Boron Trifluoride Anionic Side Groups in Polyphosphazene Based Polymer Electrolyte with Enhanced Interfacial Stability in Lithium Batteries

Abstract

A modified polyphosphazene was synthesized using a mixed substitution at phosphorus consisting of 2-(2-methoxyethoxy)ethoxy side groups and anionic trifluoroborate groups. The primary goal was to increase the low lithium ion conductivities of the conventional lithium salt containing poly[2-(2-methoxyethoxy)ethoxy-phosphazene] (MEEP) by the immobilized anionic groups. As in previous studies, the mechanical stability was stabilized by UV induced radiation cross linking. By variation of the molar ratio between different side groups, mechanical and electrochemical properties are controllable. The polymer demonstrated large electrochemical stability windows ranging between 0 and 4.5 V versus the Li/Li+ reference. Total and lithium conductivities of 3.6 × 10−4 S·cm−1 and 1.8 × 10−5 S·cm−1 at 60 °C were revealed for the modified MEEP. When observed in special visualization cells, dendrite formation onset time and short-circuit time were determined as 21 h and 90 h, respectively, under constant current polarization (16 h and 65 h for MEEP, both with 15 wt % LiBOB), which hints to a more stable Li/polymer interface compared to normal MEEP. The enhanced dendrite suppression ability can be explained by the formation of a more conductive solid electrolyte interphase (SEI) and the existence of F-contained SEI components (such as LiF). With the addition of ethylene carbonate–dimethyl carbonate (EC/DMC) to form MEE-co-OBF3P gel polymer, both total and lithium conductivity were enhanced remarkably, and the lithium transference numbers reached reasonable values (σtotal = 1.05 mS·cm−1, σLi+ = 0.22 mS·cm−1, tLi+ = 0.18 at 60 °C).

Characterization of the Particle-Polymer Interface in Dual-Phase Electrolytes by Kelvin Probe Force Microscopy

“…The glass transition temperatures (T g ) were determined by DSC. The pure cross-linked polymer MEEP showed a glass transition temperature of −81 °C which is in accordance to literature 58 and the addition of conducting salt increased the glass transition temperature up to −46 °C. However, the addition of LLZ particles leads to a decrease of T g and a kind of plasticizing effect can be observed.…”

A BF3‐Doped MXene Dual‐Layer Interphase for a Reliable Lithium‐Metal Anode

“…In parallel, the doped functional groups containing B and F further stabilize the SEI by suppressing the growth of lithium dendrites. [ 31,37 ] To better understand the lithium dendrite prevention, mossy/dendritic Li growth/depletion upon lithium plating and stripping inside a capillary cell was observed real time. During the plating (0–40 min) and stripping (40–80 min) on D‐Li at 5 mA cm −2 , homogeneous Li deposition/depletion without any mossy/dendritic lithium was observed (Figure S8a, Supporting Information).…”

Increasing the Lithium Ion Mobility in Poly(Phosphazene)-Based Solid Polymer Electrolytes through Tailored Cation Doping

“…Salt analytics.-All experiments were carried out using alkali metal BOB salts as both conducting agent and additive instead of other typical alkali metal salts due to the reportedly high interfacial compatibility of MEEP polymers with LiBOB content towards lithium metal as well as LiBOB's positive influence on the formation of an effective SEI. [72][73][74][75][76] A known synthesis route for LiBOB was modified to prepare KBOB in this work, which has not been described in literature thus far. 67 The synthesized KBOB was analyzed with 13 C and 11 B NMR spectroscopy to confirm the successful synthesis; the spectra are depicted in Fig.…”