Advancement of solid-state batteries through the understanding of cathode-solid electrolyte interactions

Abstract

The aim of this thesis was to obtain fundamental insights in the interaction between the polymeric eutectogel (P-ETG) and NMC622 positive electrode materials on the one hand, and obtaining expertise in the integration of the P-ETG electrolyte in solid-state batteries on the other hand, with the final goal the realization of a high-performance positive electrode for solid-state batteries. The flexible P-ETG electrolytes were introduced by Joos et al. at the DESINe group (UHasselt) as a hybrid solid-state electrolyte in which a lithium-ion conducting deep eutectic solvent (DES) is confined within a polymeric backbone. They demonstrate a broad electrochemical stability window (1.5 − 4.5 V vs. Li+/Li) and a high ionic conductivity at room temperature (0.76 mS cm−1). Therefore, the P-ETG holds potential as an inexpensive and flexible electrolyte for the nextgeneration solid state Li-ion batteries with high-potential positive electrode materials. Given the availability of this interesting candidate solid electrolyte, the bottleneck of solid-state battery development today is not only limited to maximize the ionic conductivity. Another main issue of solid-state batteries is the integration of the solid-state electrolyte and electrodes. The challenge of cell integration is associated mainly with maintaining chemical and mechanical stability between the electrodes and the solid-state electrolyte during battery operation, besides controlling the properties of the electrode/electrolyte interface. A good interface between a solid electrolyte and electrode requires fast ion transport, optimum contact area, and chemical stability during cycling. However, experimentally, it was found that the reported P-ETG containing 4-acryloyl morpholine (ACMO) backbone units has limited chemical stability in contact with high-potential positive electrode materials, such as NMC622. Therefore, the already existing P-ETG needed to be optimized to be electrochemical compatible with the commercially relevant NMC622 positive electrode materials. Chapter 3 is was seen that the (electro)chemical compatibility between solid electrolyte and positive electrode active material is very important and has a significant effect on the battery performance. The incompatibility between the ACMO-based P-ETG and high-nickel NMC622 positive electrode material, resulting in fast capacity fading, was tackled by adapting the polymer backbone. To this end, replacing the ACMO backbone with N-isopropyl acrylamide (NIPAM) was successful. The NIPAM was selected because it does not contain any ether functions, which was hypothesized to be responsible for limiting the cycle life in combination with high-energy, positive electrodes. This NIPAM-based P-ETG shows interesting properties, such as relatively high ionic conductivity (0.82 mS cm-1) and a broad electrochemical stability window (1.5 – 5.0 V vs. Li+/Li). The PETG also displays a higher thermal stability compared to conventional liquid electrolytes (i.e., 1 M LiPF6 in EC/EMC (3/7) + 2% VC), leading to a suppressed flammability which results in increased safety in the use of a battery applications, such as electric vehicles. Physico-chemical characterization techniques, such as PXRD, ATR-FT-IR, ICP-AES, and EIS, indicated that the NIPAM-based P-ETG electrolyte and NMC622 active positive electrode material are chemically compatible. However, a chemical instability of the P-ETG-85/Li interface leads to the formation of resistant films. The Li|NMC622 coin cells assembled with this NIPAM-based P-ETG as the electrolyte were found to deliver a capacity of 134, 110, and 97 mAh g-1 over 90 cycles at C/5, C/2, and 1C. The coulombic efficiency is exceeding 95% for C/5, C/2, and 1C. In contrast to the former generation of PETG, which had the limitation that it could only function in conjunction with LiFePO4, a low energy density cathode material, our novel composition of P-ETG overcomes this limitation and elevates the P-ETG up to the league of the realworld high voltage batteries built with high Ni-NMC, such as NMC622. In conclusion, the NIPAM-based P-ETG shows a great promise as an inexpensive and safe electrolyte for the next generation solid state Li/Li-ion batteries. Nonetheless, further research was performed in chapter 4 to develop new stateof-the-art P-ETGs focussing on the interaction of the polymeric framework and the enclosed DES. The properties of polymeric backbone eutectogel (P-ETG) electrolytes for LIBs, created by DES confined inside various polymeric networks are studied. Different P-ETG compositions were synthesized by varying the monomer and crosslinker. The aim of this chapter was to study the effect of the monomer and crosslinker by obtaining a more in-depth knowledge on the interaction between the polymer and the DES. First, it was found that the composition of the DES plays an important role on its electrochemical properties. The improved anodic stability of DES composition with high LiTFSI concentrations indicates that also in the Li-based DES hydrogen bonds between MAc molecules are replaced with strong ionic interactions between MAc, Li+ and TFSI- , lowering the concentration of free and electrochemically unstable MAc molecules. Secondly, the DES was incorporated into a polymer matrix. LSV and NMR experiments indicated that the incorporation of the DES into the polymeric matrix affects the interaction of LiTFSI and MAc inside the DES. To obtain a P-ETG with a high ionic conductivity and anodic stability, a high vol% DES is necessary. Thirdly, varying the type of functional groups in the polymeric backbone (monomer or crosslinker) resulted in limited changes in interactions between DES and the various polymeric backbones according to NMR spectroscopy. However, this could be explained by the fact that only acrylamide-based monomers were studied. In future work, the properties of the P-ETG electrolytes could be improved to facilitate the transport of more Li+ ions by investigating the effect of immobilization of the DES components, such as MAc and TFSI- by means of hydrogen bonding to functional groups present in the polymeric backbone matrix. In a second part of chapter 4, the effect of the various P-ETGs on the electrochemical properties was studied. The ionic conductivities depend on the monomer and crosslinker, reaching values from 0.400 mS cm-1 for ACMO-EGDMA to 1.011 mS cm-1 for DEAA-EGDMA at 25°C. The choice of the crosslinker has a significant effect on the ionic conductivity as amides (MBAA) result in less mobile chains compared to esters (EGDMA) due to increased rotational barriers in MBAA, a shorter chain length, and the possibility of the MBAA N-H to hydrogen bond with MAc, all reducing the mobility of the DES in the polymeric matrix. Using several P-ETGs as the electrolyte, the Li | NMC622 coin cells were assembled. The cells demonstrated initial specific discharge capabilities of 163, 136, 136, and 24 mAh g-1 at C/10 for NIPAM, HEAA, DEAA, and ACMO in combination with MBAA crosslinker, making the NIPAM-MBAA P-ETG composition the most promising electrolyte. EIS indicated that the P-ETGs with the NIPAM, HEAA, and DEAA backbones are electrochemically compatible with high-voltage NMC622 active positive electrode materials. However, resistant surface films arise as a result of the chemical instability of all P-ETG | Li interfaces, limiting the life-time of the batteries. However, the development of solid-state batteries is not only limited to the development of new solid electrolytes. The replacement of the conventional liquid electrolyte by the solid electrolytes also introduces new challenges for cell production processes, and its steps need to be adapted or even redesigned. In the previous chapters, we showed that the P-ETG shows a great promise as an inexpensive and safe electrolyte for the next generation solid state Li/Li-ion batteries. Nonetheless further research was required to investigate the long-term stability of the solid-solid contacts between the P-ETG and the electrodes by improving the processing method and developing a composite electrode containing the NMC622 active material and P-ETG electrolyte. For commercial application of solid electrolytes and their electrodes for high capacity solid-state batteries, a wet/solution process using a slurry coating method is preferable. The aim of chapter 5 was the preparation of a composite positive electrode for high-energy density Li-ion batteries by infiltrating the P-ETG electrolyte precursor into the pores of the positive electrode. Knowledge was obtained on the realisation of the composite positive electrode, and performance limiting factors, such as electrolyte thickness, positive electrode active material loading, porosity, and electrolyte composition. The preparation of a composite positive electrode for high-energy density lithium-ion batteries was studied by infiltrating the liquified P-ETG electrolyte into the pores of the positive electrode. The P-ETG solution infiltration shows uniform distribution of the P-ETG electrolyte inside the pores of the NMC622 positive electrode, even deep inside the electrode. This solution infiltration process, called the in-situ synthesis method, enables the formation of intimate ionic contacts between electrodes and electrolyte as well as densification the positive electrodes. Long-term cycling tests verify the reliability of the P-ETG-infiltrated composite NMC622 electrodes in Li NMC622 coin cells. However, increasing the areal capacity of the NMC622 positive electrode from 0.11 mAh cm-2 to 0.61 mAh cm-2 resulted in significant capacity fading which could be explained by the stability issues between our P-ETG electrolyte and the Li metal anode at higher current densities. A stable discharge capacity was obtained by using LTO negative electrode materials and the in-situ synthesis method for the P-ETG electrolyte resulting in a more stable discharge capacity with an initial discharge capacity of 110 mAh g-1 , and 76.6% of the initial capacity is remained after 90 cycles, compared to 56.8% for the noninfiltrated NMC622 positive electrodes. The effect of the positive electrode porosity and electrolyte thickness was studied using design of experiments. It could be seen that the effect of the electrode porosity was not significant, but the electrolyte thickness has a significant effect on the initial discharge capacity. Decreasing the electrolyte thickness results in higher initial discharge capacities. The effect of the polymeric backbone on the battery performance using the in-situ P-ETG synthesis method developed in this work was studied in LTO | P-ETG | NMC622 cells. It was seen that the DEAA-MBAA monomer-crosslinker combination resulted in the highest and most stable discharge capacity at all C-rates, with even a discharge capacity around 20 mAh g-1 at 3C. Therefore, it is concluded that in this chapter knowledge was obtained on the realisation of a promising composite positive electrode for the use in the next-generation solid state Li-ion batteries. Increasing the areal capacity of the NMC622 positive electrodes even further to 0.98 mAh cm-2 resulted in a decreased discharge capacity. Possible issues that could explain this reduced discharge capacity could be the poor conduction in the positive electrode or conduction over the interphase between electrode and electrolyte. To solve this issue, the P-ETG electrolyte could also be processed in the electrode slurry. This could result in a higher, and more homogeneous Li-ion conduction in the positive electrode, combined with a better interphase contact between electrode and electrolyte. Although there is lots of experience in processing of porous electrode sheets for lithium-ion battery applications with conventional liquid electrolytes, there is little experience in processing of compact electrodes optimized for solid-state batteries. One of the challenges is the formulation of a slurry recipe containing the active material and the solid electrolyte. Many parameters of the electrode fabrication process have an influence on the final battery performance, which makes the electrode fabrication process a critical and complicated step in battery assembly. Therefore, the last aim of this thesis was the preparation of a composite positive electrode for highenergy density lithium-ion batteries by the development of one-pot method in which the NMC622 active material, the P-ETG precursor solution electrolyte, and carbon additives are mixed and tape-casted onto a current collector. Different processing procedures were tested in this work, and knowledge on the slurry recipe and processing parameters was be obtained by studying the effect of the loading of the active material and solid electrolyte in the composite positive electrode. One important finding was that the polymerization of the P-ETG processed in the positive electrode was hindered by the presence of carbon black. To solve this issue, the UV-initiator was replaced by a thermal-initiator, allowing to start the polymerization by a heat pulse. However, this thermal polymerization step was responsible for the loss of a part of the MAc in the DES. Applying the insitu synthesis method to fill the pores of the positive electrodes with P-ETG precursor, made it possible to obtain dense positive electrodes. The performance was tested in LTO | P-ETG | NMC66 cells, resulting in reasonably high and stable discharge capacities for 30 cycles. Almost all composition of positive electrodes synthesized using the one-pot method, mixing the NMC622 active material and PETG electrolyte, result in a high initial discharge capacity compared to the infiltrated NMC622 electrodes using the in-situ synthesis method described in chapter 5. This indicates the importance of the processing of the solid electrolyte inside the electrode. Decreasing the NMC622 active material loading resulted in the highest discharge capacity. However, increasing the NMC622 active material loadings resulted in a decreased initial discharge capacity, indicating the complexity of an electrode preparation process and the importance of the knowledge on the performance limiting factors. Therefore, additional research on the processing of on-pot positive electrodes containing the NMC622 active material and the P-ETG electrolyte is needed to identify the processing limiting factors and to improve the NMC622 active material loading, discharge capacity and cycle life of the cell. 2. Impact for battery/solid electrolyte community Since the publication of the first silica-based eutectogel as a new class of solid composite electrolytes by Joos and co-workers [1] in 2018, there has been an increase in research into the use of liquid DES electrolytes as non-flammable, hybrid solid-state electrolytes for lithium-ion batteries. Especially, the introduction of the polymer-based eutectogel electrolytes showed a great potential due to their facile synthesis method and promising properties, such as a high ionic conductivity at room temperature and relatively broad electrochemical stability window. [2] Many compositions of DES-containing hybrid electrolytes were described in literature during the past years. [3]–[8] However, most publications focus only on the development of a new electrolyte composite and its properties, such as increasing the ionic conductivity even to higher values. This are indeed very interesting and needed topics, however, given the availability of many promising solid electrolytes, the bottleneck of solid-state battery development is no longer to maximize the ionic conductivity or the anodic stability limit. The main issue has now shifted towards the integration of the solid electrolyte and the electrode materials in a battery. This PhD focusses on the combination of the synthesis of the P-ETG electrolyte and its integration within a porous electrode, which is mostly overlooked in the existing literature. A part of the results presented in chapter 3 and 4, resulted in two peer-reviewed publications, indicating its scientific relevance for the battery field and its possible contribution to the development of the next generation solid-state batteries. In summary, the work presented in this PhD thesis provides valuable insights for the battery and solid electrolyte community for the development of P-ETG as solid electrolytes, focussing both on their design and their integration into a battery.