Further improving aqueous superconcentrated-based LIB by expanding the ESW limit at the negative electrode side
Several strategies are employed to solve the cathodic challenge. On the one hand, the tuning of the electrolyte properties with the removal of free water molecules from the electrolyte149, which would potentially open the path for increased ESW, or the use of additive to suppress water from Li+ solvation sheath, as reports pointed towards the greater reactivity of water molecules participating to the Li+ solvation sheath compared to free water molecules144 are envisioned. On the other hand, several research groups focused their efforts on the use of coatings capable of preventing water to access the interface. In binary mixtures, the electrolyte concentration is found to increase to greater values than the solubility limit of each of the two salts, thus reducing the amount of free water. Indeed, a hydrated salt can dissolve a non-hydrated salt which possess similar chemical properties. Mix-anions WiBS electrolytes58,122,180,181, i.e. two salts based on the same cation but different anions, were first introduced in 2016 by Suo et al.122 and Yamada et al.58, using 21 m LiTFSI : 7 m LiOtf and Li(TFSI)0.7(BETI)0.3 2∙H2O, respectively. These systems are generally based on the use of stable chaotropic anions, i.e. disrupting the bulk-like water interactions, and good SEI-former anions, as rationalized by Reber et al.182. However, despite the decrease of free water molecule in the bulk, the increase in ESW for mix-anion systems is very limited, if not inexistent at the negative electrode (see Figure 1. 23).
Mix-cation electrolytes183,184, i.e. two salts based on one anion but using two different cations (Na-Li, K-Li or Na-K-based ones), enable to achieve greater solubility than mix-anion162, further minimizing the water to salt ratio. Nonetheless, a major drawback of this strategy is the co-intercalation184 of both alkali cations that leads to fast performances decay. Thus, ammonium inert co-cations (such as tetraethylammonium (TEA+) or trimethylethylammonium (Me3EtN+)) with larger radii were used162,183 (see Table 1. 1, for a comparison of cation radii).
Reducing water activity by polymer electrolyte
The incorporation of WiBS in a polymer matrix enables to reduce water activity197 by coordinating water with the polymer moieties. UV-curable gel-polymer electrolyte (GPE) (see Table A.1. 3, at the end of the chapter for details) using low viscosities polymers was proposed to enable good wetting of the porous electrodes197–199. Moreover, in the spirit of suppressing the use of fluorinated salts and developing low-cost electrolyte, He et al.200 reported a Water-in-ionomer electrolyte based on 50 wt% LiPAA but constraining the negative material selection to the use of TiO2. Furthermore, the suppression of the classical H2O-H2O H-bonds structure and the reduction of the amount of free water is promoted upon addition of PEG201,202 or PEO203. Such strategy enables decreasing the concentration of LiTFSI down to 2 m LiTFSI in a 2 m LiTFSI∙PEGx(H20)(1-x), 5371 < x (wt%) < 94, leading to a decrease in cost and toxicity, as reported by Xie et al.201 and Li et al.202 However, alike for classical WiSE, a Li(H2O)4+-rich domain and a polymer-LiTFSI-rich domain are found, suggesting that the cathodic challenge remains unsolved203. Besides, in these GPE, alike previously observed for organic/aqueous electrolytes, the ESW and the transport properties were found to be directly related to the water content201,203. Indeed, increasing the water concentration from 6 wt% to 29 wt% in a LiTFSI-PEG-based GPE enhances the conductivity from 0.9 mS/cm to 3.4 mS/cm, while unfortunately reducing the ESW by ≈ 500 mV201. Eventually, in 2017, Yang et al.204 reported the assembly of batteries using metallic Li or graphite in combination with high potential positive electrode using a WiBS-gel polymer based on 21 m LiTFSI : 7 m LiOtf with 10 wt% of polyvinyl alcohol (PVA). Nonetheless, these outstanding performances rely more on an extra organic polymer coating layer used to protect the negative electrode from the HER, as shown in Figure 1. 29, than on the use of WiBS.
interface optimization : inotganic or organic coatings on the negative electode to from artifical SEL
The use of metallic Li or graphite in WiSE-based electrolyte were shown to be enabled by the use of organic-coatings that (i) provides good mechanical properties, (ii) prevents water from accessing the interface and (iii) may contribute to the SEI formation. The first type of organic coating integrates a GPE consisted of a HFE-LiTFSI-PEO GPE (HFE stands for highly fluorinated ether, here 1,1,2,2-Tetrafluoroethyl 2,2,2-Trifluoroethyl Ether)204. During first charges, the GPE undergoes reductive decomposition to form a LiF-based SEI which properties are enhanced by the contribution from organic-based compounds. Using this system, graphite and metallic Li electrodes were cycled 50 cycles at 0.3C with ≈ 99 % of Coulombic efficiency. Similarly, a UV-induced GPE coating using 1 m LiTFSI in fluoroethylene carbonate:trifluoroethyl methyl carbonate (FEC:FEMC, 1:1 vol%) was found to passivate the graphite electrode199. However, the graphite/LCO cell performances show a rapid capacity decay (see Figure 1. 30). Besides, as proposed by Dubouis et al.134, TFSI- anions undergo a chemical degradation in presence of HO- anion to form the SEI. Therefore, Zhang et al.198 cycled a LTO-based cell for 200 cycles at 0.5C using a strongly basic solid polymer electrolyte (SPE), LiTFSI-PEO-KOH, to enable the formation of a LiF-Li2CO3-containing SEI that incorporates polymeric decomposition products.
Figure 1. 30 (a) Scheme of a cell based on an organic coating. (b) Capacity and Coulombic efficiency as a function of cycle number for a graphite/LCO cell cycled in hybrid organic/aqueous electrolyte and using a polymer coating as a protection of the negative electrode. Adapted from Ref199.
Inorganic coatings were also tested in WiSE. The propensity of a coating to suppress or, at least, reduce HER depends on its intrinsic electrocatalytic activity205. Aluminum oxide (Al2O3) coating was one of the most widely used205–207 as it shows the lower HER activity (see Figure 1. 31a.) and it is also known to be insoluble in water. However, LTO-Al2O3-coated electrodes were found to initially deliver 84.5 % of Coulombic efficiency in a LTO-Al2O3/LMO battery, suggesting that even though a conformal inorganic coating is deposited, some defects are present causing some electrolyte consumption. After 60 cycles, the LTO-Al2O3/LMO cell could deliver 99 % of Coulombic efficiency even though a smooth capacity decay is observed (see Figure 1. 31b). The use of Al2O3 coating was also reported to suppress the oxygen reaction reduction (ORR)207 and thus enable the use of open-air cells (see Equation 1. 6 for ORR reactions) by suppressing self-discharge caused by the presence of dissolved O2107 (Figure 1. 31c). However, such design restricts the use of WiSE to LTP-like negatives.
Figure 1. 31 (a) Cathodic limits evaluated by linear sweep voltammetry on LTO surfaces coated with different materials (inset shows the enlarged view). Counter electrode: activated carbon; reference electrode: Ag/AgCl, scan rate: 1 mV/s. (b) The cycling performance of the full cell using an Al2O3-coated LTO negative and LMO positive in 21 m LiTFSI. Adapted from Ref205. (c) Effects of the ORR on the self-discharge of the lithiated Li3Ti2(PO4)3 electrodes in a three-electrodes open-cell configuration. The open-circuit potential curve of Li3Ti2(PO4)3 in the 1m Li2SO4 over 10 h of relaxation at open-circuit (orange). The open-circuit potential profile of the lithiated Al2O3@Li3Ti2(PO4)3 electrode in 21 m LiTFSI : 8 m LiOtf (yellow) in the 28m WiSE over 10 h of relaxation at open-circuit. These tests were conducted in an open-cell configuration with exposure to the ambient air. Adapted from Ref207. 2+ 2 +2∙ −= 2−+ − 2−+2=22+ − 2 2+ 2∙ −+2∙ += 2 2+2∙ 2 Moreover, AlF3-Al2O3206, LTP208 and carbon 122,208,209 coatings were also reported but limiting the negative electrode choice to the use of TiO2 electrode, far above graphite or even LTO ones. Altogether, polymeric coating was reported to be the most efficient strategy as only this strategy enabled to cycle metallic Li or graphite negative electrodes.
Table of contents :
GENERAL INTRODUCTION AND THESIS OUTLINE
CHAPTER 1 –INTRODUCTION TO AQUEOUS SUPERCONCENTRATED ELECTROLYTE AND THEIR USE IN LI-ION BATTERY (LIB)
AQUEOUS SUPERCONCENTRATED ELECTROLYTE: CAN THE MODIFICATION OF THE PHYSICO-CHEMICAL PROPERTIES AND THE INTERFACIAL REACTIVITY UNLOCK THE COMPETITIVENESS OF AQUEOUS LI-ION BATTERIES?
FURTHER IMPROVING AQUEOUS SUPERCONCENTRATED-BASED LIB, EXPANDING THE ESW LIMIT AT THE NEGATIVE ELECTRODE SIDE
CONCLUSION OF THE CHAPTER
CHAPTER 2 – CYCLING VIABILITY OF AQUEOUS SUPERCONCENTRATED ELECTROLYTES BASED ON 20 MOL/KG LITFSI AND 20 MOL/KG LITFSI : 8 MOL/KG LIBETI
ORIGIN OF THE PERFORMANCES DECAY: A GAS MONITORING STUDY .
SELF-DISCHARGE PROTOCOL TO ASSESS AQUEOUS SUPERCONCENTRATED ELECTROLYTES VIABILITY DURING RESTING PERIOD
CYCLING VIABILITY ON THE POSITIVE SIDE: A GAS MONITORING STUDY
CONCLUSION OF THE CHAPTER
CHAPTER 3 – INSTABILITY OF NATIVE SEI LEADS TO THE DRYING OUT OF AQUEOUS SUPERCONCENTRATED LI-ION BATTERY
IMPACT OF WATER CONSUMPTION ON ELECTROLYTE CRYSTALLIZATION
ACTIVATION ENERGY OF DIRECT AND INDIRECT HER IN WISE
DISCUSSION AND CONCLUSION OF THE CHAPTER
CHAPTER 4 –MIMICKING INORGANIC-BASED SEI WITH LIF-COATING. UNDERSTANDING OF INORGANIC SEI LIMITATIONS IN WATER-IN-SALT ELECTROLYTE.
USING LI/LIF-COATING TO MIMIC INORGANIC-BASED SEI. EXPOSURE TO ATMOSPHERE ENVIRONMENT, AQUEOUS SUPERCONCENTRATED ELECTROLYTE AND COMPARISON WITH THE BEHAVIOR OBSERVED IN ORGANIC ELECTROLYTE
COMPARISON OF LIF BEHAVIOR WITH AL2O3-COATED LI SAMPLE ..
FILLING THE STRUCTURAL DEFECTS BY PRESOAKING IN ORGANIC ELECTROLYTE: ASSESSMENT OF THE IMPORTANCE OF AN ORGANIC-INORGANIC BASED SEI
CONCLUSION OF THE CHAPTER
GENERAL CONCLUSION AND PERSPECTIVES