Theoretical dependences of the free energyand chemical potential upon compositionin intercalation systems with repulsiveinteraction between guest ions

An all manganese-based oxide nanocrystal cathode and anode for high performance lithium-ion full cells

Manganese oxide nanocrystals are of great interest for producing advanced high-performance lithium ion batteries owing to the shortened lithium ion diffusion length and accelerated interfacial charge transfer rate. Here we have developed a well-controlled generic method to synthesize monodisperse MnO nanocrystals, and present a comparative study regarding the effect of crystallite size on electrochemical stability. Nanocrystalline MnO with a size of about 10 nm shows the optimal lithium-storage performance. Notably, Mn-based nanocrystals retain their stable cyclability and excellent high-rate performance as both the anode and cathode. The all-nanocrystal MnO/C//LMO Li-ion full cells not only significantly improve the electrochemical properties of Mn-based materials but also open up avenues for the future development of various energy devices.

Quantifying structure dependent responses in Li-ion cells with excess Li spinel cathodes: matching voltage and entropy profiles through mean field models

Measurements of the open circuit voltage of Li-ion cells have been extensively used as a non-destructive characterisation tool. Another technique based on entropy change measurements has also been applied for this purpose. More recently, both techniques have been used to make qualitative statements about aging in Li-ion cells. One proposed cause of cell failure is point defect formation in the electrode materials. The steps in voltage profiles, and the peaks in entropy profiles are sensitive to order/disorder transitions arising from Li/vacancy configurations, which are affected by the host lattice structures. We compare the entropy change results, voltage profiles and incremental capacity (dQ/dV) obtained from coin cells with spinel lithium manganese oxide (LMO) cathodes, Li1+yMn2-yO4, where excess Li y was added in the range 0 ≤ y ≤ 0.2. A clear trend of entropy and dQ/dV peak amplitude decrease with excess Li amount was determined. The effect arises, in part, from the presence of pinned Li sites, which disturb the formation of the ordered phase. We modelled the voltage, dQ/dV and entropy results as a function of the interaction parameters and the excess Li amount, using a mean field approach. For a given pinning population, we demonstrated that the asymmetries observed in the dQ/dV peaks can be modelled by a single linear correction term. To replicate the observed peak separations, widths and magnitudes, we had to account for variation in the energy interaction parameters as a function of the excess Li amount, y. All Li-Li repulsion parameters in the model increased in value as the defect fraction, y, increased. Our paper shows how far a computational mean field approximation can replicate experimentally observed voltage, incremental capacity and entropy profiles in the presence of phase transitions.

Identifying the Origin and Contribution of Surface Storage in TiO2 (B) Nanotube Electrode by In Situ Dynamic Valence State Monitoring

Fundamental insight into the surface charging mechanism of TiO2 (B) nanomaterials is limited due to the complicated nature of lithiation behavior, as well as the limitations of available characterization tools that can directly probe surface charging process. Here, an in situ approach is reported to monitor the dynamic valence state of TiO2 (B) nanotube electrodes, which utilizes in situ X-ray absorption spectroscopy (XAS) to identify the origin and contribution of surface storage. A real-time correlation is elucidated between the rate-dependent electrode performance and dynamic Ti valence-state change. A continuous Ti valence state change is directly observed through the whole charging/discharging process regardless of charging rates, which proves that along with the well-known non-faradaic reaction, the surface charging process also originates from a faradaic reaction. The quantification of these two surface storage contributions at different charging rates is further realized through in situ dynamic valence state monitoring combined with traditional cyclic voltammetry measurement. The methodology reported here can also be applied to other electrode materials for the real-time probing of valence state change during electrochemical reactions, the quantification of the faradaic and non-faradaic reactions, and the eventual elucidation of electrochemical surface charging mechanisms.

High Cycling Stability and Extreme Rate Performance in Nanoscaled LiMn2O4 Thin Films

Ultrathin LiMn2O4 electrode layers with average crystal size of ∼15 nm were fabricated by means of radio frequency sputtering. Cycling behavior and rate performance was evaluated by galvanostatic charge and discharge measurements. The thinnest films show the highest volumetric capacity and best cycling stability, retaining the initial capacity over 70 (dis)charging cycles when manganese dissolution is prevented. The increased stability for film thicknesses below 50 nm allows cycling in both the 4 and 3 V potential regions, resulting in a high volumetric capacity of 1.2 Ah/cm3. It is shown that the thinnest films can be charged to 75% of their full capacity within 18 s (200 C), the best rate performance reported for LiMn2O4. This is explained by the short diffusion lengths inherent to thin films and the absence of phase transformation.

Fast Li-Ion insertion into nanosized LiMn(2)O(4) without domain boundaries

The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn(2)O(4). Accurate size control of nanocrystalline LiMn(2)O(4), which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn(2)O(4) with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process.

Nanosize effect on high-rate Li-ion intercalation in LiCoO2 electrode

Recently, battery technology has come to require a higher rate capability. The main difficulty in high-rate charge-discharge experiments is kinetic problems due to the slow diffusion of Li-ions in electrodes. Nanosizing is a popular way to achieve a higher surface area and shorter Li-ion diffusion length for fast diffusion. However, while various nanoelectrodes that provide excellent high-rate capability have been synthesized, a size-controlled synthesis and a systematic study of nanocrystalline LiCoO2 have not been carried out because of the difficulty in controlling the size. We have established the size-controlled synthesis of nanocrystalline LiCoO2 through a hydrothermal reaction and, for the first time, clarified the structural and electrochemical properties of this intercalation cathode material. Lattice expansion in nanocrystalline LiCoO2 was found from powder X-ray diffraction measurements and Raman spectroscopy. Electrochemical measurements and theoretical analyses on nanocrystalline LiCoO2 revealed that extreme size reduction below 15 nm was not favorable for most applications. An excellent high-rate capability (65% of the 1 C rate capability at 100 C) was observed in nanocrystalline LiCoO2 with an appropriate particle size of 17 nm.