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The ionic transport in solid-state composite electrolytes based on poly(trimethylene carbonate) (PTMC) with LiTFSI salt and garnet-type ion-conducting Li6.7Al0.3-La3Zr2O12 (LLZO) ceramic particles is here investigated for a range of different compositions. Positive effects on ionic conductivity have previously been reported for LLZO incorporated into poly(ethylene oxide) (PEO), but the origin of these effects is unclear since the inclusion of particles also affects polymer crystallinity. PTMC is, in contrast to PEO, a fully amorphous polymer, and therefore here chosen for the design of a more straight-forward composite electrolyte (CPE) system to study ionic transport. With LLZO loadings ranging from 5 to 70 wt%, the CPE with 30 wt% of LLZO exhibits the highest ionic conductivity with a cationic transference number of 0.94 at 60 degrees C. This is significantly higher than for the pristine PTMC polymer electrolyte. Generally, low to moderate LLZO loadings display a gradual increase of the ionic conductivity, transference number and also of the polymer-cation coordination number. The combined contributions of ionic transport along polymer-ceramic interfaces and Lewis acid-base interaction between the LLZO particles and the LiTFSI salt can explain this enhancement. With loadings of LLZO above 50 wt%, a detrimental effect on the ionic conductivity was however observed. This could be explained by agglomeration of ceramic particles, and by a partial coverage of LLZO particles with a Li2CO3 layer. Consequently, inner polymer-particle interfaces become more resistive, and Li+conduction is prevented along interfacial pathways. The presence of Li2CO3 has more detrimental impact at higher LLZO loadings, since inter-particle connectivity will be hampered, and this is vital for efficient ionic transport. This suggests that there is an interplay between the LLZO particle surface chemistry with its loading, which ultimately controls the Li-ion transport.

Poor ionic conductivity, low Li+ transference number, and limited electrochemical stability plague all-solid-state Li-metal batteries based on solid polymer electrolytes (SPEs). One strategy to overcome these hurdles is the insertion of ceramic fillers to generate composite polymer electrolytes (CPEs). These are based either on active (ion-conductive) fillers like Li7La3Zr2O12 or passive (non-conductive) fillers like Al2O3. In this work, the effect of passive Li-containing fillers is showcased, exemplified by a CPE platform of poly(trimethylene carbonate) (PTMC:LiTFSI) with LiAlO2 particles. The inclusion of such fillers shows a strikingly positive effect. The ionic conductivity is greatly improved by one order of magnitude at 20 wt% of LiAlO2 compared to the pristine PTMC SPE. Moreover, the Li+ transference number is significantly boosted and reaches values close to unity (T + = 0.97 at 20 wt% of LiAlO2), effectively rendering the material a single-ion conductor. The CPEs show outstanding cycling stability vs Li-metal, and electrochemical stability of up to 5 V vs Li+/Li. When implemented in a solid-state battery cell with LiNi0.33Mn0.33Co0.33O2 (NMC111) and Li-metal, a stable cycling performance for over 100 cycles is observed. This demonstrates the potential of using microsized and cost-effective LiAlO2 fillers in CPEs for applications in all-solid-state Li-metal batteries.

Solid composite polymer electrolytes (CPEs) are complex mixtures of ceramics, polymers, and lithium salts, where the interfaces between the different phases play an important role for stability, conductivity, and compatibility with electrode materials. In this study, two interfacial phenomena of CPEs consisting of lithium lanthanum zirconium oxide (LLZO) ceramic fillers in poly(trimethylene carbonate) (PTMC) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt are studied. First, the LLZO-polymer electrolyte interfaces are investigated. Second, the stability of this CPE material vs a Li-metal electrode is explored, by employing soft X-ray photoelectron spectroscopy (PES) in combination with in situ deposition of Li. Three different LLZO loadings in PTMC are investigated: 30, 50, and 70 wt %. The concentration of LiTFSI follows that of the particle concentration at the surface of the samples, where the CPE with 50 wt % bulk content of LLZO exhibits the highest surface concentrations of both salt and ceramic. This shows an affinity for the salt at the LLZO surface. Furthermore, the stability of the CPEs against Li is studied after in situ Li deposition and shows that PTMC can decompose, potentially forming polypropylene at the CPE|Li interface, with the CPE at 50 wt % of LLZO showing the most pronounced PTMC and TFSI breakdown. This is in agreement with the observed properties for the polymer-ceramic interfaces and highlights the decisive role of LiTFSI accumulation on the surface of the ceramic particles, both for ionic transport and chemical stability.

Despite being promoted as a superior Li-ion conductor, lithium lanthanum zirconium oxide (LLZO) still suffers from a number of shortcomings when employed as an active ceramic filler in composite polymer–ceramic solid electrolytes for rechargeable all-solid-state lithium metal batteries. One of the main limitations is the detrimental presence of Li₂CO₃ on the surface of LLZO particles, restricting Li-ion transport at the polymer–ceramic interfaces. In this work, a facile way to improve this interface is presented, by purposely engineering the LLZO particle surfaces for a better compatibility with a PEO:LiTFSI solid polymer electrolyte matrix. It is shown that a surface treatment based on immersing LLZO particles in a boric acid solution can improve the LLZO surface chemistry, resulting in an enhancement in the ionic conductivity and cation transference number of the CPE with 20 wt % of boron-treated LLZO particles compared to the analogous CPE with non-treated LLZO. Ultimately, an improved cycling performance and stability in Li//LiFePO₄ cells was also demonstrated for the modified material.