Researchers from Sweden, Italy and Germany have proposed and demonstrated a novel full Li-ion cell that is able to cycle for thousands of cycles at 1000 mAg−1 with a capacity retention of 65% at cycle 2000.
The cell uniquely combines a nanostructured TiO2-based anode with a tailored 1-D tubular morphology; a LiNi0.5Mn1.5O4-based cathode (LNMO) with a finely tuned stoichiometry and a surface layer obtained through a single-stage, simple, cheap and easy-scalable mechanochemical milling route followed by high temperature annealing in air; and a composite liquid electrolyte formed by a mixture of LiPF6, ethylene carbonate, dimethyl carbonate and Py1,4PF6 ionic liquid. An open access paper on the work is published in Scientific Reports.
A very large number of possible alternative configurations for next generation lithium-ion cells have been proposed, among them, the concept of a 3–3.5 V Li-ion cell made by coupling LNMO spinel and TiO2-based anodes.
Titanium oxide-based anodes are attractive because the working potential falls within the thermodynamic stability window of the standard organic carbonate electrolytes (>0.8 V vs. Li). Further, the easily obtained materials have an energy density two times larger than graphite; thus the volumetric performance can double compared to a standard graphite-based Li-ion cells.
However, the team notes, the high operating potential of the mateirals (1.5 V vs Li) is an important drawback for full cell energy density. TiO2-based anodes thus need to be coupled with high-potential cathodes such as LNMO to achieve competitive performance with respect to the state-of-the-art formulations.
LNMO spinel oxide is a promising cathode material due to its large reversible capacity, high thermal stability, low cost—and lack of toxic, high-cost cobalt. To achieve excellent power performance from this material requires obtaining well-formed particles with optimal morphology.
However, the authors note, adoption of a simple and single-step synthesis strategy to optimize the crystallinity, composition, morphology and surface properties to be able to fully address the serious capacity fading of LNMO cathodes, especially at high rate and at elevated temperatures, has never been reported.
To address the shortcomings at high potentials … and to improve the safety of the battery we developed a composite solution, made by mixing an ionic liquid (IL) component, Py1,4PF6, with a conventional LiPF6-alkyl carbonate based electrolyte … to obtain an innovative electrolyte able to operate at high potentials and with improved thermal stability.
… the coupling of this innovative electrolyte with a high potential positive electrode material, i.e. LNMO, and an intrinsically safe negative electrode material, i.e. TiO2, allows a final cell configuration with intrinsic chemical safety comparable with the safest carbonate-based Li-ion cell on market, i.e. the LiFePO4/Li4Ti5O12 (LFP-LTO) one, but with a mean working voltage of about 2.7–3 V, well above the 1.9 V of the LFP-LTO cell.
In summary, the here-proposed full lithium ion cell formulation exploits three simultaneous innovations, on the two electrodes sides as well as for the electrolyte, to disclose outstanding and unpreceded power performance and cycling life compared to the state-of-the-art, with a parallel improvement in the intrinsic safety of the device through the use of an ionic liquid component and the full environmental benignity by replacing cobalt in the cathode active material.—Agostini et al.
M. Agostini, S. Brutti, M. A. Navarra, S. Panero, P. Reale, A. Matic & B. Scrosati (2017) “A high-power and fast charging Li-ion battery with outstanding cycle-life” Scientific Reports 7, Article number: 1104 doi: 10.1038/s41598-017-01236-y