To obtain the local structure in glassy, nano, disordered and amorphous materials, having unresolved, weak or broad signals, neutron total scattering, or/and inelastic neutron scattering (INS) or quasielastic neutron scattering (QENS) can be powerful tools. Total scattering allows extraction of the local structure in terms of interatomic distances, bond angles and coordination numbers. In this case scattering is detected over a wider Q-range and short-range interactions of a sample are probed and modelled. In addition, rotations and vibrations picked up by INS and QENS are very sensitive to local distortions and allow otherwise difficult to detect relevant species such as protons, OH, water and in rare cases Li to be studied.

The negative-anode carbon is a good example of where neutron total scattering, in conjunction with other neutron-based methods, has been able to quantify important, previously ill-defined, aspects of the material’s function [20, 123, 124], as demonstrated with a range of low-crystallinity C negative electrodes. Addi­tionally, C-based anodes can be analysed in the lithiated and delithiated states and over the course of phase transitions. Typical neutron total-scattering data for graphite is presented as a radial distribution function (or pair distribution function) as illustrated in Fig. 7.10, and peak positions are indicative of interatomic distances. Often, neutron total scattering is combined with INS data to provide supporting information concerning the short-range order in C. For further information on total scattering the reader is directed to a review on the structure and dynamics of ionic liquids [125], and total scattering is likely to become increasingly used as the range of nano-sized active electrode materials increase.

Total scattering and INS are particularly attractive for disordered C where conventional diffraction provides limited information and more generally for Li arrangements in C. Disordered C where a large amount of H is present can exhibit significant Li capacity (one ‘excess’ Li per H) and studies have investigated how Li is taken into these materials [123, 124, 126]. Studies have shown that these materials exhibit randomly-arranged graphene fragments of different sizes with edges terminated by a single H, similar to Si with H at the surface. The spectra also contain a boson peak, an indicator of disorder, and distinct similarities to polycyclic aromatic hydrocarbon (PAH) spectra exist, some of which feature two or three edge-terminating H. Additionally, comparison with PAH spectra allowed the determination of methyl groupings when higher H concentrations are used. The boson peak is at the same position in samples with different concentrations of H and changes in position and intensity with Li insertion. This shows that the Li interacts with the C environment, contrary to the idea of Li accumulation in voids. These findings agree with two models of Li insertion: One where Li resides on both sides of the graphene layers (the so-called ‘house-of-cards’ model) and the other where Li is bonded to the H-terminated C at the edge of the graphene layers (and reside in interstitial sites). INS data also illustrate that the Li-Li interlayer and intralayer

interactions are comparable in strength. Computer modelling showed that there is insignificant energy difference between interstitial Li and those that are bonded to the terminal H. Other models include the formation of covalent Li2 molecules, but no evidence was found in support of these. The key aspect in these studies is that all models satisfy the observed capacity of LiC6. Finally, QENS [124] was used to show that Li jumps between nearest or second-nearest neighbour interstitial sites.

Related work investigated the entropy of intercalation into C [127]. This study shows how the sign of entropy changes from low Li concentrations on initial charge (x < 0.2 in LixC6) to higher concentrations (x > 0.2) indicating that multiple pro­cesses are occurring and that one of these is vibrational in origin. In graphite the entropy remains negative, but reduces in magnitude as lithiation progresses. Similar entropy information from INS data during lithiation of LiCoO2 cathodes has also been reported [128].

Cathode materials pertinent to Li-ion batteries based on olivine LiMPO4 have also been probed with INS, but for magnetic properties (low temperature) rather than Li-ion diffusion or lattice dynamic studies. Studies of LiFePO4 [129], LiNi1-x-FexPO4 [130], and LiMnPO4 [131], show spin-wave dispersions and allow characterisation of magnetic-exchange interactions. Further INS work was moti­vated by the need to understand the electronic conductivity in LiFePO4 and probed the thermodynamics and vibrational entropy of the phase transition in Li0 6FePO4 [121]. The oxidation state of Fe influences its neighbouring O atoms and the polyhedral distortions can characterize the motion of carrier hopping between Fe sites, which results in relaxations or displacements that can in turn be considered as the sum of longitudinal phonons. Similarly, occupation or vacancy of Li can result in distortions of atom positions and are expected to alter the frequency of phonons, in particular longitudinal optical phonons.

The phase evolution of Li0.6FePO4 as a function of temperature, via a two-phase transition to a disordered solid-solution transition at 200 °C [121], can shed light on the reaction mechanism during charge/discharge of this cathode. This is particularly pertinent as the two-phase or solid-solution mechanism of LiFePO4 is a topical issue as discussed above. The difference in two-phase and solid-solution LiFePO4 optical modes above 100 meV (higher energies) was found, with broadening evi­dent for the solid-solution sample. The low-energy region features mostly acoustic lattice modes, translations and librations of PO4 and translations of Fe. By com­parison with infrared (IR) and Raman data, it was found that the PO4 stretching vibrations are damped in the solid-solution sample. The difference in INS data of solid-solution and two-phase samples at higher energy mostly involve optical modes that can arise from motion of Li-ions, charge hopping between Fe-ions, and heterogeneities. The entropy was found to be larger in the solid-solution phase in conjunction with the subtle differences in the dynamics due to different optical modes. The similarity in two-phase and solid-solution phonon density of states (Fig. 7.11) agrees with the ease with which LiFePO4 seems to undergo either transition, and the difficulty in pinning down the experimental evidence related to the reaction-mechanism evolution.

Arguably the most studied materials using INS are manganese oxides and lithiated manganese oxides, predominantly due to the ease of using H as a probe for Li. These compounds are used for both primary and Li-ion batteries and ion-exchange methods have been used to show where Li may reside in these compounds. Although indirect, this information can provide further answers to some of the problems in this field of research. Attempts are also being made to use INS to provide comparisons between H and Li where H is used as a calibrated probe for Li [132].

One approach is to replace structural or surface water present on manganese oxides with protons, which can in turn be exchanged for Li to see how Li might displace water in these compounds. This was undertaken for spinel Li133-x/3CoJ1Mn;L67-2x/3O4 [133] which shows, as is the case in many compounds of this family, that protons are inserted as hydroxyl groups giving a strong incoherent INS signal. The hydroxyl groups are located on the O atoms neighbouring the vacant 16d sites and aligned with the 8a sites in the spinel structure. Conversely, studies on undoped spinels have shown that the Li extraction from the 16d sites allows the insertion of protons. The main features of the INS spectra are strong y(OH) modes, ahighly ordered proton site, a shoulder and smaller features between 300-700 cm 1 showing riding of protons on the oxide lattice and some librational water modes. The hydroxyl groups have characteristic signals around 908 cm-1 and their orientations are also determined using INS [134-137] of spinel-derivative compounds. Interestingly, IR data shows features between 950 and 1300 cm-1 which were considered to arise from protons, but the absence of these features at corresponding frequencies in the INS data indicate a manganese oxide lattice origin. Notable discoveries of this and related studies include the finding that in undoped spinels 40 % of protons cannot be exchanged and form disordered water, the chemical re-insertion of Li in Li-rich spinel Li16Mn16O4 removes most of the hydroxyl groups [137], that generally the reversible Li amount is 50 % in both undoped and doped spinels, and that fewer protons are re-exchanged as

the Co concentration increases. The latter is an interesting way to tune the Li-proton exchange capacity of these materials.

Figure 7.12 shows the INS data from a series of Li-rich Li1.6Mn16O4 spinels formed through various methods. The pure sample (bottom of Fig. 7.12) shows some evidence of protons, OH and water, whilst the acid-treated version, where acid results in H-Li exchange, shows strong characteristic peaks for protons and y(OH) groups. Finally, the acid-treated sample undergoes a chemical Li re-insertion step and results in the loss of the proton and OH signatures. However, the re­inserted material does not replicate the pure sample suggesting some protons remain as structural water and hydroxyl groups [137]. Relative comparisons of the INS intensity can be made between the acid-treated and re-inserted samples, with the 909 cm 1 peak showing a larger drop in intensity compared to the 1,087 cm 1 peak, which is attributed to an H site being easier to depopulate. A comparison of INS data for two Li-rich variants, Li133Mn167O4 and Li16Mn16O4, shows that the proton stability is higher in Li16Mn16O4 than in Li133Mn167O4. This suggests the reason that Li16Mn16O4 has a larger Li-ion exchange capacity than Li133Mn1 .67O4 concerns the stability of the inserted species (or more specifically the stabilized proton sites).

Studies of ^-MnO2 [135] illustrate subtle differences in INS spectra depending on synthesis precursors, noting that precursors and conditions are both important. This work again highlights the need to focus on the protons (often disordered). A related study investigated proton-exchanged spinels that form ^-MnO2 showing that the proton diffusion was dependent on octahedral Mn vacancies [136]. In this study, certain features in the INS spectrum were found to disappear in the highly crys­talline sample, suggesting that motion can be perturbed with crystallinity. Researchers have also looked at the proton and water environments in bare and lithiated MnO2 [132] to demonstrate how lithiation influences the proton and water motions, which can then be used to extract information on lithiation processes. Using neutron total scattering from oxidized and lithiated versions of ^-MnO2 researchers derived models for oxidation and lithiation [138].

Further work on the spinel LiMn2O4 system investigated the cubic to ortho­rhombic phase transition near room temperature, which is associated with Mn3+/ Mn4+ charge ordering [139]. Excess Li was introduced at the 16c site to study why the phase transition is suppressed in this situation. QENS was used here, where data were found to be dominated by magnetic contributions rather than that from Li hopping, with the slight narrowing of the elastic line near room temperature leading to the preliminary conclusion that electrons are localized on the Mn. A dynamic transition in Li-rich compounds seems to coincide with the structural transition in the parent. The magnetic properties of Li096Mn2O4 were explored in a related INS study [140] showing that two short-range magnetic transitions are present and related to spin ordering of Mn3+ and Mn4+.