Mahalingam Balasubramanian

Hollow iron oxide nanoparticles for application in lithium ion batteries.

Material design in terms of their morphologies other than solid nanoparticles can lead to more advanced properties. At the example of iron oxide, we explored the electrochemical properties of hollow nanoparticles with an application as a cathode and anode. Such nanoparticles contain very high concentration of cation vacancies that can be efficiently utilized for reversible Li ion intercalation without structural change. Cycling in high voltage range results in high capacity (∼132 mAh/g at 2.5 V), 99.7% Coulombic efficiency, superior rate performance (133 mAh/g at 3000 mA/g) and excellent stability (no fading at fast rate during more than 500 cycles). Cation vacancies in hollow iron oxide nanoparticles are also found to be responsible for the enhanced capacity in the conversion reactions. We monitored in situ structural transformation of hollow iron oxide nanoparticles by synchrotron X-ray absorption and diffraction techniques that provided us clear understanding of the lithium intercalation processes during electrochemical cycling.

Nanostructured Bilayered Vanadium Oxide Electrodes for Rechargeable Sodium-Ion Batteries

Tailoring nanoarchitecture of materials offers unprecedented opportunities in utilization of their functional properties. Nanostructures of vanadium oxide, synthesized by electrochemical deposition, are studied as a cathode material for rechargeable Na-ion batteries. Ex situ and in situ synchrotron characterizations revealed the presence of an electrochemically responsive bilayered structure with adjustable intralayer spacing that accommodates intercalation of Na(+) ions. Sodium intake induces organization of overall structure with appearance of both long- and short-range order, while deintercalation is accompanied with the loss of long-range order, whereas short-range order is preserved. Nanostructured electrodes achieve theoretical reversible capacity for Na(2)V(2)O(5) stochiometry of 250 mAh/g. The stability evaluation during charge-discharge cycles at room temperature revealed an efficient 3 V cathode material with superb performance: energy density of ~760 Wh/kg and power density of 1200 W/kg. These results demonstrate feasibility of development of the ambient temperature Na-ion rechargeable batteries by employment of electrodes with tailored nanoarchitectures.

Surface changes on LiNi0.8Co0.2O2 particles during testing of high-power lithium-ion cells

LiNi0.8Co0.2O2 particles from high-power lithium-ion cells were examined to determine material changes that result from accelerated aging tests. X-ray absorption spectroscopy (XAS) and transmission electron microscope (TEM) data indicated a LixNi1−xO-type layer on the particle surfaces. The greater thickness on particles from high-power fade cells indicate that these surface layers are a significant contributor to cathode impedance rise observed during cell tests.

Fe/N/C composite in Li-O2 battery: studies of catalytic structure and activity toward oxygen evolution reaction.

Atomically dispersed Fe/N/C composite was synthesized and its role in controlling the oxygen evolution reaction during Li-O(2) battery charging was studied by use of a tetra(ethylene glycol) dimethyl ether-based electrolyte. Li-O(2) cells using Fe/N/C as the cathode catalyst showed lower overpotentials than α-MnO(2)/carbon catalyst and carbon-only material. Gases evolved during the charge step contained only oxygen for Fe/N/C cathode catalyst, whereas CO(2) was also detected in the case of α-MnO(2)/C or carbon-only material; this CO(2) was presumably generated from electrolyte decomposition. Our results reiterate the catalytic effect in reducing overpotentials, which not only enhances battery efficiency but also improves its lifespan by reducing or eliminating electrolyte decomposition. The structure of the Fe/N/C catalyst was characterized by transmission electron microscopy, scanning transmission electron microscopy, inductively coupled plasma optical emission spectroscopy, and X-ray absorption spectroscopy. Iron was found to be uniformly distributed within the carbon matrix, and on average, Fe was coordinated by 3.3 ± 0.6 and 2.2 ± 0.3 low Z elements (C/N/O) at bond distances of ~1.92 and ~2.09 Å, respectively.

Microscopy and spectroscopy of lithium nickel oxide-based particles used in high power lithium-ion cells

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy (XAS) revealed trivalent Ni and Co ions in the bulk LiNi{sub 0.8}Co{sub 0.2}O{sub 2} powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a <5 nm thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered R{bar 3}m structure. This structural change was accompanied by oxygen loss and a lowering of the Ni- and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells.

Unique behaviour of nonsolvents for polysulphides in lithium–sulphur batteries

Combination of a solvent–salt complex [acetonitrile(ACN)2–LiTFSI] with a hydrofluoroether (HFE) co-solvent unveil a new class of Li–S battery electrolytes. They possess stability against Li metal and viscosities which approach that of conventional ethers, but they have the benefit of low volatility and minimal solubility for lithium polysulphides while exhibiting an uncharacteristic sloping voltage profile. In the optimal system, cells can be discharged to full theoretical capacity under quasi-equilibrium conditions while sustaining high reversible capacities (1300–1400 mA h g−1) at moderate rates, and capacities of 1000 mA h g−1 with almost no capacity fade at fast discharge rates under selected cycling protocols. A combination of operando X-ray absorption spectroscopy at the S K-edge, and electrochemical studies demonstrate that lithium polysulphides are indeed formed in these ACN-complexed systems. Their limited dissolution and mobility in the electrolyte strongly affect the speciation and polysulphide equilibria, leading to controlled precipitation of Li2S.

Investigation of the local structure of the LiNi0,5Mn0,5O2 cathode material during electrochemical cycling by X-ray absorption and NMR spectroscopy

In situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges and 6 Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy have been carried out during the first charging and discharging process for the layered LiNi 0 . 5 Mn 0 . 5 O 2 cathode material. The Ni K-edge structure in the X-ray absorption near-edge structure (XANES) spectrum exhibits a rigid positive energy shift with increased Li deintercalation level, while the Mn XANES spectra do not show any substantial energy changes. The Ni edge shifts back reversibly during discharge. Further Li-ion intercalation at ∼1 V (vs. Li) could be accomplished by reduction of the Mn 4 + ions. The 6 Li MAS NMR results showed the presence of Li in the Ni 2 + /Mn 4 + layers, in addition to the expected sites for Li in the lithium layers. All the Li ions in the transition metal layers are removed on the first charge, leaving residual lithium ions in the lithium layers.

Formation of SEI on Cycled Lithium-Ion Battery Cathodes: Soft X-ray Absorption Study

The formation of a solid electrolyte interface (SEI) on LiNi 0 . 8 5 Co 0 . 1 5 O 2 cathodes from lithium-ion cells cycled at 40 and 70°C was observed and characterized using soft X-ray absorption spectroscopy (XAS). XAS measurements were made in the energy region between 500 and 950 eV, encompassing the Ni and Co L 3 - and L 2 -edges and at the K-edges of O and F. Measurements, obtained in the total electron yield mode, are surface sensitive, probing to a depth of ∼5 nm. XAS at the F K-edge demonstrates the presence of poly(vinylidene fluoride) (PVdF) in addition to LiF on the surface of cycled electrodes. The results indicate that the PVdF in the cycled electrodes is largely intact and that the LiF comes from decomposition of LiPF 6 from the electrolyte. XAS also suggests Fe contamination of cycled cathodes.

In Situ X‐Ray Absorption Studies of a High‐Rate LiNi0.85Co0.15 O 2 Cathode Material

We have performed an in situ X-ray absorption spectroscopy (XAS) study to investigate the evolution of the local electronic and atomic structure of a high-surface-area Li 1-x Ni 0.85 Co 0.15 O 2 (0 ≤ x ≤ 1) cathode material during electrochemical delithiation. We have measured the changes in the oxidation state, bond distance, coordination number, and local disorder of Ni and Co absorbers as a function of the state of charge of the material. The X-ray absorption near edge spectra shows that delithiation of Li 1-x Ni 0.85 Co 0.15 O 2 leads to the oxidation of Ni 3+ to Ni 4+ . Ni atoms oxidize during the initial stages of charge and attain a maximum oxidation state of Ni 4+ well before the end of charge (x 0.85). On the other hand, Co atoms do not oxidize during the initial stages of charge but oxidize close to the end of charge. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that the oxidation of Ni 3+ to Ni 4+ leads to the expected reduction in the Jahn-Teller effect. Also, to within the accuracy of the EXAFS technique, Co absorbers occupy Ni-type sites in the NiO 2 slabs. Furthermore, Co doping has a strong effect on the overall structural evolution and leads to a slight expansion of the a and b axes close to the end of charge.

Combined NMR and XAS Study on Local Environments and Electronic Structures of Electrochemically Li-Ion Deintercalated Li1 − x Co1 / 3Ni1 / 3Mn1 / 3 O 2 Electrode System

Combined 6 Li magic-angle spinning (MAS) NMR, in situ metal K-edge (hard) X-ray absorption spectroscopy (XAS), and O K-edge (soft) XAS have been carried out during the first charging process for layered Li 1 - x Co 1 / 3 Ni 1 / 3 Mn 1 / 3 O 2 cathode material. The 6 Li MAS NMR results showed the presence of Li in the Ni 2 + /Mn 4 + layers, in addition to the expected sites for Li in the lithium layers. On charging, Li ions in both the transition metals and lithium layers are removed and no new resonances are observed. The metal K-edge XAS results suggest that the major charge compensation at the metal site during charge is achieved by oxidation of Ni 2 + ions, while manganese ions remain mostly unchanged in the Mn 4 + state. From observation of O K-edge XAS results, one can conclude that a large portion of the charge compensation during charge is achieved in the oxygen site. This work provides the possibility of larger capacity of the electrode material using Li in the transition metal layers and contribution of oxygen during charge.