Mechanism of Li intercalation/deintercalation into/from the surface of LiCoO2

A synergetic promotion of surface stability for high-voltage LiCoO2 by multi-element surface doping: a first-principles study

The utilization of high-voltage LiCoO2 is an effective approach to break through the bottleneck of practical energy density in lithium ion batteries. However, the structural and interfacial degradations at the deeply delithiated state as well as the associated safety concerns impede the application of high-voltage LiCoO2. Herein, we present a synergetic strategy for promoting the surface stability of LiCoO2 at high voltage by Ti-Mg-Al co-doping and systematically study the effects of the dopants on the surface stability, electronic structure and Li+ diffusion properties of the LiCoO2 (104) surface using first-principles calculations. It is found that Ti, Mg and Al dopants can be facilely introduced into the Co sites of the LiCoO2 (104) surface. Furthermore, the co-doping could significantly stabilize the surface oxygen of LiCoO2 at a high delithiation state. Particularly, by aggregating Ti-Mg-Al co-dopant distribution in the surface layer, surface oxygen loss is dramatically suppressed. In addition, analysis of the electronic structure indicates that Ti-Mg-Al co-doping can enhance the electronic conductivity of the LiCoO2 (104) surface and greatly inhibit the charge deficiency of the superficial lattice O atoms at a highly delithiated state. In spite of a negligible improvement in the surface Li+ diffusion kinetics, the Ti-Mg-Al surface-modified LiCoO2 is expected to exhibit improved electrochemical performance at high voltage due to its superior surface stability. Our results suggest that aggregating Ti, Mg and Al co-dopant distribution in the surface layer is a promising modulation strategy to synergistically promote the surface oxygen stability of LiCoO2 at high voltages.

First-principles study on small polaron and Li diffusion in layered LiCoO2

Li-ion conductivity is one of the essential properties that influences the performance of cathode materials for Li-ion batteries. Here, using density functional theory, we investigate the polaron stability and its effect on the Li-ion diffusion in layered LiCoO2 with various magnetic orderings. We show that the local magnetism promotes the localized Co4+ polaron with the Li-diffusion barrier of ∼0.34 eV. While the Li-ion diffuses, the polaron migrates in the opposite direction to the Li movement. In the non-magnetic structure, on the other hand, the polaron does not form, and the Li diffusion barrier is lowered to 0.21 eV. Although the presence of the polaron raises the diffusion barrier, the magnetically ordered structures are energetically more stable during the migration than the non-magnetic case. Thus, our work advocates the hole polaron migration scenario for Li-ion diffusion. We further demonstrate that the strong electron correlation of Co ions plays an essential role in stabilizing the Co4+ polaron.

In Situ Preparation of Three-Dimensional Porous Nickel Sulfide as a Battery-Type Supercapacitor

A one-step sulfurization method to fabricate Ni₃S₂ nanowires (Ni₃S₂ NWs) directly on a Ni foam (NF) was developed as a simple, low-cost synthesis method for use as a supercapacitor (SC), aimed at optimizing energy storage. Ni₃S₂ NWs have high specific capacity and are considered a promising electrode material for SCs; however, their poor electrical conductivity and low chemical stability limit their applications. In this study, highly hierarchical three-dimensional porous Ni₃S₂ NWs were grown directly on NF by a hydrothermal method. The feasibility of the use of Ni₃S₂/NF as a binder-free electrode for achieving high-performance SCs was examined. Ni₃S₂/NF exhibited a high specific capacity (255.3 mAh g^-1 at a current density of 3 A g^-1), good rate capability (2.9 times higher than that of the NiO/NF electrode), and competitive cycling performance (capacity retention of specific capacity of 72.17% after 5000 cycles at current density of 20 A g^-1). Owing to its simple synthesis process and excellent performance as an electrode material for SCs, the developed multipurpose Ni₃S₂ NWs electrode is expected to be a promising electrode for SC applications. Furthermore, the synthesis method of self-growing Ni₃S₂ NW electrodes on 3D NF via hydrothermal reactions could potentially be applied to the fabrication of SC electrodes using a variety of other transition metal compounds.

B-/Si- containing electrolyte additive efficiently establish a stable interface for high-voltage LiCoO2 cathode and its synergistic effect on LiCoO2/graphite pouch cells

An effective electrolyte additive, 3-(tert-Butyldimethylsilyoxy) phenylboronic acid (TBPB), is proposed to significantly improve the cycle stability of high voltage LiCoO₂ (LCO) cathode. Experimental and computational results show that TBPB has a relatively higher oxidation activity than base electrolyte, and preferentially constructs a stable cathode electrolyte interphase (CEI) containing B-/Si- components on LCO surface. Theoretical calculation, XPS and NMR data show that TBPB-derived CEI layer contains B-F species and has the function of eliminating HF. The as-formed CEI effectively inhibits the detrimental side reactions from electrolyte decomposition and LCO surface structure reconstruction. The capacity retention of LCO/Li half-cell increases from 38.92% (base electrolyte) to 83.70% after 150 cycles at 1 C between 3.0 V and 4.5 V by adding 1% TBPB. Moreover, TBPB is also reduced prior to base electrolyte, forming an ionic conducting solid electrolyte interphase (SEI) on graphite surface. Benefiting from the synergistic effect between CEI layer on LCO cathode and SEI layer on graphite anode to effectively decrease the electrolyte decomposition, the capacity retention of commercial LCO/graphite pouch cell with 1% TBPB increases from 10.44% to 76.13% after 400 cycles at 1 C between 3.0 V and 4.5 V. This work demonstrates that TBPB can act as an effective film-forming additive for high energy density LCO cathode at high voltage, and provides novel insights for its commercial application from the aspect of synergistically interfacial stability.

Novel LiAlO2 Material for Scalable and Facile Lithium Recovery Using Electrochemical Ion Pumping

In this study, α-LiAlO₂ was investigated for the first time as a Li-capturing positive electrode material to recover Li from aqueous Li resources. The material was synthesized using hydrothermal synthesis and air annealing, which is a low-cost and low-energy fabrication process. The physical characterization showed that the material formed an α-LiAlO₂ phase, and electrochemical activation revealed the presence of AlO₂* as a Li deficient form that can intercalate Li⁺. The AlO₂*/activated carbon electrode pair showed selective capture of Li⁺ ions when the concentrations were between 100 mM and 25 mM. In mono salt solution comprising 25 mM LiCl, the adsorption capacity was 8.25 mg g^-1, and the energy consumption was 27.98 Wh mol Li^-1. The system can also handle complex solutions such as first-pass seawater reverse osmosis brine, which has a slightly higher concentration of Li than seawater at 0.34 ppm.

Enhanced Cycling and Rate Capability by Epitaxially Matched Conductive Cubic TiO Coating on LiCoO2 Cathode Films

Layered lithium transition-metal oxides, such as LiCoO2 and its doped and lithium-rich analogues, have become the most attractive cathode material for current lithium-ion batteries due to their excellent power and energy densities. However, parasitic reactions at the cathode-electrolyte interface, such as metal-ion dissolution and electrolyte degradation, instigate major safety and performance issues. Although metal oxide coatings can enhance the chemical and structural stability, their insulating nature and lattice mismatch with the adjacent cathode material can act as a physical barrier for ion transport, which increases the charge-transfer resistance across the interface and impedes cell performance at high rates. Here, epitaxial engineering is applied to stabilize a cubic (100)-oriented TiO layer on top of single (104)-oriented LiCoO2 thin films to study the effect of a conductive coating on the electrochemical performance. Lattice matching between the (104) LiCoO2 surface facets and the (100) TiO plane enables the formation of the titanium mono-oxide phase, which dramatically enhances the cycling stability as well as the rate capability of LiCoO2. This cubic TiO coating enhances the preservation of the phase and structural stability across the (104) LiCoO2 surface. The results suggest a more stable Co3+ oxidation state, which not only limits the cobalt-ion dissolution into the electrolyte but also suppresses the catalytic degradation of the liquid electrolyte. Furthermore, the high c-rate performance combined with high Columbic efficiency indicates that interstitial sites in the cubic TiO lattice offer facile pathways for fast lithium-ion transport.

Structural Distortion-Induced Charge Gradient Distribution of Co Ions in Delithiated LiCoO2 Cathode

Layered LiCoO₂ has drawn tremendous attention as a modeling cathode for Li-ion batteries, while its structural instability, especially in the high delithiation region, remains unsolved. With the aim of revealing the structural fundamentals, LiCoO₂ electrodes are investigated at a long delithiation range using both in situ and ex situ techniques. In the highly delithiated LiCoO₂ electrode, the unique charge compensation process leads to a spatial charge gradient of Co²⁺/Co³⁺/Co⁴⁺ ions from surface to bulk, which can be further manipulated by structural distortion, Li extraction, and surface side reactions. The coordinated surface oxygen is shown to be electrochemically active and fully reversible in participating in the charge compensation during cycling. Moreover, the active lattice O can be significantly stabilized by introducing the undesired surface Li-Co antisites, which also play an effective role in accommodating the internal stress induced by volume changes. These findings effectively bridge the structural changes with the Li⁺/e^- migration kinetics to elucidate the degradation of LiCoO₂ cathode upon delithiation, demonstrating a rewarding avenue for improving the electrochemical performance of LiCoO₂ itself and developing high energy density cathodes for the battery community as well.

Dissolution of Complex Metal Oxides from First-Principles and Thermodynamics: Cation Removal from the (001) Surface of Li(Ni1/3Mn1/3Co1/3)O2

The rapid increase in use of Li-ion batteries in portable electronics has created a pressing need to understand the environmental impact and long-term fate of electonic waste (e-waste) products such as heavy and/or reactive metals. The type of e-waste that we focus on here are the complex metal oxide nanomaterials that compose Li-ion battery cathodes. While in operation the complex metal oxides are in a hermetically sealed container. However, at the end of life, improper disposal can cause structural transformations such as dissolution and metal leaching, resulting in a significant exposure risk to the surrounding environment. The transformations that occur between operational to environmental settings gives rise to a stark knowledge gap between macroscopic design and molecular-level behavior. In this study we use theory and modeling to describe and explain previously published experimental data for cation release from Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC) nanoparticles in an aqueous environment ( Chem. Mater. 2016 (28) 1092-1100). To better understand the transformations that may occur when this material is exposed to the environment, we compute the free energy of surface dissolution, Δ G, from the complex metal oxide NMC for a range of surface terminations and pH.

Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO(2) Nanoparticles During Electrochemical Cycling

Understanding the phase transition and Li-ion diffusion kinetics of Li-ion storage nanomaterials holds promising keys to further improve the cycle life and charge rate of the Li-ion battery. Traditional electrochemical studies were often based on a bulk electrode consisting of billions of electroactive nanoparticles, which washed out the intrinsic heterogeneity among individuals. Here, we employ optical microscopy, termed surface plasmon resonance microscopy (SPRM), to image electrochemical current of single LiCoO₂ nanoparticles down to 50 fA during electrochemical cycling, from which the phase transition and Li-ion diffusion kinetics can be quantitatively resolved in a single nanoparticle, in operando and high throughput manner. SPRM maps the refractive index (RI) of single LiCoO₂ nanoparticles, which significantly decreases with the gradual extraction of Li-ions, enabling the optical read-out of single nanoparticle electrochemistry. Further scanning electron microscopy characterization of the same batch of nanoparticles led to a bottom-up strategy for studying the structure-activity relationship. As RI is an intrinsic property of any material, the present approach is anticipated to be applicable for versatile kinds of anode and cathode materials, and to facilitate the rational design and optimization toward durable and fast-charging electrode materials.