Tailored Architectures to Stabilize Electrode-Electrolyte Interfaces in Cathode Materials for Li-Ion Batteries
Authors: Allen Eva Michelle , Kwon Bob Jin , Wolfman Mark , Cai Zhonghou , Antipova Olga , De Andrade Vincent , Cabana Jordi 
Published: May 2020

The market for electric vehicles has increased significantly in the recent decade. Thus lithium-ion batteries with high energy and power density, with the ability to withstand extreme cycling environments are necessary to compete with the flexibility of the fossil fuel reliant combustion engine. The main instabilities in the Li-ion battery can be traced to the electrode-electrolyte interface, here electroactive transition metals react with the electrolyte resulting in irreversible transitions forming insulating layers, or solid cathode electrolyte interfaces (CEI), that inhibit ion transport. Cathode material dissolution may also occur at the surface due to acidic attack from the HF byproduct from the decomposition of the LiPF6 electrolyte. During electrochemical cycling, uneven lithium diffusion causes the surface to become more reduced than the core which leads to structural changes which impedes ion transport. These structural changes cause nonuniform contraction and expansion of the lattice loosening connections between the transition metal oxide layers eventually leading to severe macroscopic cracking. This effect has been seen at lower cycling rates when particle size is increased. The cracks allow the electrolyte to permeate the particle allowing CEI formation within the particle greatly decreasing the cathode life.

A solution to these detrimental surface interactions is by replacing the electroactive transition metal content at the surface with inactive ions improving the interfacial stability. To prevent significant decreases in storage capacity the coating is a thin passivating barrier. Previously our group has introduced a strategy toward stabilizing the electrode-electrolyte interface using Ni0.25Mn0.25Co0.50O nanocrystals (NC) of active material. Each individual NC is passivated by an epitaxial grown conformal ultrathin Al2O3 shell with a final composition of LiCo0.5Ni0.25Mn0.25O2 with a concentration gradient of Al3+ ions toward the outer layers after high temperature lithiation. This material showed increased cycling stability under harsh conditions of increased cycle rate and temperature. While this research had promising results, industrial methods are currently better suited to produce well defined dense secondary particle structures with a spherical morphology, made up of agglomerated nanoparticles. While many coating methods being researched on these secondary structures require coating materials to be deposited on as-made, pre-lithiated cathode material, this method makes it difficult to completely coat all surfaces, while losing the ability to control surface chemistry and homogeneity. Here an optimized systematic study was performed to form an aluminum shell in the same process of growth of NixMnzCoy(OH)2 (NMC) precursors. This procedure affords precise control over shell thickness and concentration gradient architectures. The core-shell precursors are reacted with LiOH in a high-temperature solid-state reaction, allowing the aluminum shell to diffuse into the surface lattice which will further suppress phase distortion in the bulk. The higher concentration of Al3+ at the surface protects against side reactions with the electrolyte. These secondary structured LiNi0.25Mn0.25Co0.50O2 were electrochemically tested using both Li-metal half cells and graphite anode full cells and the results were compared to nanocrystalline primary particles of a similar composition.

While aluminum coatings have been studied on cathode material previously, it is still a challenge to determine finite spatial changes in distribution of elements within individual particles in 3D with current conventional methods available, such as SEM-EDS analysis. In this study, synchrotron-based imaging methods of X-ray fluorescence (XRF) nanoprobe cross-sectional mapping, XRF tomography, and transmission X-ray elemental tomography were used to evaluate aluminum coated gradient NMC material and other stabilized architectures of interest such as full concentration gradient NMC material. For tomographic measurements particles were loaded into 50-micron capillaries allowing for high throughput imaging. These techniques allowed for little sample manipulation which preserved sample morphology in pristine state. Increased resolution offered further insight into elemental distribution and extent of aluminum migration after high temperature lithiation. Advancements in these imaging techniques can better inform the future of stabilized cathode architecture design.

Figure 1