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Fluorinated compounds, including the polyvinylidene fluoride (PVdF) binder and lithium hexafluorophosphate salt, are considered essential components in lithium-ion batteries due to their ability to provide good performance and cycle life. However, these compounds raise potential environmental concerns, as they can lead to the formation of toxic, corrosive and persistent compounds, such as hydrofluoric acid, phosphorus pentafluoride (PF5) and per- and polyfluoroalkyl substances. In this study, the effects of a fluorine-free electrolyte consisting of lithium bis(oxalato)borate (LiBOB) salt with the vinylene carbonate (VC) additive, and an aqueous-based binder based on carboxymethyl cellulose (CMC) and latex are investigated for full cells combining a silicon–graphite composite anode with LiNi0.6Mn0.2Co0.2O2 (NMC622) cathodes. Higher capacity retention is obtained at C/2 after 500 cycles with the fluorine-free binder in the cathode (61% and 65% for the fluorine-free and fluorinated electrolytes, respectively) compared to the PVdF-based binder (58% and 56%, respectively). X-ray photoelectron spectroscopy analysis of the passivating layers on the cathode and anode across the four systems revealed a F-rich interfacial composition in the presence of the fluorinated electrolyte, while the fluorine-free electrolyte led to the formation of oxygen-rich layers at the interphases. Despite the chemical differences, both layers protect the cathode and anode during cycling, resulting in similar electrochemical performance. Moreover, the solid electrolyte interphase composition on the anode is dependent on the cathode's formulation. The aqueous-processed cathode resulted in higher coverage of the active material, which mitigated salt decomposition and facilitated the formation of a more stable passivating layer. This contributed to higher capacity retention despite a lower initial discharge capacity compared to the PVdF-based cathode. This study demonstrates the potential of fluorine-free components (electrolyte and a binder with an aqueous-processed cathode) to achieve high-energy-density full cells with comparable performance to conventional highly fluorinated lithium-ion 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.

Sodium-ion batteries are a promising alternative to lithium-ion batteries due to their potential for lower cost and greater sustainability. However, achieving stable cycling performance, particularly at high mass-loadings and elevated temperatures, remains a challenge. The stable cycling of high mass-loading sodium-ion battery cells is here made possible by addition of prop-1-ene-1,3-sultone (PES) to a non-flammable and fluorine-free electrolyte solution of sodium bis(oxalato)borate (NaBOB) salt in triethyl phosphate (TEP). This study investigates the thermal stability and electrochemical performance of such electrolyte at 40 degrees C and 55 degrees C, contrasting their performance with base NaBOB in TEP with and without ethylene sulfate (DTD) additive and with a reference carbonate electrolyte of NaPF6 in ethylene carbonate:diethylene carbonate (EC:DEC). Nuclear Magnetic Resonance spectroscopy was used to reveal degradation products formed in the electrolyte following a 4-weeks storage at 55 degrees C. Results from galvanostatic cycling at 55 degrees C demonstrated comparable performance of NaBOB-TEP + PES electrolyte and the reference carbonate electrolyte. The internal cell resistance was initially lower when cells were cycled at 55 degrees C than at 40 degrees C for all studied electrolytes.

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.

With the increasing attention to energy storage solutions, a growing emphasis has been placed on environmentally compatible electrolytes tailored for lithium-ion batteries. This study investigates the surface behavior of Si wafers as model systems cycled with a fluorine-free electrolyte based on lithium bis(oxalato)borate (LiBOB), with and without the additive vinylene carbonate (VC). By utilizing operando X-ray reflectivity (XRR) and ex situ X-ray photoelectron spectroscopy (XPS), the intricate processes involved in solid electrolyte interphase (SEI) formation is elucidated, SiO2/Si (de)lithiation, and the impact of the VC additive. Three distinct stages in SEI evolution during lithiation and delithiation are identified: SEI formation, subsequent densification and growth, and decrease in SEI thickness during delithiation, which collectively demonstrate the breathing behavior of the SEI during cycling. The addition of VC is found to mitigate LiBOB decomposition during cycling and promote a smoother SEI layer. Moreover, lithium trapping within the Si wafer post-delithiation is observed for both electrolytes but to a lesser extent with the addition of VC. This study offers structural and chemical insights into the fundamental processes governing SEI formation and Si wafer (de)lithiation in LiBOB-based electrolytes, with implications for designing environmentally friendly lithium-ion 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.

Ion transport properties such as ionic conductivity, diffusion coefficients, effective transference numbers, and interphase resistance are some of the most important electrochemical properties of polymer battery electrolytes. Precise measurement of those, particularly when polymers are of low molecular weight and in a semi-solid phase, is difficult. Here we develop an improved measurement methodology including cell development, give theoretical background necessary for the detailed understanding, and test those on high weight percent, 40-60 wt %, lithium bis(trifluormethylsulfonyl)imide salt in polyethylene oxide model system with low Mw between 2000 and 6000. We also give guidelines on how such experiments may go wrong, and the specific problems that occur.

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.

Solid polymer electrolytes (SPEs) are promising candidates for all-solid-state Li-metal batteries (ASSLMBs) due to their high safety and excellent mechanical flexibility. However, the widely used polyethers suffer from low ionic conductivity at ambient temperature and unstable electrode-electrolyte interfaces. In this work, we systematically investigate the reactivities with metallic lithium of three carbonyl-containing polymer-based SPE hosts-a polyketone (POHM), a polyester (PCL), and a polycarbonate (PTeMC)-as potential alternatives to polyethers by means of DFT calculations and AIMD simulations. Our redox potential and frontier orbital analyses indicate that introducing alkoxy oxygens connected to carbonyl groups enhances the electrochemical stability of polyester and polycarbonate, but also increases their reactivity on the Li anode surface. In particular, PTeMC shows higher electron uptake and a lower conduction band when interacting with surface Li. This increased reactivity, however, may also promote the formation of a stable solid electrolyte interphase (SEI), preventing further reduction of the electrolyte. We further summarize the possible decomposition mechanisms of the SPE polymer host and predict the resulting SEI components. The simulations revealed that POHM predominantly undergoes alpha-dehydrogenation and nucleophilic addition-elimination reactions, while PCL exhibits C_carbonyl-O_alkoxy bond cleavage, producing both saturated and unsaturated lithium alkoxides. In the case of PTeMC, breaking two C_carbonyl-O_alkoxy bonds can generate two saturated lithium alkoxides and a Li_xCO species, or it can produce a RCO₃Li species and unsaturated hydrocarbons via a C_alkoxy-O_alkoxy bond cleavage; these pathways are kinetically favorable and unfavorable, respectively. This work underscores the influence of alkoxy oxygens in carbonyl-containing polymers and provides computational insights for guiding polymer electrolyte design.

In this work, a quantitative method is developed to estimate different ion transport mechanisms in solid polymer electrolyte (SPE) systems. The well-explored poly(ethylene oxide) (PEO) is studied along with the poly(ɛ-caprolactone) (PCL) at different molecular weights and LiTFSI salt concentrations. By tracking the cation coordination changes, three transport mechanisms are categorized, i.e., ion hopping, continuous motion (successive change of the coordination sphere), and vehicular transport. The observed dominant transport mechanism is the continuous motion, and changes from polymer-mediated to anion-mediated with increasing salt concentration. Furthermore, a higher influence of polymer-mediated vehicular transport is observed in PCL systems than in PEO systems, and a correlation is found between the anion-mediated continuous motion and the cation transference number, irrespective of polymer and salt concentrations. In both systems, ion hopping is essentially absent, as can be expected in systems with strong ion–polymer interactions. The results illustrate both how the usual description of ion transport in polymer electrolytes as coupled to segmental motions is too simplistic to catch the full essence of the ion transport phenomena, whereas the frequently used notion of “ion hopping” in the majority of cases is incorrect for SPEs.