Comparison of determining the 10B and 6Li depth profiles based on NDP and SIMS analytical methods

In the study Comparison of determining the 10B and 6Li depth profiles based on NDP and SIMS analytical methods the comparison of the analytical results between SIMS (Secondary-ion mass spectrometry) and NDP (Neutron Depth Profiling) methods have been carried out with LiCoO2 and BSi samples. The NDP is an analytical method to analyze the component nuclide concentration versus depth distribution in a sample by detecting the charged particles emitted after the neutrons are absorbed.

Comparison of Determining the 10B and 6Li Depth Profiles based on

NDP and SIMS Analytical Methods

Hoang Sy Minh Tuan1, *

1Institute of Applied Technology - Thu Dau Mot University, 6, Tran Van On, Phu Hoa

Ward, Thu Dau Mot City, Binh Duong, Vietnam, 820000

*hoangsyminhtuan@tdmu.edu.vn

ABSTRACTS

Depth resolution and probing depth for 6Li in lithium thin-film batteries achievable using different ion beam analytical techniques were investigated In this study, the comparison of the analytical results between SIMS (Secondary-ion mass spectrometry) and NDP (Neutron Depth Profiling) methods have been carried out with LiCoO2 and BSi samples The NDP is an analytical method to analyze the component nuclide concentration versus depth distribution in a sample by detecting the charged particles emitted after the neutrons are absorbed The 10B and

6Li depth profiles in the BSi and LiCoO2 samples were also analyzed using a CAMECA IMS 7f SIMS instrument at the National Nanofab Center (Republic of Korea) The results from NDP analysis have been performed at the NDP system (HANARO, Republic of Korea) In comparison results for the samples, the peak depths, peak concentrations, and total dose of the NDP results are consistent with the SIMS results within 2, 6, and 11 %, respectively The NDP is useful for analyzing light elements with high neutron cross-sections for particle-producing reactions

Keywords: NDP; SIMS; Depth Profiling

Tóm tắt

Đánh giá đặc tính theo độ sâu của 6Li trong pin màng lithium mỏng có thể đạt được bằng cách sử dụng các kỹ thuật phân tích chùm ion khác nhau Trong nghiên cứu này, việc so sánh các kết quả phân tích giữa kỹ thuật SIMS (Secondary-ion mass spectrometry) và NDP (Neutron Depth Profiling) được thực hiện với các mẫu LiCoO2 và BSi Kỹ thuật NDP là một kỹ thuật phân tích

để phân tích hàm lượng nguyên tố thành phần so với phân bố độ sâu trong một mẫu bằng cách phát hiện các hạt tích điện phát ra sau khi neutron được hấp thụ Các phân bố độ sâu 10B và 6Li trong các mẫu BSi và LiCoO2 được phân tích bằng cách sử dụng thiết bị SIMS CAMECA IMS 7f tại Trung tâm Nanofab Quốc gia (Hàn Quốc) Kết quả từ phân tích NDP đã được thực hiện tại

hệ thống NDP (HANARO, Hàn Quốc) Trong kết quả so sánh cho các mẫu, độ sâu đỉnh, nồng độ cao nhất và tổng liều của kết quả NDP phù hợp với kết quả SIMS trong vòng 2, 6 và 11%, tương ứng Kỹ thuật NDP rất hữu ích để phân tích các nguyên tố ánh sáng có mặt cắt ngang neutron cao cho các phản ứng tạo ra hạt

1 Introduction

Lithium-ion rechargeable batteries are commonplace in consumer gadgets such as cell phones and laptop computers They are based on the movement of lithium ions through an electrolyte between a positive electrode (cathode), which is a lithium-containing material, and a negative electrode (anode), which is generally a porous substance The efficiency, capacity, and durability of batteries are all dependent on lithium movement during charge and discharge Energy storage technology is essential for a sustainable energy transition Understanding the regulating mechanisms is critical to meeting the ever-increasing energy storage need Different ion-beam analytical techniques were used to explore the depth resolution and probing depth for

Li in lithium thin-film batteries

SIMS (Secondary Ion Mass Spectrometry) detects very low dopant and impurity concentrations A beam of intense heavy ions erodes the surface of the material under investigation in SIMS, while secondary ions created during the sputtering process are mass assessed and recognized [1] As a result, it is no wonder that depth profiling is one of SIMS's most popular applications The method generates elemental depth profiles over a wide range of depths, from a few angstroms to tens of micrometers A beam of primary ions (typically O2+ or

Cs+) sputters/etches the sample surface, while secondary ions generated during the sputtering process are collected and studied using a mass spectrometer (quadrupole, magnetic sector, or Time-of-flight) The concentrations of secondary ions might range from matrix levels to sub-ppm trace levels SIMS methodology is limited for operando monitoring of lithium-ion batteries due to the intrinsic difficulties of studying light ions with traditional techniques As a result, SIMS depth profiling has the disadvantage of providing no information on the sample's atomic structure and is damaging On the other hand, SIMS depth profiling has the disadvantage of providing no information on the sample's atomic structure and is destructive As a result, an examined spot can no longer be used as an active device, and the SIMS results and laser performance cannot be linked

Real-time in-situ visualization and quantification of the changing lithium distribution in batteries during charging and discharging is beneficial information for their development, especially for a better understanding of the mechanisms restricting charge and discharge rates Moreover, at the present, only one technique, Neutron Depth Profiling (NDP), can accomplish such a difficult task [2] During (dis)charge, this approach offers information on the geographical and temporal lithium concentration NDP is employed in this study to shed light on critical lithium-ion battery issues The findings give a clear picture of electrode properties such as tortuosity and Li-ion transport (Figure 1) This provides for a reduction in charge times by lowering battery internal resistance or increasing the charging current

Figure 1 The principle of using Neutron Depth Profiling for Li characterization in Li-ion battery

NDP has long been used to determine the depth of lithium in a variety of materials Oudenhoven et al from the Eindhoven University of Technology in the Netherlands performed in-situ NDP on thin-film solid-state micro-batteries, seeing the lithium concentration profile evolve [3] C Co et al from Ohio State University in the United States collected one NDP spectrum every 5 minutes during charge and discharged to perform real-time in-situ quantification of Li transport in a Li-ion cell [4] This offered previously unobtainable information on lithium incorporation rates and losses in various battery regions, establishing operando NDP as a viable tool for understanding Li transport in Li-ion batteries The lowering of battery weight was one of the issues that were eventually overcome A copper sheet, which is heavy and expensive, is used to capture the current in the anode Aluminum is substantially

lighter and less expensive than steel Aluminum's application in Li-ion batteries has been hampered so far by a reaction with lithium that removes the lithium ions from the anode With operando NDP in a cell where the aluminum was covered with a protective 500 nm thick Sn layer, the viability of Al as the anode current collector in a lithium-ion cell was demonstrated [5] The Sn worked as a protective layer that did not react with the aluminum by integrating some lithium Yuping et al from Binghamton University in the United States used a neutron beam with an 11 mm2 aperture to measure the Li depth profile for each pixel on the surface of a Li-ion cell with a LeFePO4 electrode [6] The 3D lithium distribution for the total sample is obtained by combining all depth profiles They compared electrodes with only one charge/discharge cycle to electrodes that had 5000 charge/discharge cycles The Li concentration dropped by roughly 40% after 5000 cycles, whereas lateral inhomogeneity rose considerably This is a direct observation

of one of the causes of battery degeneration and failure

2 Method

If the target isotope captures the neutron, the compound nucleus emits a charged particle, and the leftover nucleus recoils in 1 ps Depending on the isotope, these reactions create a proton

or an alpha particle and a recoiling nucleus Figure 2 depicts the process of neutron captures in a sample deposited to a thickness of x, as well as the emission of charged particles (a) Some charged particles created isotopically in the sample pass through the sample material, reach the sample surface, and then pass through a charged particle detector Figure 2 depicts the NDP spectrum acquired as a result of the procedure (b) A neutron beam illuminates the planar surface

in a vacuum chamber at an angle of θ 1 to the surface normal The detector should have an

area-plane surface with a normal angle θ 2 away from the sample normal

Figure 2 Schematic representation of the neutron-induced charged particle emission process (a), and the

resultant spectrum (b)

Then the depth of target isotope is,

2

.cos

When the average stopping power in the path length is I, the energy loss of charged particle

in the path length I, Eis the same as s S , then (2-9) is,

2

.cos

E x

S





The uncertainty of x can be defined as depth resolution, where it can describe the uncertainty

of energy loss ΔE

Using the TRIM software [7], the energies of the spectrum were converted to depths based

on the residual energy versus path length of the alpha particle Each channel's count rate was translated to a 6Li concentration When the measured spectrum has no energy broadening, the

concentration profile C(x) at depth x can be written as:

( ( ) ( ( )) ( )

cos cos

C C

n

C x

A





(3)

Where E(s) is the residual energy for particles born at depth x that travels a distance s before escaping the sample surface; N(E) is the number of ions with energy E measured by the detector

per unit time; S(E) is the stopping power of ions with energy E; f is the fractional yield for the particle of interest per charged particle production reaction; s c is the microscopic neutron

cross-section for charged particle production; ϕ c is the neutron flux; ε is the detection efficiency; Ω d is

the solid detection angle The cold neutron beam with the cross-sectional area of A n illuminates

the surface of the sample at an angle θ n to the surface normal, and the detector is assumed to

have a plane surface whose normal is at an angle θ s to the sample surface normal

Figure 3 depicts a schematic of the NDP system used in this investigation A cold neutron beam is delivered to a target chamber through a cold neutron guide The LiF beam slit in the guide is used to collimate the neutron beam, and the Cd collimator in the beam inlet port with a

10 mm diameter aperture is used to define the beam region at the sample point The average wavelength of the cold neutron beam is 5.1 A, and the actual integrated neutron flux at the sample point is 7.37  106 n/cm2s, as determined by an activation analysis of gold foil The target chamber is kept at a vacuum of less than 2 9 10-4 Pa during operation

Figure 3 The schematic of the NDP system at HANARO used in this study

Lithium-coated silicon samples were created for deposition thickness measurements The samples were made at Seoul National University's Electronic Materials Laboratory When 6Li absorbs a neutron, it undergoes the nuclear reaction shown below

(2727.88 ) (2055.51 )

Ion-beam sputtering of a LiCoO2 target with Ar+ ions was used to deposit LiCoO2 on an oxidized (SiO2 thickness: 0.1 m silicon wafer The silicon wafer had a thickness of 600 m By varying the sputtering conditions, two types of samples (LCO-1 and LCO-2) were created The

RF power, deposition time, substrate temperature, and argon gas flow rate were 200 W, 4 h, 350

oC, and 45 sccm for LCO-1; and 120 W, 9 h, room temperature, and 15 sccm for LCO-2 SIMS was used to compare the 10B and 6Li depth profiles in the BSi and LCO samples with the NDP analysis, utilizing a CAMECA IMS 7f SIMS equipment [8]

Samples were inserted in the target chamber by connecting them to the sample holder's 80

m thick Teflon tape The sample stage, which is placed above the target chamber, was used to fine-tune the location of the sample as well as the angle between the sample and the neutron beam Figure 4 depicts the boron-implanted silicon wafer, the lithium-coated silicon wafer, and the lithium-ion battery and its electrodes

Figure 4 Pictures of (a) boron implanted silicon wafer, (b) lithium-coated silicon wafer, and (c) the

lithium-ion battery and its electrodes

3 Results and Discussions

Figure 5 depicts the analysis findings for the BSi, LCO, and electrode samples SIMS analysis findings are given in the same figures for comparison The NDP error bars show the error from data smoothing with Poisson counting statistics obtained by error propagation The positive error bars are shown in the figures for clarification 10B is dispersed to a depth of 0.5 m

for the BSi-1 sample and 0.3 m for the BSi-2 sample, as shown in Fig 5a The analytical findings for the BSi-3 sample are shown in Figure 5b Three results are highly agreed upon in the region up to 0.3 m of the sample In contrast, when the sample-detector angle rises, the concentration of 10B increases in the area deeper than 0.3 m This is due to an increase in the travel length of the alpha particles released by 10B The standard deviation of energy widening owing to energy straggling grows as the route length of alpha particles increases In this case, as

explained in Eqs (3) the depth profile C(x) at depth x can be raised The NDP results' peak

depth, concentration, and total dosage are within 2%, 6%, and 11% of the SIMS values, respectively

Figure 5 10 B depth profiles of the BSi-1, BSi-2 (a), and BSi-3 samples (b) Relative depth profiles of 6 Li

in the LCO-1 and LCO-2 samples (c) 6 Li depth profiles of the electrode samples from the lithium ion

battery (d)

The SIMS technique requires a reference material with the same matrix as the sample in

order to measure the isotope concentration However, because there was no standard material for LCO samples, SIMS gave only the relative concentration of 6Li The NDP technique findings for the LCO-1 and LCO-2 samples, normalized to the SIMS results, are presented in Figure 5c The NDP and SIMS findings for LCO-1 concur well up to 0.56 m However, at the region deeper than 0.56 m, the NDP result contradicts the SIMS result In contrast, the concentration of 6Li in the SIMS result improves from 0.56 to 0.72 m, it continues to fall in the NDP result The highest depths of 6Li in the SIMS and NDP samples for the LCO-2 sample match the 0.4 m However, the NDP-determined 6Li concentration at each level contradicts the SIMS conclusion The LiCoO2 has been deposited on the Si + SiO2 substrate in these samples The SIMS approach may have exaggerated the concentration of 6Li due to the lack of information regarding the matrix of the SiO2 layer The aerial densities of 6Li in the LCO-1 and LCO-2 samples were found to (1.7 ± 0.3)  1017 and (7 ± 1)  1016 atoms/cm2, respectively, using the NDP technique The uncertainty is expressed as a 95% confidence interval that incorporates random measurement errors and systematic errors Over the entire sample spectrum, the systematic error

is estimated to be 10% The concentrations of 6Li in the plateau area LCO-1 and LCO-2 samples were determined to be (3.03 ± 3.12)  1021 and (1.36 ± 2.23)  1021 atoms/cm3, respectively As

a result, the region of channel numbers 2000  2700, where only triton contribution existed, was utilized for the conversion to concentration distribution Figure 5d depicts depth profiles of 6Li in electrode samples The concentrations of 6Li in the cathode and anode samples were 3.5  1021 and 6.7  1020 atoms/cm3, respectively)

4 Conclusions

The relative concentrations of 6Li obtained by NDP and SIMS were discordant in several sections of the samples in this investigation with the case of the lithium-coated Si samples The SIMS profiles for the lithium-coated samples are questionable since the tendency toward an apparent rise in lithium content towards the surface is highly unusual for sputter-deposited samples In the future, this issue will necessitate in-depth research and the use of a different analysis approach Additional test samples were made by removing cathode and anode material from an old lithium-ion battery The concentration of 6Li in the cathode sample of a completely drained lithium-ion battery was five times greater than in the anode sample Both samples' layer thicknesses were determined to be more than 22 m, which is the range of tritons in LiCoO2

References

[1] Magee, C W., & Honig, R E (1982) Depth profiling by SIMS—depth resolution, dynamic range and sensitivity Surface and Interface Analysis, 4(2), 35-41

[2] Verhallen, T W., Lv, S., & Wagemaker, M (2018) Operando neutron depth profiling to determine the spatial distribution of Li in Li-ion Batteries Frontiers in Energy Research, 6,

62

[3] Oudenhoven, J F M., Labohm, F., Mulder, M., Niessen, R A H., Mulder, F M., & Notten, P (2011) In situ neutron depth profiling: A powerful method to probe lithium transport in micro‐batteries Advanced Materials, 23(35), 4103-4106

[4] Liu, D X., Wang, J., Pan, K., Qiu, J., Canova, M., Cao, L R., & Co, A C (2014) In situ quantification and visualization of lithium transport with neutrons Angewandte Chemie, 126(36), 9652-9656

[5] Liu, D X., Cao, L R., & Co, A C (2016) Demonstrating the feasibility of Al as anode current collector in li-ion batteries via in situ neutron depth profiling Chemistry of Materials, 28(2), 556-563

[6] He, Y., Downing, R G., & Wang, H (2015) 3D mapping of lithium in battery electrodes using neutron activation Journal of Power Sources, 287, 226-230

[7] McDaniel, F D., Anthony, J M., Renfrow, S N., Kim, Y D., Datar, S A., & Matteson, S (1995) Depth profiling analysis of semiconductor materials by accelerator mass spectrometry Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 99(1-4), 537-540