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A Simple Method for the Complete Performance Recovery of Degraded Ni-rich LiNi0.70Co0.15Mn0.15O2 Cathode via Surface Reconstruction Binhua Huang, Dongqing Liu, Kun Qian, Lihan Zhang, Kai Zhou, Yuxiu Liu, Feiyu Kang, and Baohua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Applied Materials & Interfaces

A Simple Method for the Complete Performance Recovery of Degraded Ni-rich LiNi0.70Co0.15Mn0.15O2 Cathode via Surface Reconstruction Binhua Huang a,d,1, Dongqing Liu a,1, Kun Qian b,d, Lihan Zhang a,d, Kai Zhou a,d, Yuxiu Liu a,d, Feiyu Kang a,b,c,d, Baohua Li a,c,*

a

Engineering Laboratory for Next Generation Power and Energy Storage Batteries,

and Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China b

Shenzhen Environmental Science and New Energy Technology Engineering

Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen, 518055, China c

Shenzhen Geim Graphene Center, Shenzhen, 518055, China

d

Laboratory of Advanced Materials, School of Materials Science and Engineering,

Tsinghua University, Beijing, 100084, China 1

These authors contributed equally to this work.

*

Corresponding Author

E-mail address: [email protected] (B. Li)

Keywords: Nickel-rich cathode materials, Surface reconstruction, Storage failure, Performance recovery, Lithium-ion batteries

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Abstract The highly active surfaces of Ni-rich cathodes usually result in rapid surface degradation, which is manifested by poor cycle and rate capabilities. In this work, we propose a simple method to restore those degraded surfaces after storage. More importantly, the mechanism of surface degradation and recovery are investigated thoroughly. As storage in moist air, a lithium carbonate (Li2CO3) dominated impurity layer formed and tightly coated on the surface of the LiNi0.70Co0.15Mn0.15O2 particles. Except for the Li2CO3 layer, a NiO rock-salt structure was also found at near surface region by high-resolution transmission electron microscopy. These two inert species together impedance the transport of lithium ions and electrons, which result in no capacity at 4.3 V charge cutoff voltage of the stored material. We proposed a simple and effective method, i.e. 3 h calcination at 800 ℃ under oxygen flow. The restored LiNi0.70Co0.15Mn0.15O2 shows equivalent electrochemical performance compared to the pristine one. This is because the lithium ions in Li2CO3 layer return to the surface lattice of LiNi0.70Co0.15Mn0.15O2, and the NiO cubic phase transforms back to the layered structure with the oxidation of Ni2+. This method is not only insightful for cathode material design but also beneficial for practical application.

1. Introduction Rechargeable lithium-ion batteries (LIBs) are widely used power source in portable electronics, electric vehicles (EVs) and energy storage systems (EESs).1-9 The goal of meeting high energy density requirement of 300 Wh kg-1 by the year 2020, will 2 ACS Paragon Plus Environment

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promote new applications, such as high-performance drones. However, it is still difficult to achieve the target by only using LiNi1/3Co1/3Mn1/3O2,10-12 LiFePO413-15 or LiMn2O4 (including LiNi0.5Mn1.5O4)16, 17 as a cathode and commercial graphite as an anode. Exhilaratingly, The combination of Ni-rich LiNixCoyMn1-x-yO2 cathodes (Nirich NCM, x ≥ 0.6)18-21 with high-capacity silicon-carbon anodes,22-25 can easily reach this goal and even higher. It is well known that higher specific capacity can be obtained as the nickel content increases in NCM system.26-28 However, the practical application is limited by the structural instability, low capacity retention, poor thermal stability and insufficient safety.26, 27, 29 The active surface of Ni-rich NCM material can react with molecules in air to form residual lithium components, which becomes a major obstacle for the production and application of Ni-rich NCM materials.9 Generally, the air instability of Ni-rich NCM materials is attributed to the high alkalinity of particle surface.9, 30, 31 When the Ni-rich cathode materials are exposed to the moist air, they readily react with H2O and CO2 to form LiOH, LiHCO3 and Li2CO3, coated on the particle surface.32-41 Meantime, the unstable Ni3+ would spontaneously transform to Ni2+ with active oxygen species generating.30,

33, 34

Moreover, the NiO rock-salt

structure forms on the surface with the consumption of Li+ and release of lattice oxygen.39, 42 Recent report, it turns out that the air instability also occurs in sodium Nirich layered oxides.41 To alleviate these problems, researchers have put lots of efforts on modification, such as bulk doping1,

28, 43

and surface coating30,

32, 44

, and new

electrolyte systems to match Ni-rich NCM cathodes.8, 45-49 Nicholas et al.40 suggested that the moisture was the decisive factor to form Li2CO3, and ambient CO2 cannot 3 ACS Paragon Plus Environment


directly react with layered oxide at room temperature. They also thought that the Li2CO3 on the surface of layered compounds was not harmful to the material’s cycle life. While many researchers proposed that the Li2CO3 generation has adverse effects on the electrochemical performance. Grenier et al.50 presented that the thickness and structure of the Li2CO3 layer may be influenced by exposure time, temperature, and CO2/H2O partial pressures. In addition, the nonuniform erosion of Li2CO3 surface layer on air-exposed LiNi0.8Co0.15Al0.05 could induce reaction heterogeneity, called “twophase” behavior in first charge up to 4.1 V. Chen et al.39 reported that the LiNi0.6Co0.2Mn0.2O2 cathode suffered from severe degradation in the electrochemical performance after storage, under 55 ℃ and 80% relative humidity (RH). The changes of adsorbed species, LiOH/Li2CO3 impurities, and delithiation layer, were found in the stored material. The surface deterioration of Ni-rich cathodes is fatal to the electrochemical performance and practical application. As for the surface degraded Ni-rich NCM materials, the approach to restore the performance loss has seldom been investigated before. It is essential to find a simple and effective way to restore the failed materials in practical application. In this work, we investigated the failure and recovery behavior of the Ni-rich Li0.70Co0.15Mn0.15O2 cathodes stored in 60 ℃ and 80% RH for 30 days. The NCM at pristine, stored and calcined states were characterized by the combination of scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, together with X-ray photoelectron spectroscopy and transmission electron microscopy. It was confirmed that an impurity layer composed of Li2CO3 with ~ 90 nm thickness and NiO rock-salt 4 ACS Paragon Plus Environment

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structure was formed on the stored NCM surface, which showed no electrochemical activity. We proposed a simple method to recover the degraded NCM materials, which is high temperature calcination with oxygen flow. The effect on the processed material has been verified by its fully recovered electrochemical performance and the underlying mechanism are discussed in detail. This method is not only insightful for cathode material design, but also beneficial for industrial application.

2. Experimental section 2.1. Preparation of electrode materials The commercial Ni-rich LiNi0.70Co0.15Mn0.15O2 (NCM701515) was supplied by Hunan Changyuan Lico Co., Ltd. (China). The storage process was carried out in a constant temperature and humidity chamber (ESPEC, Japan) with 60 ℃ and 80% RH for 30 days. Then the stored NCM701515 calcined in 800 ℃ for 3 h under oxygen flow. To discuss easily, the pristine, stored and heat-treated cathodes were named P-NCM, S-NCM and CS-NCM, respectively. 2.2. Characterization The crystal structure was characterized by using X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer (Bruker, USA), which operated at 40 kV and 40 mA with Cu Kα radiation under a scan speed of 5° min-1 from 10°-80°. The morphologies of particles were observed by using scanning electron microscope (SEM) SU8010 (Hitachi Corporation, Japan). The Fourier transform infrared spectroscopy



(FTIR) was performed by the Nicolet iS 50 (Thermo Fisher Scientific, USA) with pellet method over the range of 400-4000 cm-1. X-ray photoelectron spectroscopy (XPS) was employed in PHI 5000 VersaProbe Ⅱ (Ulvac-Phi, Japan) using with an Al Kα (1486.6 eV) excitation source, and the binding energy was corrected by C 1s (284.8 eV) as a reference. The transmission electron microscopy (TEM) was performed in Tecnai G2 F30 (FEI, USA) to record the microstructure and surface lattice images of NCM. The inductively coupled plasma optical emission spectroscopy (ICP-OES) was carried out in ARCOS Ⅱ MV (Spectro, Germany) to identify the chemical composition of NCM with different treatments. The particle size distribution test of NCM701515 was measured by Mastersizer 2000Hydro 2000MU (Malvern, England) with 50 Hz of ultrasonic frequency, the deionized water is as a dispersing agent. A titration method 9, 41

was adopted to determine the residual lithium content for the series of samples.

Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) of the samples were performed on STA449 F3 (NETZSCH, Germany) at a heating rate of 10 ℃ min-1 from 30 ℃ to 850 ℃ with air flow. 2.3. Electrochemical measurements The electrochemical performances were tested in coin-type cells (CR2032). The NCM701515 powders in different stats were mixed with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 80 : 10 : 10 in N-methyl-1,2pyrrolidone (NMP) solvent. The slurry was spread onto an Al foil by doctor blade method, and dried in vacuum oven at 110 °C overnight, then punched into discs with 12 mm diameter. The active material loading level was controlled in ~3 mg cm-2. The 6 ACS Paragon Plus Environment

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CR2032 coin-type cells were assembled with the cathode electrode, Li metal anode, a separator (Celgard 2400) and the organic electrolyte (1M LiPF6, EC/DMC/EMC 1:1:1 vol%) in an argon-filled glove box (LABstar MBRAUN, Germany). The cycling performance and rate capacity varied from 0.2 C to 10 C (1 C = 170 mA g-1) were assessed on battery testing system (LAND CT2001A, China) between 2.8 V and 4.3 V at 25 ℃. The cyclic voltammetry (CV) was recorded in the VMP3 system (Bio-Logic, France) with a scan rate of 0.05 mV s-1 in a voltage window of 2.8-4.5 V. The electrochemical impedance spectra (EIS) were applied by the ModuLab XM workstation (Solartron, England) with AC voltage of a 5 mV amplitude and the frequency range from 0.01 Hz to 100 kHz.

3. Results and discussion 3.1 Material Characterization of NCM at Pristine, Stored and Calcined States. To clarify the changes of NCM701515 after storage and calcination recovery, the surface morphology, chemical composition and crystal structure have been characterized by SEM, FTIR, XRD, XPS, TEM, and ICP-OES, respectively. The microstructure of NCM701515 consists of spherical secondary particles (Figure 1a) formed by the agglomeration of primary nanoparticles with median particle size of 17.6 μm (Figure S1). The spherical morphology of the secondary particles did not experience any obvious change after storage (Figure 1b) and recovery calcination (Figure 1c). However, in the magnified images of Figure 1d-f, there is obvious



difference between the pristine material (P-NCM), the stored material (S-NCM) and calcined material (CS-NCM). The P-NCM shows very smooth surfaces with clear boundaries among the primary particles. In contrast, the surfaces of S-NCM turn to be very rough, coated with an impurity layer as seen obviously from the red circled area (Figure 1e). In Figure 1f of CS-NCM, the surface is almost the same as P-NCM and the impurity surface can be removed by the simple heat-treatment from morphology perspective. Whether the impurity layer have been removed completely with the high temperature calcination will be discussed in combination with other characterization.

Figure 1. SEM images of (a, d) P-NCM, (b, e) S-NCM, and (c, f) CS-NCM.

FTIR was applied to investigate the surface chemical composition of all the three samples as shown in Figure 2a. A set of peaks was observed in the S-NCM instead of the other two samples. The peak at 530 cm-1 represents the metal-oxygen bond (M-O,


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M = Ni, Co, Mn), which is seen is all three samples. The peak at 1488 cm-1 and 1438 cm-1 are caused by the antisymmetric stretching vibration of -CO3 group, and the peak at 868 cm-1 is due to the out-of-plane bending vibration of -CO3 group.37-40 These peak signals indicate that the impurities are composed of carbonate. FTIR spectra of P-NCM and CS-NCM are almost identical, suggesting successful removal of the carbonate impurity layer. The XRD patterns are shown in Figure 2b, all the diffraction peaks in the patterns were indexed to the layered α-NaFeO2 structure with 𝑅3̅𝑚 space group.20, 51 Detailed 2θ degree range of 18.5-19.0° and 20-34° are also displayed. Rietveld refinements of XRD patterns are shown in Figure S2 with the lattice parameters listed in Table S1. The (003) reflection of the S-NCM shifted toward lower 2θ compared to what of the PNCM and CS-NCM samples. An expansion along the c-axis is expected from the shift of (003) to lower angle,52 also seen from Table S1. This is due to the migration of Li+ from lattice, which increases the electrostatic repulsion between the oxygen layers. It is very similar to the de-lithiation process during charging.53, 54 In addition, the diffraction peaks of lithium carbonate appear in range of 20-34° of the S-NCM pattern, which is in agreement with previous publication.34, 37, 39 While no diffraction peaks of lithium carbonate are observed in the P-NCM and the CS-NCM. This demonstrate clearly the existence of impurity phase as Li2CO3 on S-NCM. To avoid the fake shift of the lab source XRD on peak positions, the high purity silicon (99.99%) is mixed with NCM powder as the standard to calibrate the peak position (Figure S3). The result is consistent with Figure 2b. 9 ACS Paragon Plus Environment


Furthermore, the chemical states of the surface elements were investigated by XPS (Figure S4). The atomic percentage of the main elements are displayed in Figure 2c with their fine sections shown in Figure 2d-h. The surface of all three samples is dominant by C and O. To get a better understanding of the surface chemistry, the C 1s and O 1s spectra are analyzed with MultiPak software as shown in Figure 2d and e. The C 1s spectra can be divided into three peaks corresponding to CO32- (289.8 eV), C=O (286.5 eV) and C-C (284.8 eV). The area proportion of peak at 289.8 eV (CO32-) is 21%, 33% and 24% for the P-NCM, S-NCM and CS-NCM, respectively. The O 1s spectra can be fitted with two peaks, indexed to the Oimpurity (531.9 eV) and the Olattice (529.5 eV).42, 55, 56 The Oimpurity is mainly originated from the active oxygen species O-, O2or/and CO32- of the Li2CO3 impurity layer. The Olattice is derived from the lattice oxygen O2- of the M-O (M=Ni, Co, Mn) bond. Noted that the area percentage of Olattice is 19% for the P-NCM and 13% for the CS-NCM, while only 2% can be detected from the SNCM. It infers that the surface is covered by impurity layer, mainly composed of Li2CO3. This is in consistent with the weak signals of transition metal elements in Figure 2f-h. The peaks of Ni 2p (873, 855 eV), Co 2p (796, 780 eV) and Mn 2p (654, 643 eV) can be clearly seen in P-NCM and CS-NCM, but not apparent in S-NCM. The reducing in signal is mainly caused by the thick impurity layer on the surface.


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Figure 2. (a) FTIR spectra of P-NCM, S-NCM and CS-NCM. (b) XRD patterns of the P-NCM, S-NCM and CS-NCM, including the detail magnification of the degree range 18.5-19.0° and 20-34°. (c) Atomic percentage of the main elements of P-NCM, S-NCM and CS-NCM through XPS analysis. XPS sepctra of (d) C 1s, (e) O 1s, (f) Ni 2p, (g) Co 2p, and (h) Mn 2p.

Figure 3 displays the TEM images of P-NCM, S-NCM and CS-NCM. The P-NCM and CS-NCM have clean and smooth surfaces (Figure 3a and c), while a ~ 90 nm impurity layer was observed on the surface of S-NCM (Figure 3b). Much detail 11 ACS Paragon Plus Environment


structure near surface region of S-NCM are clarified in the high-resolution TEM images (Figure 3d-f). Different from the P-NCM and CS-NCM, in which only one rhombohedral phase was observed, the S-NCM not only has an Li2CO3 layer on the surface, but also forms a rock-salt structure near the surface during storage.34 The Li2CO3 layer is formed by the interaction between the active Li+ in the near surface lattice and H2O/CO2 in the air, which is in agreement with the peak shift of XRD.34, 42 During this process, Ni3+ can spontaneously reduce to Ni2+ to produce the NiO rocksalt structure.34, 42 The thick Li2CO3 layer and the inactive NiO phase will become dual barriers to the kinetics of bulk NCM material. The TEM analysis also suggest strongly that the high-temperature treatment in O2 can restore the disordered surface structure.

Figure 3. TEM images of (a, d) P-NCM, (b, e) S-NCM and (c, f) CS-NCM. The lattice fringes of P-NCM have an interplanar distance of 4.734 Å, which is assigned to the (003) planes in a rhombohedral phase. The CS-NCM also shows a pure rhombohedral


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phase with interplanar spacing of 2.448 Å indexed to (101) planes. The S-NCM displays two phase in surface region, including rhombohedral and cubic phase, the latter shows an interplanar spacing of 2.410 Å indexed to (111) planes. The insets of (d), (f), and (e) are FFT images of the R3̅m layered structures and Fd3̅m cubic phase, respectively.

3.2 Electrochemical Performance of P-NCM, S-NCM and CS-NCM. Because of the impurity layers, the stored material S-NCM shows quite different electrochemical performance compared with what of P-NCM and CS-NCM. The NCM/lithium half cells were first cycled at 0.2 C rate for three cycles of activation and then cycled at 1 C rate between 2.8 V and 4.3 V. The fist charge-discharge curves at 1 C rate are shown in Figure 4a. The P-NCM delivers a discharge capacity of 169.8 mAh g-1, CS-NCM with almost the same discharge capacity of 168.1 mAh g-1, and only 0.7 mAh g-1 for S-NCM. The capacity of P-NCM faded slowly to 153.2 mAh g-1 after 100 cycles at 1 C with a capacity retention of 90.2%, and the CS-NCM displays an equivalent capacity retention of 90.4% (Figure 4b). In contrast, the S-NCM did not deliver any capacity with irregular coulomb efficiency at the same charge-discharge conditions. The “zero capacity” behavior of S-NCM also appeared in the rate property test, while the P-NCM and CS-NCM delivered available capacities under each rates from 1 C to 10 C (Figure 4c). The inactive behavior of S-NCM are broken when cycled at higher cut-off voltage during the activation stage. Figure 4d shows the cycling performance of S-NCM at 0.2 C from 2.8 V to 4.5 - 4.8 V for the first three cycles, and



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then cycled at 1 C between 2.8 V and 4.3 V. It can be seen that higher capacity is obtained at higher cut-off voltage. Although the S-NCM could deliver certain capacity in this case with poor cycling stability, it is much lower than that of P-NCM and CSNCM. CV curves were recorded for the first three cycles at a scan rate of 0.05 mV s-1 with upper cut off voltage of 4.5 V for P-NCM and S-NCM as described in Figure 4e and f. The CV curves of CS-NCM was almost exactly the same as what of P-NCM (Figure S5a). Three pairs of redox peaks are observed from 2.8 V to 4.5 V corresponding to the different phase transformations, as reported in other previous literatures.27, 29, 57 There is an activation process comparing the first and following cycles. The activation potential is 4.4 V for S-NCM and 3.9 V for P-NCM. According to previous literature, the activation could be caused by either bulk structure rearrangement of the material or the Li2CO3 surface film.50,

58

In this experiment

conduction, S-NCM sample with more surface impurity layer displays much more obvious activation process. The much larger activation potential of NCM should be mainly caused by the surface impurity film. The impurity surface film serves as barrier for the lithium (de-)insertion reaction’s kinetics. When the cut off voltage is as low as 4.3 V, the poorly conductive Li2CO3 layer and inactive NiO layer impede the electrochemical reaction, so the Li+ could hardly be extracted from the bulk material (Figure S5b). When the potential is large enough, the Li2CO3 is gradually break down through oxidation to create conduction paths, the “zero capacity” is overcome and a decrease of potential is seen in the subsequence cycles. However certain amount of active Li+ are still lost during the impurity layer formation and the performance 14 ACS Paragon Plus Environment

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recovery is limited.

Figure 4. (a) The charge-discharge curves of P-NCM, S-NCM and CS-NCM. (b) Cycling and (c) rate performance of P-NCM, S-NCM and CS-NCM. (d) Cycling performance of S-NCM active in high voltage. CV curves of (e) P-NCM and (f) SNCM.



3.3 Possible Surface Reconstruction Methods and Electrochemical Performance In order to further understand whether the activation barrier is caused by the Li2CO3 surface film or NiO rock-salt structure, or both, we have designed and compared different treatment methods to remove the impurity layer. The S-NCM was treated with water washing, calcining in Ar flow (800 ℃, 3 h), and Air flow (800 ℃, 3 h), denoted as S-NCM-WW, CS-NCM-Ar and CS-NCM-Air respectively. As shown in Figure 5, the Li2CO3 layer can be removed via all the three different treatments to obtain a smooth surface from SEM observation (Figure 5a-c). For cycling test in Figure 5d, the S-NCMWW still cannot deliver capacity in the whole cycling process. The CS-NCM-Ar displays very low specific capacity of 109.3 mAh g-1 in the initial cycle with only 56.5% capacity retention after 100 cycles. While the CS-NCM-Air displayed excellent recoverability with an initial capacity of 166.1 mAh g-1 and 88.6% capacity retention. The ICP-OES technique was also combined to analyze the relative proportion of Li, Ni, Co and Mn, especially on distinguishing the content of Li. Figure 5e displays the chemical composition of NCM was analyzed using ICP-OES (detail value in Table S3), which gives the total amount of each element regardless of its spatial distribution and phase. In addition, the content of residual lithium on the surface was analyzed by a titration method as shown in Figure 5f. The proportion of Ni : Co : Mn is close to 0.70 : 0.15 : 0.15 for all the samples. Nevertheless, the content of Li element is quite different. Specifically, the Li is reduced by 13.1% in the S-NCM-WW compared to the P-NCM. This is caused by the removal of surface Li2CO3 after water washing, as demonstrated


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by the decrease of lithium residues. However, this cannot help to restore the inactive NiO rock-salt phase, which explains the poor electrochemical performance. Notably, the Li element of P-NCM, S-NCM and CS-NCM are almost the same, while the content of Li residues obviously increased in the stored material compared to the pristine one. It indicates the calcination will induce the lithium ions in the Li2CO3 layer back to the lattice, and the rock-salt phase is restored to the layered crystal structure. Therefore, it manifests that the rock-salt phase plays an important role upon the degradation of NCM701515, which agrees with previous reports.34, 56 Calcination at high temperature is an effective method to restore the phase disorder, however the lithium residues didn’t decrease but increase with calcination under Ar flow. The Li2CO3 impurity decomposed to Li2O during this process, the inert atmosphere would induce oxygen loss and lattice lithium extraction to create more lithium residues. So calcination with O2 atmosphere is necessary for the oxidation of NiO rock salt layer and insertion of residual lithium back to the lattice structure. Oxygen flow with high partial pressure is more beneficial to restore the failed materials, even complete recovery, as displayed by the HR-TEM of CS-NCM (Figure 3f).



Figure 5. SEM images of (a) S-NCM-WW, (b) CS-NCM-Ar, (c) CS-NCM-Air. (d) Cycling performance of samples with different treatments. (e) ICP-OES result for the chemical composition of the samples with different treatments. (f) Concentration of residual lithium on the samples with different treatments (the concentration is calculated based on lithium mass only). Here, the CS-NCM-O2 and CS-NCM refer to the same sample.

3.4 Selection of Calcination Temperature with O2 Atmosphere. In addition to the possible surface reconstruction methods, we have proposed a series of calculation temperature study for further procedure refinement. The stored NCM can be fully recovered by calcining at 800 ℃ under O2 flow as mentioned above. It would be more meaningful for practical application if calcination at lower temperature could achieve the same purpose. TG & DSC tests were performed for the pristine NCM, stored NCM and pure Li2CO3 materials (Figure S6). The stored NCM 18 ACS Paragon Plus Environment

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shows much larger mass drop (3.01%) than that of pristine NCM (0.85%). In addition, two mass drop stage were observed for the S-NCM sample (Figure 6a and b). Three characteristic calcination temperatures, i.e. 350 °C, 500 °C, 650 °C, were chosen to verify if they have similar recovery effect as what of 800 °C, labeled CS-NCM-350, CS-NCM-500, and CS-NCM-650, respectively. Through the combination analysis of SEM (Figure S7), FTIR (Figure 6c), and electrochemical tests including cycling performance (Figure 6d), cyclic voltametry and impedance spectroscopy (Figure S8), it’s found that calcination temperature increasing to 650 °C could remove detectable Li2CO3, however the electrochemical performance could hardly be comparable with that at 800 °C. It indicates that full recovery of the stored material requires a sufficient high temperature to decompose the surface impurity, oxide the NiO layer, and provide driving force high enough achieve the surface reconstruction.



Figure 6. TG and DSC tests for (a) P-NCM and (b) S-NCM. (c) FTIR spectra of SNCM calcined at different temperatures. (d) Cycling performance of P-NCM and SNCM calcined at different temperatures.

3.5 Mechanisms of Surface degradation and Reconstruction. Herein, we propose the failure and recovery mechanism of the stored NCM701515, which is shown in Scheme 1. With the growth of surface Li2CO3 layer, the lithium ions would escape from the lattice near the surface, resulting in the transformation of Ni3+ to Ni2+ and the layered structure to the NiO rock-salt structure. So there are two inactive layers formed near the surface. Wash the aged NCM with water, only the surface Li2CO3 layer could be cleared. The S-NCM was calcinated at high temperature with inert Ar flow, the Li2CO3 layer can also be removed. However the surface lithium could hardly insert back to the lattice with the NiO layer largely retained at oxygen deficient environment. Further, the stored NCM701515 is calcined at high temperature with oxidizing gas (e.g. O2, air) flow, the lithium ions could return to the lattice with the pyrolysis of Li2CO3 impurity. At the same time, Ni2+ in the NiO phase can be reoxidized to Ni3+ and restored back to the layered structure. To a large degree, the lattice near surface can be restored like the fresh materials and achieve a complete recovery in electrochemical performance.


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Scheme 1. The failure and recovery behaviors of the stored NCM701515.

4. Conclusions In this study, the failure and recovery behaviors of the stored NCM701515 were carefully investigated. After storage, an impurity layer was strictly coated on the surface of particles through SEM observation. Furthermore, FTIR, XRD, XPS and TEM confirmed that the chemical composition of the impurity is mainly Li2CO3 with ~ 90 nm thickness. In addition, the high-resolution TEM revealed that the NiO rock-salt structure was formed in the near surface region. The ICP-OES and residual lithium results verified that the generation of Li2CO3 layer consumed some active lithium ions from lattice. The inert Li2CO3 layer together with NiO rock-salt structure are obstacles for the delithiation reaction of bulk NCM and damage the electrochemical performance seriously. More importantly, we found a simple and effective method to remove these barriers by high temperature calcination with oxygen flow (e.g. 800 ℃ for 3 h). During 21 ACS Paragon Plus Environment


this process, the lithium ions in the Li2CO3 layer can return to the lattice, and the rocksalt phase transform back to the original layered structure with the oxidation of Ni2+ to Ni3+. It is found that the inactivated NCM701515 was fully restored, as manifested by the electrochemical performance. This work deeply enhances the understanding upon the failure and recovery mechanisms of electrode material, which is beneficial for the practical production and application of Ni-rich layered oxide cathodes.

Acknowledgments This work was supported by National Nature Science Foundation of China (No. 51872157), Shenzhen Technical Plan Project (No. KQJSCX20160226191136, JCYJ20170412170911187 and JCYJ20170817161753629), Guangdong Technical Plan Project (No. 2015TX01N011) and Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01N111), Special Fund Project for Strategic Emerging Industry Development of Shenzhen (No. 20170428145209110), and Shenzhen Key Laboratory of Security Power Battery Research (No. ZDSYS201707271615073).

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