Olesia M. Karakulina

The adoption of silicon-graphite composites as anode materials for the next generation of lithium-ion batteries with enhanced specific capacity requires complex technological efforts in order to mitigate the problem of the quick performance fading of electrodes due to the mechanical degradation of materials. The matter is currently being addressed in terms of electrolyte components, polymer binders, materials structure and morphology itself, as well as current collector design, which differ greatly in cost and scalability. The present work describes the efficacy of Cu foil perforation—a simple, low-cost, and easily scalable approach—as a means of Si/C composite anode performance stabilization during extensive charge-discharge cycling. The NMC||Si/C pouch-type full cells demonstrated over 90% of initial capacity retention after 100 charge-discharge cycles in the case of a 250 µm perforated Cu foil used as a current collector, compared to only 60% capacity left in the same conditions for plain Cu foil as an anode. The obtained result is related to the prevention of anode material delamination off the foil surface as a result of silicon expansion and contraction, which is achieved through the formation inter-penetrating metal-composite structure and the presence of “stitches”, connecting and holding both sides of the electrode tightly attached to the current collector.

Silicon-graphite anodes offer a practical route to increase the energy density of lithium-ion batteries (LIBs), but their widespread adoption is hampered by cyclic instability due to huge volume changes of silicon during lithiation/delithiation process. Another fallout of LIBs capacity gain is growing safety concerns due to fire risks, associated with the uncontrolled release of chemical energy. Herein, we test a hexakis(fluoroethoxy)phosphazene (HFEPN) as a multifunctional electrolyte additive designed to mitigate both issues. The flammability of HFEPN-containing electrolytes was evaluated using a self-extinguishing time test, while the electrochemical performance was assessed in Si/C composite||NMC pouch cells under a progressively accelerated cycling protocol. It is shown that the additive fully imparts flame-retardant properties to the electrolyte at a 15 wt% loading. Despite forming a more stable solid–electrolyte interphase (SEI) with enhanced interfacial kinetics the additive did not improve the cycling stability of the Si/C-based cells. The cells with 15 wt% HFEPN retained 43% of capacity after 70 cycles, comparable to 46.5% for the reference electrolyte. The diffusion limitations imposed by the increased electrolyte viscosity are assumed to offset the interfacial benefits of the additive. Thus, alongside the improved synthetic route, this study demonstrates that while HFEPN functions as an effective flame retardant and SEI modifier, its practical benefits for silicon anodes are limited at high concentrations by detrimental effects on electrolyte transport properties and should be improved in future molecular design efforts.

The success and continued expansion of research on metal-oxo clusters owe largely to their structural richness and wide range of functions. However, while most of them known to date are negatively charged polyoxometalates, there is only a handful of cationic ones, much less functional ones. Here, we show an all-inorganic hydroxyiodide [H _10.7 Sb _32.1 O ₄₄ ][H _2.1 Sb _2.1 I ₈ O ₆ ][Sb _0.76 I ₆ ] ₂ ·25H ₂ O ( HSbOI ), forming a face-centered cubic structure with cationic Sb ₃₂ O ₄₄ clusters and two types of anionic clusters in its interstitial spaces. Although it is submicrometer in size, electron diffraction tomography of HSbOI allowed the construction of the initial structural model, followed by powder Rietveld refinement to reach the final structure. The cationic cluster is characterized by the presence of acidic protons on its surface due to substantial Sb ³⁺ deficiencies, which enables HSbOI to serve as an excellent solid acid catalyst. These results open up a frontier for the exploration and functionalization of cationic metal-oxo clusters containing heavy main group elements.

The Sn⁴⁺ substitution limit in Li_1.2Ni_0.13Co_0.13Mn_0.54−xSn_xO₂ is around x = 0.045. For x = 0.027 the honeycomb ordering and O₃ structure is preserved. For x = 0 and x = 0.027 similar voltage fade has been obtained in the 3 V–4.55 V vs. Li/Li⁺ window.

We demonstrate that changes in the unit cell structure of lithium battery cathode materials during electrochemical cycling in liquid electrolyte can be determined for particles of just a few hundred nanometers in size using in situ transmission electron microscopy (TEM). The atomic coordinates, site occupancies (including lithium occupancy), and cell parameters of the materials can all be reliably quantified. This was achieved using electron diffraction tomography (EDT) in a sealed electrochemical cell with conventional liquid electrolyte (LP30) and LiFePO₄ crystals, which have a well-documented charged structure to use as reference. In situ EDT in a liquid environment cell provides a viable alternative to in situ X-ray and neutron diffraction experiments due to the more local character of TEM, allowing for single crystal diffraction data to be obtained from multiphased powder samples and from submicrometer- to nanometer-sized particles. EDT is the first in situ TEM technique to provide information at the unit cell level in the liquid environment of a commercial TEM electrochemical cell. Its application to a wide range of electrochemical experiments in liquid environment cells and diverse types of crystalline materials can be envisaged.

LiNi _0.5 Mn _1.5 O _4−δ surface is doped with Ti ion maintaining the spinel structure at 500 °C, higher annealing temperatures cause Ti diffusion from surface towards the core.

Today researches in the field of cathode materials for Li-ion batteries are directed towards the development of new materials with high power and energy densities, especially to answer the requirements of large scale applications. In this respect, fluoride-phosphate materials are very attractive as they demonstrate wide structural diversity. Furthermore, they are expected to exploit the advantages of both anion species: the increase in the working potential due to inductive effect of the polyanion group and higher ionicity of the M−F bond, faster kinetics arising from lower affinity of lithium to the F-anion.¹ In the search for new cathode materials for Li-ion batteries we have explored the A₂MPO₄F and AMPO₄F (A=Li, Na, K, and M=Co, Mn, Fe, V) fluorophosphates families. The fluoride-phosphate phases were prepared through different synthetic routes depending on the chemistry of transition metal. Combination of X-ray/neutron diffraction and electron microscopy were applied for a thorough structural characterization. It was shown that in these systems the substitution on the alkali metal site leads to the wide range of adopted structures. Thus, in the Li₂FePO₄F system synthesis of sodium-containing counterpart, NaLiFePO₄F, followed by subsequent electrochemical exchange of Na by Li leads to the cathode material with a 3D “stacked” structure.² In the (Li,Na)₂CoPO₄F system it provides a new low-temperature polymorph adopting a complex layered structure, in contrast to the high-temperature phase with 3D “stacked” arrangement. The introduction of large potassium into the AMPO₄F system gives a raise to a new cathode material with a KTP structure, which exhibits excellent rate capability.³ Reversible electrochemical activity demonstrated by these fluoride-phosphate materials in Li-cell will be discussed. A special attention will be given to the structure peculiarities and their impact on electrochemical properties, which is important for possible practical applications of these materials. The work was partly supported by Russian Science Foundation (grant 16-19-00190), Skoltech Center for Electrochemical Energy Storage and Moscow State University Development Program up to 2020. References Antipov, E. V.; Khasanova, N. R.; Fedotov, S. S. International Union of Crystallography Journal 2015, 2, 85-94 Khasanova N. R., Drozhzhin O. A., Storozhilova D. A., Delmas C., Antipov E. V. Chem. Mater., 2012, 24, 4271–4273 Fedotov, S. S.; Khasanova, N. R.; Samarin, A. Sh.; Drozhzhin, O. A.; Batuk, D.; Karakulina, O. M.; Hadermann, J.; Abakumov, A. M; Antipov, E. V. Chem. Mater. 2016, DOI: 10.1021/acs.chemmater.5b04065

In a hunt for high-energy cathode materials for Li-ion batteries, different families of fluoride-containing materials such as fluoride-phosphates A_xMPO₄F (A = Li, Na; M = V, Mn, Fe, Co, Ni; x = 1, 2)^1,2,3have been extensively examined. Introduced to the lattice fluorine brings about not only an increase in operating voltages due to its highest electronegativity, but also a richer structural variety. In these systems the nature of alkali and transition metals strongly affects the adopting crystal structure, which in turn determines electrochemical properties. Due to a larger ionic radius compared to Li and Na and somewhat different crystal chemistry, K provides ample opportunities in searching and stabilizing new frameworks for reversible ion de/intercalation. Here we report on a novel vanadium-based AVPO₄F (A = Li, K) cathode material adopting a KTP-type framework for high power rechargeable batteries with enhanced specific energy and excellent rate capability⁴. KVPO₄F was prepared via freeze-drying assisted solid-state route in two steps. The composition of the material was studied by TEM-EDX, ICP-AES and FTIR spectroscopy. The structure of KVPO₄F was refined from powder XRD data [S.G. Pna2₁, a = 12.8200(3) Å, b = 6.3952(1) Å, c = 10.6115(2)] using the KFeSO₄F structure as an initial model⁵. The refined structure was confirmed with HAADF-STEM imaging and electron diffraction. Vanadium +3 oxidation state was confirmed with EELS. For the electrochemical evaluation the initial KVPO₄F material was oxidized by charging up to 4.8 V vs Li/Li⁺ at C/20 rate and holding at this potential for 5 hours. According to the EDX data, potassium in not completely removed and about 15% remains in the structure giving rise to a K_0.15VPO₄F formula. The obtained K_0.15VPO₄F material was tested within the 2.0–4.7 V vs Li/Li⁺ potential range at different rates from C/5 to 40C showing an uptake of 0.7 Li per formula unit at the average voltage of ~4 V. K_0.15VPO₄F exhibits a sloping voltage profile indicating a solid-solution-like de/intercalation mechanism. Ex-situ XRD and electron diffraction tomography were applied to examine the crystal structure of the recovered oxidized and lithiated electrode materials. In contrast to the noncentrosymmetric parent KVPO₄F phase (Pna2₁), the crystal structures of both oxidized and lithiated electrode materials possess a centrosymmetric space group (Pnan); however, KTP-type structure is preserved. The difference in the unit cell volume of all three phases is small and does not exceed 2.2%. Li_xK_0.15VPO₄F shows remarkable capacity retention at 40C maintaining more than 50% of theoretical (156 mAh/g) or 75% of initial specific capacity (111 mAh/g at C/5). Rapid kinetics of the Li de/intercalation is also evidenced by rate capability measurements (figure 1). Synthesis, structure and electrochemical properties of the obtained cathode material will be discussed in detail. A special focus will be given on structural peculiarities and substitution in the metal sublattice and their effect on the electrochemical performance in a Li anode cell. Figure 1. Rate capability measurements of (K,Li)VPO₄F. References Barker, J.; Saidi, M. Y.; Swoyer, J. L. J. Electrochem. Soc. 2003, 150, A1394−A1398. Okada, S.; Ueno, M.; Uebou, Y.; Yamaki, J. J. Power Sources 2005, 146, 565−569. Ellis, B. L.; Makahonouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. Nat. Mater. 2011, 10, 772−779. Fedotov, S. S.; Khasanova, N. R.; Samarin, A. Sh.; Drozhzhin, O. A.; Batuk, D.; Karakulina, O. M.; Hadermann, J.; Abakumov, A. M; Antipov, E. V. Chem. Mater. 2016, DOI: 10.1021/acs.chemmater.5b04065 Recham, N.; Gwenaelle, R.; Sougrati, M. T.; Chotard, J.-N.; Frayret, C.; Mariyappan, S.; Melot, B. C.; Jumas, J.-C.; Tarascon, J.-M. Chem. Mater. 2012, 24, 4363–4370. Figure 1