–discharge tests were carried out on a LAND battery program–control test system (LAND-V34, Land Electronic Co., Ltd., Wuhan) between 2.0 and 4.8 V at 30 °C. All in situ XRD cells charge and discharge at 40 mA g-1 or 200 mA g-1 under 2.0 - 4.8 V. Material Characterization: X–ray diffraction (XRD) data were collected on a Bruker D8 discover powder diffraction with Cu Kα radiation at 40 kV and 40 mA. All ex situ XRD data were recorded by LYNXEYE Detector (1-D detector) at a scan rate of 1 ° min-1, from 10° to 90°. All in situ XRD data were collected by VANTEC500 detector (2-D detector) with 1 mm collimator at a scan rate of 20 °/


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step from 20° to 60° on the same instrument. Each spectrogram took 15 minutes with 5 min per step. The surface crystal morphology and element energy spectrum were characterized by field– emission scanning electron microscopy (FE–SEM, SIGMA, ZEISS microscope, 15 KV or 20KV). Raman spectra were recorded on a Raman spectrometer (Dilor-Raman, LabRam I) coupled with microscope in a reflectance mode with a 532 nm excitation laser source. All the OEMS experiments conducted in a custom-made cell. The cells were assembled in an argon-filled glove box to avoid any negative effect from air. The OEMS experiment procedures are listed as follows. Firstly, the slurry with 80 wt% active materials, 10 wt% conductive carbon and 10 wt% of PVDF binder was coated on a 16 mm diameter carbon paper and then dried (loading mass of active material: 10~15 mg ) to prepare the cathode films,. Secondly, 16 mm diameter disks and 2 mm thick Lithium metal foils were used as counter electrodes. 19 mm diameter Celgard 2400 disks were used as separator. Thirdly, the cathode electrodes were assembled into a 2025 coin cell in which a designed hole was punched at the side of cathode electrodes. Fourthly, after the coin cell was assembled into the custom-made cell, it will be joined up to the test system. The pre-dehydrate Helium (99.999%) was used as carrier gas to carry out the mixture. High flow rate will be used to purge the pipeline to remove the argon gas. Then the flow rate will be controlled at 8 mL/min by a gas controller. The gas product during the charge-discharge process will be carried out to pass through a cold trap (mixture of dry ice and ethanol, -78.5 °C, 1atm) to condense the electrolyte vapor before entering the mass spectrometer


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(modified 5975C mass selective detector, Agilent). The schematic diagram of OEMS cell and OEMS system is show in Figure S11. Transmission electron microscopy (TEM, JEOLJEM-2100 microscope or TECNAI-F30 microscope) were employed to characterize the morphology and crystal structure of the products.

3. RESULTS AND DISCUSSION 3.1. Material Characterization and Electrochemical Properties The morphology and composition for these materials were characterized by SEM and EDS (Figure 1). SEM images show that all the four materials are spherical secondary particles (15-20 µm) consisting of nanoparticles (~100 nm). EDS results show that Co/Ni ratios of four cathodes are 1:0, 2:1, 1:1, 1:2, respectively, which is consistent with the feed ratios. The layer structure of HE-NCM materials are illustrated in Figure S2, which is usually considered as an integration of LiMO2 (R-3m) and Li2MnO3 (C/2m) at an atomic level, crystal lattice parameters a = b ~ 2.8 Å, c ~ 14.2 Å. The layered structure of these materials were confirmed by XRD (Figure S9). Coin cells were assembled to evaluate the electrochemical performance of those HE-NCM materials, which is recorded in Figure 2 (cycle performance) and Figure 3 (voltage information).


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Figure 1. (a)-(d) exhibit the SEM images and (c)-(d) exhibit the the corresponding EDS results. SEM images prove that all the four materials are spherical secondary particles containing nanoparticles. EDS results show Co/Ni value of all the four samples are 1:0, 2:1, 1:1, 1:2 which is the same as was designed.


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Figure Figure 2. (a) The first

charge-discharge curves for Li-rich materials with different Co/Ni ratios at

0.2 C; (b) cycling performance of Li-rich materials with different Co/Ni ratios at 0.5 C; (c) cycling performance of Li-rich materials with different Co/Ni ratios at 1.0 C; (d) the initial discharge capacity and capacity retention at 100th cycles at the current of 0.5 C and 1.0 C. Figure 2a compares the initial charge-discharge curves of materials with different Co/Ni ratio at the current of 40 mA g-1 (0.2 C) from 2.0 to 4.8 V. As the Co/Ni ratio decreases from 2.0 to 0.0, the charge capacity contribution below 4.40 V goes up, whereas the platform length related to Li2MnO3 becomes shorter. What’s more, apparent medium discharge voltage increase can be observed (Figure 2a). The cycling performance of materials with different Co/Ni ratio are explained in Figure 2b and 2c. For 1 C rate, the specific capacity of the electrode with Co/Ni=2.0 at the 3rd cycle and after 100 cycles stands at 232 mAh g-1 and 170 mAh g-1, respectively. That


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is to say, this material suffers both relative high capacity degradation and fast voltage decay upon cycling (Figure 3).

Figure 3. Normalized discharge profiles of the 3rd, 25th, 50th, 75th and 100th cycles of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0, and (e) medium discharge voltage corresponding to Figure 2 (b).


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The 3rd and 100th medium discharge voltage is 4.544 V and 3.055 V at the expense of 0.400 V voltage decay. When Co/Ni=1.0, the specific capacity at the 3rd cycle and after 100 cycles turn out to be 226 mAh g-1 and 181 mAh g-1. Though capacity degradation and voltage decay were improved, the performance is still unsatisfactory. The 3rd and 100th medium discharge voltage is 3.501 V and 3.127 V, respectively, with 0.374 V voltage decay. The electrode with Co/Ni=0.5 exhibits high discharge capacity and capacity retention rate with 238 mAh g-1 and 207 mAh g-1 at the 3rd cycle and after 100 cycles, respectively. What’s more, the initial medium discharge voltage at the current of 200 mA g-1 were improved to 3.623 V, much higher than the electrode with Co/Ni=2.0 and 1.0. But the voltage decay of the electrode with Co/Ni=0.5 is still astonishing, i.e. 3.294 V at 100th cycles with 0.358 V degradation. A further decrease of cobalt substitution ratio (Co/Ni=0), though giving lower specific capacity than the electrode with Co/Ni=0.5 (with 226 mAh g-1 and 194 mAh g-1 at the 3rd cycle and after 100 cycles, correspondingly), yields higher initial medium discharge voltage (3.710 V) and better suppression on voltage decay (3.586 V after 100 cycles with only 0.124 V decline). Seeing from the above results with the substitution of nickel by cobalt, the initial medium discharge voltage decreases. The Li1.13Ni0.275Mn0.580O2 material with no cobalt substitution exhibits the highest initial medium discharge voltage of 3.710 V at the current of 200 mA g-1, and 3.455 V after 100 cycles with the lowest voltage decay of 0.255 V. 3.2. Mechanism analysis for initial medium discharge voltage decline and voltage decay upon cycling OEMS and in situ XRD were employed to have a better understanding over the mechanism for initial medium discharge decline and voltage decay upon cycling when nickel is substituted by cobalt.


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3.2.1. OEMS Experiments One of the most outstanding differences between the HE-NCM and traditional layered NCM materials is the initial charge-discharge cycle. For HE-NCM materials, the initial charge process contains two regions, one below 4.40 V which is almost the same as NCM materials, and another

Figure 4. Plots of CO2 (m/z=44) and O2 (m/z=32) evolution for the first cycle. The electrodes with Co/Ni = (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0. above 4.40 V which is attributed to the activation of Li2MnO3. Because of the particularity of the first cycle of the HE-NCM, especially the initial charge process, the mechanism of the first charge-discharge process has been widely concerned. In the last few years, OEMS has been applied to track the gaseous productions during the initial cycle. And oxygen gas was observed at the end of the first charge process. The observation of the oxygen gas directly evidences its involvement in the redox reaction during the activation of the Li2MnO317-19. The OEMS results


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of the electrodes with Co/Ni= 0.0, 0.5, 1.0, 2.0 are shown in Figure 4 (a)-(d). The red lines of m/z is 32, which can be ascribed to oxygen gas. The oxygen gas in the electrode with Co/Ni= 0.0 and 0.5 is very low. Compared with little oxygen gas in the electrode with Co/Ni= 0.0, small amount of oxygen gas can be observed in the electrode with Co/Ni= 0.5 after amplifying the high voltage regions around 4.80 V. In the electrode with Co/Ni= 1.0, the peak of oxygen gas can be clearly observed. When the Co/Ni= 2.0, large amount of oxygen can be observed. An OEMS experiment on LiNi0.8Co0.1Mn0.1O2 was also made by using the same electrolyte and chargedischarge between 2.8-4.8 V at the first cycle (Figure S3).Unfortunately, no oxygen gas was observed in this experiment, which means the oxygen gas observed in HE-NCM electrodes comes from the HE-NCM materials but not the electrolyte. By tracking the amount of oxygen gas observed from different materials, it can be found that the more nickel is substituted by cobalt, the more lattice oxygen is likely to be divorced from the materials in the form of oxygen gas. The loss of oxygen may result in initial medium discharge voltage decline. As-synthesized and 1st cycled HE-NCM materials are denoted as Li1.13NixCoyMnzO2 and Li1.13-aNixCoyMnzO2-b, accordingly. According to the previous studies, the valence of nickel and cobalt will be reduced to +2 and +3 at the end of the first cycle, respectively, and the valence of manganese will be reduced to +3.5 and +420-22. The initial charge-discharge columbic efficiencies of the electrodes with Co/Ni= 0.0, 0.5, 1.0, 2.0 are 88%, 86%, 90%, 90%, respectively, which is not quite different from each other. That means the lithium ions content in Li1.13-aNixCoyMnzO2-b is almost the same after the first cycle. Our viewpoint is that the reduction of Mn4+ in the discharge process will be affected by the activation of Li2MnO3. Oxygen loss in the charge process will reduce the oxygen content in cathode material, when lithium ions intercalate to the cathode at low discharge voltage, Mn4+ will be reduced. Based on the similar columbic efficiency and


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charge balance in Li1.13-aNixCoyMnzO2-b, we infer that the more oxygen loss in the charge process, the more Mn4+ will be reduced in the discharge process. It has been proved that the reduction voltage of manganese from +4 to +3 occurred between 3.6 V to 2.0 V at the charge and discharge current of 10 mA g-1

22

. Therefore, it is believed that the increased manganese ions

reduction at low discharge voltage is an important reason of the initial medium discharge voltage decline of the electrodes with Co/Ni= 1.0 and 2.0. What is more, it is widely accepted that the nickel can be oxidized from +2 to +421 while cobalt can only be partially oxidized from +3 to +3.623. With the substitution of nickel by cobalt, the totally contribution of nickel and cobalt in discharge voltage will fade. The reduction reaction of nickel and cobalt is believed to take place between 4.00 V to 3.30 V21 (in our experiment, this region is determined from 4.00 V to 3.50 V at a current of 40 mA g-1 and from 4.00 V to 3.35 V at a current of 200 mA g-1), but manganese reduction voltage is lower. As a result, the substitution of nickel by cobalt will also reduce the initial medium discharge voltage. All in all, the initial medium discharge voltage decline is affected by the manganese ions reduction at relative low discharge voltage and the nickel ions and cobalt ions reduction at relative high discharge voltage. Articles explaining CO2 evolution about Li-rich materials are rare. In our research, three peaks of CO2 are observed during the first charge-discharge process. The first peak appeared at the voltage between 4.2~4.5 V. The second peak appeared at the end of the first charge process. And the third peak appeared at the end of the first discharge process. The CO2 evolution voltage of NCM811 cathode material began at 4.2 V (Figure S3). The CO2 evolution of Li-rich cathode materials appeared at the same voltage (Figure 4). This means the first peak of CO2 caused by the oxidation of the electrode, is related to the deintercalation of lithium ions from the traditional layered component. According to Imhof and Novak’s report24,


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in EC/DMC electrolyte, CO2 evolution was detected only at LiNiO2 electrodes at the voltage of 4.2 V. Whereas no CO2 was observed at LiCoO2 and LiMn2O4 electrodes. The information above implies that the nickel ions with high valence around the interface have a stronger oxidizing ability to the electrolyte. While manganese and nickel ions with high valence have little effect on oxidation to the electrolyte at the voltage of 4.2 V. So the first peak of CO2 evolution between 4.2~4.5 V is considered to be caused by the high nickel ions’ strong oxidizing ability to the electrolyte. When the voltage reaches 4.5 V, the Li2MnO3 component is activated, resulting in the oxidation of oxygen ions and the suppression of the electrolyte decomposition. That is why the amount of CO2 evolution drops. At the end of the charge process, the activation of the Li2MnO3 component will finish and the oxidation of the electrolyte by nickel ions will be activated again, giving rise to second peak of CO2 evolution. The third peak at the end of the first discharge process may be caused by the decomposition of electrolyte at the surface of lithium metal foil. 3.2.2. In situ XRD Experiments In situ XRD experiments were carried out to investigate the structure change of HE-NCM materials during the charge/discharge processes. In situ XRD spectra were collected during the first three cycles at the current of 40 mA g-1 for all the four electrodes. Figure 5 shows contour maps of in situ XRD and the corresponding charge-discharge curves of the electrodes with Co/Ni= 0.0, 0.5, 1.0 and 2.0 at the first three cycles. The change of


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intensity and angle is clearly presented in the contour maps, The unchanged peaks at 41.3°, 45.8°, 51.0°, 52.9° can be assigned to BeO or Be of in situ XRD cell window. The four contour maps unveil a huge difference between peaks intensity at low angle and peaks shift at high angle. To better understand the rationale behind the marked change regions, partial enlarged 3D view


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Figure 5. Contour maps and 3D view graphics between 15° to 22° and 57° to 70° of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0.


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graphics between 15° to 22° and 57° to 70° are listed (the full graphics are shown in Figure S5). Compared with the full charge states of the four samples, not only (003) peak positions of the electrodes with Co/Ni= 1.0 and 2.0 apparently shift to higher degrees, which means the value of parameter c shrinks dramatically in the full charge station, but also the peak intensity of these two electrodes plummet. What’s more, (018) peak intensity of the electrodes with Co/Ni= 1.0 and 2.0 decreases more severely or even disappear. The XRD patterns of the electrodes with Co/Ni= 1.0 and 2.0 at the full charge states are similar to spinel structure (which can be seen more clearly by 1D view in Figure S4). It is believed that the above-mentioned dramatic change of the peaks may increase material instability and is easier to result in a spinel-like or spinel structure upon cycling. And the large structure changes of the electrodes with Co/Ni= 1.0 and 2.0 might be caused by the oxygen released, which is observed by OEMS during the initial charge process. All the data of four materials were refined by TOPAS software. The refined cell parameters of a, c, V and corresponding charge-discharge voltage are drawn in Figure 6. The value change of c and a will be discussed. In the charge region below 4.45 V, lithium ions are deintercalated from Li layers, which intensifies the electrostatic repulsion among oxygen layers, making c value 25. In the 4.5 V plateau region where Li2MnO3 component is active, the lithium ions will migrate from metal layer to Li layer to make up for the lithium ions loss in the Li layer, keeping the c value constant. At the end of fully charged stage, large-scale lithium ions will be deintercalated from the layered structure. Without or with small amount of lithium ions in the layer, lithium layer structure would be compromised and shrink, leading to a sudden drop of c value26. The change of parameter a is attributed to the increase or decrease of the metal-metal distance which is caused by the oxidization or reduction of metal ions in the metal layer in layer cathode


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materials. As a result, the valence state of metal ions can also be inferred from the change of parameter a and its different slopes during the charge-discharge process indicating different redox process. The mechanism of initial voltage decline and voltage decay upon cycling may be better understood when the discharge curves are more accurately divided and specified.

Figure 6. Change in lattice parameters as a function of electrochemical charge/discharge profile during the first three cycles. The electrode with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0.

The change of parameter a is explained separately during the charge-discharge process as follows. Initial charge process: It was confirmed that the value of manganese remains +4 throughout the first charge process20-21. As a result, the change of parameter a is mainly caused by the


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oxidization of nickel and cobalt in this region. With the extraction of lithium ions from the lithium layer, a value drops as a result of the oxidation of Ni2+ (0.69 Å) and Co3+ (0.61 Å) to Ni3+ (0.56 Å)/Ni4+ (0.48 Å) and Co4+ (0.53 Å), respectively. It continues until the voltage arrives at 4.4 V. Particularly, parameter a is almost decreasing in a linear fashion in this region. However, the value of parameter a will remain as a constant when the potential reaches 4.40 V. That said, metal-metal distance stays unchanged and the state of all metal ions remains the same at 4.40 V in this region. Seeing from these data, it is reasonable to speculate that the oxygen must have been oxidized from -2 to a higher state or even oxygen gas has been released from the material, which was confirmed by the OEMS experiment rather than some researchers’ inference that manganese is oxidized to a high valence state27-28. Initial discharge process: During the discharge process, parameter a went up because of the reduction of Ni4+, Co4+ and Mn4+ to Ni3+/Ni2+, Co3+ and Mn3+ (Mn4+ will be only partial reduced to Mn3+

21, 29

). Based on the in situ XRD experiment, the discharge curves can be divided into

three regions (region I, II, III) for all the samples and two sub-regions (I1, I2) for the electrodes with Co/Ni= 1.0 and 2.0 according to the different slope of parameter a. Region III covers 4.80 V to 4.00 V, region II includes 4.00 V to 3.50 V and region I extends from 3.50 V to 2.00 V. Region I1 is from 3.50 V to 3.06 V while I2 3.06 V to 2.00 V. In region III, parameter a remains constant, which means the capacity in this region stems from the reaction at oxygen sites. In region II, parameter a goes up steadily where the capacity contribution in this region can be assigned to the reduction of Ni4+ and Co4+ to Ni3+/Ni2+and Co3+ (it will be explained below), respectively. In region I, parameter a continues to increase but with a much smaller slope where the capacity contribution can be assigned to the partial reduction of Mn4+ to Mn3+.


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For a more detailed description in region I and II of the first discharge process, the slopes in these regions were fitted and compared with these in the first charge process. Here the electrode with Co/Ni=1.0 at the current of 40 mA g-1 is taken as an example as is shown in Figure 7a (the fitting data are listed in Figure S6). The absolute slope value of charge process is 1.4465E-4 and that of region II is 1.10761E-4, closed to the absolute slope value of charge process. As is general consent that the charge process below 4.40 V in the first cycle is caused by the oxidization of nickel and cobalt, it can be inferred that region II can be assigned to the reduction of Ni4+ to Ni3+/Ni2+ and Co4+ to Co3+, respectively. The slope of parameter a value in region I is dramatically different from that in region II. Region I can be assigned to the reduction of Mn4+ to Mn3+. The slope of parameter a value in region I1 and I2 is 6.29365E-6 and 1.61047E-4. Different slopes mean reduction reaction at different rates. It is inferred that there are two different reduction reactions of manganese in region I, which may be caused by layer manganese reduction and/or spinel manganese reduction. Based on the previous reports that the redox potentials are placed in the order of O2- > Ni4+/Ni3+ > Ni3+/Ni2+ > Mn4+/Mn3+ 21, 30-32. The order of redox potentials

is fairly consistent with our results. Yang’s experiment by in situ X-ray

absorption spectra 21 is better matched the divided regions of I, II, and III in this experiment by in situ XRD. To divide the discharge process at 1 C (200 mA g-1) more accurately, the electrode with Co/Ni= 1.0 in the first four cycles of in situ XRD experiment at 1 C is further analyzed in Figure 7b (the fitting data are shown in Figure S7). The absolute slope value of charge process is 6.85833E-4. It is 6.63483E-4 in region II, which is closed to that of charge process. The absolute slope of a value in region I is 3.5565E-4. This is a distinct difference from the experiment done at 0.2 C as only one region can be observed at the current of 1 C. There are no sub-regions in


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region I, which may be caused by the polarization of large current density. It means that there is only one reduction reaction of Mn4+ to Mn3+ at the rate of 1 C. Concerning the slopes, the three regions of the discharge process can be divided as follows: region III from 4.80 V to 4.00 V, region II from 4.00 V to 3.35 V, and region I from 3.35 V to 2.00 V.

Figure 7. Change in lattice parameters as a function of electrochemical charge/discharge profile at (a) 40 mA g-1 and (b) 200 mA g-1 during the first three and four cycles of the electrode with Co/Ni=1.0.


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Manthiram and his co-workers15 believed the length of platform region during the first charge process is the main factor leading to voltage decay. In this research, with the substitution of nickel by cobalt, the activation length of Li2MnO3 at ~4.5 V platform extends. The capacity contribution of manganese during the first discharge process also increases, which means more manganese ions will be reduced. The more Mn4+ be reduced to Mn3+, the easier layer structure manganese can transform to spinel-like/spinel structure manganese. This might be the main reason of the voltage decay during the cycling. 3.2.3. The differential capacity curves In order to understand the structure changes of the cathode materials during the chargedischarge process at 1 C more clearly, the differential capacity curves (dQ/dV vs E) in the 3rd, 25th and 100th cycles of the four types of cathode materials were analyzed (Figure 8). According to the in situ XRD analysis, dQ/dV curves can also be divided into three regions. The redox reaction peaks associated with nickel and cobalt can be clearly divided at the 3rd cycle. Though the intensity of oxidation process at ~3.8 V declines during the cycling, the peaks can be distinguished easily. The intensity of reduction process of the electrodes with Co/Ni=0.0 at ~3.7 V only has a small changes, which means the reaction in this region is relatively stable and reversible. But with more nickel being substituted by cobalt, the intensity in this region decreases faster and faster. For the electrodes with Co/Ni=0.5, only a small peak can be recognized at 100th cycle. But there are no peaks for the electrodes with Co/Ni=1.0 and 2.0 at 100th cycle. Thackeray33 believed the different intensity changes between charge and discharge process in region II is caused by different mechanisms of lithium extraction or insertion process. The most dominant cathodic peaks appearing below 3.35 V are associated with the reduction of Mn4+/3+. The peaks above 3.0 V are ascribed predominantly to a layer-structure with some spinel-like


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compositions. And the peaks at ~2.8 V is just the same as spinel’s reduction voltage 33. But it is hard to distinguish the spinel from layer-spinel in the discharge region, which might be caused by the high testing current (200 mA g-1) shown in Figure 8. While in the charge process, they can be easily distinguished at ~ 3.1 V. After 97 cycles at 1 C, the dQ/dV curves of each material after one more cycle at 0.2 C are shown in Figure S8.

Figure 8. Differential capacity of dQ/dV vs E plots for the 3rd, 25th and 100th cycles of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0. In region I, the peaks around 3.1 V, 3.0 V, and 2.8 V can be assigned to the Mn4+/3+ reduction of layer structure, layer/spinel-like structure and spinel structure. In summary, there will come two main influences when the cobalt content increases, which form the main reasons for the voltage


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decay upon cycling. The first influence is the increase of irreversible capacity in region II upon cycling. The second influence is the increase of spinel in region I upon cycling. 3.2.4. ex situ analysis for different cycles To penetrate into the electrochemical performance of those materials, the behavior of the electrodes are recorded by ex situ XRD, Raman spectroscopy, and high–resolution TEM (HRTEM). First, to inspect the existence of the spinel phase after long cycles, ex situ XRD patterns of assynthesized, the 1st and 101th cycles of the four samples are shown in Figure S9. According to the XRD results, no significant new peaks were observed. But the satellite peaks due to the presence of Li2MnO3 phase around 21° appear again after one cycle though its intensity is very weak. It suggests that the activation of Li2MnO3 process is partially reversible. This phenomenon also has been reported by Yoon et al.34. Among the cycled satellite peak intensity of Li-rich cathode materials with different Co/Ni ratios, lower Co/Ni ratio will lead to stronger peak intensity. Yet it has not been observed in in situ XRD experiment of this research, which may be caused by relatively low spatial resolution of 2D testing system and strong x-ray absorption at relative low degree by the window of in situ cell. A weak characteristic peak of the Li2MnO3 component at the 101th cycle of the electrode with Co/Ni=0.0 is also observed by ex situ XRD. For the electrodes with Co/Ni=0.5, the intensity of this peak is weaker. But no peaks in this region were detected for the electrodes with Co/Ni=1.0 and 2.0 at 101th cycle. Considering against the OEMS experiments, it is believed that the reversibility of Li2MnO3 may have a strong connection with the loss of oxygen gas (or lattice oxygen). During the first charge process, less or no oxygen loss will be contributed to the reversibility of Li2MnO3. Upon cycling, almost all peaks get lower,


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which justifies continuous structure changes of the materials. For the electrodes with Co/Ni=0.0 and 0.5, the splitting degree of (006)/(012) and (108)/(110) change little which indicates good characteristics of the layered structure. But for the electrodes with Co/Ni=1.0 and 2.0, the splitting degree of (006)/(012) and (108)/(110) become less obvious at 101th cycle, indicating the layered structure getting deteriorated upon cycling. Therefore, the partially reversibility of Li2MnO3 may also be an important reason for suppressing the structure change and voltage decay of HE-NCM. Second, Raman spectroscopy is a powerful tool to probe the short-range environment of transition metals, even in the absence of extensive long-range order35-36. Raman spectroscopy was applied to comparing the changes in four cathode materials at different cycles (Figure S10). Two major peaks at ~597 and 490 cm-1 of the four as-synthesized samples are attributed to A1g and Eg modes of Li2MnO3 or LiMO2. A weak peak at ~430 cm-1 belongs to Li2MnO3 component which is associated with Li-O bonding. After 100 cycles, Raman spectroscopy of electrodes with Co/Ni=0.0 only shows very small peaks around 657 cm-1. But in the electrodes with high Co/Ni ratio, shoulder peaks with higher intensity were observed, which owes to spinel Li4Mn5O1237. Peak position of Li4Mn5O12 is similar to HE-NCM in XRD pattern which is difficult to distinguish. But it is easy to be detected by Raman spectroscopy. After the activation of Li2MnO3 at the first cycle, the peak intensity for all the samples at 657 cm-1 grows more or less, and continues growing upon cycling. Figure S10 shows the Raman plots of all the samples in assynthesized, after 1st and 100th cycle. It can be clearly observed that the intensity of the electrodes with high Co/Ni ratio at 657 cm-1 grows faster, which means more Li4Mn5O12 spinel forms but not for the electrodes with low Co/Ni ratio.


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Finally, HRTEM was carried out to investigate the crystal structure of the electrode with Co/Ni=1.0 at 101th cycle because HRTEM could provide more intuitional observation than XRD and Raman spectroscopy, especially for local structure. Figure 9 (a)-(d) show the HRTEM image and IFFT (inverse fast Fourier transformed) diffraction patterns taken from different regions. In Figure 9 (a), two different phases could be obviously observed, and some local structure damage in bulk could also be clearly observed. IFFT of region I and II marked by yellow and green squares have well-defined lattice fringes with a d-spacing of 0.288 nm and 0.247 nm, corresponding to the (220) plane of spinel phase (Fd3m) and (101) plane of layer phase (R3m). Figure 9 (d) is the IFFT of region III. This picture shows the gradual evolution from layer structure transforming into spinel structure. Figure 9 (e) shows another HRTEM image and corresponding FFT (fast Fourier transformed) diffraction patterns from different regions. FFT pattern of green region could be own to [010] zone axis of layer R3m. In FFT pattern of red region, except for the


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Figure 9. (a) HRTEM image and corresponding IFFT patterns of (b) region I, (c) region II, (d) region III, (e) HRTEM image and corresponding FFT patterns of the electrode with Co/Ni=1.0 after 100 cycles. same diffraction spots of layer structure, extra diffraction spots originating from cubic spinel or spinel-like structure were observed. The TEM data indicate that the activated Li-rich cathode materials’ structure in surface region is easier to transform from layer to spinel structure than that in bulk region. 3.3. Proposed mechanism for structure evolution Based on above results and analysis, we proposed a mechanism for structure evolution of the lithium-rich layered cathode materials during the electrochemical cycling, as is shown in Figure 10. During the first charge process, lithium-ions are removed from the lithium layer below 4.40 V. When it comes to the 4.5 V platform, lithium-ions in metal layer will be activated and enter the


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Figure 10. Schematic showing the proposed mechanism for structure evolution during the electrochemical cycling. lithium layer. Also lattice oxygen ions in Li2MnO3 will be oxidized to higher valence state at the same time. What’s more, some lithium dumbbell may form at 4.5 V platform7, 38 and will remain stable during the charge-discharge process, which may result in relative low columbic efficiency at the first cycle. For the electrodes with relative high Co/Ni ratio, oxygen at the surface will be oxidized to a higher valence state and runs out as oxygen gas at the end of the first charge process, which has been confirmed by OEMS. After oxygen loss at the surface of the materials, metal ions in metal layer will migrate into lithium layer to form spinel-like or spinel phase at the surface of the materials, which is confirmed by HRTEM and Raman spectroscopy. What’s different for the electrodes with low Co/Ni ratio is that the activation platform at 4.5 V of Li2MnO3 is relative shorter, and oxygen elements have a relative lower oxidation station where


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they are unable to be oxidized to oxygen. This will be helpful to form less spinel-like or spinel phase at the surface of the cathode materials. When discharged to 2.0 V, some lithium ions will go back to metal layers in the electrodes with low Co/Ni ratio, which is confirmed by ex situ XRD. Although the formation of Li2MnO3 is restricted at the end of the first cycle and no apparent platforms can be observed during the second charge process, it does play an important role in stabilizing the HE-NCM structure. For the electrodes with high Co/Ni ratio, structure evolution will occur upon cycling, which is confirmed by in situ, ex situ XRD and Raman spectroscopy, and the continuous structure evolution results in the voltage decay. All in all, the voltage decay could be suppressed by decrease Co/Ni ratio. 4. CONCLUSION All the cathodes were synthesized by carbonate co-precipitation method, among which Li1.13Ni0.275Mn0.580O2 cathode shows high electrochemical performance. Its capacity after 3 cycles and 100 cycles at the current of 200 mAh g-1 is 226 mAh g-1 and 194 mAh g-1, respectively. It also shows high initial medium discharge voltage of 3.710 V at the current of 200 mA g-1, and 3.455 V after 100 cycles with the lowest voltage decay of 0.400 V. The OEMS results indicate that the electrodes with no cobalt release no oxygen. The substitution of cobalt by nickel and the suppression of the oxygen release increase the initial medium discharge voltage. The suppression of the oxygen release at the first charging process will stabilize the structure of the cathode material, reduce the reduction of manganese and enhance the reversibility of Li2MnO3, which effectively suppress the voltage decay upon cycling. The differential capacity curves during the discharge process could be divided into three regions according to the analysis of in situ XRD data. The relative high reversibility of Ni4+/2+ region and O2-/O2- and the suppression of peak shift with Mn4+/3+ region also contribute to the suppression of the voltage


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decay. Inspired by the results of this research, changing Ni/Co ratios of HE-NCM cathodes is efficient and feasible for suppressing the voltage decay. However, to achieve a better suppression effect, more efforts should be made on controlling the continuous transformation of Manganese from +4 to +3 during the discharge process upon cycling.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: am-2018-00919v. The SEM images (a) - (h) of the precursors of Co/=0.0, 0.5, 1.0, 2.0 and corresponding EDS results (e) - (h). Schematic illustration of the structure in HE-NCM cathode. Plots of CO2 (m/z=44) and O2 (m/z=32) evolution of LiNi0.8Co0.1Mn0.1O2 ( NCM811 ) for the first cycle. Selected 1D view in in situ XRD patterns of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0. 3D view graphics between 15° to 50° and 50.4° to 70.4° of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0. The value change of a at 40 mA g-1 during the first three cycles with Co/Ni=1.0, (a) a value in the first charging process before 4.4 V and corresponding fitting line, (b) a value in the first discharging process II and corresponding fitting line, (c) a value in the first discharging process I1 and corresponding fitting line, (d) a value in the first discharging process I2 and corresponding fitting line. The value change of a at 200 mA g-1 during the first three cycles with Co/Ni=1.0, (a) a value in the first charging process before 4.4 V and corresponding fitting line, (b) a value in the first discharging process II and corresponding fitting line, (c) a value in the first discharging process I and corresponding fitting line, (d) Change in


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lattice a parameter and corresponding fitting line as a function of electrochemical charge/discharge profile at 200 mA g-1 during the first cycle. Differential capacity of dQ/dV vs E plots for the 101th cycles of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0 at the current of 40 mA g-1. Ex situ XRD plots for the powder, 1st and 101th cycles of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0 and corresponding enlarged plots. Ex situ Raman plots for the powder, 1st and 101th cycles of the electrodes with Co/Ni= (a) 0.0, (b) 0.5, (c) 1.0, (d) 2.0. (a) Schematic diagram of OEMS cell for Li-ion battery; (b) Schematic diagram of the OEMS system; (c) Physical diagram of the OEMS system. Relative amounts of Li, Ni, Co, Mn in the materials with different Co/Ni ratio.

AUTHOR INFORMATION Corresponding Authors Prof. L. Huang, E-mail: [email protected]

Prof. S. G. Sun E-mail: [email protected]

Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was jointly supported by the NSFC (Grants Nos. 21673194, 21621091, 21403157, 21773176 and 21373008), the National Key Research and Development Program (No. 2016YFB0100202), and NFFTBS (Grant No. J1310024). ABBREVIATIONS OEMS, Online continuous flow differential electrochemical mass spectrometry.

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