Deconvolution of Intermixed Redox Processes in Ni-based Cation-Disordered Li-Excess Cathodes

Cation-disordered rock-salt transition-metal oxides or oxyfluorides (DRX) have emerged as promising cathode materials for Li-ion batteries due to their potential to reach high energy densities and accommodate diverse, lower cost transition-metal chemistries compared to conventional layered oxide materials. However, the intricate local coordination environment in DRX also results in complex electrochemical electron transfer involving parallel mechanisms of transition-metal (TM) redox and oxygen (anionic) redox. Without decoupled and quantitative information of these intermixed redox processes, the origin of irreversibility, voltage hysteresis, and capacity fading is obscured, which impedes the development of strategies to address these issues. Here we deconvolute the mixed redox processes in a Ni-based DRX, Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15, by combining 18O isotopic enrichment, differential electrochemical mass spectrometry (DEMS), and ex-situ acid titration. The summation of TM-redox and oxygen-redox capacities measured through our approach agrees with the net electron transfer measured by the potentiostat. This study reveals much less Ni oxidation efficiency (59.5%) than its initially designed efficiency (100%) due to competition of oxygen redox, which can occur at potentials as low as 4.1 V (vs. Li/Li+). We propose that the chemical approach presented in this work and its future extension can resolve and quantify various mixed redox processes in different DRX, which allows clear correlations among material design, deconvoluted redox capacities, and battery performance

An Experimental Approach to Assess Fluorine Incorporation into Disordered Rock Salt Oxide Cathodes.

Disordered rock salt oxides (DRX) have shown great promise as high-energy-density and sustainable Li-ion cathodes. While partial substitution of oxygen for fluorine in the rock salt framework has been related to increased capacity, lower charge-discharge hysteresis, and longer cycle life, fluorination is poorly characterized and controlled. This work presents a multistep method aimed at assessing fluorine incorporation into DRX cathodes, a challenging task due to the difficulty in distinguishing oxygen from fluorine using X-ray and neutron-based techniques and the presence of partially amorphous impurities in all DRX samples. This method is applied to Li1.25Mn0.25Ti0.5O1.75F0.25 prepared by solid-state synthesis and reveals that the presence of LiF impurities in the sample and F content in the DRX phase is well below the target. Those results are used for compositional optimization, and a synthesis product with drastically reduced LiF content and a DRX stoichiometry close to the new target composition (Li1.25Mn0.225Ti0.525O1.85F0.15) is obtained, demonstrating the effectiveness of the strategy. The analytical method is also applied to Li1.33Mn0.33Ti0.33O1.33F0.66 obtained via mechanochemical synthesis, and the results confirm that much higher fluorination levels can be achieved via ball-milling. Finally, a simple and rapid water washing procedure is developed to reduce the impurity content in as-prepared DRX samples: this procedure results in a ca. 10% increase in initial discharge capacity and a ca. 11% increase in capacity retention after 25 cycles for Li1.25Mn0.25Ti0.50O1.75F0.25. Overall, this work establishes new analytical and material processing methods that enable the development of more robust design rules for high-energy-density DRX cathodes

Total Ankle Arthroplasty Survivorship: A Meta-analysis.

The gold standard for management of end-stage ankle arthritis was previously ankle arthrodesis; however, improvements in total ankle replacements are making this a more viable treatment option. The primary aim of this meta-analysis was to evaluate the survivorship of total ankle replacement implants currently in use. An extensive search strategy initially captured 20,842 citations that were evaluated for relevance. Abstract screening produced 97 articles to be read in entirety, of which 10 articles studying 1963 implants met all prospective inclusion criteria for analysis. Overall survivorship of all implants was 93.0% (95% confidence interval, 85.2-96.9) using a random effect model. There was significant heterogeneity between the studies (Q = 131.504). Meta-regression identified an inverse relationship between survivorship and study follow-up duration (p \u3c .0001). Furthermore, age (p = .36) and implant type (fixed-bearing [95.6%, 95% confidence interval, 85.9-98.7] versus mobile-bearing ]89.4%, 95% confidence interval, 79.6%-94.8%]) did not have a statistically significant impact on survivorship, p = .213. However, patients with higher preoperative functional scores had improved survivorship (p = .001). Complications were inconsistently reported with varied definitions. In order of reported frequency, complications were classified into technical error (28.15%), subsidence (16.89%), implant failure (13.28%), aseptic loosening (6.3%), intraoperative fracture (5.67%), wound problems (4.3%), deep infection (1%), and postoperative fracture (0.0001%). Overall study quality was low, with only 10% being prospective and 90% from nonregistry data. The results from this meta-analysis revealed a promising overall survivorship of current implants in use for total ankle replacement; however higher quality studies with standardized outcomes measures are needed

Tuning Bulk Redox and Altering Interfacial Reactivity in Highly Fluorinated Cation-Disordered Rocksalt Cathodes

Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode-electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are combined to quantify the extent of bulk redox and surface degradation reactions for a set of Mn2+/4+-based DRX oxyfluorides during initial cycling with a high-voltage charging cutoff (4.8 V vs Li/Li+). Increasing the fluorine content from 7.5 to 33.75% is shown to diminish oxygen redox and suppresses high-voltage O2 evolution from the DRX surface. Additionally, electrolyte degradation processes resulting in the formation of both gaseous species and electrolyte-soluble protic species are observed. Subsequently, DEMS is paired with a fluoride-scavenging additive to demonstrate that increasing fluorine content leads to increased dissolution of fluorine from the DRX material into the electrolyte. Finally, a suite of ex situ spectroscopy techniques (X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and solid-state nuclear magnetic resonance spectroscopy) are employed to study the change in DRX composition during charging, revealing the dissolution of manganese and fluorine from the DRX material at high voltages. This work provides insight into the degradation processes occurring at the DRX-electrolyte interface and points toward potential routes of interfacial stabilization

Irreversible anion oxidation leads to dynamic charge compensation in the Ru-poor, Li-rich cathode Li2Ru0.3Mn0.7O3

Conventional cathodes for Li-ion batteries are layered transition-metal oxides that support Li+ intercalation charge-balanced by redox on the transition metals. Oxidation beyond one electron per transition metal can be achieved in Li-rich layered oxides by involving structural anions, which necessitates high voltages and complex charge compensation mechanisms convoluted by degradation reactions. We report a detailed structural and spectroscopic analysis of the multielectron material Li2Ru0.3Mn0.7O3, chosen due to its low Ru content. Ex situ and operando spectroscopic data over multiple cycles highlight the changing charge compensation mechanism. Notably, over half of the first-cycle capacity is attributed to O2 gas evolution and reversible O redox is minimal. Instead, reduced Ru and Mn species are detected in the bulk and on the surface, which then increasingly contribute to charge compensation as more metal reduction occurs with cycling. Permanent structural changes linked to metal migration are observed with EXAFS and Raman analysis

Quantitative Decoupling of Oxygen‐Redox and Manganese‐Redox Voltage Hysteresis in a Cation‐Disordered Rock Salt Cathode

Pronounced voltage hysteresis in Li-excess cathode materials is commonly thought to be associated with oxygen redox. However, these materials often possess overlapping oxygen and transition-metal redox, whose contributions to hysteresis between charge and discharge are challenging to distinguish. In this work, a two-step aqueous redox titration is developed with the aid of mass spectrometry (MS) to quantify oxidized lattice oxygen and Mn3+ /4+ redox in a representative Li-excess cation-disordered rock salt—Li1.2Mn0.4Ti0.4O2 (LMTO). Two MS-countable gas molecules evolve from two separate titrant-analyte reactions, thereby allowing Mn and O redox capacities to be decoupled. The decoupled O and Mn redox coulombic efficiencies are close to 100% for the LMTO cathode, indicating high charge-compensation reversibility. As incremental Mn and O redox capacities are quantitatively decoupled, each redox voltage hysteresis is further evaluated. Overall, LMTO voltage hysteresis arises not only from an intrinsic charge-discharge voltage mismatch related to O redox, but also from asymmetric Mn-redox overvoltages. The results reveal that O and Mn redox both contribute substantially to voltage hysteresis. This work further shows the potential of designing new analytical workflows to experimentally quantify key properties, even in a disordered material having complex local coordination environments

High-throughput Li plating quantification for fast-charging battery design

Fast charging of most commercial lithium-ion batteries is limited due to fear of lithium plating on the graphite anode, which is difficult to detect and poses considerable safety risk. Here we demonstrate the power of simple, accessible and high-throughput cycling techniques to quantify irreversible Li plating spanning data from over 200 cells. We first observe the effects of energy density, charge rate, temperature and state of charge on lithium plating, use the results to refine a mature physics-based electrochemical model and provide an interpretable empirical equation for predicting the plating onset state of charge. We then explore the reversibility of lithium plating and its connection to electrolyte design for preventing irreversible Li accumulation. Finally, we design a method to quantify in situ Li plating for commercially relevant graphite|LiNi0.5Mn0.3Co0.2O2 (NMC) cells and compare with results from the experimentally convenient Li|graphite configuration. The hypotheses and abundant data herein were generated primarily with equipment universal to the battery researcher, encouraging further development of innovative testing methods and data processing that enable rapid battery engineering

Oxygen Activities Governing Structural Reversibility in Industrial Ni-Rich Layered Cathodes

The chemical reactions and phase transitions at high voltages determine the electrochemical properties of high voltage layered cathodes such as Ni-rich rhombohedral materials. Here, we performed a comprehensive and comparative study of the cationic and anionic redox reactions, as well as the structural evolution of a series of industrial Ni-rich layered cathode materials with and without Al doping, which are being utilized in the cells made by LG Energy Solutions Co.. We combined the results from X-ray spectroscopy, operando electrochemical mass spectrometry, and neutron diffraction with electrochemical properties, and revealed the different oxygen activities associated with structural and electrochemical degradations. We show that Al doping suppresses the irreversible oxygen release thereby enhancing the reversible lattice oxygen redox resulting from the interplay between static (doped Al) and dynamic disorders (reversible oxygen redox). With this modulated oxygen activity, the Ni-rich cathode\u27s notorious H2-H3 structural phase transition becomes highly reversible. Our findings disentangle the different oxygen activities during high-voltage cycling and clarify the role of dopants in the Ni-rich layered cathodes in terms of structural and electrochemical stability finally making all the cell makers get back to the fundamental investigation regarding whether high-Ni NCM chemistry (NCM811 or NCM 91/2 1/2) is substantially beneficial compared to its mid-Ni homologues (NCM622)

Chemical and structural evolutions of Li–Mn-rich layered electrodes at different current densities

Although the two active redox centers in Li-rich cathodes, including the anionic and cationic contributions, can enable Li-ion batteries to achieve outstanding specific energy, their behaviors at different current densities have not been clarified. Here, we provide a comparative study of transition metals (TMs) and oxygen redox activities by directly accessing their oxidation states in Li-rich materials operated at very different current rates. Our data reveal that the oxidation of oxygen in the near-surface region is at the same level for electrodes cycled with a wide range of current rates, indicating a reaction gradient of lattice oxygen redox reactions. The oxidation process of lattice oxygen is found to be dynamically compatible with that of the TMs. Combining the results of first principles calculations and complementary experimental findings, we propose a detailed mechanism of structural distortion from octahedral Li to tetrahedral Li and the role of oxygen vacancy in Li+ diffusion. It is found that fast delithiation occurring at high current densities can easily cause local structural transformation, leading to a limited Li+ diffusion rate and consequently suppressing rate capability