Quantifying Degradation Parameters of Single-Crystalline Ni-Rich Cathodes in Lithium-Ion Batteries.

Single-crystal LiNix Coy Mnz O2 (SC-NCM, x+y+z=1) cathodes are renowned for their high structural stability and reduced accumulation of adverse side products during long-term cycling. While advances have been made using SC-NCM cathode materials, careful studies of cathode degradation mechanisms are scarce. Herein, we employed quasi single-crystalline LiNi0.65 Co0.15 Mn0.20 O2 (SC-NCM65) to test the relationship between cycling performance and material degradation for different charge cutoff potentials. The Li/SC-NCM65 cells showed >77 % capacity retention below 4.6 V vs. Li+ /Li after 400 cycles and revealed a significant decay to 56 % for 4.7 V cutoff. We demonstrate that the SC-NCM65 degradation is due to accumulation of rock-salt (NiO) species at the particle surface rather than intragranular cracking or side reactions with the electrolyte. The NiO-type layer formation is also responsible for the strongly increased impedance and transition-metal dissolution. Notably, the capacity loss is found to have a linear relationship with the thickness of the rock-salt surface layer. Density functional theory and COMSOL Multiphysics modeling analysis further indicate that the charge-transfer kinetics is decisive, as the lower lithium diffusivity of the NiO phase hinders charge transport from the surface to the bulk.

Gas induced formation of inactive Li in rechargeable lithium metal batteries.

The formation of inactive lithium by side reactions with liquid electrolyte contributes to cell failure of lithium metal batteries. To inhibit the formation and growth of inactive lithium, further understanding of the formation mechanisms and composition of inactive lithium are needed. Here we study the impact of gas producing reactions on the formation of inactive lithium using ethylene carbonate as a case study. Ethylene carbonate is a common electrolyte component used with graphite-based anodes but is incompatible with Li metal anodes. Using mass spectrometry titrations combined with 13C and 2H isotopic labeling, we reveal that ethylene carbonate decomposition continuously releases ethylene gas, which further reacts with lithium metal to form the electrochemically inactive species LiH and Li2C2. In addition, phase-field simulations suggest the non-ionically conducting gaseous species could result in an uneven distribution of lithium ions, detrimentally enhancing the formation of dendrites and dead Li. By optimizing the electrolyte composition, we selectively suppress the formation of ethylene gas to limit the formation of LiH and Li2C2 for both Li metal and graphite-based anodes.

Understanding the failure process of sulfide-based all-solid-state lithium batteries via operando nuclear magnetic resonance spectroscopy.

The performance of all-solid-state lithium metal batteries (SSLMBs) is affected by the presence of electrochemically inactive (i.e., electronically and/or ionically disconnected) lithium metal and solid electrolyte interphase (SEI), which are jointly termed inactive lithium. However, the differentiation and quantification of inactive lithium during cycling are challenging, and their lack limits the fundamental understanding of SSLMBs failure mechanisms. To shed some light on these crucial aspects, here, we propose operando nuclear magnetic resonance (NMR) spectroscopy measurements for real-time quantification and evolution-tracking of inactive lithium formed in SSLMBs. In particular, we examine four different sulfide-based solid electrolytes, namely, Li10GeP2S12, Li9.54Si1.74P1.44S11.7Cl0.3, Li6PS5Cl and Li7P3S11. We found that the chemistry of the solid electrolyte influences the activity of lithium. Furthermore, we demonstrate that electronically disconnected lithium metal is mainly found in the interior of solid electrolytes, and ionically disconnected lithium metal is found at the negative electrode surface. Moreover, by monitoring the Li NMR signal during cell calendar ageing, we prove the faster corrosion rate of mossy/dendritic lithium than flat/homogeneous lithium in SSLMBs.

Sieving carbons promise practical anodes with extensible low-potential plateaus for sodium batteries.

Non-graphitic carbons are promising anode candidates for sodium-ion batteries, while their variable and complicated microstructure severely limits the rational design of high-energy carbon anodes that could accelerate the commercialization of sodium-ion batteries, as is the case for graphite in lithium-ion batteries. Here, we propose sieving carbons, featuring highly tunable nanopores with tightened pore entrances, as high-energy anodes with extensible and reversible low-potential plateaus (<0.1 V). It is shown that the tightened pore entrance blocks the formation of the solid electrolyte interphase inside the nanopores and enables sodium clustering to produce the plateau. Theoretical and spectroscopic studies also show that creating a larger area of sodiophilic pore surface leads to an almost linearly increased number of sodium clusters, and controlling the pore body diameter guarantees the reversibility of sodium cluster formation, producing a sieving carbon anode with a record-high plateau capacity of 400 mAh g-1. More excitingly, this approach to preparing sieving carbons has the potential to be scalable for modifying different commercial porous carbons.

A machine learning protocol for revealing ion transport mechanisms from dynamic NMR shifts in paramagnetic battery materials.

Solid-state nuclear magnetic resonance (ssNMR) provides local environments and dynamic fingerprints of alkali ions in paramagnetic battery materials. Linking the local ionic environments and NMR signals requires expensive first-principles computational tools that have been developed for over a decade. Nevertheless, the assignment of the dynamic NMR spectra of high-rate battery materials is still challenging because the local structures and dynamic information of alkali ions are highly correlated and difficult to acquire. Herein, we develop a novel machine learning (ML) protocol that could not only quickly sample atomic configurations but also predict chemical shifts efficiently, which enables us to calculate dynamic NMR shifts with the accuracy of density functional theory (DFT). Using structurally well-defined P2-type Na2/3(Mg1/3Mn2/3)O2 as an example, we validate the ML protocol and show the significance of dynamic effects on chemical shifts. Moreover, with the protocol, it is demonstrated that the two experimental 23Na shifts (1406 and 1493 ppm) of P2-type Na2/3(Ni1/3Mn2/3)O2 originate from two stacking sequences of transition metal (TM) layers for the first time, which correspond to space groups P63/mcm and P6322, respectively. This ML protocol could help to correlate dynamic ssNMR spectra with the local structures and fast transport of alkali ions and is expected to be applicable to a wide range of fast dynamic systems.

Mitigating the Surface Reconstruction of Ni-Rich Cathode via P2-Type Mn-Rich Oxide Coating for Durable Lithium Ion Batteries.

Ni-rich materials have received widespread attention as one of the mainstream cathodes in high-energy-density lithium-ion batteries for electric vehicles. However, Ni-rich cathodes suffer from severe surface reconstruction in a high delithiation state, constraining their rate capabilities and life span. Herein, a novel P2-type NaxNi0.33Mn0.67O2 (NNMO) is rationally selected as the surficial modification layer for LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode, which undergoes a spontaneous Na+-Li+ exchange reaction to form an O2-type LixNi0.33Mn0.67O2 (LNMO) layer revealed by combining X-ray diffraction and solid-state nuclear magnetic resonance techniques. Owing to the specific oxygen stacking sequence, O2-type LNMO significantly prevents the initial layered structure of NCM811 from transforming to the spinel or rock-salt phases during cycling, thus effectively maintaining the integral surficial structure and the Li+ diffusion channels of NCM811. Eventually, the NNMO@NCM811 electrode yields enhanced thermal stability, outstanding rate performance, and long cycling stability with 80% capacity retention after 294 cycles at 200 mA g-1, and its life span is further extended to 531 cycles while enhancing the mechanical stability of the bulk material.

Enabling Fast Na+ Transfer Kinetics in the Whole-Voltage-Region of Hard-Carbon Anodes for Ultrahigh-Rate Sodium Storage.

Efficient electrode materials, that combine high power and high energy, are the crucial requisites of sodium-ion batteries (SIBs), which have unwrapped new possibilities in the areas of grid-scale energy storage. Hard carbons (HCs) are considered as the leading candidate anode materials for SIBs, however, the primary challenge of slow charge-transfer kinetics at the low potential region (<0.1 V) remains unresolved till date, and the underlying structure-performance correlation is under debate. Herein, ultrafast sodium storage in the whole-voltage-region (0.01-2 V), with the Na+ diffusion coefficient enhanced by 2 orders of magnitude (≈10-7 cm2 s-1 ) through rationally deploying the physical parameters of HCs using a ZnO-assisted bulk etching strategy is reported. It is unveiled that the Na+ adsorption energy (Ea ) and diffusion barrier (Eb ) are in a positive and negative linear relationship with the carbon p-band center, respectively, and balance of Ea and Eb is critical in enhancing the charge-storage kinetics. The charge-storage mechanism in HCs is evidenced through comprehensive in(ex) situ techniques. The as prepared HCs microspheres deliver a record high rate performance of 107 mAh g-1 @ 50 A g-1 and unprecedented electrochemical performance at extremely low temperature (426 mAh g-1 @ -40 °C).

Size-Dependent Chemomechanical Failure of Sulfide Solid Electrolyte Particles during Electrochemical Reaction with Lithium.

The very high ionic conductivity of Li10GeP2S12 (LGPS) solid electrolyte (SE) makes it a promising candidate SE for solid-state batteries in electrical vehicles. However, chemomechanical failure, whose mechanism remains unclear, has plagued its widespread applications. Here, we report in situ imaging lithiation-induced failure of LGPS SE. We revealed a strong size effect in the chemomechanical failure of LGPS particles: namely, when the particle size is greater than 3 µm, fracture/pulverization occurred; when the particle size is between 1 and 3 µm, microcracks emerged; when the particle size is less than 1 µm, no chemomechanical failure was observed. This strong size effect is interpreted by the interplay between elastic energy storage and dissipation. Our finding has important implications for the design of high-performance LGPS SE, for example, by reducing the particle size to less than 1 µm the chemomechanical failure of LGPS SE can be mitigated.

Quantitatively analyzing the failure processes of rechargeable Li metal batteries.

Practical use of lithium (Li) metal for high­energy density lithium metal batteries has been prevented by the continuous formation of Li dendrites, electrochemically isolated Li metal, and the irreversible formation of solid electrolyte interphases (SEIs). Differentiating and quantifying these inactive Li species are key to understand the failure mode. Here, using operando nuclear magnetic resonance (NMR) spectroscopy together with ex situ titration gas chromatography (TGC) and mass spectrometry titration (MST) techniques, we established a solid foundation for quantifying the evolution of dead Li metal and SEI separately. The existence of LiH is identified, which causes deviation in the quantification results of dead Li metal obtained by these three techniques. The formation of inactive Li under various operating conditions has been studied quantitatively, which revealed a general "two-stage" failure process for the Li metal. The combined techniques presented here establish a benchmark to unravel the complex failure mechanism of Li metal.

Insights of the Electrochemical Reversibility of P2-Type Sodium Manganese Oxide Cathodes via Modulation of Transition Metal Vacancies.

Among cathode materials for sodium-ion batteries, Mn-based layered oxides have attracted enormous attention owing to their high capacity, cost-effectiveness, and fast transport channels. However, their practical application is hindered by the unsatisfied structural stability and the deficient understanding of electrochemical reaction mechanisms. Among these issues, the research of transition metal (TM) vacancy remains highly active due to their modulation roles on the anionic redox reactions, but their effects on structural and electrochemical stability remain obscure. Herein, based on Al-substituted P2-type Na2/3MnO2, we comprehensively investigate the effects of TM vacancies on the corresponding layered oxides. With several characterization techniques such as neutron diffraction, superconducting quantum interferometry, in situ X-ray diffraction, ex situ solid-state nuclear magnetic resonance techniques, and X-ray photoelectron spectroscopy, we determined the TM vacancy content and further revealed that higher content of TM vacancies (7.8%) in the transition layer is beneficial to mitigate the structure evolutions and maintain the P2 structure during cycling in voltage range 1.5-4.5 V, while the oxides with lower content of TM vacancies (1.6%) deliver higher discharge capacity but experience complicated phase transition, including stacking faults and P2-P2' transitions. It is demonstrated that regulating the contents of TM vacancies can be utilized as an effective strategy to tune the structure stability and electrochemical performances of layered sodium oxide cathodes.

Engineering Na+-layer spacings to stabilize Mn-based layered cathodes for sodium-ion batteries.

Layered transition metal oxides are the most important cathode materials for Li/Na/K ion batteries. Suppressing undesirable phase transformations during charge-discharge processes is a critical and fundamental challenge towards the rational design of high-performance layered oxide cathodes. Here we report a shale-like NaxMnO2 (S-NMO) electrode that is derived from a simple but effective water-mediated strategy. This strategy expands the Na+ layer spacings of P2-type Na0.67MnO2 and transforms the particles into accordion-like morphology. Therefore, the S-NMO electrode exhibits improved Na+ mobility and near-zero-strain property during charge-discharge processes, which leads to outstanding rate capability (100 mAh g-1 at the operation time of 6 min) and cycling stability (>3000 cycles). In addition, the water-mediated strategy is feasible to other layered sodium oxides and the obtained S-NMO electrode has an excellent tolerance to humidity. This work demonstrates that engineering the spacings of alkali-metal layer is an effective strategy to stabilize the structure of layered transition metal oxides.

Constructing a High-Energy and Durable Single-Crystal NCM811 Cathode for All-Solid-State Batteries by a Surface Engineering Strategy.

Single-crystal LiNi0.8Co0.1Mn0.1O2 (S-NCM811) with an electrochemomechanically compliant microstructure has attracted great attention in all-solid-state batteries (ASSBs) for its superior electrochemical performance compared to the polycrystalline counterpart. However, the undesired side reactions on the cathode/solid-state electrolyte (SSE) interface causes inferior capacity and rate capability than lithium-ion batteries, limiting the practical application of S-NCM811 in the ASSB technology. Herein, it shows that S-NCM811 delivers a high capacity (205 mAh g-1, 0.1C) with outstanding rate capability (175 mAh g-1 at 0.3C and 116 mAh g-1 at 1C) in ASSBs by the coating of a nano-lithium niobium oxide (LNO) layer via the atomic layer deposition technique combined with optimized post-annealing treatment. The working mechanism is verified as the nano-LNO layer effectively suppresses the decomposition of sulfide SSE and stabilizes the cathode/SSE interface. The post-annealing of the LNO layer at 400 °C improves the coating uniformity, eliminates the residual lithium salts, and leads to small impedance increasing and less electrochemical polarization during cycling compared with pristine materials. This work highlights the critical role of the post-annealed nano-LNO layer in the applications of a high-nickel cathode and offers some new insights into the designing of high-performance cathode materials for ASSBs.

O3-Type NaCrO2 as a Superior Cathode Material for Sodium/Potassium-Ion Batteries Ensured by High Structural Reversibility.

O3-type NaCrO2 is attracting increasing attention as potential cathode material for sodium-ion batteries (SIBs). Bare NaCrO2 is usually synthesized by a solid-state reaction and suffers from serious capacity decay and poor power capability. Modification by coating is an effective method to improve the electrochemical properties, but it inevitably reduces the energy density. To avoid the decrease of energy density and optimize the electrochemical performance, a specific route, i.e., a freeze-drying-assisted sol-gel method, has been adopted to synthesize bare NaCrO2 in this work. Three-phase coexistence during charging is confirmed for the first time, which contributes to delaying the disappearance of the O3 phase and then improving the structural reversibility, resulting in superior cycle stability (∼50% capacity retention after 3000 cycles at 5C). Meanwhile, as-synthesized NaCrO2 delivers an outstanding rate capability (82.1 mAh g-1 at 50C), which is attributed to the fast Na+ diffusivity and high electronic conductivity proved by density functional theory (DFT) calculations. It is worth mentioning that NaCrO2 also exhibits excellent electrochemical properties when used as a cathode for potassium-ion batteries (PIBs). This work provides new perspectives on the structural evolution of NaCrO2, and the results are expected to contribute to the development of SIBs and PIBs.

Solid-State NMR and MRI Spectroscopy for Li/Na Batteries: Materials, Interface, and In Situ Characterization.

Enhancing the electrochemical performance of batteries, including the lifespan, energy, and power densities, is an everlasting quest for the rechargeable battery community. However, the dynamic and coupled (electro)chemical processes that occur in the electrode materials as well as at the electrode/electrolyte interfaces complicate the investigation of their working and decay mechanisms. Herein, the recent developments and applications of solid-state nuclear magnetic resonance (ssNMR) and magnetic resonance imaging (MRI) techniques in Li/Na batteries are reviewed. Several typical cases including the applications of NMR spectroscopy for the investigation of the pristine structure and the dynamic structural evolution of materials are first emphasized. The NMR applications in analyzing the solid electrolyte interfaces (SEI) on the electrode are further concluded, involving the identification of SEI components and investigation of ionic motion through the interfaces. Beyond, the new development of in situ NMR and MRI techniques are highlighted, including their advantages, challenges, applications and the design principle of in situ cell. In the end, a prospect about how to use ssNMR in battery research from the perspectives of materials, interface, and in situ NMR, aiming at obtaining deeper insight of batteries with the assistance of ssNMR is represented.

Unravelling the Fast Alkali-Ion Dynamics in Paramagnetic Battery Materials Combined with NMR and Deep-Potential Molecular Dynamics Simulation.

Solid-state nuclear magnetic resonance (ssNMR) has received extensive attention in characterizing alkali-ion battery materials because it is highly sensitive for probing the local environment and dynamic information of atoms/ions. However, precise spectral assignment cannot be carried out by conventional DFT for high-rate battery materials at room temperature. Herein, combining DFT calculation of paramagnetic shift and deep potential molecular dynamics (DPMD) simulation to achieve the converged Na+ distribution at hundreds of nanoseconds, we obtain the statistically averaged paramagnetic shift, which is in excellent agreement with ssNMR measurements. Two 23 Na shifts induced by different stacking sequences of transition metal layers are revealed in the fast chemically exchanged NMR spectra of P2-type Na2/3 (Mg1/3 Mn2/3 )O2 for the first time. This DPMD simulation auxiliary protocol can be beneficial to a wide range of ssNMR analysis in fast chemically exchanged material systems.

Visualizing the growth process of sodium microstructures in sodium batteries by in-situ 23Na MRI and NMR spectroscopy.

The growth of sodium dendrites and the associated solid electrolyte interface (SEI) layer is a critical and fundamental issue influencing the safety and cycling lifespan of sodium batteries. In this work, we use in-situ 23Na magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) techniques, along with an innovative analytical approach, to provide space-resolved and quantitative insights into the formation and evolution of sodium metal microstructures (SMSs; that is, dendritic and mossy Na metal) during the deposition and stripping processes. Our results reveal that the growing SMSs give rise to a linear increase in the overpotential until a transition voltage of 0.15 V is reached, at which point violent electrochemical decomposition of the electrolyte is triggered, leading to the formation of mossy-type SMSs and rapid battery failure. In addition, we determined the existence of NaH in the SEI on sodium metal with ex-situ NMR results. The poor electronic conductivity of NaH is beneficial for the growth of a stable SEI on sodium metal.

The stability of P2-layered sodium transition metal oxides in ambient atmospheres.

Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable batteries. The promising P2-layered sodium transition metal oxides (P2-NaxTmO2) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-NaxTmO2 have not yet been clearly elucidated. Herein, taking P2-Na0.67MnO2 and P2-Na0.67Ni0.33Mn0.67O2 as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-NaxTmO2 in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-NaxTmO2 is determined by the amount of extracted Na+, which is mainly compensated by Na+/H+ exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-NaxTmO2 is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable NaxTmO2 cathodes.

High-Efficiency Lithium Metal Anode Enabled by a Concentrated/Fluorinated Ester Electrolyte.

Lithium (Li) metal anode (LMA) has received growing attention due to its highest theoretical capacity (3860 mA h g-1) and lowest redox potential (-3.04 V versus standard hydrogen electrode). However, practical application of LMA is obstructed by the detrimental side reactions between Li metal and organic electrolytes, especially when cycled in traditional carbonate ester electrolytes. Herein, we propose a novel fluorinated carbonate ester-based electrolyte by combining diethyl fluorocarbonate (ETFEC) solvent and 5 M LiFSI concentration (M = mol L-1). Using this electrolyte, an ultrahigh Li plating/stripping Coulombic efficiency (CE) of 99.1% can be obtained in Li||Cu cells and a stable cycle performance of Li||LiFePO4 is achieved under the conditions of limited Li metal (5 mA h cm-2), moderate loading LiFePO4 (7-8 mg cm-2), and lean electrolyte (40 uL). The fundamental functioning mechanism of this novel electrolyte has been carefully investigated by scanning electronic microscopy (SEM), operando optical microscopy (OM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and solid state nuclear magnetic resonance (SS-NMR). The results demonstrate that this optimized electrolyte facilitates formation of a high Li+ conductive SEI layer enriched with LiF and inorganic sulfur-containing species, which can effectively suppress the side reactions between electrolyte and Li metal and prevent formation of dead Li.

Elucidating and Mitigating the Degradation of Cationic-Anionic Redox Processes in Li1.2Mn0.4Ti0.4O2 Cation-Disordered Cathode Materials.

Cation-disordered rock-salt oxides with the O2-/O2n- redox reaction, such as Li1.2Mn0.4Ti0.4O2 (LMTO), are critical Li-rich cathode materials for designing high-energy-density batteries. Understanding the cationic-anionic redox accompanying the structural evolution process is really imperative to further improve the performance. In this work, the cationic-anionic redox and capacity degradation mechanism of carbon-coated LMTO during (dis)charge processes are elucidated by combining in situ X-ray diffraction, X-ray absorption near-edge spectroscopy, differential electrochemical mass spectrometry, transmission electron microscopy, and electrochemical analyses. It is concluded that the redox reaction of Mn2+/Mn4+ is quite stable, while the severe degradation is mainly caused by the O2-/O2n- redox process. Moreover, we clearly clarify how the cationic-anionic interplay governs sluggish kinetics, large polarization, and capacity fading in LMTO, and reveal for the first time that a certain amount of carbon coating is capable of suppressing the irreversible lattice oxygen loss and results in an encouraging cycling performance. In summary, we elucidate the degradation of cationic-anionic redox processes in cation-disordered cathode materials and propose strategies for adjusting the electronic/ionic conductivity of the electrodes to modulate the oxygen redox reactions, setting a new direction for the design of better cation-disordered oxides.

3D Lithiophilic "Hairy" Si Nanowire Arrays @ Carbon Scaffold Favor a Flexible and Stable Lithium Composite Anode.

Lithium metal anode is considered to be a promising candidate for high-energy-density lithium-based batteries. However, the safety issue induced by uncontrollable dendrite growth hinders the commercialization of a Li anode. Herein, self-supported three-dimensional flexible carbon cloth covered with a lithiophilic silicon nanowire array is constructed as the host for loading of molten Li to achieve the C/SiNW/Li composite anode. The electrode component of the carbon cloth provides the flexible and conductive substrate to accommodate the volume change during the stripping/plating of Li and facilitate more efficient electron transport, while silicon nanowires improve the wettability of the carbon host to liquefied Li and render uniform Li deposition on the surface of the composite electrode. The as-prepared C/SiNW/Li composite anode exhibits enhanced cycling stability with a low hysteresis of 40 mV for more than 200 h and a better rate tolerance even at a current density of up to 5 mA cm-2. When coupling with the LiNi0.5Mn1.5O4 cathode, the full cells using the C/SiNW/Li composite anode demonstrate a remarkable electrochemical performance with an exceptional rate performance of up to 10 C and stable long-term cycling (the capacity retention of 62% at a 5 C rate over 2000 cycles), which is much higher than the cells with pure Li anode. This work provides a universal strategy to fabricate the flexible and stable carbon-based Li metal anode toward high-energy-density batteries.