A. Robert Armstrong

Silicate cathodes for lithium batteries: alternatives to phosphates?

Polyoxyanion compounds, particularly the olivine-phosphate LiFePO4, are receiving considerable attention as alternative cathodes for rechargeable lithium batteries. More recently, an entirely new class of polyoxyanion cathodes based on the orthosilicates, Li2MSiO4 (where M = Mn, Fe, and Co), has been attracting growing interest. In the case of Li2FeSiO4, iron and silicon are among the most abundant and lowest cost elements, and hence offer the tantalising prospect of preparing cheap and safe cathodes from rust and sand! This Highlight presents an overview of recent developments and future challenges of silicate cathode materials focusing on their structural polymorphs, electrochemical behaviour and nanomaterials chemistry.

Nanoparticulate TiO2(B): an anode for lithium-ion batteries.

Titanates are being intensively investigated as anodes for lithium-ion batteries due to their superior safety and rate capability compared with graphite, although their higher voltage lowers the overall energy density of the lithium-ion cell. Li4Ti5O12 spinel is now used in commercial lithium-ion batteries. TiO2 possesses twice the theoretical specific capacity (335 mAh g ) compared with Li4Ti5O12 (175 mAhg ), i.e., is comparable to graphite, rendering TiO2 potentially attractive as an anode for Li-ion batteries. 3] TiO2(B) can accommodate more lithium than any other TiO2 polymorph as a bulk material (micrometer-sized particles). 4] It has been shown that nanostructured forms of TiO2(B) enhance rate capability compared to the bulk, with nanotubes of TiO2(B) or TiO2(B)/anatase microspheres exhibiting the highest rate capability to date. Their performance is exceeded by nanoparticulate TiO2(B) described here. Nanoparticulate TiO2(B) was synthesized by a procedure described in the Experimental Section. Briefly, Ti metal is dissolved in a mixture of H2O2 and NH3 in water, to which glycolic acid is added, forming a titanium glycolate complex. This is subjected to hydrothermal treatment at 160 8C for 30 min. The resulting solid was finally calcined in dry air at 300 8C for 1 h. The powder X-ray diffraction pattern (PXRD) of nanoparticulate TiO2(B) is shown in Figure 1, where it is compared with standard TiO2(B). The small particle dimensions necessarily result in peak broadening but the powder diffraction pattern follows the same intensity distribution as that of the bulk material. Confirmation that the nanoparticulate powder is TiO2(B) was obtained by high-resolution TEM (Figure 2); lattice spacings of 0.357 nm and 0.619 nm corresponding to the (110) and (001) reflections from the TiO2(B) structure (ICDD 046-1237) are observed. O NMR spectra of O-enriched TiO2 clearly show characteristic resonances from the three oxygen coordination environments OTi2, OTi3, and OTi4 in intensity ratios of approximately 1:2:1, which is characteristic of the TiO2(B) polymorph (Supporting Information, Figure S2). Rutile and anatase both contain only OTi3 environments. The TEM data demonstrate that the material is composed of nanoparticles of ca. 2.5 4.3 nm size (based on analysis of 100 nanoparticles), with a relatively narrow size distribution, and that form agglomerates of 0.3–3 mm (Figure 2a and S1). The BET surface area determined from N2 adsorption is 251 m g 1 (pore volume 0.12 cm g ) whereas, based on the primary particle size of 2.5 2.5 4.3 nm, the predicted surface area is 567 m g , i.e., more than twice the observed value and consistent with aggregation of the primary particles into porous agglomerates. One problem that often besets nanoparticles is the need to employ molecules (e.g. surfactants) in the synthesis to inhibit particle size growth. Such molecules, if they remain on the Figure 1. PXRD pattern of the TiO2(B) nanoparticles. Bulk TiO2(B) from the ICDD database is shown for comparison.

Influence of Size on the Rate of Mesoporous Electrodes for Lithium Batteries

High power rechargeable lithium batteries are a key target for transport and load leveling, in order to mitigate CO(2) emissions. It has already been demonstrated that mesoporous lithium intercalation compounds (composed of particles containing nanometer diameter pores separated by walls of similar size) can deliver high rate (power) and high stability on cycling. Here we investigate how the critical dimensions of pore size and wall thickness control the rate of intercalation (electrode reaction). By using mesoporous beta-MnO(2), the influence of these mesodimensions on lithium intercalation via single and two-phase intercalation processes has been studied in the same material enabling direct comparison. Pore size and wall thickness both influence the rate of single and two-phase intercalation mechanisms, but the latter is more sensitive than the former.

The lithium intercalation process in the low-voltage lithium battery anode Li1+xV1−xO2

Lithium can be reversibly intercalated into layered Li(1+x)V(1-x)O(2) (LiCoO(2) structure) at ~0.1 V, but only if x>0. The low voltage combined with a higher density than graphite results in a higher theoretical volumetric energy density; important for future applications in portable electronics and electric vehicles. Here we investigate the crucial question, why Li cannot intercalate into LiVO(2) but Li-rich compositions switch on intercalation at an unprecedented low voltage for an oxide? We show that Li(+) intercalated into tetrahedral sites are energetically more stable for Li-rich compositions, as they share a face with Li(+) on the V site in the transition metal layers. Li incorporation triggers shearing of the oxide layers from cubic to hexagonal packing because the Li(2)VO(2) structure can accommodate two Li per formula unit in tetrahedral sites without face sharing. Such understanding is important for the future design and optimization of low-voltage intercalation anodes for lithium batteries.

Structure and Lithium Transport Pathways in Li2FeSiO4 Cathodes for Lithium Batteries

The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li(2)FeSiO(4). The structure of Li(2)FeSiO(4) undergoes significant change on cycling, from the as-prepared γ(s) form to an inverse β(II) polymorph; therefore it is important to establish the structure of the cycled material. In γ(s) half the LiO(4), FeO(4), and SiO(4) tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse β(II) polymorph on cycling involves inversion of half the SiO(4), FeO(4), and LiO(4) tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li(+) transport paths and corresponding Li-Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO(4) tetrahedra.

Dependence of Li2FeSiO4 Electrochemistry on Structure

Small differences in the FeO(4) arrangements (orientation, size, and distortion) do influence the equilibrium potential measured during the first oxidation of Fe(2+) to Fe(3+) in all polymorphs of Li(2)FeSiO(4).

TiO2(B) nanotubes as negative electrodes for rechargeable lithium batteries

TiO 2 (B) nanotubes were investigated as anodes for rechargeable lithium batteries. They can accommodate up to 338 mAh g -1 of charge, equivalent to Li 1.01 TiO 2 (B) at a potential of ∼1.5 V vs Li + (1 M)/Li. Rate capability is good with a capacity of 95 mAh g -1 at 2000 mA g -1 (21C). Capacity fade is 0.16% per cycle compared with 0.10% for the corresponding nanowires. There is an irreversible capacity loss of 29% on the first cycle but thereafter charge/discharge efficiency is close to 100%.

Na0.67Mn1−xMgxO2 (0 ≤ x ≤ 0.2): a high capacity cathode for sodium-ion batteries

Earth-abundant Na0.67[Mn1−xMgx]O2 (0 ≤ x ≤ 0.2) cathode materials with the P2 structure have been synthesized as positive electrodes for sodium-ion batteries. Na0.67MnO2 exhibits a capacity of 175 mA h g−1 with good capacity retention. A Mg content of 5% is sufficient to smooth the charge/discharge profiles without affecting the capacity, whilst further increasing the Mg content improves the cycling stability, but at the expense of a lower discharge capacity (∼150 mA h g−1 for Na0.67Mn0.8Mg0.2O2). It was observed that the cooling process during synthesis, as well as Mg content, have an influence on the structure.

β-NaMnO2: A High-Performance Cathode for Sodium-Ion Batteries

There is much interest in Na-ion batteries for grid storage because of the lower projected cost compared with Li-ion. Identifying Earth-abundant, low-cost, and safe materials that can function as intercalation cathodes in Na-ion batteries is an important challenge facing the field. Here we investigate such a material, β-NaMnO2, with a different structure from that of NaMnO2 polymorphs and other compounds studied extensively in the past. It exhibits a high capacity (of ca. 190 mA h g(-1) at a rate of C/20), along with a good rate capability (142 mA h g(-1) at a rate of 2C) and a good capacity retention (100 mA h g(-1)after 100 Na extraction/insertion cycles at a rate of 2C). Powder XRD, HRTEM, and (23)Na NMR studies revealed that this compound exhibits a complex structure consisting of intergrown regions of α-NaMnO2 and β-NaMnO2 domains. The collapse of the long-range structure at low Na content is expected to compromise the reversibility of the Na extraction and insertion processes occurring upon charge and discharge of the cathode material, respectively. Yet stable, reproducible, and reversible Na intercalation is observed.

New intercalation compounds for lithium batteries: layered LiMnO2

The mechanism of lithium intercalation in layered LiMnO 2 has been investigated by combining data from a variety of techniques, including powder X-ray and neutron diffraction, cyclic voltammetry and galvanostatic cycling. Whereas the diffraction data indicate the coexistence of layered and spinel phases at Li 0.5 MnO 2 after 5 charge(extraction)-discharge(insertion) cycles, the electrochemical data only change significantly on the first charge(extraction), near Li 0.5 MnO 2 . A rationale is provided by a model in which, on first extracting 0.5 Li from layered LiMnO 2 , displacement of Mn ions occurs into the lithium layers, forming regions with the local structure and composition of spinel. This can explain the presence of a 4 V peak in the cyclic voltammogram on the first charge. Long range order only develops on more extended cycling and since this does not alter significantly the Li + or e – energies, the electrochemistry does not change further. Load curves show significant hysteresis and this is linked to a domain-like microstructure with spinel imbedded in layered material. The marked difference between load curves for this material and LiMn 2 O 4 spinel indicates that the former does not convert to ‘normal’ spinel on cycling. By doping LiMnO 2 with as little as 10% Co the cooperative Jahn-Teller distortion due to localised high spin Mn 3+ (3d 4 ) disappears despite the high concentration of Mn 3+ and a substantial improvement in the ability to cycle lithium is obtained from 130 mAh g –1 to 200 mAh g –1 at 100 µA cm –2 .