New Organic Complex for Lithium Layered Oxide Modification: Ultra

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Letter

New Organic Complex for Lithium Layered Oxide Modification: Ultra-thin Coating, High-Voltage and Safety Performances Yingqiang Wu, Hai Ming, Mengliu Li, Junli Zhang, Wandi Wahyudi, Leqiong Xie, Xiangming He, Jing Wang, Yuping Wu, and Jun Ming ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00032 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Energy Letters

New Organic Complex for Lithium Layered Oxide Modification: Ultra-thin Coating, High-Voltage and Safety Performances Yingqiang Wu #§†, Hai Ming &†, Mengliu Li ¶, Junli Zhang ∥*, Wandi Wahyudi ¶, LeqiongXie ⊥,

#

Xiangming He ⊥, Jing Wang #, Yuping Wu #* and Jun Ming §¶*

State Key Laboratory of Materials-oriented Chemical Engineering & School of Energy

Science and Engineering, Nanjing Tech University Nanjing, 211816, P. R. China. §

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. ∥

Key Laboratory of Magnetism and Magnetic Materials of the Ministry of Education,

Lanzhou University, Lanzhou 730000, P. R. China. &

¶

Research Institute of Chemical Defense, Beijing 100191, P. R. China. Physical Sciences and Engineering Division, King Abdullah University of Science and

Technology, Thuwal, 23955-6900, Saudi Arabia. ⊥

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, P.

R. China. † These

authors contribute equally

1 ACS Paragon Plus Environment

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

* To whom correspondence should be addressed: [email protected]; [email protected]; [email protected].


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ACS Energy Letters

Abstract: Surface modification of cathode (e.g., lithium layered oxide, NCM) has become ever more important in lithium-ion batteries, particularly for pursuing higher energy densities and safety at high voltage. This is because structural degradation of cathode can be mitigated significantly. Herein, an organic complex is introduced for metal phosphates (e.g., AlPO4) modification through a new film-forming process in non-aqueous solution. This general strategy overcomes the challenge of non-uniform coating in current precipitation method, then opens a new avenue towards ultra-thin surface modification on molecular scale. As one of examples, as-prepared AlPO4-coated NCM exhibits much improved structural and electrochemical stability; meanwhile, thermal runaway can be suppressed significantly in over-charged cell using the modified NCM, demonstrating higher and reliable safety features. The great improvements benefit from the uniform and ultrathin AlPO4 coating, which inhibits the collapse and conversion of layered structure to spinel especially to rock salt structure at high-voltage conditions, as confirmed by HRTEM and EELS. TOC

Clear solution

High-Voltage 4.6V

5.52 V Film coating

MPO4-based Organic Complex 5.06 V


Thermal runway


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Rechargeable lithium-ion batteries (LIBs) have dominated the energy market from hand-held electronic devices to electric vehicles (EVs) since its commercialization, but the energy density and reliability are still far from satisfied.1-5 The cathode, as the core component in LIBs, have a large room to improve in capacity and stability (vs. the anode of graphite,6, 7 silicon8,

9

or Sn-based compound10,

xCoxMnyO2,

NCM).12,

13

11),

especially for the lithium layered oxide (LiNi1-

This is ascribed to its higher theoretical capacity (~276 mAh g-1)

than those of LiFePO4 (175 mAh g-1) and LiMn2O4 (145 mAh g-1), respectively, and the superior cycle stability and lower cost than the commercial LiCoO2.14-17 However, the practical capacities of most NCM are only around 155-170 mAh g-1 at present, because the charged cut-off voltage is restricted at 4.2 V for a good stability. Although a high capacity of ~200 mAh g-1 can be achieved as increasing the cut-off voltage (e.g., 4.5 V vs. graphite anode),18,

19

the capacity fading and safety concerns become more serious.20,

21

This is

because the structural degradation, triggered from the electrode/electrolyte interphase, become aggravated at the high voltage conditions.22-27 Thus, developing an efficient strategy to modify the NCM with robust characteristics remains challenging.28-38 To date, the metal phosphates have become an efficient coating layer to improve the NCM performance because of their great anti-corrosive feature to the acidic compound (e.g., HF) in the electrolyte.39,

40

In addition, the metal phosphates are more robust than metal

oxides41, 42 (i.e., the higher binding energy of “M-OP” than that of “M-O”),43 and also much easier to handle than fluorides.44, 45 However, there is only one soultion-based precipitation method developed in the past two decades,46-50 where a uniform and ultrathin coating is still a great challenge. A thick, rough and/or scattered coating layer was always caused by the aforementioned precipitation method, where the metal phosphate suspension (e.g., AlPO4) was prepared first by the precipitation of metal nitrate and (NH4)2HPO4, and then used as precursors to coat the NCM.39, 43 As a result, an irregular dispersion and aggregation of metal


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ACS Energy Letters

phosphate on NCM were induced due to the inhomogeneous suspension and the high surface tension of aqueous solution. Unfortunately, this situation is hard to be improved at present, because the metal hydroxide may be formed on the alkaline surface of NCM if the metal phosphate was not precipitated first. This is because the solubility product constant (Ksp) of metal hydroxide is much lower than that of metal phosphate51 (e.g., 3×10-34 of Al(OH)3 vs. 6.3 ×10-19 of AlPO4). In addition, the low solubility of (NH4)2HPO4 also limit its applications in other solvents apart from the water. Note that the unique atomic layer deposition (ALD) strategy can achieve an uniform and ultrathin AlPO4 coating,52 but the cost and efficiency of deposition need to be addressed before a scale-up application. Thus, design a soluble metal phosphate-based molecular compound, rather than a precipitate suspension, is urgently needed to guarantee a uniform and thin coating, which could suppress the structural degradation and also reduce the effect for Li+ ion transfer constant at the electrode/electrolyte interphase. Herein, a novel organic ligand coordination complex is introduced to stabilize the metal phosphate in non-aqueous solution, then used for the surface modification through a film-forming, rather than the precipitation process for the first time. A new coordination compound (i.e., M(OEt)xPOy(OEt)z, M = cations, e.g., Al3+) with the core of M/P and the ligand of ethyl ester/carbonyl is prepared through the reaction of phosphorus oxide, ethanol and metal nitrate (e.g., Al(NO3)3·9H2O) in ethanol (EtOH). In this way, the soluable complex containing the main component of MPO4 can be coated on target (e.g., LiNi1/3Co1/3Mn1/3O2, NCM) as a uniform film when the ethanol was removed in the dry process. Thus, an ultrathin MPO4 (e.g., AlPO4) can be formed after the thermal treatment. This general strategy not only overcomes the non-uniform coating of conventional precipitation method, but also opens a new avenue towards ultra-thin surface modification feasible to control on molecular scale. As one of the example, the modified NCM demonstrates much higher high-voltage



performances in lihtium (ion) batteries, and the differences of electrode with and without coating during the cycling are also further analyzed in detail. Features of organic ligand coordination complex. A typical synthetic routine of organic ligand coordination complex is shown in Figure 1a, which can be used for AlPO4 coating directly. Firstly, the solid P2O5 powders react with ethanol to form coordinated molecules of PO(OH)2(OEt)/PO(OH)(OEt)2. Then, a soluble mixture of AlPO3(OEt)2/AlPO2(OEt)4 and HNO3 was formed after the reaction with Al(NO3)3. Finally, a uniform, transparent AlPO3(OEt)2/AlPO2(OEt)4 ethanol solution, abbreviated as AlPO4-based solution, is obtained when the HNO3 was precipitated by the NH4NO3. Herein, the pH-sensitive Al3+ and metal phosphate are stabilized for the first time by the phosphate ester in the ethanol as coordinated complex. Thus, there is no any precipitate (e.g., AlPO4 or Al(OH)3) formed even the pH value was adjusted to around 7.0 (Figure 1b, Figure S1). More importantly, a transparent, ultra-thin film can be formed after removing the ethanol after drying (Figure 1b). This result fully demonstrates that the AlPO4-based film can be also formed on any target, and then a uniform AlPO4 coating layer can be achieved by calcination (Figure 1c). The great features of this strategy superior to the current precipitation method are summarized as below (Figure 1d-e, Figure S2): (i) a more convenient coating process, which can be achieved through mixing the AlPO4-based solution and target (e.g., NCM) directly without any precipitation step; (ii) availability to get a more uniform coating, because the ethanol solution has a low surface tension and the coordination complex can disperse on the target on molecular-scale; (iii) feasibility to tune an ultrathin coating, where the complexes concentration can be controlled accurately and then an ultra-thin AlPO4-based film can be formed on target after the dry; (iv) universality of this strategy for most materials, in which the mild and/or neutral condition can avoid any possible corrosion, especially for the pHsensitive materials (e.g., Ni-rich NCM) during the modification process (Figure S3); (v) a 6 ACS Paragon Plus Environment

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ACS Energy Letters

green process feasible for practical applications. This is because there is no harmful chemicals generated in synthetic routine, whereas the NH4NO3 and ethanol can be recycled by filtration and evaporation, respectively. The schematic illustration of coating procedure and varied chemical compositions of coating layer is presented in Figure 2a. Firstly, the AlPO4-based solution could disperse on the NCM particles uniformly on molecular-scale and form a thin film after drying, which was then converted into amorphous AlPO4 layer after sintering. The fine distribution of elemental Al/P corresponding to the AlPO4 and the ultra-thin coating on NCM is confirmed by the SEM/EDS (Figure 2b) and HRTEM/EDXA mapping, respectively. We can find that the coating layer is about 2.5 nm, as shown in the BFTEM (Figure 2c). In addition, the appearance of AlPO4 can be judged from the new peaks of Al2p (5.9 wt%) and P2p (6.3 wt%) in XPS spectrum (Figure 2d), respectively. The amount of metal elements (Ni+Co+Mn) reduces from 22.7 wt% to 7.4 wt%, further demonstrating the coverage of AlPO4 on the NCM particles (Figure 2e). Note that the amount of AlPO4 in this study is feasible to control as low as 0.2 wt%, which is much lower than most previous literature.39, 40 This benefits from the thin film-formation feature of the organic ligand coordination complex; meanwhile the electrochemical performance can be much improved as discussed later. Note that the ultrathin coating layer is important. This is because the ultrathin coating layer can minimize the negative effect for the lithium ion diffusion constant, electric conductivity and energy density when a coating layer was coated on the active cathode (e.g., NCM). Particularly, the ultrathin feature is efficient to reduce the internal resistance of electrode and suppress the thermal runaway, because most coating layer (e.g., AlPO4, Al2O3, ZrO2, AlF3) are semiconductor or insulator which could lead to the increment of resistance if the coating layer is too thick.52-54 High-voltage and safety performances. The high-voltage electrochemical performance of AlPO4-coated NCM is evaluated first in lithium battery using the metallic lithium anode (4.6 7 ACS Paragon Plus Environment


V vs. Li/Li+). The AlPO4-coated NCM delivers a reversible capacity of 198.5 mAh g-1 with an initial coulombic efficiency (CE) of 89.5%, which is higher than that of pristine one (i.e., 195.0 mAh g-1, 89.2%) (Figure 3a). Importantly, the cycle stability of AlPO4-coated NCM is much improved. We find that a high capacity of 177.1 mAh g-1 can be maintained with the capacity retention of 95.6% after 100 cycles under 0.5C (1C = 200 mA g-1) (Figure 3b). These values are much higher than 151.0 mAh g-1 of pristine NCM, where the capacity retention is only 83.2% (Figure 3b, Figure S4a-b). The superior stability of the AlPO4 coated NCM over that of the pristine NCM can be further confirmed in the prolonged cycles (Figure S5). Another great feature of the modified NCM is the reduced polarization. For example, the average value of the decreased discharge voltage (i.e., Vdischarge = E/C, E, the energy (Wh); C, the discharge capacity (Ah) of the battery) for AlPO4-coated NCM is only 0.2 mV per cycle (i.e., from 3.92 V to 3.90 V), while the value is as large as 1.4 mV per cycle (i.e., from 3.92 V to 3.78 V) for the pristine NCM (Figure S4c-d). The comparative CV curves in Figure 3c-d also corroborate this result. We find that the peak shift of anodic reaction is only 0.07 V (i.e., from 3.93 V to 4.0V) for the AlPO4-coated NCM, while the value is as high as 0.16 V (i.e., from 3.89 V to 4.05 V) for the pristine one. Note that, herein we further confirm that the 0.2wt% of AlPO4 coating amount is close to be an optimum value (Figure S6). In addition, amorphous AlPO4 is good for achieving a high performance compared to the crystalline one (Figure S7). The higher stability of AlPO4-coated NCM at high-voltage conditions is further confirmed in lithium ion full battery (i.e., 4.5 V vs. graphite) (Figure 4a, b). First, we find that the battery using AlPO4-coated NCM shows much better cycle performance. The capacity retention can maintain at 87.2% after 100 cycles, which is 17.0% higher than that of the battery using the pristine NCM (Figure 4c). Note that the capacity decay of battery using pristine NCM mainly occurs at 3.5 V (Figure S8a-b), and this phenomena is not obvious


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ACS Energy Letters

when the AlPO4-coated NCM was used (Figure S8c-d), as confirmed by the dq/dv plots. The reason should be ascribed to a good structural maintenance and the suppressed the deactivation of Ni3+/2+ after the AlPO4 coating.55 Another great feature of AlPO4-coated NCM is the higher rate capability. For example, the discharge capacity of AlPO4-coated NCM is 1.3%, 2.4%, 3.9%, 5.3% and 6.5% higher than those of the pristine one under 0.5C, 1.0C, 2.0C, 4.0C and 5.0C (1C = 200 mA g-1) (Figure 4d), respectively. This should be ascribed to the advantages of ultrathin AlPO4 layer, which not only protects the NCM but also maintains the rate of lithium ion diffusion. In addition, the attractive safety performance is confirmed in the overcharge experiment, where the 1.0 Ah pouch cell is charged to 10.0 V under 1.0C (Figure 4e). We find that the temperature is only about 36 ºC when the battery using AlPO4-coated NCM is overcharged to the voltage as high as 5.5 V. By contrast, a serious thermal runaway occurs only at 5.06 V for the battery using the pristine NCM, where the temperature increases sharply to ~90 °C within few seconds (Figure 4e). The significant enhancement should be ascribed to the high stability of AlPO4 coating layer, which has an extraordinary anticorrosive and anti-oxidized capability due to the strong covalent bond between the Al3+ and PO43-.46, 56-58 Thus, the AlPO4 layer can improve the structural stability of NCM (e.g., partial Al doping in the crystal structure59) and reduce the contact area between NCM and electrolyte,60-62 thereby suppressing the NCM structural degradation (e.g., release O2) and electrolyte decomposition efficiently.63-65 The improved thermal stability of AlPO4 coated NCM was further confirmed in high-temperature storage, where a much smaller selfdischarge (i.e., voltage decay) is observed. For example, the voltage of the battery with AlPO4-coated NCM can maintain at 4.0 V for 6 days under 100 °C, but this value drops to 3.93 V for the battery using the pristine NCM (Figure 4f). Electrode analysis and characterizations. The significant improvements of electrochemical 9 ACS Paragon Plus Environment


performance are ascribed to the unique, uniform and ultrathin AlPO4 coating layer. The electrochemical impedance spectroscopy (EIS) analysis support our results. Firstly, the increment of RSEI and Rct of the battery with the AlPO4-coated NCM is only about 1.5 Ω and 9.6 Ω after 100 cycles (Figure 5a, Figure S9c), respectively. By contrast, these values are as high as 10.0 Ω and 24.5 Ω for the battery using the pristine NCM (Figure 5b). In addition, we find that the increased resistance of the battery mainly results from the NCM cathode (Figure 5c-d), because there is no obvious change for the resistance of graphite anode after the cycling (Figure S9a-b). The results demonstrate that the stability of NCM and the NCM/electrolyte interface at high voltage was greatly improved after the AlPO4 coating. These results can be further confirmed by the Rietveld refinement of XRD before and after cycling (Figure S10). Although both NCM cathodes can mainly maintain the layered-type structure (space group R-3m) after 100 cycles, the amount of the antisite defect (i.e., Li/Ni cation ions mixing) is almost the same before and after the cycling for the AlPO4-coated NCM (i.e., 2.82% of fresh NCM vs. 2.83% of cycled NCM). By contrast, this value increases from 2.82% to 2.96% for the pristine NCM after the cycling (Table S1). This result reveals that more interfacial degradation (e.g., phase transition) exists for the pristine NCM during the cycling at high voltage conditions. The structural information and phase evolution of the NCM is further investigated by HRTEM. Firstly, the high-magnification HADDF-STEM image shows that the fresh NCM has an integral layered structure with R-3m space group66 in both surface and bulk of the crystal particle (Figure 6a). Then, an obvious crystal boundary with the presence of new rock salt structure can be found on the particle surface of the pristine NCM after 100 cycles (Figure 6b). We find that the layered structure on the surface of the crystal particle collapsed along the (012) plane, as confirmed by the magnified HRTEM images of two neighboring phases (Figure 6c-d). In addition, the electronic states of the transition metal sites (i.e.,


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ACS Energy Letters

electronic states at the metal site) before and after cycling were further characterized by the EELS (Figure 6e), where the L3 and L2 white-lines indicate the electrons transitions from 2p3/2 and 2p1/2 to unoccupied 3d states, respectively.24, 67 Generally the L3/L2 intensity ratio represents the varied number of electrons in the 3d-band,68 which value decreases as the variation from 3d0 to 3d5 or increases from 3d5 to 3d10 conﬁguration for transition metal elements. Thus, we can judge the variatio of valence based on the configuration of the outer electrons,69 where the electron configuration for metallic Ni, Co and Mn are 3d84s2, 3d74s2 and 3d54s2, respetively. Quantitative analysis of the EELS spectrum reveals that the major changes of charge compensation at the metal sites after cycling mainly located at the Ni ions (i.e., 3d8, wherein the L3/L2 ratio decreases from 2.69 to 2.27), while the Mn and Co ions almost remain their initial states (Figure S11, Table S2). This result demonstrates that the valence of Ni ions changed after cycling, which should be ascribed to the structural degradation (i.e., LiMO2→M4+O2→M2+O + O2).70,

71

These data are consitent with the

observations in Figure 6b-d, which demonstrates that the surficial layers of pristine NCM probably have collapsed and converted into the rock salt structure after the cycling under high voltage, as illustrated in Figure 6f. By contrast, there is only a tiny new phase (about 5 nm) at the outer layer of the cycled AlPO4-coated NCM (Figure 7a). The magnified HRTEM images suggest the new phase is spinel-like structure after cycling, where the layered structure collapsed along the (006) plane (Figure 7c). In addition, only a slight change of Ni/Mn/Co L-edge EELS spectrum (Figure 7d) is observed on the charge compensation before and after the cycling, which is different from those of the pristine NCM after the cycling. Thus, the Ni, Mn and Co ions almost remain the initial state (Figure S11, Table S2). These results confirm that the AlPO4 coating layer on layered NCM can efficiently prevent structure collapse and convert into spinel structure especially to the rock salt structure at high-voltage conditions, thereby



achieving the goal of higher stability, longer lifetime and more reliable safety. Conclusion. In summary, a novel organic ligand coordination complex is introduced to stabilize the metal phosphate in non-aqueous solution, then used for the metal phosphate (e.g., AlPO4) modification of lithium layered oxide (e.g., NCM). This approach can disperse the MPO4-based complexes on target materials on molecular-scale, and then in-situ form a new film without any precipitation process, thereby achieving a precise, uniform and ultrathin coating layer. This strategy not only overcomes the challenge of non-uniform coating in current precipitation method, but also opens a new avenue towards ultra-thin surface modification feasible to control on molecular scale. As a result, a significant improvement of NCM performance at high voltage conditions, including the higher stability, better cycle ability and more reliable safety features, are demonstrated in both lithium (ion) batteries. We hope that the advantages of this strategy can be widely applied in many other fields for diverse surface functionalization; meanwhile, the electrode analysis and characterizations could be significant to understand and design electrode with greater performance.


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ACS Energy Letters

Figure 1. Features of the organic ligand coordination complex and film-forming process. (a) Chemical reaction for coordination complex, where the pH-sensitive Al3+ was stabilized by the ligand. (b) Digital-photo of the uniform, thin film formed from the transparent coordination complex (AlPO4-based non-aqueous solution) after the drying. (c) XRD analysis of the film component after drying and sintering under different temperatures. (d) Research trend and limitations of precipitation method in the past two decades and (e) digital-photo of scattered metal phosphate (e.g., AlPO4) powders after drying the metal phosphate suspension in dish, which was prepared by the precipitation between the metal nitrate (e.g., Al(NO3)3·9H2O) and (NH4)2HPO4 in aqueous solution.


ACS Energy Letters

a

H2O and CO2

Ethanol

AlPO4 solution

Sintering

Drying

Infiltration

AlPO4-based Film

Pristine Al Kα1

b

AlPO4-coated NCM

P Kα1

c

20 nm Al: 2p

1200

Pristine Coating

2000

1000 800 600 78

Figure

2.

76 74 72 Binding Energy / eV

Surface

70

e

P: 2p

1600 1200 800

modification

Al

P

Content by weight / %

1400

0.1 µm

Counts / s-1

d Counts / s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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138

by

135 132 129 Binding Energy / eV

the

organic

126

40

Pristine cample coating sample

30 20 10 0

Me

coordination

Li

C

complex

O

Al+P

and

characterizations (a) Schematic illustration of surface modification for NCM. (b) SEM/EDS and (c) TEM/EDXA mapping of Al and P for AlPO4 coated NCM. (d) XPS spectrum of Al2p and P2p peak for AlPO4 coated NCM. (e) Comparison of elemental content determined by the XPS before and after the modification, where Me = Ni + Co + Mn.


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a

b 220 4.5 -1

200

Capacity / mAh g

Voltage / V

4.0

3.5

0

50

100

150

Capacity / mAh g

c 7.5

AlPO4 coated NCM

180 160 140 120

Pristine NCM AlPO4 coated NCM

3.0

200

100

250

Pristine NCM AlPO4 coated NCM 0

20

40 60 Cycle number

-1

d

0.07 V

Pristine NCM

80

100

0.16 V

5.0

Voltage / V

5.0

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5 0.0 1st cycle 100th cycle

-2.5 2.7

3.0

3.3

2.5 0.0 1st cycle 100th cycle

-2.5 3.6

3.9

4.2

Capacity / mAh g

4.5

4.8

2.7

-1

3.0

3.3

3.6

3.9

4.2

Capacity / mAh g

4.5

4.8

-1

Figure 3. Comparative performance of NCM before and after the modification in lihtium battery. (a) Comparison of the 1st cycle (dis-)charge curves under 0.1C (i.e., 20 mA g-1) and (b) cycle performance of the pristine and AlPO4-coated NCM under 4.6 V (vs. Li/Li+) and 0.5C (1C = 200 mA g-1). Comparative CV of (c) AlPO4-coated and (d) pristine NCM upon cycling.


ACS Energy Letters

a

b

4.8

4.5 4.0

LiNCM LiC

2.4 1.6

Voltage / V

3.2

N/P = 1.1

Voltage / V

4.0

0.8

3.5 3.0 2.5


0.0

c

2.5

Capacity / mAh

0.0

2.0

0.5

1.0 1.5 Capacity / mAh

2.0

2.0

2.5

d

-1

1.5 1.0 Pristine NCM AlPO4 coated NCM 0

20

40

0.0

0.5

1.0 1.5 Capacity / mAh

2.2

1.8

0.5C 0.8C

80

2.0C 3.0C

4.0C

5.0C

0.5C

1.4 1.2

0.8 0

100

1.5C

1.6


1.0 60

1.0C

3

6

9

12

15

18

21

24

27

Cycle number

Cycle number

e

2.0

2.0 Capacity / mAh

Cycle at 1.0 C, 200 mA g

0.5

f

100 Over charge to 10 V

4.15

Storage under 100 C

4.10

80 Voltage / V

Temperature / C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 5.52 V

20 0

5.06 V Thermal runaway 4

5

6

7 8 Voltage / V

4.00 3.95


9

4.05

3.90

10

Pristine NCM AlPO4 coated NCM 0

25

50

75

100

125

150

Time / hours

Figure 4. Comparative electrochemical and safety performance of NCM before and after modification in lithium ion full battery with graphite anode. (a) Typical (dis-)charge curve of NCM and graphite. Performance of batteries with pristine and AlPO4-coated NCM in (b) the 1st cycle at 0.1C, (c) cycle performance at 1C (1C=200 mA g-1) and (d) rate capability. Comparison of the anti-overcharge and high temperature storage performance, (e) temperature vs. voltage curves of the pouch cell and (f) Self-discharge performance, where the battery was stored under 100 °C from the fully charged state. The N/P (i.e., negative/positive electrode) ratio is controlled at 1.1.


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50

30

15

Rct

5

20

0

20

40 60 80 Cycle number

100

10

20

30

40

50

60

0

70

Z' / 

d

50 40 30

Rct

20

0

20

100

Cycles: 1st, 25th, 75th, 100th

0

10

20

30

40 Z' / 

RSEI / Ω

Rct / Ω

1st

2.1

7.6

100th

18.5

12.8

20

40

60 Z' / 

80

100

120

Pristine NCM from the full battery

40

1st cycle 100th cycle

cycle

RSEI / Ω

Rct / Ω

1st

6.7

8.1

100th

19.5

39.1

20

10 0

0

60

40 60 80 Cycle number

cycle

1st cycle 100th cycle

10 0

20

80

RSEI

30

-Z'' / 

Resistance / 

b

0

40 20

st th th th 10 Cycles: 1 , 25 , 75 , 100

0

AlPO4 coated NCM from the full battery

60

10

0

80

RSEI

-Z'' / 

40

-Z'' / 

c

20

Resistance / 

a

-Z'' / 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

50

60

0

70

0

20

40

60 Z' / 

80

100

120

Figure 5. Electrochemical impedance analysis. Electrochemical impedance spectroscopy (EIS) of the full battery with (a) AlPO4 coated and (b) pristine NCM cathode. Impedance analysis of the NCM cathode taken from the full battery (c) AlPO4 coated and (d) pristine NCM. All the EIS were measured under 50% SOC of the batteries.


ACS Energy Letters

a

Layered structure

b

cc

Layered structure

2.44 Å

[42-1]

2.304 Å

1 nm 0.5 nm

2.02 Å 2.36 Å

d

Fm-3m [01-1]

Rock salt

1 nm 2.02 Å

R-3m [0-11] Li O Ni/Co/Mn

e

R-3m

Pristine Cycled

2 nm O

2.362 Å

1 nm

Mn

Co

Ni

f Layered structure

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

Boundary

520 533 546 559 Energy loss (eV)

624 636 648 660 Energy loss (eV)

767 780 793 806 845 858 871 884 Energy loss (eV)

Energy loss (eV)

Rock salt

Figure 6. Structural analysis of pristine NCM before and after cycling. (a) HADDFSTEM image of the fresh NCM. (b) High-resolution TEM of the cycled NCM, showing the presence of segregation layer with rock salt structure. The boundary between two phases was marked by rose red line. (c, d) Magnified HRTEM image and corresponding crystal structure of the area marked by the blue square and yellow square in (b). (e) EELS spectrum of NCM before and after the cycling. (f) Schematic illustration of the structure for the cycled NCM, showing the structral degradation from layered structure to rock salt structure.


Page 19 of 29

a

Layered structure

b

[0 -10]

e

Layered structure

2.46 Å 1 nm 2.36 Å

c

2.47 Å Spinel structure 2 nm

d

R-3m [0 1 1]

Boundary

A site metal Bsite metal

1 nm

Spinel Coating Cycled

O

Mn

Co

Ni

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

520 533 546 559

624 637 650 663

767 780 793 806

845 858 871 884

Energy loss (eV)

Energy loss (eV)

Energy loss (eV)

Energy loss (eV)

Figure 7. Structural analysis of the AlPO4-coated NCM before and after cycling. (a) HADDF-STEM image of the cycled AlPO4 coated NCM, revealing only a thin surface segregation layer is built up with spinel structure. (b, c) Magnified HADDF-STEM image and corresponding crystalline structure of the areas marked by the blue square and orange square in (a). (d) EELS spectrum of AlPO4 coated NCM before and after the cycling. (e) Schematic illustration of the crystal structure of the cycled AlPO4 coated NCM, showing the structural degradation of surficial layers into a spinel structure.



Page 20 of 29

Supporting Information. The following files are available free of charge. Experimental section, Figures S1-S11 and Table S1-S2 are included. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Zhang) *E-mail: [email protected] (Y. Wu) *E-mail: [email protected] (J. Ming) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †Y. Wu and H. Ming contributed equally. ACKNOWLEDGMENT The research was supported by the funding of National Materials Genome Project (2016YFB0700600),

National

Natural

Science

Foundation

Committee

of

China

(Distinguished Youth Scientists Project of 51425301, U1601214, 51573013, 51773092 and 51772147). This work is supported by the National Natural Science Foundation of China (21521092, 21703285 and 11604130) and the Independent Research Project of the State Key Laboratory of Rare Earth Resources Utilization (110005R086), Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. The authors also thank the great support from the King Abdullah University of Science and Technology (KAUST).

References 20 ACS Paragon Plus Environment

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New Organic Complex for Lithium Layered Oxide Modification: Ultra

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