Correlating structural changes of the improved cyclability upon Nd-substitution in LiNi0.5Co0.2Mn0.3O2 cathode materials

Author’s Accepted Manuscript Correlating Structural Changes of the improved cyclability upon Nd-Substitution in LiNi0.5Co0.2Mn0.3O2 Cathode Materials Yan Mo, Lingjun Guo, Bokai Cao, Yiguang Wang, Liao Zhang, Xiaobo Jia, Yong Chen www.elsevier.com/locate/ensm

PII: DOI: Reference:

S2405-8297(18)30829-8 https://doi.org/10.1016/j.ensm.2018.09.003 ENSM496

To appear in: Energy Storage Materials Received date: 4 July 2018 Revised date: 3 September 2018 Accepted date: 5 September 2018 Cite this article as: Yan Mo, Lingjun Guo, Bokai Cao, Yiguang Wang, Liao Zhang, Xiaobo Jia and Yong Chen, Correlating Structural Changes of the improved cyclability upon Nd-Substitution in LiNi0.5Co0.2Mn0.3O2 Cathode Materials, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2018.09.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Correlating Structural Changes of the improved cyclability upon Nd-Substitution in LiNi0.5Co0.2Mn0.3O2 Cathode Materials Yan Moa, Lingjun Guoa, Bokai Caob, Yiguang Wanga, Liao Zhangb, Xiaobo Jiab, Yong Chenb* a

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China

b

State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key

Laboratory of Research on Utilization of Si-Zr-Ti Resources, College of Materials and Chemical Engineering, Hainan University, 58 Renmin Road, Haikou 570228, China [email protected] Abstract: Spherical LiNi0.5Co0.2Mn0.3O2 (NCM523), cycling to voltages greater than 4.3 V, often suffers from structure instability and the resultant inferior cyclability. Here, Nd is used as dopant into NCM523 to address this long-standing issue. The mechanism of Nd substitution effect on the structural evolution of NCM523 is also investigated. In-situ X-ray diffraction reveals that volume variation of the cathode could be alleviated due to the Nd doping effect. The larger-diameter Nd3+, integrating into the crystal lattice of NCM523 as a positively charged center, is beneficial to the diffusion of Li ion, stability of crystal phase and physical structure upon cycling. In-situ Raman spectroscopic measurements verify that partial Nd substitution can lead sustainable structure evolution during the first cycle. More importantly, the stable cut-off voltage could be enhanced to as high as 4.6 V.

Keywords: Li-ion battery, cathode material, layered oxide, Nd-doping, in-situ X-ray diffraction

1. Introduction Since the commercialization of Li-ion batteries (LIBs) in the 1990s, the application of Li-ion battery has been extended from small electronics to electric vehicles and large-scale energy storage system where stable, safe and high-energy-density cathode materials are urgently needed[1]. Among various cathode materials, layered-NCM523 cathode material shows great potential owing to its relatively high operating voltage (above 4.3 V) and large theoretical capacity (270 mAh g-1). The

ideal energy density could get to 800Wh kg-1, far better than that of the LiCoO2 (580 Wh kg-1) and LiFePO4 (500 Wh kg-1)[2, 3]. Meanwhile, NCM523 is a typical α-NaFeO2 structure belongs to the R 3 m space group with large 2D interstitial space, which ensures rapid Li+ mobility[4, 5]. However, the commercialized NCM523 for high energy LIBs is still restricted because of the dramatic capacity fading over numerous cycles. Studies show that the presence of the microcracking from cyclic volume changes and spontaneous side reactions at the cathode/electrolyte interfaces cause the capacity loss over cycling[6, 7]. These critical issues are dependent on several factors, including the particle morphology, cation migration related to Ni2+, residual species on the surface, synthesis strategies and operation environment. To solve the performance degradation and enhance battery safety, substantial efforts have been proposed and demonstrated. In terms of stable electrochemical behavior, the structural integrity needs to be maintained during cycling. But the cation mixing of Li/Ni usually causes not only the nonstoichiometric structure, but also the capacity fading upon repeated Li+ extraction/insertion especially at high temperature. Cationic doping, (Al[8, 9], Mg[10], Ti[11], Zr[12], Cr[13] and V[14]), has

been

proved

to

be an effective strategy to enhance the electrochemical

performance of the cathode. The substitution Al for Co gave rise to a minimal effect on the defect concentration and realized a capacity of 60.5 mAh g−1 at 5C under very low temperature (-20 ℃)[8]. Chen et al. reported that when V5+ occupied the Li sites in Li[Ni0.5Co0.2Mn0.3]1-xVxO2 material, cycling stability was enhanced to 93.9% in 50 cycles between 2.7 and 4.4 V at 60 °C[14]. Recently, it’s believed that the inserted Na+ could enlarge Li layer spacing in Li0.97Na0.03Ni0.5Co0.2Mn0.3O2, contributing to the excellent rate capability (60.09 mA h g-1 at 50 C)[15]. These results suggest that introducing extrinsic ions into the host structure could reduce the number of unstable elements, stabilize the valence of Ni ion or increase the bonding strength between cationic ions and oxygen, thus reduce the cationic disordering and enhance the electrochemical property of the electrodes. However, understanding of deep mechanism is not sufficient and the long-term cyclability is required to be improved especially when the voltage is larger than 4.5 V. Previous research progresses have proved that lanthanides, possessing large-diameter and unique electrical property, could be used as co-dopants to improve the performance of the relevant pristine cathode materials[16-19]. But there are less reports on using lanthanides as single dopant, and their doping effects are still unclarified. Here, a novel lightly Nd-NCM cathode material is

synthesized, replacing Li by Nd in Li0.992Nd0.008Ni0.5Co0.2Mn0.3O2 material. The Nd-doped material can efficiently work even at 60 ℃ under a high cut-off voltage of 4.6 V, and significantly improve its capacity retention from 52% to73% with an exceptionally high capacity of 103 mAh g–1 at 5C. To further reveal the relevant mechanism, in-situ X-ray diffraction (in-situ XRD) technology is conducted to probe the structural change upon (de)lithiation. The Nd dopants that served as pillars in NCM523 could suppress the undesired hexagonal H3 phase that incurs volume change and capacity loss. It is noteworthy that this is the first time that the difference of phase transformation from hexagonal H2 to H3 between the pristine and cation-doped NCM523 cathode materials is illustrated.

2. Experimental 2. 1 Materials Synthesis The P-NCM523 powders were synthesized by coprecipitation method followed by high-temperature calcination. A mixed solution of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O was simultaneously added into a stirred reactor at a constant rate. A NaOH solution (10 mol L-1) and a dilute solution of NH4OH were separately poured into the reactor to adjust the pH value at 11.0. The precipitate was washed with distilled water several times to remove any dissolved salts, and then was dried at 60 °C for 12 h. The obtained Ni0.5Co0.2Mn0.3(OH)2 powder was mixed with excess LiOH, the atomic ratio of Li/(Ni+Co+Mn) were 1.05. The blended powders were preliminarily annealed in air at 480 °C for 6 h, and then heated at 900 °C in air for 12 h. The Nd-NCM compounds were synthesized in the same procedure above in which the LiOH was changed into a mixture of excess LiOH and an amount of Nd2O3. The stoichiometric ratio of LiOH: Nd2O3 is 0.992: 0.008. 2.2 Material characterization The X-ray diffraction (XRD) measurements were operated on the cathode powder using a Bruker D2 Advance diffractometer, with wavelength of Cu Kα (λ=1.541 Å) at an accelerating voltage of 40 kV. For Raman experiments, the in-situ diffraction spectroscopy of the samples was carried out on a Thermo Fisher DXRxi (USA) spectroscope with an excitation source of 532.00 nm in the wavenumber range of 200-800 cm-1. High-resolution transmission electron microscope (HRTEM) were carried out on a Tecnai G2 F30, S-TWIN instrument with an accelerating voltage of 200 kV. The crystal structure was studied by scanning transmission electron microscopy (STEM,

FEI Titan 80-300). For differential scanning calorimetry (DSC), the cells were fully charged to 4.6 V at 45 ℃ and opened in a glove box. After the electrolyte had been carefully removed from the surface by soaking in salt-free dimethyl carbonate, the cathode material was recovered from the current collector. Measurements were performed in a DSC (Netzsch STA 449C) using a temperature scan rate of 5 °C min-1. 2.3 Electrochemical Measurement The positive electrodes for electrochemical evaluation were prepared with slurry consisting of 80 wt% P-NCM (or Nd-NCM), 10 wt% Ketjen Black (KB), and 10 wt% PVDF binder in N-methyl-2-pyrrolidinone (NMP). Subsequent drying resulted in an active material of approximately 2-3 mg on each disc with 10 mm in diameter. All the working electrodes were assembled in the Ar-filled glove box using 2025-type coin cell using Li foil as the counter electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (in a volume ratio of 1:2). Neware-BTS9 battery testing system was employed to perform the charge-discharge cycling of the cathodes. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were carried out on the electrochemical workstation (Biologic VSP-300) device. CV measurements were measured with a scan rate of 0.1 mV s-1 between 4.7 and 2.7 V vs Li/Li+. EIS was

used in

the frequency range from

0.1 to

200 kHz

with

the amplitude of 5 mV. 2.4 In-situ XRD and in-situ Raman studies Cells for in-situ XRD studies were designed based a normal 2032 coin cell. The cell case on the positive side was punched with a 16 mm diameter hole. The hole was covered on the inside of the case with a thin Be window (containing a small quantity of BeO) to allow penetration of the X-rays through the cap of the cell, held in place with a thermoplastic film. Meanwhile, in order to improve the signal quality, the active mass was loading from 3-5 mg up to 20-30 mg. The in-situ Raman coin cell is similar to the in-situ XRD coin cell, except for an optical window of quartz. Each scan in Fig. 4 and Fig. 7, and in Fig. 6 took about 1 h and 2 h, respectively.

3. Result and discussion XRD studies of the P-NCM and Nd-NCM compounds are shown in Fig. 1. Here, ~5% graphite is added to calibrate the peak position. As expected, the main diffraction peaks of the two samples can be indexed to a hexagonal a-NaFeO2 layered structure with R 3 m space group[20]. The obvious

splitting of (006)/(102) and (108)/(110) peaks for all samples demonstrate that these materials have a well-developed layered structure[21]. No extra diffraction peaks from related impurities exist. These results illustrates that the Nd substitution do not change the crystal structure of NCM523. Subtle structural variations after doping are seen due to the larger size of the Nd3+ in an octahedral environment (rNd3+=0.098 nm) as compared to the lower Li+ (rLi+=0.076 nm) it replaces. As observed from the enlarged region of I, the XRD peaks of Nd-NCM sample shifts to lower angle than that of the pristine one, while the position of (002) of the graphite is well-kept at 26.60 ° in both samples, which is also a clear evidence of the incorporation of Nd[22]. Further peak fitting has been carried out by using GSAS/EXPGUI. From the comparison of the P-NCM (a = b =2.8666 Å, c = 14.2259 Å) and Nd-NCM (a = b =2.8669 Å, c = 14.2298 Å), we believe that Nd doping leads to the expansion of the space for both TMO2 (TM=Ni, Co and Mn) layers and Li layers (Table S1). The ratio c/a associated with the ordering degree of hexagonal structure is thus increased by Nd doping, showing the presence of Nd in NCM523 oxides have positive effect on the mobility of lithium ion[23, 24]. The intensity ratio of I003/I104 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. The I003/I104 values of the P-NCM is larger than Nd-NCM sample in Table S1, indicating that the cation mixing can be restrained to some extent by Nd substitution. The refinement results also confirm that Li/Ni mixing of P-NCM and Nd-NCM is 3.37% and 2.19%, respectively. The low Li/Ni mixing is usually favorable for reversibility of repeated Li+ insertion/extraction.

Fig. 1 XRD patterns of the P-NCM and Nd-NCM samples (a), the enlarged XRD patterns of (I) (003) peak in (a), and corresponding XRD Rietveld refinements of P-NCM (b) and Nd-NCM (c), respectively. The galvanostatic charge-discharge tests have been performed at 45 ℃ to compare the electrochemical properties of the P-NCM and Nd-NCM samples (Fig. 2a-b). Obviously, typical potential plateaus obtained at 3.8 V for both samples are much closer to the oxidation of Ni2+ to Ni3+ occurred in the previous study[25]. The first discharge capacity of Nd-NCM is 169 mAh g-1, which is lower than the pristine 174 mAh g-1. The decrease of the capacity could be ascribed to the electrochemically inactive Nd substitution for active Li ions. But the fading for Nd-NCM is greatly alleviated (Fig. 2a). The 89% retention of the initial capacity after 100 cycles for Nd-NCM is much better than 78% for the pristine sample. It’s noted that the capacity retention of Nd-doped sample is higher than most reported results[14, 26-29]. The contribution of Nd doping can be further proved by CV in Fig. 2c. The P-NCM have two oxidation peaks; one is centered at 3.918 V for Ni2+/Ni3+

and the other is 4.466 V for Co3+/Co4+, and the corresponding reduction peaks move toward 3.564 V and 4.248 V, respectively. In the case of Nd-NCM sample, the oxidation peaks locate at 3.868 V and 4.230 V, and the reduction peaks at 3.606 V and 4.276 V, respectively. The potential gap between the oxidation peak and reduction peak (ΔV) is known to indicate the electrode polarization. Obviously, the main ΔV for the P-NCM is 0.354 V, which is reduced when compared with that of Nd-NCM sample to 0.262 V. The variation of ΔV suggests that the incorporation of Nd into NCM523 suppresses the electrode polarization and enhances the reversibility of Li+ during cycling. This result that matches well with the observation from Fig. 2a, Fig. 2d and Fig. S2 shows the rate performance of the two samples. The differences in the plateau potentials and discharge capacity scanned at 0.5C and 1C are negligible. However, Nd-NCM exhibits a greater capacity and a higher discharge potential (Fig. S2) than the P-NCM with increasing C rates from 2C, 5C to 10C. The specific capacity of the P-NCM sample at 2C, 5C and 10 C is only 135.6, 84.3 and 20.1 mA h g-1, respectively. In contrast with Nd-doped one, they are significantly enhanced to 146.8, 111.8, and 75.4 mAh g-1, respectively. Moreover, a large reversible capacity of 188.6 mAh g-1, as high as that of the first discharge, is maintained for Nd-NCM sample when the current rate is back to 0.2C. But for the P-NCM, the discharge capacity could only regain to 166.7 mAh g-1 with a capacity retention of 89%, indicating Nd doping can significantly enhance the rate performance of NCM523 cathode material. Cycling behavior in galvanostatic mode when charged at various temperatures (25 ℃, 45 ℃, and 60 ℃) is also presented. As shown in Fig. 2e-f, as increasing in the temperature, the capacity retention decreases. Nd-NCM exhibits better cycling stability than P-NCM. Even under a high temperature of 45 ℃, favorable capacity retention of 82% Nd-NCM sample could be obtained, while that of P-NCM is 63%. Moreover, at the elevated temperature measurements, a discharge capacity of 103 mAh g-1 with acceptable capacity retention of 73% after 100 cycles under 5C rate is achieved. This improvement is also confirmed by the results of DSC (Fig. S3) that the exothermic reaction temperature of P-NCM is higher than that of Nd-NCM. Therefore, these results clearly demonstrate that thermal stability of the charged cathode material is improved by Nd substitution.

Fig. 2 Comparison of electrochemical properties of the P-NCM and Nd-NCM electrodes at 45 ℃, cycle abilities (a) and the corresponding initial charge and discharge profiles at 1C (200 mA g-1) (b), CV at 0.1 mV s-1 for the second cycle (c) and rate capabilitiy measured at various charge-discharge rates (d). Cyclabilities at 5C under various temperatures (e-f). Fig. 3 shows EIS results in the charged state (4.6 V) after a given number of cycles at 5C rate. All plots essentially comprise of a high frequency semicircle and a medium frequency semicircle. The semicircle in the high frequency region is associated with surface film resistance (Rsf), related to lithium-ion migration in the cathode electrolyte interface (CEI). The semicircle in the medium frequency region represents the charge transfer resistance (Rct). The Rsf is always lower in the Nd-NCM sample than P-NCM. The Rsf values in the P-NCM sharply increase from 50.81 Ω to 85.90 Ω and 92.36 Ω, respectively (Table S2). The increase in Rsf can be attributed to the passive effect of the oxidative decomposition of the electrolyte. By contrast, the Nd-NCM electrode shows mild increase from 25.67 Ω to 73.91 Ω after 100 cycles. It is apparent that the Nd influences the

formation of the CEI film, and then suppresses the decomposition of electrolyte after long-term cycling. In addition, the Nd-NCM delivers a much lower Rct value when compared with that of Nd-NCM. The Nd-NCM delivers a Rct of 71.64 Ω at 100 th, while P-NCM shows that of 110.5 Ω. It is known that the increasing Rct value is mainly caused by the side reactions that give rise to the structural degradation. Consequently, Nd-substitution contributes to maintaining structural stability of the NCM523.

Fig. 3 Equivalent circuit used for fitting the experimental results (a). Nyquist plots of the (b) P-NCM523 and (c) Nd-NCM523 samples after the 1st, 50 th and 100 th cycles.

Fig. 4 shows the structural evolution of the two samples at different potential steps during the initial charge and discharge process. The corresponding charge-discharge profile between 2.8 V and 4.6 V at a current density of 20 mA g-1 (0.1C) are presented on the right side of XRD patterns. Three separate segments are shown for each material during OCV (black), charge (red) and discharge (blue) process. All patterns can be well indexed to a hexagonal NaFeO2 structure. The additional strong diffraction peaks, appearing at approximately 41.63 °, 46.23 °, 53.18 ° and 71.22 °, are ascribed to the use of metal Be window for X-ray transmission. The structural change as lithium ion extraction and intercalation is evidenced by the obvious peak shifting and splitting. All peaks of the samples exhibit slightly change upon lithium ion extraction. As the lithium ion is extracted, the (003) peaks move toward lower angle and (101), (104), (105) and (110) peaks shift to higher angle, reflecting expansion and contraction of the c axis and a plane, respectively. It can be attributed to the increased electrostatic repulsion between adjacent oxygen layers and smaller ionic radius,

respectively, by Ni2+ to Ni3+ and Co3+ to Co4+[30]. Interestingly, we also discover that the (003) peak exhibits slightly different on further increasing the voltage. The (003) peak of the P-NCM sample initially shifts to lower angle and then move toward higher angle following the potential above 4.05 V, while that of Nd-doped one turns over to higher angle over the potential 4.5 V. This sudden increase of the position in (003) peaks also affects the evolution of lattice parameter c, which will discuss in the following part. The selected regions contour plots of the (003), (101), (104), (105), (107), (108) and (110) Bragg peaks are presented along with the c axis during the first cycle (Fig. 4c-d). There is an obvious consistency that all the Bragg intensities of peaks decrease during lithium extraction process and return to the original state after lithium ion intercalation. The difference is that the un-doped sample deviates more seriously from the origin state at the end of discharge than the un-doped sample in intensities and shapes. It indicates that the structural variation of Nd-doped one is significantly restrained and exhibits more reversible than un-doped sample after lithium ions are inserted back to the crystal lattice.

Fig. 4 In-situ XRD patterns during the first charge and discharge process of (a) the P-NCM and (b) Nd-NCM samples under a current rate of 0.1 C between 2.8-4.6 V. Contour plots of the (003), (101), (104), (105), (107), (108) and (110) of (c) the P-NCM and (d) Nd-NCM samples. Values of x correspond to the amount of lithium extracted (red)/lithium intercalated (blue).

Fig. 5 Calculated lattice parameters change of the (a) P-NCM and (b) Nd-NCM samples during the first charge and discharge process between 2.8-4.6 V. The detailed changes and corresponding normalized variation in the lattice parameters during the intercalation/deintercalation process are shown in Fig. 5 and Fig. S4, respectively. The results show that both samples suffer from a contraction within the a-plane, which matches well with the results from Fig. 4. Comparative analysis suggests that, in some extent, incorporation of Nd limits the changes in a parameter. The value ofΔa at the end of charge is 1.94% and 1.73% for the pristine and Nd-doped sample, respectively. A 2.73%Δa at the end of discharge for the P-NCM is decreased to1.67% after doping. Another significant difference exists in the evolution of c between the doped and un-doped samples. The lattice parameter c of the pristine sample gradually increases at the beginning of delithiation, reaching a maximum value 14.492 Å at 4.4 V, and then abruptly decreases to 14.338 Å after fully charge. The occurrence of “turning point” is often linked with the deep-degree removal of lithium ions, which indicates the formation of hexagonal H3 phase. As we all known, the layered-type LiMO2 material experiences a series of phase variation during the Li + extraction process, going from hexagonal phase H1, to H2 and H3[24, 31]. In contrast with H1 phase, H2 phase owns higher hexagonal property owing to more open paths within TMO2 slabs for Li-ion diffusion. However, it usually suffers undesired H3 phase conversion in high charge state, which causes severe volume change and capacity loss[24, 32]. Contrary to the P-NCM, the lattice parameters c of the Nd-doped cathode changes linearly upon further Li+ extraction, increasing from

14.245 Å to 14.507 Å. It seems plausible that the charge state when c starts to contract (the presence of H3 phase) is shifted to higher voltage. The volume V for both samples, primarily determined by the variation of a and c, gets to contract during charge, and then expands in the reverse the discharge process. With the differences inΔa andΔc, the maximum value ofΔV for Nd-doped sample is 1.66%, much smaller than that of the pristine one (3.17%). Moreover, the volume change of Nd-NCM sample returns closer to the zero-strain characteristics at the end of discharge as compared with that of the pristine one. This result is also in accordance with the variation in intensity and shape of the peaks (Fig. 4c-d). In total, partial Nd substitution for Li could limit the variation of crystal structure and lead to more sustainable structure evolution in the voltage range of 2.8-4.6 V. Therefore, more reversible volume variation is achieved under Li+ intercalation, resulting in better structural stability and long-term cyclability. By tracking the (003) peak and corresponding evolution of lattice parameter c, hexagonal phases (H1, H2 and H3) for Nd-NCM sample could be identified during charge process (Fig. 6)[24, 31, 33]. The distinct separation of (003) peak is not observed due to the long test time of each scan resulted from the sensitivity of the XRD device. But it is believed that the phase transition from H1 to H2 occurs from 3.70 V to 4.02 V, during the process of the (003) peak moving toward lower angle (Fig. 6a). As plotted in Fig. 6c, the lattice parameter c starts to increase at 3.70 V, indicating the separation of (003) peak into (003)H1 and (003)H2 peak. The (003)H2 peak increases its intensity step by step as lithium ion is removed, and shifts away from the original (003)H1 peak and becomes dominant at 4.02 V. At the same time, the lattice parameter c reaches its maximum, which indicates H1 phase is completely transformed into the H2 phase. With increasing the voltage above 4.02 V, the lattice parameter c gets reduced and the (003) peak move toward higher angle at 4.47 V, indicating the appearance of H3 phase[24]. Then, towards the end of charge, the (003) peak shifts to higher angle and deviates completely from the original (003)H2 after 4.54 V, a clear evidence of the conversion of H2 to H3 phase[31]. This observation is highly consistent with the calculated results of lattice parameters in Fig. 5a. A quite different evolution of (003) peak for NCM523 have been observed with Nd doping. Fig. 6b shows that the (003) peak of the H1 phase moves toward lower angle with the continuously increased lattice parameter c in the voltage region 3.71-4.46 V (Fig. 6d). Upon further charging, the lattice parameter c gets reduced with a major shift toward higher angle (4.54-4.98 V), suggesting the emergence of H3 phase. At the end of charge, a dominate peak

(003)H3 of H3 phase apart from the original (003)H2 has been detected at 4.98 V. The comparison of the evolution for (003) peak between the pristine and Nd-NCM shows that the potential at which the formation of H3 phase is pushed to 4.98 V with Nd doping.

、 Fig. 6 Selected region for (003) peak in the in-situ XRD patterns (Fig. S5) for (a) the P-NCM and (b) Nd-NCM samples during phase transformation processes of H1to H2 and H2 to H3, the corresponding calculated lattice parameter c of (c) the P-NCM and (d) Nd-NCM samples. In-situ Raman spectra is also a versatile tool to study the structure change and especially the surface chemical composition of the electrodes at a spatial resolution of ~2 μm. The in-situ Raman measurements associated with the first charge and discharge process of the samples between OCV-4.6 V and 4.6-2.8 V are shown in Fig. 7. The Raman spectra in present study features typical characteristic of layered-type LiMO2 material. Two major peaks at OCV in the Raman spectrum can be seen at 598 cm-1 and 489 cm-1, respectively. These two peaks features typical characteristic of layered-type LiMO2 material, which can be assigned to the characteristic modes A1g and Eg, attributing to the TM-O symmetrical stretching and O-TM-O bending vibrations, respectively[13, 34]. As the lithium ion is extracted step by step, a drastic change in the spectra is the decrease in intensity of the 598 cm-1 peak to a minor signature, which indicates the change in the short-range local environment as the removal of the lithium ions (Fig.7a-b). In addition, a new peak at 542 cm−1 has been observed at 4.02 V for P-NCM and 4.00 V for Nd-NCM, it pronounces well upon charging the cell above the voltage near 4.11 V. This peak can be interpreted as Raman active modes of LiNi3+O2, which is caused by the oxidation of Ni2+ ions in the structure[35]. Another obvious change in the spectra of P-NCM is that the 490 cm−1 peak shifts toward lower wavenumber utill

4.11 V, originating from the lattice expansion in the c axis caused by the extraction of Li+ ions (Fig.7a)[36]. But the situation is different above 4.27 V where this peak returns toward higher wavenumber and its shoulder becomes much broader. The sudden decrease in the intensity of the signal has also been observed at the beginning of the discharge of the sample (Fig. 7c). This phenomenon indicates the rearrangement of the ions and local distortion in the structure due to the appearance of the H3 phase or the passive effect of the electrolyte[37]. As for Nd-doped sample, the 490 cm−1 peak continuously shifts to lower wavenumber and still pronounced at the end of charge (Fig.7b), showing more sustainable structure evolution. These results that are in accordance well with our in-situ XRD results show that the Nd-doped strategy leads to more stable structure and results in the superior electrochemical performance.

Fig. 7 In-situ Raman spectra of during first cycle charging (a) the P-NCM sample of and (b) Nd-NCM samples, and corresponding discharging (c) and (d), respectively. To identify the morphology change of the particles triggered by the aforementioned H3 phase, the SEM images of two samples before and after cycling are provided in Fig. S8. All of the particles before cycling are composed of densely aggregated primary particles. The morphology for the P-NCM sample drastically changes after 100 cycles. Obvious pulverizations on its surface and interior are seen from Fig. S8b. But the fractures in the primary particle during the cycling are restrained in the Nd-doped sample (Fig. S8d). This distinct difference further confirms that the mechanical stress of NCM523 associated with phase transitions during cycling is considerably alleviated by Nd doping. According to the above results and analysis, we may infer that O2--O2- repulsions could be screened by Nd doping, different to the pristine one. Fig. 6c shows that P-NCM undergoes severe

lattice variation at a high charge state. The lattice c is gradually expanded upon the potential of 3.7 V, and then shrinks following the potential from 4.54 V (Fig. 8d). The initial expanding results from the repulsive force of among neighbor oxygen layers (Fig. 8b). The subsequent shrinkage comes from the TM-O2 layers gliding toward the adjacent Li planes due to the instability caused by deep delithiation, which results in phase distortion to H3 phase. The H3 phase severely stresses the structure, which leads to mechanical degradation and micro-cracks within the particles, and dramatic capacity fading in the deeply charged electrode. However, the H3 phase of Nd-NCM sample is difficult to obtain and the lattice contraction requires a high charge cutoff to realize. This distinct difference can be attributed to the residence of Nd3+ in the crystal lattice of NCM523. The Nd3+ acts as a pillar to link the M-O slabs, screening the electrostatic repulsive force between the oxygen layers (Fig. 8f-h). This screening effect can maintain the distance of among (003) planes containing Nd3+, which gives rise to the reduction of lattice stress and the suppression of phase transition. Therefore, Nd incorporation can maintain the structure without severe interslab collapse, even though the material suffers from high delithiation state.

Fig. 8 Schematic diagram of phase transitions in the P-NCM (a-d) and Nd-NCM (e-h) samples

4. Conclusion In summary, Nd-NCM materials have been synthesized by a simple solid route followed by high-temperature calcinations. The combined analysis of in-situ XRD and SEM reveals that NCM523 material experiences a series of phases during the Li+ extraction process, going from hexagonal phase H1, H2 to H3. The presence of undesired H3 phase in high-voltage regions (near 4.5 V) severely stresses the structure, resulting in mechanical strains and micro-cracks within the particles. Partial Nd substitution for Li is capable of limiting the volume variation by suppressing H2 to H3 phase transition and then mitigating the mechanical stress of the particles upon cycling. Thus, this doped NCM523 can achieve outstanding cycling stability even at a cut-off voltage of 4.6 V (with 73% capacity retention after 100 cycles at 60 ℃) and significantly enhanced rate capability (42% capacity retention at 10 C). Our study offers new insights into the rational design of high-voltage, high-rate and safe layered cathode materials for the design of advanced LIBs cathode. Acknowledgements This work was financially supported by Hainan Provincial Natural Science Foundation of China (2018CXTD332), Science and technology development special fund project (ZY2018HN09-3), the NSFC (No.51362009 and No. 21603048), and 111 Project (B12015).

References [1] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature, 458 (2009) 190-193. [2] P.K. Nayak, E.M. Erickson, F. Schipper, T.R. Penki, N. Munichandraiah, P. Adelhelm, H. Sclar, F. Amalraj, B. Markovsky, D. Aurbach, Review on Challenges and Recent Advances in the Electrochemical Performance of High Capacity Li‐ and Mn‐ Rich Cathode Materials for Li‐ Ion Batteries, Adv. Energy Mater., 8 (2018) 1702397. [3] A. Manthiram, B. Song, W. Li, A perspective on nickel-rich layered oxide cathodes for lithium-ion batteries, Energy Storage Mater., 6 (2017) 125-139. [4] W. Liu, P. Oh, X. Liu, M.-J. Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Nickel-Rich Layered Lithium Transition-Metal Oxide for High-Energy Lithium-Ion Batteries, Angew. Chem., 54 (2015) 4440-4457. [5] A. Manthiram, J.C. Knight, S.-T. Myung, S.-M. Oh, Y.-K. Sun, Nickel-Rich and Lithium-Rich Layered Oxide Cathodes: Progress and Perspectives, Adv. Energy Mater., 6 (2016) 1501010. [6] S.K. Jung, H. Gwon, J. Hong, K.Y. Park, D.H. Seo, H. Kim, J. Hyun, W. Yang, K. Kang, Understanding the Degradation Mechanisms of LiNi0.5Co0.2Mn0.3O2 Cathode Material in Lithium Ion Batteries, Adv. Energy Mater., 4 (2014) 94-98. [7] Y. Cho, P. Oh, J. Cho, A new type of protective surface layer for high-capacity Ni-based cathode materials: nanoscaled surface pillaring layer, Nano Lett., 13 (2013) 1145-1152.

[8] G. Li, Z. Zhang, R. Wang, Z. Huang, Z. Zuo, H. Zhou, Effect of Trace Al Surface Doping on the Structure, Surface Chemistry and Low Temperature Performance of LiNi0.5Co0.2Mn0.3O2 Cathode, Electrochim. Acta, 212 (2016) 399-407. [9] D. Aurbach, O. Srur-Lavi, C. Ghanty, M. Dixit, O. Haik, M. Talianker, Y. Grinblat, N. Leifer, R. Lavi, D.T. Major, G. Goobes, E. Zinigrad, E.M. Erickson, M. Kosa, B. Markovsky, J. Lampert, A. Volkov, J.Y. Shin, A. Garsuch, Studies of Aluminum-Doped LiNi0.5Co0.2Mn0.3O2: Electrochemical Behavior, Aging, Structural Transformations, and Thermal Characteristics, J. Electrochem. Soc., 162 (2015) A1014-A1027. [10] H. Kondo, Y. Takeuchi, T. Sasaki, S. Kawauchi, Y. Itou, O. Hiruta, C. Okuda, M. Yonemura, T. Kamiyama, Y. Ukyo, Effects of Mg-substitution in Li(Ni,Co,Al)O2 positive electrode materials on the crystal structure and battery performance, J. Power Sources, 174 (2007) 1131-1136. [11] I.M. Markus, F. Lin, K.C. Kam, M. Asta, M.M. Doeff, Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1-2x-yTiy)O2 for Lithium Ion Batteries, J. Phys. Chem. Lett., 5 (2014) 3649-3655. [12] H. Chen, Q. Hu, Z. Huang, Z. He, Z. Wang, H. Guo, X. Li, Synthesis and electrochemical study of Zr-doped Li[Li0.2Mn0.54Ni0.13Co0.13]O2 as cathode material for Li-ion battery, Ceramint., 42 (2016) 263-269. [13] G. Singh, R. Thomas, A. Kumar, R.S. Katiyar, Electrochemical Behavior of Cr-Doped Composite Li2MnO3-LiMn0.5Ni0.5O2 Cathode Materials, J. Electrochem. Soc., 159 (2012) A410. [14] H. Zhu, T. Xie, Z. Chen, L. Li, M. Xu, W. Wang, Y. Lai, J. Li, The impact of vanadium substitution on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2, Electrochim. Acta, 135 (2014) 77-85. [15] W. Hua, J. Zhang, Z. Zheng, W. Liu, X. Peng, X.D. Guo, B. Zhong, Y.J. Wang, X. Wang, Na-doped Ni-rich LiNi0.5Co0.2Mn0.3O2 cathode material with both high rate capability and high tap density for lithium ion batteries, Dalton Trans., 43 (2014) 14824-14832. [16] Y. Xie, R. Yang, L. Yan, Q. Lu, K. Dai, P. He, Synthesis and electrochemical characterization of Li1.05RExCryMn2−x−yO4 spinel as cathode material for rechargeable Li-battery, J. Power Sources, 168 (2007) 272-277. [17] X. Sun, X. Hu, Y. Shi, S. Li, Y. Zhou, The study of novel multi-doped spinel Li1.15Mn1.96Co0.03Gd0.01O4+δ as cathode material for Li-ion rechargeable batteries, Solid State Ionics, 180 (2009) 377-380. [18] D. Arumugam, G.P. Kalaignan, P. Manisankar, Development of structural stability and the electrochemical performances of ‘La’ substituted spinel LiMn2O4 cathode materials for rechargeable lithium-ion batteries, Solid State Ionics, 179 (2008) 580-586. [19] Q. Liu, X. Su, D. Lei, Y. Qin, J. Wen, F. Guo, Y.A. Wu, Y. Rong, R. Kou, X. Xiao, F. Aguesse, J. Bareño, Y. Ren, W. Lu, Y. Li, (2018) Approaching the capacity limit of lithium cobalt oxide in lithium ion batteries via lanthanum and aluminium doping, Nat. Energy, https://doi.org/10.1038/s41560-41018-40180-41566. [20] T. Ohzuku, A. Ueda, Solid‐ State Redox Reactions of LiCoO2 (R3̅m) for 4 Volt Secondary Lithium Cells, J. Electrochem. Soc., 141 (1994) 2972-2977. [21] E. Zhao, X. Liu, Z. Hu, L. Sun, X. Xiao, Facile synthesis and enhanced electrochemical performances of Li2TiO3-coated lithium-rich layered Li1.13Ni0.30Mn0.57O2 cathode materials for lithium-ion batteries, J. Power Sources, 294 (2015) 141-149. [22] D. Li, Y. Sun, X. Liu, R. Peng, H. Zhou, Doping-induced memory effect in Li-ion batteries: the case of Al-doped Li4Ti5O12, Chem. Sci., 6 (2015) 4066-4070. [23] F. Wu, J. Tian, N. Liu, Y. Lu, Y. Su, J. Wang, R. Chen, X. Ma, L. Bao, S. Chen, Alleviating structural degradation of nickel-rich cathode material by eliminating the surface Fm3 ¯m phase, Energy Storage Mater., 8 (2017) 134-140. [24] J. Shu, R. Ma, L. Shao, M. Shui, K. Wu, M. Lao, D. Wang, N. Long, Y. Ren, In-situ X-ray diffraction study on the structural evolutions ofLiNi0.5Co0.3Mn0.2O2 in different working potential windows, J. Power Sources, 245 (2014) 7-18. [25] J. Zhang, Z. Yang, R. Gao, L. Gu, Z. Hu, X. Liu, Suppressing the Structure Deterioration of Ni-rich

LiNi0.8Co0.1Mn0.1O2 through Atom-scale Interfacial Integration of Self-forming Hierarchical Spinel Layer with Ni Gradient Concentration, ACS Appl. Mater. Interfaces, 9 (2017) 29794-29803. [26] Y. Huang, F.M. Jin, F.J. Chen, L. Chen, Improved cycle stability and high-rate capability of Li3VO4-coated Li[Ni0.5Co0.2Mn0.3]O2 cathode material under different voltages, J. Power Sources, 256 (2014) 1-7. [27] Z. Wang, S. Huang, B. Chen, H. Wu, Y. Zhang, Infiltrative coating of LiNi 0.5Co0.2Mn0.3O2microspheres with layer-structured LiTiO2: towards superior cycling performances for Li-ion batteries, J. Mater. Chem. A, 2 (2014) 19983-19987. [28] D. Wang, X. Li, Z. Wang, H. Guo, Y. Xu, Y. Fan, J. Ru, Role of zirconium dopant on the structure and high voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries, Electrochim. Acta, 188 (2016) 48-56. [29] Y. Mo, B. Hou, D. Li, X. Jia, B. Cao, L. Yin, Y. Chen, Enhanced high-rate capability and high voltage cycleability of Li2TiO3-coated LiNi0.5Co0.2Mn0.3O2 cathode materials, RSC Adv., 6 (2016). [30] P.F. Wang, H.R. Yao, X.Y. Liu, Y.X. Yin, J.N. Zhang, Y. Wen, X. Yu, L. Gu, Y.G. Guo, Na +/vacancy disordering promises high-rate Na-ion batteries, Sci. Adv., 4 (2018) eaar6018. [31] X.Q. Yang, X. Sun, J. Mcbreen, New findings on the phase transitions in Li 1−xNiO2: in situ synchrotron X-ray diffraction studies, Electrochem. Commun., 1 (1999) 227-232. [32] Y.-N. Zhou, J. Ma, E. Hu, X. Yu, L. Gu, K.-W. Nam, L. Chen, Z. Wang, X.-Q. Yang, Tuning charge–discharge induced unit cell breathing in layer-structured cathode materials for lithium-ion batteries, Nat. Commun., 5 (2014). [33] P.Y. Liao, J.G. Duh, J.F. Lee, H.S. Sheu, Structural investigation of Li 1−xNi0.5Co0.25Mn0.25O2 by in situ XAS and XRD measurements, Electrochim. Acta, 53 (2007) 1850-1857. [34] H. Guo, Z. Wei, K. Jia, B. Qiu, C. Yin, F. Meng, Q. Zhang, L. Gu, S. Han, Y. Liu, H. Zhao, W. Jiang, H. Cui, Y. Xia, Z. Liu, Abundant nanoscale defects to eliminate voltage decay in Li-rich cathode materials, Energy Storage Mater., 16 (2019) 220-227. [35] P. Kalyani, N. Kalaiselvi, Various aspects of LiNiO2 chemistry: A review, Sci. & Technol. Adv. Mater., 6 (2005) 689-703. [36] C.V. Rao, J. Soler, R. Katiyar, J. Shojan, W.C. West, R.S. Katiyar, Investigations on Electrochemical Behavior and Structural Stability of Li1.2Mn0.54Ni0.13Co0.13O2 Lithium-Ion Cathodes via in-Situ and ex-Situ Raman Spectroscopy, J. Phys. Chem. C, 118 (2014) 14133–14141. [37] G. Singh, W.C. West, J. Soler, R.S. Katiyar, In situ Raman spectroscopy of layered solid solution Li 2MnO3 –LiMO2 (M=Ni, Mn, Co), J. Power Sources, 218 (2012) 34-38.

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