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Zhang et al. Energy Mater 2021; Volume:NumberDOI: 10.20517/energymater.2021.xx

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Energy Materials1

Note: This version is accepted and the final version will be published shortly.2

3

Article4

5

Enhancing cycle life of nickel-rich LiNi0.9Co0.05Mn0.05O2 via a highly6

fluorinated electrolyte additive - pentafluoropyridine7

8

Xiaozhen Zhang, Gaopan Liu, Ke Zhou, Tianpeng Jiao, Yue Zou, Qilong Wu,9

Xunxin Chen, Yong Yang, Jianming Zheng*10

11

State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry12

and Chemical Engineering, Xiamen University, Xiamen 361005, China.13

14

*Correspondence to: Prof. Jianming Zheng, State Key Laboratory of Physical15

Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen16

University, Xiamen 361005, China. E-mail: [email protected]

18

How to cite this article: Zhang X, Liu G, Zhou K, Jiao T, Zou Y, Wu Q, Chen X, Yang19

Y, Zheng J. Enhancing cycle life of nickel-rich LiNi0.9Co0.05Mn0.05O2 via a20

highly fluorinated electrolyte additive - pentafluoropyridine. Energy21

Mater 2021;1:[Accept]. https://dx.doi.org/10.20517/energymater.2021.0722

23

Received: 7 Sep 2021 Revised: 1 Oct 2021 Accepted: 8 Oct 2021 First online: 8 Oct24

202125

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of 23 Zhang et al. Energy Mater 2021; Volume:Number│http://dx.doi.org/10.20517/energymater.2021.xx

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Abstract31

A highly fluorinated additive, pentafluoropyridine (PFP), has been investigated to32

enhance the interfacial stability of Ni-rich LiNi0.9Co0.05Mn0.05O2 (NCM90) cathode33

electrode at a cut-off voltage of 4.3 V vs. Li/Li+ at 30 oC. The capacity retention of34

NCM90||Li cell is obviously improved from 72.3% to 80.3% after 200 cycles at 1C (1C35

= 180 mA/g) when 0.2% PFP is introduced into the baseline electrolyte (1 mol/L LiPF636

in EC/DEC). The improvement in electrochemical performance could be attributed to37

the formation of a compact and uniform cathode electrolyte interphase (CEI) layer38

enriched with F-containing polypyridine moieties and LiF species on the NCM9039

particles, which prevents the side reactions between the electrode and electrolyte and40

hinders the corrosion of cathode causing by HF attacking. In addition, the formation of41

internal particle cracks is somewhat suppressed by the robust CEI, thus prohibiting the42

irreversible phase transformation, and better maintaining the superior lithium-ion43

diffusion kinetics.44

45

Keywords: Pentafluoropyridine, electrolyte additive, cathode electrolyte interphase,46

LiNi0.9Co0.05Mn0.05O2, lithium-ion batteries47

48

49

INTRODUCTION50

Lithium-ion batteries (LIBs) have been widely investigated and deployed as the power51

sources of electric vehicles (EVs) owing to their relatively high energy density, long52

cycle life and environmentally friendliness.[1-4] However, up to now, the energy density53

of the LIBs is still not sufficient enough to achieve EV driving mileage that could54

compete the conventional cars driven by internal combustion engineers, thus impeding55

the complete substitution of EVs for conventional cars in the automobile market.56

Consequently, boosting the energy density of LIBs to relieve the “range anxiety”57

become an urgent demand to be met. Nickel-rich (Ni-rich) LiNixCoyMn1-x-yO2 (NCM, x58

≥ 0.9) layered oxides have been considered one of the most promising cathode materials59

Zhang et al. Energy Mater 2021; Volume:Number│http://dx.doi.org/10.20517/energymater.2021.xx Page 2 of 23

for next generation LIBs due to their high specific capacity and high achievable energy60

density, in comparison with LiCoO2 and NCM analogues with lower Ni content.[5-7]61

However, there are still some key problems unsolved for the Ni-rich cathode materials,62

especially the rapid capacity deterioration when operated with high charge cut-off63

voltages ≥ 4.3 V vs. Li/Li+. The reasons causing the rapid capacity deterioration of the64

Ni-rich NCM include (1) irreversible structure transformation from layered to65

disordered rock-salt phase during repeated charging and discharging process with66

excessive lithium utilization,[8] (2) the interfacial degradation resulted from the parasitic67

side reactions between Ni-rich NCM and electrolyte.[9, 10]68

69

Structural doping and surface modification are mostly adopted measures to improve the70

structural and interfacial stability of Ni-rich NCM.[11-14] Sim et al. coated the surface of71

LiNi0.9Co0.05Mn0.05O2 (NCM90) particles with 0.5 wt% tungsten oxide, realizing a72

capacity retention of 84.6% at 1C after 80 cycles, which surpassed the 76.6% for73

pristine NCM90.[15] Park et al.’s result showed that the 1 wt% boron doped NCM9074

could deliver a discharge capacity of 237 mAh g-1 at 4.3 V, with outstanding capacity75

retention of 91% after 100 cycles at 55 oC, which is 15% higher than the undoped76

counterpart.[16] Besides, manipulation of the material particle morphology and77

orientation, as well as the elemental distribution is also an alternative approach to78

overcome the degradation.[4] However, these strategies could not address all the issues79

existing with Ni-rich NCM, because the side reactions between active cathode materials80

and electrolyte could not be completely prevented.81

82

In this regard, incorporating functional additive into electrolyte is considered a feasible83

and scalable approach to ameliorate the electrochemical performance of the Ni-rich84

NCM electrode due to its cost effectiveness. It has been well established that the85

electrolyte with appropriate additive could generate a unique and protective cathode86

electrolyte interphase (CEI) film on the surface of cathode particles during initial87

formation process, thus improving the cycling performance of the cell.[9, 10, 17-19] Of note,88

research attention has been paid to N-containing heterocyclic molecules as functional89


electrolyte additives, due to its unique property and positive effect in CEI layer90

formation. For instance, Liao et al. utilized a 1-(2-cyanoethyl) pyrrole (CEP) to91

construct a protecting CEI film on LiNi0.8Co0.1Mn0.1O2, significantly boosting its cycling92

performance.[20] Besides, extensive research has conducted in exploiting pyridine93

derivatives, such as fluoropyridine family (2-fluoropyridine, 3-fluoropyridine, 4-94

fluoropyridine),[21] 2-vinylpyridine.[22] These additives are able to produce an effective95

and stable CEI on LiMn2O4 cathode and solid electrolyte interphase (SEI) on carbon96

anode, respectively. Particularly, LiF could be simultaneously formed from the break-97

down of fluoropyridine additive, generating LiF-containing robust CEI layer, which is98

beneficial to extend cycle life. However, these additives only contain one fluorine in the99

molecule structure, limiting the content of LiF in the CEI layer. Besides, to date, it is100

still unknown about the effect and functioning mechanism of the highly fluorinated101

pyridine for high energy density Ni-rich NCM.102

103

In this work, a highly fluorinated molecule, i.e., pentafluoropyridine (PFP, Figure 1)104

was explored for the first time as a functional electrolyte additive to enhance the105

electrochemical performance of Ni-rich NCM90 electrode. By introducing 0.2% PFP106

into the standard electrolyte (1 mol L-1 LiPF6 in EC/DEC), the capacity retention of107

NCM90 can reach 80.3% after 200 cycles at 1C, which is much higher than that of the108

standard electrolyte (72.3%). Electrochemical impedance spectroscopy (EIS),109

transmission electron microscope (TEM), scanning electron microscope (SEM), and X-110

ray photoelectron spectroscopy (XPS) were performed to explore the interfacial111

chemical environment and microstructural evolution of NCM90 electrode to obtain deep112

insight into the essential functioning mechanism for the PFP additive.113

114


A B

Figure 1.A: Molecular structure and B: three-dimensional ball-stick model of PFP.115

116

117

EXPERIMENTAL118

Calculation method119

HOMO and LUMO energies of EC, DEC, and PFP with and without solvation with Li+120

ions were calculated on the basis of density functional theory (DFT) with the method of121

B3LYP in Gaussian 16 package with the 6-311++G (d, p) basis set.[23] Oxidation122

potentials of EC, DEC, and PFP with and without solvation with Li+ ion were computed123

based on Equation (1), where the Eox is calculated oxidation potential of solvents or124

additive, Gsolv(X) and Gsolv(X+) are the solvation free energies of molecule X (X = EC,125

DEC, PFP) and its cation (X+), respectively, F is the Faraday constant (96485 C mol-126

1).[24] The bond dissociation energy (BDE) was calculated to evaluate the strength of the127

C-F bond, and the basis-set superposition error (BSSE) was corrected at the same time.128

129

Eox V vs. Li Li+ = Gsolv X+ -Gsolv(X)F

-1.46 (1)

130

Electrode and electrolyte preparation131

The single crystalline NCM90 and polycrystalline LiNi0.92Co0.05Mn0.03O2 (P-NCM92)132

materials were provided by Ningbo Ronbay Technology Co., Limited (Ningbo, China)133

with its basic information shown in our previous report.[10] To prepare the NCM90 and134

P-NCM92 electrodes, a slurry containing 80 wt% active material, 10 wt% acetylene135

black as conductive agent, and 10 wt% poly (vinylidene fluoride) (HSV900, provided136


by Arkema) as a binder was coated onto the Al foil ( 16 um in thickness) and then dried137

at 110 oC in an air-convection oven. After that, the electrode was punched into certain138

circular disks of 1.54 cm-2 (14 mm in diameter) and baked in a 110 oC vacuum oven for139

1 hr, and then the as-prepared electrodes with loading of 2.5~3.0 mg cm-2 were140

transferred into an argon-filled glovebox (Shanghai Mikrouna Co., Ltd.) with141

moisture/oxygen contents controlled below 0.1 ppm. Lithium metal (15.8 mm in142

diameter, 2 mm in thickness, from China Energy Lithium Co., Ltd., Tianjin) was used as143

the negative electrode.144

145

The standard electrolyte (STD) was 1 mol L-1 LiPF6 dissolved in ethylene carbonate146

(EC) and diethyl carbonate (DEC) (3:7 by weight ratio), as provided by Zhangjiagang147

Guotai Huarong new material Co., Ltd. PFP, which was explored as a functional148

additive was purchased from Shanghai Bide Pharma Technology Co., Ltd. (Shanghai)149

and used without further purification. The optimized electrolytes were prepared by150

adding the PFP additive to the STD electrolyte with weight percentages of 0.2%, 0.5%151

and 1.0%, respectively.152

153

Electrochemical measurements154

CR2025 coin cells were fabricated with the as-prepared electrodes, Celgard2400 as the155

separator, Li metal as the negative electrode, along with 100 uL electrolyte in the above-156

mentioned argon-filled glovebox. For long-term cycling performance evaluation, the157

fabricated coin cells were rested for 5 hrs at 30 oC and then charged/discharged158

galvanostatically at 0.1C (1C = 180 mA g-1) for the initial 3 formation cycles and at 1C159

charge / 1C discharge for the subsequent cycles in the voltage range of 3.0~4.3 V at 30160

oC with Neware CT-4008 battery testers. Rate performance was assessed at ascending161

rates of 0.5C, 1C, 2C, 3C, 5C, 7C and 10C for 5 cycles, respectively, with the same162

charging rate of 0.2C after 3 formation cycles at 0.1C in the voltage range consistent163

with the long cycling performance test. EIS was performed on CHI760e (Chenhua,164

Shanghai) with a ±5 mV potential amplitude at the frequency between 100 kHz and165


0.001 Hz. Linear sweep voltammetry (LSV) was also recorded on CHI760e at a166

scanning rate 0.1 mV s-1 from open circuit potential to 7.0 V vs. Li/Li+ at room167

temperature utilizing a graphite working electrode and a lithium metal as counter and168

reference electrode.169

170

Characterizations171

For post-mortem analysis, cycled NCM90 electrodes were obtained from the172

disassembled cells, then washed by dimethyl carbonate (DMC) for 3 times to eliminate173

the residual electrolyte. The morphology of NCM90 electrodes were characterized by174

SEM (S-4800, HITACHI). The characterization of CEI layer formation on NCM90175

particle surface was carried out by TEM (Tecnai F30 TWIN, FEI). The difference in176

chemical components on the surface of NCM90 electrode was explored by XPS (PHI-177

5000, ULVAC-PHI) using Al K X-ray (1486.7 eV) source for excitation, with the178

binding energy referring to residual Carbon (C-C) at 284.8 eV.179

180

RESULTSAND DISCUSSION181

Oxidative Stability of PFP182

By means of Gauss calculation, the HOMO and LUMO energies of EC, DEC and PFP183

before and after solvation with Li+ ions were calculated and the results are summarized184

in Supplementary Table 1. The HOMO energy of PFP molecules solvated with Li+185

ions is determined to be ca. -7.96 eV, in comparison to -9.17 eV of EC and -8.89 eV of186

DEC, as shown in Figure 2A, indicating that PFP could be oxidized preferentially on187

the cathode surface. In addition, LSV curves suggest an onset of current response at188

about 4.0 V (with a current peak at ca. 4.2 V) when 0.5% PFP is added into the STD189

electrolyte, which can be ascribed to the oxidation of PFP, as shown in Figure 2B. This190

LVS result is in alignment with the theoretical calculation result (Supplementary Table191

2) that PFP additive possesses a lower oxidation potential than EC and DEC solvents.192

The reduction behavior of PFP was also explored with SiC working electrode. The193

differential capacity vs. voltage (dQ/dV) curves (Supplementary Figure 1) of SiC194

electrode derived from the initial lithiation process shows no reduction peak that could195


be assigned to PFP, which is consistent with the theoretical calculation result.196

197

A B

198

Figure 2. A: Calculated HOMO/LUMO energies (eV) of EC, DEC, and PFP solvated199

with Li+ ions; B: LSV curves of STD and 0.5% PFP-containing electrolytes measured200

with a graphite working electrode and Li metal as counter electrode at the scanning rate201

of 0.1 mV s-1.202

203

Electrochemical performance of NCM90 and P-NCM92 electrodes in Li half cells204

Figure 3A shows the cycling performance of NCM90 electrode in lithium half cells205

with STD and electrolytes containing various contents of PFP in the voltage range206

3.0~4.3 V at 30 oC. Three formation cycles at 0.1C (1C = 180 mA g-1) were performed207

before the subsequent cycling under higher charge/discharge current (1C). The capacity208

retention of cell with STD electrolyte is found to be only 72.3% after 200 cycles,209

whereas the capacity retentions of 0.2%, 0.5%, and 1.0% PFP-containing electrolytes210

are 80.3%, 81.0%, and 68.4%, respectively. Careful comparison indicates that the 0.2%211

PFP-containing electrolyte shows superior electrochemical performance in terms of212

reversible capacity and capacity retention compared with the other electrolytes. The213

initial charge/discharge curves of cells with and without PFP additive show that the214

specific capacity is almost identical for both cells, implying that the PFP additive has no215

apparent effect on the initial discharge capacity though with slightly higher electrode216

polarization at the initial stage of charge, as displayed in Supplementary Figure 2. It is217


implied that the CEI layer formed on the NCM90 surface may be too thick in the218

presence of excessive PFP additive such as 0.5% or higher, which restricts the219

movement of Li+ ions through electrode-electrolyte interface, causing apparent decrease220

in specific capacity at 1C. Besides, the average Coulombic efficiency of cell with 0.2%221

PFP-containing electrolyte is 99.6% among 200 cycles, which is higher than the one222

with PFP-free electrolyte (99.0%), as shown in Supplementary Figure 3. The positive223

influence of PFP is further affirmed by the superior cycling stability when the NCM90224

electrodes were cycled at higher cut-off voltages of 4.4 V and 4.5 V (Supplementary225

Figures 4 and 5), and when higher active mass loading NCM90 electrodes were226

evaluated (Supplementary Figure 6), respectively.227

228

In addition, compared with PFP-containing electrolyte, the obvious polarization229

increase of charge/discharge plateaus of cell with STD electrolyte during prolonged230

cycling suggests that the degradation of electrode/electrolyte interface has a significant231

effect on cycling performance, as displayed in Figure 3B and C. It is believed the232

decomposition of electrolyte caused by the unwanted parasitic reactions on the NCM90233

particle surface contributing to this degradation during cycling. This phenomenon could234

be further affirmed by the dQ/dV curves (Supplementary Figure 7) during repeated235

charge and discharge process, indicating that the cell with PFP additive exhibits slower236

shrinkage of redox peaks ca. 4.0~4.2 V, related to the H2~H3 phase transformation.[25]237

Higher average discharge voltage and lower average charge voltage during cycling238

process ascertains the stabilized electrode redox reaction process for cell with 0.2%239

PFP-containing electrolyte, as shown in Figure 3D. In addition, chronoamperometry240

test (i.e., floating charge at 4.3 V) result further evidences the stabilized interfacial CEI241

as generated with PFP additive, as reflected by the lower leakage current compared with242

the one without PFP additive, as shown in Figure 3E.243

244


A

B C

D E

Figure 3. A: Cycling performance and Coulombic efficiency of NCM90||Li cells with245

different PFP contents in the voltage range of 3.0~4.3 V at 30 oC; B, C:246

Charge/discharge curve evolution of cells (B) without and (C) with PFP; D: Average247

charge/discharge voltage during cycling; E: Chronoamperometry (floating charge at 4.3248

V) results of NCM90||Li cells with STD and 0.2% PFP-containing electrolyte.249

250

Furthermore, the rate performance of NCM90||Li cells was also investigated at different251

discharge rates (0.5/1/2/3/5/7/10C) with the same charge rate of 0.2C in the voltage252


range of 3.0~4.3 V at 30 oC, as shown in Figure 4A. It is presented that the introduction253

of 0.2% PFP additive starts to show positive effect when the discharge rate is above 3C.254

Interestingly, when the discharge rate is increased (i.e., 5C, 7C, and 10C), the discharge255

capacity of cell with PFP-containing electrolyte becomes higher than the cell with PFP-256

free electrolyte. Figure 4B and C display the charge and discharge curves of cell with257

and without PFP additive at 5C and 10C rates, respectively. For the cell with PFP258

additive, the NCM90 delivers discharge capacities of 165 mAh g-1 at 5C and 152 mAh259

g-1 at 10C, well above that discharges in PFP-free electrolyte (163 mAh g-1 at 5C and260

143 mAh g-1 at 10C, respectively). The results indicate that the cell with PFP additive261

possesses the capability of fast discharging at high C rates, especially those above 3C,262

implying the enhanced NCM90 interface in the presence of PFP additive.263

264

It is worth mentioning that the effectiveness of PFP additive has been also proved on265

other Ni-rich cathodes, such as the polycrystalline NCM92 (P-NCM92). Similar to the266

case for NCM90, with 0.2% PFP-containing electrolyte, the P-NCM92 electrode shows267

obviously improved discharge capacities at high C rates, as displayed in268

Supplementary Figure 8. Meanwhile, high temperature (45 oC) cycling performance269

of the P-NCM92||Li cells is also enhanced when introducing PFP into the electrolyte, as270

shown in Figure 4D. The result suggest that the PFP additive could be broadly applied271

for promoting the interfacial stability of a wide spectrum of NCM cathode electrodes,272

depending on the specific demand for practical application.273

274

275

276

277


A B

C D

Figure 4. A: Discharge capacity of NCM90||Li cells at various discharge rates at 30 oC;278

B, C: Charge/discharge curves of cells discharging at (B) 5C and (C) 10C rates with and279

without PFP additive. D: Cycling performance of P-NCM92||Li cells with STD and280

0.2% PFP-containing electrolytes in the voltage range of 3.0~4.3 V at 45 oC.281

282

Kinetics of the cathode interfacial film283

EIS characterization of NCM90||Li cells during cycling was carried out to understand284

the interfacial reaction kinetics and the results are displayed in Figure 5A and B. Two285

semicircles could be obviously observed at high to medium frequency, which represent286

the resistance of Li+ migration through the surface CEI film (Rsf) and the charge transfer287

resistance (Rct). The corresponding fitting results of Rsf and Rct are illustrated in Figure288

5C and D. It is apparent demonstrated that the Rsf (Figure 5C) for the PFP-containing289

cell (44.8 Ohm) is higher than the STD electrolyte (22.8 Ohm), which could be290

contributed to the formation of the protective CEI film on the cathode electrode. It is291

worth to notice that after 50 cycles, the Rct (Figure 5D) of the cell with STD electrolyte292

increases nearly twice (from 137.0 to 245.6 Ohm) that of the 1st cycle, while minimal293


increase in Rct is observed for the PFP-containing cell (from 152.4 to 153.5 Ohm),294

suggesting that the PFP-derived CEI facilitates to maintain lower interfacial resistance,295

therefore, effectively favoring the transfer of Li+ ions through the electrode/electrolyte296

interface.297

298

A B

C D

E F

Figure 5.A, B: Nyquist plots of NCM90||Li cells at various cycles at room temperature,299

A: STD electrolyte, B: 0.2% PFP-containing electrolyte; C, D: Corresponding fitted300

results of Rsf and Rct for NCM90||Li cells; E, F: Relationship between Zre and ω-1/2 at301

low frequency region for cells with (E) STD and (F) 0.2% PFP-containing electrolyte.302

303


It is well established that the slope of EIS spectra at low frequency is associated with the304

lithium-ion diffusion in the solid electrode (Warburg impedance), which could be used305

to determine the lithium-ion diffusion coefficients (DLi+) based on the Equations (2)306

and (3).[26]307

308

Zre=Rsf+Rct+σω-1/2 (2)

DLi+=R2T2 2A2n4F4C2σ2 (3)

309

where σ represents Warburg impedance coefficient and ω (ω=2π f) is the function of310

frequency (f) for Equation (2) and (3), R stands for the ideal gas constant (8.314 J mol-311

1 K-1), T is thermodynamic temperature in K and T=298.15 K at room temperature, A312

relates to the surface area of the electrode, n is number of electrons, F is Faraday313

constant (96485 C mol-1), and C is the concentration of Li+ for Equation (3). The314

relationships between Zre and ω-1/2 at low frequency region are presented in Figure 5E315

and F.316

317

The calculated results of lithium-ion diffusion coefficients at different cycles are listed318

in Table 1. For PFP-containing electrolyte, the lithium-ion diffusion coefficient is319

higher than that of the STD electrolyte. Particularly, the lithium-ion diffusion coefficient320

of the NCM90 electrode is identified to be 2.25×10-10 cm2 s-1 after 50 cycles in PFP-321

containing electrolyte, which is higher than that in the STD electrolyte (2.13×10-10 cm2322

s-1). This better maintained diffusion kinetics in the presence of PFP additive evidences323

the enhanced stability of CEI layer which prevents the interfacial side reactions and324

suppresses surface structure degradation.325

326

Table1. Calculated results of DLi+ of NCM90||Li cells at different cycles327

Cycle No.STD

DLi+ (cm2 s-1)

0.2% PFP

DLi+ (cm2 s-1)

1st 4.74 x 10-10 5.62 x 10-10


10th 1.85 x 10-10 6.74 x 10-10

50th 2.13 x 10-10 2.25 x 10-10

328

Interfacial microstructure evolution of the NCM90 electrodes329

Figure 6A-C presents the TEM images of fresh NCM90 electrode, and those cycled in330

STD and 0.2% PFP-containing electrolytes at 3.0~4.3 V after three formation cycles at331

0.1C at 30 oC. As shown in Figure 6A, a smooth surface on the fresh NCM90 cathode332

can be observed. In comparison, a thick and ununiform interfacial film with a thickness333

up to ca. 30 nm covers the surface of NCM90 when cycled with STD electrolyte334

(Figure 6B), which is unfavorable for the lithium-ion transportation and the interfacial335

charge transfer reactions.[10] In contrast, in the existence of PFP additive, a much thinner336

and compact CEI with a limited thickness of ca. 15 nm can be clearly identified on the337

surface of NCM90 (Figure 6C). It is convinced that the improvement of cycling338

performance was attributed to the thin and compact CEI layer, which could enhance the339

interfacial structure, restrain unwanted side reactions between electrode and electrolyte340

and depress the attack by the acidic species (e.g., HF etc.) from the electrolyte.341

342

A B C

Figure 6. A-C: TEM images of (A) fresh NCM90 and the NCM90 cycled in the (B)343

STD and (C) 0.2% PFP-containing electrolytes after three formation cycles at 0.1C344

between 3.0 and 4.3 V at 30 oC.345

346

SEM characterization was adopted to explore the morphology evolution of NCM90347

particle surface after long-term cycling, and the results are shown in Figure 7A-F. The348


surface of the fresh NCM90 (Figure 7A and D) is smooth and well-defined. However,349

the surface of NCM90 electrode particles is obviously wrapped with byproducts350

generated from the decomposition of electrolyte when cycled in the STD electrolyte351

(Figure 7B and E). Slight cracks could also be observed on NCM90 particles for the352

electrode cycled in the STD electrolyte (red arrows in Figure 7B), indicating that the353

particle structure was destructed during cycling, which may be resulted from the HF354

corrosion and consequent dissolution of transition-metal ions. On the contrary, the355

surface of the electrode cycled in 0.2% PFP-containing electrolyte, as shown in Figure356

7C and F, keep smooth and intact, which is analogous to the fresh electrode. This357

improvement could again be ascribed to the formation of robust and stable CEI358

generated in the assistance of PFP additive, which inhibits the attack of HF species in359

the electrolyte.360

361

A B C

D E F

Figure 7.A-F: SEM images of (A, D) fresh NCM90 electrode and those cycled in (B, E)362

STD and (C, F) 0.2% PFP-containing electrolytes after 200 cycles at 1C between 3.0363

and 4.3 V at 30 oC. (Magnification: (A-C) 10k times, (D-F) 20k times)364

365

Characterization of CEI chemical composition366

In order to understand more on the functioning mechanism of PFP on the NCM90367


electrode, the chemical components of the surface byproducts were diagnosed by XPS.368

Figure 8 presents XPS spectra of cycled NCM90 cathode harvested from NCM90||Li369

cells after three formation cycles at 0.1C at cut-off voltage of 4.3 V. In C 1s spectra370

(Figure 8A), a peak appearing at 284.8 eV can be attributed to C-C/C-H bond,[9] while371

the peaks centered at 286.5 and 291.0 eV are corresponding to the existence of C-O372

bond and CO32- species, respectively,[10] which are derived from the decomposition of373

electrolyte carbonate solvents. The presence of C-O bond and CO32- species is further374

confirmed in O 1s spectra (Figure 8C) at 533.0 eV and 531.3 eV,[10] which could be375

considered as ROCO2Li originated from the decomposition of carbonate solvents.[27]376

377

In F 1s spectra (Figure 8E and F), two peaks located at 685.0 and 688.0 eV were378

observed, which were related to the presence of LiF and C-F/P-F species, respectively.[9]379

As reported in the previous literature, LiF is usually identified as one of the major380

components of the surface film formed in both cathode and anode surface. Nevertheless,381

there are still disputes existing with the function of LiF, depending on the origin sources382

of the LiF species, which is typically considered to be formed in two different reaction383

pathways. Generally, it could be generated by the attack of HF toward NCM90384

electrode surface, which not only devastates the structure of NCM90 cathode but also385

hinders the lithium-ion diffusion due to its poor conductivity for both electrons and386

lithium ions, leading to the reduction in the charge-transfer resistance.[28-30] As an387

alternative pathway, LiF could be produced from the break-down of electrolyte additive.388

The LiF-rich CEI layer as constructed on cathode surface is beneficial for enhancing the389

interfacial stability against electrolyte attack.[21] In the present study, more LiF is390

detected on the surface of NCM90 cycled with PFP compared with the one without PFP391

additive.(Figure 8F). Based on the superior electrochemical performance with PFP392

additive, it is suggested that the LiF could be mainly derived from the preferential393

oxidation of highly fluorinated PFP additive, rather than from the attacks by HF acidic394

species in the electrolyte.395

396


A B

C D

E F

G H

Figure 8. A-H: XPS of NCM90 electrodes after 3 formation cycles at 0.1C with (A, C,397

E, G) STD and (B, D, F, H) 0.2% PFP- containing electrolytes. (A, B) C 1s spectra, (C,398

D) O 1s spectra, (E, F) F 1s spectra, (G, H) N 1s spectra.399


400

Comparison of the P 2p spectra in the cycled NCM90 with and without PFP additive401

shows that the peak intensity of LixPOyFz decomposed from LiPF6 is lower in the402

presence of PFP than that without additive, as illustrated in Supplementary Figure 9A403

and B. This result further consolidates that the higher LiF peak intensity as detected for404

the CEI film is generated by the PFP additive other than the decomposition of LiPF6. In405

the N 1s spectra (Figure 8G and H), N signal is absent for NCM90 cycled with PFP-406

free electrolyte (Figure 8G). In comparison, there is an obvious N 1s peak located at407

400.5 eV found for NCM90 cycled in PFP-added electrolyte (Figure 8H), which could408

be attributed to pyridinic nitrogen[21] derived from PFP additive. The binding energy of409

the interfacial nitrogen species is higher than that detected for regular pyridinic nitrogen410

at 398.7 eV,[31] which could be explained that a fraction of fluorine atoms are preserved411

during the electrochemical polymerization of PFP additive.412

413

Furthermore, to deeply investigate the mechanism for electrochemical performance414

improvement, the binding dissociation energies (BDE) of C-F located at different415

position of pyridine ring were calculated based on the DFT, as listed in Table 2. The416

results shows that the fluorine atoms at C2 position (Figure 1B) possesses the lowest417

binding dissociation energy, followed by those at C3 and C5 positions, in the case that418

one electron is extracted from the PFP molecule. Therefore, the fluorine atoms at C2,419

C3, C5 are considered more inclined for electrochemical polymerization, leading to the420

formation of F-containing polypyridine and LiF as proposed in Figure 9. Similar421

electrochemical polymerization mechanism has been previously reported for nitrogen-422

heterocyclic compounds[20] and pyrimidine derivatives.[22]423

424

Table2. Calculated BDE results of C-F bonds of PFPmolecule425

Location of C-F BDE (kcal mol-1)

C2-F7 102.5

C3-F11 103.5

C5-F10 103.5


C1-F8 104.8

C6-F9 104.8

426

Figure 9. Electrochemical polymerization pathway as proposed for the functioning427

mechanisms of PFP additive.428

429

Finally, The Coulombic efficiency of Li||Cu cells was also studied to explore the effect430

of PFP on SEI on anode surface. The results show that there is no significant distinction431

of Coulombic efficiency for cell with and without PFP additive (Supplementary432

Figure 10), indicating that the improved electrochemical performance could be mainly433

dictated by the enhanced NCM90 interface.434

435

CONCLUSIONS436

In this work, a highly fluorinated pyridine PFP has been systematically investigated as a437

functional electrolyte additive to enhance interfacial stability and electrochemical438

performance of Ni-rich NCM90 electrode. It is revealed that PFP additive could be439

oxidized prior to the carbonate solvents, forming a compact, uniform and protecting440

interfacial CEI layer enriched with F-containing polypyridine moieties and LiF species441


on the NCM90 cathode surface. The enhanced CEI layer could effectively inhibit the442

electrolyte decomposition and restrain the corrosion on NCM90 electrode by HF species443

in the electrolyte, giving rise to the improved cycling stability of NCM90 electrode. The444

development and application of this type of functional electrolyte additive could be445

beneficial for the industrial application of Ni-rich NCM for developing high energy446

density LIBs.447

448

DECLARATIONS449

Acknowledgments450

The authors thank Ningbo Ronbay technology Co., Ltd. and Zhangjiagang Guotai451

Huarong Co., Ltd. for kindly supplying the cathode material and electrolyte,452

respectively.453

454

Authors’ contributions455

The manuscript was written with contributions of all authors. All authors have given456

approval to the final version of the manuscript.457

458

Availability of data and materials459

The data supporting our findings can be found in the supplementary information.460

461

Financial support and sponsorship462

The project was supported by the Xiamen University Nanqiang Yang Talent Program,463

and the Natural Science Foundation of Fujian Province of China (No. 2020J06004).464

465

Conflicts of interest466

All authors declared that there are no conflicts of interest.467

468

Ethical approval and consent to participate469

Not applicable.470

471

Consent for publication472

Not applicable.473

474


Copyright475

© The Author(s) 2021.476

477

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Supplementary Material: Enhancing cycle life of Nickel-rich LiNi0.9Co0.05Mn0.05O2583

via a Highly Fluorinated Electrolyte Additive – Pentafluoropyridine584585

MAIN TEXT586

HOMO and LUMO energies of EC, DEC, and PFP with and without solvation with Li+,587

oxidation potential of EC, DEC, and PFP with and without solvation with Li+, dQ/dV588

curves of Si/C anode with different electrolytes, initial charge/discharge curves NCM90589

with different electrolytes, Coulombic efficiency of NCM90||Li cells with different590

electrolytes, cycling performance of NCM90||Li cells at cut-off voltage 4.4 V and 4.5 V，591

dQ/dV curves of NCM90 electrodes with different electrolytes, Cycle performance and592

rate capability of P-NCM92||Li cells with different electrolytes, XPS (P 2p spectra) of593

NCM90 electrodes after 3 formation cycles, Li||Cu cells cycling data, cycling594

performance of NCM90||Li cell with high loading.595

596

Supplementary Table 1. HOMO and LUMO energies of EC, DEC and PFP597

Solvent/Additive EC DEC PFP

HOMO (eV)

Without solvation with Li-8.43 -8.09 -7.70

LUMO (eV)

Without solvation with Li-0.09 -0.12 -0.06

HOMO(eV)

Solvation with Li-9.17 -8.89 -7.96

LUMO(eV)

Solvation with Li-0.66 -0.59 -1.91

598

Supplementary Table 2. Theoretically calculated oxidation potentials of EC, DEC599

and PFP600

Solvent/Additive EC DEC PFP

Potential (V vs. Li/Li+) 6.64 6.42 5.93


Without solvation with Li

Potential (V vs. Li/Li+)

Solvation with Li7.17 6.97 6.10

601

Supplementary Figure 1. The initial dQ/dV curves of Si/C electrode with STD and602

PFP-containing electrolytes at 0.1C (1C = 800 mA g-1). Si/C anode electrodes were603

prepared by casting a slurry of Si/C material, acetylene black and alginate binder with a604

mass ratio of 8:1:1 utilizing deionized water as solvent, on a Cu foil current collector605

with active material loading of ca. 0.5 mg cm-2.606

607

Supplementary Figure 2. Initial charge/discharge curves of NCM90 electrodes with608


STD and 0.2% PFP-containing electrolytes in the voltage range of 3.0~4.3 V at 30 oC.609

610

Supplementary Figure 3. Coulombic efficiency of NCM90||Li cells with STD and611

0.2% PFP containing electrolytes in the voltage range of 3.0~4.3 V at 30 oC.612

613

A B

Supplementary Figure 4. A: Initial charge/discharge curves at 0.1C; B: Cycling614

performance of NCM90||Li cells with STD and 0.2% PFP-containing electrolytes at615

charge cut-off 4.4 V at 30 oC. The NCM90 electrode is consisted of 90% active616

materials, 5% acetylene black as conductive agent, and 5% poly (vinylidene fluoride) as617

binder.618

619


A B


performance of NCM90||Li cells with STD and 0.2% PFP-containing electrolytes at621

charge cut-off 4.5 V at 30 oC. The NCM90 electrode is consisted of 90% active622

materials, 5% acetylene black as conductive agent, and 5% poly (vinylidene fluoride) as623

binder.624

625

A B


performance of NCM90 with high active material loading (8.9 mg cm-2) in the voltage627

range of 3.0~4.3 V at 30 oC.628

629


A

B

CSupplementary Figure 7. A, B: The dQ/dV curves of NCM90||Li cells at 30 oC in the630

voltage range of 3.0~4.3 V with (A) STD and (B) 0.2% PFP-containing electrolyte631

during 200 cycles; C: Comparison of dQ/dV for NCM90||Li cells with and without PFP632

additive at 30 oC at 200 cycles.633

634


Supplementary Figure 8. Rate performance of P-NCM92||Li cells with STD and 0.2%635

PFP-containing electrolytes in the voltage range of 3.0~4.3 V at 30 oC, respectively.636

The P-NCM92 electrode is consisted of 80% active materials, 10% acetylene black as637

conductive agent, and 10% poly (vinylidene fluoride) as binder.638

639

A B

Supplementary Figure 9. A, B: XPS of NCM90 electrodes after 3 formation cycles at640

0.1C with (A) STD and (B) 0.2% PFP- containing electrolytes. (A, B) P 2p spectra.641

642


Supplementary Figure 10. Coulombic efficiency of Li||Cu cells at 1 mA cm-2 with643

STD and 0.2% PFP-containing electrolytes. The Li||Cu cells were assembled by using644

Cu foil (19.0 mm in diameter, 10 um in thickness) as working electrode and Li metal645

(15.8 mm in diameter, 2 mm in thickness) as counter and reference electrode. At each646

cycle, lithium was deposited on Cu foil at 1 mA cm-2 and then stripped from Cu foil647

until the cell potential reached 1.0 V (vs. Li/Li+).648

649

:10.20517/energymater.2021.xx en ergym at ils

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