Graphitic microstructure and performance of carbon fibre Li-ion structural battery electrodes

Suppressing Lithium Migration in a Carbon Fiber Negative Electrode During Atom Probe Tomography Analysis

Carbon fibers can play dual roles, carrying mechanical load and hosting lithium (Li) simultaneously in multifunctional devices called structural batteries. It is essential to gain a detailed understanding on the interaction between Li and carbon fibers on the nanoscale. Atom probe tomography (APT) can potentially reveal individual Li and C atoms. However, lithiated carbon fibers experience massive Li migration once exposed to the electric field in the APT instrument. We show that a few nanometers of a chromium (Cr) coating on APT specimens can shield the electric field and suppress the massive Li migration. The related effects of the Cr coating, such as introduction of oxygen, enhanced mass resolving power of the mass spectrum, and increased portion of single hits, are also discussed.

Multifunctional Carbon Fiber Composites: A Structural, Energy Harvesting, Strain-Sensing Material

Multifunctional structural materials are capable of reducing system level mass and increasing efficiency in load-carrying structures. Materials that are capable of harvesting energy from the surrounding environment are advantageous for autonomous electrically powered systems. However, most energy harvesting materials are non-structural and add parasitic mass, reducing structural efficiency. Here, we show a structural energy harvesting composite material consisting of two carbon fiber (CF) layers embedded in a structural battery electrolyte (SBE) with a longitudinal modulus of 100 GPa─almost on par with commercial CF pre-pregs. Energy is harvested through mechanical deformations using the piezo-electrochemical transducer (PECT) effect in lithiated CFs. The PECT effect creates a voltage difference between the two CF layers, driving a current when deformed. A specific power output of 18 nW/g is achieved. The PECT effect in the lithiated CFs is observed in tension and compression and can be used for strain sensing, enabling structural health monitoring with low added mass. The same material has previously been shown capable of shape morphing. The two additional functionalities presented here result in a material capable of four functions, further demonstrating the diverse possibilities for CF/SBE composites in multifunctional applications in the future.

Designing Structural Electrochemical Energy Storage Systems: A Perspective on the Role of Device Chemistry

Structural energy storage devices (SESDs), designed to simultaneously store electrical energy and withstand mechanical loads, offer great potential to reduce the overall system weight in applications such as automotive, aircraft, spacecraft, marine and sports equipment. The greatest improvements will come from systems that implement true multifunctional materials as fully as possible. The realization of electrochemical SESDs therefore requires the identification and development of suitable multifunctional structural electrodes, separators, and electrolytes. Different strategies are available depending on the class of electrochemical energy storage device and the specific chemistries selected. Here, we review existing attempts to build SESDs around carbon fiber (CF) composite electrodes, including the use of both organic and inorganic compounds to increase electrochemical performance. We consider some of the key challenges and discuss the implications for the selection of device chemistries.

Structural Lithium-Ion Battery Cathodes and Anodes Based on Branched Aramid Nanofibers

Structural batteries and supercapacitors combine energy storage and structural functionalities in a single unit, leading to lighter and more efficient electric vehicles. However, conventional electrodes for batteries and supercapacitors are optimized for high energy storage and suffer from poor mechanical properties. More specifically, commercial lithium-ion battery anodes and cathodes demonstrate tensile strength values <4 MPa and Young's modulus of <1 GPa. Here, we show that using branched aramid nanofibers (BANFs) or nanoscale Kevlar fibers as a binder leads to mechanically stronger lithium-ion battery electrodes. BANFs are combined with lithium iron phosphate (LFP, cathode) or silicon (Si, anode) particles and reduced graphene oxide (rGO). Hydrogen-bonding interactions between rGO and BANFs are harnessed to accommodate load transfer within the nanocomposite electrodes. Overall, we obtained up to 2 orders of magnitude improvements in Young's modulus and tensile strength compared to commercial battery electrodes while maintaining good energy storage capabilities. This work demonstrates an efficient route for designing structural lithium-ion battery cathodes and anodes with enhanced mechanical properties using BANFs as a binder.

Structural Batteries: A Review

Structural power composites stand out as a possible solution to the demands of the modern transportation system of more efficient and eco-friendly vehicles. Recent studies demonstrated the possibility to realize these components endowing high-performance composites with electrochemical properties. The aim of this paper is to present a systematic review of the recent developments on this more and more sensitive topic. Two main technologies will be covered here: (1) the integration of commercially available lithium-ion batteries in composite structures, and (2) the fabrication of carbon fiber-based multifunctional materials. The latter will be deeply analyzed, describing how the fibers and the polymeric matrices can be synergistically combined with ionic salts and cathodic materials to manufacture monolithic structural batteries. The main challenges faced by these emerging research fields are also addressed. Among them, the maximum allowable curing cycle for the embedded configuration and the realization that highly conductive structural electrolytes for the monolithic solution are noteworthy. This work also shows an overview of the multiphysics material models developed for these studies and provides a clue for a possible alternative configuration based on solid-state electrolytes.