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Investigations on the Surface Degradation of LiNi1/3Co1/3Mn1/3O2 after Storage Binhua Huang, Kun Qian, Yuxiu Liu, Dongqing Liu, Kai Zhou, Feiyu Kang, and Baohua Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00621 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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ACS Sustainable Chemistry & Engineering

Investigations on the Surface Degradation of LiNi1/3Co1/3Mn1/3O2 after Storage Binhua Huang a,b,1, Kun Qian b,c,1, Yuxiu Liu a,b, Dongqing Liu a, Kai Zhou a,b, Feiyu Kang a,b,c,d, Baohua Li a,d,*

a

Engineering Laboratory for Next Generation Power and Energy Storage Batteries, and

Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, No.2279, Lishui Road, Nanshan District, Shenzhen, 518055, China b

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

University, No. 30, Shuangqing Road, Haidian District, Beijing, 100084, Chinas c

Shenzhen Environmental Science and New Energy Technology Engineering Laboratory,

Tsinghua-Berkeley Shenzhen Institute, Xili University Town, Nanshan District, Shenzhen, 518055, China d

Shenzhen Geim Graphene Center, Xili University Town, Nanshan District, Shenzhen, 518055,

China ∗

Corresponding author

E-mail address: [email protected] (B. Li) 1

These authors contributed equally to this work.

Keywords: Surface degradation, Storage property, LiNi1/3Co1/3Mn1/3O2, Cathode, Lithium-ion battery

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Abstract: The surface evolution of LiNi1/3Co1/3Mn1/3O2 (NCM333) during storage is investigated detailedly in this paper. Different from the traditional awareness of NCM333 with excellent storage stability, considerable deterioration is found including specific capacity loss, cycling and rate performance decline after long-time storage especially in high-humidity environment. Small flaky impurity particles are observed on the surface by scanning electron microscope, which are proved to be Li2CO3 and LiOH through Fourier-transform infrared spectroscopy tests. X-ray photoelectron spectroscopy depth profiling directly shows that the proportion of oxygen element in the lattice of NCM333 and the impurities in different depths, revealing the degree of degradation. X-ray diffraction analysis confirms that the stored NCM333 still maintains the pristine layered crystal structure, proving surface degradation leads to the performance deterioration. During storage, the lithium ions were extracted from superficial lattice to become active Li+, which gradually react with H2O and CO2 in moist air to form lithium impurities. Therefore, the humidity of air should be controlled during the storage, which is beneficial for the production and application of NCM333 cathode material.

 INTRODUCTION Lithium-ion batteries (LIBs) have been widely applied in many fields such as portable digital products (mobile phone, laptop, and camera et al.), electric vehicles (EVs), energy storage systems (ESSs) and smart grid.1-10 However, the cathode material


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is still the bottle-neck, which greatly impedes the development of high-performance LIBs. Cathode materials possess much lower specific capacity (100~160 mAh g-1) when compared with the anodes ( 300 mAh g-1), with weight and cost contributing ~50% and ~40% of the whole battery, respectively.11 Fortunately, the layered LiNixCoyMn1-x-yO2 (0 < x, y < 1, NCM) material has been discovered and successfully applied in LIBs with higher energy density, lower cost and much more environmentfriendly than LiCoO2.12-17 Moreover, an improved electrochemical performance of NCM material can be obtained by tuning the ratio of transition metal. The NCM material with equal molar ratio of Ni, Co and Mn, namely x = y = 1/3, was firstly synthesized by Ohzuku’s group18 in 2001, which combined the high specific capacity of LiNiO2, high conductivity and satisfactory cycling performance of LiCoO2 and low cost of LiMnO2.19 After that, numerous research keep focusing on this type of layered oxides. It is found that the elements of Ni, Co and Mn play different roles in the (de)lithiation process of NCM333. Specifically, the chemical valence of Ni changes between +2 and +4 to provide the charge/discharge capacity. The Co3+ is beneficial to reduce the Li/Ni disorder, and improve the cycling and rate performance. The chemical valence of Mn always keeps at +4 during charge-discharge process to maintain the stable structure.20 In recent years, many novel synthetic and modified methods, such as sol-gel method,21,

22

hydro/solvothermal method,23,

coating,12,

32-39

24

bulk phase doping,25-31 and surface

have been proposed, aiming to improve the performance of NCM.

However, the storage property of NCM material lacks attention to a large degree. 3 ACS Paragon Plus Environment


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Storage in air has serious negative effect for NCM materials, harmful for electrode fabrication and practical production.12, 38, 40, 41 Based on work of Liu et al.,42 a series of chemical processes occurred on the surface of the LiNiO2 during the storage including the formation of Li2CO3 on the surface, Ni3+-Ni2+ transformation and the evolution of active oxygen species. The chemical equations of the surface reaction during storage are as follows: Ni3+ + O(lattice)2- → Ni2+ + O-

(1)

O- + O- → O(active)2- + O

(2)

O- + O → O2-, or O + O → O2↑

(3)

O(active)2- + CO2/H2O → CO32-/2 OH-

(4)

2 Li+ + CO32-/2 OH- → Li2CO3/2 LiOH

(5)

Meanwhile, Zhang’s group 43 drew a similar conclusion on the investigation of LiNi0.8Co0.2O2 storage in air. In a further step, they discovered that two thin layers existed on the surface of stored material, namely NiO-like species close to bulk and top layer consist of adsorbed hydroxyl, bicarbonate and crystalline Li2CO3. They believed that the formation of lithium impurity on the surface would damage the electrochemical performance seriously because of their chemical inertness.43-45 While Li et al.46 proposed that the surface reaction of LiNi0.4Co0.3Mn0.3O2 was negligible when stored in air with CO2/H2O and faintly affect the electrochemical performance. But the oxygen may release from the surface of LiNi0.4Co0.2Mn0.4O2 during storage in oxygen deficient environment like Ar-filled glove box, which can lead to poor cycling performance. Traditionally, NCM333 cathodes have been considered to possess outstanding 4 ACS Paragon Plus Environment

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storage performance due to their excellent chemical stability. The Ni keeps stale chemical valence of +2 without spontaneous transformation of Ni3+-Ni2+ during storage,46 which is different from the stored LiNiO2 as mentioned before. However, in this work, we found that the NCM333 also undergone serious degeneration after longterm storage in air, particularly in high-humidity surroundings. Herein, we focused on the influence of storage time and humidity on the deterioration of NCM333. The degradation mechanism has been investigated by material characterization of scanning electron microscope, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction in combination with electrochemical cycling and rate performance. It revealed that the performance deterioration originates from surface degradation with impurities formed by consumption of active Li+ and CO2/H2O, which is accelerated at high humidity environment. This work will provide helpful guidance for the storage of high-performance cathode materials.

 EXPERIMENTAL SECTION Storage tests The commercial NCM333 was provided by Amperex Technology Limited (ATL, China). Part of the NCM333 material was stored in lab for one year’s time, exposure in air with ~ 45% relative humidity (RH) at 25 ℃, which is labeled as “Aged NCM333”. The rest NCM333 was stored under 50% RH, 25 ℃ and 80% RH, 25 ℃ for six months in a constant temperature and humidity chamber (ESPEC, Japan), labeled as “50% RHNCM333” and “80% RH-NCM333” respectively. The reference NCM333 was kept in 5 ACS Paragon Plus Environment


glass desiccator with silica gel self indicator. The content of CO2 remains the same as that in the ambient air, i.e. 400 ppm, for all the storage conditions. Material characterization The crystal structure was analyzed by X-ray diffraction (XRD) under a Bruker D8 Advance diffractometer (Bruker, USA) operated at 40 kV and 40 mA with Cu Kα radiation (λ = 1.541 Å, step size = 0.02°, 2θ = 10°-80° with 5° min-1). The morphologies were observed in scanning electron microscope (SEM) SU8010 (Hitachi Corporation, Japan). The Fourier-transform infrared spectroscopy (FTIR) was carried out the Nicolet iS 50 (Thermo Fisher Scientific, USA) in Attenuated Total Reflection (ATR) mode over the range of 400-4000 cm-1. X-ray photoelectron spectroscopy (XPS) was performed in PHI 5000 VersaProbe Ⅱ (Ulvac-Phi, Japan) using an Al Kα monochromatization of radiation (hν = 1486.6 eV) beam (100 μm, 25 W, 15 kV), the samples were charge-neutralized with an electron and Ar+ beam. All binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon to correct the shift caused by charge effect. The microstructure and surface lattice images of NCM samples were observed by Tecnai G2 F30 (FEI, USA). The inductively coupled plasma optical emission spectroscopy (ICP-OES) was utilized in ARCOS Ⅱ MV (Spectro, Germany) to verify the chemical element compositions of NCM333 materials. Electrochemical measurements The electrochemical performances were tested in coin-type cells (CR2032). A mixture of 80 wt% active cathode material, 10 wt% acetylene black, 10 wt% 6 ACS Paragon Plus Environment

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polyvinylidene fluoride (PVDF) binder was dissolved in N-methyl-1,2-pyrrolidone (NMP) solvent to obtain a uniform slurry. The slurry was coated onto aluminum foil current collector using doctor blade method, and dried in vacuum oven at 110 °C overnight, then punched into discs with 12 mm diameter. The mass loading of the active material was controlled in ~3 mg cm-2. The coin cells were assembled with the cathode electrode, Li metal counter electrode, a separator (Celgard 2300) and the organic electrolyte (1M LiPF6, EC/DMC/EMC 1:1:1 vol%) in an argon-filled glove box (LABstar MBRAUN, Germany). The charge-discharge profiles and rate capacity varied from 0.2 C to 10 C (1 C = 170 mA g-1) were performed on battery testing system (LAND CT2001A, China) between 2.8 V and 4.3 V at room temperature. The cyclic voltammetry (CV) was recorded in the VMP3 system (Bio-Logic, France) with a scan rate of 0.05 mV/s over the range of 2.8-4.3 V. The electrochemical impedance spectra (EIS) were carried out by the ModuLab XM workstation (Solartron, England) with an amplitude of 5 mV and the frequency range from 100 kHz to 0.01 Hz.

 RESULTS AND DISCUSSION Long-time storage influence on the performance degradation of NCM333 Unlike the traditional opinion that NCM333 possesses a stable chemical state and is easy to preserve, evident performance degradation was observed on the NCM333 sample after storage. Figure 1 displays the electrochemical performance of the reference and aged NCM333 with coin-type cells. In the initial charge-discharge profiles at 0.2 C rate (Figure 1a), the aged NCM333 delivers less discharge capacity of 145.3 mAh g-1 7 ACS Paragon Plus Environment


and lower initial coulomb efficiency (ICE) of 86.6%, compared with the 153.6 mAh g-1 capacity and 87.8% ICE of the reference NCM333. Besides, the cycling performance and rate capability also get impaired for the aged NCM333. The reference NCM333 delivers a discharge capacity of 141.4 mAh g-1 initially with capacity retention of 96.7% after 200 cycling at 1 C rate, while the capacity of aged NCM333 drops from 132.7 mAh g-1 to 122.1 mAh g-1 with a poor capacity retention of 89.7% (Figure 1b). The rate performance in Figure 1c shows that the available capacities of aged NCM333 are lower than that the reference sample at each rate. In order to find the reason why the aged NCM333 experienced such an electrochemical degradation, the crystal structure, surface morphology, chemical composition and surface chemical states were carefully examined by XRD, SEM, FTIR and XPS, respectively.


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Figure 1. (a) The initial charge-discharge curves of Reference NCM333 and Aged NCM333 at 0.2 C rate; (b) cycling performance and (c) rate performance of Reference NCM333 and Aged NCM333 with rates varied from 0.2 C to 10 C; (d) XRD patterns of Reference NCM333 and Aged NCM333.

Figure 1d displays the XRD patterns of the reference and aged NCM333. The diffraction peaks were well maintained at the original 2θ positions and no additional peaks appear, which suggests that the aged NCM333 still keeps the hexagonal layer αNaFeO2 crystal structure after storage, and the crystal structure is not the reason for performance decline. More information of crystal structures are provided by the Rietveld refinements (Figure S1 and Table S1). The chemical element compositions of reference and aged NCM333 have verified by ICP-OES, showing no distinct difference (Table S2). Nevertheless, an obvious change on surface morphology is found between the reference NCM333 and the aged sample by SEM. In Figure 2, the NCM333 materials show ~10 μm diameter spherical particles in form of agglomerated primary particles. There is no obvious difference for reference and aged NCM333 samples at low magnification (Figure 2a and c). While in the magnified images of Figure 2b and d, 5-20 nm flake-like grains are observed at the boundary area among the primary particles of the aged NCM333. Those impurities already filled and covered the tiny gap between the primary particles, which may be the reason of the electrochemical degradation. The obvious differences of these samples were also revealed by TEM (Figure 2c and f). 9 ACS Paragon Plus Environment


Figure 2. SEM images of (a, b) Reference NCM333 and (c, d) Aged NCM333; TEM images of (c) Reference NCM333 and (f) Aged NCM333.

FTIR was employed to identify the chemical composition of the impurities as shown in Figure 3a. The peak at 2362 cm-1 is caused by the CO2 in the air, and the peak at 537 cm-1 is indicative of the metal-oxygen bond (M-O, M = Ni, Co, Mn). Notably, there is considerable expansion on several peaks located at 1111 cm-1, 1400 cm-1, 1632 cm-1, 3141 cm-1 and 3436 cm-1. According to previous literatures, the peaks at 3436 cm-1 and 3141 cm-1 belong to the peaks of hydroxyl from LiOH.41, 47 While, the peaks at 1632 cm-1, 1400 cm-1 and 1111 cm-1 are caused by the C=O anti-symmetric stretching vibration and C-O symmetrical stretching vibration in CO32- group.48 It suggests that the NCM333 surface is covered by LiOH and Li2CO3 impurities both before and after storage. However, the amount of those impurities increased during storage, which is in


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agreement with the results observed by SEM. Furthermore, XPS analysis confirmed the surface chemical composition before and after storage. In Figure 3b of the O 1s spectrum, it clearly shows two distinctive peaks corresponding to lattice oxygen O-M (M = Ni, Co, Mn) at ~529 eV and active oxygen species O-, O2-, OH- and the CO32- near 532 eV.42 The binding energy of oxygen in hydroxide –OH is close to that of carbonate group CO32-. Here, we take both of them as surface impurity, i.e. Oimpurity. The information of carbonate group can be recognized more easily from the C 1s spectrum (Figure S2). The growth of surface impurity on aged NCM333 is verified strongly by the obvious increase of Oimpurity peak in the O 1s and CO32- peak in the C 1s. In comparison, the chemical states of Ni, Co and Mn keep their original valence of +2, +3 and +4 after storage (Figure S3). The depth profiling of XPS was employed to evaluate the thickness of the impurity layer through the change of Olattice and Oimpurity. In Figure 3c and d, an increase of Olattice peak and decrease of Oimpurity peak can be seen with sputtering time. The atomic concentration of Oimpurity decreases from 68% to a relatively stable state of 48% in around 3 min for reference NCM333, while it takes 6 min for aged NCM333 due to the higher initial concentration around 85% (Figure 3e). Here, the applied sputtering rate is 2.5 nm min-1 by calibrating with standard SiO2 wafer. Therefore, the thickness of the impurity layer can be roughly determined according to the sputtering rate. In this case, the impurity layer of aged NCM333 samples is ~7.5 nm thicker than the reference NCM333, which result in higher energy barrier for the charge transfer at the interface. Meantime, the XPS depth-profiling of Ni, Co, and Mn was also performed (Figure S4). 11 ACS Paragon Plus Environment


In the reference and aged NCM333, the peak positions are stable but the peak intensity increases with the removal of impurities layer by Ar+ sputtering.

Figure 3. (a) FTIR spectra and (b) XPS spectra of O 1s from Reference NCM333 and Aged NCM333; Selective XPS spectra of O 1s with Ar+ ion sputtering at different times for (c) Reference NCM333 and (d) Aged NCM333; (e) Chang of the atomic concentration for Oimpurity with sputtering time for NCM333 before and after storage.

Figure 4 shows the first three CV curves of the reference and aged NCM333 between 2.8 V and 4.3 V at scan rate of 0.05 mV/s. The first cycle is different from the subsequent cycles for the two samples. The oxidation peak of Ni2+ to Ni4+ decrease from 3.82 V in the first cycle to 3.78 V with cycles for the reference NCM333. It’s an activation process caused by the surface impurity layers of NCM333.12, 49 The aged sample process higher potential at 3.89 V and larger potential difference towards 3.81 12 ACS Paragon Plus Environment

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V because of much thicker surface impurity layer. In addition, the aged NCM333 shows two obvious oxidation peaks at 3.89 V and 3.97 V, these two peaks are ascribed the oxidation of Ni2+,50 but influenced by the nonuniform erosion of surface layer.51 More specifically, the impurity layer is distributed randomly on the surface, and this nonuniform distribution would induce quite large reaction heterogeneity of the sample, which is manifested by oxidation peaks at different potentials. For reference NCM333 with very thin impurity surface layer, this effect is not obvious as the oxidation peaks with difference polarization overlay. The EIS measurements were carried out at an electrochemical stable state of 4 V for each cell (Figure 4c). The much larger interfacial resistance (Rsei) and charge transfer resistance (Rct) of the aged NCM333 imply the hindrance of ion and electron migration through the impurity layer. This is in agreement with the increased polarization observed in CV results.

Figure 4. CV profiles of (a) Reference NCM333 and (b) Aged NCM333; (c) electrochemical impedance spectra of Reference NCM333 and Aged NCM333, the inset is the equivalent circuit model and value of Rs, Rsei and Rct by fitting.

Impact of humidity on the surface deterioration 13 ACS Paragon Plus Environment


The effect of humidity on the surface degradation is less known compared with storage in air so far. The NCM333 was stored at 25 oC with precise control of the relative humidity of 50% and 80% for six months. Figure 5 displays the surface morphology of NCM333 after storage, considerable tiny impurities are found on the two samples, which indicates water vapor accelerated the formation of the LiOH/Li2CO3. From the O 1s of XPS spectra, the Oimpurity raising to 71% for 50% RH and 75% for 80% RH conditions, suggesting that higher humidity could induce more impurity formation and is detrimental to the storage of NCM333 material.

Figure 5. SEM images of NCM333 after storage under (a) 50% RH and (d) 80% RH; TEM images of NCM333 after storage under (b) 50% RH and (e) 80% RH; XPS spectra of O 1s on aged NCM333 under (c) 50% RH and (f) 80% RH.

Figure 6 summarized the electrochemical performance including CV, cycling performance and EIS data of samples stored in high humidity. It can be seen that 14 ACS Paragon Plus Environment

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samples stored under 50% and 80% RH both display distinct peaks at around 4.0 V (indicated by arrow in Figure 6a and b), and this phenomenon is similar to that in Figure 4b, which is caused by the surface impurity induced reaction heterogeneity. However, larger potential is observed in the aged NCM333 at 80% RH, proving thicker impurity layer formation induced slower reaction kinetics. This is manifest in the cycling performance, the sample under 80% RH storage experienced more severe capacity fading at 1 C rate (Figure 6c). The EIS spectra also verified that the 80% RH-NCM333 sample has larger Rsei and Rct values (Figure 6d), which also supported the conclusion that surface impurity is easy to form in high-RH environment. A much comprehensive summary of the electrochemical parameters measured after different storage conditions are listed in Table S3.

Figure 6. The first and second CV cycle of NCM333 after storage under (a) 50% RH



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and (b) 80% RH; (c) long cycling performance of reference, 50% RH-NCM333 and 80% RH-NCM333; (d) electrochemical resistance of NCM333 half cells with 50% RHNCM333 and 80% RH-NCM333, the inset is the equivalent circuit model and value of Rs, Rsei and Rct by fitting.

Here we propose the surface degradation mechanism on NCM333 material. When NCM333 material is stored in air, CO2 and H2O are adsorbed on the particle surface to form carbonic acid or carbonate group. The surface of NCM333 is alkaline and it is prone to react with those surface adsorption groups. CO32- ions can capture Li+ from the crystal lattice to generate Li2CO3. Meanwhile, some O2- ions in the crystal structure may combine with H+ to keep charge balance and produce LiOH in the end. The LiOH can further react with CO2 to form Li2CO3. These impurity components impede the pathways of electron and ion migration and result in considerable capacity loss, poor cycling and rate performance. At higher humidity condition, this reaction process can be accelerated, leading to much worse electrochemical performance. The degenerative reaction mechanism are described the following equations 6, 7 and 8). In sum, it is necessary to control the humidity for long-time storage of NCM333, using methods such as vacuum package, inert gas protection, dry environment and etc. CO2 + H2O → H2CO3

(6)

LiMO2 + x/4 H2CO3 → Li1-xMO2-x/2 + x/2 LiOH + x/4 Li2CO3 (M=Ni, Co, Mn)

(7)

2 LiOH + CO2 → Li2CO3 + H2O

(8)


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 CONCLUSIONS In this study, surface degradation is observed on the NCM333 cathode after storage. This surface degradation is proved to be much easier to in high-humidity environment. The degradation manifests itself in the formation of many sheet-like impurities on the surface observed by SEM. FTIR tests confirm the impurities consisting of Li2CO3 and LiOH. In a further step, the XPS depth profiling reveals the evolution of Oimpurity and Olattice at different depths, which clearly reflects the thickness of impurities. While the bulk material preserved its orignal crystal structure through XRD analysis, proving that the degradation only took place on the surface. Different from the storage failure of LiNiO2, Ni did not experience the valence change while the impurity mainly came from the reaction of active Li+ with CO2/H2O. The as formed of inert impurities block the (de)lithiation channels and result in the specific capacity loss, cycling and rate performance deterioration. Higher humidity can accelerate this surface aging process, therefore dry environment is necessary to preserve the high performnce of NCM materials.

ASSOCIATED CONTENT Supporting Information. Rietveld refinements of XRD patterns (Figure S1), XPS (Figure S2-4), structure parameters from Rietveld refinements (Table S1), ICP (Table S2), summary of electrochemical parameters (Table S3) (PDF)



AUTHOR INFORMATION Corresponding authors E-mail address: [email protected] (B. Li)

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS 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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Table of Contents Synopsis: The LiNi1/3Co1/3Mn1/3O2 experience surface degradation, it not only consumes lithium ions in lattice but also blocks transport channels of lithium ions, leading to worse electrochemical performance than reference one.


3O2

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