Fast Li-ion Storage and Dynamics in TiO2 Nanoparticle Clusters Probed by Smart Scanning Electrochemical Cell Microscopy

Anatase TiO₂ is a promising material for Li-ion (Li⁺ ) batteries with fast charging capability. However, Li⁺ (de)intercalation dynamics in TiO₂ remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO₂ nanoparticle clusters. Directly probing active nanoparticles revealed that TiO₂ with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li⁺ storage in TiO₂ particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

Electrochemically induced amorphous-to-rock-salt phase transformation in niobium oxide electrode for Li-ion batteries

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb₂O₅ electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li⁺. This electrode can reversibly cycle three lithiums per Nb₂O₅, corresponding to a capacity of 269 mAh g^-1 at 20 mA g^-1, and retains a capacity of 191 mAh g^-1 at a high rate of 1 A g^-1. It exhibits superb cycling stability with a capacity of 225 mAh g^-1 at 200 mA g^-1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials.

Memory Effects' Mechanism in the Intercalation Batteries: The Particles' Bipolarization

To develop energy-storage devices, understanding their charge-discharge behaviors and their underlying mechanisms is mandatory. Memory effect (ME) is among the most important behaviors that should be understood, influencing the batteries' applications. In this paper, the intercalation batteries' ME and their features are justified and explained by employing the particles' bipolarization mechanism. Diffuse regions, located in both sides of the reactant/product phases, turn the particles into dipoles (bipolarized particles) during/after the processes. This bipolarization and subsequent neutralization can explain many charge-discharge behaviors, including the ME. Here, the mechanism explains and justifies all the known features and some aspects of the phenomena which have not been considered so far. According to the proposed mechanism, the aged-neutralized particles react later and in a higher voltage than the fresh-neutralized particles, causing a bump in the curve called the ME. It is the same mechanism that causes the increase in the charge voltage by increasing the open-circuit voltage rest time. Our experiments sufficiently verified the mechanism. In the paper, impacts of the average particle size, relaxation/rest time, discharge cutoff voltage of the memory-writing cycle (MWC), Li-mobility kinetics, current rate, state of charge, depth of discharge of the MWC, boundaries of the charge-discharge curve, and so forth are considered, and their influences on the ME are explained. This mechanism sheds light on the relevant characteristics of the batteries and helps design, tune, control, and engineer the behaviors.

Preparation of LiFePO4 Powders by Ultrasonic Spray Drying Method and Their Memory Effect

The memory effect of lithium-ion batteries (LIBs) was first discovered in LiFePO₄, but its origin and dependence are still not clear, which is essential for regulating the memory effect. In this paper, a home-made spray drying device was used to successfully synthesize LiFePO₄ with an average particle size of about 1 μm, and we studied the influence of spray drying temperature on the memory effect of LiFePO₄ in LIBs. The results showed that the increasing of spray drying temperature made the memory effect of LiFePO₄ strengthen from 1.3 mV to 2.9 mV, while the capacity decreased by approximately 6%. The XRD refinement and FTIR spectra indicate that the enhancement of memory effect can be attributed to the increment of Li–Fe dislocations. This work reveals the dependence of memory effect of LiFePO₄ on spray drying temperature, which will guide us to optimize the preparation process of electrode materials and improve the management system of LIBs.

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

Nanocrystalline anatase TiO₂ is a robust model anode for Li insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li insertion and electron accumulation, respectively, and these optical signals are used to deconvolute bulk Li insertion from other electrochemical responses, such as double-layer capacitance, pseudocapacitance, and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO₂ systematically tunes the Li-insertion potentials with the particle size. However, the particle size does not affect the kinetics of Li insertion in ensemble electrodes. Rather, the Li-insertion rates depend on the applied overpotential, electrolyte concentration, and initial state of charge. We conclude that Li diffusivity and phase propagation are not rate limiting during Li insertion in TiO₂ nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO₂ particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute the nonequilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations, and spectroelectrochemistry.

Reversible Al-Site Switching and Consequent Memory Effect of Al-Doped Li4Ti5O12 in Li-Ion Batteries

Among many electrode materials, only a small amount of two-phase electrode materials were found to possess the memory effect, for instance, olivine LiFePO₄, anatase TiO₂, and Al-doped Li₄Ti₅O₁₂, in which the underlying mechanism is still not clear beyond the electrochemical kinetics. Here, we further studied the memory effect of Al-doped Li₄Ti₅O₁₂ to reveal the microstructure and the microprocess. By controlling the potentiostatic step after discharging, we found that the memory effect of Al-doped Li₄Ti₅O₁₂ was closely related to the discharged lattice parameters and the subsequent charge capacity. According to the ex situ magic-angle spinning (MAS) NMR results, we first revealed that the Al ions would move from 8a to 16c sites, when the electrode was discharged and potentiostatic at a low potential, and then move back through charging in the spinel structure of Al-doped Li₄Ti₅O₁₂, which would contribute to the capacity as the Li ions. Therefore, the reversible Al-ion switching between 8a and 16c sites should be the origin of memory effect in Al-doped Li₄Ti₅O₁₂, which would inspire us to explore the memory effect of other electrode materials in Li-ion batteries (LIBs), as well as optimize the performance of electrode materials by controlling the ionic switching.

Designing Hierarchical Assembly of Carbon-Coated TiO2 Nanocrystals and Unraveling the Role of TiO2/Carbon Interface in Lithium-Ion Storage in TiO2

Despite the many benefits of hierarchical nanostructures of oxide-based electrode materials for lithium-ion batteries, it remains a challenging task to fully exploit the advantages of such materials partly because of their intrinsically poor electrical conductivities. The resulting limited electron supply to primary particles inside secondary microparticles gives rise to significant variation in the lithium-ion (Li⁺) storage capability within the nanostructured particles. To address this, facile annealing, where in situ generated carbon-coated primary particles were assembled into porous microagglomerates, is demonstrated to prepare nanostructured titanium dioxide (TiO₂). A systematic study on the effect of the carbon coating reveals that it is exclusively governed by the characteristics of the TiO₂/carbon interface rather than by the nature of the carbon coating. Depending on their number, oxygen vacancies created by carbothermal reduction on the TiO₂ surface are detrimental to Li⁺ diffusion in the TiO₂ lattice, and structural distortion at the interface profoundly influences the Li⁺ (de)intercalation mechanism. This new insight serves as a stepping stone toward understanding an important yet often overlooked effect of the oxide/carbon interface on Li⁺ storage kinetics, thereby demanding more investigations to establish a new design principle for carbon-coated oxide electrode materials.

Memory-effect-induced electrochemical oscillation of an Al-doped Li4Ti5O12 composite in Li-ion batteries

Memory effects and electrochemical oscillation have recently been found in modified Li4Ti5O12, but the correlation between these two phenomena has not been reported yet. Here, we found that these two phenomena could simultaneously occur in an Al-doped Li4Ti5O12 composite, and the electrochemical oscillation can be controlled by regulating the memory effect.

Size-Dependent Memory Effect of the LiFePO4 Electrode in Li-Ion Batteries

In Li-ion batteries, the phase transition usually determines the electrochemical kinetics of some two-phase electrode materials, and it can be adopted to excellently interpret the memory effect of Li-ion batteries, therefore the size dependence of phase transition was expected to affect the memory effect significantly. In this work, we investigated the memory effect and phase transition of olivine LiFePO₄ in Li-ion batteries. Through electrochemical measurements, we found that the memory effect of LiFePO₄ was dependent on the particle size, especially after a long-time relaxation. By using the in situ X-ray diffraction, we found that the phase transition of nano-LiFePO₄ was ahead of the charging and discharging processes, while it took place concurrently or later for micro-LiFePO₄, which might be attributed to the high-specific two-phase boundary of nano-LiFePO₄. Furthermore, the phase-transition diagram was adopted to interpret the size-dependent memory effect schematically. Notably, it is the first time to report the phase transition ahead of (dis)charging for nano-LiFePO₄, which is significant to understand the phase transition of two-phase electrode materials, as well as the relevant phenomena, such as the memory effect.

Relaxation-Induced Memory Effect of LiFePO4 Electrodes in Li-Ion Batteries

In Li-ion batteries, memory effect has been found in several commercial two-phase materials as a voltage bump and a step in the (dis)charging plateau, which delays the two-phase transition and influences the estimation of the state of charge. Although memory effect has been first discovered in olivine LiFePO₄, the origination and dependence are still not clear and are critical for regulating the memory effect of LiFePO₄. Herein, LiFePO₄ has been synthesized by a home-built spray drying instrument, of which the memory effect has been investigated in Li-ion batteries. For as-synthesized LiFePO₄, the memory effect is significantly dependent on the relaxation time after phase transition. Besides, the voltage bump of memory effect is actually a delayed voltage overshooting that is overlaid at the edge of stepped (dis)charging plateau. Furthermore, we studied the kinetics of LiFePO₄ electrode with electrochemical impedance spectroscopy (EIS), which shows that the memory effect is related to the electrochemical kinetics. Thereby, the underlying mechanism has been revealed in memory effect, which would guide us to optimize two-phase electrode materials and improve Li-ion battery management systems.