The failure mechanism of silicon-based electrodes has been studied only in a half-cell configuration so far. Here, a combination of 7Li, 19F MAS NMR, XPS, TOF-SIMS, and STEM-EELS, provides an in-depth characterization of the solid electrolyte interphase (SEI) formation on the surface of silicon and its evolution upon aging and cycling with LiNi1/3Mn1/3Co1/3O2 as the positive electrode in a full Li-ion cell configuration. This multiprobe approach indicates that the electrolyte degradation process observed in the case of full Li-ion cells exhibits many similarities to what has been observed in the case of half-cells in previous works, in particular during the early stages of the cycling. Like in the case of Si/Li half-cells, the development of the inorganic part of the SEI mostly occurs during the early stage of cycling while an incessant degradation of the organic solvents of the electrolyte occurs upon cycling. However, for extended cycling, all the lithium available for cycling is consumed because of parasitic reactions and is either trapped in an intermediate part of the SEI or in the electrolyte. This nevertheless does not prevent the further degradation of the organic electrolyte solvents, leading to the formation of lithium-free organic degradation products at the extreme surface of the SEI. At this point, without any available lithium left, the cell cannot function properly anymore. Cycled positive and negative electrodes do not show any sign of particles disconnection or clogging of their porosity by electrolyte degradation products and can still function in half-cell configuration. The failure mechanism for full Li-ion cells appears then very different from that known for half-cells and is clearly due to a lack of cyclable lithium because of parasitic reactions occurring before the accumulation of electrolyte degradation products clogs the porosity of the composite electrode or disconnects the active material particles.

Continuous solid electrolyte interface (SEI) formation remains the limiting factor of the lifetime of silicon nanoparticles (SiNPs) based negative electrodes. Methods that could provide clear diagnosis of the electrode degradation are of utmost necessity to streamline further developments. We demonstrate that electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM can be used to quickly map SEI components and quantify LiSi alloys from single experiments with resolutions down to 5 nm. Exploiting the low-loss part of the EEL spectrum allowed us to circumvent the degradation phenomena that have so far crippled the application of this technique on such beam-sensitive compounds. Our results provide unprecedented insight into silicon aging mechanisms in full cell configuration. We observe the morphology of the SEI to be extremely heterogeneous at the particle scale but with clear chemical evolutions with extended cycling coming from both SEI accumulation and a transition from lithium-rich carbonate-like compounds to lithium-poor ones. Thanks to the retrieval of several results from a single data set we were able to correlate local discrepancies in lithiation to the initial crystallinity of silicon as well as to the local SEI chemistry and morphology. This study emphasizes how initial heterogeneities in the percolating electronic network and the porosity affect SiNPs aggregates along cycling. These findings pinpoint the crucial role of an optimized formulation in silicon-based thick electrodes.

Operando Raman spectroscopy and synchrotron X-ray diffraction were combined to probe the evolution of strain in Li-ion battery anodes made of crystalline silicon nanoparticles. The internal structure of the nanoparticles during two discharge/charge cycles was evaluated by analyzing the intensity and position of Si diffraction peaks and Raman TO-LO phonons. Lithiation/delithiation of the silicon under limited capacity conditions triggers the formation of "crystalline core-amorphous shell" particles, which we evidenced as a stepwise decrease in core size, as well as sequences of compressive/tensile strain due to the stress applied by the shell. In particular, we showed that different sequences occur in the first and the second cycle, due to different lithiation processes. We further evidenced critical experimental conditions for accurate operando Raman spectroscopy measurements due to the different heat conductivity of lithiated and delithiated Si. Values of the stress extracted from both operando XRD and Raman are in excellent agreement. Long-term ex situ measurements confirmed the continuous increase of the internal compressive strain, unfavorable to the Si lithiation and contributing to the capacity fading. Finally, a simple mechanical model was used to estimate the sub-nanometer thickness of the interfacial shell applying the stress on the crystalline core. Our complete operando diagnosis of the strain and stress in SiNPs provides both a detailed scenario of the mechanical consequences of lithiation/delithiation in SiNP and also experimental values that are much needed for the benchmarking of theoretical models and for the further rational design of SiNP-based electrodes.

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Titanium oxides and carbides are often considered as electrode materials in energy conversion and storage devices due to their high potential conductivity and good stability. Titanium monoxide and titanium carbide have structures that can both be described as rocksalt with the same cubic close packed titanium sublattice with oxygen and carbon respectively occupying the octahedral interstices; however the oxide is characterised by extensive defects on both sublattices whilst the carbide is stoichiometric and might be considered as an interstitial metal. Despite the anticipated very different natures of the oxide and carbide sublattices, these two phases actually form a complete solid solution. In the present investigation, we carefully characterise this titanium oxycarbide solid solution, reporting on crystal structure, magnetic and electronic conduction properties. Titanium oxycarbide powders (TiO1-xCx with x = 0 ≤ x ≤ 1) have been prepared by solid state reaction of TiO and TiC powder under controlled environments at elevated temperatures. X-ray diffraction and pycnometric density measurements illustrate the gradual transition of crystal structure of titanium oxycarbides from vacancy containing rock-salt structure of TiO to the fully occupied TiC with increase in carbon content in the oxycarbide lattice. The variation of lattice parameter of oxycarbide crystal as a function of carbon content has been found to be non-linear which can be attributed to variations in the level of vacancies present in metal as well as non-metal sub-lattices.The existence of short-range ordering of anion vacancies in oxycarbide with a nominal composition of TiO0.5C0.5 where half of oxygen of TiO is replaced by carbon has been confirmed by selected-area electron diffraction studies. Low temperature magnetic and conductivity measurements confirm that all oxycarbide compositions are Pauli paramagnetic and good metallic conductors.

Probing the structural changes that electrode materials undergo during electrochemical cycling while monitoring their spatial distribution within the volume gives valuable insights on dynamic processes, i.e. side reactions and evolution of phase migration barriers, often associated to capacity and power limitation. In this work, we present an electrochemical cell to perform spatial and time resolved operando synchrotron X-ray diffraction on Lithium (Li) metal polymer batteries operating at 80 °C. A 3.2 mm diameter battery made of a Li metal anode, a LiFePO4 based cathode, and a solid polymer electrolyte acting as separator, is placed inside a glass-based casing and cycled at a beamline. The cylindrical cell geometry with its small size enables to follow the phase transformations occurring at different states of charge and at different cathode heights. It is possible to create spatially resolved phase distribution plots and to differentiate active material structural changes occurring close to the interface with the electrolyte from those at the current collector vicinity. The results provide a direct observation of the Li diffusion in the LiFePO4 and FePO4 phase distribution. In addition, synchrotron X-ray diffraction computed tomography (XRD-CT) measurements were performed to obtain phase distribution maps at different heights of the battery assembly.

Silicon represents one of the most promising anode materials for next generation lithium-ion batteries. However its colossal volume expansion (up to 300%) upon electrochemical reaction with lithium repeatedly exposes fresh surfaces to electrolyte solvent oxidation. This leads to very high irreversible capacities1. Deeper insight into these degradation phenomena is critical to engineer adequate electrodes and/or electrolytes. Little is known about the SEI’s morphology at the particle scale. Attempts to characterize this system through transmission electron microscopy (TEM) have been severely limited by the radiolysis and sputtering damage respectively undergone by the SEI and lithium-silicon alloys (LixSi). In this work we demonstrate the possibility to map major SEI phases2 as well as quantifying LixSi compositions3 and Si crystallinity from a single dataset by combining scanning transmission electron microscopy and low-loss electron energy loss spectroscopy2 (STEM-EELS)4. The protocol we developed allows for large spectrum image acquisitions within short timeframes (~10 ms/voxel), making this method a robust and practical diagnostics tool for battery electrodes and other beam-sensitive nanostructured systems. Results on silicon nanoparticle (SiNP)-based electrodes shed light on the SEI’s deposition mechanism and morphological as well as chemical evolution along cycling (figure 1). Revealing the morphology of lithium fluoride (LiF) - large chunk-like deposits, lithium carbonates (such as Li2CO3) - thin conformal layers and carbon black allows us to get unprecedented insight into the SEI's formation mechanism at the particle scale, while mapping lithium content in Li-Si alloys shed light on the lithiation mechanisms at the particle and aggregate scale. Strong correlations between the SEI's local chemistry and our nanoparticles cycling performances were observed. Alloy formation appeared to make particles fuse together into what became a-Si/c-Si composite networks after delithiation (figure 2d), as revealed through careful examination of the EELS low-loss data. Furthermore, lithiation is shown to proceed in a very heterogeneous manner on the aggregate and particle scale, with particles at the aggregate/porosity boundary exhibiting higher lithium content (figure 2e), pristine and lithiation particles side to side (figure 2a), and composition gradients across single shells. Lithiation was also observed to proceed preferentially along grain boundaries (figure 2c), resulting in different behaviours between mono- and polycrystalline silicon powders, with polycristalline silicon showing higher lithium contents on average in charged electrodes. These tools were used to study the influence of the electrolyte solvent on the SEI's morphology and chemistry along cycling. Observations from electrodes cycled in pure carbonate electrolyte (1M LiPF6 in EC:DMC 1:1 + 10%FEC) at the 1st, 10th and 100th cycle were obtained and show clear trends in SEI accumulation and chemical evolution. These insights into both the SEI and the behavior of SiNPs are an exceptional asset in understanding the rapid capacity decay of silicon-based electrode. Reference s : [1] Delpuech, N.; Dupre, N.; Moreau, P.; Bridel, J.-S.; Lestriez, B.; Guyomard, D. ChemSusChem 2016. [2] Yakovlev, S.; Libera, M. Micron 2008, 39 (6), 734–740. [3] Danet, J.; Brousse, T.; Rasim, K.; Guyomard, D.; Moreau, P. Phys. Chem. 2010, 12 (1), 220–226. [4] Boniface, M., Quazuguel, Guyomard, D., Moreau, P., & Bayle-Guillemaud, P. (2016). Nano Letters. Figure 1

Silicon represents one of the most promising anode materials for next generation lithium‐ion batteries. However its colossal volume expansion (up to 300%) upon electrochemical reaction with lithium repeatedly exposes fresh surfaces to electrolyte solvent oxidation 1‐2 . This leads to very high irreversible capacities, compounded by the fact that parts of the silicon‐based electrodes are progressively disconnected from both electrical and ionic transport networks as the solid electrolyte interface (SEI) accumulates. Deeper insight into these degradation phenomena is critical to engineer adequate electrodes and/or electrolytes. Characterization of these electrodes has so far mostly focused on either bulk or surface analysis, both lacking spatial resolution. Little is known about the SEI's morphology in silicon nanoparticles (SiNPs) aggregates that make the electrode, or about the evolution of these nanoparticles themselves with cycling. Attempts to characterize this system through electron microscopy have been severely limited by the radiolysis and sputtering damage, respectively, undergone by the SEI and lithium‐silicon alloys (Li x Si). In this work we demonstrate the possibility to map major SEI and electrode components such as lithium carbonate (Li 2 CO 3 ), lithium fluoride (LiF) and lithium oxide (Li 2 O) as well as quantifying lithium‐silicon alloys compositions 4 and Si crystallinity from a single dataset by combining scanning transmission electron microscopy and low‐loss electron energy loss spectroscopy 5 (STEM‐EELS) (fig. 1). The low‐loss part of the EEL spectrum is considerably more intense than its high energy counterpart and contains both the Li K‐edge and plasmons. Fine tuning of the experimental parameters allows us to acquire low‐loss spectrum images with good signal‐noise ratios within timeframes compatible with minimal sample degradation. Plasmons can then either be used as unique molecular signatures for the SEI, or directly for quantification in the case of Li x Si compounds (fig. 2 inset). This can yield unique insight into electrode degradation phenomena through careful data processing (MLLS, Drude model fit…). Large spectrum images can be acquired within short timeframes (~10 ms/voxel), making this method a powerful and practical diagnostics tool for battery electrodes and other beam‐sensitive nanostructured systems. Results on electrodes disassembled from full cells at their 1 st , 10 th and 100 th charge and discharge, with a limited capacity of 1200 mAh/g, shed light on the SEI's deposition mechanism and morphological as well as chemical evolution along cycling for different electrolytes. Strong correlations were observed between the SEI's local chemistry and our nanoparticles cycling performance (fig. 2). Lithiation was also observed to proceed preferentially along grain boundaries, resulting in different behaviours between mono‐ and polycrystalline silicon powders.