A unique method to “see” Li-ion concentration profiles is provided by neutron depth profiling (NDP), Fig. 7.32. Previously it has been shown that NDP is capable of determining Li concentration gradients in optical waveguides [200], electrochromic devices [201], and under ex situ conditions in thin film battery electrodes and electrolytes [202]. NDP uses a neutron-capture reaction for 6Li resulting in:

6Li + nthermal!4 He (2.06MeV) +3 H(2.73MeV) (7.1)

The kinetic energy of the products due to AE = Amc2, where E is the energy, m is the rest mass of the particles and c is the speed of light, is distributed over the tritium (3H) and the alpha particle (4He), while the incoming thermal energy of the neutron at *25 meV, is negligible. Due to the small particle flux and the inher — ently-low interaction of neutrons with matter, NDP is a totally non-destructive technique. When such a capture reaction takes place in a Li-ion battery electrode, the particles produced (4He and 3H) lose part of their kinetic energy due to the scattering by the electrode material, referred to as stopping power. The stopping power is directly related to the composition and density of the electrode and hence is a known quantity. Therefore, by measuring the energy of the 4He and 3H ions when they exit from the electrode, the depth of the capture reaction can be reconstructed. Typically, the spatial resolution of NDP for well-defined homoge­neous layers is on the order of tens of nano-meters, Currently, the main restrictions of the NDP technique is the maximum depth that can be probed and the time resolution which, depending on the material investigated and the in situ cell design, are approximately 5-50 microns and 10-20 min, respectively. Ideal solid-state batteries can be designed with high spatial homogeneity for initial experiments, before proceeding to more complicated systems.

NPD has only been applied occasionally to Li-ion battery research, however, in these ex situ studies [202-205] NDP is very powerful in identifying Li-ion transport and aging mechanisms. The possibilities of NDP in Li-ion battery research is demonstrated with the first in situ study on thin film solid-state batteries probing the kinetic processes in these Li-ion batteries.

Oudenhoven et al. brought NDP one step further, demonstrating that Li depth profiles can be measured in situ in an all solid-state micro battery system during (dis)charging [206]. The Li-ion distribution was studied in a thin film solid state battery stack containing a monocrystalline Si substrate with a 200 nm Pt current

collector, 500 nm LiCoO2 positive electrode, 1.5 ^ N-doped Li3PO4 (LiPON) electrolyte, and a 150 nm Cu top current-collector. The basic setup of the experi­ment is shown in Fig. 7.33. By subtracting the NDP spectrum of the as-prepared electrode from the charged and discharged spectra, the changes in Li-distributions can be observed directly, see Fig. 7.33. Upon charging, Li in the positive LiCoO2 electrode is depleted and increased at the negative Cu current collector.

The development of large concentration-gradients in both the LiCoO2 electrode and the LiPON solid electrolyte, Fig. 7.34, reveals that in this system ionic transport in both electrolyte and electrode limit the overall charge-rate. The cathode was enriched with 6Li to highlight the redistribution of 6Li and the natural abundance of 6,7Li in the electrolyte during time-dependent experiments. In this case, the NDP intensity increases by approximately a factor of 13. Apart from being able to observe

Energy (keV)

where 6Li is going, the expected diffusive equilibration of the 6Li concentration within the electrolyte was observed during a 2 h equilibration period. Interestingly, the enriched Li remains in the LiCoO2 electrode, even though the exchange current that establishes the dynamic equilibrium would be expected to redistribute the 6Li equally throughout the LiCoO2 electrode and the LiPON electrolyte. The absence of vacancies at the initial stage probably makes the exchange-current extremely small.

As the battery is charged at 0.5 °C, this results in a large decrease in the Li-ion signal of the LiCoO2 electrode, as shown in Fig. 7.34.

The stronger decrease in Li-ion signal near the interface with the electrolyte suggests an inhomogeneous Li-ion distribution in the electrode. Although this may be the case, a redistribution of the 6Li ions due to exchange with the electrolyte will lead to a lower Li-ion signal in the electrode. That this is indeed part of the explanation is clear from the almost 70 % decrease in Li-ion signal. This decrease is more than would be expected under the mild electrochemical conditions that should lead to Li05-CoO2, and hence at most a factor of two decrease in the Li-ion signal is to be expected. However, the inhomogeneous signal from the electrode indicated that the exchange does not reach the back part of the electrode that is closest to the current collector. The inhomogeneous distribution of the Li-ion signal originating from the electrolyte indicates the presence of an inhomogeneous 6Li and Li-ion distribution. The evolution of this non-equilibrium situation was investigated by relaxing the system after charging during a period of 2 h and taking NDP spectra, shown in Fig. 7.34. After 2 h the 6Li gradient almost vanishes in the electrolyte, whereas it remains in the LiCoO2 electrode. Clearly, Li-ions are much more mobile in the electrolyte compared to in the electrode. The work of Oudenhoven et al. shows for the first time that the evolution of the Li distribution and gradient under dynamic conditions can be studied.