Routes towards high-capacity and high-rate Li ion insertion batteries Miran Gaberšček1,2 1National Institute of Chemistry, Ljubljana, Slovenia 2Faculty of Chemistry and Chem. Technol., University of Ljubljana, Slovenia
Development of Li-ion batteries Higher energy density Small electronic devices and small batteries Higher power density Environmentally friendly Higher safety
Principle of Li ion battery operation Anode: Cathode: C6 + xLi+ + xe- LixC6LiCoO6 LixCoO6 + (1-x) Li+ + (1-x)e-
CATHODES (+) ANODES (-) From J.-M. Tarascon, M. Armand, Nature, 2001, vol 414, p.359. Overview of Li ion battery materials
Promising candidates for high capacity LixM2-yMyO4 M = transition element Good candidates: Ge, Sb, As, P, Si, Ti Li2MSiO4 Our choice No 1: M = Si Our choice No 2: M = Ti Li2MTiO4 M=Fe, Mn, Ni, V
STRUCTURE 1 Orthorhombic unit cell, space group Pmn21
STRUCTURE 2 (cubic rock-salt structure with cationic disorder, space group Fm3m) • Li2FeTiO4 - LFT • Li2MnTiO4 - LMT • Li2NiTiO4 - LNT - oxygen Multicoloured balls - Li, Fe, Ti
Do we observe exchange of more than 1 Li in Li2MSiO4or in Li2MTiO4?
Note: Capacity depends on the voltage window Increasing voltage Increasing capacity
Cycling stability is moderate
Time-limited experiment above 3.8 V additional capacity was detected no structural changes observed reversible SEI formation? In-situ XRD nanosized LFT Voltage-limited experiment single phase insertion mechanism red. 1.4 %contraction of unit-cell volume ox.
as-prepared material (19.5 at.% of Fe3+) reduction process introduces higher disorder structural changes are not fully reversible In-situMössbauer spectroscopy for nanosized LFT Time-limited experiment complete oxidation Fe2+ to Fe3+ + additional capacity above 3.9 V was observed end of reduction no change in Fe ox. state in last two spectra reversible SEI formation
oxygen neigh. Fe2+ oxidation Fe3+ In-situ XAS for nanosized LFT (EXAFS spectra) 3+3 octahedral deformation (as-prepared sample) Jahn-Teller octahedral deformation on the induvidualcrystallographic site 4+2 octahedral deformation (completely oxidized sample)
Partial conclusion • Silicates and titanates can give high • reversible capacities (< 300 mAh/g) • No direct proof that more than 1 e- is exchanged • (example: in Li2MnTiO4 only 1 Li is exchanged = 150 mAh/g) 3. At high and low potential additional reversible capacity is observed 4. The origin of additional capacity is still unclear
PROBLEM OF BATTERY POWER Electric power: P = UI Special case: UI , U=f(I) Electric power density: p = m URI 1C =170 mA/g Polarization (due to internal “resistance”)
Two transport steps: 1. Toward active particles 2. Inside active particles 1 2 2 1 Major sources of internal resistance
50-200nm L2 = Dchem How to decrease resistance inside solid particles? 1-30 m
4,0 U1 3,8 LiFePO4 (600 nm) /Li] 3,6 + 3,4 U [V vs. Li 3,2 3,0 2,8 0 20 40 60 80 100 120 140 Capacity [mAh/g] Rev. capacity U2 100 nm 600 nm Impact of particle size (example: LiFePO4) LiFePO4 (ca. 100 nm) Reversible capacity
Measured Electrode Resistance, Rm (U/I) as a function of particle diameter THEORY: LiFePO4 with ad-mixed carbon black carbon-coated LiFePO4 with ad-mixed carbon black
50-200 nm 1-30 m L2 = Dchem ? 5-10 nm Can wefurther decrease particle size?
Problem Heating leads to pronounced particle growth and agglomeration TiO2 nanotubes TiO2 anatase nanoparticles heating 15-20 nm 8-10 nm 10 nm
10 nm Problem solution TiO2 nanotubes TiO2 anatase nanoparticles 8-10 nm 4-7 nm heating TEOS (silica precursor)
Impact of surface treatment on morphology development
Impact of morphology on power capability and capacity Small particles, + loose structure Smaller polarization= BIGGER POWER! Bigger particles + more compact structure J. Jamnik, R. Dominko, B. Erjavec, M. Remskar, A. Pintar, M. Gaberscek, Adv. Mater.21,2715 (2009).
Impact on cycling and on current density Increasing current density J. Jamnik, R. Dominko, B. Erjavec, M. Remskar, A. Pintar, M. Gaberscek, Adv. Mater.21,2715 (2009).
Why do we always see hysteresis between charge and discharge? W.Dreyer, J. Jamnik, ..and M. Gaberscek, Nature Materials 9, 448 (2010).
Model assumptions Two main assumptions: (i)The chemical potential of the individual particle is a non-monotone function of the Li mole fraction. (ii) Network consists of manyparticles. W.Dreyer, J. Jamnik, ..and M. Gaberscek, Nature Materials 9, 448 (2010).
