Impact of the metal electrode size in half-cells studies: Example of graphite/Li coin cells

International audienceWhen half-cells are used to evaluate and understand electrochemical performance and interface properties of a given electrode, the metal electrode size is rarely mentioned. To evaluate such impact on the electrochemical performance and interphase formation, graphite/Li coin cells were used. Undersizing the Li metal electrode led to significantly lower charge/discharge capacities even at C/20 rate due to incomplete lithiation/delithiation processes of the graphite electrode edge. It also led to the formation of non-uniform Li metal deposits as well as to a graphite SEI film with a thickness and composition gradient across the electrode. Oversized Li led, however to homogeneous graphite SEI film. Overall, these results highlight the critical role of the metal electrode size in half-cells and should apply to all half-cells studies using other metal electrodes such as Na, K, Mg

Exploring Interactions between Electrodes in Li[Ni x Mn y Co 1-xy ]O 2 /Graphite Cells through Electrode/Electrolyte Interfaces Analysis

International audienceInteractions occurring between positive and negative electrodes during constant current-constant voltage cycling in Li[NixMnyCo1-xy]O2/graphite pouch cells at various upper cutoff voltages was investigated. Below 4.3 V, good capacity retention, high/stable coulombic efficiency (CE) and low gas production were observed. At 4.5 V, however, extensive capacity loss, dramatic/unstable CE and large gas generation were observed due to extensive electrolyte degradation at the NMC electrodes. XPS experiments highlighted that solvents (-CO containing species) and salt (mostly LiF) degradation products including gas and NMC dissolution products are produced at the NMC electrodes then migrate to the graphite electrodes surface where they are either reduced or simply deposited. Although these phenomena were greatly accelerated at high voltage, the dominant failure mechanism was the formation of a Li-ion insulator/electron conductor surface layer (maybe rock salt) due to irreversible structural change of the NMC particles surface. At low voltage, the failure mechanism was explained by accumulation of SEI at the graphite electrode surface that hinder the ionic transport through the electrode porosity. Overall, both failure mechanisms are driven by oxidative parasitic reactions at the positive electrode and interaction with the negative electrode. Passivation of the positive electrode appears therefore crucial to promote long lifetime of Li-ion cells

Optimized electrode formulation for enhanced performance of graphite in K-ion batteries

International audiencePotassium ion batteries (KIBs) are emerging notably due to the reversible K+ intercalation into graphite. So far, optimization of graphite electrode formulation remains, however, to be investigated. This work thus proposes to optimize ionic (porosity) and electronic percolation networks as well as mechanical properties of graphite electrodes for KIBs. To do so, carbon black amount (CB) and electrode calendering is first adjusted. Then carboxylated styrene butadiene rubber (SBR) is used, with different amount and functional groups contents, as co-binder with carboxymethyl cellulose (CMC–Na). The best electrodes, made of 95/5/8/2 wt.% of graphite(SLP30)/CB(C65)/CMC–Na/SBR(Synthomer) with 2.4 mg cm−2 loading and 35% porosity, deliver ∼250 mAh g−1 during depotassiation at 5C with only ∼0.3 V polarization and 205 mAh g−1 during potassiation at 1C (3-electrodes configuration). In addition, no capacity loss is observed after 55 cycles at C/5 potassiation/1C depotassiation due to a low electrode volume expansion (11%) compared to 24% without SBR after only 35 cycles. These results highlight that appropriate SBR significantly improves the electrode structure/cohesion/elasticity (i.e. volume expansion management), leading to better power performance and capacity retention of graphite electrodes for KIBs. Finally, considering that further improvements are expected by tuning electrolyte formulation, this work will benefit to the development of high energy KIBs

Interest of molecular functionalization for electrochemical storage

International audienceDespite great interests in electrochemical energy storage systems for numerous applications, considerable challenges remain to be overcome. Among the various approaches to improving the stability, safety, performance, and cost of these systems, molecular functionalization has recently been proved an attractive method that allows the tuning of material surface reactivity while retaining the properties of the bulk material. For this purpose, the reduction of aryldiazonium salt, which is a versatile method, is considered suitable; it forms robust covalent bonds with the material surface, however, with the formation of multilayer structures and sp3 defects (for carbon substrate) that can be detrimental to the electronic conductivity. Alternatively, non-covalent molecular functionalization based on π–π interactions using aromatic ring units has been proposed. In this review, the various advances in molecular functionalization concerning the current limitations in lithium-ion batteries and electrochemical capacitors are discussed. According to the targeted applications and required properties, both covalent and non-covalent functionalization methods have proved to be very efficient and versatile. Fundamental aspects to achieve a better understanding of the functionalization reactions as well as molecular layer properties and their effects on the electrochemical performance are also discussed. Finally, perspectives are proposed for future implementation of molecular functionalization in the field of electrochemical storage

Impact of the Salt Anion on K Metal Reactivity in EC/DEC Studied Using GC and XPS Analysis

International audienceTo develop K-ion batteries, the potassium metal reactivity in a half-cells must be understood. Here, it is shown first that the K metal leads to the migration of the electrode degradation species to the working electrode surface so that half-cells' solid electrolyte interphase (SEI) studies cannot be trusted. Then, the K metal reactivity was studied by combining gas chromatography (GC)-mass spectrometry, GC/Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis after storage in ethylene carbonate/diethylene carbonate (EC/DEC) wo/w 0.8 M KPF6 or KFSI. A comparison with Li stored in EC/DEC wo/w 0.8 M LiPF6 was also performed. Overall, full electrolyte degradation pathways were obtained. The results showed a similar alkali reactivity when stored in EC/DEC with the formation of a CH3CH2OCO2M-rich SEI. For a MPF6-based electrolyte, the reactivity was driven by the PF6- anion (i) forming mostly LiF (Li metal) or (ii) catalyzing the solvent degradation into (CH2CH2OCOOK)2 and CH3CH2OCOOK as main SEI products with additional C2H6 release (K metal). This highlights the higher reactivity of the K system. With KFSI, the reactivity was driven by the FSI- anion degradation, leading to an inorganic-rich SEI. These results thus explain the better electrochemical performance often reported in half-cells with KFSI compared to that with KPF6. Finally, the understanding of these chemically driven electrolyte degradation mechanisms should help researchers to design robust carbonate-based electrolyte formulations for KIB

Methodological developments to expose and analyse buried interfaces in lithium solid-state batteries using

Lithium solid-state batteries (SSBs) are a promising technology for electrochemical energy storage systems. So far, the performance of SSBs are mainly governed by the electro-chemo-mechanical properties of the diverse solid/solid interfaces and their evolution upon cycling. However, as these interfaces are buried in the battery stack, their comprehensive understanding remains a challenge. Here, we thus provide some advances in methodological developments for ex situ, in situ and operando cycling/analysis of these buried interfaces. It is showed that noble gaz ion milling at liquid nitrogen temperature is a suitable and reproducible method to prepare cross-section without any chemical/physical change even for polymer-based SSBs. In addition, innovative operando cycling using Auger analysis was proposed for the first time on a model Li/Li6PS5Cl stack. The interest of this approach is to be able to proceed without a dedicated electrochemical cell and to use the fully adjustable electron beam of the auger to create a surface potential difference followed by lithium migration then SEI (Solid Electrolyte Interface) formation and Li plating. Overall, this work should greatly benefits to all researchers working on buried interfaces study in lithium solid-state batteries

Operando Auger/XPS using an electron beam to reveal the dynamics/morphology of Li plating and interphase formation in solid-state batteries

International audienceInterfaces and their understanding/control are the key to pave the way for the development of solid-state batteries. This work focuses on the development of operando Auger cycling using an electron beam to investigate the Li/solid electrolyte (SE) interphases. To do so, the fully tunable electron gun of the Auger was applied on top of a model Li/Li6PS5Cl(Arg) stack, allowing charge build up at the Arg surface and Li+ migration from the lithium electrode followed by SE interphase formation and Li plating. Overall, it is found that (i) Li6PS5Cl is first reduced to Li2S, LiCl and Li3P while (ii) Li plating occurs almost concomitantly and (iii) proceeds until the end of the operando cycling. These results were then confirmed by operando XPS using an electron beam. Importantly, this study highlights that operando Auger is more powerful than operando XPS as it provides visual observation of the dynamics/morphology of both Li/solid electrolyte interphase formation and Li plating together with reliable chemical information. This study thus opens the door for future development of operando Auger cycling using an electron beam as a powerful approach to better understand the interfaces in solid-state batteries

Cross-section nano-Auger/SEM analysis to reveal bulk chemical/morphological properties of composites for energy storage

International audienceComposite materials for energy storage such as powders, electrodes or battery stack often require probing their bulk chemical/morphological properties, which remains challenging so far with conventional analytical methods. In this work, Ar + milling cross-section is proposed to reveal the intrinsically buried bulk information of three different composites without physical/chemical change. Then, nano-Auger/scanning electron microscopy (SEM) analysis is proposed to investigate their bulk properties at both micro-and nano-scales. For MnCo-based powders with micrometric particles, it allowed revealing the bulk porosity and the bulk nano-or micro-Mn/Co distribution. For micrometer thick TiSnSb-based electrodes, it allowed proving the conversion reaction over long term cycling (i.e. the participation of the electrochemically inactive Ti) while revealing the TiSnSb particles morphological evolution (shell to core spreading/pulverization into porous structure) and SEI formation inside the porous TiSnSb. For PEO-based solid battery stacks, the cross-section allowed revealing well-defined interfaces so that reliable interfaces analysis can thus be perform. Advantage/limitation of this cross-section nano-Auger/SEM approach are also discussed. Overall, this work opens the door for future development of Ar + milling cross-section and Auger analysis as powerful tools to reveal/study buried chemical/morphological properties at micro-and nano-scales even beyond the energy storage field

Use of a li-ion battery comprising an anode which contains an alloy based on tin and antimony

The present invention relates to the use of a Li-ion battery comprising an anode which contains an alloy based on tin andantimony, at a high temp., in order to reduce the loss of capacity during cycling of said battery

Use of a li-ion battery comprising an anode which contains an alloy based on tin and antimony

The present invention relates to the use of a Li-ion battery comprising an anode which contains an alloy based on tin andantimony, at a high temp., in order to reduce the loss of capacity during cycling of said battery