Sn2+ bridging enhanced oxidation stability of polyether solid-state electrolyte for stable sodium metal batteries
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摘要: 聚醚类固态电解质因本征电化学氧化窗口较窄而难以匹配高电压正极材料，同时钠金属负极在循环过程中面临枝晶不可控生长和固态电解质界面（SEI）持续破裂-重构的挑战，二者协同限制了固态钠金属电池的实际应用。研究使用“离子桥联”结构策略，以三氟甲磺酸锡（Sn(OTf)2）为路易斯酸引发剂，通过对1,3-二氧戊环（DOL）原位开环聚合并结合纤维素三乙酸酯（CTA），构建了SnPDOL-CTA双网络固态电解质。以不含金属离子的三（五氟苯基）硼烷（B(C6F5)3）引发体系作为对照，系统分析了电解质的聚合物结构、热性能、离子传输性能、电化学稳定性以及钠金属负极界面的沉积形貌和SEI化学组成，揭示了Sn2+离子桥联结构对电解质性能和负极界面稳定性的协同调控机制。Sn2+同时与PDOL链段和CTA骨架中的醚氧官能团配位形成双网络交联结构，体系最高占据分子轨道（HOMO）能级由−7.96 eV降低至−13.01 eV，电化学稳定窗口拓展至超过4.5 V。SnPDOL-CTA电解质的室温离子电导率为3.35×10−4 S/cm，Na+迁移数达0.64，钠对称电池稳定循环超过
1100 h，临界电流密度达1.1 mA/cm2。Sn2+在钠金属负极表面被原位还原，形成富含NaF和Na-Sn合金的有机-无机复合SEI层，有效引导钠的致密沉积并抑制枝晶生长。搭配磷酸钒钠和氟磷酸钒钠正极的半电池均表现出良好的循环稳定性与倍率性能。-
关键词:
- 钠金属电池 /
- 固态电解质 /
- 原位聚合 /
- 聚二氧戊环(PDOL)
Abstract: Polyether-based solid-state electrolytes suffer from an intrinsically narrow electrochemical oxidation window that limits their compatibility with high-voltage cathode materials, while sodium metal anodes face the challenges of uncontrollable dendrite growth and continuous rupture-reconstruction of the solid electrolyte interphase (SEI) during cycling. These two issues jointly hinder the practical application of solid-state sodium metal batteries. In this work, an "ion-bridging" design strategy is proposed, in which tin(II) trifluoromethanesulfonate (Sn(OTf)2) serves as a Lewis acid initiator to trigger the in situ ring-opening polymerization of 1,3-dioxolane (DOL) combined with cellulose triacetate (CTA), constructing a SnPDOL-CTA dual-network solid-state electrolyte. Using a metal-ion-free tris(pentafluorophenyl)borane (B(C6F5)3)-initiated system as the control, we systematically analyzed the polymer structure, thermal properties, ionic transport, and electrochemical stability of the electrolyte, together with the deposition morphology and SEI chemical composition at the sodium metal anode. This comparison reveals the synergistic modulation mechanism of the Sn2+ ion-bridging structure in electrolyte performance and anode interfacial stability. Sn2+ coordinates simultaneously with the ether oxygen functional groups in both the PDOL chains and the CTA backbone, forming a dual-network crosslinked structure that lowers the highest occupied molecular orbital (HOMO) energy level of the system from −7.96 eV to −13.01 eV, expanding the electrochemical stability window beyond 4.5 V. The SnPDOL-CTA electrolyte exhibits a room-temperature ionic conductivity of 3.35×10−4 S·cm−1 and a Na+ transference number of 0.64. Na||Na symmetric cells achieve stable cycling for over 1100 h with a critical current density of 1.1 mA·cm−2. Sn2+ is in-situ reduced on the sodium metal anode surface, forming an organic-inorganic composite SEI layer rich in NaF and Na-Sn alloys, which effectively guides dense sodium deposition and suppresses dendrite growth. The half cells assembled with Na3V2(PO4)3 and Na3V2(PO4)2F3 cathodes both deliver good cycling stability and rate capability. -
图 1 原位聚合固态电解质的设计与制备
(a)原位聚合实现电池一体化制备；(b) SnPDOL-CTA“离子桥联”结构示意；(c)分别由B(C6F5)3和Sn(OTf)2引发的DOL聚合过程实物照片
Figure 1. Design and preparation of the in-situ polymerized solid-state electrolytes
图 2 DOL聚合机理与电解质热性能
(a)B(C6F5)3引发DOL聚合的反应机理; (b)Sn2+引发DOL聚合的反应机理；(c)不同PDOL基电解质的TGA曲线；(d)液态DOL、BPDOL-CTA和SnPDOL-CTA的1H NMR谱图；(e)对应的13C NMR谱图；(f)不同PDOL基电解质的DSC曲线
Figure 2. DOL polymerization mechanism and thermal properties of the electrolytes
图 3 电解质的结构与形貌表征
(a)FTIR光谱；(b)拉曼光谱；(c)BPDOL-CTA表面的SEM图像及元素分布；(d)BPDOL-CTA截面的SEM图像及元素分布；(e)SnPDOL-CTA表面的SEM图像及元素分布；(f)SnPDOL-CTA截面的SEM图像及元素分布
Figure 3. Structural and morphological characterization of the electrolytes
图 4 电解质的离子传输与电化学稳定性
(a) EIS谱图；(b) SnPDOL-CTA和(c)BPDOL-CTA的Na||Na对称电池极化曲线(插图为极化前后EIS)；(d)四种电解质的Na||Na对称电池CV曲线；(e)对应的Tafel曲线；(f)四种电解质中Na||Cu半电池的恒流沉积曲线和(g) LSV曲线；(h) NVPF正极体系的电化学浮充测试；(i)不同分子的HOMO能级及优化几何构型
Figure 4. Ionic transport and electrochemical stability of the electrolytes
图 5 Cu||Na电池中Cu表面钠沉积形貌
BPDOL-CTA: (a) 1 mA/cm2, (b) 3 mA/cm2, (c) 5 mA/cm2,； SnPDOL-CTA: (a) 1 mA/cm2, (b) 3 mA/cm2, (c) 5 mA/cm2
Figure 5. SEM images of Na deposits on Cu surface in Cu||Na cells
图 6 钠对称电池测试与循环后负极表征
临界电流密度测试: (a)BPDOL-CTA, (b) SnPDOL-CTA；(c) 0.1 mA/cm2下的长循环性能; 钠对称电池循环后钠金属表面的SEM图像及对应元素分布: (d) BPDOL-CTA, (e) SnPDOL-CTA
Figure 6. Sodium symmetric cell tests and post-cycling anode characterization
图 7 半电池性能
(a) Na||NVPF电池的倍率性能；(b) SnPDOL-CTA基Na||NVPF电池的充放电曲线；(c) Na||NVP电池的循环性能；(d) SnPDOL-CTA基Na||NVP电池的充放电曲线
Figure 7. Half-cell performance
图 8 SnPDOL-CTA 钠对称电池循环后SEI的XPS谱图
(a)全谱；(b) C 1s；(c) F 1s；(d) Na 1s；(e) Sn 3d；(f) N 1s；(g) B 1s；(h) O 1s；(i) S 2p
Figure 8. XPS spectra of the SEI after cycling in a SnPDOL-CTA sodium symmetric cell
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