Acquiring a comprehensive understanding of interfacial reactions in batteries is essential for the design of new materials [4]. However, due to the difficulty in probing the interfaces, relatively little is known about the relevant processes. What is known is that the stability of electrolytes at the electrode interface plays a key role in determining the cycle life and safety of batteries. The most studied interface is between the carbon negative electrode and carbonate organic electrolytes. The instability of carbonate electrolytes with respect to the carbon chemical-potential results in deposition of the electrolyte Li-containing inorganic and organic decomposition products on the electrode surface, otherwise known as the SEI layer. This reduces the amount of active Li available to the cell and degrades the elec­trolyte. Typically electrolytes contain two components, one for the Li-salt disso­lution, and one that assists in the formation of a protective layer on the anode preventing continuous electrolyte-reduction and self-discharge, e. g. ethylene car­bonate. This requires the formation of a stable SEI film showing good ionic — conductivity and poor electronic-conductivity. In this way the SEI can passivate against further electrolyte decomposition without severely influencing battery performance. The SEI formation and maintenance during further cycling is expected to play an essential role in the cycle life and stability of batteries, however, the growth mechanism under variable battery-conditions is largely unexplored. In addition, the charge-transfer reaction at the interface, most likely influenced by the SEI development [169], is an essential parameter that in many cases limits the power of batteries. Also, interfaces within the electrode material can establish upon phase transitions during (de)lithiation. From an applications point of view these transitions have the favourable property of being associated with a constant potential that is independent of the composition, but the disadvantage of being associated with volumetric changes that may restrict the cycle life. Probing such interfaces under in situ conditions is possible using neutron reflectometry.

One of the first reported neutron-reflectometry studies on a Li-ion battery system determined the Li insertion and extraction mechanism in thin film anatase TiO2, a negative electrode material operating around 1.7 V versus Li/Li+. Lithiation of the tetragonal anatase TiO2 leads, via a first-order phase transition, to the orthorhombic Li05TiO2 Li-titanate phase. The aim of the neutron-reflectometry study was to discover the phase-evolution scheme in this electrode material, which is of more general value for electrode materials undergoing first-order phase transitions. Pre­vious studies suggested the establishment and movement of a diffusion-controlled phase boundary, parallel to the electrode surface, between the Li-rich Li-titanate and the Li-poor anatase phase [170]. This is in contrast to, for instance, a perco­lation scheme where the Li-titanate phase would penetrate the original anatase layer only at certain regions of the thin film. Further intercalation would increase both the perpendicular and the lateral dimension of these percolation paths, eventually leading to a homogeneously-intercalated film. Van de Krol et al. suggested a specific scheme in order to explain the more facile Li-ion extraction rate compared to the insertion rate [170]. Based on the assumed faster Li-diffusion in the Li — anatase phase [171] one might expect fast depletion of Li in the near-surface region of the Li-titanate phase containing electrode, which is in contact with the electro­lyte. As a result, during Li extraction, the Li-anatase phase starts to grow at the electrolyte surface into the layer at the expense of the Li-titanate phase.

The contrast difference between the Li0.5TiO2 Li-titanate phase and the TiO2 anatase phase for neutrons should make it possible for neutron reflectometry to determine the phase-evolution scheme both during Li insertion and extraction.

An approximately 25 nm smooth anatase TiO2 electrode was deposited on a thin *20 nm Au current collector on a 10 mm thick single-crystal quartz block that served as the medium for the incoming and reflected neutron beam. The latter is practically transparent to thermal neutrons allowing approximately 70 % trans­mission over 10 cm path length. The TiO2 electrode is exposed to a 1 M solution of LiClO4 in propylene carbonate electrolyte using Li metal both as counter and as reference electrode. Li was galvanostatically inserted in two steps and extracted in two steps using 10 mA (C/5) in the same voltage window. Neutron-reflection experiments were performed after each step when a constant equilibrium-potential was achieved. The results, including the fit and the associated scattering-length density (SLD) profiles are shown in Fig. 7.23. The profound change observed in the neutron reflection from the virgin state and the state after the electrochemistry can be explained by the formation of a SEI layer on the TiO2 surface.

For the half lithiated state the best fit of the neutron reflection data was achieved by assuming a Li-rich Li-titanate phase (Li05TiO2) in contact with the electrolyte. As a result of the negative coherent neutron-scattering length of Li the SLD of lithiated TiO2 being is smaller than that of pure TiO2. Neutron reflectometry proved the

establishment of a phase boundary parallel to the electrode surface rather than a percolation model, which would lead to a homogeneous change of the SLD. The fully-lithiated state was fitted with a single electrode layer with the SLD corre­sponding to the composition Li052TiO2. The neutron reflection data after half de — lithiation indicated that the phase front moves back via the way in which it came, with the Li-titanate phase being in contact with the electrolyte. This is in contrast to the expected Li depletion during extraction, which should lead to TiO2 formation at the electrolyte interface. This symmetric phase-front movement does not immedi­ately explain the difference in insertion and extraction rate. However, nuclear magnetic resonance experiments show the diffusion over the phase boundary to be the rate-limiting step [172, 173], giving a rationale for the more sluggish lithiation of TiO2, which as opposed to delithiation, requires diffusion over the phase boundary.

Information related to the structure and composition of the SEI layers is mostly based on ex situ spectroscopic and microscopic studies [174, 175], but because of the reactive and delicate nature of these layers, in situ analysis is essential to improve our understanding. Being relatively sensitive to the light organic and inorganic species present in the SEI and to the surface layers ranging from a few to hundreds of nanometers, neutron reflectometry is an exceptionally suitable tech­nique for in situ studies of the growth, composition, and the structure of the SEI.

Owejan et al. [176] used neutron reflectometry to study the formation and structure of the SEI layer in a Li battery. A requirement for neutron reflectometry is a flat and smooth surface as it probes the average in-plane SLD profile. A Li half-cell was configured with Cu as the ‘counter’ electrode to prevent Li reaction with the electrode, so that all electrochemical charge can be attributed to decomposition of the electrolyte and SEI layer formation. The use of a non-intercalating electrode, such as Cu, as model electrode for electrolyte decomposition appears to be justified by the similarity of the SEI layers formed by C materials at low potentials in Li-salt con­taining electrolytes [46, 47]. Additionally, the thermodynamics of electrolyte reduction appear to be governed by the cation that is used in the electrolyte [43]. The scattering contrast of the electrolyte was increased by preparation of a 1 M LiPF6 solution in a 1:2 (v/v) ratio of deuterated ethylene carbonate and isotopically-normal diethyl carbonate. The deuterated ethylene carbonate also offers the opportunity to identify the possible preferential decomposition of cyclic (ethylene) over acyclic (diethyl) carbonates. By deuterating selected components in the electrolyte solution researchers can access which component contributes or forms the SEI layer.

In Fig. 7.24 the neutron reflectivity versus Q is shown for the pristine Cu electrode immersed in the electrolyte at the open-cell potential. This electrode underwent 10 cyclic voltammogram sweeps between 0.05 and 3 V at a 10 mVs-1 rate, followed by holding the potential at 0.25 V versus Li/Li+ (potentiostatic reducing conditions). A clear difference between the peak amplitudes and oscilla­tions (positions) in the reflectivity of the fresh and electrochemically-cycled elec­trode is observed. Initially, at the Cu-electrolyte interface, copper carbonate/ hydroxide ligand-containing layers appear to be present which are removed after the cyclic voltammetry. Interestingly, after 10 cyclic voltammetry sweeps and the potential-hold step under reducing conditions, a 4.0 nm thick SEI layer at the

interface had developed with a SLD much lower than that of the electrolyte. A further 10 additional cyclic voltammetry sweeps led to only a small growth of the SEI layer. For completeness, the authors then took a number of data sets at different potentials by slowly ramping the potential at 10 mVs-1 to the next potential value and holding the potential during neutron reflectometry data collection. The results

are summarized in Fig. 7.25. During the first two points d and e, using an oxidation current, a small decrease in the SEI layer thickness was observed and the SLD suggests a shrinking of the SEI layer due to solubility of SEI components. How­ever, at the next point, holding at a reducing current the SEI layer grows signifi­cantly up to 8.9 nm. Most of the neutron reflection measurements indicate rather homogeneous SLD profiles with little roughness, in contrast to proposed structures in literature. After point f even the lowest potentials do not lead to further SEI layer growth, illustrating the passivating nature of the SEI layer. The systematic decrease of the SLD at lower potentials indicates that the SEI is increasingly composed of low SLD elements, which indicate Li-rich molecules.

Further insight into the composition of the SEI layer was obtained by combining X-ray photoelectron spectroscopy-derived compositions with the neutron reflec — tometry results for the SEI layer. This indicated an increase in LiOH and LiF molecules, and the decrease of lithium alkyl carbonates at the lower reducing — potentials. This study demonstrated the advantage of neutron reflectometry in giving direct insight into the growth and composition of the SEI layer and its relationship to the electrochemical conditions.