Composite Cathode Design for High-Energy All-Solid-State Lithium Batteries with Long Cycle Life

All-solid-state batteries (ASSBs) consisting of a 4 V class layered oxide cathode active material (CAM), an inorganic solid-state electrolyte (SE), and a lithium metal anode are considered the future of energy storage technologies. To date, aside from the known dendrite issues at the anode, cathode instabilities due to oxidative degradation of the SE and reactivities between the SE and CAM as well as loss of mechanical integrity are considered to be the most significant barriers in ASSB development. In the present study, we address these challenges by developing composite cathode structures featuring two key design elements: (1) a halide SE with high oxidative stability to enable direct use of an uncoated 4 V class CAM and (2) a single-crystal (SC) CAM to eliminate intergranular cracking associated with volume changes and mechanical instability. We demonstrate exceptional performance achieved on such ASSB cells incorporating an uncoated SC-LiNi0.8Co0.1Mn0.1O2 (NMC811) CAM, a Li3YCl6 (LYC) SE, and a Li–In alloy anode, delivering a high discharge capacity of 170 mAh/g at C/5 and an impressive capacity retention of nearly 90% after 1000 cycles. Through comparative studies on polycrystalline and single-crystal NMC811 composite cathodes, we reveal the working mechanism that enables such stable cycling in the latter cell design. The study highlights the importance of proper cathode composite design and provides key insights in the future development of better-performing ASSB cells

The Formation Mechanism of Fluorescent Metal Complexes at the Li_<i>x</i>Ni_0.5Mn_1.5O_4−δ/Carbonate Ester Electrolyte Interface

Electrochemical oxidation of carbonate esters at the Li_<i>x</i>Ni_0.5Mn_1.5O_4−δ/electrolyte interface results in Ni/Mn dissolution and surface film formation, which negatively affect the electrochemical performance of Li-ion batteries. Ex situ X-ray absorption (XRF/XANES), Raman, and fluorescence spectroscopy, along with imaging of Li_<i>x</i>Ni_0.5Mn_1.5O_4−δ positive and graphite negative electrodes from tested Li-ion batteries, reveal the formation of a variety of Mn^II/III and Ni^II complexes with β-diketonate ligands. These metal complexes, which are generated upon anodic oxidation of ethyl and diethyl carbonates at Li_<i>x</i>Ni_0.5Mn_1.5O_4−δ, form a surface film that partially dissolves in the electrolyte. The dissolved Mn^III complexes are reduced to their Mn^II analogues, which are incorporated into the solid electrolyte interphase surface layer at the graphite negative electrode. This work elucidates possible reaction pathways and evaluates their implications for Li⁺ transport kinetics in Li-ion batteries

Lithium Diffusion in Graphitic Carbon

Graphitic carbon is currently considered the state-of-the-art material for the negative electrode in lithium ion cells, mainly due to its high reversibility and low operating potential. However, carbon anodes exhibit mediocre charge/discharge rate performance, which contributes to severe transport-induced surface structural damage upon prolonged cycling and limits the lifetime of the cell. Lithium bulk diffusion in graphitic carbon is not yet completely understood, partly due to the complexity of measuring bulk transport properties in finite-sized nonisotropic particles. To solve this problem for graphite, we use the Devanathan−Stachurski electrochemical methodology combined with ab initio computations to deconvolute and quantify the mechanism of lithium ion diffusion in highly oriented pyrolytic graphite (HOPG). The results reveal inherent high lithium ion diffusivity in the direction parallel to the graphene plane (∼10⁻⁷−10⁻⁶ cm² s⁻¹), as compared to sluggish lithium ion transport along grain boundaries (∼10⁻¹¹ cm² s⁻¹), indicating the possibility of rational design of carbonaceous materials and composite electrodes with very high rate capability

Mesoscale Phase Distribution in Single Particles of LiFePO₄ following Lithium Deintercalation

The chemical phase distribution in hydrothermally grown micrometric single crystals of LiFePO₄ following partial chemical delithiation was investigated. Full field and scanning X-ray microscopy were combined with X-ray absorption spectroscopy at the Fe and O K-edges, respectively, to produce maps with high chemical and spatial resolution. The resulting information was compared to morphological insight into the mechanics of the transformation by scanning transmission electron microscopy. This study revealed the interplay at the mesocale between microstructure and phase distribution during the redox process, as morphological defects were found to kinetically determine the progress of the reaction. Lithium deintercalation was also found to induce severe mechanical damage in the crystals, presumably due to the lattice mismatch between LiFePO₄ and FePO₄. Our results lead to the conclusion that rational design of intercalation-based electrode materials, such as LiFePO₄, with optimized utilization and life requires the tailoring of particles that minimize kinetic barriers and mechanical strain. Coupling TXM-XANES with TEM can provide unique insight into the behavior of electrode materials during operation, at scales spanning from nanoparticles to ensembles and complex architectures

Dependence on Crystal Size of the Nanoscale Chemical Phase Distribution and Fracture in Li_<i>x</i>FePO₄

The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptychographic microscopy combined with X-ray absorption spectroscopy and electron microscopy creates a powerful suite of tools that we use to assess the chemical and morphological changes in lithium iron phosphate (LiFePO₄) micro- and nanocrystals that occur upon delithiation. All sizes of partly delithiated crystals were found to contain two phases with a complex correlation between crystallographic orientation and phase distribution. However, the lattice mismatch between LiFePO₄ and FePO₄ led to severe fracturing on microcrystals, whereas no mechanical damage was observed in nanoplates, indicating that mechanics are a principal driver in the outstanding electrode performance of LiFePO₄ nanoparticles. These results demonstrate the importance of engineering the active electrode material in next generation electrical energy storage systems, which will achieve theoretical limits of energy density and extended stability. This work establishes soft X-ray ptychographic chemical imaging as an essential tool to build comprehensive relationships between mechanics and chemistry that guide this engineering design