Effect of Fluorination on Lithium Transport and Short‐Range Order in Disordered‐Rocksalt‐Type Lithium‐Ion Battery Cathodes

Fluorine substitution is a critical enabler for improving the cycle life and energy density of disordered rocksalt (DRX) Li‐ion battery cathode materials which offer prospects for high energy density cathodes, without the reliance on limited mineral resources. Due to the strong Li–F interaction, fluorine also is expected to modify the short‐range cation order in these materials which is critical for Li‐ion transport. In this work, density functional theory and Monte Carlo simulations are combined to investigate the impact of Li–F short‐range ordering on the formation of Li percolation and diffusion in DRX materials. The modeling reveals that F substitution is always beneficial at sufficiently high concentrations and can, surprisingly, even facilitate percolation in compounds without Li excess, giving them the ability to incorporate more transition metal redox capacity and thereby higher energy density. It is found that for F levels below 15%, its effect can be beneficial or disadvantageous depending on the intrinsic short‐range order in the unfluorinated oxide, while for high fluorination levels the effects are always beneficial. Using extensive simulations, a map is also presented showing the trade‐off between transition‐metal capacity, Li‐transport, and synthetic accessibility, and two of the more extreme predictions are experimentally confirmed.     

Improved Cycling Performance of Li‐Excess Cation‐Disordered Cathode Materials upon Fluorine Substitution

The recent discovery of Li‐excess cation‐disordered rock salt cathodes has greatly enlarged the design space of Li‐ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn³⁺–Nb⁵⁺‐based cation‐disordered oxyfluorides, Li_1.2Mn³⁺_0.6+0.5xNb⁵⁺_0.2?0.5xO_2?xF_x (x = 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition‐metal redox (Mn³⁺/Mn⁴⁺) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn³⁺‐induced Jahn–Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn‐redox capacity to the overall capacity.     

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.     

A Fluorination Method for Improving Cation‐Disordered Rocksalt Cathode Performance

In Li‐rich cation‐disordered rocksalt oxide cathodes (DRX), partial fluorine substitution in the oxygen anion sublattice can increase the capacity contribution from transition‐metal (TM) redox while reducing that from the less reversible oxygen redox. To date, limited fluorination substitution has been achieved by introducing LiF precursor during the solid‐state synthesis. To take full advantage of the fluorination effect, however, a higher F content is desired. In the present study, the successful use of a fluorinated polymeric precursor is reported to increase the F solubility in DRX and the incorporation of F content up to 10–12.5 at% into the rocksalt lattice of a model Li‐Mn‐Nb‐O (LMNO) system, largely exceeding the 7.5 at% limit achieved with LiF synthesis. Higher F content in the fluorinated‐DRX (F‐DRX) significantly improves electrochemical performance, with a reversible discharge capacity of ≈255 mAh g^?1 achieved at 10 at% of F substitution. After 30 cycles, up to a 40% increase in capacity retention is achieved through the fluorination. The study demonstrates the feasibility of using a new and effective fluorination process to synthesize advanced DRX cathode materials.     

Anion Reactivity in Cation‐Disordered Rocksalt Cathode Materials: The Influence of Fluorine Substitution

The demand for high energy‐density, mass‐producible cathode materials has spurred the exploration of new material structures and compositions. Lithium‐excess, cation‐disordered rocksalt (DRX) materials are a new class of transition metal oxides that display high capacity and environmental friendly composition. These materials achieve their high capacities partially through oxygen redox, which leads to oxygen loss and detrimental reactivity with the electrolyte. It has previously been shown that oxygen loss can be suppressed by partial substitution of the lattice oxygen for fluorine, but the explicit mechanism behind this effect remains unknown. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are used to quantify the primary electrochemical reactions occurring during the first cycle in DRX materials. Comparing a DRX oxide and a DRX oxyfluoride, it is shown that fluorination limits oxygen redox and suppresses oxygen loss. Additionally, DEMS is coupled with fluoride‐scavenging to demonstrate that small amounts of fluorine dissolve from DRX oxyfluorides during the first cycle. Finally, these techniques are extended over the first several cycles, demonstrating that CO₂ evolution persists and fluoride dissolution continues to a diminishing extent during the first few cycles. These findings motivate surface modifications to control interfacial reactivity and improve long‐term cycling.     

Unraveling the Rapid Redox Behavior of Li‐Excess 3d‐Transition Metal Oxides for High Rate Capability

Li‐excess 3d‐transition metal layered oxides are promising candidates in high‐energy‐density cathode materials for improving the mileage of electric vehicles. However, their low rate capability has hindered their practical application. The lack of understanding about the redox reactions and migration behavior at high C‐rates make it difficult to design Li‐excess materials with high rate capability. In this study, the characteristics of the atomic behavior that is predominant at fast charge/discharge are investigated by comparing cation‐ordered and cation‐disordered materials using X‐ray absorption spectroscopy (XAS). The difference in the atomic arrangement determines the dominance of the transition metal/oxygen redox reaction and the variations in transition metal–oxygen hybridization. In‐depth electrochemical analysis is combined with operando XAS analysis to reveal electronically and structurally preferred atomic behavior when a redox reaction occurs between oxygen and each transition metal under fast charge/discharge conditions. This provides a fundamental insight into the improvement of rate capability. Furthermore, this work provides guidance for identifying high‐energy‐density materials with complex structural properties.     

Computational Design and Preparation of Cation‐Disordered Oxides for High‐Energy‐Density Li‐Ion Batteries

Cation‐disordered lithium‐excess metal oxides have recently emerged as a promising new class of high‐energy‐density cathode materials for Li‐ion batteries, but the exploration of disordered materials has been hampered by their vast and unexplored composition space. This study proposes a practical methodology for the identification of stable cation‐disordered rocksalts. Here, it is established that the efficient method, which makes use of special quasirandom structures, correctly predicts cation‐ordering strengths in agreement with accurate Monte‐Carlo simulations and experimental observations. By applying the approach to the composition space of ternary oxides with formula unit LiA_0.5B_0.5O₂ (A, B: transition metals), this study discovers a previously unknown cation‐disordered structure, LiCo_0.5Zr_0.5O₂, that may function as the basis for a new class of cation‐disordered cathode materials. This computational prediction is confirmed experimentally by solid‐state synthesis and subsequent characterization by powder X‐ray diffraction demonstrating the potential of the computational screening of large composition spaces for accelerating materials discovery.     

Ambient‐Pressure Relithiation of Degraded LixNi0.5Co0.2Mn0.3O2 (0 < x < 1) via Eutectic Solutions for Direct Regeneration of Lithium‐Ion Battery Cathodes

With the rapid growth of the lithium‐ion battery (LIBs) market, recycling and re‐use of end‐of‐life LIBs to reclaim lithium (Li) and transition metal (TM) resources (e.g., Co, Ni), as well as eliminating pollution from disposal of waste batteries, has become an urgent task. Here, for the first time the ambient‐pressure relithiation of degraded LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathodes via eutectic Li⁺ molten‐salt solutions is successfully demonstrated. Combining such a low‐temperature relithiation process with a well‐designed thermal annealing step, NCM523 cathode particles with significant Li loss (≈40%) and capacity degradation (≈50%) can be successfully regenerated to achieve their original composition and crystal structures, leading to effective recovery of their capacity, cycling stability, and rate capability to the levels of the pristine materials. Advanced characterization tools including atomic resolution electron microscopy imaging and electron energy loss spectroscopy are combined to demonstrate that NCM523's original layered crystal structure is recovered. For the first time, it is shown that layer‐to‐rock salt phase change on the surfaces and subsurfaces of the cathode materials can be reversed if lithium can be incorporated back to the material. The result suggests the great promise of using eutectic Li⁺ molten–salt solutions for ambient‐pressure relithiation to recycle and remanufacture degraded LIB cathode materials.     

Ti‐Gradient Doping to Stabilize Layered Surface Structure for High Performance High‐Ni Oxide Cathode of Li‐Ion Battery

High‐Ni layered oxide cathodes are considered to be one of the most promising cathodes for high‐energy‐density lithium‐ion batteries due to their high capacity and low cost. However, surfice residues, such as NiO‐type rock‐salt phase and Li₂CO₃, are often formed at the particle surface due to the high reactivity of Ni³⁺, and inevitably result in an inferior electrochemical performance, hindering the practical application. Herein, unprecedentedly clean surfaces without any surfice residues are obtained in a representative LiNi_0.8Co_0.2O₂ cathode by Ti‐gradient doping. High‐resolution transmission electron microscopy (TEM) reveals that the particle surface is composed of a disordered layered phase (≈6 nm in thickness) with the same rhombohedra structure as its interior. The formation of this disordered layered phase at the particle surface is electrochemically favored. It leads to the highest rate capacity ever reported and a superior cycling stability. First‐principles calculations further confirm that the excellent electrochemical performance has roots in the excellent chemical/structural stability of such a disordered layered structure, mainly arising from the improved robustness of the oxygen framework by Ti doping. This strategy of constructing the disordered layered phase at the particle surface could be extended to other high‐Ni layered transition metal oxides, which will contribute to the enhancement of their electrochemical performance.     

Fundamental Understanding and Material Challenges in Rechargeable Nonaqueous Li–O2 Batteries: Recent Progress and Perspective

Energy storage challenges have triggered growing interest in various battery technologies and electrocatalysis. As a particularly promising variety, the Li–O₂ battery with an extremely high energy density is of great significance, offering tremendous opportunities to improve cell performance via understanding catalytic mechanisms and the exploration of new materials. Furthermore, focus on nonaqueous electrolyte‐based Li–O₂ batteries has markedly intensified since there could be a higher probability of commercialization, compared to that of solid‐state or aqueous electrolytes. The recent advancements of the nonaqueous Li–O₂ battery in terms of fundamental understanding and material challenges, including electrolyte stability, water effect, and noncarbon cathode materials are summarized in this review. Further, the current status of water impact on discharge products, possible mechanisms, and parasitic reactions in nonaqueous electrolytes are reviewed for the first time. The key challenges of noncarbon oxygen electrode materials, such as noble metals and metal oxides‐based cathodes, transition metals, transition metal compounds (carbides, oxides) based cathodes as well as noncarbon supported catalysts are discussed. This review concludes with a perspective on future research directions for nonaqueous Li–O₂ batteries.     

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.     

New Lithium Salt Forms Interphases Suppressing Both Li Dendrite and Polysulfide Shuttling

Lithium–sulfur batteries (LSBs) are considered promising candidates for the next‐generation energy‐storage systems due to their high theoretical capacity and prevalent abundance of sulfur. Their reversible operation, however, encounters challenges from both the anode, where dendritic and dead Li‐metal form, and the cathode, where polysulfides dissolve and become parasitic shuttles. Both issues arise from the imperfection of interphases between electrolyte and electrode. Herein, a new lithium salt based on an imide anion with fluorination and unsaturation in its structure is reported, whose interphasial chemistries resolve these issues simultaneously. Lithium 1, 1, 2, 2, 3, 3‐hexafluoropropane‐1, 3‐disulfonimide (LiHFDF) forms highly fluorinated interphases at both anode and cathode surfaces, which effectively suppress formation of Li‐dendrites and dissolution/shuttling of polysulfides, and significantly improves the electrochemical reversibility of LSBs. In a broader context, this new Li salt offers a new perspective for diversified beyond Li‐ion chemistries that rely on a Li‐metal anode and active cathode materials.     

Lithium Polyacrylate (LiPAA) as an Advanced Binder and a Passivating Agent for High‐Voltage Li‐Ion Batteries

Intensive studies of an advanced energy material are reported and lithium polyacrylate (LiPAA) is proven to be a surprisingly unique, multifunctional binder for high‐voltage Li‐ion batteries. The absence of effective passivation at the interface of high‐voltage cathodes in Li‐ion batteries may negatively affect their electrochemical performance, due to detrimental phenomena such as electrolyte solution oxidation and dissolution of transition metal cations. A strategy is introduced to build a stable cathode–electrolyte solution interphase for LiNi_0.5Mn_1.5O₄ (LNMO) spinel high‐voltage cathodes during the electrode fabrication process by simply using LiPAA as the cathode binder. LiPAA is a superb binder due to unique adhesion, cohesion, and wetting properties. It forms a uniform thin passivating film on LNMO and conducting carbon particles in composite cathodes and also compensates Li‐ion loss in full Li‐ion batteries by acting as an extra Li source. It is shown that these positive roles of LiPAA lead to a significant improvement in the electrochemical performance (e.g., cycle life, cell impedance, and rate capability) of LNMO/graphite battery prototypes, compared with that obtained using traditional polyvinylidene fluoride (PVdF) binder for LNMO cathodes. In addition, replacing PVdF with LiPAA binder for LNMO cathodes offers better adhesion, lower cost, and clear environmental advantages.     

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.     

Thermodynamic Aspects of Cathode Coatings for Lithium‐Ion Batteries

Metal oxide cathode coatings are capable of scavenging the hydrofluoric acid (HF) (present in LiPF₆‐based electrolytes) and improving the electrochemical performance of Li‐ion batteries. Here, a first‐principles thermodynamic framework is introduced for designing cathode coatings that consists of four elements: i) HF‐scavenging enthalpies, ii) volumetric and iii) gravimetric HF‐scavenging capacities of the oxides, and iv) cyclable Li loss into coating components. 81 HF‐scavenging reactions involving binary s‐, p‐ and d‐block metal oxides and fluorides are enumerated and these materials are screened to find promising coatings based on attributes (i‐iv). The screen successfully produces known effective coating materials (e.g., Al₂O₃ and MgO), providing a validation of our framework. Using this design strategy, promising coating materials, such as trivalent oxides of d‐block transition metals Sc, Ti, V, Cr, Mn and Y, are predicted. Finally, a new protection mechanism that successful coating materials could provide by scavenging the wide bandgap and low Li ion conductivity LiF precipitates from the cathode surfaces is suggested.     

Ni and Co Segregations on Selective Surface Facets and Rational Design of Layered Lithium Transition‐Metal Oxide Cathodes

The chemical processes occurring on the surface of cathode materials during battery cycling play a crucial role in determining battery's performance. However, the understanding of such surface chemistry is far from clear due to the complexity of redox chemistry during battery charge/discharge. Through intensive aberration corrected STEM investigation on ten layered oxide cathode materials, two important findings on the pristine oxides are reported. First, Ni and Co show strong plane selectivity when building up their respective surface segregation layers (SSLs). Specifically, Ni‐SSL is exclusively developed on (200)_m facet in Li–Mn‐rich oxides (monoclinic C2/m symmetry) and on (012)_h facet in Mn–Ni equally rich oxides (hexagonal R‐3m symmetry), while Co‐SSL has a strong preference to (20?2)_m plane with minimal Co‐SSL also developed on some other planes in Li–Mn‐rich cathodes. Structurally, Ni‐SSLs tend to form spinel‐like lattice while Co‐SSLs are in a rock‐salt‐like structure. Second, by increasing Ni concentration in these layered oxides, Ni and Co SSLs can be suppressed and even eliminated. The findings indicate that Ni and Co SSLs are tunable through controlling particle morphology and oxide composition, which opens up a new way for future rational design and synthesis of cathode materials.     

Narrowing the Gap between Theoretical and Practical Capacities in Li‐Ion Layered Oxide Cathode Materials

Although layered lithium oxides have become the cathode of choice for state‐of‐the‐art Li‐ion batteries, substantial gaps remain between the practical and theoretical energy densities. With the aim of supporting efforts to close this gap, this work reviews the fundamental operating mechanisms and challenges of Li intercalation in layered oxides, contrasts how these challenges play out differently for different materials (with emphasis on Ni–Co–Al (NCA) and Ni–Mn–Co (NMC) alloys), and summarizes the extensive corpus of modifications and extensions to the layered lithium oxides. Particular emphasis is given to the fundamental mechanisms behind the operation and degradation of layered intercalation electrode materials as well as novel modifications and extensions, including Na‐ion and cation‐disordered materials.     

A New Spinel‐Layered Li‐Rich Microsphere as a High‐Rate Cathode Material for Li‐Ion Batteries

Li‐rich layered materials are considered to be the promising low‐cost cathodes for lithium‐ion batteries but they suffer from poor rate capability despite of efforts toward surface coating or foreign dopings. Here, spinel‐layered Li‐rich Li‐Mn‐Co‐O microspheres are reported as a new high‐rate cathode material for Li‐ion batteries. The synthetic procedure is relatively simple, involving the formation of uniform carbonate precursor under solvothermal conditions and its subsequent transformation to an assembled microsphere that integrates a spinel‐like component with a layered component by a heat treatment. When calcined at 700 °C, the amount of transition metal Mn and Co in the Li‐Mn‐Co‐O microspheres maintained is similar to at 800 °C, while the structures of constituent particles partially transform from 2D to 3D channels. As a consequence, when tested as a cathode for lithium‐ion batteries, the spinel‐layered Li‐rich Li‐Mn‐Co‐O microspheres obtained at 700 °C show a maximum discharge capacity of 185.1 mA h g^?1 at a very high current density of 1200 mA g^?1 between 2.0 and 4.6 V. Such a capacity is among the highest reported to date at high charge‐discharge rates. Therefore, the present spinel‐layered Li‐rich Li‐Mn‐Co‐O microspheres represent an attractive alternative to high‐rate electrode materials for lithium‐ion batteries.     

Mixed Metal Difluorides as High Capacity Conversion‐Type Cathodes: Impact of Composition on Stability and Performance

With the most recent development of ultrahigh capacity anodes, such as Li‐ or Si‐based anodes, metal fluorides hold promise as complementary high‐capacity conversion cathode materials for next‐generation energy storage devices. Despite their higher theoretical energy density compared to cells with sulfur cathodes, these materials have received dramatically less attention and little is understood about the origins of their electrochemical behavior. Here, the successful methodology to produce highly uniform size‐controlled mixed metal difluoride nanocomposites is reported. It is discovered that such materials undergo reduction in a single step with a reduction potential intermediate to those for the corresponding single‐metal difluorides and that a solid solution is reformed upon charging, which is advantageous for practical applications. For the first time the progressive formation of metal trifluorides upon repeated cycling of difluorides is reported. Systematic electrochemical measurements in combination with postmortem analyses lead to the conclusion that the cathode stability strongly depends on the ability to prevent formation and growth of a resistive cathode solid electrolyte interphase, which, in turn, strongly depends on the metal composition. This methodology and new findings will help to elucidate a path to developing metal fluoride–based commercial Li‐ion batteries and provide guidelines for material selection.     

Insights into the Effects of Electrolyte Composition on the Performance and Stability of FeF2 Conversion‐Type Cathodes

As an alternative to commercial Ni‐ and Co‐based intercalation‐type cathode materials, conversion‐type metal fluoride (MF_x) cathodes are attracting more interest due to their promises to increase cell‐level energy density when coupled with lithium (Li) or silicon (Si)‐based anodes. Among metal fluorides, iron fluorides (FeF₂ and FeF₃) are regarded as some of the most promising candidates due to their high capacity, moderately high potential and the very low cost of Fe. In this study, the impacts of electrolyte composition on the performance and stability of nanostructured FeF₂ cathodes are systematically investigated. Dramatic impacts of Li salt composition, Li salt concentration, solvent composition, and cycling potential range on the cathode's most critical performance parameters—stability, capacity, rate, and voltage hysteresis are discovered. In contrast to previous beliefs, it is observed that even if the Fe²⁺ cation dissolution could be avoided, the dissolution of F^? anions may still negatively affect cathode performance. Formation of the more favorable cathode solid electrolyte interface (CEI) is found to minimize both processes.