Advances in the Cathode Materials for Lithium Rechargeable Batteries

Real-space measurement of orbital electron populations for Li1-xCoO2

The operation of lithium-ion batteries involves electron removal from and filling into the redox orbitals of cathode materials, experimentally probing the orbital electron population thus is highly desirable to resolve the redox processes and charge compensation mechanism. Here, we combine quantitative convergent-beam electron diffraction with high-energy synchrotron powder X-ray diffraction to quantify the orbital populations of Co and O in the archetypal cathode material LiCoO₂. The results indicate that removing Li ions from LiCoO₂ decreases Co t_2g orbital population, and the intensified covalency of Co-O bond upon delithiation enables charge transfer from O 2p orbital to Co e_g orbital, leading to increased Co e_g orbital population and oxygen oxidation. Theoretical calculations verify these experimental findings, which not only provide an intuitive picture of the redox reaction process in real space, but also offer a guidance for designing high-capacity electrodes by mediating the covalency of the TM-O interactions.

A Competitive Solvation of Ternary Eutectic Electrolytes Tailoring the Electrode/Electrolyte Interphase for Lithium Metal Batteries

The development of electrolytes with high safety, high ionic conductivity, and the ability to inhibit lithium dendrites growth is crucial for the fabrication of high-energy-density lithium metal batteries. In this study, a ternary eutectic electrolyte is designed with LiTFSI (TFSI = bis(trifluoromethanesulfonyl)imide), butyrolactam (BL), and succinonitrile (SN). This electrolyte exhibits a high ion conductivity, nonflammability, and a wide electrochemical window. The competitive solvation effect among SN, BL, and Li⁺ reduces the viscosity and improves the stability of the eutectic electrolyte. The preferential coordination of BL toward Li⁺ facilitates the formation of stable solid electrolyte interphase films, leading to homogeneous and dendrite-free Li plating. As expected, the LiFePO₄/Li cell with this ternary eutectic electrolyte delivers a high capacity retention of 90% after 500 cycles at 2 C and an average Coulombic efficiency of 99.8%. Moreover, Ni-rich LiNi_0.8Co_0.1Al_0.1O₂/Li and LiNi_0.8Co_0.1Mn_0.1O₂/Li cells based on the modified ternary eutectic electrolyte achieve an outstanding cycling performance. This study provides insights for understanding and designing better electrolytes for lithium metal batteries and analogous sodium/potassium metal batteries.

Structural Stabilization of Cation-Disordered Rock-Salt Cathode Materials: Coupling between a High-Ratio Inactive Ti4+ Cation and a Mn2+/Mn4+ Two-Electron Redox Pair

Cation-disordered rock-salt cathode materials are featured by their extraordinarily high specific capacities in lithium-ion batteries primarily contributed by anion redox reactions. Unfortunately, anion redox reactions can trigger oxygen release in this class of materials, leading to fast capacity fading and major safety concern. Despite the capability of absorbing structural distortions, high-ratio d⁰ transition-metal cations are considered to be unfavorable in design of a new cation-disordered rock-salt structure because of their electrochemically inactive nature. Herein, we report a new cation-disordered rock-salt compound of Li_1.2Ti_0.6Mn_0.2O₂ with the stoichiometry of Ti⁴⁺ as high as 0.6. The capacity reducing effect by the low-ratio active transition-metal center can be balanced by using a Mn²⁺/Mn⁴⁺ two-electron redox couple. The strengthened networks of strong Ti-O bonds greatly retard the oxygen release and improve the structural stability of cation-disordered rock-salt cathode materials. As expected, Li_1.2Ti_0.6Mn_0.2O₂ delivers significantly improved electrochemical performances and thermal stability compared to the low-ratio Ti⁴⁺ counterpart of Li_1.2Ti_0.4Mn_0.4O₂. Theoretical simulations further reveal that the improved electrochemical performances of Li_1.2Ti_0.6Mn_0.2O₂ are attributed to its lower Li⁺ diffusion energy barrier and enhanced unhybridized O 2p states compared to Li_1.2Ti_0.4Mn_0.4O₂. This concept might be helpful for the improvement of structural stability and electrochemical performances of other cation-disordered rock-salt metal oxide cathode materials.

Mesoporous Polyimide-Linked Covalent Organic Framework with Multiple Redox-Active Sites for High-Performance Cathodic Li Storage

Covalent organic frameworks (COFs) are gaining increasing attention as renewable cathode materials for Li-ion batteries. However, COF electrodes reported so far still exhibit unsatisfying capacity due to their limited active site density and insufficient utilization. Herein, a new two-dimensional polyimide-linked COF, HATN-AQ-COF with multiple redox-active sites for storing Li⁺ ions, was designed and fabricated from a new module of 2,3,8,9,14,15-hexacarboxyl hexaazatrinaphthalene trianhydrides with a 2,6-diaminoanthraquinone linker. HATN-AQ-COF possessing excellent stability, good conductivity, and a large pore size of 3.8 nm enables the stable and fast ion transport. This, in combination with the abundant redox active sites, results in a high reversible capacity of 319 mAh g^-1 at 0.5 C (1 C=358 mA g^-1 ) for the HATN-AQ-COF electrode with a high active site utilization of 89 % and good cycle performance, representing one of the best performances among the reported COF electrodes.

Hysteresis Induced by Incomplete Cationic Redox in Li-Rich 3d-Transition-Metal Layered Oxides Cathodes

Activation of oxygen redox during the first cycle has been reported as the main trigger of voltage hysteresis during further cycles in high-energy-density Li-rich 3d-transition-metal layered oxides. However, it remains unclear whether hysteresis only occurs due to oxygen redox. Here, it is identified that the voltage hysteresis can highly correlate to cationic reduction during discharge in the Li-rich layered oxide, Li_1.2 Ni_0.4 Mn_0.4 O₂ . In this material, the potential region of discharge accompanied by hysteresis is apparently separated from that of discharge unrelated to hysteresis. The quantitative analysis of soft/hard X-ray absorption spectroscopies discloses that hysteresis is associated with an incomplete cationic reduction of Ni during discharge. The galvanostatic intermittent titration technique shows that the inevitable energy consumption caused by hysteresis corresponds to an overpotential of 0.3 V. The results unveil that hysteresis can also be affected by cationic redox in Li-rich layered cathodes, implying that oxygen redox cannot be the only reason for the evolution of voltage hysteresis. Therefore, appropriate control of both cationic and anionic redox of Li-rich layered oxides will allow them to reach their maximum energy density and efficiency.

Ion Substitution Strategy of Manganese-Based Layered Oxide Cathodes for Advanced and Low-Cost Sodium Ion Batteries

Sodium ion batteries (SIBs) have recently been promising in the large-scale electric energy storage system, due to the low cost, abundant sodium resources. Mn-based layered oxide cathode materials have been widely investigated, because of the high theoretical specific capacity, low cost, and abundant reserves. However, their development is limited by the problems of Jahn-Teller distortion, Na⁺ /vacancy ordering, complex phase transitions, and irreversible anionic redox during cycling. Ion substitution strategy is one simple and effective way to regulate the crystal structure and boost sodium-storage performances of Mn-based cathode materials. In this review, we summarize the progress and mechanism of ion-substituted Mn-based oxides, establish a composition-crystal structure-electrochemical performance relationship, and also offer perspectives for guiding the design of high-performance Mn-based oxides for SIBs.

A Molten-Salt Method to Synthesize Ultrahigh-Nickel Single-Crystalline LiNi0.92 Co0.06 Mn0.02 O2 with Superior Electrochemical Performance as Cathode Material for Lithium-Ion Batteries

Ni-rich layered oxides have been intensively considered as promising cathode materials for next-generation Li-ion batteries. Nevertheless, the performance degradation caused by intergranular cracks and electrode/electrolyte interface parasitic reactions restricts their further application. Compared with secondary particles, single-crystal (SC) materials have better mechanical integrity and cycling stability. However, the preparation of ultrahigh-nickel layered SC cathode still remains a serious challenge. Herein, a novel LiOH-LiNO₃ -H₃ BO₃ molten-salt method is proposed to synthesize SC LiNi_0.92 Co_0.06 Mn_0.02 O₂ with considerable crystallinity and uniformity. The critical impacts of calcination temperature and boric acid on the microstructure and electrochemical property of Ni-rich layered oxides are systematically investigated. The results show that the crystal growth is promoted and the stability of crystal structure is improved by this synthesis method. In particular, the optimal electrode demonstrates a superior initial discharge capacity of 214.8 mAh g^-1 with a high capacity retention of 86.3% over 300 cycles as tested by pouch-type full cells at 45 ºC. This work not only prepares an ultrahigh-nickel layered CS cathode with superior electrochemical performances, but also provides a feasible method for the synthesis of other CS layered cathode materials.

N-doped engineering of a high-voltage LiNi0.5Mn1.5O4 cathode with superior cycling capability for wide temperature lithium-ion batteries

Spinel LiNi_0.5Mn_1.5O₄ (LNMO) is one potential cathode candidate for next-generation high energy-density lithium-ion batteries (LIBs). However, serious capacity decay from its poor structural stability, especially at high operating temperatures, shadows its application prospects. In this work, N-doped LNMO (LNMON) was synthesized by a facile co-precipitation method and multistep calcination, exhibiting a unique yolk-shell architecture. Concurrently, N dopants are introduced into a LNMO lattice, endowing LNMON with a more stable structure via stronger Ni-N/Mn-N bindings. Benefiting from the synergistic effect of the yolk-shell structure and N-doped engineering, the obtained LNMON cathode exhibits an impressive rate and the state-of-the-art cycling capability, delivering a high capacity of 103 mA h g^-1 at 25 °C after 8000 cycles. Even at a high operating temperature of 60 °C, the capacity retention remains at 92% after 1000 cycles. The discovery of N dopants in improving the cycling capability of LNMO in our case offers a prospective approach to enable 5 V LNMO cathode materials with excellent cycling capability.

Layered Cathode Materials: Precursors, Synthesis, Microstructure, Electrochemical Properties, and Battery Performance

The exploitation of clean energy promotes the exploration of next-generation lithium-ion batteries (LIBs) with high energy-density, long life, high safety, and low cost. Ni-rich layered cathode materials are one of the most promising candidates for next-generation LIBs. Numerous studies focusing on the synthesis and modifications of the layered cathode materials are published every year. Many physical features of precursors, such as density, morphology, size distribution, and microstructure of primary particles pass to the resulting cathode materials, thus significantly affecting their electrochemical properties and battery performance. This review focuses on the recent advances in the controlled synthesis of hydroxide precursors and the growth of particles. The essential parameters in controlled coprecipitation are discussed in detail. Some innovative technologies for precursor modifications and for the synthesis of novel precursors are highlighted. In addition, future perspectives of the development of hydroxide precursors are presented.

Atomically Intimate Solid Electrolyte/Electrode Contact Capable of Surviving Long-Term Cycling with Repeated Phase Transitions

The electrode-electrolyte contact issue within the composite electrode layer is a grand challenge for all-solid-state Li batteries. In order to achieve cycling performances comparable to Li-ion batteries based on liquid electrolyte, the aforementioned solid-solid contact not only needs to be sufficiently thorough but also must tolerate repeated cycling. Simultaneously meeting both requirements is rather challenging. Here, we discover that epitaxy may effectively overcome such bottlenecks even when the electrode undergoes repeated phase transitions during cycling. Through epitaxial growth, the perovskite Li_0.33La_0.56TiO₃ solid electrolyte was found capable of forming atomically intimate contact with both the spinel Li₄Ti₅O₁₂ and rock-salt Li₇Ti₅O₁₂. In contrast to conventional expectations, such epitaxial interfaces can also survive repeated spinel-to-rock-salt phase transitions. Consequently, the Li₄Ti₅O₁₂-Li_0.33La_0.56TiO₃ composite electrode based on epitaxial solid-solid contact delivers not only a rate capability comparable to that of the surry-cast one with solid-liquid contact but also an excellent long-term cycling stability.

Regulating Pseudo-Jahn-Teller Effect and Superstructure in Layered Cathode Materials for Reversible Alkali-Ion Intercalation

The Jahn-Teller effect (JTE) is one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge; however, many reports have shown that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (pseudo-JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and antibonding molecular orbitals in transition-metal octahedra. As a model case, a P2-type Mn-rich cathode (Na_3/4MnO₂) was investigated in detail. MnO₆ octahedra are well known to undergo either elongation or contraction in a specific direction due to JTE. Here, the substitution of Li for Mn (Na_3/4(Li_1/4Mn_3/4)O₂) helped to oxidize Mn³⁺ to Mn⁴⁺ suppressing JTE; however, the MnO₆ octahedra remained asymmetric with a clear trace of the superstructure. With various advanced analyses, we disclose the pseudo-JTE as a general reason for the asymmetric distortions of the MnO₆ octahedra. These distortions lead to the significant electrochemical degradation of Na_3/4Li_1/4Mn_3/4O₂. The suppression of the pseudo-JTE modulates phase transition behaviors during Na intercalation/deintercalation and thereby improves all of the electrochemical properties. The insight obtained by coupling a theoretical background for the pseudo-JTE with verified layered cathode material lattice changes implies that many previous approaches can be rationalized by regulating pseudo-JTE. This suggests that the pseudo-JTE should be thought more important than the well-known JTE for layered cathode materials.

Enabling the High-Voltage Operation of Layered Ternary Oxide Cathodes via Thermally Tailored Interphase

Layered ternary oxides LiNi_x Mn_y Co_z O₂ are promising cathode candidates for high-energy lithium-ion batteries (LIBs), but they usually suffer from the severe interfacial parasitic reactions at voltages above 4.3 V versus Li⁺ /Li, which greatly limit their practical capacities. Herein, using LiNi_1/3 Mn_1/3 Co_1/3 O₂ (NMC111) as the model system, a novel high-temperature pre-cycling strategy is proposed to realize its stable cycling in 3.0-4.5 V by constructing a robust cathode/electrolyte interphase (CEI). Specifically, performing the first five cycles of NMC111 at 55 °C helps to yield a uniform CEI layer enriched with fluorine-containing species, Li₂ CO₃ and poly(CO₃ ), which greatly suppresses the detrimental side reactions during extended cycling at 25 °C, endowing the cell with a capacity retention of 92.3% at 1C after 300 cycles, far surpassing 62.0% for the control sample without the thermally tailored CEI. This work highlights the critical role of temperature on manipulating the interfacial properties of cathode materials, opening a new avenue for developing high-voltage cathodes for Li-ion batteries.

Lithium-rich sulfide/selenide cathodes for next-generation lithium-ion batteries: challenges and perspectives

The extraordinarily high capacity exhibited by lithium-rich oxides has motivated intensive investigations towards both the cationic and anionic redox processes. With recent main focus on the anionic redox behavior, the anionic redox chemistry has emerged as a new orientation to pursue higher-energy cathodes for lithium-ion batteries. However, the key practical issues such as voltage decay, voltage hysteresis, and irreversible oxygen loss of lithium-rich oxides have triggered researchers to act on the ligand by designing novel lithium-rich sulfides/selenides. In light of this, we herein provide a timely and in-depth perspective on the development of these lithium-rich sulfides/selenides with various structures and coordinations. We highlighted both the variations of phases and structures, lithium storage mechanism, detailed change of sulfur/selenide through anionic redox, and potentials for higher energy densities. We also outlined the main academic and commercial obstacles or challenges for these Li-rich sulfides/selenides for next-generation lithium-ion batteries.

Screening LiMn2O4 Surface Modification Schemes under Theoretical Guidance

Mn dissolution is one of the most important factors for the failure of LiMn₂O₄ batteries. Doping has been widely adopted in the modification of LiMn₂O₄ cathodes; however, there is still a lack of theoretical guidance on screening the dopants. Here, through first-principles calculations, we systematically investigated the effects of all 3, 4d transition metals as well as Mg, Ca, Sr, Al, Ga, and In on the surface oxygen stability of LiMn₂O₄ cathodes, which has been proved to be correlated with the stability of the surface Mn atoms. Six competitive dopants, namely Nb, Ru, Mo, V, Tc, and Ti, were screened out. Besides, for three dopants in low valence states (Mg, Cu, and Zn), their Li-site doping can more effectively stabilize the surface oxygen atoms compared with Mn-site doping. Finally, we synthesized LiMn₂O₄ samples with Mg, Mo, and Nb surface doping to validate the rationality of the computational results. We found that particle morphology should also be considered in addition to surface oxygen stability for controlling Mn dissolution. Moreover, the electrochemical performance of LiMn₂O₄ batteries is a more complex issue and cannot be solely regulated by Mn dissolution. During the experiments, we have explored novel efficient binary chromogenic reagents for ultraviolet-visible spectroscopy analysis that can be used for rapid and low-cost Mn dissolution detection. This work provides a paradigm for the systematic design of the surface modification of the LiMn₂O₄ cathode under theoretical guidance.

Construction of Li3PO4 nanoshells for the improved electrochemical performance of a Ni-rich cathode material

A Li₃PO₄ nanocoating around a nickel-rich cathode material was successfully constructed via controlling the reaction between the electrode material and a preformed phosphorus-containing polymeric nanoshell; this not only effectively tackles the alkali residue challenge, but it also contributes to much-improved electrochemical performance being shown by a high-energy cathode.

Photoinduced Rechargeable Lithium-Ion Battery

Lithium-ion battery (LIB) design is the predominant technology to power portable and mobile electric devices/equipment. Fast charging and self-powering of LIBs are significant but challenging features to be addressed for meeting the fast-paced society and emerging demands. Herein, we report a rational photorechargeable lithium-ion battery (photo-LIB) design using LiV₂O₅ as a photocathode by directly modifying a commercial LIB without using any additives, which works in both photoassisted fast charging and photo-only charging modes. The photo-LIB attains a high specific capacity of 185 mAh g^-1 in as fast as 5 min under illumination, an enhancement of 270% referring to that in dark. Under the photo-only charging mode, the device achieves a highest full-spectrum photo-energy conversion efficiency of 9% so far, demonstrating a highly efficient self-powering mode. We reveal that the improved performance of photo-LIB is due to reversible vanadium intervalence charge transfer and Li⁺ insertion/deinsertion in LiV₂O₅ under illumination.

Surface Stabilization of Ni-Rich Layered Cathode Materials via Surface Engineering with LiTaO3 for Lithium-Ion Batteries

Recently, Ni-rich layered cathode materials have become the most common material used for lithium-ion batteries. From a structural viewpoint, it is crucial to stabilize the surface structures of such materials, as they are prone to undesirable side reactions and particle cracking in which intergranular microcracks form at the particle surfaces and then propagate inside. As a simplified engineering technique for obtaining Ni-rich cathode materials with high reversibility and long-term cycling stability, we propose a facile surface coating of piezoelectric LiTaO₃ onto a Ni-rich cathode material to enhance the charge transfer reaction and surface structural integrity. Based on theoretical and experimental investigation, we demonstrate that this surface protection approach is effective at enhancing the reversibility and mechanical strength of Ni-rich cathode materials, leading to a stable cycle performance at up to 150 cycles, even at 60 °C. Furthermore, the piezoelectric characteristics of the surface LiTaO₃ can enhance the rate capability of Ni-rich cathode materials at current densities of up to 2.0C. The results of this study provide a practical insight on the development of Ni-rich cathode materials for practical use in electric vehicle applications.

Preparation of the nanorods-assembled and CNTs-embedded LiMnPO4 hollow microspheres for enhanced electrochemical performance of lithium ion batteries

LiMnPO4 is a promising cathode material for lithium-ion batteries, but it has the drawback of poor electronic conductivity and low Li+ diffusivity. Here, we expect to simultaneously improve electron transfer...

Architecting a Hydrated Ca0.24V2O5 Cathode with a Facile Desolvation Interface for Superior-Performance Aqueous Zinc Ion Batteries

Vanadium-based materials are promising cathode candidates for low-cost and high-safety aqueous zinc-ion batteries (AZIBs). However, they suffer from inferior rate capability and undesirable capacity fading due to their intrinsic poor conductivity and structural instability. Herein, we synthesize hydrated Ca_0.24V₂O₅·0.75H₂O (CaVOH) nanoribbons with in situ incorporations of the carbon nanotubes via a one-step hydrothermal method, achieving an integrated architecture hybrid cathode (C/CaVOH) design. Benefitting from the robust structure and low desolvation interface, the prefabricated C/CaVOH cathodes deliver a high capacity of 384.2 mA h g^-1 at 0.5 A g^-1 with only 5.6% capacity decay over 300 cycles, enable an ultralong cycling life of 10,000 cycles at 20.0 A g^-1 with 80.2% capacity retention, and exhibit an impressive rate capability (165 mA h g^-1 at 40.0 A g^-1) with a high mass loading of ∼4 mg cm^-2. Moreover, through the theoretical calculations and a series of ex situ characterizations, we demonstrate the Zn²⁺/H⁺ co-intercalation storage mechanism, the key role of the gallery water, and the function of the induced C-O groups in promoting kinetics of the C/CaVOH electrode. This work highlights the strategy of in situ implanted high conductivity materials to engineer vanadium-based or other cathodes for high-performance AZIBs.

Deformation and Failure Properties of High-Ni Lithium-Ion Battery under Axial Loads

To explore the failure modes of high-Ni batteries under different axial loads, quasi-static compression and dynamic impact tests were carried out. The characteristics of voltage, load, and temperature of a battery cell with different states of charge (SOCs) were investigated in quasi-static tests. The mechanical response and safety performance of lithium-ion batteries subjected to axial shock wave impact load were also investigated by using a split Hopkinson pressure bar (SHPB) system. Different failure modes of the battery were identified. Under quasi-static axial compression, the intensity of thermal runaway becomes more severe with the increase in SOC and loading speed, and the time for lithium-ion batteries to reach complete failure decreases with the increase in SOC. In comparison, under dynamic SHPB experiments, an internal short circuit occurred after impact, but no violent thermal runaway was observed.

SnS2-SnS pn hetero-junction bonded on graphene with boosted charge transfer for lithium storage

Despite the advantage of high capacity, practical implementation of the tin disulfide (SnS₂) anode for lithium-ion batteries is still plagued by the inferior rate performance due to its low intrinsic electronic conductivity and mediocre ion transport in the bulk. Herein, to address these issues, a peculiar heterojunction of SnS₂-SnS quantum dots (QDs) closely coupled with reduced graphene oxide (rGO) sheets was developed. Because of the typical n-type and p-type semiconductor characteristics of SnS₂ and SnS, respectively, the formed pn junction at the SnS₂/SnS interface will induce a built-in electric field, which can significantly accelerate lithium-ion transport through the SnS₂/SnS interface. The ultrafine SnS₂ and SnS nano-domains with superlong pn junction interfacial length construct an accelerated lithium-ion diffusion network, while the conductive rGO nanosheets provide a high-speed electron conduction pathway. Meanwhile, the flexible rGO chemically coupled with SnS₂/SnS buffers the volumetric variation during repeated lithiation/delithiation processes and guarantees robust structural durability. These merits afford the designed SnS₂-SnS/rGO electrode with fast electrode reaction kinetics and good structural durability upon cycling. Consequently, the delicate SnS₂-SnS/rGO electrode harvests a superlative rate capability of 926 and 865 mA h g^-1 at 5 and 10 A g^-1, respectively, and excellent long-term cycling stability with a high reversible capacity of 1075 mA h g^-1 at 1 A g^-1 up to 1000 cycles with negligible degradation.

A New Co-Free Ni-Rich LiNi0.8Fe0.1Mn0.1O2 Cathode for Low-Cost Li-Ion Batteries

In recent years, with the rapid development of electric vehicles, the ever-fluctuating cobalt price has become a decisive constraint on the supply chain of the lithium-ion (Li-ion) battery industry. To address these challenges, a new and unreported cobalt-free (Co-free) material with a general formula of LiNi_0.8Fe_0.1Mn_0.1O₂ (NFM) is introduced. This Co-free material is synthesized via the coprecipitation method and examined by using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) to investigate the morphological, crystal-structure, and electrochemical properties. The NFM cathode can deliver a specific capacity of 202.6 mA h g^-1 (0.1C, 3.0-4.5 V), a specific energy capacity of 798.8 W h kg^-1 in material level (0.1C, 3.0-4.5 V), a reasonable rate capability, and a stable cycling performance (81.1% discharge capacity retention after 150 cycles at 10C, 3.0-4.3 V). Although the research on this subject is still in its early stage, the capability of this novel cathode material as a practical candidate for applications in next-generation Co-free lithium-ion batteries (LIBs) is highlighted in this study.

Challenges and Recent Advances in High Capacity Li-Rich Cathode Materials for High Energy Density Lithium-Ion Batteries

Li-rich cathode materials have attracted increasing attention because of their high reversible discharge capacity (>250 mA h g^-1 ), which originates from transition metal (TM) ion redox reactions and unconventional oxygen anion redox reactions. However, many issues need to be addressed before their practical applications, such as their low kinetic properties and inefficient voltage fading. The development of cutting-edge technologies has led to cognitive advances in theory and offer potential solutions to these problems. Herein, a recent in-depth understanding of the mechanisms and the frontier electrochemical research progress of Li-rich cathodes are reviewed. In addition, recent advances associated with various strategies to promote the performance and the development of modification methods are discussed. In particular, excluding Li-rich Mn-based (LRM) cathodes, other branches of the Li-rich cathode materials are also summarized. The consistent pursuit is to obtain energy storage devices with high capacity, reliable practicability, and absolute safety. The recent literature and ongoing efforts in this area are also described, which will create more opportunities and new ideas for the future development of Li-rich cathode materials.

Fe3+xCr3+2-xCr6+4O15: A High-Capacity Cathode Material Synthesized Using an Ion-Exchange Chromatographic Method for Li-Ion Batteries

An advanced ion-exchange method using resin was employed to produce a novel cathode material, Fe³⁺_xCr³⁺_2-xCr⁶⁺₄O₁₅ (0 ≤ x ≤ 2), where some of the Cr³⁺ ions at the octahedral sites of Cr₂O₅ were substituted with Fe³⁺ ions. The battery cell test and X-ray photoelectron spectroscopy analysis of Cr₂O₅, Cr₈O₂₁, and Fe_1.5Cr_4.5O₁₅ indicate a change in the capacity from 210 to 280 and 350 mA h g^-1 with a change in the Cr⁶⁺/Cr³⁺ atomic ratio from 2 to 3 and 8 for Cr₂O₅, Cr₈O₂₁, and Fe_1.5Cr_4.5O₁₅, respectively. The discharge capacity of the compound with the crystallographic formula Fe³⁺_1.5Cr³⁺_0.5(Cr⁶⁺O₄)₂(Cr⁶⁺₂O₇) is, by far, the highest reported capacity for transition metal oxide electrodes in the voltage range of 2.0-4.5 V vs Li⁺/Li⁰.

Challenges and Strategies toward Cathode Materials for Rechargeable Potassium-Ion Batteries

With increasing demand for grid-scale energy storage, potassium-ion batteries (PIBs) have emerged as promising complements or alternatives to commercial lithium-ion batteries owing to the low cost, natural abundance of potassium resources, the low standard reduction potential of potassium, and fascinating K⁺ transport kinetics in the electrolyte. However, the low energy density and unstable cycle life of cathode materials hamper their practical application. Therefore, cathode materials with high capacities, high redox potentials, and good structural stability are required with the advancement toward next-generation PIBs. To this end, understanding the structure-dependent intercalation electrochemistry and recognizing the existing issues relating to cathode materials are indispensable prerequisites. This review summarizes the recent advances of PIB cathode materials, including metal hexacyanometalates, layered metal oxides, polyanionic frameworks, and organic compounds, with an emphasis on the structural advantages of the K⁺ intercalation reaction. Moreover, major current challenges with corresponding strategies for each category of cathode materials are highlighted. Finally, future research directions and perspectives are presented to accelerate the development of PIBs and facilitate commercial applications. It is believed that this review will provide practical guidance for researchers engaged in developing next-generation advanced PIB cathode materials.

A Simple Gas-Solid Treatment for Surface Modification of Li-Rich Oxides Cathodes

Li-rich layered oxides with high capacity are expected to be the next generation of cathode materials. However, the irreversible and sluggish anionic redox reaction leads to the O₂ loss in the surface as well as the capacity and voltage fading. In the present study, a simple gas-solid treatment with ferrous oxalate has been proposed to uniformly coat a thin spinel phase layer with oxygen vacancy and simultaneously realize Fe-ion substitution in the surface. The integration of oxygen vacancy and spinel phase suppresses irreversible O₂ release, prevents electrolyte corrosion, and promotes Li-ion diffusion. In addition, the surface doping of Fe-ion can further stabilize the structure. Accordingly, the treated Feox-2 % cathode exhibits superior capacity retention of 86.4 % and 85.5 % at 1 C and 2 C to that (75.3 % and 75.0 %) of the pristine sample after 300 cycles, respectively. Then, the voltage fading is significantly suppressed to 0.0011 V per cycle at 2 C especially. The encouraging results may play a significant role in paving the practical application of Li-rich layered oxides cathode.