Energy Storage Materials Lab

Cobalt-free layered lithium-rich nickel manganese oxides, Li[LixNiyMn1-x-y]O-2 (LLNMO), are promising positive electrode materials for lithium rechargeable batteries because of their high energy density and low materials cost. However, substantial voltage decay is inevitable upon electrochemical cycling, which makes this class of materials less practical. It has been proposed that undesirable voltage decay is linked to irreversible structural rearrangement involving irreversible oxygen loss and cation migration. Herein, the authors demonstrate that the voltage decay of the electrode is correlated to Mn4+/Mn3+ redox activation and subsequent cation disordering, which can be remarkably suppressed via simple compositional tuning to induce the formation of Ni3+ in the pristine material. By implementing our new strategy, the Mn4+/Mn3+ reduction is subdued by an alternative redox reaction involving the use of pristine Ni3+ as a redox buffer, which has been designed to be widened from Ni3+/Ni4+ to Ni2+/Ni4+, without compensation for the capacity in principle. Negligible change in the voltage profile of modified LLNMO is observed upon extended cycling, and manganese migration into the lithium layer is significantly suppressed. Based on these findings, we propose a general strategy to suppress the voltage decay of Mn-containing lithium-rich oxides to achieve long-lasting high energy density from this class of materials.

Organic rechargeable batteries, which use organics as electrodes, are excellent candidates for next-generation energy storage systems because they offer design flexibility due to the rich chemistry of organics while being eco-friendly and potentially cost efficient. However, their widespread usage is limited by intrinsic problems such as poor electronic conductivity, easy dissolution into liquid electrolytes, and low volumetric energy density. New types of organic electrode materials with various redox centers or molecular structures have been developed over the past few decades. Moreover, research aimed at enhancing electrochemical properties via chemical tuning has been at the forefront of organic rechargeable batteries research in recent years, leading to significant progress in their performance. Here, an overview of the current developments of organic rechargeable batteries is presented, with a brief history of research in this field. Various strategies for improving organic electrode materials are discussed with respect to tuning intrinsic properties of organics using molecular modification and optimizing their properties at the electrode level. A comprehensive understanding of the progress in organic electrode materials is provided along with the fundamental science governing their performance in rechargeable batteries thus a guide is presented to the optimal design strategies to improve the electrochemical performance for next-generation battery systems.

Redox-active organic materials (ROMs) have recently attracted significant attention for redox flow batteries (RFBs) to achieve green and cost-efficient energy storage. In particular, multi-redox ROMs have shown great promise, and further tailoring of these ROMs would yield RFB technologies with the highest possible energy density. Here, we present a phenazine-based catholyte material, 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ), that undergoes two single-electron redox reactions at high redox potentials (-0.29 and 0.50 V versus Fc/Fc(+)) with enhanced solubility (0.5 M in acetonitrile), remarkable chemical stability, and fast kinetics. Moreover, an all-organic flow battery exhibits cell voltages of 1.2 and 2.0 V when coupled with 9-fluorenone (FL) as an anolyte. It shows capacity retention of 99.94% per cycle over 200 cycles and 99.3% per cycle with 0.1 M and 0.4 M BMEPZ catholyte, respectively. Notably, the BMEPZ/FL couple results in the highest energy density (similar to 17 Wh L-1) among the non-aqueous all- organic RFBs reported to date.

The use of high-energy-density lithium-rich layered-oxide electrodes in batteries is hindered by voltage decay on cycling. Improving the reversible cation migration by altering oxygen stacking is shown to suppress voltage decay and redox asymmetry in lithium-rich nickel manganese oxides.Despite the high energy density of lithium-rich layered-oxide electrodes, their real-world implementation in batteries is hindered by the substantial voltage decay on cycling. This voltage decay is widely accepted to mainly originate from progressive structural rearrangements involving irreversible transition-metal migration. As prevention of this spontaneous cation migration has proven difficult, a paradigm shift toward management of its reversibility is needed. Herein, we demonstrate that the reversibility of the cation migration of lithium-rich nickel manganese oxides can be remarkably improved by altering the oxygen stacking sequences in the layered structure and thereby dramatically reducing the voltage decay. The preeminent intra-cycle reversibility of the cation migration is experimentally visualized, and first-principles calculations reveal that an O2-type structure restricts the movements of transition metals within the Li layer, which effectively streamlines the returning migration path of the transition metals. Furthermore, we propose that the enhanced reversibility mitigates the asymmetry of the anionic redox in conventional lithium-rich electrodes, promoting the high-potential anionic reduction, thereby reducing the subsequent voltage hysteresis. Our findings demonstrate that regulating the reversibility of the cation migration is a practical strategy to reduce voltage decay and hysteresis in lithium-rich layered materials.

Lithium-rich layered oxides (LLOs) are considered promising cathode materials for lithium-ion batteries because of their high reversible capacity, which is attributed to the exploitation of the novel anionic redox in addition to the conventional cationic redox process. Transition metal (TM) migration, which is known to be the main cause of the voltage decay in LLOs, is now understood to also be the critical factor triggering anionic redox, although this origin is still under debate. A better understanding of the specific TM migration behavior and its effect during charge/discharge would thus enable further development of this class of materials. Herein, we demonstrate that the unique TM migration during charge/discharge significantly alters the lithium diffusion mechanism/kinetics of LLO cathodes. We present clear evidence of the much more sluggish lithium diffusion occurring during discharge (lithiation) than during charge (de-lithiation), which contrasts with the traditional lithium diffusion model based on simple topotactic lithium intercalation/deintercalation in the layered framework. The reversible but asymmetric TM migration in the structure, which originates from the non-equivalent local environments around the TM during the charge and discharge processes, is shown to affect the lithium mobility. This correlation between TM migration and lithium mobility led us to propose a new lithium diffusion model for layered structures and suggests the importance of considering TM migration in designing new LLO cathode materials.

Organic electrode materials hold great potential due to their cost-efficiency, eco-friendliness, and possibly high theoretical capacity. Nevertheless, most organic cathode materials exhibit a trade-off relationship between the specific capacity and the voltage, failing to deliver high energy density. Herein, it is shown that the trade-off can be mitigated by utilizing the multi-redox capability of p-type electrodes, which can significantly increase the specific capacity within a high-voltage region. The molecular structure of 5,10-dihydro-5,10-dimethylphenazine is modified to yield a series of phenoxazine and phenothiazine derivatives with elevated redox potentials by substitutions. Subsequently, the feasibility of the multi-redox capability is scrutinized for these high-voltage p-type organic cathodes, achieving one of the highest energy densities. It is revealed that the seemingly impractical second redox reaction is indeed dependent on the choice of the electrolyte and can be reversibly realized by tailoring the donor number and the salt concentration of the electrolyte, which places the voltage of the multi-redox reaction within the electrochemical stability window. The results demonstrate that high-energy-density organic cathodes can be practically achieved by rational design of multi-redox p-type organic electrode materials and the compatibility consideration of the electrolyte, opening up a new avenue toward advanced organic rechargeable batteries.

Flame-spray-pyrolysis (FSP) is a robust and scalable process to synthesize particles at the commodity-scale. FSP has been used to produce the precursor powders which were converted to the layered structure (R (3) over barm phase) by a postannealing step in making nickel-rich cathode materials (NCMs). Theoretically, the high flame temperature (normally >1500 K) in FSP can provide adequate energy for the phase conversion from rock-salt to layered structures and potentially enables one-step synthesis. However, the high flame temperature is a critical issue to cause lithium loss and structural degradation, preventing the formation of the layered phase. In this work, guided by the gaseous nucleation theory, we implemented several FSP processes with different solution recipes. The layered phase concentration in the as-burned products can be increased with the solution enthalpies. By adding a rapid quench step to suppress the lithium loss and phase degradation, the layered phase can be further increased. This work contributes new ideas to innovating process regarding the process efficiency and throughput of manufacturing cathode materials at a large scale.

The exceptionally high gravimetric capacity of lithium-excess layered cathodes (LLCs) has generated interest in their use in lithium-ion batteries (LIBs) for high-capacity applications. Their unique electrochemical and structural properties are responsible for this high capacity, which exceeds the theoretical redox capability of transition metal oxides and have been intensively investigated. However, various fundamental and practical challenges must be overcome before LLCs can be successfully commercialized. The structure of pristine LLCs, which varies with the composition and type of transition metal species used, remains unclear. In addition, the structure continuously changes during electrochemical cycling, which further complicates its understanding. In this review, we discuss the current understanding of LLCs, including their pristine structures, redox chemistries, and structural evolution during cycling, and suggest future research directions to address the critical issues. (C) 2015 The Electrochemical Society. All rights reserved.