PEO-based Polymer Electrolytes For High-energy-density And Flexible Lithium Batteries

Lithium batteries are considered as efficient energy storage and conversion devices which are commonly used in mobile phones,computers,wearable devices and electric vehicles.However,with the increase of the demand for high-energy-density lithium batteries,safety concerns,such as combustion and explosion,have become the bottleneck,which limit the development and application of lithium batteries.Solid-state electrolytes are promising alternatives to separators and traditional flammable liquid electrolyte,which is a new solution to solve the contradiction between energy density and safety.Solid-state electrolytes can be divided into solid-state inorganic electrolytes and solid-state polymer electrolytes.Compared with the brittle solid-state inorganic electrolytes,solid-state polymer electrolytes have several advantages,such as superior processability and readily-regulated mechanical properties,and low electrode-electrolyte interface resistance.Meanwhile,solid-state polymer electrolytes can be employed to process flexible and stretchable batteries which are the power sources for flexible and wearable devices.PEO-based solid-state polymer electrolytes are considered to be the most promising solid-state polymer electrolytes for practical applications,because of the superior lithium-ion transport performance of the ether-based repeating units.Nevertheless,current commercial solid-state batteries based on PEO-based polymer electrolytes are operated between 70 and 80°C,illustrating that the ionic conductivity of the PEO-based solid-state polymer electrolytes should be significantly enhanced.In addition,the development of PEO-based solid-state polymer electrolytes for high-energy-density and high-safety lithium batteries and flexible lithium batteries faces the following important issues:(1)Poor compatibility with high-voltage cathodes.PEO-based polymers decompose at the voltage higher than 3.8 V,which limits the compatibility of PEO-based solid-state polymer electrolytes with high-voltage cathode and application in high-energy-density lithium battery.(2)Poor ability to suppress lithium dendrite.The lithium metal batteries,assembled PEO-based polymer electrolytes with good compatiblility for the lithium metal,have poor long-term stability.The reason is that the PEO-based electrolyte is not enough to completely inhibit lithium dendrite growth.(3)Few stretchable electrolytes have been reported.To meet the needs of the next-generation flexible electronic devices,the PEO-based flexible and stretchable polymer electrolytes should be developed to prepare the high-performance flexible and stretchable lithium batteries,to meet the needs of the new generation of flexible electronic devices.The following research works have been carried out in this thesis around the above issues:(1)The PEO-based polymer electrolytes are difficult to match with high-voltage cathode materials due to the decomposition of PEO at the high voltage(>3.8 V).We prepared PEO-based solid-state polymer electrolytes with high-voltage compatibility based on a"one-pot"strategy which is beneficial for mass production.The resulting PEO-based high-voltage polymer electrolytes can effectively extend the cycle life of 4V-class solid-state lithium batteries,which is of great significance for the application of PEO-based polymer electrolytes in high-energy-density lithium batteries.Siloxane groups were grafted in the PEO chain segments through ring-opening reaction,and the a highly cross-linked polymer network was obtained by hydrolysis/condensation reaction of siloxane groups.Due to incorporation of the high content of lithium salt in the PEO cross-linked network,a large amount of Li⁺was bonded to solvent molecules(N,N-dimethylformamide),resulting that the bonded solvent molecules were retained even after heating under vacuum.Through the above reaction process,the PEO-based solid polymer electrolyte was obtained,which is compatible with high-voltage cathodes.The ionic conductivity of as-prepared PEO-based solid polymer electrolyte is as high as 3.4×10^-4 S cm^-1,which can meet the requirements of lithium batteries operating at room temperature.The excellent high-voltage stability of lithium fluoride avoids the oxidative decomposition of PEO-based polymer electrolytes under high voltage and achieves compatibility with high-voltage cathodes.As a result,Li Co O₂||Li batteries assembled with the PEO-based polymer electrolyte can not only operate at room temperature but also be stably cycled at higher rate compared to previously reported PEO-based polymer electrolytes.The capacity retention of the Li Co O₂||Li battery is 87%after 200 cycles at room temperature at 1C.The PEO-based electrolytes can also be applied to rich-nickel layered NCM cathode.NCM₆₂₂||Li cells can be cycled over 200cycles at 0.5C at room temperature.This work provides a simple and effective method to enhance the compatibility between PEO-based polymer electrolyte and high-voltage cathode,which is expected to improve the energy density of lithium battery and solve the current"mileage anxiety"problem of electric vehicles.(2)The PEO-based polymer electrolytes possess low room-temperature ionic conductivity and the poor ability to inhibit lithium dendrites.We report a highly stretchable,elastic and adhesive PEO-based polymer electrolyte possessing high ionic conductivity.The PEO-based polymer electrolyte in lithium metal batteries has obvious inhibition effect on the growth of lithium dendrites,and provides a strong guarantee for the stable operation of lithium metal batteries.The polymer electrolyte was fabricated by in-situ incorporation of lithium salts in the elastomeric polymer network including PEO chain segment.The as-prepared polymer electrolyte with excellent mechanical properties exhibits a room-temperature ionic conductivity as high as 4.8×10^-4 S cm^-1as well as good adhesiveness to the electrode of the lithium metal battery.The excellent elastic self-recovery in combination with high robustness and adhesiveness of the polymer electrolyte gives rise to the self-adaptive and tightly bonded electrolyte-electrode interfaces in the Li metal battery.As a result,the polymer electrolyte can significantly suppress the Li dendrite growth and lower the interfacial resistance in the Li metal battery.The Li Fe PO₄||Li battery assembled with the polymer electrolyte can be stably cycled for 450 cycles at 0.5C at room temperature and deliver steady and high discharge capacity of ca.133.0 m Ah g^-1.The as-developed polymer electrolyte shows not only promise for application in high-performance Li metal battery,but also high potential as an elastic electrolyte for stretchable lithium-ion batteries.(3)Developing flexible and stretchable power sources is important for the flexible and stretchable electronic devices.We prepared stretchable all-in-one lithium batteries with the excellent healability which can continuously and stably power the devices after stretching or being healed the mechanical damage,which is important for promoting the development of next-generation wearable devices.The polymer electrolyte was fabricated by in-situ incorporation of lithium salts in the PEO-based polymer network,wherein the PEO chains are crosslinked by dynamic imine bonding.We prepared highly stretchable,elastic and healable PEO-based polymer electrolytes and positive/negative binders by regulating the content of lithium salts.Due to the dynamic reversibility of imine bond,the prepared cathode,electrolyte and cathode can be integrated to build a stretchable all-in-one lithium battery.The stretchable and resilient all-in-one lithium battery possesses a high elongation-at-break of ca.286%.Importantly,the structure of the dynamic imine-bond crosslinked network endows the all-in-one battery with the excellent healability.The all-in-one battery exhibits an autonomous self-healing behavior,making the cut sample easily recover its original mechanical performance and battery performance.The assembled all-in-one lithium battery delivers a high discharge capacity of 136.6 m Ah g^-1 up to 40^th cycles at 0.1C.The all-in-one lithium battery can be healed after a complete break,and recover to 90.7%of the capacity of the original battery.This research work provides a new idea and methodvfor the development of reliable and safe lithium batteries with the stretchability and healability.