Slow Li-ion Diffusion Research Articles

Multi-Directional 3D Laser Scanning of Lithium-Ion Cells to Detect Inhomogeneity during Cycling and Aging

Introduction Measuring the thickness of lithium-ion batteries is a reliable method to detect the intercalation stages of the electrodes, the state of charge and irreversible effects during aging [1]. When using tactile high-precision displacement sensors, the measurement is limited to only a few positions of the cell at the same time [2]. Experimental In this work, we developed a test stand, which enables simultaneous precise thickness measurement at 100 positions of the cell. This allows for investigations of the homogeneity of mechanical expansion during charging and discharging, as well as non-destructive state-of-health (SOH) mapping. In a first step, inhomogeneity of mechanical expansion during cycling is studied. In a second step, the effects of expansion inhomogeneity during cycling on the aging behavior of the cells are investigated by determining the irreversible and reversible thickness change at each of the 100 positions every 100th cycle. Results We found that the cell contracts homogeneously during discharge, whereas a significant overshoot in the thickness increase during high-rate charging at the current collector tabs is observed at 25 °C. When the ambient temperature is increased to 40 °C, this inhomogeneity is no longer observable. This effect has been ascribed in literature to non-homogeneous intercalation of Li-ions due to the interplay of fast and slow Li ion diffusion processes [3]. The SOH distributions reveal a homogeneous aging of the cells cycled at low charging rate and high charging rate at 40 °C. Compared to that, the cells cycled at high charging rate at 25 °C show inhomogeneous degradation in areas close to the current collector tabs which show the largest degradation. Literature [1] J. Cannarella, C.B. Arnold, Journal of Power Sources 269 (2014) 7–14. [2] K.-Y. Oh, J.B. Siegel, L. Secondo, S.U. Kim, N.A. Samad, J. Qin, D. Anderson, K. Garikipati, A. Knobloch, B.I. Epureanu, C.W. Monroe, A. Stefanopoulou, Journal of Power Sources (2014). [3] L.W. Sommer, P. Kiesel, A. Ganguli, A. Lochbaum, B. Saha, J. Schwartz, C.-J. Bae, M. Alamgir, A. Raghavan, Journal of Power Sources 296 (2015) 46–52. Figure 1

Fast and slow ion diffusion processes in lithium ion pouch cells during cycling observed with fiber optic strain sensors

Cell monitoring for safe capacity utilization while maximizing pack life and performance is a key requirement for effective battery management and encouraging their adoption for clean-energy technologies. A key cell failure mode is the build-up of residual electrode strain over time, which affects both cell performance and life. Our team has been exploring the use of fiber optic (FO) sensors as a new alternative for cell state monitoring. In this present study, various charge-cycling experiments were performed on Lithium-ion pouch cells with a particular class of FO sensors, fiber Bragg gratings (FBGs), that were externally attached to the cells. An overshooting of the volume change at high SOC that recovers during rest can be observed. This phenomenon originates from the interplay between a fast and a slow Li ion diffusion process, which leads to non-homogeneous intercalation of Li ions. This paper focuses on the strain relaxation processes that occur after switching from charge to no-load phases. The correlation of the excess volume and subsequent relaxation to SOC as well as temperature is discussed. The implications of being able to monitor this phenomenon to control battery utilization for long life are also discussed.

Facile Synthesis of Anatase TiO2 Quantum Dot/ Graphene Nanosheet Composites with Enhanced Electrochemical Performance for Lithium-Ion Batteries

1. Introduction With rising interest in green electrode materials for lithium-ion batteries (LIBs), increasing attention has been paid to titanium dioxide (TiO2) anode material in recent years because of its long cycle life, low cost, and minimum environmental impact. Moreover, the relatively high lithium insertion /extraction voltage of a TiO2 anode (higher than 1.5 V vs Li+/Li) can efficiently avoid the formation of SEI layers and lithium plating on the anode, which improves the safety of the batteries as compared with its carbon-based counterparts. However, many potential electrode materials (e.g., TiO2) in Li-ion batteries are limited by poor electron transport, slow Li-ion diffusion in electrodes, and increased resistance at the interface of electrode/electrolyte at high chargedischarge rates. Graphene, which has exceptional electrical, mechanical, optical, and surface properties, is widely utilized to prepare various hybrid materials. The graphene substrate itself can be contributory to the improved electrochemical performance because it may enhance the electronic conductivity of the overall electrode[1]. It is noteworthy, on the other hand, that the size and dispersion of nanoparticles on graphene are crucial factors for improving cell performance because small particle size plus good dispersion (e.g., down to several nanometers) can endow the composite electrode a superior high surface area to buffer the volume change of the particles, but it could also bring the required conductivity to individual nanoparticles and shorten the diffusion length for Li ions, which are beneficial for high lithium storage and rate capability, respectively. However, most metal oxide/graphene composites prepared so far have the high level of metal oxides accompanied with the partial aggregation of particles may also result in the rapid capacity loss and poor cycle performance. Therefore, it remains a challenge to develop a facile and general approach for the synthesis of well-dispersed MOx (e.g., TiO2) Quantum Dots/Graphene composites with favored structures for high-performance lithium-ion batteries (LIBs). 2. Experimental Section 2.1 Synthesis of graphene oxide (GO) The graphite oxide was synthesized from natural graphite flake (Alfa Aesar, 325 mesh) by a modified Hummers method. As-prepared graphite oxide was dispersed in water by ultrasonication for 30 min, followed by a low-speed centrifugation to get rid of any aggregated GO.2.2 Preparation of TiO2 quantum dots /Graphene nanosheets (TiO2-QDs/GNs) Composites In a typical experimental procedure, 5.8 g of CTAB was dissolved in a mixture of 10 ml of n-pentanol and 60 ml of n-hexane; Then, the 10 ml GO aqueous dispersion (1 mg mL-1) was slowly poured and intensely stirred for 30 min at room temperature. Subsequently, with the formation of a golden water-in-oil emulsion. Then 0.8 ml of Titanium(III) chloride was added to golden water-in-oil emulsion while stirring. The achieved transparent microemulsions were poured into a Teflon-lined stainless steel autoclave (100 ml), and then placed in an oven maintaining 200°C for 6 h. The collected precipitates were treated under reduced pressure in a rotary evaporator to remove the volatile organic reagents and then repeatedly washed with water and ethanol to remove surfactants and other impurities. The final samples were dried at 80 °C for 2 h for further characterizations. 3. Results and Discussion In summary, we report a facile method to synthesize well-dispersed TiO2 quantum dots (6±2 nm) on graphene nanosheets (TiO2-QDs/GNs) in a water-in-oil (W/O) emulsion system (Figure 1). The prepared TiO2/graphene composites displayed high performance as an anode material for lithium-ion battery, such as high reversible lithium storage capacity (190 mA h g-1 after 100 cycles), high Coulombic efficiency (over 96%), excellent cycling stability and high rate capability (as high as 144 mA h g-1 at 10 C, 135 mA h g-1 at 20 C, 124 mA h g-1 at 30 C and 101 mA h g-1 at 50 C, respectively). Very significantly, the preparation method employed can be easily adapted and may offer an attractive alternative approach for preparation of the highly dispersed nanosized graphene-based nanostructured composites as promising applications in various energy-storage devices high performance electrodes for various energy-storage devices.Figure 1 Electrochemical measurements of (a) TiO2-QDs/GNs and (b) TiO2-QDs electrodes. Inset in Fiure 1 is the Schematic of synthesis steps for TiO2-QDs/GNs composite.

Growth and characterization of LiNiVO 4 thin film cathode by pulsed laser deposition

Crystallized LiNiVO 4 thin films have been prepared by pulsed laser deposition and their physical and electrochemical properties have been studied. With the increase of deposition temperatures and oxygen pressures, the crystallization became better, but accompanied with large sizes of grains. The initial discharge capacity of the film deposited at 873 K and 40 Pa of oxygen was just around 7.2 μA h/cm 2 μm when it was cycled between 3.0 and 4.8 V with a current density of 10 μA h/cm 2. Cyclic voltammetry at a sweep rate of 0.1 mV/s showed a main anodic peak at 4.20 V, a weak anodic peak at 4.59 V and a cathodic peak located at 3.73 V. Based on the linear relationship between the peak currents of cathodic peaks and the square roots of scan rates, the diffusion coefficient was estimated to be about 2.3 × 10 − 15 cm 2/s. Electrochemical impedance spectra revealed high charge-transfer resistance of Li-ion, such as about 9000 Ω at 4.0 V. The extremely slow Li-ion diffusion and high charge-transfer resistance indicate that the electrochemical reaction in LiNiVO 4 thin films is sluggish.