Carboxyl functionalized carbon fibers with preserved tensile strength and electrochemical performance used as anodes of structural lithium-ion batteries
Graphical abstract
Introduction
The use of structural batteries could effectively decrease the whole mass of devices by acting as integrated load-carrying parts (e.g., car roof) [1], [2], [3]. By combining electrochemical and mechanical properties in the same material, a structural battery could function both as a structural element and as energy storage, having the possibility to become a fully integrated part of a device. In addition, the power and energy density could be increased on a system level. Carbon fiber (CF) is a promising lightweight material used as electrode material in structural power batteries because of its high specific tensile properties and carbonaceous microstructure, which enables reversible lithium-intercalation reactions [4], [5], [6]. However, CF used in lithium-ion batteries display poor capacity, only about 100 mAh g−1 at moderate lithiation rates [7]. Moreover, surfaces are chemically inert and manifest weak interfacial interactions [8]. Therefore CF cannot be easily used to form composites with high-capacity materials (e.g. silicon [9], ZnCo2O4 [10] and WO3 [11]) to effectively improve electrical properties [12], [13], and there is poor interfacial strength between CF and solid polymer electrolytes [14], [15], [16]. Therefore the surface carboxyl functionalization of pristine carbon fibers is necessary.
There are a number of existing functionalization methods are studied to address different application requirements of functionalizing carbon fibers, but there are few is aimed at carbon fibers as structure electrode material. Different with the other application requirements, carbon fibers used as structure electrode material have a high demands on both tensile strength and electrochemical properties. Hence the appropriate functionalization method is necessary and urgently needed. In this study, chemical oxidation method is improved to address the high demands for carbon fibers as structure electrode materials because of the following reasons. First, compared with other functional groups, carboxyl groups are proved more suitable for adhesion with nanoparticles [12], the addition of surface polar functional groups effectively increases surface wettability of fibers, and the chemical oxidation using strong oxidizing acids is easier to obtain more carboxyl groups than the others [17]. Second, chemical oxidation method is simpler and cost less, and has less potential to damage electrochemical properties. Third, although surface functional group grafting procedures could preserve tensile strength of CF [18], [19], grafting reactions using solid organic matters as secondary materials have great potential to damage electrode electrical properties. Besides, grafting reactions using carbon nanotube (CNT) [20], carbon aerogel or graphene as a secondary material suffer high cost and low productive efficiency on the synthesis of CNT or graphene and grafting processes.
However, chemical oxidation is usually accompanied by the cleavage of CC bond, and creates surface defects, which are undesirable for fiber tensile strength, and cannot be ignored. The negative effect of chemical oxidation method should be decreased. Otherwise, the application of carbon fibers would be influenced. A previous study indicated that the use of phosphoric acid as secondary acid can protect the vicinal diols by forming a cyclic structure, thereby preventing or retarding the over-oxidation of the CNT and nanoribbon [21]. Therefore, the presence of phosphoric acid may be used to prevent CC from breaking. Meanwhile the tensile strength of carbon fibers is determined by the turbostratic structure. Hence, we hypothesize that the addition of phosphoric acid could preserve the strength of carbon fibers.
In this study, we developed a carboxyl functionalization method with mixed phosphoric/sulfuric/nitric acids (H3PO4/H2SO4/HNO3) for carbon fiber surface. The effectiveness of this proposed method on increasing the concentration of surface acid functional groups, tensile strength and electrochemical properties of carbon fibers was assessed. The proposed functionalization mechanism was also verified by several tests. The PAN-based T800 carbon fibers were viewed as a highly promising structural electrode material and have a promising prospect in further researching during the process of developing a functional structural battery in the previous study [22]. Hence PAN-based T800 carbon fibers are utilized in this study.
Section snippets
Experimental
PAN-based carbon fibers were coated particularly with epoxy or polyurethane resin. PAN-based T800 carbon fibers were desized through solvent extraction using a superfluous volume of acetone for 72 h at room temperature to avoid the influence of surface resin on tensile strength and electrochemical properties especially the reversible capacity. The desized carbon fiber (DCF), was considered as the unmodified carbon fiber. The carbon fibers were then filtered and dried at 60 °C for 12 h to remove
Tensile properties and concentration of surface acid groups
The measured data were integrated through a two-parameter Weibull model [25]. This model is based on the weakest link theory, and it is widely used to analysis and evaluate fiber tensile strength. The failure of carbon fiber can be attributed to the presence of random flaws. In this model, the failure probability of each single-filament carbon fiber P was given by Eq. (1):
Where n is the data point number and i is the rank of the ith data point ranked from smallest to largest. The failure
Conclusion
Carbon fibers were functionalized with mixed H3PO4/H2SO4/HNO3. This kind of functionalized carbon fibers had preserved tensile strength and improved electrochemical properties. The functionalization degree of carbon fibers was not lower than that of carbon fibers functionalized with existing functionalization method. The PFCF fibers attained good reversibility, lower charge-transfer impedance and higher stable capacity than DCF in the charging and discharging process. Moreover, the proposed
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.
References (28)
- et al.
Structural power composites
Compos. Sci. Technol.
(2014) - et al.
Expansion of carbon fibres induced by lithium intercalation for structural electrode applications
Carbon
(2013) - et al.
The effect of lithium-intercalation on the mechanical properties of carbon fibres
Carbon
(2014) - et al.
Piezo-Electrochemical effect in lithium-intercalated carbon fibres
Electrochem. Commun.
(2013) - et al.
Electrochemical properties of PAN-based carbon fibers as anodes for rechargeable lithium ion batteries
Carbon
(2001) - et al.
The surface analytical characterization of carbon fibers functionalized by H2SO4/HNO3 treatment
Carbon
(2008) - et al.
Study of electrochemically treated PAN based carbon fibres by IGC and XPS
Carbon
(2007) - et al.
Impact behavior and fractographic study of carbon nanotubes grafted carbon fiber-reinforced epoxy matrix multi-scale hybrid composites
Compos. A
(2015) - et al.
Improving the interfacial properties of carbon fiber-reinforced epoxy composites by grafting of branched polyethyleneimine on carbon fiber surface in supercritical methanol
Compos. Sci. Technol.
(2015) - et al.
Carboxyl functionalization of carbon fibers through a grafting reaction that preserves fiber tensile strength
Carbon
(2011)
Interface enhancement of carbon fiber reinforced methylphenylsilicone resin composites modified with silanized carbon nanotubes
Mater. Des.
Surface acidity of carbons characterized by their continuous pK distribution and boehm titration
Carbon
The effect of siO2-doped boron nitride multiple coatings on mechanical properties of quartz fibers
Appl. Surf. Sci.
Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites, Part I: Fibre characterisation
Carbon
Cited by (29)
Fabrication of carboxylated tubular carbon nanofibers as anode electrodes for high-performance lithium‐ion batteries
2023, Fabrication and Functionalization of Advanced Tubular Nanofibers and their ApplicationsPreparation of multifunctional structural P-CF@ZnCo<inf>2</inf>O<inf>4</inf> composites used as structural anode materials
2020, Journal of Alloys and CompoundsPreparation of multifunctional P-CF@SnO<inf>2</inf>-MOF composite used as structural anode materials
2020, Journal of Electroanalytical Chemistry