A film maturation process for improving the cycle life of Si-based anodes for Li-ion batteries

doi:10.1016/j.elecom.2015.10.014

Electrochemistry Communications

Volume 61, December 2015, Pages 102-105

https://doi.org/10.1016/j.elecom.2015.10.014 Get rights and content

Highlights

•
Storing Si–C–CMC electrode in humid air for a few days improves its cycling behavior.
•
The ester bonds between Si and CMC are partially converted into hydrogen bonds.
•
The covalent/hydrogen bond ratio impacts on the electrode cracking behavior.

Abstract

This study shows that storage for a few days in humid air before cell assembling of Si-carboxymethyl cellulose (CMC) composite electrode prepared with a slurry buffered at pH 3 has a major positive impact on its cycle life and coulombic efficiency. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy analysis shows that water molecules in humid air partly convert the ester bonds between Si particles and CMC binder into less rigid hydrogen bonds. Complementary to cycling tests, scanning electron microscopy (SEM) observations suggest that the mechanical integrity of the film is better maintained for an optimal ratio of ester bonds to hydrogen bonds between Si particles and the CMC chains. Such a favorable impact of storage in humid air on the cycling behavior of a composite electrode for lithium battery was unexpected when compared to standard practices that show a detrimental aging of active electrode materials when exposed to water.

Graphical abstract

Introduction

Silicon is a very attractive active material for Li-ion battery anodes due to its ~ 10 times higher gravimetric capacity and ~ 3 times higher volumetric capacity than conventional graphite anodes. However, obtaining Si-based anodes with long cycle life is very challenging due to the large volume change of Si upon cycling (~ 280% from Si to Li₁₅Si₄) [1]. The role of the binder is very critical for Si electrodes to maintain the electronic network in the composite electrode despite the large Si volume change and thereby to achieve long cycle life [2].

Although carboxymethyl cellulose (CMC) is not an elastomeric binder, it has been shown to significantly improve the cycling performance of Si electrodes. The efficiency of CMC can be attributed to its extended conformation in solution that facilitates a networking process of the conductive additive and Si particles during the elaboration of the composite electrode [3]. Moreover, hydrogen bonding between carboxyl groups in CMC and hydroxyl groups on the Si surface may exhibit a self-healing behavior that would be favorable to the long-term cycling stability [4], [5]. When a pH 3 buffer solution is used for the electrode preparation, i.e. at a pH value lower than the isoelectric point (IEP) of Si particles (3.5) and pKa of CMC (3.5), a grafting esterification reaction between SiOH groups present at the surface of Si particles and COOH groups of CMC is greatly favored upon the electrode drying procedure [6]. Such a covalent bonding at the Si/CMC interface may also be achieved through high temperature drying [7]. As a result, the mechanical strength of particle to particle Si/carbon black (CB) contacts is assumed to be stronger than for Si/CMC/CB composite electrodes prepared in neutral water, which could explain the improvement in the electrode cycle life [6]. However, it is generally assumed that only the hydrogen-type Si–CMC interaction (i.e. not the covalent bonds) may allow the preservation of the network through the self-healing process [4], [5]. It was also demonstrated by comparing binders having different types of interactions with Si (no/weak, covalent, or self-healing bonds) that self-healing effect is most critical for both cycling stability and high initial Coulombic efficiency [8].

In the present study, it is shown that the storage for a few days in humid air before cell assembling of the Si–C–MC composite electrode (prepared at pH 3) has a major positive impact on the electrode cycle life. A film maturation mechanism based on the conversion of some covalent bonds between Si and the CMC binder into hydrogen bonds, leading to an optimal ratio of ester bonds and hydrogen bonds, which apparently optimizes the mechanical properties of the film, is proposed to explain the improvement of the electrode cycle life.

Section snippets

Experimental

Ball-milled Si (325 mesh Si powder milled for 40 h using an Union Process attritor) was used as active material, with carbon black (Super P grade, Timcal) as conductive agent, and carboxymethyl cellulose (CMC) (Aldrich, DS = 0.7, Mw = 90,000) as binder. Details on the morphological, structural, and chemical characteristics of ball-milled Si powder are presented elsewhere [9]. 200 mg of Si/C/CMC materials in a mass ratio of 80/12/8 were mixed in 0.5 mL of a pH 3 buffered solution (0.173 M citric acid + 0.074

Results and discussion

Fig. 1A shows the evolution with cycling of the discharge capacity of the Si electrodes depending of their storage conditions. The corresponding Coulombic efficiencies (CE) are shown in Fig. 1B. After 6 days of storage in humid air, a significant improvement of the electrode cycle life is observed with a remaining capacity after 50 cycles of 2500 mAh g^− 1 compared to ~ 750 mAh g^− 1 for the no-stored and air-stored electrodes. Note that storing the electrode in humid air for a longer period of time does

Conclusion

This study showed that storing Si/C/CMC composite electrodes in humid air for a few days significantly improves their electrochemical performance. Such a result was unexpected because storage in humid air of electrodes is generally detrimental to their cycle life [14], [15]. It was shown that during this storage step, some of the ester bonds between Si particles and CMC binder are converted into less rigid hydrogen bonds. This may increase the deformability of the electrode, which can better

Conflict of interest

There is no conflict of interest.

Acknowledgements

The authors thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for supporting this work.

References (15)

D. Mazouzi et al.
Critical roles of binders and formulation at multiscales of silicon-based composite electrodes
J. Power Sources
(2015)
B. Lestriez et al.
On the binding mechanism of CMC in Si negative electrodes for Li-ion batteries
Electrochem. Commun.
(2007)
M. Cuisinier et al.
Moisture driven aging mechanism of LiFePO₄ subjected to air exposure
Electrochem. Commun.
(2010)
J. Ma et al.
Structural and electrochemical stability of Li-rich layer structured Li₂MoO₃ in air
J. Power Sources
(2014)
M.N. Obrovac et al.
Alloy negative electrodes for Li-ion batteries
Chem. Rev.
(2014)
J.S. Bridel et al.
Key parameters governing the reversibility of Si/carbon/CMC electrodes for Li-ion batteries
Chem. Mater.
(2010)
C. Wang et al.
Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries
Nat. Chem.
(2013)

There are more references available in the full text version of this article.

Cited by (21)

Silicon-based electrodes formulation in buffered solution for enhanced electrode-electrolyte interfaces
2021, Journal of Power Sources
Citation Excerpt :
They also showed a stronger carbon-CMC film adhesion with a silicon wafer surface at neutral pH, which indicates that silicon/CMC interaction could be maximized by limiting CMC interchain interactions. Also, enhanced coulombic efficiency and cyclability was observed by modulating the ratio hydrogen/covalent bond between silicon particles and binder, through a post treatment called maturation (80% of humidity), the electrode showed an optimum between stiffness and flexibility maintaining electronic contacts and buffering the volumetric changes [16]. Finally, the pH = 3 formulation showed an impact on the reorganization of the polymer at the contacts points between silicon particles and CC limiting electrode delamination, and promoting the formation of a thinner and less resistive SEI [17,18].
Silicon nanoparticles based composite electrodes for Li-ion batteries reach high specific capacities and enhance cyclability compared with larger silicon particles. Such electrodes are foreseen for the next generation Li-ion batteries (LiB). Playing on binder and conductive additives in the formulation of the Si based electrodes is a well-known strategy to enhance performance. Aqueous electrode formulations using carboxymethyl cellulose as environmentally friendly binder has already showed great cyclability enhancements, especially when employed in acidic conditions. In this study, pH = 1, 3 and 7 buffered solutions were studied as solvent to prepare the Si/C/CMC composite electrodes.
The influence of the formulation pH on the electrode components interactions were followed through a combined ATR-FTIR/XPS experiment and discussed in relation with the electrode electrochemical performance. Interestingly, the pH = 7 buffered solution show increased capacity retention and coulombic efficiency during 100 cycles compared with the electrode obtained in acidic conditions. Post mortem analysis and EIS study highlighted differences of electrode/current collector (CC) adhesion and SEI deposition.
Towards efficient binders for silicon based lithium-ion battery anodes
2021, Chemical Engineering Journal
Silicon (Si)-based anodes have been intensively pursued as one of the most promising candidates for next-generation high-capacity electrodes in lithium-ion batteries (LIBs) due to their ultra-high theoretical capacity. However, huge volumetric change associated with the insertion and extraction of lithium-ions is a major impediment to the deployment of Si anodes, which result in the pulverization of Si particles and the loss of electric contact, as well as parasitic side reactions; these transformations cause poor cyclic stability and fast capacity fading. Binders, with minor content in electrode systems, have demonstrated their pivotal role in electrodes, particularly in Si-based anodes. Present review aims at providing a thorough analysis of existing and emerging binders, which are either synthetic or biomass polymers, for Si-based LIB anodes in terms of their structures, properties, advantages, limitations, working principles, design rules, as well as associated electrochemical mechanisms to address the challenges of Si-based anodes. On the basis of current progress, a short perspective is provided on existing challenges and research directions on Si-based anodes for commercialization in the near future.
Effects of polymeric binders on the cracking behavior of silicon composite electrodes during electrochemical cycling
2019, Journal of Power Sources
Citation Excerpt :
During lithiation/delithiation, the volumetric expansion/contraction of Si particles causes microstructural changes in Si composite electrodes. Recent research has found that Si composite electrodes, even made with Si nanoparticles, crack at the micro-meter length scale during electrochemical cycling [20–26]. Moreover, binders influence the cracking features, such as crack density and crack spacing, of Si composite electrodes [20,21,23].
Mechanical degradation caused by lithiation/delithiation-induced stress and large volume change is the primary cause of fast capacity fading of silicon (Si)-based electrodes. Although intensive efforts have been devoted to understanding electromechanically induced fractures of electrodes made of Si alone (e.g., Si particles, Si thin films, and Si wafers), the cracking behavior of Si/polymeric binders/carbon black composite electrodes is unclear and poorly understood. Here, we investigate, by in situ and ex situ techniques, the cracking behavior of Si composite electrodes made with different binders, including polyvinylidene fluoride (PVDF), sodium-alginate (SA), sodium-carboxymethyl cellulose (Na-CMC), and Nafion. We found that extensive cracks form during the 1st delithiation process, which periodically open and close during subsequent lithiation/delithiation cycles at the same locations in the Si composite electrodes made with SA, Na-CMC, and Nafion. In contrast, a significantly fewer number of cracks form in the Si/PVDF electrodes after electrochemical cycling. A possible mechanism is proposed to help understand the effects of binders on the cracking behavior (e.g., crack spacing and island size) of Si composite electrodes. We also suggest possible approaches, including reducing the electrode thickness, patterning electrodes, and using highly recoverable binders, to inhibit cracks and improve the mechanical integrity of Si composite electrodes.
Interactions of silicon nanoparticles with carboxymethyl cellulose and carboxylic acids in negative electrodes of lithium-ion batteries
2019, Journal of Power Sources
Citation Excerpt :
Our results suggest that a major fraction of the chemically bound species on the Si surface is attributed to the buffer rather than the binder. It should be highlighted that each of the reactions discussed above, namely pre-SEI coatings of carboxylic acids [13] or CMC-Na/CMC-H [8], crosslinking [17,39] and binder grafting [19,20], have the potential to positively affect the cycle life and capacity retention of Si electrodes. The slurry preparation itself, however, gives no control over the reactions occurring, although more control could probably be achieved by further optimizing the concentration of the buffer and the pH of the solution.
Slurry preparation in a citric-acid-buffered aqueous solution at low pH has been established as a viable strategy for tackling the poor capacity retention of silicon electrodes. A number of studies ascribed the improved capacity retention to the formation of a silyl ester between the Si surface and carboxymethyl cellulose (CMC-Na). Most recent findings suggest that the citric acid itself interacts with the Si surface. Moreover, cross-linking reactions between the carboxylic acid and the binder can occur. In order to provide a comprehensive overview and to gain a better understanding of the reactions on the Si surface during slurry preparation, we review here previous results and interpretations and revisit earlier infrared (IR) studies, whose findings we link to our own IR studies of the impact of the slurry components, individually and combined. Specifically, we studied the interactions between the carboxylic acid, CMC-Na and Si particles, with the aim to clarify the effects of different amounts of carboxyl groups in carboxylic acids, namely glycolic, malic and citric acids with 1, 2 and 3 carboxyl groups, respectively. Furthermore, we demonstrate that the capacity retention of Si electrodes can be improved considerably with any of the acids studied.
CMC-citric acid Cu(II) cross-linked binder approach to improve the electrochemical performance of Si-based electrodes
2019, Electrochimica Acta
Maturation, which consists of storing the electrode under humid atmosphere before drying and cell assembly, improves very significantly the cyclability of silicon-based composite electrodes made with an acidic binder. During maturation, the atmospheric-induced corrosion of the current collector releases oxidized copper species that migrate into the electrode to physically crosslink the binder phase, substantially modifying its resiliency to the silicon volume variation. The pre-addition of a small amount of copper salt in the electrode slurry allows to reach similar improvement of electrochemical performance than maturation.
One-step combustion method for pomegranate Si/Ni compostie anode
2018, Materials Letters
Silicon is considered as a promising candidate for next-generation lithium-ion battery anodes due to its high theoretical capacity and low charge potential. However, large volume change during Li-ion insertion and extraction hinders its practical application. In this work, we develop a facile approach to fabricate pomegranate Si/Ni composite to solve the aforementioned problem. The pomegranate composites are linked together by Ni nanofoams, which can be directly converted into electrode by simple mechanical compression without the use of current collector, conductive agent, and binder. The pomegranate Si/Ni composite exhibits good cycling stability after 1000 cycles and a high reversible capacity of 0.84 mAh. This combustion method is a facile and economical approach, which provides a new idea for the commercial application of Si anode materials.

View all citing articles on Scopus

View full text

A film maturation process for improving the cycle life of Si-based anodes for Li-ion batteries

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Conflict of interest

Acknowledgements

J. Power Sources

Electrochem. Commun.

Electrochem. Commun.

J. Power Sources

Alloy negative electrodes for Li-ion batteries

Chem. Rev.

Key parameters governing the reversibility of Si/carbon/CMC electrodes for Li-ion batteries

Chem. Mater.

Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries

Nat. Chem.