Lignin-based carbon fibers (LCFs) from the renewable resource softwood kraft lignin were synthesized via oxidative thermostabilization of pure melt-spun lignin and carbonization at different temperatures from 1000 degrees C to 1700 degrees C. The resulting LCFs were characterized by tensile testing, scanning electron microscopy (SEM), X-ray diffraction (XRD) and confocal Raman spectroscopy. The microstructure is mainly amorphous carbon with some nanocrystalline domains. The strength and stiffness are inversely proportional to the carbonization temperature, while the LCFs carbonized at 1000 degrees C exhibit a strength of 628 MPa and a stiffness of 37 GPa. Furthermore, the application potential of LCFs was evaluated as negative electrodes in a lithium-ion battery (LIB) by electrochemical cycling at different current rates in a half-cell setup. The capacity drops with the carbonization temperature and the LCFs carbonized at 1000 degrees C have a capacity of 335 mAh g(-1). All LCFs showed good cycling stability. Because of the mechanical integrity and conductivity of the LCFs, there is no need to apply current collectors, conductive additives or binders. The advantage is an increased gravimetric energy density compared to graphite, which is the most common negative electrode material. LCFs show a promising multifunctional behavior, including good mechanical integrity, conductivity and an ability to intercalate lithium for LIBs.

Flexible Li-ion batteries attract increasing interest for applications in bendable and wearable electronic devices. TEMPO-oxidized cellulose nanofibrils (TOCNF), a renewable material, is a promising candidate as binder for flexible Li-ion batteries with good mechanical properties. Paper batteries can be produced using a water-based paper making process, avoiding the use of toxic solvents. In this work, finely dispersed TOCNF was used and showed good binding properties at concentrations as low as 4 wt %. The TOCNF was characterized using atomic force microscopy and found to be well dispersed with fibrils of average widths of about 2.7 nm and lengths of approximately 0.1-1 mu m. Traces of moisture, trapped in the hygroscopic cellulose, is a concern when the material is used in Li-ion batteries. The low amount of binder reduces possible moisture and also increases the capacity of the electrodes, based on total weight. Effects of moisture on electrochemical battery performance were studied on electrodes dried at 110 degrees C in a vacuum for varying periods. It was found that increased drying time slightly increased the specific capacities of the LiFePO4 electrodes, whereas the capacities of the graphite electrodes decreased. The Coulombic efficiencies of the electrodes were not much affected by the varying drying times. Drying the electrodes for 1 h was enough to achieve good electrochemical performance. Addition of vinylene carbonate to the electrolyte had a positive effect on cycling for both graphite and LiFePO4. A failure mechanism observed at high TOCNF concentrations is the formation of compact films in the electrodes.

Carbon fibers have the combined mechanical and electrochemical properties needed to make them particularly well suited for usage as electrodes in a structural lithium-ion battery, a material that simultaneously works as a battery and a structural composite. Presented in this paper is an evaluation of commercial polyacrylonitrile-based carbon fibers in terms of capacity and coulombic efficiency, as well as a microstructural and surface evaluation. Some polyacrylonitrile based carbon fibers intercalate lithium ions, resulting in a similar capacity as state-of-the-art graphite based electrodes, presently the most commonly used negative electrode material. Using high precision coulometry, we found a capacity of around 250-350 mAh/g and a very high coulombic efficiency of over 99.9% after ten cycles, which is even higher than a commercial state-of-the art graphitic electrode evaluated as reference. The high coulombic efficiency is attributed to the very low surface area of the carbon fibers, resulting in a small and stable solid-electrolyte interface layer. A highly graphitized ultra high modulus carbon fiber was evaluated as well and, compared to the other fibers, less lithium was inserted (corresponding to approximately 150 mAh/g). We show that the use of carbon fibers as an electrode material in a structural composite battery is indeed viable.

The industrial lignin used here is a byproduct from Kraft pulp mills, extracted from black liquor. Since lignin is inexpensive, abundant and renewable, its utilization has attracted more and more attention. In this work, lignin was used for the first time as binder material for LiFePO4 positive and graphite negative electrodes in Li-ion batteries. A procedure for pretreatment of lignin, where low-molecular fractions were removed by leaching, was necessary to obtain good battery performance. The lignin was analyzed for molecular mass distribution and thermal behavior prior to and after the pretreatment. Electrodes containing active material, conductive particles and lignin were cast on metal foils, acting as current collectors and characterized using scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge cycles. Good reversible capacities were obtained, 148 mAhg(-1) for the positive electrode and 305 mAhg(-1) for the negative electrode. Fairly good rate capabilities were found for both the positive electrode with 117 mAhg(-1) and the negative electrode with 160 mAhg(-1) at 1C. Low ohmic resistance also indicated good binder functionality. The results show that lignin is a promising candidate as binder material for electrodes in eco-friendly Li-ion batteries.

A MEMS-based amperometric nitric oxide (NO) gas sensor is reported in this paper. The sensor is designed to detect NO gas for the purpose of asthma monitoring. The unique property of this sensor lies in the combination of a microporous high-surface area electrode that is coated with Nafion (TM), together with a liquid electrolyte. The sensor is able to detect gas concentrations of the order of parts-per-billion (ppb) and has a measured NO sensitivity of 0.045 nA/ppb and an operating range between 25 and 65% relative humidity. The settling time of the sensor is measured to 8s. The selectivity to interfering gases such as ammonia (NH3) and carbon monoxide (CO) was high when placing an activated carbon fiber filter above the sensor. The ppb-level detection capability of this sensor combined with its relatively fast response, high selectivity to CO and NH3 makes the sensor potentially applicable in gas monitoring for asthma detection.

In this article a novel energy-storing composite electrode is investigated with regards to its mechanical and electrochemical properties. This composite electrode consists of carbon fibers, which provide both the mechanical reinforcement and the negative electrode in the battery cell. Also, this carbon fiber composite electrode consists of a polymer matrix that can conduct lithium ions, in order to simultaneously act as the electrolyte in the battery cell.

Electrochemical tests were performed on the manufactured composite electrode and show extremely promising results for the battery performance. Furthermore, mechanical tests show that the composite electrode has acceptable mechanical properties for structural use.

It is shown that the internal distances in the composite are large, and volume fraction of fibers is low. This is not only significantly limiting the mechanical properties of the composite, but also the electrochemical properties.

Overall, the carbon fiber composite electrode is found to have suitable characteristics for further research, where many further research topics are found in order to improve and characterize the composite further. 

In this paper we show how one can functionalise carbon fibres by using them as an electrochemicalelectrode. The electrochemical process is the same as in a lithium-ion battery cell so that the carbonfibres act as an active electrode in future structural battery concepts. The functionalization of carbonfibres using lithium ion intercalation reveals three novel and interesting possibilities enabling carbonfibres composites to obtain several other multi-functionalities. These are strain sensing, actuation andenergy harvesting.We have found that by intercalating lithium ions into the nano-/micro-structure of carbon fibres apiezo-electrochemical effect is revealed. This is observed as a change in the potential of the carbon fibreelectrode when applying a mechanical load. The response is direct and easily measurable being in theorder of several mV. This can be utilised as a strain sensor since there is a relation between the potentialchange and the strain in the carbon fibre.Secondly, we have measured substantial axial expansion of carbon fibres when intercalated withlithium ions. The strain measured is as high as 1%. Since the stiffness of carbon fibres is very high, thiscorresponds to very large forces. This can be used for actuation or morphing.Thirdly, the newly found piezo-electrochemical effect can be used to harvest energy by convertingmechanical work to electrical energy. Applying a tensile force to carbon fibre bundles used as Liintercalatingelectrodes results in a response of the electrode potential of a few mV which allows, at lowcharge rates, discharge at higher electrode potential than at charge. More electrical energy is therebyreleased from the cell at discharge than provided at charge, harvesting energy from the mechanical workof the applied force.

The mechanical and electrochemical properties are coupled through a piezo-electrochemical effect in Li-intercalated carbon fibers. It is demonstrated that this piezo-electrochemical effect makes it possible to harvest electrical energy from mechanical work. Continuous polyacrylonitrile-based carbon fibers that can work both as electrodes for Li-ion batteries and structural reinforcement for composites materials are used in this study. Applying a tensile force to carbon fiber bundles used as Li-intercalating electrodes results in a response of the electrode potential of a few millivolts which allows, at low current densities, lithiation at higher electrode potential than delithiation. More electrical energy is thereby released from the cell at discharge than provided at charge, harvesting energy from the mechanical work of the applied force. The measured harvested specific electrical power is in the order of 1 muW/g for current densities in the order of 1 mA/g, but this has a potential of being increased significantly.

This paper introduces the concept of structural battery composite materials and their possible devices and the rationale for developing them. The paper presents an overview of the research performed in Sweden on a novel structural battery composite material. The research areas addressed include: carbon fibre electrodes, structural separators, multifunctional matrix materials, device architectures and material functionalization. Material characterization, fabrication and validation are also discussed. The paper focuses on a patented battery composite material technology. Here, carbon fibres are employed as combined negative battery electrodes and reinforcement, coated with a solid polymer electrolyte working simultaneously as electrolyte and separator with ability to transfer mechanical loads. The coated fibres are distributed in a conductive positive cathode material on an aluminium electron collector film. Efficient Li-ion transport between the electrodes is achieved by the solid polymer electrolyte coating being only a few hundred nanometres thick. Finally some outstanding scientific and engineering challenges are discussed. Such challenges, calling for further research are related to manufacture, development of new solid polymer electrolytes for improved multifunctionality and the lack of material models.

As a route to increase the efficiency of electric vehicles, weight reductions through composite building materials are constantly being introduced. To further aid this effort focus has been put on structural batteries, where the composite is multifunctional serving both as energy storing as well as load bearing unit. In an attempt to reduce the high ionic resistances solid polymer electrolytes introduces, carbon fibres have been individually coated with polymeric layers ranging from <500 nm to >3 mu m in thickness. This study investigates the feasibility of using such coatings in structural applications with respect to mechanical load cycling. The coated fibres were subjected to cyclic load up to approximately 1 % strain for up to 70,000 cycles. The polymer coatings were found not to be visibly affected by the prolonged mechanical fatigue. No cracks were observed in the coatings which makes the coating technique promising for future structural battery applications.