Although Li-rich layered oxides (Li1+xNiyCozMn1-x-y-zO2 > 250 mAh g(-1)) are attractive electrode materials providing energy densities more than 15% higher than today's commercial Li-ion cells, they ...suffer from voltage decay on cycling. To elucidate the origin of this phenomenon, we employ chemical substitution in structurally related Li2RuO3 compounds. Li-rich layered Li2Ru1-yTiyO3 phases with capacities of ~240 mAh g(-1) exhibit the characteristic voltage decay on cycling. A combination of transmission electron microscopy and X-ray photoelectron spectroscopy studies reveals that the migration of cations between metal layers and Li layers is an intrinsic feature of the charge-discharge process that increases the trapping of metal ions in interstitial tetrahedral sites. A correlation between these trapped ions and the voltage decay is established by expanding the study to both Li2Ru1-ySnyO3 and Li2RuO3; the slowest decay occurs for the cations with the largest ionic radii. This effect is robust, and the finding provides insights into new chemistry to be explored for developing high-capacity layered electrodes that evade voltage decay.
Li-ion batteries have contributed to the commercial success of portable electronics and may soon dominate the electric transportation market provided that major scientific advances including new ...materials and concepts are developed. Classical positive electrodes for Li-ion technology operate mainly through an insertion-deinsertion redox process involving cationic species. However, this mechanism is insufficient to account for the high capacities exhibited by the new generation of Li-rich (Li(1+x)Ni(y)Co(z)Mn(1-x-y-z)O₂) layered oxides that present unusual Li reactivity. In an attempt to overcome both the inherent composition and the structural complexity of this class of oxides, we have designed structurally related Li₂Ru(1-y)Sn(y)O₃ materials that have a single redox cation and exhibit sustainable reversible capacities as high as 230 mA h g(-1). Moreover, they present good cycling behaviour with no signs of voltage decay and a small irreversible capacity. We also unambiguously show, on the basis of an arsenal of characterization techniques, that the reactivity of these high-capacity materials towards Li entails cumulative cationic (M(n+)→M((n+1)+)) and anionic (O(2-)→O₂(2-)) reversible redox processes, owing to the d-sp hybridization associated with a reductive coupling mechanism. Because Li₂MO₃ is a large family of compounds, this study opens the door to the exploration of a vast number of high-capacity materials.
Li-ion batteries have empowered consumer electronics and are now seen as the best choice to propel forward the development of eco-friendly (hybrid) electric vehicles. To enhance the energy density, ...an intensive search has been made for new polyanionic compounds that have a higher potential for the Fe²⁺/Fe³⁺ redox couple. Herein we push this potential to 3.90 V in a new polyanionic material that crystallizes in the triplite structure by substituting as little as 5 atomic per cent of Mn for Fe in Li(Fe(1-δ)Mn(δ))SO₄F. Not only is this the highest voltage reported so far for the Fe²⁺/Fe³⁺ redox couple, exceeding that of LiFePO₄ by 450 mV, but this new triplite phase is capable of reversibly releasing and reinserting 0.7-0.8 Li ions with a volume change of 0.6% (compared with 7 and 10% for LiFePO₄ and LiFeSO₄F respectively), to give a capacity of ~125 mA h g⁻¹.
The electrochemical activity of the CoP conversion electrode was investigated through the combination of computational and experimental techniques. The carbon-free CoP electrode shows better ...performances than the carbon-coated electrode, in sharp contrast with the beneficial role of carbon coating reported in many insertion materials. A two-step insertion/conversion process associated with the exchange of 3Li is predicted for this system from the T = 0 K phase stability diagram performed on bulk structures within the DFT framework. The voltage hystereses measured for these two processes through a seven-day relaxation procedure (GITT) are 1 order of magnitude higher for the conversion process (ΔV conv exp = 0.44 V) than for the insertion process (ΔV ins exp = 0.04 V). The various elementary reactions susceptible to occur at the surface of the electrode were investigated by means of surface DFT calculations. This mechanistic study shows that the insertion mechanism is not significantly affected by the electrode nanosizing (ΔV ins th = 0.04 V), while the conversion reaction does. Asymmetric responses are expected upon charge and discharge for this system, due to the growth of different interfaces. This induces different electrochemical equilibriums and then different voltages in charge and discharge. The hysteresis voltage computed for the conversion of LiCoP into Li3P + Co0 is again in very good agreement with experiments (ΔV conv th = 0.41 V). Such results are very encouraging and open new routes to the rationalization of the microscopic mechanisms acting as limiting reactions in electrode materials for Li-ion batteries.
The voltage delivered by rechargeable Lithium- and Sodium-ion batteries is a key parameter to qualify the device as promising for future applications. Here we report a new formulation of the cell ...voltage in terms of chemically intuitive quantities that can be rapidly and quantitatively evaluated from the alkaliated crystal structure with no need of first-principles calculations. The model, which is here validated on a wide series of existing cathode materials, provides new insights into the physical and chemical features of a crystal structure that influence the material potential. In particular, we show that disordered materials with cationic intermixing must exhibit higher potentials than their ordered homologues. The present method is utilizable by any solid-state chemist, is fully predictive and allows rapid assessement of material potentials, thus opening new directions for the challenging project of material design in rechargeable batteries.
We report the electrochemical study of cubic and monoclinic NiP2 polymorphs toward Li, as a candidate for anodic applications for Li-ion batteries. We found that the monoclinic form is the most ...attractive one performance-wise. Monoclinic NiP2 can reversibly uptake 5 lithium per formula unit, leading to reversible capacities of 1000 mAh/g at an average potential of 0.9 V vs Li+/Li°. From complementary X-ray diffraction (XRD) and HRTEM (high-resolution transmission electron microscopy) measurements, it was shown that, during the first discharge, the cubic phase undergoes a pure conversion process (NiP2 + 6 Li+ + 6e- → Ni° + 2Li3P) as opposed to a sequential insertion−conversion process for monoclinic NiP2. Such a different behavior rooted in subtle structural changes was explained through electronic structure calculations. Once the first discharged is achieved, both phases were shown to react with Li through a classical conversion process. More importantly, we report a novel way to design NiP2 electrodes with enhanced capacity retention and rate capabilities. It consists in growing the monoclinic NiP2 phase, through a vapor-phase transport process, on a commercial Ni-foam commonly used in Ni-based alkaline batteries. These new self-supported electrodes, based on chemically made interfaces, offer new opportunities to fully exploit the capacity gains provided by conversion reactions.
In the field of energy storage and Li-ion batteries, searching for new (positive) electrode materials with better electrochemical performances than those of transition-metal oxides is of permanent ...concern. To that aim, very simple concepts of chemical bonding can be used to find out the origin of the electrode limitations and to guide experimentalists for the design of new promising materials. This local approach was recently applied to hybrid architectures, such as metal−organic frameworks (MOFs), and allowed some of us to demonstrate the first reversible lithium insertion into the MIL53(Fe) positive electrode. In this paper, we combine first-principles density functional calculations and local chemical bond analyses to fully interpret the redox mechanism of this material. Its reactivity versus elemental lithium is investigated as a function of (i) the lithium composition from xLi/Fe = 0−1, (ii) the lithium distribution over the most probable Li sites, and (iii) the OH/F substitution ratio along the redox chains. The results show that the MIL53(Fe) is a weak antiferromagnet at T = 0 K with iron ions in the high-spin state (Fe3+, S = 5/2). It reacts with lithium through a two-step insertion/conversion mechanism. The insertion reaction is perfectly reversible and proceeds in two steps: first, a single-phase reaction whose capacity increases linearly with the fluorine content in the starting material, then a two-phase reaction that ends around xLi/Fe = 0.5 due to the stabilization of a localized Fe2+/Fe3+ mixed-valence state along the inorganic chains. Further lithium insertion into Li0.5MIL53(Fe) is shown to provoke an irreversible conversion reaction due to a complete loss of the local interactions between the inorganic and organic networks of the MOF architecture. On the basis of this interpretation, several alternatives to improve the capacity of these materials can be proposed by means of appropriate ligand functionalization and/or use of electrochemically active molecules within the large open space occurring in such porous materials.
FeP y (y = 1, 2, 4) anodes all react with lithium through a conversion reaction FeP y + 3yLi → yLi3P + Fe0 in their first discharge, leading to nanocomposite discharged electrodes described by ...nanosized Fe0 particles embedded in yLi3P matrixes. From electrochemical and complementary in situ X-ray diffraction and high-resolution transmission electron microscopy studies, we deduce that the conversion reaction occurring during the first discharge is followed by two successive insertion and conversion processes in further cycles for the FeP electrode. The insertion process is highly reversible, leading to a capacity retention of 300 mA h g-1 and 1900 mA h cm-3 after 100 cycles, and corresponds to the formation of an intermediate tetragonal LiFeP phase as deduced from first-principles T = 0 K phase diagram calculations and preliminary Mössbauer analyses. We expect the kinetics of this reaction to be strongly limited by the increase in y, thus leading to an increasing capacity fading when increasing the y P/Fe ratio.
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