Preparation And Properties Of Lignin-based Polymer Electrolytes

In recent years,with the global environmental pollution and the depletion of fossil fuels,environmental energy storage systems with high electrochemical performance have become more and more important.Lithium-ion batteries（LIBs）have occupied the market of plug-in hybrid electric vehicles and pure electric vehicles,as well as electronic watches,mobile phones and wireless earphones because of their outstanding advantages,such as high operating voltage,long cycle life,low probability of self-discharge and no memory effect.However,safety problems,such as electrolyte leakage,flammability and explosion caused by the addition of liquid electrolyte,have become the key to the application of high-power rechargeable lithium battery.In order to overcome these shortcomings and maintain the high-performance LIBs,polymer electrolytes（SPEs）with high flexibility and high safety have been developed.Compared with liquid electrolyte,SPEs have the advantages of low flammability,high mechanical strength,high thermal stability and high cycle stability.It can not only enhance the safety and accelerate the energy density,is an important direction for the development of the next generation of lithium-ion batteries!Lignin is one of the most abundant natural polymers next to cellulose in nature.It is mainly extracted from wood and is an environmentally friendly,sustainable,biodegradable and biocompatible low-cost material.In this study,lignin-based lithium salt（L-Li）containing lithium sulfonate group and lithium trifluoromethylsulfonimide group was synthesized using lignosulfonate as raw material.Then lithium lignin salt（L-Li）and polyvinylidene-hexafluoropropylene（PVDF）were used to prepare lignin-based three-dimensional network single ion polymer electrolyte（L-Li/PVDF）with high porosity,which can promote the infiltration of plasticizer,improve the absorption rate,reduce the internal impedance of the battery,and make Li⁺transmission more efficient.The conductivity of L-Li/PVDF can reach2.12 m S cm^-1 with only a little plasticizer at room temperature.The anions were fixed on the lignin skeleton,and Li⁺became the only mobile active substance in the electrolyte,which effectively increased the transfer number of Li⁺,and inhibited the lithium dendrites effectively.Under 0.05 m A cm^-2,the assembled symmetrical Li//L-Li/PVDF//Li cells had no obvious lithium dendrites after cycling for 2000 h with a polarization voltage of only 30 m V.Lignin-based lithium salt（L-Li）can provide a large number of Li⁺that were easy to dissociate,so that it can ensure the high capacity and high efficiency only with the plasticizer,avoiding the safety problems caused by electrolyte.The initial discharge capacity of the assembled Li//L-Li10/PVDF//LFP cells can reach 157 m Ah g^-1 at 0.2 C with the coulomb efficiency about 100%,and the capacity retention rate reached 82.8%after 550 cycles（up to 6400 h）.Based on the research of L-Li/PVDF,an ultra-thin and high energy density all-solid composite polymer electrolyte（CPEs）（L-Li/PVDF-PEO/Li）was prepared by electrospinning/electrostatic spraying process in order to further improve the energy density of the membrane and solve the problems resulted by electrolyte leakage.Using L-Li/PVDF as the nanofiber skeleton,PEO/Li TFSI was infiltrated in the electrospun membrane through the electrostatic spraying.The ultra-thin film thickness of only 13μm was not only conducive to the interface contact between the membrane and the electrode for good interfacial compatibility,but also conducive to shorten the transmission distance of Li⁺.Its ionic conductivity can reach0.13 m S cm^-1 at room temperature,which was comparable to that of gel electrolyte.In addition,the 3D L-Li/PVDF supporting skeleton not only provided good mechanical properties for the membrane,but also reduced the crystallinity of polyethylene oxide（PEO）,avoided the agglomeration of Li TFSI particles,and formed a continuous and rapid transmission channel of Li⁺,significantly increased energy density.The specific capacity of the assembled Li//L-Li10/PVDF/PEO/Li//Li Fe PO₄ cells reached 158 m Ah g^-1 at 0.2 C with the coulomb efficiency about 100%,and the reversible capacity was 106 m Ah g^-1 after 730 cycles.Meanwhile,the obtained CPEs had good interfacial stability.Under the current density of 0.5 m Acm^-2,the assembled lithium symmetric cells achieved overlength cycling of 4000 h with a minimum voltage of 13 m V,which significantly inhibited the growth of lithium dendrites.