Preparation And Electrochemical Performance Of Lithium-rich Flexible Lithium-ion Battery

With the rapid development of electronic technology,people’s demand for miniaturization,portability,intelligence and diversified electronic products is increasing.Therefore,in recent years,the development of flexible wearable electronic technology has been very rapid.To meet the needs of wearable,portable and bendable electronic products,it is particularly important to develop safe,high-energy-density flexible lithium-ion batteries（LIBs）.During use,flexible lithium-ion batteries need to withstand bending,twisting,stretching and even folding.However,flexible lithium ions still have problems such as low specific capacity of cathode materials,low active material loading,poor mechanical performance of flexible electrodes,and unstable electrochemical performance during bending deformation.In addition,the current preparation methods of flexible electrodes are cumbersome,which is not conducive to industrialization.In order to solve the above problems,this article has done the following work,the main contents are as follows:（1）In view of the low specific capacity of flexible lithium ion battery cathode materials,lithium-rich cathode（LLOs）materials with high specific capacity were selected as the research object.However,the cycle performance and rate performance of lithium-rich batteries are poor.In this paper,lithium-ion conductor Li Fe PO₄ is used to modify the surface of lithium-rich.A one-step milling method was used to prepare Li Fe PO₄-coated lithium-rich cathode material.The 2 wt.%Li Fe PO₄-coated LLOs delivers a high initial discharge specific capacity of 287.8 m Ah g^-1 at 1 C and retains 149.1 m Ah g^-1 after 100 cycles at 1 C,whose cycle performance is higher than the pristine material（the discharge specific capacity increase47.2%）.In addition,the rate performance of lithium-rich materials has also been greatly improved.The discharge specific capacity is 89.6 m Ah g^-1 at5C（the discharge specific capacity increase 48.8%）,which is much higher than the original lithium-rich material of 60.2 m Ah g^-1.The improvement of the electrochemical performance of lithium-rich cathode materials is mainly due to the truth that Li Fe PO₄ can avoid the direct contact between the lithium-rich material and the electrolyte,reduce the appearance of side reactions,and reduce the interface impedance to increase the diffusion rate of ions.Besides,Li Fe PO₄can also provide additional reversible capacity for lithium-rich cathode materials to improve the cycle performance and rate performance of LLOs（2）In view of the problems of low electrode capacity,unstable bending cycle performance,and poor flexibility in flexible lithium-ion batteries,nanofibrillated celluloses（NFCs）is selected as the binder and stabilizer,and carbon nanotubes（CNTs）are used as the conductive agent.The three-dimensional conductive network structure composed of NFCs and CNTs facilitates the deposition of active particles and both have high mechanical strength.The flexible composite cathodes with different loadings were prepared from 2 wt.%Li Fe PO₄@LLOs、CNTs and NFCs using the vacuum filtration method.The prepared flexible electrodes has high flexibility under repeated.2wt.%Li Fe PO₄@LLOs/CNTs/NFCs electrode owns a high initial discharge specific capacity of 285 m Ah g^-1 at0.1 C and retains 171.5 m Ah g^-1 after 100 cycles at 1 C.the specific discharge capacity is 247.3 m Ah g^-1 after 35 cycles.As the load increases,the electrochemical performance of the flexible electrode decreases.Even at a high load（13 mg/cm²）,the electrode delivers a high initial discharge specific capacity of 276.4 m Ah g^-1 at 1 C and retains 134.6 m Ah g^-1 after100 cycles at 1 C.Its excellent electrochemical performance is owing to the coating of lithium-rich cathode material by Li Fe PO₄,and the NFCs with excellent mechanical properties and the high-conductivity CNTs form a conductive three-dimensional porous structure,which is a combination of electrons and ions.Migration provides a fast track.（3）Aiming at the problems of low discharge capacity,poor bending cycle performance,and poor flexibility of flexible lithium-ion batteries,the prepared flexible electrodes were assembled into lithium-rich flexible lithium-ion batteries（abbreviated as lithium-rich flexible LIBs）,and explore the flexible bending performance of lithium-rich flexible lithium-ion batteries.The initial discharge specific capacity and capacity retention rate of the lithium-rich flexible lithium-ion battery are 179.6 m Ah/g and83.6%at 1 C before bending,respectively;after bending,which were 178m Ah g^-1 and 78.2%.After being repeatedly bent at an angle of 60°for 200times and recovered,the electrode owns a high initial discharge specific capacity of 168.2 m Ah g^-1 at 1 C and a good capacity retention rate of76.5%at 1 C after 100 cycles.The first discharge specific capacity of the lithium-rich flexible lithium-ion battery is 213.1 m Ah g^-1 at 0.1 C after being bent 1000 times.The lithium-rich flexible lithium-ion battery has high flexibility and electrochemical performance stability,thanks to the uniform and continuous high-conductivity three-dimensional porous network structure constructed by CNTs and NFCs.Lithium-rich lithium-ion batteries can not only work stably under normal conditions,but also maintain safe and stable work before and after 60°bending or even during multiple bending processes,which proves the practical application value of lithium-rich flexible LIBs and is expected to be used in wearable flexible electronic devices.