Design And Application Of Polyethylene Oxide-based Electrolytes For All-Solid-State Lithium Metal Batteries

In recent years,all-solid-state lithium batteries have attracted much attention due to their high safety and energy density.As an important component,solid-state electrolytes are crucial to achieve high performance all-solid-state battery.Polyethylene oxide（PEO）-based electrolytes are considered one of the most promising solid-state electrolyte system for large-scale applications due to their high lithium salt solubility,low cost,and good processability.However,the development of PEO-based electrolytes is hindered by issues such as high crystallinity at room temperature,poor ion transfer ability,poor interface stability with lithium metal,and poor compatibility with high-voltage cathodes.Taking account of the above problems,this paper conducts research on improving the performance of PEO-based electrolytes through multifunctional fillers and electrolyte structure design,thereby improving the performance of all-solid-state lithium batteries.The specific work is as follows:1.To solve the problem of poor ion transport ability in PEO-based electrolytes,nickel-based two-dimensional metal-organic framework nanosheets（NMS）were selected as fillers and the mechanism of interaction between NMS and PEO-based electrolytes was explored.It is found that the porous nanosheet structure and abundant Lewis acid sites on the surface of NMS can effectively reduce the crystallinity of PEO and promote the dissociation of lithium salts,improving the ion conductivity of the electrolyte.Thanks to the ultra-thin nanosheet structure of NMS,the transport path of lithium ions at the filler/polymer interface is shortened,and a relatively continuous ion transport network is formed.Based on the above advantages,the Li Fe PO₄/Li battery assembled with this composite electrolyte exhibits good cycling stability at 30 ^oC and 0.1 C,with a discharge specific capacity of 130 m Ahg^-1 after 50 cycles.2.To solve the problem of lithium dendrite growth in PEO-based electrolyte,we introduced graphite phase carbon nitride nanosheets（GCN）into PEO-based electrolyte and studied the working mechanism of GCN and PEO-based electrolytes.The high specific surface area and ultra-thin layered structure of GCN fillers effectively promote the chain segment movement of PEO and the transport of lithium ions in the electrolyte.At the same time,GCN has excellent stability and rich nitrogen active sites,which promotes the uniform deposition of lithium ions.The assembled lithium symmetric battery can be stably cycled for 1500 hours at30 ^oC and 0.05 m Acm^-2.The Li Fe PO₄/Li battery assembled with this composite solid electrolyte has a discharge specific capacity of 141 m Ahg^-1 at 30 ^oC and 0.1 C,and a capacity retention rate of 83.9%after 200 cycles.The Li Ni_0.6Co_0.2Mn_0.2O₂/Li battery assembled with this composite solid electrolyte has a discharge specific capacity of 104 m Ahg^-1 at 30 ^oC and0.1 C.3.To solve the problem of rapid battery capacity degradation caused by the incompatibility between PEO-based electrolyte and Li Co O₂ cathode,a layer of PEO/Li DFOB electrolyte（D-PEO）with a thickness of only 1μm was added between the Li Co O₂ cathode and PEO/Li TFSI（T-PEO）electrolyte to construct a double-layer PEO-based electrolyte（D-PEO/T-PEO）.During the charging and discharging process of the battery,Li DFOB in the D-PEO decomposes to in situ form an interface layer containing Li_xB_xO_y and Li F on the surface of Li Co O₂ cathode,inhibiting the oxidative decomposition of PEO and promoting lithium ion transport at the electrolyte/Li Co O₂ electrode interface.The capacity retention rate of Li Co O₂/Li battery assembled with this double-layer electrolyte increased to 75%after 100 cycles at 0.1 C,significantly higher than that of 15%using single-layer electrolyte.The design of this double-layer electrolyte has also been extended to Li Ni_0.6Co_0.2Mn_0.2O₂/Li battery.After 500 cycles at0.5 C,the capacity retention rate of Li Ni_0.6Co_0.2Mn_0.2O₂/Li battery using D-PEO/T-PEO is 60%,significantly higher than that using T-PEO,which is 17%,demonstrating the universality of this strategy in high-voltage lithium metal batteries.4.To solve the problem of low energy density of PEO-based all-solid-state lithium batteries,PEO-based electrolyte co-modified by alkali metal iodides（such as lithium iodide,potassium iodide,sodium iodide）and GCN was designed,and the synergistic mechanism of composite filler was explored.GCN can adsorb and fix the position of iodine species to reduce the migration of iodine ions and promote the redox reaction kinetics of iodine ions.In addition to reducing the crystallinity of PEO and improving the ion conductivity of the electrolyte,the addition of GCN-assisted alkali metal iodide functional filler promotes the migration of lithium ions and constructs an iodine rich interface layer on the surface of the lithium metal anode,which is conducive to the stable deposition/stripping of lithium.The reversible redox reaction of iodine ions can provide additional capacity for the Li Fe PO₄/Li batteries.Taking GCN/Li I composite filler as an example,an Li Fe PO₄/Li battery based on this composite electrolyte can provide a discharge capacity of 228.4 m Ahg^-1 at 0.1 C.After 900 cycles at 0.5 C,the capacity retention rate is as high as 87.3%.The battery also achieves high discharge capacity of 99.2m Ahg^-1 at 10 C.