Nano Energy 70 (2020) 104450

Available online 7 January 20202211-2855/© 2020 Elsevier Ltd. All rights reserved.

Full paper

Crack-free single-crystalline Ni-rich layered NCM cathode enable superior cycling performance of lithium-ion batteries

Xinming Fan a, Guorong Hu a, Bao Zhang a, Xing Ou a,**, Jiafeng Zhang a, Wengao Zhao b,c,***, Haiping Jia d, Lianfeng Zou e, Peng Li b, Yong Yang c,*

a School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China b Department of Energy Engineering, Hanyang University, Seoul, 04763, South Korea c School of Energy Research, Xiamen University, Xiamen, Fujian, 361005, PR China d Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, 99354, United States e Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99354, United States

A R T I C L E I N F O

Keywords: Intergranular crack Electrode/electrolyte interface Single-crystalline NCM Cycling capability Thermal stability

A B S T R A C T

Ni-rich layered oxides are extensively employed as a promising cathode material in lithium ion batteries (LIBs) due to their high energy density and reasonable cost. However, the hierarchical structure of secondary particles with grain boundaries inevitably induces the structural collapse and severe electrode/electrolyte interface parasitic reactions as the intergranular crack arises from the anisotropic shrink and expansion. Herein, the single- crystalline LiNi0.83Co0.11Mn0.06O2 (SC-NCM) with primary particles of 3–6 μm diameter is developed and comprehensively investigated, which exhibits superior cycling performance at both room temperature and elevated temperature (55 �C) as well as significantly improved structural integrity after long-term cycling. Remarkably, the SiO-C||SC-NCM pouch-type full cell with a practical loading (8.7 mAh cm� 2) delivers a capacity retention of 84.8 % at 45 �C after 600 cycles at a current rate of 1C (1C ¼ 200 mA g� 1), retaining a high specific energy density of 225 Wh/kg. Using a combination of X-ray photoelectron spectroscopy, time-of-flight second-ary-ion mass spectrometry and scanning transmission electron microscopy, we reveal that SC-NCM particles with micron-sizes effectively mitigate the undesired electrode/electrolyte side interactions and prevent the generation of intergranular cracks, thereby alleviating irreversible structural degradation. The strategy of developing single- crystalline micron-sized particles may offer a new path for maintaining the structural stability and improving cycling life of Ni-rich layered NCM cathodes even under high temperature.

1. Introduction

As the mushrooming of electric vehicles (EVs), lithium-ion batteries (LIBs) have stood out as the major power supplies that possibly meet the demand of high energy density [1,2]. In particular, the Ni-rich layered LiNixCoyMnzO2 (NCM, x � 0.8, x þ y þ z ¼ 1) cathodes have involved into viable choice in the market, because of their overwhelming merits of high energy density and reasonable cost [3]. Generally, in order to pursuit the higher packing density, the design principle of a normal LiNixCoyMnzO2 (N-NCM) cathode material is to integrate it into micron-sized secondary particles to achieve the relatively higher

packing density and reversible capacity [4], which are aggregated by hundreds of nanosized primary particles [5].

However, it is always observed that the N-NCM (Ni > 0.8) electrode shows severe electrochemical performance degradation and thermal instability during the long-term cycling, which should be well-addressed to further boost the larger-scale EV application [6]. Mostly, the inter-granular crack is responsible for the exasperating electrochemical per-formance and structural integrity of N-NCM electrodes during long-term cycling [7]. It is generally considered that the intergranular cracks are typically initiated among the grain boundaries as the primary particles are randomly orientated and the volume change is anisotropic [8]. Also

* Corresponding author. College of Energy, State Key Lab for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, PR China. ** Corresponding author. *** Corresponding author. Department of Energy Engineering, Hanyang University, Seoul, 04763, South Korea.

E-mail addresses: ouxing@csu.edu.cn (X. Ou), zhaowgno1@stu.xmu.edu.cn (W. Zhao), yyang@xmu.edu.cn (Y. Yang).

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https://doi.org/10.1016/j.nanoen.2020.104450 Received 6 November 2019; Received in revised form 17 December 2019; Accepted 31 December 2019

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the intergranular cracks are easily propagated to the surface of particles during high voltage operation and long-term cycling, resulting in the structural collapse and cycling degradation [9]. Moreover, due to the electrolyte penetration into the secondary microsphere along the intergranular crack, the electrode/electrolyte interphase area is greatly enhanced, which typically leads to the drastic accumulation of side re-actions and phase transformation on the cycled N-NCM [10–13]. In addition, the phase transformation from layered to disordered spinel/rock-salt structure is easily induced on the electrode/electrolyte interphase, resulting in the oxygen loss accompanied with undesired heat release [14,15]. Even worse, the transition metal dissolution and the electrolyte decomposition are exacerbated at elevated temperatures, leading to the profoundly detrimental influences on the electro-de/electrolyte interfacial interactions and the structural stability asso-ciated with the exothermic reactions [16,17]. Consequently, the accumulated thermal runaway will unavoidably arouse the horrible safety issues [18]. It is significantly important to deeply understand the degradation mechanism of Ni-rich N-NCM cathodes and explore an effective strategy to improve the calendar life and guaranteed safety of LIBs.

In the absence of grain boundary in the intrinsic structure, the single- crystalline (SC) primary particles efficiently alleviate the formation of micro/nano-cracks and maintain the structural integrity of Ni-rich SC- NCM. Moreover, the unique SC structure is also beneficial to alleviate the accumulation of side reaction products and unexpected phase transformation due to the limited electrode/electrolyte interphase area [19–21]. Therefore, the Ni-rich SC-NCM primary particles can be one of the most promising strategies to strengthen the structural stability and suppress the interphase parasitic reactions, leading to the enhanced cycling performance and thermal stability [22,23]. There have been a few reports in the academic papers exhibiting the SC-NCM materials were successfully synthesized. Enlightened by the superior performance of SC LiCoO2 [24], LiNi0.5Mn0.3Co0.2O2 [25,26], LiNi0.6Mn0.2Co0.2O2 [17,23], and LiNi0.88Co0.09Al0.03O2 [27], the SC LiNi0.83Co0.11Mn0.06O2 (SC-NCM) with micron-sized primary particle is developed in this work, which has not been reported and systematically investigated yet. Compared with the conventional N-NCM particles, the SC-NCM exhibits superior cycling capability and even no crack can be observed after long-term cycling. Furthermore, the thermal stability is thoroughly evaluated between delithiated SC-NCM and N-NCM, demonstrating the higher safety and better stability for SC-NCM at the elevated tempera-ture. The evolution of structural integrity and the accumulation of parasitic reactions on the cycled cathodes (SC-NCM and N-NCM) are also systematically investigated to fundamentally understand the degrada-tion mechanism generated by intergranular cracks and severe electro-de/electrolyte interphase side reactions.

2. Experimental section

2.1. Synthesis of SC-NCM material

Ni0.83Co0.11Mn0.06(OH)2 precursor with a spherical size was pre-pared via a co-precipitation method. The aqueous solutions of NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅5H2O (all are 2 mol L� 1) with molar ratio of Ni: Co: Mn ¼ 83: 11: 06 were simultaneously pumped into a continuously stirred tank reactor (CSTR, 200 L) under N2 atmosphere. Meanwhile, appropriate amount of NaOH solution (5 mol L� 1, used as the precipitation agent and controlled by the pH meter) and NH3⋅H2O solution (4 mol L� 1, used as chelating agent) were fed into the reactor separately. The temperature (50 �C), pH value (11.5), and stirring speed (500 rpm) of the solution were carefully controlled and maintained constant. Then, Ni0.83Co0.11Mn0.06(OH)2 particles were obtained through washing, filtering, and drying in a vacuum oven at 110 �C overnight. Next, this precursor was thoroughly mixed with LiOH⋅H2O (Li:M ratio ¼ 1.06:1), and excess lithium was used for the compensation of lithium loss during the sintering process. After that, the mixture was

preliminarily annealed at 500 �C for 5 h, and subsequently calcined at 830 �C for 10 h in oxygen atmosphere, finally the single-crystalline LiNi0.83Co0.11Mn0.06O2 (SC-NCM) was prepared. For comparison, the normal LiNi0.83Co0.11Mn0.06O2 (N-NCM) was obtained by sintering commercial Ni0.83Co0.11Mn0.06(OH)2 precursor (from Power New En-ergy Co., Ltd., China) at 750 �C in oxygen atmosphere.

2.2. Material characterizations

The chemical compositions of the SC-NCM particles were identified by ICP (OPIMA 8300, PerkinElmer). Crystalline structure was deter-mined by a Rint-2000, Rigaku type X-ray diffractometer, where the Bragg angle was scanned over a range of 5�–120� at a scan rate of 2�

min� 1. The collected intensity data of XRD was analyzed by the Rietveld refinement program-General Structure Analysis System (GSAS) software package. The morphology and the microstructure of the samples were observed by scanning electron microscopy (SEM, JSM 6400, JEOL) equipped with an energy dispersive spectrometer (EDS), transmission electron microscopy (TEM, JEOL 2100F, JEOL), and the spherical ab-erration corrected transmission electron microscopy (ACTEM) was performed on FEI Titan G2 80–200 ChemiSTEM. Besides, the samples should be pretreated by focused ion beam (FIB, SCIOS, FEI) before TEM and ACTEM tests. X-ray photoelectron spectroscopy (XPS) was con-ducted by Thermo Fisher ESCALAB 250Xi. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS, ION-TOF) was used for depth profiling and chemical analysis.

2.3. Electrochemical measurements

For 2032-type coin cells, the cathode electrodes were fabricated with 89 wt% active material, 5 wt% super P and 3.5 wt% KS-6 as conductive additive, and 2.5 wt% polyvinylidene fluoride (PVDF) as binder, dis-solving in N-methyl-1,2-pyrrolidone solvent (NMP) to obtain the ho-mogeneous slurry. The cells were assembled in an argon-filled glovebox with Li metal and 1 M LiPF6 in ethyl carbonate/diethylene carbonate (EC/DEC, 1:1 in volume) as the counter electrode and the electrolyte solution, respectively. The mass loadings of active cathode materials for all cells were controlled at 8.5 � 0.15 mg cm� 2.

For pouch cells, the cathode electrodes were prepared by mixture slurry of active material (96.4 wt%), carbon black (1.5 wt%), CNT (0.8 wt%) and PVDF (1.3 wt%). the silicon oxide/graphite composite (BTR New Energy Material Ltd) was used as an anode in this work. The loading weights of cathode and anode (SiO/C, 10 wt% SiO) were around 47.0 mg cm� 2 (ca. 8.7 mAh cm� 2) and 21.2 mg cm� 2 (ca. 9.54 mAh cm� 2), respectively. The designed capacity of anode and cathode is 450 and 185 mAh g� 1, respectively. The capacity balance of anode to cath-ode was approximately 1.08 for pouch cells. The pouch cells were assembled with stacked process. Each pouch cell contained 13 layers anode (8.5 cm � 15.6 cm in size) and 12 layers cathode (8.2 cm � 15.2 cm in size), injected with around 36.6 g electrolyte (1.05 mol L� 1 LiPF6 in ethylene carbonate-ethyl methyl carbonate-diethyl carbonate (EC: EMC:DEC, 3:5:2 by weight) with addition of 5 wt% fluorinated ethylene carbonate (FEC).

Galvanostatic charge/discharge tests were conducted between 2.75 and 4.4 V (or 4.2 V) at various temperature of 25 and 55 �C (Land CT2001A). A CHI660E electrochemical workstation was applied to characterize the electrochemical impedance spectroscopy (EIS) over the frequency range of 100000–0.1 Hz.

3. Results and discussion

The morphologies of precursors and their corresponding cathode materials were observed via scanning electron microscope (SEM). As presented in Fig. S1, the N-NCM sample displays an obvious secondary particle of 10–15 μm assembled by densely irregular primary nano-particles, whereas the SC-NCM precursor demonstrates a good

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secondary flower morphology with the particle size of 3–5 μm (Fig. 1a). After the modified solid-state reaction at high temperature and crushing technology, the SC-NCM precursor turns into micron-sized “single crystal” particles with a smooth surface (Fig. 1b and c). Although there is a substantial morphology change, the Ni, Co and Mn elements are ho-mogeneously distributed through the entire particle, examined by the mapping energy-dispersive X-ray spectroscopy (EDS) (Fig. 1d). Ac-cording to the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) test, it is confirmed that the stoichiometric ratios of Ni-Co-Mn for both samples are very close to the anticipated values of 83:11:6 as listed in Table S1, suggesting the well-consistence of high Ni-content during the formation procedure.

Moreover, the SC-NCM particle was further investigated by trans-mission electron microscopy (TEM). Fig. 1e displays a typical SC particle with a diameter of 4 μm, which is in an agreement with SEM observa-tion. As shown in Fig. 1f, the high resolution TEM (HRTEM) image re-veals the clear and uniform interplanar distances, and the magnified interplanar distance is measured to be 0.47 nm, corresponding to the (003) crystal planes of the layered structure (Fig. 1g). Taking the selected area electron diffraction (SAED) result into account (Fig. 1h), the natural hexagonal layered structure for the as-prepared SC-NCM is further confirmed. Additionally, the X-ray diffraction (XRD) patterns were collected and are presented in Fig. S2 and the Rietveld refinement results are listed in Tables S2 and S3. All peaks of both SC-NCM and N- NCM samples in XRD patterns can be indexed to the rhombohedral layered structure with space group R3m (PDF#87–1562) without any impurity phase, indicating the successful fabrication of Ni-rich SC-NCM cathode material.

Fundamental electrochemical performances of N-NCM and SC-NCM were evaluated at 2.75–4.4 V and 25 �C in 2032-type coin cells. As presented in Fig. 2a and Fig. S3, both N-NCM and SC-NCM cathodes display a similar initial discharge capacity of approximately 200 mAh g� 1 at 1C rate (1C ¼ 200 mA g� 1) and similar first columbic efficiency (88.6 % vs 88.9 %). However, the SC-NCM cathode exhibits higher discharge capacity of 162.6 mAh g� 1 after 150 cycles, maintaining a satisfied capacity retention of 84.5%, which is better than that of the N- NCM cathode (135.7 mAh g� 1 and 68.3%, respectively). The charge/ discharge curves at selected cycle numbers are displayed in Figs. S4a and

b. It is apparent that the N-NCM cathode exhibits obvious decay of voltage plateau with increasing cycles, indicating the severe structural degradation. In contrast, the average voltage plateau still retains above 3.58 V for SC-NCM electrode (Fig. S5a). As a result, the SC-NCM cathode delivers larger specific energy density of 583.3 Wh kg� 1 (based on the cathode material) after 150 cycles due to its higher reversible capacity and more stable voltage platform, compared with that of N-NCM (461.8 Wh kg� 1) as demonstrated in Fig. 2c.

The thermal stability is a key challenge for the development of Ni- rich layered NCM cathodes. The cycling performances of N-NCM and SC-NCM are further evaluated at an elevated temperature of 55 �C (Fig. 2b). Obviously, the N-NCM cathode suffers from rapid capacity decay (78.1 mAh g� 1), displaying a capacity retention of 35.3% after 100 cycles. In contrast, the SC-NCM cathode still maintains a reversible capacity of 168.8 mAh g� 1 at the same cycle, which is twice higher than that of N-NCM cathode. Generally, the reaction kinetic is improved when escalating the operating temperature, yet accompanied with the accelerated electrolyte erosion, enhanced side reactions and the severe dissolution of the active material [28,29]. Therefore, the N-NCM elec-trode undergoes more serious deteriorations on energy density at elevated temperature than room temperature (Fig. 2c and d). Remark-ably, the SC-NCM cathode still maintains the voltage plateau of 3.54 V despite operating at elevated temperature (Fig. S5b), leading to the higher energy density of 598.4 Wh kg� 1 after 100 cycles. Obviously, the SC-NCM cathode delivers superior cycling stability compared to the N-NCM, which may be attributed to the suppressed electro-de/electrolyte interface parasitic reactions and the unique SC structural integrity [26,27].

Generally, in order to broaden the application of EVs to the longer driving range even under the elevated temperature condition, the bat-tery life should be extended to the outstanding capacity retention with advanced thermal safety. To evaluate the real cycling life in the practical application, pouch-type full cells with N-NCM or SC-NCM cathodes and commercial silicon oxide/graphite (SiO/C) anodes were assembled and tested at 1C at the elevated temperature of 45 �C. As presented in Fig. 3a, the SC-NCM displays the slightly lower specific capacity at the initial cycling compared with the N-NCM (184.1mAh g� 1 vs 188.2 mAh g� 1), however, it maintains much higher capacity retention after 600 cycles.

Fig. 1. Morphology and elemental distribution in SC-NCM. SEM images of (a) precursor and (b, c) sintered SC-NCM. (d) EDS elemental mapping of Ni, Co, Mn, O for SC-NCM. TEM (e), HRTEM images (f, g), and SAED pattern (h) of the SC-NCM.

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The discharge capacity of the cell using N-NCM fades quickly to 108.0 mAh g� 1 after 600 cycles accompanied with the detrimental voltage drop due to its increased polarization and structural decay during cycling (Fig. 3b) [30–32]. By comparison, the SC-NCM based cell just delivers inappreciable drop with ultrahigh reversible capacity of 156.1 mAh g� 1 after 600 cycles, maintaining a capacity retention of 84.8 % (Fig. 3c), which is nearly 1.5 times as the cell using N-NCM. Moreover, the discharge mid-point voltage remains super stable at around 3.60 V for SC-NCM, compared with the gradually drop to 3.46 V for N-NCM (Fig. 3d). As a result, the SC-NCM cell can maintain the specific energy of 225 Wh kg� 1 after 600 cycles, which is much higher than that of N-NCM with only 150 Wh kg� 1 (Fig. 3e). In order to comprehensively evaluate the electrochemical performance of SC-NCM, the rate capabilities comparison at different current densities ranging from 0.1 C to 5 C are displayed in Fig. S6. There is no obvious difference of rate capability below 2C between the N-NCM and SC-NCM. However, the SC-NCM electrode exhibits a slightly lower discharge capacity of 175 mAh g� 1

at the rate of 5C, slightly lower than that of N-NCM electrode (177 mAh g� 1). Thus, the rate capability of single-crystalline cathode will be optimized in our future work. To our best knowledge, this is a pioneering report of single-crystalline Ni-rich layered NCM cathode presenting such stable cycling performance at elevated temperature. Overall, the

SC-NCM electrode demonstrates greatly improved cycling performance at elevated temperature compared with the N-NCM material, which sheds light on the commercial application of Ni-rich NCM with a unique single crystalline structure.

It is reported that the seriously degraded cycling performance of Ni- rich NCM is attributed to the intergranular cracks and unwanted side reactions (electrode/electrolyte interactions) [18,23]. Thus, to confirm the amelioration effect of SC particles, both cycled pouch-type cells were disassembled after 600 cycles at high-temperature of 45 �C, and the chemical compositions of the outer cathode-electrolyte interphase (CEI) species for harvested cathodes and anodes were examined by XPS [33–35].

The peaks corresponding to C–C, C–H, C–O, C––O, and OCO2 bonds are observed in the C 1s spectra for both N-NCM and SC-NCM electrodes (Fig. 4a, d). The presence of C–C and C–H bonds is mainly assigned to the binder and conductive additive, whereas C–O, C––O, and OCO2 peaks are related to lithium alkyl carbonates (ROCO2Li), ROLi, and Li2CO3 as the decomposition of carbonate electrolyte solvents. The C–O, C––O, CO3

2� and M-O species are observed in the O 1s spectra for both N-NCM and SC-NCM electrodes (Fig. 4b, e). The peaks of C–O, C––O, and CO3

2�

are related to lithium alkyl carbonates (ROCO2Li), ROLi, and Li2CO3 as the decomposition of carbonate electrolyte solvents [30,36,37].

Fig. 2. Electrochemical performance comparison of N-NCM and SC-NCM electrodes in a coin-cell. (a) Cycling performance and (c) energy density at 1C in the voltage range of 2.75–4.4 V for150 cycles. (c) Cycling performance and (d) energy density at 1C in the voltage range of 2.75–4.4 V for 100 cycles.

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Compared with the SC-NCM cathode, the stronger peak intensity of C–O, C––O, and CO3

2� bonds is observed for the N-NCM electrode, suggesting more carbonate solvent decompositions on the CEI films induced by the intergranular cracks. Moreover, a stronger intensity peak of C–O, C––O, and OCO2 in C1s spectra (Fig. S9a) is observed in the cycled N-NCM electrode, which further validates that more carbonated solvent decomposed on the interphase of N-NCM/electrolyte [38]. In addition, for F 1s spectra (Fig. 4c, f), the greatly decreased peak intensity of LiF (685.0 eV) and LixPOyFz (687.8 eV) are shown on the cycled SC-NCM, which indicates the fewer decompositions of LiPF6 on its surface than that of cycled N-NCM electrode [39,40]. Notably, the XPS spectra of C 1s, F 1s, O1s, Li 1s and P 2p on cycled silicon oxide/graphite electrodes further confirm that less decomposition formed in the SC-NCM cell system. For the C 1s spectra, the peak at 289.0 eV is ascribed to C––O species (Li2CO3 and ROCO2Li) with stronger intensity on the cycled anode in the N-NCM cell (Fig. S7a) than that of the SC-NCM based cell (Fig. S7b), suggesting the severer degradation of solvent on the cycled anode, which can be further confirmed by the stronger intensity of

Li-species in Li 1s spectra (Fig. S7g). Besides, the cycled anode in the N-NCM based cell exhibits higher intensity for both peaks of LixPOyFz and LiF in the F 1s spectrum (Fig. S7c) compared with that of SC-NCM, indicating the thinner SEI film formed at outmost surface of cycled sil-icon oxide/graphite in the SC-NCM cell system (Fig. S7d). Therefore, we can safely conclude that single-crystalline structure can efficiently mitigate the electrolyte decomposition at the elevated temperature operation [26,27].

Additionally, the CEI films consisted of the outer layer and inner layer were analyzed by the time-of-flight secondary ion mass spec-trometry (TOF-SIMS) depth profiling of both cycled electrodes. It is re-ported that the organic species (C2HO� , POF2

� , C2F� ) usually localize at the surface of the CEI film (i.e. the outer layer), which are originated from the decomposition of salt and solvent (Fig. 4g–i). On the contrary, the transition-metal fluorides (6LiF2

� , NiF3� , and MnF3

� ) exist in the inner layer with high-concentrations (Fig. 4j-l), which are derived from the active material dissolution aggravated by acidic species attack (e.g. HF) and destabilized migration of transition-metal during the intrinsic phase

Fig. 3. Electrochemical capability comparisons of N-NCM and SC-NCM electrodes in pouch-type full-cells. The testing was carried out at 1C rate and 45 �C with the voltage range of 2.5–4.2 V. (a) Cycling performances and (b–c) corresponding charge/discharge profiles of both electrodes for the 1st, 100th, 400th, 600th cycles. (d) Mid-point voltage and (e) energy density comparisons of both electrodes for 600 cycles.

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transformation (from layered to rock-salt structure) [41]. It is apparent that all these fragments of the SC-NCM electrode exhibit significantly decreased signal compared with the N-NCM electrode, especially the less content of transition-metal fluorides (NiF3

� and MnF3� ), suggesting the

suppressed cathode/electrolyte parasitic reactions (Fig. 4k and l). However, the strong signal of C2HO� , POF2

� , C2F� , and 6LiF2� still can be

observed after 1500s Csþ etching on the N-NCM electrode (Fig. 4g–j), indicating the unexpected electrolyte penetration into the bulk through generating intergranular cracks [33,42]. In contrast, the SC-NCM significantly alleviate the electrolyte decomposition and the transition metal dissolution because of the suppressed electrode/electrolyte side

reactions and the avoided crack formation. The above results are further supported by the electrochemical impedance spectroscopy (EIS) varia-tions (Fig. S8) [43–45]. The charge transfer resistance (Rct) of N-NCM is obviously increased after 100 cycles, whereas the Rct of SC-NCM dem-onstrates a limited increase at the same testing protocol. The values of Rs, Rct and DLi are summarized in Table S4.

Figs. S10a–b clearly displays the severely pulverized secondary particles of the N-NCM electrode, which is a main reason to account for the poor capacity retention (Fig. 3b). To further demonstrate the structure evolution, cross-sections of the cycled cathode were also conducted to check the fractured bulk. Numerous intergranular cracks

Fig. 4. The chemical composition at the cycled electrode surface for N-NCM and SC-NCM after 600 cycles. XPS spectra of O 1s and F 1s elements for (a, b) N-NCM and (c–d) SC-NCM electrodes. The TOF-SIMS depth profiles of (e) C2HO-, (f) POF2

� , (g) C2F� , (h) 6LiF2� , (i) NiF3

� , and (j) MnF3� for both cycled electrodes with a cutoff

voltage of 4.2 V, demonstrating the compositions of the CEI layer on the cycled electrode.

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distributed over the microsphere are clearly observed for the cycled N- NCM (Fig. 5a and b), which were generated along the grain boundaries from the core and propagated to the surface, resulting from the inner anisotropic strain. The broken particles derived from the severe pul-verization and intergranular crack lead to the decrease of the mechan-ical integrity of the secondary particles and thus the loss of active materials. Besides, the intergranular cracks result in the electrolyte penetration and formation of new electrode/electrolyte interphase. Therefore, the side reactions accumulate, leading to the continuous ca-pacity degradation of the N-NCM cathode during extended cycling.

In contrast, the SC-NCM electrode exhibits well-preserved rectan-gular-like morphology and maintains mechanically intact without visible cracks (Fig. 5c and d), owing to unique integrated structure of single-crystalline primary particles. Moreover, the chemical species variation also confirms the severe interphase side reactions as the fluorine (F) enriches on the cross-section of the cycled N-NCM [34,35]. As displayed in Fig. 5e, the F-element is full-spatially distributed within the sectional surface of the cycled microsphere electrode, indicating the severe electrolyte penetration along the microcracks in the cycled N-NCM particles, which is consistent with the XPS and TOF-SIMS results (Fig. 4). In addition, the HRTEM image in Fig. 5i indicates that the

Fm3m rock-salt phase is clearly observed from the outmost surface to the inner of the primary particle, which should be ascribed to the severe corrosion because of the undesirable electrode/electrolyte interphase side reactions [14]. In contrast, F-element is not detectable in sectional surface of the SC-NCM electrode (Fig. 5f), illustrating the structural integrity of single-crystalline particles without electrolyte infiltration. Compared with the formation of disordered rock-salt NiO phase in the cycled N-NCM electrode, the R3m-layered structure of SC-NCM is well-maintained without irreversible transformation (Fig. 5m).

To get fundamental understanding of the relationship between crack formation and surface deterioration, the internal morphological and structural evolution difference for cycled N-NCM and SC-NCM elec-trodes in pouch-type cell at high temperature were thoroughly analyzed by dark-field scanning transmission electron microscopy (HAADF- STEM). After 600 cycles, numerous intergranular microcracks are observed from the inner bulk of the N-NCM electrode (Fig. S14a), resulting in the interparticle crack network. Obviously, the primary-like particles are also fractured by the anisotropic strain after prolonged cycling, generating the apparent internal nanocracks (Fig. S11c), which further facilitates the electrolyte penetration. As is revealed in Fig. 6a, the crystal structure near the nanocrack can be divided into four regions

Fig. 5. Morphology and elemental distribution on cycled N-NCM and cycled SC-NCM after 600 cycles at 1C rate and 45 �C in the voltage range of 2.5–4.2 V. Cross- sectional SEM images and EDS mapping of Ni, Co, Mn, F elements for (a, b, e) N-NCM and (c, d, f) SC-NCM electrodes. TEM, HRTEM images and corresponding the FFT for (g–j) N-NCM and (k–n) SC-NCM electrodes.

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on the cycled N-NCM. The R3m-layered structure is still maintained in the central bulk far away from the nanocrack (Fig. 6a1). However, the approached parts gradually deteriorate into disordered layered phase (Fig. 6a2) and defect rock-salt phase (Fig. 6a3), eventually transfers into the disordered rock-salt phase at the nanocrack surface (Fig. 6a4). On the contrary, this phase evolution and structural deformation is almost immunized for the SC-NCM electrode. As a result, the intergranular nano/micro-cracks for the SC-NCM electrode are not observed (or detected) both inner bulk and outer surface (Fig. S11f). The structural integrity of the micron-sized particle is well maintained. Furthermore,

there is only a thin layer of disordered phase as shown on the surface after long-term cycling at the elevated temperature (Fig. 6b4) while the bulk structure maintains original R3m layered structure (Fig. 6b1-b3).

The exothermic oxygen evolution is tightly associated with the structural deformation, especially at elevated temperature condition [46]. Based on the aforementioned superior electrochemical perfor-mance, the thermal stability of SC-NCM cathode is anticipated to be significantly enhanced. Thus, differential scanning calorimetry (DSC) measurement was conducted on both charged electrodes within the electrolyte [47,48]. As displayed in Fig. S12, the N-NCM electrode

Fig. 6. The interior morphology and structure evolution in N-NCM and SC-NCM after 600 cycles. TEM and corresponding HAADF-STEM images from the interior region to the intergranular crack surface for (a) N-NCM and (b) SC-NCM electrodes. (c) Schematic illustration of crack evolution and the internal morphological difference for N-NCM and SC-NCM cathodes during prolong cycling.

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exhibits an exothermic peak at 216.16 �C with a heat generation of 405.22 J g� 1, while SC-NCM electrode exhibits a peak at a much higher temperature of 232.08 �C with a lower heat generation of 201.99 J g� 1. The delayed onset temperature demonstrates that the unique single-crystalline structure can suppress thermal runaway and oxygen release, maintaining the structural stability and improve the thermal safety.

Based on the above results, it is demonstrated that the fabrication of single-crystalline structure is beneficial to alleviate the intergranular cracks and reduce the undesired electrode/electrolyte interface in-teractions. To fundamentally understand the relationship between phase transformation and intergranular crack, the crack and internal structure evolution are schematically illustrated for N-NCM and SC-NCM cathodes during the prolonged cycling, as presented in Fig. 6c. For conventional microsphere of N-NCM, the micron-sized intergranular crack formed within secondary particles during the prolonged cycling, resulting from the anisotropic volume changes accompanied with localized stress contractions [49]. Moreover, the appearance of nanocracks among the nano-sized primary particle aggravate the electrolyte penetration in the interior of the primary particle, especially under the high voltage operation [50,51]. And the severe electrode/electrolyte interphase side reactions will result in the irreversible phase transformation from layered structure to rock-salt phase and aggravate the structural collapse [52,53]. It is noted that the disordered structure or rock-salt phase will block the Li-ion diffusion and accelerate the capacity deterioration. In contrary, the mechanical connection is strengthened by its micron-sized integral single-crystalline structure in SC-NCM cathode, which prevents the intergranular nano/micro-cracks formation, leading to the decreased accumulation of electrolyte/electrode side reactions species and disordered rock-salt phase formation [54]. Therefore, the single-crystalline architecture strengthens the intrinsic microstructure of bulk materials due to mitigated anisotropic stress in the charge/di-scharge process, resulting in the remarkable enhancement of cycling stability, especially at elevated temperature.

4. Conclusions

The integrated micron-sized single-crystalline LiNi0.83-

Co0.11Mn0.06O2 is successfully fabricated and comprehensively investi-gated. The cycling stability of SC-NCM at elevated temperature (45 �C) is significantly improved. A satisfied energy density of 225 Wh kg� 1 can be achieved at a rate of 1C after 600 cycles, with the capacity retention of 84.8 % compared with the initial capacity. The cycling degradation on the N-NCM is mainly attributed to the severe intergranular crack prop-agation, undesired parasitic products accumulation and disordered rock-salt phase formation. The single-crystalline particles significantly prevent the formation of intergranular crack induced by anisotropic stress with oriented volume changes, thus alleviating the undesired electrode/electrolyte interactions. The enhanced interfacial stability effectively prevents the irreversible phase transformation from layered to rock-salt phase, dramatically enhancing the structural stability and cycling performance. Therefore, these unique merits of single-crystalline Ni-rich NCM pave a new avenue to boost the energy density and improve the cycling life of LIB through preventing the intragranular crack for-mation and alleviating the electrode/electrolyte interphase parasitic reactions.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grand No. 51902347, 51822812, 51772334, 51778627, 21761132030, 21621091), and the Graduate

Innovation Project of Central South University (Grand No. 502221908).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2020.104450.

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Xinming Fan obtained his Master’s degree in Central South University in 2013. After 4-years work in ATL battery company. He is currently a Ph.D. candidate in the School of Metallurgy and Environment at Central South University. His current research focuses on the design, synthesis as well as performance improvement and structural characterization of cathode mate-rials for high performance Li and sodium ion batteries.

Prof. Guorong Hu received his Ph.D. degree from China Uni-versity of Geosciences in 1998. He is now a full professor in the School of Metallurgy and Environment at Central South Uni-versity. His research group mainly focuses on nonferrous-based materials and corresponding resources for energy storage, such as lithium ion batteries, sodium ion batteries. He has published about 300 SCI papers with more than 3000 citations.

Dr. Bao Zhang received his Ph.D. degree from Central South University in 2005. He is now a professor of materials physical chemistry in the School of Metallurgy and Environment, CSU. His main research interests include phosphate-based materials and heterostructured materials for lithium storage, EV cell, extractive metallurgy.

Dr. Xing Ou received his Ph.D. degree in environmental engi-neering from South China University of Technology in 2018. He is now working as an associate professor at school of metallurgy and environment, Central South University. His current research interests are electrochemical energy storage system and renewable resources regeneration.

Dr. Jiafeng Zhang is an associate professor of metallurgy and environmental engineering at Central South University, PR China. He received Ph. D. degree from Central South Univer-sity. His research interests include energy-related materials and environmental engineering.

Dr. Wengao Zhao is currently working as a BK 21 postdoctoral research associate at Hanyang University, South Korea. He received his Ph. D degree in 2018 from Xiamen University. He studied at Pacific Northwest National Laboratory from October 2016 to June 2018 as a joint Ph. D student sponsoring by the CSC scholarship. His research focuses on the synthesis and application of advanced cathode materials with high perfor-mance for rechargeable Li/Na ion/ metal batteries. He also explores the degradation mechanisms of advanced cathode materials (especially the Ni-rich NCM) with XRD, STEM, Ex/in- situ ss-NMR and Ex/in-situ XAS.

Dr. Haiping Jia is currently a lithium-ion batteries expert in Mercedes-benz AG. She was a research associate of the Pacific Northwest National Laboratory from January 2017 to August 2019. She received her Ph.D. in 2016 from Meet battery research center, University of Muenster (Germany). Dr. Jia has 9 years of electrochemistry and advanced anode material research experience with emphasis on lithium-ion batteries. She also has experience of in the development of electrolytes for high voltage cathodes and silicon-based anodes.

Dr. Lianfeng Zou received his MS and Ph. D degree from State University of New York (SUNY) Binghamton, focused on studying the surface and interface dynamics of metals and metal oxides using environment transmission electron micro-scopy (ETEM). He is currently a research associate at Pacific Northwest National Laboratory and his research interests are in situ and ex situ TEM characterizations of Li ion batteries.

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Nano Energy 70 (2020) 104450

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Peng Li received his B.S. and M.S. degree from the Department of Chemical Engineering in China University of Petroleum in 2013 and 2016, respectively. He is presently a Ph.D. candidate in the Department of Energy Engineering at Hanyang Univer-sity, under the supervision of Professor Yang-Kook Sun. His research focuses on design novel electrode materials and their applications in Li-ion batteries and K-ion batteries.

Prof. Yong Yang is a distinguished professor in Chemistry in the State Key Lab for Physical Chemistry of Solid Surface at Xiamen University since 1997. He also serves as Editor for J Power Sources and Board Member of International Battery Materials Association (IBA) and International Meeting of Lithium Battery (IMLB). His main research interests are new electrode/electrolyte materials for Li/Na-ion batteries, in-situ spectroscopic techniques, and interfacial reaction mechanism study in electrochemical energy storage and conversion system. He published 300þ papers in referred journals and edited a book entitled on “Solid State Electrochemistry” (Chinese Chemical Industry Press, Beijing, 2017).

X. Fan et al.