An Ion‐Exchange Promoted Phase Transition in a Li‐Excess Layered Cathode Material for High‐Performance Lithium Ion Batteries

A new approach to intentionally induce phase transition of Li‐excess layered cathode materials for high‐performance lithium ion batteries is reported. In high contrast to the limited layered‐to‐spinel phase transformation that occurred during in situ electrochemical cycles, a Li‐excess layered Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ is completely converted to a Li₄Mn₅O₁₂‐type spinel product via ex situ ion‐exchanges and a post‐annealing process. Such a layered‐to‐spinel phase conversion is examined using in situ X‐ray diffraction and in situ high‐resolution transmission electron microscopy. It is found that generation of sufficient lithium ion vacancies within the Li‐excess layered oxide plays a critical role for realizing a complete phase transition. The newly formed spinel material exhibits initial discharge capacities of 313.6, 267.2, 204.0, and 126.3 mAh g^?1 when cycled at 0.1, 0.5, 1, and 5 C (1 C = 250 mA g^?1), respectively, and can retain a specific capacity of 197.5 mAh g^?1 at 1 C after 100 electrochemical cycles, demonstrating remarkably improved rate capability and cycling stability in comparison with the original Li‐excess layered cathode materials. This work sheds light on fundamental understanding of phase transitions within Li‐excess layered oxides. It also provides a novel route for tailoring electrochemical performance of Li‐excess layered cathode materials for high‐capacity lithium ion batteries.     

P2‐K0.75[Ni1/3Mn2/3]O2 Cathode Material for High Power and Long Life Potassium‐Ion Batteries

Rationally designed P2‐K_0.75[Ni_1/3Mn_2/3]O₂ is introduced as a novel cathode material for potassium‐ion batteries (KIBs). P2‐K_0.75[Ni_1/3Mn_2/3]O₂ cathode material designed through electrochemical ion‐exchange from P2‐Na_2/3[Ni_1/3Mn_2/3]O₂ exhibits satisfactory electrode performances; 110 mAh g^?1 (20 mA g^?1) retaining 86% of capacity for 300 cycles and unexpectedly high reversible capacity of about 91 mAh g^?1 (1400 mA g^?1) with excellent capacity retention of 83% over 500 cycles. According to theoretical and experimental investigations, the overall potassium storage mechanism of P2‐K_0.75[Ni_1/3Mn_2/3]O₂ is revealed to be a single‐phase reaction with small lattice change upon charge and discharge, presenting the Ni^4+/2+ redox couple reaction. Such high power capability is possible through the facile K⁺ migration in the K_0.75[Ni_1/3Mn_2/3]O₂ structure with a low activation barrier energy of ≈210 meV. These findings indicate that P2‐K_0.75[Ni_1/3Mn_2/3]O₂ is a promising candidate cathode material for high‐rate and long‐life KIBs.     

Lithium/Oxygen Incorporation and Microstructural Evolution during Synthesis of Li‐Rich Layered Li[Li0.2Ni0.2Mn0.6]O2 Oxides

As promising cathode materials, the lithium‐excess 3d‐transition‐metal layered oxides can deliver much higher capacities (>250 mAh g^?1 at 0.1 C) than the current commercial layered oxide materials (≈180 mAh g^?1 at 0.1 C) used in lithium ion batteries. Unfortunately, the original formation mechanism of these layered oxides during synthesis is not completely elucidated, that is, how is lithium and oxygen inserted into the matrix structure of the precursor during lithiation reaction? Here, a promising and practical method, a coprecipitation route followed by a microwave heating process, for controllable synthesis of cobalt‐free lithium‐excess layered compounds is reported. A series of the consistent results unambiguously confirms that oxygen atoms are successively incorporated into the precursor obtained by a coprecipitation process to maintain electroneutrality and to provide the coordination sites for inserted Li ions and transition metal cations via a high‐temperature lithiation. It is found that the electrochemical performances of the cathode materials are strongly related to the phase composition and preparation procedure. The monoclinic layered Li[Li_0.2Ni_0.2Mn_0.6]O₂ cathode materials with state‐of‐the‐art electrochemical performance and comparably high discharge capacities of 171 mAh g^?1 at 10 C are obtained by microwave annealing at 750 °C for 2 h.     

A New P2‐Type Layered Oxide Cathode with Extremely High Energy Density for Sodium‐Ion Batteries

Herein, a new P2‐type layered oxide is proposed as an outstanding intercalation cathode material for high energy density sodium‐ion batteries (SIBs). On the basis of the stoichiometry of sodium and transition metals, the P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂ cathode is synthesized without impurities phase by partially substituting Ni and Fe into the Mn sites. The partial substitution results in a smoothing of the electrochemical charge/discharge profiles and thus greatly improves the battery performance. The P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂ cathode delivers an extremely high discharge capacity of 221.5 mAh g^?1 with a high average potential of ≈2.9 V (vs Na/Na⁺) for SIBs. In addition, the fast Na‐ion transport in the P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂ cathode structure enables good power capability with an extremely high current density of 2400 mA g^?1 (full charge/discharge in 12 min) and long‐term cycling stability with ≈80% capacity retention after 500 cycles at 600 mA g^?1. A combination of electrochemical profiles, in operando synchrotron X‐ray diffraction analysis, and first‐principles calculations are used to understand the overall Na storage mechanism of P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂.     

Structural Insight into Layer Gliding and Lattice Distortion in Layered Manganese Oxide Electrodes for Potassium‐Ion Batteries

Potassium‐ion batteries (PIBs) are an emerging, affordable, and environmentally friendly alternative to lithium‐ion batteries, with their further development driven by the need for suitably performing electrode materials capable of reversibly accommodating the relatively large K⁺. Layer‐structured manganese oxides are attractive as electrodes for PIBs, but suffer from structural instability and sluggish kinetics of K⁺ insertion/extraction, leading to poor rate capability. Herein, cobalt is successfully introduced at the manganese site in the K_xMnO₂ layered oxide electrode material and it is shown that with only 5% Co, the reversible capacity increases by 30% at 22 mA g^‐1 and by 92% at 440 mA g^‐1. In operando synchrotron X‐ray diffraction reveals that Co suppresses Jahn–Teller distortion, leading to more isotropic migration pathways for K⁺ in the interlayer, thus enhancing the ionic diffusion and consequently, rate capability. The detailed analysis reveals that additional phase transitions and larger volume change occur in the Co‐doped material as a result of layer gliding, with these associated with faster capacity decay, despite the overall capacity remaining higher than the pristine material, even after 500 cycles. These results assert the importance of understanding the detailed structural evolution that underpins performance that will inform the strategic design of electrode materials for high‐performance PIBs.     

Countering Voltage Decay and Capacity Fading of Lithium‐Rich Cathode Material at 60 °C by Hybrid Surface Protection Layers

Li‐rich layered metal oxides have attracted much attention for their high energy density but still endure severe capacity fading and voltage decay during cycling, especially at elevated temperature. Here, facile surface treatment of Li_1.17Ni_0.17Co_0.17Mn_0.5O₂ (0.4Li₂MnO₃·0.6LiNi_1/3Co_1/3Mn_1/3O₂) spherical cathode material is designed to address these drawbacks by hybrid surface protection layers composed of Mg²⁺ pillar and Li‐Mg‐PO₄ layer. As a result, the surface coated Li‐rich cathode material exhibits much enhanced cycling stability at 60 °C, maintaining 72.6% capacity retention (180 mAh g^?1) between 3.0 and 4.7 V after 250 cycles. More importantly, 88.7% average discharge voltage retention can be obtained after the rigorous cycle test. The strategy developed here with novel hydrid surface protection effect can provide a vital approach to inhibit the undesired side reactions and structural deterioration of Li‐rich cathode materials and may also be useful for other layered oxides to increase their cycling stability at elevated temperature.     

Mn‐Rich P′2‐Na0.67[Ni0.1Fe0.1Mn0.8]O2 as High‐Energy‐Density and Long‐Life Cathode Material for Sodium‐Ion Batteries

Herein, P′2‐type Na_0.67[Ni_0.1Fe_0.1Mn_0.8]O₂ is introduced as a promising new cathode material for sodium‐ion batteries (SIBs) that exhibits remarkable structural stability during repetitive Na⁺ de/intercalation. The O? Ni? O? Mn? O? Fe? O bond in the octahedra of transition‐metal layers is used to suppress the elongation of the Mn? O bond and to improve the electrochemical activity, leading to the highly reversible Na storage mechanism. A high discharge capacity of ≈220 mAh g^?1 (≈605 Wh kg^?1) is delivered at 0.05 C (13 mAg^?1) with a high reversible capacity of ≈140 mAh g^?1 at 3 C and excellent capacity retention of 80% over 200 cycles. This performance is associated with the reversible P′2–OP4 phase transition and small volume change upon charge and discharge (≈3%). The nature of the sodium storage mechanism in a full cell paired with a hard carbon anode reveals an unexpectedly high energy density of ≈542 Wh kg^?1 at 0.2 C and good capacity retention of ≈81% for 500 cycles at 1 C (260 mAg^?1).     

Insight of a Phase Compatible Surface Coating for Long‐Durable Li‐Rich Layered Oxide Cathode

Li‐rich layered oxides (LLOs) can deliver almost double the capacity of conventional electrode materials such as LiCoO₂ and LiMn₂O₄; however, voltage fade and capacity degradation are major obstacles to the practical implementation of LLOs in high‐energy lithium‐ion batteries. Herein, hexagonal La_0.8Sr_0.2MnO_3?y (LSM) is used as a protective and phase‐compatible surface layer to stabilize the Li‐rich layered Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LM) cathode material. The LSM is Mn? O? M bonded at the LSM/LM interface and functions by preventing the migration of metal ions in the LM associated with capacity degradation as well as enhancing the electrical transfer and ionic conductivity at the interface. The LSM‐coated LM delivers an enhanced reversible capacity of 202 mAh g^?1 at 1 C (260 mA g^?1) with excellent cycling stability and rate capability (94% capacity retention after 200 cycles and 144 mAh g^?1 at 5 C). This work demonstrates that interfacial bonding between coating and bulk material is a successful strategy for the modification of LLO electrodes for the next‐generation of high‐energy Li‐ion batteries.     

Radially Oriented Single‐Crystal Primary Nanosheets Enable Ultrahigh Rate and Cycling Properties of LiNi0.8Co0.1Mn0.1O2 Cathode Material for Lithium‐Ion Batteries

Ni‐rich Li[Ni_xCo_yMn_1?x?y]O₂ (x ≥ 0.8) layered oxides are the most promising cathode materials for lithium‐ion batteries due to their high reversible capacity of over 200 mAh g^?1. Unfortunately, the anisotropic properties associated with the α‐NaFeO₂ structured crystal grains result in poor rate capability and insufficient cycle life. To address these issues, a micrometer‐sized Ni‐rich LiNi_0.8Co_0.1Mn_0.1O₂ secondary cathode material consisting of radially aligned single‐crystal primary particles is proposed and synthesized. Concomitant with this unique crystallographic texture, all the exposed surfaces are active {010} facets, and 3D Li⁺ ion diffusion channels penetrate straightforwardly from surface to center, remarkably improving the Li⁺ diffusion coefficient. Moreover, coordinated charge–discharge volume change upon cycling is achieved by the consistent crystal orientation, significantly alleviating the volume‐change‐induced intergrain stress. Accordingly, this material delivers superior reversible capacity (203.4 mAh g^?1 at 3.0–4.3 V) and rate capability (152.7 mAh g^?1 at a current density of 1000 mA g^?1). Further, this structure demonstrates excellent cycling stability without any degradation after 300 cycles. The anisotropic morphology modulation provides a simple, efficient, and scalable way to boost the performance and applicability of Ni‐rich layered oxide cathode materials.     

Understanding Performance Degradation in Cation‐Disordered Rock‐Salt Oxide Cathodes

The collective redox activities of transition‐metal (TM) cations and oxygen anions have been shown to increase charge storage capacity in both Li‐rich layered and cation‐disordered rock‐salt cathodes. Repeated cycling involving anionic redox is known to trigger TM migration and phase transformation in layered Li‐ and Mn‐rich (LMR) oxides, however, detailed mechanistic understanding on the recently discovered Li‐rich rock‐salt cathodes is largely missing. The present study systematically investigates the effect of oxygen redox on a Li_1.3Nb_0.3Mn_0.4O₂ cathode and demonstrates that performance deterioration is directly correlated to the extent of oxygen redox. It is shown that voltage fade and hysteresis begin only after initiating anionic redox at high voltages, which grows progressively with either deeper oxidation of oxygen at higher potential or extended cycling. In contrast to what is reported on layered LMR oxides, extensive TM reduction is observed but phase transition is not detected in the cycled oxide. A densification/degradation mechanism is proposed accordingly which elucidates how a unique combination of extensive chemical reduction of TM and reduced quality of the Li percolation network in cation‐disordered rock‐salts can lead to performance degradation in these newer cathodes with 3D Li migration pathways. Design strategies to achieve balanced capacity and stability are also discussed.     

Construction of Hierarchical K1.39Mn3O6 Spheres via AlF3 Coating for High‐Performance Potassium‐Ion Batteries

Potassium‐ion batteries are attracting great interest for emerging large‐scale energy storage owing to their advantages such as low cost and high operational voltage. However, they are still suffering from poor cycling stability and sluggish thermodynamic kinetics, which inhibits their practical applications. Herein, the synthesis of hierarchical K_1.39Mn₃O₆ microspheres as cathode materials for potassium‐ion batteries is reported. Additionally, an effective AlF₃ surface coating strategy is applied to further improve the electrochemical performance of K_1.39Mn₃O₆ microspheres. The as‐synthesized AlF₃ coated K_1.39Mn₃O₆ microspheres show a high reversible capacity (about 110 mA h g^?1 at 10 mA g^?1), excellent rate capability, and cycling stability. Galvanostatic intermittent titration technique results demonstrate that the increased diffusion kinetics of potassium‐ion insertion and extraction during discharge and charge processes benefit from both the hierarchical sphere structure and surface modification. Furthermore, ex situ X‐ray diffraction measurements reveal that the irreversible structure evolution can be significantly mitigated via surface modification. This work sheds light on rational design of high‐performance cathode materials for potassium‐ion batteries.     

Grafting Benzenediazonium Tetrafluoroborate onto LiNixCoyMnzO2 Materials Achieves Subzero‐Temperature High‐Capacity Lithium‐Ion Storage via a Diazonium Soft‐Chemistry Method

Subzero‐temperature Li‐ion batteries (LIBs) are highly important for specific energy storage applications. Although the nickel‐rich layered lithium transition metal oxides(LiNi_xCo_yMn_zO₂) (LNCM) (x > 0.5, x + y +z = 1) are promising cathode materials for LIBs, their very slow Li‐ion diffusion is a main hurdle on the way to achieve high‐performance subzero‐temperature LIBs. Here, a class of low‐temperature organic/inorganic hybrid cathode materials for LIBs, prepared by grafting a conducting polymer coating on the surface of 3 µm sized LiNi_0.6Co_0.2Mn_0.2O₂ (LNCM‐3) material particles via a greener diazonium soft‐chemistry method is reported. Specifically, LNCM‐3 particles are uniformly coated with a thin polyphenylene film via the spontaneous reaction between LNCM‐3 and C₆H₅N₂⁺BF₄^?. Compared with the uncoated one, the polyphenylene‐coated LNCM‐3 (polyphenylene/LNCM‐3) has shown much improved low‐temperature discharge capacity (≈148 mAh g^?1 at 0.1 C, ?20 °C), outstanding rate capability (≈105 mAh g^?1 at 1 C, ?20 °C), and superior low‐temperature long‐term cycling stability (capacity retention is up to 90% at 0.5 C over 1150 cycles). The low‐temperature performance of polyphenylene/LNCM‐3 is the best among the reported state‐of‐the art cathode materials for LIBs. The present strategy opens up a new avenue to construct advanced cathode materials for wider range applications.     

Controlled Atomic Solubility in Mn‐Rich Composite Material to Achieve Superior Electrochemical Performance for Li‐Ion Batteries

The quest for high energy density and high power density electrode materials for lithium‐ion batteries has been intensified to meet strongly growing demand for powering electric vehicles. Conventional layered oxides such as Co‐rich LiCoO₂ and Ni‐rich Li(Ni_xMn_yCo_z)O₂ that rely on only transition metal redox reaction have been faced with growing constraints due to soaring price on cobalt. Therefore, Mn‐rich electrode materials excluding cobalt would be desirable with respect to available resources and low cost. Here, the strategy of achieving both high energy density and high power density in Mn‐rich electrode materials by controlling the solubility of atoms between phases in a composite is reported. The resulting Mn‐rich material that is composed of defective spinel phase and partially cation‐disordered layered phase can achieve the highest energy density, ≈1100 W h kg^?1 with superior power capability up to 10C rate (3 A g^?1) among other reported Mn‐rich materials. This approach provides new opportunities to design Mn‐rich electrode materials that can achieve high energy density and high power density for Li‐ion batteries.     

Al Doping for Mitigating the Capacity Fading and Voltage Decay of Layered Li and Mn‐Rich Cathodes for Li‐Ion Batteries

Li and Mn‐rich layered cathodes, despite their high specific capacity, suffer from capacity fading and discharge voltage decay upon cycling. Both specific capacity and discharge voltage of Li and Mn‐rich cathodes are stabilized upon cycling by optimized Al doping. Doping Li and Mn‐rich cathode materials Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ by Al on the account of manganese (as reflected by their stoichiometry) results in a decrease in their specific capacity but increases pronouncedly their stability upon cycling. Li_1.2Ni_0.16Mn_0.51Al_0.05Co_0.08O₂ exhibits 96% capacity retention as compared to 68% capacity retention for Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ after 100 cycles. This doping also reduces the decrease in the average discharge voltage upon cycling, which is the longstanding fatal drawback of these Li and Mn‐rich cathode materials. The electrochemical impedance study indicates that doping by Al has a surface stabilization effect on these cathode materials. The structural analysis of cycled electrodes by Raman spectroscopy suggests that Al doping also has a bulk stabilizing effect on the layered LiMO₂ phase resulting in the better electrochemical performance of Al doped cathode materials as compared to the undoped counterpart. Results from a prolonged systematic work on these cathode materials are presented and the best results that have ever been obtained are reported.     

Site‐Selective In Situ Electrochemical Doping for Mn‐Rich Layered Oxide Cathode Materials in Lithium‐Ion Batteries

Various doped materials have been investigated to improve the structural stability of layered transition metal oxides for lithium‐ion batteries. Most doped materials are obtained through solid state methods, in which the doping of cations is not strictly site selective. This paper demonstrates, for the first time, an in situ electrochemical site‐selective doping process that selectively substitutes Li⁺ at Li sites in Mn‐rich layered oxides with Mg²⁺. Mg²⁺ cations are electrochemically intercalated into Li sites in delithiated Mn‐rich layered oxides, resulting in the formation of [Li_1?xMg_y][Mn_1?zM_z]O₂ (M = Co and Ni). This Mg²⁺ intercalation is irreversible, leading to the favorable doping of Mg²⁺ at the Li sites. More interestingly, the amount of intercalated Mg²⁺ dopants increases with the increasing amount of Mn in Li_1?x[Mn_1?zM_z]O₂, which is attributed to the fact that the Mn‐to‐O electron transfer enhances the attractive interaction between Mg²⁺ dopants and electronegative O^δ? atoms. Moreover, Mg²⁺ at the Li sites in layered oxides suppresses cation mixing during cycling, resulting in markedly improved capacity retention over 200 cycles. The first‐principle calculations further clarify the role of Mg²⁺ in reduced cation mixing during cycling. The new concept of in situ electrochemical doping provides a new avenue for the development of various selectively doped materials.     

Review on Challenges and Recent Advances in the Electrochemical Performance of High Capacity Li‐ and Mn‐Rich Cathode Materials for Li‐Ion Batteries

Li and Mn‐rich layered oxides, xLi₂MnO₃·(1–x)LiMO₂ (M=Ni, Mn, Co), are promising cathode materials for Li‐ion batteries because of their high specific capacity that can exceed 250 mA h g^?1. However, these materials suffer from high 1^st cycle irreversible capacity, gradual capacity fading, low rate capability, a substantial charge‐discharge voltage hysteresis, and a large average discharge voltage decay during cycling. The latter detrimental phenomenon is ascribed to irreversible structural transformations upon cycling of these cathodes related to potentials ≥4.5 V required for their charging. Transition metal inactivation along with impedance increase and partial layered‐to‐spinel transformation during cycling are possible reasons for the detrimental voltage fade. Doping of Li, Mn‐rich materials by Na, Mg, Al, Fe, Co, Ru, etc. is useful for stabilizing capacity and mitigating the discharge‐voltage decay of xLi₂MnO₃·(1–x)LiMO₂ electrodes. Surface modifications by thin coatings of Al₂O₃, V₂O₅, AlF₃, AlPO₄, etc. or by gas treatment (for instance, by NH₃) can also enhance voltage and capacity stability during cycling. This paper describes the recent literature results and ongoing efforts from our groups to improve the performance of Li, Mn‐rich materials. Focus is also on preparation of cobalt‐free cathodes, which are integrated layered‐spinel materials with high reversible capacity and stable performance.     

Layered Na‐Ion Cathodes with Outstanding Performance Resulting from the Synergetic Effect of Mixed P‐ and O‐Type Phases

Herein, the synthesis of new quaternary layered Na‐based oxides of the type Na_xMn_yNi_zFe_0.1Mg_0.1O₂ (0.67≤ x ≤ 1.0; 0.5≤ y ≤ 0.7; 0.1≤ z ≤ 0.3) is described. The synthesis can be tuned to obtain P2‐ and O3‐type as well as mixed P‐/O‐type phases as demonstrated by structural, morphological, and electrochemical properties characterization. Although all materials show good electrochemical performance, the simultaneous presence of the P‐ and O‐type phases is found to have a synergetic effect resulting in outstanding performance of the mixed phase material as a sodium‐ion cathode. The mixed P3/P2/O3‐type material, having an average elemental composition of Na_0.76Mn_0.5Ni_0.3Fe_0.1Mg_0.1O₂, overcomes the specific drawbacks associated with the P2‐ and O3‐type materials, allowing the outstanding electrochemical performance. In detail, the mixed phase material is able to deliver specific discharge capacities of up to 155 mAh g^?1 (18 mA g^?1) in the potential range of 2.0–4.3 V. In the narrower potential range of 2.5–4.3 V the material exhibits high average discharge potential (3.4 V versus Na/Na⁺), exceptional average coulombic efficiencies (>99.9%), and extraordinary capacity retention (90.2% after 601 cycles). The unexplored class of P‐/O‐type mixed phases introduces new perspectives for the development of layered positive electrode materials and powerful Na‐ion batteries.     

Native Vacancy Enhanced Oxygen Redox Reversibility and Structural Robustness

Cathode materials with high energy density, long cycle life, and low cost are of top priority for energy storage systems. The Li‐rich transition metal (TM) oxides achieve high specific capacities by redox reactions of both the TM and oxygen ions. However, the poor reversible redox reaction of the anions results in severe fading of the cycling performance. Herein, the vacancy‐containing Na_4/7[Mn_6/7(?_Mn)_1/7]O₂ (?_Mn for vacancies in the Mn? O slab) is presented as a novel cathode material for Na‐ion batteries. The presence of native vacancies endows this material with attractive properties including high structural flexibility and stability upon Na‐ion extraction and insertion and high reversibility of oxygen redox reaction. Synchrotron X‐ray absorption near edge structure and X‐ray photoelectron spectroscopy studies demonstrate that the charge compensation is dominated by the oxygen redox reaction and Mn³⁺/Mn⁴⁺ redox reaction separately. In situ synchrotron X‐ray diffraction exhibits its zero‐strain feature during the cycling. Density functional theory calculations further deepen the understanding of the charge compensation by oxygen and manganese redox reactions and the immobility of the Mn ions in the material. These findings provide new ideas on searching for and designing materials with high capacity and high structural stability for novel energy storage systems.     

Li2O:Li–Mn–O Disordered Rock‐Salt Nanocomposites as Cathode Prelithiation Additives for High‐Energy Density Li‐Ion Batteries

The irreversible loss of lithium from the cathode material during the first cycles of rechargeable Li‐ion batteries notably reduces the overall cell capacity. Here, a new family of sacrificial cathode additives based on Li₂O:Li_2/3Mn_1/3O_5/6 composites synthesized by mechanochemical alloying is reported. These nanocomposites display record (but irreversible) capacities within the Li–Mn–O systems studied, of up to 1157 mAh g^?1, which represents an increase of over 300% of the originally reported capacity in Li_2/3Mn_1/3O_5/6 disordered rock salts. Such a high irreversible capacity is achieved by the reaction between Li₂O and Li_2/3Mn_1/3O_5/6 during the first charge, where electrochemically active Li₂O acts as a Li⁺ donor. A 13% increase of the LiFePO₄ and LiCoO₂ first charge gravimetric capacities is demonstrated by the addition of only 2 wt% of the nanosized composite in the cathode mixture. This result shows the great potential of these newly discovered sacrificial additives to counteract initial losses of Li⁺ ions and improve battery performance.     

Highly Reversible Oxygen‐Redox Chemistry at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn Vacancy)

Increasing the energy density of rechargeable batteries is of paramount importance toward achieving a sustainable society. The present limitation of the energy density is owing to the small capacity of cathode materials, in which the (de)intercalation of ions is charge‐compensated by transition‐metal redox reactions. Although additional oxygen‐redox reactions of oxide cathodes have been recognized as an effective way to overcome this capacity limit, irreversible structural changes that occur during charge/discharge cause voltage drops and cycle degradation. Here, a highly reversible oxygen‐redox capacity of Na₂Mn₃O₇ that possesses inherent Mn vacancies in a layered structure is found. The cross validation of theoretical predictions and experimental observations demonstrates that the nonbonding 2p orbitals of oxygens neighboring the Mn vacancies contribute to the oxygen‐redox capacity without making the Mn?O bond labile, highlighting the critical role of transition‐metal vacancies for the design of reversible oxygen‐redox cathodes.