Solid‐State Electrochemical Thermal Switches with Large Thermal Conductivity Switching Widths

Abstract Thermal switches that switch the thermal conductivity (κ) of the active layers are attracting increasing attention as thermal management devices. For electrochemical thermal switches, several transition metal oxides (TMOs) are proposed as active layers. After electrochemical redox treatment, the crystal structure of the TMO is modulated, which results in the κ switching. However, the κ switching width is still small (<4 W m−1 K−1). In this study, it demonstrates that LaNiO x ‐based solid‐state electrochemical thermal switches have a κ switching width of 4.3 W m−1 K−1. Fully oxidized LaNiO3 (on state) has a κ of 6.0 W m−1 K−1 due to the large contribution of electron thermal conductivity (κ ele, 3.1 W m−1 K−1). In contrast, reduced LaNiO2.72 (off state) has a κ of 1.7 W m−1 K−1 because the phonons are scattered by the oxygen vacancies. The LaNiO x ‐based electrochemical thermal switch is cyclable of κ and the crystalline lattice of LaNiO x . This electrochemical thermal switch may be a promising platform for next‐generation devices such as thermal displays.

thermal transistor may be a promising platform for next-generation devices such as thermal displays.

Introduction
Reuse of waste heat resulting from the low conversion rate of primary energy is crucial for sustainable development.Low-to medium-temperature (100−300 °C) waste heat is the most difficult to reuse; the temperature is too low to generate jet steam for power generation.
Although thermoelectric energy conversion technology is a solution, its efficiency is too low in this temperature range in air [1][2][3][4] .Thermal management technologies [5] such as thermal diodes [6][7][8] and thermal transistors [9][10][11][12][13][14][15][16] have recently attracted attention.Thermal diodes rectify the heat flow; thermal transistors electrically switch the heat flow on and off.We expect that thermal displays that visualise heat contrast using infrared cameras can be realised using thermal transistors.Thus, thermal transistors may be useful for reuse of waste heat.
For this purpose, electrical control of thermal conductivity (κ) in the active materials of thermal transistors is paramount.It necessitates a switch between the on state (high κ) and off state (low κ).Electrochemical [10][11][12][13][14] and electrostatic [15,16] approaches offer pathways to govern the κ of active materials.Although electrostatic methods provide rapid κ control, their suitability for thermal display applications is limited by the requirement of an extremely thin active material around the heterointerface between the gate dielectric and the active material.
We focus on electrochemical methods because they control the κ of entire materials.Many studies have used ionic liquids such as organic electrolytes and water for electrochemical modulation of materials [10][11][12]14] . Howevr, this method is incompatible with integrated circuits, limiting its application.In our pursuit of thermal displays with substantial thermal conductance differences between the on and off states, we used all-solid-state electrochemical thermal transistors [13,17,18] .All-solid-state electrochemical thermal transistors, a cornerstone of advanced thermal management, harness the redox modulation of transition metal oxides (TMOs).Among many candidates [9][10][11][12][13][14][15][16] , TMOs have emerged as promising materials.When subjected to electrochemical redox treatment by inserting and extracting metal ions [10] and oxide ions [12][13][14] , TMOs undergo structural transformations leading to a switch in their κ width.Despite these advancements, the challenge lies in achieving a substantial κ switching width, a critical factor for practical application that is often limited (< 4 W m −1 K −1 ) [10,[12][13][14]19] .
To overcome this limitation, we chose LaNiO3 as the active material for solid-state electrochemical thermal transistors.Bulk LaNiO3 has comparably high electrical and thermal conductivity [20] , indicating its potential for thermal conductivity modulation (Supplementary Information S1).The schematic depicted in Fig. 1 introduces LaNiOx as the active layer, offering a wide κ switching width.In the on state, LaNiO3 has heightened electrical conductivity when fully oxidised, resulting in a significant contribution from the electron thermal conductivity.Conversely, the off state achieved through electrochemical reduction leads to oxygen vacancies, reduced electrical conductivity, and negligible electron thermal conductivity.Scattering of phonons by these vacancies manifests as a low thermal conductivity.
This study focuses on use of LaNiOx-based electrochemical thermal transistors to address the limitations in κ switching width.Our investigations indicated a κ switching width of 4.3 W m −1 K −1 .Fully oxidised LaNiO3 (on state) exhibited a high κ of 6.0 W m −1 K −1 , primarily attributed to the contribution of electron thermal conductivity (3.1 W m −1 K −1 ).Conversely, reduced LaNiO2.72(off state) had a low κ of 1.7 W m −1 K −1 due to scattering of phonons by oxygen vacancies.Both the reduction and oxidation processes exhibited a nearly linear change in thermal conductivity.Furthermore, our study investigates the cyclability of κ and the crystalline lattice of LaNiOxbased thermal transistors.The exceptional performance of these electrochemical thermal transistors positions them as viable candidates for integration into next-generation devices, particularly thermal displays.

Electrochemical Thermal Transistor Fabrication and Operation
Figure 2a shows a schematic of the thermal transistor device structure, which is similar to those of our previous thermal transistors [13,17,18] .In this study, we inserted an extremely thin SrCoOx layer between the LaNiO3 and solid electrolyte (Gd-doped CeO2/YSZ) (Supplementary Information S2).As shown in Supplementary Fig. S2, when LaNiO3 was grown on the SrCoOx/GDC-buffered (001) YSZ, the crystallographic orientation was stronger than that without a buffer layer.X-ray reciprocal space mapping (RSM) around 113 YSZ diffraction spots (data not shown) confirmed that LaNiO3 grown on SrCoOx/GDC-buffered (001) YSZ demonstrated the highest quality.Moreover, there was a positive correlation between the quality of LaNiO3 and its thermal conductivity (Supplementary Table S3).
Thus, use of the thermal transistor structure shown in Fig. 2a is crucial.
The setup for operation of the LaNiOx thermal transistor is shown in Fig. 2b.Electrochemical redox treatment was performed at 280 °C in air by applying a constant current of ±10 μA.
During the redox reaction, we controlled the flown electron density Q = (I•t)/(e•V) through the current application time (Figs.2c and 2d), with a step of Q = 2 × 10 21 cm −3 marked as A -K, where I is the flown current, t is the application time, e is the electron charge, and V is the volume of the LaNiOx layer in the thermal transistor.In this study, electrochemical redox treatments were performed according to Faraday's law of electrolysis.
Electrochemical redox treatment was initiated by applying a negative current to reduce LaNiO3 to LaNiO2.72 (Fig. 2c).As Q increased, the absolute value of the voltage increased.
The slope decreased after 2 × 10 21 cm −3 and slightly increased after 2 × 10 21 cm −3 .When the current became positive, LaNiO2.72 was oxidised to LaNiO3 (Fig. 2d).As Q increased, fluctuations occurred at 2 × 10 21 cm −3 and 8 × 10 21 cm −3 .Overall, there was still an increasing trend and an absolute value between 3 V and 5 V, with minimal variations.The absence of steps in the process curve indicates that there were no new thermodynamically stable phases.

Crystalline Lattice and Thermal Conductivity Changes during Redox Treatment
Redox treatment induced reversible changes in the crystalline lattice of LaNiOx step by step (reduction A → F, oxidation: F → K), as evidenced by the out-of-plane x-ray diffraction patterns (Fig. 3a).Slight shifts in the 002 diffraction peak indicated modulations in the crystal structure, with lattice expansion observed after reduction and shrinkage observed after oxidation.The change in the lattice parameter c is shown in Fig. 3b. Figure 4 and Supplementary Fig. S3 illustrate the significant changes in the κ of the LaNiOx during redox treatment.Time-domain thermoreflectance (TDTR) decay curves show a decrease in κ from 5.9 W m −1 K −1 to 1.8 W m −1 K −1 after reduction (A → F) and an increase from 1.8 W m −1 K −1 to 5.9 W m −1 K −1 after oxidation (F → K).Both the reduction and oxidation processes exhibited a nearly linear change in thermal conductivity.The slight variations in lattice constants between the oxidised and reduced states are attributed to the disappearance and generation of oxygen vacancies.Through phonon scattering, oxygen vacancies contribute to a decrease in the lattice thermal conductivity.
However, there is a significant contrast in electrical conductivity (σ) between the oxidised (4250 S cm −1 ) and reduced (9 S cm −1 ) states (Supplementary Table S4).We estimated the electron thermal conductivity (κele) by assuming the Wiedemann-Franz law; κele where L is the Lorentz number (2.44 × 10 −8 W Ω K −2 ), and T is the absolute temperature.The κele of the oxidised state reached 3.1 W m −1 K −1 .According to the principle that observable thermal conductivity is the sum of lattice thermal conductivity (κlat) and κele [22] , we estimated the κlat of LaNiO3 (on state) to be 2.9 W m −1 K −1 .This value aligns with previously reported values for bulk LaNiO3 [20] , confirming the consistency of our findings.This substantial difference enabled the LaNiOx thermal transistor to modulate its thermal conductivity over a wide range.Furthermore, the linear and reversible changes in thermal conductivity indicate the potential of the LaNiOx-based thermal transistor for precise thermal modulation.

Cycle Properties of Thermal Transistor
As shown in Fig. 5 and Supplementary Fig. S4, the LaNiOx-based thermal transistor exhibits exceptional cycling properties.The on state (LaNiO3) has a higher average thermal conductivity (6.0 W m −1 K −1 ) than the off state (LaNiO2.72,1.7 W m −1 K −1 ).Seven cycles are shown in the figure.The TDTR decay of LaNiO3 was faster than that of LaNiO2.72 (Fig. 5a).
The TDTR curves for each cycle overlapped significantly, indicating excellent repeatability.

Conclusion
This study presents a breakthrough with LaNiOx-based electrochemical thermal transistors, with an exceptional κ switching width of 4.3 W m −1 K −1 .The fully oxidised LaNiO3 (on state) produced a κ of 6.0 W m −1 K −1 , primarily attributed to a substantial contribution from electron thermal conductivity (3.1 W m −1 K −1 ).In contrast, the reduced LaNiO2.72(off state) had a low κ of 1.7 W m −1 K −1 due to negligible electron thermal conductivity (0.007 W m −1 K −1 ) and phonon scattering caused by oxygen vacancies.The LaNiOx-based electrochemical thermal transistor demonstrated outstanding κ cyclability while maintaining the structural integrity of the LaNiOx crystalline lattice, making it a promising candidate for integration into nextgeneration devices, particularly thermal displays.
Subsequently, an approximately 80-nm-thick LaNiO3 film was heteroepitaxially grown on the GDC film at 625 °C in an oxygen atmosphere (25 Pa).The laser fluence was approximately 1.6 J cm −2 pulse −1 .After film growth, the sample was cooled to room temperature in a PLD chamber in an oxygen atmosphere (25 Pa).An approximately 50-nm-thick Pt film was sputtered on the top surface of the LaNiO3 epitaxial film, followed by an approximately 50nm-thick Pt film sputtered on the backside of the YSZ substrate.Pt sputtering was performed at room temperature.The samples were cut into four squares (5 mm × 5 mm).
Electrochemical redox treatment: thermal transistor (5 mm × 5 mm) was placed on a Ptcoated glass substrate and heated at 280 °C in air.Electrochemical redox treatment was performed by applying a constant current of ±10 μA, after which the sample was immediately cooled to room temperature.

Crystallographic analyses:
The crystalline phase, orientation, and lattice parameters of the resultant films were analysed using high-resolution x-ray diffraction (Cu Kα1, λ = 1.54059Å, ATX-G, Rigaku).Out-of-plane Bragg diffraction patterns and reciprocal space mappings (RSMs) were measured at room temperature to clarify changes in the crystalline phase of LaNiOx.The lattice parameters were calculated from the diffraction peaks.The atomic arrangements of the LaNiO3 films were visualised using a STEM (JEM-ARM200CF, JEOL) operated at 200 keV.

Measurement of electrical properties of LaNiOx layers:
To measure the electrical conductivity (σ) of the LaNiOx layers after redox treatment, we mechanically attached Au foil on the film surface while Pt films were deposited only on the backside of the YSZ substrate [13] .The LaNiOx films were oxidised and reduced electrochemically at 280 °C in air using the Au foil as the electrode.The σ of the LaNiO3 (on state) and LaNiO2.72 (off state) films was measured using the DC four-probe method with a van der Pauw electrode configuration at room temperature in air.

S1. Selection of LaNiO3 as the active layer
In this study, we focused on perovskite-structured LnNiO3 (Ln = La, Nd, and Sm) as the active layer for the solid-state electrochemical thermal transistor.Table S1 summarizes the electrical conductivity (σ) and the thermal conductivity (κ) of bulk LnNiO3 (Ln = La, Nd, and Sm) at room temperature [20] .At room temperature, bulk LnNiO3 (Ln = La, Nd, and Sm) shows the following κ; LaNiO3: 10.7 W m −1 K −1 , NdNiO3: 6.5 W m −1 K −1 , and SmNiO3: 4.0 W m −1 K −1 .Thus, LaNiO3 is a promising candidate as the active layer of the thermal transistors.It should be noted that the σ of LaNiO3 is 10500 S cm −1 .We assumed the Wiedemann-Frantz law for the estimation of electron contribution to the observed thermal conductivity (κele = L•σ•T, where L is the Lorentz number of 2.44 × 10 −8 W Ω K −2 and T is the absolute temperature of 298 K) and obtained κele of 7.6 W m −1 K −1 for LaNiO3.This reflects the lattice thermal conductivity (κlat Table S1.The electrical conductivity (σ) and the thermal conductivity (κ) of bulk LnNiO3 (Ln = La, Nd, and Sm) at room temperature.The ionic radius data is from Shannon's report. [2].-S.Zhou et al., PRB 67, 020404(R) (2003) [1] LaNiO3 NdNiO3 SmNiO3 Ionic radius of Ln 3+ ion (Å) (C.N. = 12) 1.36 1.27 1.24 Electrical conductivity at RT (S cm −1 ) 10500 3400 ---Total thermal conductivity, κ (W m −1 K −1 ) 10.7 6.5 4.0 Electron thermal conductivity, κele (W m −1 K −1 ) 7.6 2.5 0 Lattice thermal conductivity, κlat (W m −1 K −1 ) 3.1 4.0 4.0 To check the potential of LaNiO3 epitaxial films as the active layer of the thermal transistors, we fabricated LaNiO3 epitaxial films on (001) SrTiO3 substrates and measured the electrical and thermal conductivity of the resultant films at room temperature (Table S2).The σ of the resultant LaNiO3 film was only 135 S cm −1 , two orders of magnitude smaller than that of bulk.The κ in the out-of-plane of the LaNiO3 film was 7.3 W m −1 K −1 .If we assumed the Wiedemann-Frantz law for the estimation of κele, the κele of the LaNiO3 film was only 0.1 W m −1 K −1 , reflecting the κlat of LaNiO3 is 7.2 W m −1 K −1 .
Since the estimated κlat of the LaNiO3 film on (001) SrTiO3 substrate is higher than that of the bulk, there is a possibility that we underestimated the κele of the LaNiO3 film.To clarify the origin of it, we observed the microstructure using Cs-corrected scanning transmission electron microscopy (Fig. S1).The columnar structure is visualized in the LaNiO3 film (Fig. S1a).The magnified image (Fig. S1b) reveals that there are many planer defects due to the formation of Ruddlesden-Popper phases.Since the electron transport is suppressed by the planer defects, the observed electrical conductivity in the in-plane direction would be lower than that in the out-of-plane direction.

S2. Fabrication of LaNiO3-based solid-state thermal transistors
Firstly, we fabricated LaNiO3 films directly on (001) YSZ substrates.Figure S2a shows the out-of-plane XRD pattern of the resultant film.Together with 001 and 002 diffraction peaks of LaNiO3, 110 diffraction peaks of LaNiO3 are seen with 002 YSZ, indicating the mixed orientation of the film.The out-of-plane rocking curve of the 002 LaNiO3 (Fig. S2d) is broad (the full width at half maximum, FWHM ~3.1°).Then, we fabricated LaNiO3 films on GDCbuffered (001) YSZ substrates.As shown in Fig. S2b, intense diffraction peaks of 001 and 002 LaNiO3 are seen together with 002 GDC and 002 YSZ.The FWHM of the 002 LaNiO3 is 1.6° as shown in Fig. 2e.The reciprocal space mapping (RSM, not shown) of the film revealed that the LaNiO3 is heteroepitaxially grown on the GDC-buffered YSZ substrate.To further improve the crystallographic orientation of the film, we inserted thin (~2 nm) SrCoOx layer between the LaNiO3 and GDC-buffered substrate.The out-of-plane XRD peaks became stronger (Fig. S2c) and the tilting became small (1.2°) as shown in Fig. S2f.

Figure 6
Figure 6 compares the thermal conductivity switching widths of different TMOs, indicating the unparalleled performance of LaNiOx-based thermal transistors, with a thermal conductivity switching width of approximately 4.3 W m −1 K −1 , significantly greater than that of other TMOs.The wide switching range indicates the exceptional versatility of LaNiOx in modulating its thermal conductivity.The inherent ability of LaNiOx-based thermal transistors to linearly transition between high thermal conductivity in the fully oxidised state and low thermal conductivity in the reduced state positions them at the forefront of TMOs for thermal management applications.The nuanced control over thermal conductivity and the stability

Figure 1 .
Figure 1.Strategy of electrochemical thermal transistors with transition metal oxide active layer with large thermal conductivity switching width.(Left) Diagram of on state.Fully oxidised TMOs show high electrical conductivity.Both electrons and phonons carry heat.The thermal transistor shows high thermal conductivity.(Right) Diagram of off state.Electrochemical reduction treatment produces oxygen vacancies.The reduced TMOs show low electrical conductivity.The electron thermal conductivity is negligible.Phonons are scattered by oxygen vacancies.The thermal transistor shows low thermal conductivity.

Figure 2 .
Figure 2. LaNiOx-based thermal transistor operation.(a) Structure of thermal transistor composed of six layers: 50-nm-thick Pt film, 80-nm-thick LaNiOx film, 2-nm-thick SrCoOx film, 10-nm-thick Gd-doped CeO2 (GDC) film, 0.5-mm-thick (001) YSZ single-crystal substrate, and 40-nm-thick Pt film.(b) Schematic of LaNiOx thermal transistor.The transistor was placed on a Pt-coated glass substrate and heated at 280 °C in air.A K-type thermocouple was used to monitor the transistor surface temperature.The transistor measured 5 mm × 5 mm.A constant negative current (−10 μA) was applied for reduction; a constant positive current (+10 μA) was applied for oxidation.(c)(d) Changes in observed DC voltage of thermal transistor during (c) reduction from LaNiO3 to LaNiO2.72 and (d) oxidation from LaNiO2.72 to LaNiO3 with a step of Q = 2 × 10 21 cm −3 .

Figure 3 .
Figure 3. Change in crystalline lattice of LaNiOx layer after redox treatment.(a) Change in out-of-plane XRD patterns after redox treatment with a step of Q = 2 × 10 21 cm −3 .The 002 diffraction peak shifted to a smaller qz side after reduction and a larger qz side after oxidation.These shifts were entirely reversible.(b) Changes in lattice parameter c of LaNiOx as a function of Q.A lattice expansion of approximately 0.6% occurred after reduction.

Figure 4 .
Figure 4. Change in thermal conductivity (κ) of LaNiOx layer after redox treatment.(a) TDTR decay curves of thermal transistor after (upper) reduction and (lower) oxidation treatment.(b) Changes in thermal conductivity of LaNiOx layer after (left) and (right) oxidation treatment with a step of Q = 2 × 10 21 cm −3 .The κ of LaNiO3 was 5.9 W m −1 K −1 ; it decreased almost linearly with Q after reduction.After oxidation, the κ increased almost linearly with Q and returned to 5.9 W m −1 K −1 .

Figure 5 .
Figure 5. Cycle properties of LaNiOx-based thermal transistor.(a) Changes in TDTR decay curves (seven cycles overlapped).The TDTR decay of the LaNiO3 layer was faster than that of the LaNiO2.72 layer.(b) Change in thermal conductivities of LaNiO3 and LaNiO2.72 layers after redox cycling.The average thermal conductivities of the LaNiO3 (on state) layer and LaNiO2.72 (off state) layer were 6.0 W m −1 K −1 and 1.7 W m −1 K −1 , respectively.The electron thermal conductivity of the LaNiO3 (on state) layer was high (3.1 W m −1 K −1 ).The electron thermal conductivity of the LaNiO2.72 (off state) layer was negligible.

Figure 6 .
Figure 6.Comparison of thermal conductivity switching widths of several transition metal oxides.LaNiOx-based thermal transistors exhibited large thermal conductivity switching widths (~4.3 W m −1 K −1 ).Data for LiCoO2 ↔ Li1−δCoO2 are from Ref. 10 [10] , entry should be 50-60 words long and should be written in the present tense.The text should be different from the abstract text.Z. Bian, M. Yoshimura, A. Jeong, H. Li, T. Endo, Y. Matsuo, Y. Magari, H. Tanaka, H. Ohta* Solid-State Electrochemical Thermal Transistors with Large Thermal Conductivity Switching Widths ToC figure ((Please choose one size: 55 mm broad × 50 mm high or 110 mm broad × 20 mm high.Please do not use any other dimensions))

Figure S3 .
Figure S3.Change in the thermal conductivity of the LaNiO3 layer in the thermal transistor.a, b, TDTR phase signal decay curves during (a) reduction and (b) oxidation.The reduction treatment was performed in the order of A, B, C,…F with a step of Q = 2 × 10 21 cm −3 .The oxidation treatment was performed in the order of F, G, H,…K with a step of Q = 2 × 10 21 cm −3 .

Figure S4 .
Figure S4.TDTR decay cycle of the LaNiO3 layer in the thermal transistor.a, b, TDTR phase signal decay curves after (a) reduction and (b) oxidation.