A new photon-in/photon-out endstation at beamline 02B02 of the Shanghai Synchrotron Radiation Facility for studying the electronic structure of energy materials has been constructed and fully opened to users. The endstation has the capability to perform soft x-ray absorption spectroscopy in total electron yield and total fluorescence yield modes simultaneously. The photon energy ranges from 40 eV to 2000 eV covering the K-edge of most low Z-elements and the L-edge of 3d transition-metals. The new self-designed channeltron detector allows us to achieve good fluorescence signals at the low photon flux. In addition, we synchronously collect the signals of a standard reference sample and a gold mesh on the upstream to calibrate the photon energy and monitor the beam fluctuation, respectively. In order to cross the pressure gap, in situ gas and liquid cells for soft x-ray absorption spectroscopy are developed to study the samples under realistic working conditions.

The hierarchical structure of the composite cathodes brings in significant chemical complexity related to the interfaces, such as cathode electrolyte interphase. These interfaces account for only a small fraction of the volume and mass, they could, however, have profound impacts on the cell-level electrochemistry. As the investigation of these interfaces becomes a crucial topic in the battery research, there is a need to properly study the surface chemistry, particularly to eliminate the biased, incomplete characterization provided by techniques that assume the homogeneous surface chemistry. Herein, we utilize nano-resolution spatially-resolved x-ray spectroscopic tools to probe the heterogeneity of the surface chemistry on LiNi_0.8Mn_0.1Co_0.1O₂ layered cathode secondary particles. Informed by the nano-resolution mapping of the Ni valance state, which serves as a measurement of the local surface chemistry, we construct a conceptual model to elucidate the electrochemical consequence of the inhomogeneous local impedance over the particle surface. Going beyond the implication in battery science, our work highlights a balance between the high-resolution probing the local chemistry and the statistical representativeness, which is particularly vital in the study of the highly complex material systems.

In all-solid-state lithium batteries, the impedance at the cathode/electrolyte interface shows close relationship with the cycle performance. Cathode coatings are helpful to reduce the impedance and increase the stability at the interface effectively. LiTi₂(PO₄)₃ (LTP), a fast ion conductor with high ionic conductivity approaching 10^-3 S·cm^-1, is adopted as the coating materials in this study. The crystal and electronic structures, as well as the Li⁺ ion migration properties are evaluated for LTP and its doped derivatives based on density functional theory (DFT) and bond valence (BV) method. Substituting part of Ti sites with element Mn, Fe, or Mg in LTP can improve the electronic conductivity of LTP while does not decrease its high ionic conductivity. In this way, the coating materials with both high ionic conductivities and electronic conductivities can be prepared for all-solid-state lithium batteries to improve the ion and electron transport properties at the interface.

It is important for the electrolytes to maintain and enhance the lithium ion battery electrochemical performance, and solvation of Li⁺ is a key parameter for the property of the electrolytes. The comparative study on Li⁺ solvation structures, energy, enthalpy, Gibbs free energy, infrared and Raman spectra in common organic electrolyte solvents is completed by density functional theory (DFT) method. The solvation reaction energy results suggest that the Li⁺ solvation priority order is propylene carbonate (PC) > ethylene carbonate (EC) > ethyl methyl carbonate (EMC) > diethyl carbonate (DEC) > tetrahydrofuran (THF) > dimethyl carbonate (DMC) > 1,3-dioxolane (DOL) > dimethoxyethane (DME) to form 5sol-Li⁺. It is also indicated that the most innermost solvation shell compounds formations by stepwise spontaneous solvation reaction are four cyclic solvent molecules and three linear solvent molecules combining one Li⁺ forming 4sol-Li⁺ and 3sol-Li⁺, respectively, at room temperature. Besides, the vibration peaks for C=O and C-O bonds in carbonate ester solvents-Li⁺ compounds shift to lower frequency and higher frequency, respectively, when the Li⁺ concentration increases in the solvation compounds. All Li-O stretching vibration peaks shift to higher frequency until forming 2solvent-Li⁺ complexes, and C-H stretching also shifts to higher frequency except for nDME-Li⁺ solvation compounds. The Raman spectrum is more agile to characterize C-H vibrations and IR is agile to C=O, C-O, and Li-O vibrations for Li⁺ solvation compounds.

Silicon is an important high capacity anode material for the next generation Li-ion batteries. The electrochemical performances of the Si anode are influenced strongly by the properties of the solid electrolyte interphase (SEI). It is well known that the addition of flouroethylene carbonate (FEC) in the carbonate electrolyte is helpful to improve the cyclic performance of the Si anode. The possible origin is suggested to relate to the modification of the SEI. However, detailed information is still absent. In this work, the structural and mechanical properties of the SEI on Si thin film anode in the ethylene-carbonate-based (EC-based) and FEC-based electrolytes at different discharging and charging states have been investigated using a scanning atomic force microscopy force spectroscopy (AFMFS) method. Single-layered, double-layered, and multi-layered SEI structures with various Young's moduli have been visualized three dimensionally at nanoscale based on the hundreds of force curves in certain scanned area. The coverage of the SEI can be obtained quantitatively from the two-dimensional (2D) project plots. The related analysis indicates that more soft SEI layers are covered on the Si anode, and this could explain the benefits of the FEC additive.

In anode free batteries (AFBs), the current collector acts as anode simultaneously and has large volume expansion which is generally considered as a negative effect decreasing the structural stability of a battery. Moreover, despite many studies on the fast lithium diffusion in the current collector materials of AFB such as copper and aluminum, the involved Li diffusion mechanism in these materials remains poorly understood. Through first-principles calculation and stress-assisted diffusion equations, here we study the Li diffusion mechanism in several current collectors and related alloys and clarify the effect of volume expansion on Li diffusion respectively. It is suggested that due to the lower Li migration barriers in aluminum and tin, they should be more suitable to be used as AFB anodes, compared to copper, silver, and lead. The Li diffusion facilitation in copper with a certain number of vacancies is proposed to explain why the use of copper with a thickness ≤ 100 nm as the protective coating on the anode improves the lifetime of the batteries. We show that the volume expansion has a positive effect on Li diffusion via mechanical-electrochemical coupling. Namely, the volume expansion caused by Li diffusion will further induce stress which in turn affects the diffusion. These findings not only provide in-depth insight into the operating principle of AFBs, but also open a new route toward design of improved anode through utilizing the positive effect of mechanical-electrochemical coupling.