Rock salt type LiTiO2 @ LiNi0.5Co0.2Mn0.3O2 as cathode materials

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Applied Chemistry

Rock salt type LiTiO2 @ LiNi0.5Co0.2Mn0.3O2 as cathode materials with high capacity retention rate and stable structure Hao Chen, Li Xiao, Pengcheng Liu, Han Chen, Zhimei Xia, Longgang Ye, and Yujie Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03276 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

Rock salt type LiTiO2 @ LiNi0.5Co0.2Mn0.3O2 as cathode materials with high capacity retention rate and stable structure Hao Chen, Li Xiao*, Pengcheng Liu, Han Chen, Zhimei Xia, Longgang Ye, Yujie Hu School of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou 412007, PR China *Corresponding author E-mail address: [email protected] (Li Xiao)

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

Abstract:

Herein, a strategy is presented for improving the electrochemical

performance of a LiNixCoyMn1-x-yO2 layered oxide involving the coating of rock salt-type LiTiO2 on the surface via a hydrothermal method followed by a calcining process. Thickness of LiTiO2 coated on the surface of LiNi0.5Co0.2Mn0.3O2 (NCM523) of approximately 10 nm has been achieved, and the optimal molar ratio (LiTiO2:NCM523) is found to be 1.0 mol.%. The LiTiO2 @ LiNi0.5Co0.2Mn0.3O2 when used as a cathode material exhibits not only a higher capacity retention, but also a more enhanced rate performance in comparison with that of pristine NCM523. The LiTiO2 layer protects the NCM523 active particles from being eroded by the electrolyte and reduces the volume change of the electrode active particles before and after lithium ion intercalation/deintercalation during the cycling. These new findings contribute towards the design of a stable structured nickel-based cathode material for lithium ion batteries. Keywords: Lithium-ion batteries, LiTiO2, coating, Energy storage and conversion

1. INTRODUCTION Goodenough et al. first proposed layered LiCoO2 oxides as cathode materials, which were then successfully commercialised in the early 1990s.1 LIBs are extensively used in digital products, electric vehicles, medical instruments, energy storage and conversion and national defence industries owing to their fast charge– discharge rates and stable working voltage, among others.2 With the popularisation of electric vehicles, there is an increase in the demand for better energy storage materials. Recently, owing to their lower cost and toxicity and high discharge capacity, layered 2


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transition metal oxides, including Li(Ni1/3Co1/3Mn1/3)O2, Li(Ni0.5Co0.2Mn0.3)O2, Li(Ni0.8Co0.1Mn0.1)O2 (NCM) and LiNi1-x-yCoxAlyO2 (NCA), have been considered as promising alternatives to conventional LiCoO2.3-8 However, NCM materials currently have many limitations associated with their use,9 listed as follows: (1) The material needs to be charged at a high voltage to obtain a high energy density, while the material suffers irreversible phase change, resulting in poor electrochemical performance.10 (2) A high voltage state accelerates the oxidation of the electrolyte and reduces the stability of the electrode/electrolyte interface, leading to a severe degradation in capacity.11 (3) Under high temperature, the dissolution velocity of positive ions is accelerated, and the collapse of the layered structure leads to a deterioration in the thermal stability of NCM, resulting in the generation of oxygen (which may cause thermal runaway and fire in a battery).12 (4) During cycling, the viscosity of the electrolyte increases upon the loss of electrolyte solvent, leading to an increase in the charge transfer resistance and significant decrease in the capacity retention.13 These disadvantages in the cathodes of LIBs lead to poor electrochemical performance, hindering the wide application of NCM materials. To overcome this series of limitations, several modification methods have been proposed, such as introducing a concentration gradient, doping with positive and negative ions and introducing a surface coating, among others. However, the synthesis of concentration gradient materials involves extremely strict production techniques, which are not advantageous for large-scale production. Although doping can bring about some improvement in performance, it cannot prevent the structural damage 3



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caused by electrolyte erosion of the electrode material. Modification by coating is considered to be an effective and practical way of overcoming the difficulties associated with NCM. Commonly used coating materials include metal fluorides (AlF3),14 metal oxides (Al2O3,4 TiO2,15 V2O5,5 ZrO216 and Co3O417), phosphates (FePO4,18 Co3(PO)419 and Ni3(PO)420), lithium salts (Li3PO4,21 Li2MnO3,22 Li2TiO3,23 LiAlO224 and Li2ZrO325), carbon materials (carbon,26 graphene27 and carbon nanotubes28)

and

conductive

polymers.29

However,

most

coatings

are

electrochemically inert materials, making lithium ion diffusion difficult, increasing the diffusion distance and charge transfer impedance. Moreover, the outer coating and inner core expand and shrink to different degrees in most cases during the intercalation/deintercalation process of lithium ions, leading to the separation of the core–shell structure and increasing the impedance of lithium ion diffusion, which is also the problem in the case of many coated layers.30 Therefore, appropriate coating materials take a key part in improving the performance of NCM materials. Rock salt LiTiO2 with a stable layer-structure is associated with the similar radii of Ti3+ and Li+ in the six-fold O-coordination at a lower constant potential (1.3 eV) in a Li-insertion reaction.31 LiTiO2 has a 3D lithium ion diffusion path, suggesting that lithium ions can easily intercalate/ deintercalate within its structure.32, 33 Meanwhile, previous research has shown that the rate capability of NCM materials doped with titanium is significantly improved.34 Herein, LiTiO2 was successfully coated onto the surface of LiNi0.5Co0.2Mn0.3O2 (NCM523) as a cathode material for LIBs, resulting in a high capacity retention rate 4


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and stable structure. This study details the hydrothermal reaction and post calcining process that were used to obtain a tight LiTiO2 coating on the surface of the NCM523 material. Results show that titanium does not enter the bulk structure of the material and the thickness of the LiTiO2 coated on the surface of NCM523 of approximately 10 nm.

2. EXPERIMENT 2.1. Materials synthesis Spherical NCM523 particles were synthesised by solid state reaction using LiOH·H2O

(99%,

Aladdin)

and

commercial

Ni0.5Co0.2Mn0.3(OH)2

powder

(Guangdong Jiana energy technology Co. Ltd, China) as raw materials. A mixture of Ni0.5Co0.2Mn0.3(OH)2 powder and LiOH·H2O in a molar ratio of 1:1.05 was heated at 500°C for 5 h and 850°C for 15 h in air at a heating rate of 3°C min−1 to obtain pristine NCM523. 2.2. Surface modification LiTiO2-coated NCM523 (LTO–NCM) materials were synthesised through a hydrothermal method followed by calcining, as illustrated in Figure 1. To obtain a NCM523 suspension, 5.0 g of NCM523 powder was stirred and dispersed by ultrasonication in 60 mL of deionised water. A solution prepared using an appropriate amount of tetrabutyl titanate (Ti(OC4H9)4 or TBT) dissolved in 20 mL of absolute alcohol was slowly added to NCM523 suspension under stirring at 80°C for 2 h to hydrolyse and form a sol. The sol and precise amounts of KOH and LiOH·H2O mixed solution were added in a hydrothermal Teflon-lined autoclave at 200°C for 16 h to 5



produce a precipitate that was then washed with ethanol and deionised water and dried at 100°C for 12 h to obtain the precursor (The molar concentration of LiOH·H2O is 8 times that of TBT , and the molar concentration of KOH is 1.5 times that of LiOH·H2O). Finally, the precursor was calcined at 500°C for 5 h in air at a heating rate of 3°C min−1 to form LiTiO2-coated NCM523 cathode materials (labelled as LTO-0.5 for 0.5 mol.%, LTO-1.0 for 1.0 mol.%, LTO-1.5 for 1.5 mol.% and LTO-2.0 for 2.0 mol.%). To investigate the LiTiO2 coating formed on the surface of NCM523, a 15 mol.% LiTiO2-coated NCM523 sample was prepared using the above process to analyse the crystal structure of LiTiO2 on the surface of NCM523.

Figure 1. Schematic diagram of the preparation procedure. 2.3. Materials characterization The samples were identified by powder X-ray diffraction (XRD, Rigaku Ultima 6


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IV, Japan with CuKα radiation) from 10° to 90° at a scanning rate of 5° min−1. The morphology was analysed by scanning electron microscopy (SEM, Phenom ProX) with an accelerating voltage of 15 kV and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-TWIN working at 200 kV). X-ray photoelectron

spectroscopic

(XPS,

Thermo

Fisher

Scientific

K-Alpha+)

measurements were conducted to obtain surface information of the materials. 2.4. Electrochemical measurement The cathode electrode was prepared by mixing active powder (80 wt.%), carbon black (Super P, 10 wt.%) and polyvinylidene fluoride (PVDF, 10 wt.%). The mixture was made into a slurry using N-methyl-2-pyrrolidone (NMP) as a solvent and the slurry was pasted onto Al foils, followed by drying at 120 °C for 12 h. To investigate the electrochemical performance of the pristine NCM523 and LTO–NCM cathode materials, CR2025 coin-type batteries were assembled in an argon-filled glove box, with Li metal as the anode, Celgard 2400 as a separator and 1.0 mol L−1 LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DEC) and ethyl methyl carbonate (EMC) (1:1:1 by volume) as an electrolyte. Automatic galvanostatic charge–discharge cycling tests were performed at 2.7–4.3 V (vs. Li+/Li) at room temperature (25°C) using a NEWARE battery testing system (Shenzhen, China). The electrochemical impedance spectra (EIS) of the coin cells were obtained within a frequency range of 1 mHz–100 kHz with an amplitude of 5 mV on an electrochemical workstation (CHI660E, Shanghai, China).

3. RESULTS AND DISCUSSION 7



Figure 2 shows the XRD patterns of the pristine NCM523 and LTO–NCM materials. From Figure 2a, the NCM523 and LTO–NCM samples have a well-defined crystal structure, wherein the peaks at 2θ values can be indexed to characteristic peaks (JCPDS card No. 89-3601), and the LTO–NCM samples do not have any impurity peaks. Meanwhile, distinct peak splittings of (006)/(012) and (018)/(110) can be found in those patterns and the intensity ratio of I(003)/I(104) is greater than 1.2, which means that all of the materials have an α-NaFeO2 ordered layered structure.35 Since no other impurity peaks in Figure 2a are visible, it can be speculated that the coating method maintained the bulk structure of the NCM523. To analyse if the LiTiO2 was formed on the surface of the sample, the ratio of Ti:(Ni+Co+Mn) was increased to 15 mol.% (denoted as LTO-15) using the same synthetic route as that described above. The XRD pattern of LTO-15 is shown in Figure 2b; Figures 2c and 2d show the partial enlarged views of Figure 2b. As the content of Ti increased, pristine NCM523 diffraction peaks at 2θ values associated with peaks that can be indexed to the crystal phase of LiTiO2 (JCPDS card No. 16-0223) were observed.36 In addition, the hydrothermal method was conducted in the absence of any NCM523 material to prove that the procedure can be used to synthesise rock salt-type LiTiO2 (see the Supporting Information Figure S1).

8


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(113)

(018) (110)

 LiTiO2

LTO-15-hydrothermal-temper

LTO-1.0

LTO-0.5

Intensity (a.u.)

LTO-1.5

Intensity (a.u.)

(107)

(104)

(015)

(101)

LTO-2.0

(006)/(012)

(003)

(018) (110)

(b)

(113)

(107)

(104)

(101)

(006)/(012)

(003)

(a)



 LTO-15-hydrothermal

NCM JCPDS No. 16-0223

NCM JCPDS No. 89-3601

20

30

(c)

40

50

2 (degree)

60

70

80

JCPDS No. 89-3601

90 10

20

(d)

 LiTiO2

30

 LiTiO2

40

50

60

70

80

90

64.0

64.5

65.0

65.5

66.0

2 (degree)

(220)

10



 

Intensity (a.u.)

(111)

(200)



Intensity (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


(015)

Page 9 of 29





35

36

37

38

39

40

41

2 (degree)

42

43

44

4562.0

62.5

63.0

63.5

2 (degree)

Figure 2. XRD patterns of (a) the pristine NCM523 and samples coated with different amount of LiTiO2, (b) the pristine NCM523 and 15 mol% LiTiO2 coated NCM523, (c) the partial enlarged view of Figure 2b at 35º-45º and (d) the partial enlarged view of Figure 2b at 62º-66º. XPS was applied to analyse the surface elemental state of LTO-1.0 and the related spectra are shown in Figure 3. All the binding energies were referenced to the C 1s peak (set at 284.6 eV). Figures 3a-3c show that the Ni 2p3/2, Co 2p3/2/Co 2p1/2 and Mn 2p3/2/Mn 2p1/2 peaks appear at 854.28, 779.58/794.68 and 642.08/653.78 eV, respectively, consistent with the Ni2+, Co3+ and Mn4+ valence states of the metal ions. Also, peaks of Ti 2p3/2/Ti 2p1/2 can be respectively found at 458.28 and 464.08 eV, corresponding to the Ti3+ valence state in the LiTiO2 crystal structure.33 Therefore, these results provide evidence that LiTiO2 is coated on the NCM523 powder (see 9



Supporting Information for the overall XPS spectrum for the Ni elements of LTO-1.0 shown in Figure S2).

(a) Ni 2p

Ni 2p

3/2 854.3 eV

865

860

855

850

Binding energy (eV)

(c) Mn 2p

Experiment Fitting

Mn 2p

3/2 642.1 eV

Mn 2p

660

(b) Co 2p

Intensity (a.u.) 650

645

Binding energy (eV)

640

635

Co 2p

3/2 779.6 eV

1/2 794.7 eV

800

795

790

785

Binding energy (eV)

(d) Ti 2p

1/2 653.8 eV

655

805

845

Experiment Fitting

Co 2p

Intensity (a.u.)

Intensity (a.u.)

Experiment Fitting

Satellite peak 860.6 eV

870

Intensity (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 10 of 29

Experiment Fitting

780

775

Ti 2p 3/2 458.3 eV

Ti 2p

1/2 464.1 eV

470 468 466 464 462 460 458 456 454 452 450

Binding energy (eV)

Figure 3. XPS results for (a) Ni, (b) Co, (c) Mn and (d) Ti of LTO-1.0. Figure 4 presents the SEM of the Ni0.5Co0.2Mn0.3(OH)2 precursor, NCM523 and LTO–NCM. As shown in Figure 4a, the precursor particles are uniform and spherical shaped with the size of the secondary spherical particles about 5 -8 μm. The NCM523 secondary particles also exhibit a uniform spherical shape and the primary particles are denser and more rounded than those of the precursor (Figure 4b). The LTO–NCM particles are shown in Figures 4c–4f. EDS mapping was employed to inspect the composition and distribution of LTO-1.0 surface elements. Figure 3g shows that the distribution of Ti, Ni, Co, and Mn are completely overlapped.37 The uniform 10


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distribution of Ti elements has a positive effect on the electrochemical performance of LTO-1.0.

Figure 4. SEM images of materials: (a) Ni0.5Co0.2Mn0.3(OH)2, (b) NCM523, (c) LTO-0.5, (d) LTO-1.0, (e) LTO-1.5, (f) LTO-2.0 and (g) the EDS mapping of LTO-1.0. For further investigation, high-resolution TEM (HRTEM) and fast Fourier transform (FFT) were employed to investigate the structural features of NCM523 and LTO-1.0. Figure 5b reveals that pristine NCM523 has an inter-planar crystal spacing of 0.144 nm (marked in red) and the obtained planes correspond well to the (110) inter-planar distance of hexagonal structured LiNi0.5Co0.2Mn0.3O2. Upon comparing the TEM images in Figures 5a and 5c, the coating thickness of approximately 10 nm can be observed. Obvious lattice orientation fringes can be observed in Figure 5d with an inter-planar spacing of 0.120 nm, corresponding to the (222) crystal plane of LiTiO2 based on the FFT and XRD patterns. Combined with the surface analysis 11



results of the XPS and HRTEM, it can be confirmed that the thickness of the LiTiO2 on the surface of the LTO–NCM core–shell structured material obtained herein is approximately 10 nm.

Figure 5. TEM and HRTEM images of the pristine NCM523 (a, b) and LTO-1.0 (c, d) samples. The electrochemical performances of the pristine NCM523 and LTO–NCM materials were compared to prove that the LiTiO2 improved the stability of NCM523. Figure 6a presents the initial charge/discharge profiles of the five materials at a rate of 0.1 C (1 C = 170 mAg−1) in the voltage range of 2.7–4.3 V at 25°C. It can be observed from Figure 6a that the charge/discharge specific capacities of NCM523, LTO-0.5, LTO-1.0, LTO-1.5 and LTO-2.0 are 227.88/187.54, 218.83/185.18, 215.86/180.37, 12


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209.67/173.45 and 199.71/162.73 mAhg−1, respectively. Meanwhile, it can be found that the LiTiO2 is mainly responsible for the reduction in the discharge capacity. However, the initial Coulombic efficiencies of NCM523, LTO-0.5, LTO-1.0, LTO-1.5 and LTO-2.0, which were 82.30, 84.62, 83.56, 82.73 and 81.48%, respectively, increased in comparison with that of pristine NCM523 (Table 1. Initial charge/discharge capacity and capacity retention rate of pristine and LiTiO2 coated NCM523 samples at a rate of 0.1 C). This can be explained by the fact that the LiTiO2 has a 3D lithium ion diffusion path and low lithium ion intercalation potential on the surface

of

the

LTO–NCM

samples

improves

the

lithium

ion

intercalation/deintercalation efficiency. However, an excessive coating thickness results in a decrease in the discharge capacity.38 Therefore, the optimum coating ratio was found to be LTO-1.0. Figure 6b presents the cycling stability of the batteries during 100 cycles. The batteries were first charged/discharged 2 times at 0.1 C, mainly because a small current charge/discharge is beneficial for the activation of the batteries. Then, the batteries were cycled 100 times at 1.0 C, and the initial discharge specific capacities of NCM523, LTO-0.5, LTO-1.0, LTO-1.5 and LTO-2.0 were 167.96, 166.04, 163.03, 156.41 and 145.97 mAhg−1, respectively. The capacity retention rate of pristine NCM523 sample after 100 cycles was 70.70%, the lowest value among the measured samples and it showed a significant decline in its discharge capacity. The highest capacity retention rate among the LTO–NCM materials was 86.08%, for LTO-1.0. In comparison with NCM523, the retention rate showed an obvious increase for each of 13



the core–shell structured materials. Therefore, the LiTiO2 coating was found to increase the capacity retention of the NCM523 materials. Figure 6c exhibits a comparison of the rate performances of all of the samples cycled at various rates from 0.1 to 10 C in the range of 2.7–4.3 V at 25°C. The discharge capacity of the pristine NCM523 material decreased obviously after several high-rate cycles. On the contrary, LTO-1.0 has the advantage in high-rate cycling performance because of its relative coating thickness, even at 10 C, a discharge capacity of 125.49 mAhg−1 was still retained. The discharge capacity of LTO-1.0 recovered to 171.53 mAhg−1 once the current density returned to 0.1 C (Table S1 the discharge capacity of NCM523 modified by different coating materials at rate of 0.1C, 0.5C, 1.0C, 5C and 10C). Figure 6d shows the EIS measurements of the as-prepared batteries before cycling. EIS is a widely used method for evaluating the electrochemical properties of materials,24,

39

which reveals the charge transfer

impedance and diffusion impedance of lithium ion intercalation/deintercalation in electrode materials. The charge transfer resistance, i.e. the values corresponding to the semicircle in the high frequency region, of NCM523 was the lowest and that of LTO-2.0 was the highest among the samples. This corresponds to the highest initial discharge capacity of NCM523 in the first cycle and the lowest initial discharge capacity of LTO-2.0. Figure 6e shows the EIS plots of the NCM523 and LTO-1.0 batteries have been measured after 50 and 100 cycles charged to 4.3 V, respectively. All the curves consist of three parts, a semicircle in the high frequency region, a semicircle in the 14


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intermediate frequency region and a straight line in the low frequency region. Re is the starting point of the semicircle in the high frequency region represents the diffusion resistance of lithium ions in the electrolyte. Rsf and CPEsf are the diffusion resistances of Li ions through the solid electrolyte interface layer and the corresponding constant phase element (CPE). Rct and CPEct are the charge transfer impedance of the lithium ions entering electrode material and the corresponding CPE. Wo (Warburg impedance) has a corresponding relation with the solid state diffusion of lithium ions in the electrode material.40, 41 Figure 6f shows the relationship between Z' and ω

−1/2

corresponding to the Nyquist plots in Figure 6e. The slope of the line in

Figure 6f represents the Warburg factor σ. Table 2 lists the fitted resistance values for the NCM523 and LTO-1.0 cells after 50 and 100 cycles. The Re of each sample in Table 2 is very small and an increase in the Re value is not obvious after 100 cycles in comparison to that after 50 cycles. On the contrary, the Rsf and Rct values significantly increased42 and the increase in the trend of Rct was greater. Therefore, the electrochemical performance of the pristine NCM523 batteries deteriorated after 100 cycles mainly due to the increase in the Rct value. At the same time, the Rsf and Rct values of LTO-1.0 are evidently smaller than those of NCM523 after cycling, and the Rct value of NCM523 is 2.3 times higher than that of LTO-1.0 after 100 cycles. Meanwhile, the DLi+ of LTO-1.0 is 10.6 times that of the NCM523. (The DLi+ coefficient equation in the Supporting Information).

15



(a)

200

4.4

(b)

4.2

0.1 C

180 3.8 3.6 3.4 NCM523 LTO-0.5 LTO-1.0 LTO-1.5 LTO-2.0

3.2 3.0 2.8 2.6

0

200

(c)

Discharge capacity (mAhg-1)

Voltage (V)

4.0

50

1.0 C 160 140 NCM - 70.70% LTO-0.5 - 74.98% LTO-1.0 - 86.08% LTO-1.5 - 84.62% LTO-2.0 - 84.96%

120 100

100

150

200

Capacity (mAhg-1)

80

250

0

0.1C

1.0C


160

200

60

- Z / ohm

5.0C

140

10.0C

0

5

60

10

100

100 50

15

20

Number

600

25

30

0

35

(f)

(e)

0

50

100

900

150

Z / ohm

200

250

300

800

500

700

300

- Z' / ohm

400

NCM523-50th NCM523-100th LTO-1.0-50th LTO-1.0-100th

200

600 500 400

Sample

Slope

NCM-50th

32.01

NCM-100th

93.63

LTO-50th

18.84

LTO-100th

28.85

NCM523-50th NCM523-100th LTO-1.0-50th LTO-1.0-100th Fitting

300

100 0

80

150

NCM LTO-0.5 LTO-1.0 LTO-1.5 LTO-2.0

80

Number

NCM523 LTO-0.5 LTO-1.0 LTO-1.5 LTO-2.0

250

2.0C

100

40

(d)

0.2C

120

20

300

0.1C

180

- Z / ohm

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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200

0

100

200

300

Z / ohm

400

500

600

100 1.5

2.0

2.5

3.0

 -1 / 2

3.5

4.0

4.5

Figure 6. (a) Initial charge/discharge curves of pristine and LiTiO2 coated NCM523 samples at a rate of 0.1 C between 2.7 and 4.3 V, (b) Cycling performance of the pristine and LiTiO2 coated NCM523 samples at a rate of 1.0 C between 2.7 and 4.3 V, (c) Discharge capacities of all samples at various rate between 2.7 and 4.3 V, (d) Nyquist plots of the as-prepared batteries before cycling, (e) EIS plots of the NCM523 and LTO-1.0 batteries after 50 and 100 cycles charge to 4.3 V and (f) the relationships 16


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between Z' and ω-1/2 based on 50 and 100 cycles. Table 1 Initial charge/discharge capacity and capacity retention rate of pristine and LiTiO2 coated NCM523 samples at a rate of 0.1 C. Charge capacity

Discharge capacity

(mAhg-1)

(mAhg-1)

NCM523

227.88

187.54

82.30%

LTO-0.5

218.83

185.18

84.62%

LTO-1.0

215.86

180.37

83.56%

LTO-1.5

209.67

173.45

82.73%

LTO-2.0

199.71

162.73

81.48%

Capacity retention

Table 2 EIS impedance data and DLi+ for the pristine and LiTiO2 coated NCM523 batteries. Sample

Re

Rsf

Rct

σ

DLi+

NCM523 50th

5.228

90.62

198.5

32.01

2.17×10-14

NCM523 100th

14.29

209.5

404.3

93.63

2.54×10-15

LTO-1.0 50th

5.664

50.93

95.69

18.84

6.27×10-14

LTO-1.0 100th

8.719

69.54

172.4

28.85

2.68×10-14

Figures 7a and 7b show cyclic charge/discharge profiles for materials at a rate of 1.0 C between 2.7 and 4.3 V. Under the same number of cycles, LTO-1.0 material has a higher charge and discharge capacity, which is mainly due to the lower charging platform and the higher discharge platform. Figures 7c and 7d are the differential capacity vs voltage plots (dQ / dV vs V). The redox peaks of NCM523 appear at 3.75 / 3.72 V, respectively, while LTO-1.0 appear at 3.76 V and 3.71 V. As the number of cycles increases, the oxidation peak shifts to a high voltage, and the reduction peak shifts to a low voltage. The ΔV of the redox peaks after 100 cycles were 0.191 and 0.151 V, respectively. The significant difference between NCM523 and LTO-1.0 after 100 cycles mainly occurred at the shoulder shrinkage of reduction peaks of 3.8 V and 4.2 V. Therefore, LTO-1.0 has a more stable voltage platform and less polarization, 17



resulting in better electrochemical performance. 4.4

dQ / dV (mAhg-1 V-1)

4.0 3.8 3.6 3.4

1st 10th 20th 50th 100th

3.2 3.0 2.8 2.6

0

4.4

20

40

60


800 400

80

100 120 140 160 180 200

Number

-800 2.6

1600

3.667V

 V = 0.191V

2.8

3.8 3.6 3.4


3.2 3.0 2.8 0

20

40

60

3.0

3.2

100 120 140 160 180 200

800 400

2.6

3.8

4.0

4.2

4.4

4.2

4.4

3.821V

0

-800

Number

3.6


3.670V

-400

80

3.4

Voltage (V)

(d)

1200

4.0

3.858V

0

-400

(b)

4.2

2.6

(c)

1200

dQ / dV (mAhg-1 V-1)


1600

(a)

4.2


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

 V = 0.151V

2.8

3.0

3.2

3.4

3.6

Voltage (V)

3.8

4.0

Figure 7. Charge/discharge profiles for (a) NCM523 and (b) LTO-1.0, and dQ / dV vs V plots for (c) NCM523 and (d) LTO-1.0. The SEM images of the pristine NCM523 and LTO-1.0 cathode materials after 100 cycles at 1.0 C were obtained (Figure 8). From Figure 8a, the NCM523 cathode material was severely damaged, as obvious cracks can be seen on the surface. This is mainly due to the inevitable reaction of the electrolyte with the metal ions in the electrode material during cycling, which causes the transition metal ions to dissolve and at the same time, due to the volume change of the active material before and after lithium ion intercalation/deintercalation during the cycling, the dual action causes the material to be evidently damaged and collapsed.43 As the damaged surface area of the particles increased, the electrolyte was able to more rapidly attack the electrode 18


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material, thereby causing an obvious decrease in the discharge specific capacity and cycling performance.44 On the contrary, there are hardly any cracks on the surface of the LTO-1.0 electrode and morphology of the spherical particles remained intact, as can be observed in Figures 8d–8f. We believe that this is mostly because the LiTiO2 coating prevents contact between the active materials and electrolyte. Therefore, the core–shell structure of the cathode material is stable, and LiTiO2 with its 3-dimensional lithium ion diffusion path can improve the diffusion efficiency of Li+, leading to the better electrochemical performance of the LTO-1.0 material. To evaluate the elemental distribution and composition on the surface of the LTO-1.0 electrode after 100 cycles, the energy-dispersive X-ray spectroscopy (EDS) mapping images of the LTO-1.0 sample are exhibited in Figure 8g. From the images that the Ni, Co, Mn and Ti elements are uniformly distributed on the surface of the particles, and that the coincidence of Ti and the other three elements is good (Figure S3 shows a schematic diagram of the mechanism of the LiTiO2 coating to improve the stability of the NCM523 electrode).

19



Figure 8. SEM images of the pristine NCM523 (a, b, and c), the LTO-1.0 (d, e, and f) 20


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electrodes, and (g) the EDS mapping of LTO-1.0 after 100 cycles. Figure 9 shows the XPS spectra of F1s and O1s after 100 cycles of NCM523 and LTO-1.0 electrodes. It can be seen from Figures 9a and 9b that F1s is composed of two characteristic peaks, the peak position near 688 eV is caused by PVDF, and the binding energy of about 685 eV is interpreted as LiF phase,21 and the area ratio of NCM523 material is 18.14. %, while LTO-1.0 material is only 3.1%. It is well known that the LiF is an electrochemically inert material, and the LiF phase adheres to the surface of the active material, which hinders the transfer process of lithium ions, resulting in deterioration of electrochemical performance. It is believed that the LiF is derived from the reaction of HF with the lithium salt on the surface of the active material. HF is the product of the side reaction of the electrolyte.21 On the contrary, LTO-1.0 has less LiF content, indicating that the modified material LTO-1.0 has better erosion resistance of the electrolyte, so the LiTiO2 coating can inhibit the electrolyte from the positive electrode material. In other words, the modified material has better structural stability, and this result also shows that LTO-1.0 has better electrochemical performance. In Figures 9c and 9d, the binding energy of O1s is between 528 and 538 eV, and the peak position near 532 eV is considered to be related to the composition of the SEI film on the surface of the active material (ROCO2Li, polyether, Li2CO3, and LiOH).15 LTO-1.0 has a smaller proportion of area, indicating that the decomposition of the electrolyte of the modified material is inhibited, and near 529 eV is the binding energy of oxygen-metal, corresponding to the characteristics of oxygen atoms in the layered metal crystal network.15 The area 21



ratio of LTO-1.0 is larger, indicating that the modified material has better stability.

(a) F 1s

F 1s PVDF F 1s LiF

96.90 %

Intensity / (a.u.)

Intensity / (a.u.)

(b) F 1s

F 1s PVDF F 1s LiF

81.86 %

18.14 %

3.10 %

680

682

684

686

688

690

692

694

696

Binding energy (eV)

(c) O 1s

O 1s Li2CO3 / ROCO2Li O 1s M - O (M = Ni Co Mn)

93.89 %

680

Intensity / (a.u.) 6.11 %

526

528

530

532

534

536

682

684

(d) O 1s

Intensity / (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 22 of 29

538

526

686

688

690

Binding energy (eV)

692

694

696

O 1s Li2CO3 / ROCO2Li

89.20 %

O 1s M - O (M = Ni Co Mn)

10.80 %

528

530

532

534

536

538

Binding energy (eV)

Binding energy (eV)

Figure 9. F1s XPS spectra of NCM523 (a) and LTO-1.0 (b) after 100 cycles; and O1s XPS spectra of NCM523 (c) and LTO-1.0 (d) after 100 cycles. To analyse the changes in the crystal structure of the electrode material after 100 cycles, XRD was used to examine the NCM523 and LTO-1.0 materials. The diffraction peaks of materials before and after 100 cycles are shown in Figure 10a. Figure 10b is (003) peak of materials, and the fluctuated magnitude of (003) peak indicates the variation of c-axis.45 The NCM523 has a 0.16° shift to the left after 100 cycles, and the LTO-1.0 offset is only 0.04° after 100 cycles, this can be explained by the fact that the LTO-1.0 material greatly inhibits the migration of (003) during the cycling as compared to NCM523. Furthermore, as shown in Figure 10c, (101) peak of 22


Page 23 of 29

pristine NCM523 moves toward a higher angle, indicating lattice expansion along the c-axis and contraction along the a-axis and the b-axis.45 The peak position of LTO-1.0 barely moved after 100 cycles, while the NCM523 moved 0.12° after 100 cycles. From the above, it could be demonstrated that the LiTiO2 coating successfully stabilized the structure of the NCM523 material.

(a)

 Al  C







(003)

(c)

(101)



 



60

70

80





(113)

50

2 (degree)

(018) (110)

(107)

40

(104)

(101)

30

NCM before cycle NCM after 100cycle (015)

20

(006)/(102)

 (003)

10

(b)



LTO-1.0 before cycle LTO-1.0 after 100 cycle

Intensity (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


90 17

18

19

20 35.5 36.0 36.5 37.0 37.5

Figure 10. (a) XRD patterns of NCM523 and LTO-1.0 cathode materials before and after 100 cycles at a rate of 1.0 C between 2.7 and 4.3 V, (b) (003) homologous magnified diffraction peak and (c) (101) homologous magnified diffraction peak.

4. CONCLUSION In summary, this study details a 200°C hydrothermal method and 500°C low temperature annealing process to prepare a LiTiO2 tightly-coated NCM523 material. The LTO-1.0 battery has excellent electrochemical performance at 1.0 C in 2.7–4.3 V for charge and discharge, showing a capacity retention rate of 86.08% after 100 cycles. Furthermore, the discharge capacity of the material was obviously improved at various rate. Most importantly, the charge transfer impedance value of the LTO-1.0 material after 100 cycles is only 43% that of NCM523 and lithium ion diffusion coefficient of LTO-1.0 is 10.6 times that of the latter. This can be mainly attributed to 23



the outstanding stability of rock salt-type LiTiO2 and its 3-dimensional lithium ion diffusion path, which not only inhibits the erosion of the electrode material by the electrolyte but also improves the diffusion efficiency of the lithium ions. The results of this study are beneficial for the industrial application of NCM transition metal oxides in energy storage. ASSOCIATED CONTTENT Supporting Information X-ray diffraction pattern of the LiTiO2 without NCM523 was synthesized through a hydrothermal method (Figure S1); XPS unfit data and fit data for Ni elements of LTO-1.0 (Figure S2); Schematic diagram of the mechanism of LiTiO2 coating to improve the stability of the NCM523 electrode (Figure S3); The discharge capacity of NCM523 modified by different coating materials at rate of 0.1C, 0.5C, 1.0C, 5C and 10C (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors

E-mail address: [email protected] (Li Xiao) Note The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS This work is supported by funds from the National Natural Science Foundation of China (No. 51774127).

24


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Rock salt type LiTiO2 @ LiNi0.5Co0.2Mn0.3O2 as cathode materials

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