Neutron Depth Profiling (NDP) in Battery Research

• Neutron Depth Profiling (NDP) is a non-destructive element-specific technique that measures lithium isotope distributions via thermal neutron-induced charged particle reactions.
• It achieves depth resolutions from 50 nm to 30 μm by converting detected energy losses of alpha particles and tritons into spatial profiles.
• NDP is pivotal for characterizing buried interfaces in battery systems and complements neutron reflectometry by accessing thicker layers.

Neutron depth profiling (NDP) is a non-destructive, element-specific technique for measuring near-surface and sub-surface distributions of certain isotopes—primarily lithium—via analysis of charged particles emitted from nuclear reactions induced by thermal neutrons. In contemporary solid-state battery research, NDP is particularly applied to resolve buried interfaces between lithium metal and solid electrolytes, where conventional surface-sensitive probes are limited by the depth and encapsulation of the interphases (Westover et al., 6 Dec 2025).

1. Physical Mechanism and Principle of Operation

NDP relies on the nuclear reaction between thermal neutrons ($n_{th}$) and the isotope ${}^6$Li, described by:

$${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$

where $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$ at $E_n \approx 0.025~\mathrm{eV}$ and natural lithium contains $7.5\%$ ${}^{6}$Li. Incident thermal neutrons penetrate the material and are captured whenever ${}^{6}$Li is present. Each capture releases an $\alpha$-particle and a triton (${}^3\mathrm{H}$), which subsequently traverse the overlying material and lose kinetic energy at a rate determined by the local stopping power. The emergent charged particles are detected and the measured energy spectrum is quantitatively mapped to the original depth of emission, thereby yielding a position-resolved concentration profile for lithium.

2. Depth Resolution and Quantitative Reconstruction

The spatial resolution, ${}^6$0, in NDP is governed by both the energy resolution of the ${}^6$1-particle detector (${}^6$2) and the stopping power (${}^6$3) for the charged particles in the host matrix:

${}^6$4

Typical detector energy resolutions are a few keV, while the stopping power for ${}^6$5-particles in Li metal and LiPON is of order ${}^6$6. This sets the attainable depth resolution to ${}^6$7 on the lower bound, extending to ${}^6$8 for ${}^6$9-particles. Using triton detection, the probed thickness can be extended to $${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$0, albeit at coarser depth resolution. In experimental practice and simulation (Westover et al.), effective depth-resolution limits fall within this 50 nm–5 μm range for $${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$1-based NDP, and 100 nm–30 μm for triton-based profiles (Westover et al., 6 Dec 2025).

3. Experimental Procedures in Li Metal–LiPON Systems

Westover et al. (Westover et al., 6 Dec 2025) detail a representative NDP implementation:

• Neutron Source and Beam: LVR-15 research reactor (CANAM, Nuclear Physics Institute, Řež, Czech Republic); collimated thermal-neutron beam ($${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$2 eV) illuminates sample area $${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$31 cm$${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$4.
• Sample Preparation: LiPON films (100 nm or 500 nm, with nominal composition Li$${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$5PO$${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$6N$${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$7) sputtered onto lithium metal ($${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$8500 nm); some configurations include an $${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$950 nm Ni(O) artificial interphase.
• Detection and Spectroscopy: Charged-particle detectors arranged around the sample capture $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$0-particles (2.05 MeV initial energy) and tritons (2.73 MeV). The measured spectrum encodes the energy loss (hence, emission depth).
• Quantification: Energy-to-depth conversion for each host phase uses tabulated stopping powers; witness LiPON-only samples anchor the $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$1 peak references at $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$22050 eV (LiPON) versus $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$32026 eV (Li metal). No absolute neutron flux or detailed geometry numbers are reported.

4. Analytical Regimes, Sensitivity, and Practical Limits

NDP’s detection depth, resolution, and sensitivity are determined by both physics and instrumental configuration:

• Detection Limits: $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$4-particles probe up to 5 μm; tritons up to 30 μm (if $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$5-blocking is used).
• Li Sensitivity: Sub-percent changes in lithium concentration over $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$6–$$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$7m are resolvable, limited ultimately by 6Li(n,α)T event statistics.
• Simulated Interphase Identifiability: Simulations of Li$$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$8O interlayers $$\sigma({}^{6}\mathrm{Li}(n,\alpha)T)\approx 940~\mathrm{b}$$9 nm drive only minor ($E_n \approx 0.025~\mathrm{eV}$0 eV) shifts in the Li $E_n \approx 0.025~\mathrm{eV}$1-peak. High-Z interlayers (e.g., Ni or AuLi, $E_n \approx 0.025~\mathrm{eV}$2–20 nm) yield distinguishable features in the energy spectrum. In actual LiPON–Li stacks, no distinct “natural” interphase $E_n \approx 0.025~\mathrm{eV}$3 nm is observed by NDP; models with zero interphase suffice to fit the data.
• Overlapping Analytical Windows: NDP with $E_n \approx 0.025~\mathrm{eV}$4-particles covers 50 nm–5 μm; with tritons, 100 nm–30 μm. In contrast, neutron reflectometry (NR) spans 0.1–200 nm, requiring ultra-smooth ($E_n \approx 0.025~\mathrm{eV}$51 nm rms) and thin ($E_n \approx 0.025~\mathrm{eV}$6500 nm) samples.

5. Comparative Assessment and Complementarity

The distinctive operating regimes and sample requirements of NDP and NR are summarized in Table 3 of Westover et al. (Westover et al., 6 Dec 2025):

Parameter	NDP (Neutron Depth Profiling)	NR (Neutron Reflectometry)
Typical interphase resolution	50 nm–5 μm (α); 100 nm–30 μm (T)	0.1–200 nm
Surface/roughness tolerance	up to tens of nm	$E_n \approx 0.025~\mathrm{eV}$71 nm rms
Max. stack thickness	$E_n \approx 0.025~\mathrm{eV}$85 μm (α), 30 μm (triton)	$E_n \approx 0.025~\mathrm{eV}$9500 nm
Elemental sensitivity	Li (via $7.5\%$0Li(n,α)T), ³He, ¹⁰B, etc.	All elements via SLD contrast
Area probed	$7.5\%$11 cm$7.5\%$2	%%%%53$${}^6\mathrm{Li} + n_{th} \rightarrow \alpha (2.05~\mathrm{MeV}) + T (2.73~\mathrm{MeV})$$054%%%%cm$7.5\%$5

NDP enables direct, element-specific lithium profiling through thick ($7.5\%$61–10 μm) stacks and is tolerant of moderate surface roughness, making it suitable for realistic battery electrodes. NR, while providing sub-nanometer to few-tens-of-nanometer resolution, is sensitive to sample smoothness and thinness and yields indirect (model-based) compositional information. NDP cannot resolve interphases thinner than 50–100 nm unless they comprise high-$7.5\%$7, Li-free layers; NR excels for ultrathin ($7.5\%$8200 nm) structures but has limited penetration depth. Used together, NDP and NR span length scales from $7.5\%$9 to ${}^{6}$0, enabling a comprehensive, non-destructive characterization of buried solid–solid interfaces.

6. Advantages, Limitations, and Significance in Interface Analysis

Advantages:

• Element-specific lithium depth profiling using a single, well-characterized reaction.
• Penetration through several microns of overlay, allowing direct interrogation of buried interfaces.
• Intolerance only to extreme surface roughness or total stack thickness, as compared to NR’s stricter requirements.
• Large-area signal averaging (${}^{6}$11 cm${}^{6}$2), permitting study of representative samples and real-world device dimensions.

Limitations:

• Diminished resolution for ultrathin (${}^{6}$350–100 nm), low-${}^{6}$4 interphases; only pronounced for high-${}^{6}$5 or thicker features.
• Maximum total stack thickness set by overlap of ${}^{6}$6/triton signals (${}^{6}$75–30 μm).
• Sensitivity is mostly to lithium; profiling other elements requires variant nuclear reactions such as ³He, ¹⁰B, or ¹⁴N.

Contextual Importance:

In the context of next-generation solid-state Li metal batteries, where buried electrolyte–electrode interfaces are critical yet challenging to access, NDP provides unique, non-destructive access to lithium distribution and interphase features inaccessible by surface probes. The complementary use of NDP and NR, as demonstrated for the Li metal–LiPON interface, delivers multi-decadal, cross-validated insight into buried chemistries across the 0.1 nm to 10 μm range (Westover et al., 6 Dec 2025).