Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage Cathode–Electrolyte Interface in Lithium-Ion Batteries

Current developments of electrolyte additives to stabilize electrode–electrolyte interface in lithium-ion batteries highly rely on a trial-and-error search, which involves repetitive testing and intensive amount of resources. The lack of understandings on the fundamental protection mechanisms of the additives significantly increases the difficulty for the transformational development of new additives. In this study, we investigated two types of individual protection routes to build a robust cathode–electrolyte interphase at high potentials: (i) a direct reduction in the catalytic decomposition of the electrolyte solvent; and (ii) formation of a “corrosion inhibitor film” that prevents severely attack and passivation from protons that generated from the solvent oxidation, even the decomposition of solvent cannot be mitigated. Effect of two exemplary electrolyte additives, lithium difluoro­(oxalato)­borate (LiDFOB) and 3-hexylthiophene (3HT), on LiNi_0.6Mn_0.2Co_0.2O₂ (NMC 622) cathode were investigated to validate our hypothesis. It is demonstrated that understandings of both electrolyte additives and solvent are essential and careful balance between the cathode protection mechanism of additives and their side effects is critical to obtain optimum results. More importantly, this study opens up new directions of rational design of functional electrolyte additives for the next-generation high-energy-density lithium-ion chemistries

Cathode Material with Nanorod StructureAn Application for Advanced High-Energy and Safe Lithium Batteries

We have developed a novel cathode material based on lithium–nickel–manganese–cobalt oxide, where the manganese concentration remains constant throughout the particle, while the nickel concentration decreases linearly and the cobalt concentration increases from the center to the outer surface of the particle. This full concentration gradient material with a fixed manganese composition (FCG–Mn-F) has an average composition of Li­[Ni_0.60Co_0.15Mn_0.25]­O₂ and is composed of rod-shaped primary particles whose length reaches 2.5 μm, growing in the radial direction. In cell tests, the FCG–Mn-F material delivered a high capacity of 206 mAh g^–1 with excellent capacity retention of 70.3% after 1000 cycles at 55 °C. This cathode material also exhibited outstanding rate capability, good low-temperature performance, and excellent safety, compared to a conventional cathode having the same composition (Li­[Ni_0.60Co_0.15Mn_0.25]­O₂), where the concentration of the metals is constant across the particles

Conductive Polymer Binder-Enabled SiO–Sn_<i>x</i>Co_<i>y</i>C_<i>z</i> Anode for High-Energy Lithium-Ion Batteries

A SiOSnCoC composite anode is assembled using a conductive polymer binder for the application in next-generation high energy density lithium-ion batteries. A specific capacity of 700 mAh/g is achieved at a 1C (900 mA/g) rate. A high active material loading anode with an areal capacity of 3.5 mAh/cm² is demonstrated by mixing SiOSnCoC with graphite. To compensate for the lithium loss in the first cycle, stabilized lithium metal powder (SLMP) is used for prelithiation; when paired with a commercial cathode, a stable full cell cycling performance with a 86% first cycle efficiency is realized. By achieving these important metrics toward a practical application, this conductive polymer binder/SiOSnCoC anode system presents great promise to enable the next generation of high-energy lithium-ion batteries

Role of Manganese Deposition on Graphite in the Capacity Fading of Lithium Ion Batteries

Lithium ion batteries utilizing manganese-based cathodes have received considerable interest in recent years for their lower cost and more favorable environmental friendliness relative to their cobalt counterparts. However, Li ion batteries using these cathodes combined with graphite anodes suffer from severe capacity fading at high operating temperatures. In this paper, we report on how the dissolution of manganese impacts the capacity fading within the Li ion batteries. Our investigation reveals that the manganese dissolves from the cathode, transports to the graphite electrode, and deposits onto the outer surface of the innermost solid-electrolyte interphase layer, which is known to be a mixture of inorganic salts (e.g., Li₂CO₃, LiF, and Li₂O). In this location, the manganese facilitates the reduction of the electrolyte and the subsequent formation of lithium-containing products on the graphite, which removes lithium ions from the normal operation of the cell and thereby induces the severe capacity fade

Storage and Effective Migration of Li-Ion for Defected β‑LiFePO₄ Phase Nanocrystals

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, α-LiFePO₄ (αLFP). However, the new phase β-LiFePO₄ (βLFP), which can be transformed from αLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO₄] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in αLFP disappear, while the Fe ions in the β-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that βLFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of βLFP cathode achieved a capacity of 128 mAh g^–1 at a rate of 0.1 C (1C = 170 mA g^–1) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as Fe_LiLi_Fe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density

Exploring Highly Reversible 1.5-Electron Reactions (V³⁺/V⁴⁺/V⁵⁺) in Na₃VCr(PO₄)₃ Cathode for Sodium-Ion Batteries

The development of highly reversible multielectron reaction per redox center in sodium super ionic conductor-structured cathode materials is desired to improve the energy density of sodium-ion batteries. Here, we investigated more than one-electron storage of Na in Na₃VCr­(PO₄)₃. Combining a series of advanced characterization techniques such as ex situ ⁵¹V solid-state nuclear magnetic resonance, X-ray absorption near-edge structure, and in situ X-ray diffraction, we reveal that V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples in the materials can be accessed, leading to a 1.5-electron reaction. It is also found that a light change on the local electronic and structural states or phase change could be observed after the first cycle, resulting in the fast capacity fade at room temperature. We also showed that the irreversibility of the phase changes could be largely suppressed at low temperature, thus leading to a much improved electrochemical performance

Nanostructured Black Phosphorus/Ketjenblack–Multiwalled Carbon Nanotubes Composite as High Performance Anode Material for Sodium-Ion Batteries

Sodium-ion batteries are promising alternatives to lithium-ion batteries for large-scale applications. However, the low capacity and poor rate capability of existing anodes for sodium-ion batteries are bottlenecks for future developments. Here, we report a high performance nanostructured anode material for sodium-ion batteries that is fabricated by high energy ball milling to form black phosphorus/Ketjenblack–multiwalled carbon nanotubes (BPC) composite. With this strategy, the BPC composite with a high phosphorus content (70 wt %) could deliver a very high initial Coulombic efficiency (>90%) and high specific capacity with excellent cyclability at high rate of charge/discharge (∼1700 mAh g^–1 after 100 cycles at 1.3 A g^–1 based on the mass of P). In situ electrochemical impedance spectroscopy, synchrotron high energy X-ray diffraction, ex situ small/wide-angle X-ray scattering, high resolution transmission electronic microscopy, and nuclear magnetic resonance were further used to unravel its superior sodium storage performance. The scientific findings gained in this work are expected to serve as a guide for future design on high performance anode material for sodium-ion batteries

Modifying the Surface of a High-Voltage Lithium-Ion Cathode

Ni-rich lithium nickel manganese cobalt oxides (LiNi_<i>x</i>Mn_<i>y</i>Co_{1–<i>x</i>–<i>y</i>}O₂, NMCs) suffer from poor cycling stability at potentials above 4.2 V vs Li/Li⁺. This degraded cyclability at high potentials has been largely ascribed to the parasitic reactions between the delithiated cathode and the nonaqueous electrolyte. In this study, we mitigated the performance degradation of high-voltage NMC 622 by designing a functional interfacial layer that consists of a surface doping by Ti⁴⁺ and a TiO₂ coating at the same time. The doping of Ti⁴⁺ near the surface of NMC can suppress the irreversible phase transformation, while the TiO₂ coating can kinetically reduce the rate of the electron-transfer reaction between the delithiated cathode and the solvent. It is revealed that this interfacial engineering approach significantly enhanced both the cycling stability and the rate performance of NMC 622

Revisiting the Corrosion of the Aluminum Current Collector in Lithium-Ion Batteries

The corrosion of aluminum current collectors and the oxidation of solvents at a relatively high potential have been widely investigated with an aim to stabilize the electrochemical performance of lithium-ion batteries using such components. The corrosion behavior of aluminum current collectors was revisited using a home-build high-precision electrochemical measurement system, and the impact of electrolyte components and the surface protection layer on aluminum foil was systematically studied. The electrochemical results showed that the corrosion of aluminum foil was triggered by the electrochemical oxidation of solvent molecules, like ethylene carbonate, at a relative high potential. The organic radical cations generated from the electrochemical oxidation are energetically unstable and readily undergo a deprotonation reaction that generates protons and promotes the dissolution of Al³⁺ from the aluminum foil. This new reaction mechanism can also shed light on the dissolution of transitional metal at high potentials

Tuning of Thermal Stability in Layered Li(Ni_<i>x</i>Mn_<i>y</i>Co_<i>z</i>)O₂

Understanding and further designing new layered Li­(Ni_<i>x</i>Mn_<i>y</i>Co_<i>z</i>)­O₂ (NMC) (<i>x</i> + <i>y</i> + <i>z</i> = 1) materials with optimized thermal stability is important to rechargeable Li batteries (LIBs) for electrical vehicles (EV). Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM­(Ni, Mn, Co)₃-O-Li_{3–<i>x</i>′}): each O atom bonds with three transition metals (TM) from the TM-layer and three to zero Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed. The synergistic effect between Li vacancies and raised valence state of Ni during delithiation process can aggravate instability of oxygen, and oxygen coordinated with more nickel (especially with high valence state) in LSCU becomes more unstable at a fixed delithiation state. The Ni/Li mixing would decrease the thermal stability of the “NiMn” group NMC materials but benefit the thermal stability of “Ni-rich” group, because the Ni in the Li layer would form 180° Ni–O–Ni super exchange chains in “Ni-rich” NMC materials. Mn and Co doping can tune the initial valence state of Ni, local coordination environment of oxygen, and the Ni/Li disorder, thus to tune the thermal stability directly