No abstract

Spinel LiMn2O4, whose electrochemical activity was first reported by Prof. John B. Goodenough's group at Oxford in 1983, is an important cathode material for lithium‐ion batteries that has attracted continuous academic and industrial interest. It is cheap and environmentally friendly, and has excellent rate performance with 3D Li+ diffusion channels. However, it suffers from severe degradation, especially under extreme voltages and during high‐temperature operation. Here, the current understanding and future trends of the spinel cathode and its derivatives with cubic lattice symmetry (LiNi0.5Mn1.5O4 that shows high‐voltage stability, and Li‐rich spinels that show reversible hybrid anion‐ and cation‐redox activities) are discussed. Special attention is given to the degradation mechanisms and further development of spinel cathodes, as well as concepts of utilizing the cubic spinel structure to stabilize high‐capacity layered cathodes and as robust framework for high‐rate electrodes. “Good spinel” surface phases like LiNi0.5Mn1.5O4 are distinguished from “bad spinel” surface phases like Mn3O4.

A new “full fluorosulfonyl” (FFS) electrolyte is developed for highly reversible 4 V class lithium-metal batteries (LMBs).

Li‐rich metal oxide (LXMO) cathodes have attracted intense interest for rechargeable batteries because of their high capacity above 250 mAh g−1. However, the side effects of hybrid anion and cation redox (HACR) reactions, such as oxygen release and phase collapse that result from global oxygen migration (GOM), have prohibited the commercialization of LXMO. GOM not only destabilizes the oxygen sublattice in cycling, aggravating the well‐known voltage fading, but also intensifies electrolyte decomposition and Mn dissolution, causing severe full‐cell performance degradation. Herein, an artificial surface prereconstruction (ASR) for Li1.2Mn0.6Ni0.2O2 particles with a molten‐molybdate leaching is conducted, which creates a crystal‐dense anion‐redox‐free LiMn1.5Ni0.5O4 shell that completely encloses the LXMO lattice (ASR‐LXMO). Differential electrochemical mass spectroscopy and soft X‐ray absorption spectroscopy analyses demonstrate that GOM is shut down in cycling, which not only stabilizes HACR in ASR‐LXMO, but also mitigates the electrolyte decomposition and Mn dissolution. ASR‐LXMO displays greatly stabilized cycling performance as it retains 237.4 mAh g−1 with an average discharge voltage of 3.30 V after 200 cycles. More crucially, while the pristine LXMO cycling cannot survive 90 cycles in a pouch full‐cell matched with a commercial graphite anode and lean (2 g A−1 h−1) electrolyte, ASR‐LXMO shows high capacity retention of 76% after 125 cycles in full‐cell cycling.

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The development of Li‐excess disordered‐rocksalt (DRX) cathodes for Li‐ion batteries and interpretation through the framework of percolation theory of Li diffusion have steered researchers to consider “Li‐excess” (x > 1.1 in LixTM2−xO2; TM = transition metal) as being critical to achieving high performance. It is shown that this is not necessary for Mn‐rich DRX‐cathodes demonstrated by Li1.05Mn0.90Nb0.05O2 and Li1.20Mn0.60Nb0.20O2, which both deliver high capacity (>250 mAh g−1) regardless of their Li‐excess level. By contextualizing this finding within the broader space of DRX chemistries and confirming with first‐principles calculations, it is revealed that the percolation effect is not crucial at the nanoparticle scale. Instead, Li‐excess is necessary to lower the charging voltage (through the formation of condensed oxygen species upon oxygen oxidation) of certain DRX cathodes, which otherwise would experience difficulties in charging due to their very high TM‐redox potential. The findings reveal the dual roles of Li‐excess – modifying the cathode voltage in addition to promoting Li diffusion through percolation – that must be simultaneously considered to determine the criticality of Li‐excess for high‐capacity DRX cathodes.

Nanocrystalline materials with superior properties are of great interest. Much is discussed about obtaining nanograins, but little is known about maintaining grain‐size uniformity that is critical for reliability. An especially intriguing question is whether it is possible to achieve a size distribution narrower than what Hillert theoretically predicted for normal grain growth, a possibility suggested—for growth with a higher growth exponent—by the generalized mean‐field theory of Lifshitz, Slyozov, Wagner (LSW), and Hillert but never realized in practice. Following a rationally designed two‐step sintering route, it has been made possible in bulk materials by taking advantage of the large growth exponent in the intermediate sintering stage to form a uniform microstructure despite residual porosity, and freezing the grain growth thereafter while continuing densification to reach full density. The bulk dense Al2O3 ceramic thus obtained has an average grain size of 34 nm and a size distribution much narrower than Hillert's prediction. Bulk Al2O3 with a grain‐size distribution narrower than the particle‐size distribution of starting powders is also demonstrated, as are highly uniform bulk engineering metals (refractory Mo and W‐Re alloy) and complex functional ceramics (BaTiO3‐based alloys with superior dielectric strength and energy capacity).

The emergence of nanomaterials in the past decades has greatly advanced modern energy storage devices. Nanomaterials can offer high capacity and fast kinetics yet are prone to rapid morphological evolution and degradation. As a result, they are often hybridized with a stable framework in order to gain stability and fully utilize its advantages. However, candidates for such framework materials are rather limited, with carbon, conductive polymers, and Ti-based oxides being the only choices; note these are all inactive or intercalation compounds. Conventionally, alloying-/conversiontype electrodes, which are thought to be electrochemically unstable by themselves, have never been considered as framework materials. This concept is challenged. Successful application of conversion-type MnO nanorod as a anode framework for high-capacity Mo 2 C/MoO x nanoparticles has been demonstrated in sodium-ion batteries. Surprisingly, it can stably deliver 110 mAh g −1 under extremely high rate of 8000 mA g −1 (≈70 C) over 40 000 cycles with no capacity decay. More generally, this is considered as a proof of concept and much more alloying-/conversion-type materials are expected to be explored for such applications.