In-Situ Development of Elastic Solid Electrolyte Interphase via Nano

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In-Situ Development of Elastic Solid Electrolyte Interphase via Nano-Regulation and SelfPolymerization of Sodium Itaconate on Graphite Surface Shuai Heng, Qiang Shi, Yan Wang, Qunting Qu, Jingyu Zhang, Guobin Zhu, and Honghe Zheng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01912 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Energy Materials

In-Situ Development of Elastic Solid Electrolyte Interphase via Nano-Regulation and Self-Polymerization of Sodium Itaconate on Graphite Surface

Shuai Heng, Qiang Shi, Yan Wang, Qunting Qu*, Jingyu Zhang, Guobin Zhu, Honghe Zheng*

College of Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu, 215006, P. R. China.

Corresponding authors: *[email protected] (Qunting Qu) *[email protected](Honghe Zheng)

KEYWORDS: Lithium ion batteries; graphite anode; elastic SEI film; sodium Itaconate; self-polymerization.

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Abstract: The regulation of solid electrolyte interface (SEI) on electrode material surface is one of the most crucial fundamental issues in lithium ion batteries. A desired SEI film is characterized by high homogeneity, flexibility and ductility, which is able to stand up with the volume change of the active material during repeated electrochemical cycles.

More than 10% volume change of graphite particle is easy

to cause brittle fracture and rearrangement of the SEI film, resulting in continuous consumption of the active Li ion and capacity decline of the cell.

Herein, we

proposed an in-situ development of elastic SEI film via nano-regulation and self-polymerization of sodium itaconate (SI) on graphite surface. The nano-tuned uniform SI nano-layer on the graphite surface, as a template for SEI film, contributes to a polymer-reinforced SEI film through self-polymerization between the carbon double bonds. With 20 nm of the SI nano-layer template, the overall electrochemical performances including the first coulombic efficiency, rate capability, cycling stability and even the high temperature performance are remarkably improved. Moreover, the impedance rise of the electrode with electrochemical cycles is effectively suppressed.

As the result, cycle-life of the full cell based on LiFePO4

cathode and the SI-decorated graphite anode is greatly enhanced.


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1. Introduction High performance lithium ion batteries (LIBs) are booming for wide applications in electric vehicles (EVs) and large-scale energy storage systems (ESSs) [1-4]. As the matter of fact, some electrochemical properties of the state-of-the-art LIBs are still waiting for further improvement.

The major challenges are maximizing the life-span

and improving the reliability of the cell during operation. In essence, an electrochemical cell is not just a simple superposition of the key materials including the cathode, anode, electrolyte, and separator [5-8]. There are many fundamental issues involved in the battery engineering and technologies.

Interactions between

these key materials during cell operation strongly affect the overall properties. Interface is the most important place for the interactions between the key materials. Therefore, interfacial optimization is of great significance for developing high performance LIBs of long cycle-life and high reliability. For commercial LIBs, the capacity decline within its life-span can be divided into two different electrochemical stages. The first stage is from the cell formation to the 50-100th cycle.

In this stage, the variation of the cell capacity with the cycle

number is in accordance with the following power-law equation: Q = Q0 η1n

(1)

Where Q0 is the initial cell capacity, η1 is the cycling efficiency in this stage and n is the cycle number.

η1 can be obtained by fitting the capacity fading curve with cycle

number. The origin of the capacity-fading at this stage is mainly related to the growth and reformation of the solid electrolyte interphase (SEI) on the electrode, particularly on the graphite surface, resulting in an irreversible consumption of the reversible



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lithium ions [9-13]. After 50~100 cycles, the capacity decline turn to another different trend, i.e. a linear decrease of the cell capacity with cycle number.

The variation of the cell

capacity with the cycle number obeys a linear equation below: Q = Q1η2(n-n1)

(2)

Where Q1 is the cell capacity after stage 1, n1 refers to the cycle number where the stage 1 ends, and η2 is the cycling efficiency of the cell in stage 2. Capacity-fading in this stage is related to the instability and rearrangement of the developed SEI layer. The SEI formed during stage 1 is not immutable and frozen. As the cycle going on, there is a continuous damage and repair of the passivation film due to the volume expansion and contraction of the graphite particles. For this reason, active lithium consumption is the most important cause for the capacity-fading of LIBs. [14-16] It is seen the SEI stability is a critical and fundamental issue deciding the cycling and storage property of LIBs, particularly at high temperature condition.

To explain

the instability of the SEI layer on graphite surface, researchers have proposed several different mechanisms: (I) escape of electrons to the electrode surface through the electronic insulated SEI film [14], (II) dissolution of the SEI components into the electrolyte [15], and (III) continuous rapture and repair of the SEI layer [16, 17]. Among the three factors, rapture and repair of the SEI film has been confirmed to be the most important cause for the SEI growth and the continuous active lithium loss with electrochemical cycles [17].

Graphite is known exhibiting more than 10%

volume expansion at its fully lithiated state while the naturally grown SEI layer, mainly consisting of inorganic components such as Li2CO3 [18], LiF [19], and Li2O [20] salts, is fragile. The mechanical mismatch between the passivation layer and the substrate explains the damage of the SEI film during the cell operation.


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Rearrangement and growth of the SEI film causes a ceaseless consumption of the active lithium and the electrolyte components, and thus results in a steady or linear lithium inventory loss with electrochemical cycles.

In this sense, SEI stability and

the related lithium inventory loss is a common and important issue for full cells based on lithium concept.

Enhancing the SEI stability on graphite surface is very critical

for further improving the electrochemical performance of the-state-of-the-art LIBs. To date, a variety of studies have been attempted to stabilize the SEI film through graphite modification and electrolyte optimization.

For example, mechanically

robust material coatings with metal oxides such as TiO2, Al2O3, ZrO2, TiO2, ZnO, and Li4Ti5O12 have been reported to effectively reduce interfacial side reaction between the active material and liquid electrolyte [21-26]. Meanwhile, some inorganic coating with Li2CO3, LiF, Na2CO3, and K2CO3 have also been applied on graphite surface [27-30]. However, it should be noted that the inorganic layer on graphite surface is always incomplete, fragile and easy to peel off.

Polymer coatings with conductive

polymers such as polyimide (PI) and polypyrrole (PPy) have been attempted to optimize the electrode/electrolyte interface, but the results are still less satisfactory. [31, 32]

Various electrolyte additives including VC, FEC, DTD, LiBOB and

LiODFB have shown good promise to stabilize the SEI on graphite and enhance the cell performances [33-37].

However, it should be noted that rare of them attempts to

solve the mechanical mismatch by improving the flexibility and elasticity of the SEI film.

Addressing the fundamental issue is not only of great significance for

commercial LIBs based on graphite anodes, but also shed new light on the use of silicon anode, which shows 300% volume change during electrochemical cycles.



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Interestingly, it is known that carbon double bond in organic molecules is able to get an electron and turns into a radical [38, 39]. The radicals can initiate polymerization between the double bonds and thereby produce polymer skeleton for the SEI layer [40, 41].

If a regulated nano-layer containing unsaturated carbon

bonds is applied on the graphite surface, acting as a template for the SEI growth, the SEI would be developed via in-situ polymerization between the carbon double bonds. On the one hand, the polymeric skeleton contribute to high strength and flexibility characteristics of the SEI film. On the other hand, thickness of the organic template can be easily regulated by changing the amount of the template material on the graphite surface.

Meanwhile, carboxyl functional groups have been known playing

a positive role for the formation of SEI film by enhancing the ionic transport properties. According to earlier studies [42,43], the fast ion transport mechanism for the SEI film containing carboxyl groups is via hopping of lithium ions between the neighboring carboxylic cites.

If the two important concepts are incorporated into the

design and regulation of the SEI film on graphite surface, a desired SEI film of high elasticity and high ionic conductivity are expected. In this work, we proposed a new concept of SEI design and regulation through nano-regulation and self-polymerization of an electro-active organic nano-layer applied on graphite surface. Sodium itaconate, (SI, see Fig. S1), containing both unsaturated bonds and carboxyl functional groups, is unprecedentedly prescribed as a template layer for SEI formation on graphite particles.

In-situ polymerization

between the unsaturated carbon bonds produces polyitaconate compound, which acts


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a reinforced material for the SEI film while the carboxyl groups help to improve the Li transport properties. Inorganic lithium salt species aroused from the reduction of solvated Li ions are inherently bonded to the polymeric skeleton.

The exquisite

organic-inorganic composite SEI film is thus of high elasticity and ionic conductivity properties.

The new strategy for SEI design provides a new avenue stabilizing

graphite surface and realizing the next-generation LIBs of long life-span and high reliability.

2. Experimental 2.1 Material preparation The natural graphite (NG, 10-25 m in diameter, d002 = 0.3356 nm, the specific surface area=10.2 m2 g-1) was obtained from Beiterui New Energy Materials Co. Ltd. China. Itaconic acid was supplied by Adamas-beta Reagent Co. Ltd.

The inactive

materials including acetylene black (AB, 40 nm diameter) and polyvinylidene fluoride (PVDF) binder (Kureha Battery Materials Shanghai, Inc.) were used as received. To increase the solubility of the itaconic acid in water and eliminate the impact of the H proton on the graphite surface, a certain amount of itaconic acid aqueous solution is neutralized by a certain amount of NaOH aqueous solution. ratio between itaconic acid and NaOH was set at 1:1.98. pH value of the solution finally reaches 7.

The molar

With vigorous stirring, the

To realize a uniform distribution of the

sodium itaconate (SI) onto the graphite surface, 2g graphite was dispersed into 20mL SI aqueous solution of different concentrations. While vigorous stirring, the



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temperature was increased from room temperature to 80 °C in order to slowly evaporate the solvent. surface.

After completely drying, SI is applied onto the graphite

Finally, the obtained product was subject to further heat-treatment at 120

°C for 12 h under vacuum. This is to remove the trace water and make the SI layer more compact. The weight ratio between the graphite and the SI layer was set at 100:0, 98:2, 96:4 and 94:6 and the obtained samples are denoted as NG@SI-0%, NG@SI -2%, NG@SI -4%, and NG@SI -6%, respectively. 2.2 Electrode preparation and the electrochemical testing The graphite electrode laminate is prepared by casting the homogeneous slurries consisting of 88.8 wt% graphite, 8 wt% PVDF binder and 3.2 wt% acetlylene black (AB) dispersed in N-methylpyrrolidone (NMP) solvent. The reason for adopting PVDF binder and NMP dispersant in this work is to avoid the dissolution of the SI nano-layer from the graphite surface into the slurry during the electrode processing. The slurry is cast onto a copper foil (12 μm-thick) by using a doctor blade.

Similarly,

LiFePO4 cathode laminate is obtained by casting a slurry containing LiFePO4, PVDF binder and AB conductive additive with a weight ratio of 8:1:1 onto Al foil (18 μm-thick).

After visible drying, the electrode laminates were roll-pressed and

punched out into small discs. The graphite electrode has a diameter of 14mm while the LiFePO4 cathode has a diameter of 13mm.

The capacity match between the

LiFePO4 cathode and the graphite anode is realized by controlling the mass loading of the electrode laminate.

The mass loading of the graphite anode is controlled at 4.5

mg cm-2 while the LiFePO4 cathode is around 10.0 mg cm-2.


Prior to use, all the

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electrode discs were thoroughly dried at 120 °C for 16 hrs under vacuum. The electrochemical properties of the graphite anode were evaluated in CR2032 coin cells. The cells were carefully assembled in a glove box filled with argon atmosphere (dew point≤ -80 ◦C). The half cells were fabricated in which the graphite laminate is the working electrode and the counter electrode is Li foil.

The separator

adopted is the Celgard 2500 and 1 mol L−1 LiPF6/EC + DEC (1:1) is used as the electrolyte. All the cells were cycled on a Maccor S4000 battery cycler at 303 K unless otherwise specified. The half cells were formed at C/20 charge and discharge rate between 0.01 and 2 V vs. Li/Li+.

From the results, the first coulombic efficiency

and the reversible capacity of the graphite are obtained.

Rate performance of the

graphite anode was compared by discharging at various rates of 0.1C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, 20 C, 30 C and 50 C, respectively. A charge (lithiation) to 0.01V at 0.1C rate preceded each discharge. Cycling test of the graphite anodes was conducted at 0.5C charge and 0.5C discharge rate.

Full cells with LiFePO4 cathode were

assembled and formed at 0.05 C between 2.5 and 3.9 V. Thereafter, the cells were cycled at 0.5 C charge/discharge rate for 400 cycles. At different electrochemical stages, electrochemical impedance spectra (EIS) of the graphite anodes were collected on a Zahner Elektrik IM6 electrochemical workstation. At 60% depth of discharge (DOD), the measurements were carried out with an alternating amplitude of 5 mV in the frequency of 105 HZ ~ 100 mHz at 30 oC. 2.3

Morphological and spectroscopic characterizations

Transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN, 200 KV)



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was adopted to observe the SI layer applied on the graphite surface. Morphological images of the graphite anode at different electrochemical stages were observed with SEM (Hitachi S-800, 10 or 15 kV).

Energy dispersive X-ray spectroscopy (EDX,

Hitachi S-8010) was employed to see the distributions of typical elements on the graphite surface.

XRD patterns of the samples were collected by powder X-ray

diffraction (XRD, Rint-2000, Rigaku with Cu Kα). The scan rate is 3 ° min-1 from 10° to 80°.

Fourier transform infrared spectroscopy (FTIR, Bruker, Tensor 27) was

employed to characterize the graphite surface. cm-1.

The wavenumber range is 400-4000

To further analyze the graphite surface, X-ray photoelectron spectroscopy

(XPS, Escalab 250Xi, Thermo Fisher, America, Pass Energy 30.0 eV) analyses were conducted at different electrochemical stages.

3. Results and discussion The XRD patterns and FTIR spectra for the graphite with different thicknesses of the SI layer is depicted in Fig. 1.

All the typical XRD peaks are in good accordance

with the standard Power Diffraction Card of NG (JCPDS NO.75-1621) [44].

For all

the samples, no diffraction peak ascribed to the SI compound is observed. This is because both the content and the crystallinity of the SI in the sample are very low. However, the intensity decline of the characteristic 002 peak with increasing SI content reveals the thickening of SI layer on the graphite. For the FTIR spectra in Fig.1b, a characteristic strong peak at 1620 cm-1 obtained from the pure SI film is assigned to the C=O stretching vibration [45]. The other peak at around 1450 cm-1 is


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assigned to methylene (-CH2) symmetrical deformation mode [46]. With increasing amount of SI on the graphite surface, a slight increase of the typical peaks at around 1600-1 and 1400 cm-1 is observed.

Meanwhile, no new peak is detected in the whole

region, demonstrating that the SI nano-layer neither changes the crystal structure of the graphite nor brings about any chemical reactions. The SEM and TEM images of the graphite with different amounts of the SI salt are provided in Fig. 2.

The pristine graphite shows spherical shape and clear edges

of the graphite pallets can be seen. decoration. distinct.

The surface condition is changed by the SI

With increasing SI content, the edges of graphite pellets become less

When the SI content reaches 6 wt% (see Fig. 2d), the graphite edges are

hard to distinguish, implying the SI salt is applied onto the graphite particle surface. TEM images shows the variation of the SI thickness on the graphite surface with increasing SI amount (see Fig.2 e to h).

For the 2 wt% SI-wrapped sample

(NG@SI-2%), thickness of the SI layer is seen to be ca. 10 nm.

The thickness of the

SI layer gets to ca. 20 nm with 4 wt% SI content and ca. 30 nm with 6 wt% SI salt. According to literature [17, 47], the typical SEI thickness developed on graphite surface is around 20 nm.

In this sense, the 4 wt% SI nan-layer is ideal as the SEI

template for graphite anode. To examine the physical integrity of the SI nano-layer before and after the electrode processing, morphologies and the element mappings for the 4 wt.% SI-wrapped graphite are compared as shown in Fig.3. As Na is the representative element of the SI, we can clearly see the distribution of SI salt simply from the



distribution of Na element in the sample.

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Either before or after the electrode

processing, it is seen the Na element mainly appears on the graphite surface. hard to detect Na element in between the graphite particles.

It is

In other words, the

electrode processing does not destroy or damage the SI nano-layer on the graphite surface.

This is because the SI salt is intrinsically insoluble in NMP solvent. Of

course, SI salt is also insoluble in the organic electrolyte. Therefore, we do not need to worry about the dissolution of the SI nano-layer from the graphite particle into the organic electrolyte and its possible effect on the cathode side. The electrochemical performances of the graphite anodes with different thicknesses of the SI nano-layer are compared in Fig. 4. Overall, all the graphite anodes can be effectively cycled and the lithiation and delithiation behavior seems quite identical.

The first charge/discharge capacity and the corresponding coulombic

efficiency for the graphite anodes with different thicknesses of the SI nano-layer are summarized in Table 1.

For the bare graphite without SI nano-layer, the first charge

and discharge capacity is obtained to be 413.1 and 362.3 mAh g-1, respectively, corresponding to the first coulombic efficiency of 87.7%. For the graphite with 20 nm SI nano-layer, a decrease of the irreversible capacity contributes to an increased coulombic efficiency of 90.98%. The decrease of the first irreversible capacity implies the reduced side reactions relating to the SEI formation. The SEI film developed with the SI template layer is more effective.

Thick SI layer brings about a

slight increase of the reversible capacity. This infers the electrochemical lithiation property of the SI salt. Wang et al. have shown that the carbon double bond and the


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carbonyl oxygen are active sites for lithium storage, which may contribute an extra-capacity to the electrode. [48] The differential capacity plots (dQ/dV vs. potential) in the first cycle for the graphite with different thicknesses of the SI template layer are shown in Fig. 4b. The first cathodic process between 0.8 V and 0.6 V is assigned to the SEI formation. From the insert figure, it is seen the irreversible decomposition reactions relating to the build-up of SEI on the graphite surface is changed by the SI nano-layer. On the one hand, the SI layer slightly enhanced the onset potential of the SEI formation reactions, indicating the SI template facilitates the SEI development on the graphite. On the other hand, the SI template effectively reduced the intensity of the SEI formation reactions. The subsequent electrochemical processes located at 0.22, 0.12, and 0.08 V are attributed to the 4-stage, 2-stage, and 1-stage graphite intercalation compound (GIC), respectively. By comparison, less polarization and fast electrochemical kinetics of the graphite anode with the SI nano-layer is achieved. The principal reason for the electrochemical enhancement of the graphite anode is related to the unsaturated carbon bond in the SI nano-layer.

The applied SI

nano-layer acts as the template of the SEI film on the graphite surface.

During the

charge process, the C=C double bond is able to get an electron and converts to a radical which is able to initiate self-polymerization between the itaconate monomers, and thereby produces polymeric skeleton for the SEI film. The in-situ developed polymeric network is of high mechanical strength and flexibility compared to the inorganic Li salt components from the reduction of the solvated Li ions on graphite



surface.

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Besides, the abundant carboxyl group in the SI facilitates the formation of

compact and conducting SEI film. The fast ion conducting property is realized via hopping of lithium ions between the neighboring carboxylic cites.

From these

viewpoints, the concept of SEI design based on the itaconate template on the graphite surface contributes to a significant enhancement of the electrochemical properties. Fig. 5 shows the rate performance of the graphite anode with different thicknesses of the SI nano-layer.

In the 0.1C-10C discharge rate range, no distinct

capacity retention difference is observed for all the samples.

However, at discharge

rate above 20 C, the graphite with 20 nm SI template (4 wt%) exhibits much improved capacity retention.

At 50 C rate, capacity retention of the electrode is

obtained to be ca. 80%, significantly higher than that of 58% for the bare graphite. As the rate capability of graphite anode is greatly affected by the composition, thickness and Li ion conductivity of the SEI layer, the new approach of in-situ growth of SEI by the SI template effectively improved the Li ion diffusion kinetics [19]. Further increasing the thickness of the SI nano-layer leads to a decline of the rate capability.

This is believed to be associated with the insulating property of the SI

nano-layer, which contributes to higher interfacial resistance of the electrode. The Nyquist plots of the graphite anode with different thicknesses of the SI nano-layer at 40% DOD after the rate test are illustrated in Fig. 5d. In this figure, the high frequency semicircle corresponds to the SEI resistance (RSEI) while the subsequent semicircle in intermediate frequency refers to the charge transfer resistance (Rct) of the graphite anode [49-51]. Clearly, the 20 nm SI (4 wt%) wrapped


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graphite exhibits a significant impedance decrease. The results are in good accordance with the rate capability of the graphite with SI nano-layer of different thicknesses. The graphite with 30 nm SI (6 wt%) template exhibits an impedance rise, which is related low electronic conductivity of the thick SI interfacial layer. The cycling stability of the graphite anode with SI nano-layer of different thicknesses at different temperatures are compared in Fig. 6. For the bare graphite, there is a clear capacity-fading with electrochemical cycle. After 200 cycles, only ca. 83.2% of the reversible capacity is retained and the main reason for the capacity-fading is related to the SEI instability.

The instability of SEI means

ceaseless side reactions at the electrode/electrolyte interface, which have detrimental effects on the surface and even the structure of the graphite. With SI template of different thicknesses, the SEI mechanism is altered and the cycling stability of the graphite electrode is significantly enhanced. With 20 nm of the SI nano-layer (4 wt%), almost no observable capacity loss is detected in the 200 electrochemical cycles. When cycled at high temperature condition of 60 oC (see Fig.6b), the bare graphite shows even worse cycling performance, its capacity dives after ca. 50 electrochemical cycles.

This is mainly attributed to the destruction or decomposition of the naturally

grown SEI components at high temperature.

Decomposition of the SEI components

on graphite surface has been observed at high temperature conditions ( 60 oC ) [52]. By contrast, the SI layer remarkably enhanced the cycling performance of the graphite anode at high temperature.

10 nm SI interfacial layer (2 wt%) postponed the onset

of the capacity drop from the 50th to 80th cycle.

20 nm (4 wt%) and 30 nm (6wt%)



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itaconate layer enables the graphite anode effectively cycle in the tested 200 cycles and the capacity retention is obtained to be more than 92% for the graphite with 30 nm of the itaconate layer.

To the best of our knowledge, the obtained cycling

performance at the high temperature for the natural graphite anode with SI template layer is obviously superior to those of many other modified strategies on graphite anodes. [53, 54]

A comparison of the Nyquist plots for the graphite anodes with

different thicknesses of the SI template nano-layer after 200 cycles under 60 oC is seen in Fig. 6c. A significant high impedance for the bare graphite is obtained due to the evolution of the SEI film during prolonged electrochemical cycles. Interestingly, the electrode impedance is remarkably decreased by applying the itaconate nano-layer. This infers the stable SEI film developed via the itaconate template layer, which explains the improved cycling stability of the graphite at 60 oC condition. The SEM images of the graphite anodes with different thicknesses of the SI nano-layer after the cell formation and after 200 electrochemical cycles are presented in Fig.7. It is seen that the morphology of the bare graphite changed a lot after 200 cycles. The graphite particles are covered with thick surface film, revealing the continuous growth of the SEI layer on the graphite surface.

The main components

on graphite have been known to be various lithium salts including ROCO2Li, Li2CO3, (CH2OCO2Li)2, LiF, LiPFx, and Li2C2O4 etc. As the Li salt film is brittle and prone to cracking with the volume expansion and contraction of the graphite particle, the repair and growth of SEI will occur on the refreshed graphite surface exposed to the electrolyte. The ceaseless electrolyte decomposition precipitates more and more


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insoluble Li salts on the graphite surface, which explains the evolution of the graphite surface film with prolonged electrochemical cycles. By contrast, the morphology is not significantly changed for the graphite with 20 nm SI nano-layer (4 wt%) after 200 cycles. Graphite sheets is still clearly seen after 200 cycles, manifesting the rapture of the SEI film is effectively suppressed.

This is attributed to the improved SEI

stability by enhancing its mechanical properties through the in-situ polymerization of the SI template. XPS was conducted to analyze the surface compositions of the graphite without and with 20 nm SI wrapping layer (4 wt%) after formation and after 200 cycles. results are shown in Fig. 8.

The

By contrast, Fig. 8a and b shows the C1s spectra of the

graphite without and with 20 nm (4wt%) SI-layer after the cell formation while Fig. 8c and d present the C1s spectra after 200 cycles.

In the C 1s spectra, the strong

peak at 284.8 eV is assigned to the C-C bond of the graphite structure [55]. The peak at 286.6 eV is ascribed to the C-O bond of the carbonates, while the peak at 288.4 eV corresponds to the C=O bond relating to Li2CO3/ROCO2Li. [56, 57]

The peak at

284.5 eV is attributed to the C=C bond in the SI molecules and the peak at 289.8 eV is ascribed to the CO32- in the SEI film.

Comparison between Fig 8a and b shows the

intensity of C-C bond and CO32- for the 20 nm SI wrapped graphite is lower than those obtained from the bare graphite, implying the reduction of the carbonate solvent on the graphite surface is alleviated.

Also, it is worth mentioning that there is a

strong C=C peak for the 20nm SI-wrapped graphite. Existence of the strong C=C peak means the polymerization between the C=C in SI involved in the formation of



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SEI film is a slow process, which is not completed during the cell formation. As the cycle progresses, the intensity of the typical C-O, C=O and LiCO3 aroused from the reduction reactions of the carbonate is significantly increased (by comparison between Fig. 8a and Fig. 8c), manifesting the continuous interfacial reactions in the prolonged electrochemical cycles.

With 20 nm itaconate layer, all the typical peaks relating to

the decomposition of carbonate is not significantly intensified. Meanwhile, the C=C signal is greatly reduced as seen in Fig. 8d.

This confirms the self-polymerization

between the unsaturated double bonds in the electrochemical cycles as described below: H

COOH m H2C

C

CH2COOH

.

COOH COOH

m C

C

H

H

COOH

CH2COOH

+

n H2C

C

polymerization

CH2COOH

*

CH2

C

* m+n

(3)

CH2COOH

The XPS result infers that the self-polymerization reaction of SI template is not completed in the cell formation.

Instead, it may last for many electrochemical cycles.

Anyway, the produced robust and elastic polymeric framework on the graphite effectively stabilize the SEI on the graphite surface. Accordingly, Fig. 8e-h show the F 1s spectra of the graphite anodes without and with 20 nm SI wrapping layer (4 wt%) after formation and after 200 cycles.

The

peak at 684.5 eV is ascribed to LiF salt and the 687 eV peak is related to P-F species [58, 59].

After the cell formation, the LiF signal for the bare graphite is similar with

that obtained from the 20 nm (4wt%) SI-wrapped sample. However, after 200 electrochemical cycles, LiF signal for the 20 nm (4wt%) SI-decorated graphite is significantly low.

As LiF is generated by the reduction of the electrolyte, the result


Page 19 of 40 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


illustrates that the reactions of electrolyte components on the graphite surface during the repeated charge-discharge cycles is effectively inhibited by the SI polymerization. To get conclusive evidence for the elastic SEI on the operation of LIBs, full cells based on LiFePO4 cathode and the graphite without and with 20 nm SI nano-layer were assembled. Fig. 9 demonstrates the galvonostatic profiles for the full cells based on the graphite anode without and with 20 nm itaconate layer.

As seen in Fig. 9a, 20

nm SI template contributes to an improvement of the first coulombic efficiency and the cell capacity. This is attributed to the relatively low irreversible capacity of the graphite anode by SI wrapping as obtained in Fig. 4. Not only that, the cycle life of the cell with 20 nm SI-decorated graphite anode is also remarkably enhanced (Fig.9b). After 400 deep charge-discharge cycles, the full cell with bare graphite retained only 72.4% of its initial capacity.

By comparison, the full cell with the 20 nm

SI-decorated graphite anode shows a capacity retention of 92.2%.

Shift of the

discharge profile end toward the left id related to the rapid consumption of reversible Li ions for the cell with the bare graphite (Fig.9c). This phenomenon is effectively suppressed by the 20 nm SI layer applied on the graphite as show in Fig. 9d.

The

cycle-life is therefore dramatically extended. This result intuitively illustrates the lithium inventory loss is successfully suppressed by the concept of elastic SEI layer. Based on the discussions above, a schematic illustration of the role of the SI template layer on graphite surface is depicted in Fig. 10.

Compared with the natural

grown SEI on the bare graphite, which is always incomplete, brittle, and easy to peel off, the electrochemical enhancement resulted from the SI template is ascribed to



Page 20 of 40

following reasons: (1) in-situ development of an elastic SEI layer via self-polymerization of the SI template enhances the SEI stability in electrochemical cycles and even under high temperature condition; (2) with enhanced flexibility of the SEI layer, mechanical failure of the SEI aroused from the large repeated volume changes of the graphite particles due to lithium intercalation and extraction is effectively controlled or alleviated; (3) lithium inventory loss is greatly reduced during long-term electrochemical cycles; (4) the growing impedance of the graphite anode with electrochemical cycles due to the SEI growth is effectively controlled. From all these aspects, the new strategy of designing robust SEI by in-situ polymerization between unsaturated carbon bonds provides a novel avenue for realizing next-generation LIBs of long cycle-life and high reliability.

4. Conclusion In summary, constructing elastic SEI film on graphite surface is of great significance to suppress the lithium inventory loss in the state-of-the-art LIBs.

A

functional organic salt, SI is evenly applied onto a natural graphite surface, which acts as a template for SEI formation. Polymeric network is generated via in-situ self-polymerization between the carbon double bonds of the itaconate.

Inorganic

components coming from the electrolyte decomposition and the carboxyl groups are intrinsically bonded to polymeric network.

The composite SEI with improved

elasticity effectively avoids the rapture or damage due to the volume change induced by Li intercalation and deintercalation.

The electrochemical properties of the


Page 21 of 40 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


graphite anode is significantly improved in terms of the first columbic efficiency, rate performance, and cycling behavior. is greatly prolonged.

Cycle-life of the full cell with LiFePO4 cathode

This elastic SEI construction on graphite provide a new way

for substantially prolonging the life-span of LIBs in the future.

ASSOCIATED CONTENT Supporting Information Available: [Sodium Itaconate and its in-situ polymerization on graphite surface acting as the SEI template in electrochemcial process]

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Qunting Qu) *E-mail: [email protected](Honghe Zheng) ORCID Qunting Qu: 0000-0003-2590-2695 Honghe Zheng: 0000-0001-9115-0669 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are greatly indebted to the Natural Science Foundation of China



Page 22 of 40

(NSFC, contract no. 21875154 and 21473120) and the Ministry of Science and Technology of the People's Republic of China (2015AA034601).

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Page 28 of 40

Table 1. The first charge-discharge capacity and the first coulombic efficiency for the graphite anodes with different thicknesses of the SI nano-layer

Samples

Charge capacity

Discharge capacity

Irreversible capacity

First coulombic

(mAh g-1)

(mAh g-1)

(mAh g-1)

efficiency (%)

Bare NG

413.1

362.3

52.8

87.7

NG@SI-2%

406.2

363.4

42.8

89.46

NG@SI-4%

403.6

367.2

36.4

90.98

NG@SI-6%

413.9

371.8

41.1

89.82


Page 29 of 40

Fig. 1 500

NG@SI-6% NG@SI-4% NG@SI-2% NG JCPDS NO.75-1621

(a)

Intensity / a.u.

400

300

200

100

0 20

30

40

50

60

70

80

2θ/degree

SI NG@SI-6% NG@SI-4% NG@SI-2% NG

(b)

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


500

1000

1500

2000

2500

3000

3500

4000

-1

Wave number (cm )

Fig.1 XRD patterns (a) and FTIR spectra (b) of the graphite anodes with different thicknesses of the SI nano-layer: 0 nm (0 wt%), 10nm (2 wt%), 20nm (4 wt%), and 30nm (6 wt%), respectively.



Page 30 of 40

Fig. 2 (a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig.2 SEM (Up) and TEM (down) images of the graphite anodes with different thicknesses of the SI nano-layer, (a) (e): 0 nm (0 wt%); (c) (g): 20nm (4 wt%) and (d) (h): 30nm (6 wt%).


(b) (f): 10nm (2 wt%);

Page 31 of 40 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


Fig. 3 Graphite particle

Graphite electrode

Na Mapping

Na Mapping

Fig.3 EDX mappings of the graphite with 20nm SI (4wt%) nano-layer before and after the electrode processing.



Fig. 4 (a) NG NG@SI-2% NG@SI-4% NG@SI-6%

2.0

Voltage (V)

1.5

1.0

0.5

0.0 0

100

200

300

400

Specific Capacity (mAh/g)

20

(b)

NG NG@SI-2% NG@SI-4% NG@SI-6%

10

dQ/dV (Ah/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

Page 32 of 40

0 0.00

-10

-0.02

-0.04

-0.06

-20

-0.08

-0.10

-30 0.0

-0.12 0.6

0.2

0.4

0.7

0.6

0.8

0.8

0.9

1.0

Voltage (V)

Fig.4 the electrochemical properties of the graphite anodes with different thicknesses of the SI-template layer. (a) The initial charge–discharge profiles, (b) differential capacity plots (dQ/dV vs. electrode potential) at the first cycle,


Page 33 of 40

Fig. 5 110

(a)0.1C

(a)

0.2C 0.5C

2C

1C

5C 10C

(a)

0.1C

50C,30C,20C,10C,5C,2C,1C,0.5C,0.2C,0.1C

(b)

2.0

20C 30C

90

Voltage (V)

Capacity Retention (%)

100

50C

80

1.5

Bare NG 1.0

70 NG NG@SI-2% NG@SI-4% NG@SI-6%

60

50

0

5

0.5

10

15

20

25

30

0.0

35

0

100

Cycle Number

2.0

200

300


50C,30C,20C,10C,5C,2C,1C,0.5C,0.2C,0.1C

(c)

40

-ZIm(Ohm)

1.5

NG@SI-4% 1.0

(d)

30

20

10

0.5


0 0.0

400

50

(b)

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


0

100

200

300

400


0

10

20

30

40

ZRe(Ohm)

Fig.5 (a) Rate capability; (b) discharge curves of the bare graphite anode; (c) discharge curves of the graphite with 20nm SI ( 4wt%) nano-layer, and (d) the Nyquist plots of the graphite anodes with different thicknesses of the SI nano-layer.


50

60

70


Fig. 6 120

(a)

Capacity Retention(%)

100

80

60

40 NG NG@SI-2% NG@SI-4% NG@SI-6%

20

0

0

50

100

150

200

Cycle Number NG NG@SI-2% NG@SI-4% NG@SI-6%

(b)

140

Capacity Retention (%)

120 100 80 60 40 20 0

0

50

100

150

200

600

800

Cycle Number

300


(c)

200

-ZIm(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

100

0

0

200

400

ZRe(Ohm)

Fig.6 Cycling behavior of the graphite anodes with different thicknesses of the SI nano-layer (a) at room temperature, (b) at 60 oC, and (c) Nyquist plots after 200 cycles at 60 oC.


Page 34 of 40

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Fig. 7 (a)

(e)

(i)

(b)

(f)

(j)

(c)

(g)

(k)

(d)

(h)

(l)

Fig.7 SEM images of the graphite anode with different thicknesses of SI nano-layer (a, b, c, and d) after the cell formation and (e, f, g, and h) after 200 cycles at room temperature and (I, j, k, l) after 200 cycles at high temperature condition of 60 oC. From top to the bottom: 0nm (0 wt%), 10nm (2 wt%), 20nm (4 wt%), and 30nm (6 wt%) , respectively.



Fig.8

Intensity / a.u.

Experiment Fitting C-C C-O C=O CO3

282

284

286

288

290

(b)

(b)

Experiment Fitting C-C C=C C-O C=O CO3

Intensity / a.u.

(a)

282

292

284

(c)Experiment

284

288

290

(d)

(d)

286

288

292

Experiment Fitting C-C C=C C-O C=O CO3

Intensity / a.u.

Fitting C-C C-O C=O CO3

282

286

Binding energy / eV

Binding energy / eV

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 36 of 40

290

292

Binding energy / eV


282

284

286

288

Binding energy / eV

290

292

Page 37 of 40

Experiment Fitting LiF P-F


(f) Intensity / a.u.

Intensity / a.u.

(e)

682

684

686

688

690

682

684

Binding energy / eV

686

688

690

692

Binding energy / eV Experiment Fitting LiF P-F


(h) Intensity / a.u.

(g) 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


682

684

686

688

690

Binding energy / eV

682

684

686

Binding energy / eV

Fig.8 XPS spectra of the graphite anodes without and with 20 nm (4wt%) SI layer : (a) and (b) C1s spectra after the cell formation, (c) and (d) C1s spectra after 200 electrochemical cycles, (e) and (f) F 1s spectra after the cell formation, (g) and (h) F1s spectra after 200 cycles.


688

690


Fig.9 4.5

105

(a)

NG//LFP NG@SI-4%//LFP

(b)

100


4.0


Voltage (V)

95 3.5

3.0

2.5

90 85 80 75 70

2.0

65 1.5 -20

0

20

40

60

80

100

120

140

160

180

60

0

50

100

150


3.2 3.1

NG//LFP

NG@SI-4%//LFP

3.0

2.8 60

80

400

100

1st 40th 80th 120th 160th 200th 240th 280th 320th 360th 400th

3.1

2.8 40

350

3.2

2.9

20

300

3.3

2.9

0

250

(d)

3.4

Voltage (V)

1st 40th 80th 120th 160th 200th 240th 280th 320th 360th 400th

3.3

3.0

200

Cycle Number

(c)

3.4

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

Page 38 of 40

120


0

20

40

60

80


Fig.9 The electrochemical performances of the full cell with LiFePO4 cathode and the graphite anode without and with 20 nm SI (4 wt%) nano-layer: (a) the initial charge-discharge profiles; (b) cycling behavior; (c) discharge curves of the cell with bare graphite and (d) discharge curves of the cell with the 20nm SI-wrapped graphite at different cycles.


100

120

Page 39 of 40 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


Fig.10

SEI template

In-situ polymerization

Development of elastic SEI

Naturally grown SEI

Fig.10 Schematic illustration of the role of the SI template layer on the graphite surface.



TOC/Abstract Graphic

Unsaturated polymerization

Liquid coating

105


100 95


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 40 of 40

90 85 80 75 70 65 60

Bare graphite

Hydrogen

0

50

100

150

200

250

Cycle Number

Carbon

Sodium

Oxygen


300

350

400

In-Situ Development of Elastic Solid Electrolyte Interphase via Nano

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