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Surface Heterostructure Induced by PrPO4-modifying in Li1.2[Mn0.54Ni0.13Co0.13]O2 cathode material for high performance lithium ion batteries with Mitigating Voltage Decay Feixiang Ding, Jianling Li, Fuhai Deng, Guofeng Xu, Yanying Liu, Kai Yang, and Feiyu Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07221 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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ACS Applied Materials & Interfaces

Surface

Heterostructure

Induced

by

PrPO4-modifying

in

Li1.2[Mn0.54Ni0.13Co0.13]O2 cathode material for high performance lithium ion batteries with Mitigating Voltage Decay Feixiang Ding†, Jianling Li†,*, Fuhai Deng†, Guofeng Xu†, Yanying Liu†, Kai Yang‡, Feiyu Kang§ †

School of Metallurgical and Ecological Engineering, University of Science and

Technology Beijing, No. 30 College Road, Haidian District, Beijing 100083, China ‡

China Electric Power Research Institute, Haidian District, Beijing, 100192, China

§

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

Tsinghua University, Haidian District, Beijing, China 100084, China *

Corresponding author: Jianling Li

E-mail: [email protected](J.Li) ABSTRACT: Li-rich layered oxides (LLOs) have been attractive cathode materials for lithium-ion batteries due to their high reversible capacity. However, they suffer from low initial coulombic efficiency and capacity/voltage decay upon cycling. Herein, facile surface modification of Li1.2Mn0.54Ni0.13Co0.13O2 cathode material is designed to overcome these defects by the protective effect of surface heterostructure composed of induced spinel layer and PrPO4 modification layer. As anticipated, a sample modified with 3 wt.% PrPO4 (PrP3) shows an enhanced initial coulombic efficiency of 90% compared to 81.8% for the pristine one, more excellent cycling stability with a capacity retention of 89.3% after 100 cycles compared to only 71.7% for the pristine one, and less average discharge voltage fading from 0.6353V to 0.2881V. These results can be attributed to the fact that the modification nanolayers have moved amount of oxygen and lithium from the lattice in the bulk crystal structure, leading to a chemical activation of Li2MnO3 component previously and forming a spinel interphase with a 3D fast Li+ diffusion channel and stable structure. Moreover, the elaborate surface heterostructure on Li-rich cathode material can effectively curb the undesired side reactions with the electrolyte and may also extend to other layered oxides to improve their cycling stability at high voltage.

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KEYWORDS: Li-ion batteries, Li-rich layered oxides, surface heterostructure, structural transformation, voltage decay

1. Introduction With the development of the large-scale hybrid electric vehicles (HEVs), electric vehicles (EVs) and smart grids, the demand for rechargeable Li-ion batteries with high-energy density is increasing rapidly over the years.1,2 Lithium-rich layered oxides xLi2MnO3•(1–x)LiMO2 (M = Mn, Ni, Co), deliver a discharge capacity higher than 250 mAh g-1 within the voltage window of 2.0–4.8 V (vs. Li/Li+), making it a possible cathode material for substituting the most widely positive electrode materials (LiMO2) used in commercialized LIBs. 2-4 Recently, monoclinic Li2MnO3 (C2/m) existing in the oxide grains at the atomic scale along with rhombohedral LiTMO2 (R-3m) were directly revealed by Yu et.5 The Li2MnO3 component has a characteristic activated voltage plateau around 4.5 V during initial charge, which is accompanied by the simultaneous Li extraction and oxygen release from local structure.6,7 Although the electrode capacity is drastically improved after initial activation process, it apparently leads to instability among the electrode system and material crystal structure. On the one hand, activated oxygen finally leads to the hydrolysis of the electrolyte and the formation of Li2O, resulting in a large irreversible capacity loss. And oxidation product of the electrolyte will facilitate the formation of insulating solid electrolyte interface (SEI) on the surface, leading to an increasing polarization. On the other hand, the transition metal (TM) ions coordinated with released oxygen ions are destabilized and likely to migrate to the vacant octahedral sites in Li layers, and causing a phase transformation gradually from the surface to the interior of the material that will lead to sluggish lithium ions transportation and the gradual voltage fade during cycling. In the past decades, lots of efforts and attempts have been applied to improve the electrochemical performance of Li-rich layered oxides. These concluded doping or substitution with cationic ions (Na+,8 Mg2+,9 Al3+,10 Ti4+,11 Ru4+,12 Y3+,13 etc.) and polyanion doping (PO43-,14,15 BO33-/BO45-,16 etc.), surface coatings using metal oxides,17–19 fluorides,20 phosphates21–23 and even other cathode materials (such as LiMn2O4,24 LiFePO425), and specific nanoarchitecture.26,27 Among all the above strategies, surface modification has been extensively studied, because the coating layer can protect the particles from the erosion of electrolytes, enhance the Li+ and electronic conductivity, especially some of them are electrochemically active.25 Furthermore, in our previous work18 and Cho et al.23 (LiMgPO4) have reported that apart from the surface protection effect of the coating layer, there is an advantage of surface-doping. Nevertheless, the oxygen evolution and structural transformation issues still exist and can’t be terminated, it is still supposed to find a novel surface


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structure with further improved performance. Therefore in this work, we have investigated a novel rare-earth metal phosphate PrPO4 as the covering layer of Li-rich layered Li1.2Mn0.54Ni0.13Co0.13O2, which were synthesized by a PVP-assisted wet chemical deposition route and a low-temperature calcination process (450°C) to enhance the interaction at the modified interface. However, to our surprise, a spinel phase layer was formed homogeneously between the covering layer and host particle, as depicted in Fig 1. High-resolution transmission electron microscopy (HRTEM), Raman and dQ/dV plots have been used to confirm the existence of spinel structure. And an elaborate evolution process of layer to spinel phase transformation is described boldly. The off-stoichiometric, spinel interphase, characteristic of enrichment of Ni and Mn (Li1+yNi0.5-x/2CoxMn1.5-x/2O4), along with outside covering layer can protect the LLO material from the corrosion of electrolytes and accelerate Li+ and electron transport. Therefore, the initial coulombic efficiency, rates performance and cycling properties of the PrPO4 modified samples were significantly improved, especially, with a mitigating voltage decay.

Fig 1. Schematic illustration of surface heterostructure and it’s protective effect on Li-rich cathode material particle.

2. Results and Discussion The morphologies of the pristine and modified Li1.2Mn0.54Ni0.13Co0.13O2 particles were characterized by SEM. As shown in Fig 2, all samples are consisted with well-crystallized nanoparticles with a diameter in range of 200-300nm. Furthermore, it can be seen that the pristine and modified samples have regular polyhedron morphologies. However, the crystal faces and boundaries of the cathode particles



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become slight blurry after modifying. Especially, with the increase of PrPO4 addition content, the PrPO4-modified samples have an increase of roughness, which is due to the deposition of nanoscale PrPO4 particles on the cathode material. When the content of PrPO4 increased to 5wt%, the crystal faces of the sample have been covered and the PrPO4 particles agglomerated to larger particle size, which implies the amount of PrPO4 have exceeded the anticipation.

Fig 2. SEM images of the prepared samples: (a) LNCM, (b) PrP2, (c) PrP3 and (d) PrP5.

The XRD patterns of the four samples (Fig 3) were indexed to the R-3m space group of the layered α-NaFeO2 structure. Additional weak peaks located between 20 and 25° are attributed to the LiMn6 superstructures of monoclinic Li2MnO3 phase with space group C2/m. When the covering content of PrPO4 increased to 5wt.%, several minor peaks belong to hexagonal phase PrPO4 (JCPDS card: 20-0966)28 were found (as can be seen in Fig S1 as well). In comparison, we can note that the (003) peaks of the modified samples move to lower angle (Fig 3b). It suggests an expansion in the c axis of the crystal structure. The full spectrum fitting with fullprof software package have been operated (Fig S2), basing on main phase (hexagonal R-3m space group), and the obtained lattice parameters are listed in Table 1. It is interesting that a slight increase in lattice parameter c and a is observed with increasing modifying amount of PrPO4, whereas the c/a value are a bit of decreasing. Furthermore, the value of the I(003)/I(104) intensity ratio (R), which indicates the extent of cation mixing between Ni2+ and Li+, have decreased from 1.30 for the bare LMNC to1.22, 1.22, and 1.15 for the PrP2, PrP3 and PrP5 samples respectively. All of the data show that the modified samples have an increasing degree of cation mixing and expanded crystal cell parameters, suggesting that part of transition metal ions have migrated to Li layer and led a phase transformation, despite no impurity phase peaks. Indeed, according to the Raman spectrum of sample PrP3 (Fig S3), a prominent band emerges around 670 cm-1 24,29(as indicated by the arrow), which indicates that the cubic spinel phase actually arises after PrPO4-modifying. It is also verified directly by TEM analysis and the electrochemical charge/discharge curves in the following discussion.


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Fig 3. X-ray diffraction patterns (a) of the LNCM, PrP2, PrP3 and PrP5. The partial enlarged detail of (003) diffraction (b) is chosen to be shown on the right side. Table 1. The lattice parameters of the four samples.



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Fig 4. TEM images (a, b) and HRTEM (c, d) images of the pristine and PrP3 samples. Corresponding FFT pattern (e) and SAED image (g) of image (c) and (d) within the red rectangle area. (f) crystal model diagram of the surface heterostructure.

To further observe the modification layer detailly, the pristine, and PrP3 samples were characterized using TEM. The images clearly show the bulk particles of the pristine sample and the PrP3 sample with the deposition of nanoscale PrPO4 particles in Fig 4a and b, respectively. The high-resolution TEM (HRTEM) micrograph (Fig 4c) of the pristine sample exhibits clear lattice fringes from surface regions to bulk region with a d spacing equal to 0.47 nm, which corresponds to the (003) planes of the layered structure (R-3m). Fig 4e shows the fast Fourier transformation (FFT) pattern, and the diffraction spots can be indexed to the (003), 011, and 014 planes of the layered structure, which is consistent with the R-3m space group along [100] zone axis. HRTEM analysis demonstrates that the PrP3 particles have a core of typical layered structure, and a uniformly modification layer thickness of 6-8 nm with a


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lattice spacing of 0.31nm indexed to the (002) plane of PrPO4 (Fig 4d), which is consistent with the XRD pattern. And it is interesting that a thin spinel phase layer with an interplanar spacing of 0.24 nm (enlarged images) coexists between the crystal lattices of LNCM and PrPO4. Moreover, the SAED pattern of primary PrP3 particles (Fig 4g), clearly showing the diffraction spots indexed to the [100] zone axis of the layered structure, and the diffraction spots (as marked by yellow cycles) can be indexed to the [110] zone axis of the cubic spinel phase (Fd-3m), indicative of a phase transformation induced by the PrPO4 surface-modifying. Thus, it’s not easy to observe the XRD peaks of the spinel interphase in Fig 3 because of their similar XRD patterns, as well as the small content of the spinel phase (2-3nm). In fact, surface spinel phase was also observed in AlPO4 coating reported by Xiao et al,6 where they attributed the structural change to the ALD process, but the mechanism and evolution progress of this transformation were not provided detailedly. According to the HRTEM images and SAED pattern, an integrated crystal model diagram of cubic spinel and layered phase is suggested (Fig 4f). And the evolution progress is described carefully in next part. As is known, Li+ transport in LNCM is two-dimensional parallel to the Li+ layers along the a (or b) axis, in spinel phase is a 3D Li+ transportation route. However, the pristine and modified samples show a (003) plane parallel to the surface, which means that the Li transportation route perpendicular to particle surface has been impeded. Therefore, in-situ induced spinel interphase has opened the passageway of Li ions insertion and de-insertion, and has more stable structure and electrochemical properties at the highly charged state than other electrochemical delithiated phases30, which implies enhanced high rates and cycling performance.

Fig 5. Schematic drawing (a-f) illustrates the transformation of monoclinic layered phase to cubic spinel phase induced by PrPO4 modifying.

To understand the formation mechanism of the spinel interphase, an elaborate evolution process of layer to spinel phase transformation is described in Fig 5. The



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hexagonal layered, monoclinic layered phase and cubic spinel are based on the cubic close packing (CCP) array of the oxygen ions, where the transition metal ions of these phases are located in octahedral sites. The difference between the layer and the spinel (set as LiMn2O4 spinel for example) structure is that in the LiMn2O4 spinel, lithium atoms are present in the tetrahedral sites, whereas in the layered phases Li ions are in the octahedral sites of the Li layers. For simplicity, we choose 4-unit cells of Li2MnO3 shown in Fig 5b from a monoclinic layer (C2/m space group, Fig 5a). During the calcination process of PrPO4 modified samples, it is speculated that some lattice O and several Li ions (3Li2O) has been extracted or chemically leached by PrPO4(Fig 5c). Once oxygen and lithium vacancies have been formed, the activation barriers for transition-metal migration is significantly reduced and causes massive micro-structural changes31,32. As shown in Fig 5d, a fraction of Mn cations from the octahedral positions in the TM layers migrate to the octahedral positions in the Li layers through an empty tetrahedral site in the latter, and the Li ions migrate from the octahedral sites of the Li layers into the adjacent tetrahedral sites in the same layer. The Li tetrahedral occupancy in the spinel structure increases the symmetry from R-3m or C2/m to Fd-3m space group. Finally, a spinel lattice has been formed based on the layered host structure and underneath the PrPO4 nanoparticles. To further observe the element distribution of the PrP3 sample, EDX map, line and points scanning are conducted in STEM mode. It can be seen that O, Ni, Co and Mn of the PrP3 are uniformly distributed among the randomly selected particle (Fig S4), whereas Pr and P have enriched on the surface of the particle. The line and points scanning (Fig 6b and c) further confirm the enrichment of Pr and P in the edge of the particle, and a certain amount of bulk transition metal ions and modification layer ions diffuse to each other due to the concentration gradient. Furthermore, paying attention to the ratio of Ni:Co, there is a slight increase especially almost to the particle’s marginal area. It should be pointed out that Gu et al33 have detailedly illustrated that nickel can preferentially move along the fast diffusion channels of Li layer with a calcination process. Therefore, during the phase transformation process, Ni ions most probably migrate to the Li layers of the surface regions, ideally formed a spinel phase of enrichment of Ni and Mn (Li1+yNi0.5-x/2CoxMn1.5-x/2O4).


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Fig 6. (a) STEM images of PrP3 particles; the EDS results of (b) line canning and (c) A, B and C points scanning on the edge of particle in (a).

Fig 7. XPS spectra (a, b) of O 1s and Pr 3d of the prepared samples LNCM, PrP2, PrP3 and PrP5.

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface compositions and the elements oxidation state of the as-prepared samples (Fig 7 and Fig S5). As expected, the Pr 3d, 4p, 4d and P 2s, 2p photoemission peaks in Fig S5a, are found for the modified samples but not for the pristine sample. The Ni 2p3/2 Co 2p3/2 and Mn2p3/2 binding energy of pristine and PrPO4-modified samples are located at 854.6, 780 and 642.2eV without obvious chemical shift, in accordance with Ni2+, Co3+ and Mn4+ in LNCM compounds respectively. The P 2p central peaks of the modified samples are located at 132.9 eV (Fig S5b), which is attributed to the emergence of phosphate groups in PrPO4 and consistent with other rare-earth metals phosphate.30 The O1s core spectra show an obvious change between the pristine and modified samples. The prominent O1s peak at 529.4 eV, corresponding to the lattice oxygen of the LLOs, which gradually shifts to a lower binding energy (529.3, 529.2, and 529.2 eV) in the PrPO4 modified samples ascribed to the local environment change derived from the spinel phase transformation. Whereas, two weakly adsorbed surface oxidized species (oxygen-containing species) are clearly visible due to two



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extra peaks at 531.4eV and 533eV.34 Besides, the peak at 530.5 eV, which is consistent with that of oxygen lattice in the PrPO4 crystal network (Fig S6a), appears and is stronger with the increase of PrPO4 covering thickness. Turning to the Pr 3d spectrum of the surface modified samples (Fig 7b), two main peaks characteristic of Pr3+ located at 933.3 (3d5/2) and 954 (3d3/2) are clearly visible. While, a pair of accompanying peaks at 929.4 (3d5/2) and 949.8 (3d3/2) show up, which can be assigned to Pr4+.35 Similar circumstance arises in pure PrPO4 (Fig S6b), indicating that certain amount of Pr in the PrPO4 compound have been oxidized to Pr4+. It can be deduced that when the modified samples are subjected to the low-temperature calcination, a slight amount of lattice oxygen can be removed by the oxidation of praseodymium ions. The first charge/discharge curves of the pristine and PrPO4 modified LNCM materials measured between 2 and 4.8 V at 0.05C are plotted in Fig 8a. It is obviously observed that all of the cathodes typically display a smoothly sloping voltage profile, which is ascribed to the oxidation of Ni2+ to Ni4+, Co3+ to Co4+, below 4.45 V, and followed by a long plateau above 4.45 V, which is attributed to the simultaneous removal of Li+ and O2− (as Li2O) from Li2MnO3. The corresponding differential capacity versus voltage (dQ/dV) curves are compared in Fig 8c. Interestingly, a pair of cathodic/anodic peak at around 2.7/2.9 V, which refers to the Li+ in/deintercalation 16c octahedral sites of the spinel structure,36,37 is observed in the dQ/dV curves of the modified samples not the pristine one Fig 8d. It indicates that a layer-to-spinel phase transformation has been induced by the PrPO4 surface modification, in agreement with the HRTEM images. Initial charge/discharge capacities and coulombic efficiency of the four samples are tabulated in Table 2. Compared to the pristine, the modified samples have a gradually decreased charging capacity with the increasing PrPO4 modifying amount. The data reveal that the capacity of the plateau region (above 4.45V) decreases as PrPO4-modifying increases, which gives further evidence that the Li2MnO3 phase has been activated in the modification process. On the other hand, the sloping region decreases as well, which is attributed to the reduction of transition metals, moreover, it can be deduced that part of Ni2+ might migrate to induced spinel phase (Li1+xNi0.5Mn1.5O4) and be oxidized at a higher voltage (4.7V) plateau.37 The first discharge specific capacities are found to be about 286.3, and 286.9 mAh g−1, respectively, for PrP2 and PrP3 electrodes, a slight increase compared to the pristine one (282.8 mAh g−1), which benefit from both the stabilization of the cathode/electrolyte interface and the newly formed spinel phase can intercalate more Li+ at around 2.7 V. Therefore, the coulombic efficiencies are found to be 81.8%, 87.1%, and 90%, respectively, for LNCM, PrP2 and PrP3 in the 1st cycle. However, for sample PrP5, the decrease in the discharge capacity is expected that excess layer to spinel transformation will lead to irregular cation mixing phase (NiO phase) formed,


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which is electrochemically inactive and can interfere the diffusion of Li+ insertion/extraction the cathode material.30 Thus the thickness of the cation mixing layer should be controlled by controlling the thickness of modification layer. Table 2. The initial charge-discharge data of LMCN, PrP2, PrP3 and PrP5 at 0.05C.

Sample

Charge capacity

Charge capacity

Charge

Discharge

Coulomb

below 4.45V

above 4.45 V

capacity

capacity

Efficiency

(mAh g-1)

(mAh g-1)

(mAh g-1)

(mAh g-1)

(%)

LNCM

134.5

211.1

345.6

282.8

81.8

PrP2

126.5

202.1

328.6

286.3

87.1

PrP3

120.2

198.6

318.8

286.9

90.0

PrP5

106.4

183.1

289.5

258.2

89.19

Fig 8. The 1st (a) and 2nd (b) charge–discharge curves for the four samples between 2.0–4.8 V at 0.05C; (c), (d) Corresponding dQ/dV curves for the four samples.



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Fig 9. (a) Cycling performance, (b) average discharge voltages and voltage profiles from galvanostatic cycling of (c) LNCM, (d) PrP2, (e) PrP3, and (f) PrP5 electrodes at 0.5 C-rate in the potential range of 2.0–4.8 V after two activation cycles.

The cycling performance of the bare and modified samples were compared with charge-discharge rates at 0.5 C for 100 cycles, after two activation cycles at 0.05C. As we can see from Fig 9a, PrP3 sample show a much-improved capacity retention of 89.3% with a higher discharge capacity of 210.1 mAh g-1, while the pristine sample fades to 165.3 mAh g-1 with a capacity retention of 71.7%. The charge-discharge curves and the related dQ/dV plots with the different cycles of the four types of electrodes are shown in Fig 9 and Fig S7. It is obvious that the polarization effects upon charge and discharge are distinctly reduced during 100 cycles. In general, these curves in differential capacity plots are characterized by three broad, hardly separated them, corresponding to the redox behavior of the three transition metals involved. Besides, the anodic peaks at around 2.9 V related to the spinel interphase of modified


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samples are gradually disappeared due to the increasing polarization. The most remarkable cathodic peak appearing between 3.2 and 3.5 V belong to the Mn4+-Mn3+ redox activity,17,19,20 is gradually shifting toward a lower voltage at 2.7V for the pristine, which indicated the transformation of layered to spinel and cubic rock-salt phase.30,38 The disordering rock-salt phase is electrochemical inert and has relatively lower Li+ diffusion kinetics, which in a large amount may cause dramatic capacity fading and severe polarization, even collapse of the crystal structure. But the shifts of modified samples have been suppressed by the stabilization effects of the modification layer and spinel interphase. And more directly, the plots of the average discharge voltages of these four cathode materials regarding cycle number are presented in Fig 9b. It can be seen that the average discharge voltage of PrP2 only decays 0.2881V after 100cycles, compared with 0.6353V for LNCM. As is known, both layered phases (Li2MnO3 and LiMO2) undergo layer-to-spinel transformation and then rock-salt phase upon cycling in large potential range. The layered LiMO2 transforms to spinel through migration of TM ions to the Li sites without destroying the lattice, whereas the transition from Li2MnO3 to spinel involves the removal of Li+ and O2− with a destruction of the parent lattice.10 However, modifying the LLOs with PrPO4 have previously moved amount of oxygen and lithium from the lattice leading to a chemical activation of Li2MnO3 component and forming a spinel interphase. Different from the spinel phase formed during the long term cycling, the induced spinel phase based on the oxygen lattice of host layered materials have a stable structure and fast Li+ diffusion channel, which can stabilize the crystal structure of the host layered structure and significantly enhance the Li+ storage/diffusion. On the other hand, the less release of oxygen will certainly weaken the decomposition of the electrolyte under high voltage and suppresses the formation of the unstable SEI layer. The rate capability further proves the advantage of the PrPO4 modifying. The cells were charged galvanostatically between 2.0 and 4.8 V with a current density of 25 mA g−1 (0.1 C-rate, except for first circle at 0.05C-rate) before discharged at current densities from 25 mA g−1 (0.1 C rate) to 2500 mA g−1 (10 C rate) with four cycles per rate. As can be seen in Fig S8, the discharge capacities of PrP2 and PrP3 cells are higher than that of the pristine cell, and the capacity differences become more noteworthy with increasing discharge rates. Especially when discharged at 10 C, the PrP3 sample delivered a higher discharge capacity of 124.2 mAh g-1, whereas the pristine sample showed a lower capacity of 80.6 mAh g-1. In addition, when returning to a low current rate of 0.1 C again, the PrP3 sample gets a better performance compared with the pristine one. This result suggests that the induced spinel phase can stabilize the layered structure and open the Li+ channels to facilitate fast migration of Li+ ions. To further understand the positive effects of spinel interphase and PrPO4



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modification layer on LNCM cathode cycle performance, the electrochemical impedance spectroscopic (EIS) of the fabricated coin cells consisting of the four samples were measured and analyzed at first and 51st cycles charged to 4.0 V during cycling (mentioned above). In general, all the spectra (Fig S8) comprise two well-separated semicircles and a very short slanted line. And the equivalent circuit in the inset of Fig S8 was used to fit the EIS. According to previous reports, the semicircle at the higher frequency range assigns to the Li+ ion migration through surface films (Li+ diffusion resistance Rsf), the other one at lower frequency range corresponds to the charge transfer process (charge transfer resistance Rct), and the slanted line at low-frequency region reflects Li+ ion diffusion in the particles of the electrode material (Warburg diffusion impedance Zw).10,39 Moreover, CPEs and Rs indicate the corresponding constant phase elements and the solution resistance, respectively. The fitted values of Rsf and Rct for all samples are tabulated in Table S1. It is obvious that the PrP2 and PrP3 electrodes exhibit lower Rsf values than the pristine, suggesting that reduced oxygen release has a suppressive effect on the formation of SEI layer. After 50 cycles, the differences of Rsf values for PrP2, PrP3 and LNCM electrodes are bigger, reflecting that PrPO4 modification layer protect the electrode surface from HF attack in electrolyte and reduce metal dissolution.39 Similar situation has been found in medium frequency, after cycling, the increases of charge transfer resistance in PrP2 and PrP3 is much lower, indicating a more stabilized bulk and surface structure. By contrast, PrP5 show relatively large surface films resistance and charge transfer resistance due to the formation of irregular cation mixing (NiO phase), which results in the opposite effect in electrochemical properties.


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Fig 10. TEM images (a), (b) and HRTEM images of the pristine LNCM and PrP3 samples after 100 cycles.

To evaluate structural changes in Bare-LNCM and PrP3 after cycling process, the cycled cells were dissembled in a glove box, and the cathode sheets were collected and thoroughly washed with DMC several times for TEM analysis. HRTEM images (Fig 10) show that the bare electrode exhibits uneven surface (similar to cubic rock-salt facets40) and complex structural characteristics. It is clearly observed that near surface region and the bulk region have two different kinds of cubic lattice, the cubic rock-salt phase with Fm-3m space group and spinel-like phase with Fd-3m space group. This observation probably indicates that the outermost surface layers may firstly transform to spinel-like structure, then convert to cubic rock-salt, which may gradually extend to the interior of the particle. The increase of the disordered rock-salt structure during cycling is closely associated with the capacity fade because of the electrochemically inactive characteristic of the disordered structure.30,41 In comparison with the drastic phase transition of the pristine electrode, the PrP3 retains an unaltered layered structure with a modification layer of PrPO4, except that the spinel interphase has extended slight thicker. The observation here also gives immediate proof that formation of surface heterostructure can achieve the high surface and bulk crystal structure stability and suppress the erosion of electrolytes, contribute to enhanced capacity retention and voltage profile.



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3 Conclusions In summary, a PVP-assisted wet chemical deposition route with a low-temperature calcination process is successfully applied to obtain PrPO4-modified Li-rich layered Li1.2Mn0.54Ni0.13Co0.13O2 material. After the surface modification and subsequent calcination, a surface heterostructure is formed on the host particles, containing spinel phase layers distributed homogeneously between the modification layers and host particles. It is confirmed that the spinel phase has a stable structure and fast Li+ diffusion channel, which can stabilize the crystal lattice of the host material and significantly enhance the Li+ storage/diffusion. Meanwhile, along with the PrPO4 modification layer, the spinel interphase can effectively prevent side reactions between the LNCM cathode and electrolytes during high voltage cycling. These results suggest that the surface heterostructure on LLOs, induced by PrPO4 modifying, is an excellent construction for protecting the electrode from the erosion of electrolytes, mitigating the voltage decay, and improving the long-term cycling stability.

ASSOCIATED CONTENT Supporting Information Experimental details, XRD pattern of synthesized pure PrPO4 and Reitveld fitting of the XRD patterns of the four samples, Raman spectra, STEM mapping, XPS spectra, dQ/dV curves, rate performance comparison, detailed Nyquist plots of the samples(PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected](J.L)

ORCID Jianling Li: 0000-0002-3915-9540

Acknowledgements


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This work is financially supported by the National Natural Science Foundation of China (51572024) and the science and technology project of State Grid Corporation of china (DG71-16-025).

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