Boris Markovsky

With the increasing energy density and expanding applications of lithium-ion batteries, the demand for enhanced high-temperature performance has grown significantly. Although previous studies have attempted to improve the high-temperature stability through electrolyte modifications, the underlying failure mechanisms and the rational design principles for suitable electrolyte systems remain insufficiently understood. This study focuses on electrolytes containing boron-based additives, particularly lithium tetraborate, which exhibits excellent rate capability and impressive low-temperature performance but suffers from instability at elevated temperatures. Our investigation reveals that high-temperature battery failure is not solely attributed to aluminum current collector corrosion and the thermal instability of the bulk electrolyte but also to interphasial instability. These include detrimental side reactions catalyzed by Ni-rich cathodes and compromised electron-blocking capabilities of interphasial films. Based on these findings, we propose a new design guideline for high-temperature-stable electrolyte solutions, which is validated by the successful application of tris(2,2,2-trifluoroethyl) borate and 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane as functional additives. These additives effectively address the identified degradation pathways, resulting in significantly enhanced high-temperature performance. This comprehensive framework provides valuable insights into the rational design of advanced electrolyte systems for lithium-ion batteries that can be operated across a broad temperature range.

Lithiated oxides like Li[Ni _x Co _y Mn _z ]O ₂ ( x + y + z = 1) with high nickel content ( x ≥ 0.8) can possess high specific capacity ≥200 mA h g ⁻¹ and have attracted extensive attention as perspective cathode materials for advanced lithium-ion batteries.

High-energy cathode materials that are Li- and Mn-rich lithiated oxides—for instance, 0.35Li₂MnO₃^.0.65LiNi_0.35Mn_0.45Co_0.20O₂ (HE-NCM)—are promising for advanced lithium-ion batteries. However, HE-NCM cathodes suffer from severe degradation during cycling, causing gradual capacity loss, voltage fading, and low-rate capability performance. In this work, we applied an effective approach to creating a nano-sized surface layer of Li₂SO₄ on the above material, providing mitigation of the interfacial side reactions while retaining the structural integrity of the cathodes upon extended cycling. The Li₂SO₄ coating was formed on the surface of the material by mixing it with nanocrystalline Li₂SO₄ and annealing at 600 °C. We established enhanced electrochemical behavior with ~20% higher discharge capacity, improved charge-transfer kinetics, and higher rate capability of HE-NCM cathodes due to the presence of the Li₂SO₄ coating. Online electrochemical mass spectrometry studies revealed lower CO₂ and H₂ evolution in the treated samples, implying that the Li₂SO₄ layer partially suppresses the electrolyte degradation during the initial cycle. In addition, a ~28% improvement in the thermal stability of the Li₂SO₄-treated samples in reactions with battery solution was also shown by DSC studies. The post-cycling analysis allowed us to conclude that the Li₂SO₄ phase remained on the surface and retained its structure after 100 cycles.

In this work, we continued our systematic investigations on synthesis, structural studies, and electrochemical behavior of Ni-rich materials Li[Ni_xCo_yMn_z]O₂ (x + y + z = 1; x ≥ 0.8) for advanced lithium-ion batteries (LIBs). We focused, herein, on LiNi_0.85Co_0.10Mn_0.05O₂ (NCM85) and demonstrated that doping this material with high-charge cation Mo⁶⁺ (1 at. %, by a minor nickel substitution) results in substantially stable cycling performance, increased rate capability, lowering of the voltage hysteresis, and impedance in Li-cells with EC-EMC/LiPF₆ solutions. Incorporation of Mo-dopant into the NCM85 structure was carried out by in-situ approach, upon the synthesis using ammonium molybdate as the precursor. From X-ray diffraction studies and based on our previous investigation of Mo-doped NCM523 and Ni-rich NCM811 materials, it was revealed that Mo⁶⁺ preferably substitutes Ni residing either in 3a or 3b sites. We correlated the improved behavior of the doped NCM85 electrode materials in Li-cells with a partial Mo segregation at the surface and at the grain boundaries, a tendency established previously in our lab for the other members of the Li[Ni_xCo_yMn_z]O₂ family.

This work continues our systematic study of Li- and Mn- rich cathodes for lithium-ion batteries. We chose Li₂MnO₃ as a model electrode material with the aim of correlating the improved electrochemical characteristics of these cathodes initially activated at 0 °C with the structural evolution of Li₂MnO₃, oxygen loss, formation of per-oxo like species (O₂²⁻) and the surface chemistry. It was established that performing a few initial charge/discharge (activation) cycles of Li₂MnO₃ at 0 °C resulted in increased discharge capacity and higher capacity retention, and decreased and substantially stabilized the voltage hysteresis upon subsequent cycling at 30 °C or at 45 °C. In contrast to the activation of Li₂MnO₃ at these higher temperatures, Li₂MnO₃ underwent step-by-step activation at 0 °C, providing a stepwise traversing of the voltage plateau at >4.5 V during initial cycling. Importantly, these findings agree well with our previous studies on the activation at 0 °C of 0.35Li₂MnO₃·0.65Li[Mn_0.45Ni_0.35Co_0.20]O₂ materials. The stability of the interface developed at 0 °C can be ascribed to the reduced interactions of the per-oxo-like species formed and the oxygen released from Li₂MnO₃ with solvents in ethylene carbonate–methyl-ethyl carbonate/LiPF₆ solutions. Our TEM studies revealed that typically, upon initial cycling both at 0 °C and 30 °C, Li₂MnO₃ underwent partial structural layered-to-spinel (Li₂Mn₂O₄) transition.

We demonstrate a novel surface modification of Li- and Mn-rich cathode materials 0.33Li₂MnO₃·0.67LiNi_0.4Co_0.2Mn_0.4O₂ for lithium-ion batteries (high-energy Ni–Co–Mn oxides, HE-NCM) via their heat treatment with trimesic acid (TA) or terephthalic acid at 600 °C under argon. We established the optimal regimes of the treatment—the amounts of HE-NCM, acid, temperature, and time—resulting in a significant improvement of the electrochemical behavior of cathodes in Li cells. It was shown that upon treatment, some lithium is leached out from the surface, leading to the formation of a surface layer comprising rock-salt-like phase Li_0.4Ni_1.6O₂. The analysis of the structural and surface studies by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy confirmed the formation of the above surface layer. We discuss the possible reactions of HE-NCM with the acids and the mechanism of the formation of the new phases, Li_0.4Ni_1.6O₂ and spinel. The electrochemical characterizations were performed by testing the materials versus Li anodes at 30 °C. Importantly, the electrochemical results disclose significantly improved cycling stability (much lower capacity fading) and high-rate performance for the treated materials compared to the untreated ones. We established a lower evolution of the voltage hysteresis with cycling for the treated cathodes compared to that for the untreated ones. Thermal studies by differential scanning calorimetry also demonstrated lower (by ∼32%) total heat released in the reactions of the materials treated with fluoroethylene carbonate (FEC)−dimethyl carbonate (DEC)/LiPF₆ electrolyte solutions, thus implying their significant surface stabilization because of the surface treatment. It was established by a postmortem analysis after 400 cycles that a lower amount of transition-metal cations dissolved (especially Ni) and a reduced number of surface cracks were formed for the 2 wt % TA-treated HE-NCMs compared to the untreated ones. We consider the proposed method of surface modification as a simple, cheap, and scalable approach to achieve a steady and superior electrochemical performance of HE-NCM cathodes.