Sunny Hy

Direct In situ Observation of Li2O Evolution on Li-Rich High-Capacity Cathode Material, Li[NixLi(1–2x)/3Mn(2–x)/3]O2 (0 ≤ x ≤0.5)

High-capacity layered, lithium-rich oxide cathodes show great promise for use as positive electrode materials for rechargeable lithium ion batteries. Understanding the effects of oxygen activating reactions on the cathodes surface during electrochemical cycling can lead to improvements in stability and performance. We used in situ surfaced-enhanced Raman spectroscopy (SERS) to observe the oxygen-related surface reactions that occur during electrochemical cycling on lithium-rich cathodes. Here, we demonstrate the direct observation of Li2O formation during the extended plateau and discuss the consequences of its formation on the cathode and anode. The formation of Li2O on the cathode leads to the formation of species related to the generation of H2O together with LiOH and to changes within the electrolyte, which eventually result in diminished performance. Protection from, or mitigation of, such devastating surface reactions on both electrodes will be necessary to help realize the potential of high-capacity cathode materials (270 mAhg(-1) versus 140 mAhg(-1) for LiCoO2) for practical applications.

Performance and design considerations for lithium excess layered oxide positive electrode materials for lithium ion batteries

The Li-excess oxide compound is one of the most promising positive electrode materials for next generation batteries exhibiting high capacities of >300 mA h g−1 due to the unconventional participation of the oxygen anion redox in the charge compensation mechanism. However, its synthesis has been proven to be highly sensitive to varying conditions and parameters where nanoscale phase separation may occur that affects the overall battery performance and life. In addition, several thermodynamic and kinetic drawbacks including large first cycle irreversible capacity, poor rate capability, voltage fading, and surface structural transformation need to be addressed in order to reach commercialization. This review will focus on the recent progress and performance trends over the years and provide several guidelines and design considerations based on the library of work done on this particular class of materials.

Gas-solid interfacial modification of oxygen activity in layered oxide cathodes for lithium-ion batteries

Lattice oxygen can play an intriguing role in electrochemical processes, not only maintaining structural stability, but also influencing electron and ion transport properties in high-capacity oxide cathode materials for Li-ion batteries. Here, we report the design of a gas–solid interface reaction to achieve delicate control of oxygen activity through uniformly creating oxygen vacancies without affecting structural integrity of Li-rich layered oxides. Theoretical calculations and experimental characterizations demonstrate that oxygen vacancies provide a favourable ionic diffusion environment in the bulk and significantly suppress gas release from the surface. The target material is achievable in delivering a discharge capacity as high as 301 mAh g−1 with initial Coulombic efficiency of 93.2%. After 100 cycles, a reversible capacity of 300 mAh g−1 still remains without any obvious decay in voltage. This study sheds light on the comprehensive design and control of oxygen activity in transition-metal-oxide systems for next-generation Li-ion batteries.

Room-Temperature All-solid-state Rechargeable Sodium-ion Batteries with a Cl-doped Na3PS4 Superionic Conductor.

All-solid-state sodium-ion batteries are promising candidates for large-scale energy storage applications. The key enabler for an all-solid-state architecture is a sodium solid electrolyte that exhibits high Na+ conductivity at ambient temperatures, as well as excellent phase and electrochemical stability. In this work, we present a first-principles-guided discovery and synthesis of a novel Cl-doped tetragonal Na3PS4 (t-Na3−xPS4−xClx) solid electrolyte with a room-temperature Na+ conductivity exceeding 1 mS cm−1. We demonstrate that an all-solid-state TiS2/t-Na3−xPS4−xClx/Na cell utilizing this solid electrolyte can be cycled at room-temperature at a rate of C/10 with a capacity of about 80 mAh g−1 over 10 cycles. We provide evidence from density functional theory calculations that this excellent electrochemical performance is not only due to the high Na+ conductivity of the solid electrolyte, but also due to the effect that “salting” Na3PS4 has on the formation of an electronically insulating, ionically conducting solid electrolyte interphase.

Insights into the Performance Limits of the Li7P3S11 Superionic Conductor: A Combined First-Principles and Experimental Study

The Li7P3S11 glass-ceramic is a promising superionic conductor electrolyte (SCE) with an extremely high Li(+) conductivity that exceeds that of even traditional organic electrolytes. In this work, we present a combined computational and experimental investigation of the material performance limitations in terms of its phase and electrochemical stability, and Li(+) conductivity. We find that Li7P3S11 is metastable at 0 K but becomes stable at above 630 K (∼360 °C) when vibrational entropy contributions are accounted for, in agreement with differential scanning calorimetry measurements. Both scanning electron microscopy and the calculated Wulff shape show that Li7P3S11 tends to form relatively isotropic crystals. In terms of electrochemical stability, first-principles calculations predict that, unlike the LiCoO2 cathode, the olivine LiFePO4 and spinel LiMn2O4 cathodes are likely to form stable passivation interfaces with the Li7P3S11 SCE. This finding underscores the importance of considering multicomponent integration in developing an all-solid-state architecture. To probe the fundamental limit of its bulk Li(+) conductivity, a comparison of conventional cold-press sintered versus spark-plasma sintering (SPS) Li7P3S11 was done in conjunction with ab initio molecular dynamics (AIMD) simulations. Though the measured diffusion activation barriers are in excellent agreement, the AIMD-predicted room-temperature Li(+) conductivity of 57 mS cm(-1) is much higher than the experimental values. The optimized SPS sample exhibits a room-temperature Li(+) conductivity of 11.6 mS cm(-1), significantly higher than that of the cold-pressed sample (1.3 mS cm(-1)) due to the reduction of grain boundary resistance by densification. We conclude that grain boundary conductivity is limiting the overall Li(+) conductivity in Li7P3S11, and further optimization of overall conductivities should be possible. Finally, we show that Li(+) motions in this material are highly collective, and the flexing of the P2S7 ditetrahedra facilitates fast Li(+) diffusion.

Avalanching strain dynamics during the hydriding phase transformation in individual palladium nanoparticles

Phase transitions in reactive environments are crucially important in energy and information storage, catalysis and sensors. Nanostructuring active particles can yield faster charging/discharging kinetics, increased lifespan and record catalytic activities. However, establishing the causal link between structure and function is challenging for nanoparticles, as ensemble measurements convolve intrinsic single-particle properties with sample diversity. Here we study the hydriding phase transformation in individual palladium nanocubes in situ using coherent X-ray diffractive imaging. The phase transformation dynamics, which involve the nucleation and propagation of a hydrogen-rich region, are dependent on absolute time (aging) and involve intermittent dynamics (avalanching). A hydrogen-rich surface layer dominates the crystal strain in the hydrogen-poor phase, while strain inversion occurs at the cube corners in the hydrogen-rich phase. A three-dimensional phase-field model is used to interpret the experimental results. Our experimental and theoretical approach provides a general framework for designing and optimizing phase transformations for single nanocrystals in reactive environments.

Stabilizing Nanosized Si Anodes with the Synergetic Usage of Atomic Layer Deposition and Electrolyte Additives for Li-Ion Batteries

A substantial increase in charging capacity over long cycle periods was made possible by the formation of a flexible weblike network via the combination of Al2O3 atomic layer deposition (ALD) and the electrolyte additive vinylene carbonate (VC). Transmission electron microscopy shows that a weblike network forms after cycling when ALD and VC were used in combination that dramatically increases the cycle stability for the Si composite anode. The ALD-VC combination also showed reduced reactions with the lithium salt, forming a more stable solid electrolyte interface (SEI) absent of fluorinated silicon species, as evidenced by X-ray photoelectron spectroscopy. Although the bare Si composite anode showed only an improvement from a 56% to a 45% loss after 50 cycles, when VC was introduced, the ALD-coated Si anode showed an improvement from a 73% to a 11% capacity loss. Furthermore, the anode with the ALD coating and VC had a capacity of 630 mAh g(-1) after 200 cycles running at 200 mA g(-1), and the bare anode without VC showed a capacity of 400 mAh g(-1) after only 50 cycles. This approach can be extended to other Si systems, and the formation of this SEI is dependent on the thickness of the ALD that affects both capacity and stability.

Capacity retention of lithium sulfur batteries enhanced with nano-sized TiO2-embedded polyethylene oxide

The shuttle effect of polysulfides in Li/S batteries causes the loss of active materials and capacity decay. This phenomenon has retarded the practical application of Li/S batteries. Herein, we demonstrate that the undesired shuttle mechanism owing to dissolved lithium polysulfides can be effectively suppressed by the incorporation of TiO2 nanoparticles (NPs) within a solid nanocomposite polymer electrolyte of polyethylene oxide (PEO). The approach shows enhanced capacity retention, whereby a cell with a hybrid solid electrolyte of TiO2 NPs embedded in PEO has delivered more than 1450 mA h g−1 (first cycle discharge capacity) with ∼87% capacity retention after 100 cycles, compared to only 38% capacity retention in the absence of TiO2. Density Functional Theory (DFT) computational results based on bonding interactions and Raman characterization of the different components reveal that the undesired diffusion shuttle mechanism causing Li anode passivation can be effectively reduced by the use of nano-sized TiO2 and polyethylene oxide.

Nucleation of dislocations and their dynamics in layered oxide cathode materials during battery charging

Lithium-rich layered oxides (LRLO) are among the leading candidates for the next-generation cathode material for energy storage, delivering 50% excess capacity over commercially used compounds. Despite excellent prospects, voltage fade has prevented effective use of the excess capacity, and a major challenge has been a lack of understanding of the mechanisms underpinning the voltage fade. Here, using operando three-dimensional Bragg coherent diffractive imaging, we directly observe the nucleation of a mobile dislocation network in LRLO nanoparticles. The dislocations form more readily in LRLO as compared with a classical layered oxide, suggesting a link between the defects and voltage fade. We show microscopically how the formation of partial dislocations contributes to the voltage fade. The insights allow us to design and demonstrate an effective method to recover the original high-voltage functionality. Our findings reveal that the voltage fade in LRLO is reversible and call for new paradigms for improved design of oxygen-redox active materials.Voltage fade is a major obstacle for the efficient use of lithium-rich layered oxide materials in batteries. Here, the authors reveal the link between voltage fade and nucleation of a mobile dislocation network in the oxide nanoparticles, offering design ideas to restore the voltage.

Identifying the chemical and structural irreversibility in LiNi0.8Co0.15Al0.05O2 – a model compound for classical layered intercalation

In this work, we extracted 95% of the electrochemically available Li from LiNi0.8Co0.15Al0.05O2 (NCA) by galvanostatically charging the NCA/MCMB full cell to 4.7 V. Joint powder X-ray and neutron diffraction (XRD & ND) studies were undertaken for NCA at highly charged states at the first cycle, and discharged states at different cycles. The results indicate that the bulk structure of NCA maintains the O3 structure up to the extraction of 0.90 Li per formula unit. In addition, we found that the transition metal layer becomes more disordered along the c-axis than along the a- and b-axes upon charging. This anisotropic disorder starts to develop no later than 4.3 V on charge and continues to grow until the end of charge. As Li is re-inserted during discharge, the structure that resembles the pristine NCA is recovered. The irreversible loss of Li and the migration of Ni to the Li layer have been quantified by the joint XRD and ND refinement and the results were further verified by solid state 7Li NMR and magnetic measurements. Our work clearly demonstrates that the NCA bulk retains a robust, single phase O3 structure throughout the wide delithiation range (up to 0.9 Li per formula unit of NCA) and is suitable for higher energy density usage with proper modifications.