Component Design And Performance Regulation Of Flexible Self-Supporting Paper-based Lithium Iron Phosphate Electrodes

The development of new energy vehicles,smart grids,and flexible electronics has brought greater demands and challenges for traditional electrochemical energy storage devices and electrode technologies.Paper,as a fibrous material,possesses numerous advantages such as lightweight,costeffectiveness,environmental friendliness,and renewability.Its porous,flexible,and self-supported nature provides convenience in constructing high-energydensity and flexible energy storage electrodes.However,the efficient integration of active materials,conductive additives,and papermaking fibers,especially in the development of high-energy-density paper-based electrodes(e.g.,lithium-ion battery electrodes),often presents difficulties,leading to compromises in the mechanical,electrochemical,and papermaking filtration properties of the electrodes.Additionally,during the assembly of paper-based electrodes in battery cells,there is a lack of sufficient strength in the conventional metal tab welding,making it challenging to prepare multilayer high-capacity paper-based batteries.These factors hinder the subsequent scaling-up and practical applications of paperbased electrodes.In this study,we focus on paper-based lithium-iron phosphate(LiFePO4,LFP)electrodes for lithium-ion batteries.Various aspects,including the composition design of electrode slurry,enhancement through nanofiber additives,and optimization of assembly,are investigated to explore the mechanisms of component coordination,structural optimization,and their impacts on filtration performance,mechanical strength,electrochemical performance,and welding strength in the paper-based LFP electrode.The main findings are as follows:(1)By investigating the composition design and preparation process of paperbased electrodes,the factors that influence the filtration efficiency,mechanical strength,and electrical properties of these electrodes are identified.A component opyimization and preparation process that achieve a balance between electrode mechanics and electrochemical performance while ensuring filtration efficiency are established.The paper-based electrode LFP100/CNT20/SWF20,prepared using the optimized ratio,exhibits a filtration time of 2.7 min,a tensile strength of 4.28 MPa,a fracture elongation of 1.92%,and good physical properties.A multi-layer battery assembled with three layers of this electrode achieves a surface capacity of 3.37 mAh cm-2,outperforming traditional commercial LFP electrodes,with excellent high-current charge-discharge performance.(2)By refining plant fibers into nanofibers and using bacterial cellulose nanofibers and nanofibrillated cellulose fibers as filling fibers,a three-level gradient-enhanced fiber support network is constructed,increasing the fiber binding area and the number of hydrogen bonds,thereby further improving the mechanical strength of the paper-based electrode.The prepared LFP100/CNT20/FWF15-BNF5 paper-based electrode demonstrates a filtration time of 3.7 min,achieving an increased tensile strength of 7.40 MPa and fracture elongation of 3.71%.It also exhibits good foldability and electrolyte wettability.The initial discharge capacity of a single electrode reaches 1.10 mAh cm-2,and after 200 cycles,the capacity retention rate is 88.2%.After three layers of assembly,the initial discharge capacity of the electrode reaches 3.32 mAh cm-2,showing excellent cycling performance at current densities of 0.22 mA cm-2/0.66 mA cm-2 and 1.1 mA cm-2/3.3 mA cm-2.(3)Based on the functional three-dimensional porous structure of the selfsupporting paper-based electrode,the LFP loading in the paper-based electrode was further improved by controlling the composition and structure.The LFP content was increased from 3.3 g L-1 to 5 g L-1 in the fabricated LFP150/CNT20/FWF15-BNF5 paper-based electrode.The electrode exhibited a tensile strength of 7.59 MPa,an elongation at break of 3.52%,a thickness of 70μm,and a conductivity of 5.17 S cm-1.The filtration time was 3.9 min.The volumetric energy density of the electrode reached 82.9 Wh L-1.The initial discharge areal capacity was 1.75 mAh cm-2,and after 200 cycles at current densities of 0.22 mA cm-2/0.66 mA cm-2 and 1.1 mA cm-2/3.3 mA cm-2,the discharge areal capacities were 1.71mAh cm-2 and 1.38 mAh cm-2,respectively.After three-layer stacking,the initial discharge areal capacity reached 5.02 mAh cm-2,demonstrating excellent cycling performance of the electrode.(4)The scalability of the designed paper-based electrode for papermaking processes was validated by enlarging the electrode through a paper forming machine.Resin was coated on the paper-based electrode carrier fibers on the surface and also impregnated within the internal fiber network.With the assistance of ultrasonic welding machine,effective "welding" between the paper-based electrode(non-metallic)and the tab(metal)was achieved.The tensile strength of the welded single-layer paper-based electrode with Ni tab reached 7.5 MPa,and the tensile strength of the welded 5-layer paper-based electrode and tab reached 13.5 MPa.The welding process did not affect the conductivity and electrochemical performance of the electrode.A pouch cell of paper-based LFP battery assembled with 5-layer electrode sheets achieved an initial discharge capacity of 170.4 mAh and demonstrated good cycling performance at different current densities,exhibiting excellent electrochemical performance.This work achieves a balance between the composition design and performance control of the paper-based LFP electrode,resulting in improved efficiency,mechanical strength,and electrical performance of the paper electrode.It demonstrates good adaptability to the papermaking process in preparing LFP paper-based electrode.By optimizing the impregnation and coating of resin on the paper-based electrode and studying the "welding" process,effective connection between the non-metallic paper-based electrode and the metal current collector is achieve,addressing the welding issue faced by paper-based electrodes during the assembly of flexible lithium-ion batteries.The research provides an experimental foundation for the scalable production of self-supporting flexible electrodes using the papermaking process and their application in commercial lithium-ion batteries.