The significance of aqueous binders in lithium-ion batteries
Graphical abstract
Introduction
Lithium-ion batteries (LIBs) are the most progressive energy technology, providing the power source for consumer electronics and electric vehicles [1]. The global market for LIBs surpassed USD 44.2 billion in 2020 and is anticipated to increase at a compound annual growth rate of 16.4% by 2025 [2]. The enormous growth of the LIB market is likely to be driven by automobile giants such as Tesla, Nissan, BMW, Ford, Chevrolet, and Toyota, mainly in the United States, the United Kingdom, Germany, and China [3]. With the rapid growth of the battery market, scientific and technological challenges are now being recognized. The energy density and cycle life of the existing LIBs are need to be improved for future electric vehicles application [4]. Moreover, the cost of LIBs based on each unit of energy stored ($/kWh) is still very high. Besides performance and cost, well-publicized incidents, including Galaxy Note 7 battery explosions, Boeing 787 Dreamliner battery failures, and Tesla Model S battery degradation, have also raised concerns over the safety of the batteries [[5], [6], [7]]. Battery failure is the leading culprit behind these accidents, which could have occurred due to weak mechanical design, inadequate pressure seals and vents, imprecise cell fabrication, and electrode component failures [8].
A typical LIB contains several major components: cathode, anode, membrane separator, electrolyte, and current collector, and auxiliary components such as polymeric binders and conductive carbon, as shown in Fig. 1. Battery electrodes (both anodes and cathodes) are generally fabricated by mixing conductive carbon and polymeric binders and subsequently dispersing them in an organic solvent to obtain a viscous slurry. The resulting slurry is then cast onto a metallic current collector using the doctor blade technique. The fabrication process of the electrode slurry has significant influence over electrochemical performance since it determines the distribution of active materials as well as the overall structure of the electrode layers [9]. The optimization of the electrode composition also plays a vital role in manufacturing a high-quality electrode slurry [10].
Polymeric binder is one of the major components in a cell and has been identified as influencing ageing, irreversible capacity loss, and coulombic efficiency (CE) of a cell [[11], [12], [13]]. Binders provide no electrochemical benefits to the electrodes, but rather function to interconnect the active materials and carbon additive and adhere the electrode slurry to the current collector. In general, the primary reason for the rapid degradation of a cell is the loss of lithium and active materials due to dissolution, poor interconnectivity, and electrical breakdown between the electrode components, which inevitably causes the delamination of electrode components from the current collector [14]. A battery management system is usually deployed to monitor the performance and safety of the cell [15]. Electrical contact loss and delamination are generally induced by imprecise electrode fabrication (manufacturing failure), drastic volume expansion, weak mechanical properties of active materials, and dense coating thickness of active materials (exceeding the optimal level of the prescribed limit). Furthermore, self-agglomeration of active materials also triggers the rapid decline in capacity [16,17]. Utilizing a prominent binder with an optimal weight ratio with respect to the active materials can mitigate electrical contact loss and also prevent the self-agglomeration of active materials. Another pivotal role of binders is an ability to create an “artificial solid-electrolyte interface (SEI) layer,” which is beneficial for cycling performance of electrode materials [18]. Moderate binder content (5%) forms a thin and stable SEI layer and maintains the best cycling stability compared to the lower (2%) and larger (10%) binder content [19]. Therefore, choosing an ideal binder that exhibits promising electrochemical performance even with minimal content is a prerequisite in battery technology.
Although few reviews on aqueous-based binders have been reported in the literature [20], [21], [22], [23], [24], [25]; however, the specific focus on the electrochemical performance as well as limitations of PVDF and other aqueous-based binders in association with the most successful electrode materials—such as graphite, Li-rich metal oxide cathodes, and Si anodes—remain elusive. Herein we aim to provide an extensive overview of the advantages and limitations of conventional polyvinylidene fluoride (PVDF) as well as other aqueous-based binders. In the first section, we present a succinct introduction to the fundamentals of binders and their function in LIBs. In the next section, we summarize the advantages and critical limitations associated with PVDF binder. We also discuss the influence of PVDF binder over capacity decay in an electrochemical cell. Next, we discuss the stringent requirements for successful binders in future LIBs. We then highlight the recent advances in electrochemical performance of graphite anodes, Li-rich cathodes, and Si anodes enabled by several aqueous-based binders. Finally, we discuss the challenges and prospects binders present for future battery technology.
Section snippets
Binders
The primary function of binders is to interconnect the active material and conductive additives and adhere the electrode materials to the current collectors, thereby preventing the active material from disintegrating due to chemical and mechanical stress that occurs during the continuous charge/discharge process. The binding function of the polymer matrix to the active materials can be achieved in two different ways: direct binding (covalent) or indirect binding (noncovalent). In direct
History of fluorinated (PTFE and PVDF) binders
In the 1980s, PTFE resin was mainly used as a binder for both anodes and cathodes in LIBs because of its high chemical and thermal resistances and excellent binding properties. The aqueous dispersion of PTFE shows fibrillation characteristics, offering excellent binding capability with electrode materials. However, the excessive fibrillation hinders the formation of homogeneous dispersion, resulting in poor interconnection with the electrode materials. Another difficulty is the incompatibility
PVDF binder failure mechanisms
The rapid capacity loss of electrode materials is generally influenced by numerous factors, such as poor electronic conductivity, large volume expansion, poor mechanical strength, and active material dissolution in the electrolyte. Imprecise cell fabrication and improper operating conditions also provoke capacity loss. Additionally, the inherent limitations of conductive and binder additives are detrimental to the efficiency of batteries. Sanyo Corporation clearly pointed out the technical
Requirements for successful binders in future LIBs
An ideal binder should meet the following stringent requirements to justify its massive adoption in large-scale commercialization for future applications. The chemical stability of the binders, especially at a fully charged state, is considered the foremost requirement for use in LIBs. Binders should be chemically inert to separators, electrolytes, and electrode materials. Any reaction between the binder and other components eventually leads to new complex chemistry that results in degradation
Aqueous-based polymeric binders
Advances in polymer technologies have resulted in the discovery of high-quality polymers that can serve as environmentally benign and cost-effective binders for a wide range of electrode materials. Aqueous-based binders have several advantages: they are low cost and pollution free, they do not have strict processing requirements with respect to air and humidity, and they feature fast evaporation of solvent. A small quantity (around 5%) of binder is required as compared to traditional PVDF-based
Conclusions and future perspectives
Aqueous-based electrode manufacturing strategy has successfully reformed the conventional harmful and expensive technology to cost-effective and environmentally friendly technology—without compromising the existing electrochemical performance. The binder for specific active materials should be carefully selected to achieve remarkable specific capacity and prolonged cycle life. Binders comprising a higher density of carboxylic acid groups exhibit reasonable electrochemical cycling performance
Declaration of competing interest
The authors declare no competing interest.
Acknowledgement
The authors would like to thank the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland and the more than 150 companies and organizations that support its research annually.
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