In situ neutron reflectometry analysis of electrode/electrolyte interface for lithium batteries
Tokyo Institute of Technology, Ryoji Kanno
1. Introduction
Neutron reflectometry is a powerful technique that can provide an atomistic understanding ofsolid/liquid interfaces as a function of the depth, since neutrons have a sufficiently small absorption crosssection for penetrating electrochemical cells so that electrochemical interfaces can be probed.

As a lightelement, lithium has a relatively high neutron scattering power compared to that of X-rays, and the depth
profile of the neutron scattering length density (SLD) obtained by curve fitting can reveal the surface
structure changes of lithium battery electrode films during electrochemical processes.

Nanosizing ofLi4Ti5O12 has recently been the focus for improving the rates of lithium deintercalation by reducing the path
lengths for electronic and lithium ion transport in the electrode.

We have been investigating the interfacial

reactions of Li4Ti5O12 using an epitaxial-film model electrode the discharge-charge measurements.

The

discharge-charge measurements, which were conducted with 1 mol dm-3 LiPF6 containing propylene
carbonate (PC), revealed that a 23.8-nm-thick Li4Ti5O12 film exhibited a small capacity of 115 mAh g-1
compared to the theoretical value of 175 mAh g-1.

Recently, the neutron reflectometry analyses revealed

that the phase change occurs in the 10 nm surface region from the Li4Ti5O12/1 mol dm-3 LiPF6 PC interfaces
during the initial reaction processes.

This implies that the surface-reconstructed phase formed in the LiPF6

containing PC electrolytes limits the lithium intercalation.

on this proposal, we analyzed the interfacialreactions of Li4Ti5O12 with the 1 mol dm-3 LiPF6 ethylene carbonate–diethyl carbonate (EC:DEC/3:7 vol.%)electrolyte to clarify the effects of the electrolyte species on the surface structure and the electrochemical
properties of Li4Ti5O12 electrolyte.

2. Experiment
Epitaxial Li4Ti5O12(111) thin films were grown on 0.5%Nb-doped SrTiO3(111) substrates (20 × 20 × 5 mm,
Crystal Base Co., Ltd.) using a KrF excimer laser with a wavelength of 248 nm and pulsed laser deposition(PLD) apparatus (PLAD131, AOV Inc.).

The following optimal synthetic conditions were selected based on

results of our previous studies : oxygen pressure, pO2 = 6.6 Pa; distance between substrate and target, d = 60mm; laser frequency, f = 10 Hz; deposition time, td = 5-25 min; energy density, E = 0.3-0.4 J cm−2; temperature,t = 650 °C.

In situ NR measurements were performed on SOFIA (BL-16), which is a time-of-flightreflectometer.

An electrochemical cell was composed of a Li4Ti5O12/SrTiO3 cathode, a Li anode anddeuterated EC/DEC electrolytes containing 1 mol dm-3LiPF6 as a supporting electrolyte, with the purposeof the deuterated electrolytes being to prevent incoherent scattering (80.27 and 2.05 for 1H and 2H,respectively).

Deintercalation and intercalation were performed via the potentiostatic method using a

potentiostat/galvanostat (Ivium Tech., Compactstat).

The NR spectra were collected using the single-frame

mode at 0.3, 0.6, and 1.2 °, and the individual data points were combined.

The cell conditions weremonitored after the cell construction (OCV), at 1.6, 1.4, and 1.2 V (vs. Li/Li+) at the first discharge, 3.0 V at thefirst charge, and after discharging and charging in the fifth cycle.

3. Results
1Fig. 1 shows neutron reflectometry analysis results of Li4Ti5O12(111) films in pristine condition andfollowing soaking in the 1.0 mol dm-3 LiPF6 EC/DEC (3:7 vol.).

The reflectivity spectra were plotted asfunctions of the scattering vector, Qz = 4psinq/l where l is the neutron wavelength and q is the glancing angle.
A two-layer model consisting of surface impurity/Li4Ti5O12/SrTiO3 fitted closely with the reflectivity curvefor the pristine film.

The SLD value of the pristine Li4Ti5O12 films was 2.03±0.04 × 10-4 nm-2, which isconsistent with the theoretical value of 2.04 × 10-4 nm-2 as calculated with the ICSD data (#15787).

The

pristine film was covered with a surface impurity layer with a thickness of 2.0±0.3 nm, and the surface
impurity had a higher neutron SLD value than the Li4Ti5O12 layer.

Considering the SLD values of typicallithium containing oxides (3.46 × 10-4 nm-2 for Li2CO3, 8.15 × 10-5 nm-2 for Li2O, and 6.02 × 10-6 nm-2 for LiOH),
the surface layer consisted of Li2CO3.

NR spectra collected under the OCV condition were analyzed using

the three-layer model (SrTiO3/Li4Ti5O12-bulk/Li4Ti5O12-surface/SEI/electrolyte), which was the same model
used for the Li4Ti5O12(111) film in the 1.0 mol dm-3 LiPF6 PC electrolyte.

The Li4Ti5O12 layer showed nosignificant changes in the SLD using 1.0 mol dm-3 LiPF6 EC/DEC electrolyte.

Furthermore, no surface

reconstruction proceeded at the Li4Ti5O12 surface following soaking in the EC/DEC-based electrolyte.

In theprevious study, the film electrode was soaked in the 1.0 and 2.4 mol dm-3 LiPF6 PC electrolyte, the SLD
increased in the entire region of the 16 nm-thick Li4Ti5O12 film.

This result clarified that the surface

reconstruction of Li4Ti5O12 was influenced by the solvent and the solute species in the organic electrolytes.

Fig. 1(a) In situ reflectometry analysis results and (b) neutron SLD profiles of epitaxial Li4Ti5O12(111) films inpristine conditions and after soaking in an electrolyte of 1 mol/dm3 LiPF6 EC:DEC (3:7 vol.).

4. Conclusion
The neutron reflectometry analyses revealed that the phase change occurs in the 10 nm surface region fromthe Li4Ti5O12/organic electrolyte interfaces during the initial reaction processes.

The surface-reconstructed

phase formed in the LiPF6 containing PC electrolytes limits the lithium intercalation although the surfacereconstruction behavior is highly influenced by the electrolyte composition.

The Li4Ti5O12 surface does notshow transitions to the low-active phase in the EC/DEC based electrolytes, which leads to highly-reversiblelithium intercalation.

Our results have demonstrated the importance of surface reactions in thedevelopment of Li4Ti5O12 anodes for lithium ion batteries of high power density.

In future works, we willconduct further NR experiments to detect interfacial structures at the electrolyte-side which should also
affect the electrochemical properties of intercalation electrodes.

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