XANES Investigation of Dynamic Phase Transition in Olivine Cathode for Li‐Ion Batteries

Dynamic phase transformation in olivine LiFePO₄ involving formation of one or more intermediate or metastable phases is revealed by an in situ time‐resolved X‐ray absorption near edge structure (XANES) technique. The XANES spectra measured during relaxation immediately after the application of relatively high overpotentials, where metastable phases are expected, show a continuous shift of the Fe K‐edge toward higher energy. Surprisingly, the Fe K‐edge relaxes to higher energies after current interrupt regardless of whether the cell is being charged or discharged. This relaxation phenomenon is superimposed upon larger shifts in K‐edge due to changes in Fe^2+/Fe³⁺ ratio due to charging and discharging, and implies an intermediate phase of larger Fe? O bond length than any of the known crystalline phases. No intermediate crystalline phases are observed by X‐ray diffraction (XRD). A metastable amorphous phase formed during dynamic cycling and which structurally relaxes to the equilibrium crystalline phases over a time scale of about 10 min after cessation of charging/discharging current is consistent with the experimental observations.     

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.     

A High‐Energy NASICON‐Type Cathode Material for Na‐Ion Batteries

Over the last decade, Na‐ion batteries have been extensively studied as low‐cost alternatives to Li‐ion batteries for large‐scale grid storage applications; however, the development of high‐energy positive electrodes remains a major challenge. Materials with a polyanionic framework, such as Na superionic conductor (NASICON)‐structured cathodes with formula Na_xM₂(PO₄)₃, have attracted considerable attention because of their stable 3D crystal structure and high operating potential. Herein, a novel NASICON‐type compound, Na₄MnCr(PO₄)₃, is reported as a promising cathode material for Na‐ion batteries that deliver a high specific capacity of 130 mAh g^?1 during discharge utilizing high‐voltage Mn^2+/3+ (3.5 V), Mn^3+/4+ (4.0 V), and Cr^3+/4+ (4.35 V) transition metal redox. In addition, Na₄MnCr(PO₄)₃ exhibits a high rate capability (97 mAh g^?1 at 5 C) and excellent all‐temperature performance. In situ X‐ray diffraction and synchrotron X‐ray diffraction analyses reveal reversible structural evolution for both charge and discharge.     

A Stable Layered Oxide Cathode Material for High‐Performance Sodium‐Ion Battery

As one of the most promising cathode candidates for room‐temperature sodium‐ion batteries (SIBs), P2‐type layered oxides face the challenge of simultaneously realizing high‐rate performance while achieving long cycle life. Here, a stable Na_2/3Ni_1/6Mn_2/3Cu_1/9Mg_1/18O₂ cathode material is proposed that consists of multiple‐layer oriented stacking nanoflakes, in which the nickel sites are partially substituted by copper and magnesium, a characteristic of the material that is confirmed by multiscale scanning transmission electron microscopy and electron energy loss spectroscopy techniques. Owing to the optimal morphology structure modulation and chemical element substitution strategy, the electrode displays remarkable rate performance (73% capacity retention at 30C compared to 0.5C) and outstanding cycling stability in Na half‐cell system couple with unprecedented full battery performance. The underlying thermal stability, phase stability, and Na⁺ storage mechanisms are clearly elucidated through the systematical characterizations of electrochemical behaviors, in situ X‐ray diffraction at different temperatures, and operando X‐ray diffraction upon Na⁺ deintercalation/intercalation. Surprisingly, a quasi‐solid‐solution reaction is switched to an absolute solid‐solution reaction and a capacitive Na⁺ storage mechanism is demonstrated via quantitative electrochemical kinetics calculation during charge/discharge process. Such a simple and effective strategy might reveal a new avenue into the rational design of excellent rate capability and long cycle stability cathode materials for practical SIBs.     

New O2/P2‐type Li‐Excess Layered Manganese Oxides as Promising Multi‐Functional Electrode Materials for Rechargeable Li/Na Batteries

A new and promising P2‐type layered oxide, Na_5/6[Li_1/4Mn_3/4]O₂ is prepared using a solid‐state method. Detailed crystal structures of the sample are analyzed by synchrotron X‐ray diffraction combined with high‐resolution neutron diffraction. P2‐type Na_5/6[Li_1/4Mn_3/4]O₂ consists of two MeO₂ layers with partial in‐plane √3a × √3a‐type Li/Mn ordering. Na/Li ion‐exchange in a molten salt results in a phase transition accompanied with glide of [Li_1/4Mn_3/4]O₂ layers without the destruction of in‐plane cation ordering. P2‐type Na_5/6[Li_1/4Mn_3/4]O₂ translates into an O2‐type layered structure with staking faults as the result of ion‐exchange. Electrode performance of P2‐type Na_5/6[Li_1/4Mn_3/4]O₂ and O2‐type Li_x[Li_1/4Mn_3/4]O₂ is examined and compared in Na and Li cells, respectively. Both samples show large reversible capacity, ca. 200 mA h g^?1, after charge to high voltage regardless of the difference in charge carriers. Structural analysis suggests that in‐plane structural rearrangements, presumably associated with partial oxygen loss, occur in both samples after charge to a high‐voltage region. Such structural activation process significantly influences electrode performance of the P2/O2‐type phases, similar to O3‐type Li₂MnO₃‐based materials. Crystal structures, phase‐transition mechanisms, and the possibility of the P2/O2‐type phases as high‐capacity and long‐cycle‐life electrode materials with the multi‐functionality for both rechargeable Li/Na batteries are discussed in detail.     

High‐Voltage Oxygen‐Redox‐Based Cathode for Rechargeable Sodium‐Ion Batteries

Recently, anionic‐redox‐based materials have shown promising electrochemical performance as cathode materials for sodium‐ion batteries. However, one of the limiting factors in the development of oxygen‐redox‐based electrodes is their low operating voltage. In this study, the operating voltage of oxygen‐redox‐based electrodes is raised by incorporating nickel into P2‐type Na_2/3[Zn_0.3Mn_0.7]O₂ in such a way that the zinc is partially substituted by nickel. As designed, the resulting P2‐type Na_2/3[(Ni_0.5Zn_0.5)_0.3Mn_0.7]O₂ electrode exhibits an average operating voltage of 3.5 V and retains 95% of its initial capacity after 200 cycles in the voltage range of 2.3–4.6 V at 0.1C (26 mA g^?1). Operando X‐ray diffraction analysis reveals the reversible phase transition: P2 to OP4 phase on charge and recovery to the P2 phase on discharge. Moreover, ex situ X‐ray absorption near edge structure and X‐ray photoelectron spectroscopy studies reveal that the capacity is generated by the combination of Ni²⁺/Ni⁴⁺ and O^2?/O^1? redox pairs, which is supported by first‐principles calculations. It is thought that this kind of high voltage redox species combined with oxygen redox could be an interesting approach to further increase energy density of cathode materials for not only sodium‐based rechargeable batteries, but other alkali‐ion battery systems.     

A Versatile Pyramidal Hauerite Anode in Congeniality Diglyme‐Based Electrolytes for Boosting Performance of Li‐ and Na‐Ion Batteries

For the first time, environmentally friendly sulfur‐rich pyramidal MnS₂ synthesized via a single‐step hydrothermal process is used as a high‐performance anode material in Li‐ion and Na‐ion batteries. The superior electrochemical performance of the MnS₂ electrode along with its high compatibility with ether‐based electrolytes are analyzed in both half‐ and full‐cell configurations. The reversible capacities of ≈84 mAh g^?1 and ≈74 mAh g^?1 at a current density of 50 mA g^?1 are retained in the Li‐ion and Na‐ion full‐cells, respectively, over 200 cycles with excellent capacity retentions. Moreover, important findings regarding activation processes in the presence of a new phase transition and protective electrolyte interphase layer are revealed using ab initio density function theory calculation and in situ potentio‐electrochemical impedance spectroscopy. The detailed complex redox mechanism of MnS₂ in Li/Na half‐cells is also elucidated by ex situ X‐ray photoelectron spectroscopy.     

Elucidating the Effect of Iron Doping on the Electrochemical Performance of Cobalt‐Free Lithium‐Rich Layered Cathode Materials

The eco‐friendly and low‐cost Co‐free Li_1.2Mn_0.585Ni_0.185Fe_0.03O₂ is investigated as a positive material for Li‐ion batteries. The electrochemical performance of the 3 at% Fe‐doped material exhibits an optimal performance with a capacity and voltage retention of 70 and 95%, respectively, after 200 cycles at 1C. The effect of iron doping on the electrochemical properties of lithium‐rich layered materials is investigated by means of in situ X‐ray diffraction spectroscopy and galvanostatic intermittent titration technique during the first charge–discharge cycle while high‐resolution transmission electron microscopy is used to follow the structural and chemical change of the electrode material upon long‐term cycling. By means of these characterizations it is concluded that iron doping is a suitable approach for replacing cobalt while mitigating the voltage and capacity degradation of lithium‐rich layered materials. Finally, complete lithium‐ion cells employing Li_1.2Mn_0.585Ni_0.185Fe_0.03O₂ and graphite show a specific energy of 361 Wh kg^?1 at 0.1C rate and very stable performance upon cycling, retaining more than 80% of their initial capacity after 200 cycles at 1C rate. These results highlight the bright prospects of this material to meet the high energy density requirements for electric vehicles.     

Hollow CuS Nanoboxes as Li‐Free Cathode for High‐Rate and Long‐Life Lithium Metal Batteries

Transition metal sulfides hold promising potentials as Li‐free conversion‐type cathode materials for high energy density lithium metal batteries. However, the practical deployment of these materials is hampered by their poor rate capability and short cycling life. In this work, the authors take the advantage of hollow structure of CuS nanoboxes to accommodate the volume expansion and facilitate the ion diffusion during discharge–charge processes. As a result, the hollow CuS nanoboxes achieve excellent rate performance (≈371 mAh g^?1 at 20 C) and ultra‐long cycle life (>1000 cycles). The structure and valence evolution of the CuS nanobox cathode are identified by scanning electron microscopy, transmission electron microscopy, and X‐ray photoelectron spectroscopy. Furthermore, the lithium storage mechanism is revealed by galvanostatic intermittent titration technique and operando Raman spectroscopy for the initial charge–discharge process and the following reversible processes. These results suggest that the hollow CuS nanobox material is a promising candidate as a low‐cost Li‐free cathode material for high‐rate and long‐life lithium metal batteries.     

An Air‐Stable and Dendrite‐Free Li Anode for Highly Stable All‐Solid‐State Sulfide‐Based Li Batteries

Li metal is a promising anode material for all‐solid‐state batteries, owing to its high specific capacity and low electrochemical potential. However, direct contact of Li metal with most solid‐state electrolytes induces severe side reactions that can lead to dendrite formation and short circuits. Moreover, Li metal is unstable when exposed to air, leading to stringent processing requirements. Herein, it is reported that the Li₃PS₄/Li interface in all‐solid‐state batteries can be stabilized by an air‐stable Li_xSiS_y protection layer that is formed in situ on the surface of Li metal through a solution‐based method. Highly stable Li cycling for over 2000 h in symmetrical cells and a lifetime of over 100 cycles can be achieved for an all‐solid‐state LiCoO₂/Li₃PS₄/Li cell. Synchrotron‐based high energy X‐ray photoelectron spectroscopy in‐depth analysis demonstrates the distribution of different components within the protection layer. The in situ formation of an electronically insulating Li_xSiS_y protection layer with highly ionic conductivity provides an effective way to prevent Li dendrite formation in high‐energy all‐solid‐state Li metal batteries.     

Study of Thermal Decomposition of Li1‐x(Ni1/3Mn1/3Co1/3)0.9O2 Using In‐Situ High‐Energy X‐Ray Diffraction

Safety has been a major technological concern hindering the deployment of lithium‐ion batteries for automobile applications. We investigated the decomposition mechanism of delithiated cathode materials at thermal abuse conditions using Li_1.1[Ni_1/3Mn_1/3Co_1/3]_0.9O₂ as a model cathode material. An in‐situ high‐energy X‐ray diffraction technique was established as an alternative to conventional thermal analysis techniques like differential scanning calorimetry and accelerating rate calorimetry. The X‐ray diffraction data revealed that the thermal decomposition pathway of delithiated Li_1‐x[Ni_1/3Mn_1/3Co_1/3]_0.9O₂ strongly depended on the exposed chemical environment, like solvents and lithium salts. A phase transformation of dry delithiated Li_1‐x[Ni_1/3Mn_1/3Co_1/3]_0.9O₂ was observed at about 278 °C, and its onset temperature was reduced to about 197°C with the presence of the electrolyte. It is suggested that the reduction in thermal stability is possibly related to proton intercalation into the delithiated material.     

Understanding the Rate Capability of High‐Energy‐Density Li‐Rich Layered Li1.2Ni0.15Co0.1Mn0.55O2 Cathode Materials

The high‐energy‐density, Li‐rich layered materials, i.e., xLiMO₂(1‐x)Li₂MnO₃, are promising candidate cathode materials for electric energy storage in plug‐in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). The relatively low rate capability is one of the major problems that need to be resolved for these materials. To gain insight into the key factors that limit the rate capability, in situ X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) studies of the cathode material, Li_1.2Ni_0.15Co_0.1Mn_0.55O₂ [0.5Li(Ni_0.375Co_0.25 Mn_0.375)O₂·0.5Li₂MnO₃], are carried out. The partial capacity contributed by different structural components and transition metal elements is elucidated and correlated with local structure changes. The characteristic reaction kinetics for each element are identified using a novel time‐resolved XAS technique. Direct experimental evidence is obtained showing that Mn sites have much poorer reaction kinetics both before and after the initial activation of Li₂MnO₃, compared to Ni and Co. These results indicate that Li₂MnO₃ may be the key component that limits the rate capability of Li‐rich layered materials and provide guidance for designing Li‐rich layered materials with the desired balance of energy density and rate capability for different applications.     

Solid Solution Domains at Phase Transition Front of LixNi0.5Mn1.5O4

Nickel‐substituted manganese spinel LiNi_0.5Mn_1.5O₄ (LNMO) is a promising 5 V class positive electrode material for lithium‐ion batteries. As micron‐sized LNMO particles show high rate capability in its two‐phase coexistence regions, the phase transition mechanism is of great interest in understanding the electrode behavior at high rates. Here, the phase transition dynamics of Li_xNi_0.5Mn_1.5O₄ is elucidated on high rate charging–discharging using operando time‐resolved X‐ray diffraction (TR‐XRD). The TR‐XRD results indicate the existence of intermediate states, in addition to the thermodynamically stable phases, and it is shown that the origin of such intermediate states is ascribed to the solid‐solution domains at the phase transition front, as supported by the analysis using transmission electron microscopy coupled with electron energy‐loss spectroscopy. The phase transition pathways dependent on the reaction rate are shown, together with possible explanation for this unique transition behavior.     

Hollandite‐Type VO1.75(OH)0.5: Effective Sodium Storage for High‐Performance Sodium‐Ion Batteries

Nanosized hollandite‐type VO_1.75(OH)_0.5 is introduced as a novel cathode material for Na‐ion batteries. Structural investigation based on X‐ray diffraction and Rietveld refinement suggests the presence of numerous vacant sites for Na⁺ intercalation in the VO_1.75(OH)_0.5 structure. All of the possible Na⁺ sites and tunnel‐type Na⁺ diffusion pathways along the c‐axis are confirmed by bond‐valence‐sum analyses. The nanosized hollandite‐type VO_1.75(OH)_0.5 delivers an unexpectedly high specific capacity of ≈351 mAh g^?1 at 15.5 mA g^?1 in the voltage range of 1.0–3.7 V (vs Na⁺/Na), which agrees well with the results predicted by first‐principles calculations. In addition, combined studies using first‐principles calculations and several experimental techniques including in situ operando X‐ray diffraction and ex situ X‐ray absorption spectroscopy confirm that the nanosized hollandite‐type VO_1.75(OH)_0.5 undergoes a single‐phase reaction with a capacity retention of 71% over 200 cycles. Furthermore, the open structure and nanosized particles of hollandite‐type VO_1.75(OH)_0.5 contribute to its excellent power capability with 56% of the capacity measured at 0.05 C being delivered at 7 C.     

Direct Observation of Li‐Ion Transport in Electrodes under Nonequilibrium Conditions Using Neutron Depth Profiling

One of the key challenges of Li‐ion electrodes is enhancement of (dis)charge rates. This is severely hindered by the absence of a technique that allows direct and nondestructive observation of lithium ions in operating batteries. Direct observation of the Li‐ion concentration profiles using operando neutron depth profiling reveals that the rate‐limiting step is depended not only on the electrode morphology but also on the cycling rate itself. In the LiFePO₄ electrodes phase nucleation limits the charge transport at the lowest cycling rates, whereas electronic conductivity is rate limiting at intermediate rates, and only at the highest rates ionic transport through the electrode is rate limiting. These novel insights into electrode kinetics are imperative for the improvement of Li‐ion batteries and show the large value of in situ NDP in Li‐ion battery research and development.     

Structure Distortion Induced Monoclinic Nickel Hexacyanoferrate as High‐Performance Cathode for Na‐Ion Batteries

Prussian blue analogs with an open framework are ideal cathodes for Na‐ion batteries. A superior high‐rate and highly stable monoclinic nickel hexacyanoferrate (NiHCF‐3) is synthesized via a facile one‐step crystallization‐controlled co‐precipitation method. It gives a high specific capacity of 85.7 mAh g^?1, nearly to its theoretical value. It also exhibits an excellent rate capability with a high capacity retention ratio of 78% at 50 C and a stable cycling performance over 1200 cycles. Through the ex situ X‐ray diffraction and pair distribution function measurements, it is found that the monoclinic structure with distorted framework is greatly related to the high Na content. The electronic structure studies by density functional theory (DFT) calculation demonstrate that NiHCF‐3 deformation promotes the framework conductivity and improves the electrochemical activity of Fe, which results in an ultrahigh‐rate performance of monoclinic phase. Furthermore, the high‐quality monoclinic (NiHCF‐3) exhibits excellent compatibility with both hard carbon and NaTi₂(PO₄)₃ anodes in full cells, which shows great prospects for the application in the large‐scale energy storage systems.     

Solid‐State Prelithiation Enables High‐Performance Li‐Al‐H Anode for Solid‐State Batteries

Lithium alanates exhibit high theoretical specific capacities and appropriate lithiation/delithiation potentials, but suffer from poor reversibility, cycling stability, and rate capability due to their sluggish kinetics and extensive side reactions. Herein, a novel and facile solid‐state prelithiation approach is proposed to in situ prepare a Li₃AlH₆‐Al nanocomposite from a short‐circuited electrochemical reaction between LiAlH₄ and Li with the help of fast electron and Li‐ion conductors (C and P6₃mc LiBH₄). This nanocomposite consists of dispersive Al nanograins and an amorphous Li₃AlH₆ matrix, which enables superior electrochemical performance in solid‐state cells, as much higher specific capacity (2266 mAh g^?1), Coulombic efficiency (88%), cycling stability (71% retention in the 100th cycle), and rate capability (1429 mAh g^?1 at 1 A g^?1) are achieved. In addition, this nanocomposite works well in the solid‐state full cell with LiCoO₂ cathode, demonstrating its promising application prospects. Mechanism analysis reveals that the dispersive Al nanograins and amorphous Li₃AlH₆ matrix can dramatically enhance the lithiation and delithiation kinetics without side reactions, which is mainly responsible for the excellent overall performance. Moreover, this solid‐state prelithiation approach is general and can also be applied to other Li‐poor electrode materials for further modification of their electrochemical behavior.     

Cooling Induced Surface Reconstruction during Synthesis of High‐Ni Layered Oxides

Transition metal layered oxides have been the dominant cathodes in lithium‐ion batteries, and among them, high‐Ni ones (LiNi_xMn_yCo_zO₂; x ≥ 0.7) with greatly boosted capacity and reduced cost are of particular interest for large‐scale applications. The high Ni loading, on the other hand, raises the critical issues of surface instability and poor rate performance. The rational design of synthesis leading to layered LiNi_0.7Mn_0.15Co_0.15O₂ with greatly enhanced rate capability is demonstrated, by implementing a quenching process alternative to the general slow cooling. In situ synchrotron X‐ray diffraction, coupled with surface analysis, is applied to studies of the synthesis process, revealing cooling‐induced surface reconstruction involving Li₂CO₃ accumulation, formation of a Li‐deficient layer and Ni reduction at the particle surface. The reconstruction process occurs predominantly at high temperatures (above 350 °C) and is highly cooling‐rate dependent, implying that surface reconstruction can be suppressed through synthetic control, i.e., quenching to improve the surface stability and rate performance of the synthesized materials. These findings may provide guidance to rational synthesis of high‐Ni cathode materials.     

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.     

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.