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Solid state lithium batteries appear as viable options for next-generation energy storage systems addressing the issues related to the safety risks associated with conventional liquid electrolytes. The use of inorganic fillers in polymer electrolyte matrices significantly boost battery performance, making composite polymer electrolytes (CPEs) a subject of growing interest. This study serves as a proof of concept, investigating lithium phosphate (Li₃PO₄) as a reinforcing agent in poly(ethylene oxide)-lithium bis(trifluoromethanesulfonyl)imide (PEO-LiTFSI) electrolytes. A range of composite electrolytes with varying Li₃PO₄ concentrations (0-30 wt%) were prepared using solvent casting. The addition of Li₃PO₄ markedly improved the electrochemical properties, with the 20 wt% composition demonstrating the highest lithium-ion transference number (T₊ ∼0.33 at 60°C) and ionic conductivity (6.59 x 10^-5 S/cm at 60°C), as well as extended electrochemical stability up to 5.2 V. Despite its higher degree of crystallinity, this composition exhibited useful cycling stability in Li|Li cells for over 400h, attributed to improved interfacial contacts and mechanical properties, inhibiting lithium filaments growth. When tested in an NMC₈₁₁cell configuration, the composite electrolyte displayed stable cycling performance, achieving a coulombic efficiency of 95 %. These results advance the development of phosphate-based composite polymer electrolytes for safer, high-performance solid-state lithium batteries.

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.

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.

While polymer-based solid-state sodium batteries promise both safe operation and utilization of sustainable materials, they are held back by the insufficient ionic conductivity of the involved solid polymer electrolytes (SPEs). In this study, the conductivity and cation transference number are significantly improved through the construction of a composite polymer electrolyte (CPE) system based on poly(trimethylene carbonate) (PTMC) with sodium bis(trifluorosulfonylimide) (NaTFSI), combined with NaAlO₂ (NAO) ceramic filler at loadings ranging from 10 to 40 wt%. The NAO-based CPEs show the highest conductivity at 20 wt% NAO, with a Na⁺ transference number of ∼0.9 at 60 °C also being obtained for the same material, which is notably higher than that for the NAO-free SPE. Solid-state batteries composed of a Prussian white cathode and a Na metal anode and employing these CPEs reach a cycling performance of ∼100–150 mA h g⁻¹ at C/10 and 55 °C for more than 200 cycles without additives or plasticizers, thus opening the door to the potential exploration of CPEs for Na-based battery chemistries.

Since the state-of-the-art Li-ion batteries are close to reaching their theoretical limit in energy density, it becomes crucial to develop next-generation batteries that enable better safety, higher energy density, and longer lifetime. One such next-generation technology is solid-state batteries, employing solid-state electrolytes. Both polymer and inorganic electrolytes are well-explored in this context. While polymers are flexible and easily processable, their ionic conductivities are generally insufficient. Inorganic ceramics can be good ionic conductors, but display interfacial issues. Therefore, combining polymeric and ceramic material in composites polymer electrolytes (CPEs) can – in principle – be beneficial to merge the advantages of both categories. However, it is still unclear how to best construct such systems, and how the ions are actually transported in them. 

This thesis explores ionic transport in CPEs, both with ion-conducting (“active”) and non-ion-conducting (“passive”) ceramic fillers. The focus is on the amorphous polymer material poly(trimethylene carbonate) (PTMC), the active ceramic filler Li₇La₃Zr₂O₁₂ (LLZO), and the passive ceramic fillers LiAlO₂ (LAO) and NaAlO₂ (NAO). The ionic transport mechanism in PTMC:LLZO CPEs is determined to be dependent on two main factors: particle loading and surface chemistry. An increase in ionic conductivity up to 30 wt% of Li₇La₃Zr₂O₁₂ is seen due to formation of additional transport pathways along the polymer-ceramic interfaces, while higher loadings affect the ionic conductivity negatively. While this can partly be explained by particle agglomeration, the presence of Li₂CO₃ on the Li₇La₃Zr₂O₁₂ surface also contributes to retard the ionic movement along the interfaces. Therefore, boric acid treatment is explored as a strategy to enable a Li₂CO₃-free surface of Li₇La₃Zr₂O₁₂ particles, which renders improved ionic transport and battery performance. Boron-treated Li₇La₃Zr₂O₁₂ shows formation of LiBO₂, which yields a negative zeta-potential, indicative of interactions between the ceramic particles and Li⁺ ions. That the surface chemistry – rather than the bulk – of the ceramic filler ultimately controls the overall transport, opens the door towards employment of passive fillers. It is shown that LiAlO₂  particles can increase the ionic conductivity by one order of magnitude and the Li⁺ transference number to almost 1, effectively rendering the LiAlO₂-based CPE a single-ion conductor. These enhanced ionic transport properties can be explained by the ability of LiAlO₂  particles to promote better ion-ion separation through the attraction of negatively charged TFSI anions to the surface. This renders considerably improved battery performance, enabling cycling in Li||NMC cells. Similar effects are also seen for the analogous Na-ion battery system. 

Thereby, considering that the bulk conductivity of active fillers does not contribute to the overall ionic conduction in CPEs, and that passive fillers such as LiAlO₂  can greatly enhance the ionic transport because of its surface chemistry enabling greater ion-ion separation and favorable transport pathways, this thesis provides guidelines for future design of solid-state conductors for Li- and Na-batteries. 

Constructing a Z-scheme heterojunction with enhanced photocatalytic hydrogen evolution for graphitic carbon nitride-based (g-C3N4) composites is challenging because integrating g-C3N4 with other semiconductors, without specific band structure design, typically results in type I or type II heterojunctions. These heterojunctions have lower redox ability and limited enhancement in photocatalysis. Herein, we select highly crystalline carbon nitride (HCCN) as a proof-of-concept substrate. For the first time, we develop a AgBr nanosphere/HCCN composite photocatalyst that features an all -solid -state direct Z-scheme heterojunction for visible-light photocatalytic hydrogen evolution. The electron transfer mechanism is initially studied from the band structures and Fermi levels of HCCN and AgBr. It is subsequently confirmed by X-ray photoelectron spectroscopy (XPS), and electron microscopy. The close heterojunction contact and the built-in electron field of the Z-scheme heterojunction promote the migration and separation of photogenerated electrons and holes in the composite photocatalyst. Due to the redistribution of charge carriers, the photocatalyst shows superior redox capability and a markedly enhanced hydrogen evolution performance compared to its individual components. Combining all the advantages, AgBr nanosphere/HCCN reached an apparent quantum efficiency (AQE) of 6 % under the illumination of 410 nm, which is 4 times higher than that of the single HCCN component.

A limiting factor for solid polymer electrolyte (SPE)-based Li-batteries is the functionality of the electrolyte decomposition layer that is spontaneously formed at the Li metal anode. A deeper understanding of this layer will facilitate its improvement. This study investigates three SPEs – polyethylene oxide:lithium tetrafluoroborate (PEO:LiBF₄), polyethylene oxide:lithium bis(oxalate)borate (PEO:LiBOB), and polyethylene oxide:lithium difluoro(oxalato)borate (PEO:LiDFOB) – using a combination of electrochemical impedance spectroscopy (EIS), galvanostatic cycling, in situ Li deposition photoelectron spectroscopy (PES), and ab initio molecular dynamics (AIMD) simulations. Through this combination, the cell performance of PEO:LiDFOB can be connected to the initial SPE decomposition at the anode interface. It is found that PEO:LiDFOB had the highest capacity retention, which is correlated to having the least decomposition at the interface. This indicates that the lower SPE decomposition at the interface still creates a more effective decomposition layer, which is capable of preventing further electrolyte decomposition. Moreover, the PES results indicate formation of polyethylene in the SEI in cells based on PEO electrolytes. This is supported by AIMD that shows a polyethylene formation pathway through free-radical polymerization of ethylene.

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.

A correlation between the crystal structure, the ionic conductivity and the electrochemical performance in Lithium-ion batteries was established for a series of NASICON-type phosphates Li0.5M0.5Ti1.5Fe0.5(PO4)(3) (M = Mn, Co, Mg). These electrode materials, where the M1 site contains both lithium and the divalent cation M, were prepared using a simple sol-gel process while controlling the pH and the final synthesis temperature. The three phosphates crystallize in the rhombohedral system (S.G. R-3c) with comparable unit cell parameters but with slight difference in the local distortion of the PO4 tetrahedra as confirmed by the Raman study. The ionic conductivities of the Li0.5M0.5Ti1.5Fe0.5(PO4)(3) materials were measured at different temperatures using a wide range of frequencies. Mn-based phosphate shows the best features for application as electrode material for Li-ion batteries in term of the conductivity at room temperature and the activation energy of Li+ conduction process. The initial discharge capacity of 100 mAh.g(-1) was obtained for the Mg-based phosphate, 104.3 mAh.g(-1) for the Co-based material while the Mn-based material delivers the best first discharge capacity of 125.3 mAh.g(-1) with the lowest polarization in relation with its better conduction properties. This result was also confirmed by the rate capability tests where Mn-based phosphate shows enhanced electrochemical performance even at fast rate of 5C.