Xiaohe Song

Kinetics Tuning of Li-Ion Diffusion in Layered Li(NixMnyCoz)O2

Using ab initio calculations combined with experiments, we clarified how the kinetics of Li-ion diffusion can be tuned in LiNixMnyCozO2 (NMC, x + y + z = 1) materials. It is found that Li-ions tend to choose oxygen dumbbell hopping (ODH) at the early stage of charging (delithiation), and tetrahedral site hopping (TSH) begins to dominate when more than 1/3 Li-ions are extracted. In both ODH and TSH, the Li-ions surrounded by nickel (especially with low valence state) are more likely to diffuse with low activation energy and form an advantageous path. The Li slab space, which also contributes to the effective diffusion barriers, is found to be closely associated with the delithiation process (Ni oxidation) and the contents of Ni, Co, and Mn.

Janus Solid–Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries

LiFePO4 has long been held as one of the most promising battery cathode for its high energy storage capacity. Meanwhile, although extensive studies have been conducted on the interfacial chemistries in Li-ion batteries,1-3 little is known on the atomic level about the solid-liquid interface of LiFePO4/electrolyte. Here, we report battery cathode consisted with nanosized LiFePO4 particles in aqueous electrolyte with an high charging and discharging rate of 600 C (3600/600 = 6 s charge time, 1 C = 170 mAh g(-1)) reaching 72 mAh g(-1) energy storage (42% of the theoretical capacity). By contrast, the accessible capacity sharply decreases to 20 mAh g(-1) at 200 C in organic electrolyte. After a comprehensive electrochemistry tests and ab initio calculations of the LiFePO4-H2O and LiFePO4-EC (ethylene carbonate) systems, we identified the transient formation of a Janus hydrated interface in the LiFePO4-H2O system, where the truncated symmetry of solid LiFePO4 surface is compensated by the chemisorbed H2O molecules, forming a half-solid (LiFePO4) and half-liquid (H2O) amphiphilic coordination environment that eases the Li desolvation process near the surface, which makes a fast Li-ion transport across the solid/liquid interfaces possible.

High-performance NaFePO4 formed by aqueous ion-exchange and its mechanism for advanced sodium ion batteries

Room-temperature sodium ion batteries (SIBs) have attracted tremendous attention recently as cheaper alternatives to lithium ion batteries (LIBs) for potential application in large-scale electrical energy storage stations. Among the various classes of iron phosphate cathodes used in SIBs, olivine NaFePO4 is one of the most attractive host materials for advanced sodium ion batteries owing to its electrochemical profile and high theoretical capacity. As an alternative to the organic-based electrochemical ion-exchange process which is disadvantaged by sluggish dynamics and co-intercalation of Li+, we investigated an aqueous-based, electrochemical-driven ion-exchange process to transform olivine LiFePO4 into highly pure olivine NaFePO4, which shows superior electrochemical performance. Using a combination of ab initio calculations and experiments, we demonstrate that the mechanism is attributed to the much faster Na+/Li+ ion-exchange kinetics of NaFePO4 at the aqueous electrolyte/cathode interface compared to the organic electrolytes. Operando Fe K-edge XANES and XRD were also carried out to study the staged evolution of phases during the sodiation/desodiation of NaFePO4 nanograins.

Storage and Effective Migration of Li-Ion for Defected β-LiFePO4 Phase Nanocrystals

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, α-LiFePO4 (αLFP). However, the new phase β-LiFePO4 (βLFP), which can be transformed from αLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO4] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in αLFP disappear, while the Fe ions in the β-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that βLFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of βLFP cathode achieved a capacity of 128 mAh g(-1) at a rate of 0.1 C (1C = 170 mA g(-1)) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as FeLiLiFe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density.

Metal and F dual-doping to synchronously improve electron transport rate and lifetime for TiO2 photoanode to enhance dye-sensitized solar cells performances

A general strategy to synchronously improve electron transport rate and lifetime for TiO2 photoanode by metal and F− dual doping is proposed and demonstrated for dye-sensitized solar cells (DSSCs) for the first time. Tin and fluorine dual-doped TiO2 nanoparticles are prepared and X-ray photoelectron spectroscopy (XPS) analysis indicates that the Sn atoms and the F atoms locate mainly in the TiO2 lattice and on the TiO2 particles surface, respectively. The DSSC based on Sn/F–TiO2 sample shows a high photoconversion efficiency of 8.89% under an AM 1.5 solar condition (100 mW cm−2), which is higher than those for the undoped TiO2 nanoparticles (7.12%) and the solely Sn (8.14%) or F doped (8.31%) samples. This improvement is attributed to the combined effects of a faster electron transport rate and a longer electron lifetime in the dual-doped TiO2 film. Following this strategy, we also prepare Ta/F, Nb/F, and Sb/F dual-doped TiO2 nanoparticles and find that the performance of DSSCs based on all the dual-doped samples is further improved compared with the single doping cases. Finally, through density functional theory (DFT) calculations, the mechanism behind the improvement by tin and fluorine dual-doping is discussed in detail.

Excess Li-ion storage on reconstructed surfaces of nanocrystals to boost battery performance

Because of their enhanced kinetic properties, nanocrystallites have received much attention as potential electrode materials for energy storage. However, because of the large specific surface areas of nanocrystallites, they usually suffer from decreased energy density, cycling stability, and effective electrode capacity. In this work, we report a size-dependent excess capacity beyond theoretical value (170 mA h g-1) by introducing extra lithium storage at the reconstructed surface in nanosized LiFePO4 (LFP) cathode materials (186 and 207 mA h g-1 in samples with mean particle sizes of 83 and 42 nm, respectively). Moreover, this LFP composite also shows excellent cycling stability and high rate performance. Our multimodal experimental characterizations and ab initio calculations reveal that the surface extra lithium storage is mainly attributed to the charge passivation of Fe by the surface C-O-Fe bonds, which can enhance binding energy for surface lithium by compensating surface Fe truncated symmetry to create two types of extra positions for Li-ion storage at the reconstructed surfaces. Such surface reconstruction nanotechnology for excess Li-ion storage makes full use of the large specific surface area of the nanocrystallites, which can maintain the fast Li-ion transport and greatly enhance the capacity. This discovery and nanotechnology can be used for the design of high-capacity and efficient lithium ion batteries.

The formation and mechanism of nano-monocrystalline γ-Fe2O3 with graphene-shell for high-performance lithium ion batteries

Using a sintering process with Prussian Blue (PB) and 20 wt% glucose at high temperature (950 °C for 6 hours in Ar/H2) with oxidation in the air at room temperature, we synthesized a nano-monocrystalline γ-phase iron oxide (γ-Fe2O3) compound coated with carbon comprising a number of graphene layers, which was named as core–shell nano-monocrystalline γ-Fe2O3@graphene. It can be noted that the formation of nano-monocrystal is different from forming core–shell nano-polycrystalline hollow γ-Fe2O3@graphene sintered at lower temperature (650 °C 6 hours in Ar) via a simple Kirkendall process with oxidation at room temperature as reported in our previous study. We further investigate how nano-monocrystalline γ-Fe2O3 is formed by controlling the synthesis process and testing with TEM and SEM. We confirmed that the nano-monocrystalline γ-Fe2O3 is grown from nano-monocrystalline Fe with interface catalysis of O2 and the related mechanism is discussed through comparing the structures of γ-Fe2O3 and the Fe crystals. The core–shell nano-monocrystalline γ-Fe2O3@graphene shows high performance as an anode material in Li-ions batteries (much better than nano-polycrystalline hollow γ-Fe2O3@graphene reported in previous study). For example, the cycling stability and rate performance are remarkable as an anode material for lithium ion batteries with a high reversible capacity of 848.08 and 782.54 mA h g−1 at 1C and 5C for 600 cycles, respectively, and a high rate performance (284.42 mA h g−1 at 20C). Another interesting performance is that during the first 80 cycles, the specific capacity increases, which may result from more interface area being generated by the γ-Fe2O3 nano-monocrystal crushing with protection of the graphene-shell during the initial charging/discharging cycles. This synthesis method and mechanism can be used as a guide to produce γ-Fe2O3 as an anode material for lithium ion batteries with high performance on a large scale.

Controllable synthesis of LiFePO4 in different polymorphs and study of the reaction mechanism

Lithium iron phosphate, a widely used cathode material in Lithium Ion Batteries (LIBs), crystallizes typically in an olivine-type phase, alpha-LiFePO4 (aLFP). However, the new phase beta-LiFePO4 (b ...

Excess lithium storage in LiFePO4-Carbon interface by ball-milling

As one of the most popular cathode materials for high power lithium ion batteries (LIBs) of the electrical-vehicle (EV), lithium iron phosphate (LiFePO4 (LFP)) is limited to its relatively lower theoretical specific capacity of 170mAh g−1. To break the limits and further improve the capacity of LFP is promising but challenging. In this study, the ball-milling method is applied to the mixture of LFP and carbon, and the effective capacity larger than the theoretical one by 30mAh g−1 is achieved. It is demonstrated that ball-milling leads to the LFP-Carbon interface to store the excess Li-ions.