Maximising the signal from materials of interest and maintaining acceptable elec­trochemical performance has been the overriding factor in designing neutron — friendly batteries. Initial designs were plagued by the need for large quantities of electrode materials and the associated requirement to use low current to ensure the reaction of the bulk of the electrode, for example studies of LiMn2O4 electrodes used 5 g of material as shown in Fig. 7.27 (left) [190, 191]. This design has evolved to designs shown in Fig. 7.27 (right) [178] which increasingly resemble their commercial equivalents, allowing high current to be used and a more direct com­parison with commercial performance. For most of these examples the polyethylene separator is replaced with a separator containing a smaller amount of H, e. g. polyvinylidene difluoride, and the electrolyte solution is replaced with deuterated equivalents. By using the design in Fig. 7.27 it was possible to show the loss of long-range order of the MoS2 anode during its first discharge [178], the composite nature of the TiO2/Li4Ti5O12 anode [192], relaxation phenomena in LiCo0.16Mn184O4 cathodes [193], evolution of LiMn2O4 structure [190, 191], and the reaction mechanism evolution of LiFePO4 [188, 194].

Alternate designs have been developed for in situ neutron diffraction experiments and these include coin-type cells [195-197] which still feature relatively-thick electrodes but have been used to successfully investigate Li4Ti5O12, graphite, and LiFePO4. Similarly, pouch-type cells with alternate layers of cathode and anode — coated current collectors are applicable for investigating full cells, as opposed to the use of Li metal in the majority of the previous examples. Studies have been con­ducted on Li[Ni1/3Mn1/3Co1/3]O2llgraphite, Li[Li0.2Ni018Mn0.53Co01]O2Hgraphite [198], and LiNi0 5Mn15O4lLi4Ti5O12 [199] full cells.

The motivation for designing these neutron-friendly cells is that any electrode material can be tested in situ in a real cell. Effectively, some of these designs can be manufactured using relatively-small electrode sizes (0.5-1 g) allowing a variety of materials to be investigated, and the interplay between structure, electrochemistry, and reaction mechanism can be elucidated. This information can then be used to direct the choice of future electrode-materials.

Some of these cells have been used to extract time-dependent information which reveals the rate of reactions as a function current applied, relating structural per­turbations to electrochemical factors [188, 193]. Of particular note has been the study of the reaction mechanism of LiFePO4 [188]. The evolution of LiFePO4, either by a single-phase solid-solution reaction, or a two-phase reaction, during charge/discharge has been extensively discussed in the literature (see [188]). Some parameters that lead to a particular type of reaction mechanism being favoured have been detailed. However, there was a lack of time-resolved infor­mation concerning bulk-electrode behaviour in a commercially-equivalent cell to definitively establish the working mechanism of LiFePO4. Time-resolved in situ NPD data showed the evolution of the reaction mechanism of LiFePO4 during charge/discharge processes. This is significant because the experiment probed the material under real working-conditions at the bulk-electrode scale. It should be noted that the LiFePO4 sample used was expected to have only two-phase behaviour, and this work revealed a solid-solution reaction mechanism region during charge/discharge which is followed by a two-phase reaction mechanism. Moreover, the transition between the ‘competing’ reaction mechanisms was iden­tified and characterized to be a gradual transition with solid-solution reactions persisting into the two-phase reaction region, rather than an abrupt transition. Figure 7.28 details this evolution and the co-existing reaction mechanism region.

Therefore, in situ NPD not only provides information on the evolution of electrode structure, but also on the evolution of the (de)lithiation reaction mechanisms of the electrode. This information can be time-dependent and as a function of the electrochemical process, and can be used to design alternative electrodes that avoid, or undergo, certain reaction mechanisms to enhance battery performance.