Surface Structural Disordering in Graphite upon Lithium Intercalation/Deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in LixC (0 \leq x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li+). Steep Li+ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the LixC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.Comment: 12 pages, 5 figure

An electrochemical investigation of oxygen adsorption on Pt single crystal electrodes in a non-aqueous Li+ electrolyte

Cyclic voltammetry has been used to probe the initial stages of oxygen reduction and oxidation in lithium-containing dimethyl sulfoxide at well-defined Pt single crystal electrodes in order to elucidate any catalytic effects ascribable to surface structure. In contrast to previous work involving sodium-oxygen, lithium-oxygen studies did not yield any significant differences for reaction on the three basal planes of platinum. Rather, all three planes generated a similar voltammetric response. However, by judicious use of various potential sweep limits, the formation of superoxide together with both a “conformal” or surface adlayer of lithium peroxide (Li2O2) together with a “microcrystallite” surface Li2O2 phase was resolved. Voltammetric peak intensity versus sweep rate measurements confirmed that superoxide electrooxidation was diffusion limited whereas electrooxidation of the two Li2O2 phases displayed behaviour typical of a surface-confined process. Under steady-state conditions for the formation of superoxide, it was found that for both the conformal and microcrystallite Li2O2 phases, electrooxidation followed zero-order kinetics, pointing to the importance of free surface sites in facilitating these reactions. A marked change in the rate of Li2O2 formation was found to coincide with a coverage of 0.25 monolayers, as measured by the charge density of the conformal Li2O2 electrooxidation peak. We postulate that electron tunnelling through both the conformal Li2O2 layer and microcrystallites deposited on this surface layer coincides with this coverage and accounts for such behaviour. This phenomenon of electron tunnelling through single conformal and mixed conformal/microcrystallite structures should prove vitally important in governing the overall electrooxidation rate

Time-resolved SERS study of the oxygen reduction reaction in ionic liquid electrolytes for non-aqueous lithium-oxygen cells

We use the Raman active bands of O2˙− to probe its changing Lewis basicity through its interaction with various ionic liquid electrolytes at the electrode surface.</p

Growth and dissolution of NaO2 in an ether-based electrolyte as the discharge product in the Na-O-2 cell

The deposition and dissolution of sodium superoxide (NaO2) was investigated by atomic force microscopy.</p

Localised degradation within sulfide-based all-solid-state electrodes visualised by Raman mapping

The distribution of degradation products, before and after cycling, within common sulfide-based solid electrolytes (β-Li$_3$PS$_4$, Li$_6$PS$_5$Cl and Li$_{10}$GeP$_2$S$_{12}$) was mapped using Raman microscopy. All composite electrodes displayed the appearance of side reaction products after the initial charge-discharge cycle, located at the site of a LiNi$_{0.6}$Mn$_{0.2}$Co$_{0.2}$O$_2$ particle

Institutions and economic policies for pro-poor agricultural growth

"This paper draws together findings from different elements of a research project examining critical components of pro-poor agricultural growth and of policies that can promote such growth in poor rural economies in South Asia and Sub-Saharan Africa. Agricultural growth, a critical driver in poverty reducing growth in many poor agrarian economies in the past, faces many difficulties in today's poor rural areas in South Asia and Sub-Saharan Africa. Some of these difficulties are endogenous to these areas while others result from broader processes of global change. Active state interventions in 'kick starting' markets in 20th century green revolutions suggest that another major difficulty may be current policies which emphasize the benefits of liberalization and state withdrawal but fail to address critical institutional constraints to market and economic development in poor rural areas. This broad hypothesis was tested in an analysis of the returns (in agricultural growth and poverty reduction) to different government spending in India over the last forty years. The results reject the alternate hypothesis underlying much current policy, that fertilizer and credit subsidies, for example, depressed agricultural growth and poverty reduction in the early stages of agricultural transformation. The results show initially high but then declining impacts from fertilizer subsidies; high benefits from investment in roads, education and agricultural R&D during all periods and varying benefits from credit subsidies over four decades; low impacts from power subsidies; and intermediate impacts from irrigation investments. These findings demand a fundamental reassessment of policies espousing state withdrawal from markets in poor agrarian economies. Given widespread state failure in many poor agrarian economies today, particularly in Africa, new thinking is urgently needed to find alternative ways of 'kick starting' markets ways which reduce rent seeking opportunities, promote rather than crowd-out private sector investment, and allow the state to withdraw as economic growth proceeds. Authors' AbstractAgricultural growth ,Poverty, Rural ,South Asia Rural poor ,Africa, Sub-Saharan ,Agrarian economies ,Globalization ,Green Revolution ,Poverty alleviation ,Government spending policy India ,

Approach to Wide-Frequency Battery Impedance Measurements in Commercial Applications

Electrochemical Impedance Spectroscopy (EIS) is a well-established diagnostic technique for the characterization and monitoring of electrochemical devices, such as batteries. As different electrochemical processes have very different dynamics, a complete characterization of batteries requires measuring their impedance over a very wide frequency range, from less than 1 mHz to more than 1 kHz. However, significant measurement challenges affect the extreme frequencies and may limit the practically achievable frequency range, particularly in case of in situ EIS. This paper analyzes the main measurement challenges that appear at low and high frequencies, respectively, and proposes possible strategies, suitable to be used in commercial applications. A compensation for the effect of the state-of-charge variation is proposed to improve the accuracy of EIS at low frequencies, while a strategy to measure high-frequency perturbations with a lower sampling frequency is presented to achieve high-frequency EIS with affordable instrumentation. Experimental results obtained on a lithium iron phosphate cell are reported, used to validate the proposed methods