For increasing the energy density, lithium metal is considered the "holy grail" anode material due to its high theoretical capacity (3860 mA h g-1) and lowest electrochemical potential (-3.04 V vs the normal hydrogen electrode) ​[1]​. However, the commercialisation of a lithium metal anode is hindered by several issues including safety concerns and rapid lithium inventory loss ​[2]​. To address these challenges, research has focused on electrolyte modification strategies and, more recently, 3D current collectors ​[3]​. Here we propose using free-standing, electro-spun, lignin-derived carbon fibre as a sustainable, high performance lithium anode host structure to replace the conventional current collector. 

Optimisation of the fibre fabrication method, in particular the carbonisation process, resulted in an average coulombic efficiency greater than 99% over 100 cycles, demonstrating low lithium inventory loss. In addition, these fibres promote cycling stability and inhibit dendrite formation as demonstrated by extended lifetimes, and a low and stable voltage hysteresis.  

Increasing the carbonisation temperature from 700°C to 1000°C decreases the amount of solid-electrolyte interface (SEI) formed initially on the fibre surface, with electrochemical impedance spectroscopy (EIS) confirming a decrease in SEI resistance as well as an improvement in SEI stability for fibres carbonised at higher carbonisation temperatures. Increasing the carbonisation temperature from 700°C to 1000°C further increases the pre-plating plateau capacity, defined as the capacity in the plateau region of the voltage curve before the lithium metal nucleation point is reached. Ex-situ nuclear magnetic resonance spectroscopy (NMR) indicates that, in this region, lithium intercalates in ordered crystal regions, while small lithium clusters form in the closed porosity. An increased pre-plating plateau capacity appears to have a positive effect on the fibre lithiophilicity.  

Overall, an improvement in cycling performance at increased carbonisation temperatures has been attributed to a combination of decreased SEI resistance and increased fibre lithiophilicity. In detail, the reduced SEI resistance ensures homogeneous lithium-ion flux and low deposition overpotential, which favours large, uniform and low surface area lithium deposits. The increased lithiophilicity further homogenises the lithium-ion flux by electrostatic interaction ​[4]​.  

Finally, the lithium deposition behaviour and cycling performance at different carbonisation temperatures were studied in relation to the varying fibre properties such as heteroatom content, structural order, as well as open and closed porosity ​[5]​. This knowledge will help us to tune the fibre structure to optimise performance, and ultimately minimise the amount of excess lithium needed in the electrode for improved safety and sustainability.