Nano-Scale Lithium Titanium Oxide As Anode Material for Nanoelectrofuel Flow Batteries

Nanoelectrofuel (NEF) flow battery is new transformational technology that merges the advantages of high energy density solid materials and the convenience of flowable battery format. Traditional flow batteries rely on solubility of redox salts and suffer from the low energy densities. New nanoelectrofuel format resolves this issue by using stable suspensions of solid battery-active nanoparticles in the electrolyte at high loading that can be charged and discharged multiple times when passing through customized flow battery cells. The flowable format of this technology provides a key advantage over conventional batteries: its energy storing material - nanoelectrofuel - can be separated from its charging device, the flow cell. The latter allows for highly flexible battery system designs and a wide range of applications form fueling the electric vehicles to supplying electricity from renewable sources to homes and more.

This presentation will focus on the development of nanoelectrofuel anodes with Li-ion battery chemistries. Amongst the known Li-ion battery chemistries we chose nanosized Li₄Ti₅O₁₂ (LTO) for nanoelectrofuel anode development. Its theoretical capacity is 175 mAh/g and typical experimental capacity values are up to 150 mAh/g [1]. The main reason for selection of this material for NEF anode however is its relatively high and flat lithiation/delithiation voltage (around 1.55 V vs. Li/Li⁺), which can minimize the formation of solid-electrolyte interphase layer (SEI) affecting the anode performance and its excellent capacity reversibility due to its minimal decrease in unit cell volume of 0.2% as transitions between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂ [2]. It was previously demonstrated that electrochemical performance of electrode material could be effectively improved by changing particle sizes and microstructures because decreasing grain size to nano-scale can shorten the Li⁺diffusion pathway [3,4].

In our work we compare performance of LTO nanoparticles in solid casted electrodes to the electrochemical performance of the same nanoparticles in form of suspension electrodes. For solid casted electrodes LTO nanoparticles are deposited on Cu substrate for electrochemical study in coin cells vs. Li-metal foil (half-cell configuration). Cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy are used in characterization of the coin cells. The LTO nanomaterials show good cycling performance and the initial discharge capacity of 150 mAh/g. After charge/discharge 100 cycles at C/4 rate the discharge capacity is 96% of initial value. For comparison we tested commercial pre-deposited LTO electrode with micron-sized LTO particles in the same conditions and it showed LTO nanoparticles exhibit higher reversible capacity and better cycling performance. These effects are most likely related to the advantages of nano-scale materials.

Next we investigate suspensions of LTO nanoparticles dispersed in commercial Li-ion electrolyte. The capacities are studied by measuring charge/discharged curves at different C-rates. Various weight percentage active materials are studies as well as the ratio required between active material and the current collectors, the materials of the current collectors and effects of the different additives and surface modifications on the suspension electrochemical performance.  The LTO nanoparticles and the current collectors are separated from suspension after the electrochemical tests and examined with X-ray diffraction and scanning electron microscopy to study the phase changes, agglomeration and residue on the current collectors after multiple charge/discharge cycles.

Additionally this presentation will discuss characterization of solid casted and suspension electrodes with X-ray absorption fine structure (XAFS) that reveals local structure changes around Ti atoms in charged and discharged LTO nanoparticles. 

References:

[1] Crain, D. J.; Zheng, J. P.; Roy, D. Solid State Ionics 240 (2013) 10-18.

[2] Borghols, W. J. H.; Wagemaker, M.; Lafont, U.; Kelder, E. M.; Mulder, F. M. J. Am. Chem. Soc. 2009, 131, 17786-17792

[3] Lia, X.; Hua, H.; Huanga, S.; Yub, G.; Gaoa, L.; Liub, H.; Yu, Y. Electrochimica Acta 112 (2013) 356-363

[4] Kavan, L.; Prochazka, J.; Spitler, T. M.; Kalbac, M.; Zukalova, M. T.; Drezen, T.; Gratzel, M. J. Electrochem. Soc. 2003, 150, A1000-A1007.