Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g. In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a β-LiIrO phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (M) and anionic (O) redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, β-LiIrO presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir at potentials as low as 3.4 V versus Li/Li, as equivalently observed in the layered α-LiIrO polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

Progress over the last two decades in positive electrode materials for Li-ion batteries has generated a variety of novel materials. The Li-rich rocksalt oxides Li 2 MO 3 (M = 3d/4d/5d transition metal) are especially promising, displaying capacities exceeding 300 mAh/g thanks to the participation of the oxygen non-bonding O(2p) orbitals in the redox process.Understanding the oxygen redox limitations and the role of O/M ratio is therefore crucial for the rational design of materials with improved electrochemical performances. Herein, we push oxygen redox to its limits with the discovery of a new Li 3 IrO 4 compound (O/M = 4) which can reversibly uptake and release 3.5 e-per transition metal; the highest capacity ever reported for any positive insertion electrode, via the cumulative activation of cationic and anionic redox processes. By quantitatively monitoring the oxidation process, we demonstrate the material instability against O 2 release upon removal of all Li. Accordingly; we find the fully-delithiated phase to undergo irreversible amorphization producing a new a-IrO 3 phase with a local structure made of threefold connected IrO 6 octahedra. Our results show that the O/M parameter delineates the boundary between the material's maximum capacity and its stability, hence providing valuable insights for further high capacity materials developments.

Recent findings revealed that surface oxygen can participate in the oxygen evolution reaction (OER) for the most active catalysts, which eventually triggers a new mechanism for which the deprotonation of surface intermediates limits the OER activity. We propose in this work a "dual strategy" in which tuning the electronic properties of the oxide, such as LaSrCoO, can be dissociated from the use of surface functionalization with phosphate ion groups (P) that enhances the interfacial proton transfer. Results show that the P functionalized LaSrCoO gives rise to a significant enhancement of the OER activity when compared to LaSrCoO and LaCoO. We further demonstrate that the P surface functionalization selectivity enhances the activity when the OER kinetics is limited by the proton transfer. Finally, this work suggests that tuning the catalytic activity by such a "dual approach" may be a new and largely unexplored avenue for the design of novel high-performance catalysts.

Li-rich NMC are considered nowadays as one of the most promising candidates for high energy density cathodes. One significant challenge is nested in adjusting their synthesis conditions to reach optimum electrochemical performance, but no consensus has been reached yet on the ideal synthesis protocol. Herein we revisited the elaboration of Li-rich NMC electrodes by focusing on the science involved through each synthesis steps using carbonate Ni 0.1625 Mn 0.675 Co 0.1625 CO 3 precursor co-precipitation combined with solid state synthesis. We demonstrate the effect of precursor's concentration on the kinetics of the precipitation reaction and provide clues to obtain spherically agglomerated NMC carbonates of different sizes. Moreover, we highlight the strong impact of the Li 2 CO 3 /NMC carbonate ratio on the morphology and particles size of Li-rich NMC and subsequently on their electrochemical performance. Ratio of 1.35 was found to reproducibly give the best performance with namely a 1 st discharge capacity of 269 mAh.g -1 and capacity retention of 89.6% after 100 cycles. We hope that our results, which reveal how particle size, morphology and phase composition affect the material's electrochemical performance, will help in reconciling literature data while providing valuable fundamental information for up scaling approaches.

Sodium ion battery technology is gradually advancing and can be viewed as a viable alternative to lithium ion batteries in niche applications. One of the promising positive electrode candidates is P2 type layered sodium transition metal oxide, which offers attractive sodium ion conductivity. However, the reversible capacity of P2 phases is limited by the inability to directly synthesize stoichiometric compounds with sodium to transition metal ratio equals to 1. To alleviate this issue, we report herein the in-situ synthesis of P2-NaxMO2 (x≤ 0.7, M= transition metal ions) -Na2CO3 composites. We find that sodium carbonate acts as a sacrificial salt, providing Na + ion to increase the reversible capacity of the P2 phase in sodium ion full cells, and also as a useful additive that stabilizes the formation of P2 over competing P3 phases. We offer a new phase diagram for tuning the synthesis of the P2 phase under various experimental conditions and demonstrate, by in-situ XRD analysis, the role of Na2CO3 as a sodium reservoir in full sodium ion cells. These results provide insights into the practical use of P2 layered materials and can be extended to a variety of other layered phases.Introduction:

Searching for novel high-capacity electrode materials combining cationic and anionic redox processes is an ever-growing activity within the field of Li-ion batteries. In this respect, we report on the exploration of the Li 3 Ru y Nb 1−y O 4 (0 ≤ y ≤ 1) system with an O/M ratio of 4 to maximize the number of oxygen lone pairs, responsible for the anionic redox. We show that this system presents a very rich crystal chemistry with the existence of four structural types, which derive from the rocksalt structure but differ in their cationic arrangement, creating either zigzag, helical, jagged chains or clusters. From an electrochemical standpoint, these compounds are active on reduction via a classical cationic insertion process. The oxidation process is more complex, because of the instability of the delithiated phase. Our results promote the use of the rich Li 3 MO 4 family as a viable platform for a better understanding of the relationships between structure and anionic redox activity.

Stabilizing new host structures through potassium extraction from K-based polyanionic materials has been proven to be an interesting approach to develop new Li/Na insertion materials. Pursuing the same trend, we here report the feasibility of preparing langbeinite "Fe(SO)" via electrochemical and chemical oxidation of KFe(SO). Additionally, we succeeded in stabilizing a new KCu(SO) phase via a solid-state synthesis approach. This novel compound crystallizes in a complex orthorhombic structure that differs from that of langbeinite as deduced from synchrotron X-ray and neutron powder diffraction. Electrochemically, the performance of this new phase is limited, which we explain in terms of sluggish diffusion kinetics. We further show that KCu(SO) decomposes into KCuO(SO) on heating, and we report for the first time the synthesis of fedotovite KCuO(SO). Finally, the fundamental attractiveness of these S = / systems for physicists is examined by neutron magnetic diffraction, which reveals the absence of a long-range ordering of Cu magnetic moments down to 1.5 K.

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