Engineering topics
Allen, Joshua L.
Publications and source records attributed to Allen, Joshua L..
Changes in Electronic Structure upon Li Deintercalation from LiCoPO 4 Derivatives
On the path toward the design of Li-ion batteries with increased energy densities, efforts are focused on the development of positive electrodes that can maximize the voltage of the full cell. However, the development of novel materials that operate at high voltage, while also showing high efficiency and meeting strict safety standards, is an ongoing challenge. LiCoPO 4 is being explored as a possible candidate, as the Co 2+/3+ redox couple operates at 4.8 V versus Li + /Li 0 . The presence of phosphate groups is typically expected to stabilize the compound against oxygen loss, yet the changes in Co-O bonding upon Li extraction have not been ascertained. In addition, LiCoPO 4 is riddled with problems relating to poor transport and strain in the crystal structure of the delithiated phase, which handicap its use as a high-voltage electrode. In this work, substituting ions to generate Li 1.025 Co 0.84 Fe 0.10 Cr 0.05 Si 0.01 (PO 4 ) 1.025 is found to stabilize both the electronic structure and crystal structure and, therefore, substantially improve the ability to fully utilize the redox capacity of the material. A thorough study by spectroscopic tools, combined with computations of the electronic structure, was used to probe changes in chemical bonding. The measurements revealed the existence of redox gradients between surface and bulk that are common in other materials that react at high potential. Lastly, the study offers a comprehensive understanding of the fundamental reactions in LiCoPO 4 -type frameworks, while further demonstrating that ion substitution is an effective tool for improving their performance.
Cr and Si Substituted-LiCo0.9Fe0.1PO4: Structure, full and half Li-ion cell performance
Abstract not provided
Competitive lithium solvation of linear and cyclic carbonates from quantum chemistry
The composition of the lithium cation (Li + ) solvation shell in mixed linear and cyclic carbonate-based electrolytes has been re-examined using Born–Oppenheimer molecular dynamics and Li + (EC) n (DMC) m cluster calculations.
Combined Quantum Chemical/Raman Spectroscopic Analyses of Li+ Cation Solvation: Cyclic Carbonate Solvents - Ethylene Carbonate and Propylene Earbonate
Combined computational/Raman spectroscopic analyses of ethylene carbonate (EC) and propylene carbonate (PC) solvation interactions with lithium salts are reported. It is proposed that previously reported Raman analyses of (EC)n-LiX mixtures have utilized faulty assumptions. In the present studies, density functional theory (DFT) calculations have provided corrections in terms of both the scaling factors for the solvent's Raman band intensity variations and information about band overlap. By accounting for these factors, the solvation numbers obtained from two different EC solvent bands are in excellent agreement with one another. The same analysis for PC, however, was found to be quite challenging. Commercially available PC is a racemic mixture of (S)- and (R)-PC isomers. Based upon the quantum chemistry calculations, each of these solvent isomers may exist as multiple conformers due to a low energy barrier for ring inversion, making deconvolution of the Raman bands daunting and inherently prone to significant error. Thus, Raman spectroscopy is able to accurately determine the extent of the EC...Li+ cation solvation interactions using the provided methodology, but a similar analysis of PC...Li+ cation solvation results in a significant underestimation of the actual solvation numbers.
Structural Interactions within Lithium Salt Solvates: Cyclic Carbonates and Esters
Only limited information is available regarding the manner in which cyclic carbonate and ester solvents coordinate Li+ cations in electrolyte solutions for lithium batteries. One approach to gleaning significant insight into these interactions is to examine crystalline solvate structures. To this end, eight new solvate structures are reported with ethylene carbonate, γ-butyrolactone and γ-valerolactone: (EC)3:LiClO4, (EC)2:LiClO4, (EC)2:LiBF4, (GBL)4:LiPF6, (GBL)1:LiClO4, (GVL)1:LiClO4, (GBL)1:LiBF4 and (GBL)1:LiCF3SO3. The crystal structure of (EC)1:LiCF3SO3 is also re-reported for comparison. These structures enable the factors which govern the manner in which the ions are coordinated and the ion/solvent packing—in the solid-state—to be scrutinized in detail.
Solvate Structures and Computational/Spectroscopic Characterization of LiBF 4 Electrolytes
Crystal structures have been determined for both LiBF 4 and HBF 4 solvates—(acetonitrile) 2 :LiBF 4 , (ethylene glycol diethyl ether) 1 :LiBF 4 , (diethylene glycol diethyl ether)1:LiBF 4 , (tetrahydrofuran) 1 :LiBF4, (methyl methoxyacetate) 1 :LiBF 4 , (suc-cinonitrile) 1 :LiBF 4 , (N,N,N',N",N"-pentamethyldiethylenetriamine)1:HBF 4 , (N,N,N',N'-tetramethylethylenediamine) 3/2 :HBF 4 and (phenanthroline) 2 :HBF 4 . These, as well as other known LiBF 4 solvate structures, have been characterized by Raman vibrational spectroscopy to unambiguously assign the anion Raman band positions to specific forms of BF 4 -...Li + cation coordination. In addition, complementary DFT calculations of BF 4 -...Li + cation complexes have provided additional insight into the challenges associated with accurately interpreting the anion interactions from experimental Raman spectra. This information provides a crucial tool for the characterization of the ionic association interactions within electrolytes.
Lithium bis(2-methyllactato)borate monohydrate
The title compound {systematic name: poly[[aqualithium]-μ-3,3,8,8-tetramethyl-1,4,6,9-tetraoxa-5 4-borataspiro[4.4]- nonane-2,7-dione]}, [Li(C 8 H 12 BO 6 )(H 2 O)]n (LiBMLB), forms a 12-membered macrocycle, which lies across a crystallographic inversion center. The lithium cations are pseudotetrahedrally coordinated by three methyllactate ligands and a water molecule. The asymmetric units couple across crystallographic inversion centers, forming the 12-membered macrocycles. These macrocycles, in turn, cross-link through the Li + cations, forming an infinite polymeric structure in two dimensions parallel to (101).