【作者】 李欣；

【导师】 谢海明；

【作者基本信息】 东北师范大学 ， 物理化学， 2021， 硕士

【摘要】 采用不燃、不漏液、易封装的固态电解质代替传统的有机液态电解质有望从根本上解决传统锂离子电池的安全问题。在目前的固态电解质体系中,有机-无机复合电解质（CSEs）结合了固态聚合物电解质（SPEs）和无机固态电解质（ISEs）的优点,兼具较好的离子导电性和较高的机械强度,与单一的SPEs和ICEs相比,电化学性能被大幅提升。然而,要实现CSEs的实际应用仍有诸多挑战,如界面稳定性差、界面电阻高等。此外,其与高电压正极较差的相容性,导致了现有的CSEs通常只能与低电压平台正极如Li Fe PO₄配合使用,这严重限制了其在高比能量密度固态锂电池中的应用。向固态电解质|电极界面添加少量液态电解质形成固液混合电解质是解决这类问题的有效办法之一。然而,传统碳酸酯类液态电解质的加入不仅牺牲了固态电池的安全优势,且其与固态电解质（SEs）之间的化学稳定性差,易形成新的固液电解质界面杂质相层（SLEI）,从而导致电池的整体内阻增大,影响其电化学性能。因此,探究SLEI的组分,分析SLEI的形成机理,对实现固液混合电解质在高电压固态锂电池中的应用具有重要意义。基于此,本论文主要进行了以下研究:（1）将纳米Li_6.4La₃Zr_1.4Ta_0.6O₁₂（nano-LLZTO）粒子作为无机填料加入到聚偏氟乙烯-共六氟丙烯（PVDF-HFP）聚合物中,用溶液法流延法制备了nano-LLZTO/PVDF-HFP超薄（25μm）复合固态电解质。将不燃的高氟化含硼电解液（FEC:HFE:PFEMC=1:1:4 0.95 M Li TFSI+0.05 M Li BOB）添加到高电压正极Li Ni_0.6Co_0.2Mn_0.2O₂（NCM622）与CSEs界面作为界面改性剂,提升界面化学、电化学稳定性。电化学实验结果表明,25℃时,在1 C倍率下,NCM622|B,F-CSE|Li电池在截止电压为4.3 V下的首圈放电比容量为152.3 m Ah g^-1,电池循环300圈后,放电比容量仍可达143.8 m Ah g^-1。即使在2 C高倍率下,放电比容量依然可达146 m Ah g^-1。在4.4 V截止电压下,以0.1 C倍率循环100圈后,电池的容量保持率为97.9%。当截止电压增加到4.5 V时,初始放电比容量为194m Ah g^-1,在循环50圈后容量衰减率仅为0.6%。（2）通过监测NCM622|NCM622对称电池中的界面阻值变化,证明了B,F-CSE与NCM622正极具有良好的界面兼容性。通过计算电极|电解质界面的界面Li⁺迁移活化能（Ea）,发现B,F-CSE的Ea值仅为7.98 k J mol^-1。通过非原位表征技术XPS、TEM、SEM和NMR手段对SLEI的形成机理进行了分析。结果表明,在高氟溶剂环境下,Li BOB的引入能够抑制SLEI层中Li F的过量生成,在SLEI层中生成具有高离子导电性的Li_xBO_yF_z,不仅促进了界面Li⁺的快速运输,还可以对正极起到的一定的保护作用。最终,使固态电池在高截止电压下的循环稳定性和倍率性能均得到显著提升。我们的研究提供了一种全新的界面构建策略,将SLEI从“真正的罪魁祸首”转变为“救世主”,为获得高稳定性、低阻抗的SLEI开辟了一条全新的途径。更多还原

【Abstract】 Solid-state electrolyte with non-flammability,no leakage problem and easy encapsulation instead of conventional organic liquid electrolyte is expected to fundamentally solve the safety problem of the traditional lithium ion battery.Currently,organic-inorganic composite solid electrolytes（CSEs）combine the advantages of solid polymer electrolytes（SPEs）and inorganic solid electrolytes（ISEs）,exhibit both good ionic conductivity and high mechanical strength.Compared with single SPEs and ICEs,the electrochemical performances of CSEs are greatly improved.However,there are still many challenges to realize the practical application of CSEs,such as poor interface stability and high interface resistance.In addition,its poor compatibility with high-voltage cathodes has resulted in existing CSEs that can only be used with low-voltage platform anodes such as Li Fe PO₄,which severely limits their application in high specific energy density solid-state lithium batteries.Adding a small amount of liquid electrolyte to the solid electrolyte|electrode interface to form a solid-liquid hybrid electrolyte is one of the effective ways to solve the interfacial issues.However,the addition of traditional carbonate-based liquid electrolytes not only sacrifices the safety advantages of solid-state batteries,but also shows the poor chemical stability with solid-state electrolytes.Typically,solid electrolyte and liquid electrolyte are generally（electro）-chemically incompatible and the resistance of the newly-formed solid-liquid electrolyte interphase（SLEI）appears as an additional contribution to the overall battery resistance.Therefore,SLEI plays a significant role in regulating interfacial stability and Li⁺transportation,further profoundly affecting the morphology and structural stability of electrode,and the regulating the SLEI chemistry by selection of functional solvent and salt additive is of great significance to realize the application of solid-liquid hybrid electrolyte in high-voltage solid-state lithium batteries.Based on the above,the research contents of this thesis are as follows:Nano Li_6.4La₃Zr_1.4Ta_0.6O₁₂（nano-LLZTO）particles were added as inorganic fillers to the polyvinylidene fluoride-co-hexafluoropropylene（PVDF-HFP）polymer,and an ultra-thin（25μm）composite solid electrolyte（nano-LLZTO/PVDF-HFP）was prepared by solution casting method.A non-flammable high-fluorinated boron-containing electrolyte（FEC:HFE:PFEMC=1:1:4 0.95 M Li TFSI+0.05 M Li BOB）is added to the interface between the high-voltage cathode Li Ni_0.6Co_0.2Mn_0.2O₂（NCM622）and the CSEs as an interface modifier to improve the stability of chemical、electrochemical interface.At a cut-off voltage of 4.3 V（25℃）,the initial discharge specific capacity of NCM622|B,F-CSE|Li batteries is 152.3 m Ah g^-1 at a current density of 1 C.After 300 cycles,the specific discharge capacity can still reach 143.8m Ah g^-1.Even at a high rate of 2 C,the specific discharge capacity can still reach 146 m Ah g^-1.At a cut-off voltage of 4.4 V,after 100 cycles,the battery capacity retention at 0.1 C was97.9%.When the cut-off voltage is increased to 4.5 V,the initial discharge specific capacity is194 m Ah g^-1,and the capacity decay rate is only 0.6%after 50 cycles.By monitoring the interface resistance changes in the NCM622|NCM622 symmetric battery,it is proved that B,F-CSE and NCM622 positive electrode have good interface compatibility.By calculating the interface Li⁺migration activation energy（Ea）of the electrode|electrolyte interface,it is found that the Ea value of B,F-CSE is only 7.98 k J mol^-1.The formation mechanisms of SLEI were analyzed by ex-situ XPS,TEM,SEM and NMR characterization techniques.The results show that Li BOB decomposes into Li DFOB,and further into Li_xBO_yF_z ionic conducting under F^-excessive condition,which dynamically inhibits side reactions,maintains the initial structure of NCM622 cathode and thus reinforces interfacial stability.Moreover,Li BOB additive can effectively regulate the Li F content in high fluorinated solvent condition,which avoid over-accumulation of highly resistive Li F(10^-9～10^-11 S cm^-1)in the SLEI to sluggish Li⁺transport kinetics.We innovatively proposed an interfacial engineering strategy to turn SLEI from“real culprit”into“savior”,which opens up a new way to obtain a high-stability,low-impedance SLEI.更多还原