Fresnel transmission coefficients for thermal phonons at solid interfaces

Interfaces play an essential role in phonon-mediated heat conduction in solids, impacting applications ranging from thermoelectric waste heat recovery to heat dissipation in electronics. From a microscopic perspective, interfacial phonon transport is described by transmission and reflection coefficients, analogous to the well-known Fresnel coefficients for light. However, these coefficients have never been directly measured, and thermal transport processes at interfaces remain poorly understood despite considerable effort. Here, we report the first measurements of the Fresnel transmission coefficients for thermal phonons at a metal-semiconductor interface using ab-initio phonon transport modeling and a thermal characterization technique, time-domain thermoreflectance. Our measurements show that interfaces act as thermal phonon filters that transmit primarily low frequency phonons, leading to these phonons being the dominant energy carriers across the interface despite the larger density of states of high frequency phonons. Our work realizes the long-standing goal of directly measuring thermal phonon transmission coefficients and demonstrates a general route to study microscopic processes governing interfacial heat conduction

Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite

Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane directions are well known, less understood are the microscopic properties of the thermal phonons responsible for heat conduction. In particular, recent experimental and computational works indicate that the average phonon mean free path (MFP) along the c-axis is considerably larger than that estimated by kinetic theory, but the distribution of MFPs remains unknown. Here, we report the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation. Our study provides important new insights into heat transport and phonon scattering mechanisms in graphite and other anisotropic van der Waals solids

Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium–oxygen cell

Lithium–oxygen cells in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer–sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied here could sustain charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here. Lithium-oxygen cells in which the cathode reaction of lithium peroxide formation takes place in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to oxidise it. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide particles in the pores of the cathode that are disconnected from the surface. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for operation of the lithium-oxygen cell. While molecules with fast electron transfer at the electrode surface are common, it is also essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain relatively fast charging. Here we investigate the kinetics of lithium peroxide oxidation by several classes of redox mediators, with varying electrochemical and structural properties (amines, nitroxy and thiol compounds). The rates range from 0.025 to 7.9 x10—3 cm s—1 with the nitroxy compounds exhibiting the highest rates. We show that the reaction is not a simple outer sphere electron transfer and that the nature of the mediator molecule plays an important role for example the steric hindrance around the active redox centre of the mediator. The fastest mediator studied here could sustain an areal current density on charging of up to 1.9 A cm—2, based on a model for a porous electrode described in the paper

Quasiballistic Thermal Transport from Nanoscale Heaters and the Role of the Spatial Frequency

Quasiballistic heat conduction from nanoscale heat sources of size comparable to phonon mean free paths has recently become of intense interest both scientifically and for its applications. Prior work has established that, in the quasiballistic regime, the apparent thermal properties of materials depend both on intrinsic mechanisms and the characteristics of the applied thermal gradient. However, many aspects of this regime remain poorly understood. Here, we experimentally study the thermal response of crystals to large thermal gradients generated by optical heating of nanoline arrays. Our experiments reveal the key role of the spatial frequencies and Fourier series amplitudes of the heating profile for thermal transport in the quasiballistic regime, in contrast to the conventional picture that focuses on the geometric dimensions of the individual heaters. Our work provides the insight needed to rationally mitigate local hot spots in modern applications by manipulating the spatial frequencies of the heater patterns

A rechargeable lithium–oxygen battery with dual mediators stabilizing the carbon cathode

At the cathode of a Li–O2 battery, O2 is reduced to Li2O2 on discharge, the process being reversed on charge. Li2O2 is an insulating and insoluble solid, leading ultimately to low rates, low capacities and early cell death if formed on the cathode surface. Here we show that when using dual mediators, 2,5-Di-tert-butyl-1,4-benzoquinone [DBBQ] on discharge and 2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO] on charge, the electrochemistry at the cathode surface is decoupled from Li2O2 formation/decomposition in solution. Capacities of 2 mAh cmareal−2 at 1 mA cmareal−2 with low polarization on charge/discharge are demonstrated, and up to 40 mAh cmareal−2 at rates ≫1 mA cmareal−2 are anticipated if suitable gas diffusion electrodes can be devised. One of the major barriers to the progress of Li–O2 cells is decomposition of the carbon cathode. By forming/decomposing Li2O2 in solution and avoiding high charge potentials, the carbon instability is significantly mitigated ( < 0.008% decomposition per cycle compared with 0.12% without mediators)

Promoting solution phase discharge in Li-O-2 batteries containing weakly solvating electrolyte solutions

On discharge, the Li–O2 battery can form a Li2O2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li2O2 particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the LiO2 intermediate involved in the formation of Li2O2. However, the characteristics that make high donor or acceptor number solvents good (for example, high polarity) result in them being unstable towards LiO2 or Li2O2. Here we demonstrate that introduction of the additive 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) promotes solution phase formation of Li2O2 in low-polarity and weakly solvating electrolyte solutions. Importantly, it does so while simultaneously suppressing direct reduction to Li2O2 on the cathode surface, which would otherwise lead to Li2O2 film growth and premature cell death. It also halves the overpotential during discharge, increases the capacity 80- to 100-fold and enables rates >1 mA cmareal−2 for cathodes with capacities of >4 mAh cmareal−2. The DBBQ additive operates by a new mechanism that avoids the reactive LiO2 intermediate in solution

EKF-Based a Novel SOC Estimation Algorithm of Lithium-ion Battery

State of charge (SOC) is an essential parameter for battery management system (BMS). Accurate estimation of SOC ensures battery work within a reasonable range, which can prevent over-charge or over-discharge damage to extend battery life. The third-order RC equivalent circuit model is established to describe the characteristics of battery, in which the parameters can be identified by the discharge experiment. For the multiple state variables, strong coupling, stochastic noise, and wild values in the battery system, the principle of superposition is used to decompose the measurement equation so that the separately estimating for state variables to eliminate the coupling relationship between them. A novel SOC estimation method based on Extended Kalman Filtering (EKF) is proposed in this paper. The simulation and experimental results show the validity of the established third-order RC equivalent circuit model, SOC estimation has a high accuracy