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A New Avenue for Limiting Degradation in NanoLi4Ti5O12 for Ultrafast Charge Lithium-Ion Batteries: Hybrid Polymer-Inorganic Particles Jean-Christophe Daigle, Yuichiro Asakawa, Mélanie Beaupré, René Vieillette, Dharminder Laul, Michel L. Trudeau, and Karim Zaghib Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03119 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Nano Letters

A New Avenue for Limiting Degradation in NanoLi4Ti5O12 for Ultra-fast Charge Lithium-Ion Batteries: Hybrid Polymer-Inorganic Particles

Jean‐Christophe Daigleac, Yuichiro Asakawabc, Mélanie Beaupréac, René Vieilletted, Dharminder Laulad, Michel Trudeaud and Karim Zaghibac* a

Center of Excellence I transportation electrification and energy storage (CETEES), Hydro‐

Québec, 1800, Lionel‐Boulet blvd., Varennes, Qc., Canada, J3X 1S1 b

Sony Corporation, 1‐7‐1 Konan, Minato‐ku, Tokyo, 108‐0075, Japan

c

Esstalion Technologies Inc., 1804, Lionel‐Boulet blvd., Varennes, Qc., Canada, J3X 1S1

d

Institut de recherche d’Hydro‐Québec (IREQ), Unité science des matériaux, 1800, Lionel‐

Boulet blvd., Varennes, Qc., Canada, J3X 1S1 Abstract

Lithium titanium oxide (Li4Ti5O12)-based cells are a very promising battery technology for ultrafast charge-discharge and long-cycle-life batteries. However, surface reactivity of lithium titanium oxide in the presence of organic electrolytes continues to be a problem that may cause expansion of pouch cells. In this study, we report on the development of a simple and economical ``grafting to`` method for forming hybrid polymer-Li4Ti15O12 nanoparticles, which 1

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can be successfully applied in lithium-ion batteries. This method utilizes a low-cost and scalable hydrophobic polymer that is applicable in industrial processes. The hybrid materials demonstrated exceptional capability for preventing degradation of cells in accelerated aging, and operating over 150 cycles at 1C and 45oC.

Keywords: Lithium-ion battery, hybrid nanoparticle, energy storage, lithium titanium oxide

Introduction

One of the promising approaches to counter global warming is to integrate alternate and greener sources of energy (wind, solar, etc.) into national power grids. However, as the production of electricity from these sources fluctuate, an economic way to store electricity is required. Therefore, the introduction of energy storage from wind power, solar plants, etc. requires a new generation of batteries in order to optimize the integration of these alternate sources. To do so, the development of a battery with a high rate of charge and discharge is necessary and the operation temperature must be sufficiently wide for deployment under different climatic conditions. Moreover, it is necessary to develop a battery with a long-cycle-life to optimize returns on investments for customers.1, 2

Lithium titanium oxide (LTO) is a rising candidate as an anode material for high-rate charge-discharge batteries. The absence of electrode expansion after numerous charge-discharge cycles makes it a perfect candidate for a long cycle-life anode.3, 4 This was demonstrated through

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the efficient use of carbon-coated LiFePO4 and carbon-coated LTO cylindrical 18,650 size-cells at sub-zero and higher than 60oC temperatures.5, 6 However, LTO has a very reactive surface and can induce electrolyte degradation upon cycling. During battery operation, the lithium titanium oxide and electrolytes based on carbonate derivatives react with residual water to form gases. These products cause the pouch cell to expand, which could be a safety issue.7, 8 The water is a residual contamination from the cathode, especially when using olivine-based cathode materials or a water-based binder.9

A common way of limiting degradation is to employ a protective coating at the electrode interface. This coating, referred to as the Solid-Electrolyte-Interphase (SEI), prevents contact between the electrolyte and the active surface of the electrodes. For example, the decomposition of vinylene carbonate as an additive in the electrolyte forms a film. A myriad of compounds was tested for creating the SEI. However, this technique is based on the decomposition/degradation of single organic molecules to form the SEI during the operation of the cell.10

We have

previously demonstrated the use of polymers as the SEI by a new strategy such as ring opening polymerization of propylene carbonate on the surface of LTO.11 This approach will pave the way to identifying well-defined polymeric film coatings.

We propose to use hydrophobic poly(styrene)-based polymers to form the shell on the particle. A new simple method "grafting to" for anchoring the polymer on the surface of LTO was developed. The particles will be protected by a shell that is able to limit the degradation of the electrode and battery by stabilizing the electrode resistance during accelerated aging and

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increasing retention capacity. The polymer shell minimizes the undesirable reaction between electrolyte and residual water at the particle surface, which generates the formation of gases.

Results and Discussion

Formation of hybrid polymer-inorganic particles Generally speaking, the absorption of polymers on the surface of particles is an important topic because of its role in improving the stability of minerals in water or organic solvents. The absorption is due to the affinity of the polymer with the particle surface and the interfacial energies of mineral/solvent, solvent/polymer and polymer/mineral. However, most of the dispersions are done in water because the difference between interfacial energies (water/minerals) allows better stabilisation of the slurry. These strategies resulted in the development of specific polymers together the mineral or/and the solvent.12-15 In this case, absorption is only physical and cannot resist mechanical treatment. This approach is difficult to implement during electrode fabrication because the process involves several mechanical manipulation steps that may alter the interfacial equilibrium of the film.

Other methods consist of attaching the polymer to the particle surface by covalent bonding.16 The polymer is anchored on the particle surface, which improves the stability of the shell over aggressive mechanical and chemical manipulations.17 Usually, these methods require modification of the mineral surface to increase the affinity with the polymers. In other words, the surface must be more "organic" in order to improve the bonding between the polymer and the

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mineral. The drawback is that slight modifications of the mineral can modify its desirable physical properties. We divide this approach into two well-known methods such as ``grafting from`` and ``grafting to``. The method ``grafting from`` was widely used to decorate the surface of silica wafers or nanoparticles.16, 18, 19 This method allows good control of the grafting density, but needs strict reaction conditions and modification of the surface before polymerization.20, 21 Although, most of the publications report on the use of controlled radical polymerization to produce the desirable properties of the shell, these methods are difficult to scale-up for industrial applications.15, 22, 23

SiO2 nanoparticles grafted with poly(ethylene oxide) chains were successfully used as a Solid Polymer Electrolyte (SPE) in the Li-Metal-Polymer Battery. This material exhibited a remarkable ionic conductivity of 1.2 x 10-3 Scm-1 at 60oC.24 Also, it was reported that a poly(pyrrole) coating on Si nanoparticles (SiNPs) has a beneficial effect towards limiting the volume change during the cycling of the cells. The cells became more durable due to an increased cyclability and retention capacity.25 Similar results were also reported by Assresahegh and Bélanger for SiNPs grafted with poly(acrylic acid) using atom-transfer radical polymerization (ATRP).17 However, no instance of polymers grafted on LTO was reported in the literature.

We propose a simple method to bond the shell to the surface based on a ``grafting to`` method. This method does not require pre-modification of the surface, and the polymer is formed beforehand and grafted on the unmodified surface of the particle with a strong covalent bond that is resistant to the aggressive environment in the battery. This procedure is carried out under mild

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reaction conditions, but does not produce a uniform graft. Because the surface of LTO had a high density of hydroxyl groups, it was possible to promote a reaction between the polymer and these functional groups. The poly(styrene) polymer was selected for this application because its hydrophobicity prevents contact between the residual water from the electrolyte and the LTO surface. Moreover, because it is prepared by free radical polymerization and is compatible with a SBR/CMC binder for electrode manufacturing, an advantage of poly(styrene) is a low-cost and ease of scale up for industrial production. We can also incorporate the graft of 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) in order to improve the affinity of the polymer with the active materials, and to form in-situ a second shell during the operation of the cell.11 Scheme 1 highlights the formation steps of the hybrid polymer inorganic particle.

Scheme 1. Reaction steps for forming hybrid polymer inorganic particles

The first step involves the free radical copolymerization of styrene and vinyl benzyl chloride. The molecular weight of the polymers and PDIs was determined by GPC in THF at 40oC (See Figure S1 in SI and Table 1). The ratio between styrene and vinyl benzyl chloride in the 6


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copolymer microstructure was calculated from the NMR 1H spectrum (See Figure S2 in SI and Table 1) and was determined to be approximately 50 mol% for enhancing the probability of grafting. Low molecular weight polymers were selected in order to enhance the diffusion of the polymer in the solution, and therefore, grafting on LTO was more favorable, as demonstrated before by many groups showing the preferential grafting of low molecular weight polymers.13, 18 In step 2, which is optional, nucleophilic substitution of DBU on the methyl chloride moieties is done using a strong base reported by Ochiai et al.26 The DBU is efficient for ring-opening polymerization of propylene carbonate, which is used in the electrolyte. For this reason, we expect the in situ formation of a second layer of polymer on the LTO surface. Moreover, the DBU makes the polymer more polar, which is favorable for absorption on the polar LTO surfaces. The amount of DBU grafted was not determined with precision because of poor solubility of polymers in any solvent used to isolate it from the reaction medium. (step 2, intra cross-linked occurs). However, the presence of DBU was confirmed by FTIR (Figure S3 in SI) by the presence of the distinctive signal at 1630 cm-1 which is correlated with C=N from DBU moieties.26 The reaction product was directly used in step 3 without any purification; this avoids intra cross-linking formation between DBU and chloride moieties. Step 3 allows easy grafting of the polymer on LTO using Williamson ether synthesis with lithium hydroxide in a mixture of THF and DMF at reflux for 48 hours; C-O ether bonds were formed on the surface by the reaction between –OH groups from the LTO and chloride moieties from the polymer. The method involves polymer grafting by a "grafting to" method which permits a less dense and a non-uniform grafting to the LTO surface. This method was carefully chosen because it was imperative to permit lithium ions to efficiently diffuse during cell operation; densely grafted polymer can hinder the insertion-deinsertion of lithium ions. The ratio between the polymer and

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LTO is directly connected to the amount of polymer anchored on the composite (Ratio P/LTO in Table 1). Also, based on entries PS1 and PSD2, the DBU positively contributes towards enhancing the grafting for the same ratio of polymer-LTO (P/LTO). This method is efficient for molding the polymer shell, and the residual polymer solution recovered after step 3 can be recycled to use for grafting again.

Table 1 reports the physical characteristics of the hybrid particles selected for this study. The particle with a shell poly(styrene-co-vinyl benzyl chloride) is designated PS, and the particles with a shell of poly(styrene-co-vinyl benzyl chloride-g-DBU) are designated PSD.

Table 1. Physical characteristics of polymers and hybrid particles

a

ID

Polymer

Mn (gmol‐1)a

PDIa

x (mol%)b

Ratio P/LTO

Polymer (wt%)c

PS1 PSD1 PSD2 PSD3

PS

6200 6400 6400 5100

3.3 2.5 2.5 3.0

55 48 48 65

1:2 1:3 1:2 1:1

1.0 1.3 2.5 8.5

PS‐DBU

Determined by GPC in THF at 40oC; b Styrene content, calculated from 1H NMR spectrum; c

Determined by TGA.

The presence of a polymer grafted on LTO was confirmed by TGA, FTIR spectrum, TEM images and XPS analysis. FTIR clearly shows peaks typical of methyl stretching at 2900 cm-1 and C=N stretching from DBU at 1610 cm-1 for samples PSD3 (Figure 1a). The signal at 1210 cm-1 associated with ether bonds is also observed by FTIR. The amount of polymer in the composite after extensive washing was determined by TGA (Figure 1b).

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Figure 1. a) FTIR spectrum of PSD3. b) TGA thermogram of PSD3 under air.

The weight loss between 250-550oC is correlated with the amount of polymer because pure LTO has no weight loss between these temperatures. Also, the surface of pure LTO is oxidised at 800oC and above, therefore a weight increase is observed at higher temperatures. Because the polymer is bonded to –OH groups on the surface, hydrolysis at high temperature did not occur during the analysis of LTO-polymer particles. Consequently, we suspect that the side reaction which occurs on the surface of LTO will be suppressed. The composition of the surface of PSD3 was determined by high resolution XPS (see Figure S4 in SI). The amount of oxygen, 9


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nitrogen, and carbon was 29.3%, 3.0%, and 59.1%, respectively. However, the C1s spectrum shows a percentage of 5.8 % with the peak at 286.4 eV for the C-O/C-N bonds, which is even more remarkable. This value demonstrates covalent bonding of the polymer with the hydroxyl groups from LTO, but probably is an overestimate due to the contribution of C-N bonds from DBU. The images recorded by STEM shows the effectiveness for forming a polymer layer on the nanoparticle surfaces. However, this layer is not uniform as expected (see Figures 2b and 2c), and some of the surfaces are also not covered; PSD1 had coverage of 80% estimated by TEMEELS. In fact, polymer coverage is connected with the method used and the presence of several edges on the LTO particles. The LTO is a micro-particle of 10-20 m dimensions that is composed of an aggregation of 10-20 nm nanoparticles (see Figure S5 in SI) with a surface area of 4 m2g-1. This aggregation has many edges which also induces a non-uniform coverage of the nanoparticles. A nano-layered sized coating is essential for maintaining good ionic and electronic diffusion from the electrolyte to the LTO surface.27 Also, the partial covering by the polymer is helpful for charge-discharge efficiency at 1C and 2C, but is a disadvantage at 4C. This topic will be discussed later.

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Figure 2. STEM Images of PSD1. a) Homogeneous coating. b) Non-homogeneous coating. c) Surface free of coating. d) Agglomeration of polymer on the surface. The star shows an agglomeration of polymers.

Electrochemical Evaluations In order to evaluate the capacity of hybrid LTO that avoids gas evolution in the cell, we performed a float test using half cells (LTO-Li). The electrochemical conditions of the float tests 11


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are described in the Experimental section. The impedance measurements were performed on LTO-Li half cells before and after the float test. Figure 3a and 3b show the first and second cycles of the samples and references. Figure 3a shows the superiority of a water-based binder compared with PVDF for LTO electrodes. Figure 3b shows better capacities for shell-bearing DBU moieties; indeed PS1 has less capacity comparing with hybrid particles bearing DBU moieties. We explain that behavior by the better wettability of those samples. Figure 3c and 3d show the Nyquist plots for PSD1 and PSD3. After the first cycle, before the float test (Figure 3c), the resistance of the PSD1 and PSD3 with SBR/CMC binder is higher compared to the references (commercial bare LTO). The references were made using PVDF and SBR/CMC binders. We explain this behavior by the physical barrier induced by the polymeric shell. However, after the float test, the resistance decreased drastically towards the same level as the references; this phenomenon is probably related to the formation of channels in the polymer which facilitate lithium transport from the electrolyte to the LTO surface. The wetting or swelling of the polymer shells by the electrolyte facilitates the formation of channels and thus the diffusion of lithium ions. PSD3 had a lower resistance compared with PSD1, as observed in Figure 3c and 3d with the high concentration of DBU moieties.

For the Solid Polymer

Electrolyte (SPE) used in Lithium-Metal-Polymer batteries, this "formation" step was previously reported.28

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3.0

PVDF

Voltage (V)

2.5

b)

SBR/CMC

3.0 PS1

2.5

PVDF

1st charge 1st discharge 2nd charge 2nd discharge

Voltage (V)

a)

2.0

1.5

1st charge 1st discharge 2nd charge 2nd discharge

2.0

1.5

0.5

0.5 0

1

2

3

0

4

1

120

d)

100

3

4

25

20

PVDF SBR/CMC PSD1 PSD3

Z'' (Ohm)

80

2

Capacity (mAh)

Capacity (mAh)

c)

PSD1 PSD3 PSD2

1.0

1.0

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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60

PVDF SBR/CMC PSD1 PSD3

15

10

40 5

20

0

0 0

20

40

60

80

100

120

0

Z' (Ohm)

5

10

15

20

25

Z' (Ohm)

Figure 3. a) Voltage profiles of LTO-Li cells (references). b) Voltage profiles of hybrid particles-Li cells. Nyquist plots of LTO-Li half cells before (c) and after float test during 72 hours (d).

These results demonstrate the viability of the concept. Coin cells with carbon-coated LiFePO4 cathodes and LTO anodes were assembled to evaluate the efficiency of the system. Table 2 reports the capacity for the first and second cycles at 0.2C and 25oC. The results for hybrid LTO are the same order of magnitude as the standards, and no major effect of the polymer shell was observed for the charge-discharge efficiency. Only a slight capacity decrease is

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observed after the nominal cycles, which are proportionally to the amount of polymer in the shell.

Table 2. Electrochemical Performances of coin-cells

First Cycle ID

PVDF SBR/CMC PS1 PSD1 PSD2 PSD3

Nominal Cycle

Charge Charge Charge Discharge Charge Discharge discharge discharge capacity capacity capacity capacity efficiency efficiency (mAhg-1) (mAhg-1) (mAhg-1) (mAhg-1) (%) (%) 166.1 168.4 172.5 173.4 166.2 162.9

159.8 165.0 166.8 166.7 160.6 157.1

96.2 98.0 96.7 96.1 96.7 96.4

160.2 165.4 167.3 166.9 161.2 157.5

157.7 164.5 165.9 166.2 159.5 156.5

98.4 99.5 99.2 99.6 98.9 99.4

Figure 4a and 4b show the cells profile after two cycles at 0.2C and 25oC. No major degradation or side reactions occurred after two cycles. More information about the capability of the hybrid particles for preventing degradation were collected using the float test. The float test permits evaluation of the resistance of materials with aging of the cells; the tests at high temperature and constant current promote undesirable side reactions. We can adequately evaluate the propensity of our materials for avoiding those reactions by these tests. The results of the charge-discharge efficiency after the float test are shown in Figure 4c. The diagram bars demonstrate the positive impact of the polymer shell to limit degradation of the cells.

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a)

b)

2.5

2.5

2.0

Voltage (V)

2.0

Voltage (V)

1.5 1st charge 1st discharge 2nd charge 2nd discharge

1.0

SBR/CMC

0.5

1.5 1st charge 1st discharge 2nd charge 2nd discharge

1.0

0.5

PVDF

PSD3 PSD2

PSD1 PS1

0.0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0.5

c)

1.0

1.5

2.0

2.5

3.0

Capacity (mAh)

Capacity (mAh)

100

Charge-discharge efficiency (%)

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95

90

85

80 PVDF

SBR/CMC

PS1

PSD1

PSD2

PSD3

Figure 4. a) Voltage profiles of LFP-LTO cells (references). b) Voltage profiles of LFP-hybrid particles cells. c) Charge-discharge efficiency at 0.2C after the float test at 45oC, 2.4V and 72 hours.

There is no major difference between PS1, PSD1 and PSD2, however the effect of the amount of polymer is slightly noticeable with PSD3. An increased hydrophilicity of the polymer by integrating a large amount of DBU moieties may reduce the repelling effect of the shell to water. This effect can be a cause of degradation. Major degradation is observed for standard LTO with PVDF binder, and the hazardous HF formed by degradation of LiPF6 with water is highly detrimental to the operation of the cell.29-31 Also, the surface of bare LTO can undergo some side reactions that accentuate this phenomenon.7 It appears that the hydrophobic polymer is very 15


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effective for covering the reactive hydroxyl groups on the LTO surface and preventing contact with residual water. However, when a large amount of polymer is present (see for example PSD3), the physical barrier due to the polymer can impede the diffusion of lithium and decrease efficiency after the float test. This effect was observed at 4C with the hybrid LTOs, and the retention capacities during charge and discharge are lower with 2 to 10% polymer (see Table S1 in SI). However, the effect on Li diffusion is negligible compared to the tremendous positive effect of the polymer for preventing degradation during the float test. This is apparent in the measurements recorded at 1C and 2C. The hybrid LTO is still preferable, when compared to the references. SBR/CMC binder appears to be better for preventing degradation, and the best system is composed of hybrid LTO with 1-3 wt% polymeric shell and SBR/CMC binder. Accordingly, a low percentage of polymers is preferable for optimising the efficiency of the cell. Because SBR/CMC is an aqueous binder, it is more eco-friendly compared with classical NMPbased binder electrodes.

In order to evaluate degradation after long cycling, coin-cells composed of PS1, PSD1 and PSD2 (anode) and carbon-coated LFP (cathode) were assembled and cycled at 45oC and 1C for 150 cycles (Figure 5).

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105

Retention capacity (%)

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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PVDF SBR/CMC PS1 PSD1 PSD2

100

95

90

85 0

20

40

60

80

100

120

140

160

Number of cycles Figure 5. Cycle life of LFP-LTO full cells at 45oC and 1C.

The major degradation occurred during the initial cycles. The PVDF reference showed significant degradation, but the hybrid particles are effective in suppressing it. We believe that no further major degradation occurs after the initial cycles. The sample PSD1 shows the best retention capacity, which is in agreement with the result obtained in the float test. The PSD1 has 1.3% more retention capacity compared to the standard LTO and PVDF binder, and 2.6% compared with standard LTO and SBR/CMC binder. The presence of a thin layer of polymer on the surface is advantageous because it prevents degradation without impeding lithium transport. These studies demonstrate that residual water in SBR/CMC binder has an adverse effect after

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extending cycling because more undesirable side reactions occur with aging. However, we notice the limited effect of water-based binders compared with PVDF on the hybrid particles. This result is probably due to the limited contact between the residual water and the particle surface, which is a significant advantage of the hydrophobic coating. In order to fabricate a viable system, we made a compromise between performance and durability by having a fraction of the particle surface exposed to the electrolyte in order to enhance lithium diffusion.

Conclusion In this article, we discussed a new strategy for grafting polymers on the surface of LTO. The principle of grafting is based on the bonding between the hydroxyl groups from the surface and the polymer. This grafting process is compatible with a wide range of particles. Furthermore, it is also appealing because it is compatible with the harsh conditions involved in the fabrication and cycling of cells. The use of poly(styrene), which is a low-cost polymer, permits the production of hybrid particles on a large scale, which is a plus for the battery industry. We demonstrated the effectiveness of using hybrid polymer-inorganic particles as active materials for lithium-ion batteries. Our materials are effective for preventing capacity fading of LTO-based batteries, and, serendipitously, limit gas evolution,32 which is a major drawback for the industrial development of LTO-based batteries. For this reason, this new strategy can pave the way for the development of a new class of materials that prevents degradation caused by reactions between the electrolyte and the active surface. We conclude that the use of very active inorganic particles with a polymer shell offers a viable option for extending the cycle life of lithium-ion batteries.

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Experimental Section General information. Styrene and vinyl benzyl chloride (VBzCl) were passed over a bed of basic Al2O3. Azobisisobutyronitrile (AIBN) was purified by recrystallization in methanol and dried under vacuum for 12 hours. The monomers were used immediately after purification. The carbon-coated lithium iron phosphate (LFP) was purchased from Sumitomo Osaka Cement and the lithium titanium oxide (LTO) from Posco. The PVDF, LIPF6 and the carbonate solvents were obtained from BASF. The carbon Denka Black was obtained from Denka. The SBR latex and CMC were obtained from Zeon Co. and DKS Co. respectively. All other chemicals from Sigma Aldrich were used as-received.

Polymerization of VBzCl and Styrene. 100 mL toluene, 7.2 g. (47 mmol) VBzCl and 5.7 g. (55 mmol) styrene were mixed in a 250 mL round bottomed flask. The solution was stirred for 30 min. under a flow of nitrogen. 500 mg AIBN was added and the solution was heated at 95oC under nitrogen. The flask was connected to a condenser. After 8 hours, the solution was cooled down and slowly poured in 10 volumes of methanol. The polymer precipitated and the supernatant was decanted. The polymer was washed 3 times with methanol, and the polymer was dried at 60oC under vacuum for 12 hours to yield 8.2 g of polymer (yield = 62%) of the copolymer.

Grafting of DBU moieties on polymer backbone. The method was based on the report by Orchiai et al.26 The synthesis was carried out under a nitrogen atmosphere using standard Shlenk techniques. 2.7 mL DBU and 50 mL of dried THF were introduced in a 150-mL Shlenk flask. The flask was cooled to 4oC and stirred for 15 min. 8.6 mL of 2.5M n-BuLi in hexane was added 19


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with a dropper. After completion of the addition, the solution was stirred for 1 hour to 4oC. The flask was warmed to room temperature, then a solution of polymers (10.0 g. of polymers dissolved in 50 ml of dried THF) was added dropwise. The solution was stirred at room temperature under nitrogen for 24 hours. The solution from this step is used directly.

Grafting of the polymer on LTO particles. The solution was poured into the 500 mL round bottom flask containing 100 mL DMF, 2.8 g. LiOH.H2O and 10.0 g. LTO. The flask was connected to a condenser and the slurry was stirred for 48 hours at 70oC. After completion, the slurry is cooled and filtrated on a Buchner funnel. The solid was washed 3 times with methanol and transferred to the 250 mL Erlenmeyer flask containing 100 mL dichloromethane (DCM) to ensure the complete removal of free polymers. The slurry was stirred for 3 hours and filtrated on a Buchner funnel. The solid was washed 3 times with DCM and 3 times with acetone then transferred into the 250 mL Erlenmeyer flask with 100 mL water to ensure the complete removal of free LiOH.H2O. The slurry was stirred for 12 hours and filtrated on a Buchner funnel. The solid was washed 4 times with water and 3 times with acetone, and the final product was dried at 60oC under vacuum for 12 hours.

Experimental Characterization. FTIR measurements were conducted on the CARY 630 from Agilent. The NMR analyses were performed on the Varian Inova 300. The chemical shift of deuterated chloroform was used as internal reference. The TGA analysis was performed with a heating rate of 10oCmin-1 from 25-800oC under air in a TGA 550 (TA Instruments). The micrograph images were recorded with a High-Resolution STEM (Hitachi HD2700 dedicated STEM with CEOS corrector). The molecular weight distribution was determined with a gel

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permeation chromatograph (GPC) equipped with double detection (LS at 690.0 nm and RI), and operating at 40oC with THF as eluent. The system was a combination of Viscotek GPCmax VE2001 GPC auto sampler with a Wyatt Optilab RI detector and Wyatt DAWN EOS Multi Angle Light Scattering detector. The columns were Agilent (Varian) PL-Gel mixed-B 10 mm (heated at 40oC, the same temperature as the detectors). The chemical composition of the surface was investigated by X-ray Photolectron Spectroscopy (XPS) using a PHI 5600-ci spectrometer (Physical Electronics, Eden Prairie, MN). The main XPS chamber was maintained at a base pressure of < 8*10-9 Torr. A standard aluminium X-ray source (Al k = 1486.6 eV) was used to record survey spectra (1400-0 eV, 10min), while pure magnesium was used for high resolution spectra, both without charge neutralization. The detection angle was set at 45º with respect to the surface normal and the analyzed area was 0.05 cm2 (aperture 4). High resolution spectra were obtained for C1s with 40 sweeps.

Cells Assembly and Electrochemical Measurements. LFP-LTO cells were fabricated as described here. The electrode dimension is 30 mm x 40 mm. The cathodes contain carbon-coated LiFePO4 (LFP), carbon black and poly(vinylene difluoride) (PVDF) binder in a proportion of 90: 5: 5. The slurry was coated on 15 m aluminium collector by a doctor blade method. The anodes contain Li4Ti5O12 (LTO) or hybrid-LTO, carbon black, and PVDF binder in a proportion of 90: 5: 5. The slurry was coated on 15 μm aluminium collector by a doctor blade method. The poly(ethylene) separator thickness is 16 m. When SBR/CMC was used as binder, the composition is a proportion of 91: 5: 2.5/1.5. The electrolyte was 1 molkg-1 LiPF6 with carbonates as solvents. Prior to the cycle test, the batteries were charged and discharged twice at 0.2 C at 25 °C (xC is the current that can fully charge/discharge cell capacity in 1/x hour). Other

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test parameters include charge: CC-CV (Constant current constant-voltage) mode; voltage: 2.4 V, current 0.2 C, cut-off current 0.03 mA; discharge: CC (Constant current) mode, cut-off voltage: 0.5 V; current: 0.2 C. To accelerate gas evolution in the cell, we performed a float test at 2.4 V for 72 hours in the climate chamber with the ambient temperature set to 45 °C. The cells were charged at 25 °C to 2.4 V with constant current at 0.2C, and then charged at constant voltage at 2.4 V for 72 hours at 45 °C. For cycle life testing, the cells were charged at 25 °C to 2.4 V with constant current at 0.2C, and then charged-discharged at 1C and 45oC for 150 cycles.

ASSOCIATED CONTENT The supporting information is available free of charge. Additional FTIR, NMR, GPC, XPS spectra, tables, and TEM images are available.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGEMENT This work was supported by Hydro-Québec and Sony. The authors also wish to thank Dr. Pascale Chevalier from Laval University for the XPS analysis.

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Reference

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